What is a light stabilizer, and what types are there?
Light stabilizers used by plastic manufacturers are substances that can inhibit or reduce the degradation of plastics caused by light, thereby improving their light resistance. Most of them can absorb ultraviolet light, so they are also called UV absorbers. Ultraviolet rays in sunlight are the primary cause of aging in polymer materials. Although UV rays account for only about 5% of sunlight, their energy is very high. This energy is sufficient to break chemical bonds in polymers, causing molecular chains to break or cross-link, leading to deterioration of mechanical properties and changes in color.

(1) Classification of Light Stabilizers
There are many types of light stabilizers. According to their mechanisms of action, they can be divided into four categories:

1. Light Shielding Agents Also known as light blockers, these agents can reflect and absorb ultraviolet rays, thereby shielding, isolating, or reducing the UV radiation reaching the plastic products. This prevents the inner parts of the products from being harmed by UV rays and effectively inhibits photodegradation. Common light shielding agents include carbon black, zinc oxide, and titanium dioxide. Among these, carbon black is the most effective light shielding agent.
2. Ultraviolet Absorbers These are currently the most widely used light stabilizers and constitute the main body of light stabilizers. They strongly and selectively absorb UV rays and convert the energy into heat or harmless low-energy radiation, thereby releasing or consuming this energy. Common UV absorbers include salicylate esters and benzophenone compounds, with the latter being produced in the largest quantities and used most widely. Benzophenones absorb a broad range of UV wavelengths, hardly absorb visible light, and are compatible with most resins.
3. Quenchers UV rays that are not shielded or absorbed are absorbed by polymer molecules, putting them in an unstable "excited state." To prevent further damage to the polymer chains, this excited state must be rapidly and effectively "quenched" to return to a low energy state, thus protecting the polymer's stability. Common quenchers are mainly organic complexes of nickel.
4. Free Radical Scavengers This is a new type of light stabilizer, consisting of pyridine-derivative stabilizers with a steric hindrance effect, abbreviated as hindered amine light stabilizers (HALS). These compounds absorb almost no UV light but can capture free radicals. The free radicals are formed when polymers absorb UV rays and decompose, triggering an autocatalytic oxidation reaction. Free radical scavengers capture these active free radicals to stop the chain reaction and form stable compounds. Common free radical scavengers include sebacate esters and trialkyl phosphites.

(2) Principles for Selecting Light Stabilizers
In practical applications, selecting light stabilizers is often not easy, but attention can generally be paid to the following points:

1. Choose types with the highest possible absorption coefficient within the polymer's most sensitive wavelength range.
2. Consider the effects of other additives such as antioxidants, heat stabilizers, lubricants, and colorants on the performance of the light stabilizer. These additives should not diminish the effectiveness of the light stabilizer.
3. Some light stabilizers have certain toxicities. Pay careful attention to their use, and avoid adding them to plastics that will come into contact with food.