Water vapor permeability coefficient testing, commonly known as water vapor transmission rate (WVTR) testing, measures how effectively plastic films, laminates, sheets, and functional barrier materials resist the passage of moisture. This testing is essential for quality control and product design in food packaging, pharmaceutical packaging, photovoltaic encapsulation, electronics, and construction materials. This article explains the test principle, applicable sample types, key test items, standard testing methods, typical test conditions, and answers to frequently asked questions.
Technical Overview
Water vapor permeability coefficient testing quantifies the steady-state rate at which water vapor diffuses through a material under controlled temperature and humidity conditions. The most commonly reported result is the water vapor transmission rate (WVTR), expressed in grams of water vapor passing through one square meter of material per 24 hours, i.e., g/(m²·24 h). For engineering and material comparison purposes, the result can be normalized into a permeability coefficient that is independent of specimen thickness. Accredited third-party laboratories routinely perform these tests to support product development, supplier qualification, incoming inspection, and regulatory compliance for packaging and barrier materials.
Key Parameters
- Water Vapor Transmission Rate (WVTR): the mass of water vapor transmitted per unit area per unit time, typically in g/(m²·24 h) or g/(m²·day).
- Water vapor permeance: the transmission rate divided by the water vapor partial pressure difference across the specimen, often reported alongside WVTR.
- Water vapor permeability coefficient: the material-intrinsic property obtained by normalizing permeance for specimen thickness, commonly expressed in g·cm/(m²·24 h·kPa) or in SI units of g·cm/(cm²·s·Pa).
Test Principle
A conditioned specimen is mounted as a barrier between two chambers: a high-humidity side and a low-hidity (dry) side. The resulting water vapor partial pressure gradient drives diffusion through the specimen. The amount of transmitted water vapor is measured either gravimetrically, by tracking the mass change of a dish containing desiccant or water, or instrumentally, by detecting water vapor in a dry carrier gas stream using infrared, electrolytic, or humidity sensors. Once steady-state transmission is reached, the WVTR is calculated, and the permeability coefficient is derived by factoring in specimen thickness and the partial pressure difference.
Barrier Performance Classification
- Low barrier (general packaging films such as PE and PP): WVTR typically above 10 g/(m²·24 h).
- Medium barrier (PET, PA, coated films): approximately 1–10 g/(m²·24 h).
- High barrier (PVDC, EVOH structures, aluminized and aluminum foil laminates): approximately 0.01–1 g/(m²·24 h).
- Ultra-high barrier (transparent oxide-coated films for OLED and perovskite photovoltaic encapsulation): below 10⁻² g/(m²·24 h), down to 10⁻⁴–10⁻⁶ g/(m²·24 h) in advanced applications.
Testing Samples
Plastic Films and Flexible Laminates
Single-layer plastic films (PE, PP, PET, PA, PS, PC, PVDC, EVOH) and multi-layer composite laminates used in flexible packaging are the most frequently tested sample types. Aluminum foil laminates, metallized films, and transparent oxide-coated (SiOx, AlOx) high-barrier films are tested to verify barrier layer integrity and lamination quality.
Sheets and Rigid Polymer Materials
Thermoforming sheets, pharmaceutical blister materials, rigid PVC, PP, and PET sheets, as well as injection-molded or extruded polymer plaques, can be evaluated, provided a flat, defect-free test area of sufficient size is available.
Textiles, Paper, and Coated Materials
Breathable waterproof fabrics, nonwovens, paper and paperboard, coated papers, and vapor-permeable construction membranes are tested to characterize moisture management performance. Inverted-cup configurations are often used for hydrophobic breathable membranes.
Functional Barrier Materials
Photovoltaic encapsulant films (EVA, POE), display encapsulation films, lithium battery barrier films, waterproofing membranes, and medical device packaging materials require precise WVTR characterization because moisture ingress directly affects long-term reliability.
Sample Preparation and Conditioning
Specimens must be flat, crease-free, and free of pinholes, and thickness is measured at multiple points to calculate the permeability coefficient accurately. Prior to testing, samples are typically conditioned in a standard atmosphere (for example, 23 °C and 50% relative humidity in accordance with ISO 291 or GB/T 2918) for a defined period to reach moisture equilibrium.
Testing Items
- Water vapor transmission rate (WVTR), g/(m²·24 h)
- Water vapor permeance
- Water vapor permeability coefficient, g·cm/(m²·24 h·kPa) or g·cm/(cm²·s·Pa)
- Comparative testing under multiple temperature and humidity conditions
- Barrier performance verification after aging, sterilization, or environmental stress
Common test conditions include 23 °C / 50% RH, 23 °C / 85% RH, 38 °C / 90% RH (tropical condition widely used per GB/T 1037 and ASTM E96/E96M), and 40 °C / 90% RH for accelerated evaluation of high-barrier materials. Photovoltaic encapsulants are frequently evaluated under elevated temperature and humidity conditions relevant to module service environments referenced in IEC 61215 and IEC 61730 qualification frameworks.
