Hydrophobic silica powder and method for producing the same

Abstract

To provide a hydrophobic silica powder having excellent fluidity and hydrophobicity and capable of being used in a variety of applications.SOLUTION: The hydrophobic silica powder with a silica surface modified with a chemical structure (-Si(CH3)2-O-)n, to have the number of surface silanol groups determined by an active hydrogen quantitative method of equal to or less than 2.0 per 1 nm2, and to have the amount of free oil of 4.0-13.0 mass%, can be obtained through a first surface treatment step where a fumed silica powder and cyclic dimethylsiloxane are mixed in a closed reactor at a temperature of 250 to 400°C and a second surface treatment step where a silicone oil is mixed.SELECTED DRAWING: None

Description

[Technical Field] 【0001】 This invention relates to hydrophobic silica powder and a method for producing the same. More specifically, the surface has a chemical structure (-Si(CH3)2-O-) n The objective is to provide a powder that is modified with a low number of surface silanol groups and has excellent fluidity. [Background technology] 【0002】 Silica powder is widely used in applications such as imparting thixotropy to resins (as a thickener), as a reinforcing filler for rubber and elastomers, and as a fluidizing agent for powdered materials. Among these, hydrophobic silica powder, whose surface has been hydrophobicized, is particularly suitable due to its excellent environmental stability, dispersibility, solvent resistance, and water resistance. For example, when hydrophobic silica powder is used as a thickener or reinforcing filler, the change in wettability with the matrix results in improved thickening and dispersibility. In these applications, a high degree of hydrophobicity is required because the above effects are reduced by hygroscopic silica absorption and the detachment of hydrophobic groups in the resin. 【0003】 Furthermore, using hydrophobic silica powder as a fluidizer for powdered materials improves fluidity because the hydrogen bonding adhesion due to silanol groups on the silica surface is reduced compared to hydrophilic silica powder. 【0004】 As a method for producing hydrophobic silica powder, for example, Patent Document 1 reports a hydrophobic treatment using cyclic dimethylsiloxane, and Patent Document 2 reports a hydrophobic treatment in which a low molecular weight trimethylsilylation agent such as hexamethyldisilazane is brought into contact with hydrophilic silica powder in gaseous form. Furthermore, Patent Document 3 reports a hydrophobic treatment using silicone oil. [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] Japanese Patent Publication No. 2004-352606 [Patent Document 2] Japanese Patent Publication No. 2000-264621 [Patent Document 3] Special Publication No. 57-2641 [Overview of the Initiative] [Problems that the invention aims to solve] 【0006】 However, while the hydrophobic silica powder obtained by contacting hydrophilic silica powder with a gaseous hydrophobic agent, as described in Patent Documents 1 and 2, is excellent in that it can uniformly hydrophobize the surface, the entire silica surface has a similar degree of hydrophobicity, and it has high fluidity, it also has the drawback that the modifying groups of the hydrophobic agent have a sterically bulky structure, so it is not possible to modify all of the silanol groups on the silica surface, which are the reaction sites, and unreacted silanol groups remain. Therefore, further improvement of hydrophobicity was required to prevent a decrease in affinity between the silica powder and the matrix and a decrease in fluidity due to the remaining silanol groups on the silica surface. 【0007】 Furthermore, while the hydrophobicization method described in Patent Document 3 is superior in that it uses silicone oil for hydrophobicization and thus exhibits high hydrophobicity, silicone oil cannot uniformly treat the surface, resulting in areas where the silanol groups on the silica surface are not covered by the silicone oil. Consequently, there is a problem in that the silica powder aggregates due to hydrogen bonding interactions between silanol groups between particles, leading to a decrease in fluidity. In addition, since silicone oil is a polymer, it has high viscosity, and there is a problem in that fluidity decreases when an excessive amount of silicone oil is added. This invention has been made in view of the above circumstances and aims to provide a hydrophobic silica powder that can be applied to various applications. [Means for solving the problem] 【0008】 The inventors of the present invention conducted intensive studies to solve the above problems. As a result, under the condition that the reaction temperature is 250 °C or higher, the hydrophobic silica powder treated with silicone oil after being treated with cyclic dimethylsiloxane has a high affinity between the site modified with cyclic dimethylsiloxane and silicone oil, so it was found that silicone oil is likely to adhere to the silica surface. Therefore, by adhering silicone oil to the silica surface, it was possible to obtain a hydrophobic silica powder with improved fluidity while maintaining the uniformity of the hydrophobization treatment and high hydrophobicity, which are the advantages of the conventional method, and the present invention was completed. 