I n d . E n g . Chern. Res. 1988,27, 65-70
65
Comparison of Photostabilization in Acrylic/Urethane and Acrylic/Melamine Coatings Containing Hindered Amines and Ultraviolet Absorbers David R. Bauer,* Mary J. Dean, and John L. Gerlock Research S t a f f , Ford M o t o r C o m p a n y , Dearborn, Michigan 48121
The photostabilization chemistry of a two-package acrylic/urethane coating containing a benzotriazole UV absorber and/or a hindered amine light stabilizer has been studied by using FTIR and ESR. FTIR has been used to follow the rate of chemical change on exposure of the coatings to UV light and humidity. The coatings are found to undergo photooxidation and loss of urethane cross-links during exposure. In contrast to melamine cross-linked coatings, humidity plays only a minor role in the degradation of the urethane coating. The addition of a benzotriazole UV absorber reduces the rate of degradation by reducing the rate of formation of free radicals. The addition of hindered amine greatly reduces the rate of oxidation and urethane loss. The hindered amine is significantly more effective in the urethane coating than in similar melcmine coatings. ESR measurements of nitroxide concentrations in hindered amine containing coatings reflect the differences in stabilization chemistry. In addition to being more effective, the effective life of the hindered amine stabilizer is also longer in the urethane than in the melamine coating.
I. Introduction Two-package coatings based on aliphatic isocyanate cross-linkers are widely used as topcoats particularly for plastic substrates which require low bake temperatures (Potter et al., 1984). The cure and photodegradation chemistry of two-package coatings based on acrylic polyols and a biuret of hexamethylene diisocyanate has been recently studied by using Fourier transform infrared spectroscopy (FTIR) and cross-polarization magic-angle nuclear magnetic resonance (CP-MAS-NMR)(Bauer et al., 1986a). It was found in this study that exposure to ultraviolet (UV) light results in an increase in carbonyl intensity and a loss of amide I1 and amide IV bands in the infrared spectrum. The increase in carbonyl intensity results from oxidation of the coating. The loss of amide band intensity was interpreted as a scission of acrylic-urethane cross-links followed by loss of the biuret cross-linker. The loss of acrylic-urethane cross-links was found to occur even in the absence of humidity, indicating that the loss is not a hydrolytic process. This was confirmed by the observed retention of urethane cross-links during exposure to condensing humidity. It was also found in the earlier study that the addition of a hindered amine light stabilizer (HALS) (bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate) greatly reduced the rate of increase of carbonyl intensity and loss of urethane cross-links (Bauer et al., 1986a). This paper is an extension of our earlier study. FTIR has been used to measure rates of carbonyl formation and urethane cross-link loss as a function of exposure conditions and the presence of stabilizers. Both a benzotriazole UV absorber as well as the above-hindered amine were studied. Electron spin resonance (ESR) has been used to follow the nitroxide concentration in HALS containing coatings as a function of exposure time and conditions. The results are compared with previous studies of melamine cross-linked coatings. 11. Experimental Section The two acrylic copolymers used in this study were prepared by conventional free-radical polymerization. Both were composed of butyl methacrylate, hydroxyethyl acrylate, and acrylic acid. Copolymer 1 has a slightly higher molecular weight and level of hydroxyethyl acrylate than that for copolymer 2. The isocyanate cross-linker is a conventional biuret of hexamethylene diisocyanate
(L2291A from Mobay Chemical). An idealized structure for this cross-linker is U
II
OCN(CH2)eN
/
CNH(CH2),NCO
'CNH~CH~~CO
II
0
The melamine cross-linker used for comparison purposes was a partially alkylated melamine (Cyme1325 from American Cyanamid). Urethane coatings were formulated with an OH-to-NCO ratio of 1:l. The melamine coating was formulated using 30% by weight melamine. The UV absorber used, 2-(2-hydroxy-3,5-bis(l,l-dimethylbenzyl)phenyl)-2H-benzotriazole,was obtained from Ciba-Geigy (TIN 900). The HALS used, bis(2,2,6,6-tetramethyl-4piperidinyl) sebacate, was also obtained from Ciba-Geigy (TIN 770). All coatings were cured for 20 min at 130 OC. The most significant difference between coatings prepared with copolymers 1 and 2 was that the rate of formation of free radicals (photoinitiation rate) was roughly 4 times higher in the coatings prepared from copolymer 2 (Gerlock et al., 1984). Samples for the FTIR measurements were cast on KRS-5 salt plates at film thicknesses of roughly 5 pm. Spectra were measured in transmission by using a Nicolet FTIR. Samples for the ESR measurements were cast on quartz slides by using a spinning technique. Cured film thicknesses were roughly 25 pm. Nitroxide concentrations were measured with a IBM-Brucker ER 200 D ESR spectrometer using previously developed techniques (Gerlock, 1983). Coatings were exposed in modified Atlas UV2 weathering chambers. The chambers were modified to allow simultaneous control of both air temperature and relative humidity (Gerlock et al., 1984). The air temperature was maintained at 60 f 1 "C. One chamber had a dew point of 25 "C, while the other had a dew point of -40 "C. FS-20 UV-B fluorescent lamps were used in both chambers. The UV-B bulbs produce UV light over the wavelength range 275-350 nm centered at 310 nm. The total UV light intensity at the sample was approximately 1.5 mW/cm2. No dark condensing humidity cycle was employed.
