PARTICULATE WATER-ABSORBING AGENT AND METHOD FOR PRODUCING THE SAME

Abstract

[Problem] In an embodiment involving addition of a chelating agent in an upstream process of the process for production, such as the polymerization step, the residual ratio of the chelating agent in the final product, a particulate water-absorbing agent, is improved.

[Solution] A particulate water-absorbing agent having a poly(meth)acrylic acid (salt)-based water-absorbing resin as a main component, containing a chelating agent having a nitrogen atom and an inorganic reducing agent having a sulfur atom, wherein the particulate water-absorbing agent has a chelating agent ratio of 0.8 to 1.8, as calculated by the following procedures (a) to (c): (a) subjecting the particulate water-absorbing agent to a predetermined impact test; (b) sieving the particulate water-absorbing agent subjected to the impact test into a particle group 1 with a particle size of less than 300 μm and a particle group 2 with a particle size of 300 μm or more and less than 850 μm using a JIS standard sieve; and (c) quantifying a content C1 of the chelating agent present in the particle group 1 and a content C2 of the chelating agent present in the particle group 2, and then dividing the C1 by the C2.

Claims

1. A particulate water-absorbing agent having a poly(meth)acrylic acid (salt)-based water-absorbing resin as a main component, comprising a chelating agent having a nitrogen atom and an inorganic reducing agent having a sulfur atom, wherein the particulate water-absorbing agent has a chelating agent ratio of 0.8 to 1.8, as calculated by the following procedures (a) to (c): (a) subjecting the particulate water-absorbing agent to a predetermined impact test; (b) sieving the particulate water-absorbing agent subjected to the impact test into a particle group 1 with a particle size of less than 300 μm and a particle group 2 with a particle size of 300 μm or more and less than 850 μm using a JIS standard sieve; and (c) quantifying a content C1 of the chelating agent present in the particle group 1 and a content C2 of the chelating agent present in the particle group 2, and then dividing the C1 by the C2.

2. The particulate water-absorbing agent according to claim 1, wherein a color tone YI is 12 or less in an initial color tone test.

3. The particulate water-absorbing agent according to claim 1, wherein a content C7 of the chelating agent in the particulate water-absorbing agent is 10 to 5,000 ppm.

4. The particulate water-absorbing agent according to claim 1, wherein the chelating agent is at least one compound selected from the group consisting of amino-polyvalent carboxylic acids and amino-polyvalent phosphoric acids.

5. The particulate water-absorbing agent according to claim 1, wherein the inorganic reducing agent is a sulfite or a hydrogen sulfite.

6. The particulate water-absorbing agent according to claim 1, wherein a residual ratio of the chelating agent is 50% by mass or more.

7. The particulate water-absorbing agent according to claim 1, wherein a content of a residual monomer is 700 ppm or less.

8. The particulate water-absorbing agent according to claim 1, wherein a content of the inorganic reducing agent is 1 to 3,000 ppm.

9. The particulate water-absorbing agent according to claim 1, wherein a proportion of particles with a particle size of less than 850 μm is 90 to 100% by mass of the total.

10. A method for producing a particulate water-absorbing agent having a poly(meth)acrylic acid (salt)-based water-absorbing resin as a main component, comprising a chelating agent having a nitrogen atom and an inorganic reducing agent having a sulfur atom, the method comprising: a step of making a chelating agent ratio to be 0.8 to 1.8.

11. A method for producing a particulate water-absorbing agent, comprising: a step of preparing an aqueous monomer solution containing (meth)acrylic acid (salt) and persulfuric acid (salt); a polymerization step of polymerizing monomers contained in the aqueous monomer solution to obtain a hydrogel; optionally, a gel-crushing step of crushing the hydrogel to obtain a particulate hydrogel; a drying step of drying the hydrogel or the particulate hydrogel to obtain a dry polymer; a step of pulverizing or classifying the dry polymer to obtain a water-absorbing resin powder; and a surface crosslinking step of subjecting the water-absorbing resin powder to surface crosslinking to obtain a water-absorbing resin particle, the method comprising: (i) a step of adding a chelating agent having a nitrogen atom to the aqueous monomer solution at a concentration of 10 to 5,000 ppm of a monomer component (based on monomer mass); and/or (ii) a step of adding the chelating agent to the hydrogel or the particulate hydrogel at a concentration of 10 to 5,000 ppm of a solid content (mass) of the hydrogel or the particulate hydrogel; and (iii) a step of adding an inorganic reducing agent having a sulfur atom to the hydrogel or the particulate hydrogel at a concentration of 10 to 50,000 ppm of the solid content (mass) of the hydrogel or the particulate hydrogel.

12. The method according to claim 11, wherein the particulate water-absorbing agent is obtained by polymerizing the monomers in the aqueous monomer solution containing the chelating agent having a nitrogen atom, an amount C3 of the chelating agent added to the aqueous monomer solution (relative to a monomer solid content) is 50% by mass or more relative to a total amount C5 of the chelating agent added in the step of producing the particulate water-absorbing agent, and a content C6 of the chelating agent relative to a solid content of the particulate water-absorbing agent is 50% by mass or more of the C5.

13. The method according to claim 11, wherein the particulate water-absorbing agent is obtained by adding the chelating agent having a nitrogen atom to the hydrogel obtained by polymerizing the monomers in the aqueous monomer solution or the particulate hydrogel made by crushing the hydrogel, an amount C4 of the chelating agent added to the hydrogel or the particulate hydrogel is 50% by mass or more relative to the total amount C5 of the chelating agent added in the step of producing the particulate water-absorbing agent, and a content C6 of the chelating agent relative to a solid content of the particulate water-absorbing agent is 50% by mass or more of the C5.

14. The method according to claim 10, wherein the addition of the chelating agent in (ii) is performed in a form of a 0.01 to 20% by mass aqueous solution.