【0009】 That is, the hydrophobic silica powder of the present invention is a hydrophobic silica powder whose silica surface is modified with a chemical structure (-Si(CH3)2-O-) n and the number of surface silanol groups by the active hydrogen quantification method is 2.0 or less per 1 nm 2 and the free oil content is 4.0 to 13.0% by mass. Further, the method for producing the hydrophobic silica powder of the present invention includes a first surface treatment step of mixing fumed silica powder and cyclic dimethylsiloxane at a temperature of 250 to 400 °C in a sealed container, and a second surface treatment step of further mixing silicone oil. 【Effects of the Invention】 【0010】 The hydrophobic silica powder of the present invention has a small number of silanol groups on the surface and exhibits a high degree of hydrophobicity. Also, due to the high affinity between the silica surface treated with cyclic dimethylsiloxane and silicone oil, the amount of free oil is reduced, and it exhibits excellent fluidity and high stability of the silica surface. Therefore, it is possible to exhibit good performance in applications such as thickeners and reinforcing fillers for resins, etc., and its industrial value can be said to be extremely high. 【Best Mode for Carrying Out the Invention】 【0011】 [Hydrophobic Silica Powder] The hydrophobic silica powder of the present invention is a hydrophobic silica powder whose silica surface is modified with a chemical structure (-Si(CH3)2-O-) nA hydrophobic silica powder modified with, having a surface silanol group number of 2.0 or less per 1 nm 2 and a free oil content of 4.0 to 13.0% by mass. This will be described in detail below. 【0012】 〈Chemical Structure of Silica Surface〉 The hydrophobic silica powder of the present invention has a chemical structure (-Si(CH3)2-O-) n chemically modified on the surface. The presence of (-Si(CH3)2-O-) chemically modified on the surface n can be confirmed by Si DD / MAS NMR measurement. For example, if a hydrophobic silica powder from which free oil remaining on the silica surface has been removed and dried with an organic solvent such as hexane is measured, a peak is detected around -21.0 ppm. 29 【0013】 〈Silanol Group Number on Silica Surface〉 The surface silanol group number determined by the active hydrogen quantification method on the silica surface of the hydrophobic silica powder of the present invention is 2.0 or less per 1 nm 2 This is the case. The surface silanol group number determined by the active hydrogen quantification method of the hydrophobic silica powder of the present invention may be 2.0 or less per 1 nm 2 but is preferably 1.9 or less, more preferably 1.8 or less. Generally, the surface silanol group number of hydrophilic fumed silica powder is about 5.0 per 1 nm 2 On the other hand, the lower the surface silanol group of the hydrophobic silica powder, the higher the degree of hydrophobization. When the silanol group number exceeds 2.0, the hydrogen bonding interaction due to the silanol group becomes strong, and the fluidity may decrease. 【0014】 〈Free Oil Content〉 ​The lower limit of the free oil content of the hydrophobic silica powder of the present invention is 4.0% by mass, and the upper limit is 13.0% by mass. The silicone oil present on the surface of the silicone oil-treated silica powder can be divided into two types: silicone oil that is bonded to the silica powder and silicone oil that is simply adhering to the surface by physical adsorption. The silicone oil bonded to the silica powder is immobilized by chemical bonding due to the high affinity between the cyclic dimethylsiloxane-modified portion of the silica powder's surface and the silicone oil. Furthermore, the silicone oil simply adhering to the surface can be released from the hydrophobic silica powder using a hydrocarbon-based organic solvent such as hexane. Silicone oil that can be released from hydrophobic silica powder using an organic solvent in this manner is called free oil. The amount of free oil can be determined by measuring the amount of silicone oil that dissolves when hydrophobic silica powder is immersed in n-hexane. The lower limit of the free oil content of the hydrophobic silica powder of the present invention is 4.0% by mass, and the upper limit may be 13.0% by mass, but preferably the lower limit is 4.5% by mass or more, more preferably 5.0% by mass or more. The upper limit is preferably 12% by mass or less, more preferably 10% by mass or less. If the free oil content is less than 4.0% by mass, the silica surface may not be completely covered, and the hydrophobicity may decrease. Also, silanol groups may remain on the silica surface, which may reduce environmental stability due to moisture absorption, etc. On the other hand, if the free oil content is more than 13.0% by mass, the powder may aggregate due to the excess free oil, and its fluidity and dispersibility may deteriorate. 