Assessing the Variability of Adhesion Promoter Adhesion Performance

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Ind. Eng. Chem. Res. 1996, 35, 1771-1776

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Painted Thermoplastic Olefin System: Assessing the Variability of Adhesion Promoter Adhesion Performance following UV Exposure E. Nu ´n ˜ ez, P. J. Schmitz,* and J. W. Holubka Ford Research Laboratories, MD3083, SRL, P.O. Box 2053, Dearborn, Michigan 48121-2053

The development of test methods for assessing fundamental properties of materials which influence in-service coating adhesion is important for screening potential material candidates. The main focus of this study was to develop a methodology for screening adhesion-promoting primer materials used on thermoplastic olefin (TPO) systems as to their susceptibility to UV degradation. This work assesses the adhesion performance of a single unfortified (no UVA or HALS additives) 2K polyester-urethane topcoat over a number of commercially available adhesion promoter systems following ambient UV exposure. Adhesion performance is correlated with the relative photodegradation of the adhesion promoter materials. This study was specifically designed to simulate conditions of thin topcoat application and/or loss of light stabilizers in the paint by adventitious processes that would render the adhesion promoter/ topcoat interface exposed to ambient UV radiation. Results of the adhesion testing showed that there is a wide range in the performance of the adhesion promoters used in this study. Adhesion loss following ambient exposure to UV radiation was observed in as little as 100 h to greater than 6300 h of exposure. The variation in the performance of the adhesion promoters was found to correlate with the degree of photooxidation of the materials. These results suggest that this type of a methodology would be useful in assessing the relative sensitivity of polymers used in adhesion promoter formulations when exposed to ambient weathering conditions. Introduction Thermoplastic olefins (TPO) are becoming increasingly popular as a substrate material for painted automotive fascia because of price and recyclability. However, the adhesion of coatings to TPO is generally poor and, therefore, requires additional substrate preparation for proper paint adhesion. Adhesion promoters (AP’s), which are spray-applied primers, are commonly used to improve the adhesion of a coating to TPO. A number of studies have focused on various aspects of the AP-TPO system (Aoki (1968), Dechent et al. (1993, 1994a,b), Grundke and Jacobasch (1992), Lawniczak et al. (1993a,b, 1994), Ryntz (1991), Schmitz and Holubka (1995)). The mechanism involved in adhesion improvement has not been unequivocally determined; however, it appears to be associated with interpenetration of polymer materials at the TPO-AP interface. The AP can, therefore, be regarded as a coupling agent in the painted TPO system. AP’s generally are composed of a chlorinated polyolefin (CPO) formulated into a primer resin package. It is generally believed that the CPO is the primary component of the AP which binds the primer to the TPO surface. Although the specific formulation of the resin matrix differs from manufacturer to manufacturer, the CPOs themselves are generally a chlorinated polypropylene (Lawniczak (1993a,b)). As is evident, the function of the AP is very important to the performance of the painted TPO system. Therefore, determining potential shortfalls of the AP, which may compromise long-term performance of the paint system as a whole, is of importance. The stability of coatings, with regard to photodegradation, is probably the principal factor compromising long-term paint durability. AP primer systems should, theoretically, be in the UV dark for a stabilized paint system containing UV packages and proper film builds and, therefore, are * Author to whom correspondence is addressed. E-mail: [email protected]. FAX: (313) 594-2923.

not themselves UV stabilized. However, in the event of low film builds or improper UV stabilization of the basecoat/clearcoat system, UV penetration to the AP may occur. Therefore, to fully understand the robustness of the paint system as a whole, it is of importance to develop methods to evaluate the resiliency of AP primer systems to photodegradation which, in certain circumstances, may affect long-term topcoat adhesion performance. In this study an attempt has been made to develop a methodology for assessing the variability of topcoatadhesion promoter adhesion performance and relative adhesion promoter degradation following UV exposure. The adhesion loss of a single unfortified (no UVA or HALS additives) 2K polyester-urethane coating to a number of commercially available uniquely formulated AP’s was evaluated as a function of UV light exposure. An unfortified coating was utilized to assure UV penetration to the AP layer. The photodegradation of the adhesion promoter materials themselves was also followed as a function of UV exposure, in a separate experiment, using transmission FTIR. Correlations are made between the extent of change of the adhesion promoters following UV exposure by FTIR, and adhesion performance in the top-coated systems. In addition, the effect of process variation (coating and AP bake temperatures) on the performance of the systems studied was also evaluated. The main focus of this work is to develop a means of screening adhesion promoter materials for inefficiencies, such as in weatherability, which may affect the overall performance of the paint system. Experimental Section Materials. Five commercially available adhesion promoters were used in this study. Since the formulations of these adhesion promoter systems are proprietary and the main focus of this work is to assess a method of determining differences in performance and not link performance to any specific material formula-

