Analysis of weathering effects in automotive coatings using thermoluminescence.

Introduction

Thermally stimulated luminescence (TL) can be used for the early diagnosis of aging, caused by the inherent high sensitivity to structural, morphological, and chemical changes in macromolecular chains. (1) TL is emitted following electronic excitations, chemical reactions, and recombination of trapped charges. Such processes occur predominantly at chromophoric groups such as carbonyls or other [pi]-electron containing groups. In many cases, the polymer itself does not emit any significant luminescence, and the observed emission derives from impurities. Therefore, this method is well-suited for the detection of additives in polymers--particularly stabilizers. The detected luminescence can be caused by charge-trapping defects, irregularities, and radical sites, either in polymers or in additives.

In the TL experiment, emitted photons are recorded as a function of temperature during a thermal scan (glow curve). These photons result from radiative transitions of free electrons that are released from traps to recombination centers. The electrons are trapped by structural or chemical defects during the low-temperature irradiation of the polymers. Traps in polymers can be of different natures--either physical or chemical. For example, electrons can be localized in the cavities or voids of structural imperfections. Free radicals are the most important kind of chemical trap. Moreover, electrons can be also attached to double bonds. Thus, the position and the intensity of the TL peaks can be associated with structural and phase changes, or with temperature-induced changes at which relaxation processes occur (glass transitions, crosslinking, plasticization, preorientations, etc.). (2)

Fundamental work for the phenomenon of TL was developed at end of the 1940s by Randall and Garlick. (3,4) In the 1950s it resulted--due to the possibility of the use of inorganic materials in the dosimetry--in a rapid development in the field of thermoluminescence. TL of polymers was first reported in 1963 by Charlesby. (5) A basic review article was published in 1972 by Partridge. (6) Since this time, reports have pointed out that the behavior of light emission is related to the structure and the variations of polymeric materials. (7-12) TL is a kind of fingerprint for polymers. For this reason, glow curves can be used for the early detection of damages in automotive coatings after weathering. However, studies of the degradation of polyurethane automotive coatings via thermoluminescence are not reported in the literature. In contrast to that, many authors describe the degradation of polyurethane automotive coatings using other techniques like IR spectroscopy, (13,14) thermal analysis, (15) or chemiluminescence. (16-18) Here, for the first time, TL measurements were used to detect damages in various well-defined automotive coatings after artificial weathering at the earliest stage of degradation, dependent upon specific sample parameters like matrices, hardeners, and stabilizers.

Experiment

Method

The principal course of a TL experiment can be described as follows (9): A sample is bombarded with high-energy radiation to generated electrons. After the excitation is switched off, there is a spontaneous recombination of the electrons with cations at constant temperature (isothermal TL, ITL), and this is recorded as a decay curve as a function of time. After this emission has levelled off, the heating-up process is started at 80 K and works up to ambient temperature. In the glow curve, the thermally stimulated luminescence (TL) monitors photons as a function of temperature during a thermal scan.

In an in-house constructed vacuum- and light-tight chamber, the samples were subjected to a pressure of [10.sup.-4] Pa, and the polymer sample was cooled to 80 K. For the irradiation of the polymer sample, an electron gun (7.5 keV; electron gun EGPS-14 B, Kimball Physics Inc.) equipped with a computer controllable shutter was used. After 5 m of excitation, the irradiation was stopped and the emitted radiation from the sample was collected and transmitted by flexible fiber optics to a monochromator. The wavelength of the emitted light is in the UV/Vis range (about 350-650 nm). (19)

Samples

The white automotive coatings we investigated were model compounds prepared by Bayer MaterialScience AG (Leverkusen, Germany). They contain the following components from bottom to top: aluminum plate, primer (35 [micro]m), white basecoat (15 [micro]m), and finally a clearcoat (40 [micro]m) as a topcoat. The eight clearcoat systems are differing in matrices (polyacrylate polyol PAP or polyester polyol PEP), stabilizers (with or without HALS) and hardeners (HDI, HDI/IPDI blocked or not blocked). The HALS is a mixture of Bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate and Methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate, and the polyacrylate polyole is commercial matrices. The composition of systems is detailed in Table 1. For the TL measurements, artificially weathered samples of 100 mm x 75 mm size were cut in pieces of 10 mm x 20 mm each.

Artificial weathering exposure

The artificial weathering was carried out using a commercially available Xenon arc lamp weathering device (ATLAS Company, Germany, type Weather-Ometer Ci 5000 with an inner Quartz filter and an outer Type S Boro filter) by Bayer MaterialScience. The weathering cycle used was the CAM 180 test described in SAE J 1960. The following exposure stages were used for the eight systems: (a) without weathering, (b) after 1000 h of artificial weathering, and (c) after 2000 h of artificial weathering. For the TL measurement, the sample size was 10 mm x 20 mm.

Results and discussion

The weathering effects were investigated for eight automotive clearcoat systems as a function of a variation of matrices, stabilizers, and hardeners. The resulting effects of artificial weathering were analyzed using TL, mostly with the goal of ranking different coating systems relative to their performance in terms of weatherability against subsequent degradation.

