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4. Two-part Silicone (Two-Part)
Like epoxy, silicone encapsulation is in widespread use in the
high voltage industry. Poor adhesion is perhaps the biggest
shortcoming of silicones. However, preparing the surface properly
can alleviate this. Some silicones are adversely affected by certain
materials. In the presence of these materials, the curing process is
inhibited, and the silicone never cures.
In addition, most silicones do not have good thermal
conductivity, as compared to most epoxies.

5. One-part Silicone
One part silicones are available in “self leveling” and
thixotropic form. Thixotropic materials do not flow and remain in
whatever shape they emerge in from the tube. It is somewhat like
shaving cream in that respect. Prior to curing, the thixotropic
silicone can be formed into a desired shape. Air bubbles can be
trapped in cured thixotropic silicones, and thus their use is limited.
One more thing to watch for is that some one-part silicones will
corrode electronic assemblies.

6. Oil
Oil is in widespread use in part due to its relatively low cost and
thermal characteristics.
Prior to 1977, some oils were dangerous due to the inclusion of
toxic PCB’s (polychlorinated biphenyls), but this is thankfully an
issue of the distant past. Considerations include the need to have a
sealed housing with a provision for expansion and contraction.
Oil can easily become contaminated by arcing and with the
buildup of foreign materials. In addition, it can be messy to work
with oil.

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