Two sets of nanocomposites were synthesized; namely SWNT/NR and SiC/NR nanocomposites. The synthesis of SWNT/NR composite system consists of dissolving SWNTs (0.5 mg/mL) and natural rubber gum in toluene (4 mg/mL) in two separate beakers. The SWNTs were provided by CNT Inc., (Houston, TX), pure natural rubber was supplied by the Goodyear Tire and Rubber Company (Akron) which was used as received. The SWNTs/toluene was subsequently dispersed in the natural rubber/toluene with ultrasonication at room temperature for 2 h. Vulcanizing agents were then added to the natural rubber solution, and the solution was mixed for 1 h. Finally, the SWNT/NR was prepared by evaporating the solvent off at 50 °C for approximately 6 h. The composition of NR used in the work is given in Table 1.
The vulcanization was carried out at 150 °C for 20 min using a hot press. The pressure of the hot press was adjusted (∼300 kPa) to obtain rubber composite sheets with the desired thickness, usually in the order of 200–300 μm. The SWNT/NR nanocomposites were black. Natural rubber nanocomposite thin sheets with SWNT reinforcement were prepared with a filler content of 1%, 1.5% and 2% by weight. However, there was not enough SWNT/NR sample at 2% filler content for the tensile testing, thus no test results are provided for mechanical properties in Section 3.1 for composite with 2% SWNT filler.
The SiC/NR nanocomposites were prepared using SiC nanoparticles 29 nm in size purchased from MTI Corporation. Natural rubber nanocomposite thin sheets were reproduced with 1% and 1.5% and 2% filler content by weight. The SiC/NR samples appeared to be lighter in color. Sample strips were prepared by cutting the sheets to 3 mm × 10 mm × 0.3 mm size for testing.
Mechanical properties of SWNT/NR and SiC/NR nanocomposites were measured by using an Instron tensile testing machine model 1100. At least six samples of each nanocomposite systems were used to perform the test. SEM micrographs of the nanocomposites were obtained using SEM operating at 15.0 kV. Raman spectrum of SWNTs from 500 to 3000 cm−1 was obtained by using the 748.5 nm line of a diode laser. The laser beam was focused onto the sample surface with a 50× objective lens to a spot of 2 μm size. Two sets of SWNT nanocomposite samples were subjected to cyclic loading and ageing and tested for mechanical properties and Raman band shift. For the cyclic loading, the samples were subjected to cyclic tensile force using an Instron machine at a maximum strain of 200% and a maximum stress of 1.25 MPa for six cycles. The other set of samples were aged in distilled water at ambient temperature for 30 days, and resulting Raman band shifts were quantified.