Specification

Barrier properties: reading and specifying OTR and WVTR

Shelf life is decided by two numbers. Both are quoted in units nobody uses in conversation, both are meaningless without the test conditions printed beside them, and both are routinely compared across data sheets that were measured differently.

What the two numbers measure

One tracks oxygen getting in, the other tracks moisture crossing in either direction.

OTR — oxygen transmission rate. How much oxygen passes through a given area of film in a given time. Usually quoted as cc/m²/day, sometimes cc/100in²/day. Lower is better. Oxygen is what oxidises fats, fades pigment and lets aerobic spoilage organisms work, so OTR is the governing number for coffee, nuts, snacks, cured meat and anything with a high fat content.

WVTR — water vapour transmission rate. How much water vapour passes through, quoted as g/m²/day. Lower is better. It governs whether a dry product goes soft and whether a moist product dries out. Note the direction runs both ways: WVTR matters as much for keeping moisture in as for keeping it out.

A film can be excellent at one and poor at the other. EVOH is an outstanding oxygen barrier and a poor moisture barrier — and its oxygen performance degrades as humidity rises, which is why it is normally buried between polyolefin layers that keep water away from it. PE is the reverse: a good moisture barrier and a weak oxygen one.

Why two data sheets are usually not comparable

Both figures change by an order of magnitude with the conditions they were measured under, and the conditions are the small print.

OTR is measured at a stated temperature and relative humidity. A figure taken at 23°C and 0% RH will look far better than the same film measured at 23°C and 90% RH, particularly for any structure containing EVOH or nylon. WVTR is likewise measured at a stated temperature and humidity gradient — 38°C at 90% RH is a common tropical condition and gives a much higher number than a temperate one.

Thickness also has to be stated. Barrier through a homogeneous layer is broadly inversely proportional to its thickness, so a figure without a thickness attached is not a specification.

Three things to check on any barrier data sheet before comparing it with another:

Test temperature and relative humidity for each figure.
The film thickness the figure applies to.
The test standard used — ASTM and ISO methods do not always produce identical results.

Where those are missing, the number is marketing rather than data. It is fair to ask for the full report, and we would rather be asked.

Working backwards from shelf life

The useful direction is from the shelf life you need to the barrier that delivers it, not from a barrier figure to a hope.

The chain is: how much oxygen or moisture your product can absorb before it is unacceptable, divided by how fast the pack lets it in, gives you the time you get. To work through it you need four things:

The failure criterion — the peroxide value, moisture content or sensory threshold at which the product is no longer sellable.
The headspace — how much oxygen is inside the pack at the moment it is sealed. Flushing with nitrogen changes the answer completely.
The surface area to volume ratio — a small pack has proportionally more film per gram of product and therefore a shorter shelf life at the same barrier.
The storage conditions — ambient tropical distribution is a different problem from chilled temperate.

A worked calculation gets you into the right band; accelerated and then real-time shelf-life testing on the actual pack is what confirms it. Anyone who promises a shelf life from a structure alone, without the product data, is guessing — including us.

What each structure gives you, roughly

Enough of an ordering to know which conversation you are in before you ask for numbers.

PE monolayer — good moisture barrier, poor oxygen barrier. Suits dry non-oxygen-sensitive goods.
PET/PE laminate — moderate on both, good stiffness and print. The general-purpose workhorse.
PET/AL/PE with foil — effectively a total barrier to both. Opaque, and it is not recyclable in any normal stream.
Metallised PET structures — high barrier, much of the foil benefit at lower cost and weight, but the metallised layer is vulnerable to flex cracking.
EVOH-containing co-extrusions — very high oxygen barrier, humidity-sensitive, needs protecting layers on both sides.
High-barrier coated mono-material PE or PP — the recyclable route. Barrier has improved substantially and now covers many applications that previously needed foil, though not the most demanding ones.

That last line is the one worth planning around. If a recyclability requirement is coming, the question is whether a coated mono-material structure meets your barrier target, and the answer is application-specific. It is a test, not an opinion.

Common questions

What is a good OTR for coffee?

Roasted coffee needs a high oxygen barrier and a degassing valve, because it releases CO2 after roasting while still needing oxygen kept out. The specific target depends on roast, grind and the shelf life you are claiming, so it is set from the product data rather than from a standard figure.

Does a thicker film always give better barrier?

Within one material, broadly yes and roughly in proportion. Between materials, no — a thin EVOH or metallised layer outperforms a much thicker polyethylene one on oxygen by orders of magnitude.

Why does my EVOH structure underperform in humid conditions?

EVOH absorbs moisture and its oxygen barrier falls as it does. It has to be shielded between polyolefin layers, and the data sheet figure has to be read at the humidity your product will actually see.

Can a recyclable mono-material pouch match a foil laminate?

Not at the top end. High-barrier coated mono-material structures now cover a large share of applications that used to need foil, but the most oxygen-sensitive long-shelf-life products still sit outside what they can do.

Where to go next

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