JPH11349625A - Preparation of water absorbent and water absorbent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a water absorbent wherein the water retention and the absorption under a load are balanced at a high level by improving the surface- crosslinking process. SOLUTION: A solution containing a crosslinking agent in the state of droplets with an average particle size of 200 microns or smaller is sprayed onto and mixed with a water-absorbing resin particle which is not surface- crosslinked and has an average particle size of from 200 to 500 μm, wherein the content of particles of 100 μm or smaller is 3 wt.% or lower and wherein the water retention against physiological saline is 50 g/g or larger, and heated to obtain a water absorbent wherein the water retention and the absorption under a load are well balanced and high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a water absorbing agent and a water absorbing agent. More specifically, the present invention relates to a method for producing a water-absorbing agent having a high water-holding power and excellent absorption performance under load, and a water-absorbing agent.

[0002]

The water-absorbing resin or the water-absorbing agent used in absorbent articles such as disposable diapers has a water retention capacity when the weight of the wearer is not increased and a load under the weighted state. It is desired that both of the absorption amounts be balanced and at a high level. In response to this request,
As a method for increasing the amount of absorption under a load, many so-called surface cross-linking methods for cross-linking the vicinity of the surface of the water-absorbent resin particles with a cross-linking agent have been proposed.

The important points in this surface cross-linking process are to control the degree of penetration of the cross-linking agent (the depth of cross-linking from the particle surface) and to uniformly cross-link the water-absorbent resin particles. Techniques for controlling the degree of penetration of the cross-linking agent include a method in which cross-linking agents having different solubility parameters are used in combination (for example, JP-A-6-184320).
And a method for controlling the water content of the water-absorbent resin particles. As a technique for uniformly cross-linking the respective particles, a method in which the inner wall surface of a mixer is made of a specific material and an aqueous cross-linking agent liquid is added and mixed under high-speed stirring (for example, see Japanese Patent Application Laid-Open No.
14738); a method of crosslinking the surface of a polymer powder having a specific range of average particle size and particle size distribution (for example, JP-A-2-196802, JP-A-9-309)
No. 916) has been proposed.

[0004]

However, in the above-mentioned surface cross-linking technique, since the amount of water retention is greatly reduced as the amount of absorption under load is increased, there is a limit in balancing the two at a high level. It was not always satisfactory. . Conventionally, the disposable diapers for infants have been the main use of the water-absorbent resin, so that the above-described water-absorbent resin can also be used. However, as the application of water-absorbent resin to adult paper diapers progresses, the ability to efficiently retain urine excreted in large quantities at a time and the ability to absorb under high loads, such as when the weight of an adult is added, are increasing. However, the appearance of a water-absorbing resin or a water-absorbing agent at a high level in a balanced manner is desired.

[0005]

Means for Solving the Problems The present inventors have intensively studied to improve the surface cross-linking process to obtain a water-absorbing agent in which both the amount of water retention and the amount of absorption under load are balanced at a high level. As a result, the present invention has been achieved.

That is, the present invention provides an average particle size of 200 to 50.
0 micron, content of particles less than 100 micron is 3
% By weight or less, and the water retention capacity with respect to physiological saline is 50 g.
/ G or more of particles of the water-absorbent resin (A1) which has not been surface-crosslinked and spray-mixed with the crosslinking agent-containing liquid (B) in the form of droplets having an average particle diameter of 200 μm or less, followed by heating. A method for producing a surface-crosslinked water-absorbing agent; and the vicinity of the surface of the water-absorbent resin (A1) particles is crosslinked by spraying the crosslinking agent-containing liquid (B) in the form of droplets having an average particle diameter of 200 μm or less. Water absorbing agent (A2), wherein the water absorbing resin (A1)
Difference between the water holding capacity of the water absorbent and the water holding capacity of the water absorbing agent (A2) is 20 g / g
And a water-absorbing agent satisfying the following requirements. Water absorption capacity for physiological saline is 35 g / g or more; absorption capacity for physiological saline under 20 g / cm ² load is 25 g / g or more; absorption capacity for physiological saline under 40 g / cm ² load is 15 g / g or more; Absorption amount to physiological saline under a load of 60 g / cm ² is 10 g / g or more.

