Polymerization method for water-absorbent resin

Abstract

The present invention provides: a polymerization method for a water absorbent resin, the polymerization method enabling improvement of the polymerization rate without deteriorating the water absorption performance; and a water absorbent resin which is produced using this polymerization method. One embodiment of the present invention relates to a polymerization method for a water absorbent resin, the polymerization method essentially using an aqueous monomer solution containing an acrylic acid (salt) and an internal crosslinking agent, wherein polymerization is performed in the presence of (i) a polyacrylic acid (salt) polymer and (ii) a fibrous material.

Description

[Technical Field]

[0001] The present invention relates to a method for polymerizing a water-absorbent resin, and more particularly to a method for polymerizing a water-absorbent resin that improves the polymerization rate of the water-absorbent resin and reduces the amount of residual monomer at the end of polymerization. [Background technology]

[0002] Water-absorbent resins are resins with excellent water absorption capacity, and are used in a wide range of applications by taking advantage of these properties. For example, water-absorbent resins are used in sanitary products such as disposable diapers and sanitary napkins, soil water retention agents, etc. Since the amount of such water-absorbent resins used in sanitary products, soil water retention agents, etc. is relatively large, this has a significant impact on the cost competitiveness of products using the water-absorbent resins. Therefore, there is a demand for the development of technology to reduce the production costs of water-absorbent resins.

[0003] When producing a water-absorbent resin by aqueous solution polymerization, a large amount of energy is required to dry the hydrous polymer gel. Therefore, a method has been disclosed in which an aqueous solution containing a high concentration of acrylic acid or its sodium salt is polymerized while evaporating water by setting the polymerization initiation temperature high (see Patent Document 1). Also, methods for improving productivity by continuous aqueous solution polymerization have been disclosed (Patent Documents 2 to 6).

[0004] Although the manufacturing methods mentioned above exist, further improvements in productivity are required. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2002-212204 [Patent Document 2] U.S. Patent No. 4,893,999 [Patent Document 3] U.S. Patent No. 6,241,928 [Patent Document 4] US Patent Application Publication No. 2005 / 215734 etc. [Patent Document 5] U.S. Patent No. 6,987,151 [Patent Document 6] U.S. Patent No. 6,710,141 Summary of the Invention [Problem to be solved by the invention]

[0006] However, if the polymerization concentration and / or temperature are increased in order to further improve productivity (to increase the polymerization rate), there is a limit to how high the polymerization concentration and / or temperature can be because this has an adverse effect on the water absorption characteristics. Therefore, it is necessary to improve the polymerization rate (to increase productivity) by a method that does not rely on these methods. [Means for solving the problem]

[0007] One embodiment of the present invention relates to a method for polymerizing a water-absorbent resin, in which polymerization is carried out in the presence of an aqueous monomer solution, an internal crosslinking agent, and the following i) and ii): i) Polyacrylic acid (salt) polymer ii) Fibrous materials That is, one embodiment of the present invention relates to a method for polymerizing a water absorbent resin, which essentially comprises an aqueous monomer solution containing acrylic acid (salt) and an internal crosslinking agent, and which is performed in the presence of the following i) and ii): i) Polyacrylic acid (salt) polymer ii) Fibrous materials [Effects of the Invention]

[0008] According to the method for polymerizing a water-absorbent resin of one embodiment of the present invention, the polymerization rate of the water-absorbent resin can be improved without relying on conventional methods of increasing the polymerization concentration and / or the polymerization temperature. Furthermore, according to the method for polymerizing a water-absorbent resin of the present invention, the water-absorbing properties are not impaired, and therefore productivity can be improved. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described in detail below, but the scope of the present invention is not limited to these descriptions, and in addition to the examples given below, the present invention can be implemented by making appropriate modifications within the scope that does not impair the spirit of the present invention.

[0010] Furthermore, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the claims. Other embodiments obtained by appropriately combining the technical means described in each of the embodiments are also included in the technical scope of the present invention.

[0011] [1] Definitions (1-1) “Water-absorbing resin” In one embodiment of the present invention, the term "water-absorbent resin" refers to a water-swellable, water-insoluble polymer gelling agent that satisfies the following physical properties: "Water-swellable" refers to a polymer gelling agent that satisfies the physical properties of having a CRC of 5 g / g or more as defined in ERT441.2-02, and "water-insoluble" refers to a polymer gelling agent that satisfies the physical properties of having an Ext of 50 mass % or less as defined in ERT470.2-02.

[0012] The water-absorbent resin can be designed appropriately depending on its application and is not particularly limited, but is preferably a hydrophilic cross-linked polymer obtained by cross-linking an unsaturated monomer having a carboxyl group. The carboxylic acid group is partially neutralized with sodium hydroxide, potassium hydroxide, or the like. A preferred configuration of the water-absorbent resin is a hydrophilic cross-linked polymer (polyacrylic acid-based water-absorbent resin) obtained by cross-linking an acrylic acid (salt) as an unsaturated monomer. Furthermore, the water-absorbent resin is not limited to a form in which the entire amount (100% by mass) is a polymer, and may be a water-absorbent resin composition containing additives and the like within a range that satisfies the above physical properties (CRC, Ext).

[0013] Furthermore, the water-absorbent resin in one embodiment of the present invention is not limited to a final product, and may refer to an intermediate in the manufacturing process of the water-absorbent resin, and all of these, together with the water-absorbent resin composition, are collectively referred to as "water-absorbent resin". Examples of the intermediate include a hydrogel-like crosslinked polymer after polymerization, a dried polymer after drying, and a water-absorbent resin powder before surface crosslinking. Note that, examples of the shape of the water-absorbent resin include a sheet, fiber, film, particle, gel, etc., but a particle-like water-absorbent resin is preferred in one embodiment of the present invention.

[0014] (1-2) "Polyacrylic acid (salt) polymer" In one embodiment of the present invention, a "polyacrylic acid (salt) polymer" refers to a polymer obtained by polymerizing a polyacrylic acid (salt) monomer as the main component, and may contain an internal crosslinking agent as an additional component. The "polyacrylic acid (salt) polymer" may be in the form of a water-soluble aqueous solution, a water-swellable, water-insoluble swollen gel, a swollen gel solution containing these in combination, or a dried powder.

[0015] (1-3) "Polyacrylic acid (salt)" In one embodiment of the present invention, "polyacrylic acid (salt)" refers to polyacrylic acid and / or its salt, specifically a polymer containing acrylic acid and / or its salt (hereinafter referred to as "acrylic acid (salt)") as a repeating unit as a main component, and optionally containing a graft component. The term "main component" refers to the amount (content) of acrylic acid (salt) used relative to the total monomers (excluding the internal crosslinking agent) used in the polymerization, typically 50 to 100 mol%, preferably 70 to 100 mol%, 80 to 100 mol%, more preferably 90 to 100 mol%, and even more preferably substantially 100 mol%. In other words, the "polyacrylic acid (salt)" contains structural units derived from acrylic acid (salt) in an amount of 50 to 100 mol%, preferably 70 to 100 mol%, 80 to 100 mol%, more preferably 90 to 100 mol%, and even more preferably substantially 100 mol% relative to the total structural units of the polymer.

[0016] (1-4) "Induction time until polymerization begins" The "induction time until the initiation of polymerization" refers to the time until the initiation of polymerization after mixing an aqueous monomer solution and an aqueous initiator solution in the production of a water-absorbent resin. The initiation of the polymerization can be confirmed, for example, by the occurrence of expansion and foaming of the solution, the occurrence of cloudiness in the solution, or a large increase in the temperature of the aqueous monomer solution. The shorter the "induction time until the initiation of polymerization", the faster the polymerization rate is considered to be.

[0017] (1-5) "Residual monomer amount" The "amount of residual monomer" refers to the amount of monomer dissolved in the aqueous solution relative to the polymer solid content in the polymer.

[0018] (1-6) "Fiber-like materials" In one embodiment of the present invention, the term "fibrous material" refers to a material having a shape in which the length is at least twice its thickness (width). Due to this shape, the material often has a larger specific surface area than a particulate material of the same weight. The fibrous material used in one embodiment of the present invention may be either hydrophilic or hydrophobic, and may be made of either natural or synthetic polymers.

[0019] (1-7) "EDANA" and "ERT" "EDANA" is an abbreviation for European Disposables and Nonwovens Associations. "ERT" is an abbreviation for EDANA Recommended Test Methods, a European standard (almost a global standard) for measuring the properties of water-absorbent resins. In this specification, unless otherwise specified, the physical properties of water-absorbent resins are measured in accordance with the original ERT (revised in 2002 / publicly known document).

[0020] (1-8) "CRC" (ERT441.2-02) "CRC" is an abbreviation for Centrifuge Retention Capacity, and means the water absorption capacity of a water-absorbent resin under no load (sometimes referred to as "water absorption capacity"). Specifically, it refers to the water absorption capacity (unit: g / g) after 0.2 g of a water-absorbent resin is placed in a nonwoven bag, immersed in a large excess of 0.9 mass % sodium chloride aqueous solution for 30 minutes to allow free swelling, and then drained using a centrifuge (250 G).

