Accelerated sulfur vulcanization of silica-filled natural rubber analyzed by rubber-state NMR spectroscopy

An optimal concentration (in phr) of vulcanization accelerator was determined for the preparation of silica-filled natural rubber (NR) vulcanizates without the use of a silane coupling agent, achieving outstanding mechanical properties. The silica-filled NR vulcanizates were prepared from 50 phr silica filled NR by accelerated sulfur vulcanization with N - tert -butyl-2-benzothiazyl sulfenamide (TBBS) at concentrations ranging from 0.7 to 12.0 phr, in combination with stearic acid, zinc oxide, and sulfur. The resulting vulcanizates were characterized by swelling method, uniaxial tensile test, and rubber-state nuclear magnetic resonance (NMR) spectroscopy. The optimal vulcanization time decreased with increasing TBBS concentration, reaching its minimum at 5.5 phr of TBBS. Crosslink density and the stresses at 100% and 300% strains increased with increasing TBBS concentration. The stress at break also depended on the TBBS concentration, reaching a maximum value of approximately 34.6 MPa at 5.5 phr. Rubber-state NMR spectroscopy revealed that accelerated sulfur vulcanization occurred at TBBS concentrations above 3.5 phr. Based on these results, the optimal TBBS concentration for the preparation of silica-filled NR vulcanizates was determined to be 5.5 phr. • Accelerated sulfur vulcanization of silica-filled NR • Occurrence of accelerated sulfur vulcanization at 3.5 and 6.0 phr of TBBS • Formation of uncrosslinked pendant sulfidic linkages at 0.7, 2.0 and 12 phr of TBBS