Overview

The first polyols used for rigid polyurethane (PU) foams were low molecular weight polyesters based on adipic acid, phthalic anhydride (PA) and various glycols or polyols. One example of a polyester of this type is the polycondensation product between adipic acid (AA), PA and trimethylolpropane (TMP) [1-3].

Sometimes, this polyester is modified with oleic acid in order to improve its compatibility with blowing agents. The chemistry for the synthesis of rigid polyester polyols is absolutely the same as the chemistry for the synthesis of polyester polyols used in elastic PU, described in detail in the Chapter 8.

These polyester polyols were of minor importance and during the period 1960-1970, they were replaced rapidly by low cost, low viscosity and high functionality polyether polyols, which lead to superior physico-mechanical properties of the resulting rigid PU foams and superior miscibility with fluorocarbon blowing agents. The polyether polyols proved to be superior in rigid PU foam fabrication, when the isocyanate index was low, around 105-115. The characteristics of two representative polyester polyols for rigid PU foams based on AA, PA and TMP are presented in Table 16.1.