Water based polyurethane is the base material and key component of water based PU adhesive.According to the type of charge carried by particles, water-based polyurethane can be divided into three types: anion, cationic and non-ionic.The preparation of waterborne polyurethane is usually the synthesis of polyurethane prepolymer with a certain molecular weight, and then the prepolymer is dispersed in water under shear force.At present, the main preparation methods are hydrophilic monomer chain extender and self-emulsification (the so-called hydrophilic monomer chain extender method), that is, hydrophilic chain extender is introduced into the molecular structure of polyurethane prepolymer, and the resulting polymer can be directly dispersed in water to form waterborne polyurethane without additional emulsifier.The synthesis processes of hydrophilic monomer chain extender include solvent method, prepolymer dispersed water chain extender method, melt dispersed polycondensation method, etc.
As early as in the 1940s and 1950s, there had been a small amount of research on waterborne polyurethane, but due to poor storage stability and other reasons, the research made little progress until 1972, when Bayer company in Germany officially adopted polyurethane dispersion as leather coating, it began to develop rapidly.The average annual growth rate of waterborne polyurethane was reported to be 8% between 1992 and 1997.
China has been studying waterborne polyurethane since 1970, and the research work has been very active in recent years.The development has the following characteristics:

(1) from the product structure, the main type is emulsion, water-soluble second, latex is not common.
(2) from the perspective of raw materials, polyols are mainly polyether type, polyester type second, polycarbonate is rarely seen.The varieties of isocyanates are less, and only TDI, HDI and MDI have been reported.Chain extenders are mostly alcohols, amines are rarely used.
(3) from the perspective of preparation methods and types, it is generally self-emulsifying, carboxylic acid type, anion system;External emulsification, sulfonic acid type, quaternary ammonium salt type emulsification system is less;Melt dispersion, solid spontaneous dispersion method is not involved.
(4) from the perspective of theory and application, emphasis is placed on application development, with little theoretical research.
To prepare high performance waterborne polyurethane adhesive, high performance waterborne polyurethane must be prepared.Most waterborne polyurethanes are mainly linear thermoplastic polyurethanes. Due to their lack of crosslinking and low molecular weight, they have poor water resistance, solvent resistance and adhesive film strength.In order to improve the properties of waterborne polyurethane, it must be modified.