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Forensic aspects of the weathering and ageing of spray paints
Forensic Science International 258 (2016) 32–40
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Forensic aspects of the weathering and ageing of spray paints Ce´dric Jost, Cyril Muehlethaler *, Genevie`ve Massonnet Ecole des Sciences Criminelles, Institut de Police Scientifique, Universite´ de Lausanne, 1015 Lausanne-Dorigny, Switzerland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 August 2015 Received in revised form 12 October 2015 Accepted 3 November 2015 Available online 21 November 2015
This paper presents a preliminary study on the degradation of spray paint samples, illustrated by Optical, FTIR and Raman measurements. As opposed to automotive paints which are specifically designed for improved outdoor exposure and protected using hindered amine light absorbers (HALS) and ultra-violet absorbers (UVA), the spray paints on their side are much simpler in composition and very likely to suffer more from joint effects of solar radiation, temperature and humidity. Six different spray paint were exposed to outdoor UV-radiation for a total period of three months and both FTIR and Raman measurements were taken systematically during this time. These results were later compared to an artificial degradation using a climate chamber. For infrared spectroscopy, degradation curves were plotted using the photo-oxidation index (POI), and could be successfully approximated with a logarithmic fitting (R2 > 0.8). The degradation can appear after the first few days of exposure and be important until 2 months, where it stabilizes and follow a more linear trend afterwards. One advantage is that the degradation products appeared almost exclusively at the far end (3000 cm1) of mid-infrared spectra, and that the fingerprint region of the spectra remained stable over the studied period of time. Raman results suggest that the pigments on the other side, are much more stable and have not shown any sign of degradation over the time of this study. Considering the forensic implications of this environmental degradation, care should be taken when comparing samples if weathering is an option (e.g. an exposed graffiti compared to the paint from a fresh spray paint can). Degradation issues should be kept in mind as they may induce significant differences between paint samples of common origin. ß 2015 Elsevier Ireland Ltd. All rights reserved.
Keywords: Infrared Raman Photo-oxidation Degradation Graffiti
1. Introduction The study of paint degradation and paint weathering is of particular interest to forensic science. Every coating material, albeit providing some optical properties such as color, luminosity and brightness, is mostly designed to protect the substrate from environmental conditions. Yet this external environment can be particularly harsh when combined heat, moisture and solar radiation act at the same time. The chemical modifications, scission of chemical bonds, or creation of crosslinkings all provide significant changes in the molecular structure of the film. As mentioned by Adamsons [1], ‘‘the chemical effects can be quickly identified at early stages of degradation using surface/near-surface analytical measurements in order to establish photo-oxidative transformation of flexible chain segments into more rigid structures, cross-link type or density changes, and/or formation of hydrophilic groups capable of adsorbing more water’’. The
* Corresponding author. Current address: Department of Chemistry, City College of New York, 10031 New York (NY), United States. Tel.: +1 41216924628. E-mail address: [email protected] (C. Muehlethaler). http://dx.doi.org/10.1016/j.forsciint.2015.11.001 0379-0738/ß 2015 Elsevier Ireland Ltd. All rights reserved.
outcomes for the painted layer can be opticals such as color fading, mechanicals with crackings, delamination or peelings, but also chemicals and directly influent on the results of instrumental analyses. For this purpose many different analytical techniques were applied to all kind of polymers for characterizing this eventual degradation. Among the forensic community, the majority of published papers about paint degradation concentrated on automotive coatings. The study of multi-layered original equipment manufacturer (OEM) allows chemical changes to be detected based on appearance changes, and be used to predict resistance and other time-to-failure considerations [2]. The clearcoat is obviously the most affected layer in automotive paint systems, because it is (when present) the only one in direct contact with the environment. The majority of automotive clearcoats are based on acrylic-melamine polymers and various articles concentrated on this particular composition, either analyzed by Pyrolysis GC–MS, FTIR or UV–vis spectroscopy [3–5]. Color fading, delamination of the basecoat-clearcoat layers system (either adhesive or cohesive failures [1]), and the paint mechanical deterioration were also deeply investigated [4,6,7]. Most of these studies involved surface or near-surface analyses mostly carried out by attenuated
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total reflectance infrared spectroscopies (ATR-IR). As the degradation penetration depth is time-correlated, the surface will always present the most important modifications. To increase stability over time the industries have developed solutions for protecting the painted film. Ultraviolet Absorber (UVA) and Hindered Amines Light Absorbers (HALS) were both used to absorb the excess of photons and avoid their associated energy to be used for breaking the chemical bonds [6,8–10]. These were proved very useful in various situations and drastic increase of the stability can be reached [11,12]. The second main field to have been investigated deeply for paint degradation is the artistic and cultural heritage. One particularity of ancient artistic paintings is the potential use of proteinaceous binders, for which the degradation has been studied with the objective to be able to predict their long-term behavior [13]. Others have compared synthetic binders such as polyvinyl acetate and acrylic [14], or acrylic and alkyd paints [15,16]. Different binders were studied by Papliaka and coworkers, who demonstrated that the risks of chemical modification due to UV exposure are superior to both heat and humidity factors [17]. Finally the domestic paints encountered in forensic cases have not been studied much because the problems of degradation associated were not as essential as in the industry or in the conservation fields. From the forensic point-of-view however, it might be decisive to prove that a degradation has occurred. Demonstrating that specific photo-oxidation products appeared in the paint film is necessary when unexplained spectral differences are to be used as evidence of uncommon origin. Trace materials have acquired characteristics, and it is crucial to be able to infer an explanation for these differences. Concerning the domestic paints, the use of UVA or HALS is also less likely as in most cases they are cheaper to produce and not necessarily designed for extended outdoor exposure. The different binders used also reflect this predominance of cheaper materials. Recent studies on spray paints have shown that a vast majority of the binders were based on alkyd modified polyesters (orthophthalic and isophthalic acids mostly) [18]. These binders do not have the same properties as acrylic monomers and do not react similarly under strong UV-light exposure. The main modifications in infrared spectra of acrylic paints were pointed out to be a decrease and broadening of the
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carbonyl peak at around 1730 cm1 [15,19], and also around 2800– 3400 cm1 for which the methylenic, hydroxy, amine and carboxylic acid envelops are present. Alkyd paints on the other side were shown to be mostly affected around the same 2800– 3400 cm1 region [5]. This area, as pointed out by Adamsons [2], is particularly interesting for quantifying the degree of degradation and the accumulation of oxidized products, as the ratio of the two envelops increases with UV exposure. The so-called photooxidation index (POI) is calculated as follow, based on data previously normalized to the methylenic –CH2 stretch at 2930 cm1 and baseline corrected (Fig. 1): POID ¼
½OH; NH; COOH ½CH
(1)
As most of the spray paints are based on alkyd polyesters binders [18], it is therefore necessary to better understand their chemistry in order to predict the eventual degradation and the consequences on subsequent instrumental analyses. The various binders, pigments and/or extenders do not have the same stability in regard to solar radiation or moisture, as are the different kinds of paints (artistic, domestic or automotive). The following sections will present some theoretical aspects of the paint degradation. It is followed by an example of application using a sample set of 6 spray paints, measured systematically after days/weeks/months, and left to degrade for a total period of 3 months. These results are later compared to an artificial weathering using a climate chamber. Both photo-oxidation indexes and PCAs are used to evaluate the results and estimate the degree of degradation observed on these samples. 2. Theory on paint degradation Solar radiation is often considered the main and most important factor of the paint degradation. In fact, it is the UV portion of the electromagnetic spectrum that is mostly responsible for the degradation (UV-A, UV-B and UV-C). These short wavelengths and highly energetic radiations possess photons sufficiently energetic to catalyze and supply the photo-oxidation process in the presence of oxygen. Different mechanisms of the chain photo-oxidation process for polymers were described in the literature [20,21].
