Method for producing quaternary ammonium-modified silica dispersion

Abstract

To provide a method for producing a quaternary ammonium-modified silica dispersion under an acidic condition suppressed in coagulation of silica in a surface modification process with a silane coupling agent.SOLUTION: A method for producing a quaternary ammonium-modified silica dispersion includes the following processes (1) and (2). Process (1): a process of adding an acidic compound b to silica a dispersed in water and / or an organic solvent to prepare a mixture. Process (2): a process of adding a quaternary ammonium group-containing silane coupling agent c to the mixture prepared in the process (1) to obtain a quaternary ammonium-modified silica dispersion.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a quaternary ammonium modified silica dispersion. [Background technology]

[0002] With the recent advances in semiconductor device performance, there is a need for technologies to build multilayer wiring on wafer surfaces and manufacture highly integrated VLSIs (Large Scale Integration). CMP (Chemical Mechanical Polishing) is one such technology, and is used to selectively polish and flatten thin films for multilayer wiring, such as oxide films, metal films, and ceramic films, formed on wafer surfaces.

[0003] During CMP, polishing compositions containing various additives such as polishing accelerators and pH adjusters are commonly used in addition to the main component, the abrasive. Silica is widely used as the abrasive, and surface modification treatments are carried out on silica to impart properties suited to various applications. One such surface modification treatment is silane coupling agent treatment. In silane coupling agent treatment, silanes or their hydrolysis condensates having various functional groups undergo a condensation reaction with silanol groups on the silica surface to form covalent bonds (siloxane bonds).

[0004] For example, Patent Document 1 describes a technique for producing a quaternary ammonium-modified silica dispersion by performing a surface modification treatment using a quaternary ammonium group-containing silane coupling agent or the like. However, the surface modification conditions were not carefully examined, and there was a problem that silica aggregated significantly during the modification process. There is a concern that this aggregated silica will deteriorate the polishing performance.

[0005] Patent Document 2 describes a technique for producing a quaternary ammonium-modified silica dispersion by performing a surface modification treatment with an epoxy group-containing silane coupling agent and simultaneously ring-opening the epoxy group with a tertiary amine. This method can suppress silica aggregation and halogen inclusion, but has the problem that the reaction cannot be completed unless the pH is adjusted with an acid after surface modification. In addition, the pH adjustment must be performed instantly, and there is a concern that the silica aggregation may occur due to poor mixing during scale-up. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5843613 [Patent Document 2] Patent No. 7284573 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a method for producing a quaternary ammonium-modified silica dispersion under acidic conditions, which suppresses silica aggregation during the surface modification process with a silane coupling agent. [Means for solving the problem]

[0008] [Concept 1] The method for producing a quaternary ammonium modified silica dispersion according to the present invention comprises the steps of: The method may include the following steps (1) and (2). Step (1): A step of adding an acidic compound b to silica a dispersed in water and / or an organic solvent to prepare a mixed liquid. Step (2): A step of adding a quaternary ammonium group-containing silane coupling agent c to the mixture prepared in step (1) to obtain a quaternary ammonium-modified silica dispersion.

[0009] [Concept 2] In the manufacturing method according to Concept 1, It is not necessary to adjust the pH with an acid after step (2).

[0010] [Concept 3] In the manufacturing method according to Concept 1 or 2, In step (1), the average secondary particle size of silica a may be 300 nm or less.

[0011] [Concept 4] 4. A method of manufacturing according to any one of concepts 1 to 3, In step (1), the pH of the resulting mixture may be less than 6.5.

[0012] [Concept 5] 5. A method of manufacturing according to any one of concepts 1 to 4, In the step (2), the amount of the quaternary ammonium group-containing silane coupling agent c added may be 0.1 to 100% by mass relative to the silica a.

[0013] [Concept 6] 6. A method of manufacturing according to any one of concepts 1 to 5, Furthermore, the following step (1') may be included before step (1) or step (2). Step (1'): A step of preparing a quaternary ammonium group-containing silane coupling agent c.

