JP7187220B2 - Absorbent resin particles and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to absorbent resin particles and a method for producing the same.

2. Description of the Related Art Absorbent resins mainly composed of hydrophilic fibers such as pulp and acrylic acid (salt) are widely used as absorbers for sanitary materials such as disposable diapers, sanitary napkins and incontinence pads. In recent years, from the viewpoint of improving QOL (quality of life), the demand for these sanitary materials has shifted to lighter and thinner ones, and along with this, it has become desirable to reduce the amount of hydrophilic fibers used. . Therefore, there is a demand for more highly functional absorbent resins. Absorption performance desired for the above-mentioned absorbent resin includes high absorption rate, high water retention capacity, high absorption capacity under pressure, high gel elasticity, and high liquid permeability. ing.

On the other hand, hot air drying is the main method of drying hydrogel in the past. Over-drying is inevitable. As a result, the polymer is partially thermally degraded, leading to a decrease in absorbency under pressure and gel strength, an increase in residual monomer amount and water-soluble content, and the like. In addition, since the distribution of the weight loss on drying in the absorbent resin becomes non-uniform, the reaction rate of the cross-linking agent in the surface cross-linking step also becomes non-uniform, resulting in variations in the above performances. Moreover, it invites a decrease in whiteness. Furthermore, since a large amount of undried gel remains due to uneven drying of the hydrous gel, troubles such as a decrease in yield and adhesion of the undried gel to the interior of the pulverization equipment occur.

To solve this problem, a method has been proposed in which the undried gel in the dryer is separated and additionally dried (see, for example, Patent Documents 1 and 2). The uniformity is insufficient, and there are also drawbacks such as increased utility costs and equipment expansion. A method that enables uniform and rapid drying by controlling the primary particles of the hydrous gel after pulverization to a specific particle size distribution (see, for example, Patent Document 3) or a method of gently drying in multiple temperature zones (for example, , see Patent Document 4) are also known. A method of applying a second heat source such as an electromagnetic wave from the opposite direction to the hot air inlet has also been proposed (see, for example, Patent Document 5), but the problem of thermal deterioration due to hot air drying remains. It does not teach or suggest the effect of suppressing variations in absorption performance brought about by drying by irradiation.

Japanese translation of PCT publication No. 2009-516043 Japanese translation of PCT publication No. 2015-526577 JP-A-11-349687 JP 2013-053315 A JP 2007-224224 A

The object of the present invention is to reduce the variation in absorption performance, especially the water retention amount, and further to improve the coefficient of variation of the absorption characteristics under pressure, and to achieve a high whiteness with an excellent coefficient of variation and a low whiteness with an excellent coefficient of variation. An object of the present invention is to provide absorbent resin particles having a residual monomer content and a method for producing the same.

The present invention provides a crosslinked polymer (A) comprising a water-soluble vinyl monomer (a1) and/or a vinyl monomer (a2) that becomes a water-soluble vinyl monomer (a1) by hydrolysis and an internal cross-linking agent (b) as essential structural units. The surface of the absorbent resin particles is surface-crosslinked with the surface-crosslinking agent (d), and the average value (number average value; the same shall apply hereinafter) of the water retention amount measured at the analysis point N = 100 is 30 g / g or more , absorptive resin particles having a coefficient of variation determined from the unbiased standard deviation and the average value of 3% or less, and a method for producing the same, comprising a drying step of irradiating a crosslinked polymer hydrous gel with electromagnetic waves.

The absorbent resin particles of the present invention are particularly excellent in each coefficient of variation of water retention capacity and absorption characteristics under pressure, and furthermore, whiteness with high and excellent coefficient of variation and residual monomer with low and excellent coefficient of variation. It has a large amount and little variation in absorption performance. Moreover, the absorbent resin particles of the present invention have a high neutralization uniformity after drying and are excellent in absorption properties under pressure. In the production method of the present invention, electromagnetic waves are used in the drying process to uniformly dry the outside and inside of the hydrous gel, effectively suppressing thermal deterioration of the polymer and the generation of undried gel. In addition, the drying weight loss of the crosslinked polymer after drying can be made uniform, and in the absorbent resin obtained after surface cross-linking, the absorption performance can be improved and variation can be suppressed. Furthermore, as compared with the conventional drying method, it is possible to crush the polymer while the loss on drying is still high, thereby further suppressing the thermal deterioration of the polymer.

When the above products are applied to thin sanitary materials and absorbent articles with a high absorbent resin ratio, they exhibit stable and excellent absorption performance (e.g., liquid diffusibility, absorption rate, and absorption amount) under various conditions of use. and less likely to cause rashes.

The water-soluble vinyl monomer (a1) in the present invention is not particularly limited. vinyl monomers having saturated groups (e.g., anionic vinyl monomers, nonionic vinyl monomers and cationic vinyl monomers), anionic vinyl monomers disclosed in paragraphs 0009 to 0024 of JP-A-2003-165883, nonionic a carboxy group, a sulfo group, a phosphono group, a hydroxyl group, a carbamoyl group, an amino group and an ammonio group disclosed in paragraphs 0041 to 0051 of JP-A-2005-75982; Any vinyl monomer having at least one of the following can be used.

A vinyl monomer (a2) that becomes a water-soluble vinyl monomer (a1) by hydrolysis [hereinafter also referred to as a hydrolyzable vinyl monomer (a2). ] is not particularly limited, and is known {for example, a vinyl monomer having at least one hydrolyzable substituent that becomes a water-soluble substituent by hydrolysis disclosed in paragraphs 0024 to 0025 of Japanese Patent No. 3648553; At least one hydrolyzable substituent [1,3-oxo-2-oxapropylene (-CO-O-CO-) group, acyl group and cyano A vinyl monomer having a group, etc.} can be used. The term "water-soluble vinyl monomer" is a concept well known to those skilled in the art, but if expressed in terms of quantity, it means, for example, a vinyl monomer that dissolves in 100 g of water at 25° C. in an amount of at least 100 g. In addition, the hydrolyzability of the hydrolyzable vinyl monomer (a2) is a concept well known to those skilled in the art. It means the property of being hydrolyzed and becoming water soluble. The hydrolysis of the hydrolyzable vinyl monomer (a2) may be carried out during polymerization, after polymerization, or both, but from the viewpoint of the absorption performance of the resulting absorbent resin particles, it is preferably carried out after polymerization.

Among these, the water-soluble vinyl monomer (a1) is preferable from the viewpoint of absorption performance and the like, and more preferable are the above-mentioned anionic vinyl monomers, carboxy (salt) group, sulfo (salt) group, amino group, carbamoyl group, A vinyl monomer having an ammonio group or a mono-, di- or tri-alkylammonio group, more preferably a vinyl monomer having a carboxy (salt) group or a carbamoyl group, particularly preferably a (meth)acrylic acid (salt) and (Meth)acrylamide, especially preferred (meth)acrylic acid (salt), most preferred acrylic acid (salt).

