JP2016112474A - Method for producing water-absorbing agent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide means for producing a surface-crosslinked water-absorbing resin (water-absorbing agent) which has an excellent balance between non-pressure water absorption magnification and water absorption magnification under pressure and has no problem in safety at low temperatures for a short time with a safe method at low cost.SOLUTION: A method for producing a water-absorbing agent containing a poly(meth)acrylic acid (salt)-based water-absorbing resin as a main component includes: a) a step of mixing the water-absorbing resin with 50 pts.wt. or less of a treatment liquid containing a radical polymerizable monomer and an organic-based crosslinking agent with respect to 100 pts.wt. of the water-absorbing resin; and b) a step of crosslinking and polymerizing the radical polymerizable monomer and the organic-based crosslinking agent. The organic-based crosslinking agent has a plurality of functional groups selected from the group consisting of functional groups that can react with a (meth)acrylate group or a carboxyl group, and has a molecular weight of less than 200.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a water-absorbing agent comprising a poly (meth) acrylic acid (salt) -based water-absorbing resin as a main component. More specifically, in the present invention, a poly (meth) acrylic acid (salt) water-absorbing resin is mixed with a surface treatment liquid containing a radical polymerizable monomer and an organic crosslinking agent and subjected to cross-linking polymerization, thereby adding no additive. The present invention relates to a technique for producing a water-absorbing agent having an excellent balance between a reduced water absorption ratio and a pressurized water absorption ratio and improved safety.

  Conventionally, a water-absorbing agent mainly composed of a water-absorbing resin has been used as one of the constituent materials of sanitary materials that absorb sanitary cotton, disposable diapers, or other body fluids. Examples of the water-absorbing resin constituting such a water-absorbing agent include starch-acrylonitrile graft polymer hydrolyzate, starch-acrylic acid graft polymer neutralized product, vinyl acetate-acrylic acid ester copolymer kenken. Products, hydrolysates of acrylonitrile copolymers or acrylamide copolymers, cross-linked products thereof, and cross-linked products of partially neutralized poly (meth) acrylic acid. All of these have a crosslinked structure and are insoluble in water.

  Properties desired for such a water-absorbent resin include a high water absorption ratio, a high water absorption speed, a high gel strength, an excellent suction force for sucking water from the substrate, and a high liquid permeability.

  As a technique for improving various water absorption characteristics of the water absorbent resin, conventionally, a cross-linking agent having a plurality of functional groups capable of reacting with a carboxyl group present in the water absorbent resin is used in the vicinity of the surface of the water absorbent resin. A method of forming a cross-linked structure and increasing the surface cross-linking density of the water-absorbent resin (surface cross-linking) is performed (Non-patent Document 1).

  For example, as a typical surface cross-linking agent for a water-absorbent resin, alkylene carbonate (Patent Document 1), mono- or polyvalent oxazolidinone (Patent Document 2), polyol (Patent Document 3), oxetane, thiourea (Patent Document 4), and the like In addition, in Patent Document 5, a plurality of crosslinking agents (for example, glycerin and diepoxy compound) having different solvent parameters are mixed in a water-absorbent resin and heated at 160 ° C. or higher to effect surface crosslinking. It is exemplified that a water-absorbing agent excellent in diffusion absorption capacity is obtained.

  However, these surface cross-linking techniques of Patent Documents 1 to 5 require heating at a high temperature for a long time (for example, several tens of minutes at 160 ° C. or higher) for the dehydration esterification reaction or dehydration amidation reaction between the crosslinking agent and the water absorbent resin. In addition to the increase in production costs and equipment costs associated with high-temperature surface crosslinking, there is a problem of coloring (slightly yellowing) of the resulting water-absorbent resin due to high-temperature heating, and low water content (for example, 0.00). %), It is brittle and has a problem that it becomes weak against dust generation and mechanical damage. Therefore, a technique for improving water content by adding water after surface crosslinking in order to improve dust generation and mechanical damage due to low moisture content after surface crosslinking (Patent Document 6) is also known. In addition to making the addition process complicated, surface cross-linking was destroyed when water was added, and physical properties were sometimes lowered. Moreover, since the crosslinking agent which requires high temperature for reaction is generally low in reactivity, the crosslinking agent tends to remain on the surface of the final product.

  On the other hand, polyvalent metal salts (for example, aluminum sulfate) and polyglycidyl compounds (for example, ethylene glycol diglycidyl ether) are also known as surface cross-linking agents that react at relatively low temperatures (for example, 150 ° C. or less, further 120 ° C. or less). However, in the case of these low-temperature reactive crosslinking agents, the polyvalent metal salt has a problem of physical properties, and the polyglycidyl compound has a problem of safety of the remaining crosslinking agent.

  In order to improve the conventional problems of surface crosslinking at high temperature or low temperature, instead of surface crosslinking by reacting the carboxyl group of the water-absorbent resin with the surface crosslinking agent, the second surface crosslinking method is a low temperature reaction. A technique for surface cross-linking by polymerizing a monomer on the surface of the water-absorbent resin (surface cross-linking by monomer cross-linking polymerization), which is surface cross-linking that can obtain a water-absorbing resin having high physical properties, has also been proposed. Specifically, a radically polymerizable monomer, an organic crosslinking agent, and a radical polymerization initiator are mixed in a water-absorbing resin, and the mixture is heated or irradiated with active energy rays such as ultraviolet rays to crosslink the surface. The technique which performs is disclosed (patent documents 7 to 13).

  As a polymerization method after mixing these water-absorbing resin with a radical polymerizable monomer, an organic crosslinking agent, and a radical polymerization initiator, in Patent Documents 7 to 9, surface crosslinking is performed at a low temperature by heating at about 100 ° C. Things are illustrated. Patent Documents 10 to 12 exemplify performing surface crosslinking under conditions of about room temperature by irradiating ultraviolet rays. According to the disclosure in these patent documents, on the surface of the water-absorbent resin, radically polymerizable monomers and the like are polymerized, and the surface cross-linking density of the water-absorbent resin is increased, and various water absorption characteristics can be improved. Yes.

  However, in these techniques, surface crosslinking with a relatively high water content (several percent to several tens of percent in terms of water content) is achieved by a low-temperature reaction. However, conventional surface crosslinking (particularly, Patent Documents 1 to 5) Compared to high-temperature surface cross-linking), the balance between water absorption capacity without pressure and water absorption capacity under pressure is not sufficient, or in order to achieve high cross-linking for the purpose of high physical properties, it is necessary to use a large amount of cross-linking agent. After crosslinking, the amount of remaining crosslinking agent was very large. In addition, amide-based cross-linking agents such as N, N′-methylenebisacrylamide have problems such as mutagenesis and reproductive toxicity, and amide-based cross-linking agents are not 100% safe as raw materials for water-absorbing resins. Met.

  Furthermore, in addition to the first surface crosslinking method (surface crosslinking by a crosslinking agent that reacts with a carboxyl group) and the second surface crosslinking method (surface crosslinking by monomer crosslinking polymerization), as a third surface crosslinking method Surface cross-linking with a radical polymerization initiator (Patent Documents 14 and 15) has also been proposed, but it is less than physical properties compared to the first and second surface cross-linking methods.

  Moreover, as an organic type crosslinking agent containing a plurality of (meth) acrylate groups, a crosslinking agent having a molecular weight of 200 or more, for example, polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9) conventionally used (molecular weight 522. 66), when using trimethylolpropane triacrylate (molecular weight 296.35), glycerin dimethacrylate (molecular weight 228.27), etc., it has been found that there is a problem of remaining in surface crosslinking.

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Modern Superabsorbent Polymer Technology (1998) pages 55-60, pages 97-103.

  Therefore, the problem of the present invention is low cost (low temperature and short time), excellent balance and safety between the absorption ratio without load and the absorption ratio under pressure, and further, the moisture content does not decrease excessively. To provide a method for producing a surface-crosslinked water-absorbing resin (water-absorbing agent) that is resistant to mechanical damage and has a small amount of remaining crosslinking agent.

  As a result of intensive studies in view of the above problems, the present inventors have found that, among the first to third surface crosslinks, the second surface crosslink method described in Patent Documents 7 to 13 (monomer It has been found that the use of a specific cross-linking agent solves the above problems in surface cross-linking by cross-linking polymerization). Many of the crosslinking agents conventionally used for the polymerization of water-absorbing resins have been proposed (for example, Patent Documents 16 to 31 described later), but the present invention has a molecular weight of 200 in surface crosslinking by monomer crosslinking polymerization. By using less specific crosslinking agent, it was found that the above problems were solved and the effects of the present invention (reduction of residual crosslinking agent and improvement of absorption capacity under pressure) were obtained, and the present invention was completed.

  That is, when the surface-crosslinking is performed by mixing the radically polymerizable monomer and the organic crosslinking agent with the poly (meth) acrylic acid (salt) water-absorbing resin described in Patent Documents 7 to 13, the molecular weight of the crosslinking agent and It was found that the functional group greatly affects the remaining cross-linking agent and physical properties, and when a specific cross-linking agent having a molecular weight of less than 200 is used, the remaining cross-linking agent is extremely low, excellent in safety, and water absorption capacity without pressure. The inventors have found that a surface-crosslinked water-absorbing resin (water-absorbing agent) that is excellent in balance of water absorption ratio under pressure and cost performance can be produced, and the present invention has been completed.

  That is, in the second surface cross-linking method (surface cross-linking by monomer cross-linking polymerization) described in Patent Documents 7 to 13 and the like, the present invention solves the above problems by using a specific cross-linking agent having a molecular weight of less than 200. Was found. Although the water-absorbing resin (base polymer) used for surface cross-linking may be polymerized using these specific cross-linking agents having a molecular weight of less than 200, the cross-linking agent may be used for polymerization of the water-absorbing resin. The effect (reduction of residual crosslinking agent, improvement of absorption capacity under pressure) is not particularly obtained, and the effect of the present invention is demonstrated for the first time by using a specific crosslinking agent for surface crosslinking by monomer crosslinking polymerization.

  The present invention solves the above problems by using a specific cross-linking agent in the second surface cross-linking method (surface cross-linking by monomer cross-linking polymerization) described in Patent Documents 7 to 13 among conventional surface cross-links. I found something to do.

That is, the method for producing a water absorbing agent according to the present invention includes:
a) A poly (meth) acrylic acid (salt) -based water-absorbing resin containing a surface treatment liquid containing a radical polymerizable monomer and an organic crosslinking agent at 50 parts by weight or less with respect to 100 parts by weight of the water-absorbing resin. Mixing, and
b) a step of crosslinking polymerization of the radical polymerizable monomer and the organic crosslinking agent, wherein the organic crosslinking agent comprises a (meth) acrylate group or a functional group capable of reacting with a carboxyl group. It has a plurality of functional groups selected from the group and has a molecular weight of less than 200.

  The water-absorbing agent of the present invention is a water-absorbing agent mainly composed of a surface-crosslinked poly (meth) acrylic acid (salt) water-absorbing resin, and the residual amount of the cross-linking agent is 100 ppm by weight or less. The absorption capacity under pressure (AAP 4.83 kPa) for physiological saline of .83 kPa is 10 to 30 g / g.

  According to the production method of the present invention, it is possible to produce a water-absorbing agent excellent in safety and water-absorbing performance at low temperature, in a short time and at low cost. In addition, the water-absorbing agent of the present invention has high physical properties and a small amount of internal and surface residual cross-linking agent (particularly vinyl cross-linking agent), so that it has excellent safety and excellent hygiene products (for example, diapers, incontinence pads, Sanitary napkins).

  Hereinafter, the production method of a water-absorbing agent mainly comprising a poly (meth) acrylic acid (salt) -based water-absorbing resin according to the present invention will be described in detail, but the scope of the present invention is not limited to these descriptions, Other than the following examples, changes and implementations can be made as appropriate without departing from the spirit of the present invention. Specifically, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the claims, and technical means disclosed in different embodiments are appropriately combined. Embodiments obtained in this manner are also included in the technical scope of the present invention.

[1] Definition of terms (1-1) “Water absorbing agent”
The “water-absorbing agent” in the present invention means an aqueous liquid absorption gelling agent mainly composed of a surface-crosslinked water-absorbing resin. The content of the water-absorbing resin in the water-absorbing agent is preferably 70 to 100% by weight, more preferably 75 to 97% by weight, still more preferably 80 to 95% by weight, and particularly preferably 85 to 95% by weight. The other components (0 to 30% by weight, 3 to 25% by weight, 5 to 20% by weight, and 5 to 15% by weight) preferably contain water as a main component, and further include inorganic fine particles and cationic polymers. Compounds, water-soluble polyvalent metal cation-containing compounds, surfactants, anti-coloring agents, urine resistance improvers, deodorants, fragrances, antibacterial agents, foaming agents, pigments, dyes, fertilizers, oxidizing agents, reducing agents, etc. , 0 to 10% by weight, preferably 0.1 to 1% by weight.

  Further, the aqueous liquid is not limited to water, but may contain water such as urine, waste water, blood, saline, waste liquid, deodorant, and fragrance. Further, the form of the aqueous liquid is not limited to liquid, but may be solid or gaseous (for example, water vapor or moisture).

