SILICONES — BASS 231 



heat and moisture, the principal enemies which conspire to reduce the 

 life of electric equi^jment. 



WATER REPELLENT 



In addition to heat stability, other useful properties are inherent in 

 the silicone chemical structure. All silicones are water repellent, and 

 the methyl silicones are particularly so. The methyl silicone oils are 

 long-chain structures of silicon atoms carrying two methyl groups 

 each. The silicon atoms are joined to each other through an oxygen 

 atom, the hydrocarbon portion of the molecule acting like a paraflinic 

 umbrella for the rest of the molecule. Applied to the surfaces of glass 

 or ceramic insulators, the silicon-oxygen-silicon portion of the mole- 

 cule attaches itself to the surface, leaving the hydrocarbon portion 

 upward. When water or moisture condenses on this surface, it does 

 not do so in a continuous film, as in the case of an untreated glass 

 surface, but in distinct droplets. This silicone treatment of insulators 

 in radio sets, for example, prevents leakages across the surface because 

 no continuous film of moisture forms a conducting path. 



The waterproof property of silicones was utilized in a translucent 

 and nonmeltable silicone paste, which proved to be quite essential to 

 the proper operation of aircraft ignition systems and disconnectible 

 joints in radar systems during the war. It served as an auxiliary 

 dielectric and waterproofing seal for the ignition cable insulation 

 where it entered the spark plug wells and magneto plugs of military 

 aircraft. It prevented condensation of moisture at these points and 

 kept the ignition system from being short-circuited. Another water- 

 repellent application of silicone oils was initiated during the war and 

 is still being developed — the production of a water-repellent filler 

 made of fine glass fibers for use in life jackets and as thermal insulation. 



STABLE VISCOSITY 



One of the fundamental properties of long-chain silicone molecules 

 is their resistance to associating with each other in ordered arrange- 

 ment at low temperatures. These silicone fluids are characterized by 

 an exceptionally flat viscosity-temperature slope. They do not thin 

 out at elevated temperatures or thicken at low temperatures to so 

 great an extent as do the petroleum fluids. Another manifestation 

 of this property is found in the flexibility and resilience of silicone 

 rubber at low temperatures. As a matter of fact, this silicone product 

 retains its flexibility at temperatures only slightly above that of dry ice. 

 Because of its heat resistance it is useful at temperatures up to 200° 

 centigrade. It is, therefore, useful over a wider range of temperature 

 than any other material having rubberlike properties. 



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