Control of free-radical reactions during processing

3 Control of free-radical reactions during processing 

In the inhibition of the oxidation of polymer melts, the types of antioxidant may be characterized by their mode of action. 

These stabilizers react stoichiometrically with the alkyl radicals and are thus consumed in the process, unless there is a subsequent reaction to liberate the quinone again. In the absence of this, the kinetic relation for the CB-A stabilizer may be determined by simply replacing the usual termination reaction of the free-radical reaction scheme with that for the reaction of the inhibitor with the alkyl radical, R-, which would otherwise be propagating the chain. Thus, if reaction with the inhibitor can occur at the diffusion-controlled rate, then the rate of termination, rt is dominated by the reaction with inhibitor of concentration [Inh] and becomes, in the steady state, 

If the inhibitor is able to destroy more than one radical (as described below for chain-breaking redox antioxidants) then the efficiency of inhibition will be increased by this ratio. 

These antioxidants include the hindered phenols and are considered to be most effective when the chain-carrying (propagation) radical is an -oxy radical such as alkyl peroxy, R02. Thus, in reactive processing, they would be expected to be of value in suppressing the oxidation reactions which can occur in the earlier zones of a reactive extruder. The chemistry of these systems has been studied in detail (Al-Malaika, 1989, Scott, 1993b), and it has been found in the case of hindered phenols that the effectiveness of these stabilizers is dependent on the chemistry of the oxidation product rather than the simple donor reaction of the phenol hydrogen atom to the propagating radical. 

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