Chemically Assisted Thermo-Mechanical Devulcanization of Passenger Car Tire Rubber for SBS-Composite-modified Asphalt: Mechanisms, Rheological Properties, and Dosage Effects

Passenger car waste tires (PCWTs) are difficult to recycle as asphalt modifiers due to their high styrene-butadiene rubber (SBR) content and poor compatibility with asphalt. This study investigates whether hexadecylamine (HDA)-assisted thermo-mechanical devulcanization can transform PCWT rubber into an effective modifier for styrene-butadiene-styrene (SBS)-composite-modified asphalt, and how rubber dosage influences the performance balance. PCWT rubber was devulcanized using HDA-assisted twin-screw extrusion and incorporated into 90# asphalt with 3 wt% SBS at 20 wt% and 30 wt% dosages, alongside non-devulcanized PCWT and truck tire rubber for comparison. Comprehensive characterization, including sol fraction, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), dynamic shear rheometer (DSR), multiple stress creep recovery (MSCR), and bending beam rheometer (BBR), was conducted. The results demonstrate that HDA-assisted devulcanization effectively breaks sulfur crosslinks while preserving the rubber backbone, significantly improving compatibility and low-temperature performance. Increasing the devulcanized rubber dosage from 20 wt% to 30 wt% compensates for the reduced high-temperature stiffness, achieving a balanced performance profile comparable to truck tire rubber. The novelty of this work lies in establishing HDA-assisted thermo-mechanical devulcanization as a viable pathway for converting otherwise underutilized PCWT rubber into a high-performance asphalt modifier, thereby expanding the feedstock options for crumb rubber-modified asphalt.