Coating Adhesion Keeps Failing on PP and PE — Why the Fix Starts Before You Spray- Suzhou Qingtian New Material Co., Ltd.

Substrate Pre-Treatment Technology

When adhesion on PP, PE, POM, EPDM rubber, or stainless steel consistently falls short — even after switching coatings or raising crosslinker ratio — the problem usually does not live in the coating. It lives in the gap between a chemically inert substrate surface and a coating system that was designed to bond to a much more reactive one.

Why Low-Surface-Energy Substrates Resist Bonding

Surface energy is the measure of how readily a liquid or coating can spread on and interact chemically with a solid surface. Polyolefins such as PP and PE have surface energies of around 29–36 mN/m — well below the 38–40 mN/m threshold at which most coating systems can form adequate contact and bonding. Stainless steel, POM, and EPDM rubber each present their own barriers: passivated oxide layers, crystalline surfaces, or chemically inert elastomeric chemistry that conventional coatings cannot penetrate or bond to reliably.

PP & PE

Non-polar polyolefin surfaces with surface energy below 36 mN/m — most coating binders cannot wet or bond effectively without surface activation

POM (Polyacetal)

Highly crystalline polymer surface with low chemical reactivity — bonding sites are scarce and coatings have limited ability to anchor

EPDM Rubber

Elastomeric surface with non-polar saturated backbone — inherently incompatible with the polar functional groups most coating adhesion mechanisms rely on

Stainless Steel

Passive chromium-oxide surface layer that is chemically stable and low in reactive bonding sites — the same property that prevents corrosion also limits adhesion

Optimising the coating formulation improves what happens above the surface. Pre-treating the substrate changes the surface itself — which is where the adhesion bond is actually formed.

DH-7208: Halogen-Free Graft Copolymer Substrate Treatment Agent

DH-7208 is a halogen-free special graft copolymer designed for surface pre-treatment of difficult-to-coat substrates — PP, PE, POM, EPDM rubber, and stainless steel — before the application of PU coatings, thermoplastic coatings, or vacuum plating processes. By treating the substrate surface prior to coating, it creates a chemically compatible interface that the topcoat or plating layer can bond to reliably and durably.

Performance Contributions

Coating Adhesion

Significantly improved adhesion on PP, PE, POM, EPDM, stainless steel after pre-treatment

Water-Boil Resistance

Adhesion maintained after water-boil testing — a common qualification for automotive and consumer goods

Vacuum Plating Stability

Provides a stable adhesion base for vacuum plating processes applied over the treated substrate

Shrinkage Control

Reduces the adhesion-reducing effects of substrate dimensional change during and after cure

Substrate & Process Coverage

PP (Polypropylene)	Primary target substrate — provides the surface activation that PP's low surface energy cannot offer on its own
PE (Polyethylene)	Similar mechanism to PP; HDPE and LDPE both respond to pre-treatment with improved coating adhesion
POM (Polyacetal / Delrin)	Crystalline surface becomes more receptive to coating after pre-treatment, improving adhesion consistency
EPDM Rubber	Elastomeric substrate pre-treatment improves bonding for coatings and overlays on rubber seals and components
Stainless Steel	Reduces the barrier presented by the passive oxide layer, supporting better interfacial bonding for topcoats
PU & Thermoplastic Coatings	Compatible with standard PU topcoat and thermoplastic coating systems applied after pre-treatment
Vacuum Plating	Creates a stable base for vacuum metallisation processes, improving plating adhesion and durability

Compliance

DH-7208 is halogen-free and formulated to meet RoHS and PFAS-related requirements — supporting the regulatory compliance needs of automotive, consumer electronics, and medical device coating applications.

Frequently Asked Questions

Why doesn't adding an adhesion promoter to the topcoat achieve the same result?

Adhesion promoters in the topcoat improve the coating's ability to bond — but the substrate surface still needs to present adequate bonding sites for those interactions to occur. On surfaces like PP and PE where bonding sites are essentially absent, a surface-level pre-treatment that modifies the substrate itself is the more effective approach.

How is DH-7208 applied to the substrate?

It is typically applied by spray, wipe, or immersion to the cleaned substrate surface and allowed to dry before the topcoat is applied. The specific application method and drying conditions depend on the substrate type and downstream process — technical support is available for process design.

Does it replace the need for flame treatment or plasma treatment on PP?

For many production environments, chemical pre-treatment with DH-7208 can provide a more consistent and processable alternative to flame or plasma treatment — particularly for complex 3D geometry where uniform plasma or flame exposure is difficult to achieve. The optimal approach depends on the specific production environment and adhesion specification.

Is it compatible with water-based topcoats as well as solventborne?

DH-7208 has been evaluated for use with PU and thermoplastic topcoat systems in both solventborne and relevant waterborne formats. Compatibility with a specific topcoat system should be confirmed with a trial before production use.

Key Takeaway

Adhesion failures on PP, PE, POM, EPDM, and stainless steel are fundamentally substrate surface problems — not coating problems. The solution starts before the spray gun is picked up.

DH-7208 modifies the substrate surface chemistry to create bonding sites the coating can anchor to
Improves adhesion, water-boil resistance, shrinkage control, and vacuum plating stability
Halogen-free, meets RoHS and PFAS-related requirements
Complementary to coating formulation optimisation — the two approaches address different parts of the adhesion system