(1-2) “Water Absorbent Resin”
The “water-absorbing resin” in the present invention means a water-swellable, water-insoluble polymer gelling agent and has the following physical properties. That is, as an indicator of “water swellability”, the CRC (water absorption capacity under no pressure) defined by ERT441.2-02 (2002) is 5 (g / g) or more, and as an indicator of “water insolubility”, It means a polymer gelling agent having a physical property of Ext (water-soluble content) defined by ERT470.2-02 (2002) of 50% by weight or less.

  The water-absorbent resin can be appropriately designed according to its use and is not particularly limited, but is preferably a hydrophilic cross-linked polymer obtained by cross-linking an unsaturated monomer having a carboxyl group.

  Moreover, the whole amount (100 weight%) is not limited to the form which is a polymer, The water absorbing resin composition containing the additive etc. may be sufficient in the range which satisfies the said physical property (CRC, Ext).

  In addition, the water absorbent resin in the present invention is a term encompassing intermediates in the production process of the water absorbent resin. If there is a need for distinction separately, “hydrogel after polymerization”, “dry polymer after drying”, “water absorbent resin before surface crosslinking (base polymer)”, “water absorbent resin after surface crosslinking ( "Water-absorbing agent)" may be distinguished.

  Examples of the shape of the water-absorbing resin include a sheet shape, a fiber shape, a film shape, a particle shape, and a gel shape. In the present invention, a particulate water-absorbing resin is preferable.

(1-3) “Poly (meth) acrylic acid (salt) water-absorbing resin”
The “poly (meth) acrylic acid (salt) -based water-absorbing resin” in the present invention optionally includes a graft component, and (meth) acrylic acid and / or a salt thereof (hereinafter referred to as “(meth) acrylic” as a repeating unit. It is a polymer mainly composed of “acid (salt)”.

  The “main component” means that the content (use amount) of (meth) acrylic acid (salt) is usually 50 to 100 mol based on the whole monomer (excluding the internal crosslinking agent) used for polymerization. %, Preferably 70 to 100 mol%, more preferably 90 to 100 mol%, still more preferably substantially 100 mol%. The poly (meth) acrylate as a polymer essentially contains a water-soluble salt, preferably a monovalent salt, more preferably an alkali metal salt or an ammonium salt.

(1-4) “EDANA” and “ERT”
“EDANA” is an abbreviation for European Disposables and Nonwovens Associations, and “ERT” is an abbreviation for EDANA Recommended Test Methods. It is. In the present invention, unless otherwise specified, the physical properties of the water-absorbent resin are measured according to the original ERT (revised in 2002 / known literature).

(A) "CRC" (ERT441.2-02)
“CRC” is an abbreviation for Centrifugation Retention Capacity (centrifuge retention capacity) and means water absorption capacity without pressure (sometimes referred to as “water absorption capacity”). Specifically, 0.2 g of the water-absorbent resin in the nonwoven fabric was freely swollen for 30 minutes with respect to a large excess of 0.9 wt% sodium chloride aqueous solution, and then drained with a centrifuge (250 G). It is a water absorption magnification (unit; g / g).

(B) "Ext" (ERT470.2-02)
“Ext” is an abbreviation for Extractables and means a water-soluble component. Specifically, for 1.0 g of water-absorbing resin, a value obtained by measuring the amount of dissolved polymer after stirring for 16 hours at 500 rpm with respect to 200 ml of 0.9 wt% aqueous sodium chloride solution (unit: wt%) ).

(C) “Residual Monomers” (ERT410.2-02)
“Residual Monomers” means the amount of monomer (monomer) remaining in the water-absorbent resin (hereinafter referred to as “residual monomer”). Specifically, with respect to 1.0 g of the water-absorbing resin, the amount of residual monomer dissolved after stirring for 1 hour at 500 rpm with respect to 200 ml of 0.9 wt% sodium chloride aqueous solution was measured by high performance liquid chromatography (HPLC). Value (unit: ppm by weight).

(D) “PSD” (ERT420.2-02)
“PSD” is an abbreviation for Particle Size Distribution and means a particle size distribution measured by sieve classification. The weight average particle size (D50) and the particle size distribution width are described in US Pat. No. 7,638,570, columns 27 and 28, “(3) Mass-Average Particle Diameter (D50) and Logical Standard Deviation (σζ)”. The measurement is performed in the same manner as the “of Particle Diameter Distribution”.

(E) "AAP" (ERT442.2-02)
“AAP” is an abbreviation for Absorption Against Pressure, which means water absorption capacity under pressure. Specifically, 0.9 g of water-absorbing resin was swollen under a load of 2.06 kPa (21 g / cm 2, 0.3 psi) for 1 hour against a large excess of 0.9 wt% sodium chloride aqueous solution. Of water absorption (unit: g / g). In ERT442.2-02, “Absorption Under Pressure” is described, but it has substantially the same content as AAP. In the present invention, since the load condition may be changed to 4.83 kPa (0.7 psi, 50 [g / cm 2]), the measurement result under a load of 2.06 kPa is described as AAP 2.06 kPa. The result measured under a load of 4.83 kPa is expressed as AAP 4.83 kPa.

(1-5) Molecular Weight In this specification, “molecular weight” means the third decimal place for each atom based on the atomic weight table (2012 version) defined by the International Union of Pure and Applied Chemistry. Is a value calculated using the atomic weight obtained by rounding off. If the atomic weight of a typical atom is mentioned, it will be 1.01 of a hydrogen atom, 12.01 of a carbon atom, 14.01 of a nitrogen atom, and 16.00 of an oxygen atom.

(1-6) Others In this specification, “X to Y” indicating a range means “X or more and Y or less”. Further, unless otherwise noted, “t (ton)” as a unit of weight means “Metric ton”, and “ppm” means “weight ppm” or “mass ppm”. Further, “weight” and “mass”, “parts by weight” and “parts by mass”, “% by weight” and “% by mass” are treated as synonyms. Further, “˜acid (salt)” means “˜acid and / or salt thereof”, and “(meth) acryl” means “acryl and / or methacryl”.

  For convenience, “liter” may be described as “l” or “L”, and “wt%” may be described as “wt%”. Furthermore, in the case of measuring a trace component, the N.sub. D. (Non Detected).

[2] Production method of poly (meth) acrylic acid (salt) -based water absorbent resin The production method of poly (meth) acrylic acid (salt) -based water absorbent resin used for surface crosslinking in the present invention is not particularly limited, The method for producing a water-absorbent resin described in Patent Literatures 1 to 15 (further, Patent Literatures 8 to 13, typically Patent Literature 9) can be widely applied.

  Although it does not specifically limit below, it shows about the typical manufacturing process (2-1)-(2-5) of the water absorbing resin concerning this invention.

(2-1) Monomer aqueous solution preparation step This step is a step of preparing an aqueous solution (monomer aqueous solution) containing (meth) acrylic acid (salt) as a main component. The monomer aqueous solution may be a part of the monomer on the emulsion and / or slurry as long as the water absorption performance of the resulting water-absorbing agent is not impaired. Included as an aqueous solution.

((Meth) acrylic acid)
The (meth) acrylic acid used as the monomer of the present invention is not limited in raw materials and production methods, and usually contains trace components such as a polymerization inhibitor and impurities. As the polymerization inhibitor, phenols are preferable, methoxyphenols are more preferable, and the concentration of the polymerization inhibitor is preferably 1 to 200 ppm from the influence on the polymerizability of the monomer and the color tone of the obtained water absorbing agent. More preferably, it is 10-160 ppm. Examples of the impurities include compounds described in US Patent Publication No. 2008/0161512.

(Other monomers)
In the present invention, other monomers may be used in combination with (meth) acrylic acid (salt), and examples of the other monomers include water-soluble or hydrophobic unsaturated monomers. Specific examples include monomers (excluding (meth) acrylic acid) described in paragraph [0035] of US Patent Application Publication No. 2005/215734. The water-absorbing resin may contain the other monomer as a copolymerization component.

(Monomer neutralization)
For neutralization of the monomer, it is preferable to use a monovalent salt such as an alkali metal salt, ammonium salt or alkanolamine salt, particularly an alkali metal salt, and further a sodium salt. Furthermore, a polyvalent metal salt compound may be used in a very small amount (for example, about 0 to 5 mol% with respect to the monomer).

  The neutralization rate of the monomer aqueous solution is preferably 10 to 90 mol%, more preferably 40 to 85 mol%, still more preferably 50 to 80 mol%, and particularly preferably 60 to 75 mol%. The neutralization rate is represented by the amount of acid groups in the aqueous monomer solution, particularly the amount of basic compound added to the carboxylic acid group derived from the monomer.

  The neutralization may be performed on the hydrogel after polymerization in addition to the monomer and / or the monomer aqueous solution before polymerization, or both may be used in combination. In addition, when performing several times, it is preferable to adjust to the range of the said neutralization rate in consideration of the addition amount of all the basic compounds.

(Internal crosslinking agent)
The internal cross-linking agent used in the present invention is preferably a compound having two or more functional groups capable of reacting with (meth) acrylic acid, such as the cross-linking agents described in Patent Documents 16 to 35, for example, US Pat. No. 6,241,928. The crosslinking agent as described in 14 columns is mentioned.

  In consideration of the water-absorbing properties of the resulting water-absorbing resin, the crosslinking agent is a compound having two or more radical polymerizable functional groups, particularly two or more radical polymerizable compounds having a (poly) alkylene glycol structural unit. A compound having a functional group (preferably an allyl group, a (meth) acrylate group, particularly an acrylate group) is preferable. Specifically, it is preferable to use (poly) alkylene glycol di (meth) acrylate or tri (meth) acrylate during polymerization. Preferable examples of the alkylene glycol unit include polyethylene glycol having an n number of 1 to 100, and further 6 to 50. Of these, one or more are used.

  The molecular weight of the internal cross-linking agent may be from 50,000 to 50,000, and is preferably a molecular weight larger than the cross-linking agent (less than 200 molecular weight) used for surface cross-linking described later in view of the physical properties of the resulting water-absorbing agent. Specifically, 200 to 40,000 are preferable, 300 to 30,000 are more preferable, and 400 to 10,000 are more preferable.

  The amount of the internal cross-linking agent used is preferably 0.005 to 2 mol%, more preferably 0.01 to 1 mol%, still more preferably 0.05 to 0.5 mol%, based on the monomer. is there. By setting the amount of the internal cross-linking agent used within the above range, desired water absorption characteristics can be obtained.

  In the present invention, in addition to the method of crosslinking by adding an internal crosslinking agent before polymerization, a method of adding an internal crosslinking agent after polymerization or post-crosslinking after polymerization, a method of radical crosslinking with a radical polymerization initiator, an electron Although a method of radiation cross-linking with a wire or the like can be adopted, a method in which a predetermined amount of an internal cross-linking agent is previously added to the monomer for polymerization and a cross-linking reaction at the same time as the polymerization is more preferable.

(Other substances added to monomers)
In the present invention, the following substances can be added in addition to the monomer aqueous solution and the substances described above. Specifically, for the purpose of improving various properties of the water-absorbent resin, the water-soluble resin or the water-absorbent resin is preferably added in an amount of 0 to 50% by weight, more preferably 0 to 20% by weight. A foaming agent (for example, carbonate, azo compound, bubbles, etc.), surfactant, chelating agent, chain transfer agent, etc. is preferably added in an amount of 0 to 5% by weight, more preferably 0 to 1% by weight. I can do it. These substances may be added during the polymerization reaction as well as in the form of adding to the monomer aqueous solution adjustment step.

  The use of the water-soluble resin or water-absorbing resin gives a graft polymer or a water-absorbing resin composition (for example, starch- (meth) acrylic acid polymer, PVA- (meth) acrylic acid polymer, etc.). These polymers and water-absorbing resin compositions are also collectively referred to as poly (meth) acrylic acid (salt) -based water-absorbing resins in the present invention.

(Concentration of monomer component)
In this step, a monomer aqueous solution is prepared by mixing the above-described substances. At that time, the concentration of the monomer component in the aqueous monomer solution (specified by the weight ratio of monomer / (monomer + water)) is not particularly limited, but is preferably from the viewpoint of the physical properties of the water-absorbent resin. It is 10 to 80% by weight, more preferably 20 to 75% by weight, still more preferably 30 to 70% by weight.

(2-2) Polymerization step The polymerization method applied in the present invention is not particularly limited, but from the viewpoint of water absorption characteristics and ease of polymerization control, spray droplet polymerization, aqueous solution polymerization, reverse phase suspension polymerization are preferable, Aqueous solution polymerization and reverse phase suspension polymerization are more preferred, and aqueous solution polymerization is more preferred. Among them, continuous aqueous solution polymerization is particularly preferable, and either continuous belt polymerization or continuous kneader polymerization may be used.

  As specific polymerization forms, continuous belt polymerization is disclosed in U.S. Pat. Nos. 4,893,999 and 6,241,928 and U.S. Patent Application Publication No. 2005/215734, and continuous kneader polymerization is disclosed in U.S. Pat. Nos. 6,987,151 and 6,710,141. , Respectively. By adopting such continuous aqueous solution polymerization, the production efficiency of the water-absorbent resin is improved.

