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Studying the effects of the chemical structure of an automotive clearcoat on its biological degradation caused by tree gums.

Abstract The effects of artificial and natural tree gums on the mechanical, chemical, and aesthetic performances of two automotive acrylic/melamine clearcoats were studied. To this end, two clearcoats with different acrylic/melamine ratios were investigated. Biological experiments were performed under post-aging conditions using an accelerated weathering lest. Analytical techniques including optical microscopy, scanning electron microscopy (SEM), gloss measurement, FTIR, and DMTA analyses were utilized to reveal the responses of the coating system upon exposure to the aforementioned biological materials. Contact angle measurements were also conducted to estimate the surface energy of the coatings. Greater crosslinking density, together with a higher Tg and damping behavior of the clearcoat, indicative of a greater degree of cure, were obtained as the ratio of melamine crosslinker increased. It was shown that both Arabic and natural tree gums could strongly attach to the clearcoats' surface, imposing a significant stress during the drying process, thereby leading to a physical failure. In addition, the acidic nature of these biological materials leads to a chemical alteration in the clearcoats' structure. The greater crosslinking density and lower hydrophilicity of the clearcoats containing higher melamine crosslinker were responsible for the weaker interaction of gums with the surface. This showed a greater capability for stress damping. Small surface cracks with fracture morphology on the coatings exposed to biological materials at higher exposure times (in the xenon test) were also observed. This is discussed based on the adhesion of the coatings to gums at longer exposure times, because of significant stress.

Keywords Arabic gum, Natural tree gum, Physical degradation, Biological materials

Introduction

Hydrolylic degradations and photodegradations are the two common phenomena causing most coatings to fail during their exposure to outdoor conditions. These include mainly hot-cold shocks, humidity, and UV radiation. (1-3) In addition, a rarely reported type of degradation can also be observed in coatings exposed to various pollutants originating from different natural sources. (4-8)

When coatings are exposed to a natural environment, chemical and/or physical influences can occur which may lead to degradation. Bird droppings are good examples of these conditions. This biological material contains various kinds of enzymes, i.e., amylase, lipase, and protease. (4), (5) It has been shown that, when an automotive coating is exposed to such enzymes, a severe chemical degradation may take place. Consequently, a significant etching can occur on the coating surface because of a catalyzed hydrolytic degradation in the presence of these enzymes. Moreover, the coating's mechanical properties have been shown to be negatively affected. The overall effect was the depreciation of the crosslinking density. (4) In another study, (6) the effects of such biological materials on two different clearcoats with different degrees of cure were investigated. It was revealed that the greater curing extent could not necessarily lead to an improved biological performance. Based on the results shown in our recent studies, the lowered surface hydrophilicity of the coatings with a greater degree of cure resulted in an inferior interaction between the coating and these biological materials. This caused a profound degradation. Accordingly, it was concluded that a balance in surface energy (hydrophobicity and hydrophilicity) seemed plausible for a coating with better biological resistance. (6) On the other hand, some of these pollutants could strongly attach to the coating surface and impose a significant stress, upon which a physical degradation seemed more probable.

Most car owners keep their cars under tree shadows to prevent them being exposed to direct UV light. However, the car may sometimes be impacted by the gums extracted from the leaves and the tree trunk. A few systematic studies have been reported on the mechanism of automotive coatings exposed to tree gums. Haagen described this as a biological degradation, (9) and reported the visual performance of affected coatings. In another study performed by Gaszner et al., the influences of Arabic and insect gums on an automotive coating was studied by optical microscopy. To our best knowledge, no systematic study on the mechanism of such degradations has been reported. The general degradation of coatings exposed to these materials is a physical phenomenon. Based on the general information reported in the literature, (9), (10) the visual appearance of degraded coatings includes the formation of cracks. No mechanism for this type of degradation, however, has been explained.

