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T. C. WILLIAMS, R. A. PIKE, and F. FEKETE'
Silicones Division, Union Carbide Corp., Tonawanda, N. Y.
Cyanosilicone Elastomers A New Class of Solvent-Resistant High-Temperature Rubbers f o r . . . . 0
adhesives
electric cable components
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solvent barriers
sealing compounds
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enca psu1at ing compositions
automotive and aircraft components
Cyanosilicone elastomers show new combinations of solvent resistance and low-temperature flexibility, retain characteristic high-temperature oxidative stability of silicones
c
ONVENTIONAL dimethylsilicone elastomers are highly useful largely because they remain rubberlike over a wide temperature range. But with only moderate resistance to solvent swelling, their utility is limited when solvent contact is a prominent factor. To fill this gap, a new class of silicone elastomers, the cyanoalkylsilicones, has been prepared, which not only shows high resistance to the swelling action of many solvents, but also has improved low-temperature flexibility properties and excellent oxidative stability up to 250' C.
Preparation Cyanoalkylsilicone elastomers are prepared by methods similar to those used for dimethylsilicone elastomers. The cyanoalkyl function can be attached to silicon by adding silanic hydrogen to olefinic nitriles, as in the addition of trichlorosilane to acrylonitrile ( 4 ) . By choice of suitable catalysts and reaction conditions, particular isomers can be prepared ( 5 ) . The cyanoalkylchlorosilanes are hydrolyzed alone or with other organochlorosilanes to form partially condensed siloxane polymers, from which cyanoalkylsilicone gums are prepared by further condensation. Families of cyanoalkylsilicone copolymers, varying from free-flowing fluids to viscous gums, are made by adjusting molecular weight, structure of the cyanoalkyl group, and concentration of the cyanoalkylsilicone component in the copolymer. Fully condensed cyanoalkylsilicone gums are clear, viscous polymers, similar in appearance to dimethylsilicone gums. Compounding with fillers and curing agents is accomplished by any of the wellknown rubber processing techniques. The compounded stoc 1 Present address, Mellon Institute, Pittsburgh, Pa.
able for further fabrication by molding, extruding, solvent coating, etc.
Effect of Structure The structure of the cyanoalkyl group definitely affects the oxidative thermal stability of the corresponding silicone polymers (Table 1). Substitution of a cyano group at beta or gamma positions markedly increases the oxidative stability over that of the parent alkyl group or of a-substituted structures. T h e stabilizing effect of the cyano group is sufficiently great in certain structure'sj .such as the y-cyanopropyl group, that the oxidative thermal stability of the polysiloxane is of the same order as that of methylsilicone. By use of suitable structures, cyanoalkylsilicone elastomers possessing a high degree of oxidative thermal stability have been prepared.
Table I. Oxidative Thermal Stability of Organosiloxanes Depends on Structure of Cyanoalkyl Group (Air, 250' C., 100 hours) yo Organic Carbon Organosiloxane Retained 96.7 Methyl Ethyl 6.7 a-C yanoethyl
