SYNOPSISThe effects of copper, mercury, nickel, zinc, cadmium, indium, magnesium, and calcium stearates on the course of N-cyclohexyl-2-benzthiazylsulphenamide-accelerated sulfur vulcanization of natural rubber have been investigated on the basis of curemeter measurements at 145OC. The differences in the efficiencies of these metal activators of accelerated sulfur vulcanization have been discussed from the points of view of the electron configurations of the metals and their affinities to sulfur. We attempted to determine why zinc oxide is generally accepted as the best metal vulcanization activator. 0 1993 John Wiley & Sons, Inc.

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SynopsisThe crosslinking of poly(viny1 chloride) in the presence of zinc oxide, magnesium oxide, ethylenethiourea (ETU), tetramethylthiuramdulfide (TMTD), and sulfur at 160 and 180°C has been studied. The effects of ETU and sulfur on the crosslinking of PVC in the presence of TMTD have been individually studied. It has been found that zinc dimethyldithiocarbamate (ZnDMDC) is the actual crosslinking agent and heat stabilizer of PVC, formed in the cured polymer mix. Chemical Structure and Thermal Stability of Poly(viny1 Chloride)The field of investigation of the chemical structure and thermal stability of PVC is at present so wide that we can give here only a short survey of the

SynopsisOn the basis of Mooney scorch measurements a t 12OOC and of continuous measurements of vulcanization in a Vuremo curemeter at 145OC, sulfenamides which function as inhibitors of the sulfur-free thiuram vulcanization have been investigated. It was found that the actual prevulcanization inhibitor is 2-mercaptobenzothiazole. Its formation from sulfenamides is activated by zinc dimethyldithiocarbamate since the vulcanization of rubber by thiuram disulfides in the presence of zinc oxide is always accompanied by the formation of zinc dithiocarbamate. It seems probable that the inhibition effect of Zmercaptobenzothiazole is attributable to the formation of a complex with zinc dimethyldithiocarbamate which decreases the reaction rate in the initial steps of the sulfur-free thiuram vulcanization and thus causes the scorch delay.

synopsisSulfur-free thiuram vulcanization has been investigated a t temperatures from 160" to 190°C over 0.5 to 600 min. Continuous measurements in a W R E M O curemeter were used to estimate the extent of crosslinking, which was plotted against cure time.Simultaneously the values of the network chain density were calculated from swelling measurements on the vulcanizates. The cure curves show clearly an induction period (t?), then fast crosslinking, a partial degradation, a "long-time" crosslinking, and finally a slow, limited degradation. Apart from the induction period, the kinetic graphs are satisfactory represented by a rate equation asuming three independent firsborder reactions:The rate equation contains seven kinetic parameters. Over the temperature range studied, there is no difference between the values of activation energy for fast crosslinking, for degradation, for slow crosslinking, and for t t -l . Due to the presence of thiourea, the values of the induction period, the rate constant, and the extent of slow crosslinking are decreased. Simultaneously the activation energies calculated from degradation and slow crosslinking are significantly increased. On the basis of the above results, the mechanism of the sulfur-free thiuram vulcanization, in which ionic and radical reactions take place, is discussed. fast crosslinking, degradation, and slow crosslinking.