Oil–rubber compatibility as a design rule for de-crosslinking efficiency and storage stability in rubberized asphalt

The compatibility between swelling agents and crumb rubber (CR) plays a pivotal role in determining de-crosslinking efficiency and the storage stability of CR-modified asphalt. This study explores the mechanistic differences between a petroleum-based agent, naphthenic oil (NO), and a bio-based agent, soybean oil (SO), focusing on their effects on CR swelling behavior, network disruption, and asphalt performance. The molecular structures and thermal stabilities of NO and SO were characterized using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) analysis. Compatibility with CR was assessed through polarity similarity theory, Hansen solubility parameters, and swelling kinetics. De-crosslinking behavior was quantified by measuring sol content, crosslink density, and through Horikx analysis, supplemented by scanning electron microscopy (SEM) for microstructural evaluation. Results show that NO exhibits greater thermodynamic affinity with CR, resulting in higher equilibrium swelling and more substantial network degradation compared to SO. An increased liquid-to-solid mass ratio further enhances de-crosslinking, with random main-chain scission identified as the dominant degradation mechanism. After Soxhlet extraction, naphthenic oil–de-crosslinked crumb rubber (SNCR) and soybean oil–de-crosslinked crumb rubber (SSCR) exhibited enhanced compatibility with asphalt. Compared with CR–modified asphalt, the storage stability increased by 67.9% and 55.4% for SNCR- and SSCR-modified asphalt, respectively, with SNCR-modified asphalt showing the best storage stability. These findings highlight the critical role of molecular compatibility in the design of effective swelling agents for stable CR-modified asphalt systems.