Method for producing silica sol and silica sol

Abstract

To provide a method for producing silica sol capable of increasing the proportion of irregularly shaped silica particles.SOLUTION: A method for producing silica sol comprises a silica nucleus particle preparation step of preparing a first liquid comprising silica nucleus particles having an average maximum diameter of primary particles of 20 nm or less, and a bonding step of holding the first liquid for 72 hours or more to prepare a second liquid comprising connected silica nucleus particles.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a silica sol and a silica sol. [Background technology]

[0002] Chemical mechanical polishing (CMP) using a polishing composition has been used to polish the surfaces of materials such as metals, semi-metals, non-metals, and their oxides. This polishing composition generally contains a mixture and dispersion of an aqueous solution with chemical polishing properties and particles (abrasive grains) with mechanical polishing properties, and it is known to use silica sol as the abrasive grains. In this case, by deforming the silica particles, high friction can be achieved during polishing, further improving the polishing rate.

[0003] Patent Document 1 discloses a method for deforming silica particles, which comprises a step of preparing a reaction liquid by mixing a liquid (A) containing an alkali catalyst, water, a first organic solvent, and silica particles for association with a liquid (B) containing at least one of tetramethoxysilane and its condensate and a second organic solvent, and the addition rate of the liquid (B) is 8.5 × 10 in terms of silicon atoms per 1 mol of water contained in the liquid (A). -4 ~5.6×10 -3 mol / min. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-67969 Summary of the Invention [Problem to be solved by the invention]

[0005] The technique of Patent Document 1 makes it possible to obtain a silica sol containing highly aggregated silica particles. However, for example, to eliminate the surface roughness of a wafer, it is necessary to increase the proportion of irregularly shaped silica particles contained in the silica sol.

[0006] Therefore, the present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a method for producing silica sol that can increase the proportion of irregularly shaped silica particles. [Means for solving the problem]

[0007] In view of the above-mentioned problems, the present inventors have conducted extensive research and have found that the above-mentioned effects can be achieved by a method for producing a silica sol, which includes a silica core particle preparation step of preparing a first liquid containing silica core particles having an average longest diameter of primary particles of 20 nm or less, and a bonding step of holding the first liquid for 72 hours or more to prepare a second liquid containing linked silica core particles, thereby completing the present invention. [Effects of the Invention]

[0008] According to the present invention, there is provided a method for producing a silica sol that can increase the proportion of irregularly shaped silica particles. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a diagram illustrating a method for measuring the longest diameter of the primary particles of the silica core particles. DETAILED DESCRIPTION OF THE INVENTION

[0010] An embodiment of the present invention will be described below. The present invention is not limited to the following embodiment, and various modifications can be made within the scope of the claims. The embodiments described in this specification can be combined in any manner to form other embodiments.

[0011] In this specification, the range "X to Y" means "X or more and Y or less." Unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0012] <Method of manufacturing silica sol> One aspect of the present invention relates to a method for producing a silica sol, which includes a silica core particle preparation step of preparing a first liquid containing silica core particles having an average longest diameter of primary particles of 20 nm or less, and a bonding step of holding the first liquid for 72 hours or more to prepare a second liquid containing linked silica core particles. This method provides a silica sol containing a large amount of irregularly shaped silica particles.

[0013] In the present invention, a silica sol containing irregularly shaped silica particles means that the silica particles in the silica sol satisfy one of an average degree of association of 1.7 or more, an average aspect ratio of 1.5 or more, and an average circularity of 0.80 or less. The average degree of association, average aspect ratio, and average circularity will be explained later in the description of silica sol.

[0014] The reason why the manufacturing method according to the present invention exhibits the above-mentioned effects is not entirely clear, but is thought to be as follows.

[0015] In the method for producing silica sol according to the present invention, the surfaces of the silica core particles contained in the first liquid are thought to be activated due to the large amount of hydroxyl groups. The inventors therefore hypothesized that the hydroxyl groups on the surfaces of the silica core particles would undergo a condensation reaction with the hydroxyl groups on the surfaces of other silica particles, thereby bonding the silica core particles together to obtain irregularly shaped silica particles. As a result of their investigations, the inventors found that the silica core particles can be bonded together by holding the first liquid for a long period of time (72 hours or more). Furthermore, the inventors found that if the average longest diameter of the primary particles of the silica core particles exceeds a certain value (20 nm), irregularly shaped silica particles cannot be obtained even if the first liquid is held for a long period of time. Based on these findings, the inventors have completed the present invention.

[0016] The above mechanism is based on speculation, and whether it is correct or incorrect does not affect the technical scope of the present invention.

[0017] The constituent elements of the method for producing silica sol according to the present invention will be described below.

[0018] (Silica core particle preparation process) In the silica core particle preparation step, a first liquid containing silica core particles having an average longest diameter of primary particles of 20 nm or less is prepared.

[0019] In this specification, the silica core particles contained in the first liquid include silica particles in which two or more primary particles are bonded together.

[0020] In this specification, the average longest diameter of primary particles of silica core particles refers to the value obtained by measuring the longest diameter of the primary particle of each silica core particle in a photographed scanning electron microscope (SEM) image, as shown by the double-headed arrow in Figure 1, and calculating the average longest diameter of the primary particles of all silica core particles in the SEM image. The average longest diameter of primary particles is a value calculated by the method described in the Examples.

[0021] In this specification, the average value of the longest diameter of the primary particles of the silica core particles is also simply referred to as the "average longest diameter of the silica core particles."

[0022] The first liquid according to the present invention contains silica core particles having an average longest diameter of primary particles of 20 nm or less. If the average longest diameter of the primary particles of the silica core particles exceeds 20 nm, irregularly shaped silica particles cannot be obtained in the bonding step described below (see Comparative Example 4). The upper limit of the average longest diameter of the silica core particles is preferably 18 nm or less. The lower limit of the average longest diameter of the silica core particles is not particularly limited and is, for example, 2 nm or more, preferably 5 nm or more, and more preferably 10 nm or more. The average longest diameter of the silica core particles is preferably 2 nm or more and 20 nm or less, more preferably 5 nm or more and 20 nm or less, even more preferably 10 nm or more and 20 nm or less, and particularly preferably 10 nm or more and 18 nm or less.

[0023] In the silica core particle preparation step, the method for preparing the first liquid is not particularly limited, and a conventionally known method can be used. For example, the first liquid containing silica core particles can be obtained by reacting an alkoxysilane or its condensate in an organic solvent containing water and an alkali catalyst.

[0024] Hereinafter, one embodiment of the silica core particle preparation step will be described.

[0025] In one embodiment, the silica core particle preparation step includes adding and mixing a liquid (B1) containing at least one of an alkoxysilane and its condensate and a second organic solvent to a liquid (A) containing an alkali catalyst, water, and a first organic solvent, or a liquid (C1) containing the liquid (B1) and water but not containing an alkali catalyst, and terminating the addition when the average longest diameter of the primary particles of the silica core particles is 20 nm or less to prepare the first liquid.

[0026] The liquid (A) according to this embodiment contains an alkali catalyst, water, and a first organic solvent. In addition to the alkali catalyst, water, and first organic solvent, the liquid (A) may contain other components as long as the effects of the present invention are not impaired.

