Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/113—Silicon oxides; Hydrates thereof
  • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
  • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK  ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
  • C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
  • C09C1/28—Compounds of silicon
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK  ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
  • C09C3/00—Treatment in general of inorganic materials, other than fibrous fillers, to enhance their pigmenting or filling properties
  • C09C3/12—Treatment with organosilicon compounds
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2006/00—Physical properties of inorganic compounds
  • C01P2006/90—Other properties not specified above

Definitions

• the present invention relates to hydrophobic silica and a method for producing the same. More specifically, the present invention relates to hydrophobic silica in which release of silicone oil is suppressed and a method for producing the same.
• This application claims the priority of Japanese Patent Application No. 2014-250355 filed on Dec. 10, 2014, the entire description of which is specifically incorporated herein by reference.
• hydrophobic silica hydrophobic wet-process synthetic silica whose surface is hydrophobized with silicone oil is widely used as an antifoaming agent, paint matting agent, film antiblocking agent and the like. ing.
• hydrophobic silica When hydrophobic silica is used in paints, it can improve the water repellency of the coating surface by adding the conventional matte performance, and can provide antifouling, alkali and chemical resistance functions. It becomes possible to improve the scratch resistance of the coating surface. Due to these characteristics, recently, the use of hydrophobic silica for matting agents for plastic coatings for coating on the surface of home appliances and electronic devices is increasing.
• the average particle diameter of hydrophobic silica fine particles it is used for the purpose of imparting antiglare property in addition to the above functions in applications for transparent films and glass surface coating agents (for example, Patent Document 1).
• Hydrophobic silica having a hydrophobic function can be produced simply by mixing raw material synthetic silica and silicone oil, but various methods are being studied for the purpose of improving hydrophobic performance.
• Patent Document 2 a silicone oil having a viscosity of 50 centistokes (unit: cSt) is sprayed on wet synthetic silica and heated at a temperature of 250 to 350 ° C. for about 1/2 to 2 hours to make it hydrophobic. Silica has been obtained.
• Patent Document 3 describes a method in which an alkali metal hydroxide (sodium hydroxide) is added to wet-method synthetic silica and used as a catalyst.
• Patent Document 4 proposes a method of performing heat treatment at a relatively low temperature of 60 to 150 ° C. using ammonia or amine as a catalyst.
• Patent Document 5 proposes a method of hydrophobizing at a low temperature of about room temperature to about 100 ° C. using silica containing 0.5 part by weight or more of aluminum in terms of Al 2 O 3 with respect to 100 parts by weight of SiO 2 .
• the above method for imparting and improving hydrophobic performance to hydrophilic wet-process silica is realized by covering hydrophilic silanol groups (-SiOH) present on the surface of wet-process silica with hydrophobic silicone oil. is doing.
• DBA di-n-butylamine
• M value a value for knowing the degree of hydrophobicity of the surface of hydrophobic silica.
• DBA adsorption amount unit: m ⁇ mol / kg
• Normal silica sica before hydrophobization
• DBA is ion-bonded to the silanol groups on the surface of the silica particles.
• silanol groups present on the silica surface are coated with silicone oil or reacted to disappear.
• silanol group remaining on the surface of the hydrophobic silica it binds to DBA, and the amount of the remaining silanol group can be determined from the amount bound. Therefore, it is used as a means for knowing the degree of hydrophobicity of silica.
• the M value is a concentration expressed in volume% of methanol at which the treated powder begins to wet in a mixed solution of water and methanol.
• Higher M value means higher degree of hydrophobicity (maximum value is about 70%).
• DBA values and M values both represent the hydrophobic state of hydrophobic silica, but the values do not show a unique correlation.
• hydrophobic silica obtained by mixing hydrophilic silica with a large amount of silicone oil or hydrophobic silica obtained by hydrophobizing only the surface of the aggregate has a large amount of silanol groups and a high DBA adsorption amount.
• the M value is high. Therefore, in order to properly grasp the hydrophobic state of hydrophobic silica, it is preferable to monitor both the DBA value and the M value.
