Carbon nanotube/conductive carbon black-filled natural rubber composites for strain sensing

Strain sensors were developed using conductive natural rubber (NR) and epoxidized natural rubber (ENR) composites filled with carbon nanotubes (CNT) and carbon black (CCB) to evaluate their rapid recovery after elastic deformation. The CCB concentration was set at 1.5 times the CNT concentration, with CNT and CCB contents maximized at 25 and 37.5 parts per hundred rubbers (phr), respectively. The crosslink propagation, mechanical and dynamic mechanical properties, and morphological characteristics were analyzed to elucidate the effect of the CNT/CCB hybrid filler within the NR and ENR matrices. Increasing the CNT/CCB content above 5/7.5 phr reduced the tensile strength and elongation at break due to filler agglomeration. However, the crosslink density, modulus, and production time improved. Compared to NR composites, ENR composites exhibited superior properties, attributed to the improved dispersion and distribution of fillers facilitated by the polarity of the ENR molecular chains. The selected composites were subjected to piezoresistive testing to evaluate the performance of strain sensors under stretching, compression, and bending protocols. The results revealed that CNT/CCB-filled ENR composites exhibited high sensitivity in tension and bending tests, while NR composites demonstrated appropriate signal responses in compression modes. This behavior was attributed to the critical CNT:CCB ratio of 20:30 phr, which enabled an optimal correlation between modulus and extensibility in the rubber matrix. This balance facilitated interactions between rubber molecular chains and the hybrid filler surface under multidirectional forces. These findings offer valuable insights for motion detection applications, particularly in scenarios involving deformation in multiple directions.