[0027] In a preferred embodiment, liquid (A) comprises an alkaline catalyst, water, and a first organic solvent. By using liquid (A) comprising an alkaline catalyst, water, and a first organic solvent, the impurities contained in the first liquid can be reduced to a minimum. This reduces the impact of impurities on polishing when the silica sol obtained by the manufacturing method of the present invention is used as a polishing slurry. Furthermore, the polishing slurry can be used in applications where metal impurities are undesirable, such as silicon wafers and device wafers, providing a polishing slurry with a wide range of applications.

[0028] The alkali catalyst contained in liquid (A) may be a conventionally known catalyst. From the viewpoint of minimizing the inclusion of metal impurities and the like, examples of the alkali catalyst include ammonia, tetramethylammonium hydroxide and other ammonium salts, ethylenediamine, diethylenetriamine, triethylenetetraamine, urea, monoethanolamine, diethanolamine, triethanolamine, and tetramethylguanidine. Among these, from the viewpoint of excellent catalytic activity, ammonia, tetramethylammonium hydroxide and other ammonium salts are more preferred, and ammonia is even more preferred. Ammonia is highly volatile and can be easily removed during the silica sol production process. The alkali catalyst may be used alone or in combination of two or more. The alkali catalyst may also be in the form of an aqueous solution.

[0029] The water contained in liquid (A) is preferably pure water or ultrapure water from the viewpoint of minimizing the inclusion of metal impurities and the like. When the alkali catalyst is in the form of an aqueous solution, the water contained therein is the water contained in liquid (A). Therefore, the water contained in the aqueous solution of the alkali catalyst is also preferably pure water or ultrapure water.

[0030] The first organic solvent contained in liquid (A) is preferably a hydrophilic organic solvent, and specific examples thereof include alcohols such as methanol, ethanol, n-propanol, isopropanol, ethylene glycol, propylene glycol, and 1,4-butanediol; ketones such as acetone and methyl ethyl ketone; etc. The first organic solvent may be used alone or in combination of two or more.

[0031] The first organic solvent according to this embodiment is preferably an alcohol. The use of an alcohol has the advantage that the alcohol can be easily substituted with water by thermal distillation during the water substitution step described below. Furthermore, from the viewpoint of recovery and reuse of the organic solvent, it is preferable to use the same type of alcohol as that produced by hydrolysis of the alkoxysilane.

[0032] Among alcohols, at least one of methanol, ethanol, isopropanol, etc. is particularly preferred. When tetramethoxysilane is used as the alkoxysilane, the first organic solvent is preferably methanol.

[0033] The contents of the alkali catalyst, water, and first organic catalyst in the liquid (A) are not particularly limited, and can be adjusted appropriately so that the average longest diameter of the silica core particles has a desired value.

[0034] The lower limit of the content of the alkali catalyst (e.g., ammonia) in liquid (A) is, for example, 0.1% by mass or more, preferably 0.3% by mass or more, based on the total amount (100% by mass) of liquid (A), from the viewpoint of its function as a hydrolysis catalyst or the growth of silica particles. The lower limit of the content of the alkali catalyst (e.g., ammonia) may be 0.5% by mass or more, 1.0% by mass or more, or 2.0% by mass or more, based on the total amount (100% by mass) of liquid (A). The upper limit of the content of the alkali catalyst (e.g., ammonia) is not particularly limited. From the viewpoint of productivity and cost, the upper limit of the content of the alkali catalyst (e.g., ammonia) is preferably 50% by mass or less, based on the total amount (100% by mass) of liquid (A). The upper limit of the content of the alkali catalyst (e.g., ammonia) may be 40% by mass or less, 20% by mass or less, 15% by mass or less, or 10% by mass or less, based on the total amount (100% by mass) of liquid (A). The content of the alkali catalyst (e.g., ammonia) can be 0.1% by mass or more and 50% by mass or less, 0.3% by mass or more and 40% by mass or less, 0.5% by mass or more and 20% by mass or less, 1.0% by mass or more and 15% by mass or less, or 2.0% by mass or more and 10% by mass or less, relative to the total amount (100% by mass) of liquid (A).

[0035] The water content in liquid (A) is adjusted according to the amount of alkoxysilane or its condensate used in the reaction. From the viewpoint of hydrolysis of the alkoxysilane, the lower limit of the water content is preferably 5% by mass or more, based on the total amount (100% by mass) of liquid (A). From the viewpoint of compatibility with liquid (B1), the upper limit of the water content is preferably 50% by mass or less, more preferably 40% by mass or less, based on the total amount (100% by mass) of liquid (A). The upper limit of the water content may be 20% by mass or less, based on the total amount (100% by mass) of liquid (A). The water content may be 5% by mass or more and 50% by mass or less, 5% by mass or more and 40% by mass or less, or 5% by mass or more and 20% by mass or less, based on the total amount (100% by mass) of liquid (A).

[0036] The lower limit of the content of the first organic solvent (e.g., methanol) in liquid (A) is preferably 10% by mass or more, more preferably 20% by mass or more, based on the total amount (100% by mass) of liquid (A), from the viewpoint of compatibility with liquid (B1). The lower limit of the content of the first organic solvent (e.g., methanol) may be 50% by mass or more, or 75% by mass or more, based on the total amount (100% by mass) of liquid (A). The upper limit of the content of the first organic solvent (e.g., methanol) is preferably 98% by mass or less, more preferably 95% by mass or less, based on the total amount (100% by mass) of liquid (A), from the viewpoint of dispersibility. The upper limit of the content of the first organic solvent (e.g., methanol) may be 90% by mass or less, or 85% by mass or less, based on the total amount (100% by mass) of liquid (A). The content of the first organic solvent (e.g., methanol) can be 10% by mass or more and 98% by mass or less, 20% by mass or more and 95% by mass or less, 50% by mass or more and 90% by mass or less, or 75% by mass or more and 85% by mass or less, relative to the total amount of liquid (A) (100% by mass).

[0037] The method for producing the liquid (A) is not particularly limited, and for example, a method can be used in which an alkali catalyst, water, a first organic solvent, and, if necessary, other components are mixed by stirring.

[0038] The liquid (B1) according to this embodiment contains at least one of an alkoxysilane and its condensate, and a second organic solvent. In addition to the at least one of an alkoxysilane and its condensate, and the second organic solvent, the liquid (B1) may contain other components within a range that does not impair the effects of the present invention.

[0039] In this specification, "at least one of an alkoxysilane and a condensate thereof" is also collectively referred to simply as "alkoxysilane, etc."

[0040] In a preferred embodiment, the liquid (B1) comprises at least one of an alkoxysilane and its condensate, and a second organic solvent. By using the liquid (B1) comprising at least one of an alkoxysilane and its condensate, and a second organic solvent, the impurities contained in the first liquid can be reduced as much as possible. This reduces the impact of impurities on polishing when the silica sol obtained by the manufacturing method of the present invention is used as a polishing slurry. Furthermore, the polishing slurry can be used in applications where metal impurities are undesirable, such as silicon wafers and device wafers, providing a polishing slurry with a wide range of applications.

[0041] Examples of the alkoxysilane or condensate thereof contained in the liquid (B1) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and condensates thereof. These may be used alone or in combination of two or more. Among them, tetramethoxysilane is preferred from the viewpoint of having suitable hydrolysis reactivity.

