Ind. Eng. Chem. Prod. Res. Dev. 198SV2 5 , 449-453
449
Formaldehyde Release from Acrylic/Melamine Coatings during Photolysis and the Mechanism of Photoenhanced Cross-Link Hydrolysis J. L. Gerlock, M. J. Dean, T. J. Kornlskl, and D. R. Bauer” Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
A previous thin film infrared spectroscopy study on acrylic/melamine coatings revealed that cross-link scission accelerates during exposure to ultraviolet light. Cross-link scission results in regeneration of copolymer hydroxyl consumed during cure and production of melamine methyloi groups. These products suggested a hydrolytic process. Condensation of melamine methylol groups was found to yield melamine-melamine cross-links, releasing formaldehyde as a byproduct. The present work reexamines the nature of the cross-link scission process. Fourier transform infrared spectroscopy is used to follow formaldehyde release during exposure as a function of exposure conditions and coating composition. Coating doped with a hindered amine light stabilizer based nitroxide is found to release formaldehyde at a greatly reduced rate. This result indicates that the mechanism for cross-link scission during ultraviolet light exposure is free-radical in nature. Comparing formaldehyde release rate with cross-link scission rate during photolysis reveals that >75% of the formaldehyde formed as the result of melamine methylol condensation fails to escape the coating. The implications of low levels of formaldehyde on coating oxidation chemistry are discussed.
Introduction Melamine formaldehyde resins are widely used as cross-linking agents in coatings intended for outdoor use. The chemistry of melamine/hydroxyl copolymer cross-link formation during cure is well understood and has been recently reviewed (Santer, 1984). There can be little doubt that cross-link chemistry during outdoor exposure plays a major role in determining long-term durability. A better understanding of the chemistry set in motion during outdoor exposure could improve the intrinsic weatherability of coatings and assist in the selection of appropriate stabilizing additives. The study of the chemistry of acrylic/melamine cross-links subjected to the photooxidative and hydrolytic stresses existent during outdoor exposure has only recently begun. The hydrolysis chemistry of acrylic/melaminecross-links during exposure to condensing humidity has been examined by Bauer (1982) using thin film infrared spectroscopy (IR). Hydrolysis could be followed by measuring the reappearance of copolymer hydroxyl consumed during cure and the disappearance of residual melamine methoxy groups. Cross-link hydrolysis results in formation of melamine methylol groups which can self-condense to yield melamine-melamine cross-links with the release of formaldehyde. The rates of these reactions are consistent with solution measurements (Berge et al., 1970). These reactions are summarized below; the following abbreviations are used: Me1 = melamine ring, P = copolymer, and X = H, -CH,OCH,, or -CH20H. Mel-NXCH,OP
HOH
Mel-NXCH20CH3
Mel-NXCH20H
HOH
Mel-NXCH20H
2Mel-NXCH20H Mel-NXCH2NX-Me1 -+
+ POH
(1)
+ CH,OH (2)
+ HzCO + HOH
(3)
The net effect of hydrolysis in the absence of ultraviolet (UV) light is conversion of acrylate/melamine cross-links to melamine/melamine cross-links. The coatings studied here were cross-linked with Cymel 325. Under the conditions of cure used here, the ratio of H to -CH20CH3 to 0196-4321/86/1225-0449$01.50/0
-CHzOH to -CHzOP is 1.1:0.5:0.2:1 (Bauer and Budde, 1981). There is also a small amount (0.2) of melaminemelamine cross-link (-NCH2N-) formed on cure. From the above distribution of groups, it can be seen that for the majority of groups X is an amine group. When X = H, the mechanism of reactions 1 and 2 is given by (Berge et al., 1970) Mel-NHCH20P HA Mel-NHCH,O+(H)P + A(4) Mel-NHCH20+(H)P + AMel-N=CH2 POH + HA (5)
+
-
-
-
+