• the wet method synthetic silica used for the raw material is synthesized by neutralization reaction of sodium silicate and mineral acid, and roughly divided, precipitation method synthetic silica synthesized in the alkali region and gel method synthetic silica synthesized in the acidic region. There is.
• Patent Document 1 Japanese Patent No. 3504338
• Patent Document 2 Japanese Patent Publication No. 42-26179
• Patent Document 3 Japanese Patent Publication No. 47-12770
• Patent Document 4 Japanese Patent Publication No. 57-2641
• Patent Document 5 Japanese Patent Application Laid-Open No. 6-316408
• Patent Document 6 Japanese Patent Application Laid-Open No. 8-176462 Patent Documents 1 to 6 are all incorporated herein by reference.
• Hydrophobic silica is in a situation where there are many cases where it is difficult to cope with the above-mentioned conventional technology due to diversification of user needs and higher functionality.
• hydrophobic silica when used as a matting agent in paint applications, it becomes possible to impart antifouling function, alkali resistance function, chemical resistance function, and scratch resistance as described above.
• antifouling function alkali resistance function
• chemical resistance function chemical resistance function
• scratch resistance when a coating film is formed and cured as a paint, a part of unreacted oil is eluted on the coating film surface, and so-called color unevenness occurs.
• hydrophobic silica when used as an anti-blocking agent for films, the use of hydrophobic silica has the advantage of improving transparency, but due to the bleed of silicone oil, parts of the film having a different haze (cloudiness) are generated. It is also a cause of problems such as loss of overall quality.
• the hydrophobic silica that caused the above problem has a high M value and a sufficiently low DBA adsorption amount.
• the M value and the DBA adsorption amount are used as indices, the degree of hydrophobicity is high.
• the actual condition is that the amount of unreacted silicone oil that affects the performance of hydrophobic silica cannot be grasped only by M value and DBA adsorption amount. Further, if it is intended to reduce the amount of unreacted silicone oil, a long-time heat treatment is required, which is disadvantageous for commercial production.
• silica is alkaline in a method in which alkali metal hydroxide, ammonia, or amines are added to wet synthetic silicic acid as described in Patent Documents 3 and 4 and used as a catalyst. Therefore, wet synthetic silica which is weak against alkali has a problem that it cannot exhibit stable performance due to changes in surface properties such as a decrease in specific surface area over time.
• Patent Document 5 has an advantage that surface treatment can be performed at a low temperature.
• the purpose is to improve the M value and the DBA value, and the hydrophobic silica treated by such a method tends to increase the amount of silicone oil eluted.
• elution of silicone oil can be reduced by heat treatment for 6 hours or more at a high temperature of 350 to 400 ° C in order to complete the chemical bond between silicone oil and silanol group. Turned out.
• a part of the silicone oil itself is decomposed and deteriorated by the heat of treatment, so that the desired performance cannot be exhibited.
• the high temperature and long time treatment is not practical because it only causes a significant deterioration in production efficiency and an increase in cost.
• silica particles of raw powder have a high BET specific surface area, have pores, and have a particle size of several microns, so that silicone oil with viscosity is uniformly treated at the molecular level.
• the present inventors after the hydrophobization treatment, release and elute silanol groups on the silica surface and unreacted silicone while maintaining the hydrophobization degree represented by the M value and DBA adsorption amount at the target values.
• hydrophobic silica capable of reducing oil
• intensive studies were conducted.
• the present invention is as follows.
• Hydrophobic silica obtained by surface treatment of wet synthetic silica with silicone oil The hydrophobic silica is (i) The elution amount of silicone oil in toluene is less than 0.2% with respect to the mass of hydrophobic silica, provided that the elution amount is dispersed in toluene at a concentration of 2%, and after 24 hours at 20 ° C.
• the hydrophobic silica which has a silicone oil elution rate compared with that before dispersion.
• [5] A process for producing hydrophobic silica obtained by surface treatment of a wet process synthetic silica with silicone oil, Silicone oil is coated on the surface of the wet process synthetic silica in which a strong acid strong base type neutral salt compound is present, and then subjected to the conditions under which the coated silicone oil and the silanol group of the wet process synthetic silica form a siloxane bond. Obtaining said hydrophobic silica.
