What You Need to Know About O-Rings

1. The temperatures it will be exposed to, especially if likely to involve extended periods below 0º F or over 200º F (-18º C -93º C); otherwise, it is unlikely that temperature ratings will be much of a factor.
2. Any chemical, fluids, gases, lubricants, or other elements that will contact the rubber seal, including ordinary things like water, sunlight, and atmospheric ozone.
3. The maximum pressure (psi) the seal will be subject to, especially if high pressure is involved.
4. Whether it is for a dynamic or static seal, the former type generally increasing the importance of properties like tensile strength, abrasion resistance, elongation, etc.
5. Any additional specification, like material type, durometer, color, tensile strength, elongation, or standardized composites such as published by organizations like the FDA, NSF, Military, AMS, ASTM, DIN, ISO.

Most applications citing specifications such as those named in #5 above already specify material type and can be purchased by vendors selling products designed to meet the same, bypassing the need for the more detailed investigation outlined below.

The First Question

Will Standard Commercial Grade Buna-n O-Rings Work?

Since it is more likely than not that you won’t need to look any further for a suitable material, we begin with the simple question, will Buna-n o-rings work?   Not only are largely standardized Buna-n O-Rings the lowest cost and most widely available type by a wide margin across the market, it is also generally quite resistant to the kind of fluid and gas environment typically associated with o-ring seals like hydrocarbon fuels, petroleum-based fluids and lubricants, and vegetable fats.  In contrast to other chemical groups of elastomers, Nitrile Rubber (or Buna-n, as more generically known), is a good quality elastomer in terms of tensile strength, elongation, and resistance to compression sets, tears and abrasion.

(By “good quality” we mean in very general terms in contrast to other common chemical groups of elastomers, not that there isn’t a full range of good, better, and even what might be considered sub-standard products available in the highly competitive o-ring market.)

In most cases, two more questions are all that’s necessary to know whether looking further at Buna-n is or investigating other options is warranted:

What temperature range will the o-rings be subject to?

Buna-n rubber compounds are generally rated between -40 F and +250 F service temperatures (be sure to check particular brand ratings before buying).  Application-specific testing is strongly recommended where o-rings could be subjected to extended periods of time at or near the manufacturer’s stated limits.

Rubber service temperature ratings are seldom ever stated as absolute, unconditional specifications.   In most cases, rubber will degrade as a result of extended exposure to temperatures near or above the stated operating ranges, which can be exacerbated by the fluid and gas environment.   This is a result of a breakdown in the molecular bond that gives rubber its elastic properties, so the damage is generally permanent, even if very slow in development.

On the other hand, low temperature crystallization causes rubber materials to become brittle rather than elastic, but doesn’t normally cause permanent damage in the same way as excessive heat.   However, crystallized rubber can be damaged or broken much easier, and won’t exhibit any real elastic qualities in such a state.  Because of this, applications involving extended exposure near, but not beyond, the actual crystallization or “glass” temperature don’t generate as great a risk of untimely failure as high temperature ranges.

What chemical fluid and gas elements will it contact?

Buna-n is known to exhibit generally good resistance to petroleum-based oils, greases and hydraulic fluids, aliphatic hydrocarbons like propane, butane, petroleum oil, mineral oil, diesel fuel and fuel oils, vegetable oils, organic fats, most silicone-based fluids and lubricants, alcohols, most HFA, HFB and HFC fluids, alkali and salt solutions, and some other chemicals.   For more information, see related:

308 Chemicals that are generally COMPATIBLE with Buna-n rubber – These chemicals (fluids, gases) tend to have little or no affect on commercial Buna-n rubber compounds which are, therefore, very likely to perform well in both dynamic and static seals and gaskets in environments containing them.

147 Chemicals that are generally SOMEWHAT COMPATIBLE with Buna-n rubber – These chemicals are listed in two categories, Good and Fair, and tend to have a minor to moderate (“Good”) or moderate to severe (“Fair”) affects on commercial Buna-n rubber compounds.  Buna-n is considered generally suitable for some dynamic and most static sealing applications involving the first group, while only suitable for some static sealing applications with the latter group of chemicals.

On the other hand, Buna-n rubbers generally have poor resistance to sunlight, ozone, weathering, phosphate esters, ketones, glycol-based brake fluids, high-aromatic fuels and hydrocarbons like benzene, chlorinated hydrocarbons like trichlorethylene, polar solvents like ketone, acetone, acetic acid and ethylene-ester, and strong acids.  For more information, see related:

237 Chemicals that are generally INCOMPATIBLE with Buna-n rubber – These chemicals tend to have a severe affect on commercial Buna-n rubber compounds which are, therefore, very unlikely to perform well in sealing applications where they are present.

If the above temperature and chemical resistance information indicates that Buna-n O-Rings likely suit your requirements, there is no need to look any further; there aren’t any more commonly available or lower cost options.  The next step would be to compare available brands, for which we recommend the O-Rings on the Web section of our affiliate internet outlet Standard-Gasket.com as a good starting point.

However, if, based on the above, Buna-N is not a likely material for your o-ring requirements, then we’ll need to look a little further…

The Next Question

If not Buna-N, then What?

The same factors apply, but if Buna-N doesn’t appear to be a good option, a more broad array of materials must be considered.  The next level of more or less standard materials, from the least to the most costly (in general), include Silicone, Viton® (Fluoro-elastomer), EPDM, and Kalrez® (perfluoro-elastomer).  These or similar materials easily meet 90% of the remaining o-ring sealing requirements overall (after Buna-n) and are widely available through numerous sources, usually with feature-enhanced varieties as well as standard commercial grades.

As fluid and gas resistance is mostly determined by the chemical grouping of a compound, commonly published references are fairly reliable gauges of what can be expected in application, even if the information is general in nature.   The following resources will usually provide sufficient information on this factor, though testing is always necessary for absolute certainty:

Complete Chart (689 chemicals) of Chemical Compatibility Ratings for Common Commercial Elastomers – Useful cross-compound comparisons of 14 common rubber types by 689 chemicals with four-level compatibility ratings.

Buna-n Chemical Compatibility Ratings (Efunda) – Includes fewer chemicals than the above (around 350), but useful utility allows easy cross-referencing of chemicals by material and, conversely, materials by chemical.

Keep in mind that temperature ranges don’t normally mean that the rubber won’t degrade by continuous subjection to the high and low ends of the stated range; usually quite the contrary.  Some vendors provide standardized criteria that specifies physical property changes under defined time, temperature, and environmental conditions.

Physical properties vary greatly between material classes and the compounds within each.   When stated, most of the standard commercial varieties of o-rings are made of good quality elastomers and often accompany variations designed for enhanced properties and/or chemical compatibility, though always by special quote only.   In high pressure, dynamic friction, or compression applications, properties like tensile strength, elongation, compression resistance, tear resistance, and abrasion resistance can be relative to seal performance.  Harder or higher durometer variations, though not “better” per se, may also improve the durability of a seal, but may also decrease the effectiveness of it, as harder rubbers will not conform to surfaces to form a seal as readily under the same compression.

If the application merits spending more for special or higher grade compounds, one way of finding one might be through various standardized specifications that have been defined by military, aerospace, ASTM, ISO, or other recognized agencies.   Many of these have been formulated for specific and often demanding applications, and are available through many sources by special quotation.   Having such a specification reference could make obtaining competitive and easily comparable quotations much easier, as well as lend assurance of performance and longevity.

As far as getting general bearings on what chemical class of rubber is most likely to suit your application, the following chart should be helpful: