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.
2 = 5 {[exp(KAi -__ AHA)]-' RT + [ exp ( Kui -__ A: ' ) ] -' }-' VgE i = l AH$, and AH$, are the respective experimental activation energies, KA, and Ku, are the respective natural logarithms of the experimental collision term, R and T have their usual meaning. Macrokinetical interpretation: Each thermal transition (step) is caused by a sum A,+B, ofconsecutive reactions. The slowest reaction determines the rate of the whole transition. The different thermal transitions (steps) are concurrent reactions summarized from i= 1 to m. The fastest step determines which thermal transition occurs. Microkinetical interpretation: Process B, in Eq.(1) is a chain segment motion that occurs with low activation energy. The high activation energy process A, in Eq. (1) occurs in advance and makes the process B, possible perhaps by producing free volume.
Temperature Dependence of the Shift Factor (Isomodulus)The logarithmic complex tensile modulus log E* or the storage modulus log E of amorphous polymers plotted against the logarithmic frequency log vat constant temperature (isothermal) results in an S-shaped curve. If the temperature as parameter increases the similar isotherms are shifted along the logarithmic frequency coordinate to higher frequencies. The shift factor %/ vgt of each isotherm is found to be the frequency y of the considered isotherm at a fixed value of the E-modulus normalized by the frequency ugE of the glass transition isotherm. Because the temperature dependence of the shift factor is considered at a constant E-modulus it is called "isomodud,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.