I. Reaction Principle
Polyester polyurethane is formed from the reaction between polyester and isocyanate. Polyester is produced through the polycondensation reaction of polyacids (such as adipic acid, phthalic acid, maleic acid, etc.) and polyols (such as ethylene glycol, propylene glycol, trimethylolpropane, etc.). It can be categorized into hydroxyl polyester and carboxyl polyester. The polyester used for polyurethane foam is hydroxyl polyester, where polyols are in excess.
Hydroxyl Polyester (excess polyols):
2OH-R-OH + HOOC-R’-COOH → HO-R-OCO-R’-COO-R-OH
Carboxyl Polyester (excess polyacids):
OH-R-OH + 2HOOC-R’-COOH → HOOC-R’-COO-R-OCO-R’-COOH
Polyether polyurethane is produced from the reaction between polyether polyols and isocyanates. Polyether polyols are obtained through the ring-opening polymerization of alkylene oxides (such as ethylene oxide, propylene oxide) with initiators containing active hydrogen (such as alcohols, amines).
Polyether Polyol:
R-OH + nPO → R-(-O-CH2-CH3-CH2-O)n-H
Both types of polyurethane are ultimately formed by the reaction between hydroxyl groups and isocyanates to produce urethanes.
R-NCO + R’-OH → RNHCOOR’
Therefore, from the reaction mechanism perspective, the primary difference between the two types of polyurethane is whether the soft segment molecular chains mainly contain ester bonds (-COO-) or ether bonds (-C-O-C-).
II. Reaction Result
Polyester polyurethane exhibits high mechanical strength and excellent oil and heat resistance. Thus, it is mainly used in microcellular foam shoe soles, elastomers, coatings, and synthetic leather. However, due to the presence of ester bonds (unsaturated double bonds), polyester polyurethane is less stable against hydrolysis, low temperatures, oxidation, acids, and bases compared to polyether polyurethane.
III. Development Trend
Due to the high viscosity of polyester polyols, poor miscibility with other components, difficulty in processing, and high raw material costs, their application in the polyurethane field is limited. Conversely, polyether polyurethane finds extensive use, primarily in the synthesis of foam plastics.
In recent years, to improve the processing performance of polyester polyols, alkylene oxide compounds (such as PO, EO) have been introduced into the polyester polyol molecules. This results in soft segment molecular chains containing both ester bonds (-COO-) and ether bonds (-C-O-C-). Polyurethanes made from these new polyols possess characteristics of both polyester and polyether polyurethanes.
