Natural Rubber/Carbon Black Nanocomposites Prepared by Ultrasonically Aided Extrusion

Significant efforts were made in rubber research to improve dispersion of carbon black (CB) in rubbers to achieve better processibility and performance of tires and rubber products. In addressing these issues the present study is an attempt to further improve the processibility and dispersion by means of application of ultrasonic waves. Natural rubber (NR)/CB nanocomposites at loadings from 15 to 60 phr were prepared by ultrasonically aided extrusion at ultrasonic amplitudes up to 7.5 μm. A die pressure significantly decreased with an increase of amplitude, especially at higher loadings indicating an improvement in processibility. Ultrasonic power consumption was almost insensitive to loadings. The complex dynamic viscosity, storage and loss moduli of compounds and vulcanizates at loadings of 15, 25, 35 and 60 phr were reduced by the ultrasonic treatment at an amplitude of 7.5 μm, indicating NR chain scission. Bound rubber in compounds decreased by the ultrasonic treatment. The maximum torque in curing curves, crosslink density, gel fraction, hardness, M100, M300, tensile strength and abrasion resistance of vulcanizates at loadings of 15, 25, 35 and 60 phr decreased at an amplitude of 7.5 μm, due to the NR chain scission, while the elongation at break increased. AFM studies of vulcanizates showed a penetration of rubber chains into agglomerates at an amplitude of 7.5 μm, indicating an improvement of dispersion of CB. Based on AFM images, a dispersion index was introduced, showing that the ultrasonic treatment at an amplitude of 7.5 μm led to a better dispersion of CB in vulcanizates. Comparison of NR/CB compounds and vulcanizates with those of NR/CNT of an earlier study was carried out. In general, the CB-containing NR showed significantly lower modulus and abrasion resistance, but higher tensile strength and bound rubber, than CNT-containing NR.