SPHERICAL SILICA PARTICLE, AND METHOD FOR PRODUCING SAME

Abstract

Provided are porous spherical silica particles whose oil absorption is suppressed while the porous spherical silica particles have a large specific surface area; and a method for manufacturing such spherical silica particles.

According to the present invention, provided are spherical silica particles whose specific surface area obtained by employing a BET method is 300 m.sup.2/g or more, total pore volume is 0.3 ml/g or less, and oil absorption is 50 ml/100 g or less, the spherical silica particles obtained by subjecting silica gel particles obtained by employing a sol-gel method, for example, in which an alkali silicate is emulsified and coagulated, to only drying at a low temperature without subjecting the silica gel particles to calcination at a high temperature; and a method for manufacturing such spherical silica particles.

Claims

1. Spherical silica particles whose specific surface area being obtained by employing a BET method is 300 m.sup.2/g or more, total pore volume is 0.3 ml/g or less, and oil absorption is 50 ml/100 g or less.

2. The spherical silica particles according to claim 1, wherein a content percentage of structural water is 1.6% or more.

3. The spherical silica particles according to claim 1, wherein the spherical silica particles are not subjected to calcination processing at a temperature of 1000° C. or more.

4. The spherical silica particles according to claim 1, wherein one kind or more of a metal oxide or metal oxides selected from the group consisting of a titanium oxide, a zinc oxide, an iron oxide, and an aluminum oxide is or are compounded.

5. The spherical silica particles according to claim 4, a content rate of the metal oxide or metal oxides to a whole of the silica particles is 0.5 wt. % to 30 wt. %.

6. A method for manufacturing spherical silica particles comprising: (1) a step of forming a W/O type emulsion in which an alkali silicate aqueous solution is a dispersed phase and a liquid which does not mix with the alkali silicate aqueous solution is a continuous phase; (2) a step of generating a spherical silica gel by mixing the W/O type emulsion with a mineral acid aqueous solution; (3) a step of performing separation into two layers which are an O phase and a W phase and removing the O phase by heating a reaction liquid including the generated spherical silica gel; (4) a step of washing the W phase including the spherical silica gel from which the O phase has been removed; and (5) a step of drying the washed spherical silica gel.

7. The method for manufacturing spherical silica particles according to claim 4, further comprising a step of crushing the dried spherical silica particles.

8. The method for manufacturing spherical silica particles according to claim 4, wherein no calcination step is included after the (5) step.

9. The method for manufacturing spherical silica particles according to claim 6, wherein the liquid which does not mix with the alkali silicate aqueous solution is a non-polar solvent.

10. The method for manufacturing spherical silica particles according to claim 6, further comprising, before the (1) step, a step of adding, to the alkali silicate aqueous solution, one kind or more of a metal oxide or metal oxides selected from the group consisting of a titanium oxide, a zinc oxide, an iron oxide, and an aluminum oxide or a precursor compound or precursor compounds of one kind or more of the metal oxide or metal oxides selected from the group consisting of the titanium oxide, the zinc oxide, the iron oxide, and the aluminum oxide.

11. A texture improver for cosmetics, comprising the spherical silica particles according to claim 1.

12. A cosmetic in which the spherical silica particles according to claim 1 are compounded.

Description

DESCRIPTION OF EMBODIMENT

[0052] Hereinafter, with reference to specific examples, spherical silica particles and a method for manufacturing spherical silica particles according to the present invention will be described in detail. Note that the present invention is not limited to an embodiment set forth below, and a variety of modifications can be made without departing from the technical ideas of the present invention.

EXAMPLES

Example 1

<Step 1: Preparation of Emulsion>

[0053] 100 grams of a non-polar solvent (xylene), 4 grams of an emulsifying agent (sorbitan monostearate), and 400 grams of a No. 1 silicate soda aqueous solution (having a concentration of 10 wt. % expressed in terms of SiO.sub.2) were stirred by using an emulsifying apparatus (T. K. ROBOMIX manufactured by PRIMIX Corporation) at the number of revolutions of 4500 rpm for five minutes, thereby preparing emulsion.

