Barrier films and shelf life: OTR and WVTR in plain words

In our homes, an earthen surahi sweats on a hot day. Water seeps through the clay wall so slowly that you cannot see it moving, only the damp outside.

Plastic film lets gases through in a similar way. Oxygen and water vapour pass straight through the plastic itself, molecule by molecule, even when there is not a single hole in it. Barrier films are designed to slow that down. Let us see how that is measured.

The short answer is this. OTR, the oxygen transmission rate, is how much oxygen passes through one square metre of film in one day. WVTR, the water vapour transmission rate, is the same for moisture. Lower numbers mean a better barrier. But a number only means something with its test conditions beside it, and the barrier you need depends on what actually spoils your product.

I run Jagannath Polymers, and we make multilayer blown film.

So let me admit one thing about my own trade first.

Data sheets are written by people who sell film, and we are no different. Read the small print beside every number, every time.

What does OTR mean?

Let us start with oxygen. OTR is measured in cubic centimetres of oxygen per square metre of film per day, written cc/(m²·day) or cm³/(m²·day).

The common test is ASTM D3985 (ASTM International, current edition D3985-24). It measures oxygen passing through film, sheeting, laminates and co-extrusions with a coulometric sensor, a sensor that turns oxygen into a tiny electric current. The 2024 edition says the method has been shown to work from about 0.06 to 64 cc/(m²·day).

There is a second method you should know, ASTM F1927 (ASTM International, F1927-20). It measures OTR at a set humidity. Its own scope says why it exists: humidity plays an important role in the oxygen transmission of many materials. Keep that sentence in mind, because it comes back when we talk about EVOH.

Older American data sheets, which many of us still receive, give OTR per 100 square inches. One square metre is 1,550 square inches, so multiply that figure by 15.5 to get cc/(m²·day).

What does WVTR mean?

WVTR is measured in grams of water vapour per square metre per day, written g/(m²·day).

Two methods are common:

1. ASTM F1249 (ASTM International, current edition F1249-25) uses an infrared sensor. It covers single and multilayer films and foils up to 3 mm thick, and the current edition says it has been shown to work from about 0.05 to 26 g/(m²·day).
2. ASTM E96/E96M (ASTM International, E96/E96M-24a) is the older cup method. A dish of desiccant or water is sealed under the film and weighed over time. The standard leaves the temperature and humidity to the user, but says they must be reported with the result.

Read the conditions before you read the number

An OTR of 5 measured at room temperature in dry air and an OTR of 5 measured hot and humid describe two different films. Before you compare anything, I would check these six things on the data sheet:

1. Temperature. Gases pass through plastic faster when it is warmer.
2. Relative humidity, and on which side of the film. This matters most for films with EVOH or nylon.
3. Thickness and structure. A thicker layer of the same material lets less through. But do not scale multilayer numbers yourself. ASTM F1249 itself warns that the simple link between thickness and transmission may not always apply.
4. The test gas. Ask whether the OTR test used pure oxygen. Your pack sits in air, which is about one-fifth oxygen, so the oxygen entering a real pack is lower than a pure-oxygen figure suggests. That is fine, as long as we all compare like with like.
5. Flat film or a creased one. Data sheets are made on flat, fresh film. Folds and creases can damage thin barrier layers. ASTM F392 (ASTM International, F392/F392M-23) is a standard way to flex a film before testing it, if you want to know what handling does to the barrier.
6. The method, the lab and the date of the test.

If any of these is missing, ask for it. We would, before approving any film.

Common barrier layers, in plain words

Most of the dairy films we see are built from a few materials. Each one has a strong side and a weak side:

1. Polyethylene (PE) is a good moisture barrier and a poor oxygen barrier. It is the usual sealant layer in dairy pouches.
2. PET (polyester) is a better oxygen barrier than polyolefins such as HDPE. A CFTRI chapter on oils, fats and vanaspati in ICPE's book Plastics in Food Packaging (undated) says HDPE is inferior to PET in oxygen barrier.
3. Nylon (polyamide) is used inside co-extruded films when oxygen matters. The CFTRI chapter on milk products in the same book found nylon-based co-extruded films and PET laminates good for ghee, where plain PE was not.
4. EVOH is an excellent oxygen barrier when it is dry, and much weaker when it is wet. A 2021 study in the journal Polymers (Melendez-Rodriguez and others) explains that EVOH is used as thin inner layers, typically well below 10 micron, protected from moisture by outer layers such as PE, PP or PET. This is why EVOH sits in the middle of the film and not on the outside. If a data sheet shows an EVOH layer, I would ask for its OTR at a humid condition too, by ASTM F1927.
5. Metallised film and aluminium foil block oxygen, moisture and light together. ICPE's note on packaging of milk and dairy products (undated) reports that a PET, foil and PE pouch for whole milk powder, flushed with gas, was found as effective as a can.

Match the barrier to what actually spoils your product

This is the part a data sheet cannot tell you. A barrier only helps against something that comes through the wall of the pack. If your product spoils for another reason, a better film will not save it. We can put the common dairy products side by side:

The milk powder line comes from the ICPE note, which describes milk powder as hygroscopic (it pulls moisture from the air) and whole milk powder as highly sensitive to oxygen. For the others, we have separate guides on milk pouch film, curd and buttermilk packs and ghee packaging.

My own view is simple. For pasteurised milk sold within days, I would spend on light protection and the cold chain long before I spent on an oxygen barrier. For ghee and milk powder, OTR and WVTR are exactly the numbers to ask for.

From data sheet to shelf life: a rough sum, then a real trial

A data sheet will not give you a shelf life. But a rough sum tells us whether a film is in the right range.

Let us do the arithmetic with made-up numbers, only to show the method. Say a pouch has 0.05 m² of film, and the film has an OTR of 10 cc/(m²·day) at conditions close to your storage. That is 0.05 × 10 = 0.5 cc of oxygen a day, or about 15 cc in a month. If that OTR was measured with pure oxygen, a pack sitting in air takes in roughly a fifth of it. Whether 15 cc is a problem depends entirely on your product. Only a trial can tell us that.

So run one.

Pack the same batch in two or three candidate films. Store one set at your normal warehouse conditions and one set somewhere warmer. Check them on a fixed schedule against a control, by smell, taste and, if you can, a lab test for oxidation or moisture.

We should not forget two more things that decide the result as much as the film does:

1. Seals and closures. A film with a perfect OTR is wasted if the seal leaks. Our guide on why milk pouches leak covers the seal tests.
2. The food-contact layer. The barrier layer can sit in the middle of the film, but under regulation 3(12) of the FSSAI Packaging Regulations, 2018 (compendium version IV, 9 September 2022), the layer that touches the food must meet the listed Indian standards, and regulation 3(14) asks for a NABL lab certificate for it.

If you send us the product, the pack size and the shelf life you want to print, we will suggest film structures to trial and tell you which test numbers to ask for.