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Screening the weathering stability of automotive coatings by chemiluminescence.


The development of ever more stable coating systems on the one side, and the need to reduce development cycle times on the other side, has led to a bottleneck in the testing of new systems. Demands to shorten testing times are becoming ever stronger.

One solution could be the use of a more sensitive detection technique since most weathering tests have to be continued up until weathering effects can be detected. The time needed for the breakdown or deterioration in a certain property to occur is often the basic result of a test. This detection is commonly focused on bulk properties such as haze and gloss, discoloration, or cracking. However, degradation usually begins with small changes on the molecular level, spreads to the outer surface layers, and finally reaches the bulk of coating material. A technique that only detects at the final stage wastes valuable exposure time in comparison to a technique that could detect the starting stages of degradation at the molecular level, or at least at the microscopic level. Such a sensitive technique in the latter sense is chemiluminescence. (1) Therefore, an attempt was made to combine artificial weathering with CL--a technique that could reduce the usual exposure duration of 2000 h by at least half.

CL is part of a group of different luminescence techniques. (2) The CL produced by oxidative processes is a highly sensitive indicator of the thermo-oxidative stability of polymers. (1,3,4) Because practically all degradative aging reactions are linked to oxidation of the material, it enables one to follow the ageing process by measurement of light emissions as follows,

[I.sub.CL] = K x [PHI] x r

with [I.sub.CL] as the CL emission, K a detection device specific constant, [PHI] as the material specific quantum efficiency of the CL process, and r as the reaction rate of the oxidation reaction.

First studies of the chemiluminescence of polymers were published in the 1960s by Ashby (5) and Russell. (6) Russell's pioneer work showed that the oxidation of polymers was closely connected with the formation of peroxy radicals. According to the Russell mechanism, recombination of secondary peroxy radicals leads to the formation of singlet oxygen and the excited triplet state of carbonyl groups in the polymer chains. (6) The transfer of these excited species to their grounded state can produce light emission called the chemiluminescence.

2[R.sub.1][R.sub.2]CHO[O.sup.[dot]] [right arrow] [R.sub.1][R.sub.2]CHOH + [.sup.1.O.sub.2] = [R.sub.1][R.sub.2](C = O)* [right arrow][right arrow][right arrow] [R.sub.1][R.sub.2](C = O) + hv

Scheme 1: The Russell mechanism

With the development of more efficient photomultipliers, the number of the publications in this field increased rapidly. In the last five years, about 150 papers were published. CL is one of the few techniques that directly measure the rate of oxidation at a particular time of exposure, as opposed to direct or indirect measurement of the accumulated concentration of oxidation products that is used by a much bigger number of established techniques. A number of similar techniques--including photoinitiation rate measurements and hydroperoxide concentration measurements--were investigated by Gerlock et al. (7-12) They compared them to techniques that follow accumulated degradation products such as FTIR. (11) The detection of hindered amine light stabilizer (HALS) in automotive coatings was featured in a review article. (13,14) The role of hydroperoxides in the photo-oxidation of cross-linked polymer coatings is described in Mielewski. (15) CL is measuring the concentration of peroxy radicals in the coatings because its emission is assumed to be derived from the recombination of peroxy radicals described by the Russell mechanism.

In unexposed coatings, this concentration is very low and their CL emission curves reflect a thermal oxidative stability. In weathering exposed coatings, the peroxy-radical level depends on the concentration of hydroperoxides in the coating that are formed in the photo-oxidation due to weathering exposure. Their decomposition is the rate-determining step, and the starting point, of the so-called autoxidation scheme of polymer oxidation, (16) and also is the main focus of stabilization by HALS. The autoxidation scheme--a free radical chain process--is assumed to describe the oxidation of saturated polymers. The hydroperoxide level starts at a low value during a weathering exposure, increases rapidly after leaving the intial so-called induction period for the autocatalytic acceleration phase of autoxidation, eventually reaches a plateau value, and usually levels off when radical concentration reaches levels high enough for recombination reactions to overcompensate initiation reactions. The role of stabilizers is to scavenge active radicals formed after hydroperoxide decomposition and in the radical chain process. In comparison to nonstabilized systems, they result in a lower radical concentration during the induction period and mainly delay the passing of the induction period into the acceleration reaction.

