Elastomers aid in defense applications

Failure in any component is not an option in critical defense applications. When it comes to some silicone-based components, such as sealings and coatings, degradation can lead to a range of negative consequences, from more frequent repairs to increased downtime or premature failure.

In the early years of the defense industry, it was standard to use dimethyl silicones, which are naturally permeable, easily absorbing fuels, crude oil, de-icing fluids, and other hydrocarbon-based solvents. But when dimethyl silicone absorbs these substances, it swells, weakening the silicone network and degrading mechanical properties and dimensional shape. In addition, while dimethyl silicone offers thermal stability in many applications, prolonged exposure to sunlight can impact the formulation and cause cracking.

In the 1960s, it was discovered that cracking and stripping of these standard silicones lead to increased frequency of repairs and downtime. To reduce these risks and ensure the defense equipment could continue to serve safely and effectively, a material solution was required that could resist the cold, the heat of engine exhaust, and UV exposure – without sacrificing performance.

The solution proved to be fluorosilicones. Formulated to be less soluble to fuels and organic solvents, these elastomers resist swell and maintain their physical properties and dimensional shape, making the material more resistant to mechanical failure caused by hydrocarbon exposure. In addition, fluorosilicones resist breakdown at very high temperatures and remain flexible at extremely low temperatures.

Since then, fluorosilicones have accumulated decades of defense service, demonstrating the material provides reliable protection from breakdown, even against prolonged exposure. Defense manufacturers now widely use these elastomers for many functional areas.

Their versatility makes them ideal as:

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

Fluorosilicones are available in various forms and cure chemistries, and are ideal as molding compounds, paints, coatings, and one- or two-part adhesives. With the level of versatility these elastomers offer, defense manufacturers can optimize processing and application efficiency while providing options based on the chemical and physical end-use requirements.

How fluorosilicones are evaluated

Since fluorosilicones are used in place of dimethyl silicones specifically because of the unique operating environment, the material’s functional characteristics must be validated. Fluorosilicones are evaluated against the military specification Mil-DTL-25988C, which the Air Force developed for elastomers included in this specification.

These specifications outline heat aging and mechanical properties after the elastomer has been exposed to heat and hydrocarbons. Fluorosilicones meeting these specifications can withstand temperature extremes and hydrocarbon exposure longer without breaking down.

Mil-DTL-25988C focuses on three specific areas to ensure the formulation demonstrates superior stability while resisting degradation from harsh temperature extremes and fuel exposure.

Swell: The swell test measures the percentage of mass change occurring when a material is exposed to hydrocarbons, over a set time.

Thermal stability: This evaluates the fluorosilicone’s ability to resist mechanical breakdown, ensuring the elastomer can function properly during operation while reducing repairs and downtime. Thermogravimetric analysis (TGA), which measures mass changes when exposed to incremental temperature shifts through time, is widely used to evaluate thermal stability of silicones and fluorosilicones.

Weight loss: Conducted in a convection oven, this evaluation measures how fluorosilicone reacts when exposed to high heat for an extended time.

When identifying a silicone solution, collaborate with a fluorosilicone partner that can provide elastomers with proven superior performance, stability, and mass change resistance.

Determining the right choice

Two key factors dictate whether a fluorosilicone is needed: the operating environment and/or the hydrocarbons the fluorosilicone may be exposed to during the operation, fueling, or maintenance. Consider whether the material solution requires:

• Resistance to swell when exposed to hydrocarbons
• Resistance to breakdown when subjected to temperatures above 200°C (392°F)
• Resistance to cracking when exposed to UV rays
• Flexibility at temperatures below -40°C (-40°F)
• Retaining softness and pliability to reduce stress

Selecting a fluorosilicone

Several questions can help determine if a fluorosilicone is needed and which type is required to suit the manufacturing process and end-use application.

The most efficient way to identify the right fluorosilicone is to collaborate with an experienced silicone technology partner. In addition to possessing a deep understanding of silicone chemistry, a formulator must offer extensive expertise in unique applications, key performance requirements, and manufacturing processes. Formulations can also be customized to include the addition of functional fillers, adjustable cure times, and other processing requirements, as well as tailored with AMS-STD-595 or PMS color matching.

An expert partner can produce novel fluorosilicone solutions customized for flow rate, viscosity, conductivity, and cure rate. For example, if a defense company needs to significantly reduce the cure rate for adhesives, spray-on coatings, or gap-fill materials, an experienced silicone provider can formulate an elastomer with faster cure times so the company can reduce downtime, operational complexity, and costs.

In addition to choosing a fluorosilicone partner with an established heritage, it’s important to select a provider that follows quality management standards, including certifications with AS9100 and ISO 9001 with all relevant regulations.

Fluorosilicones are ideal for mission-critical defense systems. With many forms and cure chemistries, their versatility makes them ideal for the conditions aircraft must safely and effectively endure. Understanding fluorosilicones, how these elastomers perform, and what to consider when selecting a formulation can help pinpoint a solution providing excellent stability and superior degradation resistance in some of the world’s harshest environments.

About the author: Michelle Velderrain, is an applications engineer at NuSil – a division of Avantor.

NuSil – a division of Avantor https://nusil.avantorsciences.com/nusil