Storage stable dispersions of finely divided solids in organosiloxane compositions

Abstract: A stable dispersion of a finely divided solid such as a pigment in an organosiloxane composition is prepared as a homogeneous thixotropic liquid, paste or gel by blending the solid with a finely divided, partially treated silica, a specified class of liquid diorganosiloxane/oxyalkylene copolymers and, as a diluent, at least one liquid organosiloxane oligomer having a density lower than the density of the dispersed solid. The resultant gels and pastes convert to flowable liquids when subjected to shearing forces.

Description: This invention relates to organosiloxane compositions containing a thixotropic matrix in the form of a liquid, paste or gel and a uniformly suspended solid phase that does not precipitate during storage of the compositions for extended periods of time. When subjected to shearing forces the matrices in the form of pastes and gels will convert to flowable liquids. One of the problems associated with low viscosity organosiloxane compositions containing pigments or other insoluble solid materials, having a higher density than that of the liquid phase, is the tendency of the solid phase to settle during storage of the compositions. When the solid is a pigment and the stored compositions are used to form shaped articles by molding or extrusion, this tendency of the pigment to separate can result in uneven coloration of the final product. The prior art discloses a variety of additives yielding stable dispersions of pigments and other solids in liquid compositions. This art is generally represented by the following: JP-A 4/110,302; US-A 5,006,597 and US-A 5,036,121. Thixotropic agents that have been used in curable organosiloxane compositions include silica treated with various organosilicon compounds and organosiloxane/oxyalkylene copolymers. To achieve adequate storage stability some curable organosiloxane compositions, particularly those containing tin compound catalysts, are packaged with the tin compound and crosslinkable polyorganosiloxane in separate containers. The crosslinking agent is typically packaged together with the curing catalyst. Because only a small volume of crosslinking agent relative to the volume of crosslinkable polyorganosiloxane is required to achieve the desired degree of crosslinking, the portion of the composition containing the crosslinking agent typically includes a quantity of diluent that will allow this part of the composition to be blended with the part containing the crosslinkable polyorganosiloxane in a volume ratio of 1 part of curing agent portion to 20 parts of the portion containing the crosslinkable polyorganosiloxane. The diluent is a solvent for the crosslinking agent and the catalyst. Commonly assigned patent applications JP-A 8/50040 (3/7/96) and EP 96 301311.5 (2/27/96) describe two-part, moisture curable organosiloxane compositions. The portion of these compositions containing the crosslinking agent and curing catalyst is thixotropic and comprises, in addition to the crosslinking agent and catalyst, a specified class of liquid diorganosiloxane/ oxyalkylene copolymers as the thixotropic agent together with a specified class of non-reactive organosiloxane oligomers containing silicon-bonded phenyl radicals as a diluent. The examples in those applications describe preparation of the thixotropic portion of the two-part composition by combining all of the ingredients together, blending these ingredients to homogeneity and combining the resultant mixture with the other ingredients of the curable composition. We subsequently found that when prepared according to the examples of the aforementioned applications, the thixotropic portion of the two-part composition would separate into a very soft gelatinous material and a supernatant liquid. When this two-phase thixotropic portion contained a pigment that had a higher density than the density of the liquid, a portion of the pigment settled to the bottom of the liquid phase. When the pigmented portion containing the catalyst was later combined with the base portion comprising a crosslinkable liquid polyorganosiloxane and fillers, the pigment could not be homogeneously blended into the resultant mixture to form a uniformly pigmented, homogeneous composition. One objective of this invention is to provide uniform dispersions of finely divided solids, including but not limited to pigments, in low molecular weight organosiloxane compositions that will not settle upon standing and a method for preparing these compositions. A second objective is to provide multi-part moisture curable organosiloxane composition wherein the portion containing the crosslinking agent also contains a stable dispersion of a finely divided solid exhibiting a higher density than that of the organosiloxane compounds present in the composition. A final objective is to provide a method for preparing organosiloxane compositions containing a dispersed form of a finely divided solid that does not settle during long term storage of the compositions. A stable dispersion of a finely divided solid in a liquid organosiloxane composition, exhibiting a lower density than the density of the solid, is prepared as a homogeneous thixotropic paste or gel by blending the solid with a finely divided, treated silica, a specified class of liquid diorganosiloxane/oxyalkylene copolymers and, as a diluent, at least one liquid organosiloxane oligomer having a density lower than the density of the pigment. The resultant paste or gel converts to a flowable liquid when subjected to shearing forces. In preferred embodiments of our present compositions, the dispersed solid is a pigment. Other ingredients such as curable polyorganosiloxanes, crosslinking agents and curing catalysts are subsequently blended with our claimed dispersions to form homogeneous curable, organosiloxane compositions that do not separate into liquid and solid