The present invention is a process for preparing cyclic organohydrogensiloxanes. The process comprises contacting an organohydrogendichlorosilane with about a stoichiometric equivalent of water to form a hydrolyzate. The hydrolyzate is diluted in an inert solvent and contacted with an acidic rearrangement catalyst to effect formation of cyclic organohydrogensiloxanes.
Cyclic organohydrogensiloxanes such as cyclic methylhydrogensiloxane are useful as crosslinkers in silicone coatings and encapsulating materials used in the electronic industry and can be used as intermediates to form SiH functional siloxanes. In typical processes for preparing organohydrogensiloxanes a first step involves the hydrolysis of an organohydrogendichlorosilane to form an equilibrium mixture containing cyclic organohydrogensiloxanes and short-chain linear organohydrogensiloxanes. Generally, the weight percent of cyclic organohydrogensiloxanes in the equilibrium mixture is small in relation to the weight percent of linear organohydrogensiloxanes present. Consequently when demand for cyclic organohydrogensiloxanes is high, an excess of linear organohydrogensiloxanes may be produced. Therefore, it is an objective of the present process to provide a method to increase the proportion of cyclic organohydrogensiloxane species in the equilibrium mixture. Another objective is to provide a method where the linear species recovered from the equilibrium method can be recycled to the process for conversion to cyclic organohydrogensiloxanes. Still another objective is to provide a process where hydroxyl substitution on the silicon of the cyclic and linear species is minimized. This is necessary to prevent gelling of the linear species during conduct of the process and to provide cyclic organohydrogensiloxanes of consistent reactivity for use in other processes.
Sokolov et al., All-Union Electrotechnical Institute, Mar. 5, 1955, p. 1061-1063, teach the reaction of methydichlorosilane with water to form cyclic methylhydrogensiloxanes.
Numerous procedures have been reported in the art for improving the yield of cyclic organohydrogensiloxanes from hydrolysis processes similar to that taught by Sokolov et al.
Lacefield, U.S. Pat. No. 3,590,064, issued Jun. 19, 1971, teaches that halogen endblocked linear polysiloxanes can be reacted with at least a stoichiometric amount of an alkali metal carbonate salt in the present of a suitable polar solvent to form cyclic organopolysiloxanes. Lacefield indicates the process is suitable for forming cyclic organopolysiloxanes having hydrogen bonded to silicon.
Miller et al., U.S. Pat. No. 3,714,213, issued Jan. 30, 1973, describe a process for preparing cyclic methylhydrogensiloxanes by contacting linear methylhydrogen siloxanes with an acid catalyst absorbed on a carrier. The process requires the presence of high molecular weight chain termination groups. The yield of tetramethylcyclotetrasiloxane is reported to be about 73 percent.
Crivello et al., U.S. Pat. No. 4,895,967, issued Jan. 23, 1990, describe a method for making cyclic organohydrogensiloxanes by contacting a linear organohydrogensiloxane with a heated bed of a cracking catalyst at reduced pressure. The resulting volatile cyclic organohydrogensiloxane is then recovered. A typical yield for the method is reported to be about 85 percent.
Wright et al., Polymer, Vol. 11, p. 464-471, 1969 teach the equilibration of tetramethylcyclotetrasiloxane at 0.degree. C. using n-butyl lithium as catalyst and about 2% tetrahydrofuran as promoter. Wright et al. report that the equilibration has to be carried out at low temperature with exclusion of moisture and oxygen in order to prevent crosslinking of the formed linear siloxanes. Wright et al. teach that if a diluent is used in the equilibration process, the weight fraction of cyclics should increase with increasing dilution up to a critical point beyond which linear polymers will be effectively absent.
The present inventors have found that when a hydrolyzate is formed by contacting an organohydrogendichlorosilane with about stoichiometric water, that this hydrolyzate can be rearranged in the presence of an acid catalyst and an inert solvent to form cyclic organohydrogensiloxanes. The process allows for linear organohydrogensiloxanes separated from the cyclic organohydrogensiloxanes to be continuously recycled to the rearrangement process, reducing the amount of linears produced by the process. Furthermore, under the conditions of the process the formation of hydroxyl substituted siloxanes is minimized as well as scission of hydrogen and organic substituents from the silicon atoms. Thus a process is provided where minimal loss of siloxanes occurs due to crosslinking of the siloxanes to high molecular weight byproducts having less utility and which may serve to plug the processing apparatus.