Industrial Photoinitiators · G.A. Rist et al, Macromolecules 1992, 25, 4182. O Cl Cl Cl Important...
Transcript of Industrial Photoinitiators · G.A. Rist et al, Macromolecules 1992, 25, 4182. O Cl Cl Cl Important...
Industrial Photoinitiators
The most important radical photoinitiators applications and relevant topics
Jean-Luc Birbaum
Consortium Meeting Mainz 15102019
Prologue
J-L Birbaum 2
ldquoLightacts chemically on substances [hellip]
It is absorbed it combines with themhellip it even solidifies
them and makes them more or less insoluble according to
the duration or the intensity of its action This is in short the
principle of my discoveryrdquo
JN Nieacutepce Notice sur lrsquoHeacuteliographie
1829
The same year the German chemist August Kekuleacute is born
He will identify the chemical structure of benzene in 36 years
Photopolymerization vision of a
new technology
The invention of photolithography
J-L Birbaum 3
The worldrsquos oldest lsquophotocopyrsquo
obtained by Nieacutepce in 1825
Bought for euro 450rsquo000 by the French National
Library in 2002
Sunlight
Bitumen of Judea the first photoresist
Copper plate
Exposure
through mask
Development
by lavender oil
Etching
by acid
crosslinked
bitumen
Modern restart of radiation curing
4J-L Birbaum
After 100 years Sleeping Beauty woke up
in the 1940rsquos with the first photoinitiator patents
Illustration by G Doreacute (1867)
US 2rsquo406rsquo878 (1946) Interchemical Co
first UV-curable printing ink
US 2rsquo423rsquo520 (1947) DuPont
solution photopolymerization of styrene MMA etc
Attractive features of radiation curing
5J-L Birbaum
Time
Vis
cosity
Thermally curable formulation
shelf life few days at RT
UV-curable formulation
several months in the dark
UV light
Long shelf life in the dark
No light no viscosity increase
Fast cure after UV irradiation
Spatial control by imaging
through mask
No light no curing
Cure on demand control of time and space
Photoresist for printed circuit
boards
Added bonus no VOC
Contents
6
1 Basic photochemistry
Types of photoinitiators
Excited states and the generation of active radicals
2 Most important radical initiators
Spectral characteristics pros and cons uses
3 Selected topics of interest
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration emissions and the law
Water soluble photoinitiators
4 Conclusion
J-L Birbaum
times Cationic photoinitiators
times Photolatent bases
times Radical specialties for microelectronics
Out of scopeOslash
7J-L Birbaum
Basic photochemistry
1
Two main types of photoinitiators
Excited states and the generation of active radicals
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Prologue
J-L Birbaum 2
ldquoLightacts chemically on substances [hellip]
It is absorbed it combines with themhellip it even solidifies
them and makes them more or less insoluble according to
the duration or the intensity of its action This is in short the
principle of my discoveryrdquo
JN Nieacutepce Notice sur lrsquoHeacuteliographie
1829
The same year the German chemist August Kekuleacute is born
He will identify the chemical structure of benzene in 36 years
Photopolymerization vision of a
new technology
The invention of photolithography
J-L Birbaum 3
The worldrsquos oldest lsquophotocopyrsquo
obtained by Nieacutepce in 1825
Bought for euro 450rsquo000 by the French National
Library in 2002
Sunlight
Bitumen of Judea the first photoresist
Copper plate
Exposure
through mask
Development
by lavender oil
Etching
by acid
crosslinked
bitumen
Modern restart of radiation curing
4J-L Birbaum
After 100 years Sleeping Beauty woke up
in the 1940rsquos with the first photoinitiator patents
Illustration by G Doreacute (1867)
US 2rsquo406rsquo878 (1946) Interchemical Co
first UV-curable printing ink
US 2rsquo423rsquo520 (1947) DuPont
solution photopolymerization of styrene MMA etc
Attractive features of radiation curing
5J-L Birbaum
Time
Vis
cosity
Thermally curable formulation
shelf life few days at RT
UV-curable formulation
several months in the dark
UV light
Long shelf life in the dark
No light no viscosity increase
Fast cure after UV irradiation
Spatial control by imaging
through mask
No light no curing
