JP2010053296A - Method for producing water-absorbing resin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently producing a particle type water-absorbing resin having excellent physical properties at low cost while securing high productivity. <P>SOLUTION: In the method for producing a water-absorbing resin, a process (1) for polymerizing a monomer aqueous solution, a process (2) for drying an obtained hydrous gel polymer, a process (3) for crushing a dried product or crushing and classifying the dried product for controlling grain size, and a process (5) for performing surface crosslinking on water-absorbing resin powder with controlled grain size are carried out successively. In the method, a process (4) for performing second heating and drying additionally on the water-absorbing resin powder with controlled grain size is also carried out before the process (5) for performing surface crosslinking. The water content (a loss on drying for 3 hours at 180°C) of the water-absorbing resin fed to the surface crosslinking process (5) is 0-3 wt.%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a water absorbent resin. More specifically, the present invention relates to a method for producing a water-absorbent resin capable of efficiently obtaining a particulate water-absorbent resin having excellent physical properties at a low cost while ensuring high productivity.

  Water-absorbing resin is widely used in various applications such as sanitary materials such as paper diapers, sanitary napkins, incontinence products for adults, and soil water retention agents because of its ability to absorb a large amount of aqueous liquid, several to several hundred times its own weight. Being produced and consumed in large quantities. Such a water-absorbing resin (also referred to as a superabsorbent resin or a water-absorbing polymer) is described in, for example, Japanese Industrial Standard (JIS) K7223-1996, and is also introduced in many commercially available reference books. Is already known.

  In recent years, especially in sanitary products such as paper diapers, sanitary napkins, and incontinence products for adults, there is a tendency to increase the amount of water-absorbent resin and reduce the amount of pulp fiber used to make the product thinner. Thereby, it becomes necessary for the water-absorbent resin to perform functions such as liquid permeability and diffusibility of liquids that have been conventionally performed in the absorbent body. As a publicly known index for evaluating the superiority or inferiority of such a function, the water absorption capacity and liquid permeability under pressure of the water absorbent resin are proposed, and those having a large value are desired. On the other hand, this tendency to reduce the thickness leads to an increase in the amount of water-absorbing resin used per sanitary product, and therefore demand for a low-cost water-absorbing resin increases.

  In general, the water-absorbent resin is produced by drying a water-containing gel-like polymer obtained by polymerizing an aqueous solution containing a hydrophilic monomer and a cross-linking agent and performing surface cross-linking. The hydrated gel polymer is obtained as a mass or an aggregate of hydrated gel particles, and is usually roughly pulverized (coarsely crushed) to a particle size of about 1 to 10 mm using a pulverizer such as a kneader or meat chopper. The The coarsely crushed (coarse crushed) hydrogel is dried to a solid content of about 95% by weight.

  In the pulverization step after drying, the pulverization is performed by a pulverizer so that the value of the weight average particle diameter is 300 μm or more and 600 μm or less. At this time, particles outside the target particle diameter (particle diameter) range are also generated. Therefore, the pulverized product after drying is sieved with a classifier to adjust particles having a target particle size range. As a result, a particulate water-absorbing resin is obtained. Although there are differences depending on the application, as the particulate water-absorbing resin used for hygiene products, those having a particle diameter in the range of 150 μm or more and less than 850 μm are preferably used.

  The water-absorbent resin is subjected to a surface cross-linking step to obtain physical properties such as absorption capacity under pressure and liquid permeability, which are desirable for sanitary agents (hygiene products). The surface crosslinking step is usually a step of providing a highly crosslinked layer in the vicinity of the surface by reacting the water-absorbing resin with a surface crosslinking agent or a polysynthetic monomer.

  The surface cross-linking techniques that have been studied so far include, for example, a combination of surface cross-linking agents (Patent Document 1), a device for mixing a water-absorbing resin and a surface cross-linking agent (Patent Document 2), For a heating device for reacting a resin and a surface cross-linking agent (Patent Document 3), for heating control of a heating temperature for reacting a water absorbent resin and a surface cross-linking agent (Patent Document 4), Examples of the surface cross-linking treatment of a water-absorbent resin having a high water content (Patent Document 5). Further, unlike normal surface cross-linking treatment, there is an example in which a water-absorbing resin is modified by applying heat without using a surface cross-linking agent (Patent Documents 6 and 7).

  Various surface cross-linking agents have also been proposed, such as oxazoline compounds (Patent Document 8), vinyl ether compounds (Patent Document 9), epoxy compounds (Patent Document 10), oxetane compounds (Patent Document 11), polyhydric alcohol compounds (Patent Document 12). ), Polyamide polyamine-epihalo adduct (Patent Documents 13 and 14), hydroxyacrylamide compound (Patent Document 15), oxazolidinone compound (Patent Document 16), bis or poly-oxazolidinone compound (Patent Document 17), 2-oxotetrahydro- 1,3-oxazolidine compounds (Patent Document 18), alkylene carbonate compounds (Patent Document 19), and the like are known. Further, a technique for polymerizing monomers to surface crosslink (Patent Document 20) and a technique for radical crosslinking with persulfate (Patent Documents 21 and 22) are also known.

  Further, a technique of using an additive in combination with the surface cross-linking agent has also been proposed. As an additive, a water-soluble cation such as an aluminum salt (Patent Documents 23 and 24), an alkali (Patent Document 25), an organic acid or an inorganic acid (patent Document 26) is known. In addition, a technique (Patent Document 27) that uses a specific mixer as a mixer for the surface cross-linking agent is also known.

  Further, in the heating process, a technique for performing surface crosslinking twice (Patent Document 28), a technique for using a plurality of heat treatment apparatuses (Patent Document 29), and a technique for heating a water-absorbing resin before surface crosslinking in advance (Patent Document 30). 31) has also been proposed.

US Pat. No. 5,422,405 (published June 6, 1995) Japanese Patent Publication “Japanese Patent Laid-Open No. 4-214734 (Publication Date: August 5, 1992)” Japanese Patent Publication “Japanese Patent Laid-Open No. 2004-352941 (Publication Date: December 16, 2004)” US Pat. No. 6,514,615 (published February 4, 2003) US Pat. No. 6,875,511 (published April 5, 2005) US Pat. No. 5,206,205 (published on Apr. 27, 1993) [Japanese published patent gazette “Japanese Patent Laid-Open No. 5-194762 (published: Aug. 3, 1993)”] European Patent No. 0603292 (released on June 29, 1994) US Pat. No. 6,297,319 (published August 2, 2001) US Pat. No. 6,372,852 (published April 16, 2002) US Pat. No. 6,265,488 (published July 24, 2001) US Pat. No. 6,809,158 (published August 26, 2004) US Pat. No. 4,734,478 (published March 29, 1988) US Pat. No. 4,755,562 (published July 5, 1988) US Pat. No. 4,824,901 (published April 25, 1989) US Pat. No. 6,239,230 (published March 29, 2001) US Pat. No. 6,559,239 (published March 6, 2003) US Pat. No. 6,472,478 (published October 29, 2002) US Pat. No. 6,657,015 (published December 2, 2003) US Pat. No. 5,672,633 (published September 30, 1997) US Patent No. 2005-48221 (published March 3, 2005) US Pat. No. 4,783,510 (published November 8, 1988) European Patent 1824910 (published August 29, 2007) US Pat. No. 6,605,673 (published August 12, 2003) US Pat. No. 6,620,899 (published September 16, 2003) US 2004-106745 (published on June 3, 2004) US Pat. No. 5,610,208 (published March 11, 1997) US Patent No. 6071976 (published on June 6, 2000) US Pat. No. 5,672,633 (published September 30, 1997) US 2007-0149760 (published June 28, 2007) Japanese Patent Publication “Japanese Patent Laid-Open No. 7-242709 (Publication Date: September 19, 1995)” Japanese Patent Publication “Japanese Patent Laid-Open No. 7-224204 (Publication Date: August 22, 1995)”

  However, many of the above-mentioned surface cross-linking agents (see Patent Documents 18 to 32) and their combined use (see Patent Document 11), mixing devices (see Patent Documents 12 and 37), surface cross-linking aids (see Patent Documents 33 to 36). ), Although there are many heat treatment methods (see Patent Documents 13, 14, 38 to 41) and the like, the surface cross-linking technique alone provides the absorption capacity or liquid under pressure of the water-absorbent resin from the user. It has been difficult to meet the increasing demand for physical properties such as permeability.

  Further, with the change of the surface cross-linking agent or the use of a new auxiliary agent, there are cases where cost increases, safety decreases, and other physical properties decrease (eg, decrease in coloring).

In addition, the above method shows a certain effect in laboratory-scale small-scale and batch-type (batch-type) manufacturing, but in industrial scale (for example, 1000 kg or more per unit time) continuous production is as small as small scale. In some cases, the effect was not shown.
The present invention has been made in view of the above-described conventional problems, and the object thereof is to efficiently obtain a surface-crosslinked water-absorbing resin having excellent physical properties while ensuring high productivity at low cost. Another object of the present invention is to provide a method for producing a water absorbent resin.

  In order to solve the above-mentioned problems, the present inventors have made various studies and as a result, focused on the “influence on the surface cross-linking of the drying of the water-absorbent resin powder”, which has not been noticed in the past, and the specific drying before the surface cross-linking The present inventors have found that a relatively low water content greatly affects the results of surface crosslinking. The present invention focused on specific drying before surface cross-linking and low water content provides the following two methods for producing a water-absorbent resin.

