Introduction: Fluorosilicones in the Aerospace Industry Nathan Lipps, NuSil Technology Fluorosilicone s advent into the commercial marketplace offers opportunities for manufacturers to pursue new applications for silicones in the automotive, aircraft and general markets. The unique properties of fluorosilicones provide a solution for products that need broad operating temperatures, fuel resistance and long-term reliability. Conventional dimethyl silicones cannot survive the harsh environments that fluorosilicones can endure. Similar to other silicones, fluorosilicones can be provided as high consistency rubbers (HCRs), liquid silicone rubbers (LSRs), dispersions, gels and even foams. Silicone manufacturers offer a variety of fluorosilicone solutions that fit the unique processes of any end-user. Whether fluorosilicones are used as adhesives, molded parts or protective coatings, flexibility of application is possible. Purpose: The intent of this whitepaper is to outline the chemistry of silicone and fluorosilicone; demonstrate the different types of fluorosilicones available for end-users processes; and highlight their performance in hydrocarbon, as well as high- and low-temperature environments. Silicone and Fluorosilicone Chemistry Figure 1 shows a chemical structure of a typical polysiloxane polymer a polymer backbone of alternating silicon and oxygen units with R groups on the silicon atom represent functional organic groups such as methyl, phenyl or trifluoropropyl. Figure 1. Typical polysiloxane polymer backbone. Silicones have highly unique properties compared to organic-based rubbers, as their ability to remain elastic at low temperatures and resistance to breakdown at high temperatures or in UV light make them valuable in harsh environments. The typical glass transition point (Tg) of dimethyl silicones is less than -65 ºC, and -115 ºC for a diphenyl silicone. Other properties are low modulus, resistance to moisture (<.4 %), high dielectric strength of typically 5 V/mil, low shrinkage for vinyl end-blocked silicones (< 1%), low ionic content (< 2 ppm) and formulation flexibility. Initially, the silicone polymer is produced, and silica is added as a reinforcing filler to improve the physical properties of the elastomer. This mixture is called a base. Acting Fluorosilicones in the Aerospace Industry Page 1 of 9
similarly to gravel in concrete, silica reinforces the cured silicone polymers through van der Waals forces and hydrogen bonding between hydroxyl groups on the silica surface and siloxane backbone of polymer. These weak interactions, multiplied by millions, increase the strength of the cured silicone. The silica is typically treated with organosilicones to help increase its compatibility in the polymer, stabilizing the viscosity to a certain degree and reducing crepe aging. Addition-cure silicones, based on a two-part platinum catalyst system, do not require moisture to cure. The cure involves the direct addition of the hydride-functional crosslinker to the vinyl-functional polymer forming an ethylene bridge crosslink (See Figure 2). Figure 2. Addition-cure system mechanism. Unlike one-part moisture cure silicone adhesives, the two-part platinum catalyst system involves no leaving group, allowing it to cure in closed environments. Most platinum systems can fully cure at room temperature in 24 hours, or the cure can be accelerated with heat. Fluorosilicones can be formulated with both platinum- and moisture-cure systems. Types of Fluorosilicones Within the flurosilicone family, several formulations are available that can be adjusted to fit specific applications. The breadth of material choices for these types of applications is attributable to advances in fluorosilicone technology based on trifluoropropyl methylpolysiloxane polymers (See Figure 3). While some fluorosilicones contain 1% trifluoropropyl methylpolysiloxane repeating units, other systems contain a combination of the fluorosiloxane units and dimethyl units to form a co-polymer. Adjusting the amount of trifluoropropyl methyl siloxane units in the polymer can modify the glass transition temperature, solvent resistance and other properties. Fluorosilicones in the Aerospace Industry Page 2 of 9
Fluorosilicones and Process Improvement Figure 3. Trifluoropropyl methylpolysiloxane The collaboration between silicone manufacturers and product engineers continues to result in more successes as advanced products are brought to market. Some of the key benefits of fluorosilicones include: Hydrocarbon solvent resistance Broad temperature ranges, compared to other polymeric materials Good tensile, tear and % elongation Fluorosilicones are offered in a variety of forms: Liquid injection molding (LIM) materials HCRs for extrusion, compression molding or calendering LSRs packaged in convenient side-by-side kits with static mix tips Greases, gels, fluids, foams and dispersions While most packaging provides manufacturers a way to process fluorosilicones in largescale environments, small side-by-side kits accommodate use with a way to easily process smaller quantities and prevent mixing errors. No hand mixing No de-airing No errors in mix ratio Less possibility of contamination when dispensed directly into the intended application Fluorosilicones in the Aerospace Industry Page 3 of 9
