Elucidating the roles of carbon black in filler- and rubber-filler interactions using synchrotron Nano-CT and STXM

Abstract

To investigate the correlation of carbon black (CB) surface activity to filler-filler and rubber-filler interactions, the CB underwent heat treatment with high-temperature and oxidation with concentrated nitric acid. With synchrotron radiation-based X-ray Nano-CT and scanning transmission X-ray microscopy (STXM), the dispersing structures and rubber-filler interactions of CB with different surface activities in natural rubber (NR) were studied. The results showed that the heated CB with inactive surface tended to gather together in small sizes and dispersed homogeneously in NR, while the oxidized CB aggregated in big sizes and dispersed inhomogeneously in non-polar NR. Meanwhile, oxygen-containing functional groups (C–O, CO) on CB surface were strongly conjugated with C–H groups from NR and induced C–H group orientations, and the more activities on the CB surface, the stronger rubber-filler interactions. With the measured stress-strain curves and the stress-bearing and strain-bearing double network models, the moduli of the CB fillers in NR were calculated. A nonlinear increase of the CB filler moduli was observed as compared to the CB content in NR. Specifically, with the CB content increasing 5 times, the CB filler moduli increased about 10 times or more. This suggest that the CB filler-filler interactions (filler network structures) are more effective in NR reinforcement than rubber-filler interactions. Thus, the modulus of the heated CB with weak rubber-filler interactions was larger than that of the oxidized CB with strong rubber-filler interactions. The insightful information provides a valuable perspective on the CB reinforcement mechanisms and the design of new rubber composites with specific properties.