Preventative antioxidants

Preventive antioxidants which prevent the formation of radicals by reactions (7.1) and (7.3). 

Any agent which functions to prevent the formation of radicals by these processes may be considered as a preventive antioxidant. In practice three classes are generally recognised ... 

Another method for slowing oxidation of rubber adhesives is to add a compound which destroys the hydroperoxides formed in step 3, before they can decompose into radicals and start the degradation of new polymer chains. These materials are called hydroperoxide decomposers, preventive antioxidants or secondary antioxidants. Phosphites (phosphite esters, organophosphite chelators, dibasic lead phosphite) and sulphides (i.e. thiopropionate esters, metal dithiolates) are typical secondary antioxidants. Phosphite esters decompose hydroperoxides to yield phosphates and alcohols. Sulphur compounds, however, decompose hydroperoxides catalytically. 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

Preventive antioxidants reduce the rate of chain initiation, and they can be subdivided into three main categories. 

The mechanisms of inhibition by peroxide decomposers, metal deactivators, and ultraviolet absorbers are fairly well understood in general terms, although many details of the individual reactions remain to be elucidated. Classifying a preventive antioxidant into one of the three categories above will only rarely describe its entire function. The dual behavior of dialkyl dithiophosphates in the liquid phase has been mentioned. Many other phosphorus- and sulfur-containing antioxidants commonly classified as peroxide decomposers can also act as chain breakers. Similarly, the structure of many metal deactivators and ultraviolet absorbers indicates that they must also have some chain-breaking activity. 

High in vitamin C, potassium, and cancer-preventing antioxidants and bioflavonoids... 

We will not go in depth into the subject of antioxidants (12), which is more a part of preformulation than a stress test, but the autoxidation mechanism does suggest that oxidation can be inhibited by peroxy radical scavengers (chain-breaking antioxidants) like phenol antioxidants, by heavy metal chelating agents, and by peroxide inactivating substances (preventive antioxidants). 

Shelton J R (1981) Organic sulfur compounds as preventive antioxidants. In Developments in polymer stabilization-4, Scott G (Ed), Applied Science Publishers London, pp 23-70. 

Preventive antioxidants reduce the rate of the chain initiation. The most important initiation suppressors are metal deactivators that chelate metal ions. Metal deactivators used for stabilizing edible fat and lipid-containing foods include citric, phosphoric, tartaric acids, and phospholipids. Peroxide destroyers also are preventive... 

Shelton, J.R. Organic sulphur compounds as preventive antioxidants. In Developments in Polymer Stabilisation, Scott, G., Ed. Applied Science London, 1981 Vol. 4, 23-69. 

Some of the most powerful uv stabilisers belong to the class of peroxide decomposing preventive antioxidants and it has been suggested that the mechanism of this type of uv stabiliser is not distinguishable from their behaviour as thermal antioxidants although all peroxide decomposers do not behave as uv stabilisers (11). Of paramount importance in the peroxide decomposer -uv stabiliser class are the metal dithiocarbamates (III) (9,22,23,24) the dithiophosphates (IV) (11,24) and the... 

Secondary, or preventive antioxidants destroy hydro-peroxide groups, responsible for chain initiation and chain branching. 

There are many types of antioxidants. The most popular in composite materials are primary antioxidants or chain breaking antioxidants, and secondary antioxidants or preventative antioxidants. ... 

Preventative antioxidants, 526 Primary antioxidants, 526, 527 Primary macroalkyl radical, 497, 498... 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Preventive antioxidants (a) light absorbers, (b) metal deactivators, (c) peroxide decomposers (nonradical products). 

Since stabilization against photooxidation and metal-catalyzed oxidation are covered elsewhere in this symposium, this discussion is restricted to protection against thermal autoxidation. I will first review the mechanism by which typical chain-breaking antioxidants function and then describe some of our current studies on the way in which certain organic sulfur compounds act as preventive antioxidants. 

We have thus established that sulfides, disulfides, and their initial oxidation products are not the actual preventive antioxidants. The active peroxide decomposers are the sulfenic acid from sulfoxide decomposition, the thiosulfoxylic acid from thiolsulfinate decomposition, and the acidic products formed when they react with hydroperoxides. The catalytic... 

Studies of the kinetic deuterium isotope effects which established the chain-breaking mechanism of antioxidant action by hydrogen donation were carried out in our laboratories by E. T. McDonel, J. C. Crano, and D. N. Vincent. Studies of sulfoxides, sulfenic acids, thiolsulfinates, and their reactions with hydroperoxides which illustrate the chemistry of the processes involved in their activity as preventive antioxidants were done by K. E. Davis, J. V. Webba, E. R. Harrington, and D. M. Kulich. 

The Role of Certain Organic Sulfur Compounds as Preventive Antioxidants... 

Thiolsulfinates and their reaction products play an important role in the preventive antioxidant activity observed with organic sulfides and disulfides. An investigation of the decomposition of cumene hydroperoxide in benzene at 25°C in the presence of tert -butyl tert-butanethiolsulfinate has shown that the actual peroxide decomposer is an acidic species whose activity is affected by the basic character of the S-O group in the parent thiolsulfinate and in the sulfoxides. Alternative mechanisms for generating the acidic species are discussed. Although hydroperoxide decomposition occurs primarily by a polar process, the results also indicate the involvement of radical generating processes. 

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