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Home / News / Industry News / PP and PE Reject Coatings Chemically — Formulation Changes Alone Are Never Enough
Industry News
Polyolefin Coating Adhesion · DH-7208
The adhesion problem on PP and PE is not a coating formula problem. It is a surface energy problem. Polypropylene and polyethylene have surface energies of 29–36 mN/m — below the critical threshold at which standard coating binders can spread, wet, and bond to the substrate. Changing the resin, adding adhesion promoters to the coating, or adjusting the crosslinker ratio can improve adhesion within the limited range that the surface energy gap allows — but cannot close the gap itself. That requires changing the surface.
Coating-Side Optimisation
Modifies how the coating reaches and interacts with the surface. Effective for marginal improvements in adhesion on substrates with some surface reactivity — limited by the substrate's inherent surface energy.
Works on: mildly challenging substrates
Cannot solve: PP / PE fundamental surface energy deficit
Cannot solve: PP / PE fundamental surface energy deficit
VS
Substrate Pre-Treatment (DH-7208)
Modifies the substrate surface chemistry before the coating is applied — creating reactive bonding sites that the coating can anchor to. Addresses the root cause independent of the coating formulation.
Works on: PP, PE, POM, EPDM, stainless steel
Resolves: the surface energy gap directly
Resolves: the surface energy gap directly
What Changes After DH-7208 Pre-Treatment
| Cross-Cut Adhesion (GT0) | Adhesion after pre-treatment meets GT0 specification on substrates where untreated results were GT3–GT5 |
| Water-Boil Resistance | Adhesion maintained after water-boil test — a common qualification requirement in automotive and consumer goods |
| Shrinkage Accommodation | Reduces the adhesion reduction caused by dimensional change in the substrate during and after cure |
| Vacuum Plating Compatibility | Pre-treated surface provides a stable base for vacuum metallisation without delamination |
| Subsequent Process Stability | Treated parts retain consistent adhesion performance through downstream assembly, handling, and use conditions |
In Production: What DH-7208 Pre-Treatment Replaces or Reduces
Flame treatment and plasma treatment are the most common alternatives for PP/PE adhesion improvement. Both are effective but have practical limitations in production:
- Complex 3D geometry: flame and plasma cannot reach interior surfaces, recesses, and undercuts consistently — chemical pre-treatment can
- Process consistency: flame treatment intensity varies with gas flow, distance, and operator technique — liquid pre-treatment is more controllable
- Post-treatment window: flame and plasma activation decays over hours — chemical pre-treatment creates more durable surface modification
Can DH-7208 be used for both PP/PE and stainless steel parts on the same production line?
Yes — it is designed for mixed-substrate production environments where PP, PE, POM, EPDM, and stainless steel parts may all require coating adhesion improvement. The same pre-treatment process can be applied to all substrate types, simplifying the production workflow.
How long after pre-treatment should the coating be applied?
The pre-treated surface should be coated within the recommended time window after drying — contact with contaminants or extended standing time before coating can reduce the effectiveness of the surface modification. Specific guidance on the application window is available from the technical team.
PP and PE adhesion failures are not coating problems — they are substrate surface problems. DH-7208 pre-treatment changes the substrate's surface chemistry, providing the bonding sites that the coating system needs to develop lasting adhesion independent of any further formulation change.
