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Elastomeric Gaskets for KF, ISO & ASA

O-rings are common seals used in many manufacturing industries. With such a broad range of materials available, it is important to select the appropriate material for any given project.

Elastomeric gaskets, often o-rings of circular cross-section, are convenient, easy-to-use vacuum seals. With flange surface finishes of 32 rms or better, compressing the o-ring by 25% to 35% will form a vacuum seal that should be easily compatible with 10-7 Torr or mbar and, with better design/preparation, perhaps 10-9 Torr or mbar

To give you a clear insight into the advantages and disadvantages of common O-ring materials we have explained some of the most common properties of o-rings.

Color

Rubbers are usually formulated with the addition of various fillers including the most common which is carbon black fillers. Hence, most compounds are black in color. Rubbers gain much of their strength and heat resistance from the addition of fillers which enhance the performance of the o-rings. The standardized color of each elastomer o-ring type offered by the Kurt J. Lesker Co. is noted below. Please note that the color of the compound is subject to change without notice based on availability.

Hardness

The term hardness is the measure of a material's resistance to a set deforming force exerted by a given standard indentation implement over a defined length of time. Hardness is measured in Shore Durometer. Standard compounds Durometer are nominally 70.

Compression Set

Compression set is the permanent deformation an o-ring exhibits when released from a flange seal. Sealing force retention is a measure of the elastomer's resilience when held under a constant compressive force.

When an elastomeric material fails to return to its original size after release from a constant compressive load, the amount of material that remains compressed is called a compression set.

Gas Permeability

Elastomers allow gas to enter the structure. They will diffuse or permeate through and escape via the low-pressure side.

The rate of permeation is governed by temperature, pressure, gas type and elastomer type. This may be critical for vacuum and gas containment.

The drawbacks are the elastomer's gas permeability, outgassing rate, and limited temperature compatibility. At elevated temperatures (but well below those causing pyrolysis), elastomeric gaskets have greatly increased permeation rates and have their sealing characteristics slowly compromised by compression set and lack of sealing force retention.

The elastomeric materials most commonly used in vacuum seals are Buna-N (Nitrile-NBR), Fluorocarbon (FKM), Perfluorocarbon (FFKM), Ethylene Propylene (EPDM), and Silicon Rubber (Si). The specific properties depend on the compounding, the inert materials mixed with the pure elastomer to give it acceptable mechanical properties, and the cross-linking agent used to "cure" it.

The general properties of o-rings made from these materials are:

Buna-N

  • Grade: 4900 - 70 Nitrile
  • Color: Black
  • Durometer: 70
  • Temperature Range: -35°C to 210°C
  • Buna-N (Nitrile) O-rings are a popular type of rubber seal gasket that's used in a wide range of applications and environments - they're broadly seen as a general-purpose O-ring type, with reasonably resistant sealing properties against water, gasoline, petroleum oils, crude oil, propane and some hydraulic fluids.
  • Least expensive of all standard o-rings for vacuum service.
  • Permeation rate for water vapor ~20 times that of Fluorocarbon.
  • Its chemical properties roughly match natural rubber and is just as readily attacked by ozone.
  • Must not be used in any O2 plasma process.
  • Shelf-life is 15 years, but shorter if stored, or used, near an ozone source such as an electric motor with brushes.

Fluorocarbon (FKM)

  • Grade:9505-75 Fluorocarbon
  • Color:Brown
  • Durometer:75
  • Temperature Range:-26°C to 210°C
  • Often referred to as Viton. Viton is a brand name which is a registered trademark of DuPont Performance Elastomers L.L.C.
  • Fluorocarbon (FKM) is the most used elastomer in vacuum service.
  • Fluorocarbon (FKM) has excellent resistance to high temperatures, ozone, oxygen, mineral oil, synthetic hydraulic fluids, fuels, aromatics and many organic solvents and chemicals. Low temperature resistance is normally not favorable and for static applications is limited to approximately -26°C.
  • Reasonably low gas permeability (except for helium, which is double that of Buna-N).
  • Good performance at high temperatures. The upper temperature limit depends on the compounding with 180°C to 210°C being cited. DuPont Dow Elastomers tests on Fluorocarbon indicate a compression set of ~50%, and a sealing force retention of ~80% after 70 hours at 204°C.

