Ultraviolet absorbers can absorb strong high-energy ultraviolet rays, convert them in the form of energy, release the absorbed energy, and then consume it through thermal energy or harmless low-energy radiation to avoid skin damage caused by ultraviolet rays, and to prevent The photophysical and photochemical decomposition reactions of high molecular polymers due to the absorption of ultraviolet rays. At present, the commonly used UV absorbers mainly include benzophenones, benzotriazoles, salicylates, substituted acrylonitriles, triazines and others. There are some differences. The following Shanghai Jingyan Chemical will introduce the mechanism of action of different types of UV absorbers.
1. Benzophenones
Benzophenone UV absorbers are the most widely used class of UV absorbers, which absorb UV-A, UV-B and UV-C slowly. The ketone group and hydroxyl group in the molecule can form an intrinsic hydrogen bond to form a butyl chelate ring. After absorbing ultraviolet light energy, the molecule undergoes thermal vibration, the internal hydrogen bond is broken, the chelate ring is opened, and the ultraviolet energy is converted into heat energy and released. In addition, the carbonyl group in the molecule will be excited by the absorbed ultraviolet light energy, resulting in mutation and enol structure, which also consumes a part of the energy. In this type of UV absorber, the strength of intramolecular hydrogen bond is related to its light stabilization effect. The stabilization effect of the UV absorber is also related to the length of the alkoxy chain on the benzene ring. If the length is compatible with the polymer, the stabilization effect is just right. In the benzophenone UV absorber, the ortho position of the carbonyl must contain 10 hydroxyl groups, otherwise it cannot form intrinsic hydrogen and cannot be used as a UV absorber. Coloring, good compatibility with high molecular polymers. If there are two hydroxyl groups in the ortho position of the carbonyl group, it can absorb ultraviolet rays of 300-400 fm, and can also absorb part of the visible light. Due to the absorption of visible light, the complementary light is unbalanced, and articles incorporating this UV absorber appear yellow. The compatibility with high molecular polymers is also poor, so its application range is small. Although benzophenone without o-hydroxyl group also has the ability to absorb ultraviolet rays, it will cause self-decomposition when exposed to light, so it is not suitable for use as ultraviolet absorbers.
2. Salicylate
Salicylate UV absorbers are the first to be used, and salicylates also have intrinsic hydrogen bonds in the molecule. This kind of ultraviolet absorber has a very low ability to absorb ultraviolet rays at the beginning, and the absorption range is very narrow (less than 340vm), but after a certain period of ultraviolet irradiation, its absorption gradually increases until the maximum absorption value, which is due to the rearrangement of molecules under ultraviolet irradiation. , forming a benzophenone structure with strong ultraviolet absorption ability, thereby enhancing its ultraviolet absorption. Therefore, it is called Pioneer UV Absorber. After dihydroxybenzophenone and its derivatives undergo molecular rearrangement, they can absorb part of the visible light and appear yellow, resulting in the yellowing of the substances added with this ultraviolet absorber.
3. Benzotriazoles
The mechanism of action of benzotriazole ultraviolet absorbers is similar to that of benzophenone. Visible light above meters, so the product will not be colored;
4. Substituted acrylonitrile and triazine
In addition, the substituted acrylonitrile and triazine ultraviolet absorbers, according to the mechanism of action, are also cis-trans isomerization, so that the energy of light is converted into harmless energy and released, and the ultraviolet absorbers substituted by acrylonitrile can absorb 290-320 fm Ultraviolet light, does not absorb visible light, and will not cause the application to flood, and the ultraviolet absorber can absorb the chromogenic gene and chromogenic group with a wavelength below 400-M, which are all connected to the aromatic core.
5. Other classes
Organickel can also be used as a UV absorber, but is generally classified as a quencher, which has a lower ability to absorb UV light than the previous one, but prevents dissociation of the polymer around absorbing UV light. The organonickel polymer reacts with the molecules in the polymer that are excited to the excited state under ultraviolet light irradiation, so that the excited state returns to the excited state. In the ground state, the ultraviolet energy is converted into a low-energy spectrum without causing its destruction. To protect the polymer from being destroyed, it is called a quencher because its mechanism of action is different from that of general UV absorbers. UV absorbers change their molecular structure, and quenchers only dissipate energy through intermolecular energy transfer.
In addition, carbon black, titanium dioxide, zinc oxide, zinc barium and other sunscreen agents are also a class of substances that can absorb or reflect ultraviolet rays. A blocking layer is set between the polymer and the light source to absorb or reflect ultraviolet rays. When it reaches the polymer surface, Absorbed and reflected, preventing UV light from penetrating into the interior of the polymer, with carbon black being the most efficient. Free radical scavengers are also a class of substances that can produce light stabilization, and are piperidine derivatives with steric hindrance effect, called hindered amine light stabilizers.
