Production of Polyether-Based Prepolymers and Their Foaming Process

In industrial applications, the manufacturing methods for prepolymers of various grades differ based on the requirements of the foaming process and the final product performance. In addition to variations in raw materials and formulations, the manufacturing techniques for polyether-based prepolymers generally fall into three distinct types: biuret-branching, carbamate-branching, and urethane-branching.

(1) Biuret-Branching Prepolymers

This method involves adding a specific amount of water (e.g., 0.4%) to polypropylene glycol, which reacts with isocyanate to form urea groups. The reaction of isocyanate with active hydrogen then creates biuret branches, enhancing the gel strength during foaming.

A typical formulation involves mixing 100 parts of difunctional polyether with an appropriate amount of water to achieve a total water content of 0.4%. The mixture is heated at 35–40°C for 30 minutes. Subsequently, the first batch of TDI (toluene diisocyanate) is added based on the hydroxyl value of the polyether and water content, maintaining a molar ratio of NCO/OH at 1.25/1 and NCO/H₂O at 1.0/1.0. After mixing, the exothermic reaction occurs. Once complete, the temperature is increased to 120°C at a rate of 2°C/min and maintained for 90 minutes.

After cooling to 80°C, the second batch of TDI is added, with the quantity determined by the NCO content of the initial reaction product and the target NCO content of the prepolymer for foaming (generally about 25 parts, resulting in a final NCO content of approximately 9.5%). After thorough stirring, the prepolymer is cooled to 40°C, packaged, and sealed.

(2) Carbamate-Branching Prepolymers

In this method, isocyanates react with the active hydrogen of urethane segments on the main chain to form carbamate branches.

A typical preparation involves mixing 100 parts of polypropylene glycol with water (adjusted to a water content of 0.15%) at 35–40°C for 30 minutes. The first batch of TDI is added to achieve NCO/OH at 1.5/1.0 and NCO/H₂O at 1.0/1.0. After the exothermic reaction, the temperature is increased to 120°C and maintained for 2 hours. The second batch of TDI (approximately 25 parts, yielding a final NCO content of 9.5%) is then added and reacted at 120°C for 1 hour. The product is cooled to 40°C to obtain the carbamate-branching prepolymer.

Foams made from this type of prepolymer exhibit good physical properties. However, the gel strength of the foam during foaming is inferior to that of biuret-branching prepolymers.

(3) Urethane-Branching Prepolymers

This method introduces a small amount of multifunctional (tri- or higher functional) polyether and polyisocyanates into difunctional polyether to form crosslinks via urethane bonds. This crosslinking increases the foam's gel strength during foaming.

These fully urethane-branching prepolymers can employ high-molecular-weight, low-molecular-weight, neutral, or alkaline polyethers as crosslinking agents.

This approach enhances the foam's structural stability and gel strength, making it suitable for applications requiring robust foams.