Cure on demand control of time and space
Photoresist for printed circuit
boards
Added bonus no VOC
Contents
6
1 Basic photochemistry
Types of photoinitiators
Excited states and the generation of active radicals
2 Most important radical initiators
Spectral characteristics pros and cons uses
3 Selected topics of interest
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration emissions and the law
Water soluble photoinitiators
4 Conclusion
J-L Birbaum
times Cationic photoinitiators
times Photolatent bases
times Radical specialties for microelectronics
Out of scopeOslash
7J-L Birbaum
Basic photochemistry
1
Two main types of photoinitiators
Excited states and the generation of active radicals
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
The invention of photolithography
J-L Birbaum 3
The worldrsquos oldest lsquophotocopyrsquo
obtained by Nieacutepce in 1825
Bought for euro 450rsquo000 by the French National
Library in 2002
Sunlight
Bitumen of Judea the first photoresist
Copper plate
Exposure
through mask
Development
by lavender oil
Etching
by acid
crosslinked
bitumen
Modern restart of radiation curing
4J-L Birbaum
After 100 years Sleeping Beauty woke up
in the 1940rsquos with the first photoinitiator patents
Illustration by G Doreacute (1867)
US 2rsquo406rsquo878 (1946) Interchemical Co
first UV-curable printing ink
US 2rsquo423rsquo520 (1947) DuPont
solution photopolymerization of styrene MMA etc
Attractive features of radiation curing
5J-L Birbaum
Time
Vis
cosity
Thermally curable formulation
shelf life few days at RT
UV-curable formulation
several months in the dark
UV light
Long shelf life in the dark
No light no viscosity increase
Fast cure after UV irradiation
Spatial control by imaging
through mask
No light no curing
Cure on demand control of time and space
Photoresist for printed circuit
boards
Added bonus no VOC
Contents
6
1 Basic photochemistry
Types of photoinitiators
Excited states and the generation of active radicals
2 Most important radical initiators
Spectral characteristics pros and cons uses
3 Selected topics of interest
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration emissions and the law
Water soluble photoinitiators
4 Conclusion
J-L Birbaum
times Cationic photoinitiators
times Photolatent bases
times Radical specialties for microelectronics
Out of scopeOslash
7J-L Birbaum
Basic photochemistry
1
Two main types of photoinitiators
Excited states and the generation of active radicals
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Modern restart of radiation curing
4J-L Birbaum
After 100 years Sleeping Beauty woke up
in the 1940rsquos with the first photoinitiator patents
Illustration by G Doreacute (1867)
US 2rsquo406rsquo878 (1946) Interchemical Co
first UV-curable printing ink
US 2rsquo423rsquo520 (1947) DuPont
solution photopolymerization of styrene MMA etc
Attractive features of radiation curing
5J-L Birbaum
Time
Vis
cosity
Thermally curable formulation
shelf life few days at RT
UV-curable formulation
several months in the dark
UV light
Long shelf life in the dark
No light no viscosity increase
Fast cure after UV irradiation
Spatial control by imaging
through mask
No light no curing
Cure on demand control of time and space
Photoresist for printed circuit
boards
Added bonus no VOC
Contents
6
1 Basic photochemistry
Types of photoinitiators
Excited states and the generation of active radicals
2 Most important radical initiators
Spectral characteristics pros and cons uses
3 Selected topics of interest
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration emissions and the law
Water soluble photoinitiators
4 Conclusion
J-L Birbaum
times Cationic photoinitiators
times Photolatent bases
times Radical specialties for microelectronics
Out of scopeOslash
7J-L Birbaum
Basic photochemistry
1
Two main types of photoinitiators
Excited states and the generation of active radicals
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Attractive features of radiation curing
5J-L Birbaum
Time
Vis
cosity
Thermally curable formulation
shelf life few days at RT
UV-curable formulation
several months in the dark
UV light
Long shelf life in the dark
