Polyolefin copper-catalyzed oxidation

Implications for the Copper-Catalyzed Oxidation of Polyolefins. Correlation of Contributing Mechanisms. The susceptibility of a polyolefin/copper composite to enhanced oxidation rates will depend critically on the rate of dissolution of copper ions from the copper (oxide) surface and the subsequent rates at which catalytically active concentrations of copper ions can be established within significantly large regions of the polymer matrix. Dissolution of copper with formation of interfacial salt layers appears to occur at very early times (4) (during the induction period) and can be described by overall reactions such as Equation 7 ... 

The presence of the above-mentioned metal ions increases the decomposition rate of hydroperoxides and the overall oxidation rate in the autoxidation of a hydrocarbon to such an extent that even in the presence of antioxidants, the induction period of oxygen uptake is drastically shortened. In such a case, sterically hindered phenols or aromatic amines even at rather high concentrations, do not retard the oxidation rate satisfactorily. A much more efficient inhibition is then achieved hy using metal deactivators, together with antioxidants. Metal deactivators are also known as copper inhihitors, because, in practice, the copper-catalyzed oxidation of polyolefins is by far of greatest importance. This is due to the fact that polyolefins are the preferred insulation material for communication wire and power cables, which generally contain copper conductors. 

The main function of metal deactivators (MD) is to retard efficiently metal-catalyzed oxidation of polymers. Polymer contact with metals occur widely, for example, when certain fillers, reinforcements, and pigments are added to polymers, and, more importantly when polymers, such as polyolefins and PVC, are used as insulation materials for copper wires and power cables (copper is a pro-oxidant since it accelerates the decomposition of hydroperoxides to free radicals, which initiate polymer oxidation). The deactivators are normally poly functional chelating compounds with ligands containing atoms like N, O, S, and P (e.g., see Table 1, AOs 33 and 34) that can chelate with metals and decrease their catalytic activity. Depending on their chemical structures, many metal deactivators also function by other antioxidant mechanisms, e.g., AO 33 contains the hindered phenol moiety and would also function as CB-D antioxidants. 

Though there are metals other than copper (such as iron, manganese and cobalt) that can accelerate thermal oxidation of polyolefins and related polymers such as EPDM, in practice, however, the inhibition of copper-catalyzed degradation of polyolefins is of paramount importance because of the steadily increasing use of polyolefin insulation over copper conductors. Among polyolefins, polyethylene is still the most common primary insulation material for wire and cable. In the United States, high-density polyethylene and ethylenepropylene copolymers are used in substantial amounts for communications wire insulation. 

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