  Moreover, high temperature initiation polymerization and high concentration polymerization are mentioned as a preferable example of the said continuous aqueous solution polymerization. “High temperature initiation polymerization” means that the temperature of the monomer aqueous solution is preferably 30 ° C. or higher, more preferably 35 ° C. or higher, still more preferably 40 ° C. or higher, particularly preferably 50 ° C. or higher (the upper limit is the boiling point of the monomer). The “high concentration polymerization” means that the monomer concentration is preferably 30% by weight or more, more preferably 35% by weight or more, still more preferably 40% by weight or more, particularly preferably. Means a polymerization method in which the polymerization is carried out at 45% by weight or more (the upper limit is a saturated concentration). These polymerization methods can be used in combination.

(2-3) Gel crushing step (gel crushing step)
In aqueous solution polymerization, gel pulverization (formation of hydrous gel) is performed before or after polymerization, before drying. When the polymerization process is kneader polymerization, the polymerization process and the gel grinding process are performed simultaneously.

  Although it does not specifically limit in the gel grinding | pulverization process of this invention, The gel grinding | pulverization method disclosed by the international publication 2011/126079 pamphlet is applied preferably.

(2-4) Drying step This step is a step of obtaining a dry polymer by drying the particulate hydrogel obtained in the polymerization step and / or the gel grinding step to a desired resin solid content. The resin solid content is determined from loss on drying (weight change when 1 g of water-absorbing resin is heated at 180 ° C. for 3 hours), and is usually 70% by weight or more, preferably 80% by weight or more, more preferably 85 to 85%. 99% by weight, more preferably 90-98% by weight, particularly preferably 92-97% by weight (the remainder is mainly water).

  The dried water-absorbent resin may contain water in the above range inside, but unless otherwise specified, the weight-based water-absorbent resin and water-absorbing agent including water are described below. (For example, in the weight ratio described later, 100 parts by weight of the water-absorbing resin refers to the weight including water contained in the resin).

  In the drying step of the present invention, although not particularly limited, for example, heat drying, hot air drying, vacuum drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, Various drying methods such as high-humidity drying using high-temperature steam are applied.

  Among the above drying methods, hot air drying is preferable because of high efficiency, and band drying in which hot air drying is performed on a belt is particularly preferable. The hot air temperature is preferably 100 to 250 ° C., more preferably 120 to 220 ° C. from the viewpoint of color tone and efficiency.

(2-5) Pulverization step, classification step In this step, the dried polymer obtained in the drying step is pulverized (pulverization step), adjusted to a predetermined particle size (classification step), and particulate water absorption This is a step of obtaining a resin.

  Examples of the equipment used in the pulverization process of the present invention include a high-speed rotary pulverizer such as a roll mill, a hammer mill, a screw mill, and a pin mill, a vibration mill, a knuckle type pulverizer, and a cylindrical mixer. Is done.

  In addition, the particle size adjustment method in the classification step of the present invention is not particularly limited, and examples thereof include sieve classification using JIS standard sieve (JIS Z8801-1 (2000)), airflow classification, and the like. The particle size adjustment of the water-absorbing resin is not limited to the above pulverization step and classification step, but is a polymerization step (especially reverse phase suspension polymerization or spray droplet polymerization) or other steps (for example, granulation step, fine powder recovery step). Therefore, it can be carried out as appropriate.

  As a particle size of the water-absorbent resin in the present invention, the weight average particle size (D50) is preferably 200 to 600 μm, more preferably 200 to 550 μm, still more preferably 250 to 500 μm, and particularly preferably 300 to 450 μm. The ratio of particles having a particle diameter of less than 150 μm is preferably 0 to 10% by weight, more preferably 0 to 5% by weight, and still more preferably 0 to 1% by weight. The ratio of particles having a particle diameter of 850 μm or more is preferably It is 0-5 weight%, More preferably, it is 0-3 weight%, More preferably, it is 0-1 weight%. Furthermore, the logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 to 0.50, more preferably 0.25 to 0.40, and still more preferably 0.27 to 0.35. In addition, these particle sizes are measured using a standard sieve according to the measuring method disclosed in US Pat. No. 7,638,570 and EDANA ERT420.2-02.

  The particle size described above is applied not only to the water-absorbing resin before surface crosslinking but also to the water-absorbing resin after surface crosslinking and the water-absorbing agent as the final product. Therefore, it is preferable that the fluctuation of the particle diameter is small before and after the surface crosslinking step. Specifically, the ratio of the average particle size after surface crosslinking to that before surface crosslinking is preferably 0.8 to 1.5, more preferably 0.9 to 1.2.

[3] Water-absorbing agent manufacturing process (surface cross-linking process)
This step is a step for obtaining a water-absorbing agent using a water-absorbing resin that is not surface-crosslinked, and a water-absorbing resin obtained according to the method for producing a poly (meth) acrylic acid (salt) -based water-absorbing resin. It is preferable to use it. The surface cross-linking step may be repeated not only once but also a plurality of times. The surface cross-linking step is preferably performed on the water-absorbing resin before surface cross-linking, but the surface cross-linking is already performed on the water-absorbing resin surface cross-linked by the methods described in Patent Documents 1 to 5. Alternatively, the surface crosslinking may be further performed by the methods described in Patent Documents 1 to 5 after the surface crosslinking.

  The surface cross-linking is a technique for increasing the cross-linking density of the surface layer of the water-absorbent resin (part of several μm to several tens μm from the surface of the water-absorbent resin). This step includes two steps: a mixing step of mixing a water-absorbing resin and a surface treatment liquid (a treatment liquid containing a radical polymerizable monomer and an organic crosslinking agent) and a crosslinking polymerization step of crosslinking and polymerizing the mixture. It can be divided into processes.

  In the present invention, the organic crosslinking agent has at least one functional group selected from the group consisting of a (meth) acrylate group or a functional group capable of reacting with a carboxyl group, and has a molecular weight. Is a compound of less than 200. By using the surface cross-linking agent, there is very little residual cross-linking agent, excellent safety, and a surface-crosslinked water-absorbing resin (water-absorbing water) that is excellent in balance between water absorption capacity without pressure and water absorption capacity under pressure and cost performance. Agent).

(3-1-1) Mixing Step This step is a step of obtaining a water absorbent resin mixture (hereinafter referred to as “mixture”) by adding and mixing the surface treatment liquid to the water absorbent resin described above.

(Surface treatment liquid)
The surface treatment liquid contains a radical polymerizable monomer and an organic crosslinking agent, and optionally contains a radical polymerization initiator, a solvent, a mixing aid and the like.

  The total amount of the surface treatment liquid is more than 0 and not more than 50 parts by weight, preferably 0.1 to 40 parts by weight, more preferably 1 to 30 parts by weight with respect to 100 parts by weight of the water-absorbing resin (including water if necessary). Parts, more preferably 3 to 20 parts by weight, particularly preferably 5 to 15 parts by weight. If the total amount of the surface treatment liquid is small, surface crosslinking may be insufficient. If the total amount exceeds 50 parts by weight, problems such as adhesion and clogging of the water-absorbent resin mixture occur, and there is no water-absorbing agent. The absorption capacity under pressure (CRC) is undesirably lowered, and the handleability of the mixture of the water-absorbent resin and the surface treatment liquid is lowered.

(Radical polymerizable monomer)
The radical polymerizable monomer means a compound that can be polymerized by radical polymerization, and specifically, a monomer having an (preferably terminal) unsaturated double bond, such as the poly (meta) described above. ) (Meth) acrylic acid (salt) used in the production of acrylic acid (salt) -based water-absorbent resin and / or monomers used in combination may be preferably used.

  The radical polymerizable monomer preferably contains an acid group and has a predetermined neutralization rate (mol% of neutralized acid groups in all acid groups). Among monofunctional radical polymerizable monomers, a compound containing an acid group is very excellent in terms of water absorption characteristics. Examples of the acid group include a carboxyl group, a sulfone group, and a phosphoric acid group.

  The acid group-containing monofunctional radical polymerizable monomer is preferably a monomer containing an acid group. Specifically, (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, styrene Examples include sulfonic acids and / or their salts. Among these, (meth) acrylic acid and 2- (meth) acrylamido-2-methylpropanesulfonic acid are more preferable, and acrylic acid is particularly preferable in terms of water absorption characteristics.

  The ratio of the total amount of acrylic acid (salt) in the total acid group-containing radical polymerizable monomer is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, and particularly preferably 90 to 100 mol%. is there. The acid group-containing radical polymerizable monomer may be used alone or in a mixture of two or more.

  When the acid group-containing radically polymerizable monomer is neutralized (in the form of a salt), the monomer is preferably a monovalent salt selected from alkali metal salts, ammonium salts, and amine salts. Among these, an alkali metal salt is more preferable, and a salt selected from a sodium salt, a lithium salt, and a potassium salt is particularly preferable.

  The neutralization rate of the acid group-containing radical polymerizable monomer is appropriately determined in the range of 0 to 100 mol%, and may be the same as or different from the neutralization rate of the water-absorbent resin as the base polymer. It may be. The “total acid groups of the acid group-containing radical polymerizable monomer” and “neutralized acid group” are all acid groups when there are two or more acid group-containing radical polymerizable monomers. The total of the acid group and the neutralized acid group in a containing radically polymerizable monomer is pointed out. For example, when acrylic acid and sodium acrylate are used in a molar ratio of 1: 1 as the acid group-containing radical polymerizable monomer, the neutralization rate of the acid group-containing radical polymerizable monomer is 50 mol. %.

  In the case where the surface crosslinking of the water absorbent resin is performed in the presence of the radical polymerizable monomer, the amount of the radical polymerizable monomer present in the crosslinking polymerization reaction system is preferably 100 parts by weight of the water absorbent resin. It is 0.1-20 weight part, More preferably, it is 1-15 weight part, More preferably, it is 2-10 weight part, Especially preferably, it is 3-7 weight part. If the amount of the monomer is less than 0.1 parts by weight or unused (0), the water absorption performance under pressure of the surface-crosslinked water-absorbing resin is not improved to a sufficient value, which is not preferable. On the other hand, when the amount of the radically polymerizable monomer exceeds 20 parts by weight, the water absorption capacity without load of the surface-crosslinked water-absorbing resin is remarkably lowered, which is not preferable.

(Organic crosslinking agent)
The organic crosslinking agent used in the surface crosslinking step of the present invention is one or more carboxyl groups, particularly from the viewpoint of the survivability of the crosslinking agent, the safety of the crosslinking agent alone, and the efficiency of surface crosslinking with respect to the amount of crosslinking agent used. It has two or more functional groups capable of reacting with (meth) acrylate groups and has a molecular weight of less than 200.

  Here, the organic cross-linking agent is an organic covalent cross-linking agent, and is a cross-linking agent that cross-links with a functional group of a monomer or a water absorbent resin through a covalent bond. In addition, a crosslinking agent that forms a crosslink with an ionic bond such as aluminum acrylate (for example, a polyvalent metal salt of an unsaturated organic acid) is not included in the organic crosslinker in the present invention, and is an ionic crosslink only with a polyvalent metal salt. However, the effect of the present invention is not exhibited in terms of improving the absorption capacity under pressure.

  The molecular weight of the organic crosslinking agent is less than 200, preferably less than 190, more preferably less than 180. When the functional groups in the cross-linking agent are the same, the smaller molecular weight is preferable because the cross-linking efficiency is excellent. On the other hand, when the molecular weight is 200 or more, the crosslinking agent molecule becomes large and it is difficult to penetrate from the surface of the water-absorbent resin, so that the CRC is difficult to be lowered. This is not preferable because the residual crosslinking agent in the resin increases. The lower limit of the molecular weight is sufficient if it has a plurality of functional groups capable of crosslinking with the radical polymerizable monomer and / or the water-absorbent resin, and is usually 50 or more, more preferably 80 or more, and 100 or more.

  In the present invention, examples of functional groups that can react with a carboxyl group include cyclic functional groups, hydroxy groups, primary amino groups, secondary amino groups, and isocyanate groups. Examples of cyclic functional groups include an epoxy group, an aziridine group, an oxetane group, an ethylene carbonate group, an oxazoline group, and an oxazolidinone group.

  The reaction between the functional group of the cross-linking agent and the carboxyl group of the water-absorbent resin may be performed at the time of mixing with the surface treatment liquid containing the organic cross-linking agent, and may be simply performed in the (3-1-2) cross-linking polymerization step described later. Along with polymerization of the monomer, the functional group of the cross-linking agent and the carboxyl group of the water-absorbing resin may be reacted. Before or after the cross-linking polymerization step, if necessary, the functional group of the cross-linking agent and the water-absorbing resin can be reacted. A carboxyl group may be reacted.

  As for the organic crosslinking agent that can be used, for example, an internal crosslinking agent used in the production of the poly (meth) acrylic acid (salt) -based water-absorbing resin, U.S. Pat. Nos. 6,228,930 and 6071976, You may select the organic type crosslinking agent which satisfy | fills the said range from the surface crosslinking agent illustrated by the said 6254990 specification etc. For example, a crosslinking agent having a molecular weight of less than 200 and having a plurality of (meth) acrylate groups, a crosslinking agent having functional groups that can react with (meth) acrylate groups and carboxyl groups, and two functional groups that can react with carboxyl groups. It is a crosslinking agent having the above.

