Current Development and Modified Varieties of Benzotriazole UV Absorbers

With the expansion of application fields for polymer materials, the role of ultraviolet absorbers in the overall family of polymer additives has become increasingly prominent. The pursuit of new UV-absorber varieties and new structures that are efficient, safe, inexpensive and able to meet demanding processing and application conditions has always been the goal of industry. Benzotriazole UV absorbers are the second-largest category after hindered amines (HALS) among plastic light stabilizers, with the greatest number of varieties and the largest output, and they occupy an important position among polymer material stabilizers. As early as the 1960s, China began research on such products. In 1964, the Tianjin Synthetic Material Research Institute began work in this field, and in 1968, based on that institute's results, the state invested heavily to produce the UV absorber UV-327 at the Tianjin Lisheng Chemical Plant. With economic development, there are now about 10 domestic manufacturers of such products, with an annual output of about 2,000 t.

The Tianjin Synthetic Material Research Institute conducted in-depth research [1-4] on this field and brought it to industrial production. East China University of Science and Technology, the Chemical Engineering Research Institute of the Beijing Chemical Defense Command Engineering College, Tongji University, and Beijing University of Chemical Technology also carried out research in this field. The following will discuss the modified varieties and synthesis technologies of benzotriazole UV absorbers.

1. Modified varieties

The structure of these compounds can be represented by Figure 1.

Different products are formed with different R groups. Table 1 lists some structures and trade names of these products.

To meet complex and demanding polymer processing and service conditions, modifying traditional benzotriazole light stabilizers is the development trend in this field. Such plastic additives are moving toward higher molecular weight, multi-functionality and reactivity. Considerable progress has been made in recent years; the technology has moved from the research stage to the application stage, and some varieties have been industrialized and successfully applied in the plastics industry.

1.1 Higher molecular weight

Plastics processing is generally carried out at relatively high temperatures (200-300°C), at which plastic additives volatilize to varying degrees, polluting the environment on the one hand and affecting the health of operators on the other, while also increasing cost. Increasing the relative molecular mass of the plastic additive is an important way to solve this problem, and it has long been a subject of research in the additive industry.

A Japanese patent reported the condensation of two molecules of 329 (R1=R2=H, R3=tert-octyl in Figure 1) with one molecule of formaldehyde to produce a high-molecular-weight compound designated UV-360, whose structure is shown in Table 1. The product has very low volatility and good compatibility with many polymers, and is especially suitable for the high-temperature processing requirements of polycarbonate; it can also be used in polyoxymethylene, polyamide, polystyrene and other polymers. The benzotriazole UV absorber UV-P (R=R1=H, R2=methyl) was subjected to chloromethylation to introduce a chloromethyl group into the molecule, which was then condensed with cyanuric acid to eliminate hydrogen chloride, combining three molecules of UV-P with one molecule of cyanuric acid to form the following high-molecular-weight compound:

Furthermore, introducing a methyl acrylate group into the benzotriazole molecule and then transesterifying it with polyethylene glycol produces a high-molecular-weight reactive stabilizer. Ciba used this method to produce a product designated UV-213, a yellow viscous liquid suitable for styrenic polymers, elastomers, PVC and PMMA.

1.2 Multi-functionality

Plastic additives include light stabilizers, antioxidants, etc., which need to be added separately in use, which is inconvenient and increases cost. Endowing a single stabilizer molecule with groups of different functions to provide multiple stabilization effects has long been pursued by industry. Japan's Johoku Kagaku Co. reacted formaldehyde with the benzotriazole UV absorber (UV-329) and the antioxidant 2-tert-octyl-4-methylphenol in the presence of an alkaline catalyst to produce a multi-functional additive designated JAST-500 [5] with both light-stabilizing and antioxidant functions—a successful model in this field, whose structure is shown in Table 1.

