Effects of silica, calcium carbonate, and SiO2/CaCO3 blends on some properties of cellular NR compounds

Objective of this work was to study the effect of silica (SiO2) and calcium carbonate (CaCO3) on compounding torque, mechanical, physical, morphological properties, and swelling behaviour of cellular natural rubber, using a compression moulding and conventional vulcanization (CV) system with oxybisbenzenesulphonylhydrazine (OBSH) as a blowing agent, the moulding cure temperature and time being 160 deg C and 7 mins respectively. The cellular rubber vulcanisates were characterised with respect to tensile strength, abrasion resistance, thermal aging, swelling and deswelling beehaviours. The results indicated that the addition of 20 phr SiO2 in the rubber compound was the most preferable for the tensile strength and abrasion resistance, this being associated with reinforcing effects between the silica and rubber molecules, which also promoted the swelling resistance in toluene during equilibrium state. In the case of CaCO3 filler, added into the rubber compounds having 20 phr SiO2, the optimum CaCO3 content to maintain the overall properties was between 20 and 30 phr. The CaCo3 above 30 phr reduced the tensile and abrasive properties of the compounds due to the excessive fillers (silica and calcium carbonate) contents being greater than 50 phr. Addition of CaCO3 had no effect on the swelling behaviour of the compounds. Considering the effect of thermal aging in the SiO2 filled rubber compounds, it was observed that the mechanical properties of non-filler cellular rubber tended to decrease whereas the swelling resistance increased with aging, this being due to the desulphuration of the surphur crosslinks. However, the properties of the NR compounds containing 20 phr SiO2 and various contents of CaCO3 were unaffected by thermal aging. In summary, the recommended mixed fillers contents for SiO2 and CaCO3 in this work were 20 phr and 20-30 phr, respectively.