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A non-destructive study on the degradation of polymer coating I: Step-scan photoacoustic FTIR and confocal Raman microscopy depth profiling
Polymer Testing 31 (2012) 855–863
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Analysis method
A non-destructive study on the degradation of polymer coating I: Step-scan photoacoustic FTIR and confocal Raman microscopy depth profiling W.R. Zhang a, *, T.T. Zhu a, R. Smith a, C. Lowe b a b
School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London E1 4NS, UK The Long Term Development Group, Becker Industrial Coatings, Goodlass Road, Speke, Liverpool L24 9HJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 May 2012 Accepted 3 July 2012
A polyester-melamine coating was exposed to accelerated weathering (i.e., cycle of high UV followed by condensation). The degradation depth profile in the coating was investigated using non-destructive step-scan photoacoustic fourier transform infrared spectroscopy (SSPA-FTIR) and confocal Raman microscopy (CRM). The degradation at different sampling depths was then evaluated following the method developed in this work. The results obtained using the two techniques correlate well with each other and suggest coating surface is more likely to undergo moisture enhanced photo-oxidation degradation. This may be due to the higher moisture level near the coating surface. It is also found that melamine side chains degrade prior to the melamine ring. The quantification methods developed in this work were found to be very suitable to monitor the degradation depth profile in polymeric coatings. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Polyester-melamine coating Degradation Step-scan photoacoustic FTIR Confocal Raman microscopy Depth profiling
1. Introduction Coil coating is a high-speed continuous process for the application of paint onto metal substrates prior to fabrication. Most coil coated products are used outdoors, which can mean harsh environments (e.g. high UV light intensity and high moisture level). Such conditions may lead to both moisture induced and photo-induced degradation. Among various coil coating systems (e.g., polyester-melamine, polyurethane, polyvinylidene fluoride, polyvinyl chloride, etc.), polyester-melamine coil coatings still dominate the market due to their low cost, good formability and adequate weathering performance. The chemistry of polyester-melamine coil coating involves a cross-linking (trans-etherification) reaction of hydroxyl groups on polyester molecules and methoxy groups on melamine side
* Corresponding author. Present address: AkzoNobel, Wexham Rd., Slough SL2 5DS, UK. Tel.: þ44 (0)1753 879286. E-mail address: [email protected] (W.R. Zhang). 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2012.07.002
chains. The molecular structures of most commonly used melamine (i.e., hexa-methoxy methyl melamine (HMMM) and melamine with lower degree of methylation) are shown in Scheme 1. It is known that both melamine and polyester molecules can be sensitive to UV light and moisture; the degradation of polyester and melamine or even polyester-melamine linkages may result in the collapse of cross-link network and finally lead to coating failure. If durability is to be improved, it is very important to investigate the mechanisms behind the coating degradation in detail. This has stimulated the development and application of new techniques to study polymer coating degradation. Many useful techniques have been previously applied to the study of coating degradation. The use of FTIR with different sampling techniques, for example, transmission, attenuated total reflection (ATR) and photoacoustic (PA), to interrogate polymer coatings has long been common practice [1]. Raman spectroscopy has also demonstrated some advantages for studying polymeric materials [2]. X-ray photo-electron spectroscopy (XPS) [3] and time-of-flight
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Scheme 1. Molecular structure of melamine; (a) HMMM; (b) Melamine with lower degree of methylation.
secondary ion mass spectroscopy (ToF-SIMS) have been proven excellent for polymer coating surface chemistry investigations [4]. More recently, the focus of polymer coating research has moved towards small volume analysis and depth profiling studies. This is because coating formulations are mixtures of different components; the distribution of certain components within the final film usually controls the coating properties (e.g., chemical resistance, adhesion and mechanical properties). Techniques such as ATR-FTIR [5] and angle resolved XPS [6] were previously utilised to achieve depth profiles of polymeric coatings in the near surface region (from microns down to nanometres). Step-scan photoacoustic FTIR (SSPA-FTIR) and confocal Raman microscopy (CRM) are both able to provide a high signal strength from the bulk of polymers, the sampling depths ranging from a few microns to tens of microns. Detailed investigations into the use of SSPA-FTIR and CRM in the assessment of polyester-melamine coil coatings have been previously published [7–10]; the results show that both SSPA-FTIR and CRM are suitable for depth profiling of coil coatings. Works reported by Gonon et al. [11] and Dupuie et al. [12] also show that both SSPA-FTIR and CRM are suitable for the depth profiling analysis of polymer degradation. Quantification methods utilising traditional FTIR and Raman spectroscopic techniques have been well established. For example, Bauer et al. found the carbonyl band area in the transmission FTIR spectra to be linearly correlated to the coating degradation level. Moreover, the area ratio of the carbonyl band before and after UV exposure was used to monitor the coating photo-oxidation rate [13,14]. Decker and Zahouily introduced a method that involved using the ratios of band areas in the IR spectra before and after QUV-A exposure to evaluate the formation/decomposition of various functional groups (e.g. –COOH, –C]O, –C–NH) in the coating. They also found a strong correlation between the band area ratio and photo-degradation rate [15]. Work reported by Nichols et al. and Adamsons suggests that the amount of degradation in coatings can be also analysed using the ratio of the IR absorbance in the 3800–2000 cm1 region (–OH and –NH regions) to that in the 3100–2800 cm1 region (CH2/ CH3 regions) [16–18]. Nguyen et al. showed that the band absorption intensity in IR spectra could be used as an indication of the concentration of functional groups in polymer coatings [19]. Moreover, Raman spectroscopy was previously demonstrated to be suitable for chemical
concentration analysis; both the band area [20] and intensity ratios [21] could be used to monitor the distribution of functional groups in polymer coatings. Although extensive progress has been made in the use of both SSPA-FTIR and CRM, quantitative depth profiling analysis of degraded coatings is less well developed. Most of the applications using SSPA-FTIR and CRM are based on qualitative analysis of the distribution of components in layered polymer films. Polymer degradation quantification methods using traditional IR & Raman techniques have been well established, but lack of depth profiling capabilities has restricted their further application in this field. This paper is mainly focused on developing and applying new SSPA-FTIR and CRM depth profiling methods to a polymer coating before and after degradation. Both band area and height ratio were used to quantify the degradation trend of functional groups in the coatings. At first sight, the results demonstrate the obvious: that degradation takes place primarily in the surface region of the coating. In the past it has been associated with photo-oxidation and moisture enhanced photo-oxidation. Our results show that the degradation occurs in the side chains of the melamine first followed by the rest of the molecule. The main conclusion, however, is that degradation appears to be limited to the top 7-8 microns for the first 2000 h of exposure in an accelerated weathering cabinet. Only subsequently does catastrophic degradation occur throughout the coating. The depth profiling methods developed in this work were found to be very suitable to monitor the degradation depth profile throughout the polymer coating. 2. Experimental 2.1. Materials and method The clear coil coating investigated in this work was mainly based on an aliphatic polyester resin and melamine crosslinking agent. The polyester to melamine weight ratio was 4:1 (higher than stoichiometric ratio) to ensure adequate cross-link density in the cured thermoset coating. Hence, it is expected that around 20-30% of methoxy groups are not reacted with polyester during the curing process. The melamine used was a mixture of HMMM and lower methylation melamine (Scheme 1) with a ratio of 2:1. The melamine with lower degree of methylation is expected to react quicker with polyester.
