Symposium on Lecture Demonstrations*
LECTURE DEMONSTRATIONS WITH SILICONES DAVID P. SPALDING General Electric Company, Waterford, New York
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its resilience for long periods at an elevated temperature (100 hours a t 400°F.) whereas natural rubber decomposes rapidly under these conditions (10hoursat400°F.). StGSi-0-Si-0--Si Although the rubber will burn at a high temperature i t residual silicone valences are satisfied with alkyl or differs from other rubbers in that it produces sand as aryl groups. The commercial process for producing the residue. This makes it particularly suitable as a these polymers is the hydrolysis of monomeric chloro cable insulation material, since even in case of fire the alkyl silanes, %SiCI,,, to the corresponding hydroxy rubber burns to non-conducting sand. To demonstrate compounds which immediately split out water to prc- these properties, silicone rubber strips and natural duce the basic siloxane network. In the silicones of rubber strips are immersed in a dry ice-acetone bath and commerce the R represents either the methyl or the flexed. The silicone lubber remains flexible while the phenyl radical. natural rubber becomes sufficiently brittle to be shatSilicones are available in three basic forms or states: tered mith a blow from a hammer. Similar strips are namely oils or fluids, resins,elastomer gum or compound. placed in a Bunsen flame. If the polymerization is run with difunctiooal mateOther typical examples of unusual high- and lom-temrial (&SiC12) with sufficient monofunctional material perature properties are demonstrated by: (1) Small (R3SiC1)present to stop the chain growth at relatively change of viscosity with temperature exhibited by the low levels, oils or fluids are obtained. fluids, making them excellent damping fluids; (2) high If the polymerization is carried out with considerable temperature resistance of the resins, which may be used trifunctional material (RSiCl,) present, cross linking as improved electrical insulating varnishes permitting occurs and resinous materials are produced. the design of lighter, more compact electrical motors If the polymerization is carried out with practically and transformers. pure difunctional material so that high-molecular-weight One of the properties of the silicones which has relong chains are produced, the product is a gum which ceived much prominence is the property of outstanding with suitable compounding and curing can be formed release from sticking. A mixture of sand and phenolic into a rubber-like substance. resin serves as a molding shell in foundry work. The Thus the silicones are available in three basic forms, mixture is cured in a master mold, and then itself used fluids, resins, and elastomers, all of which exhibit to as a mold. The release property of silicone may be different degrees the unusual properties discussed here- exhibited with ordinary household molds. One mold is after. treated with silicone oil and the other unt.reated, and The basic properties of the silicones which make them both of them are placed in the oven for a few minutes unusual substances may be classified as follows: (1) mith the molding compound to cur: the resin. Resistance to high and low temperatures, (2) ontstandSilicone release material shave found many application^ ing release from sticking, (3) unusual surface proper- in industry. In the molding of rubber tires mannties, (4) chemical inertness. facturers had sought a release material which would not I should like to illustrate these properties with a few decompose at the temperatures used in the molding of typical examples of their application and point out the product. With the advent of silicone emulsions several other applications which may serve as a guide such a release agent was realized, along with added for demonstrations. advantages such as longer mold life and less down time A typical example of the high- and low-temperature for cleaning. A similar application has been found in properties of the silicones is exhibited by silicone rubber. the food baking business. Silicone-resin-treated pans It remains elastic and pliable a t temperatures as low provide release of baked breads by merely tipping the as-10O0F., whereas natural rubber becomes brittle pan over. The same pan can be used as many as 500 under these conditions. Silicone rubber maintains times before retreatment is required. Besides this 'Presented at the 120th Meeting of the American Chemical obvious advantage, the messy job of cleaning grease blackened pans is eliminated, as is also much of the Society, New York, September 7, 1951. silicones are polymeric substance, the basic skeleton of which is the siloxane structure, where the
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general cleaning problem of the baking room. Thus the amazing release properties of the silicones have found many practical useful applications in American industry. The silicones exhibit a variety of unusual surface properties such as water repellency, antifoam action, and low surface tension. Probably one of the best known surface phenomenon is that which is exhibited in combination with waxes in the production of the new car and furniture polishes. Here the silicone gives the easy mb;out to the wax and produces a lustrous oil-freefinish. The water-repellency-producing property of the silicones is one which is attracting more and more attention. Thus it is possible t o produce water repellency
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with small quantities of silicones. We speak of water repellency as distinguished from waterproofness. A water repellent does not produce an ahsolute barrier, but presents a porous surface which can still "breathe" even though it repels water. This property can be demonstrated with laboratory filter paper. A water repellent is produced on its surface by treatment with chlorosilane vapors (silicone monomers) which react with the surface fibers of the paper to produce the silicone polymer "in situ." The water repellency of this material is easily shown. That the paper is still porous is demonstrated by filtering a mixture of hydrocarbon solvent and water. The hydrocarbon passes through the paper and the water is held behind.
