JP3389130B2 - Surface cross-linking method for water absorbent resin - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for surface-crosslinking a water absorbent resin. More specifically, the present invention relates to a method for surface-crosslinking a water-absorbent resin to obtain a water-absorbent agent (water-absorbent resin having a specific physical value or more) that has a high absorption rate and an excellent absorption capacity under load. The present invention also relates to a water-absorbing agent having a small amount of fine powder and having excellent absorption capacity under pressure, liquid permeability under pressure, and impact resistance, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, a water-absorbent resin that absorbs several tens to several hundreds of times its own weight of water has been developed. Various water-absorbent resins have been used for applications requiring water absorption or water retention, such as fields, industrial fields such as dew condensation prevention, and cold insulation materials.

Examples of such water absorbent resin include, for example,
Hydrolyzate of starch-acrylonitrile graft polymer (JP-B-49-043395), neutralized product of starch-acrylic acid graft polymer (JP-A-51-125)
No. 468), a saponified product of vinyl acetate-acrylic acid ester copolymer (JP-A No. 52-014689),
Hydrolyzate of acrylonitrile copolymer or acrylamide copolymer (Japanese Patent Publication No. 53-015959), crosslinked products thereof, and self-crosslinking sodium polyacrylate obtained by reverse phase suspension polymerization (Japanese Patent Laid-Open No.
3-046389), crosslinked polyacrylic acid partially neutralized products (JP-A-55-084304), and the like.

The required properties of the water-absorbent resin differ depending on the application in which it is used. It has a high absorption rate and a large liquid permeability. However, the relationship between these characteristics does not always show a positive correlation, and it has been difficult to improve these characteristics at the same time.

Among these characteristics, two characteristics of water absorption rate and absorption capacity under load are desired for the water absorbent resin as basic physical properties. For example, water absorption resin having high water absorption rate and high absorption capacity under load. Sanitary material containing 60% by weight or more of the core in a high concentration (US Pat. No. 5,149,335), 60 g
A water-absorbent resin having a high absorbency against pressure of 12 g / g under a load of 1 / cm ² and a high water absorption rate (US Pat.
No. 16, European Patent Publication No. 707603) and the like have been proposed.

Therefore, as an attempt to increase the absorption rate of the water-absorbent resin, for example, in order to increase the surface area, an attempt has been made to reduce the particle size, granulate or scale. However, in general, when the water-absorbent resin is formed to have a small particle size, the water-absorbent resin forms a so-called "mamako" upon contact with the aqueous liquid, and the absorption rate is rather lowered. Further, when the water-absorbent resin is formed on the granulated product, a phenomenon in which the granulated product itself individually changes into a "mamako" state by contacting with the aqueous liquid, and the absorption rate is rather reduced. Further, when the water-absorbent resin is formed into a scaly form, its absorption rate is improved, but the absorption rate is not sufficient to induce gel blocking, and further the water-absorbent resin is formed into a scaly form. This is uneconomical because the water absorbent resin produced is necessarily bulky and requires larger transportation and storage facilities.

Therefore, as a means other than improving the surface area of the water-absorbent resin, the molecular chains in the vicinity of the surface of the water-absorbent resin are crosslinked to increase the crosslink density of the surface layer, that is, to prevent the formation of mamako by surface crosslinking. Techniques for improving the absorption rate have also been proposed. Further, such surface cross-linking is particularly important for improving the absorbency against pressure of the water absorbent resin. Such techniques are disclosed in, for example, JP-A-57-44627, JP-A-58-42602, JP-B-60-18690, JP-A-58-180233, and JP-A-59-62665.
JP-A-61-16903, US Pat. No. 5,422,405
U.S. Pat. No. 5,597,873, U.S. Pat.
1, European Patent 450923, European Patent 450924
And European Patent 668080. Furthermore, in order to achieve surface cross-linking with an improved water absorption rate, a method of performing granulation simultaneously with surface cross-linking of the water absorbent resin (WO91 /
17200, Japanese Patent Publication No. 6-216042, U.S. Pat. No. 50
02986, US Pat. No. 5,122,544, US Pat. No. 54
86569 and European Patent 695763) are also known. Further, a technique for keeping the particle size constant during surface cross-linking (all Examples of JP-A-58-42602) is also known. Further, a technique of adding a cross-linking agent to a hydrogel and drying it, and then subdividing and further cross-linking it (US Patent No. 51459).
06, US Pat. No. 5,385,983, US Pat.
727, U.S. Pat. No. 563316).

Indeed, the surface cross-linking certainly improves the water absorption rate of the water absorbent resin to some extent. However, the water absorption rate of the water absorbent resin basically largely depends on the contact area with the liquid to be absorbed. It is necessary to increase the specific surface area of the resin.
Therefore, a technique for surface-crosslinking the foamed water-absorbent resin (Japanese Patent Publication No. 8-509521, Japanese Patent Laid-Open No. 5-237378)
No. 63/88410, WO96 / 17884
US Pat. No. 5,314,420, US Pat.
No. 1, US Pat. No. 5,451,613, US Pat.
72, European Patent 574435, European Patent 70760
No. 3, European Patent No. 744435) has also been proposed. Further, a technique of finely controlling the average particle diameter is also known.

However, when the specific surface area of the surface-crosslinked water-absorbent resin is increased by increasing the specific surface area of the water-absorbent resin by foaming or making the average particle diameter fine, the water-absorbent resin which is surface-crosslinked becomes surface-crosslinked. Since the added crosslinking agent is absorbed instantly, it becomes difficult to uniformly coat the surface of the water-absorbent resin with the surface crosslinking agent. Therefore, in general, a water-absorbent resin having a large specific surface area has a problem that uniform surface cross-linking becomes difficult due to its too fast absorption rate and the absorption capacity under load is low.

Further, the control of the average particle diameter causes a problem of fine powder. That is, from the viewpoint of liquid permeability, dust generation, workability, and physical properties of the absorbent article, it is generally preferable that the content of the fine powder having a particle diameter of less than 150 μm is smaller in the water absorbent resin. However, if the average particle diameter is finely controlled in order to increase the surface area, a large amount of fine powder is eventually produced as a by-product, and the physical properties of the water absorbent resin are reduced and the cost of fine powder collection is increased with the increase in fine powder. Further, it is difficult to industrially finely control the particle size because of the stability of the particle size, and as a result, there is a problem that the physical properties of the water absorbent resin such as the absorption capacity under pressure and the water absorption speed are varied.

That is, the two most basic characteristics of the water-absorbent resin are the water-absorption rate and the absorption capacity under load, because it becomes difficult to uniformly cross-link the surface as the specific surface area of the water-absorbent resin increases. , The characteristics are contradictory.

[0012]

SUMMARY OF THE INVENTION Therefore, the object of the present invention is to provide a water absorbent resin having a large specific surface area, although a water absorbent resin having a high water absorption rate and a high absorption capacity under pressure is strongly demanded. However, since it is difficult to uniformly coat the surface cross-linking agent, the surface cross-linking is difficult, that is, in the present situation where the water absorption speed and the absorption capacity under load are contradictory. Therefore, an object of the present invention is to obtain a water-absorbing agent under a high pressure with a high water-absorption rate.

Another object of the present invention is to stably obtain a water-absorbing agent which has a small amount of fine powder, is excellent in strength in a dry state, and has a high absorption capacity under pressure and liquid permeability under pressure. That is, it is possible to stably provide a water-absorbing agent having excellent affinity to an aqueous liquid, improved water absorption capacity under no pressure and under pressure compared with conventional ones, and improved liquid permeability and swelling gel strength. is there.

[0014]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result, conventionally, a technique of granulating at the time of surface cross-linking (WO91 / 17200, Japanese Patent Publication No.
No. 216042) and a technique for keeping the particle size constant before and after surface cross-linking (all Examples of JP-A-58-42602) have been known, but surprisingly, a cross-linking agent was added to the water-absorbent resin particles. Then, in the method of cross-linking the vicinity of the surface, it was found that both the water absorption rate and the absorption capacity under load are simultaneously satisfied by surface cross-linking while pulverizing at least a part of the particles, and the present invention has been completed. It was

Further, by controlling the particle size of the water-absorbent resin powder before surface cross-linking to be large coarse particles, and further cross-linking the vicinity of the surface while crushing at least a part of the particles, the amount of fine powder is small and the absorption capacity under pressure is small. It was found that a water-absorbing agent excellent in dry strength was obtained, and the present invention was completed. That is, the method for surface-crosslinking the water-absorbent resin of the present invention is a dry bridge.
In the method of adding a surface cross-linking agent to the water-absorbent resin powder having a bridge structure to cross-link the vicinity of the surface, the proportion of fine powder of less than 150 μm is 10% by weight or less, and the weight average particle diameter is The water-absorbent resin powder having a particle size of 200 to 1000 μm is ground and simultaneously surface-crosslinked.

The surface-crosslinking method of the water-absorbent resin of the present invention is such that the water-absorbent resin powder after the surface-crosslinking reaction is pulverized by grinding the water-absorbent resin powder during the surface-crosslinking, as compared with the water-absorbent resin powder before the addition of the surface-crosslinking agent. It is preferred that the weight average particle size be reduced by 1-50%. The surface cross-linking method of the water-absorbent resin of the present invention, by pulverization of the water-absorbent resin powder during surface cross-linking, 150 μm in the water-absorbent resin powder after the surface cross-linking reaction as compared with the water-absorbent resin powder before the addition of the surface cross-linking agent. It is preferable that the proportion of fine particles of less than 10% is increased and the generation amount thereof is 10% by weight or less.

In the surface cross-linking method of the water-absorbent resin of the present invention, the BET specific surface area of the water-absorbent resin powder coated with the surface cross-linking agent is smaller than that before the reaction by pulverizing the water-absorbent resin powder during the surface cross-linking. After the crosslinking reaction 1.
It is preferable to increase it by 05 to 10 times. In the surface cross-linking method of the water-absorbent resin of the present invention, it is preferable that the pulverization of the water-absorbent resin powder at the time of surface cross-linking is performed at the time of mixing the surface cross-linking agent or at the time of heat-crosslinking reaction. In the method for surface-crosslinking the water-absorbent resin of the present invention, the surface-pressure applied to the water-absorbent resin powder is 20 g / cm when the water-absorbent resin powder is pulverized at the time of surface cross-linking under the mixer or heat treatment machine for the surface cross-linking agent.
It is preferable to carry out the mixing or heat treatment under a load of ² (corresponding to about 1.96 kPa) or more and / or in the presence of a ball mill.

[0018] surface crosslinking process for a water-absorbent resin of the present invention, be pre-Symbol grinding is carried out in the grinding index over 1000
preferable. However, (grinding index) = (surface pressure A to the water absorbent resin powder) x (times
Number of rotations B) x (stirring time C) A: Water absorbent resin powder in the lower part of the mixer or heat treatment machine
Such pressure (g / cm ² ) B: Number of rotations per minute of mixer or heat treatment machine (rp
m) C: Retention of water-absorbent resin powder in the mixer or heat treatment machine
Time

Another surface-crosslinking method for the water-absorbent resin of the present invention is to add a surface-crosslinking agent to a water-absorbent resin powder having a dry crosslinked structure, and crosslink the vicinity of the surface while pulverizing as necessary. In, the powder is 15
The proportion of fine powder of less than 0 μm is 10% by weight or less, the weight average particle diameter is 300 to 600 μm, the absorption capacity for physiological saline is 35 (g / g) or less, and low absorption capacity is exhibited, and 25% by weight or more of powder is 600 μm or less
It is characterized in that it is coarse particles having an upper diameter of less than 1000 μm. Moreover, in the surface cross-linking method for all the water-absorbent resins of the present invention described above, the surface cross-linking agent preferably contains a polyhydric alcohol, and further, the polyhydric alcohol has 3 to 8 carbon atoms.
It is preferable to use a plurality of polyhydric alcohols.

