January 1965 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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.