Gas Permeability: What It Is, How It Is Measured and Why It Matters

Gas permeability is an important property of materials used in packaging and other applications where controlling gas transmission is essential. It describes how a gas can pass through a material and is particularly relevant when evaluating films, coatings and barrier structures designed to limit the transfer of oxygen, carbon dioxide or other gases.

Understanding gas permeability helps manufacturers, material developers and laboratories compare materials, assess barrier performance and select suitable solutions for specific applications.

But what exactly is gas permeability, how is it measured, and which parameters should be considered when evaluating a material?

What Is Gas Permeability?

Gas permeability describes the ability of a material to allow a gas to pass through it under a difference in gas pressure or chemical potential.

At a material level, gas transport generally involves several steps: the gas interacts with one side of the material, enters the material, moves through it and exits on the opposite side.

The extent to which a gas passes through a material depends on several factors, including:

• The chemical and physical properties of the material.
• The type of gas being measured.
• The material’s thickness and structure.
• The temperature and, where relevant, relative humidity.
• The difference in gas partial pressure across the material.
• The measurement method and test conditions.

Different materials can behave very differently when exposed to the same gas. A material that provides a good oxygen barrier, for example, may not provide the same level of resistance to water vapor or carbon dioxide.

For this reason, barrier properties should be evaluated using measurements relevant to the intended application.

Gas Permeability vs. Gas Transmission Rate

Gas permeability and gas transmission rate are related concepts, but they are not interchangeable.

Gas permeability is a material property that describes how readily a gas moves through a material under defined conditions. Depending on the measurement approach, permeability calculations take material thickness and the driving force for gas transport into account.

Gas transmission rate describes the amount of a particular gas passing through a specified area of material over a given period under defined test conditions.

For packaging films, gas transmission is commonly characterized using parameters such as the oxygen transmission rate (OTR) and carbon dioxide transmission rate (CO₂TR).

When comparing test results, it is important to distinguish between permeability coefficients and transmission rates. The reported value, units, material thickness and test conditions all affect how results should be interpreted.

Why Is Gas Permeability Important?

Gas transmission can influence the performance of packaging materials and other barrier applications. Measuring it helps determine whether a material’s properties are suitable for a particular purpose.

Product Protection

Some products are sensitive to oxygen or other gases. Characterizing the gas barrier properties of packaging materials can help manufacturers evaluate material options for protecting product quality.

Packaging Development

During material development, permeability measurements can be used to compare alternative polymers, coatings, laminates and multilayer structures.

Quality Control

Defined test procedures and consistent environmental conditions help laboratories monitor material performance and investigate changes between samples or production batches.

Material Selection

Gas permeability data can support the selection of materials for specific applications. The most suitable material depends on the product requirements, the packaging design and the relevant barrier parameters.

Research and Product Improvement

Measuring gas transmission can help researchers investigate how material composition, thickness, processing and structure influence barrier performance.

A permeability result is one element of a broader material evaluation. It should be interpreted alongside other relevant properties and application requirements.

Which Gases Are Commonly Measured?

The gas of interest depends on the material and the intended application.

Oxygen Permeability and OTR

Oxygen transmission rate (OTR) is an important parameter for materials used in packaging applications where oxygen transfer needs to be controlled or characterized.

OTR testing measures the rate at which oxygen passes through a material under specified test conditions. Results can help compare the oxygen barrier performance of different films and packaging structures.

Learn more in Oxygen Permeability and OTR: Measurement and Analysis.

Carbon Dioxide Permeability and CO₂TR

Carbon dioxide transmission rate (CO₂TR) characterizes the transmission of carbon dioxide through a material under defined conditions.

CO₂TR measurements can be useful when carbon dioxide exchange is relevant to the intended application. As with oxygen, results depend on the material, test conditions and measurement method.

Learn more in CO₂ Permeability and CO₂TR: Measurement and Analysis.

Water Vapor Transmission and WVTR

Water vapor is not usually classified as a permanent gas in the same sense as oxygen or carbon dioxide, but its transmission is another important aspect of packaging barrier performance.

The water vapor transmission rate (WVTR) measures the rate at which water vapor passes through a material under specified conditions. It is particularly relevant when moisture transfer needs to be evaluated.

Learn more in Water Vapor Permeability and WVTR: Measurement and Analysis.

How Is Gas Permeability Measured?

to detect or measure the gas that passes through the sample.

The measurement system records the transmission behavior over time. The resulting data can then be used to determine a transmission rate or, where the required inputs and assumptions are available, calculate a permeability coefficient.

The specific apparatus, detection method and calculation procedure depend on the instrument and the applicable test method.

Important Test Conditions

To obtain useful and comparable results, several factors need to be considered.

Temperature: Gas transport through a material can change with temperature, so the test temperature should be defined and controlled.

Relative humidity: Humidity can affect the barrier properties of some materials, particularly those sensitive to moisture. Where relevant, humidity conditions should be controlled and documented.

