Butyl (IIR) O-Rings

Butyl is a copolymer of isobutylene with a small proportion of isoprene. That “small proportion” is the whole story. Isoprene is what puts double bonds in the main chain, and double bonds are what ozone, oxygen, and heat attack. Butyl has very few of them, so it weathers and ages well for the same structural reason HNBR does: there is less on the backbone for ozone to grab.

But nobody buys Butyl for its ozone resistance. They buy it for one number.

Butyl has the lowest gas and water vapor permeability of the common seal elastomers. Gas does not diffuse through it the way it wanders through nitrile or silicone. That single property is the entire reason to choose this material, and it is why Butyl holds the tire inner tube, the vacuum flange, and the pharmaceutical stopper. If gas containment is not your problem, Butyl is probably not your answer. If it is, nothing common does it better.

Its second job is chemical: it handles glycol brake fluids, phosphate esters, acids, bases, and steam much like EPDM does, and it fails on the same things EPDM fails on.

Quick specs

Common names

All of these mean the same family:

• IIR, the ASTM D1418 designation for butyl rubber
• Isobutylene isoprene rubber, the mouthful the letters stand for
• Butyl or butyl rubber, what everyone actually says
• CIIR / chlorobutyl and BIIR / bromobutyl, the halogenated versions, made by a later reaction of the base polymer with elemental chlorine or bromine

Mechanical properties

• The lowest gas and water vapor permeability of the common seal elastomers. This is the property that sells the material.
• Excellent resistance to weathering, ageing, and ozone, a direct result of the low proportion of double bonds in the main chain.
• Very good low-temperature flexibility, serviceable well below where many rubbers stiffen.
• High damping and slow elastic recovery. Butyl absorbs energy rather than springing back: useful in vibration mounts, unhelpful in a fast dynamic seal.
• Good resistance to hot water and steam.
• Media compatibility comparable to that of tire rubbers, which is fair shorthand. It lives where natural rubber lives, not where the oil-resistant rubbers live.

Limitations

The honest list. Butyl is a specialist, and it is worth knowing exactly where it stops:

• Never petroleum oils, fuels, greases, or aromatic hydrocarbons. It swells the same way EPDM swells, and a swollen O-Ring is a failed O-Ring. Use Nitrile (Buna-N) or FKM (Viton®) instead, and see chemical swell or shrink for what that failure looks like.
• Poor resistance to chlorinated hydrocarbons. This is not a marginal call. Go FKM.
• Slow elastic recovery and higher damping than most seal rubbers. Butyl is not a fast dynamic seal. In a reciprocating or rotary gland it lags, heats, and wears. Keep it static.
• Modest heat ceiling. It runs out well below FKM at +400°F or Silicone at +450°F. If your job is hot, Butyl is not in the conversation.
• Niche availability. Butyl is not stocked in the size and durometer depth of nitrile. We carry the common inch sizes and quote many of the rest to order. Plan lead time.
• For most water, steam, and brake fluid jobs, EPDM is the more available and usually cheaper answer. We would rather tell you that than sell you a quoted-to-order Butyl ring that does the same work. Butyl earns its keep when gas permeability is the deciding factor. That is the line.

Chemical compatibility

Handles well:

• Gases and air, where its low permeability is the point
• Vacuum service (see Special Applications)
• Glycol brake fluids (DOT 3, 4, 5.1)
• Phosphate-ester hydraulic fluids
• Acids and bases, including many at concentration
• Hot water and steam
• Ketones, esters, and alcohols
• Ozone, weather, sunlight, and long-term ageing
• Silicone fluids and greases

Attacks it:

• Petroleum oils, greases, fuels, and hydraulic oils
• Aromatic hydrocarbons (benzene, toluene, xylene)
• Chlorinated hydrocarbons

The pattern is the same one EPDM follows, and Freudenberg describes Butyl’s media compatibility as comparable to that of tire rubbers. Remember “oil kills it, gas cannot escape it” and you have most of Butyl. Confirm your exact fluid on the Chemical Compatibility page or in the interactive Chemical Compatibility Chart.

A word on temperature ranges

Our published range for Butyl is -75°F to +250°F (-59°C to +121°C), and that is what we stand behind for the compounds we sell.

