JP6959030B2 - Water-absorbing agents, absorbent articles, and methods for manufacturing water-absorbing agents - Google Patents

Description

The present invention relates to a water-absorbing agent and an absorbent article using the water-absorbing agent, and more particularly, it is particularly excellent when used in an absorbent body of an absorbent article such as a disposable diaper, a sanitary napkin, or an incontinence pad. It relates to a water absorbing agent capable of providing absorption characteristics and further reducing an unpleasant odor at the time of swelling, and an absorbent article.

Water-absorbent resins and water-absorbing agents are water-swellable, water-insoluble polymer gelling agents, such as absorbent articles such as disposable diapers and sanitary napkins, soil water-retaining agents for agriculture and horizons, and industrial water-stopping agents. , Used for various purposes. Although many types of monomers and hydrophilic polymers are used as raw materials for the water-absorbent resin and the water-absorbing agent, acrylic acid and / or a salt thereof (hereinafter referred to as "salt") are used from the viewpoint of water absorption performance and manufacturing cost. Polyacrylic acid (salt) -based water-absorbent resin using (denoted as "acrylic acid (salt)") as a monomer is most often used.

In recent years, in absorbent articles such as disposable diapers and sanitary napkins, the amount of hydrophilic fibers such as crushed wood pulp has been reduced by increasing the amount of water-absorbent resin and water-absorbing agent used per absorbent article. , There is a tendency to reduce the thickness of absorbent articles. Along with this tendency, the functions that hydrophilic fibers have been responsible for, for example, the function of taking in and diffusing a liquid into an absorbent article, are also required for water-absorbent resins and water-absorbent agents, and high performance is achieved. It has come to be required. Therefore, for example, a water-absorbent resin having both an absorption ratio under pressure and a water absorption rate, which are contradictory physical properties, and a water-absorbent resin having both an absorption ratio and a liquid permeability have been developed.

However, if the liquid permeability (SFC, GBP) required for the water-absorbent resin or water-absorbing agent is increased in order to make the absorbent article thinner, the elastic modulus of the gel after swelling increases, so that the gel after urine swelling As a result, the gel becomes bulky and voids are easily formed, so that the odor is easily volatilized from the gel after absorption, which makes it easier for the wearer and his / her caregiver to detect it. As a countermeasure, various attempts have been made to give the resin an additional function by adding a deodorant and antibacterial compound to the water-absorbent resin. In particular, deodorization of absorbent articles after urine absorption is one of the desired performances, and studies have been made to improve the urine deodorization performance of water-absorbent resins. For example, a method of containing activated charcoal (Patent Document 1), a method of containing a leaf extract of a camellia family plant (Patent Document 2), a method of containing a coniferous tree extract (Patent Document 3), and a method of containing tea making (Patent Document 3). Patent Document 4), a method of containing a tannic acid and a composite silicate compound (Patent Document 5), a method of containing a natural antibacterial component extracted from grapefruit resin or herbs (Patent Document 6), a method of containing a drug component. (Patent Documents 7 and 8) and the like have been proposed.

Japanese Unexamined Patent Publication No. 59-10548 Japanese Unexamined Patent Publication No. 60-158861 Japanese Unexamined Patent Publication No. 11-24130 Japanese Unexamined Patent Publication No. 2-4-1155 Japanese Unexamined Patent Publication No. 11-116829 Japanese Unexamined Patent Publication No. 9-208787 Japanese Unexamined Patent Publication No. 63-265963 Japanese Unexamined Patent Publication No. 11-5847 International Publication No. 2006/042704 Pamphlet International Publication No. 2017/057709 Pamphlet

However, it is hardly known that the water-absorbent resin or the water-absorbing agent itself emits an odor at the time of swelling. Therefore, even in Patent Documents 1 to 5, the malodorous component derived from urine after urine absorption is reduced or deodorized. However, we did not pay attention to the reduction of odor derived from the water-absorbing resin or the water-absorbing agent.

Under such circumstances, a specific post-crosslinking agent (so-called surface cross-linking agent) is selected, and the water-absorbent resin or the water-absorbent resin does not perceive the odor of organic compounds such as alcohol, aldehyde, ketone, and aromatic when the liquid swells. Proposing Patent Document 9) or selecting a specific substance when using a cationic organic polymer (Patent Document 10) suppresses the odor generated by the water-absorbent resin or the water-absorbent agent when the liquid swells. It was one of the few attempts to try.

Therefore, an object of the present invention is to provide a water absorbing agent, an absorbent article, and a method for producing a water absorbing agent, which makes it difficult to detect an unpleasant odor even if the gel becomes bulky when the liquid is absorbed and the gaps between the gel particles increase. There is.

The present inventors have diligently studied from the viewpoint of reducing the unpleasant odor perceived by combining with an aromatic substance, rather than reducing the unpleasant odor itself emitted when the water-absorbent resin swells. As a result, the present invention has been found that by controlling the swelling gel characteristic range by combining the aromatic substance and the water-absorbent resin, the volatile state of the aromatic substance from the gel can be controlled and the above object can be achieved. It came to be completed.
That is, the present invention is as follows.
[1] A water-absorbing agent containing a surface-crosslinked polyacrylic acid (salt) -based water-absorbent resin and an aromatic substance.
The water-absorbent resin is 90% by mass or more based on the total amount of the water-absorbing agent.
The aromatic substance is present on the surface of the water-absorbent resin,
A water absorbing agent having a gel bulk density of 0.50 g / cm ³ or more and 1.00 g / cm ³ or less.
[2] The water-absorbing agent according to the above [1], which contains the aromatic substance in the range of 0.001 ppm (mass-based) to 50 ppm (mass-based) with respect to the water-absorbent resin.
[3] The water absorbing agent according to the above [1] or [2], wherein the aromatic substance is at least one compound selected from a terpene-based aromatic compound and a phenol-based aromatic compound.
[4] Any of the above [1] to [3], wherein the polyacrylic acid (salt) -based water-absorbent resin is surface-crosslinked with a surface-crosslinking agent containing at least one compound selected from a polyhydric alcohol and an alkylene carbonate. The water absorbing agent described in one.
[5] The proportion of particles having a mass average particle diameter (D50) of 300 μm or more and 600 μm or less and a particle diameter of less than 150 μm is 5% by mass or less in 100% by mass of the water absorbent. ] The water absorbing agent according to any one of.
[6] The water absorbing agent according to any one of the above [1] to [5], which has a specific surface area of 25 m ² / kg or more and 50 m ^{2 / kg or less.}
[7] The water absorbing agent according to any one of the above [1] to [6], which further satisfies at least one of the following physical properties.
-Unpressurized absorption ratio (CRC) is 25-40 g / g-Pressurized absorption ratio (AAP) is 20 g / g or more-Pressurized permeation-dependent absorption ratio (PDAUP) is 3 g / g or more [ 8] The water absorbing agent according to any one of the above [1] to [7], wherein the water absorption rate (Vortex method) is 10 seconds or more and 50 seconds or less.
[9] The water absorbing agent according to any one of the above [1] to [8], wherein the polyhydric alcohol is 1.5% by mass or less based on the total amount of the water absorbing agent.
[10] The water absorbing agent according to any one of the above [1] to [9], wherein the residual monomer is 1000 ppm or less with respect to the total amount of the water absorbing agent.
[11] The water-absorbing agent according to any one of the above [1] to [10], which is used for an absorbent article.
[12] An absorbent article comprising an absorbent layer containing the water-absorbing agent according to any one of the above [1] to [10], a liquid-permeable front sheet, and a liquid-impermeable back sheet.
[13] A characteristic is that an aromatic substance is arranged on the surface of a surface-crosslinked polyacrylic acid (salt) water-absorbent resin having a gel bulk density of 0.50 g / cm ³ or more and 1.00 g / cm ^{3 or less.} A method for producing a water absorbing agent containing 90% by mass of the water absorbing resin.
[14] The method for producing a water-absorbing agent according to the above [13], wherein the aromatic substance is arranged in the range of 0.001 ppm (mass-based) to 50 ppm (mass-based) with respect to the water-absorbent resin.
[15] The method for producing a water-absorbing agent according to the above [13] or [14], wherein the water-absorbent resin further satisfies at least one of the following requirements.
-Mass average particle size (D50) is 300 μm or more and 600 μm or less.-The proportion of particles with a particle size of less than 150 μm is 5% by mass or less in 100% by mass of the water-absorbent resin.-Specific surface area is 25 m ² / kg or more. 50 m ² / kg or less ・ Non-pressurized absorption ratio (CRC) is 25-40 g / g ・ Pressurized absorption ratio (AAP) is 20 g / g or more ・ Pressurized permeation-dependent absorption ratio (PDAUP) 3 g / g or more ・ Water absorption rate (Vortex method) is 10 seconds or more and 50 seconds or less [16] The aromatic substances are mixed with the surface-crosslinked water-absorbent resin [13] to [15]. The method for producing a water absorbing agent according to any one of the above.
[17] The method for producing a water absorbing agent according to any one of the above [13] to [16], wherein the aromatic substance is at least one compound selected from a terpene-based aromatic compound and a phenol-based aromatic compound. ..

Since the water absorbent of the present invention exhibits a specific gel bulk density and an aromatic substance is arranged on the surface thereof, even if an unpleasant odor is emitted after absorbing a liquid and swelling, the wearer or a caregiver It is possible to reduce the discomfort of the odor perceived by the user. The present invention is particularly suitable for a water-absorbing agent having high gel strength, in which the gel bulk density after swelling becomes small and the space between gel particles tends to increase. It can be particularly preferably used for absorbents having a sex resin concentration, disposable diapers containing such absorbents, sanitary napkins, incontinence pads for adults, absorbent articles such as diapers for adults, and the like.

Hereinafter, the water-absorbing agent according to the present invention, the absorbent article using the water-absorbing agent, and the method for producing the water-absorbing agent will be described in detail. Can be appropriately changed and implemented as long as the gist of the present invention is not impaired. Specifically, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims, and the technical means disclosed in the different embodiments can be appropriately used. Embodiments obtained in combination are also included in the technical scope of the present invention.

[1] Definition of terms [1-1] Water-absorbent resin, base polymer, water-absorbent agent The "water-absorbent resin" in the present invention means a water-swellable water-insoluble polymer gelling agent, and is generally a powder. It is a shape. Further, "water swellability" means that the absorption ratio (CRC) under no pressure defined in EDANA WSP241.3 (10) described later is 5 g / g or more, and "water insoluble" means WSP270. It means that the soluble content (Ext) defined in 3 (10) is 50% by mass or less, respectively.
The "water-absorbent resin" is preferably a hydrophilic crosslinked polymer (so-called internal crosslinked polymer) formed by crosslinking and polymerizing an unsaturated monomer having a carboxyl group, but the total amount (100% by mass) thereof is It does not have to be a crosslinked polymer, and additives and the like can be contained within a range that satisfies the above performance (CRC, Ext).
In general, a "water-absorbent resin" is a "polymer in which only the inside is crosslinked (that is, a polymer in which the crosslink density between the inside and the surface is substantially the same)" or "a polymer in which the inside and the surface are crosslinked. In some cases, it refers to a polymer that has been cross-linked (that is, a polymer whose surface cross-linking density is relatively high relative to the internal cross-linking density). The polymer is referred to as "base polymer", and the polymer in which the inside and the surface are crosslinked is referred to as "water-absorbent resin".
The "water-absorbing agent" in the present invention means a water-absorbent resin that is ready to be shipped as a final product. Therefore, when the "water-absorbent resin" contains an aromatic substance, it becomes a "water-absorbing agent", but the "water-absorbent resin" and the "water-absorbing agent" may be treated synonymously.

[1-2] Polyacrylic acid (salt) -based water-absorbent resin The "polyacrylic acid (salt) -based water-absorbent resin" in the present invention refers to acrylic acid and / or a salt thereof (hereinafter, "acrylic acid (salt)". It means a water-absorbent resin made from (denoted) as a raw material. That is, the polyacrylic acid (salt) -based water-absorbent resin is a water-absorbent resin having a structural unit derived from acrylic acid (salt) in the polymer and having a graft component as an optional component.
Specifically, the polyacrylic acid (salt) -based water-absorbent resin is preferably 50 mol% to 100 mol%, more than the total amount of the monomers involved in the polymerization reaction (excluding the internal cross-linking agent). A water-absorbent resin preferably containing 70 mol% to 100 mol%, more preferably 90 mol% to 100 mol%, and particularly preferably substantially 100 mol% of acrylic acid (salt).

[1-3] "EDANA" and "WSP"
"EDANA" is an abbreviation for European Disposables and Nonwovens Associations. Further, "WSP" is an abbreviation for Worldwise Strategic Partners, and indicates a world standard measurement method for water-absorbent resin provided by EDANA. In the present invention, unless otherwise specified, the physical characteristics of the water-absorbent resin are measured in accordance with the WSP original (revised in 2010 / publicly known literature).

[1-3-1] "CRC" (WSP241.3 (10))
"CRC" which means the absorption ratio under no pressure is an abbreviation for Centrifuge Retention Capacity, and means the absorption ratio of a water absorbing agent or a water absorbing resin under no pressure. Specifically, 0.2 g of the water-absorbent resin is placed in a non-woven fabric bag and then immersed in a large excess of 0.9 mass% sodium chloride aqueous solution for 30 minutes to freely swell the water-absorbent resin, and then centrifuged. It is the absorption ratio (unit: g / g) after dehydration using a separator (250 G).

