Introduction

Linear polyurethanes (PU) are polymers in which the principal chain structure is composed of aliphatic or aromatic sections R1 and R2 fi xed together with polar urethane groups (formula 1):

wherein R1 makes an aliphatic, aromatic or alicyclic radical derived from the isocyanate monomer, and R2 is a more complex group derived from the polyol component (polyether or polyester). The commercial manufacture of polyurethanes is nearly solely based on the polyaddition process which involves diisocyanates and bi-functional or poly-functional polyols with hydroxyl terminal groups (equation 2):

(2)

The possible presence of poly-functional components, e.g. triisocyanates obtained from trimerisation of isocyanate monomers, or the use of branched hydroxyl polyols, will yield polyurethanes with three-dimensional cross-linking.