Fig. 1. Illustration of photo-oxidation index calculation, using the two envelops of the methylenic (CH), and hydroxy, amine and carboxylic acid (OH, NH, COOH). Spectra are normalized to the methylenic –CH2 stretch (2950 cm1) and one can observe an increase of oxidized products over the radiation received.
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Table 1 Spray paint cans used for the degradation experiments. The samples were sprayed onto clean microscope glass slides after homogeneous shaking of the cans. The composition is given by the major contributions in FTIR and Raman spectra. ACR: Acrylic, STY: Styrene, CC: Calcium carbonate, ALK OPH: Alkyd orthophthalic, MS: Magnesium silicate, NCL: Nitrocellulose. PB15: Pigment Blue 15, PO34: Pigment Orange 34, PR63: Pigment Red 63, PG7: Pigment Green 7. Code
Color
Brand
Description
Production
Composition
B1 B2 R1 R2 G1 G2
Blue Blue Red Red Green Green
Ironlak DayColors DayColors Mtn Mtn Belton
Phat1 true royal blue Artists Artists Nitro 2G colors 94 Era green by Montana colors Molotow leaf green
Australia Spain Spain Spain Spain Germany
ACR, STY, ALK OPH, ALK OPH, ALK OPH, ALK OPH, ALK OPH,
Moisture and water uptake is another factor of influence of the degradation. However, unlike the UV radiation, it affects both the paint and the support. In most cases it is the support itself that reacts the most to water humidity. Corrosion of metallic substrates causes cracks and peelings of the paint that cannot adhere anymore to the substrate [22]. In addition, paints adsorb and resorb constantly the water from the environment and normally remain in equilibrium with it. In some cases and with a high concentration of oxygen in the surroundings, hydrolysis can happen and create chemical modifications in the painted film. The intensity of the hydrolysis is directly dependent on the density of chemical bonds and crosslinking in the polymer layer. Finally chemical and biological agents can also cause a chemical deterioration of the paint. Acidic environments are good catalysts for both hydrolysis and photo-oxidation. Schulz et al. mentioned the acidic rain (sulfur dioxide uptake (SO2 $ H2SO4)) as one of the most important factors of acidic catalyst [23]. Sampers also exposed the fact that the performance of HALS can be drastically reduced in the presence of acids or pesticides [24]. Finally Gaylarde et al. published a comprehensive review on biodeterioration of architectural finishes [25]. Among the biological agents, the most common cause is birds excrements that act as catalyzer for the scission of ether–ester bonds. 3. Material and methods 3.1. Samples The sample set is composed of 6 spray paint cans purchased in specialized graffitis shops (Table 1). Reference samples were prepared by spraying paints onto clean microscope glass slides at a distance of about 30 cm. 3 min shaking time using an IKA laboratory shaker were completed to ensure a homogeneous distribution of the pigments into solution [26]. Samples were then placed on a wood holder side by side. The glass slides were slightly tilted to allow for rain and moisture flow. Three different sets of samples were prepared; one for exterior exposure experiments, one for the artificial exposure in climate chamber, and one left in the dark as a control. 3.2. Exposure conditions For the natural weathering experiments, the glass slides on the holder were placed on the rooftop of the building, where sun exposition is maximal throughout the day. Meteorological data were collected during the three months of the experiment. The samples were exposed to a mean solar radiation of 908,615,000 W s/m2, which accounts for about a fifth of the annual solar radiation observed in Switzerland1. To consider the contribution of UV only, it was estimated from the literature that about 6.8% of this radiation is due to the ultraviolet range of the spectrum (295–400 nm), hence a total of 61,786,000 W s/m2 UV radiation for each sample. 1
www.meteolausanne.com (last accessed 8th May 2015).
CC, PB15 MS, PB15 MS, unidentified pigment PO34, PR63 PG7 NCL, PG7
For the artificial weathering experiments, the glass slides were put in a Vo¨tsch VC3 climate chamber, using a constant 20 8C temperature and 55% humidity. The samples were at about 30 cm of a UV Ultra-Vitalux 300 W light. At this distance the samples were exposed to UVA/UVB radiation of about 51.89 W/m2. For the 2 months of artificial exposition, the samples received an equivalent of 10 months of natural UV radiation (255,361,000 W s/m2), according to [27]. 3.3. Measurement conditions All samples were first observed under a Leica DMRX microscope with 50 magnification. Illustrations were recorded under a standardized MacBeth daylight illumination for color comparison (X-Rite MacBeth lighting Judge II). Analytical measurements were made after, respectively, 1, 2, 3, 4, 8, 15, 22, 29 (1 month), 36, 43, 50, 57 (2 months) and 85 days (3 months) of exposition for the natural weathering conditions. Measurements after 2 months of artificial weathering (10 months natural equivalent) were compared to these latter. Infrared measurements were made in transmission mode by scraping and flattening a small amount of paint with a micro-roller. Transmission mode was selected over attenuated total reflectance (ATR) because it reflects the practice used in forensic laboratories to obtain better quality infrared spectra. More variability among consecutive measurements is awaited due to the flattening and ‘‘mixing’’ of the first few mm of the surface. For these reasons seven replicated measurements were made from random positions over the painted surface. The instrument used was a Nicolet 6700 FTIR Spectrometer, equipped with a Nicolet Continuum FT-IR Microscope from Thermo Electron Corp. with a 32 Reflachromat objective, a mercury cadmium telluride (MCT/A) detector and the software OMNIC 9.1. The sampling method was transmittance on KBr pellets. The measurement parameters were as follow: 150 150 mm window size, 4.0 cm1 resolution, 32 co-added spectra, and 4000–650 cm1 range. Raman measurements were made in situ on the glass slides. Seven replicated measurements were made from random positions over the painted surface. The instrument used is a Renishaw InVia and a DM 2500 M Leica Microscope with a 50 objective. Different laser lines were tested and optimized with laser power for every samples. 785 nm Renishaw NIR diodes were used for samples B2, R1 and R2. 633 nm He–Ne Renishaw was used for samples B1, G1 and G2. The other parameters were fixed at 1 accumulation, 10s integration time and a 200–2000 cm1 range. The potential organic binder signals between 2000 and 4000 cm1 were not visible and neglected of further analyses. 4. Results and discussion 4.1. Physical aspect The first examination concerned the visual aspect of the samples. After about 1 month of natural exposition, cracks began to
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Fig. 2. Color comparison of the paint samples after 3 months in the dark (control) and 3 months in an accelerated weathering chamber (artificial). Illustrations recorded under a standardized McBeth excitation light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
appear on the sample G2. Adherence of the painted film over the glass surface is known to be dependent on the water uptake [22]. However this effect was not observed on other samples, which remained strongly bound to the substrate. No significant color fading was observed for the natural weathering conditions. However, the comparison of the control samples left in the dark with the artificially weathered ones, showed a fade of the tint when observed under a standardized MacBeth light. These differences were more pronounced for the red sample R2 and green sample G2 as illustrated within Fig. 2. 4.2. Infrared spectra Visual examination of every spectra showed no actual differences of absorption in the fingerprint 650–1700 cm1 range. The broadening of the carbonyl peak at 1730 cm1, as mentioned
by [15,19], has not been observed in that particular situation. The only significant changes were detected around 2850–3600 cm1 for which the methylenic –CH2 stretch peak decreased with higher exposure times (Fig. 3). The photo-oxidation indexes (POI) were calculated for every paints and every days of the experiment. Diminution of stretch – CH2 together with an accumulation of oxidization products such as –OH, –NH and –COOH led to ratios quantifying the degree of degradation of the paints. An increase in POI is awaited for every sample. Fig. 4 presents the POI degradation curves as a function of the cumulated solar radiation received by the samples. Every point represents the mean of all 7 replicated measurements. The curves from Fig. 4 were then approximated by a log function with a relatively good coefficient of determination (R2 ranging from 0.56 to 0.87). Only samples B1 and G2 present a value below 0.8. The degradation is rapid during the first two
Fig. 3. FTIR spectra of the 6 spray paints after, respectively 0, 1, 2, 3 months of natural weathering and 2 months of artificial exposition in a climate chamber.
