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The role of humidity in the photooxidation of acrylic melamine coatings
Polymer Degradation and Stability 40 (1993) 349-355
The role of humidity in the photooxidation of acrylic melamine coatings D. R. Bauer & D. F. Mielewski Ford Motor Company, Research Laboratory, PO Box 2053, Dearborn, MI 48121, USA (Received 5 May 1992; revised version received 6 July 1992; accepted 26 August 1992)
Predicting the weatherability of acrylic melamine coatings commonly used as enamel clearcoats requires a detailed understanding of each of the factors that influence photooxidation kinetics. Previous work t has shown that the photooxidation rate in coatings can be written as the following function of hydroperoxide concentration: photooxidation rate = K[YOOH] + M. The existence of a measurable photooxidation rate in the absence of hydroperoxide (i.e. a non-zero value of the intercept, M) has been observed only in melamine crosslinked coatings. It has also been observed that the photooxidation rate in acrylic melamine coatings increases with increasing humidity. In contrast, for urethane crosslinked coatings the value of M is zero, and the photooxidation rate is independent of humidity. In this paper, infrared spectroscopic measurements of functional group changes (e.g. carbonyl growth and crosslink scission) are used to measure photooxidation rates in acrylic melamine coatings during UV exposures at different humidities. Comparisons of these rates to measured hydroperoxide concentrations for the same coatings and exposures reveal that the increase in photooxidation rate with humidity is due to the fact that the intercept M increases with increasing humidity. Since the intercept is zero under dry conditions, the chemical reactions responsible for the intercept in melamine crosslinked coatings must involve both UV light and moisture. These results confirm the importance of accurately controlling the humidity during UV exposure for predicting the weatherability of melamine crosslinked coatings.
INTRODUCTION
A number of previous studies have indicated that the humidity level during UV exposure plays a key role in the rates of chemical change in acrylic melamine coatings. For example, when an acrylic melamine coating was exposed to UV light from FS-40 fluorescent bulbs, the rate of crosslink scission increased by 70% when the dew point was raised from -40°C to q-25°C. 4 The rate of erosion of thin films as measured by the decrease in the C - H stretch band of the infrared spectrum with UV exposure also increased with humidity level. 5 Film erosion is a result of both crosslink scission and carbonyl growth. In contrast to these results, photooxidation and photostabilization in acrylic urethane coatings was found to be virtually independent of the humidity of the UV exposure." Several mechanisms may contribute to acrylicmelamine crosslink scission. Acid catalyzed hydrolysis does account for the observed rate of
When acrylic melamine clearcoats are exposed to ultraviolet (UV) light they undergo a number of chemical reactions which ultimately lead to the physical failure of the coating. Two main types of chemical change can be identified using infrared spectroscopy. 2'3 The first is the formation of a variety of carbonyl groups as well as other photooxidation products. The second is the scission of acrylic-melamine crosslinks. This scission is followed by formation of new melamine-melamine crosslinks and the evolution of formaldehyde. The rates of carbonyl growth and crosslink scission have been found to be related to one another. Coatings with high rates of crosslink scission for a given exposure typically have high rates of carbonyl growth as well. Both measurements are useful for measuring relative photooxidation rates in these coatings. 349
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D. R. Bauer, D. F. Mielewski
crosslink scission in the absence of UV light. 7 When both UV light and humidity are present, the rate of crosslink scission increases dramatically. 4 Absorption of UV light by melamine followed by excited-state hydrolysis4 and free-radical attack on the acrylic-melamine crosslink have been proposed to account for the increase in crosslink scission with UV light intensity. 8 Another important aspect of acrylic melamine photodegradation chemistry is that formaldehyde is released as a by-product of crosslink scission. 8 Formaldehyde emission rates are related to crosslink scission rates and increase with both UV light intensity and humidity level. Formaldehyde can be oxidized to performic acid, a strong oxidant. It has been proposed that this formaldehyde-based chemistry is responsible for the observed increase in photooxidation rate with increasing humidity. Formaldehyde-based chemistry could also account for some of the effects of humidity observed in the hindered amine light stabilizer (HALS) chemistry of stabilized acrylic melamine coatings. 9'1° For example, the observation that the rate of conversion of HALS to nitroxide increases with humidity level in these coatings could be a consequence of the fact that peracids rapidly convert hindered amines to nitroxides. Recently, a technique has been developed to measure the concentration of hydroperoxides in coatings as a function of weathering time. H'~2 Hydroperoxides are key intermediates in the free-radical oxidation cycle. In general, hydroperoxide levels in coatings rapidly rise to a maximum, and then decrease to a plateau value that is maintained throughout the rest of the exposure period. The addition of HALS lowers the hydroperoxide level. The plateau hydroperoxide level tends to be lower in melamine crosslinked coatings than in urethane crosslinked coatings. The following relationship between the hydroperoxide level and the photooxidation (i.e., carbonyl growth) rate has been observed in both acrylic melamine and urethane coatings: 13 photooxidation rate = K[hydroperoxide] + M (1) where K is a 'collective' rate constant for all reactions which consume hydroperoxide. Similar relationships are observed between hydroperoxide concentration and crosslink scission rate. The linear relationship between photooxidation rate and hydroperoxide concentration is consis-
