PARTICULATE MATERIAL COMPRISING A PLATINUM-CONTAINING HYDROSILYLATION CATALYST AND A DIORGANOPOLYSILANE

Abstract: This invention provides particulate catalyst compositions comprising a platinum-containing hydrosilation reaction catalyst and a polydiorganosilane exhibiting a softening point of from 50 to 200 o C. The average particle size of the particulate does not exceed 100 micrometers, and the surface layer of the particulate is substantially free of said catalyst. Curable organopolysiloxane compositions containing the present catalyst compositions exhibit excellent storage stability.

Description: The present invention relates to a composition in particulate form comprising a platinum-containing hydrosilylation reaction catalyst and an organosilicon material. More particularly, the present invention relates to a particulate form of a platinum-containing hydrosilylation catalyst that will not cause premature curing when incorporated into polyorganosiloxane compositions which cure by a hydrosilylation reaction. From EP-A-0 300 645 it is known to microencapsulate platinum group metals and compounds thereof within one or two layers of thermoplastic organic polymers. The layer or layers are deposited using known prior art microencapsulation techniques. The microcapsules can be incorporated into storage-stable one-part polyorganosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. The average diameter of that microcapsules is less than 500 microns. One objective of the present invention is to provide a platinum-type hydrosilylation catalyst in particulate form. When used as the curing catalyst in a hydrosilylation-curing polyogranosiloxane composition, this catalyst composition provides an excellent storage stability while inducing a rapid curing upon heating. A second objective is to provide a method for the preparation of said particulate. The particulate compositions comprise a platinum-containing hydrosilylation catalyst and a polydiorganosilane having a softening point of from 50 to 200°C. The average particle size of the particulate material does not exceed 100 microneters, and the surface layer of the particulate is substantially free of the hydrosilylation catalyst. This invention provides a particulate composition comprising a platinum-containing hydrosilylation reaction catalyst dispersed in a polydiorganosilane having a softening point within the temperature range of from 50 to 200 degrees Centigrade, where the average particle size of said particulate does not exceed 100 micrometers and the surface layer of said particulate is substantially free of said catalyst. To explain the present compositions in greater detail, the polydiorganosilane ingredient of the present compositions must have a softening point in the range of 50 to 200 degrees Centigrade. A softening point below 50 degrees Centigrade will result in a substantial reduction in storage stability after addition to the hydrosilylation-curing polyorganosiloxane composition. At softening points in excess of 200 degrees Centigrade, the temperature at which the platinum-containing catalyst becomes active is too high, and the catalytic function of the present compositions is essentially not realized. Viewed from these standpoints, it is preferred that the polydiorganosilane's softening point fall within the temperature range of 70 to 150 degrees Centigrade. The molecular structure and chemical structure of the polydiorganosilane are not particularly critical. The only requirement is that the polydiorganosilane be impermeable with respect to the platinum-containing hydrosilylation catalyst and that it not cause any deterioration or quality changes in the presence of the catalyst. For example, when a platinum/olefin complex catalyst is used, polydiorganosilanes containing silicon-bonded hydrogen atoms should be avoided. Suitable polydiorganosilanes can be represented by the average unit formula R¹ a R² (2-a) Si, where R¹ is methyl, R² is phenyl, and the value of a is from 0.3 to 1.7, inclusive. Specific examples of suitable polydiorganosilanes include but are not limited to polymethylphenylsilane, methylphenylsilane-dimethylsilane copolymers, and polymethylcyclohexylsilane. The platinum-containing hydrosilylation catalysts can be a metal from the platinum group of the periodic table of the elements, compounds these elements and compositions based on these metals and compounds which exhibit a catalytic activity with respect to hydrosilylation reactions. Examples of suitable platinum-containing hydrosilylation reaction catalysts include but are not limited to finely divided platinum, chloroplatinic