The raw materials for polyurethane synthesis are polyester polyols, typically produced by reacting organic polybasic acids with organic polyols. The production process mainly consists of five steps: pre-production confirmation, material charging, temperature increase, catalyst addition, and further temperature rise.

I. Process Overview: Polyester polyols are essential raw materials for the synthesis of polyurethane materials. As feedstock for polyester resins, these compounds—generally hydroxyl-containing polyester compounds—are also referred to as polyester polyols. In industrial applications, polyester polyols with lower molecular weights, typically ranging from 1,000 to 3,000, are commonly used in alkyd resins and unsaturated polyesters. Polyester polyols derived from adipic acid have a wide range of applications, including the production of soft and rigid polyurethane foams, rubber adhesives, synthetic leather, and coatings. By optimizing process conditions and experimenting with different raw material ratios, we have successfully synthesized adipic-acid-based polyester polyols with molecular weights around 1,500 to 2,000. Using these polyester polyols and employing the casting method, we can produce polyurethane materials with a broad spectrum of performance characteristics. After numerous explorations and improvements, we have refined the conventional nitrogen-dehydration process by identifying a specific equilibrium temperature, enhancing vacuum levels, and carefully controlling the appropriate holding time. These measures have significantly reduced reaction times, lowered energy consumption, achieved cost savings, and ensured consistent product quality.

2. Before production begins, confirm that the reactor is empty, the bottom valve is closed, and there is an adequate stock of raw materials—enough for the batch to be fed. Also confirm that the pipelines have been thoroughly purged. Pipeline purging is critical, as it can affect subsequent dehydration processes and may even impact the product’s color shade. Ensure that all valves on the feed lines to each reactor are closed, that the reactor pressure is normal, that the reactor agitator is functioning properly, and that the circulating cooling water for the agitator is operating normally. The bypass (vacuum line) SV valve and manual valve next to the distillation column should be closed; the vacuum control valve should also be closed, while the SV valve and manual valve on the distillation column should be open. The drainage pipeline should be fully open for drainage, and both the drainage pipeline drain valve and the drainage pipeline gas-phase equalization valve should be open. The recovered alcohol in the recovery tank has been discharged, and the valves have been restored to their normal production positions. The boiler’s temperature and pressure are within normal ranges, and the boiler system is in good working condition. The hot oil inlet and outlet valves are functioning properly.

3. After the feeding and purging are completed, open the drain line drain valve and the equalization valve, and then open the manhole to further confirm that the reactor is indeed empty. Begin adding the low-molecular-weight alcohol. First, record the flow rate indicated by the low-molecular-weight alcohol flowmeter. Make sure that the feed valves of all other reactors are closed, then open the hand valve of the reactor you intend to fill. Set up the program on the computer: first, reset the cumulative amount to zero, enter the exact quantity of alcohol to be added, and verify that the program matches the order requirements. Start the feeding procedure on the standalone computer. First, check whether the pump in the filling area has started. In winter, when the weather is cold, the pump might fail during the feeding process, which would halt the feeding operation and cause significant trouble, disrupting production. Next, check the instantaneous flow rate—it should be between 6 and 8 m²/h. Also, monitor the alcohol flow for any air bubbles; if bubbles persist for more than 30 minutes, stop the current procedure immediately, release the material, and recalculate before resuming feeding. After completing the feeding, close the hand valve, record the data, and double-check to ensure that the amount fed matches the order requirements. If the amount is insufficient, promptly add more. Check the liquid level inside the reactor. Weigh out 14 grams of OB-1 and pour it into the reactor—this is mainly for whitening purposes. Be careful not to add too much, as excessive amounts could affect the color shade of the material. Start stirring and stir for about 30 minutes. Then, add the adipic acid. Confirm from the order which type of adipic acid needs to be added. Before feeding, count the adipic acid again to make sure you have enough. If you don't have sufficient quantities, do not proceed with the addition and contact your supervisor immediately. To add the adipic acid: Place the funnel over the reactor, use a hoist to lift the bag of adipic acid onto the funnel, and during the hoist’s movement, avoid letting the bag swing excessively. Use a knife to cut the bottom of the bag. To ensure product quality and meet the required color standards, never add adipic acid from bags that appear dirty. After adding the adipic acid, securely tie up the bag and double-check the amount you’ve added to prevent overfeeding or underfeeding, which could impact subsequent production steps. Finally, seal the opening, put the manhole cover back in place, and tighten the screws diagonally. Increase the pressure gradually until it reaches 0.015 MPa, then shut off the nitrogen supply. Perform an air-tightness test using soapy water. Check for any leaks—if there are leaks, verify that the connections were properly tightened and retighten them if necessary. If the gasket seems worn or improperly positioned, replace it promptly. Air-tightness is extremely important; poor air-tightness could cause the material inside the reactor to spray out during heating, seriously disrupting production.

