Sustainable Application and Valorization of Toner Waste as a Functional Filler in Styrene–Butadiene Rubber (SBR) Composites

This study investigates the incorporation of toner residue (TR), derived from post-consumer printing cartridges, as an alternative filler in styrene–butadiene rubber (SBR) composites, with emphasis placed on solid waste valorization and the promotion of a circular economy. TR consists predominantly of fine particles containing thermoplastic polymers, carbon black, metal oxides, and additives, exhibiting functional potential as a partially reinforcing filler material. Composites containing 0 to 50 phr of TR were prepared and characterized in terms of rheometric properties, dispersion degree, elemental composition by X-ray fluorescence (XRF), crosslink density, scanning electron microscopy (SEM), infrared spectroscopy, Shore A hardness, abrasion resistance, tensile strength, and tear resistance. Rheometric results indicated modifications in vulcanization kinetics and a reduction in maximum torque for formulations with high TR contents, suggesting a possible diluent effect or interference with elastomeric network formation. Conversely, moderate TR concentrations promoted increased hardness, improved tensile strength, and higher crosslink density, associated with adequate particle dispersion within the matrix, as confirmed by SEM analysis. However, excessive TR loading led to increased abrasion loss and an overall reduction in mechanical performance. It is concluded that TR demonstrates technical feasibility as a partial substitute for conventional fillers in SBR composites, with potential industrial application, such as in footwear sole prototypes, combining functional performance with environmental impact mitigation.