Further, drying the water-absorbent resin powder of the present invention, dry
The vicinity of the surface of the water-absorbent resin powder having a dried crosslinked structure,
The surface-crosslinked water-absorbent resin powder further crosslinked with a plurality of polyhydric alcohols having 3 to 8 carbon atoms has a weight of
The average particle diameter is 300 to 600 μm, and 1 in the powder
The ratio of fine powder of less than 50 μm is 10% by weight or less, and 1
The proportion of particles of 000 μm or more is 5% by weight or less, and 3
Proportion of 00μm or more particles, wherein this <br/> and is 70 to 99 wt%. According to the above configuration, since the water-absorbent resin before the surface-crosslinking agent is mixed has a large particle size and a small amount of fine powder, it is easy to uniformly mix the surface-crosslinking agent. To do. That is, in the present invention, the water-absorbent resin powder after the surface cross-linking agent is uniformly mixed is further pulverized, so that the water-absorbent resin having a high water absorption rate / a high absorption capacity under pressure can be stably provided.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below. With the water absorbent resin in the present invention,
A hydrophilic cross-linked polymer that swells by absorbing 5 times or more of its own weight of physiological saline under no load, and its water absorption capacity (water absorption)
Is preferably 10 times or more, more preferably 20 to 2
The range is 00 times.

The water-absorbent resin used in the present invention may be a water-absorbent resin obtained by post-crosslinking an aqueous solution of an uncrosslinked water-soluble polymer, but is preferably polymerized simultaneously with crosslinking in an aqueous solution. It is a water-absorbent resin obtained by Examples of the monomer used for the cross-linking polymerization include a ring-opening polymerizable monomer, an acid group-containing unsaturated monomer, a nonionic unsaturated monomer, and a cationic unsaturated monomer. In the above, an acid group-containing unsaturated monomer (salt), more preferably acrylic acid (salt), is essentially used as a monomer.

Further, in the present invention, during the polymerization, only a monomer other than acrylic acid (salt) is used and / or acrylic acid (salt) is used.
Water-absorbent resin may be obtained by polymerizing together with.
The monomer other than acrylic acid (salt) used in the present invention is not particularly limited, and specifically, for example, methacrylic acid, maleic acid, crotonic acid, sorbic acid, itaconic acid, silica. Cinnamic acid, maleic anhydride, vinylsulfonic acid, allylsulfonic acid, vinyltoluenesulfonic acid, styrenesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, 2- (Meth) acryloyl propane sulfonic acid, 2-hydroxyethyl (meth)
Acid group-containing unsaturated monomers such as acryloyl phosphate and salts thereof; acrylamide, methacrylamide, N
-Ethyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, methoxypolyethylene glycol (meth)
Acrylate, polyethylene glycol mono (meth) acrylate, vinyl pyridine, N-vinyl pyrrolidone,
1 such as a nonionic hydrophilic group-containing unsaturated monomer such as N-acryloylpiperidine or N-acryloylpyrrolidine
Or two or more unsaturated monomers. A water-absorbent resin may be obtained by polymerizing only unsaturated monomers other than acrylic acid (salt), but when used in combination with acrylic acid (salt), 50 mol% or less of all monomers, and further 30 It is preferably used in an amount of not more than mol%.

In the present invention, the acid group-containing unsaturated monomer (salt) /
Or when obtaining a water-absorbent resin using an acid group-containing polymer,
From the viewpoint of water absorption characteristics such as water absorption ratio and water absorption speed, and safety, the neutralization ratio of acid groups of the water absorbent resin is preferably 30.
To 100 mol%, more preferably 60 mol% to 90 mol%, and even more preferably 65 mol% to 75 mol%. The neutralization of the acid group may be carried out as an acid group-containing monomer in an aqueous solution before the polymerization, or may be carried out by neutralizing the aqueous solution of the polymer, that is, the polymer gel, or both may be used in combination. May be. Also, when a cationic monomer is used, the monomer or polymer may be neutralized.

In the present invention, the neutralizing agent used for neutralizing the monomer or polymer is not particularly limited, and a conventionally known inorganic or organic base or acid can be used. As the base of the neutralizing agent for the acid group,
Specifically, for example, sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, lithium carbonate, sodium carbonate, potassium carbonate, ammonium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, ammonium hydrogen carbonate, sodium phosphate. , Potassium phosphate, ammonium phosphate, sodium borate, potassium borate, ammonium borate, sodium acetate, potassium acetate, ammonium acetate, sodium lactate, potassium lactate, ammonium lactate, sodium propionate, potassium propionate, ammonium propionate And the like. Examples of the acid as the neutralizing agent for the basic group include acids such as acetic acid, propionic acid, hydrochloric acid, sulfuric acid and phosphoric acid.

When a water-absorbent resin is obtained in the present invention, an uncrosslinked water-soluble polymer may be obtained and then cross-linked in an aqueous solution to form a water-absorbent resin. Crosslinking takes place simultaneously with the polymerization of the monomers. As a method of crosslinking at the time of polymerization, self-crosslinking may be performed at the time of polymerization without using an internal crosslinking agent, but in the present invention, a water-absorbent resin polymerized in the presence of an internal crosslinking agent is preferably used.

The internal cross-linking agent used in the present invention is a compound having a plurality of substituents copolymerizable with and / or reacting with the unsaturated monomer in one molecule to form a cross-linking structure, and is used without particular limitation. To be Specifically, N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane Di (meth) acrylate, glycerin tri (meth) acrylate, glycerin acrylate methacrylate, ethylene oxide modified trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, triallyl cyanurate , Triallyl isocyanurate, triallyl phosphate, triallyl amine, poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether , Glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl (meth) acrylate, acetals such as tetraallyloxyethane, pentaerythritol tetraallyl ether, pentaerythritol triallyl ether Ether, pentaerythritol diallyl ether, trimethylolpropane triallyl ether, trimethylolpropane diallyl ether, ethylene glycol diallyl ether, diethylene glycol diallyl ether, triethylene glycol diallyl ether, monosaccharides, disaccharides, polysaccharides, cellulose Polyary derived from a compound having two or more molecules in the molecule Ethers such as ether, triallyl isocyanurate, the like triallyl null rate and the like, but is not particularly limited. Among these internal cross-linking agents, in the present invention, by using an internal cross-linking agent having a plurality of polymerizable unsaturated groups in one molecule, it is possible to further improve the absorption characteristics and the like of the resulting water-absorbent resin, which is preferable. .

These internal crosslinking agents may be used alone or during polymerization.
Are used in combination, also may be added together, dividing may be added. The amount used depends on the kind of the cross-linking agent and the desired cross-linking density, but it is 0.
It is preferably within the range of 005 mol% to 3 mol%, and 0.01
More preferably in the range of mol% to 1.5 mol%, 0.0
It is more preferably within the range of 5 to 1 mol%, and more preferably 0.06 to 0.5 mol%. If it deviates from this range, there is a fear that a water-absorbent resin having desired absorption characteristics may not be obtained.

The method of polymerizing the above-mentioned monomer is not particularly limited, and known methods such as aqueous solution polymerization, reverse phase suspension polymerization, bulk polymerization, precipitation polymerization and the like can be adopted. Among them, the method of polymerizing the monomer component in an aqueous solution, that is, the aqueous solution polymerization and the reverse phase suspension polymerization is preferable from the viewpoint of easy control of the polymerization reaction and performance of the water-absorbent resin to be obtained. The aqueous solution polymerization and the reverse phase suspension polymerization are described in, for example, US Pat. Nos. 4,625,001, 4,769,427, 4,873,299, and 40.
No. 93776, No. 4377323 , No. 44462.
No. 61, No. 4683274, No. 4690996
No. 4,721,647, No. 4,738,867, No. 4,748,076 and the like.

When the monomer is polymerized as an aqueous solution in the present invention, the monomer concentration is preferably 5 to 70% by weight, more preferably 10% to 50% by weight, and most preferably 15 to 40% by weight. Is the range. Too high concentration,
If it is too low, the effects of the present invention may not be easily exhibited. The reaction conditions such as reaction temperature and reaction time may be appropriately set according to the composition of the monomer used and are not particularly limited, but are usually 10 ° C to 110 ° C, preferably 15 ° C. Polymerization is carried out within a temperature range of ˜90 ° C. During the polymerization, a hydrophilic polymer such as starch, starch derivative, cellulose, cellulose derivative, polyvinyl alcohol, polyacrylic acid (salt), polyacrylic acid (salt) cross-linked product; hypophosphorous acid (salt), etc. The chain transfer agent described above, or a foaming agent such as an inert gas or a carbonate described below may be added. Further, for the initiation of polymerization, for example, radical polymerization of potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide, 2,2′-azobis (2-amidinopropane) dihydrochloride, etc. An initiator or an active energy ray such as ultraviolet ray or electron beam can be used. When an oxidizing radical polymerization initiator is used, redox polymerization may be carried out using a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid together. The amount of these polymerization initiators used is 0.001 with respect to the amount of the monomer.
It is preferably in the range of mol% to 2 mol%, and 0.01 mol%
Is more preferably in the range of 0.5 mol%, and the polymerization initiator is preferably added after being dissolved or dispersed in a solvent such as water.

In the present invention, the hydrogel polymer obtained above is essentially dried before adding the surface crosslinking agent,
Further, the water-absorbent resin powder is obtained by crushing / classifying if necessary. When the addition of the surface cross-linking agent is not a powder, for example, in the case of a hydrous gel (hydrogel), even if the pulverization is performed after the addition of the cross-linking agent of the present invention or during the reaction of the cross-linking agent, it is excellent in absorption capacity under pressure and water absorption rate. The water absorbing agent aimed at by the invention cannot be obtained.

Therefore, with respect to the drying which is essentially performed before the addition of the surface cross-linking agent, the drying temperature is not particularly limited, but is, for example, in the range of 100 to 250 ° C., more preferably in the range of 120 to 200 ° C. It should be inside. The drying time is not particularly limited, but is 10
About 2 to 5 hours is preferable. In addition, before drying
The water-containing gel polymer may be neutralized, or may be further crushed and fragmented.

The drying method used is heat drying,
Various methods such as hot air drying, reduced pressure drying, infrared drying, microwave drying, drum dryer drying, dehydration by azeotropic distillation with a hydrophobic organic solvent, and high-humidity drying using high-temperature steam can be adopted. It is not limited. In the present invention, the surface of the water-absorbent resin powder obtained above is crosslinked.

In the present invention, the water-absorbent resin powder used for surface cross-linking has a practical upper limit of less than 1000 μm and an average particle diameter of 200 to 1000 μm, preferably 300 to 600 μm, more preferably 400 to 50.
It is preferably 0 μm particles, particularly less than 150 μm fine powder with a small amount of fine powder, for example, the amount of fine powder is 10% by weight or less, further 5% by weight or less, even more preferably 1% by weight or less, and particularly preferably substantially zero. . When the amount of fine powder is large, granulation is likely to occur, the pulverization referred to in the present invention does not occur, and the physical properties are difficult to improve. Fine powder, that is, the present invention is characterized by achieving uniform surface cross-linking by reducing the amount of fine powder before surface cross-linking, and improving the water absorption rate by crushing during surface cross-linking.

