NuSil's high-purity silicones are taking off

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).

"There is a big family here from which to choose," Steckler said, adding that NuSil has 150 off-the-shelf silicones and customizable options available.

The initial and most important question that needs answered must come from the customer: What is the material intended to do?

What is the bonding or adhesion value that is required? Is the silicone material to be used for electrical conductivity, thermal management or vibration damping?

Is it to be applied to metals or composites?

How sensitive and what is the geometry of the area where the silicone is to be used?

"If it is an area where any outgassing or contamination could cause failure, then maybe ultra-low outgassing would be better for mission-critical applications," Steckler said.

At this point, NuSil might begin its high-purity silicone development at the monomer stage with low volatility materials, moving to the polymer stage where long-chain and short-chain polymers are developed.

There are two functional groups of polymer chains, Steckler said, one of which has a more functional (or dense) concentration and another that is less functional.

"Functionality at the chain ends dictates the functional chemistry," he said.

Most importantly for high-purity, space-grade silicones, all low molecular weight species must be removed early in the process—the types of polymers that reared their oily heads during the early space missions.

The next step is the filler, with fume silicas most often used in designing for space since they exhibit different particle sizes that can be applied to different surface areas, giving the silicone its mechanical strength.

"We can treat it to allow control of the interaction between the filler and the polymer—and by that control the mechanical properties," Steckler said.

The final step is establishing the crosslink system, which gives the silicone its elastomeric properties through either condensation or addition cure methods.

"The processing conditions—time and shear, temperature all are important," he said. "When this is all done under control, then we can move to the testing phase, where we can receive some regulatory support as well."

From a cure chemistry perspective, addition and condensation cures are two-part systems.

The condensation cure method can produce corrosive byproducts, while addition cure does not emit byproducts, Steckler said.

Using silicone to coat thick sections usually requires the addition cure method, and bonding and sealing silicones can cure via either method (but sometimes require a primer to increase adhesion).

"In many cases, the cure chemistries are wide open for either thermal management or electric conductivity management, for shielding of electromagnetic or ultraviolet radiation for the most sensitive areas," Steckler said.

As NASA, the U.S. Department of Defense and other global agencies look to the next horizon in space exploration, Steckler said NuSil is prepared.

"Designing a satellite and working in the extreme condition of space has its challenges, but finding the right material shouldn't be one," he said. "Remember to think about form, function, design and process.

"From our founding heritage in 1979 to our first space-grade product in 1981, we have had long and lasting relationships with the space industry for a number of years. And we have the knowledge and technical expertise to be a partner to meet those desired applications."