Light Stabilizers are used to protect plastics and other materials against degradation from long term exposure to UV radiation. Exposure of plastics to the UV radiation in sunlight initiates degradation through a photo-oxidative process. This can produce a number of undesirable effects including changes in appearance (discoloration, changes in gloss, and/or chalking), deterioration of mechanical properties, and the formation of visible defects such as cracks. Fluorescent lamps used for indoor lighting also emit UV radiation, but at a much lower intensity than sunlight.
As in the case of thermal oxidation, photo-oxidation is a chain reaction process involving free radical and hydroperoxide intermediates. Light stabilizers inhibit this process either by scavenging the reactive intermediates or by competitive absorption of damaging UV radiation.
Hindered amine light stabilizers (HALS) function mainly by scavenging the free radical intermediates in the photo-oxidation process. HALS are highly effective for the light stabilization of polyolefins, styrenic polymers, and some engineering plastics.
Ultraviolet (UV) absorbers act by shielding the interior of plastic parts from damaging UV radiation. These products are characterized by a very strong UV absorbance and a high degree of inherent photostability. UV absorbers are effective for the stabilization of many plastics, including PVC and engineering plastics. They are also used in plastic packaging applications to protect the package content from the effects of UV radiation.
Combinations of HALS and UV absorbers often show a synergistic performance superior to either type of light stabilizer used alone.