Space Grade Silicone

Stockwell Elastomerics offers space grade silicone and fabricates silicone gaskets for spacecraft and space vehicles. These specialized elastomers remain flexible at low temperatures and have low levels of outgassing. Stockwell Elastomerics uses various cutting methods to convert space grade solid silicone sheet and can also use compression molding to manufacture space grade gaskets for satellites, rockets, telescopes, and other applications. These materials are actively stocked to help meet customers’ development timelines. Additionally, these materials are available with robust certificates of analysis with data that is critical for space applications.

Space Grade Silicone Material with Extreme Low Temperature Flexibility

The first material, SSP4716, is a 65 durometer, phenyl-based silicone that remains flexible at extreme temperatures as low -116°C (-177°F). Compared to conventional dimethyl silicone, phenyl-based silicone offers enhanced thermal stability, improved radiation resistance, better oxidation and chemical resistance, and superior optical and refractive properties. SSP4716 has a tensile strength of 1350 psi and an elongation percentage of 600%. It is black in color as a standard but can be pigmented to meet customer specifications. It meets ASTM E595* low outgassing specifications.

Space Grade Silicone with Low Temperature Flexibility

The second group of materials, SSP4734-50LV and SSP4734-70LV, are methyl-based silicone materials that are available in either 50 or 70 Shore A. SSP4734-50LV is the 50 durometer version and SSP-4734-70LV is 70 durometer version.

• SSP4734-50LV has a tensile strength of 975 psi and an elongation percentage of 400%
• SSP4734-70LV has a tensile strength of 1080 psi and an elongation percentage of 300%

Both versions are available in black or gray color, but can be pigmented with other colors. They remain flexible in the low temperatures of space but do not have the extreme low temperature resistance of SSP4716. SSP4734-50LV and SSP4734-70LV also meet ASTM E595* low outgassing specifications.

Space Grade Silicone Materials Technical Specifications

Key Technical Concepts about Space Grade Silicone Material Selection

Material selection of space grade silicone involves these key concepts:

• Outgassing
• Emissivity
• Low Temperature and High Temperature Performance
• Compression Set and Force Deflection in Space Environments
• Inertness
• Atomic Oxygen
• Space Radiation

Outgassing

Outgassing refers to the release of gases or volatile compounds from a material that is exposed to heat or vacuum conditions. This a problem in space applications because optical sensors, camera lenses, and sensitive electronics need to remain free from contamination. Silicones support a wide temperature range and have low leakage rates, but they can release gases in the vacuum of space.

To minimize outgassing, special low outgassing silicone is used. These materials are tested according to ASTM E595, Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials for Outgassing in a Vacuum Environment. NASA’s low outgassing test references this standard test method and establishes limits for Total Mass Loss (TML) and Collected Volatile Condensable Materials (CVCM).

Historically, NASA has used a TML of 1.00 % and a CVCM of 0.10 % as screening levels for the rejection of spacecraft materials. TML quantifies the total amount of material that evaporates or releases gases from a sample during the test. CVCM determines the amount of volatile material that condenses on a cooler surface within the test chamber.

Stockwell Elastomerics maintains an inventory of low outgassing silicones can provide samples for testing or validation. The list below describes some of the most popular low outgassing materials.

• NORSEAL® R10480M (formerly COHRlastic R10480M), closed cell silicone sponge, medium – TML: 0.61, CVCM: 0.31
• BISCO® HT-800 silicone foam, medium, TML: 1.04, CVCM: 0.26
• NORSEAL® R10404M (formerly COHRlastic R10404M), thermally conductive silicone sponge, medium, TML: 0.26, CVCM: 0.11
• 3M™ Kapton® 92 tape, TML: 2.74, CVCM: 1.06

Emissivity

Emissivity is a measure of how effectively a surface emits thermal radiation. Values range from 0 to 1, where 0 represents a perfect reflector and 1 represents a perfect emitter. Surface properties like roughness and color affect a rubber material’s emissivity. Hardness is also a factor, and harder rubber tends to have higher emissivity than softer rubber.

Emissivity is important for accurate thermal measurements, especially with infrared thermometers or thermal cameras. In space, infrared thermometers are used for studying the temperature of stars and planets and monitoring spacecraft components. Thermal cameras are used to detect heat signatures and do not require visible light to function.

Low Temperature and High Temperature Performance

In space, materials must withstand extreme temperatures. High temperatures can cause materials to weaken, melt or degrade. Low temperatures can make materials brittle and prone to failure instead. Some general-purpose silicones have a temperature range from -50°C to 200°C (-58°F to 392°F). However, the vacuum of space is usually colder than -50°C and heat from solar arrays, nuclear power generators and atmospheric re-entry can raise temperatures well above 200°C.

Stockwell Elastomerics sources specialized silicone that can resist temperatures beyond what general purpose silicone can provide. Three of these materials also provide low outgassing.

• SSP4716 is a 65 durometer, phenyl-based silicone that remains flexible at extreme temperatures as low -116°C (-177°F).
• SSP4734-50LV is a 50 durometer, methyl-based silicone that remains flexible at low temperatures.
• SSP2734-70LV is a 70 durometer, methyl-based silicone that remains flexible at low temperatures.

Compression Set and Force Deflection in Space Environments

Spacecraft seals and gaskets need to prevent the leakage of fluids and gases in vacuum or pressurized environments. Compression set and force deflection are key concepts.

• Compression Set: Materials with a low compression set resist permanent deformation and continue to provide effective sealing even after prolonged compression. A low compression set also ensures that the seal won’t lose its elasticity when compressed for extended periods of time, such as long duration missions for satellites.
• Force Deflection: This measures the force required to compress a material by a specific amount. In space applications, a seal must provide enough “pushback” force to create a tight seal against a vacuum or pressurized environment. However, too much force can cause warping or damage to the seal or to surrounding components.

Inertness

Inert materials do not easily undergo chemical reactions or physical changes under normal use. However, space presents harsh and unusual conditions. Because silicone is inert, it won’t readily combine with other compounds or elements to form new substances. The inertness of silicone also ensures that it retains its mechanical properties (such as flexibility and elasticity) over time, even under extreme conditions. That’s critical for seals, gaskets, and other space industry parts that need to maintain their integrity and prevent leaks.

Atomic Oxygen

Atomic oxygen (O) is a highly reactive form of oxygen where a single oxygen atom exists independently. In other words, this atom is not bonded with another oxygen atom to form the more common diatomic oxygen (O₂). Atomic oxygen is abundant in Earth’s upper atmosphere and is a concern for spacecraft materials in low Earth orbit (LEO) because of its reactivity. Silicone rubber offers good resistance to atomic oxygen because of its inorganic Si-O-Si backbone and organic side chains.

Space Radiation

Space radiation is the result of energized particles and electromagnetic waves that originate from sources like the Sun. This form of ionizing radiation can increase the hardness of rubber by promoting cross-linking between polymer chains. In turn, this reduces the rubber’s flexibility and makes it more brittle. Space radiation can also affect tensile strength and cause cracks to form. Silicone provides greater radiation resistance than other rubbers.

* ASTM E595 – Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment

** NASA’s description of its low outgassing test using ASTM E595, sometimes referred to as NASA E595

BISCO® is a registered trademark of Rogers Corporation.
NORSEAL® and COHRlastic® are registered trademarks of Saint-Gobain.
3M™ is a registered trademark of 3M Company.