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Eckert, C. A.; Van Aleten, J. G.; Stoicos, T. Supercritical Fluid Proceseing. Environ. Sci. Technol. 1986,20, 319. Foster, N. R.; Macnaughton, S. J.; Chaplin, R. P.; Wells, P. A. Critical Locus and Partial Molar Volume Studies of the Benzaldehydecarbon Dioxide Binary System. Znd. Eng. Chem. Res. 1989,28, 1903.
Gurdial, G. So;Foster, N. R. Solubility of o-HydroxybenzoicAcid in Supercritical Carbon Dioxide. Znd. Eng. Chem. Res. 1991,30,575. Gurdial, G. S.; Wells, P. A.; Foster, N. R.; Chaplin, R. P. The Role of Polarity in Correlations of Solid-Supercritical Fluid Phase Systems. J. Supercrit. Fluids 1989, 2, 85. Johnston, K. P.; Ziger, D. H.; Eckert, C. A. Solubilities of Hydrocarbon Solids in Supercritical Fluids. The Augmented van der Waals Treatment. Znd. Eng. Chem. Fundam. 1982,21, 191. Johnston, K. P.; Barry, S. E.; Read, N. K.; Holcomb, T. R. Separation of Isomers Using Retrograde Crystallization from Supercritical Fluids. Znd. Eng. Chem. Res. 1987, 26, 2372. Krukonis, V. J.; Kurnik, R. T. Solubility of Solid Aromatic Isomers in Carbon Dioxide. J. Chem. Eng. Data 1985,30, 247. Kumar, S. K.; Johnston, K. P. Modelling the Solubility of Solids in Supercritical Fluids with Density as the Independent Variable. J. Supercrit. Fluids 1988, I, 15. Kurnik, R. T.; Holla, S. J.; Reid, R. C. Solubility of Solids in Supercritical Carbon Dioxide and Ethylene. J. Chem. Eng. Data 1981, 26, 47.
McHugh, M.; Yogan, T. Three-phase Solid-Liquid-Gas Equilibria for Three Carbon Dioxide-Hydrocarbon-Solid Systems, Two Ethane-Hydrocarbon-Solid Systems, and Two Ethylene-Hydro-
carbon-Solid Systems. J. Chem. Eng. Data 1984,29, 112. Paulaitis, M. E.; Krukonis, V. J.; Kurnik, R. T.; Reid, R. C. Supercritical Fluid Extraction. Rev. Chem. Eng. 1982, 1, 179. Schmitt, W. J.; Reid, R. C. Solubility of Monofunctional Organic Solids in Chemically Diverse Supercritical Fluids. J. Chem. Eng. Data 1986, 31, 204. Tsekhanskaya, Y. V.; Iomtev, M. B.; Mushkina, E. V. Solubility of Naphthalene in Ethylene and Carbon Dioxide Under Pressure. Russ. J. Phys. Chem. 1964,38, 1173. Van Leer, R. A.; Paulaitis, M. E. Solubilities of Phenol and Chlorinated Phenols in Supercritical Carbon Dioxide. J. Chem. Eng. Data 1980,25, 257. Wells, P. A.; Chaplii, R. P.; Foster, N. R. Solubility of Phenylacetic Acid and Vanillan in Supercritical Carbon Dioxide. J. Supercrit. Fluids 1990,3, 8. Williams, D. F. Extraction with Supercritical Gases. Chem. Eng. Sci. 1981,36, 1769.