Studies of the degradation of polyurethane automotive coatings has been described by many authors who used various techniques like IR spectroscopy (17,18) or thermal analysis. (19) The results of these investigations are reflected in the following equations:

[R.sub.1]-C[H.sub.2]-NH-C(O)-O-C[H.sub.2]-[R.sub.2] + [P.sup.[dot]] [right arrow] [R.sub.1]-CH-NH-C(O)-O-C[H.sub.2]-[R.sub.2] + PH

[R.sub.1]-[[dot][dot].C]H-NH-C(O)-O-C[H.sub.2]-[R.sub.2] + [O.sub.2] [right arrow] [R.sub.1] - C(O[O.sup.[dot][dot]])H-NH-C(O)-O-C[H.sub.2]-[R.sup.2]

[R.sub.1]-C(O[O.sup.[dot][dot]])H-NH-C(O)-O-C[H.sub.2]-[R.sub.2] + [O.sub.2] + PH [[right arrow].-[P.sup.[dot]]] [R.sub.1]-C(O)-NH-C(O)-O-C[H.sub.2]-[R.sub.2] + [H.sub.2]O [right arrow] [R.sub.1]-C(O)-OH + N[H.sub.2]-C(O)-O-C[H.sub.2]-[R.sub.2]

Scheme 2: Oxidation of polyurethane

The CL as a means to study the photostabilization of polyurethane automotive coatings in connection with weathering exposure was first reported in 1998 by Dudler and Bolle. (20) They showed that the CL method is suitable for the study of polyurethane degradation. A new artificial weathering test for automotive coatings was published in 2000 by Schulz et al., (21) and was used by Wachtendorf and Geburtig, (22) in combination with CL as a sensitive detector of weathering effects to shorten the duration of the tests. The residual stability of poly(ester urethanes) and poly(acrylic urethanes) automotive coatings was studied in 2006 by Fratricova et al. (23) However, only a few CL studies on the stability of polyurethane coatings have been published so far. The aim of this work is to investigate the thermo-oxidative stability and the efficiency of light stabilizers in various well-defined automotive coatings after artificially weathering them with a dependence on specific sample parameters such as variation of matrices, hardeners, and stabilizers.



The sample is heated in a gas-tight cell. Under a constant flow of oxygen, samples are heated in 10 K steps from ambient temperature up to a temperature of 410 K, keeping each respective new temperature level for a duration of 60 min. As a result, the sample is oxidized. Chemilumincscence uses the weak emission of light emitted in the oxidation process to monitor the kinetics of an oxidation reaction. In the first approximation, the CL emission is proportional to the rate of the oxidation reaction, with high emission meaning rapid oxidation reaction and low oxidative stability. The CL cell consists of a BAM (Federal Institute for Materials Research and Testing) built heating sample chamber attached to a photomultiplier, and commercial equipment for single photon counting. (24) Heating is performed electrically and controlled by a Lake Shore (Westerville, OH, USA) 340 temperature controller. The gas-tight sample chamber has a gas inlet and outlet with the sample located in its middle, and a quartz window on its top that connects it to a photomultiplier that is a side-on, low-noise photomultiplier tube (HAMAMATSU, Herrsching, Germany, Type R1527P select). The signal of the photomultiplier tube is processed by a Perkin Elmer/EG & G/ORTEC (Oak Ridge, TN, USA) single-photon counting device consisting of pre-amplifier VT120C, discriminator 935, and counter 994. The ORTEC counter and the Lake Shore temperature controller are connected via an IEEE bus to a personal computer (PC) that uses programmed Turbo Pascal software to read out and control.


