. using ASTM D and identification of volatiles by analytical techniques.

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1 c The work described in this paper vu not funded by the US. Envirormcntd Rotmion Agency. The contents do not necessarily reflect the views of the Agency urd no official mdrsement should be inferred VOLATILE CONTENTS OF UV CATIONICALLY CURABLE EPOXY COATINGS J. Wells 'carter Jagdeesh Bandekar Mark Jupina J... Linda A. Kosensky 1. W. Perry Union Carbide Corporation P.O. Box 67 Bound Brook, NJ 885 INTRODUCTION Ultraviolet cationically curable epoxide coatings used are for protecting metal and inks in rigid packaging applications and as overprint varnishes for plastic tubes. Excellent adhesion to metal and plastic and toughness during post-forming are benefits of cationic epoxide chemistry and requirements of these applications. The cycloaliphatic epoxy resins used in these coatings are low viscosity and low odor. Coatings can be formulated with low application viscosities without adding solvenc therefore, the coatings have very low volatile organic contents. This paper reports findings from measurtmerrts of the volatile contents of starting-point formulations. using ASTM D and identification of volatiles by analytical techniques. EXPERMENTAL DETAILS Coating volatile contents were measured in triplicate following ASTM D Test Method A, including Note 3 for UV cationic epoxide coatings. Coating weights ranged.2- to.4-g and the substrate was 4- X 12-in. aluminum panels for volatile content measurements. Coatings samples for the FT-WGA, GWS and water titration experiments were obtained by curing the coatings on glass plates and scraping coating samples off the glass. The coated aluminum panels and coated glass plates wen cured using 25 mj/cm? UV dose supplied by a LWEI UV cure unit operated with a 4 Wh. mercury-vapor arc lamp and the conveyor at 9 fpm IT-IWGA is a hyphenated technique that simultaneously records thermopvimemc (TG) weight loss and the FT-IR spectra of effluent gases without operator intervention.1 The FT-IFUI'GA instrument was from Bio-Rad, Cambridge, MA and consisted of a Bio-Rad FTS6 IT-IR spectrometer and an Omnithem TG analyzer. The Bio-Rad minicomputer, in addition to performing its normal FT-IR functions, controlled the operation of the TGA. This enabled the TG and n-ir data to be coordinated in time in one computer, and for the spectral data to be comlated.with the observed weight losses. A temperature program was semp which raised the temperature from 2 to 11 OC at 1 OUmin, held it there for 35 min lxfore continuing at 2 OC/min to 8 OC for 7 min. The ramp to 8 "C was to prepare the equipment for the next sample. The samples were analyzed using Headspace Gas Chromatography. The instruments were an AutoSystem gas chromatograph interfaced with HS-4 Headspace Sampler, both from Perkin Elmer. The samples were heated in 22-mL headspace vials in the sampler at 11 "C for 1-h. A DB-1 (3- X.53-mm Ld., 5-pm film)megabore column was used with a split of 5/1 and a. column flow of 8 Wmin helium. The oven propram was 6 "C for 4 min, ramp at 1 "C/min to 26 "C. The injection pon temperature was 2 OC and the flame ionization detector temperature was26 OC. The HS-4 needle and transfer temperatures were 13 OC, and the pressurization time and injection time was.2 min. Gas chromatography/mass spectrometry 3-26