In addition, "carboxy (salt) group" means "carboxy group" or "carboxylate group", and "sulfo (salt) group" means "sulfo group" or "sulfonate group". (Meth)acrylic acid (salt) means acrylic acid, acrylate, methacrylic acid or methacrylate, and (meth)acrylamide means acrylamide or methacrylamide. Examples of salts include alkali metal (lithium, sodium, potassium, etc.) salts, alkaline earth metal (magnesium, calcium, etc.) salts, ammonium (NH ₄ ) salts, and the like. Among these salts, alkali metal salts and ammonium salts are preferable, alkali metal salts are more preferable, and sodium salts are particularly preferable, from the viewpoint of absorption performance and the like.

When either the water-soluble vinyl monomer (a1) or the hydrolyzable vinyl monomer (a2) is used as a structural unit, each one type may be used alone as a structural unit, and if necessary, two or more types may be used as a structural unit. good. The same applies to the case where the water-soluble vinyl monomer (a1) and the hydrolyzable vinyl monomer (a2) are used as structural units. Further, when the water-soluble vinyl monomer (a1) and the hydrolyzable vinyl monomer (a2) are used as structural units, the molar ratio [(a1)/(a2)] of these contents is preferably 75/25 to 99/1. , more preferably 85/15 to 95/5, particularly preferably 90/10 to 93/7, most preferably 91/9 to 92/8. Within this range, the absorption performance is further improved.

In addition to the water-soluble vinyl monomer (a1) and the hydrolyzable vinyl monomer (a2), other vinyl monomers (a3) copolymerizable therewith are used as structural units of the crosslinked polymer (A). can be done. Other vinyl monomers (a3) may be used alone or in combination of two or more.

Other copolymerizable vinyl monomers (a3) are not particularly limited, and are known (for example, hydrophobic vinyl monomers disclosed in paragraphs 0028 to 0029 of Japanese Patent No. 3648553, Japanese Patent Laid-Open No. 2003-165883, 0025 paragraph and the vinyl monomers disclosed in JP-A-2005-75982 paragraph 0058) can be used, and specifically, for example, the following vinyl monomers (i) to (iii), etc. can be used. (i) Aromatic ethylenic monomer having 8 to 30 carbon atoms Styrene such as styrene, α-methylstyrene, vinyltoluene and hydroxystyrene, and halogen-substituted styrene such as vinylnaphthalene and dichlorostyrene. (ii) Aliphatic ethylenic monomers having 2 to 20 carbon atoms Alkenes (ethylene, propylene, butene, isobutylene, pentene, heptene, diisobutylene, octene, dodecene, octadecene, etc.); and alkadienes (butadiene, isoprene, etc.). (iii) C5-C15 alicyclic ethylenic monomers monoethylenically unsaturated monomers (pinene, limonene, indene, etc.); and polyethylene vinyl monomers [cyclopentadiene, bicyclopentadiene, ethylidenenorbornene, etc.], and the like.

The content (mol %) of the other vinyl monomer (a3) units is, from the viewpoint of absorption performance and the like, based on the total number of moles of the water-soluble vinyl monomer (a1) units and the hydrolyzable vinyl monomer (a2) units. It is preferably 0 to 5, more preferably 0 to 3, particularly preferably 0 to 2, and most preferably 0 to 1.5. From the viewpoint of absorption performance etc., the content of other vinyl monomer (a3) units is 0 mol % is most preferred.

The internal cross-linking agent (b) is not particularly limited and is known (for example, a cross-linking agent having two or more ethylenically unsaturated groups disclosed in paragraphs 0031 to 0034 of Japanese Patent No. 3648553, a water-soluble substituent and A cross-linking agent having at least one functional group capable of reacting with at least one ethylenically unsaturated group and a cross-linking agent having at least two functional groups capable of reacting with a water-soluble substituent, JP-A-2003-165883 A cross-linking agent having two or more ethylenically unsaturated groups, a cross-linking agent having an ethylenically unsaturated group and a reactive functional group, and a cross-linking having two or more reactive substituents disclosed in paragraphs 0028 to 0031 of the publication crosslinkable vinyl monomers disclosed in paragraph 0059 of JP-A-2005-75982 and crosslinkable vinyl monomers disclosed in paragraphs 0015-0016 of JP-A-2005-95759). can. Among these, from the viewpoint of absorption performance and the like, cross-linking agents having two or more ethylenically unsaturated groups are preferable, and more preferable are triallyl cyanurate, triallyl isocyanurate and polyols having 2 to 40 carbon atoms ( Meta)allyl ethers, particularly preferred are triallyl cyanurate, triallyl isocyanurate, tetraallyloxyethane, polyethylene glycol diallyl ether and pentaerythritol triallyl ether, most preferred is pentaerythritol triallyl ether. The internal cross-linking agent (b) may be used alone or in combination of two or more.

The content (mol%) of the internal cross-linking agent (b) unit is the water-soluble vinyl monomer (a1) unit and the hydrolyzable vinyl monomer (a2) unit, and when other vinyl monomers (a3) are also used, (a1 ) to (a3), based on the total number of moles, preferably 0.001 to 5, more preferably 0.005 to 3, and particularly preferably 0.01 to 1. Within this range, the absorption performance is further improved.

Examples of the method for producing the crosslinked polymer (A) include known solution polymerization (adiabatic polymerization, thin film polymerization, spray polymerization, etc.; JP-A-55-133413, etc.), known suspension polymerization and reverse phase suspension. Drying a water-containing gel polymer (consisting of a crosslinked polymer and water) obtained by turbidity polymerization (Japanese Patent Publication No. 54-30710, Japanese Patent Publication No. 56-26909, Japanese Patent Publication No. 1-5808, etc.), It can be obtained by grinding. The crosslinked polymer (A) may be used alone or as a mixture of two or more.

The crosslinked polymer (A) is obtained by polymerizing a monomer composition comprising a water-soluble vinyl monomer (a1) and/or a hydrolyzable vinyl monomer (a2) and an internal crosslinking agent (b) as essential components. However, the preferred polymerization method is the solution polymerization method, which is advantageous in terms of production cost because it does not require the use of an organic solvent, etc., so the aqueous solution polymerization method is particularly preferred. In addition, the aqueous solution adiabatic polymerization method is most preferable because an absorbent resin composition with a small amount of water-soluble components can be obtained and temperature control during polymerization is unnecessary.

When carrying out aqueous polymerization, a mixed solvent containing water and an organic solvent can be used. A mixture of When aqueous solution polymerization is carried out, the amount (% by weight) of the organic solvent used is preferably 40 or less, more preferably 30 or less, based on the weight of water.