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Fig. 4. Degradation curves plotted from the photo-oxidation index (POI) values given the cumulated UV radiation received. Linear (red) and logarithmic fittings (green) are superimposed to the data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
months, then stabilizes and follows a more linear trend. Approximation by a linear function is illustrated as well, with R2 ranging from 0.53 to 0.83. In every case except B1, the log functions performed better than a linear function fitted to the data, mostly because of the rapid degradation during the first few days. It was supposed, however not confirmed, that the degradation would eventually stop after a few months when all oxidation possibilities are drained. The results also confirm that degradation is present even in the early stages of weathering and might be rapidly visible in infrared spectra of spray paints. The artificially weathered samples are not plotted on the curves because of different experimental conditions that cannot be compared directly to an outside environment. They are instead mentioned in Table 2 together with the POI values from fresh and control samples. Compared to the initial situation, naturally weathered samples increase around 1.5 over the time of the experiments, while artificially weathered samples is around 2. The control samples left in the dark for 3 months have values
Table 2 Photo-oxidation indexes calculated for fresh samples (24 h in the dark), control samples (3 months in the dark), naturally weathered (3 months outside) and artificially weathered (2 months in a climate chamber, equivalent to 10 months natural weathering). Every sample is represented by the mean of 7 replicated measurements. Photo-oxidation indexes (POI)
Fresh
Control
Natural
Artificial
Conditions
24 h (dark)
3 months (dark)
3 months (outside)
2 months (climate chamber, 10 months natural)
B1 B2 R1 R2 G1 G2
0.761 1.376 1.449 1.016 2.129 3.256
0.977 1.546 1.780 1.352 2.446 3.346
1.332 2 2.177 1.676 3.157 3.792
1.612 2.67 3.061 2.532 4.37 4.777
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Fig. 5. PCAs of the individual paints considering the FTIR replicated measurements after 0, 1, 2, and 3 months of natural weathering, and 2 months of artificial exposition in a climate chamber. * Sample B2 is illustrated by PC1 vs PC3 (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
Fig. 6. PCAs of the individual paints considering the FTIR replicated measurements of fresh samples (0 months), after 2 months of artificial exposition in a climate chamber (2 m.a.) and after 3 months as control in the dark (3 m.c.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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in-between the fresh and 3 months naturally weathered samples. This observation means that other factors might have an effect on the POI in the absence of solar radiation, although at a much slower rate. It also demonstrates that UV light acts as an accelerating factor. The differentiation between the degraded paints was further studied by PCA on previously pretreated FTIR data (SNV + Detrending, 650 1800 + 2700 3800 cm1 variable selection). As the selected measurement technique was transmission on KBr pellets, more variability among the replicated measurements is awaited compared to ATR measurements. However, better quality spectra with a more stable baseline and more significant relative intensities are expected. Compared to the POI degradation curves that were considering only the 2800–3400 cm1 variables, these PCA take into account all of the absorption bands that shape the spectra by only removing the regions of poor interest. Fig. 5 presents the individual PCAs of every sample measured by FTIR after 0, 1, 2 and 3 months. Artificially degraded samples are also included for comparison. We observe that the measurements from sample B1 are randomly distributed over the score space, with no clear trends or separations. This sample was also the one with the least good fitted curves over the POI values. From these results it seems that sample B1 is more stable than the others and do not degrade much over the time of this study even for the artificially degraded set. It is also the only one having an acrylic binder, compared to alkyd for other samples. For the others, the artificially weathered samples are more easily separated than the rest of the measures. They always present differences along PC1, except for the blue samples. The loadings highlight a few variables, mostly being the envelops
at around 3000 cm1, as well as some of the peaks of binders or extenders. None of them is however strong enough to be considered a main contributor to the differences observed. Sample B2 presents a separation between artificially and naturally degraded samples along the third principal component (PC3), influenced in part by the methylenic peaks (2800 cm1) and some of the binders peaks. The second principal component (PC2) accounted for the carbonyl peak at 1730 cm1 only but did not provide any separation. Apart from the artificially degraded samples, the distinction between the different months is less noticeable. There is not a consistent trend and it appears that these variations cover mostly some heterogeneity from the paint samples rather than a chemical modification of any type. In only two situations (R2, G1) the fresh samples (0 months) are separated from the others but this is the only substantial observation made. These results would suggest that when considering the whole spectrum, very few modifications permit to discriminate degraded from fresh samples. The effect of pretreatment might also have an important effect on these PCAs. The normalization over the whole spectrum as it is done with SNV will tend to diminish the importance of the hydroxy, amine and carboxylic acid envelop. Only by normalizing to the methylenic peak at 2930 cm1 and considering this restrained zone will permit to highlight the modifications in POI as presented in Fig. 1. Finally, the comparison between fresh (24 h), controls in the dark (3 months), and artificially degraded samples (10 months equivalent of a natural degradation) was also considered by PCA and is presented in Fig. 6. As was the case for Fig. 5 the results are very dependent on the type of samples. The blue sample 1 (B1) do not exhibit significant differences, and with all three conditions the
Fig. 7. Raman spectra of the 6 spray paints after, respectively 0, 1, 2, 3 months of natural weathering and 2 months of artificial exposition in a climate chamber.
