Ultrasonic Aided Extrusion of CNT- and CNF-Filled SBR Compounds and Properties of Their Vulcanizates

Ultrasonically assisted single screw extrusion of SBR compounds filled with multi-wall carbon nanotubes (CNT) up to 20 phr and carbon nanofibers (CNF) up to 35 phr was carried out. The effects of ultrasonic amplitude on processing characteristics, rheology, extractable amount, vulcanization behavior, mechanical properties, abrasion, electrical resistivity, and morphology were investigated. Ultrasonically treated compounds showed an increase in viscosity and unextractable content in both CNT- and CNF-filled compounds. A decrease in the induction time of curing with amplitude was observed in CNT-filled compounds. However, the induction time of CNF-filled compounds showed more complex behavior with amplitude. The final cure torque, crosslink density, modulus and tensile strength showed a maximum at an intermediate amplitude in filled vulcanizates. An increase in the glass transition temperature (T_g) and a decrease in tan δ at T_g with amplitude were observed in CNF- and CNT-filled vulcanizates. At the same time, tan δ in the range of 0 to 60 °C of these vulcanizates showed, respectively, a minimum and a continuous increase with amplitude. CNF-filled vulcanizates had improved in abrasion resistance with optimum results at an amplitude of 5 um. The effect diminished with increasing CNF loadings. Improvements in abrasion resistance of CNT-filled vulcanizates were observed at certain amplitudes at low CNT loadings only. Ultrasonic treatment caused a significant reduction in the electrical percolation threshold of CNF- and CNT-filled vulcanizates. AFM analysis showed a generation of unique CNT agglomerates by ultrasonic treatment, suggesting an increase of rubber-filler interaction. HR-SEM analysis of vulcanizates showed better dispersion of ZnO by ultrasonic treatment.