The Vulcanization of Elastomers. 20. Sulfur Vulcanization Accelerated with Thiuram Compounds. I

Abstract

The present paper deals with the results of an orientating, quantitative investigation of sulfur vulcanization accelerated by thiuram disulfide, with tetramethylthiuram disulfide as the representative example. It was found: In the sulfur cure of natural rubber with tetramethylthiuram disulfide at different TMTD:S ratios, the rates of TMTD decrease and dithiocarbamate formation increase with increasing sulfur concentration, the TMTD content being kept constant. The rates practically do not change any further when the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The peak value of dithiocarbamate formation increases with the increase of sulfur concentration and reaches a constant end value of about 90 mole per cent based on the amount of original thiuram disulfide, when the stocks contain 4 gram atoms of sulfur per mole thiuram disulfide. This end value is identical to the end value of dithiocarbamate formation in the reaction of thiuram disulfide with zinc oxide (in the absence of rubber). The crosslinking, as measured by the change of reciprocal equilibrium swelling per time unit is also a reaction whose rate increases with the sulfur concentration to the point where the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The optimum degrees of crosslinking are roughly proportional to the sulfur concentration; at high sulfur levels the vulcanizates tend to revert. As in the pure TMTD vulcanization, the TMTD decrease as well as the dithiocarbamate formation are always first order reactions. The reversion at higher sulfur levels as well as the complicated course of the increase of combined sulfur during vulcanization render all but impossible an accurate determination of the reaction order for the crosslinking at higher sulfur levels. Nevertheless, in vulcanizations with 1 mole TMTD per 1 or 2 gram atoms of sulfur the crosslinking is a first order reaction.

The Effect of Accelerator/Sulphur Ratio on the Cure Time and Torque Maximum of Epoxidized Natural Rubber

“…In the case of TMTD, the dependance of cure time is more obvious than that of ZDMC. This may be attributed to the slower rate of crosslinking by TMTD due to the formation of ZDMC as the reaction intermediate in TMTD accelerated vulcanization [23,24]. The accelerators can be arranged in the order of increasing cure time as shown for the CV system.…”

Evaluation of vulcanization nanoactivators with low zinc content: characterization of zinc oxides, cure, physico‐mechanical properties, Zn2+ release in water and cytotoxic effect of EPDM compositions

“…During the first step, ZnO 30 and ZnO 40 have smaller particle sizes compared to sZnO; thus, ZnO 30 and ZnO 40 could penetrate easily into the elastomer matrix and react like polar pathway with the co‐activator, accelerator and sulfur molecules embedded between the EPDM chains . On the other hand, the samples without ZnO and with sZnO react mainly through a radical pathway due to the difficulty of dispersion of sZnO particles in the rubber matrix . Additionally, ZnO 30 and ZnO 40 increases the amount of sulfur absorbed on the activator specific surface area and accelerates the crosslink formation reactions in the second stage of cure process.…”

Diverse Veranstaltungen

“…Kawaoka24 reported a vigorous exothermic reaction beween TMTD and ZnO. Duchacek et al25 also reported the formation of zinc bis(dimethyldithiocarbamate) (ZDMC) and kinetic studies by Scheele and Franck26 provided evidence for a reaction between TMTD and ZnO as the first step in thiuram‐based vulcanization. Coleman et al,1 using Raman spectroscopy, reported the formation to TMTP in addition to a number of zinc‐containing species, including ZDMC, and possibly zinc (dimethylmonocarbamate–dimethyldithiocarbamate) and zinc bis(dimethylmonocarbamate), as well as COS, as main products on heating TMTD/ZnO and TMTD/sulfur/ZnO at vulcanization temperatures.…”