JP2004359943A - Water absorption resin and its producing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water absorption resin which shows an excellent balance in water absorption performance, and is optimum for an absorption product such as a diaper. <P>SOLUTION: The water absorption resin comprises polymerizing an acrylic acid (salt) and crosslinking the resulting product, the resin having a mass average particle diameter of 300-600 μm, proportion of powders below 150 μm in a particle diameter to the resin being 0-10 mass%, total absorption magnification of the resin being ≥70 g/g, and an absorption efficiency under pressure thereof being ≥70%. A producing method of the resin comprises the steps (1) to (4) of: (1) polymerizing a monomer component comprising an acid group-containing unsaturated monomer as an essential component to obtain a water-containing gelling polymer, (2) drying and grinding the water-containing gelling polymer to obtain a water absorption resin powder, (3) adding a surface crosslinking treating agent comprising a surface crosslinking agent and water as essential components thereto, and (4) making a surface crosslinking treatment by heating the resulting product, and a time between the finalizing time of the step (3) and the starting time of the step (4) being ≤5 min. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a water absorbent resin and a method for producing the same. More specifically, the present invention relates to a water-absorbing resin having excellent performance and a method for producing a water-absorbing resin which is subjected to a specific surface crosslinking treatment.

In recent years, for the purpose of absorbing aqueous liquids such as body fluids, water-absorbent resins have been widely used as constituent materials of sanitary materials such as disposable diapers, sanitary napkins and incontinence pads.
Examples of the water-absorbing resin include a crosslinked product of a partially neutralized polyacrylic acid, a hydrolyzate of a starch-acrylonitrile copolymer, a neutralized product of a starch-acrylic acid graft polymer, and a vinyl acetate-acrylic ester. Saponified polymer, hydrolyzate of acrylonitrile copolymer or acrylamide copolymer or crosslinked product thereof, crosslinked carboxymethylcellulose, crosslinked copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS), polyethylene Crosslinked oxides, crosslinked polyallylamine, crosslinked polyethyleneimine and the like are known.

The characteristics that the water-absorbent resin should have include: excellent water absorption and absorption rate when in contact with an aqueous liquid such as a body fluid, liquid permeability, gel strength of a swollen gel, and water from a substrate containing an aqueous liquid. For example, a suction force for sucking up.
The relationship between these various properties does not necessarily show a positive correlation. For example, the higher the absorption characteristics under no pressure, the lower the absorption characteristics under pressure.
As a method for improving various properties of the water-absorbent resin in a well-balanced manner, a so-called surface cross-linking treatment technique for cross-linking the vicinity of the surface of the water-absorbent resin is known.
As crosslinking agents used in the surface crosslinking treatment, polyhydric alcohols, polyhydric glycidyl ethers, haloepoxy compounds, polyhydric aldehydes, polyvalent amines, polyvalent metal salts, and the like are known. As a method of cross-linking the vicinity of the surface of the water-absorbent resin by using, as a typical method, a method of mixing a surface cross-linking agent in which a cross-linking agent is dissolved in water and a hydrophilic organic solvent with a water-absorbent resin and heating. (See, for example, Patent Documents 1 to 3), a method of dispersing a water-absorbent resin in a mixed solvent of water and a hydrophilic organic solvent, adding a crosslinking agent, and reacting (for example, see Patent Document 4) is known. I have.

On the other hand, due to the problem of environmental pollution due to waste liquid and waste gas during manufacturing, and the application circumstances that water-absorbent resin is used for sanitary materials that come into direct contact with the human body, surface crosslinking treatment without using any hydrophilic organic solvent It is desired to do.
However, when a water-absorbent resin is produced using only water as a solvent without using a hydrophilic organic solvent during the surface cross-linking treatment (for example, see Patent Document 5), the water-absorbent resin is compared with a case where a hydrophilic organic solvent is used. There is a problem that the various characteristics, especially the absorption characteristics, are deteriorated.
Furthermore, until now, the manufacturing conditions of the water-absorbent resin itself and the surface treatment conditions as post-processing for the water-absorbent resin have been studied, and many water-absorbent resins have the above-described water absorption, absorption speed, liquid permeability, and gel. It has been designed or manufactured by focusing on various physical properties such as strength and suction power, but recently considering the actual use of urine and blood absorption applications such as diapers, it is desirable to satisfy these physical properties only to achieve a high level desired. It has been found that there is a case where the absorption performance cannot be exhibited. That is, there has been a problem that an optimal water-absorbing resin has not yet been designed or manufactured for a water-absorbing resin used for an absorbent article for use in excrement and blood absorption.
JP 2001-270948 A Japanese Patent Publication No. 9-502221 Japanese Patent No. 3305718 JP-B-61-48521 JP-A-4-246403

Therefore, the problem to be solved by the present invention is a water-absorbent resin exhibiting an excellent balance in water-absorbing performance, and in producing a water-absorbent resin, a hydrophilic organic solvent is not used or extremely small during the surface crosslinking treatment. Even so, an object of the present invention is to provide a method capable of producing a water-absorbing resin having excellent absorption characteristics.
Another object of the present invention is to provide a water-absorbent resin most suitable for absorbent articles such as diapers.

  The present inventor has conducted intensive studies in order to solve the above problems. As a result, to the water-absorbent resin powder, a surface cross-linking treatment agent containing a surface cross-linking agent and water as essential components and having a reduced content of a hydrophilic organic solvent in the treatment agent is added. When the step of performing a surface cross-linking treatment by heating is included in the production line of the water-absorbent resin, from the end of the operation of adding the surface cross-linking treatment agent to the water-absorbent resin powder to the start of the operation of heating. Focusing on the intermediate step, it has been found that the time required for the intermediate step greatly affects the absorption characteristics of the produced water-absorbent resin. Then, it has been found that the above problem can be solved by setting this time to a short time of 5 minutes or less, which cannot be expected from the conventionally known embodiments.

Generally, in a water-absorbent resin production plant, various reactors, processing apparatuses, and the like are connected in an intermediate step of transporting and storing as necessary (for example, see Patent Documents 1 to 3 described above), As the plant scale increases, the time spent on intermediate processes inevitably increases. And, in the case of a conventional manufacturing plant having a general production scale (production amount of tens of thousands to hundreds of thousands of tons per year), the intermediate process is designed on the scale of residence time in the range of tens of minutes to several hours. Was normal.
The present invention provides a surface cross-linking agent containing a surface cross-linking agent and water as essential components during the surface cross-linking treatment, wherein the surface cross-linking agent has a reduced content of a hydrophilic organic solvent in the processing agent. Using, to produce a water-absorbent resin having excellent absorption characteristics, the time spent in the intermediate process from the end of the operation of adding the surface crosslinking agent to the water-absorbent resin powder to the start of the operation of heating It was completed only after the knowledge that it was important to set a very short time.

According to the findings of the present inventor, the water-absorbent resin used for absorbent articles such as diapers, as is known so far, takes into account the particle size factor (that is, mass average particle size, and particle size). Needless to say, the ratio of powder having a diameter of less than 150 μm) is necessary, but in addition to this, it is also important to consider two entirely new parameters: total absorption capacity and absorption efficiency under pressure. According to the above-described manufacturing method, a water-absorbing resin that satisfies the above two parameters can be easily manufactured while satisfying the design criteria focusing on the particle diameter element.
That is, the method for producing a water-absorbent resin according to the present invention,
A step (1) of polymerizing a monomer component containing an acid group-containing unsaturated monomer as an essential component to obtain a hydrogel polymer.
Drying and pulverizing the obtained hydrogel polymer to obtain a water-absorbent resin powder (2);
The resulting water-absorbent resin powder has a surface cross-linking treatment agent containing a surface cross-linking agent and water as essential components, and the content of the hydrophilic organic solvent in the treatment agent is 0 to 10 mass with respect to the treatment agent. % Adding a surface cross-linking treatment agent,
Step (4) of performing a surface cross-linking treatment by heating;
Including
The time from the end of the step (3) to the start of the step (4) is within 5 minutes.

The water-absorbent resin according to the present invention is a water-absorbent resin obtained by polymerizing and crosslinking a monomer component containing acrylic acid and / or a salt thereof (neutralized product) as a main component, The particle size is 300 to 600 μm, the proportion of powder having a particle size of less than 150 μm is 0 to 10% by mass based on the water-absorbent resin, the total absorption capacity is 70 (g / g) or more, and the absorption under pressure is The efficiency is 70% or more.
However, the total absorption capacity and the absorption efficiency under pressure are defined by the following formula based on the value of 1 hour after absorbing a 0.90% by mass aqueous sodium chloride solution (25 ° C.).
Total absorption capacity (g / g) = Absorption capacity under no pressure (g / g) + Absorption capacity under single layer pressure (g / g)
Absorption efficiency under pressure (%) = Single layer absorption capacity under pressure (g / g) × 100 / absorption capacity under no pressure (g / g)
Furthermore, an absorbent article according to the present invention is an absorbent article for absorbing manure and blood, which contains the water-absorbent resin of the present invention.