[0014] [Concept 7] 7. The method of manufacturing according to any one of concepts 1 to 6, The method may further include the following step (3). Step (3): A step of substituting the organic solvent and / or acidic compound b in the quaternary ammonium-modified silica dispersion obtained in step (2) with water.

[0015] [Concept 8] 8. A method of manufacturing according to any one of concepts 1 to 7, The silica concentration in the quaternary ammonium-modified silica dispersion may be 50% by mass or less.

[0016] [Concept 9] 9. A method of manufacturing according to any one of concepts 1 to 8, comprising: The increase in average secondary particle size due to surface modification may be 10% or less. [Effects of the Invention]

[0017] According to the present invention, a quaternary ammonium-modified silica dispersion is provided under acidic conditions with little silica aggregates during the surface modification process. It is also possible to adopt an embodiment in which pH adjustment with an acid is not performed, and in that case, productivity can be increased. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described.

[0019] The inventors of the present application have discovered that by treating a silica dispersion prepared with an acidic compound with a quaternary ammonium group-containing silane coupling agent, a quaternary ammonium-modified silica dispersion in which silica aggregation is suppressed can be obtained.

[0020] In this embodiment, there is provided a method for producing a quaternary ammonium-modified silica dispersion, which includes the following steps (1) and (2). Step (1): A step of adding an acidic compound b to silica a dispersed in water and / or an organic solvent to prepare a mixed liquid. Step (2): A step of adding a quaternary ammonium group-containing silane coupling agent c to the mixture prepared in step (1) to obtain a quaternary ammonium-modified silica dispersion.

[0021] The method for producing a quaternary ammonium-modified silica dispersion in this embodiment may further include the following step (3). Step (3): A step of substituting the organic solvent and / or acidic compound b in the quaternary ammonium-modified silica dispersion obtained in step (2) with water.

[0022] The method for producing a quaternary ammonium-modified silica dispersion in this embodiment may further include the following step (1') before step (1) or step (2). Step (1'): A step of preparing a quaternary ammonium group-containing silane coupling agent c.

[0023] The silica in the silica a dispersion liquid used in the present embodiment is not particularly limited, and can be synthesized by hydrolysis of alkoxysilane (sol-gel silica), ion exchange of sodium silicate (ion-exchange silica), flame hydrolysis of silicon tetrachloride (fumed silica), etc. Among these, sol-gel silica is preferred because it can produce high-purity silica in terms of suppressing metal impurities.

[0024] Surface modification treatments include treatment with a silane coupling agent (covalent bond) and treatment with a polymer (ionic bond, hydrogen bond, etc.). When a quaternary ammonium group is immobilized on the silica surface by a covalent bond, the strong covalent bond is advantageous in that the functional group is hardly detached even under strongly acidic or strongly basic conditions, and stable surface-modified silica can be obtained.

[0025] In consideration of the solubility of the silane coupling agent, the silica a dispersion may contain an organic solvent other than water. Examples of the organic solvent include alcohol. The organic solvent is preferably the same as the by-product alcohol in the hydrolysis of the silane coupling agent, more preferably methanol or ethanol, and even more preferably methanol.

[0026] The water concentration of the silica a dispersion is not particularly limited, but from the viewpoint of rapid hydrolysis of the silane coupling agent, it is preferably 5 to 99 mass %, more preferably 10 to 95 mass %.

[0027] The silica concentration of the silica a dispersion is not particularly limited, but is preferably 1 to 60 mass %, more preferably 5 to 50 mass %. When the lower limit of the silica concentration is within the above range, it is advantageous in that productivity can be increased.

[0028] The average secondary particle size of silica a is not particularly limited, but from the viewpoint of enabling a high polishing rate, it is preferably from 1 to 500 nm, more preferably from 2 to 400 nm, and even more preferably from 5 to 300 nm.