Sample thickness and preparation: Thickness, surface condition and sample preparation can influence the measurement and the interpretation of results.

Gas and pressure conditions: The gas being measured and the pressure difference across the sample are important factors in defining the test.

Measurement range and method: The instrument must be suitable for the expected transmission values and the material being tested.

Repeatability: Consistent preparation and test conditions help laboratories compare samples and monitor changes over time.

When reviewing a result, always consider the reported parameter, units, test conditions and measurement method rather than comparing numerical values in isolation.

What Factors Affect Gas Permeability?

The gas barrier properties of a material are influenced by its composition and structure, as well as by the conditions under which it is evaluated.

Material Type

Different polymers and barrier materials have different molecular structures and interactions with gases. These differences influence how readily a particular gas can enter, move through and exit a material.

Thickness

For otherwise comparable materials tested under the same conditions, thickness affects the path that gas must travel through the material. However, thickness alone does not determine barrier performance.

Multilayer Structures

Packaging materials may combine multiple layers to achieve a balance of barrier performance and other functional properties.

The overall behavior of a multilayer structure depends on its constituent materials, their thicknesses, interfaces and manufacturing characteristics. Testing the complete structure provides information about the performance of that structure as a whole.

See [Permeability Testing of Multilayer Films and Laminates] for more information.

Temperature and Humidity

Environmental conditions can affect material behavior and therefore the measured transmission rate. Meaningful comparisons require clearly defined and consistent conditions.

Gas Type

A material’s interaction with oxygen may differ from its interaction with carbon dioxide or water vapor. A result for one gas should not be used as a substitute for a measurement of another.

Gas Permeability in Packaging Applications

Packaging is one of the main areas where gas barrier performance is evaluated.

Food Packaging

Gas transmission measurements can help characterize materials used to package products that are sensitive to oxygen, moisture or changes in the surrounding atmosphere.

OTR, WVTR and CO₂TR measurements provide different types of information, and the relevant parameters depend on the product and packaging requirements.

Explore Permeability Testing for Food Packaging.

Plastic Films

Plastic films are used in a wide range of packaging structures. Permeability testing helps compare their barrier properties under defined conditions.

The results can support material selection and development, particularly when different film formulations or thicknesses are being evaluated.

Explore Permeability Testing of Plastic Films.

Flexible Packaging

Flexible packaging may contain single-layer films, laminates, coatings or multilayer structures. Testing can help characterize the barrier performance of the selected material structure.

Explore Permeability Testing of Flexible Packaging.

The appropriate measurement depends on the application, the sample and the test configuration. Results from a material specimen do not automatically describe the performance of an entire finished package.

How to Choose a Gas Permeability Tester

Choosing an instrument begins with defining the measurement objective and the properties of the samples to be tested.

Before selecting a system, consider the following questions:

• Which gas or transmission parameter needs to be measured?
• What range of transmission values is expected?
• Which materials, thicknesses and sample structures will be tested?
• What temperature and humidity conditions are required?
• Which test method will be used?
• How many samples need to be evaluated?
• What level of data recording and reporting is required?

If more than one transmission parameter is needed, a system that supports multiple measurements may help streamline laboratory workflows.

Instrument compatibility should always be checked against the actual sample type, test method and measurement requirements.

ExtraSolution Instruments for Gas and Water Vapor Transmission Testing

ExtraSolution offers solutions for characterizing oxygen, carbon dioxide and water vapor transmission through films and barrier materials. The appropriate system depends on the parameters required and the intended application.

MultiPerm O2-CO2

Designed for oxygen transmission rate (OTR) and carbon dioxide transmission rate (CO₂TR) measurements.

Explore MultiPerm O2-CO2.

MultiPerm O2-H2O

Designed for oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) measurements.

Explore MultiPerm O2-H2O.

MultiPerm H2O-CO2

Designed for water vapor transmission rate (WVTR) and carbon dioxide transmission rate (CO₂TR) measurements.

Explore MultiPerm H2O-CO2.

TotalPerm

Combines OTR, CO₂TR and WVTR measurement capabilities in one integrated system.

Explore TotalPerm.

PermeH2O

Provides a dedicated solution for water vapor transmission rate (WVTR) measurement.

Explore PermeH2O.

The suitability of a specific instrument depends on the sample, required measurement range, test method and configuration. Contact ExtraSolution to identify the most appropriate solution for your application.

Conclusion

Gas permeability is an important consideration when characterizing materials designed to control gas transmission. Understanding the difference between permeability and transmission rate, selecting the relevant gas parameter and defining appropriate test conditions are essential for interpreting results correctly.

OTR, CO₂TR and WVTR measurements provide complementary information about the barrier properties of packaging materials. Together with a clear understanding of the intended application, these measurements can support material comparison, product development and quality control.

ExtraSolution provides solutions for measuring oxygen, carbon dioxide and water vapor transmission through films and barrier materials.

Contact ExtraSolution to discuss your measurement requirements and identify a suitable testing solution.