Polyester polyols, which are organic substances, are usually formed by condensation (or transesterification) of an organic dicarboxylic acid (anhydride or ester) with a polyol (including a diol) or by polymerization of a lactone with a polyol. The dibasic acid is phthalic acid or phthalic anhydride or an ester thereof, adipic acid, halogenated phthalic acid or the like. The polyhydric alcohol is ethylene glycol, propylene glycol, diethylene glycol, trimethylolpropane, pentaerythritol or the like. Different types of polyester polyols have different properties due to different types or different preparation processes. Several indicators that are important for polyester polyols are hydroxyl number, acid value, moisture, viscosity, molecular weight, density, and color. Characteristics and uses of polyester polyols: Polyester-based polyurethane contains a large number of polar groups such as ester groups and amino groups in the molecule, and has strong cohesive strength and adhesion, and