PROCESS OPTIMIZATION OF PHOTOCURABLE POLYESTER GEL COAT AND LAMINATE

Transcription

1 PROCESS OPTIMIZATION OF PHOTOCURABLE POLYESTER GEL COAT 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 UNIVERSITY Cleveland, Ohio May, 014

2 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 i

3 I grant to Case Western Reserve University the right to use this work irrespective of any copyright, for the University s own purposes without cost to the University or to its 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 from microforms, to the public for the cost of reproduction. ii

4 List of Tables. Table of Contents Table of Contents..... List of Tables List of Figures Acknowledgements..... vi xii xvi xviii Chapter I The Composite Open Molding Process.. 1 Chapter II UV Curing Equipment and Radiometry Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems.. Chapter IV Characterization of Resin / Coating State of Cure Chapter V Modeling the Degree of Cure of a D 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 iii

5 List of Tables Table of Contents Chapter I The Composite Open Molding Process. 1 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. 16 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 iv

6 List of Tables Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems.. 34 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 v

7 List of Tables 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 vi

8 List of Tables Chapter V Modeling the Degree of Cure of a D UV Curing 75 Process a. Studies of coatings and laminate resin related variables i. Experiment 1 Effect of pigmentation-screening study involving ten different colors ii. Experiment Effect of TiO concentration iii. Experiment 3 Effect of gel coat film thickness iv. Experiment 4 Factorial study of photoinitiator concentration, UV energy, and film thickness v. Experiment 5 - Binder / reactive diluent selection vi. Experiment 6 Light transmission studies in the laminate resin b. Studies of UV curing equipment variables i. Experiment 7 Reciprocal law for UV energy, independence of irradiance and line speed ii. Experiment 8 DSC cure studies in clear and white gel coat effect of film thickness and UV energy iii. Experiment 9 Temperature-Energy-Irradiance map for several UV light sources iii. Experiment 10 Variations in energy and irradiance of a single 600 W/inch lamp as function of distance from the lamp centerline iv. Experiment 11 Measurement of energy and irradiance from a bank of five 600 W/inch lamps as a function of lateral position vi. Experiment 1 Testing the additive law for UV energy using two 600 W/inch lamps vii

9 List of Tables vii. Experiment 13 Effect of UV energy and irradiance level on the surface temperature of the coating viii. Experiment 14 Validation of the cosine law for non-perpendicular exposure conditions ix. Experiment 15 Effect of lamp height on UV energy and irradiance x. Experiment 16 Evaluation of dichroic reflectors c. Studies of the reflectivity of the mold surface Experiment 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 - The effect of lamp spacing on the irradiance and energy distribution iv. Simulation 3 The effect of a lamp failure on the irradiance and energy distribution v. Simulation 4 The effect of lamp height on energy level and uniformity viii

10 List of Tables Chapter VI Process Optimization - Defining the Process 14 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 ix

11 List of Tables Table # Description Page Chapter I - The Composite Open Molding Process U.S. Markets and Applications for Unsaturated Polyester Based Composites Cone and plate rheometer programming sequence to simulate the rheological 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 kinetic model 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 D UV Process Factorial study of photoinitiator concentration, UV energy, and TiO concentration Binder / reactive diluent selection Reciprocal law for UV energy, independence of irradiance and line speed Variations in energy and irradiance of a single 600 W/inch lamp as function of distance from the lamp centerline Gaussian fit parameters to model light dispersion a 600 W/inch lamp x

12 List of Tables Measurement of energy and irradiance from a bank of five 600 W/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 surface temperature of the substrate Validation of the cosine law for non-perpendicular exposure conditions UV Energy and irradiance measurements at various lamp heights static one minute exposure The irradiance from a point source of light varies with the square of the distance from the source Evaluation of dichroic reflectors White UV curable gel coat results on a reflective and nonreflective 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) xi

13 List of Figures Figure # 1 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 Page Chapter II - UV Curing Equipment and Radiometry Reflector designs to focus (elliptical), collimate (parabolic), and disperse (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 redox initiator system UPR-styrene monomer Effect of initiator concentration on the gel point time redox initiator 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, and films commonly used for UV cure applications (path length=10 mm UV cell, 100% concentration) xii

14 List of Figures Absorption-scattering characteristics of titanium dioxide pigment Effect of the absorptivity on light transmission characteristics in a film Fraction of incident energy absorbed in the top 1% and bottom 1% 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 film measured by FTIR (T=5C) Chapter V - Modeling the Degree of Cure of a D 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 an opaque 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 laminate containing 35% short fiber E-glass reinforcement (0.75% BAPO photoinitiator) Effect of photoinitiator concentration on the cure of a UPR laminate containing 35% short fiber E-glass reinforcement xiii

15 List of Figures (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 several commercially available UV lamps UV lighting set-up for experiment 10 a single 600 W/inch Fusion UV lamp Energy distribution for a Fusion 600 W/in lamp UV lighting set-up for experiment 11 a bank of five 600 W/inch Fusion UV lamps Measured UV Energy Bank of five 600 W/inch Fusion UV lamps UV lighting set-up for experiment 1 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 surface temperature of a part being cured with UV lamps Validation of the cosine law for non-perpendicular exposure conditions 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 various colors xiv

16 List of Figures UV-Visible reflection from metal molds and aluminum flake filled polyester tooling gel coat Schematic of a conveyor line with the coordinate system indicated Lamp height dependence of the pre-exponential multiplier and dispersion 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 UV energy The impact of a lamp failure on the UV energy and irradiance distribution The effect of lamp height on the level and uniformity of UV energy Chapter VI - Process Optimization - Defining a Process Window Process window for UV curable gel coat 138 xv

17 List of Figures Photograph # Description Page 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 () Application of barrier skin coat laminate on the white gel coat Completed hull after being removed from the mold Spray pattern test prior to applying the gel coat on a deck mold Clear gel coat applied to a mold used to make a synthetic marble 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 () Spray-up process using an external mix chopper gun with continuous E-glass roving Vacuum infusion lamination of a small boat hull Hybrid process open mold wet lay-up followed by press molding to cure electrical panels Wet lay-up compression molding Open molding process automated lamination of roofing panels Closed molding resin transfer molding (RTM) of toy locomotive xvi

18 List of Figures Dry reinforcement charged in to RTM mold prior to mold closure 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 and the bulb exposed Top view of UV lamp housing. The red hoses are used for water cooling during operation Electrode style medium pressure mercury vapor lamp ( the bulb is energized by applying an electric current across 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 dimpled reflector capable of producing diffusely reflected light Modular microwave UV lamp Bottom view note the metal mesh designed to prevent leakage of the RF waves produced by the magnetron heating source Modular microwave UV lamp Bottom view electrodeless style bulb and elliptical reflector - note RF leakage monitor interlocked to the power supply Modular microwave UV lamp side view - note the 6 diameter air cooling hose Bench scale UV conveyor and 6 modular microwave electrodeless lamp Pilot scale UV conveyor fitted with two 10 modular microwave electrodeless lamps Industrial robotic curing xvii

19 List of Figures Industrial robotic curing Photodiode radiometer with dry air purge line UV lamp housing with a process radiometer mounted on the lamp housing to monitor the lamp output Traveling radiometer top view photodiode array is visible Traveling process radiometer bottom view controls and readout are visible Chapter III - Chemistry of Thermosetting Systems. Alligatoring phenomena the top 1-3 mils is cured while the balance of the film is wet Uncured coating material which remains after the cured surface 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 D 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 xviii

20 List of Figures 5 6 UV curing the laminate Cured laminate xix

21 Acknowledgement I would like to thank my wife Ruth for giving me the many uninterrupted hours needed to prepare this paper. xx

22 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 xxi

23 irradiance distribution for a D 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. xxii

24 Chapter I - The Composite Open Molding Process 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 Market 1. 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 Construction (664 MM lbs) Consumer and Recreational (73 MM lbs) Electrical / Electronic (61 MM lbs) Marine (314 MM lbs) Transportation (160MM lbs) Non-Reinforced Market Transportation / Body Putty (69 MM lbs) Construction (0.4 MM lbs) Consumer Goods (36 MM lbs) Gel Coats (10 MM lbs) Other (53 MM lbs) Other (15MM lbs) Total ( 1.9 B lbs) MM=million B=billion Total (0.55 B lbs) Table 1 U.S. Markets and Applications for Unsaturated Polyester Based Composites 1 1

25 Chapter I - The Composite Open Molding Process 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 inches (0-30 mils). This film thickness is a 10-0 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 6-34 dyne/cm prior to coating application to aid in the separation of final composite article from the mold. 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.

26 Chapter I - The Composite Open Molding Process Photograph 1 Application of white gel coat to a large hull mold Photograph - Application of white gel coat to a large hull Photograph 3 Application of barrier skin coat laminate on the white gel coat Photograph 4 Completed hull after being removed from the mold Gel coat application 55 foot luxury yacht 3

27 Chapter I - The Composite Open Molding Process Photograph 5 Spray pattern test prior to applying the gel coat on a deck mold Photograph 6 Clear gel coat applied to a mold used to make a synthetic marble sink. Photograph 7 Black gel coat applied to a cowling mold for a small tractor Photograph 8 Gel coat applied to a tub/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 treatments 3 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 4

28 Chapter I - The Composite Open Molding Process (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 gel coat Mold surface energy= 0 dynes/cm Coating surface tension= 41 dynes/cm Initial film thickness = 0 mils Photograph 10 De-wetting (crawling) of a white gel coat Mold surface energy= 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 - 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). 5

29 Chapter I - The Composite Open Molding Process M=torque =angle r=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 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 below. The results of the experiment are shown in figure. Maximum* Maximum Collection # Points Sequence # Function Stress Duration Interval Collected Sequence 1 equilibration 9 Pa 6000 sec 1 point/60 sec 10 Sequence pumping 80 Pa 15 sec 1 point/30 sec 4 Sequence 3 spraying 34 Pa 15 sec 1 point/30 sec 4 Sequence 4 recovery (fast) 9 Pa 45 sec points/sec 99 Sequence 5 recovery (slow) 9 Pa 750 sec 1 point/3 sec 50 Table Cone and plate rheometer programming sequence to simulate the rheological lifecycle of a commercial gel coat. 6

30 Chapter I - The Composite Open Molding Process Rheological Performance Requirement No sedimentation of fillers and pigments Ease of pumping from the container to a spray gun Ease of atomization at the spray gun Sag resistant at the applied film thickness Gel Coat Solvent Based Paint similar requirement for both types of coatings similar requirement for both types of coatings Commonly used equipment: Commonly used equipment: airless spray gun with a tip air atomizing pressurized opening of 0.00 and fluid pot spray gun with a tip pressure of 1000 psi. The opening of an a fluid fluid lines are frequently pressure of psi. heated to 100F to lower the viscosity under high shear. Typical application thickness: Typical application 0-30 mils thickness: 1-3 mils h Shear stress calculation gh 10 lb/gal.=100 kg/m 3 g=9.8 m/s h=30 mils=7.63 x 10-4 m Shear stress calculation gh 10 lb/gal.=100 kg/m 3 g=9.8 m/s h= mils=5.08 x 10-5 m 9.8 kg/m-sec = 9.8 Pa 9.8 kg/m-sec = 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. 7

31 Chapter I - The Composite Open Molding Process Table 3 Application flow requirements of gel coat and solvent based paint Pump - Spray - Sag Simulation for Gel Coat and Polyurethane Paint 10, , Gel Coat Viscosity Polyurethane Paint Viscosity Shear Stress Viscosity (Pa-s) Shear Stress (Pa) Time (sec) 0 Figure Experiment to simulate the shear history of a commercial polyester gel coat and 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 34 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 8

32 Chapter I - The Composite Open Molding Process figure 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. 9

33 Chapter I - The Composite Open Molding Process methyl ethyl ketone peroxide monomer (MEKP) As a conventional free radical polymerization, the kinetic mechanism of the styreneunsaturated 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. ROOH + Co + RO* + R* + OH - + Co 3+ ROOH + Co 3+ RO* + R* + H + + Co + Redox decomposition of organic peroxide initiator in the presence of cobalt salts 4. 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 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 bonds 5 being 80-90%. Following the film cure of the gel coat the laminate may be applied. 10

34 Chapter I - The Composite Open Molding Process 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 bagging 6,7. 11

35 Chapter I - The Composite Open Molding Process OPEN MOLDING LAMINATE PROCESSES Process Part Volume Low<1000 High>10,000 Are gel coats commonly used with this process? Glass Content Low 36% High 50% Surface Quality (gloss, smoothness) Hand lay-up Low Yes Low High Yes Spray-Up laminate Process Spray-Up laminate Process - automated 1 Possible use of coring materials? Low Yes Low High Yes High Yes Low High Yes Casting Medium Yes None High No Filament winding Medium No High Low No Wet lay-up compression molding Medium No Medium Low No CLOSED MOLDING LAMINATE PROCESSES Are gel coats commonly used with this process? Process Part Volume Low<1000 High>10,000 Glass Content Low 36% High 50% Surface Quality (gloss, smoothness) Vacuum bag / infusion Low Yes High High Yes SCRIMP/ZIP Pultrusion High No High Low No Compression molding SMC / BMC Resin transfer molding (RTM) Reinforced Reaction injection molding (SRIM) High No Low Varies with use of LPA Possible use of coring materials? Medium Yes Medium High Yes High No Low Low No Table 4 - Summary of lamination process features and limitations No

36 Chapter I - The Composite Open Molding Process Photograph 11 Hand lay-up process Photograph 1 Hand lay-up process () Photograph 13 Spray-up process using an external mix chopper gun with continuous E- glass roving 13

37 Chapter I - The Composite Open Molding Process Photograph 14 Vacuum infusion lamination of a small boat hull Photograph 15 Hybrid process open mold wet lay up followed by press molding to cure electrical panels Wet lay-up compression molding Photograph 16 Open molding process automated lamination of roofing panels Photograph 17 Closed molding resin transfer Photograph 18 Dry reinforcement charged in 14

38 Chapter I - The Composite Open Molding Process molding (RTM) of toy locomotive to RTM mold prior to mold closure Photograph 19 Casting molding of synthetic marble sink ( non-reinforced part) Photograph 0 Demolding the cured sink bowl (see photo. 6 for the clear gel coat application) 15

39 Chapter II UV Curing Equipment and Radiometry 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 lamps 1,,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 assemblies 3-6 consist of a bulb, a reflector, a housing, a cooling source, and a power supply. A conventional lamp assembly is shown in photographs 1-. 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 required 8. 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 16

40 Chapter II UV Curing Equipment and Radiometry 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. reflector shutter arc style bulb Photograph 1 Bottom view of UV lamp housing with the shutter open and the bulb exposed Photograph Top view of UV lamp housing. The red hoses are used for water 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) 17

41 Chapter II UV Curing Equipment and Radiometry Photograph 4 Electrodeless style medium pressure mercury vapor bulb ( the bulb is energized with microwave heating) Photograph 5 UV lamp with an electrode style bulb and a dimpled reflector capable of producing diffusely reflected light (ref. 5-6) Photograph 6 Modular microwave UV lamp Bottom view note the metal mesh designed to prevent leakage of the RF waves produced by the magnetron heating source RF Detector Photograph 7 - Modular microwave UV lamp Bottom view electrodeless style bulb and elliptical reflector - note RF leakage monitor interlocked to the power Photograph 8 Modular microwave UV lamp side view - note the 6 diameter air cooling hose (air flows from top to bottom through the lamp) 18

42 Chapter II UV Curing Equipment and Radiometry 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 (70 o arc) 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. Ultraviolet Light Reflectors Elliptical Parabolic Dimpled Focal Plane Figure 1 Reflector designs to focus (elliptical), collimate (paraboloic), and disperse (dimpled) light energy 19

43 Chapter II UV Curing Equipment and Radiometry 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 nonfocused 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 0

44 Chapter II UV Curing Equipment and Radiometry the surface temperature of the fused quartz UV bulb is approximately 800 o 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. UV Visible IR Dielectric Series Absorbing Layer Thermally Conductive Substrate Figure IR absorbing dichroic reflector 1

45 Chapter II UV Curing Equipment and Radiometry The thickness of a single layer of a dichroic coating and its refractive index will determine the reflected and non-reflected (transmitted) wavelengths. REFLECTED 4nt ' odd' ; TRANSMITTED 4nt ' even' 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 ). The stainless steel reflector base is thermally conductive and the heat is easily removed by cooling it. The ratio of UV energy ( nm) to IR energy ( nm) from an electrodeless mercury UV bulb is E UV /E IR = 1.73 where the UV band is nm and the IR band is nm. The radiant energy from the bulb reaching the target, E TARGET, can be determined from the energy balance below: E REFLECTED E DIRECT E TARGET