【0015】 <Cohesion degree> The degree of aggregation of the hydrophobic silica powder of the present invention is preferably 60% or less. A smaller degree of aggregation indicates that the hydrophobic silica powder has excellent fluidity. The degree of aggregation is calculated by preparing a three-tiered sieve consisting of a sieve with a mesh size of 355 μm, a sieve with a mesh size of 250 μm, and a sieve with a mesh size of 150 μm (all sieves with a diameter of 75 mm and conforming to JIS Z8801), stacked in this order from top to bottom at 2 cm intervals, placing 5 g of hydrophobic silica powder on the top sieve, and shaking it up and down for 15 seconds with an amplitude of 1 mm and a frequency of 60 Hz, and then calculating the amount of particles remaining on each sieve using the following formula (1). Cohesion degree (%) = {(Upper sieve residue + Middle sieve residue × 0.6 + Lower sieve residue × 0.2)} ÷ Initial mass of hydrophobic silica powder × 100 ... (1) The degree of aggregation of the hydrophobic silica powder of the present invention is preferably 60% or less, more preferably 58% or less, and even more preferably 56% or less. If the degree of aggregation is greater than 60%, when used as a thickener, reinforcing filler, or fluidizer, the aggregation of the silica powder particles themselves may make it difficult to uniformly disperse the silica powder in resins, etc., and the function may be reduced. 【0016】 <BET specific surface area> The lower limit of the BET specific surface area of ​​the hydrophobic silica powder of the present invention is 25 m². 2 The value is / g, and the upper limit is 270m 2 The value is / g. The BET specific surface area is measured by the nitrogen adsorption BET single-point method. The lower limit of the BET specific surface area of ​​the hydrophobic silica powder of the present invention is 25 m². 2 The value is / g, and the upper limit is 270m 2 It is acceptable if it is / g, but preferably the lower limit is 70m 2 / g or more, more preferably 80m 2 It is 160mg or more. The upper limit is preferably 160mg. 2 / g or less, more preferably 120m 2 The BET specific surface area of ​​the hydrophobic silica powder is 25 m² or less. 2 If the BET specific surface area is smaller than / g, the particle size of the silica powder may become too large, which can reduce its fluidity and dispersibility. 2If the value is greater than / g, the particle size of the silica powder becomes too small, which can lead to excessively high viscosity when dispersed in resins, making mixing impossible, or it can reduce the reinforcing properties as a reinforcing filler. 【0017】 <Hydrophobicity (M value)> The degree of hydrophobicity (M value) of the hydrophobic silica powder of the present invention, as measured by methanol titration, is preferably 60% to 80% by volume. A higher degree of hydrophobicity (M value) indicates higher hydrophobicity, while a lower value indicates higher hydrophilicity. The degree of hydrophobicity (M value) of the hydrophobic silica powder of the present invention, as measured by methanol titration, is preferably 60% to 80% by volume, more preferably 62% or higher by volume, and even more preferably 64% or higher by volume. A higher degree of hydrophobicity (M value) tends to be less affected by humidity and is advantageous in terms of environmental stability. 【0018】 <Carbon content> The lower limit of the carbon content present on the surface of the hydrophobic silica powder of the present invention is 3.0% by mass, and the upper limit is preferably 10.5% by mass. The carbon content is measured using a total nitrogen / total carbon analyzer (Sumigraph NC-22F manufactured by Sumika Analysis Center). The lower limit of the carbon content present on the surface of the hydrophobic silica powder of the present invention is 3.0% by mass, the upper limit is preferably 10.5% by mass, more preferably the lower limit is 3.5% by mass or more, and even more preferably 4.0% by mass or more. Furthermore, the upper limit is more preferably 8.0% by mass or less, and even more preferably 6.0% by mass or less. When the content is within the above range, aggregation of the powder due to excess silicone oil can be suppressed, and both high hydrophobicity and high fluidity can be achieved. 【0019】 [Method for producing hydrophobic silica powder] The manufacturing method of the present invention comprises a first surface treatment step of mixing fumed silica powder and cyclic dimethylsiloxane in a closed reactor at a temperature of 250 to 400°C, and a second surface treatment step of further mixing in silicone oil. The details will be described below. 【0020】 Fumed Silica Powder The hydrophilic silica powder used in the manufacturing method of the present invention is fumed silica powder, which is produced by supplying a silicon compound into a flame. Because such silica powder has low moisture content and coarse particles, it is excellent at imparting thixotropy to resins and other materials, and at improving the fluidity of powder materials. The fumed silica powder is not particularly limited and any known powder can be used, but silica powder produced by the flame decomposition of chlorosilane is preferred. In this embodiment, in order to have the above BET specific surface area, the BET specific surface area of ​​the fumed silica powder is 30 to 450 m². 2 A value of / g is preferred, and more preferably 50 to 380m 2 The value is / g. Furthermore, it is acceptable to use one type of fumed silica powder or to use two or more types in combination. 【0021】 Reactor The form of the hydrophobic treatment reaction for hydrophobic silica powder of the present invention is not particularly limited and may be, for example, a batch type or a continuous type. The reaction apparatus may also be a fluidized bed type, a fixed bed type, or a stirrer, mixer, or even a static type. However, considering the uniformity and acceleration of the reaction, a closed mixer that can process the material in a fluidized state by pressurization and stirring is preferred, and it is preferable to select the rotation speed of the stirring and the shape of the stirring blades so that the silica powder becomes fluid and a stable stirring state is obtained during stirring in the mixer. Furthermore, the first surface treatment step and The second surface treatment step may be carried out in a different reactor. In that case, it is preferable to transport the silica powder without contacting it with air. 