• the strong acid strong base type neutral salt compound is sodium sulfate, sodium chloride, potassium sulfate, potassium chloride, or a mixture thereof.
• the hydrophobic silica has an elution amount of silicone oil in toluene of less than 0.2% based on the mass of the hydrophobic silica, provided that the elution amount is dispersed in toluene at a concentration of 2% and is heated at 20 ° C. for 24 hours.
• the hydrophobic silica is (i) M value is 20% or more and / or (ii) The production method according to any one of [5] to [7], wherein the DBA adsorption amount is less than 100 m ⁇ mol / kg. [10] The method according to any one of [5] to [9], wherein the silicone oil is a silicone oil having a kinematic viscosity of 500 centistokes or less.
• a hydrophobic surface treated with a silicone oil can be obtained in which a stable coating surface can be obtained without the silicone oil eluting to the surface even after being added to the coating material and applied to form a coating film.
• Silica can be provided.
• the hydrophobic silica of the present invention can be favorably used for metal paints, matting agents for plastic paints, coating on antiglare films, resin fillers, film antiblocking agents, and the like.
• the hydrophobic silica of the present invention is a hydrophobic silica obtained by surface-treating wet-process synthetic silica with silicone oil, and satisfies the following (i).
• the amount of silicone oil eluted in toluene is less than 0.2% with respect to the mass of hydrophobic silica.
• the elution amount is a silicone oil elution rate as compared with that before dispersion after dispersion for 24 hours at 20 ° C. in toluene.
• the hydrophobic silica of the present invention is dispersed in a toluene solution at a concentration of 2%, and the amount of silicone oil eluted in the toluene solution when 24 hours have passed at room temperature is higher than that in the state before dispersion. It is less than 0.2%. If it is 0.2% or more, part of the unreacted oil will elute on the surface of the paint film when it is formed and cured as a paint, causing color unevenness, or coating the film and then winding the film. In addition, the silicone oil released during winding is transferred to the back of the film and contaminated, that is, the deterioration of oil bleed characteristics and the various problems described above are caused. In order to further solve the problem, the elution amount of the silicone oil is preferably less than 0.15%, more preferably 0.10% or less, and even more preferably 0.08% or less.
• silicone oil eluted when resin is blended when dispersed in paint, etc. it is difficult to analyze directly, but hydrophobic silica is dispersed in toluene solvent.
• the amount of unreacted silicone oil can be measured easily and accurately by selecting a method for determining the amount of silicone oil eluted in
• the hydrophobic silica of the present invention is a hydrophobic silica satisfying the above (i), and further preferably satisfies the following (ii) and / or (iii).
• M value is 20% or more.
• the DBA adsorption amount is less than 100 m ⁇ mol / kg.
• the surface treatment state of the hydrophobic silica can be expressed by using the DBA adsorption amount and the M value.
• the hydrophobic silica of the present invention has an M value of 20% or more and a DBA adsorption amount of From the viewpoint of satisfying the physical properties of conventional hydrophobic silica, it is preferably less than 100 m ⁇ mol / kg.
• M value expressed as the volume% of methanol at which the treated powder begins to wet in the mixed solution of water and methanol, is less than 20%
• the degree of hydrophobicity of the silica itself is generally low (hydrophilic groups remain) This is preferably 20% or more.
• the M value of the hydrophobic silica of the present invention is more preferably 40% or more, and more preferably 55% or more.
• ⁇ DBA adsorption amount which represents the amount of silanol groups (hydrophilic groups) remaining on the surface of the raw material hydrophilic silica, is 100 m ⁇ mol / kg or more, in general, many hydrophilic groups remain on the hydrophobic silica. It will be a thing. (Almost occurs when heat treatment described below is insufficient, and also leads to elution of silicone oil.) If the DBA adsorption amount is less than 100 m ⁇ mol / kg, the hydrophilic groups are dispersed when dispersed in a solvent-based paint.
• the DBA adsorption amount of the hydrophobic silica of the present invention is more preferably less than 60 m ⁇ mol / kg, and further preferably less than 20 m ⁇ mol / kg.
