UV absorbents are a class of compounds that can selectively and strongly absorb ultraviolet light. The molecular structure of the UV absorbent contains chromophore groups that can absorb ultraviolet light, such as -N-N-, -C-N-, -C-O-, -N-O-, etc. Therefore, UV absorbents can convert high-energy ultraviolet light into harmless low-energy radiation to protect other substances from ultraviolet light. The UV absorbent mainly realizes the ultraviolet shielding function through the following four ways. The first way is to scatter or reflect ultraviolet light waves to reduce the transmission of ultraviolet light. The second way is to strongly absorb high-energy ultraviolet light and convert its energy into harmless thermal energy, chemical energy or long-wave light energy and so on. The third way is to quickly quench the excited state of the polymer that has been excited by ultraviolet rays back to the ground state. The fourth way is to efficiently capture the free radicals generated by ultraviolet-induced polymers, thereby protecting the polymer materials from ultraviolet rays. The UV absorbents currently developed are mostly oil-soluble. When using this type of UV absorbent, an organic solvent needs to be added to dissolve or disperse in the aqueous solution with the aid of dispersant, emulsifier and other auxiliary agents.
Because UV absorbents have the advantages of rich variety and excellent resistance to ultraviolet radiation, they are widely used in leather industry, cosmetics, biodegradable film and other fields.
- Leather industry: Leather products contain macromolecular substances, and the energy carried by light waves will cause the chemical bonds of macromolecules in the material to be excited and cracked. After the polymer chain breaks, the leather material becomes brittle and loses its toughness and impact resistance. With the further development of photoaging, unsaturated bonds will oxidize and cause leather products to quickly turn yellow. Therefore, in the leather industry, chemicals such as dyes, fatliquors and finishing agents are required to have good UV stability. UV absorbent UV-P has a keto-enol interconversion structure, which can undergo an isomerization reaction, thereby efficiently absorbing external ultraviolet rays and avoiding the aging of polymer materials caused by excessive ultraviolet radiation. UV absorbent UV-P is widely used in the leather industry.
Figure 1. Structural formula of UV absorbent UV-P
- Sunscreen cosmetics: The destruction of the ozone layer has caused a significant increase in skin diseases worldwide due to excessive sunlight exposure. Skin sunscreen has attracted widespread attention. At present, most sunscreen products on the market achieve broad-spectrum UV protection by adding physical sunscreens and chemical sunscreens. Nano-zinc oxide, nano-titanium dioxide and rare-earth nano-oxide are inorganic UV absorbents commonly used in cosmetics.
- Biodegradable film: Some biodegradable films are more prone to photo-oxygen degradation due to the presence of unsaturated groups under ultraviolet irradiation, thereby reducing the service life of the film. If UV absorbents are added to the preparation of biodegradable films, the retention rate of the mechanical properties of the film can be increased, the change of the crystallinity of the film can be reduced, and the gel content of the film during the UV aging process can be reduced, thereby delaying the biodegradable film degradation rate. For example, polylactic acid (PLA) is a biodegradable plastic with high strength, good biocompatibility and easy melt processing. Polyterephthalic acid-succinyl adipate (PBAT) is a biodegradable plastic with good toughness. Since polylactic acid and polybutylene terephthalate-butylene adipate are complementary in mechanical properties, PLA/PBAT fully biodegradable composite materials have become one of the most favored research systems in academia and industry. There are unsaturated groups in the PLA/PBAT film, so it is easily degraded under ultraviolet irradiation. Adding benzophenone UV absorbents to PLA/PBAT films can effectively absorb harmful ultraviolet rays, thereby delaying the aging and degradation of the film.
Figure 2. Diphenylketone UV absorbent UV-531
According to the chemical structure, UV absorbents can be divided into salicylate UV absorbents, benzophenone UV absorbents, benzotriazole UV absorbents, substituted acrylonitrile UV absorbents, triazine UV absorbents and hindered amine UV absorbents.
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