Eight different automotive coatings were investigated. All of these are model compounds of Bayer MaterialScience AG (Leverkusen, Germany), containing the following components from bottom to top: aluminum-panel, primer-surfacer (35 [micro]m), white-pigmented base coat (15 [micro]m), and finally clearcoat (40 [micro]m). Samples were cut to a size about of 15 x 15 [mm.sup.2]. The eight clearcoat systems differ in matrix (polyacrylate polyol PAP or polyester polyol PEP), stabilizer (with or without HALS) and hardener (HDI, HDI/IPDI unblocked or 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 matrix of systems and their components is shown in Table 1.

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 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.

Results and discussion

The degradation effects due to the preceding artificial weathering exposure were analyzed using CL. The aim was to allow a ranking of different clearcoat systems in terms of weathering resistance at an earlier stage than using established detection techniques. The CL investigations showed that the thermo-oxidative stability of the different automotive coatings is dependent on the sample composition--like matrix, hardener, and stabilizer. Therefore, results are presented on the oxidative stability of these automotive coatings as a function of matrices, stabilizers, and hardeners.

Before (0 h) and during the weathering exposure, samples were taken after 1000 h and after 2000 h, respectively, and the subsequent CL measurement outside the weathering chamber was used to determine the effective stabilization at the respective exposure duration. CL was used this way to titrate the momentary oxidation state of the polymer and its residual stabilization under weathering exposure at a particular exposure time.

The photo-oxidative degradation that a polymer undergoes during weathering might be sketched as it is in Fig. 1. Lower oxidative stability results in a shorter induction period and a faster degradation (see Fig. 2). The stability is mostly influenced by the matrix and the stabilizer on the one hand, and its degradation due to exposure on the other.

The CL conducted at a certain stage of weathering exposure starts with the level of photo-oxidative degradation obtained up to that point and adds the thermo-oxidative degradation on those polymer species that were not already oxidized during the weathering process. Because thermo-oxidative and photo-oxidative degradation of saturated systems usually are considered to proceed via radical intermediate states, CL investigations are relevant for photo-oxidative degradations such as weathering exposures, as well.



Influence of artificial weathering

The CL experiments on all automotive coatings show a significant effect after artificial weathering compared to the CL of unexposed samples. The CL emission increases as a function of the duration of artificial weathering. The CL emission of System 1 (PAP/HALS/HDI) during the heating up to 410 K for a weathering period of 1000 h and 2000 h is shown in Fig. 3 (left graph). The weathering time of 1000 h resulted in an increase of CL emission and a lowering of the temperature needed to cause significant CL emissions in comparison with the unexposed sample.

Looking at the behavior during heat-up (left graph in Fig. 3), the difference between 1000 h and 2000 h of exposure is small over the investigated temperature range, with systems after 2000 h of exposure showing slightly higher emissions. This situation is to be expected because, after the longer photo-oxidative exposure, less stabilization should be left in the system and therefore it should oxidize more readily in the CL experiment. The difference in emission levels between 1000 h and 2000 h of exposure depends on the composition of the systems. Looking beyond this example, if the systems contain HALS, the CL emission increases or remains constant after 2000 h of exposure compared to the CL after 1000 h. The HALS stabilizer slows the oxidative degradation down and causes less oxidative degradation following oxidative exposure. It seems that after about 2000 h of weathering exposure, the limit of the stabilizer's effectiveness seems to be reached.


In contrast to the stabilized systems, those without HALS show a much higher absolute emission level. However, in the comparison of emissions after 1000 h and those after 2000 h, slightly higher CL emissions show up. This might be explained by an already advanced stage of photo-oxidative degradation during the weathering process (for instance Point C in Fig. 1) that already was reached because no stabilizer protects the system. Furthermore, the decay of the isothermal CL at 410 K differs between the exposed and the unexposed systems, which is shown in Fig. 3 (right graph). The decay of CL emissions in the isothermal period is due to consumption of reactive species and/or rate control of diffusing oxygen.