  According to the present invention, it is also possible to provide a water-absorbent resin exhibiting an excellent balance in water-absorbing performance, and, when producing a water-absorbent resin, does not use a hydrophilic organic solvent during the surface crosslinking treatment or extremely. Even if the amount is small, it is possible to provide a method for producing a water-absorbent resin having excellent absorption properties, and it is possible to provide a water-absorbent resin most suitable for an absorbent article such as a diaper.

Hereinafter, the present invention will be described in detail. However, the scope of the present invention is not limited to the description, and modifications other than the following examples may be appropriately made without departing from the spirit of the present invention.
The method for producing a water-absorbent resin according to the present invention,
A step (1) of polymerizing a monomer component containing an acid group-containing unsaturated monomer as an essential component to obtain a hydrogel polymer.
Drying and pulverizing the obtained hydrogel polymer to obtain a water-absorbent resin powder (2);
The resulting water-absorbent resin powder has a surface cross-linking treatment agent containing a surface cross-linking agent and water as essential components, and the content of the hydrophilic organic solvent in the treatment agent is 0 to 10 mass with respect to the treatment agent. % Adding a surface cross-linking treatment agent,
Step (4) of performing a surface cross-linking treatment by heating;
including.

The water-absorbent resin in the present invention is a water-swellable, water-insoluble, crosslinked polymer capable of forming a hydrogel. The term "water swelling" means that the ion-exchanged water absorbs a large amount of water at least five times its own weight, preferably 50 to 1000 times. The term "water-insoluble" means that the uncrosslinked water-soluble component (water-soluble polymer) in the water-absorbent resin specified in US Pat. No. 6,187,872 is preferably 0 to 50% by mass based on the water-absorbent resin. , More preferably 0 to 25% by mass, further preferably 0 to 20% by mass, particularly preferably 0 to 15% by mass, and most preferably 0 to 10% by mass.
As the water-swellable water-insoluble crosslinked polymer constituting the water-absorbent resin in the present invention, polyacrylic acid partially neutralized polymer, starch-hydrolyzate of acrylonitrile graft polymer, starch-acrylic acid graft polymer, Saponified vinyl acetate-acrylic ester copolymer, hydrolyzed acrylonitrile copolymer or acrylamide copolymer or cross-linked product thereof, carboxyl group-containing cross-linked polyvinyl alcohol-modified product, cross-linked isobutylene-maleic anhydride copolymer Or the like, but preferably, a polyacryl obtained by polymerizing and crosslinking a monomer component containing acrylic acid and / or a salt thereof (neutralized product) as a main component. It is an acid partially neutralized polymer.

In the method for producing a water-absorbent resin according to the present invention, the hydrogel polymer has an acid group and / or a salt thereof, and is preferably a monomer containing an acid group-containing unsaturated monomer as an essential component. Obtained by polymerizing the components. In addition, as the acid group-containing unsaturated monomer, a monomer that becomes an acid group by hydrolysis after polymerization (for example, acrylonitrile) is also included, but preferably, the monomer already contains an acid group at the time of polymerization. Acid group-containing unsaturated monomer.
In the present invention, it is preferable that acrylic acid and / or a salt thereof be a main component as a monomer component.
When acrylic acid and / or its salt are used as a main component as a monomer component, other monomers may be used in combination. The monomer that can be used in combination is not particularly limited as long as the effects of the present invention can be exhibited even when used in combination. For example, methacrylic acid, maleic acid (anhydride), fumaric acid, crotonic acid, itaconic acid, vinylsulfonic acid , 2- (meth) acrylamide-2-methylpropanesulfonic acid, (meth) acryloxyalkanesulfonic acid and its alkali metal salt, ammonium salt, N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol (meth) acrylate, isobutylene, lauryl (meth) acrylate Water-soluble or hydrophobic unsaturated monomers and the like.

When a monomer other than acrylic acid and / or its salt is used in the present invention, the monomer other than acrylic acid and / or its salt is the total amount of acrylic acid and / or its salt used as a main component. Is preferably 0 to 30 mol%, more preferably 0 to 10 mol%. By using such a ratio, the effect of the present invention is sufficiently exhibited, and the absorption performance of the obtained water-absorbent resin is further improved, and the water-absorbent resin can be obtained at a lower cost.
In the method for producing a water absorbent resin according to the present invention, the hydrogel polymer has a crosslinked structure. Such a cross-linked structure may be a self-cross-linking type that does not use a cross-linking agent, but a cross-linking agent having two or more polymerizable unsaturated groups or two or more reactive groups in one molecule (water-absorbing agent) A crosslinked structure obtained by copolymerizing or reacting an internal crosslinking agent of a resin) is more preferable.

  Specific examples of the internal cross-linking agent include, for example, N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) A) acrylate, glycerin tri (meth) acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri (meth) acrylate, pentaerythritol hexa (meth) acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine , Poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene Recall, propylene glycol, glycerol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethylenimine, and glycidyl (meth) acrylate.

The internal cross-linking agent may be used alone, or two or more types may be appropriately used in combination. Further, the internal crosslinking agent may be added to the reaction system all at once, or may be added in portions. When an internal crosslinking agent is used, it is preferable to use a compound having two or more polymerizable unsaturated groups at the time of polymerization in order to sufficiently exert the effects of the present invention.
The amount of the internal crosslinking agent to be used is preferably 0.001 to 2 mol%, more preferably 0.005 to 0.5 mol%, and still more preferably, the monomer component (excluding the crosslinking agent). It is 0.01 to 0.2 mol%, particularly preferably 0.03 to 0.15 mol%. When the use amount of the internal cross-linking agent is less than 0.001 mol% or more than 2 mol%, the effect of the present invention may not be sufficiently exerted, and the obtained water-absorbing resin may not be sufficient. It is not preferable because the absorption characteristics may not be exhibited.

When introducing a cross-linked structure into the polymer using an internal cross-linking agent, the internal cross-linking agent is added to the reaction system before or during polymerization of the monomer component, or after polymerization, or after neutralization. do it.
In the method for producing a water-absorbent resin according to the present invention, a method for polymerizing a monomer component to obtain a hydrogel polymer is not particularly limited, and examples thereof include aqueous solution polymerization, reverse phase suspension polymerization, and bulk polymerization. , Precipitation polymerization, etc., from the viewpoint of performance, easy control of polymerization, absorption characteristics of swollen gel, etc. Is preferred.

When the monomer component is used as an aqueous solution, the concentration of the monomer component in the aqueous solution (hereinafter sometimes referred to as an aqueous monomer solution) is determined by the temperature of the aqueous solution and the type of the monomer component, and is particularly limited. However, the content is preferably in the range of 10 to 70% by mass, and more preferably in the range of 20 to 60% by mass, based on the aqueous monomer solution. In addition, when performing the aqueous solution polymerization, a solvent other than water may be used in combination as needed, and the type of the solvent used in combination is not particularly limited.
Reverse phase suspension polymerization is a polymerization method in which an aqueous monomer solution is suspended in a hydrophobic organic solvent, and examples thereof include U.S. Pat. Nos. 4,093,776, 4,667,323, 4,446,261, 4,683,274, and 5,244,735. It is described in US patents. The aqueous solution polymerization is a method of polymerizing an aqueous monomer solution without using a dispersion solvent. For example, U.S. Pat. Nos. 4,625,001, 4,873,299, 4,286,082, 4,973,632, 4,985,518, 5,124,416 and 5,250,640. No. 5,264,495, 5,145,906 and 5,380,808, and European patents such as EP0811636, EP0955086, and EP0922717. The monomer components and initiators exemplified in these polymerization methods can also be applied to the present invention.

When the above polymerization is initiated, for example, potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide, 2,2′-azobis (2-amidinopropane) dihydrochloride, etc. A radical polymerization initiator or a photopolymerization initiator such as 2-hydroxy-2-methyl-1-phenyl-propan-1-one can be used. The amount of these polymerization initiators used is preferably 0.001 to 2 mol% (based on the total monomer components), and more preferably 0.01 to 0.1, in view of the physical properties of the resulting water-absorbent resin. Mol% (based on the total monomer components).
As described above, a hydrogel polymer is obtained by the step of polymerizing a monomer component containing an acid group-containing unsaturated monomer as an essential component.

The hydrogel polymer obtained by the above polymerization is subdivided if necessary, then dried and pulverized. The pulverization may be performed before, simultaneously with, or after drying. Pulverization is preferably performed after drying.
In the present invention, drying is preferably performed on a particulate hydrogel polymer (for example, having a mass average particle diameter of 2 cm or less, preferably 1 cm or less, more preferably 5 mm or less). In the present invention, as a fragmentation method for converting the hydrogel polymer into particles, the fragmentation may be performed simultaneously with the polymerization using a kneader or the like, or may be separately fragmented after the polymerization, Subdivision during polymerization and subdivision after polymerization may be used in combination. When the hydrogel polymer is not dried in the form of particles, for example, in the case of a film, the physical properties are poor and the particle size may not be in a preferable range.

