Temperature
The temperature of materials exposed to solar radiation has a significant influence on the effect of the radiation. The destructive effects of light are usually accelerated at elevated temperatures as a result of the increased rate of secondary reactions, with reaction rates about doubling with each 10°C rise; this may not be true of all materials but is often found with polymers. At high temperatures molecules have greater mobility. Therefore, the rate of oxygen diffusion increases and free radical fragments formed in primary photochemical processes are more readily separated. Thus, the chance of recombination is reduced and secondary reactions are promoted. Reactions may take place at higher temperatures that occur at a very low rate or not at all at lower temperatures.
In the presence of sunlight the surface temperature of an object is usually considerably higher than the temperature of the air. Solar absorptivity is closely related to color, varying from about 20% for white materials to 90% for black materials; thus samples of different colors will reach different on-exposure temperatures. Because the thermal conductivity and heat capacity of polymeric materials are generally low, much higher temperatures can be obtained on the surface than in the bulk of the material. Therefore, both the surface temperatures of the samples, produced largely by infrared radiation absorption which varies by material color, and ambient air temperature and its fluctuations during exposure do play a role.
Diurnal and seasonal variations occur in solar radiation. Temperature cycling can cause mechanical stress, particularly in composite systems consisting of materials with widely differing temperature coefficients of expansion. Temperature and its cycles are also closely linked with water in all of its forms. Drops in temperature can cause water to condense on the material as dew, a rise in temperature causes evaporation, and sudden rainfall can cause thermal stress.
Read more about this topic: Factors Of Polymer Weathering
Famous quotes containing the word temperature:
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—Henry David Thoreau (1817–1862)
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—E.B. (Elwyn Brooks)
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—Hermann Hesse (1877–1962)