This work aims at investigating the degradation of coatings in the presence of tree gums. Natural and Arabic gums were utilized for this purpose. In order to simulate the effects of outdoor conditions on the biological attack, a biological test was performed at different exposure times in an accelerated weathering cabinet. The visual effects, appearance, and mechanical changes of the coatings exposed to these biologicals were used to reveal the mechanism by which these materials affected the coatings system.

Experimental

Sample preparation

A multilayer automotive coating system applied over carefully acid-washed steel metal plates was used. The steel sheets of D7-21.2 type, 1.2 mm thick, were provided by Foolad Mobarake Co. (Iran). The substrate was treated by a three-cationic phosphating conversion coating supplied by Irankhodro Co. (Iran). The phos-phated substrates were coated by a cationic waterborne electrodeposition coating (ED) containing an amine-modified epoxy resin supplied by PPG. Coated samples by ED layer were then finished by a primer surfacer, followed by curing at 140[degrees]C for 20 min. The surfacer layer was based on a mixture of a saturated polyester resin and melamine curing agent at 70:30 w/w ratio, on top of which a black basecoat layer was applied, followed by application of a clearcoat layer through a wet-on-wet method. Two different clearcoats (Cl-1 and CI-2) differing in acrylic:melamine ratios being 70:30 and 80:20, respectively were utilized. The information for each coating layer, including thickness and curing condition, is schematically represented in Fig. 1. The acrylic used in this study had a hydroxyl content of 4.5%. The melamine resin was a partially alkylated one. Both (JVAs and HALS were used as a stabilizer. The UVA and HALS used were Tinuvin 1130 (1-3 wt%) and Tinuvin 292 (0.5-1.0 wt%), all procured from Ciba Specialty Chemicals.

[FIGURE 1 OMITTED]

Biological test

Arabic and natural tree gums were used as biological materials in this study. The latter were obtained from a cypress tree planted in the northern part of Iran, and the former were purchased from the Merck Company. The pHs of tree and Arabic gums in water were 4.28 and 4.7, respectively. Because of the presence of some impurities in the natural gum, such as wood particulates, a simple filtration process was performed.

Arabic and tree gums were exposed to 1 [cm.sup.2] of the clearcoat surface, followed by maintaining samples at 100 and 300 h in a xenon chamber. The gum removal was carried out according to the PSA Peugeot-Citroen D27 5415 standard. Accordingly, samples were kept in DI water at 40[degrees]C for 1 h to remove the dissolved gum from the surface.

Instrumentation

An Atlas Xenotest Beta LM weather-o-meter, utilizing a xenon arc light source, with inner and outer borosilicate/borosilicate filter was used to simulate the outdoor aging conditions. This test was performed according to the Peugeot D27 1389-95 standard at 300 h. Gloss and DOI measurements were measured using a BYK-Gardner micro-tri glossmeter and a BYK-Gardner macro wave-scan VER 1.25 instrument. To further study the mechanism of degradation, digital photographs were taken from the samples using a Canon digital camera. For more detailed studies, back-scattered electron micrographs, as well as optical images, were also recorded, using a Phillips SEM and a Leica DMR optical microscope, respectively. A DME scanner AFM microscope DS 95-50 was also used to investigate the effect of biological materials at the nanoscopic scale. Micro hardness measurements were performed by a Leica VMHTMOT, at a load and loading times of 19.6 N and 20 s, respectively. A Tritec2000 DMTA instrument was used at the frequency, temperature, and heating rate of 1 Hz, -50 to 180[degrees]C, and 5[degrees]C/min, respectively, to study the crosslinking density and Tg of the samples. The surface energy of the clearcoats was measured using a Kruss G40 type contact angle measuring system.

Results and discussion

Clearcoat properties

DMTA and FTIR analyses were utilized to investigate the mechanical properties and chemical differences of two clearcoats. The DMTA curves of clearcoats (Cl-1 and Cl-2) representing storage moduli and loss peak changes are shown in Fig. 2. The data deduced from Fig. 2 are listed in Table 1.