  That is, the method for producing a water-absorbent resin of the present invention includes a step (1) of polymerizing an aqueous monomer solution, a step (2) of drying the hydrogel polymer obtained by the step (1), and the step (2). Step (3) of controlling the particle size by pulverizing, or pulverizing and classifying the dried product obtained by the above), and the step of surface cross-linking the particle size-controlled water-absorbent resin powder (Step (3)) 5) is a method for producing a water-absorbent resin, comprising: sequentially performing the second heat drying on the water-absorbent resin powder whose particle size is controlled in the step (3) before the step (5) of performing the surface crosslinking. Step (4) is included.

  Moreover, the manufacturing method of the water absorbing resin of this invention is the process (1) which superposes | polymerizes monomer aqueous solution, the process (2) which dries the hydrogel polymer obtained by the said process (1), and the said process (2). Step (3) of controlling the particle size by pulverizing, or pulverizing and classifying the dried product obtained by the above), and the step of surface cross-linking the particle size-controlled water-absorbent resin powder (Step (3)) 5), wherein the water content of the water-absorbent resin (specified by loss on drying at 180 ° C. for 3 hours) is 0 to 3% by weight. It is characterized by being.

  According to the above invention, in the surface cross-linking of the water-absorbent resin, the physical properties of the water-absorbent resin after the surface cross-linking (for example, the absorption capacity under pressure AAP, the pass through) are used without changing the surface cross-linking agent or using a new auxiliary agent. Liquid SFC, etc.) can be improved. Conventionally, scale-up in a production process has been accompanied by a decrease in physical properties. However, according to the above invention, there is almost no decrease in physical properties even in continuous production or scale-up.

  Other objects, features, and advantages of the present invention will be fully understood from the following description. The advantages of the present invention will become apparent from the following description with reference to the accompanying drawings.

  Hereinafter, the present invention will be described in detail. However, the scope of the present invention is not limited to these descriptions, and other than the following examples, the present invention can be appropriately modified and implemented without departing from the spirit of the present invention. It is. 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. In this specification, “mass” and “weight” are synonymous.

[Embodiment 1]
Embodiments of the present invention will be described below. In addition, although the manufacturing method of a typical water-absorbing resin is shown in the following (1) to (5) and the surface cross-linking method is in the following (7), the “particulate particle-controlled water-absorbing resin powder” is a characteristic part of the present invention. Furthermore, the step (4) of performing the second heat drying ”and the“ water content of the water-absorbing resin used in the step of performing the surface crosslinking (5) ”are the following (6), (6-2) and (6-3). Show. Each of these steps may be batch or continuous, but industrially, it is preferable that the steps are connected and continuously produced as a whole.

(I) Particulate water-absorbing resin (1) Polyacrylate-based water-absorbing resin The water-absorbing resin obtained by the method for producing a water-absorbing resin of the present invention can be widely applied to various polymer structures. Polyacrylate water-absorbent resin, preferably 30 to 100 mol%, preferably 50 to 100 mol%, more preferably 70 to 100 mol of acrylic acid (salt) in the repeating unit (excluding the crosslinking agent). It is a water-swellable and water-insoluble crosslinking agent polymer containing mol%, particularly preferably 90 to 100 mol%. The water swellability refers to a water absorption capacity (GV) described later of 5 g / g or more, and further 10 g / g or more. The term “water-insoluble” means that a water-soluble component described later is 50% or less, further 30% or less, particularly 20% or less.

  The acrylic acid salt or acrylic acid group as a repeating unit of the polymer is a monovalent salt, preferably an alkali metal salt or an ammonium salt, more preferably an alkali metal salt, particularly preferably a sodium salt, 0 to 100 mol%, preferably Is neutralized in the range of 20 to 100 mol%, more preferably 50 to 99 mol%, still more preferably 60 to 90 mol%.

  Examples of unsaturated monomers that can be used include acrylic acid, methacrylic acid, (anhydrous) maleic acid, fumaric acid, crotonic acid, itaconic acid, vinyl sulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, (meta ) Acryloxyalkanesulfonic acid, N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) Hydrophilic monomers such as acrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol (meth) acrylate, and salts thereof. Among these monomers, for example, acrylic acid alone, or a combination of acrylic acid and a monomer other than acrylic acid, or only a monomer other than acrylic acid, a water absorbent resin is appropriately obtained. be able to.

  Of the unsaturated monomers, acrylic acid and / or a salt thereof are preferable from the viewpoint of physical properties of the water-absorbent resin (water absorption capacity, soluble content, residual monomer, liquid permeability, etc.). When acrylic acid and / or a salt thereof is used as the unsaturated monomer, acrylic acid (salt) composed of 1 to 50 mol% of acrylic acid and 50 to 99% of an alkali metal salt of acrylic acid is most preferably used. .

  Examples of the crosslinking agent that can be used include N, N′-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, and (polyoxyethylene) trimethylolpropane. Tri (meth) acrylate, trimethylolpropane di (meth) acrylate, polyethylene glycol di (β-acryloyloxypropionate), trimethylolpropane tri (β-acryloyloxypropionate), poly (meth) allyloxyalkane, etc. A compound having at least two polymerizable double bonds in the molecule; polyglycidyl ether (ethylene glycol diglycidyl ether), polyol (ethylene glycol, polyethylene glycol, glycerin, sorbitol), etc. One type or two or more types of compounds capable of reacting with the carboxyl group to form a covalent bond can be exemplified.

  In the case of using a crosslinking agent, it is preferable to essentially use a compound having at least two polymerizable double bonds in the molecule in consideration of the water absorption characteristics of the resulting water absorbent resin. Moreover, a crosslinking agent is used in the range of 0.0001-5 mol% with respect to the said monomer from a physical property surface, Preferably it is 0.005-2 mol%.

  These monomers are usually polymerized in an aqueous solution, and the monomer concentration is usually 10 to 90% by weight, preferably 20 to 80% by weight, more preferably 30 to 70% by weight, and particularly preferably 40 to 40% by weight. It is in the range of 60% by weight. The aqueous solution contains a surfactant, polyacrylic acid (salt) or a crosslinked product thereof (water absorbent resin), a polymer compound such as starch, polyvinyl alcohol, various chelating agents, various additives, and the like. You may use together 0-30 weight% with respect to. In the present application, the aqueous solution includes a dispersion exceeding the saturation concentration, but is preferably polymerized at a saturation concentration or less.

(2) Polymerization process The water-absorbent resin of the present invention is produced by crosslinking the unsaturated monomer to obtain a water-containing crosslinked polymer. The polymerization method is usually carried out by spray polymerization, drop polymerization, aqueous solution polymerization or reverse phase suspension polymerization in view of performance and ease of control of polymerization.

  From the standpoint of further exerting the effects of the present invention, preferably aqueous solution polymerization or reverse phase suspension polymerization, more preferably aqueous solution polymerization, more preferably continuous aqueous solution polymerization, particularly preferably continuous belt polymerization or continuous kneader polymerization is applied. From the viewpoint of physical properties and drying efficiency, it is preferable to volatilize at least a part of the polymerization solvent by the polymerization heat at the time of the polymerization. For example, the solid content is increased by 0.1% by weight or more, preferably 1 to 40% before and after the polymerization. What is necessary is just to raise about 3 to 20 weight%, More preferably, it is 2 to 30 weight% more preferably. The rise in solid content is appropriately determined by the temperature at the time of polymerization (for example, polymerization at the boiling point), the air flow and the shape (particle size and sheet thickness of the polymer gel), and the like.

  Although these polymerizations can be carried out in an air atmosphere, they are carried out in an inert gas atmosphere such as nitrogen or argon, for example, at an oxygen concentration of 1% or less. The monomer component is preferably used for polymerization after the dissolved oxygen is sufficiently substituted with an inert gas and the oxygen concentration becomes less than 1 ppm.

  Reverse phase suspension polymerization is a polymerization method in which an aqueous monomer solution is suspended in a hydrophobic organic solvent. For example, U.S. Pat. Nos. 4,093,763, 4,367,323, 4,446,261, 4,683,274, and 5,244,735 can be used. It is described in US patents. On the other hand, aqueous solution polymerization is a method of polymerizing an aqueous monomer solution without using a dispersion solvent. For example, U.S. Pat. Nos. 4,462,001, 4,873,299, 4,286,682, 4,973,632, 4,985,518, 5,124,416, No. 5,250,640, No. 5,264,495, No. 5,145,906 and No. 5,380,808, and other European patents such as European Patent Nos. 081636, 09555086, 0922717, and No. 1178059. In the polymerization, monomers, cross-linking agents, polymerization initiators, other additives and the like described therein can be used in the present invention.

  Examples of the polymerization method for aqueous solution polymerization include a static polymerization method in which an aqueous monomer solution is polymerized in a static state, and a stirring polymerization method in which polymerization is performed in a stirring device. In the static polymerization method, it is preferable to use an endless belt. In the stirring polymerization method, a uniaxial stirrer can be used, but a multi-stirrer stirrer such as a kneader is preferably used. More specific examples of the polymerization method in the present invention include a continuous polymerization method at a high monomer concentration using an endless belt as described in JP-A-2005-307195. Such continuous belt polymerization or continuous kneader polymerization is also suitably applied to the present invention.

