01Select a good water resistance resin and curing agent
The resin dispersion with good water resistance has the following indicators
» Higher hydroxyl value;
» Try to have a low acid value;
» Higher glass transition temperature (less alcohol with soft chain)
» Higher molecular weight, narrower molecular weight distribution;
» Minimal polar monomers;
» Monomers with urea cyclic amines that enhance wet adhesion;
» Suitable salt forming auxiliaries.


For isocyanate curing agents:
    HDI aliphatic isocyanate trimers are most commonly used. Previously, polyether chain ends were generally modified, but the polyether segments remaining in the coating film after film formation greatly reduced the water resistance of the coating film; The modified HDI is made into a salt by using ammonia water. After the coating film is dried, the ammonia water is volatilized, and the reduction of the water resistance of the small amount of the sulfonic acid group is much smaller than that of the polyether modified HDI.


02Select a good hydrophobic filler


» silicate fillers with obvious hydrophilic swelling should be used with caution, such as montmorillonite and kaolin;
» In addition, the metal ions in the montmorillonite have a destructive effect on the stability of the dispersion. If it is not necessary, it can be modified with a volatile amine.
» For talc powder and precipitated barium sulfate type fillers, it is better to choose hydrophobic treatment;
» For organic pastes, attention must be paid to their chemical structure, chargeability and the surfactants used;
» Coating hydrophobicity can also be obtained by adding nanomaterials, especially nanosilicon oxides.

03
Reduce the amount of "hydrophilic structure" in the formulation
» Reduce the use of additives with more hydrophilic groups;
» Here, we emphasize the co-solvent (or the film-forming auxiliaries we call). The alcohol ethers with too high boiling point and too low volatilization will remain in the coating film for a long time after the coating water is completely dried. The water resistance of the coating film.

04
Increase the crosslink density and degree of cure of the film
» Select a resin dispersion with high hydroxyl value, a suitable catalyst, and ensure a high crosslink density after curing of the film;
» Studies have shown that an appropriate increase in the amount of curing agent can enhance the water resistance of the film (NCO / - OH ratio is 1.5 - 2.0 water resistance is best), of course, too much curing agent will be excessive, but the water resistance is reduced.
» High cross-linking density must first ensure that the degree of cross-linking is complete, otherwise too high a hydroxyl group means that the hydroxyl groups that do not have time to react will also increase;
» In addition, too high crosslink density can degrade impact, bending and other properties. Therefore, the best cross-linking state of the resin is the appropriate cross-linking density, as much as possible.