Introduction
Most adhesion problems blamed on ink, adhesive, or coating are actually surface energy problems. A film that leaves the extruder at 31 mN/m will not hold a water-based ink no matter how the press is set up. Corona treatment exists to raise that surface energy to a level the downstream process can grip, and the only reliable way to know whether it worked is to measure the dyne level.
This guide explains what dyne level measures, the target values for common substrates, how to test correctly, and how to troubleshoot a corona treatment system that measures lower than it should. It is written for operators, quality engineers, and converters who need numbers rather than reassurance.
What Dyne Level Actually Measures
Dyne level - more precisely, surface energy or wetting tension - describes how strongly a liquid will spread on a solid surface. It is expressed in mN/m (millinewtons per metre), which is numerically identical to the older dyne/cm, so the two terms are used interchangeably in most plants.
Untreated polyolefins are non-polar and low in surface energy. Ink, adhesive, and coating are polar. When the surface energy is lower than the liquid's surface tension, the liquid beads up instead of spreading, and the bond that forms is mechanical rather than chemical - weak, and prone to failing in the field.
Why Dyne Level Matters More Than Machine Power
Buyers routinely specify a corona treatment system by power output in kilowatts and treat that figure as a proxy for performance. It is not. Achieved dyne level depends on the combination of power density, electrode design, dwell time, web temperature, humidity, and the formulation of the film itself.
Two machines rated at the same power can deliver very different dyne levels on the same substrate. The specification that matters is the dyne level achieved at a stated line speed on a stated film - not the kilowatt rating on the nameplate.
Target Dyne Levels by Substrate
The following values are practical working targets for common packaging and converting substrates. Requirements vary with the specific ink or adhesive, so always confirm with your supplier as well as with the measurements below.
| Substrate | Typical target dyne level | Common downstream process |
|---|---|---|
| LDPE / LLDPE film | 38–42 mN/m | flexo printing, lamination |
| HDPE film | 40–44 mN/m | labels, packaging |
| BOPP film | 38–42 mN/m | labels, adhesive coating |
| PET film | 44–50 mN/m | metallising, coating |
| PVC film | 38–42 mN/m | shrink sleeves |
| Aluminium foil | 38–44 mN/m | lamination |
Two rules follow from this table. First, over-treatment is a real fault: pushing PET far above 50 mN/m can embrittle the surface and cause blocking or seal failure. Second, treat to the target, not to the maximum - the goal is a process window, not a record number.
How to Test Dyne Level
The standard methods are simple and inexpensive. ASTM D2578 covers wetting tension testing of polyethylene and polypropylene films, and ISO 8296 defines the equivalent method for plastics films and sheeting. Both rely on the same principle: apply a series of test liquids of known surface tension and observe which one just wets the surface instead of beading.
Dyne pens are the fastest field method, but they have limits. The pen tip can be contaminated, the ink can age, and the result is subjective if the operator is in a hurry. For anything customer-facing, confirm with a fresh set of test inks on a clean sample, and record the temperature and humidity at the time of measurement.
Practical rules for a trustworthy reading:
- Test the treated side, and never touch the test area with bare hands - skin oils read as a false low.
- Measure across the web width, not only in the centre lane.
- Test within minutes of treatment on the line, and again after storage to see how the level decays.
- Use a fresh specimen; cut a clean sample rather than testing a torn edge.
- Record the number, the substrate, the line speed, and the date.
Reading Results: Common Patterns
A single number tells you little. The pattern across the web tells you a lot.
Level correct in the centre, low at the edges points to electrode wear, an edge gap, or uneven electrode-to-roll contact. Level high at the start of the run and falling over hours usually indicates increasing web temperature or a drifting power supply. Level correct on one line and low on another running the same film suggests a mechanical setup difference rather than a formulation problem.
Level correct immediately after treatment but low when the roll is unwound days later is normal decay, and it is why a corona treatment system should always be positioned as close to the downstream process as the layout allows. For some films, a re-treatment station before the press is a sensible investment rather than a luxury.
Troubleshooting a Corona Treatment System
Work through these checks in order when measured dyne level is below target:
- Confirm the measurement first. Check pen age, operator technique, and sample cleanliness before touching the machine.
- Inspect the electrodes and roll. Contamination, oxide build-up, and worn electrodes reduce treatment efficiency dramatically.
- Check the gap and contact. Electrode-to-roll clearance and wrap angle decide dwell time; small mechanical errors cause large losses.