phases upon standing. This invention provides an organosiloxane composition comprising a solid phase uniformly dispersed within a matrix phase, wherein the solid phase comprises A) a finely divided material other than silica exhibiting a density greater than the density of said matrix phase; and B) a finely divided fume type reinforcing silica, and said matrix phase comprises C) a liquid organosiloxane copolymer exhibiting a general formula selected from the group consisting of and GR 1 2 SiO(R 1 2 SiO) b SiR 1 2 G wherein each R 1 is individually selected from unsubstituted or substituted monovalent hydrocarbon radicals, Q is R 1 or G, G is a polyoxyalkylene group having an average structure selected from the group consisting of -R 2 (OCH 2 CH 2 ) c OZ and wherein R 2 represents a divalent hydrocarbon radical containing from 2 to 20 carbon atoms; Z is selected from the group consisting of hydrogen atom, alkyl radicals containing from 1 to 4 carbon atoms and the -(O)CR group, wherein R is selected from monovalent hydrocarbon radicals or monovalent hydrocarbon radicals containing the -C(O)OH group as a substituent; a, b, c and d are individually selected from positive integers; and D) a liquid organosiloxane oligomer that does not react with the ingredients in said dispersion and is compatible with said organosiloxane copolymer, wherein the silicon-bonded hydrocarbon radicals present in said oligomer are selected from monovalent unsubstituted or substituted hydrocarbon radicals; and the number average molecular weight of said oligomer is sufficient to solubilize said organosiloxane copolymer in said matrix phase; wherein said silica constitutes from 2 to 15 weight percent, said copolymer constitutes from 0.1 to 10 weight percent, the concentrations of said silica and said copolymer are based on the combined weights of said finely divided solid, silica, organosiloxane copolymer and organosiloxane oligomer and wherein the consistency of said composition at 25°C. is selected from the group consisting of liquids, pastes and gels and said composition is flowable when subjected to shear. Depending upon the diameter and contour of the silica particles and the concentration of this ingredient, the present compositions can be a liquid, paste or gel. As used in this specification, the terms "paste" and "gel" describe the consistency of the present compositions at 25°C. in the absence of shearing forces. Some of these compositions flow slowly if at all under their own weight and all will convert to flowable liquids when subjected to shearing forces. In preferred pastes or gels, the shearing forces typically generated by manual stirring of the composition are sufficient to achieve this conversion. Some of the present compositions are in the form of flowable liquids wherein the solid materials, including silica, remain uniformally dispersed in the matrix for at least seven days. The stability of the present dispersions is typically determined by noting the time required for the solid materials to precipitate after the composition has been sufficiently blended to uniformly disperse the solid materials throughout the composition. The dispersion is considered stable if no settling of solid material is apparent seven days after the composition has been blended and allowed to remain undisturbed. An interaction between the organosiloxane/oxyalkylene copolymer, referred to hereinafter as ingredient C, and the finely divided fume silica, referred to hereinafter as ingredient B, is believed responsible for the formation of the thixotropic matrix in which the finely divided silica and other solid materials are uniformly dispersed. The organosiloxane/oxyalkylene copolymers suitable for use as ingredient C are represented by a general formula selected from the group consisting of and GR 1 2 SiO(R 1 2 SiO) b SiR 1 2 G In these formulae, each R 1 is individually selected from unsubstituted or substituted monovalent hydrocarbon radicals, Q is R 1 or G, G is a polyoxyalkylene group having an average structure selected from the group consisting of -R 2 (OCH 2 CH 2 ) c OZ and In these formulae, R 2 represents a divalent hydrocarbon radical containing from 2 to 20 carbon atoms, Z is selected from the group consisting of hydrogen atom, alkyl radicals containing from 1 to 4 carbon atoms, -(O)CR groups wherein R is selected from unsubstituted monovalent or substituted monovalent hydrocarbon radicals containing a -C(O)OH group as a substituent; a and b are individually selected from positive integers; and c and d are individually selected from positive integers. R preferably contains from 1 to 20 carbon atoms. R 2 preferably contains from 3 to 6 atoms, the positive integers represented by a and b are preferably from 1 to 100, preferably from 1 to 10, inclusive; and the positive integers represented by c and d are preferably from 1 to 20. R 2 is most preferably propylene, based on the commercial availability of the corresponding allyl-terminated polyethers used to prepare the copolymer. The monovalent hydrocarbon radicals represented by R 1 that are bonded to the silicon atoms of ingredient C contain from 1 to 12 carbon atoms. R 1 is alkyl, including but not limited to, methyl, ethyl, propyl, butyl and octyl; cycloalkyl, including but not limited to, cyclopentyl and cyclohexyl; or aryl, including phenyl, naphthyl, benzyl and tolyl. When atoms other than carbon and hydrogen are present on the hydrocarbon radicals represented by R 1 and R 2 , these substituents can be any that will not react with the other ingredients of the present dispersions, adversely affect the stability of dispersions or interact with ingredients that the dispersions subsequently come in contact with. Preferred substituents are halogen atoms such as chlorine, bromine and fluorine. Preferred substituted hydrocarbon radicals include chloromethyl, 3-chloropropyl and 3,3,3-trifluoropropyl. Preferably all of the hydrocarbon