Peroxide decomposing antioxidants

Peroxide-decomposing antioxidants destroy hydroperoxides, the sources of free radicals in polymers. Phosphites and thioesters such as tris(nonylphenyl) phosphite, distearyl pentaerythritol diphosphite, and dialkyl thiodipropionates are examples of peroxide-decomposing antioxidants. 

The kinetics of the zinc diisopropyl dithiophosphate-in-hibited oxidation of cumene at 60°C. and Tetralin at 70°C. have been investigated. The results cannot be accounted for solely in terms of chain-breaking inhibition by a simple electrow-transfer mechanism. No complete explanation of the Tetralin kinetics has been found, but the cumene kinetics can be explained in terms of additional reactions involving radical-initiated oxidation of the zinc salt and a chain-transfer step. Proposed mechanisms by which zinc dialkyl dithiophosphates act as peroxide-decomposing antioxidants are discussed. 

The proposed cyclic peroxide bands are also observed In BR containing about 5 percent by weight of a peroxide-decomposing antioxidant (D-2), dl-t-butyl sulfoxide, shown In the top difference spectrum of Figure 2. The sulfoxide functions by thermal decomposition to products which destroy hydroperoxides (1, 23, 24),but It does not do so at 25°C. After three days at room temperature, stabilized BR exhibited trans methlne at 975 cm l as well as cyclic peroxide at 1080 and 820 cm. Apparently, the cyclic peroxide was not decomposed In the presence of the sulfoxide and must be fairly stable at 25°C since no carbonyls were detected. 

The action of a chain breaking antioxidant inhibits the formation of peroxides and will reduce the load on any peroxide-decomposing antioxidants. At the same time, preventive antioxidants decompose peroxides and prevent the formation of the radicals which destroy the chain-breaking antioxidants. The consequence is that mixtures of the two types of antioxidant can be much more effective than the effects of the two antioxidants in isolation. This effect is... 

The new free radical A- is unreactive because of aromatic stability and unable to propagate an oxidative chain. Commonly, such chainbreaking antioxidants are used in conjunction with peroxide-decomposing antioxidants such as dilauryl P, jS -thiodipropionate (DLTP) (IV). These antioxidants decompose hydroperoxides in a complex series of non-radical reactions (see Reference 6 for an account of this mechanism) so that initiation of the oxidative process does not occur. 

Most conventional antioxidants either trap the oxy radicals or decompose the hydroperoxides (COOH). Radical traps forperoxy and alkoxy radicals are the familiar amines and phenols. Examples of peroxide decomposing antioxidants are phosphate stabilizers such as tris(nonylphenyl) phosphite (Naugard P), thioester stabilizers such as dilauryl thiodiproprionate (DLTDT), and dithiocarbamate stabilizers such as nickel di-n-butyldithiocarbamate (Naugard NBC). 

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