[1-3-2] "AAP" (WSP242.3 (10))
"AAP" is an abbreviation for Absorption Against Pressure, and means the absorption ratio under pressure of a water absorbing agent or a water absorbing resin. Specifically, 0.9 g of a water-absorbing agent or a water-absorbent resin ^{is swollen with a large excess of 0.9 mass% sodium chloride aqueous solution for 1 hour under a load of 4.83 kPa (49 g / cm 2} , 0.7 psi). It refers to the absorption ratio (unit: g / g) after being allowed to grow.

[1-3-3] "PSD" (WSP220.3 (10))
"PSD" is an abbreviation for Particle Size Distribution, which means the particle size distribution of a water-absorbing agent or a water-absorbent resin as measured by sieving. The mass average particle size (D50) and the logarithmic standard deviation (σζ) of the particle size distribution are described in “(3) Mass-Average Particle Diameter (D50) and Logistic Standard Deviation (σζ)” described in US Pat. No. 7,638,570. It is measured by the same method as "Particle Diameter Deviation".

[1-3-4] "PDAUP" (WSP243.3 (10))
"PDAUP" is an abbreviation for Permeability Dependent Absorption Under Pressure, and means a permeation-dependent absorption ratio of a water-absorbing agent or a water-absorbent resin under pressure. Specifically, 5.0 g of a water-absorbing agent or a water-absorbent resin ^{is swollen with a large excess of 0.9 mass% sodium chloride aqueous solution for 1 hour under a load of 4.83 kPa (49 g / cm 2} , 0.7 psi). It refers to the absorption ratio (unit: g / g) after being allowed to grow.

[1-3-5] "Amount of residual monomer" (WSP210.3 (10))
The amount of residual monomer (Amount of Resinical Acrylate Monomers) is the amount of unreacted acrylic acid and a salt thereof in a water-absorbing agent or a water-absorbent resin. Specifically, it refers to the amount of extracted monomer (unit: ppm (mass basis)) after 1.0 g of a water-absorbing agent or a water-absorbent resin is added to 200 ml of a 0.9 mass% sodium chloride aqueous solution and stirred for 1 hour. .. The amount of residual monomer is measured by using high performance liquid chromatography.

[1-4] Liquid permeability The "liquid permeability" of the water-absorbing agent or water-absorbent resin in the present invention means the flowability of the liquid passing between the particles of the swollen gel under load or no load. do. Typical measurement methods for "liquid permeability" include SFC (Saline Flow Conducity / Saline Flow Inducibility) and GBP (Gel Bed Permeability / Gel Bed Permeability).
The "SFC" is the liquid permeability of a 0.69 mass% sodium chloride aqueous solution with respect to a water absorbing agent or a water absorbing resin under a load of 2.07 kPa, and is described in the SFC test method described in US Pat. No. 5,669,894. It is a value measured in accordance with it.
The "GBP" is the liquid permeability of a 0.69 mass% sodium chloride aqueous solution to a water-absorbing agent or a water-absorbent resin under load or free swelling, and is the GBP test described in International Publication No. 2005/016393. It is a value measured according to the method.

[1-5] Specific Surface Area In the present specification, the “specific surface area” means the surface area (unit: m ² / kg) per unit mass of the water-absorbing agent or the water-absorbent resin. The "specific surface area" can be obtained by performing an image analysis of the result measured by a three-dimensional analyzer using the X-ray CT method with three-dimensional analysis software. In the unlikely event that there is a water-absorbing agent or water-absorbent resin to which the X-ray CT method cannot be applied, the gas adsorption (BET) method can be used, and if it is difficult to apply the gas adsorption method (BET) method, air permeation The method, mercury intrusion method, etc. can be used.

[1-6] Water absorption rate (Voltex method) (Japanese Industrial Standard JIS K 7224 (1996))
The water absorption rate (Vortex method) is the time (unit) until the stirrer chip is covered with the test solution by adding 2.0 g of a water absorbent or a water absorbent resin to 50 g of a 0.9 mass% sodium chloride aqueous solution stirred at 600 rpm. ; Seconds).

[1-7] Amount of residual polyhydric alcohol The amount of residual polyhydric alcohol is the amount of unreacted polyhydric alcohol in the water-absorbing agent or the water-absorbent resin among the polyhydric alcohols used for surface cross-linking of the water-absorbent resin. Is. Specifically, it refers to the amount of extracted polyhydric alcohol (unit: mass%) after adding 1.0 g of a water-absorbing agent or a water-absorbent resin to 200 ml of a 0.9 mass% sodium chloride aqueous solution and stirring for 1 hour. The amount of residual polyhydric alcohol is measured by using high performance liquid chromatography. The polyhydric alcohol is often used as a surface cross-linking agent, and the residual amount of such a polyhydric alcohol is specified.

[1-8] Others In the present specification, "X to Y" indicating a range means "X or more and Y or less". Unless otherwise noted, "ppm (based on mass)" means "mass ppm (based on mass)". Further, "-acid (salt)" means "-acid and / or a salt thereof", and "(meth) acrylic" means "acrylic and / or methacryl".

[2] Method for producing a water-absorbing agent The water-absorbing agent according to the present invention contains 90% by mass of a polyacrylic acid (salt) -based water-absorbing resin having ^{a gel bulk density of 0.50 g / cm 3} or more and 1.00 g / cm ^{3 or less.} It is produced by arranging an aromatic substance on the surface of the water-absorbent resin (particularly, a particulate water-absorbent resin) containing% or more.

First, a preferred method for producing the water-absorbent resin will be described in detail.
[2-1] Preparation Step of Aqueous Monomer This step is a step of preparing a monomer aqueous solution containing a monomer containing acrylic acid (salt) as a main component and at least one polymerizable internal cross-linking agent. be. The "main component" means that the amount (content) of acrylic acid (salt) used is usually 50 mol% with respect to the entire monomer (excluding the internal cross-linking agent) subjected to the polymerization reaction. As mentioned above, it means that it is preferably 70 mol% or more, more preferably 90 mol% or more (upper limit is 100 mol%). Although a monomeric slurry liquid can be used within a range that does not affect the water absorption performance of the water absorbing agent obtained as the final product, the monomer aqueous solution will be described here for convenience.

(Acrylic acid (salt))
In the present invention, it is preferable to use a known acrylic acid (salt) as a monomer (also referred to as a polymerizable monomer) from the viewpoint of physical properties and productivity of the water absorbing agent. Known acrylic acids contain trace amounts of components such as polymerization inhibitors and impurities. As the polymerization inhibitor, methoxyphenols are preferably used, and p-methoxyphenols are more preferably used. The content (concentration) of the polymerization inhibitor in acrylic acid is preferably 200 ppm (mass standard) or less, more preferably 10 ppm (mass standard) to 160 ppm, from the viewpoint of the polymerizable property of acrylic acid and the color tone of the water absorbent. (Mass basis), more preferably 20 ppm (mass basis) to 100 ppm (mass basis). As the impurities, in addition to organic compounds such as acetic acid, propionic acid, and furfural, each compound described in US Patent Application Publication No. 2008/0161512 is also contained in acrylic acid used in the present invention.
In addition, examples of the acrylate include a salt obtained by neutralizing the above-mentioned acrylic acid with the following basic compound. The acrylate may be a commercially available acrylate (for example, sodium acrylate) or a salt obtained by neutralizing acrylic acid.

(Basic compound)
The basic compound in the present invention refers to a compound exhibiting basicity, and specifically, sodium hydroxide or the like is applicable. Commercially available sodium hydroxide contains heavy metals such as zinc, lead, and iron on the order of ppm (mass basis), and can be expressed as a composition in a strict sense. In the present invention, such a composition is also treated as being included in the category of basic compounds.
Specific examples of the basic compound include alkali metal carbonates and hydrogen carbonates, alkali metal hydroxides, ammonia, and organic amines. Among them, a strongly basic compound is selected from the viewpoint of the water absorption performance of the water absorbing agent. Therefore, hydroxides of alkali metals such as sodium, potassium and lithium are preferable, and sodium hydroxide is more preferable. From the viewpoint of handleability, the basic compound is preferably an aqueous solution.

(Neutralization)
When a salt obtained by neutralizing acrylic acid is used as the acrylate, the time for neutralizing the salt is not particularly limited, and may be any of before, during, and after polymerization, and there are a plurality of times. Neutralization can also be done at the time or place of. Further, from the viewpoint of the production efficiency of the water absorbing agent, it is preferable to neutralize in a continuous manner.
When acrylic acid (salt) is used in the present invention, its neutralization rate is preferably 10 mol% to 90 mol%, more preferably 40 mol% to 85 mol%, and further, with respect to the acid group of the monomer. It is preferably 50 mol% to 80 mol%, particularly preferably 60 mol% to 75 mol%. By setting the neutralization rate within the range, it is possible to suppress a decrease in the water absorption performance of the water absorbing agent.
The range of the neutralization rate is applied to any of the above-mentioned neutralization before, during, and after the polymerization. The same applies to water absorbing agents as final products.

(Other monomers)
In the present invention, a monomer other than the above-mentioned acrylic acid (salt) (hereinafter, referred to as “other monomer”) can be used in combination with acrylic acid (salt), if necessary.
Specific examples of the other monomer include (anhydrous) maleic acid, itaconic acid, acrylic acid, vinyl sulfonic acid, allyltoluenesulfonic acid, vinyltoluenesulfonic acid, styrenesulfonic acid, 2- (meth). Anionic unsaturated monomers such as acrylamide-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2-hydroxyethyl (meth) acryloyl phosphate and the like. Salt; mercaptan group-containing unsaturated monomer; phenolic hydroxyl group-containing unsaturated monomer; amide group-containing unsaturated such as (meth) acrylamide, N-ethyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, etc. Monomer; Amino group-containing unsaturated monomer such as N, N-dimethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide Can be mentioned. In addition, the other monomer includes a water-soluble or hydrophobic unsaturated monomer. When the other monomer is used, the amount used is preferably 30 mol% or less, more preferably 10 mol% or less, still more preferably 10 mol% or less, based on the entire monomer (excluding the internal cross-linking agent). Is less than 5 mol%.

(Internal cross-linking agent)
In this preferred production method, an internal cross-linking agent is used. Specific examples of the internal cross-linking agent include N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, and trimethylolpropane. Di (meth) acrylate, trimethylolpropane tri (meth) acrylate, glycerin tri (meth) acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa ( Meta) acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene Glycol, glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl (meth) acrylate and the like can be mentioned. From these internal cross-linking agents, at least one kind of internal cross-linking agent is selected in consideration of reactivity and the like. Further, from the viewpoint of water absorption performance of the water absorbing agent, an internal cross-linking agent having two or more polymerizable unsaturated groups is preferable, an internal cross-linking agent having thermal decomposition property at a drying temperature described later is more preferable, and (poly) is more preferable. ) An internal cross-linking agent having two or more polymerizable unsaturated groups having an alkylene glycol structure is selected.

Specific examples of the polymerizable unsaturated group include an allyl group and a (meth) acrylate group, and a (meth) acrylate group is more preferable. Specific examples of the (poly) alkylene glycol structure include polyethylene glycol. The number of alkylene glycol units (hereinafter, may be referred to as n) is preferably 1 to 100, more preferably 6 to 50, still more preferably 6 to 20, and most preferably 6. It is 10.

The amount of the internal cross-linking agent used is preferably 0.0001 mol% to 10 mol%, more preferably 0.001 mol% to 5 mol%, based on the entire monomer (excluding the internal cross-linking agent). , More preferably 0.01 mol% to 1 mol%. By setting the amount to be used within the above range, a water absorbing agent having a desired water absorbing performance can be obtained. Further, it is important to adjust the amount of the internal cross-linking agent in order to keep the gel bulk density of the water-absorbent resin or the water-absorbent agent within a predetermined range. On the other hand, if the amount used is outside the range, an increase in the water-soluble component and a decrease in the absorption ratio may be observed as the gel strength decreases.

The internal cross-linking agent is preferably added in advance at the time of preparing the aqueous monomer solution, and in this case, the cross-linking reaction is carried out at the same time as the polymerization reaction. On the other hand, the polymerization reaction can be started without adding the internal cross-linking agent, and the cross-linking reaction can be carried out by adding the internal cross-linking agent during or after the polymerization reaction. Moreover, these methods can be used together.

(Substances added to monomeric aqueous solution)
In the present invention, at the time of preparing the monomer aqueous solution, during the polymerization reaction and the cross-linking reaction, or at any one or more places after the polymerization reaction and the cross-linking reaction, from the viewpoint of improving the physical properties of the water absorbing agent, the following The substance can be added to the monomeric aqueous solution.
Specific examples of the substance include hydrophilic polymers such as starch, starch derivative, cellulose, cellulose derivative, polyvinyl alcohol (PVA), polyacrylic acid (salt), and crosslinked product of polyacrylic acid (salt); carbon dioxide. Examples thereof include salts, azo compounds, compounds such as foaming agents that generate various bubbles, surfactants, chelating agents, and chain transfer agents.

The amount of the hydrophilic polymer added is preferably 50% by mass or less, more preferably 20% by mass or less, still more preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on the aqueous monomer solution. There is (the lower limit is 0% by mass). The amount of the compound added is preferably 5% by mass or less, more preferably 1% by mass or less, still more preferably 0.5% by mass or less, based on the aqueous monomer solution (the lower limit is 0% by mass). ).
When a water-soluble resin or a water-absorbent resin is used as the hydrophilic polymer, a graft polymer or a water-absorbent resin composition (for example, a starch-acrylic acid (salt) copolymer, a PVA-acrylic acid (salt) copolymer) is used. Etc.) is obtained. These graft polymers or water-absorbent resin compositions are also included in the category of the polyacrylic acid (salt) -based water-absorbent resin according to the present invention.