<Step 2: Generation of Silica Gel and Removal of Impurities>500 grams of the emulsion obtained in step 1 were added while 500 grams of a sulfuric acid aqueous solution having a concentration of 40 wt. % was being stirred; the resultant was stirred for 30 minutes under a room temperature; and thereafter, the resultant was heated to 90° C. under stirring and was retained for 30 minutes, thereby separating reaction liquid in an emulsion state to an oil phase and a water phase including a spherical silica gel. The oil phase was removed from the separated reaction liquid; the water phase including the silica gel was washed by pure water until electric conductivity in the water phase reached 80 μS/cm or less; and thereafter, dehydration was conducted, thereby obtaining the silica gel.

<Step 3: Drying of Silica Gel>

[0054] The silica gel obtained in step 2 was dried at 120° C. for 24 hours, thereby preparing spherical silica particles in Example 1.

Example 2

[0055] Similar operations were conducted under the same conditions in Example 1 except that as a silicate soda aqueous solution, a No. 2 silicate soda aqueous solution was used, thereby preparing spherical silica particles in Example 2.

Example 3

[0056] Similar operations were conducted under the same conditions in Example 1 except that as a silicate soda aqueous solution, a No. 3 silicate soda aqueous solution was used, thereby preparing spherical silica particles in Example 3.

Example 4

[0057] Similar operations were conducted under the same conditions in Example 2 except that the heating temperature in step 2 was 70° C., thereby preparing spherical silica particles in Example 4.

Example 5

[0058] Similar operations were conducted under the same conditions in Example 2 except that the heating temperature in step 2 was 50° C., thereby preparing spherical silica particles in Example 5.

Example 6

[0059] Similar operations were conducted under the same conditions in Example 2 except that a concentration of the used silicate soda aqueous solution expressed in terms of SiO.sub.2 was 5 wt. %, thereby preparing spherical silica particles in Example 6.

Example 7

[0060] Similar operations were conducted under the same conditions in Example 2 except that a concentration of the used silicate soda aqueous solution expressed in terms of SiO.sub.2 was 20 wt. %, thereby preparing spherical silica particles in Example 7.

Example 8

[0061] Similar operations were conducted under the same conditions in Example 2 except that a concentration of the used sulfuric acid aqueous solution was 30 wt. %, thereby preparing spherical silica particles in Example 8.

Example 9

[0062] Similar operations were conducted under the same conditions in Example 2 except that a concentration of the used sulfuric acid aqueous solution was 50 wt. %, thereby preparing spherical silica particles in Example 9.

Example 10

[0063] Similar operations were conducted under the same conditions in Example 2 except that as an emulsifying agent, sorbitan monopalmitate was used, thereby preparing spherical silica particles in Example 10.

Example 11

[0064] Similar operations were conducted under the same conditions in Example 2 except that as an emulsifying agent, sorbitan monolaurate was used, thereby preparing spherical silica particles in Example 11.

Example 12

[0065] Similar operations were conducted under the same conditions in Example 2 except that as an emulsifying agent, sorbitan distearate was used, thereby preparing spherical silica particles in Example 12.

Example 13

[0066] Similar operations were conducted under the same conditions in Example 2 except that as an emulsifying agent, sorbitan tristearate was used, thereby preparing spherical silica particles in Example 13.

Example 14

[0067] Similar operations were conducted under the same conditions in Example 2 except that as an emulsifying agent, sorbitan monooleate was used, thereby preparing spherical silica particles in Example 14.

Example 15

[0068] Similar operations were conducted under the same conditions in Example 14 except that an amount of a used emulsifying agent was 4.8 grams, thereby preparing spherical silica particles in Example 15.

Example 16

[0069] Similar operations were conducted under the same conditions in Example 14 except that an amount of a used emulsifying agent was 3.2 grams, thereby preparing spherical silica particles in Example 16.

Example 17

[0070] Similar operations were conducted under the same conditions in Example 2 except that as an emulsifying agent, sorbitan sesquioleate was used, thereby preparing spherical silica particles in Example 17.

Example 18

[0071] Similar operations were conducted under the same conditions in Example 2 except that as an emulsifying agent, sorbitan trioleate was used, thereby preparing spherical silica particles in Example 18.

Example 19

[0072] Similar operations were conducted under the same conditions in Example 2 except that the silica particles obtained in step 3 was further dried at a temperature of 350° C. for three hours, thereby preparing spherical silica particles in Example 19.

