Millable Urethane (EU) O-Rings

Millable urethane is urethane’s toughness in a compound that is mixed, milled, and molded like ordinary rubber instead of poured as a liquid. That one processing difference is the whole story. You keep the two properties urethane is famous for, abrasion resistance and tensile strength, in a part that behaves like a normal O-Ring on the shop floor: standard AS568 sizes, standard durometers, standard grooves, no casting shop.

The trade is heat. Millable urethane tops out at +180°F where cast urethane reaches +250°F, and that ceiling is why this material lives where it lives: air guns, paintball markers, and pneumatics, where a seal takes rapid, repeated impact and abrasion at moderate temperature and has to hold gas while doing it. In that market it is not a compromise. It is the known standard.

Quick specs

Common names

Urethane naming is a mess: the same word covers several different materials. All of these mean this page’s material:

• EU, the ASTM D1418 designation for polyether urethane rubber
• Millable urethane, millable polyurethane, or millable PU
• Simply “urethane” on most air gun and paintball parts lists
• Not to be confused with AU (polyester urethane), cast urethane (Disogrin), or TPU, the thermoplastic version

Urethane rubber is built from polyols, isocyanates, and chain extenders, and the polyol splits the family in two: polyester urethane (AU) and polyether urethane (EU). That distinction drives most of what follows.

Mechanical properties

• High abrasion resistance. The headline property, and the reason it survives impact and reciprocating service that chews up general-purpose rubber.
• High tensile strength, well above nitrile or EPDM at the same durometer.
• High elasticity, so it takes a hit and returns rather than tearing.
• Low gas permeability. Worth featuring: in an air gun or a pneumatic valve the seal is holding a gas column, and a low-permeability polymer holds it better.
• Good resistance to high-energy radiation.
• Resistant to oxygen and ozone.
• Better hydrolysis resistance than polyester (AU) grades, because the polyether backbone is less vulnerable to water attack. A real advantage of EU over the polyester cast grades, but a difference in degree, not a license for water service. See the limitations below.

Limitations

The honest list. Millable urethane is a specialist, and it is easy to put in the wrong hole:

• Narrowest temperature range of anything we stock. -30°F to +180°F. That is the price of the toughness. This is not a hot-service material, and no grade of it is.
• Hot water and steam cause hydrolysis. The polyether (EU) chemistry resists it better than polyester (AU) cast grades, but urethane rubbers are only partially resistant to aqueous media generally, and are not resistant in hot water or water vapor. For water and steam, use EPDM.
• Not for acids, bases, or amines, and not for polar solvents, chlorinated hydrocarbons, or aromatics.
• Not for glycol ether-based brake fluids. Use EPDM.
• Short shelf life: 5 years per ARP5316, the shortest class we stock. Plan stocking around it. See storage and shelf life.
• Higher compression set than nitrile in long static service. On a seal that sits bolted and unmoving for years, urethane’s strength buys nothing and its set works against you.

Positioning it honestly: for high-pressure hydraulics and the toughest abrasion, cast urethane is stronger and takes more heat. For ordinary oil at ordinary temperature, nitrile is cheaper and more available. Millable urethane is for impact and abrasion at moderate temperature, and for the air gun and paintball market, where it is what the guns were designed around.

Chemical compatibility

Handles well:

• Mineral oils and greases
• Water-oil mixtures
• Silicone oils and greases
• Oxygen and ozone
• Air and compressed gases, helped by its low permeability
• High-energy radiation
• Cold and ambient water, better than polyester (AU) grades, though only partially resistant

Attacks it:

• Hot water and water vapor (hydrolysis)
• Polar solvents
• Chlorinated hydrocarbons and aromatics
• Glycol ether-based brake fluids
• Acids and bases
• Amines

Urethane’s chemical personality is narrow but well defined: an oil-and-air material that does not like water when hot and does not like aggressive chemistry at all. 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 this grade is -30°F to +180°F (-34°C to +82°C), and it is the narrowest window of any compound we stock. Some context around that number:

• The top end is limited by two things at once: the polymer softens as it heats, and hydrolysis accelerates in any moisture present. Heat and moisture together are far worse than either alone.
• Freudenberg publishes -31°F to +230°F for urethane rubber generally, wider than our figure and describing the family rather than this millable grade. We publish the conservative number because it is the one we will stand behind on a part we ship.
• If you need more heat, cast urethane reaches +250°F while keeping most of the strength story.

