Corona treatment: why ink and adhesive stick to plastic film

What corona treatment actually is

Corona treatment is a surface-modification process that raises the surface energy of a plastic film so that inks, coatings, adhesives, and laminating bonds will actually stick to it. The name comes from the faint blue-violet corona — a controlled electrical discharge — that plays across the web as film passes through the treater at the front of a printing press, a laminator, or a stand-alone treating station.

The reason it exists is chemistry. The workhorse films of flexible packaging — polyethylene (PE), polypropylene including biaxially-oriented BOPP, and to a lesser degree PET — are polyolefins or near-inert polymers with smooth, non-polar, low-energy surfaces. A drop of water-based or solvent-based ink placed on untreated PE beads up like rain on a waxed car instead of spreading and keying into the surface. Corona treatment briefly oxidizes that surface, planting polar, oxygen-containing groups that ink and adhesive can grab onto. It is a foundational input to flexographic printing, the process that lays ink onto film at speed.

Crucially, treatment is a surface effect measured in molecular layers. It does not change the film's thickness, clarity, or bulk strength — only the top few nanometers, which is exactly the zone where a printed film either bonds or fails. That makes corona treatment one of the quiet, load-bearing steps behind almost every printed pouch, shrink sleeve, and poly mailer.

Surface energy and dyne level, explained

Whether a liquid spreads on a solid or beads up is governed by the balance between the surface energy of the solid and the surface tension of the liquid. If the film's surface energy is higher than the liquid's surface tension, the liquid wets out — it spreads into a thin, continuous film and makes intimate contact. If it is lower, the liquid retracts into droplets and never forms a proper bond.

Surface energy is reported in dynes per centimeter (dyne/cm), usually shortened to a "dyne level." In practice, a dyne level is the surface tension of the highest test fluid that will still wet the film. The higher the number, the more printable and bondable the surface. Untreated polyethylene typically measures around ~30–31 dyne/cm — below what most inks and adhesives need — which is why it prints poorly straight off the roll.

How a corona treatment station works

A corona treater is deceptively simple. The web of film passes over a roller — the treater roll — covered in a dielectric (an insulating sleeve, often silicone or ceramic). Above the roll sit one or more electrodes, separated from it by a small air gap. A high-frequency, high-voltage power supply drives that gap, ionizing the air into a visible corona discharge. As the film travels through the discharge, the energized, oxygen-rich plasma bombards its upper surface and oxidizes it.

Three parts do the work:

1. The electrode — the high-voltage bar that creates the discharge across the air gap.
2. The dielectric-covered roll — the film rides on this insulated roller, which carries the web through the discharge zone and keeps the machine from arcing.
3. The discharge and film path — the ionized air gap the film passes through; only the exposed face is treated, so film is treated on the side you intend to print or bond.

Because only the exposed face is treated, converters are careful about which side of the web faces the electrode. Treating the wrong side, or bleeding treatment through to the back of the web ("backside treatment"), is a real defect. Treatment intensity is a function of power and dwell — how much energy is delivered per unit area as the film moves through. Too little and the surface never reaches its target dyne level; too much can degrade the surface, create weak low-molecular-weight material that actually reduces the bond, or cause blocking. That power-and-dwell relationship — electrode, dielectric roll, film path, and the resulting dyne level — is the heart of how a treater is set up.

Dyne targets by substrate and process

There is no single "correct" dyne level — the target depends on the substrate and on what you are doing to it. The values below are industry-typical rules of thumb, not universal specifications; your ink, adhesive, and film suppliers give you the exact number to hit for your job. Wetting tension is commonly measured to the method described in ASTM D2578.

Substrate / process	Typical untreated surface energy	Typical target for printing or lamination
Polyethylene (PE / LDPE / HDPE)	~30–31 dyne/cm	~38–42 dyne/cm for surface printing
Polypropylene (PP / BOPP)	~29–31 dyne/cm	~38–42 dyne/cm for surface printing
PET (polyester)	~40–44 dyne/cm	often printable as-is; frequently boosted for lamination
Adhesive lamination (any film)	—	often higher than print, e.g. ~40–44 dyne/cm

A few honest caveats about those numbers:

• They are typical figures reported across the converting industry, not guarantees. Films ship with different additive packages, and slip agents in particular can lower the effective dyne level.
• Lamination and specialty coatings usually demand a higher dyne level than surface printing, because a buried bond has to survive the whole life of the package.
• The only number that matters for your job is the one your ink, adhesive, and film suppliers specify — treat the table as a starting point, not a spec.

Why corona treatment fades over time

Corona treatment is not permanent. From the moment film leaves the treater, its dyne level begins decaying back toward the untreated state — a phenomenon called hydrophobic recovery, or simply "dyne decay." The oxidized surface groups slowly reorient into the bulk, and mobile additives such as slip and anti-block agents migrate to the surface and bury the treatment.

How fast this happens depends on the film chemistry, the additive package, the storage temperature, and the roll tension. Warmer storage and additive-rich films decay faster. That is why converters follow two rules of thumb:

• Treat close to converting. Film is often treated — or "bump" re-treated — in-line, right before printing or laminating, so the surface is fresh when ink or adhesive meets it.
• Do not stockpile treated film. A roll that tested fine months ago may have drifted below target by the time it reaches the press.

If your film was treated by the supplier and then sat in a warehouse, do not assume the dyne value printed on the box still holds. Re-check it before you run.

How to check surface energy: dyne pens and inks

You do not need a lab to get a working read on surface energy. The everyday shop tools are dyne pens (also called corona pens) and dyne test inks — calibrated fluids with a known surface tension.

• Dyne pens: draw a line of the calibrated fluid across the film. If the line holds together as a continuous film for a couple of seconds, the film's surface energy is at or above that pen's rating; if it beads up or retracts into droplets, the film is below it. You step up or down through pen values to bracket the dyne level.
• Dyne test solutions (per ASTM D2578) — the same principle with poured solutions read against a stopwatch, for a more repeatable number.

Two cautions: dyne fluids are a go/no-go indicator, not a precision instrument, and the test itself can contaminate the film — so test on a sacrificial edge, never on product, and always test the side you will print. When ink adhesion is marginal, a fresh dyne check is the fastest, cheapest diagnostic you can run.

When the ink won't stick: under-treated or decayed film

"Why won't ink stick to my plastic film?" almost always has one of two answers on the surface-energy side: the film was under-treated and never reached its target dyne level, or it was decayed — fine once, but the treatment faded before printing. Either way the ink or adhesive cannot wet out, and you see scuffing, poor rub resistance, ink that lifts with tape, or a lamination that peels.

Surface energy is only one of several adhesion variables — cure, ink and adhesive chemistry, primer or coating choice, and contamination all matter — but it is the first one to rule out because it is quick and cheap to check. Adhesion failures that appear after lamination, as a bond separating between layers, are a related problem we cover in depth in our guide to delamination in flexible packaging. If you are still choosing a structure, our overview of flexible packaging types explains where these printable films fit, and this whole topic is part of our broader guide to how flexographic printing works.

How PackOS flags surface-energy risk

You cannot read a dyne level off a PDF — surface energy is a physical property of the film on press day. What PackOS can do is catch the upstream conditions that make it matter: recognizing that a job is a printed polyolefin film, spotting where an ink or coating has to bond to a low-energy substrate, and flagging lamination structures where treatment and adhesion are load-bearing. Those checks live alongside the rest of the preflight and quality tooling on our quality technology page, so a surface-energy-sensitive job gets the right questions asked before it ever reaches the press. When you are ready for a real number on a printed flexible-packaging job, Quick Quote reads your artwork and prices it.