- Verify power supply output. A failing transformer or generator can lose output without any visible symptom.
- Look at film formulation. Slip and anti-block additives migrate to the surface and suppress treatment; a reformulated resin may need different settings.
- Check the environment. High humidity and low web temperature both reduce achieved dyne level.
Documenting the results of these checks turns a recurring mystery into a short, repeatable procedure.
Maintenance and Electrode Care
A corona treatment system rarely fails suddenly. It drifts, and the drift is invisible until a bond fails downstream. Electrode condition is the main variable, and it deserves a scheduled routine rather than attention only when trouble appears.
Clean electrodes on a fixed interval - weekly in dusty converting environments, monthly in clean ones - using the procedure and solvents the manufacturer specifies. Oxide build-up and ink mist on the electrode surface reduce the effective treatment area, so power stays constant while corona treatment output quietly falls. Ground roll surfaces need the same attention: a contaminated roll changes the dielectric barrier and shifts results across the web.
Record the cleaning date, the measured dyne level before and after, and any electrode replacement. Over a few months that log tells you the real replacement interval for your line, instead of the one on the datasheet. It also gives you evidence when a customer asks why a batch behaved differently: a corona treatment fault that appears in the log is a solved problem, while one that appears only in the field is a dispute.
Keep spare electrodes, a set of fresh test inks, and a calibrated measurement routine on site. Downtime spent waiting for an electrode to arrive costs far more than the part.
Specification Checklist
When you request a quotation for a corona treatment system, include these points:
- Substrate type, thickness, and line speed range.
- Required dyne level at the slowest and fastest line speeds, not just in the middle.
- Web width, including any edge trim, and treatment width required.
- Existing process: printing, lamination, coating, or metallising, and the ink or adhesive chemistry.
- Electrode access for cleaning and expected maintenance interval.
- Measurement method and who will verify compliance on acceptance.
A supplier who asks for these details is specifying a treatment result. A supplier who quotes only kilowatts is selling a box.
FAQ
Q1: What is a good dyne level for printing on BOPP?
38–42 mN/m is the usual working range for BOPP before flexo printing or adhesive coating. Confirm the exact figure with your ink or adhesive supplier, since solvent and water-based systems differ.
Q2: How long does corona treatment last?
It decays over time, faster on some films and in warm storage. Many converters aim to use treated film within days, and re-treat immediately before printing for critical work. Where the corona treatment station sits in the line matters: the closer it is to the press, the less the level decays before use.
Q3: Can you over-treat a film?
Yes. Excessively high dyne levels can cause blocking, seal failure, and surface embrittlement, and they waste energy. Treat to the target window rather than as high as possible.
Q4: What is the difference between dyne level and surface energy?
They describe the same property. Dyne/cm and mN/m are numerically equal, and both express wetting tension. The terminology differs between industries, not the physics.
Q5: Which standard covers dyne testing?
ASTM D2578 covers wetting tension testing of polyethylene and polypropylene films, and ISO 8296 covers plastics films and sheeting. Both are based on test liquids of known surface tension.
Q6: Why does my dyne level drop at the edges of the web?
Usually electrode wear, an uneven gap, or insufficient treatment width. Check electrode condition and the mechanical alignment before increasing power.
Q7: Is corona treatment suitable for aluminium foil?
Yes, and it is widely used to improve adhesion before lamination and coating. Foil needs careful thermal management because it conducts heat efficiently, so treatment settings differ from plastic films. Confirm that the corona treatment system is specified for metallic substrates, since electrode and roll configurations are not identical.
Conclusion
Dyne level is the only honest measure of whether a corona treatment has done its job. Set a target for each substrate, test it with fresh inks using a documented method, watch how the number behaves across the web and over time, and keep the machine clean and correctly gapped. Treat the dyne level as part of production control rather than a one-off acceptance test, and the corona treatment specification finally means something on the shop floor instead of on a certificate. Measure the corona treatment result on a schedule and the corona treatment stays repeatable, batch after batch.
Shenzhen Hefengjiada Technology Co., Ltd. manufactures corona treatment systems, plasma treaters, web cleaning machines, and static eliminators for film, foil, and sheet converters, including narrow-web coronastations, floor-standing and cabinet-style corona generators, and integrated and hanging configurations. Send us your substrate, line speed, and required dyne level, and our team will specify the right system and support your acceptance test.