The unexposed system expectedly shows low emissions and has not reached its maximum of CL emission during the isothermal heating within the 10 h CL investigation, although it is still rising. The latter might be explained by enough radical scavenging stabilizers in the system to suppress higher oxidation rates because it has not undergone photo-oxidative degradation in a preceding exposure stage, and thus exhibits the original stabilizer level at the moment it is introduced in the CL experiment.

Influence of the matrices

Comparing different polymer systems involves a complicating factor for the interpretation of CL results in terms of stability against oxidation--that is, the quantum efficiency [PHI] is an additional factor between CL emission and reaction rate. As mentioned above, this factor is specific for any material, and therefore has to be taken into consideration for a comparison of CL results from different systems. While the determination is difficult, a normalization of the emission of the respective unexposed systems is proposed instead and is used in the following section for any comparison of different systems. (22)

When varying the matrix, a significant influence on normalized CL intensities can be observed. The heating up to 410 K, and the isothermal phase at 410 K of Systems 3 and 4 are depicted in Fig. 4 (left and right graph). The intensities shown are normalized again to the respective unexposed system's emission at 410 K to make up for the different quantum efficiencies of different materials. The compared systems only differ in the matrix. The CL intensities of all PAP systems are higher than the CL intensities of the PEP systems. Therefore, the temperature needed before considerable CL emissions can occur are lower for PAP samples than for PEP systems, which is also shown in Fig. 4. This indicates that the oxidative stability of PEP systems is higher in comparison to the PAP systems.

Influence of the stabilizer

The influence of the stabilizer is as dramatic as expected. The CL emission of the PAP/HDI system with the HALS stabilizer is considerably lower than without the stabilizer, which is represented in Fig. 5. This means that--as expected--the HALS stabilized system shows more stability than the unstabilized one. The stabilizer effects an improvement of the thermo-oxidative stability of the systems.



The CL intensities of all unstabilized systems are significant higher than those of the stabilized systems. The temperature needed to show considerable CL intensity is also higher for all stabilized systems than for the unstabilized systems. Furthermore, for the heat-up behavior (left graph in Fig. 5), and during the isothermal CL experiment (right graph in Fig. 5), the CL emission of the stabilized systems increases with the duration of weathering (from 1000 h to 2000 h). In contrast, the emission of the unstabilized systems after 2000 h is lower than after 1000 h of artificial weathering--the oxidative degradation of the matrix is probably already in an advanced stage.

Also notable is the characteristic shape of the isothermal CL curve at 410 K. It is dependent upon the presence or absence of the stabilizer. This is particularly the case for all PEP systems. The stabilized PEP systems show a smaller decay in CL emission during the isothermal phase at 410 K than the unstabilized systems, which is demonstrated in Fig. 5. The CL emission of the stabilized PEP systems runs a wide maximum during the isothermal phase after approximately 1 h--in contrast to all other investigated systems.

Influence of the hardener

The hardener did not have a great influence on CL intensities or the temperatures needed to show noticeable CL emissions of exposed systems. Notably, the decrease of the CL emission during the isothermal phase depends on the type of hardener--e.g., HDI hardener or HDI/IPDI blocked hardener--which is demonstrated in Fig. 6. The included hardener HDI caused a smaller decay during the isothermal phase at 410 K because, for example, hardener HDI/IPDI blocked.


Influence of the oxygen partial pressure

In another experiment, the coatings were investigated in different gas atmospheres, which are represented in Fig. 7. It showed that the coatings, which were measured after exposure to oxygen, showed the highest CL intensity. Emission at exposure to air is already lower and the intensities measured after exposure to nitrogen were negligible.

Therefore, it could be concluded that the CL emission during the (isothermal) heating is the result of an initial reaction of the polymer with oxygen. Experiments regarding exposure to nitrogen do not show CL intensities, which are caused by the absence of oxygen. The experiments regarding air show only half the intensities. If the experiments take place in oxygen, the oxidability of the systems--and consequently the CL emission--increases, because more oxygen is available for the reaction.