  The polymerization initiator used in the present invention is appropriately selected depending on the form of polymerization. Examples of such a polymerization initiator include a photodegradable polymerization initiator, a thermal decomposable polymerization initiator, and a redox polymerization initiator. Examples of the photodegradable polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives, azo compounds, and the like. Examples of the thermal decomposition type polymerization initiator include persulfates such as sodium persulfate, potassium persulfate and ammonium persulfate; peroxides such as hydrogen peroxide, t-butyl peroxide and methyl ethyl ketone peroxide; azonitrile compounds , Azoamidine compounds, cyclic azoamidine compounds, azoamide compounds, alkylazo compounds, 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydro Examples include azo compounds such as chloride. Examples of the redox polymerization initiator include a system in which a reducing compound such as L-ascorbic acid or sodium hydrogen sulfite is used in combination with the persulfate or peroxide, and the both are combined.

  In the present invention, it is also preferable to use a photodegradable polymerization initiator and a thermal decomposable polymerization initiator in combination. The amount of the polymerization initiator is 0.0001 to 1 mol%, preferably 0.001 to 0.5 mol%, based on the monomer.

  More specific examples of the polymerization method in the present invention include a continuous polymerization method at a high monomer concentration using an endless belt as described in JP-A-2005-307195. The continuous belt polymerization and the continuous kneader polymerization are preferable because a highly concentrated water-containing crosslinked polymer that can be suitably applied to the present invention is easily obtained.

(3) Gel atomization step From the viewpoint of physical properties and drying efficiency, it is preferable that the hydrogel polymer before drying is finely divided during polymerization or after polymerization.

  When aqueous polymerization, particularly continuous belt polymerization, is used for the polymerization in the present invention, the water-containing crosslinked polymer obtained by aqueous polymerization in the polymerization step, for example, in the form of a lump or sheet, is pulverized by a pulverizer to form a particulate water-containing crosslinked polymer. And then dried. In addition, in spray polymerization, drop polymerization, and reverse phase suspension polymerization, a particulate water-containing crosslinked polymer is obtained by polymerization, but the particulate water-containing crosslinked polymer after polymerization may be dried as it is, and further if necessary. The particle size may be adjusted by pulverization or granulation. As a preferable particle diameter of the particulate water-containing crosslinked polymer, the weight average particle diameter determined by measurement (standard sieve classification) described later is in the range of 0.5 to 10 mm, in the range of 1 to 5 mm, It is further more preferable that it is 1-3 mm, especially 1-2 mm. When the weight average particle diameter exceeds 10 mm, it is difficult to fluidize in a fluidized bed, which will be described later, and it is necessary to increase the air flow rate into the layer.

  In addition, as a method of controlling within the above range in the gel atomization step, US Pat. No. 6,906,159, US Pat. No. 5,275,773, US Pat. No. 6,100,305, US Pat. No. 6,140,395, US Pat. No. 6,875,511, US Publication No. 2004 / No. 234607, US Publication No. 2005/46069, etc. are adopted.

(4) Drying step (first drying)
The above hydrogel polymer, preferably the particulate hydrogel polymer is dried to a pulverizable solid content. Here, the form of the hydrophilic cross-linked polymer used in the drying step is a crushed hydrous gel and its aggregates by a kneader, a meat chopper and a cutter, and a sheet-like hydrogel. You may put the crushing process and the crushing process of an aggregate suitably in this drying process. As such a technique, for example, US Pat. No. 6,187,902 is adopted.

  The particle size (weight average particle size) of the crushed hydrous gel is preferably 1 to 5 mm. Drying methods include heat drying, hot air drying, vacuum drying, infrared drying, microwave drying, dehydration by azeotropy with a hydrophobic organic solvent, high moisture drying using high-temperature steam, and the desired moisture content. As described above, various methods can be employed, and the method is not particularly limited.

  When drying by hot air drying, as a drying method to be used, a method of drying in a stationary state, a method of drying in a stirring state, a method of drying in a vibrating state, a method of drying in a fluidized state, an air current There is a method of performing drying. The drying temperature is usually 60 to 250 ° C., preferably 100 to 220 ° C., more preferably 120 to 200 ° C. (hot air temperature). The drying time depends on the surface area of the polymer, the moisture content, and the type of dryer, and is selected to achieve the desired moisture content. For example, the drying time may be appropriately selected within the range of 1 minute to 5 hours. By this drying, the solid content of the hydrophilic cross-linked polymer is preferably increased to 70 to 95%, more preferably 80 to 95%.

  Compared to the case where the solid content is less than 90%, when the solid content is 90% or more, the solid content rises slowly in the drying process, and when the solid content exceeds 95%, the solid content almost increases. No longer. This is considered because the solid content is 90 to 95% and the constant rate drying period is changed to the decreasing rate drying period. Therefore, it is not efficient that the solid content exceeds 95% in the drying step before the pulverization step. Although described in the following (6) and (6-2), for this reason, the conventional surface crosslinking has been performed at a solid content of 95% or less (in other words, a moisture content of 5% or more).

  In order to suppress coloring of the water-absorbent resin, it is desirable to reduce the oxygen partial pressure in the drying atmosphere. This is because the water-absorbent resin is colored due to a reaction caused by the presence of oxygen during heating. Here, in order to achieve the present invention, it is preferable that the moisture content of the dried product (specified by loss on drying at 180 ° C. for 3 hours) is 5 to 15% by weight. The weight average particle diameter (specified by sieving classification) of the dried product is preferably 1 to 5 mm, and more preferably 1.5 to 4 mm.

(5) Step of controlling particle size Water-absorbent resin particles obtained by drying are pulverized and classified as necessary for particle size control. These methods are described in, for example, WO 2004/69915 (US Patent No. 2006024755).

  If it is for sanitary materials, Preferably a weight average particle diameter is 100-1000 micrometers, More preferably, it is 200-800 micrometers, Most preferably, it is 300-600 micrometers.

  The fine powder having a particle size of 150 μm or less generated in this step deteriorates the physical properties of the water-absorbent resin and causes problems in safety and hygiene, and is classified and removed. The step of classifying and removing fine powder may be performed during or after the heating and drying step, as will be described later. This fine powder is appropriately collected, formed into a granular shape again, and passed through a process such as being recovered in a monomer aqueous solution.

(6) Heat drying step (second drying after particle size control)
The water-absorbent resin powder obtained by the above-described method has been further surface-crosslinked in the above-mentioned Patent Documents 11 to 41, etc., but “surface cross-linking of drying water-absorbent resin powder” has not been noted in the present invention. Focusing on the “influence on the surface”, it was found that specific drying and low water content before surface cross-linking greatly affect the result of surface cross-linking, and the present invention was completed.

  That is, the method for producing a water-absorbent resin of the present invention includes a step (1) of polymerizing an aqueous monomer solution, a step (2) of drying the hydrogel polymer obtained by the step (1), and the step (2). Step (3) of controlling the particle size by pulverizing, or pulverizing and classifying the dried product obtained by the above), and the step of surface cross-linking the particle size-controlled water-absorbent resin powder (Step (3)) 5) is a method for producing a water-absorbent resin, comprising: sequentially performing the second heat drying on the water-absorbent resin powder whose particle size is controlled in the step (3) before the step (5) of performing the surface crosslinking. Step (4) is included.

  Hereinafter, the second heat drying step (second drying after particle size control) will be described.

  In the present invention, the water-absorbent resin whose particle size is controlled is further dried by heating through a drying step, pulverization, and, if necessary, a classification step. As a specific embodiment, the particle size of the water-absorbent resin subjected to heat drying is preferably 100 to 1000 μm, more preferably 200 to 800 μm, and particularly preferably 300 to 600 μm in weight average particle size (specified by sieve classification). . In addition, when water-absorbing resin particles having a particle size exceeding 1 mm (specified by sieving classification) are present in an aggregate of particles having a particle size close to the above-mentioned weight average particle size, the increase in solid content is slow for large particles. It tends to be non-uniform drying between. Therefore, the content of particles having a particle size exceeding 1 mm is preferably 0 to 20% by weight, more preferably 0 to 10% by weight, and particularly preferably 0 to 5% by weight.

  Examples of the heat drying method that can be used in the present invention include stationary drying, fluidized bed drying, and stirring drying. The heat drying apparatus may be an ordinary drier that can heat the water-absorbent resin particles, and any of known drying methods, batchwise or continuous, direct heating and / or indirect heating, may be used. May be used. For example, belt-type stationary dryer, ventilated vertical dryer, cylindrical stirring dryer, grooved stirring dryer, rotary dryer, rotary dryer with water vapor tube, aerated rotary dryer, fluidized bed dryer, cone-type dryer Machine, vibration fluidized bed dryer, airflow dryer and the like. Among them, it is desirable to use a fluidized bed dryer in order to prevent uneven drying. More preferably, a heat transfer tube is used to efficiently heat the water absorbent resin particles. Industrially, it is desirable to use a continuous fluidized bed dryer having a plurality of drying chambers. Moreover, airflow classification can be performed by using fluidized bed drying. That is, by adjusting the air volume of the hot air used for drying, fine particles having a particle size equal to or smaller than a preferable particle size for the product can be blown off by the wind to control the particle size.

  As in the drying step, it is desirable that the atmosphere during the heat drying is a low oxygen partial pressure in order to suppress the coloring of the water absorbent resin.