The solid content of the water-absorbent resin powder is preferably more than 85% by weight, more preferably more than 90% by weight, and even more preferably 95% by weight in order to achieve uniform pulverization during surface cross-linking. Over particles are used. If the water content is high, uniform / efficient pulverization may not be achieved, and the mixing property of the surface cross-linking agent may be deteriorated. Especially in place of powder, European Patent No. 509708 / US Pat. No. 56333
No. 16, US Pat. No. 5,145,906, etc., the water-containing gel polymer having a water content of 15% by weight or more, 15 to 90% by weight, and particularly 30-45% by weight is used. On the other hand, even if the method of the present invention is applied, the effect of the present invention is not sufficiently achieved. Further, its shape is preferably an irregular crushed shape adjusted to a predetermined particle size in the crushing step after drying. Further, as in US Pat. No. 5,385,983, the object of the present invention cannot be achieved unless the particle size is controlled in advance.

Further, as a feature of the present invention, in order to carry out stable surface cross-linking, and in particular to improve the absorbency under pressure, it is preferable that the absorption capacity of the water-absorbent resin powder under no load is as low as possible. It has a low absorption capacity of 50 (g / g) or less, more preferably 35 (g / g) or less. Conventionally, it is natural that the water absorption resin preferably has a high water absorption capacity, but it has been surprisingly found that the lower the absorption capacity, the more preferable is 35 g / g or less. When the absorption capacity in the present invention is high, it may be difficult to stably obtain a water absorbing agent having the desired physical properties, the surface cross-linking may be uneven, and the absorption capacity under pressure and the liquid permeability under pressure may be reduced. It has been found that a low absorption capacity of 35 g / g or less, even 33 to 27 g / g is particularly preferably used in the present invention.

The water content and particle size of the water-absorbent resin powder are as described above, and further, the powder is (a) porous, (b) coarse powder having a constant particle size, (c) granulated particles, Either of these is preferably used. That is, the present invention is a method of adding a cross-linking agent to a water-absorbent resin powder to cross-link the vicinity of the surface thereof, wherein the powder having a weight average particle diameter of 200 to 1000 μm is simultaneously surface-crosslinked while being ground. Although this is a method for surface-crosslinking a resin, it is preferable that the water-absorbent resin (a) used is porous in order to efficiently perform pulverization during surface-crosslinking. Such a porous water-absorbent resin, mechanical force more than specific at the time of surface cross-linking described later,
A specific device having a crushing function is preferable because it can crush stably and efficiently starting from the pores and folds of the porous water-absorbent resin powder.

The porous water-absorbent resin preferably used in the present invention is obtained by foaming the water-absorbent resin in at least one of the above-mentioned polymerization, crosslinking and drying steps of the water-absorbent resin. The porous water absorbent resin is preferably obtained by foaming polymerization during polymerization. Hereinafter, the method for producing (a) the porous water absorbent resin preferably used in the present invention will be further described.

As a foaming method for obtaining the preferred porous water-absorbent resin of the present invention, the aqueous solution polymerization according to the present invention is
Preferably, it is carried out in a state in which bubbles are dispersed in the aqueous monomer solution disclosed in European Patent Application 97306642.2. The volume of the aqueous monomer solution in which the bubbles are dispersed is 1.02 times or more, preferably 1.08 times or more, more preferably 1.11 times or more, and most preferably 1 times the volume of the non-dispersed state. 2 times or more.

Even in the conventional polymerization reaction operation under stirring, some bubbles may be mixed in. However, according to the confirmation by the inventors, even if bubbles are mixed in in the normal operation, it is caused by it. The volume change is less than 1.01 times.
The volume change of 1.02 times or more is the result of intentionally mixing the bubbles, and the performance of the resin obtained by this operation was confirmed to be improved, and the porous water-absorbent resin suitable for the present invention was obtained. To be Since the volume change of the aqueous monomer solution in the reaction vessel appears only in the height of the waterline, the volume change rate can be easily confirmed. Furthermore, as a result of the intentional mixing of bubbles, the transparency of the aqueous solution is reduced and the solution becomes white, so that the dispersed state of bubbles in the aqueous solution can be visually confirmed.

The preferred foaming method for obtaining the porous water-absorbent resin in the present invention is to mix the aqueous monomer solution and the gas by fluid mixing in order to disperse the bubbles in the aqueous monomer solution. After obtaining the monomer aqueous solution in which bubbles are dispersed,
It is a manufacturing method in which the monomer is polymerized in a state where the bubbles are dispersed. In the present invention, as one method for obtaining a porous water-absorbent resin, the monomer aqueous solution and gas are fluidly mixed. The monomer aqueous solution or gas is made into a fluid state by, for example, being jetted or sucked from a nozzle. By mixing both in a fluid state, the gas can be uniformly and stably dispersed in the aqueous monomer solution. Then, by polymerizing the monomer in a state where the gas is previously dispersed in the aqueous monomer solution, the pore size can be easily controlled, and a porous water-absorbent resin having a high absorption rate can be obtained.

As a method for mixing fluids, there is a method in which either one of the aqueous monomer solution and the gas is flowed, and the other fluid is injected from a nozzle to mix the two. Specifically, for example, a method in which a gas of a monomer aqueous solution ejected from a nozzle is co-flowed with a gas from another nozzle to mix them, or a gas of a gas ejected from a nozzle is separated from another nozzle by a single flow. A method may be mentioned in which the monomer aqueous solution is caused to flow in parallel and the both are mixed. Alternatively, the gas may be directly blown into the fluid of the aqueous monomer solution. When fluids are mixed, they can be injected cocurrently, countercurrently, or vertically. It is preferable to inject in parallel flow. Bubbles can be dispersed uniformly by injecting in parallel flow. When it is jetted in countercurrent, the droplets will adhere to the walls,
There is a risk of polymerization. Examples of the fluid mixing device include an aspirator and an ejector.

In the present invention, the polymerization reaction is preferably carried out in the presence of a surfactant in order to obtain a porous water-absorbent resin. Bubbles can be stably dispersed by using a surfactant. In addition, the pore diameter and water absorption rate of the resulting water absorbent resin can be controlled by appropriately controlling the type and amount of the surfactant. Examples of such surfactants include anionic surfactants, nonionic surfactants, cationic surfactants and amphoteric surfactants.

As the anionic surfactant used, fatty acid salts such as mixed fatty acid sodium soap, semi-cured tallow fatty acid sodium soap, sodium stearate soap, potassium oleate soap, castor oil potassium soap; sodium lauryl sulfate and higher alcohols. Alkyl sulfate ester salts such as sodium sulfate and lauryl sulfate triethanolamine; alkylbenzene sulfonates such as sodium dodecylbenzenesulfonate; alkylnaphthalenesulfonates such as sodium alkylnaphthalenesulfonate; alkylsulfosuccinates such as sodium dialkylsulfosuccinate Alkyl diphenyl ether disulfonates such as sodium alkyl diphenyl ether disulfonate; alkyl phosphates such as potassium alkyl phosphate; polyoxy Polyoxyethylene alkyl (or alkylallyl) sulfate ester salts such as sodium ethylene lauryl ether sulfate, sodium polyoxyethylene alkyl ether sulfate, triethanolamine polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate; special reaction type Anionic surfactant; special carboxylic acid type surfactant; sodium salt of β-naphthalenesulfonic acid formalin condensate, sodium salt of special aromatic sulfonic acid formalin condensate, etc. naphthalenesulfonic acid formalin condensate; special polycarboxylic acid type Polymer surfactants: polyoxyethylene alkyl phosphate ester and the like.

Examples of the nonionic surfactant used include polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl ether, polyoxyethylene higher alcohol ether and other polyoxyethylene alkyl ethers. , Polyoxyethylene alkylaryl ethers such as polyoxyethylene nonylphenyl ether; polyoxyethylene derivatives; sorbitan monolaurate, sorbitan monopalmitate, sorbitan monostearate, sorbitan tristearate, sorbitan monooleate, sorbitan trioleate, Sorbitan fatty acid esters such as sorbitan sesquioleate and sorbitan distearate; polyoxyethylene sorbitan mono Polyoxyethylene sorbitan fatty acid ester such as urate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan tristearate, polyoxyethylene sorbitan monooleate, polyoxyethylene sorbitan trioleate; tetra Polyoxyethylene sorbitol fatty acid ester such as oleic acid polyoxyethylene sorbitol; glycerol monostearate, glycerol monooleate, glycerol fatty acid ester such as self-emulsifying glycerol monostearate; polyethylene glycol monolaurate, polyethylene glycol monostearate, Polio such as polyethylene glycol distearate, polyethylene glycol monooleate Shiechiren fatty acid ester, polyoxyethylene alkylamine;
Polyoxyethylene hydrogenated castor oil; alkylalkanolamides and the like.

As the cationic surfactant and double-sided surfactant used, coconut amine acetate,
Alkylamines such as stearylamine acetate; quaternary ammonium salts such as lauryltrimethylammonium chloride, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride, distearyldimethylammonium chloride, alkylbenzyldimethylammonium chloride; laurylbetaine, stearylbetaine, Alkyl betaines such as lauryl carboxymethyl hydroxyethyl imidazolinium betaine; amine oxides such as lauryl dimethyl amine oxide. In addition, by using such a cationic surfactant, it is possible to impart antibacterial properties to the resulting water absorbent resin.

Further, as the surfactant used,
There are fluorochemical surfactants. By using the fluorine-based surfactant, it is possible to stably disperse the bubbles of the inert gas in the aqueous monomer solution for a long time. In addition, it is easy to control the amount of bubbles and the pore size. The resulting water absorbent resin becomes a porous foam and has a high absorption rate. In addition, antibacterial properties can be imparted. As the fluorine-based surfactant used in the present invention, there are various ones. For example, the hydrogen of the lipophilic group of a general surfactant is replaced with fluorine to form a perfluoroalkyl group. Is much stronger.

The surfactant used in the present invention is not limited to the above surfactants. These surfactants are used in an amount of 0.0001 to 10 parts by weight, preferably 0.0 to 100 parts by weight, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer used.
003 to 5 parts by weight. That is, if the amount is less than 0.0001 parts by weight, the gas may be insufficiently dispersed. On the other hand, if it exceeds 10 parts by weight, the effect corresponding to the added amount may not be obtained, which is uneconomical.

Although it has been conventionally known to use a surfactant in aqueous solution polymerization, such a known technique does not improve the water absorption rate at all. In the present invention, it is preferable to polymerize the monomer in a state where bubbles are dispersed. As the gas mixed with the aqueous monomer solution, nitrogen, argon,
Examples of the inert gas include helium and carbon dioxide. When a gas containing oxygen is mixed, when a sulfite such as sodium hydrogen sulfite is used as the polymerization initiator, water-absorbent resins having various molecular weights can be obtained by appropriately controlling the ratio of oxygen and sulfite. When an oxidizing agent is used as the polymerization initiator, it is possible to start the polymerization by mixing sulfur dioxide gas.

The viscosity of the aqueous monomer solution is not particularly limited, but bubbles can be more stably dispersed by adjusting the viscosity to 10 cP or more. Preferably 100 to 100,000 c
P, more preferably 20 to 3000 cP. By setting the viscosity of the monomer aqueous solution to 10 cP or more, bubbles can be stably dispersed in the monomer aqueous solution for a long time. still,
When the viscosity is higher than 100,000 cP, bubbles in the aqueous monomer solution become large and it may be difficult to obtain a water absorbent resin having a high water absorption rate.

