High-Quality Elastomeric Gasket Technical Notes for Optimal Performance

Elastomeric Gaskets for KF, ISO & ASA O-Rings

Introduction to O-Rings

O-rings serve as essential seals across various manufacturing sectors. With a diverse selection of materials, selecting the right one for your specific application is crucial.

Elastomeric gaskets, typically circular in cross-section, function as user-friendly vacuum seals. To achieve an effective vacuum seal compatible with pressures around 10^-7 Torr or mbar, the o-ring should be compressed by 25% to 35% when used with flange surface finishes of 32 rms or better. With optimal design and preparation, seals can potentially reach 10^-9 Torr or mbar.

Key Properties of O-Ring Materials

Understanding the strengths and weaknesses of different O-ring materials is important for effective applications. Below are some fundamental properties to consider.

Color and Composition

Most elastomer compounds incorporate various fillers, with carbon black being the most prevalent. Consequently, O-rings are often black. These fillers enhance the rubber’s strength and heat resistance. While the standard color of each elastomer type can vary, changes in compound availability may affect the color.

Hardness

Hardness refers to the material’s ability to resist deformation under pressure, measured using Shore Durometer. Standard elastomeric compounds typically have a durometer value of around 70.

Compression Set

Compression set indicates how much an O-ring remains deformed after being released from a flange seal. This measure reflects the elastomer’s resilience under a constant compressive load. If an elastomer fails to regain its original shape, the remaining compressed material is termed the compression set.

Gas Permeability

Elastomers can allow gas diffusion, permitting gases to escape through the material under low pressure. Factors such as temperature, pressure, and the type of gas influence the rate of permeation, which is critical for applications requiring vacuum or gas containment.

However, elastomers exhibit drawbacks, including gas permeability, outgassing rates, and limited temperature tolerance. High temperatures, although below pyrolysis levels, can significantly increase permeation rates and compromise sealing efficiency due to compression set and diminished sealing force retention.

Common elastomers used in vacuum seals include Buna-N (Nitrile-NBR), Fluorocarbon (FKM), Perfluorocarbon (FFKM), Ethylene Propylene (EPDM), and Silicone Rubber (Si). Each material’s specific characteristics depend on the compounding agents and cross-linking agents employed during manufacture.

Properties of Common O-Ring Materials

Buna-N (Nitrile)

Grade: 4900 – 70 Nitrile
Color: Black
Durometer: 70
Temperature Range: -35°C to 210°C

Buna-N O-rings are widely used due to their general-purpose sealing properties, particularly resistant to water, gasoline, petroleum oils, and some hydraulic fluids. They are the most affordable option for vacuum applications, but they have a higher permeation rate for water vapor compared to Fluorocarbon. Notably, these O-rings should not be used in oxygen plasma processes, and their shelf life can extend up to 15 years if stored away from ozone sources.

Fluorocarbon (FKM)

Grade: 9505-75 Fluorocarbon
Color: Brown
Durometer: 75
Temperature Range: -26°C to 210°C

Commonly known as Viton, Fluorocarbon is a leading elastomer in vacuum applications, providing excellent resistance to high temperatures, ozone, and a wide range of chemicals. While it has low permeability to most gases, helium presents an exception with a permeability rate double that of Buna-N. The material performs well at elevated temperatures, with tests indicating a compression set of approximately 50% after prolonged exposure.

Perfluoroelastomer (FFKM) Kalrez®

Grade: Kalrez® Grade 4079
Color: Black
Durometer: 75
Temperature Range: -19°C to 316°C

Kalrez® 4079 is designed for semiconductor processing environments, exhibiting exceptional chemical resistance and thermal stability. It can withstand temperatures up to 316°C and shows low weight loss in reactive plasma environments. However, it’s not suitable for applications involving hot water or specific chemicals.

Ethylene Propylene (EPDM)

Grade: 5600-70 EPDM
Color: Black
Durometer: 70
Temperature Range: -51°C to 150°C

EPDM O-rings demonstrate outstanding resistance to weathering, ozone, and steam, making them versatile for various applications. While they excel in heat resistance and flexibility at low temperatures, they are not compatible with petroleum products.

Silicone Rubber (Si)

Grade: 1200-70 Silicone
Color: Brick Red
Durometer: 70
Temperature Range: -50°C to 230°C

Silicone O-rings offer good resistance to ozone and various chemicals but have limited shear strength. They maintain a broad temperature range; however, prolonged exposure to high temperatures can lead to a significant compression set.

Best Practices for Using Elastomeric Seals

When specifying or designing O-ring vacuum seals, consider these guidelines:

A double O-ring seal is ineffective unless the space between the O-rings is differentially pumped.
Ensure the mating surfaces are well-finished; scratches across the seal’s footprint can lead to leaks.
Before installation, vacuum-bake O-rings to improve outgassing characteristics.
The use of grease on O-rings remains a topic of debate. While some prefer it for smoother sealing, others recommend avoiding it unless necessary. If grease is applied, use gloves to handle the rings and apply only a minimal amount.
Selecting the right grease is crucial; it should have appropriate vapor pressure for the temperature and avoid liquefying into the vacuum chamber.

At TFM, we offer a wide range of O-rings rated for vacuum use, available in various sizes and materials. For common dimensions used in KF, ISO, and ASA components, check our website listings for details.