Cast Urethane (Disogrin®) O-Rings

Cast urethane is the mechanical champion of the seal world. Nothing else in this handbook comes close on abrasion resistance, tensile strength, or resistance to extrusion and shock loading. Where a nitrile O-Ring would wear away, tear, or squeeze into the gland gap and get nibbled off, a cast urethane O-Ring simply keeps sealing. That is why urethane owns high-pressure hydraulics.

The reason it is not in everything is the mirror image of that strength. Urethane is chemically narrow: superb in oil, terrible in hot water. It has a modest heat ceiling, the shortest shelf life of anything we stock, and a longer list of fluids that destroy it than of fluids it survives. So the framing for this whole page is simple: buy urethane when the seal is being physically destroyed, not chemically attacked.

Quick specs

Common names

Urethane collects names the way few materials do. All of these point at the same family:

• Polyurethane, urethane, PU, or PUR in general use
• AU, the ASTM D1418 designation for polyester urethane rubber
• EU, the ASTM D1418 designation for polyether urethane rubber
• TODI-based urethane, named for the diisocyanate used in the high-performance cast grades
• Disogrin®, the cast urethane brand we stock
• TPU, the thermoplastic cousin, which is a different processing class entirely

Mechanical properties

This is the section urethane exists for:

• The highest abrasion resistance of any common O-Ring elastomer. Not marginally better. In a different class.
• High tensile strength, which is what carries it through shock and impact loading
• High elasticity, so it takes the hit and comes back
• Excellent extrusion resistance, the property that lets it survive high pressure and wide gland gaps
• Good resistance to high-energy radiation
• Low gas permeability
• Resistant to oxygen and ozone, so weathering and shelf exposure are not the problem here (age control is, but for different reasons)

Limitations

This is the heart of the page. Read it before you buy.

• Hydrolysis is the killer. Hot water and water vapor degrade urethane: the polymer backbone comes apart and the part turns soft, sticky, or crumbly. Urethane rubbers are only partially resistant to aqueous media even at ambient, and in hot water and steam they are not resistant at all. This is the single most important thing to know about urethane and it is the number one urethane failure we see. Polyester (AU) grades hydrolyze faster than polyether (EU) grades, which is the whole reason the AU/EU distinction exists. For water and steam service, stop here and use EPDM.
• Not for acids, bases, or amines.
• Not for glycol ether-based brake fluids, polar solvents, chlorinated hydrocarbons, or aromatics. That list eliminates a large share of general industrial chemistry.
• Modest heat ceiling. It sits below nitrile in practical hot service and far below FKM (Viton®).
• The shortest shelf life we stock: 5 years per ARP5316, or 10 with inspection. Every other compound in the handbook gets more. See Storage and Shelf Life.
• Higher compression set than nitrile in long static service. Urethane is a mechanical material, not a set-and-forget gasket.

The honest positioning: buy urethane for abrasion, extrusion resistance, and shock. If the job is chemical, buy the compound that matches the fluid instead. We would rather sell you the right EPDM than the wrong urethane twice.

Chemical compatibility

Handles well:

• Mineral oils and greases, including hydraulic oil
• Water-oil mixtures
• Silicone oils and greases
• Oxygen and ozone
• High-energy radiation

Attacks it:

• Hot water and water vapor (hydrolysis, the primary failure mode)
• Aqueous media generally, where resistance is only partial
• Polar solvents
• Chlorinated hydrocarbons
• Aromatics
• Glycol ether-based brake fluids
• Acids and bases
• Amines

Notice how short the first list is and how long the second. That asymmetry is urethane’s personality: a specialist in petroleum-based mechanical service. Confirm your exact fluid on the Chemical Compatibility page or in the interactive Chemical Compatibility Chart before committing.

A word on temperature ranges

Our published range for the cast urethane grades we stock is -115°F to +250°F (-82°C to +121°C), and that is the headline number for this page. It deserves some honesty around the edges.

• The very low cold figure reflects the cast urethane grades we stock, not urethane as a class. Do not assume every urethane reaches it.
• Freudenberg publishes a much narrower -31°F to +230°F for urethane rubber generally. If you are working from a generic “urethane” callout rather than a known compound, that is the safer window to design to.
• The top end is limited by hydrolysis and softening, not by burning. Urethane does not char at its limit; it goes soft and, in the presence of moisture, it comes apart.
• Heat plus moisture is far worse than heat alone. A urethane seal at +200°F in dry hydraulic oil can live a long life. The same seal at +200°F in wet service can be finished in weeks. Treat temperature and the water question as one combined answer, not two.