No light no viscosity increase
Fast cure after UV irradiation
Spatial control by imaging
through mask
No light no curing
Cure on demand control of time and space
Photoresist for printed circuit
boards
Added bonus no VOC
Contents
6
1 Basic photochemistry
Types of photoinitiators
Excited states and the generation of active radicals
2 Most important radical initiators
Spectral characteristics pros and cons uses
3 Selected topics of interest
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration emissions and the law
Water soluble photoinitiators
4 Conclusion
J-L Birbaum
times Cationic photoinitiators
times Photolatent bases
times Radical specialties for microelectronics
Out of scopeOslash
7J-L Birbaum
Basic photochemistry
1
Two main types of photoinitiators
Excited states and the generation of active radicals
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Contents
6
1 Basic photochemistry
Types of photoinitiators
Excited states and the generation of active radicals
2 Most important radical initiators
Spectral characteristics pros and cons uses
3 Selected topics of interest
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration emissions and the law
Water soluble photoinitiators
4 Conclusion
J-L Birbaum
times Cationic photoinitiators
times Photolatent bases
times Radical specialties for microelectronics
Out of scopeOslash
7J-L Birbaum
Basic photochemistry
1
Two main types of photoinitiators
Excited states and the generation of active radicals
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
7J-L Birbaum
Basic photochemistry
1
Two main types of photoinitiators
Excited states and the generation of active radicals
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Basic Photochemistry type I
8J-L Birbaum
RC
OC
O
RRC
O
+
T
excited photoinitiator
UV light
2 initiating free radicals
Type I = Cleavage type unimolecular bond cleavage upon
irradiation
Main mechanism is cleavage of a-bond (next to C=O)
Minor mechanism is cleavage of b-bond
S C
O
N O
S C
O
C N O
S C O
C N O
+
GA Rist et al Macromolecules 1992 25 4182
O
Cl
Cl
Cl
Important for
a-haloketones
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Fast photocleavage from the triplet state
9J-L Birbaum
IC Internal Conversion (non radiative)
ISC Inter-System Crossing
Flu
ore
sce
nce
Energy
kISC
ISCkIC
S0
S1
T1Initiating radicals via
C-C bond cleavagekcleavage
The same triplet state T1 (np or pp) is populated and the same triplet photochemistry
takes place independently of the excitation wavelength
The rate constants kisc and kcleavage are very fast and compete efficiently with fluorescence
phosphorescence and radiationless deactivation in typical photoinitiators
Example for BAPO kisc = 8 times109 s-1 and kcleavage = 1010 s-1 (J Am Chem Soc 2002 124 14952)
Polymer
reaction with O2
disproportionation
recombination
rearrangement
Quenching(by O2 or monomer)
By-products
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Basic Photochemistry type II
10J-L Birbaum
Type II = Abstraction type bimolecular reaction of excited
photoinitiator with coinitiator needed for radical generation
Two mechanisms
With tertiary amines electron transfer followed by hydrogen transfer
With other H-donors direct hydrogen transfer
1 initiating
free radical
inactive
charge transfer
exciplex
H-
transferUV light
inactive
(dimerizes)
electron
transfer
no cleavage
C
O
OC
N
H
R
R
R
COH N
R
R
R
N
H
R
R
R
CO
T
+ +
T
C O H
Donorketyl
radical +
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Special case of type IIPhenylglyoxylates
11J-L Birbaum
C
O
C
O
O
H
C
O
C
O
O CH3
O
H
C
OH
C
O
O CH3
O
- CH2O
- CO
C
O
C
O
O CH3
C
OH
C
O
O CH3
C
O
C
O
O CH2
C
C
O
OHCH
2O
T
C
O
C
O
O
CH2
H
C
O
h
H-donor(eg ether acrylate)
+
dimerization
+
+
initiation
dimerization initiation
initiation
(at high initiator conc)
-Habstraction
Some benzoyl radicals formed
but not by direct a-cleavage
DCNeckers et al Macromolecules 2000 33 4030
Norrish II
Darocur MBF
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
The efficiency of a photoinitiator is the result of a chain of events
12J-L Birbaum
A high absorption in the near UV (good match with the emission spectrum of the light
source is desirable