Any published range assumes the compound, the media, the pressure, the hardware, and the duty cycle all cooperate. A seal cycling in dry air at 150°F and one sitting in warm damp air at 150°F are not the same job. If your application sits near an edge, send us the conditions.

Shop Millable Urethane O-Rings

Every Millable Urethane variation we stock, by grade and size:

Inch (AS568):

• Millable Urethane EU 70 Duro
• Millable Urethane EU 90 Duro

Metric:

• Urethane Metric O-Rings

Need a size, durometer, or color that is not listed? Ask us.

Variations and grades explained

Urethane is a family, and the members are genuinely different materials.

EU (polyether) versus AU (polyester)

The polyol decides it:

Neither is a water-service material. But if there is ambient moisture in play, EU is the better bet. See cast urethane for the AU side of the family.

Millable versus cast processing

• Millable urethane is compounded on a mill and press cured like conventional rubber: normal molded O-Rings, normal tooling, normal AS568 sizes, normal lead times. Best for impact, abrasion, and pneumatics at moderate temperature.
• Cast urethane is liquid processed, reaching higher strength, better extrusion resistance, and +250°F. Best for high-pressure hydraulics and the most abrasive service. See cast urethane (Disogrin).

By hardness (durometer)

• 70 Shore A: the general-purpose choice, and the one most air gun and pneumatic seals use. Easier to install, seals better at low pressure.
• 90 Shore A: firmer, for higher pressure and to resist extrusion through the gland gap. Urethane at 90 duro is stiffer than most rubbers at the same reading, so check your groove design and stretch.

TPU, the thermoplastic relative

Thermoplastic polyurethane (TPU) is a thermoplastic elastomer rather than a cured rubber, widely used for seals in pneumatics and hydraulics and processed by injection molding rather than milling and curing. Same chemical family, different manufacturing world.

Standards and specifications

Millable Urethane O-Rings are supplied across the full AS568 inch size range and common metric sizes. Useful callouts:

• ASTM D1418: designates the polymer as EU (polyether urethane) or AU (polyester urethane).
• ASTM D2000 / SAE J200: the line-callout system most commercial urethane is bought to.
• ARP5316: the shelf-life standard. Urethane is a 5 year class, the shortest we stock. See storage and shelf life.

No dedicated military O-Ring material spec exists for urethane. Unlike nitrile, FKM, silicone, fluorosilicone, and EPR, it has no M-number part standard of its own, so it is bought to ASTM D2000 or to a customer drawing. See the mil-spec reference for the compounds that do have one.

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 Design & Engineering for the gland around the seal.

Is Millable Urethane the right choice?

Three questions settle it:

1. Does the seal take impact or abrasion? That is what this material is for. Nothing general-purpose survives it as long.
2. Does it stay under +180°F and out of hot water, steam, acids, and solvents? If yes, the narrow window is not a problem. If no, it is a dealbreaker, not a detail.
3. Is the media air, oil, or grease? Low gas permeability and oil resistance make urethane a natural. Aggressive chemistry sends you elsewhere.

If the first answer is no and the fluid is ordinary oil at ordinary temperature, Buna-N does the job for less money and ships from more shelves. If you need more pressure or more heat, cast urethane is the stronger member of the family. We will tell you that rather than sell you the wrong one. Browse the full materials guide to compare.

Not sure whether millable urethane covers your fluid and temperature? Send us the conditions: the media, the temperature, the pressure, and whether the seal moves. We will spec the compound and the size with you. We make getting O-Rings easy.

Urethane polymer chemistry, the AU/EU polyol distinction and its hydrolysis behavior, property set including low gas permeability, media resistance, and the -31°F to +230°F range: Freudenberg FST 2025 Technical Manual, ch. 2 (Materials and Process Engineering), §3.4, p. 61. Our published -30°F to +180°F range reflects the millable grade we stock. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.