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1772 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996

tion, we will treat all systems as generic materials. That is, the five adhesion promoters will be referred to as AP-1 through AP-5, and only generic information is supplied. The first material, AP-1, was a chlorinated polyolefin material and was not formulated as a complete primer package. AP-1 is the same type of CPO formulated in the AP-2...AP-5 adhesion promoter systems. The other adhesion promoters AP-2...AP-5 can be referred to generically as a polyester-acrylic and a CPO component, with each AP having its own unique filler and additive packages. The topcoat applied over all the AP’s was a commercially available two-component (2K) polyester/ urethane clearcoat which was prepared without UVA or HALS additives. All materials were used without additional purification. The TPO used was a commercial product and was in the form of molded plaques cut into 10 × 15 cm sections. Prior to coating application, the plaques were washed with a commercial aqueous acidic cleaner. The cleaner system simulated a production power wash process and is designed to remove low molecular weight olefinic and/or extraneous materials (dirt and greases) introduced during the molding and handling of the parts. Coated plaques used for UV exposure and adhesion testing were prepared in the following manner. Powerwashed panels of TPO were sprayed with adhesion promoter (as received) and subsequently sprayed with an unfortified 2K polyester-urethane clearcoat. The dry thicknesses of the adhesion promoter and topcoat were approximately 8 µm (0.3 mil) and 46 µm (1.8 mil), respectively. The two different processing conditions evaluated consisted of (1) a 3 min adhesion promoter flash at 25 °C (77 °F) followed by topcoat application and baking at 88 °C (190 °F) for 30 min and (2) a 15 min adhesion promoter bake at 88 °C (190 °F) followed by topcoat application and baking at 88 °C (190 °F) for 30 min. Laboratory Weathering and Adhesion Performance Testing. The painted plaques were tested for initial adhesion using a “cross-hatch tape pull” method. After testing for initial adhesion, the panels were placed in the QUV weatherometer and exposed to UV light under “near ambient conditions” (40 °C, air; 25 °C, water; UVA 340 bulbs). The panels were periodically removed to check adhesion loss as a function of UV light exposure. Adhesion failure was reported when greater than 5% adhesion loss from the cross-hatch area was observed. Spectroscopic Analysis of Adhesion Promoter Films. Fourier Transform Infrared (FTIR). It was of interest to look for any bulk changes (degradation) of the adhesion promoters with UV exposure that could be used to correlate differences observed in the adhesion performance testing. This was accomplished by using transmission FTIR to look at the changes induced by UV irradiation of films of the individual adhesion promoter materials. To facilitate transmission IR analysis, samples of adhesion promoter were centrifuged to remove the filler materials (pigments) from the organic solution containing the CPO and remaining organic resin. The supernatant solutions formed were then used in the FTIR studies. Specimens for FTIR analysis were obtained by casting the above-mentioned solutions onto silicon disks (Wilmad Glass Co., Buena, NJ). FTIR spectra were obtained in transmission using a Mattson 5020 Fourier

Table 1. Summary of Adhesion Promoter Systems, Preparation Conditions, and Adhesion Performance Results Paint Application Conditions supplier AP-1 AP-2 AP-3 AP-4 AP-5 a

adhesion promoter bake

topcoat bake

time of failure (h)

3 min @ RT 15 min @ 190 °F 3 min @ RT 15 min @ 190 °F 3 min @ RT 15 min @ 190 °F 3 min @ RT 15 min @ 190 °F 3 min @ RT 15 min @ 190 °F

