Special Polyether Polyols - Polyurea Polyols

Polyurea particles are dispersed in polyether through the reaction of diamines or hydrazine with diisocyanates in polyether polyols, forming polyurea polyols (also referred to as "PHD dispersions" in German). These polyurea polyols are a type of polymer-modified polyol used to enhance the load-bearing capacity of foam materials. During the formation of polyurea, some terminal isocyanate groups on the urea chains react with polyether hydroxyl groups, creating urea-urethane copolymers. Thus, polyurea-modified polyols contain three structural components: unmodified polyether, polyurea dispersions, and urea-urethane polymers.

The process to prepare polyurea polyols in situ via stepwise polymerization of hydrazine and TDI in polyether polyols involves heating polyols with high primary hydroxyl content, mixing them with an aqueous solution of hydrazine in a stirred reactor, adding TDI in an amount stoichiometrically equivalent to hydrazine, allowing the reaction mixture to reflux under the reaction heat, cooling the reactor, and using a vacuum to remove excess water. The solid content of polyurea polyols (proportion of TDI and hydrazine in the polyol) is typically 20%.

The polyether polyols most commonly used as the base for synthesizing polyurea polyols are highly active polyether polyols with a functionality of 3, a hydroxyl value around 34 mgKOH/g, and a primary hydroxyl molar fraction greater than 70%. Polyesters have also been reported as base polyols in the literature. Ethylenediamine, hexamethylenediamine, p-phenylenediamine, hydrazine, N,N'-dimethylhydrazine, and their hydrates can be used to synthesize polyurea polyols, though industrial production commonly uses hydrazine hydrate. To prevent sedimentation during storage due to the higher density of polyurea particles compared to polyether, a slight excess of isocyanate relative to amino groups can be used to generate additional urea-urethane copolymers, which stabilize the dispersion.

Polyurea polyols can be prepared using either batch or continuous methods. In the batch method, diamine is first dispersed in polyether polyols, and then isocyanate is slowly added under vigorous stirring. This method is time-consuming, challenging to control, and results in higher viscosity but is easier to implement for production. In the continuous method, isocyanates, polyamines, and polyols are pumped at a fixed ratio into a mixing chamber for rapid blending. The key to ensuring product quality is the thorough dispersion of diamines in the polyether and precise control over the extent of urethane reaction. The contact time between isocyanates and polyether before mixing with the diamine dispersion must be strictly controlled. Foreign industrial production of polyurea polyols primarily uses the continuous method, although it requires higher equipment investment.

Polyurea polyols are suitable for high-resilience soft foams, semi-rigid foams, flexible foams, and rigid foams. When used in high-resilience foam plastics, they provide similar benefits to polymer polyols, including improved foam system stability, open-cell formation, and enhanced foam load-bearing capacity. They also increase the initial gelation rate of the foam, reducing the required amount of catalyst, and give the foam flame-retardant properties, enabling the final product to achieve self-extinguishing flame resistance even without added flame retardants.

However, the high production cost of polyurea polyols has limited their application, resulting in relatively low production volume.

Another type of modified polyether polyol, called PIPA, is created by reacting polyether polyols with triethanolamine (TEA) at 20°C, then rapidly adding TDI to form polyurethane-modified polyether polyols. The reaction, catalyzed by dibutyltin dilaurate, completes within 3–5 minutes. The isocyanate amount added is generally lower than required to react with TEA hydroxyl groups, with a typical TEA/TDI ratio of approximately 1:1.