[0021] (1-9) "AAP" (ERT442.2-02) "AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption capacity under pressure of a water-absorbent resin. Specifically, 0.9 g of a water-absorbent resin is absorbed under pressure of 2.06 kPa (21 g / cm) for 1 hour in a large excess of a 0.9 mass % aqueous sodium chloride solution. 2 It refers to the water absorption capacity (unit: g / g) after swelling under a load of 0.3 psi.

[0022] (1-10)Other In this specification, the range "X to Y" means "X or more and Y or less."

[0023] In this specification, unless otherwise specified, "ppm" means "ppm by weight."

[0024] In this specification, "acid (salt)" means "acid and / or its salt." "(Meth)acrylic" means "acrylic and / or methacrylic."

[0025] In this specification, the unit of volume "liter" may be written as "l" or "L".

[0026] In this specification, "weight" and "mass," "wt %" and "mass %," and "parts by weight" and "parts by mass" are treated as synonyms.

[0027] [2] Polymerization method for water-absorbent resin The polymerization steps (2-1) and (2-2) for the water-absorbent resin according to one embodiment of the present invention will be described below.

[0028] (2-1) Preparation of aqueous monomer solution This step is a step of preparing an aqueous solution containing acrylic acid (salt) as a main component (hereinafter referred to as "aqueous monomer solution"). Note that, although a slurry liquid of a monomer can also be used within a range in which the water absorption performance of the obtained water absorbent resin is not reduced, for the sake of convenience, in this section, an aqueous monomer solution will be described.

[0029] Further, the above-mentioned "main component" means that, as described in the above-mentioned (1-3), the amount (content) of acrylic acid (salt) used is usually 50 mol % or more, preferably 70 mol % or more, 80 mol % or more, more preferably 90 mol % or more (upper limit: 100 mol %), based on the whole of the monomers (excluding the internal crosslinking agent) to be subjected to the polymerization reaction of the water absorbent resin.

[0030] (acrylic acid) In one embodiment of the present invention, from the viewpoint of the physical properties and productivity of the resulting water-absorbent resin, acrylic acid and / or a salt thereof (hereinafter referred to as "acrylic acid (salt)") is used as the monomer.

[0031] The "acrylic acid" may be a known acrylic acid, and may contain, as a polymerization inhibitor, preferably a methoxyphenol, more preferably p-methoxyphenol. From the viewpoint of the polymerizability of acrylic acid and the color tone of the water-absorbent resin, the polymerization inhibitor may be contained in an amount of preferably 200 ppm or less, more preferably 10 to 160 ppm, and even more preferably 20 to 100 ppm. Furthermore, with regard to impurities in acrylic acid, compounds described in U.S. Patent Application Publication No. 2008 / 0161512 are also applicable to one embodiment of the present invention.

[0032] Furthermore, the above-mentioned "acrylate salt" is obtained by neutralizing the above-mentioned acrylic acid with the following basic composition, and the acrylate salt may be a commercially available acrylate salt (e.g., sodium acrylate), or may be one obtained by neutralization in a production plant for a water-absorbent resin.

[0033] (Basic composition) In one embodiment of the present invention, the term "basic composition" refers to a composition containing a basic compound, such as a commercially available aqueous sodium hydroxide solution.

[0034] Specific examples of the basic compound include carbonates and / or hydrogen carbonates of alkali metals, hydroxides of alkali metals, ammonia, organic amines, etc. Among these, from the viewpoint of the physical properties of the obtained water absorbent resin, it is desirable that the basic compound be strongly basic. That is, alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide are preferred, and sodium hydroxide is more preferred.

[0035] (neutralization) In one embodiment of the present invention, neutralization can be performed by either neutralizing acrylic acid (before polymerization) or neutralizing a hydrogel-like crosslinked polymer obtained by crosslinking and polymerizing acrylic acid (after polymerization) (hereinafter referred to as "post-neutralization"). When the above-mentioned "polyacrylic acid (salt) polymer" is used to produce a water-absorbent resin, it is likely that the polymer contains a basic compound. This basic compound neutralizes the acrylic acid before polymerization or the hydrogel-like crosslinked polymer after polymerization. Therefore, as described below, the neutralization rate of the acrylic acid and the final water-absorbent resin product is adjusted to a predetermined range by taking this basic compound into consideration. Furthermore, these neutralization processes may be performed continuously or batchwise, but continuous processes are preferred from the viewpoint of production efficiency, etc. Regarding the conditions for neutralization, such as the apparatus, neutralization temperature, and residence time, the conditions described in International Publication No. 2009 / 123197 and U.S. Patent Application Publication No. 2008 / 0194863, etc., also apply to one embodiment of the present invention.

[0036] In one embodiment of the present invention, the neutralization rate is preferably 10 to 90 mol%, more preferably 40 to 85 mol%, even more preferably 50 to 80 mol%, and particularly preferably 60 to 75 mol%, based on the acid groups of the monomer. If the neutralization rate is less than 10 mol%, the water absorption capacity may be significantly reduced. On the other hand, if the neutralization rate exceeds 90 mol%, a water-absorbent resin with a high water absorption capacity under pressure may not be obtained. For example, a neutralization rate of 75 mol% means a mixture of 25 mol% of acrylic acid and 75 mol% of an acrylic acid salt. This mixture may also be referred to as a partially neutralized product of acrylic acid. The above neutralization rate is the same in the case of post-neutralization. The above neutralization rate also applies to the neutralization rate of the water-absorbent resin as a final product. The present invention is characterized in that the "polyacrylic acid (salt)-based polymer" is used to produce a water-absorbent resin. When a basic compound is contained in the polyacrylic acid (salt) polymer, the neutralization of acrylic acid (before polymerization) and / or the post-neutralization of the hydrogel-like crosslinked polymer is appropriately adjusted so that the neutralization rate of the water-absorbent resin as a final product, including the basic compound, falls within a predetermined range.

[0037] (other monomers) In one embodiment of the present invention, the term "other monomer" refers to a monomer other than the acrylic acid (salt) and can be used in combination with the acrylic acid (salt) to produce a water-absorbent resin. Examples of the other monomer include water-soluble or hydrophobic unsaturated monomers. Specifically, the compounds described in U.S. Patent Application Publication No. 2005 / 0215734 (excluding acrylic acid) are also applicable to one embodiment of the present invention.

[0038] (internal crosslinking agent) As the internal crosslinking agent used in one embodiment of the present invention, the compounds described in U.S. Pat. No. 6,241,928 are also applicable to one embodiment of the present invention. Among these, one or more compounds are selected in consideration of reactivity. Furthermore, as the internal crosslinking agent, a compound having two or more polymerizable unsaturated groups is preferred from the viewpoint of the water absorption performance of the obtained water absorbent resin. More preferably, a compound having thermal decomposition properties at the drying temperature in the drying step described below, even more preferably a compound having two or more polymerizable unsaturated groups having a (poly)alkylene glycol structural unit, is used as the internal crosslinking agent.

[0039] The polymerizable unsaturated group is preferably an allyl group or a (meth)acrylate group, more preferably a (meth)acrylate group. The (poly)alkylene glycol structural unit is preferably polyethylene glycol, and the number n is preferably 1 to 100, more preferably 6 to 50.

[0040] The amount of the internal crosslinking agent used is preferably 0.0001 to 10 mol %, more preferably 0.001 to 1 mol %, based on the total amount of monomers. By setting the amount used within the above range, a desired water-absorbent resin can be obtained. If the amount used is too small, the gel strength tends to decrease and the water-soluble content tends to increase, while if the amount used is too large, the water absorption capacity tends to decrease, which is not preferable. The mol % based on the total amount of monomers refers to the percentage of the number of moles of the internal crosslinking agent relative to the total number of moles of monomers contained in the aqueous monomer solution.

[0041] In one embodiment of the present invention, a method is preferably used in which a predetermined amount of internal cross-linking agent is added to an aqueous monomer solution in advance, and a cross-linking reaction is carried out simultaneously with polymerization. Other methods that can be used include a post-cross-linking method in which an internal cross-linking agent is added during and / or after polymerization; a radical cross-linking method using a radical polymerization initiator; and a radiation cross-linking method using active energy rays such as electron beams or ultraviolet rays. These methods can also be used in combination.

[0042] (Mixture of polyacrylic acid (salt) polymer and fibrous material) In one embodiment of the present invention, the following "mixture of a polyacrylic acid (salt) polymer and a fibrous material" is added from the viewpoint of improving the polymerization rate of the water-absorbent resin. The polymerization method for a water-absorbent resin according to one embodiment of the present invention can improve the polymerization rate of the water-absorbent resin without adversely affecting the water-absorbing properties, thereby improving productivity. Furthermore, as an unexpected additional effect, the amount of residual monomers contained in the hydrogel after polymerization can be reduced.

[0043] The "polyacrylic acid (salt) polymer" in one embodiment of the present invention is as described above in (1-2). The function of the "polyacrylic acid (salt) polymer" in one embodiment of the present invention is presumed to be to improve the dispersion state of the "fibrous material" described below in an aqueous monomer solution.

