JPH0629308B2 - Manufacturing method of paper filler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing a filler for paper. Specifically, when blended with raw pulp to make paper, a urea-formaldehyde cross-linked resin that can improve optical properties such as whiteness, opacity, and opacity after printing, and has a high yield rate during paper making The present invention relates to a method for producing a paper filler comprising

(Prior Art and Problems) Conventionally, in the paper manufacturing industry, inorganic fillers such as kaolin, talc, and white carbon have been widely used to improve optical properties such as whiteness and opacity of paper. . However, these inorganic fillers do not sufficiently improve the whiteness and opacity of the paper, and because they have a low fixing property to the paper, they easily generate paper dust, and easily damage the printing equipment due to abrasion. There is a drawback of.

On the other hand, it is also known to use a urea-formaldehyde cross-linking resin as an alternative to the inorganic filler. For example,
JP-B-51-23601 discloses an insoluble, infusible and non-porous fine urea formaldehyde polymer having a urea formaldehyde molar ratio of 1: 1 to 1: 2 and a BET specific surface area of 5 to 100 m ² / g. Is used as a filler for paper. However, the conventionally known filler composed of a urea-formaldehyde cross-linking resin has a large effect of improving whiteness and opacity of paper, but has a problem that the effect of improving opacity after printing is not sufficient.

Generally, when the amount of the filler added is increased, the degree of improvement in the above optical properties is also increased, but on the other hand, not only the strength of the paper is decreased, but also with the conventional filler, the whiteness, opacity and post-printing non-printing property of the paper are reduced. It was difficult to improve the transparency in a balanced manner.

Further, the urea-formaldehyde cross-linking resin as a filler for paper has a particle size of 0.1 to 1.0 from the viewpoint of good dispersion in paper.
It is suitable to use secondary particles having an average particle diameter of 1 to 10 μm formed by agglomeration of extremely fine particles (primary particles) having an average particle diameter of 1 to 10 μm. It has been difficult to manufacture a filler in this range and having a high ratio of particles having a particle size of 1 to 10 μm continuously and in high yield on an industrial scale.

In addition, the conventional filler has a low yield rate at the time of papermaking, and particularly in a papermaking machine such as a twin-wire papermaking machine that applies a high shear force at a high speed, it is difficult to uniformly and highly perform the filling.

(Means for Solving Problems) The inventors of the present invention have improved a conventionally known paper filler composed of a urea-formaldehyde cross-linking resin to improve optical properties of a paper product, particularly opacity after printing. As a result of diligent studies on a method for industrially producing a filler that can be used, by combining specific means, without mixing other components, urea and formaldehyde alone, the optical properties of paper products The inventors achieved the present invention by knowing that a filler having an excellent improvement effect and a high yield rate during papermaking can be obtained.

That is, the gist of the present invention is that urea and formaldehyde are
The molar ratio of formaldehyde to urea is 1: 1.9-
1: 2.4, a mixture mainly composed of an initial condensate obtained by reacting in a neutral to weakly alkaline aqueous medium, carboxymethylcellulose or a salt thereof, 5 to 15 times by weight of water and an acidic catalyst based on the initial condensate. Was stirred so that the whole became uniform, and then the stirring was stopped for at least 5 minutes to react at 35 to 65 ° C. to produce a solid urea-formaldehyde cross-linking resin, and then to obtain the obtained solid urea-formaldehyde cross-linking resin. A method for producing a filler for paper, which comprises pulverizing an aqueous slurry containing s by using an ultrasonic dispersion pulverizer.

Next, the present invention will be described in detail. In the following description, "%" means "% by weight" unless otherwise specified.

The paper filler produced by the method of the present invention described in detail below is composed of finely pulverized urea formaldehyde crosslinked resin particles, and has a linseed oil absorption of 700% or more. According to the study by the present inventors, a filler comprising conventionally known urea-formaldehyde crosslinked resin particles is
Both have a small amount of linseed oil absorption. The amount of linseed oil absorption is one measure of the absorbability of the printing ink, and it is considered that the larger the linseed oil absorption, the greater the absorbability of the printing ink. Therefore,
If the same kind of filler is used, paper made from paper with a large mania oil absorption will be better absorbed by the filler in which the printing ink is dispersed in the paper, so optical properties such as opacity after printing Is expected to improve. Particularly preferred is a urea-formaldehyde crosslinked resin having a linseed oil absorption of 800% or more.