4. Heating and Temperature Raising: First, use steam to raise the temperature. The purpose is that the steam has a relatively low temperature, which allows the adipic acid in the reactor to melt completely without burning or sticking to the heating tubes. Once the temperature reaches 128℃ and you see water flowing through the drain sight glass—indicating that the material inside the reactor has melted—turn off the steam supply. Then, check whether the pressure of the hot oil is within the proper range; typically, it should be around 0.15 MPa. Next, start the hot oil circulation and set the program to automatically maintain the temperature at 128℃. The upper temperature limit should be adjusted according to the specific material being reacted. After maintaining this temperature for one hour, proceed to the next step. Dehydration and Temperature Raising: Activate the dehydration and temperature-raising program. During this phase, pay close attention to the temperature at the top of the distillation column—it must remain within the specified upper and lower limits. If the temperature exceeds these limits, alcohol from the reactor could vaporize and escape, affecting the acid value. Therefore, during dehydration, regularly inspect the system: monitor the flow of liquid in the sight glass and check the pressure indicated on the process diagram. Generally, the maximum pressure should be around 0.001 MPa. If the pressure fluctuates during the heating process, it could indicate a blockage in the pipeline; in such cases, use steam to clear the blockage. When the temperature becomes too high, you can introduce nitrogen gas at the top of the reactor—this is a temporary solution. Additionally, appropriately reducing the heating rate can also help alleviate the phenomenon of foaming at the top of the column.

5. After the acid value of the catalyst has been confirmed to meet the specifications, add an appropriate amount of catalyst according to the grade of the material. Adding too much or too little catalyst will both affect the reaction and result in a substandard product. During this process, discard the recycled alcohol produced from the previous reactor batch. It will take about ten minutes to achieve vacuum; then, start the vacuum pump: Close the bottom valve of the vacuum pump’s buffer tank and close the top vent valve. Turn on the circulating water—but don’t set it too high; generally, open it to about one-third. Then, turn on the vacuum pump and check whether the current is normal and whether there are any issues with the pressure in the buffer tank.

6. After the cooling sample has been approved, begin the cooling process: Close the hot oil outlet valve, open the chilled oil outlet valve, open the chilled oil inlet valve, and start the chilled oil pump. Adjust the outlet temperature to 40°C, and monitor the pressure—typically around 0.2 MPa. Observe whether the hot oil temperatures in each reactor have changed. After cooling is complete, shut down the system.

VII. Conclusion

1. Extending the holding time—i.e., the high-temperature reaction time—can reduce the total amount of extractables, and the overall reaction time will also be extended accordingly.

2. Raising the temperature is beneficial to the polycondensation reaction of alcohols and acids; it can reduce the acid value before vacuum extraction, shorten the vacuum extraction time, increase the amount of water produced, and decrease the amount of substances extracted. However, the reaction temperature should not be too high either, as excessively high temperatures can easily lead to decomposition reactions and result in the loss of carboxyl groups.

3. The amount of acid used, whether in excess or insufficient, affects the acid value before and after vacuum extraction. In a simple alcohol-acid polycondensation reaction, the appropriate addition of organic acid anhydrides can help control the degree of branching. Increased water output leads to a shorter overall reaction time. Thus, adding organic acid anhydrides at a specific degree of branching has an impact on both the vacuum extraction time and the amount and temperature of water produced.

4. Theoretical Influence of Hydroxyl Value: The hydroxyl value has a significant impact on both polyester synthesis processes and polyurethane elastomers. When the hydroxyl value is high, the pre-vacuum acid value is low, and the amount of material subjected to vacuum treatment is greater. The actual hydroxyl value of the polyester increases as the theoretical calculated value rises. Both the vacuum time and the viscosity of the polyester are markedly influenced by the hydroxyl value, particularly when the degree of branching is 1/1. As the hydroxyl value increases, the vacuum time significantly decreases, and the viscosity of the polyester drops accordingly. When the degree of branching is 1/2, increasing the hydroxyl value also leads to a decrease in the water-evaporation temperature.