Water vapor transmission coefficient testing of plastic food cling film is a critical quality control procedure that measures how effectively food wrap resists moisture migration, directly impacting food freshness, shelf life, and packaging compliance. This article provides a comprehensive technical guide covering testing principles, international standards such as ASTM F1249, ISO 15106, and GB 1037, sample preparation requirements, instrumental methods, and practical applications for manufacturers and packaging engineers.
Technical Overview
Plastic cling film, commonly known as food wrap or preservation film, functions primarily as a moisture and gas barrier that protects perishable food products from dehydration, oxidation, and microbial contamination. The water vapor transmission coefficient (also expressed as water vapor permeability) quantifies the rate at which water molecules permeate through a unit area of film material under specified conditions of temperature and relative humidity. This parameter is fundamental to predicting how long wrapped food will retain moisture, crispness, and overall quality during storage and distribution.
The water vapor transmission rate (WVTR) is typically expressed in grams per square meter per 24 hours, g/(m²·24h), while the water vapor permeability coefficient is expressed in g·cm/(cm²·s·Pa), which normalizes the transmission rate against film thickness and the partial pressure differential of water vapor. Accurate determination of these values requires controlled environmental conditions, precise gravimetric or sensor-based instrumentation, and strict adherence to internationally recognized test standards.
For food-grade cling films, water vapor barrier performance is a decisive factor in product classification and market acceptance. Films manufactured from different polymers—polyethylene (PE), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), and polymethylpentene (PMP)—exhibit permeability values that can differ by more than an order of magnitude, making reliable testing essential for material selection, quality assurance, and regulatory compliance.
Testing Samples
Polyethylene (PE) Cling Film
PE cling film, typically produced from low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE), is the most widely used household and commercial food wrap. It offers moderate water vapor barrier properties with typical WVTR values in the range of 10 to 40 g/(m²·24h) depending on thickness, which generally ranges from 8 to 15 micrometers. PE samples are non-self-adhesive and usually require mechanical sealing or sufficient tack generated by additives.
Polyvinyl Chloride (PVC) Cling Film
PVC cling film is widely used for wrapping fresh produce, meat, and seafood in supermarkets due to its excellent transparency, elasticity, and self-cling characteristics. PVC films typically contain 25% to 35% plasticizer content, which influences both mechanical properties and permeability. Their water vapor transmission rate is comparatively high, which is actually beneficial for fresh produce packaging where some moisture exchange prevents condensation and spoilage.
Polyvinylidene Chloride (PVDC) Cling Film
PVDC cling film provides the highest barrier performance among common cling film materials, combining low water vapor permeability with excellent oxygen and aroma barrier properties. It is frequently used for premium food preservation, cured meats, and cheese products where maximum moisture and gas protection is required.
Sample Preparation Requirements
Test specimens must be flat, free from wrinkles, pinholes, creases, and visible defects. Samples should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours prior to testing, unless otherwise specified by the applicable standard. A minimum of three replicate specimens is recommended, with each specimen sealed securely into the test cell to eliminate edge leakage that could artificially inflate results.
Testing Items
The core testing program for water vapor barrier performance of cling film includes the following measurable parameters:
- Water Vapor Transmission Rate (WVTR): the mass of water vapor passing through a unit area of film per unit time, expressed in g/(m²·24h)
- Water Vapor Permeability Coefficient (Pv): the normalized material property accounting for film thickness and vapor pressure differential, expressed in g·cm/(cm²·s·Pa)
- Water Vapor Permeance: the transmission rate divided by the water vapor pressure differential, expressed in g/(m²·24h·kPa)
- Thickness measurement: performed at multiple points using a precision micrometer or non-contact gauge, as thickness directly affects permeability calculations
- Temperature and humidity conditioning verification to ensure test environment stability
Testing Methods
Gravimetric Cup Method (ASTM E96 / GB 1037)
The gravimetric method, also known as the dish or cup method, seals the test film over a cup containing either a desiccant (dry cup method) or distilled water (wet cup method). The assembled cell is placed in a controlled-humidity chamber, and the cup is weighed periodically to determine mass change caused by water vapor transmission. The wet cup method maintains a high vapor pressure inside the cup with the external environment held at lower humidity, while the dry cup method reverses this gradient. The transmission rate is calculated from the linear portion of the mass-versus-time curve. This method is simple, low-cost, and widely accepted, though it requires long test durations—often several days to reach equilibrium.
