Polyethylene PE is a widely used polymer in various applications due to its low cost, ease of processing, and excellent mechanical properties. However, it is susceptible to degradation under UV light exposure, which can lead to a reduction in its mechanical properties and service life. The mechanisms of degradation of polyethylene under UV light exposure are as follows:1. Photo-oxidation: When polyethylene is exposed to UV light, the high-energy photons can break the carbon-hydrogen C-H bonds in the polymer chain, generating free radicals. These free radicals can react with oxygen molecules to form peroxy radicals, which can further react with the polymer chain, leading to chain scission and the formation of carbonyl and hydroperoxide groups. This process is known as photo-oxidation and results in the degradation of the polymer's mechanical properties.2. Chain scission: The free radicals generated during photo-oxidation can cause chain scission, leading to a reduction in the molecular weight of the polymer. This can result in a decrease in the polymer's mechanical properties, such as tensile strength and elongation at break.To improve the stability of polyethylene under UV light exposure, various additives can be used:1. UV stabilizers: These additives, such as hindered amine light stabilizers HALS and UV absorbers, can help protect the polymer from UV-induced degradation. Hindered amine light stabilizers HALS work by scavenging free radicals and converting them into stable nitroxyl radicals, which do not participate in the degradation process. UV absorbers, such as benzophenones and benzotriazoles, can absorb the UV radiation and convert it into harmless heat, thereby protecting the polymer from photo-oxidation.2. Antioxidants: Antioxidants, such as phenolic antioxidants and phosphite antioxidants, can be used to prevent the oxidation of the polymer. They work by reacting with the peroxy radicals generated during photo-oxidation, thereby inhibiting the chain scission process.3. Pigments and fillers: Inorganic pigments and fillers, such as titanium dioxide and carbon black, can be added to the polymer to provide protection against UV light exposure. These additives can absorb or scatter the UV radiation, thereby reducing the amount of UV light that reaches the polymer surface.4. Coatings and films: Applying a protective coating or film on the surface of the polyethylene can help shield it from UV light exposure. These coatings can be made from materials that have high UV resistance, such as acrylics, polyurethanes, or silicone-based materials.In conclusion, the degradation of polyethylene under UV light exposure occurs mainly through photo-oxidation and chain scission processes. To improve its stability, various additives, such as UV stabilizers, antioxidants, pigments, and fillers, can be used. Additionally, protective coatings and films can also be applied to the polymer surface to provide further protection against UV-induced degradation.