46 Chapter II UV Curing Equipment and Radiometry E TARGET E REFLECTED E DIRECT E REFLECTED MAX MIN E. R.( ) where E REFLECTED is the energy reflected which reaches the target, and E DIRECT 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. E REFLECTED represents the energy that reaches the target after being reflected, and E DIRECT 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 70 o collecting approximately 75% of the light emitted from the bulb. A 90% IR absorbing dichroic reflector can increase the E UV /E IR ratio by decreasing the reflected IR waves. E E UV IR dichroic 1.73 (1 0.75) (0.75)(1 0.9) 5.3 direct reflected ) 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: E 4 eat 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 3

47 Chapter II UV Curing Equipment and Radiometry o K. Electrodeless style UV bulbs utilized in the microwave lamps are reported 8 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 (5mm and 11 mm respectively): E E IR ARC STYLE IRELECTRODELESS ea ea IR ARC STYLE T IRELECTRODELESS 4 T 4 A A IR ARC STYLE IRELECTRODELESS D D IR ARC STYLE IRELECTRODELESS 5 mm 11mm.3 Thus the smaller diameter bulb produces less heat. A recent patent application 9 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 galliumindium (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 4

48 Chapter II UV Curing Equipment and Radiometry 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. Spectral Output of UV Lamps Gallium Doped Lamp Relative Energy Watts / Inch Iron Doped Lamp Mercury Lamp gy Wavelength (nm) 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 (<300 nm). Most polymers absorb light below 90 nm limiting the mercury bulb to surface cure and thin film cure applications. Another shortcoming of the mercury lamp is the 5

49 Chapter II UV Curing Equipment and Radiometry generation of ozone arising from the peak around 50nm. The iron doped mercury lamp provides significant energy in the nm range. The gallium doped mercury bulb provides significant energy in the 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 9-1) Lighting systems can be designed with linear, rotational, and complex programmed motion paths to address a wide range of curing requirements. Rotation Linear 6 Linear with Rotation Complex Motion Industrial Robot

50 Chapter II UV Curing Equipment and Radiometry 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 conveyor and 6 modular microwave lamp Photograph 10 Pilot scale UV conveyor fitted with two 10 modular microwave lamps (built by the author) Photograph 11 Industrial robotic curing Photograph 1 Industrial robotic curing b. Radiometers and radiametric characterization of a UV curing process 7

51 Chapter II UV Curing Equipment and Radiometry 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 the light transmitted through it. Numerically, it is the logarithm of the ratio of incident spectral irradiance to the transmitted spectral irradiance. It is a unitless number. Absorbance implies monochromatic radiation, although it is sometimes 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 to the mercury emissions. This term is preferred over the term doped lamps. Cosine response Description of the spatial response to the incident energy where 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 of dynamic exposures, energy is the time integral of the irradiance profile. Effective energy density Radiant energy, within a specified wavelength range, arriving at a surface per unit area, usually expressed in Joules per square centimeter or millijoules per square centimeter (J/cm or mj/cm ). Alternate terms are exposure, or energy. Irradiance Radiant power, within a specified wavelength range, arriving at a surface per unit area. It is expressed in watts or milliwatts per square centimeter (W/cm, or mw/cm ). 8

52 Chapter II UV Curing Equipment and Radiometry Irradiance profile The irradiance pattern of the lamp; or, in the case of dynamic exposure, the varying irradiance at a point on a surface that passes through 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 the electrical power input divided by the effective length of the bulb. (It does not have 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 instantaneous signal output that is proportional to the radiant flux over a wavelength range. Static exposure Exposure to a constant irradiance for a controlled period of time. Contrast with dynamic exposure. UV Ultraviolet Radiant energy in the 100 nm to 450 nm range. Radiant energy in the 100 nm to 00 nm is referred to as vacuum UV (VUV), because it does not transmit in air VUV, UVA, UVB, UVC, UVV UVA is commonly referred to as long wavelength UV. UVC is commonly referred to as short wavelength UV. UVV is very long wavelength UV. VUV: nm UVC: nm UVB: nm UVA: nm UVV: 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. 9

53 Chapter II UV Curing Equipment and Radiometry 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). E ( ) ( 1 t t 1 1 ) 0 I dt 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. 30

54 Chapter II UV Curing Equipment and Radiometry The radiometer shown in photographs is mounted on the side of the UV curing lamp and measures the instantaneous irradiance (mw/cm ) 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/cm ) arriving at the surface. This instrument also serves as a dosimeter, with the capability of reporting the energy (mj/cm ) which is the time integral of the irradiance. Photograph 13 Photodiode radiometer with dry air purge line Photograph 14 UV lamp housing with a process radiometer mounted on the lamp housing to monitor the lamp output 31

55 Chapter II UV Curing Equipment and Radiometry Photograph 15 Traveling process radiometer top view sensor is visible Photograph 16 Traveling process radiometer bottom view controls and readout 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/cm ) and radiant energy (J/cm ). 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.5 feet/minute would take a measurement every a measurement every 1/1 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 10 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. 3

56 Chapter II UV Curing Equipment and Radiometry 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 radiometers to be used so alternatives must be found. A common method is to use radiachromic tags 14 (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. 33

57 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems Unsaturated Polyesters a. Synthesis of thermosetting polyester and acrylate oligomers (condensation polymerization) -7 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 The unsaturated polyester resin solutions used in the production of gel coats and laminating resins are low molecular weight condensation oligomers (Mn ) which have been diluted in a reactive diluent such as styrene or methyl methacrylate. SATURATED DIBASIC ACIDS UNSATURATED DIBASIC ACIDS GLYCOLS HYDROCARBON MODIFIERS PHTHALIC ANHYDRIDE ISOPHTHALIC ACID ADIPIC ACID TERPHTHALIC ACID CHLORENDIC ANHYDRIDE MALEIC ANHYDRIDE FUMARIC ACID PROPYLENE GLYCOL DIETHYLENE GLYCOL ETHYLENE GLYCOL DIPROPYLENE GLYCOL NEOPENTYL GLYCOL OTHER GLYCOLS DICYCLOPENTADIENE ESTERIFICATION UNSATURATED POLYESTER CONDENSATE REACTIVE MONOMERS PROMOTORS, INHIBITORS, ETC. STYRENE METHYL METHACRYLATE VINYL TOLUENE PARA-METHYL STYRENE ALPHA-METHYL STYRENE FREE RADICAL INITIATOR CROSSLINKED UNSATURATED POLYESTER 34

58 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems 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 0-40% 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 (,,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 alphahydrogen 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 35

59 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems 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, propane diol, and - butyl, - 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. 0 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 36

60 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems weight (Mn<300) fragments can be easily quantified by several techniques (GC-MS, NMR). The average molecular weight and molecular weight distribution of the unsaturated polyester greatly affect the properties of the cured styrene-crosslinked network. Below the critical molecular weight of entanglement the melt viscosity of the polyester varies linearly with the molecular weight. Mechanical properties improve sharply initially with increases in molecular weight. Above a certain molecular weight, the mechanical properties become relatively insensitive to further increases in molecular weight. During the synthesis of polyester resins, the diacids and glycols are charged to the reaction kettle in a non-stoichiometric ratio of COOH:OH to preclude the gel point. A common charge ratio is 1.05 moles of OH per 1.0 moles of COOH. The presence of excessive acid or alcohol end groups are known to have a detrimental effects on the properties of the cured network. The presence of a low molecular weight tail in the distribution may adversely affect the water resistance and mechanical properties. The number of end groups may be reduced by charging monofunctional acids and alcohols to the latter stages of the cook, effectively capping the polymer chain ends. In addition to the considerations above, the physiochemical characteristics of the cured polyester resin depend on the choice of crosslinking monomer, concentration of maleic anhydride in the UPR, and the ratio of the polyester carbon-carbon double bonds to monomer carbon-carbon double bonds. A generalized coding approach to 37

61 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems 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 :.1 :.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 density 4. The specific ratio of monomer to unsaturated diacid, M:UA, depends on the reactivity ratios of the monomers. As shown in figure, 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 98 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. 38

62 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems Double Bond Conversion Reactivity - UPR / monomer C=C by FTIR Fumarate C=C (UPR-Styrene) STYRENE C=C (UPR-Styrene) Fumarate C=C (UPR-Acrylate) Acrylate C=C (UPR-Acrylate) Time Since Initiation (minutes) Figure 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>. Rapid copolymerization of styrene with unsaturation present in the prepolymer occurs 3 when the maleic acid (cis isomer) is isomerized to fumaric acid (trans isomer). 39

63 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems HO HO OH O C O C C O C C C C fumaric acid (trans isomer) C O maleic acid (cis isomer) OH 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 bond 6,7. HO HO O C H O C + C C C O OH fumaric acid (trans isomer) CH 3 H O C H CH propylene glycol OH C C O C O CH 3 C CH OH OH H Ordelt Reaction - double bond saturation 40

64 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems 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. CH 3 O CH O CH CH O C CH 3 O CH CH O CH + HO C C CH 3 CH bisphenol A epoxy O methacrylic acid H C O C C CH 3 CH 3 O CH CH CH O C O CH CH CH O C CH C OH CH 3 OH CH 3 epoxy diacrylate (vinyl ester resin) Cured vinyl ester resins exhibit corrosion resistance and thermal-mechanical properties that are superior to UPR resins. Tensile strength, elongation and Tg values are 0-50% higher with vinyl ester resins than with UPR resins. Due to the presence of the bisphenol moiety, the exterior durability of vinyl ester resins is extremely poor and should not be used for exterior coating applications requiring color or gloss stability. 41

65 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems b. Network formation of thermosetting polyester and acrylate oligomers (free-radical polymerization) i. Microgel formation and macrogelation For a free radical crosslinking copolymerization of a monovinyl-divinyl system, cyclization or intramolecular reaction has been reported 8. The cyclization was thought to be a major reason to explain that many experimental gellation results deviated greatly from the theoretical prediction by classical Flory-Stockmayer theory. It has been proposed that in the early stages of the reaction, the occurrence of cyclization leads to the formation of internally crosslinked and rigid structures in the primary polymer chain called microgels. Many of the vinyl or vinylene groups are buried within the core of the microgels and show a reduced or completely suppressed reactivity. Unsaturated polyester resins contain between -15 vinylene groups per polyester molecule depending on the molecular weight and the level of unsaturation. Low molecular weight DCPD modified polyesters will possess a low number vinylene groups per molecule. High molecular weight propylene glycol-maleate polyesters contain a large number of vinylene groups per polyester molecule. The cyclization effect and the microgel formation is more important for the UPR-styrene resins than the monovinyl-divinyl system 8. Coagulated polyester nodules have been found in the pre-gel phase of curing UPR resins 44. The microgel particles are composed of densely crosslinked domains 46. 4

66 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems Growth of Free Radicals Microgel Particles * (a) I* (b) Figure 3 Formation of a UPR-styrene microgel (a) Growth of the free radical - I*: free radical initiator, *:growing free radical, : UPR molecule, : styrene chain (b) Schematic drawing of the microgel particle The mechanism of network formation in UPR-styrene systems is reported to occur in four stages 8. During the first stage, the induction stage, the initiators produce free radicals which trigger the polymerization. At the beginning of the reaction, all the free radicals formed from the initiators are immediately consumed by the inhibitorretarder additives 9,10 in the resin solution. Additives commonly used in include hydroquinone derivatives and tertiary butyl catechol. At this point, no polymerization has occurred due to the inhibition effect and the viscosity is relatively constant. The second stage, microgel formation, follows the induction stage. During the microgel formation stage the initiator continues to produce free radicals which can react with styrene and UPR vinylene unsaturations to form polymer chains through 43

67 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems intermolecular and intramolecular reactions. The intramolecular reaction produces crosslinked coils, i.e. microgels. Given the high degree of crosslinking, free radicals on the polymer chain which are buried in the interior of the microgel particle are occluded. This implies that termination among polymeric radicals may not be important and may be neglected. As shown in figure 4, the crosslinks between individual microgel particles may also be neglected because of the low concentration of microgels. Accordingly, the viscosity changes very little during stage II, the microgel formation stage. Macrogelation VISCOSITY, (or 0 ) Transition Microgel Formation Induction TIME, t (or t/t gel = R*/R c ) I II III IV Figure 4 Microgel formation and macrogelation in UPR-styrene system 44

68 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems The pendent carbon-carbon double bonds on the unsaturated polyester molecule are either buried within the microgel particle where they may increase the article crosslink density, or on the surface of the particle where they may react with surrounding comonomers and other microgel particles. The third stage, the transition stage, is characterized by the simultaneous formation of new microgel particles and the formation of inter-microgel crosslinks. The inter-particle covalent bonding produces a viscosity rise that is much more pronounced than in the previous stages. During the final stage, the macrogelation stage, the concentration of microgel particles is high and the inter-particle microgel crosslinking reaction is favored over the formation of new particles. The viscosity rises very rapidly during the macrogelation stage until the gel point. ii. Kinetics of redox initiated polymerization of UPR-styrene 8,43-47 The decomposition of peroxides such as methyl ethyl ketone peroxide in the presence of a metal salt such as cobalt octoate produces the primary free radicals for the initiation step. The peroxide may also decompose at higher temperature resulting from the laminate exotherm.\ (1) Decomposition of Peroxides (i) Thermal decomposition k ROOH ( I) 1 RO OH (R) (1) (ii) Redox decomposition k 3 ROOH ( I) Co RO( R) OH Co () k 3 ROOH ( I) Co ROO( R) H Co (3) (I) =peroxides (R ) =primary free radicals k 1, k rate constants of thermal and redox decomposition 45

69 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems () Inhibition R Z P (4) (3) Initiation R S S R E E (5) (4)Propagation S S S then eqns. (5) and (6) become (6) S E E E S S k p R M R E E E (7) where S stands for styrene monomer E stands for polyester vinylene groups where R = all free radicals (i.e. primary, plus S, plus E and M = all reactive groups (i.e. S and E) ) (5) Termination As previously discussed the termination step among polymeric radicals may be neglected because of the formation of highly crosslinked microgels RATE EQUATIONS Combining eqns. 1,,3, and 7, we obtain the rate equations for the peroxide decomposition and the radical formation in eqns. 8 and 9. d [ I] 0 0 Ea / RT [ I]( k1 k[ Ia]) [ I]( k1 k [ Ia]) e dt (8) d [ R] 0 0 Ea / RT f [ I](k1 k[ I a ]) f [ I](k1 k [ I a ]) e dt (9) where the activation energies (E a ) of the two decomposition rate constants are assumed to be the same Integrating eqns. (8) and (9) at isothermal conditions one obtains the concentrations of peroxides and free radicals at time t, dv recalling lnv and ln( a / b) ln( a) ln( b) v ( k1k Ia ]) t [ I] [ I o ] e [ (10) 46

70 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems Combine eqn. (9) and (10) and integrate k1 k[ I a ] ( k1 k [ Ia ]) t) [ R ] f [ ] [ Io ][1 e ] (11) k k [ I ] 1 a Where [Io] is the initial concentration of peroxide, [Ia] is the concentration of accelerator (cobalt), and f is the initiator efficiency Rearranging eqn(11) f [ I o](k1 k[ Ia ]) [ R]( k1 k[ Ia ]) ( k 1 k[ Ia ]) t ln (1) f [ Io](k1 k[ Ia ]) ln1 [ R]( k 1 k[ I a ]) f [ I o ](k1 k[ I a ]) If [ R ] f [ Io], recalling -ln(1-x/y) ~ x/y as x/y 0 then ( k 1 Solving for t k [ I a ]) t [ R]( k1 k[ Ia ]) f [ I ](k k [ I ]) o 1 a t [ R]/ f [ R] f exp[ E / RT] 0 o a (13) [ I ](k k [ I ]) [ I ](k k [ I ]) o 1 a o 1 a At the gel point, t= t gel [ R] R c R I R II R III RI IV Where Rc = sum of radicals generated during phases I-IV Thus t R / f R / c c Ea / RT gel 0 o (14) [ Io](k1 k[ Ia]) [ Io](k1 k[ Ia]) f e 47

71 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems By ratio of eqn 13 and 14 t [ R ] t gel R c (15) which implies that the pre-gel region may be simply expressed as a function of [R ]/R c, independent of polymerization conditions such as T, [I a ], [I o ] The following data set will be used to demonstrate the practical application of eqn. 14 to predict the gel point time by modeling the influence of: (a) curing temperature (b) initiator concentration (c) accelerator concentration Sample % Cobalt Temp (C) Temp (K) % MEKP Gel Time (minutes) Table 1 Gel Point Time data set used to validate the redox cure kinetic model Eqn. 14 can be rearranged to study (a) curing temperature 48

