How AlOx BOPP carries a mono-material PP pack

A coating, not a foil

AlOx stands for aluminium oxide. It is deposited onto BOPP in a vacuum chamber as a layer thin enough to read as a sheen rather than as a film, and it bonds to the base instead of sitting on it as a separate sheet. The barrier comes from that oxide layer — not from thickness, and not from the polymer underneath it.

Because the layer is an oxide and not a metal, the pack does not turn silver. The product stays visible, which is why the film is specified for clear packs where a metallized web would hide what is inside. Two process properties follow: an AlOx layer can pass a microwave field, and it does not trip a metal detector, so packs built with it can run through inspection lines that reject metallized film.

The base is BOPP. That is what puts the finished pack in the PP stream: coated film, sealant and print web are all polypropylene, so the pack sorts as one material instead of as a laminate of three. The grade ships at 18 µm, one gauge, in two variants that differ only in the protective coating.

Where the film sits in the structure

In a PP pack the oxide film is the barrier web. The common build is PP / AlOx-BOPP / PP: a printed outer web, the coated film carrying the barrier, and a PP sealant inside doing the sealing. Flow-wrap and some pouch builds drop one layer and run AlOx-BOPP / PP sealant, with the ink on the outside of the coated web.

Either way, the film is not the sealant. It supplies barrier and, on the printing grade, the print surface; the seal comes from the PP web laminated to it. That division keeps the structure simple to convert — and makes the grade decision a print decision.

Grade A carries a protective coating over the oxide and is specified for printing and lamination: the coat gives the press a stable surface to key to and protects the oxide during handling. Grade B has no coat and is specified for lamination only; it goes in as the barrier layer under a separately printed web. Both are 18 µm, and grade A carries the tighter barrier.

Reading the barrier numbers

The two figures that decide most jobs are oxygen transmission rate (OTR) and water vapour transmission rate (WVTR). For grade A, OTR is ≤ 1 cm³/(m²·24h) with a typical 0.63 at 23 °C / 0 % RH to GB/T 19789 / ASTM D3985; WVTR is ≤ 1 g/(m²·24h) with a typical 0.89 at 38 °C / 90 % RH to GB/T 26253 / ASTM E398. Grade B is specified at ≤ 2 for both, with typical values of 1.29 and 1.32 under the same conditions.

Two words carry the meaning. “≤” is the limit the grade is specified to; “typical” is what the grade sheet shows for a production roll. A converter who needs the guarantee quotes the limit and asks for the lot certificate, which records what was measured on the roll that ships.

The condition is part of the number. An OTR at 23 °C / 0 % RH is not comparable with one stated at another humidity, and WVTR at 38 °C / 90 % RH is harsher than a room-condition figure. A comparison that does not state temperature, humidity, method and unit compares test methods, not films.

These are film values. Lamination, ink, adhesive and sealant all change what the pack transmits; only a structure trial gives the pack's number.

When AlOx BOPP is the wrong call

BOPP has a temperature ceiling, and the TDS does not rate this grade for retort. A pack that is retorted or boiled does not start here; that job belongs to a different base film.

The film is also not the highest barrier available. Where the target is near-zero oxygen ingress over a long ambient shelf life — coffee, long-life fatty foods, some nutraceuticals — foil or an EVOH-based laminate still does more than an 18 µm oxide film. Say so before a mono-material claim is made.

The PP-stream argument only holds if the rest of the structure is PP. If the pack keeps a PET or foil layer for another reason, AlOx BOPP may still be the right barrier web, but it is no longer the reason the pack recycles.

The TDS is also explicit about aggressive fills: acids, heavy oils, alcohol and preservative systems need a trial before production. On grade B the oxide has no protective coat, so unwind and lamination need clean, unscratched rollers.

None of these limits is unusual for an 18 µm oxide film; each one rules a job out before the trial does.