Use of Pottery Stone as Reinforcing Filler In Natural Rubber

The objective of this research was to study the possibility of using pottery stone (PS) as reinforcing filler in natural rubber (NR). PS is the commercial name for rhyolite, a light colored igneous rock. Besides its low cost and ready availability in Thailand, its major chemical component is SiO₂, which is known to be suitable as a NR filler and so was the motivation to conduct this study. The effects of the amount and the particle size of PS on the curing and mechanical properties of NR were studied. 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 microns (PS(<106)) and less than 38 microns  (PS(<38)).  The other two were ground PS, one by current jet milling of the PS(<106) to obtain PS(JM), and the other by wet ball milling crude PS and then 400-mesh sieving  to obtain PS(BM).  For each different PS, they were added into NR at 0, 10, 20, 30, 40 and 50 phr.

From the chemical composition analysis, the PS used in this study, obtained from within the Lopburi province of Thailand, consisted of SiO₂ at 77.1% (w/w), whilst the rest was composed of various metal oxides. The particle size distribution of the four different PS preparations prior to ultrasonication were in the ranked order (largest to smallest size), based upon their d(0.5) and d(0.9) values, 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, whilst the remaining three PS groups showed nearly the same d(0.5) being within the range of 3 - 5 microns. The presence of PS shortened the optimum cure time, with PS(BM) inducing the greatest decrease in the cure time, whilst 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 vulcanizate was higher than that of the unfilled one, and this was PS dose-dependent. The higher the PS loading was, the higher the increase in the delta torque was, but the addition of PS had no significant effect on the number of crosslinks in the rubber vulcanizate. With respect to the mechanical properties of the NR vulcanizates filled with PS, it was generally observed that NR vulcanizates filled with PS(BM) at 20 to 50 phr gave a higher tensile strength, tear strength and abrasion resistance than both the unfilled NR vulcanizate and also the NR vulcanizates filled with the other remaining three PS groups. The optimum PS(BM) loading was at 30 phr. For NR filled with the remaining three PS groups, their mechanical properties were slightly higher or lower than, but comparable with, those of gum NR depending on the type of mechanical properties and also the PS loading.