Some Important factors to polyester coating degradation
Polyester degradation is affected by solar radiation, photocatalytic admixtures, water and moisture, chemicals, oxygen, ozone, temperature, abrasion, internal and external stress, and pigment fading.Out of all these, the following factors, all present in outdoor weathering, are the most important to coating degradation:
moisture, temperatures, oxidation ,UV radiation.
Hydrolysis occurs when a plastic is exposed to water or humidity.This chemical reaction may be a major factor in the degradation of condensation polymers such as polyesters , where the ester group is hydrolyzed.
When a polymer is subjected to a thermal energy greater than the bond energy that holds the atoms together, it is readily cleaved. As a result, two macroradicals, or electron-deficient molecules, are generated.
Most plastics are generally evaluated in three conditions of elevated-temperature exposure: elevated temperature over a long period of time, elevated temperature over a short period of time, or cyclic exposure to elevated and lowered temperatures, such as may occur during alternating day and night exposure.When temperature is increased, the destructive effects are further enhanced by additional UV exposure.
Due to the presence of oxygen in the atmosphere, the most common type of radiation degradation, photooxidation, occurs on all organic polymers. Again, the process will be accelerated by UV radiation and higher temperatures. The chemical reaction can be described by the oxygen attack on the bond in a polymer chain, which may either form carbonyl groups or crosslink.Different stabilizers may be used to reduce polymer degradation: antioxidants, thermostabilizers, photostabilizers, etc.
When it comes to outdoor applications, the primary concern in the material evaluation is the potential sunlight degradation when plastics are meant for outdoor use.Of the total radiation that reaches the earth’s surface, about 5-6% of the light is in the UV region of the spectrum and usually will vary with daily environmental weather conditions.
The photochemical effect of sunlight on a plastic material depends on the superficial absorption properties and the chemical bond energies of the material. Light wavelengths that have the most affect on plastics range from 290 to 400 nm. The wavelength of UV radiation whose photon energy corresponds to a particular bond energy in the polymer chain can break the chemical bonds (through a chain scission), changing the properties, and therefore the performance of the polymer.6 The most damaging wavelength for polyesters is believed to be 325 nm