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PROCESS OPTIMIZATION OF PHOTOCURABLE POLYESTER GELCOAT AND LAMINATE
by
L. SCOTT CRUMP
Submitted in partial fulfillment of the requirements
For the Degree of Master of Science
Thesis Advisor: Professor Alex. M. Jamieson
Department of Macromolecular Science and Engineering
CASE WESTERN RESERVE UNIVERSITYCleveland, Ohio
May , 2014
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i
CASE WESTERN RESERVE UNIVERSITY
GRADUATE STUDIES
We hereby approve the thesis of Larry Scott Crump
candidate for the Master of Science-Macromolecular Science & Engineering degree.
(signed) Professor A. Jamieson __________________________________
Professor H. Ishida______________________________________
Professor D. Schiraldi (chair)_____________________________
date ______________________
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I grant to Case Western Reserve University the right to use this work irrespective ofany copyright, for the University’s own purposes without cost to the University or toits students, agents, and employees. I further agree that the University may reproduce
and provide single copies of the work, in any format other than in or frommicroforms, to the public for the cost of reproduction.
_________________________________________
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
iii
. Table of Contents
Table of Contents……….……………………………..…………..
List of Tables……………….…………………………..……….....
List of Figures………………………………………………………
Acknowledgements………………………………………..………...
Chapter I The Composite Open Molding Process………………..
Chapter II UV Curing Equipment and Radiometry………………
Chapter III Chemistry of Thermosetting Unsaturated Polyester andAcrylate Systems…………………………….…….
Chapter IV Characterization of Resin / Coating State of Cure…...
Chapter V Modeling the Degree of Cure of a 2D UV Curing Process
Chapter VI Process Optimization - Defining the Process Window –
Balancing Safety, Throughput, Capital Investment and
Operating Costs
Chapter VII Case Study – Flat Construction Panel Laminate……..
Bibliography………………………………………………………..
vi
xii
xvi
xviii
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16
34
70
75
142
150
156
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
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Table of Contents
Chapter I The Composite Open Molding Process……………….
a. Unsaturated Polyester Resin Based Composite Products
b. Application of the In-Mold Coating (Gel Coat)
i. transfer to the mold
ii. rheology of gel coat
iii. curing the gel coat film on the mold
c. Reinforced Laminate Application
Chapter II UV Curing Equipment and Radiometry………….
a. Lighting systems
i. bulb design
ii. reflector design – light ray management
iii. temperature management in UV curing applications-dichroic reflectors-bulb diameter
iv. metal halide doping to modify the spectral power
distribution
v. lamp motion relative to the target
b. Radiometers and radiometric characterization of a UV
curing process
1
16
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
v
Chapter III Chemistry of Thermosetting Unsaturated Polyester
and Acrylate Systems…………………………………………..
a. Synthesis of thermosetting polyester and acrylate
oligomers (condensation polymerization)
b. Network formation of thermosetting polyester and acrylate
oligomers (free-radical polymerization)
i. Microgel formation and macrogelation
ii. Kinetics of redox initiated polymerization of UPR
styrene
iii. Kinetics of light induced polymerization involving
multifunctional monomers
c. Formulation of conventional gel coat and resins
d. The Case for UV Curable Composite Materials
e. Formulation of UV curable gel coat and UP resins
. i. Historical work in the area of UV curable composites
ii. Classification of photoinitiators photolysis
mechanisms
iii. Physical Concepts of UV Curing – interaction of light
with the photocurable material
iv. Photobleaching and high radical yield-impact of
acylphosphine oxide photoinitiators- curing thick films
containing titanium dioxide pigment
v. Light scattering within a coating or laminate
. vi. Commercial applications for UV curable composites
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
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Chapter IV Characterization of Resin / Coating State of Cure 70
a. Qualitative methods for estimating cure
i. Probing techniques to assess cure – hardness
development, dry-to-touch assessment
ii. Limitation of probing techniques to assess cure
b. Quantitative methods for cure characterization
i. Analytical methods used to study cure during the
product development cycle( DSC, FTIR )
ii. Process quality control methods to measure cure –
NIR, dielectric spectroscopy
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
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Chapter V Modeling the Degree of Cure of a 2D UV Curing 75Process
a. Studies of coatings and laminate resin related variables
i. Experiment 1 – Effect of pigmentation-screeningstudy involving ten different colors
ii. Experiment 2 – Effect of TiO2 concentration
iii. Experiment 3 – Effect of gel coat film thickness
iv. Experiment 4 – Factorial study of photoinitiatorconcentration, UV energy, and filmthickness
v. Experiment 5 - Binder / reactive diluent selection
vi. Experiment 6 – Light transmission studies in thelaminate resin
b. Studies of UV curing equipment variables
i. Experiment 7 – Reciprocal law for UV energy,independence of irradiance and linespeed
ii. Experiment 8 – DSC cure studies in clear andwhitegel coat – effect of film thickness and UVenergy
iii. Experiment 9 – Temperature-Energy-Irradiance mapfor several UV light sources
iii. Experiment 10 – Variations in energy andirradiance of a single 600 W/inch lamp as functionof distance from the lamp centerline
iv. Experiment 11 – Measurement of energy andirradiance from a bank of five 600 W/inch lampsas a function of lateral position
vi. Experiment 12 – Testing the additive law for UVenergy using two 600 W/inch lamps
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
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vii. Experiment 13 – Effect of UV energy and irradiancelevel on the surface temperature of the coating
viii. Experiment 14 – Validation of the cosine law fornon-perpendicular exposure conditions
ix. Experiment 15 – Effect of lamp height on UVenergy and irradiance
x. Experiment 16 – Evaluation of dichroic reflectors
c. Studies of the reflectivity of the mold surfaceExperiment 17 – Effect of reflectivity on cure
d. Integrated mathematical model for a UV conveyor line
i. Mathematical model development
ii. Simulation 1 - Validation of the mathematical model
iii. Simulation 2 - The effect of lamp spacing on theirradiance and energy distribution
iv. Simulation 3 – The effect of a lamp failure on theirradiance and energy distribution
v. Simulation 4 – The effect of lamp height on energylevel and uniformity
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
ix
Chapter VI Process Optimization - Defining the Process 142
Window. Balancing Safety, Throughput,
Environmental Impact,
Capital Investment and Operating Costs.
a. Safety considerations
b. Throughput considerations
c. Environmental benefits of UV curable composites
c. Economic considerations
Chapter VII Case Studies – Flat Construction Panel 150
Laminate
Bibliography 156
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
x
Table # Description Page
1
2
3
4
1
2
3
1
1
2
3
4
5
Chapter I - The Composite Open Molding Process
U.S. Markets and Applications for Unsaturated Polyester BasedComposites
Cone and plate rheometer programming sequence to simulate therheological lifecycle of a commercial gel coat.
Application flow requirements of gel coat and solvent based paint
Summary of lamination process features and limitations
Chapter III - Chemistry of Thermosetting Systems.