72 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems R c / f Ea 1 ln( tgel ) ln A B 0 o 1 1 [ I o ](k1 k [ I a ]) RT T eqn. 16 Effect of Curing Temperature on the Gel Point Time (0.1% cobalt octoate, 0.4% MEKP) ln [ gel time (minutes) ] R = (1/T) K -1 Figure 5 Effect of curing temperature on the gel point time redox initiator system UPR-styrene monomer (b) initiator concentration (MEKP) 1 t gel 0 o f (k1 k [ I a ]) [ Io ] B[ Io ] eqn. 17 R exp[ E / RT] c a Effect of Initiator Concentration on the Gel Point Time (0.1% cobalt octoate, 303K) [ Gel Time (minute) ] R = % Initiator Figure 6 Effect of initiator concentration on the gel point time redox initiator system UPR-styrene monomer 49

73 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems (c) accelerator concentration (cobalt salt) 1 t gel f k R e c [ I 0 1 o E / RT a ] f R c 0 k [ I Ea / e ] [ I o RT a ] A 3 B [ I 3 a ] eqn. 18 Effect of Accelerator Concentration on the Gel Point Time ( T=303 K, MEKP=0.4%) [ Gel Time (minutes) ] R = % Cobalt Accelerator Figure 7 Effect of cobalt accelerator on the gel point time redox initiator system UPR-styrene monomer iii. Kinetics of light induced polymerization involving multifunctional monomers For the chemist accustomed to working with conventional MEKP cured gel coats and UPR resins which have cure times of hours at room temperature, the most surprising features in UV curing is that the polymerization can develop so extensively in less that 1- minutes of UV light exposure. At first glance it would appear that the only between UV curing and thermal or redox curing is the initiation step. Redox Initiation M+, MEKP Thermal Initiation H R* R R* Photoinitiation h Structure capable of producing radicals 50 Free Radical R*

74 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems We now consider the kinetics of UV curing below. Initiation: h A b[ PI ] PI R Ri I ai I 0 (1 10 ) i I 0 (1 10 ) i (1) Propagation: k p R M RM or P R p k p[ R][ M ] () Termination: t P P k products n n R t k [ P] R (3) t combining (1)-(3) R p t i k p k 0.5 p b[ PI ] 0.5 ( I ) [ M ] ( I0(1 10 ) ) [ M ] 0.5 ai 0.5 i k k With conventional addition polymerization of a monofunctional monomer such as styrene, where termination occurs by bimolecular interaction of radicals to form a dead polymer, a large in crease in the rate of initiation will result in a decrease in the polymerization efficiency since the formation of radicals in large concentration will favor termination over propagation. It is expected that the kinetic chain length and the quantum yield for polymerization, p, will decrease with increasing light irradiance t levels. While this conclusion is reported 54 to be valid for monofunctional monomers, it no longer holds for multifunctional monomers where a close to first order relationship between R p and irradiance level is observed. The explanation for this effect is the progressive occlusion of polymer radicals in the network under formation. As soon as one of the pendant double bonds starts to polymerize, the radical becomes attached to the network and loses its mobility. The radical will 51

75 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems continue to grow as long as monomer molecules can pass close by, but it has little chance to encounter another crosslinked radical to undergo bimolecular termination. c. Formulation of conventional gel coat and resins Specific commercial gel coat and laminating resin formulations are the property of material suppliers and will not be disclosed. Disclosures limited to those readily available in public documents (patents, articles, conference proceedings, etc.) provide sufficient information for the purpose of this report. \ d. The Case for UV Curable Composite Materials UV curable composite resins and coatings offer the potential for environmental waste reductions, productivity gains through decreased cycle time and controlled operating conditions, as well as gains in exterior durability 11, Low temperature UV curable powder coating provide manufacturers with greatly reduced cycle times and cure energy demands. 1 Low VOC liquid UV curable coatings for plastic substrates produce less air pollution requirement, outstanding mar resistance, better space utilization due to rapid cure rates(less curing space required), and less curing energy when compared to conventional thermally cured coatings 13. UV curable pultruded laminates offer lower air pollution (low VOC), elimination of dimensional change associated with elevated temperature processing, increased resin storage stability, and 5

76 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems lower energy costs 14. Additional benefits of UV curable materials include the elimination of metering and mixing plural component systems, and the re-use of resin waste. e. Formulation of UV curable gel coat and UP resins. i. Historical work in the area of UV curable composites 0,1,5 Initial work conducted in the mid-1960 s discussed the UV and electron beam (EB) curing of UPR prepolymers diluted in styrene monomer. Deninger reports curing UPR resins with low pressure mercury vapor fluorescent lamps using a diphenyl disulphide photoinitiators. Cure was achieved with three minutes of exposure 5. Fuhr reports curing thin unpigmented unsaturated polyester ( phthalic anhydride, propylene glycol, maleic anhydride) castings under fluorescent lighting using a bezoin ether photoinitiator. The exposure time was minutes. The spatial and temporal control afforded by photopolymerization make it attractive for rapid and inexpensive processing of polymeric composites. However, photopolymerization of thick and fiber-filler polymers is more challenging than curing of thin-film systems due to the exponential reduction in light intensity through the sample resulting from absorption and scattering. Therefore, proper selection of the initiator formulation and illumination wavelength are imperative to ensure that the samples cure throughout. With the advent of high irradiance (600 W/inch) lamps and specialized photoinitiators it is now possible to cure gel coats and laminates in a continuous manner at several feet per minute. 53

77 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems ii. Classification of photoinitiators photolysis mechanisms 11, -4 The various photoinitiators in use today can be classified into three major catregories, depending on the mechanism involved in their photolysis. Radical formation by photocleavage This class includes aromatic carbonyl compounds that undergo Norrish type 1 fragmentation when exposed to UV light O OCH3 C C OCH3 Dimethoxybenzil ketal h O C + Free Radicals OCH3 C OCH3 The benzoyl radical was shown to be the major initiating species while the other fragments contribute to a lesser extent. Radical generation by hydrogen abstraction Aromatic ketones like benzophenone or thioxanthone when promoted to their excited statesby UV radiation do not undergo fragmentation, but rather a hydrogen abstraction from a proton donor molecule to generate a ketyl radical and the donor radical. The initiation of polymerization usually occurs through the donor radical with the ketyl radical disappearing by bimolecular coupling. Tertiary amines are commonly used as hydrogen donors. 54

78 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems O C O h * C R-H OH C * + R* Cationic Photoinitiators diaryliodonium salts (ArI+X-) generate strong Bronsted acids in the presence of a hydrogen donor which are efficient initiators for the polymerization of epoxy monomers. Table below lists some of the common commercially available photoinitiators which will be referred to in chapter V. 55

79 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems Type α- Hydroxy Ketone α- Hydroxy Ketone Dimethyl Ketal Common Name Irgacure 184 Irgacure 959 Irgacure 651 Chemical Name 1-Hydroxycyclohexyl phenyl ketone - Hydroxy-1-[4-(- hydroxyethoxy)phenyl]- -methyl -1-propanone α, α-dimethoxy- α- phenylacetophenone Structure O OH C HO O O CH 3 C O O O C CH 3 CH 3 OH α-amino Ketone α-amino Ketone BAPO Irgacure 907 Irgacure 369 Irgacure Methyl-1-[4- (methylthio)phenyl]- 4- morpholinyl) -1- propanone benzyl-- (dimethylamino)-1-[4- (4-morpholinyl) phenyl]-1-butanone Phenylbis(,4,6- trimethylbenzoyl)- phosphine oxide S H 3 C O H 3 C N O C C N O O CH 3 CH 3 CH O 3 CH 3 C N CH 3 O O P C MAPO Lucrin TPO Diphenyl(,4,6- trimethylbenzoyl)- phoshpine oxide O C O P BAPO / α- Hydroxy Ketone Irgacure % CGI 403 bis(,6- Dimethoxybenzoyl)(,4, 4-trimethylpentyl) phosphine oxide / 75% Irgacure 184 O O C O 5% O P O C O O + O C CH 3 O 75% O BAPO / α- Hydroxy Ketone Irgacure % CGI 403 bis(,6- Dimethoxybenzoyl)(,4, 4-trimethylpentyl) phosphine oxide / 50% Irgacure 184 O O C O 50% O P O C O 50% O + H 3 C O C O CH 3 O H 3 C Table Commercially available photoinitiators 56

80 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems iii. Physical Concepts of UV Curing interaction of light with the coating - curing thick UPR films containing titanium dioxide pigment The effective generation of free radicals in a thick film such as a gel coat or unsaturated polyester laminate requires (1) a light source with sufficient irradiance in a wavelength band(s) consistent with the absorption characteristics of the photoinitiator and the transmission characteristics of the remaining components of the coating, () adequate transmission/scattering of the irradiance through the thickness direction of the film, (3) absorption of photons by the initiating molecule, and (4) conversion of absorbed energy to radical generation by the photoinitiator. A review of the physics of the interaction of light with matter provides the following general sequence leading to the formation of free radicals from a photoinitiator: Light Energy X Transmission X Absorbance X Quantum Yield Wavelength Wavelength Wavelength Wavelength # radicals/sec = I() x T() x A() x () IRRADIANCE TRANSMISSION ABSORPTION QUANTUM YIELD # photons second # photons transmitted through the coating film # photons absorbed by the photoinitiator # radicals formed # photons # photons transmitted # photons absorbed through the coating by the photoinitiator Figure 8 Light induced free radical formation in a coating film 57

81 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems The quantum yield of a radiation-induced process is the number of times that a defined event such as a chemical reaction step occurs per photon absorbed by the system. Thus, the quantum yield, is a measure of the efficiency with which absorbed light produces some effect. For the successful initiation of the chemical curing reaction there must be some overlap of the susceptibility of the photoinitiator, A() x (), and the transmission of energy to the photoinitiator molecule E() x T() from the light source. Medium pressure mercury lamps (H- bulb) have a spectral power distribution that is not well suited for thick UPR systems (see figure 3 in chapter II). The majority of the energy for the H- bulb is below 300 nm and is therefore absorbed in the surface layers of the film. The UV transmission spectrum for a large number of UV curable monomers and oligomers is presented in figure 9 below. The majority of these substances are aliphatic in nature and allow complete light transmission above 350 nm. The styrene monomer (thick red curve) also shown little absorption above 350 nm. The transmission characteristics of the UPR prepolymer as well as clear films of UPR prepolymer crosslinked with styrene show a marked increase in absorption of light between nm. This is thought to be due to conjugation of the carbonyl groups and the aromatic ring present in the diacid (isophthalic acid) used to prepare the UPR prepolymer. Likewise, the scattering/absorption characteristics of rutile titanium dioxide do not favor the use of short wavelength H-bulbs (see figure 10). The excellent visual opacity of films containing titanium dioxide is due to the ability of the pigment to scatter light in the visible portion of the electromagnetic spectrum ( nm). The UV absorption of 58

82 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems titanium dioxide below 370 nm is high. The spectral band of nm is window available to thick cure coatings containing titanium dioxide 50,53,54. Metal halide bulbs are used in many types of applications whose photochemistry requires different UV wavelengths to initiate the photopolymerization process due to light transmission barriers associated with mercury bulbs. The gallium doped medium pressure mercury vapor lamp is a much better choice for UV cure of thick white coating films since the spectral power distribution of the lamp overlaps the transmission window of the coating. %T UV Spectrophotomer Scan Data Wave Lengths 10 mil film clear gel coat 30 mil film clear gel coat Styrene monomer (thick red line) 5-7 mil UPR prepolymer (uncrosslinked resin solids) (thin black line) 1 0 mi l s Cl ear Gel Coat 30 mi l s Cl ear Gel Coat Styr ene MMA BYK-306 Heloxy 505 EPON 88 SR 30 SR 31 SR 38 SR 306 SR 351 SR 50 SR 506 Cyr acur e UVR-61 8 Cyracure UVR-6110 Acetoni tr i l e Ebecr yl 70 Ebecr yl 30 Ebecr yl 3700 CN963E80 Ebecr yl 4830 Ebecr yl 840 Ebecr yl 8804 Uvacur e K-16 K-16 UPR Resin prepolymer Solids B Ebecr yl 8804 Epi -Cur e 370 Epi -Cur e 955 Uvacur e Ebecr yl 70 MP MP 10-5 Figure 9 UV Transmission characteristics of monomers, oligomers, and films commonly used for UV cure applications (pathlength=10 mm UV cell, 100% concentration) 59

83 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems 100 Reflectance and Absorbance of Rutile Titanium Dioxide 90 % Absorbance, % Reflectance Absorbance Reflectance Wavelength (nanometers) Figure 10 Absorption-scattering characteristics of titanium dioxide pigment iv. Photobleaching and high radical yield-impact of acylphosphine oxide photoinitiators If we imagine an ideal clear, non-pigmented film (i.e. no scattering centers) of unit thickness, the irradiance at the top surface of the film, I 0, from an overhead lamp will be attenuated through absorption I a or transmitted I t I 0 I 0 I a I t I a I t 60

84 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems According to the Beer-Lambert law, the transmission, T, is related to the absorbance T I t I 10 Abs bc / 0 10, the fraction of light absorbed, f a is f a T 10 Abs A 1 1, and I a I 10 ). 0 (1 Using a numerical simulation in which the film is divided into 100 layers we can repeatedly apply the calculation below to each layer to estimate the relative light intensity within each layer as a function of the absorption characteristics of the film. A/100 I a I (1 10 ). 0 The results of the simulation point out that while the concentration of photoinitiator may be uniform, the distribution of UV intensity is not uniform. For strongly absorbing systems the light reaching the bottom of the film is negligible. Simulation of Light Intensity Transmitted through a Film Relative Light Intensity Abs=0.1 Abs=0.5 Abs=1.0 Abs= Fractional Film Depth Figure 11 Effect of the absorptivity on light transmission characteristics in a film 61

85 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems For most photoinduced free radical polymerizations, the uniformity of double bond conversion as a function of coating thickness and the overall degree of conversion are crucial to achieve the performance characteristics of the coating. Further simulation results to determine the fraction of energy absorbed within the top 1% and bottom 1% of the coating film are presented in table and figure 1 below. The results obtained agree with literature results 48. The maximum fraction of light absorbed in the bottom layer occurs when the absorbance is between Even under this best case, the energy absorbed in the top layer of the film is.5 to 3 times greater than the energy absorbed in the bottom. The film has been defined to be optically dense when the ratio of A Fraction Absorbedtop 1% Fraction Absorbedbottom 1% Ratio Top:Bottom Table 3 Energy absorbed in the top 1% and bottom 1% of a coating film 6

86 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems % Energy Absorbed in the top and bottom 1% of a coating film % Energy Absorbed in the 1% Layer Absorbed-top 1% Absorbed-bottom 1% Absorbance value Figure 1 Fraction of incident energy absorbed in the top 1% and bottom 1% of a film the light absorbed in the top layer and bottom layer is greater than 10. Consider a strongly absorbing layer (Abs.=3) in which the ratio is fraction absorbed in the top fraction absorbed in the bottom 1% 1% 993 In this case we see poor through-cure of the film. This is illustrated in photographs 1- below where a white gel coat was drawn down to a thickness of 10 mils and exposed to a medium pressure mercury (H-bulb) vapor lamp. The photoinitiator used, Irgacure 184, is an α-hydroxy ketone which undergoes Norrish type I fragmentation when exposed to UV light. This combination of lamp and photoinitiator produced an effect known as alligatoring. The effect is so named because the coating has a pronounced wrinkled surface appearance. The gel coat shrinks approximately 7% by volume during cure. The alligatoring effect occurs 63

87 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems when the top surface layer cures with commensurate shrinkage. The bulk of the film located beneath the surface remains completely uncured. Photograph 1 Alligatoring phenomena the top 1-3 mils is cured while the balance of the film is wet Photograph Uncured coating material which remains after the cured surface film in photograph 1 is peeled away Referring back to the review of the physics of the interaction of light with matter presented in figure 8, # radicals/sec = I() x T() x A() x () IRRADIANCE TRANSMISSION ABSORPTION QUANTUM YIELD # photons second # photons transmitted through the coating film # photons absorbed by the photoinitiator # radicals formed # photons # photons transmitted # photons absorbed through the coating by the photoinitiator we can improve the cure of the gel coat film by switching from the short wavelength H-bulb to the long wavelength gallium V-bulb. This change provides irradiance in a wavelength band that is consistent with the transmission characteristics of the film. Unfortunately, this change alone will not solve the issue of poor through-cure of the 10 mil film since the photoinitiator itself strongly absorbs the UV light. The solution 64

