1.What are the key factors affecting wet adhesion?
Resin type: Resins such as fluorocarbon and silicon-modified polyester have dense molecular structures, low water vapor permeability, excellent moisture resistance and adhesion, and are suitable for high humidity environments.
Ordinary polyester and acrylic coatings have low crosslinking degrees, and long-term moisture may cause slight swelling of the coating, thereby reducing adhesion.
Coating thickness and uniformity: When the coating thickness is insufficient or there are pinholes or missed coatings, water vapor can easily penetrate directly into the substrate interface through defects, causing corrosion of the coating and indirectly destroying adhesion; while a uniform thick coating can extend the water vapor penetration path and delay interface failure.
2.How does the quality of substrate surface preparation and primer adhesion affect moisture resistance performance?
Surface treatment:
The substrate needs to be chromate treated or chromium-free passivated to form a conversion film to enhance the chemical bonding with the primer; if the pretreatment is not thorough, the humid environment will accelerate the interface delamination.
Primer type:
The primer needs to have both "water resistance" and "adhesion" characteristics: Epoxy primer has strong bonding with the metal substrate and excellent water resistance, and is the first choice for high humidity scenes; while alkyd primer has poor water resistance and is easy to emulsify in long-term humidity, resulting in reduced adhesion.
3.Will the humidity, temperature, and presence of corrosive media in a humid environment aggravate adhesion corrosion?
High humidity + high temperature: Water vapor can more easily penetrate through the pores of the coating to the interface, causing slight corrosion of the substrate. The corrosion products will form a "wedge effect" at the interface, pushing the coating away, resulting in decreased adhesion.
Humid environment containing salt: Chloride ions in salt spray will accelerate the corrosion of the substrate and destroy the chemical bond between the coating and the substrate, causing the adhesion to be significantly reduced in a short period of time.
4.What are the evaluation methods for wet adhesion?
Wet heat test
Test process: Place the color-coated coil sample in a constant temperature and humidity chamber, expose it for 500h and 1000h, then take it out and dry it.
Boiling test (accelerated water penetration test)
Test process: Immerse the sample completely in boiling distilled water (100℃), keep it for 1-2 hours, then take it out and cool it to room temperature.
Condensation test (simulating condensation environment)
Test process: Place the sample at a 45° tilt, heat the back (e.g. 50°C), and expose the front to normal temperature air to form continuous condensation (simulating condensation on the exterior wall of a building), and continue to expose for 1000 hours.
5.What are the measures to improve wet adhesion?
Coating system upgrade: give priority to resins with excellent water resistance, and increase the coating thickness to more than 25μm to reduce water vapor penetration channels.
Substrate pretreatment enhancement: use chromium-free passivation or phosphating treatment to form a denser conversion film on the substrate surface and enhance the chemical bonding with the primer.
Primer optimization: use epoxy primer to improve adhesion and inhibit corrosion of the substrate in a humid environment.
Process control: ensure that the coating is fully cured and reduce the residual pores inside the coating.