[FIGURE 2 OMITTED]
Table 1: Mechanical parameters deduced from Fig. 2

Sample     Crosslinking density      Peak height of     [T.sub.g]
          (mol/[cm.sup.3]) (100x)     tan [delta]      ([degrees]C)

CI-1               10.6                   0.07             68.4
CI-2                6.64                  0.04             54.2


The results shown in Fig. 2 and Table 1 can clearly reveal the noticeable difference in thermal-mechanical properties between two clearcoats. The clearcoat having greater melamine content (Cl-1) shows a greater crosslinking density and therefore a higher [T.sub.g]. In addition, a greater loss of peak height and a higher storage modulus of Cl-1 demonstrate the enhanced elastic behavior and improved damping ability of this clearcoat against external stresses. In turn, a lower [T.sub.g] and crosslinking density shows a greater viscoplastic behavior, leading to a greater ability of the coating to plastically deform and heal. As was previously shown, the biological attack can be directly influenced by the viscoelastic properties of the coating.

The chemical composition of Cl-1 and Cl-2 (after curing) was studied before. (7) Results showed a higher reaction of acrylic to melamine as the melamine increased (sample Cl-1). Accordingly, a more complete curing degree of Cl-1 is expected. This can be clearly deduced from data given in Fig. 2 and Table 1.

Consequently, the surface composition of these clearcoats may vary due to the difference in curing. At lower ratios of acrylic to melamine, the presence of greater unreacted -OH groups at the surface is expected. This may result in a higher hydrophilicity of Cl-2. Therefore, the interaction of Arabic gum with the clearcoat surface can be expected to vary for these clearcoats.

Microscopic analysis

Optical and electron microscopes were utilized to investigate the effects of Arabic and natural tree gums on the clearcoats' surfaces. In order to investigate the effects of such materials under natural outdoor conditions, biologically exposed samples were kept under sunlight. The results are shown in Figs. 3 and 4.

[FIGURE 3 OMITTED]

According to Fig. 3, severe surface cracks were formed on the clearcoats exposed to Arabic and natural gums. This observation reveals that biological materials may significantly affect the visual performance of clearcoat. In addition, unlike the other common types of biological materials such as bird droppings, in which a chemical effect appears, (4) the failures occurring on the coatings exposed to gums seem to be physical. The cracks produced by natural gum on both Cl-1 and Cl-2 samples were wider and deeper in comparison to the one attacked by Arabic gum. The etched surface of clearcoats exposed to natural gum also differs from the surface properties of samples exposed to natural and simulated gums. Accordingly, natural tree gum can also affect clearcoat surface properties chemically. In addition, a greater number of cracks, larger in width and depth, together with higher etched areas of Cl-2, were obtained. These observations could result from the different mechanical and chemical properties presented between these two clearcoats. It can be seen that a higher resistance of the clearcoat to the stress from Arabic or tree gum can be obtained on the clearcoat showing greater elastic behavior. The same was also true for the clearcoat with improved crosslinking density and a lower hydrophilicity. As with the results observed in Fig. 3, a more severe effect of Arabic gum on the Cl-2 sample is observed from SEM micrographs. In addition, by comparing the secondary and back-scattered (not shown here) SEM images of different samples, dark and light areas around the degraded parts of the clearcoats can be observed. The contrast between the light and the dark areas in SEM micrographs (Fig. 4), represents the differences in the chemical composition of the affected areas. The dark area can be attributed to the presence of lower atomic number elements, whereas the light regions correspond to the higher atomic elements. These observations are in agreement with the results shown in our previous study, indicating the presence of metal compounds around the degraded parts. (4) Based on the above explanations, natural tree gums, like other kinds of biological materials such as bird droppings, may also consist of metal compounds. More discussion on these metal compounds will be found later in this paper. These results are similar to the results obtained in our previous study, (7) representing the same effect of Arabic and natural gums on the clearcoats' performance.

