1. What is a light stabilizer
Sunlight is an electromagnetic wave. When it passes through space and the ozone layer, almost all rays below 290nm and above 3000nm are filtered out. The electromagnetic waves that actually reach the ground are 290nm~3000nm, of which the wavelength range is 400~800nm (about 40 %) is visible light, the wavelength of about 800-3000nm (about 55%) is infrared, and the wavelength of about 290-400nm (only 5%) is ultraviolet.
Ultraviolet rays in sunlight are the main reason for the aging effect of polymer materials. Although ultraviolet light only accounts for about 5% of sunlight, it has a lot of energy. After the polymer product absorbs ultraviolet rays, it is enough to cause the polymer to self-oxidize and degrade, destroy the chemical bonds of the polymer, break and cross-link it, and cause the appearance and physical and mechanical properties of the polymer product to deteriorate, and the strength is reduced. , shortened life. This process is called photoredox or photoaging. Ultraviolet rays can also pass through the surface layer of a person's skin, destroying skin cells, causing the skin to gradually harden and lose its elasticity, appear wrinkled and accelerate aging.
Light stabilizer is an additive for polymer products (such as plastics, rubber, coatings, synthetic fibers), which can shield or absorb ultraviolet energy, quench singlet oxygen, and decompose hydroperoxides into inactive substances. , under the irradiation of light, the polymer can exclude or slow down the possibility of photochemical reaction, prevent or delay the process of photoaging, so as to achieve the purpose of prolonging the service life of polymer products.
2. Types of light stabilizers
l Light shielding agent: This is a kind of substance that can shield or reflect ultraviolet rays, so that light cannot penetrate into the interior of the polymer, thereby protecting the polymer. Light shielding agents include inorganic pigments such as carbon black and titanium oxide, and organic pigments such as phthalocyanine blue and phthalocyanine green, among which carbon black has the best shielding effect.
l Ultraviolet absorber: Its function can effectively absorb ultraviolet rays with a wavelength of 290-410nm, but rarely absorb visible light, and it has good thermal stability and light stability. According to its chemical structure, it can be divided into: o-hydroxybenzophenones, benzotriazoles, salicylates, triazines and substituted acrylonitriles. Used as auxiliary light stabilizer and hindered light stabilizer, especially in polyolefins or coatings.
l Quencher: It can accept the energy absorbed by the chromophore in the plastic and emit the energy in the form of heat, fluorescence or phosphorescence, thereby protecting the polymer from UV damage. It has a good stabilization effect on polymers and is mostly used in films and fibers. Mainly some divalent organic nickel chelates. Organonickel light stabilizers have good performance, but may be replaced by other nontoxic or low toxicity quenchers due to the toxicity of heavy metal ions.
l Free radical scavenger: This type of light stabilizer can capture the active free radicals generated in the polymer, thereby inhibiting the photo-oxidation process and achieving the purpose of light stabilization. Mainly hindered amine light stabilizers (HALS). It is the most promising new type of high-efficiency light stabilizer, and the average annual demand growth rate in the world is 20% to 30%.
l Hydroperoxide decomposer: It is one of hindered amine light stabilizers. Polymers can generate hydroperoxides during storage and processing, resulting in photo-oxidative degradation of polymers, and hydroperoxide decomposers can decompose peroxides, generate stable nitrogen-oxygen radicals, and further capture free radicals , thereby inhibiting polymer degradation.
3. Disadvantages of light stabilizers
l The light shielding agent has good protective effect and low price, but has light-shielding and coloring properties, and is only suitable for opaque materials;
l UV absorbers have a wide range of applicability, but due to their protective effect, they cannot effectively protect the surface of products and thin products. At the same time, because they are pure organic compounds, there are also volatile, frosting, migration, and solvent extraction. Disadvantages, which not only affect the persistence of its performance, but also lead to pollution of the environment;
l Excited state quencher and hydroperoxide decomposer have high photostability, low volatility, little blooming and migration, and are resistant to extraction, which can effectively protect the surface of products and thin products. However, the color is dark, the toxicity and environmental pollution are large, and it will decompose and change color at high temperature, which has a counteracting effect with sulfur-containing additives;
l The free radical scavenger is light in color, has outstanding light stabilization effect, and can effectively protect the surface of products and thin products. However, due to its alkalinity, it has an antagonistic effect with acidic substrates and additives. The acidic environment will affect its performance. At the same time, like the UV absorber, because it is a pure organic compound, it also has disadvantages such as volatile, blooming, migration, and extraction.
4. Examples of the mechanism of action of light stabilizers
1. UVA
2. HALS
