JP4903493B2 - Method for producing composite particles - Google Patents

Description

Detailed Description of the Invention

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for producing a novel composite filler.

Conventional technology

In recent years, there has been a need for lighter paper from the standpoint of environmental load reduction, including forest resource protection, resource saving problems, and garbage problems. When aiming to reduce the weight of paper, especially in the fields of printing paper and wrapping paper, various fillers are internally added to increase whiteness, opacity, and printability. Conventionally, as a method for improving the whiteness and opacity of paper by adding filler, a method of increasing scattering efficiency by internally adding a filler having a large refractive index such as titanium dioxide, white clay, talc, calcium carbonate, organic pigment, etc. A method has been adopted in which a filler having a refractive index of about 1.5 is internally added to suppress adhesion between pulp fibers and increase the scattering surface area.

In the past, methods such as simply lowering the basis weight or increasing the deinked pulp ratio have been used in order to cope with lighter paper. However, with this method, the paper is thinned, and the whiteness and opacity are lowered, resulting in poor printability such as back-through. Opacity and strikethrough are closely related to the thickness of the paper, and so far, bulky paper has been produced by using extra pulp or using bulky pulp.

However, the filler such as calcium carbonate having a small particle diameter as described above has a problem that most of the filler flows out into the white water at the time of paper making and is very poorly retained in the paper layer. Further, such small filler particles are distributed between pulp fibers, so that the bonding between the fibers is hindered and the paper strength is lowered.

The object of the present invention is to improve the yield of the filler, and the interfiber bonding is not hindered by the filler particles distributed between the fibers, and even a small amount of pulp can be bulked, and at the same time, the whiteness and opacity can be improved. The object is to provide a bulky filler manufacturing method and a filler-added paper capable of producing “light and thick paper”.

The above problem is that after adding and dispersing inorganic fine particles in an alkali silicate aqueous solution to prepare a slurry, while heating and stirring, the liquid temperature is maintained in the range of 60 to 100 ° C., and an acid is added to form a silica sol. This was solved by preparing inorganic fine particles / silica composite particles by adjusting the pH in the range of neutral to weakly alkaline and internally adding them as a filler to the pulp slurry.

In the present invention, silica sol fine particles of about several nanometers formed by adding an acid such as dilute sulfuric acid to a sodium silicate solution are thinly attached to the entire surface of the inorganic fine particles. Bonding occurs between the fine particles and the silica sol fine particles on the surface of the other inorganic fine particles, and an aggregate of inorganic fine particles and silica composite particles is formed.

For this reason, the pH of the final reaction solution is an important factor, and it is necessary to control the pH so that hydrated silicic acid (white carbon) is not generated in the reaction system, with the pH ranging from neutral to weakly alkaline. A preferred pH is in the range of 8-11. If sulfuric acid is added until the pH reaches an acidic condition of less than 7, white carbon is generated instead of silica sol, and the white carbon surrounds the aggregates of inorganic fine particles in a spherical shape. The optical characteristics appear preferentially, and the optical characteristics of the inorganic fine particles in the core are not exhibited at all. When the pH exceeds 11, the generation of silica sol becomes insufficient, and it is difficult to obtain an aggregate of the inorganic fine particles / silica composite of the present invention.

The silica sol produced during the reaction in the present invention is prepared by reacting sodium silicate (water glass) as a raw material with a dilute solution of mineral acid such as sulfuric acid, hydrochloric acid, nitric acid, etc. at a high temperature, and by hydrolysis reaction and polymerization of silicic acid. Silica sol particles having a particle diameter of 10 to 20 nm are obtained.

The alkali silicate solution used in the present invention is not particularly limited, but a sodium silicate solution (No. 3 water glass) is desirable from the viewpoint of availability. The concentration of the alkali silicate solution is preferably 3 to 10% by weight in terms of the silicic acid content in the aqueous solution (in terms of SiO2). When the content exceeds 10% by weight, the formed composite is not an inorganic fine particle / silica composite aggregate, but the inorganic fine particles are encapsulated with white carbon, and the optical properties of the inorganic fine particles in the core are not exhibited at all. . On the other hand, when the content is less than 3% by weight, the silica component in the composite particles is lowered, so that aggregate particles are difficult to form.

