The reactions of 2-(4-morpholinothio)- and 2-(4-morpholinodithio) benzothiazole in the absence of polyisoprene

Abstract

2-(4-morpholinothio)benzothiazole (MOR) and 2-(4-morpholinodithio) benzothiazole (MDB) were heated with sulfur and ZnO in a DSC. The products formed at various temperatures were identified and analyzed by HPLC. At temperatures below 200ЊC, decomposition of the accelerator in the absence of other curatives was slow, degradation products being mainly 2-bisbenzothiazole-2,2 -disulfide (MBTS) and 2-mercaptobenzothiazole (MBT). A rapid exothermic decomposition above 200ЊC resulted in the formation of MBT (or its amine salt) and 2-(4-morpholino)benzothiazole (MB). MOR and MDB reacted with sulfur to form higher polysulfides. MDB was shown to react more readily with sulfur than MOR and the delayed action of MOR in rubber can therefore not be ascribed to a stable polysulfide as suggested by other authors. Neither MOR nor MDB was found to react with ZnO. A limited reaction between MBT and ZnO was observed.

Sulfur Vulcanization of Natural Rubber for Benzothiazole Accelerated Formulations: From Reaction Mechanisms to a Rational Kinetic Model

“…The formation of MBTP can occur via the recombination of radicals; [45][46][47] specifically, MBTS can form via R.14:…”

“…In contrast to the conclusions of Son et al, 40 other researchers postulate that a radical mechanism controls the formation of MBTS from MBS. [35][36][43][44][45] The free radical reactions are assumed to proceed by (i) S-N bond scission in MBS (generically represented as Bt-S-NR 2 for an arbitrary amine) yielding Bt-S • and NR 2…”

“…In order to gain better insight into the interaction of the curatives during the early stages of vulcanization, Gradwell et al 35,36,45 studied the reactions of MBS, CBS, TBBS and MDB in the absence of polyisoprene both with and without sulfur in a DSC, where the products formed at 130 to 178 o C were analyzed by HPLC. They found that on heating MBS in the absence of sulfur, MBT, or possibly an MBT-amine salt, and MBTS 2 2 606 RUBBER CHEMISTRY AND TECHNOLOGY VOL.…”

“…and although not explicitly proposed by Gradwell et al, 45,46 consistency demands that the exchange reactions R.12 and R.13 and the recombination reaction R.14 be generalized to include longer length polysulfidic species which will yield MBTPs in addition to MBTM as shown below:…”

“…The experimental results of Geyser and McGill 59 are shown in Figure 6, where TMTPs with lower sulfur length are produced in higher concentrations than TMTPs with higher sulfur lengths. Similar trends in the production of MBTPs of different polysulfidic lengths were also observed by Gradwell et al 45 A careful examination of the data in Figure 6 indicates that early in the reaction (i.e. times less than a minute) short chain length TMTPs are formed prior to the TMTPs with longer chain lengths.…”