Metal ion deactivating agents

The basic objective in the selection of antioxidants is to obtain the best stabilization with no effect on optimum values of technical properties of polymers. The stabilizing effect on oxidative degradation is based on protection against the formation of active free radicals and on nonradical decomposition of hydroperoxides. Optimum stabilization is obtained by the combination of different compounds that function with different mechanisms as metallic ion deactivating agents, UV absorbers, and so on. 

Metal Deactivation. Compounds capable of forming coordination complexes with metal ions are needed for this purpose. A chelating agent such as ethylene-diaminetetraacetic acid (EDTA) is a good example. 

Chelants, or chelating agents, typically are organic chemicals (although inorganic chelants exist) that react with polyvalent metal ions to form stable ring structures that incorporate the metal ion within the molecule. Chelants tie up metals and deactivate them. 

Retard efficiently oxidation of polymers catalysed by metal impurities. Function by chelation. Effective metal deactivators are complexing agents which have the ability to co-ordinate the vacant orbitals of transition metal ions to their maximum co-ordination number and thus inhibit co-ordination of hydroperoxides to metal ions. Main use of stabilisation against metal-catalysed oxidation is in wire and cable applications where hydrocarbon materials are in contact with metallic compounds, e.g. copper. 

Synergy between primary and secondary anti-oxidants occurs and often a mixture is employed. Also included are metal complexing agents, e.g., EDTA (ethylenediaminetetraacetic acid), citric acid, the purpose of which is to deactivate extraneous metal ions that catalyse polymer oxidation. 

Complexing agent that deactivates or reduces the ability of metal ions to initiate or to catalyze the degradation of a polymer. 

In some cases (e.g., gasoline), autoxidation of hydrocarbons is undesirable, and trace amounts of metal catalysts may often be deactivated by the addition of suitable chelating agents. The latter affect the catalytic activity of metal complexes by hindering or preventing the formation of catalyst-hydroperoxide or catalyst-substrate complexes by blocking sites of attack or by altering the redox potential of the metal ion. 

In chain termination reactions, the cocatalyst acts as a transfer-agent (see below). It can also re-initiate the propagation centers subsequent to other chain termination processes i) after deactivation of the centers by the impurities present in the polymerization medium ii) after monomer insertion, binding a secondary carbon atom to the metal ion instead of the normal primary insertion ... 

The reactions responsible for the catalytic initiation [i.e., reactions (k) and (1)] probably involve coordination of ROOH to the metal ion catalyst (inner-sphere electron transfer). Deactivation of such catalysts by chelating agents such as ethylenediaminetetraacetic acid may result from the blocking of this coordination . 

Metal deactivators chelate the transition metal ions, which otherwise would catalyze the oxidation reaction. Some effective chelating agents for latexes are ethylenediaminetetraacetic acid and its salts, and citric acid. A list of commercial antioxidants is given in ref. 115. 

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