88 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems lies in the use of a photobleaching photoinitiator 4-3. These initiators are in the phosphine oxide family and function mechanistically via photocleavage. Several commercially available phosphine oxide initiators appear in table above. By replacement of the α-hydroxy ketone photoinitiator with bisacyl phosphine oxide (Irgacure 819) it is possible to achieve rapid through-cure of the 10 mil gel coat using a gallium V-bulb. The photochemical reason behind the improved through-cure of the optically dense white coating lies in the very different photolysis process of the two photoinitiators. The phosphine oxide types of photoinitiators have extended absorption into the UV visible region. The primary photolysis at longer wavelength causes an α-cleavage of one of the CO-P bonds. The radical-radical recombination of the benzoyl and phosphinoyl radical is inefficient due to steric and planarization restraints 49. The same mechanism is also controlling the second cleavage of the primary formed acylphosphinoyl radical now attached to a growing chain end. The photolysis results in a loss of absorption at 350 < < 410 nm. Therefore, increased photon absorption will occur in the deeper layers of the coating as the irradiation continues. The time-lapsed UV absorption spectrum of the phosphine oxide 51 is shown in figure 13. The photobleaching effect in the depth direction of the coating film is presented schematically in figure

89 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems 0.5 Photobleaching Effect of Phosphine Oxide Photoinitiator 40 sec 0 sec 0 sec irradiation time 60 sec 30 sec 10 sec 0.4 Absorbance concentration=1.0 x 10-3 molar in methanol Wavelength (nanometers) Figure 13 Time-lapsed UV absorption spectrum of phosphine oxide photoinitiator Photobleaching LIGHT INTENSITY LIGHT INTENSITY LIGHT INTENSITY MOLD MOLD MOLD Non-Photobleaching LIGHT INTENSITY LIGHT INTENSITY LIGHT INTENSITY MOLD Exposure Time MOLD MOLD Figure 14 Comparison of photobleaching and non-photobleaching photoinitiators 66

90 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems R R R n O P + H 3 C O C H 3 C CH 3 h R n O P H 3 C O C H 3 C CH 3 R H 3 C CH 3 O C O P O C H 3 C CH 3 h H 3 C CH 3 O C + O P O C H 3 C CH 3 CH 3 H 3 C CH 3 H 3 C R bisacylphosphine oxide CH 3 H 3 C O C R n CH 3 Reaction Photolysis of BAPO While UV curing of UPR resins was first reported in the mid-1960 s 0,1,5 the commercial availability of phosphine oxide photoinitiators in the late 1980 s led to a renewed interest in UV technology for UPR based composites applications starting in the mid-1990 s. 67

91 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems Publication of Documents (patents, articles) Related to Photocuring of UPR Glass-Reinforced Composites # Documents Published Publication Year Figure 15 UV Composites publications Source - Chemical Abstracts Services SciFinder-keyword: photocure, unsaturated polyester, composite v. Light scattering within a coating or laminate Several authors report improved cure reaction in UV formulations containing particles designed to scatter the incident light, effectively lengthening the path of the UV light within the film. In one study 34 a UV curable powder coating containing light-refractive microbeads was reported to exhibit improved through-cure properties. In a separate report 35 a polyester acrylate coating containing silica nanoparticles was studied by photo-dsc and UV-visible and FTIR spectroscopy. As the concentration of silica nanoparticles was increased up to 5%, the exotherm, ultimate percentage conversion, and cure rate increased gradually, whereas they decreased above 10%. 68

92 Chapter III Chemistry of Thermosetting Unsaturated Polyester and Acrylate Systems. vi. Commercial applications for UV curable composites 36-4 UV curable composite applications reported in the literature include: Simulated marble containing clear urethane acrylate 36 Flat UPR-multifunctional acrylate gel coated laminates prepared using a transparent mold with bidirectional cure 37 Roofing panel laminates with good weather resistance 38 39, 40 Gel coated laminates used as wood paneling replacements for spas Protective gel coats applied to filament wound high pressure tanks 41 Residential doors prepared entirely from composite materials 4 69

93 Chapter IV - Characterization of Resin / Coating State of Cure Chapter IV - Characterization of Resin / Coating State of Cure a. Qualitative methods for estimating cure i. Probing techniques to assess cure hardness development, dry-to-touch assessment Several probing methods to have been used to determine the state of cure of gel coats and resin laminates. All of these methods involve touching the surface of the curing material to assess the level of cure. One pass/fail method involves the use of a small wooden spatula to test the rigidity of the curing film. In another approach the curing is assessed by pressing a cotton ball on the coating surface. The coating is said to be cured when the cotton fibers do not remain stuck on the coating 1. In yet another method known as the circular drying time recorder (ASTM 5895), a small weighted stylus inscribes the curing film in a circular motion that is approximately three inches in diameter. Once sufficient cure is developed the moving stylus tears the coating film. With a basic knowledge of the angular velocity of the stylus and the distance traveled from the starting point to the point the film is torn it is possible to determine the curing time of the film. Finally, there is the thumb pressure test. A common ranking scale used with the thumb test is 1=wet, =very tacky, 3=tacky, 4=slight tack, 5=dry. 70

94 Chapter IV - Characterization of Resin / Coating State of Cure ii. Limitation of probing techniques to assess cure The first obvious limitation of probing methods to determine the state of cure of a thermosetting film is that they are not quantitative. Secondly, probing techniques are biased toward the surface of the film or laminate. As it turns out this is a serious limitation with gel coat films where it is possible to develop a lacquer dry on the surface if (a) the solvents/reactive diluents are sufficiently volatile, and (b) the glass transition temperature of the uncrosslinked polymer melt is sufficiently high. The evaporative loss of reactive monomer is shown in figure 1 where a UPR prepolymermonomer blend was spray-applied to a small glass mold and allowed to sit for specific intervals. Samples were collected and analyzed to determine compositional changes arising from evaporative losses of styrene and methyl methacrylate monomer. 71

95 Chapter IV - Characterization of Resin / Coating State of Cure b. Quantitative methods for cure characterization FTIR Absorbance 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 Compositional Changes In Conventional Gel Coat Resulting From Evaporative Losses of HAPS Monomers PEAK HT. Pre Post Post Post Post Post % CHANGE Resin Styrene MMA WEIGHT Pre Post Post Post Post Post CHANGE Resin (g) Styrene (g) MMA (g) Total (g) % Loss Styrene Pre-spray composition Post-spray composition Post-spray + 5 minutes Post-spray + 10 minutes Post-spray + 15 minutes Post-spray + 30 minutes Post-spray + 60 minutes Post-spray + 10 minutes Resin - isophthalate ring bending mode 0.1 Methyl Methacrylate wavenumber (cm -1 ) Figure 1 Evaporative losses of reactive monomers in gel coat film measured by FTIR (T=5C) Peaks at 731 cm -1 (UPR prepolymer-isophthalate ring bending mode), 815 cm (methyl methacrylate C=C bond), and 911 cm -1 (styrene monomer C=C bond) were monitored to follow the change in composition. The spectra were normalized using the prepolymer peak since the resin solids is non-volatile. After thirty minutes on the glass plate roughly one-half of the styrene monomer and nearly all the methyl methacrylate evaporated. 7

96 Chapter IV - Characterization of Resin / Coating State of Cure Unsaturated polyester resins are well known to be air-inhibited due to the formation of stable peroxide radicals at the air-resin interface. A lacquer dry effect in which the back side of the film appears to be cured is observed if the Tg of the uncured film is greater than 15C. To better illustrate this point the tack-tg data for a series of three UPR prepolymers is given in table 1. The lacquer dry effect may also be seen by conducting an acetone wipe test. The cured gel coat (mold side) is not dissolved by the solvent while the back side is easily removed. Attempts 9-1 to overcome oxygen inhibition of UPR resins include the use of: insoluble paraffin waxes forming a surface barrier to preclude oxygen modification with oxidative curing moieties such as conjugated fatty acids, dicyclopentadiene, and allylic ethers oxygen scavenging additives inert gas blanketing Resin Designation Tg (C ) of the uncured Surface Tackiness Level prepolymer A 14.3 dry no tackiness B 11.3 slightly tacky to the touch C -5 very tacky to the touch Table 1 Effect of Tg on the surface tackiness of UPR prepolymer b. Quantitative methods for estimating cure i. Analytical methods used to study cure state during the product development cycle (DSC, FTIR) While numerous methods have been used to study cure including touch 1, Koenig pendulum hardness, Sward rocker hardness, radiachromic labels, differential 73

97 Chapter IV - Characterization of Resin / Coating State of Cure scanning calorimetry (DSC) 3-5, FTIR 6, NIR 7-8, gel fraction measurements 0, and laser nephelometry 1, the most commonly used methods for development and process definition are DSC and FTIR. dielectric ii. Process quality control methods to measure cure NIR, spectroscopy The conversion of double bonds in UV-cured acrylic coatings on various substrates was followed in-line by near-ir (NIR) reflection spectroscopy 7-8. Quantitative data were obtained directly from the intensity of the acrylic overtone band at 160 nm, which allows very easy calibration of the method. The custom-made probe head of the NIR spectrometer was fitted to several pilot-scale coatings and curing lines, and the conversion was determined in clear and pigmented coatings. It was shown that reasonable conversion data can be obtained at line speeds of at least 10 m/min. Another in-line sensing technique, dielectric spectroscopy, places the curing resin in an oscillating electric field. The ionic mobility (ion viscosity) is measured and correlated with the state of cure. This method is useful until the point of vitrification, but not sensitive to changes in the later stages of cure. 74

98 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Chapter V Modeling the Degree of Cure of a D UV Curing Process a. Studies of coatings and laminate resin related variables -11 i. Experiment 1 Effect of pigmentation-screening study involving ten different colors Previously, the criteria set forth for effective generation of free radicals in a thick film of gel coat was stated to be (1) a light source with sufficient irradiance in a wavelength band(s) consistent with the absorption characteristics of the photoinitiator and the transmission characteristics 1 of the remaining components of the coating, () adequate transmission/scattering of the irradiance through the thickness direction of the film, (3) absorption of photons by the initiating molecule, and (4) conversion of absorbed energy to radical generation by the photoinitiator. Photocrosslinking of nonpigmented glass fiber-reinforced composites, based on vinyl ester and unsaturated polyester has been reported for mm thick laminates, containing 30-40% glass fiber mats. As the concentration of photoinitiator increases, the degree of polymerization reaches a maximum and then decreases. This effect is very marked for film thicknesses >1 mm. The optimum level of photoinitiator decreases as the sample depth increases 1. Photoinitiators with a high extinction coefficient at the principal wavelength of the UV source give rise to optimum concentrations at lower initiator levels. Several studies reported in the literature 3,5,6 for white thermosetting coatings containing rutile TiO. The degree of cure depended strongly on the film thickness and concentration of pigment. With the proper selection of lamp and photoinitiator 75

99 Chapter V - Modeling the Degree of Cure of a D UV Curing Process (electrodeless V and D bulbs and,4,6-trimethylbenzoyldiphenoylphosphine oxide) it was possible to properly cure films 1 mils in thickness. The physical interactions between electromagnetic radiation and color pigments in pigmented UV coatings complicate the through-cure of such coating films. Certain pigments 7 do not allow adequate transmission/scattering of the irradiance through the thickness direction of the film and therefore cannot be cured in thick with UV curing. An experiment was conducted in which a series of different color pigments were added to a UV curable gel coat base formula and cured. In this screening experiment the gel coats were drawn down on a white mold at 0 mils thick. All formulas contained 1% bisacyl phosphine oxide/1-hydroxycyclohexyl phenyl ketone (Irgacure 819/Irgacure 184, 1:3 ratio), and were irradiated under two rows of Honle Gallium metal halide lamps at a line speed of feet per minute. None of the samples reached full cure throughout the entire film thickness of 0 mils. At the conclusion of cure test the cured portion of the film was peeled off of the uncured coating beneath. The cured film thickness was measured along with the degree of cure by DSC. Photograph 1 Colored gel coat films before UV curing 76

100 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Photograph Colored gel coat films after UV curing The results of the screening experiment are summarized in figure. Four of the ten colored films were cured to a depth greater than 5 mils with 75% or more conversion of double bonds ( blue, white, almond, and light tan). The curing response of the remaining six samples were less responsive to the UV exposure. 100 Mils Cured Degree of Cure (%) 10 Degree of Cure % (DSC) Thickness of Cured Film, x (mils) x lamp cured gel coat wet gel coat mold 9 inches 0 mils 0 Blue Orange Green Red Yellow Black Grey White Lt Tan Almond Color of the Gel Coat Figure 1 Screening experiment to evaluate the effect of color pigmentation on the degree of UV cure 77 0 ft/min

101 Chapter V - Modeling the Degree of Cure of a D UV Curing Process The results obtained in the screening experiment can be explained in terms of the scattering and absorption of light within the film. As mentioned in chapter III several authors report improved cure reaction in UV formulations containing particles designed to scatter the incident light, effectively lengthening the path of the UV light within the film. chap III ref The incident incident on an opaque film is either scattered or absorbed. The relative amount of absorption and scattering can be determined for a given wavelength by measuring the reflectance spectrum R(). The Kubelkla-Munk theory states that K S (1 R) equation 1.1 R where K= the absorption coefficient, S=scattering coefficient, and R=reflectance. For a mixture of colorants such as the case for the screening experiment K S mixture K S mixture mixture c K c S 1 ck c S c3k c S equation 1. Relationship Between Measured Reflectance and the Absorption (K) and Scattering (S) Coeffcients in an Opaque Film 10 8 K/S Reflectance absorption dominates scattering dominates Figure Kubelka-Munk prediction of light absorption and scattering in an opaque pigmented film 11 The reflectance spectrum were measured for the ten pigmented gel coat samples at 0 nm intervals over the visible portion of the spectrum ( nm) figure 4. 78

102 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Recall that the gallium doped mercury bulb provides the most significant energy in the nm range. The absorption spectra of the photoinitiator solution that is presented in figure 5 indicates a maximum in absorption of light at 40 nm. This peak location is an ideal match for the gallium type metal halide electrodeless UV lamp for the purposes of maximizing the photons absorbed by the photoinitiator. The reflectance curves in figure 4 clearly show that the four colors with the best cure response also have the lowest K/S ratio i.e. greatest scattering in the wavelength range of interest. Conversely, the colored films with the poorest cure response (yellow, red, green, orange, black) have the highest light absorption in the wavelength range of interest. From this experiment we can conclude that UV curing is not a viable option for curing films containing tinctorial pigments with strong absorbance in the nm range. During the cure of a UPR laminating resin, the resin shrinks Reflectance Curve for Several Gel Coat Colors 5-8 % by volume. 100 white 80 almond(off white) orange % Relectance light tan yellow red grey 0 green blue Wavelength (nanometers) Figure 3 Reflectance spectra for the ten pigmented gel coats shown in photograph 1 79 black

103 Chapter V - Modeling the Degree of Cure of a D UV Curing Process One consequence of the shrinkage of the matrix resin around the reinforcing fiber is the appearance of an uneven surface known as fiber prominence or fiber printing at the surface of the laminate. A major function of a gel coat is to serve as a fiber-print blocking layer. The maximum cured film thickness is approximately four mils in this case, which is too thin for gel coat applications requiring good surface smoothness. Absorbance Calibration Curves - Photoinitiator Solution %PI solids Absorbance Units = -log(t) Wavelength nm Figure 4 Absorption spectra of the photoinitiator solution 80

104 Chapter V - Modeling the Degree of Cure of a D UV Curing Process ii. Experiment Effect of TiO concentration Objective: The objective of the experiment is to determine the correct level of titanium dioxide to use in a white gel coat formula. Approach: Gel coat formulas were prepared with increasing levels of pigment and exposed in a UV curing application. The cured films were evaluated visually for surface quality and cure. INPUT FACTORS 1) Type of experimental design or comparative strategy: simple ladder study FIXED FACTORS these factors will be held constant UNITS LEVEL A) Photoinitiator level and type % Irgacure 819 at 0.5% B) Line speed fpm 0 C) Coating thickness mils 16 D) Coating na UPR diluted in styrene, rutile TiO E) Lighting na 600 W/in Fusion lamp, std. reflector, at 4 inches, V bulb ) How many independent factors are to be studied? 1 How many Tests will be conducted? FACTOR Variable these factors will be systematically varied UNITS LEVELS A) concentration of titanium dioxide % 10, 15, 0 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME A) visual assessment of cure and surface quality na UNITS visual TEST METHOD TO USE RESULTS Run # % TiO Visual Assessment of the Film 1 10 cure: slightly tacky on the surface, wet below the surface appearance: the surface is smooth no alligatoring texture noted 15 cure: slightly tacky on the surface, wet below the surface appearance: the surface is smooth no alligatoring texture noted 3 0 sample was not run based on the results of run # 81

