Polyurethane O-rings stand up to tough conditions

Aug. 3, 2000
Designs that call for rugged, durable seals might benefit from polyurethane O-rings.

Designs that call for rugged, durable seals might benefit from polyurethane O-rings. That's because polyurethane generally outperforms other elastomeric sealing materials in terms of toughness, abrasion resistance, and extrusion resistance.

Polyurethane O-rings handle pressures to 5,000 psi even with a 0.010-in. extrusion gap. The material also resists explosive decompression and handles temperatures from -90 to 225°F. Polyurethane O-rings are used in a variety of products, including quick-disconnect hydraulic fittings, hydraulic cylinders and valves, pneumatic tools, and CO2 firearms.

Designers have many options when selecting polyurethane O-rings for a given application. Material properties stem from polyol and diisocyanate in the compound and the molding method used to form the seal. Design choices in each of these areas determine the characteristics of the finished O-ring. Here's a quick look at some options.

Nearly 200 polyols are available, but the most common types are polyester, polyether, and polycaprolactone. Each has its own unique advantages. Polyester polyols resist tearing, abrasion, oil, and long-term heat, but they break down when exposed to water. Poly-ether polyols have good water resistance, but their mechanical and heat properties suffer. Polycaprolactone polyols are mechanically better than polyethers and better in water than polyesters.

There are also three diisocyanates commonly used in poly-urethane O-rings, namely MDI, TODI, and PPDI. MDI is environ-mentally safer than other diisocyanates and is most widely used in millable-gum polyurethanes. TODI resists compression set and temperatures to 250°F, but is relatively expensive and hard to find. PPDI has good dynamic properties and resilience and resists temperatures to 275°F, but the material is also expensive and in short supply.

Polyurethane is typically processed as either a compression-molded millable gum, a compression-molded cast thermoset, or an injection-molded thermoplastic. Millable-gum O-rings offer good flexibility at low temperatures and resist sunlight, ozone, hydrocarbon fuels, and petroleum-based oils. Specifically, thermoset-compounds 9272 and 9292 combine polycaprolactone and TODI to provide exceptional resistance to heat, compression set, and water. And although injection-molded polyurethane O-rings have historically offered relatively unfavorable properties, PPDI-based compound 9800 provides a solution. The O-rings combine good dynamic properties, resilience, and resistance to heat and compression set.

This information supplied by R. L. Hudson & Co., Tulsa, Okla.

Sponsored Recommendations

The entire spectrum of drive technology

June 5, 2024
Read exciting stories about all aspects of maxon drive technology in our magazine.


May 15, 2024
Production equipment is expensive and needs to be protected against input abnormalities such as voltage, current, frequency, and phase to stay online and in operation for the ...

Solenoid Valve Mechanics: Understanding Force Balance Equations

May 13, 2024
When evaluating a solenoid valve for a particular application, it is important to ensure that the valve can both remain in state and transition between its de-energized and fully...

Solenoid Valve Basics: What They Are, What They Do, and How They Work

May 13, 2024
A solenoid valve is an electromechanical device used to control the flow of a liquid or gas. It is comprised of two features: a solenoid and a valve. The solenoid is an electric...

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!