Carbon nanotube (CNT)-filled natural rubber (NR) composites were prepared by melt and by latex mixing methods. Also in situ functionalization of CNTs with a silane coupling agent, namely bis(triethoxysilylpropyl)tetrasulfide (TESPT), was done to improve the filler–rubber interactions between CNT surfaces and rubber molecules. The grafting of TESPT molecules on CNT surfaces was confirmed by attenuated total reflection (ATR)–Fourier transform infrared (FTIR) spectroscopy and by the improvement of composite properties. Tensile properties were determined to assess the reinforcement efficiency of the CNTs in the composites. Also, electrical conductivity of the composites was measured to assess the formation of CNT networks (or connected conductive CNT pathways) in the rubber matrix. The results indicate that the composites prepared by latex mixing, in particular with the TESPT, had better tensile properties and electrical conductivities than the composites made by melt mixing. The lowest percolation threshold concentration, about 0.55 phr of CNTs, was observed in the latex–CNT composites, and three-dimensional network formations of CNTs in the rubber matrix were found with added TESPT, used by in situ functionalization. The improvement of filler–rubber interactions with the addition of TESPT was also examined by temperature scanning stress relaxation measurements, revealing the relaxation modulus, the relaxation spectrum, and an estimate of the cross-link density.

Electrically conductive epoxidized natural rubber filled with conductive carbon black was prepared. The AC conductivity ( σ_AC), dielectric constant ( ɛ′), loss factor (tan δ*) and dynamic mechanical properties of the composites were studied. It was found that the epoxide groups in epoxidized natural rubber molecules positively contributed to AC conductivity, dielectric constant, and tan δ* of the composites. Especially, the composite with epoxidized natural rubber containing 50% mol epoxide (epoxidized natural rubber–50) showed better electrical and dynamic mechanical properties than the composites with epoxidized natural rubber–25 or NR. The effects of conductive carbon black loading level on electrical conductivity and dielectric constant of the epoxidized natural rubber–50/conductive carbon black composites was also studied. The percolation threshold was found at very low content of conductive carbon black at volume fraction of 0.07 with the critical exponent value 2.04. Furthermore, the glass transition temperatures of epoxidized natural rubber–50/conductive carbon black composites were higher than those of epoxidized natural rubber–25/conductive carbon black or NR/conductive carbon black composites, and increased with conductive carbon black content.