Thesis -Gel-L Scott Crump

8/15/2019 Thesis -Gel-L Scott Crump

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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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ii

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

1

16

34

70

75

142

150

156


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iv

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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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

34


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vi

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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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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viii

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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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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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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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


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


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

56


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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)


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

58

59

62

63

64

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72

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74

75

84

84

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xiv

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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xv

23

24

25

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27

28

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30

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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

119

121

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126

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132

134

138


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Photograph # Description Page

1

2

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8

9

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11

12

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15

16

17


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

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3

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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


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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12

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16

1

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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


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


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

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5

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UV curing the laminate

Cured laminate

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Process Optimization of Photocurable Polyester Gel Coat and Laminate

xx

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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xxi

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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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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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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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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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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(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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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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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



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


45/187



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


47/187



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


48/187



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).


52/187



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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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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Process Optimization of Photocurable Polyester Gel Coat and LaminateChapter III – Chemistry of Thermosetting Unsaturated Polyester and AcrylateSystems


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

Documents

Thesis -Gel-L Scott Crump