Antioxidants
Authors: Dexter Martin 
Published: August 2004

Abstract

Antioxidants are used to retard the reaction of organic materials with atmospheric oxygen. Such reaction can cause degradation of the mechanical, aesthetic, and electrical properties of polymers; loss of flavor and development of rancidity in foods; and an increase in the viscosity, acidity, and formation of insolubles in lubricants. The need for antioxidants depends on the chemical composition of the substrate and the conditions of exposure. Relatively high concentrations of antioxidants 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. The mechanism by which an organic material (RH) undergoes autoxidation involves a free‐radical chain reaction. The usual concentrations of hydrogen‐donating antioxidants (AH) in saturated thermoplastic polymers range from 0.01 to 0.05%, based on the weight of the polymer. Higher concentrations, ie, ca 0.5–2%, are required in substrates that are highly sensitive to oxidation, such as unsaturated elastomers and 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. Nearly all polymeric materials require the addition of antioxidants to retain physical properties and to ensure an adequate service life. The selection of an antioxidant or system of antioxidants is dependent upon the polymer and the anticipated end use. Low concentrations of stabilizers (

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) are added to polyethylene and polypropylene after synthesis and prior to isolation to retard oxidation of the polymers exposed to air. A phosphite, such as tris(2,4‐di‐
tert
‐butylphenyl)phosphite, in combination with a phenolic antioxidant such as octadecyl 3,5‐di‐
tert
‐butyl‐4‐hydroxyhydrocinnamate, may be used. Polyunsaturated elastomers are sensitive to oxidation. Stabilizers are added to the elastomers prior to vulcanization to protect the rubber during drying and storage. Nonstaining antioxidants such as butylated hydroxytoluene or a phosphite such as tris(nonylphenyl)phosphite may be used. Staining antioxidants such as
N
‐isopropyl‐
N
′‐phenyl‐
p
‐phenylenediamine 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 scavengers such as 2,4‐dimethyl‐6‐
tert
‐butylphenol or 2,6‐di‐
tert
‐butyl‐
p
‐cresol 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. Up to 1% of a mixture of hindered phenols, of the type used for gasoline, and secondary aromatic amines, such as alkylated diphenylamine and alkylated phenyl‐α‐naphthylamine, are used as the primary antioxidants. The oxidation of food containing fats and oils results in a loss of sensory appeal and eventually rancidness. FDA regulations covering the use of direct food additives are stringent and few new materials have been regulated. A number of products in use today, such as citric acid, and α‐tocopherol, are in the GRAS list. The most commonly used materials are butylated hydroxytoluene
n
‐propyl gallate, α‐tocopherol, and butylated hydroxyanisole. Safety is assessed by subjecting the antioxidant to a series of animal toxicity tests, eg, oral, inhalation, eye, and skin tests. Mutagenicity tests are also carried out to determine possible or potential carcinogenicity. A number of antioxidants have been accepted by the FDA as indirect additives for polymers used in food applications. Acceptance is determined by subchronic or chronic toxicity in more than one animal species and by the concentration expected in the diet.