30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F 30 min @ 190 °F

40 200 92 68 6300a 6300a 2800 3500 3400 4200

No failure observed, exposure terminated at the time listed.

transform spectrometer; 16 interferograms of doublesided symmetry (4 cm-1 resolution) were coadded. Single-beam spectra were ratioed against a reference spectrum taken on an uncoated silicon disk in a chamber purged with dry air. Film thicknesses were such that the maximum absorbance at 1730 cm-1 was less than 1.5. FTIR spectra were first taken on the unexposed samples and then taken periodically during weathering exposure by removing the silicon disks briefly from the QUV machine. Weathering conditions were identical to those used for the painted TPO panels. Photochemically induced adhesion promoter degradation was followed by comparison of the spectra of the weathered and unweathered samples. X-ray Photoelectron Spectroscopy (XPS). XPS was used to identify differences in the surface composition of the unweathered AP films. Analyses were performed on the same coated silicon disks used for the FTIR analyses. XPS analyses were performed using an SSX-101 spectrometer manufactured by Fisons Surface Science, Mountain View, CA. All spectra were acquired using monochromatic Al KR radiation (1486.6 eV) focused to a 600 µm spot and operated at 100 W. Charging effects were minimized by use of a low-energy (1-3 eV) flood gun in conjunction with a Ni charge neutralization screen. All data reduction routines utilized were supplied by the instrument manufacturer. Results and Discussion Adhesion Performance Testing. The stability of the painted TPO system to photochemically induced paint adhesion loss was evaluated by examining the adhesion of an unfortified (formulated without UV light absorbers and HALS stabilizers) 2K polyester-urethane coating over the series of adhesion promoter materials. No modification was performed on any of the adhesion promoters prior to application, all adhesion promoters were applied as production materials. This study simulates conditions of thin topcoat application and/or loss of light stabilizers in the paint by adventitious processes that would render the adhesion promoter/ topcoat interface exposed to ambient UV radiation. To enable the direct comparison of adhesion performance during weathering, a single 2K polyesterurethane coating was evaluated over each of the five adhesion promoters. All the samples were prepared and tested in the same manner. Summarized in Table 1 are the various adhesion promoter materials and process conditions that were examined in this study. A painted system prepared without any adhesion promoter (i.e.,

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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1773 Table 2. XPS Measured Surface Compositions of Adhesion Promoter Films elemental composition, atomic %

Figure 1. Exposure time to ambient UV radiation required to induce adhesion loss in topcoat-adhesion promoter systems on TPO.

topcoat applied directly to the TPO plaque) showed no initial adhesion. All other systems passed the initial adhesion testing. The adhesion performance results for the different systems following UV exposure are compared in Table 1 and Figure 1. The AP-2 adhesion promoter based system showed the most significant adhesion loss. Complete “sheeting off” (delamination of intact free film) of the topcoat from the adhesion promoter was observed. Coating adhesion loss occurred in less than 100 h of UV light exposure, whether the adhesion promoter had been subjected to a room temperature flash or baked at 88 or 121 °C (190 or 250 °F). The paint system prepared using the AP-1 material as an adhesion promoter failed at what appeared to be the topcoat/adhesion promoter interface after 200-400 h of laboratory weathering exposure. After ∼3000 light hours, the AP-4 adhesion promoter sample failed interfacially between the adhesion promoter and the substrate. The AP-5 adhesion promoter system also failed at the adhesion promoter/substrate interface after ∼4000 h of UV light exposure. The AP-3 system was weathered for 6300 h and showed no deterioration of properties other than some embrittlement of the topcoat. No large differences were observed in the performance of any of the adhesion promoter systems with adhesion promoter bake temperature. The results of the adhesion testing show that there can be a wide fluctuation in the performance of the adhesion promoters when exposed to UV radiation. The performance of the adhesion promoters suggests that the AP-1 adhesion promoter material is very sensitive to light and likely prone to photochemically induced topcoat adhesion loss should the adhesion promoter be exposed to UV light. It is clear from these results that this type of an approach should be useful in assessing the relative performance of adhesion promoters that are exposed to UV. In addition to the adhesion testing, it was of interest to see if the degree of change induced in the bulk coating exposed to UV correlates with the relative adhesion performance of the topcoat-adhesion promoter systems. FTIR analyses were conducted on films of the adhesion promoter materials themselves both prior to and following exposure to UV. FTIR Studies of Adhesion Promoters. FTIR examination of the series of adhesion promoters was conducted after the pigments were removed by centrifugation. No evidence of precipitation of a less dense,

