Preparation method of polyester polyols

The preparation of polyester polyols adopts an intermittent method.
In the first stage, polyols (ethylene glycol, propylene glycol, diethylene glycol, trimethylolpropane, pentaerythritol, 1,4-butanediol, etc.) are subjected to esterification and condensation reactions with binary acids (phthalic acid, adipic acid, halogenated phthalic acid, etc.) or anhydrides (phthalic anhydride, etc.) at 140-200 ℃. The top temperature of the fractionator is controlled at 100-102 ℃. After removing the majority of the generated by-product water under atmospheric pressure, it is kept at 200-230 ℃ for 1-2 hours, At this point, the acid value has generally decreased to 20-30mg KOH/g.
The second stage involves vacuuming and gradually increasing the vacuum degree to remove trace amounts of water and excess glycol compounds under reduced pressure, allowing the reaction to proceed in the direction of generating low acid value polyester polyols, which can be called the "vacuum melting method". Inert gases such as nitrogen can also be continuously introduced to carry water, known as the "carrier gas melting method". Alternatively, azeotropic solvents such as toluene can be added to the reaction system, and the resulting water can be slowly carried out by a water separator during toluene reflux. This method is called "azeotropic distillation".
Due to different types or preparation processes, different types of polyester polyols have different properties. The important indicators for polyester polyols are hydroxyl value, acid value, moisture, viscosity, molecular weight, density, and chromaticity.