(Concentration of monomer component)
A monomer aqueous solution is prepared by selecting various substances (components) described above according to the purpose, defining the respective amounts so as to satisfy the above range, and mixing them with each other. In the present invention, in addition to using the monomer as an aqueous solution, it can also be used as a mixed solution of water and a hydrophilic solvent.
The total concentration of each substance (component) (hereinafter referred to as "monomer component") is preferably 10% by mass to 80% by mass, more preferably 20% by mass, from the viewpoint of the physical properties of the water absorbing agent. It is ~ 75% by mass, more preferably 30% by mass to 70% by mass. The concentration of the monomer component is calculated from the following formula (1).
Concentration of monomer component (mass%) = [(mass of monomer component) / (mass of aqueous monomer solution)] × 100 ... Equation (1)
In the above formula (1), (mass of the monomer aqueous solution) does not include the mass of the graft component, the water-absorbent resin, and the hydrophobic organic solvent in the reverse phase suspension polymerization.

[2-2] Polymerization Step This step contains a monomer containing acrylic acid (salt) as a main component and at least one polymerizable internal cross-linking agent obtained in the step of preparing the aqueous monomer solution. This is a step of polymerizing a monomer aqueous solution to obtain a hydrogel-like crosslinked polymer (hereinafter, referred to as “hydrogen gel”).

(Polymerization initiator)
In the present invention, a polymerization initiator is used at the time of polymerization. Examples of the polymerization initiator include a thermal decomposition type polymerization initiator, a photodegradable polymerization initiator, and a redox-based polymerization initiator in which a reducing agent that promotes the decomposition of these polymerization initiators is used in combination. Specific examples of the polymerization initiator include sodium persulfate, potassium persulfate, ammonium persulfate, t-butyl hydroperoxide, hydrogen peroxide, and 2,2'-azobis (2-amidinopropane) dihydrochloride. Examples include radical polymerization initiators. From these polymerization initiators, at least one type of polymerization initiator is selected in consideration of the polymerization form and the like. Further, from the viewpoint of the handleability of the polymerization initiator and the physical properties of the water absorbing agent, a peroxide or an azo compound is preferably selected, a peroxide is more preferable, and a persulfate is more preferably selected as the polymerization initiator. When an oxidizing radical polymerization initiator is used, for example, a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid may be used in combination to perform redox polymerization.

The amount of the polymerization initiator used is preferably 0.001 mol% to 1 mol%, more preferably 0.001 mol% to 0.5, based on the entire monomer (excluding the internal cross-linking agent). It is mol%, more preferably 0.01 mol% to 0.1 mol%. The amount of the reducing agent used is preferably 0.0001 mol% to 0.02 mol%, more preferably 0.0005 mol% to the total amount of the monomer (excluding the internal cross-linking agent). It is 0.015 mol%. By setting the amount to be used within the above range, a water absorbing agent having a desired water absorbing performance can be obtained.
Further, in the present invention, the polymerization reaction may be started by irradiation with active energy rays such as radiation, electron beam, and ultraviolet rays. Further, irradiation with active energy rays and the polymerization initiator may be used in combination.

(Polymerization form)
Examples of the polymerization form applied to the present invention include aqueous solution polymerization, reverse phase suspension polymerization, spray polymerization, droplet polymerization, bulk polymerization, and precipitation polymerization. Among them, from the viewpoint of ease of polymerization control and water absorption performance of the water absorbing agent, aqueous solution polymerization or reverse phase suspension polymerization is preferable, aqueous solution polymerization is more preferable, and continuous aqueous solution polymerization is more preferable. Reversed phase suspension polymerization is described in WO 2007/004529, WO 2012/023433 and the like. Further, the continuous aqueous solution polymerization can produce a water absorbing agent with high productivity, and specific examples thereof include US Pat. No. 4,893,999, US Pat. No. 6,906,159, US Pat. No. 7091253, and US Pat. No. 7,741400. Examples thereof include continuous belt polymerization described in US Pat. No. 8519212, JP-A-2005-36100, and continuous kneader polymerization described in US Pat. No. 6,987,151.

Preferred forms of the continuous aqueous solution polymerization include high temperature start polymerization, high concentration polymerization, foam polymerization and the like. In the high temperature start polymerization, the temperature of the aqueous monomer solution at the start of polymerization is preferably 30 ° C. or higher, more preferably 35 ° C. or higher, further preferably 40 ° C. or higher, and particularly preferably 50 ° C. or higher (upper limit is a single amount). Boiling point of body aqueous solution) Polymerized form. In high-concentration polymerization, the monomer concentration at the start of polymerization is preferably 30% by mass or more, more preferably 35% by mass or more, still more preferably 40% by mass or more, and particularly preferably 45% by mass or more (upper limit is 45% by mass). Saturation concentration of monomer aqueous solution) Polymerization form. Foam polymerization is a polymerization form in which the monomer aqueous solution containing a foaming agent or bubbles is polymerized. In addition, each of these polymerization forms may be carried out individually, or two or more may be carried out in combination.

The method for dispersing bubbles in the foam polymerization is not particularly limited, but a method for dispersing the gas dissolved in the monomer aqueous solution as bubbles by reducing the solubility, a method for introducing a gas from the outside and dispersing as bubbles, Examples thereof include a method of adding a foaming agent to a monomer aqueous solution to cause foaming. Further, the dispersion method may be appropriately used in combination depending on the physical properties of the target water absorbing agent.
When a gas is introduced from the outside, specific examples of the gas include oxygen, air, nitrogen, carbon dioxide, ozone and the like, and a mixed gas of these gases, preferably nitrogen, carbon dioxide and the like. An inert gas is used, more preferably nitrogen is used from the standpoint of polymerizable and cost.
Effervescent agents that can be used include azo compounds and solutions of organic or inorganic carbonates, dispersions or powders (eg, particle size 0.1-1000 μm), preferably inorganic carbonates, typically sodium carbonate, ammonium carbonate. ， Carbonates such as magnesium carbonate and hydrogen carbonates can be used.

Further, each of the above-mentioned polymerization forms can be carried out in an air atmosphere, but from the viewpoint of the color tone of the water absorbing agent, it should be carried out in an atmosphere of an inert gas such as nitrogen or argon (oxygen concentration is 1 volume% or less). Is preferable. It is also preferable that the dissolved oxygen in the monomer aqueous solution is sufficiently replaced with an inert gas (the amount of dissolved oxygen is less than 1 mg / L).
By using a water-containing gel having a foamed shape (also known as porous), a water-absorbing resin, or a water-absorbing agent by foam polymerization, the water-absorbing rate of the water-absorbing agent can be improved, and the water-absorbing agent can be easily immobilized on the absorbing article. The foamed shape can be confirmed by the holes on the particle surface (for example, holes having a diameter of 1 to 100 μm) with an electron microscope. The number of pores is preferably 1 or more, more preferably 1 to 10000 or 10 to 1000 per water absorbing agent, and can be controlled by the foam polymerization.
The foam polymerization is a preferable technique for increasing the specific surface area of the water-absorbent resin and the water-absorbent agent described later.

[2-3] Gel crushing step This step is a step of gel crushing the hydrous gel obtained in the polymerization step to obtain a particulate hydrogel (hereinafter, referred to as “particulate hydrogel”). .. In addition, in order to distinguish from "crushing" in the crushing step described later, this step is referred to as "gel crushing".
The gel crushing refers to adjusting the hydrous gel to a predetermined size by using a gel crusher such as a kneader, a meat chopper, or a cutter mill.

Regarding the embodiment of gel crushing, operating conditions, etc., the contents described in Pamphlet No. 2011/126079 of International Publication No. 2011/126079 are preferably applied to the present invention. When the polymerization form is kneader polymerization, the polymerization step and the gel crushing step are carried out at the same time. When a particulate hydrogel is obtained in the polymerization step such as reverse phase suspension polymerization, spray polymerization or droplet polymerization, it is considered that the gel pulverization step is carried out at the same time as the polymerization step. Further, by undergoing the gel crushing step in the present invention, an amorphous crushed water-absorbent resin powder or a water-absorbing agent can be obtained.
The gel granulated by the gel pulverization step is generally preferably in the range of 0.1 to 10 mm. If the gel is finer than 0.1 mm, the physical properties of the obtained water-absorbent resin may be low. If it is larger than 10 mm, it may be difficult to dry. The mass average particle size (D50) of the particulate hydrogel is preferably 500 μm to 2000 μm, more preferably 550 μm to 1500 μm, and even more preferably 600 μm to 1000 μm.

In order to increase the specific surface area of the water-absorbent resin or water-absorbing agent described later, it is preferable to use the gel pulverization method described in Pamphlet No. 2011/126079. Further, the gel pulverization technique may be combined with the above-mentioned foam polymerization.
The method for measuring the mass average particle size (D50) of the particulate hydrogel is carried out by the method described in International Publication No. 2011/126079.

[2-4] Drying Step This step is a step of drying the hydrous gel and / or the particulate hydrogel obtained in the polymerization step and / or the gel pulverization step to a desired resin solid content to obtain a dry polymer. be. The resin solid content is determined from the weight loss by drying (mass change when 1 g of the water-absorbent resin is heated at 180 ° C. for 3 hours), preferably 80% by mass or more, more preferably 85% by mass to 99% by mass, still more preferably. Is 90% by mass to 98% by mass, particularly preferably 92% by mass to 97% by mass.

The drying method of the hydrogel and / or the particulate hydrogel is, for example, heat drying, hot air drying, vacuum drying, fluidized layer drying, infrared drying, microwave drying, drum dryer drying, co-boiling dehydration with a hydrophobic organic solvent. Drying by, high humidity drying using high temperature steam, etc. can be mentioned. Above all, from the viewpoint of drying efficiency, hot air drying is preferable, and band drying in which hot air drying is performed on a ventilation belt is more preferable.

The drying temperature (temperature of the hot air) in the hot air drying is preferably 120 ° C. to 250 ° C., more preferably 150 ° C. to 200 ° C. from the viewpoint of the color tone of the water-absorbent resin and the drying efficiency. Drying conditions other than the drying temperature, such as the air velocity and drying time of hot air, may be appropriately set according to the water content and total mass of the particulate hydrogel to be dried and the target resin solid content. When drying the band, the conditions described in the international publication 2006/100300 pamphlet, 2011/025012 pamphlet, 2011/0250513 pamphlet, 2011/111657 pamphlet, etc. are applied as appropriate. Will be done.

The drying temperature in the present invention is preferably 120 ° C. to 250 ° C., more preferably 130 ° C. to 230 ° C., and even more preferably 150 ° C. to 200 ° C. from the viewpoint of drying efficiency and color tone of the water absorbing agent. The drying time is preferably 10 minutes to 2 hours, more preferably 20 minutes to 1.5 hours, and even more preferably 30 minutes to 1 hour. By setting the drying temperature and drying time within the above range, the physical characteristics of the obtained water absorbing agent can be set within a desired range. Further, the physical properties of the water-absorbent resin powder and the water-absorbent resin particles as intermediate products can also be set within a desired range.

[2-5] Crushing Step, Classification Step In this step, the dry polymer obtained through the drying step is crushed (crushing step) and adjusted to a desired particle size (classifying step) to obtain a base polymer (classification step). This is a step of obtaining a water-absorbent resin before surface cross-linking. By going through the pulverization step after drying, an amorphous crushed water-absorbent resin or a water-absorbing agent can be obtained.

Examples of the crusher used in the crushing step include a high-speed rotary crusher such as a roll mill, a hammer mill, a screw mill, and a pin mill, a vibration mill, a knuckle type crusher, and a cylindrical mixer. Among them, a roll mill is preferably selected from the viewpoint of pulverization efficiency. Further, a plurality of these crushers can be used in combination.

Examples of the method for adjusting the particle size in the classification step include sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)), air flow classification, and the like. Above all, sieving classification is preferably selected from the viewpoint of classification efficiency. The adjustment of the particle size of the water absorbing agent is not limited to the pulverization step and the classification step, but is not limited to the polymerization step (particularly reverse phase suspension polymerization, droplet polymerization, etc.) and other steps (for example, granulation step). It can also be carried out in the fine powder recovery step).

The particle size distribution of the base polymer has a mass average particle size (D50) of 300 to 600 μm, and the proportion of particles less than 150 μm is 5% by mass or less. The mass average particle size (D50) is preferably 300 to 500 μm, more preferably 300 to 450 μm, and even more preferably 300 to 400 μm. The proportion of particles less than 150 μm is more preferably 4% by mass or less, further preferably 3% by mass or less, and particularly preferably 2% by mass or less. Further, the logarithmic standard deviation (σζ) indicating the narrowness of the particle size distribution is preferably 0.25 to 0.50, more preferably 0.25 to 0.40, still more preferably 0.27 to 0.35. .. The logarithmic standard deviation (σζ) of the particle size distribution has the advantage that the smaller the value, the more uniform the particle size and the less segregation of the particles. However, in order to make the logarithmic standard deviation (σζ) of the particle size distribution less than 0.2, it is necessary to repeat pulverization and classification to remove coarse particles and fine particles, which is disadvantageous from the viewpoint of productivity and cost. May bring.
The above-mentioned particle size is applied not only to the water-absorbent resin after surface cross-linking but also to the water-absorbent agent as a final product. Therefore, it is preferable that the surface cross-linking treatment (surface cross-linking step) is performed so as to maintain the particle size in the above range adjusted by the base polymer, and the particle size can be adjusted by providing a sizing step after the surface cross-linking step. preferable.