AP system

C

O

Cl

Si

N

Al

AP-1 AP-1 AP-2 AP-2 AP-3 AP-3 AP-4 AP-4 AP-5 AP-5

90.1 89.7 84.1 83.2 87.6 86.9 95.8 95.9 81.9 83.5

1.0 1.0 10.5 10.6 11.0 11.8 4.2 4.1 8.3 7.4

8.4 9.4 3.5 3.2 1.4 1.3

0.4 2.2

0.7

0.8 0.7

4.1 4.1

4.3 3.5

1.4 1.5

organic component (as shown by an inhomogeneous precipitate) was observed in any adhesion promoter sample. Although details of the adhesion promoter formulations are unavailable, the FTIR results suggest that the initial formulations of these adhesion promoter materials are quite different (Figures 2a-6a). A cursory XPS analysis of the adhesion promoters also appears to show significant variations in surface composition (Table 2), consistent with unique formulations (resin, filler, and additive packages) for the different AP systems. Photochemically induced changes in the adhesion promoter films were assessed qualitatively by following the growth and change of carbonyl moieties (∼1730 cm-1). Carbonyl growth, indicative of photooxidative degradation of acrylic topcoats, (Bauer et al. (1990)), was followed as a function of UV exposure time. The changes in the 1000-1500 cm-1 region of the FTIR spectra were used to assess other changes in the adhesion promoter materials. A band at about 700 cm-1 that is characteristic of the C-Cl bond of the CPO was also examined as a function of exposure, but the characteristic weakness of this band as well as the modest concentration of the CPO in the adhesion promoter formulations made accurate monitoring of this band difficult. The FTIR spectra for the adhesion promoters are shown as a function of exposure time in Figures 2-6. The adhesion promoters were observed to weather the same when baked at 88 °C (190 °F) or flashed at room temperature. Therefore, only the results of the baked films are depicted in Figures 2-6. All adhesion promoters showed growth and broadening in the carbonyl region (∼1730 cm-1 ) as a function of UV exposure. This is indicative of oxidation of the polymer and formation of degradation products. More evidence of oxidation products comes in the formation of -OH bands in the 2500-3500 cm-1 region of the spectra for all materials. The adhesion promoter materials were found to exhibit qualitatively similar photochemically induced changes when weathered. Because details of the adhesion promoter formulations are unavailable, it is difficult to speculate on the changes induced by the UV exposure. However, it can be generalized that the spectra from all the materials experienced a growth and in some cases broadening or splitting of the carbonyl region, growth of -OH bands (2500-3500 cm-1), a general broadening of the 1000-1500 cm-1 region, and a simultaneous attenuation of the -CH stretching bands (2900 cm-1) with exposure. In an attempt to compare the relative degradation occurring in each adhesion promoter system with UV exposure, the changes in the carbonyl growth and CH stretch intensities were monitored. The intensity of the carbonyl band at 1730 cm-1 was ratioed to the intensity

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Figure 2. FTIR transmission spectra of AP-1, as received and following exposure to UV light.




of the CH stretching band at 2900 cm-1. The increase in this ratio, relative to the ratio measured from the unweathered materials, is shown in Figure 7. The data of Figure 7 show the change following 1000 h of weathering. As is evident, the relative change in this ratio correlates well with the relative adhesion performance of the adhesion promoter systems. That is, from the adhesion performance testing the relative performance can be ranked as AP-3 > AP-5, AP-4 > AP-1, AP-2. From the change in carbonyl growth in the IR spectra the relative degradation of the AP systems can

be ranked as AP-3, AP-2 > AP-5, AP-4 > AP-1. The only discrepancy that is observed is for the AP-2 system, the poorest performer in the adhesion testing which gives one of the smallest changes in carbonyl growth. However, when one takes a closer look at the IR spectra of AP-2, in addition to the general changes in spectral features observed for all the other adhesion promoters, the spectra for AP-2 also show an abrupt decrease in absorbance following exposure that was not apparent with the other adhesion promoters. This is evident following even very short exposures of about 200 h.

