Almost all plastics are prone to degradation when exposed to ultraviolet light, such as sunlight or fluorescent lighting. This degradation can occur during processing or once the product is in use, and its symptoms include brittleness, discoloration and loss of physical properties such as impact strength, tensile strength and color – all of which lead to a shortened lifespan of the plastic. An everyday example of UV degradation is a lawn chair that can become discolored and brittle over time due to plastic degradation.
UV stabilization additives are compounds added to prevent plastic deterioration and significantly extend the life of the end 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 manufacturing or prepared as part of a masterbatch. There are many additives on the market for UV stabilization including Amshield, Amcor's Ultra Violet Inhibitor stabilizer.
What is Photooxidation?
Plastic degradation caused by UV light is the result of a chemical process called photooxidation or photodegradation. UV light degrades existing chemical bonds in the polymer chains, weakening the plastic by reducing its molecular weight, leading to loss of strength and other undesirable symptoms, as previously described. Molecular degradation, such as photooxidation, often causes plastics to fail, so it is important to understand these processes and how to avoid them!
UV resistance of unprotected polymers varies, depending on structure and composition. Some plastics are more susceptible to photooxidation due to their structure and functional groups. Bonds most susceptible to photooxidation include carbon-nitrogen bonds such as nitriles, amides, and amines; carbon-oxygen bonds such as ethers, esters, ketones, and carboxylic acids; carbon-chlorine bonds; oxygen-oxygen bonds such as peroxides; and nitrogen-hydrogen bonds such as amides and amines.
Types of UV Stabilizing Additives
To inhibit the photooxidative process and protect plastics from harmful UV rays, UV stabilizers are used. UV protection can be provided in different ways depending on the specific UV stabilizer used. Typically, the most common types of additives act as UV absorbers, quenchers, or HALS, and in some cases more than one additive can be used to provide the desired level of UV stability.
UV absorbers: Upon sufficient light input, sensitive functional groups (called chromophores) in polymers undergo a series of reactions to generate free radicals. This form of UV stabilizer, as the name suggests, absorbs UV radiation to prevent the initiation of photooxidative reactions. Once the UV heat is absorbed, it is dissipated through the polymer chains. Black is an excellent UV absorber, so paints, dyes, or elemental carbon black 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 multiple reactions that ultimately generate free radicals that react with multiple bonds in the polymer chain, destroying the integrity of the plastic. Quenchers work by quenching the energy generated during photooxidation reactions, thus, returning excited molecules to the ground state where they are less likely to propagate the free radical-generating photooxidation reaction. A nickel quencher is an example of this form of UV stabilizer.
HALS: Hindered Amine Light Stabilizers (HALS) are UV stabilizers that target and trap free radicals generated during photo-oxidation, preventing them from reacting with the polymer structure. The structure of HALS varies, but generally has a 2,2,6,6-tetramethylpiperidine ring structure.