As the particle diameter of the hydrogel polymer before drying, from the viewpoint of drying efficiency and physical properties, the mass average particle diameter is preferably in the range of 45 to 4000 μm, more preferably in the range of 50 to 2000 μm, and still more preferably 100 to 1500 μm. And particularly preferably in the range of 200 to 1000 μm. If the weight average particle diameter of the hydrogel polymer is out of the above range, the resulting water-absorbent resin may have a reduced water absorption capacity or an increased water-soluble component.
Apparatus suitable for subdivision include, for example, a kneader, a vertical slitter with a cutter blade, a horizontal slitter with a cutter blade, a cutter-type crusher with a rotary blade, a meat chopper with a predetermined hole diameter, and the like. Can be exemplified.

In the method for producing a water absorbent resin according to the present invention, the hydrogel polymer is essentially dried. The drying in the present invention means that the above-mentioned hydrogel polymer has a solid content (defined by a loss on drying at 180 ° C. for 3 hours) of at least 80% by mass, preferably at least 80% by mass, based on the polymer after drying. 85% by mass or more, more preferably 90% by mass or more, particularly preferably 93% by mass or more. The drying in the present invention does not necessarily need to be a dry polymer having a solid content of 100% by mass (zero moisture).
The drying method that can be used in the present invention is not particularly limited, and is, for example, one of drying methods such as hot air drying, thin film drying using a drum dryer, a reduced pressure drying method, stirring drying, and fluidized bed drying. Alternatively, two or more kinds can be used, and continuous or batch drying is not particularly limited. From the viewpoint of physical properties and drying efficiency, in the present invention, hot-air drying, particularly continuous hot-air drying, is preferably used. For this purpose, for example, it is preferable to dry by standing on a belt.

  From the viewpoint of drying efficiency, such hot air drying includes, for example, laminating a particulate hydrogel polymer on a wire mesh / or a punching metal having holes or slits, and then moving the gel vertically or horizontally, preferably vertically. In the direction, hot air may be ventilated into the gaps between the stacked particles. As a wire mesh or a hole diameter to be used, for example, in the case of a hole or a wire mesh, a vent hole having preferably about 0.1 to 5 mm, more preferably about 0.2 to 2 mm may be provided. The lamination of the gel on a wire mesh or a punched metal is preferably carried out in terms of physical properties after drying, preferably from 1 to 20 cm, more preferably from 1.5 to 10 cm, and still more preferably from 2 to 8 cm. The gel polymer may be laminated.

  The drying temperature at the time of drying the above-mentioned hydrogel polymer is usually preferably 100 ° C. or higher, more preferably 110 to 230 ° C., further preferably 130 to 200 ° C., and particularly preferably, from the viewpoint of physical properties and productivity. May be set to 150 to 190 ° C. The drying temperature is defined by the material temperature or the temperature of the heat medium (such as hot air), but is preferably specified by the heat medium temperature. Further, the drying temperature during the drying period may be constant, or may be appropriately changed during the drying in the above temperature range. Furthermore, when drying with hot air, the dew point of the hot air is preferably in the range of 40 to 100 ° C, more preferably 50 to 90 ° C, and still more preferably 60 to 85 ° C from the viewpoint of physical properties and energy efficiency.

It should be noted that the particulate hydrogel polymer that has been laminated and dried is likely to become a block-shaped dried product having lost fluidity due to aggregation between particles after drying. Such a block is an aggregate of dried polymer particles, but has continuous voids and air permeability to the inside of the block. However, since the aggregate has no fluidity, it is pulverized (crushed). It is necessary to keep.
In the present invention, the hydrogel polymer is dried as described above and further pulverized. The pulverization may be before, simultaneously with or after drying, but is preferably performed after drying. More preferably, it is ground and further classified.
In the present invention, drying and pulverization, and if necessary, classification are preferably performed in a continuous step, and the time from the dryer outlet to the pulverizer inlet is preferably within 10 minutes, more preferably within 5 minutes, and further preferably. Within 2 minutes.

In the present invention, the pulverization method is not particularly limited as long as the dried polymer or its aggregate (block-like substance) can be converted into a fluid powder, preferably a powder having a mass average particle diameter of 2 mm or less. A method of pulverizing using a hammer type pulverizer, a roll type pulverizer, a jet stream type pulverizer, or the like, and one or more of various conventionally known pulverization or pulverization methods can be used. When the agglomeration during drying is weak, the agglomeration of the polymer may be loosened and crushed by vibrating and classifying the dried polymer without using a pulverizer.
In the present invention, after the above-mentioned pulverization, if necessary, classification is further preferably carried out to remove coarse particles and fine powder. The mass-average particle diameter of the water-absorbent resin powder thus obtained is determined according to the purpose, but in order to sufficiently exhibit the effects of the present invention, the water-absorbent resin powder has a mass-average particle diameter of preferably 300. ６００600 μm, more preferably 300 to 550 μm, particularly preferably 380 to 550 μm. Further, the proportion of the powder having a particle diameter of less than 150 μm is preferably 0 to 10% by mass, more preferably 0 to 8% by mass, still more preferably 0 to 5% by mass, based on the water-absorbent resin powder. Preferably it is 0-3 mass%. Further, the total of particles having a particle size of less than 150 μm to 850 μm or more is preferably 0 to 15% by mass, more preferably 0 to 10% by mass, and still more preferably 0 to 5% by mass, based on the water-absorbent resin powder. is there.

In particular, in the present invention, the mass average particle diameter of the water-absorbent resin powder is 300 to 600 μm, and the ratio of the powder having a particle diameter of less than 150 μm is preferably 0 to 10% by mass relative to the water-absorbent resin powder. , More preferably 0 to 5% by mass, and most preferably 0 to 3% by mass.
The bulk specific gravity of the water-absorbent resin powder varies depending on the true specific gravity (g / cm ³ ) uniquely determined by the monomer composition. For example, when the water-absorbent resin is sodium polyacrylate, particularly, the neutralization ratio is 50 to 90 mol. % Of sodium polyacrylate, and further, in the case of sodium polyacrylate having a neutralization ratio of 60 to 80 mol%, the bulk specific gravity is usually preferably 0.63 g / ml or more, particularly preferably 0.65 g / ml or more. It is preferable that Incidentally, the bulk specific gravity may be measured by an apparatus according to JIS K-3362. In the present invention, since the water-absorbent resin powder after pulverization has few scales and is more rounded and has a uniform shape, the bulk specific gravity tends to be high, and the bulk specific gravity is preferably 0.65 to 0.89 g / ml. , More preferably 0.67 to 0.88 g / ml, still more preferably 0.73 to 0.87 g / ml, still more preferably 0.74 to 0.86 g / ml, and still more preferably 0.75 to 0.85 g. / Ml. If the bulk specific gravity of the pulverized water-absorbent resin powder is out of the above range, the effects of the present invention may not be sufficiently exerted.

  After the above-mentioned pulverization, coarse particles (for example, 850 μm on product) and fine powder (for example, 150 μm pass product) may be appropriately recycled in some cases. The particle size distribution may be obtained by re-grinding coarse particles and removing or collecting fine particles. However, the sharpness of the particle size distribution in the present invention greatly reduces the need for such recycling. In addition, the recycling method of the fine powder of the water absorbent resin is disclosed in U.S. Pat. Nos. 4,950,692, 5,064,582, 5,264,495 and 5,478,879, and European Patents 0812873, 0888517, 0844270, and the like. It is also possible to apply the fine powder recycling method of the present invention to the present invention. The amount of the fine powder recycled is preferably 30% by mass or less, more preferably 15% by mass or less, particularly preferably 1 to 10% by mass, and most preferably 2 to 8% by mass. In the present invention, a water-absorbent resin powder having a sharp particle size distribution can be obtained by pulverization with high productivity, and thus a small amount of fine powder can be recycled to obtain a water-absorbent resin powder having a sharp particle size distribution.

The water-absorbent resin powder obtained as described above preferably has an absorption capacity under a no-pressure condition of not less than 40 g / g, more preferably not less than 45 g / g, with respect to a 0.90% by mass aqueous sodium chloride solution (physiological saline). , More preferably at least 50 g / g, particularly preferably 55 g / g. In order to obtain a water-absorbent resin having excellent absorption characteristics, it is preferable that the water-absorbent resin powder as the base polymer has a high absorption capacity.
In the method for producing a water-absorbent resin according to the present invention, the obtained water-absorbent resin powder is a surface cross-linking agent containing a surface cross-linking agent and water as essential components, and a hydrophilic organic solvent in the processing agent. The method includes a step (3) of adding a surface crosslinking agent having a content ratio of 0 to 10% by mass with respect to the treating agent, and a step (4) of performing a surface crosslinking treatment by heating.

