Alan K. Schrock

Modeling Mechanical Properties of Segmented Polyurethanes

We review recent developments in modeling mechanical properties of segmented polyurethanes. Polyurethanes, poly(urethaneureas), polyureas, and polyisocyanurate materials are widely used in a variety of industries due to the versatility of their properties. We concentrate on segmented polyurethane elastomers and discuss the relationship between formulation variables, polymer morphology, and polymer mechanical properties (Young’s modulus, tensile stress-strain curves, etc.). Specifically, we show how one can derive predictive structure—property relationships by combining thermodynamic block copolymer theory with micromechanical models. The resulting predictions for Young’s modulus and stress-strain curves compare favorably with

Volume Rise During Flexible Urethane Foaming

ABSTRACTDuring 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 microphase separation during foaming. A mechanism is presented to correlate temperature and volume rise profiles w...