[0007]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, a water-absorbent resin (A
Examples of 1) include, for example, a crosslinked product of a partially neutralized salt of polyacrylic acid, a partially neutralized salt of a self-crosslinkable polyacrylic acid, a crosslinked product of a partially neutralized salt of a starch-acrylic acid graft copolymer, and a starch-acrylonitrile graft polymer. Combined hydrolyzate, saponified vinyl acetate-acrylate copolymer, cross-linked acrylate-acrylamide copolymer, cross-linked polyacrylamide or hydrolyzate thereof, acrylic acid and 2-acrylamide-2-methylpropane One or more water-absorbing resins such as a crosslinked product of a sulfonic acid copolymer salt, a crosslinked product of an isobutylene-maleic anhydride copolymer salt, and a crosslinked carboxymethylcellulose salt are exemplified.

In the above, the salt is a sodium salt,
Salts such as potassium salt, ammonium salt and amine salt (alkylamine salt such as methylamine and trimethylamine; alkanolamine salt such as triethanolamine and diethanolamine) are included. Preferred salts are sodium and potassium salts.

[0009] Among the above, preferred water-absorbing resin (A1)
Considering the absorption performance of the finally obtained water-absorbent resin, a crosslinked polymer of an ethylenically unsaturated monomer having acrylic acid and / or a salt thereof as a main constituent unit (a partially neutralized salt of polyacrylic acid) Crosslinked products, neutralized salts of self-crosslinked polyacrylic acid, crosslinked products of partially neutralized starch-acrylic acid graft copolymer salts). Particularly preferred resin is a polymerizable component comprising a radical polymerizable monomer having an acid group and / or a radical polymerizable monomer capable of forming an acid group by hydrolysis, a crosslinking agent and, if necessary, a grafting agent, and if necessary, hydrolysis. This is a particulate water-absorbing resin obtained by neutralizing, drying and pulverizing. More preferably, acrylic acid is used as a radical polymerizable monomer, and two or more radical polymerizable double bonds are used as a cross-linking agent in that a resin having excellent absorption performance and a small amount of a water-soluble component is obtained. It is a water-absorbing resin obtained by polymerizing using a compound and then neutralizing.

The method for producing the water-absorbent resin (A1) is not particularly limited, and may be an aqueous solution polymerization method using a radical polymerization initiator, a suspension polymerization method, a reversed-phase suspension polymerization method, a photoinitiated polymerization method, an electron beam polymerization method, or the like. It is manufactured by a conventionally known polymerization method such as a method of initiating polymerization by ultraviolet rays or the like. Preferably, the aqueous solution adiabatic polymerization method is relatively inexpensive in polymerization equipment and does not require temperature control during polymerization.

The shape of the particles of (A1) is not particularly limited, and may be any of crushed, scaly, pearl, granulated, etc., and may be fibrous materials such as pulp for use in disposable diapers. Crushed particles obtained by drying and pulverizing are preferred in that they have good entanglement and are less likely to fall off from the fibrous material.

In the method of the present invention, the particle size of (A1) and the content of fine particles are important factors. Regarding the particle diameter of (A1), the average particle diameter is usually 200 to 500 microns, preferably 250 to 450 microns. When the average particle size is less than 200 microns, the uniform mixing property between the droplets of the crosslinking agent-containing liquid (B) and (A1) is reduced, and the reduction in water retention after surface crosslinking may be increased. on the other hand,
Even in the case where the average particle diameter exceeds 500 microns, the uniform mixing property of the droplets of the crosslinking agent-containing liquid (B) and (A) is reduced, and the water retention after surface crosslinking is greatly reduced.
Another problem arises in that the absorption rate is reduced. (A1)
Is also an important factor, and the content of fine particles of 100 microns or less is 3% by mass or less, preferably 2% by mass or less.
% By mass or less. When the content of the particles having a size of 100 μm or less exceeds 3% by mass, the uniformity of the mixture of the droplets of the crosslinking agent-containing liquid (B) and (A1) becomes insufficient. (A) having an average particle size as described above and a fine particle content of 100 microns or less.
In order to obtain 1), a usual method (for example,
The particle size is adjusted by sieving or air classification.

The water-absorbing resin (A1) having no surface cross-linked used in the method of the present invention has a water retention capacity for physiological saline of usually 50 g / g or more, preferably 55 g / g or more. Further, the amount of absorption in physiological saline under a load of 20 g / cm ² is 10 g / g or less. When the water retention of (A1) is less than 50 g / g, even if a resin having a large water absorption under load is obtained by surface crosslinking, the water retention is reduced, so that the balance between the water retention and the absorption under load is reduced. It becomes difficult to obtain an excellent resin.