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3 (GC/MS) was conducted using an HP-589 gas chromatograph interfaced with an HP-597 mass Selective Detector, both from Hcwlett Packard. A Mitsubishi Moisture Meter Model CA-5 and Water Vaporizer Model VA-5 were used to analyze coating volatiles for moisture. Approximately.3-g samples were introduced into the boat and purified and dried nitrogen was passed over the ~amples With a flow rate of 2 d/min for 1-h. The VA-5 was maintained at 11 OC during this time. The water was determined by coulometric titration. The reagents were Mitsubishi Aquamicron AU and Aquamicron.CK. Replicates were tested. All the water was evolved from the Coating Samples within the first 3 min. Two DEFENSOR 31 humidifiers were used to maintain constant high relative humidity (RH) when UV curing coatings for volatile content and surface-curt rate measurements requiring constant high humidity. Sealed chambers containing desiccant or a saturated ZnS4 solution were used to store coated panels during the 48-h quilibration for volatile content measurements conducted with the quilibration close to % and at 9% RH, respectively. Surface-cure rates were measured by lightly touching coating surfaces with a cotton ball immediately after UV curing. The conveyor speeds, measured in feet per minute (fpm), at which no cotton fibers adhered to coating surfaces were recorded as the surface-cure rates. Coatings were applied to aluminum foil-coated paper and cured using a 4 Whn. mexury-vapor arc lamp for surface-cure rate measurements. The cycloaliphatic epoxide resins 3.4-epoxy cyclohexylmethyl-3,4-epoxy cyclohexyl carboxylate (EEC) and bis(3,4-epoxy cyclohexylmethyl) adpate (BEA), polyester diol (hydroxyl no. 212) and mol (hydroxyl no. 56); epoxidized linseed oil (LOE), and mixed maryl sulfonium hexafluorophosphate salts 5 wt8 solution in propylene carbonate (PI solution), were obtained from Union Carbide Corporation. Dipropylene glycol diglycidyl ether (DPGDGE) was obtained from The Dow Chemical Company. Castor oil was obtained from CasChem, Inc. Indicating DRIERITEo (anhydrous calcium carbonate) was obtained from W. A. Hammond Drierite Company and ZnSOq7H2 from Mallinckrodt All materials were used as received. RESULTS AND D.ISCUSSION ASTM Method B Note 3 specifies that cationic epoxide coatings which gain weight while heating I-h at 11 OC should equilibrate for 48-h at room temperature after UV curing and before heating. The coatings pnsented in Tables I-N gained weight as a result of UV curing and two gained weight as a result of heating (coatings G and H in Table n). The weight gain has been attributed to atmospheric moisture reacting with cationic coatings.* The superacid (WF6), which is generated from photolysis of the cationic photoinitiator (Ar$+PF6- + HPF6), is hygroscopic and its appearance greatly increases coating hygroscopicity. The FT-IFUT GA method was used to identify the major volatiles from coatings-a and C, the inments of which are provided in Table 1. Within the first 15 min of each FT-IlUfGA experiment, an evolved gas was detected whose weight fraction varied between.1 and 3% of the total weight. Volatile loss was complete within 35 min of heating at 11 C during the TGA experiments, Figure 3 shows a rcpresentative profile of the evolved gas. The FT-IR spectra of sample A and C were similar, Figure 4 shows a typical specmm. The main bands are observed at 1858 cm-1, due to carbonyl stretch, and near 117 cm-1, due to ester type C- stretching band. The FT-IR spectrum corresponds to propylene carbonate (PC) gas, the PI solvent