For a given chromophore substituent effects (electron donating or withdrawing) can tune
the absorption spectrum of the photoinitiator
However red-shifting can affect the spectroscopic nature of the triplet state (for instance
from np to pp) and reduce the efficiency of photocleavage
Efficiency of
light absorption
Overall
efficiency of
photoinitiator
Efficiency of
radical generation
Reactivity of
generated radical=
Absorbance
extinction molar
coefficient e at l
Quantum yield of triplet
formation and photocleavage
fISC fcleavage
Addition constant to
monomer kA
Not independent from each other
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
A red-shifted absorption is not always beneficial
13J-L Birbaum
photoinitiatorlmax
(nm)t T (ns) F cleavage
triplet
naturekadd (M-1∙ s -1)n-butylacrylate
Darocur 1173
243 037 038 np 27∙105
Darocur 2959
273 12 029 nppp 35∙105
302 7rsquo000 001 pp 55∙105
335 12rsquo000 003 pp(41∙105)4-morpholino
S Jokusch F Landis N Turro Macromolecules 2001 34 1619
C S Colley et al J Am Chem Soc 2002 124 14952
13∙107
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
14J-L Birbaum
Most important radical
photoinitiators
2
Spectral characteristics pros and cons uses
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Absorption range of main classes of photoinitiators
15J-L Birbaum
Absorbance
c = 01 in acetonitrile
0
02
04
06
08
1
12
14
16
300 350 400 450 500 550Wavelength (nm)
a-Aminoketones
Acylphosphine oxides
Titanocenes and specialties
a-Hydroxyketones
Clear formulations and surface cure
Pigmented formulations
Pigmented and white formulations
Type II Initiators(benzophenones and thioxanthones)
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Spectral overview of commercial Type I products
16J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
c = 0001 in acetonitrile
C
O
OH
OHO C
O
O H
S C
O
N O
N C
O
NO
C
O
P
O
C
O
BAPO
Irgacure 369
Irgacure 907
Daroccur 2959
Daroccur 1173
c = 001
N
F
F
N
F
F
Ti
Irgacure 784
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Main photoinitiatorsfor clear coats and surface cure (1)a-Hydroxyketones phenylglyoxylate benzilketal
17J-L Birbaum
Compound Darocur 1173 Irgacure 184 Irgacure 2959 Darocur MBF Irgacure 651
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 7473-98-5 947-19-3 106797-53-9 15206-55-0 24650-42-8
Properties liquid good solvent
properties good for
blends with other
photoinitiators
efficient surface cure
Volatile
low yellowing
medium volatility
highly efficient
surface cure
low yellowing low
odor low volatility
low migration FDA
approval for indirect
food contact
(adhesives)
(type II initiator)
liquid good surface
cure low emission
low residual odor
low yellowing after
cure
extremely short tT
yellowing after
curing
Applications clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal and
plastics co-initiator
in pigmented
systems
clear top-coats for
paper metal
plastics also in
waterbased
UV dispersions
floor coatings wood
parquets furniture
plastic metal
coatings
flexo printing
plates fillers and
topcoats based on
UPESstyrene
C
O
OHOH
O C
O
O H
OOH
O
O
OOO
O
200 250 300 350 400 200 250 300 350 4000
05
1
15
200 250 300 350 400 200 250 300 350 400 200 250 300 350 400
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Main photoinitiatorsfor clear coats and surface cure (2)Dimeric a-Hydroxyketones phenylglyoxylate
18J-L Birbaum
Compound Irgacure 127 Esacure ONE Esacure KIP 160 Irgacure 754
Structure
+30 monoester
Spectrum
005
0001
in CH3CN
CAS Regno 474510-57-1 163702-01-0 71868-15-0211510-16-6
442536-99-4
Properties high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
after curing
high reactivity low
sensitivity to
oxygen inhibition
low
emissionmigration
after curing
high reactivity low
sensitivity to oxygen
inhibition low
emissionmigration
can show some
yellowing after cure
very good surface
cure low emission
low residual odor
lowest yellowing
after cure
Applications pigmented UV-inks
thin clear lacquers
food packaging
thin clear lacquers
food packaging
pigmented UV-inks
thin clear lacquers
food packaging
clear coats PVC
floors wood
parquetsfurniture
O
OH
O
OH
0
05
1
15
200 250 300 350 400
