Tear strength of filled rubbers

The strength of black-filled rubbers has been investigated under a variety of test conditions using various compounding formulations, of both strain-crystallizing and non-strain-crystallizing elastomers. The enhancement in the tear strength is substantial when knotty tearing occurs. In knotty tearing, the crack tip grows perpendicularly to the general direction of propagation. This effectively increases the tip diameter and thus the tear strength. A strong correlation between tearing energy and knot diameter (measured in the unstrained state) has been found in the present investigation. Factors which affect the development of knotty tearing were investigated. It was found that knotty tearing is affected by the degree of strain-crystallization, molecular mobility, nature and concentration of crosslinks, the type and concentration of carbon black, temperature and tear rate. The onset of knotty tearing appears to be related to the development of strength anisotropy at the tip of the tear. The effect of this anisotropy on the energy to propagate tearing in the direction of pre-straining was investigated using split tear test-pieces. The tearing energy for crack propagation in the direction of molecular orientation gives a quantitative measure of strength anisotropy developed in the vulcanizate as a consequence of pre-straining. It was found that, in a stretched vulcanizate, the tearing energy to propagate tearing in this direction was a factor of about 20 lower than the tearing energy of the unstretched vulcanizate. In a certain type of black-filled vulcanizate, the anisotropy introduced during pre-stressing to large extension still persisted even after the pre-stretch was removed. The present investigation shows that the anisotropy introduced by pre-stressing is associated with the set. The tearing energy of a pre-stressed vulcanizate was found to correlate strongly with the set

Peroxide prevulcanization of natural rubber latex

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 polyesters 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 - 100 °C. The effectiveness of the prevulcanization systems was characterized by the rate and efficiency of crosslinking achieved by these systems. Fructose-activated peroxyester and fructose-activated 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 partly explains 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 peroxide-prevulcanized NR latex indicate that a preventive antioxidant is an essential component for an effective antioxidant system for these films