15. The method according to claim 11, wherein polymerization time for polymerizing the monomers contained in the aqueous monomer solution is 30 minutes or less.

16. The method according to claim 15, wherein the polymerization time is 10 minutes or less.

17. The method according to claim 11, wherein in the drying step, drying is performed with hot air at 120 to 250° C.

18. The method according to claim 11, wherein in the drying step, drying time until an amount of solid component of the dry polymer becomes 80% by mass is 30 minutes or less.

19. The method according to claim 11, wherein the addition of the inorganic reducing agent is performed in a form of a 0.01 to 20% by mass aqueous solution.

20. The method according to claim 11, wherein in the drying step, drying is performed using a dryer selected from static dryers and band dryers.

21. The method according to claim 20, wherein the hydrogel or the particulate hydrogel stacked has a layer height of 1 to 10 cm.

22. The method according to claim 11, wherein a time period from the addition of the inorganic reducing agent to initiation of the drying is 30 minutes or less, or a temperature of the hydrogel from the addition of the inorganic reducing agent to the initiation of the drying is 0 to 55° C.

23. The method according to claim 11, comprising the gel-crushing step of crushing the hydrogel.

24. The method according to claim 23, wherein the inorganic reducing agent is added at the same time of the crushing of the hydrogel.

25. The method according to claim 23, wherein the chelating agent is added at the same time of the crushing of the hydrogel.

26. The method according to claim 11, wherein 0.02 mol % or more of the persulfuric acid (salt) relative to the monomer is added.

27. The method according to claim 11, wherein water insoluble inorganic fine particles with a volume-average particle size of 0.2 to 3 μm are added to the particulate water-absorbing agent.

28. The water-absorbing agent according to claim 1, having water insoluble inorganic fine particles with a volume-average particle size of 0.2 to 3 μm added to a particle surface of the particulate water-absorbing agent.

29. A water absorbent article, comprising the particulate water-absorbing agent according to claim 1.

30. A water absorbent article, comprising the particulate water-absorbing agent obtainable by the production method according to claim 10.

31. A water absorbent article, comprising the particulate water-absorbing agent obtainable by the production method according to claim 11.

Description

EXAMPLES

[0168] Unless otherwise specified, various physical properties described in the claims and Examples of the present invention were determined under the conditions of room temperature (20 to 25° C.) and humidity of 50 RH % according to the EDANA method and the following measurement methods. In addition, the electrical equipment presented in Examples and Comparative Examples was powered by 200 V or 100 V, and 60 Hz. Note that “liters” and “% by weight” are sometimes written as “L” and “wt %”, respectively, for convenience. Unless otherwise stated, “%” means“% by mass”.

[0169] [Production of Particulate Water-Absorbing Agent]

Example 1

[0170] (Step for Preparing an Aqueous Monomer Solution)

[0171] A solution (A) in which 10% by mass aqueous polyethylene glycol diacrylate (weight-average molecular weight; 523) solution (3.13 g, 0.02 mol % relative to acrylic acid) as an internal crosslinking agent and an aqueous solution of sodium salt of diethylenetriaminepentaacetic acid (1.32 g) as a chelating agent were added into acrylic acid (216.2 g), and a solution (B) in which 48.5% by mass aqueous sodium hydroxide solution (180.6 g) were diluted with deionized water (209.9 g) adjusted to 50° C. were each prepared in polypropylene containers with a capacity of 1.5 L and an inner diameter of 80 mm. While stirring the solution (A) using a magnetic stirrer, the solution (B) was added thereto and mixed to prepare a solution (C). Note that the temperature of the solution (C) was increased to 101° C. due to the heat of neutralization and the heat of dissolution generated during the mixing process.

[0172] Subsequently, the stirring of the solution (C) was continued, and when the temperature of the solution (C) reached 95° C., a 10% by mass of an aqueous sodium persulfate solution (3.6 g, 0.05 mol % relative to acrylic acid (sodium), 0.12 g/mol) was added to the solution (C) as a polymerization initiator and stirred for about 3 seconds to make an aqueous monomer solution (1). Note that the chelating agent content of the aqueous monomer solution (1) when converted to an acid type, in other words, the content of diethylenetriaminepentaacetic acid is 856 ppm (relative to the monomer).

[0173] (Polymerization Step)

[0174] The aqueous monomer solution (1) had a rate of neutralization of 73 mol % and a monomer concentration of 43% by mass. Next, the aqueous monomer solution (1) was poured into a vat-type container in an open-air system. Note that the vat-type container had a bottom surface of 250 mm×250 mm, a top surface of 640 mm×640 mm, a height of 50 mm, a trapezoidal central cross section, and a TEFLON (R) sheet attached to the inner surface. In addition, the vat-type container was placed on a hot plate heated to 100° C. and preheated. After the aqueous monomer solution (1) was poured into the vat-type container, the polymerization reaction started about 5 seconds later, raising the temperature of the aqueous monomer solution. The polymerization initiation temperature was 93° C. The polymerization reaction proceeded with the aqueous monomer solution (1) expanding and foaming upward in all directions while generating water vapor, and then the polymerization reaction was completed with the resulting hydrogel (1) shrinking to a size slightly larger than the bottom surface of the vat-type container. Note that the polymerization reaction (expansion and shrinkage) was completed within about 1 minute, but the hydrogel (1) was kept in the vat-type container for 2 minutes afterwards. A bubble-containing hydrogel (1) was obtained by this polymerization reaction. The polymerization time was 3 minutes.

[0175] (Gel-Crushing Step)

[0176] Next, the hydrogel (1) obtained by the above polymerization reaction was divided into 16 equal parts, and then a tabletop meat chopper (MEAT-CHOPPER TYPE: 12VR-400KSOX, manufactured by Iizuka Industry Co., Ltd.) having a perforated plate was used to crush the gel by adding 25 parts by mass of 0.04% by mass aqueous sodium sulfite solution at 25° C. to the hydrogel relative to 100 parts by mass (265 g) of the solid content of the hydrogel at the same time as the hydrogel was fed. Therefore, 100 ppm of sodium sulfite was added relative to the solid content of the hydrogel.

