Double-sided corona treatment is widely used to prepare PET film before metallization, printing, or lamination. It improves adhesion by changing the film surface—but the effect is not explained by roughness alone. Surface energy, polar functional groups, removal of weak surface layers, and treatment uniformity usually matter more than the microscopic texture of the film.
This article explains what corona treatment does to PET, which mechanisms contribute to aluminum adhesion, and what should be controlled and tested. Final treatment specifications should be confirmed for the specific film, metallizer, and downstream process.
What double-sided corona treatment does
A corona treater applies a high-frequency, high-voltage discharge across an air gap at the film surface. The discharge ionizes the air and creates a non-thermal plasma containing electrons, ions, and reactive species. These species interact with the PET surface, producing several effects:
- Oxidation and polar groups: reactive oxygen species introduce carbonyl, hydroxyl, and other polar groups, increasing surface energy.
- Chain scission: some surface polymer chains are broken, which can improve wetting but can also create low-molecular-weight material if treatment is excessive.
- Surface cleaning: contamination and weakly bound material can be removed or altered, reducing a weak boundary layer.
- Topography changes: treatment can modify surface roughness, although the effect is usually secondary and depends on film grade and treatment intensity.
Treating both sides is useful when both surfaces require adhesion—for example, metallization on one side and printing or lamination on the other. If only one surface is later bonded or printed, single-sided treatment may be sufficient; unnecessary treatment can add cost and handling risk.
Why the “roughness” explanation is incomplete
A roughened surface can provide more area and mechanical anchoring points, and this can contribute to adhesion. However, vacuum-deposited aluminum atoms and the native oxide on the aluminum surface interact with PET primarily through surface chemistry and wetting.
Two points are often overlooked:
- Surface energy and wetting determine how well the aluminum layer wets and anchors to the PET surface.
- Over-treatment can create a weak, low-molecular-weight boundary layer, which may reduce adhesion even if roughness increases.
For these reasons, adhesion should be evaluated by measuring both surface energy and practical peel or tape tests—not by roughness alone.
Mechanisms that improve aluminum adhesion
Surface cleaning and weak boundary layer removal
Corona treatment can remove or modify surface contamination and weakly bound material. A clean, stable surface allows the aluminum layer to bond more uniformly and reduces the risk of localized delamination.
Polar groups and chemical interactions
Corona treatment introduces polar groups such as carbonyl and hydroxyl groups. These groups can interact with the aluminum surface and its native oxide through acid–base interactions, coordination, and hydrogen bonding with surface hydroxyl groups on the oxide. These interactions contribute to adhesion, especially in the presence of moisture and during aging.
Surface topography and mechanical anchoring
The treated surface may have a modified topography that provides additional anchoring sites. This mechanical contribution is real but should not be treated as the dominant mechanism. A smooth but chemically active surface can outperform a rough surface with a weak boundary layer.
Uniformity across the web
Adhesion problems are often local: streaks, untreated edges, or inconsistent treatment across the roll width. Electrode condition, web tension, line speed, and air gap should be controlled to keep treatment uniform.
Effect on printability
Corona treatment is also used to improve ink adhesion. Higher surface energy and polar groups help the ink wet the film and form a more uniform layer, which can improve print definition and resistance to rubbing or peeling.
Some practical limits:
- Treatment is not a drying method. Drying and curing are governed by the ink system, solvent, oven, and press conditions.
- Ozone is generated. Corona discharge in air produces ozone and nitrogen oxides; adequate exhaust, ventilation, and ozone control are required.
- Treatment can decay. Surface energy may decrease during storage, so the interval between treatment and printing or metallization should be defined and verified.
- More is not always better. Excessive treatment can cause blocking, surface damage, or a weak boundary layer.
What to control and validate
Before production, define and verify:
- Treatment level in the appropriate units (for example, W·min/m²) for each side
- Surface energy or contact angle, measured across the roll width and over time
- Aluminum adhesion with an agreed peel or tape test before and after lamination
- Treatment uniformity and electrode condition
- Storage interval between treatment and metallization or printing
- Print and lamination performance on the actual production line
- Ozone exhaust and workplace safety controls
Applications
Corona-treated metallized PET is used in packaging, printing, and industrial laminates where aluminum adhesion, print quality, and consistent surface properties are required. The treatment should be specified as part of the film and converting process—not as a standalone guarantee of performance.
Environmental note
Corona treatment does not make a film inherently “green,” and the process is not emission-free: it generates ozone and requires appropriate controls. Environmental claims should be based on the complete material and process, supported by market-specific evidence.
Next step
Review our metallized PET film platform or send your treatment, adhesion, and print requirements through our technical inquiry form. We can discuss a starting grade and the test methods used for evaluation.
Disclaimer
This article is provided for general technical information only. It is not regulatory, medical, or legal advice, and it does not confirm that any film, laminate, or package is suitable for a particular application or market. Final material selection, processing conditions, and compliance requirements must be validated for the intended product, production line, and destination market. Performance values are typical and must be confirmed against a grade-specific TDS and finished-package testing.