Como escolher um absorvedor de UV?
Have you ever noticed that plastic chairs left by the window for a long time become brittle and crack, or that the coating on outdoor railings becomes mottled and faded? This is because ultraviolet (UV) light is silently “decomposing” the materials. Absorvedores de UV are like “invisible sunscreen” specifically designed for polymer materials. They absorb harmful UV rays from sunlight and convert them into harmless heat, thus protecting plastics, coatings, rubber, and other materials from “sunburn” and aging.
Choosing the right “sunscreen” is crucial. It needs to meet several core conditions: strong absorption of harmful UV rays in the 290-400nm range, resistance to high-temperature processing, good compatibility with the material, and it must be stable, safe, and not leach out. Different materials require different “sunscreen” protection. Below, we will review several mainstream types of UV absorbers and see what each is best suited to protect.
Category 1: Benzophenone-based compounds (most commonly used, wide range of applications)
This type of UV absorber is the most widely used. Its core characteristics are good compatibility and excellent heat resistance. Most of them can be used in light-colored or transparent products, making them the “standard sunscreen” in the plastics and coatings industries.
2,4-Dihydroxybenzophenone
Appears as light-colored needle-shaped crystals or white powder, almost insoluble in water, but easily soluble in organic solvents such as acetone and ethanol. It can absorb ultraviolet light with a wavelength of 280-340 nm, has a wide range of applications, and can be used in various materials such as polyvinyl chloride, polystyrene, epoxy resin, coatings, and synthetic rubber. The conventional addition amount is 0.1%-1%. A drawback is that its light stabilization effect is not outstanding and usually needs to be used in combination with other additives.
• 2-Hydroxy-4-methoxybenzophenone (UV-9)
A light yellow or white crystalline powder, odorless, with good heat resistance (does not decompose at 200°C), and hardly absorbs visible light, making it particularly suitable for light-colored and transparent plastic products. It is easily soluble in most organic solvents, insoluble in water, and can absorb 280-340 nm ultraviolet light. It is suitable for polyvinyl chloride, polymethyl methacrylate (plexiglass), ABS resin, cellulose resin, etc., with an addition amount of 0.1%-1.5%. It can also be used in paints and synthetic rubber. It has high safety, and in some countries, the maximum addition amount is limited to 0.3% when used in products that come into contact with food.
• 2-Hydroxy-4-n-octyloxybenzophenone (UV-531)
A light yellow or white crystalline powder with low density and low volatility. It has excellent compatibility with various plastics and strongly absorbs 270-330 nm ultraviolet light. It is mainly suitable for polyethylene, polypropylene, polystyrene, ABS resin, polycarbonate, etc., with an addition amount of 0.1%-1%. When used with antioxidants, the sun protection effect is significantly improved. It has very low toxicity, and several countries allow its use in food-contact plastic products (such as polyethylene and polypropylene containers), with specific usage limits for different materials (0.2%-1%).
Category 3: Salicylate Esters (High Cost-Effectiveness, Suitable for Low-End Plastics)
These UV absorbers are inexpensive and readily available, but they have a narrow absorption wavelength range and provide moderate sun protection. They are mainly used in plastic products that do not require high weather resistance, making them an “economical sun protection option.”
1. Phenyl Salicylate
A colorless crystalline powder with a faint wintergreen oil scent, easily soluble in organic solvents such as ether and benzene, and almost insoluble in water and glycerol. It has a narrow absorption wavelength range and is mainly used in plastic products due to its high cost-effectiveness. Approved by the US Food and Drug Administration (FDA) for use in acrylic resin products that come into contact with food (such as acrylic tableware).
2. Resorcinol Monobenzoate
A white crystalline powder, slightly soluble in water and benzene, and easily soluble in acetone and ethanol. Its sun protection performance is similar to that of benzophenone derivatives. Mainly used in polyvinyl chloride, cellulose resins, and polystyrene, with a typical addition amount of 1%-2%. Suitable for low-end products where cost is a concern and high weather resistance is not required.。
Category 4: Other Special Types (Adapted to Specific Scenarios)
This category of Absorvedores de UV has special absorption bands or different mechanisms of action, and is mainly used for specific materials or scenarios with special requirements, making them highly targeted.
