Butyl (IIR) O-Rings
The lowest gas permeability of any elastomer — ideal for vacuum seals, gas-tight joints and brake fluid systems.
Overview
Butyl Rubber (IIR — Isobutylene Isoprene Rubber) is a synthetic elastomer prized for one property above all others: extremely low gas permeability. IIR transmits gases at approximately 1/10th to 1/20th the rate of natural rubber and 1/5th the rate of EPDM, making it the material of choice for vacuum seals, gas-tight enclosures, tire inner tubes, and pneumatic accumulator bladders.
Butyl O-rings operate from -40°C to +120°C and exhibit excellent resistance to polar solvents, glycol-based brake fluids (DOT 3, DOT 4, DOT 5.1), phosphate ester hydraulic fluids (Skydrol), and dilute acids and alkalis. This chemical profile is almost the inverse of NBR: IIR resists what NBR cannot, and fails where NBR excels.
The low gas permeability of IIR originates from its molecular structure: polyisobutylene is a saturated polymer with methyl groups packed densely along the backbone. This structure creates a tortuous diffusion path for gas molecules that is far more restrictive than the open chains of natural rubber, NBR, or silicone. Helium permeability of IIR is approximately 10–20 × 10⁻⁸ cm³·cm/cm²·s·atm — an order of magnitude lower than EPDM and two orders of magnitude lower than VMQ.
Critical limitation: IIR is incompatible with mineral oils, petroleum fuels, and aliphatic hydrocarbons. Contact with gasoline, diesel, or mineral oil causes rapid and severe swelling (100–300% volume increase). IIR must never be used in fuel or standard hydraulic oil systems. For applications requiring both low gas permeability and oil resistance, specify halobutyl (BIIR / CIIR) grades or consider FKM for moderate permeation resistance.
Lead time: 10–20 days; 3–5 days for stocked sizes. MOQ: 1 piece. ISO 9001 certified.
Material Properties
| Temperature Range | -40°C to +120°C (-40°F to +248°F); short-term to +150°C |
| Hardness Range | 40–80 Shore A (ASTM D2240) |
| Tensile Strength | 7–17 MPa (ASTM D412) |
| Elongation at Break | 300–700% (ASTM D412) |
| Gas Permeability (Helium) | 10–20 × 10⁻⁸ cm³·cm/cm²·s·atm — lowest of any elastomer |
| Compression Set (70 h/100°C) | 20–35% (ASTM D395 Method B) |
| Specific Gravity | 0.91–0.93 (lighter than water) |
| Color (standard) | Black or gray |
| Standards | ASTM D2000 BA, DIN ISO 1629 IIR, SAE AMS3208 |
Typical Applications
Vacuum Systems
Vacuum chamber seals, vacuum line couplings and pump seals where low outgassing and minimal gas permeation are required for achieving and maintaining high vacuum.
Automotive Braking
Master cylinder seals, wheel cylinder seals and brake system O-rings in DOT 3, DOT 4 and DOT 5.1 glycol-ether brake fluid systems.
Pneumatics
Accumulator bladders, air spring seals and pneumatic system static seals where air retention over long periods is critical.
Chemical Processing
Seals in dilute acid and alkali handling, polar solvent systems, and phosphate ester hydraulic fluid (Skydrol) lines.
Pharmaceutical
Gas-tight vial seals, sterile packaging seals and containment seals where gas barrier properties prevent oxidation or moisture ingress.
Chemical Compatibility Summary
Compatible With
- - Glycol-based brake fluids
- - Phosphate ester hydraulic fluids (Skydrol)
- - Polar solvents
- - Dilute acids and alkalis
- - Water and steam
Incompatible With
- - Mineral oils and hydrocarbon fuels
- - Gasoline and diesel
- - Aromatic hydrocarbons
- - Chlorinated solvents
- - Ketones
Key Advantages
Lowest Gas Permeability of Any Elastomer
IIR has helium permeability 5–20× lower than EPDM and 50–100× lower than VMQ silicone. This makes butyl O-rings the standard for vacuum seals, gas-tight enclosures, and any application where gas loss or ingress must be minimized. No other elastomer provides comparable gas barrier performance at comparable cost.
Excellent Brake Fluid and Polar Solvent Resistance
IIR resists glycol-based brake fluids (DOT 3/4/5.1), phosphate ester hydraulic fluids (Skydrol), and polar solvents including ketones and alcohols. ASTM D471 volume swell in glycol brake fluid is typically <5% — well within acceptable limits for brake system seals. This is the inverse of NBR, which swells severely in glycol fluids.
Good Weather and Ozone Resistance
The saturated polyisobutylene backbone provides excellent resistance to ozone cracking and UV degradation. Butyl O-rings tolerate outdoor exposure without the surface cracking that destroys natural rubber and standard NBR. Service life in outdoor pneumatic applications exceeds that of NBR by 3–5×.
Excellent Vibration Damping
IIR's high internal hysteresis makes it an excellent vibration isolator and shock absorber. Butyl O-rings used as machinery mounts and vibration isolators absorb energy more effectively than NBR, EPDM, or FKM — reducing transmitted vibration by 20–40% in equivalent configurations.
Good Acid and Base Resistance
IIR tolerates dilute sulfuric acid, hydrochloric acid, sodium hydroxide, and most inorganic salt solutions. It is not suitable for concentrated oxidizing acids (concentrated HNO₃, H₂SO₄) or strong halogenating agents, but handles the dilute acids and alkalis common in water treatment and chemical processing.
