Function, purity, and process are key considerations when researching aerospace materials. Daniel Hess, applications engineer at NuSil, explains silicone’s unique characteristics and guidance for choosing the right material.
The SpaceX Crew Dragon Endeavour took this picture of the International Space Station during a fly-around of the orbiting lab in November 2021. Image: NASA
Humans have used silicon-based materials for thousands of years, with archaeological evidence from ancient Chinese, Egyptian, and Phoenician cultures. However, Jöns Jakob Berzelius, known as “The Father of Swedish Chemistry,” is credited as being the first to isolate the silicon element in 1823. Silicones advanced significantly during World War II, starting as greases and reinforcing agents for composites. Natural rubbers were in short supply at the time, so the push for researching and manufacturing synthetic rubbers quickened.
“A carbon-based grease at cold temperature seen at high elevation can seize up and become more solid, whereas silicone can remain non-solid at the same temperature range,” said Daniel Hess, applications engineer at NuSil, a brand of Avantor. “After World War II, more silicone manufacturing took place to get scalable processes and increase the volume. Ever since, its utility and research for new materials and form factors have been increasing, and it is becoming ubiquitous.”
Today, silicones are ever-present in our everyday lives. Aerospace, automotive, medical, and consumer goods applications abound.
“I like to think that before you leave the door to go to work in the morning, you probably touch 10 different things that are either made of silicone or made with silicone in some capacity,” said Hess. “Before silicones became ubiquitous, there were traditional natural rubbers, neoprenes or polyolefins in elastomers, or for adhesives, you also had polyurethanes and epoxies, which have different uses and different utilities, but they’re generally not as soft or elastomeric. Cyanoacrylates, or super glue, that’s another one. It’s a thermoset elastomer, but it doesn’t really have that softness or stretchiness like a silicone adhesive does.”
Silicones have very low modulus, making them an excellent adhesive and creating compliant bonds and unions that experience low stress when flexed or encountering low or high temperatures. NuSil, which Avantor acquired in 2017, has been manufacturing silicone materials for the International Space Station to help address such temperature challenges.
“Our broad operating temperature portfolio of silicones is a classification of materials that has what we call glass transition temperature that goes down to about -115 or -120° C for operational temperatures,” said Hess. “That’s the point at which it will remain soft and compliant, whereas traditional plastics or other types of elastomers have a much higher temperature, so they can’t be as cold. After that point, these materials will become stiff and glassy.”
For decades, silicones have been used in aerospace for adhesives, coatings, foams, elastomers, tapes, and potting compounds. All these materials are critical for building a functional and reliable device or assembly suitable for outer space or terrestrial vehicles in low atmosphere for the aircraft industry.
Hess explained that gaseous atomic oxygen is a predominant concern in aerospace and is found abundantly in low Earth orbit atmospheres. It’s a very aggressive gas that attacks carbon-based materials and erodes and compromises the integrity of structures. Therefore, silicones act as a sacrificial coating and help prevent the degradation of critical materials underneath. This is particularly important for the exteriors of capsules and components such as solar panel arrays.
“That’s where one of our longest-standing products has been used for atomic oxygen coatings on the International Space Station,” he said. “Other popular coatings are ablative coatings that will help a craft survive reentry into the atmosphere, so the silicone will absorb the heat and glow white. If you ever see a rover or some kind of shuttle descending into the atmosphere that’s glowing red and white, that’s ablative silicone coating absorbing the heat to help the materials and composites underneath survive.”
NuSil’s silicones characteristically have low volatility in their cured form. The company manufactures two different grades of products in this area, CV (controlled volatility) and SCV (super controlled volatility), with specific outgassing characteristics based on international standards and ASTM E595 testing, which is critical for most aerospace applications. NuSil is on the ASTM committee and manufactures, maintains, and sells equipment that ASTM E595 is based on. The team has been working with NASA and other international regulatory bodies to establish and maintain those standards for ASTM E595. They also implement additional specialized processing to develop super-low volatility materials necessary for aerospace.
“One of the reasons why very low volatility is necessary is that in a vacuum environment like space, any residual volatile materials that come from the original synthesis of the polymer can stay in there and then be pulled out in a vacuum, and that’s going to go somewhere. It can collect on sensitive surfaces like sensor housings and other glass or plastic surfaces that end up either fogging it up or making adhesive bonding a problem later,” said Hess. “So, we address that in the very beginning of the polymer synthesis and remove those low-volatile and low-molecular-weight compounds to ensure that upstream problems are mitigated.”
When selecting adhesives, coatings, or elastomers, Hess advises engineers or designers to consider three criteria: function, purity, and process. First, decide what the material should do — bonding, sealing, thermal management, electrical protection, and so on. Then, investigate the required mechanical properties and the environment to which the materials will be subject. For example, consider the purity necessary in a vacuum environment near sensors, optics, or other extremely sensitive components.
“The CV and SCV have two main measurements of what’s important: the total mass loss and the collected volatile condensable materials (CVCM). The total mass loss is just what it sounds like. It’s the amount of mass that’s removed from a cured material. And in these test conditions, it’s 24 hours under vacuum at 125° C,” Hess said. “The international standard for CV grade material is 1%, and the CVCM is 0.1%. On the SCV grade, it’s just 10 times lower. So, those are even 10 times more pure than the traditional CV grades of materials.”
Engineers must consider function, purity, and process when selecting silicones for aerospace applications. Image: NuSil
Lastly, on the process side, Hess advises engineers to think about scale and construction.
“If you’re making just a tiny little unit, that’s going to have different processing considerations, as opposed to building the whole aircraft. So, you might need to look for materials with specific packaging, like either side-by-side kits with easy dispensing, all the way up to drums, which might need specific types of equipment to meter it out, mix it, and then dispense it,” he said.
“There are also different types of rheologies that silicones will have, or flow characteristics that are better suited for certain areas. If you have vertical joints or curved surfaces, you might need a thixotropic material that won’t slump or fall over and hold a nice bead shape once it’s been dispensed so that you can join two parts without it falling on your head. Other complex geometries or large surface areas might need really flowable and self-leveling adhesives that will find the level and flow out into all the nooks and crannies of a geometry.
“There’s also a certain class of adhesives called film adhesives that are either in a sheet or a roll, and after preparing a surface, you can apply the film and it will cure into a permanent elastomeric film, which is kind of like a permanent tape, if you will. This is really useful for large array manufacturing. That’s going to give you a nice, consistent bond line across a big array. And these arrays can be quite large, several 100 square feet at a minimum. So, this also avoids any mixing and dispensing operations, so you can instead stick a film over it and allow it to cure at room temperature, instead of having any mixing or cleanup for liquid adhesives.”
Hess asserts that silicones are here to stay for the foreseeable future. They have broad uses in aerospace for constellation satellites, space research missions with crewed capsules and extraterrestrial rovers, deep space satellite imaging programs, and beyond. The material needs for each application are unique and require early collaboration and appropriate material selections to help ensure mission longevity and success.
NuSil
nusil.avantorsciences.com
Filed Under: Aerospace + defense, Adhesives • epoxies, Materials • advanced
