OTR-Wert bei Verpackungsfolien: Sauerstoffdurchlässigkeit verstehen – Aluminiumfolie, Blister, Laminate

The OTR value, displayed Oxygen transmission rate, Describes the amount of oxygen that occurs in a stationary state through a specific packaging film per area and time. The common unit is cm³/(m²·d): cubic centimeters of oxygen per square meter of film per day. Under the same measurement conditions, a lower OTR value indicates less oxygen permeability and thus a higher oxygen barrier. ^{[1] [2]}

However, the number alone is not enough for technical pre-selection. An OTR value does not assess a material in a general way nor does it prove the durability of a packaged product. Only units, measurement conditions, and the structure of the film show whether two test values are truly comparable and whether they describe the subsequent packaging design.

What the OTR value measures and how to read its units

What is crucial is the stationary measurement condition: Only when the oxygen permeability has stabilised under the specified test conditions does the value describe a reproducible rate of penetration of the sample. The gas volume indicated in the unit is also not to be interpreted independently; it is based on defined reference conditions. A datasheet value is therefore always the result of specific film, defined test conditions and a defined volume reference – not a freely transferable material statement. ^{[1] [2]}

Data sheets also include notations such as cc/(m²·day) or other area measurements. Therefore, it must be clear before any comparison which area and period the notation refers to. A possible unit of measurement is particularly important: an addition such as „/atm“ does not simply indicate a variant spelling, but a different reference unit.

OTR

Amount of oxygen per area and time through the specific tested film, for example cm³/(m²·d).

Permeability

The permeability is also related to the oxygen partial pressure difference Δp. A pressure unit in the unit indicates this relationship.

Permeability coefficient P

A material-related parameter that additionally includes the film thickness l. In the usual technical relationship, P = OTR·l/Δp; the SI unit is mol/(m·s·Pa).

The boundary condition prevents a common error in thinking: An OTR without a pressure sensor describes the examined film under documented test conditions. The permeability additionally takes into account the driving partial pressure difference of oxygen. The permeability coefficient P, on the other hand, is intended to represent a material property. Its thickness normalization is only meaningful for homogeneous materials if the thickness dependence is experimentally confirmed. Therefore, the permeability coefficient of a multilayer composite is not to be equated with the material permeability of a single layer. ^{[3] [4]}

If a pressure gauge is marked with a symbol, it must not be placed directly next to a pure OTR and read as a better or worse barrier. Even the designation „OTR“ is not sufficient to clarify the matter, because technical documents can also name pressure-standardized sizes in this way. What is decisive is the unit listed along with the reference size. ^[5]

Which measurement conditions are relevant for comparison

Two OTR values can only be directly compared if they describe the same measurement variable under appropriate conditions. Same numerical formats or same unit of area are not sufficient. Temperature, relative humidity on both sides of the film, oxygen partial pressures, and the testing method determine which statement the test value allows.

The moisture measurement must cover both sides of the sample. One film can be exposed to a different atmosphere on one side than on the other. Therefore, when combined with moisture-sensitive barrier layers, the shortened „measured at humidity“ statement is not sufficient if it remains unclear how the moisture was distributed.

Even the mere mention of „oxygen as a test gas“ does not replace the reference to pressure. For the comparison decision, the test report must indicate which partial oxygen pressures are present on both sides of the film or which partial pressure difference was used. Only this indication explains whether a pressure sensor represents the same reference value and whether the driving force of the two measurements is consistent. ^[5]

The consequence is clear: If two candidates encounter different temperature, humidity, or pressure conditions, the data sheets first compare measurement situations rather than film performance under the same conditions. A blanket conversion would not be valid because the change depends on the material and film structure. The next step that is acceptable is a common, in-use comparison measurement with documented conditions.

Why film construction and processing change the test value

In multi-layer systems, the total thickness does not sufficiently explain the oxygen barrier. What is crucial is which layer forms the barrier, how thick that individual layer is, and how the other layers shield it from moisture. The OTR value therefore describes the entire tested construction, not a uniformly acting material mass.

This can be seen in a combination with EVOH as a barrier layer. For classification, not only the thickness and location of the EVOH layer are important. Humidity inside and outside as well as the thickness and water vapor permeability of the adjacent layers can also influence the assessment. The surrounding layers determine how strongly moisture reaches the barrier layer. ^[5]

Therefore, a thicker composite is not automatically the better oxygen barrier. Two films with the same overall thickness can differ significantly in layer sequence, barrier layer thickness, and moisture protection. Conversely, the OTR of a specific composite cannot be transferred solely by converting the overall thickness to another structure.

During thermoforming, the layer distribution in the mold zone can change compared to the flat film. This does not automatically imply a specific direction of the barrier change. However, there is a clear boundary to interpretation: if a shaped packaging is to be evaluated later, the processed structure is the appropriate subject for analysis.

Two data sheets for reliable pre-selection

Hypothetical example: Two multilayer composites are currently being pre-selected. Composite A exhibits an OTR under dry conditions, while Composite B exhibits an OTR under controlled humidity. A report also leaves open the question of whether the unit contains a pressure sensor. Although the numbers are different, this does not result in a reliable ranking or a statement regarding product durability.

1. Measuring units are separated. First, it must be clarified whether both statements describe a pure OTR or a pressure-normalized permeability. Different reference sizes must not be combined in a ranking order.
2. Compare test conditions. Temperature, relative humidity on the inside and outside, partial oxygen pressures, testing method and stationary measurement conditions are collected for both samples. Deviations explain why the numbers do not form a direct basis for comparison.
3. Create a common measurement base. Both connections are tested at the same temperature, humidity distribution and oxygen partial pressure that are suitable for the intended use. A computational adjustment of dry and wet values does not replace this measurement. ^[1]
4. Document the build. In the case of an EVOH composite, complete layer sequence and individual layer thicknesses are factors to be taken into account. The total thickness alone would not be a viable basis for decision-making.
5. Take into account the processed state. Following the flat film, thermoforming is performed; the shaped structure is then examined under the relevant conditions. Only this test value applies to the packaging configuration that must be decided upon. ^[7]

Conclusion: The credibility decides

The OTR value is a useful indicator of oxygen permeability, provided it is read correctly. A lower value indicates a higher oxygen barrier only when the units, measurement conditions, and comparable film structures are identical. Different test climates, unclear pressure reference, or a different manufacturing process make a seemingly simple comparison of numbers an open technical question.

The next decision therefore depends on the specific data gap: whether to supplement the data provider with missing test conditions, to conduct a common in-service comparison measurement, or to consider the processed packaging after transformation. From the OTR alone, neither a universal limit value nor the shelf life of a product can be derived.