0888-5885/88/2627-006~~01.~0~0 0 1988 American Chemical Society
66
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 UNEXPOSED bl
A -A-
II
P T ACRYLIC CH URETHANE
llr
CH
?
I
600
0 CARBONYL
i 3500
L 3000
A/
2 ,
I 2600
1200
1800
2400
3000
EXPOSUAE TIME ( h r s ) I
2000
I
I
1500
1000
WAVENUMBERS
Figure 1. FTIR spectra of cured and ultraviolet light exposed acrylic/urethane coating 1. The bands of interest are marked by arrows.
Figure 2. Increase in FTIR carbonyl intensity with exposure time for acrylic/urethane coating 1. The open symbols are for UV exposure a t 25 "C dew point, while the filled symbols are for UV exposure unstabilized coating; (0) coating containing at -40 "C dew point: (0) 2% by weight TIN 900 UV absorber; and (A)coating containing 2% by weight TIN 770 HALS.
111. Results and Discussion
FTIR Measurements. In the earlier study on these coatings, it was found that FTIR and CP-MAS-NMR provided similar information concerning the photodegradation chemistry (Bauer et al., 1986a). FTIR spectra of cured and degraded acryliclurethane coating 1 are shown in Figure 1. The band assignments have been discussed in detail by Bauer et al. (1986a). The acrylic CH stretching band at 2960 cm-' was used as an internal standard. Changes that are observed on degradation include a slow erosion of the film (as evidenced by a loss of total hydrocarbon intensity), a decrease in the intensity of the methylene CH stretching band from the cross-linker (2940 cm-') relative to that from the acrylic copolymer (indicating a loss of cross-linker), an increase in the carbonyl intensity (indicating oxidation), and a loss of intensity in the amide I1 and amide IV bands (indicating rupture of the acrylic-urethane cross-links). The amide IV band is due solely to acrylic-urethane cross-links, while the amide I1 band is composed of signals from both acrylic-urethane cross-links and urea groups from the biuret cross-linker. The rate of increase of carbonyl intensity is proportional to the oxidation rate. The rate of loss of acrylic-urethane cross-links can be determined from the decrease in the amide IV band. The decrease in the amide I1 band intensity reflects both the rupture of acrylic-urethane cross-linksand the subsequent loss of cross-linker from the coating. This behavior can be compared with that of melamine cross-linkers in similar resins. It has been found that melamine cross-linkers self-condense during degradation and are retained in the coating (Bauer and Briggs, 1984). There is no equivalent reaction in the isocyanate cross-linker. After the urethane cross-links are broken, the cross-linker can be lost from the coating. Carbonyl intensity is plotted as a function of exposure time for acryliclurethane coating 1 exposed to humid and dry environments in Figure 2. Oxidation is roughly linear with time and independent of humidity. The rate of oxidation in the unstabilized coating is similar to that for a melamine coating prepared from similar polymers except that the rate of oxidation is found to increase with increasing humidity in the melamine coating (Bauer and Briggs, 1984). It should be noted that at high levels of oxidation, the apparent rate of oxidation slows. This is likely due to the fact that coating material is being lost. Initially, the oxidation products are retained. As the degradation proceeds, however, oxidation products are lost, leading to an apparent decrease in the oxidation rate. Plots
0 0
'
b
0
,,I:;o,;;
io00
,
,
2000
3000
,
,
4000
5000
,
EXPOSURE TIME ( h i s 1
Figure 4. Decrease in intensity of amide I1 band relative to acrylic CH band versus exposure time. The symbols are the same as in Figure 2.