radicals represented by R 1 are alkyl radicals containing from 1 to 4 carbon atoms. Most preferably all of these hydrocarbon radicals are methyl, based on the availability of the copolymers and the compatibility of the copolymers with the diluents of the present compositions, referred to hereinafter as ingredient C. The factors determining the selection of the values for a, b, c and d are 1) the level of thixotropic properties to be imparted to our pigment dispersion by the copolymer, 2) the ability of the copolymer to be solubilized by the particular diluent selected, and 3) the requirement that the copolymer be a liquid at the use temperature of the composition, which can be as low as 0°C. The copolymers suitable for use in the present compositions are prepared by reacting 1) an organohydrogensiloxane with concentrations of diorganosiloxane (R 1 2 SiO) and organohydrogensiloxane (R 1 HSiO) units corresponding to the values for a and b in the preceding formulae and 2) at least one polyether corresponding to one of the following general formulae: R 2' (OCH 2 CH 2 ) c OZ and In these formulae, R 2' represents a terminally unsaturated alkenyl radical containing the same number and configuration of carbon atoms as R 2 . For example, when R 2 is -CH 2 CH 2 CH 2 -, R 2' is CH 2 =CHCH 2 -. The copolymer is typically isolated as a solution in the polyether used to prepare the copolymer and is used in this form to prepare the dispersions of the present invention. The diluent in the present compositions, ingredient D, is a liquid organosiloxane oligomer containing at least one siloxane unit per molecule. To achieve the desired level of thixotropy the organosiloxane/oxyalkylene copolymer (ingredient C) should constitute from 0.1 to 10 weight percent of the combined weight of ingredients A, B, C and D. The concentration of preferred copolymers is typically from 0.5 to 4 weight percent. We have determined that the solubility of a given copolymer suitable for use as ingredient C in a diluent appears to be inversely proportional to 1) the molecular weight of the organosiloxane portion of the copolymer, which is, in turn, directly proportional to the sum of a and b , and 2) the number of oxyethylene and oxypropylene units in the copolymer, represented by c and d , respectively. Most of the commercially available types of fume silica with surface areas greater than 250 m 2 /gram are suitable for use in our claimed compositions. The average diameter of the particles preferably is less than one micrometer, most preferably less than 100 nanometers. Fume silicas are typically in the form of agglomerates that are separated into aggregated particles during blending with the liquid ingredients of the present compositions. To facilitate preparation of the present dispersions, a portion, preferably up to 50 percent, of the silanol groups initially present on the silica are reacted to form trihydrocarbylsiloxy groups. It appears that at least half of the silanol groups initially present on the silica are necessary to provide the requisite degree of thioxotropy that will maintain the silica and other solid materials as a stable dispersion. The organosilicon compound that is reacted with the silanol groups present on the silica to form the trihydrocarbylsiloxy groups is a silane of the general formula R 3 3 SiX or a compound of the formula (R 3 3 Si) 2 NH. In these formulae, each R 3 is individually selected from monovalent hydrocarbon radicals and X is a hydrolyzable group such as alkoxy. The silica treating agent is preferably a hexaorganodisilazane wherein each of the substituents represented by R 3 is lower alkyl, most preferably methyl. To obtain a matrix phase exhibiting the desired consistency and thixotropic character the concentration of fume silica is from 2 to 15 weight percent, preferably from 5 to 10 weight percent, based on the total weight of the present dispersions. The diluent in the present pigment dispersions is an organosiloxane oligomer that is a liquid at 25°C. and contains at least two siloxane units per molecule. The type of monovalent hydrocarbon radicals bonded to the silicon atoms of the diluent is not critical so long as the diluent is miscible with the organosiloxane/oxyalkylene copolymer, ingredient C. The hydrocarbon radicals present in the diluent, ingredient D, are one or more of alkyl containing from 1 to 8 carbon atoms; alkenyl; cycloalkyl; aryl such as phenyl; aralkyl such as beta-phenylethyl and alkaryl such as tolyl or xylyl. These hydrocarbon radicals can also contain substituents such as halogen atoms that will not interfere with the stability of the present dispersions. Preferred embodiments of ingredient D include 1) cyclic and linear oligomers containing dimethylsiloxane, phenylalkylsiloxane and/or (phenylalkyl)alkylsiloxane units and 2) linear oligomers comprising at least one phenylsilsesquioxane unit and trialkylsiloxane units. We have found that diluents wherein at least 50 percent of the silicon atoms contain a phenyl or other aromatic hydrocarbon radical are more compatible with the preferred organosiloxane/oxyalkylene copolymers used as ingredient C. They also yield dispersions of fume silica that are optically clear. Organosiloxane oligomers wherein all of the silicon-bonded hydrocarbon radicals are alkyl, such as methyl, appear to be less compatible with the organosiloxane/oxyalkylene copolymer (ingredient C) and the dispersions formed by adding fumed silica to a mixture of these ingredients are typically cloudy. When the siloxane units of ingredient D contain phenyl radicals, these units are represented by the formula R 4 e Ph f (C g H 2g )SiO (4-e-f)/2 , wherein Ph represents phenyl, R 4 represents an alkyl radical containing from 1 to 10 carbon atoms, e is 0, 1 or 2, f is 1 or 2, the sum of e and f does not exceed 3 and g is 0 or an integer from 1 to 4. R 4 preferably contains from 1 to 3 carbon atoms, is most preferably methyl and f is