Water vapor permeability coefficient testing by the infrared sensor method is a high-precision analytical technique used to quantify the moisture barrier performance of plastic films, laminates, sheets, and high-barrier materials. Compliant with ASTM F1249, ISO 15106-2, and GB/T 26253, this method delivers rapid and sensitive water vapor transmission rate (WVTR) results that are essential for quality control and product development in food, pharmaceutical, photovoltaic, and electronics packaging. This guide explains the test principle, applicable sample types, standard test conditions, instrumentation, data processing, and typical applications of infrared-based WVTR measurement.
Technical Overview
The water vapor transmission rate (WVTR), also known as water vapor permeability, describes the steady-state flow of water vapor through a unit area of material per unit time under specified temperature and humidity conditions, typically expressed in g/(m²·24h). The water vapor permeability coefficient further normalizes this value by specimen thickness and the water vapor partial pressure difference across the specimen, expressed in g·cm/(cm²·s·Pa), which allows intrinsic comparison between materials of different thicknesses.
The infrared (IR) detection method is one of the most sensitive and widely adopted instrumental techniques for this measurement. In a two-chamber permeation cell, one side of the specimen is maintained at a controlled high relative humidity while the other side is swept by a stream of dry, high-purity nitrogen carrier gas. Water vapor that permeates through the specimen is carried by the gas stream to an infrared sensor, where the concentration of water vapor is quantified based on the absorption of infrared radiation at characteristic wavelengths (approximately 1.9 μm to 2.7 μm) in accordance with the Beer-Lambert law. Once the transmission reaches a steady state, the WVTR is calculated directly from the calibrated sensor signal and the exposed test area.
Compared with the traditional gravimetric cup method (ASTM E96, GB/T 1037, ISO 2528), the infrared method offers significantly higher sensitivity, with detection limits down to approximately 0.001 g/(m²·24h), full automation, shorter test cycles, and superior repeatability. It is therefore the preferred technique for characterizing medium- to high-barrier materials used in demanding packaging and industrial applications.
Testing Samples
Plastic Monolayer Films and Sheets
Common materials include LDPE, LLDPE, HDPE, CPP, BOPP, PET, BOPA, PVC, EVA, CPE, PVDC-coated films, EVOH films, and PLA/PBAT biodegradable films. Rigid plastic sheets within the instrument-permitted thickness range can also be tested using appropriate adapters.
Multilayer and Laminated Structures
Typical laminates include PET/AL/PE, BOPP/VMPET/PE, PA/PE, PET/PE, paper/AL/PE, and similar flexible packaging structures used for food, pharmaceuticals, and consumer products. For multilayer structures, the result is generally reported as WVTR (permeance) rather than as a permeability coefficient.
High-Barrier Materials
The infrared method is particularly suited to aluminum foil laminates, vacuum-metallized films (VMPET, VMCPP), transparent oxide-coated films (SiOx, AlOx), and ultrahigh-barrier films for OLED and flexible electronics, where gravimetric methods lack the required sensitivity.
Textiles, Paper, and Functional Materials
Waterproof-breathable membranes, nonwovens, paper and paperboard, photovoltaic backsheets (TPT, KPK), and EVA/POE encapsulant films can be evaluated, provided the specimen can be hermetically sealed in the test cell.
Testing Items
- Water vapor transmission rate (WVTR), expressed in g/(m²·24h) under specified temperature and humidity conditions
- Water vapor permeability coefficient, expressed in g·cm/(cm²·s·Pa), calculated for homogeneous monolayer materials
- Water vapor permeance for multilayer constructions and coatings, expressed in g/(m²·24h)
- Comparative barrier ranking and customized test conditions according to client specifications
Testing Methods
Reference Standards
- ASTM F1249: Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor
- ISO 15106-2: Plastics — Film and sheeting — Determination of water vapour transmission rate — Part 2: Infrared detection sensor method
- GB/T 26253: Determination of water vapor transmission rate of plastic films and sheeting — Infrared detector method
Standard Test Conditions
- 38°C / 90% RH — tropical condition, widely applied in packaging qualification
- 37.8°C / 100% RH — tropical condition referenced in ASTM F1249
- 23°C / 85% RH — temperate condition for general-purpose materials
- 40°C / 90% RH or higher — accelerated aging conditions commonly used for photovoltaic backsheets and encapsulants in support of IEC 61215 and IEC 61730 evaluations
Specimen Preparation
Specimens are conditioned at 23±2°C and 50±5% RH for at least 48 hours in accordance with ISO 291 or GB/T 2918. The specimen, typically a disc of approximately 100 mm diameter, is mounted between the two halves of the permeation cell and sealed with grease or gaskets to eliminate edge leakage. Thickness is measured at multiple points according to ASTM D6988 or GB/T 6672, and the average value is used in permeability coefficient calculations. High-barrier specimens may require a reduced test area and extended conditioning to reach reliable equilibrium.