Gel Point Time data set used to validate the redox cure kineticmodel
Commercially available photoinitiators
Energy absorbed in the top 1% and bottom 1% of a coating film
Chapter IV - Characterization of Resin / Coating State of
Cure
Effect of Tg on the surface tackiness of UPR prepolymer
Chapter V - Modeling the Degree of Cure of a 2D UV
Process
Factorial study of photoinitiator concentration, UV energy, andTiO2 concentration
Binder / reactive diluent selection
Reciprocal law for UV energy, independence of irradiance andline speed
Variations in energy and irradiance of a single 600 W/inch lampas function of distance from the lamp centerline
Gaussian fit parameters to model light dispersion a 600 W/inchlamp
1
6
7
12
46
53
59
68
80
82
86
95-96
98
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Process Optimization of Photocurable Polyester Gel Coat and LaminateList of Tables
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6
7
8
9
10
11
12
13
1
1
Measurement of energy and irradiance from a bank of five 600W/inch lamps as a function of lateral position
Testing the additive law for UV energy using two 600 W/inch
lamps
Effect of UV energy and irradiance level on the surfacetemperature of the substrate
Validation of the cosine law for non-perpendicular exposureconditions
UV Energy and irradiance measurements at various lamp heights – static one minute exposure
The irradiance from a point source of light varies with the squareof the distance from the source
Evaluation of dichroic reflectors
White UV curable gel coat results on a reflective and non-reflective mold
Chapter VI - Process Optimization - Defining a Process
Window
Effect of lamp spacing on UV energy level and uniformity
Chapter VII Case Study – Flat Construction Panel Laminate
Summary of UV Curing Knowledge (from Experiments 1- 18)
102
106
109
113
114
115
116
120
129
143
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
xii
Figure # Page
1
2
1
2
3
4
1
2
3
4
5
6
7
8
9
Chapter I - The Composite Open Molding Process
Controlled stress cone and plate rheometer
Experiment to simulate the shear history of a commercial polyester gel coat and automotive polyurethane paint
Chapter II - UV Curing Equipment and Radiometry
Reflector designs to focus (elliptical), collimate (parabolic), anddisperse (dimpled) light energy
IR absorbing dichroic reflector
Relative spectral power distribution of commonly used UV bulbs
UV curing lighting systems
Chapter III - Chemistry of Thermosetting Systems.
Production of unsaturated polyester resin solutions
Polymerization of UPR prepolymer and crosslinking monomer
Formation of a UPR-styrene microgel
Microgel formation and Macrogelation in UPR-Styrene System
Effect of curing temperature on the gel point time – redoxinitiator system – UPR-styrene monomer
Effect of initiator concentration on the gel point time – redoxinitiator system – UPR-styrene monomer
Effect of cobalt accelerator concentration on the gel point time –
redox initiator system – UPR-styrene monomer
Light induced free radical formation in a coating film
UV Transmission characteristics of monomers, oligomers, andfilms commonly used for UV cure applications (path length=10mm UV cell, 100% concentration)
6
8
19
21
25
26
33
37
41
42
46
47
47
54
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
xiii
10
11
12
13
14
15
1
1
2
3
4
5
6
7
Absorption-scattering characteristics of titanium dioxide pigment
Effect of the absorptivity on light transmission characteristics ina film
Fraction of incident energy absorbed in the top 1% and bottom1% of a film
Time-lapsed UV absorption spectrum of phosphine oxide photoinitiator
Comparison of photobleaching and non-photobleaching photoinitiators
UV Composites publications
Chapter IV - Characterization of Resin / Coating State of
Cure
Evaporative losses of reactive monomers in gel coat filmmeasured by FTIR (T=25C)
Chapter V - Modeling the Degree of Cure of a 2D UV
Process
Screening experiment to evaluate the effect of color
pigmentation on the degree of UV cure
Kubelka-Munk prediction of light absorption and scattering in anopaque pigmented film
Reflectance spectra for the ten pigmented gel coats shown in photograph 1
Absorption spectra of the photoinitiator solution
Effect of UV energy on cure of a UPR laminate containing 35%
short fiber E-glass reinforcement (0.75% BAPO photoinitiator)
Effect of UV energy on surface temperature of a UPR laminatecontaining 35% short fiber E-glass reinforcement (0.75% BAPO photoinitiator)
Effect of photoinitiator concentration on the cure of a UPRlaminate containing 35% short fiber E-glass reinforcement
57
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84
85
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
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8
9
10
11
11.5
12
12.5
13
14
15
16
17
18
19
20
21
22
(BAPO photoinitiator) – 5 minute static exposure
Energy and irradiance vs. conveyor speed
Energy requirements to cure a clear gel coat and white gel coat
Energy and surface temperature profiles for severalcommercially available UV lamps
UV lighting set-up for experiment 10 – a single 600 W/inchFusion UV lamp
Energy distribution for a Fusion 600 W/in lamp
UV lighting set-up for experiment 11 – a bank of five 600W/inch Fusion UV lamps
Measured UV Energy – Bank of five 600 W/inch Fusion UVlamps
UV lighting set-up for experiment 12
Output from two 600 W/inch UV lamps
Effect of lamp type, lamp height, line speed, and reflector type
on UV energy, irradiance, and exit temperature
Correlation of UV energy and irradiance with the surfacetemperature of a part being cured with UV lamps
Validation of the cosine law for non-perpendicular exposureconditions
Schematic of lighting set-up for experiment 15
UV Energy and irradiance measurements at various lamp heights
– static one minute exposure
Inverse square law validation
Interactions of UV light with the coating and mold surface
UV-Visible reflection from polyester tooling gel coat – variouscolors
87
89
91
93
97
100
103
104
107
110
111
112
113
115
115
117
119
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
xv
23
24
25
26
27
28
29
30
31
1
UV-Visible reflection from metal molds and aluminum flakefilled polyester tooling gel coat
Schematic of a conveyor line with the coordinate system
indicated
Lamp height dependence of the pre-exponential multiplier anddispersion parameter
Schematic of an industrial UV curing line
Ten lamp UV curing conveyor – Two rows of five lamps
Validation of the predictive model to estimate UV energy and
irradiance levels
The effect of lamp spacing on the level and uniformity of UVenergy
The impact of a lamp failure on the UV energy and irradiancedistribution
The effect of lamp height on the level and uniformity of UVenergy
Chapter VI - Process Optimization - Defining a Process
Window
Process window for UV curable gel coat
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126
127
128
130
132
134
138
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
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Photograph # Description Page
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Chapter I - The Composite Open Molding Process
Application of white gel coat to a large hull mold
Application of white gel coat to a large hull mold (2)
Application of barrier skin coat laminate on the white gelcoat
Completed hull after being removed from the mold
Spray pattern test prior to applying the gel coat on a deckmold
Clear gel coat applied to a mold used to make a syntheticmarble sink.
Black gel coat applied to a cowling mold for a small tractor
Gel coat applied to a tub/shower mold
Severe de-wetting of a clear gel coat
De-wetting (crawling) of a white gel coat
Hand lay-up process
Hand lay-up process (2)
Spray-up process using an external mix chopper gun withcontinuous E-glass roving
Vacuum infusion lamination of a small boat hull
Hybrid process – open mold wet lay-up followed by pressmolding to cure electrical panels – Wet lay-up compression
molding
Open molding process – automated lamination of roofing panels
Closed molding – resin transfer molding (RTM) of toylocomotive
3
3
3
3
4
4
4
4
5
5
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
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18
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20
1
2
3
4
5
6
7
8
9
10
11
Dry reinforcement charged in to RTM mold prior to moldclosure
Casting molding of synthetic marble sink ( non-reinforced
part)
demolding the cured sink bowl
Chapter II - UV Curing Equipment and Radiometry
Bottom view of UV lamp housing with the shutter open andthe bulb exposed
Top view of UV lamp housing. The red hoses are used forwater cooling during operation
Electrode style medium pressure mercury vapor lamp( the bulb is energized by applying an electric currentacross the metal electrodes)
Electrodeless style medium pressure mercury vapor lamp( the bulb is energized with microwave heating)
UV lamp with an electrode style bulb and a dimpledreflector capable of producing diffusely reflected light
Modular microwave UV lamp – Bottom view – note themetal mesh designed to prevent leakage of the RF waves produced by the magnetron heating source
Modular microwave UV lamp – Bottom view electrodelessstyle bulb and elliptical reflector - note RF leakage monitorinterlocked to the power supply
Modular microwave UV lamp – side view - note the 6”diameter air cooling hose
Bench scale UV conveyor and 6” modular microwaveelectrodeless lamp
Pilot scale UV conveyor fitted with two 10” modularmicrowave electrodeless lamps
Industrial robotic curing
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18
18
18
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
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12
13
14
15
16
1
2
1
2
1
1
2
3
4
Industrial robotic curing
Photodiode radiometer with dry air purge line
UV lamp housing with a process radiometer mounted onthe lamp housing to monitor the lamp output
Traveling radiometer – top view – photodiode array isvisible
Traveling process radiometer – bottom view – controls andreadout are visible
Chapter III - Chemistry of Thermosetting Systems.