Examples of the inorganic fine particles used in the present invention include light calcium carbonate, heavy calcium carbonate, talc, kaolin, clay, calcined kaolin, titanium dioxide, and aluminum hydroxide, which are fillers for papermaking. The particle size is preferably 0.05 to 50 microns in view of the particle size of the composite aggregate particles to be formed.

Since these inorganic fine particles have a function as a dispersant in the sodium silicate solution, they are used after being previously dispersed in the sodium silicate solution. However, if the dispersibility of the particles is not good during dispersion, the sodium silicate solution may be added after the filler is dispersed in the water to which the dispersant has been added. In particular, since titanium dioxide has a small particle size and easily aggregates, it is desirable to use a dispersant. Examples of the dispersant include sodium hexametaphosphate, sodium pyrophosphate, and polycarboxylic acid soda.

The above inorganic fine particles may be used alone, but a composite filler having high functionality can also be produced by using two or more kinds of inorganic fine particles in combination.

Examples of the acid used in the present invention include dilute sulfuric acid, dilute hydrochloric acid, dilute nitric acid and other mineral acids, acetic acid, carbon dioxide, and the like, but dilute sulfuric acid is most desirable in terms of price and handling. Furthermore, the concentration when adding dilute sulfuric acid is preferably 0.2 to 1.0 molar.

Regarding the reaction temperature during the production of the inorganic fine particles / silica composite particles in the present invention, a range of 60 to 100 ° C. is desirable. The reaction temperature affects the formation of silica sol, the crystal growth rate, and the mechanical strength of the formed inorganic fine particles / silica composite aggregate particles. When the reaction temperature is less than 60 ° C, the formation and growth rate of silica sol is slow, and the bond strength of the formed inorganic fine particles and silica composite aggregate particles is weak. . If it exceeds 100 ° C, the reaction process becomes complicated because an autoclave must be used because it is an aqueous reaction. The optimum reaction temperature is 70-90 ° C.

In addition, when producing inorganic fine particles / silica composite particles, inorganic fine particles are added to and dispersed in an aqueous alkali silicate solution to prepare a slurry. The slurry concentration is preferably 3 to 35% by weight. By adjusting the slurry concentration, the particle size of the formed inorganic fine particles / silica composite aggregate particles is controlled, and at the same time, the composition ratio of the inorganic fine particles and silica is determined.

Furthermore, by making the average particle size of the inorganic fine particle / silica composite aggregate particles large particles of 10 to 60 microns, it is possible to increase the yield of the filler. The following three methods are mentioned as a method of particle size control. (1) The filler slurry concentration obtained by adding inorganic fine particles to a 3.61% concentration alkali silicate aqueous solution as the silicic acid content is set to 3.2 to 9.6% by weight. (2) Use inorganic fine particles having a large particle size of 2 to 10 microns as a raw material. (3) Dry or wet pulverize a solid obtained by air drying or heat drying a slurry of inorganic fine particles / silica composite aggregate particles. By the above method, it is possible to control the particle size to a size of 10 to 60 microns.

In the present invention, inorganic fine particles are added and dispersed in an aqueous alkali silicate solution to prepare a slurry, and while stirring, the liquid temperature is maintained in the range of 60 to 100 ° C., and an acid is added to form a silica sol. By adjusting the pH to neutral to weakly alkaline, preferably 8 to 11, inorganic fine particles / silica composite particles are produced, and the slurry is filtered and washed with water to obtain a wet cake.

