Two film problems get confused constantly, and because the fixes are different additives, confusing them means you dose the wrong one, see no improvement, and dose more of it.

Blocking is two layers of film sticking to each other. You unwind the reel and it peels apart reluctantly, or tears. Slip is friction — how easily film slides against itself or against machine parts. A film can block without having a slip problem, and vice versa.

Getting the diagnosis right takes about a minute, and saves a lot of wasted additive.

Why film blocks

Blocking happens because two smooth polymer surfaces in intimate contact, under pressure, at slightly elevated temperature, begin to behave like one surface. All three conditions are present in a freshly wound reel: the film is still warm from the line, the winding tension presses the layers together, and a good film has a very smooth surface.

Which is the irony — the better your film surface, the worse it blocks. LLDPE and LDPE block more readily than HDPE, and thin film blocks more than thick.

The fix: anti-block

Anti-block additives are fine inorganic particles — typically silica, talc or synthetic silicates — that migrate to the film surface and sit slightly proud of it. They hold the two layers microscopically apart, so there is no intimate contact and no blocking.

The trade-off is optical. Those particles scatter light, so haze goes up and gloss goes down as you increase anti-block loading. On a film sold on clarity — a window in a package, a laminate face — that trade-off is the whole design problem. Particle size and particle size distribution matter here: a well-graded anti-block gives you separation at lower loading and costs you less clarity.

Why film has friction problems

Slip is a different property: the coefficient of friction between film and film, or film and metal. High COF shows up as film that will not feed cleanly through a form-fill-seal machine, bags that will not open on a filling line, or reels that are hard to handle.

The fix: slip

Slip additives are usually fatty acid amides — erucamide and oleamide are the common ones — that bloom to the surface and form a very thin lubricating layer. Erucamide migrates more slowly and is more thermally stable, so it tends to be preferred where the film will be stored before use or processed at higher temperature. Oleamide blooms faster.

Two things to know about slip. First, it is time-dependent. The additive has to migrate to the surface, so COF drops over hours or days after production. Testing a reel straight off the line will give you a reading that is not representative of what your customer receives. Second, slip can interfere with printing, sealing and lamination, because a lubricating layer on the surface is exactly what an ink or adhesive does not want to bond to. If the film is being printed or laminated, tell your supplier before they formulate.

So which do you need?

Diagnose it this way:

  • Layers stick and resist separating when unwinding — blocking. You need anti-block.
  • Layers separate fine, but film drags through machinery or bags will not open — friction. You need slip.
  • Both — which is the common case in packaging film, and why anti-block and slip are frequently supplied as a combined additive masterbatch.

A useful quick test: take two pieces of the film, press them together firmly by hand, and try to pull them apart. Resistance to separating is blocking. Then slide one piece across the other. Resistance to sliding is friction. They are genuinely independent, and you can have one without the other.

Common mistakes

Adding more slip to fix a blocking problem. Slip reduces friction; it does not hold layers apart. You will spend money and still have a blocked reel — and you may now have a film your printer cannot use.

Over-dosing anti-block on a clarity film. Haze is largely irreversible once specified into the formulation. Start low, test, increase — do not start at the loading someone used on an opaque film.

Testing too early. Both additives need time to migrate. A COF or block test on film an hour off the line tells you very little about the reel your customer opens next week.

Not telling your supplier about downstream printing or lamination. This is the expensive one. A slip package that works beautifully for handling can ruin ink adhesion, and that failure surfaces after you have made tonnage.

Winding hot. Sometimes the fix is not an additive at all. Film wound before it has cooled sufficiently, or at excessive tension, will block regardless of formulation. Check the line before reformulating.

Getting the specification right

When you enquire, give your supplier: the polymer and grade, film gauge, whether mono or multi-layer, target COF if you hold one, acceptable haze, and — critically — whether the film will be printed, laminated or heat-sealed.

That last item changes the formulation more than anything else on the list, and it is the one most often left out.

Both additives are available as single-function masterbatches or combined into one product, which removes a dosing unit and one more opportunity for let-down error. See additive masterbatches for the full range, including anti-fog and antistatic packages that often belong in the same film.

Send us your film structure and the problem you are seeing, and we will tell you which additive it actually is before you buy any.