This study was initiated in an attempt to investigate dithiocarbamic acid accelerated sulfur vulcanisation. This was, however, found impossible due to the innate instability of dithiocarbamic acids. The focus of the study was then shifted toward thiuram disulfide accelerated sulfur vulcanisation, with emphasis being placed on a rate comparison. Three groups of accelerators were investigated, namely the aromatic, linear aliphatic and cyclic aliphatic thiuram disulfide adducts. The analysis methods that were employed were conventional rubber (cis-1,4-polyisoprene) techniques coupled to model (squalene) compound investigations. The data that was collected consisted of rheometrical torque vs. time data in the rubber system while the data obtained in the model compound study consisted of sulfur and accelerator concentration data as determined by means of high performance liquid chromatography (HPLC). The aromatic accelerators were synthesised in our labs by means of an addition reaction between the aromatic amine and CS2 in basic medium and subsequent oxidation with K3Fe(CN)6, all in a 1:1 molar ratio. The reaction yield was low due to the instability of the dithiocarbamate intermediates and a sluggish oxidation reaction. In the rate constant determination a first order mathematical approach was used for the rubber system as crosslinking is considered to roughly obey first order kinetics. The model compound data was also found to more accurately fit the first order rate law, with an initial slopes method also being applied to the system to determine secondary rate constants and relative rates for the system. The determination of vulcanisation rate constants in the cis-1,4-polyisoprene system was a success, while the rate data determined by means of the squalene model was more related to the rate of accelerator and sulfur consumption as opposed to the rate of crosslinking as is the case with the rubber rate data. The sulfur first order rate data mirrored the rate data derived from the rubber system more closely than the corresponding accelerator rate data, the relative rate data determined by means of initial slopes method, proved that the homolytic cleavage of thiuram disulfides and the subsequent formation of accelerator polysulfides were not limiting steps. This is seen in the similar relative rate data derived from both the raw sulfur and accelerator data in systems that exhibit vastly different vulcanisation rates. Squalene was deemed a suitable model for the cis-1,4-polyisoprene system, although one should consider the extent of charring and solution effects in the individual systems to account for possible incongruities that may be observed between the rubber and simulated system. The lack of agreement between the rubber and model compound rate constant data lies in the fact that the rate of crosslinking is not simplistically related to the rate at which accelerator and sulfur is consumed, this being especially true for the rate at which the accelerator is consumed. Thus the discussion over the acceleratory rates in the various accelerator systems was limited to observations made in the rubber system, with the model compound data was used exclusively to elucidate mechanistic processes. It was discovered that the groups of accelerators examined, namely linear, cyclic and aromatic thiuram disulfide adducts, produced vastly varied rate data. The aromatic thiuram disulfide adducts had only a slight acceleratory effect on the rate of vulcanisation as compared to the unaccelerated sulfur system. The morpholine adduct had a moderately larger rate of acceleration followed by tetramethyl and tetrethylthiuram disulfide, with N’N-dicyclopentamethylenethiuram disulfide having the fastest rate of acceleration.

The peroxide prevulcanization of NR latex using a range of commercially-available organic peroxides and an inorganic peroxide (potassium peroxydisulphate), in both activated and non-activated systems, has been investigated. A range of reducing agents and compounds that are known to promote peroxide-initiated emulsion polymerization and peroxide curing of po1yesters have been evaluated as promoters for the peroxide prevulcanization of NR latex. A few reactive peroxyesters have been found to be effective as prevulcanizing agents at temperatures in the range 80 °C-lOO °C. the effectiveness of the prevulcanization systems was characterized by the rate and efficiency of crosslinking achieved by these systems. Fructose-activated peroxyester and fructoseactivated hydroperoxide systems were found to effect prevulcanization at temperatures in the range 50 °C-80 °C. There is no clear correlation between the structure/reactivity of peroxyesters and the effectiveness of fructose-activated prevulcanization systems. The relative reactivity of the alkoxy radicals generated by the commercial hydro peroxides PQ~tly exp\a~~s the differences in the effectiveness of various fructose-activated hydroperoxide prevulcanization systems. The prevulcanization kinetics for the fructose-activated t-butyl peroxyisobutyrate (tBPIB) system have been investigated. The overal rate of tBPIB decomposition in NR latex, in both non-activated and fructose-activated systems was found to be first-order reaction with respect to tBPIB concentration. However, investigation of initial rate of tBPIB decomposition in NR latex indicates that the initial rate of tBPIB decomposition in NR latex is half order with respect to initial tBPIB concentration. This is probably a consequence of induced decomposition of tBPIB by certain non-rubber substances, and_termination by recombination of radicals derived from tBPIB. But, the reason for the difference in the reaction order with respect to tBPIB concentration, at the initial stage of the reaction and during the run is not clear. The prevulcanization kinetics also exhibit a number of other peculiar characteristics. Thus at temperatures greater than 70°C, and using a high fructose concentration, the rate coefficient for crosslink formation tends to be greater than that for peroxide decomposition. This is probably attributed to the differences in the temperature-coefficients of the various competing reactions during peroxide prevulcanization of NR latex. The instantaneous crosslinking efficiency was found to increase linearly with prevulcanization time. At temperatures greater than 70°C, the instantaneous crosslinking efficiency can attain values greater than 50%, indicating the involvement of alkyl radicals as well as the alkoxy radicals in the crosslinking reaction. The experimental activation energies for peroxide decomposition and crosslink formation were found to decrease to apparently constant values with increasing fructose/ peroxide concentration ratio. The rate of tBPIB decomposition was found to be significantly determined by activation free energy and not just activation energy for the decomposition. The factors which influence the physical properties of films from peroxide-prevulcanized NR latex have been investigated. The crosslink concentration was found to be the most important factor in determining the physical properties of films from peroxide-prevulcanized NR latex. Factors that account for the differences in the physical properties of films from peroxide- and sulphur-prevulcanized latices, and peroxide gum NR vulcanizates have been discussed. Attempts to improve the ageing properties of films from peroxideprevulcanized NR latex indicate that a preventive antioxidant is an essential component for an effective antioxidant system for these films.

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Thesis (Ph. D.)--University of Wisconsin--Madison, 1967. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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