of the loss of amide IV and amide I1 band intensities for acryliclurethane coating 1 with exposure time are shown in Figures 3 and 4. The loss of urethane cross-link (amide IV) obeys simple first-order kinetics, and the rate is independent of humidity. Again this behavior is in contrast to the acrylic/melamine coating where the rate of loss of acrylic-melamine cross-links is a strong function of humidity. The initial rate of loss of amide I1 intensity is similar to that for the amide IV band. At long times, however, the amide I1 band intensity does not go to zero but instead approaches a plateau value. Since all the acrylic-urethane cross-links have been broken by this time, this residual intensity must represent the signal from urea
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 67 ,OOr
Table I. Relative Rates of Degradation: AcrylicAJrethane Coating 1 % uv % HALS oxidation cross-link scission 0 0 I1 2 0 0.56 0.50 0 2 0.17 0.052 1 1 0.125 0.056 ~~
. . , ,
NELANINE.'Z% H A L 5
'
---
'
~
biuret groups retained in the coating. The level of retained cross-linker is higher under dry exposure conditions as evidenced by the higher level of amide I1 intensity As shown in Figures 2 and 3, the addition of 2% by weight TIN 900 UV absorber reduces the rate of carbonyl formation and the rate of loss of acrylic-urethane crosslinks by roughly a factor of 2. Similar results have been observed for acrylic/melamine coatings (Bauer and Briggs, 1984). Some but not all of the reduction is due to the reduction in average light intensity in the film. The rest of the reduction is likely due to quenching of excited states (Bauer et al., 1986b). Both processes reduce the rate of formation of free radicals in the coating. The addition of 2% by weight HALS TIN 770 to the acrylic/urethane coating greatly reduces the rates of carbonyl formation and acrylic-urethane cross-link loss. The relative effectiveness of the stabilizers at controlling carbonyl formation and acrylic-urethane cross-link scission are reported in Table I. The effectiveness of the different stabilizers is independent of the humidity of exposure. The effectiveness of the stabilizers is relatively constant throughout the exposure period (>5000 h). Some synergism between HALS and UV absorbers is observed since the combination of 1% TIN 900and 770 reduces the oxidation rate by more than 2% of either TIN 900 or 770 alone. The hindered amine is significantly more effective in the urethane coating than it is in typical melamine cross-linked coatings where at most reductions in oxidation rate of a factor of 2 are observed (Gerlock et al., 1985). These observations are completely consistent with measurements of physical property changes (such as gloss loss) in coatings. Typically, the addition of hindered amines improves gloss retention by at most a factor of 2-3. In urethane coatings, gloss retention is typically improved by factors of 5 or more. In addition, synergism between HALS and UV absorbers is typically observed. (Gloss retention curves for a variety of coatings with and without HALS are available in Ciba-Geigy Stabilizer Technical Bulletins). This correlation is consistent with earlier observations relating the rate of degradation chemistry to the rate of loss of appearance and other physical properties (Gerlock et al., 1985). It also suggests that a more detailed understanding of the photostabilization chemistry that occurs in coatings could lead to more durable materials. ESR Measurements of Nitroxide Concentration. HALS additives are generally thought to function by interfering with free-radical chain oxidation (Gerlock et al., 1986a,b; Bauer and Gerlock, 1986). Hindered amines are oxidized to nitroxides by peroxy and other radicals. Nitroxides can react with a variety of radicals to form amino ethers. Amino ethers can be converted back to nitroxides by free-radical reactions. All of these reactions effectively terminate free-radical chains, shortening the chain length and reducing oxidation. The chemistry of stabilization by HALS compounds can be followed either by monitoring the rate of consumption of HALS (Gerlock et al., 1986b) or by following the concentration of nitroxide (Gerlock et al., 1986a). In earlier work (Gerlock et al., 1986b), it was shown that TIN 770 is rapidly (a few hundred hours of exposure) consumed in both urethane and melamine cross-linked acrylic coatings. The main stabilization re-
LIGHT INTENS!TY FULL 0
_i10
4
EXPOSURE TIME ( h i s !
