Almost all plastics degrade easily when exposed to ultraviolet light, such as sunlight or fluorescent lights. This degradation can occur during processing or after the product is put into service, and its symptoms include brittleness, discoloration, and loss of physical properties such as impact strength, tensile strength, and color—all of which contribute to a shortened lifespan of plastics. UV rays An everyday example of degradation is lawn chairs, which discolor and become brittle over time due to the deterioration of plastic.
UV stabilizing additives are compounds added to combat plastic deterioration and significantly extend the life of the final product. UV stabilizers are added at very low levels, typically 0.1 – 0.5% of the polymer. These additives can often be combined with the base polymer during the manufacturing process or prepared as part of a masterbatch. There are many additives on the market for UV stabilization, including Amcor's UV inhibitor stabilizer Amshield.
What is photooxidation?
The deterioration of plastics caused by UV light is the result of a chemical process called photooxidation or photodegradation. UV light degrades the existing chemical bonds in the polymer chain, weakening the plastic by reducing its molecular weight, resulting in loss of strength and other undesirable symptoms, as discussed earlier. Molecular degradation, such as photo-oxidation, often causes plastics to fail, so understanding these processes and how to avoid them is important!
The UV resistance of unprotected polymers varies by structure and composition. Some plastics are more susceptible to photo-oxidation due to their structure and functional groups. The bonds most susceptible to photooxidation include carbon-nitrogen bonds, such as nitriles, amides, and amines; carbon-oxygen bonds, such as ethers, esters, ketones, carboxylic acids, etc.; carbon-chlorine bonds; oxygen-oxygen bonds, such as peroxides; and nitrogen Hydrogen, such as amides and amines.
Types of UV Stabilizers
To inhibit the photo-oxidation process and protect the plastic from harmful UV rays, UV stabilizers are used. Depending on the specific UV stabilizer used, UV protection can be provided in different ways. In general, the most common types of additives appear as UV absorbers, quenchers, or HALS, and in some cases more than one additive may be used to provide the desired level of UV stabilization.
UV absorbers: With enough light input, sensitive functional groups (called chromophores) in the polymer generate free radicals through a series of reactions. As the name suggests, this form of UV stabilizer absorbs UV radiation to prevent the initiation of photo-oxidative reactions. Once the heat from the UV light is absorbed, it is dissipated through the polymer chains. Black is an excellent UV absorber, so paints, dyes or elemental carbon blacks are often added to protect plastic products from UV rays. Benzotriazoles and hydroxyphenyltriazines are also examples of UV absorbers.
Quenchers: The photo-oxidation process involves several reactions that ultimately produce free radicals that react with multiple bonds in the polymer chain, destroying the integrity of the plastic. Quenchers act by quenching the energy generated during the photo-oxidation reaction and, therefore, are excited back to the ground state where they are less likely to propagate the photo-oxidation reaction that produces free radicals. Nickel quenchers are an example of this form of UV stabilizer.
HALS: Hindered Amine Light Stabilizer or HALS is a UV stabilizer that targets and traps free radicals generated during photo-oxidation, preventing them from reacting with the polymer structure. The structure of HALS varies, but usually has a 2,2,6,6-tetramethylpiperidine ring structure.