The surface cross-linking agent for performing the surface cross-linking treatment includes various types, and is not particularly limited.From the viewpoint of improving the physical properties of the obtained water-absorbing resin, a polyhydric alcohol compound, an epoxy compound, a polyamine compound or Preferred are condensates with the haloepoxy compound, oxazoline compounds, mono-, di- or polyoxazolidinone compounds, cyclic urea compounds, polyvalent metal salts, alkylene carbonate compounds and the like.
The surface cross-linking agent that can be used in the present invention is not particularly limited, and for example, the surface cross-linking agents exemplified in U.S. Patent Nos. 6,228,930, 6,071,976, and 6,254,990 can be used. For example, mono, di, tri, tetra or polyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin , 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol Alcohol compounds; Epoxy compounds such as ethylene glycol diglycidyl ether and glycidol; Ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, polyethylene imine, polyamide polyamine, etc. Polyvalent amine compounds; haloepoxy compounds such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin; condensates of the above polyvalent amine compounds with the above haloepoxy compounds; oxazolidinone compounds such as 2-oxazolidinone; Compound; an alkylene carbonate compound such as ethylene carbonate and the like, and the like, and only one type of these compounds may be used, or two or more types may be used in combination. In order to sufficiently exert the effects of the present invention, it is preferable to use a polyhydric alcohol compound among these surface crosslinking agents. As the polyhydric alcohol compound, those having 2 to 10 carbon atoms are preferable, and those having 3 to 8 carbon atoms are more preferable.

The amount of the surface cross-linking agent used depends on the compound to be used and the combination thereof, but is preferably in the range of 0.001 to 10% by mass, and more preferably in the range of 0.01 to 5% by mass, based on the water-absorbent resin powder. Is more preferable.
The surface cross-linking agent used in the method for producing a water-absorbent resin according to the present invention is a surface cross-linking agent containing the surface cross-linking agent and water as essential components, and the content ratio of the hydrophilic organic solvent in the processing agent. Is a surface cross-linking treatment agent in an amount of 0 to 10% by mass based on the treatment agent.
In the present invention, the amount of water contained in the surface crosslinking agent depends on the water content of the water-absorbing resin powder used, but is preferably in the range of 0.5 to 20% by mass based on the water-absorbing resin powder. Preferably it is in the range of 0.5 to 10% by weight, particularly preferably 0.5 to 7% by weight, most preferably 0.5 to 4% by weight.

The content ratio of the hydrophilic organic solvent in the surface cross-linking treatment agent in the present invention is 0 to 10% by mass based on the treatment agent. It is preferably 0 to 8% by mass, more preferably 0 to 5% by mass, still more preferably 0 to 3% by mass, particularly preferably 0 to 1% by mass, and most preferably 0% by mass (substantially free). is there. In the method of the present invention, various physical properties are improved, and a novel water-absorbent resin described later is obtained.In addition, by using a surface-treated cross-linking agent in which the hydrophilic organic solvent is reduced, a waste liquid or a waste gas during the production is used. The problem of environmental pollution is solved, and the water-absorbent resin is used in a sanitary material that is in direct contact with the human body.
Here, the hydrophilic organic solvent may be any organic compound that does not cause a cross-linking reaction with an acid group, preferably a carboxyl group, of the water-absorbing resin (usually, 0 or 1 functional group). For example, alcohols such as ethyl alcohol, propyl alcohol, and isopropyl alcohol; ketones such as acetone; ethers such as dioxane, alkoxy (poly) ethylene glycol and tetrahydrofuran; amides such as ε-caprolactam; dimethyl sulfoxide and the like Sulfoxides; and the like.

In the present invention, the hydrophilic organic solvent refers to an organic compound that can be dissolved in liquid water at normal temperature, and usually has a solubility of 1 g or more, preferably 10 g or more in 100 g of water at normal temperature. In the present invention, it is essential that the hydrophilic organic solvent is a liquid at room temperature, but as a solvent that maximizes the effect intended by the present invention, a volatile hydrophilic organic solvent, for example, preferably has a boiling point of A hydrophilic organic solvent having a temperature of 50 to 170C, more preferably 60 to 150C, and particularly preferably 60 to 130C.
When the crosslinking agent is an organic compound, the distinction between the crosslinking agent and the hydrophilic organic solvent is as follows. Usually, a compound that does not substantially cause a cross-linking reaction with the water-absorbent resin under a selected reaction condition (a compound that does not substantially change the physical properties of the water-absorbent resin) is classified as a hydrophilic organic solvent. In the case where a water-absorbent resin and an organic compound or an aqueous solution of an organic compound are mixed and heated to carry out a crosslinking reaction, the organic compound is defined as a crosslinking agent.

The surface crosslinking agent in the present invention may further contain components other than those described above as long as the effects of the present invention are not impaired. Examples of such a component include a water-insoluble fine particle powder and a surfactant.
In the method for producing a water-absorbent resin according to the present invention, the method for adding the surface cross-linking agent to the water-absorbent resin powder is not particularly limited. Method.
When adding the surface crosslinking agent to the water-absorbent resin powder, it is preferable to use a mixing device in order to add the agent effectively. The mixing device that can be used is not particularly limited, but preferably has a large mixing force in order to uniformly and surely mix the water-absorbing resin powder and the surface crosslinking agent. Examples of the mixing device include a cylindrical mixer, a double-walled conical mixer, a V-shaped mixer, a ribbon-type mixer, a screw-type mixer, a fluid-type rotary desk-type mixer, a gas-flow type mixer, Examples include a double-arm kneader, an internal mixer, a pulverizing kneader, a rotary mixer, and a screw-type extruder. The mixing speed may be high or low.

In the method for producing a water-absorbent resin according to the present invention, the temperature of the water-absorbent resin powder when the surface crosslinking agent is added is preferably in the range of 40 to 80 ° C, more preferably 45 to 80 ° C. Within the range, more preferably within the range of 50 to 70C. If the temperature of the water-absorbent resin powder at the time of adding the surface cross-linking treatment agent is lower than 40 ° C., the water-absorbent resin is not easily produced in a high-humidity environment when water-absorbent resin is produced, which is not preferable because it causes trouble. . If the temperature of the water-absorbent resin powder at the time of adding the surface cross-linking treatment agent is higher than 80 ° C., the surface cross-linking treatment tends not to be uniform, which is not preferable.
In the method for producing a water-absorbent resin according to the present invention, the temperature of the surface-crosslinking agent when the surface-crosslinking agent is added is not particularly limited, but substantially exceeds the temperature of the water-absorbing resin powder to be added. It is preferable that the temperature is not so as to sufficiently exert the effects of the present invention.

In the method for producing a water-absorbent resin according to the present invention, a surface cross-linking treatment is performed by adding a surface cross-linking treatment agent to the water-absorbent resin powder and then heating.
The heating temperature (material temperature to heating medium temperature) is preferably in the range of 60 to 260 ° C, more preferably in the range of 80 to 240 ° C, further preferably in the range of 100 to 220 ° C, and particularly preferably 120 to 220 ° C. The heating time is preferably in the range of 1 minute to 120 minutes, more preferably in the range of 10 minutes to 100 minutes, further preferably in the range of 20 minutes to 90 minutes, and particularly preferably. It is within the range of 30 to 60 minutes. Preferred examples of the combination of the heating temperature and the heating time are, for example, 1 minute to 90 minutes at 180 ° C and 1 minute to 60 minutes at 200 ° C.

  In the method for producing a water-absorbent resin according to the present invention, it is important to start heating within 5 minutes more than 0 second after the addition of the surface crosslinking agent to the water-absorbent resin powder. That is, it is important that the time from the end of the step (3) to the start of the step (4) exceeds 0 seconds and is within 5 minutes. It is preferably within 4 minutes, more preferably within 3 minutes, further preferably within 2 minutes, particularly preferably within 1 minute. By starting heating within 5 minutes after the addition of the surface crosslinking agent to the water-absorbent resin powder, the effects of the present invention are exhibited. If the time from completion of the addition of the surface crosslinking agent to the water-absorbent resin powder to the start of heating exceeds 5 minutes, the effect of the present invention cannot be exhibited.

  In the present invention, the time from completion of the addition of the surface crosslinking agent to the water-absorbent resin powder to the start of heating is, specifically, the step of adding the surface crosslinking agent to the water-absorbent resin powder ( It refers to the time from the end of 3) to the start of the step (4) of performing the surface cross-linking treatment by heating, for example, the time to enter the apparatus of the step (4). In the case of continuous production, the amount of feed (supply amount) of the water-absorbent resin per unit time, the residence time of the water-absorbent resin in an apparatus for adding and mixing a surface cross-linking agent, and the surface cross-linking treatment by heating. Can be easily calculated based on the transport time required to enter the apparatus of the step (4) for performing the above. Specifically, this time may be determined by actually measuring or calculating the movement time of the powder from the mixer outlet to the heater inlet.

As a method for cross-linking the surface of the water-absorbent resin, a treatment agent (surface cross-linking agent) is continuously mixed with the water-absorbent resin powder fed at 1000 kg / h in the examples of JP-A-8-508517, and then temporarily stored. And the like for continuous heating. As described above, in the conventional technique, generally, a water-absorbing resin is temporarily provided between the mixer of the surface cross-linking agent and the subsequent reactor (heating machine) as a cushion connecting the respective steps. The storage was done.
Further, in the case of continuous production of the water-absorbent resin, each process is connected using a transporter, but as the production scale of the water-absorbent resin increases from several tens of thousands to several hundred thousand tons annually, In addition to storage, a certain transport distance (transport time) between each process is required, and an intermediate process (transport and storage) of about 10 minutes to several tens minutes, and sometimes several hours, exists between each process. That was the fact.

