Rheological Properties and Compatibility of Polyphosphoric Acid/Low-Density Polyethylene Composite Modified Asphalt

Asphalt pavements are increasingly challenged by high-temperature rutting and low-temperature cracking from heavy traffic loads. Waste low-density polyethylene (LDPE) is a sustainable modifier that enhances high-temperature performance, but its compatibility with asphalt is limited, causing phase separation and reduced low-temperature ductility. Polyphosphoric acid (PPA) is a cost-effective modifier that improves viscoelasticity, yet the synergistic effects of PPA and LDPE on asphalt properties are not well understood. This study investigates the performance and modification mechanisms of PPA/LDPE composite modified asphalt and identifies the optimal modifier dosages. Fifteen sample groups with varying LDPE and PPA dosages were tested through conventional performance, rheological, and compatibility analyses. Key findings include: (1) Both PPA and LDPE synergistically enhanced high-temperature performance. LDPE increased the softening point by 16.8% to 38.3% compared to base asphalt through physical network formation, while PPA improved the rutting factor ( G ∗ / sin δ ) by 21.1% to 47.4% compared to LDPE-modified asphalt through esterification reactions. (2) PPA reduced nonrecoverable creep compliance ( J n r ) by 35.6% to 62.8% under 3.2 kPa, indicating better resistance to permanent deformation. (3) Composite modification increased yield stress by 23.9% to 44.3% compared to base asphalt, extending fatigue life. (4) LDPE increased creep stiffness ( S ) by 15.2% to 29.5% at –18°C, while PPA reduced S by 8.3% to 11.6%. (5) Compatibility analyses showed that PPA decreased the segregation rate to less than 0.2 from 0.8 for 4% LDPE-modified asphalt and promoted uniform LDPE dispersion, with fluorescence microscopy revealing a 40% to 55% reduction in particle size. Fourier transform infrared spectroscopy (FTIR) analysis identified a new peak at 1,010 cm − 1 (P─O–C/P=O bonds), confirming chemical synergy between PPA and LDPE. This study provides a comprehensive data set on PPA/LDPE composite modified asphalt, clarifying the synergistic mechanisms between the modifiers and offering guidance for incorporating waste LDPE into sustainable pavement materials. The findings also establish a foundation for further optimization of polymer–acid composite modified asphalt systems.