CARPINTERIA, Calif.—It is no coincidence that a period of great growth occurred in the silicone industry in concert with the golden age of space exploration, beginning 70 years ago with the Mercury Program and carrying through the 1950s and 1960s with the Gemini and Apollo programs.
But the seals and coatings used in these early missions were not made from high-purity silicone, and they tended to leak when they broke down on re-entry—emitting an oily residue, a low molecular weight silicone species that did not crosslink into the matrix—causing unsafe condensations on windshields and contaminating other areas of the space vehicle.
Fast forward to 1979 and the onset of NASA's iconic Space Transport System, otherwise known as the Space Shuttle, and the birth of NuSil Technology L.L.C. in California. The silicone industry accelerated once again, as NuSil produced its first certified space-grade silicone in 1981—the same year that STS-1 blasted off from Kennedy Space Center.
"NuSil bridged that gap—it was created to meet the need for ultra-high purity materials for the space industry," said Timothy Steckler, an applications engineer with NuSil who spoke about silicones in space during a webinar offered by Carpinteria-based NuSil, an Avantor brand, late last year. "Once NASA introduced a test that could grade the silicone materials, NuSil really took off, so to speak.
"Now there are silicones in deep space, in crew capsules and launch vehicles—it is all over every vehicle."
Silicones can be found on satellites, within their solar panels and near antennas and sensors. They form protective coatings against vacuum forces, atomic oxygen, particle and ultraviolet radiation and micrometeoroid and orbital debris, Steckler said. They remain the only class of space-qualified elastomeric material that can span such an incredible temperature range, from between -75oC to 125oC, according to NASA.
When a customer like NASA, the European Space Agency or SpaceX requests a space-grade silicone, Steckler noted that they need to consider function, purity and process, as there are "many levers to pull" to make it all work.
"At a high level, these are the things we want to think about," Steckler said.
Designing silicones for space: Fundamentals
At its most basic level, space travel requires low viscosity elastomers like liquid silicone rubber and high consistency rubber, polymers that have low outgassing properties, meaning they lose only a tiny fraction of their total mass when subjected to high temperatures.
Prior to the launch of Columbia in 1981, NASA introduced a process (a vacuum environment at 125oC for 24 hours) to test these properties, a process that could measure volatile, condensible materials as well as the recovered water vapor. Silicones still need to pass this test to be considered for space flight, Steckler said.
So what makes a space-grade silicone?
They come in many forms, Steckler said, from primers to adhesives, foams, resins, gels and greases. Within elastomers alone, there are LSRs (typically used in injection molding), HCRs (typically used in extrusions and compression molding) and dispersions (used for coatings).