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How to Choose the Right O-Ring Material: 7 Factors to Consider

NBR vs. FKM vs. EPDM vs. Silicone—and why O-ring material selection is rarely as simple as picking a rubber type from a chart

O Rings and Materials Selection

10 min read

Choosing an O-ring often starts with a seemingly simple question:

What material do I need?

NBR? FKM? EPDM? Silicone? HNBR?

There are plenty of O-ring material charts that can point you in the right direction. But choosing a material based on a single property—temperature, pressure, chemical resistance, or even price—can create problems later.

The reality is that O-ring material selection is an application-specific decision.

A material that performs well with a particular fluid may behave differently when that same fluid is combined with elevated temperature. A material with excellent high-temperature capability may not provide the low-temperature flexibility an application requires. Pressure capability can also depend on factors such as hardness, clearance, gland geometry, and the use of backup rings.

That is why the better question is not simply:

“What is the best O-ring material?”

It is:

“What material and compound are appropriate for this combination of operating conditions?”

Here are seven factors to consider before selecting an O-ring material.

Important: The information below is intended as general educational guidance. Material properties vary by compound and manufacturer, and actual seal performance depends on the complete application. Published manufacturer data, applicable specifications, and application-specific requirements should always be reviewed before final material selection.

1. What Media Will the O-Ring Contact?

Chemical compatibility is one of the first considerations in O-ring material selection.

Start by identifying everything the seal may contact, not just the primary process fluid.

That could include:

 

Why does this matter?

Elastomers can react differently when exposed to different chemicals. Depending on the material and fluid, exposure may contribute to swelling, shrinkage, softening, hardening, loss of physical properties, or other changes that can affect sealing performance.

Chemical compatibility guides are extremely useful as a starting point, but they should not automatically be treated as an approval for a specific application.

The Parker O-Ring Handbook, for example, includes extensive compatibility information for elastomers exposed to fluids, gases, and other media—but material selection still requires consideration of the broader operating environment.

Don’t forget mixtures

This becomes especially important when the application contains more than one fluid.

A seal might encounter:

hydraulic oil + an additive package + cleaning solution

or

fuel + lubricant + elevated temperature

rather than one pure chemical in a controlled environment.

When compatibility is critical, identify the actual media as precisely as possible and evaluate the specific compound, not just the general elastomer family.

2. What Is the Actual Temperature Range?

Temperature is another major factor—but this is where an O-ring material chart can sometimes create false confidence.

It is tempting to look at a chart, find a material’s stated temperature range, and assume:

“Our application is below that number, so we’re good.”

Not necessarily.

Temperature capability can vary considerably among compounds within the same polymer family.

More importantly, ask:

 

Published application limits are typically developed under defined conditions.  Temperature, speed, and media limits can represent maximum values established under laboratory conditions.

That is an important distinction.

Temperature and media should be considered together

An elastomer’s resistance to a fluid at room temperature does not automatically tell you how that same combination will behave at substantially higher temperatures.

This is one reason material selection should be based on the combined application, rather than independently checking boxes for:

The interaction between those conditions matters.

3. What Pressure Will the Seal Experience?

Customers understandably ask questions such as:

“What PSI can this O-ring handle?”

Unfortunately, there usually is not one universal pressure rating for an O-ring.

Pressure capability can depend on the entire sealing system, including:

 

As pressure increases, the elastomer may be forced toward the clearance gap between mating components. If the combination of pressure, material properties, and clearance becomes unfavorable, extrusion or nibbling can occur.

A harder compound or backup ring may help in certain designs—but that does not mean that simply moving from a 70 Shore A O-ring to a 90 Shore A O-ring creates a universal pressure rating.

Pressure needs to be evaluated together with the gland and hardware design.

The takeaway

Instead of asking only:

“What PSI is this O-ring rated for?”

provide as much information as possible about:

pressure + gland + clearance + material + temperature + motion.

That gives manufacturers, distributors, and engineers much better information to work from.

4. Is the Application Static or Dynamic?

An O-ring sitting between two stationary components has a different job from an O-ring sealing a moving shaft or reciprocating piston.

That distinction can significantly affect material selection.

Static applications

In a static application, the sealing surfaces generally do not move relative to one another during operation.

Examples might include:

Dynamic applications

Dynamic seals experience relative motion, such as:

Now additional factors become important, including friction, lubrication, wear, surface finish, speed, and heat generation.

A material that looks excellent on a chemical compatibility chart is not automatically the best choice for a dynamic application.

In some cases, the seal design itself may also need to change.

The right question may no longer be:

“Which O-ring material?”

It may be:

“Is an O-ring the appropriate sealing solution for this motion and operating environment?”