The water content of the water-absorbent resin (A1) is not particularly limited, but is usually 10% by mass or less, preferably 7% by mass or less. When the water content exceeds 10% by mass, penetration of the crosslinking agent-containing liquid (B) into the inside of the particles of (A1) is promoted, and the degree of penetration of the crosslinking agent into the vicinity of the surface of the particles of (A1) is controlled. It becomes difficult to perform efficient surface crosslinking.

In the present invention, as the crosslinking agent-containing liquid (B), a mixed aqueous solution comprising a crosslinking agent, water and an organic solvent is usually used. As the crosslinking agent, a crosslinking agent having two or more groups capable of reacting with the functional group of (A1) is usually used. As the crosslinking agent, for example, ethylene glycol diglycidyl ether, glycerol diglycidyl ether, polyglycidyl compounds such as polyglycerol polyglycidyl ether, glycerin, polyhydric alcohols such as ethylene glycol, ethylene carbonates, polyamide polyamine epichlorohydrin resin and the like Examples thereof include polyamine resins, polyamine compounds such as ethylenediamine, and aziridine compounds. Preferred are polyglycidyl compounds and polyamine resins in that the crosslinking reaction can be carried out at a relatively low temperature. These crosslinking agents may be used alone or in combination of two or more.

The amount of the crosslinking agent to be used for (A1) can be variously changed depending on the kind of the crosslinking agent, the performance target of the obtained water-absorbing agent (A2) and the like, but is usually 0.1% based on the mass of (A1). 005 to 2% by mass, preferably 0.
The content is from 01 to 1% by mass, particularly preferably from 0.05 to 0.5% by mass. If the amount of the crosslinking agent used is less than 0.005% by mass, the degree of crosslinking on the surface is insufficient, and the effect of improving the absorption under load becomes insufficient. On the other hand, when the amount of the cross-linking agent exceeds 2% by mass, the absorption under load can be improved, but the cross-linking of the surface becomes excessive and the water retention decreases, and the absorption under load and the water retention increase to a high level. Makes it difficult to balance.

In the present invention, the amount of water added to (A1) is controlled to control the degree of penetration of the crosslinking agent into the interior of the particles of (A1) and to control the reactivity of (A1) with the crosslinking agent. At the same time, an organic solvent is used in combination. The amount of water used can be variously changed depending on the target performance of the obtained water-absorbing agent (A2), and may be smaller than the conventional surface crosslinking method. 1 to 8% by mass, preferably 2 to 6% by mass
It is. When the amount of water used is less than 1% by mass, the cross-linking agent (A1) does not sufficiently penetrate inside the particles, and the amount of absorption under load, particularly under high load (for example, 60 g / cm)
^The effect of improving the amount of absorption in ² ) becomes poor. On the other hand, if the amount of water used exceeds 8% by mass, the penetration of the cross-linking agent (A1) into the inside of the particles becomes excessive, and although the amount of absorption under load is improved, the water retention is reduced. This makes it difficult to balance the absorption under load and the water retention at a high level.

The type of the solvent used in combination with water is not particularly limited, and a conventionally known solvent can be used, and the degree of penetration of the crosslinking agent into the inside of the particles (A1), the reaction of the crosslinking agent Although it can be appropriately selected and used in consideration of properties and the like, it is preferable to use an organic solvent containing at least one alcohol-based organic solvent having a relative dielectric constant of 25 or more. Examples of such an organic solvent include ethylene glycol, diethylene glycol, propylene glycol, glycerin, and methanol. Such solvents may be used alone or in combination of two or more. The amount of the solvent to be used can be changed depending on the type of the solvent, but is usually 1 to 8% by mass, preferably 2 to 6% by mass, based on the mass of (A1). Also,
The ratio of the solvent to water can also be arbitrarily changed, and is usually 20 to 80% by mass, preferably 30 to 70% by mass on a mass basis.

In the present invention, it is an essential requirement that the average particle size of the droplets of the crosslinking agent-containing liquid (B) be 200 microns or less. It is preferably at most 100 microns, more preferably at most 50 microns, particularly preferably at most 30 microns. If the average particle size of the droplets exceeds 200 microns, the uniform mixing of (A1) and (B) becomes insufficient,
Not only does the decrease in water retention increase, but the effect of improving the absorption performance under load becomes insufficient. Further, the dispersion of the water retention amount and the absorption amount under load increases. As a method for controlling the droplet diameter in (B), a method in which (B) is ejected from a liquid pressurized nozzle, a method in which (B) is ejected from a nozzle, and a method in which air is mixed and sprayed to form a mist, etc. Is mentioned. Preferably, a method in which (B) is ejected from a nozzle and, at the same time, air is mixed and sprayed in that a droplet having a fine average particle diameter can be obtained.