preferably 1. Examples of suitable phenyl-containing siloxane units are methylphenylsiloxane units, methyl-beta-phenylethylsiloxane units, methyl-2-methyl-2-phenylethylsiloxane and phenylsilsesquioxane units. When the siloxane units in ingredient D are arranged in a linear configuration, the two terminal triorganosiloxane units present in each molecule preferably contain three monovalent hydrocarbon radicals bonded to the silicon atom. These hydrocarbon radicals can be all alkyl, such as methyl, or a combination of two alkyl and one phenyl radical(s). Because the compatibility of the diluent with the thixotropic agent typically decreases with increasing molecular weight of the diluent, the diluent should contain no more than an average of 30 repeating units, preferably no more than 10 repeating units, per molecule. To achieve optical clarity, it appears that at least 20 percent of the siloxane units in the diluent should be phenylsiloxane units. To ensure solubility of the higher molecular weight thixotropic agents, which may either be required or the only ones available, when the oligomer(s) used as ingredient D contain silicon-bonded phenyl radicals, the oligomer should have the lowest possible number average molecular weight commensurate with the cost and availability of the diluent. If cost and availability were not considerations, cyclic dialkylsiloxanes, such as dimethylsiloxanes and methylphenylsiloxanes, would be the preferred diluents, particularly for higher molecular weight thixotropic agents; followed by trimethylsiloxy-terminated dimethylsiloxane and polymethylphenylsiloxanes; and linear oligomers containing trimethylsiloxy and either methylsilsesquioxane or phenylsilsesquioxane units. Conversely, if only higher molecular weight oligomers are available, this limits the choice of organosiloxane/oxyalkylene copolymers (ingredient C) to those containing an average of fewer than 10 organosiloxane units per molecule. It should be apparent that solubility of the organosiloxane/oxyalkylene copolymer (ingredient C) in the diluent is maximized by using the lowest molecular weight embodiment of ingredient C that will provide the desired level of thixotropy and by minimizing the molecular weight of the diluent. The diluent constitutes from 10 to 90 weight percent of the matrix phase of the present compositions. Any commercially available finely divided solid with a density greater than the combined densities of the organosiloxane/oxyalkylene copolymer (ingredient C) and the diluent (ingredient D) can be dispersed in the present compositions, so long as the solid does not contain groups that react with these ingredients, or any of the additional ingredients, which are present when the dispersion is incorporated into other compositions such as curable organosiloxane compositions, a preferred end-use application for the present dispersions. The types of solid materials that are blended with ingredients B, C and D to form the present thixotropic dispersions include metals such as magnesium, titanium, tin, zinc, iron and copper; oxides and other compounds of these metals; compounds of metals such as calcium, barium, lead, sodium, potassium, cerium and antimony; and compounds of non-metallic elements such as phosphorus and sulfur. Preferred solids are pigments that are used to impart color to compositions varying in consistency from liquids to gels to gums. The finely divided solid typically constitutes from 0.1 to 50 weight percent of the total dispersion, preferably from 1 to 10 weight percent. When the finely divided solid is a pigment, the relative concentration of this ingredient will depend upon the upon the shade and intensity of the color desired in the final article. This concentration is typically from 0.1 to 5 weight percent, based on the combined weight of ingredients B, C and D. The maximum particle size of the solid material will be dependent at least in part on the density of the material, the viscosity of the matrix under no-shear conditions and how rapidly this viscosity is achieved after the solid has been initially dispersed in the composition. The particle size of solid materials other than the fume silica are preferably less than 1 mm. The thixotropic dispersions of the present invention are prepared by blending the desired amounts of the organosiloxane/oxyalkylene copolymer, fume silica and diluent to homogeneity. The pigment or other finely divided solid forming the major portion of the dispersed solid phase is preferably included together with these ingredients. However, it can be added at a later time without any adverse effect on the stability of the final dispersion. Irrespective of whether or not the finely divided solid is present, the initial mixture of ingredients A, B, C and D is a relatively thin liquid. If left undisturbed, some of the claimed compositions will increase slightly in viscosity and remain flowable while others will gradually thicken to the consistency of a flowable or non-flowable, gelatinous paste or gel within 15 minutes under ambient conditions (25°C. and atmospheric pressure) if allowed to remain undisturbed. When subjected to shear by moderate to vigorous stirring, these pastes and gels are converted to flowable liquids that revert to their non-flowable state when stirring is discontinued. The period required for this reversion will vary from several minutes to 24 hours or longer. A preferred end use application for our stable dispersions is in curable organosiloxane compositions to impart color or other properties such as flame retardancy or heat stability to these compositions, depending upon the types of solid materials present, other than fume silica, that are present in the solid phase of the dispersion. When the present dispersions are used as ingredients in curable organosiloxane compositions, the ingredients present in the curable compositions are not critical, so long as none of these ingredients