Water-based coatings use water-based polymers as film-forming substances, which not only reduces VOC emissions, but also has the advantages of safety, no fire hazard, construction in humid environments, and convenient cleaning.
The water resistance of the paint refers to the resistance of the paint film to the action of water. It means that the paint film is immersed in water and resists the erosive effect of water on it (including dissolution, corrosion, etc.). Blisters, shedding, etc.
The influence of water on the paint film mainly includes two aspects: First, the paint film has strong water absorption and is easy to regain moisture (or slow drying, not easy to completely dry). The second is that the paint film is prone to physical or chemical interactions with water, such as ether bonds or ester bonds that are easily hydrolyzed. The water absorption of paint film is universal, let’s discuss how to improve the water resistance of water-based paint.
First, we can start with water-based resins, such as the polymer structure of water-based polyurethane and water-based acrylic acid. The surfactants usually used in emulsion polymerization are all amphiphilic, that is, one end is lipophilic, and the other end is hydrophilic.
After the polymerization is completed, these surfactants still remain in the emulsion, and remain in the paint film after film formation. After encountering water, the hydrophilic group interacts with water, which promotes more penetration, diffusion and migration of water in the paint film, increasing the amount of water. water absorption. In order to overcome the adverse effects of surfactants, emulsions can be prepared by soap-free polymerization, and the water resistance of the synthesized emulsions is significantly improved during emulsion polymerization.
The water absorption of different emulsions or water dispersions is very different. The low water absorption rate is less than 10%, and the high water absorption rate can exceed 100%, which is mainly determined by the structure of the polymer. The hydrophilic structure in the polymer molecule and the hydrophilic monomer used in the synthesis have a great influence on the water absorption and water resistance of the paint film.
Hydrophilic monomers such as (meth)acrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, maleic anhydride, dimethylolpropionic acid, and methylolbutyric acid can improve polymerization The dispersibility of the paint in water is beneficial to the formation and stability of the emulsion or water dispersion, and at the same time, it brings about the disadvantages of increased water absorption and decreased water resistance of the paint film after film formation.
When there is a polyethylene glycol segment in the polyurethane structure, the water absorption rate will increase; while the polyurethane with a polyester structure has a high possibility of hydrolysis and degradation, and the water resistance will be affected. The introduction of fluorocarbon monomer or organosilicon compound during emulsion synthesis can improve the water resistance of the paint film, and the water resistance increases with the increase of the amount of fluorocarbon and silicon compound.
Secondly, it can be cross-linked by adding a cross-linking agent, and cross-linking can significantly improve the water resistance of the paint film. After the same paint is cross-linked by adding a cross-linking agent, no matter whether it is cross-linked with isocyanate curing agent, aziridine cross-linking or epoxy compound cross-linking, although the water absorption rate is different, it can usually be reduced to 15. %the following.
The stronger the hydrophilicity of the hydrophilic segment in the hydrophilically modified isocyanate curing agent, the higher the water absorption rate of the paint film after crosslinking and curing. The water absorption rate of the emulsion or water dispersion with good water resistance is very low, and the water absorption rate decreases little after cross-linking. On the contrary, for the paint film with high hydrophilicity, the water absorption rate of the paint film decreases greatly after the crosslinking agent.
In addition to the above two ways, the water resistance of the paint film can also be improved by physical methods. Adding hydrophobic compounds to the paint formulation can improve the hydrophobicity of the paint film and reduce or reduce the infiltration of the paint film.
For example, carnauba wax emulsion buy and wax powder can effectively increase the interfacial contact angle of the paint film. In addition, certain hydrophobic additives are also more effective and can produce results similar to the lotus leaf effect. This hydrophobic effect is caused by the regular aggregation of hydrophobic additives on the surface of the paint film. The migration of the hydrophobic agent will make the lotus leaf effect disappear, and the hydrophobic effect of the paint film added with the hydrophobic agent will gradually weaken after multiple scrubbing.
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