Table 1. Structures and trade names of some main benzotriazole UV absorber products

JAST-500 contains groups with both light-stabilizing and antioxidant functions. Compared with the traditional UV absorber UV-326, it has higher heat resistance: under conditions of 2500 (keeping for 40 min), the heat loss of UV-326 is about 90%, while that of JAST-500 is only 6%. It has high solubility in solvents such as toluene, MEK, styrene and butyl acetate, and good compatibility with many base polymers. In antioxidant tests, JAST-500 showed almost the same antioxidant effect as the commonly used Irganox 1010, while UV-326 has no antioxidant groups and therefore no antioxidant property. Japan's Asahi Chemical used JAST-500 in high-impact polystyrene, greatly improving the product's light stability [6].

The stabilization and flame retardancy of polymer materials often present problems, usually addressed by adding the two types of additives mentioned above. A Japanese patent [7] reported reacting the UV absorber UV-P with a flame-retardant phosphorus compound to produce a multi-functional additive with both light stability and flame retardancy in one molecule, whose structure is as follows:

Using the above additive combining light stabilization and flame retardancy to treat PBT achieved good light-stabilizing and flame-retardant effects.

1.3 Reactivity

A stabilizer with reactivity is a reactive (also called polymer-bound) stabilizer. Such stabilizers contain reactive groups in the molecule that, through chemical reaction during processing or polymerization, bond into the base polymer, giving the final product heat resistance, extraction resistance and non-staining properties. With increasing performance requirements for plastic products and growing environmental awareness, reactive stabilizers have gradually become a research hotspot in the polymer field.

Noramco's [8] reactive light stabilizer designated Norbloc 7966 was produced by reacting methacryloyl chloride with the corresponding benzotriazole compound (structure in Table 1); Japan's Hitachi [9] used 7966 to stabilize rubber with good results. For reactive UV absorbers, Norbloc 7966 is representative: this benzotriazole structure containing a methacrylate group can copolymerize with vinyl double bonds or be grafted onto the backbone of unsaturated polymers, giving durable stabilization without affecting product color. Its main applications are PVC, acrylic resins, PS, MBS and SAN. Noramco has industrialized this product.

IOLAB's [10] patent reported that 4-hydroxypropoxy-2-nitroaniline and 2-tert-butyl-4-methoxyphenol are diazotized, coupled and reduced to the corresponding benzotriazole compound, which is then esterified with acrylic acid to form the following compound:

As can be seen from the formula, this compound links two benzotriazole molecules and introduces an acryloyl group, so it combines high molecular weight and reactivity and has excellent performance. It is said that such compounds can be copolymerized with polymerizable monomers such as styrene, acrylic acid, acrylamide, acrylonitrile, vinyl acetate and vinyl chloride to produce polymers with light stability. Materials containing this stabilizer do not volatilize during high-temperature processing, do not exude from the surface of the molded body, and have high long-term weather resistance and heat resistance; moreover, they do not color even in the presence of alkali or metal ions. Furthermore, because the benzotriazole group replaces an ordinary alkyl group, the molar absorptivity of the stabilizer is higher than that of ordinary products. UV-curable coatings are a widely used class of polymer materials; to improve their light resistance, additive stabilizers are ineffective, but when this class of stabilizer is added, it bonds with the base resin during UV curing to give excellent results. The company's reactive UV absorber designated RUVA-93 (same structure as Norbloc 7966) is also a benzotriazole compound containing an acryloyl group.

As early as the 1970s, a research group led by Vogl [11-15] at Polytech Institute of New York began work in this field; China's Fudan University, the Chinese Academy of Sciences and the Beijing Institute of Photochemistry also participated. More recently, John Cranford of the United States' Rohm & Haas [16-17] reviewed the field.

Chen Lixin et al. [18] of Northwestern Polytechnical University synthesized benzotriazole from o-nitrobenzene and resorcinol, then reacted it with acryloyl chloride to produce benzotriazole (BDHA) containing an acryloyl group. The above reactive benzotriazole light stabilizers can be copolymerized with polymerizable monomers such as styrene, acrylic acid, acrylamide, acrylonitrile, vinyl acetate, vinyl chloride and olefins to produce polymers with light-stabilizing function.

East China University of Science and Technology [19-20] used resorcinol instead of traditional alkylphenol to synthesize a type-(II) benzotriazole light stabilizer, said to have a molar extinction coefficient more than 1.5 times that of traditional benzotriazoles.