[2-6] Surface Crosslinking Step This step is a step of providing a portion having a higher crosslink density on the surface layer of the base polymer obtained through each of the above steps, and includes a mixing step, a heat treatment step, a cooling step and the like. It is composed. In the surface cross-linking step, radical cross-linking, surface polymerization, a cross-linking reaction with a surface cross-linking agent and the like occur on the surface of the base polymer to obtain a surface-crosslinked water-absorbent resin.

[2-6-1] Mixing Step This step is a step of obtaining a humidified mixture by mixing a solution containing a surface cross-linking agent (hereinafter referred to as "surface cross-linking agent solution") with a base polymer in a mixing device. Is.
(Surface cross-linking agent)
In the present invention, a surface cross-linking agent is used at the time of surface cross-linking. Specific examples of the surface cross-linking agent include the surface cross-linking agent described in US Pat. No. 7,183,456. From these surface cross-linking agents, at least one kind of surface cross-linking agent is selected in consideration of reactivity and the like. Further, from the viewpoint of the handleability of the surface cross-linking agent, the water-absorbing performance of the water-absorbing agent, and the like, preferably, a surface cross-linking agent having two or more functional groups that react with a carboxyl group and an organic compound in which a covalent bond is formed is selected. Will be done.

More specifically, as the surface cross-linking agent,
Ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, propylene glycol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, 2,3 -Butandiol, 1,5-pentanediol, 1,4-pentanediol, 1,3-pentanediol, 1,2-pentanediol, 2,3-pentanediol, 2,4-pentanediol, dipropylene glycol, Polypropylene glycol, glycerin, polyglycerin, 1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 1,3-hexanediol, 1,2-hexanediol, 2,3-hexanediol, Polyhydric alcohol compounds such as 2,4-hexanediol, diethanolamine, triethanolamine,
Polyvalent amine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyallylamine, polyethyleneimine, etc.
Halo epoxy compound,
Condensate of polyvalent amine compound and haloepoxy compound,
Oxazoline compounds such as 1,2-ethylenebisoxazoline,
Oxazolidinone compound,
1,3-Dioxolane-2-one (ethylene carbonate), 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl- 1,3-Dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxane-2-one, 4 Alkylene carbonate compounds such as -methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, 1,3-dioxopan-2-one,
Polyvalent glycidyl compounds such as ethylene glycol diglycidyl ether, polyethylene diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycidyl, etc.
Oxetane compound,
Vinyl ether compound,
Cyclic urea compounds and the like can be mentioned. Preferred surface cross-linking agents are polyhydric alcohol compounds such as 1,3-propanediol, alkylene carbonate compounds such as ethylene carbonate, and polyhydric glycidyl compounds such as ethylene glycol diglycidyl ether. In order to improve PDAUP, which is one of the preferable features of the water absorbing agent according to the present invention, it is preferable to use at least one of a polyhydric alcohol and an alkylene carbonate.

The amount of the surface cross-linking agent used (the total amount when a plurality of types are used) is preferably 0.01 parts by mass to 10 parts by mass, and more preferably 0.01 parts by mass with respect to 100 parts by mass of the base polymer. It is ~ 5 parts by mass, more preferably 0.01 part by mass to 2 parts by mass. By setting the amount of the surface cross-linking agent to be within the above range, an optimum cross-linked structure can be formed on the surface layer of the base polymer, and a water-absorbent resin or a water-absorbing agent having high physical characteristics can be obtained. Further, adjusting the amount of the surface cross-linking agent according to the type is also effective for optimizing the gel bulk density of the water-absorbent resin and the water-absorbent agent.

The surface cross-linking agent is preferably added to the base polymer as an aqueous solution. In this case, the amount of water used is preferably 0.1 parts by mass to 20 parts by mass, more preferably 0.3 parts by mass to 15 parts by mass, and further preferably 0.5 parts by mass with respect to 100 parts by mass of the base polymer. 10 parts by mass. By setting the amount of water used within the above range, the handleability of the surface cross-linking agent solution is improved, and the surface cross-linking agent can be evenly mixed with the base polymer.

Further, a hydrophilic organic solvent may be used in combination with the water as needed to prepare the surface cross-linking agent solution. In this case, the amount of the hydrophilic organic solvent used is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and further preferably 1 part by mass or less with respect to 100 parts by mass of the base polymer. Specific examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol; ketones such as acetone; ethers such as dioxane; amides such as N, N-dimethylformamide; sulfoxides such as dimethyl sulfoxide. ; Polyhydric alcohols such as ethylene glycol; and the like. However, since the use of these hydrophilic organic solvents may cause an unpleasant odor of the swollen gel, which is a subject of the present invention, it is preferable to limit the amount to be used as small as possible.
In addition, various additives added in the following "[2-9] Liquid permeability improving agent or the same component addition step" and "[2-10] Other steps" are within the range of 5 parts by mass or less. Therefore, it can be added to the surface cross-linking agent solution, or can be added separately in the mixing step.

(Mixing method, mixing conditions)
For mixing the base polymer and the surface cross-linking agent solution, a surface cross-linking agent solution is prepared in advance, and the solution is preferably sprayed or dropped onto the base polymer, and more preferably sprayed and mixed. The method is selected.
It is preferable that the mixing device for mixing has the torque required to uniformly and reliably mix the base polymer and the surface cross-linking agent. The mixing device is preferably a high-speed stirring type mixer, and more preferably a high-speed stirring type continuous mixer. The rotation speed of the high-speed stirring type mixer is preferably 100 rpm to 10000 rpm, more preferably 300 rpm to 2000 rpm.
The temperature of the base polymer supplied to this step is preferably 35 ° C. to 80 ° C., more preferably 35 ° C. to 70 ° C., still more preferably, from the viewpoint of mixability with the surface cross-linking agent solution and cohesiveness of the humidified mixture. The temperature is 35 ° C to 60 ° C. The mixing time is preferably 1 second to 1 hour, more preferably 5 seconds to 10 minutes.

[2-6-2] Heat Treatment Step This step is a step of applying heat to the humidified mixture obtained in the mixing step to carry out a crosslinking reaction on the surface of the base polymer.
In the heat treatment of the humidified mixture, the humidified mixture may be heated in a stationary state or may be heated in a fluid state by using a power such as stirring, but stirring may be performed in that the entire humidified mixture can be heated evenly. It is preferable to heat underneath. From the above viewpoint, the heat treatment apparatus that performs the heat treatment includes a paddle dryer, a multi-fin processor, a coward dryer and the like.
The heating temperature in this step is preferably 150 ° C. to 250 ° C., more preferably 170 ° C. to 250 ° C., still more preferably 180 ° C. to 230, from the viewpoint of the type and amount of the surface cross-linking agent and the water absorption performance of the water absorbing agent. ℃. The heating time is at least 5 minutes, preferably at least 7 minutes.
By controlling the heating temperature and the heating time within the above range, the water absorption performance of the obtained water absorbing agent is improved, which is preferable.

[2-6-3] Cooling step This step is an arbitrary step provided as needed after the heat treatment step. This step is a step of forcibly cooling the high-temperature water-absorbent resin that has completed the heat treatment step to a predetermined temperature to promptly terminate the surface cross-linking reaction.
The water-absorbent resin may be cooled in a stationary state or in a fluid state using power such as stirring, but the water-absorbent resin as a whole can be cooled evenly under stirring. It is preferable to cool it. From the above viewpoint, the cooling device for cooling includes a paddle dryer, a multi-fin processor, a coward dryer and the like. It should be noted that these cooling devices may have the same specifications as the heat treatment devices used in the heat treatment step. This is because it can be used as a cooling device by changing the heat medium of the heat treatment device to a refrigerant.
The cooling temperature in this step may be appropriately set according to the heating temperature in the heat treatment step, the water absorption performance of the water absorbing agent, etc., but is preferably 40 ° C. to 100 ° C., more preferably 40 ° C. to 90 ° C., and even more preferably. It is 50 ° C. to 70 ° C.

[2-7] Characteristics of Surface Crosslinked Water Absorbent Resin The water absorbent resin produced as described above has a gel bulk density of 0.50 g / cm ³ or more and 1.00 g / cm ³ or less. Further, the water-absorbent resin preferably has one or more of the following properties, and more preferably has all of the following properties.
-Mass average particle size (D50) is 300 μm or more and 600 μm or less.-The proportion of particles with a particle size of less than 150 μm is 5% by mass or less in 100% by mass of the water-absorbent resin.-Specific surface area is 25 m ² / kg or more. 50 m ² / kg or less-Unpressurized absorption ratio (CRC) is 25-40 g / g-Pressurized absorption ratio (AAP) is 20 g / g or more-Pressurized permeation-dependent absorption ratio (PDAUP) 3 g / g or more ・ The water absorption rate (Vortex method) is 10 seconds or more and 50 seconds or less. As will be described later, the water absorbent of the present invention also has the same gel bulk density as the water-absorbent resin, and is preferable. Has one or more of the above-mentioned properties, more preferably all of the above-mentioned properties, and its technical significance, a range more preferable than the numerical range, and the like are common to the water-absorbent resin and the water-absorbent agent. The details of the various properties will be described in the section of the water-absorbing agent, and the description of the water-absorbing agent will be referred to as it is in the description of the present water-absorbent resin.
[2-8] Arrangement of Aromatic Substances The procedure for arranging an aromatic substance on the surface of a surface-crosslinked water-absorbent resin is not particularly limited, and for example, the base polymer before surface cross-linking and the aromatic substance are mixed. Then, the base polymer may be surface-crosslinked, but in order to increase the surface presence probability of the aromatic substance, it is preferable to mix the surface-crosslinked water-absorbent resin and the aromatic substance.

Examples of the aromatic substance include terpene-based aromatic compounds and phenol-based aromatic compounds, which may be used alone or in combination.
As a terpene-based aromatic compound,
Chain monoterpenes; for example, citronellol, citronellal, geraniol, geranial, neral, nerol, linalool and derivatives thereof, etc.
Monocyclic 6-membered ring monoterpine; for example, α-terpinene, terpinene, limonene, α-terpinene, β-terpinene, γ-terpinene, α-phellandrene, β-phellandrene, menthol, menthone, pregon, piperitone, piperitone. , Carvone, diosphenol, thymol, carpachlor, cuminaldehyde and derivatives thereof, etc.
7-membered ring monoterpenes; for example, α-glossine, β-glossy (hinokitiol), β-drabulin, etc.
Monoterpenes containing an oxygen ring; for example, rose oxide, 1,4-cyonel, 1,8-cyonele, rosefuran and derivatives thereof, etc.
Can be mentioned.

On the other hand, examples of phenolic aromatic compounds include cinnamaldehyde, eugenol, vanillin, aviol, coumarin, methyl salicylate, anethole, gingerlon, raspberry ketone, 3-hydroxy-4-phenyl-2-butanone, 2-phenylethanol, shogaol and derivatives thereof. Can be mentioned.

Among them, preferable aromatic substances are selected from the group consisting of chain monoterpenes, monocyclic 6-membered ring monoterpenes, 7-membered ring monoterpenes, and phenolic aromatic compounds (particularly 7-membered ring monoterpenes and phenolic compounds). One or more substances selected from geranial, neral, terpinolene, limonene, β-zybricin (hinokitiol), vanillin (particularly β-zybricin (hinokitiol), vanillin) 1 selected from the group consisting of aromatic compounds. One or more substances).

A pure aromatic substance may be mixed with the water-absorbent resin, or a composition containing the aromatic substance (including the extract) may be mixed with the water-absorbent resin. In other words, the aromatic substance may be a synthetic substance or may be contained in an extract obtained from a natural product. The extraction method from natural products is not particularly limited, and any essential oil, distillate, etc. obtained by steam distillation or alcohol extraction may be used.
Natural products include herbs such as basil, lavender, bergamot, lemongrass, hops and thyme, citrus fruits such as lemon, orange and lime, cedar, cypress and hiba (extracts include hiba oil, and the aroma contained therein. (The sex substance is hinoki thiol), trees such as tsuga, todomatsu, karamatsu, ezomatsu, red pine, fir, cherry blossoms, roses, lavender, wisteria flowers, carnations, hyacinths, alleppo pine, ylang ylang, geranium, neroli, kinkouboku, etc. And leaves, citrus beans, vanilla beans (extract includes vanilla essence, and the aromatic substance contained is vanillin), beans such as coffee beans, and the like.

The amount of the aromatic substance is, for example, 0.001 ppm (mass standard) or more, preferably 0.005 ppm (mass standard) or more, and 0.01 ppm (mass standard) or more with respect to the water-absorbent resin. It is more preferably 0.02 ppm (mass basis) or more, particularly preferably 0.05 ppm (mass basis) or more, and most preferably 0.3 ppm (mass basis). The upper limit of the aromatic substance is not particularly limited, and even if the amount is increased, the malodor from the water-absorbent resin can be prevented. Therefore, for example, it may be 1% by mass or less, or 0.1% by mass or less. However, if there is a large amount of aromatic substances, the odor of the aromatic substances itself is perceived as a foul odor, so it is preferable not to increase the amount more than necessary. From this point of view, the upper limit of the aromatic substance is preferably 50 ppm (mass basis) or less, more preferably 30 ppm (mass basis) or less, particularly preferably 10 ppm (mass basis) or less, and most preferably 5 ppm (mass basis). ) It is as follows.
The above-mentioned quantitative relationship between the aromatic substance and the water-absorbent resin is the same as the quantitative relationship between the aromatic substance and the water-absorbent resin contained in the water-absorbing agent.

