Butyl O-Rings

What butyl is, and where it is chosen

Butyl is a copolymer of isobutylene with a little isoprene, a saturated backbone with almost no unsaturation to attack. Gas molecules move through it far more slowly than through nitrile or EPDM, which is why it seals vacuum chambers, gas-holding equipment, tank vents and instrument housings that must not breathe. It also serves glycol brake fluids, phosphate-ester hydraulic fluids, ketones, some acids and bases, and outdoor service, since ozone and weather leave it alone. The Parker handbook puts the family at -59 C to 121 C, a cold limit most oil-resistant rubbers cannot reach.

Where butyl fails

Petroleum oils, greases, fuels and chlorinated solvents swell butyl badly, so it is never the seal for a hydraulic system on mineral oil or a fuel line. Its recovery is slower than nitrile's and its compression set is higher, which limits it in dynamic glands. Where an oil-resistant seal must also hold gas, nitrile or HNBR is the usual compromise; where the gas is the whole job and no oil is present, butyl is the usual fit. The compatibility guide carries the media families.

How to specify

Send the AS568 dash number or the inside diameter and cross-section, the durometer, the medium and the temperature, and say whether the seal holds vacuum or pressure. Name any ASTM D2000 line callout the print carries. The quote names the compound your print or your engineering specifies and sends its datasheet.

Sizes and forms we supply

All 369 AS568 sizes (349 general-purpose dash sizes plus the 20 boss-seal sizes) and the ISO 3601 metric series, in the durometers listed above; O-ring cord for rings spliced and vulcanized by the maker in odd diameters (static service only), where the compound is made as cord. The AS568 chart and the metric chart carry the dimensions, and the dash number guide explains the numbering.

Sources: Parker O-Ring Handbook ORD 5700, section 2.2 (read 2026-09-14) (§2.2.5); Apple Rubber, O-ring material selection guide (what the family is and where it is used, qualitative), read 2026-09-14; ASTM D1418; ASTM D2000 (documents named, not reproduced).

Parker is a trademark of Parker Hannifin Corporation. No affiliation or endorsement is implied.

Reviewed by Grant Midstokke, Conquest Seal Corporation. Last updated September 2026. Material ranges are the published references named beside them; the compound datasheet and your application govern the final choice.