Chloride reduction in polysiloxanes

Abstract: The instant invention is a process for reducing residual chloride content of polysiloxane fluides, without detrimental effects on the polysiloxane polymer. The process comprises contacting the polysiloxane fluid, containing residual chloride, with selected weakly-basic alkaline metal compounds at a temperature less than 100°C. After an appropriate contact time, the polysiloxane fluid is separated from the solid alkaline metal compounds. In a preferred embodiment of the present invention, water is added to the process to facilitate removal of residual chloride.

Description: The present invention is a process for removing residual chloride from polysiloxane fluids. The process involves contacting a weakly-basic alkaline metal compound with a chloride containing polysiloxane fluid at a temperature of 100°C. or less. In a preferred embodiment of the present invention, water is added along with the alkaline metal compound to facilitate removal of residual chloride. The treated polysiloxane fluid, reduced in residual chloride content, is filtered to remove solid alkaline metal compounds. Residual chloride may occur in polysiloxane fluids in a variety of forms. For example the chloride may be present as ionic chloride, as an alkyl chloride, or as chloride bound to silicon. Sources of chloride may be the feed-stocks used to prepare the polysiloxanes, equipment contamination, and added chlorosilanes. The presence of chloride in polysiloxane fluids has been shown to be correlated with such detrimental processes as color changes, viscosity changes, and increased electrical conductivity. To avoid these detrimental processes it is often desirable to reduce residual chloride content of polysiloxanes. The reduction of residual chloride must be achieved without detrimental effect on the polysiloxane. EP-A 0 215 470 teaches a process for removing phophorous compounds from organopolysiloxanes by adsorption onto the solids of metal oxide, metal hydroxide or metal carbonate. The solid materials are then removed from the organopolysiloxanes by filtration or centrifugation. This document is silent with regard to the ultimate fate of the chloride component of the phosphonitrilic chloride catalyst. To our knowledge, no one before this time has taught that, at a temperature of 100°C. or less, selected weakly-basic alkaline metal compounds can be used to reduce residual chloride in a polysiloxane fluid without detrimental effects on the polysiloxane polymer. Furthermore, we know of no teaching that the reduction of residual chloride in polysiloxane fluid can be facilitated by the addition of water to the process. The present invention is a process for reducing residual chloride content of polysiloxane fluids without detrimental effects on the polysiloxane polymer. The polysiloxane fluid containing residual chloride is contacted with selected weakly-basic alkaline metal compounds at a temperature less than 100°C. After an appropriate contact time, the polysiloxane fluid is separated from the solid alkaline metal compounds. In a preferred embodiment of the present invention, water is added to the process to facilitate removal of residual chloride. The present invention is a process for reducing residual chloride content of a polysiloxane fluid. The process comprises contacting the polysiloxane fluid with an alkaline metal compound selected from magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, barium oxide, and barium hydroxide. The process is conducted at a temperature of 0°C. to 100°C. In a preferred embodiment of the instant invention, a stoichiometric excess of about 10 mole % of water in relation to hydrolyzable chloride is added to the process. The polysiloxane fluid consists primarily of organo-substituted polysiloxanes of varying degrees of polymerization (dp). The only requirement regarding the dp, for the instant process, is that the viscosity of the fluid be such that the alkaline metal compound can be mixed with the polysiloxane fluid adequate to insure good contact. In general, the process of the present invention is effective for fluids with a viscosity less than 10,000 mPa.s (centipoise) Higher viscosity fluids may be employed, but mixing and separation of the alkaline metal compounds may be difficult. The polysiloxanes can be totally methylated fluids, methyl and phenyl substituted fluids, or either type fluid further substituted with minor components of pendant hydroxyl or vinyl groups on silicon atoms. By minor component, is meant less than about 10 mole percent of the pendant group. The polysiloxanes can be either linear or cyclic in structure. The polysiloxanes can be dimethyl and methylphenyl substituted copolymers. The polysiloxanes can be, for example, trimethylsilyl end-blocked methyl and phenyl substituted polysiloxanes, trimethylsilyl end-blocked dimethylpolysiloxanes, dimethylhydroxylsilyl end-blocked methyl and phenyl substituted polysiloxanes, vinyldimethylsilyl end-blocked dimethylpolysiloxanes, vinyldimethylsilyl end-blocked vinyl and methyl substituted polysiloxanes, trimethylsilyl end-blocked vinyl and methyl substituted polysiloxanes, trimethylsilyl end-blocked methyl and hydroxyl substituted polysiloxanes, methylphenylhydroxysilyl end-blocked methyl and phenyl substituted polysiloxanes dimethylcyclopoly-siloxanes, and methyl and phenyl substituted cyclopolysiloxanes. The term "residual chloride content," as used in the present invention refers to the total chloride present in the polysiloxane fluid. This may include both ionic and non-ionic chloride species. Ionic chloride species include, for example, free hydrogen chloride and chloride substituted on silicon atoms. The non-ionic species include reactive organic chloride materials. The weakly-basic alkaline metal compound is selected from a group consisting of magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, barium oxide, and barium hydroxide. Magnesium oxide is the preferred alkaline metal compound. It is important that the alkaline metal compound be a weak base in solution, since strongly basic compounds can cause depolymerization of the polysiloxanes or if the polysiloxanes contain silanol groups, condensation may be catalyzed resulting in molecular weight and viscosity changes. Particle size of the alkaline metal compound is important to insure adequate contact with the residual chloride within the polysiloxane fluid. In general, a particle size with an average diameter less than two µm is preferred. Larger particle sizes up to 0,125mm (120 mesh) can be used, but effectiveness for removing residual chloride content may be reduced. The lower limit for particle size is limited only by the