When a liquid aromatic substance or a composition thereof (hereinafter collectively referred to as a liquid aromatic component) is mixed with a water-absorbent resin, the liquid aromatic component is sprayed on the water-absorbent resin and a sufficient torque is applied. It is preferable to uniformly and surely mix the water-absorbent resin and the liquid aromatic component. On the other hand, when a solid (powder or the like) aromatic substance or a composition thereof (hereinafter, these are collectively referred to as a solid aromatic component) is mixed with a water-absorbent resin, it may be dry-blended and may be water-based such as water. The liquid may be used as a binder. In the solid aromatic component, the aromatic substance may be supported on a carrier such as organic fine particles or inorganic fine particles.

Examples of the device used for the mixing include a stirring type mixer, a cylindrical type mixer, a double-walled conical type mixer, a V-shaped mixer, a ribbon type mixer, a screw type mixer, and a fluid type rotary desk type. A mixer, an air flow type mixer, a double-armed kneader, an internal mixer, a crushing type kneader, a rotary mixer, a screw type extruder and the like are suitable. When a stirring type mixer is used, the rotation speed is preferably 5 rpm to 10000 rpm, more preferably 10 rpm to 2000 rpm.

[2-9] Step of Adding Liquid Permeability Improver or Component Agent By the way, to any one or more of the base polymer before surface cross-linking, the water-absorbent resin after surface cross-linking, and the water-absorbent resin on which aromatic substances are arranged. On the other hand, a liquid permeability improving agent may be added. In other words, the water absorbing agent may contain a liquid permeability improving agent in addition to the above-mentioned aromatic substance and water absorbing resin. Examples of the method for measuring the liquid permeability include saline flow inducibility (SFC) and gel bed permeability under load or no load (GBP). In the present invention, the liquid permeability greatly affects the value. The reduction permeation-dependent absorption ratio (PDAUP) is measured. In the present specification, the liquid permeability improving agent refers to a compound having a function of improving the liquid permeability of the water-absorbent resin (for example, improving SFC and GBP by 0.1% or more).
The liquid permeability improver used in the present invention is preferably at least one compound selected from a polyvalent metal salt, a cationic polymer, and inorganic fine particles. Two or more of these liquid permeability improving agents are used in combination, if necessary. These are not intended to improve the liquid permeability, and may be added as other functions such as an Anti-Caking agent under moisture absorption, a powder flow control agent, a binder of a water-absorbent resin, and the like. When added for the purpose of other functions, it is referred to as the same component agent in this specification. The amount of the liquid permeability improving agent or the component agent added is appropriately set according to the selected compound.

(Multivalent metal salt)
When a polyvalent metal salt is used, the polyvalent metal cation of the polyvalent metal salt is preferably divalent or higher, more preferably divalent to tetravalent, and even more preferably trivalent or tetravalent. Examples of the multivalent metal that can be used include aluminum and zirconium. Therefore, examples of the polyvalent metal salt that can be used in this step include aluminum lactate, zirconium lactate, aluminum sulfate, and zirconium sulfate. Of these, aluminum lactate or aluminum sulfate is more preferable, and aluminum sulfate is even more preferable, from the viewpoint of improving SFC.
The amount of the polyvalent metal salt added is preferably 0 mol or more and ^{less than 3.6 × 10 -5} mol, more preferably 0 mol or more and 1.4 × 10 ^{-5 with respect to 1 g of the base polymer or the water-absorbent resin.} It is less than a mole, more preferably 0 mol or more and ^{less than 1.0 × 10 -5} mol.

(Cationic polymer)
When a cationic polymer is used, examples of the cationic polymer include substances described in US Pat. No. 7,098284. Of these, vinylamine polymers are more preferable from the viewpoint of improving SFC and GBP. The mass average molecular weight of the cationic polymer is preferably 5000 to 1000000.
The cationic polymer is preferably 0 parts by mass or more and less than 2.5 parts by mass, more preferably 0 parts by mass or more and less than 2.0 parts by mass, and further preferably 0 parts by mass with respect to 100 parts by mass of the water-absorbent resin particles. It may be added so as to be less than 1.0 part by mass.

(Inorganic fine particles)
When inorganic fine particles are used, examples of the inorganic fine particles include substances described in US Pat. No. 7,638,570. Of these, silicon dioxide is preferable from the viewpoint of improving SFC and GBP.
When the primary particle diameter is less than 20 nm, the inorganic fine particles preferably have 0 parts by mass or more and less than 1.2 parts by mass, and more preferably 0 parts by mass or more and 1.0 part by mass with respect to 100 parts by mass of the water-absorbent resin particles. It may be added so as to be less than 10 parts by mass, more preferably 0 parts by mass or more and less than 0.5 parts by mass. When the primary particle diameter is 20 nm or more, it is preferably 0 parts by mass or more and less than 2.0 parts by mass, and more preferably 0 parts by mass or more and less than 1.5 parts by mass with respect to 100 parts by mass of the water-absorbent resin particles. More preferably, it may be added so as to be 0 parts by mass or more and less than 1.0 part by mass.

[2-10] Other Steps In the present invention, in addition to the above-mentioned steps, a granulation step, a granulation step, a fine powder removal step, a fine powder recovery step, a fine powder reuse step, a step of adding other additives, and removal of the fine powder. The iron process and the like can be carried out as needed. Further, at least one kind of process selected from a transportation process, a storage process, a packing process, a storage process, and the like may be further included.
The granulation step includes a step of classifying and removing fine powder after the surface cross-linking step, and a step of classifying and pulverizing when the water-absorbent resin aggregates and exceeds a desired size. Further, the step of reusing the fine powder includes a step of converting the fine powder as it is or making it into a large water-containing gel in the granulation step and adding it to the water-containing gel or the like as a raw material in any step of the manufacturing step of the water-absorbent resin.
Further, in the step of adding the liquid permeability improving agent or the same component agent, in order to impart various functions to the water absorbing agent, a chelating agent, an inorganic reducing agent, an organic reducing agent, a hydroxycarboxylic acid compound, a surfactant, and phosphorus. One or more of a compound having an atom, an oxidizing agent, an organic powder such as a metal soap, a deodorant, an antibacterial agent, and other additives selected from pulp, thermoplastic fibers, etc. It can also be added together with the agent or in place of the liquid permeability improving agent or the same component agent. In addition, these other additives can be mixed at the same time as or separately from the surface cross-linking agent. That is, the water absorbing agent of the present invention may also contain such other additives.

In the present invention, a chelating agent is preferably added (particularly amino polyvalent carboxylic acid to amino polyvalent phosphoric acid), and typically, JP-A-11-060975, International Publication No. 2007/004529 Pamphlet, International Publication No. 2011 / Pamphlet No. 126079, Pamphlet No. 2012/023433, Japanese Patent Application Laid-Open No. 2009-509722, Japanese Patent Application Laid-Open No. 2005-097519, Japanese Patent Application Laid-Open No. 2011-074401, Japanese Patent Application Laid-Open No. 2013-076073, Japanese Patent Application Laid-Open No. 2013-213083 The chelating agent described in Japanese Patent Publication No. 1 to 4 and the like is added or contained in an amount of 0.001 to 1% by mass (with respect to a monomer or a water absorbing agent).

[3] Water-absorbing agent The water-absorbing agent of the present invention that can be produced as described above contains a surface-crosslinked polyacrylic acid (salt) -based water-absorbing resin and an aromatic substance, and the water-absorbing resin is added to the total amount of the water-absorbing agent. On the other hand, it is 90% by mass or more, and the aromatic substance is present on the surface of the water-absorbent resin.

The amount of the water-absorbent resin is preferably 95% by mass or more, more preferably 98% by mass or more, and most preferably 99% by mass or more, based on the total amount of the water-absorbing agent. The remaining components include aromatic substances, and if necessary, the above-mentioned liquid permeability improver, the same component, and other additives may be included. In addition, various additives may be included by improving the process.

[3-1] Gel Bulk Density The water absorbing agent of the present invention that can be produced as described above has a gel bulk density of 0.50 g / cm ³ or more and 1.00 g / cm ³ or less. When the gel bulk density is larger than the above range, the gap between the gels after swelling becomes small, and the added aromatic substance cannot be effectively volatilized within a range that does not cause an unpleasant odor. This is not preferable because the water-absorbent resin of the above perceives a swelling and volatilizing unpleasant odor. On the other hand, if the gel bulk density is smaller than the above range, there is a high possibility that other absorption characteristics will be adversely affected, which is not preferable. The gel bulk density is preferably 0.55 g / cm ³ or more, more preferably 0.60 g / cm ³ or more, particularly preferably 0.65 g / cm ³ or more, preferably 0.98 g / cm ³ or less, and more. It is preferably 0.95 g / cm ³ or less, and ^{may be 0.90 g / cm 3} or less.
The gel bulk density of the water-absorbing agent according to the present invention is such that the water-absorbing agent is swollen 6 times under no pressure using 0.9% by mass of physiological saline, and then compressed under a load of ^{20 g / cm 2.} Be measured. The reason for measuring the gel bulk density by 6-fold swelling depends on the literature to be investigated, but it is generally said that the amount of urination of an infant up to about 3 years old is 50 to 70 mL (that is, about 60 g). It is based on the fact that the average water-absorbent resin content of disposable diapers for children, which is a general absorbent article, is about 10 g. In the present invention, it is set that 60 g of urine is absorbed by 10 g of a water-absorbent resin or a water-absorbing agent, and a state in which a liquid (physiological saline) 6 times the mass of the water-absorbing agent is absorbed is used as a model.

[3-2] Aromatic substances Specific examples of aromatic substances and preferable amounts thereof are as described in the section of addition of aromatic substances.

[3-3] Other Properties of Water Absorbent The water absorbent (and the above-mentioned water-absorbent resin) of the present invention has at least one of the following properties shown in "[3-3] Other properties of water absorbent". Is preferable. When two or more of the following characteristics are provided, for example, the ratio of the mass average particle diameter (D50) and the particles having a particle diameter of 150 μm may be provided in the same manner, and the non-pressurized absorption ratio (CRC) and the pressurized absorption ratio (CRC) may be provided. It may be provided with two or more of AAP) and permeation-dependent absorption ratio under pressure (PDAUP), and in particular, it may be provided with the absorption ratio under no pressure (CRC) and the absorption ratio under pressure (AAP) as necessary. It may have a permeation-dependent absorption factor (PDAUP) under pressure. It is preferable that the amount of polyhydric alcohol and the amount of residual monomer are also prepared. It is most preferable to have all the characteristics.

(Mass average particle size (D50))
The mass average particle size (D50) of the water-absorbing agent (and the above-mentioned water-absorbent resin) of the present invention is preferably 300 to 600 μm, more preferably 300 to 500 μm, and further preferably 300 to 450 μm. It is preferably 300 to 400 μm, and particularly preferably 300 to 400 μm.
By setting the mass average particle size (D50) of the water absorbing agent (and the above-mentioned water absorbing resin) within the above range, (a) gel bulk density, preferable absorption characteristics such as absorption ratio under pressure (AAP), and permeation under pressure The dependent absorption ratio (PDAUP) and water absorption rate (Voltex method) can be controlled in a well-balanced manner. When the mass average particle size (D50) is smaller than the above range, the gel bulk density becomes too high, or the absorption ratio under pressure (AAP) and the permeation-dependent absorption ratio under pressure (PDAUP), which are preferable absorption characteristics, are low. There is a risk of becoming too much. On the other hand, when the mass average particle size (D50) is larger than the above range, the gel bulk density may become too small, or the water absorption rate (Vortex method), which is a preferable absorption characteristic, may become too long. In addition, the roughness of the particles of the water absorbing agent becomes conspicuous, and when it is used for an absorbent article such as a disposable diaper or a sanitary napkin, the feel and wearing feeling may be deteriorated.

(Ratio of particles with a particle size of less than 150 μm)
The proportion of particles of less than 150 μm in 100% by mass of the water absorbing agent (or the above-mentioned water absorbing resin) according to the present invention is preferably 5% by mass or less, more preferably 4% by mass or less, and 3% by mass. It is more preferably% or less, and particularly preferably 2% by mass or less. The lower limit is 0% by mass, but in continuous commercial production, it may be very difficult to reduce the proportion of particles smaller than 150 μm to 0% by mass from the viewpoint of production efficiency. Therefore, in that case, the lower limit is preferably 0.1% by mass, more preferably 0.2% by mass, and even more preferably 0.3% by mass.
By setting the proportion of particles less than 150 μm within the above range, the gel bulk density and preferable absorption characteristics such as the absorption ratio under pressure (AAP), the permeation-dependent absorption ratio under pressure (PDAUP), and the water absorption rate (Vortex method) can be obtained. It can be controlled in a well-balanced manner. If the proportion of particles smaller than 150 μm is larger than the above range, not only may the absorption ratio under pressure (AAP) and the permeation-dependent absorption ratio under pressure (PDAUP), which are preferable absorption characteristics, become too low. , It is not preferable because the working environment may be deteriorated due to the scattering of dust in the place where the water absorbing agent is handled, or the handling may become difficult due to the accumulation of fine particles in the device.

Further, the logarithmic standard deviation (σζ) indicating the narrowness of the particle size distribution is preferably 0.25 to 0.50, more preferably 0.25 to 0.45, and 0.25 to 0. It is more preferably .40, and particularly preferably 0.27 to 0.35. The logarithmic standard deviation (σζ) of the particle size distribution has the advantage that the smaller the value, the more uniform the particle size and the less segregation of the particles. However, in order to make the logarithmic standard deviation (σζ) of the particle size distribution less than 0.2, it is necessary to repeat pulverization and classification to remove coarse particles and fine particles, which is disadvantageous from the viewpoint of productivity and cost. May bring.