In the present invention, a thickener may be added to the above aqueous monomer solution, if necessary. The thickener may be a hydrophilic polymer, and for example, polyacrylic acid (salt), polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, polyethylene oxide, hydroxyethyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose and the like can be used. As the thickener, a water absorbent resin such as colloidal silica or a crosslinked polyacrylic acid (salt) can also be used. It is preferable that these hydrophilic polymers used as a thickener have an average molecular weight of 10,000 or more, preferably 100,000 or more. If the average molecular weight is less than 10,000, the amount of the thickener added will increase, and the water absorption performance may decrease, which is not preferable. Further, the addition amount of the thickener is such that the viscosity of the aqueous monomer solution is 10 cP.
The amount is generally 0.01 to 10% by weight, preferably 0.1 to 5% by weight, based on the above-mentioned monomers, though not particularly limited. If the added amount of the thickening agent is less than 0.01% by weight, the viscosity may not reach 10 cP or more, and if it exceeds 10% by weight, the water absorption performance may decrease.

Next, if necessary, the obtained hydrogel containing air bubbles is shredded, further dried, and the dried product obtained is pulverized to obtain a water-absorbent resin having a high water absorption rate and a high dissolution rate in a powder form. . Although a porous water absorbent resin can be obtained by the above method, it may be obtained by other foaming polymerization. Examples of the foaming agent used include sodium carbonate, potassium carbonate, ammonium carbonate, magnesium carbonate, calcium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, ammonium hydrogen carbonate, magnesium hydrogen carbonate, calcium hydrogen carbonate, zinc carbonate, barium carbonate and the like. , A water-soluble azo polymerization initiator such as azobisamidinopropane dihydrochloride, dicarboxylic acids such as malonic acid, and volatile organic solvents such as trichloroethane and trifluoroethane.
When a foaming agent is added, its amount is 0 to 5 parts by weight, more preferably 0 to 1 part by weight, based on 100 parts by weight of the total amount of the water-soluble unsaturated monomer and the water-soluble crosslinkable monomer. Is appropriate.

The hydrogel polymer obtained by the above-mentioned foaming polymerization is crushed into pieces of about 0.1 mm to about 50 mm by a predetermined method during or after the reaction, if necessary. Next, in order to foam more efficiently, the bubble-containing hydrogel polymer is dried. In addition,
The foaming with the foaming agent can be performed not at the time of reaction but at the time of drying.

In the present invention, the hydrogel polymer obtained above is essentially dried before adding the surface crosslinking agent,
Further, it is crushed if necessary to obtain a water-absorbent resin powder. The drying method used is as described above and is not particularly limited. Among the drying methods exemplified above, hot air drying and microwave drying are more preferable.
When the bubble-containing hydrogel is irradiated with microwaves, the bubbles expand several times to several tens of times, so that it is possible to obtain a water absorbent resin having a further improved water absorption rate.

When the hydrogel polymer containing bubbles is subjected to microwave drying, the crushed hydrogel has a thickness of 3 m.
It is preferably m or more, more preferably 5 mm or more, still more preferably 10 mm or more. When the hydrogel is dried by microwave, it is particularly preferable to form the hydrogel into a sheet having the above thickness.

By the above-mentioned polymerization, that is, by the above-mentioned production method, the porous water-absorbent resin according to the present invention can be obtained at low cost and easily. The water-absorbent resin has an average pore diameter of 10 to 500 μm, more preferably 20 to 40 μm.
It is in the range of 0 μm, more preferably in the range of 30 to 300 μm, and most preferably in the range of 40 to 200 μm. The average pore size is determined by image analysis of a cross section of the dried water absorbent resin with an electron microscope. In other words, an average pore diameter is obtained by performing image analysis to create a histogram showing the pore diameter distribution of the water-absorbent resin, and calculating the number average of pore diameters from the histogram. The water-absorbent resin obtained as described above, preferably a porous water-absorbent resin, has a bulk specific gravity smaller than that of a conventional one, specifically 0.6 to 0.1 g / cc, and further 0.5 to 0. It is within the range of 2 g / cc. Also, B
ET specific surface area is larger than the conventional one, namely 0.02
It is 5 m ² / g or more, more preferably 0.03 m ² / g or more, even more preferably 0.04 m ² / g. The object of the present invention can be further achieved by using such a water-absorbent resin and pulverizing the powder simultaneously with surface cross-linking.

Further, in the present invention, in addition to the above-mentioned (a) porous water-absorbent resin powder, (b) water-absorbent resin powder having a constant particle size can be used, and it is also preferable to be able to grind at the same time as crosslinking near the surface. . When the porous water-absorbent resin is not used, for example, in the case of water-absorbent resin powder having a bulk specific gravity of 0.6, the particle size of the water-absorbent resin powder used is 150
The proportion of particles having a size of μm or more is 90% by weight, more preferably 95% by weight or more, still more preferably 98% by weight or more, and further preferably, the particles having a size of 600 μm or more are 20% by weight or more, further 25% by weight or more. Further, it is preferable to contain 25 to 50% by weight. The preferred particle size is 1
The proportion of particles of 000 μm or more is 5% by weight or less, and 3
The proportion of particles having a size of 00 μm or more is preferably 70 to 99% by weight. This 150μm, 300μm, 6
Crosslinking in the vicinity of the surface of coarse-grained particles containing a certain proportion of particles of 00 μm or more is carried out while pulverizing, so that the amount of fine powder is small, and the water absorption rate, absorption capacity under pressure, liquid permeability under pressure, and impact resistance. A water absorbent having excellent properties can be stably obtained. When the particle size is out of this range, the crushing referred to in the present invention becomes difficult.

Further, in the present invention, in addition to (a) porous powder and (b) coarse powder having a constant particle size,
(C) Granulated particles may be used. (B) to (c)
In, preferably, in (b) and (c), the fine powder in the water-absorbing resin powder that crosslinks in the vicinity of the surface is reduced in advance, and the fine powder is mixed with the surface crosslinker at the same time so that the fine powder does not mix with the crosslinker at the same time. Surface cross-linking, a high water absorption rate due to pulverization at the time of surface cross-linking, and the impact resistance of the resulting water-absorbing agent after surface cross-linking when dry. The present invention reduces the fine powder before surface cross-linking by granulating in advance and destroys a part of the granulated particles, so that the fine powder is small, the liquid permeability under pressure is excellent, and the absorption capacity under high pressure is high. And achieve high absorption rate.

When the granulated particles are used in the present invention, it is preferable to granulate the water-absorbent resin fine powder with water to a constant particle size. The granulation may be carried out at the time of polymerization of the water-absorbent resin, at the time of drying, or at the powder after drying, preferably in the form of fine powder (for example, particles of 150 μm or less). From the viewpoint of physical properties, it is preferable that only fine powder be removed by classification or the like and granulated separately.

The method of obtaining the granulated particles using the aqueous liquid in the present invention is not particularly limited, and the methods at that time include the rolling granulation method, the compression granulation method, the stirring granulation method and the extrusion granulation method. The granulation method, the crush type granulation method, the fluidized bed granulation method, the spray drying granulation method and the like can be mentioned. From the granulation strength, granulated particles further dried with water or an aqueous liquid are preferable. The granulated particles (c) are preferable, as in the case of (a) and (b), because some of them are efficiently destroyed at the time of surface cross-linking when the particles are crosslinked. The amount of water that can be used for granulation of the fine powder is 2 to 300 parts by weight of water, further 30 to 250 parts by weight, 70 to 200 parts by weight, and 100 to 200 parts by weight with respect to 100 parts by weight of the water absorbent resin. In addition, it is preferable to use a granulated powder obtained by drying and crushing if necessary. When the amount of water deviates from the above range, the absorption capacity under pressure and the liquid permeability under pressure tend to be poor.

After mixing the above-mentioned aqueous liquid, in the present invention, in the granulation, the aqueous liquid is preheated before granulation in order to further improve the granulation strength and the water absorption capacity under pressure.
To the boiling point range, and more preferably, the powder before granulation is heated to 40 ° C. or higher, further 50 to 100 ° C., and then mixed. Further, upon granulation, it is more preferable that the heated aqueous liquid and the fine powder are mixed at high speed, preferably 3 minutes or less, more preferably 1 minute or less, and most preferably 1 second to 60 seconds. If the mixing time is long,
Uniform mixing becomes difficult, resulting in huge agglomerates, and increase in water-soluble components and decrease in water absorption capacity under pressure. Further, the obtained granulated product, especially the hydrous gel-like granulated product, is inevitably dried and, if necessary, pulverized and classified to be a water-absorbent resin powder. The drying temperature and method after mixing the aqueous liquid, and the particle size and water content of the powder after drying are the same as above.

When the granulated product is used in the present invention, only the above-mentioned granulated product may be used, but crosslinking in the vicinity of the surface is carried out while pulverizing with a mixture of the primary particles of the water absorbent resin and the granulated product. In this case, the weight ratio of the primary particles to the granulated particles is preferably 50/50 to 99/1, more preferably 60/40 to 9
It is in the range of 8/2, and further 70/30 to 95/5. Within this range, a fine powder is reduced in advance by granulation, and then surface cross-linking is carried out while pulverizing, whereby a water absorbing agent excellent in absorption capacity under pressure and impact resistance can be obtained.

As described above, in the present invention, as the water absorbent resin powder, the above-mentioned (a) porous powder, (b) coarse powder having a constant particle size, and (c) granulated powder are preferably used, but other powders Can also be used as appropriate. Hereinafter, in the method of adding a cross-linking agent to the water-absorbent resin powder of the present invention to cross-link the vicinity of the surface thereof, the weight average particle diameter is 200 to 10
The surface cross-linking method of the water-absorbent resin, which is characterized in that the powder having a particle diameter of 00 μm is ground and simultaneously cross-linked, will be further described.

Conventionally, a technique of combining a plurality of water-absorbent resins at the time of surface cross-linking to granulate particles simultaneously with surface cross-linking (W
O91 / 17200, Tokuyohei 6-216042) is also known. Further, a technique for keeping the particle size constant during surface cross-linking (all Examples of JP-A-58-42602) is also known. Further, a technique for surface-crosslinking the foamed water-absorbent resin (Japanese Patent Laid-Open No. 8-509521, Japanese Patent Laid-Open No. H5-509521).
-237378, JP-A-63-88410, WO9
6/17884) is also known. Further, it is also known that surface destruction due to mechanical stress on the water-absorbent resin after surface-crosslinking is a problem (European Patent No. 812873).

However, in the present invention, contrary to the conventional wisdom that when a mechanical stress is exerted on the water-absorbent resin, the resin is adversely affected, the water-absorbent resin powder is mechanically stressed to a certain level or more until it is crushed. It was found that a water-absorbent resin satisfying both the water absorption rate and the absorption capacity under load can be obtained by surface-crosslinking the water-absorbent resin powder simultaneously with pulverization. Further, in view of pulverization efficiency and physical properties, the water-absorbent resin powder before surface cross-linking to be pulverized at that time is used as the above-mentioned (a) porous powder, (b) coarse particles having a constant particle size, or (c) granulation. Particles are preferred. (A) Surface cross-linking by pulverizing a porous foam gives a water-absorbing agent having a satisfactory water absorption rate and absorption capacity under pressure, and (b) Surface cross-linking by pulverizing coarse particles absorbs under pressure or under pressure. A water absorbent having excellent liquid permeability and impact resistance can be obtained.

