Nitroxide-Functionalized Green Silica with Radical Capturing Potential as a Filler in the Styrene–Butadiene Rubber Compound

In this work, a potent functionality to capture active polymer radicals, a nitroxide functionality, has been introduced to silica derived from rice husk ash. The thiol–ene reaction with surface mercaptopropyl functionality and 4-(acryloyloxy)TEMPO [4-acryloyloxy(2,2,6,6-tetramethylpiperidin-1-yl)oxy] resulted in nitroxide-functionalized silica particles. Three different types of mercaptopropyl-functionalized silica particles were synthesized via co-condensation of sodium silicate derived from rice husk ash (RHA) and via postmodification of commercial precipitated silica (CPS) and commercial RHA silica (CRHA). The polymer radical capturing potential of surface nitroxide radicals was determined by using polystyrene grafting studies. The nitroxide-functionalized silica samples were subjected to compounding studies with styrene–butadiene rubber to determine the effect of surface nitroxide functionality on capturing the elastomer radicals formed during mixing and compounding. The effects of surface nitroxide radicals on curing time and mechanical properties have been discussed with respect to the three different types of nitroxide-functionalized silica samples.