How to Select the Right Silicone for Space Applications

From traditional satellites to crew capsules to exploration vehicles, aerospace manufacturers have long relied on silicone adhesives, coatings and elastomers for applications that move science forward, into the deepest reaches of space. Silicones offer broad operating temperatures, the ability to maintain elasticity over these temperatures and low modulus, thus making them a popular choice for bonding and protecting sensitive components and electronics (i.e., sensors, antennas, cameras and solar panels) and for general spacecraft assembly.

Space exploration presents unique environmental challenges, particularly related to extreme temperatures and vacuum conditions. These challenges can cause volatiles in a material to outgas, which can potentially damage sensitive sensors, lenses, electronics and other surfaces. Ultraviolet (UV) and atomic oxygen (AO) exposure can further degrade outgassed materials, a process that can cause adverse effects, shorten the operational lifetime of hardware and, at worst, trigger mission failure.

However, silicone manufacturers can formulate highly purified, low outgassing silicones that aerospace manufacturers can use to avoid contamination and prevent material degradation.

Formulating Low Volatility Silicones for Space

Silicone manufacturers have a variety of tools at their disposal to create specialized materials. Starting with the polymer, having control over the molecular weight, purity and functionality allows the silicone to be tailored to achieve the material properties needed for the end application (cure chemistry, kinetics and rheology) as well as performance qualities (elasticity, thermal stability, volatility, optical clarity, etc.). Addition of fillers can help determine a final material’s rheological and mechanical properties, such as flow, hardness, strength and other characteristics. In addition, specialty fillers can be added to enhance thermal or electrical conductivity, increase thermal stability or reduce weight.

Whether the application requires a next-generation silicone film adhesive for advanced assembly or an electrically conductive silicone to protect vital components, when formulating space-grade silicones it is imperative to minimize the volatiles that might outgas. Formulating with the right materials will ensure the final silicone material meets the low volatility requirements and the application-specific performance characteristics.

Evaluating Space-Grade Silicones

It is essential to consider material requirements early in the design process by evaluating a range of different factors, including the following. Function: The first consideration is what the material is intended to do. Space applications might require any one of a wide range of functions, including:

  • Adhesives (either in liquid or tape form) to bond components to substrates, like metals, plastics, glass and more

  • Thermally or electrically conductive materials to transfer heat from electronic devices or form conductive bonds

  • Coatings to protect against atomic oxygen and provide radiation or thermal stress resistance

  • Potting or encapsulating materials to protect sensitive electronic components from vibration, shock and external contaminants like moisture and debris

As part of function evaluation, it is important to consider what materials the silicone will be in contact with: metals, polymers or composites. If the selected silicone requires heat to cure, that could negatively impact adjacent materials, requiring the use of a room temperature vulcanizing (RTV) silicone instead. Purity: Based on the application and where the silicone will be used, it is important to understand how the risk of out-gassing relates to mission success. For the most sensitive applications, even minimal amounts of outgassed species can create the potential for mission failure. These situations might dictate more stringent volatility requirements where higher purity ultra-low outgassing silicones would be the best option. Process: Once the material function and purity levels have been determined, consider where and how the material will be applied. For example, a liquid adhesive may be a better choice than a tape for an application with very tight and complex geometries. If the application requires silicone to be applied to a vertical surface, a material that is thixotropic/non-slump would be preferred over a low viscosity system.

Process time is also a key consideration. Cure chemistry can influence the pot life and cure time, with additional cure formulations being able to cure more quickly than condensation cure formulations. Other factors that play a role in determining process time include temperature, humidity and the need for alternate cure schedules. When all these factors — function, purity and process — are considered holistically, the proper space-grade silicone solution can be identified and implemented.

Comparing Controlled Volatility Silicones

NASA developed the ASTM E595 test method to evaluate the volatile content of materials used in space applications. ASTM E595 measures outgassing after a sample is heated under vacuum (< 10-5 torr) at 125 °C for 24 hours and provides data on total mass loss (TML), collected volatile condensable material (CVCM) and water vapor regained (WVR). The ASTM E595 standard establishes acceptance limits at ≤ 1.0 percent TML of the material and ≤ 0.1 percent CVCM.

Materials used on a spacecraft’s more sensitive areas may require more stringent and detailed testing, with acceptance limits at ≤ 0.01 percent CVCM. In these cases, NASA recommends ASTM E1559 testing, which is a more exhaustive testing process that records out-gassing levels as a function of time and temperature, thus providing outgassing kinetic data.

A silicone manufacturer used both ASTM E595 and E1559 to analyze a suite of its products, including condensation and addition cure materials and sheeting products. The purpose was to give an overview of the company’s low outgassing (CV product designation) and ultra-low outgassing (SCV product designation) products and how the outgassing kinetics differ between them, to aid design engineers in material selection. The ultra-low outgassing materials were developed specifically to meet the most stringent standards for space applications, where even low amounts of volatiles in traditional space-grade materials might outgas and cause potential problems. These materials are expected to demonstrate a TML of ≤ 0.1 percent and CVCM of ≤ 0.01 percent by ASTM E595.

Figure 1. Comparison of the percentage of TML at the end of testing by ASTM E595 vs. E1559.
Figure 2. Cure chemistries and applications of the tested low outgassing and ultra-low outgassing silicones for space applications.

Figure 1 shows the percent of TML at the end of testing as compared using ASTM E595 versus ASTM E1559. Figure 2 lists the NuSil® space-grade silicones included in the test, their cure chemistry and the application for each tested formulation.

The percentage of volatile condensable materials (VCM) measured by ASTM E1559 testing was just as important as TML since these are the higher molecular weight materials with lower volatility, which can lead to mission failure if found in sensitive areas. In Figure 4, the results illustrate the substantial difference in outgassing of volatiles over time for low outgassing and ultra-low outgassing silicones. While all materials had CVCM of ≤ 0.04 percent over 72 hours of testing, ultra-low outgassing silicones showed CVCM values of ≤ 0.001 percent, a full order of magnitude less compared to the low outgassing materials.

Figure 3. Comparison of the TML percentage over time for silicones tested by ASTM E1559.
Figure 4. Comparison of the VCM percentage (298 K QCM) over time for products tested by ASTM E1559.

The testing demonstrates that ultra-low outgassing silicones are highly suited for sensitive applications that require minimum contamination to ensure mission success.

Making the Right Material Choices

Selecting the right silicone manufacturer with a proven track record of extensive experience in the application and customization of space-grade silicones is as important as considering the materials’ function, purity and process requirements when choosing a silicone for space applications. The manufacturer must provide advanced low outgassing and ultra-low outgassing materials that can withstand the rigorous environmental demands of space.

In addition, the ideal silicone manufacturer should have deep regulatory experience, customization capabilities, a comprehensive quality system and the ability to test space-grade silicones on a lot-to-lot basis to ensure reliable, compliant performance, ultimately allowing for mission success anywhere in the universe.

This article was written by Timothy Steckler, Ph.D., Applications Technology Manager, NuSil — a brand of Avantor. For more information, visit here .