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Ziger, D. H.; Eckert, C. A. Correlation and Prediction of Solid-Supercritical Fluid Phase Equilibria. Znd. Eng. Chem. Process Des. Deu. 1983,22, 582. Received for review December 5, 1990 Revised mantcscript received July 8, 1991 Accepted July 15, 1991
Photodegradation and Photostabilization of Urethane Cross-Linked Coatings David R. Bauer,* John L. Gerlock, Deborah F. Mielewski, Michelline C. Paputa Peck, and Roscoe 0. Carter I11 Research Staff, Ford Motor Company, P.O. Box 2053,Dearborn, Michigan 48121
Infrared spectroscopy has been used to monitor photodegradation chemistries in urethane cross-linked coatings as a function of copolymer (acrylate versus polyester), cross-linker (biuret versus triisocyanurate), and exposure condition. Results are compared with measurements of the photoinitiation rate of free radicals and with hydroperoxide concentrations. Cross-link scission and oxidation are the main degradation chemistries in all of the coatings studied. The rates of degradation correlate well with free radical photoinitiation rates and hydroperoxide concentrations, confirming that photodegradation is controlled by free radical chemistry. The addition of hindered amine light stabilizer to these coatings inhibits free radical oxidation and reduces the rate of degradation. The extent of inhibition depends on the coating composition and on the exposure condition.
Introduction Urethane cross-linked coatings are finding increased applications in a variety of areas (Potter et al., 1984). They provide the low cure temperaturea and flexibility neceaSary for coating many plastic parts. In addition, they have the potential for improved appearance and resistance to acidic deposition. In many cases, the implementation of new coating systems based on isocyanate or other novel cross-linking chemistry is limited by the lack of proven outdoor weatherability. In principle, weatherability can be predicted from measurementa of the chemical changes that occur during exposure to both outdoor and accelerated laboratory weathering tests (Bauer et al., 1987). For predictions based on such comparisons to be successful, several criteria must be met. First, the types of chemical change that occur in the accelerated test(s) used must be the same as that which occurs outdoors. Second, if multiple types of chemical change are observed, the relative extent of the changes must be roughly the same in the accelerated and outdoor exposures. Finally, it is necessary that the kinetics of degradation parallel one another in the
two testa since extrapolation is based on the ratio of the rates of chemical changes measured in the initial stages of accelerated and outdoor testing. It is essential to the success of this approach that the themistry of photodegradation be well understood. There have been relatively few studies of the photodegradation of urethane cross-linked coatings (Bauer et al., 1986;Bauer et al., 1988). Exposure of unstabilized acrylic copolymers cross-linked with the biuret of hexamethylene diisocyanate to short wavelength ultraviolet light (UV-B, FS-40 bulbs) results in the loss of urethane cross-links and formation of carbonyl groups. These chemical changes were identified by using both infrared spectroscopy and solids I3C NMR spectroscopy. The rate of crw-link lcw obeys simple fmt-order kinetics, implying a constant rate of photooxidation throughout the exposure. In contrast, photooxidation in unstabilized polyester urethane coatings during near-ambient exposure, as measured by the rate of formation of free radicals, was found to be autocatalytic (Gerlock.et al., 1989). In this paper, earlier studies of the photodegradation and stabi-
0888-5885191/263O-2482$O2.5O/O 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 11,1991 2483 lization of urethane cross-linked coatings are extended. Infrared spectroscopy is used to follow the degradation chemistry of different acrylic and polyester urethane coatings using exposure conditions more closely resembling outdoors. Coatings with different intrinsic stabilities and different cross-linker chemistries (biuret versus triisocyanurate) have been studied. The data reported here are compared to previous results using harsh exposure and to recent measurements of the free radical formation rates and of hydroperoxide concentrations in these coatings (Mielewski et al., 1989, 1990, 1991). The effect on photooxidation kinetics of the addition of a hindered amine light stabilizer (HALS) has also been determined for the different coatings. Again, results are compared to previous harsh exposure results.