When an initiator is used for polymerization, conventionally known initiators for radical polymerization can be used. hydrochloride, etc.], inorganic peroxides (hydrogen peroxide, ammonium persulfate, potassium persulfate, sodium persulfate, etc.), organic peroxides [benzoyl peroxide, di-t-butyl peroxide, cumene hydroperoxide, succinate acid peroxides and di(2-ethoxyethyl)peroxydicarbonate, etc.] and redox catalysts (alkali metal sulfites or bisulfites, reducing agents such as ammonium sulfite, ammonium bisulfite and ascorbic acid and alkali metal persulfates salts, ammonium persulfate, hydrogen peroxide, organic peroxides, etc.). These catalysts may be used alone, or two or more of them may be used in combination. The amount (% by weight) of the radical polymerization initiator used is the water-soluble vinyl monomer (a1) and the hydrolyzable vinyl monomer (a2), or (a1) to (a3) when another vinyl monomer (a3) is also used. , preferably 0.0005 to 5, more preferably 0.001 to 2, based on the total weight.

At the time of polymerization, a polymerization control agent represented by a chain transfer agent may be used in combination as necessary. Specific examples thereof include sodium hypophosphite, sodium phosphite, alkyl mercaptans, alkyl halides. , thiocarbonyl compounds and the like. These polymerization control agents may be used alone, or two or more of them may be used in combination. The amount (% by weight) of the polymerization control agent used is the water-soluble vinyl monomer (a1) and the hydrolyzable vinyl monomer (a2), or (a1) to (a3) when the other vinyl monomer (a3) is also used. , preferably 0.0005 to 5, more preferably 0.001 to 2, based on the total weight.

When a suspension polymerization method or a reverse-phase suspension polymerization method is used as the polymerization method, polymerization may be carried out in the presence of a dispersant or a surfactant, if necessary. Moreover, in the case of the reversed-phase suspension polymerization method, the polymerization can be carried out using a hydrocarbon solvent such as xylene, normal hexane and normal heptane.

The polymerization initiation temperature can be appropriately adjusted depending on the type of catalyst used, but is preferably 0 to 100°C, more preferably 5 to 80°C.

When a solvent (organic solvent, water, etc.) is used for polymerization, it is preferred to distill off the solvent after polymerization. When the solvent contains an organic solvent, the content (% by weight) of the organic solvent after distillation is preferably 0 to 10, more preferably 0 to 5, particularly preferably 0 to 5, based on the weight of the crosslinked polymer (A). is 0-3, most preferably 0-1. Within this range, the absorption performance of the absorbent resin particles is further improved.

When the solvent contains water, the loss on drying (%) after distillation is preferably 0 to 20, more preferably 1 to 10, particularly preferably 2 to 9, based on the weight of the crosslinked polymer (A). Most preferably 3-8. Within this range, the absorption performance is further improved.

The crosslinked polymer (A) obtained by the above-described polymerization method is a hydrous gel containing water (i.e., a crosslinked polymer (A) that is a hydrous gel). ) can be obtained, and a dried crosslinked polymer (A) can be obtained by drying the hydrous gel. When using an acid group-containing monomer such as acrylic acid or methacrylic acid as the water-soluble vinyl monomer (a1), the water-containing gel may be neutralized with a base. The degree of neutralization of acid groups is preferably 50 to 80 mol %. If the degree of neutralization is less than 50 mol %, the resulting hydrous gel polymer will have high adhesiveness, and workability during production and use may deteriorate. Furthermore, the water retention capacity of the resulting absorbent resin particles may decrease. On the other hand, if the degree of neutralization exceeds 80%, the resulting resin will have a high pH and may be unsafe for human skin. The neutralization may be carried out at any stage after the polymerization of the crosslinked polymer (A) in the production of the absorbent resin particles. exemplified. As the neutralizing base, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, and alkali metal carbonates such as sodium carbonate, sodium hydrogencarbonate and potassium carbonate can usually be used.

The hydrous gel obtained by polymerization can be chopped before drying, if desired. The size (maximum diameter) of the gel after shredding is preferably 50 μm to 10 cm, more preferably 100 μm to 2 cm, particularly preferably 1 mm to 1 cm. Within this range, the drying property in the drying process is further improved.

Shredding can be performed by a known method, and can be performed using a shredding device (eg, Vex mill, rubber chopper, farmer mill, mincing machine, impact pulverizer, roll pulverizer) and the like.

The content of the organic solvent and the loss on drying were measured using an infrared moisture meter [JE400 manufactured by KETT Co., Ltd., etc.: 120 ± 5 ° C., 30 minutes, atmospheric humidity before heating 50 ± 10% RH, lamp specification 100 V, 40 W].

Electromagnetic waves are preferably used in the drying step of distilling off the solvent (including water) of the hydrous gel and drying it. Therefore, the production method of the present invention includes a step of drying by irradiating the crosslinked polymer hydrous gel with electromagnetic waves. The wavelength of the electromagnetic wave can be in the range of 0.01 nm to 1×10 ¹⁷ nm, preferably microwaves of 1×10 ⁵ nm to 1×10 ⁹ nm. Also, electromagnetic waves with a single wavelength may be applied, or electromagnetic waves with a plurality of wavelengths may be applied.

The drying loss of the hydrous gel upon irradiation with electromagnetic waves is preferably 1 to 90% by weight, particularly preferably 2 to 80% by weight.

The irradiation intensity and irradiation time of the electromagnetic wave may be appropriately set according to the drying weight loss of the hydrogel before drying, the desired drying weight loss of the hydrogel, and the like. Specifically, for example, the examples of the specification can be referred to. Further, the irradiation may be divided into a plurality of times, and for example, the same or different energy amounts can be irradiated at intervals of several seconds to several minutes.

Known generators such as magnetrons, klystrons, gyrotrons, and semiconductors can be used as electromagnetic wave generators. The generated electromagnetic wave may be irradiated using a single waveguide or a plurality of waveguides, and it is preferable to irradiate using a plurality of waveguides from the viewpoint of uniformly irradiating the hydrous gel.

The air velocity supplied to the hydrous crosslinked polymer gel in the drying step is preferably 0.01 to 30 m/sec, more preferably 0.02 to 20 m/sec, and particularly preferably 0.02 to 20 m/sec, from the viewpoint of improving absorption performance and suppressing variations. is 0.03 to 15 m/sec. If the wind speed is less than 0.01 m/sec, the hydrous gel of the crosslinked polymer may be scorched and the absorption performance may be lowered. At that time, the temperature of the air stream is preferably 15 to 450°C, more preferably 20 to 400°C. If the temperature is lower than 15°C, sufficient drying may not be achieved, and if the temperature is higher than 450°C, the crosslinked polymer may be thermally deteriorated. Moreover, in the drying step, other drying methods may be used in combination as necessary, as long as the effects of the present invention are not impaired. Other drying methods that can be applied include a thin film drying method using a drum dryer heated to 100 to 230° C., a (heating) reduced pressure drying method, a freeze drying method, an infrared drying method, decantation and filtration. At that time, it is preferable to carry out while paying attention to excessive drying and local drying.

After drying the hydrous gel, it can be further pulverized. The method of pulverization is not particularly limited, and a pulverizer (for example, a hammer pulverizer, an impact pulverizer, a roll pulverizer, and a jet pulverizer) or the like can be used. The pulverized crosslinked polymer particles can be classified by sieving or the like to adjust the particle size.

