Curing kinetic of natural rubber under conventional vulcanization system with incorporation of sodium bicarbonate

This research examines the curing kinetics, activation energy (Ea), tensile properties, chemical structure, and surface morphology of natural rubber (NR) composites prepared via a conventional sulfur vulcanization (CV) system, with and without sodium bicarbonate (NaHCO₃) acting as a blowing agent. Two NR composites were prepared: a control sample (0 phr NaHCO₃/NR) and a modified compound containing 8 phr NaHCO₃. Curing behaviour was also assessed through Moving Die Rheometer (MDR) but the interpretation of kinetic behaviour was done using pseudo-first-order and pseudo-second-order models. Arrhenius relationship was used to calculate the Ea values. Analysis of MDR torque showed that NaHCO3 addition substantially shortened scorch time (ts2) and raised the cure rate index (CRI), which suggests a decrease in the cure initiation rate. But it has lower maximum torque (MH) and torque changes (ΔM) value, indicating density of crosslinks formation reduced in the process, as the porous structure is formed during vulcanization. Both NR composite curing profile were established to be pseudo-first-order in nature. As 8 phr NaHCO3 was added, the Ea of the vulcanization increased from 118.23 kJ/mol to 157.59 kJ/mol, indicating that more thermal energy was needed to form the crosslinks in the modified composite. This was explained by the endothermic reaction of NaHCO3 and reaction with the activators, which modified the curing process. Though NaHCO3 increased curing window stability at elevated temperatures, the tensile strength and elongation at break (Eb) decreased as a result of the formation of microvoids that caused local stress concentration. Moreover, FTIR analysis showed no other dominant peaks showing that the initial chemical structure of NR did not change. Peaks at 1440cm-1 and 1640 cm-1 were attributed to CH2 scissoring vibrations and stearic acid-zinc interactions respectively which overlapped the carbonate and carbonyl signals from NaHCO3 breakdown. The optical microscopic observation revealed the control sample had more dense and uniform morphology, while the NaHCO3-modified composite had closed cell structures which were uniformly distributed throughout the rubber matrix. This suggests that there was control in the release of the gases during vulcanization. Overall, NaHCO₃ primarily influenced the kinetic and physical properties of NR vulcanization without causing significant chemical alteration. The findings highlight the potential of NaHCO₃ as a controlled blowing agent for developing foamed NR composites.