Effects of MDI–PPG molecular weight on the thermorheological behaviour of MDI–isocyanate based bituminous foams

Dimensionally stable bituminous foams were prepared by using 4,4′-diphenylmethane diisocyanate/polypropylene glycol (MDI–PPG) quasi-prepolymers, with different molecular weights. From the results obtained, it may be concluded that, after foam formation, the rheological properties are remarkably affected, especially at high temperatures. Paradoxically, the greater the molecular weight of MDI–PPG, the lower the values of viscosities at 60 °C, and the linear viscoelasticity functions of the resulting foams. This result seems to be related to the improved ability of small MDI–PPG molecules to interact strongly with polar bitumen compounds, leading to the formation of larger molecular-sized compounds.