Moisture
Moisture can take the form of humidity, dew, rain, snow, frost or hail, depending on the ambient temperature. Moisture, in combination with solar radiation, contributes significantly to the weathering of many materials. This is due both to the mechanical stresses imposed when moisture is absorbed or desorbed and to the chemical participation of moisture in the chemical evolution (and in some instances physical effects such as impact). The span of time over which the precipitation occurs and the frequency of wetness are more important in the weathering of materials than the total amount of precipitation. The mechanical stresses induced by freeze/thaw cycling can cause structural failures in some systems, or accelerate degradation already initiated.
Moisture participates both physically and chemically in degradation. Water absorption by synthetic materials and coatings from humidity and direct wetness is a diffusion controlled process. This hydration of the surface layers produces a volume expansion which places mechanical stress on the dry subsurface layers. A following drying out period signifies a desorption of water. The drying out of the surface layers would lead to a volume contraction; the hydrated inner layers resist this contraction, leading to surface stress cracking. This oscillation between hydrated and dehydrated states may result in stress fractures. Because of diffusion rates in organic materials, it may takes weeks or months to reach a moisture equilibrium.
The chemical effects of moisture can be seen in the chalking of titanium dioxide (TiO2) pigmented coatings and polymers; the anatase form is particularly sensitive to wavelengths below about 405 nm while the rutile forms absorb energy above that wavelength. Chalking results from the degradation of the binding material resulting in a release of the TiO2 pigment particles. These particles form a dull layer on the surface which may be wiped off. Experience shows that chalking is strongest where more water is available on the surface; little to no chalking occurs in dry atmospheres. TiO2 is a semiconductor where electron transitions from the valence band to the conduction band result from the absorption of light at wavelengths in the near UV range, below 400 nm. Ultraviolet radiation causes electron-hole pairs to be created in the TiO2 lattice. These react with the hydroxide groups on the surface and the Ti4+ ions. Hydroxyl and perhydroxyl radicals are formed through the conversion of oxygen and a water molecule whereby the TiO2 surface again resumes the initial form and acts as a catalyst for continued activity, thus repeating the chalking cycle. The hydroxide and perhydroxyl radical then cause oxidative decomposition of the binder with the subsequent release of TiO2 particles.
Read more about this topic: Factors Of Polymer Weathering