Chemical Pathways for Polymer Restoration in SBS-Modified Binders: Strong Crosslinking or Compatibilising-Crosslinking Systems?

ABSTRACTSBS-modified binders are widely used in surface layers for noise reduction and rutting resistance, but once severely aged, reclaimed asphalt containing these binders is often downcycled or blended at low levels. Conventional rejuvenation, largely relying on softening oils, can partially restore stiffness and improve workability, yet leaves the oxidatively severed SBS network only incompletely rebuilt. Mixtures produced with such “bitumen-only” rejuvenation therefore show limited recovery of elastic response and fatigue resistance. This presentation focuses on polymer restoration in SBS-modified binders and asks how far chemical routes can go in rebuilding the aged polymer network. Drawing on three recent studies, two representative strategies are examined: isocyanate-based systems (e.g. MDI/TDI), which promote strong crosslinking between aged SBS and polar bitumen fragments, and epoxied systems (e.g. TMPGE-based formulations), which primarily provide compatibilisation and more moderate crosslinking. Isocyanate-based chemo–physical rejuvenator systems, combining oil, fresh polymer and isocyanate, can drive elastic recovery and rutting resistance of aged binders towards, or beyond, virgin levels. However, this gain is accompanied by higher stiffness, reduced low-temperature and fatigue performance, tight processing windows and occupational-health constraints in hot-mix production. Epoxied systems offer a more favourable balance between softening, compatibility and workability and show benefits for low-temperature and fatigue behaviour, yet they appear to have an inherent ceiling in the degree of network rebuilding and high-temperature strength, especially in highly modified binders. By combining FTIR/GPC-based chemical analysis, microscopy of phase morphology and rheology (DSR, MSCR, LAS, BBR) with plant-like recycling experiments, the contrasted behaviour of these two approaches is synthesised into a mechanism-oriented design framework for chemo–physical rejuvenators. The framework shows how isocyanate and epoxy functionalities can be tuned and combined with oils and fresh polymer to enable high-value recycling of RAP containing SBS-modified binders, and outlines research and specification challenges for transfer into routine practice.