Payne effect of carbon black filled natural rubber compounds and their carbon black gels

Carbon black (CB) is able to reinforce natural rubber (NR) but also enhance nonlinear viscoelastic behaviors significantly. Herein, the reinforcement and nonlinearity of NR/CB compounds, their unextractable gels, and swollen and oil-extended products are investigated systematically. The results show that the formation of bound rubber could facilitate the recovery and eliminate the hysteresis during cyclic deformation while the presence of sol fraction is able to improve the onset strain amplitude of the Payne effect. Both the highly filled compounds and their unextractable gels may demonstrate weak strain overshoot. It is revealed for the first time that the overshoot is either triggered or its intensity is improved by paraffin swelling. On the other hand, paraffin introduced during the compounding process would lower the content of glassy chains in the oil-extended compounds and cause a two-step softening with increasing strain amplitude. The results provide new insights into the Payne effect and allow gaining recognition of the important roles played by the bound rubber, free rubber and the filler.