2. Synthesis technology research

These compounds are prepared by diazotizing an o-nitroaromatic amine with inorganic acid to form a diazonium salt, coupling it with a phenolic compound to form an azo-pigment intermediate, and then reducing and ring-closing:

2.1 Synthesis of the azo pigment

The synthesis of the azo pigment includes preparation of the diazonium salt and the coupling reaction. The diazonium salt can be prepared by conventional methods. The coupling reaction is relatively complex: when the coupling component is soluble in alkaline solution, the reaction proceeds smoothly (for example, p-cresol used in synthesizing UV-P), but when the coupling component has a larger molecular weight and is insoluble in alkali and only soluble in organic solvent, how to emulsify it becomes an important issue. Sodium dodecylbenzenesulfonate (ABS) is commonly used as an emulsifier, but the effect is not good and the resulting pigment purity is only about 90%. When synthesizing UV-326, we selected a special emulsification system and the synthesized pigment purity reached over 98%, creating the condition for synthesizing a high-purity final product.

The Shenyang Research Institute of Chemical Industry [21] described Ciba Spec's continuous diazotization unit, which uses a polarized-voltage method to control the rate of nitrite addition during continuous diazotization; the system consists of a pair of platinum electrodes that automatically control the reaction temperature at the diazotization endpoint. The continuous coupling unit feeds the coupling component vertically in laminar flow from the bottom of the reactor, with the diazo liquor added through several ports in the vessel wall, and the reaction endpoint is automatically controlled by potentiometric determination.

2.2 Reduction of the azo pigment

Reduction technology has long been a research hotspot in industry. The difficulty lies in the fact that reduction of the azo body easily causes cleavage of the azo bond to generate by-product amine compounds; the key to the reduction reaction is to create conditions that minimize side reactions and thereby improve yield and quality. The reduction and ring-closure of an o-nitroazo compound into a benzotriazole compound is a very complex and subtle process. The reaction likely proceeds by the o-nitroazobenzene first opening the azo bond and reducing to a hydrazine compound, which intramolecularly dehydrates and ring-closes to form an N-oxide, which is further reduced to the final product; this is the main reaction (reductive ring-closure). Side reactions are: the hydrazine compound splitting into two molecules of amine, and the oxide opening its ring to form two molecules of amine.

The N-oxide in the above reaction can be isolated or reduced directly to product without isolation. According to the reducing agent used, reduction of benzotriazole compounds can be divided into one-step and two-step methods. The so-called one-step method uses a single reducing agent to reduce the azo pigment to product; the two-step method first uses one reducing agent to reduce the pigment to the N-oxide (which may or may not be isolated), as described below.

2.2.1 One-step reduction method

(1) Zinc dust reduction

This is the earliest studied method and is still used in industrial production. Early processes generally carried out the reduction in alkaline alcohol solution; after reaction, the zinc sludge was filtered off and the product separated from the reaction liquor, yielding a product with many impurities that required purification to obtain a refined grade.

In recent years extensive research has brought this method to a new level, marked by the use of water-insoluble aromatic hydrocarbons instead of water-soluble alcohols; both the azo pigment and the product are soluble in such solvents, allowing the reaction to proceed in a heterogeneous system. After reaction, the product remains dissolved in the solvent and crystallizes out upon cooling, giving a higher-quality product.

In 2004, Tongji University synthesized UV-327 by the zinc-dust reduction method with an 86% crude yield; recrystallization from ethyl acetate gave the refined product.

(2) Hydrazine hydrate reduction

The hydrazine hydrate reduction method gradually releases hydrogen from hydrazine hydrate under alkaline conditions, allowing the nascent hydrogen to perform the reduction; it can be regarded as a variant of catalytic hydrogenation. Early solvents for this method were diethylene glycol diethyl ether; the author once experimented with it and obtained good yield and quality, but the solvent is too expensive for industrial use. The Shanghai Reagent Research Institute [22] used the lower-cost 2-methoxyethanol as solvent to synthesize UV-326.