tent with a model in which hydroperoxides establish a steady state concentration via standard free-radical oxidation reactions, a3 The existence of a significant rate of photooxidation at negligible hydroperoxide concentration (i.e. a non-zero value of the intercept M) is not predicted by free-radical oxidation schemes. A non-zero value of M suggests pathways for photodegradation that do not depend upon free-radical chemistry. For urethane coatings, M is found to be zero within experimental error. ~3 That is, all of the photooxidation chemistry can be related directly to hydroperoxide chemistry. In melamine crosslinked coatings, on the other hand, M is not zero and a substantial fraction of the photooxidation chemistry is independent of measured hydroperoxide level. The addition of HALS reduces the hydroperoxide level but does not affect M. Since HALS function primarily by free-radical scavenging, this result is consistent with a non-zero M value being due to non-free radical photooxidation. It is somewhat surprising that the types of chemical changes observed in acrylic melamine coatings are the same when hydroperoxide levels are low (photooxidation dominated by non-free radical chemistry) as when they are high (photooxidation dominated by free radical chemistry).14 The results described above clearly demonstrate that photooxidation in acrylic melamine coatings has a number of unique aspects. Of particular interest are the dependence upon humidity, the emission of formaldehyde, and the existence of photooxidation chemistry which is not directly related to free-radical oxidation (i.e. independent of hydroperoxide concentration). A question arises as to whether or not these different effects are related, and if so, how. Interpretation of all of the results described above is complicated by the fact that the experiments were performed on different coatings using a variety of exposure conditions. Many of the early experiments employed FS-40 UV fluorescent bulbs. These bulbs emit UV light at wavelengths that are much shorter than those observed outdoors. Results from such exposures do not always correlate with actual outdoor exposures. In order to have more reliable laboratory weathering tests, it is necessary to determine the relationships between photooxidation rates, hydroperoxide levels, and humidity under more realistic light exposure conditions. In this paper, we report the relationship between
Role of humidity in the photooxidation of acrylic melamine coatings hydroperoxide levels, carbonyl growth, and crosslink scission for an acrylic melamine coating exposed to 'outdoor-like' UV light at two different humidity levels. The implications of the results to the mechanisms for photooxidation in acrylic melamine coatings and to the design of appropriate accelerated tests are discussed.
EXPERIMENTAL Material The synthesis of acrylic copolymer A has been described in detail elsewhere. '5 Copolymer A contains 68% by weight butylmethacrylate, 30% hydroxyethylacrylate, and 2% acrylic acid and has a number-average molecular weight of 1700. Coating A is prepared by combining copolymer A and Cymel 325 melamine formaldehyde crosslinker in a ratio of 70:30 and curing the coating for 20 min at 130°C. Coatings based on copolymer A have a relatively high rate of initiation of free radicals due to the presence of a small concentration of ketone end-groups on the polymer? 6 The hindered amine light stabilizer bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate (TIN 770 from Ciba Geigy) was added at a level of 2% by weight to some coatings.
Exposure conditions Samples were exposed in a modified Atlas UV2 weathering chamber. UV-340 fluorescent bulbs were used as the light source. The wavelength distribution of UV light for these bulbs is similar to sunlight over the range 300-350 nm. The air temperature in the chamber was maintained at 40°C for both experiments. Two different humidity conditions were employed. For the humid exposure, the bottom of the chamber was filled with water maintained at 25°C. Air was bubbled through the water and circulated in the chamber using fans to insure that a uniform dew point of 25 + 3°C was achieved. Dry conditions were achieved by blowing air through a Pneumatech compressed air drier (PHS-10) and into the chamber. The measured dew point under dry conditions was -10°C. The modifications to this chamber for humidity control required the removal of one of the four fluorescent bulbs. For
351
this reason, the overall light intensity on the samples in these exposures is somewhat lower than for experiments reported previously. ,3
Infrared spectroscopy Coatings were cast and cured on Si infrared plates. Infrared spectra were obtained in transmission using a Mattson Galaxy 5000 FTIR spectrometer.
Hydroperoxide titration Coatings were cast and cured on 4 in x 12 in glass plates. After weathering, the coating was removed and cryogenically ground to a fine powder. Methylene chloride was used to swell the crosslinked particles. Hydroperoxide determinations were performed using a previously described procedure based on iodometric titration, tj Titration values for HALS stabilized coatings were corrected for contribution from nitroxide. 12 In all cases, this correction was negligible.
RESULTS AND DISCUSSION Infrared spectra of unweathered coating A, coating A weathered under humid conditions and the difference between the two are shown in Fig. 1. A similar set of spectra for coating A weathered under dry conditions is shown in Fig.
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Fig. 1. Infrared spectra of unstabilized acrylic melamine coating A before exposure and after 1134 h of UV exposure under humid conditions. A difference spectrum is also shown.
352
D. R. Bauer, D. F. M i e l e w s k i 35
COATING A, UNWEATHERED
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Fig. 2. Infrared spectra of unstabilized acrylic melamine coating A before exposure and after 1690 h of UV exposure under dry conditions. A difference spectrum is also shown.
