ASTM F1307 Oxygen Transmission Rate through Dry Packs Using a Coulometric Sensor

Developed by the American Society for Testing and Materials (ASTM),ASTM F1307, "Standard Test Method for Oxygen Transmission Rate through Dry Packages Using a Coulometric Sensor," describes a test method for determining the steady-state transmission rate of oxygen gas into packages. More specifically, this method applies to packages that will surround a dry environment under normal use.

For the purposes of this standard, the following terms and definitions apply:

• The oxygen gas transmission rate is the amount of oxygen gas that passes through the surface of a package per unit time when applied to it. Test conditions, including temperature, oxygen partial pressure, and humidity on both sides of the package, should be specified. Conventional film To distinguish it from oxygen gas transmission rate results, the term package is used in the unit of transmission rate. Unlike film tests, where the test area is always known, a package oxygen transmission rate measurement often involves complex systems, including seals and closures, through which oxygen transmission occurs. A package oxygen gas transmission rate test is generally a packaging system test rather than a simple component test.
• The oxygen permeability coefficient is the product of the barrier's permeability and its thickness. Permeability is only meaningful for homogeneous materials, in which case it is a property characteristic of the bulk material. This quantity should not be used unless the relationship between thickness and permeability has been verified in tests using various material thicknesses.
• Oxygen permeability expresses the ratio of the oxygen gas transmission rate to the difference between the partial pressure of oxygen on both sides of the package wall.

This test method uses a coulometric oxygen sensor and associated equipment in an arrangement similar to that described in ASTM D3985, "Standard Test Method for Oxygen Gas Transmission Rate through Plastic Film and Sheets Using a Coulometric Sensor." The oxygen gas transmission rate is determined after the package is mounted in a test fixture and equilibrated in the test environment.

The package is mounted so that the outside of the package is exposed to a known oxygen concentration while the inside of the package is slowly purged with a stream of nitrogen. The package can be exposed to ambient room air containing 20,8 percent oxygen or immersed in a 100 percent oxygen atmosphere. As the oxygen diffuses through the package walls into the nitrogen carrier gas, it is transported to the coulometric detector, where it generates an electric current proportional to the amount of oxygen flowing to the detector per unit time.

Oxygen gas transmission rate is an important determinant of the protection provided by barrier materials. However, it is not the sole determinant, and additional empirical testing should be used to correlate package performance with oxygen gas transmission rate. This test method is suitable as a peer-reviewed test method if the user and source agree on sampling procedures, standardization procedures, test conditions, and acceptance criteria.

The presence of certain interfering substances in the carrier gas stream can lead to undesirable electrical outputs and error factors. Interfering substances include free chlorine and some strong oxidizing agents. Exposure to carbon dioxide should also be minimized to prevent damage to the sensor by reacting with the potassium hydroxide electrolyte.

The oxygen gas transmission apparatus used in the tests consists of the following parts:

• Package test stations provide a means of inlet and outlet of nitrogen carrier gas streams without significant loss or leakage. Experience has shown that arrangements using multiple package test stations are a practical way to increase the number of measurements available from a coulometric sensor. A valve manifold connects the carrier gas side of each test station to the sensor in a predetermined pattern. The carrier gas continuously purges the carrier gas sides of the packages not connected to the sensor. The appropriate test gas (100 percent oxygen) or normal room air (20,8 percent oxygen) contacts the outside of the package.
• A diffusion cell is a piece of metal, consisting of two halves that, when closed over the film used for system calibration, will accurately define the circular area of ​​that film. Typical diffusion cell areas are 100 cm² and 30 cm². The volumes within the cell above and below the closed film are not critical; they should be small enough to allow rapid gas exchange, but not so small that an overhanging or bulging unsupported film would contact the top or bottom of the cell. Instruments are provided for measuring cell temperature.
• A suitably sized groove machined into the oxygen (or test gas) side of the diffusion cell holds a neoprene O-ring. The test area is defined as the area created by the internal contact diameter of the compressed O-ring when the diffusion cell is compressed against the test sample. This area can be determined by measuring the internal diameter of the impression left by the O-ring on the sample after removal from the diffusion cell.
• The nitrogen (or carrier gas) side of the diffusion cell should have a flat, raised edge. This edge is the sealing surface against which the test sample is pressed, so it should be smooth and flat, free of scratches that could promote leakage. Both halves of the diffusion cell should have suitable connections for the inlet and outlet of the gas without significant loss or leakage. Thermostatic control of the diffusion cell is desirable. A simple resistance heater connected to the carrier gas side of the cell to ensure good thermal contact is sufficient for this purpose. A thermistor sensor and a suitable control circuit will regulate the cell temperature unless measurements are taken near ambient temperature. In this case, it is desirable to provide cooling coils to remove some of the heat.
• The catalyst bed is a small metal tube with fittings for connecting to the inlet of the nitrogen gas pneumatic fitting containing 3 percent to 5 percent platinum or palladium catalyst on alumina and provides an essentially oxygen-free carrier gas to the diffusion cell and each package test station.
• A flow meter with an operating range of 5 to 100 mL/min is needed to monitor the flow rate of nitrogen carrier gas passing through each test station.
• Flow diverter valves are two or more valves for switching the nitrogen and test gas flow streams.
• To monitor the amount of oxygen delivered, an oxygen-sensitive coulometric sensor is used, which operates at an essentially constant efficiency.

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