【0022】 《First Surface Treatment Process》 The manufacturing method of the present invention involves mixing fumed silica powder and cyclic dimethylsiloxane in a closed reactor at a temperature of 250 to 400°C as a first surface treatment step. The lower limit of the hydrophobicity (M value) of the silica powder obtained in this step, measured by methanol titration, is preferably 47% by volume and the upper limit is preferably 63% by volume. More preferably, the lower limit is 50% by volume or more, and even more preferably 55% by volume or more. By satisfying the above conditions, the first surface treatment step is sufficiently performed, the number of silanol groups on the silica surface can be reduced, and ultimately, a hydrophobic silica powder with high fluidity can be obtained. 【0023】 <Cyclic Dimethylsiloxane> In the manufacturing method of the present invention, there are no particular restrictions on the cyclic dimethylsiloxane used in the first surface treatment step, and known cyclic dimethylsiloxanes can be used without particular limitation. For example, hexamethylcyclotrisiloxane (hereinafter also referred to as "D3"), octamethylcyclotetrasiloxane (hereinafter also referred to as "D4"), decamethylcyclopentasiloxane (hereinafter also referred to as "D5"), dodecamethylcyclohexasiloxane (hereinafter also referred to as "D6"), etc. Among the above cyclic dimethylsiloxanes, those with a boiling point of 300°C or lower are preferred due to their good reactivity and ease of handling. Furthermore, D4 ​​is preferred because it is readily available on the market. In addition, a mixture of the above cyclic dimethylsiloxanes may be used as the treatment agent. The amount of cyclic dimethylsiloxane added should be within the range that satisfies the aforementioned degree of hydrophobicity for the silica powder obtained in the first surface treatment step. Furthermore, although it varies depending on the cyclic dimethylsiloxane used, generally, the lower limit is... 40 The amount is in parts by mass, preferably with an upper limit of 150 parts by mass. More preferably, the lower limit is 40 parts by mass and the upper limit is 130 parts by mass. 【0024】 <Reaction conditions> (Reaction atmosphere) The first surface treatment step of the manufacturing method of the present invention is carried out in a closed system under high pressure in order to efficiently carry out the catalytic reaction between fumed silica powder and gaseous cyclic dimethylsiloxane. Furthermore, in order to prevent side reactions, it is preferable to carry out the process in a nitrogen atmosphere after thoroughly purging with an inert gas such as nitrogen. In addition, after the first surface treatment step, it is preferable to thoroughly replace any unreacted cyclic dimethylsiloxane remaining in the reactor with an inert gas such as nitrogen, and then carry out the following second surface treatment step. 【0025】 (Reaction temperature) In the manufacturing method of the present invention, the lower limit of the reactor temperature in the first surface treatment step is 250°C, and the upper limit is 400°C, but preferably the lower limit is 270°C or higher, and more preferably 290°C. The upper limit is preferably 380°C or lower, and more preferably 360°C. If the reactor temperature is lower than 250°C, the reactor temperature will drop below the boiling point of cyclic dimethylsiloxane due to endothermic heat from the vaporization of the sprayed cyclic dimethylsiloxane, which may prevent the gas-phase solid-phase reaction from occurring uniformly. On the other hand, if the reactor temperature is higher than 400°C, the types of reactors that can be used will be limited by the reactor's pressure resistance and heating method. Also, if the second surface treatment step is performed immediately after the first surface treatment step, cooling will be required to lower the reactor temperature, which will reduce productivity. 【0026】 (Reaction time) The reaction time for the first surface treatment step is not particularly limited and should be appropriately selected so that the hydrophobicity (M value) of the silica powder obtained after the first surface treatment step is 47% by volume or more. Generally, it is 30 minutes to 6 hours, but a reaction time of 1 hour or more is preferable to increase the hydrophobicity of the silica powder, and 3 hours or less is preferable considering productivity. 