Permeation FAQ

Permeation is the movement of gas and vapor through a barrier such as the wall of a bottle. It is driven by the permeant concentration gradient that always happens from the permeant’s high concentration side to the low concentration side.

         Fig 1.       Permeation a Natural Process         

For example, imagine there is a bottle of carbonated soft drink (Fig 1). The CO2 within the product is about 4 atm (when it is freshly filled) while the CO2 concentration in environment is less than 0.5% in the air. Therefore, the CO2 will permeate from inside bottle towards the outside. Similarly, the oxygen from room air will permeates from the outside of the bottle to inside. Although permeation process is invisible, it can be detected by the CO2 concentration loss over time. Or, simply you can taste the soft drink. The flat taste of the liquid indicates the loss of CO2 which lead to the loss of its premier quality or shelf life.

     
       Fig 2.      Solution-Diffusion Mechanism

The permeation mechanism has three steps (Fig 2):

• Permeant molecules absorb into the surface (high concentration side)
• Permeant molecules move or diffuse through the barrier material
• Permeant molecules desorb out of the other side (low concentration side)

Therefore, permeation is related to both solubility (S) and diffusivity (D) with the following math equation:

P = permeability coefficient
D = diffusion coefficient
S = solubility coefficient
q = quantity of permeant transferred by a unit of area, A, in a time t, is the thickness of material and Δp is the partial pressure difference.

In practical applications, the transmission rate (TR), is the most common way to report the “flux” of gas moving through a polymer. It also makes the most sense, as many polymers are multi-layered or coated. The “net flux” of oxygen, water vapor, carbon dioxide…etc. is what’s important to a product’s shelf-life.

Permeation and Transmission Rate have the following units:


Above equations demonstrate that the permeation rate is the thickness and driving force normalized transmission rate.

Related Links:

• Why Choose Us for Permeation Analysis?
• Permeation Glossary
• Permeation FAQ

The permeation of a material is obtained via the transmission rate test. The transmission rates for oxygen (OTR), water vapor (WVTR) and CO2 (CO2TR) can be measured by permeation instruments with different sensor technologies. The setup for analyzing a film (Fig 3) for transmission rate is analogous to the diagram shown for the diffusion mechanism, i.e. test gas on one side of a film and a carrier gas on the other side. This setup is so called Iso-static method (aka Equal Pressure Method). The carrier gas (usually Nitrogen) carries the permeated test gas to the sensor for quantification.
Some instrument examples are:

• MOCON OX-TRAN® 2/22for OTR testing
• PERMATRAN-W® 3/34 for WVTR
• PERMATRAN-C® for CO2TR

Related Links:

What is Permeation?

Why Choose Us for Permeation Analysis?

Permeation Glossary

Permeation FAQ

Permeation is affected by the environment factors such as temperatures, relative humidity (RH), as well as the driving force around it. Here are some important generalities:

• For every 10°C increase in temperature, transmission rate doubles.
• Humidity will cause Non-Fickian behavior for hydrophilic materials (e.g., EVOH with moisture)
• Transmission rate is proportional to the driving force
• Transmission rate is reversely proportional to the material thickness

Therefore, testing at controlled test temperature and relative humidity is crucial for obtaining accurate transmission rate results.

Permeation is also influenced by the nature of polymer and permeant and their interactions. Some factors include but not limited to:

• Chemical substituent on polymer backbone
• Degree of packing, crystallinity, and orientation of molecular chains
• Susceptibility to moisture and other possible interactants with the chains (e.g., EVOH with moisture)
• Polymer surface contacting permeant
• Additives used in manufacturing or modifying polymer
• The polarities of the polymer and the permeant.

Related Links:

• What is Permeation? 
• Permeation Glossary
• Permeation FAQ

ASTM has approved two test methods to track water vapor transmission from materials:

• ASTM E96 – Water Vapor Transmission of Materials Using Gravimetric Method (adopted in 1941), and
• ASTM 1249 – Water Vapor Transmission Rate Through Plastic Film Sheeting Using a Modulated Infrared Sensor (adopted in 1990).

So, what would you choose from these two methods to meet today’s packaging needs? Let’s have a look at the comparison of those two methods:

Summary table of method comparison ASTM F1249 vs E96

To explore detailed interesting facts regarding these methods, please read the full white paper here.

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• What is Permeation?
• Why Choose Us for Permeation Analysis?
• Permeation Glossary
• Permeation FAQ
• Products
• Test Methods

To protect the product’s quality, oxygen barrier packaging materials are widely used for products that are sensitive to oxygen. Consequently, accurate Oxygen Transmission Rate (OTR) measurement is important when assessing oxygen barrier properties during the selection of packaging materials, as well as QA/QC process down the road.

