Recycling of Post-Consumer Silica Gel Desiccants as Reinforcing Filler in Natural Rubber Composites: The Effect of Coupling Agents and Comparison with Commercial Silicas

This study presents, for the first time, a systematic investigation of the use of micronized post-consumer silica gel as a reinforcing filler in natural rubber composites, in direct comparison with commercial silicas. Thirteen formulations were prepared using three types of silica (recycled, Copasil, and ZC-185P) and three coupling agents (TESPT, VTMO, and Chartwell C-515.71HR®). The recycled silica exhibited high purity (97.33% Si) and irregular morphology but resulted in lower crosslink density (0.47–0.59 × 10−4 mol·cm−3) and inferior mechanical performance, with tensile strength up to 7.9 MPa and high abrasion loss (878–888 mm3). In contrast, ZC-185P silica combined with TESPT achieved the best results, with a tensile strength of 18.5 MPa, tear resistance of 99.36 N·mm−1, and minimum abrasion loss of 170 mm3. Although less efficient in reinforcement, composites containing recycled silica were successfully applied in the production of a functional rubber mat, demonstrating their practical viability. The results confirm the potential for valorization of spent silica gel as an alternative raw material for sustainable composites, contributing to the circular economy.