MVTR Explained: What Moisture Vapour Transmission Rate Means for Blister Packaging

What MVTR actually measures

Moisture vapour transmission rate (MVTR), sometimes written WVTR, describes how much water vapour passes through a given area of material in a given time. It is normally quoted in grams per square metre per 24 hours, for example g/m2/24h at a stated temperature and relative humidity. The value matters because moisture ingress is one of the main reasons a solid oral dose loses potency, changes appearance or fails at the end of shelf life.

MVTR is a property of a structure, not of a single resin. A 250 micron PVC film, a PVC/PVDC-coated film and an aluminum laminate will all give different numbers, and the same film tested under two different condition sets will also give different numbers.

Why test conditions matter more than the number

Two data sheets can quote very different MVTR values for products that behave similarly in service, simply because the tests ran under different conditions. Tropical conditions (38 degrees Celsius and 90 percent relative humidity) drive more vapour through a film than temperate conditions (25 degrees Celsius and 75 percent relative humidity), so a value quoted at the milder condition is not directly comparable.

Always establish the condition set before comparing two materials. If a supplier quotes a single number without conditions, ask for the test method and the conditions.

Data sheet values are not pack values

A material data sheet describes a flat sheet. A formed blister cavity has drawn walls that are thinner than the starting film, and the sealed pack adds a lidding material, a seal layer and an edge. The finished pack therefore behaves differently from the flat film, and a pack design can pass or fail on the quality of the seal rather than on the film's own transmission rate.

This is why barrier discussions should cover the finished pack: forming ratio, film thickness after drawing, seal width and the lidding material all belong in the same conversation.

Comparing structures on an equal basis

For practical selection, group candidate structures by the barrier family they belong to and then compare within that group:

• Uncoated PVC films: the reference standard for many blister packs, suitable where moisture sensitivity is modest
• Coated and high-barrier PVC films: a coated layer or an alternative polymer reduces transmission while keeping the familiar forming behaviour
• Cold-form aluminum laminates: a metallic barrier for the most moisture-sensitive and hygroscopic formulations
• Metallized and laminated films: used for sachets, stick packs and pouches where a flexible format is wanted

Our high-barrier PVC films and composite laminated film cover the coated, metallized and laminated options, while tropical aluminum foil and cold-form structures serve the highest-barrier end of the range.

Linking barrier data to shelf life

MVTR on its own does not give a shelf life. Shelf life is calculated from the moisture sensitivity of the formulation, the headspace and fill weight, the cavity geometry and the pack's total transmission. Teams normally combine supplier data with a moisture sorption study for the product, then confirm the result with a stability trial on the finished pack.

Where a stability trial is planned in a hot, humid market, ask for barrier data measured under tropical conditions so that the calculation starts from the right base.

What to ask a supplier for

When you request barrier information, ask for:

• The test method and the exact condition set
• The structure that was tested, including layer thicknesses
• Whether the value refers to flat film or to a formed and sealed pack
• Whether the grade is covered by the compliance documents needed for your market

If any of these are missing, the comparison is incomplete and the safest step is to run a line trial on your own equipment before committing.

Summary

MVTR is a useful comparison tool only when the conditions, the structure and the point of measurement are known. Treat data sheet values as a screening input, take the finished pack into account, and confirm the design with a stability trial. For help matching a barrier family to a specific formulation or market, contact our team or review the material range for your industry.