acid, alcohol-modified chloroplatinic acid, platinum/diketone complexes, platinum/olefin complexes, chloroplatinic acid/alkenylsiloxane complexes, and any of the preceding catalysts supported on a carrier such as alumina, silica or carbon black. Among these suitable catalysts platinum/olefin complexes and chloroplatinic acid/alkenylsiloxane complexes are preferred, based on their high catalytic activity in hydrosilylation reactions. Platinum/ alkenylsiloxane complexes of the type disclosed in Japanese Patent Publication Number 42-22924 [22,924/67] are particularly preferred. The particulate compositions of the present invention comprise a polydiorganosilane and a platinum-containing hydrosilylation reaction catalyst as described in the preceding specification. It is essential that the average diameter of the particles constituting the present compositions not exceed 100 micrometers in order for the catalytic activity to develop satisfactorily and in order to maintain a stable dispersion upon addition to polyorganosiloxane compositions. The relative concentrations of polydiorganosilane and platinum-type hydrosilylation reaction catalyst should be equivalent to a concentration of platinum metal in the composition of from 0.01 to 5 weight percent. The particulate composition of this invention consists essentially of a mixture of polydiorganosilane and a platinum-containing hydrosilylation catalyst that is encapsulated by a layer of polydiorganosilane which essentially does not contain any catalyst. As a consequence of this, hydrosilylation- curing polyorganosiloxane compositions which contain this particulate composition display an excellent storage stability. One of the characterizing features of the present compositions, namely the absence of platinum-containing catalyst in the surface layers of the particles, is achieved by washing a particulate catalyst composition with a liquid which is a solvent for said platinum-containing hydrosilylation reaction catalyst and a non-solvent for said polydiorganosilane. The procedure for preparing a mixture of a polydiorganosilane with a softening point of from 50 to 200 degrees Centigrade and a platinum-containing hydrosilylation catalyst is not particularly critical. Example of such procedures include but are not limited to mixing the polydiorganosilane and platinum-containing hydrosilylation catalyst on a hot roll, or dissolving the polydiorganosilane and catalyst in a co-solvent and then removing the co-solvent by evaporation. Procedures for preparing a particulate form of such mixtures include known methods for grinding or pulverizing of solid mixtures using mechanical means as well as methods in which a solvent solution of the mixture is sprayed into a spray drier to rapidly evaporate the solvent. A preferred method for preparing the present composition involves the following sequence of steps: A) Preparing a solution of a platinum-containing hydrosilylation reaction catalyst and a polydiorganosilane with a softening point in the temperature range of 50 to 200 degrees Centigrade by dissolving said polydiorganosilane and catalyst in a water-immiscible liquid, and then preparing an aqueous dispersion of said solution by adding said solution to surfactant-containing water while stirring or shaking the water. (B) Evaporatively removing the water-immiscible liquid from said aqueous dispersion at a temperature below the softening point of the polydiorganosilane to yield an aqueous dispersion of a particulate comprising the polydiorganosilane and the platinum-containing hydrosilylation catalyst, and then separating the particulate material from the aqueous dispersion. (C) Washing the particulate with a solvent in which the catalyst is soluble and in which said polydiorganosilane is insoluble. To explain the preceding method in greater detail, in step (A) a homogeneously mixed solution of polydiorganosilane and a platinum-containing hydrosilylation reaction catalyst is dispersed in microparticulate form in water to prepare an aqueous or water-based dispersion. The solvent used here should homogeneously dissolve both the polydiorganosilane and the platinum-type hydrosilylation reaction catalyst, and must also be immiscible with water. Furthermore, in order to facilitate removal of the solvent in step (B), it is preferred that the boiling point of this solvent under ambient pressure not exceed 100 degrees Centigrade. Most preferably the boiling point of this solvent is from 30 to 70 degrees Centigrade. Suitable solvents for step (A) are for example methylene chloride, chloroform, tetrahydrofuran, and diethyl ether. The particular surfactant