Silica and carbon black reinforcement of natural rubber

Recent investigations on filled natural rubber are reported. They include a mechanical characterization as well as a molecular analysis based on measurements on chain orientation. It is demonstrated that, at intermediate strains, the increase in the moduli can be explained by the inclusion of rigid particles in the soft matrix and from molecular interactions between the rubber and the filler. These interactions can be evaluated by equilibrium swelling and by orientational measurements. With regard to the unfilled formulation, carbon black- and silanized-filled natural rubber exhibit increases in the cross-linking density ascribed to filler-polymer links while a large decrease in the orientational level, evidenced by birefringence and by infrared dichroism, is observed when silica is added without any couplig agent.