Experimental Section Materials. The two acrylic copolymers (A and N) used in this study have been described previously (Gerlock et al., 1987). Copolymer A was synthesized by using tertbutyl perbenzoate and cumene hydroperoxide as coinitiators in 2-heptanone. Copolymer A contained 68% by weight butyl methacrylate (BMA), 30% hydroxyethyl acrylate (HEA), and 2% acrylic acid (AA). Copolymer N was synthesized by using azobis(iaobutyronite) in xylene. Copolymer N contained 58% by weight BMA, 40% HEA, and 2% AA. Coatings formulated with copolymer A have much higher initial rates of radical formation than do coatings formulated with copolymer N. This is due to the fact that the synthesis conditions used for copolymer A (peroxy initiators and ketone solvent) result in incorporation of a significant concentration of ketone-functional end groups on the polymer chain (Carduner et al., 1988). Copolymer N has no detectable ketone end groups. T w o different isocyanate cross-linkers were obtained from Mobay. Cross-linker N3200 is a biuret of hexamethylene diisocyanate while N3300 is the triisocyanurate. Structurea of these cross-linkers have been given in Potter et al., 1984. Acrylic urethane coatings were formulated by using a weight solids ratio of 65:35 copolymer to crosslinker. No external catalysts were added. The coatings were cured a t 78 "C for 5 h. Coatings formulated with copolymer A or N and the biuret cross-linker will be referred to as A-Ure and N-Ure while coatings formulated with the triisocyanurate cross-linker will be referred to as A-Tri and N-Tri. The hindered amine light stabilizer used in these studies was bis(2,2,6,64etramethylpiperidinyl) sebacate (TIN 770 from Ciba-Geigy). The polyester urethane coatings used in this study were proprietary (Gerlock et al., 1989). They consisted of both aliphatic and aromatic polyesters cross-linked with both biuret and triisocyanaurate cross-linkers. Weathering Conditions. Coatings were cast on NaCl plates and exposed in a modified Atlas Ci35 Xenon arc weathering chamber. The light source was a xenon arc lamp with borosilicate inner and outer fiiters. The acrylic urethanes were exposed directly to this light at an intensity of 0.35 W/m2 (340nm). This corresponds to the %earambient" exposure conditions used in previous studies (Bauer et al., 1990a). The spectral profile of borosilicate filtered xenon arc light contains a small amount of light at wavelengths shorter than 295 nm. This light, not present outdoors, has been shown to induce rapid degradation in aromatic polyesters (Gerlock et al., 1989). In the case of the polyester urethane coatings, the short wavelength UV light was removed by placing the samples behind a 320-nm UV cutoff filter (Schott Glass). These samples were placed somewhat closer to the light source than the acrylic samples so that while the light intensity
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Figure 1. Infrared spectra from unstabilized A-Ure. The top spectrum is from unweathered A-Ure, while the middle spectrum is from the eame coating after 871 h of near-ambient weathering. The bottom spectrum is the normalized difference spectrum. lllrnr
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at wavelengths shorter than 320 nm was less for the polyesters, the intensity at wavelengths longer than 320 nm was greater. The samples were exposed at an air temperature of 40 "C and a dew point of 25 "C. No water spray or condensing humidity cycles were used. Infrared Spectroscopy. Infrared spectra were obtained in transmission with use of a Mattson Sirius 100 FTIR spectrometer, as previously described (Bauer et al., 1990a). Spectra were obtained for the various coatings as a function of weathering time.
Results and Discussion Degradation Chemistry. Infrared spectra from coating A-Ure before weathering and after 871 h of near-ambient exposure are shown in Figure 1. Similar spectra for A-Tri are shown in Figure 2. Difference spectra are also shown. The difference spectra were obtained by normalizing the spectra at the acrylic C-H stretch frequency (2960 cm-') and subtracting the spectrum of the unweathered coating from that of the weathered coating. The difference spectra
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2484 Ind. Eng. Chem. Res., Vol. 30, No. 11,1991 1.1
