UV absorber is a type of substance that can prevent polymer degradation caused by sunlight or other artificial ultraviolet light. It is a light-stable agent. Since sunlight contains a large amount of ultraviolet light that is harmful to colored objects, its wavelength is about 290-460nm. These harmful ultraviolet light causes the color molecules to eventually decompose and fade through chemical oxidation and reduction, and the use of ultraviolet absorbers can protect the protected objects. Objects can be implemented to effectively prevent or weaken the damage to color. After adding ultraviolet absorbers, this high-energy ultraviolet ray can be selectively absorbed and turned into harmless energy for release or consumption. Due to different types of polymers, the length of the ultraviolet wave diluent that causes them to deteriorate is also different. Different UV environmentally friendly plasticizer 168 ray absorbers can absorb ultraviolet rays of different wavelengths. When used, the ultraviolet absorber should be selected according to the type of polymer. agent.
Mechanism of UV absorbers
1. The reason why ultraviolet absorbers can absorb ultraviolet light is because the molecules of this type of compounds contain a structure of a conjugated π electron system and a structure that can move hydrogen atoms. Due to two parts. Some only have the first part.
2. The structure of ultraviolet absorbers contains at least one ortho-hydroxyphenyl substituent in the molecule. This type of compound consists of ortho-hydroxyl groups and nitrogen atoms or oxygen atoms. A chelate ring is formed. After absorbing ultraviolet light, the hydrogen bond breaks and molecular isomerization occurs. The intramolecular structure undergoes thermal vibration, the hydrogen bond is destroyed, the chelate ring opens, and the intramolecular structure changes, thus changing harmful ultraviolet light into Harmless heat energy is released, thus protecting the material,
3. In this process, the chelate ring formed in the molecule is the key to its ability to absorb ultraviolet light. The energy sensitive range of opening this ring is exactly 290 ~nm wavelength ultraviolet energy range.
4. In addition, as an ultraviolet absorber, it must be able to resist photochemical reactions under the action of ultraviolet light or visible light; it must have good stability against chemicals and water, Good thermal stability; low volatility; good compatibility with polymer materials and not extracted by solvents, etc.
Conditions that UV absorbers should meet
1. Can strongly absorb ultraviolet rays (especially the wavelength of 290-nm);
2. Good thermal stability, will not change due to heat even during processing, and has low thermal volatility;
3. Good chemical stability and no adverse reactions with material components in the product;
4. Good miscibility, can be evenly dispersed in the material, no blooming, no bleeding;
5. The absorber itself has good photochemical stability and does not decompose or discolor;
6. Colorless, non-toxic, odorless;
7. Resistant to soaking and washing;
8. Cheap and easy to obtain;
9. Insoluble, or difficult to dissolve in water.
Classification of UV absorbers
According to chemical structure, UV absorbers can be divided into salicylates, benzophenones, benzotriazoles, substituted acrylonitriles, and triazines and hindered amines.
Benzotriazoles
This type of compound is the product with the largest output and the most varieties among UV absorbers, and research at home and abroad is also very active. The synthesis of benzotriazole ultraviolet absorbers is generally made by diazotizing aromatic amines to form diazonium salts. The diazonium salts undergo a coupling reaction with phenolic compounds to form intermediate azo pigments, which are then ring-closed through reduction.
Benzophenones
The output and variety of this type of UV absorber is second only to benzotriazole.
The synthesis route is basically a two-step reaction, namely the synthesis of 2,4-dihydroxybenzophenones and the synthesis of alkoxybenzophenones.
UV absorber research direction
1. High molecular weight
Reduce the volatility of additives and reduce losses during the reaction
2. Multifunctional
Make a stabilizer molecule have groups with different functions at the same time, playing a variety of stabilizing effects (thermal stabilizer, antioxidant…)
3. Reactivity
Introduce reactive groups into the light stabilizer molecules so that they can bond with the base polymer during processing and thus permanently exist in the polymer material
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