What is a permeation? – A study on fuel permeation of various seal materials

Permeation is the process in which molecules pass through a material. In many industries, this is sought to be minimised to prevent loss from within a system. In this guide from TRP, we discuss what permeation is, the various permeation processes, the results of our own permeation tests and which elastomers are best for your processing as a result.  

What is permeation?

Permeation is the method by which molecules transfer through a material, normalised to the pressure gradient. Permeation happens through a process called the ‘solution diffusion mechanism’, or ‘Graham’s colloidal diffusion’. This happens in these steps:​

  • The fluid is absorbed into the face of the material and dissolves into it
  • This dissolvent then diffuses through the membrane​
  • The fluid then desorbs at the other side of the membrane​

Permeation is closely related to solubility and diffusion, which is often modelled using Fick’s law.

permeation molecules diffusing

How can permeation be modified?

In many industries and processes, permeation is sought to be minimised to keep substances within certain mechanisms. For example, within chemical processes, ensuring the integrity and efficiency of a system means that permeation is kept to a minimum. In the case of industrial environments and fuel systems, the reduction of permeation is essential to reduce environmental impact through the release of pollutants. 

To minimise permeation the properties of rubber can be altered in several different methods. These include:

  • Adding specialised additives​
  • Altering the elastomer thickness
  • Modifying the chemical structure of the polymer​
  • Modifying the surface texture of the elastomer or applying a coating​
  • Applying chemically-resistant veneers, such as PTFE

How is permeation measured?

To test the suitability of various rubbers for their use within industry, permeation can be tested in controlled environments in a variety of different ways. These include the gravimetric method and the isostatic method. We will discuss these in further detail below.

What is the gravimetric method?

The Gravimetric method of measuring permeation can be done in two different ways: a dry cup for weight gain and a wet cup for weight loss.

The dry cup method

The dry cup method is used to analyse the transmission of water. A desiccant is inserted into a cup while a rubber membrane is placed over the top. The resulting humidity is then measured on the outside of the cup in addition to the weight gain. This then allows permeation through the cup to be detected.

The wet cup method

The wet cup method to measure weight loss, can be used for the transmission of various fluids. A fluid is placed into a cup and, like the dry cup method, a rubber membrane sits over the top. The weight loss is measured and the permeation is calculated when a steady state is reached.

At TRP, we use a variation on this method to monitor fuel permeation through various elastomers. We carry out this method by placing the cup upside down, so that the fluid is in contact with the rubber membrane. This more closely simulates production environments.

gravimetric method

What is the isostatic method?

The isostatic method is done by placing a gas inside a vessel. A membrane is then placed over the top of the vessel and fixed into place. A carrier gas, such as nitrogen, will then pass over the membrane and collect any resulting gas. The permeation rate can then be calculated. This method is quicker and provides more accurate results than the Gravimetric method. However, it does require specialised equipment to carry out.

We also use the isostatic method at TRP to establish hydrogen permeation at elevated temperatures. We have done so with a variety of rubber materials, including EPDM, FKM, EDPM with a PTFE veneer and butyl. The results of this can be seen in our article on gaskets for use in hydrogen electrolysis.

Isostatic permeation measurement

In-house permeation testing at TRP Polymer

To see which rubber grades offer the best permeation reduction, we tested a 70IRHD FVMQ (fluorosilicone) with a temperature range of -50°C to +200°C against a specialised ultra-low temperature FKM (F207), with a temperature range of -55°C to +200°C.

We did so as FVMQ is often specified where high fuel resistance is required in low temperatures or arctic conditions. Using this experiment, we intended to show FKM’s increased capabilities in comparison to fluorosilicone such as its reduced loss of fluid due to permeation, improved low-temperature properties and chemical resistance.

The results of the experiment showed that mass loss of fuel through fluorosilicone was calculated to be 0.0452g/hour – or around 1g/day – when tested at room temperature using the upside-down wet cup Isostatic method on a 1mm thick sheet.

Under the same conditions, for FKM F207, the mass loss of fuel was calculated to be 4.43×10 – 4g/hour – or around 0.01g/day – once a steady state was reached.​

Therefore, if fluorosilicone is specified within your application, you should strongly consider using our FKM F207. Its superior low-temperature properties, notably reduced permeation and high chemical resistance make it a dependable choice. These qualities and the overall reliability of this FKM make it suitable for many industrial applications.

Low permeation rubber moulding at TRP

As demonstrated in the above studies, our FKM grades are proven to have low permeation, in addition to the added benefits of extreme temperature and chemical resistance. These properties make them an excellent choice for a wide variety of industrial applications.

For dependable rubber moulding, FKM and FFKM materials choose TRP Polymer Solutions, we can provide you with high-performance ‘O’-rings, custom moulded rubber seals and high-temperature ‘O’ rings made specifically for your application. With custom colours, metal detectability and certifications including FDA, USP Class VI & Norsok M710, be sure to get in touch with an enquiry.

For more information on our products and how TRP can assist your business, please contact us. Our technical team is ready to provide expert advice and solutions tailored to your needs.