The crosslinked polymer particles after classification may contain some other components such as residual solvent and residual crosslinked components depending on the case. The weight average particle diameter (μm) of the crosslinked polymer particles is preferably 100 to 800, more preferably 200 to 700, next preferably 250 to 600, particularly preferably 300 to 500, most preferably 350 to 450. . Within this range, the absorption performance is further improved.

In addition, the weight average particle size was measured using a low-tap test sieve shaker and standard sieve (JIS Z8801-1: 2006), Perry's Chemical Engineers Handbook 6th Edition (McGrow Hill Book Company, 1984). , page 21). That is, JIS standard sieves of 1000 μm, 850 μm, 710 μm, 500 μm, 425 μm, 355 μm, 250 μm, 150 μm, 125 μm, 75 μm and 45 μm and a saucer are combined in this order from the top. About 50 g of the particles to be measured are placed in the top sieve and shaken for 5 minutes with a Rotap test sieve shaker. The weight of the particles to be measured on each sieve and the tray is weighed, the total is 100% by weight, and the weight fraction of the particles on each sieve is obtained. ) and the weight fraction on the vertical axis], draw a line connecting the points, determine the particle diameter corresponding to a weight fraction of 50% by weight, and take this as the weight average particle diameter.

In addition, since the smaller the content of the fine particles contained in the crosslinked polymer (A) after pulverization, the better the absorption performance, the fine particles of 106 μm or less (preferably 150 μm or less) in the total weight of the crosslinked polymer (A) is preferably 3 or less, more preferably 1 or less. The content of fine particles can be determined using the graph created when determining the weight-average particle size.

The shape of the crosslinked polymer (A) after pulverization is not particularly limited, and examples thereof include irregular pulverized form, scaly form, pearl form and rice grain form. Of these, irregularly crushed forms are preferred from the viewpoints of good entanglement with fibrous materials for use in paper diapers and the like, and no fear of falling off from fibrous materials.

The absorbent resin particles of the present invention have a structure in which the surface of the crosslinked polymer (A) is crosslinked with the surface crosslinking agent (d). By cross-linking the surface of the crosslinked polymer (A), the gel strength of the absorbent resin particles can be improved, and the desired water retention capacity and absorption capacity under load of the absorbent resin particles can be satisfied. As the surface cross-linking agent (d), known (polyvalent glycidyl compounds, polyvalent amines, polyvalent aziridine compounds and polyvalent isocyanate compounds described in JP-A-59-189103, etc., JP-A-58-180233 And the polyhydric alcohol of JP-A-61-16903, the silane coupling agent described in JP-A-61-211305 and JP-A-61-252212, the JP-A-5-508425 described Alkylene carbonate, polyvalent oxazoline compounds described in JP-A-11-240959, and polyvalent metals described in JP-A-51-136588 and JP-A-61-257235) are used as surface cross-linking agents. can. These surface cross-linking agents include those falling under covalent surface cross-linking agents or ionic surface cross-linking agents, e.g. Cross-linking agents such as polyvalent metal salts and polyvalent amines are ion-binding surface cross-linking agents. Among these surface cross-linking agents, covalent surface cross-linking agents or ionic surface cross-linking agents are preferable from the viewpoint of economy and absorption characteristics. At least one selected from the group consisting of a functional alcohol, an alkylene carbonate, or an oxazoline is preferable, and the ion-binding surface cross-linking agent is preferably either a polyvalent metal salt or a polyvalent amine.

The surface cross-linking step is preferably carried out after the pulverization step (that is, it is preferred to have the pulverization step before the surface cross-linking step). Moreover, it is preferable to carry out after the drying step, preferably after the drying step of irradiating radio waves.

In the case of surface cross-linking, the amount (% by weight) of the surface cross-linking agent (d) used is not particularly limited because it can be varied depending on the type of surface cross-linking agent, cross-linking conditions, target performance, etc. From the viewpoint of absorption properties, etc., it is preferably 0.001 to 3, more preferably 0.005 to 2, and particularly preferably 0.01 to 1.5, relative to 100 parts by weight of the crosslinked polymer (A).

Surface cross-linking of the cross-linked polymer (A) can be carried out by mixing the cross-linked polymer (A) and the surface cross-linking agent (d) and, if necessary, heating. Examples of the method for mixing the crosslinked polymer (A) and the surface cross-linking agent (d) include a cylindrical mixer, a screw mixer, a screw extruder, a turbulizer, a Nauta mixer, a twin-arm kneader, a fluid Crosslinked polymer (A) and surface crosslinking using a mixing device such as a mixer, a V-shaped mixer, a mincing mixer, a ribbon mixer, an airflow mixer, a rotating disc mixer, a conical blender and a roll mixer A method of uniformly mixing the agent (d) can be mentioned. At this time, the surface cross-linking agent (d) may be diluted with water and/or any solvent before use.

The temperature at which the crosslinked polymer (A) and the surface cross-linking agent (d) are mixed is not particularly limited, but is preferably 10 to 150°C, more preferably 20 to 100°C, and particularly preferably 25 to 80°C. .

After mixing the crosslinked polymer (A) and the surface cross-linking agent (d), heat treatment is usually performed for surface cross-linking. The heating temperature is preferably 100 to 200°C, more preferably 110 to 175°C, particularly preferably 120 to 170°C, from the viewpoint of breaking resistance of the resin particles. If the heating temperature is 200°C or less, indirect heating using steam is possible, which is advantageous in terms of facilities. The heating time can be appropriately set depending on the heating temperature, but from the viewpoint of absorption performance, it is preferably 5 to 60 minutes, more preferably 10 to 40 minutes. It is also possible to further surface-crosslink the absorbent resin obtained by surface-crosslinking using a surface-crosslinking agent that is the same as or different from the surface-crosslinking agent used first.

After cross-linking the surface of the cross-linked polymer (A) with the surface-cross-linking agent (d), the particle size is adjusted by sieving if necessary. The average particle size of the obtained particles is preferably 100-600 μm, more preferably 200-500 μm. The content of fine particles is preferably as small as possible, the content of particles of 100 μm or less is preferably 3% by weight or less, and the content of particles of 150 μm or less is more preferably 3% by weight or less.

The absorbent resin particles of the present invention may further contain water-insoluble inorganic particles (f), and the production method of the present invention may include a step of mixing the water-insoluble inorganic particles (f). By mixing the water-insoluble inorganic particles (f), the surfaces of the absorbent resin particles are surface-treated with the water-insoluble inorganic particles (f), thereby improving blocking resistance and liquid permeability of the absorbent resin particles.

Examples of the water-insoluble inorganic particles (f) include colloidal silica, fumed silica, clay and talc. Colloidal silica and silica are preferred and more preferred from the viewpoint of availability, ease of handling and absorption performance. is colloidal silica. The water-insoluble inorganic particles (f) may be used singly or in combination of two or more.