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The substrate used involved a layer of primer and a layer of white topcoat on the hot-dip galvanised (HDG) steel substrate. The white topcoat was based on the same resin system as the clear coating but with titanium dioxide pigment. The primer coating was based on standard polyester-melamine primer coating system with strontium chromate pigment. The white coating was applied to avoid early primer coating failure (primer is usually based on a polyester resin with lower durability compared to that used in topcoat formulations), which may affect the accuracy of clear coating durability evaluation. For reasons of commercial confidentiality, only a general description of the samples involved in this work is provided (Table 1). The wet clear coating was applied using a standard laboratory draw down method. A wire wound steel bar with suitable diameter was used to draw down the wet coating to achieve a dry film thickness of w20 mm. The panel was then cured in a convection oven for 30 s to achieve a peak metal temperature (PMT) of 232 C, in order to ensure a correct degree of cure. The white and primer coatings were both cured separately prior to the clear coat application. Samples with a diameter of w8 mm were punched from the panel for SSPA-FTIR and CRM studies. 2.2. SSPA-FTIR instrumentation All SSPA-FTIR spectra were recorded using a Nicolet 8700 research grade FTIR spectrometer (Madison, USA) with a MTEC model 200 photoacoustic (PA) cell (Ames, USA). The PA cell was purged with dry helium at a rate of 20 cm3/s for 5 min before any spectra acquisition. The step-scan experiments were performed by varying the modulation frequency of IR beam at constant amplitude of 3.5lHeNe. Spectra were collected at mid-IR range (400-4000 cm1) with a resolution of 8 cm1. The correlation between the IR modulation frequency and sampling depth is provided in Table 2; based on taking the thermal diffusivity of clear coat as 0.01 105 m2 s1 (a general value for most polymeric materials, a more detailed sampling depth calculation can be found elsewhere [7]). Note that the thermal diffusivity value may change with weathering due to molecular degradation; however, such change is minor and won’t affect the main discussion in this paper. A carbon black reference material was used as a strong surface absorber to calibrate the instrument. The in-phase (I) spectra were used as a direct indication of the chemical composition within the corresponding sampling depths. All SSPA-FTIR spectral data processing was accomplished using Omnic 7.3 software. 2.3. CRM instrumentation All Raman spectra were recorded using a Nicolet Almega research grade visible dispersive spectrometer (Madison, USA) with 785 nm laser excitation. A 100 dry objective Table 1 Sample information. Coating layer Resin system Clear White Primer
Pigment Thickness
Aliphatic Polyester þ Melamine N/A Aliphatic Polyester þ Melamine TiO2 Phthalate Polyester þ Melamine SrCrO4
w20 mm w20 mm w3 mm
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Table 2 IR sampling depth at different modulation frequencies. Modulation frequency (Hz)
IR sampling depth (mm)
50 100 200 400 600 800 1000
25 18 13 9 7 6 5.6
(NA ¼ 0.90, Olympus) was used to perform non-destructive CRM depth profiling experiments. All Raman spectral data processing was accomplished using Ominc 7.3 software. 2.4. Accelerated weathering exposure The clear coating was exposed in a QUV-A chamber (The Q-Panel Company, USA) equipped with QUV-A-340nm fluorescent tubes (295 nm – 400 nm) each with peak irradiance at 340 nm. The sample panel was irradiated for 8 h each 12 h cycle at 60 C. During the dark period (4 h), the panel was subjected to a moist environment at a temperature of 50 C with moisture level reaching 100% so that condensation appeared on the panels. During the light period, the moisture level decreases slowly from 100% to around 30%. A 8 mm diameter disc was punched from the sample panel after total exposure times of 500, 1000, 2000, 2500 and 3000 h. The sample discs were used for the SSPA-FTIR and CRM analyses. 3. Results and discussion 3.1. SSPA-FTIR depth profiling The SSPA-FTIR 800 Hz spectra (sampling depth: w6 mm) of the clear coat before (STD) and after of QUV-A exposure are shown in Fig. 1. It is observed that the clear coating generally undergoes hydrolysis and photo-oxidation degradation during the QUV-A exposure. The increase in –OH/–NH group region (w3600-3000 cm1) is mainly due to the hydrolysis/photo-oxidation of both the polyester and the melamine. The broadening of the carbonyl band increase of amine/amide band (w1730 cm1), (w1630 cm1) intensity and decrease of melamine (w1550 cm1) and residual methoxy (w1085 cm1 and 913 cm1, unreacted methoxy groups on melamine side chain) band intensities, indicate photo-induced degradation of the polyester and especially the melamine side groups and melamine-polyester linkages. The observations are in good agreement with previous findings [19,22,23,25]. A general band assignment is provided in Table 3 to assist the spectral interpretation. It is also observed from Fig. 1 that the hydrocarbon bands in the 2700 cm1 – 3000 cm1, 1450 cm1 and 1390 cm1 regions are relatively stable after exposure. Therefore, they are considered as the internal reference bands when evaluating the changes of intensity of the other bands (i.e., hydroxyl and melamine bands). The SSPA-FTIR spectra collected at different sampling depths after 3000 h QUV-A exposure are shown in Fig. 2.
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Fig. 1. SSPA-FTIR spectra of clear coating before (STD) and after QUV-A exposure.
The (I) spectra were taken as a direct indication of the chemical composition of clear coating at different sampling depths; the detailed development of this depth profiling approach can be found in a previous paper [7]. The hydrocarbon bands (1450 cm1 and 1390 cm1) in each spectrum are normalised to have approximately the same intensity. It is then observed that the melamine band at 1550 cm1 shows increasing intensity with the sampling depth. This observation suggests the degradation of melamine is more significant near the coating surface region. It is also observed that the bands in the region 1300 cm1 – 1175 cm1 show rapidly decreasing intensity with sampling depth. However, this is not related to the functional group distribution in the coating but due to the signal saturation (i.e., the carbonyl band at 1730 cm1 and C–O bands at 1300 cm1 – 1175 cm1 signals saturate after approaching approximately 6 mm below sample surface, due to their stronger IR absorption). When the C]O and C– O signals saturate, the detector can no longer sense the change in their thermal propagation at different sampling depths. However, the signal of the other bands are not
saturated so that the detector can still sense the change in band intensity at a deeper sampling depth, this makes it appear as C–O band intensity decrease when the intensity of hydrocarbon bands are normalised in Fig. 2. However, this effect will not affect the discussion in this work as the hydroxyl and melamine bands together with reference bands selected (hydrocarbon at 2800 cm1 – 3000 cm1, 1450 cm1 and 1390 cm1) are not saturated. Moreover, as the internal reference bands selected in this work are all adjacent to the band of interest (e.g., hydroxyl and melamine bands); this will further minimise any instrument related effects. Although differences in melamine degradation can be observed from spectra shown in Fig. 2, this is only a general qualitative comparison. A quantitative depth profile is preferable to reveal the coating degradation. As reviewed
Table 3 Band assignment [22–24]. Band position (cm1)
Assignment
3600-3000
Hydroxyl (OH) and amine/amide (NH) vibration Hydrocarbon (CH2/CH3) stretching Carbonyl (C]O) stretching Amine/amide vibration Melamine ring and side chain C–N stretching Hydrocarbon (CH2/CH3) bending Residual methoxy group (O–CH3) stretching
3000-2700 1730 1630 1550 1450/1390 1085/913
Fig. 2. SSPA-FTIR spectra of clear coating after 3000 h QUV-A exposure, as collected at different sampling depth.
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previously, the hydroxyl and melamine band intensities can be used to track hydrolysis and photo-induced degradation in coatings. The amount of hydroxyl functionality in the coating is determined using a band area normalisation method described in Eq. (1). It needs to address that, as the –NH band (3300-3500 cm1) is usually narrower than the –OH band, the broadened band region after weathering is considered to be mainly due to hydroxyl groups. The ACH and AOH were determined following the baseline fitting method as shown in Fig. 3 (note AOH is the area between hydrocarbon and hydroxyl groups). In order to minimise the effect of signal saturation, the RatioOH was calculated for both the unexposed (RatioOH-STD) and degraded (RatioOHQUVA) clear coat at each of the selected sampling depths. The OH% (hydroxyl group band increment) values at different sampling depth are then calculated individually following Eq. (2) to represent the degradation depth profile in the coating.
RatioOH ¼ AOH =ACH
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that the hydroxyl groups increase with QUV-A exposure time. The increment rate within the first 1000-2000 h (w200-400%) is faster than the following 1000 h (w50100% per 1000 h). This indicates that more UV and moisture sensitive groups in the coating were degraded in the initial QUV-A exposure stage. This is very likely to be partly due to them becoming exhausted after longer periods of exposure (i.e., more than
(1)
where, RatioOH – ratio of hydroxyl group area to hydrocarbon group area; AOH – hydroxyl group band area (approximately 3600 cm1 – 2200 cm1); ACH – hydrocarbon band area (approximately 3100 cm1 – 2750 cm1).
OH% ¼ ðRatioOHQUVA RatioOHSTD ÞRatioOHSTD %
(2)
where, OH% – formation of hydroxyl group due to degradation (i.e., percentage increase in hydroxyl group band region in spectrum of an exposed coating compared to that in the spectrum of an unexposed coating); RatioOH-QUVA – hydroxyl group to hydrocarbon band area ratio in the exposed coating; RatioOH-STD– hydroxyl group to hydrocarbon band area ratio in the unexposed coating. It is known that error might be introduced through the baseline fitting or IR spectrometer itself. In order to verify the accuracy of the quantification, ten spectra were collected from the same coating sample; the RatioOH values were found to be consistent (i.e. 2.82 0.03). The increase of hydroxyl groups after QUV-A exposure, as calculated using the method demonstrated above, is shown in Fig. 4a; the OH% values were recorded at different QUV-A hours and IR sampling depths. It is firstly observed
Fig. 3. Hydroxyl and hydrocarbon band area measurement in SSPA-FTIR spectra.
Fig. 4. SSPA-FTIR quantitative depth profile of the clear coating (colour control bar represents the value of vertical axis): (a) 3-D plot of increase of OH% (vertical axis), QUV-A exposure time and sampling depth; (b) 3-D plot of loss of MEL% (vertical axis), QUV-A exposure time and sampling depth; (c) 3-D plot of increase of amine/amide% (vertical axis), QUV-A exposure time and sampling depth.
