Structure-property relationships in the development of bituminous foams from MDI based prepolymers

Stable bituminous foams based on diphenyl methane-4,4' diisocyanate (MDI-isocyanate prepolymers) that combine bituminous membranes and polyurethane foams into one material are promising new materials. They are produced in a two-step process: firstly, a reactive modified bitumen is obtained by mixing a bitumen and a prepolymer, and, secondly, a bituminous foam is formed by a reaction of an excess of water with the remaining isocyanate groups. The prepolymer was formed from a polypropylene glycol (PPG) polymer of a molecular weight of about 2,000 g/mol and MDI. The main goal of this study was to investigate the effect that both free MDI (not bonded to the PPG) and polyol (soft segment) exert on the thermorheological behaviour of bituminous polyurethane materials. The results obtained show that free MDI molecules are mostly responsible of the improvement of the viscoelastic and viscous response at high in-service temperatures due to the chemical reaction that give rise to rigid polyurethane and polyurea rigid blocks. PPG molecules partially inhibit the abovementioned effect but improve the low-temperature properties (lower glass transition temperatures). In general, for the used polyol, a good balance of rigidity and flexibility can be obtained if enough amount of a prepolymer containing a molar ratio of MDI/PPG of 5 is used.