Alligatoring phenomena – the top 1-3 mils is cured whilethe balance of the film is wet
Uncured coating material which remains after the curedsurface film is peeled away
Chapter IV - Characterization of Resin / Coating Cure
Colored gel coat films – before UV curing
Colored gel coat films – after UV curing
Chapter V - Modeling the Degree of Cure of a 2D UV
Process
UV Curing line used to develop the irradiance and energy process model
Chapter VII Case Study – Flat Construction Panel
Laminate
Application of the white UV curable gel coat to the
reflective mold
UV curing of the white gel coat
Cured white gel coat film
Hand lay-up of the laminate
27
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31
31
31
60
60
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72
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146
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Process Optimization of Photocurable Polyester Gel Coat and Laminate – List ofFigures
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5
6
UV curing the laminate
Cured laminate
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Process Optimization of Photocurable Polyester Gel Coat and Laminate
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Acknowledgement
I would like to thank my wife Ruth for giving me the many uninterrupted hours
needed to prepare this paper.
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Process Optimization of Photocurable Polyester Gel Coat and Laminate
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Abstract
by
L. SCOTT CRUMP
It is the purpose of this project to develop the basic process data and approach
needed to produce photocurable gel coated laminates. A review of the composite
open molding process is made describing the application steps used to produce
conventional composite parts prepared from unsaturated polyester resins. A summary
of the current state of the art in ultraviolet (UV) curing equipment and process
radiometers is given to develop the basis for the experimental portion of the report.
The basic chemistry of thermosetting polyester and acrylate oligomers is reviewed
with particular emphasis given to redox and photoinitiation processes. The physical
concepts of UV curing related to the interaction of light(transmission, absorption, and
scattering) within the coating film and photoinitiating molecules is discussed along
with the analytical methods to characterize the degree of cure of the
photopolymerizing system. Material and process design data are generated through
systematic experimentation. The material variables studied include the selection of
pigmentation, photoinitiator type and concentration, and resin / reactive diluents
chemistry. Process variables studied include coating thickness, lamp type and
placement (height, spacing, orientation), and throughput. A rigorous mathematical
model and associated software is developed and used to simulate the UV energy and
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Process Optimization of Photocurable Polyester Gel Coat and Laminate
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irradiance distribution for a 2D panel conveyor curing station. General considerations
are discussed to optimize the throughput of a production curing station while
maintaining a safe operation. The material and process data and the simulation
software are then tested and validated by constructing a pilot scale UV curing station
and producing large scale UV cured gel coated composite laminates.
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter I - The Composite Open Molding Process
1L. Scott Crump – May 2014
1) The Composite Open Molding Process
a. Unsaturated Polyester Resin Based Composite Products
The leading trade organization for the U.S. Composites Industry, the American
Composites Manufacturers Association (ACMA), classifies unsaturated polyester resin
(UPR) based composite materials within markets – Reinforced Market, and Non-
Reinforced Market1. Products within the reinforced market contain some form of
continuous or short fiber reinforcement, normally E-glass. Reinforced composite
materials are used in processes such as sheet molding compound (SMC), resin transfer
molding (RTM), reaction injection molding (RIM), pultrusion, filament winding, vacuum
bagging, and open molding hand lay-up lamination. Non-reinforced products include
casting resins and gel coats. The total U.S. market for gel coat is approximately 100 MM
lbs/year. The market division and end-use application of UPR composites is summarized
in table 1.
Reinforced Market Non-Reinforced Market
Construction (664 MM lbs) Transportation / Body Putty (69 MM lbs)
Consumer and Recreational (73 MM lbs) Construction (0.4 MM lbs)
Electrical / Electronic (61 MM lbs) Consumer Goods (36 MM lbs)
Marine (314 MM lbs) Gel Coats (102 MM lbs)
Transportation (160MM lbs) Other (253 MM lbs)
Other (15MM lbs)
Total ( 1.29 B lbs) Total (0.55 B lbs)
MM=million B=billion
Table 1 – U.S. Markets and Applications for Unsaturated Polyester Based Composites 1
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter I - The Composite Open Molding Process
2L. Scott Crump – May 2014
b. Application of the In-Mold Coating (Gel Coat)
A gel coat is a formulated in-mold coating typically based on unsaturated polyester
resin (UPR). The formulation building blocks of a gel coat consist of:
the polymeric binder ( unsaturated polyester oligomer)
fillers and pigments ( impart color and rheological modification)
additives (impart flow control, curing, storage stability, exterior durability, etc.)
solvent/reactive diluent ( typically styrene monomer)
Transfer to the Mold
The gel coat is spray or brush applied onto a high gloss (≥85) open mold to a film
thickness of 0.020 – 0.030 inches (20-30 mils). This film thickness is a 10-20 fold
increase over conventional painting applications such as automotive paints. The mold is
constructed of either fiber reinforced polyester (FRP) tooling materials, epoxy, or
polished metal. The mold surface is treated with a release agent to lower the mold surface
energy to 26-34 dyne/cm prior to coating application to aid in the separation of final
composite article from the mold 2. The gel coat application process is shown for a variety
of applications including marine market, construction market, and the sanitary market in
photographs 1- 8 below.
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter I - The Composite Open Molding Process
3L. Scott Crump – May 2014
Photograph 1 – Application of white gel coat toa large hull mold
Photograph 2 - Application of white gel coat to large hull
Photograph 3 – Application of barrier skin coatlaminate on the white gel coat
Photograph 4 – Completed hull after beingremoved from the mold
Gel coat application – 55 foot luxury yacht
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter I - The Composite Open Molding Process
4L. Scott Crump – May 2014
Photograph 5 – Spray pattern test prior toapplying the gel coat on a deck mold
Photograph 6 – Clear gel coat applied toa mold used to make a synthetic marblesink.
Photograph 7 – Black gel coat applied to acowling mold for a small tractor
Photograph 8 – Gel coat applied to atub/shower mold
The reduced surface energy mold represents a significant departure from substrates
encountered in the conventional painting process in which the applied coating is meant to
permanently adhere to the substrate. The surface energy of primed surfaces and surfaces
treated with chemical conversion treatments3 such as phosphates and chromates have
surface energies ≥50 dynes/cm. High surface energy substrates such as these are easily
wetted by the applied coating due to the high work of adhesion. Not surprisingly a
common problem with in-mold coatings is de-wetting of the low surface energy mold
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter I - The Composite Open Molding Process
5L. Scott Crump – May 2014
(photograph 9-10). De-wetting is best addressed by modification of the gel coat surface
tension, film thickness, and rheology.
Photograph 9 - Severe de-wetting of a clear gelcoat
Mold surface energy= 20 dynes/cm Coating surface tension= 41 dynes/cm Initial film thickness = 20 mils
Photograph 10 – De-wetting (crawling) of awhite gel coat
Mold surface energy= 22 dynes/cm Coating surface tension= 33 dynes/cm Initial film thickness = 16 mils
Rheology of Gel Coat
Conventional polyester based gel coats have rheological performance requirements that
differ substantially from those of solvent based paints . The gel coat is first pumped from
a container to a high pressure airless spray gun. Typical gel coat fluid delivery rates of 2-
5 pounds per minute are 3-10 times those of solvent based paints. The fluid pressure at
the tip of the spray gun needed to achieve these delivery rates is approximately 1000 psi.
The gel coat should resist sagging at 30 mil film thickness. The rheological lifecycle of a
commercial gel coat and commercial automotive polyurethane paint have been simulated
using a controlled stress cone and plate rheometer (figure 1).
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter I - The Composite Open Molding Process
6L. Scott Crump – May 2014
The rheometer’s applied shear stress has been programmed to simulate pumping,
spraying and post-spray recovery of the viscosity. The programming sequence used to
control the rheometer is provided in table 2 below. The results of the experiment are
shown in figure 2.
