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

Abstract

ABSTRACT:The effect of benzoic and stearic acids on 2-bisbenzothiazole-2,2Ј-disulfide (MBTS)-and tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of polyisoprene compounds, containing ZnO, was studied at 150°C. In the presence of ZnO, the acids are rapidly converted to their zinc salts that, unlike the acids, do not attack accelerator polysulfides and, instead, promote vulcanization. The addition of accelerator polysulfides to the chain is not affected by zinc salts, but the rate of crosslinking of pendent groups is facilitated. In MBTS vulcanization, the promotion of crosslinking reactions reduces the number of pendent groups involved in cyclization reactions and higher crosslink densities are attained. In TMTD compounds, crosslink densities are increased on addition of ZnO and of ZnO/carboxylic acids. A mechanism is suggested to account for the promotion of crosslinking by the zinc salts. Bis(2-mercaptobenzothiazolo)zinc(II) and bis(dimethyldithiocarbamato)zinc(II) can act similarly to zinc benzoate or stearate to promote the crosslinking of pendent groups, but are less efficient. With benzoic acid, a limited attack by zinc benzoate on TMTP leads to a slightly lower increase in crosslink density than is achieved with the addition of ZnO alone.

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

“…Thus it is proposed that the progressive decrease in initial density of compounds, heated for longer periods, results from an increase in free volume in compounds occasioned by the formation of pendent groups on the polymer chain. HPLC data confirm earlier findings30, 42 that 80% of the TMTD is attached to the chain by the time crosslinking commences.…”

“…(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)].…”

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

“…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.…”

“…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.…”

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

“…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.…”