Nanocomposites of Natural Rubber with Carbon Nanotubes Prepared by Ultrasonically Aided Extrusion

Nanocomposites of natural rubber with multi walled carbon nanotubes (NR/MWCNT) at a loading range of 2 to 25 phr were prepared by ultrasonically aided single screw extrusion process at ultrasonic amplitudes up to 7.5 μm. Die pressure was found to significantly decrease with an increase of ultrasonic amplitude, especially at higher MWCNT loadings. Power consumption was almost insensitive to the MWCNT loading and increased with an increase of ultrasonic amplitude. Complex dynamic viscosity, storage and loss moduli of compounds and vulcanizates measured at high strain amplitudes decreased with ultrasonic amplitude, indicating a molecular chain scission by ultrasonic treatment. Bound rubber in compounds decreased with increasing ultrasonic amplitude. The induction time during vulcanization decreased with MWCNT loading with the largest decrease being at the lowest loading, and was not affected by ultrasonic treatment. The minimum and maximum torque during vulcanization of compounds generally decreased with increasing ultrasonic amplitude with the largest decrease observed at 25 phr loading and the highest ultrasonic amplitude. This finding was in agreement with the crosslink density measurement. The modulus at a strain of 100 and 300 %, and tensile strength of vulcanizates decreased with increasing ultrasonic amplitude. The electrical volume resistivity of vulcanizates in the percolation region corresponding to 3.5 phr of MWCNT loading increased with an increase of ultrasonic amplitudes up to 5.0 μm and decreased with its further increase. The quantitative analysis of optical microscope images of this vulcanizate was performed to evaluate the level of dispersion. The correlation between the dispersion level and electrical volume resistivity was found, showing that a better dispersion led to a lower resistivity. Morphological studies by optical microscope and AFM indicated that ultrasonic treatment does not improve the dispersion of CNT in NR matrix at the percolation region. The vulcanizate of ultrasonically treated compounds at a loading of 25 phr at the highest amplitude led to the strong Payne effect and an increase in the hardness. Also, AFM images showed a better distribution of CNT and an increased CNT-CNT interaction.