Water vapor permeability determination, commonly expressed as the water vapor transmission rate (WVTR), is a critical quality control test for evaluating the moisture barrier performance of plastic films used in food, pharmaceutical, and industrial packaging. This article provides a comprehensive technical overview of WVTR testing principles, internationally recognized standards such as ASTM E96, ASTM F1249, and ISO 15106, sample requirements, test procedures, and typical applications across the flexible packaging industry.
Technical Overview
Water vapor permeability determination measures the rate at which water vapor passes through a plastic film under controlled conditions of temperature and relative humidity. The most widely reported result is the water vapor transmission rate (WVTR), defined as the steady-state mass of water vapor transmitted per unit area of film per unit time, typically expressed in grams per square meter per 24 hours, or g/(m²·24h).
Water vapor permeation through a polymer film follows the solution–diffusion mechanism. Water molecules are first absorbed onto the high-humidity side of the film, then dissolve into the polymer matrix, diffuse through it along a concentration gradient, and finally desorb from the low-humidity side. The overall transmission rate therefore depends on the polymer chemistry, degree of crystallinity, molecular polarity, film thickness, presence of coatings or barrier layers, and any pinholes or defects.
Accurate determination of water vapor permeability is essential because moisture ingress is one of the leading causes of product degradation. For hygroscopic foods, effervescent tablets, dry pharmaceuticals, moisture-sensitive electronics, and sterilized medical devices, the WVTR of the packaging film directly determines shelf life, product efficacy, and regulatory compliance. The test is routinely performed by accredited third-party laboratories, packaging converters, and material manufacturers as part of incoming inspection, product development, and quality release testing.
Testing Samples
Mono-Layer Plastic Films
Single-layer films are the most common test specimens. These include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), cast polypropylene (CPP), biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene terephthalate (BOPET), polyamide (PA), polyvinyl chloride (PVC), and specialty barrier resins such as ethylene-vinyl alcohol copolymer (EVOH) and polyvinylidene chloride (PVDC). Specimens must be flat, free of creases, pinholes, and visible defects, and should be representative of the production lot.
Multilayer and Laminated Films
Flexible packaging structures are frequently composed of two or more layers bonded by adhesives, such as PET/PE, BOPP/CPP, PET/AL/PE, or PA/EVOH/PE laminates. The water vapor barrier of a laminate is governed by the complete structure, so testing is performed on the finished laminate rather than individual plies when the end-use performance is being evaluated.
Coated, Metallized, and Foil-Containing Structures
Vacuum-metallized films (for example, aluminum-metallized PET or BOPP), transparent oxide-coated films (SiOx or AlOx coatings), and aluminum foil laminates provide enhanced moisture barrier properties. Because extremely thin barrier layers can be damaged by handling, these specimens require careful preparation and are often tested with high-sensitivity sensor-based methods capable of resolving WVTR values below 0.1 g/(m²·24h).
Sheets, Membranes, and Other Flat Materials
In addition to thin films, the same test methods apply to thicker plastic sheets, rubber membranes, paper-plastic composites, and coated textiles, provided the specimen can be hermetically sealed into the test cell without leakage.
Testing Items
The following parameters are typically reported in a water vapor permeability determination:
- Water Vapor Transmission Rate (WVTR) – the steady-state quantity of water vapor transmitted through a unit area of film per unit time under specified temperature and humidity conditions, expressed in g/(m²·24h).
- Water Vapor Permeance – the WVTR normalized by the water vapor pressure difference across the specimen, expressed in g/(m²·24h·kPa).
- Water Vapor Permeability Coefficient – a material property calculated for homogeneous mono-layer films by multiplying permeance by thickness, expressed in g·cm/(cm²·s·Pa) or equivalent units.
- Condition dependence data – WVTR values measured at multiple temperature and humidity combinations to characterize barrier behavior under different climates and distribution environments.
Testing Methods
Several standardized methods are available, and the choice depends on the expected barrier level, required throughput, and applicable regulatory or customer specification.
Gravimetric Cup Method (Dish Method)
The traditional and most widely referenced technique, standardized in ASTM E96/E96M, ISO 2528, GB 1037, JIS Z 0208, and DIN 53122-1. The film specimen is sealed across the mouth of a permeability cup containing either a desiccant (dry-cup procedure) or water (wet-cup procedure). The assembly is placed in a controlled-humidity chamber, and the cup is weighed periodically. The change in mass is recorded until a steady state is reached, and the slope of mass versus time is used to calculate the WVTR. The gravimetric method is robust, economical, and suitable for WVTR values above approximately 0.5–1 g/(m²·24h).