[0177] The particulate hydrogel (1) obtained by the gel crushing had a weight average particle diameter (D50) of 1.2 mm.

[0178] (Drying Step)

[0179] Next, 450 g of the particulate hydrogel (1) was spread out on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a static dryer. At that time, the layer height of the particulate hydrogel stacked in the static dryer (through-flow circulating dryer model 71-S6 (manufactured by SATAKE MultiMix Corporation)) was measured at five points and averaged, resulting in an average height of 3 cm.

[0180] On the other hand, the addition timing of an inorganic reducing agent was measured beforehand in the gel-crushing step, and the ventilation of hot air at 180° C. was started 5 minutes after the addition of the inorganic reducing agent. Note that the temperature of the hydrogel immediately after the gel crushing was confirmed to be 50° C.

[0181] Note that in the same drying step, it has been found that the amount of solid component can reach 80% by mass in 10 minutes by repeating an operation of temporarily pulling the semi-dry material out of the dryer and measuring its mass.

[0182] The ventilation of hot air was terminated 30 minutes after the start of the ventilation, thereby obtaining a dry polymer (1) in the form of particles. At this point, the amount of solid component of the dry polymer was 95% by mass.

[0183] (Pulverizing and Classification Step)

[0184] The dry polymer (1) was then fed into a roll mill (WML roll mill, manufactured by Inokuchi Giken) and pulverized, followed by classifying with a ro-tap sieve classifier using two types of JIS standard sieves with a mesh size of 850 μm and 150 μm to obtain a water-absorbing resin (1) in an irregularly crushed shape.

[0185] (Surface Crosslinking Step)

[0186] Next, a surface crosslinking agent solution (1) consisting of ethylene glycol diglycidyl ether (0.05 parts by mass), propylene glycol (1 part by mass), deionized water (3.0 parts by mass), and isopropyl alcohol (1 part by mass) was added to the water-absorbing resin (1) (100 parts by mass) in an irregularly crushed shape and mixed with a spatula until the mixture became homogeneous to obtain a humidified mixture (1). Then, the humidified mixture (1) was uniformly placed in a stainless-steel container (9 cm in diameter and about 1 cm in height) and subjected to heat treatment at 180° C. for 40 minutes to obtain a water-absorbing resin (1) subjected to the surface crosslinking.

[0187] Subsequently, the water-absorbing resin (1) subjected to the surface crosslinking was passed through a JIS standard sieve with a mesh size of 850 μm to obtain the final product, a particulate water-absorbing agent (1). The results are shown in Table below.

Example 2

[0188] A particulate water-absorbing agent (2) was produced in the same manner as in Example 1, except that the concentration of the aqueous sodium sulfite solution was set to 0.2% by mass and the amount of sodium sulfite added was set to 500 ppm relative to the solid content of the hydrogel in Example 1. The results are shown in Table below.

Example 3

[0189] A particulate water-absorbing agent (3) was produced in the same manner as in Example 1, except that the concentration of the aqueous sodium sulfite solution was set to 0.8% by mass and the amount of sodium sulfite added was set to 2,000 ppm relative to the solid content of the hydrogel in Example 1. The results are shown in Table below.

Example 4

[0190] A particulate water-absorbing agent (4) was produced in the same manner as in Example 1, except that the concentration of the aqueous sodium sulfite solution was set to 2.0% by mass and the amount of sodium sulfite added was set to 5,000 ppm relative to the solid content of the hydrogel in Example 1. The results are shown in Table below.

Example 5

[0191] A particulate water-absorbing agent (5) was produced in the same manner as in Example 1, except that the amount of sodium salt of diethylenetriaminepentaacetic acid added was changed to 43 ppm (relative to the monomer) as acid type relative to the aqueous monomer solution (1), the concentration of the aqueous sodium sulfite solution was set to 0.2% by mass, the amount of sodium sulfite added was changed to 500 ppm relative to the solid content of the hydrogel, 813 ppm of sodium salt of diethylenetriaminepentaacetic acid (relative to the solid content of the hydrogel) was added as an acid type to the aqueous sodium sulfite solution to perform the gel-crushing step in Example 1. The results are shown in Table below.

Example 6

[0192] A particulate water-absorbing agent (6) was produced in the same manner as in Example 5, except that a JIS standard sieve with a mesh size of 150 μm was not used for classification in Example 5. The results are shown in Table below.

Example 7

[0193] A particulate water-absorbing agent (7) was produced in the same manner as in Example 5, except that the concentration of the aqueous sodium sulfite solution was set to 2.4% by mass and the amount of sodium sulfite added was set to 6,000 ppm relative to the solid content of the hydrogel in Example 5. The results are shown in Table below.

Example 8

[0194] A particulate water-absorbing agent (8) was produced in the same manner as in Example 6, except that a JIS standard sieve with a mesh size of 150 μm was not used for classification in Example 7. The results are shown in the table below.

Example 9

[0195] A particulate water-absorbing agent (9) was produced in the same manner as in Example 1, except that the amount of the aqueous sodium persulfate solution added was changed to 0.055 mol % (0.13 g/mol) relative sodium acrylate, the polymerization initiation temperature was changed to 77° C., the mass of the polymerization raw material was doubled and polymerized, and instead of 100 ppm of sodium sulfite added to the hydrogel relative to the solid content of the hydrogel, 10,000 ppm of sodium hydrogen sulfite was added relative to the solid content of hydrogel at a concentration of 4% by mass of the aqueous sodium hydrogen sulfite solution to stack and dry 1 kg of a particulate hydrogel in Example 1. The results are shown in Table below. Note that the particulate hydrogel gel had a layer height of 5 cm.

