Silicone
ASTM D1418, ISO 1629 Designations: MQ, PMQ, VMQ, PVMQ
ASTM D2000 / SAE J200 Type/Class: FC, FE, GE
Standard Color: Red
Trade Names:
- Baysilone® (Bayer Corp.)
- KE® (Shincor Silicones)
- Silastic® (Dow Corning Corp.)
- Silplus® (General Electric)
- Tufel® (General Electric)
Relative Cost: Moderate
General Temperature Range: -65° to +450° F
Though carbon and hydrogen are part of their chemistry, silicones are primarily based on a sequence of silicon and oxygen atoms rather than a long chain of carbon atoms. This silicon-oxygen backbone is much stronger than a carbon-based backbone, making silicones more resistant to extreme temperatures (-65° F to +450° F, -54° C to +232° 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.
There are four different silicone formulations in use today. Standard methyl silicone is known simply 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 lower 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 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.
Silicones are not well suited for dynamic use due to their high friction characteristics, low abrasion resistance, and poor tear and tensile strength. Many silicones also suffer from above average mold shrinkage. Though they can be utilized in high aniline point oils, silicones are considered non-resistant to petroleum oils. Silicones swell considerably in both aliphatic and aromatic hydrocarbon fuels unless a special compound is formulated. Silicones are also very gas permeable.
MQ performs well in:
- Chlorinated diphenyls
MQ does not perform well in:
- Concentrated acids
- Ketones (MEK, acetone)
- Water