105 Chapter V - Modeling the Degree of Cure of a D UV Curing Process CONCLUSIONS The level of pigment should be held to a maximum of 10% to prevent the alligatoring texture which can occur with a cure and shrinkage gradient in the thickness direction. iii. Experiment 3 Effect of gel coat film thickness Objective: The objective of the experiment is to determine the process window for the gel coat film thickness. Approach: Gel coat formulas were applied at varying film thickness using a draw down bar on a mylar film and exposed in a UV curing application. The cured films were evaluated visually for surface quality and cure. INPUT FACTORS 1) Type of experimental design or comparative strategy: simple ladder study FIXED FACTORS these factors will be held constant UNITS LEVEL A) Photoinitiator level and type % Irgacure 819 at 1.0% B) Line speed fpm 10 C) Coating type na UPR-styrene, rutile TiO-10% D) Lighting na 600 W/in Fusion lamp, std. reflector, at 4 inches, V bulb ) How many independent factors are to be studied? 1 How many Tests will be conducted? FACTOR Variable these factors will be systematically varied UNITS LEVELS A) Coating thickness mils 7, 10, 16 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME A) visual assessment of cure and surface quality na UNITS visual TEST METHOD TO USE 8

106 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS Run # Film Visual Assessment of the Film Thickness mils 1 7 cure: dry on the surface, good thru cure appearance: the surface is smooth no alligatoring texture noted 10 cure: very slight tack on the surface, good thru good appearance: the surface is smooth no alligatoring texture noted 3 16 cure: slightly tacky on the surface, good thru good appearance: the surface is smooth slight alligatoring texture noted CONCLUSIONS The wet film thickness should be held to 10 mils to prevent the alligatoring texture which can occur with a cure and shrinkage gradient in the thickness direction. 83

107 Chapter V - Modeling the Degree of Cure of a D UV Curing Process iv. Experiment 4 Factorial study of photoinitiator concentration, UV energy, and TiO concentration Objective: The objective of the experiment is to determine the process window for the gel coat film thickness. Approach: Gel coat formulas were applied at varying film thickness using a draw down bar on a mylar film and exposed in a UV curing conveyor application. The cured films were evaluated visually for surface quality and cure. INPUT FACTORS 1) Type of experimental design or comparative strategy: full factorial 3 design FIXED FACTORS these factors will be held constant UNITS LEVEL A) Coating type na UPR-styrene, rutile TiO-10% B) Lighting na 600 W/in Fusion lamp, std. reflector, at 4 inches, V bulb ) How many independent factors are to be studied? 3 How many Tests will be conducted? 8 FACTOR Variable these factors will be systematically varied UNITS LEVELS A) Coating thickness mils 7, 10, 16 B) Photoinitiator concentration % 0.1%, 0.5% Irgacure 819 C) Line speed fpm 5,0 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME A) visual assessment of cure and surface quality na UNITS visual TEST METHOD TO USE 84

108 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS Run % PI Line Film Visual Assessment of the Film # Speed (fpm) Thickness (mils) cure: tack free, good thru cure appearance: no alligatoring texture cure: tacky surface, incomplete thru cure appearance: some are smooth, other areas show texture cure: tacky surface, incomplete thru cure appearance: no alligatoring texture wet film cure: well cured appearance: no alligatoring texture cure: dry on the surface, dry below appearance: alligatoring texture is present cure: tacky surface appearance: no alligatoring texture cure: tacky surface, wet below the surface appearance: no alligatoring texture Table 1 - Factorial study of photoinitiator concentration, UV energy, and film thickness CONCLUSIONS All films at 16 mils were had a textured appearance while the 4 mil film is easily cured with proper selection of line speed. It is necessary to use 0.5% PI to achieve good surface cure with this lamp-pi combination. 85

109 Chapter V - Modeling the Degree of Cure of a D UV Curing Process v. Experiment 5 - Binder / reactive diluent selection Objective: The objective of the experiment is to study the differences in UV cure response using different binder chemistry (UPR-styrene, thermosetting acrylic) Approach: Gel coat formulas were using two different binder chemistries and applied at varying film thickness using a draw down bar on a mylar film and exposed in a UV curing conveyor application. The cured films were evaluated visually for surface quality and cure. INPUT FACTORS FIXED FACTORS these factors will be held constant UNITS LEVEL A) Photoinitiator concentration % 1.0% Irgacure 819 B) TiO type and concentration % rutile, 10% C) Lighting na 600 W/in Fusion lamp, std. reflector, at 4 inches, V bulb FACTOR Variable these factors will be systematically varied UNITS LEVELS A) Coating thickness mils 7, 10, 16 B) Line speed fpm 5, 10, 0, 40, 80 C) Binder chemistry na UPR-styrene, thermosetting acrylate diluted with multifunctional acrylate monomers LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME A) visual assessment of cure and surface quality na UNITS visual TEST METHOD TO USE 86

110 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS Run Binder Line Film Visual Assessment of the Film # chemistry Speed (fpm) Thickness (mils) 1 UPR 10 7 cure: tack free, good thru cure appearance: no alligatoring texture UPR cure: tacky surface, complete thru cure appearance: some areas show texture 3 UPR 5 16 cure: slight tacky surface appearance: alligatoring texture 4 UPR 5 1 cure: tack free, good thru cure appearance: no alligatoring texture 5 UPR cure: v. slight surface tack, good thru cure appearance: no alligatoring texture 6 Acrylic 0 10 cure: tack free, good thru cure appearance: no alligatoring texture 7 Acrylic cure: tack free, good thru cure appearance: no alligatoring texture Acrylic cure: tack free, good thru cure appearance: no alligatoring texture 8 Acrylic 0 16 cure: tack free, good thru cure appearance: no alligatoring texture Table - Binder / reactive diluent selection CONCLUSIONS The difference in cure response is dramatic. The thermosetting acrylic is cured at eight times the line speed of the UPR based gel coat. The film is also observed to cure without the surface wrinkling issue at a thickness 16 mils. 87

111 Chapter V - Modeling the Degree of Cure of a D UV Curing Process vi. Experiment 6 UV Curing studies of a UPR laminate Objective: The objective of the experiment is to determine the process conditions necessary to cure a fiber reinforced UPR laminate Approach: Photoinitiator was added to a general purpose orthophthalic laminating resin and exposed in a static UV curing application. The laminate was prepared using three plies of 1.5 ounce/ft chopped strand mat. INPUT FACTORS 1) Type of experimental design or comparative strategy: full factorial 3 design FIXED FACTORS these factors will be held constant UNITS LEVEL A) Laminate materials na UPR-styrene, 35% E-glass, 3 plies of 1.5 oz/ft CSM B) Lighting na 100 W/in Honle UVASPOT 400 lamp, dimpled diffuse reflector, at 14 inches, V bulb FACTOR Variable these factors will be systematically varied UNITS LEVELS A) Exposure time minutes 0-0 B) Photoinitiator concentration % 0-% Irgacure 819 LIST OF RESPONSE VARIABLES TEST METHOD TO USE RESPONSE VARIABLE NAME UNITS A) Barcol hardness Barcol units Barber-Coleman stylus hardness tester B) Double bond conversion % DSC C) Surface temperature C embedded thermocouple D) UVV energy J/cm visual 88

112 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS conversion and barcol hardness ( ) exposure time (s) Barcol hardness (exposed surface) Barcol hardness (Mylar film side) Figure 5 Effect of UV energy on cure of a UPR laminate containing 35% short fiber E-glass reinforcement (0.75% BAPO photoinitiator) maximum reached temperature and exposure time at Tmax Conversion by thermal DSC analysis (%) total received light energy (J/cm) , maximum reached temperature ( C): ,18 Tmax. 8 exposure time at Tmax (s): tmax 6 0 3,4 4,3 total received light energy (J/cm) 4,16 1, exposure time (s) total received energy total received energy 89

113 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Figure 6 Effect of UV energy on surface temperature of a UPR laminate containing 35% short fiber E-glass reinforcement (0.75% BAPO photoinitiator) , Barcol hardness and conversion , 56, , BARCOL hardness (exposed side) 10 BARCOL hardness (Mylar film side) 7 4 conversion by thermal DSC analysis (%) ,1 0, 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1 1, 1,3 1,4 1,5 1,6 1,7 1,8 1,9,1 % of BAPO / formulation Figure 7 Effect of photoinitiator concentration on the cure of a UPR laminate containing 35% short fiber E-glass reinforcement (BAPO photoinitiator) 5 minute static exposure CONCLUSIONS From the results presented in figures 5-7 we can conclude that full cure as measured by hardness development and DSC double bond conversion can be achieved under these curing conditions in approximately three minutes of exposure (~ 4 J/ cm UV energy). The maximum surface temperature reached is 60 C. The minimum level of photoinitiator needed to achieve full cure is approximately 0.3%. 90

114 Chapter V - Modeling the Degree of Cure of a D UV Curing Process b. Studies of UV curing equipment variables i. Experiment 7 Reciprocal law for UV energy, independence of irradiance and line speed Objective: The objective of the experiment is to determine the dependence of UV energy and irradiance on the line speed in a conveyor curing apparatus. Approach: An EIT Power Puck radiometer / dosimeter was used to monitor the UV energy and irradiance levels. INPUT FACTORS 1) Type of experimental design or comparative strategy: full factorial 3 design FIXED FACTORS these factors will be held constant UNITS LEVEL A) Lighting na 400 W/in Fusion lamp, std. reflector, at 9 inches, V bulb FACTOR Variable these factors will be systematically varied UNITS LEVELS A) Conveyor speed fpm 0-40 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS A) Irradiance W/cm EIT Power Puck B) UV energy J/cm EIT Power Puck RESULTS TEST METHOD TO USE UVA UVB UVC UVV UVA UVB UVC UVV Line Speed W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm Table 3 - Reciprocal law for UV energy, independence of irradiance and line speed 91

115 Chapter V - Modeling the Degree of Cure of a D UV Curing Process UVA Energy(J/cm) UV Energy and Irradiance vs. Conveyor Speed Energy (J/cm) Irradiance (W/cm) Line Speed (fpm) ~ (Exposure Time) -1 Figure 8 Energy and Irradiance vs. Conveyor Speed CONCLUSION The UV energy varies with the reciprocal of line speed. The irradiance (photon flux) is fixed for a given lighting configuration UVA Irradiance (W/cm) E 1 v (equation 7.1) 9

116 Chapter V - Modeling the Degree of Cure of a D UV Curing Process ii. Experiment 8 DSC cure studies in clear and white gel coat role of film thickness and UV energy Objective: The objective of the experiment is to determine the UV energy requirements needed to cure a clear gel coat and a white gel coat. Approach: Photoinitiator was added to a general purpose orthophthalic laminating resin and exposed in a static UV curing application. The laminate was prepared using three plies of 1.5 ounce/ft chopped strand mat. INPUT FACTORS FIXED FACTORS these factors will be held constant UNITS LEVEL A) Laminate materials na UPR-styrene, 35% E-glass, 3 plies of 1.5 oz/ft CSM B) Lighting na 400 W/in Fusion lamp, std. reflector, at 19 inches, V bulb C) Irradiance level W/cm UVV=0.14 FACTOR Variable these factors will be systematically varied UNITS LEVELS A) UV Energy J/cm 0-6 B) Film thickness mils 0-0 LIST OF RESPONSE VARIABLES TEST METHOD TO USE RESPONSE VARIABLE NAME UNITS A) Double bond conversion % DSC 93

117 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS Dose-Cure Response for White UV Curable, Ultra Low HAPS Weather Resistant Gel Coat, and Conventional UPR Clear Gel Coat % Cure Measured by DSC DoseUVV wht vs cure wht 0 White - 10 mils White 15 mils White 0 mils Clear 10 mils Clear 15 mils Clear 0 mils UVA Dosage (Joules / cm ) NCLUSIONS UVV Dosage (Joules / cm ) Figure 9 Energy requirements to cure a clear gel coat and white gel coat CONCLUSIONS The chemical conversion as measured by DSC is dependent on the film thickness and pigmentation. All clear gel coats reach 90-95% of double bond conversion with J/cm of UVV energy. The thick (0 mil) pigmented film requires ~ 5 J/cm to reach full cure. 94

118 Chapter V - Modeling the Degree of Cure of a D UV Curing Process iii. Experiment 9 Temperature-Energy-Irradiance map for several UV light sources Objective: Determine the relationship between lamp height and the UVV and UVA energy and maximum surface temperature. Approach: The distance from the lamp to the radiometer is systematically varied in a static exposure test. INPUT FACTORS FIXED FACTORS these factors will be held constant UNITS LEVEL A) Exposure time min UPR-styrene, 35% E-glass, 3 plies of 1.5 oz/ft CSM B) Lighting na 400 W/in Fusion lamp, std. reflector, at 19 inches, V bulb C) Irradiance level W/cm UVV=0.14 FACTOR Variable these factors will be systematically varied UNITS LEVELS A) Lighting source na (1) Honle UVASPOT 400 with gallium metal halide lamp () Honle UVASPOT 400 with iron metal halide lamp (3) Fusion 400 W/cm lamp with gallium metal halide lamp (4) Fusion 400 W/cm lamp with iron metal halide lamp (5) Phillips VHO fluorescent lamp with 1500 ma ballast B) Lamp height inches 0-34 LIST OF RESPONSE VARIABLES TEST METHOD TO USE RESPONSE VARIABLE NAME UNITS A) UV Energy J/cm EIT Power Puck radiometer-dosimeter B) Surface temperature F Fluke IR thermometer 95

119 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS Light (UVV & UVA) Dosage Comparison for Several Illumination Sources (exposure interval = 1 minute) 100 =137 =169F Dosage of Light (J/cm ) F 106F 10F UVA Light from Honle UV Lamp Gallium Doped Bulb UVV Light from Honle UV Lamp Gallium Doped Bulb =10F =8F UVV Light from Fusion UV Lamp 400 W/inch with Gallium Doped Bulb =97F UVA Light from Fusion UV Lamp 400 W/inch with Gallium Doped Bulb =94F UVA Light from Honle UV Lamp Iron Doped Bulb 0.1 UVV Light from VHO Flourescent uo Lamp = RT for VHO UVV Light from Honle UV Lamp Iron Doped Bulb = Lamp Distance from the Surface of the Mold (Inches) Figure 10 - Energy and surface temperature profiles for several commercially available UV lamps CONCLUSIONS The irradiance from a point source of light varies with the inverse square of distance. As shown in figure 10 the UV energy falls sharply as expected with distance from the lamp. The power of the lamps increases in the following order: Phillips VHO fluorescent< Honle < Fusion. 96

120 Chapter V - Modeling the Degree of Cure of a D UV Curing Process The VHO and Honle lamps have reflectors which produce diffusely reflected light. The energy increases in a monotonic fashion as the distance from the lamp to the radiometer decreases. The Fusion lamp utilizes a parabolic lamp with a focal length of approximately three inches. The measured energy and irradiance actually drops slightly for the Fusion lamp when the distance from the lamp to the radiometer is less than the focal length.this trend is not observed for the Honle lamp that has a dimpled reflector which produces diffusely reflected light. The data in figure 10 may be used with a dose-conversion plot for the polymerizing material (see figure 9) to select an equipment set-up (type of lamp, distance, exposure interval). A separate measurement of surface temperature after three minutes of irradiation provides additional process information that can be used to ensure safe operation. To achieve proper cure it may be important to keep the surface temperature below some critical temperature to minimize the risk of fire due to the flash point of a volatile ingredient in the polymerizing material. For example, if the energy requirement is 4 J/cm UVV and the maximum temperature requirement is 100 F, the following curing conditions will provide the required energy and temperature control. VHO lamp minutes of exposure at a distance of 4 inches Fusion ( gallium lamp) 40 seconds of exposure at a distance of inches 97

121 Chapter V - Modeling the Degree of Cure of a D UV Curing Process iv. Experiment 10 Variations in energy and irradiance of a single 600 W/inch lamp as function of distance from the lamp centerline Objective: The objective of the experiment is to develop the mathematical model needed to predict the energy and irradiance emanating from a UV curing lamp Approach: A systematic collection of radiometric measurements will be collected from a UV conveyor system built by the author and shown in photograph 1 and figure 11 fitted with Fusion UV lamps. For this experiment only one of the lamps in use. The UV irradiance and energy is measured directly below the centerline of lamp 3. The UV irradiance and energy is also measured at six inch intervals laterally from the centerline of lamp three. The data collected from this experiment is then fitted to several distribution models. Experiment #10 Irradiance and energy distribution from a single 600 W/ inch UV lamp Air plenum off off on off off 600 W/inch Fusion UV lamp Lamp 1 Lamp Lamp 3 Lamp 4 Lamp 5 h=height x Figure 11 Schematic of experiment 10 set-up Radiometric data was collected at 6 inch lateral intervals 98

122 Chapter V - Modeling the Degree of Cure of a D UV Curing Process INPUT FACTORS 1) Type of experimental design or comparative strategy: FIXED FACTORS these factors will be held constant during the experiment UNITS LEVEL A) line speed fpm 14 B) lamp reflector type and bulb position within the reflector na std. reflector, 09 position ) How many independent factors are to be studied? 3 How many Tests will be conducted? x3x11=66 FACTOR Variable these UNITS LEVELS factors will be systematically varied during the experiment A) Lamp type na iron and gallium doped metal halide medium pressure mercury lamps B) Lamp height, h inches 5, 9, 14 C) Lateral position of the radiometer relative to the centerline of lamp 3 inches -30, -4, -18, -1, -6, 0, 6, 1, 18, 4, 30 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS TEST METHOD TO USE A) Energy J/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV B) Irradiance W/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV 99