used to generate the aqueous dispersion is not critical so long as it has the ability to generate a water-based dispersion, in emulsion or suspension form, of the solution of polydiorganosilane and platinum-containing hydrosilylation reaction catalyst. The only restriction on the surfactant is that it should not contain sulfur, phosphorus or other element that can cause a loss in catalytic activity of the platinum-containing hydrosilylation reaction catalyst. Suitable surfactants are for example water soluble polymeric organic compounds such as partially hydrolyzed polyvinyl alcohol. In process (B), a particulate material comprising the polydiorganosilane and the platinum-containing hydrosilylation reaction catalyst is separated and recovered from the water-based solvent dispersion generated by process (A). Evaporative removal of the solvent from aqueous dispersion must be conducted at a temperature below the softening point of the polydiorganosilane in order to avoid damage to the shape of the particulate. Evaporative solvent removal can be efficiently carried out, for example, by injecting an inert gas such as nitrogen into the water-based solvent dispersion while maintaining the dispersion under ambient or reduced pressure. Separation of the particulate from the water can be carried out by filtration or centrifugal separation. In process (C), the surface of the isolated particulate material is washed with a solvent to wash out and remove the platinum-containing hydrosilylation reaction catalyst present in the surface layer of the particulate. This washing solvent must be capable of dissolving the platinum-containing hydrosilylation reaction catalyst while at the same time it must not dissolve the polydiorganosilane. Selection of a particular washing solvent is based on the type of platinum-containing hydrosilylation reaction catalyst and diorganopolysilane used, however this washing solvent is typically an alcohol, preferably methyl alcohol or ethyl alcohol, or a low molecular weight polyorganosiloxane. Low molecular weight polyorganosiloxanes are particularly suitable washing solvents when a platinum/alkenylsiloxane complex catalyst is used as the platinum-containing hydrosilylation reaction catalyst. Siloxane oligomers such as hexamethyldisiloxane and cyclic diorganosiloxanes such as octamethylcyclotetrasiloxane are particularly preferred. The objective of the washing step, namely removal of catalyst from the surface of the particulate material, can be satisfactorily achieved simply by dispersing the particulate material in a large quantity of the washing solvent with stirring. The stirring efficiency can be boosted using, for example, ultrasound and the like, as long as such tactics do not cause a deterioration of the platinum-containing hydrosilylation reaction catalyst or polydiorganosilane. It is recommended that the wash temperature not exceed 30 degrees Centigrade. The particulate obtained following the washing step is converted to a micropulverulent platinum-containing hydrosilylation catalyst-containing particulate by evaporative removal of the washing solvent. The structure of the present particulate composition is characterized by a mixture of platinum-containing hydrosilylation reaction catalyst and polydiorganosilane that is encapsulated by a layer of polydiorganosilane having a softening point of from 50 to 200 degrees Centigrade and which is essentially free of said catalyst. This composition does not exhibit hydrosilylation reaction-promoting catalytic activity particulate at temperatures close to room temperature because the platinum-containing hydrosilylation reaction catalyst cannot leak to the surface at such temperatures. Moreover, the particulate material can develop a hydrosilylation-promoting catalytic activity at temperatures in excess of the polydiorganosilane's softening point because the platinum-containing hydrosilylation reaction catalyst bleeds out at such temperatures. Accordingly, the particulate of the invention is an extremely effective curing agent or curing promoter for single-package curable polyorganosiloxane compositions which must have long-term storage stability at around room temperature. As an example, a thermosetting polyorganosiloxane composition exhibiting excellent storage stability is obtained by the addition of the particulate composition of this invention to a mixture of a polydiorganosiloxane having at least two alkenyl groups in each molecule and a quantity of SiH-containing polyorganohydrogensiloxane sufficient to crosslink the polydiorganosiloxane. The particulate composition of the present invention is also effective as a curing agent for polyether compositions comprising an polyorganohydrogensiloxane and a vinyl-terminated polyether. The following examples are intended to describe preferred embodiments of the present invention. Unless otherwise specified all parts and percentages specified in the examples are by weight and viscosities were measured at 25 degrees C. Reference Example 1 Preparation of a platinum/vinylsiloxane complex catalyst 160g of 1,3-divinyltetramethyldisiloxane and 32g chloroplatinic acid (H₂PtCl₆ . 