Degradation-Grafting Mechanochemical Treatment of Recycled Polypropylene as an Asphalt Modifier: Rheological Behavior and Microscopic Mechanisms

As a common solid waste, recycled polypropylene (RPP) has been demonstrated to be a viable asphalt modifier for enhancing pavement performance. However, its direct application still faces bottlenecks including high processing temperatures, poor dispersion, and unstable performance, which hinder its potential promotion. To address this, this study adopted a mechanochemical treatment of RPP with varying dosages of di-cumyl peroxide (DCP) and epoxidized soybean oil (ESO) to prepare a novel waste plastic asphalt modifier. This process was designed to cleave the long molecular chains of RPP via DCP, while simultaneously allowing dynamic chemical grafting reactions with ESO, yielding a degraded WPP bearing flexible functional groups. Through rheological temperature and frequency sweeps, Fourier transform infrared spectroscopy (FTIR), and mechanistic analysis, the intrinsic correlation between the microstructure and macroscopic rheological behavior of the modified binder was systematically investigated. Among the investigated formulations, the results indicated that 0.3RPP-MA showed the highest viscoelastic transition temperature, 25.7 °C, compared with 7.8 °C for the virgin binder. The dynamic shear rheology (DSR) results indicate that this formulation provided the most favorable high-temperature rheological response within the tested DCP range. FTIR spectra clearly verified the formation of ESO graft copolymers and the functional group changes induced by DCP degradation, and a reaction mechanism involving simultaneous degradation and grafting was elucidated. Overall, the preparation method proposed in this study provides a comprehensive analysis of the rheological properties of RPP-modified asphalt, providing both engineering guidance and theoretical support for the high-value utilization of waste plastics.