Elastomeric Gaskets for KF, ISO & ASA
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.
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.
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.
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.
The elastomeric materials most commonly used in vacuum seals are Buna-N (nitrile rubber-NBR), silicone, fluorocarbon, and perfluorocarbon (e.g., Kalrez and Chemraz-FFKM). 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 material are:
- Least expensive o-ring
- Maximum operating temperature of ~100°C
- 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
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.
- Most commonly used elastomer in vacuum service
- 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 200°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.
Kalrez® and Chemraz® 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%.
Using Elastomeric Seals
When designing or specifying o-ring vacuum seals, some guidelines are:
- A double o-ring seal is no better then 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.