In the early 60s, CIBA first developed the benzo** ultraviolet absorber TinuVin P, which opened the rapid development path of benzo** structure products. This kind of product has a wide range of absorption wavelengths (300-380nm), has a high molar extinction coefficient, discoloration resistance, oil resistance, low toxicity, low volatility, and good compatibility with polymers, and is widely used in various coatings and plastic products, and occupies a dominant position in ultraviolet absorbers.
Although the development of hindered amine light stabilizers in the 70s had a shock to the benzo** UV absorber market, their consumption is still increasing year by year. This is because although the performance of hindered amine light stabilizers is outstanding, because they are alkaline, they still have an absolute advantage in some acidic systems, especially acidic coating systems.
The mechanism of action of benzo** ultraviolet absorbers is similar to that of benzophenones, which also converts the energy of light into harmless energy and releases it through proton transfer. After absorbing a photon, the electron cloud density shifts from the oxygen atom of the phenyl group to the nitrogen atom, and since nitrogen is more basic than oxygen, the proton will transfer from the oxygen atom to the nitrogen atom, and then the energy conversion is achieved through the internal conversion of the intermediate.
The synthesis of benzo** ultraviolet absorbers is mainly through the synthesis of intermediate azo first, and then the intermediate azo is reduced to a closed loop into the final product, and its chemical structure is similar to that of azo dyes. Theoretically, the introduction of heteroatoms into the structure of azo dyes can change the absorption (donor) properties of the conjugated system, thereby affecting its spectral absorption performance. The molar extinction coefficient of most heterocyclic azo dyes is significantly higher than that of structurally similar benzene ring azo dyes.
By introducing various substituents on the o-hydroxyl group of benzo**, such as the 5-position chlorine substituent and the 3'- and 5'-position alkyl substituents, the red shift of the transmission spectrum can be induced, and the molar extinction coefficient can be increased. For example, by substituting the hydrogen atom at the benzo**-5- position in UV-P with chlorine, the absorption peak is redshifted compared to UV-P without chlorine, indicating that the energy between the ground state and the excited state is reduced, and the molar extinction coefficient is slightly increased, which is conducive to the absorption of ultraviolet light.
With the continuous expansion of the application field of polymer materials, the requirements for photoaging are also increasing, and therefore the requirements for ultraviolet absorbers are also increasing. At present, the main development directions of benzo** UV absorbers include high molecular weight quantification, multifunctional grouping and reactivity.
In terms of polymer quantification, UV-360 can be prepared by linking two molecules together by linking two molecules together with a long-chain compound UV-329 containing 8 carbon atoms in a molecule. The melting point of UV-329 is 101-106, while the melting point of UV-360 is 195, and this high-molecular-weight quantification method can improve the solubility and thermal stability of benzo** UV absorbers, so as to better meet the needs of polymer materials for UV protection.
In terms of multifunctional grouping, researchers are exploring the introduction of other functional groups into benzo** structures. For example, the introduction of functional groups containing epoxy, carboxylic acids, amine groups and other functional groups into benzo** molecules can make ultraviolet absorbers have more application characteristics, such as cross-linking ability, antioxidant properties, etc.
Reactive benzo** UV absorbers are a special type of benzo** compounds that can react with reactive groups in polymer systems. This UV absorber can be introduced into the polymer molecular chain as a comonomer or fixed on the polymer surface through cross-linking reactions, thereby improving the stability and durability of the UV absorber in the polymer system.
In addition, environmental and health concerns have also promoted the research and development of benzo** ultraviolet absorbers. Researchers are working to develop more environmentally friendly UV absorbers to replace some compounds with potential environmental risks.
In general, the application potential of benzo** ultraviolet absorbers in polymer materials is huge, and with the improvement of material performance requirements and the enhancement of environmental health awareness, the research and development direction of ultraviolet absorbers will be more diversified and environmentally friendly.