ASTM approved two distinct test methods for determining oxygen transmission rates of packaging materials: ASTM D3985 – 05 (Reapproved 2010) – “Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor” and ASTM F2622 – 08 – Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using Various Sensors.

Obviously just from the titles you can see the major difference of the two methods is about the sensor, “Coulometric, “ vs “Non-Coulometric”. The following table summarized the key features that either method has.

Summary table of method comparison

Coulometric and Non-coulometric methods could have different applications.

When working with high barrier materials to package foods or other products that are easily oxidized, the low OTR level packaging material requires a more accurate sensor. Best practice is to select an instrument conforming to the Coulometric method. Refinements regarding the handling of temperature, relative humidity, system leaks and other parameters means the difference between a right answer and a wrong one. When business decisions are made based on results generated from a permeation measuring device, accuracy, repeatability and reliability of the results must be dependable.

if the application is produce/fruit packaging where high levels of oxygen are needed for products’ respiration, or other products that are not oxygen sensitive, then the packaging material used is usually polyolefin or other low oxygen barrier materials. In this case, instruments with non-coulometric sensors suit the purpose. An example is the recent developed MOCON OX-TRAN® 2/12.

Related Links:

• What is Permeation?
• Why Choose Us for Permeation Analysis?
• Permeation Glossary
• Permeation FAQ
• Products
• Test Methods

ASTM E96, also called the gravimetric or cup method, is a common method used to determine the water vapor transmission rate (WVTR) of high transmitting plastic barrier or nonwoven materials. In spite of its popularity, there is a more accurate and user-friendly way to measure a high WVTR is the method described in ASTM D6701, which corresponds to the PERMATRAN® 101K produced by AMETEK MOCON.

When testing a membrane with a very high transmission rate, the air gap between the test material and the water or desiccant used in the cup method is itself a significant barrier (Figure 1). The larger the air gap, the less water vapor will permeate through it.

ASTM D6701 (Figure 2) is an instrumental method with the modified inverted cup and guard film concept, so that it eliminates the air gap issue completely, and gives more accurate WVTR results.

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• What is Permeation?
• Why Choose Us for Permeation Analysis?
• Permeation Glossary
• White Paper: A Comparison of the ASTM E96 and D6701 Test Methods

Permeation testing can be done with Equal Pressure Method , or Differential Pressure Method.

The Equal Pressure Method is also called Isostatic Method. During the test, as shown on the graph below, both sides of the film are exposed with equal pressure of test gas and carrier gas, usually atmospheric pressure. The driving force is the test gas partial pressure or concentration difference across the film. The continuous carrier gas flow takes the permeated test gas molecules to the sensor for quantification. Some popular ASTM standards commonly used to quantify transmission rate are ASTM D3985, F1249, etc.

The Differential Pressure Method is also referred to as Manometric Method. During the test, as shown on the graph below, one side of film on that side of chamber is exposed to a flow of test gas (e.g.: O2, CO2), the other side of the chamber is usually vacuumed. The gas permeation across a film is driven by a difference in absolute pressure across the film. The permeation of the test gas is determined by measuring the pressure change over time in the lower pressure side and finding the slope of the change once it becomes linear. The common ASTM example is ASTM D1434.

Here is a list of comparison for the Isostatic Method vs Manometric Method:

Related Links:

• What is Permeation?
• Why Choose Us for Permeation Analysis?
• Permeation Glossary
• Permeation FAQ

Different molecular structures of polymer films decide how the polymer reacts to moisture around it. Some barrier materials, called Fickian materials, would not be affected by the change of RH. Their OTR results obtained at different RH would be the same. These are usually polyolefins, or any material with hydrophobic nature.

The other type of materials, called Non-Fickian materials, which are sensitive to moisture or hydrophilic. Their OTR results measured at different RH could be very different. The humidity would swell up the polymer chains and make it more permeable for gases going through. In this case, OTR testing should be set with precise RH so that their worst performance can be known during real life applications. The following graph shows examples how OTR of polymer materials affected by RH.

Related Links:

• What is Permeation?
• Why Choose Us for Permeation Analysis?
• Permeation Glossary

During a package design process, the barrier properties of the packaging material meant how much protection it can provide, which is the key to a product’s shelf life.

As an essential part of the R&D process, analyzing films and/or components (bottles, closures) are useful for selecting package candidates. However, permeation rates in finished packages could be much higher due to damages caused during manufacturing, shipping, and distribution.

To obtain the true permeation rates into a package, only by analysing the “Whole Package” can one understand the package system of sealed and integrated components, along with potential “wear and tear” effects due to processing and distribution.
 
Related Links:

• What is Permeation?
• Why Choose Us for Permeation Analysis?
• Permeation Glossary
• White Paper: Shelf Life and the Importance of Testing the Whole Package