123 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Photograph 1 - UV Curing line used to develop the irradiance and energy process model Two rows of five lamps each lamp is rated at 6000W (built by the author) 100

124 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS Experiment #10 - Variation in Dose and Intensity as a Function of Distance from Centerline - Single 600 W/inch Lamp SET 1 UVA UVB UVC UVV UVA UVB UVC UVV Position (inches) Line Speed Lamp Height Bulb Type Reflector W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm " V STD SET UVA UVB UVC UVV UVA UVB UVC UVV Position (inches) Line Speed Lamp Height Bulb Type Reflector W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm " V STD SET 3 UVA UVB UVC UVV UVA UVB UVC UVV Position (inches) Line Speed Lamp Height Bulb Type Reflector W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm " V STD

125 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Results of experiment 10 (continued) Experiment #10 - Variation in Dose and Intensity as a Function of Distance from Centerline - Single 600 W/inch Lamp SET 4 UVA UVB UVC UVV UVA UVB UVC UVV Position (inches) Line Speed Lamp Height Bulb Type Reflector W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm " D STD SET 5 UVA UVB UVC UVV UVA UVB UVC UVV Position (inches) Line Speed Lamp Height Bulb Type Reflector W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm " D STD SET 6 UVA UVB UVC UVV UVA UVB UVC UVV Position (inches) Line Speed Lamp Height Bulb Type Reflector W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm " D STD Table 4 - Variations in energy and irradiance of a single 600 W/inch lamp as function of distance from the lamp centerline CONCLUSIONS The UV energy, E, and irradiance, I, at a given height can be modeled using a Gaussian distribution as shown in the plots below. The plots show measured and 10

126 Chapter V - Modeling the Degree of Cure of a D UV Curing Process predicted values. This simple model can be used to predict the energy and irradiance at any point in a lateral direction, x, from the centerline, for a given light source see figure 11. E( x) e ( x) / equation 10.1 The variance in the UV energy for a given lamp height is determined experimentally and given the symbol in the above equation. Finally, the energy (or irradiance) can be predicted for a given lamp height by adding the pre-exponential factor, E( x) / ( x) e equation 10. Light Dose as a Function of Lateral Distance from Lamp Centerline (1 F600 V Bulb), h=5" Light Dose as a Function of Lateral Distance from Lamp Centerline (1 F600 D Bulb), h=5" Light Dosage (J/cm^) Postion from Lamp Center (inches) UVA UVB UVC UVV Pred UVV Pred UVA Pred UVB Pred UVC Light Dosage (J/cm^) Postion from Lamp Center (inches) UVA UVB UVC UVV Pred UVV Pred UVA Pred UVB Pred UVC Light Dose as a Function of Lateral Distance from Lamp Centerline (1 F600 V Bulb), h=9" Light Dose as a Function of Lateral Distance from Lamp Centerline (1 F600 D Bulb), h=9 " Light Dosage (J/cm^) Postion from Lamp Center (inches) UVA UVB UVC UVV Pred UVV Pred UVA Pred UVB Pred UVC Light Dosage (J/cm^) Postion from Lamp Center (inches) UVA UVB UVC UVV Pred UVV Pred UVA Pred UVB Pred UVC Light Dose as a Function of Lateral Distance from Lamp Centerline (1 F600 V Bulb), h=15" Light Dose as a Function of Lateral Distance from Lamp Centerline (1 F600 D Bulb), h=15" Light Dosage (J/cm^) Postion from Lamp Center (inches) UVA UVB UVC UVV Pred UVV Pred UVA Pred UVB Pred UVC Light Dosage (J/cm^) Postion from Lamp Center (inches) UVA UVB UVC UVV Pred UVV Pred UVA Pred UVB Pred UVC Figure Energy distribution for a Fusion 600 W/in lamp 103

127 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Lamp Type Lamp Height (inches) UVA UVB UVC UVV V 5 V 9 V 15 D 5 D 9 D 15 Gaussian Fit Parameters mean () multiplier () deviation () Gaussian Fit Parameters mean () multiplier () deviation () Gaussian Fit Parameters mean () multiplier () deviation () Gaussian Fit Parameters mean () multiplier () deviation () Gaussian Fit Parameters mean () multiplier () deviation () Gaussian Fit Parameters mean () multiplier () deviation () Table 5 - Gaussian fit model parameters to predict UV energy for each illuminant and lamp height from experiment

128 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Lamp Type Lamp Height (inches) UVA UVB UVC UVV mean () multiplier () deviation () mean () multiplier () deviation () mean () multiplier () deviation () mean () multiplier () deviation () mean () multiplier () deviation () mean () multiplier () deviation () V 5 V 9 V 15 D 5 D 9 D 15 Table 5 cont. - Gaussian fit model parameters to predict irradiance for each illuminant and lamp height from experiment

129 Chapter V - Modeling the Degree of Cure of a D UV Curing Process v. Experiment 11 Measurement of energy and irradiance from a bank of five 600 W/inch lamps Objective: The objective of the experiment is to study the energy and irradiance pattern in the lateral direction (i.e. perpendicular to the direction of conveyor travel) from a bank of UV lamps Approach: A systematic collection of radiometric measurements will be collected from a UV conveyor system built by the author and shown in photograph 1 and figure 1 fitted with Fusion UV lamps. One row of five 600W/inch lamps are used in this experiment. All five lamps are energized during the data collection period. The UV irradiance and energy is measured directly below the centerline of lamp 3. The UV irradiance and energy is also measured at six inch intervals laterally from the centerline of lamp three. Experiment #11 Irradiance and energy distribution from a bank of five 600 W/ inch UV lamps Air plenum on on on on on Lamp Lamp Lamp Lamp Lamp W/inch Fusion UV lamp Radiometric data was collected at 6 inch lateral intervals h=height Figure 1 UV lighting set-up for experiment 11 a bank of five 600 W/inch Fusion UV lamps x 106

130 Chapter V - Modeling the Degree of Cure of a D UV Curing Process INPUT FACTORS 1) Type of experimental design or comparative strategy: FIXED FACTORS these factors will be held constant during the experiment UNITS LEVEL A) line speed fpm 14 B) lamp reflector type and bulb position within the reflector na std. reflector, 09 position C) lamp type na gallium doped metal halide D) lamp height inches 11 ) How many independent factors are to be studied? 1 How many Tests will be conducted? 17 \ FACTOR Variable these UNITS LEVELS factors will be systematically varied during the experiment A) Lateral position of the radiometer relative to the centerline of lamp 3 inches -54, -48, -4, -30, -4, -18, -1, -6, 0, 6, 1, 18, 4, 30, 36, 4, 48, 54 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS TEST METHOD TO USE A) Energy J/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV B) Irradiance W/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV 107

131 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS UVA UVB UVC UVV UVA UVB UVC UVV Position (inches) W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm Table 6 - Measurement of energy and irradiance from a bank of five 600 W/inch lamps as a function of lateral position CONCLUSIONS The first observation, one that is not unsurprising, is that the UV energy distribution is symmetric about the centerline of the center lamp (#3). Secondly, we can see that the distribution is no longer Gaussian as was the case for a single lamp. The final observation requires a basic knowledge of the lamp dimensions. The lamp housing of the Fusion Systems I600M UV lamps used in this experiment is 10.5 inches. The lamps were placed side-by-side with no space between the housings. This implies the distance from the centerline of lamp #3 to the outer edge of lamp #1 (or #5) is (.5) x (10.5 )=6.5. At the lamp height of 11 the UV energy decreases rapidly beyond the edge of the bank of lamps as shown in figure

132 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Lateral UVV Dose Uniformity for a Row of F600 Lamps 1 row - V Bulb UVV Dose (J/ cm^) lamp 1 lamp lamp Position from the Centerline of the Middle Lamp (Inches) lamp 4 lamp 5 Figure 1.5 Measured UV Energy Bank of five 600 W/inch Fusion UV lamps 109

133 Chapter V - Modeling the Degree of Cure of a D UV Curing Process vi. Experiment 1 Testing the additive law for UV energy using two 600W/inch lamps Objective: The objective of the experiment is to test the hypothesis that the total UV energy at any point from multiple light sources can be calculated as the sum of the energy form each contribution light source. Approach: Two lamps will be used for this experiment. The distance between lamps will be progressively increased and the irradiance and energy will be measured in the lateral direction (perpendicular to the direction of travel). Experiment #1 Testing the additive nature of UV energy from multiple light sources using two 600 W/ inch UV lamps y 600 W/inch Fusion UV lamp Radiometric data was collected at 10.5 inch lateral intervals Lamp width = 10.5 inches on Lamp 3 Fixed position on Lamp 1 Variable position h=height x centerline position of the stationary lamp x=location of the radiometer-dosimeter measurement y=lamp separation (centerline to centerline) h=height of the lamp from the surface Values 0 inches (fixed) -31.5, -1, -10.5, 0, 10.5, 1, 31.5, 4, 51.5, 6 inches 10.5, 1, 31.5, 4 inches 11 inches (fixed) Figure 13 UV lighting set-up for experiment 1 110

134 Chapter V - Modeling the Degree of Cure of a D UV Curing Process INPUT FACTORS 1) Type of experimental design or comparative strategy: ladder study with variables FIXED FACTORS these factors will be held constant during the experiment UNITS LEVEL A) lamp height from surface inches 11 B) conveyor speed fpm 14 C) lamp type na 1 row, gallium metal halide V bulb, std reflector ) How many independent factors are to be studied? How many Tests will be conducted? 34 FACTOR Variable these UNITS LEVELS factors will be systematically varied during the experiment A) x location (lateral position) of the radiometer-dosimeter measurement inches In intervals of 10.5 from to 5.5 B) y- separation distance between the lamps (centerline-to-centerline) inches 10.5, 1, 31.5, 4 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS TEST METHOD TO USE A) Energy J/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV B) Irradiance W/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV 111

135 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS The position of the spacing between the stationary and adjustable position lamp (y) and the position (x) of the radiometer-dosimeter are noted in table 7 below. The width of each lamp is 10.5 inches. A lamp (center-to-center) distance of 10.5 inches implies the two lamps are side-by side with no space between the lamps. UVA UVB UVC UVV UVA UVB UVC UVV Lamp Separation (y) Center to Center (inches) Position (x) of of the Radiometer- Dosimeter (relative to the stationary lamp) W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm Run # Table 7 - Testing the additive law for UV energy using two 600 W/inch lamp 11

136 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Output of Two Fusion F600 UV Lamps as a Function of Lamp-to- Lamp Centerline Distance UVV Energy (J / cm ^) Distance Between Lamp Centerlines Radiometer-Dosimeter Position (relative to the staionary lamp) Figure 14 Output from two 600 W/inch UV lamps CONCLUSIONS The total UVV energy from two parallel Fusion 600 W/in lamps is shown in figure 14. Since the light rays are highly collimated, changes in the spacing of the lamps have a pronounced effect on the distribution of energy. The area under the energy curve remains constant for all lamp configurations. 113

137 Chapter V - Modeling the Degree of Cure of a D UV Curing Process vii. Experiment 13 Effect of UV energy and irradiance level on the surface temperature of the coating Objective: The objective of the experiment is to understand the effect of UV energy and irradiance levels on the surface heating effect which occurs during UV irradiation with high power UV curing lamps. Approach: It is important to control the surface temperature of liquid coatings and resins containing flammable components for process safety reasons. The maximum temperature reached during the cure process should be maintained below the lowest flash point of the components in the mixture. In this experiment, the lighting conditions were systematically varied to provide a broad range of UV energy ( 46 J/cm ) and UV irradiance ( W / cm ). Three rows of lamps were employed within the UV conveyor to create a large heating effect. A metal Q-panel was used as the substrate. INPUT FACTORS 1) Type of experimental design or comparative strategy: full factorial FIXED FACTORS these factors will be held constant during the experiment UNITS LEVEL A) lamp type na Fusion 600 W/inch lamp B) # rows of lamps within the conveyor three rows C) substrate steel Q-panel ) How many independent factors are to be studied? 4 How many Tests will be conducted? 3 FACTOR Variable these UNITS LEVELS factors will be systematically varied during the experiment A) line speed of the conveyor fpm 5, 10, 0, 40 B) bulb type na three rows of gallium (V bulbs), two row of gallium (V bulb) plus one row of iron (D bulb) C) bulb position within the reflector na standard position, 09 position D) lamp height from the substrate inches 6,

138 Chapter V - Modeling the Degree of Cure of a D UV Curing Process LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS TEST METHOD TO USE A) Energy J/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV B) Irradiance W/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV C) Surface Temperature of the Q-panel exiting the UV conveyor F Fluke IR thermometer RESULTS UVA UVB UVC UVV UVA UVB UVC UVV TOTAL TOTAL Run # Line Speed Lamp Height Bulb Type Reflector W/cm W/cm W/cm W/cm J/cm J/cm J/cm J/cm W/cm J/cm T Exit (F) V STD V STD V STD V STD V STD V STD V STD V STD V, 1D STD V, 1D STD V, 1D STD V, 1D STD V, 1D STD V, 1D STD V, 1D STD V, 1D STD V V V V V V V V V, 1D V, 1D V, 1D V, 1D V, 1D V, 1D V, 1D V, 1D Table 8 Effect of lighting configuration and line speed on UV energy, irradiance, and surface temperature 115

139 Chapter V - Modeling the Degree of Cure of a D UV Curing Process UVA Dose Data Energy (J/cm) V,6",Std Reflector 3V,15",Std Reflector V+1D,6",Std Reflector V+1D,15",Std Reflector 3V,6",09 Reflector 3V,15",09 Reflector V+1D,6",09 Reflector V+1D,15",09 Reflector Line Speed UVV Dose Data 35 Energy (J/cm) V,6",Std Reflector 3V,15",Std Reflector V+1D,6",Std Reflector V+1D,15",Std Reflector 3V,6",09 Reflector 3V,15",09 Reflector V+1D,6",09 Reflector V+1D,15",09 Reflector Line Speed Exit Temperture (degrees F) 190 Conveyor Exit Temperature (degrees F) V,6",Std Reflector 3V,15",Std Reflector V+1D,6",Std Reflector V+1D,15",Std Reflector 3V,6",09 Reflector 3V,15",09 Reflector V+1D,6",09 Reflector V+1D,15",09 Reflector Line Speed 116

140 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Figure 15 Effect of lamp type, lamp height, line speed, and reflector type on UV energy, irradiance, and exit temperature 00 Exit Temperature (F) R = Total Energy (J/cm) Exit Temperature (F) R = Irradiance Level (W/cm) Figure 16 Correlation of UV energy and irradiance with the surface temperature of a part being cured with UV lamps CONCLUSIONS Under the test conditions used in experiment 13 the substrate temperature heating effect is well correlated with the total UV energy. The exit temperature of the substrate is not well correlated with UV irradiance levels. 117

141 Chapter V - Modeling the Degree of Cure of a D UV Curing Process viii. Experiment 14 Validation of the cosine law for non-perpendicular exposure conditions Objective: The objective of the experiment is to measure changes in UV energy and irradiance levels due to the angle of incidence of the light rays. Approach: A radiometer-dosimeter was placed under a UV lamp at two angles of incidence ( = 0 flat, and = 60 angled) as shown below in figure 17. Experiment #14 Validation of the cosine law for non-perpendicular exposure conditions PowerPuck =60 Figure 17 Validation of the cosine law for non-perpendicular exposure conditions FACTOR Variable these factors will be systematically varied during the experiment UNITS LEVELS A) angle of incidence degrees 0, 60 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS TEST METHOD TO USE A) Energy J/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV B) Irradiance W/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV 118

142 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS ENERGY (J / CM) INTENSITY (W/CM) Angle UVA UVB UVV UVA UVB UVV = = ratio arc cosine Table 9 - Validation of the cosine law for non-perpendicular exposure conditions CONCLUSIONS The UV energy and irradiance on a surface that is not perpendicular to the light source varies according to the cosine law. =E x cos() eqn 14.1 ix. Experiment 15 Effect of lamp height on UV energy and irradiance Objective: The objective of the experiment is to understand the effect of lamp distance from the curing material on the UV energy and irradiance received at the surface. Approach: A single Fusion F450 lamp is energized for one minute above a radiometer-dosimeter. The distance between the lamp and the radiometer-dosimeter is systematically varied. inches 4 inches FOCAL PLANE 8 inches 1 inches 33 inches Figure 18 Schematic of lighting set-up for experiment

