Determination of Methane Permeability of Plastics: Test Methods, Standards, and Applications

Methane permeability determination quantifies the rate at which methane gas migrates through plastic films, sheets, geomembranes, and finished packages, delivering essential data for barrier material design, safety assessment, and quality control. This technical guide explains the underlying solution–diffusion mechanism, the differential-pressure and equal-pressure test methods defined by ISO 15105, ASTM D1434, and GB/T 1038, sample requirements, reported parameters, and typical industrial applications of plastic methane permeability testing.

Technical Overview

Methane (CH4) permeability determination measures how readily methane molecules adsorb onto a plastic surface, dissolve into the polymer matrix, diffuse through the free volume, and desorb on the downstream side under a controlled partial pressure gradient. This behavior follows the classical solution–diffusion model, in which the permeability coefficient (P) is the product of the solubility coefficient (S) and the diffusion coefficient (D). Because methane is a small, non-polar molecule that moves comparatively fast through polyolefins, accurate permeability data are critical wherever plastics must contain, exclude, or monitor methane.

Testing is generally performed by accredited third-party laboratories using dedicated gas permeability analyzers operating on either the differential-pressure (manometric) principle or the equal-pressure (isostatic) principle with methane-specific detection. Results are reported as the gas transmission rate (GTR), permeance, and permeability coefficient under standardized conditions, most commonly 23 °C with a 0.1 MPa partial pressure difference and dry test gas.

• Mechanism basis: solution–diffusion transport (P = D × S)
• Primary standards: ISO 15105-1, ISO 15105-2, ISO 2556, ASTM D1434, GB/T 1038
• Standard conditions: 23 °C ± 2 °C, 0 °C to 100 % relative humidity range depending on material
• Reported outputs: transmission rate, permeance, permeability, diffusion and solubility coefficients
• Safety consideration: methane is flammable, so testing requires explosion-proof gas handling and certified detection systems

Testing Samples

Monolayer and Multilayer Films

Typical specimens include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), ethylene-vinyl alcohol copolymer (EVOH), polyvinylidene chloride (PVDC) films, and multilayer laminates used in flexible packaging. Film samples are usually cut as flat discs with a minimum diameter of about 100 mm to suit the test cell, with thickness measured at multiple points across the effective test area.

Sheets and Geomembranes

High-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), flexible polypropylene (fPP), and PVC geomembranes used in landfill covers, lagoons, and biogas facilities are tested as flat coupons cut from production rolls. Because geomembranes are relatively thick and of moderate permeability, longer stabilization times may be required before steady-state transmission is reached.

Pipes and Tubing

Coupons taken from polyethylene gas-distribution pipe walls can be evaluated for their intrinsic methane barrier quality, supporting material qualification and long-term performance modeling for natural gas networks.

Finished Packages and Containers

Complete flexible bags, liners, and rigid containers may be tested in adapted closed-chamber configurations to determine whole-package methane transmission, which integrates material permeability with the effects of seals, seams, closures, and fittings.

Testing Items

Depending on the selected method and client requirements, the following parameters are determined and reported:

• Gas transmission rate (GTR / Q): the volume of methane passing through a unit area per unit time, expressed in cm³/(m²·24h)
• Permeance: transmission rate normalized to the applied partial pressure difference, expressed in cm³/(m²·24h·0.1 MPa)
• Permeability coefficient (P): permeance multiplied by specimen thickness, expressed in cm³·mm/(m²·24h·0.1 MPa) or in SI units of cm³·cm/(cm²·s·Pa)
• Diffusion coefficient (D): derived from the time-lag method, in cm²/s
• Solubility coefficient (S): calculated as P/D, in cm³/(cm³·Pa)
• Temperature-dependent permeability profiles for activation energy analysis, when required

Testing Methods

Differential-Pressure (Manometric) Method

Per ISO 15105-1, ISO 2556, ASTM D1434, and GB/T 1038, the specimen is mounted as a sealed barrier between two chambers. The lower chamber is evacuated while the upper chamber is charged with dry methane at a defined pressure. Methane permeating through the specimen increases the pressure (or volume) in the lower chamber, and this increase is monitored continuously until a steady state is established. The method requires no detector gas, provides direct absolute measurement, and allows calculation of diffusion and solubility coefficients from the time-lag curve. It is best suited to flat films and sheets.

