The main factors affecting the quality of prepolymers is whether high-quality raw materials are used to synthesize products. For oligomer polyols, the indicators that should be controlled include hydroxyl value, acid value, moisture, metal impurities, color and unsaturation (PPG), etc. For polyisocyanates, the indicators that should be controlled include purity, acidity, color, hydrolyzed chlorine, and TDI isomer ratio. In addition, reaction equipment (such as container material, stirring), reaction conditions (reaction temperature and time) and environmental humidity all have an important impact on the quality of the prepolymer. The hydroxyl value of oligomer polyol and the NCO group content of isocyanate are the main basis for formulation design and calculation. The following will mainly discuss the influence of the reaction system moisture, pH, metal impurities, PPG unsaturation, reaction temperature and time, stirring, packaging and storage conditions on the quality of the prepolymer.

Water mainly affects the viscosity, storage stability and properties of the final product of the prepolymer. The source of moisture includes the water contained in the oligomer polyol, the dryness of the reaction vessel, and the humidity of the air. Water can have two effects: the formation of urea groups increases the viscosity of the prepolymer. Using urea groups as branching points can further react with isocyanate groups to form biuret branches or crosslinks, which reduces the storage stability of the prepolymer or even gels. The prepolymer has high viscosity, is not easy to defoam, and has poor fluidity. It is not easy to mix evenly with the chain extender, and it also affects the performance of the product. The effect of water content on the viscosity of polypropylene glycol (PPG)/toluene diisocyanate prepolymer is shown in Figure 8-4. In order to ensure the quality of the prepolymer, the moisture content in the oligomer polyol must be strictly controlled to be less than 0.05%. At present, the moisture content of formal packaging polyester and polyether produced by major companies is less than 0.05%, and the unopened ones can be used directly.