4 n GUMS identified PC as the major volatile and identified uaces of phenyl sulfide, the PI photolysis product. Phenyl sulfide was not detected in the FT-IR spectra probably because its concentration was too low to be detected. FT-WGA and GUMS experiments conducted on coatings A and C found no evidence of epoxy resin or poly1 in the volatiles. The titration method confinned the presence of water in the volatiles. In some cases in the rcsults in Tables I and I1 the amount of potentid volatiles was significantly less (EH), approximately the same (A, C and D ) and significantly more (B) than the amount of PI solution in the formulations. High coating Tgs and high boiling points of volatiles would be expected to slow diffusion of volatiles from coatings while heating. Not all of the PC and phenyl sulfide may evaporate while heating because their boiling points are high (24 and 296 OC, respectively). Water absorbed during UV curing and equilibration which reversibly hydrated the coating would most likely be evaporated completely while heating because the boiling point of water (1 "C) is less than the bake temperature (11 "C) and water absorbed during UV curing and equilibration would reside mostly athe coating surface. Coating Tg dramatically increases during UV curing and high Tg would limit diffusion of absorbed water into the lower coating layers. Water the surface could be volatilized without complications from diffusion effects. The relative amounts of PC and water in the volatile mix of a cationic epoxide coating may depend on factors such as the amount of PI solution the coating contains, coating Tg, hygroscopicity of coating ingredients and the RH during UV curing. The volatiles in excess of the weight of PI solution in coating B were most likely water. Coating Tgs probably limited the amount of volatiles by limiting diffusion in cases (E-H) where volatile contents were less than the weight of PI solution. The process weight gains and potential volatiles of coatings A-E (Table I) were significantly larger than those of coatings F-H (Table II). The differences between results in Tables I and I1 were perhaps due to differences in the amount of PI solution the coatings contained, differences in the hygroscopicity of in@ents, differences in RH at the time of the measurements or some combination of these explanations. The RH was not measured when the results in Tables I and II were.obtained. Comparing the results among Tables I and II suggests volatile contents depend on the amount of poly1 a coating contains. The amount of water absorbed during UV curing was inversely proportional to poly1 concentration in Tables I and II. Coatings A and E contained polyester diol and castor oil, respectively, and coatings B-D contained no polyol. Coatings A and E gained about half the weight coatings B-D gained during UV curing. The same trend was demonstrated by the process weight gains of coatings F-H but-the weight gain differences among these coatings wen arguably within experimental error. The potential volatiles of coatings F-H were proportional to polyol concentration. The potential volatile differences among coatings F-H were significant PC evaporated (the coatings contained 6% PI solution) and water was absorbed as a result of heating and the amount of water absorbed increased with decreasing poly1 concentration. The volatiles of coatings I, which contained 19.2% polyol, and J, which contained no polyol. were measured using controlled humidity conditions to better understand the effects of RH and the presence of poly1 on volatile content results. Coatings I and J had similar process weight gains and potential volatiles using low humidity UV cure and quilibration conditions (Table III), despite the amount of poly1 the coatings contained. Coatings I and J had significantly more process weight gain and significantly more potential ' volatiles when high humidity UV cure and equilibration were used compared to when low humidity conditions were used. The amount of water absorbed during UV cure and equilibration strongly depended on RH. The amount of potential organic volatiles was probably not affected 3-28

5 by the humidity during UV cure and equilibration; therefore, the extra potential volatiles measured under high humidity conditions compared to low humidity conditions was water vapor. Coating J, which contained no polyol, gained significantly more weight than I during UV curing and J lost less weight than I during heating using high humidity conditions. Two mechanisms;'which could occur simultaneously, best explain these finding. One mechanism is that poly1 hydroxyls and water compete for epoxides in the presence of superacid catalyst. According to this mechanism, the rate of the reaction of epoxides with water was higher in J than in I because poly1 hydroxyls were not available in J to compete with water for epoxide. The epoxides acted like a (irreversible) sponge for hydroxyls from water or polyol or both (Figure 1). The other mechanism is that polyol hydroxyls associate with cations, such as superacid (HPF6) and polymerizing cationic centers, and reduce cation hygroscopicity. Cations act like a (reversible) sponge for hydroxyls and the presence of poly1 hydroxyls decreases. the rate of hydration of cations (Figure 2). I Figure 1. Ring-opening Polymerization Initiated By Hydroxyl, Where Re = He, R'CHy (From Polyol), or R"-O, - nroh + C+ (ROH)"C+ Figure 2. Reaction Of Hydroxyls With Cationic Centers (C'), Where Re = He Or Alkyl Radical Surface-cure rate studies were conducted to better understand the effects of RH and poly1 concentration. The formulations in Table IV were used for the surface-cure rate study. A plot of surface-curc rate vs. RH is presented in Figure 5. The surface-cure rates passed through maxima and the positions of the maxima moved to higher RH with decreasing polyol concentration. Atmospheric moisture enhanced surfacecure rates at low RH as demonstrated by the increased, cure rates with increasing RH up to the maxima for each polyol concentration. This suggests water absorbed by the coatings acted as initiator for epoxide polymerization. Higher surfacecure rates of coatings containing higher polyol concentrations at low RH suggest that polyol 3-29

