Fluorosilicones

April 1, 2012

A broad operating temperature range, fuel resistance and long-term reliability are some of the unique properties of fluorosilicones that make them useful in aerospace applications. To demonstrate their effectiveness, NuSil recently compared fluorosilicones to standard dimethyl silicones in a series of tests.

Percent Swell

Figure 1. Percent mass change of fluorosilicones and dimethyl silicones.

Typically, polymeric materials will degrade and swell when exposed to hydrocarbons. In order to demonstrate the effectiveness of fluorosilicones in these environments, the NuSil Technology Swell Test, TM038, was used. Based on ASTM D471, cured samples were prepared at 1" × 1" × 0.070" and tested for specific gravity before and after 24, 48 and 72 hours in JP8 fuel. The percent swell is graphed as the percent difference in specific gravity starting at t= 0 (Figure 1).

When exposed to JP8 fuel, the dimethyl LSR and HCR that were tested both swelled above 150% of their original size, while the fluorosilicone with the greatest swell, FS-3775, underwent a comparatively low 20% change in mass. FS-3511 performed the best out of all the fluorosilicone formulations, with 5% change in mass over 72 hours in JP8. FS-3781 swelled 7% throughout the soak, and FS3-3730 (100% Mol fluorosilicone) exhibited a sustained 6% swell, with outstanding mechanical performance after thermal aging.

Thermal Stability

Figure 2. Hardness: Shore “A” durometer of FS3-3730 after exposure to 150 °C.

To test thermal stability, slabs of 100% Mol fluorosilicone FS3-3730 were suspended in an oven at 150°C (302°F) and then tested for durometer, (ASTM D 2240), 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(Figure 2).

After 192 hours at 150°C, FS3-3730’s durometer increased by 14%, while elongation, tensile and tear values did not exhibit significant trends.

Dynamic Mechanical Analysis (DMA)

As shown in Figure 3, DMA testing based on ASTM D4065, D4440 and D5279 showed that viscoelastic properties are dependent on temperature.

Figure 3. Dynamic Mechanical Analysis for FS3-3730.

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 elasticity, viscosity, and viscoelasticity of FS3-3730, respectively. This material performs down to -61ºC, the glass transition point (Tg) represented by E” peak (peak of the blue line). While the standard Tg for dimethyl silicones is typically lower, fluorosilicones champion fuel resistance and still perform well in low temperatures.

Conclusion

For projects requiring elastomers in contact with hydrocarbons, especially fuels, fluorosilicones are ideal materials. They have high and low temperature performance and can be designed to cure in a variety of configurations. Fluorosilicones offer a new solution to difficult problems involving fuel resistant elastomeric applications.