Ranking of the systems

After the preceding classification in terms of the influence of stabilizer, matrices, and hardeners, and the influence of experimental parameters, it was our goal to rank the systems in terms of performance of weathering resistance against subsequent degradation. Two different approaches (dynamic and static) were used to evaluate the systems' stability in the CL results:

A ranking of the respective levels of CL emissions at 400 K during the heat-up can be used as one criterion in evaluating the systems with highest stability, which is demonstrated in Fig. 8. According to this, the result is the following ranking of decreasing weathering stability:

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

The ranking of the systems during the isothermal phase at 410 K is another way to select the systems, which is shown in Fig. 6. Here, the respective isothermal CL emission levels after 2 h are compared. The 2 h window is necessary to get a differentiation of the CL emission of the various systems. This results in the following ranking:

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

There is a raw agreement between the rankings of the heating up to 400 K and the isothermal phase at 410 K. The difference in CL emissions is very small, but it allows us to make a more general observation that systems 4, 2 and 5 perform best, and systems 3 and 7--the PAP systems without stabilizers--perform worst. In principle, the CL method is able to divide the systems into good and bad performance, but it is difficult to find a ranking in the midfield. Because the investigated coatings are very similar, the CL method can only differentiate between the most extreme cases, while a larger field of similarly performing coatings remains.


The CL evaluation results of weathering effects reflect the chemical degradation that occurs on a molecular level. This can be related to the damage on a macroscopic scale as evaluated by visual inspection, gloss retention, and yellowing. These results, using established detection for discoloration and yellowing, show good agreement with the ranking obtained from the CL evaluation. In accordance with the CL results, the good system in that inspection was also system 2, and the worst systems were systems 3 and 7. The front-runner determined by the CL investigation of clear-coats with the highest stability is the same as detected by use of thermoluminescence, which was carried out simultaneously to this study. (25) The best system there was also system 2, and the worst ones were systems 3 and 7.


Chemiluminescence proved to give an objective measure for the degree of degradation in automotive clearcoats in the course of weathering. It showed comparable results to established means of evaluation. Its sensitivity provides the potential of assessing the damage at an early stage. It could be shown that the CL emission depends on sample-specific parameters of stabilizers, matrices, or hardeners, experimental parameters such as CL investigation temperature, and the oxygen partial pressure, as well as exposure parameters such as weathering duration.

It was demonstrated that CL has the potential to classify materials into categories of sufficient, medium, and low performance at a much earlier stage of weathering exposure than with usual macroscopic detection methods.


In ongoing studies, further exposure stages between unexposed and 1000 h weathering exposure durations were included. In the future, it is projected that CL ranking of the systems will correlate not only to physical properties, but also to actual chemical changes--for instance, in terms of IR spectroscopy experiments.

Other future studies aim to determine the possible extent of reduction in the duration of exposure time. A principle difficulty in the use of chemiluminescence for the evaluation of large numbers of samples is the extra time needed for the CL investigation of each sample. For large numbers of samples to be evaluated, it is mandatory to have several CL instruments operating in parallel.


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25. Kruger, S, Brademann-Jock, K, Rauth, W, Klimmasch, T, Kruger, P, "Analysis of Weathering Effects in Automotive Coatings by Thermoluminescence." XXVIIIth FATIPEC Congress, Proceedings, VII.P-8, Budapest, Hungary, 12-14 June 2006, ISBN: 9639319554

[c] FSCT and OCCA 2007

S. Kruger ([mailing address]), V. Wachtendorf

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


W. Rauth, T. Klimmasch, P. Kruger

Bayer MaterialScience AG, 51368 Leverkusen, Germany
Table 1: Components of the eight clearcoat systems investigated

System no. Matrix Hardener Stabilizer

2 PEP HDI blocked HALS
7 PAP HDI/IPDI blocked
8 PEP HDI/IPDI blocked
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Author:Kruger, S.; Wachtendorf, V.; Rauth, W.; Klimmasch, T.; Kruger, P.
Publication:JCT Research
Date:Mar 1, 2008
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