Secondary Factors of Weather
Ozone is produced by short wavelength (110 nm - 220 nm) UV photolysis of oxygen in the upper atmosphere. The photochemical reaction of nitrogen oxides and hydrocarbons from automobile exhausts is another source. Ozone plays a dual role in weathering. The concentrated layer in the upper atmosphere absorbs the short wavelength (≤300 nm) ultraviolet radiation emitted by the sun and thus plays a critical role in protecting terrestrial objects from this actinic radiation. Ozone is also a powerful oxidant and reported to react rapidly with elastomers and other unsaturated polymers. Ozonolysis typically results in stiffening and cracking, particularly under mechanical stress. However, the contribution of ozonolysis reactions to the overall photooxidation process is still subject to controversy.
Atmospheric pollutants (e.g., sulfur dioxide, nitrogen oxides, hydrocarbons, etc.), in combination with solar radiation, can also be responsible for severe damage. Acid-base induced chemical changes may also be responsible for much pollution-caused damage. Unsaturated alkyl and aromatic compounds may act as catalysts in the photooxidation of polymers. In the presence of sulfur dioxide and oxygen, ultraviolet radiation causes crosslinking of polyethylene and polypropylene and is responsible for the rapid loss of color in pigmented coatings.
Moisture, in combination with temperature, may also promote microbial growth. Mold, mildew and other microbiological and botanical agents may play a significant role in material degradation, particularly in tropical and subtropical climates, although they may not be generally thought of as weathering factors.
Read more about this topic: Factors Of Polymer Weathering
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