Mel-N=CH2 + H 2 0 Mel-NHCH20H (6) where the rate-limiting step in the reaction is the formation of readily hydrolyzed Schiff base. In the absence of water, the Schiff base can react with residual POH to re-form the cross-link. When acrylic/melamine coatings are exposed to both UV light and humidity, thin film IR spectra reveal that the chemical changes associated with hydrolysis occur at a greatly accelerated rate (Gerlock et al., 1983). For example, the rate of loss of melamine methoxy is 7 times greater during exposure in a conventional accelerated weather chamber (QUV, Q-Panel Co.) than it is in the absence of UV light at the same humidity. This acceleration has been termed “photoenhanced crosslink hydrolysis” by Bauer and Briggs (1984) and “solar assisted hydrolysis” by English and Spinelli (1983, 1984). Two candidate mechanisms have been proposed; neither has been verified. English and Spinelli (1983, 1984) have proposed that acceleration occurs as the result of catalysis by photooxidation-induced carboxylic acids (reaction 4). If this explanation is correct, photoenhanced cross-link hydrolysis should exhibit an induction period. None is observed. Melamine methoxy consumption is found to obey simple first-order kinetics from the onset of UV exposure (Bauer and Briggs, 1984). Furthermore, Bauer and Briggs (1984) have observed UV acceleration of melamine methoxy consumption during photolysis of hexamethoxymethylmelamine (Cymel 300, American Cyanamid) in aqueous solution. In this case photooxidation of the host, 0 1986 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev.. Vol. 25, NO. 3. 1986
water, is presumably unimportant. Nevertheless, a relationship does appear to exist between oxidation and hydrolysis in that more readily photooxidized coatings exhibit higher photoenhanced cross-link hydrolysis rates. Bauer and Briggs (1984) have proposed that photoenhanced cross-link hydrolysis arises as the result of melamine-excited-state chemistry. Melamines have weak absorption hands around 300 nm (Costa et al., 1950), and the excited state is more easily protonated and thus more easily hydrolyzed than the ground state. This explanation requires that the lifetime of the melamine excited state he sufficiently long in a cross-linked coating to do chemistry. Although this mechanism is consistent with the kinetics observed, it does not account for the parallel noted between coating photooxidation ease and hydrolysis rate. More importantly, this mechanism fails to explain the fact that addition of Tinuvin 770, a hindered amine light stabilizer, to acrylic/melamine coatings reduces the photoenhanced hydrolysis rate. The present work was undertaken to clarify the nature of photoenhanced cross-link hydrolysis in acrylic/melamine coatings and to determine whether or not formaldehyde released into the coating as a result of melaminemelamine cross-link formation could he expected to take part in coating oxidation chemistry. It has been speculated that formaldehyde plays a key role in acrylic/melamine oxidation chemistry as a formyl peroxy radical and performic acid precursor (Gerlock et al., 1983, 1985). Previous hydrolysis rate determinations were based on thin film IR measurements of functional group changes on exposure. In this work, the rate a t which volatile products are released from the coating during photolysis are followed by gas-phase Fourier transform infrared spectroscopy (FTIR) as a function of exposure conditions and coating composition variables. Data on the lifetime of the melamine excited state are also presented. Evidence is presented to suggest that acrylic/melamine cross-link scission during UV exposure is primarily a free-radical process. The implications of this result and the role that formaldehyde plays in coating photooxidation are discussed. Experimental Section Materials. Acrylic copolymer was synthesized by free-radical polymerization from the following monomers: butyl methacrylate, 58% (by weight), 2-hydroxyethyl acrylate, 40%, and acrylic acid, 2%, M. = 4ooo. Melamine cross-linker Cyme1 325 was obtained from American Cyanamid Co. This melamine contains 47% methoxy functionality, 13% methylol functionality, 36% amine functionality, and 4% melamine-melamine condensation linkages (Bauer and Budde, 1981). Triisocyanate biuret cross-linker L2291 was obtained from Mobay Chemical Co. The ratio of acrylic copolymer to melamine cross-linker was 7030. The ratio of acrylic copolymer to urethane cross-linker was 6535. Benzotriazole UV absorber CGL900 was obtained from Ciba Geigy. Synthesis of nitroxide I has been described (Gerlock, 1983). CHdCH&NHCNH
q.
nltroxide I
Procedure. Photolysis samples were prepared by brushing resin on methylene chloride-washed and tared FS20 sunlamps followed by curing a t 130 'C for 20 min. After cure, the samples were held at 100 OC for -20 h to
FS-20 SUNLAMP VlTON RING HUMIDIT CONTROLL N2.02 MIX PYREX TUBE
THERMOCOUPLE
I
FTIR
Figure 1. Design of flow reactor chamber for coated UV bulbs.
