Reading Barrier Data Sheets: Why WVTR and OTR Values Aren't Always Comparable

Barrier performance is one of the first things buyers compare when evaluating packaging materials. It is also one of the easiest things to compare incorrectly. Two data sheets can quote water vapor transmission rate (WVTR) figures that differ by a factor of three for materials that behave almost identically in service.

The Numbers Depend on the Conditions

WVTR and oxygen transmission rate (OTR) are not intrinsic constants. They are measured values that depend on the conditions of the test:

• Temperature — permeation is strongly temperature dependent. A film tested at 38°C will show a markedly higher WVTR than the same film tested at 23°C.
• Relative humidity — most methods specify a humidity gradient across the sample, for example 90% RH on one side and 50% on the other. A different gradient gives a different result.
• Test method — instrumented methods such as those described in ASTM F1249 or ISO 15106 differ in detail from cup-based gravimetric methods, and the results are not always identical.
• Sample thickness — for a homogeneous film, transmission is inversely proportional to thickness. A 250μm film and a 300μm film of the same material will not report the same WVTR.
• Structure — a coated or laminated structure behaves differently from a monolayer of the same total thickness, because the barrier is carried by one layer.

Why Supplier Comparisons Go Wrong

The most common error is comparing figures measured under different conditions as though they were equivalent. A WVTR quoted at 25°C / 75% RH cannot be set directly beside one quoted at 38°C / 90% RH.

The second most common error is comparing a film value with a laminate value, or normalizing one and not the other.

How to Compare Fairly

Four practical steps:

1. Ask for the test conditions, not just the number. Request temperature, humidity gradient, method reference, and sample thickness. 2. Normalize where possible. For homogeneous films, convert values to a common thickness basis before comparing. 3. Compare like with like. Film against film, laminate against laminate, and prefer values from the finished structure you intend to buy. 4. Request the raw report for critical projects. A summary figure without the underlying conditions cannot be verified later.

Our own published barrier values are quoted with their test conditions so that comparisons are meaningful — see the high-barrier PVC specifications, which follow the same convention as our other ranges.

What to Specify in an RFQ

When issuing a request for quotation, state the conditions you want the barrier data reported under, and ask for the method reference. This removes ambiguity at the evaluation stage and avoids discovering mid-project that two candidate materials were never measured the same way.

If you would like barrier data for a specific structure and condition, our technical team can provide measured values with the test conditions stated — contact us with your requirement.