Comparative analysis of four styrene-butadiene-styrene (SBS) structure repair agents in the rejuvenation of aged SBS-modified bitumen

At present, a large number of styrene-butadiene-styrene (SBS) block copolymer-modified bitumen pavements are approaching the end of their service life, leading to the generation of substantial amounts of reclaimed asphalt pavement (RAP) from maintenance and rehabilitation projects. Recycling RAP with rejuvenators not only mitigates waste but also offers a sustainable approach to pavement restoration. This study aims to achieve high-quality rejuvenation of aged SBS-modified bitumen (SMB) by employing aromatic oil (AO) as a colloidal structure regulate agent and using methylene-bis (4-cyclohexylisocyanate) (HMDI), hexamethylene diisocyanate (HDI), triallyl isocyanurate (TAIC), and trimethylolpropane triglycidyl ether (TMPGE) to repair the degraded SBS structure. Fourier transform infrared spectroscopy (FTIR) and fluorescence microscopy (FM) were employed to investigate the repairing mechanisms and effectiveness of rejuvenators on fracture crosslinking structure for degraded SBS. The rheological performance of the rejuvenated SMB was subsequently evaluated using dynamic shear rheometer (DSR), and the discussion of links between the microstructure and macroscopic performance was conducted. The results reveal that the isocyanate groups in HMDI and HDI react with hydroxyl and carboxyl groups in degraded SBS structure, forming carbamate and amide bonds. This reaction reconnects the SBS modifier and create cross-linked network structures. In contrast, TAIC and TMPGE show limited efficacy in reconstructing the network structures, resulting in less significant improvements. This microstructure difference is reflected in the macroscopic performance. The critical temperature of high- and low-temperature of HMDI-rejuvenated SMB are measured at 93.9°C and −22.1°C, while those of HDI-rejuvenated SMB are 90.6°C and −22.2°C. Both values consistently exceed the critical temperature of virgin SMB, which exhibit critical temperature of high- and low-temperature of 85.9°C and −21.1°C. In contrast, the performance of bitumen rejuvenated with TAIC and TMPGE is comparable to that of AO-alone rejuvenated bitumen.