[0029] The acidic compound b is not particularly limited, and examples thereof include formic acid, acetic acid, phosphoric acid, hydrochloric acid, sulfuric acid, and nitric acid. Among these, the inventors have conducted extensive studies and found that formic acid, acetic acid, and nitric acid are preferred, and formic acid and nitric acid are more preferred.

[0030] The amount of acidic compound b added varies depending on the specific surface area and silanol content of silica a, the silica concentration in the silica a dispersion, the acidity of acidic compound b, and other factors, making it difficult to define a specific amount. Since the reaction is significantly affected by pH, the silica dispersion after the addition of acidic compound b in step (1) preferably has a pH of less than 6.5, more preferably 6.4 or less. If the upper limit of the pH is within the above range at the start of the reaction, modification is carried out quickly, which is advantageous in that silica aggregation can be suppressed.

[0031] The quaternary ammonium group-containing silane coupling agent c may be used as a stock solution, or may be arbitrarily diluted with water or an organic solvent in consideration of solubility.The organic solvent may be, for example, alcohol, etc., and as mentioned above, the same as the by-product alcohol in the hydrolysis of the silane coupling agent is preferable, more preferably methanol or ethanol, and even more preferably methanol.

[0032] The concentration of the quaternary ammonium group-containing silane coupling agent c is not particularly limited, but is preferably 5 to 100 mass %, more preferably 5 to 60 mass %. From the viewpoint of excellent solubility, it is beneficial to set the upper limit at this value.

[0033] The number of quaternary ammonium groups in the quaternary ammonium group-containing silane coupling agent c is not particularly limited, but from the empirical rule that the effect of the functional group is enhanced, it is preferably 1 to 4. The position of the quaternary ammonium group is also not particularly limited, but it is preferably present at a terminal where steric hindrance is small.

[0034] The quaternary ammonium group-containing silane coupling agent c may be a commercially available product, or may be prepared in advance.

[0035] The quaternary ammonium group-containing silane coupling agent c may be used as a hydrolyzable silane or as a hydrolysis condensate thereof.

[0036] A commercially available quaternary ammonium group-containing silane coupling agent c is represented by the general formula (i). R 2 3-n Si(OR 1 ) n L-NR 3 3 + X - (i) R 1 represents an alkyl group having 1 to 6 carbon atoms, a hydrogen atom, or a silicon atom n is 1, 2 or 3 R 2 represents an alkyl group having 1 to 6 carbon atoms or a hydrogen atom L is the spacer R 3 is an alkyl group having 1 to 18 carbon atoms X - is an anion

[0037] R in general formula (i) 1 When R is an alkyl group, it preferably has 1 to 6 carbon atoms, more preferably 1 to 4 carbon atoms, and even more preferably 1 to 2 carbon atoms. Examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, and a phenyl group. A methyl group, an ethyl group, a propyl group, an isopropyl group, and a butyl group are more preferred, and a methyl group and an ethyl group are even more preferred. 1 When is a hydrogen atom or a silicon atom, it represents a hydrolysis condensation product. In general formula (i), n is preferably 2 or 3, and more preferably 3. R in general formula (i) 2 When the alkyl group is an alkyl group, it preferably has 1 to 6 carbon atoms, more preferably 1 to 4 carbon atoms, and even more preferably 1 or 2 carbon atoms.

[0038] L in general formula (i) is a spacer, which is a group that connects the silicon atom and the quaternary ammonium group. For example, it is an alkylene group, and the main chain preferably has 1 to 18 carbon atoms, more preferably 1 to 8 carbon atoms. Examples include a methylene group, an ethylene group, a propylene group, a butylene group, an octylene group, and an octadecylene group, and more preferably a methylene group, an ethylene group, a propylene group, a butylene group, and an octylene group. Furthermore, when there is a side chain, the side chain preferably has 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms. The alkylene group may have a substituent. Examples of the substituent include a hydroxyl group, an amino group, a quaternary ammonium group, and an alkyl group having 1 to 4 carbon atoms. The substituent may be one type or a combination of two or more types. The number of substituents in the main chain and the side chain is preferably 0 to 5, and more preferably 0 to 4. The alkylene group may have a heteroatom inside or at a terminal. For example, an oxygen atom, a nitrogen atom, a sulfur atom, etc. One type of heteroatom may be used alone or in combination with multiple types. The number of heteroatoms in the main chain and the side chain is preferably 0 to 4, and more preferably 0 to 3.