This wet cake is again dispersed in water to form a filler slurry, which is internally added to the pulp slurry at the time of papermaking to obtain a filler-added paper. At this time, the wet cake of inorganic fine particles / silica composite particles is made into fine dry particles by air drying or heat drying treatment, and then again using a dry pulverizer or wet pulverizer to adjust the particle size of the filler slurry during pulping. When the filler-added paper is obtained by internally adding to the above, it is possible to dramatically increase the bulkiness that is the effect of the present invention. Examples of the wet pulverizer include known homomixers, homogenizers, and sand grinders.

In the present invention, clay, silica, talc, calcined kaolin, calcium carbonate and other inorganic fillers, or vinyl chloride resins, polystyrene resins, urea formalin resins, melamine resins, styrene, as known fillers within a range not impairing the effects of the present invention. / Organic fillers produced from synthetic resins such as butadiene copolymer resins can also be used in combination.

In addition, if necessary, a paper strength enhancer such as polyacrylamide polymer, polyvinyl alcohol polymer, cationized starch, urea / formalin resin, melamine / formalin resin; salt of acrylamide / aminomethylacrylamide copolymer , Cationized starch, polyethyleneimine, polyethylene oxide, acrylamide / sodium acrylate copolymer and other drainage or yield improvers; aluminum sulfate (sulfuric acid band), water-resistant agents, UV inhibitors, fading inhibitors, etc. Etc. may be contained.

EXAMPLES Hereinafter, although this invention is demonstrated in detail according to an Example and a comparative example, this invention is not limited to these. In the description, percentage indicates weight percentage.

The neutral high-quality paper produced in Examples and Comparative Examples was measured for bulkiness, whiteness, opacity, tear length, and filler yield by the following methods.
-Bulkiness: The density of the paper was calculated from the basis weight and the paper thickness. The lower the density, the higher the bulkiness.
Measurement of whiteness: Whiteness was measured with a Hunter whiteness meter based on JIS P 8123.
-Measurement of opacity: Opacity was measured using a hunter reflectometer based on JIS P 8138.
-Filler yield: Cut out 10x10cm pieces of paper from a hand-made sheet (blank) that has not been filled with filler and a hand-made sheet that has been filled with filler. 105 ° C x 3 hours After drying, weigh the absolute dry weight. Next, this absolutely dry paper piece is baked in an electric furnace at 575 ° C. for 2 hours to obtain ash contained in the sheet. Filler yield (%) was calculated from the following formula.
Yield of filler = {(weight of sheet ash with filler / absolute dry weight−weight of blank ash / absolute dry weight)} / filler blending ratio × 100 / breaking length: JIS P 8113 was obtained by the following formula.
Breaking length = tensile strength / (test piece width × test piece basis weight) × 1000 ash: Ashing temperature was 575 ° C. based on JIS P 8128.
-Ratio of composite particles: The component ratio of the composite particles was measured by fluorescent X-ray analysis.
Overall quality evaluation was performed based on the above measurement results. Evaluation was made in the following three stages.
◎: Very good, ○: Good, ×: Inferior

<Synthesis Example 1> 30 g of calcium carbonate powder (TP-121 manufactured by Okutama Kogyo Co., Ltd.) is added to 312 g of an aqueous sodium silicate solution (3.61% as silicic acid content), and dispersed for 20 minutes at 3000 rpm using a homomixer. Treatment was carried out to prepare a calcium carbonate slurry. Next, this slurry was put into a 1 L four-necked flask equipped with a stirrer, a temperature sensor, and a reflux condenser, and heated to 75 ° C. in an oil bath while stirring. Next, while maintaining the slurry in the container at 75 ° C., 276 g of 0.36 normal sulfuric acid was dropped over 3 hours at a dropping rate of 1.53 ml / min using a micro tuning pump to obtain calcium carbonate / silica composite aggregated particles. . The pH of the reaction solution at this time was 10.3. Furthermore, a wet cake of calcium carbonate / silica composite aggregated particles was obtained by filtering, washing with water using No. 2 filter paper, and filtering again. Using a particle size distribution analyzer Mastersizer S (Malvern), 50% volume average particle size was measured by laser diffraction / scattering method. The average particle size was 8 microns, and the ratio of calcium carbonate to silica was It was 70:30.