C
O
OH
C
O
OH
200 250 300 350 400 200 250 300 350 400
O
OH
O
OH
O
O
O
O
O
O
O
O
200 250 300 350 400
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Main photoinitiators for pigmented systemsa-Aminoketones
19J-L Birbaum
Compound Irgacure Omnirad 907 Irgacure Omnirad 369 Irgacure Omnirad 379
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 71868-10-5 119313-12-1 119344-86-4
Properties White to light beige powder
rel high volatility
toxicity labeling H360
light yellow powder limited
solubility
Good through-cure of
pigmented formulations
light yellow powder much
better solubility than lsquo369rsquo
Good through-cure of
pigmented formulations
Applications for pigmented systems
photoresists (printed circuit
boards)
for pigmented systems
photoresists and printing
plates
for pigmented systems
photoresists and printing
plates
N C
O
NO N C
O
NO
S C
O
N O
200 250 300 350 400 450200 250 300 350 400 4500
05
1
15
200 250 300 350 400 450
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Main photoinitiatorsfor white and pigmented systems Acylphosphinoxides (MAPO and BAPO)
20J-L Birbaum
Compound Lucirin Omnirad TPO Lucirin Omnirad TPO-L Irgacure Omnirad 819
Structure
Spectrum
005
0001
in CH3CN
CAS Regno 75980-60-8 84434-11-7 162881-26-7
Properties low yellowing low odor
good solubility lower through-
cure performance thanrsquo819rsquo
bleaching high O2 sensitivity
liquid easy to incorporate
low yellowing bleaching
good through-cure high O2
sensitivity
low odor low volatility
photosensitivity at longer l
bleaching excellent through-
cure high O2 sensitivity
Applications Pigmented systems white
systems fibre-reinforced
coatings LED curing
White systems LED curing Pigmented systems white
systems fibre-reinforced
coatings LED curing
PC
OOPO
OO
P
O
C
O
C
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Main Type II PhotoinitiatorsBenzophenones thioxanthones etc
21J-L Birbaum
Compound Benzophenone Camphorquinone ITX CPTX Esacure 1001
Structure
Spectrum
005
0001
in CH3CN(MeOH for
CPTX)
CAS Regno 119-61-9 10373-78-1 5495-84-1 142770-42-1 272460-97-6
Properties very good surface
cure
Banned from food
packaging by Nestleacute
Absorbs in the
visible range direct
abstraction of H
from amines
Through-cure
sensitizer for Type I
PI Banned from
food pack by Nestleacute
Absorption into the
visible Sensitizer
Cl radicals formed
by photocleavage
Mixed Type II and I
(b-cleavage)
Through-cure and
surface cure
Applications General low cost in
combination with
amine synergists
Dental applications Printing inks resists
LED cure
cationics banned
from food packaging
Photoresists
cationic formulations
Pigmented
systems
O
O
O
0
05
1
15
200 250 300 350 400 450 200 250 300 350 400 450 200 250 300 350 400 450
S
O
S
OCl
O S
O
S
O
O
O
200 250 300 350 400 450
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
22J-L Birbaum
Selected topics
3
Absorption vs penetration surface cure and through-cure
laquoBleachingraquo photoinitiators conventional lamps vs LED
Photocleavage products migration polymeric photoinitiators
Water solubility of photoinitiators
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Light absorption in solutionLambert-Beer law
23J-L Birbaum
c e
II0
lI = I0 10-e c l
Absorbance A = e c l = -log II0
Transmission T = I I0
A = -log T
Absorbed light = Ia = I0 ndash I = I0 middot(1-10-e c l)
e depends on the wavelength e = e(l)
e(l) is characteristic of the electronic and optical properties
of the chromophore
Assumptions no reflection no scattering diluted solution
e molar extinction coefficient of the
dissolved substance (L mol-1 cm-1)
c concentration of the solution (mol L)
l path length (cm)
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Absorption spectrum of Irgacure 369 and overlap with emission of Hg lamp
24J-L Birbaum
0
02
04
06
08
1
200 250 300 350 400 450 500
Irgacure 369
N C
O
N
CH3
CH3
CH3
O
313 nm 366 nm
e (313nm) = 19900
e (366nm) = 1100
A
wavelength (nm)
c = 0001 = 273 x 10-5 molL
l = 1 cm
emission spectrum of medium pressure Hg lamp (arbitrary units)
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Through-cure and surface cure with a single photoinitiator at two wavelengths