[0039] R in general formula (i) 3 represents an alkyl group, preferably having 1 to 18 carbon atoms, more preferably having 1 to 4 carbon atoms, and even more preferably having 1 or 2 carbon atoms.

[0040] X in general formula (i) - is an anion, and examples thereof include chloride ion, bromide ion, iodide ion, hydroxide ion, carbonate ion, hydrogen carbonate ion, acetate ion, and nitrate ion.

[0041] On the other hand, commercially available hydrolyzable silanes include trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride, trimethyl[3-(triethoxysilyl)propyl]ammonium chloride, dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride, and POLON-V8 manufactured by Shin-Etsu Chemical Co., Ltd. Commercially available hydrolyzed condensates include VOC (volatile organic compound)-free aqueous silane coupling agents X-12-1126, X-12-1139, and X-12-1354-1 manufactured by Shin-Etsu Chemical Co., Ltd.

[0042] On the other hand, there are many methods for preparing quaternary ammonium group-containing silane coupling agents (c), such as alkylation of amino group-containing silane coupling agents, reaction of leaving group- or epoxy group-containing compounds with tertiary amines (either one or both are silane coupling agents), and bonding of quaternary ammonium group-containing compounds to silane coupling agents. Furthermore, the prepared hydrolyzable silane may be hydrolyzed, and the organic solvent may be removed. These preparation methods can lead to desired structures not found in commercially available products, and can also suppress the inclusion of metals and halogens.

[0043] The amount of the quaternary ammonium group-containing silane coupling agent c added is difficult to define uniquely because the range of the amount added varies depending on the specific surface area and silanol content of silica a, the molecular weight of silane coupling agent c, etc. However, from the viewpoint of sufficiently modifying silica a and obtaining a stable quaternary ammonium-modified silica dispersion, the amount is preferably 0.1 to 100 mass % relative to silica a, more preferably 0.5 to 75 mass %, and even more preferably 1 to 50 mass %.

[0044] After the addition of the quaternary ammonium group-containing silane coupling agent c, it is preferable to carry out heat aging to complete the reaction. The aging temperature is not particularly limited, and is preferably from 5°C to the boiling point of the silica a dispersion, and more preferably from 10°C to 70°C. However, to avoid silica aggregation due to a sudden temperature change, it is more preferable to start aging at the same temperature as the addition temperature. The aging time is also not particularly limited, but from the viewpoint of completing the reaction and obtaining a stable quaternary ammonium-modified silica dispersion, the upper limit of the aging time is preferably 24 hours or less, and in some cases may be 10 hours or less.

[0045] The pH of the quaternary ammonium-modified silica dispersion after aging in step (2) is preferably 8.0 or less, more preferably 7.5 or less. When the upper limit of the pH after aging is within the above range, the modification to silica a proceeds sufficiently, and a stable quaternary ammonium-modified silica dispersion is obtained.

[0046] The water substitution method in step (3) can be exemplified by a method in which the quaternary ammonium-modified silica dispersion is heated under normal pressure or reduced pressure and water is added in an amount equal to the fraction, a method in which the dispersion is concentrated using an ultrafiltration membrane and water is added in an amount equal to the waste liquid, etc. Among these, water substitution under normal pressure or reduced pressure is preferred from the viewpoint of rapid removal of the organic solvent and / or acidic compound b.

[0047] The quaternary ammonium-modified silica dispersion may be diluted or concentrated as necessary during or after water replacement. The silica concentration of the quaternary ammonium-modified silica dispersion is not particularly limited, but is preferably 1 to 60% by mass, more preferably 1 to 50% by mass. The upper limit of the silica concentration within the above range is advantageous in that silica aggregation can be suppressed.

