Paint & Coatings Industry - March 2009 - (Page 64) Keys to Meaningful, Long-Term Weathering Predictability This leads to the issue of standardized test methods or specifications. There are literally thousands of product specifications and test standards referencing artificial weathering. Most do not claim to predict real long-term weathering performance but do serve as a minimum basis for relative comparison and performance. It is important to realize that just as there is no single climate or weather condition, there is no ‘one size fits all’ test method able to reproduce all climates and applicable to all material chemistries. However, most standard methods and specifications act as if there were. In fact, most methods don’t purport to reproduce any specific climate, but they can serve as a common basis for com- Look forward to seeing you at the European Coatings Show 2009 Hall: 6, Booth: 345 Visit ads.pcimag.com 64 MARCH 2009 | W W W . P C I M A G . C O M parison or set minimum performance levels. By controlling the environmental factors in the laboratory artificial weathering test it is possible to simulate a variety of weather and climate end-use conditions, at least within the capabilities of the available equipment. By properly choosing the ‘boundary conditions’ of the tests, you may understand how your product will behave in real life in a shorter period of time. For example, you may find that a particular coating loses gloss under hot and humid conditions but yellows in hot dry conditions. Or you may determine that a coating fades and chalks in the tropics but cracks and crazes in the desert sun – and all without leaving the relative comfort of the laboratory. Some materials are very sensitive to a particular weather element, such as UV radiation. If the artificial weathering test is a poor match to sunlight in the UV, the test may make the product appear better or worse than it actually is. In either case we have a lack of correlation, a failure of the test to predict real-world performance. The old philosophy in testing was to achieve acceleration through unnaturally severe exposures in the belief that it would more quickly achieve the degradation that would be seen under normal conditions. While this is sometimes true, it often is not and the results can be surprising. The move of the automotive industry from monocoat paint to basecoat-clearcoat systems is a telling example. These early systems were passed as stable by very severe UV exposure tests, but quickly resulted in catastrophic clearcoat delamination on many thousands of vehicles. The unnatural conditions of the harsh test changed the degradation chemistry resulting in changes that did not result in clearcoat delamination. The lesson: do a different test and expect a different result. The particular wavelength sensitivity of a coating to solar radiation is principally a function of the binder formulation and the specific constituent chemical bonds. But in addition, any additives or contaminants may also be photoreactive and contribute to degradation. Colorants may also degrade from the effects of visible light exposure resulting in fading or hue shift. And, lastly, different coating colors and substrates will attain different surface temperatures from the infrared heating effects of the sun, which can in turn affect the rate of degradation and influencing factors such as moisture migration into the http://www.draiswerke-inc.com http://www.draiswerke-inc.com http://ads.pcimag.com http://WWW.PCIMAG.COM
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