Water vapor transmission coefficient testing measures how quickly moisture permeates through plastic films, laminates, and other barrier materials, delivering the permeability data needed for shelf-life prediction, material qualification, and quality control. This article explains the core permeation parameters, mainstream test methods such as the gravimetric cup method and modulated infrared sensor method, applicable international standards including ASTM E96, ASTM F1249, and ISO 15106, and typical applications across food, pharmaceutical, photovoltaic, and electronics packaging.
Technical Overview
Water vapor transmission coefficient testing, also known as water vapor permeability testing, quantifies the resistance of a material to the passage of moisture vapor. It underpins the design and qualification of packaging, encapsulation, and construction materials whose function depends on keeping water out, or in the case of breathable films, allowing it to pass through in a controlled manner.
Permeation through polymers follows the solution–diffusion mechanism: water molecules adsorb onto the high-humidity surface, dissolve into the polymer matrix, diffuse along a concentration gradient through the free volume, and desorb at the low-humidity side. Because both solubility and diffusivity increase with temperature and humidity, permeability exhibits strong Arrhenius-type behavior, and test conditions must always be reported together with results.
Three related parameters are commonly determined:
- Water vapor transmission rate (WVTR): the steady-state mass of water vapor permeating through unit area per unit time, expressed in g/(m²·24 h). It is the primary specification parameter for films and packages.
- Water vapor permeance: WVTR normalized by the water vapor partial pressure difference across the specimen, in g/(m²·s·Pa). It characterizes a structure of defined thickness.
- Water vapor permeability coefficient: permeance multiplied by thickness, in g·cm/(cm²·s·Pa). This is an intrinsic material property that allows comparison between materials of different thicknesses.
Barrier performance is conventionally classified by WVTR measured at 38°C under a 90% to 0% relative humidity gradient: high-barrier materials such as aluminum foil laminates and inorganic oxide coated (AlOx, SiOx) films measure below 1 g/(m²·24 h); medium-barrier materials such as PET and PA films typically fall between 1 and 20 g/(m²·24 h); and low-barrier polyolefins such as PE and PP lie above this range.
Testing Samples
Monolayer plastic films and sheets
Flat, crease-free specimens of PE, PP, PET, PA, PVC, EVOH, PVDC, and similar materials, typically 20–200 µm thick for films and up to several millimeters for sheets. Accurate thickness measurement at multiple points is essential when the permeability coefficient is to be calculated.
Multilayer and laminate structures
Coextruded films, aluminum–plastic laminates, and adhesive-bonded multilayers used in food and pharmaceutical packaging. Testing is performed on the complete structure, since interlayer interfaces, adhesives, and residual solvents all influence the finished moisture barrier.
Coated and metallized substrates
Vacuum-metallized films, transparent oxide-coated films, and PVOH- or PVDC-coated films. In these products, coating continuity and pinhole density dominate transmission, so testing verifies both coating quality and the finished barrier level.
Paper, paperboard, and fiber-based materials
Packaging papers, coated boards, and molded fiber are usually tested by the gravimetric dish method, with specimen edges carefully sealed with wax to eliminate edge leakage.
Photovoltaic and electronics encapsulation materials
Solar module backsheets, encapsulant films, and ultra-high barrier films for OLED or flexible electronic devices, where WVTR values down to 10⁻³–10⁻⁶ g/(m²·24 h) must be resolved.
Testing Items
Accredited third-party laboratories typically offer the following determinations:
- Water vapor transmission rate (WVTR), g/(m²·24 h)
- Water vapor permeance, g/(m²·s·Pa)
- Water vapor permeability coefficient, g·cm/(cm²·s·Pa)
- Supporting thickness and grammage measurement for coefficient calculation
- Multi-condition testing, for example 23°C/85% RH, 38°C/90% RH, or 40°C/90% RH
- Temperature-dependence studies for shelf-life modeling and permeation activation energy determination
Testing Methods
Gravimetric cup (dish) method
The specimen is sealed over a dish containing either a desiccant (dry-cup method) or distilled water (wet-cup method) and placed in a controlled temperature and humidity chamber. Moisture transfer causes a systematic mass change of the assembly, and the WVTR is obtained from the slope of mass versus time divided by the test area. Representative standards include ASTM E96/E96M, ISO 2528, GB/T 1037, JIS Z 0208, and DIN 53122-1. The method is robust and inexpensive and remains the reference approach for materials with WVTR above approximately 1 g/(m²·24 h), although individual tests may take several days to weeks.
