Film Formation of Waterborne Coatings

Emulsion Polymerization Processes CourseEmulsion Polymerization Processes Course16 September 2005, Donostia-San Sebastián16 September 2005, Donostia-San Sebastián

J.L. Keddie - Copyright reserved 2005J.L. Keddie - Copyright reserved 2005

Film Formation of Waterborne Coatings

Joe Keddie University of Surrey

Guildford, UK



Polymer-in-water dispersion

Close-packing of particles

Water loss

Dodecahedral structure (honey-comb)

Deformation of particles

Idealised View of Latex Film Formation

Interdiffusion and coalescence

Homogenous Film

T > MFFT

T > Tg

Optical Clarity



Stages of Latex Film Formation

Dark field optical microscopy

Atomic force microscopy

TEM on C replica



Overview• Factors affecting Minimum Film Formation

Temperature MFFT• Lateral and vertical drying• Particle packing• Fundamental driving forces for particle

deformation• Diffusion and particle coalescence• Factors influencing surfactant distribution



Typical MorphologiesParticles are flattened at their

boundaries in dry film

1.5 m x 1.5 m AFM Images5 m x 5 m

Randomly-packed array of deformable particles in dry film

Source: A. Tzitzinou et al., Macromolecules, 33 (2000) 2695.



Typical Morphologies

Voids in randomly-packed array of particles:Yet film is optically transparent



From: R. Mezzenga et al., “Templating Organic Semiconductors via Self-Assembly of Polymer Colloids,” Science, 299 (2003) p. 1872.

Percolating Phase within Deformed Particles

Latex film formation offers control at the

nanometre length scale!



Measuring MFFT

Picture courtesy of Dr P. Sperry, Rohm and Haas



Picture courtesy of Dr P. SperryRohm and Haas

Hot

Cold

Clear

Cloudy

Minimum Film Formation Temperature(MFFT)

+10°C

-10°C



Factors Affecting MFFT• MFFT has an imprecise definition - subject to human perception

• Usually is within a few degrees of the glass transition temperature of the polymer

• Optical clarity can increase over time with further coalescence of particles

• For the same polymer, MFFT decreases with particle size. Driving force for coalescence is higher for smaller particles. Also, there is an optical effect: less light scattering from smaller voids!



Effect of Particle Size on MFT

Source: D.P. Jensen & L.W. Morgan, J. Appl. Pol. Sci., 42 (1991) 2845.

Tg of the latex is

~ 37 - 40 °C



Effect of Particle Size on MFT

Blend of 63 nm and 458 nm particles with an average Tg of 38 °C.

Source: D.P. Jensen & L.W. Morgan, J. Appl. Pol. Sci., 42 (1991) 2845.



Drying of Latex Films



Experimental evidence for lateral non-uniformity

E. Sutanto et al., in Film Formation in Coatings, ACS Symposium Series 790 (2001) Ch. 10

Cryogenic SEM



Films dry first in the thinnest regions

Relevant when coating large surface areas: lateral transport of water is observed

Hard particles

Film



a = particle radius

awa

capP10

=

Capillary pressure:

Pressure of Darcy flow

Darcy

capc P

PP =

Reduced capillary pressure:

Pcap

P

x



Theory: Reduced capillary pressure controls lateral drying

• Reduced capillary pressure, pc, can pin the water at the film edge.

Ha

Ep

m

moc 2

221 137520

)(

=/

A.F. Routh and W.B. Russel, A.I.Ch.E.J., 44 (1998) 2088.

• a = particle size

• H = film thickness

• E = evaporation rate

Surface tension; viscosity;

solids fraction



MR imaging of lateral drying

2.6 mm0 hr.

6 hr.

3 hr.