  Specifically, the crosslinking agent has an organic crosslinking agent having a molecular weight of less than 200, and has (a) a polyfunctional (meth) acrylate crosslinking agent, and (b) a functional group capable of reacting with a carboxyl group ( A cross-linking agent selected from a (meth) acrylate-based cross-linking agent and (c) a cross-linking agent having a functional group capable of reacting with a plurality of carboxyl groups is used. More preferably, the (a) is a di (meth) acrylate-based crosslinking agent, the (b) is a (meth) acrylate group, and other functional groups include a cyclic functional group, a hydroxy group, a primary amine, A crosslinking agent having one or more types of functional groups selected from the group consisting of a secondary amine and an isocyanate group, the (c) as the crosslinking agent having a functional group capable of reacting with a plurality of carboxyl groups, particularly a polyol-based crosslinking Agents, polyamine crosslinking agents, polyisocyanate crosslinking agents, and crosslinking agents having one or more cyclic functional groups.

  In addition, even if a cyclic functional group such as an epoxy group or an ethylene carbonate group is used alone, it can react with another carboxyl group after once generating a hydroxyl group or an amino group by a ring-opening reaction with the carboxyl group. . Therefore, since one cyclic functional group has two reactive sites, it is a crosslinking agent capable of reacting with a plurality of carboxyl groups, and in the present invention, it has a plurality of functional groups capable of reacting with a carboxyl group.

  Among these organic crosslinking agents, (meth) acrylate groups are preferable because they are excellent in copolymerizability with radical polymerizable monomers, and epoxy groups and ethylene carbonate groups are excellent in reactivity with carboxyl groups. .

  That is, preferably, (a) a polyfunctional (meth) acrylate crosslinking agent, (b) a (meth) acrylate crosslinking agent having a functional group capable of reacting with a carboxyl group, and (c) a reaction with a plurality of carboxyl groups. A crosslinking agent selected from organic crosslinking agents having a functional group capable of being selected, more preferably a di (meth) acrylate crosslinking agent or a mono (meth) acrylate crosslinking agent having a functional group capable of reacting with a carboxyl group ( A (meth) acrylate crosslinking agent, more preferably a di (meth) acrylate crosslinking agent or a (meth) acrylate crosslinking agent having a cyclic functional group, particularly preferably a di (meth) acrylate crosslinking agent.

(A) Multifunctional (meth) acrylate-based crosslinking agent First, as the organic crosslinking agent having a plurality of (meth) acrylate groups and a molecular weight of less than 200, that is, a multifunctional (meth) acrylate-based crosslinking agent, di ( A (meth) acrylate-based cross-linking agent is used, and these di (meth) acrylate-based cross-linking agents having a molecular weight of less than 200 have a plurality of functional groups that are easy to copolymerize with the radical polymerizable monomer, and are particularly preferable because of their good cross-linking efficiency. . The polyfunctionality means that there are a plurality of acrylate groups and methacrylate groups in one molecule, and di (meth) acrylate is a general term for diacrylate, dimethacrylate, and acrylate methacrylate.

  Specifically, as a polyfunctional (meth) acrylate-based crosslinking agent, ethylene glycol diacrylate (molecular weight 170.18), ethylene glycol acrylate methacrylate (molecular weight 184.21), ethylene glycol dimethacrylate (molecular weight 198.24), Propylene glycol diacrylate (molecular weight 184.21), 1,3-propylene glycol diacrylate (molecular weight 184.21), propylene glycol acrylate methacrylate (molecular weight 198.24), 1,3-propylene glycol acrylate methacrylate (molecular weight 198.24) ), 1,3-butanediol diacrylate (molecular weight 198.24), 1,4-butanediol diacrylate (molecular weight 198.24), and the like.

(B) (meth) acrylate-based crosslinking agent having a functional group capable of reacting with a carboxyl group Next, an organic crosslinking agent having a molecular weight of less than 200 having both a (meth) acrylate group and a functional group capable of reacting with a carboxyl group, Preferably, the monofunctional (meth) acrylate crosslinking agent having a functional group capable of reacting with a carboxyl group has a functional group that is easily copolymerized with one or two radical polymerizable monomers, that is, a (meth) acrylate group. Furthermore, since it has a functional group excellent in reactivity with a carboxyl group contained in the water-absorbing resin and / or radically polymerizable monomer, it is more preferable because of its good crosslinking efficiency.

  Specifically, as a (meth) acrylate crosslinking agent having a functional group capable of reacting with a carboxyl group, carbon dioxide gas is reacted with glycidyl acrylate (molecular weight 128.14), glycidyl methacrylate (molecular weight 142.17), and glycidyl acrylate. A cross-linking agent having an epoxy group converted to ethylene carbonate (glycerol carbonate acrylate, molecular weight 172.15), and a cross-linking agent having an epoxy group converted to ethylene carbonate group by reacting glycidyl methacrylate with carbon dioxide (glycerol carbonate methacrylate, molecular weight 186) .18), 2-isocyanate ethyl acrylate (molecular weight 141.14), 2-isocyanate ethyl methacrylate (molecular weight 155.17), glycerin monoacrylate (molecular weight 146.16), glycerin Nometallate (molecular weight 160.19), hydroxyethyl acrylate (molecular weight 116.13), hydroxyethyl methacrylate (molecular weight 130.16), aminoethyl acrylate (molecular weight 115.15), aminoethyl methacrylate (molecular weight 129.18), 1 , 3-propylene glycol monoacrylate (molecular weight 130.16).

  From the physical properties and reactivity, (meth) acrylates having cyclic functional groups (especially epoxy groups or ethylene carbonate groups) as functional groups, in particular glycidyl (meth) acrylates and glycerol carbonate (meth) acrylates are (meth) acrylate-based crosslinking agents. Is more preferable.

(C) A cross-linking agent having a plurality of functional groups capable of reacting with a carboxyl group Further, an organic cross-linking agent having a plurality of functional groups capable of reacting with a carboxyl group and having a molecular weight of less than 200 is a water-absorbing resin and / or a radically polymerizable monomer. Since it has a plurality of functional groups excellent in reactivity with the carboxyl group contained in the monomer, crosslinking efficiency is good and preferable.

  Specifically, oxazolidinone (molecular weight 87.09), oxetane (molecular weight 58.09), ethylene urea (molecular weight 86.11), ethylene carbonate (molecular weight 88.07), propylene carbonate (molecular weight 102.10), ethylene glycol A crosslinking agent having a cyclic functional group such as diglycidyl ether (molecular weight 174.22), propylene glycol diglycidyl ether (molecular weight 188.25), glycerin carbonate (molecular weight 118.10), glycidol (molecular weight 74.09), Ethylene glycol monoglycidyl ether (molecular weight 118.15), propylene glycol monoglycidyl ether (molecular weight 132.18), glycerin monoglycidyl ether (molecular weight 148.18), trimethylolpropane monoglycidyl ether (Molecular weight 190.27), cross-linking agents having both a cyclic functional group and a hydroxyl group such as diethylene glycol monoglycidyl ether (molecular weight 162.21), ethylene glycol (molecular weight 62.08), diethylene glycol (molecular weight 106.14), propylene Glycol (molecular weight 76.11), triethylene glycol (molecular weight 150.20), tetraethylene glycol (molecular weight 194.26), 1,3-propanediol (molecular weight 76.11), dipropylene glycol (molecular weight 134.20) 2,2,4-trimethyl-1,3-pentanediol (molecular weight 146.26), glycerin (molecular weight 92.11), 2-butene-1,4-diol (molecular weight 88.12), 1,3- Butanediol (molecular weight 90.14), 1,4-butanedio (Molecular weight 90.14), 1,5-pentanediol (molecular weight 104.17), 1,6-hexanediol (molecular weight 118.20), 1,2-cyclohexanedimethanol (molecular weight 144.24), 1,2 A polyol-based crosslinking agent having a plurality of hydroxyl groups such as cyclohexanol (molecular weight 116.18), trimethylolpropane (molecular weight 134.20), pentaerythritol (molecular weight 136.17), sorbitol (molecular weight 182.20), diethanolamine ( Alkanolamine cross-linking agents such as molecular weight 105.16), triethanolamine (molecular weight 149.22), ethylenediamine (molecular weight 60.12), diethylenetriamine (molecular weight 103.20), triethylenetetramine (molecular weight 146.28), tetra Ethylenepentamine And polyamine crosslinking agents such as (molecular weight 189.36), and polyisocyanate crosslinking agents such as 2,4-tolylene diisocyanate (molecular weight 174.17) and hexamethylene diisocyanate (molecular weight 168.22).

  These (c) crosslinking agents having a plurality of functional groups capable of reacting with a carboxyl group are preferably a crosslinking agent selected from hydroxyl groups and cyclic functional groups, and further a functional group selected from hydroxyl groups, carbonate groups and glycidyl groups. A cross-linking agent is used.

  When the functional group of the organic crosslinking agent having a molecular weight of less than 200 is an amide group, specifically, N, N′-methylenebisacrylamide (molecular weight 154.19), N, N′-methylenebismethacrylamide (Molecular weight 182.25) and the like are not preferable in actual use because they have safety problems such as mutagenesis and reproductive toxicity.

(Crosslinking agent not covered by this application)
The surface cross-linking agent has an allyl group or a vinyl group, and another functional group is one compound, specifically, allyl acrylate (molecular weight 112.14), allyl methacrylate (molecular weight 126.17), vinyl acrylate In the case of (molecular weight 98.11), vinyl methacrylate (molecular weight 112.14) and the like, the allyl group or vinyl group is inferior in copolymerizability with the radical polymerizable monomer, so that the crosslinking efficiency of the crosslinking agent is low. This is not preferable because a water-absorbing resin having a low balance between water absorption capacity without pressure and water absorption capacity under pressure cannot be obtained.

  In addition, when the molecular weight of the organic crosslinking agent is 200 or more, the crosslinking agent molecule becomes large and it is difficult to penetrate from the surface of the water-absorbent resin, so that the CRC is difficult to be lowered. The residual cross-linking agent in the water-absorbent resin increases, which is not preferable.

  In addition, although the scope of the present invention is not limited, as a reason why the specific crosslinking agent having a molecular weight of less than 200 exhibits the above-mentioned effect specifically in the surface crosslinking polymerization, the monomer aqueous solution containing the crosslinking agent is polymerized as it is. Unlike the polymerization of the water-absorbent resin, it is presumed to be the influence of the permeability to the water-absorbent resin due to the molecular weight. That is, in surface cross-linking polymerization (surface treatment liquid of 50 parts by weight or less per 100 parts by weight of water-absorbing resin) in which a small amount of a monomer aqueous solution containing a cross-linking agent is added to the surface of the water-absorbing resin, While a specific cross-linking agent having a molecular weight of less than 200 forms an optimum surface cross-linking layer by penetrating a good water-absorbing resin, the cross-linking agent having a low penetrability and a molecular weight of 200 or more absorbs water. It is estimated that the amount remaining on the resin surface is large and the water absorption capacity under pressure is low.

(Amount of crosslinking agent used)
The amount of the organic crosslinking agent having a molecular weight of less than 200 depends on the compounds used and combinations thereof, but is 0.001 to 5 parts by weight, preferably 0.01 to 100 parts by weight with respect to 100 parts by weight of the water absorbent resin. 2 parts by weight, more preferably 0.05 to 1 part by weight. If the amount used is less than 0.001 part by weight, the water-absorbent resin is not easily cross-linked and the water absorption performance under pressure is not improved to a sufficient value, which is not preferable. On the other hand, when the amount used exceeds 5 parts by weight, not only is the water absorption capacity of the surface-crosslinked water-absorbent resin under no pressure significantly reduced, but it is also not preferable because it causes an increase in cost and an increase in the amount of the remaining crosslinking agent. . In addition to the organic crosslinking agent having a molecular weight of less than 200, one or two or more other crosslinking agents may be used in combination, if necessary. It is the range of 0-100 weight part with respect to a weight part.

  Moreover, although the ratio of the said monomer and a crosslinking agent can also be determined suitably, 1000-0.0001 weight part with respect to 100 weight part of monomers used with respect to a water absorbing resin in the said range, 100-0. The range is 001 parts by weight.

(Radical polymerization initiator)
In the present invention, the surface cross-linking step of the water absorbent resin is preferably performed in the presence of a radical polymerization initiator. There is no restriction | limiting in particular about the radical polymerization initiator which can be used in the surface crosslinking process of this invention, The polymerization initiator as described in patent documents 7-13 (especially patent documents 8-13) can be applied, and said poly (meth) acryl is mentioned above. What is necessary is just to use the thermal decomposition type radical polymerization initiator and photodecomposition type radical polymerization initiator which can be used at the time of manufacture of acid (salt) type | system | group water absorbing resin.

  The thermal decomposition type radical polymerization initiator is a compound that generates radicals by heating, and preferably has a 10-hour half-life temperature of 0 ° C. or higher and 120 ° C. or lower, more preferably 20 ° C. or higher and 100 ° C. or lower, Considering the temperature conditions for irradiating the active energy ray, those of 40 ° C. or more and 80 ° C. or less are particularly preferable. If the 10-hour half-life temperature is less than 0 ° C. (lower limit), it is unstable during storage, and if it exceeds 120 ° C. (upper limit), it may be chemically too stable to lower the reactivity.