The paper filler produced by the method of the present invention is also a finely pulverized urea formaldehyde crosslinked resin. Usually, it is pulverized so that the average particle diameter is 10 μm or less. It is preferably pulverized to have an average particle size of 2 to 9 μm, particularly preferably 3 to 6 μm.

Generally, finer pulverization improves the performance as a filler, but on the other hand, pulverization cost increases. The filler composed of the urea-formaldehyde cross-linked resin obtained by the method of the present invention is an aggregate substantially composed of primary particles having a particle size of 1 μm or less, that is, secondary particles. That is, the filler obtained by the method of the present invention is such that when the filler particles are dispersed so as not to overlap each other and observed by a scanning electron microscope, the area occupied by primary particles larger than 1 μm is the maximum projected area of the secondary particles. It is preferably 5%, usually 2% or less. Particularly preferred is a filler consisting of secondary particles which do not contain primary particles larger than 1 μm.

In addition to this, the filler of the present invention has excellent filtration characteristics as described later, and therefore has a high yield rate during papermaking.

The paper filler having these properties is produced by the method described below according to the method of the present invention.

First, an initial condensate of urea and formaldehyde obtained by a specific method is used.

That is, a mixture of urea and formaldehyde in a molar ratio of 1: 1.9 to 1: 2.4 is added in an aqueous medium containing carboxymethyl cellulose (abbreviated as CMC) or a salt thereof, for example, an alkali metal salt such as sodium salt. , Neutral to weakly alkaline, about 0.5 at a temperature of about 50 to 90 ℃
The precondensate is produced by reacting for ~ 2 hours.

The urea and formaldehyde used in this case may be those for ordinary industry. Usually 30 as formaldehyde
An aqueous solution of ˜55% strength is used, but it is also possible to use an aqueous solution of a polymer of formaldehyde such as paraformaldehyde.

The molar ratio of urea to formaldehyde is 1: 1.9 to 1: 2.4.
It is necessary to be in the range of, especially 1: 2.0 to 1: 2.2
Is preferred. When the molar ratio of formaldehyde to urea is less than 1.9, the linseed oil absorption of the filler obtained by the subsequent treatment of the invention does not reach 700%, and when it is greater than 2.4, the The filler yield obtained by the treatment of the present invention is reduced, and the linseed oil absorption does not reach 700%. As a result, when these are used as a filler for papermaking, the desired effect of improving opacity after printing cannot be obtained.

CMC or a salt thereof is usually used in an amount of about 0.1 to 10% with respect to the total amount of urea and formaldehyde. CM
C or its salt regulates the particle size of the primary particles of the crosslinked resin to be generated when polycondensing this initial condensate in the presence of an acidic catalyst to cause a crosslinking reaction so that large primary particles are not generated. Play an action. Therefore, CMC or a salt thereof may be added later to the initial condensate produced by the above reaction, instead of being added during the production of the initial condensate as described above.

The liquidity at the time of production of the initial condensate is maintained at neutral to weakly alkaline, for example, about pH 7 to 8 by adding caustic alkali or alkali carbonate.

The aqueous solution containing the initial condensate obtained by the above method,
Then, the amount of water is adjusted to a predetermined range, and if necessary, CMC or a salt thereof is added, and then an acidic catalyst is added and polycondensation is performed at a temperature within a predetermined range to obtain a urea-formaldehyde crosslinked resin as a solid. .

Examples of the acidic catalyst include mineral acids such as sulfuric acid and hydrochloric acid, formic acid,
Organic acids such as acetic acid, paratoluenesulfonic acid, and sulfamic acid can be mentioned.

The amount of water used for the polycondensation reaction of the initial condensate needs to be 5 to 15 times the amount of the initial condensate. The amount of the initial condensate is equal to the total amount of urea and formaldehyde used in the reaction, excluding those that have escaped to the outside of the system. When the amount of water is less than 5 times, the solidified product produced by polycondensation becomes hard and the linseed oil absorption of the filler obtained by the subsequent treatment of the present invention does not reach 700%. Further, when the amount of water is more than 15 times, the solidified product generated by polycondensation is too soft, and the linseed oil absorption of the filler obtained by the subsequent treatment of the present invention is similarly 700%. Not reach

As a result, in any case, when the paper is made using these as a filler, the desired effect of improving the opacity after printing cannot be obtained.

