The stability of the aqueous polyurethane dispersion increases with the increase of the concentration of the ionic group, but there is an inherent defect that the performance of the coating film is degraded and the water absorption rate of the coating film is large, and the most effective method is crosslinking. According to the conditions of cross-linking curing, it is divided into the following varieties: 1. Two-component thermosetting water-based polyurethane coating Two-component thermosetting water-based polyurethane coating refers to a system in which a polyurethane aqueous dispersion is mixed with a small amount of an external crosslinking agent, and a hydroxyl group in a linear polyurethane dispersion is usually used. The active hydrogen on the carboxyl group, the amine group, the urethane group or the urea group is reacted with various crosslinking agents to effect crosslinking curing. 2, room temperature curing two-component waterborne polyurethane coatings This type of cross-linking agent is a special aliphatic-isocyanate prepolymer containing -NCO end group, which can be stably present in various hydroxyl-containing polymers after hydrophilic treatment. In the aqueous dispersion. The -NCO group of such a crosslinking agent reacts slowly with water and thus has a small amount of reaction with water. In the process of film formation of such a coating, due to the large surface area, the evaporation rate of water is fast, and after the water evaporates, the particles of the polyisocyanate and the particles of the aqueous polymer dispersion are aggregated, and the crosslinking reaction between -NCO and -OH occurs. . Therefore, as long as the ratio of -NCO to -OH is appropriate and used within a prescribed period of time, a room temperature curing two-component waterborne polyurethane coating having excellent properties can be obtained. 3. An aqueous polyurethane coating containing a blocked isocyanate is an aqueous polyurethane resin containing a blocked isocyanate group as a film-forming substance, and an amine as a crosslinking agent. The blocked isocyanate group does not react with the crosslinking agent at room temperature, and the temperature must be raised to a certain extent. After deblocking, it can react with the crosslinking agent to crosslink and solidify. This type of coating is also called a one-component crosslinkable waterborne polyurethane coating. This type of coating can be either cationic or anionic. 4, light-curing waterborne polyurethane coatings Light-curing coatings in recent years, research and development efforts are relatively large. Photocuring is a technique of crosslinking under ultraviolet light and electron beam radiation. How to introduce unsaturated double bonds in the molecular structure is the key technology of this type of coating.
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