Table 2 – Cone and plate rheometer programming sequence to simulate the rheological
lifecycle of a commercial gel coat.
M=torque
=angler=radius
=rotational speed
=shear stress (Pa)=shear rate (1/s)=viscosity (Pa-s)
shear stress=viscosity x shear rate
= x r
r
Figure 1 – Controlled stress cone and plate rheometer
Maximum*
Maximum Collection # Points
Sequence # Function Stress Duration Interval Collected
Sequence 1 equilibration 9 Pa 6000 sec 1 point/60 sec 10
Sequence 2 pumping 80 Pa 15 sec 1 point/30 sec 4
Sequence 3 spraying 324 Pa 15 sec 1 point/30 sec 4
Sequence 4 recovery (fast) 9 Pa 45 sec 2 points/sec 99
Sequence 5 recovery (slow) 9 Pa 750 sec 1 point/3 sec 250
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7L. Scott Crump – May 2014
Rheological
Performance
Requirement
Gel Coat Solvent Based Paint
No sedimentation of
fillers and pigments
similar requirement for both types of coatings
Ease of pumping from the
container to a spray gun
similar requirement for both types of coatings
Ease of atomization at the
spray gun
Commonly used equipment:
airless spray gun with a tip
opening of 0.020 “ and fluid
pressure of 1000 psi. Thefluid lines are frequently
heated to 100F to lower the
viscosity under high shear.
Commonly used equipment:
air atomizing pressurized
pot spray gun with a tip
opening of 0.060” an a fluid pressure of 60-100 psi.
Sag resistant at the applied
film thickness
Typical application thickness:
20-30 mils
Shear stress calculation
gh
10 lb/gal.=1200 kg/m3
g=9.8 m/s2
h=30 mils=7.63 x 10 -4m
9.8 kg/m-sec2 = 9.8 Pa
Typical application
thickness: 1-3 mils
Shear stress calculation
gh
10 lb/gal.=1200 kg/m3
g=9.8 m/s2
h=2 mils=5.08 x 10-5m
9.8 kg/m-sec2 = 0.6 Pa
Leveling
Excellent leveling is required for both coatings, but for
different reasons. Proper leveling of the paint improves the
gloss and DOI. Proper leveling of the gel coat is required to
prevent a textured appearance on the mold side of the gel
coat due to uneven film thickness.
h
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Table 3 – Application flow requirements of gel coat and solvent based paint
Pump - Spray - Sag Simulation fo r Gel Coat and Polyurethane Paint
0.00
0.01
0.10
1.00
10.00
100.00
1,000.00
10,000.00
0 30 60 90 120 150 180 210 240
Time (sec)
V i s c o s i t y ( P a - s )
0
50
100
150
200
250
300
350
400
S h e a r S t r e s s ( P a )
Gel Coat Viscosity
Polyurethane Paint Viscosity
Shear Stress
Figure 2 – Experiment to simulate the shear history of a commercial polyester gel coatand automotive polyurethane paint
In the first sequence of the rheological simulation the coating is placed in the gap
between the cone and plate and allowed to recover from any shear induced viscosity
changes resulting from loading the sample by maintaining a shear stress on the gel coat of
9 Pa for a period of ten minutes (0.54 Pa for the automotive coating). The shear stress is
raised to 80 Pa for 15 seconds and then to 324 Pa for 15 seconds to simulate pumping and
spraying during the second and third sequence respectively. The final two sequences are
the viscosity recovery sequences. The shear stress is lowered to a value which represents
the shear stress for a fluid of density and thickness h applied to a vertical surface as
calculated in table 3. The actual shear stress applied to the gel coat during viscosity
recovery was 9 Pa (0.54 Pa for the automotive coating). While both coatings shown in
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figure 2 exhibit complex pseudoplastic and time-dependent behavior, the initial viscosity
recovery of the gel coat is more rapid than automotive polyurethane paint. The fully
recovered viscosity (plateau viscosity) of the gel coat is more than three hundred times
greater than the automotive paint. While rapid recovery and high plateau viscosity are
essential for the gel coat to resist sagging these conditions increase the likelihood of air
entrapment in the film if excessive fluid atomization is used during the spray process.
Trapped air bubbles which remain in the cured gel coat film are known as surface
porosity and subsurface porosity. Porosity is a very undesirable film defect due to the
reduction in exterior durability and blemished surface quality created by the voids in the
film.
Curing the gel coat film on the mold
Commercial gel coats are cured via addition of 1-3% of a free radical redox initiator
solutions such as methyl ethyl ketone peroxide (MEKP). MEKP is an organic peroxide, a
high explosive similar to acetone peroxide, and is dangerous to synthesize. Unlike
acetone peroxide however, MEKP is a colorless, oily liquid at room temperature. Dilute
solutions of MEKP, typically containing 9-11% active oxygen, are used in industry and
by hobbyists to initiate the polymerization of polyester resins. The initiator decomposes
in the presence of transition metals such as cobalt and tertiary amines such as
dimethylaniline which are added as a component of the gel coat or resin formulation.
These additives are commonly referred to as “promoter” packages.
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methyl ethyl ketone peroxide monomer (MEKP)
As a conventional free radical polymerization, the kinetic mechanism of the styrene-
unsaturated polyester reaction can be expressed by initiation, propagation, and
termination. The subject of free-radical polymerization of polyester and acrylate
oligomers will be discussed in detail in chapter three.
Redox decomposition of organic peroxide initiator in the presence of cobalt salts4.
The gel coat film cure time is the elapsed time from the addition of the initiator until
sufficient network structure develops to allow removal of an integral film from the mold.
Typical film cure times will depend upon temperature, initiator concentration, promoter
type and concentration and can vary from 10 minutes to 2 hours. Following the initial
film cure the gel coat continues to develop hardness as the reaction proceeds. The
copolymerization of styrene and fumarate polyester unsaturation is diffusion controlled
with typical room temperature conversion level of reactive double bonds5 being 80-90%.
Following the film cure of the gel coat the laminate may be applied.
ROOH + Co2+
RO* + R* + OH-
+ Co3+
ROOH + Co3+
RO* + R* + H+ + Co
2+
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c. Reinforced Laminate Application
Laminate Processes
Following the initial film cure of the gel coat a fiber reinforced laminate or cast laminate
is applied to the back side of the coating. The entire laminate may be applied and cured as
a single layer or the laminate may be built progressively, layer upon layer. The specific
laminate materials and construction sequence are known as the laminate “schedule” and
will depend on several factors including the choice of lamination process, the desired
surface smoothness, reinforcing glass content, part volume and mechanical property
design requirements such as specific strength, and stiffness which may require the
incorporation of coring materials within the laminate. A summary of laminate process
features and limitations is given in table 4.
Laminate Options for UV Curing
UV curing is a line-of-sight process. An essential requirement is the ability to directly
irradiate the gel coat or laminate being cured. The lamination processes listed in table 4
which satisfy this requirement are the open mold lay-up processes ( hand lay-up, chopped
spray-up laminate process-both manual and robotic), and the closed molding bagging
processes ( vacuum bagging, SCRIMP process, ). SCRIMP, the patented Seeman
Composite Resin Infusion Molding Process, is a variant of classical vacuum bagging6,7
.
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OPEN MOLDING LAMINATE PROCESSES
Process
Part
Volume
Low10,000
Are gel coats
commonly
used withthis process?
Glass Content
Low≤36%High≤50%
Surface
Quality
(gloss,smoothness)
Possible
use of
coringmaterials?
Hand lay-up Low Yes Low High YesSpray-UplaminateProcess
Low Yes Low High Yes
Spray-UplaminateProcess -automated
High Yes Low High Yes
Casting Medium Yes None High No
Filamentwinding Medium No High Low No
Wet lay-upcompressionmolding
Medium No Medium Low No
CLOSED MOLDING LAMINATE PROCESSES
Process
Part
Volume
Low10,000
Are gel
coats
commonly
used with
thisprocess?
Glass
Content
Low≤
36%High≤50%
Surface
Quality
(gloss,smoothness)
Possible use of
coringmaterials?
