Primers for epoxy floor coatings that serve a shielding function must possess excellent wet adhesion and fully penetrate into the micro-fissures on the floor surface. The wet adhesion of epoxy floor paint films can be improved by focusing on the following aspects:
The floor surface must be thoroughly cleaned before coating to remove all oil contamination and soluble salts. It is optimal to pretreat the floor surface to form a phosphate conversion film, followed by electrophoretic primer application. This enables the floor paint film to deliver outstanding wet adhesion performance.
The floor paint film shall maintain good integrity; otherwise, corrosion will initiate at film defects. The viscosity of the floor coating should be kept as low as possible. Adopt coatings with slow volatilization and slow crosslinking properties, so that the coating can remain in a liquid state long enough to fully infiltrate into the micropores of the substrate surface. Baking-cured primers are recommended whenever possible.
Wet adhesion requires the coating to not only bond firmly to the substrate surface, but also resist hydrolytic desorption when water penetrates the film. Baking-type epoxy primers generally provide better corrosion resistance, because higher temperature allows resin molecules to arrange themselves in a more oriented manner on the steel surface. Amino groups can enhance wet adhesion and are not easily displaced from the steel surface by water. Phosphate ester groups also promote wet adhesion, and phosphate-modified epoxy resins can effectively improve the adhesion of floor paint films.
The resin shall have excellent saponification resistance. In oxygen-absorbing corrosion, hydroxide ions (OH⁻) are generated at the cathode, with the pH value possibly reaching as high as 14. Such alkaline conditions catalyze the hydrolysis (saponification) of ester groups and reduce the wet adhesion of the paint film. Amide groups exhibit better hydrolysis and saponification resistance than ester groups. Epoxy resins feature superior wet adhesion and saponification resistance; epoxy primers show higher saponification resistance than alkyd primers. Epoxy–amine and epoxy/phenolic primers offer excellent anticorrosive performance, while epoxy/phenolic primers require high-temperature curing.
Water-soluble components should be avoided in the cured floor paint film, as residual water-soluble substances easily cause blistering. If hydrophilic solvents remain in the film, they are incompatible with the dry coating matrix and remain as a separated phase, eventually leading to blistering. Zinc oxide should not be used in primers; it reacts with water and carbon dioxide to form slightly soluble zinc hydroxide and zinc carbonate, which induce film blistering. Passive pigments such as zinc chrome yellow only work when slightly water-soluble; their presence in the coating will otherwise cause blistering defects.
