Corona Treatment for Packaging Film: Watt Density Guide

What corona treatment changes on a film surface

Corona treatment uses a high frequency electrical discharge as the film passes between an electrode and a grounded roller. The discharge changes the surface chemistry so the film's surface energy, measured in dyne/cm, rises. Higher surface energy improves wetting, which is the first step toward ink, coating or adhesive bonding. The treatment is a surface preparation step, not a substitute for choosing a compatible ink or adhesive.

Vetaphone describes the effect as breaking carbon hydrogen bonds at the surface and adding oxygen containing polar groups. The aim is to make a normally low energy polymer surface more receptive. The bulk film is not the target of the treatment. Confirm any critical mechanical, stiffness or optical requirement with the film supplier and converter rather than assuming treatment has no effect on a sensitive structure.

Why the kW readout is not the treatment dose

The number on the power supply tells you what the treater is instructed to deliver to the electrodes. It does not tell you how much treatment the film receives. The applied dose is expressed as watt density, which combines power with electrode width, line speed and the number of treated sides. A wider electrode, a faster line or a second treated side changes the dose even when the kW setting stays the same.

Use the active electrode width, not the full film width. If the electrode is segmented, add the active segments. This is the difference between a number that looks stable on the screen and a number that describes the film's exposure. The arithmetic below is a planning exercise, not a machine setup instruction.

A hypothetical watt density calculation

Suppose, hypothetically, a converter is planning a trial on a line with a 1.5 m active electrode width, a line speed of 120 m/min and one treated side. The power supply is set to 5,000 W. Watt density is power divided by the product of active electrode width, line speed and treated sides. Multiply 1.5 by 120 by 1 to get 180, then divide 5,000 by 180. The result is about 27.8 watt minutes per square metre. Enercon publishes the formula in feet, so its examples are in watt minutes per square foot. One watt minute per square foot equals about 10.76 watt minutes per square metre, so metric and imperial figures cannot be compared without converting. This is an illustrative calculation, not a recommended setpoint.

In this hypothetical example, increasing speed to 180 m/min with the same power and width makes the denominator 270 and lowers watt density to about 18.5 watt minutes per square metre. Maintaining the original dose would require 7,500 W, subject to the equipment's operating limits. For the two-sided example, assume 5,000 W total is shared equally between two treating stations, each with a 1.5 m active width, at 120 m/min. Each side then receives a calculated 13.9 watt minutes per square metre. If each station instead receives 5,000 W, the calculated dose remains 27.8 per side. Confirm how the machine reports and allocates power, and calculate each side separately when power or active widths differ.

Match the trial to material and process

Materials do not respond identically. Vetaphone notes that untreated polyolefin films can have surface energies as low as 29 to 31 dyne/cm, while many inks need roughly 38 to 42 dyne/cm to adhere reliably. Those figures are a starting point for discussion with the ink and film suppliers, not a universal target. The required level depends on the ink, coating, adhesive, substrate and end use.

Enercon's guidance makes the same point about watt density: the relationship between dose and dyne level is not linear, and it depends on system and material parameters. A dose that works on one film may be insufficient or excessive on another. Over treatment can create operational problems, so the goal is a verified window, not the highest number the treater can deliver.

Primary treatment, secondary treatment and decay

Many films are treated immediately after extrusion, often called primary treatment, so they can be wound, stored and shipped in a condition suitable for later converting. That treatment is not necessarily permanent. Vetaphone explains that surface energy can decline over time as additives migrate and the surface relaxes. Slip agents, antiblock and antistatic compounds can interfere with treatment and can speed the drop.

A secondary or bump treatment is often applied in line just before printing, laminating or coating to restore the surface energy. Do not assume the incoming roll still has the dyne level recorded at extrusion. Ask for the incoming reading, the storage conditions and the age of the roll, then confirm the reading on the actual roll used in the trial. Process conditions such as line speed, electrode design and air gap can affect uniformity.

Verify adhesion with the real ink, coating or adhesive

Dyne pens and dyne solutions are useful screening tools for surface wetting behavior, not direct measurements of adhesion or complete characterizations of surface energy. A film can show an acceptable dyne reading and still fail in a print, lamination or seal test. The practical check is to run the actual ink, coating or adhesive on the treated film under the intended process conditions and then test the result.

Record the treatment dose, the dyne reading before and after treatment, the ink or adhesive batch, the line speed and the test result. If the result is marginal, change one variable at a time. Do not use a dyne number alone to accept a production roll. The converter and material supplier should agree on the acceptance test and the sampling plan.

Ozone, equipment limits and safe operation

Corona discharge produces ozone, and Vetaphone notes that this ozone needs to be extracted. Follow the treater manufacturer's requirements for extraction, ventilation and electrical safety. Do not improvise extraction or bypass interlocks. This guide is not a service manual and does not replace the equipment maker's instructions.

Keep the trial within the treater's stated operating envelope. If a target dose appears to require a setting outside that envelope, stop and ask the treater supplier and converter to review the equipment configuration. A generic dose table cannot approve a change to electrodes, power supply or line speed.

A practical checklist for a treatment trial

This checklist is Teal Packaging's editorial planning framework for a trial. It is not a substitute for the converter's standard operating procedure or the equipment maker's instructions.

• Confirm the exact film grade, resin blend and additive package with the film supplier.
• Record the incoming dyne reading on the roll that will be used, not a generic certificate value.
• List the active electrode width, treated sides and available power range.
• Calculate watt density at the lowest and highest line speed in the trial.
• Ask whether the treater has watt density control if line speed will ramp.
• Run the actual ink, coating or adhesive and test adhesion after the agreed cure or drying step.
• Keep a sample of treated and untreated film for comparison.
• Record ozone extraction and electrical safety checks as required by the treater maker.
• Agree the acceptance test and sampling plan with the converter before production.

Scope limits and what this guide does not decide

This guide explains the planning logic for corona treatment on packaging film. It does not set a universal watt density, a universal dyne target or a production recipe. It does not approve a film, ink, coating or adhesive for a regulated application. It does not replace a trial on the actual line with the actual materials.

Use the sources as background for the principles named in each support note. Your converter, film supplier, ink supplier and treater manufacturer remain responsible for the equipment settings, safety controls and acceptance criteria. If a customer or channel has a specific requirement, confirm it in the current agreement rather than relying on a general guide.

Questions about corona treatment and watt density