6H₂O) were mixed under ambient conditions and then mixed with heating to 120 degrees Centigrade for 1 hour under a stream of nitrogen. A platinum/vinylsiloxane complex catalyst was obtained by removing the platinum black by-product by filtration and then removing the acid with a water wash. The platinum metal concentration in this catalyst was 4.25 weight %. Reference Example 2 Synthesis of polymethylphenylsilane 44g of sodium metal were added to 500 ml dry toluene. This mixture was heated to reflux temperature, at which tine 170g of methylphenyldichlorosilane were added dropwise, following which the reaction mixture was heated to reflux temperature for 8 hours. After cooling, the reaction mixture was quenched by the addition of methyl alcohol and water. The oil layer (toluene layer) was separated from the resultant reaction mixture and washed with water, and the toluene was then evaporated off. The residue was dissolved in tetrahydrofuran, and a solid was recovered by adding methyl alcohol to induce precipitation. The resultant solid was a polymethylphenylsilane with an average molecular weight of 19,000 and a softening point of 135 degrees Centigrade. Reference Example 3 Synthesis of methylphenylsilane-dimethylsilane copolymer The procedure of Reference Example 2 was followed, but using a mixture of 60g methylphenyldichlorosilane and 90g dimethyldichlorosilane in place of the methylphenyldichlorosilane used in Reference Example 2. The product was a methylphenylsilanedimethylsilane copolymer with an average molecular weight of 40,000 and a softening point of 125 degrees Centigrade. Example 1 8.0 g polymethylphenylsilane prepared as described in Reference Example 2 and 1.0 g platinum/vinylsiloxane complex catalyst, prepared as described in Reference Example 1 were dissolved in 165g methylene chloride. This methylene chloride solution was added to water containing 7.5g polyvinyl alcohol (Gosenol GL-05 from Nippon Gosei Kagasu Kogyo Kabushiki Kaisha) while stirring the water in order to prepare a water-based dispersion of the methylene chloride solution. The entire quantity of methylene chloride was then evaporated off by bubbling nitrogen gas into this aqueous dispersion while it was maintained at a temperature of from 25 to 40 degrees Centigrade over 32 hours, yielding a suspension comprising a microfine solid dispersed in water. This suspension was placed in a centrifugal separator and the microfine solid powder was separated. 10g of this powder was washed once with water, then with methyl alcohol, and then with hexamethyldisiloxane. Analysis of the particulate after washing showed it to be a powder with an average particle size of 7 micrometers containing 0.26 weight% platinum metal. The properties of this particulate as a hydrosilylation reaction catalyst were then investigated. A hydrosilylation-curing organopolysiloxane composition was prepared as follows: 100g of an alpha,omega-divinylpolydimethylsiloxane exhibiting a viscosity of 1,5x10⁻³ m²/s (1,500 centistokes) and 20g. of fumed silica having a specific surface area of 200 m2/g and previously hydrophobicized with hexamethyldisilazane were mixed. This was followed by the addition with mixing to homogeneity of 2.8g of a methylhydrogenpolysiloxane represented by the average molecular formula Me₃SiO(Me₂SiO)₃(MeHSiO)₅SiMe₃. 0.50g of the washed powder prepared as described in the first section of this example was then added to 100g of the resultant polyorganosiloxane composition with mixing to homogeneity. The storage stability and thermal curing properties of the resultant curable organopolysiloxane composition were measured, and the obtained results are reported in Table 1. The following properties were determined using a Curastometer Model 3, from the Toyo Baldwin Company, as indicators of the curing properties: the time until initiation of curing of the polyorganosiloxane composition at 150 degrees Centigrade (I t ) and the time to achieve 90% of the maximum torque value (T₉₀). With regard to the storage stability, the polyorganosiloxane composition was allowed to stand for 90 days at 25 degrees