Influence of artificial weathering

First, the influence of artificial weathering on the coatings will be presented. The glow curves show a significant effect after artificial weathering in comparison to the respective exposed system to unexposed system, which is represented in Fig. 1. The TL emission after 1000 h and 2000 h of artificial weathering decreased dramatically. This effect of weathering on TL has been found for all investigated systems and can be interpreted as a degradation effect.

In the course of weathering, chain scissions occur and the coating network is partly broken up. This allows the oxidation of molecular fragments to proceed much faster than would occur on the unexposed coating surface before any degradation. The generated oxidized fragments are able to quench TL emission. Therefore, the TL emission is reduced for weathered coatings in comparison to the unexposed ones, which was observed for all investigated systems. The only visible difference is the extent of reduction of TL emission, which can be attributed to the composition of the clearcoats (stabilizers, hardeners, matrices).

[FIGURE 1 OMITTED]

Notably, the difference in TL intensities between an artificial weathering of 1000 h and 2000 h is very small. This saturation (in terms of quenching) probably means that the extent of damages does not change very much with a higher duration of weathering. The TL emission is only occurring at the surface (typically about 4 [micro]m) of the coatings. (9)

Macroscopic properties of coatings, like yellowing and gloss changes, show only an insignificant change between 1000 h and 2000 h, as our measurements showed. (19) Most likely a steady-state of degradation was reached. Further TL experiments that shorten weathering time and parallel running infrared spectra are essential and need to be planned. To compare relative contributions of TL emissions as a function of the temperature of exposed and unexposed clearcoats, TL emissions were plotted normalized to the respective maximum in Fig. 2. The position of the main peak of the glow curves remains at about the same temperature (approximately 110 K) as for the unexposed system. However, the shoulder at approximately 170 K and the peak at 240 K of the unexposed systems disappear after 1000 h and 2000 h of artificial weathering, respectively. The duration time of 1000 h or 2000 h has no further influence on the shape of the glow curve.

The electrons can be trapped by other structural or chemical defects after artificial weathering. Therefore, the position and the intensity of the TL peaks, which can be associated with structural changes or with temperatures at which relaxation processes occur (e.g., glass transitions and crosslinking), changes after 1000 h or 2000 h of artificial weathering.

Considering other TL investigations of coatings, it can be concluded that the degradation of the matrix during the weathering is the reason for the disappearance of the shoulder near 160 K. (20)

[FIGURE 2 OMITTED]

Influence of the matrices

Furthermore, the glow curves show great influence from the matrix. The unexposed systems with a polyacrylate (PAP) matrix show essentially more TL emission than the respective systems with a polyester (PEP) matrix, which is demonstrated in Fig. 3. This relationship in TL emissions for the two matrices is essential for all investigated unexposed systems. It seems that the PAP matrix offers more trap sites for the electrons. Therefore, TL emission is higher than in the PEP matrix. However, the TL emission also is dependant on the quantum efficiency of the TL emission process, which is material specific. The lower TL emission of the PEP systems could also be affected by lower quantum efficiency, the value of which has not yet been determined.

As represented in Fig. 4, the shape of the glow curves of the unexposed systems is also dependent on the matrix. The unexposed PEP systems show a main peak at 110 K, a shoulder at approximately 170 K, and a smaller peak at 240 K. The unexposed PAP systems show the same main peak at 110 K. In contrast to that, the PAP system shows another shape of the glow curves at higher temperatures--a shoulder at 180 K. These differences are probably the result of different structures of the network and the different functional groups in the two respective matrices.

[FIGURE 3 OMITTED]

Within the same matrix, the TL emission of all exposed systems decreases dramatically in comparison with the respective unexposed systems. All glow curves of the exposed systems (within the same matrix) show roughly the same level of TL emission independent of the duration of weathering time (1000 h and 2000 h). However, the level of TL emission is influenced noticeably by the matrix, as represented in Fig. 5.

The TL intensities of the exposed PAP systems are twice as high as the PEP systems--probably caused by the different structured networks which were described above. In contrast to that, the characteristics of all glow curves of the exposed systems are nearly identical, which may be caused by the advanced degradation of the coatings. The surprising fact that all exposed systems of the respective matrix show about the same level of TL emission, independent of the duration of weathering, could mean that the duration of the weathering was too long.

Influence of the stabilizer 

As expected, the glow curves of the unexposed systems show the dramatic influence of the stabilizer. The TL emission of the unstabilized systems is significantly higher than with HALS stabilized systems, which is shown in Fig. 6. The stabilizer is responsible for the decrease in TL emission. Two causes are possible. The stabilizer may be quenching the TL emission, or the stabilizer lowers the number of trap sites in the matrix. It should be noted that the stabilizer did not influence the shape of the glow curves. It seems, therefore, that the stabilizer only acts as a quencher. Regrettably, the differences of the TL emission of the exposed systems (between 1000 h and 2000 h) were very small. It is assumed that the advanced degradation of the coatings impeded the ability to find differences (between 1000 h and 2000 h).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Influence of the hardener 

The hardener markedly affects the TL intensity of the unexposed systems. One example of the influence of the hardener is demonstrated in Fig. 7. The unexposed systems with the hardener blocked HDI/IPDI show the highest level of TL emission in comparison to other hardeners for both the PEP and PAP matrix systems. It should be added that the hardener only influences the level of the TL emission, not the shape of the glow curves. Two possible reasons for this emission behavior can be suggested. The hardener HDI quenches the TL emission, and/or the hardener HDI influences the number of trap possibilities. In contrast to the unexposed systems, the glow curves of the exposed systems do not show a significant effect from the hardener, probably because of the extended weathering time.