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measurements overlap each other and do not present any separation. For the others, a separation is visible along PC1, with the artificially degraded samples on the left, controls in the middle, and fresh on the right. The loadings of PC1 are different for every sample but all have in common a positive value for the methylenic peak at 2930 cm1. On top of that the sample B2 and R1 show a simultaneous contribution of the magnesium silicate (talc) peak at 1020 cm1 with increased UV radiations. As magnesium silicate is a very stable mineral, it is expected that the talc peak remains constant over time, while all other binder peaks are decreasing due to the photo-oxidation process. The normalization of all spectra with SNV however tend to diminish these effects by increasing all other peaks toward their normalized value and giving a falseimpression of decrease for the 1020 cm1 peak intensity. Sample G2 do not have any separation between the dark controls and the fresh samples, but the artificially degraded samples possess a diminished absorption for the 1280 cm1 nitrocellulose peak. It also appears that samples used as control in the dark during 3 months do degrade a bit, even if not initiated by ultraviolet radiation. There was no strict control of the humidity and temperature factors during the study so it seems difficult to infer any reason for that. As it was performed as a preliminary study, it would require new tests in order to estimate the extent of non-UV degradation on this kind of samples. As a summary concerning the infrared spectra, is has been shown that all six samples have very distinct compositions, and the results demonstrate that they behave very differently regarding UV degradation. The only acrylic sample from the study seems to be more stable than the others. Alkyd polyesters on the other side seem to more rapidly show a decrease of the methylenic peak, sometimes accompanied by more specific modifications in peaks
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corresponding to magnesium silicate or nitrocellulose. These modifications are minor when considered over the whole spectrum, and would very rarely lead to a false exclusion of common source. 4.3. Raman spectra Visual examination of the Raman spectra showed no differences in peaks positions and peaks intensities (Fig. 7). Raman absorptions in this sample set being mostly due to the pigments composition, it was also checked if differences in absolute intensities can happen due to pigment degradation (i.e. modifications leading to color fading). The results highlighted that over the solar radiation received no visual differences were present between the spectra collected. As the POI is not calculable for Raman spectra, PCA were studied to find differences linked to the degradation. All Raman spectra were pretreated by baseline correction and normalized by SNV. No variables selections were performed. Fig. 8 presents the individual PCAs of every sample measured by Raman after, respectively, 0, 1, 2, 3 months of natural degradation and 2 months in climate chamber (artificial). Although some separations appear punctually for some samples, none of these can be linked to the degradation or amount of radiation received. Considering that, the results support the idea that the observed variations are more analytical and due to the measurement conditions, rather than a real degradation. Raman instruments are very sensitive to the surrounding environment, as well as calibration processes (for both wavenumbers x-axis, and power intensity y-axis). For these reasons, consecutive measurements a few days apart may likely show some minor differences. The loadings associated with these PCAs mostly represent some slight
Fig. 8. PCAs of the individual paints considering the Raman replicated measurements after 0, 1, 2, and 3 months of natural weathering, and 2 months of artificial exposition in a climate chamber (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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variations in baseline that could not be corrected further by the pretreatments. Our hypothesis is that the replicated measurements are in fact very similar and close to each other. These differentiations are mostly based on instrumental or external factors, unrelated to the UV radiation or other degradation influences. It should be noted also that PCA does not have any internal standard or any way to tell at which level of differentiation we are situated. The observation of separations might potentially be significant but also part of the heterogeneity among replicated measurements. By comparing different pretreatments and methods of baseline correction or spectra normalization, no improvements on the PCAs separations were reached. As a summary, the results obtained for the Raman samples show no analytical signs of degradation. A visual examination of the spectra demonstrates very repeatable and stable measures with consistent semi-quantitative ratios between all the peaks. From that it could be guessed that the pigments are much more stable to UV radiation than the binders or extenders. These results are consistent with the theory for these kinds of molecules which are likely more stable due to absence of reactive groupings or cross-linked structures that easily degrade. Even if a fading of the color was observed, the Raman spectra were not affected and remain unchanged after months.
5. Conclusion As a preliminary study on the UV-initiated degradation of spray paints, we have shown that spectral variations due to oxidization products are likely to appear in FTIR spectra, while Raman spectra are fairly more stable. Due to their relatively simple composition, spray paints are rapidly degraded and the differences begin to appear after a few days already. These are rapidly increasing until 2 months, where the degradation becomes more stable and follow a linear trend. The differences are mostly located in the 2800– 3400 cm1 range, and can be estimated using the photo-oxidation index (POI). One advantage is that the degradation products appeared almost exclusively at the far end (3000 cm1) of midinfrared spectra, and that the fingerprint region of the spectra remained stable over the studied period of time. Even so, care should still be taken when comparing two samples to assess a common origin, and degradation issues should be kept in mind to explain any significant difference that may appear between two paint samples. References [1] K. Adamsons, Chemical depth profiling of multi-layer automotive coating systems, Prog. Org. Coat. 45 (2002) 69–81. [2] K. Adamsons, Chemical surface characterization and depth profiling of automotive coating systems, Prog. Polym. Sci. 25 (2000) 1363–1409.
