Packaging Bag Water Vapor Transmission Rate (WVTR) Testing: Methods, Standards, and Technical Guide

Water vapor transmission rate (WVTR) testing is a critical quality control procedure for evaluating the moisture barrier performance of packaging bags used in food, pharmaceutical, and electronic applications. This technical guide explains the testing principles, applicable standards such as ASTM E96, ASTM F1249, ISO 15106, and GB/T 1037, sample preparation requirements, and how to interpret test results for packaging material selection and shelf-life validation.

Technical Overview

Water vapor transmission rate (WVTR), also known as moisture vapor transmission rate (MVTR) or moisture permeability, quantifies the amount of water vapor that permeates through a packaging material per unit area over a given time. For packaging bags, this property directly determines how effectively the package protects its contents from moisture ingress, which is essential for preserving product quality, extending shelf life, and preventing degradation of moisture-sensitive goods.

The WVTR of a packaging bag is typically expressed in units of grams per square meter per 24 hours (g/m²·24h). Lower values indicate better moisture barrier performance. Monolayer polyethylene films may exhibit WVTR values of several grams per square meter per day, while high-barrier aluminum foil laminates can achieve values below 0.01 g/m²·24h. Because water vapor permeability is strongly dependent on temperature and relative humidity, all WVTR measurements must be reported together with the specific test conditions used.

Testing can be performed on flat packaging film samples or on finished packaging bags as complete packages. Whole-package testing evaluates the combined performance of the film material, seals, closures, and any structural features such as zippers or spouts, providing a more realistic assessment of real-world barrier performance.

Testing Samples

Monolayer Plastic Films

Common monolayer materials include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), and ethylene-vinyl alcohol copolymer (EVOH). These samples are usually tested as flat die-cut discs or rectangles with a defined test area. Sample thickness must be measured at multiple points because WVTR correlates with material thickness.

Multilayer Laminated Films

Laminated structures such as PET/AL/PE, BOPP/CPP, PET/PE, PA/PE, and paper/AL/PE combinations are the most widely used materials for packaging bags. Testing evaluates the barrier contribution of each functional layer, including aluminum foil, metallized films (VMPET, VMCPP), and transparent oxide coatings (SiOx, AlOx). Delamination, pinholes, and coating defects can significantly compromise the barrier performance of these structures.

Finished Packaging Bags

Complete packages, including three-side-seal pouches, stand-up pouches, side-gusseted bags, vacuum bags, and zipper pouches, can be tested as whole packages. This approach captures moisture ingress through the body material as well as through heat seals, zipper closures, and spout fittings. Special fixtures are used to mount the finished bag onto the test cell.

High Barrier and Specialty Materials

High-barrier samples such as aluminum foil laminates, coated films, and coextruded structures containing EVOH or PVDC layers require high-sensitivity test methods, typically sensor-based techniques capable of detecting transmission rates in the range of 0.001 to 0.5 g/m²·24h.

Testing Items

The following parameters are typically evaluated in a comprehensive packaging bag moisture permeability assessment:

• Water vapor transmission rate (WVTR) of flat film samples, expressed in g/m²·24h
• Whole-package water vapor transmission rate of finished bags, expressed in g/package·24h or g/day
• Water vapor permeance, expressed in g/m²·s·Pa
• Water vapor permeability coefficient for homogeneous materials, expressed in g·cm/(cm²·s·Pa)
• Barrier performance under different temperature and humidity conditions (for example, 23°C/85% RH, 38°C/90% RH, and 40°C/90% RH)
• Comparative barrier ranking of candidate packaging structures
• Verification of barrier retention after processing steps such as printing, lamination, and heat sealing

Testing Methods

Gravimetric Method (Cup Method)

The traditional gravimetric method, described in ASTM E96/E96M, GB/T 1037, and JIS Z 0208, seals the test film over a cup containing either a desiccant (dry cup method) or water (wet cup method). The assembled cup is placed in a controlled humidity and temperature environment, and the weight change of the cup is measured periodically. The weight gain of the desiccant cup, or weight loss of the water cup, is used to calculate the WVTR. This method is economical and widely accepted but requires long test durations, particularly for high-barrier materials, and has limited sensitivity.

