Isocyanate-functionalized castor oil as a novel bitumen modifier

• Sustainable bitumen modification by MDI/castor oil modifiers.
• Binders with enhanced resistance to permanent deformation.
• Important changes in the binder's microstructure corroborated by AFM pictures.
• Castor oil prepolymers with higher thermal stability than crude oil prepolymers.

The use of raw materials from renewable sources in the synthesis of polyurethane-derived polymers is lately receiving great attention from social, environmental and economic standpoints. In this work, prepolymers having different –NCO/–OH ratios were synthesised, by reaction of 4,4′-diphenylmethane diisocyanate (MDI) with castor oil (CO), to be used as the modifying agent of asphaltic bitumen. Reactions between MDI and CO, performed with –NCO/–OH molar ratios of 8:1 and 4:1, have led to suitable bitumen modifiers. Modification has been related to chemical reactions between –NCO groups, some bitumen compounds and air moisture (or added water), which gave rise to binders with enhanced resistance to permanent deformation. The results showed that a 2 wt% of MDI–CO prepolymer leads to binders with higher viscosity than that corresponding to a 3 wt% Styrene–Butadiene–Styrene (SBS) block copolymer, a polymer and concentration widely used in the paving industry. Furthermore, the resulting prepolymers are liquids that can be easily mixed with bitumen at 90 °C, which significantly lowers the typical temperature used for commercial SBS-modified bitumen (about 180 °C). This fact may represent energy savings, reduce bitumen oxidation and result in improvements of health and safety conditions during the product manufacture. However, if high processing temperatures are required, MDI–CO based modifiers have demonstrated much higher thermal stability than prepolymers derived from crude oil (e.g. based on polyethylene glycol). As a result, NCO-terminated prepolymers obtained from biomass-derived polyols (castor oil in this article) may become a promising alternative to the use of other petrochemicals in the paving industry.