Water vapor transmission rate (WVTR) testing of printed films is a critical quality control measure that verifies the moisture barrier performance of packaging materials after printing, coating, and lamination processes. This technical article explains the testing principles, applicable international standards, sample requirements, test methods, and practical applications of WVTR testing for printed films used across the food, pharmaceutical, and consumer goods packaging industries.
Technical Overview
Printed films are flexible polymer substrates, such as BOPP, PET, PE, CPP, and their laminated structures, that carry printed graphics, inks, primers, and overprint varnishes for branding and product identification purposes. The water vapor transmission rate (WVTR), also known as water vapor permeability, quantifies the steady-state rate at which water molecules diffuse through a film under specified temperature and humidity conditions. The result is typically expressed in grams per square meter per 24 hours, or g/(m²·24h), and serves as the primary indicator of a packaging material's moisture barrier capability.
Although the base film largely determines the intrinsic barrier level, the printing and converting process can significantly alter the final water vapor barrier performance. Ink layers, surface treatments, solvent residues, thermal drying, adhesive lamination, and the mechanical stress of the printing press may either partially block diffusion pathways or introduce micro-defects such as pinholes, voids, and delamination. Therefore, testing the WVTR of the printed film, rather than only the unprinted substrate, provides a realistic assessment of the moisture protection that the finished package will deliver.
WVTR testing of printed films is routinely performed by manufacturers, converters, brand owners, and accredited third-party laboratories to validate product specifications, compare ink and coating systems, troubleshoot barrier failures, and demonstrate compliance with regulatory or contractual requirements.
Testing Samples
Surface-Printed Films
Surface-printed films, where inks and varnishes are applied to the outer side of a monolayer or coextruded film, are the most common test samples. Typical substrates include printed BOPP and printed PE films used for snack bags, overwraps, and horizontal form-fill-seal packaging. Specimens should be taken from representative production rolls, with the printed surface clearly identified so that the orientation of the sample in the test cell can be documented.
Reverse-Printed and Laminated Films
Reverse-printed films, in which the graphic is printed on the inside of the outer web and then laminated to a sealing layer, are widely used in multilayer structures such as PET/ink/adhesive/CPP or BOPP/ink/adhesive/VMPET/PE. For these samples, the WVTR is evaluated on the complete laminated structure, because the combined barrier of all layers, adhesives, and inks determines the real-world performance of the package.
Printed Metallized and Coated Films
Printed metallized films, such as VMPET or VMCPP with printed graphics, and films finished with high-barrier coatings, require special attention. These structures achieve very low WVTR values, often below 1 g/(m²·24h) at 38°C and 90% RH, and demand high-sensitivity sensor-based instruments with extended conditioning times to reach a true steady state.
Sample Preparation Requirements
Specimens must be flat, wrinkle-free, and free of visible defects such as scratches, gels, or printing wrinkles within the test area. Samples are normally conditioned at 23°C ± 2°C and 50% ± 10% relative humidity for at least 24 hours in accordance with ISO 291 or GB/T 2918 prior to testing. A minimum of three replicate specimens is recommended for each condition, and the test area must be securely sealed in the test cell, using gaskets or wax sealing, to eliminate edge leakage that would falsely inflate the measured transmission rate.
Testing Items
Typical testing items for printed film WVTR evaluation include:
- Water vapor transmission rate of the finished printed film, reported in g/(m²·24h)
- Comparative WVTR between printed areas and unprinted areas of the same film
- Comparative WVTR of the film before and after printing or coating
- Influence of ink coverage, ink layer thickness, or varnish type on barrier performance
- WVTR of laminated structures after printing, lamination, and curing
- WVTR under multiple temperature and humidity conditions to characterize climate sensitivity
- Barrier retention after accelerated aging, sterilization, or transportation simulation
Testing Methods
Gravimetric Cup Method
The gravimetric method, described in ASTM E96/E96M, ISO 2528, GB/T 1037, and JIS Z 0208, seals the specimen over a cup containing either a desiccant (dry cup) or water (wet cup). The assembly is placed in a controlled humidity chamber, and the moisture gained or lost through the film is determined by periodic weighing until a steady state is reached. The WVTR is calculated from the slope of the mass change versus time divided by the test area. The cup method is simple, inexpensive, and well suited to medium-barrier printed films, but it requires several days of test time and careful sealing technique.