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UV light below 295 nm which must be eliminated by UV cutoff filters. The changes observed in the polyester coatings during exposure to xenon arc light filtered by using a 320-nm cutoff filter are similar to those observed for the acrylic urethanes (i.e., carbonyl growth and cross-link scission). Cross-link scission can be quantified by using either the 1540-cm-' band or the 1240-cm-' band. The 1240-cm-' band is specific for the urethane cross-link and is identical for both the biuret and triisocyanaurate cross-linked coatings. This band appears as a shoulder on a stronger acrylic band, which makes accurate measurements of cross-link loss somewhat difficult. The 1540-cm-' band is a result of N-H deformation (amide 11) and has contributions from both the urethane N-H and, in the case of the biuret crosslinker, the biuret N-H group. The contribution from the biuret N-H group increases the absorbance of this band relative to the triisocyanaurate cross-linked coatings. Previous studies have shown that the percent decrease in the 1240- and 1540-cm-' bands are very similar for the biuret cross-linked coatings, at least over the first 75% of loss (Bauer et al., 1988). This means that the rate of disappearance of the urethane N-H and the biuret N-H must be comparable, and that measurement of the loss of the 1540-cm-' band can be used to quantify urethane cross-link scission in both biuret and triisocyanurate cross-linked coatings. In practice, it is convenient to measure the decrease in absorbance of the 1540-cm-' band relative to the band at 1440 cm-'. As shown in the difference spectra in Figures 1 and 2, the 1440-cm-' band does not change relative to the acrylic C-H band. In previously studied acrylic melamine coatings, the extent of photooxidation was quantified by measuring the increase in absorbance of the carbonyl region of the spectrum (Bauer et al., 1990a). This was done by measuring the area of the carbonyl band in the difference spectrum relative to the area of the original C-H band. Two factors make this analysis more complicated for the urethane coatings. First, the loss of cross-linker results in a loss of carbonyl groups that is offset by the formation of carbonyl groups via photooxidation. Thus, the measured carbonyl growth is, in fact, less than the actual formation of carbonyl groups by photooxidation. Second, in the acrylic melamine coatings, the C-H stretch region did not change in shape, making it relatively straightforward to normalize the weathered and unweathered spectra. The fact that hexamethylene CH2 groups are lost during weathering of the urethane coatings makes it somewhat more difficult to accurately normalize the coatings to a constant acrylic C-H absorbance. Small errors in the normalization factor lead to large errors in the measured carbonyl growth. For this study, carbonyl growth was quantified by measuring the ratio of the area under the carbonyl band to that under the C-H band as a function of weathering time. Rates of Cross-Link Scission and Carbonyl Formation in Unstabilized Coatings. In previous harsh exposure studies of acrylic copolymers cross-linked with biuret isocyanates, the loss of cross-link was found to obey simple first-order kinetics (i.e., plots of In (retained cross-link) vs time were linear), while the increase in carbonyl area was found to be linear in exposure time (Bauer et al., 1988). Plots of retained cross-linker versus near-ambient exposure time are shown in Figure 4 for the different acrylic urethane coatings. Plots of carbonyl growth are shown for the same coatings in Figure 5. For a given copolymer, the extent of loss of cross-link and
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reveal the following chemical changes occur in both coatings on weathering: gain of OH (3500-cm-' band) and COOH (3200-cm-' band), loss of NH (3400-cm-' band), loss of hexamethylene CH2 (2940 cm-' band), increase in carbonyl (1650-1750 cm-' band), and loss of urethane and/or urea (1540-cm-' and 1240-cm-' bands). These changes are consistent with both carbonyl growth and urethane cross-link scission with loss of cross-linker. The changes on weathering for N-Ure and N-Tri are qualitatively the same as those for coatings based on copolymer A, though the rates of change are very different (see below). As shown in Figure 3, the addition of 2% by weight TIN 770 to A-Ure does not alter the types or relative extents of degradation observed. This suggests that comparisons of the rate of chemical change during near-ambient exposure can be used to evaluate HALS performance in these coatings. Comparison of Figure 1with previous weathering studies of acrylic urethanes (Figure 1 of Bauer et al., 1988, for example) under harsh short wavelength UV exposure suggests that carbonyl formation and urethane cross-link scission are the dominant photodegradation chemistries in both harsh and near-ambient exposures. The relative extents of the cross-link scission and carbonyl formation are roughly the same for the two exposures in unstabilized acrylic urethanes. Thus, the first two criteria outlined in the Introduction for the use of accelerated tests to predict service life appear to be met for unstabilized acrylics. Comparisons of the photodegradation kinetics in the different exposures (the third criteria) are discussed below. In contrast to the acrylic coatings in which short wavelength UV light does not alter the type of chemical change on exposure, it has been shown previously that small amounts of short wavelength (
