Industrial Preparation of Prepolymer and Foaming Process – Polyester-Based Prepolymer

The preparation of polyester-based prepolymers is simpler than that of polyether-based prepolymers. It can be achieved by adding isocyanates to polyester in one step for the reaction. The viscosity of the prepolymer can be controlled by varying the reaction temperature and time.

Example of Polyester-Based Prepolymer Preparation:
Polyester is added to a dry reaction kettle equipped with stirring capabilities. Isocyanates are then added at room temperature while stirring. Since the reaction between isocyanates and polyester is exothermic, the rate of isocyanate addition must be controlled based on the batch size to ensure the reaction temperature does not exceed 90°C. Once the addition is complete, the reaction is maintained at 90°C for about 1 hour. After cooling, the product is placed into a dry container. It is recommended to continuously purge the system with dry nitrogen during production to remove moisture.

The amount of isocyanate added during prepolymer preparation is determined by the hydroxyl value of the polyester and the desired density of the foam. Due to the presence of trace amounts of water in the materials and system, the actual usage typically exceeds the theoretical amount. For foam densities ranging from 32 to 48 kg/m³, the relative amounts of isocyanate added per 100 parts of polyester are shown in the table below:

Relationship Between the Density of Polyester-Based Foam and the Amount of TDI in Prepolymer

If no external blowing agents are added, and only carbon dioxide generated from the reaction between water and isocyanates is used as the blowing agent, the free isocyanate content in the prepolymer is related to the foam density, as shown in the table below:

Relationship Between NCO Content in Polyester-Based Prepolymer and Foam Density

Foaming Process:
For small-scale laboratory foaming, manual foaming is commonly used. Prepolymer, water, catalyst, surfactant, and additives are added in one step. After high-speed stirring for 2–3 seconds, the mixture is poured into a mold for foaming.

In mechanical foaming, the components—prepolymer, water, catalyst, surfactant, and additives—are delivered proportionally to a high-speed mixer using metering pumps. After thorough high-speed mixing, the mixture is continuously dispensed onto a belt conveyor with liner paper. The foam is then sent into a curing oven to produce the final foam product after maturation.