  In the present invention, the temperature for heat drying (second drying) is preferably 150 to 300 ° C, more preferably 180 to 270 ° C, and particularly preferably 200 to 250 ° C. When the temperature is low, the thermal efficiency is deteriorated, the drying speed is slowed down and the dryer becomes large, and the physical properties after the surface treatment are improved, so that the desirable change in the water-absorbent resin particles does not occur. There is. In the present invention, the heat-drying time is such that the water content of the water-absorbent resin particles is preferably 4.5% or less, 0-3%, 0-2.5%, more preferably 0-2%, 0-1. It is set to 5%, particularly preferably 0 to 1%. Note that 0% as the lower limit takes a long time and may be accompanied by deterioration of the water-absorbent resin, so the lower limit may be about 0.1%, and further about 0.2%. At this time, in the present invention, the heating and drying may be appropriately terminated when the moisture content becomes 2% or less. However, when the heating method is not uniform depending on the drying method, or between the particles in the continuous heating dryer. In the case where the dispersion of the residence time is large, it is possible to obtain a water-absorbent resin having uniform physical properties between particles by further heating. Depending on the drying method, the drying temperature and the solid content of the water-absorbent resin before drying, the heat drying time is preferably 5 minutes to 10 hours, more preferably 10 minutes to 3 hours, and particularly preferably 15 minutes to 1 Heat-dried within hours.

  In the step (4) of further heating and drying the water-absorbent resin whose particle size is controlled, it is preferable to heat until the absorption ratio (GV) per solid content increases. The increase in GV is preferably 1 to 50 g / g, more preferably 2 to 30 g / g, and particularly preferably 3 to 20 g / g. The term “per solid content” refers to a value obtained by correcting the measured value per weight of the resin solid content. For example, when the water content is 95%, the measured GV is divided by 0.95.

  In the present invention, when performing heat drying, a surface treatment agent such as a surface cross-linking agent is unnecessary, but for the purpose of improving the uniformity of heat drying or preventing the increase of fine powder due to process damage, etc. Additives may be added.

  Since the water-absorbent resin particles after heat drying are at a high temperature, a cooling chamber may be provided as appropriate to cool. In addition, a classification step may be performed after the heating and drying step, and when classification is not performed before the heating and drying step, or when water-absorbent resin particles are subjected to process damage in the heating and drying step and new fine powder is generated. It is effective for etc.

  U.S. Pat. No. 6,187,902 discloses a method for producing a water-absorbing resin in which a particulate hydrogel polymer is allowed to stand and dried, and then agglomerated parts are roughly crushed and further stirred or fluidized. Unlike the drying method, the present invention is characterized by particle size control as described in (5) above, rather than crushing aggregates to about mm, that is, pulverizing or classifying before the second drying. Thus, the particle size is controlled to improve the physical properties after surface crosslinking.

  Conventionally, when a surface cross-linking agent is added to a preheated water-absorbent resin with a heating device as in Patent Documents 30 and 31, or when surface cross-linking is performed twice as in Patent Document 28, Although sufficient physical properties cannot be obtained, in the present invention, before the step (5) of performing the surface crosslinking, the step (4) of further performing a second heat drying on the particle size-controlled water-absorbent resin powder, that is, By including a dryer separate from the mixing of the surface cross-linking agent, a dramatic improvement in physical properties is achieved.

  In the present invention, after the above (6) to (6-2), the following surface crosslinking (7) is essential, and only the heat treatment in the absence of the surface crosslinking agent as in Patent Documents 8 and 9 above. Sufficient physical property improvement is not achieved.

(6-2) Second invention in the present invention The second production method of the water-absorbent resin of the present invention includes a step (1) of polymerizing an aqueous monomer solution and a water content obtained by the step (1). By the step (2) of drying the gel polymer, the step (3) of controlling the particle size by pulverizing or pulverizing and classifying the dried product obtained by the step (2), and the step (3) A method for producing a water-absorbent resin comprising sequentially step (5) of performing surface cross-linking on a particle-controlled water-absorbent resin powder, wherein the water content (180 ° C.) of the water-absorbent resin used in the step of performing surface cross-linking (5) Stipulated by the loss on drying for 3 hours) is 0 to 3% by weight.

  As described in (4) above, since the water-absorbent resin strongly retains water, the water-absorbent resin powder after drying retains water as a polymerization solvent in a water content of about 5 to 10% by weight. Such excessive drying is very difficult in terms of cost and may cause deterioration due to long-time drying. Therefore, in the said patent documents 1-31, the surface bridge | crosslinking of a water-containing gel and the water absorbing resin of this moisture content are used for surface bridge | crosslinking as it is.

  However, in the present invention, it is found that the moisture content before surface crosslinking is important, and the physical properties of the water absorbent resin are improved by using a water absorbent resin having a moisture content lower than the conventional moisture content for surface crosslinking. It is characterized by. In order to control the moisture content, the moisture content may be adjusted by the conventional method of drying without controlling the particle size, but generally it takes a long time. (Drying) is preferably performed.

(6-3) More preferable form of heat drying step (second drying after particle size control) The water-absorbent resin after the heat drying or whose water content is controlled is surface-crosslinked, preferably surface-crosslinked In order to further improve the physical properties later, it is preferably cooled or stored as described below.

  That is, it is preferable that after the water-absorbing resin after the step (4) of performing the heat drying is cooled, it is supplied to the step (5) of performing the surface crosslinking. The temperature of the water-absorbent resin used for the step (5) of performing surface crosslinking is preferably 100 ° C. or lower, more preferably 100 to 40 ° C., further preferably 95 to 45 ° C., and particularly preferably 90 to 50 ° C. .

  Moreover, after the water-absorbing resin after the step (4) of performing the heat drying is stored, it is preferably supplied to the step (5) of performing surface crosslinking. The storage time is on average 0.1 minutes to 100 hours, further 1 minute to 50 hours, especially 5 minutes to 10 hours, and since aging is performed for a certain time by storage, more uniform surface cross-linking is achieved. it can.

  When a surface cross-linking agent described later is added during heat drying or a surface cross-linking agent described later is added immediately after heat drying (exit of the dryer), the physical properties may not be improved, and the above cooling and storage are performed. For cooling, it may be forcedly cooled by contact with a refrigerant such as cold air or a cooling transmission surface. Also, the time for leaving after heating and drying (after taking it out of the dryer) may be adjusted, or it may be dissipated during transportation or storage. Also good. Various hoppers and silos can be used for storage, pneumatic transportation and conveyor transportation can be used for transportation, and the transportation equipment and storage equipment used can be those disclosed in US Pat. No. 6,164,455. The apparatus may be kept at the cooling temperature (for example, 100 to 40 ° C.). If the cooling temperature is too low, physical properties may be deteriorated.

(7) Surface cross-linking step In the present invention, the water-absorbent resin that has undergone the above-described (6) heat-drying step is made to be a water-absorbent resin that is more suitable for sanitary materials through a conventionally known surface cross-linking step. Can do. Surface cross-linking means that a portion having a higher cross-linking density is provided in the surface layer of the water-absorbent resin (near the surface, usually around several tens of μm from the surface of the water-absorbent resin). It can be formed by a crosslinking reaction with an agent.

  In the present invention, as long as the “second drying after particle size control” in the above (6) or the “moisture content” in the above (6-2) is observed, the above-mentioned surface cross-linking agent (see Patent Documents 8 to 22) or a combination thereof (See Patent Document 1), its mixing device (see Patent Documents 2 and 27), auxiliary agent for surface crosslinking (see Patent Documents 23 to 26), its heat treatment method (see Patent Documents 3, 4, 28 to 31), etc. Is applicable. Further, the surface crosslinking may be performed once or a plurality of times.

  As the surface cross-linking agent that can be used in the present invention, various organic cross-linking agents or inorganic cross-linking agents can be exemplified, but from the viewpoint of physical properties and handleability, cross-linking agents capable of reacting with a carboxyl group are preferable. Examples thereof include polyhydric alcohol compounds, epoxy compounds, polyvalent amine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, mono-, di- or polyoxazolidinone compounds, polyvalent metal salts, alkylene carbonate compounds, and the like.

  More specifically, compounds exemplified in U.S. Pat. Nos. 6,228,930, 6071976, and 6254990 can be exemplified. For example, mono, di, tri, tetra or polyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin , 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, etc. Alcohol compounds; Epoxy compounds such as ethylene glycol diglycidyl ether and glycidol; A haloepoxy compound such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin; a condensate of the polyvalent amine compound and the haloepoxy compound; an oxazolidinone compound such as 2-oxazolidinone; an ethylene carbonate, etc. An alkylene carbonate compound; an oxetane compound; a cyclic urea compound such as 2-imidazolidinone, and the like, but is not particularly limited.

  The amount of the surface cross-linking agent used is preferably in the range of 0.001 to 10 parts by weight with respect to 100 parts by weight (parts by weight) of the water-absorbent resin particles, although it depends on the compounds to be used and combinations thereof. A range of 0.01 to 5 parts by weight is more preferable. In the present invention, water can be used together with the surface cross-linking agent. At this time, the amount of water used is preferably in the range of 0.5 to 20 parts by weight, more preferably 0.5 to 10 parts by weight with respect to 100 parts by weight of the water-absorbent resin particles. In the present invention, a hydrophilic organic solvent can be used in addition to water. The amount of the hydrophilic organic solvent used at this time is in the range of 0 to 10 parts by weight, preferably 0 to 5 parts by weight with respect to 100 parts by weight of the water-absorbent resin particles. Further, in mixing the crosslinking agent solution with the water-absorbent resin particles, the range does not hinder the effect of the present invention, for example, 0 to 10% by weight or less, preferably 0 to 5% by weight, more preferably 0 to 1% by weight. A water-insoluble fine particle powder or a surfactant may coexist. Preferred surfactants and methods for using them are exemplified in US Pat. No. 7,381,775, for example.