(Specific surface area)
The specific surface area of the water absorbing agent of the present invention (and the aforementioned water-absorbent resin) is preferably in the range of 25~50m ² / kg, is more preferably in the range of 27~48m ² / kg, 30 It is more preferably ~ 45 m ^{2 / kg.} The specific surface area can be controlled by applying the technique of excessive foam polymerization in the [2-2] polymerization step or the [2-3] gel pulverization step disclosed in WO 2011/126079.
When the specific surface area is ^{less than 25 m 2} / kg, the water absorption rate (Vortex method), which is a preferable absorption characteristic, may become long. On the other hand, ^{when it exceeds 50 m 2} / kg, it correlates with excessive foam polymerization in the [2-2] polymerization step and too fine gel crushing in the [2-3] gel crushing step, resulting in preferable absorption. The characteristic absorption ratio under pressure (AAP) and permeation-dependent absorption ratio under pressure (PDAUP) may be lowered.

(Absorption ratio under no pressure (CRC))
The absorption ratio (CRC) of the water absorbing agent (and the above-mentioned water absorbing resin) according to the present invention under no pressure is preferably in the range of 25 to 40 g / g, and more preferably in the range of 25 to 38 g / g. It is preferably in the range of 25 to 35 g / g, more preferably in the range of 25 to 32 g / g, and most preferably in the range of 25 to 30 g / g.
If the absorption ratio under no pressure (CRC) is less than 25 g / g, the absorption ratio of the water absorbing agent may decrease, and it may not be suitable for use as an absorber for absorbent articles such as disposable diapers and sanitary napkins. On the other hand, when the absorption ratio under no pressure (CRC) is 40 g / g or more, the gel strength becomes weak and the gel bulk density may not be controlled within a predetermined range.

(Absorption ratio under pressure (AAP))
The absorption ratio (AAP) of the water-absorbing agent (and the above-mentioned water-absorbent resin) according to the present invention under pressure is preferably 20 g / g or more, more preferably 21 g / g or more, and 22 g / g or more. It is more preferably 23 g / g or more, and particularly preferably 23 g / g or more. The upper limit is not particularly limited, but is preferably 30 g / g or less, more preferably 28 g / g or less, from the viewpoint of balance with other physical properties.
By setting the absorption ratio under pressure (AAP) within the above range, the amount of liquid returned when pressure is applied to the absorber is reduced, so that it can be used as an absorber for absorbent articles such as disposable diapers and sanitary napkins. It is a suitable water absorbent.

(Penetration-dependent absorption factor under pressure (PDAUP))
The PDAUP (permeation-dependent absorption ratio under pressure) of the water-absorbing agent (and the above-mentioned water-absorbing resin) according to the present invention is preferably 3 g / g or more, more preferably 8 g / g or more, and 10 g. It is more preferably / g or more, further preferably 12 g / g or more, particularly preferably 13 g / g or more, further particularly preferably 15 g / g or more, and 17 g / g or more. Is the most preferable.
On the other hand, the upper limit value is not particularly limited, but is preferably 25 g / g or less from the viewpoint of balance with other physical properties. When the permeation-dependent absorption ratio (PDAUP) under pressure is less than 3 g / g, the absorption of the liquid becomes non-uniform when pressure is applied to the obtained absorber. In particular, it is not suitable for use in an absorber having a high core concentration, which will be described later. The permeation-dependent absorption ratio (PDAUP) under pressure can be controlled by the particle size, surface cross-linking reaction, or the like.

(Water absorption rate (Vortex method))
The water absorption rate (Vortex method) of the water absorbing agent (and the above-mentioned water absorbing resin) according to the present invention is preferably 50 seconds or less, more preferably 45 seconds or less, still more preferably 40 seconds or less. The lower limit is not particularly limited, but is preferably 10 seconds or longer, more preferably 15 seconds or longer, and even more preferably 20 seconds or longer.
When the water absorption rate (Vortex method) exceeds 50 seconds, the water absorption rate of the obtained water absorbing agent such as body fluids such as urine and blood becomes long, and it is not suitable as an absorber for absorbent articles such as disposable diapers. The water absorption rate (Vortex method) can be controlled by foam polymerization, particle size distribution, or the like.

(Amount of polyhydric alcohol)
The total amount of the polyhydric alcohol remaining in the aromatic water-absorbing agent (and the above-mentioned water-absorbing resin) according to the present invention is 1.5% by mass or less with respect to the total amount of the water-absorbing agent (or the above-mentioned water-absorbing resin), and 1 It is preferably 2% by mass or less, and more preferably 1.0% by mass or less. The lower limit of the amount of residual polyhydric alcohol is preferably 0% by mass, but when a surface cross-linking agent containing at least one selected from polyhydric alcohols or alkylene carbonates is used, it is separately required to be 0% by mass. Since a removal step such as washing with water or an organic solvent is required, the production efficiency is lowered in commercial production, so the content does not have to be 0% by mass. On the other hand, when the amount of residual polyhydric alcohol exceeds 1.5% by mass with respect to the water absorbing agent, the polyhydric alcohol may give an unpleasant odor at the time of gel swelling, which is not preferable.

(Amount of residual monomer)
From the viewpoint of safety, the amount of residual polyhydric alcohol of the water-absorbing agent (and the above-mentioned water-absorbing resin) according to the present invention may be 1000 ppm (mass basis) or less with respect to the total amount of the water-absorbing agent (or water-absorbing resin). It is more preferably 500 ppm (mass basis) or less, further preferably 400 ppm (mass basis) or less, and particularly preferably 300 ppm (mass basis) or less. The lower limit is preferably about 0 ppm (based on mass), more preferably about 10 ppm (based on mass).
By setting the content of the residual monomer within the above range, irritation to the skin of the human body or the like is reduced. On the other hand, if it exceeds the above range, the residual monomer may give an unpleasant odor at the time of gel swelling, which is not preferable.

[4] Uses of water absorbent (absorbent, absorbent layer)
The water-absorbing agent according to the present invention is preferably used mainly as an absorber (absorbent layer) for absorbent articles such as disposable diapers and sanitary napkins, and is used in a large amount per absorbent article. It is more preferable to use it as an absorber (absorbent layer) of an article.
The absorber means a sheet, fibrous, tubular, or the like formed of a particulate water absorbent, and is preferably molded into a sheet to form an absorbent layer. In addition to the water-absorbing agent according to the present invention, an absorbent material such as pulp fiber, an adhesive, a non-woven fabric, or the like can be used in combination for molding. In this case, the amount of the water absorbing agent in the absorber (absorbent layer) (hereinafter, referred to as "core concentration") is preferably 50% by mass to 100% by mass, and is preferably 60% by mass to 100% by mass. More preferably, it is more preferably 70% by mass to 100% by mass, and particularly preferably 80% by mass to 100% by mass.
By setting the core concentration within the above range, when the absorber (absorbent layer) is used for an absorbent article, even if the water absorbent gels by absorbing urine, an appropriate space is created between the gel particles. By emitting a suitable fragrance through the space, the discomfort of the wearer or the caregiver can be reduced without perceiving the unpleasant odor derived from the water-absorbent resin before the addition of the aromatic substance.

[5] Absorbent Article The absorbent article according to the present invention includes the absorbent body (absorbent layer), and usually includes a liquid-permeable front sheet and a liquid-impermeable back sheet. Examples of the absorbent article include disposable diapers and sanitary napkins.
When the absorbent article is, for example, a disposable diaper, a liquid-permeable top sheet located on the side that comes into contact with human skin when worn and a liquid-impermeable back sheet located on the outside when worn. The disposable diaper is produced by sandwiching an absorber containing the water-absorbing agent of the present invention between the diapers. The disposable diaper is further provided with a member known to those skilled in the art, such as an adhesive tape for fixing the disposable diaper after mounting.

In the absorbent article according to the present invention, when the absorbent body (absorbent layer) absorbs the liquid and the water absorbing agent swells and gels, an appropriate space is created between the gel particles, and a suitable aroma is emitted through the space. By emitting, it is possible to provide an absorbent article that is comfortable for the wearer and his / her caregiver.
In addition to the disposable diapers and sanitary napkins, the water-absorbing agent according to the present invention can also be suitably used for applications such as pet urine absorbents and urine gelling agents for portable toilets.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not construed as being limited to these Examples and Comparative Examples, and the technical means disclosed in each Example will be used. Examples obtained in appropriate combinations are also included in the scope of the present invention. Unless otherwise specified, the electrical equipment used in the examples and comparative examples and the measurement of various physical properties of the water absorbing agent uses a power supply of 200 V or 100 V / 60 Hz. Unless otherwise specified, the physical characteristics of the water absorbing agent were measured under the conditions of room temperature (20 ° C. to 25 ° C.) and relative humidity of 50 ± 5% RH. For convenience, "liter" may be referred to as "l" or "L", and "mass fraction" may be referred to as "wt%".

[Physical characteristics of water absorbent]
Hereinafter, a method for measuring various physical properties of the water absorbing agent according to the present invention will be described. When the object to be measured is an object other than the water-absorbing agent, "water-absorbing agent" is read as the object to be measured and applied unless otherwise specified. For example, when the object to be measured is a base polymer or a water-absorbent resin obtained by cross-linking the base polymer, the "water-absorbing agent" in the measurement method is read as "base polymer" or "water-absorbent resin" and applied.

(A) Gel bulk density In the gel bulk density measurement of the water-absorbing agent (or water-absorbent resin), among the AAP devices described in the EDANA method (WSP242.3 (10)), the cylinder (inner diameter 60 mm, outer diameter 70 mm, high). A stainless steel mesh having a size of 50 mm and a bottom opening of 36 μm was attached) and a piston (inner diameter 59 mm, height 60 mm) were used. The weight of the piston was adjusted to 573 g so that a load of ^{20 g / cm 2} was applied to the bottom surface of the cylinder.
Further, as a container for injecting a 0.9 mass% sodium chloride aqueous solution (hereinafter referred to as physiological saline) for gelling the water absorbing agent sprayed on the mesh of the cylinder, the cylinder is smoothly immersed with almost no gap. A swelling cup having an inner diameter of 72 mm and a depth of 30 mm was prepared so as to be possible.
Further, in this measurement, in order to measure the thickness of the gel layer under a load of ^{20 g / cm 2, Mitutoyo Co., Ltd.'s Digimatic Indicator ID-U1025 was used.}
A measuring method using the measuring device having the above configuration will be described below.
First, the above-mentioned device was set in the order of a swelling cup, a cylinder, a piston, and a weight from the bottom without using a water absorbing agent, and a reference line was attached to the device so that the state could always be reproduced. In that state, the zero point of the digital indicator was adjusted. It is also preferable to fix the measurement point of the digital indicator.
Next, the swelling cup, cylinder, piston and weight were separated, and 5.0 ± 0.10 g of water absorbing agent was weighed and sprinkled evenly on the stainless mesh in the cylinder.
On the other hand, 30.0 ± 0.1 g of physiological saline was measured in the swelling cup.
The cylinder sprinkled with the water-absorbing agent was gently immersed in a swelling cup containing physiological saline along the reference line, and the water-absorbing agent was inflated for 10 minutes under no load. When the water-absorbing agent was immersed, the powder flew up into the physiological saline solution, so the entire swelling cup containing the cylinder was gently shaken to make the gel surface as uniform as possible.
After 10 minutes, the piston and cylinder were placed again on the swollen gel in line with the reference line, ^{a load of 20 g / cm 2} was applied, and the thickness of the gel layer was measured using a digital indicator in that state. ^{The volume of swelling gel (cm 3} ) in a cylinder with an inner diameter of 60 mm was calculated.
The gel bulk density is calculated by the following formula as the density of the gel volume with respect to the total amount of 35 g of 30 g of physiological saline and 5 g of water absorbing agent.
Gel bulk density (g / cm ³ ) = 35 / (gel volume under ^{20 g / cm 2 load)}

(B) Mass average particle size (D50), proportion of particles less than 150 μm, and log standard deviation (σζ)
The particle size distribution PSD of the water absorbent was measured according to the EDANA method WSP220.3 (10). The mass average particle size (D50), the proportion of particles less than 150 μm, and the logarithmic standard deviation (σζ) were measured according to the measuring method described in US Pat. No. 7,638,570. As the sieve used, sieves having meshes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 212 μm, 200 μm, 150 μm, and 45 μm (IIDA TESTING SIEVE) conforming to JIS Z 8801 were used.

(C) Specific surface area The surface area of the water absorbent is measured by using a three-dimensional analyzer using X-rays (Shimadzu Seisakusho Co., Ltd .; microfocus X-ray CT system, etc.) and is measured by three-dimensional analysis software (high-speed three-dimensional analysis). It was obtained by analysis with soft TRI / 3D-VOL-FCS64 etc.). This analyzer can measure the internal bubble ratio and the like at the same time.

(D) Absorption rate under no pressurization (CRC)
The CRC of the water absorbing agent was measured according to the EDANA method WSP241.3 (10).
(E) Absorption rate under pressure (AAP)
The AAP of the water absorbing agent was measured according to the EDANA method WSP242.3 (10).
(F) Penetration-dependent absorption ratio under pressure (PDAUP)
The water absorbing agent PDAUP was measured according to the EDANA method WSP243.3 (10).

(G) Water absorption rate (Vortex method)
The water absorption rate (Vortex method) of the water absorbing agent was measured by the following procedure in accordance with JIS K 7224 (1996).
First, 0.02 part by mass of edible blue No. 1 (CAS No. 3844-45-9), which is a food additive, was added to 1000 parts by mass of physiological saline to color it, and the liquid temperature was adjusted to 30 ° C. This was used as a test solution.
Next, 50 mL of the test solution was weighed in a beaker having a capacity of 100 mL, a cylindrical stirrer having a length of 40 mm and a diameter of 8 mm was placed in the beaker, and stirring was started at 600 rpm. Subsequently, 2.0 g of the water-absorbent resin was put into the test solution during stirring, and the time until the stirrer (stirrer tip) was covered with the test solution was measured and used as the water absorption rate (Vortex method).

