Polymer Morphology of Water-Blown Rigid Polyurethane Foams: Development of New Polyols

Unlike elastomers and flexible foams, rigid polyurethane foam polymers do not exhibit a regular hard--soft domain structure. Instead, a rather irregular "fractal" morphology is found. The latter seems to depend on mutual differences in reactivity between various polyol constituents and water. It is evident that urea formation occurs early during foam rise, followed by and in competition with urethane formation. High equivalent weight polyols of low viscosity act as diluent during the initial stage of the foaming reaction. In the final network, this type of polyol participates only marginally. For practical applications, careful optimisation of polyols is demanded to achieve maximum polymer strength without exceeding the usual polymeric MDI requirements. An example shows this to be entirely feasible.