Process for the preparation of cured silicone powder

Abstract: A highly productive process is disclosed for preparing cured silicone powder having a uniform particle size, in which a platinum-alkenylsiloxane complex catalyst is added to a water-based dispersion of a silicone composition. The silicone composition is (i) an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, and (ii) an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule. The catalyst is added and dispersed in the form of liquid particles with an average particle size in volumetric particle size distribution in water of no more than one micrometer.

Description: This invention a high-productivity process for preparing cured silicone powders having a uniform particle size. It is known that cured silicone powder can be prepared by curing liquid particles of a curable silicone composition comprising a platinum catalyst, an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, and an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule. For example, JP-A 62-243621 (1987) or US 4742142 teaches a process in which (i) a curable silicone composition comprising a platinum catalyst, an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, and an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule, is emulsified using water and a surfactant; and (ii) the curable silicone composition, dispersed in liquid particle form in this emulsion, is then cured by dispersing in hot water. In another example, JP-A 62-257939 (1987) or US 4761454, describes a process in which (i) an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, and an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule, are emulsified using water and a surfactant; (ii) a platinum catalyst is added to the emulsion; and (iii) the curable silicone composition dispersed in liquid particle form in the emulsion, is then cured by heating or spray drying. In the former process of US 4742142, however, curing occurs in the curable silicone composition prior to its homogeneous dispersion in water. This causes problems such as production of cured silicone powder with a broad particle size distribution, and the presence in the cured silicone powder of a substantial amount of large gel particles. The latter process of US 4761454 requires an extended period of time to achieve an acceptable cure, because the rate of incorporation of platinum catalyst into the organopolysiloxane emulsion is rate-determining for the process. As a result, the process in US 4761454 has poor productivity. In addition, since the emulsion breaks during the extended period of time required for curing, problems of a broad particle size distribution and the presence of large gel particles again arise. The object of this invention, therefore, is to provide a high-productivity process for preparing cured silicone powder having uniform particle size. Accordingly, in preparing cured-silicone powder by this invention, a platinum-alkenylsiloxane complex catalyst is added to a water-based dispersion of a silicone composition containing an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, and an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule. The presence of our catalyst as a dispersion in the form of liquid particles having an average particle size in their volumetric particle size distribution in water of no greater than 1 µm is unique. The process of the present invention commences with preparation of a water-based dispersion of a silicone composition which contains (i) an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, i.e., the alkenyl-functional organopolysiloxane, and (ii) an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule, i.e., the ≡SiH functional organopolysiloxane. The alkenyl-functional organopolysiloxane is the base component in the formation of the cured silicone powder. The alkenyl functionality in the organopolysiloxane is typically vinyl, allyl, butenyl, and pentenyl, with vinyl being specifically preferred. Non-alkenyl silicon-bonded groups in the organopolysiloxane can be substituted or unsubstituted monovalent hydrocarbon groups, e.g., alkyl groups such as methyl, ethyl, and propyl; aryl groups such as phenyl, tolyl, and xylyl; and haloalkyl groups such as 3,3,3-trifluoropropyl. Methyl and phenyl are preferred. The siloxane can have a straight-chain, partially branched straight-chain, branched-chain, or cyclic molecular structure, with straight-chain and partially branched straight-chain structures being preferred. The alkenyl-functional organopolysiloxane has a sufficient viscosity that permits formation of a water-based dispersion of a silicone composition with the ≡SiH functional organopolysiloxane. For example, a viscosity at 25 °C in the range of from 5 to 10,000,000mm2/S (centistoke) is preferred. The ≡SiH functional organopolysiloxane is the crosslinking component in forming our cured silicone powder. Non-hydrogen silicon-bonded groups present in the ≡SiH functional organopolysiloxane are substituted or unsubstituted monovalent hydrocarbon groups, e.g., alkyl groups such as methyl, ethyl, and propyl; aryl groups such as phenyl, tolyl, and xylyl; and haloalkyl groups such as 3,3,3-trifluoropropyl. Methyl and