(H) Amount of polyhydric alcohol The amount of polyhydric alcohol remaining in the water absorbing agent was measured by the following procedure.
First, 200 ml of physiological saline was weighed in a 250 ml beaker, and a cylindrical stirrer having a length of 40 mm and a diameter of 8 mm was placed in the beaker. 1.0 g of the water absorbing agent was weighed therein, and the polyhydric alcohol was extracted from the water absorbing agent while stirring at 250 rpm for 1 hour. After 1 hour, the filtrate was collected and the polyhydric alcohol was quantified using high performance liquid chromatography.

(I) Amount of residual monomer The amount of residual monomer in the water absorbing agent was measured according to the EDANA method WSP210.3 (10).

(J) Odor test of swelling gel The odor test of swelling gel was performed according to the following procedure.
First, 5.0 g of a water absorbent is weighed in a glass bottle (M-225 manufactured by Kashiwayo Glass Co., Ltd.) having an internal capacity of 225 ml and composed of an inner lid and an outer lid made of propylene, and 30.0 g of physiological saline is poured into the bottle. Inflated. Immediately after the saline solution was added, the inner and outer lids were closed to seal the inside of the glass bottle.
After 10 minutes, an odor test was conducted by opening the lid and having 10 adult panelists smell the gas accumulated in the upper space of the swollen gel layer.
One glass bottle for the odor test was prepared for each panelist, and each panelist reported the odor intensity and the degree of pleasantness and discomfort according to the following criteria.
[Odor intensity]
5: (comfortable odor of aromatic substances) 3: (smell of aromatic substances but a little weak), 1: (smell of aromatic substances and unpleasant odor and strong aromatic odor), -1: ( Each person describes the score based on the criteria of -3: (strong unpleasant odor), -5: (strong unpleasant odor), and the average value. Asked.

(K) Odor Test of Absorber 30 parts by mass of non-powder fibrous wood pulp and 70 parts by mass of water absorbent were dry-mixed using a mixer. Next, the obtained mixture was formed into a web having a size of 100 mm × 300 mm by air papermaking using a batch type air papermaking device on a wire screen formed in 400 mesh (mesh size 38 μm). .. Further, the web was pressed at a pressure of 196.14 kPa for 5 seconds to obtain an absorber having a basis weight of about 0.047 g / cm2.
Subsequently, this absorber was placed in a 3 L odor bag (manufactured by Omi Odo Air Service Co., Ltd.), and 60 g of physiological saline was uniformly added to the absorber to swell the absorber, and the odor bag was closed. Allowed to stand for minutes. In addition, it was filled with dry air so that the space volume in the odor bag was the same.
After 10 minutes, the odor bag was opened, and an odor test was conducted by 10 panelists sniffing the odor of the gas accumulated in the bag.
One odor bag for the odor test of the absorber was prepared for each panelist, and each panelist reported the odor intensity and the degree of comfort and discomfort according to the following criteria.
[Odor intensity]
5: (comfortable odor of aromatic substances) 3: (smell of aromatic substances but a little weak), 1: (smell of aromatic substances and unpleasant odor and strong aromatic odor), -1: ( Each person describes the score based on the criteria of -3: (strong unpleasant odor), -5: (strong unpleasant odor), and the average value. Asked.

[Manufacturing Example 1]
2.9 parts by mass of polyethylene glycol diacrylate (n = 8) as an internal cross-linking agent is dissolved in 5500 parts by mass of a 33% by mass aqueous solution of sodium acrylate (75 mol% neutralization rate) as a monomer component. It was used as a reaction solution. Next, the reaction solution was degassed in a nitrogen gas atmosphere for 30 minutes.
Next, the reaction solution was supplied to a reactor with a lid on a jacketed stainless steel twin bowl type kneader having an internal volume of 10 L having two sigma type blades, and the inside of the reactor was filled with nitrogen while keeping the reaction solution at 30 ° C. Gas replacement. Subsequently, 2.4 parts by mass of sodium persulfate and 0.12 parts by mass of L-ascorbic acid were added as aqueous solutions while stirring the reaction solution, and the polymerization started after about 1 minute. The peak temperature reached about 80 ° C. about 20 minutes after the start of the polymerization, and the particulate hydrogel-like polymer was taken out 60 minutes after the start of the polymerization while continuing stirring thereafter. The mass average particle size (D50) of the particulate hydrogel polymer was measured by the method described in International Publication No. 2011/126079, and found to be about 1800 μm.
The obtained hydrogel polymer was spread on a wire mesh having an opening of 300 μm and dried with hot air at 150 ° C. for 90 minutes. Next, the dried product was pulverized using a roll mill and further classified by a sieve having a mesh size of 850 μm and 150 μm. A crushed resin (base polymer) (1) was obtained.
Next, a surface cross-linking agent composed of 0.05 parts by mass of ethylene glycol diglycidyl ether, 1 part by mass of propylene glycol, 3 parts by mass of water, and 1 part by mass of isopropyl alcohol was stirred with respect to 100 parts by mass of the base polymer (1). It was mixed with a mixer. Then, the mixture was heat-treated at 210 ° C. for 50 minutes to obtain a surface-crosslinked water-absorbent resin (1).
The properties of the surface-crosslinked water-absorbent resin (1) are the same as those of the comparative water-absorbing agent (1) of Comparative Example 1 described later.

[Example 1]
50 ppm of Hiba oil (equivalent to about 1 ppm of hinokitiol (based on mass)) and 1.2% by mass of water with respect to the surface-crosslinked water-absorbent resin (1) obtained in Production Example 1. Hiba oil was added and mixed, and the mixture was further heated at 60 ° C. for 30 minutes. Then, the dried product was crushed and passed through a 850 μm sieve to obtain a water absorbing agent (1).
The amount of polyhydric alcohol remaining in the obtained water absorbing agent (1) was 4700 ppm (mass basis), and the amount of residual monomer was 390 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Example 2]
Vanilla essence is 160 ppm (mass standard) (equivalent to about 1 ppm (mass standard) as vanillin) and water is 0.6% by mass with respect to the surface-crosslinked water-absorbent resin (1) obtained in Production Example 1. The mixture was further mixed and heated at 60 ° C. for 30 minutes. Then, the dried product was crushed and passed through a sieve of 850 μm to obtain a water absorbing agent (2).
The amount of polyhydric alcohol remaining in the obtained water absorbing agent (2) was 4700 ppm (mass basis), and the amount of residual monomer was 390 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Comparative Example 1]
The surface-crosslinked water-absorbent resin (1) obtained in Production Example 1 was used as it was as a water-absorbent agent (1) for comparison.
The amount of polyhydric alcohol remaining in the obtained comparative water absorbent (1) was 4700 ppm (mass basis), and the amount of residual monomer was 390 ppm (mass basis). Other performances are shown in Table 1 along with the results of the odor test of the swollen gel and absorber.

[Examples 3 to 4]
To the water absorbing agent (1) or (2) obtained in Example 1 or 2, hydrophilic silica Aerosil 200 (manufactured by Nippon Aerosil Co., Ltd.) was added so that the addition amount was 0.3% by mass. They were mixed and used as water absorbing agents (3) or (4), respectively.
The amount of polyhydric alcohol remaining in the water absorbents (3) and (4) was 4700 ppm (mass basis), and the amount of residual monomer was 390 ppm (mass basis). Other performances of the water absorbents (3) and (4) are shown in Table 1 along with the odor test results of the swollen gel and absorber.

[Comparative Example 2]
The hydrophilic silica used in Example 3 was added and mixed with the surface-crosslinked water-absorbent resin (1) obtained in Production Example 1 so that the addition amount was 0.3% by mass for comparison. It was used as a water absorbing agent (2).
The amount of polyhydric alcohol remaining in the obtained comparative water absorbent (2) was 4700 ppm (mass basis), and the amount of residual monomer was 390 ppm (mass basis). Other performances of the comparative water absorbent (2) are shown in Table 1 along with the odor test results of the swollen gel and absorber.

[Manufacturing Example 2]
421.7 parts by mass of acrylic acid, 140.4 parts by mass of 48 mass% sodium hydroxide aqueous solution, 2.0 parts by mass of polyethylene glycol diacrylate (n = 8), 1.0 mass% in a container made of polypropylene having a capacity of 2 L. An aqueous solution was prepared by adding 2.7 parts by mass of an aqueous solution of diethylenetriamine 5 acetate / 3 sodium and 395.3 parts by mass of deionized water and mixing them. The deionized water was preheated to 40 ° C.
Subsequently, while stirring the aqueous solution, 211.9 parts by mass of a 48 mass% sodium hydroxide aqueous solution was added to the aqueous solution in an open state for about 30 seconds and mixed to prepare a monomer aqueous solution. .. The temperature of the monomer aqueous solution was raised to about 80 ° C. due to the heat of neutralization and the heat of dissolution generated in the mixing process.
Then, when the temperature of the monomer aqueous solution reached 78 ° C., 17.6 parts by mass of a 4 mass% sodium persulfate aqueous solution was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to prepare a reaction solution.
After stirring the initiator, the reaction solution was immediately poured into a stainless steel vat-shaped container in an open state to the atmosphere. The vat-shaped container had a bottom surface size of 200 mm × 260 mm, a top surface size of 460 mm × 560 mm, and a height of 140 mm, the cross section of the central portion was trapezoidal, and a silicone sheet was attached to the inner surface. Further, the vat-shaped container was placed on a hot plate heated to 50 ° C. and preheated before pouring the reaction solution.
The polymerization reaction was started within 1 minute after the reaction solution was poured into the vat-shaped container. By the polymerization reaction, the reaction solution expanded upward in all directions while generating water vapor, proceeded with the polymerization reaction while foaming, and then shrunk to a size slightly larger than the bottom surface of the bat-shaped container. The polymerization reaction was completed within about 1 minute. By the polymerization reaction, a sheet-like hydrogel-like crosslinked polymer was obtained. The obtained sheet-shaped hydrogel-like polymer is cut into an appropriate size, then supplied to a screw extruder and crushed into a gel, and is in the form of a particulate hydrogel having a mass average particle diameter (D50) of about 700 μm. It was made into a polymer.
Next, the obtained hydrogel polymer was spread on a wire mesh having an opening of 300 μm and dried with hot air at a temperature of 190 ° C. for 30 minutes. Next, the dried product was pulverized using a roll mill and further classified by a sieve having a mesh size of 710 μm and 150 μm to have a mass average particle size (D50) of 360 μm and a proportion of particles having a particle size of less than 150 μm of 0.5 mass. % Atypical crushed resin (prepolymer) (2) was obtained.
Next, a surface cross-linking agent consisting of 0.8 parts by mass of ethylene carbonate and 2.7 parts by mass of water was mixed with 100 parts by mass of the prepolymer (2) by a stirring mixer. Then, the mixture was heat-treated at 205 ° C. for 40 minutes to obtain a surface-crosslinked water-absorbent resin (2).
The properties of the surface-crosslinked water-absorbent resin (2) are the same as those of the comparative water-absorbing agent (3) of Comparative Example 3 described later.

[Example 5]
To the surface-crosslinked water-absorbent resin (2) obtained in Production Example 2, Hiba oil was added and mixed in the same manner as in Example 1 (50 ppm as Hiba oil (mass standard), about 1 ppm as hinokitiol (mass standard). ) Equivalent), and after further heating the mixture at 60 ° C. for 30 minutes, the dried product was crushed and passed through a 850 μm sieve to obtain a water absorbing agent (5).
The amount of polyhydric alcohol remaining in the obtained water absorbing agent (5) was 700 ppm (mass basis), and the amount of residual monomer was 300 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Example 6]
Vanilla essence was added and mixed with the surface-crosslinked water-absorbent resin (2) obtained in Production Example 2 in the same manner as in Example 2 (160 ppm as vanilla essence (mass basis), about 1 ppm as vanillin (mass basis). ) Equivalent), and after further heating the mixture at 60 ° C. for 30 minutes, the dried product was crushed and passed through a 850 μm sieve to obtain a water absorbing agent (6).
The amount of polyhydric alcohol remaining in the obtained water absorbing agent (6) was 700 ppm (mass basis), and the amount of residual monomer was 300 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Comparative Example 3]
The surface-crosslinked water-absorbent resin (2) obtained in Production Example 2 was used as it was as a water-absorbent agent (3) for comparison.
The amount of polyhydric alcohol remaining in the obtained comparative water absorbent (3) was 700 ppm (mass basis), and the amount of residual monomer was 300 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Example 7]
A mixture of 1.0 part by mass of a 50% by mass aluminum sulfate aqueous solution and 0.3 part by mass of a 60% by mass sodium lactate aqueous solution in terms of hexahydrate with respect to the water absorbing agent (5) obtained in Example 5. The liquid was added and mixed. Then, the mixture was heated at 60 ° C. for 30 minutes and then passed through a 710 μm wire mesh to obtain a water absorbing agent (7).
The amount of polyhydric alcohol remaining in the obtained water absorbing agent (7) was 700 ppm (mass basis), and the amount of residual monomer was 300 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Example 8]
To the water absorbing agent (6) obtained in Example 6, a mixed solution composed of aluminum sulfate / sodium lactate was added and mixed in the same manner as in Example 7. Then, the mixture was heated at 60 ° C. for 30 minutes and then passed through a 710 μm wire mesh to obtain a water absorbing agent (8).
The amount of polyhydric alcohol remaining in the obtained water absorbing agent (8) was 700 ppm (mass basis), and the amount of residual monomer was 300 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Comparative Example 4]
A mixed solution of aluminum sulfate / sodium lactate was added and mixed with the surface-crosslinked water-absorbent resin (2) obtained in Production Example 2 in the same manner as in Example 7. Then, the mixture was heated at 60 ° C. for 30 minutes and then passed through a 710 μm wire mesh to obtain a water absorbent (4) for comparison.
The amount of polyhydric alcohol remaining in the obtained comparative water absorbent (4) was 700 ppm (mass basis), and the amount of residual monomer was 300 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Reference Example 5]
2% by mass (about 80 ppm as hinokitiol) of the powder obtained by carrying 20% of hiba oil (containing 2% by mass of hinokitiol) on dextrin with respect to the surface-crosslinked water-absorbent resin (2) obtained in Production Example 2. (Mass-based) content) was added and mixed to obtain a reference water absorbent (5).
The amount of polyhydric alcohol remaining in the obtained reference water absorbing agent (5) was 700 ppm (mass basis), and the amount of residual monomer was 300 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Manufacturing Example 3]
In Production Example 1, the same operation was performed except that 2.9 parts by mass of the internal cross-linking agent polyethylene glycol diacrylate (n = 8) was changed to 1.5 parts by mass, and the surface-crosslinked water-absorbent resin (3) was obtained. Obtained.

