Surface treatment plays a crucial role in modern converting, printing, laminating, coating and extrusion-related applications. It may not be the most visible part of your machine line but it is often the step that determines whether the final product performs (and looks) as intended. From flexible packaging and labels to medical films and other substrates – many materials need the surface energy increased before any ink, adhesive, coatings or varnishes can bond properly.
This article helps to explain why surface treatment is needed, how corona treatment works and why getting the process right can make all the difference!
Why is surface treatment needed?
Most polymer-based films are chemically inert and have naturally low surface energy. This means that inks, adhesives, coatings and other functional layers may not properly wet or bond to the surface. The result is poor print quality, weak bonding, delamination or coatings that can reduce product performance.
Surface energy is measured in dynes/cm, and for successful adhesion, the dyne level of a material must be higher than the surface tension of the ink or coating being applied. Untreated polyolefin films, for example, may have surface energies as low as 29–31 dynes/cm, while most inks require at least 38–42 dynes/cm to adhere reliably.
The principle behind corona treatment
In simple terms, corona treatment is a high-frequency electrical discharge applied across a material’s surface as it passes between an electrode and a grounded roller. The discharge creates a cloud of ionized air which alters the surface chemistry of the material.
At a molecular level:
- The discharge breaks apart carbon-hydrogen bonds in the polymer chain.
- Oxygen from the surrounding air reacts with these free sites.
- New polar functional groups (such as carbonyls) are introduced at the surface.
The effect is confined to the outermost molecular layer (around 0.01 microns), so the bulk properties and appearance of the material remain unchanged. What changes is the surface energy, which increases significantly and improves wettability.
Primary and secondary treatment
Many films are treated immediately after extrusion (known as primary treatment). This ensures the film can be wound, stored and transported in a condition suitable for downstream processing.
However, the effect of treatment is often not permanent. Over time, surface energy levels can decline due to additives migrating, the surface relaxing and dyne level dropping. By the time the material reaches it’s next phase, the surface may no longer have the energy needed to bond reliably.
It’s at this stage that secondary treatment (often called bump treatment) is commonly applied in-line just before converting, printing, laminating or coating. This restores the dyne level and ensures consistent adhesion.
Factors that can influence results
Several variables determine how effective corona treatment will be:
- Material type – Polyethylene and polypropylene respond well; fluoropolymers and certain barrier films are more resistant.
- Additives – Slip agents, antiblock, and antistatic compounds can interfere with treatment and cause dyne levels to decay faster.
- Treatment intensity – Power density (watts per sqm/min) must be matched to the substrate and process requirements.
- Process conditions – Line speed, electrode design and air gap can all affect treatment uniformity.
Because of these factors, testing is recommended to determine the optimum treatment settings for any given material.
What are the other options?
Corona treatment is the most widely used method for raising surface energy in many industrial and converting applications, but other approaches include:
- Plasma treatment – Uses controlled gas chemistry for more specific surface modifications.
- Flame treatment – Applies a controlled flame to modify surfaces, often for rigid materials.
Each method has its advantages depending on substrate and application, but generally corona remains the most versatile and widely adopted solution for film, foil, paper, plastic and other engineered substrates.
The impact
Surface treatment is often overlooked because it’s seen as a small auxillary part of the larger process, but it is one of the most crucial as it directly impacts almost everything that follows.
If the surface isn’t properly prepared, nothing after it will perform as it should.
Want to know more? Check out our other knowledge hub resources, join us for a (virtual or in-person) showroom tour or book in to have your substrate tested in our Test Lab.
