UV absorbers are a class of substances that prevent the degradation of polymers caused by sunlight or other artificial UV light, and are a type of light stabilizer. Since the sunlight contains a large amount of ultraviolet light that is harmful to colored objects, the wavelength of which is about 290-460 nm, these harmful ultraviolet light will finally decompose and fade the color molecules through chemical redox, and the use of ultraviolet absorbers to protect the Objects can be implemented to effectively prevent or weaken their color damage. After adding a UV absorber, it can selectively absorb this high-energy UV light and turn it into harmless energy for release or consumption. Due to the different types of polymers, the wavelengths of ultraviolet rays that make them deteriorate are also different. Different ultraviolet absorbers can absorb ultraviolet rays of different wavelengths. When using, the ultraviolet absorbers should be selected according to the type of polymer.
The Mechanism and Application of Ultraviolet Absorber
UV absorber
UV absorbers are a class of substances that prevent the degradation of polymers caused by sunlight or other artificial UV light, and are a type of light stabilizer. Since the sunlight contains a large amount of ultraviolet light that is harmful to colored objects, the wavelength of which is about 290-460 nm, these harmful ultraviolet light will finally decompose and fade the color molecules through chemical redox, and the use of ultraviolet absorbers to protect the Objects can be implemented to effectively prevent or weaken their color damage. After adding a UV absorber, it can selectively absorb this high-energy UV light and turn it into harmless energy for release or consumption. Due to the different types of polymers, the wavelengths of ultraviolet rays that make them deteriorate are also different. Different ultraviolet absorbers can absorb ultraviolet rays of different wavelengths. When using, the ultraviolet absorbers should be selected according to the type of polymer.
The mechanism of UV absorbers
1. The reason why ultraviolet absorbers can absorb ultraviolet light is due to the structure of the conjugated π-electron system and the structure that can move hydrogen atoms in the molecules of these compounds. Some only have the former part.
2. The structure molecule of ultraviolet absorber contains at least one ortho-hydroxyphenyl substituent. In this type of compound, a chelate ring is formed by the ortho-hydroxyl group and nitrogen atom or oxygen atom. After absorbing ultraviolet light, the hydrogen bond is broken and the molecule is broken. Isomerization, the intramolecular structure undergoes thermal vibration, the hydrogen bond is destroyed, the chelate ring is opened, and the intramolecular structure changes, which converts harmful ultraviolet light into harmless heat energy, thereby protecting the material.
3. In this process, the chelating ring formed in the molecule is the key to its function of absorbing ultraviolet rays. The energy sensitive range of opening this ring is exactly the ultraviolet energy range of 290-400 nm wavelength.
4. In addition, as an ultraviolet absorber, it must be able to not carry out photochemical reaction under the action of ultraviolet light or visible light; good stability to chemicals and water, good thermal stability; low volatility; Good compatibility and no solvent extraction.
Conditions that UV absorbers should have
1. Can strongly absorb ultraviolet rays (especially the wavelength of 290-400nm);
2. Good thermal stability, it will not change due to heat even during processing, and the thermal volatility is small;
3. Good chemical stability, no adverse reaction with the material components in the product;
4. Good miscibility, can be evenly dispersed in the material, no frosting, no exudation;
5. The absorber itself has good photochemical stability, no decomposition, no discoloration;
6. Colorless, non-toxic and odorless;
7. Resistant to dipping;
8. Cheap and easy to obtain;
9. Insoluble or insoluble in water.
Classification of UV Absorbers
According to chemical structure, UV absorbers can be divided into salicylates, benzophenones, benzotriazoles, substituted acrylonitriles, triazines and hindered amines.
Benzotriazoles
This kind of compound is the product with the largest output and the most variety among the ultraviolet absorbers, and the research at home and abroad is also very active. The synthesis of benzotriazole UV absorbers is generally by diazotizing aromatic amines to form diazonium salts, and the diazonium salts are coupled with phenolic compounds to form intermediate azo pigments, which are then ring-closed by reduction.
Benzophenones
The output and variety of this type of UV absorber are second only to benzotriazole.
The synthetic route is basically a two-step reaction, namely the synthesis of 2,4-dihydroxybenzophenone and the synthesis of alkoxybenzophenone.
Research direction of UV absorber
1. High molecular weight
Reduce the volatility of additives and reduce losses during the reaction
2. Multifunctional
Make a stabilizer molecule with different functional groups at the same time, play a variety of stabilizing effects (thermal stabilizer, antioxidant...)
3. Reactivity
Introduce reactive groups into the light stabilizer molecule to bond with the base polymer during processing, thereby permanently present in the polymer material
Application of UV Absorbers
1. Due to the different types of polymers, the wavelengths of ultraviolet rays that degrade them are also different. Different ultraviolet absorbers can absorb ultraviolet rays of different wavelengths. When using, the ultraviolet absorbers should be selected according to the type of polymer.
2. UV absorbers used in cosmetics can reduce or completely absorb UV rays and protect the skin. However, if it is used excessively or a prohibited UV absorber is used, it will irritate the skin and cause skin allergies.