2. The changes are similar to those observed previously in this type of coating. 2'3''4 It is important to note that the types of chemical changes observed do not depend upon the humidity of the exposure. Similar but less extensive changes are observed for coating A stabilized with 2% TIN 770. Carbonyl growth is evident by comparing absorbances in the region 1550cm -' to 1750cm -1 before and after weathering. The extent of carbonyl growth was followed by measuring the increase in intensity at 1630cm -1 relative to the CH stretch band at 2940cm-L This measure has been found to correlate well with growth in total area under the carbonyl region (1750-1550cm -~) of the spectrum. Carbonyl band intensities were normalized so that they were consistent with previously published results. 2'3 Crosslink scission is evidenced by the loss of ether C - O bands at 1100 cm -~, 1016 cm -1, and 915 cm -~. The extent of crosslink scission was measured by following the disappearance of the methoxy band at 915cm -~ relative to the triazine band at 810cm-L Plots of carbonyl growth and crosslink scission are shown in Figs 3 and 4 for both exposure conditions. The carbonyl growth rate is slower under dry than under humid conditions. The addition of 2% TIN 770 reduces the rate of carbonyl growth by a factor of about 2 for both exposures. Similar results are observed for crosslink scission. The variations in hydroperoxide level with weathering time are shown in Fig. 5 for both
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Fig. 3. Carbonyl growth versus exposure time for acrylic melamine coating A. The stabilized coating contained 2% by weight TIN 770.
exposures. In all cases, hydroperoxide levels rise to a maximum and then decrease. The maximum is higher for the dry exposure and the rate of decrease is slower. The addition of TIN 770 reduces the hydroperoxide level by a factor of 2-3 for both exposures. Carbonyl growth and crosslink scission rates were determined by measuring the gradients of the lines in Figs 3 and 4 over various exposure times (e.g. 0-400 h and 400-1200 h). These rates can be compared to average hydroperoxide I a'•"l
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Fig. 4. Residual methoxy level versus exposure time for acrylic melamine coating A. The gradients of the lines were used to determine crosslink scission rates.
Role of humidity in the photooxidation of acrylic melamine coatings 50
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values as determined from Fig. 5 over the same exposure periods. The results for unstabilized and TIN 770 stabilized coating A exposed under humid and dry conditions are plotted in Figs 6 and 7. For the humid exposure, the rate of crosslink scission at a given hydroperoxide level is somewhat lower than reported in the previous study. ~3 This is, no doubt, a result of the lower light intensity used in this exposure. The humidity of the previous exposure was less well controlled and the dew point probably varied
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Fig. 6. Normalized carbonyl growth rate versus hydroperoxide level for humid and dry UV exposures. The non-zero intercept in the humid exposure is evident.
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30 H y d r o p e r o x l d e , pmol/g 7. Crosslink scission rates (× 104h 1)
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hydroperoxide level for humid and dry exposures. Again, the non-zero intercept in the humid exposure is evident.
from 15-25°C. A non-zero intercept (i.e. a non-zero M value in eqn 1) is clearly observed for both crosslink scission and carbonyl growth under humid exposure conditions. The critical finding of this paper is that the intercept (M value) is zero for both carbonyl growth and crosslink scission under dry exposure conditions. There is a clear relationship between humidity and the non-hydroperoxide-related degradation chemistry that results in a positive value of M. At any given hydroperoxide level, the rate of degradation is significantly higher under humid exposure conditions than dry conditions. This increase is only slightly offset by the fact that the hydroperoxide level itself tends to be roughly 25% lower under humid conditions. The gradient (K value) is also higher under humid than dry conditions. The increase in K with humidity suggests that humidity increases the rate of hydroperoxide decomposition in melamine crosslinked coatings. This would be consistent with a decrease in hydroperoxide level with increasing humidity. The decrease in slope for the dry exposure relative to the humid exposure is similar to that observed previously for urethane coatings relative to melamine coatings exposed under humid conditions. 13 Thus, the kinetics of photooxidation of melamine crosslinked coatings exposed under dry conditions are fundamentally the same as for urethane coatings. The basic photooxidation kinetics of
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D. R. Bauer, D. F. Mielewski
melamine and urethane coatings are only different under humid UV exposures. The effects of humidity on hydroperoxide levels and on non-free-radical photooxidation chemistry in melamine crosslinked coatings observed in this work confirms the need to control humidity during UV exposure. The level of humidity should reflect humidity levels actually encountered in service. Tests which employ humidity levels that are either very high or very low are likely to distort the photodegradation chemistry and make service life predictions meaningless. This is particularly true for stabilized coatings. The addition of HALS reduces the hydroperoxide concentration, making the non-free-radical chemistry which is most sensitive to changes in humidity, more important. It should be noted that control of humidity during the light cycle is not the same as the addition of condensing humidity or water spray during the dark cycle. The findings reported in Figs 6 and 7 suggest that under dry conditions, crosslink scission and carbonyl growth in acrylic melamine coatings is solely a result of free-radical-induced photooxidation chemistry. But, under humid conditions, an additional non-free-radical mechanism for photodegradation occurs in acrylic melamine coatings. One possibility is 'photoenhanced' hydrolysis. 4 The melamine triazine ring absorbs UV light at wavelengths below 320nm. The excited state of the melamine is more easily protonated than the ground state. This suggests that hydrolysis would be more rapid for melamine molecules in the excited state. The crosslink scission that results from this process is independent of hydroperoxide concentration and could account for the non-zero intercept in Fig. 7. Free-radical attack on the melamine crosslink could account for the increase in crosslink scission with increasing hydroperoxide concentration in Fig. 7. Together, these two mechanisms are consistent with the observed dependence of crosslink scission on hydroperoxide concentration. They do not, by themselves, account for the relationship between carbonyl growth and hydroperoxide in Fig. 6 or for the connection between carbonyl growth and crosslink scission. In theory, the formaldehydeperacid-based chemistry referred to above could provide the explanation. Formaldehyde is released as a result of crosslink scission whether or not the scission is by photoenhanced hydrolysis
or free-radical attack. Conversion of formaldhyde to performic acid could result in carbonyl growth rates that are closely related to crosslink scission rates. It should be noted that treatment of acrylic melamine coatings with p-nitro perbenzoic acid results in changes in the carbonyl region of the infrared spectrum that are similar to those observed with UV exposure. This is consistent with the fact that the photooxidation products observed in acrylic melamine coatings when free-radical chemistry dominates are similar to those observed when non-free radical chemistry dominates. ~4 Further work would be required to confirm these hypotheses.