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2000 QUV-A hours), which results in decreasing hydroxyl group formation rate in the clear coating. Moreover, the diffusion control can exert itself and, until a certain level of degradation is reached, the molecules (oxygen or water) find it more difficult to access the reaction front. It is also observed that the OH% value (w100%) obtained at 500 h exposure is relatively constant throughout the coating thickness. However, significantly higher OH% value is observed away from the coating surface region (w12 mm) only after longer exposure. In other words, the difference in OH% values between depths of 12 mm and 18 mm in the clear coating after 1000 h exposure is approximately 200%; this difference becomes 400% in the clear coating after 3000 h exposure. This is consistent with the diffusional control hypothesis presented above. A possible explanation for the above observations is moisture-enhanced degradation. Nguyen et al. has previously found that melamine methylols can be produced through the hydrolysis of melamine-acrylate (or melamine-methoxy) linkages. The melamine methylols can be then deformylated to form primary amines (–HN2) and aldehydes (–CH2O). Aldehydes undergo photo-induced decomposition and produce CHO and H radicals which will then enhance photo-oxidation of melamine side chains; moreover, photo-oxidation of aldehydes can also form peroxycaboxylic acid (HCO–OOH) which accelerates the hydrolysis of melamine-acrylate (or melaminemethoxy) linkages [22,23]. This moisture-enhanced degradation is effectively a self-catalysed degradation mechanism, which is more likely to occur near the coating surface region as it is in direct contact with moisture during the QUV-A exposure and diffusion may not be a limiting factor. For the initial 500 h, the degradation rate may be dominated by the photo-induced degradation caused by the high intensity UV irradiation. As the clear coat is transparent, UV radiation will penetrate throughout the film thickness leading to the photo-degradation, even in the deeper part of the coating. This is another fact that supports the diffusional control hypothesis. After longer exposure, a higher level of moisture is absorbed within a depth of approximately 10 mm causing moisture enhanced photo-degradation and, hence, more rapid coating degradation is observed within that region. Beyond 1000 h, the reactions result in the generation of more hydroxyl functionality both in the top 10 microns and the rest of the coating, indicating that the top few microns have broken down, reducing the limiting effect of diffusional control. In other words, moisture, oxygen and formaldehyde have a much less restricted path to the lower reaches of the film after 1000 h because the degradation has gone beyond a critical point. An estimate of melamine decomposition in the clear coating after QUV-A exposure is shown in Fig. 4b. Because the baseline at w1630 cm1 in the SSPA-FTIR changes due to the formation of amine/amide groups after melamine and/or melamine side chain decomposition, the band area measurements are highly affected by this baseline change. Thus, the heights (without baseline fitting) of the melamine band in SSPA-FTIR spectra are used to evaluate the amount of melamine remaining in clear coating; the ratio method is shown below:
RatioMelamine ¼ HMelamine =HCH
(3)
where, RatioMelamine – normalised height of melamine band; HMelamine – melamine band height at w1550 cm1; HCH – hydrocarbon band height at w1450 cm1 (spectral internal reference).
Loss of MEL% ¼ ðRatioMelamineSTD RatioMelamineQUVA Þ=RatioMelamineSTD %
(4)
where, Loss of MEL% – decomposition of melamine group due to the QUV-A exposure (i.e., percentage decrease of melamine in exposed coating comparing to the unexposed coating); RatioMelamine-QUVA – normalised melamine band height in exposed coating; RatioMelamine-STD – normalised melamine band height in unexposed coating. Using a method similar to the OH band area ratio error evaluation explained before; the RatioMelamine values are found to be relatively consistent (0.993 0.007). Although the use of uncorrected peak height ratio is usually not as accurate as the base-line fitted peak height ratio, the data accuracy won’t be affected in this case. This is mainly due to the fact that none of the individual points shown in Fig. 4b is discussed; all conclusions are based on the entire data trend. It is observed from Fig. 4b that the Loss of MEL% value decreases with sampling depth, but increases with QUV-A exposure time. However the Loss of MEL% values for coating after 2000, 2500 and 3000 h QUV-A exposure are quite similar. These observations indicate a higher melamine degradation rate in the initial QUV-A exposure stage. After 2000 h QUV-A exposure, most of the sensitive melamine groups (e.g., melamine side chains, polyestermelamine linkages) are degraded; the degradation slows down. It is also observed that the coating surface region undergoes more degradation than the deeper coating layers; this is in good agreement with the OH% results. However, the OH% and MEL% trends also show different features. For example, the OH% trends (Fig. 4a) show constantly high values within a depth of approximately 12 mm after 3000 h QUV-A exposure. On the other hand, the Loss of MEL% (Fig. 4b) shows high values only near the coating surface; this value steadily decreases with sampling depths. This observation indicates that the melamine side chain C–N bond is more sensitive to the combination of moisture and UV light than the melaminepolyester bond and the polyester backbone. The trends of amine/amide groups (amine/amide%) shown in Fig. 4c follow the Loss of MEL% method with the HMelamine replaced by HAmine/amide (height of the amine/ amide band at 1630 cm1). The error evaluation of the HAmine/amide also follows the previous method; the average value of the amine/amide to CH height ratio is 0.58 0.06. The formation of amine/amide groups is mainly due to the melamine ring and side chain decomposition. It is observed from Fig. 4c that the amine/amide% values increase with QUV-A exposure time and decrease with sampling depth. This is also correlates well with the OH% and MEL% observations and indicates more amide/amine groups are formed near the coating surface due to the moisture enhanced degradation, as expected.
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Therefore, the SSPA-FTIR depth profiling method developed here has been demonstrated to be able to monitor the general degradation trends in the coating at different depths. However, note that the SSPA-FTIR results shown in this work are bulk depth profiles; this means the signal comes from the entire sampling depth (i.e., thermal diffusion depth [7]). Moreover, the film surface roughness also changes with weathering [3,19,25]. Such change will not affect the conclusions reached in this paper as all discussions are based on the entire degradation trend. In order to obtain a higher resolution depth profile, CRM was used to monitor the degradation in clear coating layer by layer. 3.2. CRM depth profiling The CRM depth resolved spectra collected at a step size of 2 mm in clear coating after 3000 h QUV-A exposure is shown in Fig. 5. An unexposed CRM spectrum of clear coating is also included. A general band assignment for the CRM spectra of clear coating is summarised in Table 4. The band at 950 cm1 is chosen as the internal reference as it is relatively stable during exposure and is adjacent to the melamine ring band (980 cm1). It is observed from Fig. 5 that the melamine side chain C–N/C–O (1550 cm1/1390 cm1) and methoxy groups (910 cm1) are significantly degraded after 3000 h QUV-A exposure. The melamine ring is also affected but shows better stability than the melamine side chains. This suggests that the melamine side chain is more sensitive to the UV light and moisture than melamine rings. It is also observed that the melamine ring band and residual methoxy band both show increasing intensity (the internal reference band is normalised) with sampling depth. This observation suggests that the melamine ring and side groups undergo more rapid degradation near the coating
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Table 4 Band assignment for CRM spectra [26–30]. Band position (cm1)
Assignment
1550 1390 980 910
Melamine side chain C–N vibration Melamine side chain C–O vibration Melamine ring vibration Residual methoxy group (OCH3) vibration
surface. As the melamine side chain (C–N/C–O) bands are weak in the CRM spectra, their changes at different sampling depth cannot be precisely monitored. In order to monitor the trend of melamine degradation at different depths, as well as QUV-A exposure time, a quantification method is introduced here. The ratio of band area of melamine (980 cm1) and internal reference (950 cm1) were calculated to trace the trend of melamine degradation in clear coat, as shown below:
RatioMEL ¼ AMEL =AREF
(5)
where, RatioMEL is the normalised melamine band intensity; AMEL and AREF are the melamine and reference band areas, respectively. As there is no signal saturation effect in Raman spectra acquired at different sampling depths, the RatioMEL value is directly used to monitor the melamine degradation here. The RatioMEL values for the unexposed and exposed coatings at different sampling depths, as calculated using Eq. (5), are shown in Fig. 6. The data fluctuations observed here are mainly due to low Raman signal intensity. The Raman signal becomes weaker with increasing sampling depth; therefore, the signal fluctuation as observed in Fig. 6 is greater with a sampling depth over 10 mm. As discussions constructed in this paper are associated with the entire trend rather than any individual points, this signal fluctuation near surface layers won’t affect the data interpretation.
Fig. 5. Overlaid CRM spectra of the clear coating after 3000 h QUV-A exposure, as collected at different sampling depth (spectrum of the unexposed clear coating is also included).
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Fig. 6. Quantitative CRM depth profiling of clear coating before and after QUV-A exposure.
It is observed that the melamine distribution in the unexposed clear coating is relatively even; the RaitoMEL value is in a range of 1.4-1.6. During the first 500 h exposure, melamine starts to degrade especially within a depth of w10 mm from the coating surface. The RatioMEL value in the deeper layers is similar to that of the unexposed clear coating. Further drop of RaitoMEL value within a depth of 10 mm is observed in coatings after 1000 h exposure; however, no considerable drop of RatioMEL value is observed in deeper layers until 2000 h. This observation is found to be different from the SSPA-FTIR results (Fig. 4b), and is due to their different signal origins. Signals obtained using SSPA-FTIR are an average value through the entire sampling depth, when the CRM results show no melamine degradation in coating with a depth of 10 mm, considerable degradation can be found from SSPA-FTIR results due to the average signal originated from the entire sampling depth (i.e. 10 mm). On the other hand, CRM signals are originated mainly from the desired sampling depth. Therefore, CRM is expected to have higher depth resolution. The CRM results reinforce the previous hypothesis: coating surface is more likely to undergo the moisture enhanced photo-oxidation degradation. Moreover, as the deeper part of coating (10-20 mm) is not affected before 1000 h QUV-A exposure; it suggests melamine degradation is highly affected by the quantity of moisture permeated into the clear coating. It is believed that the combination of UV light and moisture can cause more rapid degradation than the UV light itself. Therefore, it is suggested that the lower degradation rate of melamine in deeper layers is mainly due to lower moisture level; in other words, the top coating layer protects deeper layers from moisture enhanced degradation. It has to be noted that the peroxide radical formation requires oxygen. Therefore, the higher oxygen level near the coating surface will also contribute to the accelerated degradation. That is the diffusional control of the degradation depends on the film retaining its integrity until a critical point, at which stage the moisture and aldehydes penetrate further into the film setting it up for catastrophic failure. Alternatively, there is possibility that melamine near the coating surface region is more likely to be removed from the coating system when all of the melamine-polyester linkages are cleaved. This may result in more significant melamine loss near the coating surface.
