Factors Affecting Phase Separation of HDPE, LDPE, and PP Modified Asphalt Binders

Abstract: Recent studies revealed that the most significant concern with the plastic-modified asphalt binder is the phase separation of plastic from asphalt binder. This was observed using the commonly used Cigar tube test where the binder is kept at 163°C in an aluminum tube for 48 hours. Therefore, the objective of this study is to investigate the factors influencing phase separation in plastic-modified asphalt binders, focusing on shear mixing, storage conditions, and container geometry so that the binder-plastic blend can be kept stable controlling the factors. The base binder used was PG 67-22, modified with varying concentrations of HDPE, LDPE, and PP plastics. Six methods including the Cigar tube method, were employed to evaluate the blends: image processing, thin film method, shear-mixing, shelf-life, reheating 2 g, and reheating 50 g. Optical microscopy and Laser Scanning Confocal Microscopy (LSCM) were used for in-depth analysis. Plastic-modified asphalt binders were prepared using a high-shear mixer and stored in quart cans for subsequent sample preparation. In the first two methods, samples were prepared directly from the stored binder without high shear mixing, whereas in the remaining four methods, binders were subjected to high shear mixing for 30 minutes prior to sample preparation. LSCM image analysis revealed phase separation with higher plastic concentrations leading to the formation of network-like structures. Large HDPE and LDPE plastic particles were observed using the thin film method, while no PP particles were detected on the pan. Additionally, no plastic particles were observed in the shear-mixed samples after multiple washes with Dichloromethane (DCM)/Toluene, a result consistent with the findings from the shelf-life method, where samples were examined at intervals of 0, 1, 3, and 7 days. The influence of storage time and container geometry was further analyzed in methods five and six, where 2 g samples were stored on vial caps and 50 g samples in aluminum tubes, both subjected to heating at 170°C for varying durations. After heating, small portions of the samples from the vial caps and tubes were washed with DCM/Toluene on PAV pans or tin can lids. No particles were visible in the vial cap samples during the first 24 hours; however, thin, ash-like particles formed afterward. In contrast, the tube samples revealed significant plastic particles, with HDPE forming tough, sheet-like chunks, LDPE developing a fibrous structure, and PP presenting as a fine powder. It can be concluded from this study that high shear mixing provides complete blending of plastic that are not detectable in an ultra thin film and the phase separation of plastic from asphalt binder can be controlled using continuous shear mixing at high temperature during storage of the blend.