Similar to the working principle of benzotriazoles and benzophenones UV absorbers. The triazine UV absorber molecule also contains an intramolecular hydrogen bond involving a hydroxyl group. And it forms a six-membered ring with N atom.
This six-membered ring and the surrounding structure form a conjugate system, and the optical band gap of the conjugate system is exactly similar to the UV energy in the UVA and UVB range. The N-H bond in this conjugated system is the weakest and most easily broken. When the molecule itself absorbs UV light, the molecular energy rises and the N-H bond breaks. The structure is unstable after the hydrogen bond is broken. It is easy to release the absorbed energy in the form of heat, fluorescence or phosphorescence, which are not harmful to the polymer material, and the molecular structure is restored to the original structure. So on and so forth, to achieve the role of UV absorption, the specific process is shown in Figure 1.
2. The Development Trend of Triazine UV Absorbers
The development trend of triazine UV absorbers is similar to that of other types of UV absorbers. At present, the most mainstream development trends are mainly as follows.
Multifunctionalization refers to the chemical modification of a molecule so that the molecule itself contains two or even more functionalized groups or fragments.
For example, UV absorbers and hindered amine light stabilizers used in combination can occur synergistic effects. A significant increase in material lifetime is achieved. However, the screening of compound formulations is a long-term process that requires extensive experiments. In contrast, the UV absorber fragment/group and the hindered amine light stabilizer fragment/group are chemically bonded together on a single molecule so that the new molecule has both UV absorber function and light stabilizer function. Such multifunctional materials can reduce the screening work for light stabilizer additive systems.
Specialized researchers in China have reported new multifunctional additives made by linking triazine UV absorbers and benzophenone UV absorbers with hindered amine light stabilizer groups, respectively. The new materials have the functions of both UV absorbers and light stabilizers.
However, whether the new multi-functionalized additives can achieve the desired performance remains to be experimentally verified. If two functions can be achieved, it can save a lot of time and unnecessary work for people to screen the light stabilizer system, so that the formulation of anti-aging additives for polymer materials becomes more simple.
2.2 Reaction type
Reactive type UV absorbers are also a more popular development trend. In recent years there have been more and more reports on the development of reactive triazine-type UV absorbers. As most of the UV absorbers are small molecules structure, after a long period of use due to surface energy or concentration differences and other reasons will migrate to the surface of polymer materials. This can cause UV absorber failure.
In addition, some polymer materials are often washed during processing or application, which will also cause the loss of UV absorbers. Caused by the decay of polymer material life.
The reactive UV absorber can react with the active site on the polymer material so that it is permanently connected to the polymer material’s molecular chain through a chemical bond. This can effectively solve the problem of UV absorber loss.
For example, professional researchers have synthesized the following molecule (Figure 3), which retains the Cl atom on the triazine ring and can be bonded to the molecular chain of the polymer material by reacting with the active sites such as hydroxyl groups on the polymer material to avoid the loss caused by migration.
Similarly, reactive UV absorbers have their own disadvantages or they are a class of “special” additives. This is because they can only be used for polymeric materials that contain active sites on the molecular chain that can react with them. Most polymer materials such as polyethylene, polypropylene, polystyrene, and other molecular chains, and there are no reactive active groups. This also greatly limits the scope of the application of reactive UV absorbers.
2.3 Development of new structures
For triazine-type UV absorbers, in recent years, such as BASF and other companies do not have new structure products, the development of new structures is mostly in universities and other research units. The whole process of structural design, process research, and performance testing of new structures is a long-term, reciprocal process. It is necessary to consider a series of issues such as the performance of the new product, whether the source of raw materials is extensive and the synthesis process is simple and feasible, and environmentally friendly.
Performance testing is performed after the new structure is developed. If the performance does not meet expectations (a probable event), the structure needs to be modified and a new compound synthesized. This is followed by a new round of evaluation. And so on and so forth. Therefore, the development of new structures is a long-distance process that requires a long-term investment in human and material resources.
2.4 Development of new processes
In contrast to the development of multifunctional, reactive, and new structures, the development of new processes is an R&D effort that can generate economic value in the short term. The development of green and efficient synthesis or production processes can significantly reduce production costs and increase profitability.
In addition to the above four trends, there are some other aspects reported in the literature. For example, UV absorbers are wrapped with polymer microspheres and prepared in the form of microcapsules, which are added to polymeric materials in the form of microcapsules for the purpose of slow release. However, these practices not only increase the cost of the additive itself but also limit the scope of application of the additive itself.
Triazine UV absorbers, not only excellent performance but also their use process has a small impact on the ecological environment, is a hot research and development direction in the field of UV absorbers. The core of R&D lies in the synthesis process and production technology of intermediates of each product.
In addition, the production process of triazine UV absorbers is accompanied by the generation of a large amount of wastewater containing aluminum trichloride. How to recover a large amount of aluminum trichloride, and reduce the discharge of high salt content wastewater is also such UV absorbers R&D and production must consider and solve the problem.
Finally, the price of triazine UV absorber products is generally higher than that of benzotriazole UV absorbers. The development of more efficient and economical synthetic routes to further reduce production costs will allow for the wider application and larger-scale production of such UV absorbers.