interfere with the stability of the present dispersions and the presence of the dispersion does not inhibit curing of the organosiloxane composition. The present dispersions are particularly useful as pigment vehicles that impart a uniform color to curable organosiloxane compositions. One suitable type of organosiloxane composition will cure in the presence of moisture and contains a polyorganosiloxane with at least two silanol groups per molecule, an organosilicon compound containing at least three alkoxy or other hydrolyzable groups per molecule as a crosslinking agent and a catalyst for the reaction of these ingredients. The polyorganosiloxanes referred to in this specification as ingredient E contain at least two silanol groups per molecule and exhibit a linear or branched structure. The polyorganosiloxanes are liquids at 25°C. The repeating units of ingredient E are represented by the general formula R 5 j SiO (4-j)/2 , where R 5 represents an unsubstituted or substituted monovalent hydrocarbon radical and j is 1 or 2. When j is 2 the hydrocarbon radicals represented by R 5 are identical or different. This ingredient contains one or more different types of repeating units. Monovalent hydrocarbon radicals that are represented by R 5 include alkyl radicals containing from 1 to 12 carbon atoms, substituted alkyl such as chloromethyl or 3,3,3-trifluoropropyl, aryl radicals such as phenyl and naphthyl, alkaryl radicals such as tolyl and xylyl and aralkyl radicals such as benzyl. Preferred radicals are alkyl such as methyl and ethyl, phenyl and 3,3,3-trifluoropropyl. Substituents other than halogen atoms can be present so long as these do not adversely affect curing of the organosiloxane composition or stability of the present dispersions. Most preferably j in the foregoing general formula is 2 and at least one of the R 5 groups on each silicon atom is a methyl radical. To achieve a useful level of tensile strength and other physical properties in the cured material, the number average molecular weight of ingredient E should be at least 20,000, preferably from 25,000 to 75,000. The viscosity of polyorganosiloxanes with number average molecular weights above 100,000 are typically too viscous for convenient processing, particularly when blending with the other ingredients of the present compositions using conventional mixing equipment. Crosslinking agents for the polyorganosiloxanes, ingredient E, of our moisture curable compositions are typically organosilicon compounds containing an average of at least three silicon-bonded hydrolyzable groups per molecule. Preferred hydrolyzable groups are alkoxy radicals containing from 1 to 4 carbon atoms, carboxy radicals containing from 2 to 4 carbon atoms and ketoximo radicals such as methylethylketoximo. The curing agent, ingredient F, is a silane, disiloxane or a polyorganosiloxane. Silanes are generally preferred, based on their cost and availability. Preferred silanes are methyltrimethoxysilane, phenyl trimethoxysilane, vinyl trimethoxysilane, methyltriethoxysilane, methyltriacetoxysilane, methyltri(methylethylketoximo)silane, alkyl orthosilicates such as tetraethyl orthosilicate and condensation products of these orthosilicates, which are typically referred to as alkyl polysilicates. The concentration of curing agent should be sufficient to convert the composition to an elastomeric or resinous material exhibiting the desired physical properties in the presence of moisture. Typical moisture curable compositions contain from 0.5 to 6 weight percent of alkoxy or other hydrolyzable group based on the total weight of the curable composition. In addition to the crosslinkable polyorganosiloxane and curing agent, our curable composition includes a catalyst for the crosslinking reaction. Examples of suitable catalysts include compounds of titanium, aluminum, zirconium, lead and tin. These catalysts promote hydrolysis of the hydrolyzable groups present on the crosslinking agent in the presence of moisture. Preferred catalysts for the present compositions when used for mold-making include divalent tin salts of carboxylic acids such as stannous acetate or stannous octoate and organotin compounds such as dimethyltin dineodecanoate, dibutyltin diacetate, dibutyltin dilaurate and dioctyltin diacetate. The carboxylic acid portion of these tin compounds contain from 1 to 20 carbon atoms. The concentration of crosslinking catalyst appears critical to achieving a useful cure rate following the inhibition period resulting from the presence of a working time extender. In preferred curable compositions, the concentration of catalyst is from 0.5 to 5 weight percent, based on the weight of the crosslinkable polyorganosiloxane (ingredient E). Curable organosiloxane compositions containing a thixotropic dispersion of this invention can optionally contain one or more of the reinforcing and non-reinforcing type fillers conventionally used in curable organosiloxane compositions. Examples of suitable fillers are reinforcing fillers, such as finely divided silica of the fume or precipitated type and non-reinforcing fillers such as alumina, titanium dioxide, silicates such as zirconium silicate and calcium carbonate. The filler or combination of fillers typically constitutes from 5 to 40 percent, preferably from 10 to 30 percent of the weight of the base portion of our preferred two part organosiloxane compositions. For some fillers, particularly, reinforcing fillers such as silica, treatment of the filler to react a portion of the hydroxyl groups present on the surface of the filler particles may be desirable to prevent a phenomenon referred to in the art as "creping" or "crepe hardening". Useful filler treating agents include silanol-terminated low molecular weight polydiorganosiloxanes and hexaalkyl disilazanes. The filler can be treated prior to being blended with the other ingredients of our curable organosiloxane composition or the treating agent