Effect of NaOH and Si69 treatments on the properties of fly ash/natural rubber composites

Natural silica in fly ash (FA) particles of 30 and 60 phr were incorporated as reinforcing filler in natural rubber (NR) compounds. Improvements of cure characteristics and mechanical properties of the FA/NR composites were expected by surface treatment of the FA particles, this being carried out using various contents of bis-(3triethoxysilylpropyl) tetrasulfane (Si69), as a chemical silane coupling agent, and sodium hydroxide (NaOH) solution. The results suggested that the Si69 coupling agent was a satisfactory property promoter in the FA/NR composites through C-S bonding with rubber molecules and siloxane linkages with FA particles. Using NaOH solution did not improve the mechanical properties of the composites. The mechanical properties of the FA/NR composites appeared to increase at low Si69 concentrations of 2.0-4.0 wt%, but decreased at high Si69 loadings of 6.0-8.0 wt%. The decrease in mechanical properties resulted from a reduction of crosslink density and a formation of flexible polysiloxane from the self-condensation reaction of the Si69. The presence of Si69 was observed to interfere with the sulphur crosslink formation. No synergetic effect on the properties of the rubber composites was observed when using a mixture of NaOH and Si69. This article recommends concentrations of 2.0-4.0 wt% Si69 to be introduced into the FA/NR composites to achieve the optimum properties.