Gas permeability testing of plastic films is a critical quality-control step for evaluating the barrier performance of flexible packaging used in the food, pharmaceutical, and electronics industries. This article explains the principles, standard test methods (ISO 15105, ASTM D1434, ASTM D3985, GB/T 1038), sample requirements, and typical applications of oxygen and gas transmission rate measurement. Understanding these procedures helps manufacturers select the appropriate testing approach for product development, compliance verification, and shelf-life optimization.
Technical Overview
Gas permeability determination measures the rate at which gases such as oxygen, carbon dioxide, nitrogen, and air pass through a plastic film under a defined pressure gradient, temperature, and humidity. Gas transport through a dense polymer film follows the solution–diffusion mechanism: gas molecules first dissolve into the high-pressure surface of the film, then diffuse through the free volume of the polymer matrix, and finally desorb from the low-pressure surface. Because the permeability coefficient equals the product of the diffusion coefficient and the solubility coefficient, the test results reflect both the molecular mobility within the polymer and the gas–polymer affinity.
The primary measurands in plastic film permeability testing include the gas transmission rate (GTR), commonly expressed in cm³/(m²·24h) under standard test conditions, the permeance, and the permeability coefficient P, expressed in cm³·cm/(cm²·s·Pa). Barrier performance varies enormously across materials: low-density polyethylene films may exhibit oxygen transmission rates of several thousand cm³/(m²·24h), while biaxially oriented PET typically falls between 50 and 130, PVDC-coated or EVOH-containing laminates reach single digits, and aluminum foil laminates can achieve values below 0.01. Accurate permeability data are therefore essential for material selection, packaging design, and shelf-life prediction, and accredited third-party laboratories routinely perform these tests against internationally recognized standards.
Testing Samples
Single-Layer Polyolefin Films
LDPE, LLDPE, HDPE, CPP, and BOPP films represent the largest sample category. These materials show moderate oxygen barrier properties but relatively high carbon dioxide permeability, and their results are strongly influenced by density, thickness, and crystallinity.
Barrier and Engineering Polymer Films
Biaxially oriented PET (BOPET), biaxially oriented polyamide (BOPA), EVOH, PVA, and PVDC-coated films are tested when high or ultra-high barrier performance is required. EVOH and polyamide samples are particularly humidity-sensitive and often require testing at multiple relative humidity levels.
Multilayer Laminated and Composite Films
Flexible packaging laminates such as PET/AL/PE, OPP/CPP, and PA/EVOH/PE structures are tested as finished systems. Since the laminate acts as a series of resistances, the measured transmission rate reflects the combined contribution of every layer, adhesive, and coating.
Metallized Films and Foil Laminates
Aluminum-metallized films (e.g., VMPET, VMCPP) and foil-based laminates demand high-sensitivity instruments capable of resolving transmission rates at the 0.01 cm³/(m²·24h) level, with extended conditioning periods to reach steady-state permeation.
Biodegradable and Specialty Films
PLA, PBAT, PBS, and cellophane films are increasingly submitted for gas permeability characterization as converters validate their suitability as sustainable packaging alternatives. Sample preparation typically involves conditioning at 23 °C and 50% relative humidity in accordance with ISO 291, cutting defect-free specimens, and measuring thickness at multiple points, since thickness directly enters the calculation of the permeability coefficient.
Testing Items
Standard testing portfolios for plastic film gas permeability determination include the following items:
- Oxygen transmission rate (OTR / O2GTR), the most frequently requested parameter for food and pharmaceutical packaging.
- Carbon dioxide transmission rate (CO2TR), essential for modified atmosphere packaging (MAP) design and carbonated-product applications.
- Nitrogen transmission rate, relevant to inert-gas flushing processes.
- Air permeance for general barrier classification.
- Permeability coefficient P, normalized for film thickness, enabling comparison between different materials.
- Diffusion coefficient D and solubility coefficient S, derived from the time-lag analysis in differential-pressure testing.
- Humidity-conditioned oxygen transmission rate, measured at controlled relative humidity (0%–90% RH) per ASTM F1927, critical for EVOH and polyamide structures.
