The simulation models provide theoretical transmission rate values for oxygen or water vapour based on following equations:

Transport equations, overall permeability in a multilayer structure, humidity/ permeability curve fitting equations (the OTR model), temperature/permeability relations (Arrhenius equations) and temperature/permeability relations between vapour pressure and temperature (the WVTR model).

Assumptions

• The top surface of the containers is assumed to be 100 % tight and is not a part of the calculations (area and transmission rate).This can be compared with an aluminium foil with perfect sealing between foil and container.
• Wall thickness of the container and IML barrier film is assumed constant throughout article surface.
• Steady state condition of permeation. The amount of permeant passing through each layer is identical, and equal to the amount passing through the total structure.
• Diffusion takes place in one direction only.
• The concentration-distance relationship through the polymer is linear (follows Henrys Law).
• The permeability coefficient is assumed to be a constant of the material and not dependent on sample thickness.
• The OTR model assumes that the level inside container is initially assumed to be 0 Vol %.
• The calculations are based on an empty container.

Limitations

• Real transmission rate values depend on a large number of factors. The estimated values are expected to show deviation from real values.
• Calculated results are expected to show highest accuracy (lowest deviation from real values) at temperatures 23 ± 5 ºC and humidity < 65 % rel H (OTR model).
• Permeability humidity corrections used in the OTR model are performed for Polyamide (PA6oriented, PA6 cast and OPA), PET and EVOH. The correction is a result of curve fitting permeability values at 20 ºC with humidity 0 – 100 % rel H. Other temperatures may show other humidity / oxygen permeability relations.
• The calculated results for the WVTR model are expected to show highest accuracy (lowest deviation from real values) at the temperature range 20 - 30 ºC.
• Permeability is a function of temperature, following an Arrhenius type of equation. Extrapolating the permeability from 20 up to 50 ºC may cause large deviations from actual test results.
• The vapour pressure of water is a function of temperature and modelled using the Antoine equation (WVTR model).
• The contribution from tie layers is not considered in this model. If the permeability and thickness of a tie layer is known, this can be included by adding a layer with custom values.
• Influence of processing conditions (e.g orientation and crystallinity) and conversion technique is not considered in detail.

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