[0042] The second organic solvent contained in the liquid (B1) is preferably a hydrophilic organic solvent, and specific examples thereof include alcohols such as methanol, ethanol, n-propanol, isopropanol, ethylene glycol, propylene glycol, and 1,4-butanediol; and ketones such as acetone and methyl ethyl ketone.

[0043] The second organic solvent according to this embodiment is preferably an alcohol. The use of an alcohol has the advantage that the alcohol can be easily substituted with water by thermal distillation during the water substitution step described below. Furthermore, from the viewpoint of recovery and reuse of the organic solvent, it is preferable to use the same type of alcohol as that produced by hydrolysis of the alkoxysilane.

[0044] Among alcohols, at least one of methanol, ethanol, isopropanol, etc. is particularly preferred. When tetramethoxysilane is used as the alkoxysilane, the second organic solvent is preferably methanol.

[0045] The contents of the alkoxysilane and the second organic solvent in the liquid (B1) are not particularly limited, and can be adjusted appropriately so that the average longest diameter of the silica core particles has a desired value.

[0046] The lower limit of the content of alkoxysilanes, etc. (e.g., tetramethoxysilane, etc.) in liquid (B1) is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 75% by mass or more. The upper limit of the content of alkoxysilanes, etc. (e.g., tetramethoxysilane, etc.) is preferably 98% by mass or less, more preferably 95% by mass or less, even more preferably 90% by mass or less, and particularly preferably 85% by mass or less. The content of alkoxysilanes, etc. (e.g., tetramethoxysilane, etc.) is preferably 50% by mass or more and 98% by mass or less, more preferably 60% by mass or more and 95% by mass or less, even more preferably 70% by mass or more and 90% by mass or less, and particularly preferably 75% by mass or more and 85% by mass or less.

[0047] The lower limit of the content of the second organic solvent (e.g., methanol) in liquid (B1) is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and particularly preferably 15% by mass or more. The upper limit of the content of the second organic solvent (e.g., methanol) is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, and particularly preferably 25% by mass or less. The content of the second organic solvent (e.g., methanol) is preferably 2% by mass or more and 50% by mass or less, more preferably 5% by mass or more and 40% by mass or less, even more preferably 10% by mass or more and 30% by mass or less, and particularly preferably 15% by mass or more and 25% by mass or less.

[0048] If the contents of the alkoxysilane and the like and the second organic solvent in the liquid (B1) are within the above ranges, the miscibility when mixed with the liquid (A) can be improved.

[0049] The alkoxysilane condensate in the liquid (B1) is, for example, a dimer to dodecamer, and preferably a tetramer to octamer.

[0050] The method for producing the liquid (B1) is not particularly limited. From the viewpoint of miscibility, the method for producing the liquid (B1) is preferably a method in which the alkoxysilane and, if necessary, other components are stirred and mixed in the second organic solvent.

[0051] The liquid (C1) according to this embodiment contains water but does not contain an alkali catalyst. The absence of an alkali catalyst in the liquid (C1) can prevent the concentration of the alkali catalyst from becoming locally high during mixing. The liquid (C1) can contain components other than the alkali catalyst within a range that does not impair the effects of the present invention.

[0052] In a preferred embodiment, the liquid (C1) is made of water. By using the liquid (C1) as water, the impurities contained in the first liquid can be reduced as much as possible. This makes it possible to suppress the influence of impurities on polishing when the silica sol obtained by the production method of the present invention is used as a polishing slurry. Furthermore, it can be used in applications where metal impurities are undesirable, such as silicon wafers and device wafers, and a polishing slurry with wide applicability can be provided.

[0053] The water contained in the liquid (C1) is preferably pure water or ultrapure water, from the viewpoint of minimizing the inclusion of metal impurities and the like.

[0054] The water content in the liquid (C1) is preferably 95% by mass or more, more preferably 98% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass.

[0055] In this embodiment, liquid (B1), or liquid (B1) and liquid (C1), are added to liquid (A) and mixed. When liquid (B1) is mixed with liquid (A), silica core particles are produced by hydrolysis and polycondensation of alkoxysilanes, etc. The addition of liquid (B1), or liquid (B1) and liquid (C1), is terminated when the average longest diameter of the silica core particles reaches 20 nm. This stops the growth of the silica core particles and allows the preparation of the first liquid. In the silica core particle preparation step, the addition of liquid (B1), or the addition of liquid (B1) and liquid (C1), may be temporarily stopped in order to measure the average longest diameter of the silica core particles.

[0056] When adding and mixing liquid (B1), or liquid (B1) and liquid (C1), to liquid (A), it is preferable to stir liquid (A). The stirring speed is not particularly limited and is, for example, 30 to 500 rpm.

[0057] When adding and mixing liquid (B1) to liquid (A), the method of adding liquid (B1) is not particularly limited, and methods such as continuous addition and divided addition (e.g., dropwise addition) can be used.

[0058] When adding and mixing the liquid (B1) to the liquid (A), the addition rate of the liquid (B1) is not particularly limited and can be adjusted as appropriate within a range that does not cause a gel-like substance to form. For example, the addition rate of the liquid (B1) is 8.5 × 10 silicon atoms per 1 mol of water contained in the liquid (A). -4 ~5.6×10 -3 The rate can be expressed in moles / min. Silicon atom conversion means that the number of moles of silicon atoms contained in the alkoxysilane and its condensate is defined as the number of moles of the alkoxysilane and its condensate. For example, if the alkoxysilane is tetramethoxysilane, 1 mole of tetramethoxysilane is equivalent to 1 mole of silicon atoms. Furthermore, if the condensate of tetramethoxysilane is a tetramer, 1 mole of the condensate is equivalent to 4 moles of silicon atoms.

[0059] When adding liquid (B1) and liquid (C1) to liquid (A) and mixing them, the method of adding liquid (B1) and liquid (C1) is not particularly limited. Approximately constant amounts of each may be added to liquid (A) simultaneously, or liquid (B1) and liquid (C1) may be added alternately to liquid (A). Liquid (B1) and liquid (C1) may also be added randomly. Among these, from the viewpoint of suppressing changes in the amount of water used in the synthesis reaction, the method of adding liquid (B1) and liquid (C1) simultaneously is preferred, and the method of adding constant amounts of liquid (B1) and liquid (C1) simultaneously is more preferred.

[0060] As a method for adding the liquid (B1) and the liquid (C1) to the liquid (A), a method of adding the liquid (B1) and the liquid (C1) in portions (for example, dropwise) or continuously to the liquid (A) is preferably used, from the viewpoint of being able to prevent the concentration of the alkali catalyst from becoming locally high.

[0061] When adding and mixing liquids (B1) and (C1) to liquid (A), the addition rates of liquids (B1) and (C1) are not particularly limited and can be adjusted as appropriate within a range that does not produce a gel. The addition rate of liquid (B1) can be the same as that described above. The addition rate of liquid (C1) is, for example, 1 mol / min or more and 3 mol / min or less, preferably 2 mol / min, calculated as water, per 1 mol (in terms of silicon atom) of alkoxysilane and its condensate added as liquid (B1).

[0062] The temperatures of liquid (A), liquid (B1), and liquid (C1) when preparing the first liquid are not particularly limited and are, for example, from 5° C. to 100° C., and preferably from 5° C. to 70° C. It is preferable that the temperatures of liquid (A), liquid (B1), and liquid (C1), and the first liquid during preparation, are the same.

