Waterborne polyurethane waterproof coating is a new type of polymer waterproof coating. Waterborne polyurethane coating has a serious disadvantage. Because it uses water as a dispersant, it will inevitably introduce hydrophilic groups such as carboxyl and hydroxyl groups into the molecular structure. Its water resistance is much worse than solvent-based polyurethane coatings. There are generally five factors that will affect the waterproof performance of waterborne polyurethane coatings.
Choice of polyol
Generally, polyether polyols are generally selected for the synthesis of waterborne polyurethane waterproof coatings since the hydrophobicity of polyether polyols is significantly better than polyester polyols. Polyether polyols have better flexibility and superior low temperature performance due to ether groups, and the hydrolysis resistance of ether groups is much better than that of ester groups. However, compared to polyester polyurethanes, the strength of polyether polyurethanes and the adhesion is poor, while the mechanical strength and water resistance of polyether polyurethane such as tetrahydrofuran ether glycol are better.
Isocyanate index (R value)
For polyurethane systems with small molecular diols as chain extenders, as the R value increases, the specific gravity of hydrophobic groups such as benzene ring, urethane bond, and urea bond in the structure increases, so the water resistance of the film is improved.
For polyurethane systems with small molecular diamines as chain extenders, as the R value increases, the residual -NCO content in the prepolymer increases, and the urea bonds formed by the reaction with water or ethylenediamine during emulsification increase. However, there are two N atoms in the urea bond and one N atom in the urethane. Therefore, the three-dimensional hydrogen bond formed by the urea bond has a greater force than that of the urethane, so the water resistance of the film becomes worse.
Hydrophilic chain extender
Hydrophilic chain extender, as a key raw material in the synthesis of waterborne polyurethane, directly affects the water resistance of waterborne polyurethane coatings. Generally, the higher the content of the hydrophilic chain extender, the better the dispersion effect of the prepolymer in water, the more transparent the obtained emulsion, the more stable the product, but the worse the water resistance.
The role of the crosslinking agent in the waterborne polyurethane waterproof coating is mainly to increase the crosslinking density of the polyurethane system, effectively preventing the decomposition of the polyurethane structure by water molecules, and significantly enhancing the water resistance of the waterborne polyurethane film. Generally speaking, the greater the degree of crosslinking, the better the waterproof performance of the system.
Take anionic waterborne polyurethane as an example. Choose volatile neutralizers such as ammonia and triethylamine, which will volatilize with the volatilization of water during the film formation process. Compared with non-volatile neutralizers such as sodium hydroxide, the volatile neutralizers cause less -COO to remain in the film system, and the water resistance of the film is obviously better.
Emulsion particle size
To a certain extent, the particle size of the water-based polyurethane emulsion reflects the uniformity of the particles in the system. From the film-forming mechanism of water-based polyurethane, the smaller the particle size, the better the film-forming property, and the better it can prevent the penetration of water molecules. But on the other hand, the smaller the particle size usually means the more hydrophilic components in the system, which reduces the water resistance of the film. Often the latter has a greater degree of influence than the former.