As the surface cross-linking agent used in the present invention,
Specifically, for example, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl-1,3-pentane. Diol, polypropylene glycol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,4-
Butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-
Polyhydric alcohol compounds such as cyclohexanedimethanol, 1,2-cyclohexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, sorbitol; ethylene glycol diglycidyl Epoxy compounds such as ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycidol; ethylenediamine, diethylenetriamine, triethylenetetramine , Tetraethylenepentamine, Polyamines such as ethylene hexamine, polyethyleneimine, polyamide polyamine; haloepoxy compounds such as epichlorohydrin, epibromhydrin, α-methylepichlorohydrin; condensation of the above polyamine compounds with haloepoxy compounds Polyisocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate;
Polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltrimethoxysilane; 1,3-dioxolan-2-one, 4
-Methyl-1,3-dioxolan-2-one, 4,5-
Dimethyl-1,3-dioxolan-2-one, 4,4-
Dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one , 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxane-
Examples thereof include alkylene carbonate compounds such as 2-one and 1,3-dioxoban-2-one; however, they are not particularly limited.

Of the above-mentioned surface cross-linking agents, polyhydric alcohol compounds, epoxy compounds, polyvalent amine compounds, condensates of polyvalent amine compounds and haloepoxy compounds, and alkylene carbonate compounds are more preferable. Furthermore, when using a polyhydric alcohol that has been known to be easily granulated in the past, surface cross-linking while pulverizing the present invention is
In suppressing the deterioration of physical properties and the occurrence of dust due to the destruction during granulation after surface cross-linking, the method of the present invention is the most preferable, so in the present invention polyhydric alcohol is most preferably used as the surface cross-linking agent or the organic solvent, Furthermore, it is more preferable to use a plurality of polyhydric alcohols.

When one or more polyhydric alcohols are used, it depends on whether the polyhydric alcohol added near the surface of the water-absorbent resin powder acts as a cross-linking agent or an organic solvent. Rate, reaction temperature, reaction time, etc. When a plurality of polyhydric alcohols are used, the reactivity or permeation with respect to the water-absorbent resin is different in each polyhydric alcohol, so that one probably acts more as a solvent and the other acts as a cross-linking agent. , Surface cross-linking with higher physical properties is achieved. Further, when a plurality of polyhydric alcohols are used in the present invention, the number of carbon atoms is C2-15, further C3-10, and particularly C3-10, in view of physical properties.
A range of 8, more preferably C3-5 is preferred.

These surface cross-linking agents may be used alone or in combination of two or more kinds. When two or more surface cross-linking agents are used in combination, US Pat.
By combining the first surface cross-linking agent and the second surface cross-linking agent having different solubility parameters (SP values) exemplified in No. 5 or the like, it is possible to obtain a water-absorbent resin having further excellent water-absorbing properties. In addition, the above-mentioned solubility parameter is a value generally used as a factor showing the breadth of a compound.

The above-mentioned first surface-crosslinking agent is a compound having a solubility parameter of 12.5 (cal / cm ³ ) ^1/2 or more, which is capable of reacting with the carboxyl group of the water-absorbent resin,
For example, glycerin, propylene glycol, etc. are applicable. The second surface cross-linking agent has a solubility parameter of 12.5 which can react with a carboxyl group of the water absorbent resin.
The compound is less than (cal / cm ³ ) ^1/2 , and examples thereof include ethylene glycol diglycidyl ether and butanediol.

The amount of the surface-crosslinking agent used with respect to the water-absorbent resin depends on the combination of the water-absorbent resin and the surface-crosslinking agent, etc., but is 0.
Within the range of 01 to 5 parts by weight, more preferably 0.05 to 3
It may be within the range of parts by weight. By using the surface cross-linking agent within the above range, it is possible to further improve the water absorption characteristics for body fluids (aqueous liquids) such as urine, sweat and menstrual blood. If the amount of the surface cross-linking agent used is less than 0.01 part by weight,
The crosslink density in the vicinity of the surface of the water absorbent resin can hardly be increased. Further, when the amount of the surface cross-linking agent used is more than 5 parts by weight, the surface cross-linking agent becomes excessive, which is uneconomical and it may be difficult to control the cross-linking density to an appropriate value.

Specific examples of the organic solvent used include methyl alcohol, ethyl alcohol, n-propyl alcohol, iso-propyl alcohol, n-butyl alcohol, iso-butyl alcohol, t-butyl alcohol and the like. Lower alcohols such as acetone; ketones such as acetone; ethers such as dioxane, ethylene oxide (EO) adduct of monohydric alcohol, tetrahydrofuran; amides such as N, N-dimethylformamide, ε-caprolactam; sulfoxides such as dimethyl sulfoxide And the like. These organic solvents may be used alone or in combination of two or more.
Moreover, you may use the above-mentioned polyhydric alcohol as an organic solvent.

The amount of the hydrophilic solvent used for the water-absorbent resin and the surface-crosslinking agent depends on the combination of the water-absorbent resin, the surface-crosslinking agent, the hydrophilic solvent, and the like.
200 parts by weight or less, more preferably 0.
001 to 50 parts by weight, more preferably 0.1
To 50 parts by weight, particularly preferably 0.5 to 20 parts by weight.

In the present invention, the water-absorbent resin powder 10 is used as described above.
Within the range of 0.01 to 5 parts by weight, more preferably 0.05 to 3 parts by weight / organic solvent 2 with respect to 0 parts by weight.
00 parts by weight or less, more preferably 0.001 to 50 parts by weight, further preferably 0.1 to 50 parts by weight, particularly preferably 0.5 to 20 parts by weight / water. The water-absorbent resin mixture mixed in the range of 1 to 30 parts by weight, preferably 0.5 to 10 parts by weight may be surface-crosslinked simultaneously with the above-mentioned pulverization.

The method of adding the cross-linking agent is not particularly limited, and the water-absorbent resin may be dispersed in an inert solvent to add the cross-linking agent, but a post-cross-linking agent capable of reacting with a carboxyl group in a hydrophilic solvent may be added. Is dissolved or dispersed, and then the solution or dispersion is sprayed or dropped onto the particle mixture and mixed. In the present invention, the pulverization carried out at the same time while surface-crosslinking does not require a separate pulverization step, and at the time of the crosslinking agent mixing or the heat crosslinking reaction, the water-absorbent resin powder has a sufficient mixing force more than the conventional one until the pulverization. Or it is sufficient to apply pressure. It should be noted that pulverization and classification after heating after mixing the crosslinking agent are not intended because physical properties are difficult to improve.

The pulverization of the present invention can be most simply defined preferably by the reduction of the weight average particle diameter of the water-absorbent resin powder before the addition of the surface-crosslinking agent and the water-absorbing agent obtained by the surface-crosslinking. In addition, it is also defined by an increase in generation of fine powder and an increase in surface area, which will be described later. In the conventional surface cross-linking, the influence of the surface cross-linking agent occurs preferentially in the granulation, and these phenomena are not found. However, the present invention is characterized by performing pulverization exceeding granulation. That is, when crushed, it shows a phenomenon that the weight average particle size decreases, which is not seen in conventional surface treatment (usually granulated and particle size up), and in addition, increase of fine powder amount and specific surface area An increase can also define the grinding of the invention.

The mixing device used for mixing the water-absorbent resin and the surface-crosslinking agent preferably has a large mixing force in order to uniformly and reliably mix the two. Examples of the above-mentioned mixing device include a cylindrical mixer, a double-walled cone mixer, a high-speed stirring mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, and a fluidized furnace rotary desk type. A mixer, an air flow type mixer, a double arm type kneader, an internal mixer, a crushing type kneader, a rotary type mixer, a screw type extruder and the like are suitable.

Then, in the present invention, the powder is ground and simultaneously surface-crosslinked. As an apparatus used for crushing at the same time as surface crosslinking in the present invention, depending on the structure of the apparatus, the mechanical strength of the water-absorbent resin used and the operating conditions of the apparatus, and the crosslinking agent composition, various crushers and crushers The mixer which has a function can be mentioned. Examples of the apparatus used include a cylindrical mixer, a double-walled cone mixer, a V-shaped mixer, a ribbon mixer, a meat chopper, a screw mixer, a fluidized furnace rotary desk mixer, and an air flow. Type mixers, double-arm kneaders, internal mixers, crushing type kneaders, rotary type mixers, screw type extruders, etc.
The shape of the stirring blade or the inner wall and the clearance thereof may have a crushing function, and the operation may be performed under the conditions until the crushing of the water absorbent resin is observed.

That is, in the present invention, the water-absorbent resin powders are sufficiently agitated by using the above mixer so that crushing exceeding granulation can be observed by comparison before and after surface cross-linking. Further, the above-mentioned pulverization at the time of surface cross-linking may be difficult at room temperature or lower, and therefore it is preferably conducted while being externally heated. The above device uses a jacket as a heat source,
It is preferable that heating can be performed from the outside using hot air, infrared rays, microwaves, or the like. The treatment temperature and treatment time when pulverizing the water-absorbent resin during surface cross-linking may be appropriately selected depending on the combination of the water-absorbent resin and the surface cross-linking agent, the desired cross-linking density, etc., and are not particularly limited. However, the treatment temperature is, for example, 50 to 250 ° C., and further 10
The range of 0-200 degreeC is suitable.

In order to achieve high physical properties by carrying out the pulverization preferably in the present invention, the pulverization of the powder at the time of surface crosslinking should be 2
It is preferably carried out under a load of 0 g / cm ² or more or in the presence of a ball mill. That is, as the ball mill for carrying out the pulverization in the present invention, a material having a heat resistance of 100 ° C. or higher, particularly 200 ° C. or higher, further 300 ° C. or higher is used. It may be agitated. The shape of the ball mill is not particularly limited as long as it can be crushed, and its size is 1 to 100 mm,
Further, 5 to 50 mm is used, and 1 to 1000% by weight, and further 10 to 100% by weight are used with respect to the water absorbent resin.

The pressure applied for crushing in the present invention is preferably 20 g / cm ² or more at the lower part, and a load may be applied by capping the upper part of the water absorbent resin,
Further, the lower part may be pressed by its own weight by vertically stacking the water-absorbent resin. When pressurizing with its own weight, the bulk specific gravity of the water-absorbent resin powder (for example, 0.6 g / c) can be obtained by stacking 50 cm or more, preferably 100 cm or more in the vertical direction.
The surface pressure can be adjusted by c).

In addition, it is preferable that the water-absorbent resins are rotated at high speed after a load (weight) is appropriately applied in the above-mentioned mixer or heat treatment machine during the pulverization. Further, the pulverization index of the water absorbent resin defined by the following formula is 100.
The rotation may be 0 or more, preferably 2000 to 100000, and more preferably 3000 to 50000. (Pulverization index) = (Surface pressure A to the water-absorbent resin powder) × (Rotation speed B) × (Stirring time C) A: Pressure applied to the water-absorbent resin powder in the lower part of the mixer or the heat treatment device (g / cm) ² ), preferably 5 or more,
More preferably, it is 20 to 500.

B: Revolutions per minute (rpm / revolutions per m) of the mixer or heat treatment machine
is), preferably 2 or more, and more preferably 8 to 2000. C: residence time (minutes) of the water-absorbent resin powder in the mixer or heat treatment machine, preferably 10 or more,
More preferably, it is 15-100. In the present invention, at least a part of the particles is pulverized by pulverizing the water-absorbent resin powder at the time of surface cross-linking. At that time, the pulverization of the powder causes generation of fine particles of less than 150 μm by 10% by weight.
The following is preferable.