Vacuum bag/ infusionSCRIMP/ZIP
Low Yes High High Yes
Pultrusion High No High Low NoCompressionmoldingSMC / BMC
High No LowVaries withuse of LPA
No
Resintransfer
molding(RTM)
Medium Yes Medium High Yes
ReinforcedReactioninjectionmolding(SRIM)
High No Low Low No
Table 4 - Summary of lamination process features and limitations
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Photograph 11 – Hand lay-up process
Photograph 12 – Hand lay-up process (2) Photograph 13 – Spray-up process using anexternal mix chopper gun with continuous E-glass roving
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Photograph 14 – Vacuum infusion lamination of a small boat hull
Photograph 15 – Hybrid process – open moldwet lay up followed by press molding to cureelectrical panels – Wet lay-up compressionmolding
Photograph 16 – Open molding process –automated lamination of roofing panels
Photograph 17 – Closed molding – resin transfer Photograph 18 – Dry reinforcement charged in
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molding (RTM) of toy locomotive to RTM mold prior to mold closure
Photograph 19 – Casting molding of syntheticmarble sink ( non-reinforced part)
Photograph 20 – Demolding the cured sink bowl(see photo. 6 for the clear gel coat application)
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter II – UV Curing Equipment and Radiometry
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Chapter II - UV Curing Equipment and Radiometry
a. Lighting systems
Numerous lighting sources have been used to photocure polymeric materials
including sunlight, fluorescent lamps, carbon-arc lamps, xenon lamps, and mercury
vapor lamps1,2,7. Mercury vapor lamps are by far the most commonly used source of
UV light for industrial applications due to the selection of intensity, spectral power
distribution, and stability. Lamps based on the mercury vapor bulb will be the focus
of the remainder of this section.
UV lamp assemblies3-6
consist of a bulb, a reflector, a housing, a cooling source, and
a power supply. A conventional lamp assembly is shown in photographs 1-2. The
bottom view of the lamp provides a clear view of the bulb, reflector, and the shutter
which can be closed to block the light from exiting the lamp. The bulb surface
temperature during operation is approximately 800C and cooling is required8.
i. Bulb Design
UV bulbs consist of an evacuated glass tube containing a small quantity of mercury.
The mercury is heated to produce an emission spectrum containing ultraviolet light.
The bulb shown in photograph 3 is an electrode arc style bulb. This type of bulb has
two electrodes located at each end of the glass tube. An excitation voltage is applied
across the electrodes to produce UV light. A shortcoming of this style of bulb arises
from the glass-metal interface design which can degrade and overheat during lamp
operation resulting in variable light intensity and ultimately bulb failure. Bulb
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degradation occurs due to oxidation of the electrodes and the metal wiring connectors
on the voltage lines resulting in poor conductivity which can lead to localized
overheating at the electrodes and bulb failure. The electrodeless bulb, shown in
photograph 4, consists of an evacuated glass tube containing a small quantity of
mercury. This type of bulb is heated using microwave energy by placing the bulb
inside a lamp housing fitted with a magnetron and radio frequency (RF) waveguide.
The RF energy is contained within the lamp housing by placing a thin metal mesh
sheet at the base of the lamp housing. A separate RF monitor is electrically
interlocked with the lamp power supply to prevent leakage of microwave energy.
Photograph 1 – Bottom view of UV lamphousing with the shutter open and the bulbexposed
Photograph 2 – Top view of UV lamphousing. The red hoses are used forwater cooling during operation
Photograph 3 – Electrode arc style medium pressure mercury vapor bulb( the bulb is energized by applying an electric current across the metal electrodes)
reflector shutter
arc st le bulb
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Photograph 4 – Electrodeless style medium pressure mercury vapor bulb( the bulb is energized with microwave heating)
Photograph 5 – UV lamp with an electrodestyle bulb and a dimpled reflector capableof producing diffusely reflected light (ref.5-6)
Photograph 6 – Modular microwave UVlamp – Bottom view – note the metalmesh designed to prevent leakage of theRF waves produced by the magnetronheating source
Photograph 7 - Modular microwave UVlamp – Bottom view electrodeless style bulb and elliptical reflector - note RFleakage monitor interlocked to the power
Photograph 8 – Modular microwave UVlamp – side view - note the 6” diameterair cooling hose (air flows from top to bottom through the lamp)
RF Detector
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supply
ii. Reflector Design – Light Ray Management
Several reflector designs may be used with the UV curing lamp. The reflector
partially circumscribes the UV bulb (270oarc) collecting approximately 75% of the
light emitted by the bulb. The elliptical reflector design produces a reflected ray
pattern that is concentrated at a fixed distance from the base of the lamp housing
known as the focal plane. The curing process is said to be “in focus” when the
material being polymerized is positioned in or near the focal plane. Maximum photon
flux, or irradiance, occurs within the focal plane of the lamp. When using an elliptical
reflector, the process is “out of focus” when the target material is located at a distance
beyond the focal plane. The focal plane is generally located at a distance of 3-7 inches
from the bulb. The exact distance can be obtained from the lamp manufacturer or
empirically by taking radiametric measurements.
Figure 1 – Reflector designs to focus (elliptical), collimate (paraboloic), and disperse(dimpled) light energy
Elliptical Parabolic Dimpled
Ultraviolet Light Reflectors
Focal Plane
Elliptical Parabolic Dimpled
Ultraviolet Light Reflectors
Focal Plane
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Reflector designs are also available to collimate the light (parabolic design) or
provide diffusely reflected light (dimpled design). Light ray management issues such
as the choice of reflector and distance from the lamp to the target will depend on the
specific factors such as the optical density (thickness, light absorption and scattering
characteristics) of the polymerizing material, curing speed requirements, and flash
point. Proper cleaning of the reflector is important to maintain the reflector efficiency.
Reflectors are usually cleaned at pre-set intervals with an alcohol solution to remove
any contamination. Consideration of equipment selection for the specific case of
curing gel coat and laminating resins will be covered in greater detail in Chapter VI.
In general, curing applications utilizing in-focus high intensity lighting are reserved
for cases involving materials with low optical density where high rates of cure are
possible. An example of this would be a graphic arts application of a UV curable ink
for a magazine advertisement. The film thickness of the ink is a fraction of a mil and
cure speeds of 300 feet per minute and greater are possible. As will be discussed in
greater detail in chapters IV and VI, gel coats and laminates are cured with non-
focused lighting to lower the light intensity for a variety of reasons such as substrate
temperature sensitivity, cure speed, safety, and the exposure time dependent
absorption characteristics of the gel coats and laminating resins.
iii. Temperature Management in UV Curing Applications
The optical efficiency of the lamp/reflector system is the ratio of light collected and
reflected versus the total light emitted in any spectral range. UV curing lamps
produce significant levels of infrared and visible radiation. As mentioned previously
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the surface temperature of the fused quartz UV bulb is approximately 800o C during
operation. It is generally desirable to maximize the ratio of UV band / IR band
radiation to keep the substrate and polymerizing coating or resin temperature as low
as possible while performing the UV curing. The primary source of infrared energy is
the hot quartz bulb itself rather than the plasma inside the bulb. In addition to proper
airflow to remove heat from the target, the following two strategies may be employed
to effectively manage the temperature:
1) Dichroic coatings on the reflector 8 – Dichroic filters operate using the principle of
interference. Alternating layers of an optical coating are built up on the reflector,
selectively reinforcing certain wavelengths of light and interfering with other
wavelengths. By controlling the thickness and number of the layers, the frequency
(wavelength) of the passband of the filter can be tuned and made as wide or narrow as
desired. A reflector having good reflectance to UV and poor reflectance to IR can
reduce the IR irradiance at the surface while providing UV irradiance. Dichroic
reflectors are sometimes referred to as “cold mirrors” due to the property of
selectively absorbing IR waves and reflecting UV waves.