  Specific thermal decomposition type radical polymerization initiators include persulfates such as sodium persulfate, ammonium persulfate and potassium persulfate; hydrogen peroxide; 2,2′-azobis (2-amidinopropane) dihydrochloride, 2 , 2′-azobis [2-2 (-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis (2-methylpropionitrile) and the like. Among them, persulfates such as sodium persulfate, ammonium persulfate, potassium persulfate, and the like, and 2,2′-azobis (2-amidinopropane) dihydrochloride having a 10-hour half-life temperature of 40 to 80 ° C., 2, Azo compounds such as 2′-azobis [2-2 (-imidazolin-2-yl) propane] dihydrochloride and 2,2′-azobis (2-methylpropionitrile) are preferred. Among these, the use of persulfate is particularly preferable in terms of excellent absorption capacity under load, liquid permeability, and free swelling capacity. Persulfate can be used in combination of not only one type but also two or more types having different counter ions.

  A photolytic initiator is a compound that generates radicals upon irradiation with light (preferably active energy rays), and examples thereof include benzoin derivatives, benzyl derivatives, benzophenone derivatives, and acetophenone derivatives. Among these, as commercially available products, hydroxycyclohexyl-phenyl ketone (trade name Irgacure (registered trademark) 184 of Ciba Specialty Chemicals), 1- [4- (2-hydroxyethoxy) -phenyl] -2-hydroxy-2-methyl- 1-propan-1-one (Irgacure (registered trademark) 2959) or the like is more preferably used.

  The amount of the radical polymerization initiator contained in the water-absorbent resin in the surface cross-linking step or mixed with the water-absorbent resin before the cross-linking polymerization reaction is preferably 0.01 to 2 with respect to 100 parts by weight of the water-absorbent resin. Part by weight, more preferably 0.05 to 1 part by weight, particularly preferably 0.1 to 0.5 part by weight. If the amount of the radical polymerization initiator is less than 0.01 parts by weight, the crosslinked structure is not effectively introduced on the surface of the water absorbent resin, which is not preferable. On the other hand, if the amount of the radical polymerization initiator exceeds 2 parts by weight, a large amount of the initiator remains or is colored, and the water absorption performance of the water absorbent resin after surface cross-linking is not preferable.

(solvent)
In the surface crosslinking step of the present invention, the radical polymerizable monomer, the organic crosslinking agent, and the radical polymerization initiator are present together with the water absorbent resin. Although the form which mixes an agent with a water absorbing resin directly as it is may be sufficient, the form mixed with a water absorbing resin in the state dissolved in the solvent is preferably adopted.

  The solvent is not particularly limited as long as the solvent described in Patent Documents 7 to 13 (especially Patent Documents 8 to 13) can be applied and dissolves these, but a radical polymerizable monomer, an organic crosslinking agent, Water is preferable as the solvent because the solubility of the radical polymerization initiator, the safety of the solvent, and the water-absorbing resin have water-absorbing properties.

  By mixing a radical polymerizable monomer, an organic crosslinking agent, and a radical polymerization initiator in the form of an aqueous solution, these can be uniformly dispersed and infiltrated on the surface of the water absorbent resin and mixed with the water absorbent resin uniformly. I can do it. At this time, the amount of water added to and mixed with the water absorbent resin is preferably 1 to 20 parts by weight, more preferably 2 to 10 parts by weight with respect to 100 parts by weight of the water absorbent resin. If the amount of water is less than 1 part by weight, the radically polymerizable monomer, the organic crosslinking agent, the radical polymerization initiator, etc. may not dissolve, and the surface treatment liquid will not easily penetrate into the water absorbent resin. Therefore, it is not preferable. On the other hand, when the amount of water exceeds 20 parts by weight, the absorption performance of the water absorbent resin after surface cross-linking is lowered, and the handleability of the resulting water absorbent resin is deteriorated, which is not preferable. In addition, the aqueous solution for adding the radical polymerizable monomer, the organic crosslinking agent, and the radical polymerization initiator is not limited to the solubility / permeability of the surface treatment liquid, and other solvents and other components (for example, And a mixing aid described later).

  In addition, the concentration of the crosslinking agent and the monomer (aqueous solution concentration) in the case of using a solvent is not particularly limited as long as it is a concentration that satisfies the above-mentioned usage amount, but is preferably 0.001 to 99% by mass, 01-90 mass% is more preferable, and 0.1-80 mass% is a still more preferable range. If the concentration is less than 0.001%, the water-absorbent resin may not be sufficiently surface-crosslinked and the absorption capacity under pressure may not be improved. If the concentration is 99% or more, a large amount of the crosslinking agent and the monomer may remain. is there.

(Mixing aid)
As described above, in the crosslinking polymerization step of the present invention, a radical polymerizable monomer, an organic crosslinking agent, a radical polymerization initiator, and water are present in the crosslinking polymerization reaction system together with the water-absorbing resin. For the purpose of improving, a mixing aid may be present in the reaction system. The mixing aid is not particularly limited as long as it can suppress aggregation of the water-absorbent resin with water and can improve the mixing property of the surface treatment liquid and the water-absorbent resin, but is not limited to Patent Documents 7 to 13 (particularly Patent Document 8). To 13) can be applied. By adding the mixing aid, the water-absorbing resin is prevented from aggregating with water, and the surface treatment liquid is more uniformly mixed with the water-absorbing resin. As a result, the entire surface of the water absorbent resin can be more uniformly cross-linked.

  Specific examples of the mixing aid include a surfactant, a water-soluble polymer, a hydrophilic organic solvent, a water-soluble inorganic compound, an inorganic acid (salt), and an organic acid (salt).

  These mixing aids may be used alone or in the form of a mixture of two or more. Further, the amount of the mixing aid in the cross-linking polymerization reaction system is not particularly limited as long as it is a form that can suppress aggregation of the water-absorbing resin with water and improve the mixing property between the surface treatment liquid and the water-absorbing resin. , Preferably from 0.0001 to 20 parts by weight, more preferably from 0.001 to 10 parts by weight, still more preferably from 0.005 to 5 parts by weight, particularly preferably from 0.01 to 100 parts by weight based on 100 parts by weight of the water absorbent resin. 1 part by weight.

  The order of addition of the mixing aid in the case of using the mixing aid is also not particularly limited, and after adding the mixing aid to the water-absorbent resin in advance, a radical polymerizable monomer or an organic crosslinking agent (in some cases) Is a method in which a surface treatment liquid containing these is added and mixed, and a method in which a mixing aid is dissolved in the surface treatment liquid and mixed with a water-absorbent resin simultaneously with a radical polymerizable monomer or an organic crosslinking agent, etc. Can be used.

  Even when the surface treatment liquid containing the radical polymerizable monomer and the organic crosslinking agent and the mixing aid are added separately, the total amount is the surface treatment liquid amount in the surface crosslinking step.

(Polymerization inhibitor / temperature / oxygen)
From the viewpoint of stability, the surface treatment liquid preferably contains a polymerization inhibitor and / or oxygen (dissolved oxygen), and is preferably stored and prepared at a predetermined temperature or lower.

  The polymerization inhibitor is preferably phenols, and more preferably methoxyphenols. Moreover, the density | concentration becomes like this. Preferably it is 1-200 ppm from the color tone of polymerizability or a water absorbing resin, More preferably, it is 10-160 ppm. The amount of oxygen (dissolved oxygen) is 1 ppm to a saturated concentration. The temperature may be 50 ° C. or lower, preferably 40 ° C. or lower, more preferably 30 ° C. or lower, and may be a temperature that does not cause the surface treatment liquid to freeze or cause components to precipitate or separate. ° C or higher is preferable, and 5 ° C or higher is more preferable.

(Mixing conditions of surface treatment liquid and water-absorbing resin)
The surface treatment liquid may be added directly to the water-absorbent resin powder, or may be added to the water-absorbent resin dispersed in a hydrophobic organic solvent (or hydrophilic organic solvent) by reverse phase suspension polymerization or azeotropic dehydration. Good.

  Regardless of the form of the additive solution such as dripping or spraying, the method for adding the surface treatment solution may be a batch addition of the surface treatment solution prepared for all components, addition for each component, or a plurality of solutions prepared for a part of each component. Addition, addition of the same surface treatment solution divided into a plurality of times, and the like. Furthermore, the addition of the surface treatment liquid may be performed using one addition port or a plurality of addition ports. For example, a radically polymerizable monomer and a radical polymerization initiator may be separately added to the water absorbent resin, or a part thereof may be added to the water absorbent resin subjected to a crosslinking polymerization reaction.

  In addition, when adding by dividing | segmenting as mentioned above, the total amount becomes the surface treatment liquid amount in a surface crosslinking process.

  The mixing conditions for mixing the surface treatment liquid and the water absorbent resin are not particularly limited, but the mixing conditions described in Patent Documents 7 to 13 (particularly Patent Documents 8 to 13) can be applied. For example, the temperature of the water absorbent resin during mixing is preferably 0 to 150 ° C, more preferably 10 to 120 ° C, still more preferably 20 to 100 ° C, particularly preferably 30 to 90 ° C, and most preferably 40 to 70 ° C. It is. The mixing time is not particularly limited, but is usually 0.1 to 60 minutes.

  There are no particular restrictions on the mixer, and ordinary mixers such as V-type mixers, ribbon-type mixers, screw-type mixers, rotating disk-type mixers, air-flow-type mixers, batch-type kneaders, and continuous types. A kneader, a paddle type mixer, a vertical type mixer, or the like can be used as the mixer. When adding to a water-absorbing resin dispersed in a hydrophobic organic solvent by reverse-phase suspension polymerization or azeotropic dehydration, a polymerization kettle with a rotary stirring function or a dehydration kettle can be used as it is as a mixer for the surface treatment liquid. The surface treatment liquid may be added after filtering the dispersed water-absorbent resin.

(3-1-2) Crosslinking polymerization step By this crosslinking polymerization step, a surface-crosslinked water-absorbing resin (water-absorbing agent) is obtained. The efficiency of surface cross-linking can be confirmed by a decrease in CRC (value corrected by resin solid content) from the water absorbent resin before the surface cross-linking step.

  The step is a step of forming a portion having a higher cross-linking density in the surface layer of the water-absorbent resin (a portion of several tens of μm from the surface of the water-absorbent resin) by cross-linking polymerization of the mixture obtained in the mixing step. is there.

  The crosslinking polymerization method is not particularly limited, but the polymerization methods described in Patent Documents 7 to 13 (particularly Patent Documents 8 to 13) can be applied. Specifically, Patent Documents 7 to 9 exemplify surface crosslinking by heating. Patent Documents 10 to 12 exemplify performing surface crosslinking by irradiating ultraviolet rays. Specifically, the radical polymerizable monomer and the organic crosslinking agent mixed in the water absorbent resin are crosslinked and polymerized on the surface of the water absorbent resin and / or in the vicinity thereof by heating and / or irradiation with active energy rays. In addition, when performing both heating and irradiation of an active energy ray with respect to a water absorbing resin, you may carry out simultaneously and may carry out separately.

  In order to advance the crosslinking polymerization reaction efficiently, it is important to maintain the water content of the water-absorbent resin in a specific range during the step, and the water content of the water-absorbent resin in the step (1 g of the water-absorbent resin is reduced). The weight loss after drying at 180 ° C. for 3 hours is preferably 1 to 30% by weight, more preferably 3 to 25% by weight, and still more preferably 5 to 20% by weight. When the amount of water contained in the water-absorbing resin during surface cross-linking is less than 1% by weight, the cross-linking polymerization reaction hardly proceeds and a large amount of radical polymerizable monomer and / or organic cross-linking agent remains, which is not preferable.

  The water content of the water absorbent resin during the crosslinking polymerization step may be adjusted by adjusting the amount of water in the surface treatment liquid in the mixing step, or by adjusting the water content of the water absorbent resin before the surface crosslinking. You can go. Further, during surface crosslinking, a method of sealing the reaction vessel during surface crosslinking, increasing the atmospheric dew point of the reaction vessel during surface crosslinking, or adding water to the water absorbent resin during surface crosslinking, etc. Thus, the water content of the water-absorbing resin can be adjusted. One of these methods may be selected and performed, or a plurality of methods may be combined.

(Heat treatment conditions)
When the polymerization is carried out by heating alone using a thermal decomposition type radical polymerization initiator, there is no need to separately provide an active energy ray irradiation device, which is preferable in terms of designing the production apparatus.

  In order to cross-link the water-absorbent resin by heat treatment, the cross-linking polymerization reaction system containing the above-described components may be heated, and preferably heated in a relatively high humidity atmosphere. There are no particular restrictions on the specific conditions of such an atmosphere and the specific conditions of the heat treatment.

  In performing the heat treatment, the mixture is heated to 50 to 150 ° C, preferably 60 to 140 ° C, more preferably 70 to 130 ° C, and still more preferably 80 to 120 ° C. The method for heating the mixture is not particularly limited, but heating by heating a heat transfer surface such as a jacket with steam, hot water, nighter, oil, etc., heating by applying hot air, and heating the atmosphere. The heating etc. by the thing is mentioned.

