Manufacturers in aerospace and aviation have used silicones since their inception, however not everyone is familiar with all of their capabilities. Silicones adhere different materials together, seal joints, coat and encapsulate electronics, and insulate and shield sensitive equipment from extreme temperatures and weather exposure. The versatility of silicone and its high-temperature resistance have made it a reliable choice for decades, and today it is still one of the most effective materials used in aerospace manufacturing.

Silicone supports engines that experience high temperatures during liftoff, as well as on the exterior of a space capsule that experiences low and high temperatures and needs shock absorption when re-entering Earth’s atmosphere. Silicones’ effectiveness and versatility make them suitable for at least 20 different areas of a launch vehicle.

Silicones defined

Silicone is a synthetic material made of polymers that has a chemical structure based on chains of alternate silicon and oxygen atoms. It can remain stable in extreme environments, whether hot or cold, and maintain its properties as temperatures fluctuate.

Physical forms include gum, room-temperature vulcanization (RTV), fluid, monomer, gel, and resin. RTV silicone is available in a variety of viscosities – ranging from low-viscosity options that can be incorporated with additives to boost the thermal protection properties, to high-viscosity formulations that can be applied to overhead surfaces. RTV silicone is used as an adhesive by manufacturers to adhere to rubber, plastics, and metal.

When bonding two substrates made of different materials, for example a composite to aluminum, each substrate expands and contracts at different rates during temperature changes. This movement, caused by a coefficient of thermal expansion (CTE) mismatch between two substrates, tends to be more pronounced at extremely high or low temperatures. These different rates of expansion can cause joints to separate, bonded materials to weaken, and critical components to fail.

Many epoxy and polyurethane adhesives can become rigid when cured and may not be able to absorb CTE mismatches between different substrates. Silicones have a low modulus that maintains flexibility even in extreme high and low temperatures, allowing silicone adhesives to more effectively cope with CTE mismatch effects.

Epoxies and polyurethanes generally offer poor electrical properties at low or high temperatures and in damp conditions. They can also be harder to work with due to toxicity and an affinity for water absorption, and they are difficult to remove or repair when equipment needs to be serviced.

The strongest attribute of an RTV silicone is its ability to maintain critical properties throughout a wide temperature range. High-temperature RTV silicones, such as those manufactured by Momentive Performance Materials, can maintain properties at temperatures ranging from -175°F to 500°F, or higher for short periods of time.

That temperature range makes RTV silicones suitable to bond lenses onto telescopes and secure optics onto satellites and other structures. Since this equipment remains in orbit for a long time, it is subjected to large temperature fluctuations due to repeated, intermittent exposure to the sun, and silicones can withstand those extreme conditions.

A silicone coating is applied to electronic components to protect against extreme environmental conditions.

Low-outgassing formulations

Materials react differently in the extreme temperatures and vacuum conditions encountered in space and other environments. Products that are stable on Earth may break down in space and emit gasses that can fog optical lenses, disrupt electronics, or damage other critical components. Because of the potential for outgassing, materials used in the manufacture and assembly of optical lenses, electronic components, avionics displays, or crew compartments that will be exposed to these conditions must meet the American Society for Testing and Materials (ASTM) E595 standard to reduce outgassing risks. The standards specify that materials must lose less than 1.0% of mass and produce less than 0.1% volatile condensable materials in a vacuum to protect critical equipment.

RTV silicones are rubber and gel silicones that cure without heat and are used for bonding, sealing, encapsulation, coating, and potting applications. Unlike the original RTV silicones used in the space program in the 1960s, today’s aerospace-grade versions are available in various low-outgassing formulations that meet ASTM E595 standards.

Electronics applications

Advanced aviation electronics have a very high-power density that generates more heat than those typically used in the past. When circuit boards that support the operation of planes and radar systems generate heat, onboard electronics are also constantly exposed to fluctuating temperature throughout their lifespan. To last longer, electronics’ coatings and adhesives must outlast the effects of repeated temperature changes.

Room-temperature vulcanizing (RTV) silicones are a prominent ingredient in aerospace manufacturing due primarily to their ability to withstand extreme temperatures while maintaining their physical properties.
All photos and images courtesy of Momentive

When protecting these delicate electronics and circuit board assemblies, or sealing power modules and sensors, manufacturers should use compounds that provide stable dielectric, thermal, and mechanical properties such as silicones. Thermally conductive silicones can also passively remove heat. There are many methods to apply liquid silicone to sensitive components, such as with pneumatic applicator guns, syringes, and metered dispense valves. The silicone is then cured into an elastomeric rubber that excels in thermal interface applications demanding good structural adhesion.

Electrostatic dissipative and electromagnetic shielding silicones can electrically ground composite structures while tolerating a low electrical charge. By creating a pathway for a current to pass through, the chance of generating a spark due to electricity build-up can be minimized.

Momentive Performance Materials http://www.momentive.com

About the authors: Clarissa Miller is senior global program manager, Industrial and Aerospace RTV Silicones, at Momentive Performance Materials. Matthew Lindberg is technical account manager, Industrial and Aerospace RTV Silicones at Momentive Performance Materials.