[0044] The amount of "polyacrylic acid (salt) polymer" used is 1 to 150 parts by mass, preferably 2 to 100 parts by mass, more preferably 3 to 75 parts by mass, and even more preferably 5 to 50 parts by mass, relative to 100 parts by mass of the monomer constituting the water absorbent resin. If the amount used is too small, the dispersibility of the fibrous material may decrease, and if it is too large, the water absorption properties of the water absorbent resin may decrease.

[0045] When the "polyacrylic acid (salt) polymer" used is a non-crosslinked polymer, the weight-average molecular weight is preferably 10,000 to 1,000,000, more preferably 20,000 to 800,000, and even more preferably 30,000 to 600,000. If the molecular weight is too low, the water absorption properties of the water-absorbent resin will decrease, and if it is too high, the viscosity of the aqueous monomer solution will increase, making it difficult to handle. The "polyacrylic acid (salt) polymer" is preferably a non-crosslinked polymer and a water-soluble polymer.

[0046] In one embodiment of the present invention, the "fibrous material" may be, for example, wood pulp fibers such as mechanical pulp, chemical pulp, semi-chemical pulp, or dissolving pulp from wood, or artificial cellulose fibers such as rayon or acetate. Synthetic polymer fibers such as polyolefin, polyester, polyacrylonitrile, or nylon may also be used. A mixture of the above-mentioned fiber types may also be used.

[0047] In one embodiment of the present invention, the "fibrous material" preferably has a fiber length of 1 to 1000 μm, and is used in an amount of 0.01 to 25 parts by mass per 100 parts by mass of the monomer. The fiber length is preferably 5 to 900 μm, and more preferably 10 to 800 μm. If the fiber length is too long, the effect of improving the polymerization rate may decrease, and if the fiber length is too short, the dispersibility of the fibrous material may decrease.

[0048] The amount of the fibers having a fiber length of 1 to 1000 μm used relative to 100 parts by mass of the monomer is preferably 0.05 to 20 parts by mass, more preferably 0.1 to 15 parts by mass. In order to improve the polymerization rate, the amount used is preferably 0.01 parts by mass or more, and in order to maintain the water absorption properties of the water absorbent resin, the amount used is preferably 25 parts by mass or less.

[0049] As long as the effect of this embodiment (the present invention) is not impaired, the material may contain fibers having a fiber length of less than 1 μm and / or more than 1000 μm.

[0050] The function of the "fibrous material" in one embodiment of the present invention is unknown, but it is presumed that the "fibrous material" acts to suppress the termination reaction of the polymerization radicals and improve the polymerization rate, for example, by increasing the viscosity of the polymerization system or by exerting some kind of interaction between the fibrous material and the polymerization radicals.

[0051] The ratio of the amount of the "polyacrylic acid (salt) polymer" to the amount of the "fibrous material" used is 99.9-80 / 0.1-20 by mass, preferably 99.8-90 / 0.2-10, and more preferably 99.5-95 / 0.5-5, from the viewpoint of dispersibility of the fibrous material.

[0052] The method for mixing the "polyacrylic acid (salt) polymer" and the "fibrous material" may be a method in which the "polyacrylic acid (salt) polymer" and the "fibrous material" are first mixed and then added to an aqueous monomer solution. Alternatively, the "polyacrylic acid (salt) polymer" and the "fibrous material" may be separately added to an aqueous monomer solution and mixed therewith. Alternatively, a method in which an already mixed state of the "polyacrylic acid (salt) polymer" and the "fibrous material" is added to an aqueous monomer solution may be used. Examples of a mixture of the "polyacrylic acid (salt) polymer" and the "fibrous material" include a mixture of recycled water-absorbent resin recovered from used absorbent articles and pulp; a mixture of a water-absorbent resin whose water-absorbency has been restored and pulp; and a mixture of a water-absorbent resin whose molecular weight has been reduced to improve its handleability and pulp. The "water-absorbent resin whose water-absorbency has been restored" is a water-absorbent resin obtained by treating the mixture of the recycled water-absorbent resin and pulp. The "treatment" is a regeneration treatment or the like that restores a water-absorbent resin with low water-absorption performance to a state with high water-absorption performance.

[0053] The "recycled water-absorbent resin" is a water-absorbent resin extracted from a used water-absorbent article, and is a resin that has been usually discarded, thermally recycled as fuel, or composted, but is made usable again for the purpose of absorbing water. In other words, the "recycled water-absorbent resin" is a water-absorbent resin whose absorption performance has been restored and which can be used for the purpose of absorbing water.

[0054] Furthermore, the mixture of the "polyacrylic acid (salt) polymer" and the "fibrous material" may be added not only in the preparation step of the aqueous monomer solution but also during the polymerization, or these forms may be used in combination.

[0055] (Other substances added to the aqueous monomer solution) In one embodiment of the present invention, from the viewpoint of improving the physical properties of the resulting water-absorbent resin, the following substances may also be added during the preparation of the aqueous monomer solution. Specifically, starch or a hydrophilic polymer of a starch derivative may be added in an amount of preferably 50% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less (the lower limit is 0% by mass); and foaming agents such as carbonates and azo compounds, surfactants, chelating agents, chain transfer agents, etc. may be added in an amount of preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less (the lower limit is 0% by mass). Furthermore, the above substances may be added not only to the aqueous monomer solution but also during polymerization, or these forms may be used in combination.

[0056] (monomer component concentration) In this step, when preparing an aqueous monomer solution, the above-mentioned substances are added. The concentration of the monomer component in the aqueous monomer solution is not particularly limited, but from the viewpoint of the physical properties of the water-absorbent resin, it is preferably 10 to 80 mass %, more preferably 20 to 75 mass %, and even more preferably 30 to 70 mass %. Furthermore, when aqueous solution polymerization or reversed-phase suspension polymerization is employed, a solvent other than water can be used in combination as necessary. In this case, the type of solvent is not particularly limited.

[0057] The "monomer component concentration" is a value calculated by the following formula (1), and the mass of the aqueous monomer solution does not include the masses of the graft component, the water-absorbent resin, the hydrophobic solvent in the reversed-phase suspension polymerization, etc.

[0058] (Concentration of monomer component (mass%)) = (mass of monomer component) / (mass of aqueous monomer solution) × 100 (1) (2-2) Polymerization process This step is a step of polymerizing the acrylic acid (salt)-based monomer aqueous solution obtained in the above-mentioned step of preparing the aqueous monomer solution to obtain a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel").

[0059] (Polymerization initiator) The polymerization initiator used in one embodiment of the present invention is not particularly limited, as it is appropriately selected depending on the polymerization form, etc., and examples thereof include thermally decomposable polymerization initiators, photodecomposable polymerization initiators, and redox-based polymerization initiators used in combination with a reducing agent that promotes the decomposition of these polymerization initiators. Specifically, one or more of the polymerization initiators disclosed in U.S. Pat. No. 7,265,190 are used. From the viewpoint of ease of handling of the polymerization initiator and / or the physical properties of the water-absorbent resin, preferably a peroxide or an azo compound is used, more preferably a peroxide, and even more preferably a persulfate.

[0060] The amount of the polymerization initiator used is preferably 0.001 to 1 mol %, more preferably 0.001 to 0.5 mol %, based on the monomer. The amount of the reducing agent used is preferably 0.0001 to 0.02 mol % based on the monomer. Note that instead of the polymerization initiator, the polymerization reaction may be carried out by irradiating with active energy rays such as radiation, electron beams, and ultraviolet rays, or these active energy rays may be used in combination with the polymerization initiator. Note that the "mol %" based on the monomer refers to the percentage of the number of moles of the reducing agent relative to the total number of moles of the monomer contained in the aqueous monomer solution.

[0061] (Mixture of polyacrylic acid (salt) polymer and fibrous material) As described above in "(2-1) Step of preparing aqueous monomer solution," the "mixture of a polyacrylic acid (salt) polymer and a fibrous material" may be added in advance in the step of preparing an aqueous monomer solution, or may be added separately from the aqueous monomer solution in the polymerization step.

[0062] (polymerization form) The polymerization mode applied to one embodiment of the present invention is not particularly limited, but from the viewpoint of water absorption characteristics, ease of polymerization control, etc., preferred examples include spray droplet polymerization, aqueous solution polymerization, and reversed-phase suspension polymerization, more preferred examples are aqueous solution polymerization and reversed-phase suspension polymerization, and even more preferred examples are aqueous solution polymerization. Among these, continuous aqueous solution polymerization is particularly preferred, and either continuous belt polymerization or continuous kneader polymerization can be applied.

[0063] As specific polymerization modes, continuous belt polymerization is disclosed in U.S. Patent Nos. 4,893,999, 6,241,928, U.S. Patent Application Publication No. 2005 / 215,734, etc., and continuous kneader polymerization is disclosed in U.S. Patent Nos. 6,987,151, 6,710,141, etc. By employing these continuous aqueous solution polymerizations, the production efficiency of a water absorbent resin is improved.