Example 10

[0196] A particulate water-absorbing agent (10) was produced in the same manner as in Example 9, except that the concentration of the aqueous sodium hydrogen sulfite solution was set to 8% by mass, and the amount of sodium hydrogen sulfite added was set to 20,000 ppm relative to the solid content of the hydrogel in Example 9. The results are shown in Table below.

Example 11

[0197] A particulate water-absorbing agent (11) was produced as the same manner as in Example 9, except that 1,910 ppm of ethylenediaminetetraacetic acid was added instead of 856 ppm of sodium salt of diethylenetriaminepentaacetic acid added as an acid type, and the amount of sodium hydrogen sulfite added was set to 3,000 ppm relative to the solid content of the hydrogel at a concentration of 1.2% by mass of the aqueous sodium hydrogen sulfite solution in Example 9. The results are shown in Table below.

Example 12

[0198] A particulate water-absorbing agent (12) was produced in the same manner as in Example 11, except that 1,250 ppm of nitrilotriacetic acid was added instead 1,910 ppm of ethylenediaminetetraacetic acid added relative to the acrylic acid in Example 11. The results are shown below.

Example 13

[0199] A particulate water-absorbing agent (13) was produced in the same manner as in Example 11, except that 799 ppm of the sodium salt of ethylenediamine tetramethylene phosphonic acid was added as acid type instead of 1,910 ppm ethylenediaminetetraacetic acid added in Example 11. The results are shown in Table below.

Example 14

[0200] A particulate water-absorbing agent (14)was produced in the same manner as in Example 1, except that, the concentration of the aqueous sodium sulfite solution was set to 1.2% by mass, the amount of sodium sulfite added was changed to 6,000 ppm relative to the solid content of the hydrogel, the time from the addition of the inorganic reducing agent to the start of hot air ventilation was changed to 120 minutes, and the particulate hydrogel from the addition of the inorganic reducing agent to the start of drying was placed in a bag to prevent moisture from evaporating and kept at 60° C. in an unventilated oven in Example 1. The results are shown in Table below. Note that the ripening step was performed only in Example 14, and in other Examples and Comparative Examples (including Examples 21 to 41 described later), the time from the point of addition of the inorganic reducing agent to the start of drying ranged from 5 to 30 minutes.

Example 15

[0201] A particulate water-absorbing agent (15) was produced in the same manner as in Example 1, except that 11,800 ppm of sodium thiosulfate pentahydrate was added to the hydrogel relative to the solid content of the hydrogel instead of 100 ppm of sodium sulfite added to the hydrogel relative to the solid content of the hydrogel at a concentration of 4.7% by mass of the aqueous sodium thiosulfate pentahydrate solution in Example 1. The results are shown in Table below.

Example 16

[0202] A particulate water-absorbing agent (16) was produced in the same manner as in Example 2, except that the amount of the aqueous sodium persulfate solution added was changed to 0.015 mol % (0.04 g/mol) relative to sodium acrylate in Example 2. The results are shown in Table below.

Example 17

[0203] A particulate water-absorbing agent (17) was produced in the same manner as in Example 16, except that the concentration of the aqueous sodium sulfite solution was set to 2.4% by mass and the amount of sodium sulfite added was set to 6,000 ppm relative to the solid content of the hydrogel in Example 16. The results are shown in Table below.

Example 18

[0204] A particulate water-absorbing agent (18) was produced in the same manner as in Example 1, except that the layer height of the particulate hydrogel was changed to 6 cm during drying in Example 1. The results are shown in Table below.

Example 19

[0205] A particulate water-absorbing agent (19) was produced in the same manner as in Example 18, except that the layer height of the particulate hydrogel during drying was set to 11 cm in Example 18. The results are shown in Table below.

Example 20

[0206] A particulate water-absorbing agent (20) was produced in the same manner as in Example 1, except that the amount of deionized water added relative to the hydrogel was changed to 6 g relative to the solid content of the hydrogel, the concentration of the aqueous sodium sulfite solution was set to 10% by mass, and the amount of sodium sulfite added was changed to 6,000 ppm relative to the solid content of the hydrogel in Example 1. The results are shown in Table below.

Comparative Example 1

[0207] A comparative particulate water-absorbing agent (1) was produced in the same manner as in Example 1, except that sodium sulfite was not added in Example 1. The results are shown in Table below.

Comparative Example 2

[0208] A comparative particulate water-absorbing agent (2) was produced according to Examples 1 to 7 described in International Publication No. WO 2011/040530. The results are shown in Table below.

Comparative Example 3

[0209] A comparative particulate water-absorbing agent (3) was produced according to Example 6 described in International Publication No. WO 2015/053372. The results are shown in Table below.

Comparative Example 4

[0210] A comparative particulate water-absorbing agent (4) was produced according to Example 1 described in Japanese Patent Laid-Open No, 2003-206305. The results are shown in Table below.

Comparative Example 5

[0211] A comparative particulate water-absorbing agent (5) was produced according to Example 2 described in Japanese Patent Laid-Open No, 2003-206305. The results are shown in Table below.

Comparative Example 6

[0212] A comparative particulate water-absorbing agent (6) was produced in the same manner as in Example 1, except that 6,000 ppm of oxalic acid dihydrate was added instead of 100 ppm of sodium sulfite added relative to the solid content of the hydrogel in Example 1. The results are shown in Table below.

Comparative Example 7

[0213] A comparative particulate water-absorbing agent (7) was produced in the same manner as in Example 5, except that the sodium salt of diethylenetriaminepentaacetic acid was not added to the hydrogel, and the surface crosslinking agent solution (2), in which the deionized water was changed to 2.9 parts by mass by adding 0.0813 parts by mass of the sodium salt of diethylenetriaminepentaacetic acid to the surface crosslinking agent solution (1) as an acid type in the surface crosslinking step, was used and added to 100 parts by mass of the water-absorbing resin in an irregularly crushed shape in Example 5. The results are shown in Table below.

