Conceptual Volume Expansion During the Reaction of Polyurethanes

Abstract

The free foaming of polyurethane is presented as the expansion of gas bubbles in a fluid. The expansion process is described by mechanical stress on the bubble walls, which have undergone a strainflow. A modification of Newton's flow law for viscous fluids, whereby the strain rate is proportional to the stress, is used.

In an extension of Newton's flow law the proportionality factor is assumed to be not a constant but, rather, a time-dependent quantity. This takes into account the changing viscosity during the foaming process.

Power law approaches for the time dependencies of the fluidity (reciprocal value of the viscosity) and the excess pressure of the bubble leads to results which reflect the measured rising curves quite well.