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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1775

Since the failure mode/mechanism was not investigated in detail and the specific formulations of the different AP systems are unknown, it is only possible to speculate on the differences observed. Failure at the topcoat/ adhesion promoter interface is likely the result of the gross photooxidation of the system, resulting in the generation of low molecular weight polar species from the adhesion promoter. Gross photooxidation of these systems is supported by the FTIR results. The accumulation of low molecular weight polar species creates a weak boundary layer which is easily disrupted by moisture, resulting in coating delamination. For the systems which endured longer exposure times and failed at the adhesion promoter/TPO interface, the mechanism of failure is surely different. As mentioned earlier, these systems did exhibit some embrittlement of the coating system. Therefore, the failure could be simply the result of changes in the mechanical properties of the film from UV-induced extended cross-linking, for example. Failure then is the result of the development of interfacial stress at the coating TPO interface. However, the formation of additional adhesion promoter photoproducts which may further react to weaken the adhesion promoter/TPO interface, and perhaps bulk TPO material itself, cannot be precluded. Figure 6. FTIR transmission spectra of AP-5, as received and following exposure to UV light.

Figure 7. Change measured in the ratio of the carbonyl to CH stretching band intensity following 1000 h of weathering, relative to the ratio for the unexposed material.

Because spectra are taken from films of the adhesion promoters, it is believed that the decrease in absorbance is due to a loss of film thickness during exposure. If this is the case, even though the carbonyl growth over time is low, the loss of film implies that the material is very susceptible to photooxidative degradation. Loss of film for these very short exposures suggests that a weak boundary layer between the topcoat and adhesion promoter forms very quickly, and, therefore, this system would be expected to perform poorly in the adhesion testing. Failure Mode/Mechanism. An interesting observation from the adhesion performance testing is the difference in the mode of failure exhibited by the different systems. Those systems performing the poorest in the adhesion testing were observed to fail at the topcoat/adhesion promoter interface, while those systems performing the best in the adhesion testing were observed to fail at the adhesion promoter/TPO interface.

Conclusions The main focus of this study was to develop a means of screening adhesion promoter materials for inefficiencies, such as weatherability, which may affect the overall performance of the paint system. This study was designed to simulate conditions of thin topcoat application and/or loss of light stabilizers in the paint by adventitious processes that would render the adhesion promoter/topcoat interface exposed to ambient UV radiation. The effect of UV light exposure on the paint system was assessed by studying (1) the effects of exposure on adhesion performance for a number of different adhesion promoters and (2) the photochemically induced changes of the adhesion promoters as measured by FTIR. Results of the adhesion testing showed that there is a wide range in performance of the adhesion promoters used for this study. Exposure times required to bring the system to failure varied from as short as 100 h to greater than 6300 h. The AP-1 and AP-2 systems were observed to fail at the adhesion promoter/topcoat interface after 400 and 100 h of exposure, respectively. The AP-4 and AP-5 systems both failed at the substrate/ adhesion promoter interface after ∼3000 and ∼4000 h, respectively. The AP-3 system never failed and testing was discontinued at 6300 h of exposure. Failure at the topcoat/adhesion promoter interface is likely the result of the gross photooxidation of the system, resulting in the generation of low molecular weight polar species from the adhesion promoter. Failure at the adhesion promoter/TPO interface is less clear. Failure could be simply the result of changes in the mechanical properties of the film or possibly from the formation of additional adhesion promoter photoproducts which further react to weaken the adhesion promoter/TPO interface. Regardless of the details of the mechanisms undermining the adhesion, it is clear that the relative adhesion performance for the systems studied is AP-3 > AP5, AP-4 > AP-1, AP-2. There did not appear to be any significant differences on the adhesion performance for the baked or flashed systems.