  As an example of the atmospheric conditions when performing the heat treatment, the temperature of the atmosphere is preferably 50 to 150 ° C, more preferably 70 to 130 ° C, and still more preferably 80 to 120 ° C. The dew point of the atmosphere is usually 20 ° C. or higher and lower than 100 ° C., preferably 30 to 95 ° C., more preferably 40 to 90 ° C., still more preferably 50 to 90 ° C., particularly preferably 60 to 90 ° C., most preferably 70. ~ 90 ° C.

  The heating time for performing the heat treatment is not particularly limited, but is preferably 1 to 90 minutes, more preferably 3 to 60 minutes, still more preferably 5 to 45 minutes, and particularly preferably 10 to 30 minutes. A heating time of 1 minute or less is not preferable because the introduction of a crosslinked structure on the surface of the water-absorbent resin becomes insufficient. On the other hand, a heating time of 90 minutes or more is not preferable because productivity deteriorates and water absorption performance decreases.

  There is no restriction | limiting in particular in the apparatus in the case of surface-crosslinking a water absorbing resin by heat processing, A well-known dryer can be used. For example, a conduction heat transfer type, a radiant heat transfer type, a hot air heat transfer type, and a dielectric heating type dryer are preferably used. Specifically, a stationary type, a belt type, a fluidized bed type, an air flow type, a paddle type, a rotation type Mold, kneading type, infrared type, electron beam type dryer and the like.

  In addition, as long as the said mixing conditions can be satisfied, a part or all of the said mixing process can be substituted by this bridge | crosslinking polymerization process. However, in order to sufficiently advance the cross-linking polymerization reaction, the time from the end of the addition of the surface treatment liquid to the end of the cross-linking polymerization step is 0.1 minutes or more, preferably 1 minute or more, more preferably 3 minutes or more, still more preferably It is necessary to adjust it to be 5 minutes or more, particularly preferably 10 minutes or more.

(Active energy ray irradiation conditions)
In the case of crosslinking polymerization with active energy rays, the active energy rays used include one or more of ultraviolet rays, electron beams and gamma rays. Among these active energy rays, ultraviolet rays and electron beams are preferable. Considering the influence of active energy rays on the human body, it is preferably ultraviolet rays, more preferably ultraviolet rays having a wavelength of 300 nm or less, and particularly preferably wavelengths of 180 to 290 nm. The irradiation conditions are preferably an irradiation intensity of 3 to 1000 mW / cm 2 and an irradiation amount of 100 to 10000 mJ / cm 2 when ultraviolet rays are used.

  Examples of the apparatus for irradiating ultraviolet rays include a high-pressure mercury lamp, a low-pressure mercury lamp, a metal halide lamp, a xenon lamp, and a halogen lamp. As long as the ultraviolet rays are irradiated, particularly as long as the ultraviolet rays in the wavelength range are irradiated, other radiations and wavelengths may be included, and the method is not particularly limited. When an electron beam is used, the acceleration voltage is preferably 50 to 800 kV and the absorbed dose is 0.1 to 100 Mrad. The time for irradiating the active energy ray depends on the amount of the water-absorbing resin to be treated, but is preferably 0.1 minutes or more and less than 60 minutes, more preferably 0.5 minutes or more and less than 20 minutes, still more preferably 0.5. Min. To less than 5 minutes, particularly preferably from 1 minute to less than 3 minutes.

  In the present invention, when the surface is crosslinked by irradiation with active energy rays, it is not necessary to heat the surface. However, when irradiation with the active energy rays is performed under heating, the surface-crosslinked water-absorbing property is excellent in water absorption characteristics. Since resin is obtained, it is more preferable. The heating temperature is preferably in the range of 0 to 150 ° C, more preferably 10 to 120 ° C, still more preferably room temperature to 100 ° C, and particularly preferably 50 to 100 ° C.

  It is preferable to stir the water-absorbent resin during irradiation with active energy rays. The active energy ray can be uniformly irradiated to the mixture of the water absorbent resin and the surface treatment liquid by stirring. There is no restriction | limiting in particular as an apparatus which can stir a water absorbing resin in the case of irradiation of an active energy ray, A well-known mixer is used. Specific examples include vibration type, vibration feeder type, ribbon type, conical ribbon type, screw extrusion type, airflow type, batch kneader, continuous kneader, paddle type, high-speed flow type, floating flow type mixer, etc. It is done.

(3-2) Heating step optionally after cross-linking polymerization Further, after heating and / or irradiation with active energy rays, adjustment of water content, radical polymerizable monomer remaining after cross-linking polymerization, and residual and / or Alternatively, the water-absorbent resin may be further heated at a temperature higher than the above temperature in the range of 50 to 250 ° C., if necessary, in order to further react with an organic crosslinking agent that has reacted only partially. By this heating, a water-absorbing agent having a desired water content can be obtained, the safety of the water-absorbing agent can be increased, and the crosslinking efficiency of the water-absorbing agent can be increased. As an apparatus for performing the heating, an apparatus similar to the apparatus described in the heat treatment is preferably used. In particular, when the functional group of the cross-linking agent is a hydroxyl group or a carbonate group, the reaction is heated to 100 to 240 ° C., more preferably 120 to 230 ° C., and 130 to 220 ° C. for 1 minute or more (further about 5 to 300 minutes). By doing so, the crosslinking reaction can be promoted.

  Of the above-mentioned crosslinking agents, (c) a crosslinking agent having a plurality of functional groups capable of reacting with a carboxyl group, particularly a polyol-based crosslinking agent, specifically a polyol such as propylene glycol, is also used as a solvent or a mixing aid. However, in the present invention, when specifically contributing to the crosslinking reaction (or when heating at a temperature sufficient for crosslinking), the polyol is an organic crosslinking agent, otherwise it is classified as a solvent or a mixing aid. The The presence or absence of a crosslinking reaction can be confirmed by a decrease in water absorption ratio (particularly 1 g / g or more in CRC), and the heating temperature sufficient for crosslinking is in the above range in the case of, for example, a hydroxyl group or a carbonate group.

(3-3) Arbitrarily cooling process The said process is a process implemented as needed for the purpose of the stop of a crosslinking reaction, the conveyance to the following process, etc. after the said crosslinking polymerization process (especially heat processing). The cooling device is not particularly limited, but is preferably a stirring device or a flow device having a cooling function by a transmission surface or an air flow. The cooling step is preferably carried out in a short time after the surface cross-linking step, immediately after the cross-linking polymerization step, preferably over 0 seconds and within 3 minutes, more preferably within 2 minutes, even more preferably within 1 minute, particularly preferably Is put into the cooling device of the cooling process within 0.5 minutes. The time until the charging can be adjusted by the layout of the apparatus (for example, the cross-linking polymerization apparatus and the cooling apparatus are directly connected or arranged with a short distance, for example, a transport distance within 10 m).

  The cooling device used in the present invention may be a heat transfer type using a refrigerant or a type in which cold air is blown. Although it does not specifically limit as said refrigerant | coolant, Preferably water, warm water, an antifreeze, etc. are mentioned. The temperature at that time (temperature of the heat transfer surface of the jacket or the like, the temperature of the cold air) is preferably 0 to 100 ° C, more preferably 20 to 90 ° C, still more preferably 40 to 80 ° C.

(3-4) Addition process of additive In addition, in order to mix an additive, you may provide the addition process of an additive as needed. The addition process of this additive can be suitably set according to the property of the additive used. For example, when there is a possibility that the additive may react with the surface treatment liquid or when it may be affected by heating, it is preferably performed during the cooling step or after the completion of the step. Since it is not necessary to provide an apparatus, it is simple and more preferable.

  Moreover, when not providing a cooling process, although an additive addition apparatus in particular is not ask | required, Preferably it is made | formed with a stirring apparatus or a fluidization apparatus. As the stirring device, a stirring mixer described in International Patent Publication No. 2008/141821 pamphlet can also be suitably used.

  The additive addition step is also preferably performed in a short time after the cross-linking polymerization step, immediately after the cross-linking polymerization step, preferably over 0 seconds within 3 minutes, more preferably within 2 minutes, more preferably within 1 minute, Particularly preferably, it is added to the stirring or fluidizing apparatus in the additive addition step within 0.5 minutes.

  The additive is at least one selected from the group consisting of the following polyvalent metal salt compounds, polycationic polymers, chelating agents, inorganic reducing agents, and hydroxycarboxylic acid compounds among the compounds not described in the mixing step. It is a seed. The additive is preferably added as an aqueous solution or a slurry solution, and may be added and mixed during the mixing step.

(Polyvalent metal salt and / or cationic polymer)
In the present invention, a polyvalent metal salt and / or a cationic polymer may be added from the viewpoint of improving the water absorption rate (Vortex), liquid permeability (SFC), fluidity at the time of moisture absorption, etc. preferable. Specifically, the polyvalent metal salt and / or cationic polymer described in “[7] Polyvalent metal salt and / or cationic polymer” of International Patent Publication No. 2011/040530 pamphlet, and the amount used thereof are It is also suitably used in the invention.

(Chelating agent)
In the present invention, a chelating agent can be further added from the viewpoint of preventing coloring and deterioration of the resulting water-absorbent resin. As the chelating agent, for example, the chelating agent described in “[2] chelating agent” of International Patent Publication No. 2011/040530 pamphlet, and the amount used thereof are also preferably used in the present invention.

(Inorganic reducing agent)
In the present invention, an inorganic reducing agent can be further added from the viewpoint of preventing coloring and deterioration of the resulting water-absorbent resin and reducing the residual monomer. As the inorganic reducing agent, for example, the inorganic reducing agent described in “[3] Inorganic reducing agent” in International Patent Publication No. 2011/040530 pamphlet and the amount used thereof are also preferably used in the present invention.

(Α-hydroxycarboxylic acid compound)
In the present invention, from the viewpoint of preventing coloring of the resulting water-absorbent resin, an α-hydroxycarboxylic acid compound can be further added. The “α-hydroxycarboxylic acid compound” is a hydroxycarboxylic acid having a hydroxy group at the α position among carboxylic acids having a hydroxy group in the molecule or a salt thereof. Examples of the α-hydroxycarboxylic acid compound include the α-hydroxycarboxylic acid compound described in “[6] α-hydroxycarboxylic acid compound” of International Patent Publication No. 2011/040530 pamphlet, and the use amount thereof. Is also preferably used.

(Other additives)
Additives other than those mentioned above can be added to impart various functions to the water-absorbent resin. Specific examples of such additives include surfactants, compounds having phosphorus atoms, oxidizing agents, organic reducing agents, and described in “[5] Water-insoluble inorganic fine particles” in International Patent Publication No. 2011/040530. Water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorizers, antibacterial agents, pulp and thermoplastic fibers. As the surfactant, the surfactant disclosed in International Patent Publication No. 2005/077500 is preferably applied. The amount of these additives used is appropriately determined according to the application and is not particularly limited, but is preferably 0 to 3% by weight, more preferably 0 to 1% by weight.

(3-5) Other steps In addition to the steps described above, a granulation step, a sizing step, a fine powder removal step, a fine powder reuse step, and the like can be provided as necessary. Moreover, you may further include 1 type, or 2 or more types of processes, such as a transport process, a storage process, a packing process, and a storage process.

  Here, the sizing step includes a fine powder removing step after the surface cross-linking step and a step in which the water-absorbent resin is agglomerated and classified when the desired size is exceeded, and further, the classified agglomerated material is pulverized as necessary. .

  Further, the fine powder recycling step includes a step of adding the fine powder as it is or making it into a large hydrogel in the fine powder granulation step and adding it in any step of the production process of the water absorbent resin.

[4] Physical properties of water-absorbing agent or poly (meth) acrylic acid (salt) -based water-absorbing resin The production method of the present invention provides a water-absorbing agent with a wide range of physical properties. Balance magnification. By taking the production method of the present invention as an example of the production method, the present invention is preferably a water-absorbing agent comprising a poly (meth) acrylic acid (salt) -based water-absorbing resin whose surface is cross-linked and polymerized, A water-absorbing agent having an absorption capacity under load (AAP 4.83 kPa) of 10 to 30 g / g with respect to physiological saline having a residual amount of 100 ppm by weight or less and 4.83 kPa is provided. Since the water-absorbing agent of the present invention has a small amount of residual crosslinking agent, it does not have various problems (for example, excessive hydrophilization or hydrophobization of the surface) derived from the residual crosslinking agent, and has high physical properties even when used. I will provide a.

  More preferably, the water-absorbing agent of the present invention has a water content of 5 to 15% by weight, and since the water content is high, it is difficult to be charged with static electricity, and has plasticity and excellent damage resistance. Provide hygiene products. Further, preferably, the water absorption capacity (CRC) is 25 to 50 g / g, and the absorption capacity under pressure (AAP 2.06 Pa) against 2.06 kPa physiological saline is 20 to 40 g / g.

  Since the water-absorbing agent according to the present invention has excellent properties that satisfy the following physical properties (4-1) to (4-7), a safe and high-performance sanitary article (especially a paper diaper) is provided. become able to do.

  Moreover, although the water absorbing agent obtained with the manufacturing method which concerns on this invention is not specifically limited about the shape, A particulate form is especially preferable. In this section, the physical properties of the particulate water-absorbing agent which is a preferred embodiment will be described. In addition, the following physical property is prescribed | regulated based on EDANA method unless there is particular notice.