[0048] From the viewpoint of suppressing scratch defects during polishing, the increase rate of the average secondary particle diameter compared to the silica before modification is preferably 10% or less, more preferably 6% or less, and even more preferably 4% or less.

[0049] The quaternary ammonium group-containing silane coupling agent c or the quaternary ammonium-modified silica dispersion may be subjected to ion exchange using an anion exchange resin. The ion exchange may be performed once or, if necessary, twice or more times. [Example]

[0050] [Silica concentration] The silica dispersion was precisely weighed into a ceramic crucible and dried at 105°C for 10 hours, and the residue was precisely weighed to calculate the silica concentration.

[0051] [Average secondary particle size] The silica dispersion was diluted with water to a silica concentration of 0.2% by mass, and a tetramethylammonium tartrate aqueous solution was added to homogenize the dispersion to prepare a measurement sample. The average secondary particle diameter (nm) of the measurement sample was measured by dynamic light scattering (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.).

[0052] [Average secondary particle size increase rate] The increase rate of the average secondary particle size before and after surface modification was calculated using the following formula. ((average secondary particle size after surface modification / average secondary particle size before surface modification)-1)*100

[0053] [pH] The pH was measured using a pH meter (Twin pH Meter II, manufactured by Horiba Ltd.).

[0054] [Zeta potential] The silica dispersion was diluted with water to a silica concentration of 1.0% by mass to prepare a measurement sample. The zeta potential (mV) of the measurement sample was measured by electrophoretic light scattering (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.).

[0055] [pH at isoelectric point] The pH at the isoelectric point (zeta potential 0 mV) was estimated from the pH at which the zeta potential was approximately ±10 mV.

[0056] [Silica a dispersion] The following silica a dispersion was used. 25nm silica dispersion water (silica concentration 10-20% by mass, average secondary particle diameter 49.1nm) Ludox CL-X silica dispersion (silica concentration 45% by mass, average secondary particle size 35.0 nm)

[0057] [Acidic compound b] The acidic compound b used is represented by a rational formula. HCOOH: Formic acid (approximately 88% by mass aqueous solution)

[0058] [Silane coupling agent c] Silane coupling agent c is expressed by product name or molecular formula. C9H 24 ClNO3Si: Trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride (48-53% by mass solution in methanol) X-12-1126 (VOC-free water-based silane coupling agent, approximately 10% by weight aqueous solution, manufactured by Shin-Etsu Chemical Co., Ltd.) C9H 20 O5Si: 3-glycidyloxypropyltrimethoxysilane

[0059] [Example 1] Process (1): 200 g of 25 nm silica dispersion water (silica concentration 10 mass %, average secondary particle diameter 49.1 nm) was placed in a flask, and 36.8 mg of formic acid (approximately 88 mass % aqueous solution) was added and mixed. The pH of the resulting mixture was 5.9.

[0060] Process (2): To the mixture obtained in step (1), 10.0 g of trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride (48-53 mass% methanol solution) was added over 18 minutes at 25°C. After the entire amount was added, stirring was maintained at 25°C for 30 minutes. Subsequently, the mixture was heated to 60°C and then stirred for an additional 180 minutes to obtain a quaternary ammonium-modified silica dispersion with a pH of 4.9.

[0061] Process (3): The quaternary ammonium-modified silica dispersion obtained in step (2) was heated to distill off the methanol, while adding water dropwise to keep the weight of the silica dispersion constant, to obtain a quaternary ammonium-modified silica dispersion with a pH of 4.8.

[0062] [Example 2] Process (1): 200 g of 25 nm silica dispersion water (silica concentration 10 mass %, average secondary particle diameter 49.1 nm) was placed in a flask, and 42.3 mg of formic acid (approximately 88 mass % aqueous solution) was added and mixed. The resulting mixture had a pH of 4.9.

