SEM of the effect of loads of CaCO3 and Cuban kaolin in rubbers made with natural rubbers

Abstract

Calcium carbonate and grey kaolin were added as fillers in 35 parts for every 100 parts of caoutchouc as a first step in the optimization of the effect of adding these mineral charges without surface activation in rubber composites with SMR-20 rubber from Malaysia or NR rubber from Brazil, Mixture and vulcanization conditions were constants during composites manufacturing. Test data of mechanical probe samples was compared with the Secondary Electron Microscopy (SEM) analysis of their surface fracture after been broken at liquid nitrogen temperature. Kaolin is better filler than calcium carbonate because of its properties: 10 times higher specific surface and also higher particle mean size; its use is only not preferable when the color of rubber articles is clear. Because of these properties, rubber composites with kaolin shows tensile strength almost three times higher than with calcium carbonate, and also better values of elongation, tear and shore hardness. SEM images allows explaining the mechanical behavior of these rubber composites. Both shows fracture surfaces very striated and stratified in areas with pieces of rubber torn off in purl or flounce form. Kaolin particles were seen isolated showing an acceptable adhesion with matrix. Even when calcium carbonate increase mechanical parameters, it does in less proportion than kaolin, because of its great tendency of particle agglutination in both rubber composites.