Influence of Accelerator Content on Cure Kinetics of Sulfur-vulcanized Natural Rubber

Based on the cure characteristics of the NR/S/TBBS system, a kinetic model with induction, curing, and post-cure periods was chosen to simulate the cure reaction of this system. Cure curves reflecting the evolution of crosslink density were recorded as a function of curing time for NR compounds using a rubber processing analyzer (RPA). The cure curves were then non-linear fitted and the kinetic parameters were determined. The results showed that the simulated curves fit well with the experimental curves. As the concentration of activated sulfurating agents, A_o, increased, the activity of crosslink precursors increased (K₂ increased), while the activity for crosslinks to degrade decreased (K₆ decreased) due to shortening of the sulfur chain both in crosslink precursors and in crosslinks, leading to the improved thermal stability of NR vulcanizate at elevated temperatures. Increased accelerator amount also increased the competition of the reaction to form crosslinks over the reaction to form dead by-products, which improved the efficiency of sulfur. Reaction activation energy E₂ and E₆, calculated from K₂ and K₆ through the Arrhenius equation, showed that E₆ is higher than E₂. The increase of accelerator dosage led to the decrease of E₂ and increase of E₆, which explained the phenomenon of improved anti-reversion.