Example 20

[0073] Similar operations were conducted under the same conditions in Example 2 except that 0.20 gram of a titanium oxide (manufactured by TAYCA CORPORATION: with a brand of MT-150AW) was added to a silicate soda aqueous solution, the resultant was sufficiently mixed, and thereafter, an operation in step 1 was conducted, thereby preparing silica particles in Example 20.

Example 21

[0074] Similar operations were conducted under the same conditions in Example 20 except that an added amount of a titanium oxide (manufactured by TAYCA CORPORATION: with a brand of MT-150AW) was 4.44 grams, thereby preparing silica particles in Example 21.

Example 22

[0075] Similar operations were conducted under the same conditions in Example 20 except that an added amount of a titanium oxide (manufactured by TAYCA CORPORATION: with a brand of MT-150AW) was 17.14 grams, thereby preparing silica particles in Example 22.

Comparative Example 1

[0076] Similar operations were conducted under the same conditions in Example 2 except that silica particles obtained in step 3 were subjected to calcination at a temperature of 1100° C. for three hours, thereby preparing silica particles in Comparative Example 1.

Comparative Example 2

[0077] Similar operations were conducted under the same conditions in Example 2 except that silica particles obtained in step 3 were subjected to calcination at a temperature of 650° C. for three hours, thereby preparing silica particles in Comparative Example 2.

Comparative Example 3

[0078] In order to compare the silica gel with one obtained by wet water-based synthesis, commercially available silica particles (manufactured by TOSOH SILICA CORPORATION: Nipsil E-743) were used as silica particles in Comparative Example 3.

Comparative Example 4

[0079] In order to make a comparison with silica particles having an average primary particle diameter and a specific surface area which are equivalent to those in Example 1, commercially available silica particles (manufactured by AGC Si-Tech Co., Ltd.: H-52) were used as silica particles in Comparative Example 4.

[0080] <Physical Property Evaluation Method>

[0081] (Average Primary Particle Diameter) Average primary particle diameters of silica particles in Examples and Comparative Examples were measured by using a scanning electron microscope.

[0082] Specifically, photographs of approximately 1000 silica particles, which were shot by using a scanning electron microscope (manufactured by Hitachi High-Tech Corporation: S-4800) were subjected to analysis by using image analysis-type particle size distribution software (manufactured by MOUNTECH Co., Ltd.: Mac-VIEW).

[0083] (Specific Surface Area)

[0084] Specific surface areas of silica particles in Examples and Comparative Examples were measured by using a specific surface area measuring device (manufactured by MOUNTECH Co., Ltd.: Macsorb HM-model 1210).

[0085] Specifically, in order to remove moisture and the like physically adsorbed on surfaces and inside pores of silica particles, as pretreatment before the measurement, the silica particles were dried under a condition of temperature of 105° C. for 12 hours and were left to be cooled in a desiccator. The cooled specimens were deaerated by nitrogen gas under a condition of a temperature of 150° C. for 20 minutes, and specific surface areas were measured.

[0086] The specific surface areas were obtained by applying a calculation formula in a BET one-point method.

[0087] (Total Pore Volume)

[0088] Total pore volumes of silica particles in Examples and Comparative Examples were measured by using a pore distribution measuring device (manufactured by MicrotracBEL Corp.: BELSORP mini).

[0089] Specifically, in order to remove moisture and the like physically adsorbed on surfaces and in pores of silica particles, first, the silica particles were subjected to vacuum drying under conditions of a degree of vacuum of 10-2 kPa and a temperature of 150° C. for 30 minutes.

[0090] Each of the total pore volumes was obtained by converting an adsorption amount of N2 gas at a relative pressure of p/p.sub.0=0.990 into a volume of N2 in a liquid state.

[0091] (Oil Absorption)

[0092] Each oil absorption of the silica particles in Examples and Comparative Examples was measured by employing a method described in JIS K 5101-13-2.

[0093] (Content Percentage of Structural Water)

[0094] Content percentages of structural water of the silica particles in Examples and Comparative Examples were measured by using a thermogravimetric/differential thermal (TG-DTA) analyzer TG/DTA 6300 (manufactured by Seiko Instrument Inc.).

