Flocculation and viscoelastic behaviour in carbon black-filled natural rubber

Abstract

The process of particulate flocculation in natural rubber melts and subsequently crosslinked samples is investigated using carbon blacks of varying surface free energy, surface area and morphology. The surface free energies are varied via thermal treatment of the carbon blacks (graphitization). Reduction in surface free energy of the particulates accelerates the flocculation processes in the melt as measured by rheological experiments and reduces the percolation volume fraction threshold as determined by D/C conductivity measurements. The consequent effects on amplification of the small strain storage moduli of the crosslinked compounds are dramatic. Reduced polymer-filler interactions result in both an increased small strain modulus versus the unmodified carbon black-filled materials and an increased mechanical fragility of the fractal networks. Examination of the dynamic elastic moduli of crosslinked carbon black-filled samples loaded at volume fractions below the onset of network development reveal a significant temperature dependence isolated at the filler-rubber interface.