<Synthesis Example 2> Calcium carbonate / silica composite aggregated particles were obtained in the same manner as in Synthesis Example 1 except that the powder of calcium carbonate (TP-121 manufactured by Okutama Kogyo Co., Ltd.) was changed to 70 g. The pH of the reaction solution at this time was 10.2. The average particle diameter of the obtained calcium carbonate / silica composite agglomerated particles was 5.4 microns, and the ratio of calcium carbonate to silica was 86:14.

<Synthesis Example 3> In Synthesis Example 1, the obtained calcium carbonate / silica composite aggregated particles were heat-dried at 105 ° C. for 5 hours. Next, this dry powder was wet-ground with a sand grinder to obtain calcium carbonate / silica composite aggregated particles having an average particle size of 2.0 microns.

<Synthesis Example 4> 18 g of powder of calcium carbonate (TP-121 manufactured by Okutama Kogyo Co., Ltd.) and 12 g of titanium dioxide (FA-50 manufactured by Furukawa Machine Metal) are added to 312 g of sodium silicate aqueous solution (3.61% as silicic acid content). Using a mixer, dispersion treatment was performed at 3000 rpm for 20 minutes to prepare a mixed slurry of calcium carbonate and titanium dioxide. Next, this slurry was put into a 1 L four-necked flask equipped with a stirrer, a temperature sensor, and a reflux condenser, and heated to 75 ° C. in an oil bath while stirring. Next, while maintaining the slurry in the container at 75 ° C., 276 g of 0.36 normal sulfuric acid was dropped over 3 hours at a dropping rate of 1.53 ml / min using a micro-tuning pump, and calcium carbonate / titanium dioxide / silica composite aggregated particles Got. The pH of the reaction solution at this time was 10.1. Furthermore, a wet cake of calcium carbonate / titanium dioxide / silica composite aggregated particles was obtained by filtering, washing with water using No. 2 filter paper, and filtering again. The average particle size was 4.9 microns and the ratio of calcium carbonate, titanium dioxide and silica was 52:34:14.

<Synthesis Example 5> 30 g of kaolin (Amazon 88SD made by CADAM) powder is added to 312 g of an aqueous sodium silicate solution (3.61% as silicic acid content), and dispersed using a homomixer for 20 minutes at 3000 rpm. A kaolin slurry was prepared. Next, this slurry was put into a 1 L four-necked flask equipped with a stirrer, a temperature sensor, and a reflux condenser, and heated to 75 ° C. in an oil bath while stirring. Next, while maintaining the slurry in the container at 75 ° C., 276 g of 0.36 normal sulfuric acid was added dropwise over 3 hours at a dropping rate of 1.53 ml / min using a micro tuning pump to obtain kaolin-silica composite aggregated particles. The pH of the reaction solution at this time was 8.5. Furthermore, a wet cake of kaolin-silica composite aggregated particles was obtained by filtering, washing with water and filtering again using No. 2 filter paper. The average particle size was 12.7 microns and the ratio of kaolin to silica was 70:30.

<Synthesis Example 6> In Synthesis Example 5, the obtained kaolin-silica composite aggregated particles were heat-dried at 105 ° C. for 5 hours. Next, the dry powder was wet pulverized with a sand grinder to obtain kaolin-silica composite aggregated particles having an average particle diameter of 2.5 microns.

<Synthesis Example 7> Calcium carbonate / silica composite aggregated particles were obtained in the same manner as in Synthesis Example 1, except that the calcium carbonate powder was changed to 20 g in Synthesis Example 1. The pH of the reaction solution at this time was 10.2. The obtained calcium carbonate / silica composite agglomerated particles had an average particle diameter of 10.0 microns, and the ratio of calcium carbonate to silica was 65:35.