Centigrade, and its change in viscosity was determined. for comparison, polyorganosiloxane compositions were prepared as described above, but using the platinum/alkenylsiloxane complex catalyst obtained in Reference Example 1 in place of the washed powder. The platinum content provided by the concentration of platinum/alkenylsilane complex was equal to the platinum content provided by the washed powder. The curing properties and storage stability of this comparison composition were also measured, and the obtained results are reported in Table 1 as Comparison Example 1. Example 2 8.0g of the methylphenylsilane-dimethylsilane copolymer prepared as described in Reference Example 3 and 1.0g of the platinum/vinylsiloxane complex catalyst prepared in Reference Example 1 were dissolved in 165g methylene chloride. This methylene chloride solution was added to water containing 7.5g polyvinyl alcohol (Gosenol GL-05 from Nippon Gosei Kagaku Kogyo Kabushiki Kaisha) while stirring the water in order to prepare a water-based dispersion of the methylene chloride solution. The entire quantity of methylene chloride was then evaporated off by injecting nitrogen gas into this aqueous dispersion at 25 to 40 degrees Centigrade over 32 hours, yielding a suspension comprising a microfine particulate dispersed in water. This suspension was placed in a centrifugal separator and the microfine particulate was separated. This powder was washed once with water, then with methyl alcohol, and then with cyclic dimethylsiloxanes. Analysis of the particulate after washing showed it to be a powder with an average particle side of 7 micrometers containing 0.26 weight % platinum, calculated as the metal. The properties of this particulate as a hydrosilylation reaction catalyst were then investigated. 100 g of an alpha,omega-divinylpolydimethylsiloxane exhibiting a viscosity of 1,5x10⁻³ m²/S (1,500 centistokes) and 20g of fumed silica exhibiting a specific surface area of 200 m²/g and that had been previously hydrophobicized with hexamethyldisilazane were mixed, and 2.8 g methylhydrogenpolysiloxane with the average molecular formula Me₃SiO(Me₂SiO)₃(MeHSiO)₅SiMe₃ was then added with mixing to homogeneity to yield a hydrosilylation-curing polyorganosiloxane composition. 0.50g of the washed particulate prepared as described in the first section of this example was added, with mixing to homogeneity, to 100g of this composition. The storage stability and thermal curing properties of the resultant curable polyorgansiloxane composition were measured as in Example 1, and the obtained results are reported below. curing properties I t (minutes) 1.5 T₉₀ (minutes) 12.0 storage stability

Claims: A particulate comprising a platinum-containing hydrosilylation reaction catalyst dispersed in a polydiorganosilane having a softening point within the temperature range of from 50 to 200 degrees Centigrade, where the average particle size of said particulate does not exceed 100 micrometers and the surface layer of said particulate is substantially free of said catalyst. A composition comprising the particulate of claim 1 and a curable polyorganosiloxane composition comprising a polydiorganosiloxane with at least two alkenyl groups in each molecule and a polyorganohydrogensiloxane with at least two silicon-bonded hydrogen atoms in each molecule, the amount of said polyorganohydrogensiloxane being sufficient to crosslink said polydiorganosiloxane. A method for preparing the particulate of Claim 1, said method comprising the following steps: (A) preparing a solution consisting of a platinum-containing hydrosilylation reaction catalyst and a polydiorganosilane with a softening point in the temperature range of 50 to 200 degrees Centigrade by dissolving said polydiorganosilane and said catalyst in a water-immiscible solvent for said polydiorganosilane and said catalyst, (B) preparing an aqueous dispersion of said solution by adding said solution to surfactant-containing water while stirring or shaking the water, (C) evaporatively removing said solvent from said aqueous dispersion at a temperature below the softening point of said polydiorganosilane to yield an aqueous dispersion of a particulate comprising said polydiorganosilane and said catalyst, (D) separating said particulate from said aqueous dispersion, (E) washing said particulate with a liquid in which said catalyst is soluble but in which said polydiorganosilane is insoluble.

Inventors: SARUYAMA, TOSHIO, TOGASHI, ATASUSHI
Applicants: TORAY SILICONE COMPANY, LIMITED
Application year: 1989
Main classification: C08J3
IPC codes: C08L83, B01J31, C08J3, C08G77, C08K3, B01J33, C08K9