[FIGURE 4 OMITTED]

To study the effects of Arabic gum, an accelerated weathering test was utilized to simulate the outdoor conditions. To this end, two exposure times of 100 and 300 h were utilized. The reason is that, at higher exposure times (greater than 300 h), the pholodegra-dation may be more effective. (11), (12) Therefore, distinguishing between the effect of biological degradations and photodegradations seems very difficult. Moreover, based on the PSA Peugeot-Citroen D275415 standard, 100 h of xenon test exposure was suggested to simulate the effect of outdoor aging. Hence, the current exposure (300 h) was used mainly because of a greater influence from biological materials compared with that from photodegradation in the course of the experiment.

As was previously shown, (7) because of the sticky behavior of gums in the slurry state, they adhere well to the clearcoat surface. Many researchers have tried to distinguish the main source of this adhesion. (8), (10) In fact, the tendency of Arabic or natural tree gum to adhere to the surface results from the polar groups existing in this material. Therefore, the adhesion of Arabic gum to coating can directly depend on the clearcoat surface energy (the balance of hydrophilicity and hydrophobicity). Another hypothesis which may be true involves the effect of metal compounds in Arabic gum structure, causing a greater adhesion of coating to gum. Based on the results shown in our previous study and other works, (4), (13) the presence of the metal compounds in natural bird droppings and pancreatin such as [Ca.sup.2+] and [Na.sup.+] can be responsible for high adhesion of biological materials to polymeric surfaces. The presence of metal compounds in both natural and Arabic gums (as will be discussed later) can possibly indicate the same effect in creating a strong interaction of gums and coating. On the other hand, although Arabic gum was able to produce a good adhesion to coating in the slurry state, a poor adhesion was observed in the dried form. This means that moisture can have a significant role in the interaction of gum and coating.

According to the above explanations, the effect of gums on the clearcoat can be directly correlated with the strong adhesion before the experiment, as well as with the weak attachment after drying. This behavior produces a great stress on the clearcoat, which in turn is responsible for the physical degradation of the coating, as shown in Figs. 2 and 3. The failure which this stress can induce on clearcoats can depend on both the clearcoats' compositions and the undercoat layers' mechanical and viscoelastic properties. In addition, the effects of this stress on coating performance can be separated into two different phenomena: (i) stress relaxation, and (ii) crack propagation. Based on coating viscoelastic properties, different behaviors of the coating in response to the inserted stress are predictable. When an external stress is imposed on coatings there are three main possible responses, i.e., plastic, elastic, and fracture deformations. As most coatings are viscoelastic materials, these responses will appear in the form of plastic/elastic or elastic/fracture deformations. The viscose part of the coating is responsible for plastic deformation (nonreversible) and the elastic part for elastic deformation (reversible). The fracture behavior of the coating can be observed when the stress applied to the coating exceeds the coating's ultimate strength. When coatings are exposed to an external stress, the initial response of the coating is elastic, which is inherently a rapid relaxation phenomenon. It means that the stress can be easily recovered after stress removal without any deformation. On the other hand, a subsequent plastic response can also be seen.

Coating Cl-1 appears to resist crack formation from biological attack more robustly than coating Cl-2. This may in part be due to the ability of coating Cl-1 to resist mechanical deformation. As seen from the data in Fig. 2, the modulus of Cl-1 is higher both in the glassy state and in the rubber plateau than that of coating Cl-2. This indicates that for a given stress, the deformation in Cl-1 will be smaller than that of Cl-2. In addition, the crosslink density of Cl-1 is higher than that of Cl-2. Previous workers have shown that the relationship between crosslink density and toughness is complex.14-16 Certainly, below some optimal level the toughness of a coating can increase as crosslink density increases. However, at some point the crosslink density becomes too large to support significant plastic deformation, resulting in a decrease in toughness. Coating Cl-1 is likely closer to the optimal level of crosslink density than coating Cl-2.

Based on the observations made in Fig. 5, it can be said that the exposure time of the xenon test can significantly influence the degradation process. This effect is more severe on the clearcoat having the lower crosslinking density.