• the type and physical properties of the wet method synthetic silica used in the present invention are not particularly limited. Either precipitated silica or gel silica can be used without problems.
• the BET specific surface area of precipitated silica is 20 to 300 m 2 / g, and the BET specific surface area of gel silica is 250 to 700 m 2 / g.
• the BET specific surface area is 100 m 2 / g.
• the throughput of silicone oil per g should be 3-9 parts.
• the treatment amount of silicone oil is preferably 9 to 27 parts.
• the surface does not necessarily have to be completely hydrophobized, and the treatment amount of the silicone oil is 3 to 9 parts per 100 m 2 / g of BET specific surface area. If it is within the range, a necessary processing amount can be selected according to the purpose of use. In general, the smaller the treatment amount, the more incomplete hydrophobized state on the silica surface, and vice versa.
• the amount of the silicone oil treated is 3 parts or more per 100 m 2 / g of BET specific surface area, the hydrophobization degree itself is lowered, so that the effect as hydrophobic silica can be sufficiently obtained.
• the treatment amount is 9 parts or less, there is no excess silicone oil relative to the silica surface, and there are sufficient silanol groups on the wet-process synthetic silica surface to bind to the silicone oil molecules. Further, the remaining amount of unreacted silicone oil and thus the amount of elution is suppressed, and the achievement of the object of the present invention is promoted.
• a range of 4 to 8 parts is preferable for a BET specific surface area of 100 m 2 / g, and within this range, the amount of treatment can be changed according to the application and purpose.
• hydrophobic silica is averaged by a laser method in the same manner as untreated wet synthetic silica.
• the particle size can be used properly according to the purpose within the range of 1-15 ⁇ m.
• the average particle size is generally adjusted by pulverization / classification, but the adjustment can be performed both at the raw material stage and after the hydrophobization treatment, and can also be adjusted in combination.
• ⁇ Silica with the desired particle size can be manufactured with higher accuracy by adjusting to some extent at the raw material stage and fine-tuning after the hydrophobization treatment to adjust the particle size.
• the type of the silicone oil used in the present invention is not limited as long as it can be mixed with the wet method synthetic silica. It is common to use commercially available dimethyl silicone oil (commonly known as straight silicone oil) with only methyl and phenyl groups, but other modified silicone oils with organic substituents on silicon atoms are also used. I can do it. As examples of substituents, many modified types of silicone oil are commercially available, including polyethers, epoxies, amines, and carboxyl groups. Examples of the modified type silicone oil include the following products.
• silicone oil is mixed with wet-process synthetic silica having a low bulk specific gravity, when silicone oil having a high molecular weight and high viscosity is used, it is necessary to devise such as diluting with a solvent or the like. Therefore, generally, silicone oil having a relatively low viscosity of 500 centistokes (500 cSt) or less is preferably used. Examples of the silicone oil having a kinematic viscosity of 500 centistokes or less include the following products.
• the present invention includes a method for producing hydrophobic silica obtained by surface treatment of wet-process synthetic silica with silicone oil.
• the surface of wet synthetic silica in which a strong acid strong base type neutral salt compound is present is coated with silicone oil, and then the coated silicone oil and the silanol group of wet synthetic silica form a siloxane bond. Subjecting to conditions to obtain hydrophobic silica.
• a strong acid strong base type neutral salt compound for example, a sodium sulfate (Na 2 SO 4 ) component, which is a substance serving as a catalyst for the wet-process synthetic silica which is the raw powder.
• the strong acid strong base type neutral salt compound may include, for example, sodium chloride, potassium sulfate, potassium chloride and mixtures thereof in addition to sodium sulfate.
• the amount of the strong acid strong base type neutral salt compound on the surface of the wet process synthetic silica is suitably in the range of 0.3 to 3.0%, for example.
• the pretreatment method is not particularly limited, but when the silica synthesis reaction is completed, the reaction by-product sodium sulfate (Na 2 SO 4 ) is washed with water until a predetermined amount is obtained, and then pH adjustment is performed before use.