phenyl are again preferred. The ≡SiH functional organopolysiloxane can have a straight-chain, partially branched straight-chain, branched-chain, or cyclic molecular structure, with straight-chain and partially branched straight-chain structures being preferred. The ≡SiH functional organopolysiloxane has a viscosity that permits formation of a water-based dispersion of a silicone composition with the alkenyl-functional organopolysiloxane. For example, a viscosity at 25 °C in the range of from 1 to 10,000,000mm2/S (centistoke) is preferred. The alkenyl-functional organopolysiloxane to ≡SiH functional organopolysiloxane blending ratio is not critical, but preferably, 0.5 to 5 moles of silicon-bonded hydrogen from the ≡SiH functional organopolysiloxane are provided, per mole of silicon-bonded alkenyl from the alkenyl-functional organopolysiloxane. The use of a surfactant is preferred for generating a highly stable dispersion of the silicone composition, i.e. the alkenyl-functional organopolysiloxane and the ≡SiH functional organopolysiloxane, in water. Any surfactant that does not impair the curing reaction is suitable. For example, nonionic surfactants such as polyoxyethylene alkyl ethers and polyoxyethylene alkylphenyl ethers; anionic surfactants such as sodium alkyl sulfates and sodium polyoxyethylene alkyl ether sulfates; or mixtures of two or more of such surfactant types are suitable. The surfactant is preferably used at from 0.1 to 10 weight parts per 100 weight parts of the silicone composition. Water is preferably used at from 10 to 200 weight parts per 100 weight parts of the silicone composition. The technique for dispersing the silicone composition, i.e. the alkenyl-functional organopolysiloxane and the ≡SiH functional organopolysiloxane, in water employs known agitators and mixers such as colloid mills or homogenizers to mix these components. The next step in the process involves addition of a platinum-alkenylsiloxane complex catalyst to the water-based dispersion of the silicone composition. The catalyst is dispersed in the water-based dispersion of the silicone composition as liquid particles with an average particle size in their volumetric particle size distribution in water of no greater than 1 µm, preferably no greater than 0.8 µm, and most preferably no greater than 0.5 µm. It is preferred that the catalyst have a particle size of no greater than 1 µm, and that it account for at least 40 weight % of the volumetric particle size distribution of catalyst dispersed in liquid particle form in the water of the silicone composition dispersion. Some suitable platinum-alkenylsiloxane complex catalysts are a platinum-1,3-divinyl-1,1,3,3 -tetramethyldisiloxane complex, a platinum-1,3-diallyl-1,1,3,3-tetramethyldisiloxane complex, a platinum-1,3-divinyl-1,3-dimethyl-1,3-diphenyldisiloxane complex, a platinum-1,3-divinyl-1,1,3,3-tetraphenyldisiloxane complex, or a platinum-1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane complex. The stability of such complexes is improved by combining the catalyst with an excess of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, 1,3-diallyl-1,1,3,3-tetramethyldisiloxane, 1,3-divinyl-1,3-dimethyl-1,3-diphenyldisiloxane, 1,3-divinyl-1,1,3 ,3-tetraphenyldisiloxane, or 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane. The platinum complex with an excess of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane is particularly preferred. The techniques for dispersing the catalyst into liquid particles with an average particle size of no more than 1 µm in their volumetric particle size distribution in water includes (i) addition of a preliminarily prepared water-based catalyst dispersion in which the catalyst is dispersed as liquid particles with an average particle size in their volumetric particle size distribution in water of no more than 1 µm; or (ii) the addition of the catalyst after its dispersion with a surfactant. The use of a surfactant in technique (i) is also preferred as it improves the stability of the water-based catalyst dispersion. Surfactants which are operative for this purpose are the same surfactants mentioned previously. In techniques (i) and (ii), the surfactant is preferably used in an amount in the range of from 0.01 to 1,000 weight parts per 100 weight parts of the catalyst. In addition to techniques (i) and (ii), and to induce a unifonn dispersion of the catalyst in water, a third technique (iii) can be used in which a water-based dispersion of the catalyst as relatively finer liquid particles is prepared by first dissolving or dispersing the catalyst in a water-soluble organic solvent, and then introducing the resulting solution or dispersion into water. Water-soluble organic solvents suitable for this purpose are alcohols having no more than four carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, and tert-butanol; or ketones such as acetone and methyl ethyl ketone. The alcohols are preferred. Actual preparation of the water-based dispersion of the catalyst is performed by mixing the components using known stirrers and mixers such as colloid mills or homogenizers. A surfactant can also be used in technique (iii), and those surfactants previously