Processing of bitumens modified by a bio-oil-derived polyurethane

Castor oil (CO) functionalized by isocyanate groups (-NCO) is proposed as a novel bio-based reactive polyurethane (PU) for bitumen modification. This work presents a comparative analysis conducted on blends of bitumen and 2 wt.% of a PU prepolymer prepared by NCO-functionalization of castor oil. Four preparation procedures were evaluated, which resulted from the combination of two processing times (1 h or 24 h, at 90 °C) followed by two different post-treatments (water addition or ambient curing for up to 6 months). It was found that the degree of modification attained after post-treatment depends on the previous processing conditions. Thus, short processing times are required if the binder is further subjected to ambient curing. Instead, the success of the water-addition modification falls on a previous long processing step. As revealed by rheological tests, ambient curing was seen to be by far a more efficient way of modification if compared to direct addition of water, and makes clear that the resulting binder evolves towards a better performance when in-service. In that sense, Thin Layer Chromatography tests, Modulated DSC and AFM images demonstrated a more complex microstructure characterized by the presence of a larger content of molecules with higher polarity, size and molecular weight.