Production of General Polyether Polyol: Polymerization Stage

The ring-opening polymerization of epoxides is an exothermic process. The heat released during the polymerization of propylene oxide is approximately 1500 kJ/kg, and for ethylene oxide, it is around 2100 kJ/kg. Therefore, the structure of the polymerization autoclave must have good heating/cooling capabilities to ensure the timely removal of reaction heat during the polymerization of the olefin oxides. Typically, an external jacket and internal serpentine coil cooling system are used. The autoclave is equipped with a propelling agitator, a nitrogen inlet pipe, and a raw material feeding pipe. The autoclave material is stainless steel or carbon steel, and the volume of polymerization vessels in industrial large-scale polyether facilities ranges from 10 to 90 cubic meters. The equipment can withstand pressures of 1.5 to 20 MPa.

Additionally, oxygen can inhibit the polymerization and cause oxidation of olefin oxides, so it is generally necessary to evacuate the polymerization vessel and replace the air with dry nitrogen before the reaction. Nitrogen purging can be done three times to ensure the reaction occurs in an oxygen-free or nitrogen atmosphere. During the reaction process, it is also crucial to prevent oxygen from entering.

Currently, batch processing is the primary method for producing polyether, with fewer applications of continuous process technology.

1.Preparation of the Catalytic System

The initiator and catalyst are melted together in a stainless steel reaction vessel at atmospheric pressure, with a melting temperature of 80-90°C. Under vacuum, dehydration occurs at 110-120°C and 1330 Pa for 1-2 hours, forming a potassium alcoholate solution. The dehydration process is crucial because water can act as an initiator, producing bifunctional polyethers. In synthesizing long-chain triol polyethers for soft foam and polyfunctional polyethers for rigid foam, the presence of water can reduce the average functionality of the polyether below 3, affecting the performance of foam products.

2.Polymerization Reaction

Generally, in laboratory setups and small-scale production, after the initiator and catalyst are dehydrated in the autoclave, a one-shot method is used, where the required monomers like propylene oxide are added at once. The vessel is then evacuated, nitrogen is introduced, and the temperature is raised with stirring for the polymerization reaction. This method is simple, has a short polymerization time, and produces stable product quality. However, due to the high polymerization heat and pressure, the reaction can be challenging to control, requiring more stringent structural requirements for the autoclave. If the heat removal is not timely, the reaction rate may become too fast, leading to concentrated heat and the potential for runaway polymerization.

For the one-shot polymerization method in industrial operations, two main types are classified based on temperature and pressure variations: isothermal operation and isobaric operation. In isothermal operation, the reaction temperature is strictly controlled to remain constant, while the system pressure changes as the reaction progresses. As the product's molecular weight increases and monomer concentration decreases towards the end of polymerization, the system pressure drops, slowing the reaction rate, and thus the reaction time increases. The main advantage of isothermal operation is that the resulting polyether polyol has a uniform molecular weight distribution, low unsaturated double bond content, and light color. Isobaric operation involves raising the material temperature to maintain constant pressure, even when system pressure begins to decrease, thereby shortening reaction time and increasing equipment capacity. However, the product quality is not as good as with the isothermal method.

For safe operation in production and to prevent excessive heat from causing runaway polymerization, batch feeding and continuous feeding methods are used.

Batch Feeding Method

This method involves adding the propylene oxide monomer in multiple batches. Initially, a portion of the monomer is added, and as the reaction progresses and the pressure starts to decrease, the remaining monomers are gradually added. The advantages of this method include safer polymerization operation, timely removal of reaction heat, and the ability to use higher reaction temperatures. However, it has the disadvantages of longer polymerization time and less stable product quality with a broader molecular weight distribution. This method is suitable for large-scale industrial batch production, particularly for initiators that are solids at room temperature (e.g., pentaerythritol, mannitol, sucrose) and low molecular weight polyether polyols. For instance, the preparation of sucrose polyoxypropylene ether uses this method. Initially, sucrose and glycerol, along with potassium hydroxide, are added to the vessel, heated, stirred, and dehydrated. The propylene oxide is then added in 4-5 batches. This ensures smooth polymerization and high-quality products. The batch process's features include simple equipment, the ability to produce multiple varieties with the same equipment, stable product quality, and high yield.

Continuous Feeding Method

This method is suitable for producing high molecular weight polyether polyols, such as copolyether triols with molecular weights ranging from 3000 to 6000. For high molecular weight polyethers, the amount of olefin oxide used far exceeds the initiator amount by 40-50 times. Without continuous feeding, there is a risk of runaway polymerization. The key to continuous feeding is to use a metering pump or nitrogen pressure to continuously feed the olefin oxide monomers into the polymerization vessel, ensuring that the feed rate matches the polymerization rate. This maintains a steady state in the vessel, with the reaction conditions influenced by the type of initiator, the specifications of the propylene oxide, the equipment structure, and the product type. Generally, the reaction temperature ranges from 80-120°C, and the pressure from 0.3-0.8 MPa. The continuous method is suitable for large-scale, single-product production.

Modifying polyoxypropylene ethers with ethylene oxide can increase the primary hydroxyl group content at the polyether's end. This is typically achieved through a batch process, where ethylene oxide is added toward the end of the propylene oxide polymerization to continue the reaction. The continuous polymerization principle is similar to the batch method, requiring two olefin oxide continuous polymerization towers.