Optimising silane coupling agent content in phenolic-resin-cured sepiolite-filled natural rubber composites

Influences of silane coupling agent loading (e.g. 0–6%) on properties of sepiolite-filled natural rubber (NR) composites were investigated in this study. The amount of silane addition on properties of phenolic resin-crosslinked NR filled with sepiolite composite was optimised. A rheometer was used to evaluate the curing characteristics of the composites while morphological properties were studied by scanning electron microscopy. Fourier-transform infrared spectrometer, temperature scanning stress relaxation, and Mooney–Rivlin model were used to assess the interactions between sepiolite filler and rubber matrix, and the tensile properties and strain-induced crystallisation behaviour were investigated by tensile testing and wide-angle X-ray scattering, respectively. Incorporating a silane coupling agent reduced the size of sepiolite aggregates from ~ 0.4–10 to 0.4–5 μm and decreased filler–filler interaction as well as the curing time of the composite. The silane also improved adhesion between the filler and the rubber by the additional interactions between silane and sepiolite filler. These interactions slowed down the rate at which NR polymer molecules relax, increased tensile strength, and facilitated strain-induced crystallisation. The strongest interactions between filler and rubber, evidenced by maximal tensile strength that was 20% over the non-silanised NR composites, were attained at 4% silane coupling agent relative to the sepiolite by mass.