The pressure of the air used as necessary during the mixing of (B) is changed depending on conditions such as the shape of the nozzle, the size of the nozzle, the diameter of the nozzle orifice, the type and viscosity of the crosslinking agent-containing liquid (B), and the flow rate. But usually 0.3-
It is in the range of 3.0 kg / cm ² . The pressure of air is 0.
If it is less than 3 kg / cm ^2, the effect of controlling the droplet diameter in (B) to be small by mixing air is insufficient. On the other hand, when the pressure of the air exceeds 3.0 kg / cm ² , the spray pressure of (B) is too large, causing the scattering of (A1), and conversely, the uniform mixing may be insufficient. In particular, in a case where such a nozzle is installed in a high-speed stirring type mixer to spray (B), excessive pressure of air may impair the stirring effect of (A1) in the high-speed stirring machine.
On the other hand, the crosslinking agent-containing liquid (B) may be supplied in any of a pressurized state and a non-pressurized state, but the pressure in the pressurized state is usually in the range of 0.3 to 3 kg / cm ² . It is preferred that

In the present invention, the mixing of the droplets (A1) and (B) is usually carried out with stirring (A1). It is preferable to spray-mix (B) while high-speed stirring (A1) using a mixer (C) having a stirring shaft with a plurality of paddles. Such a mixer (C) is not particularly limited as long as high-speed stirring and mixing can be performed. However, for example, a turbulizer (manufactured by Hosokawa Micron Corporation) or the like is preferable because a powder and a liquid can be continuously mixed. . The stirring speed during mixing is usually set at a linear speed at the tip of the paddle of 250.
To 3000 m / min, preferably 300 to 2500 m / min. Paddle tip linear velocity is 2
In the case of less than 50 m / min, even if (B) is made into fine droplets and sprayed, since the stirring effect of (A1) is small,
The uniform mixing of (A1) and (B) becomes insufficient. On the other hand, if the linear velocity at the tip of the paddle exceeds 3000 m / min, (A1) is broken by mechanical shearing force due to high-speed stirring, and the content of fine particles may be undesirably increased. Further, an excessive load is applied to the stirrer, which is not a preferable condition for the continuous mixing stirrer.

By using a mixer having a protruding portion on the inner wall surface of the mixer (C), the uniform mixing property is further enhanced.
Although this effect was surprising to the present inventors, the effect of the projecting portion was such that the projecting portion caused the flow of the water-absorbent resin (A1) and / or the mixture of (A1) and (B) in the mixer. Is presumed to be due to the improved mixing property. The shape of the protruding portion is not particularly limited as long as the above effects can be obtained, but is preferably a prismatic or columnar shape.

The size of the projection can be appropriately selected depending on the size of the mixer, the number of paddles, and the throughput per unit time. The height of the projection is preferably 0.1 to 1.0 cm, more preferably 0.2 to 0.6 c.
m. When the height of the projection is less than 0.1 cm, the effect of providing the projection is not exhibited. On the other hand, if the height of the projection exceeds 1.0 cm, it is necessary to increase the gap between the stirring portion and the inner wall of the mixer, and the uniform mixing effect due to the stirring of the mixer (C) is reduced. The upper area of the protruding portion is 0.
It is preferably from 09 to 9 cm ² . More preferably, it is 0.15 to ^2.5 cm ² . Top area is 0.09c
If it is less than m ^2, the effect of providing the projections will be poor. On the other hand, if the upper area exceeds 9 cm ² , there may be a problem that a mixture of (A) and (B) is deposited between the protrusions. May be disturbed.

Regarding the position where the projections are provided, it is preferable that each of the projections is located on the inner wall surface on the extension of the rotating circumferential surface of the paddle. The inner surface of the mixer (C)
From the viewpoint of preventing the mixture of (A1) and (B) from adhering to the inner surface of (C), it is preferable that at least the material of the surface portion other than the protruding portion is a material having low adhesion such as Teflon. More preferably, the surface of the projecting portion is also made of a material having low adhesion such as Teflon. When a material having high adhesiveness, for example, stainless steel is used as the inner surface material, hard aggregates may adhere and accumulate on the inner wall surface during long-term operation, which is not preferable.

