4.1 Introduction

The term polyurethane covers a wide range of materials produced by the reaction of polyfunctional isocyanates with substances containing at least two hydroxyl groups. The main chemical moiety resulting from this reaction is the urethane group:

Other groups which may be formed include urea, amide, biuret, allophanate, ether and ester linkages, depending on reaction conditions and the particular catalysts used. Hence, the term polyurethane is a very general one covering materials having very different properties and including both expanded and non-expanded products. The materials discussed in this chapter are hard, expanded polyurethanes having low flexibility and a high load bearing capacity in relation to their density. Compression deflection curves of these materials show a pronounced yield point and hence the foams have a significant permanent set after compression. In contrast to the open cell structure of flexible polyurethane foams, rigid foams have a mainly closed cell structure. They are expanded using physical blowing agents which are retained in the closed cells and which have a significant influence on some of the foam properties, particularly thermal insulation.

Isocyanate-based rigid foams for insulation applications were introduced in the early 1950s and can be made by both one shot or prepolymer processes using either toluene di-isocyanate (TDI) or di-phenyl methane di-isocyanate (MDI). In the one shot process, isocyanate and polyol are reacted to provide the final foam in a single step. In the prepolymer process, some polyol and excess isocyanate are reacted to provide an isocyanate tipped prepolymer which...