mentioned are suitable. The quantity of catalyst used is not critical; however, it is added in an amount sufficient to provide from 0.1 to 1,000 weight ppm of platinum metal in the silicone composition. A preferred amount is from 0.1 to 500 weight ppm of platinum metal, while the most preferred amount is 1 to 50 weight ppm platinum metal, respectively, in the silicone composition. The curing reaction of the components of the silicone composition, i.e., the alkenyl-functional organopolysiloxane and the ≡SiH functional organopolysiloxane, will begin once the catalyst has been added to the water-based dispersion of the silicone composition. A curing reaction will proceed at temperatures from 0 to 100 °C, but excessively low temperatures result in a slow curing reaction, while excessively high temperatures impair the stability of the water-based silicone composition dispersion. The preferred temperature range is therefore from 5 to 70 °C. Examples The process for preparing cured silicone powder is explained in more detail in the following examples. In these examples, viscosity values were measured at 25 °C. The particle size distribution of cured silicone powder was measured by dispersing the water-based silicone powder dispersion in methanol, and then carrying out measurements using a laser diffraction-based instrument for measuring particle size distributions, i.e., a Model LA-500 device manufactured by Horiba™ Seisakusho. In this regard, the following characteristics were determined: (i) the median size of the cured silicone powder, i.e., the particle size corresponding to 50 % in the cumulative distribution, (ii) the content in volume % of the cured silicone powder with a particle size no greater than 10.0 µm, and (iii) the particle size of the cured silicone powder at 90 volume % cumulative distribution. With respect to the volumetric particle size distribution of the catalyst, the water-based catalyst dispersion was measured using a submicron particle analyzer employing laser scattering, i.e., a Coulter™ Model N4 device manufactured by Coulter Electronics. The device was used to determine the average particle size of the catalyst, and the content, i.e., the volume % of the catalyst with a particle size no greater than 1 µm. The cure status of the silicone composition was determined by adding 5 g ethanol to 2 g of the water-based dispersion of the silicone composition. In this test, cured silicone powder separated when the silicone composition had cured at the point of ethanol addition, while the silicone composition separated when the silicone composition had not cured at the point of ethanol addition. Reference Example 1 A water-based silicone composition dispersion was prepared as described below, and it was used in the following examples. 561.4 g of a dimethylvinylsiloxy-endblocked dimethylpolysiloxane with a viscosity of 400mm2/S (centistoke), and 38.6 g of a trimethylsiloxy-endblocked dimethylsiloxane-methylhydrogensiloxane copolymer with a viscosity of 50mm2/S (centistoke), in which the molar ratio of silicon-bonded hydrogen in the copolymer to vinyl in the dimethylpolysiloxane was 1.1, were placed in a one L beaker and mixed with a blade stirrer to homogeneity. An aqueous surfactant solution of 2.4 g of a nonylphenyl polyether surfactant in which the moles of ethylene oxide addition were 9.5, dissolved in 120 g of ion-exchanged water, was added to the beaker with stirring. The resulting water-based dispersion was emulsified in a colloid mill to provide a water-based silicone composition dispersion. The water-based silicone composition dispersion was transferred to a two L beaker, 460 g of ion-exchanged water was added, and the water-based silicone composition dispersion was stirred for 30 minutes with a motor stirrer to homogeneity. Reference Example 2 - Preparation of catalyst (A) 1 g of a solution containing platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in isopropyl alcohol and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, was homogeneously dispersed in 97 g of ion-exchanged water and 8 g of a polyoxyethylene nonylphenyl ether surfactant in which the moles of ethylene oxide addition were 9.5, to provide a water-based catalyst dispersion. The water-based catalyst dispersion had an average particle size in its volumetric particle size distribution of 0.25 µm, and contained 95 weight % of the catalyst with a particle size of one µm or less in the volumetric particle size distribution. Reference Example 3 - Preparation of catalyst (B) 1 g of a 1,3-divinyl-1,1,3,3-tetramethyldisiloxane solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex was homogeneously dispersed in 97 g of ion-exchanged water and 8 g of a polyoxyethylene nonylphenyl ether surfactant in which the moles of ethylene oxide addition were 9.5, to provide a water-based catalyst dispersion having an average particle size in its volumetric particle size distribution of 0.25 µm. The water-based catalyst dispersion contained 79 weight % of the catalyst which had a particle size of one µm or less in the volumetric particle size distribution. Reference Example 4 - Preparation of catalyst (C) 1 g of a solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in