• the Na component and S component contained in the wet method synthetic silica which is the raw powder, are approximately in the target range of 0.3 to 1.4 wt% in terms of Na 2 O and 0.36 to 1.44 wt% in terms of SO 3 , respectively. adjust. These values indicate the amount of impurities in the raw powder, and tend to be slightly different from the amount of impurities contained in the final product, hydrophobic silica.
• S and SO 3 are acidic components for suppressing the formation of basicity by Na, and by setting the above range, it is possible to suppress the bias to basicity.
• wet-process synthetic silica which is a raw powder
• an alkaline component such as sodium hydroxide (NaOH)
• an acidic component such as sulfuric acid (H 2 SO 4 )
• Wet process synthetic silica components are not preferable because they become alkaline and acidic, respectively, and both components must be present in a well-balanced manner and the pH should be near neutral.
• the pH of the wet-process synthetic silica, which is the raw powder is preferably in the range of 5.5 to 8.0 in the vicinity of neutrality from the viewpoint of use and purpose.
• the alkali becomes stronger, the surface of the silica is altered or modified with time, which is not preferable because a problem different from the purpose of this study occurs.
• the amount of aluminum contained in the wet process synthetic silica is preferably less than 0.5 wt% in terms of Al 2 O 3 .
• the wet component synthetic silica that is the raw powder thus produced, Na component and S component are each converted to Na 2 O in 0.20 ⁇ 1.20 wt%, it should be 0.25 ⁇ 1.30 wt% converted to SO 3.
• the difference between the raw powder and the content range is because the weight percentage of impurities in the entire silica is reduced by adding the weight of the silicone oil during the hydrophobization treatment.
• the surface treatment method of hydrophobizing with silicone oil is not particularly limited with respect to the wet method synthetic silica that is the raw powder, but treatment by dry mixing is common as exemplified below.
• a dry mixing and dispersing device such as an FM mixer (Henschel mixer) to perform surface treatment.
• FM mixer Hexschel mixer
• it is effective to perform heat treatment after the surface treatment, but the specific heat treatment method is not particularly limited.
• silicone oil is chemically bonded to the silanol groups on the silica surface by heat treatment at 250 to 400 ° C for about 0.5 to 4.0 hours using a heat treatment device such as an electric furnace, Nesco heater, or kiln. Long-term hydrophobicity can be maintained.
• Silicone oil has organic groups such as methyl group (-CH 3 ), so after hydrophobizing, the amount of carbon (C amount) in hydrophobic silica is measured. It is also possible to obtain the silicone oil treatment amount by back calculation.
• hydrophobic silica of the present invention hydrophobic silica with very little elution silicone oil can be efficiently produced even at a low temperature and in a short time due to the catalytic effect of the strong acid strong base type neutral salt compound.
• the use of the hydrophobic silica of the present invention is not limited, it can be used mainly as a matting agent for paints, a resin filler, an anti-blocking agent for films, and the like.
• it is a solvent type for matting agents for coatings on plastic surfaces (soft-feel coatings) and film coating applications that require both transparency and matting properties (for example, antiglare films and antiglare films to be applied to liquid crystal surfaces).
• the solventless type can be used more suitably.
• M value A mixed solution with water in which the concentration of methanol was changed at intervals of 5% by volume was prepared, and 5 ml thereof was put into a test tube having a volume of 10 ml. Next, 0.1 to 0.2 g of the test powder is added, shaken and allowed to stand, and observed to find the minimum concentration of methanol in which the powder is suspended, and this is taken as the M value.
• Toluene-eluting silicone oil amount Silicone oil amount before toluene elution (%)-Silicone oil amount after toluene elution (%)
• Carbon content analysis used for the measurement of silicone oil elution is 1,250 ° C and oxygen inflow pressure 0.07 MPa using a solid carbon analyzer (device name: carbon analyzer, model EMIA-110, manufactured by Horiba, Ltd.). The measurement was performed under the condition of a measurement time of 90 s. The amount of silicone oil treated was calculated from the obtained carbon amount using the following formula.