CONCLUSION Hydroperoxide concentrations in an unstabilized and HALS stabilized acrylic melamine coating have been compared to rates of carbonyl growth and crosslink scission using 'outdoor-like' UV exposures at two different humidities. The hydroperoxide level is somewhat higher under dry conditions, especially at relatively short exposure times. A HALS stabilizer (TIN 770) reduces the hydroperoxide level by a factor of 2.5-3 for both humid and dry exposures. Both carbonyl growth and crosslink scission depend linearly upon the hydroperoxide level. Under dry exposure, carbonyl growth and crosslink scission are directly proportional to hydroperoxide level with a zero intercept. Under humid conditions, the gradient is somewhat higher, and there is a non-zero intercept. This additional oxidation chemistry is responsible for the overall increase in photooxidation rate with increasing humidity. The fact that both the photooxidation rate and the hydroperoxide level depend upon the humidity during UV exposure re-emphasizes the importance of humidity control during laboratory exposure, and also explains why acrylic melamine coatings weather faster under humid conditions.
REFERENCES
1. Bauer, D. R., Mielewski, D. F. & Gerlock, J. L., Polym. Deg. Stab., 38 (1992) 57. 2. Bauer, D. R., Gerlock, J. L., Mielewski,D. F., Paputa Peck, M. C. & Carter, R. O. III, Polym. Deg. Stab., 27 (1990) 271. 3. Bauer, D. R., Gerlock, J. L., Mielewski,D. F., Paputa Peck, M. C. & Carter, R. O. III, Polym. Deg. Stab., 28 (1990) 39.
Role of humidity in the photooxidation of acrylic melamine coatings 4. Bauer, D. R. & Briggs, L. M., In Characterization of Highly Cross-Linked Polymers, ed. S. S. Labana & R. A. Dickie. American Chemical Society Symposium Series No. 243, Washington, DC, 1984, p. 271. 5. Gerlock, J. L., Bauer, D. R. & Briggs, L. M., In Polymer Stabilization and Degradation, ed. P. P. Kiemchuk. American Chemical Society, Symposium Series No. 280, Washington DC, 1985, p. 119. 6. Bauer, D. R., Dean, M. J. & Gerlock, J. L. Ind. Eng. Chem., 27 (1988) 65. 7. Bauer, D. R., J. Appl. Polym. Sci., 27 (1982) 3651. 8. Gerlock, J. L., Dean, M. J., Korniski, T. J. & Bauer, D. R., Ind. Eng. Chem., Prod. Res. Dev., 25 (1986)
449. 9. Gerlock, J. L., Bauer, D. R. & Briggs, L. M., Polym.
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Deg. Stab., 14 (1986) 53. 10. Gerlock, J. L., Riley, T. & Bauer, D. R., Polym. Deg. Stab., 14 (1986) 73. 11. Mielewski, D. F., Bauer, D. R. & Gerlock, J. L., Polym. Deg. Stab., 33 (1991) 93. 12. Mielewski, D. F., Bauer, D. R. & Gerlock, J. L., Polym. Deg. Stab., (in press). 13. Bauer, D. R., Mielewski, D. F. & Gerlock, J. L., Polym. Deg. Stab., 38 (1992) 57. 14. Bauer, D. R., Gerlock, J. L. & Mielewski, D. F., Polym. Deg. Stab., 36 (1992) 9. 15. Gerlock, J. L., Bauer, D. R., Briggs, L. M. & Hudgens, J. K., Prog. Org. Coat., 15 (1987) 197. 16. Gerlock, J. L., Mielewski, D. F., Bauer, D. R. & Carduner, K. R., Macromolecules, 21 (1988) 1604.