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FTIR spectra of all adhesion promoter samples showed signs of photooxidation upon exposure in the weatherometer. There was a good correlation observed between the changes in the carbonyl growth, relative to the CH stretching band, and the relative adhesion performance of the adhesion promoter systems. These results suggest that this type of an exercise would be useful in assessing inefficiencies, such as weatherability. AP primer systems should, theoretically, be in the UV dark for a stabilized paint system containing UV packages and proper film builds. However, in the event of low film builds, especially around highly contoured areas, or improper UV stabilization of the basecoat/clearcoat system, UV penetration to the AP may occur. Using this type of method for screening adhesion promoters would be expected to result in the choice of a robust adhesion promoter system which would provide performance benefits in the event of UV exposure. Special Abbreviations and Symbols AP:

adhesion promoter

CPO:

chlorinated polyolefin

FTIR:

Fourier transform infrared

HALS:

hindered amine light stabilizer

mil:

one one-thousandth of an inch

TPO

thermoplastic olefin

UV:

ultraviolet

UVA:

ultraviolet absorber

µm:

micrometer

W:

watts

XPS:

X-ray photoelectron spectroscopy

2K:

two component

Bauer, D. R.; Gerlock, J. L.; Mielewski, D. F.; Paputa Peck, M. C.; Carter, R. O., III, The Role of Hydroperoxides in the Photooxidation of Crosslinked Polymer Coatings. Polym. Degrad. Stab. 1990, 27, 271. Dechent, W. L.; Stoffer, J. O. Waterborne Chlorinated Polyolefin Adhesion Promoters. Polym. Mater. Sci. Eng. 1993, 69, 380. Dechent, W. L.; Giles, J. A.; McMillan, C. A.; Stoffer, J. O. Adhesion Promoters for Exterior Applications using Chlorinated Polyolefins. Polym. Mater. Sci. Eng. 1994a, 70, 172. Dechent, W. L.; Giles, J. A.; Sitaram, S. P.; Stoffer, J. O. Environmental Effects on Chlorinated Polyolefin Enhanced TopCoat to Plastic Adhesion. Polym. Mater. Sci. Eng. 1994b, 70, 617. Grundke, K.; Jacobasch, H. J. The Mechanism of Coating Adhesion to the Surfaces of Plastics. Farbe + Lack 1992, 98, 934-942. Lawniczak, J. E.; Clemens, R. J.; Batts, G. N.; Middleton, K. P.; Sass, C. How do Chlorinated Polyolefins Promote Adhesion of Coatings to Polypropylene? Proceedings of the 3rd Annual ESD Advanced Coating Conference, Dearborn, MI, Nov 9-11, 1993a, p 205. Lawniczak, J.; Sass, C.; Stoffer, J. O.; Dechent, W. L. Chlorinated Polyolefins as Adhesion Promoters for Plastics. Polym. Mater. Sci. Eng. 1993b, 68, 28. Lawniczak, J. E.; Sass, C.; Evans, R. Effects of Formulation Variables on the Performance of CPO-Based Adhesion Promoters on Polypropylene Substrates. Proceedings of the 4th Annual ESD Advanced Coating Conference, Dearborn, MI, Nov 8-11, 1994, p 305. Ryntz, R. A. Selection of Weatherable Coatings for Thermoplastic Olefins. J. Coat. Techn. 1991, 63, 63-8. Schmitz, P. J.; Holubka, J. W. Investigation of the “Surface” and “Interphase” Composition of Adhesion Promoter/Thermoplastic Olefin Systems: The Effect of Adhesion Promoter Bake Temperature. J. Adhes. 1995, 48, 137.

Received for review September 12, 1995 Accepted January 16, 1996X IE9505672

Literature Cited Aoki, Y. The Role of Crystallinity of Polymer in the Adhesion between Chlorinated Isotactic Polypropylene and Isotactic Polypropylene. Polym. Sci., Part C 1968, 28, 855.

X Abstract published in Advance ACS Abstracts, March 1, 1996.

Assessing the Variability of Adhesion Promoter Adhesion Performance

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