Figure 2 – IR absorbing dichroic reflector
UV
Visible
IR
Dielectric Series
Absorbing Layer
Thermally Conductive Substrate
UV
Visible
IR
Dielectric Series
Absorbing Layer
Thermally Conductive Substrate
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The thickness of a single layer of a dichroic coating and its refractive index will
determine the reflected and non-reflected (transmitted) wavelengths.
''
4;
''
4
even
nt
odd
nt DTRANSMITTE REFLECTED
Where t is the thickness of the film, n is its refractive index, ‘odd’ and ‘even’ are
integers. When the film thickness is a multiple of the quarter-wavelength in the film,
that wavelength will be reflected.
Industrial dichroic reflectors are produced by vacuum deposition coating of a large
number (fifty or more) of thin layers of hard, transparent dielectric materials on the
conventional polished stainless steel reflector. Each layer has a different refractive
index from its adjacent layer. The coatings are formed using various inorganic oxides
such as aluminum oxide and silicon dioxide. The coating thickness of each layer is
very precisely controlled to achieve the cumulative constructive interference over the
UV spectral range of interest. The initial coating has an absorbing (black) coating in
which visible and IR waves are converted into heat (figure 2). The stainless steel
reflector base is thermally conductive and the heat is easily removed by cooling it.
The ratio of UV energy (200-450 nm) to IR energy (700-2500 nm) from an
electrodeless mercury UV bulb is
EUV/EIR = 1.73 where the UV band is 200-450 nm and the IR band is 700-2500 nm.
The radiant energy from the bulb reaching the target, ETARGET, can be determined
from the energy balance below:
ETARGET
EREFLECTED
EDIRECT
ETARGET
EREFLECTED
EDIRECT
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MAX
MIN
R E E
E E E
REFLECTED
DIRECT REFLECTEDTARGET
2
).(.
where EREFLECTED is the energy reflected which reaches the target, and EDIRECT is the
energy traveling directly from the bulb to the target without being reflected. E is the
spectral irradiance from the bulb at wavelength , R is the reflectance from the
surface of the lamp reflector, is the angle subtended by the reflector, and is the
sector of the reflector that is obscured by the bulb itself. EREFLECTED represents the
energy that reaches the target after being reflected, and EDIRECT is the energy radiating
directly from the bulb to the target. The reflector of an electrodeless lamp wraps
about the bulb including an angle of approximately 270o collecting approximately
75% of the light emitted from the bulb. A 90% IR absorbing dichroic reflector can
increase the EUV/EIR ratio by decreasing the reflected IR waves.
reflected direct
dichroic IR
UV
E
E
3.5
)9.01)(75.0()75.01(
73.1
2) Bulb diameter – Infrared energy is also focused via the reflector as well as being
directly radiated to the target. The primary source of infrared energy is the hot quartz
bulb envelope itself rather than from the plasma inside the bulb. The energy radiated
by the bulb is described by the Stefan-Boltzmann law:
4 AT e E where e is the emissivity of the surface, A is the surface area of
the bulb, is the Stefan-Boltzmann constant, and T is the temperature of the bulb in
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oK. Electrodeless style UV bulbs utilized in the microwave lamps are reported8 to
emit less IR radiation than conventional arc type bulbs due to their smaller surface
area. A comparison of arc-style and electrodeless bulbs is given below. Both lamps
are made of fused quartz and they will have the same emissivity. The only term that
will differ is the bulb surface area.
The ratio of surface area of the bulbs is the same as the ratio of their outer diameter
(25mm and 11 mm respectively):
3.21125
4
4
mmmm
D D
A A
T AeT Ae
E E
ESS ELECTRODEL IR
STYLE ARC IR
ESS ELECTRODEL IR
STYLE ARC IR
ESS ELECTRODEL IR
STYLE ARC IR
ESS ELECTRODEL IR
STYLE ARC IR
Thus the smaller diameter bulb produces less heat. A recent patent application9
reports good temperature management using LED lamps to perform photocuring.
iv. Metal halide doping to modify the spectral power distribution
Metal halide lamps are mercury vapor bulbs with the addition of metal halogens. The
metal halogens are added to create specific wavelength lines of ultraviolet radiation to
match the sensitivity of the photopolymer and photoinitiators being exposed. Metal
halogens are compounds composed of metal and halogen elements combined within a
curing bulb to form salts. Common metals added to the mercury bulb include gallium-
indium (known as gallium bulbs or “V” bulbs) and iron-cobalt (known as iron bulbs
or “D” bulbs). The electronegative halogens chemically react within the UV curing
bulb to cause a reaction in which the metals take on a positive charge. As the internal
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temperature of the metal halide lamp increases to the vaporization point of the metals,
the positive ions being produced allow the metals to release their outer electrons
causing ultraviolet radiation output at specific wavelengths.
The relative spectral power distributions of the mercury, iron, and gallium bulbs are
shown in figure 3.
Figure 3 – Relative spectral power distribution of commonly used UV bulbs(data obtained from Fusion UV Systems)
The mercury lamp provides the greatest output in the far UV (
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generation of ozone arising from the peak around 250nm. The iron doped
mercury lamp provides significant energy in the 340-400 nm range. The gallium
doped mercury bulb provides significant energy in the 390 – 440 nm range. The
interaction of light within a thick film of photocuring material will be discussed in
greater detail in Chapter III. It is critical to match the spectral power distribution
of the light source with the transmission-absorption characteristics of the coating ,
and photoinitiator.
v. Lamp motion relative to the target
UV curing process typically offer several advantages over oxidative, thermal, and
peroxide cure coating systems such as cure speed, energy utilization, and the abilityt
formulate with non-polluting multifunctional acrylate monomers and oligomers. To
realize these benefits however it is usually necessary move the lamp over the coating
or move the coating under the lamp to perform the UV curing step. Conveyors and
industrial robots are used move the lamp relative to the surface of the coating
providing control of the cure speed and energy exposure that is not possible with
fixed lamps (see figures 9-12)
Lighting systems can be designed with linear, rotational, and complex programmed
motion paths to address a wide range of curing requirements.
Linear with
RotationRotation Linear
Complex Motion
Industrial Robot
Linear with
RotationRotation Linear
Complex Motion
Industrial Robot
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Figure 4 – UV curing lighting systems
UV curing is a “line-of-sight” curing process. The material to be cured must be
capable of being directly illuminated by the light source or possible to illuminate with
the use of reflectors.
Photograph 9 – Bench scale UV conveyorand 6” modular microwave lamp
Photograph 10 – Pilot scale UVconveyor fitted with two 10” modularmicrowave lamps (built by the author)
Photograph 11 – Industrial robotic curing Photograph 12 – Industrial robotic curing
b. Radiometers and radiametric characterization of a UV curing process
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UV curing of coatings and composite laminates requires precise control of the process
variables related to energy exposure to insure safety and full development of
properties resulting from complete cure. Process radiometers are widely used in UV
curing applications to develop the process window, monitor and control levels of
energy exposure. Some of the basic terminology used for ultraviolet curing process
design and measurement is presented below. A more complete listing of terms may be
found in reference 10.
TERMINOLOGY
Absorbance – An index of the light absorbed by a medium compared to thelight transmitted through it. Numerically, it is the logarithm of the ratio ofincident spectral irradiance to the transmitted spectral irradiance. It is aunitless number. Absorbance implies monochromatic radiation, although it issometimes used as a average applied over a specific wavelength range.
Additive lamps – Medium pressure mercury vapor lamps (arc or microwave)that have had small amounts of metal halides added to the mercury within the buld. These materials will emit their characteristic wavelengths in addition tothe mercury emissions. This term is preferred over the term doped lamps.
Cosine response – Description of the spatial response to the incident energywhere the response is proportional to the cosine of the incident angle.
Dynamic exposure – Exposure to varying irradiance, such as when a lamp passes over a surface or a surface passes under a lamp or lamps. In the case ofdynamic exposures, energy is the time integral of the irradiance profile.
Effective energy density – Radiant energy, within a specified wavelengthrange, arriving at a surface per unit area, usually expressed in Joules per
square centimeter or millijoules per square centimeter (J/cm2
or mJ/cm2
).Alternate terms are exposure , or energy.