[0063] The preparation of the first liquid can be carried out under any pressure condition, such as reduced pressure, normal pressure, or increased pressure, although from the viewpoint of production costs, it is preferably carried out under normal pressure.

[0064] In this manner, the first liquid can be prepared.

[0065] (Joining process) In the bonding step, the first liquid is held for 72 hours or more to prepare a second liquid containing linked silica core particles.

[0066] In this specification, the term "connected silica core particles" refers to silica particles in which two or more primary particles, preferably three or more primary particles, are bonded together.

[0067] As used herein, "maintaining" refers to maintaining the first liquid in a stirred or stationary state. The maintaining may be carried out in the same device after the preparation of the first liquid, or may be carried out after the first liquid has been transferred to another device. The maintaining is preferably carried out in the same device after the preparation of the first liquid.

[0068] In this specification, "holding the first liquid for 72 hours or more" means holding the first liquid for 72 hours or more after the completion of preparation of the first liquid, and more specifically means holding the first liquid for 72 hours or more after the completion of addition of the above-mentioned liquid (B1), or liquid (B1) and liquid (C1).

[0069] The lower limit of the time for which the first liquid is retained is not particularly limited as long as it is 72 hours or more. The lower limit of the time for which the first liquid is retained may be 100 hours or more or 120 hours or more. The upper limit of the time for which the first liquid is retained is preferably 200 hours or less, from the viewpoint of being able to suppress particle aggregation. The upper limit of the time for which the first liquid is retained may be 170 hours or less or 150 hours or less. The time for which the first liquid is retained may be 72 hours or more and 200 hours or less, 100 hours or more and 170 hours or less, or 120 hours or more and 150 hours or less.

[0070] When the first liquid is held, the first liquid may be stirred or left to stand.

[0071] The temperature at which the first liquid is maintained is not particularly limited and is, for example, from 5° C. to 100° C., preferably from 5° C. to 70° C., and more preferably from 5° C. to 60° C. The temperature at which the first liquid is maintained may be constant or may vary.

[0072] The first liquid can be retained under any pressure condition, such as reduced pressure, normal pressure, or increased pressure, although normal pressure is preferred from the viewpoint of production costs.

[0073] In this manner, the second liquid can be prepared.

[0074] In the production method according to the present invention, the prepared second liquid can be used as a silica sol as it is. The prepared second liquid may be subjected to a growth step, post-processing, etc., which will be described later, and the resulting liquid may be used as a silica sol. In a preferred embodiment, a water substitution step is performed in which the organic solvent present in the second liquid is substituted with water, from the viewpoint of being able to suppress aggregation of silica particles.

[0075] (growth process) The method for producing silica sol according to the present invention can include, after the above-mentioned bonding step, a growth step in which a liquid (B2) containing at least one of an alkoxysilane and its condensate and a third organic solvent, or a liquid (C2) containing the liquid (B2) and water but not an alkali catalyst, is added to and mixed with the second liquid to prepare a third liquid containing grown linked silica core particles.

[0076] In this specification, the term "grown linked silica core particles" means that at least one of the average primary particle size and the average secondary particle size of the grown linked silica core particles contained in the third liquid is larger than that of the grown linked silica core particles contained in the second liquid.

[0077] The liquid (B2) contains at least one of an alkoxysilane and its condensate, and a third organic solvent. In addition to the at least one of an alkoxysilane and its condensate, and the third organic solvent, the liquid (B2) may contain other components within a range that does not impair the effects of the present invention.

[0078] In a preferred embodiment, liquid (B2) comprises at least one of an alkoxysilane and its condensate, and a third organic solvent. By using liquid (B2) comprising at least one of an alkoxysilane and its condensate, and a third organic solvent, the impurities contained in the third liquid can be reduced as much as possible. This reduces the impact of impurities on polishing when the silica sol obtained by the manufacturing method of the present invention is used as a polishing slurry. Furthermore, the polishing slurry can be used in applications where metal impurities are undesirable, such as silicon wafers and device wafers, providing a polishing slurry with a wide range of applications.

[0079] The explanation for the liquid (B2) is the same as that for the liquid (B1) described above, and therefore will be omitted. The explanation for the liquid (C2) is the same as that for the liquid (C1) described above, and therefore will be omitted.

[0080] In the growth step, liquid (B2), or liquid (B2) and liquid (C2), are added to the second liquid and mixed. When liquid (B2) is mixed with the second liquid, alkoxysilane and the like undergo hydrolysis and polycondensation, causing the linked silica core particles contained in the second liquid to grow. The growth of the linked silica core particles is stopped when the addition of liquid (B2), or liquid (B2) and liquid (C2), is terminated. This stops the growth of the linked silica core particles, and the third liquid can be prepared.

[0081] The conditions for adding and mixing liquid (B2) or liquid (B2) and liquid (C2) to the second liquid are the same as the conditions for adding and mixing liquid (B1) or liquid (B1) and liquid (C1) to the above-mentioned liquid (A), and therefore explanations will be omitted. The second liquid, liquid (B2), and liquid (C2) correspond to liquid (A), liquid (B1), and liquid (C1), respectively.

[0082] In this manner, the third liquid can be prepared.

[0083] In the production method according to the present invention, the prepared third liquid can be used as a silica sol as it is. The prepared third liquid may be subjected to a post-process, as described below, and the resulting liquid may be used as a silica sol. In a preferred embodiment, a water substitution step is performed in which the organic solvent present in the third liquid is substituted with water, from the viewpoint of suppressing aggregation of silica particles.

[0084] (post-process) In the method for producing a silica sol of the present invention, after the above-mentioned bonding step or growing step is performed, a post-step described below may be performed.

[0085] Specifically, at least one of the following steps may be performed: a water replacement step in which the organic solvent present in the second or third liquid is replaced with water; or a concentration step in which the second or third liquid is concentrated. More specifically, only the concentration step in which the second or third liquid is concentrated may be performed; or only the water replacement step in which the organic solvent in the second or third liquid is replaced with water may be performed; or the concentration step may be followed by a water replacement step in which the organic solvent in the concentrated liquid is replaced with water; or the water replacement step may be followed by a concentration step in which the water-replaced liquid is concentrated. Furthermore, the concentration step may be performed multiple times, and in such cases, a water replacement step may be performed between concentration steps. For example, after the concentration step, a water replacement step in which the organic solvent in the concentrated liquid is replaced with water may be performed, and a concentration step in which the water-replaced liquid is concentrated may be further performed.

[0086] (Water replacement process) As one embodiment of the present invention, the method for producing a silica sol of the present invention may include a step of substituting the organic solvent contained in the second liquid or the third liquid with water (also referred to simply as a "water substituting step" in this specification). The second liquid or the third liquid in this form also includes a form in which the second liquid or the third liquid has been subjected to a concentration step.

[0087] When ammonia is selected as the alkaline catalyst by replacing the organic solvent in the second or third liquid with water, the pH of the produced silica sol can be adjusted to a neutral range, and by removing unreacted substances contained in the second or third liquid, a silica sol that is stable for a long period of time can be obtained.

