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

Rubber and the model compound 2,3-dimethyl-2-butene (TME) are vulcanized with zinc dimethyldithiocarbamate [Zn 2 (dmtc) 4 ] accelerated sulfur formulations. When heated in dry nitrogen, Zn 2 (dmtc) 4 is stable at vulcanization temperatures. However, it shows a mass increase when heated in moist nitrogen, indicating strong coordination with water; in a nitrogen/H 2 S atmosphere rapid degradation to dimethyldithiocarbamic acid (Hdmtc) and ZnS occurs. Model compound studies show that crosslinked sulfides are essentially bis(alkenyl) and confirm the absence of accelerator terminated pendant groups in the vulcanizates, while the ease with which rubber vulcanizates crystallize on cooling in a density column also suggests that pendant groups are largely absent. However, the rates of crystallization, measured as the time for the crystallization process to go to 50% completion, are slower in lightly crosslinked gels than in peroxide cures of similar crosslink density, particularly in the vulcanizates cured in a vacuum; this is interpreted as an indication that some residual pendant groups are present in Zn 2 (dmtc) 4 vulcanizates. Water promotes the rate of crosslink formation in both rubber and TME systems, and it is suggested that the strong coordination of water with zinc in Zn 2 (dmtc) 4 promotes its reactivity. The H 2 S liberated in the vulcanization process promotes decomposition of Zn 2 (dmtc) 4 to Hdmtc, and this reaction makes an important contribution to the amount of Hdmtc that is formed in situ. The importance of Hdmtc as an accelerator and its role in providing alternative routes to crosslink formation in Zn 2 (dmtc) 4 accelerated sulfur vulcanization are discussed. It is suggested that water, which is liberated when Hdmtc reacts with ZnO to form Zn 2 (dmtc) 4 , activates newly formed Zn 2 (dmtc) 4 molecules; and this accounts for the beneficial influence of ZnO in Zn 2 (dmtc) 4 formulations.

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

Shumane

Gradwell

McGill

2002

J of Applied Polymer Sci

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Crystallization of vulcanizates. II. Low‐temperature crystallization as a function of extent of cure for polybutadiene vulcanized with dicumyl peroxide, tetramethylthiuram disulfide, 2‐bisbenzothiazole‐2,2′‐disulfide, and zinc–accelerator complexes

Merwe¹,

Gradwell²,

McGill³

2001

J of Applied Polymer Sci

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Polybutadiene compounds, vulcanized to various degrees of cure, were crystallized in a density column at Ϫ16°C. The percentage crystallinity of vulcanizates was also determined by differential scanning calorimetry where samples, precooled at a programmed rate, were reheated. Curing with peroxides has little effect on either the rate or the extent of crystallization, except at very high crosslink densities, although the induction period prior to crystallization increases progressively with increased crosslink density. Tetramethylthiuram disulfide (TMTD)/sulfur and 2-bisbenzothiazole-2,2Ј-disulfide (MBTS)/sulfur vulcanizates, cured for progressively longer periods, were found to have lower densities, a result attributed to an increase in free volume occasioned by the formation of accelerator-terminated pendent groups on the polymer chain. The induction period for crystallization increases and both the rate and the extent of crystallization decrease with extent of cure. These changes are more marked for MBTS vulcanizates that do not crystallize once a gel has formed. Formulations with zinc stearate develop higher crosslink densities and crystallize to a greater extent on cooling, showing the effect of zinc stearate in the crosslinking of pendent groups. The densities of both zinc dimethyldithiocarbamate [Zn 2 (dmtc) 4 ]-and zinc mercaptobenzothiazole [Zn(mbt) 2 ]-accelerated sulfur vulcanizates increase with cure time, a result attributed to the formation of ZnS in the compounds. Zn 2 (dmtc) 4 compounds crystallize extensively on cooling, pointing to limited main-chain modification. It is suggested that main-chain modification in these vulcanizates may comprise cyclic sulfide formation. Zn(mbt) 2 compounds crystallize less readily than Zn 2 (dmtc) 4 compounds, but to a greater extent than MBTS/sulfur compounds. The crystallization of the vulcanizates is discussed in terms of vulcanization reactions that give rise to crosslinking with the different formulations used.

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Influence of pendent groups on the tensile properties of polyisoprene vulcanizates

Merwe

Gradwell

McGill

2001

J of Applied Polymer Sci

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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.

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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

Cited by 4 publications

References 39 publications

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

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

Crystallization of vulcanizates. II. Low‐temperature crystallization as a function of extent of cure for polybutadiene vulcanized with dicumyl peroxide, tetramethylthiuram disulfide, 2‐bisbenzothiazole‐2,2′‐disulfide, and zinc–accelerator complexes

Influence of pendent groups on the tensile properties of polyisoprene vulcanizates

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