Antioxidants

Antioxidants can be divided in two basic classifications: primary and secondary antioxidants. Primary antioxidants interrupt oxidation degradation by tying up the free radicals. They react rapidly with peroxy radicals to break growing chains. Secondary antioxidants destroy the unstable hydroperoxides that function as sources of free radicals during oxidative degradation. They react with hydroperoxides to yield nonreactive produtcs. Thus, secondary antioxidants are also known as hydroperoxide decomposers.

The two major groupings among the primary antioxidants are hindered phenolics and aromatic amines. Primary antioxidants function by donating their reactive hydrogen to the peroxy free radicals so that the propagation of subsequent free radicals does not occur.

The most widely used antioxidants for polymer protection are phenolics. These products generally resist staining or discoloration. However, they may form quinoid structures upon oxidation, which leads to yellowing.

Amines, normally arylamines, may be more effective than phenolic, but most are staining and discoloring and lack FDA approval for use in contact with food. Amines are commonly used in the rubber industry, but they also ind uses in applications such as wire and cable formulations and in polyurethance polyols.

Secondary antioxidants. Secondary antioxidants are used in conjunction with primary antioxidants to provide added stability to the polymer. They function by decomposing hydroperoxides to form nonreactive products. Typical secondary antioxidant compounds contain sulfur or phosphorous. The more popular secondary antioxidants are thioesters (thiodipropinonic acid derivatives and polythiodipropionates) and organophosphites.

Organophosphites provide color stability, but have a hygroscopic tendency. Hydrolysis of phosphites can ultimately lead to the formation of phosphoric acid, which can corrode processing equipment. Thioesters have high heat stability, but have an inherent odor, which can be transferred to the host polymer.

Antioxidant Blends. Every stabilizer has a specific temperature range in which it develops its optimum properties. For this reason, antioxidant system (stabilizer packages) combining two or more materials are offen used. Such blends are commercially available as a single additive. The most effective mixture will combine a free radical inhibitor (primary antioxidant) with a peroxide decomposer (secondary antioxidant). The free radical inhibitor retards the initiation of reaction chains, but some hydroperoxide is nevertheless formed. A peroxide decomposer available to react with the hydroperoxide prevents it from decomposing with free radicals.

Hindered Amine Light Stabilizers (HALS) are extremely efficient stabilizers against light-induced degradation of most polymers.

They do not absorb UV radiation, but act to inhibit degradation of the polymer, thus extending its durability. Significant levels of stabilization are achieved at relatively low concentrations.

HALS' high efficiency and longevity are due to a cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process. They also protect polymers from thermal degradation and can be used as thermal stabilizer.

The unavoidable presence of catalyst residues in polymers or other chromophores coming from polymerization and processing can initiate the degradation process.


UV absorbers (UVA's) are often used to compete with these contaminant chromophores and preferentially absorb UV light.

UV absorbers are since long time well known as stabilizers of transparent or translucent plastics to protect the bulk of thick applications.

Color fading of full shade or tint colored plastics with organic pigments can be significantly prolonged by the use of UV absorbers.