Reactions of 2-(4-morpholinothio)- and 2-(4-morpholinodithio)benzothiazole in the presence of polyisoprene

Abstract

2-(4-Morpholinothio)benzothiazole (MOR) and 2-(4-morpholinodithio)-benzothiazole (MDB) were reacted, in combination with sulfur and ZnO, in the presence of polyisoprene (IR). Samples were heated in a DSC at 2.5ЊC/min and characterized by swelling experiments. The products formed at various temperatures were analyzed by HPLC. Crosslinking only occurred once all the benzothiazole sulfenamide had been consumed, the onset of vulcanization characterized by a considerable increase in 2-mercaptobenzothiazole (MBT) concentration. Crosslinking occurred earlier in all corresponding MDB formulations. Higher crosslink densities were recorded with addition of ZnO. The delayed action experienced in MOR systems was attributed to an exchange reaction between benzothiazole-terminated pendent groups and MOR and not due to the stability of the disulfide, MDB.

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

“…R.68) and, thus, delay the formation of MBTS and the polysulfidic accelerators; 40,121 alternatively, CTP is assumed not to delay the formation of the crosslinks but only to inhibit the rate of crosslink formation. 33,120 The proposed reaction of the polysulfidic crosslink precursors with CTP requires a trimolecular reaction (i.e. Step 3 in R.73) which seems rather unlikely for bulky molecules.…”

“…Many researchers 2,34,81,94,95 believe that MBT is created during the formation of crosslink precursors via R.34. However, Gradwell et al 33 dispute this conclusion, since they did not observe any MBT during crosslink precursor formation and thus associate the formation of MBT with the crosslinking step. Consequently, they postulate that CTP does not delay the onset of crosslinking, rather it inhibits the crosslinking reactions once 632 RUBBER CHEMISTRY AND TECHNOLOGY VOL.…”

“…Gradwell et al, 33 however question the formation of MBT at this step of the reaction, since for polyisoprene vulcanization accelerated with MBS and MDB, they did not observe any MBT formation prior to crosslinking. They propose that in the case of MBS accelerated systems, polysulfides of MBS are formed which are then added to the chain as benzothiazole pendant groups liberating free amine as shown below; first (R. 35) and then (R.36) R.36 depends upon R.35; however, as discussed earlier Bt-S-NR 2 readily converts to BtS-SBt via R.5 and R.6 in preference to sulfur activation to form Bt-S x -NR 2 8 as required in R. 35.…”

“…For zinc-complexed sulfurating species, a polar mechanism with the associated liberation of MBT proposed by Bateman et al 2 (i.e. R.37) is widely accepted; however, Gradwell et al 33,120 question the formation of MBT at this stage of the reaction and argue that the precursor formation in MBS accelerated systems proceeds via the direct reaction of MBS-polysulfides with rubber (i.e. R.36).…”

“…This scavenging of MBT by the retarder delays the formation of the critical precursor MBTS from MBS and hence the delay of the vulcanization reaction. Although R.68 is generally accepted as the reaction responsible for the delay, Gradwell et al 33,120 argue that MBT is primarily released by the formation of crosslinks (i.e. R.72); thus, the retarder does not delay the initial formation of crosslinks, but only inhibits the rate of crosslink formation.…”