  The water-absorbent resin after mixing the surface cross-linking agent is preferably subjected to a heat treatment and, if necessary, a cooling treatment thereafter. The heating temperature is in the range of 70 to 300 ° C, preferably 120 to 250 ° C, more preferably 150 to 250 ° C. The heating time is preferably in the range of 1 to 120 minutes. The heat treatment can be performed using a normal dryer or a heating furnace.

  The surface cross-linking agent can be added by various techniques. However, a method in which the surface cross-linking agent is preliminarily mixed with water and / or a hydrophilic organic solvent, if necessary, and then sprayed or dropped into the particulate water-absorbing resin is preferable, and a spraying method is more preferable. In the case of the spraying method, the size of the droplets to be sprayed is preferably in the range of 0.1 to 300 μm, more preferably in the range of 0.1 to 200 μm, in terms of average particle diameter.

  The mixing device used when mixing the particulate water-absorbing resin, the surface cross-linking agent, and water or a hydrophilic organic solvent has a large mixing force in order to mix these substances uniformly and reliably. It is preferable. Examples of the mixing apparatus include a cylindrical mixer, a double wall conical mixer, a high-speed stirring mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a double-arm kneader, and a pulverizing kneader. Rotating mixers, airflow mixers, turbulators, batch-type Redige mixers, continuous-type Redige mixers, and the like are suitable.

  Moreover, as another form of the surface cross-linking treatment in the present invention, there is a method of performing surface cross-linking treatment by irradiating active energy after adding a treatment liquid containing a radical polymerizable compound to the particulate water-absorbing resin. And Japanese Patent Application No. 2003-303306 (US Pat. No. 7,201,941). Moreover, a surface active agent can also be added to the said process liquid, and an active energy can be irradiated and surface crosslinking can also be performed. Furthermore, another form of the surface cross-linking treatment in the present invention includes a method in which an aqueous solution containing a peroxide radical initiator is added to the particulate water-absorbing resin and then heated to carry out the surface cross-linking treatment. No. 7-8883 (US Pat. No. 4,783,510).

(8) Liquid permeability improver It is preferable that a liquid permeability improver is further added to the particulate water absorbent resin obtained by the method for producing a water absorbent resin of the present invention after surface cross-linking treatment. By adding the liquid permeability improver, the particulate water-absorbing resin has a liquid permeability improver layer. Thereby, the particulate water-absorbing resin is further excellent in liquid permeability.

  Examples of the liquid permeability improver include polyamines, polyvalent metal salts, and water-insoluble fine particles. In particular, polyvalent metal salts such as aluminum sulfate, particularly water-soluble polyvalent metal salts are preferable. US Pat. No. 7,179,862, European Patent No. 1165631, U.S. Pat. No. 7,157,141, U.S. Pat. No. 6,831,142, U.S. Publication No. 2004/176557, U.S. Publication No. 2006/204755, U.S. Publication No. 2006/73969, U.S. Publication No. 2007/106013. Is done. Polyamines and water-insoluble fine particles are exemplified in WO2006 / 082188A1, WO2006 / 082189A1, WO2006 / 082197A1, and the like.

  The amount of the liquid permeability improver used is preferably in the range of 0.001 to 5 parts by weight and in the range of 0.01 to 1 part by weight with respect to 100 parts by weight of the particulate water-absorbing resin. More preferred. If the usage-amount of a liquid-permeability improver is in the said range, the absorption capacity | capacitance under pressure (AAP) and physiological saline flow-inductivity (SFC) of particulate water-absorbing resin can be improved.

  The liquid permeability improving agent is preferably added by mixing with water and / or a hydrophilic organic solvent as necessary, and then spraying or dropping and mixing the particulate water-absorbing resin, more preferably spraying. The liquid permeability improver is preferably added in the cooling step in the fluidized bed of the particulate water absorbent resin.

(9) Other processes In addition to the above processes, a granulation process, a fine powder removal process, a fine powder recycling process, and the like may be provided as necessary. Examples of the process include those described in US Pat. No. 5,264,495, US Pat. No. 5,369,148, US Pat. No. 5,478,879, US Pat. No. 6,228,930, US Publication No. 2006/247351, and International Publication No. WO 2006/101271.

(10) Other substances added to the particulate water-absorbing resin The particulate water-absorbing resin is a surface cross-linking agent, liquid permeability improver, lubricant, chelating agent, deodorant, antibacterial agent, during or after polymerization. Water, a surfactant, water-insoluble fine particles, a reducing agent, and the like can be added to and mixed with the water-absorbent resin at about 0 to 30%, and further about 0.01 to 10%. In the case of addition and mixing after polymerization, the addition and mixing can be performed before drying, after drying, before pulverization, or after pulverization. In addition, other materials may be added to the particulate water absorbent resin as long as the properties of the water absorbent resin are not impaired. The method for adding other substances is not particularly limited. In the present invention, even when the water-absorbent resin contains a small amount of additives (for example, more than 0 and 30% by weight), that is, when it is a water-absorbent resin composition, it is generically called a water-absorbent resin.

(II) Physical properties of particulate water-absorbing resin obtained by the method for producing a water-absorbing resin Weight (mass) average particle diameter (D50) of the particulate water-absorbing resin obtained by the method for producing a water-absorbing resin of the present invention, Saline flow conductivity (SFC / Saline Flow Conductivity) and absorption capacity under pressure (AAP / Absorbency Against Pressure) will be described. In addition, each specific measuring method is shown in the Example mentioned later.

<Weight average particle diameter (D50)>
The weight average particle diameter (D50) is a particle diameter of a standard sieve corresponding to 50% by weight of the whole particle with a standard sieve having a constant opening, as described in US Pat. No. 5,051,259.

  For example, the particles are sieved with a JIS standard sieve (z8801) such as 8000 μm, 5600 μm, 3350 μm, 2800 μm, 2000 μm, 1000 μm, 850 μm, 710 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm, 75 μm, etc. Is plotted on a log-probability paper. Thus, the weight average particle diameter (D50) is read from the particle diameter corresponding to R = 50%.

  The final water-absorbing resin preferably has a weight average particle size of 300 to 600 μm, more preferably 350 to 500 μm, and a ratio of 850 to 150 μm is 90 to 100% by weight, and more preferably 95 to 95%. It is preferably controlled to 100% by weight, particularly 98 to 100% by weight.

<Saline Flow Conductivity (SFC)>
The physiological saline flow inductivity is a value indicating the liquid permeability when the particulate water-absorbing resin swells, and indicates that the larger the value, the higher the liquid permeability.

The particulate water-absorbent resin obtained by the method for producing a water-absorbent resin of the present invention has a saline flow conductivity (SFC) of preferably 10 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ). More preferably, it is in the range of 20 or more, particularly preferably 50 or more, and most preferably 80 or more. Thereby, the particulate water-absorbing resin is excellent in liquid permeability.

<Absorbency Against Pressure (AAP)>
The absorption capacity under pressure indicates the absorption capacity of the particulate water-absorbent resin in a state where a load is applied. The particulate water-absorbing resin obtained by the method for producing a water-absorbing resin of the present invention has an absorption capacity under pressure (AAP) of preferably 10 g / g to 28 g / g, more preferably 15 to 27 g / g, particularly It is in the range of 20 to 26 g / g. Thereby, the particulate water-absorbing resin has excellent absorption characteristics.

<Applicable water-absorbing resin>
The production method of the present invention is particularly suitable for obtaining the water absorbent resin. That is, the relationship between the water absorption ratio (GV), the water absorption capacity under pressure (AAP), and the liquid permeability (SFC) is improved as compared with the conventional method, that is, when the same one kind of physical property is constantly compared (for example, The same water absorption ratio) and other physical property values are improved (for example, SFC and AAP are improved), which is preferable.

Therefore, in the present invention, the water absorption capacity (GV) is 20 to 100 g / g, preferably 25 to 50 g / g, and 27 to 45 g / g, and the absorption capacity under pressure (AAP) is preferably 10 to 28 g / g. The physiological saline flow conductivity (SFC) is preferably 10 or less (10 ⁻⁷ · cm ³ · s · g ⁻¹ ). Preferably, two or more, more preferably three of these physical properties are satisfied simultaneously. The solid content is 85 to 99.9%, more preferably 90 to 99.9%, and particularly 95 to 99.9%. If the solid content (in other words, 100-moisture content) is deviated, the physical properties decrease. Sometimes.

(III) Absorber and / or absorbent article The particulate water-absorbent resin obtained by the method for producing a water-absorbent resin of the present invention is used for applications intended to absorb water, and is widely used as an absorber or absorbent article. However, it is particularly preferably used as a sanitary material for absorbing body fluids such as urine and blood.

  Specifically, a surface cross-linking agent is added to the particulate water-absorbent resin obtained by the method for producing a water-absorbent resin of the present invention to perform surface cross-linking treatment, and then a liquid permeability improver, a surfactant, a lubricant, etc. A particulate water-absorbing agent is produced by adding other substances. And an absorber and an absorptive article are manufactured using this particulate water absorbing agent. The method for adding other substances is not particularly limited.