[0063] Process (2): To the mixture obtained in step (1), 1.0 g of trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride (48-53 mass% methanol solution) was added over 2 minutes at 25°C. After the entire amount was added, stirring was maintained at 25°C for 30 minutes. Subsequently, the mixture was heated to 60°C and then stirred for an additional 180 minutes to obtain a quaternary ammonium-modified silica dispersion with a pH of 4.3.

[0064] Process (3): The quaternary ammonium-modified silica dispersion obtained in step (2) was heated to distill off the methanol, while adding water dropwise to keep the weight of the silica dispersion constant, to obtain a quaternary ammonium-modified silica dispersion with a pH of 4.4.

[0065] [Example 3] Process (1): 200 g of 25 nm silica dispersion water (silica concentration 10 mass %, average secondary particle diameter 49.1 nm) was placed in a flask, and 111.4 mg of formic acid (approximately 88 mass % aqueous solution) was added and mixed. The resulting mixture had a pH of 3.5.

[0066] Process (2): To the mixture obtained in step (1), 80.0 g of X-12-1126 (approximately 10% by mass aqueous solution) was added over 80 minutes at 25°C. After the entire amount was added, stirring was maintained at 25°C for 30 minutes. Subsequently, the mixture was heated to 60°C and then stirred for an additional 180 minutes to obtain a quaternary ammonium-modified silica dispersion with a pH of 7.1.

[0067] Process (3): The quaternary ammonium-modified silica dispersion obtained in step (2) was heated to distill off the trace amount of remaining methanol, while adding water dropwise to keep the weight of the silica dispersion constant, to obtain a quaternary ammonium-modified silica dispersion with a pH of 7.0.

[0068] [Example 4] Process (1): 200 g of 25 nm silica dispersion water (silica concentration 10 mass %, average secondary particle diameter 49.1 nm) was placed in a flask, and 83.5 mg of formic acid (approximately 88 mass % aqueous solution) was added and mixed. The pH of the resulting mixture was 3.9.

[0069] Process (2): To the mixture obtained in step (1), 20.0 g of X-12-1126 (approximately 10% by mass aqueous solution) was added over 20 minutes at 25°C. After the entire amount was added, stirring was maintained at 25°C for 30 minutes. Subsequently, the mixture was heated to 60°C and then stirred for an additional 180 minutes to obtain a quaternary ammonium-modified silica dispersion with a pH of 5.9.

[0070] Process (3): The quaternary ammonium-modified silica dispersion obtained in step (2) was heated to distill off the trace amount of remaining methanol, while adding water dropwise to keep the weight of the silica dispersion constant, to obtain a quaternary ammonium-modified silica dispersion with a pH of 5.9.

[0071] [Comparative Example 1] A flask was charged with 89.0 g of Ludox CL-X silica dispersion (silica concentration 45% by mass, average secondary particle size 35.0 nm), 28.4 g of water, and 71.0 g of methanol. The pH of the resulting mixture was 9.9. 12.4 g of trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride (48-53% by mass methanol solution) was added to the mixture, and the mixture was heated at 68°C for 24 hours. The pH of the resulting quaternary ammonium-modified silica dispersion was 8.3, but a large amount of gel was generated.

[0072] Comparative Example 2 A flask was charged with 900 g of a 25 nm silica dispersion (silica concentration 20% by mass, average secondary particle diameter 49.1 nm), and 16.0 g of triethylamine was added. The pH of the resulting dispersion was 11.6. This dispersion was heated to an internal temperature of 55°C, and then a mixture of 12.5 g of 3-glycidyloxypropyltrimethoxysilane and 49.9 g of methanol was added over 60 minutes while adjusting the temperature to prevent fluctuations in the internal temperature. After the entire amount was added, the mixture was maintained at 55°C for 30 minutes. 345 g of the resulting dispersion with a pH of 11.5 was placed in another flask, and 3.1 g of acetic acid was added at 20°C. The pH of the resulting mixture was 6.0. To distill off the methanol from this mixture, the mixture was heated under normal pressure, and water was added dropwise while maintaining the volume constant, replacing the methanol with water, yielding a quaternary ammonium-modified silica dispersion with a pH of 5.7.