The amount (parts by weight) of the water-insoluble inorganic particles (f) used is preferably 0.01 to 5 parts by weight, more preferably 0.05 to 1 part by weight, and particularly It is preferably 0.1 to 0.5.

When the water-insoluble inorganic particles (f) are included, it is preferable to mix the absorbent resin particles and the water-insoluble inorganic particles (f), and the mixing method includes a cylindrical mixer, a screw mixer, and a screw extruder. , turbulizer, Nauta type mixer, double arm type kneader, fluidized type mixer, V type mixer, mincing mixer, ribbon type mixer, air flow type mixer, rotating disk type mixer, conical blender and roll mixer A method of uniform mixing using a known mixing device such as.

The temperature at which the water-insoluble inorganic particles (f) are mixed is not particularly limited, but is preferably 10 to 150°C, more preferably 20 to 110°C.

The mixing time for mixing the water-insoluble inorganic particles (f) is not particularly limited, but is preferably within 1 hour, more preferably within 30 minutes.

After performing the step of mixing the water-insoluble inorganic particles (f), a step of adjusting the particle size of the absorbent resin particles may be performed. The particle size adjustment can be performed in the same manner as the particle size adjustment described above, and the particle size after the particle size adjustment is the same.

The absorbent resin particles of the present invention may optionally contain additives (for example, known (described in JP-A-2003-225565 and JP-A-2006-131767) preservatives, antifungal agents, antibacterial agents, oxidation inhibitors, ultraviolet absorbers, colorants, fragrances, deodorants, liquid permeability improvers, organic fibrous substances, etc.). When these additives are contained, the content (% by weight) of the additives is preferably 0.001 to 10, more preferably 0.01 to 5, especially 0.01 to 5, based on the weight of the crosslinked polymer (A). It is preferably between 0.05 and 1, most preferably between 0.1 and 0.5.

The absorbent resin particles of the present invention may further contain a polyvalent metal salt (e). By containing the polyvalent metal salt (e), the blocking resistance and liquid permeability of the absorbent resin particles are improved. Examples of the polyvalent metal salt (e) include salts of at least one metal selected from the group consisting of magnesium, calcium, zirconium, aluminum and titanium and the following inorganic or organic acids.

Examples of organic acids include carboxylic acids, sulfonic acids, and organic phosphoric acids. Carboxylic acid: saturated monocarboxylic acid having 1 to 30 carbon atoms (acetic acid, propionic acid, capric acid, lauric acid, myristic acid, stearic acid, behenic acid, etc.), unsaturated monocarboxylic acid (acrylic acid , methacrylic acid, oleic acid, etc.), aliphatic oxycarboxylic acids (glycolic acid, lactic acid, gluconic acid, etc.), aliphatic polycarboxylic acids (oxalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, maleic acid, fumar acid, itaconic acid, etc.), aromatic carboxylic acids (phthalic acid, trimellitic acid, pyromellitic acid, etc.); sulfonic acid, etc.), aromatic sulfonic acids (p-toluenesulfonic acid, naphthalenesulfonic acid, etc.), etc.; acids, aliphatic alkylphosphonic acids having 1 to 30 carbon atoms, and the like; Preferred among organic acids are carboxylic acids, and more preferred are monocarboxylic acids, aliphatic oxycarboxylic acids, aliphatic polycarboxylic acids, and amino(poly)carboxylic acids. Examples of inorganic acids include hydrofluoric acid, hydrochloric acid, bromic acid, iodic acid, sulfuric acid, nitric acid, phosphoric acid, carbonic acid and boric acid. Preferred inorganic acids are hydrochloric acid, sulfuric acid, phosphoric acid, carbonic acid and boric acid.

The water retention amount (g/g) of the absorbent resin particles of the present invention is calculated as an average value of 100 analysis points (N=100), and the water retention amount (g/g) is 30 or more from the viewpoint of absorption characteristics. , preferably 30 to 50, more preferably 32 to 48, particularly preferably 34 to 46. The coefficient of variation is determined from the unbiased standard deviation and average value and is 3% or less, preferably 2% or less, and particularly preferably 1% or less. The average of the analysis points N=100 for the water retention capacity (g/g) of the crosslinked polymer (A) is preferably 35 to 70, more preferably 37 to 68, and particularly preferably 40, from the viewpoint of absorption characteristics. ∼65, and the coefficient of variation is preferably 5% or less, more preferably 4% or less, and particularly preferably 3% or less.

The absorption under pressure (g/g) of the absorbent resin particles of the present invention at 0.3 psi is calculated as the average value of 100 analysis points (N = 100), and the absorption under pressure (g/g) is From the viewpoint of absorption characteristics, it is preferably 25 or more, more preferably 25 to 50, still more preferably 27 to 48, and particularly preferably 29 to 46. The coefficient of variation is determined from the unbiased standard deviation and average value, and is preferably 3% or less, more preferably 1.5% or less, and particularly preferably 1% or less.

In the absorbent resin particles of the present invention, the average value of the drying weight loss (%) of the crosslinked polymer in the drying step is calculated as the average value of 100 analysis points (N=100), and is preferably 1 or more, more preferably 2-10. The coefficient of variation is determined from the unbiased standard deviation and average value, and is preferably 10% or less, more preferably 8% or less, and particularly preferably 6% or less.

The sharper the particle size distribution of the absorbent resin particles of the present invention, the higher the uniformity, which is more desirable from the viewpoint of reducing variations in absorption performance. Specifically, the average value of the ratio (%) of particles having a diameter of 300 to 500 μm in the total number of particles is calculated as the average value of 100 analysis points (N=100), and is preferably 30 to 99, more preferably 32. to 97, particularly preferably 34 to 95. The coefficient of variation is determined from the unbiased standard deviation and average value, and is preferably 5% or less, more preferably 4% or less, and particularly preferably 3% or less.

In the absorbent resin particles of the present invention, the average value of the whiteness of the crosslinked polymer is calculated as the average value of 100 analysis points (N=100), and is preferably 65 or more, more preferably 65 to 94, and still more preferably. is 68 to 94, particularly preferably 70 to 94, and the coefficient of variation is determined from the unbiased standard deviation and the average value, and is preferably 3% or less, more preferably 2% or less, and particularly preferably 1% or less. Within this range, when the absorbent resin is applied to water absorbent articles such as disposable diapers and sanitary napkins, the absorbent resin is less visible and the absorbent articles are visually less uncomfortable. .

In the absorbent resin particles of the present invention, the residual monomer (ppm) of the crosslinked polymer has an average value of preferably 300 or less, more preferably 280 or less, still more preferably 260 or less, from the viewpoint of anti-rash of the absorbent article. is. The average value is calculated as the average value of 100 analysis points (N=100). The coefficient of variation obtained from the unbiased standard deviation and average value is preferably 10% or less, more preferably 8% or less, and particularly preferably 6% or less.