Figure 5. Nitroxide concentration versus exposure time for acrylic/melamine coating 1 containing 2% by weight TIN 770 HALS exposed to three UV light intensities at a dew point of 25 "C. The light intensity in the weathering chamber was reduced with neutral density filters. I
"
I
"""
4 0 0 1 URETHANE 2% HALS
-
,E.
,001
l
LIGHT INTENSITY FULL 3 HALF TENTH A
?
/
\ 4 1
EXPOSURE TIME ( b r s 1
Figure 6. Nitroxide concentration versus exposure time for acrylic/urethane coating 1 containing 2% by weight TIN 770 HALS exposed to three UV light intensities at a dew point of 25 "C.
actions involve reactions of nitroxide and amino ethers. The nitroxide concentration has been measured as a function of exposure time, humidity, light intensity, and HALS concentration for both the urethane and melamine cross-linkedacrylic copolymer 1. Typical curves are shown in Figures 5 and 6. In both coatings, the nitroxide concentration rises to a maximum and slowly decreases. In both coatings, the time to reach maximum nitroxide concentration increases with decreasing light intensity. The maximum nitroxide concentration increases with decreasing light intensity in both coatings; however, the magnitude of the increase is much larger in the urethane coating. Decreasing the light intensity by a factor of 10 results in a 35% increase in the nitroxide concentration in the melamine coating and a 130% increase in the urethane coating. This observation may have significance toward accelerated test design. Higher than normal light intensities may result in unnatural stabilization chemistry in acrylic/urethane coatings. Since HALS stabilization is of key importance to the durability of urethane coatings, a test which changes the balance of the stabilization reactions may not correctly predict the durability. In addition to being more sensitive to light intensity, the nitroxide concentration in the urethane coating can also be more sensitive to the concentration of HALS. In melamine coatings studied previously, the nitroxide maximum was roughly proportional to the initial HALS level. In acrylic/urethane coating 2, on the other hand, doubling the HALS concentration resulted in an increase in the maximum nitroxide concentration of a factor of 3. The reason for this is unclear but may suggest that a minimum HALS
,
2% PALS
m
5
I/,
:
I
/
ESR SPECTRUM FROM HALS DOPED
EX'OSJ:E
-IVE
Prs
J
MELAMINE wet,ory
0 8
URETHANE wet,dry
0
1
40
1000
2000
3000
4000
5000
" \I
COATING A F T E R SHORT UV EXPOSURE
Figure 7. Nitroxide concentration versus exposure time for the acrylic/urethane (circles) and acrylic/melamine coatings (squares) exposed to UV light a t dew points of 25 O C (open symbols) and -40 "C (filled symbols). Both coatings were prepared with copolymer 1 with a HALS concentration of 2 % .
0'
PEROXY RADICAL SPECTRUM
6000
7000
BO00
EXPOSURE TIME (hrs 1
Figure 8. Nitroxide concentration versus exposure time. Symbols and coatings are the same as in Figure 7 .
