AbstractStarch xanthide is an effective reinforcing agent for vulcanized elastomers. On prolonged exposure to water, however, the effectiveness of the starch xanthide as a reinforcing agent is greatly reduced. The xanthide crosslink density is too low to prevent plasticization of the filler by water. The introduction of additional crosslinks into the filler by treatment with diisocyanates increases the modulus of the filler, significantly improving the physical properties both of the dry vulcanizate and of the vulcanizate after a 70‐hr water immersion test. Studies, which included the use of model systems, showed that the diisocyanates crosslinked the starch filler and did not introduce starch–rubber or rubber–rubber crosslinks. Several diisocyanates worked satisfactorily in the final vulcan
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