Comparison of the systems

After considering the parameters like the hardener, stabilizer, and matrix, another goal of this work was to find a correlation of TL results with established detection modes--for instance, discoloration and yellowing--to rank the various automotive systems. The latter was very difficult, because the weathering time was too long and the degradation of the coatings was too advanced to characterize weathering effects with the highly sensitive TL method. Therefore, this ranking is only a simply attempt to find correlations between macroscopic properties and TL emission of coating systems. The difference between the TL emission of one system at 1000 h and one at 2000 h is too small and cannot used for the ranking. However, the differences between the systems after weathering--e.g., after 2000 h--can offer the possibility of comparing the coatings. There are two possibilities for ranking the investigated systems. The first one is analyzing the intensity at the TL maximum at 110 K, and the second one is analyzing the integral intensities of the glow curves. These two criteria were used to try to rank the various automotive coating systems. The first ranking possibility is shown in Fig. 8.

[FIGURE 7 OMITTED]

As different systems are compared, it is essential to normalize the TL emission to correct for sample-specific emission efficiencies. Exposed systems were normalized for the emission of the respective unexposed systems. Using this procedure the following ranking was received:

[FIGURE 8 OMITTED]

e.g., after 2000 h of weathering:

2 > 6 > 1 > 4 > 8 > 5 > 3 > 7

The second possibility for ranking the systems is represented in Fig. 9. It was analyzed using the normalized integral TL emissions of the whole glow curves that were normalized in respect to the unexposed systems. Comparing the integrals of the TL emission of the various systems, the following ranking order results:

e.g., after 2000 h of weathering:

1 > 2 > 6 > 4 > 8 > 3 > 5 > 7

Both ranking possibilities show a good correlation. It can be concluded that the best systems are 2, 6, and 1, and the worst systems are 5, 3, and 7. The best systems (2 and 6) are both stabilized polyester automotive coatings, and the worst systems (3 and 7) are both unstabilized polyacrylate automotive coatings. As expected, the stabilizer is the critical part for weathering stability, but the matrix also has an important influence on stability. In contrast to that, the influence of the hardener on the TL signal is marginal.

These results are in accordance with the chemiluminescence (CL) results, which are published in Kruger et al. (18) Using CL, the best system was also 2 and the worst were 3 and 7. Thus, it shows a good agreement between both methods.

Furthermore, a good correlation could be found (ranking correlation factor 0,9) between visual inspection (yellowing, color change) and the TL emission, but in the case of TL it was at much earlier stages of weathering exposure. That would be an advantage, but further TL experiments are needed, and infrared spectra must be run in parallel to confirm this ranking. This work is the first step in finding a correlation between the relatively unknown TL method and macroscopic characteristics of the automotive coatings.

[FIGURE 9 OMITTED]

Conclusion

The degradation effects due to the preceding artificial weathering exposure can be analyzed very sensitively using TL. The presented TL investigations show a clear dependence on parameters like stabilizers, matrices, hardeners, temperature, and weathering time, and provide a potential opportunity to monitor the early stages of damage to polymers. The TL results showed an agreement with established evaluation techniques. However, in contrast to these established methods, the TL could allow the evaluation at significantly earlier degradation stages and could therefore enable one to reduce the time required for weathering tests. Future studies are aimed at finding a finer differentiation within the ranking of automotive coatings and at determining the possible extent of a further reduction in the duration of the weathering exposure. That would be an advantage, but it needs still more TL experiments and it must be made parallel to running infrared spectra to confirm this ranking. This work is the first step to finding a correlation between the relatively unknown TL method and macroscopic characteristics of automotive coatings.

References

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[c] FSCT and OCCA 2007

S. Kruger ([mailing address]), K. Brademann-Jock

Federal Institute for Materials Research  and Testing, BAM VI.3, Unter den Eichen 87, Berlin 12205, Germany

e-mail: [e-mail]

W. Rauth, T. Klimmasch, P. Kruger

Bayer MaterialScience AG, Leverkusen 51368, Germany

Table 1: Components of the eight clearcoat systems investigated

System No.  Matrix  Hardener          Stabilizer

1           PAP     HDI               HALS
2           PEP     HDI blocked       HALS
3           PAP     HDI
4           PEP     HDI
5           PAP     HDI/IPDI blocked  HALS
6           PEP     HDI/IPDI          HALS
7           PAP     HDI/IPDI blocked

8           PEP     HDI/IPDI blocked