Polyester polyols for elastic polyurethanes

Polyester polyols for elastic polyurethanes (PU) are low molecular weight (MW) polymers (1000-4000 daltons) characterised by the presence of ester groups and terminal hydroxyl groups. The general formula of a polyester polyol is [1-8]  

Polyester polyols are the second most important group of oligo-polyols for production of polyurethanes after polyether polyols. They represent around 18% of the polyols used globally in polyurethanes, corresponding to a total worldwide production of around 600,000 tones/year. About 60% of polyester polyols being produced in Europe [1]. 

Polyester polyols were the only polyols available to the polyurethanes industry at the beginning, but now, in spite of the leading position of polyethers in the total polyols usage in the polyurethane industry (around 80%), polyester polyols have stable and specific practical applications due to some superior characteristics of the resulting polyurethanes [1,2,3,6-11]. 

The superior characteristics of polyester polyol based polyurethanes are explained by a better crystalline structure [1, 7] in the urethane segment, compared to the majority of poly ether polyols which are amorphous [except polytetrahydrofuran (PTHF)], due to the superior secondary forces between the polyester chains [8] and also due to a superior thermal and fire resistance, compared to polyether polyol based polyurethanes. Polyester-based polyurethanes (flexible foams, coatings), have a superior solvent resistance compared to the polyether-based polyurethanes [8]. 

The most important segments of polyester polyol applications are those of polyurethane elastomers (43% of global polyester polyols consumption), flexible foams (15-18%), coatings, adhesives, rigid foams, synthetic leather, and sealants. 

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Table 8.4 presents the main characteristics of some representative polyester polyols for elastic polyurethanes. 

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