Perfluoroelastomer (FFKM)

  • Grade: Kalrez® Grade 4079
  • Color: Black
  • Durometer: 75
  • Temperature Range: -19°C to 316°C
  • Kalrez® 4079, which is a low compression set compound for use in many dry and wet semiconductor process environments. It has excellent chemical resistance, good mechanical properties and outstanding thermal stability.
  • Kalrez® 4079 exhibits low weight loss in reactive plasmas and has good response to temperature cycling effects. A maximum continuous service temperature of 316°C is suggested. Short excursions to higher temperatures are also possible.
  • Kalrez® 4079 is not recommended for use in hot water/steam applications or in contact with certain hot aliphatic amines, ethylene oxide, or propylene oxide.
  • Kalrez® o-rings have the best high temperature performance. DuPont Dow Elastomers tests on o-rings made from a particular Kalrez compound showed a compression set of ~50%, and a sealing force retention of ~80% after 672 hours at 204°C. After 672 hours at 250°C, the compression set is still only ~63%.
  • The Kurt J. Lesker Co. standardizes on Kalrez® Grade 4079 O-rings, however other more cost-effective grades such as Kalrez® 9300 are available upon request.

Silicone Rubber (Si)

  • Grade: 1200-70 Silicone
  • Color: Brick Red
  • Durometer: 70
  • Temperature Range: -50°C to 230°C
  • Silicone O-rings offer very good resistance to ozone, acids, water, and decent resistance to weathering and heat but offer little shear resistance or tensile strength.
  • Standard silicone O-ring temperature range is generally quite wide, from very low to reasonably high.
  • It is commonly thought that silicone has good high temperature characteristics; however, tests (on one unspecified silicone rubber compound) by DuPont Dow Elastomers indicate a compression set of 87% and a sealing force retention of ~80% after 70 hours at 204°C. In prolonged high temperature operation, much anecdotal evidence points to silicone losing it resilience, hardening, and sticking to the flange surfaces, making replacement difficult. Silicone's permeation for water vapor is extremely high, ~200 times that of Fluorocarbon.

USING ELASTOMERIC SEALS

When designing or specifying o-ring vacuum seals, some guidelines are:

  • A double o-ring seal is no better than a single o-ring if the volume between the two o-rings is not differentially pumped.
  • The surface finish of metal parts mating to the o-ring is critical. Scratches on the metal (or glass) that cross from the air-side to the vacuum-side of the o-ring's footprint will cause leaks.
  • Before installing o-rings, they should be vacuum baked (without compression). This greatly improves outgassing characteristics.
  • Greasing o-rings:
    • Purists tend to avoid it. Pragmatists tend to use it, claiming surface finishes on metal mating to the o-rings are frequently too scratched or rough to support the purist view.
    • Do not apply grease with a bare fingertip. Use polyethylene gloves to handle the rings and apply the grease. Apply only the thinnest film possible.
    • Choose the grease carefully. Consider its vapor pressure at the joint's temperature, and whether it will liquefy and "surface creep" into the chamber.
    • The debate about greasing o-rings is ongoing. Typically, the same vacuum level can be achieved with or without grease. However, when not using grease, the o-ring must be dust- and fiber-free, and the metal surfaces touching the o-ring must have at least a 32 rms finish. Grease permits sealing in less pristine conditions.
    • Greasing o-rings that will reach high temperatures is not recommended. Most greases have a significant vapor pressure at ~200°C and almost certainly a grease film will form on every surface of a baked system.

At the Kurt J. Lesker Co., we stock hundreds of O-rings rated for vacuum use in a variety of diameters, cross sectional areas, and materials. For common sizes typically used in KF, ISO, and ASA components, please see listings in those sections of our web site.

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