Curing Characteristics of Natural Rubber Filled with Gypsum

This research aims at reutilizing discarded plaster molds by using waste gypsum obtained from these molds as an inert filler in natural rubber. Its effects on curing characteristics were investigated in comparison with commercial gypsum and calcium carbonate. These fillers were characterized by an X-ray diffractometer before being mixed with natural rubber at various amounts. Several ingredients necessary for curing via sulfur system were also incorporated. The rubber compounds were then tested for their curing behaviors using a moving die rheometer to investigate the effects of the fillers. The results show that when filler loadings increased the maximum and minimum torques of the compounds increased accordingly while the scorch time and 90% cure time steadily decreased. The cure rate index of the NR-filled compounds was clearly affected by the filler especially when commercial and waste gypsums were used.