The droplets of (B) are sprayed onto the particles of (A1) and mixed to carry out a heating reaction. The heating reaction can be performed by a conventionally known method and apparatus. For example, a rotary dryer, a paddle dryer, a Nauter-type dryer, a rotary kiln, or the like can be used. The conditions for the heating reaction are as follows: the type and amount of the cross-linking agent, and the obtained water-absorbing agent (A2).
Although it can be appropriately selected depending on the performance goal of, for example, when using a polyglycidyl compound as a crosslinking agent,
It is usually carried out by heating at a temperature of 100 to 160 ° C. for 20 to 60 minutes. In the method of the present invention, (B)
A crosslinking reaction accelerator (for example, an organic carboxylic acid compound such as adipic acid and acetic acid; a thiol compound such as mercaptoethanol, thiomalic acid, and mercaptoacetic acid; and a compound such as triethanolamine and triisopropanolamine).
(E.g., a divalent amine compound), the efficiency of the crosslinking reaction can be improved. Further, the water-absorbing agent (A2) obtained by surface cross-linking may be subjected to additional surface cross-linking, and the surface cross-linking may be performed in two stages.

According to the practice of the present invention, the difference between the water retention of the water-absorbent resin (A1) before surface crosslinking and the water retention of the water-absorbing agent (A2) obtained by surface-crosslinking the particles of (A1) is 20 g / g. A method for producing a water-absorbing agent which is the following and satisfies the following requirements: Water absorption capacity for physiological saline is 35 g / g or more; absorption capacity for physiological saline under 20 g / cm ² load is 25 g / g or more; absorption capacity for physiological saline under 40 g / cm ² load is 15 g / g or more; Absorption amount to physiological saline under a load of 60 g / cm ² is 10 g / g or more. Preferably, the water retention amount of the water absorbing resin (A1) and the water absorbing agent (A
This is a method for producing a water-absorbing agent in which the difference in water retention of 2) is 15 g / g or less and satisfies the following requirements. Water absorption capacity for physiological saline is 37 g / g or more, absorption capacity for physiological saline under 20 g / cm ² load is 30 g / g or more, absorption capacity for physiological saline under 40 g / cm ² load is 20 g / g or more, Absorbed amount to physiological saline under a load of 60 g / cm ² is 15 g / g or more.

The method of the present invention is superior to the conventional surface cross-linking technique in terms of uniformity of absorption performance. That is, the crosslinking agent-containing liquid (B) is converted into fine droplets (A)
Since it is a process that can be performed continuously by spray-mixing and heating reaction in 1),
When the obtained water-absorbing agent (A2) is sampled 100 times at intervals of 5 minutes for each 100 g or more, the standard deviation σ of the water retention and the absorption under load is within 3% of the average value. This is a method for producing a water-absorbing agent having excellent properties. Further, even when the surface crosslinking operation is continuously performed for a long period of time, there is a feature that generation of aggregates due to aggregation of particles is extremely small. Further, in the production method of the present invention, since the reaction efficiency of surface crosslinking is improved,
The remaining cross-linking agent used is small and substantially zero.

The water absorbing agent (A) obtained by carrying out the present invention
2) is a water retention amount of the water absorbent resin (A1) before surface crosslinking,
Difference in water retention of water absorbing agent (A2) after surface crosslinking is 20 g / g
And excellent absorption properties satisfying the following requirements. Water absorption capacity for physiological saline is 35 g / g or more; absorption capacity for physiological saline under 20 g / cm ² load is 25 g / g or more; absorption capacity for physiological saline under 40 g / cm ² load is 15 g / g or more; Absorption amount to physiological saline under a load of 60 g / cm ² is 10 g / g or more.

At any stage in the process of the present invention, preservatives, fungicides, antibacterial agents, antioxidants, ultraviolet absorbers, antioxidants, coloring agents, fragrances, deodorants, inorganic fine powders, organic A fibrous substance or the like can be added, and the amount is usually 5% by mass or less, preferably 3% by mass or less based on the obtained water-absorbing agent. Further, the water absorbing agent of the present invention may be further granulated or molded.

[0030]

The present invention will be further described below with reference to examples and comparative examples, but the present invention is not limited to these examples. Water holding capacity for physiological saline, 20 g / cm ² , 40
The absorption amount under a load of g / cm ² or 60 g / cm ² was measured by the following method. Below, unless otherwise specified,%
Indicates mass%.

