What are the differences in reaction principles and results between polyester-based polyurethane and polyether-based polyurethane?

1. Reaction Principles

Polyester-based polyurethane is obtained by the reaction of polyester and isocyanate. The polyester is synthesized by a condensation reaction of polyfunctional acids (such as adipic acid, phthalic acid, succinic acid, etc.) and polyols (such as ethylene glycol, propylene glycol, trimethylolpropane, etc.). Polyester can be divided into hydroxyl polyester and carboxyl polyester. The polyurethane foam is made using hydroxyl polyester with an excess of polyols.

Hydroxyl polyester (excess polyol): 2OH-R-OH + HOOC-R'-COOH → HO-R-OCO-R'-COO-R-OH

Carboxyl polyester (excess polyacid): OH-R-OH + 2HOOC-R'-COOH → HOOC-R'-COO-R-OCO-R'-COOH

Polyether-based polyurethane is obtained by the reaction of polyether polyols and isocyanate. The polyether polyols are obtained by ring-opening polymerization of oxirane compounds (such as ethylene oxide, propylene oxide) using initiators containing active hydrogen (such as alcohols, amines).

Polyether polyols: R-OH + nPO → R-(-O-CH-CH3-CH2-O)n-H

Both types of polyurethanes are finally formed by the reaction of hydroxyl groups with isocyanates to produce urethane groups:

R-NCO + R'-OH → RNHCOOR'

So, the major difference in the reaction mechanism between the two types of polyurethane lies in whether the soft segment molecular chain mainly contains ester bonds (-COO-) or ether bonds (-C-O-C-).

2. Reaction Results

Polyester-based polyurethane has high mechanical strength, good oil resistance, and heat resistance. Therefore, it is mainly used in microporous foam shoe soles, elastomers, coatings, and synthetic leather. However, due to the presence of ester bonds (unsaturated double bonds), polyester-based polyurethane is not as stable in terms of hydrolysis resistance, low-temperature resistance, oxidation resistance, acid resistance, and alkali resistance as polyether-based polyurethane.

3. Development Trends

Due to the high viscosity of polyester polyols, poor compatibility with other components, and difficulty in construction, coupled with high raw material costs, its application in the field of polyurethanes is limited. On the other hand, polyether-based polyurethane has a wide range of applications, mainly in synthetic foam plastics.

In recent years, to improve the processability of polyester polyols, oxirane compounds (such as PO/EO) have been introduced into the polyester polyol molecules. This results in soft segment molecular chains that contain both ester bonds (-COO-) and ether bonds (-C-O-C-). Polyurethanes made from these new polyols have characteristics of both polyester-based and polyether-based polyurethanes.