[0088] The organic solvent in the second or third liquid can be replaced with water by a conventional method, for example, by adding water dropwise while maintaining the volume of the second or third liquid at a certain level or more, followed by distillation under heating. In this case, the replacement operation is preferably carried out until the liquid temperature and the column top temperature reach the boiling point of the replaced water.

[0089] The water used in this step is preferably pure water or ultrapure water, from the viewpoint of minimizing the inclusion of metal impurities and the like.

[0090] An example of a method for replacing the organic solvent in the second or third liquid with water is to separate the silica particles (including linked silica core particles or grown linked silica core particles) from the second or third liquid by centrifugation, and then redisperse them in water.

[0091] (concentration process) As one embodiment of the present invention, the method for producing a silica sol of the present invention may further include a step of concentrating the second or third liquid (also referred to simply as a "concentration step" in this specification). Note that the second or third liquid in this embodiment also includes an embodiment in which the second or third liquid has been subjected to a water substitution step.

[0092] The method for concentrating the second or third liquid is not particularly limited, and any conventionally known method can be used, such as a heat concentration method or a membrane concentration method.

[0093] In the heat concentration method, the second or third liquid is heated and concentrated under normal pressure or reduced pressure, thereby obtaining a concentrated second or third liquid.

[0094] In the membrane concentration method, for example, the second liquid or the third liquid can be concentrated by membrane separation using ultrafiltration, which can filter silica particles (including linked silica core particles or grown linked silica core particles). The molecular weight cutoff of the ultrafiltration membrane is not particularly limited, but the molecular weight cutoff can be selected to match the particle size to be produced. The material constituting the ultrafiltration membrane is not particularly limited, but examples include polysulfone, polyacrylonitrile, sintered metal, ceramic, and carbon. The form of the ultrafiltration membrane is not particularly limited, but examples include spiral, tubular, and hollow fiber types. In the ultrafiltration method, the operating pressure is not particularly limited, but can be set to be equal to or lower than the operating pressure of the ultrafiltration membrane used.

[0095] In the production of the silica sol according to the present invention, the physical properties (average primary particle size, average secondary particle size, average degree of association, average aspect ratio, and average circularity) of the silica particles in the produced silica sol will be described later in the section on silica sol, which is another embodiment of the present invention.

[0096] <Silica sol> Another aspect of the present invention relates to a silica sol containing silica particles having an average aspect ratio of 1.50 or more, and the ratio of the number of silica particles having an aspect ratio of 1.50 or more to the number of all silica particles is 40% or more. Because the silica sol of the present invention contains a high proportion of irregularly shaped silica particles, it can be used to eliminate surface roughness.

[0097] The silica sol according to the present invention can be obtained by the above-mentioned method for producing a silica sol.

[0098] In this specification, the aspect ratio refers to the value obtained by measuring the long and short sides of the smallest rectangle circumscribing a silica particle and calculating the ratio of the long and short sides (long side value / short side value). The average aspect ratio is the value obtained by calculating the average aspect ratio of a predetermined number of silica particles (e.g., 100 or more). The aspect ratio and average aspect ratio can be determined, for example, by observation with a scanning electron microscope (SEM). More specifically, the aspect ratio and average aspect ratio are values ​​measured by the method described in the Examples.

[0099] The ratio of the number of silica particles having an aspect ratio of 1.50 or more to the total number of silica particles can be determined by observing with a scanning electron microscope (SEM), checking the aspect ratio of all silica particles (for example, 100 or more) in the SEM image, and calculating the ratio (%) of the number of silica particles having an aspect ratio of 1.50 or more to the total number of silica particles in the SEM image (= number of silica particles having an aspect ratio of 1.50 or more in the SEM image / total number of silica particles in the SEM image × 100). Details of the measurement method will be described in the Examples.

[0100] In the silica sol according to this embodiment, the lower limit of the average aspect ratio of the silica particles is 1.50 or more, preferably 1.55 or more, and more preferably 1.60 or more. The upper limit of the average aspect ratio of the silica particles is, for example, 5.00 or less, preferably 4.00 or less, and more preferably 3.00 or less. The average aspect ratio of the silica particles is preferably 1.50 or more and 5.00 or less, more preferably 1.55 or more and 4.00 or less, and even more preferably 1.60 or more and 3.00 or less.

[0101] In the silica sol according to this embodiment, the lower limit of the ratio of the number of silica particles having an aspect ratio of 1.50 or more to the total number of silica particles is 40% or more, preferably 50% or more. The upper limit of the ratio of the number of silica particles having an aspect ratio of 1.50 or more to the total number of silica particles is not particularly limited, and may be, for example, 80% or less or 70% or less. The ratio of the number of silica particles having an aspect ratio of 1.50 or more to the total number of silica particles is preferably 40% or more and 80% or less, more preferably 50% or more and 70% or less.

[0102] In the silica sol according to this aspect, the average circularity of the silica particles is preferably 0.80 or less. A preferred embodiment of the present invention relates to a silica sol containing silica particles having an average aspect ratio of 1.50 or more and an average circularity of 0.80 or less, wherein the proportion of the number of silica particles having an aspect ratio of 1.50 or more to the number of all silica particles is 40% or more.

[0103] In this specification, the average circularity refers to the calculated average circularity of all silica particles contained in the silica sol. The average circularity is measured by the method described in the Examples.

[0104] The upper limit of the average circularity of the silica particles is more preferably 0.75 or less, and even more preferably 0.70 or less.The lower limit of the average circularity of the silica particles is not particularly limited, and can be, for example, 0.30 or more, 0.40 or more, or 0.50 or more.The average circularity of the silica particles is preferably 0.30 or more and 0.80 or less, more preferably 0.40 or more and 0.75 or less, and even more preferably 0.50 or more and 0.70 or less.

[0105] In the silica sol according to this embodiment, the ratio of the number of silica particles having a circularity of 0.90 or more to the number of all silica particles is preferably 40% or less. A preferred embodiment of the present invention relates to a silica sol containing silica particles having an average aspect ratio of 1.50 or more, wherein the ratio of the number of silica particles having an aspect ratio of 1.50 or more to the number of all silica particles is 40% or more, and the ratio of the number of silica particles having a circularity of 0.90 or more to the number of all silica particles is 40% or less. A more preferred embodiment of the present invention relates to a silica sol containing silica particles having an average aspect ratio of 1.50 or more and an average circularity of 0.80 or less, wherein the ratio of the number of silica particles having an aspect ratio of 1.50 or more to the number of all silica particles is 40% or more, and the ratio of the number of silica particles having a circularity of 0.90 or more to the number of all silica particles is 40% or less.

[0106] The ratio of the number of silica particles with a circularity of 0.90 or more to the number of all silica particles can be obtained by carrying out scanning electron microscope (SEM) observation, measuring the circularity of each silica particle in SEM image, and calculating the ratio (%) of the number of silica particles with a circularity of 0.90 or more to the number of all silica particles (for example, 100 or more) in SEM image (= the number of silica particles with a circularity of 0.90 or more in SEM image / the number of all silica particles in SEM image × 100).The details of the measurement method will be described in Examples.

[0107] In the silica sol according to this embodiment, the upper limit of the ratio of the number of silica particles having a circularity of 0.90 or more to the total number of silica particles is more preferably 30% or less.The lower limit of the ratio of the number of silica particles having a circularity of 0.90 or more to the total number of silica particles is not particularly limited, and may be, for example, 5% or more or 10% or more.The ratio of the number of silica particles having a circularity of 0.90 or more to the total number of silica particles is preferably 5% or more and 40% or less, more preferably 10% or more and 30% or less.