22 mm

Packed particlebed filled with water

Wet, colloidaldispersion

J.M. Salamanca et al., Langmuir, 17 (2001) 3202.MR Image



Water is pinned at the film edge when there is a high PcPc = 1.0 Pc = 420

H = 1.2 mm and a = 25 nm H = 0.32 mm and a = 4.4 m

22 mm1.1mm

22 mm2.4mm



Experiments support theory

• Lower thickness, larger particle size, and slower evaporation rate encourage uniform lateral drying

Pc



Experimental Evidence for Vertical Non-Uniformity

E. Sutanto et al., in Film Formation in Coatings, ACS Symposium Series 790 (2001) Ch. 10

Densely-packed particle layer

Cryogenic SEM



Theory: Peclet number for vertical drying uniformity

• Competition between Brownian diffusion that re-distributes particles and evaporation that causes particles to accumulate at the surface



Experimental Observation of Brownian Movement

Phenomenon was first reported by a Scottish botanist named Brown (19 cent.)

Brown observed the motion of pollen grains but realised that they were not living.

Brownian motion



H

E

Pe << 1

ODHE

Pe =R

RkT

Do 6= Dilute limit

Peclet number for vertical drying uniformity E

Pe >> 1



21

~ Pedzd pol

Scaling Relation for Vertical Drying Uniformity

A.F. Routh and W.B. Zimmerman, Chem. Eng. Sci., 59 (2004) 2961-68.

z



Simulations of the Vertical Distribution of Particles

pol

Vertical Position

Pe = 0.2 Top

Close-packed

m

z




pol

Vertical Position

Pe = 1Close-packed




Vertical Position

pol

Pe = 10



GARField depth profiling magnet

Characteristics :• 0.7 T permanent magnet

(B0)• 17.5 T.m-1 gradient in the

vertical direction (Gy)

Abilities :• accommodates samples of 2 cm

by 2 cm area• achieves better than 10 m pixel

resolution!

B0

GyB1

Film Sample

Coverslip RF Coil

posi

tion

Signal intensity

Gravity

for planar samples

P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.



Experiments partially agree with simulations

• Slow evaporation rate, small particle size, low film thickness and low serum viscosity favor uniform vertical drying.

H = 255 m, E = 0.2 x 10-8 ms-1, D = 3.23 x 10-12 m2s-1

High humidity Pe 0.2

w

z

J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421



• High evaporation rate, large particle size, high film thickness and high serum viscosity favor non-uniform vertical drying.

There is no discontinuity in the water

concentration.

H = 340 m, E = 15 x 10-8 ms-1, D = 3.23 x 10-12 m2s-1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-50 0 50 100 150 200 250 300 350 400 450Height (m)

Mag

netis

atio

n (A

rbitr

ary

Uni

ts)

2 minute7 minutes13 minutes31 minutes

Flowing Air Pe 16

w

z

w=0.15

Experiments partially agree with simulations



Mixed modes of dryingFlowing air: High E and

vertical uniformity of waterStatic air: Low E and non-uniformity of water vertically

Time



Particle Packing



FCC

BCC

Particles of “wrong” size

Particle Packing DefectsRequires monodisperse particle sizes

Slow drying favours ordering

Packing defects are often associated with particles of differing size!



Solids Fraction of Packed Particles

• If monosized particles pack into a face-centered array, the volume fraction of solids, , is 0.74 - the densest possible for hard spheres.

• If the particles are randomly-packed, 0.6

• If smaller particles fit into the void space between larger particles, then will be higher.

• Also, if an electric double-layer prevents particle-particle contact, then will be lower at “close packing”.



Effect of Double-Layers

Confinement of particles but without particle/particle contact



Solids Fractions of Hard Particles

••• ••••••

••••••

••••••

•••••••••

If small particles fit into the voids between large particles, the packing

fraction can be increased!

Solids fraction is 74 vol% for FCC

packing of both small and large particles



These size ratios are required to create various types of colloidal crystal:

This strategy requires tight control of particle sizes and controlled drying conditions.

