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The Influence of Production Processes on Polyester Polyol Synthesis

Polyester polyols, as a crucial component of polyurethane materials, directly influence the quality of the final product. In polyurethane synthesis, the molecular weight distribution, functionality, and acid value of polyester polyols have a significant impact on the hardness, elasticity, and weatherability of the polyurethane.

Therefore, in-depth research into the effects of synthesis processes on polyester polyol properties is essential for optimizing the performance of polyurethane materials and expanding their applications in various fields.

Polyester Polyol Synthesis Processes

The synthesis processes of polyester polyols can vary depending on the specific grade, with different operational procedures and preparation conditions. However, they all essentially involve a polycondensation reaction. Polycondensation processes can be broadly categorized into two types: solvent azeotropic dehydration and melt polycondensation.

Solvent Azeotropic Dehydration

In this process, toluene (or xylene) is added to the polycondensation reaction. The azeotropic behavior of toluene and water allows the removal of water produced during the reaction, ensuring the continuation of the polycondensation process.

The advantages of this method include a relatively stable reaction, lower temperature, ease of operation, and no need for fractional distillation equipment, resulting in a better product color. However, a water-separating reflux device is required, and the introduction of solvents like toluene makes the polycondensation process more susceptible to explosion hazards, complicating operations and increasing production costs.

Melt Polycondensation

This method is characterized by its simplicity, requiring only the addition of raw materials without the need for any additional components. By leveraging the boiling point differences between substances and introducing inert gases (or creating a vacuum), the water generated during the reaction can be efficiently removed.

Due to its simple equipment and short processing time, melt polycondensation has become the primary method for producing polyester polyols.

Main Steps of the Melt Polycondensation

  • Raw Material Proportioning: The acid-to-alcohol ratio in the reaction is typically 1:1.12. Specifically, the ratio of adipic acid to ethylene glycol and glycerol is 1:2:0.223.
  • Equipment: A stainless steel polymerization kettle is used, heated by electrical induction. A thermometer is installed at the top outlet of the condenser to control the temperature of the effluent, which is usually maintained at around 120°C to ensure the removal of generated water and reflux of volatile lower alcohols.
  • Reaction Process: The raw materials are sequentially added to the reactor, and the system is purged with nitrogen to remove air. The temperature is then raised to 134°C, and stirring and water removal are initiated. Over the next 6 hours, the temperature is increased to 225°C.
  • Monitoring and Sampling: At 220°C, samples are taken for analysis. If the acid value is around 40, vacuum dehydration can begin. The entire dehydration process takes approximately 4 hours.
  • Final Product: When the sample’s acid value is below 3 and the molecular weight is within the specified range, the product can be discharged.

The Primary Influencing Factor in Polyester Polyol Synthesis

1. Reaction Temperature

The higher the temperature, the faster the acid value decreases. In the first four hours of the reaction, a higher temperature leads to a faster decrease in acid value (which indicates the acidity of the polyester). However, as the reaction proceeds, the rate of decrease in acid value slows down in the higher temperature system. Generally, controlling the temperature at 250°C is more appropriate.

2. The Ratio of Alcohol to Acid

The amount of alcohol used is typically in excess of the theoretical value. The actual ratio of alcohol to acid used is usually 1.10 to 1.25 times the theoretically calculated value. When the ratio is 1.164, the actual value is closest to the theoretical value. This is because when synthesizing polyester, a slightly excess amount of alcohol is needed to ensure that the final product reaches the desired molecular weight.

3. Vacuum Degree

Increasing the vacuum degree is beneficial to the reaction. Enhancing the vacuum during the reaction can strengthen polyester synthesis. However, it is not advisable to use a very high vacuum initially, as it may remove small molecule polymers. A gradual increase in vacuum degree is usually adopted. When the vacuum degree is greater than 0.09 MPa, the hydroxyl value (indicating the number of hydroxyl groups in the polyester) decreases slowly, while when it is less than 0.09 MPa, the acid value decreases more rapidly.

4. Effluent Rate/Water Removal Rate

An excessive effluent rate can lead to losses. If water is removed too quickly, it may cause the loss of diol. When the effluent rate exceeds 65 ml/h, there is a significant difference between the actual and theoretical molecular weight. The ideal effluent rate is around 50 ml/h, which allows for better control of the final product quality.

5. Catalyst Dosage/Amount

Catalysts accelerate reactions. Under the same reaction conditions, as the amount of catalyst added increases, the molecular weight gradually increases. However, this does not mean that the catalyst determines the molecular weight itself. Rather, increasing the catalyst amount accelerates the reaction and shortens the reaction time, thus reducing both the acid value and hydroxyl value, which ultimately leads to an increase in the polyester’s molecular weight. When the catalyst amount is between 0.01% and 0.03%, the catalytic effect is not significant. When it is greater than 0.05%, side reactions increase. Using 0.04% is considered more appropriate.

While these parameters are widely recognized as effective in literature, they still require adjustment and optimization based on specific circumstances in practical applications. Different production environments, raw material characteristics, and equipment conditions can all affect the final results of polyester polyol synthesis. Therefore, flexibility and experimental validation are crucial in actual production.

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