The durability of OEM coatings on plastic substrates for automotive interior as well as exterior applications has been an important focus area due to the increasing role of plastic parts in the transportation industry. 1

However, performance of identical coatings on metal and plastic parts is known to differ due to the migration effect of the additives. 2 Nonetheless, due to the high cost of commonly used non-migrating light stabilizers, most coatings formulators prefer to use standard UV absorbers and HALS additives to protect the coatings on plastic substrates. 3 This compromise tends to limit the OEM’s ability to incorporate more coated plastic parts, as they wish to avoid customer complaints.

Over the years, manufacturers have acknowledged the consumer’s need for their automobiles to be more reliable, of higher quality and more aesthetically pleasing. Consequently, more interior automotive plastic parts need to be coated, especially dashboards, to meet these consumer demands. 4,5

To support the automotive coatings industry, Clariant has developed a comprehensive range of light stabilizer dispersions that respond to these market challenges. This publication is focused primarily on waterborne coatings.

 

Experiment

In this study, we demonstrate the improved weathering performance observed in a waterborne topcoat applied on a plastic substrate, formulated with both Clariant’s UV absorber and unique polymeric radical scavenger (HALS), compared to the industry-standard light stabilizer package available on the market. Properties measured in this study during the accelerated weathering tests were:

  • Color measurements (delta E, L*, a* and b*) with a Minolta spectrophotometer, model CM 3600A, and reflectance mode SCI.
  • Gloss measurements with a gloss meter BYK Gardner, micro-TRI-gloss and measuring 60º angle gloss.

Products

STD UVA

Currently, the standard UV absorber “STD UVA” used in most waterborne coatings is a liquid, 90% active product, chemically named hydroxyphenyl benzotriazole, with the following key features:

  • Compatible with solventborne and waterborne coatings. Significant mixing and often the addition of co-solvent(s) and a defoamer are required for waterborne coatings.
  • Lowest absorptivity amongst all benzotriazole chemical class.
  • Weak photopermanence 6 with approximate relative loss or rate of 3.5 under UV light. 7
  • Partially functionalized with reactable hydroxy groups.

STD HALS

Regarding the standard radical scavenger “STD HALS” used in this study, it is a 100% active liquid mixture composed of two basic active tertiary amines. STD HALS is compatible with solventborne and waterborne coatings. Significant mixing and often the addition of co-solvent(s) and a defoamer are required for waterborne coatings.

For this experiment, 2% STD UVA and 1% STD HALS were added to the coating formulation in active ingredient on solid binder. The Clariant UV stabilizers used in this study were:

  • The recently launched Hostavin® 3315 DISP is a 52% aqueous dispersion of a high-efficiency benzotriazole UV absorber, designed for UV stabilization of paints and clearcoats. The solution is characterized by its ease of incorporation, broad compatibility, broader UV absorbance profile than hydrophenyl triazine while exhibiting approximately 60% higher photo-permanence 6 vs. STD UVA.
  • Hostavin 3070 DISP is a 52% aqueous dispersion of an oligomeric hindered amine light stabilizer (HALS), designed for UV stabilization of paints and clearcoats. The solution is characterized by its ease of incorporation, broad compatibility, high resistance to migration and low extraction, making it attractive for the use in plastic coatings.

Both Clariant products are easy-to-use dispersions, requiring no milling or dissolving steps before use in a waterborne coating. They can be incorporated at any stage of the paint production process. So far, there are no indications that the grades impact the hazardous labelling of any final coating formulations. These two products benefit from hazards-free classification (in Europe and North America) and have been awarded with the Clariant EcoTain® label for sustainability excellence. 8

Weathering Tests

The goal of this study was to determine the weathering performance of the standard UV additives in a waterborne acrylic topcoat on a black plastic substrate in comparison with Clariant’s light stabilizer solution. As shown in Table 1, for the accelerated weathering test, a xenon-arc artificial weathering instrument set to ISO 4892-2 E2013 Wet/Dry was used.

Equipment settings for Xenon-arc artificial weathering exposure test.
TABLE 1 Equipment settings for Xenon-arc artificial weathering exposure test.

 

Results

Weathering tests took place at intervals of 1,500 hrs, 2,000 hrs and 3,000 hrs. The initial results at 1,500 hrs proved positive, giving optimism to carry on the exposure.

Consequently, after 2,000 hrs and 3,000 hrs, the combination of Hostavin 3315 DISP and Hostavin 3070 DISP confirmed the initial hypothesis and observations, that this new light stabilizer package leads to a lower color change as well as better gloss retention than the conventional UV protection packages.

In Figure 1, minimum color change is observed after 3,000 hrs with Hostavin 3315 DISP and Hostavin 3070 DISP, and in Figure 2, gloss retention is also improved versus STD UVA and STD HALS.

Color change (∆Ε) of waterborne plastic-coated panels after WOM exposure.
FIGURE 1 Color change (∆Ε) of waterborne plastic-coated panels after WOM exposure.
Gloss (at 60º angle) of waterborne plastic-coated panels after WOM exposure.
FIGURE 2 Gloss (at 60º angle) of waterborne plastic-coated panels after WOM exposure.

In fact, while the gloss retention measurements and ΔΕ (color change) are better for the new formulation, it is only visually that we can fully appreciate the significant improvement obtained with the new stabilizers package. The visual confirmation of the data provided above can be seen in Figure 3.

Photo of the coated panels after weathering.
FIGURE 3 Photo of the coated panels after weathering.

 

Conclusion

In this study, we demonstrated the weathering improvement of automotive waterborne coatings on plastic substrates. The color and gloss retention in the visual assessment for the panels stabilized with the Clariant water-dispersed UV package (with Hostavin 3315 DISP and Hostavin DISP 3070) performed significantly better than the conventional light stabilizer package.

Such improvement was achieved by formulating the coating with the best-in-class mixture of light stabilizer dispersions. The key was to select a high-end benzotriazole with a good balance of photo-permanence and broad UV absorption, combined with a non-migrating oligomeric HALS.

In addition to their technical performance, these two light stabilizer additives have been awarded with the Clariant EcoTain label for sustainability excellence.

For more information, visit www.clariant.com.

Disclaimer: The performance of a product depends on the conditions of use. Users should always determine the suitability of a product for its particular application prior to use.

References

1 Schönmayr, D. Automotive Recycling, Plastics, and Sustainability: The Recycling Renaissance. Germany: Springer International Publishing, (2017).

2 Adamsons, K.; Cliff, N.; Kanouni, M.; Peters, C.; Yannef, P.V.; Use of Reactable Light Stabilizers to Prevent Migration and to Improve Durability of Coatings on Plastic Substrate, JCT Res., 2005, 2, 371-387.

3 Biron, M. Material Selection for Thermoplastic Parts: Practical and Advanced Information. Netherlands: Elsevier Science, 533, (2015).

4 Trends in Automotive Plastics, Plastics Decorating Magazine, 2018.

5 Toensmeier, P. High Style for Automotive Interiors, Plastics Engineering, September 2015.

6 Handbook of Coating Additives. (2004). Switzerland: Taylor & Francis, 198.

7 Handbook of Coating Additives. (2004). Switzerland: Taylor & Francis, 200.

8 Clariant – EcoTain ® , https://www.clariant.com/en/Sustainability/Discover-Ecotain.