Influence of Sulfenamide Accelerators on Cure Kinetics and Properties of Natural Rubber Foam

The effect of molecular structure of sulfenamide accelerator, i.e., 2-morpholinothiobenzotiazole (MBS), N-t-butylbenzothiazole-2-sulfenamide (TBBS) and N-cyclohexyl benzothiazole-2-sulfenamide (CBS) on cure kinetics and properties of natural rubber foam was studied. It was found that the natural rubber compound with CBS accelerator shows the fastest cure rate and the lowest activation energy (E_a) because CBS accelerator produces higher level of basicity of amine species than other sulfenamide accelerators, further forming a complex structure with zinc ion as ligands in sulfur vulcanization. Because of the fastest cure rate of CBS accelerator, natural rubber foam with CBS accelerator shows the smallest and narrowest bubble size distribution. Natural rubber foam with CBS accelerator exhibits the lowest bulk density, thermal conductivity and thermal expansion coefficient as well as the highest compression set as a result of fast crosslink reaction.