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Home / News / Industry News / Plastic Part Substrate Has Been Treated — Why Does the Coating Still Peel Off After Use?
Industry News
1
Plastic Surface Treated Does Not Mean Surface State Is Permanently Stable
Plastic differs from metal — its surface state may change over time. Especially in injection moulded parts, release agents and lubricants are used during production. Even after just completing cleaning or surface activation, some low-molecular substances may still gradually migrate from inside the plastic toward the surface. The coating looks fine right after spraying, but after a period, the interface state between coating and substrate changes, causing adhesion to decline.
2
Why the Same Coating on a Different Plastic Readily Causes Peeling?
Different plastics have very different surface energies, polarity, and chemical structures. For example, common PP and PE materials inherently have lower surface energy — the coating finds it difficult to spread naturally as on high-surface-energy substrates. Even if a complete film forms, the actual contact area may still be insufficient. This situation may show no obvious problem on a simple adhesion test immediately after application, but after water immersion, cold-heat cycling, or long-term use, interface weak points gradually become apparent.
3
Excessive Surface Treatment Is Not Always Better
Surface activation is not the stronger the better. If treatment conditions are too strong, the plastic surface structure may be changed, or even surface damage caused. Additionally, some surface activation effects have a certain time validity — if the standing time after treatment is too long before spraying, the activated state may gradually diminish, causing originally improved adhesion to decline. Treatment strength, uniformity, post-treatment waiting time, and actual spray window all need to be considered.
4
Why Adhesion Test Qualified at Delivery, But Peeling After a Period?
Adhesion testing is usually performed at a specific time and environment. But actual products may need to undergo high temperature, low temperature, humid heat, bending, friction, and hot-cold cycling. Plastic and film have different degrees of thermal expansion and contraction — after temperature change, substrate and film produce different magnitudes of dimensional change. If the bond between coating and plastic is insufficient, or the film itself has large internal stress, the interface may gradually develop micro-cracks after multiple cycles.
5
If the Coating Is Too "Hard," Plastic Part Peeling May Also Result
Plastic parts typically have a certain degree of flexibility. If the coating is designed too hard and brittle, while the substrate itself easily undergoes slight bending or deformation, the coating needs to bear greater stress. When the deformation capability difference between film and plastic is large, interface stress concentration may appear during long-term use. Therefore, plastic part coatings do not have higher hardness is better — a suitable balance between adhesion, flexibility, and film strength needs to be found.
6
Why Adhesion Promoter Can Help but Is Not a Complete Solution?
For low-surface-energy plastics, adhesion promoters can improve the interfacial bonding between coating and substrate. But they cannot replace substrate treatment. If obvious oil contamination, release agent, or other contamination is present on the plastic surface, simply increasing adhesion promoter cannot solve the fundamental problem. The adhesion promoter also needs to match the resin system — too low a dosage may produce insufficient effect; too high may affect film hardness, water resistance, and recoat performance.
7
How to Determine Whether the Problem Is the Substrate or the Coating?
If adhesion is very good right after treatment but declines noticeably after standing for a period, focus on surface activation stability and low-molecular substance migration. If different plastic substrates show very different performance with the same coating, focus on substrate surface energy and coating wetting. If initial adhesion qualifies but peeling occurs after cold-heat cycling, focus on coating flexibility, internal stress, and thermal expansion differences between substrate and film. If adding adhesion promoter clearly improves the situation, further optimise the promoter type and addition ratio.
8
How to Improve the Long-Term Adhesion of Plastic Part Coatings?
First confirm the plastic substrate type and actual surface state. Then select a coating system matched to the substrate surface characteristics, allowing the coating to fully wet the substrate. For low-surface-energy plastics, also combine an appropriate adhesion promotion system. Simultaneously balance film hardness and flexibility, avoiding overly brittle coatings that generate excessive interfacial stress during cold-heat variation or mechanical deformation. In actual evaluation, not only initial adhesion testing — also combine water resistance, humid heat, cold-heat cycling, and bending tests to observe long-term performance.
Summary
Plastic part substrate already treated but still peeling does not mean surface treatment has no value. The true problem may come from unstable surface state after treatment, low plastic surface energy, release agent migration, insufficient coating wetting, or mismatch in the deformation capability between film and plastic. The more rational approach is to let the substrate surface, coating wetting, interfacial bonding, and film flexibility mutually match — only when both initial adhesion and long-term use remain stable is the plastic part peeling problem truly resolved.
Frequently Asked Questions
How long is the effective window after flame or plasma treatment of PP before wettability begins to diminish?
The surface activation from flame and plasma treatment is not permanent. For PP, the elevated surface energy typically remains substantially above the untreated baseline for approximately 24–72 hours under normal storage conditions, after which low-molecular-weight surface migration progressively reduces the activation benefit. For best adhesion results, coating application within a few hours of surface treatment is recommended — the specific window depends on the treatment intensity, substrate grade, and storage conditions. Measuring water contact angle at intervals after treatment provides a quantitative indication of how quickly the surface energy is declining for a specific substrate and treatment combination.
Is chlorinated polyolefin (CPO) primer still the standard approach for adhesion to PP and polyolefin substrates?
CPO-based primers remain widely used for polyolefin adhesion because CPO chains can physically entangle with and chemically interact with polyolefin substrate surfaces, providing adhesion that is difficult to achieve with conventional adhesion promoters. However, environmental and regulatory pressure on chlorinated materials has driven development of alternative approaches — including specific adhesion promoter additives, modified resin systems, and substrate surface energy optimisation — some of which can reduce or eliminate the need for CPO. The appropriate approach depends on the specific polyolefin type, coating system, performance requirements, and applicable regulations.