When comparing the melamine degradation after 2500 and 3000 h in Fig. 6; it is observed that the surface degradation is similar, most of degradation occurs in the deeper layers. This may well indicate that as more moisture permeates into coating, deeper layers start to be affected by the moisture enhanced photo-oxidation that leads to more rapid degradation. Moreover, the formaldehyde formed near the surface can permeate into deeper layers together with moisture; this will enhance the concentration of free radicals in deeper layers and leads to coating degradation. On the other hand, the coating surface degradation slows down; mainly due to the fact that most of the radical sensitive functional groups are already degraded. This observation is different from SSPA-FTIR results shown in Fig. 4 (i.e. sample exposed for 2500 and 3000 h shows similar degradation profiles) and is again due to the different signal origins of the two techniques explained before. Another observation from the above results is that the second coating layer (i.e., the bottom of clear coating) shows relatively good stability. It is known that titanium dioxide can catalyse photo-oxidation in spite of silica coating stabilisation [3]. Titanium dioxide can produce free radicals when exposed in the environment with UV light and moisture; this will lead to accelerated coating degradation. However, the titanium dioxide pigment in the second coating layer may be effectively stabilised due to the lower moisture level. Overall, the above SSPA-FTIR and CRM findings suggest the higher degradation rate found near the coating surface is mainly due to the higher level of moisture. That is the low molecular weight chromophores (e.g., formaldehyde) formed through the hydrolysis of melamine may absorb UV light and produce peroxycarboxylic acids. This may make the coating surface more hydrophilic, which leads to more moisture absorption (i.e. hydrolysis enhanced photo degradation). The decomposition of the peroxycarboxylic acids and formaldehyde may also produce free radicals that enhance photo-oxidation of coating; on the other hand, the acids formed may enhance the hydrolysis of polyesters (i.e. photo enhanced hydrolysis degradation). As the QUV-A cabinet was set up to simulate a natural environment (i.e. dry/light – wet/dark) cycles; the coating actually undergoes the following degradation stages: (a). dark period – hydrolysis degradation (note, the photooxidation degradation products can make coating surface more hydrophilic that can accelerate the hydrolysis process); (b). “wet” light period – moisture enhanced photo degradation (during the initial light period, the moisture absorbed by coating evaporates slowly); (c). “dry” light period – photo-oxidation (most of moisture absorbed by coating evaporates leaving a relatively dry film). It is widely agreed UV light is playing a more important role in polymer coating degradation than moisture when only one factor is considered. It is then believed that after the degradation of most UV sensitive molecules in the clear coat during the light period, the overall surface degradation slows down as photo enhanced hydrolysis no longer plays a key role in accelerating degradation process. This effectively slows down the hydrolysis degradation process (as coating degrades very slowly with moisture only) during the dark
W.R. Zhang et al. / Polymer Testing 31 (2012) 855–863
period and inhibits hydrolysis enhanced photo degradation mechanism during the “wet” light period. Hence, the higher overall degradation rate near coating surface observed during the initial QUV-A test is due to the higher concentration of radicals and acids that cause accelerated radical attack and hydrolysis of coating.
Acknowledgements
4. Conclusions
References
The SSPA-FTIR and CRM have been demonstrated to be very suitable for monitoring degradation depth profile of clear coating before and after weathering. The nondestructive nature of the two techniques allows analyses to be carried out on the same sample. Moreover, the effect of different signal origins of SSPA-FTIR and CRM on their accuracy is also explained. It is generally found that the degradation depth profile in the coating can be precisely monitored using the band area/height ratio methods developed in this work. The results obtained from the SSPA-FTIR and CRM are in agreement in demonstrating coating degradation depth profile trend. Inconsistency between them is also observed and can be explained by their different signal natures. It is then concluded that CRM has a higher depth resolution than SSPA-FTIR. However, the collection of data using CRM at 1 mm interval though the coating thickness is time consuming. A model developed to predict CRM degradation trend with minimum data collection required, will be discussed in a forthcoming paper [31]. All findings suggest that significant degradation only occurs near the coating surface; deeper layers are less affected. This is explained by the moisture-enhanced photooxidation that is more likely to affect the coating surface in direct contact with moisture. Hence, the deeper layers are well protected due to the lower level of moisture. After a certain period of exposure (in this case, 2500 h), surface degradation slows down and deeper layer degradation speeds up. This is believed to be due to the permeation of moisture and surface degradation by-products (e.g. formaldehyde) leading to increasing concentration of moisture and radicals that accelerate degradation in deeper layers. The degradation mechanisms are also proposed and explained in this paper. It is also found that melamine side groups degrade prior to the melamine ring degradation. Moreover, the second white coating layer shows better durability. This may be due to the inhibition of the titanium dioxide activity when the titanium dioxide particles are not in direct contact with moisture; hence, preventing the formation of hydroxyl and peroxide radicals.
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The authors would like to thank Mr. James Maxted (Becker Industrial Coatings, UK) for useful helps and discussions.
[1] M.W. Urban, Prog. Org. Coat. 16 (1998) 321–353. [2] H.A. Szymanski (Ed.), Raman Spectroscopy: Theory and Practice, Plenum Press, New York, 1967. [3] W.R. Zhang, S.J. Hinder, R. Smith, C. Lowe, J.F. Watts, J. Coat. Tech. and Res. 8 (2011) 329–342. [4] S.R. Leadley, J.F. Watts, M.M. Chehimi, C. Lowe, Surf. Interface. Anal. 26 (1998) 444–454. [5] T. Hirayama, M.W. Urban, Prog. Org. Coat. 20 (1992) 81–96. [6] C. Perruchot, J.F. Watts, C. Lowe, G. Beamson, Int. J. Adhes. Adhes. 23 (2003) 101–113. [7] W.R. Zhang, C. Lowe, R. Smith, Prog. Org. Coat. 65 (2009) 469–476. [8] W.R. Zhang, C. Lowe, R. Smith, Prog. Org. Coat. 66 (2009) 141–148. [9] W.R. Zhang, R. Smith, C. Lowe, J. Coat. Tech. and Res. 6 (2009) 315–328. [10] W.R. Zhang, T.T. Zhu, R. Smith, C. Lowe, Prog. Org. Coat. 69 (2010) 376–383. [11] L. Gonon, J. Mallegol, S. Commereuc, V. Verney, Vib. Spectrosc. 26 (2001) 43–49. [12] J.L. Dupuie, W.H. Weber, D.J. Scholl, J.L. Gerlock, Polym. Degrad. Stab. 57 (1997) 339–348. [13] D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C. Paputa Peck, R.O. Cater III, Polym. Degrad. Stab. 27 (1990) 271–284. [14] D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C. Paputa Peck, R.O. Cater III, Polym. Degrad. Stab. 28 (1990) 39–51. [15] C. Decker, K. Zahouily, Polym. Degrad. Stab. 64 (1999) 293–304. [16] M.E. Nichols, J.L. Gerlock, C.A. Smith, Polym. Degrad. Stab. 56 (1997) 81–91. [17] M.E. Nichols, J.L. Gerlock, Polym. Degrad. Stab. 69 (2000) 197–207. [18] K. Adamsons, Prog. Polym. Sci. 25 (2000) 1363–1409. [19] T. Nguyen, J. Martin, R. Byrd, N. Embree, Polym. Degrad. Stab. 77 (2002) 1–16. [20] C. Mura, J. Yarwood, R. Swart, D. Hodge, Polymer 41 (2000) 8659– 8671. [21] P. Schmidt, M.R. Fernandez, J.M. Pastor, J. Roda, Polymer 38 (1997) 2067–2075. [22] T. Nguyen, J. Martin, R. Byrd, J. Coat. Technol. 75 (2003) 37–50. [23] T. Nguyen, F. Martin, R. Byrd, N. Embree, Polym. Degrad. Stab. 77 (2002) 1–16. [24] P. Delorme, J. Lemair, F. Carrara, C. Bonnebat, Photooxidation Mechanisms of Aliphatic Crosslinked Polyesters Used for Flat Steel Coatings. American Institute of Physics, Proceeding of the 53rd International Meeting of Physical Chemistry, Paris, France, 354 (Jan 2–6 1996) 413–423. [25] X. Gu, D. Raghavan, T. Nguyen, M.R. VanLandingham, D. Yebassa, Polym. Degrad. Stab. 71 (2001) 139–149. [26] W.J. Jones, W.J. Orville-Thomas, Trans. Faraday. Soc. 55 (1959) 203–210. [27] M.L. Scheepers, R.J. Meier, J.M. Gelan, D.J. Vanderzande, B.J. Kip, Vib. Spectrosc. 9 (1995) 139–146. [28] P.J. Larkin, M.P. Makowski, N.B. Colthup, L.A. Flood, Vib. Spectrosc. 17 (1998) 53–72. [29] M.K. Marchewka, Mater. Sci. Eng. B 95 (2002) 214–221. [30] M.K. Marchewka, Bull. Kor. Chem. Soc. 25 (2004) 466–470. [31] W.R. Zhang, T.T. Zhu, R. Smith, C. Lowe, submitted to Polym. Test. 2012.
