ASTM D 1418 Designation: VMQ

ASTM D 2000, SAE J200 Type / Class: FC, FE, GE

RELATIVE COST: Medium

GENERAL TEMPERATURE RANGE: -65° to +300° F

Though carbon and hydrogen are part of their chemistry, silicones are primarily based on a strong sequence of silicon and oxygen atoms (see Figure 39) rather than a long chain of carbon atoms (as with many hydrocarbons). This silicon-oxygen backbone is much stronger than a carbon-based backbone, making silicones more resistant to extreme temperatures (-65° F to +300° F, -54° C to 149° C), chemicals, and shearing stresses.

Due to saturation in the polymer’s main chain, silicones are very resistant to oxygen, ozone, and UV light. Of course, this same saturation also demands that the material be peroxide cured since it is not possible to sulfur cure a saturated polymer. In addition to being generally inert (non-reactive), silicones are odorless, tasteless, non-toxic, and fungus resistant. They also have great flexibility retention and low compression set.

As the compositions of modern lubricants have become more complex (i.e. incorporating more additives), use of silicone as a material in radial lip seals has declined. In actuality, silicones are not well suited for dynamic use due to their high friction characteristics, low abrasion resistance, and poor tensile and tear strength; silicones see only limited use as shaft seals for these reasons. (Fluoroelastomers and hydrogenated nitriles are often used instead.) Silicones typically undergo a large amount of shrinkage after molding, so special mold designs are required to compensate for this. Silicones swell considerably in both aliphatic and aromatic hydrocarbon fuels unless a special compound is formulated. Silicones are also very gas permeable.

SILICONE PERFORMS WELL IN:
• Mineral oils with low additive contents
• Ozone
• Dry heat

SILICONE DOES NOT PERFORM WELL IN:
• Concentrated acids
• EP fluids
• Fuels
• Gear oils
• Ketones (MEK, acetone)
• Steam

“Silicones are primarily based on a strong sequence of silicon and oxygen atoms rather than a long chain of carbon atoms.”

Figure 39