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ASTM
D 1418 Designations: MQ, PMQ, VMQ, PVMQ
ASTM
D 2000 / 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: Medium
GENERAL
TEMPERATURE RANGE: -65° to +450° 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
48) 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° to +450° F,
-54° 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 (see Figure 48).
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.
SILICONE
PERFORMS WELL IN:
• Engine & transmission
oils (mineral oils)
• Ozone
• Dry heat
SILICONE
DOES NOT PERFORM WELL IN:
• Petroleum oils & fuels
• Ketones (MEK,
acetone)
• Steam
• Concentrated
acids
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