Conventionally, it has been known that fine particles of less than 150 μm are not preferable for the surface cross-linking of the water absorbent resin, but there is also a problem that the water absorption rate decreases when the fine powder is reduced. On the other hand, in the present invention, before the addition of the surface cross-linking agent, the amount of fine powder of less than 150 μm is small, specifically, 10% by weight or less is used to perform uniform surface cross-linking, and by increasing the generation of fine powder during surface cross-linking, The problem of water absorption rate was solved. From the viewpoint of physical properties, the amount of fine powder generated is preferably 0.
The range is 5 to 6% by weight, and the range is 1 to 5% by weight. Further, the amount of fine powder of the water absorbing agent obtained after pulverization is also preferably 10% by weight or less.

When the amount of fine powder generated is too large, the absorption capacity under load decreases, and when it is too small, the water absorption rate decreases. In addition, in order to reduce the fine powder before surface crosslinking,
As a preferred method, it is preferable to use (b) coarse particles having a constant particle size and (c) granulated powder as the surface-crosslinked water-absorbent resin powder. In the case of (b), the generation amount of fine powder due to pulverization is small, and in the case of (c), fine powder can be granulated.

In the present invention, the pulverization of the powder during surface cross-linking as described above suppresses the generation of fine particles of less than 150 μm to 10% by weight or less. At that time, the BET specific surface area of the water absorbent resin is To increase. By pulverizing the powder by surface cross-linking, the BET specific surface area is 1.05 to 10
And further increase 1.05 to 2 times. If the increase rate of the surface area is too small, it is disadvantageous in improving the water absorption rate, and if the increase rate is too large, it is disadvantageous in the absorbency against load. Such an increase in surface area requires a certain amount of mechanical stress before being pulverized,
The water-absorbent resin powder before adding the surface cross-linking agent has a bulk specific gravity of 0.6 to
It is more easily achieved because the resin is 0.1 (g / cc) and the water-absorbent resin powder before addition of the surface crosslinking agent is porous. The specific surface area can be determined by comparing the BET specific surface areas of the water-absorbent resin powder coated with the surface cross-linking agent before and after the reaction.

Further, as the most simple method for defining the pulverization of the present invention, there is a decrease in the weight average particle diameter. The weight average particle diameter can be compared by performing sieving classification on the water-absorbent resin powder before the addition of the surface crosslinking agent and the water-absorbent resin powder after the surface crosslinking reaction . Conventionally, it has been known that surface cross-linking of a water absorbent resin increases the average particle size, but in the present invention, conversely, the surface cross-linking reduces the weight average particle size. The reduction of the weight average particle diameter in the present invention is preferably 1 to 50%, more preferably 2 to 20%, even more preferably 3 to 15%, and most preferably 4 to 10%. If the weight-average particle size is reduced too much, the absorption capacity under load will not be improved, and if it is too small, the water absorption speed or impact resistance will not be improved. Furthermore, the weight average particle diameter of the water absorbing agent obtained by pulverization at the time of surface cross-linking is 100 to 60.
It may be appropriately adjusted so as to be in the range of 0 μm, more preferably 300 to 600 μm, and particularly preferably 400 to 600 μm.

In the present invention, the particles are ground at the same time as the surface cross-linking in this way, but since the particle size is controlled in advance, no classification is carried out. Further, in the surface cross-linking method of the present invention, it is preferable that the water-absorbent resin powder is ground and granulated at the same time. In order to carry out granulation, the surface cross-linking agent composition is appropriately adjusted, for example, water is used in a predetermined amount or more, and the crushing conditions and the crushing device at the time of surface cross-linking are appropriately adjusted so that all agglomerated particles are not destroyed. Good. By granulating after pulverizing at the time of surface cross-linking, the water absorption rate can be further improved and the particle size distribution can be optimized.

The amount of water used in the present invention is 0.1 with respect to 100 parts by weight of the water-absorbent resin powder, from the viewpoint of absorption capacity under pressure, and from the viewpoint of pulverization efficiency and granulation efficiency of the water-absorbent resin powder. -30 parts by weight, preferably 0.5-10 parts by weight. If the amount of water is too large, pulverization is difficult, and if it is too small, granulation is difficult. Further, for efficient granulation, it is preferable to essentially use a polyhydric alcohol as a surface cross-linking agent in the present invention.

The present invention is characterized in that the water-absorbent resin powder is ground and simultaneously surface-crosslinked, and more preferably granulation is also carried out at the same time.
Changes in particle size distribution before and after surface cross-linking, changes in specific surface area,
It can be confirmed by an electron micrograph (about 30 to 50 times magnification) of the water absorbent resin. In addition to the electron micrograph, the granulation is performed by crushing the water-absorbent resin powder as a blank with a mixer without using the surface cross-linking agent which is a binder of the granulation and its solution, and comparing the particle sizes with each other. Grain ratio is required. The granulation rate determined from the particle size distribution is preferably 1% or more, 1
10 to 10% by weight.

In the water-absorbent resin of the present invention obtained above,
Furthermore, if necessary, deodorants, fragrances, various inorganic powders, foaming agents, pigments, dyes, hydrophilic short fibers, plasticizers, adhesives, surfactants, fertilizers, oxidizing agents, reducing agents, water, Various functions may be added to the water absorbent resin by adding salts and the like.

[0092]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the scope of the present invention is not limited to these Examples. The physical properties of the water absorbent resin were measured by the following methods. (1) Water absorption amount of water-absorbent resin under no load 0.2 g of water-absorbent resin is added to a tea bag type bag (6 cm x 6 c
m) and heat seal the opening, and then
It was immersed in a 9 wt% sodium chloride aqueous solution (physiological saline). After 60 minutes, the tea bag type bag was pulled up, drained at 250 G for 3 minutes using a centrifuge, and then the weight W1 (g) of the bag was measured. Further, the same operation was performed without using the water absorbent resin, and the weight W0 (g) at that time was measured. Then, from these weights W1 and W0, the amount of water absorption (g / g) under no load was calculated according to the following equation a.

Formula a; Water absorption (g / g) = (W1-W
0) / weight of water-absorbent resin (g) (2) Water-absorption rate of water-absorbent resin 1.0 g of the water-absorbent resin was placed in a bottomed cylindrical polypropylene cup having an inner diameter of 50 mm and a height of 70 mm. Next, 28 g of physiological saline was poured into the cup. Then, the time from when the physiological saline was poured to when the physiological saline was completely absorbed by the water absorbent resin and disappeared was measured. The measurement was repeated 3 times, and the average of these values was taken as the water absorption rate (seconds).
And

(3) Solid Content (Water Content) of Water-Absorbent Resin About 1.000 g of the water-absorbent resin was placed in an aluminum cup (inner diameter 53 mm * height 23 mm) and dried in a windless oven at 180 ° C. for 3 hours. The water content / solid content (% by weight) of the water absorbent resin was actually measured and calculated from the loss on drying. (4) The average particle diameter and the particle size distribution average particle diameter of the water absorbent resin are JIS standard sieves (850 μm, 600 μm,
(300 μm, 150 μm, 106 μm), the water absorbent resin was sieve classified, and the residual percentage R was plotted on a logarithmic probability paper, and the particle diameter corresponding to R = 50% by weight was taken as the average particle diameter. When obtaining the weight average particle size, it is preferable to use four or more sieves, and if necessary, 710 μm,
500 μm, 425 μm, 355 μm, 200 μm, 7
A sieve of 5 μm or the like may be used.

(5) Specific Surface Area of Water-Absorbent Resin The specific surface area of the water-absorbent resin is determined by the "BET one-point method (Brun
auer-Emmett-Teller adsorption method) ". As the measuring device, "Specimen fully automatic specific surface area measuring device 4-sorb UC" (manufactured by Yuasa Ionics Co., Ltd.) was used. First, a water-absorbent resin (preliminarily separated by a JIS standard sieve and used as a sample) was a microcell (TYP
E: QS-400) and the sample-containing microcell was heated to 150 ° C. under a nitrogen gas stream to sufficiently degas and dehydrate the sample. Next, helium gas and 0.1%
The microcell containing the sample was cooled to −200 ° C. under a mixed gas flow consisting of krypton gas, and the mixed gas was adsorbed to the sample until equilibrium was achieved. Then, the temperature of the microcell containing the sample was returned to room temperature, the mixed gas was desorbed from the sample, and the specific surface area of the water absorbent resin was determined from the desorbed amount of the krypton mixed gas. Incidentally. The adsorption-desorption step of the microcell containing the sample was performed three times, and the specific surface area (m ² / g) of the water absorbent resin was determined from the average amount.

(6) Absorption capacity under pressure of water-absorbent resin First, the measuring apparatus used for measuring the absorption capacity under pressure will be briefly described below with reference to FIG. Figure 1
As shown in FIG. 1, the measuring device includes a balance 1, a container 2 having a predetermined volume placed on the balance 1, an outside air suction pipe 3,
The conduit 4, the glass filter 6, and the glass filter 6
It comprises a measuring unit 5 placed on top. The container 2 has an opening 2a at its top and an opening 2a at its side.
b, and the outside air intake pipe 3 is fitted into the opening 2a, while the conduit 4 is attached to the opening 2b. In addition, the container 2 contains a predetermined amount of physiological saline solution 11.
Is included. The lower end of the outside air suction pipe 3 is submerged in 0.9 wt% physiological saline 11. The glass filter 6 has a diameter of 70 mm. The container 2 and the glass filter 6 are in communication with each other through the conduit 4. Further, the upper portion of the glass filter 6 is fixed so as to be at a position slightly higher than the lower end of the outside air suction pipe 3.

The above-mentioned measuring unit 5 comprises a filter paper 7 and a supporting cylinder 8.
And a wire mesh 9 attached to the bottom of the support cylinder 8 and a weight 10. Then, in the measuring unit 5, the filter paper 7 and the supporting cylinder 8 (that is, the wire net 9) are placed in this order on the glass filter 6, and the weight 10 is placed inside the supporting cylinder 8, that is, on the wire net 9. It has been done. Support cylinder 8
Has an inner diameter of 60 mm. The wire net 9 is made of stainless steel and has a JIS 400 mesh (mesh size 38 μm).
m). Then, a predetermined amount of the water absorbent resin is evenly spread on the wire net 9. Weight 1
0 is 50 g / cm with respect to the wire net 9, that is, the water absorbent resin
The weight is adjusted so that the load of ² can be applied uniformly.

The water absorption capacity under pressure was measured using the measuring apparatus having the above-mentioned structure. The measuring method will be described below. First, the container 2 is filled with a predetermined amount of physiological saline solution 11.
A predetermined preparatory operation such as fitting the outside air intake pipe 3 into the above was performed. Next, the filter paper 7 was placed on the glass filter 6. On the other hand, in parallel with these mounting operations, inside the support cylinder,
That is, 0.9 g of the water-absorbent resin was evenly spread on the wire net 9, and the weight 10 was placed on the water-absorbent resin. Then filter paper 7
The wire net 9, that is, the support cylinder 8 on which the water-absorbent resin and the weight 10 are placed, is placed thereon. Then, from the time when the supporting cylinder 8 is placed on the filter paper 7, the weight W2 (g) of the physiological saline solution 11 absorbed by the water absorbent resin for 60 minutes.
Was measured using the balance 1. And the above weight W2
From this, the water absorption capacity under pressure (g / g) 60 minutes after the start of absorption was calculated according to the following equation b, and the water absorption capacity under pressure (50 g / cm) was calculated.
The water absorption capacity (g / g) of the water absorption capacity (g / g) of ² ) was used.

