Ultrasonically aided extrusion of rubber nanocomposites and rubber blends

Dispersion of nanofillers in rubber nanocomposites and distribution of components and their interaction in rubber blends is known to significantly affect their rheology and performance characteristics. Application of ultrasonic waves during extrusion is considered a possible means to influence compounding of polymer nanocomposites and polymer blends. The present study is devoted to the ultrasonically aided extrusion of natural rubber (NR) and styrene butadiene rubber (SBR) compounds filled with carbon black (CB), carbon nanotube (CNT) and carbon nanofiber (CNF) and NR/SBR blends. The ultrasonic treatment was found to provide a better processibility leading to a significant reduction of the die pressure during extrusion. For SBR nanocomposites, an increase of the ultrasonic amplitude up to certain values caused an increase of the gel fraction and viscosity of compounds, minimum and maximum torques during their curing. Due to the gel formation in SBR under ultrasonic treatment, an increase of the crosslink density, modulus and tensile strength of vulcanizates was observed. In contrast to SBR nanocomposites, the gel formation in NR nanocomposites under ultrasonic treatment was absent, and their properties were little affected by the treatment till an amplitude of 5.0 mum.;The ultrasonic treatment of SBR and NR nanocomposites at an amplitude of 7.5 mum led to a decrease of their properties, due to the dominant effect of chain scission. The ultrasonic treatment of SBR/CB and SBR/CNT nanocomposites led to a decrease of the electrical percolation threshold of their vulcanizates due to the ultrasonically induced uneven distribution of fillers as observed by the optical microscopy. The latter was caused by the gel formation in SBR affecting flow behavior of their compounds. The AFM analysis of SBR vulcanizates indicated that the ultrasonic treatment of SBR/CB and SBR/CNT nanocomposites led to a penetration of SBR chains into filler agglomerates. Interestingly, at the same apparent crosslink density, the modulus at 300 % of the ultrasonically treated SBR/CB and SBR/CNT vulcanizates was higher than that of the untreated vulcanizates. The ultrasonic treatment at an amplitude of 5.0 mum led to an increase of both the tensile strength and elongation at break of SBR/CB vulcanizates at a loading of 60 phr. These observations are an indication of a reinforcing effect by the ultrasonic treatment, possibly due to a better SBR-filler interaction via the chemical reaction between SBR macroradicals created by the ultrasonic treatment and active sites of fillers.;For ultrasonically treated NR/CB and NR/CNT nanocomposites, the uneven distribution of fillers and penetration of NR chains into agglomerates were not observed. The optical microscopy and AFM studies showed that the ultrasonic treatment of NR/CNT nanocomposite at a loading of 25 phr led to a breakdown of agglomerates and a better distribution of individual CNTs. Such morphological changes are associated with the strong Payne effect and the increase in hardness. A quantitative AFM analysis of vulcanizates showed that the ultrasonic treatment of NR/CB nanocomposites led to a better micro-dispersion. It is believed that the ultrasonically induced pulsating cavities in compounds transfer the vibration energy to surrounding molecules, leading to a breakdown of agglomerates. At the same time, it is believed that the ultrasonic treatment induces oxidation of NR leading to a chain scission and preventing a rubber gel formation and rubber-filler chemical reaction.;The ultrasonic treatment of NR/SBR blends at an amplitude of 5.0 mum led to an increase of both the tensile strength and elongation at break, with the modulus at 100 % being maintained. The phase contrast optical microscopy and AFM analysis of ultrasonically treated NR/SBR blend indicated a significant reduction of size of rubber phases. However, the ultrasonic treatment at an amplitude of 10 mum led to a significant chain scission, decreasing the modulus, tensile strength and elongation at break.