Contribution to the Mechanism of Devulcanization

(1) Three types of natural-rubber vulcanizates have been treated under specific reclaiming conditions. The resultant products have been examined particularly for the manner of sulfur combination and for chloroform extract. The normally accepted, non-reverting tetramethylthiuram disulfide type stock was found to be by far the most amenable to plasticization by thermal agencies. This observation is more or less in line with conclusions of van Amerongen, drawn from a recent study of the oxidative and non-oxidative thermal degradation of rubber. A natural pure-gum type, cured with tetramethylthiuram disulfide only, when heated in the absence of oxygen, at temperatures up to 175° C, was found to revert much more than a similar type cured with 1.0 per cent Santocure and 2 per cent sulfur. Swelling in benzene, after heating 3 days at 150° C, increased considerably with the former, whereas with the latter it remained unchanged. This greater “devulcanizing tendency” is explained on the basis of the tetramethylthiuram disulfide type having comparatively few crosslinks and no free sulfur to form additional ones, hence, “… if any links are broken down, a seemingly unvulcanized rubber results. …” The access of air under the reclaiming conditions used in our work would certainly be limited, and the highly plastic stocks obtained are to be expected in the light of the reference cited. Finally, van Amerongen concludes that in the absence of oxygen, it is inadvisable to use tetramethylthiuram disulfide vulcanizates where thermal stability is important. Mercaptobenzothiazole and rubber/sulfur types did not show any very significant differences in plasticization. (2) Considerable increases in zinc sulfide resulted in all cases, even when free sulfur was virtually removed before reclaiming, and in the latter instance there appears to be a reduction in rubber combined sulfur. In general, the changes are in line with those reported elsewhere in the reversion of natural and Butyl vulcanizates. (3) In all reclaims made, the ratio of sulfur in the chloroform-insoluble rubber to that in the soluble portion is very much lower than that previously published elsewhere. The comparison is not necessarily sound since the products tested are not technical reclaims, and generally the chloroform extracts were of a low order. (4) Under the highly reverting thermal conditions employed in this work, plasticization was generally inferior. This perhaps supports the theory that reclaiming is essentially a depolymerization process associated with traces of oxygen. The line of demarcation between reclaiming and reversion is by no means well defined. Since reclaiming treatments are normally much more severe than those needed to induce the characteristics of reversion, it may well be that reversion is only one factor in the mechanism of reclaiming, but it is a factor which cannot be ignored in any comprehensive consideration of devulcanization.