Photooxidation and Fading of Composite Building Materials

Photooxidation of plastics and wood-plastic composites (WPCs) was described in principal detail in the preceding Chapter 15. It was emphasized that photooxidation acts in a synergism with thermooxidation of the materials, speeding up an oxidative degradation of WPC products, particularly being exposed to direct sunlight. 

However, photooxidation has its own specific manifestations, and, first of all, changing the material s color or fading (Fig. 16.1). Hence, we will focus in this chapter primarily on fading, what causes it, how to characterize it, and how to minimize it. We will also show how much commercial composite deck boards differ in terms of fading. 

Despite many similarities between thermooxidation and photooxidation of plastics, on one hand, and WPCs, on the other, there are some important differences. Photodegradation of plastics typically occurs in oxygen starvation conditions, while photodegradation of WPC, which are always porous, occurs in the presence of plenty of oxygen in the 

Wood-Plastic Composites, by Anatole A. Klyosov Copyright 2007 John Wiley Sons, Inc. 

Upper layers as well as in the bulk of the material. Hence, different kinetics and sometimes different chemistry of photodegradation of WPC compared with those of neat plastics. 

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