[3] J.L. Gerlock, et al., On the use of Fourier transform infrared spectroscopy and ultraviolet spectroscopy to assess the weathering performance of isolated clearcoats from different chemical families, Polym. Degrad. Stab. 62 (1998) 225–234. [4] C.M. Seubert, et al., The long-term weathering behavior of UV curable clearcoats: I. Bulk chemical and physical analysis, Polym. Degrad. Stab. 81 (2003) 103–115. [5] H. Yari, M. Mohseni, B. Ramezanzadeh, Comparisons of weathering degradation of two automotive refinish coatings: a case study, J. Appl. Polym. Sci. 111 (6) (2009) 2946–2956. [6] J.L. Gerlock, A. Kucherov, M.E. Nichols, On the combined use of UVA: HALS, photooxidation, and fracture energy measurements to anticipate the long-term weathering performance of clearcoat/basecoat automotive paint systems, J. Coat. Technol. 73 (918) (2001) 45–54. [7] M.E. Nichols, C.A. Darr, Effect of weathering on the stress distribution and the mechanical performance of automotive paint systems, J. Coat. Technol. 70 (1998) 141–149. [8] D.R. Bauer, M.J. Dean, J.L. Gerlock, Comparison of photostabilization in acrylic/ urethane and acrylic/melamine coatings containing hindered amines and ultraviolet absorbers, Ind. Eng. Chem. Res. 27 (1988) 65–70. [9] C.A. Smith, J.L. Gerlock, R.O. Carter III, Determination of ultraviolet light absorber longevity and distribution in automotive paint systems using ultraviolet microspectroscopy, Polym. Degrad. Stab. 72 (2001) 89–97. [10] W. Stoecklein, H. Fujiwara, The examination of UV-absorbers in 2-coat metallic and non-metallic automotive paints, Sci. Justice 39 (3) (1999) 188–195. [11] M. McCusker, A UVA/HALS primer: everything you’ve ever wanted to know about light stabilizers. Part I, Met. Finishing 97 (5) (1999) 51–53. [12] M. McCusker, A UVA/HALS primer: everything you’ve ever wanted to know about light stabilizers. Part II, Met. Finishing 97 (6) (1999) 83–86. [13] E. Manzano, et al., Preliminary study of UV ageing process of proteinaceous paint binder by FT–IR and principal component analysis, Talanta 77 (5) (2009) 1724–1731. [14] M.T. Domenech-Carbo, et al., Study of behaviour on simulated daylight ageing of artists’ acrylic and poly(vinyl acetate) paint films, Anal. Bioanal. Chem. 399 (9) (2011) 2921–2937. [15] F.X. Perrin, et al., Artificial aging of acrylurethane and alkyd paints: a micro-ATR spectroscopic study, Polym. Degrad. Stab. 70 (2000) 469–475. [16] F.X. Perrin, et al., Evaluation of accelerated weathering tests for three paint systems: a comparative study of their aging behaviour, Polym. Degrad. Stab. 72 (2001) 115–124. [17] Z.E. Papliaka, K.S. Andrikopoulos, E.A. Varella, Study of stability of a series of synthetic colorants applied with styrene-acrylic copolymer: widely used in contemporary paintings, concerning the effects of accelerated ageing, J. Cult. Heritage 11 (4) (2010) 381–391. [18] C. Muehlethaler, G. Massonnet, P. Esseiva, Discrimination and classification of FTIR spectra of red, blue and green spray paints using a multivariate statistical approach, Forensic Sci. Int. 244 (2014) 170–178. [19] V. Pintus, M. Schreiner, Characterization and identification of acrylic binding media: influence of UV light on the ageing process, Anal. Bioanal. Chem. 399 (9) (2011) 2961–2976. [20] D. Feldman, Polymer weathering: photo-oxidation, J. Polym. Environ. 10 (4) (2002) 163–173. [21] S.I. Kuzina, A.I. Mikhailov, Chain and photochain mechanisms of photooxidation of polymers, High Energy Chem. 44 (1) (2010) 37–51. [22] J. Kittel, et al., Influence of the coating-substrate interactions on the corrosion protection: characterisation by impedance spectroscopy of the inner and outer parts of a coating, Prog. Org. Coat. 46 (2003) 135–147. [23] U. Schulz, et al., The effects of acid rain on the appearance of automotive paints systems studied outdoors and in a new artificial weathering test, Prog. Org. Coat. 40 (1-4) (2000) 151–165. [24] J. Sampers, Importants of weathering factors other than UV radiation and temperature in outdoor exposure, Polym. Degrad. Stab. 76 (3) (2002) 455–465. [25] C.C. Gaylarde, et al., Biodeterioration of external architectural paint films—a review, Int. Biodeterior. Biodegrad. 65 (2011) 1189–1198. [26] C. Muehlethaler, G. Massonnet, P. Buzzini, Influence of the shaking time on the forensic analysis of FTIR and Raman spectra of spray paints, Forensic Sci. Int. 237 (2014) 78–85. [27] M. McGreer, Practical Applications, in Weathering Testing Guidebook, ATLAS Material Testing Solutions, Chicago (IL), 2001, pp. 60–72.
Contents lists available at ScienceDirect
Forensic Science International journal homepage: www.elsevier.com/locate/forsciint
Forensic aspects of the weathering and ageing of spray paints Ce´dric Jost, Cyril Muehlethaler *, Genevie`ve Massonnet Ecole des Sciences Criminelles, Institut de Police Scientifique, Universite´ de Lausanne, 1015 Lausanne-Dorigny, Switzerland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 August 2015 Received in revised form 12 October 2015 Accepted 3 November 2015 Available online 21 November 2015
This paper presents a preliminary study on the degradation of spray paint samples, illustrated by Optical, FTIR and Raman measurements. As opposed to automotive paints which are specifically designed for improved outdoor exposure and protected using hindered amine light absorbers (HALS) and ultra-violet absorbers (UVA), the spray paints on their side are much simpler in composition and very likely to suffer more from joint effects of solar radiation, temperature and humidity. Six different spray paint were exposed to outdoor UV-radiation for a total period of three months and both FTIR and Raman measurements were taken systematically during this time. These results were later compared to an artificial degradation using a climate chamber. For infrared spectroscopy, degradation curves were plotted using the photo-oxidation index (POI), and could be successfully approximated with a logarithmic fitting (R2 > 0.8). The degradation can appear after the first few days of exposure and be important until 2 months, where it stabilizes and follow a more linear trend afterwards. One advantage is that the degradation products appeared almost exclusively at the far end (3000 cm1) of mid-infrared spectra, and that the fingerprint region of the spectra remained stable over the studied period of time. Raman results suggest that the pigments on the other side, are much more stable and have not shown any sign of degradation over the time of this study. Considering the forensic implications of this environmental degradation, care should be taken when comparing samples if weathering is an option (e.g. an exposed graffiti compared to the paint from a fresh spray paint can). Degradation issues should be kept in mind as they may induce significant differences between paint samples of common origin. ß 2015 Elsevier Ireland Ltd. All rights reserved.
Keywords: Infrared Raman Photo-oxidation Degradation Graffiti
1. Introduction The study of paint degradation and paint weathering is of particular interest to forensic science. Every coating material, albeit providing some optical properties such as color, luminosity and brightness, is mostly designed to protect the substrate from environmental conditions. Yet this external environment can be particularly harsh when combined heat, moisture and solar radiation act at the same time. The chemical modifications, scission of chemical bonds, or creation of crosslinkings all provide significant changes in the molecular structure of the film. As mentioned by Adamsons [1], ‘‘the chemical effects can be quickly identified at early stages of degradation using surface/near-surface analytical measurements in order to establish photo-oxidative transformation of flexible chain segments into more rigid structures, cross-link type or density changes, and/or formation of hydrophilic groups capable of adsorbing more water’’. The
* Corresponding author. Current address: Department of Chemistry, City College of New York, 10031 New York (NY), United States. Tel.: +1 41216924628. E-mail address: [email protected] (C. Muehlethaler). http://dx.doi.org/10.1016/j.forsciint.2015.11.001 0379-0738/ß 2015 Elsevier Ireland Ltd. All rights reserved.