<Water Retention> 1.000 g of a water-absorbent resin was placed in a tea bag (length 20 cm, width 10 cm) made of a 250-mesh nylon net, and placed in a physiological saline solution (an ion exchange aqueous solution having a NaCl concentration of 0.90%). After being immersed in water for 60 minutes, it is hung for 15 minutes and drained. Then, the whole tea bag is put into a centrifugal dehydrator and centrifugally dehydrated at 150 G for 90 seconds to remove excess water. The increased mass after the centrifugal dehydration was measured and defined as the water retention. <Absorption under load> A plastic cylinder with a 250 mesh nylon mesh attached to the bottom surface (inner diameter 25 mm, height 30 m)
m) into 0.160 g of a water-absorbent resin, and evenly
On this water-absorbent resin is placed a weight of 100 g that moves up and down smoothly in the cylinder with an outer diameter of 25 mm. The load at this time corresponds to about 20 g / cm ² . A plastic cylinder containing a water-absorbent resin and a weight is immersed in a petri dish (diameter: 12 cm) containing 60 ml of physiological saline with the nylon mesh side facing down, and left. The mass of the water-absorbent resin increased by absorbing the physiological saline was measured after 60 minutes, and the value was measured as 1 g of the water-absorbing agent.
It was converted into the value per unit and the absorption amount under a load of 20 g / cm ² was obtained. The absorption amounts under a load of 40 g / cm ² and 60 g / cm ² were 200 g and 300 g of the same outer diameter, respectively.
Is determined by performing the same measurement using a weight of

Production Example 1 of Water Absorbent Resin Acrylic acid (2,000 parts), pentaerythritol triallyl ether (6 parts) as a crosslinking agent, and ion exchange water (5377 parts) were mixed to prepare an aqueous polymerizable monomer solution.
After cooling to ℃, this solution was charged into a polymerization tank capable of adiabatic polymerization. The amount of dissolved oxygen in the solution was reduced to 0.2 ppm or less by introducing nitrogen gas into the solution. To this polymerization solution, 10 parts of a 1% aqueous hydrogen peroxide solution, 0.5% L
-10 parts of an aqueous solution of ascorbic acid and 10 parts of a 10% aqueous solution of V-50 (azo catalyst manufactured by Wako Pure Chemical Industries) were added. After about 20 minutes, a temperature rise indicating initiation of polymerization was confirmed.
The maximum temperature was reached after 5 hours. After further aging for 4 hours, a hydrogel polymer was obtained. The hydrogel polymer was cut into small pieces using a small kneader, and 50% Na was added thereto.
1600 parts of an aqueous OH solution were added, and about 72 mol% of the carboxyl groups in the polymer were converted into sodium salts. The neutralized hydrogel polymer is heated with hot air at 150 ° C. to a water content of about 4
%, And then pulverized, passed through a 22-mesh wire mesh, and removed from a portion not passing through a 119-mesh wire mesh, to obtain a particulate water-absorbent resin (1). As a result of measuring the particle size distribution of the water-absorbent resin (1), the average particle size was 380 microns, and contained 0.4% of fine particles of 100 microns or less. Table 1 shows the performance evaluation results of the water absorbent resin (1).

Production Example 2 of Water Absorbent Resin To a solution consisting of 2000 parts of acrylic acid and 4100 parts of water, 1600 parts of a 50% aqueous NaOH solution was added under cooling to convert about 72 mol% of acrylic acid into a sodium salt. After adding and dissolving 6 parts of N, N'-methylenebisacrylamide as a crosslinking agent, the solution was further cooled to a solution temperature of 5 ° C. This polymerization solution is charged into a polymerization tank capable of adiabatic polymerization,
The amount of dissolved oxygen in the solution was reduced to 0.2 ppm or less by introducing nitrogen gas into the solution. In this polymerization solution, 1
10 parts of a 10% aqueous hydrogen peroxide solution, 10 parts of a 0.5% L-ascorbic acid aqueous solution, and 10 parts of a 10% V-50 aqueous solution were added. After about 30 minutes, a temperature rise indicating the start of polymerization was confirmed, and the maximum temperature was reached after about 3 hours. After further aging for 4 hours, a hydrogel polymer was obtained. The hydrogel polymer was crushed into small pieces using a small kneader, dried with hot air at 150 ° C. until the water content became about 4%, and crushed to obtain a powder.
A portion passing through a 2-mesh wire mesh and not passing through a 119-mesh wire mesh was taken to obtain a particulate water-absorbent resin (2). As a result of measuring the particle size distribution of the water-absorbent resin (2), the average particle size was 375 microns, and contained 0.7% of fine particles of 100 microns or less. Table 1 shows the performance evaluation results of the water absorbent resin (2).