[0095] Specifically, in order to remove moisture and the like physically adsorbed on surfaces and in pores of silica particles, as pretreatment before the measurement, the silica particles were dried under a condition of temperature of 105° C. for 12 hours and were left to be cooled in a desiccator. A difference between a weight reduction percentage (%) from a room temperature to a temperature of 500° C. and a weight reduction percentage (%) from a room temperature to a temperature of 1100° C., measured when a temperature of the cooled silica particles was increased in the air (a flow rate: 200 ml/minute) at a temperature increasing speed of 10° C./minute from a room temperature to 1200° C., was defined as a content percentage of the structural water (%).

[0096] (Texture Evaluation of Each of Powders upon Applying to Skin)

[0097] As to each of powders in Examples and Comparative Examples, adherability to skin and texture were evaluated by a sensory test by five monitors. Specifically, the adherability to skin and the texture sensed when a small amount of each of the powders was taken and applied to the back of the hand with his or her finger were evaluated with the following criteria and an average value was calculated.

[0098] <Criteria of Evaluation Points>

[0099] Five points: extremely excellent

[0100] Four points: excellent

[0101] Three points: fair

[0102] Two points: inferior

[0103] One point: extremely inferior

[0104] Manufacturing conditions of each of the powders in Examples and Comparative Examples are shown in Table 1 and evaluation results thereof are shown in Table 2.

TABLE-US-00001 TABLE 1 Manufacturing conditions Preparation of emulsion Concentration Added of silicate amount of soda Kind of emulsifying Kinds of (expressed Composite Oil emulsifying agent silicate in terms of metal phase agent (to SiO.sub.2) soda SiO.sub.2) oxide Example 1 Xylene Sorbitan 10% No. 1 10% — monostearate Example 2 Xylene Sorbitan 10% No. 2 10% — monostearate Example 3 Xylene Sorbitan 10% No. 3 10% — monostearate Example 4 Xylene Sorbitan 10% No. 2 10% — monostearate Example 5 Xylene Sorbitan 10% No. 2 10% — monostearate Example 6 Xylene Sorbitan 10% No. 2  5% — monostearate Example 7 Xylene Sorbitan 10% No. 2 20% — monostearate Example 8 Xylene Sorbitan 10% No. 2 10% — monostearate Example 9 Xylene Sorbitan 10% No. 2 10% — monostearate Example 10 Xylene Sorbitan 10% No. 2 10% — monopalmitate Example 11 Xylene Sorbitan 10% No. 2 10% — monolaurate Example 12 Xylene Sorbitan 10% No. 2 10% — distearate Example 13 Xylene Sorbitan 10% No. 2 10% — tristearate Example 14 Xylene Sorbitan 10% No. 2 10% — monooleate Example 15 Xylene Sorbitan 12% No. 2 10% — monooleate Example 16 Xylene Sorbitan  8% No. 2 10% — monooleate Example 17 Xylene Sorbitan 10% No. 2 10% — sesquioleate Example 18 Xylene Sorbitan 10% No. 2 10% — trioleate Example 19 Xylene Sorbitan 10% No. 2 10% — monostearate Example 20 Xylene Sorbitan 10% No. 2 10% Titanium monostearate dioxide Example 21 Xylene Sorbitan 10% No. 2 10% Titanium monostearate dioxide Example 22 Xylene Sorbitan 10% No. 2 10% Titanium monostearate dioxide Comparative Xylene Sorbitan 10% No. 2 10% — Example 1 monostearate Comparative Xylene Sorbitan 10% No. 2 10% — Example 2 monostearate Comparative — — — — — — Example 3 Comparative — — — — — — Example 4 Manufacturing conditions Preparation of emulsion Content rate Synthesis of silica gel (to the Concentration Drying Calcination whole of sulfuric Heating Drying Calcination particles) acid temperature temperature temperature Example 1 — 40% 90° C. 120° C. — Example 2 — 40% 90° C. 120° C. — Example 3 — 40% 90° C. 120° C. — Example 4 — 40% 70° C. 120° C. — Example 5 — 40% 50° C. 120° C. — Example 6 — 40% 90° C. 120° C. — Example 7 — 40% 90° C. 120° C. — Example 8 — 30% 90° C. 120° C. — Example 9 — 50% 90° C. 120° C. — Example 10 — 40% 90° C. 120° C. — Example 11 — 40% 90° C. 120° C. — Example 12 — 40% 90° C. 120° C. — Example 13 — 40% 90° C. 120° C. — Example 14 — 40% 90° C. 120° C. — Example 15 — 40% 90° C. 120° C. — Example 16 — 40% 90° C. 120° C. — Example 17 — 40% 90° C. 120° C. — Example 18 — 40% 90° C. 120° C. — Example 19 — 40% 90° C. 350° C. — Example 20 0.5%  40% 90° C. 120° C. — Example 21 10% 40% 90° C. 120° C. — Example 22 30% 40% 90° C. 120° C. — Comparative — 40% 90° C. 120° C. 1100° C. Example 1 Comparative — 40% 90° C. 120° C.  650° C. Example 2 Comparative — — — — — Example 3 Comparative — — — — — Example 4