143 Chapter V - Modeling the Degree of Cure of a D UV Curing Process INPUT FACTORS 1) Type of experimental design or comparative strategy: FIXED FACTORS these factors will be held constant during the experiment UNITS LEVEL A) lamp type na Fusion F450 lamp, V bulb B) exposure time minutes 1 ) How many independent factors are to be studied? How many Tests will be conducted? FACTOR Variable these UNITS LEVELS factors will be systematically varied during the experiment A) lamp height (h) inches, 4, 8, 1, 33.5 LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS TEST METHOD TO USE A) Energy J/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV B) Irradiance W/ cm PowerPuck process radiometerdosimeter UVA, UVB, UVC, UVV RESULTS Irradiance Energy Height (inches) UVA UVB UVC UVV UVA UVB UVC UVV Table 10 UV Energy and irradiance measurements at various lamp heights static one minute exposure 10

144 Chapter V - Modeling the Degree of Cure of a D UV Curing Process UVV Energy (J/cm) UV Irradiance and Energy as a Function of Distance from the Light Source Energy (J/cm) Irradiance (W/cm) Irradiance (W/cm) Distance (inches) Figure 19 - UV Energy and irradiance measurements at various lamp heights static one minute exposure Distance (h) Irradiance (W/cm ) Energy (J/cm ) 1/h Table 11 The irradiance from a point source of light varies with the square of the distance from the source Inverse Square Law for Irradiance UV Irradiance (W/cm) E-08 in focus inside the focal plane / (lamp height) Figure 0 Inverse square law validation CONCLUSIONS As noted in figures 19 and 0 the inverse square law is validated for the UV lamps at distances greater the focal length of the UV reflector. 11

145 Chapter V - Modeling the Degree of Cure of a D UV Curing Process x. Experiment 16 Evaluation of dichroic reflectors Objective: The objective of the experiment is to evaluate the use of dichroic reflectors to lower the surface temperature of the substrate. Approach: Direct comparison of a standard reflector and a dichroic reflector using a Q-panel steel substrate. INPUT FACTORS 1) Type of experimental design or comparative strategy: FIXED FACTORS these factors will be held constant during the experiment UNITS A) conveyor speed fpm 15 LEVEL B) lamp type na two rows of gallium lamps, plus one row of iron lamps 600 W/inch Fusion C) lamp height inches 4 ) How many independent factors are to be studied? How many Tests will be conducted? FACTOR Variable these factors will be systematically varied during the experiment UNITS LEVELS A) reflector type na standard reflector, dichoric reflector LIST OF RESPONSE VARIABLES RESPONSE VARIABLE NAME UNITS TEST METHOD TO USE A) surface temperature F Fluke IR thermometer RESULTS Run # Line Speed Lamp Height T Exit (F) STD dichroic 140 Table 1 Evaluation of dichroic reflectors CONCLUSIONS The dichroic cold mirror reflector did not appear to reduce the surface temperature. 1

146 Chapter V - Modeling the Degree of Cure of a D UV Curing Process c. Studies of the reflectivity of the mold surface Experiment 17 Effect of reflectivity on cure Objective: The objective of the experiment is to evaluate the effect of the mold reflectivity on the degree of cure of a photocurable white gel coat. Approach: Direct comparison of the extent of cure on a polished stainless steel mold and a composite mold with an orange polyester tooling gel coat. As noted in figure 1 below there are several possibilities for the interaction of UV light with the coating and mold surface. Case 1 the light is completely absorbed within the coating film. This case is unacceptable since it would result in incomplete thru-cure of the coating. Case the light is partially absorbed in the coating film. Some of the light passes through the coating film and is absorbed within the mold surface. Case 3 - the light is partially absorbed in the coating film. A portion of the light passes through the coating film and is absorbed within the mold surface. A portion of the light reaching the mold surface is reflected back into the coating film further promoting UV curing of the coating. Case 1 The incident light is absorbed within the coating film I 0 Case The incident light is partially absorbed within the coating film. The light that is transmitted through the film is absorbed by the mold I 0 Case 3 The incident light is partially absorbed within the coating film. The light that is transmitted through the film is reflected by the mold back into the coating film I 0 Coating I A I T I A I T I A IA I R I A Mold I 0 I A I T I R = incident light = absorbed light = transmitted light =reflected light Figure 1 Interactions of UV light with the coating and mold surface 13

147 Chapter V - Modeling the Degree of Cure of a D UV Curing Process INPUT FACTORS 1) How many independent factors are to be studied? 1 How many tests will be conducted? 7 FACTOR Variable these factors will be systematically varied during the experiment UNITS LEVELS A) mold type-color na polyester molds (1) orange () green (3) black (4) white (5) silver-reflective Al flake pigment other molds (6) polished aluminum (7) polished stainless steel LIST OF RESPONSE VARIABLES TEST METHOD TO USE RESPONSE VARIABLE UNITS NAME A) Mold reflectance % Perkin-Elmer Lambda 6 UV spectrophotometer with an integrating sphere accessory 14

148 Chapter V - Modeling the Degree of Cure of a D UV Curing Process RESULTS Multi-colored composite Total %R (specular reflectance included) Figure UV-Visible reflection from polyester tooling gel coat various colors Stainless Steel, Aluminum, and Reflective Composite Total %R (specular reflectance included) Figure 3 UV-Visible reflection from metal molds and aluminum flake filled polyester tooling gel coat. 15

149 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Run # 1 Lamp Height Lamps (inches) 1 row V, 1 row D row V, 1 row D 7.5 Coating Coating Thickness (mils) Line Speed (fpm) white, UPR white, UPR Mold UVV (J/cm) UVA (J/cm) Result polished stainless steel coating is well cured polished stainless steel coating is well cured 3 3 rows V rows V rows V rows V, 1 row D 5 3 rows V, 1 row D 5 1 row V, 1 row D 9 white, UPR 8 0 white, UPR 8 1 white, UPR 8 1 white, UPR 6 8 white, UPR 6 1 white, UPR 1 1 composite mold with orange tooling gel coat 7.4. composite mold with orange tooling gel coat composite mold with orange tooling gel coat composite mold with orange coating is dry on the top surface but wet against the mold the coating film is rubbery, not completely cured, marginal surface gloss the coating film is rubbery, not completely cured, marginal surface gloss - better than run 4 tooling gel coat coating is well cured composite mold with orange tooling gel coat coating is dry on the top surface but there is some slight haze on the mold following de-mold indcating incomplete thru-cure polished stainless steel coating is well cured Table 13 White UV curable gel coat results on a reflective and non-reflective mold ( V lamp=gallium metal halide, D lamp=iron metal halide) CONCLUSIONS The reflectance characteristics of various pigmented polyester tooling gel coats is presented in figure. Only the white tooling gel coat has any reflectance below 400 nm. As noted in figure 3 the polished stainless steel, polished aluminum, and aluminum flake filled polyester mold exhibit significant reflection in the wavelength range of interest for BAPO photoinitiators ( nm). The data in table 13 provides a practical energy requirement to cure the film. The extent of cure is highly dependent upon the film thickness as might be expected. A direct comparison of the non-reflective orange composite mold and the reflective polished stainless steel mold is given in runs #7 and #8. The coating cured on the stainless steel mold is better cured than the coating on the orange composite mold when the UV energy is approximately equal. Separate tests with the white composite mold resulted in slightly better coating cure than the orange mold, but not as well as the stainless steel mold. 16

150 Chapter V - Modeling the Degree of Cure of a D UV Curing Process 18) d. Integrated mathematical model for a UV conveyor line (experiment i. Mathematical model development The results provided in the preceding experiments have provided the components of a mathematical model to predict UV energy and irradiance levels. An integrated model capable predicting UV exposure levels for a conveyor system with multiple banks (rows) of UV curing lamps will now be developed. Several simulations will be conducted using a software version of the model prepared by the author. The model will be validated using with actual process data from an industrial scale UV conveyor system discussed in Chapter 7. Consider a line with conveyor h-direction Lamp height Y-direction Direction of travel X-direction Perpendicular to travel Figure 4 Schematic of a conveyor line with the coordinate system indicated The distribution of UV energy, E (J/cm ) and irradiance (W/cm ) from a high powered industrial UV lamp (Fusion UV Systems 6000 watt lamp) were shown from experiment 10 to be approximated in the x-direction according to a Gaussian distribution: 17

151 Chapter V - Modeling the Degree of Cure of a D UV Curing Process 18 / ) ( ) ( E x E e x E eqn / ) ( ) ( I x I e x I eqn. 18. Where E, I, E, and I are the pre-exponential and dispersion parameters developed in experiment 10. An additional finding of experiment 10 is that the preexponential and dispersion parameters are dependent on the lamp height, h. ) ( ) ( h h eqns. 18.3, 18.4 Combining eqns we obtain ] ) ( / ) ( [ ) ( ), ( h x E E e h h x E eqn ] ) ( / ) ( [ ) ( ), ( h x I I e h h x I eqn The functional form of the lamp height (h) dependence on the pre-exponential multiplier,, and the dispersion term, in equations was chosen to be a simple quadratic model.

152 Chapter V - Modeling the Degree of Cure of a D UV Curing Process ( h) h h ( h) h h To illustrate the technique to model the height dependence of (h) and (h), an example is provided below using the UVV band irradiance level as the response variable. The irradiance level for each UV band (UVV, UVA, UVB, UVC) was collected at three lamp height settings (5, 9, 15 ). The data was fitted to the model shown in equation 18. and summarized in the table below. Model Parameters for the UVV band to predict irradiance levels from a single 600W/inch Fusion UV lamp Lamp Height h Dispersion parameter Pre-exponential term ( h) h h ( h) h h The pre-exponential multiplier decreases with increasing lamp height and the dispersion term increases with increasing lamp height as expected see figure 5. 19

153 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Parameter Value h) = 0.134h h (h) = h h h = Lamp Height (inches) Figure 5 Lamp height dependence of the pre-exponential multiplier and dispersion parameter Next, we will add a term to the model to account for the line speed of the conveyor system v(fpm). A finding of experiment 7 is that the energy, E (J/cm) varies with the inverse of the line speed. The irradiance, I (W/cm) is independent of the line speed. 1 E I v v The reference line speed, v ref, used to produce the results for experiment 10 was 14 fpm. A term will now be added to equation 18.5 to allow computation of UV energy for any line speed, V. The instantaneous photon flux (i.e. irradiance) does not depend on line speed. 130

154 Chapter V - Modeling the Degree of Cure of a D UV Curing Process 131 ] ) ( / ) ( [ ) ( ),, ( h x E ref E e h v v v h x E eqn Based on the findings of experiment 14 a term will now be added to equation 18.7 to adjust the predicted UV energy and irradiance level at non-normal angles of incidence. ] ) ( / ) ( [ ) ( ) cos( ),, ( h x E ref E e h v v v h x E eqn ] ) ( / ) ( [ ) ( ) cos( ), ( h x I I e h h x I eqn The UV energy and irrandiance at a point in the x direction (see fig. 4) from a conveyor with multiple lighting sources is the sum of the energy from each source i m i TOT E E E E eqn i m i TOT I I I I eqn The UV irradiance at a point in the x-direction is not cumulative. The UV energy for a bank of two lamps then may be determined from

155 Chapter V - Modeling the Degree of Cure of a D UV Curing Process 13 ] ) ( ) ( )[ cos( ),,, ( ] ) ( / ) ( [ ] ) ( / ) 1 ( [ 1 1 h x E h x E ref TOT E E e h e h v v v h x E eqn A common equipment configuration for an industrial curing line includes multiple rows of UV lighting (figure 6). High levels of UV energy are often needed to achieve satisfactory cure of optically thick materials such as pigmented gel coats. Multiple rows of lighting may be needed for one of two reasons: 1) to meet the energy requirements needed to complete the photopolymerization while maintaining acceptable conveyor line speed, and/or ) to allow for different spectral power distributions required to activate multiple photoinitiators such as a surface cure photoinitiator (shorter wavelength required) and a through-cure photoinitiator (longer wavelength required). The generalized form of the model for n rows of m lamps within each row is given below: ] ) ( / ) ( [ 1 ) ( ) cos( ),,, ( h x E i m i ref rows TOT Ei i i e h v v n v h x E eqn ] ) ( / ) ( [ 1 ) ( ) cos( ),, ( h x I i m i TOT Ii i i e h h x I eqn where i is the centerline position of lamp i.

156 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Y-direction Direction of travel h-direction Lamp height 1 X-direction Perpendicular to travel Figure 6 Schematic of an industrial UV curing line The complete mathematical model to predict the UV energy and irradiance levels is given in equations 18.13, and respectively. The model will now be used in the following four simulations. ii. Simulation 1 - Validation of the mathematical model The purpose of the first simulation is to validate the predictive capability of the model. The lighting configuration will be designed to support a four foot wide conveyor with two rows of lamps. Each row will contain five lamps. The first row will consist of iron doped metal halide (D-bulb) 600 W/inch Fusion UV lamps. The second row will consist of gallium doped metal halide (V- 133

157 Chapter V - Modeling the Degree of Cure of a D UV Curing Process bulb) 600 W/inch Fusion UV lamps. Each lamp is 10.5 inches in width. The lighting system used to validate the predictive model is shown in figure 7. Figure 7 - Ten lamp UV curing conveyor Two rows of five lamps Energy (J/cm ) UV Energy Distribution Energy (J/cm) - UVA Energy (J/cm) - UVB Energy (J/cm) - UVC Energy (J/cm) - UVV Line Speed: 5 fpm Angle : 0 degrees Lamp Height: 7.5 inches Lamp Spacing: 0 inches V Lamps: 1 row D Lamps: 1 row Position from the left edge (inches) 134

158 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Irradiance (W/cm) Irradiance Profile Irradiance (W/cm) - UVA Irradiance (W/cm) - UVB Irradiance (W/cm) - UVC Irradiance (W/cm) - UVV Position from the left edge(inches) Figure 8 - Validation of the predictive model to estimate UV energy and irradiance levels (predicted values shown) Energy (J/ cm ) Irradiance (W/cm) PREDICTED VALUES MEASURED VALUES UVV UVA UVB UVC UVV UVA UVB UVC The predicted values the key UV bands (UVV, UVA, UVB) are within 10% of the measured values. iii. Simulation - The effect of lamp spacing on the UV energy and irradiance level The purpose of the simulation is to study the effect of lamp spacing on the level and uniformity of the UV light energy delivered to the surface. The lighting configuration will be designed to support a ten foot wide conveyor with up to three rows of lamps. Each row will contain eleven lamps. The first row will consist of iron doped metal halide (D-bulb) 600 W/inch Fusion UV lamps. Subsequent rows will consist of gallium doped metal halide (V-bulb) 600 W/inch Fusion UV lamps. Each lamp is 135

159 Chapter V - Modeling the Degree of Cure of a D UV Curing Process 10.5 inches in width. The lamp spacing for simulation 1 (figure 8) was zero inches. This is the optimum condition to maximize energy delivery if equipment costs are not considered. By incorporating space between the lamps within a row it should be possible to cover more area with a limited number of lamps. The spacing between the lamps is gradually increased in figure 9 from 3 inches to 6 inches, and finally 1 inches. The result of the increasing spacing between the lamps is a commensurate increase in coverage area, decrease in energy and decrease in the uniformity of UV energy and irradiance levels. The results are summarized below: Spacing Between Lamps Relative Area Illuminated Relative Energy Level 0 inches 100% 100% 0% 3 inches 137% 78% 3.1% 6 inches 181% 50% 4.6% 1 inches 44% 47% 105% Table 1 Effect of lamp spacing on UV energy level and uniformity Lateral Variation in Energy Based on the dispersion pattern of these lamps at the height of 7.5 inches the spacing between lamps should be limited to less than 3-4 inches to maintain adequate uniformity. 136

160 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Energy (J/cm ) UV Energy Distribution Energy (J/cm) -UVA Energy (J/cm) -UVB Energy (J/cm) -UVC Energy (J/cm) -UVV Line Speed: 5 fpm Angle : 0 degrees Lamp Height: 7.5 inches Lamp Spacing: 3 inches V Lamps: 1 row D Lamps: 1 row Position from the left edge (inches) Energy (J/cm ) UV Energy Distribution Energy (J/cm) -UVA Energy (J/cm) -UVB Energy (J/cm) -UVC Energy (J/cm) -UVV Line Speed: 5 fpm Angle : 0 degrees Lamp Height: 7.5 inches Lamp Spacing: 6 inches V Lamps: 1 row D Lamps: 1 row Position from the left edge (inches) Energy (J/cm ) UV Energy Distribution Energy (J/cm) -UVA Energy (J/cm) -UVB Energy (J/cm) -UVC Energy (J/cm) -UVV Line Speed: 5 fpm Angle : 0 degrees Lamp Height: 7.5 inches Lamp Spacing: 1 inches V Lamps: 1 row D Lamps: 1 row Position from the left edge (inches) Figure 9 - The effect of lamp spacing on the level and uniformity of UV energy 137