You will see other numbers. Freudenberg publishes -40°F to +266°F for its butyl. The spread is two different questions being answered, not a contradiction. The cold end depends heavily on the compound: Butyl has genuinely good low-temperature flexibility, and a seal compound formulated for it reaches well below a conservative general-purpose figure. The hot end depends on how long you expect the seal to live there. The +266°F reflects short-term capability in cooperative media; +250°F is the realistic continuous number.

The halogenated grades take slightly more heat than plain IIR, along with slightly better setting behavior and fatigue resistance. If you are sitting at the top of the range, chlorobutyl or bromobutyl is the version to ask for.

Any published range assumes the compound, the media, the pressure, the hardware, and the duty cycle all cooperate. Heat plus an aggressive fluid shortens life faster than heat alone. If your application sits near an edge, send us the conditions and we will pull the datasheet for the actual compound rather than guess from a chart.

Shop Butyl O-Rings

Inch (AS568):

• Butyl Rubber (IIR) Compounds

For metric sizes, metric Butyl is available on request. Just contact us.

Butyl is more a made-to-order world than a shelf-stock one. If the size or durometer you need is not listed, ask. We quote it.

Variations and grades explained

By polymer: plain, chlorinated, brominated

All three keep the low-permeability character that defines the family. The halogenated types are chosen when you need a little more heat, a little better set recovery, or the cure chemistry they enable. They are what tubeless tire inner layers, tire-vulcanizing heating bellows, and pharmaceutical closures are made from.

By hardness (durometer)

• 70 Shore A: the standard, and what we stock. It covers almost every Butyl job.
• Harder compounds resist extrusion through the gland gap at higher pressure. Softer compounds seal better on rough or low-pressure flanges. Both are quoted to order in Butyl.

Pharmaceutical grades

Stoppers and closures are a classic butyl application, and not by accident: low permeability is exactly what a drug vial needs. Pharmaceutical grades are formulated and documented for that service. If your job needs the paperwork, see USP Class VI and FDA food contact, and tell us what the drawing calls for.

Standards and specifications

Butyl O-Rings are supplied in the AS568 inch size range and in common metric sizes on request. The callouts that matter:

• ASTM D1418: designates the polymers as IIR, CIIR, and BIIR.
• ASTM D2000 / SAE J200: the line-callout system Butyl is actually bought to. This is the spec to put on your drawing.

There is no dedicated military O-Ring material spec for Butyl. Worth stating plainly, because it gets cited wrong. MIL-G-21569 is sometimes called a butyl spec. It is not. It is a Navy specification for diesel engine cylinder-liner seal gaskets, and its material classes are Class 1 nitrile and Class 2 silicone. Butyl does not appear in it. If a drawing hands you MIL-G-21569 and says butyl, somebody copied it from the wrong place and it is worth a phone call before you buy. For the compounds that do have an M-number part standard, see the mil-spec reference.

If your drawing calls out a specific spec or compound, tell us and we will match it, with a Certificate of Conformance and lot traceability under our ISO 9001:2015 system.

Typical applications by industry

See O-Rings by Industry for the wider picture, or all compounds to compare against the rest of the shelf.

Is Butyl the right choice?

Three questions settle it:

1. Is gas or vapor permeation your actual problem? If gas diffusing through the rubber is what costs you (vacuum base pressure, gas loss, vial contents), Butyl is the answer and there is not a close second among common elastomers.
2. Is there any petroleum oil, fuel, or aromatic in the system? If yes, stop. Go Nitrile or FKM. Butyl will swell.
3. Is the seal fast and dynamic? Butyl’s slow recovery makes it a poor choice. Look at HNBR or FKM depending on the fluid.

Then the fourth question we will ask even though you did not: is this really a permeability job, or is it a water and brake fluid job? If it is the latter, EPDM does the same chemistry, is stocked deeper, ships faster, and costs less. Butyl is worth its lead time when the gas number is the one that matters.

Not sure whether Butyl is doing something EPDM could do for less? Send us the conditions: the media, the temperature, the pressure, whether it is vacuum, and whether the seal moves. We will spec the compound and the size with you. We make getting O-Rings easy.

Polymer chemistry, the halogenated CIIR and BIIR types, the low main-chain double bond count behind the ozone and ageing resistance, the low gas and water vapor permeability, media compatibility comparable to tire rubbers, the -40°F to +266°F thermal application range, and the listed applications: Freudenberg FST 2025 Technical Manual, ch. 2 (Materials and Process Engineering), §3.4, p. 53. Our published -75°F to +250°F range reflects the commercial seal compounds we supply. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.