can be combined with these ingredients to treat the filler in situ. Additional optional ingredients can be added to modify the properties of the curable organosiloxane compositions, or cured materials, prepared by using our claimed composition. These additional ingredients include liquid diluents in addition to the organosilicon oligomers referred to as ingredient F, water to accelerate curing, stabilizers to inhibit degradation in the presence of heat and/or ultraviolet light, anti-oxidants, dyes and flame retardants. Preferred two-part curable compositions containing the stabilized, uniformly dispersed, finely divided, solid materials of this invention are prepared by blending the base portion of the composition with the portion containing the crosslinking agent and ingredients A-D. Crosslinking or curing of the composition begins when ingredients E, F and G are present with water in the atmosphere and/or as an ingredient of the organosiloxane composition and they require from several minutes to several hours, depending upon the amount of water, the temperature, the type and concentration of crosslinking catalyst and the working time extender which are used. When the hydrolyzable groups present in the crosslinking agent are alkoxy groups and the crosslinking catalyst is a tin compound, our curable organosiloxane composition is preferably packaged in two parts. Ingredients A-D of the present dispersions are included with the crosslinking agent and the catalyst for the crosslinking reaction in one part, which is substantially free of hydroxyl compounds to avoid premature reaction of the crosslinking agent. The curable polyorganosiloxane, ingredient E, together with any reinforcing or non-reinforcing fillers and other optional ingredients are located in a second part, referred to as the base portion of the composition. In an alternative embodiment, a thixotropic dispersion of the present invention containing one or more pigments as the dispersed phase is packaged as a third component, separate from the base portion and the portion containing the curing agent and catalyst. This allows the user of the composition to vary the shade and intensity of color by selecting the appropriate type and amount of dispersed pigment, without affecting the concentration of crosslinking agent and crosslinking catalyst, which would affect the physical properties of any cured materials that are prepared using the present compositions. When the present dispersions containing fume silica and at least one additional finely divided solid material are blended into a portion of two-part organosiloxane compositions containing the crosslinking agent and a catalyst for the crosslinking reaction, the ability of a given diluent to dissolve the organosiloxane/oxyalkylene copolymer (ingredient C) together with the crosslinking agent and catalyst for the crosslinking reaction appears directly proportional to the molar ratio of the phenyl-containing siloxane units to other siloxane units in the molecules of diluent (ingredient D) and inversely proportional to the molecular weight of the diluent. Cured organosiloxane elastomers prepared using the thixotropic dispersions and curable compositions of this invention are suitable for a variety of end use applications, including sealants, coating materials and pottants or encapsulants for electrical and electronic devices. Our compositions are particularly useful for preparing molds by applying the composition onto the surface of a master to be replicated. The contour of the master corresponds to the shape of the articles to be prepared using the mold. The shaped articles are prepared by pouring a curable composition into the mold, allowing the ingredients of the composition to react and form a solid, crosslinked material and then stripping the mold away from the resultant shaped article. The following examples describe preferred thixotropic dispersions and curable organosiloxane compositions suitable for use as our claimed invention. Unless otherwise indicated, all parts and percentages in the examples are by weight and the viscosities are the values measured at 25°C. Example 1 This example describes preparation of a preferred pigment dispersion. A thixotropic pigment dispersion was prepared by blending the following ingredients to homogeneity: as the diluent, 94 parts of a mixture containing 18 weight percent of tris(trimethylsiloxy)phenylsilane, 48 weight percent of [(Me 3 SiO) 2 PhSi] 2 O and 34 weight percent of (Me 3 SiO) 2 PhSiOSiPh(OSiMe 3 )OSiPh(OSiMe 3 ) 2 , referred to hereinafter as ingredient C1; 6 parts of a fume silica that had been previously treated with hexamethyldisilazane; 2 parts of a dimethylsiloxane/oxyethylene copolymer of the formula as an 85 weight percent solution in CH 2 =CHCH 2 (OCH 2 CH 2 ) 7 OH and 1 part each of Irgalite Red, a pigment manufactured by Ciba Geigy and Heliogen Blue, a pigment manufactured by BASF. The initial mixture was a liquid that formed a gelatinous paste within 15 minutes. The paste was converted to a flowable liquid by stirring it for several seconds with a metal spatula. There was no noticeable separation of pigment when the dispersion was allowed to remain undisturbed for 30 days. As a comparative example, a dispersion was prepared by blending to homogeneity the amounts of ingredient C1, silica and pigments described in the preceding section of this example. The resultant mixture was a viscous liquid that did not appear to thicken over 24 hours, at which time no appreciable amount of pigment appeared to be present on the bottom of the container. Most of the pigment settled to the bottom of the container when the composition was allowed to remain undisturbed for 30 days. Example 2 To determine the effect on the stability of a dispersion of the present invention, by adding conventional crosslinking agents and catalysts for moisture curable organosiloxane compositions, 10 parts of ethyl polysilicate