Irradiance – Radiant power, within a specified wavelength range, arriving at asurface per unit area. It is expressed in watts or milliwatts per squarecentimeter (W/cm2, or mW/cm2).
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Irradiance profile – The irradiance pattern of the lamp; or, in the case ofdynamic exposure, the varying irradiance at a point on a surface that passesthrough the field of illumination of a lamp or lamps.
Peak irradiance – The intense peak of focused power directly under a lamp.
The maximum point of the irradiance profile.
Power – The operating power of tubular UV lamps is commonly reported in“watts per inch” or “watts per centimeter”. This is derived simply from theelectrical power input divided by the effective length of the bulb. (It does nothave a direct meaning to the output efficiency of the lamp, to the curing performance, nor to the irradiance delivered to a work surface).
Radiometer – A device that senses irradiance incident on its sensor element.The construction consists of a photonic diode detector with an instantaneoussignal output that is proportional to the radiant flux over a wavelength range.
Static exposure – Exposure to a constant irradiance for a controlled period oftime. Contrast with dynamic exposure.
UV – Ultraviolet – Radiant energy in the 100 nm to 450 nm range. Radiantenergy in the 100 nm to 200 nm is referred to as vacuum UV (VUV), becauseit does not transmit in air
VUV, UVA, UVB, UVC, UVV – UVA is commonly referred to as longwavelength UV. UVC is commonly referred to as short wavelength UV. UVVis very long wavelength UV.
VUV: 100 - 200 nmUVC: 200 – 280 nmUVB: 280 – 315 nmUVA: 315 – 400 nmUVV: 400 – 450 nm
The key optical and physical characteristics of the curing equipment are:
UV Irradiance – the radiant power, within a stated wavelength range, arriving at the
surface per unit area. Irradiance varies with lamp output power, efficiency, and focus
of the reflector system. Irradiance is a characteristic of the lamp geometry and power
and does not vary with line speed.
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UV Energy Density – the radiant energy, within a stated wavelength range, arriving
at a surface per unit area. The energy, sometimes referred to as “dose”, is the total
accumulated photon quantity. Energy is inversely proportional to line speed under
any given light source, and proportional to the number of exposures (for example,
rows of lamps).
dt I E
t
t
1
0
2121 )()(
Spectral Distribution – is the radiant energy as a function of wavelength or
wavelength range. It may be expressed in power units or in relative terms
(normalized). The radiant energy from a bulb is presented by grouping the data in 10
nanometer bands in the form of a distribution plot.
Irradiance Profile – is the irradiance as a function of distance from the centerline of
the lamp. This profile takes the form of a Gaussian distribution. The peak irradiance
value occurs at the centerline. The irradiance profile is characteristic of the lamp
design. Increasing the power to the lamp does not change the ratio of peak irradiance
to total energy (at any speed). The e profile of a lamp can change if the bulb sags out
of the focused position, or if the reflector has been deformed.
Infrared Radiance – the heating effect from infrared energy emitted by the hot
quartz bulb.
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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter II – UV Curing Equipment and Radiometry
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The radiometer shown in photographs 13-14 is mounted on the side of the UV curing
lamp and measures the instantaneous irradiance (mW/cm2) from the lamp. The fixed
mounted (static) radiometer is used to monitor the output stability of the lamp. The
distance and angle of the radiometer with respect to the lamp must be held constant.
A second type of radiometer, the traveling (dynamic) process radiometer (see
photographs 15-16), is placed on the conveyor belt and used to measure the irradiance
(mW/cm2) arriving at the surface. This instrument also serves as a dosimeter, with the
capability of reporting the energy (mJ/cm2) which is the time integral of the
irradiance.
Photograph 13 – Photodiode radiometerwith dry air purge line
Photograph 14 – UV lamp housing with a process radiometer mounted on the lamphousing to monitor the lamp output
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Photograph 15 – Traveling processradiometer – top view – sensor is visible
Photograph 16 – Traveling processradiometer – bottom view – controls andreadout are visible
Modern instruments measure multiple UV bands (UVC, UVB, UVA, UVV). The
responsivity of a radiometer is the amplitude of the response of a detector to different
wavelengths. Radiometers need to be calibrated periodically due to solarization of the
sensing element which can affect the responsivity of the radiometer. Other important
information that should be known to avoid errors include:
The dynamic range of the radiometer – The range of the instrument must be
adequate for the irradiance to which it is exposed. If the light intensity exceeds
the radiometer limit the result will be an under reporting of irradiance
(W/cm2) and radiant energy (J/cm2).
The sampling rate of the radiometer / dosimeter – the dosimeter calculates the
accumulated photon count by measuring the irradiance at specific sampling
intervals. The sampling rate should be adequate for the process being
measured. For example, assume the irradiance profile of a lamp was 3 inches
wide. A traveling radiometer with a sampling rate of 10 samples/second
moving at a line speed of 2.5 feet/minute would take a measurement every a
measurement every 1/12 of an inch (i.e. 36 measurements within the
irradiance profile). This would provide a reliable measure of the lamp energy.
On the other hand, if the line speed was 120 feet/minute, the radiometer would
collect one measurement for every four inches of travel. This condition would
produce a reporting serious error for the lamp irradiance and energy.
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Additional information that should be known about the radiometer includes the
spatial response, the threshold response (minimum irradiance), and temperature
tolerance limits.
Lamp monitoring is a critical process control parameter for a UV cure process in a
production environment. In many cases, however, equipment design does not allow
conventional radiometers11-13 to be used so alternatives must be found. A common
method is to use radiachromic tags14
(a film or paper strip coated with a UV sensitive
dye that undergoes a photochemical color change upon exposure). The extent of the
color change can be correlated with the exposure conditions. Radiachromic tags
function as dosimeters and can be very useful under the right conditions and provide
extremely reliable process control information.
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Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate
Systems
a. Synthesis of thermosetting polyester and acrylate oligomers(condensation polymerization)
2-7
Unsaturated Polyesters
The first reported synthesis of polyester resins was carried out by Julian Hill, a
member of Wallace Carothers team, at the Dupont Research Labs in 19331. The
unsaturated polyester resin solutions used in the production of gel coats and
laminating resins are low molecular weight condensation oligomers (Mn 1000 –
5000) which have been diluted in a reactive diluent such as styrene2 or methyl
methacrylate.
SATURATED
DIBASIC ACIDS
PHTHALIC ANHYDRIDE
ISOPHTHALIC ACID
ADIPIC ACID
TERPHTHALIC ACID
CHLORENDIC ANHYDRIDE
UNSATURATED
DIBASIC ACIDS
MALEIC ANHYDRIDE
FUMARIC ACID
GLYCOLS
PROPYLENE GLYCOL
DIETHYLENE GLYCOL
ETHYLENE GLYCOL
DIPROPYLENE GLYCOL
NEOPENTYL GLYCOL
OTHER GLYCOLS
HYDROCARBON
MODIFIERS
DICYCLOPENTADIENE
REACTIVE MONOMERS
STYRENE
METHYL METHACRYLATE
VINYL TOLUENEPARA-METHYL STYRENE
ALPHA-METHYL STYRENE
UNSATURATED
POLYESTER
CONDENSATE
CROSSLINKED
UNSATURATED
POLYESTER
PROMOTORS,
INHIBITORS,
ETC.
FREE RADICAL
INITIATOR
ESTERIFICATION
SATURATED
DIBASIC ACIDS
PHTHALIC ANHYDRIDE
ISOPHTHALIC ACID
ADIPIC ACID
TERPHTHALIC ACID
CHLORENDIC ANHYDRIDE
UNSATURATED
DIBASIC ACIDS
MALEIC ANHYDRIDE
FUMARIC ACID
GLYCOLS
PROPYLENE GLYCOL
DIETHYLENE GLYCOL
ETHYLENE GLYCOL
DIPROPYLENE GLYCOL
NEOPENTYL GLYCOL
OTHER GLYCOLS
HYDROCARBON
MODIFIERS
DICYCLOPENTADIENE
REACTIVE MONOMERS
STYRENE
METHYL METHACRYLATE
VINYL TOLUENEPARA-METHYL STYRENE
ALPHA-METHYL STYRENE
UNSATURATED
POLYESTER
CONDENSATE
CROSSLINKED
UNSATURATED
POLYESTER
PROMOTORS,
INHIBITORS,
ETC.
