Preparation Technology Of Polyurethane Prepolymerization

The most commonly used method for synthesizing prepolymers is the NCO terminated polyurethane prepolymer. The general method for preparing NCO based prepolymerization systems is to first remove a small amount of water from oligomer polyols (such as polyether polyols or polyester polyols), and then add oligomer polyols to excess polyisocyanates while stirring in a nitrogen atmosphere, and promptly remove the heat generated by the reaction to control the reaction temperature within a certain limit. Sometimes, according to the needs of the reaction, appropriate solvents can be added to adjust the viscosity of the system, and catalysts can be added to control the speed of the prepolymerization reaction.
The synthesis method of OH terminated polyurethane prepolymers is similar to that of NCO terminated prepolymers, generally controlling a slight excess of hydroxyl containing raw materials. Prepolymers used for adhesives and other main agents are generally obtained by further extending the chain with slightly excess binary alcohols on the basis of NCO terminated prepolymers. There are various factors that affect the prepolymerization reaction. In addition to the properties of isocyanates and oligomer polyols (such as hydroxyl value, acid value, water content, and metal ion content), the operating process is also the main influencing factor. The stability of NCO based polyurethane prepolymers is also affected by the sealing of the storage container, the properties of the container wall, and the gas inside the container. Sometimes the presence of a small amount of air in the container can affect the stability of prepolymers, especially MDI type prepolymers. Sometimes, in order to improve the storage stability and reduce the chances of viscosity increase and gel, it is necessary to add a small amount of stabilizer such as benzene sulfonyl chloride.