At present, most of the aqueous PU is mainly prepared by a self-emulsification method, and the hydrophilic group-containing PU is mainly used as a main curing component. When the coating film is dried, if the hydrophilic component cannot effectively enter the crosslinked network, the formed coating film is dried. It is easy to swell when exposed to water. In addition, it lacks crosslink density and high molecular weight which can be obtained by a two-component solvent-based PU coating film, and thus the water-dispersion, solvent resistance, heat resistance and gloss of these aqueous dispersion coating films are poor, and serious The ground limits the scope of its use. Therefore, it is often used to increase the crosslinking density of the coating film to improve the water resistance of the emulsion coating film. There are two commonly used crosslinking methods: one is to add a polyhydroxy compound having a functionality of more than 2 when synthesizing a PU prepolymer, directly forming a crosslinked PU prepolymer, and dispersing the above prepolymer well in water. And chain extension to form macromolecules, and finally form an emulsion.
This method is also called pre-crosslinking method. The disadvantage is that the viscosity of the prepolymer is easily increased, and it is difficult to disperse in water, which affects the stability of the emulsion. Active research on the screening and synthesis of novel cross-linking agents and multi-functional chain extenders has become one of the main ways to improve the physical and mechanical properties and water resistance of aqueous PU. Another method is the diplomatic method, which uses an anionic PU emulsion with a carboxyl group for cross-linking. The cross-linking reaction takes place on the carboxyl group of the PU molecule, with aziridine, carbimimine and metal salt compounds, which are carried out at room temperature. Cross-linking. Such a crosslinking agent is generally added when a PU emulsion is used, because the crosslinking reaction rate is fast, and a gel is generated in a short time to break the emulsion. The diplomatic law can successfully solve the hydrophilicity problem of PU emulsion coating film, but due to the addition of cross-linking agent, the composition of two-component finishing agent is inconvenient for construction, and this method is less used.
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