An alternative approach is to disperse large particles in a continuous phase of small particles.

Formation of Colloidal Crystals

Cubic # Nearest Large/Small Crystal Structure Neighbors

Ratio .

CsCl (Simple) 8 1.37:1

NaCl (Face-centred) 6 2.41:1

ZnS (Diamond) 4 4.45:1



MRS Bulletin,

Feb 2004, p. 86

Ordered Arrays of Particle Blends



Critical Volume of Particle to Achieve a Continuous Phase

Large/Small Ratio

Enough small particles to percolate around

larger particles



Example of Morphology from the Packing of Bimodal Particles

Atomic force microscopy images of a latex film made by blending 80 wt% 300 nm particles with 20 wt% 50 nm particles

Source: A. Tzitzinou et al., Macromolecules, 33 (2000) 2695.

1.5 m x 1.5 m



In Latex with Bimodal Size Distribution: Number Fraction

Weight FractionExample: 10:1 ratio of Large:Small ParticlesWeight/Vol. Fraction Large Number Fraction Large

0.01 0.00001

0.10 0.00011

0.50 0.00100

0.95 0.01865

0.97 0.03132

0.99 0.90082Actual sizes are irrelevant. Calculations assume large and small particles have the same density.



Effects of Shear Stress on Colloidal Dispersions

With no shear Under a shear stressConfocal Microscope ImagesMRS Bulletin, Feb ‘04, p. 88



Mechanisms for Particle Deformation

and Coalescence




1

2 34

56 •

Face-centered cubic array of particles:

12 neighbours for each particle

•••

••1

2 3

4

56




Particles are deformed to fill all available space: dodecahedra

Y. Wang et al., Langmuir 8 (1992) 1435.



Particle Coalescence

Surface area of N particles: 4Nr2

Surface area of particle made from coalesced particles:

4R2

Same polymer volume before and after coalescence:

Rr

Change in area, A = - 4r2(N-N2/3)In 1 L of latex (50% solids), with a particle diameter of 200 nm, N is ~ 1017 particles. Then A = -1.3 x 104 m2



Driving Force for Coalescence: Reduction in Free Energy

Decrease in Gibbs Free Energy, G, with particle coalescence:

G = A = interfacial energy (J m-2)

A = change in interfacial area

Coalescence is favorable when G is reduced (G < 0).

For coalescence of N = 1017 particles with a 200 nm diameter, with = 3 x 10-2 J m-2, G = - 390 J.



Concept of Energy Balance

Energy “gained” by the reduction in surface area with particle deformation is “spent” on the deformation of particles:

Deformation is either elastic, viscous (i.e. flow) or viscoelastic (i.e. both)



Typical Values of Interfacial Energy

InterfaceWater/Air

Polymer/Water

Polymer/Air

(10-3 J m-2)72

5 - 10

20 - 35



Particle Deformation Mechanisms

Dry Sintering: pa

Wet Sintering: pw

Water recedes before particles are deformed. Reduction of the polymer/air interfacial energy is the driving force.

Particles are deformed before water has evaporated. Reduction of the polymer/water interfacial energy is the driving force.




Capillary Action: wa

rP wa9.12

r

For wa = 3 x 10-2 Jm-2 and r = 150 nm,

P is ~ 3 MPa!



Skin Formation


In those cases in which the water distribution is non-uniform AND in which wet sintering is favoured, skin formation is predicted to occur.



Scaling Prediction for Particle Deformation Mechanism

HREwa

0

A single parameter has been proposed to predict which mechanism of deformation is operative.

It represents the ratio of time for viscous deformation (Ro/wa) over the evaporation time (H/E):

where o is the zero-shear viscosity of the polymer.



Theory: Deformation mechanism is a function of the dimensionless parameters, and Pe

HREwa

0

kTERH6Pe

100

10000

1

Wet Sintering: pw

Capillary Deformation: wa

Receding Water Front

Dry Sintering: pa

1

0Skinning

Partial Skinning

A.F. Routh & W.B. Russel, Langmuir, 15 (1999) 7762-73.