[0064] Furthermore, preferred forms of the continuous aqueous solution polymerization include "high-temperature initiated polymerization" and "high-concentration polymerization." "High-temperature initiated polymerization" refers to a form in which polymerization is initiated at a temperature of the aqueous monomer solution of preferably 30°C or higher, more preferably 35°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher (upper limit is the boiling point). "High-concentration polymerization" refers to a form in which polymerization is carried out at a monomer concentration of preferably 30% by mass or higher, more preferably 35% by mass or higher, even more preferably 40% by mass or higher, and particularly preferably 45% by mass or higher (upper limit is the saturated concentration). These polymerization forms can also be used in combination.

[0065] In one embodiment of the present invention, the polymerization can be carried out in an air atmosphere, but from the viewpoint of the color tone of the obtained water-absorbent resin, it is preferable to carry out the polymerization in an inert gas atmosphere such as nitrogen or argon. In this case, for example, it is preferable to control the oxygen concentration to 1% by volume or less. It is also preferable to replace the dissolved oxygen in the aqueous monomer solution with an inert gas (for example, dissolved oxygen: less than 1 mg / L).

[0066] In one embodiment of the present invention, foam polymerization can also be carried out by dispersing bubbles (particularly the above-mentioned inert gases or the like) in an aqueous monomer solution.

[0067] In one embodiment of the present invention, the solid content concentration may be increased during polymerization. The degree of increase in solid content, which serves as an index of such an increase in solid content concentration, is defined by the following formula (2). The degree of increase in solid content concentration is preferably 1% by mass or more, and more preferably 2% by mass or more.

[0068] (Increase in solid content (mass%)) = (solid content concentration (mass%) of hydrogel after polymerization) - (solid content concentration (mass%) of aqueous monomer solution) (2) Here, the solid content concentration of the aqueous monomer solution is a value calculated by the following formula (3), and the components in the polymerization system are the aqueous monomer solution, the graft component, the water-absorbent resin, and other solids (for example, water-insoluble fine particles), but do not include the hydrophobic solvent in the reversed-phase suspension polymerization.

[0069] (Solid content of monomer aqueous solution concentration (mass%) = (mass of (monomer component + graft component + water-absorbent resin + other solids)) / (mass of components in the polymerization system) × 100 (3) [3] Manufacturing method of water-absorbent resin (after polymerization process) Hereinafter, a method for producing a water-absorbing resin according to one embodiment of the present invention (from the polymerization step onwards) will be described.

[0070] (3-1) Gel crushing process In this step, the hydrogel obtained in the polymerization step is pulverized using a gel pulverizer such as a screw extruder (e.g., a kneader or a meat chopper) or a cutter mill to obtain a particulate hydrogel (hereinafter referred to as "particulate hydrogel"). When the polymerization step is kneader polymerization, the polymerization step and the gel pulverization step are carried out simultaneously. When the particulate hydrogel is obtained directly during the polymerization process, such as in gas-phase polymerization or reversed-phase suspension polymerization, the gel pulverization step may not be carried out. Regarding gel pulverization conditions and forms other than those described above, the contents disclosed in WO 2011 / 126079 are preferably applied to the present invention.

[0071] (3-2) Drying process This step is a step of obtaining a dried polymer by drying the particulate hydrogel obtained in the polymerization step and / or gel crushing step to a desired resin solid content. The resin solid content is determined from the loss on drying (the change in mass when 1 g of the water-absorbent resin is heated at 180°C for 3 hours), and is preferably 80 mass% or more, more preferably 85 to 99 mass%, even more preferably 90 to 98 mass%, and particularly preferably 92 to 97 mass%.

[0072] The method for drying the particulate hydrogel is not particularly limited, and examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, high-humidity drying using high-temperature water vapor, etc. Among these, from the viewpoint of drying efficiency, hot air drying is preferred, and band drying in which hot air drying is performed on a ventilated belt is more preferred.

[0073] The drying temperature (hot air temperature) in the hot air drying is preferably 120 to 250°C, more preferably 150 to 200°C, from the viewpoints of the color tone of the water-absorbent resin, drying efficiency, etc. Drying conditions other than the drying temperature, such as the hot air speed and drying time, may be appropriately set depending on the water content and total mass of the particulate hydrogel to be dried and the target resin solid content. When band drying is performed, the conditions described in WO 2006 / 100300, WO 2011 / 025012, WO 2011 / 025013, WO 2011 / 111657, etc. are appropriately applied.

[0074] (3-3) Grinding process, classification process

[0113] This step is a step of pulverizing the dried polymer obtained in the above drying step (pulverizing step), and adjusting the particle size to a predetermined range (classifying step), to obtain a water absorbent resin powder (for convenience, a powdery water absorbent resin before being subjected to surface cross-linking is referred to as "water absorbent resin powder").

[0075] Examples of equipment used in the pulverization step in one embodiment of the present invention include high-speed rotary pulverizers such as roll mills, hammer mills, screw mills, and pin mills, vibration mills, knuckle-type pulverizers, and cylindrical mixers, which are used in combination as necessary. Furthermore, the particle size adjustment method in the classification step in one embodiment of the present invention is not particularly limited, and examples thereof include sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)), airflow classification, and the like. The particle size adjustment of the water-absorbent resin is not limited to the above-mentioned pulverization step and classification step, and can be appropriately carried out in a polymerization step (particularly reversed-phase suspension polymerization and spray-droplet polymerization) or other steps (for example, a granulation step, a fine powder recovery step).

[0076] The water absorbent resin powder obtained in one embodiment of the present invention has a weight average particle diameter (D50) of preferably 200 to 600 μm, more preferably 200 to 550 μm, even more preferably 250 to 500 μm, and particularly preferably 350 to 450 μm. The proportion of particles having a particle diameter (D50) of less than 150 μm is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less. The proportion of particles having a particle diameter (D50) of 850 μm or more is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. The lower limit of the proportion of these particles is preferably as small as possible, and 0% by mass is desirable, but it may be about 0.1% by mass. These particle sizes are measured using standard sieves in accordance with the measurement methods disclosed in U.S. Pat. No. 7,638,570 and EDANA ERT 420.2-02.

[0077] The above-mentioned particle size is applied not only to the water absorbent resin after surface cross-linking (hereinafter, for convenience, may be referred to as "water absorbent resin particles"), but also to the water absorbent resin as a final product. Therefore, in the water absorbent resin particles, it is preferable to perform a surface cross-linking treatment (surface cross-linking step) so as to maintain a particle size in the above range, and it is more preferable to adjust the particle size by providing a particle size regulation step after the surface cross-linking step.

[0078] (3-4) Surface crosslinking process This step is a step of providing a portion with higher crosslink density on a surface layer (a portion several tens of μm from the surface of the water absorbent resin powder) of the water absorbent resin powder obtained through the above-mentioned steps, and is composed of a mixing step, a heat treatment step and a cooling step (optional).

[0079] In the surface cross-linking step, a surface-cross-linked water-absorbent resin (water-absorbent resin particles) is obtained by radical cross-linking, surface polymerization, cross-linking reaction with a surface cross-linking agent, etc. on the surface of the water-absorbent resin powder.

[0080] (Surface cross-linking agent) The surface cross-linking agent used in one embodiment of the present invention is not particularly limited, and examples thereof include organic or inorganic surface cross-linking agents. Among these, organic surface cross-linking agents that react with carboxyl groups are preferred from the viewpoints of the physical properties of the water-absorbent resin, the handleability of the surface cross-linking agent, and the like. For example, one or more surface cross-linking agents disclosed in U.S. Patent No. 7,183,456 may be used. More specifically, examples thereof include polyhydric alcohol compounds, epoxy compounds, haloepoxy compounds, polyamine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, oxazolidinone compounds, polyvalent metal salts, alkylene carbonate compounds, and cyclic urea compounds.

[0081] The amount of the surface crosslinking agent used (when a plurality of agents are used, the total amount used) is preferably 0.01 to 10 parts by weight, more preferably 0.01 to 5 parts by weight, relative to 100 parts by weight of the water absorbent resin powder. Further, the surface crosslinking agent is preferably added as an aqueous solution, and in this case, the amount of water used is preferably 0.1 to 20 parts by weight, more preferably 0.5 to 10 parts by weight, relative to 100 parts by weight of the water absorbent resin powder. Furthermore, when a hydrophilic organic solvent is used as necessary, the amount used is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, relative to 100 parts by weight of the water absorbent resin powder.

[0082] In addition, each of the additives to be added in the "re-moistening step" described later can be added by mixing it with the surface cross-linking agent (aqueous solution) in a range of 5 parts by weight or less, or can be added separately in this mixing step, etc.

[0083] (Mixing process)

[0044] This step is a step of mixing a water absorbent resin powder with the above-mentioned surface crosslinking agent. A method of mixing the surface crosslinking agent is not particularly limited. For example, a method of preparing a surface crosslinking agent solution in advance, and mixing the solution with the water absorbent resin powder by preferably spraying or dropping, more preferably spraying, can be mentioned.

[0084] The device for carrying out the mixing is not particularly limited, but is preferably a high-speed stirring mixer, more preferably a high-speed stirring continuous mixer.

[0085] (heat treatment process) This step is a step in which heat is applied to the mixture discharged from the above-mentioned mixing step to cause a crosslinking reaction on the surface of the water-absorbent resin powder.