Testing Methods
Gravimetric Cup Method
The specimen is sealed over a dish containing either a desiccant (dry-cup method) or water (wet-cup method). Water vapor transfer causes a measurable mass change in the dish, which is monitored over time until a steady rate is achieved. The method is governed by ASTM E96/E96M, ISO 2528, GB/T 1037, and JIS Z 0208. It is simple, economical, and suitable for medium-barrier and permeable materials, but equilibrium times are long and sensitivity is limited at very low transmission rates.
Infrared Sensor Method
A dry carrier gas (typically nitrogen) sweeps the low-humidity side of the cell, and transmitted water vapor is quantified by an infrared sensor, often a modulated infrared sensor for high sensitivity. This method, covered by ASTM F1249, ISO 15106-2, and GB/T 26253, offers rapid measurement and detection limits down to approximately 10⁻³ g/(m²·24 h), making it the preferred technique for high-barrier films and quality control in production environments.
Electrolytic (Coulometric) Sensor Method
Water vapor carried by the dry gas stream is electrolyzed in a phosphorus pentoxide-based cell, and the electrolysis current is directly proportional to the water vapor transmission rate. The method is specified in ISO 15106-3 and GB/T 21529 and provides accurate, continuous measurement across a wide range, including low transmission rates. Sensor-based procedures are also incorporated in the Chinese YBB pharmaceutical packaging standard YBB 00092003 for water vapor transmission determination of drug packaging materials.
Humidity Sensor Method
The relative humidity change on the dry side of the test cell is monitored by a humidity sensor, and the WVTR is derived from the rate of humidity increase. The method is standardized in ISO 15106-1 and GB/T 30412 and is well suited to automated multi-cell testing.
Ultra-High Barrier Techniques
For transparent ultra-high barrier films used in OLED displays and perovskite or organic photovoltaic devices, where transmission rates are below the detection limits of conventional instruments, specialized techniques such as the optical calcium degradation test and tracer-based methods are employed to resolve values in the 10⁻⁴ to 10⁻⁶ g/(m²·24 h) range.
Method Selection Considerations
- Expected barrier level: gravimetric methods for higher transmission rates; infrared or electrolytic sensor methods for high-barrier materials.
- Material form: films, sheets, and laminates are mounted in cells; textiles and membranes may require special support or inverted-cup configurations.
- Throughput requirements: sensor methods reach steady state faster and support multi-specimen parallel testing.
- Regulatory context: packaging for food and pharmaceuticals should be tested according to the standards referenced by the applicable specification or pharmacopoeial packaging requirements.
Applications
- Food packaging: shelf-life modeling for snacks, coffee, powdered beverages, dried foods, and retort pouches, and verification of barrier layer performance in multi-layer laminates.
- Pharmaceutical packaging: WVTR determination for blister foils, composite pouches, infusion bags, and desiccant-containing bottle systems in accordance with YBB and related packaging standards.
- Photovoltaics: qualification of EVA and POE encapsulant films, backsheets, and edge-seal materials to control moisture ingress and prevent module degradation.
- Electronics and displays: barrier film development for OLED and micro-LED encapsulation, flexible electronics, and lithium battery components.
- Construction: water vapor permeability characterization of vapor barriers, breathable roof membranes, and waterproofing sheets in accordance with EN 1931 and related European standards.
- Textiles and medical products: moisture management evaluation of breathable fabrics, wound dressings, and sterile barrier systems.
FAQ
What is the difference between WVTR and the water vapor permeability coefficient?
WVTR describes the transmission through a specific specimen of a given thickness under defined conditions, while the permeability coefficient is a material property obtained by normalizing the WVTR for thickness and the water vapor partial pressure difference. The permeability coefficient enables fair comparison between materials of different thicknesses.
Which test method should be selected?
For general and medium-barrier materials, the gravimetric cup method (ASTM E96/E96M, GB/T 1037, ISO 2528) is usually sufficient. For high-barrier films or where fast turnaround is required, the infrared sensor method (ASTM F1249) or the electrolytic sensor method (ISO 15106-3, GB/T 21529) is recommended. Laboratories can advise on method selection based on the expected transmission range.
What test conditions are typically used?
The most common condition is 38 °C with 90% relative humidity, which simulates a tropical environment. Other frequently used conditions include 23 °C / 50% RH, 23 °C / 85% RH, and 40 °C / 90% RH. Higher temperature and humidity increase the transmission rate and shorten test duration.
What sample size and preparation are required?
Typical test cells accept circular specimens with a test diameter of roughly 50 to 100 mm, depending on the instrument. Specimens must be flat and free of folds, pinholes, and contamination. Thickness is measured at several points, and samples are conditioned in a standard atmosphere before testing.
How long does a test take?
Permeable materials may reach steady state within one to two days using sensor-based methods. High-barrier materials with very low transmission rates can require several days to a few weeks, particularly with the gravimetric method. Testing is continued until a statistically steady transmission rate is confirmed.
How should results be reported?
Reports should include the test standard, method, temperature and humidity conditions, specimen thickness, test area, WVTR with units of g/(m²·24 h), and, where required, the calculated permeability coefficient. Test conditions must always accompany the results because transmission rates vary strongly with temperature and humidity.