[FIGURE 5 OMITTED]

According to Fig. 5, no visual failures can be observed on the blank sample. In addition, no visual effects can be observed on the samples exposed only 300 h to xenon testing. This observation may reveal that xenon testing is not able to show any detectable surface failure at the micro scale. It can be clearly seen that the cracks formed on the clearcoats experiencing 300 h of xenon is greater than for the samples exposed to 100 h. Such observations mostly correspond to the number of cracks and their size. The greater exposure time of Arabic gum in the xenon test caused more cracks to be shorter and narrower, compared with the samples exposed to 100 h of xenon.

To explain the greater surface cracks occurring on the clearcoat exposed to 300 h of xenon, two different hypotheses may be useful, as will be discussed later. As was previously shown, (7) because of the brief gum drying stage the stress is imposed on the clearcoat very rapidly. Therefore, the rate of stress applied to the clearcoat during drying of the Arabic gum decreases. However, in a real outdoor condition, different wet (humidity and rain) and dry (sunlight) conditions are continuously applied to the gum in contact with the clearcoat surface. In such conditions, gum drying can occur whenever it is exposed to sunlight. However, the dried gum may also be in contact with humid conditions afterward. This indicates that the gum periodically absorbs water and then becomes dry. This can lead to formation of stress and consequently more cracks. To simulate the effect of this environmental condition on gum behavior and coating degradation, in each 100 h of exposure, water was sprayed on dried Arabic gum. This action changed the gum to a slurry state again and created a strong interaction with the clearcoat surface. In each step, the interaction of coating and gum can occur, producing significant stress on the clearcoat. Therefore, the clearcoat covered by Arabic gum and exposed to 300 h of xenon testing is more affected than the sample exposed to 100 h of xenon. Another probable hypothesis for explaining the greater surface cracks in coating at higher exposure times involves the aging behavior of coating under UV radiation. According to the results shown in previous studies, (11), (12) although slight photodegradation was observed at low exposure times of xenon testing, clearcoat surface energy increased. This process was related to the increase of surface polar groups produced on the coating exposed to UV light. This means that UV radiation can age the clearcoat, especially at higher exposure times. Based on the explanations made previously, the greater surface polarity of the coating can cause a stronger interaction between the coating and the Arabic gum in the slurry state. Therefore, the biological performance of the clearcoat due to the aging process can be negatively influenced. Hence the greater attack occurring on the sample exposed to longer xenon testing can be explained. The same phenomenon can be observed in the results obtained with two clearcoats. The more intensive failure observed on Cl-1 can be ascribed to the differences between both the surface polarity and the viscoelastic properties of this sample compared with those of Cl-2. As was previously shown in Fig. 2 and our previous study, Cl-1, because of its more complete curing degree, showed a greater elastic behavior. In addition, lower hydrophilicity of this clearcoat due to the lower unreacted remaining -OH groups is expected. For Cl-1, however, a weaker interaction of gum and clearcoat surface is more probable, as this clearcoat is less hydrophilic. The stress stored in the clearcoat structure can gradually be relaxed. On the other hand, stronger gum interaction with the Cl-2 surface, due to its greater hydrophilicity, can be expected. This can be responsible for the greater stress produced during the gum drying process. Moreover, the lower stress damping behavior of Cl-2, due to its viscoplastic character, can cause initiation and propagation of some microcracks. This can start with the parts of the clearcoat having lower cohesion.

In order to compare the irreversible effects of this kind of degradation with those of other types, the surface morphology of the coatings exposed to Arabic gum and different xenon testing conditions (not exposed to Arabic gum) was studied using AFM microscope images as shown in Fig. 6.