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Analysis method
A non-destructive study on the degradation of polymer coating I: Step-scan photoacoustic FTIR and confocal Raman microscopy depth profiling W.R. Zhang a, *, T.T. Zhu a, R. Smith a, C. Lowe b a b
School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London E1 4NS, UK The Long Term Development Group, Becker Industrial Coatings, Goodlass Road, Speke, Liverpool L24 9HJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 May 2012 Accepted 3 July 2012
A polyester-melamine coating was exposed to accelerated weathering (i.e., cycle of high UV followed by condensation). The degradation depth profile in the coating was investigated using non-destructive step-scan photoacoustic fourier transform infrared spectroscopy (SSPA-FTIR) and confocal Raman microscopy (CRM). The degradation at different sampling depths was then evaluated following the method developed in this work. The results obtained using the two techniques correlate well with each other and suggest coating surface is more likely to undergo moisture enhanced photo-oxidation degradation. This may be due to the higher moisture level near the coating surface. It is also found that melamine side chains degrade prior to the melamine ring. The quantification methods developed in this work were found to be very suitable to monitor the degradation depth profile in polymeric coatings. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Polyester-melamine coating Degradation Step-scan photoacoustic FTIR Confocal Raman microscopy Depth profiling
1. Introduction Coil coating is a high-speed continuous process for the application of paint onto metal substrates prior to fabrication. Most coil coated products are used outdoors, which can mean harsh environments (e.g. high UV light intensity and high moisture level). Such conditions may lead to both moisture induced and photo-induced degradation. Among various coil coating systems (e.g., polyester-melamine, polyurethane, polyvinylidene fluoride, polyvinyl chloride, etc.), polyester-melamine coil coatings still dominate the market due to their low cost, good formability and adequate weathering performance. The chemistry of polyester-melamine coil coating involves a cross-linking (trans-etherification) reaction of hydroxyl groups on polyester molecules and methoxy groups on melamine side
* Corresponding author. Present address: AkzoNobel, Wexham Rd., Slough SL2 5DS, UK. Tel.: þ44 (0)1753 879286. E-mail address: [email protected] (W.R. Zhang). 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2012.07.002
chains. The molecular structures of most commonly used melamine (i.e., hexa-methoxy methyl melamine (HMMM) and melamine with lower degree of methylation) are shown in Scheme 1. It is known that both melamine and polyester molecules can be sensitive to UV light and moisture; the degradation of polyester and melamine or even polyester-melamine linkages may result in the collapse of cross-link network and finally lead to coating failure. If durability is to be improved, it is very important to investigate the mechanisms behind the coating degradation in detail. This has stimulated the development and application of new techniques to study polymer coating degradation. Many useful techniques have been previously applied to the study of coating degradation. The use of FTIR with different sampling techniques, for example, transmission, attenuated total reflection (ATR) and photoacoustic (PA), to interrogate polymer coatings has long been common practice [1]. Raman spectroscopy has also demonstrated some advantages for studying polymeric materials [2]. X-ray photo-electron spectroscopy (XPS) [3] and time-of-flight
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Scheme 1. Molecular structure of melamine; (a) HMMM; (b) Melamine with lower degree of methylation.
secondary ion mass spectroscopy (ToF-SIMS) have been proven excellent for polymer coating surface chemistry investigations [4]. More recently, the focus of polymer coating research has moved towards small volume analysis and depth profiling studies. This is because coating formulations are mixtures of different components; the distribution of certain components within the final film usually controls the coating properties (e.g., chemical resistance, adhesion and mechanical properties). Techniques such as ATR-FTIR [5] and angle resolved XPS [6] were previously utilised to achieve depth profiles of polymeric coatings in the near surface region (from microns down to nanometres). Step-scan photoacoustic FTIR (SSPA-FTIR) and confocal Raman microscopy (CRM) are both able to provide a high signal strength from the bulk of polymers, the sampling depths ranging from a few microns to tens of microns. Detailed investigations into the use of SSPA-FTIR and CRM in the assessment of polyester-melamine coil coatings have been previously published [7–10]; the results show that both SSPA-FTIR and CRM are suitable for depth profiling of coil coatings. Works reported by Gonon et al. [11] and Dupuie et al. [12] also show that both SSPA-FTIR and CRM are suitable for the depth profiling analysis of polymer degradation. Quantification methods utilising traditional FTIR and Raman spectroscopic techniques have been well established. For example, Bauer et al. found the carbonyl band area in the transmission FTIR spectra to be linearly correlated to the coating degradation level. Moreover, the area ratio of the carbonyl band before and after UV exposure was used to monitor the coating photo-oxidation rate [13,14]. Decker and Zahouily introduced a method that involved using the ratios of band areas in the IR spectra before and after QUV-A exposure to evaluate the formation/decomposition of various functional groups (e.g. –COOH, –C]O, –C–NH) in the coating. They also found a strong correlation between the band area ratio and photo-degradation rate [15]. Work reported by Nichols et al. and Adamsons suggests that the amount of degradation in coatings can be also analysed using the ratio of the IR absorbance in the 3800–2000 cm1 region (–OH and –NH regions) to that in the 3100–2800 cm1 region (CH2/ CH3 regions) [16–18]. Nguyen et al. showed that the band absorption intensity in IR spectra could be used as an indication of the concentration of functional groups in polymer coatings [19]. Moreover, Raman spectroscopy was previously demonstrated to be suitable for chemical
concentration analysis; both the band area [20] and intensity ratios [21] could be used to monitor the distribution of functional groups in polymer coatings. Although extensive progress has been made in the use of both SSPA-FTIR and CRM, quantitative depth profiling analysis of degraded coatings is less well developed. Most of the applications using SSPA-FTIR and CRM are based on qualitative analysis of the distribution of components in layered polymer films. Polymer degradation quantification methods using traditional IR & Raman techniques have been well established, but lack of depth profiling capabilities has restricted their further application in this field. This paper is mainly focused on developing and applying new SSPA-FTIR and CRM depth profiling methods to a polymer coating before and after degradation. Both band area and height ratio were used to quantify the degradation trend of functional groups in the coatings. At first sight, the results demonstrate the obvious: that degradation takes place primarily in the surface region of the coating. In the past it has been associated with photo-oxidation and moisture enhanced photo-oxidation. Our results show that the degradation occurs in the side chains of the melamine first followed by the rest of the molecule. The main conclusion, however, is that degradation appears to be limited to the top 7-8 microns for the first 2000 h of exposure in an accelerated weathering cabinet. Only subsequently does catastrophic degradation occur throughout the coating. The depth profiling methods developed in this work were found to be very suitable to monitor the degradation depth profile throughout the polymer coating. 2. Experimental 2.1. Materials and method The clear coil coating investigated in this work was mainly based on an aliphatic polyester resin and melamine crosslinking agent. The polyester to melamine weight ratio was 4:1 (higher than stoichiometric ratio) to ensure adequate cross-link density in the cured thermoset coating. Hence, it is expected that around 20-30% of methoxy groups are not reacted with polyester during the curing process. The melamine used was a mixture of HMMM and lower methylation melamine (Scheme 1) with a ratio of 2:1. The melamine with lower degree of methylation is expected to react quicker with polyester.