Modulated infrared sensor method
Dry nitrogen sweeps the low-humidity side of the specimen and carries the permeated water vapor to a modulated infrared detector whose signal is proportional to vapor concentration. The method reaches equilibrium quickly, offers resolution down to about 0.001 g/(m²·24 h), and is specified in ASTM F1249, ISO 15106-2, and GB/T 26253. It is the industry workhorse for high-barrier flexible packaging and photovoltaic backsheets.
Electrolytic (coulometric) sensor method
A carrier gas transports permeated vapor into an electrolytic cell based on phosphorus pentoxide, where water is quantitatively electrolyzed; the electrolysis current is directly proportional to the amount of water transferred. The technique provides excellent linearity across a wide range and is covered by ISO 15106-3, GB/T 21529, and DIN 53122-2.
Relative humidity sensor and other methods
Dynamic methods using calibrated relative humidity sensors (ASTM E398, ISO 15106-1) shorten conditioning time and suit routine quality-control screening, while gas-chromatographic detection (ISO 15106-4) is available for specialized applications. Paper and board are commonly tested gravimetrically per TAPPI T 464 or ISO 2528.
Calculation of the permeability coefficient
From the steady-state WVTR, permeance is obtained by dividing by the water vapor partial pressure difference across the specimen, and the permeability coefficient is obtained by multiplying permeance by thickness. Because saturation vapor pressure depends strongly on temperature, approximately 2.8 kPa at 23°C and 6.6 kPa at 38°C, the applied humidity gradient must be explicitly stated in every test report.
Applications
Moisture barrier data directly drive material selection, structure design, and shelf-life modeling across many industries:
- Food packaging: dry foods, snacks, powders, and condiments, where WVTR combined with product moisture sorption isotherms predicts moisture gain or loss over the target shelf life.
- Pharmaceutical packaging: blister films, sachets, and bottle liners protecting effervescent tablets, powders, and moisture-sensitive active ingredients.
- Medical device packaging: sterile barrier systems that must remain dry while permitting sterilant removal.
- Photovoltaics: backsheets and encapsulants whose moisture exclusion correlates with long-term module reliability.
- Electronics: high-barrier films for OLED encapsulation, flexible displays, and moisture-sensitive components.
- Construction: vapor control layers, breathable membranes, and roofing underlays specified by their water vapor permeance.
- Textiles and hygiene: breathable films and laminates engineered for controlled moisture transmission.
FAQ
What is the difference between WVTR and the water vapor transmission coefficient?
WVTR describes the moisture flux through a specific structure under specific conditions, in g/(m²·24 h). The permeability coefficient removes both thickness and pressure-gradient effects, expressing an intrinsic material property in g·cm/(cm²·s·Pa), so it can be compared across materials and used to model multilayer structures.
Which test method should be chosen?
For routine barrier films, the infrared sensor method (ASTM F1249 or ISO 15106-2) offers the best balance of speed, repeatability, and sensitivity. The gravimetric cup method (ASTM E96 or GB/T 1037) suits thick, low-barrier, or absorbent materials such as paper and textiles. Electrolytic and humidity sensor methods are selected where instrument availability or a specific product standard dictates.
What are the standard test conditions?
The most common condition is 38°C with a 90% to 0% relative humidity gradient for general packaging; 23°C/85% RH, 37°C/90% RH, and 40°C/90% RH are also widely specified, and higher temperatures are used for accelerated evaluation. Results obtained under different conditions are not interchangeable because of the strong temperature dependence of permeation.
How long does testing take?
Sensor-based methods typically reach equilibrium within a few hours for medium-barrier materials and may require 24–72 hours for high-barrier laminates; gravimetric tests generally require three to fourteen days depending on the transmission level.
How are ultra-high barrier films measured?
When the WVTR approaches 10⁻⁴–10⁻⁶ g/(m²·24 h), extended conditioning and accumulation times, larger test areas, and high-sensitivity coulometric or infrared detection are applied. Calcium corrosion testing is also used as a corroborating technique for OLED-grade barriers.
How should results be reported?
A compliant report states the test method and standard, temperature and humidity gradient, specimen thickness and conditioning history, test area, individual and mean WVTR values, and, where required, the calculated permeance and permeability coefficient together with measurement uncertainty.