The temperature of the polycondensation reaction of the initial condensate must be 35 to 65 ° C., preferably 40 to 60 ° C., and the temperature lower than 35 ° C. can be obtained by the subsequent treatment of the present invention. The linseed oil absorption of the filler does not reach 700%. Also, 65 ℃
At higher temperatures, the yield of filler obtained by subsequent treatment is reduced and the linseed oil absorption does not reach 700%.

In the polycondensation reaction, an acidic catalyst is added to an aqueous solution containing 5 to 15 times the amount of water relative to the initial condensate, CMC or a salt thereof and the initial condensate, and the mixture is stirred so that the whole becomes uniform, and the temperature is 35 to 65 ° C. Hold it at. At this time, stirring should be carried out in the shortest possible time, and after stopping stirring, at least 5
It is necessary not to stir for a minute. That is, when an acidic catalyst is added and stirred, the initial condensate solidifies into a gel, but the stirring is stopped before solidifying into a gel, and the polycondensation reaction is independently performed in each part of the gel solid for at least 5 minutes. Make progress.

As a result, the polycondensation reaction in each part proceeds only with the reaction agent present in that part, and a crosslinked resin in which primary particles smaller than 1 μm are bonded is produced. After the lapse of 5 minutes, stirring may be carried out, but usually the reaction is completed as it is without stirring. The reason why the crosslinked resin has a structure consisting of such an aggregate of primary particles is unknown, but the polycondensate formed with the progress of the polycondensation reaction from the aqueous solution of the initial condensation product precipitates to form primary particles, It is considered that these are aggregated in the process of further growth to generate secondary particles. Since the skeletal structure of such primary particles-secondary particles is formed relatively quickly, it is considered that the structure of the crosslinked resin produced does not be affected even if stirring is restarted after 5 minutes.

In the present invention, the solidified product of the urea-formaldehyde cross-linked resin obtained as described above is roughly crushed in advance and water is added to the slurry to have a slurry concentration of 3 to 10%, preferably about 4 to 8% (average particle size 50 ˜100 μm), and then pulverization treatment using an ultrasonic dispersion pulverizer is required.

The ultrasonic dispersion type crusher belongs to a siren type converter in classification and converts mechanical energy into ultrasonic waves.
As a dispersing mechanism, for example, as shown in FIG. 1, a dispersing type crusher having two or more stages of comb-shaped concentric rings, a generator including a rotor and a stator.

When the ultrasonic dispersion type pulverizer is applied to the pulverization process of the slurry by the mechanism described above, shearing force and ultrasonic waves are generated by the mechanical action of the motion of the rotor and the stator, and the ultrasonic waves cause cavitation. generate. Pulverization is performed by the synergistic effect of the shearing force and cavitation, and urea formaldehyde crosslinked resin particles having an average particle diameter of 10 μm or less, particularly about 2 to 9 μm, which is suitable as a filler for paper, can be obtained.

Moreover, since the ultrasonic dispersion type pulverizer also has a pumping action, it can be continuously operated, and it is industrially advantageous to produce the particulate urea-formaldehyde crosslinked resin as a filler for paper.

One of the features of the particulate urea-formaldehyde crosslinked resin obtained by pulverizing with an ultrasonic dispersion pulverizer according to the method of the present invention is that it has excellent filtration characteristics.

As the filtration characteristics, the filtration rate and the coverage of ultrafine particles are important, but the urea-formaldehyde crosslinked resin particles obtained by the method of the present invention have a high filtration rate and the coverage of ultrafine particles. Is small, which results in a high yield rate at the time of paper making, and accordingly, the optical characteristics of the paper product can be improved by using a relatively small amount.

(Examples) Hereinafter, the present invention will be described in more detail with reference to Examples.
The present invention is not limited to the following examples unless it exceeds the gist. In the description of Examples and Reference Examples, "%" and "parts" means "% by weight" and "parts by weight" excluding evaluation of paper.

Further, in the present invention, the amount of linseed oil absorption, the particle size of the crosslinked resin, the yield of the crosslinked resin, the filler yield, the filtration rate, and the eye retention of the ultrafine particles were measured by the following methods.