Lightweight and Buoyant
Specific gravity of 0.91–0.93 means IIR floats in water — the lightest common O-ring material. This is advantageous in marine and underwater applications where seal weight affects buoyancy, and in pneumatic systems where low mass reduces inertial effects during rapid valve cycling.
Frequently Asked Questions - Butyl
What makes butyl rubber ideal for vacuum sealing?
Butyl rubber (IIR) has the lowest gas permeability of any commercial elastomer — approximately 1/10th to 1/20th that of natural rubber and 1/5th that of EPDM. In vacuum systems, every elastomer allows some atmospheric gas to permeate through its bulk. IIR minimizes this permeation rate, allowing the vacuum pump to achieve lower base pressures and maintain them with less pumping capacity. For high vacuum (10⁻⁶ torr), IIR is suitable for roughing lines and chamber seals; for ultra-high vacuum below 10⁻⁸ torr, FFKM or metal seals are required.
Can I use butyl O-rings with mineral oil or gasoline?
No. IIR is severely incompatible with petroleum-based oils, fuels, and aliphatic hydrocarbons. Contact with gasoline or mineral oil causes volume swell of 100–300% within hours, destroying the seal. IIR must only be used in systems where the fluid is water, glycol, polar solvents, or phosphate esters. For fuel or oil service, specify NBR (to +120°C), HNBR (to +150°C), or FKM (to +200°C).
What is the difference between butyl (IIR) and EPDM?
Both IIR and EPDM are saturated elastomers with excellent ozone and weather resistance. The key differences: (1) Gas permeability — IIR is 5× lower than EPDM, making IIR superior for vacuum and gas-tight seals. (2) Brake fluid resistance — both resist glycol brake fluids, but IIR has lower swell and better long-term stability. (3) Oil resistance — both are poor in petroleum oil, but EPDM is slightly worse. (4) Temperature — EPDM extends to +150°C; IIR is limited to +120°C. (5) Cost — IIR is typically 20–30% more expensive than EPDM. Choose IIR for vacuum, brake fluid, and maximum gas retention; choose EPDM for hot water, steam, and outdoor static seals above +120°C.
Are butyl O-rings suitable for brake fluid systems?
Yes. IIR is one of the best elastomers for glycol-ether brake fluids (DOT 3, DOT 4, DOT 5.1). Volume swell is typically <5%, and compression set remains acceptable after years of service. IIR has been used in automotive master cylinder and wheel cylinder seals for decades. However, IIR is not compatible with silicone brake fluid (DOT 5) — for DOT 5, specify EPDM or VMQ.
What is halobutyl (BIIR / CIIR) and when is it used?
Halobutyl is butyl rubber modified by adding bromine (BIIR) or chlorine (CIIR) to the polymer chain. This modification enables sulfur or peroxide curing and improves adhesion to other elastomers and metals. Halobutyl retains IIR's low gas permeability while offering better cure versatility and co-vulcanization with other rubbers. For O-ring applications, standard IIR is usually sufficient; halobutyl is specified when the seal must be bonded to metal inserts or co-molded with other elastomers.
What hardness should I specify for butyl O-rings?
For general-purpose static seals in vacuum and brake fluid, 60–70 Shore A is standard — providing good conformability and sealing force without excessive compression set. For higher-pressure applications (up to 15 bar) or larger clearance gaps, 80 Shore A improves extrusion resistance. 40–50 Shore A is available for low-contact-force seals and vibration damping mounts. Avoid using IIR above 15 bar without backup rings — its moderate tensile strength limits high-pressure performance.
How does butyl compare to FKM for vacuum applications?
For rough and medium vacuum (10⁻³ to 10⁻⁵ torr), IIR is a cost-effective alternative to FKM with lower gas permeability. FKM has better outgassing characteristics and tolerates higher bake-out temperatures (+200°C vs +120°C for IIR). For high vacuum (10⁻⁶ to 10⁻⁸ torr), FKM is preferred due to lower outgassing and wider temperature range. For ultra-high vacuum below 10⁻⁸ torr, neither IIR nor FKM is suitable — use FFKM or metal seals. Choose IIR for budget-conscious vacuum systems operating at ambient temperature; choose FKM for high-vacuum, high-temperature, or chemically aggressive vacuum environments.
Do butyl O-rings meet automotive or brake system certifications?
Yes. IIR compounds are available meeting SAE J200 specifications for brake system elastomers, FMVSS 116 brake fluid compatibility requirements, and various OEM brake system material specifications (Ford, GM, VW). Compound-specific testing includes brake fluid immersion (SAE J1703 / FMVSS 116), compression set, hardness change, and tensile retention after aging. For critical brake applications, request a compound with specific OEM approval and full material certification.
What is the shelf life of butyl O-rings?
When stored in original packaging away from direct sunlight, ozone, and heat sources above +25°C, IIR O-rings maintain properties for 5–7 years. The saturated backbone resists oxidative aging better than natural rubber or NBR. Halobutyl grades (BIIR/CIIR) may have slightly shorter shelf life due to the reactive halogen groups. Store per SAE ARP 5316: sealed opaque bags, 15–25°C, <75% relative humidity, no ozone-generating electrical equipment nearby.
Can butyl O-rings be used in dynamic applications?
IIR has moderate abrasion resistance and tear strength — better than VMQ silicone but worse than NBR and PU. It can be used in low-speed, low-pressure dynamic applications such as slow-rotating valve stems and low-cycle pneumatic actuators. For high-speed reciprocating or rotary dynamic service, specify NBR (general purpose), HNBR (fuel/oil), or PU (high abrasion). IIR's primary advantage is static sealing in vacuum, brake fluid, and gas-tight applications — not dynamic motion.