(3) Catalytic hydrogenation reduction

With increasingly strict environmental protection requirements, clean and friendly reduction methods have received attention in recent years. Catalytic hydrogenation reduction is a research hotspot in this field; the by-product of the reduction is water, making it an ideal reduction method that may be industrialized. Catalysts used for catalytic hydrogenation reduction are Pd/C and Raney Ni. The former is more expensive; the latter is cheaper. Pd/C is a porous material with palladium salt supported on activated carbon; Raney Ni is produced by treating a nickel-aluminum alloy with sodium hydroxide solution to remove the aluminum, forming a honeycomb nickel. Both are characterized by a very large surface area and strong adsorption of hydrogen sources and organic matter, reducing the organic compound through complex reactions.

UV-P among the benzotriazoles is a representative low-cost, widely used variety. The Ivanov Institute of Chemical Technology [23-26] in Russia conducted extensive research on the hydrogenation reduction synthesis of UV-P using Raney Ni as catalyst. The results showed that, in addition to the target product, o-phenylenediamine, 4-methyl-2-aminophenol and hydroxylamine compounds are also generated, indicating azo-bond cleavage during reduction. The institute also studied the effect of titanium-modified Raney Ni on the reduction; the results showed the best effect at a titanium content of 1.5%. When using an isopropanol-water mixed solvent, the highest yield was obtained at an alkali concentration of 0.0025%-0.005%. The institute's patent [27] states that using a titanium-modified nickel catalyst with a Ni:Ti ratio of 1:0.15-0.25 and methanol-water as solvent gave a crude yield (based on o-nitroaniline) of 79.2%; after treatment with activated carbon, the refined yield was 63.5%, with a melting point of 130-130.5°C and a light transmittance of 97.5% (500 nm). Japan's Yoray Co. used Raney Ni as catalyst and methanol as solvent to prepare UV-P with a 93.5% yield. China's Chemical Defense Command Engineering College [28] used Pt/C as catalyst at 1 MPa and 35-37°C for hydrogenation to synthesize UV-326 with a 70% crude yield.

Ciba Geigy used 5% Pd/C as catalyst in a sodium hydroxide medium at 40-60°C / 470-530 kPa for 2.5 h to synthesize UV-P with an 84% crude yield. Japan's Toray used Pd/C as catalyst and methanol-sodium hydroxide solution as reaction medium to synthesize UV-327 with a 95% yield. Ciba used xylene and n-butylamine as reaction medium and Pt/C as catalyst for hydrogenation reduction to produce UV-326 with a 91% yield and a melting point of 138-140°C. Although industry has conducted extensive research on catalytic hydrogenation, no industrial-scale report has yet appeared.

(4) Hydrogen-transfer-agent reduction

After the 1980s, reduction methods appeared that use formaldehyde or aldehyde-containing compounds as reducing agents and aromatic ketones as catalysts. In this reduction, the aromatic ketone first obtains hydrogen from the reducing agent and then transfers it to the substance being reduced; such catalysts are therefore called "hydrogen-transfer agents." Substances that can serve as hydrogen-transfer agents include fluorenone, benzanthrone, naphthoquinone, diphenoquinone, and 2,3-dichloro-1,4-naphthoquinone. The advantage of this reduction method is reduced environmental pollution and avoidance of heavy metals (zinc, etc.) remaining in the product. Its disadvantage is long reaction time, large consumption of the hydrogen-transfer agent, and higher cost.

Japan's Sumitomo reported heating the UV-326 intermediate azo pigment in alkaline isopropanol at 30-50°C for 1 h, adding 2,3-dichloro-1,4-naphthoquinone and charging polyoxymethylene batchwise, reacting the mixture for 2 h, and acidifying to obtain a 97.6% yield of the oxide; then raising the temperature to 75-80°C and reacting for 2 h to obtain UV-326 at 93.5%. Another Japanese patent reported reacting the UV-328 pigment, sodium hydroxide and methanol at 65°C for 30 min, adding 9-fluorenone and glucose, and reacting the mixture for 4 h to obtain an 85% yield of the UV-328 oxide. In another example, the UV-328 pigment was added to a methanol-sodium hydroxide mixture, reacted at 65°C for 30 min, 9-fluorenone was added, glucose was added slowly, the mixture was refluxed for 4 h, water was added, the reaction liquor was neutralized with sulfuric acid, cooled, crystallized, filtered, washed with methanol and dried to give UV-328 at 85%.