In the absorbent resin particles of the present invention, the post-drying neutralization uniformity (%) is preferably 90 or more, more preferably 92 or more, and still more preferably 95 or more, from the viewpoint of absorbency under pressure. The post-drying neutralization uniformity (%) is preferably 95 or more, more preferably 92 or more, still more preferably 90 or more, from the viewpoint of absorption under pressure. The average value is calculated as the average value of 100 analysis points (N=100).

The absorbent body of the present invention contains the absorbent resin particles of the present invention. As the absorbent, the absorbent resin particles may be used alone, or may be used together with other materials to form an absorbent. Other materials include fibrous materials and the like. The structure and manufacturing method of the absorbent when used with fibrous materials are the same as those of known ones (Japanese Patent Laid-Open Nos. 2003-225565, 2006-131767 and 2005-097569). be.

Cellulose fibers, organic synthetic fibers, and mixtures of cellulosic fibers and organic synthetic fibers are preferable as the fibrous material.

Examples of cellulosic fibers include natural fibers such as fluff pulp, and cellulosic chemical fibers such as viscose rayon, acetate and cupra. The raw material (softwood, hardwood, etc.), manufacturing method (chemical pulp, semi-chemical pulp, mechanical pulp, CTMP, etc.), bleaching method, and the like of this cellulose-based natural fiber are not particularly limited.

Examples of organic synthetic fibers include polypropylene fibers, polyethylene fibers, polyamide fibers, polyacrylonitrile fibers, polyester fibers, polyvinyl alcohol fibers, polyurethane fibers, and heat-fusible composite fibers (the above fibers having different melting points). (sheath-and-core type, eccentric type, side-by-side type, etc.), fibers obtained by blending at least two of the above fibers, and fibers obtained by modifying the surface layer of the above fibers.

Among these fibrous base materials, cellulosic natural fibers, polypropylene fibers, polyethylene fibers, polyester fibers, heat-fusible conjugate fibers and mixed fibers thereof are preferred, and more preferred are They are fluff pulp, heat-fusible conjugate fiber, and mixed fiber of these, in that they are excellent in shape retention after absorption of water.

The length and thickness of the above-mentioned fibrous material are not particularly limited, and a length of 1 to 200 mm and a thickness of 0.1 to 100 denier can be suitably used. The shape is not particularly limited as long as it is fibrous, and examples include a thin cylindrical shape, a split yarn shape, a staple shape, a filament shape, a web shape, and the like.

When the absorbent resin particles are used as an absorber together with fibrous materials, the weight ratio of the absorbent resin particles to the fibers (weight of absorbent resin particles/weight of fibers) is preferably 40/60 to 90/10, more preferably. is 70/30 to 80/20.

The absorbent article of the present invention uses the above absorbent. The absorbent article can be applied not only to sanitary goods such as paper diapers and sanitary napkins, but also to various uses such as absorption of various aqueous liquids, retention agent use, and gelling agent use, which will be described later. The manufacturing method of the absorbent article and the like are the same as those known (described in JP-A-2003-225565, JP-A-2006-131767, JP-A-2005-097569, etc.).

EXAMPLES The present invention will be further described with reference to Examples and Comparative Examples below, but the present invention is not limited to these. Hereinafter, unless otherwise specified, parts indicate parts by weight and % indicates % by weight.

<Production Example 1> Water-soluble vinyl monomer (a1-1) {acrylic acid, manufactured by Mitsubishi Chemical Corporation, purity 100%} 155 parts (2.15 mol parts), cross-linking agent (b1) {pentaerythritol triallyl ether, Daiso Co., Ltd.) 0.415 parts (0.0016 mol parts) and 340.48 parts of deionized water were kept at 3° C. while stirring and mixing. After nitrogen was introduced into this mixture to make the dissolved oxygen content 1 ppm or less, 0.62 parts of a 1% aqueous hydrogen peroxide solution, 1.1625 parts of a 2% ascorbic acid aqueous solution and 2% 2,2'-azobis [ 2-Methyl-N-(2-hydroxyethyl)-propionamide] aqueous solution (2.325 parts) was added and mixed to initiate polymerization. After the temperature of the mixture reached 90° C., the mixture was polymerized at 90±2° C. for about 5 hours to obtain hydrous gel (1).

<Example 1> Next, 128.42 parts of a 48.5% sodium hydroxide aqueous solution was added and mixed while shredding 502.27 parts of this hydrous gel (1) with a mincing machine (12VR-400K manufactured by ROYAL). and shredded gel (2) was obtained. The resulting shredded gel (2) was dried for 30 minutes in a microwave oven NE-710GP made by Panasonic (output: 500 W, wind speed: 0.05 m/sec, wind temperature: 25°C) modified to allow air to be introduced and exhausted. , to obtain a dry form. The dried product was pulverized with a juicer mixer (Osterizer Blender manufactured by Oster), and then adjusted to a particle size of 150 to 710 μm using sieves with mesh openings of 150 and 710 μm to obtain a crosslinked polymer (A-1). . The average loss on drying in the drying process measured with an analysis score of N = 100 was 1 to 10%, and the coefficient of variation obtained from the unbiased standard deviation and the average value was 10% or less. While 100 parts of this crosslinked polymer (A-1) was stirred at high speed (high-speed stirring turbulizer manufactured by Hosokawa Micron: 2000 rpm), a 2% water/methanol mixed solution of ethylene glycol diglycidyl ether (weight ratio of water/methanol) was obtained. = 70/30) was added while spraying and mixed, and the mixture was allowed to stand at 160°C for 20 minutes for surface cross-linking to obtain absorbent resin particles (D-1) of the present invention. Various physical properties of the resulting crosslinked polymer (A-1) and absorbent resin particles (D-1) were determined by the methods described below. The results are shown in Tables 1 and 2.

<Example 2> While 100 parts of the crosslinked polymer (A-1) prepared in Example 1 was stirred at high speed (high-speed stirring turbulizer manufactured by Hosokawa Micron: 2000 rpm), ethylene glycol diglycidyl ether in 2% water/ 4 parts of a mixed solution of methanol (weight ratio of water/methanol=70/30) was added while spraying and mixed, and left standing at 160° C. for 20 minutes for surface cross-linking. Subsequently, 1 part of a 30% aqueous solution of potassium alum is added while spraying and mixed, left to stand at 80° C. for 20 minutes for surface cross-linking, and then 0.3 part of inorganic particles {Aerosil 200 (manufactured by Nippon Aerosil Co., Ltd.). } to obtain the absorbent resin particles (D-2) of the invention. Various physical properties of the resulting absorbent resin particles (D-2) were determined by the methods described below. Table 2 shows the results.