  Here, the absorber is an absorbent formed mainly of a particulate water absorbent (water absorbent resin) and hydrophilic fibers. The absorbent body is manufactured by being molded into, for example, a film shape, a cylindrical shape, or a sheet shape using a particulate water-absorbing agent and hydrophilic fibers. In the absorbent body, the content (core concentration) of the particulate water-absorbing agent with respect to the total mass of the particulate water-absorbing agent and the hydrophilic fiber is preferably 20 to 100% by weight, more preferably 30 to 100% by weight, even more preferably. Is in the range of 40-100% by weight. In the absorbent body, as the core concentration of the particulate water-absorbing agent is higher, the effect of reducing the absorption characteristics of the particulate water-absorbing agent at the time of production of the absorbent body, a paper diaper or the like becomes more prominent. Moreover, it is preferable that the said absorber is thin with a thickness of 0.1-5 mm.

The said absorbent article is an absorbent article provided with the said absorber, the surface sheet which has liquid permeability, and the back sheet | seat which has liquid impermeability. The manufacturing method of the said absorbent article first produces an absorber (absorption core) by blending or sandwiching, for example, a fiber material and a particulate water-absorbing agent. Next, the absorbent body is sandwiched between a liquid-permeable top sheet and a liquid-impermeable back sheet, and if necessary, equipped with an elastic member, a diffusion layer, an adhesive tape, etc. Absorbent articles, especially adult paper diapers and sanitary napkins. The absorber is used by being compression-molded in a range of density 0.06 to 0.50 g / cc and basis weight 0.01 to 0.20 g / cm ² . Examples of the fiber material used include hydrophilic fibers such as pulverized wood pulp, cotton linters and crosslinked cellulose fibers, rayon, cotton, wool, acetate, and vinylon. Preferably, they are airlaid.

  The water-absorbent article exhibits excellent absorption characteristics. Specific examples of such absorbent articles include diapers for children, sanitary napkins, sanitary materials such as so-called incontinence pads, as well as adult paper diapers that have been growing rapidly in recent years. However, it is not limited to them. The above-mentioned water-absorbent article has a small amount of return due to the excellent absorption characteristics of the particulate water-absorbing agent present in the absorbent article, has a remarkably dry feeling, and greatly reduces the burden on the wearer and the caregiver. be able to.

  Hereinafter, the present invention will be described more specifically with reference to production examples, examples, and comparative examples, but the present invention is not limited thereto. Embodiments obtained by appropriately combining technical means disclosed in different embodiments are also included in the scope of the present invention.

  In addition, as long as there is no designation | designated, the electric equipment used in the Example was all used at 200V or 100V. Further, the water-absorbing resin was used under the conditions of 25 ± 2 ° C. and relative humidity 50% RH unless otherwise specified. Reagents and instruments exemplified in the following measurement methods, production examples, examples, and comparative examples may be appropriately replaced with equivalent products.

[Method for measuring physical properties]
<Absorption capacity under no load (GV)>
0.2 g of the particulate water-absorbing resin is uniformly placed in a non-woven bag (60 mm × 60 mm, Nankoku Pulp Industries Co., Ltd. Heaton Paper GS-22), and after heat sealing, 0.9 wt% aqueous sodium chloride solution (physiological) It was immersed in (saline solution). After 30 minutes, the bag was pulled up, drained at 250 × 9.81 m / s ² (250 G) for 3 minutes using a centrifuge, and then the mass W ₁ (g) of the bag was measured. Moreover, the same operation was performed without using particulate water-absorbing resin, and the mass W ₀ (g) at that time was measured. Then, GV (absorption capacity under no load) was calculated from these masses W ₁ and W _{0 according} to the equation (5).
GV (g / g) = ((W ₁ -W ₀ ) / mass of particulate water-absorbing resin) -1 Formula (5)
However, for the GV measurement of the particulate water-absorbing resin prior to the heat drying step, the above method was followed except that 0.2 g of the solid content was used and the solid content was corrected when calculating the GV.

<GEX value>
When the pH soluble content is x (% by weight) and the GV is y (g / g), the GEX value is defined by equation (6).
GEX value = (y−15) / ln (x) (6)
ln (x): represents the natural logarithm of x.

<Saline flow conductivity (SFC)>
Saline flow conductivity (SFC) was performed according to the saline flow conductivity (SFC) test of US Published Patent US2004-0106745 and JP-T 9-509591.

  Specifically, 0.90 g of the particulate water-absorbing resin uniformly placed in the cell was swollen for 60 minutes under pressure of 0.3 psi (2.07 kPa) in artificial urine, and the height of the gel layer was recorded. . Next, under a pressure of 0.3 psi (2.07 kPa), a 0.69 wt% sodium chloride aqueous solution was passed through the gel layer swollen from the tank at a constant hydrostatic pressure.

  A glass tube is inserted in the tank. The glass tube is arranged by adjusting the position of the lower end so that the level of the 0.69 wt% sodium chloride aqueous solution in the cell is maintained at a height of 5 cm above the bottom of the swollen gel. The 0.69 wt% sodium chloride aqueous solution in the tank is supplied to the cell through an L-shaped tube with a cock. A collection container for collecting the liquid that has passed is arranged under the cell, and the collection container is placed on an upper pan balance. The inner diameter of the cell is 6 cm. A 400 stainless steel wire mesh (aperture 38 μm) is installed. There is a hole in the lower part of the piston sufficient for the liquid to pass through, and a glass filter with good permeability is attached to the bottom so that the particulate water-absorbing resin or its swelling gel does not enter the hole. The cell is placed on a table on which the cell is placed, and this table is installed on a stainless steel wire mesh so as not to prevent the liquid from passing therethrough.

  The artificial urine is composed of 0.25 g of calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, 2 hydrogen phosphates. What added 0.15g of ammonium and 994.25g of pure waters is used.

The SFC test was performed at room temperature (20 to 25 ° C.). Using a computer and a balance, the amount of liquid passing through the gel layer at 20 second intervals as a function of time was recorded for 10 minutes. The flow rate Fs (t) passing through the swollen gel (mainly between the particles) was determined in units of g / s by dividing the increased mass (g) by the increased time (s). The time when a constant hydrostatic pressure and a stable flow rate were obtained was taken as ts, and the flow rate obtained during 10 minutes from ts was used to calculate the value of Fs (t = 0), that is, the first flow rate through the gel layer. . Fs (t = 0) was calculated by extrapolating the result of least squares of Fs (t) with respect to time to t = 0. And the physiological saline flow-inducible SFC (pressurization flow rate under pressure) was calculated | required using Formula (7). In addition, the unit of the liquid flow rate under pressure is (10 ⁻⁷ × cm ³ × s × g ⁻¹ ).
Liquid passage speed under pressure (10 ⁻⁷ × cm ³ × s × g ⁻¹ ) = Fs (t = 0) × L ₀ / (ρ × A × ΔP) (7)
Fs (t = 0): flow rate L ₀ expressed in g / s: height of gel layer expressed in cm ρ: density of NaCl solution (1.003 g / cm ³ )
A: Area above the gel layer in the cell (28.27 cm ² )
ΔP: hydrostatic pressure applied to the gel layer (4920 dyne / cm ² )
<Absorption capacity under pressure (AAP)>
A load prepared to a pressure of 4.83 kPa (0.7 Psi) was prepared. Then, 0.90 g of the particulate water-absorbing resin was uniformly dispersed on a plastic cylindrical wire net having a diameter of 60 mm and having a 400 mesh (38 μm mesh) wire net attached to the bottom. On top of that, the above load was placed, and the mass W ₂ (g) of this measuring device set was measured.

  Next, a glass filter with a diameter of 90 mm (manufactured by Mutual Riken Glass Co., Ltd., pore diameter: 100 to 120 μm) is placed inside a petri dish with a diameter of 150 mm, and a 0.90% by weight sodium chloride aqueous solution (20 to 25 ° C.). ) Was added to the same level as the top surface of the glass filter.

  On top of that, one sheet of filter paper having a diameter of 90 mm (trade name: “JIS P 3801 No. 2”, manufactured by ADVANTEC Toyo Co., Ltd., thickness 0.26 mm, reserved particle size 5 μm) was placed so that the entire surface was wetted. Excess liquid was removed.

The set of measuring devices was placed on the wet filter paper, and the liquid was absorbed under load. After 1 hour (60 minutes), the measuring device set was lifted and its mass W ₃ (g) was measured. And from these masses W ₂ and W ₃ , the absorption capacity under pressure (g / g) was calculated according to the formula (8).
Absorption capacity under pressure (g / g) = (W ₃ −W ₂ ) / mass of particulate water-absorbing resin (g) (8)
The absorption capacity under pressure under the pressure (under load) of 4.83 kPa (0.7 Psi) was used for absorbent articles such as absorbent bodies or disposable diapers when the infant was sleeping and sitting. Is assumed to be used.

<Moisture content>
A 1 g hydrated gel or a water-absorbing resin was spread thinly on a 6 cm aluminum dish and dried in a windless oven at 180 ° C. for 3 hours to measure the loss on drying (% by weight), and the moisture content was measured. In addition, solid content (%) is prescribed | regulated by (100- moisture content) (%).

