Production of Polyester Polyols - Vacuum Melting Method

Process Overview

The measured amounts of dicarboxylic acid and diol are added to a reaction kettle and heated under a nitrogen atmosphere. The reaction begins at 140–160°C, producing low molecular weight polyester polyols and by-product water. The water is gradually distilled out through a distillation column. As the water is removed, the temperature in the kettle gradually increases. At 170–230°C, the vacuum pressure is progressively reduced to about 500 Pa, evaporating excess diols and small amounts of by-products (low-molecular-weight polyesters, aldehydes, and ketones) along with residual water.

In China, polyester polyols for polyurethane resins are typically produced using the vacuum melting method, which is suitable for small- to medium-scale manufacturers. This method involves low equipment investment, supports smaller production scales, and accommodates a variety of products. Manufacturers using polyester polyols can produce them in-house to reduce product costs.

Equipment

The vacuum melting method employs equipment including a reaction kettle (heated by thermal oil or electricity), a mechanical-seal agitator, a distillation column, a condenser, a water receiver, a buffer tank, and a vacuum pump. To avoid trace metal contamination in the polyester polyols, equipment materials must have high corrosion resistance to acidic reaction media and distillates. Key equipment is typically made from stainless steel.

Operation Procedure

Initial Setup:

Add the dicarboxylic acid to the reaction kettle, followed by the diol.

Secure the feed cap and heat the mixture.

Once the reaction materials have melted, start the agitator and add a suitable amount of catalyst.

Introduce nitrogen gas. When the kettle temperature reaches approximately 150°C, water begins to distill out.

Water Removal:

Maintain the top temperature of the distillation column at 100–102°C.

Continue heating the reaction kettle until the temperature reaches around 220°C. By this point, the amount of water distilled typically matches the theoretical yield.

Depending on the size of the reaction kettle, pipe diameters, and distillation efficiency, water removal takes 5–12 hours.x

Post-Water Removal:

Maintain the kettle temperature for 2 hours before stopping the nitrogen flow.

Sample and analyze the acid value (typically 20–30 mg KOH/g).

Vacuum Application:

Begin stepwise vacuum extraction, gradually increasing vacuum pressure:

-0.02 MPa for 0.5 hours.

-0.04 MPa for 1 hour.

-0.06 MPa for 1 hour.

-0.08 MPa for 1 hour.

Sample and analyze acid and hydroxyl values.

High Vacuum Polymerization:

Increase the vacuum to -0.099 to -0.1 MPa while maintaining the kettle temperature at 220–230°C.

Depending on the required molecular weight of the polyester polyols, high vacuum polymerization lasts 2–6 hours.

During this phase, the molecular weight of the polyester increases rapidly. Monitor acid and hydroxyl values continuously until the target values are achieved, and then discharge the product.

Catalyst Use

Adding a catalyst can shorten the polycondensation reaction time, facilitating industrial production. However, catalysts are challenging to remove from the polyester, potentially adversely affecting subsequent polyurethane reactions. The catalyst should be used sparingly to minimize these effects. Antimony trioxide and organic titanium compounds are commonly used as condensation catalysts.

Adjustments in Feed Ratios

During both atmospheric and vacuum water removal stages, a small amount of diol may evaporate with the water or be distilled during the vacuum stage. Therefore, when calculating feed ratios, an appropriate excess of diol should be added based on the reaction kettle and process parameters.