STABLE FREE RADICAL–ASSISTED MECHANICAL DEVULCANIZATION OF CARBON BLACK–FILLED NATURAL RUBBER VULCANIZATES
ABSTRACT The issue of recombining of the radicals formed from cross-link scission and the consequent reformation of once-cleaved cross-links during mechanical devulcanization of carbon black–filled NR vulcanizates was investigated in a two-roll mill by the addition of the stable free radical 4-hydroxy-TEMPO (4HT), formerly 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine-N-oxyl free radical. The effect of concentration of 4HT (0–6 pounds per hundred of rubber [phr]) and the effect of cure system, that is, conventional, semi-efficient (EV), and EV, used for vulcanization of the original sample on the efficiency of devulcanization were determined from the residual cross-link density of the gel part of the devulcanized rubber and the percent devulcanization, and further from revulcanizate properties. Chemical probe analysis of the devulcanized sample disclosed that (i) mechanical shearing predominantly breaks the majority cross-link type that decides the associated percent devulcanization; (ii) shear-induced chain-shortening reactions of polysulfidic cross-links occur at ambient conditions, increasing the absolute value of monosulfidic links irrespective of the significant reduction in total cross-link density due to cross-link scission; (iii) re–cross-linking of once cleaved bonds is being arrested in the presence of 4HT, thereby increasing the percent devulcanization. Both the bond energy of the cross-link type to be cleaved and the recombining capacity of the broken cross-links seem to play a decisive role in percent devulcanization, and the probable mechanism involved is suggested.