Electrical Corona Discharge
Key Takeaways
- Corona ionises ambient air across a small gap between an electrode and a grounded surface, forming a low-temperature plasma.
- Typical input is 10–30 kV AC applied to a sharp or rolled electrode.
- It raises surface energy so inks and adhesives wet out on polyethylene, polypropylene and polyester films.
- It has virtually no cleaning capability — contaminants must be removed before treating, not by it.
Electrical Corona Discharge
Electrical corona discharge treatment raises the surface energy of plastic films, sheets and webs by passing them through a high-voltage, high-frequency electrical discharge in air at atmospheric pressure. It is the dominant pretreatment in continuous web converting — extrusion coating, flexible packaging, label stock and film printing — because it is fast, inexpensive per square metre and integrates directly into the line.
How a Corona Treater Is Built
The web passes through a narrow gap between a high-voltage electrode and an earthed roller. One of the two surfaces carries a dielectric covering. Applying high-frequency high voltage across the gap — typically in the region of 10–20 kV at 10–30 kHz — ionises the air, producing the visible violet discharge that gives the process its name.
Reactive oxygen species formed in that discharge oxidise the polymer surface, grafting hydroxyl, carbonyl and carboxyl groups onto the hydrocarbon backbone. Polyethylene and polypropylene films, which are essentially unprintable untreated, routinely move from around 30 dyne/cm to 40–46 dyne/cm in a single pass.
Dose and Its Control
Corona treatment is quantified as watt density — power delivered per unit area per unit time, usually expressed in W·min/m²:
Watt density = Power (W) ÷ [ web width (m) × line speed (m/min) × number of treated sides ]
Holding watt density constant is what keeps dyne level constant when line speed changes. A treater set correctly at 100 m/min will under-treat at 200 m/min unless power is raised proportionally.
Characteristic Problems
- Treatment decay. Corona-treated film loses surface energy over days to weeks as polymer chains reorient and low-molecular-weight oxidised material migrates. Slip and antiblock additives accelerate the loss. Roll stock should be printed or laminated within its validated window, not months later.
- Backside treatment. Discharge can wrap around thin or narrow webs and treat the reverse face, causing blocking in the wound roll. Correct roller covering and gap setting control this.
- Pinholing and micro-perforation. Excessive watt density on thin film can puncture it or create a weak, over-oxidised boundary layer that fails cohesively — more treatment is not better.
- Ozone. Generated continuously; extraction and destruction are required.
- Geometry. The process suits two-dimensional webs. Moulded three-dimensional parts call for flame plasma, atmospheric plasma or cold gas plasma instead.
Measurement
Dyne pens and wetting tension solutions to ASTM D2578 remain the standard shop-floor check, applied across the web width to confirm uniformity rather than at a single point. Edge-to-centre variation is a common and easily missed defect. For validation work, contact angle measurement gives the quantitative surface energy figure — the approach set out in the science of solving plastics adhesion problems.
Watt-Density Targets in Practice
Watt density is the control variable, but the number that matters is the dyne level it
produces on a given substrate. These are the ranges commonly encountered in web
converting; treat them as a starting point for a trial, not as a specification, because
slip and antiblock packages move the requirement substantially:
| Substrate | Untreated surface energy | Usual target after treatment | Notes |
|---|---|---|---|
| LDPE film | About 31 dyne/cm | 38–44 dyne/cm | Slip additives migrate quickly and pull the treated level back down |
| HDPE film | About 31 dyne/cm | 38–44 dyne/cm | Similar to LDPE; decay is generally slower |
| BOPP | About 29 dyne/cm | 38–42 dyne/cm | The classic corona substrate; over-treatment causes blocking readily |
| PET film | About 41 dyne/cm | 46–52 dyne/cm | Starts higher, so the increment needed is smaller |
| Nylon film | About 40 dyne/cm | 46–52 dyne/cm | Hygroscopic — ambient humidity affects the reading |
| Polyethylene extrusion coating | Low, and falling with melt temperature | Set by the lamination bond requirement | Ozone treatment of the melt curtain often accompanies corona on the substrate |
Set the requirement from the ink, adhesive or coating actually being used rather than
from a generic dyne target. A specification that says 42 dyne/cm because the industry
often says 42 dyne/cm is a specification nobody has tested.
