Covulcanisation of natural rubber and chlorobutyl rubber - Effect of curative diffusion between phases and ZnCl2 addition

Natural rubber (NR) and chlorobutyl rubber (CIIR) were compounded with vari ous formulations containing tetramethylthiuram disulphide (TMTD). sulphur, and ZnO and masterbatches of these compounds were blended in a 70:30 NR/CHR ratio and vulcanised in a press at 150 degreesC. Crosslink densities of vu lcanisates were determined by swelling and tensile properties measured. in formulations in which the concentration gradient permitted the diffusion of TMTD and sulphur to a CHR phase containing ZnO, tensile strengths were sli ghtly better than 70% of the values of NR compounds of similar crosslink de nsities. In formulations in which TMTD and sulphur diffused to the Faster c uring NR phase, blend properties were also better than 70% of those of NR c ompounds, but at higher crosslink densities, tensile strengths and elongati on at break decreased in parallel. This was attributed to failure in a laye r of more highly crosslinked material formed within the NR phase close to t he interface. Although ZnCl2 can crosslink CIIR to NR, thus ensuring good i nterfacial bonding, the addition of ZnCl2 to the CHR masterbatch led to att ack on the TMTD accelerator and a significant reduction in crosslink densit y and tensile properties of blends.