【0027】 《Second Surface Treatment Process》 The second surface treatment step of the manufacturing method of the present invention involves mixing the silica powder obtained in the first surface treatment step with silicone oil. It is preferable to heat the silicone oil sprayed in this step to 75-150°C. Heating the silicone oil to this temperature reduces its viscosity, allowing it to be sprayed in a sufficiently finely atomized state, thus enabling uniform treatment of the silica surface with silicone oil. As a result, a hydrophobic silica powder with fewer aggregates and good fluidity can be obtained. The method for spraying the silicone oil is not particularly limited, and includes a method (wet treatment method) in which silicone oil is dissolved in a solution such as toluene, silica powder is dispersed in the solution, the solvent is evaporated to adhere the silicone oil to the silica surface, and then a predetermined heat treatment is performed; and a method (dry treatment method) in which silicone oil is sprayed onto silica powder while mixing in a mixer or fluidized bed to adhere the silicone oil to the silica surface, and then a predetermined heat treatment is performed. Of the wet and dry processing methods described above, the dry processing method is preferred because it yields more uniformly processed silica powder and is superior in terms of cost, safety, and environmental impact. In the second surface treatment step of the manufacturing method of the present invention, when the silicone oil treatment is performed by the dry treatment method described above, it is preferable that the spray particle size of the silicone oil is 80 μm or less. Keeping the spray particle size within the above range facilitates more uniform treatment. The silicone oil spraying device can be a one-fluid nozzle, a two-fluid nozzle, etc., attached to the top of the reactor so as not to come into direct contact with the silica powder. It is preferable to use a two-fluid nozzle because it allows for spraying with a smaller particle size. 【0028】 <Silicone oil> The silicone oil used in the second surface treatment step of the manufacturing method of the present invention is not particularly limited, and known silicone oils can be used without restriction. Specifically, examples include dimethyl silicone oil, methylphenyl silicone oil, amino-modified silicone oil, and epoxy-modified silicone oil. Among the above silicone oils, dimethyl silicone oil, which has two methyl groups in its main chain, is preferred in order to maintain hydrophobicity while immobilizing the silicone oil on the hydrophobic silica surface after surface treatment with cyclic dimethylsiloxane. The viscosity of the silicone oil is not particularly limited, but a viscosity of 20 to 500 cSt is preferably used. If the viscosity of the silicone oil is lower than the above range, the silicone oil becomes volatile, making it difficult to adhere a predetermined amount to the silica surface. On the other hand, if the viscosity of the silicone oil is higher than the above range, the treatment tends to become uneven. Furthermore, two or more types of silicone oil with different functional groups may be mixed and used, or two or more types of silicone oil with the same functional group but different viscosity and molecular weight distribution may be mixed and used. The amount of silicone oil added varies depending on the specific surface area of ​​the silica powder, but it is sufficient if the amount of free oil in the hydrophobic silica powder obtained after the second surface treatment step falls within the aforementioned range. Generally, 5 to 20 parts by mass per 100 parts by mass of fumed silica powder is preferred, but 8 to 13 parts by mass is more preferable to achieve both hydrophobicity and fluidity in the hydrophobic silica powder. The silicone oil may also be dissolved in a solvent beforehand before use. 【0029】 (Reaction atmosphere) The second surface treatment step of the manufacturing method of the present invention may be carried out in either an open or closed system. Furthermore, in order to prevent side reactions as in the first treatment step, it is preferable to perform the procedure in a nitrogen atmosphere after thoroughly purging with an inert gas such as nitrogen. 【0030】 (Reaction temperature) The reactor temperature in the second surface treatment step of the manufacturing method of the present invention is not particularly limited, but it is preferable to perform a predetermined heat treatment after the silicone oil treatment. For example, the lower limit of the reactor temperature in the second surface treatment step is preferably 250°C, and the upper limit is preferably 400°C. More preferably, the lower limit is 280°C, and even more preferably 300°C. The upper limit is even more preferably 380°C, and even more preferably 360°C. If the reactor temperature is lower than 250°C, the affinity between the cyclic dimethylsiloxane-modified portion and the silicone oil decreases, and the amount of free oil may increase. On the other hand, if the reactor temperature is higher than 400°C, decomposition of the silicone oil occurs, making it difficult to uniformly adhere a predetermined amount to the silica surface. Therefore, by processing within the above range, it is possible to suppress the decomposition of the silicone oil while immobilizing it on the silica surface, thereby obtaining a hydrophobic silica powder with high hydrophobicity and silica surface stability. 【0031】 (Reaction time) The reaction time for the second surface treatment step of the manufacturing method of the present invention is not particularly limited and can be appropriately selected so that the amount of free oil in the hydrophobic silica powder obtained after the second surface treatment step is 4.0 to 13.0% by mass. However, a sufficient reaction rate can usually be obtained within 24 hours, and considering productivity, 6 hours or less is preferable, and more preferably 3 hours or less. [Examples] 【0032】 Examples and comparative examples are shown below to illustrate the present invention in detail, but the present invention is not limited to these. The physical properties of the prepared samples were evaluated by the following methods. 