Testing Methods
Differential-Pressure (Manometric) Method
The differential-pressure method is specified in ISO 15105-1, ASTM D1434, GB/T 1038, and JIS K 7126-1. A circular specimen is clamped as a barrier between two sealed chambers, both of which are evacuated. The test gas is then admitted to the upstream chamber at a constant pressure while the downstream chamber remains under vacuum. As gas permeates through the film, the pressure increase in the lower chamber is recorded continuously by a high-sensitivity manometer. The steady-state slope of the pressure–time curve yields the gas transmission rate, while the intercept of the extrapolated steady-state line (the time lag) allows calculation of the diffusion and solubility coefficients. The method accommodates virtually any pure test gas, requires no carrier gas, offers excellent sensitivity, and delivers the full set of permeation parameters, although most classical configurations operate at low humidity rather than ambient conditions.
Equal-Pressure (Coulometric Sensor) Method
The equal-pressure method is covered by ISO 15105-2, ASTM D3985, ASTM F1927, GB/T 19789, and JIS K 7126-2. Both sides of the specimen are maintained at atmospheric pressure: the upstream chamber is purged with oxygen (at controlled relative humidity where required), while the downstream chamber is swept by a high-purity nitrogen carrier gas. Oxygen molecules that permeate through the film are transported to a coulometric sensor, where they are electrochemically reduced and quantified, giving a direct reading of the oxygen transmission rate in cm³/(m²·24h). Because the test runs at ambient pressure and can incorporate precise humidity control, it is the preferred technique for evaluating humidity-dependent barrier polymers such as EVOH and polyamide, and for measuring finished packages and lidding films under realistic conditions.
Instrumentation, Calibration, and Data Reporting
Modern permeation instruments integrate thermostated test cells (typically 23 °C ± 0.1 °C or 37 °C for special applications), automatic purge and evacuation sequences, and software that verifies steady-state permeation before reporting. Calibration is performed using certified reference films traceable to national metrology institutes. Results are reported together with the test gas, temperature, relative humidity, specimen thickness, mean and standard deviation of at least three replicate specimens, and the applicable standard. Temperature dependence follows Arrhenius behavior, so any comparison between laboratories is valid only when test conditions are identical.
Applications
Food Packaging and Shelf-Life Modeling
Oxygen and carbon dioxide transmission data are the core inputs for MAP design and shelf-life modeling. Dairy products, processed meats, nuts, coffee, and snack foods require carefully matched barrier structures; combining measured OTR and package headspace data allows converters and brand owners to predict oxidation-limited shelf life quantitatively.
Pharmaceutical and Medical Packaging
Blister laminates, lidding foils, and sterile barrier systems for moisture- and oxygen-sensitive dosage forms are verified against permeability specifications. Regulatory submissions frequently require OTR data generated under controlled humidity by accredited third-party laboratories using pharmacopeia-referenced film test methods.
Industrial, Agricultural, and Specialty Applications
Permeability testing supports vacuum insulation panels, desiccant packaging, silage and greenhouse films, and ultra-high-barrier encapsulation films for OLED and photovoltaic devices. It is also applied in research and development for comparing new resin grades, evaluating coating integrity, and monitoring product consistency in routine factory quality control.
FAQ
What is the difference between gas transmission rate and permeability coefficient?
The gas transmission rate describes the volume of gas passing through a specific film of a given thickness per unit area per day, while the permeability coefficient is normalized by thickness and pressure difference, making it an intrinsic material property suitable for comparing different polymers independently of specimen geometry.
How do I choose between the differential-pressure and equal-pressure methods?
Choose the differential-pressure method when multiple gases (O2, CO2, N2) must be tested, when diffusion and solubility coefficients are needed, or when following ISO 15105-1, ASTM D1434, or GB/T 1038. Choose the equal-pressure coulometric method for oxygen transmission rate measurements under controlled humidity, particularly for EVOH and polyamide films, in line with ASTM D3985, ASTM F1927, ISO 15105-2, or GB/T 19789.
Why does humidity strongly affect some barrier films?
Polar barrier polymers such as EVOH and polyamide absorb water molecules that plasticize the polymer matrix, increasing chain mobility and raising the oxygen transmission rate by one to two orders of magnitude at high relative humidity. This is why humidity-controlled OTR testing is indispensable for realistic performance assessment.
How long does a typical permeability test take?
High-permeability polyolefin films may reach steady state within a few hours, whereas high-barrier metallized or foil-containing structures can require 24 to 72 hours or longer. Test duration always includes chamber purging, temperature equilibration, and verified steady-state permeation before the result is accepted.
Which sample size and conditioning are required?
Standard test cells typically accept circular specimens with an effective test area of around 50 cm² or smaller mask openings for ultra-barrier materials. Specimens must be flat, crease-free, and free of pinholes, and are normally conditioned at 23 °C and 50% relative humidity for at least 24 hours before testing unless the applicable standard specifies otherwise.