Infrared Sensor Method

The modulated infrared sensor method, standardized in ASTM F1249 and ISO 15106-2, passes dry carrier gas through a chamber separated by the test film from a controlled high-humidity side. Water vapor permeating through the film is carried to an infrared sensor that quantifies the vapor concentration. This method offers high sensitivity, short test times, and full automation, making it suitable for high-barrier laminated films used in packaging bags.

Electrolytic Sensor Method

The electrolytic (coulometric) sensor method, covered by ISO 15106-3 and GB/T 21529, uses a phosphorus pentoxide-based electrolytic cell to detect permeated water vapor. Carrier gas transports the permeated moisture to the electrolytic detector, where it is quantified electrically. This technique provides excellent accuracy for low transmission rates and is commonly used for pharmaceutical and high-barrier flexible packaging applications.

Dynamic Relative Humidity Method

The humidity sensor method, described in ISO 15106-1, monitors the change in relative humidity in a dry chamber as water vapor permeates through the test film. Two-chamber instruments with calibrated humidity sensors allow precise measurement across a wide range of transmission rates and can simulate realistic storage conditions for packaging bags.

For all methods, test specimens must be conditioned prior to testing, free from wrinkles, folds, and pinholes, and securely mounted to eliminate edge leakage. Test conditions, sample thickness, and instrumentation details must be documented in the test report to ensure reproducibility and comparability of results.

Applications

Moisture permeability testing of packaging bags supports quality control and product development across many industries:

• Food packaging: verifying the barrier performance of bags for snacks, baked goods, coffee, powdered beverages, dried foods, and seasonings, and supporting shelf-life studies.
• Pharmaceutical packaging: evaluating moisture protection for tablets, capsules, powders, and effervescent products in compliance with stability testing requirements.
• Electronics packaging: confirming the moisture barrier of moisture-sensitive device (MSD) barrier bags and static-shielding laminated bags used for semiconductors and electronic components.
• Industrial and chemical packaging: assessing the performance of bags for hygroscopic powders, desiccants, fertilizers, and agrochemical products.
• Material development: comparing new film structures, coatings, and bio-based materials against conventional barrier laminates.
• Supplier verification and incoming inspection: confirming that delivered packaging materials meet contractual barrier specifications.

FAQ

What is a good WVTR value for a packaging bag?

The acceptable WVTR depends on the product sensitivity and intended shelf life. For general food applications, WVTR values below 5 g/m²·24h are common, while moisture-sensitive products typically require high-barrier structures below 0.5 g/m²·24h, and aluminum foil laminates can achieve values below 0.01 g/m²·24h under 38°C/90% RH conditions.

Which test condition should be selected?

Common conditions include 23°C/85% RH for temperate climates, 38°C/90% RH for tropical conditions, and 40°C/90% RH for accelerated evaluation. The selected condition should reflect the intended distribution and storage environment of the packaged product.

Why test whole bags instead of only flat film?

Finished bags include heat seals, folds, and closures that can create moisture ingress pathways not present in flat film. Whole-package testing provides a more accurate prediction of actual package performance and helps identify sealing defects.

How long does a WVTR test take?

Cup method tests typically require 7 to 14 days to reach equilibrium. Sensor-based methods using infrared or electrolytic detection generally complete within a few hours to two days, depending on the barrier level of the sample.

Which standard is most appropriate?

ASTM E96/E96M and GB/T 1037 suit general gravimetric testing, while ASTM F1249, ISO 15106, and GB/T 21529 are preferred for high-barrier materials requiring higher sensitivity and faster turnaround. Selection should align with customer specifications and target market requirements.

What factors can affect test results?

Sample thickness, pinholes, mounting seal quality, temperature and humidity stability, conditioning time, and instrument calibration all influence results. Testing should be performed by accredited third-party laboratories using calibrated equipment and validated procedures to ensure reliable and repeatable data.