【0033】 (Measurement of surface structure) Surface structure of hydrophobic silica powder (-Si(CH3)2-O-) n teeth 29The measurement was performed by Si DDMAS NMR. For the measurement sample, hydrophobic silica powder was used, which was dried after removing the free oil remaining on the silica surface with n-hexane. The measurement instrument used was a Bruker Biospin AVANCE II. Specifically, 0.5 g of the sample and 32 ml of n-hexane were placed in a 50 ml centrifuge tube and ultrasonically dispersed for 30 minutes using an ultrasonic cleaner (Yamato Scientific ultrasonic cleaner 1510JMTH) to suspend the sample. The resulting suspension was centrifuged to separate and recover the solid phase (silica). To the recovered silica, another 32 ml of n-hexane was added, and the ultrasonic dispersion and centrifugation operations were repeated a total of three times. The silica was then dried under reduced pressure (120°C, 12 hours) to obtain a dried powder. Next, this powder was analyzed using a 4mm MAS probe, with the nuclide 29Si, MAS rotation speed 7kHz, pulse program hpdec, repetition time 20 sec, and more than 30,000 integrations. The external standard was the polydimethylsilane peak (34 ppm), and the analysis was calculated using the Bruker Topspin application software (Version 3.2). The surface structure of the hydrophobic silica powder in this example is (-Si(CH3)2-O-). n A peak is detected around -21.0 ppm. 【0034】 (Measurement of the number of silanol groups on the silica surface) The number of silanol groups on the silica surface of hydrophobic silica powder was measured by the active hydrogen quantification method. In the active hydrogen quantification method, the number of silanol groups can be determined from the amount of methane gas obtained by the reaction of Grignard reagent (CH3MgI) with the silanol groups on the silica surface and the specific surface area of ​​the silica. Details are described in Bull. Chem. Soc. Jpn., Volume 46, pp. 2000-2003 (1973) by T. Morimoto et al. Specifically, 1.5 g of hydrophobic silica powder was placed in a thin-walled glass sphere and dried at 110°C for 4 hours under 0.1 mmHg. Subsequently, the glass sphere was joined to a glass tube, and a gas burette, manometer, and dry box were attached. After attachment, the stopcock on the dry box side was opened, and nitrogen was purged, and Grignard reagent (CH3MgI) was added to the Erlenmeyer flask. Next, the rotor was struck against a glass sphere to break it, and the amount of methane generated by the reaction between hydrophobic silica powder and Grignard reagent was measured using a gas burette. The amount of methane gas V (mL / g) generated per gram of hydrophobic silica powder under standard pressure and temperature conditions was calculated using the measured amount of methane gas and the following formula (2). From the calculated amount of methane gas (mL / g), the amount of silanol groups (mol / g) per gram of silica powder was calculated, and further, the specific surface area of ​​the hydrophobic silica powder was used to determine the amount of methane gas on the surface of the hydrophobic silica powder at 1 nm. 2 Number of silanol groups per nm 2 ) was calculated. V={V1+(V2-W / ρ)} 1 / W 273 / (273+t) Formula (2) V1, V2, W, ρ, and t are as follows. V1: Volume change (mL) measured by gas burette V2: Volume of the glass sphere (mL) W: Weight of silica powder (g) ρ: Density of silica powder (2.2 g / mL) t: Measured temperature (°C) (This measurement was performed within the range of 17-20°C.) The specific surface area of ​​the silica powder used to calculate the number of silanol groups by the activated hydrogen quantitative method is the BET specific surface area of ​​calcined silica powder obtained by pre-calcining hydrophobic silica powder at 600°C for 4 hours to remove organic groups from the silica surface. 【0035】 (Measurement of free oil content) The amount of free oil in hydrophobic silica powder was determined by measuring the amount of silicone oil dissolved when the powder was immersed in n-hexane. Specifically, 0.5 g of hydrophobic silica powder and 32 ml of n-hexane were placed in a 50 ml centrifuge tube and ultrasonically dispersed for 30 minutes using an ultrasonic cleaner (Yamato Scientific 1510JMTH) to suspend the powder. The resulting suspension was centrifuged to separate and recover the solid phase (silica). To the recovered silica, another 32 ml of n-hexane was added, and the ultrasonic dispersion and centrifugation operations were repeated a total of three times. The powder was then dried under reduced pressure (120°C, 12 hours) to obtain a dried powder. The carbon content of this powder was measured using a total nitrogen / total carbon analyzer (Sumika Analysis Center Sumigraph NC-22F). The total carbon content in 0.5 g of the sample was measured beforehand, and the amount of extracted free oil was calculated from the difference between this total carbon content and the measured amount. 【0036】 (Measurement of cohesion) The degree of aggregation of hydrophobic silica powder was measured using a Hosokawa Micron PT-R powder tester. Specifically, a sieve with a mesh size of 355 μm was placed on top, a sieve with a mesh size of 250 μm was placed in the middle, and a sieve with a mesh size of 150 μm was placed on the bottom (all sieves conforming to JIS Z8801 with a diameter of 75 mm) were stacked with a spacing of 2 cm between each sieve. 