Manufacturing Methods for Flexible Polyurethane Foam

The manufacturing methods for flexible polyurethane foam are classified into three types: the prepolymer method (also known as the two-step method), the semi-prepolymer method, and the one-shot method.

Prepolymer Method (Two-Step Method)

The prepolymer method divides foam production into two steps, hence the name "two-step method."

Step 1: Polymeric polyols (polyether or polyester with two or more terminal hydroxyl groups) react with diisocyanates to form low-molecular-weight polymers with terminal isocyanate groups (prepolymers).

Step 2: Water is added to the prepolymer under high-speed stirring. The reaction produces urea groups, forming a high polymer. During the reaction, carbon dioxide is generated, facilitating both chain extension and foaming, ultimately producing the foam material. Catalysts and surfactants are added to adjust the reaction rate and foam cell size as needed.

Semi-Prepolymer Method

In this method, part of the polymeric polyol (polyether or polyester with hydroxyl groups) reacts with an excess of diisocyanate to form a low-molecular-weight oligomer with a specific viscosity. The remaining polyol and water in the formulation are then added to the prepolymer. This mixture is stirred at high speed in the presence of catalysts and surfactants to produce foam.

This method is more commonly used for rigid or semi-rigid foams and is less frequently applied to flexible foams. Its primary purpose is to adjust the viscosity of the materials.

One-Shot Method

The one-shot method involves adding all components of the formulation—polymeric polyols, diisocyanates, and other additives—at once, followed by high-speed stirring and foaming. Initially used for polyester-based foams, this method has become widely adopted for polyether-based foams due to advancements in high-efficiency catalysts and silicone-based foam stabilizers.

The one-shot process offers simplicity, as raw materials can be used without preprocessing, and it results in superior product performance. Currently, this method dominates the production of flexible polyurethane foam in industrial applications.