Long time UV irradiation will not only affect human skin health, but also accelerate the degradation of polymer materials. This eventually leads to malignant changes in the physical and mechanical properties of polymer products, such as reduced strength and shortened life. It is common practice to add light stabilizers to achieve the best protection for polymers. The following are three common types of light stabilizers and their mechanism of action.
UV absorber protects polymers by absorbing UV radiation and converting it into harmless thermal energy.
Quencher accepts the energy absorbed by the chromophores in the plastic and dissipates this energy as heat, fluorescence, or phosphorescence, thus protecting the polymer from UV damage.
HALS enters into various reactions which inhibit photooxidation of polymers and is a so-called multifunctional light stabilizer.
The properties of light stabilizers make them important in many fields, such as pigmented coatings, agricultural films, and construction materials.
Light stabilization of coatings has been a challenge for the industry for long time. Automotive clearcoats are, for example, expected to ensure gloss retention and retain a colorless appearance for many years, while protecting the basecoat or coated plastic parts. Throughout their service life, these coatings are not only subject to UV radiation, but also to heat, temperature changes and mechanical stress. This series of factors can lead to a significant change in the color of the coating. In addition, there is the possibility of surface cracking and even delamination. To ensure the long life of coatings and to prevent its decorative and protective character, it has been common practice to combine UV absorbers and HALS.
The most important agricultural applications of plastic films are greenhouse, walk-in tunnel and low tunnel covers, and mulching. The raw materials are usually low-density polyethylene (LDPE) and ethylene-vinyl acetate (EVA) or ethylene-butyl acrylate (EBA) copolymers for the covers and linear low-density polyethylene (LLDPE) for mulching. These plastic films are exposed to strong solar radiation and the choice of a light stabilizer to protect them is necessary. The main classes of light stabilizers used in greenhouse films are: UV absorbers (usually organic molecules of the benzophenone, benzotriazole or triazine type), nickel derivatives (often called quenchers) and polymeric HALS.
Polymers such as plastic shingles, composite roofing tiles, and siding used in construction applications can deteriorate when exposed to continuous sunlight or outdoor conditions. Therefore, the working life span of these polymers is related to their ability to resist UV degradation to a large extent. Many additives possess absorptive characteristics in the UV spectrum region, which contributes to the protection of polymer. The UV absorbers lightfastness can be increased by addition of HALS, so that a better effectiveness in protection can be obtained using combination of UV absorbers with HALS, which contribute to polymer protection by different activity mechanisms.
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