Background and overview[1][2]
As the amount of ultraviolet radiation increases and becomes shorter-wavelength, it has significant harm and impact on human production and life. Protecting the human body from excessive and ultraviolet radiation and reducing photoaging of polymer materials has become one of the goals of developing new products in many industries today. During the production and processing of polymer materials, it is generally necessary to add UV-stabilizing absorbers and other additives. In particular, automotive parts, building materials and other fields have put forward stringent requirements for weather resistance, oxidation resistance, heat resistance, etc. In fields such as medicine and food packaging, stabilizers are required to be non-toxic and not migrate into medicines and food.
In addition, plastic carpets, artificial turf, synthetic fiber products, etc. are required not to be extracted when the stabilizer is exposed to rain or washed. Furthermore, the impact of plastic additives on the environment during processing has also been put on the agenda. For this reason, the industry has conducted a lot of research on the above issues in recent years to adapt to the development of industry.
With the expansion of outdoor applications of polymer material products, stabilizers (especially light stabilizers) have become increasingly prominent among the entire polymer additives. The pursuit of new varieties and new structures of stabilizers that are efficient, hygienic, cheap and meet harsh processing and application conditions has always been the goal pursued by the industry. Ultraviolet absorber is a kind of light stabilizer. It is a kind of substance that can absorb the ultraviolet part of sunlight or fluorescence source without changing its structure. It can convert high-energy ultraviolet light energy into heat energy or non-destructive relatively Long wavelengths of light are released, thereby protecting substances to which UV absorbers have been added from UV damage.
Mainly used as a light stabilizer for plastics, rubber, coatings, dyes, etc. to prevent their photodecomposition when exposed to sunlight for a long time. Generally, the requirements are light color, good miscibility, heat resistance and chemical stability, low volatility, etc. When used as food packaging materials, it also needs to be non-toxic and non-migrating. Various substances have different sensitive wavelengths to ultraviolet rays, and the range of light waves that various ultraviolet absorbers can absorb are also different. Only by appropriate selection can a satisfactory light stabilization effect be achieved.
Category[1]
According to chemical structure, UV absorbers can be divided into benzotriazole, benzophenone, triazine, salicylate and cyanoacrylate.
Mechanism of action[2]
The reason why ultraviolet absorbers can absorb ultraviolet light is that the molecules of this type of compounds contain two parts: a structure of a conjugated electron system and a structure that can move hydrogen atoms. There are also cases where only the first part is present. The structural molecules of UV absorbers contain at least one ortho-hydroxyphenyl substituent. In this type of compound, the ortho-hydroxyl group forms a chelate ring with a nitrogen atom or an oxygen atom. After absorbing ultraviolet light, the hydrogen bond breaks and molecular isomerism occurs. , the intramolecular structure undergoes thermal vibration, the hydrogen bonds are destroyed, the chelating ring is opened, and the intramolecular structure changes, thus converting harmful ultraviolet light into harmless heat energy released by the Bayer curing agent, thus protecting the material.
For commonly used benzophenone and benzotriazole UV absorbers, the above mechanism can be expressed by the following formula:
In this process, the chelate ring formed in the molecule is the key to its ultraviolet function. The energy sensitive range of opening this ring is exactly the ultraviolet energy range of 290-n chemical m wavelength. In addition, as an ultraviolet absorber, it must also be able to not undergo photochemical reactions under the action of ultraviolet light or visible light; have good stability to chemicals and water, good stability to heat; have low volatility; and be compatible with polymer materials. It has good properties and is not extracted by solvents. Only compounds that meet the above conditions can be used as UV absorbers.
Classification and synthesis[2][3]
1. Benzotriazoles
This type of compound is the product with the largest output and the most varieties among ultraviolet absorbers, and research at home and abroad is also very active. The synthesis of benzotriazole ultraviolet absorbers generally involves the diazotization of 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:
In order to adapt to complex and harsh polymer material processing and use conditions, it is a development trend in this field to modify traditional benzotriazole light stabilizers. This type of plastics is moving toward high molecular weight, multifunctional, and Reactivity and other aspects of development. In recent years, great progress has been made in this area, and it has entered the application stage from the research stage. Some varieties have been industrialized and successfully used in the plastics industry.
2. Benzophenones
The output and variety of this type of UV absorber is second only to benzotriazole. The synthetic route is basically a two-step reaction, namely the synthesis of 2,4-dihydroxybenzophenone (UV-0, UV-214) and the synthesis of alkoxybenzophenones:
3. Salicylates
This type of compound itself does not absorb ultraviolet light, but under the long-term effect of light, it undergoes photo-Fries rearrangement to form 2-hydroxybenzophenone with strong absorption effect. Therefore, this type of compound can be used as an ultraviolet absorber.�Used, the rearrangement reaction can be expressed by the following formula:
There are few varieties of this type of UV absorber. In the 1970s, the Tianjin Institute of Synthetic Materials Industry developed the UV absorber BAD. This product was made by reacting salicylic acid with bisphenol A after hydrogenation. In 1987, this product was The products were transferred to Tianjin Organic Chemical Factory No. 1 for production today.
4. Cyanoacrylates
A typical product of this type of light stabilizer is BASF’s Uvinnl N-35, whose structure is as follows:
This product is mainly used in automobile repair coatings and high-rise building coatings. In order to meet the needs of industry, high molecular weight products with the following structures have been reported:
5. Triazines
Triazine derivatives have many important uses in industry and medicine. They can be used as bactericides, pharmaceutical intermediates, antioxidants and ultraviolet absorbers for polymers. Recently, some triazine UV absorbers have appeared, such as Ciba{ast P, Tinuvin, CyasorbUV 1164, Tinuvin 1577 FF, etc.
It is said that T 1577 FF is much more effective than traditional UV-234 when used on polyester fibers. Domestic Shanxi Institute of Chemical Industry has done a lot of work. After successfully developing triazine-5, it synthesized triazine-425 using cyanuric chloride, m-xylene, and resorcinol as raw materials. This product has ultraviolet absorption. With the advantages of high efficiency, light color and low dosage, it is a new highlight of domestic ultraviolet absorbers.
6. Others
UV-418, UV-419, GW-628, and GW-4611 have recently been developed by Shanxi Chemical Industry Research Institute. The first two are efficient UV absorbers and can be used in conjunction with hindered amine light stabilizers. Excellent synergy. Among them, UV-418 is mainly used in engineering plastics such as PC and PET. It has the characteristics of large molecular weight and non-volatility.
UV-419 can be used in polymers such as ABS, PC, PS, PVC, etc. It has the effect of strongly absorbing medium and low wavelengths of ultraviolet rays. GW-628 is a monolithic light stabilizer with a new structure. It is mainly used for injection molded products of PE and PP that require high weather resistance. GW-4611 is a compound of UV-419 and GW-628. It is mainly used for paints and coatings PE agricultural films. The introduction of the above-mentioned new varieties by the institute has greatly enriched the variety and structure of the domestic light stabilizer market.
Main reference materials
[1] Concise Dictionary of Fine Chemicals
[2] Ding Zhuming, Liu Lixiang, Zhou Shujing. Research progress of ultraviolet absorbers[J]. Fine and Specialty Chemicals, 2005, 13(13): 5-10.
[3] Jiao Lin, Zhu Liangjie, Fan Rumei. Synthesis and application of ultraviolet absorber UV-9[J]. Printing and Dyeing Auxiliaries, 2001, 18(3): 7-9.
