Degradation of polymers by solar radiation

From the solar spectrum passing through the earth s atmosphere, it is evident that no radiation with a wavelength 300 nm is transmitted by the atmosphere. This has important practical implications in that the number and type of photochemical reactions which may occur are thereby severely restricted, in comparison to the case when polymer samples are exposed to radiation from UV radiation sources. Since the energy of (sun)light (E ) is inversely proportional to the wavelength, i.e.  

A further restriction on possible reactions is imposed by the elementary photochemical principle that light must be absorbed by the polymer if a reaction is to occur. However, most pure polymers do not absorb at wavelengths longer than 300 nm, and hence should not be affected by solar radiation. However, most polymers degrade when subjected to solar radiation, and this has been attributed to the presence of small amounts of internal and/or external impurities which absorb light and initiate oxidative chain reactions within the polymer. A detailed study of such reactions is difficult because of the frequently unknown nature of the trace impurities which initiate the reactions and of the multiplicity of products formed in the photo-oxidative degradation (cf. section 2.21). 

From the point of weathering damage the most important of the internal chromophores absorbing in the near ultraviolet are those containing oxygen (C=0, OOH), since they are most likely to be formed in polymers degraded in the atmosphere. 

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