[0108] In the silica sol according to this embodiment, the average primary particle diameter of the silica particles is, for example, 5 nm to 40 nm, preferably 10 nm to 30 nm. The average primary particle diameter of the silica particles can be calculated, for example, based on the specific surface area (SA) of the silica particles calculated by the BET method and the density of the silica particles. More specifically, the average primary particle diameter of the silica particles is a value measured by the method described in the Examples.

[0109] In the silica sol according to this embodiment, the average secondary particle diameter of the silica particles is, for example, 30 nm or more and 70 nm or less, preferably 40 nm or more and 60 nm or less. The average secondary particle diameter of the silica particles can be measured, for example, by a dynamic light scattering method, such as a laser diffraction scattering method. More specifically, the average secondary particle diameter of the silica particles is a value measured by the method described in the Examples.

[0110] In the silica sol according to this embodiment, the average degree of association of silica particles (ratio of average secondary particle size to average primary particle size) is, for example, 1.8 or more and 5.5 or less, preferably 2.0 or more and 5.0 or less, and more preferably 2.2 or more and 4.5 or less.

[0111] The pH of the silica sol according to this embodiment is not particularly limited as long as gelation does not occur. For example, the pH of the silica sol obtained by the production method according to the present invention may be 5.0 or more and 8.0 or less, or 6.5 or more and 7.5 or less. The pH of the silica sol can be measured with a pH meter.

[0112] In the silica sol according to this embodiment, the concentration (content) of the silica particles can be appropriately adjusted within a range that does not cause gelation. The method for adjusting the concentration of the silica particles is not particularly limited, and examples thereof include a method of diluting with water and a method of performing the above-mentioned concentration step.

[0113] The silica sol according to the present invention may contain water. The concentration (content) of water in the silica sol is not particularly limited and can be adjusted appropriately depending on the concentration (content) of silica particles.

[0114] Components other than water that may be contained in silica sol include components derived from the above-mentioned manufacturing process. Examples of components other than water include alkali catalysts, organic solvents, alkoxysilanes and their condensates, and metal impurities. It is preferable to remove as many components as possible other than silica particles and water. The content of components other than silica particles and water in silica sol is preferably 0.0001% by mass or less, and more preferably 0% by mass.

[0115] <Application> The silica sol of the present invention can be used in a variety of applications. It is particularly suitable as an abrasive for polishing objects such as semiconductor substrates. Examples of objects to be polished include metals or semimetals, such as silicon, aluminum, nickel, tungsten, steel, tantalum, titanium, and stainless steel, or alloys thereof; glassy materials, such as quartz glass, aluminosilicate glass, and glassy carbon; ceramic materials, such as alumina, silica, sapphire, silicon nitride, tantalum nitride, and titanium carbide; compound semiconductor substrate materials, such as silicon carbide, gallium nitride, and gallium arsenide; and resin materials, such as polyimide resins. Furthermore, the silica sol produced by the method of the present invention can be used as a filler for resins (e.g., a filler for sealing semiconductor elements), a hard coating agent, a resin modifier, a surface treatment agent, a paint, a pigment, a catalyst, an anti-slip agent, a spacer for liquid crystal display devices, a fiber treatment agent, a binder, an adhesive, a polymer flocculant, a toner, a cleaning agent, a cosmetic product, a dental material, a nanocomposite, a thermal recording medium, a photosensitive film, and a suspending agent.

[0116] The present invention encompasses the following aspects and embodiments. [1] a silica core particle preparation step of preparing a first liquid containing silica core particles having an average longest diameter of primary particles of 20 nm or less; a bonding step of holding the first liquid for 72 hours or more to prepare a second liquid containing linked silica core particles. [2] The method for producing a silica sol according to [1] above, wherein in the silica core particle preparation step, a liquid (B1) containing at least one of an alkoxysilane and its condensate and a second organic solvent, or a liquid (C1) containing the liquid (B1) and water but not an alkali catalyst, is added to and mixed with a liquid (A) containing an alkali catalyst, water, and a first organic solvent, and the addition is terminated to prepare the first liquid when the average longest diameter of the primary particles of the silica core particles is 20 nm or less. [3] The method for producing a silica sol according to [2] above, wherein the alkali catalyst is ammonia. [4] The method for producing a silica sol according to the above [2] or [3], wherein the first organic solvent and the second organic solvent are methanol. [5] A method for producing a silica sol according to any one of the above [1] to [4], comprising a growth step of, after the bonding step, adding to and mixing with the second liquid a liquid (B2) containing at least one of an alkoxysilane and a condensate thereof and a third organic solvent, or a liquid (C2) containing the liquid (B2) and water but not containing an alkali catalyst, to prepare a third liquid containing the grown linked silica core particles. [6] silica particles having an average aspect ratio of 1.50 or greater; A silica sol in which the ratio of the number of silica particles having an aspect ratio of 1.50 or more to the number of all silica particles is 40% or more. [7] The silica sol according to [6] above, wherein the average circularity of the silica particles is 0.80 or less. [8] The silica sol according to [6] or [7] above, wherein the ratio of the number of silica particles having a circularity of 0.90 or more to the number of all silica particles is 40% or less. [Example]

[0117] The present invention will be described in more detail using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples. Unless otherwise specified, "%" and "parts" mean "% by mass" and "parts by mass," respectively. In the following examples, unless otherwise specified, the operations were carried out under the conditions of room temperature (20 to 25°C) and relative humidity of 40 to 50% RH.

[0118] Example 1 (Silica core particle preparation process) A liquid (A) prepared by mixing 3266 g of methanol (manufactured by Kanto Chemical Co., Inc.), 335 g of pure water, and 359 g of 29 mass % aqueous ammonia was added to a 5 L reaction vessel equipped with a stirrer and having a cooling function. The liquid temperature in the reaction vessel (reaction liquid temperature) was kept at 55°C, and while stirring at 300 pm, a liquid (B1) prepared by dissolving 88 g of tetramethoxysilane (TMOS, manufactured by Tama Chemicals Co., Ltd.) in 22 g of methanol and a liquid (C1) of 21 g of pure water were simultaneously added to prepare a reaction liquid, and the synthesis reaction was allowed to proceed.

[0119] The addition rate of liquid (B1) is 1.2 x 10 silicon atoms per 1 mol of water contained in liquid (A). -3 The liquid (C1) was added at a rate of 2 mol / min in water equivalent to 1 mol (in terms of silicon atom) of tetramethoxysilane added as the liquid (B1).

[0120] When the average longest diameter of the primary particles of the synthesized silica core particles reached 17.8 nm, the addition of liquid (B1) and liquid (C1) was stopped to complete the synthesis reaction and prepare the first liquid.

[0121] (Joining process) The prepared first liquid was held in the same reaction vessel while stirring at a rotation speed of 300 rpm. The liquid temperature at the start of holding was 55°C, and was then kept at room temperature. At the end of holding, the temperature was 25°C. Stirring was stopped 136 hours after the completion of the addition of liquid (B1) and liquid (C1), and a second liquid was prepared.