outcomes for the painted layer can be opticals such as color fading, mechanicals with crackings, delamination or peelings, but also chemicals and directly influent on the results of instrumental analyses. For this purpose many different analytical techniques were applied to all kind of polymers for characterizing this eventual degradation. Among the forensic community, the majority of published papers about paint degradation concentrated on automotive coatings. The study of multi-layered original equipment manufacturer (OEM) allows chemical changes to be detected based on appearance changes, and be used to predict resistance and other time-to-failure considerations [2]. The clearcoat is obviously the most affected layer in automotive paint systems, because it is (when present) the only one in direct contact with the environment. The majority of automotive clearcoats are based on acrylic-melamine polymers and various articles concentrated on this particular composition, either analyzed by Pyrolysis GC–MS, FTIR or UV–vis spectroscopy [3–5]. Color fading, delamination of the basecoat-clearcoat layers system (either adhesive or cohesive failures [1]), and the paint mechanical deterioration were also deeply investigated [4,6,7]. Most of these studies involved surface or near-surface analyses mostly carried out by attenuated
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total reflectance infrared spectroscopies (ATR-IR). As the degradation penetration depth is time-correlated, the surface will always present the most important modifications. To increase stability over time the industries have developed solutions for protecting the painted film. Ultraviolet Absorber (UVA) and Hindered Amines Light Absorbers (HALS) were both used to absorb the excess of photons and avoid their associated energy to be used for breaking the chemical bonds [6,8–10]. These were proved very useful in various situations and drastic increase of the stability can be reached [11,12]. The second main field to have been investigated deeply for paint degradation is the artistic and cultural heritage. One particularity of ancient artistic paintings is the potential use of proteinaceous binders, for which the degradation has been studied with the objective to be able to predict their long-term behavior [13]. Others have compared synthetic binders such as polyvinyl acetate and acrylic [14], or acrylic and alkyd paints [15,16]. Different binders were studied by Papliaka and coworkers, who demonstrated that the risks of chemical modification due to UV exposure are superior to both heat and humidity factors [17]. Finally the domestic paints encountered in forensic cases have not been studied much because the problems of degradation associated were not as essential as in the industry or in the conservation fields. From the forensic point-of-view however, it might be decisive to prove that a degradation has occurred. Demonstrating that specific photo-oxidation products appeared in the paint film is necessary when unexplained spectral differences are to be used as evidence of uncommon origin. Trace materials have acquired characteristics, and it is crucial to be able to infer an explanation for these differences. Concerning the domestic paints, the use of UVA or HALS is also less likely as in most cases they are cheaper to produce and not necessarily designed for extended outdoor exposure. The different binders used also reflect this predominance of cheaper materials. Recent studies on spray paints have shown that a vast majority of the binders were based on alkyd modified polyesters (orthophthalic and isophthalic acids mostly) [18]. These binders do not have the same properties as acrylic monomers and do not react similarly under strong UV-light exposure. The main modifications in infrared spectra of acrylic paints were pointed out to be a decrease and broadening of the
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carbonyl peak at around 1730 cm1 [15,19], and also around 2800– 3400 cm1 for which the methylenic, hydroxy, amine and carboxylic acid envelops are present. Alkyd paints on the other side were shown to be mostly affected around the same 2800– 3400 cm1 region [5]. This area, as pointed out by Adamsons [2], is particularly interesting for quantifying the degree of degradation and the accumulation of oxidized products, as the ratio of the two envelops increases with UV exposure. The so-called photooxidation index (POI) is calculated as follow, based on data previously normalized to the methylenic –CH2 stretch at 2930 cm1 and baseline corrected (Fig. 1): POID ¼
½OH; NH; COOH ½CH
(1)
As most of the spray paints are based on alkyd polyesters binders [18], it is therefore necessary to better understand their chemistry in order to predict the eventual degradation and the consequences on subsequent instrumental analyses. The various binders, pigments and/or extenders do not have the same stability in regard to solar radiation or moisture, as are the different kinds of paints (artistic, domestic or automotive). The following sections will present some theoretical aspects of the paint degradation. It is followed by an example of application using a sample set of 6 spray paints, measured systematically after days/weeks/months, and left to degrade for a total period of 3 months. These results are later compared to an artificial weathering using a climate chamber. Both photo-oxidation indexes and PCAs are used to evaluate the results and estimate the degree of degradation observed on these samples. 2. Theory on paint degradation Solar radiation is often considered the main and most important factor of the paint degradation. In fact, it is the UV portion of the electromagnetic spectrum that is mostly responsible for the degradation (UV-A, UV-B and UV-C). These short wavelengths and highly energetic radiations possess photons sufficiently energetic to catalyze and supply the photo-oxidation process in the presence of oxygen. Different mechanisms of the chain photo-oxidation process for polymers were described in the literature [20,21].
Fig. 1. Illustration of photo-oxidation index calculation, using the two envelops of the methylenic (CH), and hydroxy, amine and carboxylic acid (OH, NH, COOH). Spectra are normalized to the methylenic –CH2 stretch (2950 cm1) and one can observe an increase of oxidized products over the radiation received.
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Table 1 Spray paint cans used for the degradation experiments. The samples were sprayed onto clean microscope glass slides after homogeneous shaking of the cans. The composition is given by the major contributions in FTIR and Raman spectra. ACR: Acrylic, STY: Styrene, CC: Calcium carbonate, ALK OPH: Alkyd orthophthalic, MS: Magnesium silicate, NCL: Nitrocellulose. PB15: Pigment Blue 15, PO34: Pigment Orange 34, PR63: Pigment Red 63, PG7: Pigment Green 7. Code
Color
Brand
Description
Production
Composition
B1 B2 R1 R2 G1 G2
Blue Blue Red Red Green Green
Ironlak DayColors DayColors Mtn Mtn Belton
Phat1 true royal blue Artists Artists Nitro 2G colors 94 Era green by Montana colors Molotow leaf green
Australia Spain Spain Spain Spain Germany
ACR, STY, ALK OPH, ALK OPH, ALK OPH, ALK OPH, ALK OPH,
Moisture and water uptake is another factor of influence of the degradation. However, unlike the UV radiation, it affects both the paint and the support. In most cases it is the support itself that reacts the most to water humidity. Corrosion of metallic substrates causes cracks and peelings of the paint that cannot adhere anymore to the substrate [22]. In addition, paints adsorb and resorb constantly the water from the environment and normally remain in equilibrium with it. In some cases and with a high concentration of oxygen in the surroundings, hydrolysis can happen and create chemical modifications in the painted film. The intensity of the hydrolysis is directly dependent on the density of chemical bonds and crosslinking in the polymer layer. Finally chemical and biological agents can also cause a chemical deterioration of the paint. Acidic environments are good catalysts for both hydrolysis and photo-oxidation. Schulz et al. mentioned the acidic rain (sulfur dioxide uptake (SO2 $ H2SO4)) as one of the most important factors of acidic catalyst [23]. Sampers also exposed the fact that the performance of HALS can be drastically reduced in the presence of acids or pesticides [24]. Finally Gaylarde et al. published a comprehensive review on biodeterioration of architectural finishes [25]. Among the biological agents, the most common cause is birds excrements that act as catalyzer for the scission of ether–ester bonds. 3. Material and methods 3.1. Samples The sample set is composed of 6 spray paint cans purchased in specialized graffitis shops (Table 1). Reference samples were prepared by spraying paints onto clean microscope glass slides at a distance of about 30 cm. 3 min shaking time using an IKA laboratory shaker were completed to ensure a homogeneous distribution of the pigments into solution [26]. Samples were then placed on a wood holder side by side. The glass slides were slightly tilted to allow for rain and moisture flow. Three different sets of samples were prepared; one for exterior exposure experiments, one for the artificial exposure in climate chamber, and one left in the dark as a control. 3.2. Exposure conditions For the natural weathering experiments, the glass slides on the holder were placed on the rooftop of the building, where sun exposition is maximal throughout the day. Meteorological data were collected during the three months of the experiment. The samples were exposed to a mean solar radiation of 908,615,000 W s/m2, which accounts for about a fifth of the annual solar radiation observed in Switzerland1. To consider the contribution of UV only, it was estimated from the literature that about 6.8% of this radiation is due to the ultraviolet range of the spectrum (295–400 nm), hence a total of 61,786,000 W s/m2 UV radiation for each sample. 1
www.meteolausanne.com (last accessed 8th May 2015).