were added to a dispersion prepared by blending to homogeneity the types and amounts of organosiloxane/oxyethylene copolymer and silica described in Example 1 together with 94 parts of the diluent referred to as ingredient C1 in Example 1. The initially flowable liquid thickened to form a gelatinous paste when it was allowed to remain undisturbed for 15 minutes. The same thickening effect was observed when 10 parts of n-propyl orthosilicate were used in place of the ethyl polysilicate. Example 3 This example demonstrates the effect of varying the type and concentration of organosiloxane/oxyalkylene copolymer, silica and diluent on the type of dispersion containing the silica as the dispersed solid phase. The various ingredients used in Table 2 are identified as follows: Silica (Ingredient B): B1 - a fume silica wherein 50 percent of silanol groups were endblocked with trimethylsiloxy groups, the median particle size was 7 nanometers and the surface area was 260 m 2 /g. The degree of surface structuring, i.e., contour variation, was relatively high. B2 - a fume silica wherein 50 percent of silanol groups were endblocked with trimethylsiloxy groups and the surface area was 130 m 2 /g. The degree of surface structuring was lower than B1. B3 (comparative example) - a precipitated silica wherein 50 percent of the silanol groups were endblocked with trimethylsiloxy groups, the median particle size was 11 micrometers and the surface area was 200 m 2 /g. The degree of surface structuring was considerably lower than either B1 or B2. Organosiloxane/oxyalkylene Copolymers (Ingredient C) (CH 3 ) 3 SiO[Si(CH 3 ) 2 SiO] x [Si(A)(CH 3 )O] y Si(CH 3 ) 3 A= -CH=CHCH 2 (OCH 2 CH 2 ) m [OCH 2 CH(CH 3 )] n X The Diluent (ingredient D) D1 - A mixture containing 18 weight percent (25 mole percent) of tris(trimethylsiloxy)phenylsilane; 48 percent by weight (50 mole percent) of [(CH 3 ) 3 SiO] 2 PhSiOSiPh[OSi(CH 3 ) 3 ] 2 ; and 34 weight percent (25 mole percent) of [(CH 3 ) 2 SiO] 2 PhSiOSiPh[OSi(CH 3 ) 3 ]OSiPh[OSi(CH 3 ) 3 ] 2 D2 - A trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 50 mm 2 /sec. D3 - A trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 350 mm 2 /sec. D4 - A trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 500 mm 2 /sec. D5 - a dimethylvinylsiloxy-terminated polydimethylsiloxane having a viscosity of 0.4 Pa·s. The appearance of the dispersions are summarized in Table 3. Sample No. Appearance 1 Opaque flowable gel 2 Opaque gel, less flowable than sample 1 3 Opaque gel, thicker and more less flowable than sample 2 4 Clear gel, about as flowable as 1, entrapped bubbles 5 Opaque gel, similar to 3 6 Hazy flowable liquid, no entrapped bubbles, silica completely dispersed 7 Liquid, silica settled to bottom of container, no entrapped bubbles 8 Clear gel, flowability similar to sample 4, entrapped bubbles 9 Clear gel, similar to sample 8, entrapped bubbles 10 Very flowable, slightly viscous liquid, no entrapped bubbles 11 Clear flowable gel, similar to samples 8 and 9, but with more residue on container wall, entrapped bubbles 12 Clear flowable gel,more flowable than sample 11, less residue on container wall 13 Clear gel, very slightly flowable in absence of stirring 14 Cloudy gel, very slightly flowable in absence of stirring 15 Flowable liquid, less viscous than sample 10, no entrapped bubbles 16 Low viscosity liquid, solid gel at bottom of container that could be redispersed by stirring, gel particles on container wall 17 Clear flowable gel with entrapped bubbles Example 4 This example describes the preparation and curing of an organosiloxane composition containing a pigment dispersion of the present invention. The base portion of a curable organosiloxane composition was prepared by blending the following ingredients to homogeneity: 42 parts of a silanol-terminated polydimethylsiloxane exhibiting a viscosity of 50 Pa·s; 21 parts of a trimethylsiloxy-terminated polydimethyl-siloxane exhibiting a viscosity of 0.325 Pa·s; 3 parts of hexamethyldisilazane; 21 parts of a precipitated silica exhibiting an average particle size of 4 micrometers; 1 part of water; 11 parts of zirconium silicate exhibiting an average particle size of 5 micrometers; 42 parts of a silanol-terminated polydimethylsiloxane exhibiting a viscosity of 10 Pa·s; 0.8 part of an emulsion prepared by blending 35 parts of water, 60 parts of a liquid trimethylsiloxy-terminated polydimethylsiloxane exhibiting a viscosity of 0.35 Pa.s, 4 parts of a non-ionic surfactant, 2,6,8-trimethyl-4-nonyloxypolyethyleneoxyethanol, and 1 part of sodium polyethylene oxide octylphenoxy sulfonate. A crosslinking agent/catalyst mixture was prepared by blending the following ingredients to homogeneity: 2 parts of the organosiloxane/oxyalkylene copolymer referred to as C1 in Example 3; 67 parts of the diluent referred to as D1 in Example 3; and 4 parts of the silica identified as B1 in Example 3; and as pigments, 0.5 part each of Irgalite Red, a pigment manufactured by Ciba Geigy and Heliogen Blue, a pigment manufactured by BASF. The resulting mixture was blended to homogeneity with 10 parts of ethyl polysilicate and 8 parts of n-propyl orthosilicate as crosslinking agents; and as the curing catalyst 10 parts of dibutyltin dineodecanoate. Curable compositions were prepared by adding 1 part of this pigmented crosslinking agent/catalyst mixture to 10 parts of the base portion of the composition. The viscosity of the freshly-prepared composition was 55,000 Pa.s, measured using a Brookfield™ viscometer equipped with a number 6 spindle rotating at a speed of 10 revolutions per minute. The curable composition was poured into a chase on the bed of a hydraulic press and allowed to cure for 2 hours under ambient conditions and a gauge pressure of 9,000 psi (62 MPa). The cured sheet of elastomer exhibited a tensile strength of 400 psi (2.8 MPa) an elongation at break of 456% and a hardness value of 20 on the shore A scale.

