Three suppliers quote the same structure and send three datasheets that look alike. The numbers are usually similar. What differs is what the sheet does not say: the conditions behind the WVTR figure, whether optical density was measured on the roll or on the converted web, and whether adhesion was measured at all. This guide is about reading a metallized film specification in the order that matters.
The four numbers that matter — and the two that are usually missing
A usable metallized film datasheet answers six questions. Most answer four.
| Parameter | What it is | Why it decides |
|---|---|---|
| Base film and gauge | PET 12 µm, OPP 20 µm, CPP 25 µm, PE 40 µm | Mechanical behaviour, sealability and cost per square metre |
| Optical density (OD) | log10 of incident over transmitted light | Aluminium coverage — a process control number, not a barrier number |
| WVTR | g/(m²·24 h) at stated conditions | Moisture barrier; comparable only at identical conditions |
| Adhesion | How firmly the aluminium layer is bonded | Lamination, printing and flex-crack survival |
| Surface treatment | Wetting tension of the metallized face | Print and adhesive anchorage |
| Test method | Which standard and which instrument | Whether the figure can travel at all |
A datasheet that states WVTR without the test conditions is not a specification. It is an aspiration.
Optical density: the input, not the output
Optical density measures how much light the aluminium layer blocks. OD 2.3 means roughly 0.5% transmission, or about 99.5% blocked. It is the number our metallized film line runs to as standard, because it correlates with a dense, continuous layer rather than a thin scatter of aluminium.
OD is useful because it is cheap to measure on every roll, which is why it appears on process records. It is also the number buyers over-read. A higher OD gives you more aluminium, and up to a point more moisture barrier — but OD says nothing about pinholes, nothing about adhesion, and nothing about what the barrier will be after lamination, printing and sealing. Two rolls at OD 2.3 can behave very differently in a finished pack.
WVTR: read the conditions before you read the number
WVTR is water vapour transmission rate: grams of water vapour passing through one square metre per 24 hours at a defined temperature and relative humidity. The units hide the two variables that matter. 38 °C at 90% RH and 23 °C at 85% RH give different numbers for the same film, and neither is wrong.
Our WVTR table on the metallized films page carries its own caveat: those figures are historical, taken at 40 °C, and the table tells you to ask for measured data on the exact structure. Our PE metallized film page publishes 2.1 g/(m²·24 h) on a 40 µm base, measured in-house at 20–28 °C and 40–60% RH. The two numbers are not comparable — and that is the point: a figure without its condition is not a specification. If a quotation gives you a WVTR with no conditions, ask for them before you compare anything.
There is a second trap, and it is the more expensive one: the WVTR on a datasheet is measured on flat film, not on your finished pack. Folding, sealing, laminating and filling all create routes for moisture that the flat sample never had. The figure is a starting point, not a promise — we set out the reasoning in why the datasheet figure is not the figure in the pack.
Adhesion: the number most datasheets leave out
The aluminium layer is tens of nanometres thick. It survives because it is bonded to the base film, and the bond is what converting tests. A metallized film with an excellent WVTR and poor adhesion will delaminate in a lamination nip, lift under print, or crack along a fold.
Two tests are worth asking for. The tape test is qualitative and fast: press a defined tape to the metallized face, strip it, and inspect how much metal leaves the film. Peel or bond strength is quantitative and needs a stated method — which side, which adhesive, which speed. A datasheet that reports neither has not measured the property that most often fails first.
Our practice is to confirm adhesion on the structure you will actually run, because the answer depends on your adhesive and your line, not only on our roll. That is why a sample roll comes before a volume quotation.
What a specification cannot tell you
No datasheet covers pack-level performance. Seal integrity, pinholes after handling, flex-crack at the fold, print adhesion, lamination bond, and shelf life in the finished pack all depend on the structure and the machine. Where a customer needs a coated alternative — for a chlorine-free structure, or where the barrier target is oxygen and aroma rather than moisture — the comparison starts on our coated films page instead, because a coating is specified differently from a metallized layer.
The practical consequence is simple: trial the structure. A short roll on your own line answers in an hour what a datasheet cannot answer at all.
A reading checklist
Before comparing two quotations, get these eight answers in writing.
- Base film, gauge and which side is metallized
- Optical density, and whether it was measured on the roll
- WVTR with temperature, relative humidity, sample thickness and method
- Adhesion: tape test result, peel strength and method
- Surface treatment or wetting tension of the metallized face
- Roll width, length, core and maximum number of splices
- Recommended storage conditions and roll shelf life
- Whether the figures are from a production roll or a laboratory sample
Two quotations that agree on all eight are genuinely comparable. Two that agree on the headline WVTR only are not.
Comparing two quotations fairly
Match the structure before you match the price. The same nominal film at 12 µm and 15 µm is not the same product, and a higher OD at a thinner gauge can deliver a better barrier at lower cost per square metre — or the reverse. Compare yield per square metre, not price per kilogram, then add the conversion losses that a thin or poorly bonded roll will create on your line.
For a two-ply build, the relevant comparison is against a three-ply laminate rather than against another metallized roll: that is the argument set out for PE metallized film. For a moisture-sensitive dry good where barrier is the constraint, the comparison is usually against metallized PET at a stated OD and gauge.
FAQ
Is a higher optical density always better?
No. Higher OD means more aluminium, which usually helps moisture barrier, but it also raises cost and can affect adhesion if the process window is pushed. OD 2.3 is a working standard for a reason: it delivers a continuous layer without over-coating.
Can I convert a WVTR from one set of conditions to another?
Not reliably. Transmission rate depends on temperature and humidity non-linearly, and on the film. Ask for the measurement you need, or test the structure at the conditions your shelf-life trial uses.
Why is my pack’s barrier worse than the datasheet?
Because the datasheet describes flat film. Folds, seals, lamination adhesive, print and handling all add transmission paths. The finished pack is the only number that decides shelf life.
Does a metallized layer need a coating?
Not usually. A coating is a different way to deliver barrier — chlorine-free options, oxygen and aroma barrier, or a heat-seal surface — and it is specified on its own terms.
Getting a comparable specification
If you are comparing metallized films, send us the structure you are running now, the shelf-life target and the test conditions you need the barrier quoted at. We will send a specification in the same format as this checklist, plus a sample roll to trial on your line. Start from the metallized film range or contact our technical team.