W.R. Zhang et al. / Polymer Testing 31 (2012) 855–863
The substrate used involved a layer of primer and a layer of white topcoat on the hot-dip galvanised (HDG) steel substrate. The white topcoat was based on the same resin system as the clear coating but with titanium dioxide pigment. The primer coating was based on standard polyester-melamine primer coating system with strontium chromate pigment. The white coating was applied to avoid early primer coating failure (primer is usually based on a polyester resin with lower durability compared to that used in topcoat formulations), which may affect the accuracy of clear coating durability evaluation. For reasons of commercial confidentiality, only a general description of the samples involved in this work is provided (Table 1). The wet clear coating was applied using a standard laboratory draw down method. A wire wound steel bar with suitable diameter was used to draw down the wet coating to achieve a dry film thickness of w20 mm. The panel was then cured in a convection oven for 30 s to achieve a peak metal temperature (PMT) of 232 C, in order to ensure a correct degree of cure. The white and primer coatings were both cured separately prior to the clear coat application. Samples with a diameter of w8 mm were punched from the panel for SSPA-FTIR and CRM studies. 2.2. SSPA-FTIR instrumentation All SSPA-FTIR spectra were recorded using a Nicolet 8700 research grade FTIR spectrometer (Madison, USA) with a MTEC model 200 photoacoustic (PA) cell (Ames, USA). The PA cell was purged with dry helium at a rate of 20 cm3/s for 5 min before any spectra acquisition. The step-scan experiments were performed by varying the modulation frequency of IR beam at constant amplitude of 3.5lHeNe. Spectra were collected at mid-IR range (400-4000 cm1) with a resolution of 8 cm1. The correlation between the IR modulation frequency and sampling depth is provided in Table 2; based on taking the thermal diffusivity of clear coat as 0.01 105 m2 s1 (a general value for most polymeric materials, a more detailed sampling depth calculation can be found elsewhere [7]). Note that the thermal diffusivity value may change with weathering due to molecular degradation; however, such change is minor and won’t affect the main discussion in this paper. A carbon black reference material was used as a strong surface absorber to calibrate the instrument. The in-phase (I) spectra were used as a direct indication of the chemical composition within the corresponding sampling depths. All SSPA-FTIR spectral data processing was accomplished using Omnic 7.3 software. 2.3. CRM instrumentation All Raman spectra were recorded using a Nicolet Almega research grade visible dispersive spectrometer (Madison, USA) with 785 nm laser excitation. A 100 dry objective Table 1 Sample information. Coating layer Resin system Clear White Primer
Pigment Thickness
Aliphatic Polyester þ Melamine N/A Aliphatic Polyester þ Melamine TiO2 Phthalate Polyester þ Melamine SrCrO4
w20 mm w20 mm w3 mm
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Table 2 IR sampling depth at different modulation frequencies. Modulation frequency (Hz)
IR sampling depth (mm)
50 100 200 400 600 800 1000
25 18 13 9 7 6 5.6
(NA ¼ 0.90, Olympus) was used to perform non-destructive CRM depth profiling experiments. All Raman spectral data processing was accomplished using Ominc 7.3 software. 2.4. Accelerated weathering exposure The clear coating was exposed in a QUV-A chamber (The Q-Panel Company, USA) equipped with QUV-A-340nm fluorescent tubes (295 nm – 400 nm) each with peak irradiance at 340 nm. The sample panel was irradiated for 8 h each 12 h cycle at 60 C. During the dark period (4 h), the panel was subjected to a moist environment at a temperature of 50 C with moisture level reaching 100% so that condensation appeared on the panels. During the light period, the moisture level decreases slowly from 100% to around 30%. A 8 mm diameter disc was punched from the sample panel after total exposure times of 500, 1000, 2000, 2500 and 3000 h. The sample discs were used for the SSPA-FTIR and CRM analyses. 3. Results and discussion 3.1. SSPA-FTIR depth profiling The SSPA-FTIR 800 Hz spectra (sampling depth: w6 mm) of the clear coat before (STD) and after of QUV-A exposure are shown in Fig. 1. It is observed that the clear coating generally undergoes hydrolysis and photo-oxidation degradation during the QUV-A exposure. The increase in –OH/–NH group region (w3600-3000 cm1) is mainly due to the hydrolysis/photo-oxidation of both the polyester and the melamine. The broadening of the carbonyl band increase of amine/amide band (w1730 cm1), (w1630 cm1) intensity and decrease of melamine (w1550 cm1) and residual methoxy (w1085 cm1 and 913 cm1, unreacted methoxy groups on melamine side chain) band intensities, indicate photo-induced degradation of the polyester and especially the melamine side groups and melamine-polyester linkages. The observations are in good agreement with previous findings [19,22,23,25]. A general band assignment is provided in Table 3 to assist the spectral interpretation. It is also observed from Fig. 1 that the hydrocarbon bands in the 2700 cm1 – 3000 cm1, 1450 cm1 and 1390 cm1 regions are relatively stable after exposure. Therefore, they are considered as the internal reference bands when evaluating the changes of intensity of the other bands (i.e., hydroxyl and melamine bands). The SSPA-FTIR spectra collected at different sampling depths after 3000 h QUV-A exposure are shown in Fig. 2.
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Fig. 1. SSPA-FTIR spectra of clear coating before (STD) and after QUV-A exposure.
The (I) spectra were taken as a direct indication of the chemical composition of clear coating at different sampling depths; the detailed development of this depth profiling approach can be found in a previous paper [7]. The hydrocarbon bands (1450 cm1 and 1390 cm1) in each spectrum are normalised to have approximately the same intensity. It is then observed that the melamine band at 1550 cm1 shows increasing intensity with the sampling depth. This observation suggests the degradation of melamine is more significant near the coating surface region. It is also observed that the bands in the region 1300 cm1 – 1175 cm1 show rapidly decreasing intensity with sampling depth. However, this is not related to the functional group distribution in the coating but due to the signal saturation (i.e., the carbonyl band at 1730 cm1 and C–O bands at 1300 cm1 – 1175 cm1 signals saturate after approaching approximately 6 mm below sample surface, due to their stronger IR absorption). When the C]O and C– O signals saturate, the detector can no longer sense the change in their thermal propagation at different sampling depths. However, the signal of the other bands are not
saturated so that the detector can still sense the change in band intensity at a deeper sampling depth, this makes it appear as C–O band intensity decrease when the intensity of hydrocarbon bands are normalised in Fig. 2. However, this effect will not affect the discussion in this work as the hydroxyl and melamine bands together with reference bands selected (hydrocarbon at 2800 cm1 – 3000 cm1, 1450 cm1 and 1390 cm1) are not saturated. Moreover, as the internal reference bands selected in this work are all adjacent to the band of interest (e.g., hydroxyl and melamine bands); this will further minimise any instrument related effects. Although differences in melamine degradation can be observed from spectra shown in Fig. 2, this is only a general qualitative comparison. A quantitative depth profile is preferable to reveal the coating degradation. As reviewed
Table 3 Band assignment [22–24]. Band position (cm1)
Assignment
3600-3000
Hydroxyl (OH) and amine/amide (NH) vibration Hydrocarbon (CH2/CH3) stretching Carbonyl (C]O) stretching Amine/amide vibration Melamine ring and side chain C–N stretching Hydrocarbon (CH2/CH3) bending Residual methoxy group (O–CH3) stretching
3000-2700 1730 1630 1550 1450/1390 1085/913
Fig. 2. SSPA-FTIR spectra of clear coating after 3000 h QUV-A exposure, as collected at different sampling depth.
W.R. Zhang et al. / Polymer Testing 31 (2012) 855–863
previously, the hydroxyl and melamine band intensities can be used to track hydrolysis and photo-induced degradation in coatings. The amount of hydroxyl functionality in the coating is determined using a band area normalisation method described in Eq. (1). It needs to address that, as the –NH band (3300-3500 cm1) is usually narrower than the –OH band, the broadened band region after weathering is considered to be mainly due to hydroxyl groups. The ACH and AOH were determined following the baseline fitting method as shown in Fig. 3 (note AOH is the area between hydrocarbon and hydroxyl groups). In order to minimise the effect of signal saturation, the RatioOH was calculated for both the unexposed (RatioOH-STD) and degraded (RatioOHQUVA) clear coat at each of the selected sampling depths. The OH% (hydroxyl group band increment) values at different sampling depth are then calculated individually following Eq. (2) to represent the degradation depth profile in the coating.
RatioOH ¼ AOH =ACH
859
that the hydroxyl groups increase with QUV-A exposure time. The increment rate within the first 1000-2000 h (w200-400%) is faster than the following 1000 h (w50100% per 1000 h). This indicates that more UV and moisture sensitive groups in the coating were degraded in the initial QUV-A exposure stage. This is very likely to be partly due to them becoming exhausted after longer periods of exposure (i.e., more than
(1)
where, RatioOH – ratio of hydroxyl group area to hydrocarbon group area; AOH – hydroxyl group band area (approximately 3600 cm1 – 2200 cm1); ACH – hydrocarbon band area (approximately 3100 cm1 – 2750 cm1).
OH% ¼ ðRatioOHQUVA RatioOHSTD ÞRatioOHSTD %
(2)
where, OH% – formation of hydroxyl group due to degradation (i.e., percentage increase in hydroxyl group band region in spectrum of an exposed coating compared to that in the spectrum of an unexposed coating); RatioOH-QUVA – hydroxyl group to hydrocarbon band area ratio in the exposed coating; RatioOH-STD– hydroxyl group to hydrocarbon band area ratio in the unexposed coating. It is known that error might be introduced through the baseline fitting or IR spectrometer itself. In order to verify the accuracy of the quantification, ten spectra were collected from the same coating sample; the RatioOH values were found to be consistent (i.e. 2.82 0.03). The increase of hydroxyl groups after QUV-A exposure, as calculated using the method demonstrated above, is shown in Fig. 4a; the OH% values were recorded at different QUV-A hours and IR sampling depths. It is firstly observed
Fig. 3. Hydroxyl and hydrocarbon band area measurement in SSPA-FTIR spectra.
Fig. 4. SSPA-FTIR quantitative depth profile of the clear coating (colour control bar represents the value of vertical axis): (a) 3-D plot of increase of OH% (vertical axis), QUV-A exposure time and sampling depth; (b) 3-D plot of loss of MEL% (vertical axis), QUV-A exposure time and sampling depth; (c) 3-D plot of increase of amine/amide% (vertical axis), QUV-A exposure time and sampling depth.