level is required to control the oxidation chemistry. Even though the maximum nitroxide levels are qualitatively similar in urethane and melamine coatings, the initial accumulation rate of nitroxide is much larger in the melamine coating than in the urethane coating using the same copolymer (1) (Figure 7). In addition, the accumulation rate of nitroxide increases with increasing humidity of exposure in the melamine coating but is almost independent of humidity in the urethane coating. This behavior has been attributed to the fact that melamine coatings release formaldehyde during exposure (Gerlock et al., 1986~).Formaldehyde can be converted to performic acid which can rapidly oxidize hindered amine to nitroxide. More formaldehyde is released during humid exposures accounting for the dependence on humidity in the melamine coating. The rate at which the nitroxide concentration approaches its maximum value is also different in the melamine and urethane coatings. In the melamine coating, the rate of increase of nitroxide slows with increasing nitroxide concentration above 30-50% of its maximum. In contrast, the rate of increase of nitroxide in the urethane coating is roughly constant or even increases slightly until just before the maximum is reached. Interpretation of these results requires a better understanding of the details of the chemistry (particularly the amino ether reactions) in the two coatings. The decrease in nitroxide concentration at long times is also different in the two coatings. As shown in Figure 8, the rate of loss of nitroxide at very long times is less in the urethane coating than it is in the melamine coating. The concentration of nitroxide in the melamine coating drops steadily from its maximum. In contrast, the nitr-
NlTROXlDE SPECTRUM
Figure 9. ESR spectrum from HALS-doped acrylic/urethane coating 2 exposed for 3 h to UV light. This coating had a much higher photoinitiation rate than the other acrylic/urethane coatings studied here. The ESR spectrum observed is the sum of coating based (peroxy) and nitroxide signals as shown.
oxide concentration in the urethane coating drops rapidly from its maximum to a value roughly 20-50% of the maximum (depending on the humidity). At this point, the rate of loss of nitroxide slows dramatically. The concentration of nitroxide at long times is smaller for dry exposure conditions than it is for humid conditions for both acrylic/urethane and acrylic/melamine coatings. The reason for this is unclear particularly in the urethane case where the degradation chemistry is basically independent of humidity (except for the retention of cross-linker). The effectivenessof HALS at long times is an important question since HALS derivatives are primarily responsible for the low oxidation rate in the acrylic/urethane coating. As noted above, HALS function by interfering with the propagation of free-radical oxidation. By reacting with other free radicals, HALS and their derivatives reduce the ambient concentration of peroxy and other radicals. A measure of the persistence of stabilizers can be determined in principle by monitoring the steady-state concentration of these other (peroxy) radicals in the coating. Unfortunately, in coatings using copolymer 1, the concentration of other (peroxy) radicals during exposure is too low to measure with the techniques employed here (< 2 X mol/g). This is probably due to the low photoinitiation rate in this coating (-2 X mol/(gmin)). An ESR signal from coating based radicals has been observed during the early stages of photolysis of melamine coatings which had higher rates of formation of free radicals (Gerlock et al., 1986a). Photolysis of a 2% HALS doped acryliclurethane coating (coating 2) which had a photoinitiation rate of -7 X mol/(g.min) produced signals from both coating based (peroxy) and nitroxide radicals at short exposure times, Figure 9. The signals from the nitroxide and other (peroxy) radicals can be separated and the intensities of both components determined. It is impossible to assign the coating based radical signal to a specific radical. Based on the oxidation chemistry, it seems likely that most of the coating based radicals are peroxy. The concentrations of both radicals are plotted in Figures 10 and 11 for acryliclurethane coating 2 exposed under wet and dry conditions. Initially, the peroxy concentration is higher than the nitroxide concentration. As the nitroxide concentration increases, the peroxy concentration decreases and eventually is impossible to quantify in the presence of the strong nitroxide signal. At long exposure times, the coating based radical signal reappears after the nitroxide concenmol/g. The retration has dropped to around 10 x
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 69
:XF3SUFcE
T M E lhrs
Figure 10. Nitroxide (0) and peroxide ( 0 )concentrations versus exposure time for acrylic/urethane coating 2 containing 2 % by weight TIN 770 HALS exposed to UV light at a dew point of 25 "C. 5C
I
I
FXFOS,RE
TME
1
(h,s)
Figure 11. Nitroxide (0) and peroxide ( 0 )concentrations versus exposure time for acrylic/urethane coating 2 containing 2 % by weight TIN 770 HALS exposed to UV light at a dew point of -40 "C.
