O’ring Elastomer Application Guide

Other materials

XNBR: Carboxylated Nitrile

XNBR is a type of nitrile rubber (NBR) with a carboxylic acid group added to increase mechanical strength and temperature resistance. XNBR exhibits increased tear and abrasion resistance over standard nitrile and is commonly used in dynamic applications as u-cup seals and rod wipers in hydraulic cylinders. XNBR has a similar chemical resistance to NBR.

ACM, PA: Polyacrylate

Polyacrylate rubbers are often used in lieu of nitrile in high temperature applications (up to 177°C). ACM offers excellent oil, oxygen, ozone and weathering resistance compared to nitrile, but lack cold flexibility (-15°C) and water compatibility. ACM is often used for rotary shaft seals in automotive applications.

AEM, (Vamac): Ethylene Acrylate

A copolymer of ethylene and methyl acrylate with an added carboxylic group, AEM is often used as a higher grade alternative to nitrile (NBR) and is a cost effective alternative to HNBR and ACM in automotive applications, AEM offers resistance to automatic transmission fluid, ozone and oxidising media. Temp range: -29°C to 149°C

IRR (Butyl): Isobutylene Isoprene Rubber

Very good resistance to permeability, but seldom used in the seal industry nowadays as NBR provides better mechanical properties and ED resistant FKM (Viton) provides better options for temperature and chemical compatibility. Commonly used for tyre inner-tubes, gas masks, bladders for footballs and a binding agent in C-4 explosive.

CSPE, CSM (Hypalon): Chlorosulfonated Polyethylene

Known by its DuPont trade name Hypalon, CSPE offers good permeability resistance and excellent weathering and UV resistance. Has poor to fair compression set characteristics and is rarely used in the sealing industry. It is mainly found in use as a replacement for neoprene in the manufacture of inflatable watercraft, snowshoes, roofing materials and radar domes. Hypalon has been discontinued by DuPont but still available from alternative manufacturers.

SBR (Buna-S): Styrene-Butadiene Rubber

Developed as a replacement for natural rubber during World War II, SBR rubber is now one of the most commonly used rubbers worldwide. Uses range from paper coating, loudspeaker damping to chewing gum and building construction. The most common use is SBR blended with natural rubber, butadiene rubber and silica to produce a hard wearing, high friction rubber for automotive tyres and shoe soles. SBR is not commonly used in sealing due to poor weathering, aromatic and oil resistance.

Virgin PTFE (Teflon): Polytetrafluoroethylene

Known by its DuPont trade name Teflon. Noted for its broad chemical resistance, low friction coefficient and broad temperature range (upper limit continuous @ 260°C). But being a thermoplastic, it lacks elasticity of an elastomer which can make installation difficult, has very poor compression set characteristics and requires high clamping forces to achieve the same sealing efficiency as a more pliable elastomer.

FEP: Fluorinated Ethylene Propylene

Marketed by DuPont as Teflon FEP®, it differs from PTFE by being able to be injection moulded in the manufacturing process. FEP can match PTFE’s chemical resistance and friction coefficient with a broad temperature range between -270°C to 204°C (-454°F to 400°F). FEP o’rings are often called encapsulated o’rings (encaps) because of the two-piece structure of the o’ring; a transparent hollow FEP outer casing with an elastomeric core. This rubber core and FEP jacket gives the o’ring the resistance and friction coefficient of Teflon, but with some elasticity of a rubber o’ring.

PFA: Perfluoroalkoxy polymer resin

Often used for extended service in harsh environments, PFA o’rings are basically the same design and construction as encapsulated FEP o’rings, but with an elevated service temperature limit @ 260°C (500°F) and superior resistance to mechanical stresses, especially at high temperatures where FEP can fail.

Note: Encapsulated o’rings like the FEP and PFA above are commonly available with either, an FKM (Viton®), silicone or EPDM rubber core. The most common type having an FKM core. Care should be taken as some chemicals can permeate the outer jacket and come into contact with the elastomeric core, so chemical compatibility and temperature range of the core should also be considered.

Metal

Commonly soft alloys like copper, brass or nickel plated steel. For use in extremely harsh environments where temperature and pressures are beyond the capabilities of elastomeric seals. Sometimes used in forced induction race engines to seal engine head to the block; the circular section of the o’ring (sometimes triangular section) creating a high efficiency seal where a normal flat section head gasket alone, would fail.