FREE RADICAL
INITIATOR
ESTERIFICATION
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Figure 1 – Production of unsaturated polyester resin solutions
Reactants are chosen on the basis of the specific properties needed for the application.
Laminating resins based on phthalic anhydride / maleic anhydride/ propylene glycol
(PAn/MA/PG) are in frequent use due to the combination of low cost, good balance
of thermal mechanical properties (Tg, strength, elongation), and the ability to conduct
the condensation with glycol with a short cycle time in a single processing step. In
recent years the use of dicyclopentadiene (DCPD) has been incorporated into
laminating resins to lower volatile organic content (VOC) due to increasing
regulations (NESHAP – national Emission Standards for Hazardous Air Pollutants).
The use of dicyclopentadiene allows resin producers to prepare resins with lower
solution viscosities and therefore higher solids content. The primary drawbacks of
DCPD based resins are 20-40% lower thermal-mechanical properties, high resin
color, and poor secondary bonding.
Gel coat resins based on isophthalic acid / maleic anhydride / neopentyl glycol
(IPA/MA/NPG) offer an excellent balance of thermal-mechanical properties needed
to preclude cracking, provide surface hardness, and prevent fiber printing. Neopentyl
glycol (2,2,dimethyl-1,3 propane diol) imparts excellent hydrolytic stability due to
steric hindrance of the ester group by the methyl groups and the absence of alpha-
hydrogen atoms. Condensation reactions carried out with IPA/MA/NPG have the
disadvantage of greater production cycle times than PAn/MA/PG condensation
polymers. The former polymer requires a two step synthesis due to the unequal
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reactivity of isophthalic acid and maleic anhydride with alcohol. Resins with high
levels of neopentyl glycol are commonly modified with a non-linear glycol or a
glycol with bulky side groups to improve the solubility in styrene. Common glycols
used for this purpose include 1,2 propane diol, and 2- butyl, 2- ethyl 1,3 propane diol.
Adipic acid may be used as a flexibilizing diacid in gel coat and laminating resins
when water resistance and Tg can be compromised.
Terephthalic acid (TPA), a configurational isomer of isophthalic acid (IPA), offers
slightly improved thermal resistance and favorable costs compared to isophthalic acid
in UPR resin applications. Unfortunately the reactivity of the carboxylic acid groups
on TPA is lower than those of IPA resulting in a 50% increase in cycle time (30 hours
vs. 20 hours). Polyester resins based on TPA can be prepared via alkoxylation by
reacting the terephthalic acid with ethylene oxide or propylene oxide when pressure
reaction vessels are available.
While liquid samples of UPR resins are easily analyzed, the chemical structure of
samples of cured polyester resins are not readily elucidated in solid form by
spectroscopic techniques (H-NMR, FTIR) or chromatography (GPC/HPLC) since the
cured polymer is not soluble in organic solvents. Certain features of the cured
network such as the fraction of maleic anhydride carbon-carbon double bonds
reacting into the network may be analyzed by C-NMR 3. The most common method
to analyze cured polyester resins involves hydrolysis of the ester group followed by
condensation with monofunctional reactants such as acetic acid. The low molecular
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weight (Mn
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specify the prepolymer composition is :SA:UA:G:M , where SA= moles of saturated
acid, UA=moles of unsaturated diacid, G=moles of glycol, M=moles of crosslinking
monomer C=C. Specific resin compositions used by gel coat and resin suppliers are
confidential and are not disclosed in this paper. Typical resin formulations are
available in the patent and trade literature listed in the references. A commercially
useful gel coat may have the following molar ratio of reactants SA:UA:G:M = 1.0 :
1.0 : 2.1 : 2.0. High temperature molding resins useful for sheet molding compounds
(SMC) may have the approximate molar ratio SA:UA:G:M – 0 : 1 : 1: 1.5. Higher
levels of saturated diacid to unsaturated diacid, SA:UA, will result in a lower glass
transition temperature due to a reduction in crosslink density4. The specific ratio of
monomer to unsaturated diacid, M:UA, depends on the reactivity ratios of the
monomers. As shown in figure 2, the disappearance of C=C double bonds was
studied by transmission FTIR for an unsaturated polyester prepolymer diluted with
styrene monomer. A second sample was prepared in which the prepolymer was
diluted a methacrylate monomer, 1,6 hexanediol diacrylate. Peaks centered at 911
cm-1 (styrene C=C), 815cm-1 (methacrylate C=C), and 982 cm-1 (UP C=C) were
used to follow the reaction. The sample was cured at room temperature using a cobalt
octoate promoter and methyl ethyl ketone peroxide initiator.
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Reactivi ty - UPR / monomer C=C by FTIR
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140
Time Since Initiation (minu tes)
D o u b l e B o n d C o n v e r s i o n
Fumarate C=C (UPR-Styrene)
STYRENE C=C (UPR-Styrene)
Fumarate C=C (UPR-Acrylate)
Acrylate C=C (UPR-Acrylate)
Figure 2 – Polymerization of UPR prepolymer and crosslinking monomer
As predicted from published reactivity ratios the fumarate-ester copolymerizes more
readily with styrene monomer than with methacrylate monomer. The conversion of
double bonds during the formation of the crosslinked network is diffusion controlled
and based on monomer mobility. Using FTIR it is possible to determine the amount
of unreacted double bonds which remain trapped in the polymer due to vitrification of
the system. It has been shown experimentally for styrene monomer:maleate-ester
combinations, where copolymerization is favored over homopolymerization, the
maximum fraction of the maleic anhydride carbon-carbon double bonds in the
polyester is reacted into the network 3,5
when M/UA>2. Rapid copolymerization of
styrene with unsaturation present in the prepolymer occurs3 when the maleic acid (cis
isomer) is isomerized to fumaric acid (trans isomer).
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C C
C CO O
OHHO
maleic acid (cis isomer
CC
C
C
O
O
OH
HO
fumaric acid (trans isomer)
In practice, the cis-trans isomerization is accomplished by the sequence of addition of
reactants to the reaction vessel as well as proper temperature control during heat-up.
With good synthesis processing controls in place it is possible to achieve >90%
isomerization to the trans isomer. High levels of isomerization are particularly
important for prepolymers used in exterior gel coat formulations where unreacted
double bonds may lower exterior durability. In addition to the degree of
isomerization, it is important to allow for reductions in prepolymer reactivity
associated the double bond saturation reaction, also known as the Ordelt reaction,
which occurs when an alcohol adds directly to the fumarate-ester carbon-carbon
double bond6,7
.
C C
C
C
O
O
OH
HO
fumaric acid (trans isomer)
CH3
HO
CH2 OH
H
C
C C
C
C
O
O
OH
HO
CH3
O
CH2 OH
H
C
H
propylene glycolOrdelt Reaction - double bond saturatio
+
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Vinyl Ester Resins
A second major class of thermosetting polymers used to produce recreational
composite products is based the acrylation of bisphenol A epoxides, or vinyl ester
resins
(VE). The prepolymer is diluted with styrene monomer to produce the VE resin.
CH2CH
CH3
CH2
CH3
CO HO
CH3
CH2
O
O
O
O
CH2CHC
O
C
CH2+
bisphenol A epoxy methacrylic acid
CH2CH
CH3
CH2CO
CH3
CH2 O CH2CH
OH OH CH3
OC
O
C
CH2
CH3
CC
O
H2C
epoxy diacrylate (vinyl ester resin)
Cured vinyl ester resins exhibit corrosion resistance and thermal-mechanical
properties that are superior to UPR