Effect of sodium bicarbonate loading on the curing kinetics, tensile and morphological properties of peroxide-cured natural rubber

This study investigates the effect of sodium bicarbonate (NaHCO₃) loading on the curing behavior, kinetic response, tensile and morphological properties of natural rubber composites crosslinked with dicumyl peroxide (DCP). NaHCO₃, acting as a chemical blowing agent, decomposes upon heating to release gases that can interfere with the crosslinking process. Results showed that increasing NaHCO₃ loading had minimal impact on minimum and maximum torque, indicating that its incorporation does not affect initial mixing or crosslinking density. Scorch time increased due to the endothermic decomposition of NaHCO3, while the optimum cure time decreased, likely from enhanced heat transfer during the evolution of gases. The cure rate index also increased, suggesting a synergistic effect between NaHCO₃ and DCP. Pseudo-first-order kinetic modeling yielded high correlation coefficients (R² > 0.9964), confirming its suitability. Pseudo-second-order plots showed distinct phase behavior attributed to volatile loss of water and carbon dioxide with the increasing NaHCO3. Tensile testing revealed a decrease in tensile strength with increasing NaHCO₃, attributed to microvoid formation. Elongation at break shows improvement due to increased chain mobility but then declined from excessive voids and microstructural disruptions. Microscopy confirmed the presence of isolated, closed-cell structures (∼4 µm) without interconnected pores, while EDX analysis detected non-uniform sodium residues consistent with Na₂CO₃ distribution. Overall, the results demonstrate the complex interplay between foaming and crosslinking in peroxide-cured NR systems, with implications for tuning properties through controlled NaHCO₃ loading.