Influence of pendent groups on the tensile properties of polyisoprene vulcanizates

Abstract

Gum compounds of polyisoprene were vulcanized with a number of different curing systems to give networks with crosslink densities in two different ranges. Stress-strain curves were obtained upon rapid (500 mm/min) and slow (0.5 mm/min) extension. In tetramethylthiuram disulfide (TMTD)/sulfur and zinc dimethyldithiocarbamate/sulfur vulcanizates, which crystallize readily, failure occurred at higher tensile values upon rapid than upon slow extension and this is attributed to a greater contribution to tensile strength by a larger amount of stress-induced crystallites. X-ray diffraction showed that 2-benzothiazole-2,2Ј-disulfide (MBTS)/sulfur vulcanizates did not stress-crystallize and failure occurred at lower tensile values. Furthermore, samples extended rapidly failed at lower tensile values than did slowly extended samples. These differences, compared to TMTD vulcanizates, are attributed to extensive mainchain modifications (pendent groups), causing delays in the movement of sections of the chain, leading to the load being unequally distributed between chains. The fewer load-bearing chains ensure earlier failure. The addition of zinc stearate to TMTD/sulfur and MBTS/sulfur formulations increases the ability of vulcanizates of similar crosslink density to crystallize and enhances tensile properties of vulcanizates with similar crosslink densities, outcomes that are attributed to zinc stearate's promoting crosslinking of pendent groups and reducing impediments to crystallization and chain movement. Dicumyl peroxide-cured networks crystallize readily and exhibit a very rapid upturn in the stress-strain curve. However, failure occurs at lower stress values than apply to accelerated sulfur networks and it is suggested that the distribution of subchain lengths between crosslinks may contribute to their inferior properties.

Effect of water and hydrogen sulfide in zinc dimethyldithiocarbamate accelerated sulfur vulcanization

“…6, 7; Tables V, VI). This is in agreement with the work of van der Merwe et al 37,52 who reported a progressive increase in the density of vulcanizates heated for longer times and attributed the increase to the formation in the compound of ZnS, which has a higher density (4.09 g/mL) than the Zn 2 (dmtc) 4 (1.79 g/mL) from which it forms. It is well known that compounds shrink on vulcanization, a reaction leading to some physical bonds between the components being replaced by chemical bonds in which the bond distance is shorter.…”

“…27,35 In formulations containing ZnO, the dimethyldithiocar- 4 . 24 However, no thiuram pendant groups were observed in model compound studies with Zn 2 (dmtc) 4 accelerated sulfur vulcanization, 21,22,29 and the ease with which Zn 2 (dmtc) 4 vulcanizates of polybutadiene (BR) crystallize on cooling 37 also points to the absence of pendant groups. Thus, the proposals for the involvement of pendant groups in the vulcanization process are based on circumstantial evidence.…”

“…35,51 The crystallization of vulcanizates was followed on a density column at Ϫ16°C. 37,52 Crystallization was allowed to occur over a period of 3000 -6000 min, although only the initial portions of the crystallization curves are reproduced in the figures. The TME formulations were prepared as described in two studies 24,50 and reacted in evacuated sealed tubes fully immersed in an oil bath at 150 Ϯ 0.5°C for various times; the reaction was then quenched and the products analyzed by HPLC on a C18 reverse phase -Bondapak column with 80/20 (v/v) methanol/water as the eluent.…”

“…6, Table V); in TMTD and 2-bisbenzothiazole-2,2Ј-disulfide (MBTS) accelerated vulcanizates, where crosslinking occurs via pendant groups, the initial densities of compounds heated for short periods decreased with the cure time. 37,52 This decrease was attributed to pendant groups increasing the free volume of the vulcanizates. At longer heating times, where high crosslink densities develop, further changes in the initial density of TMTD and MBTS vulcanizates were minimal, while the initial density of Zn 2 (dmtc) 4 accelerated compounds cured at both at 130 and 140°C were noticeably increased (Figs.…”

“…Kruger and McGill 27 suggested that much of the sulfur was present in pendant groups, which they thought were highly sulfurated in Zn 2 (dmtc) 4 formulations. However, van der Merwe et al 37 reported that Zn 2 (dmtc) 4 vulcanizates of BR crystallize readily on cooling, although less rapidly than peroxide cures with similar crosslink density. This implies that if cyclic sulfides formation occurs it must be limited, and it certainly conflicts with the suggestion that sulfur is present in pendant groups.…”