[FIGURE 6 OMITTED]

Different roughness parameters of the samples shown in Fig. 6 are calculated and listed in Table 2. It seems that samples which are exposed to Arabic gums under 300 h of xenon testing are affected more significantly than the samples which only experienced the xenon test conditions. In addition, by comparing the surface morphology and roughness of the sample exposed to Arabic gum and 300 h of xenon testing to those of the one exposed very much longer (even for 1000 h of xenon), a greater surface roughness can be seen. This can indicate that a greater degree of biological degradation on the coating at nanoscale is more plausible than for the samples which only experience photodegradation. The greater surface roughness was obtained for Cl-2 samples. This can again show the lower resistance of this clearcoat to Arabic gum.
Table 2: Roughness parameters obtained from samples shown in Fig. 6

Sample                           [S.sub.y] (nm)    [S.sub.a] (nm)

CM                                    243                7.49
CI-2                                  127                5.50
CI-1 (300 h xenon)                    460               18.8
CI-1 (1000 h xenon)                   679               17.8
CI-2 (300 h xenon)                    594               22.4
CI-2 (1000 h xenon)                   743               13.4
CI-1 (Arabic gum-300 h xenon)         804               11
CI-2 (Arabic gum-300 h xenon)        4103              491


Effect of Arabic gum on clearcoat chemical structure

The IR spectra of the clearcoats before and after the weathering are shown in Fig. 7.

[FIGURE 7 OMITTED]

According to this figure, the vibration intensity of the hydroxyl groups (3380 [cm.sup.-1]) for both clearcoats has increased, indicative of coating degradation during the weathering test. A greater increase in intensity of OH for Cl-2 can be observed after the weathering. This can reveal the greater possibility of etheric linkage break under UV irradiation. Alternatively, the lower resistance of sample Cl-2 to photodegradation may be due to the lower curing degree of this sample, the effect of which is a greater hydrolytic degradation and photodegradation during the weathering test.

The surface chemistry of clearcoats (before and after the weathering test) was also studied using contact angle measurements, as shown in Table 3.
Table 3: Polar and disperse components of surface energy of samples

Sample                    Polar part  Disperse part  Surface energy
                            (mN/m)        (mN/m)         (mN/m)

CI-1 (fresh sample)          3.40          33.0           36.4
CI-1 (after 300 h xenon)    14.8           32.0           46.8
CI-2 (fresh sample)          5.20          37.0           42.2
CI-2 (after 300 h xenon)    26.8           27.4           54.3


A lower value of the polar component as well as the total surface energy of Cl-1 compared to Cl-2 can be observed in Table 3. These exhibit the greater hydro-phobicity of Cl-1. This finding is in agreement with the results obtained from DMTA analysis. Therefore, it may mean that the effect of decreased unreacted hydroxyl groups during curing of the Cl-1 sample has counterbalanced the tendency of melamine groups to increase the coating polarity. The polar component and therefore the total surface energy of both clearcoats decreased after 300 h weathering. This increase is more pronounced for the Cl-2 sample, a finding which is completely in agreement with the results shown in Fig. 7. This can also indicate that the polarity of Cl-2 has significantly increased during UV exposure, leading to an increase in gum interaction with the clearcoat surface.

The chemical structure of clearcoats (before and after biological attack) was studied using the FTIR technique. The normalized FTIR spectra of Cl-1 and Cl-2 samples are shown in Fig. 8. The normalization of spectra was done by taking the -C-H peak at 2825 [cm.sup.-1] as an internal reference.

[FIGURE 8 OMITTED]

In our previous study, (7) it was shown that gums affected coating properties not only physically, but also chemically. Due to the acidic nature of gums, similar to acid rain, (14) they catalyze the hydrolysis reaction. The more acidic behavior of the natural tree gum is responsible for a greater effect on etching of the coating. This etching behavior seems different for two clearcoats. To show this behavior, the quantified bond intensities of each clearcoat, obtained from Fig. 8, are shown in Table 4.
Table 4: Deduced data obtained from DMTA and FTIR results for different
clearcoats

Sample             [DELTA] [NH/N[H.sub.2] and OH]   [DELTA] [Etheric
                      /[CH] (3380 [cm.sup.-1])     bands] (1108 cm-1)

CI-1 (Arabic gum)              +0.34                      -0.10
CI-2 (Arabic gum)              +0.47                      -0.09

Sample                   [v.sub.e]           Peak    [T.sub.9]
                   (mol/[cm.sup.3]) (100x)  height  ([degrees]C)