[0122] (growth process) 3570 g of the prepared second liquid was added to a 5 L reaction vessel equipped with a stirrer and cooling function, and while maintaining the liquid temperature in the reaction vessel at 55°C and stirring at 300 rpm, a liquid (B2) prepared by dissolving 324 g of tetramethoxysilane (TMOS) in 83 g of methanol and a liquid (C2) containing 77 g of pure water were simultaneously added to prepare the third liquid.

[0123] The addition rate of liquid (B1) is 1.2 x 10 silicon atoms per 1 mol of water contained in the second liquid. -3 The liquid (C1) was added at a rate of 2 mol / min in water equivalent to 1 mol (in terms of silicon atom) of tetramethoxysilane added as the liquid (B1).

[0124] (Water replacement process) The resulting third liquid was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and heated to distill the mixture, replacing the methanol in the reaction liquid with pure water to obtain a silica sol.

[0125] <Example 2> (Silica Core Particle Preparation Process and Bonding Process) The second liquid was prepared in the same manner as in Example 1.

[0126] (growth process) 4016 g of the prepared second liquid was added to a 5 L reaction vessel equipped with a stirrer and cooling function, and while maintaining the liquid temperature in the reaction vessel at 55°C and stirring at 300 rpm, a liquid (B2) prepared by dissolving 127 g of tetramethoxysilane (TMOS) in 33 g of methanol and a liquid (C2) of 30 g of pure water were simultaneously added to prepare a third liquid.

[0127] The addition rate of liquid (B1) is 1.2 x 10 silicon atoms per 1 mol of water contained in the second liquid. -3The liquid (C1) was added at a rate of 2 mol / min in water equivalent to 1 mol (in terms of silicon atom) of tetramethoxysilane added as the liquid (B1).

[0128] (Water replacement process) The resulting third liquid was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and heated to distill the mixture, replacing the methanol in the reaction liquid with pure water to obtain a silica sol.

[0129] Example 3 (Silica core particle preparation process) A liquid (A) prepared by mixing 3266 g of methanol (manufactured by Kanto Chemical Co., Inc.), 335 g of pure water, and 359 g of 29 mass % aqueous ammonia was added to a 5 L reaction vessel equipped with a stirrer and having a cooling function. The liquid temperature in the reaction vessel (reaction liquid temperature) was kept at 55°C, and while stirring at 300 pm, a liquid (B1) prepared by dissolving 88 g of tetramethoxysilane (TMOS, manufactured by Tama Chemicals Co., Ltd.) in 22 g of methanol was added to prepare a reaction liquid, and the synthesis reaction was allowed to proceed.

[0130] The addition rate of liquid (B1) is 1.2 x 10 silicon atoms per 1 mol of water contained in liquid (A). -3 mol / min.

[0131] When the average longest diameter of the primary particles of the synthesized silica core particles reached 17.8 nm, the addition of liquid (B1) was stopped to prepare a first liquid.

[0132] (Joining process) The prepared first liquid was held in the same reaction vessel while stirring at a rotation speed of 300 rpm. The liquid temperature at the start of holding was 55°C, and then kept at room temperature. At the end of holding, the temperature was 25°C. Stirring was stopped 110 hours after the completion of the addition of liquid (B1), and a second liquid was prepared.

[0133] (Water replacement process) The resulting second liquid was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and heated to distill the mixture, replacing the methanol in the reaction liquid with pure water to obtain a silica sol.

[0134] Example 4 (Silica core particle preparation process) The first liquid was prepared in the same manner as in Example 1.

[0135] (Joining process) The prepared first liquid was held in the same reaction vessel while stirring at a rotation speed of 300 rpm. The liquid temperature at the start of holding was 55°C, and then kept at room temperature. At the end of holding, the temperature was 25°C. Stirring was stopped 72 hours after the addition of liquid (B1) and liquid (C1) was completed, and a second liquid was prepared.

[0136] (Water replacement process) The resulting second liquid was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and heated to distill the mixture, replacing the methanol in the reaction liquid with pure water to obtain a silica sol.

[0137] <Comparative Example 1> A liquid (A') prepared by mixing 2945 g of methanol with 375.92 g of pure water, 108 g of 29 mass% ammonia water, and 91 g of colloidal silica (silica concentration: 12 mass%, average secondary particle diameter: 25 nm, average longest diameter of primary particles: 18.9 nm) was added to a 5 L reaction vessel equipped with a stirrer and cooling function. The temperature of the liquid in the reaction vessel was kept at 20°C, and while stirring at 300 rpm, a liquid (B') prepared by dissolving 309 g of tetramethoxysilane (TMOS) in 79 g of methanol was added to produce a reaction liquid.

[0138] The addition rate of liquid (B') is 1.9 x 10 silicon atoms per 1 mol of water contained in liquid (A'). -3 mol / min.

[0139] The resulting reaction solution was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and distilled by heating to replace the methanol in the reaction solution with pure water, thereby obtaining a silica sol.

[0140] <Comparative Example 2> A liquid (A') prepared by mixing 2945 g of methanol with 411.36 g of pure water, 108 g of 29 mass % ammonia water, and 46.5 g of colloidal silica (silica concentration: 4 mass %, average secondary particle diameter: 8 nm, average longest diameter of primary particles: 8 nm or less) was added to a 5 L reaction vessel equipped with a stirrer and cooling function. The liquid temperature in the reaction vessel was kept at 20°C and stirred at 300 rpm. Liquid (B') prepared by dissolving 309 g of tetramethoxysilane (TMOS) in 79 g of methanol was added to produce a reaction liquid.

[0141] The addition rate of liquid (B') is 1.3 x 10 silicon atoms per 1 mol of water contained in liquid (A'). -4 mol / min.

[0142] The resulting reaction solution was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and distilled by heating to replace the methanol in the reaction solution with pure water, thereby obtaining a silica sol.

[0143] <Comparative Example 3> A liquid (A) prepared by mixing 1633 g of methanol, 167 g of pure water, and 180 g of 29 mass % ammonia water was added to a 5 L reaction vessel equipped with a stirrer and having a cooling function. The temperature of the liquid in the reaction vessel was kept at 55°C, and while stirring at 300 pm, a liquid (B1) prepared by dissolving 88 g of tetramethoxysilane (TMOS) in 22 g of methanol was added to prepare a reaction liquid, and the synthesis reaction was allowed to proceed.

[0144] The addition rate of liquid (B1) is 1.2 x 10 silicon atoms per 1 mol of water contained in liquid (A). -3 mol / min.

[0145] When the average longest diameter of the primary particles of the synthesized silica core particles reached 19.7 nm, the addition of liquid (B1) was stopped, and a first liquid was prepared.

[0146] The resulting first liquid was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and heated to distill the mixture, replacing the methanol in the reaction liquid with pure water to obtain a silica sol.

[0147] <Comparative Example 4> A liquid (A) prepared by mixing 1633 g of methanol, 167 g of pure water, and 180 g of 29 mass % ammonia water was added to a 5 L reaction vessel equipped with a stirrer and cooling function. The temperature of the liquid in the reaction vessel was kept at 55°C, and while stirring at 300 pm, a liquid (B1) prepared by dissolving 175 g of tetramethoxysilane (TMOS) in 45 g of methanol was added to prepare a reaction liquid, and the synthesis reaction was allowed to proceed.

