Ultraviolet absorber is a very important additive in plastic products. It is a kind of stabilizer that acts as a light stabilizer. It mainly produces a stabilizing effect by absorbing and converting the destructive ultraviolet part of sunlight. Ultraviolet absorbers are mainly used in plastic products, such as plates, cables, pipes and other thick products. In many cases, ultraviolet absorbers are also added to plastic films to suppress or delay the negative effects of ultraviolet light on protected objects or substances. effect.
Ultraviolet rays and their damaging effects on polymers such as plastics
We all know that the electromagnetic waves emitted from the sun are very broad, and the radiation that can reach the ground mainly includes ultraviolet rays, visible light and infrared rays. The energy of radiation is inversely proportional to the wavelength, the shorter the wavelength, the greater the energy. Among them, ultraviolet rays in the wavelength range of 290nm-400nm have the shortest wavelength and the highest energy, and are also the most destructive to polymers such as plastics. The bond energy of organic polymers is usually in the range of 290~400kJ/mol, so it is easy for ultraviolet rays destroyed. Therefore, ultraviolet rays are the main factor affecting the photoaging of plastics. However, it should be noted that polymer compounds with different structures have different sensitivity to various long and short wavelength bands of ultraviolet rays.
The reason why polymer materials such as plastics have many excellent properties is due to their long enough macromolecular chains. However, due to the short wavelength and high energy of ultraviolet rays, under the action of ultraviolet rays (mostly with the participation of oxygen), polymer materials absorb After ultraviolet rays, it is easy to form an electronically excited state. Molecules in this excited state can cause a series of photochemical reactions, that is, free radical chain reactions, and at the same time, "photooxygen aging" or "photooxidation reaction" occurs with oxidation. The macromolecular chains of the polymer are cut off or cross-linked to a certain extent, which directly affects the performance of the polymer material, so a series of aging phenomena appear. Such as darkening, brittleness, hardening, surface cracking, and degradation of mechanical and electrical properties, so that it eventually loses its use value.
The concept and function of UV absorber
Polymer materials have different molecular structures, and their resistance to ultraviolet light is naturally different, which means that different polymer materials have different "photostability". For polymer materials containing unsaturated bonds such as double bonds such as ABS and SBS, they are easy to absorb ultraviolet light and undergo photooxidation reactions, so their photostability is not good. But for a pure polymer containing only saturated chemical bonds, it does not absorb or hardly absorbs ultraviolet light, so it is stable to ultraviolet light.
But the actual situation is that this kind of pure polymer with saturated bonds does not exist, because in the industrial production process of polymers, such as polymerization, extrusion granulation, injection molding, storage, etc., inevitably introduce catalyst residues, Some impurities such as trace hydroperoxides are sensitive to light, and after absorbing ultraviolet rays, they will trigger the photoaging reaction of the material.
UV absorbers are currently the most widely used class of light stabilizers. It can strongly and selectively absorb high-energy ultraviolet light, and release or consume the absorbed energy as heat energy or harmless low-energy radiation in the form of energy conversion, thereby preventing the chromophore in the plastic from absorbing ultraviolet energy Excitation ensues.
Classification of UV Absorbers
UV absorbers include a wide variety of compounds. According to the structure, it mainly includes benzophenones, salicylates, benzotriazoles, substituted acrylonitriles, triazines, etc. The most widely used in the plastics industry are benzophenones and benzotriazoles.
Benzophenone UV absorbers are currently the most widely used type of UV absorbers. They absorb almost the entire UV range. The photostable mechanism is the hydroxyl hydrogen on the benzene ring and the adjacent carbonyl in the molecule. Intramolecular hydrogen bonds are formed between oxygen, forming a chelation ring, and when the energy of ultraviolet light is absorbed, the molecule undergoes thermal vibration, the hydrogen bond is broken, and the chelation ring is opened, so that the harmful ultraviolet light can be turned into harmless Harmful heat energy is released. In the diphenyl ketone ultraviolet absorber, there must be an ortho hydroxyl group, otherwise it cannot be used as a light stabilizer for polymers. Typical representative varieties of this type of ultraviolet absorber include UV-531, UV-9 and so on.
Salicylate UV absorbers are the earliest type of UV absorbers. It can form hydrogen bonds in the molecule, its own ability to absorb ultraviolet light is very low, and the wavelength range of absorption is extremely narrow (less than 340nm). However, after absorbing a certain amount of energy, molecular rearrangement occurs. A benzophenone structure with a strong ability to absorb ultraviolet light is formed, thereby producing a strong photostabilization effect.
The light stabilization mechanism of benzotriazole ultraviolet absorbers is similar to that of benzophenones, and there is also a hydrogen bond chelating ring in the molecule, which is formed by hydroxyl oxygen and nitrogen on the triazole group. When ultraviolet light is absorbed, the hydrogen bond is broken or turned into a phototautomer, turning harmful ultraviolet energy into harmless heat energy. Typical representative varieties of this type of ultraviolet absorber include UV-P, UV-326, UV-327, UV-329 and so on.
