Antioxidants, Polymers

Abstract

AbstractAntioxidants are used to retard the reaction of organic materials with atmospheric oxygen. Such reaction can cause degradation of the properties of the polymer; loss of flavor and development of rancidity in foods; and a change in lubricants. The need for antioxidants depends on the chemical composition of the substrate and the conditions of exposure. High concentrations of antioxidants, ie, 0.5–2% based on the weight of he polymer, are used to stabilize polymers such as natural rubber and polyunsaturated oils. Saturated polymers have greater oxidative stability and require relatively low concentrations of stabilizers; ie, 0.01–0.05%, based on the weight of the polymer. The mechanism by which an organic material (RH) undergoes autoxidation involves a free‐redical chain reaction. High concentrations are also required in substrates that are highly sensitive to oxidation, such as unsaturated elastomers and acrylonitrile–butadiene–styrene (ABS). Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. The selection of an antioxidant or system of antioxidants is dependent on the polymer and the anticipated end use. Low concentrations of antioxidants ( < 0.01%) are added to polyethylene and polypropylene after synthesis and prior to isolation to retard oxidation of the polymers exposed to air. Polyunsaturated elastomers are sensitive to oxidation. Antioxidants are added to the elastomers prior to vulcanization to protect the rubber during drying and storage. Nonstaining antioxidants may be used. Staining antioxidants are preferred for the manufacture of tires. Gasoline and jet engine fuels contain unsaturated compounds that oxidize on storage, darken, and form gums and deposits. Radical scanvengers and alkylated paraphenylene diamines are used as stabilizers. Lubricants for gasoline engines are required to withstand harsh conditions. Relatively high concentrations of primary antioxidants and synergists are used to stabilize lubricating oils. The oxidation of food containing fats and oils results in a loss of sensory appeal and eventually rancidness. The U.S. Food and Drug Administration (FDA) regulations covering the use of direct food additives are stringent and few new materials have been regulated. The most commonly used materials are butylated hydroxytoluenen‐propyl gallatte, α‐tocopherol, and butylated hydroxyanisole. A number of antioxidants have been accepted by the FDA as indirect additives for polymers used in food applications.