Modulated Infrared Sensor Method
The infrared sensor method, standardized in ASTM F1249, ISO 15106-2, and GB/T 26253, uses a two-chamber permeation cell in which dry carrier gas flows beneath the film while the upper side is exposed to a controlled high-humidity atmosphere. Water vapor permeating the film is swept to an infrared detector, and the steady-state signal is converted directly into a WVTR value. This method offers high sensitivity, short test cycles, and automated operation, and it is the preferred technique for most printed film qualification work, including high-barrier metallized structures.
Electrolytic Sensor Method
The electrolytic detection method, specified in ISO 15106-3 and GB/T 21529, quantifies permeated water vapor by electrolyzing it in a phosphorus pentoxide cell and measuring the resulting electrolytic current, which is proportional to the moisture transmission rate. This method provides excellent precision at low transmission levels and is frequently used for printed films intended for pharmaceutical or electronics applications where moisture control is critical.
Humidity Sensor Method
The humidity sensor method, described in ISO 15106-1, tracks the relative humidity increase in a sealed dry chamber as water vapor permeates the specimen. It is a versatile option for films of various barrier grades and supports multi-specimen testing with independent monitoring of each cell.
Test Conditions and Data Evaluation
Commonly applied test conditions include 38°C with 90% RH, which represents a tropical storage scenario and is the reference condition in most Chinese national and international specifications, as well as 23°C with 85% RH and 40°C with 90% RH for specialized comparisons. Because WVTR increases nonlinearly with temperature and humidity, the test conditions must always be reported together with the result. Reported data should include the mean value, standard deviation, specimen thickness, test area, conditioning history, and the specific standard applied.
Applications
WVTR testing of printed films supports quality assurance and product development across many sectors:
- Food packaging: moisture barrier validation for printed snack films, coffee pouches, confectionery wrappers, dry food laminates, and barrier overwraps
- Pharmaceutical packaging: barrier verification for printed blister lidding foils, sachet laminates, and medical device pouches requiring low moisture ingress
- Consumer and daily chemical packaging: qualification of printed films for detergents, cosmetics refills, and hygiene products sensitive to humidity
- Electronics packaging: assessment of printed moisture-barrier bags and protective wraps for circuit boards and electronic components
- Material development: comparison of ink systems, coatings, adhesives, and film structures during R&D and supplier qualification
- Regulatory and contractual compliance: generation of test reports by accredited third-party laboratories for customer audits, tenders, and import-export documentation
FAQ
Why should WVTR be tested on printed films instead of only on the base film?
Printing, varnishing, surface treatment, and lamination all modify the diffusion path of water vapor. Inks and coatings can slightly improve the barrier by adding dense layers, while processing defects such as micro-pinholes, poor ink adhesion, or insufficient curing can degrade it. Testing the actual printed film reflects the true performance of the finished package.
Which test method is most suitable for printed films?
For routine qualification of medium-barrier printed films, the infrared sensor method under ASTM F1249 or ISO 15106-2 offers the best balance of speed, precision, and automation. The gravimetric cup method under ASTM E96/E96M or GB/T 1037 remains valuable for verification and for structures that are difficult to seal. High-barrier metallized printed films are best measured with infrared or electrolytic sensor instruments.
How long does a WVTR test take?
Sensor-based tests on typical printed films usually stabilize within several hours to one day, while high-barrier laminates may require two to five days to reach steady state. Gravimetric cup tests generally require three to ten days depending on the barrier level and test conditions. Multiple specimens are tested in parallel to improve throughput.
What sample size and quantity are required?
Most laboratories request film samples of at least 200 mm by 200 mm per condition, taken from at least three different positions across the web width. A minimum of three testable specimens is required, and five or more replicates are recommended when statistical comparison between printed and unprinted areas is needed.
What are typical WVTR values for common printed film structures?
Representative values at 38°C and 90% RH include approximately 4 to 6 g/(m²·24h) for 20 µm BOPP-based printed films, 15 to 25 g/(m²·24h) for 12 µm PET-based printed films, below 1 g/(m²·24h) for metallized printed laminates, and below 0.5 g/(m²·24h) for foil-containing printed structures. Actual values depend on thickness, structure, printing coverage, and test conditions.
Can test results from different methods be compared directly?
Results obtained under the same temperature and humidity conditions are generally comparable, but systematic differences between gravimetric and sensor-based methods can occur due to differences in test geometry and sealing. For compliance purposes, the specified standard method and conditions stated in the product specification or purchase agreement should always be followed.