However, the present inventor, when using a specific surface cross-linking treatment agent that is a surface cross-linking treatment agent containing a surface cross-linking agent and water as essential components, wherein the content of the hydrophilic organic solvent in the treatment agent is reduced. Focusing on the intermediate step between the mixer for the surface cross-linking agent and the subsequent reactor (heater), it is important to omit or simplify the intermediate step in order to improve the physical properties of the water-absorbent resin. I found something. This makes it possible to not only improve the performance of the water-absorbent resin but also reduce the capital investment by omitting or simplifying the intermediate steps.
As means on manufacturing equipment for achieving the present invention, for example, there is an equipment layout in which a reactor (heater) is installed under an apparatus for adding and mixing a surface crosslinking agent. In other words, the water-absorbent resin mixed with the surface crosslinking agent is discharged from the apparatus for performing the step (3) of adding the surface crosslinking agent to the water-absorbent resin powder, falls freely, and is subjected to a surface treatment reactor (heater). It is thrown into. Thereby, the time from the end of the step (3) to the start of the step (4) is reduced.

The above-described method for producing a water absorbent resin of the present invention provides a novel water absorbent resin described below.
That is, a water-absorbent resin obtained by polymerizing and crosslinking a monomer component containing acrylic acid and / or a salt thereof (neutralized product) as a main component, and having a mass average particle diameter of 300 to 600 μm, The ratio of the powder having a particle diameter of less than 150 μm is 0 to 10% by mass relative to the water-absorbent resin, the total absorption capacity is 70 (g / g) or more, and the absorption efficiency under pressure is 70% or more. A characteristic feature of the present invention is to provide a water-absorbent resin of the present invention.
The fact that the water-absorbent resin of the present invention is novel is that, in addition to the analysis of the current state of many commercially available products (water-absorbent resin and diapers) in the world range, many technologies that have been proposed so far are compared and evaluated. Has been confirmed.

The two new parameters discovered by the present inventors, namely, the total absorption capacity and the absorption efficiency under pressure, are defined by the following formula based on the value of one hour after absorbing a 0.90% by mass aqueous sodium chloride solution (25 ° C.). The measurement of the one-hour value will be described in detail in Examples described later.
Total absorption capacity (g / g) = Absorption capacity under no pressure (g / g) + Absorption capacity under single layer pressure (g / g)
Absorption efficiency under pressure (%) = Single layer absorption capacity under pressure (g / g) × 100 / absorption capacity under no pressure (g / g)
The novel water-absorbent resin according to the present invention is a water-absorbent resin having the above-described particle diameter, total absorption capacity, and absorption efficiency under pressure, preferably, a surface-crosslinked resin, and particles described below. It has a diameter element (mass average particle diameter, ratio of powder having a particle diameter of less than 150 μm, particle size distribution), absorption capacity under no pressure, absorption capacity under single layer pressure, total absorption capacity, absorption efficiency under pressure. . The novel water absorbent resin according to the present invention is preferably used in combination with additives such as a deodorant and an inorganic powder.

The water-absorbent resin according to the present invention is not limited even if it is controlled in the particle size factor (mass average particle size and the ratio of powder having a particle size of less than 150 μm), the total absorption capacity, and the absorption efficiency under pressure. The production method is not limited, but the particle size is controlled within the above range after polymerization and crosslinking of a monomer component containing acrylic acid and / or a salt thereof (neutralized product) as a main component, and further surface crosslinking is performed. In this case, in particular, it can be easily obtained by performing the novel surface crosslinking treatment described above for the production method of the present invention.
Hereinafter, the water absorbent resin obtained by the production method of the present invention or the novel water absorbent resin of the present invention will be further described.

As described above, the water-absorbing resin obtained by performing the surface crosslinking treatment is preferably adjusted to a specific particle size in order to sufficiently exhibit the effects of the present invention.
As such a particle size, preferably, particles having a particle size of less than 850 μm and 150 μm or more account for 90% by mass or more and particles having a particle size of 300 μm or more account for 60% by mass or more, and more preferably particles having a particle size of less than 850 μm and 150 μm or more. It is at least 95% by mass, more preferably at least 98% by mass. Further, the particle size of 300 μm or more is more preferably 65% by mass or more, further preferably 70% by mass or more, and particularly preferably 75% by mass or more.
The mass average particle diameter of the water-absorbent resin in the invention is preferably from 200 to 700 µm, more preferably from 300 to 600 µm, particularly preferably from 380 to 550 µm, most preferably from 400 to 500 µm. The mass average particle diameter of the water-absorbent resin may be adjusted by granulation, if necessary.

In the water-absorbent resin of the present invention, the ratio of the powder having a particle size of less than 150 μm is preferably 0 to 10% by mass, more preferably 0 to 8% by mass, and still more preferably 0 to 10% by mass with respect to the water-absorbent resin. It is 5% by mass, particularly preferably 0 to 3% by mass.
The water-absorbent resin in the present invention has a logarithmic standard deviation (σζ) of the particle size distribution obtained as described below in the range of 0.25 to 0.50. Preferably, it is more preferably in the range of 0.27 to 0.48, and even more preferably in the range of 0.30 to 0.45.
Logarithmic standard deviation of particle size distribution; The water-absorbent resin was sieved with a JIS standard sieve having openings of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, and 45 μm, and the residual percentage R was plotted on log probability paper. particle size X ₁ and when R = 84.1 mass% from the results, determine the particle diameter X ₂ when the R = 15.9 mass%, obtained by introducing these values into the following formula values It is.

σζ = 0.5 × In (X ₂ / X ₁ )
When σζ exceeds 0.50, it means that the particle size distribution of the powder is wide, and this may cause segregation and errors in the amount of powder supplied in the handling of the powder. Not so desirable. On the other hand, when σζ is less than 0.25, it means that the particle size distribution is narrow, which is preferable in handling powder. However, the amount of coarse particles and fine powder to be recycled increases for particle size adjustment, and It is not so preferable because it causes the production amount to decrease.
The water-absorbent resin in the present invention preferably has an absorption capacity under a non-pressurized condition of not more than 33 g / g, more preferably not less than 35 g / g, and still more preferably not less than 38 g / g with respect to a 0.90 mass% aqueous sodium chloride solution (physiological saline). It is particularly preferably at least 40 g / g, most preferably at least 45 g / g. If the absorption capacity under no pressure to a 0.90% by mass aqueous sodium chloride solution (physiological saline) is less than 33 g / g, the effect of the present invention may not be sufficiently exerted. For example, when used in diapers, The absorption amount is low, and leakage in actual use increases. In addition, setting the absorption capacity under no pressure to exceed 80 g / g leads to reducing the amount of the internal cross-linking agent or the surface cross-linking agent, and the effect of the present invention may not be sufficiently exerted. So less preferred.

  The water-absorbing resin in the present invention preferably has an absorption capacity under a single-layer pressure of 1.9 kPa with respect to a 0.90% by mass aqueous sodium chloride solution (physiological saline) of 25 g / g or more, more preferably 30 g / g. And more preferably 35 g / g or more. When the absorption capacity under pressure of a single layer at 1.9 kPa with respect to a 0.90% by mass aqueous sodium chloride solution (physiological saline) is smaller than 25 g / g, the effect of the present invention may not be sufficiently exerted. During use, rash on the skin (buttocks) may occur due to the liquid on the diaper surface. There is no particular upper limit to the absorption capacity under pressure in a single layer, and a higher absorption capacity is preferable. However, increasing the absorption capacity to a certain level or more is extremely low in productivity, increases in water-soluble components in physical properties, and gel strength. In some cases, the upper limit may be about 60 g / g from the balance between cost performance (production cost per absorption ratio) and physical properties.

  The water-absorbent resin in the present invention preferably has a total absorption capacity of 70 g / g or more, more preferably 74 g / g or more, further preferably 78 g / g or more, and particularly preferably 82 g / g or more. The total absorption capacity is the absorption capacity under no pressure for a 0.90 mass% aqueous sodium chloride solution (physiological saline) and the absorption under a single layer pressure of 1.9 kPa for a 0.90 mass% aqueous sodium chloride solution (physiological saline). This is the sum of the magnification and the total absorption magnification in the non-pressurized state and the pressurized state. If the total absorption ratio is less than 70 g / g, the effect of the present invention may not be sufficiently exhibited. For example, when used in a diaper, the absorption amount is low and the leakage in actual use increases, During use, rash on the skin (buttocks) may occur due to the liquid on the diaper surface. It should be noted that setting the total absorption ratio to more than 140 g / g leads to remarkably lowering the productivity, leading to steps of polymerization, drying, pulverization, particle size adjustment, surface cross-linking, and the like, which is not preferable in terms of cost. .

  The water absorbing resin in the present invention preferably has an absorption efficiency under pressure of 70% or more, more preferably 74% or more, further preferably 78% or more, and particularly preferably 82% or more. The absorption efficiency under pressure refers to the absorption capacity under no pressure with respect to a 0.90 mass% aqueous sodium chloride solution (physiological saline) and a single-layer pressurization at 1.9 kPa with respect to a 0.90 mass% aqueous sodium chloride solution (physiological saline). This is a ratio expressed as a percentage of the absorption capacity, and represents the absorption characteristics under pressure. When the absorption efficiency under pressure is high, the change in absorption capacity with respect to pressure is small. If the absorption efficiency under pressure is less than 70%, the effects of the present invention may not be sufficiently exhibited. For example, when used in diapers, the absorption amount is low, the leakage in actual use increases, During use, rash on the skin (buttocks) may occur due to the liquid on the diaper surface. Making the absorption efficiency under pressure more than 150% leads to carrying out processes such as polymerization, drying, pulverization, particle size adjustment, surface cross-linking and the like with a remarkable decrease in productivity, which is not preferable in terms of cost. .