5 g of the hydrophobic silica powder sample was placed on the top sieve, and after shaking it up and down for 15 seconds at an amplitude of 1 mm and a frequency of 60 Hz, the degree of aggregation (%) was calculated from the amount of sample remaining on each sieve using the following formula (3). A smaller degree of aggregation value indicates that the hydrophobic silica powder has superior fluidity. Cohesion degree (%) = {(upper sieve residue (g) + middle sieve residue (g) × 0.6 + lower sieve residue (g) × 0.2) ÷ 5 (g)} × 100... (3) 【0037】 (Measurement of BET specific surface area) The BET specific surface area of ​​hydrophobic silica powder, fumed silica powder, and calcined silica powder used for calculating the number of silanol groups by the activated hydrogen quantitative method was measured using a BET specific surface area meter (SA-1000 specific surface area measuring device manufactured by Shibata Scientific Instruments Co., Ltd.) by the BET single-point method based on nitrogen adsorption amount. 【0038】 (Measurement of hydrophobicity) The degree of hydrophobicity (M value) was measured by methanol titration. First, 0.2 g of hydrophobic silica powder was added to a 250 mL beaker containing 50 mL of water. Methanol was then added dropwise from a burette while stirring with a magnetic stirrer until the entire volume of the mixture was wet. The degree of hydrophobicity (M value) was defined as the percentage of methanol relative to the total volume of the mixture and the added methanol at the end of the dropwise addition. A higher degree of hydrophobicity (M value) indicates higher hydrophobicity. 【0039】 (Measurement of carbon content) The carbon content of the hydrophobic silica powder was measured using the total nitrogen and total carbon analyzer (Sumigraph NC-22F, manufactured by Sumika Analysis Center). 【0040】 Example 1 (First surface treatment process) As a hydrophilic fumed silica powder, Rheoroseal QS-10 (manufactured by Tokuyama Corporation, BET specific surface area 145 m²) is used. 2 ( / g, bulk density 50g / L) 50kg in volume 2m³ 3 The mixture was placed in a mixer and stirred while nitrogen was supplied to create a nitrogen atmosphere inside the container, and the mixture was heated to 330°C. The container was sealed, and 40 kg of cyclic dimethylsiloxane D4 (manufactured by Shin-Etsu Silicone Co., Ltd., product name: KF-994) was added. After the addition, the mixture was stirred and held for 1 hour while maintaining the above atmosphere and temperature. (Second surface treatment process) After depressurizing the container, nitrogen was supplied in an open system to create a nitrogen atmosphere inside the container. Then, with the container heated to 330°C in an open system, 5 kg of silicone oil KF-96-100cSt (manufactured by Shin-Etsu Silicone Co., Ltd., viscosity 100cSt), heated to 110°C, was sprayed using a two-fluid nozzle. After spraying, the mixture was stirred for 1 hour while maintaining the above atmosphere and temperature to obtain hydrophobic silica powder. The manufacturing conditions are shown in Table 1, and the physical property evaluation results are shown in Table 2 (the same applies hereafter). 【0041】 Example 2 Hydrophilic fumed silica powder QS-102 (manufactured by Tokuyama Corporation, BET specific surface area 200 m²) 2 Hydrophobic silica powder was prepared in the same manner as in Example 1, except that the weight ( / g, bulk density 50 g / L) was used. 【0042】 Example 3 Hydrophilic fumed silica powder is used with QS-30 (manufactured by Tokuyama Corporation, BET specific surface area 300 m²). 2 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the silicone oil used in the second surface treatment step was KF-96-50cSt (manufactured by Shin-Etsu Silicone Co., Ltd., viscosity 50cSt), with a viscosity of 50 g / L. 【0043】 Example 4 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the cyclic dimethylsiloxane used in the first surface treatment step was a mixture of D3 (manufactured by Shin-Etsu Silicone Co., Ltd., product name: LS-8120), D4 (manufactured by Shin-Etsu Silicone Co., Ltd., product name: KF-994), D5 (manufactured by Shin-Etsu Silicone Co., Ltd., product name: KF-995), and D6 (manufactured by Shin-Etsu Silicone Co., Ltd., product name: LS-9060) in a volume ratio of D3:D4:D5:D6 = 1:7:1:1. 【0044】 Example 5 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the amount of silicone oil added in the second surface treatment step was 2.5 kg. 【0045】 Example 6 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the amount of D4 added in the first surface treatment step was 20 kg. 【0046】 Example 7 Hydrophilic fumed silica powder QS-05 (manufactured by Tokuyama Corporation, BET specific surface area 60 m²) 2 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the amount of silicone oil added in the second surface treatment step was 5 kg, with a density of 50 g / L. 【0047】 Example 8 Hydrophilic fumed silica powder is used with QS-40 (manufactured by Tokuyama Corporation, BET specific surface area 380 m²). 2 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the amount of silicone oil added in the second surface treatment step was 12.5 kg, with a density of 50 g / L. 【0048】 Example 9 Hydrophobic silica powder was produced in the same manner as in Example 2, except that the temperatures for the first and second surface treatment steps were set to 280°C. 【0049】 Example 10 Hydrophobic silica powder was produced in the same manner as in Example 2, except that the temperatures for the first and second surface treatment steps were set to 310°C. 