Overview

Polyether polyols for rigid polyurethane (PU) foams are low molecular weight (MW) adducts of propylene oxide (PO) [sometimes together with ethylene oxide (EO)] to polyols having 3-8 hydroxyl groups/mol or to polyamines having 2-3 amino groups/mol, the chain derived from one hydroxyl group being very short, around 0.5-2 alkylene oxide units. As mentioned previously, the hydroxyl number of these polyols is high, generally in the range 300 - 800 mg KOH/g [1-4] (rarely in the range 600-800 mg KOH/g). It is observed that the equivalent weight (EW) of polyether polyols for rigid foams is low, around 60 - 200, as compared with polyether polyols for flexible PU foams which have a much higher EW, of around 1000-2000.

The general synthesis reaction of polyether polyols for rigid PU foams by polymerisation of alkylene oxides (PO, EO) initiated by polyolic starters is presented in reaction 13.1.

(13.1)

The most important low molecular weight polyols used as starters for polyether polyols destined for rigid PU foams synthesis are: glycerol, trimethylolpropane (TMP), triethanolamine, pentaerythritol, dipentaerythritol, ?-methyl glucoside, xylitol, sorbitol and sucrose [1-27]. The main properties of these starter polyols, which are of interest for polyurethane chemistry, are presented in Table 13.1.