In industry, the main production of polyester polyols is batch process.
In the first stage, the polyols (ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, 1,4-butanediol, etc.) are reacted with the diacids (phthalic acid, adipic acid, halogenated phthalic acid, etc.) or anhydrides (phthalic anhydride, etc.) at 140 - 200℃ through esterification and polycondensation reactions. The temperature at the top of the fractionation tower is controlled at 100 - 102℃. After stripping away most of the by-product water at normal pressure, the reaction is maintained at 200 - 230℃ for 1 - 2 hours. At this point, the acid value has generally decreased to 20 - 30 mg KOH/g.
In the second stage, vacuum is applied, and the vacuum degree is gradually increased. The trace water and excess polyols are removed 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". Alternatively, inert gases such as nitrogen can be continuously introduced to remove the water, known as the "carrier gas melting method". In the reaction system, a co-boiling solvent such as toluene can be added, and the water generated is slowly removed using a water separator during the reflux of toluene. This method is called the "co-boiling distillation method".
Different varieties of polyester polyols have different properties due to their different types or preparation processes. The several important indicators for polyester polyols are the hydroxyl value, acid value, moisture content, viscosity, molecular weight, density, and color.
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