[0214] [Various Measurements]

[0215] [Amount of Solid Component of Water-Absorbing Resin]

[0216] The proportion of components that do not volatilize at 180° C. in a water-absorbing resin is expressed as the amount of solid component. The relationship between the amount of solid component and the moisture content is as follows:

[0217] Amount of solid component (% by mass)=100−moisture content (% by mass).

[0218] The method for measuring the amount of the solid component is as follows. About 1 g of a water-absorbing resin was weighed (mass W1) in an aluminum cap (mass W0) with a bottom diameter of about 5 cm, and the resin was left to stand for 3 hours in a still dryer at 180° C. and dried. The mass of the “aluminum cap+water-absorbing resin” after drying (W2) was measured to determine the amount of solid component by the following formula:

Amount of solid component(% by mass)=((W2−W0)/W1)×100.

Note that for the block-shaped dry polymer, five pieces of samples were taken from various positions, crushed so that each particle size was 5 mnm or less, and then measured to adopt the average value.

[0219] [Amount of Solid Component of Hydrogel]

[0220] The solid content of the hydrogel in the form of particles was measured in the same method as that for measuring the “solid content of water-absorbing resin”. However, the amount of hydrogel was changed to about 2 g, the drying temperature to 180° C., and the drying time to 24 hours.

[0221] [Residual Monomer]

[0222] Residual monomers were measured in accordance with the EDANA method (ERT410.2-02).

[0223] [Content, Residual Amount, and Residual Ratio of Chelating Agent]

[0224] The chelating agent in the particulate water-absorbing agent was extracted and analyzed in a method described in International Publication No. WO 2015/053372.

[0225] Specifically, 1 g of the particulate water-absorbing agent obtained in each of Example and Comparative Examples was added to 100 g of a 0.9% mass of aqueous sodium chloride solution and stirred at room temperature for 1 hour (stirring speed: 500±50 rpm) to extract the chelating agent into the 0.9% mass of the aqueous sodium chloride solution. Then, the swollen gel was filtered using filter paper (No. 2/reserved particle size defined in JIS P 3801; 5 μm/TOYO ROSHI KAISHA, Ltd.). The resulting filtrate was then passed through an HPLC sample pretreatment filter (Chromatdisk 25A/aqueous type, pore size; 0.45 μm/Kurabo Industries Ltd.), and the content of the chelating agent in the filtrate was measured using high-performance liquid chromatography (HPLC). The content of the chelating agent in the particulate water-absorbing agent was determined by using the calibration curve obtained by measuring a standard chelating agent liquid of known concentration as an external standard and considering the dilution ratio of the particulate water-absorbing agent to a 0.9% by mass of the aqueous sodium chloride solution. The measurement conditions of HPLC were changed according to the type of chelating agent as appropriate. Specifically, diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA) were quantified according to the following measurement condition 1, and ethylenediamine tetra (methylenephosphonic acid) (EDTMP) was quantified according to the following measurement condition 2.

[0226] Measurement Condition 1:

[0227] <Eluent>; A mixture of 0.3 ml of 0.4 mol/L alum solution, 450 ml of 0.1N potassium hydroxide solution, 3 ml of 0.4 mol/L tetra-n-butyl ammonium hydroxide solution, 3 ml of sulfuric acid, 1.5 ml of ethylene glycol, and 2,550 ml of ion-exchanged water

[0228] <Column>; LichroCART 250-4 Superspher 100 Rp-18e (4 μm) (manufactured by Merck KGaA)

[0229] <Column temperature>; 23±2° C.

[0230] <Flow rate>; 1 ml/min

[0231] <Detector>; UV at wavelength of 258 nm.

[0232] Measurement Condition 2:

[0233] <Eluent>; 0.003 mol/L aqueous sulfuric acid solution

[0234] <Column>; Shodex IC NI-424 (manufactured by Showa Denko K.K.)

[0235] <Column temperature>; 40° C.

[0236] <Flow rate>; 1 ml/min

[0237] <Detector>; RI.

[0238] Since the content of the chelating agent above is affected by a moisture content, the content of the chelating agent in the present invention, C6, is a value corrected for the moisture content and converted relative to the solid content of the particulate water-absorbing agent. Contents of the chelating agent of the present invention, C1, C2, and C7 are values that are not corrected for the moisture content. The chelating agent is considered present as an acid type in the water-absorbing resin and the particulate water-absorbing agent when it is an anionic type. The calibration curve is prepared using the standard chelating agent liquid to which the chelating agent is added, and the conversion to the acid type is made by calculation using the molecular weight.

[0239] [Concentration of Inorganic Reducing Agent]

[0240] The inorganic reducing agent in the particulate water-absorbing agent was extracted and analyzed in a method described in International Publication No. WO 2011/040530.

[0241] Specifically, 50 g of ion exchange water and 0.5 g of a water-absorbing resin are put in a 200 ml beaker and left for 1 hour. After adding 50 g of methanol, 2.5 g of a solution in which 2 mmol of malachite green dissolved in an eluent described later is then added thereto. This solution is stirred for about 30 minutes and filtered, and the filtrate is analyzed by high-performance liquid chromatography to determine the amount of reducing agent contained in the particulate water-absorbing agent. Note that the eluent was prepared in the ratio of 400 ml of methanol, 6 ml of n-hexane, and 100 ml of 0.0 M-2-N-morpholino-ethanesulfonic acid, sodium salt. The calibration curve was prepared by analyzing the particulate water-absorbing agent without reducing agent spiked with the reducing agent. The detection limit of the inorganic reducing agent by the above analysis was 5 ppm.

