United States Patent Office Patented Feb

3,168,542 United States Patent Office Patented Feb. 2, 1965 2 It is also often desired to separate mixtures of chloro 3,168,542 silanes having different organic substitutents attached to PROCESS FOR SEPARATENG MEXTURES OF silicon thereof into individual chlorosilane portions where CHOROSLANES EEgyd H. Saaffer, Sayder, N.Y., assignor to Unioia Car in each portion contains specific chlorosilanes having the bide Corporation, a corporation of New York same organic substituents attached to silicon thereof. No Drawing. Fied May 15, 1957, Ser.3. No. 659,204 Illustratively, in the manufacture of vinyl silicone elas 23 Claims. (C. 260-44S.2) tomers the proportion of vinyl groups contained by the elastomer affects the properties of the elastomers. By This invention relates to chlorosilanes and in particular controlling the amount of vinyl siloxane groups com to a process for separating specific chlorosilanes from mix O bined in the elastomers, the properties of such elasto tures of different chlorosilanes. mers can be controlled substantially as desired. This It is often desirable to separate mixtures of chloro can be advantageously accomplished by regulating the silanes of different functionalities into chlorosilane por relative amounts of substantially pure vinyl chlorosilanes tions such that each portion contains only chlorosilanes and coreacting chlorosilanes. When low purity chloro of a specific functionality and is substantially uncontami 5 silanes are employed additional variables are encountered nated by chlorosilanes of other functionalities. Illus thus complicating an otherwise simple operation. Simi tratively, it has been found that dimethylpolysiloxane larly, substantially pure diphenyldichlorosilane, dimethyl gums that are suitable for conversion to silicone elastom dichlorosilane, methylphenyldichlorosilane, and the like ers cannot be prepared directly by the hydrolysis of di are highly desirable. - methyldichlorosilane prepared in the usual manner with 20 Various processes had been suggested heretofore for out further treatment. One explanation for the difficulty Separating mixtures of different chlorosilanes such as, encountered is that the starting dimethyldichlorosilane is mixtures of dimethyldichlorosilane and methyltrichloro rarely, if at all, obtained in a pure form. That is, it silane. These processes were not found to be entirely contains methyltrichlorosilane in an amount of 0.3 to Satisfactory for several reasons including incomplete sepa 0.5 mole percent or more based on the total moles of 25 rations, low yields and/or inability to separate low con methyltrichlorosilane and dimethyldichlorosilane even centrations of chlorosilanes from the mixtures. Known when careful purification procedures have been employed. physical methods, such as distillation, are often not well Thus, upon hydrolysis of the starting silane and dehy Suited for the separation of mixtures of different chloro dration of the resulting silanol, a pure difunctional silox silanes. By way of illustration, tetrachlorosilane and ane product is rarely obtained. Any trifunctional silane 30 trimethylchlorosilane form an azeotrope and hence can impurities present are converted to trifunctional silox not be separated from each other by simple distillation. ane groups, such as monomethylsiloxane groups, result As a further illustration, some chlorosilanes, such as ing in the cross-linking of the siloxane chains of the prod dimethyldichlorosilane and methyltrichlorosilane, boil at. ucts. Such cross-linking of the siloxane chains of the close to the same temperature and also are not readily products due to the presence of the trifunctional silane 35 separated from each other by simple distillation. prevents the production of the soft gums into which fillers In order to separate mixtures of chlorosilanes, it had can be milled. It has been found that amounts of com been proposed, heretofore, that a mixture containing two bined monomethylsiloxane groups of more than from or more chlorosilanes and a compound such as a tertiary about 0.01 part to about 0.02 part by weight per 100 amine, a diorganodiacyloxysilane or an alkali metal hy parts by weight of the monomethylsiloxane groups, di 40 droxide can be mixed and subjected to such conditions methylsiloxane groups and trimethylsiloxane groups that that one or more, but not all of the chlorosilanes react are combined in the siloxane products prevent the use with the amine, the diorganodiacyloxysilane or the hy of the product as as siloxane gum which can be subse droxide to form products that are appreciably higher qently converted to desirable silicone elastomers. boiling than the remaining unreacted . chlorosilane or Amounts of combined monomethylsiloxane groups equal 45 chlorosilanes. The remaining