Blown film is where masterbatch quality stops being an opinion.

A 25-micron film is a thin, transparent, back-lit test specimen running at high speed. Every agglomerate becomes a visible gel. Every incompatibility becomes a fish-eye. Every screen-pack problem becomes a pressure spike. Material that performs perfectly in a 2 mm moulded part can be unusable in film.

If you are buying masterbatch for film, you are buying to a different standard than the rest of the industry – and you should be asking different questions.

Carrier resin compatibility is the whole game

The single largest cause of film defects traceable to masterbatch is a carrier that does not belong in the host polymer.

A masterbatch carrier is a polymer. When it enters the melt it must fully melt, mix and become indistinguishable from the film resin. If it does not, it stays as a discrete domain – and in a thin film, a discrete domain is a fish-eye: a lens-shaped defect with an unmelted core, visible against the light, and a weak point in the web.

The rules that matter:

  • Polyolefin into polyolefin, matched to family. A PE-carrier masterbatch belongs in PE film. A PP-carrier masterbatch in a PE film is a classic source of gels – PP and PE are not miscible, and a PP domain in a PE film will not go away no matter how hard you work the screw.
  • Universal carriers are a compromise, not a magic solution. They exist, they are useful when a converter runs many polymers on one line, and they are almost always second best to a matched carrier for a demanding thin-gauge film.
  • The carrier’s MFI should be equal to or higher than the host resin’s. A carrier that is stiffer than the polymer around it does not spread; a carrier that flows more easily distributes readily. This is the reverse of what people often assume.

Ask for the carrier polymer and its MFI on every quote. If a supplier will not tell you what the carrier is, you cannot predict how it will behave in your film.

Filter pressure value: the number film converters should demand

Filter pressure value (FPV) is the objective measure of how clean a masterbatch is. The concentrate is extruded through a fine screen and the pressure rise is recorded, normalised against the mass of pigment that passed through it.

A low FPV means the pigment is fully dispersed and the material is free of oversize particles and contamination. A high FPV means agglomerates and hard particles are blinding the screen – which in your factory means screen changes, pressure spikes, gels in the film and, on a gauge-sensitive line, web breaks.

No other test tells a film converter as much about a masterbatch as FPV does, and it is routinely absent from Indian technical data sheets.

Ask for it. Ask for the mesh size and test conditions alongside the number, because an FPV without its conditions is not comparable to anything. A supplier who measures FPV is running a quality system. A supplier who has never heard of it is selling on price.

We go deeper on this in black specks in moulded parts – is it your masterbatch?.

What actually changes between LDPE, LLDPE and HDPE

The three polyethylenes run differently and they stress the masterbatch differently.

LDPE. Highly branched, high melt strength, forgiving. A big stable bubble, low output pressure, wide processing window. LDPE is the easiest of the three for masterbatch to perform in, and it is where marginal material hides.

LLDPE. Linear with short branches. Much better mechanical properties and much less forgiving to run – lower melt strength, higher back pressure, narrower die gaps, and a strong tendency toward melt fracture if the shear rate at the die is pushed. LLDPE film is usually run thinner than LDPE film for the same duty, which means the same masterbatch gets a harder examination. Marginal dispersion that passes in LDPE fails here.

HDPE. Run at high stalk – a long neck above the die before the bubble expands – which produces very high molecular orientation and very thin, stiff, high-strength film. HDPE film is often the thinnest of the three and shows every defect. Carrier compatibility becomes critical because HDPE’s higher crystallinity is less tolerant of a mismatched amorphous domain.

Practical summary: qualify masterbatch on your thinnest, fastest LLDPE or HDPE grade. If it works there, it will work in LDPE. The reverse is not true, and approving a supplier on an easy grade is how converters end up with a problem six months later.

Why colour looks weaker in film than in a moulded part

This is not a masterbatch fault and it causes a lot of unnecessary argument.

Colour is a function of the optical path length. Light entering a 2 mm moulded part travels through eighty times more pigment than light entering a 25-micron film at the same loading. The film will look dramatically paler, and no amount of complaint changes the physics.

The consequences are practical:

  • A shade matched on a plaque will not match in film. Match on the actual gauge, on the actual process.
  • Dosage recommendations are gauge-specific. Moving a shade from 50 micron to 25 micron generally needs substantially more masterbatch – and past a point, a reformulation rather than a dosage increase.
  • White is the extreme case. Opacity in thin film is genuinely hard, which is why film whites run at high TiO₂ loading and high addition rates. Use our let-down ratio calculator to convert an opacity target into a hopper setting.

Always give your supplier the film gauge, the polymer, the process and a physical reference before asking for a match.

Where colour meets the other additives

Blown film rarely runs colour alone. It runs colour alongside slip, anti-block, processing aid, and often UV.

Two interactions worth knowing:

Anti-block and pigment compete for surface. Anti-block works by creating microscopic surface roughness that stops layers welding on the reel. Some pigment and filler packages change the surface enough to alter how the anti-block performs, so changing colour can change your blocking behaviour.

Slip additives migrate, and their migration rate is not constant. Coefficient of friction on a freshly wound reel is not the COF a week later. If you change masterbatch and your COF moves, look at whether the new carrier is changing the migration rate before blaming the slip.

If you are running combinations, get them formulated together rather than dosed as three separate concentrates from three suppliers.

What to send your masterbatch supplier

  • Polymer and grade, with MFI and density
  • Film gauge and layflat width
  • Number of layers and structure if co-extruded
  • Line speed and output
  • Die gap and blow-up ratio
  • Screen pack mesh
  • What else is being dosed – slip, anti-block, processing aid, UV
  • End use, and whether the film is printed, laminated or food contact
  • A physical colour reference at the actual gauge

FAQs

What causes gels and fish-eyes from masterbatch in blown film? Usually an incompatible or unmelted carrier resin – most classically a PP-carrier masterbatch in a PE film. Poorly dispersed pigment agglomerates and contamination in the concentrate cause the same visible defect.

What is filter pressure value and why does it matter for film? FPV measures the pressure rise when a masterbatch is forced through a fine screen, normalised for pigment mass. It is the objective measure of dispersion quality and cleanliness. High FPV means screen blinding, pressure spikes and gels in thin film.

Should a masterbatch carrier have a higher or lower MFI than the film resin? Equal or higher. A carrier that flows more easily than the host disperses readily; a stiffer carrier resists distribution and can survive as a discrete domain.

Why does my colour look lighter in film than on the sample plaque? Optical path length. Light passes through far less pigment in a 25-micron film than in a thick plaque, so the same loading looks much weaker. Always match colour at the actual film gauge.

Can I use the same masterbatch for LDPE, LLDPE and HDPE film? Sometimes, with a universal carrier, but it is a compromise. Qualify any masterbatch on your thinnest and fastest grade – usually LLDPE or HDPE – because material that passes there will pass in LDPE, not the other way round.

Does changing colour affect slip and anti-block performance? It can. Pigment and filler packages change the film surface, which affects how anti-block performs, and a different carrier can change how quickly slip additive migrates to the surface. Test COF after a colour change.