Linseed oil absorption: Measured according to the method of JIS K5101 (1978). That is,
Approximately 1 g of the dried sample is placed on a glass plate, boiled linseed oil is added drop by drop to the center of the sample from a buret, and the mixture is kneaded with a spatula on the glass plate. The amount of boiled linseed oil added until the sample becomes fluid with one drop of boiled linseed oil is read, and the oil absorption (%) is calculated by the following formula. Perform all operations at room temperature (about 25 ° C).

G = H / S × 100 [G: oil absorption amount (%), H: linseed oil addition amount (ml), S: sample mass (g)] In addition, the sample is prepared by making it wet. This is poured into 10 times volume of methanol and stirred at room temperature for 30 minutes.
Then, suction filtration is performed, and the filter cake is dried at 40 ° C and 750 mmHg for 8 hours.
This is done by drying for an hour.

Particle diameter of crosslinked resin: Measured by a light transmission method using Stokes' law.
That is, the crosslinked resin particles are dispersed in distilled water, and the change in the turbidity due to the sedimentation of the particles is measured as the change in the light transmittance, and the weight cumulative distribution curve (one logarithm) is calculated from this measured value, and 50% of that is obtained.
The diameter is defined as the average particle diameter. In calculating the weight cumulative distribution curve, the respective values of the true specific gravity of the sample: 1.45, the specific gravity of distilled water (20 ° C): 0.998, and the viscosity: 1.005 are used.

In the following examples, as a measuring device, Micron Fort Sizer (SKC-2000S) manufactured by Seishin Co., Ltd. was used, and the maximum particle size was 50 μm. 50 ml of distilled water and 100 mg of sample are put into a beaker and subjected to an ultrasonic disperser (output 20 W, frequency 40 KHz) for 5 minutes for dispersion treatment. When this was placed in the measuring cell, the absorbance [-log (I ₀ /
I)] was diluted to 1.3 to 1.4 with distilled water to prepare a measurement sample.

Yield of crosslinked resin: The weight of the wet cake that has undergone the steps of pulverization-neutralization-filtration-washing with water is K, and the yield when the wet cake is dried at 120 ° C for 2 hours is A%. The total amount of formaldehyde and urea used is B, and is calculated by the following formula.

Yield (%) of crosslinked resin = K × A / B Filler yield: The nitrogen content (weight) in the paper and papermaking slurry is measured by the Kjeldahl method and calculated by the following formula.

Filler yield = nitrogen in paper / nitrogen in slurry × 100 Filtration speed: The slurry after pulverization is adjusted so that the solid content concentration is 5%. 200 g of the above slurry is put into a filter device (filtering area: 9.62 cm ² , filter cloth: 400 mesh stainless plain woven wire mesh), constant pressure filtration is performed with an aspirator while maintaining a water column differential pressure of 80 mm, and it is necessary for the amount of filtrate to reach 100 ml. Let the filtration rate be time.

Eye leak rate: A sample is taken from the filtrate when the amount of the filtrate reaches 100 ml in the above measurement of the filtration rate, and the light transmittance is measured by the light transmission method. On the other hand, a relationship curve between the concentration and the light transmittance is prepared using a urea-formaldehyde cross-linked resin having an average particle size of 4.7 μm, and the light transmittance obtained above is applied to this relationship curve to calculate the solid concentration of the sample. .

Eye leak rate (%) = solid amount (g) / 10 × 100 in 100 ml of filtrate As a transmittance measuring device, SPECTROPHOTOMETER 101 type, a product of Hitachi, Ltd., was used and the wavelength was measured at 450 μm.

Example 1 41.0 parts (0.5 mol) of 37% strength formaldehyde aqueous solution
And 43.1 parts of water are mixed, and 0.7 part of CM is added to this while stirring.
C sodium salt [CMC Daicel 1193] was dispersed, and caustic soda was further added to adjust the pH to 7.4 (BTB test paper). Then, 15.2 parts (0.25 mol) of urea was added to raise the temperature, and the temperature was increased to 70 ° C at 1.
The reaction was carried out for 5 hours to obtain an aqueous solution of the initial condensate. The pH of the obtained aqueous solution of the initial condensate as measured by BTB test paper was 7.2.

100 parts of the aqueous solution of the initial condensate obtained above was heated to 40 ° C.,
When 104 parts of 3.6% sulfuric acid at the same temperature was rapidly added and mixed, it became cloudy after 30 seconds and solidified after 45 seconds. The amount of water coexisted during the solidification reaction was 5.6 times the amount of the initial condensate.

