Ultrasonically assisted single screw extrusion of carbon black (CB)-filled styrene–butadiene rubber (SBR) compounds up to 60 phr was carried out and its processing characteristics were measured. The effects of ultrasonic amplitude on rheology, extractable amount, vulcanization behavior, mechanical properties, abrasion, electrical resistivity, and morphology were investigated. Increasing ultrasonic amplitude led to a reduction of extractable content with the effect diminishing with increasing CB loading. Treated compounds showed an increase in viscosity at certain amplitudes. Cure curves revealed reduced induction times with increasing amplitude, except at the highest CB content. The maximum torque of the cure curve, crosslink density, and mechanical properties of vulcanizates all showed an increase at certain ultrasonic amplitudes. An increase in the glass transition temperature (Tg) and a decrease in tan δ at Tg were observed with increasing amplitude. Ultrasonically treated unfilled and CB-filled compounds led to vulcanizates with significant and slight improvements in abrasion, respectively. Ultrasonic treatment caused a significant reduction in the electrical percolation threshold of vulcanizates. Morphological study by atomic force microscope revealed a creation of unique CB agglomerates, suggesting the strong interactions between the SBR and CB aggregates in treated compounds, leading to reinforcing effects in vulcanizates.

Ultrasonically assisted single screw extrusion of styrene–butadiene rubber (SBR) compounds filled with multiwall 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 (Tg) and a decrease in tan δ at Tg with amplitude were observed in CNF- and CNT-filled vulcanizates. At the same time, tan δ in the range of 0–60 °C of these vulcanizates showed, respectively, a minimum and a continuous increase with amplitude. CNF-filled vulcanizates exhibited improved abrasion resistance with optimum results at an amplitude of 5.0 μm. 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. Atomic force microscope analysis showed a generation of unique CNT agglomerates by ultrasonic treatment, suggesting an increase of rubber–filler interaction. High resolution scanning electron microscopy analysis of vulcanizates showed better dispersion of ZnO by ultrasonic treatment.

Natural rubber/multiwalled carbon nanotube (MWCNT) nanocomposites at loadings from 2 to 25 phr were prepared by ultrasonically aided extrusion at ultrasonic amplitudes up to 7.5 μm. Die pressure significantly decreased with an increase of ultrasonic amplitude, especially at higher loadings. Power consumption increased with ultrasonic amplitude and was almost insensitive to loadings until an amplitude of 5.0 μm. However, lower values of power consumption were observed at an amplitude of 7.5 μm for loadings of 10 and 15 phr. At these loadings, complex viscosity, storage, and loss moduli of compounds and vulcanizates were unaffected by ultrasonic treatment, whereas at other loadings, these properties were reduced with ultrasonic amplitude, indicating chain scission. Bound rubber in compounds decreased with treatment. The induction time during vulcanization was unaffected by ultrasonic treatment but decreased with loading, with the largest decrease observed at the lowest loading. The maximum torque in curing, cross-link density, and gel fraction of vulcanizates at loadings of 2, 3.5, 5, 7.5, and 25 phr decreased with ultrasonic amplitude, with the largest decrease observed at loading of 25 phr at an amplitude of 7.5 μm. The modulus at 100% and 300% and tensile strength of vulcanizates at loadings up to 15 phr were unaffected by the ultrasonic treatment, but these properties significantly decreased at a loading of 25 phr at an amplitude of 7.5 μm. Atomic force microscope studies of vulcanizates at a loading of 3.5 phr showed a deagglomeration at an amplitude of 7.5 μm, although dispersion analysis from an optical microscope at loadings of 3.5 and 7.5 phr did not show an improvement of dispersion by ultrasonic treatment. Optical microscope and AFM studies of vulcanizates at a loading of 25 phr indicated that ultrasonic treatment at an amplitude of 7.5 μm significantly improved dispersion of MWCNT. Such a behavior led to a strong Payne effect and an increase in hardness of vulcanizates.

Significant efforts have been made in rubber research to improve the 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, cross-link 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, whereas the elongation at break increased. Atomic force microscope (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/carbon nanotube (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.

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