• Silicone oil treatment amount (%) Carbon analysis measurement value (%) x silicone oil molecular weight / carbon amount in silicone oil
• BET specific surface area Measurement was carried out by a one-point method using a fully automatic specific surface area measuring device (model: Macsorb (R) HM model-1200, manufactured by Mountec Co., Ltd.).
• Example 1 Precipitated silica with a BET specific surface area of 130 m 2 / g was synthesized by reacting sodium silicate and sulfuric acid. The synthesized precipitated silica was filtered and washed with water to obtain a silica cake. While adding water to this silica cake and performing redispersion with a reciprocating rotary stirrer (manufactured by Shimazaki Engineering Co., Ltd.), sodium sulfate (Na 2 SO) was added so that the concentration would be 0.50 wt% in terms of Na 2 O and 0.60 wt% in terms of SO 3.
• silica slurry having a pH of 7.5.
• This silica slurry was dried with a spray dryer (manufactured by Okawara Chemical Co., Ltd.), and pulverized and classified to obtain precipitated silica having an average particle size of 6.3 ⁇ m.
• Table 1 shows the physical properties of the raw powder, the properties after hydrophobization, and the amount of elution.
• Example 2 In Example 1, the same method as in Example 1 except that sodium sulfate (Na 2 SO 4 ) was added so that the concentration was 0.32 wt% in terms of Na 2 O and 0.38 wt% in terms of SO 3. Hydrophobic silica was obtained.
• sodium sulfate Na 2 SO 4
• Example 3 Hydrophobic silica was obtained in the same manner as in Example 1, except that the amount of silicone oil treated in Example 1 was changed to 3.8 parts per 100 m 2 / g of BET specific surface area.
• Example 4 In Example 1, hydrophobic silica was added in the same manner as in Example 1 except that sodium sulfate (Na 2 SO 4 ) was added so that it was 0.90 wt% in terms of Na 2 O and 1.08 wt% in terms of SO 3. Obtained.
• sodium sulfate Na 2 SO 4
• Example 5 Implemented except that the raw material silica was changed to the commercially available precipitation silica nip seal N-300A (BET specific surface area 150 m 2 / g) and the amount of silicone oil treated was changed to 6.7 parts per 100 m 2 / g BET specific surface area Hydrophobic silica was obtained in the same manner as in Example 1.
• Example 6 Hydrophobic silica was obtained in the same manner as in Example 1 except that the amount of silicon oil treated in the precipitated silica of Example 5 was changed to 3.3 parts with respect to a BET specific surface area of 100 m 2 / g.
• Example 7 Implemented except that the silica of the raw material was changed to a commercially available gel method silica nip gel AZ-200 (BET specific surface area 300 m 2 / g) and the amount of silicone oil treated was changed to 5.3 parts per 100 m 2 / g of BET specific surface area Hydrophobic silica was obtained in the same manner as in Example 1.
• Example 8 Implemented except that the raw material silica was changed to a commercially available gel-process silica nip gel BY-601 (BET specific surface area 500 m 2 / g) and the amount of silicone oil treated was changed to 3.0 parts per 100 m 2 / g BET specific surface area Hydrophobic silica was obtained in the same manner as in Example 1.
• Comparative Example 2 In Comparative Example 1, hydrophobic silica was obtained in the same manner as in Comparative Example 1 except that sodium hydroxide (NaOH) was added to adjust the pH to obtain a silica slurry having a pH of 10.8.
• NaOH sodium hydroxide
• Example 5 hydrophobic silica was obtained in the same manner as in Example 5 except that sodium sulfate (Na 2 SO 4 ) was not added and the pH was not adjusted.
• Comparative Example 4 In Comparative Example 3, hydrophobic silica was obtained in the same manner as in Comparative Example 3 except that sulfuric acid (H 2 SO 4 ) was added to adjust the pH to obtain a silica slurry having a pH of 3.2.
• sulfuric acid H 2 SO 4
• Example 7 hydrophobic silica was obtained in the same manner as in Example 7 except that sodium sulfate (Na 2 SO 4 ) was not added and the pH was not adjusted.
• Nipseal SBY-61 was used as an example of commercially available hydrophobic silica
• the present invention is useful for a method for producing hydrophobic silica.

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