25J-L Birbaum
The photoinitiator has large differences of e at main wavelengths of Hg lamp
Since light penetration varies with wavelength production of radicals also varies with depth
Concentration of radicals ~ fRmiddotIa
At 313nm good surface cure
At the same time good throughcureat 366 nm
In practice a mixture of several photoinitiators may be superior to a single one at the same overall loading 0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
e (313nm) = 19900
e (366nm) = 1100
Ra
dic
al co
nce
ntr
ation
(a
rbitra
ry u
nits)
2 Irgacure 369
0
02
04
06
08
1
200 250 300 350 400 450 500
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
0 20 40 60 80 100
Effect of concentrationon light penetration at single wavelength
26J-L Birbaum
0
50
mm
25
0
50
mm
25
0
50
mm
25
2
02
1
Example Irgacure 369 at 313 nm in a 50 mm clear coat
conc
Good surface cure no bottom cure1 absorbed
by bottom half
68 absorbed
by top 5 mm
total absorption
682
11 absorbed
by top 5 mm
5 absorbed
by bottom half
44 absorbed
by top 5 mm
Fair surface cure poor bottom cure
Poor surface cure good bottom cure
Relative absorption ()
Assuming no competitive absorption from formulation and e = 19900 at 313 nm
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
The challenge of through-cure under LED light Bleaching photoinitiators
27J-L Birbaum
0
20
40
60
80
100
5 10 15 20 25 30 35 40 45 50
coating depth (mm)
313 nm
366 nm
Rad
ical co
nce
ntr
ation 2 Irgacure 369
medium pressure Hg lamp
0
02
04
06
08
1
200 250 300 350 400 450 500
LED UV-A Nichia NSHU550E
Emax 375 nm
wavelength (nm)
Irg 369
A good balance of surface cure and through-cure
requires in principle differential absorbance at 2
wavelengths (polychromatic light)
LED have a narrow emission spectrum therefore
little differential absorbance over the range
overlapping with the photoinitiator sensitivity
Option I use a combination of LED of 2 different wavelengths (eg 365 and 395 nm)
Option II use a bleaching photoinitiator
The chromophore of a bleaching photoinitiator is destroyed during radical generation
this results in an absorbance loss and allows light penetration in the deeper layers
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Bleaching photoinitiatorsBAPO under irradiation
28J-L Birbaum
0
02
04
06
08
325 350 375 400 425 450
Wavelength (nm)
Ab
so
rban
ce
CM
O
P MM
O
PM C
OO
O
CO R
C M
O
C
O
P C
OO
O
CO R
h
m
h
n n o p+ +
( m + n ) ( o + p )
The chromophore of a bleaching
photoinitiator is destroyed during
radical generation this results in
a decrease of absorbance with
time and allows light penetration
in the deeper layers
Curing in the presence of white
colored pigments or UV absorber
Curing of glass fiber reinforced
coatings and thick layers
Wavelength-dependent photo-
chemistry allows synthesis of
diblock copolymers
h lt410nm
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Use of bleaching photoinitiators in pigmented formulations
29J-L Birbaum
0
02
04
06
08
1
12
240 280 320 360 400 440 480
0001
TiO2
005
Ab
so
rban
ce
Irgacure 819
Lucirin TPO
In pigmented formulations (especially white) most of the UV light is
absorbed by the pigment The only light available is in the visible
range
However photoinitiators absorbing blue light are necessary yellow
The solution is to use a yellow photoinitiator that will bleach after
doing the job
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Present options for LED curing
30J-L Birbaum
Acylphosphine oxidesPros Cons
PC
OO
C
O
P
O
C
O
PO
OO
Thioxanthones
Excellent through-cure
Good spectral overlap
with 385 and 395 nm
LED
Photobleaching low YI
Slow surface cure
High oxygen inhibition
S
O
S
O
S
O
O
Cl
TPO TPO-L BAPO
ITX DETX CPTX Acceptable surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Ban in food packaging
O
NN
Benzophenones
O
NN
Michlerrsquos ketone EMK Excellent surface cure
Good spectral overlap
with 385 and 395 nm
LED
Limited through-cure
No photobleaching
high YI
Human carcinogens
Typical surface cure photoinitiators a-hydroxyketones have poor spectral overlap with LED
Poor surface cure not only impacts mechanical and chemical resistance but also causes migration
and odor problems due to incomplete crosslinking