[Production Example 1]
48.5 wt% aqueous sodium hydroxide solution 9.7 g / sec, acrylic acid 12.1 g / sec, 30 wt% polyethylene glycol diacrylate (average molecular weight 523) aqueous solution (flow rate 0.0513 g / sec) and 46 wt% Continuously supplying the mixed solution with trisodium diethylenetriaminepentaacetic acid aqueous solution (flow rate 0.0016 g / sec) to the mixer at a flow rate of 0.0529 g / sec and water at 5.286 g / sec. Thus, an aqueous monomer solution was prepared. At this time, the temperature of the monomer aqueous solution was 103 ° C.

  An endless belt running at a speed of 230 cm / min, kept at about 100 ° C., is further added to this adjusted aqueous monomer solution at a flow rate of 0.589 g / sec. The monomer aqueous solution was continuously supplied. The aqueous monomer solution continuously supplied onto the belt started to polymerize quickly, and a band-shaped hydrogel sheet (hydrogel polymer) was obtained.

  This hydrogel sheet was continuously roughly crushed (subdivided) using a cutter mill having a screen with a diameter of 8 mm (trade name: “U-280”, manufactured by Horai Co., Ltd.). A particulate water-containing crosslinked polymer (a) having a temperature of about 35 ° C. and a size of about 2 to 5 mm was obtained. At this time, the water content of the particulate water-containing crosslinked polymer (a) was 29% by weight.

[Example 1]
21.5 kg of a particulate water-containing crosslinked polymer (a), a conduction flow dryer (trade name: “Production name:”, in which the hot air temperature is set to 180 ° C., the wind speed is set to 2.4 m / second, and the heat transfer tube temperature is set to 180 ° C. FCA-2 ", manufactured by Okawara Manufacturing Co., Ltd., fluidized bed length 850 mm / fluidized bed width 240 mm = 3.54). And it dried for 11 minutes and obtained the polymer dried material (1st step drying). This operation was performed 5 times to obtain 90 kg of a dried product (A1) having a moisture content of 10.0% by weight. The weight average particle diameter of the dried product (A1) was 2.4 mm.

  Subsequently, the dried product (A1) was pulverized using a roll granulator (clearance 10 mm / 5 mm), and the particulate pulverized product (B1) having a weight average particle size of 410 μm (particle size controlled water-absorbent resin powder) Got.

  Next, 20.0 kg of the obtained particulate pulverized product (B1) was charged into a conduction flow dryer in which the hot air temperature was set to 180 ° C. and the wind speed was set to 0.5 m / second in advance. Then, drying was performed for 40 minutes (second stage drying) to obtain a dried product (C1) having a water content of 4.3% by weight.

[Comparative Example 1]
The same operation as in Example 1 was performed to obtain a dried product (A1) (first stage drying).

  Subsequently, 14.0 kg of the dried product (A1) was charged into a conduction flow dryer in which the hot air temperature was set to 180 ° C. and the wind speed was set to 2.4 m / sec. And it dried for 40 minutes and obtained the coarse grain dried material (D1) with a moisture content of 3.6 weight% (2nd step | paragraph drying).

[Example 2] ... Heat drying after particle size control The pulverized and dried product (C1) obtained in Example 1 is classified by using a sieve screen having an opening size of 850 µm and 150 µm, so that the majority is 150 Particles having a particle diameter of ˜850 μm and a weight average particle diameter of 414 μm were obtained. To this classified particle (400 g), a surface crosslinking agent-containing aqueous solution (14.04 g) having a ratio of 1,4-butanediol: propylene glycol: water = 0.34: 0.56: 3.0 was sprayed with a Redige mixer. Mixing was performed. Thereafter, heat treatment was performed for 40 minutes with a mortar mixer in which the oil bath temperature was set to 205 ° C. in advance. After the heat treatment, it was classified with a 850 μm sieve to obtain a surface-treated product (E1) (surface-crosslinked water-absorbent resin).

  35 g of the surface-treated product (E1) and 10 g of glass beads were placed in a 225 ml mayonnaise bottle and shaken with a paint shaker for 30 minutes. Next, 0.36 g of a liquid permeability improver having a ratio of 50% aluminum sulfate: propylene glycol: sodium lactate = 1.0: 0.025: 0.167 was added to and mixed with 30 g of the powder after shaking. Curing was performed for 30 minutes with a dryer at 60 ° C. After curing, it was put into a 225 ml mayonnaise bottle containing 10 g of glass beads and shaken with a paint shaker for 10 minutes to obtain an aluminum surface treated product (F1).

[Comparative Example 2] ... No heat drying after particle size control The coarse particle dried product (D1) was pulverized using a roll granulator (clearance 10 mm / 5 mm). Subsequently, the particles were classified using a sieve screen having an opening of 850 μm and 150 μm, whereby particles having a particle diameter of 150 to 850 μm and a weight average particle diameter of 397 μm were obtained. The surface-treated product (G1) and the aluminum surface-treated product (H1) were obtained by carrying out the same operations as in Example 5 except that the heat treatment for 40 minutes in Example 5 was changed to heat treatment for 55 minutes. .

<Physical property comparison>
Table 1 summarizes the physical properties of the aluminum surface treated product (F1) and the aluminum surface treated product (H1).

  As shown in Table 1, when comparing Example 2 (with second drying after particle size control, moisture content 4.3%) and Comparative Example 2 (without second drying, moisture content 3.6%), In the aluminum surface treatment product (F1), the physical properties of AAP0.7Psi and SFC in the case of GV30 were improved as compared with the aluminum surface treatment product (H1). That is, it has been clarified that by performing the second drying after the particle size control, the physical properties of the aluminum surface treatment product are improved as compared with the case where the second drying is not performed.

[Production Example 2]
A neutralized solution in which 13.3 g of 48.5% by weight sodium hydroxide aqueous solution, 45.5 g of acrylic acid, and 19.8 g of industrial pure water were mixed was continuously prepared.

  The neutralization solution is 78.6 g / sec, the 48.5 wt% sodium hydroxide aqueous solution is 23.3 g / sec, and the 20 wt% polyethylene glycol diacrylate (average molecular weight 523) is 0.199 g / sec. The monomer aqueous solution was adjusted by continuously supplying to the mixer. At this time, the temperature of the monomer aqueous solution was 95 to 100 ° C.

  After adding 46 wt% diethylenetriaminepentaacetic acid trisodium aqueous solution (flow rate 0.0278 g / sec) and 4.0 wt% sodium persulfate aqueous solution to the prepared monomer aqueous solution at a flow rate of 0.635 g / sec. The monomer aqueous solution was continuously supplied to an endless belt running at a speed of 7 m / min. The monomer aqueous solution continuously supplied onto the belt started to polymerize rapidly, and a band-shaped hydrogel sheet (hydrogel polymer) was obtained.

  This hydrogel sheet is continuously coarsely crushed (subdivided) using a cutter mill having a screen with a diameter of 8 mm (trade name: “RC450”, manufactured by Yoshiko), and is about 1 to 3 mm in size. A crosslinked polymer (b) was obtained. At this time, the water content of the particulate water-containing crosslinked polymer (b) was 29% by weight.

Particulate hydrous crosslinked polymer (b) in an amount of 280 kg / h, fluidized bed dryer (trade name: “CT-FBD-0.91 m ² ”, Nara Machinery Co., Ltd., fluidized bed length 800 mm / fluidized bed A width of 550 mm = 1.45) was continuously charged and dried. The fluidized bed has a three-chamber configuration with two partition plates separated in the length direction at a ratio of approximately 2 to 1: 1. The velocity of hot air in the dryer was 2.4 m / sec, and the temperature of the hot air was 130 ° C. in the first chamber and 190 ° C. in the second and third chambers. The outlet of the dryer was provided in the upper part and the lower part of the fluidized part, and the ratio of the amount of dry matter extracted was upper outlet / lower outlet = 4/1. The retention amount of the particulate hydrous crosslinked polymer (b) was controlled by the height of the weir at the upper outlet of the dryer. The height of the fluidized bed during drying was about 900 mm, and the average time for the particulate water-containing crosslinked polymer (b) to stay in the dryer was about 50 minutes.

  The dried product (A2) exiting the dryer is pneumatically transported, pulverized by a roll mill, and further continuously classified using a sieve having openings of 850 μm and 213 μm, so that the particle size is mostly 150 to 850 μm. A particulate pulverized product (B2) having a weight average particle size of 480 μm was obtained. The particulate pulverized product (B2) was sampled and the water content was measured and found to be 8.7% by weight.

Example 3
500 g of the particulate pulverized product (B2) was charged into a fluidized bed dryer (trade name: “Spray dryer GB-22” manufactured by Yamato Co., Ltd.) in which the hot air temperature was set to 220 ° C. in advance. A mantle heater was attached to the glass cell in the fluidized bed portion of this dryer, and the temperature was set at 200 ° C. in advance. After heating and drying for 40 minutes, the heat-dried product (C2) was taken out and the water content was measured and found to be 1.3% by weight. Further, the heat-dried product (C2) had an increase in the absorption capacity GV (solid content correction value) of 7.3 g / g in the portion excluding the water content, compared to the particulate pulverized product (B2).

Example 4
500 g of the particulate pulverized product (B2) was charged into the fluidized bed dryer of Example 14 in which the hot air temperature and the mantle heater temperature were set to 180 ° C. After heating and drying for 40 minutes, the heat-dried product (C3) was taken out and the water content was measured and found to be 2.7% by weight. Further, the heat-dried product (C3) had an increase in the absorption capacity GV (solid content correction value) of 2.5 g / g in the portion excluding the moisture content, compared to the particulate pulverized product (B2).