5. What Does O-Ring Durometer Actually Tell You?

Most elastomer O-rings are identified with a hardness measurement, commonly expressed as Shore A durometer.

You may see descriptions such as:

Higher numbers generally indicate a harder elastomer.

But hardness should not be confused with an overall measure of material quality.

A 90-durometer O-ring is not simply a “better” or “stronger” version of a 70-durometer O-ring.

Hardness influences how an elastomer responds to deformation, pressure, installation, and the gland around it. Different applications may benefit from different hardness levels.

More importantly, durometer does not define the compound.

Two black, 75-durometer FKM O-rings can still be made from different formulations and have different:

That leads to one of the most important lessons in seal sourcing:

Polymer + Color + Durometer Does Not Fully Define an O-Ring Material

“Black FKM 75” may sound specific.

From a material engineering standpoint, it is still a relatively broad description.

6. What Physical Properties Matter?

Material data sheets contain a lot of numbers. Understanding what those values represent can help explain why seemingly similar compounds may perform differently.

Some of the most common properties include:

Hardness

Tensile Strength

Elongation

Compression Set

 

But even here, comparisons require context.

Test method, temperature, time, specimen geometry, and other conditions can affect reported values.

Why material data sheets matter

A material data sheet helps answer:

“What are the measured characteristics of this specific compound?”

That is different from asking:

“Is this compound guaranteed to work in my application?”

7. Does the Application Require a Specification, Approval, or Specific Compound?

Sometimes the question is not simply whether an elastomer could work.

The application may require a particular:

This distinction is critical.

For example, identifying an O-ring as FKM tells you its general polymer family.

It does not necessarily tell you:

This is why substitutions deserve careful review.

Two materials can appear similar on a basic product description while having meaningful differences in formulation, physical properties, testing, approvals, or documented performance.

If a drawing, purchase specification, OEM requirement, or customer specification calls out a specific material or compound, do not assume another material is equivalent simply because the polymer family and hardness match.

NBR vs. FKM vs. EPDM vs. Silicone: A Quick Comparison

There is no universal “best O-ring material,” but several elastomer families appear frequently in industrial sealing applications.

Material Family Often Considered For Important Factors to Investigate
NBR / Buna-N Petroleum-based oils, hydraulic fluids, general industrial applications Temperature, weathering/ozone, specific fluid compatibility
FKM / Fluorocarbon Elevated temperatures, oils, fuels and many chemical environments Low-temperature requirements and specific chemical exposure
EPDM Water, weathering and certain steam or chemical applications Generally unsuitable for many petroleum oils and fuels
Silicone Applications requiring flexibility across a broad temperature range Mechanical strength, wear and fluid compatibility
HNBR Applications requiring improved heat and mechanical performance compared with conventional NBR in certain environments Actual media, temperature and compound-specific properties
FFKM Highly aggressive chemical or elevated-temperature applications where specialized performance is required Significant compound differences, application requirements and cost

This table should be used as a screening tool—not a final material specification.

Specialty formulations within each family can behave differently, which is why compound-specific manufacturer data should be reviewed before making a final selection. Parker’s own material offering literature distinguishes numerous compounds within broad elastomer families based on their properties and intended applications.

A Better Way to Specify an O-Ring

If you are trying to identify an O-ring for a new application, gather as much of the following information as possible:

Size

Media

Temperature

Pressure

Application

Hardware

Material requirements

The more information available at the beginning, the easier it becomes to narrow the field before a component is released into production.

So, What Is the Best O-Ring Material?

There isn’t one.

And that is really the point.

The best material for one application might be completely inappropriate for another—even when the O-rings are exactly the same size.

Successful O-ring material selection requires looking at the complete application:

Media + Temperature + Pressure + Motion + Hardware + Material Properties + Specifications

When those variables are considered together, material charts and technical data become much more useful.

And when an application involves unfamiliar chemicals, extreme operating conditions, critical equipment, regulatory requirements, or unusual seal geometry, the appropriate manufacturer or qualified application engineer should be involved in the final evaluation.

Need help navigating the options?

At Performance Seals, we work with customers to help identify available O-ring materials, compounds, sizes, specifications, and manufacturer information for consideration.

Whether you’re trying to interpret a material callout, compare available compounds, locate an unusual size, or understand what information is needed before requesting a seal, our team can help you navigate the options.

Because sometimes the most important part of selecting an O-ring isn’t immediately knowing the answer.

It’s knowing which questions need to be answered before you choose one.


Speak to our team

This article is provided for general informational purposes only and is not intended to constitute engineering advice or a guarantee of material or product performance. Operating conditions, chemical exposure, hardware design, tolerances, installation, and other factors may affect seal performance. Material compatibility and suitability should be independently verified for the specific application using applicable manufacturer data, specifications, testing, and engineering requirements.

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