Polyester Polyols: Raw Material Introduction

It is well-known that polyesters used for polyurethane synthesis differ from polyester plastics and polyester fibers. Polyester plastics and fibers are typically high-molecular-weight, high-melting-point, and highly crystalline polymers synthesized from terephthalic acid (or its esters) and ethylene glycol (or butylene glycol). In contrast, polyesters used in polyurethane materials are low-molecular-weight oligomeric polyols with ester groups (-COO-) in the main chain. These polyesters are hydroxyl-terminated, with molecular weights ranging from several hundred to several thousand and an average functionality between 2 and 3.

Polyester polyols are generally synthesized either by the condensation of organic dicarboxylic acids (or anhydrides or esters) with polyols (including diols) or by the polymerization of lactones with polyols.

The most commonly used polyester polyols in polyurethane products are aliphatic polyester polyols. However, as product performance requirements increase and raw material availability expands, aromatic polyester polyols are also increasingly used in the preparation of polyurethane foams, adhesives, and elastomers. Low hydroxyl-value polyester diols or slightly branched polyester diols are utilized for synthesizing polyester-based flexible polyurethane foams, while high hydroxyl-value, low-molecular-weight aromatic polyester polyols, along with fatty polyester polyols like rosin-based and palm oil-based polyols, are used for rigid polyurethane foams.

Conventional polyesters are obtained through the polycondensation of organic dicarboxylic acids (or their anhydrides or esters) with an excess of polyols (including diols).  

For foam synthesis, the dicarboxylic acid raw materials for polyester polyols include phthalic anhydride (PA), terephthalic acid (or dimethyl terephthalate), adipic acid, and halogenated phthalic acids. Common small-molecule polyols used are ethylene glycol (EG), diethylene glycol (DEG), 1,2-propylene glycol (PG), glycerol, 1,4-butanediol (BD), neopentyl glycol (NPG), dipropylene glycol (DPG), trimethylolpropane (TMP), and pentaerythritol.

In polyurethane foams, polyester polyols are required to have relatively low viscosity, and their costs must be controlled. Therefore, the most commonly used raw materials in polyurethane foam production are adipic acid, phthalic anhydride, diethylene glycol, ethylene glycol, and propylene glycol. Polyester polyols can also be prepared by combining two or more acids (or anhydrides) with two or more polyols.