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.
Reinvestigation of the
structure reported previously as the Ni(II) complex of TCNTA, by
EDS analysis and crystallographic refinement, failed to confirm the
presence of nickel and indicated a cobalt complex instead. It is
inferred that the crystal of interest, which appeared over a period of
months from a nitric acid solution of Ni(II) and the TCNTA ligand,
was formed by slow crystallization of cobalt impurities to form a
single crystal of the title
compound.
The interaction of sulfur, ZnO and 2-t-butylbenzothiazole sulfenamide (TBBS) was investigated by differential scanning calorimetry (DSC) in the absence of rubber. Special reference was made to the role of the amine, the distinguishing characteristic of all sulfenamides. TBBS decomposes rapidly at 211 °C, the major products being 2-bisbenzothiazole-2,2′-disulfide (MBTS) and 2-bisbenzothiazole-2,2′-monosulfide (MBTM). High mass losses occured and very little free amine was found at these high temperatures. The major degradation products of the TBBS/sulfur system were MBTS, 2-bisbenzothiazole-2,2′-polysulfides (MBTP) and 2-t-butylaminobenzothiazole polysulfides (TBBP). Little free amine and very small mass losses were found. Reaction mechanisms were proposed to account for the formation of the products.
ABSTRACT:The crystallization of polyisoprene, vulcanized to various degrees of cure with tetramethylthiuram disulfide/sulfur and 2-bisbenzothiazole-2,2Ј-disulfide (MBTS)/ sulfur formulations, was studied in a density column at Ϫ25°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 acceleratorterminated 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.
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