CC, PB15 MS, PB15 MS, unidentified pigment PO34, PR63 PG7 NCL, PG7
For the artificial weathering experiments, the glass slides were put in a Vo¨tsch VC3 climate chamber, using a constant 20 8C temperature and 55% humidity. The samples were at about 30 cm of a UV Ultra-Vitalux 300 W light. At this distance the samples were exposed to UVA/UVB radiation of about 51.89 W/m2. For the 2 months of artificial exposition, the samples received an equivalent of 10 months of natural UV radiation (255,361,000 W s/m2), according to [27]. 3.3. Measurement conditions All samples were first observed under a Leica DMRX microscope with 50 magnification. Illustrations were recorded under a standardized MacBeth daylight illumination for color comparison (X-Rite MacBeth lighting Judge II). Analytical measurements were made after, respectively, 1, 2, 3, 4, 8, 15, 22, 29 (1 month), 36, 43, 50, 57 (2 months) and 85 days (3 months) of exposition for the natural weathering conditions. Measurements after 2 months of artificial weathering (10 months natural equivalent) were compared to these latter. Infrared measurements were made in transmission mode by scraping and flattening a small amount of paint with a micro-roller. Transmission mode was selected over attenuated total reflectance (ATR) because it reflects the practice used in forensic laboratories to obtain better quality infrared spectra. More variability among consecutive measurements is awaited due to the flattening and ‘‘mixing’’ of the first few mm of the surface. For these reasons seven replicated measurements were made from random positions over the painted surface. The instrument used was a Nicolet 6700 FTIR Spectrometer, equipped with a Nicolet Continuum FT-IR Microscope from Thermo Electron Corp. with a 32 Reflachromat objective, a mercury cadmium telluride (MCT/A) detector and the software OMNIC 9.1. The sampling method was transmittance on KBr pellets. The measurement parameters were as follow: 150 150 mm window size, 4.0 cm1 resolution, 32 co-added spectra, and 4000–650 cm1 range. Raman measurements were made in situ on the glass slides. Seven replicated measurements were made from random positions over the painted surface. The instrument used is a Renishaw InVia and a DM 2500 M Leica Microscope with a 50 objective. Different laser lines were tested and optimized with laser power for every samples. 785 nm Renishaw NIR diodes were used for samples B2, R1 and R2. 633 nm He–Ne Renishaw was used for samples B1, G1 and G2. The other parameters were fixed at 1 accumulation, 10s integration time and a 200–2000 cm1 range. The potential organic binder signals between 2000 and 4000 cm1 were not visible and neglected of further analyses. 4. Results and discussion 4.1. Physical aspect The first examination concerned the visual aspect of the samples. After about 1 month of natural exposition, cracks began to
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Fig. 2. Color comparison of the paint samples after 3 months in the dark (control) and 3 months in an accelerated weathering chamber (artificial). Illustrations recorded under a standardized McBeth excitation light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
appear on the sample G2. Adherence of the painted film over the glass surface is known to be dependent on the water uptake [22]. However this effect was not observed on other samples, which remained strongly bound to the substrate. No significant color fading was observed for the natural weathering conditions. However, the comparison of the control samples left in the dark with the artificially weathered ones, showed a fade of the tint when observed under a standardized MacBeth light. These differences were more pronounced for the red sample R2 and green sample G2 as illustrated within Fig. 2. 4.2. Infrared spectra Visual examination of every spectra showed no actual differences of absorption in the fingerprint 650–1700 cm1 range. The broadening of the carbonyl peak at 1730 cm1, as mentioned
by [15,19], has not been observed in that particular situation. The only significant changes were detected around 2850–3600 cm1 for which the methylenic –CH2 stretch peak decreased with higher exposure times (Fig. 3). The photo-oxidation indexes (POI) were calculated for every paints and every days of the experiment. Diminution of stretch – CH2 together with an accumulation of oxidization products such as –OH, –NH and –COOH led to ratios quantifying the degree of degradation of the paints. An increase in POI is awaited for every sample. Fig. 4 presents the POI degradation curves as a function of the cumulated solar radiation received by the samples. Every point represents the mean of all 7 replicated measurements. The curves from Fig. 4 were then approximated by a log function with a relatively good coefficient of determination (R2 ranging from 0.56 to 0.87). Only samples B1 and G2 present a value below 0.8. The degradation is rapid during the first two
Fig. 3. FTIR spectra of the 6 spray paints after, respectively 0, 1, 2, 3 months of natural weathering and 2 months of artificial exposition in a climate chamber.
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Fig. 4. Degradation curves plotted from the photo-oxidation index (POI) values given the cumulated UV radiation received. Linear (red) and logarithmic fittings (green) are superimposed to the data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
months, then stabilizes and follows a more linear trend. Approximation by a linear function is illustrated as well, with R2 ranging from 0.53 to 0.83. In every case except B1, the log functions performed better than a linear function fitted to the data, mostly because of the rapid degradation during the first few days. It was supposed, however not confirmed, that the degradation would eventually stop after a few months when all oxidation possibilities are drained. The results also confirm that degradation is present even in the early stages of weathering and might be rapidly visible in infrared spectra of spray paints. The artificially weathered samples are not plotted on the curves because of different experimental conditions that cannot be compared directly to an outside environment. They are instead mentioned in Table 2 together with the POI values from fresh and control samples. Compared to the initial situation, naturally weathered samples increase around 1.5 over the time of the experiments, while artificially weathered samples is around 2. The control samples left in the dark for 3 months have values
Table 2 Photo-oxidation indexes calculated for fresh samples (24 h in the dark), control samples (3 months in the dark), naturally weathered (3 months outside) and artificially weathered (2 months in a climate chamber, equivalent to 10 months natural weathering). Every sample is represented by the mean of 7 replicated measurements. Photo-oxidation indexes (POI)
Fresh
Control
Natural
Artificial
Conditions
24 h (dark)
3 months (dark)
3 months (outside)
2 months (climate chamber, 10 months natural)
B1 B2 R1 R2 G1 G2
0.761 1.376 1.449 1.016 2.129 3.256
0.977 1.546 1.780 1.352 2.446 3.346
1.332 2 2.177 1.676 3.157 3.792
1.612 2.67 3.061 2.532 4.37 4.777
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Fig. 5. PCAs of the individual paints considering the FTIR replicated measurements after 0, 1, 2, and 3 months of natural weathering, and 2 months of artificial exposition in a climate chamber. * Sample B2 is illustrated by PC1 vs PC3 (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
Fig. 6. PCAs of the individual paints considering the FTIR replicated measurements of fresh samples (0 months), after 2 months of artificial exposition in a climate chamber (2 m.a.) and after 3 months as control in the dark (3 m.c.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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in-between the fresh and 3 months naturally weathered samples. This observation means that other factors might have an effect on the POI in the absence of solar radiation, although at a much slower rate. It also demonstrates that UV light acts as an accelerating factor. The differentiation between the degraded paints was further studied by PCA on previously pretreated FTIR data (SNV + Detrending, 650 1800 + 2700 3800 cm1 variable selection). As the selected measurement technique was transmission on KBr pellets, more variability among the replicated measurements is awaited compared to ATR measurements. However, better quality spectra with a more stable baseline and more significant relative intensities are expected. Compared to the POI degradation curves that were considering only the 2800–3400 cm1 variables, these PCA take into account all of the absorption bands that shape the spectra by only removing the regions of poor interest. Fig. 5 presents the individual PCAs of every sample measured by FTIR after 0, 1, 2 and 3 months. Artificially degraded samples are also included for comparison. We observe that the measurements from sample B1 are randomly distributed over the score space, with no clear trends or separations. This sample was also the one with the least good fitted curves over the POI values. From these results it seems that sample B1 is more stable than the others and do not degrade much over the time of this study even for the artificially degraded set. It is also the only one having an acrylic binder, compared to alkyd for other samples. For the others, the artificially weathered samples are more easily separated than the rest of the measures. They always present differences along PC1, except for the blue samples. The loadings highlight a few variables, mostly being the envelops
at around 3000 cm1, as well as some of the peaks of binders or extenders. None of them is however strong enough to be considered a main contributor to the differences observed. Sample B2 presents a separation between artificially and naturally degraded samples along the third principal component (PC3), influenced in part by the methylenic peaks (2800 cm1) and some of the binders peaks. The second principal component (PC2) accounted for the carbonyl peak at 1730 cm1 only but did not provide any separation. Apart from the artificially degraded samples, the distinction between the different months is less noticeable. There is not a consistent trend and it appears that these variations cover mostly some heterogeneity from the paint samples rather than a chemical modification of any type. In only two situations (R2, G1) the fresh samples (0 months) are separated from the others but this is the only substantial observation made. These results would suggest that when considering the whole spectrum, very few modifications permit to discriminate degraded from fresh samples. The effect of pretreatment might also have an important effect on these PCAs. The normalization over the whole spectrum as it is done with SNV will tend to diminish the importance of the hydroxy, amine and carboxylic acid envelop. Only by normalizing to the methylenic peak at 2930 cm1 and considering this restrained zone will permit to highlight the modifications in POI as presented in Fig. 1. Finally, the comparison between fresh (24 h), controls in the dark (3 months), and artificially degraded samples (10 months equivalent of a natural degradation) was also considered by PCA and is presented in Fig. 6. As was the case for Fig. 5 the results are very dependent on the type of samples. The blue sample 1 (B1) do not exhibit significant differences, and with all three conditions the
Fig. 7. Raman spectra of the 6 spray paints after, respectively 0, 1, 2, 3 months of natural weathering and 2 months of artificial exposition in a climate chamber.