<Synthesis Example 8> Calcium carbonate / silica composite aggregated particles were obtained in the same manner as in Synthesis Example 1 except that the calcium carbonate powder was changed to 10 g in Synthesis Example 1. The pH of the reaction solution at this time was 10.1. The obtained calcium carbonate / silica composite aggregated particles had an average particle diameter of 30.5 microns, and the ratio of calcium carbonate to silica was 60:40.

<Synthesis Example 9> Kaolin-silica composite aggregated particles were obtained in the same manner as in Synthesis Example 5, except that the powder of kaolin in Synthesis Example 5 was changed to 20 g. The pH of the reaction solution at this time was 9.8. The average particle diameter of the obtained kaolin-silica composite aggregated particles was 15.0 microns, and the ratio of kaolin to silica was 70:30.

<Synthesis Example 10> Kaolin-silica composite aggregated particles were obtained in the same manner as in Synthesis Example 5 except that the kaolin powder was changed to 10 g in Synthesis Example 5. At this time, the pH of the reaction solution was 9.6. The average particle diameter of the obtained kaolin-silica composite aggregated particles was 35.5 microns, and the ratio of kaolin to silica was 68:32.

<Synthesis Example 11> 30 g of calcium carbonate powder (TP-121 manufactured by Okutama Kogyo Co., Ltd.) was added to 312 g of an aqueous sodium silicate solution (3.61% as silicic acid content), and dispersed for 20 minutes at 3000 rpm using a homomixer. Treatment was carried out to prepare a calcium carbonate slurry. Next, this slurry was put into a 1 L four-necked flask equipped with a stirrer, a temperature sensor, and a reflux condenser, and heated to 75 ° C. in an oil bath while stirring. Next, while maintaining the slurry in the container at 75 ° C., 180 g of sulfuric acid having a concentration of 10% by weight was dropped over 3 hours at a dropping rate of 1.0 ml / min using a micro tuning pump to obtain calcium carbonate / silica composite particles. Obtained. The pH of the reaction solution at this time was 5.7. Furthermore, a wet cake of calcium carbonate / silica composite particles was obtained by filtering, washing with water and filtering again using No. 2 filter paper. The average particle size was 9 microns, the ratio of calcium carbonate to silica was 25:75, and the silica ratio was higher than calcium carbonate. When the composite particles were observed with an electron microscope, the particles were spherical and the calcium carbonate was completely covered with white carbon.

<Synthesis Example 12> 30 g of kaolin (Amazon 88SD made by CADAM) powder is added to 312 g of an aqueous solution of sodium silicate (3.61% as silicic acid), and dispersed using a homomixer at a rotational speed of 3000 rpm for 20 minutes. A kaolin slurry was prepared. Next, this slurry was put into a 1 L four-necked flask equipped with a stirrer, a temperature sensor, and a reflux condenser, and heated to 75 ° C. in an oil bath while stirring. Next, while maintaining the slurry in the container at 75 ° C., 180 g of sulfuric acid with a concentration of 10% by weight is dropped over 3 hours at a dropping rate of 1.0 ml / min using a micro tuning pump to obtain kaolin-silica composite particles. It was. The pH of the reaction solution at this time was 6.5. Furthermore, a wet cake of kaolin-silica composite particles was obtained by filtering, washing with water using No. 2 filter paper, and filtering again. The average particle size was 9.5 microns, the ratio of kaolin to silica was 20:80, and the silica ratio was higher than kaolin. When the composite particles were observed with an electron microscope, the particles were spherical and kaolin was completely covered with white carbon.

[Example 1] To a slurry (concentration: 1.00%) of hardwood bleached pulp (LBKP CSF407ml), the composite aggregate particle slurry of Synthesis Example 1 was added to 10% per pulp dry weight, stirred for 1 minute, A 1% sulfuric acid band was added per dry weight. Furthermore, after stirring for 1 minute, as a yield improver, cationic PAM (Himoloc DR-1500) was added and stirred at 100 ppm per total dry weight of pulp and filler, and a small amount of sulfuric acid band was added so that the pH was 8.0 to 8.5. did. Using this prepared pulp slurry, a square hand machine was used to make a paper with a target basis weight of 64 g / m ² and an ash content of 10% by weight. ) For 1 hour) to prepare a sheet sample. The density, whiteness, opacity and tear length of this sheet were measured and shown in Table 1.