Infrared Sensor Method
Standardized in ASTM F1249, ISO 15106-2, and GB/T 26253, this method clamps the specimen between a humid chamber and a dry carrier-gas chamber. Water vapor permeating through the film is swept by the carrier gas to an infrared detector, and the steady-state sensor signal is converted to a transmission rate. The infrared sensor method offers high sensitivity, short test times, and full automation, making it the preferred technique for high-barrier metallized and foil-laminate films with WVTR values down to 0.001 g/(m²·24h).
Electrolytic Sensor Method
Described in ISO 15106-3 and GB/T 21529, this approach uses a phosphorus pentoxide electrolytic cell to quantify the water vapor carried by the dry sweep gas. It is particularly well suited to long-duration tests on ultra-high-barrier materials because the sensor is regenerated continuously during measurement.
Humidity Sensor Method
Standardized in ISO 15106-1, this method measures the relative humidity increase in a sealed dry chamber as permeated water vapor accumulates. It is a rapid comparative technique commonly used for quality screening of medium-barrier films.
Gas Chromatographic Detection Method
Specified in ISO 15106-4, this technique quantifies permeated water vapor using gas chromatographic analysis of the carrier gas stream, providing high selectivity for multi-component permeation studies.
Test Conditions and Conditioning
Specimens are normally conditioned at 23 °C and 50% relative humidity for at least 24 hours before testing, unless otherwise specified. Common test conditions include 38 °C/90% RH (tropical accelerated condition), 23 °C/85% RH, 40 °C/90% RH, and 37 °C/90% RH for pharmaceutical applications. Because WVTR is strongly temperature dependent—typically increasing two- to three-fold with a 10 °C rise—the specified conditions must always accompany the reported result.
Applications
Water vapor permeability data support a wide range of industrial decisions:
- Food packaging: selecting barrier structures for snack foods, biscuits, powdered beverages, and dehydrated products to prevent moisture uptake, texture loss, and microbial growth during the declared shelf life.
- Pharmaceutical packaging: qualifying blister films, laminate pouches, and sachets for effervescent tablets, dry powder inhalers, and moisture-sensitive solid dosage forms in accordance with pharmacopeial stability requirements.
- Medical device packaging: verifying that sterile barrier systems protect hygroscopic devices and diagnostics from humidity during storage and transport.
- Electronics packaging: ensuring moisture-barrier bags for surface-mount components meet the requirements of moisture-sensitivity classification and prevent popcorn cracking during reflow soldering.
- Agricultural films: characterizing mulch and greenhouse films where controlled water vapor exchange influences crop microclimate.
- Construction materials: evaluating vapor retarder membranes and weather-resistive barriers used in building envelopes.
- Material development and comparison: benchmarking new resins, coatings, and biodegradable polymers against conventional barrier materials.
FAQ
What is a good WVTR value for a plastic film?
There is no universal target because requirements depend on the product and shelf life. As general guidance, general-purpose polyolefin films typically show WVTR values in the range of 5–20 g/(m²·24h) at 38 °C/90% RH, high-barrier coated or metallized films fall below 1 g/(m²·24h), and foil-containing laminates can achieve values below 0.01 g/(m²·24h). The specification should always be agreed between brand owner and converter based on product sensitivity testing.
Which test method should be chosen?
The gravimetric cup method (ASTM E96/E96M or GB 1037) is suitable for routine testing of medium-barrier films and remains the reference method in many specifications. For high-barrier materials, faster turnaround, or automated data collection, the infrared sensor method (ASTM F1249 or ISO 15106-2) or the electrolytic sensor method (ISO 15106-3) is recommended due to their superior sensitivity.
What is the difference between the dry-cup and wet-cup procedures?
In the dry-cup procedure, a desiccant inside the cup creates a near-zero humidity on one side while the controlled chamber supplies humidity on the other. In the wet-cup procedure, water inside the cup provides 100% RH and the chamber maintains a lower humidity. The two procedures impose different humidity gradients and can yield different results for the same film, so the procedure used must always be stated in the report.
How long does a test take?
Test duration depends on the barrier level of the specimen. Medium-barrier films typically reach steady state within several hours to one day using sensor-based methods, whereas high-barrier laminates may require three to seven days or longer. Gravimetric tests generally require multiple days of periodic weighing until a linear mass change is confirmed.
How does film thickness affect the result?
For a homogeneous material, WVTR is approximately inversely proportional to thickness, while the permeability coefficient remains constant. For multilayer structures, the total barrier follows the series resistance of the individual layers, which is why a thin high-barrier coating can dramatically reduce the overall transmission rate of a laminate.
Why must temperature and humidity be reported with the WVTR?
Water vapor transmission is highly sensitive to both temperature and the humidity gradient across the specimen. A WVTR value without its test conditions is not reproducible or comparable. Reports should always state the method, procedure variant, test temperature, relative humidity on both sides of the specimen, and the number of replicate specimens tested.