appearance of other (peroxy) radicals suggests an increase in oxidation rate along with a decrease in the effectiveness of the stabilizer. This occurs after roughly 4000 h of humid UV exposure and 2000 h of dry UV exposure. The peroxy concentration at long times is lower than that observed initially by roughly a factor of 4. This suggests that either the stabilizer is still partially effective or that the oxidation kinetics have changed during exposure. The reappearance of coating based radicals after the nitroxide has decreased to a low level suggests that the measurements of the nitroxide concentration can be used to estimate the effective lifetimes of HALS additives. If the exposure time at which the nitroxide concentration drops below 10 X mol/g is taken as a measure of stabilizer permanence, Figures 8,10, and 11 can be used to determine the effects of cross-linker, copolymer, and exposure humidity on the effective lifetime of TIN 770. Decreasing the humidity of exposure shortens the effective life of this stabilizer in both urethane and melamine coatings. Increasing the photoinitiation rate (coating 2 versus coating 1) shortens the effective life of the stabilizer (4000h for coating 2 versus over 8000 h for coating 1 under humid exposure). Finally, TIN 770 remains effective roughly twice as long in the urethane cross-linked coating as in the melamine coating (both coatings using copolymer 1). It would clearly be valuable to determine the effects of other exposure variables (such as condensing humidity) on stabilizer permanence. Even more important, this approach may provide the first chemical measure of effective stabilizer permanence in coatings weathered outdoors. Although the decrease in nitroxide concentration to very low levels appears to correlate with long-term stabilizer effectiveness, the nitroxide concentration at any given time
does not determine stabilizer effectiveness. This is clear from comparisons of the large nitroxide concentration changes and the relatively constant level of HALS effectiveness (as determined by the FTIR measurements) over long exposure times. The nitroxide concentration is a function of both the concentration of HALS and its derivatives and of the kinetics of the degradation and stabilization reactions. It is possible that the rapid drop in nitroxide concentration in the acrylic/urethane coating just after the maximum is caused by a change in the balance of the nitroxide and amino ether reactions and that the subsequent slow loss of nitroxide reflects a loss of stabilizer in these coatings. Mechanism of HALS Stabilization of Urethane Cross-Links. The addition of HALS reduces the oxidation rate by a factor of 6 and the urethane cross-link scission rate by a factor of 19. A number of mechanisms have been proposed for the destruction of urethane linkages formed from aromatic isocyanates including freeradical oxidation (leading, for example, to diquinoneimide structures in MDI-based urethanes), direct photoscission, and photo-Fries rearrangement (Potter et al., 1984). The coatings studied here employed an aliphatic isocyanate. The only mechanism consistent with the degree of stabilization of the acrylic-urethane cross-link provided by TIN 770 is that of free-radical oxidation. In the basic urethane moiety shown below, all of the hydrogens in the linkage are susceptible to abstraction by peroxy or other radicals: -CH2NHC(=O)OCH2-. Abstraction of the CH2 hydrogen next to the nitrogen has been observed in urethane elastomers (Lemaire et al., 1983). Abstraction of any of the above hydrogens followed by oxidation would lead to eventual rupture of the urethane cross-link. If we assume that hydrogen atom abstraction is the rate-limiting step in the loss of urethane, then the rate of loss is given by rate of urethane cross-link loss = kab,[YOO'][URE] (1) where kaba is the rate constant for abstraction, [YOO'] is the concentration of abstracting radicals, and [URE] is the concentration of urethane cross-links. This model is consistent with the observation that the loss of urethane cross-links is first order in urethane concentration. The addition of HALS reduces the rate of urethane loss by reducing the steady-state concentration of YOO' (consistent with Figures 10 and 11). Reducing the concentration of YOO' also reduces the photooxidation rate. The total oxidation rate can be given by (Emanuel et al., 1967) oxidation rate = PIR
+ k',,,[YOO'][YH]
(2)
where PIR is the photoinitiation rate and k & is also a rate constant for hydrogen atom abstraction (not necessarily the same as &). The addition of HALS does not change the photoinitiation rate. Thus, the oxidation rate is not reduced by as much as the urethane scission rate. The kinetic chain length is given by the ratio of the oxidation rate to the photoinitiation rate. The reduction by HALS of the oxidation rate by a factor of 6 and the urethane scission rate by a factor of 19 suggests that the unstabilized chain length is around 8. Comparison of the rate of increase of carbonyl with the photoinitiation rate is consistent with this value. The stabilized chain length of the coating containing 2% HALS is only 1.3. When the chain length is long, the oxidation rate is proportional to the square root of the light intensity. At very short chain lengths, the oxidation rate is proportional to the light intensity directly. This difference may explain the strong concentration dependence of nitroxide concentration on light intensity in the urethane coating relative to the
70 Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988
melamine coating. Equations 1 and 2 also explain the observed synergism between HALS and the UV absorber. The HALS reduces the oxidation rate to a value similar to the photoinitiation rate. Further reduction in the oxidation rate requires a reduction in photoinitiation rate. The UV absorber reduces the photoinitiation rate, allowing for even slower oxidation. The most significant unanswered question is why the HALS additive is so much more effective in the urethane coating than in the melamine coating. Considering the large effect that formaldehyde has been shown to play in the degradation chemistry of melamine coatings, it is tempting to speculate that formaldehyde is also responsible for the difference in effectiveness and possibly persistence of the HALS additive in the two coatings. Further efforts to explore the effect of formaldehyde on degradation and stabilization chemistry are in progress.
