Blocking is expensive because it is discovered late. The reel looks fine when it is wound; the problem appears at the customer, on the unwind, as tearing or coating transfer.
What causes it
A coating blocks when it is soft enough to flow under pressure and make intimate contact with whatever surface it is wound against — usually the reverse side of the film.
Three factors combine:
Film softness, governed mainly by glass transition temperature. A coating with Tg near or below storage temperature has enough molecular mobility to flow.
Pressure, from winding tension and reel weight. The inner layers of a large reel carry substantial pressure.
Heat, which raises effective mobility. This is why blocking complaints cluster in summer and in warm warehouses, and why a reel that shipped fine in February fails in June.
Why it fights with adhesion
This is the part that makes blocking hard to solve by formulation alone.
Adhesion requires a coating soft enough to wet and conform to the substrate surface. Block resistance requires a coating hard enough not to flow under pressure. Both are governed by the same property.
Move Tg up and blocking improves while adhesion and flexibility degrade. Move it down and the reverse happens. There is no setting that maximises both, which is why the fix usually has to come from somewhere other than the binder.
The four levers
1. Raise Tg — but only as far as adhesion allows.
The direct fix, and the one with the clearest cost. Worth doing if you have adhesion headroom. If adhesion is already marginal, this will trade one failure for another.
2. Add surface slip.
Waxes and slip additives migrate to the coating surface and create a low-friction, low-contact-area layer. The bulk of the film stays soft enough for adhesion; the surface behaves as if it were harder.
This is the most common practical answer because it decouples the two properties. The caveat is that the same additives can interfere with over-printing, lamination adhesion and heat sealing — so specify carefully if anything is applied on top later.
3. Fix the drying or cure.
An incompletely dried film contains retained solvent, which plasticises it and makes it far more prone to blocking. An incompletely cured UV film is soft at the surface for the same reason.
Before reformulating, verify:
- Drying capacity is adequate at current line speed
- Retained solvent is within specification
- For UV, surface cure is complete — surface tack from oxygen inhibition looks exactly like a blocking-prone film
This is worth checking first because it is free.
4. Change the winding.
Reduce winding tension. Allow the reel to cool before it is wound tight or stacked. Avoid storing reels in direct sun or against a warm wall. Reduce reel diameter if the inner layers are carrying too much pressure.
Process fixes are unglamorous and frequently sufficient.
A diagnostic sequence
- Test the film for retained solvent or incomplete cure. If either is out, fix that first — everything else is guesswork until it is.
- Run a block test at realistic conditions. Coated face to reverse, at the pressure and temperature the reel will actually see, for a realistic duration. A test at ambient for an hour tells you very little about a pallet in a warm warehouse for three weeks.
- If the film is fully dried and still blocks, try slip additive before changing Tg. It costs less in adhesion.
- If slip is not enough, raise Tg in small steps and re-test adhesion each time.
- In parallel, check winding tension and storage. Often the cheapest improvement available.
The thing worth saying to the customer
If a customer's blocking complaint arrives with "the ink is too soft", the useful response is to ask about storage temperature and reel tension before accepting the diagnosis.
A meaningful proportion of blocking complaints are storage conditions, not formulation. Testing that first is faster than reformulating, and it is occasionally the whole answer.