ability to separate alkaline metal compound from the polysiloxane fluid. Polysiloxane fluid is typically contacted with an amount of alkaline metal compound sufficient to reduce the chloride content of the fluid to the desired level. The alkaline metal compound is contacted with the polysiloxane fluid at about two to five times the stoichiometric equivalence of the residual chloride content. During contact of the alkaline metal compound with the polysiloxane fluid, the temperature of the contact mixture should be maintained within a range from about 0°C. to 100°C. A preferred temperature range, with or without water, is 20°C. to 75°C. Contact time of the alkaline metal compound with the polysiloxane fluid should be within the range of 10 minutes to four hours. Contact times longer than four hours may be employed, but no significant benefit is perceived. A preferred contact time for the alkaline metal compound with the polysiloxane is 0.5 to two hours. Most preferred is about 0.5 to one hour. The inventors have found that the presence of water during the contact of polysiloxane and alkali metal compound facilitates the removal of residual chloride from the polysiloxane fluid. The inventors believe that this facilitating of the process occurs by the following representative chemical reactions: HCl + MgO → MgClOH (2) MgClOH + HCl → MgCl₂ + H₂O (3) Of importance to the present invention, is the finding that water facilitates the removal of residual chloride under conditions that do not cause depolymerization or condensation of the polysiloxane polymers and do not cause reaction of pendant groups such as hydroxyl and vinyl when they are present on the polysiloxane. The water can be added to the polysiloxane fluid at the same time it is contacted with the alkaline metal compound. Any amount of water can be beneficial to the process. However, in practice, the preferred amount of water for use in the process is an amount in slight stoichiometric excess of the hydrolyzable chloride content of the polysiloxane fluid and less than the molar equivalent of the alkaline metal compound present in the process. Under these conditions, excess water is bound by the alkaline metal compound and no separate step is required to remove water from the polysiloxane fluid. Greater amounts of water may be used, but to no perceived advantage. By "slight stoichiometric excess" is meant, about a 10 mole percent stoichiometric excess of water in relation to hydrolyzable chloride in the polysiloxane fluid. The preferred procedure for conducting the instant described process is in a batch mode, as described above. The equipment and procedures utilized are those known in the art of design and operation of such batch equipment. The process can also be run in a continuous mode. When the process is run in a continuous mode a large excess of the alkaline metal compound is placed in a vessel such as a tank, column, or the like, known in the art of design of continuous reactors. The system is brought to and maintained at the desired contact temperature by preheating the polysiloxane fluid, heating the vessel in which the alkaline metal compound is contained, or a combination of both. Means for preheating or heating can be effected by conventional means known in the art. Temperature and contact times are as previously described for the batch mode of operation. If the process is to be run with added water, the water is added to the polysiloxane fluid in amounts described for the batch process. After contact of the polysiloxane fluid with the alkaline metal compound, the polysiloxane fluid is separated from residual solids, unreacted alkaline metal compound, and salts of reaction of the alkaline metal compound. Separating solids from the treated polysiloxane fluid can be effected by filtration means. Filtration means can be such means as filter presses, bag filters, and cartridge filters. So that those skilled in the art can better appreciate and understand the present invention, the following examples are given. These examples are presented to be illustrative and are not to be construed as limiting the instant invention as described herein. Example 1 The ability of magnesium oxide to reduce residual chloride content of a trimethylsilyl end-blocked methyl and phenyl substituted polysiloxane fluid was evaluated. The polysiloxane fluid was known to contain trimethylchlorosilane as one source of residual chloride. The equilibrated fluid 7,26t (16,000 lb) was cooled to a temperature below 100°C. and 181.6g 0.4 lb) of water and 454g (1.0 lb) of magnesium oxide was added to the fluid. The fluid, water, and MgO were contacted for approximately one hour with constant stirring. At the end of the contact period the fluid was filtered to remove precipitated metal salts. The MgO used was MAGOX® 98 HR Fine, C-E Refractories, Valley Forge, PA, with surface area of 38 to 62 square meters per gram and average particle size less than two microns. Chloride content of the treated polysiloxane fluid was determined by IEC. The mean chloride content for six untreated polysiloxane samples was 1506 ppb. The mean chloride content for six polysiloxane samples, contacted with MgO for 5-10 minutes prior to filtration, was 449 ppb. When the contact time was increased to one hour, the mean chloride content of the polysiloxane was 73 ppb. The data demonstrate the ability of MgO to reduce chloride content of trimethylsilyl end-blocked methyl and phenyl substituted polysiloxanes and the importance of adequate contact time.

Claims: A process for reducing residual chloride content including ionic and non ionic chloride species of a polysiloxane fluid, with the exception of those prepared using a phosphonitrilic chloride catalyst, the process comprising contacting the polysiloxane fluid with an alkaline metal compound selected from magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, barium oxide, and barium hydroxide in an amount of 2 to 5 equivalents alkaline metal compound per equivalent of residual chloride content; the alkaline metal compound having a particle size of up to 0,125 mm (120 mesh); the contact occurring at a temperature of 0°C. to 100°C. A process according to Claim 1, where an amount of water of about 10 mole percent stoichiometric excess in relation to the hydrolyzable chloride content of the polysiloxane fluid and less than molar equivalent of the alkaline metal compound is present.

Inventors: ALLEN TIMOTHY, HUNTRESS ARNOLD RAY, LANDIS DARYL DAVID, MCVANNEL DONALD EUGENE, ALLEN, TIMOTHY, HUNTRESS, ARNOLD RAY, LANDIS, DARYL DAVID, MCVANNEL, DONALD EUGENE
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1991
Main classification: C08G77
IPC codes: C08G77, C07F7