Barrier Properties: Oxygen, Water Vapor & CO2 Transmission Kinetics

Plastic packaging must protect food and pharmaceuticals from gas and moisture ingress. Oxygen causes food spoilage and oil rancidity, while moisture ingress ruins dry food texture. Selecting barrier polymers (EVOH, PVDC, PET) and sizing package thicknesses requires calculating gas transmission rates. Packaging engineers use these mathematical models to predict product shelf-life and design barrier structures.

• Explain the three-step mechanism of gas permeation through polymers (adsorption, diffusion, desorption).
• Formulate gas permeability, diffusion, and solubility coefficients.
• Solve Oxygen Transmission Rate (OTR) and Water Vapor Transmission Rate (WVTR) calculations for single and multilayer films.
• Compare barrier properties of standard packaging resins.
• Reference international permeation testing standards (ASTM D3985, ASTM F1249).

3.1 Permeation Mechanism

Permeation of small gas molecules through a polymer membrane follows three steps:

1. Adsorption: The gas dissolves into the polymer surface on the high-concentration side.
2. Diffusion: The gas diffuses through the amorphous polymer matrix driven by the concentration gradient.
3. Desorption: The gas evaporates from the low-concentration surface of the film.

3.2 Permeability Coefficient ($P$)

The Permeability ($P$) is the product of the Diffusion ($D$) and Solubility ($S$) coefficients:

For a film of thickness $L$ and surface area $A$, the gas transmission rate ($GTR$) is:

Where $\Delta p$ is the partial pressure difference across the film.

3.3 Multilayer Film Permeation

For a multilayer film containing $n$ layers of individual thickness $L_i$ and permeability $P_i$, the total permeability $P_{total}$ is calculated like electrical resistors in series:

Problem: A packaging film has a multilayer structure consisting of: (a) PP outer layer ($L_1 = 30.0$ $\mu$m, $P_1 = 1500$ cc·$\mu$m/(m²·day·atm)), and (b) EVOH core layer ($L_2 = 5.0$ $\mu$m, $P_2 = 3.0$ cc·$\mu$m/(m²·day·atm)). Calculate:

1. The effective oxygen permeability of the multilayer film ($P_{total}$, normalized to total thickness $L_{total} = 35.0$ $\mu$m).
2. The Oxygen Transmission Rate ($OTR$) of a pouch made from this film under a partial pressure difference $\Delta p = 0.21$ atm (air oxygen concentration).

Solution:

1. Calculate total resistance ($L_{total}/P_{total}$):

Solve for $P_{total}$:

1. Calculate OTR of the pouch (using $L_{total} = 35.0$ $\mu$m):

Interpretation: The inclusion of the 5 $\mu$m EVOH barrier layer yields a low OTR of 0.125 cc/m²·day, suitable for packaging oxygen-sensitive snack foods. A pure 35 $\mu$m PP film would show an OTR of $1500 \times 0.21 / 35.0 = 9.0$ cc/m²·day, showing the value of co-extrusion.

Indian packaging plants operate gas permeability test instruments. They measure OTR conforming to ASTM D3985 and WVTR conforming to ASTM F1249 at 37.8°C and 90% RH (simulating humid Indian summer conditions) to certify film batches.

• OTR: Oxygen Transmission Rate; measured in cc/m²·day.
• WVTR: Water Vapor Transmission Rate; measured in g/m²·day.
• EVOH: Ethylene Vinyl Alcohol; polar copolymer providing high gas barrier properties.
• Permeation: The physical movement of gas molecules through a solid polymer membrane.
• Series Resistance Model: Formula used to calculate the barrier properties of multilayer laminates.

1. ASTM D3985 — Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film
2. ASTM F1249 — Standard Test Method for Water Vapor Transmission Rate Through Plastic Film using Modulated Infrared Sensor

1. Explain why the gas permeability of EVOH rises dramatically when exposed to high relative humidity. Reference plasticisation and hydrogen bonding.
2. Discuss how the orientation of inorganic clay platelets in polymer nanocomposite coatings improves barrier properties. Show the tortuous path effect.
3. Calculate the WVTR of a 50-micron LLDPE film (water permeability = 10 g·$\mu$m/(m²·day)) at 37.8°C.