Polyurethane virtually crosslinked

If the diol chain extender is used in exact molar proportion to the unreacted isocyanate, then a linear polyurethane elastomer is obtained. The resulting thermoplastic elastomer may be extruded or injection-moulded, and the properties arise from the ability of the hard and soft segments to form semi-crystalline domains that act as virtual crosslinks in the polymer and give it elastomeric properties, as shown in Figure 1.17. 

V. Thomas, M. Jayabalan, Studies on the effect of virtual crosslinking on the hydrolytic stabiUty of novel aliphatic polyurethane ureas for blood contact appUcations, J. Biomed. Mater. Res. 56 (1) (2001) 144-157. 

Of the available polyurethane types, the virtually crosslinked thermoplastic polyurethane elastomers would seem to be best and ideally suited for supply and use in powdered form. Their uses include adhesive applications which exploit the advantages of the powder form. 

The concept of physical or virtual crosslinks was first introduced by Scollen-berger et al. [2,3] when they demonstrated that hnear segmented ester-based thermoplastic polyurethanes exhibited high extensibility and elasticity in a non-vulcanized form. These materials behaved as thermoplastic elastomers (TPE), as opposed to chemically crosslinked elastomers, such as vulcanized natural rubber, which cannot be thermally reprocessed. 

Scollenberger C S, Scott H and Moore G R (1962) Polyurethane VG, virtually crosslinked elastomer. Rubber Chem Tech 35 742-752. 

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