Claims: A dispersion comprising a solid phase uniformly dispersed within a matrix phase, wherein the solid phase comprises A) a finely divided material other than silica exhibiting a density greater than the density of said matrix phase, and B) a finely divided fume reinforcing silica, and said matrix phase comprises C) a liquid organosiloxane copolymer exhibiting a general formula selected from and GR 1 2 SiO(R 1 2 SiO) b SiR 1 2 G, wherein each R 1 is individually selected from unsubstituted or substituted monovalent hydrocarbon radicals, Q is R 1 or G, G is a polyoxyalkylene group having an average structure selected from the group consisting of -R 2 (OCH 2 CH 2 ) c OZ and wherein R 2 represents a divalent hydrocarbon radical containing from 2 to 20 carbon atoms; Z is selected from the group consisting of hydrogen atom, alkyl radicals containing from 1 to 4 carbon atoms and the -(O)CR group, wherein R is selected from monovalent hydrocarbon radicals or monovalent hydrocarbon radicals containing the -C(O)OH group as a substituent, a, b, c and d are individually selected from positive integers, and D) an organosiloxane oligomer having not more than an average of 30 siloxane units per molecule and being a liquid at 25°C that does not react with the ingredients in said dispersion and is compatible with said organosiloxane copolymer, wherein the silicon-bonded hydrocarbon radicals present in said oligomer are selected from monovalent unsubstituted or substituted hydrocarbon radicals, and the number average molecular weight of said oligomer is sufficient to solubilize said organosiloxane copolymer in said matrix phase, wherein said silica constitutes from 2 to 15 weight percent, said copolymer constitutes from 0.1 to 10 weight percent, the concentrations of said silica and said copolymer are based on the combined weights of said finely divided material, silica, organosiloxane copolymer and organosiloxane oligomer and wherein the consistency of said dispersion at 25°C is selected from the group consisting of liquids, pastes and gels and said composition is flowable when subjected to shear. A dispersion according to claim 1 wherein R 1 represents an alkyl radical containing from 1 to 4 carbon atoms, R 2 represents an alkylene radical containing from 3 to 6 carbon atoms, the positive integers represented by a and b are from 1 to 100, inclusive, the positive integers represented by c and d are from 1 to 20, inclusive, Q is R 1 , R contains from 1 to 20 carbon atoms, said organosiloxane copolymer constitutes from 0.5 to 4 percent of the combined weights of said finely divided solid material, silica, organosiloxane copolymer and organosiloxane oligomer, said organosiloxane oligomer constitutes from 10 to 90 weight percent of said matrix phase, wherein said siloxane units are represented by the average formula Formel R 4 e [Ph(CH 2 ) g ] f SiO (4-e-f)/2 , wherein Ph represents phenyl, R 4 represents an alkyl radical containing from 1 to 10 carbon atoms, e is 0, 1 or 2, f is 1 or 2, the sum of e and f does not exceed 3 and g is 0 or an integer from 1 to 4, and said finely divided solid constitutes from 0.1 to 10 weight percent of said composition. A dispersion according to claim 2 wherein R 1 is methyl, R 2 is propylene, the positive integers represented by j and k are from 1 to 10, inclusive, said organosiloxane oligomer contains an average of not more than ten siloxane units per molecule and is selected from the group consisting of cyclic phenylalkylsiloxanes, cyclic diorganosiloxanes containing phenylalkylsiloxane and dialkylsiloxane units. A dispersion according to claim 3 wherein said phenylalkylsiloxane units are phenylmethylsiloxane units and said dialkylsiloxane units are dimethylsiloxane units. A dispersion according to any of claims 1-4 wherein said finely divided material is a pigment. A curable organosiloxane composition comprising the dispersion according to any of claims 1-5. A composition according to claim 6 wherein said organosiloxane composition further comprises a curable polyorganosiloxane containing at least two silanol groups per molecule, a crosslinking agent for said curable polyorganosiloxane and a catalyst for the crosslinking reaction. A composition according to claim 7 wherein the repeating units of said curable polyorganosiloxane are represented by the formula R 5 j SiO (4-j)/2 wherein each R 5 is individually selected from monovalent unsubstituted or substituted hydrocarbon radicals and j is 1 or 2, the number average molecular weight of said polyorganosiloxane is from 25,000 to 75,000, said crosslinking agent is a silane containing at least three hydrolyzable groups per molecule selected from the group consisting of alkoxy containing from 1 to 4 carbon atoms, carboxy containing from 2 to 4 carbon atoms and ketoximo, and said catalyst is selected from the group consisting of compounds of titanium, aluminum, zirconium, lead and tin. A composition according to claim 7 wherein R 5 is selected from the group consisting of alkyl radicals containing from 1 to 4 carbon atoms, phenyl and 3,3,3-trifluoropropyl, the number average molecular weight of said curable polyorganosiloxane is from 25,000 to 75,000, j is 2, said hydrolyzable group is alkoxy, and said curing agent constitutes from 0.5 to 6 weight percent of said composition. A composition according to claim 9 wherein said catalyst is a tin compound and said composition is packaged in two containers, wherein the contents of the first container comprises said curable polyorganosiloxane and the contents of said second container comprises said crosslinking agent, said curing catalyst, said finely divided material, said fume silica and said matrix phase and wherein said second container is substantially free of compounds containing hydroxyl groups.

Inventors: DONATELLI JOAN MARIE, MCMAHON DANIEL FRANCIS, DONATELLI, JOAN MARIE, MCMAHON, DANIEL FRANCIS
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1996
Main classification: C08L83
IPC codes: C08L83, C08K3, C08K5