Example 1 A turbulizer (manufactured by Hosokawa Micron; Hitech Mixer HX-2) was used as a high-speed stirring mixer. The inner surface of this mixer is coated with 6 mm thick Teflon resin, has 20 paddles, and has a prismatic shape having a height of 0.2 cm and an upper area of 0.16 cm ² at opposing locations of the paddles, and has a surface. A projection covered with Teflon resin is fixed. A supply feeder (Finetron FT-25; manufactured by Hosokawa Micron) was installed to supply the water-absorbent resin to the turbulator. The crosslinker-containing liquid was prepared by previously mixing ethylene glycol diglycidyl ether, water, and diethylene glycol (dielectric constant: 31.7) at a ratio of 5/22/13. The supply of the cross-linking agent-containing liquid is inserted into the raw material inlet of the turbulaser, and about 1 c of the paddle is supplied.
m, using an air atomizing nozzle (spray setup number: SU1) fixed above. This air atomizing nozzle is connected to a dropping metering pump for supplying the cross-linking agent-containing liquid and an air pressure regulator for supplying air. The crosslinker-containing liquid was added. The droplet diameter is adjusted by the air pressure. The rotation speed of the turbulizer was set to 2160 rpm (linear velocity at the tip of the paddle: 407 m / min), and 50% of the water-absorbent resin was used.
At the same time, 0.4 kg of the crosslinker-containing liquid was added at the same time as continuous addition from the sample inlet of the turbuler at a rate of kg / hour
/ Hour, and the air pressure was controlled at 0.8 kg / cm ² , and continuously sprayed and mixed with the water absorbent resin. The average droplet size of the crosslinker-containing liquid was 15-20 microns.
The mixture of the water-absorbing resin and the cross-linking agent-containing liquid discharged from the outlet of the turbulizer was heated with a heating jacket (except that the inner surface was not coated with Teflon resin and did not have prismatic projections). (The same structure as the turbulizer) and a heating reaction was performed. At this time, the reaction temperature is 140 ° C., and the residence time in the heater is about 30 minutes. From the start of discharging the water-absorbing agent (a), sampling of about 100 g of the water-absorbing agent was performed five times at 5 minute intervals. Each sample was passed through a 22-mesh wire net to obtain water absorbing agents (A) -1 to (A) -5. In each sample,
The average value of the content of the repellent particles that did not pass through the 10 mesh was 0.2%, and generation of lumps was hardly observed. Tables 2 and 5 show the performance evaluation results of the water absorbing agents (a) -1 to (a) -5.

[0035] In Example 1, instead of the air pressure ^{0.8kg / cm 2, 0.5kg / cm} 2 except that as in the same manner as in Example 1 water-absorbing agent (b) -1 (b) -5 was obtained. At this time, the average droplet diameter of the crosslinking agent-containing liquid was 45 to 60 microns. In each sample, the average value of the content of coarse particles that did not pass through the 10 mesh was 0.4%, and generation of lumps was hardly observed. Water absorbing agent (b) -1
The evaluation results of (b) -5 are shown in Tables 3 and 5.

Example 3 A water absorbing agent (c) was obtained in the same manner as in Example 1 except that the water absorbent resin (2) was used instead of the water absorbent resin (1). In each sample, the average value of the content of the dust particles that did not pass through the 10 mesh was 0.3%, and generation of lumps was hardly observed. Table 5 shows the average value of the performance evaluation results of the water absorbing agent (c).

Comparative Example 1 In the same manner as in Example 1, except that the crosslinking agent-containing liquid was continuously dropped and supplied by the dropping metering pump directly connected to the turbulator, without using the air atomizing nozzle. Thus, comparative water absorbing agents (d) -1 to (d) -5 were obtained. The average droplet diameter of the crosslinking agent-containing liquid at this time is about 400 to
It was 600 microns. The average value of the content of coarse particles that did not pass through the 10 mesh of each sample was 5.3%, which was a hard lump. Comparative water-absorbing agent (d)
Tables 4 and 5 show the evaluation results of -1 to (d) -5.

[0038] In Comparative Example 2 Example 1, instead of the air pressure 0.8 kg / cm ^2, except that the 0.3 kg / cm ² was obtained by the same way comparative water-absorbing agent as in Example 1 (e). At this time, the average droplet diameter of the crosslinking agent-containing liquid was 210 to 250 microns. The average value of the content of coarse particles that did not pass through the 10 mesh of each sample was 2.6%, which was a hard lump. Table 5 shows the average value of the evaluation results of the comparative water absorbing agent (e).

