Metallised film and what actually creates the barrier
A metallised film is a plastic film carrying a layer of aluminium a few tens of nanometres thick, deposited in a vacuum. Published work on vacuum-deposited aluminium on polyester puts the layer between roughly 10 and 50 nm. That is thin enough to be measured in atoms rather than microns, and thin enough that it is not really a metal sheet at all.
Which leads to the thing worth understanding about it. The barrier a metallised film gives you is not decided by how much aluminium is there. It is decided by the gaps in it.
This page covers the metallised layer as a material. How it compares with EVOH and with transparent ceramic coatings, and what each costs under PPWR, is set out on barrier options and PPWR recyclability. The polyester it is usually deposited on is covered on PET film.
Metallised film properties at a glance
| Property | Typical behaviour |
| Metal layer thickness | Reported between about 10 and 50 nm for vacuum-deposited aluminium on polyester |
| Deposition | Thermal evaporation of aluminium onto a moving web, in a roll-to-roll vacuum metalliser |
| What limits the barrier | Pinhole defects in the coating, not the quantity of metal |
| How it is specified | By optical density, not by a thickness in microns |
| Light | Blocked, because the film is opaque. No other flexible barrier does this in the same layer |
| Flexing | Barrier falls as the coating is worked. Its failure mode differs from that of aluminium foil |
| Substrate | Usually PET or BOPP. The substrate’s surface decides how consistent the coating is |
The third and fourth rows are the ones that change how you buy the material, and they are the subject of the two sections after next.
How the aluminium gets there
Aluminium is heated until it evaporates, inside a chamber held under vacuum, while the film runs past on a chilled drum at high speed. The vapour condenses on the cold film surface and builds a continuous layer. The process is continuous and roll-to-roll, which is why metallised film is cheap relative to the barrier it provides.
Two consequences follow from the film being the cold surface. First, the coating is only ever on one face. Second, the quality of the coating depends heavily on the surface it lands on. Work comparing substrates found that coatings on PET deliver consistent barrier results, while BOPP shows considerable variation depending on the individual substrate. The film underneath is not a passive carrier.
This is also why the same specification bought from two suppliers can perform differently. The metal is aluminium in both cases. The surface it condensed onto was not the same, and that is where the difference sits.
The barrier is decided by pinholes, not by the metal
Aluminium is not permeable. A perfect layer of it, however thin, would stop oxygen completely. Real coatings are not perfect, and the gaps are where everything happens.
The classic study of this examined aluminium layers of 10 to 50 nm on polyester and quantified the pinhole defects in them. It found that the density of those pinholes determines the permeability of the film to oxygen. Not the thickness, not the metal. The holes.
Later work on water vapour found the same relationship and made it quantitative: permeance varies linearly with the surface fraction occupied by pinholes, and that fraction in turn varies linearly with the aluminium thickness. So thickness does matter, but indirectly. It matters because a thicker deposit closes more of the gaps, not because there is more metal in the way.
The practical version: anything that creates new gaps destroys barrier faster than anything that removes metal. Abrasion in handling, a crease, a fold, a hard nip on a converting line. A metallised web that has been handled badly can carry all of its aluminium and very little of its barrier.
Optical density: how a metallised film is actually ordered
A metal layer of a few tens of nanometres cannot sensibly be quoted in microns. So it is not. Metallised film is specified by optical density (OD), a measure of how much light the coating blocks, on a logarithmic scale. A higher OD means a denser, more continuous deposit.
It works as a proxy because the same thing that stops light also stops gas: continuity. Fewer gaps means less light through and less oxygen through. Work on laminates confirms the link, reporting that a higher OD, together with a thicker film and lamination, improved the laminate’s measured barrier.
Which means OD is the number to put in a specification, and the number to check on delivery. A metallised film ordered without one has not really been specified, and two rolls that look identical can be a long way apart.
Flexing: metallised film against aluminium foil
Aluminium foil is the reference barrier, and the assumption is usually that it beats a metallised film on every count. On flex durability that assumption is wrong, and the reason is instructive.
A study that flex-tested laminates built around foil and around metallised PET found that the foil-centred laminate kept an excellent oxygen barrier through a 20-cycle test but failed to retain it after 270 cycles. The metallised laminates were less affected by flexing on the oxygen barrier than the foil one. Water vapour barrier was less severely affected than oxygen in every case.
Why they fail differently
Foil is a self-supporting metal sheet, so bending it repeatedly cracks the metal itself. Work on bent barrier films describes exactly that split: the foil laminate failed predominantly by fracture of the aluminium layer, while the metallised polyester film failed mainly by interfacial delamination between coating and substrate.
The deposited layer is so thin that it follows the polymer rather than resisting it. That is the advantage on a pack that gets handled, and it is also the warning: what you have to protect on a metallised film is the bond between the metal and the film underneath.
What the metal layer costs you
Three costs, and none of them is about barrier performance.
- The pack is opaque. That is an advantage for light-sensitive products and a problem for anything the shopper wants to see. Transparent barrier coatings based on aluminium oxide or silicon oxide were introduced for exactly this reason, along with microwaveability and retortability.
- The metal reflects microwaves, so a conventionally metallised structure is not made for a microwave.
- It complicates sorting and recycling. A recent review in the barrier-coating field describes the elimination of metallised film as one of the major circular-economy challenges in food packaging, precisely because it is the industry standard way of reaching the required barrier.
What each alternative costs in recyclability, and where PPWR puts the pressure, is set out under barrier options and PPWR recyclability.
Frequently asked questions
What does VMPET mean?
Vacuum metallised polyethylene terephthalate. It is the standard abbreviation for a PET film that has been aluminium coated in a vacuum metalliser, and you will see VMCPP and VMOPP used the same way for cast and oriented polypropylene. The abbreviation names the substrate and the process, but not the coating weight, so a specification that says only VMPET has not yet said the thing that decides performance.
Why is a metallised film shiny on one side only?
Because the aluminium condenses onto the face that runs against the chilled drum, so only that face is coated. The other face is the bare polymer. In a finished laminate the metallised face is normally turned inward and bonded, which protects it from abrasion, and the metallic look reaches the shopper through the outer web rather than off an exposed surface. If the metal is on the outside of a pack, it is exposed to everything that scratches.
Specifying a metallised structure, optical density included
Send the product, the shelf life and whether the pack has to be seen through. We come back with a structure, an optical density and the documentation that goes with it.
Related resources
Barrier options and PPWR recyclability
The metallised layer set against EVOH and against transparent ceramic coatings, and what each option costs in the recycling stream.
PET film, the usual substrate
Why the polyester underneath decides how consistent the coating is, and what biaxial orientation and heat setting lock in.
EVOH, the transparent alternative
The oxygen barrier that keeps the pack see-through, how ethylene content scales it, and why humidity reverses it.
Modified atmosphere packaging
What the atmosphere demands of a barrier structure, and why seal integrity matters as much as the barrier itself.