TABLE-US-00002 TABLE 2 Evaluation Average Content Specific primary Total percentage of Texture surface particle pore Oil structural evaluation area diameter volume absorption water (5 points (m.sup.2/g) (μm) (ml/g) (ml/100 g) (%) maximum) Example 1 618 5.0 0.28 23.5 2.2 4.0 Example 2 462 5.0 0.22 20.5 2.0 4.6 Example 3 362 5.0 0.18 19.5 1.9 4.2 Example 4 451 5.0 0.19 19.2 2.1 4.8 Example 5 441 5.0 0.21 18.9 2.4 4.6 Example 6 429 3.9 0.19 19.3 1.8 4.8 Example 7 480 5.5 0.22 19.6 2.1 4.6 Example 8 445 5.0 0.21 19.0 2.1 4.6 Example 9 401 5.0 0.19 19.1 1.7 4.8 Example 10 359 3.3 0.21 18.5 2.1 4.0 Example 11 374 5.0 0.20 23.5 2.1 4.2 Example 12 413 5.0 0.19 21.0 2.1 4.8 Example 13 432 5.1 0.20 19.0 2.0 4.6 Example 14 508 5.4 0.26 19.0 2.1 4.2 Example 15 468 1.1 0.26 21.0 2.1 4.0 Example 16 536 11.0 0.26 17.0 2.0 4.2 Example 17 408 4.3 0.18 19.0 1.9 4.8 Example 18 435 5.0 0.21 19.0 1.9 4.6 Example 19 459 5.0 0.20 20.5 2.0 4.6 Example 20 412 5.3 0.20 19.8 1.9 4.6 Example 21 510 5.1 0.21 21.4 1.8 4.6 Example 22 620 5.0 0.24 22.5 1.7 4.2 Comparative 1 5.0 0.005 5.3 0.1 2.0 Example 1 Comparative 218 5.0 0.12 18.3 1.5 3.0 Example 2 Comparative 55 0.1 0.33 130.0 2.0 1.0 Example 3 Comparative 483 5.4 1.60 247.0 1.2 3.0 Example 4

[0105] It was found from Tables 1 and 2 that as to the spherical silica particles in each of Examples 1 to 19, while in order to obtain excellent texture evaluation, the specific surface area is 300 m.sup.2/g or more (for example, in Examples 3 and 11) and more preferably, the specific surface area is 400 m.sup.2/g or more (for example, in Examples 4, 6, 9, 12, and 17), and the pore volume is 0.3 ml/g or less (for example, in Examples 14 and 16) more preferably, the pore volume is 0.25 ml/g or less (for example, in Examples 2, 7, 8, and 18) and further preferably, the pore volume is 0.2 ml/g or less (for example, in Examples 4, 6, 9, 12, and 17), and in the comparison with Comparative Examples 3 and 4 described later in particular, the oil absorption is suppressed to 50 ml/100 g or less; more preferably, the oil absorption is suppressed to 30 ml/100 g or less; and further preferably, the oil absorption is suppressed to 20 ml/100 g or less.

[0106] It is considered that the specific surface area obtained by employing the BET method is made to be 300 m.sup.2/g or more; and more preferably, the specific area is 400 m.sup.2/g or more, thereby allowing the proportion of the area of the silica particles contacting skin to be sufficiently small and when the silica particles are applied to skin, hardness of the silica particles is hardly felt, thereby obtaining the favorable texture.

[0107] In addition, it is considered that the specific surface area obtained by employing the BET method is 300 m.sup.2/g or more; and more preferably, the specific surface area is 400 m.sup.2/g or more, and the total pore volume is 0.3 ml/g or less; more preferably, the total pore volume is 0.25 ml/g or less; and further preferably, the total pore volume is 0.2 ml/g or less (in other words, while the specific surface area is made large, the pore volume is made small), thereby allowing the oil absorption to be suppressed to a lower level such as 50 ml/100 g or less; more preferably, 30 ml/100 g or less; and further preferably, 20 ml/100 g or less and adverse influence (clumping of the silica particles and dry feeling of skin) due to excessive absorption of an oil content is hardly caused, thereby allowing excellent use feeling to be obtained.

