Choosing fluorosilicones

All photos courtesy of Avantor NuSil

The U.S. Air Force discovered a problem in the 1960s: silicone-based sealants, coatings, and other materials used around fuel-exposed surfaces required more frequent repairs or failed prematurely.

Naturally soluble, standard silicones (dimethyl silicones) easily absorb fuels, crude oil, de-icing fluids, and other hydrocarbon-based solvents, causing the materials to swell, which weakens the silicone network, diminishes mechanical properties, and distorts dimensional shape. Swelling can cause expansion and cracking that breaches a seal or gasket, and that degradation can generate unplanned downtime, increased maintenance, and even catastrophic failure.

The Air Force required a material that could withstand cold high-altitude environments, engine exhaust heat, and UV exposure without sacrificing performance. While dimethyl silicone offers thermal stability in many applications, prolonged exposure to sunlight can cause cracking.

Fluorosilicones were the solution. Less soluble to fuels and organic solvents, they resist swelling while maintaining their physical properties and dimensional shapes. The materials better resist mechanical failures caused by hydrocarbon exposure. Additionally, fluorosilicones remain flexible at extremely low temperatures and resist breakdown at very high temperatures.

With decades of flight service, fluorosilicones provide reliable protection from breakdown, even against prolonged exposure. Widely used across various military and civilian aircraft components, their versatility makes them ideal for:

• Adhesives and sealants for wire staking, bonding components, sealing joints with different coefficients of thermal expansion; attaching composite materials to aircraft exteriors
• Coatings, sheeting for aircraft outer mold line, fuel cells; thin coatings on windshields, other surfaces
• Electrically conductive gap fillers for small cavities between aircraft surfaces
• Gels, foams for encapsulating sensitive electronics for vibration, shock; potting
• Molded gaskets, bushings used in or near engine compartments, fuel tanks

Available in a variety of forms and cure chemistries, fluorosilicones are suitable as molding compounds, paints, coatings, and one- or two-part adhesives. This versatility improves aircraft manufacturer processing and application efficiency while providing options based on end-use chemical and physical requirements.

An experienced silicone technology partner can customize formulations to include additional functional fillers, adjustable cure times, and other processing requirements. Formulations can also be customized with color matching to AMS-STD-595 or Pantone Matching System.

Evaluating fluorosilicones

Because fluorosilicones must meet stringent aircraft operating environment requirements, it’s critical to properly validate functions when developing specific solutions. Fluorosilicones should be evaluated against MIL DTL 25988C, which outlines elastomer testing requirements. Softer fluorosilicones may not have the same mechanical strength as harder fluorosilicones, but they will still have excellent temperature and fuel resistance.

Testing an elastomer’s performance can ensure stability while resisting degradation from harsh temperatures and fuel exposure.

Thermal stability: The elastomer’s ability to resist mechanical breakdown, thermal stability ensures the fluorosilicone can function properly during operation as well as reduce downtime and repair. Thermogravimetric analysis (TGA) is widely accepted to evaluate the thermal stability of silicones and fluorosilicone polymeric materials. It evaluates changes in mass when exposed to incremental temperature changes.

Weight loss: Conducted in a convection oven, this test evaluates how the fluorosilicone reacts when exposed to high-heat conditions for an extended time.

Swelling: The swelling test measures the percentage of mass change when a material absorbs hydrocarbons.

Other considerations

The key to efficiently identify the right fluorosilicone is to collaborate with an experienced silicone technology partner. It’s critical that the formulator possesses deep expertise in each unique application and key performance requirements. It’s also important that a provider complies with all relevant regulations and has an established aerospace heritage.

Fluorosilicones are ideal for mission-critical aircraft systems in which failure is not an option. Understanding these elastomers, how the materials perform, and what to consider when developing a solution can help pinpoint the right solution in aircraft applications.

Avantor NuSil

About the authors: Michelle Velderrain is senior product and application specialist, and Robert Umland is director of advanced technologies, NuSil – part of Avantor. They can be reached at 805.684.8780 or silicone@nusil.com.