[Comparative Example 6]
Vanilla essence was added and mixed with the surface-crosslinked water-absorbent resin (3) obtained in Production Example 3 in the same manner as in Example 2 (160 ppm as vanilla essence (mass basis), and about 1 ppm as vanillin (mass). The mixture was further heated at 60 ° C. for 30 minutes, and then the dried product was crushed and passed through a 850 μm sieve to obtain a water absorbent (6) for comparison.
The amount of polyhydric alcohol remaining in the obtained comparative water absorbent (6) was 4500 ppm (mass basis), and the amount of residual monomer was 400 ppm (mass basis). The performance is shown in Table 1 together with the odor test results of the swollen gel and the absorber.

[Comparative Example 7]
A water absorbing agent (7) for comparison was obtained with reference to Example 4 of Patent Document 8 (Japanese Unexamined Patent Publication No. 11-5847).
Specifically, 2.9 parts by mass of the internal cross-linking agent polyethylene glycol diacrylate (n = 8) was added to 5500 parts by mass of a 33% by mass aqueous solution of sodium acrylate (75 mol% neutralization rate) as a monomer component. It was dissolved to prepare a reaction solution. Next, the reaction solution was degassed in a nitrogen gas atmosphere for 30 minutes.
Next, the reaction solution was supplied to a reactor with a lid on a jacketed stainless steel twin bowl type kneader having an internal volume of 10 L having two sigma type blades, and the inside of the reactor was filled with nitrogen while keeping the reaction solution at 30 ° C. Gas replacement. Subsequently, 2.4 parts by mass of ammonium persulfate and 0.12 parts by mass of L-ascorbic acid were added as aqueous solutions while stirring the reaction solution, and the polymerization started after about 1 minute. The peak temperature reached about 80 ° C. about 20 minutes after the start of the polymerization, and the hydrogel polymer was taken out 60 minutes after the start of the polymerization while continuing stirring thereafter.
The obtained hydrogel polymer was spread on a wire mesh having an opening of 300 μm and dried with hot air at 150 ° C. for 90 minutes. Next, the dried product was crushed using a roll mill and further classified by a sieve having a mesh size of 850 μm. Base polymer (1) for use was obtained.
Next, a surface cross-linking agent composed of 0.05 parts by mass of ethylene glycol diglycidyl ether, 1 part by mass of propylene glycol, 3 parts by mass of water, and 1 part by mass of isopropyl alcohol with respect to 100 parts by mass of the comparative base polymer (1). Was mixed with a stirring mixer. Then, the mixture was heat-treated at 210 ° C. for 50 minutes to obtain a surface-crosslinked water-absorbent resin (1) for comparison.
An aqueous solution of hinokitiol was added and mixed with the surface-crosslinked water-absorbent resin (1) for comparison so that hinokitiol was 1% by mass and water was 5% by mass, and the mixture was further heated at 80 ° C. for 30 minutes. The dried product was crushed and passed through a sieve having an opening of 850 μm to obtain a water absorbent for comparison (7).
The amount of residual polyhydric alcohol of the obtained comparative water absorbent (7) was 4600 ppm (mass basis), and the amount of residual monomer was 390 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Comparative Example 8]
A water absorbing agent (8) for comparison was obtained with reference to Example 1 of Patent Document 3 (Japanese Unexamined Patent Publication No. 11-24130).
Specifically, 4.48 parts by mass of the internal cross-linking agent polyethylene glycol diacrylate (n = 8) is added to 5500 parts by mass of a 33% by mass aqueous solution of sodium acrylate (75 mol% neutralization rate) as a monomer component. It was dissolved to prepare a reaction solution. Next, the reaction solution was degassed in a nitrogen gas atmosphere for 30 minutes.
Next, the reaction solution was supplied to a reactor with a lid on a jacketed stainless steel double-armed kneader having an internal volume of 10 L having two sigma type blades, and nitrogen was inside the reactor while keeping the reaction solution at 30 ° C. Gas replaced. Subsequently, 2.46 parts by mass of sodium persulfate and 0.12 parts by mass of L-ascorbic acid were added as aqueous solutions while stirring the reaction solution, and the polymerization started after about 1 minute. The peak temperature reached about 80 ° C. about 20 minutes after the start of the polymerization, and the hydrogel polymer was taken out 60 minutes after the start of the polymerization while continuing stirring thereafter.
The obtained hydrogel polymer was spread on a wire mesh having an opening of 300 μm and dried with hot air at 150 ° C. for 90 minutes. Next, the dried product was pulverized using a roll mill and classified with a sieve having an opening of 850 μm. The base polymer (2) was obtained.
Next, surface cross-linking consisting of 0.05 parts by mass of ethylene glycol diglycidyl ether, 1 part by mass of propylene glycol, 3 parts by mass of water, and 1 part by mass of isopropyl alcohol with respect to 100 parts by mass of the comparative base polymer (2). The agent was mixed with a stirring mixer. Then, the mixture was heat-treated at 210 ° C. for 50 minutes to obtain a surface-crosslinked water-absorbent resin (2) for comparison.
To this surface-crosslinked water-absorbent resin (2) for comparison, a powder obtained by supporting 20% of hiba oil on dextrin was added and mixed so as to be 2% by mass (containing about 80 ppm (mass standard) as hinokitiol). , A water absorbing agent (8) for comparison was obtained.
The amount of polyhydric alcohol remaining in the obtained comparative water absorbent (8) was 4400 ppm (mass basis), and the amount of residual monomer was 420 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

[Comparative Example 9]
Hiba oil was added and mixed with the surface-crosslinked water-absorbent resin (1) obtained in Comparative Example 7 in the same manner as in Example 1 (50 ppm as Hiba oil (mass basis), and about 1 ppm as hinokitiol. (Equivalent to (mass basis)), and after further heating the mixture at 60 ° C. for 30 minutes, the dried product was crushed and passed through a 850 μm sieve to obtain a water absorbent (9) for comparison.
The amount of polyhydric alcohol remaining in the obtained comparative water absorbent (9) was 4600 ppm (mass basis), and the amount of residual monomer was 390 ppm (mass basis). Other performances are shown in Table 1 along with odor test results for swollen gels and absorbers.

From Table 1, the following can be seen.
For example, from the comparison of Comparative Examples 1 and 1 and 2, Comparative Examples 2 and 3 and 4, Comparative Examples 3 and 5 and 6, and Comparative Examples 4 and 7 and 8, the gel bulk density of the present invention was obtained. If an aromatic substance is added to the water-absorbing agent of No. 1, it can be seen that the odor emitted when the water-absorbing agent and the absorber using the water-absorbing agent swell was a pleasant or acceptable odor for the panelists. On the other hand, even within the scope of the present invention, the gel bulk density is as in Example 1 and Example 3, Example 2 and Example 4, Example 5 and Example 7, and Example 6 and Example 8. As it became smaller, the intensity of the perceived aromatic substance was strongly felt. Furthermore, in Comparative Example 3 and Comparative Example 4, the gel bulk density was reduced and the aroma substance was not present, so that the perceived unpleasant odor became stronger. From these facts, it is also clear that when the gel bulk density is low and the amount of gaps between the gel particles is increased, the perceived odor becomes stronger regardless of the acceptable odor (fragrance) or unpleasant odor.
On the other hand, Comparative Example 6 is an example containing the same amount of aromatic substance as in Examples 2, 4, 6 and 8, but the aroma perceived as having a gel bulk density outside the range of the present invention. The strength was weak.
In Example 1 and Comparative Example 9, the same amount of aromatic substance was contained, but the gel bulk density was out of the range of the present invention due to the large proportion of particles less than 150 μm, and the expected aromatic substance was perceived. It turns out that it was not done.
In Comparative Examples 7 and 8, the gel bulk density is out of the range of the present invention, and referring to the results of Comparative Examples 6 and 9, even if the amount of the aromatic substance added is optimized, it is good. This is an example in which the odor test result cannot be obtained. In addition, in Comparative Examples 7 and 8, since the amount of the aromatic substance added was excessive, some panelists felt that the aromatic substance itself was foul-smelling or unpleasant, and this also adversely affected the results. .. Since Patent Document 3 citing the conditions of Comparative Example 8 is set as an addition amount for the purpose of suppressing the odor of adult urine after 24 hours, it solves a problem different from the problem of the present invention. It can be seen that this is the amount of addition for this purpose and cannot be a means for solving the present invention. In order to solve the problem of the present invention, it is preferable to control the content (or addition amount) of an appropriate aromatic substance.

Claims (12)

A water-absorbing agent containing a surface-crosslinked polyacrylic acid (salt) -based water-absorbent resin and an aromatic substance that emits aroma.
The water-absorbent resin is 90% by mass or more based on the total amount of the water-absorbing agent.
The aromatic substance is present on the surface of the water-absorbent resin,
The gel bulk density of the water absorbing agent is 0.50 g / cm ³ or more, 1.00 g / cm ³ or less ,
The aromatic substance is at least one selected from the group consisting of chain monoterpenes, monocyclic 6-membered ring monoterpenes, 7-membered ring monoterpenes, and phenolic aromatic compounds (provided that they are contained in bamboo and tea). Ingredients are excluded) ,
The content of the aromatic substance is 0.001 ppm (mass basis) to 5 ppm (mass basis) with respect to the water-absorbent resin.
The mass average particle size (D50) of the water absorbing agent is 300 μm or more, 500 μm or less,
The proportion of particles having a particle size of less than 150 μm in 100% by mass of the water absorbing agent is 2% by mass or less.
Is a water absorbent. The water-absorbing agent according to claim 1 , wherein the polyacrylic acid (salt) -based water-absorbing resin is surface-crosslinked with a surface-crosslinking agent containing at least one compound selected from a polyhydric alcohol and an alkylene carbonate. The water absorbing agent according to claim 1 or 2 , wherein the specific surface area is 25 m ² / kg or more and 50 m ² / kg or less. The water absorbing agent according to any one of claims 1 to 3 , which further satisfies at least one of the following physical properties.
-Unpressurized absorption ratio (CRC) is 25-40 g / g-Pressurized absorption ratio (AAP) is 20 g / g or more-Pressurized permeation-dependent absorption ratio (PDAUP) is 3 g / g or more The water absorbing agent according to any one of claims 1 to 4 , wherein the water absorption rate (Vortex method) is 10 seconds or more and 50 seconds or less. The water absorbing agent according to any one of claims 1 to 5 , wherein the polyhydric alcohol is 1.5% by mass or less based on the total amount of the water absorbing agent. The water absorbing agent according to any one of claims 1 to 6 , wherein the residual monomer is 1000 ppm or less with respect to the total amount of the water absorbing agent. The water absorbing agent according to any one of claims 1 to 7 , which is used for an absorbent article. An absorbent article comprising an absorbent layer containing the water-absorbing agent according to any one of claims 1 to 8, a liquid-permeable front sheet and a liquid-impermeable back sheet. An aromatic substance that emits fragrance is placed on the surface of a surface-crosslinked polyacrylic acid (salt) water-absorbent resin having a gel bulk density of 0.50 g / cm ³ or more and 1.00 g / cm ^{3 or less.}
The aromatic substance is at least one selected from the group consisting of chain monoterpenes, monocyclic 6-membered ring monoterpenes, 7-membered ring monoterpenes, and phenolic aromatic compounds (provided that they are contained in bamboo and tea). Ingredients are excluded) ,
The content of the aromatic substance is 0.001 ppm (mass basis) to 5 ppm (mass basis) with respect to the water-absorbent resin.
The mass average particle size (D50) of the water absorbing agent is 300 μm or more, 500 μm or less,
The proportion of particles having a particle size of less than 150 μm in 100% by mass of the water absorbing agent is 2% by mass or less.
Der Rukoto characterized method for producing a water absorbing agent including the water absorbent resin 90 wt%. The method for producing a water-absorbing agent according to claim 10 , wherein the water-absorbent resin further satisfies at least one of the following requirements .
- specific surface area of 25 m ² / kg or more, - at 50 m ² / kg or less, the absorption capacity without load (CRC) is a 25~40g / g · absorbency against pressure (AAP) is 20 g / g or more Permeation-dependent absorption ratio (PDAUP) under pressure is 3 g / g or more.-Water absorption rate (Vortex method) is 10 seconds or more and 50 seconds or less. The method for producing a water-absorbing agent according to claim 10 or 11 , wherein the aromatic substance is mixed with the surface-crosslinked water-absorbent resin.