Why one "oxygen transmission" carries so many names

You buy an oxygen transmission tester and the report shows OTR, oxygen transmission rate, permeance, permeability coefficient and gas transmission rate. These are not marketing words — they are the names different standards give to the same physical process of gas crossing a film under different conditions. Without knowing them you cannot compare data from different labs or instruments.

1. Oxygen Transmission Rate (OTR / O₂GTR)

The most common term. It is the volume of oxygen passing through a unit area of a plastic film per unit time under the conditions of test. The usual unit is cm³/(m²·24h); the SI unit is mol/(m²·s). Both the coulometric method (ASTM D3985, ISO 15105-2) and the differential-pressure method can measure it, but the coulometric method is oxygen-specific and far more sensitive, ideal for high-barrier materials.

2. Oxygen Permeance (P O₂)

The Chinese term "transmission coefficient" sounds like an intrinsic property, yet in ASTM D3985 and ISO 15105-2 it actually corresponds to permeance: the ratio of the O₂GTR to the oxygen partial-pressure difference across the film, with unit cm³/(m²·24h·0.1MPa). Because it removes the pressure-difference factor, permeance compares better across test conditions than bare OTR.

3. Oxygen Permeability Coefficient

Permeability coefficient equals permeance multiplied by film thickness, unit cm³·cm/(m²·24h·0.1MPa) or mol·m/(m²·s·Pa). It is the intrinsic barrier property of the material, but the standards stress it is meaningful only for homogeneous single-layer films; a "permeability coefficient" reported for a multi-layer laminate is unreliable and should not be cited casually.

4. Gas Transmission Rate (GTR) and the differential-pressure method

ASTM D1434 extends the scope from oxygen to any gas and defines the general Gas Transmission Rate (GTR): the quantity of a given gas passing through a unit of the parallel surfaces of a plastic film in unit time. The differential-pressure method (ISO 2556) commonly measures GTR and can handle oxygen, CO₂, nitrogen, hydrogen, helium and more.

5. Naming differences: Permeance vs Permeability

The standards do not use the same words. ASTM D1434 calls the pressure-divided quantity permeance and the thickness-multiplied quantity permeability; ISO 2556 calls the former simply gas transmission rate; other national standards use their own local terms for these two quantities. When comparing data, always read the unit: cm³/(m²·24h·0.1MPa) is permeance, while cm³·cm/(m²·24h·0.1MPa) is the permeability coefficient.

6. Diffusion coefficient and solubility coefficient (P = D × S)

Transport is explained by the solution-diffusion model: gas first dissolves in the film, then diffuses through it. The permeability coefficient P equals the diffusion coefficient D times the solubility coefficient S (P = D × S). Two high-barrier materials may achieve the same result by being hard to dissolve versus hard to diffuse. Knowing this tells you whether to change the formulation or the thickness.

Definitions in the standards

  • ASTM D3985: Oxygen Transmission Rate (O₂GTR) — the quantity of oxygen gas passing through a unit area of the parallel surfaces of a plastic film per unit time under the conditions of test.
  • ASTM D1434 §3.1.1: gas transmission rate (GTR) — the quantity of a given gas passing through a given unit of the parallel surfaces of a plastic film in unit time under the conditions of test.
  • ASTM D1434 §3.1.2: permeance — the ratio of the gas transmission rate to the difference in partial pressure of the gas on the two sides of the film.
  • ASTM D1434 §3.1.3: permeability — the product of the permeance and the thickness of a film; meaningful only for homogeneous materials.
  • ISO 15105-2: O₂GTR is generally expressed in cm³/(m²·24h); oxygen permeance is expressed in cm³/(m²·24h·0.1MPa).
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Selection tip: for high-barrier films such as food or pharma packaging, prefer a coulometric oxygen transmission rate tester (e.g. the LabStone OTR-E series) to measure OTR; if you must test several gases or thick sheets, turn to a differential-pressure gas transmission rate tester. When a report shows "transmission coefficient", first confirm whether it means permeance or permeability before drawing conclusions.

References

  • ASTM D3985-17, Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor
  • ASTM D1434-23, Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting
  • ISO 15105-2:2003, Plastics — Film and sheeting — Determination of gas-transmission rate — Part 2: Equal-pressure method
  • ISO 2556:2001, Plastics — Determination of the gas transmission rate of films and thin sheets under atmospheric pressure