Infrared Sensor Method (ASTM F1249 / ISO 15106-2)
The modulated infrared sensor technique passes a carrier gas (typically dry nitrogen) at controlled relative humidity across one side of the film while the other side is swept by dry nitrogen. Water vapor permeating through the film is carried to an infrared detection sensor, and the measured absorption signal is converted into a transmission rate. This method offers rapid results—often within a few hours—excellent repeatability, and the ability to test at elevated temperature and humidity conditions such as 37.8 °C and 90% RH, making it the preferred method for high-throughput quality control laboratories.
Electrolytic Sensor Method (ISO 15106-3 / GB/T 21529)
The electrolytic cell method conveys permeated water vapor into an electrolytic detection cell equipped with phosphorus pentoxide-coated electrodes. Water molecules are quantitatively absorbed and electrolyzed, and the electrolysis current is directly proportional to the amount of water vapor transported through the specimen. This method provides high sensitivity and stable long-term measurement, particularly suitable for ultra-low permeability materials such as PVDC films.
Relative Humidity Sensor Method (ISO 15106-1)
This approach monitors the change in relative humidity within a sealed test chamber as water vapor permeates the specimen. A calibrated humidity sensor tracks the humidity increase over time, from which the transmission rate is derived. The method is well suited to automated instruments and multi-specimen testing configurations.
Standard Test Conditions
Commonly specified test conditions include 23 °C with 90% to 0% RH gradient for tropical simulation, 37.8 °C with 90% RH for accelerated testing, and 38 °C with 90% RH for high-barrier materials. The selected condition must be reported alongside results, as water vapor transmission is strongly temperature-dependent, typically following Arrhenius-type behavior.
Applications
Water vapor transmission coefficient testing of cling film serves multiple purposes across the food packaging value chain:
- Incoming material inspection and supplier qualification for food packaging converters and film producers
- Product development and formulation optimization, including evaluation of plasticizer loading in PVC films and the effect of metallocene catalysts in PE grades
- Shelf-life modeling and food preservation performance prediction for fresh produce, meat, seafood, bakery, and dairy applications
- Regulatory compliance demonstration under food contact material regulations, including GB 4806 series standards, FDA 21 CFR 177 requirements, and EU Regulation 10/2011
- Comparison testing during material substitution, such as transitioning from PVC to PE or exploring biodegradable alternatives like PLA-based films
- Certification testing by accredited third-party laboratories supporting product labeling claims such as "high barrier" or "moisture-proof"
- Quality control verification for thickness reduction programs aimed at source reduction and sustainability initiatives
FAQ
What is the difference between WVTR and the water vapor permeability coefficient?
WVTR (water vapor transmission rate) describes the mass of water vapor passing through a specific film sample per unit area per day under defined conditions, and it depends on film thickness. The water vapor permeability coefficient is an intrinsic material property obtained by multiplying the transmission rate by film thickness and dividing by the water vapor pressure differential, allowing direct comparison between materials of different thicknesses.
Which test method is most accurate for cling film?
Each method has distinct advantages. The gravimetric cup method (ASTM E96) remains the reference method with broad regulatory acceptance, while the infrared sensor method (ASTM F1249) and electrolytic method (ISO 15106-3) deliver faster results, higher sensitivity, and better suitability for routine quality control of thin, high-permeability films such as household cling wrap. Consistency in method selection and test conditions is more important than the choice of a single "best" method.
Why do different laboratories sometimes report different WVTR values for the same film?
Variations commonly arise from differences in test temperature, relative humidity gradient, method type (wet cup versus dry cup versus instrumental sensor), specimen conditioning, and thickness measurement. Water vapor transmission is highly sensitive to temperature; even a 5 °C deviation can alter results significantly. Comparisons should always be made at identical, fully documented conditions.
How long does a typical water vapor transmission test take?
Instrumental methods using infrared or electrolytic sensors typically complete a test in 4 to 12 hours after the specimen reaches steady-state permeation. The gravimetric cup method generally requires 3 to 14 days of weighings to establish a stable mass-change slope, depending on the barrier level of the film.
Is a lower water vapor transmission coefficient always better for food wrap?
Not necessarily. While high-barrier films like PVDC maximize moisture retention for meat and cheese, fresh fruits and vegetables respire and benefit from moderately permeable films such as PVC or PE that allow controlled moisture and gas exchange, preventing condensation, anaerobic conditions, and accelerated decay. The optimal permeability depends on the specific food application.