dimethylvinylsiloxy-endblocked dimethylsiloxane oligomer and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane was homogeneously dispersed in 97 g of ion-exchanged water and 8 g of a polyoxyethylene nonylphenyl ether surfactant in which the moles of ethylene oxide addition were 9.5, to provide a water-based catalyst dispersion. The water-based catalyst dispersion had an average particle size in its volumetric particle size distribution of 0.8 µm, and it contained 44 weight % of the catalyst with a particle size of one µm or less in the volumetric particle size distribution. Reference Example 5 - Preparation of catalyst (D) 1 g of a solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in dimethylvinylsiloxy-endblocked dimethylsiloxane oligomer and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane was homogeneously dispersed in 97 g of ion-exchanged water and 8 g of a polyoxyethylene nonylphenyl ether surfactant in which the moles of ethylene oxide addition were 9.5, to provide a water-based catalyst dispersion having an average particle size in its volumetric particle size distribution of 1.3 µm. The water-based catalyst dispersion contained 17 weight % of the catalyst with a particle size of one µm or less in the volumetric particle size distribution. Reference Example 6 - Preparation of catalyst (E) An isopropyl alcohol solution of chloroplatinic acid was prepared as a catalyst. Reference Example 7 - Preparation of catalyst (F) 3 g of a solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in isopropyl alcohol and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane was homogeneously dispersed in 6 g of a polyoxyethylene nonylphenyl ether surfactant in which the moles of ethylene oxide addition were 9.5, to provide a surfactant-based catalyst dispersion. When this surfactant-based catalyst dispersion was dispersed in water, the average particle size in the volumetric particle size distribution of catalyst in the resulting surfactant/water-based catalyst dispersion was 0.46 µm, and the content of the catalyst with a particle size of one µm or less in the volumetric particle size distribution was 66 weight %. Examples 1 to 3 & Comparative Examples 1 and 2 Catalysts (A)-(E) were added to the water-based silicone composition dispersion prepared in Reference Example 1, in amounts sufficient to provide 16 ppm platinum metal as catalyst in the silicone composition. The cure rate of the silicone composition, and the volumetric particle size distribution of cured silicone powder were determined, and these results are shown in Table 1. Comparative Example 3 561.4 g of a dimethylvinylsiloxy-endblocked dimethylpolysiloxane with a viscosity of 400mm2/S (centistoke) and 38.6 g of a trimethylsiloxy-endblocked dimethylsiloxane-methylhydrogensiloxane copolymer with a viscosity of 50mm2/5 (centistoke), in which the molar ratio of silicon-bonded hydrogen in the copolymer to vinyl in the dimethylpolysiloxane was 1.1, were placed in a one L beaker and mixed with a blade stirrer to homogeneity. The mixture was cooled to -10 to -5°C. A curable silicone composition was prepared by addition to the mixture of a catalyst in an amount sufficient to provide 16 ppm of platinum metal as catalyst in the curable silicone composition. The catalyst used in this comparative example was a solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in 1,3-divinyl-1,1,3,3-tetramethyldisiloxane and dimethylvinylsiloxy-endblocked dimethylsiloxane oligomer. The curable silicone composition was combined and stirred with an aqueous surfactant solution prepared by dissolving 2.4 g of a nonylphenyl polyether surfactant in which the moles of ethylene oxide addition was 9.5, in 120 g of ion-exchanged water. The resulting water-based dispersion was homogenized in a colloid mill to provide a water-based dispersion containing the curable silicone composition. The water-based dispersion was transferred to a two L beaker, 460 g of ion-exchanged water was added, and the dispersion was stirred for 30 minutes with a motor stirrer to homogeneity. The cure rate of the curable silicone composition was determined by bringing the water-based dispersion to 50 °C. The volumetric particle size distribution of the cured silicone powder was also determined. The results are shown in Table 1. Example 4 Catalyst (F) prepared in Reference Example 7, was added to the water-based silicone composition dispersion prepared in Reference Example 1, in an amount sufficient to provide 16 ppm in the silicone composition of platinum metal as the catalyst. This silicone composition cured after 6 minutes at 25 °C, and it cured after 2 minutes at 50 °C. Values were determined for the volumetric particle size distribution of the cured silicone powder product. Its median size was 3.8 µm; the content of the cured silicone powder with a particle size of ten µm or less was 98.7 volume % of the volumetric particle size distribution; and the particle size of the cured silicone powder at 90 volume % of the volumetric particle size distribution was 7.0 µm. Therefore, it can be seen that the process of this invention produces a cured silicone powder of more uniform particle size in a highly productive manner.

