Crystallization of vulcanizates. I. Low‐temperature crystallization as a function of extent of cure for polyisoprene vulcanized with tetramethylthiuram disulfide/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide/sulfur

The crystallization of polyisoprene, vulcanized to various degrees of cure with tetramethylthiuram disulfide/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur formulations, was studied in a density column at ?25°C. The densities of vulcanizates before crystallization decrease progressively with cure time, which is ascribed to an increase in free volume occasioned by the formation of accelerator‐terminated pendent groups on the polymer chain. The induction period before the onset of crystallization increases and both the rate of and the degree of crystallization decrease with extent of cure. This is attributed primarily to the presence of residual pendent groups on the polymer chain and secondly to crosslink formation. The changes are more marked with MBTS formulations where pendent groups are more bulky. MBTS compounds fail to crystallize once vulcanized to the point where a gel has formed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2565–2572, 2001     

Thermoplastic elastomers based on high‐density polyethylene,ethylene–propylene–diene terpolymer,and ground tire rubber dynamically vulcanized with dicumyl peroxide

Thermoplastic vulcanizates (TPVs) based on high‐density polyethylene (HDPE), ethylene–propylene–diene terpolymer (EPDM), and ground tire rubber (GTR) were dynamically vulcanized with dicumyl peroxide (DCP). The polymer blend was composed of 40% HDPE, 30% EPDM, and 30% GTR, and the concentration of DCP was varied from 0.3 to 3.6 parts per hundred rubber (phr). The properties of the TPVs were determined by evaluation of the gel fraction content and the mechanical properties. In addition, IR spectroscopy and differential scanning calorimetry analysis were performed as a function of the DCP content. Decreases in the Young's modulus of the blends and the crystallinity of HDPE were observed when the content of DCP was greater than 1.8 phr. The results regarding the gel content indicate that the presence of DCP promoted the crosslinking of the thermoplastic matrix, and optimal properties were obtained with 1.5% DCP. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39901.     

Polyisoprene,poly(styrene‐cobutadiene) and their blends. III. Tensile properties of tetramethylthiuram disulfide/sulfur and 2‐bisbenzothiazole‐2,2'‐ disulfide/sulfur compounds

Polyisoprene (IR), poly(styrene‐cobutadiene) (SBR) and IR/SBR blends were vulcanized with tetramethylthiuram disulfide (TMTD)/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur formulations and their tensile properties were determined. MBTS vulcanized IR has inferior tensile properties to TMTD vulcanizates. This is attributed in part to main chain modification in MBTS vulcanizates decreasing the ability of chains to crystallize or to align as effective load‐bearing chains under stress. A similar discrepancy is not found in SBR compounds that cannot stress‐crystallize. Polybutadiene, which readily crystallizes on cooling, is used to demonstrate differences in the effect of MBTS and TMTD on the ability of chains in the vulcanizates to align. These differences are confirmed by X‐ray diffraction studies of stressed IR vulcanizates. The addition of zinc stearate reduces main chain modification, promotes crystallization, and improves tensile properties. Blends have inferior properties to IR, and tests involving the pulling apart of laminates and analysis of the tear surfaces are used to illustrate that failure does not occur in adhesion, but within the IR phase close to the interface. It is argued that diffusion of curatives from SBR to the faster curing IR phase, leads to the development of a layer of highly crosslinked material in IR close to the phase boundary. Failure occurs in this layer and may be attributed to a decrease in the number of effective load‐bearing chains in this region or to the shorter chains in this layer becoming taut. Less diffusion of the accelerator occurs with MBTS than with TMTD, leading to a less highly crosslinked IR zone close to the interface. Consequently, higher loads are required to initiate failure. Failure in blends is likewise considered to initiate in the highly crosslinked region in the IR phase close to the phase boundary with SBR. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2143–2149, 1999     

Effect of carboxylic acids on 2‐bisbenzothiazole‐2,2′‐disulfide‐ and tetramethylthiuram disulfide‐accelerated sulfur vulcanization. II. Vulcanization of polyisoprene in the absence of ZnO

Polyisoprene was vulcanized by 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur and tetramethylthiuram disulfide (TMTD)/sulfur in the absence and presence of benzoic and stearic acids. It was found that the crosslink density of MBTS vulcanizates is halved by the addition of carboxylic acids and this can be explained in terms of the attack of the acids on the accelerator polysulfides. TMTD polysulfides are more reactive toward polyisoprene than are MBTS polysulfides, and their addition to the polymer chain occurs before significant attack by the carboxylic acids can reduce the polysulfide concentration. Consequently, the acids have little effect on the crosslink density of TMTD vulcanizates. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1007–1012, 1999