【0050】 Example 11 Hydrophobic silica powder was produced in the same manner as in Example 2, except that the temperatures for the first and second surface treatment steps were set to 360°C. 【0051】 Comparative Example 1 Hydrophobic silica powder was produced in the same manner as in Example 2, except that the first surface treatment step was omitted and the amount of silicone oil added in the second surface treatment step was set to 10 kg. 【0052】 Comparative Example 2 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the second surface treatment step was not performed. 【0053】 Comparative Example 3 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the amount of silicone oil added in the second surface treatment step was 10 kg. 【0054】 Comparative Example 4 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the amount of D4 added in the first surface treatment step was 7.5 kg. 【0055】 Comparative Example 5 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the temperatures for the first and second surface treatment steps were set to 240°C. 【0056】 Comparative Example 6 Hydrophobic silica powder was produced in the same manner as in Example 1, except that the temperatures for the first and second surface treatment steps were set to 410°C. 【0057】 [Table 1] 【0058】 [Table 2] 【0059】 The hydrophobic silica powders in Examples 1-11 have a silanol group count of 2 / nm. 2 The following conditions are met, and the free oil content is 4.0 to 13.0% by mass. By satisfying these conditions, the degree of cohesion, which is an indicator of the fluidity of hydrophobic silica powder, is 64% or less (60% or less except for Example 7), indicating excellent fluidity, and the degree of hydrophobicity (M value) is 60% by volume or more, indicating high hydrophobicity. From this, it can be seen that both high fluidity and high hydrophobicity can be achieved by satisfying the above conditions. 【0060】 In Examples 1-11, the first surface treatment involved mixing fumed silica and cyclic dimethylsiloxane in a sealed container at a temperature of 250-400°C, which resulted in obtaining a hydrophobic silica powder that achieved both high fluidity and high hydrophobicity. On the other hand, in Comparative Example 5, the temperature of the first surface treatment step was 240°C, resulting in a silanol group count of 2.1 groups / nm. 2 Furthermore, the degree of aggregation was 66%, resulting in a hydrophobic silica powder with a large number of remaining surface silanol groups and low fluidity. In addition, the degree of hydrophobicity (M value) of the silica powder after the first surface treatment in Comparative Example 5 was 48%, which is inferior to the degree of hydrophobicity of the silica powder after the first surface treatment in Examples 1 to 11. From this, it can be seen that in order to obtain a hydrophobic silica powder that achieves both high fluidity and high hydrophobicity in this embodiment, the temperature of the first surface treatment needs to be 250 to 400°C.

Claims

[Claim 1] The surface has a chemical structure (-Si(CH ３ ) ２ -O-) ｎ A hydrophobic silica powder modified with, The number of surface silanol groups measured by the activated hydrogen quantitative method is 1 nm. ２ The number is 2.0 or less per unit. A hydrophobic silica powder characterized by having a free oil content of 4.0 to 13.0% by mass. [Claim 2] The hydrophobic silica powder according to claim 1, wherein a three-tiered sieve is prepared by stacking a sieve with a mesh size of 355 μm, a sieve with a mesh size of 250 μm, and a sieve with a mesh size of 150 μm (all sieves with a diameter of 75 mm and conforming to JIS Z8801) in this order from top to bottom at 2 cm intervals, 5 g of powder is placed on the top sieve, and after shaking up and down for 15 seconds with an amplitude of 1 mm and a frequency of 60 Hz, the degree of aggregation calculated from the amount of particles remaining on each sieve using the following formula (1) is 60% or less. Cohesion level (%) = {(Upper sieve residue + Middle sieve residue × 0.6 + Lower sieve residue × 0.2)} ÷ Initial mass of hydrophobic silica powder × 100 ... (1) [Claim 3] BET specific surface area is 25-270 m² ２ The hydrophobic silica powder according to claim 1 or 2, wherein the concentration is / g, the degree of hydrophobicity (M value) measured by methanol titration is 60 to 80% by volume, and the carbon content present on the powder surface is 3.0 to 10.5% by mass. [Claim 4] A method for producing hydrophobic silica powder, comprising a first surface treatment step of mixing fumed silica powder and cyclic dimethylsiloxane in a closed reactor at a temperature of 250 to 400°C, and a second surface treatment step of further mixing in silicone oil, A method for producing hydrophobic silica powder, characterized in that the amount of cyclic dimethylsiloxane added is 40 to 150 parts by mass per 100 parts by mass of fumed silica powder. [Claim 5] The method for producing hydrophobic silica powder according to claim 4, wherein the cyclic dimethylsiloxane has a boiling point of 300°C or less. [Claim 6] A method for producing hydrophobic silica powder according to claim 4 or 5, wherein the silicone oil is sprayed while heated to a temperature of 75 to 150°C. [Claim 7] The method for producing hydrophobic silica powder according to claim 4 or 5, wherein the second surface treatment step is carried out in a reaction vessel at a temperature of 250 to 400°C.

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