[0073] The results of the examples and comparative examples are shown in Table 1 below. [Table 1] * Expressed as the theoretical amount of quaternary ammonium group-containing silane coupling agent produced by the reaction of triethylamine with 3-glycidyloxypropyltrimethoxysilane.

[0074] In the silane coupling agent c / silica a ratio in Table 1 above, the value shown is the weight of the active ingredient of silane coupling agent c divided by the weight of silica a in the silica a dispersion (note that calculations are based on trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride being 50% by mass and X-12-1126 being 10% by mass). For example, in Example 1, silane coupling agent c / silica a 10.0g*0.5 / (200g*0.1)*100=25.

[0075] Similarly, in Comparative Example 1, calculations are made assuming that trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride is 50% by mass.

[0076] In addition, in Comparative Example 2, silane coupling agent c was prepared, but because an excess amount of triethylamine was added, the theoretical amount of silane coupling agent c produced was calculated based on 3-glycidyloxypropyltrimethoxysilane. For example, the amount of substance of 3-glycidyloxypropyltrimethoxysilane is 12.5g / 236.34g / mol=0.0529mol, and the theoretical amount of the resulting silane coupling agent c is 0.0529mol*337.53g / mol=17.86g. Therefore, the ratio of silane coupling agent c / silica a is 17.86g / (900g*0.2)*100=10.

[0077] In the examples, the addition of acidic compound b suppressed silica aggregation during the surface modification process, and a quaternary ammonium-modified silica dispersion was obtained even under acidic conditions. This method does not require pH adjustment with acid, and is therefore highly productive. Furthermore, the pH at the isoelectric point could be freely controlled by adjusting the amount of modification. After producing a quaternary ammonium-modified silica dispersion under acidic conditions, it is also possible to avoid pH adjustments using not only acid but also other methods. This approach allows for higher productivity.

[0078] On the other hand, in the production method of Comparative Example 1, since the acidic compound b was not added, a large amount of gel was generated.

[0079] In the manufacturing method of Comparative Example 2, the aggregation of silica is largely suppressed by adding a basic compound, but pH adjustment with an acid is required after surface modification, resulting in a quaternary ammonium-modified silica dispersion with low productivity.

Claims

1. A method for producing a quaternary ammonium-modified silica dispersion, comprising the following steps (1) and (2): Step (1): A step of adding an acidic compound b to silica a dispersed in water and / or an organic solvent to prepare a mixed liquid. Step (2): A step of adding a quaternary ammonium group-containing silane coupling agent c to the mixed liquid prepared in step (1) to obtain a quaternary ammonium-modified silica dispersion.

2. 2. The method according to claim 1, wherein the pH is not adjusted with an acid after step (2).

3. 3. The method according to claim 1, wherein in step (1), the silica a has an average secondary particle size of 300 nm or less.

4. 3. The method according to claim 1, wherein the pH of the mixture obtained in step (1) is less than 6.

5.

5. 3. The method according to claim 1, wherein in step (2), the amount of the quaternary ammonium group-containing silane coupling agent c added is 0.1 to 100% by mass relative to the silica a.

6. The method according to claim 1 or 2, further comprising the following step (1') before step (1) or step (2): Step (1'): A step of preparing a quaternary ammonium group-containing silane coupling agent c.

7. The method according to claim 1 or 2, further comprising the following step (3): Step (3): A step of substituting the organic solvent and / or acidic compound b in the quaternary ammonium-modified silica dispersion obtained in step (2) with water.

8. The method according to claim 1 or 2, wherein the silica concentration in the quaternary ammonium-modified silica dispersion is 50% by mass or less.

9. 3. The method according to claim 1, wherein the increase in average secondary particle size due to the surface modification is 10% or less.

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