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2000 QUV-A hours), which results in decreasing hydroxyl group formation rate in the clear coating. Moreover, the diffusion control can exert itself and, until a certain level of degradation is reached, the molecules (oxygen or water) find it more difficult to access the reaction front. It is also observed that the OH% value (w100%) obtained at 500 h exposure is relatively constant throughout the coating thickness. However, significantly higher OH% value is observed away from the coating surface region (w12 mm) only after longer exposure. In other words, the difference in OH% values between depths of 12 mm and 18 mm in the clear coating after 1000 h exposure is approximately 200%; this difference becomes 400% in the clear coating after 3000 h exposure. This is consistent with the diffusional control hypothesis presented above. A possible explanation for the above observations is moisture-enhanced degradation. Nguyen et al. has previously found that melamine methylols can be produced through the hydrolysis of melamine-acrylate (or melamine-methoxy) linkages. The melamine methylols can be then deformylated to form primary amines (–HN2) and aldehydes (–CH2O). Aldehydes undergo photo-induced decomposition and produce CHO and H radicals which will then enhance photo-oxidation of melamine side chains; moreover, photo-oxidation of aldehydes can also form peroxycaboxylic acid (HCO–OOH) which accelerates the hydrolysis of melamine-acrylate (or melaminemethoxy) linkages [22,23]. This moisture-enhanced degradation is effectively a self-catalysed degradation mechanism, which is more likely to occur near the coating surface region as it is in direct contact with moisture during the QUV-A exposure and diffusion may not be a limiting factor. For the initial 500 h, the degradation rate may be dominated by the photo-induced degradation caused by the high intensity UV irradiation. As the clear coat is transparent, UV radiation will penetrate throughout the film thickness leading to the photo-degradation, even in the deeper part of the coating. This is another fact that supports the diffusional control hypothesis. After longer exposure, a higher level of moisture is absorbed within a depth of approximately 10 mm causing moisture enhanced photo-degradation and, hence, more rapid coating degradation is observed within that region. Beyond 1000 h, the reactions result in the generation of more hydroxyl functionality both in the top 10 microns and the rest of the coating, indicating that the top few microns have broken down, reducing the limiting effect of diffusional control. In other words, moisture, oxygen and formaldehyde have a much less restricted path to the lower reaches of the film after 1000 h because the degradation has gone beyond a critical point. An estimate of melamine decomposition in the clear coating after QUV-A exposure is shown in Fig. 4b. Because the baseline at w1630 cm1 in the SSPA-FTIR changes due to the formation of amine/amide groups after melamine and/or melamine side chain decomposition, the band area measurements are highly affected by this baseline change. Thus, the heights (without baseline fitting) of the melamine band in SSPA-FTIR spectra are used to evaluate the amount of melamine remaining in clear coating; the ratio method is shown below:
RatioMelamine ¼ HMelamine =HCH
(3)
where, RatioMelamine – normalised height of melamine band; HMelamine – melamine band height at w1550 cm1; HCH – hydrocarbon band height at w1450 cm1 (spectral internal reference).
Loss of MEL% ¼ ðRatioMelamineSTD RatioMelamineQUVA Þ=RatioMelamineSTD %
(4)
where, Loss of MEL% – decomposition of melamine group due to the QUV-A exposure (i.e., percentage decrease of melamine in exposed coating comparing to the unexposed coating); RatioMelamine-QUVA – normalised melamine band height in exposed coating; RatioMelamine-STD – normalised melamine band height in unexposed coating. Using a method similar to the OH band area ratio error evaluation explained before; the RatioMelamine values are found to be relatively consistent (0.993 0.007). Although the use of uncorrected peak height ratio is usually not as accurate as the base-line fitted peak height ratio, the data accuracy won’t be affected in this case. This is mainly due to the fact that none of the individual points shown in Fig. 4b is discussed; all conclusions are based on the entire data trend. It is observed from Fig. 4b that the Loss of MEL% value decreases with sampling depth, but increases with QUV-A exposure time. However the Loss of MEL% values for coating after 2000, 2500 and 3000 h QUV-A exposure are quite similar. These observations indicate a higher melamine degradation rate in the initial QUV-A exposure stage. After 2000 h QUV-A exposure, most of the sensitive melamine groups (e.g., melamine side chains, polyestermelamine linkages) are degraded; the degradation slows down. It is also observed that the coating surface region undergoes more degradation than the deeper coating layers; this is in good agreement with the OH% results. However, the OH% and MEL% trends also show different features. For example, the OH% trends (Fig. 4a) show constantly high values within a depth of approximately 12 mm after 3000 h QUV-A exposure. On the other hand, the Loss of MEL% (Fig. 4b) shows high values only near the coating surface; this value steadily decreases with sampling depths. This observation indicates that the melamine side chain C–N bond is more sensitive to the combination of moisture and UV light than the melaminepolyester bond and the polyester backbone. The trends of amine/amide groups (amine/amide%) shown in Fig. 4c follow the Loss of MEL% method with the HMelamine replaced by HAmine/amide (height of the amine/ amide band at 1630 cm1). The error evaluation of the HAmine/amide also follows the previous method; the average value of the amine/amide to CH height ratio is 0.58 0.06. The formation of amine/amide groups is mainly due to the melamine ring and side chain decomposition. It is observed from Fig. 4c that the amine/amide% values increase with QUV-A exposure time and decrease with sampling depth. This is also correlates well with the OH% and MEL% observations and indicates more amide/amine groups are formed near the coating surface due to the moisture enhanced degradation, as expected.
W.R. Zhang et al. / Polymer Testing 31 (2012) 855–863
Therefore, the SSPA-FTIR depth profiling method developed here has been demonstrated to be able to monitor the general degradation trends in the coating at different depths. However, note that the SSPA-FTIR results shown in this work are bulk depth profiles; this means the signal comes from the entire sampling depth (i.e., thermal diffusion depth [7]). Moreover, the film surface roughness also changes with weathering [3,19,25]. Such change will not affect the conclusions reached in this paper as all discussions are based on the entire degradation trend. In order to obtain a higher resolution depth profile, CRM was used to monitor the degradation in clear coating layer by layer. 3.2. CRM depth profiling The CRM depth resolved spectra collected at a step size of 2 mm in clear coating after 3000 h QUV-A exposure is shown in Fig. 5. An unexposed CRM spectrum of clear coating is also included. A general band assignment for the CRM spectra of clear coating is summarised in Table 4. The band at 950 cm1 is chosen as the internal reference as it is relatively stable during exposure and is adjacent to the melamine ring band (980 cm1). It is observed from Fig. 5 that the melamine side chain C–N/C–O (1550 cm1/1390 cm1) and methoxy groups (910 cm1) are significantly degraded after 3000 h QUV-A exposure. The melamine ring is also affected but shows better stability than the melamine side chains. This suggests that the melamine side chain is more sensitive to the UV light and moisture than melamine rings. It is also observed that the melamine ring band and residual methoxy band both show increasing intensity (the internal reference band is normalised) with sampling depth. This observation suggests that the melamine ring and side groups undergo more rapid degradation near the coating
861
Table 4 Band assignment for CRM spectra [26–30]. Band position (cm1)
Assignment
1550 1390 980 910
Melamine side chain C–N vibration Melamine side chain C–O vibration Melamine ring vibration Residual methoxy group (OCH3) vibration
surface. As the melamine side chain (C–N/C–O) bands are weak in the CRM spectra, their changes at different sampling depth cannot be precisely monitored. In order to monitor the trend of melamine degradation at different depths, as well as QUV-A exposure time, a quantification method is introduced here. The ratio of band area of melamine (980 cm1) and internal reference (950 cm1) were calculated to trace the trend of melamine degradation in clear coat, as shown below:
RatioMEL ¼ AMEL =AREF
(5)
where, RatioMEL is the normalised melamine band intensity; AMEL and AREF are the melamine and reference band areas, respectively. As there is no signal saturation effect in Raman spectra acquired at different sampling depths, the RatioMEL value is directly used to monitor the melamine degradation here. The RatioMEL values for the unexposed and exposed coatings at different sampling depths, as calculated using Eq. (5), are shown in Fig. 6. The data fluctuations observed here are mainly due to low Raman signal intensity. The Raman signal becomes weaker with increasing sampling depth; therefore, the signal fluctuation as observed in Fig. 6 is greater with a sampling depth over 10 mm. As discussions constructed in this paper are associated with the entire trend rather than any individual points, this signal fluctuation near surface layers won’t affect the data interpretation.
Fig. 5. Overlaid CRM spectra of the clear coating after 3000 h QUV-A exposure, as collected at different sampling depth (spectrum of the unexposed clear coating is also included).
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Fig. 6. Quantitative CRM depth profiling of clear coating before and after QUV-A exposure.
It is observed that the melamine distribution in the unexposed clear coating is relatively even; the RaitoMEL value is in a range of 1.4-1.6. During the first 500 h exposure, melamine starts to degrade especially within a depth of w10 mm from the coating surface. The RatioMEL value in the deeper layers is similar to that of the unexposed clear coating. Further drop of RaitoMEL value within a depth of 10 mm is observed in coatings after 1000 h exposure; however, no considerable drop of RatioMEL value is observed in deeper layers until 2000 h. This observation is found to be different from the SSPA-FTIR results (Fig. 4b), and is due to their different signal origins. Signals obtained using SSPA-FTIR are an average value through the entire sampling depth, when the CRM results show no melamine degradation in coating with a depth of 10 mm, considerable degradation can be found from SSPA-FTIR results due to the average signal originated from the entire sampling depth (i.e. 10 mm). On the other hand, CRM signals are originated mainly from the desired sampling depth. Therefore, CRM is expected to have higher depth resolution. The CRM results reinforce the previous hypothesis: coating surface is more likely to undergo the moisture enhanced photo-oxidation degradation. Moreover, as the deeper part of coating (10-20 mm) is not affected before 1000 h QUV-A exposure; it suggests melamine degradation is highly affected by the quantity of moisture permeated into the clear coating. It is believed that the combination of UV light and moisture can cause more rapid degradation than the UV light itself. Therefore, it is suggested that the lower degradation rate of melamine in deeper layers is mainly due to lower moisture level; in other words, the top coating layer protects deeper layers from moisture enhanced degradation. It has to be noted that the peroxide radical formation requires oxygen. Therefore, the higher oxygen level near the coating surface will also contribute to the accelerated degradation. That is the diffusional control of the degradation depends on the film retaining its integrity until a critical point, at which stage the moisture and aldehydes penetrate further into the film setting it up for catastrophic failure. Alternatively, there is possibility that melamine near the coating surface region is more likely to be removed from the coating system when all of the melamine-polyester linkages are cleaved. This may result in more significant melamine loss near the coating surface.
