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JUMPING FENCES
Silicone and fluorosilicone are engineered to resist heat and weather.
by Rick Hudson
One thing that really sets RL Hudson apart from other suppliers – a Hudson Advantage, if you will – is our engineering expertise. Our highly trained and experienced engineers utilize Pro/ENGINEER® solid modeling design software, as well as proprietary automated design programs. They stress-test their designs using Abaqus® Finite Element Analysis (FEA) software to simulate service conditions.
But good part design will only take you so far; you also have to select the right materials for those parts to function effectively. The best-designed part in the world will still fail if the material it’s made from is unable to withstand, for example, the heat of a high-temperature application. With this in mind, here’s a look at two especially interesting rubber materials – silicone and fluorosilicone – and some thoughts about what makes them both so useful.
SILICONE RUBBER Most of the elastomeric materials used in the sealing industry have a chemical backbone composed primarily of carbon (C) atoms bonded together, often with hydrogens (H) attached. Silicones, on the other hand, are different. Though carbon and hydrogen are part of their chemistry, silicones are primarily based on a strong sequence of silicon (Si) and oxygen (O) atoms rather than a long chain of carbon atoms. The silicon-oxygen bond has a higher “bond disassociation energy” than a carbon-carbon bond. That is, it takes more energy to break the silicon-oxygen bonds, which gives silicone rubber its high temperature resistance. The illustration at right shows the molecular structure of a typical silicone.
Note that the bonds in silicone’s backbone are all single bonds. (For comparison, a double bond can be seen in the CH2CH vinyl group to the lower right of the image). The presence of nothing but single bonds is known as “main chain saturation” and is the primary reason why silicones are resistant to oxygen, ozone, and UV light, all of which are very damaging to many other elastomers. Double bonds are the primary attack sites for these agents, so silicone’s lack of double bonds along its main chain makes it more resistant. Silicones are odorless, tasteless, non-toxic, and fungus resistant. They also have great flexibility retention, especially at low temperatures, and low compression set, even at high temperatures.
There are four different silicone polymers in use today. Standard methyl silicone is designated under ASTM D 1418 as MQ. By replacing a small number (typically less than 1%) of the pendent methyl (CH3) groups in MQ with vinyl (CH2CH) groups, you arrive at what is known as vinyl methyl silicone, or VMQ. VMQ compounds tend to have better cure properties and undergo less compression set than standard MQ.
Replacing 5% to 10% of the methyl groups with ringed phenyl (C6H5) groups results in phenyl methyl silicone, or PMQ. PMQs have even better low temperature properties than MQ or VMQ. Finally, adding some of the aforementioned vinyl groups to PMQ results in phenyl vinyl methyl silicone, or PVMQ. PVMQs have enhanced heat and radiation resistance.
Of course, no material is perfect for every application. Though widely used in static applications (such as O-ring seals), parts made of silicone are not well suited for dynamic use due to their high friction characteristics, low abrasion resistance, and poor tensile and tear strength. Because of these poor physical properties, the use of silicone in shaft seals has declined. Also, modern lubricants have become more complex (i.e. incorporating more additives which tend to attack silicone). Silicones swell considerably in both aliphatic and aromatic hydrocarbon fuels. Silicones are also very gas permeable.
FLUOROSILICONE RUBBER Earlier I outlined the four main types of silicone polymers. In actuality, there are five, with the fifth being a combination of fluorine (F) with vinyl and methyl groups. Fluorovinylmethyl silicone rubber is more commonly known as fluorosilicone rubber. Fluorosilicone effectively combines the best properties (chemical and high temperature resistance) of fluoroelastomers (such as DuPont’s Viton®) with the good low temperature properties of silicone. Thus, fluorosilicone is notable for its resistance to solvents, fuel, and oil (similar to fluoroelastomers), as well as for its high and low temperature capabilities (as with silicones). Fluorosilicone has good resilience and excellent compression set resistance. Figure 2 shows the molecular structure of fluorosilicone.
Seals made from fluorosilicone are used extensively in aerospace fuel systems and auto fuel emission control systems, as well as in cold-weather applications such as pumps and valves used in the Alaska oilfields and pipelines. Fluorosilicone is primarily recommended for use in static seal applications. Its high friction characteristics, limited physical strength, and poor abrasion resistance make fluorosilicone inappropriate for most dynamic seals.
SAMPLE APPLICATIONS Having been part of the sealing industry for over two decades, we here at RL Hudson have seen our fair share of interesting and unique applications. Among those are many that involved use of silicone and fluorosilicone. For example, we have designed diaphragms for use in high-temperature fuel pumps, umbrella check valves such as are used in medical and automotive equipment, and O-rings for high-temperature fuel applications.
Again, the success of any given rubber part is ultimately dependent on both good design and the choice of an appropriate material. We here at RL Hudson stand ready to help you with both design and material selection. Silicone and fluorosilicone are good choices for many applications, but we offer lots of other choices, too. For detailed information about other materials, click here. Also, be sure to check out our Interactive Chemical Compatibility Guide. as well as a host of other useful features. Most importantly, don’t hesitate to call us at 1-800-722-6766 if we can answer any material questions or assist you with a design.