Formula b; Water absorption capacity under pressure (g / g) = weight W 2 (g) / weight of water-absorbent resin (g) (7) Water-soluble content of water-absorbent resin 0.5 g of water-absorbent resin 1000 ml After being dispersed in the deionized water of 1. and stirred for 16 hours, the swollen gel was filtered with a filter paper. Then, the water-soluble polymer in the obtained filtrate, that is, the water-soluble content (% by weight, against the water-absorbent resin) eluted from the water-absorbent resin was determined by colloid titration.

(8) Bulk specific gravity of water-absorbent resin An apparent density measuring device (according to JIS K3362 6.2) was placed horizontally on a stable table, and 100.0 g of the water-absorbent resin was measured.
Put it in the upper funnel of the apparent density measuring instrument, and have a known weight (g) of 100cc acrylic resin cup (JIS
Free fall to K3362 (according to 6.2). Of the dropped water-absorbent resin, the part that rises from the cup is gently scraped off with a glass rod, then the weight of the cup (g)
Was measured in units of 0.01 g to obtain the weight (g) of the water-absorbent resin per 100 cc, and this was divided by the volume of the cup (100 cc) to obtain the bulk specific gravity (g / cc). (Production Example 1) Acrylic acid 305 g, 37 wt% sodium acrylate aqueous solution 3229.5 g, polyethylene glycol (n = 8) diacrylate 1 as an internal crosslinking agent
2.4 g, interfacial polyoxyethylene sorbitan monostearate (trade name: Rheodor TW-S120, manufactured by Kao Co., Ltd.) 0.15 g, pure water 1331.1 g and 10% by weight sodium persulfate aqueous solution 20.3 g are mixed and mixed. An aqueous solution of monomers was prepared. This monomer aqueous solution and nitrogen are fluid-mixed by using Whip Auto Z manufactured by Aikosha Co., Ltd., and nitrogen gas bubbles are dispersed in the monomer aqueous solution, and the monomer aqueous solution is adjusted with the bubbles dispersed. I went. In particular,
Using the aspirator 12 ′ shown in FIG. 2, as shown in FIG. 3, the monomer aqueous solution 10 is supplied from the nozzle side at 1 kg / min, and the nitrogen gas 11 is supplied from the side pipe at 2 L / min. And mixed with each other in a fluid, and further passed through a mixing area 8 ′ having irregularities (protrusions) 9 ′ and led to a polymerization tank 16 ′. In the monomer aqueous solution 10 'that passed through the mixing area 8', nitrogen bubbles were dispersed and the volume was increased by 1.5 times. 101.6 g of a 2% by weight aqueous solution of sulfurous acid was added to the aqueous solution 13 containing bubbles to start the polymerization immediately. Subsequently, the temperature is 25 to 95 ° C with the bubbles dispersed.
Then, static polymerization was carried out for 1 hour. After the polymerization, the sponge-like hydrogel polymer containing a large amount of bubbles was cut into a square shape of 10 to 30 mm, and then dried in a hot air dryer at 160 ° C. for 2 hours. The dried material was crushed with a crusher, and the material passing through the sieve having an opening of 850 μm was collected to obtain a porous water-absorbent resin powder (1) having an average particle diameter of 310 μm. The water-absorbent resin powder (1) obtained by foaming polymerization has a water absorption of 41.8 (g / g), a water-soluble amount of 8% by weight, a solid content of 95.4% by weight, and a bulk specific gravity of 0.
It was 4 g / cc.

(Production Example 2) 60% partially neutralized sodium acrylate aqueous solution with a concentration of 33% by weight in which 75 mol% was neutralized
To 00 g, 0.03 mol% of polyethylene glycol (n = 8) diacrylate as an internal cross-linking agent (relative to monomer)
After preparing a monomer aqueous solution in which was dissolved, nitrogen was blown into the monomer aqueous solution to remove dissolved oxygen in the solution. Then, the aqueous monomer solution was poured into a double-arm kneader having an internal volume of 10 liters, and sodium persulfate 0.14 g / mol (relative to monomer) and L-ascorbic acid 0.005 g were stirred under a nitrogen stream. / Mol (to monomer) was added as an aqueous solution, and stirring polymerization was performed while pulverizing the produced polymer gel. After 1 hour, the unfoamed hydrogel polymer of about 1 mm was taken out by a kneader and then dried in a hot air dryer at 160 ° C. for 1 hour. The dried material is crushed by a crusher, and the material passing through a sieve having an opening of 850 μm is collected to obtain an average particle size of 400
A water-absorbent resin powder (2) having a size of μm was obtained. The water absorbent resin powder (2) is non-foamed and non-porous, and its water absorption amount is 50.
0 (g / g), water-soluble amount is 20.0% by weight, solid content is 9
It was 4% by weight and the bulk specific gravity was 0.66 g / cc.

(Example 1) The water-absorbent resin powder (1) obtained in Production Example 1 was classified according to JIS standard sieve 850-600 µm, and 100 parts by weight of the classified product was mixed with ethylene glycol diglycidyl ether (0.1 part). 18.09 parts by weight of a crosslinking agent solution consisting of 09 parts by weight / 3 parts by weight of propylene glycol / 9 parts by weight of water / 6 parts by weight of isopropyl alcohol were mixed. The BET specific surface area of the thus obtained mixture was 0.0275 m ² / g. Then, the mixture is added to a mortar type mixer having a crushing function depending on the shape (square shape) of the stirring blade and the clearance (about 0.1 to 10 mm), and the mixture is ground together with a pachinko ball (ball mill) as a grinding aid. Heat treatment was carried out at 210 ° C. for 35 minutes while effectively stirring at high speed (rotational motion 285 rpm / planetary motion 125 rpm).

The water absorbing agent (1) thus obtained had a specific surface area of 0.0302 m ² / g by pulverization at the time of surface cross-linking.
(1.10 times before surface cross-linking) and 150 μm
The amount of fine powder of less than 1% by weight increases, and the water absorbing agent (1)
Granulation and pulverization of the powder were confirmed in the electron micrograph of. The water absorbing agent (1) had a water absorption capacity of 34.0 (g / g), an absorption capacity under pressure of 16.3 (g / g), and a water absorption speed of 44 seconds. Further, the weight average particle diameter is 700 μm and 660
It was decreased to μm.

(Example 2) In Example 1, the high speed stirring time in the mortar type mixer having a crushing function was extended to 50 minutes. The water absorbing agent (2) thus obtained had a specific surface area of 0.0334 m ² / g (1.21 before surface cross-linking) due to pulverization during surface cross-linking.
The amount of fine powder of less than 150 μm increased by 2% by weight, and granulation and pulverization of the powder were confirmed in the electron micrograph of the water absorbing agent (2). The water absorbing agent (2) has a water absorption capacity of 32.1 (g / g) and an absorption capacity under pressure of 1
9.3 (g / g), water absorption speed is 42 seconds, water absorbing agent (1)
It was even better. The weight average particle size is 640.
It was decreased to μm.

Example 3 The water-absorbent resin powder (1) obtained in Production Example 1 was classified according to JIS standard sieve 600 to 300 μm, and then ethylene glycol diglycidyl ether of 0.1 part was added to 100 parts by weight of the classified product. 09 parts by weight / propylene glycol 3 parts by weight / water 9
18.09 parts by weight of a crosslinking agent solution consisting of 6 parts by weight / isopropyl alcohol was mixed. The BET specific surface area of the thus obtained mixture was 0.0372 m ² / g. Then, the mixture is added to a mortar type mixer having a crushing function depending on the shape (square) of the stirring blade and the clearance (about 0.1 to 10 mm), and crushed together with a pachinko ball (ball mill) as a crushing aid. Heat treatment was carried out at 210 ° C. for 45 minutes while effectively stirring at high speed (rotational motion 285 rpm / planetary motion 125 rpm). The water-absorbing agent (3) thus obtained was pulverized during surface cross-linking,
0.0391 m ² / g (1.05 times as much as before surface cross-linking), and 1% by weight of fine powder of less than 150 μm was generated, and the electron micrograph of the water absorbing agent (3) showed that Grain and crush were confirmed. The water absorption ratio of the water absorbing agent (3) is 28. 5 ( g / g), the absorption capacity under pressure is 22.9 (g
/ G), and the water absorption rate was 35 seconds. The weight average particle diameter was reduced from 450 μm to 410 μm.

(Comparative Example 1) In Example 3, the mixture obtained by mixing 18.09 parts by weight of the aqueous solution of the crosslinking agent was replaced with a mortar type mixer having a crushing function for heat treatment, and the mixture was heated at 210 ° C in a static oven. Heat treatment was performed for 50 minutes. Comparative water-absorbing agent thus obtained (1)
Has a specific surface area of 0.0359 m ² / g (0.9 before surface cross-linking) because it is not crushed during surface cross-linking and is granulated.
7 times) and no fine powder of less than 150 μm was observed. Weight average particle diameter increased to 500 μm,
The water absorption capacity of the comparative water absorbent (1) is 28.9 (g / g), the absorption capacity under pressure is 20.7 (g / g), and the water absorption speed is 41 seconds, which is much inferior to that of the water absorbent (3). It was

Comparative Example 2 The water-absorbent resin powder (2) obtained in Production Example 2 was classified by JIS standard sieve 500 to 300 μm, and then ethylene glycol diglycidyl ether of 0.1 part was added to 100 parts by weight of the classified product. 03 parts by weight / propylene glycol 1 part by weight / water 3
6.03 parts by weight of an aqueous solution of a cross-linking agent consisting of 2 parts by weight / isopropyl alcohol was mixed. The BET specific surface area of the thus obtained mixture was 0.0174 m ² / g. Then, the mixture was added to a normal mortar type mixer, and heat treatment was performed at 210 ° C. for 45 minutes while stirring at a low speed. The comparative water-absorbing agent (2) thus obtained had no pulverization at the time of surface cross-linking and was granulated, so that the weight average particle diameter increased to 450 μm and 0.0155 m ² / g (0. 89 times) and 150μ
No fine powder of less than m was also seen. The water absorption capacity of the comparative water absorbent (2) was 39.3 (g / g), and the absorption capacity under pressure was 19.
3 (g / g), the water absorption rate was 165 seconds.

(Production Example 3) 0.085 mol% of polyethylene glycol diacrylate as an internal cross-linking agent was added to 5500 g of an aqueous solution of sodium acrylate having a neutralization ratio of 71.3 mol% (monomer concentration: 39% by weight). After being dissolved and degassed with nitrogen gas for 30 minutes, the aqueous solution was supplied to a reactor equipped with a jacketed stainless steel double-armed kneader with an internal volume of 10 L and two sigma-type blades, and kept at a temperature of 20 ° C. While continuing the nitrogen substitution of the reaction system. Then, while rotating the blades, 3.5 g of sodium persulfate and 0.0.08 g of L-ascorbic acid were added as 10 wt% aqueous solutions, respectively. Polymerization started after 1 minute and the reaction system reached the peak temperature after 20 minutes. did. The hydrogel polymer produced at that time was subdivided into a size of about 5 mm. Then, stirring was further continued, and 60 minutes after the polymerization was started, the hydrogel polymer was taken out.