After mixing with sulfuric acid, the mixture was allowed to stand at 40 ° C for 30 minutes, then the solidified product was collected and crushed, and 404 parts of water was added to stir to make a slurry, which was then neutralized with a caustic soda aqueous solution to obtain a slurry concentration of about 5.0.
% Slurry was obtained.

The above slurry was finely pulverized by passing it through an ultrasonic dispersion type pulverizer (“Ebara Milder” manufactured by Ebara Corporation) having a three-stage siren generator.

The slurry was suction-filtered with a glass filter, the obtained cake was again put into 250 parts of water and stirred, and then the cake collected by filtration was dried in an oven at 120 ° C. for 2 hours. It was 6.0 μm. The particle size of the primary particles of this product was 0.1 to 0.3 μm when observed with a scanning electron microscope.

The cake obtained before drying was mixed with methanol, stirred for 30 minutes, suction-filtered with a glass filter, and the residue was dried under reduced pressure at 40 ° C for 8 hours. It was 1100%.

Further, the concentration of the finely pulverized slurry was adjusted to 5%, and the filtration rate and the clogging rate were measured. The results are shown in Table 1, respectively.

Example 2 40.3 parts (0.5 mol) of 37% aqueous formaldehyde solution
And 43.3 parts of water are mixed, and 0.7 part of CMC sodium salt [manufactured by Daicel Corp. CMC Daicel 1193] is added to the mixture while stirring.
Disperse and add caustic soda to pH 7.4 (BTB test paper)
Then, 15.7 parts (0.26 mol) of urea was added and heated, and the mixture was reacted at 70 ° C. for 1.5 hours to obtain an aqueous solution of an initial condensate. PH of the obtained aqueous solution of initial condensate measured by BTB test paper
Was 7.0.

100 parts of the aqueous solution of the initial condensate obtained above was heated to 35 ° C.,
When 136 parts of 3.0% sulfuric acid at the same temperature was rapidly added and mixed, it became cloudy after 30 seconds and solidified after 65 seconds. The amount of water coexisted during the solidification reaction was 6.6 times the amount of the initial condensate.

After mixing with sulfuric acid, the mixture was allowed to stand at 35 ° C for 30 minutes, then the solidified product was collected and crushed, 124 parts of water was added and stirred to make a slurry (slurry concentration: about 8.5%), and then a caustic soda aqueous solution was added. I made it

The above slurry was finely pulverized by passing through an ultrasonic dispersion type pulverizer [“Ebara Milder” manufactured by Ebara Corporation).

The average particle size of the particles in this slurry is 8.0 μm,
The particle size of the primary particles was 0.1 to 0.3 μm, and the linseed oil absorption was 900%.

Further, the concentration of the finely pulverized slurry was adjusted to 5%, and the results of measuring the filtration rate and the clogging rate are shown in Table 1.

Examples 3 to 4 and Comparative Examples 1 to 4 The molar ratio (F / U) of urea and formaldehyde in the production of the initial condensate in Example 1, the ratio of the initial condensate to water in the polycondensation reaction (hereinafter (Bath ratio), temperature and time were changed as shown in Table 1, and the same treatment as in Example 1 was carried out to obtain the average particle diameter, primary particle diameter, linseed oil absorption amount, and filtration shown in Table 1, respectively. A filler of speed and occlusion rate was obtained.

In addition, when the CMC sodium salt was not used in the production of the initial condensate in Example 1, and the same treatment as in Example 1 was performed otherwise (Comparative Example 1); polycondensation of the initial condensate in Example 2 When the treatment was performed according to Example 2 except that the temperature during the reaction was 75 ° C. (Comparative Example 2); the molar ratio of urea and formaldehyde (F / U) is 1.5 and the bath ratio in the polycondensation reaction is 4.
When the treatment was carried out according to Example 1 (Comparative Example 3) except 0, the time during which stirring was stopped during the polycondensation reaction of the initial condensate in Example 2 (= static time) ) Was set to 3 minutes, the average particle size, primary particle size, linseed oil absorption and eyes of the urea formaldehyde crosslinked resin particles obtained when the treatment was carried out according to Example 1 (Comparative Example 4) The leakage rate is shown in Table 1.

