Mastication. Part III. Chemical Verification of the Mechanical Degradation Mechanism of Cold Mastication

Abstract

The plasticization of natural and synthetic rubbers by cold mastication results from the degradation of the macromolecules. In Part I experimental evidence was presented that such degradation is caused by mechanical rupture of primary carbon-carbon bonds in the polymer (R—R) to give polymer radicals (R · ) which are stabilized by oxygen to give degraded polymer molecules. Such a mechanism, invoking mechanical shear, had been advocated previously, e.g., by Staudinger, most explicitly by Kauzmann and Eyring, but required systematic mastications under nitrogen with a large variety of compounds (radical acceptors) to demonstrate more critically its consistency with experimental observations. However, Part I provided no direct chemical evidence supporting the mechanism by demonstrating reaction between the rubber radical and radical-acceptor molecule. Considering that only of the order of 0.1 per cent of radical acceptor is required for radical termination to give marked degradation, especially favorable reactions and sensitive analytical techniques are required for detecting the basic chemical reactions. The incorporation of radical acceptors into two types of elastomer on mastication has been demonstrated by the use of two radical-acceptor molecules and two distinct analytical techniques: (1) the incorporation of 1,1-diphenyl-2- picryl hydrazyl into natural rubber, using a spectrometric method, and (2) the incorporation of 1,1′-dinaphthyl [S³⁵]-disulfide into GR-S, using a radiochemical method. The incorporation of the radical acceptors should be correlated with extent of degradation measured by decrease in molecular weight. Quantitative correlations have been obtained for both the above radical acceptors.