O-Ring Resistance List: Checking Media Correctly

What is an O-Ring Resistance List?

Which O-ring material is the best answer to the sum of all operating conditions? The lasting reliability of a seal depends on how well designers answer this question during its design phase. A resistance list assigns to a contact medium the materials that withstand it. It is therefore the most important tool for making a first preselection in the early design phase.

Of course, seals can be replaced with a different material during maintenance, but this is not always possible without further technical intervention. This is because some O-ring materials require differently designed installation spaces. For this reason, material selection ideally involves a comprehensive assessment of the operating conditions right from the start. The contact media are particularly important here, because the base polymer essentially determines the media resistance of an O-ring material.

If you want to check a specific media resistance, feel free to use our tool. The O-Ring Resistance List rates your medium for the most important materials at a glance.

What does media resistance mean?

Almost all O-ring materials are characterised by their elastic properties. These are responsible for allowing the sealing elements to deform flexibly in the installed position. This enables them to compensate for small irregularities in the surface of machine parts. After all, the sealing gap can change over time, for example due to mechanical influences.

The operating conditions affect not only the machine parts to be sealed, but also the O-rings themselves. Over time, they can lead to changes in the material composition. In addition to temperatures, it is mainly aggressive media that cause these processes when they come into contact with the O-rings. Two mechanisms of action can be distinguished here.

Similar to attack by aggressive media, the effect of temperature can also impair the elastic properties of O-rings. Excessive heat leads to the permanent loss of part of the elastic restoring capacity. This is shown by the compression set: after the material has been subjected to defined deformation forces for a certain time under the influence of temperature, it is relieved and returned to room temperature. The lower the permanent deformation, the more elasticity has been retained.

The contact medium and temperature mutually influence their effect on the elastomer material. Temperature can accelerate or slow down the chemical interaction between the medium and the sealing material. The compression set is tested in air according to the common standard. Other contact media such as oil or steam often lead to completely different results, depending on the media resistance of the material in question.

How do you read a resistance list correctly?

The chemical structure of the base polymer, the cross-linking and the additives: these degrees of freedom determine the resistance of O-rings to media attack. This is why media resistance also varies within a single base polymer. For example, with EPDM a peroxide cross-linking provides better durability in contact with water than a sulfur cross-linking.

Nevertheless, general resistance profiles can be given for the most important base polymers. However, they are only an initial reference point for the technical design, since media resistance only becomes meaningful in detail in conjunction with the respective operating temperatures. The following overview summarises the resistance profiles of the most important O-ring materials.

FFKM

The special feature of FFKM lies in its generally high resistance to chemicals. Together with its good mechanical strength, this property makes the material suitable for a very wide range of applications. FFKM is currently the sealing material with the most pronounced chemical resistance.

FEP

FEP has a similarly good chemical resistance to FFKM. This makes FEP an inexpensive alternative to the cost-intensive FFKM in terms of media resistance. However, FEP cannot cover the entire range of FFKM, because its resistance to aggressive media extends over a narrower temperature band. The operating range of FEP is between −60 and +205 °C. FFKM can withstand heat of over 300 °C.

FKM

FKM shares its polymer base with FFKM, but is only partially fluorinated instead of fully fluorinated like FFKM. Accordingly, its resistance to chemicals is below that of FFKM. Its high-temperature behaviour is also less good, so that 200 °C in air represents the permanent maximum. In contact with hot steam, FKM O-rings should not be exposed to temperatures exceeding 120 °C.

EPDM

With EPDM, the very good resistance to hot water and steam stands out in particular. The resistance also extends to hot air. However, oxygen attacks the material, so that the hot-air resistance also limits the durability in hot water and steam. Good-quality EPDM O-rings can be used up to 170 °C in hot water and steam. Vegetable and animal oils or fats attack the material.

NBR

NBR is mainly resistant to products based on mineral oil. These include products such as petrol and diesel fuel, among others. The upper limit of the continuous temperature is 120 °C. Water vapour is one of the most important weaknesses of the material.

Important: the following table is a highly simplified O-ring resistance list. If you want to check a specific media resistance, feel free to use our media resistance tool.

Understanding resistance levels and swelling

If a seal fails during operation, a failure analysis should always be carried out. It provides information about which interactions between the O-ring and media led to the premature failure of the seal. In principle, two types of attack can be identified from typical damage patterns.

Chemical attack: Chemical attack destroys the bonds in the O-ring material. This can be recognised by the change in the material properties. The O-ring becomes hard and brittle. This damage first appears on the side where the O-ring is directly exposed to the aggressive medium. A pronounced swelling or shrinkage as well as a sticky surface also indicate chemical media attack. In addition, permanent deformation is another characteristic.

Physical attack: The consequences of physical attack by a medium cannot always be clearly distinguished from other damage mechanisms. In principle, the medium causes the material to swell or shrink. If the O-ring shrinks, for example because plasticizers are dissolved out, the compression decreases and leakage occurs. In the case of strong swelling, the network structure can be affected. Particularly in dynamic applications, friction increases along with the volume, so that material is lost locally. In this case, swelling is the cause of failure, but the failure occurs as a result of the excessive mechanical stress.

The most important factors influencing media resistance

Basic rule: The higher the polymer content in a material compound, the higher the media resistance. The composition of the base rubber essentially determines which media the material is resistant to and which can easily attack it.

For example, NBR O-rings are not resistant to acetone, because both are polar substances. In the case of copolymers and terpolymers, the concentration of the individual monomers additionally influences the properties. This is the case with NBR, for example, where the fuel resistance increases along with the acrylonitrile content. In addition, the cross-linking mechanisms affect the media resistance, as do additional formulation components.

A high plasticizer content in combination with a corresponding medium can lead to strong shrinkage. Fillers or other additives can also cause partly undesirable interactions. Conversely, the right additives and cross-linking can also increase resistance in certain contact media.

The pH value can also influence the durability of O-rings. Every material reacts differently to the chemical environment in which it is used. Acidic environments can attack certain materials such as NBR or silicone and lead to faster embrittlement. Alkaline conditions can also be harmful, especially for materials such as EPDM. It is therefore important to select the right material for the specific application.

Because of the many influencing factors, it is essential to consider each seal individually during the design phase. Only in this way can the choice be made specifically for a material whose media resistance ideally suits the given application. You are welcome to contact us at any time regarding the media resistance of O-rings. We are happy to advise you.

The limits of the resistance list

A resistance list is an initial reference point, not a substitute for well-founded design. Its statements generally apply to a pure medium at room temperature. In practice, however, media rarely occur alone and rarely at 20 °C. Several factors limit the significance of a list.

For these reasons, a list does not replace the individual assessment of the specific installation case. It narrows down the materials in question. The final decision is made on the basis of temperature, concentration, pressure and installation situation, in case of doubt with tests or expert advice.

Check media in the tool now

Instead of searching through a static table, the fastest way to check your medium is directly in the tool. The O-Ring Resistance List from NH O-RING assigns the most important materials to your contact medium and shows you the rating at a glance. This allows you to quickly make a well-founded preselection in the design phase.

Always bear in mind the limits mentioned: also take into account temperature, concentration and installation situation. If several media, high temperatures or critical concentrations are involved, please feel free to contact us. We will help you find the right material for your seal.

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