[Example 2] A sheet was prepared under the same conditions as in Example 1 except that the composite aggregated particles of Synthesis Example 2 were used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Example 3] A sheet was prepared under the same conditions as in Example 1 except that the composite aggregated particles of Synthesis Example 3 were used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Example 4] A sheet was prepared under the same conditions as in Example 1 except that the composite aggregated particles of Synthesis Example 4 were used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Comparative Example 1] A sheet was prepared under the same conditions as in Example 1, except that calcium carbonate (TP-121 manufactured by Okutama Kogyo) was used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Comparative Example 2] A sheet was prepared under the same conditions as in Example 1 except that the composite particles of Synthesis Example 11 were used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Example 5] A sheet was prepared under the same conditions as in Example 1 except that the composite aggregated particles of Synthesis Example 5 were used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Example 6] A sheet was prepared under the same conditions as in Example 1 except that the composite aggregated particles of Synthesis Example 6 were used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Comparative Example 3] A sheet was prepared under the same conditions as in Example 1 except that kaolin (Amazon 88SD manufactured by CADAM) was used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

[Comparative Example 4] A sheet was prepared under the same conditions as in Example 1 except that the composite particles of Synthesis Example 12 were used. The physical properties were measured and evaluated in the same manner as in Example 1 with the sheet obtained, and the results are shown in Table 1.

The physical property measurement evaluation results of Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Table 1.

[Example 7] To a slurry of hardwood bleached pulp (LBKP CSF407 ml) (concentration 1.00%), the composite aggregate particle slurry of Synthesis Example 7 was added to 10% per pulp dry weight, and stirred for 1 minute. A 1% sulfuric acid band was added per dry weight. Furthermore, after stirring for 1 minute, as a yield improver, cationic PAM (Himoloc DR-1500) was added and stirred at 100 ppm per total dry weight of pulp and filler, and a small amount of sulfuric acid band was added so that the pH was 8.0 to 8.5. did. Using this prepared pulp slurry, a square hand machine was used to make a paper with a target basis weight of 64 g / m ² and an ash content of 10% by weight. ) For 1 hour) to prepare a sheet sample. The density, whiteness, opacity, tear length, and filler yield of this sheet were measured and shown in Table 2.

[Example 8] A sheet was prepared under the same conditions as in Example 7 except that the composite aggregated particles of Synthesis Example 8 were used. The physical properties were measured and evaluated in the same manner as in Example 7 with the sheet obtained, and the results are shown in Table 2.

[Example 9] A sheet was prepared under the same conditions as in Example 7 except that the composite aggregated particles of Synthesis Example 9 were used. The physical properties were measured and evaluated in the same manner as in Example 7 with the sheet obtained, and the results are shown in Table 2.

[Example 10] A sheet was prepared under the same conditions as in Example 7 except that the composite aggregated particles of Synthesis Example 10 were used. The physical properties were measured and evaluated in the same manner as in Example 7 with the sheet obtained, and the results are shown in Table 2.

The physical property measurement evaluation results of Examples 7 to 10 and Comparative Examples 1 and 2 are shown in Table 2.

As shown in Table 1, in the calcium carbonate / silica composite particles produced using the calcium carbonate of Example 1 and Example 2 as a raw material, the density of the paper is 4 to 4 in comparison with the case of only the calcium carbonate of Comparative Example 1. It was 6% lower and bulkiness was recognized. Further, the whiteness increased by 0.8 to 1.0 point, the opacity increased by 0.7 to 0.8 points, the breaking length increased by 27%, and the decrease in paper strength due to the addition of filler was small.

