Why 18 µm is the working gauge for AlOx BOPP

The short version

18 µm is a working point, not a minimum and not a maximum. It is thin enough to keep the resin content of a square metre of barrier film down, and thick enough that a clear web still runs at production speed: holding tension, holding print registration, winding to a roll the laminator can use.

Two things get confused in that sentence. The barrier is in the coating. The gauge is in the base. Adding resin does not add oxygen barrier — it changes how the film converts and what it weighs. The rest of this note keeps those two apart.

1. Where the barrier actually sits

AlOx BOPP is a BOPP base with a layer of aluminium oxide vacuum deposited on one face. The layer is nanometres deep — a small fraction of an 18 µm gauge — and it is what the oxygen and water-vapour columns on the grade sheet measure.

The base does a different job. It carries the coating through the press, the laminator and the reel, and it sets the recycling stream: the polymer here is PP, so the pack is a PP structure. Neither face of the film is a seal layer; a pack seals on the sealant web it is laminated to, and that window is confirmed on your line, not read off a film data sheet.

There are two grades on the same base and the same gauge. Grade A carries a protective coating over the oxide and is built for printing as well as lamination. Grade B is a lamination grade. Their barrier figures differ, which is a coating question — see the table below.

2. What 18 µm holds together

On a clear film there is nowhere to hide. Registration marks sit on a transparent web, so a register error is visible on the finished pack rather than lost in a printed background. The web has to stay flat and stay on tension while it runs.

Stiffness. A self-supporting web is what makes registration repeatable and keeps a printed, coated surface off the idler rolls. Lose stiffness and the first symptom is wrinkles, then register drift.

Tension and roll build. A 6,000 m standard length at 18 µm winds to a defined roll that stays inside the 440 mm outside-diameter limit our slitter is set up for. The roll weight and build are part of why the gauge is fixed: change one and the winding settings move with it.

Yield. The grade sheet gives ≈ 61 m²/kg, calculated at 18 µm. That is the figure a converter uses to put a resin cost on a square metre, and it is a calculation from density and gauge, not a measurement of a roll.

Heat at the seal bar. The seal is made between sealant faces after lamination. The carrier web is not the sealing layer, but it is what holds the seal face flat and at the right tension while the bar closes — one more reason a production line gets a vote.

3. What a thinner gauge would ask for

The resin saving on a thinner web is real and smaller than it looks. The costs arrive first on the line.

A thinner clear web wrinkles more readily, loses registration sooner, and needs tighter tension control from unwind to rewind. At the laminator the operating window narrows; on the press the job may have to run slower to keep the web flat. A trial that has to slow down to hold register has usually spent more than the resin it saved.

It also changes the roll: more metres per kilogram, a different build, and a winding set-up that has to be re-qualified. That is why the gauge is a line decision on this product, not a menu. AlOx BOPP is supplied in 18 µm only — we do not quote a thinner option, and this note is the reasoning, not a catalogue entry.

4. What more gauge would not fix

Adding thickness is the reflex when a barrier figure disappoints. On a coated film it is the wrong lever. The oxide layer that stops oxygen and water vapour is a surface layer; the polymer below it is not the barrier. More BOPP buys stiffness and abuse resistance, not oxygen transmission.

The bill for it is straightforward. Resin per square metre rises, yield falls below 61 m²/kg, a 6,000 m roll gets heavier and can hit the 440 mm outside-diameter limit at a narrower width, and freight per square metre goes up. If a pack genuinely needs more stiffness or heat resistance than BOPP gives, the answer is a different base film — not more of this one.

Supply at a glance

Thickness

18 µm single gauge

Yield

≈ 61 m²/kg calculated at 18 µm

Max width

3,000 mm slit to order

Max roll OD

440 mm

Standard roll length

6,000 m

Core

Ø76 / Ø152 mm

Minimum order

1 roll

Lead time

10–15 days by width

Samples

A4 free with lot data

6. When 18 µm is the wrong answer

18 µm AlOx BOPP is the working gauge for a clear PP structure that has to print, laminate and run fast. It is not the answer to every barrier brief, and the TDS says so as plainly as we can:

• The pack has to be retorted, or the base has to stay stiff at heat. BOPP carries no retort rating. ALOX PET is the oxide film for that job: ten models at 12.0–12.5 µm, seven of them rated for 121 °C. It is also a PET stream, not a mono-material PP one.
• The structure has to be PE, or the film has to seal. AlOx BOPP does not seal and does not belong to the PE stream. Metallized PE covers both cases: VMCPE is a sealant grade at 40 µm standard (50–80 µm made to order), and VMNCPE is a barrier core at 40–100 µm with no seal of its own.
• The question is transparent or silver. That is a different comparison, with its own test conditions — the selection guide takes it apart without ranking the two families.

What stays true across the three: the barrier is a coating, and the base is chosen for the process and the stream. Gauge is the third question, not the first.

Key facts

• ALOX BOPP is supplied in one gauge: 18 µm.
• The barrier is AlOx vacuum deposited on one face, nanometres deep. The PP base carries mechanics and stream; the coating carries OTR and WVTR.
• Grade A (protective coating, print + laminate): OTR ≤ 1 / typ. 0.63, WVTR ≤ 1 / typ. 0.89. Grade B (laminate only): OTR ≤ 2 / typ. 1.29, WVTR ≤ 2 / typ. 1.32.
• OTR at 23 °C / 0 % RH; WVTR at 38 °C / 90 % RH; “≤” is the limit, “typ.” the grade-sheet figure.
• Yield ≈ 61 m²/kg, calculated at 18 µm.
• Supply: width up to 3,000 mm slit to order, roll OD up to 440 mm, 6,000 m standard length, Ø76 / Ø152 mm cores, 1 roll minimum, 10–15 days by width, A4 samples free.