No matter it is the prepolymer synthesis reaction or various other reactions for the synthesis of CPU, it is related to the acidity and alkalinity in the reaction system. Acid and basic impurities mainly act as catalysts. The acidic medium is conducive to the chain extension reaction to generate carbamate and urea; the alkaline medium promotes the branching and cross-linking reaction to generate self-polymers of allophanate, biuret and isocyanate, thereby increasing the viscosity of the prepolymer High and even gel quickly. Therefore, when preparing the prepolymer, the reaction must be carried out under acidic conditions.

The acidity and alkalinity in the system are mainly derived from the acid value in the polyester, the hydrolysis chlorine in the isocyanate, and the basic catalyst such as KOH in the PPG. Isocyanates are produced by the phosgenation method. Hydrolyzed chlorine is the main source of isocyanate acidity. The acidity of the isocyanate plays the same role as the acid value of the oligomer polyol, inhibiting the formation of allophanate and biuret. When using PPG to prepare prepolymers, in order to avoid the appearance of gel and make the casting mixture have a proper pot life, if necessary, a small amount of acid or acid chloride can be added to adjust the acidity of the system. Prepolymers are synthesized in an acidic state. For example, the temperature of oligomer polyols is controlled at 40-50°C. Generally, the reaction exothermic temperature rise will not exceed 70°C, while the reaction exothermic temperature rise can exceed 100°C in an alkaline state. And soon gel appeared. When the residual KOH concentration exceeds the acidity of TDI, the maximum exothermic temperature of the prepolymer can rise sharply from 50°C to 130°C. This shows that the prepolymer can be synthesized in a wide range of acidity, but a small amount of base can catalyze the gel. The effect of TDI acidity on the formation of prepolymers, it can be seen that the acidity of isocyanate protects the synthesis reaction of prepolymers.

When the content of hydrolyzed chlorine is 0.01%, the reaction is normal. When the content of hydrolyzed chlorine is less than 0.005%, the reaction rate is accelerated, and the prepolymer is easy to gel. When enough adipoyl chloride was added, the total hydrolyzed chlorine increased from 0.001% to 0.01%, and the reaction proceeded normally. The hydrolyzed chlorine in isocyanates is the main source of acidity. Hydrolyzed chlorine is not always related to the reactivity of the prepolymer, but acidity has always played a major role. In summary, neither the acidity of the oligomer polyol nor the acidity of the isocyanate (hydrolyzed chlorine) plays a decisive role in the formation of prepolymers. The decisive role is the combined effect of the two, namely prepolymers. The total acidity of the system.