Beyond Boundaries: Elevating Aerospace Design with Fluorosilicones

August 1, 2024

As aerospace engineers push the boundaries of new frontiers, the need for advanced materials that can withstand the rigorous demands of these advanced applications is relentless. These materials go beyond functionality; it is about ensuring reliability in the skies, where failure is not an option. Fluorosilicone can help do exactly that.

In the 1960s, the U.S. Air Force noticed that conventional silicone-based sealants, coatings, and other components degraded rapidly when exposed to fuels, de-icing fluids, and other hydrocarbon-based solvents. Dimethyl-based silicones are non-polar and easily absorb hydrocarbon-based solvents, which may result in material swelling, mechanical weakening, and ultimately, failure.

The frequent need for repairs and the resultant short service life of these dimethyl materials initiated a search for a more durable and reliable solution robust enough to withstand the harsh environments of aerospace operation. Fluorosilicone was the answer. Engineered as an innovative solution to the application limitations of standard silicones, this family of elastomers was formulated to help resist the detrimental swelling effects caused by exposure to hydrocarbons such as jet fuels, oils, and solvents.

Bring Durability and Performance to Aerospace Applications

The introduction of fluorosilicones helped solve the immediate issues faced by the aerospace sector, but also set a new standard for material performance in challenging operational environments. Consequently, these types of silicones have become a critical component in the design and manufacturing of aircraft, contributing significantly to the reliability, safety, and longevity of aerospace systems.

Decades of flight service have since proven that fluorosilicones provide reliable breakdown protection, even when exposed to harsh environments for prolonged periods of time. Now, they are used widely across industrial, aerospace, and defense applications:

Adhesives and coatings: Fluorosilicones serve as strong adhesives and protective coatings for structural parts, offering enhanced resistance to environmental stressors while maintaining bond integrity and surface protection.
Electrical insulation: The insulating properties of these elastomers protect electrical wires and components against high temperatures and chemical exposures, preventing breakdowns in crucial electronic systems.
Engine components: Given their resistance to high temperatures and mechanical stresses, fluorosilicones are ideal for gaskets, O-rings, and seals within aircraft engines.
Environmental sealing: These elastomers can help protect components exposed to the harsh environmental conditions, such as UV radiation and oxidation encountered in flight.
Fuel systems: Fluorosilicones are extensively used in fuel tanks, lines, and seals because of their superior resistance to jet fuel and other hydrocarbon solvents.
Vibration dampening: Fluorosilicone gels and foams help absorb and dispense energy to protect mounted electronics and instrumentation.

Fluorosilicone is a versatile material that is available in a variety of forms and cure chemistries. From molding components or paints to one or two-part adhesives, it gives users the flexibility to optimize processing and application efficiency based on the application

Guide for Selecting a Fluorosilicone

Figure 1. Since fluorosilicones are versatile elastomers with a wide range of standard and custom options, it is important to consider these factors when selecting a formulation. (Image: Avantor)

If the application exposes the material to hydrocarbons during the aircraft’s flight, fueling, or maintenance, then consider fluorosilicone. Likewise, a fluorosilicone is an ideal option if the material will be subjected to extreme environments such as very high or low temperatures, or UV exposure. Consider the questions featured in Figure 1 when deciding if a fluorosilicone is needed.

If the application requires a fluorosilicone, the material should provide resistance to swelling when exposed to hydrocarbon and resistance to cracking when exposed to UV rays. Additionally, the material should provide resistance to breakdown when subjected to temperatures above 200 °C while remaining flexible at temperatures below -40 °C. It should also remain soft and pliable to reduce stress.

Validation Ensures Degradation Resistance

Figure 2. A chart showing the percentage of weight loss in air via thermogravimetric analysis (TGA) that can occur in fluorosilicones. (Image: Avantor)

Once fluorosilicone is selected as the material of choice, it is important to ensure the formulation’s functional characteristics are validated. Fluorosilicones are tested against the requirements outlines in military specification MIL-DTL-25988C, developed by the U.S. Air Force to evaluate elastomers. Fluorosilicones must demonstrate specific heat aging and mechanical properties after hydrocarbon exposure.

