The influence of viscosity buildup on the foaming dynamics of rigid polyurethane foams

The viscoelastic properties development during the reactive foaming of four standard polyurethane formulations was monitored using a flooded parallel plate rheometer. The measurements in situ demonstrated how foams of low catalyst content took longer than 10 min to increase their viscosity from 1 to 10³ Pa s, while foams with higher catalysts reached this modulus in less than 4 min. This polymerization speed deeply impacted the evolution of the cellular structure during foaming, which was demonstrated through in situ x-ray imaging and computer simulation. Foams with high catalyst stabilized in their final cell size in 4–5 min, concurring with the stage of largest viscosity buildup. In contrast, the foams of low catalyst content underwent strong degeneration as a result of the slow gelification and stiffening of the matrix. Simulation results also revealed that stabilization of the cellular structure happens faster when the catalyst and blowing agent contents are increased.