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IN PREPARATION OF THIS CHAPTER, THE CONtents of the fifteenth edition were drawn upon. The current edition will review, clarify and update the topics as addressed in the previous edition. New technology and reference materials are acknowledged and included to update the reader with the advances in this industry. Paints and coatings are used both to protect substrates and to provide an aesthetically pleasing appearance. In an outdoor environment, both of these functions can be affected by weathering. The four major factors involved in weathering are solar radiation (sunlight), moisture, oxygen, and heat. Sunlight, especially in the short wavelength/high energy ultraviolet (UV) region, has been proven to lead to discoloration, loss of gloss, scaling, embrittlement, and chalking. Moisture, in the form of rain, dew and humidity can cause blistering, flaking, loss of adhesion and promote the growth of mildew and algae. Heat exposure may cause embrittlement, cracking, peeling, and checking. Oxygen in the atmosphere participates in the oxidation of the surface of the coating, which may eventually lead to oxidation of internal layers, causing embrittlement, softening, cracking, or crazing. The oxygen is often left out of the discussion since it is more constant than the other factors, but this degradation process is a contributing factor to the other three factors of weathering. These elements contribute individually as well as in combination to cause coating failures. Naturally occurring and man-made chemicals in the environment also contribute to coating degradation and could be considered a fifth element of weathering. However, the type and levels of chemicals can vary dramatically, even over short distances. Therefore, they cannot be considered as universal in influencing the degradation process as the four factors mentioned previously. Perhaps as a consequence of this, and also partly due to tradition, chemical resistance testing is usually considered to be separate from artificial weathering. Although the effects of chemicals cannot be ignored, they are discussed elsewhere in the manual. One of the most common chemical exposure tests that is often grouped with accelerated weathering continues to be salt fog or salt spray testing, which is discussed in the corrosion section of this manual. In summary, this chapter will consider only devices that incorporate an UV light source, temperature control or monitoring, and moisture exposure monitoring.
Sherbondy, Valerie S.
Senior Chemist, KTA-Tator, Inc., Pittsburgh, PA