[0086] The apparatus for carrying out the crosslinking reaction is not particularly limited, but a paddle dryer is preferred. The reaction temperature in the crosslinking reaction is appropriately set depending on the type of surface crosslinking agent used, but is preferably 50 to 300°C, more preferably 100 to 200°C.

[0087] (cooling process) This step is an optional step that is installed as needed after the heat treatment step. The cooling device is not particularly limited, but is preferably a device with the same specifications as the device used in the heat treatment step, and more preferably a paddle dryer. This is because it can be used as a cooling device by changing the heat medium to a refrigerant. Note that the water-absorbent resin particles obtained in the heat treatment step are forcibly cooled, as needed, to preferably 40 to 80°C, more preferably 50 to 70°C, in the cooling step.

[0088] (3-5) Rehumidification process This step is a step of adding at least one kind of additive selected from the group consisting of a polyvalent metal salt compound, a cationic polymer, a chelating agent, an inorganic reducing agent, and an α-hydroxycarboxylic acid compound described below to the water absorbent resin particles obtained in the surface cross-linking step.

[0089] It should be noted that, since the additive is added as an aqueous solution or a slurry liquid, the water absorbent resin particles are swollen with water again. For this reason, this step is referred to as a "re-moistening step". In addition, as described above, the additive can also be mixed with the water absorbent resin powder simultaneously with the surface cross-linking agent (aqueous solution).

[0090] (Polyvalent metal salt and / or cationic polymer) In one embodiment of the present invention, it is preferable to add a polyvalent metal salt and / or a cationic polymer from the viewpoint of improving the water absorption rate, liquid permeability, moisture absorption fluidity, etc. of the resulting water-absorbent resin.

[0091] Specific examples of the polyvalent metal salt and / or cationic polymer that can be used in one embodiment of the present invention include the compounds disclosed in “[7] Polyvalent metal salt and / or cationic polymer” of WO 2011 / 040530 and the amounts used thereof.

[0092] (chelating agent) In one embodiment of the present invention, it is preferable to add a chelating agent from the viewpoint of the color tone (prevention of coloration) of the obtained water absorbent resin, prevention of deterioration, etc. Specifically, as the chelating agent, the compounds and the amounts used thereof disclosed in “(2) Chelating Agents” of WO 2011 / 040530 are applicable to one embodiment of the present invention.

[0093] (inorganic reducing agent) In one embodiment of the present invention, it is preferable to add an inorganic reducing agent from the viewpoint of the color tone (prevention of coloration) of the obtained water-absorbent resin, prevention of deterioration, reduction of residual monomers, and the like.

[0094] As the inorganic reducing agent, specifically, the compounds and the amounts used thereof disclosed in “(3) Inorganic Reducing Agents” of WO 2011 / 040530 are applicable to one embodiment of the present invention.

[0095] (α-hydroxycarboxylic acid compounds) In one embodiment of the present invention, it is preferable to add an α-hydroxycarboxylic acid from the viewpoint of the color tone (prevention of coloration) of the obtained water absorbent resin, etc. The "α-hydroxycarboxylic acid compound" refers to a carboxylic acid having a hydroxyl group in the molecule or a salt thereof, and is a hydroxycarboxylic acid having a hydroxyl group at the α-position.

[0096] Specifically, the compounds and amounts used thereof disclosed in "(6) α-hydroxycarboxylic acid compounds" of WO 2011 / 040530 are applicable to one embodiment of the present invention as the α-hydroxycarboxylic acid compounds.

[0097] (3-6) Addition of other additives In one embodiment of the present invention, additives other than the additives described above can be added to the water-absorbent resin to impart various functions. Specific examples of such additives include surfactants, compounds having phosphorus atoms, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, thermoplastic fibers, etc. The surfactants are compounds disclosed in International Publication No. 2005 / 075070, and the water-insoluble inorganic fine particles are compounds disclosed in "(5) Water-insoluble inorganic fine particles" in International Publication No. 2011 / 040530, both of which are applicable to one embodiment of the present invention.

[0098] The amount of the additive used (added amount) is not particularly limited since it is appropriately determined depending on the application, but is preferably 3 parts by mass or less, more preferably 1 part by mass or less, relative to 100 parts by mass of a water absorbent resin powder. In addition, the additive can also be added in a step different from the above step.

[0099] (3-7) Other processes In one embodiment of the present invention, in addition to the above-mentioned steps, a granulation step, a sizing step, a fine powder removal step, a fine powder recycling step, etc. can be provided as necessary. Furthermore, one or more steps such as a transportation step, a storage step, a packaging step, and a preservation step may be further included. The "sizing step" includes a fine powder removal step subsequent to the surface cross-linking step, a step of classifying and pulverizing when the water absorbent resin aggregates and exceeds a desired size, etc. Furthermore, the "fine powder recycling step" includes a form in which the fine powder is added as it is as in one embodiment of the present invention, as well as a step of making the fine powder into a large hydrogel and adding it to any step in the production process of the water absorbent resin.

[0100] The present invention includes the following inventions.

[0101] [1] The present invention relates to a method for polymerizing a water-absorbent resin, which essentially comprises an aqueous monomer solution containing acrylic acid (salt) and an internal crosslinking agent, and which is carried out in the presence of the following i) and ii): i) Polyacrylic acid (salt) polymer ii) Fibrous materials [2] The method for polymerizing a water-absorbent resin according to [1] above, wherein the fibrous material uses fibers having a fiber length of 1 to 1000 μm in an amount of 0.01 to 25 parts by mass per 100 parts by mass of the monomer contained in the aqueous monomer solution.

[0102] [3] The method for polymerizing a water-absorbent resin according to [1] or [2], wherein 1 to 150 parts by mass of the polyacrylic acid (salt) polymer is added to 100 parts by mass of the monomer contained in the aqueous monomer solution.

[0103] [4] The method for polymerizing a water-absorbent resin according to any one of [1] to [3], wherein a mass ratio of the polyacrylic acid (salt) polymer to the fibrous material is 99.9 / 0.1 to 80 / 20.

[0104] [5] The method for polymerizing a water-absorbent resin according to any one of [1] to [4], wherein the polyacrylic acid (salt) polymer contains 80 mol % or more of structural units derived from acrylic acid (salt) monomers based on all structural units of the polymer.

[0105] [6] The method for polymerizing a water-absorbent resin according to any one of [1] to [5] above, wherein the fibrous material is at least one selected from the group consisting of wood pulp fibers, artificial cellulose fibers, and synthetic polymer fibers.

[0106] [7] A method for producing a water-absorbent resin, comprising the polymerization method according to any one of [1] to [6] above, and a step of drying a hydrogel produced by the polymerization method.

[0107] [8] The method for producing a water-absorbent resin according to [7] above, further comprising a step of pulverizing the hydrogel before the drying step.

[0108] [9] A method for producing the water-absorbing resin according to either of [7] or [8] above, which comprises a surface cross-linking step.

[0109]

[10] A water-absorbing resin produced by the production method according to any one of [7] to [9] above.

[0110] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Example]

[0111] The present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited to these examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention.

[0112] Unless otherwise noted, the electric devices used in the examples and comparative examples were powered by a power supply of 200 V or 100 V. Furthermore, unless otherwise noted, the physical properties of the polymer and water-absorbent resin in one embodiment of the present invention were measured under conditions of room temperature (20 to 25°C) and a relative humidity of 50% RH.

[0113] [Physical property measurements] Hereinafter, measurements of physical properties related to the polymerization method for a water-absorbent resin according to one embodiment of the present invention will be described.

[0114] a) Solid content concentration The solid content concentration of the aqueous polymer solution was measured from the residue m1 (g) obtained by drying a sample m0 (g) at 180°C for 3 hours, using the following formula. Solid content concentration (mass%) = 100×m1 / m0 b) Weight average molecular weight (Mw) of the polymer (Sample preparation) The solution was diluted with a GPC solution (solvent described below: 60 mM sodium dihydrogen phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, and 400 ppm sodium azide aqueous solution) to a polymer component concentration of 0.1% by mass, and then passed through a filter (GL Sciences, GL Chromatodisc, aqueous 25A, 0.2 μm pore size). Measurements were performed on this solution under the following conditions:

[0115] (GPC measurement conditions) Measurements were carried out using a TDA302 (registered trademark) manufactured by Viscotec Co., Ltd. The instrument is equipped with a size exclusion chromatograph, a refractive index detector, a light scattering detector, and a capillary viscometer.

[0116] The measurement apparatus and conditions were as follows:

[0117] Pump and autosampler: Viscotec GPCmax Guard column: OHpak SB-G (Showa Denko K.K.) Column: Two OHpak SB-806MHQ (Showa Denko K.K.) columns connected in series Detector: Viscotec TDA302 (system temperature maintained at 30°C) Solvent: 60 mM sodium dihydrogen phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, and 400 ppm sodium azide in water (pH 6.35–6.38) Flow rate: 0.5mL / min Injection volume: 100μL The pure water used in this measurement must be thoroughly purified of impurities. Measurements must be performed with a sufficient amount of solvent flowing through the device, ensuring a stable baseline for the detected value. In particular, measurements must be performed when there are no noise peaks in the light scattering detector.