[0108] As a result, the oil absorption thereof is suppressed while the spherical silica particles in each of Examples 1 to 19 are porous and have the large specific surface area, and the spherical silica particles in each thereof can exhibit excellent adherability to skin and excellent texture.

[0109] In addition, it was found from Example 19 that even in the second drying, preferably, as drying conditions, drying is conducted at a temperature of 50° C. to 500° C.; more preferably, the drying is conducted at a temperature of 100° C. to 400° C., and preferably, retainment is conducted for one minute to 40 hours; and more preferably, the retainment is conducted for 10 hours to 30 hours, and the oil absorption thereof is thereby suppressed while the spherical silica particles have the large specific surface area, thus allowing spherical silica particles of the present invention, which exhibit excellent adherability to skin and excellent texture, to be obtained.

[0110] Furthermore, it was found from Examples 20 to 22 that as to the silica particles obtained by compounding the titanium oxide, the oil absorption thereof is suppressed while the silica particles have the large specific surface area, thereby obtaining the spherical silica particles of the present invention, which exhibit excellent adherability to skin and excellent texture.

[0111] On the other hand, it is inferred that because the silica particles in Comparative Example 1 were subjected to the calcination at the temperature of 1100° C., densification of the particles and mutual sintering of the particles proceed, thereby greatly decreasing the specific surface area and the pore volume and further causing high hardening of the particles and mutual fusion of the particles. Therefore, it was found that the silica particles in Comparative Example 1 cannot exhibit excellent adherability to skin and excellent texture, thereby worsening use feeling.

[0112] It was found that although because the silica particles in Comparative Example 2 were subjected to the calcination at the comparatively low temperature of 650° C., a decrease in the specific surface area and a decrease in the pore volume caused by proceeding of densification of the particles and mutual sintering of the particles were suppressed to some extent, a large specific surface area cannot be obtained and as a result, excellent adherability to skin and excellent texture cannot be obtained.

[0113] In addition, it was found that although the specific surface area of the silica particles in Comparative Example 3 was 55 m.sup.2/g and the pore volume thereof was also 0.33 ml/g, showing the small values, in contrast thereto, because the oil absorption thereof was high, showing 130.0 ml/100 g, the oil content of skin is, for example, excessively absorbed and as a result, mutual clumping of the silica particles and drying of a surface of skin were caused, whereby excellent adherability to skin and excellent texture cannot be obtained.

[0114] It was found that as with the silica particles in Comparative Example 1, although the silica particles in Comparative Example 4 have the specific surface area equivalent to that of the spherical silica particles in each of Examples 1 to 19, the pore volume thereof is 1.60 ml/g, showing the large value and in accordance therewith, the oil absorption thereof was 247.0 ml/100 g, showing the high oil absorption. Therefore, also the silica particles in Comparative Example 4 excessively absorb the oil content of skin and as a result, mutual clumping of the silica particles and drying of a surface of skin were caused, thereby resulting in the adherability to skin and the texture inferior to the spherical silica particles in each of Examples 1 to 19.

[0115] As described above, it was found that in order to enhance use feeling such as rolling properties, adherability, and texture to skin upon compounding silica particles in general into a cosmetic or the like, the silica particles in general are porous, having the large specific surface area and the large oil absorption (Comparative Example 4) or while the silica particles in general have the small specific surface area and the small pore volume owing to pore control, the oil absorption thereof is large (Comparative Example 3), and spherical silica particles excellent in use feeling such as adherability and texture owing to suppression in oil absorption while the spherical silica particles have the large specific surface area as in each of Examples 1 to 19 have not so far been available as the silica particles in general.

[0116] In addition, it was found by the comparison with Comparative Examples 1 and 2 that in order to obtain the spherical silica particles having characteristics like those in each of Examples 1 to 19, subjecting silica gel particles obtained by employing a sol-gel method, for example, in which an alkali silicate is emulsified and coagulated, to only drying at a low temperature without subjecting the silica gel particles to calcination at a high temperature is effective.