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Other bleaching chromophoressuitable for LED curing
31J-L Birbaum
SP h C
O
R
N O
OP h
SP h C
O
R
N O P h
O
SP h C O
Oxime esters (Irgacure OXE01)
Commercially available only for electronic applications but several patents
filed by Agfa for LED UV printing (captive use)
J Finter et al Makromol Chem
Makromol Symp 1989 24177
+ CO2 +RCN + Ph
Titanocenes (Irgacure 784)
Visible photoinitiator must be
handled under red light
N
F
F
N
F
F
Ti
Arylglyoxylates (MBF Irgacure 754)
Bleaching expected but not well investigated
Several patent applications on derivatives
suitable for LED curing
O
O
OR
R
C O
R
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Neutral by-products of photocleavage cause unwanted emissions
32J-L Birbaum
Example Irgacure 184
polymerization
OOH
C O
OR
O
OR
O
O
OR
O
OR
O
OR
O
OR
OOH
OR
OOH
OOR
O
H
O
C
C
C
C
OR
O
O
ORO
OH
C
polymerization
volatility of unreacted initiator
Benzaldehyde
Cyclohexanone
Desired chemistry Main by-products
High mw (unproblematic)
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Emission migration and set-off
33J-L Birbaum
Mobile
soluble
neutral
breakdown
products from
photoinitiator
Volatile
Set-off
Transfer by contact
Emission Release into air
Printing amp Packaging
photoinitiator Migration Contamination
of food by diffusion through
substrate
Furniture amp Flooring
Food
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Legislation pressure on photoinitiators for sensitive applications is growing
34J-L Birbaum
Emissions of some photocleavage products are regulated
France Regulation 2011-321
(introduced 01012012)
Limit values for 10 VOC released
by construction materials
Germany Introduction of LCI (Lowest
Concentration of Interest) as the
maximum permissible emission of a
given volatile organic compound (2010)
In addition IKEA guidelines forthcoming EU legislation etc hellip
Migration (Printed substrates)
EU Commission Regulation 102011Only food packaging legislation in Europe with a detailed description of migration test conditions
how to determine OML and SML values In principle only for plastics
Swiss Ordinance SR 81702321First legislation in Europe regulating ingredients of food packaging inks as non-plastic food
contact materials Inventory list (appendix 10) consisting of part A (complete risk assessment
data available) and part B (incomplete data migration limit 001 mgkg)
Exclusion list based on Swiss OrdinanceEuropean Printing Association
Positive list based on Swiss Ordinance
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Why waterborne formulations
35J-L Birbaum
In dispersions the viscosity is independent of the molecular weight of
the resin only governed by particle size and particle concentration
Thinning with water lowers viscosity enough for spraying applications
and UV curing removes the drawbacks of traditional dispersions
poor chemical and blocking resistance
Applying thin low-weight coatings onto metal or plastics by spraying
or dipping become possible
But water must be removed from coatings prior to UV cure and
therefore photoinitiators must be non steam-volatile
So does adding an extra drying step makes sense
mw
viscosity
polymer solution
dispersion
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Industrial applications of waterborne formulations
36J-L Birbaum
Wood coatings first market to adopt waterborne PUD
‒ laquo100 solidsraquo UV formulations can soak into the wood Uncured components
can migrate during lifetime of substrate
‒ Mat varnishes work best with some volumetric shrinkage
Coating with matting agent
Dried and cured good matting
Evaporation
(H2O)
only slight shrinkage more gloss
UVdrying
+ UV
Metal and plastic coatings applied by spraying or dipping
‒ Viscosity of 100 systems too high
Digital water-based UV inkjet
‒ Viscosity very important But water-soluble photoinitiators for pigmented inks
are needed
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Comparison of photoinitiators in aqueous emulsions and dispersions
37J-L Birbaum
UV cure (Hg lamp) after drying at 80degC The uncured films (20 mm) are clear