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measurements overlap each other and do not present any separation. For the others, a separation is visible along PC1, with the artificially degraded samples on the left, controls in the middle, and fresh on the right. The loadings of PC1 are different for every sample but all have in common a positive value for the methylenic peak at 2930 cm1. On top of that the sample B2 and R1 show a simultaneous contribution of the magnesium silicate (talc) peak at 1020 cm1 with increased UV radiations. As magnesium silicate is a very stable mineral, it is expected that the talc peak remains constant over time, while all other binder peaks are decreasing due to the photo-oxidation process. The normalization of all spectra with SNV however tend to diminish these effects by increasing all other peaks toward their normalized value and giving a falseimpression of decrease for the 1020 cm1 peak intensity. Sample G2 do not have any separation between the dark controls and the fresh samples, but the artificially degraded samples possess a diminished absorption for the 1280 cm1 nitrocellulose peak. It also appears that samples used as control in the dark during 3 months do degrade a bit, even if not initiated by ultraviolet radiation. There was no strict control of the humidity and temperature factors during the study so it seems difficult to infer any reason for that. As it was performed as a preliminary study, it would require new tests in order to estimate the extent of non-UV degradation on this kind of samples. As a summary concerning the infrared spectra, is has been shown that all six samples have very distinct compositions, and the results demonstrate that they behave very differently regarding UV degradation. The only acrylic sample from the study seems to be more stable than the others. Alkyd polyesters on the other side seem to more rapidly show a decrease of the methylenic peak, sometimes accompanied by more specific modifications in peaks
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corresponding to magnesium silicate or nitrocellulose. These modifications are minor when considered over the whole spectrum, and would very rarely lead to a false exclusion of common source. 4.3. Raman spectra Visual examination of the Raman spectra showed no differences in peaks positions and peaks intensities (Fig. 7). Raman absorptions in this sample set being mostly due to the pigments composition, it was also checked if differences in absolute intensities can happen due to pigment degradation (i.e. modifications leading to color fading). The results highlighted that over the solar radiation received no visual differences were present between the spectra collected. As the POI is not calculable for Raman spectra, PCA were studied to find differences linked to the degradation. All Raman spectra were pretreated by baseline correction and normalized by SNV. No variables selections were performed. Fig. 8 presents the individual PCAs of every sample measured by Raman after, respectively, 0, 1, 2, 3 months of natural degradation and 2 months in climate chamber (artificial). Although some separations appear punctually for some samples, none of these can be linked to the degradation or amount of radiation received. Considering that, the results support the idea that the observed variations are more analytical and due to the measurement conditions, rather than a real degradation. Raman instruments are very sensitive to the surrounding environment, as well as calibration processes (for both wavenumbers x-axis, and power intensity y-axis). For these reasons, consecutive measurements a few days apart may likely show some minor differences. The loadings associated with these PCAs mostly represent some slight
Fig. 8. PCAs of the individual paints considering the Raman replicated measurements after 0, 1, 2, and 3 months of natural weathering, and 2 months of artificial exposition in a climate chamber (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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variations in baseline that could not be corrected further by the pretreatments. Our hypothesis is that the replicated measurements are in fact very similar and close to each other. These differentiations are mostly based on instrumental or external factors, unrelated to the UV radiation or other degradation influences. It should be noted also that PCA does not have any internal standard or any way to tell at which level of differentiation we are situated. The observation of separations might potentially be significant but also part of the heterogeneity among replicated measurements. By comparing different pretreatments and methods of baseline correction or spectra normalization, no improvements on the PCAs separations were reached. As a summary, the results obtained for the Raman samples show no analytical signs of degradation. A visual examination of the spectra demonstrates very repeatable and stable measures with consistent semi-quantitative ratios between all the peaks. From that it could be guessed that the pigments are much more stable to UV radiation than the binders or extenders. These results are consistent with the theory for these kinds of molecules which are likely more stable due to absence of reactive groupings or cross-linked structures that easily degrade. Even if a fading of the color was observed, the Raman spectra were not affected and remain unchanged after months.
5. Conclusion As a preliminary study on the UV-initiated degradation of spray paints, we have shown that spectral variations due to oxidization products are likely to appear in FTIR spectra, while Raman spectra are fairly more stable. Due to their relatively simple composition, spray paints are rapidly degraded and the differences begin to appear after a few days already. These are rapidly increasing until 2 months, where the degradation becomes more stable and follow a linear trend. The differences are mostly located in the 2800– 3400 cm1 range, and can be estimated using the photo-oxidation index (POI). One advantage is that the degradation products appeared almost exclusively at the far end (3000 cm1) of midinfrared spectra, and that the fingerprint region of the spectra remained stable over the studied period of time. Even so, care should still be taken when comparing two samples to assess a common origin, and degradation issues should be kept in mind to explain any significant difference that may appear between two paint samples. References [1] K. Adamsons, Chemical depth profiling of multi-layer automotive coating systems, Prog. Org. Coat. 45 (2002) 69–81. [2] K. Adamsons, Chemical surface characterization and depth profiling of automotive coating systems, Prog. Polym. Sci. 25 (2000) 1363–1409.
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