(4-1) Non-pressurized water absorption capacity (CRC) (ERT441.2-02)
The water absorption capacity (CRC) of the water absorbing agent of the present invention under no pressure is 5 g / g or more, preferably 10 (g / g) or more, more preferably 15 (g / g) or more, and further preferably 25 (g / g). In addition, it is 27 g / g or more. Although an upper limit is not specifically limited, Preferably it is 70 (g / g) or less, More preferably, it is 60 (g / g) or less, More preferably, it is 50 (g / g) or less.

  When the CRC is less than 5 (g / g), the amount of absorption is small and it is not suitable as an absorbent material for sanitary goods such as paper diapers. In addition, when the CRC exceeds 70 (g / g), the speed of taking in body fluids such as urine and blood decreases, so that it is not suitable for use in a high water absorption speed type paper diaper. The CRC can be controlled by the composition and amount of the internal cross-linking agent and the surface treatment liquid.

(4-2) Water absorption capacity under pressure (AAP 2.06 kPa, AAP 4.83 kPa) (ERT442.2-02)
The water absorption capacity under pressure (AAP2.06 kPa) of the water-absorbing agent of the present invention is preferably 20 (g / g) or more, more preferably 22 (g / g) or more, still more preferably 23 (g / g) or more, particularly Preferably it is 25 (g / g) or more, most preferably 28 (g / g) or more. Although an upper limit is not specifically limited, Preferably it is 40 (g / g) or less.

  The water absorption capacity under pressure (AAP 4.83 kPa) of the water-absorbing agent of the present invention is 10 g / g or more, preferably 12 (g / g) or more, more preferably 15 (g / g) or more, and still more preferably 20 ( g / g) or more, particularly preferably 21 (g / g) or more, and most preferably 22 (g / g) or more. Although an upper limit is not specifically limited, Preferably it is 30 (g / g) or less.

  When the above AAP 2.06 kPa and AAP 4.83 kPa are less than the above ranges, the amount of return of the liquid when pressure is applied to the absorber (usually referred to as “Re-Wet”) is increased. Not suitable as an absorbent for hygiene products. AAP 2.06 kPa and AAP 4.83 kPa can be controlled by the particle size and the composition and amount of the surface treatment liquid.

(4-3) Moisture content (specified by the measurement method in the example)
The water content of the water-absorbing agent of the present invention is preferably 5 to 15% by weight, more preferably 5 to 14% by weight, still more preferably 5.5 to 13% by weight, and particularly preferably 6 to 12% by weight. By controlling the water content within the above range, it is possible to obtain a water-absorbing agent having excellent powder characteristics (for example, fluidity, transportability, damage resistance, etc.). In addition, the measurement of the moisture content was performed by the method as described in the Example mentioned later.

(4-4) Residual monomer (specified by ERT410.2-02)
From the viewpoint of safety, the residual monomer of the water-absorbing agent of the present invention is preferably 500 ppm or less, more preferably 400 ppm or less, and still more preferably 300 ppm or less. By controlling the residual monomer within the above range, a water-absorbing agent that can reduce irritation to the skin and the like can be obtained.

(4-5) Water-soluble component (Ext) (specified by ERT470.2-02)
The water-soluble component (Ext) of the water-absorbing agent of the present invention is preferably 35% by weight or less, more preferably 25% by weight or less, and still more preferably 15% by weight or less.

  If the water-soluble content exceeds 35% by weight, the gel strength may be weak and the water-absorbing agent may have poor liquid permeability. Furthermore, since Re-Wet increases, it is not suitable for use on paper diapers. The amount of water-soluble component can be controlled by the amount of the internal crosslinking agent and the composition and amount of the surface treatment liquid.

(4-6) Residual cross-linking agent (specified by the measurement method in the examples)
From the viewpoint of safety, the residual crosslinking agent of the water-absorbing agent of the present invention is preferably 100 ppm or less, more preferably 10 ppm or less, and still more preferably 1 ppm or less. By controlling the residual crosslinking agent within the above range, a water-absorbing agent having excellent safety can be obtained. In addition, the measurement of a residual crosslinking agent was performed by the method as described in the Example mentioned later.

(4-7) Particle size The particle size (particle size distribution (PSD), weight average particle size (D50) and particle size distribution width (σζ)) of the water-absorbing agent of the present invention is in the same range as the water-absorbent resin before surface crosslinking. Be controlled.

[5] Use of water-absorbing agent or poly (meth) acrylic acid (salt) water-absorbing resin The use of the water-absorbing agent of the present invention is not particularly limited, but preferably sanitary products such as paper diapers, sanitary napkins, incontinence pads, etc. It can be used for various absorbers. In particular, it can be used as an absorber for high-concentration paper diapers (those with a large amount of water-absorbing agent used per paper diaper) where odors, coloring, and the like derived from raw materials have been problems. Furthermore, a remarkable effect can be expected when used in the upper layer of the absorber.

  In addition to the water absorbing agent, an absorbent material such as pulp fiber can be used as the absorbent body. In this case, the content (core concentration) of the water-absorbing agent in the absorber is preferably 30 to 100% by weight, more preferably 40 to 100% by weight, still more preferably 50 to 100% by weight, and even more preferably 60. -100% by weight, particularly preferably 70-100% by weight, most preferably 75-95% by weight.

  By making the said core density | concentration into the said range, when the said absorber is used for the upper layer part of an absorbent article, an absorbent article can maintain the white state with a clean feeling. Furthermore, since the diffusibility of body fluids such as urine and blood is excellent, the amount of absorption can be improved by efficient liquid distribution.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not construed as being limited thereto, and combinations of technical means disclosed in each example are also included in the scope of the present invention. Shall be included.

  Unless otherwise noted, physical properties were measured under conditions of room temperature (20 to 25 ° C.) and relative humidity of 50% RH.

[Measurement method of physical properties]
(A) Absorption capacity without pressure (CRC)
The water absorption capacity (CRC) under no pressure of the water absorbent resin (water absorbent) obtained by the production method according to the present invention was measured in accordance with ERT441.2-02.

(B) Water absorption capacity under pressure The water absorption capacity under pressure (AAP 2.06 kPa) of the water absorbent resin (water absorbing agent) obtained by the production method according to the present invention was measured in accordance with ERT442.2-02. Moreover, the water absorption capacity | capacitance under pressure (AAP4.83kPa) was measured in accordance with the method similar to ERT442.2-02 except having changed the load conditions to 4.83kPa (0.7psi, 50 [g / cm2]).

(C) Solid content and water content The solid content and water content of the water-absorbent resin (water-absorbing agent) obtained by the production method according to the present invention were evaluated by the following methods. In this invention, solid content represents the ratio for which the component which does not volatilize in 180 degreeC and 3 hours in a water absorbing resin (water absorbing agent), and a moisture content is calculated | required from the following relational expressions.

Water content [wt%] = 100-solid content [wt%]
The measurement method of solid content was performed as follows.

  About 1 g of water-absorbing resin (water-absorbing agent) is weighed (weight W2 [g]) into an aluminum cup (weight W1 [g]) having a bottom diameter of about 5 cm, and allowed to stand still in a windless dryer at 180 ° C. for 3 hours. And dried. The total weight (W3 [g]) of the dried aluminum cup and the water absorbent resin (water absorbent) was measured, and the solid content was determined from Equation 1.

(Equation 1)
Solid content [% by weight] = {(W3-W1) / W2} × 100
(D) Residual cross-linking agent The amount of the residual cross-linking agent of the water-absorbent resin (water-absorbing agent) obtained by the production method according to the present invention was evaluated by the following method.

  Into a glass beaker with a capacity of 250 ml, 200 ml of a 0.9% by weight sodium chloride aqueous solution is added, and 1.0 g of a water absorbent resin (water absorbent) is added thereto, followed by stirring at 500 rpm for 16 hours to extract the remaining crosslinking agent. did. The obtained extract was filtered using one sheet of filter paper (product name: JIS P 3801, No. 2, thickness 0.26 mm, reserved particle diameter 5 μm / manufactured by ADVANTEC Toyo Co., Ltd.), and the obtained filtrate was filtered. (GL Science Disc, GL Chromatodisc, aqueous 25A, pore size 0.45 μm) was passed through, and the value (unit: ppm) measured by high performance liquid chromatography (HPLC) was defined as the amount of the remaining crosslinking agent.

  In addition, the value of the residual crosslinking agent amount is 0.005 ppm (1 ppm in terms of water-absorbing resin), 0.025 ppm (5 ppm in terms of water-absorbing resin), 0.05 ppm (0.9 wt% sodium chloride aqueous solution) Water-absorbing resin conversion 10 ppm), 0.25 ppm (vs. water-absorbing resin conversion 50 ppm), 0.5 ppm (vs. water-absorbing resin conversion 100 ppm), 2.5 ppm (vs. water-absorbing resin conversion 500 ppm), 5 ppm (vs. water-absorbing resin) Calculation was performed based on a calibration curve prepared from the dissolved solution.

(E) Particle size distribution (PSD), weight average particle size (D50), particle size distribution width (σζ)
The particle size distribution and weight average particle diameter (D50) of the water absorbent resin (water absorbent) obtained by the production method according to the present invention are determined by standard sieve classification in accordance with the measurement method disclosed in US Pat. No. 7,638,570. It was measured.

[Production Example 1]
In a 2 L polypropylene container, 351 g of acrylic acid, 0.76 g of polyethylene glycol diacrylate (molecular weight: 522.66, average number of ethylene oxide units: n = 9) as an internal cross-linking agent, 48.5 wt% sodium hydroxide aqueous solution 296 g and 337 g of water were added and mixed (stirred) to prepare an aqueous monomer solution (1). The temperature of the monomer aqueous solution (1) rose to 84 ° C. due to heat of neutralization.

  Next, when the temperature dropped to 83 ° C. while stirring the monomer aqueous solution (1), 15.4 g of a 3.8 wt% sodium persulfate aqueous solution was added. Immediately after that, the aqueous solution was poured into a stainless steel bat-type reactor (bottom; 340 × 340 mm, height: 25 mm, inner surface; Teflon (registered trademark) coating) in an open air system. Started. In addition, the surface temperature was previously set to 40 degreeC using the hot plate (Inoue Seieido Co., Ltd .; NEO HOTPLATE HI-1000) for the said stainless steel bat type | mold reactor.

  The polymerization reaction proceeded by expanding and foaming in all directions toward the upper side of the bat-type reactor while generating water vapor, and then contracted to a size slightly larger than the bottom surface of the reactor. This polymerization reaction (expansion / shrinkage) was completed in about 1 minute. After completion of the polymerization, it was kept in the reaction apparatus for 4 minutes and taken out as a hydrogel crosslinked polymer (1).

  The hydrogel crosslinked polymer (hereinafter referred to as “hydrogel”) (1) obtained by the above polymerization reaction was used as a meat chopper (manufactured by Iizuka Kogyo Co., Ltd .; MEAT-CHOPER TYPE 12VR-400KSOX / die pore diameter: 6.4 mm). And the number of holes: 38, die thickness: 8 mm), and gel pulverization gave a particulate hydrous gel (1).

  At this time, the hydrogel was added at 250 (g / min), and water adjusted to 90 ° C. in parallel was added at 50 (g / min).

  Next, the obtained particulate hydrogel (1) was spread and placed on a stainless steel wire mesh having an opening of 850 μm, and then dried at 180 ° C. for 30 minutes using a hot air dryer. Subsequently, the obtained dried polymer (1) was pulverized using a roll mill (manufactured by Inoguchi Giken Co., Ltd .; WML type roll pulverizer), and then classified using a JIS standard sieve having openings of 850 μm and 150 μm.

  According to the above operation, the weight average particle size (D50) is 350 μm, the content of particles having a particle size of 150 μm or more and less than 850 μm is 97% by weight, the 150 μm passing material is 3% by weight (of which 106 μm passing material is 1%), A regularly crushed water-absorbing resin (1) was obtained. The water-absorbing resin (1) thus obtained has a water absorption capacity (CRC) under no pressure of 51.1 (g / g), a water absorption capacity under pressure of AAP of 2.06 kPa is 7.1 (g / g), and an AAP of 4.83 kPa is 6 0.5 (g / g), the water content was 4.0% by weight. The residual crosslinking agent polyethylene glycol diacrylate in the water absorbent resin (1) is N.I. D. (The detection limit was less than 10 ppm).

[Example 1]
(Mixing process)
For 100 parts by weight of the water-absorbent resin (1) obtained in Production Example 1, 4.0 parts by weight of acrylic acid as a radical polymerizable monomer and ethylene glycol diacrylate (molecular weight 170.18, as an organic crosslinking agent) 2 acrylate groups) 0.2 parts by weight, 7.0 parts by weight of water, 0.25 parts by weight of ammonium persulfate as a radical polymerization initiator, 0.1 parts by weight of polyethylene glycol monomethyl ether (number average molecular weight of about 2000) as a mixing aid 11.55 parts by weight of the surface treatment liquid in which parts were previously mixed was uniformly dropped and mixed to obtain a mixture (1).

(Crosslinking polymerization process)
Next, 50 g of the mixture (1) obtained by the above operation was uniformly spread on a bat (manufactured by SUS304: length 158 mm × width 128 mm × depth 25 mm), and a polyethylene plastic bag with a chuck (Unipack H-4, Ltd.) Production: manufactured by Nippon Shokubai Co., Ltd .: 240 mm long × 170 mm wide × 0.04 mm thick) was sealed and heated at 100 ° C. for 30 minutes while standing still.