Comparative Examples 5 to 8 Crusher "Abara Milder" used in Example 2
In place of the above, a medium stirring type pulverizer [“Attritor” Mitsui Miike Seisakusho Model MA-01B] was used, and the same treatment as in Example 2 was performed except that the pulverization treatment was performed at 250 rpm for 15 minutes using a ceramic medium. (Comparative Example 5); Example 2
In place of the crusher "Ebara Milder" used in the above, a "Pipeline Homomixer" special machine Kako Kogyo Co., Ltd. 2S type] was used for fine pulverization, and no crushing was performed. When treated in the same manner (Comparative Example 6);
Crusher "Ebara Milder" used in Example 1
In the same manner as in Example 2 except that a colloid mill [“Mycoloyder” manufactured by Tokushu Kika Kogyo Co., Ltd.] was used (Comparative Example 7); the pulverization used in Example 2 Instead of the machine "Ebara Milder", use [West Germany Stefan's "universal high speed cutter"] at 3000 rpm 1
The primary particle size, average particle size, and linseed oil absorption of the filler composed of urea-formaldehyde crosslinked resin particles obtained when treated in exactly the same manner as in Example 2 (Comparative Example 8) except that fine pulverization treatment was carried out for 5 minutes. Table 2 shows the amount, the ratio of the particle size of 1 to 10 μm, the leakage rate, and the filtration time.

Reference Example Preparation of paper quality evaluation paper: 1% slurry of pulp with a beating degree of 300 ml (csf) containing 15 parts of NBKP, 25 parts of TMP, 30 parts of RGP and 30 parts of DIP 10
Examples 1 to 4 in which the concentration of 5.0% was adjusted to 00 parts in advance.
And 10 parts of an aqueous dispersion of the filler obtained in Comparative Examples 1 to 8 (corresponding to 5% of filler for dry pulp) was added and stirred for 2 minutes, and subsequently 3 parts of 10% aluminum sulfate aqueous solution was added to further add 2 parts. The mixture was stirred for a minute to prepare a filler-containing papermaking slurry.

Next, this is a TAPPI square sheet machine (25 cm x 25 c
m) papermaking, press dehydration at 3.5 kg / cm ² and drying for 3 minutes with a rotary dryer with a surface temperature of 105-110 ° C, calendering at a linear pressure of 40 kg / cm, and then relative Paper for paper quality evaluation was prepared by performing seasoning for 24 hours in a humidity and humidity chamber at a constant temperature of 65% and a temperature of 20 ° C.

Evaluation of paper quality: Various optical properties of the paper prepared above were examined.
That is, the whiteness is JIS P-8123 and the opacity is J
According to ISP-8138, and the opacity after printing is J. TA
It was measured according to PPI. No. 45-84. The yield of filler was measured by Kjeldahl nitrogen analysis.

Table 3 shows the whiteness, opacity, opacity after printing, and the filler retention rate of the paper products prepared by blending the fillers obtained in Examples 1 to 4 and Comparative Examples 1 to 8.

The case where no filler was added (blank) was also described as Comparative Example 9.

(Effects of the Invention) As shown in Reference Examples, when the paper filler produced by the method of the present invention is used for papermaking, the obtained paper has excellent whiteness and opacity, and The filler dispersed in the ink sufficiently absorbs the printing ink, resulting in a marked improvement in the opacity of the paper after printing. Further, since it has excellent filtration characteristics, it has a high yield rate and is efficient.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining the structure of an ultrasonic dispersion type pulverizer used in the method of the present invention, in which 1 is a stator,
2 is a rotor.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Fumiyoshi Karasawa 34, Onahama Takayama, Iwaki, Fukushima Prefecture, Nippon Kasei Co., Ltd. (72) Haruo Kusari, 34, Onahama Takayama, Iwaki, Fukushima Prefecture

Claims (1)

[Claims] 1. An initial condensate obtained by reacting urea and formaldehyde in a neutral to weakly alkaline aqueous medium at a molar ratio of formaldehyde to urea of 1: 1.9 to 1: 2.4, carboxymethyl cellulose or a salt thereof. , A mixture mainly consisting of 5 to 15 times by weight of the initial condensate and water and an acidic catalyst is stirred so that the whole becomes homogeneous, and then the stirring is stopped for at least 5 minutes, and then the reaction is carried out at 35 to 65 ° C. To produce a solid urea-formaldehyde cross-linking resin, and then pulverize the obtained aqueous slurry containing the urea-formaldehyde cross-linking resin using an ultrasonic dispersion pulverizer to produce a paper filler. .