<Example 3> In a glass reaction vessel, 77 parts of sodium acrylate, 22.85 parts of acrylic acid, 0.23 parts of N,N'-methylenebisacrylamide, 299.54 parts of deionized water and dichlorotris (triphenyl Phosphine) ruthenium (0.001 part) was charged, and the temperature of the contents was kept at 3°C while stirring and mixing. Nitrogen is flowed into the contents to make the dissolved oxygen content 1 ppm or less, 0.3 parts of a 1% aqueous solution of hydrogen peroxide, 0.8 parts of a 0.2% aqueous solution of ascorbic acid, and 2,2'-azobisamidinopropane Add and mix 0.8 parts of a 2% aqueous solution of dihydrochloride to initiate polymerization, and after the reaction solution reaches 80 ° C., polymerize for about 5 hours at a polymerization temperature of 80 ± 2 ° C. to obtain a crosslinked polymer. A hydrous gel (3) consisting of was obtained. Next, 400 parts of this hydrous gel (3) was shredded at 25° C. with a mincing machine (hole diameter of perforated plate: 6 mm, manufactured by Iizuka Kogyo Co., Ltd. 12VR-400K), and the resulting shredded gel (4) was Drying was performed for 30 minutes in a Panasonic microwave oven NE-710GP (output: 500 W, wind speed: 0.05 m/sec, wind temperature: 25° C.) modified to allow air introduction and exhaust, to obtain a dry product. The resulting dried product was pulverized with a juicer mixer (Osterizer Blender manufactured by Oster), and then adjusted to a particle size of 150 to 710 μm using sieves with mesh openings of 150 and 710 μm to obtain a crosslinked polymer (A-2). got While 100 parts of this crosslinked polymer (A-2) was stirred at high speed (high-speed stirring turbulizer manufactured by Hosokawa Micron: number of revolutions 2000 rpm), a 2% water/methanol mixed solution of ethylene glycol diglycidyl ether (water/methanol weight ratio) was added. = 70/30) were added and mixed while spraying, and the mixture was allowed to stand at 160°C for 20 minutes for surface cross-linking to obtain the absorbent resin particles (D-3) of the present invention. Various physical properties of the resulting crosslinked polymer (A-2) and absorbent resin particles (D-3) were determined by the methods described below. The results are shown in Tables 1 and 2.

<Comparative Example 1> Example 1 except that "2 parts of a 2% water/methanol mixed solution of ethylene glycol diglycidyl ether (water/methanol weight ratio = 70/30)" was not added. Absorbent resin particles (D-4) for comparison were obtained in the same manner as above. Various physical properties of the resulting absorbent resin particles (D-4) were determined by the methods described below. Table 2 shows the results.

<Comparative Example 2> The shredded gel (2) obtained in Example 1 was dried with a ventilated band dryer {160°C, 30 minutes} to obtain a dry product. After the dried product was pulverized with a juicer mixer (Osterizer Blender manufactured by Oster), a pulverized product was obtained by removing undried matter using a sieve with an opening of 4 mm. The weight of the wet matter based on the weight of dry matter was 15%. Subsequently, a crosslinked polymer (A-3) was obtained by adjusting the particle size of the crushed product to 150 to 710 μm using sieves with mesh openings of 150 and 710 μm. While 100 parts of this crosslinked polymer (A-3) was stirred at high speed (high-speed stirring turbulizer manufactured by Hosokawa Micron: 2000 rpm), a 2% water/methanol mixed solution of ethylene glycol diglycidyl ether (water/methanol weight ratio) was added. = 70/30) were added and mixed while spraying, and the mixture was allowed to stand at 160°C for 20 minutes for surface cross-linking to obtain absorbent resin particles (D-5) for comparison. Various physical properties of the resulting crosslinked polymer (A-3) and absorbent resin particles (D-5) were determined by the methods described below. The results are shown in Tables 1 and 2.

<Comparative Example 3> The shredded gel (4) prepared in Example 3 was dried with a ventilated band dryer {160°C, 30 minutes} to obtain a dry product. The dried body is a juicer mixer (OSTERIZER BL manufactured by Oster)
ENDER), and the particle size of the crushed product was adjusted to 150 to 710 μm using sieves with mesh openings of 150 and 710 μm to obtain a crosslinked polymer (A-4). While 100 parts of this crosslinked polymer (A-4) was stirred at high speed (high-speed stirring turbulizer manufactured by Hosokawa Micron: 2000 rpm), a 2% water/methanol mixed solution of ethylene glycol diglycidyl ether (water/methanol weight ratio) was added. = 70/30) were added while spraying and mixed, and the mixture was allowed to stand at 160°C for 20 minutes for surface cross-linking to obtain absorbent resin particles (D-6) for comparison. Various physical properties of the resulting crosslinked polymer (A-4) and absorbent resin particles (D-6) were determined by the methods described below. The results are shown in Tables 1 and 2.

Measurement method of crosslinked polymer and absorbent resin particles <Measurement method of water retention> A measurement sample (crosslinked weight 1.00 g of coalesced or absorbent resin particles) was added, immersed in 1,000 ml of physiological saline (salt concentration: 0.9% by weight) for 1 hour without stirring, and then hung for 15 minutes to drain. After that, the whole tea bag was placed in a centrifuge and dehydrated by centrifugation at 150 G for 90 seconds to remove excess physiological saline. The physiological saline used and the temperature of the measurement atmosphere were 25°C ± 2°C. The weight of the tea bag after centrifugal dehydration was measured in the same manner as above except that the measurement sample was not used (h2). Water Retention Amount (g/g)=(h1)-(h2) The above measurements were repeated for 100 samples, and an average value was obtained. Also, the coefficient of variation was obtained from the unbiased standard deviation and the average value.

<Absorption under pressure at 0.3 psi> Absorption under pressure was measured as follows. 0.16 g of a measurement sample sieved to a particle size of 250 to 500 μm in a cylindrical plastic tube (inner diameter: 25 mm, height: 34 mm) with a nylon net of 63 μm (JIS Z8801-1: 2006) attached to the bottom. was weighed, and the cylindrical plastic tube was set vertically so that the sample to be measured was placed on the nylon net so that it had a substantially uniform thickness. 5 mm) was placed. After measuring the weight (M1) of the entire cylindrical plastic tube, a cylindrical shape containing a measurement sample and a weight in a petri dish (diameter: 12 cm) containing 60 ml of physiological saline (salt concentration 0.9% by weight) The plastic tube was set up vertically, immersed with the nylon mesh side down, and allowed to stand for 60 minutes. After 60 minutes, lift the cylindrical plastic tube from the petri dish, tilt it diagonally, remove dripping water droplets, and weigh the entire cylindrical plastic tube containing the measurement sample and the weight (M2). The absorbency under pressure was obtained from The physiological saline used and the temperature of the measurement atmosphere were 25°C ± 2°C. Absorbance under pressure (g/g) at 0.3 psi={(M2)-(M1)}/0.16 The above measurements were repeated for 100 samples and an average value was obtained. Also, the coefficient of variation was obtained from the unbiased standard deviation and the average value.