Figure 4. Fluorosilicone materials in different packaging sizes and configurations Functional fillers can be also added to fluorosilicones when making colored coatings, molded parts, repair butters, dispersions, sheets, ribbons or other applications. Common fillers include: Thermally and electrically conductive gapfillers Pigmentation to color match sprayable dispersions Microballoons to reduce density The ability to custom-formulate and improve end-user processes has created more applications for this technology, which is supported by manufacturers knowledge of silicone and fluorosilicone chemistry. Test Methods & Results To demonstrate the effectiveness of these products, NuSil tested standard dimethyl silicone in comparison to fluorosilicone material for percent swell over time. Thermal stability measured by exposure to temperature and a Dynamic Mechanical Analysis were also used to identify fluorosilicone properties. Percent Swell Using NuSil Technology Swell Test TM38, based on ASTM D471, and Specific Gravity TM3, based on ASTM D792, cured samples were prepared at 1 x 1 x.7. Samples were tested for specific gravity before and after 24, 48 and 72 hours of exposure Fluorosilicones in the Aerospace Industry Page 4 of 9
to JP8 jet fuel. The percent swell is depicted as the percent difference in specific gravity between t=. The exposed samples are reported below: % Mass Change 2 15 1 5 Percent Mass Change of Fluorosilicones vs. Di-methyl Silicones Di-methyl HCR Di-methy LSR FS-3775 FS-3781 FS-3511 24 48 72 Time (hr) Figure 5. Percent mass change of fluorosilicones, as compared to dimethyl silicones. Percent Mass Change of Fluorosilicones % Mass Change 25 2 15 1 5 24 48 72 Time (hr) FS-3775 FS-3781 FS-3511 Figure 6. Mass change of fluorosilicones only. Fluorosilicones in the Aerospace Industry Page 5 of 9
When exposed to JP8 fuel, the fluorosilicone materials maintained their relative mass, whereas the dimethyl silicone formulations experienced extreme swelling. The dimethyl LSR and HCR both experienced swell above 15% of their original size. FS-3511 performed the best out of all the formulations, with 5% mass change over 72 hours in JP8. and FS-3781 had 6% and 7% swell, respectively, after a 72-hour soak in JP8. Of the fluorosilicones tested, FS-3775 had the greatest swell due to its copolymer formulation, and it also had the best mechanical performance when exposed to thermal stability testing. Thermal Stability To test thermal stability, slabs of were suspended in an oven at 15 C (32 o F), and then tested for durometer, Type A (ASTM D 224), elongation (ASTM D 412), tensile (ASTM D 412) and tear (ASTM D 624), at 4, 8, 24, 48, 96 and 192 hours to determine physical properties after exposure to high temperatures. For each data point three specimens were tested and averaged. To be considered a passing data point the three values had to be within a 15% range of their median. 1 Mol% Fluorosilicone After Heat Exposure: Durometer 42 41 4 39 38 37 36 35 34 33 Durometer after Exposure to 15 C 4 8 24 48 96 192 Hours of Exposure Figure 7. Durometer of after exposure to 15 C. (specify type durometer) Fluorosilicones in the Aerospace Industry Page 6 of 9
Percent Elongation 4 35 3 25 2 15 1 5 Elongation after Exposure to 15 C 4 8 24 48 96 192 Hours of Exposure Figure 8. Elongation of after exposure to 15 C. 8 Tensile after Exposure to 15 C Tensile in psi 6 4 2 4 8 24 48 96 192 Hours of Exposure Figure 9. Tensile of after exposure to 15 C. Fluorosilicones in the Aerospace Industry Page 7 of 9
Tear lbft/in. 8 7 6 5 4 3 2 1 Tear after Exposure to 15 C 4 8 24 48 96 192 Hours of Exposure Figure 1. Tear of after exposure to 15 C. After 192 hours exposure to 15 C, demonstrated adequate resistance to high temperature. While the fluorosilicone s durometer increased by 14% (See Figure 7), elongation, tensile and tear values did not exhibit significant trends. Dynamic Mechanical Analysis (DMA) DMA testing was done to show that the materials viscoelastic properties are dependent on temperature. This testing based on ASTM D465, D444 and D5279 mapped the phase transitions undergone with temperature change. Fluorosilicones in the Aerospace Industry Page 8 of 9
Figure 11. Dynamic Mechanical Analysis (DMA) for. In Figure 11 above, the E storage modulus (green line), the E loss modulus (blue line) and the tangent delta loss modulus over storage modulus (red line) demonstrate the elastic property, viscous property and the viscoelastic property of, respectively. The E peak (top of blue line) represents the glass transition point, which is -61 ºC (Tg). The standard Tg for a dimethyl silicone is usually near -115 C, whereas is closer to -6 ºC. Conclusion 1) The results from swell testing exhibit that fluorosilicones are resistant to hydrocarbon solvents where standard dimethyl systems fail. 2) Thermal stability testing demonstrated that these formulations can withstand 15 C temperatures over time and maintain elastomeric physical properties. 3) A dynamic mechanical analysis revealed that a 1 Mol% fluorosilicone performs in temperatures down to -6 ºC. Advances in silicone and fluorosilicone technology have continued to bring solutions to end-users for their applications. Equally important, the survivability of fluorosilicones in harsh environments provides the performance required for unique applications. Fluorosilicones in the Aerospace Industry Page 9 of 9