[0039]

[Table 1] * Absorption under load 20: Absorption under load of 20 g / cm ²

[0040]

[Table 2] * Absorption amount under load 40: Absorption amount under load of 40 g / cm ²

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

[Table 5]

[0044]

According to the production method of the present invention, since the average particle size of the liquid droplets of the crosslinking agent-containing liquid at the time of surface crosslinking is controlled to 200 μm or less and spray-mixed with the water-absorbing resin, the uniformity of surface crosslinking is achieved. Is remarkably improved, and hardly any lumps are formed due to aggregation of particles. Therefore, a water-absorbing agent having uniform performance with little variation can be produced, and the decrease in water retention due to surface cross-linking is small as compared with the conventional surface cross-linking technology. As a result, it is possible to produce an unprecedented water absorbing agent in which the water retention amount and the absorption amount under load are balanced at a high level. In particular, it has a feature of excellent absorption performance under a high load. Further, the amount of the cross-linking agent used for surface cross-linking is small and almost zero. Due to the above effects, the water absorbing agent of the present invention can be suitably used for absorbent articles such as disposable diapers and sanitary napkins.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Keiji Tanaka 11-1 Hitotsubashi Nohonmachi, Higashiyama-ku, Kyoto Sanyo Chemical Industry Co., Ltd.

Claims (11)

[Claims] (1) an average particle size of 200 to 500 microns,
A cross-linking agent-containing liquid (B) is added to particles of the water-absorbing resin (A1) having a surface water-absorbing resin (A1) having a content of particles of not more than 00 microns of not more than 3% by mass and having a water retention capacity of not less than 50 g / g with respect to physiological saline. And heating the mixture in the form of droplets having an average particle size of 200 μm or less, and heating the surface-crosslinked water-absorbing agent (A2). 2. The method according to claim 1, wherein air is mixed when the crosslinking agent-containing liquid (B) is ejected from the nozzle to form droplets having an average particle diameter of 200 microns or less. 3. The method according to claim 1, wherein the droplets of (B) are spray-mixed while stirring the particles of the water-absorbent resin (A1). 4. The stirring of the particles of the water-absorbent resin (A1) is performed using a mixer (C) having a stirring shaft provided with a plurality of paddles, and the linear velocity at the tip of the rotating paddle is 250.
4. The method according to claim 3, wherein the stirring is performed at a rate of 3000 m / min. 5. The mixer (C) has a plurality of prism-shaped or column-shaped projections having a height of 0.1 to 1 cm and an upper area of 0.09 to 9 cm ² on its inner wall surface. The method according to claim 4. 6. The method according to claim 5, wherein each of said projections is located on an inner wall surface on an extension of a rotating circumferential surface of the paddle. 7. The production method according to claim 4, wherein the material of at least the surface of the inner surface of the mixer other than the protruding portion is Teflon. 8. The water-absorbent resin (A1) comprises a radical polymerizable monomer having an acid group and / or a radical polymerizable monomer which forms an acid group by hydrolysis, a crosslinking agent and, if necessary, a graft base as main components. The method according to any one of claims 1 to 7, wherein the particles are obtained by subjecting a polymerization component to be subjected to aqueous polymerization, hydrolyzing and neutralizing as necessary, drying and pulverizing. 9. The water-absorbent resin (A1) has a water retention capacity of not less than 50 g / g with respect to physiological saline, and 20 g / cm.
^The method according to any one of claims 1 to 9, wherein an absorption amount under a load of ² is 10 g / g or less. 10. The difference between the water holding capacity of the water absorbing resin (A1) and the water holding capacity of the water absorbing agent (A2) obtained by surface crosslinking is 20 g.
The production method according to any one of claims 1 to 8, which is not more than / g and satisfies the following requirements. Water absorption capacity for physiological saline is 35 g / g or more; absorption capacity for physiological saline under 20 g / cm ² load is 25 g / g or more; absorption capacity for physiological saline under 40 g / cm ² load is 15 g / g or more; Absorption amount to physiological saline under a load of 60 g / cm ² is 10 g / g or more. 11. Near the surface of the particles of the water-absorbent resin (A1) is a water-absorbing agent (A2) crosslinked by spraying the crosslinking agent-containing liquid (B) in the form of droplets having an average particle diameter of 200 μm or less. The difference between the water holding capacity of the water absorbing resin (A1) and the water holding capacity of the water absorbing agent (A2) is 20 g / g or less, and
Water-absorbing agent that meets the requirements of Water absorption capacity for physiological saline is 35 g / g or more; absorption capacity for physiological saline under 20 g / cm ² load is 25 g / g or more; absorption capacity for physiological saline under 40 g / cm ² load is 15 g / g or more; Absorption amount to physiological saline under a load of 60 g / cm ² is 10 g / g or more.