[0118] The instrument was calibrated using polyoxyethylene glycol (weight average molecular weight (Mw) 21966, molecular weight distribution (Mw / Mn=1.0), differential refractive index (dn / dc)=0.132, solvent refractive index 1.33) as a standard sample.

[0119] c) Residual monomer amount The amount of residual monomer in an aqueous solution of acrylic acid (salt) polymer was measured using high performance liquid chromatography (HPLC). The amount of residual monomer refers to the amount of monomer dissolved in the aqueous solution relative to the polymer solid content in the sample solution.

[0120] d) Fiber length and mass ratio of fibrous material The fibrous material was photographed using a scanning electron microscope (Keyence Corporation: VF-9800) at a magnification of 50 to 500. From the photographed image, 50 points in the sample were randomly selected, and the fiber length of each was measured.

[0121] The mass ratio of components with fiber lengths of 1 to 1000 μm to the total mass of the 50 selected fibers was calculated using the following formula, assuming that fiber length is proportional to weight (all fiber widths are equal). Mass ratio (mass%) of components with fiber lengths of 1 to 1000 μm = {(Total fiber length of components with fiber length 1 to 1000 μm) / (Total fiber length of 50 fibers)} x 100 e) Induction time until polymerization begins In the polymerization process of the water absorbent resin, after mixing the aqueous monomer solution and the like with the aqueous initiator solution, the time from "pouring the mixed solution into a vat-type container until the solution begins to expand and foam" was measured with a stopwatch, and this was taken as the "induction time until the start of polymerization." The value was rounded off to the nearest whole number.

[0122] f) Weight average particle size The weight-average particle diameter (D50) of the water-absorbent resin is determined by the "(3) Mass-Average Particle Diameter (D50)" method described in columns 27 and 28 of U.S. Patent No. 7,638,570. 」 Measurements were made in accordance with the

[0123] g)CRC (Water absorption capacity without pressure) In one embodiment of the present invention, the CRC (absorbency without load) of the water-absorbent resin was measured in accordance with the EDANA method (ERT441.2-02).

[0124] h) AAP (water absorption capacity under pressure) In one embodiment of the present invention, the AAP (absorbency under pressure) of the water-absorbent resin was measured in accordance with the EDANA method (ERT442.2-02).

[0125] [Production Example 1] (Production of aqueous polyacrylate solution) A reaction solution was prepared by dissolving 0.28 parts of 3-mercaptopropionic acid in 340 parts of a 36% by mass aqueous solution of acrylic acid, and then degassing the reaction solution under a nitrogen gas atmosphere for 30 minutes.

[0126] The reaction solution was then fed into a jacketed stainless steel double-arm kneader equipped with two sigma blades and a retractable lid, and the system was purged with nitrogen gas while maintaining the reaction solution at 30°C. Subsequently, 0.20 parts of sodium persulfate and 0.009 parts of L-ascorbic acid were added while stirring the reaction solution, and polymerization began approximately 1 minute later. Polymerization was then carried out at 20 to 97°C, and the aqueous polymer solution was removed 60 minutes after the start of polymerization. The resulting aqueous polymer solution was allowed to stand at 60°C for 16 hours to mature the polymerization. The aged aqueous polymer solution was diluted with 400.0 g of ion-exchanged water, and then 100.6 g of a 48% by mass aqueous sodium hydroxide solution was added to neutralize the polymer, yielding an aqueous polyacrylate solution (1).

[0127] The resulting aqueous polyacrylate solution (1) had a solids concentration of 35.1% by mass, a weight-average molecular weight of 217,000, and a residual monomer amount of 0.28% by mass.

[0128] [Production Example 2] (Short-fiber long pulp production) Pulp fibers were extracted from the absorbent portion of a commercially available disposable diaper (Goon (registered trademark) manufactured by Daio Paper Co., Ltd.) and crushed in a mortar. The crushed pulp fibers were sieved through a JIS standard sieve with a mesh size of 150 μm, and the material that passed through the sieve was collected to obtain short-fiber long pulp (1). The obtained short-fiber long pulp (1) contained 83.6% by mass of pulp with a fiber length in the range of 1 to 1000 μm.

[0129] [Production Example 3] (Production of polyacrylate / short fiber long pulp mixtures) 100.0 g of the polyacrylate aqueous solution (1) obtained in Production Example 1 and 0.84 g of the short fiber pulp (1) obtained in Production Example 2 were added to a container and mixed by stirring for 10 minutes using a homomixer. This resulted in a polyacrylate / short fiber pulp mixture (1) in which the short fiber pulp was dispersed in the polyacrylate aqueous solution. The short fiber pulp (1) contained 0.70 g of pulp with a fiber length in the range of 1 to 1000 μm.

[0130] [Production Example 4] (Production of polyacrylate / short fiber long pulp mixtures) 78.0 g of the polyacrylate aqueous solution (1) obtained in Production Example 1, 3.94 g of the short-fiber pulp (1) obtained in Production Example 2, and 100.0 g of deionized water were added to a container and stirred and mixed for 10 minutes using a homomixer. This resulted in a polyacrylate / short-fiber pulp mixture (2) in which the short-fiber pulp was dispersed in the polyacrylate aqueous solution. The short-fiber pulp (1) contained 3.29 g of pulp with a fiber length in the range of 1 to 1,000 μm.

[0131] [Production Example 5] (Production of polyacrylate / short fiber long pulp mixtures) 156.1 g of the polyacrylate aqueous solution (1) obtained in Production Example 1 and 0.66 g of the short fiber pulp (1) obtained in Production Example 2 were added to a container and mixed with stirring for 10 minutes using a homomixer. This resulted in a polyacrylate / short fiber pulp mixture (3) in which the short fiber pulp was dispersed in the polyacrylate aqueous solution. The short fiber pulp (1) contained 0.55 g of pulp with a fiber length in the range of 1 to 1000 μm.

[0132] [Production Example 6] (Production of short fiber long pulp dispersion) 100.0 g of deionized water and 0.66 g of the short fiber pulp (1) obtained in Production Example 2 were added to a container and stirred and mixed for 10 minutes using a homomixer to obtain a short fiber pulp dispersion (1) in which the short fiber pulp was dispersed in deionized water. The short fiber pulp (1) contained 0.55 g of pulp having a fiber length in the range of 1 to 1000 μm.

[0133] [Production Example 7] (Manufacturing of long-fiber pulp) Pulp fibers were extracted from the absorbent portion of a commercially available disposable diaper (GOOON (registered trademark) manufactured by Daio Paper Co., Ltd.). The extracted pulp fibers were sieved through a JIS standard sieve with a mesh size of 2 mm, and the material that did not pass through the sieve was collected to obtain a long fiber pulp (1). The obtained long fiber pulp (1) contained 93.7% by mass of pulp with a fiber length of more than 1000 μm.

[0134] [Production Example 8] (Production of polyacrylate / long fiber pulp mixtures) 78.1 g of the polyacrylate aqueous solution (1) obtained in Production Example 1 and 0.54 g of the long-fiber pulp (1) obtained in Production Example 7 were added to a container and stirred and mixed for 10 minutes using a homomixer. This resulted in a polyacrylate / long-fiber pulp mixture (1) in which the long-fiber pulp was dispersed in the polyacrylate aqueous solution. The long-fiber pulp (1) contained 0.51 g of pulp with a fiber length of more than 1,000 μm.

[0135] Example 1 (Production of water-absorbent resin) A 2-liter polypropylene container was charged with 389.5 g of acrylic acid, 0.99 g of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 129.7 g of 48.5% by mass aqueous sodium hydroxide solution, and 300.0 g of deionized water, and mixed to prepare an aqueous monomer solution (a'). The temperature of the aqueous monomer solution (a') rose to 67.4°C due to the heat of neutralization in the first stage immediately after preparation.

[0136] Next, the aqueous monomer solution (a') was cooled with stirring. When the liquid temperature reached 38°C, 195.7 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (a') and mixed to produce an aqueous monomer solution (a). At this time, the temperature of the aqueous monomer solution (a) rose to 82.9°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0137] Next, 78.7 g of the polyacrylate / short fiber long pulp mixture (1) adjusted to 80° C. was added to the solution, and the mixture was stirred for 30 seconds. The polymerization concentration was 49.4% by mass.

[0138] Next, 16.5 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred mixture of the aqueous monomer solution (a) and the polyacrylate / short-fiber pulp mixture (1). The resulting mixture was immediately poured into a stainless steel batt-shaped container (bottom 340 × 340 mm, height 25 mm, inner surface: Teflon (registered trademark) coated) in an open-to-air environment. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (a) into the batt-shaped container was 55 seconds, and the batt-shaped container was heated to a surface temperature of 40°C using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.). The "start of the second-stage neutralization" refers to the point at which 195.7 g of a 48.5% by mass aqueous solution of sodium hydroxide adjusted to 40°C was started to be added to the aqueous monomer solution (a').

[0139] The polymerization reaction started 48 seconds after the aqueous monomer solution (a) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, the hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (1) was removed. Note that this series of operations was carried out in an open-air system.

