Volume Rise During Flexible Urethane Foaming

Abstract

During the formation of flexible urethane foams, isocyanate and water react to form urea dimers and oligomers which eventually segregate as a microdisperse solid phase. The role that urea segregation plays on reaction kinetics and flowability during foaming is examined in this work. Temperature and volume rise profiles were measured for a series of foam systems by varying the amount of water (2-6 pphp), isocyanate (90, 110, 130 index), and amine or tin catalysts in the formulation . Model calculations showed that 84-90% conversion of isocyanate (NCO) groups was required to form a crosslinked polymer network. However, volume rise was found to stop at 40-70% NCO conversion. This indicates that gelation is due to phase separation rather than covalent crosslinking. A sudden acceleration in the rate of reaction (dT/dt), detected before foams reached a constant volume, is thought to indicate rnicrophase separation during foaming. A mechanism is presented to correlate temperature and volume rise profiles with urea microphase segregation during reactive urethane foaming. The blowing efficiency during foaming ranges between 55-82% as the concentration of isocyanate (90-130 index) or tin catalyst (0.2-0.4 pphp) is increased in the formulation, although it appears relatively independent of the water or amine catalyst levels used in this study. It was also found that addition of glycerine to the foaming system disrupted the urea microphase separation process, and increased the blowing efficiency to 85%. tcurrent address: