CHEMICALS
New Silicones Contain Nitrile Groups Experimental polymers developed by GE have high dielectric constants, are insoluble in hydrocarbons General Electric's silicone products department, Waterford, N.Y., has turned up a new class of silicone fluids, is now offering them in experimental quantities. The fluids are copolymers of /?cyanoethylmethyldichlorosilane and dimethyldichlorosilane. These are combined in proportions to give varying amounts of carbon-functional nitrile groups. The company is now busy developing uses for the materials. Areas of application will probably be based on the fluids' electrical and solubility properties: They have high dielectric constants, are insoluble in most hydrocarbons. Four different nitrile silicones are now available in pound lots (at $11 to $16 per lb.) for testing. Although it's possible to make compounds with cyanoethylmethyl siloxy mole percentages ranging from 0 to 100, GE selected four as representative—those having 10, 25, 50, and 100%. This, GE explains, corresponds to 5, 12.5, 25, and 50% substitution of cyanoethyl units for methyl units in the dimethyl polysiloxane structure. The nitrile containing fluids are formed when the monomers copoly-
merize by means of a hydrolysis reaction. The resulting siloxanes are treated with hexamethyldisiloxane (chain stopper) in a standard equilibration step, using a catalyst, GE explains. Devolatilizing the mixture by heating under vacuum then increases the average molecular weight of the mixture. Polymer chain lengths are random, but average about 20 units. Depending on the amount of chain stopper used, however, chain lengths could go as high as 1000 units. Not Common. Interest in carbonfunctional silicone chemistry is beginning to build up, GE says. But, while some silicones with carbon-functional groups, such as glycol-silicone copolymers, are available, they aren't very common. GE thinks that one area of interest in these fluids will be due to their unusual electrical properties. The dielectric constants of this family range from a low of about 3 at 60 cycles to a high of 19.6 at 100 kilocycles, compared to about 2.75 for a typical dimethyl silicone. The fluids thus have value as high density capacitor dielectrics at high frequencies.
GENERAL FORMULA. General Electric's N. G. Holdstock (left) and Dr. E. A. Kerns, manager of research and development at the company's silicone products department, sketch the general formula for the new nitrile containing silicone fluids. Four fluids with varying amounts of nitrile are now available in experimental quantities 46
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Complicating this, however, is the dissipation factor, which is low follow nitrile containing groups, high for the high nitrile groups. This dissipation factor, though, can be an advantage when the fluids are used as antistatic agents for plastics or natural or synthetic textiles. The fluids can also serve as nonreactive, nonvolatile plasticizers. In this use, they could impart antistatic properties along with plasticizing. Another feature of the compounds is their polarity, which increases in direct proportion to nitrile percentage. The low nitrile containing fluids are soluble in such nonpolar solvents as toluene, mineral spirits, kerosine, acetone, diethyl ether, xylene, benzene, hexane, carbon tetrachloride, dioxane, butanol, methyl ethyl ketone, and gasoline. However, fluids with high amounts of nitrile are insoluble in all but dioxane and methyl ethyl ketone. These solubility properties, GE hopes, will make the fluids valuable where oil and solvent resistance is needed. Although the fluids are polar, they are not miscible with water. On the other hand, treatment with them doesn't impart water repellency. They are also stable in water, show no change in viscosity or any evidence of hydrolysis or decomposition after being boiled for 90 hr. in distilled water. Hydrolysis does occur with strong acids or bases, however. Thermal properties represent another area that could make the new fluids useful. Dimethyl silicones are stable in air at 200° C. for from 80 to 100 hr. Yet with as little as 3 to 5 mole % nitrile, the system becomes stable in air for from 1500 to 2000 hr. Where the mole percentage of the nitrile is higher than about 5%, the cyano group begins to break down, giving carbonaceous materials. This produces off-color and makes the materials unsuitable for some purposes. But it also enhances their possible use in ablative coatings for missiles. One other area GE is investigating is antifoaming agents. Because of their insolubility in hydrocarbon systems, the new fluids may work with nonaqueous systems, just as the dimethyl silicones do with aqueous ones. Distillation, dry cleaning, and crude oil handling are a few examples of such nonaqueous systems. GE plans to make the carboxy derivatives of the fluids, will offer them in experimental quantities.
BRIEFS Liquid Novalac resin for high tempera ture potting, casting, dipping, and coating compounds is available from Isochem Resins Co., Providence, R.I. Isochemrez 460 is stable from —60° to 280° C , and when used with Hard ener #63, will not degrade under long, high temperature operations, the com pany says. C1 Two liquid vinyl stabilizers that im prove polyvinyl chloride formulations for calendering, molding, and extrud ing are available from Nopco Chemi cal Co., Newark, N.J. The products, called Metasap ABC-51 and 52, are cadmium-barium-zinc liquids, light yellow in color, with specific gravi ties of 1.02 and 1.01, respectively, Nopco says. C2
Flame retardant epoxy resin for hotmelt castings and dry lay-up laminat ing systems is available from Union Carbide Plastics, New York, N.Y. The new epoxy, Bakélite ERL-0625, combines with aromatic amine or anhydride hardeners to form cured systems with excellent physical, electrical, and chemical resistance properties, the company says. Anhydride hardeners give the best high temperature stability. Semisolid at room temperature, the resin requires only moderate heating to become liquid. C 3
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