(Sizing process)
After completion of the surface cross-linking, classification was performed using a JIS standard sieve having an opening of 850 μm to obtain a water absorbing agent (1). In addition, the ON product (non-passing material) of the sieve was crushed until the entire amount passed 850 μm.

  Table 1 shows the surface crosslinking conditions of the water-absorbing agent (1) and the performance of the water-absorbing agent.

[Example 2]
In Example 1, the same operation as in Example 1 was carried out except that 0.2 parts by weight of ethylene glycol dimethacrylate (molecular weight 198.24, two methacrylate groups) was used as the organic crosslinking agent. )

  Table 1 shows the surface crosslinking conditions of the water-absorbing agent (2) and the performance of the water-absorbing agent.

[Example 3]
In Example 1, the same operation as in Example 1 was performed, except that 0.2 part by weight of 1,4-butanediol diacrylate (molecular weight 198.24, two acrylate groups) was used as the organic crosslinking agent. A water absorbing agent (3) was obtained.

  Table 1 shows the surface crosslinking conditions of the water-absorbing agent (3) and the performance of the water-absorbing agent.

[Example 4]
In Example 1, glycerol carbonate acrylate (a compound obtained by reacting an epoxy group of glycidyl acrylate with carbon dioxide to form an ethylene carbonate group) (molecular weight 172.15, one acrylate group + one carbonate ring) as an organic crosslinking agent ) After performing the same operation (polymerization of acrylate group) as in Example 1 except that 0.2 parts by weight was used, the obtained water-absorbing resin after the crosslinking polymerization step was further heated before the granulating step. A water absorbing agent (4) was obtained by heating at 140 ° C. for 30 minutes (reaction operation of carbonate ring) as a treatment step.

  Table 1 shows the surface cross-linking conditions of the water-absorbing agent (4) and the performance of the water-absorbing agent.

[Example 5]
In Example 1, the same operation as in Example 1 was performed, except that 0.05 part by weight of glycidyl methacrylate (molecular weight 142.17, one methacrylate group, one epoxy group) was used as the organic crosslinking agent. Agent (5) was obtained.

  Table 1 shows the surface crosslinking conditions of the water-absorbing agent (5) and the performance of the water-absorbing agent.

[Comparative Example 1]
In Example 1, 0.2 parts by weight of polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9) (molecular weight 522.66, two acrylate groups) used in Production Example 1 was used as the organic crosslinking agent. Except for the above, the same operation as in Example 1 was performed to obtain a comparative water-absorbing agent (1).

  Table 1 shows the surface crosslinking conditions of the comparative water-absorbing agent (1) and the performance of the water-absorbing agent.

[Comparative Example 2]
In Example 1, a comparative water-absorbing agent (2) was prepared in the same manner as in Example 1 except that 0.2 parts by weight of diethylene glycol diacrylate (molecular weight 214.24, two acrylate groups) was used as the organic crosslinking agent. )

  Table 1 shows the surface cross-linking conditions of the comparative water-absorbing agent (2) obtained and the performance of the water-absorbing agent.

[Comparative Example 3]
In Example 1, except that 0.2 parts by weight of glycerin acrylate methacrylate (Shin Nakamura Chemical Co., Ltd. NK ester 701A: molecular weight 214.24, one acrylate group + one methacrylate group) was used as the organic crosslinking agent. The same operation as in Example 1 was performed to obtain a comparative water-absorbing agent (3).

  Table 1 shows the surface cross-linking conditions of the comparative water-absorbing agent (3) obtained and the performance of the water-absorbing agent.

[Comparative Example 4]
In Comparative Example 3, a comparative water-absorbing agent (4) was obtained by performing the same operation as in Comparative Example 3 except that the amount of glycerin acrylate methacrylate was changed to 0.05 parts by weight.

  Table 1 shows the surface cross-linking conditions of the comparative water-absorbing agent (4) and the performance of the water-absorbing agent.

[Comparative Example 5]
In Example 1, Example 1 was used except that 0.2 parts by weight of glycerin dimethacrylate (NK ester 701: molecular weight 228.27, two methacrylate groups) manufactured by Shin-Nakamura Chemical Co., Ltd. was used as the organic crosslinking agent. The same operation was performed to obtain a comparative water-absorbing agent (5).

  Table 1 shows the surface cross-linking conditions of the comparative water-absorbing agent (5) and the performance of the water-absorbing agent.

[Comparative Example 6] (using the same pentaerythritol triacrylate)
In Example 1, the same operation as in Example 1 was performed except that 0.2 parts by weight of pentaerythritol triacrylate (molecular weight: 298.32, 3 acrylate groups) was used as the organic crosslinking agent. 6) was obtained.

  Table 1 shows the surface cross-linking conditions of the comparative water-absorbing agent (6) and the performance of the water-absorbing agent.

[Comparative Example 7] (using the same allyl methacrylate)
In Example 1, the same operation as in Example 1 was performed, except that 0.2 parts by weight of allyl methacrylate (molecular weight 126.17, one acrylate group + one allyl group) was used as the organic crosslinking agent. A water absorbing agent (7) was obtained.

  Table 1 shows the surface crosslinking conditions of the comparative water-absorbing agent (7) and the performance of the water-absorbing agent.

[Comparative Example 8] (using the same vinyl methacrylate)
In Example 1, the same operation as in Example 1 was performed, except that 0.2 parts by weight of vinyl methacrylate (molecular weight 112.14, one acrylate group + one vinyl group) was used as the organic crosslinking agent. A water absorbing agent (8) was obtained.

  Table 1 shows the surface cross-linking conditions of the comparative water-absorbing agent (8) and the performance of the water-absorbing agent.

(Summary)
In Comparative Examples 1 to 3, 5, and 6, since the molecular weight of the organic crosslinking agent used was as large as 200 or more, the remaining crosslinking agent was 671 to 1149 ppm, and remained in an unreacted state in the vicinity of the surface. Although not shown in the table, the residual monomer was 500 ppm or less in Examples and Comparative Examples, and a large amount of residual crosslinking agent exceeding the residual monomer (residual acrylic acid) was detected in Comparative Examples 1-3, 5, and 6. I understand that.

  On the other hand, in Examples 1-5, AAP2.06 improved to 20 g / g or more, AAP4.83 improved to 12 g / g or more, and the remaining crosslinking agent was N.P. D. It can be seen that when the molecular weight is less than 200, the amount of the remaining crosslinking agent is reduced. Here, Examples 1 to 3 are examples using a diacrylate-based crosslinking agent, Examples 4 and 5 are examples using an acrylate-based crosslinking agent having a cyclic functional group, and in Example 4 after crosslinking polymerization. Furthermore, although it includes a heat treatment step (reaction step of carbonate ring), it can be seen that any cross-linking agent gives good results.

  Further, in Production Example 1, even when polyethylene glycol diacrylate (molecular weight 522.66) is used for polymerization of the base polymer, the residual amount is N.P. D. However, as shown in Comparative Example 1, when the same crosslinking agent is used for surface crosslinking polymerization, 1149 ppm of residual crosslinking agent is detected. The effect of the present invention due to the molecular weight of the crosslinking agent is not particularly confirmed in the polymerization of a normal water-absorbent resin (crosslinking polymerization of a base polymer), and the second surface crosslinking method (monomer described in Patent Documents 7 to 13). It can be seen that it is specifically expressed in the surface cross-linking by cross-linking polymerization.

  In Comparative Example 4, when the amount of the crosslinking agent was reduced to 0.05 parts by weight in order to reduce the residual amount of the organic crosslinking agent in Comparative Example 3 (0.2 parts by weight of the crosslinking agent, 671 ppm of the remaining crosslinking agent), Although the amount of residual crosslinking agent was reduced to 15 ppm, it was found that AAP4.83 was halved (from 16.8 g / g to almost 8.8 g / g), and a sufficient crosslinking effect was not obtained.

  In Comparative Examples 7 and 8, only the CRC decreased due to surface crosslinking (from about 50 g / g of the base polymer to about 40 g / g), and AAP 4.83 kPa hardly improved. Since allyl group and vinyl group are different in polymerization reactivity from (meth) acrylate group, only (meth) acrylate group reacts, and allyl group and vinyl group do not react, so that only AAP 4.83kPa is improved. Is not formed. This shows that selection of the reactive group of the organic type crosslinking agent to be used is important.

  In Table 1, the crosslinking agents are: A: ethylene glycol diacrylate, B: ethylene glycol dimethacrylate, C: 1,4-butanediol diacrylate, D: glycerol carbonate acrylate (detection limit less than 1 ppm), E: glycidyl methacrylate (Detection limit 1 ppm), F: polyethylene glycol diacrylate (n = 9), G: diethylene glycol diacrylate, H: glycerol acrylate methacrylate, I: glycerol dimethacrylate, J: pentaerythritol triacrylate, K: allyl methacrylate (detection limit) 10 ppm), L: vinyl methacrylate (detection limit 10 ppm).

  In Table 1, the heating conditions indicate that I: 100 ° C. for 30 minutes, II: 100 ° C. for 30 minutes, and further 140 ° C. for 30 minutes.

Claims (18)

A method for producing a water-absorbing agent comprising a poly (meth) acrylic acid (salt) water-absorbing resin as a main component,
a) a surface treatment liquid mixing step of mixing a radically polymerizable monomer and a surface treatment liquid containing an organic crosslinking agent; and
b) a surface cross-linking polymerization step for surface cross-linking polymerization of the water absorbent resin composition after the surface treatment liquid mixing step,
The amount of the surface treatment liquid is 50 parts by weight or less with respect to 100 parts by weight of the water absorbent resin,
The surface treatment liquid contains a mixing aid as necessary, and the organic crosslinking agent has a plurality of functional groups selected from the group consisting of functional groups capable of reacting with (meth) acrylate groups or carboxyl groups. And a method for producing a water-absorbing agent, which is an organic compound having a molecular weight of less than 200.   The production according to claim 1, wherein the functional group capable of reacting with the carboxyl group is selected from a cyclic functional group, a hydroxy group, a primary or secondary amino group, and an isocyanate group. Method.   The production method according to claim 2, wherein the cyclic functional group is selected from an epoxy group, an aziridine group, an oxetane group, an ethylene carbonate group, an oxazoline group, and an oxazolidinone group.   The organic crosslinking agent according to any one of claims 1 to 3, wherein the organic crosslinking agent is selected from a di (meth) acrylate crosslinking agent having a molecular weight of less than 200 or a (meth) acrylate crosslinking agent having a cyclic functional group. Manufacturing method.   5. The production according to claim 1, wherein the water-absorbent resin composition used in the surface cross-linking polymerization step includes a thermal decomposition type radical polymerization initiator, and the step is a heat treatment. Method.   The manufacturing method according to claim 5, wherein the temperature of the surface crosslinking polymerization step is 50 ° C. to 150 ° C.   The manufacturing method according to any one of claims 1 to 6, wherein a main component of the radical polymerizable monomer contained in the surface treatment liquid is (meth) acrylic acid.   The amount of the radical polymerizable monomer contained in the surface treatment liquid is 0.1 to 20 parts by weight with respect to 100 parts by weight of the water absorbent resin. The production method according to claim 1.   The amount of the organic crosslinking agent contained in the surface treatment liquid is 0.001 to 5 parts by weight with respect to 100 parts by weight of the water absorbent resin. The production method according to item.   10. The production method according to claim 1, wherein water contained in the surface treatment liquid is 1 to 20 parts by weight with respect to 100 parts by weight of the water absorbent resin.   The surface treatment liquid contains the radical polymerization initiator, and the radical polymerization initiator is at least one selected from the group consisting of persulfate, hydrogen peroxide, and a water-soluble azo compound, and the amount of the radical polymerization initiator The manufacturing method according to claim 1, wherein the content is 0.01 to 2 parts by weight with respect to 100 parts by weight of the water absorbent resin.   The manufacturing method according to any one of claims 1 to 11, wherein the mixing aid is added simultaneously with or before the surface treatment liquid mixing step.   The manufacturing method according to any one of claims 1 to 12, wherein the molecular weight of the organic crosslinking agent is smaller than the molecular weight of the crosslinking agent used for the production of the water-absorbent resin.   The manufacturing method according to any one of claims 1 to 13, further comprising a heat treatment step at a higher temperature than the surface cross-linking polymerization step after the surface cross-linking polymerization step.   A water-absorbing agent composed mainly of a poly (meth) acrylic acid (salt) -based water-absorbing resin subjected to surface cross-linking polymerization, wherein the residual amount of the cross-linking agent is 100 ppm by weight or less and the absorption capacity under pressure at 4.83 kPa ( A water absorbing agent having an AAP of 4.83 kPa) of 10 to 30 g / g.   The water-absorbing agent according to claim 15, wherein the water content is 5 to 15% by weight.   The water absorption agent of Claim 15 or 16 whose water absorption capacity | capacitance (CRC) under no pressure is 5-70 g / g and whose absorption capacity under pressure (AAP2.06Pa) in 2.06 kPa is 20-40 g / g.   Sanitary goods containing the water absorbing agent according to any one of claims 15 to 17.