The role of humidity in the photooxidation of acrylic melamine coatings D. R. Bauer & D. F. Mielewski Ford Motor Company, Research Laboratory, PO Box 2053, Dearborn, MI 48121, USA (Received 5 May 1992; revised version received 6 July 1992; accepted 26 August 1992)
Predicting the weatherability of acrylic melamine coatings commonly used as enamel clearcoats requires a detailed understanding of each of the factors that influence photooxidation kinetics. Previous work t has shown that the photooxidation rate in coatings can be written as the following function of hydroperoxide concentration: photooxidation rate = K[YOOH] + M. The existence of a measurable photooxidation rate in the absence of hydroperoxide (i.e. a non-zero value of the intercept, M) has been observed only in melamine crosslinked coatings. It has also been observed that the photooxidation rate in acrylic melamine coatings increases with increasing humidity. In contrast, for urethane crosslinked coatings the value of M is zero, and the photooxidation rate is independent of humidity. In this paper, infrared spectroscopic measurements of functional group changes (e.g. carbonyl growth and crosslink scission) are used to measure photooxidation rates in acrylic melamine coatings during UV exposures at different humidities. Comparisons of these rates to measured hydroperoxide concentrations for the same coatings and exposures reveal that the increase in photooxidation rate with humidity is due to the fact that the intercept M increases with increasing humidity. Since the intercept is zero under dry conditions, the chemical reactions responsible for the intercept in melamine crosslinked coatings must involve both UV light and moisture. These results confirm the importance of accurately controlling the humidity during UV exposure for predicting the weatherability of melamine crosslinked coatings.
INTRODUCTION
A number of previous studies have indicated that the humidity level during UV exposure plays a key role in the rates of chemical change in acrylic melamine coatings. For example, when an acrylic melamine coating was exposed to UV light from FS-40 fluorescent bulbs, the rate of crosslink scission increased by 70% when the dew point was raised from -40°C to q-25°C. 4 The rate of erosion of thin films as measured by the decrease in the C - H stretch band of the infrared spectrum with UV exposure also increased with humidity level. 5 Film erosion is a result of both crosslink scission and carbonyl growth. In contrast to these results, photooxidation and photostabilization in acrylic urethane coatings was found to be virtually independent of the humidity of the UV exposure." Several mechanisms may contribute to acrylicmelamine crosslink scission. Acid catalyzed hydrolysis does account for the observed rate of
When acrylic melamine clearcoats are exposed to ultraviolet (UV) light they undergo a number of chemical reactions which ultimately lead to the physical failure of the coating. Two main types of chemical change can be identified using infrared spectroscopy. 2'3 The first is the formation of a variety of carbonyl groups as well as other photooxidation products. The second is the scission of acrylic-melamine crosslinks. This scission is followed by formation of new melamine-melamine crosslinks and the evolution of formaldehyde. The rates of carbonyl growth and crosslink scission have been found to be related to one another. Coatings with high rates of crosslink scission for a given exposure typically have high rates of carbonyl growth as well. Both measurements are useful for measuring relative photooxidation rates in these coatings. 349
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D. R. Bauer, D. F. Mielewski
crosslink scission in the absence of UV light. 7 When both UV light and humidity are present, the rate of crosslink scission increases dramatically. 4 Absorption of UV light by melamine followed by excited-state hydrolysis4 and free-radical attack on the acrylic-melamine crosslink have been proposed to account for the increase in crosslink scission with UV light intensity. 8 Another important aspect of acrylic melamine photodegradation chemistry is that formaldehyde is released as a by-product of crosslink scission. 8 Formaldehyde emission rates are related to crosslink scission rates and increase with both UV light intensity and humidity level. Formaldehyde can be oxidized to performic acid, a strong oxidant. It has been proposed that this formaldehyde-based chemistry is responsible for the observed increase in photooxidation rate with increasing humidity. Formaldehyde-based chemistry could also account for some of the effects of humidity observed in the hindered amine light stabilizer (HALS) chemistry of stabilized acrylic melamine coatings. 9'1° For example, the observation that the rate of conversion of HALS to nitroxide increases with humidity level in these coatings could be a consequence of the fact that peracids rapidly convert hindered amines to nitroxides. Recently, a technique has been developed to measure the concentration of hydroperoxides in coatings as a function of weathering time. H'~2 Hydroperoxides are key intermediates in the free-radical oxidation cycle. In general, hydroperoxide levels in coatings rapidly rise to a maximum, and then decrease to a plateau value that is maintained throughout the rest of the exposure period. The addition of HALS lowers the hydroperoxide level. The plateau hydroperoxide level tends to be lower in melamine crosslinked coatings than in urethane crosslinked coatings. The following relationship between the hydroperoxide level and the photooxidation (i.e., carbonyl growth) rate has been observed in both acrylic melamine and urethane coatings: 13 photooxidation rate = K[hydroperoxide] + M (1) where K is a 'collective' rate constant for all reactions which consume hydroperoxide. Similar relationships are observed between hydroperoxide concentration and crosslink scission rate. The linear relationship between photooxidation rate and hydroperoxide concentration is consis-