Test Procedure
- Verify instrument calibration using a certified reference film traceable to a national metrology institute
- Load the specimen, purge the system with dry high-purity nitrogen, and establish a stable zero baseline
- Set the target temperature, relative humidity, and carrier gas flow rate
- Run the test until the transmission signal reaches equilibrium, defined by an automated convergence criterion
- Record the steady-state WVTR and calculate the permeability coefficient using specimen thickness and the water vapor partial pressure difference across the specimen
Data Processing
The WVTR is obtained from the calibrated sensor output divided by the exposed test area. The water vapor permeability coefficient is then calculated as P = WVTR × d / Δp, where d is the mean specimen thickness and Δp is the water vapor partial pressure difference, derived from the saturation vapor pressure at the test temperature and the relative humidity gradient. For example, at 38°C the saturation vapor pressure is approximately 6.62 kPa, so a 90% RH to 0% RH gradient corresponds to a Δp of approximately 5.96 kPa.
Applications
- Food and beverage packaging: shelf-life modeling, barrier film qualification, and supplier incoming inspection
- Pharmaceutical packaging: blister foils, strip packs, and laminate validation supporting regulatory submissions
- Photovoltaics: moisture barrier evaluation of backsheets and encapsulant films supporting IEC 61215 and IEC 61730 certification programs
- Displays and flexible electronics: ultrahigh-barrier film characterization for OLED and thin-film device encapsulation
- Medical devices: sterile barrier performance verification for pouches andTyvek-type constructions
- Building and industrial materials: vapor-control membranes, protective coatings, and geotextiles
- Research and development: new material screening, aging studies, and humidity-temperature dependence mapping
FAQ
What is the difference between the infrared method and the gravimetric cup method?
The cup method determines WVTR by periodically weighing a desiccant or water cup sealed with the specimen. It is simple but slow, labor-intensive, and limited in sensitivity to roughly 0.5 g/(m²·24h). The infrared method automates detection, shortens test cycles, and reaches sensitivity levels of approximately 0.001 g/(m²·24h), making it the preferred choice for medium- and high-barrier materials. The two methods show good correlation when tests are performed correctly.
How does the infrared method compare with the electrolytic sensor method?
The electrolytic (coulometric) method, covered by ISO 15106-3 and ASTM F3299, electrolyzes permeated water vapor and measures the resulting current. It offers excellent low-end sensitivity but the sensor is gradually consumed during operation. The infrared sensor is a non-contact, non-consumable detector with a wide measurement range, fast response, and long service life, and modern modulated infrared sensors achieve comparable sensitivity for most industrial applications.
What is the measurement range of the infrared method?
Depending on instrument configuration, test area, and test temperature, typical instruments cover approximately 0.001 to 1000 g/(m²·24h). Using a reduced test area and extended equilibration extends the lower detection limit for ultrahigh-barrier materials.
How long does a test take?
Low-barrier films usually reach equilibrium within 2 to 6 hours, medium-barrier films within 8 to 24 hours, and high-barrier structures such as aluminum foil laminates may require 24 to 96 hours or longer. The instrument terminates the test automatically once the predefined equilibrium criterion is satisfied.
Which standard should be selected for testing?
ASTM F1249, ISO 15106-2, and GB/T 26253 describe technically equivalent instrumental approaches. The choice typically depends on the target market or regulatory framework, with ASTM preferred in North America, ISO accepted internationally, and GB/T applied in China. Test conditions and the reference standard should always be clearly stated in the report.
Does the instrument directly output the permeability coefficient?
No. The instrument directly measures the water vapor transmission rate. The permeability coefficient is subsequently calculated from WVTR, mean specimen thickness, and the water vapor partial pressure difference. For multilayer laminates and coated structures, only WVTR or permeance should be reported, since a single thickness-normalized coefficient is not physically meaningful for heterogeneous materials.