Photo-Oxidation

• The most important degradation process for vinyl siding is photo-oxidation or photodegradation. Light in the UV range of the spectrum has sufficient energy to break the bonds between chlorine and carbon atoms in the PVC, releasing reactive intermediates that participate in reactions contributing to polymer breakdown. The presence of oxygen greatly accelerates this process, and since vinyl siding outdoors is always in the presence of oxygen, this photo-oxidation process can potentially pose a problem.
  
  

Antioxidants

• Antioxidants retard photo-oxidation to some extent, but they do so by reacting with the radicals once these have already been formed. They are also used up in the process, so their effectiveness decreases over time. Consequently, PVC siding often incorporates chemicals that absorb strongly in the UV instead, since they prevent the formation of radicals in the first place. These compounds are called light stabilizers. Benzophenones are a common choice.
  
  

Effects

• As the vinyl becomes degraded, the polymer chains become cross-linked and the oxygen content of the material increases. You may observe discoloration, yellowing and cracking; sometimes the plastic will develop a chalky surface. Products of photo-oxidation include carbon dioxide, methane, carbon monoxide, hydrogen and -- at higher temperatures -- hydrogen chloride. Increasing temperature increases the rate of oxidation. The strength and flexibility of the siding deteriorate as the vinyl becomes oxidized over time.
  
  

Considerations

• PVC products are inherently susceptible to photo-oxidation. Ultimately, however, the vulnerability of vinyl siding to oxidation depends on the kinds of pigments, stabilizers and other additives the material contains. Since vinyl siding is designed for use outdoors, it will typically perform better than vinyl materials intended for indoor use. Nonetheless, when purchasing vinyl siding, you should consider stability and performance in outdoor conditions as important factors in your choice of product.