Low T: Near Tg

T- Tg > 15 °C



At later stages, water profiles depend on particle deformation

Capillary deformation:Water is always near the film surface

-40 0 40 80 120 160 200 24005

10152025303540455055

Wat

er c

once

ntra

tion

(vol

.%)

Height (m) z

w-40 0 40 80 120 160 200 240

05

10152025303540455055

Wat

er c

once

mtra

tion

(vol

.%)

Height (m)

Dry Sintering:Water recedes from the film surface

z

wTime



Water concentration profiles during latex film formation

Acrylic Latex near Tg:Uniform water recession from surface

Consistent with dry sintering with some particle deformation

Time

z

w

-50 0 50 100 150 200 2500.0

0.1

0.2

0.3

0.4

0.5

Rel

ativ

e in

tens

ity

Depth (m)Height (m)



-25 0 25 50 75 100 125

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Air

Subs

trate

7'

5'2'

Rel

ativ

e in

tens

ity

Height (m)

J. Mallégol et al., Langmuir, 18 (2002) 4478

• Tg = -45 °C

Water is “pinned” at the air surface of the film throughout the drying process!

Evidence for Capillary Deformation



Polymer Interdiffusion



Example of Good Coalescence

Immediate film formation upon drying!Hydrated film



• Particles can be deformed without being coalesced. (Coalescence means that the boundary between particles no longer exists!)

• Molecules must diffuse across the boundary between particles to achieve coalescence: analogy to crack healing.

• If the molecules entangle over a distance on the order of the radius of gyration of the polymer, then the film is stronger. Otherwise, the boundaries will be weak.

Coalescence and Interdiffusion



Entanglement at the polymer/polymer interface

Time



Factors that Influence Diffusivity, D2

1M

D ~• Molecular weight, M:

( )RTE

oDD -exp=• Temperature, T:

• Particle membranes: e.g. hydrophilic acrylic acid copolymer or serum phase at particle boundaries

• Crosslinking: Can entirely prevent diffusion!

• Molecular branching: Slows down diffusion



Coalescing Aids Increase Diffusivity• Volatile solvents can decrease the Tg and MFFT of the

polymer, enhance the rate of polymer interdiffusion and then evaporate to create a hard (high Tg) film.

• A common example of a coalescing aid is 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol TM).

• A negative aspect is that the use of coalescing aids increases the VOC concentration of a waterborne system!



Factors in Selecting a Coalescing Aid

• Evaporation rate: Determines how long the plasticizer remains in the film.

• Solvent Tg: Determines the extent of plasticization; Tg approximately 2Tm/3.

• Solubility: Determines the amount of solvent in the polymer and aqueous phases and hence the extent of plasticization.

• A balance of these factors is required to achieve the best film formation and a hard final coating.



Example Plasticizer DataSolvent K = Cw/Cp *PBMA Tg

Texanol (TPM) 0.01 10 °C

Diethylene glycol monobutyl ether (DGB) 3 13

Hexylene glycol (HG) 13 16

Benzyl alcohol (BA) 0.22 24

Diacetone alcohol (DA) 8 26

*Tg when PBMA contains 10 wt.% solvent

Source: Juhué and Lang, Macromolecules, 27 (1994) 695.



Comparison of the Evaporation Rates of Coalescing Aids

Texanol is retained in a film for an extended period of time: a “remnant plasticiser”

1 = TPM

2 = HG

3 = DGB

4 = BA

5 = DA

6 = Neat PBMA

Juhué and Lang, (1994)



Surfactants and latex serum prevent particle coalescence

Mallégol et al., Langmuir (2001) 17, 7022.

