SILICA-RICH FILLER FOR THE REINFORCEMENT IN NATURAL RUBBER

The aim of the study was to determine the reinforcing efficacy of a silica-rich filler, pottery stone (PS), in natural rubber (NR). The effects of amount and particle size of PS on curing and mechanical properties of the NR compounds were determined. The PS was first divided into four groups. Two of these were raw PS without grinding, which were sieved to particle sizes of less than 106 μm (PS(<106)) and less than 38 μm (PS(<38)). The other two were ground PS, one by current jet milling to obtain PS(JM), and the other by wet ball milling to obtain PS(BM). The particle size distribution of the four different PS prior to ultrasonication was in the ranked order (largest to smallest size), based upon their d(0.5) and d(0.9), of PS(BM), PS(<106), PS(<38), and PS(JM). However, after ultrasonication for 10 min, PS(BM) had the smallest d(0.5) at less than a micron, while the remaining three PS groups showed nearly the same d(0.5) being within the range of 3–5 μm. The presence of PS shortened the cure time, with PS(BM) inducing the greatest decrease in the cure time, while this was somewhat dose independent for at least PS(JM) and PS(<106). For all four PS groups, when present at 20 phr or more, the delta torque of the PS filled NR was higher than that of the unfilled one. However, the addition of PS had no significant effect on the number of crosslinks. With respect to the mechanical properties of the NR filled with PS, it was generally observed that NR filled with PS(BM) at 20–50 phr gave a higher tear and tensile strength, abrasion resistance, and hardness than both the unfilled NR and also the NR filled with the other three PS groups. The optimum PS(BM) loading was at 30 phr.