Why the Permeability Rate of a Polymer Material is Dependent Upon a Variety of Factors

The role of your plastic packaging materials is to protect and preserve the quality of your food product from production to end-of-use. In the process of designing your packaging, choosing the right polymer material for your product and package is vital to its success. The barrier properties of your packaging material must be considered, along with the components of your food, the interactions between the material and its environment and its permeability to various gases or vapors. Oxygen transmission rate (OTR), water vapor transmission rate (WVTR) and sometimes carbon dioxide transmission rate (CO2TR) are most common tests for evaluating the permeability of food packaging materials. It is essential to test the material's transmission rates for each of these gases individually and never assume that your material has the same level of transmission rate for every gas, as Polymer materials behave differently to certain gasses. Likewise, different gasses behave differently to the same material.

This article will explain this is in further detail to hopefully help you make the best decisions when selecting new materials for your food or beverage packaging.

Oxygen (O2) is abundant in the environment. For most food products, oxygen is destructive and will cause oxidative deterioration of the food and encourage aerobic micro-organism growth. Hence the packaging material should be a good oxygen barrier so that you can meet your desired shelf life (or even improve it) while maintaining the high quality of the food.

Moisture ingress or egress is also important factors to many food products due to the fact that the foods physical and chemical deterioration's are related to the moisture content within the package. Therefore, it is also vital to evaluate the Water vapor transmission rate (WVTR) of the packaging material.

CO2 is needed to control aerobic bacteria and mold growth, this is particularly important with food related applications. CO2 is often combined with N2 and used in Modified Atmosphere Packaging (MAP) systems to help avoid food degradation and extend shelf life. It's also critical to access the CO2 permeability of a bottle used in carbonated drink applications to halt any loss of CO2 while on the shelf, keeping it fully carbonated until consumed.

Although it may be difficult to find a mono-layer polymer material that will serve as a good barrier for all these gases remember it is possible and that there is no such thing as the perfect package. You just need to be sure that it can withstand the packaging, handling and shipping process while also maintaining the integrity of the product by properly protecting it from various gases/vapors and environments. This is where testing plays an important role in helping you choosing the right material for your package application.

For example, HDPE has the following permeation rates (relative values are shown) to the following gases:

HDPE Permeation Rates by Gas

When HDPE is compared with PET, their permeation rates (relative values) to oxygen are also different.

HDPE vs. PET Permeation Rates to Oxygen

Now that we have the basics down let's dive deeper into what makes a polymer behave so differently to specific gases/vapors. The phenomenon we see here is related to permeation theory, the chemical and physical natures of the polymer and the permeants, as well as the interaction between the polymer and the permeant.

Permeation Theory:

P = S X D

Permeability

From permeation theory, we know that P = S x D. It means that permeability (P) is directly related to diffusivity (D) and solubility (S) of a polymer material.

Here S is the solubility of a polymer material that related to "like dissolves like". "Like dissolves like" is an expression used by chemists to remember how chemical substances tend to dissolve in solvents of similar structure. It refers to "polar" and "nonpolar" solvents and solutes. Basic example: Water is polar. Oil is non-polar. Water will not dissolve oil.

When talking about gas and polymer, water is polar, so it would dissolve more into polymers that are polar such as PET, Nylon and such. While O₂ is non-polar, it would dissolve into non-polar polymers such as PE, PP and most polyolefins. Therefore, usually WVTR is higher for PET, EVOH, Nylon and alike, but OTR will be higher for polyolefins such as PP, PE, etc.

On the other hand, D is the diffusivity that is related to the permeant molecule's size and how fast it moves. Smaller molecules such as Hydrogen and Helium usually move much faster than H₂O and O₂. Hence, gases with smaller molecules will have higher transmission rate.

Permeation Affected by Environment Factors

Temperature plays a major role in the permeation rate of a material. The higher the temperature, the higher the permeation. As a Rule of Thumb, every 10°C increase in temperature, the transmission rate doubles, or every 1°C increase, the transmission rate increases 5-7%.

The relative humidity (RH) in the environment can impact polymer differently. RH can greatly affect permeation for hydrophilic materials. Proper RH generation and measurement are necessary for accurate permeation results.

Permeation Affected by Partial Pressure Differential of Permeant Gas

The driving force for permeation is the partial pressure differential of a permeant gas. An example is CO2 permeates out from a bottle of carbonated soft drink after 4atm of CO2 is filled inside the bottle to start with.

The permeation is proportional to this driving force. For example, if a film's OTR is 5 cc/(m^2·day) when the driving force is 21% oxygen (room air), its OTR becomes 23.8 cc/(m^2·day) when the driving force is 100% oxygen.

Other Factors

There are other factors (Cooksey, K.) would influence the transmission rates for different gases. To name a few, the polymer structure, crystallinity, plasticizer, co-polymer, thickness of the film, are among those affect the barrier level of a specific polymer material.

Take Away Message:

Polymer materials behave differently to certain gasses. Likewise, different gasses behave differently to the same material. Experiences tell us the following Rules of Thumb:

• For a monolayer polymer, the CO2TR is usually 3-8 times higher than OTR, N2TR is usually 3 times lower than OTR. The WVTR could be high or low depending on the polarity of the polymer.
• For a monolayer polymer, the TR is reversely proportional to the film thickness.
• Polar polymers are usually better gas barriers, while non-polar polymers are mostly better moisture barrier.
• Gases with smaller molecular sizes (H2, He, etc.) usually have much higher transmission rates than oxygen.
• For practical applications, multi-layer barrier materials or special coated materials are the solutions for products need protections from several gases.

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