Troubleshooting a Corona Line
| Symptom | Likely cause | Correction |
|---|---|---|
| Dyne level correct at centre, low at the edges | Electrode not covering full web width, or gap varying across the roll | Measure across the width every time, never at one point. Check electrode length against web width and verify gap uniformity. |
| Treatment level falls when the line speeds up | Watt density falling because power was not raised proportionally | Interlock power to line speed so watt density is held constant rather than power. |
| Roll blocks when unwound | Backside treatment — discharge wrapping around a thin or narrow web | Correct roller covering and gap. Check that web width matches electrode configuration. |
| Good dyne reading, poor ink adhesion anyway | Contamination. Corona activates whatever is on the surface, including the contaminant | Find the source — slip additive bloom, compressor oil carry-over, mould release, airborne silicone. Corona has essentially no cleaning capability. |
| Adhesion good at converting, failing weeks later | Treatment decay in stored roll stock | Establish and enforce a validated window between treatment and conversion. Re-measure dyne level on aged rolls before running them. |
| Pinholes or weak, powdery surface | Over-treatment — excessive watt density has degraded the surface into a weak boundary layer | Reduce watt density. More treatment is not better; an over-oxidised layer fails cohesively. |
| Result varies with the weather | Ambient humidity affecting the discharge and, on hygroscopic films, the reading | Log ambient conditions alongside dyne readings before concluding the treater has drifted. |
Where Corona Fits Among the Pretreatments
| Corona | Flame plasma | Atmospheric plasma | Cold gas plasma | |
|---|---|---|---|---|
| Best geometry | Two-dimensional web | Web and simple three-dimensional | Three-dimensional, selective areas | Complex three-dimensional, batch |
| Cleaning capability | Essentially none | Some | Good | Excellent |
| Cost per unit area | Lowest | Low | Moderate | Highest |
| Line integration | Direct, inline | Inline | Inline or robot-mounted | Batch, vacuum chamber |
| Treats shadowed surfaces | No | No | No | Yes |
| Durability of activation | Days to weeks | Days to weeks | Hours to days | Longest of the four |
Related Terms and Reading
- Electrical corona discharge treatment services
- All polymer surface pretreatment methods
- Advanced technologies for decorating polyethylene
- Inkjet printing and adhesion of low surface energy polymers
Applying this in production
The Sabreen Group provides independent engineering support for corona treatment optimisation and web pretreatment troubleshooting. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
What is electrical corona discharge treatment?
A surface treatment that applies a high-voltage electrical field across a small air gap, ionising the ambient air into a faint brush-like plasma near the material surface. The reactive species that forms oxidises the surface, raising its energy so inks, adhesives and coatings can wet out.
What voltage does corona treatment use?
Alternating current typically between 10 and 30 kV applied to a sharp or rolled electrode. The intense field ionises surrounding air molecules, creating reactive oxygen species and UV photons that modify the surface.
Which materials benefit most from corona treatment?
Low surface energy polymers that resist bonding — polyethylene, polypropylene and polyester films especially. It is widely used ahead of printing, lamination, coating and adhesive bonding on these substrates.
Does corona discharge clean the surface as well?
Essentially no. Corona has virtually no cleaning capability, so dirt, grease, oils, silicones, mold release and slip agents must be removed before treatment. Treating over contamination simply activates the contaminant rather than the substrate.
How long does corona treatment last?
Treatment ages, and shelf life depends on the resin, its formulation and the storage environment. Low molecular weight materials migrating to the surface progressively reduce the effect, so bonding, coating or printing should follow as soon as practical.
What watt density should I run?
Whatever holds the dyne level the ink, adhesive or coating actually requires — the number should come from a trial with the real chemistry, not from a generic industry figure. Polyolefin films commonly move from around 30 dyne/cm untreated to 38–44 dyne/cm treated; PET and nylon start higher and target 46–52. Hold watt density, not power, constant when line speed changes.
Why does adhesion fail when the dyne reading is on target?
Almost always contamination. Corona has virtually no cleaning capability, so it activates whatever is on the surface — slip additive bloom, compressor oil carry-over, mould release or airborne silicone. A dyne pen reads the surface energy of the contaminant quite happily. Find and remove the source rather than raising the treatment level, which will not help.
Can a corona-treated roll be stored before printing?
Only within a validated window. Treated film loses surface energy over days to weeks as polymer chains reorient and low molecular weight material migrates back to the surface, and slip and antiblock additives accelerate the loss. Establish the window for the specific film and storage conditions, enforce it, and re-measure dyne level across the width on any roll that has aged before running it.