Any published range assumes the compound, the media, the pressure, the hardware, and the duty cycle all cooperate. 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 Cast Urethane O-Rings

Every cast urethane grade and size we stock:

Inch (AS568):

• Cast Urethane 70 Duro (Disogrin® 6865, TODI)
• Cast Urethane 90 Duro (Disogrin® 9251, TODI)

Metric:

• Urethane Metric O-Rings

Need a durometer, a polyether grade, or a size outside the standard range? Ask us.

Variations and grades explained

Urethane rubber is made from polyols, isocyanates, and chain extenders. The polyol is the decision that matters most, because it determines which of the two families you land in.

Polyester (AU) versus polyether (EU)

This is the fork in the road:

EU rubbers are more resistant to hydrolysis than AU rubbers. If you remember one sentence from this page, make it that one. The cast Disogrin® grades here are polyester (AU) chemistry, which is exactly why we push you toward millable urethane (EU) or EPDM the moment water enters the conversation.

Cast versus millable processing

• Cast urethane (this page) is poured and cured from a liquid prepolymer. It reaches the highest hardness, tensile strength, and abrasion resistance in the family, which is why it dominates high-pressure hydraulic sealing.
• Millable urethane is processed on rubber equipment like a conventional elastomer. It is generally softer and easier to compound, and it is where you commonly find the polyether grades.

By hardness (durometer)

• 70 Shore A: the standard cast grade for general high-performance hydraulic sealing.
• 90 Shore A: harder, for the highest pressures and the greatest extrusion resistance.

One design point worth its own paragraph: urethane’s extrusion resistance is so good that a urethane O-Ring is often the alternative to adding a back-up ring to a softer compound. If you are fighting extrusion and nibbling, changing the material is sometimes cheaper than changing the gland.

TPU, the thermoplastic relative

Thermoplastic polyurethane (TPU) belongs to the thermoplastic elastomer (TPE) class rather than to vulcanized rubber. It is processed by melting rather than curing, and it is used for seals in pneumatics and hydraulics. Same chemical family, different manufacturing world. Outside sealing, the urethane family also shows up as gear wheels, rollers, and roller coverings, which tells you where its abrasion resistance gets used.

Disogrin®

Disogrin® is the cast urethane brand we stock, in TODI-cured 70 and 90 durometer, the polyester (AU) chemistry described above.

Standards and specifications

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

• ASTM D1418: designates the polymers as AU (polyester) and EU (polyether). Getting this letter right on a drawing is not pedantry; it is the hydrolysis decision.
• ASTM D2000 / SAE J200: the line-callout system most commercial urethane is bought to.
• ARP5316: sets the 5 year shelf life, extendable to 10 with inspection. See Storage and Shelf Life.
• No dedicated military O-Ring material spec exists for urethane. Unlike nitrile, FKM, silicone, and fluorosilicone, urethane 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 durometer, TODI cure, or Disogrin® 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 Cast Urethane the right choice?

Three questions settle it:

1. Is the seal being destroyed mechanically? Abrasion, wear, extrusion, tearing, shock. If yes, urethane is probably the answer and nothing else is close.
2. Is there water, steam, or humidity in the service? If yes, urethane is the wrong answer, or at minimum you need a polyether (EU) grade and a hard look at temperature. EPDM handles water and steam properly.
3. Is the fluid on the “attacks it” list? If yes, walk away and match the fluid first. See O-Ring Materials for the alternatives.

Urethane is a specialist. Inside its lane it is the toughest thing you can put in a gland. Outside it, it fails faster than the ordinary compound it replaced. We will tell you which one you have.

Not sure whether urethane survives your fluid and temperature? Send us the conditions: the media, the temperature, the pressure, whether there is any water in the picture, 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 and EU distinction and their relative hydrolysis resistance, the property set, 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. The -115°F to +250°F range reflects the cast urethane grades we stock. The datasheet for your specific compound always governs; test parts in your actual service conditions before committing to a critical seal.