Higher conversions from lsquoemulsionsrsquo due to greater molecular mobility in the soft low-
modulus polymer formed lsquoDispersionsrsquo yield harder polymers with lower conversion
C Decker and I Lorinczova JCT Research 2004 1 247
D-1173 Esa KIP I-2959 TPO-L I-819DW
D-1 70 80 84 68 68
D-2 49 52 52 50 56
D-3 32 42 40 45 40
E-1 100 99 100 100 98
E-2 78 73 81 77 79
Acrylate conversion () after one pass 5mmin
Dis
pe
rsio
ns
Em
uls
ions
D-1 Laromer 8949
D-2 Laromer 8983
D-3 Laromer 9005
E-1 Laromer PE55W
E-2 Laromer PE-22WN
Esacure KIPEM-1
water-dispersed
[photoinitiator] = 1
D-1
-1
RTIR profiles for UV curing of water-based
acrylate resins at 80degC
[ICU 819 DW] = 1 wt I = 200 mW cmndash2
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Water-based formulations and the solubility of photoinitiators
38J-L Birbaum
Irgacure 2959 has the highest solubility among usual photoinitiators
but still somewhat limited to ca 1
Low mw oil soluble photoinitiators can be incorporated using high
speed stirring Liquids respond much better than solids Omnirad 1000 (mixture of D-1173 and ICU-184
Speedcure BEM (mixture BP and MBP)
Irgacure 754 Darocur MBF
Irgacure 2100 (Mix TPO-LBAPO)
Examples
But water-soluble (~ 5) photoinitiators would be preferred
especially for UV inkjet where water-dispersed photoinitiators
are not adequate
S
N+
OH
O
Cl
Type II initiator solubility ca 20 (Great Lakes)
formerly commercial but not successful
Quantacure QTX
There is a need for mid-UV high speed photoinitiators that are
compatible with WB formulations
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Coming soon Water soluble BAPO derivatives
39J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P C
OO
CO
O
O
N a+
h
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Coming soon Water soluble BAPO derivatives
40J-L Birbaum
Chemistry developed at Ciba BASF ETH
Will it be commercialized by IGM
Polymerization of styrene in a continuous flow reactor snowballing
radical generation leads to ultrahigh mw polymers
C P C
OO
CO
O H
O
C
O
P
O
C
O
OH
C P
O
C
OOO
N a+
BAPO-AA BAPO-POH BAPO-PONa
acceptable solubility in H2O high solubility in H2O
C P
O
CO
O
O
styrene
h
styrene
styrene
P Laurino et al Macromol Rapid Commun 2012 1770
head to head
P
O
O
O
n
C O
+
P
O
O
O
O
n
n
+P
O
O
O
On
m
n
P
O
O
O
m
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
New photochemistry of BAPO derivatives in aqueous media
41J-L Birbaum M Schmallegger et al Chem Eur J 2019 25 8982
Photolysis of BAPOs in the presence of water or alcohols generates a new delocalized p-radical
which does not participate in the polymerization It either converts into a MAPO acting as a
secondary photoactive species or works as a one-electron reducing agent
C P O
R
OC
O C4
H9
O
C P
O
R
O
C P
O
R
O H
O R
C N
N C
N C
C N
C P
O
R
O
O R C P
O
R
O
O R
C O
P O
R
R O
R O Hm o n o m e r
( B A )
- e- H +
T C E -+
n ( B A )
h h
+
- H
polymerization
secondary
polymerization
electron transfer manifold
radical manifold
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
42J-L Birbaum
Summary
4
Conclusion
Bibliography
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Conclusion
43J-L Birbaum
High curing performance requires the careful match of all components
of the system the formulation the photoinitiator and the light source
But this is not enough low emission migration and sustainability
become increasingly important
Safety
vs
Sustainability
UV curing
=
Low VOC
Cure speed
vs
Migration
Environment Society
Industry
New products must show an advantage to succeed Creativity is plentiful
Commercialization is difficult The costs of a complex regulatory system
are a high hurdle for innovation
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4
Bibliography
44J-L Birbaum
WA Green (2010) Industrial Photoinitiators a Technical Guide
CRC Press Boca Raton FL
JV Crivello and K Dietliker (1998) Photoinitiators for Free Radical
Cationic amp Anionic Photopolymerization vol 3 of G Bradley (ed)
Chemistry amp Technology of UV amp EB Formulation for Coatings Inks
and Paints 2nd Ed Wiley SITA London
ldquoThree months in the laboratory can save a
couple of hours in the libraryldquo
Nick Turro The Spectrum 2004 17 4