Example 5
Particulate pulverized product (B2) was continuously charged into the conduction flow dryer of Example 13 in an amount of 40 kg / h, hot air temperature and heat transfer tube temperature set to 210 ° C., and wind speed 0.5 m / sec. And dried by heating. The fluidized bed has a four-chamber configuration in which the length direction is divided into four equal parts by three partition plates. The height of the fluidized bed during heating and drying was about 250 mm, and the average residence time of the particulate pulverized product (B2) was 40 minutes. When the temperature in the layer was stabilized, the heated dried product (C4) discharged from the outlet of the dryer was sampled, and the moisture content was measured and found to be 1.1% by weight. In addition, the heat-dried product (C4) had an increase in the absorption capacity GV of 9.4 g / g in the portion excluding the water content, compared to the particulate pulverized product (B2).

[Example 6] ... Heat drying after particle size control (water content 1.3 wt%)
1,4-butanediol: propylene glycol: water = 0.34 is added to 500 g of the heat-dried product (C2) having a water content of 1.3% by weight obtained in Example 3 (with heat-drying after particle size control). 17.55 g of a surface cross-linking agent-containing aqueous solution having a ratio of 0.56: 3.0 was sprayed and mixed with a Redige mixer. Thereafter, heat treatment was performed for 30 minutes with a mortar mixer whose oil bath temperature was set to 207 ° C. in advance. After the heat treatment, it was classified with an 850 μm sieve to obtain a surface crosslinked product (E2).

[Example 7] ... Heat drying after particle size control (moisture content 2.7% by weight)
In Example 6, the heat-dried product (C2) was changed to the heat-dried product (C3) of Example 4 (moisture content 2.7% by weight), and the heat treatment time after mixing the surface cross-linking agent was changed to 25 minutes. Except for the above, the same operation as in Example 6 was performed to obtain a surface crosslinked product (E3).

[Example 8] ... Heat drying after particle size control (water content 1.1 wt%)
In Example 6, the heat-dried product (C2) was changed to the heat-dried product (C4) of Example 5 (water content 1.1% by weight), and the heat treatment time after mixing the surface crosslinking agent was changed to 35 minutes. Except for the above, the same operation as in Example 6 was performed to obtain a surface crosslinked product (E4).

[Comparative Example 3] ... without heating and drying after particle size control To 500 g of the particulate pulverized product (B2) having a water content of 8.7 wt% obtained in Production Example 2 (without heating and drying after particle size control), 19.5 g of a surface cross-linking agent-containing aqueous solution having a ratio of 1,4-butanediol: propylene glycol: water = 0.34: 0.56: 3.0 was sprayed and mixed with a Redige mixer. Thereafter, heat treatment was performed for 30 minutes with a mortar mixer whose oil bath temperature was set to 207 ° C. in advance. After the heat treatment, it was classified with a 850 μm sieve to obtain a surface crosslinked product (I5).

<Physical property comparison>
Table 2 summarizes the physical properties of the surface crosslinked product (E2, E3, I5). As shown in Table 6 below, the heat absorption after particle size control or the water content before surface cross-linking is 3% by weight or less improves the absorption capacity under pressure (AAP), and further allows liquid permeability. (SFC) is dramatically improved.

Example 9
35 g of the surface cross-linked product (E4) and 10 g of glass beads were placed in a 225 ml mayonnaise bottle and shaken with a paint shaker for 30 minutes. Next, after adding and mixing 0.3 g of a liquid permeability improver having a ratio of 50% aluminum sulfate: propylene glycol: sodium lactate = 1.0: 0.025: 0.3 to 30 g of the powder after shaking, Curing was performed for 30 minutes with a dryer at 60 ° C. After curing, it was put into a 225 ml mayonnaise bottle containing 10 g of glass beads and shaken with a paint shaker for 10 minutes to obtain an aluminum surface treated product (F4).

  Table 3 shows the physical properties of the surface treated product (E4) and the aluminum surface treated product (F4). As shown in Table 3 below, even if continuous surface cross-linking that tends to deteriorate the physical properties is performed, the physical properties do not decrease, and even if the heating and drying process is performed with a continuous heating dryer, the batch heating shown in Table 3 is performed. Physical properties equivalent to those obtained when drying with a dryer were obtained.

  The method for producing a water absorbent resin of the present invention produces an effect that a particulate water absorbent resin having excellent physical properties can be efficiently obtained at a low cost while ensuring high productivity.

  The specific embodiments or examples made in the detailed description section of the invention are merely to clarify the technical contents of the present invention, and are limited to such specific examples and are interpreted in a narrow sense. It should be understood that the invention can be practiced with various modifications within the spirit of the invention and within the scope of the following claims.

  As described above, the method for producing a water absorbent resin of the present invention dries an object to be dried without reducing effects such as water absorption characteristics. For this reason, the particulate water-absorbing resin obtained by the method for producing a water-absorbing resin of the present invention exhibits excellent absorption characteristics and the like. Such particulate water-absorbing resin, for example, as an absorbent for hygiene materials such as adult paper diapers, children's diapers, sanitary napkins, so-called incontinence pads, which have grown significantly in recent years, flocculants, coagulants, soil conditioners, It can be widely used as a water-soluble polymer suitably used for soil stabilizers, thickeners, etc., or as a water retaining agent, dehydrating agent, etc. in the fields of agriculture and horticulture and civil engineering.

Claims (16)

A step (1) of polymerizing an aqueous monomer solution;
A step (2) of drying the hydrogel polymer obtained by the step (1),
The step (3) of controlling the particle size by pulverizing or pulverizing and classifying the dried product obtained in the step (2), and surface cross-linking to the water absorbent resin powder whose particle size is controlled by the step (3) A method for producing a water-absorbent resin comprising sequentially performing (5),
Before the step (5) of performing the surface crosslinking, a step (4) of performing a second heat drying on the water-absorbent resin powder whose particle size has been controlled in the step (3) is included. Production method.   3. The water absorption according to claim 2, wherein after the water-absorbing resin after the step (4) of performing the second heat drying is cooled, the water-absorbing resin is supplied to the step (5) of performing the surface crosslinking. For producing a functional resin.   The water-absorbing resin after the step (4) of performing the second heat drying is stored, and then supplied to the step (5) of performing the surface cross-linking. A method for producing a water-absorbent resin as described in 1. above.   In the step (4) of performing the second heat drying, heating is performed until the absorption capacity under no load (GV) per solid content is increased. 2. A method for producing a water-absorbent resin according to item 1.   The method for producing a water-absorbent resin according to any one of claims 1 to 4, wherein the step (4) of performing the second heat drying is performed at 150 to 300 ° C.   The method for producing a water-absorbent resin according to any one of claims 1 to 5, wherein the step (4) of performing the second heat drying is performed for at least 10 minutes.   The water content (specified by loss on drying at 180 ° C. for 3 hours) of the water-absorbent resin used in the surface crosslinking step (5) is 0 to 3% by weight. 7. The method for producing a water absorbent resin according to any one of items 6 to 6. A step (1) of polymerizing an aqueous monomer solution;
A step (2) of drying the hydrogel polymer obtained by the step (1),
The step (3) of controlling the particle size by pulverizing or pulverizing and classifying the dried product obtained in the step (2), and surface cross-linking to the water absorbent resin powder whose particle size is controlled by the step (3) A method for producing a water-absorbent resin comprising sequentially performing (5),
A method for producing a water-absorbent resin, characterized in that the water content of the water-absorbent resin to be subjected to the surface crosslinking step (5) (specified by loss on drying at 180 ° C. for 3 hours) is 0 to 3% by weight.   The step (4) of performing the second heat drying on the water-absorbent resin powder whose particle size is controlled in the step (3) is included before the step (5) of performing the surface crosslinking. 9. A method for producing a water-absorbent resin according to item 8.   The moisture content (specified by loss on drying at 180 ° C for 3 hours) of the dried product obtained in the step (2) is 5 to 15% by weight. The manufacturing method of the water absorbing resin of any one of Claims 1.   11. The weight average particle diameter (specified by sieving classification) of the dried product obtained in the step (2) is 1 to 5 mm. 11. A method for producing a water-absorbent resin.   The weight-average particle diameter (specified by sieving classification) of the water-absorbent resin powder whose particle size is controlled in the step (3) is 200 to 800 µm, any one of claims 1 to 11 A method for producing the water-absorbent resin according to item.   The water-absorbent resin according to any one of claims 1 to 12, wherein the water-absorbent resin used in the step (5) of performing the surface crosslinking is cooled to a temperature of 100 ° C or lower. Manufacturing method of resin.   The method for producing a water-absorbent resin according to any one of claims 1 to 13, wherein the hydrogel polymer is finely divided during polymerization or after polymerization.   16. The process according to claim 1, wherein in the step (2) of drying the hydrogel polymer or the step (4) of performing the second heat drying, the drying is performed by a fluidized bed dryer. The manufacturing method of the water absorbing resin of any one of these. 29. The water absorbing resin according to any one of claims 14 to 28, wherein the water absorbing resin is a polyacrylate water absorbing resin satisfying at least one physical property of the following (a) to (c): A method for producing the water-absorbent resin as described.
(A) Absorption capacity under load (GV) is 20 to 100 g / g.
(B) Absorption capacity under pressure (AAP) is in the range of 10 g / g or more and 28 g / g or less.
(C) Saline flow conductivity (SFC) is at least 10 (10 ⁻⁷ · cm ³ · s · g ⁻¹ ).