Transforming vulcanized styrene–butadiene waste into valuable raw material: an opportunity for high-impact polypropylene production

Polypropylene (PP) compounds with styrene–butadiene rubber residue (SBRr) from the footwear industry were produced, adding styrene-(ethylene–butylene)-styrene (SEBS) as a compatibilizer, with 20% and 30% of styrene (St.). The PP/SBRr and PP/SBRr/SEBS compounds were processed in a co-rotating twin-screw extruder and injection molded. The addition of 30% SBRr did not compromise processability, as torque and melt flow index increased slightly compared to neat PP. Significant increases in impact strength were achieved for PP/SBRr/SEBS (10–20% St.) and PP/SBRr/SEBS (10–30% St.), with gains of 316% and 248% related to PP. The elastic modulus, tensile strength, elongation at break, and Shore D hardness indicated greater flexibility for the PP/SBRr/SEBS, especially in the copolymer with 20% St. When the PP/SBRr system was made compatible with SEBS (30% St.), the elastic modulus, tensile strength, and Shore D hardness increased in relation to the PP/SBRr/SEBS (20% St.). Such behavior suggested that the SEBS copolymer (30% St.) was more effective in increasing the resistance to elastic deformation of the PP/SBRr/SEBS compounds, compared to SEBS (20% St.). The heat deflection temperature (HDT) indicated that even adding high content of SBRr to PP, the HDT was not severely affected, possibly due to its crosslinked character. PP/SBRr compatibilization with SEBS (20% St.) inhibited the PP crystalline peaks, as verified through X-ray diffraction (XRD). Stable morphology was achieved upon 10% of SEBS addition to PP/SBRr, providing proper interfacial adhesion and fine particles, contributing to toughening PP. Acquired results are promising for rubber recycling, aiming to produce high-impact polypropylene for containers and furniture accessories applications.