The obtained finely granulated product of the hydrogel polymer was spread on a wire net having an opening of 300 μm (50 mesh), and 170
It was dried with hot air at 70 ° C. for 70 minutes. The dried product is crushed with a desktop dual-purpose crusher FDS type (manufactured by Miyako Bussan Co., Ltd.), and further crushed to 85
The particles were classified with a 0 μm mesh to obtain crosslinked polyacrylate particles (B) having a solid content of 96% by weight and having an irregular crushed shape. Next, the crushed crosslinked polyacrylate particles (B) are classified using a sieve having openings of 850 μm and 150 μm, and powder (B1) having a particle size of 150 μm or more and less than 850 μm.
And a fine powder (B2) of less than 150 μm was obtained.

(Production Example 4) Particle size 1 obtained in Production Example 3
Crosslinked polyacrylate fine particles (B2) 2 having a particle size of less than 50 μm
While putting 00 g in a 5L mortar mixer (5L container is kept warm in a bath at 70 ° C) manufactured by Nishinihon Kenki Seisakusho, while rotating the stirring blades of the mortar mixer at a high speed of 60Hz / 100V (rotational motion 285rpm / planetary motion 125rpm), 90 300 g of warm water heated to ℃ was poured all at once from the funnel. The crosslinked polyacrylate fine particles (B2) and water were mixed within 10 seconds and stirred at high speed in a mortar mixer for 3 minutes.

The obtained hydrous gel-like granulated product (particle size: about 1-
3 mm) was taken out, placed on a wire mesh having an opening of 300 μm, and dried in a hot air dryer until the water content became less than 5% by weight. Then, the dried granulated product was pulverized with the tabletop dual-purpose pulverizer of Reference Example 1 to obtain a particle size of substantially less than 850 μm, 150
Granules (E) of crosslinked polyacrylate fine particles having a water absorption capacity of 30 g / g were obtained by classifying the particles into particles having a particle size of at least μm.

Next, 8% by weight of granulated particles (E) of crosslinked polyacrylate microparticles, substantially 15 obtained in Production Example 3 were used.
The primary particles (B1) having a particle size of 0 μm or more and less than 850 μm were mixed at a ratio of 92% by weight until they became uniform to obtain a particle mixture (F) having a solid content of 96% by weight. The particle size distribution of the obtained particle mixture (F) is 32.1% by weight in the range of 600 μm to less than 850 μm , and 5 in the range of 300 μm to less than 600 μm.
0.4% by weight, 15 μm or more and less than 300 μm is 15.
3% by weight, 2.2% by weight was less than 150 μm, and the weight average particle diameter was 485 μm.

Example 4 To 500 g of the particle mixture (F) of the primary particles and the granulated particles obtained in Production Example 4,
1.4-butanediol / propylene glycol / water /
Isopropanol = 0.32 / 0.50 / 2.73 /
An oil temperature of 210 ° C. with a pachinko ball (28 pieces / total 153 g) as a grinding aid in a mixer having a grinding function by mixing a cross-linking agent consisting of 0.45 (% by weight / water absorbent resin powder). The water-absorbing agent (4) was obtained by heating and stirring in the bath for 30 minutes. Water absorption capacity of water absorbing agent (4) is 28 g
/ G, the water absorption capacity under pressure was 25 g / g.

The particle size distribution of the water absorbing agent (4) thus obtained is 60
26.7% by weight of 0 μm or more and less than 850 μm , 300 μm
m or more and less than 600 μm 53.9% by weight, 150 μm or less
The particles having an upper particle size of less than 300 μm were 16.5% by weight, the particles having an particle diameter of less than 150 μm were 2.9% by weight (0.7% by weight increase), and the weight average particle diameter was 450 μm.

(Example 6) In Example 4, the water absorbent resin after mixing the cross-linking agent was laminated to a height of about 50 cm to apply a load of about 30 g / cm ² to the lower water absorbent resin.
Then, while heating at 180 ° C, the whole is rotated at 15 rpm.
By stirring for 40 minutes, the surface was cross-linked at the same time as crushing at a crushing index of 18000. The physical properties and particle size of the obtained water absorbing agent (6) were almost the same as in Example 4 (the water absorption capacity was 28 g.
/ G, water absorption capacity under pressure 25g / g, average particle size 450
μm). The liquid permeability under pressure was also the same as in Example 4.

Example 7 The water-absorbent resin powder (1) obtained in Production Example 1 was further classified and blended to obtain 600 μm or more and 850 μm or more.
Less than μm is 29.6% by weight, 300 μm or more and 600 μm
Less than 51.7% by weight, and 150 μm or more and less than 300 μm 18.7% by weight to obtain a water absorbent resin powder (1 ′) having a weight average particle diameter of 470 μm. The water-absorbent resin powder (1
′) 1.4-butanediol / 100 parts by weight
Propylene glycol / water / isopropanol = 0.9
Mixing a cross-linking agent consisting of 6 / 1.50 / 8.19 / 1.35 (wt% / water-absorbent resin powder), and in a mixer having a crushing function, pachinko balls (28 pieces) as a grinding aid. / Total 153g) and 5 in a bath with an oil temperature of 210 ° C
A water absorbing agent (7) was obtained by heating and stirring for 0 minutes. The water absorbing agent (7) has a water absorption capacity of 30 g / g and a water absorption capacity under pressure of 23.
The g / g and water absorption rate were 25 seconds.

The particle size distribution is 600 μm or more 8
Less than 50 μm 18.6% by weight, 300 μm or more 600
50.4% by weight less than μm , 150 μm or more and 300 μm
Less than 23.6% by weight, less than 150 μm is 7.4% by weight
And the weight average particle diameter was 400 μm.

[0118]

EFFECTS OF THE INVENTION According to the present invention, since the water-absorbent resin before the surface-crosslinking agent is mixed has a large particle size and a small amount of fine powder, it is easy to uniformly mix the surface-crosslinking agent. The water absorption rate is also improved. That is, in the present invention, since the water-absorbent resin powder after the surface cross-linking agent is uniformly mixed is further pulverized, it has a high absorption rate under pressure or a high absorption rate at high water absorption rate.
It is possible to provide a water-permeable resin having a lower liquid permeability .

[Brief description of drawings]

FIG. 1 is an apparatus for measuring absorption capacity under pressure, which is used in the present invention.

FIG. 2 is a cross-sectional view of an aspirator used in Production Example 1.

FIG. 3 is a cross-sectional view of a mixed region used in Manufacturing Example 1 having unevenness in a gap.

[Explanation of symbols]

1 Balance 2 containers 3 Outside air intake pipe 4 conduits 5 measuring section 6 glass filter 7 Filter paper 8 Support cylinder 9 wire mesh 10 weights 11 saline 2a opening 2b opening 1'Inlet for fluid ejected from nozzle 2'Another fluid inlet 3'nozzle 8'mixed area 9'unevenness 10 'monomer aqueous solution 11 'gas 12 'aspirator 13 'Bubble-containing monomer aqueous solution 14 'monomer preparation tank 15 'pump 16 'polymerization tank

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Nobuyuki Harada Inventor Nobuyuki Harada 1 992, Nishikioki, Akihama, Himeji-shi, Hyogo Prefecture Nihon Shatai Co., Ltd. Nippon Shokubai Co., Ltd. (56) References Patent 2675729 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C08J 3/00-3/28 B01J 20/26

Claims (11)

(57) [Claims] 1. A method of adding a surface cross-linking agent to a water-absorbent resin powder having a dry cross-linking structure to cross-link the vicinity of the surface thereof, wherein the proportion of fine powder of less than 150 μm is 10% by weight or less, and a weight average. Particle size is 200 ~ 1000μm
The method for surface-crosslinking a water-absorbent resin, wherein the water-absorbent resin powder is surface-crosslinked at the same time while being pulverized. 2. By pulverizing the water-absorbent resin powder at the time of surface cross-linking, compared with the water-absorbent resin powder before addition of the surface cross-linking agent.
The weight average particle diameter of the water absorbent resin after the surface crosslinking reaction is 1 to 5
The method for surface cross-linking a water absorbent resin according to claim 1, wherein the surface cross-linking is reduced by 0%. 3. The pulverization of the water-absorbent resin powder during surface cross-linking increases the proportion of fine particles of less than 150 μm in the water-absorbent resin powder after the surface cross-linking reaction, as compared with the water-absorbent resin powder before addition of the surface cross-linking agent. In addition, the method for surface-crosslinking the water absorbent resin according to claim 1 or 2, wherein the generation amount is 10% by weight or less. 4. A water-absorbent resin coated with a surface cross-linking agent by pulverizing a water-absorbent resin powder during surface cross-linking.
The BET specific surface area of the powder after the surface cross-linking reaction is higher than that before the reaction.
The method for surface cross-linking a water-absorbent resin according to any one of claims 1 to 3, wherein the surface cross-linking is increased 1.05 to 10 times. 5. Grinding of water-absorbent resin powder during surface cross-linking
However, when the surface cross-linking agent is mixed or the heat-crosslinking reaction is performed,
The water absorbent resin according to any one of claims 1 to 4,
Surface cross-linking method. 6. The pulverization of the water-absorbent resin powder during surface cross-linking is such that the surface pressure applied to the water-absorbent resin powder at the lower part of the surface cross-linking agent mixer or heat treatment machine is 20 g / cm ² (about 1.9).
The surface cross-linking method for a water-absorbent resin according to any one of claims 1 to 5, which is carried out by mixing or heat treatment under a load equal to or higher than 6 kPa) / or in the presence of a ball mill. 7. The surface-crosslinking method for a water-absorbent resin according to claim 6 , wherein the pulverization is performed with a pulverization index of 1000 or more. However
And, (pulverized Index) = (surface pressure A to the water-absorbent resin powder) × (times
Number of rotations B) x (stirring time C) A: Water absorbent resin powder in the lower part of the mixer or heat treatment machine
Such pressure (g / cm ² ) B: Number of rotations per minute of mixer or heat treatment machine (rp
m) C: Retention of water-absorbent resin powder in the mixer or heat treatment machine
Time 8. A method of adding a surface cross-linking agent to a dried water-absorbing resin powder having a cross-linking structure, and cross-linking the vicinity of the surface while crushing if necessary, wherein the powder is a fine powder of less than 150 μm . The proportion is 10% by weight or less,
Further, the weight average particle diameter is 300 to 600 μm , the absorption capacity against physiological saline is 35 (g / g) or less, and low absorption capacity is exhibited, and 25% by weight or more of the powder is 600 μm.
Characterized in that it is a coarse than 1000μm or more,
Surface-crosslinking method for water-absorbent resin. 9. surface crosslinking agent comprises a polyhydric alcohol, the surface crosslinking process for a water-absorbent resin according to any one of claims 1 to 8. 10. The polyhydric alcohol has 3 to 3 carbon atoms.
10. Use of a plurality of polyhydric alcohols according to claim 8,
Surface cross-linking method for water-absorbent resin. 11. A water-absorbent resin powder having a dried crosslinked structure.
The surface vicinity of the powder is further crosslinked with a plurality of polyhydric alcohols having 3 to 8 carbon atoms , and the surface-crosslinked water-absorbent resin powder has a weight-average particle diameter.
300-600 μm, less than 150 μm in the powder
Of the fine powder of 10% by weight or less and 1000 μm or less
The proportion of the upper particles is 5% by weight or less and 300 μm or more
The dry water-absorbent resin powder, wherein the proportion of the particles is 70 to 99% by weight .