When the heat-dried calcium carbonate / silica composite particles of Example 3 were pulverized with a sand grinder, the density of the paper was reduced by 10%, and extremely high bulkiness was observed. Furthermore, the whiteness increased by 1.6 points, the opacity increased by 1.3 points, the tear length increased by 47%, and the paper strength decreased very little.

In the calcium carbonate / titanium dioxide / silica composite particles of Example 4, the paper density was reduced by 6%, and high bulkiness was recognized. Furthermore, since titanium dioxide was compounded, both whiteness and opacity increased by 2.5 points, the tear length increased by 47%, and the paper strength decreased very little.

In addition, in the case where the pH of the final reaction solution of Comparative Example 2 is in the acidic region of 5.7, the ratio of calcium carbonate to silica is 25:75, the ratio of silica is higher than calcium carbonate, and the calcium carbonate is white carbon. Since it was a completely covered sphere, it was not bulky, and whiteness decreased by 0.4 points and opacity decreased by 1.4 points. The tear length is almost unchanged.

In the kaolin-silica composite particles produced using the kaolin of Example 5 as a raw material, the paper density was reduced by 8% compared to the case of only the kaolin of Comparative Example 3, and high bulkiness was recognized. Furthermore, the whiteness increased by 1.2 points, the opacity increased by 1.9 points, the tearing length increased by 40%, and the paper strength decreased little.

When the heat-dried kaolin-silica composite particles of Example 6 were pulverized with a sand grinder, the paper density was reduced by 12% as compared with the case of only the kaolin of Comparative Example 3, and high bulkiness was observed. It was. Furthermore, the whiteness increased by 1.5 points, the opacity increased by 2.4 points, the tear length increased by 47%, and the paper strength decreased little.

Further, in the case where the pH of the final reaction solution of Comparative Example 4 is in the acidic region of 6.5, the ratio of kaolin to silica is 20:80, the ratio of silica is higher than kaolin, and kaolin is completely white carbon. Since it was a covered sphere, it was not bulky and the whiteness decreased by 0.2 points and the opacity decreased by 1.0 points. The tear length is almost unchanged.

As shown in Table 2, when the particle size of the composite particles of Examples 7 to 10 is 10 microns or more, the bulkiness, whiteness and opacity are high, the decrease in paper strength is small, and the filler yield is 43 to 47%. It was very expensive.
[Effect of the invention]

After adding and dispersing inorganic fine particles in an alkali silicate aqueous solution to prepare a slurry, while heating and stirring, the liquid temperature is kept in the range of 60 to 100 ° C., and an acid is added to form a silica sol. By adding the inorganic fine particles / silica composite particles formed by adjusting to the range of low to low alkalinity to the paper, a filler-added paper having the following characteristics was obtained.
1) Light and thick paper with high bulkiness can be obtained 2) Optical properties such as whiteness and opacity are excellent 3) Paper strength (breaking length, tear strength) is excellent while being bulky 4) High yield of filler.

Claims (3)

After adding inorganic fine particles to the alkali silicate aqueous solution and dispersed to prepare a slurry, stirring under heat, and the liquid temperature kept in the range of 60 to 100 [° C. to produce a silica sol by the addition of sulfuric acid, pH of the final reaction mixture To a range of 8-11 ,
A method for producing an aggregate of inorganic fine particles and silica composite particles , comprising:
The aggregates are, the silica sol Le thinly adhered to the entire surface of the inorganic fine particles, with the crystal growth of the silica sol, that allow binding to occur between the silica sol particles in the silica sol particles and other inorganic fine particle surface on the surface of the inorganic fine particles is formed by a 50% volume average particle diameter of the aggregates was measured by a laser diffraction / scattering light is 10 to 60 microns, the method described above. The method according to claim 1, wherein the inorganic fine particles are calcium carbonate, talc, clay, kaolin, calcined kaolin, titanium dioxide, aluminum hydroxide alone or a mixture of two or more. The method according to claim 1, wherein the concentration of the alkali silicate aqueous solution is 3 to 10% by weight (in terms of SiO ₂ ).