Tg = -45 °C



Serum phase can prevent coalescenceParticles are not coalesced in this acrylic latex with a bimodal particle size

Good coalescence is achieved when the same latex has been “cleaned” via dialysis



Raman microscopy

Surfactant sometimes forms aggregates

1 m



Membranes must break-up to enable interdiffusion



Phase separation



Measuring diffusion and mixing

Phe

PhePhe

PhePhe

PhePhe

An

An

An An

AnAn

AnPhe

PhePhe

PhePhe

PhePhe

An

An

AnAn

AnAnAn

Phe = Phenanthrene

An = Anthracene

For Phe and An, energy transfer is instantaneous when r is < 12 Å and very slow when r > 50 Å.

r



Energy Transfer Technique

Time (min.)

Fraction mixing

Time (ns)

Fluorescence Intensity

Source: M.A. Winnik et al., J. Coatings Techn., 64, No. 811, (1992), 51-61.

Time



Example Diffusion Coefficients from NRET

Polymer Temp (°C) Diff. Coeff. (10-16 cm2s-1)

PBMA 70 ~1

PBMA 90 ~10

PBMA 120 ~100 - 1000

PMMA 130 6

PMMA 170 800

See J.L. Keddie, Mat. Sci. Eng. Rep. (1997) R23, 101.



Effect of Texanol on PBMA Diffusion Coefficients

Texanol Content (wt.%) *Diff. Coeff. (cm2s-1)

0 ~1 x 10-18

3 ~ 2 x 10-17

6 ~ 1 x 10-16

8 ~ 8 x 10-16

10 ~ 2 x 10-15

12 ~ 1 x 10-14

* D measured when fraction of mixing is 0.5See J.L. Keddie, Mat. Sci. Eng. Rep. (1997) R23, 101.



Surfactant Distribution



Surfactant Transport Mechanisms• During the drying stage, surfactant must be

either:• Adsorbed on particle surfaces, where it moves

along with the particles OR….• Diffusing in the latex serum OR….• Adsorbing on particles, described by adsorption

isotherm OR…• Desorbing from particles, as particles compact

together.



Changing Volume Concentration of Adsorbed Surfactant,

Rpolsurf

3

=

Micelles

R

pol increases as water evaporates and particles

pack together



surf is given by a Langmuir isotherm as a function of surfactant concentration surf, which

increases over time.

Changing Volume Concentration of Adsorbed Surfactant,

surf

surf



Surfactant desorption

• Desorption might be caused by the repulsion from anionic head groups forced into close proximity. • It might be opposed by particle rigidity.



Wide variety of surfactant distributions

Height

surf

Height

surf

Height

surf

Height

surf

HTAB SDS

HPCl NP10

hexadecyl trimethylammonium bromide sodium dodecyl sulfate

hexadecyl pyridinium chloride

polyethoxylated nonyl phenol

C.L. Zhao, et al., Coll. Polym. Sci., 265 (1987) 823



Further Reading• Books on Latex Film Formation and Wb Coatings: Film Formation in Waterborne Coatings, T. Provder, M.A. Winnik, and M.W. Urban, ed., ACS Symposium Series, Vol. 648, 1996.

Film Formation in Coatings: Mechanisms, Properties and Morphology, T. Provder and M.W. Urban, ed., ACS Symposium Series, Vol. 790, Oxford University Press, 2001.

• Review Articles on Latex Film Formation: J.L. Keddie, Mater. Sci. Eng. Reports, 21 (1997) 101.

J. Hearn, P.A. Steward, M.C. Wilkinson, Adv. Colloid Interf., 86 (2000) 195.

M.A. Winnik, Curr. Opinion Coll. Interf. Sci., 2 (1997) 192.

• Process model of film formation: A.F. Routh and W.B. Russel, Langmuir, 15 (1999) 7762.

• Review of experimental work on film formation: A.F. Routh and W.B. Russel, Ind. Eng. Chem. Res., 40 (2001) 4302.

Film Formation of Waterborne Coatings

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