6 hydroxyl also acted as initiator for epoxide polymerization. Previous reports described polyols as chain transfer agents for epoxide polym~rization.~ Hydroxyls from water and polyols could play both roles, initiators and chain transfer agents. As initiators, water and poly1 would increase cure rates. As chain transfer agent, polyol could increase the rate of gellation or become part of a dangling end. Water could enhance cure rate as an initiator but would produce dangling ends in the role of chain transfer agent. At RH values above the maxima in Figure 5 the surface-cure rates decreased probably because the rate of ring-opening competed effectively with epoxide polymerization. Ringopening can lead to dangling ends especially if water participates in the ring-opening rtaction (Figure 1). Cationic epoxide coatings with useful physical properties are generally formulated with at least a two-fold excess of epoxides relative to poly1 equivalents. Excess epoxide is required to develop high enough crosslink density and Tg via epoxide polymerization to proved useful physical properties. The viscosities reported in Tables I and III demonstrate that UV cationic coatings containing cycloaliphatic epoxides can be formulated to low viscosities required by some application techniques. The coatingsfozmulations presented also had low odor which is of benefit for plant environments. CONCLUSIONS Cationic epoxide coatings were formulated with low viscosities and were found to have very low volatile contents. The major organic volatile was identified as propylene carbonate, the photoinitiator solvent, by FT-IWGA and GCNS and traces of phenyl sulfide, the photoinitiator photolysis product, were identified by GC/MS. Water was identified in the volatiles by titration. The measurement of volatile contents of cationic epoxide coatings was complicated by the ' effect of relative humidity. Cationic epoxide coatings absorb abnospheric moisture during W curing. Coatings containing less polyol were found to absorb more moisture. Cationicepoxide coating absorbed more moisture when the volatile content measurements were conducted at higher relative humidity. Two reactions, rtaction with epoxides and hydration of cations, which could occur simultaneously, were proposed to explain the reactions of atmospheric moisture with cationic epoxide coatings. The reaction of water and epoxide was supported by cases where coatings containing no (or less) polyol absorbed more water as a result of UV curing and had less potential volatiles than coatings containing (more) polyol, and by surface-cure rau stules. Increasing RH increased surface-curc rates up to the maxima. The reversible hydration of cations was supported by the identification of water by titration as pan of the potential volatiles, by findings that the amount of volatiles depended on humidity, and by cases where the potential volatiles were greater than the amount of PI solution in the coating. References Compton, D. A. C.; Markelov, M.; Mittleman, M. L.; Grasstlli, J. G.; Appl. Specrrosc. 1985, 39,99. Strand, R. C.; Ludwigsen, R. J. SME Technical Paper FC , Society of Manufacturing Engineers. Crivello, J. V.; Conlon, D. R.; Webb, K. K. J. Radiation Curing 1986, I3(4),

7 Table 1. Volatile Contents Of UV Epoxide Coatings With And Without Poly1 Ingredients A B. C D E 67.2 EEC Diol Polyester 19.2 DPGDGE BEA 28.8 LOE 28.8 Castor Oil PI solution Surfactant Viscosity 25 O58 C (cp) VOC lave wt%l -6.5 Process Potential Total s Process Potential Total

8 I Table n. Volatile Contents Of UV Epoxide Coatings Demonstrating Weight Gain As A Result Of W Curing And Heating Ingredients F G H EEC Polyester Triol PI solution Surfactant ~ VOC (ave ~9%) Process Potential Total Standard Deviation Process Potential Total , 3-32

9 Table m. Volatile Contents Of UV Epoxide Coatings Measurtd Using Conldkd Humidity Ingredients I J EEC Polyester Diol * 14.4 DPGDGE PI solution Silicone Surfactant.2.2 Viscosity 25 OC (cp) yv cure.. co ndmons 28% RH 48-h Equilibration Conditions Desiccated VOC (ave wt%l Process Potential Total undard Deviation Process Potential Total s;v c w 48-h - E.. VOC (avt wt%l Process Potential Total Process Potential Total ev % RH.9128 '.. 9% RH

10 Polyester triol PI Solution Surfactant l2o looo t 8 - t Wt% Polyester Triol t V I Percent Relative. Humidity I I I I Figure 5. Effects Of Relative Humidity And Poly1 Concentration On Surface-cure Rate.Of UV Epoxide Coatings 3-34

11 P. r O f e Figure 1 EvoW Gas Rock From FT-IR/TCiA Expenmen1 3-35

12 .5 A b.4 5 r.3 b a 1 ".2 c