CONTROL OF HARD SEGMENT SIZE IN POLYURETHANE FORMATION

In general, segmented polyurethane elastomers are prepared by reacting an isocyanate-capped polyol prepolymer with a short-chain diol chain extender, yielding an elastomer with hard segments of uniform size. Ho wever, the hard segment size will not be uniform if the polyurethane p olymer is prepared by forming the hard segment first, followed by soft segment formation. Because the mechanical properties of polyurethane elastomers depend on the relative ratio of the hard to soft segments a s well as the effectiveness of the hard segment as a physical crosslin ker, the control of the size distribution of the hard segment is a key factor in designing polyurethane elastomers. It was found that reacti on conditions can affect the size distribution of hard segments derive d from an aliphatic diisocyanate with differential reactivity between the two isocyanate groups. Lower reaction temperatures and simultaneou s mixing of all reactants gave the preferred size distribution of hard segments. (C) 1995 John Wiley and Sons, Inc.