The water-absorbent resin in the invention preferably has a total absorption capacity of 70 g / g or more and an absorption efficiency under pressure of 70% or more.
The water content and the water-soluble component amount of the water-absorbent resin of the present invention are in the above-described range, and the residual monomer amount is usually 0 to 1000 mass ppm, preferably 0 to 500 mass ppm, more preferably 0 to 400 mass ppm. is there.
The water-absorbent resin of the present invention further includes, if necessary, a deodorant, an antibacterial agent, a fragrance, various inorganic powders, a foaming agent, a pigment, a dye, a hydrophilic short fiber, a plasticizer, an adhesive, and a surfactant. Various functions may be imparted by adding additives such as agents, fertilizers, oxidizing agents, reducing agents, chelating agents, antioxidants, water, water-soluble polymers, binders, and salts.

  In the present invention, by adding an additive to the water-absorbing resin to form a composite, the additives are substantially integrated with the water-absorbing resin (combined with the water-absorbing resin by absorption, adsorption, mixing, etc.). In this case, these are one kind of mixture, but this water-absorbing resin additive mixture (this mixture is usually called a water-absorbing agent) satisfies the above particle diameter, total absorption capacity, and absorption efficiency under pressure. As long as this water-absorbent resin additive mixture is also regarded as the water-absorbent resin in the present invention. That is, in the present invention, the water-absorbing resin obtained by polymerizing and crosslinking a monomer component containing acrylic acid and / or a salt thereof (neutralized product) as a main component is combined with the above-mentioned additive to form a complex. It is a concept that includes the so-called “water-absorbing agent”. The proportion of the water-absorbing resin in the water-absorbing agent is preferably in the range of 70 to 100% by mass, more preferably 80 to 100% by mass, and particularly preferably 90 to 100% by mass. Water may be contained as a trace component.

  Examples of the deodorant include a leaf extract of a Camellia plant exemplified in US Pat. No. 6,469,080 and WO 2003/104349, and a zinc exemplified in Japanese Patent Application No. 2003-280373. And hydrated oxides of silicon or zinc and aluminum, specific zeolites exemplified in the specification of Japanese Patent Application No. 2004-1778, and the like are used in an amount of 0.001 to 100 parts by mass of the water absorbent resin. The range is preferably from 10 to 10 parts by mass, more preferably from 0.05 to 5 parts by mass, and particularly preferably from 0.1 to 3 parts by mass. When the amount of the deodorant added is less than 0.001 part by mass, a desired deodorant effect may not be obtained, so that it is not preferable. On the other hand, adding more than 10 parts by mass of the deodorant has the advantage of having an excellent deodorizing effect, but the resulting water-absorbent resin composition itself becomes extremely expensive, and the disposable diaper / physiology For use in disposable sanitary materials / absorbent articles such as napkins for clothing, the cost is not very favorable.

  For the purpose of improving the handleability of the water-absorbent resin under high humidity, fine-particle silicon dioxide powder is added to the water-absorbent resin as an inorganic powder, or fine-particle stearic acid polyvalent metal salt powder is added as salts. The amount is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, and more preferably 0.1 to 1 part by mass, per 100 parts by mass of the water-absorbent resin. The range of parts by weight is particularly preferred. When the addition amount of the finely divided silicon dioxide powder and / or the finely divided polyvalent metal stearate powder is less than 0.01 part by mass, the desired effect of improving the handleability under high humidity may not be obtained. This is not very desirable. On the other hand, adding more than 10 parts by mass of the finely divided silicon dioxide powder and / or the finely divided stearic acid polyvalent metal salt powder has an advantage that the handleability under high humidity is excellent. The resulting water-absorbent resin composition itself becomes extremely expensive, and is not very cost-effective for use in disposable sanitary materials / absorbent articles such as disposable diapers / sanitary napkins.

The water-absorbent resin in the present invention may have a particle shape in an irregular crushed shape as obtained by the production method of the present invention, for example, as it is obtained by reverse phase suspension polymerization, that is, It may be spherical or the like.
However, in the case of reverse phase suspension polymerization, the resulting water-absorbent resin is spherical or agglomerate, so that mixing with the pulp and fixing (trapping) often become insufficient. The shape of the water-absorbent resin of the present invention is preferably irregularly crushed (that is, pulverized powder) because the water-absorbent resin is not suitable for a diaper having a high concentration.
The water-absorbent resin obtained by the production method of the present invention and the novel water-absorbent resin of the present invention include various types of water-absorbent resins for sanitary materials, civil engineering / architecture, solidification of waste liquid, agriculture / plants, food, etc. It can be used for any purpose.

  In the present invention, the absorbent article refers to a final consumable molded with a water-absorbent resin, and is represented by a disposable diaper, a sanitary napkin, and an incontinence pad. Such absorbent articles preferably use a fibrous material for molding. In the water-absorbent resin of the present invention, the mass ratio of the water-absorbent resin to the fiber material is preferably in the range of 20 to 100% by mass, more preferably 30 to 90% by mass, and particularly preferably 30 to 60% by mass. If the amount of the water-absorbent resin per absorbent article is preferably in the range of 5 to 25 g, and more preferably in the range of 8 to 15 g, the maximum effect is exhibited in such an absorbent article. Can be. Furthermore, the more the water-absorbent resin and the fibrous material are more uniformly mixed (in a state where the distribution of the water-absorbent resin is less uneven), the more the water-absorbent resin of the present invention can exert the maximum effect in such an absorbent article. it can.

Hereinafter, Examples and Comparative Examples of the present invention will be specifically described, but the present invention is not limited to the following Examples.
When a commercially available water-absorbent resin such as a water-absorbent resin in a diaper absorbs moisture during the distribution process, the water-absorbent resin is dried (for example, by drying at 60 ° C. under reduced pressure for 16 hours) and equilibrated with water (for example, the amount is , Around 5% by mass).
Hereinafter, although detailed data is omitted, the water-absorbent resin obtained in the present invention is substantially water-insoluble, has a water content of 7% by mass or less (solid content of 93% or more), and has a residual monomer content of 400% by mass. ppm or less.

Various properties of the water-absorbent resin and the absorbent article were measured by the following methods.
(A) Absorption capacity under no pressure 0.2 g of a water-absorbent resin was uniformly placed in a nonwoven fabric bag (60 mm × 80 mm), and placed in a 0.90% by mass aqueous sodium chloride solution (physiological saline) adjusted to 25 ° C. Dipped. After 60 minutes, the bag was pulled out and drained at 250 G for 3 minutes using a centrifuge, and then the mass W ₂ (g) of the bag was measured. Further, the same operation was performed without using the water-absorbing resin, and the mass W ₁ (g) at that time was measured. Then, from these masses W ₁ and W ₂ ,
Absorption capacity without load (g / g) = ((mass _W 2 (g) - Weight _W 1 (g)) / water-absorbent resin weight (g)) - 1
According to, the absorption capacity under no pressure (g / g) was calculated.

(B) Absorption capacity under pressure of a single layer A 400-mesh stainless steel wire mesh (mesh size 38 μm) was welded to one side (bottom) of a cylindrical cross section, and water absorption was performed on the wire mesh at the bottom of a plastic support cylinder having an inner diameter of 60 mm. 0.20 g of resin is evenly spread, and a piston (cover plate) on which an outer diameter is slightly smaller than 60 mm and a gap is not formed between the resin and the inner wall surface of the supporting cylinder and vertical movement is not hindered is placed thereon. Then, the mass W ₃ (g) of the support cylinder, the water-absorbent resin, and the piston was measured. On this piston, a load adjusted so that a load of 20 g / cm ² (1.96 kPa) including the piston can be uniformly applied to the water-absorbent resin is placed, and a complete measuring device is completed. Was. A glass filter having a diameter of 90 mm was placed inside a petri dish having a diameter of 150 mm, and a 0.90% by mass aqueous solution of sodium chloride (physiological saline) at 25 ° C. was added to the same level as the upper surface of the glass filter. A filter paper having a diameter of 9 cm (Toyo Filter Paper Co., Ltd. No. 2) was placed on top of it, so that the entire surface was wet, and excess liquid was removed.

The above set of measuring devices was placed on the wet filter paper, and the liquid was absorbed under a load. When the liquid level dropped from the top of the glass filter, liquid was added to keep the liquid level constant. One hour later, the complete measurement apparatus was lifted, and the mass W ₄ (g) (the mass of the support cylinder, the swollen water-absorbent resin, and the piston) from which the load was removed was measured again.
From these masses W ₃ and W ₄ ,
Monolayer absorption capacity under pressure (g / g) = (mass W ₄ (g) −mass W ₃ (g)) / mass of water absorbent resin (g)
The absorption capacity under single layer pressure (g / g) was calculated according to.