(5) Other reduction methods

Myong University [29] in South Korea has in recent years conducted research on new reduction methods for benzotriazoles, proposing synthesis using yeast as reducing agent in ethanol under alkaline conditions, a reduction method using SmI2 as reducing agent [30], and a green photochemically simulated reduction method for benzotriazole light stabilizers [31]. The Russian Research Institute of Polymer Materials Chemistry proposed an environmentally hygienic electroreduction method [32].

2.2.2 Two-step reduction method

As mentioned above, reducing the azo body to product is a very important step, and industry has conducted extensive research on it for many years. The results show that the conditions for synthesizing the oxide and for further reducing it to product are not the same. Extensive experiments show that the conditions for synthesizing the oxide are relatively mild, while those for further reducing it to product are more severe; if only one reducing agent is used it is difficult to control the reaction and achieve good results. In addition, compared with the azo body, the oxide is more stable and less prone to ring cleavage into amine compounds.

Therefore, in the 1970s, the two-step reduction method appeared: first using one reducing agent to reduce the pigment to another intermediate (the oxide), then using another reducing agent to reduce it to product, as described below.

(1) Sodium sulfide - zinc dust method

This method first uses sodium sulfide to reduce the azo body to the oxide, then zinc dust to reduce it to product. It was proposed by the American Cyanamid Company in the early 1980s. The Tianjin Synthetic Material Research Institute began research on this route in 1982, solved some key problems in the technology, and successfully used it for the synthesis of benzotriazole products. In 1987 it conducted a pilot test in cooperation with the organic plant of Lanhua Company, and in 1989 passed the ministerial-level technical appraisal of the Petrochemical Corporation, laying the foundation for the industrialization of such products. In 1998, industrial trials were conducted according to market demand, and production began in 2000.

The characteristics of this method are: when sodium sulfide is used to reduce the intermediate pigment to another intermediate oxide, the yield is close to theoretical; when zinc dust is finally used to reduce it to product, a high-quality product can be obtained without purification. This method saves half the zinc dust, has low product cost and a cost-performance advantage, and has developed considerably since its industrialization, showing strong market competitiveness.

(2) Hydrazine hydrate - zinc dust method

This method first uses hydrazine hydrate to reduce the azo body to the oxide, then zinc dust to reduce it to product. Because hydrazine hydrate has mild reaction conditions and no environmental pollution, it has received attention in recent years.

Japan's Nippon Kayaku used hydrazine hydrate in an alkaline methanol-alkali solution with a hydrogen-transfer agent to reduce the UV-P azo body to the oxide (95% yield), then used zinc dust to reduce it to product.

(3) Glucose - zinc dust method

Australian scholars used glucose to reduce the azo body to the oxide in an ethanol alkaline solution (76%-91% yield), then zinc dust to reduce it to the target (68%-88% yield).

(4) Hydrazine hydrate - catalytic hydrogenation method

As is well known, the hydrazine hydrate reduction method is a variant of hydrogenation. Patents have recently reported applying a hydrogenation catalyst to the hydrazine hydrate reduction system: first producing the oxide, then using catalytic hydrogenation to make the target. When used for UV-328, the oxide yield reached as high as 97%, and re-reduction to UV-328 gave a 92% yield with high product quality and a light transmittance of 97%.

Because the intermediate oxide can be isolated and purified to remove impurities, facilitating the next reduction step and improving product quality, many patents reduce the azo body only to the oxide and do not further reduce it. This should be regarded as part of the two-step reduction method.

3. Conclusion

After decades of research in China, benzotriazole UV absorbers have finally achieved industrial production and export, but there is still a certain gap with foreign countries in both output and quality. As the plastics industry develops toward higher temperature and higher speed, this class of products will develop correspondingly to meet the needs of the plastics industry.