Equal-Pressure (Isostatic) Method

Per ISO 15105-2, both sides of the specimen are maintained at atmospheric pressure: methane flows on one side while an inert carrier gas, typically nitrogen, sweeps the other side. The permeated methane is quantified by a downstream detector such as a flame ionization detector (FID), thermal conductivity detector (TCD), or gas chromatograph. Because there is no total pressure difference, this configuration closely simulates real service conditions and can be extended to whole-package and high-barrier applications where very low transmission rates must be resolved.

Test Conditions and Calibration

Specimens are conditioned before testing, normally at 23 °C ± 2 °C and 50 % ± 5 % relative humidity, or as dry specimens when moisture sensitivity is not a factor. Hygroscopic materials such as polyamide and EVOH can be tested under controlled humidity to reflect end-use performance. Instruments are calibrated using certified reference films with known methane transmission values, and testing continues until the transmission curve plateau satisfies the steady-state criterion of the standard. Methane supply, evacuation, and exhaust lines must comply with flammable gas safety requirements.

Applications

Methane permeability data support design, qualification, and regulatory compliance across multiple sectors:

• Environmental engineering: barrier performance of HDPE and LLDPE geomembranes for landfill covers, methane collection layers, and contaminated-site capping systems
• Biogas and renewable energy: material selection for biogas storage bladders, flexible holders, and digester covers where methane loss must be minimized
• Gas distribution: evaluation of polyethylene pipe and fitting materials for natural gas service
• Flexible packaging: verification of methane and hydrocarbon barrier layers in laminates for food, industrial, and hazardous goods packaging
• Fuel and energy systems: barrier assessment of plastic tanks, liners, hoses, and membranes in contact with methane-containing gas streams
• Quality control and R&D: incoming material inspection, batch-to-batch consistency verification, formulation and orientation optimization, and permeation modeling for shelf-life and service-life prediction

FAQ

What sample size and thickness are required?

Flat specimens are typically circular discs of at least 100 mm diameter to cover the test cell, or squares trimmed to fit, with a recommended thickness of less than 3 mm for film cells. Thickness must be uniform and is measured at several points because the permeability coefficient calculation depends directly on it.

How long does a methane permeability test take?

After conditioning, typical test durations range from several hours for permeable polyolefin films to one to three days for high-barrier multilayer structures, since the measurement must continue until a steady-state transmission plateau is reached and the time-lag interval is captured.

Which method should be chosen: differential-pressure or equal-pressure?

The differential-pressure method offers absolute measurement without a detector and suits most flat films and sheets. The equal-pressure method avoids mechanical stress from a pressure difference, simulates real barrier service, offers high sensitivity for very low transmission rates, and is preferred for finished packages. Accredited third-party laboratories can advise based on material type, expected barrier level, and the governing specification.

Can testing be performed at controlled humidity?

Yes. For moisture-sensitive materials such as polyamide and EVOH, the equal-pressure method can apply a defined relative humidity to the methane stream so that permeability is measured under representative end-use moisture conditions.

How are the results reported?

A typical report states the test standard, test method, temperature and humidity, specimen thickness, methane purity, test area, steady-state gas transmission rate, permeance, permeability coefficient, and where applicable the diffusion and solubility coefficients, together with instrument calibration information and identification of the testing laboratory's accreditation scope.

Are there safety restrictions when testing with methane?

Methane is flammable, so testing must be conducted in properly ventilated areas with explosion-proof equipment, leak-checked gas lines, and certified detectors. Reputable accredited third-party laboratories operate gas permeability analyzers with built-in safety interlocks and trained personnel for hydrocarbon gas handling.