Part 1 of this series discussed the various base oils that are used to create petroleum-based hydraulic fluids. This article will cover the two materials most commonly used in seal production and some of their variations. Part 3 will cover the interactions between hydraulic fluids and seals and how to make a preliminary prediction of compatibility.
Nitrile rubber (NBR) is probably the most commonly used material for hydraulic sealing applications. A copolymer of butadiene and acrylonitrile, NBR is relatively inexpensive and exhibits excellent resistance to petroleum-based hydraulic fluids. NBR is not resistant to weathering and ozone.
Temperature Range: -57°C to 100°C (-70°F to 212°F). NBR can be used at temperatures up to 121°C (250°F) but service life will be shortened.
Compatible Fluids: Petroleum-based hydrocarbons including propane, butane, petroleum oil, mineral oil and grease, diesel fuel, fuel oils; vegetable and mineral oils and greases; HFA, HFB and HFC hydraulic fluids; dilute acids, alkali and salt solutions at low temperatures; water and glycol-water fluids.
Incompatible Fluids: Highly aromatic or chlorinated hydrocarbons such as benzene and trichloroethylene; ketone, acetone, acetic acid, ethylene-ester and other polar solvents; strong acids; and glycol-based brake fluids.
Fluorocarbon Rubber (FPM/FKM)*
Under the DuPont trade name VITON®, fluorocarbon rubber is probably the second most common material used in seals. A compound of vinylidene Fluoride and hexafluoride propylene, Viton has much higher temperature capabilities than NBR and is compatible with most fluids. Viton is resistant to weathering and ozone.
Temperature Range: -46°C to 204°C (-50°F to 400°F). FPM / FKMan be used at higher temperatures but service life will be shortened.
Compatible Fluids: Petroleum-based hydrocarbons, chlorinated hydrocarbons, synthetic hydraulic fluids, fuels, including gasoline/alcohol fuels, aromatics; many organic solvents and chemicals
Incompatible Fluids: Glycol-based brake fluids; alkalis, amines, ammonia, formic and acetic acid; and, superheated steam.
Other materials that may be encountered in more specialized applications include:
- Highly Saturated (or Hydrogenated) Nitrile Rubber (HNBR)
- Carboxylated Nitrile Rubber (XNBR)
- Ethylene Propylene Rubber (EPDM/EPM)
- Silicone Rubbers (FMQ/FVMQ, PMQ, PVMQ, MQ and VMQ)
- Polyester and Polyether Urethane (AU and EU respectively)
- Polytetrafluoroethylene (PTFE)
- Perfluorinated Elastomer (FFKM)
- Polyacrylate ((ACM)
There are, of course, many other materials used in hydraulic sealing applications, as well as countless proprietary variations. Most seal manufacturers provide detailed information on material properties and application recommendations.
Given the range of variables present in any hydraulic system application, it is wise to seek professional guidance in selecting seal materials for either a new build, or maintenance replacement. It is particularly important to consider the likely impact of any change in hydraulic fluid on the seals already in use before proceeding with a replacement.
*When two abbreviations are given the first is DIN/ISO 1629 and the second ASTM D1418.
Viton is a registered trademark of the E.I du Pont de Nemours and Company