SILICONES

Beginning in 1942, B-17 bombers were flown from factories in the United States to airbases in Great Britain. But before the Flying Fortresses could cross the Atlantic—much less raid German factories—a critical problem had to be solved. The thin air at cruising altitudes can be ionized by the high-voltage electricity of an aircraft ignition system. The effect is not large in the relatively dry air over land, but the water vapor in damp oceanic air is highly susceptible. The moist air would work its way into tiny pores in the rubber wires insulating the ignition system. Soon the pilot would notice a blue glow around the leading edge of an engine nacelle—a corona of electricity arcing from spark plug to cylinder, shorting out the plug. The engine would start misfiring, eventually dying altogether as more pistons quit. If more than one engine went south, the crew might have to ditch, and the "Fort" would be lost at sea. A silicone polymer developed for waterproofing electrical equipment aboard submarines proved to be the answer. Applied liberally to the spark plug wires and boots, the silicone grease kept the wires dry and the bombers airborne—it was, in fact applied to all U.S. bombers throughout the war.

Silicones are polymers whose backbones are long, flexible chains of alternating silicon and oxygen atoms. Dangling from the backbone like charms from a bracelet are side chains, usually small, carbon-based units, and the choice of these side chains gives silicones a remarkable range of properties. The water-repellent grease has oily, nonpolar side chains such as methyl groups. The nonpolar side chains and the polar water molecules do not mix, repelling the water from the silicone.

Another application of silicones depends on a careful balance between polar and nonpolar side chains. Small amounts of silicone foaming agents control the bubble size in polyurethane foams. A high proportion of polar side chains makes the foam foamier. The bubbles become bigger, forming open pores and producing the soft foams found in car seats and furniture cushions. Reduce the number of polar side chains, and the bubbles remain small. These tiny bubbles do not open up to form pores, and the foam is a much stiffer solid used for insulation.

Silicones have other, seemingly contradictory properties. A silicone resin coating the bread pans in a bakery keeps fresh-baked bread from sticking in the pan, and a liquid silicone polymer on the molds in tire factories does the same thing for newly made tires. But adding a "tackifying resin" makes the silicone sticky and produces the drug-permeable contact adhesive used on those skin patches containing nicotine (for smokers who are trying to quit) or scopolamine (for seasickness sufferers who are trying not to lose it).

Silicon and oxygen are the two most abundant elements on Earth, and they combine naturally to form silicates, including glass and such minerals as quartz and granite. These two elements were first combined synthetically—as silicones—in the United States in the 1930s. They were originally expensive and unhandy to make, but the discovery of a cheaper, easier method of producing them, coinciding with the interest in their novel properties sparked by World War II, started an avalanche of research into new uses for these versatile polymers that continues unabated today.



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