tent with a model in which hydroperoxides establish a steady state concentration via standard free-radical oxidation reactions, a3 The existence of a significant rate of photooxidation at negligible hydroperoxide concentration (i.e. a non-zero value of the intercept M) is not predicted by free-radical oxidation schemes. A non-zero value of M suggests pathways for photodegradation that do not depend upon free-radical chemistry. For urethane coatings, M is found to be zero within experimental error. ~3 That is, all of the photooxidation chemistry can be related directly to hydroperoxide chemistry. In melamine crosslinked coatings, on the other hand, M is not zero and a substantial fraction of the photooxidation chemistry is independent of measured hydroperoxide level. The addition of HALS reduces the hydroperoxide level but does not affect M. Since HALS function primarily by free-radical scavenging, this result is consistent with a non-zero M value being due to non-free radical photooxidation. It is somewhat surprising that the types of chemical changes observed in acrylic melamine coatings are the same when hydroperoxide levels are low (photooxidation dominated by non-free radical chemistry) as when they are high (photooxidation dominated by free radical chemistry).14 The results described above clearly demonstrate that photooxidation in acrylic melamine coatings has a number of unique aspects. Of particular interest are the dependence upon humidity, the emission of formaldehyde, and the existence of photooxidation chemistry which is not directly related to free-radical oxidation (i.e. independent of hydroperoxide concentration). A question arises as to whether or not these different effects are related, and if so, how. Interpretation of all of the results described above is complicated by the fact that the experiments were performed on different coatings using a variety of exposure conditions. Many of the early experiments employed FS-40 UV fluorescent bulbs. These bulbs emit UV light at wavelengths that are much shorter than those observed outdoors. Results from such exposures do not always correlate with actual outdoor exposures. In order to have more reliable laboratory weathering tests, it is necessary to determine the relationships between photooxidation rates, hydroperoxide levels, and humidity under more realistic light exposure conditions. In this paper, we report the relationship between
Role of humidity in the photooxidation of acrylic melamine coatings hydroperoxide levels, carbonyl growth, and crosslink scission for an acrylic melamine coating exposed to 'outdoor-like' UV light at two different humidity levels. The implications of the results to the mechanisms for photooxidation in acrylic melamine coatings and to the design of appropriate accelerated tests are discussed.
EXPERIMENTAL Material The synthesis of acrylic copolymer A has been described in detail elsewhere. '5 Copolymer A contains 68% by weight butylmethacrylate, 30% hydroxyethylacrylate, and 2% acrylic acid and has a number-average molecular weight of 1700. Coating A is prepared by combining copolymer A and Cymel 325 melamine formaldehyde crosslinker in a ratio of 70:30 and curing the coating for 20 min at 130°C. Coatings based on copolymer A have a relatively high rate of initiation of free radicals due to the presence of a small concentration of ketone end-groups on the polymer? 6 The hindered amine light stabilizer bis (2,2,6,6-tetramethyl-4-piperidinyl) sebacate (TIN 770 from Ciba Geigy) was added at a level of 2% by weight to some coatings.
Exposure conditions Samples were exposed in a modified Atlas UV2 weathering chamber. UV-340 fluorescent bulbs were used as the light source. The wavelength distribution of UV light for these bulbs is similar to sunlight over the range 300-350 nm. The air temperature in the chamber was maintained at 40°C for both experiments. Two different humidity conditions were employed. For the humid exposure, the bottom of the chamber was filled with water maintained at 25°C. Air was bubbled through the water and circulated in the chamber using fans to insure that a uniform dew point of 25 + 3°C was achieved. Dry conditions were achieved by blowing air through a Pneumatech compressed air drier (PHS-10) and into the chamber. The measured dew point under dry conditions was -10°C. The modifications to this chamber for humidity control required the removal of one of the four fluorescent bulbs. For
351
this reason, the overall light intensity on the samples in these exposures is somewhat lower than for experiments reported previously. ,3
Infrared spectroscopy Coatings were cast and cured on Si infrared plates. Infrared spectra were obtained in transmission using a Mattson Galaxy 5000 FTIR spectrometer.
Hydroperoxide titration Coatings were cast and cured on 4 in x 12 in glass plates. After weathering, the coating was removed and cryogenically ground to a fine powder. Methylene chloride was used to swell the crosslinked particles. Hydroperoxide determinations were performed using a previously described procedure based on iodometric titration, tj Titration values for HALS stabilized coatings were corrected for contribution from nitroxide. 12 In all cases, this correction was negligible.
RESULTS AND DISCUSSION Infrared spectra of unweathered coating A, coating A weathered under humid conditions and the difference between the two are shown in Fig. 1. A similar set of spectra for coating A weathered under dry conditions is shown in Fig.
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Fig. 1. Infrared spectra of unstabilized acrylic melamine coating A before exposure and after 1134 h of UV exposure under humid conditions. A difference spectrum is also shown.
352
D. R. Bauer, D. F. M i e l e w s k i 35
COATING A, UNWEATHERED
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Fig. 2. Infrared spectra of unstabilized acrylic melamine coating A before exposure and after 1690 h of UV exposure under dry conditions. A difference spectrum is also shown.
