How to improve ink adhesion on PET film

Most adhesion failures of water-based ink on PET are diagnosed faster when you check the film surface before changing the ink. Measure surface energy, confirm corona treatment is recent and even, rule out contamination, and only then change resin type or add a primer.

This sequence mirrors how failures present on the line. A film that has lost surface energy behaves like an untreated film, and no ink modification repairs that. The reverse is just as common: re-treating a film whose failure comes from the ink–film match wastes time and can hide the real cause. Record each result as you go — the cause usually appears within one shift of structured checks.

• Step 1 — confirm surface energy at the press, not at the slitter
• Step 2 — check corona treatment age and uniformity across the web
• Step 3 — rule out contamination from slip additives, dust or moisture
• Step 4 — compare resin systems or add a primer, then verify with a drawdown trial

Why does PET reject water-based ink in the first place?

PET is a low-surface-energy, non-absorbent polyester film. Water-based ink wets the surface only when the film energy is high enough relative to the ink surface tension, so the first contact between ink and film decides most of the battle. On an untreated or decayed surface the ink beads, dries into a discontinuous film, and lifts under the first tape test.

The decorative-film context adds a second difficulty: the printed film must later survive lamination heat, aging and handling, so the ink has to stay anchored through conditions the press never sees. A clean wet-out check at the print unit is necessary but never sufficient — adhesion must be verified on the finished, conditioned part.

Step 1 — is the surface energy high enough, and is it still high at the press?

Treat the value from the film supplier as a starting point, not a fact. PET film is commonly corona- or flame-treated to a level that suits the supplier’s own tests, and that level can decay in storage. Measure with dyne test pens or a contact-angle instrument at the reel you actually run, on the side you print.

A common acceptance band in film printing is roughly 38–42 dyn/cm for water-based inks, but the right target depends on your ink system and film grade — confirm it with both suppliers. What matters more than the absolute number is consistency: record the value at the start of the shift and watch for drift between reels.

Step 2 — has corona treatment decayed or become uneven?

Corona treatment raises surface energy by oxidation, but the effect is not permanent. Decay typically accelerates with storage time, humidity and temperature, so a roll treated last month can read like an untreated roll today. Check the treatment date or energy log when your supplier provides one, and re-test reels that have waited in the warehouse.

Uneven treatment is harder to catch because one spot reading can pass while the reel fails in bands. If failures appear in repeating stripes or at fixed positions across the web, measure across the width at several points instead of at the edge, and correlate the pattern with the position in the corona station.

Step 3 — is contamination blocking the contact?

A clean, freshly treated film can still fail when something sits between ink and surface. Typical culprits are slip and antiblock additives migrating to the surface, die-cut dust from earlier converting steps, and moisture or handling residue at the unwind. Each blocks contact locally and produces pinholes, poor wet-out or localized lift.

If wiping changes the result — the wiped area prints and adheres while the unwiped area fails — contamination is confirmed, and the fix belongs to film grade and handling, not to the ink.

• Check slip and antiblock additive levels in the film specification — high-slip grades are a common root cause
• Wipe the print area and inspect for residue before blaming the ink
• Verify storage conditions: cool, dry, and away from solvents and plasticizer sources

Step 4 — should you change the resin or add a primer?

Once the surface is confirmed good, the next variable is the ink itself. Water-based ink resins differ in wetting, anchoring and post-treatment behavior: some acrylic emulsion systems give clean resolubility and printability but limited adhesion on difficult films, while polyurethane-based or specially modified systems typically anchor better on PET, with different drying or resolubility behavior. There is no universal best — the match is grade-specific.

A primer or anchor coat is the deliberate answer when no single ink satisfies both printability and end-use adhesion. Primers add a unit operation and a cost, so use them after the resin-family comparison fails, not before. Run that comparison as a drawdown or proof on the production film, condition it through the real lamination and aging steps, and judge by the end-use tape or peel test — not by a fresh print.

When should you bring the ink supplier in?

Bring the supplier in before you change chemistry, not after three failed trials. The useful data is small: a film sample, the treatment reading, the failing test and the end-use conditions. With those, an application engineer can usually separate surface causes from chemistry causes in a short bench study, and a benchmark sample can be matched to your film rather than chosen from a catalogue.

The same screening order applies to PVC and PP decorative films — the film-specific inputs are covered in the PVC decorative film ink specification guide, and the drying consequences of water-based systems in the water-based versus solvent-based gravure comparison.