CI-1 (Arabic gum)          9.05              0.05        49
CI-2 (Arabic gum)          8.40              0.03        47


According to the results shown in Fig. 8 and Table 4, a greater A [NH/[NH.sub.2] and OH]/[CH] can be observed for Cl-2. This can demonstrate a greater etching performed by Arabic gum on Cl-2. Lower crosslinking density together with higher hydrophilicity of Cl-2 are therefore responsible for the greater hydrolysis phenomenon in this sample. Accordingly, the sample having a lower curing degree not only experiences a greater physical damage, but also shows a lower chemical resistance against Arabic gum.

Effect of Arabic gum on clearcoat mechanical properties

The stress which Arabic gum can impose on clearcoats during its drying process can affect its mechanical properties. To reveal such a phenomenon, DMTA analysis was performed on the biologically degraded clearcoats (Fig. 9).

[FIGURE 9 OMITTED]

Based on the changes occurring on the storage modulus and loss peak of Cl-1 and Cl-2 samples, different parameters, including crosslinking density, [T.sub.g], and loss peak height, were calculated and are shown in Table 4. By comparing the results shown in Table 4 and Fig. 9 with the results shown in Fig. 2 and Table 1, the significant effect of Arabic gum on the mechanical properties of clearcoats can be observed. According to these results, a decreased crosslinking density and [T.sub.g], as well as loss peak height of Cl-1 can be seen. However, different results were observed for Cl-2. Although the [T.sub.g] of Cl-2 decreased after the biological attack, the crosslinking density of this sample increased. This observation is similar to those which were demonstrated in the previous study.7 During the biological attack, severe stress produced by gum drying is imposed on the clearcoats. However, the effect of this stress on these two clearcoats due to their different viscoelaslic properties is different. During the biological attack, photodegradation can also be performed on the clearcoats, and a complex effect of stress and photodegradation can occur. Cl-1, because of its greater elastic behavior, has a greater capability for stress relaxation; consequently, the stress produced by Arabic gum can be released from the clearcoat after the biological test. However, as a result of UV irradiation and the acidic nature of Arabic gum slurry, clearcoat crosslinking and [T.sub.g] can be affected by the photodegradation and hydrolytic reaction during the test. A different result, however, was seen with Cl-2. Therefore the lower stress damping behavior of this clearcoat, because of its lower elastic behavior, seems reasonable. This may cause a greater plastic deformation and undamped stress in the clearcoat layer, which can directly affect the storage modulus of the clearcoat at the rubbery plateau zone. In addition, as a result of the incomplete curing of this sample, the postcuring, which may occur during the UV irradiation (during biological testing), is plausible. The result of this is the increased crosslinking density of the clearcoat. However, decreased [T.sub.g] may reveal the effect of Arabic gum on the cohesion of the samples.

Appearance analysis

According to the results obtained by the various techniques shown in Figs. 3-9, the severe effects of such degradations on the appearance parameters were measured. The DOI and gloss retention of 75 and 20.80, respectively, were obtained for the Cl-1 sample. These were 70 and 15.6 for the Cl-2 sample, respectively, showing that the optical attributes have declined significantly. Both clearcoats' appearance parameters were negatively influenced during the biological attack. However, the lower gloss and DOI retentions were observed for Cl-2 (exposed to Arabic gum at 300 h of xenon). This clearly shows the greater effect of biological degradation (in both physical and chemical ways) on the clearcoat having lower [T.sub.g] and crosslink-ing density. The greater number of cracks, together with the higher etched areas produced on this sample during the Arabic gum attack, can be responsible for this significant change in coating appearance. These observations indicate that the degradation occurring on the coating exposed to Arabic gum can irretrievably influence the coating appearance, because of the formation of cracks. According to electron micrographs and optical images, the surface cracks produced by Arabic gum can be responsible for such low gloss and DOI retentions. These cracks have irregular shapes which can scatter the incident light diffusely from the clearcoat surface, causing a lower gloss. In addition, the results shown in Table 2 clearly indicate that the low gloss and DOI retention can be directly attributed to the high roughness produced by Arabic gum, especially at 300 h of xenon. However, the significant changes in appearance parameters of these samples may not necessarily be attributed to their crack numbers or surface roughness differences. According to the SEM micrographs shown in Fig. 10, it is clear that the surface cracks produced on the Cl-2 exposed to Arabic gum at 300 h of xenon testing have more fracture morphology compared with the cracks produced on Cl-1. The fractured cracks, due to their sharp and irregular edges, lead to greater scattering of incident light. (17), (18) In this regard, the main reason for the very low gloss retention of Cl-2 compared with Cl-1 is due to the severe fracture morphology of the cracks in the former.

