Polyester polyols are usually made by condensation (or transesterification) of organic dicarboxylic acids (anhydrides or esters) and polyols (including diols) or Lactone and polyol are polymerized. Dibasic acids include phthalic acid or phthalic anhydride or its esters, adipic acid, halogenated phthalic acid and the like. Polyols include ethylene glycol, propylene glycol, diethylene glycol, trimethylolpropane, and pentaerythritol. Different types of polyester polyols have different properties due to different types or preparation processes. The more important indicators for polyester polyols are hydroxyl value, acid value, moisture, viscosity, molecular weight, density and chroma, etc. .