ensure solvent volatilization prior to reweighing. Coating weights ranged from 1.3 to 2.8 g hut were typically in the range 1.8 f 0.2 g. This corresponds to an average film thickness of 30 f 4 pm. In a typical experiment, the sample was sealed in the Pyrex jacket reactor described in Figure 1. Humiditycontrolled N2/02dry air mix was passed over the coating overnight at 40 cm3/min to establish equilibrium between the coating and its environment. Finally, the coating was photolyzed for 30 min prior to collection of gases for FTIR analysis. The coating surface temperature was measured with a thermocouple. All experiments were conducted a t 67 3 "C. The UV light intensity of individual sample bulbs was measured with a IL 745 UV curing radiometer (Intemational Light) prior to coating. FS20 sunlamps emit UV light in the range 280-350 nm with a peak at around 310 nm. In order to determine whether formaldehyde is degraded within the air gap between the UV source and the sourrounding Pyrex jacket, coating contacting the UV source was coated with 2% benzotriazole CGL-900 UV absorber doped coating. No change in formaldehyde release rate could be detected, confirming that formaldehyde released from the coating is not consumed by photochemical reactions in the gas phase. FTIR Analysis. Analysis of gas-phase emissions was performed with a Fourier transform infrared spectrometer assembled from individual components (Maker et al., 1979; Butler et al., 1985). Principal components included an EOCOM 7001 scanning Michelson interferometer equipped with a KBr/Ge beam splitter; a liquid nitrogen cooled HgCdTe photoconductor detector from Infrared Associates; a conventional ceramic glower source; and a Wilks 20-m variable path gas sample cell used in the 14th order, resulting in an effective path length of 21.75 m. These elements together with the optics were arrayed on a granite optical table and enclosed in a Plexiglass box purged with dry nitrogen. The data acquisition/processing was accomplished with a PDP 11/34 minicomputer. Gas-phase data were collected at 700 Torr (933 mbar) and 25 2 "C. Spectral resolution of the interferogram was 0.125 cm-' (a 4-em optical retardation). Adequate signal-to-noise ratio was attained by madding 16 interferograms before Fourier
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
Table I. Formaldehyde a n d Methanol Release Ratesa conditions coating hu dew, OC T, "C 50 Me1 off 18 Me1 off 18 67b Me1 on 18 67 67 Ure on 18 Mel/>NO' on 18 67 Mel/>NOR on 18 67 Me1 on -40 67 Mel/>NO' on -40 67 Mel/N2 on 18 67
d[CH,OH]/dt measd calcd 0.1 0.1 0.25 0.25 0.65 2.6 0.04 0.21-0.3 0.21-0.25 0.5 0.15 0.5
d[ CHZO]/dt measd calcd 0.2 0.25 0.5 0.6 1.5 6.5 0.0 0.5-0.7 0.7-0.8 3.0' 0.85 0.2 1.1
k,,.,,.. h-I
X
451
IO3
0.08 0.2 2.2 1 1.2
"Units given X lo8 mol/(g.min). bExtrapolation of "dark" hydrolysis rate using activation energy of 21 kcal/mol (Bauer, 1982). 'Based on long-term exposures containing Tinuvin 770.
transform was done. After it was determined that the carrier gas contained no interfering components, the transmission spectrum of the evacuated cell was used to ratio against the transmission spectrum of the sample to generate the absorbance spectrum. The analytical spectral range included the 600-3600-~m-~ region. The concentration of a component in the sample was determined by comparing spectral line strength of a nonsaturating line to that of a known concentration of a standard spectrum (previously determined and normalized to the same pressure and path length). The detection limits for the four species measured were 0.2 ppm for both C 0 2 and CO, 0.4 ppm for CH20,and 0.8 ppm for CH30H. For the flow rates and coating weights used here these limits correspond mol/(g.min). to release rates (0.02-0.1) x Fluorescence Lifetime Measurements. Melamine resins have been found to have a weak fluorescence emission at 393 nm when excited in the 290-320-nm range. Fluorescence lifetimes were measured using Photochemical Research Associates fluorescence lifetime equipment. Lifetimes were measured for dilute melamine resin in both methanol and fully cross-linked coatings on quartz slides. The decay of the emission was found to be multiexponential. The decay curves were fit to two or three exponentials by using a nonlinear least-squares fitting program. The lifetimes reported are weighted averages of the fit lifetimes.
Results: Formaldehyde Emissions The major volatile components detected during photolysis of acrylic/melamine coatings are C 0 2 ,CO, methanol, and formaldehyde. The release rates of C 0 2 and CO range from 3.5 X to 5.0 X lo4 mol/ (gmin) for C 0 2 and from 1.0 X to 2.0 X mol/(g-min) for CO. By way of comparison, the photoinitiation rate of free radicals as measured by nitroxide decay assay in the coating studied here under these conditions is (5-6) X mol/(g.min) (Gerlock et al., 1985). Thus, release of C 0 2 and CO appears to reflect primary photochemistry. Release rates of formaldehyde and methanol from coatings are reported in Table I as a function of exposure conditions, cross-linker type, and nitroxide I dopant concentration. Methanol release rates are roughly half formaldehyde rates. The release rate of formaldehyde from triisocyanate crosslinked acrylate copolymer is zero, within experimental error, implying that all formaldehyde released during photolysis of acrylic/melamine coating arises from the melamine formaldehyde cross-linker and not the acrylate copolymer. The rate of loss of melamine methoxy group was determined by extrapolation of previously reported thin film IR data to the given experimental conditions. The formaldehyderelease rate in the absence of W light was measured for coating on a non-UV fluorescent bulb (18 "C dew point, 50 "C coating surface temperature) as
opposed to an FS-20 fluorescent sunlamp. Under these conditions, formaldehyde is released at the rate of 0.2 x mol/ (gmin). The surface temperature for coatings on FS-20 fluorescent sunlamps was 67 f 3 "C. The formaldehyde formation rate expected from dark hydrolysis at 67 "C is determined from the experimental data at 50 OC and the known activation energy for dark hydrolysis of 21 kcal/mol (Bauer, 1982). The same coating on an FS-20 fluorescent sunlamp exposed at the same air humidity and coating surface temperature releases formaldehyde at the rate of 1.5 X mol/(g.min), over 3 times greater than in the absence of UV light. Substantial formaldehyde release is maintained under these exposure conditions for over 1000 h. Rigorous exclusion of water, dew point