Claims: A process for preparing a cured silicone powder comprising adding a platinum-alkenylsiloxane complex catalyst to a water-based dispersion of a silicone composition comprising (i) an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, and (ii) an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule, the catalyst being added to the water-based dispersion of the silicone composition by dispersing the platinum-alkenylsiloxane complex catalyst in the water-based dispersion of the silicone composition as liquid particles having an average particle size in their volumetric particle size distribution in water of no more than one micrometer. The process according to Claim 1 in which the platinum-alkenylsiloxane complex catalyst with liquid particles having a particle size no more than one micrometer comprise at least 40 weight % of the volumetric particle size distribution of the platinum-alkenylsiloxane complex catalyst which is dispersed in liquid particle form in the water. The process according to Claim 1 in which the platinum-alkenylsiloxane complex catalyst has liquid particles having a particle size less than 0.5 micrometer which are present in at least 75 weight % of the volumetric particle size distribution of the platinum-alkenylsiloxane complex catalyst dispersed in liquid particle form in the water. The process according to Claim 1 in which the platinum-alkenylsiloxane complex catalyst has liquid particles having a particle size less than 0.5 micrometer which are present in at least 90 weight % of the volumetric particle size distribution of the platinum-alkenylsiloxane complex catalyst dispersed in liquid particle form in the water. A process for preparing a cured silicone powder comprising preparing a water-based dispersion of a silicone composition containing (i) an organopolysiloxane having at least two silicon-bonded alkenyl groups in each molecule, and (ii) an organopolysiloxane having at least two silicon-bonded hydrogen atoms in each molecule; the water-based dispersion being prepared by mixing organopolysiloxanes (i) and (ii) in water in the presence of a surfactant; and adding a platinum-alkenylsiloxane complex catalyst to the water-based dispersion of organopolysiloxanes (i) and (ii); the catalyst being dispersed in the water-based dispersion of organopolysiloxanes (i) and (ii) as liquid particles with an average particle size in their volumetric particle size distribution in water of no greater than one micrometer, the particles comprising at least 40 weight percent of the volumetric particle size distribution of catalyst dispersed in liquid particle form in the water-based dispersion of organopolysiloxanes (i) and (ii); and curing the organopolysiloxanes at a temperature of 5 to 70 °C. A process according to Claim 5 in which the catalyst is prepared by dissolving or dispersing it in a water-soluble organic solvent, and introducing a solution or dispersion of the catalyst into water. A process according to Claim 6 in which the water-soluble organic solvent is selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, tert-butanol, acetone, and methyl ethyl ketone. A process according to Claim 5 in which the catalyst is selected from the group consisting of a platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex, a platinum-1,3-diallyl-1,1,3,3-tetramethyldisiloxane complex, a platinum-1,3-divinyl-1,3-dimethyl-1,3-diphenyldisiloxane complex, a platinum-1,3-divinyl-1,1,3,3-tetraphenyldisiloxane complex, and a platinum-1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane complex. A process according to Claim 8 in which the catalyst also contains 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, 1,3-diallyl-1,1,3,3-tetramethyldisiloxane, 1,3-divinyl-1,3-dimethyl-1,3-diphenyldisiloxane, 1,3-divinyl-1,1,3,3-tetraphenyldisiloxane, or 1,3,5,7-tettamethyl-1,3,5,7-tetravinylcyclotetrasiloxane.

Inventors: KOBAYASHI KAZUO, MORITA YOSHITSUGAU, TACHIBANA RYUJI, KOBAYASHI, KAZUO, TACHIBANA, RYUJI, Kobayashi, Kazuo, Dow Corning Toray Sil. Co. Ltd., Morita Yoshitsugau, Dow Corning Toray Sil. Co. Ltd, Tachibana, Ryuji, Dow Corning Toray Sil. Co. Ltd.
Applicants: DOW CORNING TORAY SILICONE, DOW CORNING TORAY SILICONE COMPANY, LTD.
Application year: 1998
Main classification: C08J3
IPC codes: C08J3, C08L83, C08G77