When comparing the melamine degradation after 2500 and 3000 h in Fig. 6; it is observed that the surface degradation is similar, most of degradation occurs in the deeper layers. This may well indicate that as more moisture permeates into coating, deeper layers start to be affected by the moisture enhanced photo-oxidation that leads to more rapid degradation. Moreover, the formaldehyde formed near the surface can permeate into deeper layers together with moisture; this will enhance the concentration of free radicals in deeper layers and leads to coating degradation. On the other hand, the coating surface degradation slows down; mainly due to the fact that most of the radical sensitive functional groups are already degraded. This observation is different from SSPA-FTIR results shown in Fig. 4 (i.e. sample exposed for 2500 and 3000 h shows similar degradation profiles) and is again due to the different signal origins of the two techniques explained before. Another observation from the above results is that the second coating layer (i.e., the bottom of clear coating) shows relatively good stability. It is known that titanium dioxide can catalyse photo-oxidation in spite of silica coating stabilisation [3]. Titanium dioxide can produce free radicals when exposed in the environment with UV light and moisture; this will lead to accelerated coating degradation. However, the titanium dioxide pigment in the second coating layer may be effectively stabilised due to the lower moisture level. Overall, the above SSPA-FTIR and CRM findings suggest the higher degradation rate found near the coating surface is mainly due to the higher level of moisture. That is the low molecular weight chromophores (e.g., formaldehyde) formed through the hydrolysis of melamine may absorb UV light and produce peroxycarboxylic acids. This may make the coating surface more hydrophilic, which leads to more moisture absorption (i.e. hydrolysis enhanced photo degradation). The decomposition of the peroxycarboxylic acids and formaldehyde may also produce free radicals that enhance photo-oxidation of coating; on the other hand, the acids formed may enhance the hydrolysis of polyesters (i.e. photo enhanced hydrolysis degradation). As the QUV-A cabinet was set up to simulate a natural environment (i.e. dry/light – wet/dark) cycles; the coating actually undergoes the following degradation stages: (a). dark period – hydrolysis degradation (note, the photooxidation degradation products can make coating surface more hydrophilic that can accelerate the hydrolysis process); (b). “wet” light period – moisture enhanced photo degradation (during the initial light period, the moisture absorbed by coating evaporates slowly); (c). “dry” light period – photo-oxidation (most of moisture absorbed by coating evaporates leaving a relatively dry film). It is widely agreed UV light is playing a more important role in polymer coating degradation than moisture when only one factor is considered. It is then believed that after the degradation of most UV sensitive molecules in the clear coat during the light period, the overall surface degradation slows down as photo enhanced hydrolysis no longer plays a key role in accelerating degradation process. This effectively slows down the hydrolysis degradation process (as coating degrades very slowly with moisture only) during the dark
W.R. Zhang et al. / Polymer Testing 31 (2012) 855–863
period and inhibits hydrolysis enhanced photo degradation mechanism during the “wet” light period. Hence, the higher overall degradation rate near coating surface observed during the initial QUV-A test is due to the higher concentration of radicals and acids that cause accelerated radical attack and hydrolysis of coating.
Acknowledgements
4. Conclusions
References
The SSPA-FTIR and CRM have been demonstrated to be very suitable for monitoring degradation depth profile of clear coating before and after weathering. The nondestructive nature of the two techniques allows analyses to be carried out on the same sample. Moreover, the effect of different signal origins of SSPA-FTIR and CRM on their accuracy is also explained. It is generally found that the degradation depth profile in the coating can be precisely monitored using the band area/height ratio methods developed in this work. The results obtained from the SSPA-FTIR and CRM are in agreement in demonstrating coating degradation depth profile trend. Inconsistency between them is also observed and can be explained by their different signal natures. It is then concluded that CRM has a higher depth resolution than SSPA-FTIR. However, the collection of data using CRM at 1 mm interval though the coating thickness is time consuming. A model developed to predict CRM degradation trend with minimum data collection required, will be discussed in a forthcoming paper [31]. All findings suggest that significant degradation only occurs near the coating surface; deeper layers are less affected. This is explained by the moisture-enhanced photooxidation that is more likely to affect the coating surface in direct contact with moisture. Hence, the deeper layers are well protected due to the lower level of moisture. After a certain period of exposure (in this case, 2500 h), surface degradation slows down and deeper layer degradation speeds up. This is believed to be due to the permeation of moisture and surface degradation by-products (e.g. formaldehyde) leading to increasing concentration of moisture and radicals that accelerate degradation in deeper layers. The degradation mechanisms are also proposed and explained in this paper. It is also found that melamine side groups degrade prior to the melamine ring degradation. Moreover, the second white coating layer shows better durability. This may be due to the inhibition of the titanium dioxide activity when the titanium dioxide particles are not in direct contact with moisture; hence, preventing the formation of hydroxyl and peroxide radicals.
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The authors would like to thank Mr. James Maxted (Becker Industrial Coatings, UK) for useful helps and discussions.
[1] M.W. Urban, Prog. Org. Coat. 16 (1998) 321–353. [2] H.A. Szymanski (Ed.), Raman Spectroscopy: Theory and Practice, Plenum Press, New York, 1967. [3] W.R. Zhang, S.J. Hinder, R. Smith, C. Lowe, J.F. Watts, J. Coat. Tech. and Res. 8 (2011) 329–342. [4] S.R. Leadley, J.F. Watts, M.M. Chehimi, C. Lowe, Surf. Interface. Anal. 26 (1998) 444–454. [5] T. Hirayama, M.W. Urban, Prog. Org. Coat. 20 (1992) 81–96. [6] C. Perruchot, J.F. Watts, C. Lowe, G. Beamson, Int. J. Adhes. Adhes. 23 (2003) 101–113. [7] W.R. Zhang, C. Lowe, R. Smith, Prog. Org. Coat. 65 (2009) 469–476. [8] W.R. Zhang, C. Lowe, R. Smith, Prog. Org. Coat. 66 (2009) 141–148. [9] W.R. Zhang, R. Smith, C. Lowe, J. Coat. Tech. and Res. 6 (2009) 315–328. [10] W.R. Zhang, T.T. Zhu, R. Smith, C. Lowe, Prog. Org. Coat. 69 (2010) 376–383. [11] L. Gonon, J. Mallegol, S. Commereuc, V. Verney, Vib. Spectrosc. 26 (2001) 43–49. [12] J.L. Dupuie, W.H. Weber, D.J. Scholl, J.L. Gerlock, Polym. Degrad. Stab. 57 (1997) 339–348. [13] D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C. Paputa Peck, R.O. Cater III, Polym. Degrad. Stab. 27 (1990) 271–284. [14] D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C. Paputa Peck, R.O. Cater III, Polym. Degrad. Stab. 28 (1990) 39–51. [15] C. Decker, K. Zahouily, Polym. Degrad. Stab. 64 (1999) 293–304. [16] M.E. Nichols, J.L. Gerlock, C.A. Smith, Polym. Degrad. Stab. 56 (1997) 81–91. [17] M.E. Nichols, J.L. Gerlock, Polym. Degrad. Stab. 69 (2000) 197–207. [18] K. Adamsons, Prog. Polym. Sci. 25 (2000) 1363–1409. [19] T. Nguyen, J. Martin, R. Byrd, N. Embree, Polym. Degrad. Stab. 77 (2002) 1–16. [20] C. Mura, J. Yarwood, R. Swart, D. Hodge, Polymer 41 (2000) 8659– 8671. [21] P. Schmidt, M.R. Fernandez, J.M. Pastor, J. Roda, Polymer 38 (1997) 2067–2075. [22] T. Nguyen, J. Martin, R. Byrd, J. Coat. Technol. 75 (2003) 37–50. [23] T. Nguyen, F. Martin, R. Byrd, N. Embree, Polym. Degrad. Stab. 77 (2002) 1–16. [24] P. Delorme, J. Lemair, F. Carrara, C. Bonnebat, Photooxidation Mechanisms of Aliphatic Crosslinked Polyesters Used for Flat Steel Coatings. American Institute of Physics, Proceeding of the 53rd International Meeting of Physical Chemistry, Paris, France, 354 (Jan 2–6 1996) 413–423. [25] X. Gu, D. Raghavan, T. Nguyen, M.R. VanLandingham, D. Yebassa, Polym. Degrad. Stab. 71 (2001) 139–149. [26] W.J. Jones, W.J. Orville-Thomas, Trans. Faraday. Soc. 55 (1959) 203–210. [27] M.L. Scheepers, R.J. Meier, J.M. Gelan, D.J. Vanderzande, B.J. Kip, Vib. Spectrosc. 9 (1995) 139–146. [28] P.J. Larkin, M.P. Makowski, N.B. Colthup, L.A. Flood, Vib. Spectrosc. 17 (1998) 53–72. [29] M.K. Marchewka, Mater. Sci. Eng. B 95 (2002) 214–221. [30] M.K. Marchewka, Bull. Kor. Chem. Soc. 25 (2004) 466–470. [31] W.R. Zhang, T.T. Zhu, R. Smith, C. Lowe, submitted to Polym. Test. 2012.