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What is the reason for the adhesion failure of the coating in polyester powder coating
source：www.cnjindu.com | Release time：2026年09月19日1、 Defects in substrate pre-treatment (pre construction reasons, most common)
Powder coatings adhere to metal surfaces through mechanical anchoring and interfacial chemical forces.
Surface pollutants: oil stains, rolling oil, dust, and residual release agents on the surface of the substrate, forming a weak boundary layer. The molten resin cannot fully wet the substrate, and the inherent adhesion is poor after solidification.
Poor conversion film (phosphating/passivation): The conversion film is too thin, too thick, with coarse crystals and local leakage treatment; The conversion membrane is contaminated and has weak binding force.
Inappropriate surface roughness: insufficient roughness, few mechanical bite points; The roughness is too high, the pits contain dirt and gas, and there are gaps at the interface.
Pre rusting of substrate: Floating rust and white rust cover the substrate, and the coating adheres to the loose rust layer, and later peels off along the rust layer.
2、 Powder solidification process issues
Insufficient curing: incomplete crosslinking reaction of polyester, low crosslinking density of resin, weak cohesion, and insufficient interfacial force between coating and substrate.
Overbaking: High temperature causes thermal oxidation degradation of polyester resin, resulting in significant internal stress in the coating and brittleness of the film layer; The stress exceeds the interfacial adhesion, resulting in peeling and detachment.
Improper heating rate: If the heating is too fast, the powder melting and leveling is not sufficient, and small bubbles are wrapped around the interface, resulting in interface defects.
Uncontrolled film thickness: If the film layer is too thick, the curing shrinkage stress will significantly increase, making it easy to crack and peel off.
3、 Aging of service environment (strongly related to resin degradation, failure after long-term use)
Resin degradation does not directly break interfacial chemical bonds, but indirectly induces adhesion failure:
Polyester resin undergoes hydrolysis and chain breakage of ester bonds in humid, hot, and ultraviolet environments, resulting in micropores in the coating film. Moisture, chloride ions, and corrosive media are more likely to penetrate the coating film and reach the coating/substrate interface.
Water accumulates at the interface, disrupting the hydrogen and chemical bonds between the coating and the substrate; Metal corrodes, the volume of rust products expands, causing the coating to bubble and lose adhesion.
Cold and hot cycles: The difference in thermal expansion coefficient between polyester and metal repeatedly generates alternating thermal stress; After resin degradation, the toughness of the coating decreases, making it unable to release stress, and the interface gradually becomes detached.
4、 The internal factors brought by the formula system
Additive migration: Leveling agents and demolding agents migrate to the coating substrate interface, forming a weak interfacial layer and weakening adhesion.
Color fillers: When the pigment volume concentration exceeds the critical pigment volume concentration CPVC, the pores in the coating increase and the cohesion decreases; Some moisture absorbing fillers will accelerate internal hydrolysis.
The structure of the resin itself: ordinary outdoor polyester has weak hydrolysis resistance and is prone to degradation in humid and hot environments; Ultra weather resistant polyester has a stable molecular structure, slow hydrolysis rate, and better adhesion retention.
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