<Method for Measuring Loss on Drying> Loss on drying was measured as follows. Measurement sample before and after heating when heated with an infrared moisture meter {JE400 manufactured by KETT Co., Ltd.: 120 ± 5 ° C., 30 minutes, atmospheric humidity 50 ± 10% RH before heating, lamp specifications 100 V, 40 W} It was calculated from the weight loss of After leaving 5 g of the measurement sample to stand at an atmospheric humidity of 50 ± 10% RH for 1 hour, weighing (W1) and heating at 120 ± 5 ° C. for 30 minutes with an infrared moisture meter (JE400 manufactured by KETT Co.) The weight of the sample was weighed (W2) to determine the weight loss. Weight loss (%)=(W1-W2)/W1 The above measurements were repeated for 100 samples, and an average value was obtained. Also, the coefficient of variation was obtained from the unbiased standard deviation and the average value.

<Method for measuring particle size distribution> The particle size distribution of the crosslinked polymer in the present invention was measured using a low-tap test sieve shaker and a standard sieve (JIS Z8801-1: 2006) according to Perry's Chemical Engineers Handbook 6th Edition. (McGraw-Hill Book Company, 1984, p. 21). That is, JIS standard sieves of 1000 μm, 850 μm, 710 μm, 500 μm, 425 μm, 355 μm, 300 μm, 150 μm, 125 μm, 75 μm and 45 μm and a saucer are combined in this order from the top. About 50 g of the particles to be measured were placed in the top sieve and shaken for 5 minutes with a Rotap test sieve shaker. The weight of the particles on each sieve and the tray was weighed, and the total was taken as 100% by weight to determine the weight fraction of the particles on each sieve. Then, the ratio of particles of 300 to 500 μm was determined. The above measurements were repeated for 100 samples and an average value was obtained. Also, the coefficient of variation was obtained from the unbiased standard deviation and the average value.

<Method for measuring whiteness> In accordance with JIS Z8722: 5.3 of 2000, it was calculated from the following formula using the stimulus value (Z) measured under condition d {for example, a colorimetric color difference meter ZE-2000 ( manufactured by Nippon Denshoku Industries Co., Ltd.)}. (Whiteness)=0.847×Z The above measurements were repeated for 100 samples, and an average value was obtained. Also, the coefficient of variation was obtained from the unbiased standard deviation and the average value.

<Method for Measuring Amount of Residual Monomers> The amount of residual monomers was measured as follows. 1 g of the measurement sample was dispersed in 249 g of physiological saline, and the residual monomer was extracted with stirring at 25° C.±2° C. for 3 hours. This solution was filtered and the filtrate was analyzed by liquid chromatography according to a known method. The amount of residual monomer was obtained by converting the measured content of monomer in the filtrate to 1 g of resin. The above measurements were repeated for 100 samples and an average value was obtained. Also, the coefficient of variation was obtained from the unbiased standard deviation and the average value.

<Method for Measuring Neutralization Uniformity> A sample piece for measurement was prepared as follows. A measurement sample was sprinkled on a polyethylene sheet (5 cm x 5 cm) with one side of a double-sided tape (4 cm x 4 cm: Nichiban Co., Ltd. "Nice Tac") attached so that the surface was completely filled. The substrate was then turned over and excess unbonded measurement sample was shaken off. The surface of the substrate is turned, and a BTB solution (bromothymol blue: 400 mg, methanol: 200 mL, water: 200 mL mixed solution) is sprayed onto the substrate at 0.2 g/sec for 10 seconds. After being left for 20 seconds, it was further sprayed at 0.1 g/second for 10 seconds to obtain a sample piece. Neutralization uniformity was measured as follows. Photographs were taken in a white-lit room with no sunlight and a uniform background with pixels of 8 megapixels or more. A sample piece 20 seconds after spraying the BTB solution was used for photographing. The photographed images were analyzed using Windows image analysis software "WinROOF". First, the total area (G1) of the sample was calculated. Select [Image Processing] and [Monochrome Image] from the menu to convert the image to monochrome. Next, [Binary processing] and [Binarization with two threshold values] were selected, and the threshold values were visually adjusted so that the boundary of the region to be measured coincided with the boundary of the sample. After that, [Measurement] and [Total area/number] were selected, and the calculated numerical value was defined as the total area (G1) of the sample. Next, the area (G2) of the specified color of the sample was calculated. Binary processing was performed in a color extraction mode, and a range of 60-75 for hue, 23-120 for lightness, and 40-255 for saturation was specified. The total area/number mode was used for measurement, and the calculated numerical value was used as the area (G2) of the specified color. Neutralization uniformity was calculated as follows. Neutralization uniformity (%)=(G2)/(G1)×100 The above measurement was repeated for 100 samples, and an average value was obtained.

From the results in Table 2, it can be seen that the heat deterioration of the examples is suppressed, so that the variation coefficient of the absorption performance is excellent, the whiteness is extremely good, and the neutralization uniformity is high, so the absorption under pressure is also extremely good. there were. On the other hand, in Comparative Examples, although the water retention amount is good, the coefficient of variation is inadequate (Comparative Example 1 does not surface-crosslink the absorbent particles), and it is observed that the whiteness is lowered and the amount of residual monomer is large. rice field. As a result, it was found that the Examples had a significant difference from the Comparative Examples in terms of compatibility between uniformity of absorption performance, whiteness, and residual monomers.

Claims (5)

Obtaining a crosslinked polymer (A) having a water-soluble vinyl monomer (a1) and/or a vinyl monomer (a2) that becomes a water-soluble vinyl monomer (a1) by hydrolysis and an internal cross-linking agent (b) as essential structural units, A method for producing absorbent resin particles in which the surface is surface-crosslinked with a surface-crosslinking agent (d), wherein the absorbent resin particles have an average water retention amount of 30 g/g or more as measured by the analysis point N = 100, The coefficient of variation obtained from the unbiased standard deviation and the average value is 3% or less, and the average value of the residual monomer amount of the crosslinked polymer (A) measured at the analysis point N = 100 is 260 ppm or less, and the unbiased standard deviation and the average value is 10% or less, the surface cross-linking agent (d) is a covalent surface cross-linking agent or an ionic bond surface cross-linking agent, and has a wavelength of 1 × 10 5 nm to 1 × ¹⁰ Including a drying step by irradiating the crosslinked polymer hydrous gel with a microwave of ⁹ nm, the average value of analysis points N = 100 for the loss on drying (%) of the crosslinked polymer in the drying step is 1 to 4.5. A method for producing absorbent resin particles, comprising a step of surface-crosslinking the crosslinked polymer (A) after a drying step of irradiating with microwaves . 2. The method for producing absorbent resin particles according to claim 1 , wherein microwaves are irradiated using a plurality of waveguides. 3. The production method according to claim 1 or 2 , wherein in the drying step of irradiating with microwaves, an air flow at a wind speed of 0.01 to 30 m/sec is supplied to the hydrous crosslinked polymer gel. The production method according to any one of claims 1 to 3, further comprising a pulverization step after the drying step and before the surface cross-linking step. The production method according to any one of claims 1 to 4, wherein surface cross-linking in the absorbent resin particles is performed at a temperature of 100 to 200°C.