The synthesis of waterborne polyurethane can be divided into two stages. The first stage is pre progressive polymerization, which involves the gradual polymerization of oligomer diols, chain extenders, waterborne monomers, and diisocyanates through solution (or bulk) to generate waterborne polyurethane prepolymers with a molecular weight of 103; The second stage is the dispersion and chain expansion of the prepolymer in water after neutralization.

The early synthesis of waterborne polyurethane used forced emulsification method. Firstly, prepare a certain molecular weight polyurethane polymer, and then disperse it in water with a certain emulsifier under strong stirring. This method requires additional emulsifier, which is used in large amount, and the lotion has large particle size, wide distribution and poor stability, so it is rarely used.

Nowadays, the emulsification of waterborne polyurethane mainly adopts the internal emulsification method. This method is conducive to introducing hydrophilic ionization groups or hydrophilic blocks into the polyurethane macromolecule chain of water-borne monomers: - COO - NHEt3, SO3- Na, - N - Ac, - OCH2CH2-, etc., and self emulsifying to form lotion (or dispersions) under stirring. This lotion has good stability and stable quality. According to the different chain extension reactions, self emulsification methods mainly include acetone method and prepolymer dispersion method.

Acetone method

The acetone method uses acetone or butanone to reduce viscosity in the middle and later stages of prepolymerization. After neutralization, high-speed stirring, water is added for dispersion, and solvent is removed under reduced pressure to obtain waterborne polyurethane dispersion. This method has a simple process and good product quality, but the disadvantage is that the solvent needs to be recovered, the recovery rate is low, and it is difficult to reuse. This method is mainly used in China to synthesize ordinary aromatic waterborne polyurethane.

Prepolymer dispersion method

Firstly, a prepolymer with - NCO terminal groups is synthesized, and a small amount of N-methylpyrrolidone is usually added to adjust the viscosity. Under high-speed stirring, it is dispersed in water containing two (or more) amines, and at the same time, high molecular weight waterborne polyurethane is obtained by chain expansion. Developed countries such as the United States mainly use this method to synthesize high-grade aliphatic waterborne polyurethane.