What is a light stabilizer?
Time：2022-07-29Views：
Most organic compounds, including plastics, will change color, become brittle and have poor performance under long-term exposure to sunlight, which is the result of a series of reactions under the action of ultraviolet light and oxygen. This photooxidative aging process is also called oxidative photodegradation. The obvious effects caused by photodegradation vary with the types of polymers. For example, the oxidative photodegradation of polystyrene will cause the product to turn yellow, while the process of turning yellow to red and then black of PVC will occur. After photochemical discoloration reaction, the surface will crack, embrittlement and lose strength. At the same time, polar genes, such as ketone, alcohol and hydroxyl, will accumulate in the polymer, which will reduce its electrical insulation performance. Adding various light stabilizers can slow down the occurrence of this process and achieve the purpose of extending the service life of products.
According to the principle of light stability, it is better to include the following components in the light stabilizer: first, the ultraviolet absorber, which can absorb the radiation energy of ultraviolet light before the polymer, and convert the radiation energy into heat energy through the rearrangement of the electronic structure of its own atoms and dissipate it in the atmosphere. Once the heat energy is released, the atoms return to their original state; The other is the quenching agent (energy transfer agent), which has the function of preventing the released heat energy from damaging the polymer. It does not directly absorb the incident ultraviolet ray, but only absorbs the excessive energy that will lead to polymer degradation when the polymer has stored a certain amount of energy, and transfers this energy to the surrounding atmosphere in the form of heat; The third is the free radical trapping agent, which can capture the active free radicals generated by the action of ultraviolet light in the polymer, thereby inhibiting the photooxidation process; The fourth is UV shielding agent, which can reduce the transmission of UV to materials. Therefore, it is often not enough to only add ultraviolet absorber in the process of improving the light stability of the polymer.
Theoretically, pure polymers will not absorb ultraviolet light. Only when the polymer contains some "impurities" will it absorb ultraviolet light, and transfer the absorbed energy to the surrounding polymer in the form of heat, which will cause its molecular chain to break and deteriorate its performance. Most of these "impurities" come from the residual catalyst with metal ions during polymerization. The side reaction during polymerization leads to active groups such as hydroxyl, carboxyl, hydroperoxide, and metal ions brought in by processing aids such as thermal stabilizers, antioxidants, colorants, fillers, etc. Therefore, we should pay attention to the synergistic effect with other additives when selecting light stabilizers.
Since the end of the 1930s, phenyl salicylate was used as the ultraviolet absorber of cellulose plastics. Until the early 1950s, light stabilizers were not used in other polymers. Later, due to the rapid development of polyolefin plastics, the development of light stabilizer industry has also accelerated. At present, light stabilizers have made great progress both in variety and quantity.
