Effect of carboxylic acids on 2-bisbenzothiazole-2,2?-disulfide- and tetramethylthiuram disulfide-accelerated sulfur vulcanization. IV. Vulcanization of polyisoprene compounds with ZnO

Self Cite

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.

Section: Resultssupporting

confidence: 89%

“…(1a) and (1b)]. 15, 16, 19–24, 39 In TMTD/sulfur systems, pendent group formation is a substitutive reaction with the evolution of Hdmtc, that may escape from the system,14–17, 41 although more recent studies30, 42 have shown it to actively participate in the crosslinking process by forming thiol pendent groups [eqs. (1c) and (1d)].…”

Section: Resultsmentioning

confidence: 99%

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

Merwe¹,

Gradwell²,

McGill³

2001

Self Cite

“…Reheated, extracted samples showed an increase in crosslink density, whileereas samples reheated after swelling in zinc stearate showed a considerable increase in crosslink density, indicating the presence of pendent groups in the preheated compounds. It also shows the effectiveness of zinc stearate in crosslinking pendent groups 5, 25. The increased percentage crystallinity that develops in the latter samples, despite the increased crosslink density, points to the removal of pendent groups that impeded crystallization in the original samples and confirms the contention, stated earlier, that pendent groups more seriously impede crystallization than do crosslinks.…”

Section: Resultssupporting

confidence: 80%

“…The effect of TMTD/sulfur and MBTS/sulfur formulations on the ability of BR vulcanizates to crystallize on cooling is compared with the crystallization of similar vulcanizates of IR 3. Zinc stearate has been shown to promote the crosslinking of pendent groups25 and the effect of its inclusion on the crystallization of vulcanizates is reported. The crystallization study is also extended to vulcanizates cured with zinc–accelerator complexes.…”

Section: Introductionmentioning

confidence: 99%

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

Self Cite

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.

“…The zinc salts are less reactive toward the accelerators and accelerator polysulfides than are the acids; thus, in the presence of ZnO, carboxylic acids should not have a detriment affect on the vulcanization reaction 29. McGill and Shelver30 also investigated the effect of benzoic and stearic acids on MBTS and TMTD‐accelerated sulfur vulcanization of polyisoprene compounds containing ZnO at 150°C. In the presence of ZnO, the acids are rapidly converted to their zinc salts that, unlike the acids, do not attack the accelerator polysulfides but instead promote vulcanization.…”

Section: Resultsmentioning

confidence: 99%

Rice husk ash filled natural rubber. III. Role of metal oxides in kinetics of sulfur vulcanization

Costa

Visconte

Nunes

et al. 2003

ABSTRACT:In preceding investigations the overall rate and apparent activation energy for the vulcanization process were calculated for natural rubber compounds, assuming that vulcanization follows first-order kinetics. It was observed that the addition of white rice husk ash (WRHA) increased the rate of crosslinking and lowered the apparent activation energy more profoundly than commercial fillers, precipitated silica (Zeosil-175) and carbon black (N762), with a conventional vulcanization system. In this work, a specific model for the vulcanization process accelerated by N-cyclohexylbenzothiazole sulfenamide was used to investigate the real role of WRHA in crosslink formation. Cure studies were carried out at 150°C, and the kinetics constants were evaluated. In relation to the other fillers, WRHA seems to develop catalytic activity, resulting in a positive effect on the specific rate of crosslink formation.