[0242] [Coloration Evaluation]

[0243] The initial color tone of the water-absorbing resin was evaluated using the SZ-Σ80COLOR MEASURING SYSTEM spectrophotometer manufactured by NIPPON DENSHOKU INDUSTRIES Co., Ltd. For the measurement conditions, reflectance measurement was selected, a powder/paste sample stand with an inner diameter of 30 mm and a height of 12 mm was used, a standard round white plate No. 2 for powder/paste was used as a standard, and a 30-mm diameter floodlight pipe was used. About 5 g of the particulate water-absorbing agent obtained from Examples and Comparative Examples was filled into the provided sample stand. This filling was done in such a way that the provided sample stand was about 60% filled. The L-value (lightness: lightness index) and YI-value (degree of yellowness: yellowness index) of the surface in the Hunter Lab color system were measured with the spectrophotometer under the conditions of room temperature (20 to 25° C.) and humidity of 50 RH %. The higher the L-value, the better, and the smaller the YI-value, the lower the coloration and the closer to real white. By using the same measuring method on the same device, it is possible to measure the a- and b-values (chromaticity) of other object colors at the same time. The smaller the a and b values, the lower the coloration and the closer to real white.

[0244] [Coloring Acceleration Test]

[0245] The coloring acceleration test for water-absorbing resins was performed by putting the powder/paste sample stand containing about 5 g of water-absorbing resin, which was used in the initial coloration evaluation above, into a thermo-hygrostat adjusted to 70° C. and 65% humidity, and reserving it for 7 days. Note that, for the measurement of color tone after the coloring acceleration test, the powder/paste sample stand put into the thermo-hygrostat was disposed as it is on the same spectrophotometer as the one used for measuring the initial color tone, and the measurement was conducted under the same measurement conditions.

[0246] [Impact Test]

[0247] Thirty grams of particulate water-absorbing agent obtained in each Examples and Comparative Examples and 10 grams of glass beads with a diameter of 6 mm were put in a glass container (6 cm in diameter and 11 cm in height), and the particulate water-absorbing agent was crushed using a paint shaker (Product No. 488 test disperser, manufactured by Toyo Seiki Seisakusho, Ltd.). Note that crushing was done by vibrating the paint shaker at 800 (cycle/min) (CPM) for 30 minutes. After the vibration, the glass beads were removed using a JIS standard sieve with a mesh size of 2 mm. The details of the paint shaker (disperser) are disclosed in Japanese Patent Laid-Open No. 9-235378.

[0248] The particulate water-absorbing agent subjected to the impact test was sieved by a JIS standard sieve into a particle group 1 with a particle size of less than 300 μm and a particle group 2 with a particle size of 300 μm or more and less than 850 μm.

[0249] After quantifying a content C1 of the chelating agent present in the particle group 1 and a content C2 of the chelating agent present in the particle group 2, a chelating agent ratio (C1/C2) was obtained by dividing the C1 by the C2. The results are shown in Table below.

TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 CRC g/g 41 41 41 41 41 Residual monomer ppm 324 377 541 491 313 Content of chelating agent C7 ppm 446 469 530 604 527 Residual ratio of chelating % 53 55 63 71 62 agent Content of reducing agent ppm N.D. 15 37 89 15 Initial color tone Y1 12.0 11.5 9.8 6.5 11.4 C1/C2 1.0 1.0 1.0 1.0 1.4

TABLE-US-00002 TABLE 2 Example 6 Example 7 Example 8 Example 9 Example 10 CRC g/g 39 41 38 41 40 Residual monomer ppm 325 486 496 431 272 Content of chelating agent C7 ppm 602 693 749 676 702 Residual ratio of chelating % 71 82 88 80 83 agent Content of reducing agent ppm 17 74 74 881 2041 Initial color tone Y1 11.2 6.5 6.5 3.3 0.6 C1/C2 1.6 1.3 1.5 1.0 1.0

TABLE-US-00003 TABLE 3 Example 11 Example 12 Example 13 Example 14 Example 15 CRC g/g 41 40 41 43 39 Residual monomer ppm 492 418 485 463 1849 Content of chelating agent C7 ppm 961 679 410 637 605 Residual ratio of chelating % 51 55 52 75 71 agent Content of reducing agent ppm 55 61 53 53 N/A Initial color tone Y1 7.1 7.0 7.5 16.6 36.0 C1/C2 1.0 1.0 1.0 1.0 1.0

TABLE-US-00004 TABLE 4 Example 16 Example 17 Example 18 Example 19 Example 20 CRC g/g 40 40 40 38 39 Residual monomer ppm 762 975 458 321 630 Content of chelating agent C7 ppm 598 732 571 504 572 Residual ratio of chelating % 71 86 67 59 68 agent Content of reducing agent ppm 65 121 50 35 131 Initial color tone Y1 6.8 7.2 6.9 6.8 7.1 C1/C2 1.0 1.0 1.0 1.0 1.0

TABLE-US-00005 TABLE 5 Comparative Comparative Comparative Comparative Comparative Comparative Comparative Example Example Example Example Example Example Example 1 2 3 4 5 6 7 CRC g/g 41 28 36 45 40 40 40 Residual ppm 428 332 309 1210 29 518 255 monomer Content ppm 390 31 867 7513 480 343 831 of chelating agent C7 Residual % 46 18 98 97 62 41 98 ratio of chelating agent Content ppm — 4735 1307 N.D N.D N/A 15 of reducing agent Initial 12.9 16.7 13.0 6.2 6.6 13.5 11 color tone Y1 C1/C2 1.0 1.2 2.1 2.0 1.9 1.0 2.0

TABLE-US-00006 TABLE 6 Comparative Example 2 Example 5 Example 7 Color tone after L 78.4 77.5 72.6 coloring acceleration a 1.9 2.3 2.3 test b 9.5 10.3 12.7 YI 23.7 24.3 33.9

[0250] [Discussion]

[0251] In Examples 1 to 6, the chelating agent was added in the upstream process of the process for production, but the residual ratio of the chelating agent was good. In contrast, in Comparative Example 1, the residual ratio of the chelating agent was poor.

