For decades, silicone adhesives, coatings, and elastomers have been used in space applications due to their broad operating temperatures, ability to maintain elasticity over these temperatures, and low modulus. These advantages exemplify why silicones are widely used on spacecraft, from CubeSats and crew capsules to deep space exploration vehicles, traditional satellites, and satellite constellations. Silicones can be found in applications such as solar panels, sensors, antennas, cameras, and general assembly on these vehicles.

The extreme temperatures and vacuum conditions of space present unique environmental challenges. Unless specially formulated, these conditions can cause volatiles in a material to outgas and possibly damage sensitive sensors, lenses, electronics, and other surfaces. These outgassed materials can further degrade due to ultraviolet (UV) and atomic oxygen (AO) exposure, causing adverse effects and shortening the operational lifetime of hardware or even causing mission failure.

To counteract these conditions, silicone manufacturers have developed highly purified, low-outgassing silicones that satellite and space vehicle manufacturers can use to avoid contamination and prevent material degradation.

Creating Low-Volatility Silicones

To formulate low-outgassing and ultra-low-outgassing silicones, silicone manufacturers utilize a variety of silicone chemistry techniques. Controlling these properties starts with designing the initial silicone polymer chain, which can be made short or long, and includes different types and amounts of functional groups. Incorporating specific functional groups on the polymer (terminal and/or pendant) will dictate the type of cure chemistry used and control how the initial silicone cures to lock in the desired material characteristics.

Specialized filler materials can also be added to the formulation to aid in controlling the mechanical properties of the final material — its strength, hardness, and other properties. Beyond controlling the mechanical properties of the silicone system, other fillers can be added to enhance thermal or electrical conductivity, increase thermal stability, or decrease weight. These fillers are selected to ensure the final silicone material has the desired properties for the intended application.

When developing a space-grade silicone, the goal is to minimize to the greatest extent possible the volatiles that might outgas, while achieving the desired stability and mechanical properties required for a given application — whether it’s an adhesive, coating, or molded component.

Three Key Evaluation Criteria

When choosing the right silicones for a given application, there are several criteria to consider in three main areas:

  1. Function: What function does the material have? Is it a coating, an adhesive, or some other application?
  2. Purity: What volatility requirements should the silicone have, based on the conditions and longevity of the application?
  3. Process: How will the silicone be applied when it is used on a satellite or space vehicle?

There is a wide range of functions that silicones can be used for in space applications. They can include adhesives (either in liquid or tape form) to bond solar cells to substrates; potting or encapsulating materials to protect sensitive electronic components from vibration, shock, and other environmental factors; or coatings to protect against atomic oxygen.

Another functional consideration includes what materials the silicone will be in contact with — will they be metals, polymers, or composites? What are the temperature limitations that those materials have? If the silicone that’s been selected requires heat to cure, that could negatively impact adjacent materials; a better choice might be to select a room temperature vulcanizing (RTV) silicone instead. There are space-grade silicones available with either addition or condensation cure chemistry.

Based on the application and where the silicone will be used, it is important to understand the risk of outgassing. For the most sensitive applications, even minimal amounts of outgassed species could endanger mission success. For these situations, higher-purity ultra-low-outgassing silicones would be the best option.

Once the material function and purity levels have been determined, the focus can shift to how the material will be processed. For applications with very tight and complex geometries, it may make more sense to use a liquid adhesive instead of a tape. For applications on a vertical surface, a material that is thixotropic/non-slump would be preferred over a low-viscosity system.

These are just some examples of how the function, purity, and process can impact the selection of space-grade silicones. When all of these factors are considered holistically, the proper silicone solution can be identified and implemented.

Using Testing to Compare Low- and Ultra-Low-Volatility Silicones

To evaluate the purity and stability of space-grade silicones, the ASTM E595 test method was developed by NASA and is widely used by the industry. ASTM E595 measures outgassing at a specific point in time and reports the following:

  • TML: Total Mass Loss

  • CVCM: Collected Volatile Condensable Material

  • WVR: Water Vapor Regained

The established acceptance limits for ASTM E595 are ≤1.0% total mass loss (TML) of the material and ≤0.1% collected volatile condensable materials (CVCM). When materials are used in more sensitive areas on spacecraft, more stringent and detailed testing may be required where TML and CVCM acceptance limits may be ≤0.1% and ≤0.01%, respectively.

NASA also recommends using the ASTM E1559 test method in these instances. ASTM E1559 is a more extensive testing process that records the amount of outgassing as a function of time and temperature, thus giving outgassing kinetic data. This is crucial to help create and select silicones that can work on satellites and space vehicles that operate for extended periods of time — even decades — in space.

Recently, a silicone manufacturer analyzed a suite of its products, including condensation and addition cure materials, using both ASTM E595 and E1559 to detail and compare the purity and stability of the company’s low-outgassing (CV product designation) and ultra-low-outgassing product lines (SCV product designation) and their effects on outgassing kinetics.

For the most stringent space applications, where even low amounts of volatiles might outgas from traditional space-grade materials and cause potential problems, the manufacturer developed a suite of ultra-low-outgassing materials. These materials are held to a much higher standard, with a TML of ≤0.1% and CVCM ≤0.01% by ASTM E595.

Figure 1 - Comparison of the % TML at the end of testing by ASTM E595 vs. E1559.
Figure 2 - This table lists the cure chemistries and applications of the space-grade silicones tested by NuSil.

In Figure 1, the percent of TML at the end of testing is compared using ASTM E595 versus ASTM E1559. Figure 2 lists the NuSil ® space-grade silicones that were tested, along with their cure chemistry and the application for each silicone tested.

In Figure 3, the silicone materials are tested using ASTM E1559, evaluating TML as a function of time. This testing showed a sharp rise in TML during the first two to four hours of the testing for all samples. Then, the low-outgassing materials started to level off between 24 to 48 hours, whereas the ultra-low-outgassing materials leveled off much earlier, within four to eight hours. When comparing ultra-low-outgassing to low-outgassing materials, TML is reduced by approximately four to nine times.

Figure 3 - Comparison of the TML % over time for silicones tested by ASTM E1559.

The percentage of CVCM measured by ASTM E1559 testing was just as important as TML since the materials that condensed were higher molecular weight materials with lower volatility. Figure 4 demonstrates the difference in outgassing of volatiles over time when comparing the low-outgassing and ultra-low-outgassing materials. While all materials had CVCM of ≤0.04% over 72 hours of testing, ultra-low-outgassing silicones demonstrated their advantages by having CVCM values of ≤0.001%, a full order of magnitude less compared to the low-outgassing silicones.

Figure 4 - Comparison of the VCM % (298 K QCM) over time for products tested by ASTM E1559.

Making the Right Choices

The testing demonstrates that ultra-low-outgassing silicones offer an ideal solution for sensitive areas where contamination must be kept to a minimum to extend hardware operating life. The testing also demonstrates the value of working with a silicone supplier that is able to provide advanced materials that can stand up to the extreme environmental demands of space applications in both low-outgassing and ultra-low-outgassing options.

A spacecraft’s mission, its longevity, and the sensitivity of the spacecraft’s instruments will help guide the selection of a specific silicone, based on where it will be used. When choosing a silicone, it is important to remember the function, purity, and process requirements of the application as well as the benefits of working with silicone suppliers that have extensive experience customizing space-grade silicones to meet the needs of the most advanced spacecraft.

This article was written by Dr. Timothy Steckler, Applications Engineer, NuSil – a brand of Avantor® -- Radnor, PA. For more information, visit here .

Tech Briefs Magazine

This article first appeared in the April, 2021 issue of Tech Briefs Magazine.

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