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January 1965 Popular Electronics
 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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 As far back as 1966 electronics hobbyists knew
that silicon bathtub caulk was an excellent flexible insulator for electronics. It originally
went by the name "Silastic," which is a
portmanteau
of "silicone" and "plastic," and is a type of RTV (room temperature vulcanizing)
compound. It
has a typical voltage withstanding of over 400 V/mil, or 400 kV/inch, which
is why it is used extensively on high voltage connections (Dow Corning 4 Electrical Insulating Compound is 450 V/mil).
Dow Corning, its inventor, still sells various compounds of
Silastic both as an insulator and as a molding compound. I used it
at Westinghouse Electric in the 1980's to seal metal molds for overmolding
towed sonar transducer
arrays with a polyurethane potting compound (it made really
bouncy balls, too). Caveat: Be sure to ascertain the published voltage withstanding strength
before using a hardware store variety of silicone sealant for critical applications.
Bathtub Caulk - A Miracle on the Electronics Bench
  For some years, manufacturers of airborne electronics
gear have been using a rubbery substance called "Silastic" to moisture-proof and insulate
holes through which wires pass, fill the backs of plugs, and to cover high-voltage terminals.
The substance is spongy, stretches like a rubber band, but spreads like toothpaste.
Then "Silastic," Dow-Corning's answer to the bathtub caulk problem, hit the hardware
stores. The author purchased a big tube ($2.95) for his electronics workbench and it
quickly proved to be indispensable. The caulk is just squeezed out of the tube and onto
wires or components, and allowed to cure for 24 hours. When dry, the excess can be cut
away with a razor blade. Imagination seems to be the only limit on the number of uses
for this substance.
• A spongy pad of caulk was bonded on both sides of a piece of TV twin-lead on
which a window opened and closed. The TV antenna terminals also received a coating to
prevent rust. "Silastic" was used in place of tape to seal a splice in the twin-lead
- unlike tape, it does not unravel.
• A transistor was mounted to a board by inserting it in a glob of caulk. The
component board was shock-mounted to a chassis in the same way. A tube socket was then
shock-mounted and isolated from the chassis with "Silastic" - the leads from the socket
pass through a hole lined with a caulk-formed grommet.
• To prevent vibration from being transferred to the baffle, an even surface
gasket was formed around the mounting rim of a speaker using caulk. Nicks in insulated
leads were filled, plugs sealed, and coax fittings protected. And, of course, you can
even use "Silastic" around your bathtub!
-R. C. Apperson, Jr.
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