unreacted chlorosilane or to or less than these amounts were not found to prevent chlorosilanes can then be removed from the reaction the use of the product as a siloxane gum, which can be mixture by distillation. These latter-mentioned means subsequently converted to desirable elastomers. of separating mixtures of different chlorosilanes are not Similarly, by way of illustration, trimethylchlorosilane, entirely satisfactory because of incompiete separation, when produced by conventional means usually contains low yields and inability to remove small concentrations up to about 40 mole percent of chlorosilanes of higher of chlorosilanes from the mixtures. The method where functionality, e.g., tetrachlorosilane (i.e. silicon tetra in a tertiary amine is employed was not found to be chloride) and methyltrichlorosilane, based on the total generally applicable to separating all mixtures of chloro moles of trimethylchlorosilane and such higher func silanes but is limited to separating mixtures of inor tional chlorosilanes. Trimethylchlorosilane is utilized 55 ganic chlorosilanes. Diorganodiacyloxysilanes are not for furnishing end-blocking groups (i.e. chain-terminat readily available materials and hence the process for ing trimethylsiloxane groups) in the manufacture of sili separating mixtures of chlorosilanes using diorganodi cone oils that consist predominantly of combined di acyloxysilane entails the expenses of first synthesizing methylsiloxane units. The viscosity and tendency to gel said diorganodiacyloxysilanes. When an alkali metal of oils containing end-blocking groups furnished by tri 60 hydroxide is used in such separations, the materials ob methylchlorosilane decreases as the amounts of tri- and tained are higher boiling and are not found to be especial tetra-functional chlorosilanes in the trimethylchlorosilane ly useful because of their basic character and contami. so used are reduced. Hence, it is desirable to reduce the nation by salts. amount of tri- and tetra-functional chlorosilanes in tri It had been suggested also that zinc fluoride can be methylchlorosilane so that oils of low viscosity contain 65, added to a mixture of two chlorosilanes to selectively ing end-blocking groups furnished by trimethylchloro convert one of the chlorosilanes to the corresponding silane can be manufactured without gelation. fluorosilane. The fluorosilane so made could then be 3,168, 5 4. 3 separated from the unreacted chlorosilane by distillation. the reaction mixture leaving some of the combined nono However, the fluorosilane so separated is not a particu functional and trifunctional siloxane groups behind. larly useful material in view of the fact that when it is However, this method of producing the desired interme Subjected to hydrolysis reaction in the course of the pro diates is not adequate. By way of illustration, this meth duction of polysiloxane materials therefrom, hydrogen od produces cyclic dimethylsiloxanes that are still con fluoride is produced which is highly corrosive. taminated by materials containing combined trifunctional Still another process for separating mixtures of chlo siloxane groups. Furthermore, the high temperature rosilanes had been suggested and includes adding a phenol often employed causes decomposition and disproportion - to the mixture containing two or more chlorosilanes and ation reactions which lower the yield of the difunctional applying Such conditions that cause the chlorosilanes and O products and which produce undesired solid products that the phenol to react to form phenoxysilanes. The phen must be periodically removed from the reactor. In addi oxysilanes so made possess larger boiling point differ tion, under the conditions employed in this method, the ences than the starting chlorosilanes and hence can be equilibrium concentration of the difunctional interne separated by distillation. However, the relatively high diates is usually low and, when operated in a batchwise boiling points of these phenoxysilanes necessitate the use manner, this method inherently produces a low yield of of high temperature distillation or vacuum distiliation to the difunctional intermediates. effect a separation and regeneration of the separated This invention is based on the discovery that different phenoxysilanes to the corresponding chlorosilanes is re chlorosilanes hydrolyze at different rates and that mix quired. Hence this method of separating mixtures of tures of different chlorosilanes can be separated into chlorosilanes is often undesirable. 20 specific chlorosilane portions which are substantially free Another process for separating mixtures of methyl of other chlorosilanes by taking advantage of this dif chlorosilanes had been suggested and includes adding an ference in reactivity.

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