Packaging and printing. Printing and packaging.
The two just go together. Just about every type of primary product packaging is printed using one technology or another.
With so many types of packaging material available, some have properties that make it easy to get high-quality print results, while others can be much more difficult.
What Makes a Material Difficult to Print?
While some materials print easily, applying ink to others is rather like watching water on a waxed and polished car. The liquid ‘beads up’ rather than wet consistently across the surface.
The clean and polished car never gets consistently wet and this same effect happens with applying inks (adhesives too for other applications) on some materials.
Many packaging materials—especially plastics like polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET)—have inherently low surface energy. In practical terms, low surface energy equates to poor “wet-out” properties:
Ink Retention: Inks can bead up or fail to adhere (like the water on a polished car).
Adhesive Bonding: Glue or laminates can peel off easily due to weak interfacial bonds.
With brand graphics, product information, and crucial barcodes on the packaging, poor ink adhesion isn’t acceptable. That’s where corona treatment enters the conversation.
The Physics of Corona Treatment
Corona treatment involves using a high-voltage electrode to generate a plasma-like discharge (the “corona”) in the air gap between the electrode and the packaging material’s surface.
The image above shows how the components are aligned, with the packaging material passing close to the electrodes.
High Voltage Generation:
A power supply delivers high voltage (typically 6 kV to 20 kV, depending on the system) to the electrodes. These are often covered with a dielectric material (such as ceramic) to help regulate and control the electrical discharge.
Ionization of Air:
When the electric field is strong enough, it ionizes the air molecules in the gap, creating positively charged ions, electrons, and other reactive species (like ozone and free radicals).
Surface Activation:
As the substrate passes beneath the electrode (commonly wrapped around a grounded backing roll), these reactive species bombard the top molecular layer. This bombardment creates micro-etching (very fine roughening) and introduces polar functional groups (like hydroxyl, carbonyl, or carboxyl groups) onto the surface.
Increased Surface Energy:
The new functional groups are more receptive to ink and adhesives due to stronger intermolecular forces (e.g., hydrogen bonding, dipole interactions). Surface energy measurements, often reported in dynes/cm (or mJ/m in SI units), typically jump from a low 30s range (e.g., 30–34 dynes/cm) to 40 dynes/cm or higher—enough to make printing and bonding significantly more reliable.
Measuring Dyne Levels
A dyne pen is an easy way to measure the surface energy of a material.
The pens are sold in a set, containing pens that are calibrated for the different common dyne levels.
If a 38 dyne (for example) pen is applied to a substrate, and the ink stays as a film for three seconds, the substrate has a surface energy of at least 38 dyne/cm.
By using a range of Dyne Pens of various dyne levels, the surface energy of a substrate can be determined to a narrow range.
They say a picture is worth a thousand words, so here’s a quick demo of how to use dyne pens to determine the surface energy of a material:
Type of Corona Treatment System
Bare-Roll System
In this configuration, the treatment electrode is positioned over a conductive (usually metal) backing roll that contacts the substrate directly.
These systems are usually used to treat conductive materials, and they typically treat the entire width of the material as it passes through the station.
Covered-Roll (Sleeve) System
The backing roll is coated with a dielectric sleeve (ceramic or other insulating materials), and the electrode itself may be segmented.
This allows for zone treating of the substrate material for applications where only parts of the items need to be treated.
This system also allows for better control of the electrical discharge, which is particularly useful for delicate or specialty films.
Double-Sided (Two-Sided) Treatment
Both surfaces require corona treatment in some applications—such as when using adhesives on both sides of a film or needing strong laminate bonds. This can be achieved by having two separate treatment stations (one after the other), or a single station with multiple electrodes – depending on system configuration.
Narrow Web & Wide Web Systems
Narrow-Web Systems: Found in label printing or small packaging lines, these treat web widths of typically 34 inches. They’re designed to run at high speeds while maintaining uniform treatment.
These treatment systems are often found on the infeed of both flexo and digital printing presses so that the material is treated immediately before printing,
Wide-Web Systems: Used for large-scale flexible packaging and can treat widths exceeding several meters. These systems require higher-power electrodes, carefully managed web tension, and robust, even distribution of the corona discharge across the entire width.
Wide Web corona treatment systems are typically found in converters of packaging films, enabling corona-treated films to be delivered to their customers.
Limitation of Corona Treatment
Assuming a particular material is suitable for the corona process, something that needs to be taken into account is that the change in surface energy isn’t permanent. Materials tend to migrate back to their original service energy levels.
This is why corona stations are often integrated in line with the printing press – allowing the material to be printed immediately after treatment.
For companies that purchase pre-treated film stock, it is sometimes necessary to use an inline system on the printing line to ensure the dyne value has not reduced to the point where it can cause problems.
More Information
A good resource for info on corona treatment is this article at Science Direct.
Our article about Surface Energy of materials.