IV. Summary Stabilization chemistry in acryliclurethane coatings containing a benzotriazole UV absorber and/or a hindered amine light stabilizer have been studied by using FTIR and ESR. Benzotriazoles reduce the rates of carbonyl formation and acrylic-urethane cross-link scission by reducing the average photoinitiation rate of free radicals in the coating. Hindered amines reduce the kinetic chain length of oxidation by reducing the steady-state concentration of peroxy and other radicals. This also greatly retards the scission of urethane cross-links. Hindered amines are significantlymore effective in urethane coatings than in melamine cross-linked coatings. The stabilizer effectivenessis retained for long times in urethane coatings as evidenced by the long retention of nitroxide in HALSdoped coatings.
Registry No. TIN 900, 70321-86-7; TIN 770, 52829-07-9; (acrylic acid)(butylmethacrylate)(hydroxyethyl acrylate)(copolymer), 72259-85-9; (acrylic acid)(butylmethacrylate)(hydroxyethyl acrylate) (cyme1 325) (copolymer), 111004-74-1.
Literature Cited Bauer, D. R.; Briggs, L. M. In Characterization of Highly Crosslinked Polymers; Labana, s. s.,Dickie, R. A. Eds.; ACS Symposium Series No. 243; American Chemical Society: Washington, D.C., 1984; p 271. Bauer, D. R.; Gerlock, J. L. Polym. Degrad. Stab. 1986, 14, 97. Bauer, D. R.; Dickie, R. A.; Koenig, J. L. Ind. Eng. Chem. Prod. Res. Deo. 1986a, 25,289. Bauer, D. R.; Briggs, L. M.; Gerlock, J. L. J. Polym. S a . , Polym. Phys. 1986b, 24, 1651. Emanuel, N. M.; Denisov, E. T.; Maizus, Z. K. Liquid Phase Oxidation of Hydrocarbons; Plenum: New York, 1967. Gerlock, J. L. Anal. Chem. 1983, 54, 1529. Gerlock, J. L.; Bauer, D. R.; Briggs, L. M. In Characterization of Highly Crosslinked Polymers; Labana, S. S., Dickie, R. A., Eds.; ACS Symposium Series No. 243; American Chemical Society: Washington, D.C., 1984; p 285. Gerlock, J. L.; Bauer, D. R.; Briggs, L. M. In Polymer Stabilization and Degradation; Klemchuk, P. P., Ed.; ACS Symposium Series No. 280; American Chemical Society: Washington, D.C., 1985; p 119. Gerlock, J. L.; Bauer, D. R.; Briggs, L. M. Polym. Degrad. Stab. 1986a, 14, 53. Gerlock, J. L.; Riley, T.; Bauer, D. R. Polym. Degrad. Stab. 198613, 14, 73. Gerlock, J. L.; Dean, M. J.; Korniski, T. J.; Bauer, D. R. Ind. Eng. Chem. Prod. Res. Deu. 1986c, 25, 449. Lemaire, J.; Arnaue, R.; Gardette, J.-L. Pure Appl. Chem. 1983,55, 1603. Potter, T. A.; Schmelzer, H. G.; Baker, R. D. Prog. Org. Coat. 1984, 12, 321.
Received f o r review December 29, 1986 Accepted July 9, 1987