[0140] The hydrogel (1) obtained by the polymerization reaction was crushed using a meat chopper (HL-3225N, plate hole diameter: 10.0 mm / Remacom Co., Ltd.) to obtain a particulate hydrogel (1). The amount of residual monomer in the hydrogel (1) was measured and found to be 0.95% by mass.

[0141] The particulate hydrogel (1) obtained by the above procedure was spread on a stainless steel wire mesh with an opening of 850 μm, and dried by passing hot air through it at 180° C. for 30 minutes.

[0142] Next, the dried product was pulverized using a vibration mill, and further passed through a sieve with 850 μm openings and remained on a 106 μm opening to obtain irregularly pulverized water absorbent resin precursor (1) with a weight average particle size of 411 μm.

[0143] 100 parts by mass of the obtained water absorbent resin precursor (1) was mixed with a surface cross-linking agent composition liquid consisting of 0.04 parts by mass of ethylene glycol diglycidyl ether, 0.9 parts by mass of propylene glycol, and 3 parts by mass of water. The obtained mixture was heat-treated at 210°C for 40 minutes to obtain a white water absorbent resin (1). The water absorbent resin (1) had a weight average particle diameter of 403 μm, a CRC of 37.5 (g / g), and an AAP0.3 of 28.8 (g / g).

[0144] Comparative Example 1 A 2-liter polypropylene container was charged with 410.0 g of acrylic acid, 1.04 g of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 136.6 g of 48.5% by mass sodium hydroxide aqueous solution, and 398.0 g of deionized water, and mixed to prepare an aqueous monomer solution (b'). The temperature of the aqueous monomer solution (b') rose to 62.0°C due to the heat of neutralization in the first stage immediately after preparation.

[0145] Next, the aqueous monomer solution (b') was cooled with stirring. When the liquid temperature reached 38°C, 206.0 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (b') and mixed to produce an aqueous monomer solution (b). At this time, the temperature of the aqueous monomer solution (b) rose to 81.2°C due to the heat of neutralization in the second stage immediately after preparation. The polymerization concentration was 49.4% by mass. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0146] Next, 17.3 g of a 4.0 mass % aqueous sodium persulfate solution was added to the stirred aqueous monomer solution (b), and the solution was immediately poured into a stainless steel bat-shaped container in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (b) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate until the surface temperature reached 40°C. The "start of the second-stage neutralization" refers to the time when 206.0 g of a 48.5 mass % aqueous sodium hydroxide solution adjusted to 40°C was started to be added to the aqueous monomer solution (b').

[0147] The polymerization reaction started 62 seconds after the aqueous monomer solution (b) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (C1) was taken out. Note that this series of operations was carried out in an open-air system.

[0148] The hydrogel (C1) obtained by the polymerization reaction was crushed using a meat chopper to obtain a particulate hydrogel (C1). The amount of residual monomer in the hydrogel (C1) was measured and found to be 1.31% by mass.

[0149] The particulate hydrogel (C1) obtained by the above procedure was spread on a stainless steel wire mesh with an opening of 850 μm, and dried by passing hot air through it at 180° C. for 30 minutes.

[0150] Next, the dried product was pulverized using a vibration mill, and further passed through a sieve with 850 μm openings and remained on a 106 μm opening, to obtain an irregularly pulverized water absorbent resin precursor (C1) having a weight average particle size of 405 μm.

[0151] 100 parts by mass of the obtained water absorbent resin precursor (C1) was mixed with a surface cross-linking agent composition liquid consisting of 0.04 part by mass of ethylene glycol diglycidyl ether, 0.9 part by mass of propylene glycol, and 3 parts by mass of water.

[0152] The resulting mixture was heat-treated at 210°C for 40 minutes to obtain a white water-absorbent resin (C1). The water-absorbent resin (C1) had a weight-average particle diameter of 401µm, a CRC of 37.2 (g / g), and an AAP0.3 of 28.6 (g / g).

[0153] Example 2 (Production of water-absorbent resin) A 2-liter polypropylene container was charged with 389.5 g of acrylic acid, 0.99 g of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 129.7 g of 48.5% by mass sodium hydroxide aqueous solution, and 300.0 g of deionized water, and mixed to prepare an aqueous monomer solution (c'). The temperature of the aqueous monomer solution (c') rose to 67.4°C due to the heat of neutralization in the first stage immediately after preparation.

[0154] Next, the aqueous monomer solution (c') was cooled with stirring. When the liquid temperature reached 38°C, 195.7 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (c') and mixed to produce an aqueous monomer solution (c). At this time, the temperature of the aqueous monomer solution (c) rose to 82.9°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0155] Next, 181.9 g of the polyacrylate / short-fiber long pulp mixture (2) adjusted to 80°C was added to the solution and stirred for 30 seconds. Subsequently, 16.5 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred mixed solution of the aqueous monomer solution (c) and the polyacrylate / short-fiber long pulp mixture (2). The mixed solution containing the aqueous sodium persulfate solution was immediately poured into a stainless steel batt-shaped container (bottom 340 x 340 mm, height 25 mm, inner surface: Teflon (registered trademark) coated) in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (c) into the batt-shaped container was 55 seconds, and the batt-shaped container was heated using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.) until the surface temperature reached 40°C. The "start of second-stage neutralization" refers to the time when 195.7 g of a 48.5 mass % aqueous sodium hydroxide solution adjusted to 40°C is started to be added to the aqueous monomer solution (c').

[0156] The polymerization reaction started 46 seconds after the aqueous monomer solution (c) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, the hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (2) was removed. Note that this series of operations was carried out in an open-air system.

[0157] The hydrogel (2) obtained by the polymerization reaction was crushed using a meat chopper (HL-3225N, plate hole diameter: 10.0 mm / Remacom Co., Ltd.) to obtain a particulate hydrogel (2). The amount of residual monomer in the hydrogel (2) was measured and found to be 0.87% by mass.

[0158] The particulate hydrogel (2) obtained by the above procedure was spread on a stainless steel wire mesh with an opening of 850 μm, and dried by passing hot air through it at 180° C. for 30 minutes.

[0159] Next, the dried product was pulverized using a vibration mill, and further passed through a sieve with 850 μm openings and remained on a 106 μm opening to obtain irregularly pulverized water absorbent resin precursor (2) with a weight average particle size of 415 μm.

[0160] 100 parts by mass of the obtained water absorbent resin precursor (2) was mixed with a surface cross-linking agent composition liquid consisting of 0.04 parts by mass of ethylene glycol diglycidyl ether, 0.9 parts by mass of propylene glycol, and 3 parts by mass of water. The obtained mixture was heat-treated at 210°C for 40 minutes to obtain a white water absorbent resin (2). The water absorbent resin (2) had a weight average particle diameter of 410 μm, a CRC of 37.4 (g / g), and an AAP0.3 of 28.8 (g / g).

[0161] Example 3 (Production of water-absorbent resin) A 2-liter polypropylene container was charged with 389.5 g of acrylic acid, 0.99 g of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 129.7 g of 48.5% by mass sodium hydroxide aqueous solution, and 300.0 g of deionized water, and mixed to prepare an aqueous monomer solution (d'). The temperature of the aqueous monomer solution (d') rose to 67.5°C due to the heat of neutralization in the first stage immediately after preparation.

[0162] Next, the aqueous monomer solution (d') was cooled with stirring. When the liquid temperature reached 38°C, 195.7 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (d') and mixed to produce an aqueous monomer solution (d). At this time, the temperature of the aqueous monomer solution (d) rose to 83.1°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0163] Next, 156.8 g of the polyacrylate / short-fiber long pulp mixture (3) adjusted to 80°C was added to the solution and stirred for 30 seconds. Subsequently, 16.5 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred mixed solution of the aqueous monomer solution (d) and the polyacrylate / short-fiber long pulp mixture (3). The mixed solution containing the aqueous sodium persulfate solution was immediately poured into a stainless steel batt-shaped container (bottom 340 x 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coated) in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (d) into the batt-shaped container was 55 seconds, and the batt-shaped container was heated using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.) until the surface temperature reached 40°C. The "start of second-stage neutralization" refers to the time when 195.7 g of a 48.5% by mass aqueous solution of sodium hydroxide adjusted to 40°C is started to be added to the aqueous monomer solution (d').

[0164] The polymerization reaction started 49 seconds after the aqueous monomer solution (d) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, the hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (3) was removed. Note that this series of operations was carried out in an open-air system.

[0165] The hydrogel (3) obtained by the polymerization reaction was crushed using a meat chopper (HL-3225N, plate hole diameter: 10.0 mm / Remacom Co., Ltd.) to obtain a particulate hydrogel (3). The amount of residual monomer in the hydrogel (3) was measured and found to be 0.99% by mass.

[0166] The particulate hydrogel (3) obtained by the above procedure was spread on a stainless steel wire mesh with an opening of 850 μm, and dried by passing hot air through it at 180° C. for 30 minutes.

[0167] Next, the dried product was pulverized using a vibration mill, and further passed through a sieve with 850 μm openings and remained on a 106 μm opening to obtain irregularly pulverized water absorbent resin precursor (3) with a weight average particle size of 400 μm.