[0148] The addition rate of liquid (B1) is 1.2 x 10 silicon atoms per 1 mol of water contained in liquid (A). -3 mol / min.

[0149] When the average longest diameter of the primary particles of the synthesized silica core particles reached 23.0 nm, the addition of liquid (B1) was stopped to prepare a first liquid.

[0150] The prepared first liquid was held in the same reaction vessel while being stirred at a rotation speed of 300 rpm. The liquid temperature at the start of the holding was 55°C, and then kept at room temperature. At the end of the holding, the temperature was 25°C. Stirring was stopped 264 hours after the addition of liquid (B1) was completed, and a silica particle-containing liquid was prepared.

[0151] The resulting silica particle-containing liquid was heated to a boiling point under normal pressure. While maintaining the liquid level constant during heating, pure water was added and distilled by heating to replace the methanol in the reaction liquid with pure water, thereby obtaining a silica sol.

[0152] In the methods for producing silica sol according to Examples 1 to 4 and Comparative Examples 1 to 4, the amounts of raw materials used and the conditions are summarized in Tables 1 and 2.

[0153] [Table 1]

[0154] [Table 2]

[0155] [Methods for measuring various physical properties] In the examples and comparative examples, various physical properties were measured by the following methods. The results are shown in Table 3.

[0156] <Scanning electron microscope (SEM) image> SEM images of the silica particles in the silica sols of Examples 1 to 4 and Comparative Examples 1 to 4 and the silica core particles in the reaction solutions of Examples 1 to 4 and Comparative Examples 3 to 4 were taken using a scanning electron microscope (SEM) SU8000 (manufactured by Hitachi High-Technologies Corporation) at a magnification such that 100 to 1000 particles were photographed.

[0157] <Average longest diameter of primary particles of silica core particles> In Examples 1 to 4 and Comparative Examples 3 and 4, the longest diameter of the primary particle of each silica core particle was measured for all silica core particles in the photographed SEM images, as exemplified by the double-headed arrow in FIG. 1, and the average value of the longest diameter of the primary particles of all silica core particles was calculated.

[0158] <Average primary particle diameter> The average primary particle diameter of the silica particles in the silica sols of Examples 1 to 4 and Comparative Examples 1 to 4 was determined based on the specific surface area (SA) of the silica particles measured by the BET method using a fully automatic specific surface area measuring device Macsorb (registered trademark) HM Model-1201 (manufactured by Mountec Co., Ltd.), with the true specific gravity of silica being 2.2 g / cm. 3 The primary particle diameter was calculated using the formula: primary particle diameter = 6000 / (SA × 2.2).

[0159] <Average secondary particle diameter> The average secondary particle diameter of the silica particles in the silica sols of Examples 1 to 4 and Comparative Examples 1 to 4 was measured as the volume average particle diameter by dynamic light scattering using a particle size distribution measuring device (UPA-UT151, manufactured by Nikkiso Co., Ltd.).

[0160] <Average degree of association> The average degree of association was calculated by dividing the average primary particle size by the average secondary particle size.

[0161] <Average aspect ratio> For the silica particles in the silica sols of Examples 1 to 4 and Comparative Examples 1 to 4, the long and short side values ​​of the smallest rectangle circumscribing each silica particle in the photographed SEM image were measured, and the ratio of the calculated long side value to the short side value (long side value / short side value) was taken as the aspect ratio, and the average aspect ratio of all silica particles in the photographed SEM image was calculated.

[0162] <Proportion of the number of silica particles with an aspect ratio of 1.50 or more to the total number of silica particles> For the silica particles in the silica sols of Examples 1 to 4 and Comparative Examples 1 to 4, the long and short sides of the smallest rectangle circumscribing each silica particle in the photographed SEM images were measured, and the ratio of the calculated long and short sides (long side value / short side value) was taken as the aspect ratio. The percentage (%) of the number of silica particles having an aspect ratio of 1.50 or more in the SEM image to the total number of silica particles in the SEM image (=number of silica particles having an aspect ratio of 1.50 or more in the SEM image / total number of silica particles in the SEM image × 100) was calculated.

[0163] <Average circularity> The circularity of each silica particle in the photographed SEM images was measured for the silica particles in the silica sols of Examples 1 to 4 and Comparative Examples 1 to 4, and the average circularity of all silica particles in the photographed SEM images was calculated. The circularity was calculated from the area (S) and perimeter (L) of the silica particle using the following formula: Circularity = 4πS / L 2 (S = area of ​​circle, L = perimeter).

[0164] <Proportion of the number of silica particles with a circularity of 0.90 or more to the total number of silica particles> The circularity of each silica particle in the photographed SEM images was measured for the silica particles in the silica sols of Examples 1 to 4 and Comparative Examples 1 to 4. The ratio (%) of the number of silica particles having a circularity of 0.90 or more in the SEM image to the total number of silica particles in the SEM image (= number of silica particles having a circularity of 0.90 or more in the SEM image / total number of silica particles in the SEM image × 100) was calculated.

[0165] [Table 3]

[0166] As shown in Table 3, it can be seen that the silica particles in the silica sols of Examples 1 to 4 have a high degree of association, a high average aspect ratio, and a low circularity compared to Comparative Examples 1 to 4. Furthermore, in the silica sols of Examples 1 to 4, the number of silica particles having an aspect ratio of 1.50 or more relative to the total number of silica particles is 40% or more, and the number of silica particles having a circularity of 0.90 or more calculated based on images observed with a scanning electron microscope is 40% or less, and therefore the silica sols of Examples 1 to 4 are found to contain a large amount of irregularly shaped silica particles.

Claims

1. a silica core particle preparation step of preparing a first liquid containing silica core particles having an average longest diameter of primary particles of 20 nm or less; and a bonding step of holding the first liquid for 72 hours or more to prepare a second liquid containing linked silica core particles.

2. 2. The method for producing a silica sol according to claim 1, wherein in the silica core particle preparation step, a liquid (B1) containing at least one of an alkoxysilane and a condensate thereof and a second organic solvent, or a liquid (C1) containing the liquid (B1) and water but not the alkali catalyst, is added to and mixed with a liquid (A) containing an alkali catalyst, water, and a first organic solvent, and the addition is terminated to prepare the first liquid when the average longest diameter of the primary particles of the silica core particles is 20 nm or less.

3. The method for producing a silica sol according to claim 2, wherein the alkali catalyst is ammonia.

4. 3. The method for producing a silica sol according to claim 2, wherein the first organic solvent and the second organic solvent are methanol.

5. 2. The method for producing a silica sol according to claim 1, further comprising a growing step of adding to and mixing with the second liquid, after the bonding step, a liquid (B2) containing at least one of an alkoxysilane and a condensate thereof and a third organic solvent, or a liquid (C2) containing the liquid (B2) and water but not containing an alkali catalyst, to prepare a third liquid containing the grown linked silica microparticles.

6. silica particles having an average aspect ratio of 1.50 or greater; A silica sol in which the ratio of the number of silica particles having an aspect ratio of 1.50 or more to the number of all silica particles is 40% or more.

7. 7. The silica sol according to claim 6, wherein the average circularity of the silica particles is 0.80 or less.

8. 7. The silica sol according to claim 6, wherein the ratio of the number of silica particles having a circularity of 0.90 or more to the number of all silica particles is 40% or less.

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