2. The changes are similar to those observed previously in this type of coating. 2'3''4 It is important to note that the types of chemical changes observed do not depend upon the humidity of the exposure. Similar but less extensive changes are observed for coating A stabilized with 2% TIN 770. Carbonyl growth is evident by comparing absorbances in the region 1550cm -' to 1750cm -1 before and after weathering. The extent of carbonyl growth was followed by measuring the increase in intensity at 1630cm -1 relative to the CH stretch band at 2940cm-L This measure has been found to correlate well with growth in total area under the carbonyl region (1750-1550cm -~) of the spectrum. Carbonyl band intensities were normalized so that they were consistent with previously published results. 2'3 Crosslink scission is evidenced by the loss of ether C - O bands at 1100 cm -~, 1016 cm -1, and 915 cm -~. The extent of crosslink scission was measured by following the disappearance of the methoxy band at 915cm -~ relative to the triazine band at 810cm-L Plots of carbonyl growth and crosslink scission are shown in Figs 3 and 4 for both exposure conditions. The carbonyl growth rate is slower under dry than under humid conditions. The addition of 2% TIN 770 reduces the rate of carbonyl growth by a factor of about 2 for both exposures. Similar results are observed for crosslink scission. The variations in hydroperoxide level with weathering time are shown in Fig. 5 for both
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E x p o s u r e T i m e (hours)
Fig. 3. Carbonyl growth versus exposure time for acrylic melamine coating A. The stabilized coating contained 2% by weight TIN 770.
exposures. In all cases, hydroperoxide levels rise to a maximum and then decrease. The maximum is higher for the dry exposure and the rate of decrease is slower. The addition of TIN 770 reduces the hydroperoxide level by a factor of 2-3 for both exposures. Carbonyl growth and crosslink scission rates were determined by measuring the gradients of the lines in Figs 3 and 4 over various exposure times (e.g. 0-400 h and 400-1200 h). These rates can be compared to average hydroperoxide I a'•"l
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Fig. 4. Residual methoxy level versus exposure time for acrylic melamine coating A. The gradients of the lines were used to determine crosslink scission rates.
Role of humidity in the photooxidation of acrylic melamine coatings 50
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values as determined from Fig. 5 over the same exposure periods. The results for unstabilized and TIN 770 stabilized coating A exposed under humid and dry conditions are plotted in Figs 6 and 7. For the humid exposure, the rate of crosslink scission at a given hydroperoxide level is somewhat lower than reported in the previous study. ~3 This is, no doubt, a result of the lower light intensity used in this exposure. The humidity of the previous exposure was less well controlled and the dew point probably varied
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Fig. 6. Normalized carbonyl growth rate versus hydroperoxide level for humid and dry UV exposures. The non-zero intercept in the humid exposure is evident.
Fig.
30 H y d r o p e r o x l d e , pmol/g 7. Crosslink scission rates (× 104h 1)
40 versus
hydroperoxide level for humid and dry exposures. Again, the non-zero intercept in the humid exposure is evident.
from 15-25°C. A non-zero intercept (i.e. a non-zero M value in eqn 1) is clearly observed for both crosslink scission and carbonyl growth under humid exposure conditions. The critical finding of this paper is that the intercept (M value) is zero for both carbonyl growth and crosslink scission under dry exposure conditions. There is a clear relationship between humidity and the non-hydroperoxide-related degradation chemistry that results in a positive value of M. At any given hydroperoxide level, the rate of degradation is significantly higher under humid exposure conditions than dry conditions. This increase is only slightly offset by the fact that the hydroperoxide level itself tends to be roughly 25% lower under humid conditions. The gradient (K value) is also higher under humid than dry conditions. The increase in K with humidity suggests that humidity increases the rate of hydroperoxide decomposition in melamine crosslinked coatings. This would be consistent with a decrease in hydroperoxide level with increasing humidity. The decrease in slope for the dry exposure relative to the humid exposure is similar to that observed previously for urethane coatings relative to melamine coatings exposed under humid conditions. 13 Thus, the kinetics of photooxidation of melamine crosslinked coatings exposed under dry conditions are fundamentally the same as for urethane coatings. The basic photooxidation kinetics of
354
D. R. Bauer, D. F. Mielewski
melamine and urethane coatings are only different under humid UV exposures. The effects of humidity on hydroperoxide levels and on non-free-radical photooxidation chemistry in melamine crosslinked coatings observed in this work confirms the need to control humidity during UV exposure. The level of humidity should reflect humidity levels actually encountered in service. Tests which employ humidity levels that are either very high or very low are likely to distort the photodegradation chemistry and make service life predictions meaningless. This is particularly true for stabilized coatings. The addition of HALS reduces the hydroperoxide concentration, making the non-free-radical chemistry which is most sensitive to changes in humidity, more important. It should be noted that control of humidity during the light cycle is not the same as the addition of condensing humidity or water spray during the dark cycle. The findings reported in Figs 6 and 7 suggest that under dry conditions, crosslink scission and carbonyl growth in acrylic melamine coatings is solely a result of free-radical-induced photooxidation chemistry. But, under humid conditions, an additional non-free-radical mechanism for photodegradation occurs in acrylic melamine coatings. One possibility is 'photoenhanced' hydrolysis. 4 The melamine triazine ring absorbs UV light at wavelengths below 320nm. The excited state of the melamine is more easily protonated than the ground state. This suggests that hydrolysis would be more rapid for melamine molecules in the excited state. The crosslink scission that results from this process is independent of hydroperoxide concentration and could account for the non-zero intercept in Fig. 7. Free-radical attack on the melamine crosslink could account for the increase in crosslink scission with increasing hydroperoxide concentration in Fig. 7. Together, these two mechanisms are consistent with the observed dependence of crosslink scission on hydroperoxide concentration. They do not, by themselves, account for the relationship between carbonyl growth and hydroperoxide in Fig. 6 or for the connection between carbonyl growth and crosslink scission. In theory, the formaldehydeperacid-based chemistry referred to above could provide the explanation. Formaldehyde is released as a result of crosslink scission whether or not the scission is by photoenhanced hydrolysis
or free-radical attack. Conversion of formaldhyde to performic acid could result in carbonyl growth rates that are closely related to crosslink scission rates. It should be noted that treatment of acrylic melamine coatings with p-nitro perbenzoic acid results in changes in the carbonyl region of the infrared spectrum that are similar to those observed with UV exposure. This is consistent with the fact that the photooxidation products observed in acrylic melamine coatings when free-radical chemistry dominates are similar to those observed when non-free radical chemistry dominates. ~4 Further work would be required to confirm these hypotheses.