161 Chapter V - Modeling the Degree of Cure of a D UV Curing Process iv. Simulation 3 The effect of a lamp failure on the irradiance and energy distribution The modular microwave UV lamps with electrodless bulbs used here are reported to be highly reliable according to the manufacturer. The mean time to failure is greater than 000 hours. The purpose of the simulation is to study the effect of a lamp failure on the level and uniformity of the UV light energy delivered to the surface. of the model. The lighting configuration will be designed to support a ten foot wide conveyor with up to two rows of lamps. Each row will contain eleven lamps. The first row will consist of iron doped metal halide (D-bulb) 600 W/inch Fusion UV lamps. The second row will consist of gallium doped metal halide (V-bulb) 600 W/inch Fusion UV lamps. For the purposes of the simulation, lamp #8 on the second row is switched off within the predictive model software to simulate a gallium lamp failure as indicated in figure 30. The results of the simulation indicate a dramatic localized decrease in energy and irradiance level near the position of the lamp failure. The UVV energy and the irradiance level have decreased by 35%. If this were an actual production curing station it would be important to detect lamp failure immediately. Failure to correct the situation could result significant quality problems arising from poorly cured coating or laminate. 138

162 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Energy (J/cm ) UV Energy Distribution Energy (J/cm) - UVA Energy (J/cm) - UVB Energy (J/cm) - UVC Energy (J/cm) - UVV Line Speed: 5 fpm Angle : 0 degrees Lamp Height: 7.5 inches Lamp Spacing: 0 inches V Lamps: 1 row D Lamps: 1 row Position from the left edge (inches) Irradiance (W/cm) Irradiance Profile Irradiance (W/cm) - UVA Irradiance (W/cm) - UVB Irradiance (W/cm) - UVC Irradiance (W/cm) - UVV Position from the left edge(inches) Row of D lamps (iron-metal halide) Row of V lamps (iron-metal halide) Y-direction Direction of travel h-direction Lamp height Lamp #8 has failed in the simulation 1 X-direction Perpendicular to travel Figure 30 - The impact of a lamp failure on the UV energy and irradiance distribution 139

163 Chapter V - Modeling the Degree of Cure of a D UV Curing Process v. Simulation 4 Effect of lamp height in energy level and uniformity The purpose of the simulation is to study the effect of a lamp height on the level and uniformity of the UV light energy delivered to the surface. of the model. The lighting configuration will be designed to support a ten foot wide conveyor with up to two rows of lamps. Each row will contain eleven lamps. The first row will consist of iron doped metal halide (D-bulb) 600 W/inch Fusion UV lamps. The second row will consist of gallium doped metal halide (V-bulb) 600 W/inch Fusion UV lamps. The results presented in figure 31 show two simulations for the UV energy levels at a lamp height of 3 inches and at 0 inches. The lamps are spaced 10.5 inches apart in both simulations. Clearly the lamp height and dispersion pattern must be considered to provide adequate uniformity of the UV lighting conditions. 140

164 Chapter V - Modeling the Degree of Cure of a D UV Curing Process Energy (J/cm ) UV Energy Distribution Energy (J/cm) - UVA Energy (J/cm) - UVB Energy (J/cm) - UVC Energy (J/cm) - UVV Line Speed 5 Ref. Line Speed 14 angle of incidence 0 lamp height 0 lamp spacing 10.5 lamp width 10.5 #Rows V Lamps 1 D Lamps Position from the left edge (inches) Energy (J/cm ) UV Energy Distribution Energy (J/cm) -UVA Energy (J/cm) -UVB Energy (J/cm) -UVC Energy (J/cm) -UVV Line Speed 5 Ref. Line Speed 14 angle of incidence 0 lamp height 3 lamp spacing 10.5 lamp width 10.5 V V h ls lw #Rows V Lamps 1 n D Lamps 1 n Position from the left edge (inches) Figure 31 - The effect of lamp height on the level and uniformity of UV energy 141

165 Chapter VI Process Optimization Defining the Process Window Chapter VI Process Optimization - Defining the Process Window Balancing Safety, Throughput, Environmental Impact, Capital Investment and Operating Costs a. Safety considerations There are four safety related considerations which must be properly managed when working with UV curing equipment. 1) Eye and skin exposure The light energy from high powered UV curing lamps can burn the cornea and skin. Proper shielding through equipment design, the use of UV screens, UV blocking safety glasses and UV blocking creams is necessary. ) Fire hazard Some UV lamps emit IR radiation at levels sufficient to raise the surface temperature of the substrate or coating well above the flash point of the volatile components that are present ( see Chapter V, Experiment 9 and 13). In the case of static exposures ( i.e. no motion of the lamp relative to the part) the lamp height and exposure time should be selected to avoid overheating. Under dynamic exposure conditions a safety interlock should be used which turns off the lamps if the conveyor motion drops below a specified setting. 3) Microwave energy Lamps using the electrodeless bulb design are heated within an open sided microwave chamber. The RF energy is contained using a metal mesh screen. Potential RF leaks are detected using an interlocked RF detector (see Chapter II, photograph 7). It is important to 14

166 Chapter VI Process Optimization Defining the Process Window perform periodic inspections to insure the integrity of the mesh screen and follow the manufacturer s placement guidelines for the RF detector. 4) Breathing air There are several special conditions which may give rise to compromised breathing air quality. Each of these conditions may be remedied with proper air make-up systems. These conditions are described briefly below: a. Ozone lamps containing mercury may produce ozone due to the short wavelength UV peak near 50 nm. Fabricators should check with the equipment manufacturer to determine if this is an issue. b. Nitrogen blanketing some UV curing applications utilize localized removal of air within the curing zone through the use of a nitrogen curtain to cure oxygen inhibited systems. The equipment used to produce the oxygen free environment (< 00 ppm) consists of a fully enclosed inerting chamber. Proper air make-up systems may be needed to maintain breathing air within safe limits depending on the flow rate of nitrogen, room size, and air flow replenishment. c. Solvent vapors The air flow within high powered UV curing lamps and onto the surface of a partially cured coating can create a condition of forced convection resulting in rapid evaporation of solvent and monomer. Proper air make-up systems may be needed to maintain organic levels below the LEL (lower explosive limits) depending upon the room size, and air flow replenishment. 143

167 Chapter VI Process Optimization Defining the Process Window b. Throughput considerations 1-3 UV curing offers tremendous curing speed advantages when compared to conventional room temperature curing using redox initiators. As noted in Chapter II the curing time for a conventional gel coat can be reduced from 45 minutes to a few seconds using UV curing. There are however trade-offs with UV curing which must be factored into consideration UV curing is a line-of-sight process UV curing has thickness limitation based on the ability to transmit light through the coating or laminate. In practice, there are numerous material and process variables which must be controlled to safely produce a consistently cured part. The foundational work to define the process window is developed by conducting experiments to study the influence of pigmentation, film thickness, photoinitiator concentration, energy and irradiance levels, lamp position, and mold reflectivity on the film formation, degree of cure, and surface temperature (see chapter V, Experiments 1-4,8,9,15, and 17). Once these results are obtained, material specifications and equipment configuration details can be established to meet the design expectations. Once these controls are implemented the daily operational throughput is managed by careful control of the line speed and film thickness (figure 1). Unlike most coatings and inks which are designed to adhere to the substrate, the gel coat must release from substrate (mold). If the coating is applied too thin, the gel coat will crawl (dewetting) on the low surface energy surface of the mold (see Chapter I, photograph 10). If the gel coat is too thick the gel coat will not completely cure in the thickness direction. The resulting cure gradient will produce the alligatoring texture created by a differential shrinkage in the thickness direction (see Chapter III, photograph 1). The throughput (line speed) of 144

168 Chapter VI Process Optimization Defining the Process Window each coating chemistry will depend upon the desired thickness and reactivity. For example, as noted in Chapter V, Experiment 5, thermosetting acrylic polymers can be cured more easily (i.e. thicker, faster) than unsaturated polyester polymers. The operating line speed should chosen to allow for complete cure while simultaneously avoiding overheating. Once the process parameters are defined, periodic spot checks should be made using a process radiometer-dosimeter to assure the process is operating within the design requirements. Low Energy= Incomplete cure Conveyor Speed (fpm) Valid Process Window Excessive Heating = Fire hazard Coating Film Thickness Poor wetting of the mold Low Transmission= Incomplete cure, alligatoring texture on the surface Figure 1 - Process window for UV curable gel coat c. Environmental benefits of UV curable composites Conventional gel coats and laminating resins contain between 30% - 45% reactive diluents (typically styrene and methyl methacrylate). Presently there are two driving forces to reduce the use of styrene monomer containing products: 145

169 Chapter VI Process Optimization Defining the Process Window 1) NESHAP MACT Standard (40 CFR Part 63) (ref. Under the Clean Air Act Amendments of 1990, Congress greatly expanded the Air Toxics program, creating a list of 189 substances to be regulated as hazardous air pollutants. Rather than regulating individual pollutants by establishing health-based standards, the new Air Toxics program granted EPA the authority to regulate specific industrial major source categories with National Emission Standards For Hazardous Air Pollutants (NESHAP) based on maximum achievable control technology (MACT) for each source category. Thus, a number of NESHAP standards have been established to regulate specific categories of stationary sources that emit (or have the potential to emit) one or more hazardous air pollutants. NESHAP statndards may cover both major sources and area sources in a given source category. Major sources are defined as those facilities emitting, or having the potential to emit, 10 tons per year or more of one HAP or 5 tons per year or more of multiple HAPs. Major sources are required to comply with MACT standards. Most composite operations which use conventional gel coats and laminating resins emit more than 10 tons per year of styrene monomer based on the EPA s current formulas to compute HAPS emissions. Acceptable remedies to the styrene emission dilemma include capture and control (i.e. thermal oxidation) or reduction in source usage (i.e. alternate technologies). Either of these options present technical and financial challenges for composite fabricators. 146

170 Chapter VI Process Optimization Defining the Process Window ) EPA IRIS (Integrated Risk Information System) Carcinogenic Classification of Styrene Monomer ref. The Integrated Risk Information System (IRIS), prepared and maintained by the U.S. Environmental Protection Agency (U.S. EPA), is an electronic database containing information on human health effects that may result from exposure to various chemicals in the environment. IRIS was initially developed for EPA staff in response to a growing demand for consistent information on chemical substances for use in risk assessments, decision-making and regulatory activities. IRIS is closed to the public Scientific Process rather than a regulatory development process Viewed as a Uniquely governmental function by the EPA IRIS has no appeal process to challenge IRIS rulings The IRIS website is widely used 10,000 hits per day There is no requirement for the IRIS agency to consider downstream business impact IRIS is currently reviewing 79 chemicals The average time IRIS waits before review a prior IRIS ruling is 15 years In order to provide some measure of clarity and consistency in an otherwise freeform, narrative characterization, standard descriptors are used as part of the hazard narrative to express the conclusion regarding the weight of evidence for carcinogenic hazard potential. There are five recommended standard hazard descriptors: 1) Carcinogenic to Human, ) Likely to Be Carcinogenic to Humans 3) Suggestive Evidence of Carcinogenic Potentia, 4) Inadequate Information to Assess Carcinogenic Potential and 5) Not Likely to Be Carcinogenic to Humans. 147

171 Chapter VI Process Optimization Defining the Process Window Styrene monomer is currently under review by EPA-IRIS. If the EPA classifies styrene as Likely to Be Carcinogenic to Humans, the implications will create additional motivation to reduce or eliminate the use of styrene monomer in many products including gel coats and laminating resins. UV curable gel coat and resin formulas can be design to combine styrene with multifunctional acrylate monomers (hybrid approach), or with complete substitution for the styrene monomer. c. Economic considerations 4 The financial consequences of converting from conventional room temperature curing with redox initiators to UV curing are expressed qualitatively in terms of capital and operational expenses. The actual financial impact of UV curing must be made on a case-by-case basis. ISSUE PROS CONS Speed of Cure is greater CAPEX: fewer molds are 148

172 Chapter VI Process Optimization Defining the Process Window with UV curing Energy Usage Capital Equipment Styrene monomer partial replacement or substitution with non-regulated multifunctional acrylates required to maintain production volume OPEX: less inventory make to order OPEX: UV curing requires less energy than heating the entire shop CAPEX: allows fabricators to drop below the NESHAP 10 ton/year limit thereby eliminating the need for capture and control technology CAPEX: High power UV Curing lamps cost $ each. OPEX: Increase in raw material costs Chapter VII - Case Study Flat Construction Panel Laminate Commercially successful applications of UV curable composites include residential doors 1, and paneling for spa sidings. In this chapter a summary of the knowledge developed through experimentation (Chapter V) is presented in tabular form below. This knowledge is then applied in a case study to produce a large flat gel coated laminate. 149

173 Chapter VII Case Study Construction of a UV Curable Gel Coated FRP Panel Expt. Theme Main Points 1 Pigmentation Gel coat film cure is possible using V bulb in combination with pigments showing strong scattering ( nm), BAPO PI TiO conc. Keep TiO concentration at or below 10% to prevent alligatoring texture 3 Film Wet film thickness should be 10-1 mils maximum (UPR systems) thickness 4 PI Use 0.5% BAPO to cure white gel coat with V bulb 5 Binder The reactivity of acrylics >> UPR 6 Laminate thickness 3 plies of 1.5 oz. CSM laminate, 0.% PI, 4-5 J/cm 7 Energy vs. speed E 1/v 8 DSC cure study White UPR-acrylic system 4J/cm, UPR clear 4J/cm to reach high level of double bond conversion via DSC 9 Lamp type Lamp power : Fusion> Honle>VHO fluorescent 10 Light dispersion pattern Energy and irradiance dispersion pattern of Fusion UV lamps may be adequately modeled using a Gaussian distribution model 11 Additive nature of UV Energy 1 Lamp spacing 13 Surface temperature 14 Cosine law Energy and irradiance of a bank of shown to be additive at each point in the direction perpendicular to the conveyor travel The energy and irradiance levels drop sharply as the Fusion lamps are separated Surface temperature correlates well with UV energy; does not correlate with irradiance level E =E + cos(), I =I + cos() E 1/h 15 Inverse square law 16 Dichroics No significant temperature control observed with dichroics 17 Mold Degree of cure is superior with reflective mold due to internal reflectivity reflection 18 Simulations 1) Predictive model is validated using measured data ) Lamp spacing has a pronounced effect on uniformity 3) A single lamp failure within a bank of lamps can result in a large localized drop in energy and irradiance 4) Lamp height has a pronounced effect on uniformity 150

174 Chapter VII Case Study Construction of a UV Curable Gel Coated FRP Panel Table 1 - Summary of UV Curing Knowledge (from Experiments 1-18) Case Study Flat Panel Production The objective of this study is to provide a feasibility demonstration for the production of a series of large (4 x 8 ) gel coated FRP panels. Typical applications for this part include roofing panels, transportation panels use on large transport trucks, siding panels for recreational vehicles, etc. A UV curable polyester gel coat formulation and a dual cure laminating resin formulation (proprietary) was prepared. The curing system consisted of ten Fusion UV System 600 W/inch high output UV lamps. The lamps were arranged in two rows of five lamps with a separate air plenum to support the cooling requirements for each row. One row of lamps was fitted with the V bulb gallium doped metal halide, while the second row was fitted with the D bulb iron doped metal halide. Standard reflectors were used with the bulb placed in the 09 position to maximize the reflected energy. The lamps were placed a distance of 7.5 inches above the mold surface. The gel coat was applied manually at a thickness of 8-1 mils using a Binks spray gun (photograph 1). The gel coat was cured at a speed of 5 fpm (photograph ). The laminate was applied via the hand lay-up process (photograph 4). The laminate schedule consisted of an one layer of surfacing veil to improve the surface cosmetics and one ply of 1 oz./ft CSM chopped strand mat (Eglass). Resin : Glass Ratio (veil) 10% Resin : Glass Ratio (CSM) 3% Weight -Resin 406 g Weight - Veil 111 g Weight - CSM 915 g Weight - MEKP 57 g 151

175 Chapter VII Case Study Construction of a UV Curable Gel Coated FRP Panel The laminate was cured using a line speed of 7 fpm. The line speed, lamp spacing, and lamp height were selected to provide the energy required to achieve proper cure based on DSC measurements of residual cure energy. The part was allowed to remain on the mold for 90 seconds prior to demolding. The exit temperature on the surface of the cured laminate was measured at 168F using an infrared thermometer. A total of six successful parts were produced in series, demonstrating the feasibility of the process. 15

176 Chapter VII Case Study Construction of a UV Curable Gel Coated FRP Panel Photograph 1 Application of the gel coat on reflective mold surface Photograph UV Cure of the white gel coat 153

177 Chapter VII Case Study Construction of a UV Curable Gel Coated FRP Panel Photograph 3 Cured white gel coat film Photograph 4 Hand lay-up of the laminate 154

178 Chapter VII Case Study Construction of a UV Curable Gel Coated FRP Panel Photograph 5 UV Curing the laminate Photograph 6 Cured laminate 155