1. Reaction Product of o-Nitroaniline and p-Cresol
Colorless or light yellow crystals, insoluble in water, easily soluble in organic solvents such as gasoline, benzene, and acetone, and not easily decomposed by concentrated acids or bases. It absorbs UV light at 270-280 nm (a relatively unique wavelength). It is mainly used in resins such as polyester, chlorinated polyester, cellulose acetate, polyvinyl chloride, and polyacrylonitrile. Its stability in transparent products is better than in colored products, with an addition amount of 0.1%-0.5%.
2. 2,2′-Thiobis(4-tert-octylphenoxy)nickel (Nickel Complex)
Green powder, primarily suitable for polyolefin materials such as polyethylene and polypropylene. It is particularly effective in protecting plastic films and fibers from UV damage and can also improve the processing performance of the material. When used in combination with other UV absorbers, the sun protection effect is significantly enhanced. However, its disadvantages include its dark color, which can color the product, and the possibility of turning the product grayish-black at high temperatures. It is also toxic, requiring extra caution during use.
3. Tris(1,2,2,6,6-pentamethylpiperidinyl) phosphite (Hindered Amine)
White crystalline powder, easily soluble in organic solvents, sparingly soluble in water, and has good compatibility with polyolefins. Its unique feature is that it does not absorb UV light itself, but it can capture active free radicals generated during material degradation. Its sun protection efficacy is 2-4 times that of conventional UV absorbers, and it can also improve the material’s resistance to thermal oxidative aging. Suitable for plastics such as polyethylene and polypropylene, it has low toxicity, but poor heat resistance. The processing temperature should not exceed 270℃, and it is not suitable for long-term use in hot water.
4. 4-Benzoyloxy-2,2,6,6-tetramethylpiperidine (Hindered Amine)
White crystalline powder, with good heat resistance (decomposition temperature above 280℃), insoluble in water, and easily soluble in organic solvents such as toluene and acetone. Its mechanism of action is similar to the previous one; it does not directly absorb ultraviolet light, but efficiently scavenges active free radicals, providing several times the sun protection effect of conventional products. It is compatible with various plastics such as polypropylene, polyethylene, polystyrene, and polyamide, and is especially suitable for polyolefin products. It is non-coloring and non-polluting, and exhibits excellent synergistic effects when used in combination with antioxidants and UV absorbers.
5. 2,4,6-Tris(2′-n-butoxy-phenyl)-1,3,5-triazine
Light yellow powder, slightly soluble in n-butanol, insoluble in water, and absorbs ultraviolet light in the 300-380nm range. Its sun protection performance is superior to benzophenone-based UV-9 and UV-531. It is suitable for plastics such as polyvinyl chloride, polyoxymethylene, and chlorinated polyether, with an addition amount not exceeding 1%. Its drawbacks include slight discoloration, causing the product to have a light yellow tint, and poor compatibility with some resins.。
6. Hexamethylphosphoramide
A colorless or light yellow transparent liquid with a slightly fishy and astringent odor, it is miscible with most plasticizers and solvents and is known as a “highly effective weather-resistant agent for polyvinyl chloride.” When added to polyvinyl chloride films (2-5 parts), it not only significantly improves weather resistance and cold resistance but also lowers the processing temperature by approximately 10°C. Furthermore, it can also be used as an excellent solvent for polymers such as polyamide, polyurethane, and polyphenylene sulfide, making its applications even more extensive.
Conclusion: The Core Value and Selection Techniques of UV Absorbers
In summary, Absorvedores de UV are the “longevity secret” of polymer materials. Whether it’s everyday plastic tableware and appliance casings, or outdoor plastic pipes and coatings, their protection is indispensable. When choosing, there’s no need to blindly pursue “high performance”; the key is to match the material and application scenario: for polyolefins (polyethylene, polypropylene), benzotriazole or hindered amine types are preferred; for transparent/light-colored products, benzophenone types (UV-9, UV-531) are preferred; for low-end products, salicylate types can be used; and for products in contact with food, strict adherence to dosage limits is required.
With the development of the materials industry, UV absorbers are also upgrading towards “high efficiency, low toxicity, and broad compatibility.” In the future, they will better meet the sun protection needs of various materials, protecting every polymer product in our lives, extending their service life, and maintaining their excellent performance.