CONCLUSION Hydroperoxide concentrations in an unstabilized and HALS stabilized acrylic melamine coating have been compared to rates of carbonyl growth and crosslink scission using 'outdoor-like' UV exposures at two different humidities. The hydroperoxide level is somewhat higher under dry conditions, especially at relatively short exposure times. A HALS stabilizer (TIN 770) reduces the hydroperoxide level by a factor of 2.5-3 for both humid and dry exposures. Both carbonyl growth and crosslink scission depend linearly upon the hydroperoxide level. Under dry exposure, carbonyl growth and crosslink scission are directly proportional to hydroperoxide level with a zero intercept. Under humid conditions, the gradient is somewhat higher, and there is a non-zero intercept. This additional oxidation chemistry is responsible for the overall increase in photooxidation rate with increasing humidity. The fact that both the photooxidation rate and the hydroperoxide level depend upon the humidity during UV exposure re-emphasizes the importance of humidity control during laboratory exposure, and also explains why acrylic melamine coatings weather faster under humid conditions.
REFERENCES
1. Bauer, D. R., Mielewski, D. F. & Gerlock, J. L., Polym. Deg. Stab., 38 (1992) 57. 2. Bauer, D. R., Gerlock, J. L., Mielewski,D. F., Paputa Peck, M. C. & Carter, R. O. III, Polym. Deg. Stab., 27 (1990) 271. 3. Bauer, D. R., Gerlock, J. L., Mielewski,D. F., Paputa Peck, M. C. & Carter, R. O. III, Polym. Deg. Stab., 28 (1990) 39.
Role of humidity in the photooxidation of acrylic melamine coatings 4. Bauer, D. R. & Briggs, L. M., In Characterization of Highly Cross-Linked Polymers, ed. S. S. Labana & R. A. Dickie. American Chemical Society Symposium Series No. 243, Washington, DC, 1984, p. 271. 5. Gerlock, J. L., Bauer, D. R. & Briggs, L. M., In Polymer Stabilization and Degradation, ed. P. P. Kiemchuk. American Chemical Society, Symposium Series No. 280, Washington DC, 1985, p. 119. 6. Bauer, D. R., Dean, M. J. & Gerlock, J. L. Ind. Eng. Chem., 27 (1988) 65. 7. Bauer, D. R., J. Appl. Polym. Sci., 27 (1982) 3651. 8. Gerlock, J. L., Dean, M. J., Korniski, T. J. & Bauer, D. R., Ind. Eng. Chem., Prod. Res. Dev., 25 (1986)
449. 9. Gerlock, J. L., Bauer, D. R. & Briggs, L. M., Polym.
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Deg. Stab., 14 (1986) 53. 10. Gerlock, J. L., Riley, T. & Bauer, D. R., Polym. Deg. Stab., 14 (1986) 73. 11. Mielewski, D. F., Bauer, D. R. & Gerlock, J. L., Polym. Deg. Stab., 33 (1991) 93. 12. Mielewski, D. F., Bauer, D. R. & Gerlock, J. L., Polym. Deg. Stab., (in press). 13. Bauer, D. R., Mielewski, D. F. & Gerlock, J. L., Polym. Deg. Stab., 38 (1992) 57. 14. Bauer, D. R., Gerlock, J. L. & Mielewski, D. F., Polym. Deg. Stab., 36 (1992) 9. 15. Gerlock, J. L., Bauer, D. R., Briggs, L. M. & Hudgens, J. K., Prog. Org. Coat., 15 (1987) 197. 16. Gerlock, J. L., Mielewski, D. F., Bauer, D. R. & Carduner, K. R., Macromolecules, 21 (1988) 1604.