[0252] In Examples 1 to 4 and 9 to 20, each particulate water-absorbing agents is obtained by polymerizing monomers in the aqueous monomer solution containing a chelating agent having a nitrogen atom, and the amount C3 of the chelating agent contained in the aqueous monomer solution added (relative to the monomer solid content) is 50% by mass or more relative to the total amount C5 of the chelating agent added in the process for production of the particulate water-absorbing agent. However, the residual ratio of the chelating agent was confirmed to be 50% or more. In Examples 5 to 8, each particulate water-absorbing agent is obtained by adding having the chelating agent having a nitrogen atom to a particulate hydrogel made by crushing a hydrogel obtained by polymerizing monomers in the aqueous monomer solution, and the amount C4 of chelating agent added to the particulate hydrogel is 50% by mass or more relative to the total amount C5 of the chelating agent added in the process for production of the particulate water-absorbing agent. However, the residual ratio of a chelating agent was confirmed to exceed 50%. This suggests that a sufficient effect is obtained in the final product for the amount of the chelating agent added. The chelating agent ratios were 0.8 to 1.8 in each Example.

[0253] In contrast, in Comparative Examples 1 (no reducing agent used) and 6(oxalic acid used), each particulate water-absorbing agent is obtained by polymerizing monomers in the aqueous monomer solution containing a chelating agent having a nitrogen atom, and the amount C3 of the chelating agent contained in the aqueous monomer solution added (relative to the monomer solid content) is 50% by mass or more relative to the total amount C5 of the chelating agent added in the process for production of the particulate water-absorbing agent. This is presumably because the inorganic reducing agent having a sulfur atom was not added in the process for production of the particulate water-absorbing agent.

[0254] In Comparative Example 3, the amount C3 of the chelating agent contained in the aqueous monomer solution added is less than 50% by mass relative to the total amount C5 of the chelating agent added in the process for production of the particulate water-absorbing agent, the amount C4 of the chelating agent added to the particulate hydrogel is less than 50% by mass relative to the total amount C5 of the chelating agent added in the process for production of the particulate water-absorbing agent, and a large amount of the chelating agents having nitrogen atoms is added in the downstream process of the process for production of the particulate water-absorbing agent, thereby resulting in a high chelating agent ratio of 2.1. In Comparative Examples 4 and 5, each chelating agent is added to the hydrogel, but the chelating agent ratio is 1.9 to 2.0. This is presumably because the concentration of the chelating agent solution added is too high.

[0255] Comparing the color tone of Example 2 (chelating agent monomer added), Example 5 (chelating agent hydrogel added), and Comparative Example 7 (chelating agent added at the time of surface crosslinking) after the coloring acceleration test, it is apparent Examples 2 and 5 with chelating agent ratios in the range of 0.8 to 1.8 have less coloration compared to Comparative Example 7 with a chelating agent ratio of 2.0. In other words, it was shown that a chelating agent ratio of 0.8 to 1.8 is preferable for color stability.

Example 21

[0256] (Step for Preparing an Aqueous Monomer Solution)

[0257] A solution (A) in which 10% by mass aqueous polyethylene glycol diacrylate solution (9.19 g, 0.01 mol % relative to acrylic acid) and an aqueous solution of sodium salt of diethylenetriaminepentaacetic acid (7.74 g) as a chelating agent were added into acrylic acid (1,258 g), and a solution (B) in which 48.5% by mass aqueous sodium hydroxide solution (1,058 g) were diluted with deionized water (1,245 g) were each prepared in polypropylene containers with a capacity of 5 L and an inner diameter of 175 mm. While stirring the solution (A) using a magnetic stirrer, the solution (B) was added thereto and mixed to prepare a solution (C). Note that the temperature of the solution (C) was increased to 82° C. due to the heat of neutralization and the heat of dissolution generated during the mixing process.

[0258] Subsequently, the stirring of the solution (C) was continued, and when the temperature of the solution (C) reached 80° C., a 10% by mass aqueous sodium persulfate solution (22.3 g, 0.055 mol % relative to acrylic acid (sodium), 0.13 g/mol) was added to the solution (C) as a polymerization initiator and stirred for about 3 seconds to make an aqueous monomer solution (2). Note that the chelating agent content of the aqueous monomer solution (2) when converted to an acid type was 856 ppm (relative to the monomer).

[0259] (Polymerization Step)

[0260] The aqueous monomer solution (2) had a rate of neutralization of 73 mol % and a monomer concentration of 43% by mass. Next, the aqueous monomer solution (2) was poured into a vat-type container in an open-air system. Note that the vat-type container had a bottom surface of 520 mm X 520 mm, a top surface of 940 mm×940 mm, a height of 300 mm, a trapezoidal central cross section, and a TEFLON® sheet attached to the inner surface. In addition, the vat-type container was placed on a hot plate heated to 100° C. and heated in advance. After the aqueous monomer solution (2) was poured into the vat-type container, the polymerization reaction started about 45 seconds later, raising the temperature of the aqueous monomer solution. The polymerization reaction proceeded with the aqueous monomer solution (2) expanding and foaming upward in all directions while generating water vapor, and then the polymerization reaction was completed with the resulting hydrogel (2) shrinking to a size slightly larger than the bottom surface of the vat-type container. Note that the polymerization reaction (expansion and shrinkage) was completed within about 1 minute, but the hydrogel (2) was kept in the vat-type container for 1.5 minutes afterwards. A bubble-containing hydrogel (2) was obtained by this polymerization reaction. The polymerization time was 2.5 minutes.

[0261] (Gel-Crushing Step)

[0262] Next, the hydrogel (2) obtained from the polymerization reaction was divided into 25 equal portions, and then supplied to a screw extruder for gel crushing. The screw extruder used was a meat chopper with a screw shaft outer diameter of 86 mm comprising a perforated plate with a diameter of 100 mm, a pore size of 9.5 mm, 31 pores, and a thickness of 10 mm at the tip. While the screw shaft rotation speed of the meat chopper was set at 115 rpm, the hydrogel (2) was supplied at 4,640 g/min, and at the same time, steam was supplied at 108 g/min, so that 0.2% by mass of an aqueous sodium hydrogen sulfite solution was 100 ppm relative to the solid content of the hydrogel.