MIL-DTL-25988C tests elastomers against three specific criteria:

Swell: This evaluation measures the percentage of mass change that occurs when a material is exposed to hydrocarbons, such as jet fuel, over a specific time period.
Thermal stability: The widely used thermogravimetric analysis (TGA) measures mass changes when the material is exposed to incremental temperature shifts over time. By evaluating the fluorosilicone’s ability to resist mechanical breakdown, this test ensures the elastomer can function properly during operation while reducing repairs and downtime.
Isothermal weight loss: This test, which occurs in a convection oven, measures how fluorosilicone reacts to extended exposure to highheat conditions.

The following four different fluorosilicone materials were tested per MIL-DTL-25988C requirements:

High-temperature fluorosilicone: a 100 percent fluorosilicone, platinum-cure system designed to have better performance at high temperatures.
Standard fluorosilicone: a multi-purpose, one-part acetoxy adhesive.
Broad operating temperature fluorosilicone: a one-part acetoxy adhesive specifically designed to have better mechanical performance at lower temperatures.
Molding fluorosilicone: a 100 percent fluorosilicone, platinum-cure system designed for precision molding.

Figure 3. A chart showing the percentage of weight loss that occurs in fluorosilicones at 275 °C. (Image: Avantor)

Swell testing was conducted in JP8 jet fuel at 60 °C for 7 days; the average percent change in mass across the four fluorosilicone materials was 9 percent, and the control polydimethylsiloxane sample and polydimethyldiphenylsiloxane sample showed an average mass change of 134 percent. The test conditions of DTL 25988C are much less harsh at 24 °C for 22 hours with the specification at 1 to 30 percent max percent mass change. These fluorosilicone materials easily meet this specification.

Thermal stability testing exposes each material to a ramped heating rate of 10 °C/min from room temperature to 600 °C. The test monitors mass changes over the temperature range in ambient air, where thermo-oxidative degradation is most likely to occur. All four of the mentioned formulations lost less than 3 percent mass up to 325 °C, with the high temperature material performing at 3 percent mass loss up to 400 °C.

Figure 4. When swell testing was conducted in JP8 jet fuel at 60 °C for 7 days; the average percent change in mass across the four fluorosilicone materials, shown in the chart, was 9 percent. (Image: Avantor)

The TGA results are well-aligned with the isothermal testing of ASTM D2288, where materials were tested at 275 °C for one hour and measured for mass loss. All four materials lost less than 3 percent mass, with the high temperature fluorosilicone material posting 0.7 percent mass loss.

The results of these three test sets demonstrate the high-performance capabilities of fluorosilicones across multiple chemistries. When an elastomer meets MIL DTL 25988C specifications, it has shown that it is robust enough to withstand hydrocarbon exposure and temperature extremes for longer time periods without degradation.

Further Selection Considerations

Fluorosilicone materials are available in standard and custom formulations. An expert silicone partner can tailor a formulation to meet your application or process needs including viscosity, rheology, cure rate, conductivity, color matching, and beyond.

With versatile options for forms and cure chemistries, they are ideal for the extreme conditions of aerospace applications. By understanding how fluorosilicones perform and what to consider when choosing a formulation, aerospace manufacturers can select a solution that provides excellent degradation resistance and reliable stability for applications in which failure is not an option.

Aerospace designers can set a project up for success by keeping the following considerations in mind.

Start collaborating with a fluorosilicone partner as early as possible in the design process to ensure the material meets manufacturing and performance needs. Work with a partner that has an established flight heritage and a strict quality management standard, including certifications with AS9100 and ISO 9001 with all relevant regulations. Choose a fluorosilicone partner able to customize formulations to meet your process requirements or color matching.

This article was written by Daniel Hess, Applications Engineer, NuSil – a brand of Avantor (Bakersfield, CA). For more information, visit here .

Reference

Janecka, H. (1964). Aging of Natural and Synthetic Rubber and Rubber Products. Prevention of Deterioration Center, Division of Chemistry and Chemical Technology, National Academy of Sciences, National Research Council.