[FIGURE 10 OMITTED]

Another obvious result in the SEM micrographs in Fig. 10 is the presence of white stains inside the cracks. This observation is similar to the results shown in our previous study of bird dropping-exposed samples.4 In order to investigate the chemical compositions of these stains and their effect on the coating's biological performance, energy dispersive spectroscopy (EDS) was utilized. In Fig. 11, the results of a biologically degraded coating exposed to Arabic gum are shown.

According to the results obtained from EDS, different elemental compositions of nondegraded parts and degraded parts can be observed. The white stains seen inside the cracks were analyzed and compared with the nondegraded parts of the coating. The existence of 4.98 and 0.16 wt% of Ca and K, respectively, at white stains can clearly suggest the presence of metal compounds in degraded parts of the surface. The presence of such metal compounds on degraded parts after the removal of biological residues from the coating surface may indicate that these metal compounds have strongly attached to the coating surface. It seems that such metal compounds, especially Ca in gum structure, may affect the interaction of gum with the coating surface, and therefore contribute to the coating degradation process. However, due to the complicated chemical structure of Arabic or natural gum, further investigation of these phenomena is needed.

[FIGURE 11 OMITTED]

Conclusion

This study aimed at investigating the effect of clearcoat chemical structure on its biological performance against natural and simulated tree gums. In this regard, clearcoats containing different acrylic/melamine ratios were used.

The results revealed that the stress produced on the clearcoats caused surface cracks differing in size at various exposure times of xenon testing. Greater surface cracks having more fracture morphology, in contrast to lower surface cracks with plastic morphology, were observed on the clearcoat containing lower melamine content. It was shown that a greater adhesion between coating and gums occurred at 300 h of xenon compared with 100 h. This resulted in an increase in the clearcoats' surface polarity during exposure to UV light, causing a greater coating tendency on gum slurry. Therefore, higher stress produced on coating exposed to Arabic gum at 300 h of xenon is responsible for fracture morphology and higher crack numbers in this system. The results also revealed that biological attacks on systems exposed to xenon testing had irreversible effects on clearcoat gloss and DOI retention. However, a greater decrease was observed for the clearcoat exposed to gum at 300 h of xenon testing. Fewer mechanical and chemical changes were obtained for the clearcoat having the greater elastic behavior (CM). The clearcoat with greater crosslinking, [T.sub.g], and damping behavior showed lower physical and chemical failure. Lower hydrophilicity and a higher capability of the clearcoat with an improved degree of cure are responsible for the weaker interaction of gums with the clearcoat surface and for stress relaxation during the gum drying process. Numerous fractured cracks with sharp and irregular edges on Cl-2 are the reason for the greater light scattering and therefore the dimmer appearance of this sample.

It seems that both viscoelastic and surface characteristics of the clearcoat affect its biological performance.

Acknowledgments The authors would like to thank the Iran Khodro Company and Paint Shop 1 for providing the coating samples.

References

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B. Ramezanzadeh, M. Mohseni (El), H. Yari

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran

e-mail: mmohseni@aut.ac.ir

[C]ACA and OCCA 2011

DOI 10.1007/s11998-010-9312-z
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Author:Ramezanzadeh, B.; Mohseni, M.; Yari, H.
Publication:JCT Research
Date:May 1, 2011
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