(C) Mass (weight) average particle diameter (D50)
The water-absorbent resin was sieved with a JIS standard sieve having openings of 850 µm, 600 µm, 500 µm, 425 µm, 300 µm, 212 µm, 150 µm, 106 µm, and 75 µm, and the residual percentage was plotted on log probability paper. Thereby, the mass average particle diameter (D50) was read.
The sieving is performed by filtering 10.00 g of the water-absorbent resin powder or the water-absorbent resin into a JIS standard sieve (The IIDA TESTING SIEVE: inner diameter 80 mm) such as 850 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm, and 75 μm. The mixture was classified for 10 minutes using a low tap sieve shaker (ES-65 sieve shaker manufactured by Iida Seisakusho Co., Ltd.). Incidentally, the mass average particle diameter (D50) is a particle diameter of a standard sieve corresponding to 50% by mass of the whole particles with a standard sieve having a constant opening as disclosed in US Pat. No. 5,051,259. In the examples, the calculation was performed using a JIS standard sieve having openings of 850 μm, 600 μm, 300 μm, and 150 μm.

(D) Absorption Efficiency Under Pressure The absorption capacity under no pressure and the absorption capacity under a single layer of the water-absorbent resin are measured by the methods described in (a) and (b), and the absorption efficiency under pressure (%) is calculated according to the following equation. did.
Absorption efficiency under pressure (%) = Single layer absorption capacity under pressure (g / g) × 100 / absorption capacity under no pressure (g / g)
(E) Total absorption capacity The absorption capacity of the water absorbent resin under no pressure and the single layer absorption capacity under pressure are measured by the methods described in (a) and (b), and the total absorption capacity (g / g) is calculated according to the following equation. did.

Total absorption capacity (g / g) = absorption capacity under pressure (g / g) + absorption capacity under no pressure (g / g)
(F) Evaluation of absorbent articles (return amount test)
Using the absorbent articles obtained in Examples and Comparative Examples described below, a load of 1.96 kPa was applied to the entire absorbent article, and the absorbent article was left at room temperature. From a cylinder having a diameter of 70 mm and a height of 100 mm from the center of the absorbent article, 75 g of a 0.90% by mass aqueous sodium chloride solution (physiological saline) adjusted to 37 ° C. was charged. After leaving for 1 hour with the load applied, the same operation was performed, and a fourth physiological saline solution (a total amount including physiological saline solution of 300 g) was added. After 30 minutes, the load was removed from the absorbent article. A paper towel (manufacturer: Oji Paper Co., Ltd., kitchen towel extra dry, cut into 120 mm × 450 mm and stacked 30 sheets) is placed on the absorbent article, and a load of 37 g / cm ² (3.63 kPa) is placed thereon. Over 1 minute, the amount of liquid returning to the paper towel was measured.

[Production Example 1]
2.1 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 8) of 2.1 g was dissolved in 5,500 g of an aqueous solution of sodium acrylate having a neutralization ratio of 75 mol% (monomer concentration: 38% by mass) to prepare a reaction solution. Next, dissolved oxygen was removed from the reaction solution under a nitrogen gas atmosphere for 30 minutes. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-armed kneader having two sigma-type blades and having a capacity of 10 L and having a capacity of 10 L. The gas was replaced. Subsequently, while stirring the reaction solution, 2.46 g of sodium persulfate and 0.10 g of L-ascorbic acid were added, and polymerization started about 1 minute later. Then, polymerization was carried out at 30 to 90 ° C., and the hydrogel polymer was taken out 60 minutes after the initiation of the polymerization. The obtained hydrogel polymer was subdivided into a diameter of about 5 mm. The finely divided hydrogel polymer was spread on a wire mesh of 50 mesh (mesh size: 300 μm) and dried with hot air at 170 ° C. for 50 minutes. Next, the dried product was pulverized using a vibration mill, and further classified and blended with a 20-mesh (mesh size: 850 μm) wire mesh to obtain an irregularly crushed water-absorbent resin powder (1). The resulting water-absorbent resin powder (1) had an absorption capacity under no pressure of 57 (g / g), a mass average particle diameter of 425 μm, and a ratio of powder having a particle diameter of less than 150 μm was 2 mass with respect to the water-absorbent resin powder. %Met.

[Production Example 2]
2.2 g of polyethylene glycol diacrylate (average addition mole number of ethylene oxide: 8) was dissolved in 5500 g of an aqueous solution of sodium acrylate having a neutralization ratio of 75 mol% (monomer concentration: 36% by mass) to prepare a reaction solution. Next, dissolved oxygen was removed from the reaction solution under a nitrogen gas atmosphere for 30 minutes. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-armed kneader having two sigma-type blades and having a capacity of 10 L and having a capacity of 10 L. The gas was replaced. Subsequently, while stirring the reaction solution, 2.46 g of sodium persulfate and 0.10 g of L-ascorbic acid were added, and polymerization started about 1 minute later. Then, polymerization was carried out at 30 to 90 ° C., and the hydrogel polymer was taken out 60 minutes after the initiation of the polymerization. The obtained hydrogel polymer was subdivided into a diameter of about 5 mm. The finely divided hydrogel polymer was spread on a wire mesh of 50 mesh (mesh size: 300 μm) and dried with hot air at 170 ° C. for 50 minutes. Next, the dried product was pulverized using a vibration mill, and further classified and blended with a 20-mesh (mesh size: 850 μm) wire mesh to obtain an irregularly crushed water-absorbent resin powder (2). The resultant water-absorbent resin powder (2) had an absorption capacity under no pressure of 56 (g / g), a mass average particle diameter of 370 μm, and a ratio of powder having a particle diameter of less than 150 μm was 5 mass with respect to the water-absorbent resin powder. %Met.

[Example 1]
To 100 parts by mass of the water-absorbent resin powder (1) obtained in Production Example 1 at a powder temperature of 60 ° C., 0.4 parts by mass of propylene glycol, 0.02 parts by mass of ethylene glycol diglycidyl ether, and 1,4-butanediol 0 A surface cross-linking agent consisting of .25 parts by mass and 2.5 parts by mass of water was mixed with a Loedige mixer, and after 2 minutes, the above mixture was put into a heat treatment device adjusted to 195 ° C. and heat-treated for 45 minutes. Thereby, a water-absorbing resin (1) was obtained.
Table 1 shows the absorption capacity under no pressure, the absorption capacity under pressure of a single layer, the absorption efficiency under pressure, the total absorption capacity, the particle size distribution, and the mass average particle diameter of the obtained water-absorbent resin (1).

[Example 2]
100 parts by mass of the water-absorbent resin powder (2) obtained in Production Example 2 at a powder temperature of 60 ° C. was added with 0.4 parts by mass of propylene glycol, 0.02 parts by mass of ethylene glycol diglycidyl ether, and 1,4-butanediol 0 0.25 parts by weight of water and 2.5 parts by weight of water were mixed with a Loedige mixer, and after one minute, the mixture was placed in a heat treatment apparatus adjusted to 195 ° C. and heat-treated for 45 minutes. Thereby, a water absorbent resin (2) was obtained.
Table 1 shows the absorption capacity under no pressure, the absorption capacity under pressure of a single layer, the absorption efficiency under pressure, the total absorption capacity, the particle size distribution, and the mass average particle diameter of the obtained water-absorbent resin (2).

[Comparative Example 1]
To 100 parts by mass of the water-absorbent resin powder (1) obtained in Production Example 1 at a powder temperature of 60 ° C., 0.4 parts by mass of propylene glycol, 0.02 parts by mass of ethylene glycol diglycidyl ether, and 1,4-butanediol 0 1 hour after mixing a surface cross-linking agent consisting of .25 parts by mass and 2.5 parts by mass of water with a Loedige mixer, put the mixture into a heat treatment device adjusted to 195 ° C., and heat-treat for 55 minutes. Thereby, a comparative water absorbent resin (1) was obtained.
Table 1 shows the absorption capacity under no pressure, the absorption capacity under single layer pressure, the absorption efficiency under pressure, the total absorption capacity, the particle size distribution, and the mass average particle diameter of the obtained comparative water absorbent resin (1).

[Comparative Example 2]
100 parts by mass of the water-absorbent resin powder (2) obtained in Production Example 2 at a powder temperature of 60 ° C. was added with 0.4 parts by mass of propylene glycol, 0.02 parts by mass of ethylene glycol diglycidyl ether, and 1,4-butanediol 0 10 minutes after mixing a surface cross-linking agent consisting of .25 parts by mass and 2.5 parts by mass of water with a Loedige mixer, put the above mixture into a heat treatment device adjusted to 195 ° C., and heat-treat for 55 minutes. Thereby, a comparative water absorbent resin (2) was obtained.
Table 1 shows the absorption capacity under no pressure, single layer absorption capacity under pressure, absorption efficiency under pressure, total absorption capacity, particle size distribution, and mass average particle diameter of the obtained comparative water-absorbent resin (2).