[0168] 100 parts by mass of the obtained water absorbent resin precursor (3) was mixed with a surface cross-linking agent composition liquid consisting of 0.04 parts by mass of ethylene glycol diglycidyl ether, 0.9 parts by mass of propylene glycol, and 3 parts by mass of water. The obtained mixture was heat-treated at 210°C for 40 minutes to obtain a white water absorbent resin (3). The water absorbent resin (3) had a weight average particle diameter of 398 μm, a CRC of 37.5 (g / g), and an AAP0.3 of 28.6 (g / g).

[0169] Comparative Example 2 (Production of water-absorbent resin) A 2-liter polypropylene container was charged with 389.5 g of acrylic acid, 0.99 g of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 129.7 g of 48.5% by mass sodium hydroxide aqueous solution, and 300.0 g of deionized water, and mixed to prepare an aqueous monomer solution (e'). The temperature of the aqueous monomer solution (e') rose to 67.2°C due to the heat of neutralization in the first stage immediately after preparation.

[0170] Next, the aqueous monomer solution (e') was cooled with stirring. When the liquid temperature reached 38°C, 195.7 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (e') and mixed to produce an aqueous monomer solution (e). At this time, the temperature of the aqueous monomer solution (e) rose to 82.3°C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0171] Next, 78.1 g of a polyacrylate aqueous solution (1) adjusted to 80°C was added to the solution, and the mixture was stirred for 30 seconds. Subsequently, 16.5 g of a 4.0% by mass aqueous sodium persulfate solution was added to the stirred mixture of the monomer aqueous solution (e) and the polyacrylate aqueous solution (1). The mixture containing the sodium persulfate aqueous solution was immediately poured into a stainless steel bat-shaped container (bottom 340 x 340 mm, height 25 mm, inner surface: Teflon (registered trademark) coated) in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the monomer aqueous solution (e) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.) until the surface temperature reached 40°C. The "start of second-stage neutralization" refers to the time when 195.7 g of a 48.5% by mass aqueous solution of sodium hydroxide adjusted to 40°C starts to be added to the aqueous monomer solution (e').

[0172] The polymerization reaction started 62 seconds after the aqueous monomer solution (e) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (C2) was taken out. Note that this series of operations was carried out in an open-air system.

[0173] The hydrogel (C2) obtained by the polymerization reaction was crushed using a meat chopper (HL-3225N, plate hole diameter: 10.0 mm / Remacom Co., Ltd.) to obtain a particulate hydrogel (C2). The amount of residual monomer in the hydrogel (C2) was measured and found to be 1.28% by mass.

[0174] The particulate hydrogel (C2) obtained by the above procedure was spread on a stainless steel wire mesh with 850 μm openings and dried by passing hot air through it at 180° C. for 30 minutes.

[0175] Next, the dried product was pulverized using a vibration mill, and further passed through a sieve with 850 μm openings and remained on a 106 μm opening, to obtain an irregularly pulverized water absorbent resin precursor (C2) with a weight average particle size of 409 μm.

[0176] 100 parts by mass of the obtained water-absorbent resin precursor (C2) was mixed with a surface cross-linking agent composition liquid consisting of 0.04 parts by mass of ethylene glycol diglycidyl ether, 0.9 parts by mass of propylene glycol, and 3 parts by mass of water. The obtained mixture was heat-treated at 210°C for 40 minutes to obtain a white water-absorbent resin (C2). The water-absorbent resin (C2) had a weight-average particle diameter of 404 μm, a CRC of 37.2 (g / g), and an AAP0.3 of 28.5 (g / g).

[0177] Example 4 (Production of water-absorbent resin) A 2-liter polypropylene container was charged with 389.5 g of acrylic acid, 0.99 g of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 129.7 g of 48.5% by mass aqueous sodium hydroxide solution, and 300.0 g of deionized water, and mixed to prepare an aqueous monomer solution (f'). The temperature of the aqueous monomer solution (f') rose to 67.5°C due to the heat of neutralization in the first stage immediately after preparation.

[0178] Next, the aqueous monomer solution (f') was cooled with stirring. When the liquid temperature reached 38°C, 195.7 g of a 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added to the aqueous monomer solution (f') and mixed to produce an aqueous monomer solution (f). At this time, the temperature of the aqueous monomer solution (f) rose to 82.6°C due to the heat of neutralization in the second stage immediately after production. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution.

[0179] Next, 78.6 g of the polyacrylate / long-fiber pulp mixture (1) adjusted to 80°C was added to the solution and stirred for 30 seconds. Subsequently, 16.5 g of a 4.0% by mass aqueous solution of sodium persulfate was added to the stirred mixed solution of the aqueous monomer solution (f) and the polyacrylate / long-fiber pulp mixture (1). The mixed solution containing the aqueous sodium persulfate solution was immediately poured into a stainless steel batt-shaped container (bottom 340 × 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coated) in an open-to-air system. The time from the start of the second-stage neutralization to the pouring of the aqueous monomer solution (f) into the batt-shaped container was 55 seconds, and the batt-shaped container was heated using a hot plate (NEO HOTPLATE HI-1000 / Iuchi Seieido Co., Ltd.) until the surface temperature reached 40°C. The "start of second-stage neutralization" refers to the time when 195.7 g of a 48.5% by mass aqueous solution of sodium hydroxide adjusted to 40°C starts to be added to the aqueous monomer solution (f').

[0180] The polymerization reaction started 59 seconds after the aqueous monomer solution (f) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (C3) was taken out. Note that this series of operations was carried out in an open-air system.

[0181] The hydrogel (C3) obtained by the polymerization reaction was crushed using a meat chopper (HL-3225N, plate hole diameter: 10.0 mm / Remacom Co., Ltd.) to obtain a particulate hydrogel (C3). The amount of residual monomer in the hydrogel (C3) was measured and found to be 1.33% by mass.

[0182] The particulate hydrogel (C3) obtained by the above procedure was spread on a stainless steel wire mesh with an opening of 850 μm, and dried by passing hot air through it at 180° C. for 30 minutes.

[0183] Next, the dried product was pulverized using a vibration mill, and further passed through a sieve with 850 μm openings and remained on a 106 μm opening, to obtain an irregularly pulverized water absorbent resin precursor (C3) with a weight average particle size of 415 μm.

[0184] 100 parts by mass of the obtained water-absorbent resin precursor (C3) was mixed with a surface cross-linking agent composition liquid consisting of 0.04 parts by mass of ethylene glycol diglycidyl ether, 0.9 parts by mass of propylene glycol, and 3 parts by mass of water. The obtained mixture was heat-treated at 210°C for 40 minutes to obtain a white water-absorbent resin (C3). The water-absorbent resin (C3) had a weight-average particle diameter of 408 μm, a CRC of 37.0 (g / g), and an AAP0.3 of 28.5 (g / g).

[0185] Table 1 shows the polymerization results of the examples and comparative examples, and the physical properties of the obtained water-absorbent resins.

[0186] [Table 1] The results in Table 1 show that Examples 1 to 4, in which polymerization was carried out by adding a mixture of polyacrylic acid (salt) polymer and pulp fiber, had a shorter induction time and a higher polymerization rate than Comparative Examples 1 and 2, in which polymerization was carried out without adding these. In particular, Examples 1 to 3, in which a mixture of polyacrylic acid (salt) polymer and short-fiber long pulp was used, showed a greater effect of improving the polymerization rate. Furthermore, an unexpected effect of reducing the amount of residual monomer in the hydrogel was also observed.

[0187] Furthermore, the water-absorbing properties of the water-absorbent resins obtained by subjecting the obtained hydrogel to the steps of drying, pulverization, and surface treatment were also found to be slightly superior in Examples 1 to 3 to those in Comparative Examples 1 and 2.

[0188] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

Claims

1. A method for polymerizing a water-absorbent resin, which essentially comprises an aqueous monomer solution containing acrylic acid (salt) and an internal crosslinking agent, and which initiates polymerization in the presence of the following i) and ii): i) Polyacrylic acid (salt) polymer ii) Fibrous materials The fibrous material is a fiber having a fiber length of 1 to 1000 μm, and is used in an amount of 0.01 to 25 parts by mass per 100 parts by mass of the monomer contained in the aqueous monomer solution. The fibrous material is wood pulp fiber.

2. 2. The method for polymerizing a water-absorbent resin according to claim 1, wherein 1 to 150 parts by mass of the polyacrylic acid (salt) polymer is added to 100 parts by mass of the monomer contained in the aqueous monomer solution.

3. 2. The method for polymerizing a water-absorbent resin according to claim 1, wherein a mass ratio of the polyacrylic acid (salt) polymer to the fibrous material is 99.9 / 0.1 to 80 / 20.

4. 2. The method for polymerizing a water-absorbent resin according to claim 1, wherein the polyacrylic acid (salt) polymer contains 80 mol % or more of structural units derived from acrylic acid (salt) monomers based on all structural units of the polymer.

5. A method for producing a water-absorbent resin, comprising the polymerization method according to any one of claims 1 to 4, and also comprising a step of drying a hydrogel produced by the polymerization method.

6. The method for producing a water-absorbent resin according to claim 5, further comprising a step of pulverizing the hydrogel before the drying step.

7. The method for producing the water-absorbent resin according to claim 5, further comprising a surface cross-linking step.

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