How Does Corona Value (Dyne Level) Affect Packaging Printing?
In flexible packaging printing, ink delamination (ink peeling or ink drop-off) is a long-standing and persistent quality issue. Printed packaging may show ink flaking, fading graphics, or partial image loss during post-processing, transportation, storage, or even retail display.
In mild cases, the issue leads to rework or full reprinting. In severe cases, entire batches may be scrapped, causing delivery delays and significant cost losses.
Many printing companies initially blame ink quality and switch suppliers multiple times without improvement. Others focus on printing equipment accuracy and repeatedly adjust machine settings, yet the problem still remains.
1. Why Do Plastic Films Naturally Resist Ink Adhesion?
To understand ink delamination, it is important to first understand the nature of plastic films.
Most commonly used packaging substrates such as PE (polyethylene) and PP (polypropylene) films are non-polar materials. Their surfaces are smooth, highly dense, and lack porous structures that would allow ink penetration or anchoring.
For example, untreated BOPP film typically has a surface tension of only around 31 dynes/cm.
Printing ink, however, behaves like a liquid system. For proper adhesion, ink must be able to wet, spread, and anchor onto the film surface. This requires the substrate’s surface energy to be equal to or higher than the ink’s surface tension.
When surface tension is too low, ink cannot spread evenly. Instead, it contracts into droplets—similar to water on a waxed surface. As a result, the ink layer cannot form a stable bond and will easily peel off later.
This is why untreated PE and PP films are generally not suitable for direct printing.
2. What Is Corona Treatment and How Does It Improve Ink Adhesion?
To overcome the low surface energy of plastic films, the industry widely uses corona treatment as a surface modification process.
The working principle is based on high-voltage discharge. When the film passes through a high-voltage electrode field, the surrounding air becomes ionized, generating plasma and high-energy particles. These particles interact with the film surface and introduce polar functional groups such as carbonyl (-C=O) and hydroxyl (-OH).
At the same time, the treatment slightly etches the surface at a microscopic level, increasing surface roughness that improves mechanical anchoring.
As a result, the surface becomes more polar and chemically active. The surface tension (dyne level) increases significantly, allowing better wetting and adhesion of inks and coatings.
In simple terms, corona treatment transforms a “non-stick” plastic surface into one that allows ink to firmly grip and stay attached.
3. What Dyne Level Is Required for Printing? It Depends on the Material
Corona treatment is not just a pass-or-fail process. It must reach a sufficient and stable dyne level to ensure reliable print performance.
In general, for good ink adhesion in flexible packaging printing, the surface tension should be at least 38 dynes/cm. Below 36 dynes/cm, ink adhesion problems such as delamination and poor wetting become much more likely.
For UV printing applications, requirements are even higher, typically 42 dynes/cm or above.
Different materials also have different target levels:
Freshly treated films usually reach 40 dynes/cm or higher immediately after corona treatment. However, after storage, the surface energy gradually decreases and should remain above 38 dynes/cm for stable printing performance.
It is also important to understand that higher is not always better. Excessive corona treatment can damage the surface structure of the film and lead to instability in printing performance. The practical rule is simple: meet the requirement without over-processing.
4. Why Does Dyne Level Decrease Over Time? The Hidden Risk in Storage
One of the most overlooked issues in flexible packaging printing is that corona treatment is not permanent.
After treatment, the improved surface energy gradually declines over time—a process known as dyne decay.
Under normal storage conditions (cool, dry, and well-ventilated), the effective dyne retention period is typically 3 to 6 months. However, higher temperatures and humidity levels can significantly accelerate this decline.
In controlled warehouse environments, a drop of about 2–3 dynes/cm over six months is common.
Why does this happen?
The reason lies in molecular migration and surface instability. After corona treatment, the surface layer of the polymer becomes more reactive but also less stable. Over time, internal additives such as slip agents and anti-block agents gradually migrate to the surface, forming a weak boundary layer.
In addition, environmental factors such as humidity, dust, and airborne contaminants further reduce surface energy.
This explains a very common situation in production: the same batch of film performs well when newly delivered, but shows ink adhesion failures after a few months of storage. The dyne level has quietly dropped below the required threshold.
5. Frequently Asked Questions (FAQ)
Not necessarily. While 38 dynes/cm is widely considered the minimum requirement, real-world performance depends on multiple factors, including ink type, printing speed, substrate, and environmental conditions.
For example, UV inks typically require at least 42 dynes/cm or higher. Water-based inks also demand higher surface energy compared to solvent-based inks.
In addition, dyne level only reflects surface energy—it does not fully represent actual ink adhesion strength. For this reason, print testing before mass production is strongly recommended.
The effectiveness of corona treatment is time-sensitive. In general, dyne levels remain stable for 3 to 6 months, depending on storage conditions.
For PE films, immediate use after production is recommended whenever possible. Storage should ideally not exceed 3 months. If storage exceeds 1 month, dyne testing is necessary before printing.
Proper storage conditions include a cool, dry, and well-ventilated environment. High temperature and humidity will significantly accelerate surface energy loss.
There are several solutions available:
The most direct method is re-corona treatment, which restores surface tension before printing. If the printing line is equipped with in-line corona systems, this can even be done during production.
Another option is primer coating, which applies a functional layer to improve ink adhesion on the film surface.
However, both methods add extra processing steps and cost. From a production efficiency perspective, the best approach is still preventive: selecting freshly treated films with stable dyne levels and using them within their effective storage period.
Conclusion
Ink delamination in packaging printing is rarely caused by a single factor. Instead, it is the result of a chain of variables across film production, storage conditions, and printing processes.
Among all these factors, dyne level (corona treatment surface energy) plays a central role. It directly affects ink wetting, adhesion, and long-term print stability.
From proper corona treatment control during film manufacturing, to controlled storage environments, to routine dyne testing before printing—every step is essential to ensuring consistent print quality and reducing production risks.