[Comparative Example 3]
100 parts by mass of the water-absorbent resin powder (2) obtained in Production Example 2 at a powder temperature of 60 ° C. was added with 0.4 parts by mass of propylene glycol, 0.02 parts by mass of ethylene glycol diglycidyl ether, and 1,4-butanediol 0 25 parts by mass and 2.5 parts by mass of water were mixed with a Loedige mixer, and after 20 minutes, the above mixture was put into a heat treatment device adjusted to 195 ° C. and heat-treated for 55 minutes. Thereby, a comparative water absorbent resin (3) was obtained.
Table 1 shows the absorption capacity under no pressure, absorption capacity under pressure, absorption efficiency under pressure, total absorption capacity, particle size distribution, and mass average particle diameter of the obtained comparative water-absorbent resin (3).

[Example 3]
50 parts by mass of the water-absorbent resin (1) obtained in Example 1 and 50 parts by mass of the ground wood pulp were dry-mixed using a mixer. Next, the obtained mixture was air-formed on a wire screen formed into a 400 mesh (mesh size of 38 μm) using a batch-type air-forming device to form a web having a size of 120 mm × 400 mm. . Further, the web was pressed at a pressure of 196.14 kPa for 5 seconds to obtain an absorbent having a basis weight of about 0.05 g / cm ² .
Subsequently, a so-called back sheet (liquid-impermeable sheet) made of liquid-impermeable polypropylene, the above-described absorber, and a nonwoven surface sheet (liquid-permeable sheet) made of liquid-permeable polypropylene are applied to a double-sided tape. Were adhered to each other in this order to obtain a sanitary absorbent article (that is, a paper diaper) (1). The mass of the absorbent article (1) was 45 g.

A return amount test of the obtained absorbent article (1) was performed. The results are summarized in Table 2.
[Example 4]
In Example 3, an absorbent article (2) was obtained in the same manner as in Example 3, except that the water absorbent resin (1) was changed to the water absorbent resin (2) obtained in Example 2.
A return amount test of the obtained absorbent article (2) was performed. The results are summarized in Table 2.
[Comparative Examples 4 to 6]
Example 3 was the same as Example 3 except that the water absorbent resin (1) was changed to the comparative water absorbent resins (1), (2), and (3) obtained in Comparative Examples 1, 2, and 3. In the same manner, comparative absorbent articles (1), (2) and (3) were obtained.

  A return amount test of the obtained comparative absorbent articles (1), (2) and (3) was performed. The results are summarized in Table 2.

[Example 5]
2.5 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 8) of 2.5 g was dissolved in 5500 g of an aqueous solution of sodium acrylate having a neutralization ratio of 75 mol% (monomer concentration: 38% by mass) to prepare a reaction solution. Next, after removing dissolved oxygen from the reaction solution in the same manner as in Production Example 1, the solution was supplied to the same reactor as in Production Example 1, and the system was purged with nitrogen gas while maintaining the temperature of the reaction solution at 30 ° C. . Subsequently, while stirring the reaction solution, 2.98 g of sodium persulfate and 0.015 g of L-ascorbic acid were added, and polymerization started about 1 minute later. Then, polymerization was carried out at 25 to 90 ° C., and the hydrogel polymer was taken out 60 minutes after the polymerization was started. The obtained hydrogel polymer was finely divided into particles having a diameter of about 1 to 5 mm. This hydrogel polymer was dried in the same manner as in Production Example 1. Next, the dried product is pulverized using a roll mill, classified by a wire mesh having an aperture of 850 μm, and further classified by a wire mesh having an aperture of 150 μm to remove fine powder from the classified material. Powder (3) was obtained. The water-absorbent resin powder (3) thus obtained had an absorption capacity under no pressure of 55 g / g, a mass average particle diameter of about 420 μm, and a ratio of powder having a particle diameter of less than 150 μm was 0.8 mass relative to the water-absorbent resin powder. %Met.

Subsequently, 0.5 parts by mass of propylene glycol, 0.02 parts by mass of ethylene glycol diglycidyl ether and 1,4-butanediol 0 part by mass were added to 100 parts by mass of the water-absorbent resin powder (3) obtained above at a powder temperature of 60 ° C. An aqueous solution of a surface cross-linking agent consisting of 0.3 parts by mass and 2.5 parts by mass of water was mixed with a Loedige mixer, and after 1 minute, the resulting mixture was placed in a heat treatment device adjusted to 195 ° C. and heated for 40 minutes. By performing the treatment, a water absorbent resin (3) was obtained.
The obtained water-absorbent resin (3) is sieved with an absorption capacity under no pressure, a single layer absorption capacity under pressure, an absorption efficiency under pressure, and a total absorption capacity with a JIS standard sieve having openings of 850 μm, 600 μm, 300 μm, and 150 μm. Table 3 shows the mass average particle diameter (D50) determined by the above method. Table 4 shows the logarithmic standard deviation (σζ) of the particle size distribution obtained by sieving with a JIS standard sieve having openings of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm and 45 μm.

  Table 4 also shows the particle size distribution of the water-absorbent resins obtained in Examples 1 and 2 and Comparative Examples 1 to 3 and their logarithmic standard deviation (σ （).

[Example 6]
In Example 3, an absorbent article (3) was obtained in the same manner as in Example 3, except that the water absorbent resin (1) was changed to the water absorbent resin (3) obtained in Example 5.
When a return amount test of the obtained absorbent article (3) was performed in the same manner as in Example 3, the return amount was 8 g.
[Example 7]
35 parts by mass of the water-absorbent resin (3) obtained in Example 5 and 65 parts by mass of the ground wood pulp were dry-mixed using a mixer. Next, the obtained mixture was formed into a web having a size of 120 mm × 400 mm using the same apparatus as in Example 3. Further, by pressing this web under the same conditions as in Example 3, an absorbent having a basis weight of about 0.04 g / cm ² was obtained.

Subsequently, the same operation as in Example 3 was performed to obtain an absorbent article (4) having a mass of 37 g.
When a return amount test of the obtained absorbent article (4) was performed in the same manner as in Example 3, the return amount was 25 g.
[Comparative Example 7]
A comparative absorbent article (4) was prepared in the same manner as in Example 7, except that the water absorbent resin (3) was changed to the comparative water absorbent resin (1) obtained in Comparative Example 1. Got.
A return amount test of the comparative absorbent article (4) obtained was performed in the same manner as in Example 3, and the return amount was 39 g.

  The water-absorbent resin according to the present invention has excellent absorption properties and can be used for a wide range of applications.In particular, the water-absorbent resin obtained by the production method of the present invention uses a hydrophilic organic solvent in the surface crosslinking treatment. Since it is not available, it is suitable for sanitary materials / absorbent articles such as disposable diapers / sanitary napkins, and can be preferably used as a sanitary material by being compounded with a hydrophilic fiber material such as pulverized pulp.

Claims (9)

A step (1) of polymerizing a monomer component containing an acid group-containing unsaturated monomer as an essential component to obtain a hydrogel polymer.
Drying and pulverizing the obtained hydrogel polymer to obtain a water-absorbent resin powder (2);
The resulting water-absorbent resin powder is a surface cross-linking agent containing a surface cross-linking agent and water as essential components, and the content of the hydrophilic organic solvent in the processing agent is 0 to 10% by mass with respect to the processing agent. (3) adding a surface cross-linking agent which is
Step (4) of performing a surface cross-linking treatment by heating;
Including
The time from the end of the step (3) to the start of the step (4) is within 5 minutes;
A method for producing a water absorbent resin.   The method for producing a water-absorbent resin according to claim 1, wherein the temperature of the water-absorbent resin powder at the time of adding the surface crosslinking agent is in the range of 40 to 80C.   The method for producing a water-absorbent resin according to claim 1 or 2, wherein the water-absorbent resin powder has an absorption capacity under no pressure of 40 g / g or more. A water-absorbent resin obtained by polymerizing and crosslinking a monomer component containing acrylic acid and / or a salt thereof (neutralized product) as a main component, having a mass average particle diameter of 300 to 600 μm and a particle diameter of The ratio of the powder having a particle size of less than 150 μm is 0 to 10% by mass relative to the water-absorbent resin, the total absorption capacity is 70 (g / g) or more, and the absorption efficiency under pressure is 70% or more. Water absorbent resin.
However, the total absorption capacity and the absorption efficiency under pressure are defined by the following formula based on the value of 1 hour after absorbing a 0.90% by mass aqueous sodium chloride solution (25 ° C.).
Total absorption capacity (g / g) = Absorption capacity under no pressure (g / g) + Absorption capacity under single layer pressure (g / g)
Absorption efficiency under pressure (%) = Single layer absorption capacity under pressure (g / g) × 100 / absorption capacity under no pressure (g / g)   The water-absorbent resin according to claim 4, wherein the proportion of the powder having a particle size of less than 150 µm is 0 to 3% by mass based on the water-absorbent resin.   The water-absorbent resin according to claim 4, wherein the mass average particle diameter is 380 to 550 μm.   The water-absorbing resin according to any one of claims 4 to 6, wherein the absorption capacity under no pressure is 35 g / g or more.   The water-absorbent resin according to any one of claims 4 to 7, wherein the particle shape is irregularly crushed.   An absorbent article for absorbing manure and blood, comprising the water absorbent resin according to any one of claims 4 to 8.