[0263] The particulate hydrogel (2) obtained by the gel crushing had a weight average particle diameter (D50) of 3.0 mm.

[0264] (Drying Step)

[0265] Next, 1 kg of the particulate hydrogel (2) was spread out on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a static dryer. At that time, the layer height of the particulate hydrogel stacked in the static dryer was measured at five points, resulting in an average height of 5 cm.

[0266] On the other hand, the addition timing of an inorganic reducing agent was measured beforehand in the gel-crushing step, and the ventilation of hot air at 190° C. was started 5 minutes after the addition of the inorganic reducing agent. Note that the temperature of the hydrogel immediately after the gel crushing was confirmed to be 50° C.

[0267] Note that in the same drying step, it has been found that the amount of solid component can reach 80% by mass in 10 minutes by repeating an operation of temporarily pulling the semi-dry material out of the dryer and measuring its mass.

[0268] The ventilation of hot air was terminated 30 minutes after the start of the ventilation, thereby obtaining a dry polymer (2) in the form of particles. At this point, the solid component of the dry polymer was 95% by mass.

[0269] (Pulverizing and Classification Step)

[0270] The dry polymer (2) was then fed into a roll mill (WML roll mill, manufactured by Inokuchi Giken) and ground, followed by classifying with a ro-tap sieve classifier using two types of JIS standard sieves with a mesh size of 850 μm and 150 μm to obtain a water-absorbing resin (2) in an irregularly crushed shape.

[0271] (Surface Crosslinking Step)

[0272] Next, a surface crosslinking agent solution (1) consisting of ethylene glycol diglycidyl ether (0.05 parts by mass), propylene glycol (1 part by mass), deionized water (3.0 parts by mass), and isopropyl alcohol (1 part by mass) was added to the water-absorbing resin (2) (100 parts by mass) in an irregularly crushed shape and mixed with a spatula until the mixture became homogeneous to obtain a humidified mixture (2). Then, the humidified mixture (2) was uniformly placed in a stainless-steel container (width: about 22 cm, depth: about 28 cm, and height: about 5 cm) and subjected to heat treatment at 100° C. for 40 minutes to obtain a water-absorbing resin (2) subjected to the surface crosslinking.

[0273] Subsequently, the water-absorbing resin (2) subjected to the surface crosslinking was passed through a JIS standard sieve with a mesh size of 850 μm to obtain the final product, a particulate water-absorbing agent (21). The results are shown in Table below.

Examples 22 to 41

[0274] Particulate water-absorbing agents (22 to 41) were produced in the same manner as in Example 21, except that the preparation of the aqueous monomer solution, the agent (chelating agent) used therein, and the inorganic reducing agent added in the gel-crushing step were changed to values shown in Tables. The results are shown in Table below.

TABLE-US-00007 TABLE 7 Example 21 22 23 24 25 26 (Step for preparing an aqueous monomer solution) Acrylic acid g 1258 1258 1258 1258 1258 1258 Internal crosslinking agent (10% by mass aqueous polyethylene glycol diacrylate solution) Amount added relative to mol % 0.010 0.010 0.015 0.030 0.020 0.020 acrylic acid Chelating agent species 1 (sodium salt of diethylenetriaminepentaacetic acid) Amount added relative to ppm 856 856 856 856 1712 2569 monomer solid content (as acid type) Chelating agent species 2 (sodium salt of ethylenediamine tetramethylene phosphonic acid) Amount added relative to ppm — — — — — — monomer solid content (as acid type) 48.5% by mass aqueous g 1058 1058 1058 1058 1058 1058 sodium hydroxide solution Deionized water g 1245 1245 1245 1245 1245 1245 (Gel-crushing step) Inorganic reducing agent species Sodium Sodium Sodium Sodium Sodium Sodium hydrogen sulfite sulfite hydrogen sulfite hydrogen sulfite sulfite sulfite Amount added relative to ppm 100 1000 2000 4000 5000 10000 solid content of hydrogel Concentration of inorganic % by 0.2 2.0 4.0 8.0 10 20 reducing agent aqueous mass solution

TABLE-US-00008 TABLE 8 Example Example 27 28 29 30 31 32 (Step for preparing an aqueous monomer solution) Acrylic acid g 1258 1258 1258 1200 1200 1317 Internal crosslinking agent (10% by mass aqueous polyethylene glycol diacrylate solution) Amount added mol % 0.020 0.010 0.010 0.015 0.020 0.020 relative to acrylic acid Chelating agent species 1 (sodium salt of diethylenetriaminepentaacetic acid) Amount added ppm 2569 — — — — — relative to monomer solid content (as acid type) Chelating agent species 2 (sodium salt of ethylenediamine tetramethylene phosphonic acid) Amount added ppm — 799 799 799 799 799 relative to monomer solid content (as acid type) 48.5% by mass g 1058 1058 1058 1009 1009 1107 aqueous sodium hydroxide: solution Deionized water g 1245 1245 1245 1354 1354 1135 (Gel-crushing step) Inorganic reducing Sodium Sodium Sodium Sodium Sodium Sodium agent species sulfite sulfite hydrogen sulfite sulfite hydrogen sulfite sulfite Amount added ppm 1000 100 1000 2000 4000 5000 relative to solid content of hydrogel Concentration of % by 2.0 0.2 2.0 4.0 8.0 10 aqueous mass inorganic reducing agent

TABLE-US-00009 TABLE 9 Example 33 34 35 36 37 38 (Step for preparing an aqueous monomer solution) Acrylic acid g 1142 1200 1142 1142 1200 1142 Internal crosslinking agent (10% by mass aqueous polyethylene glycol diacrylate solution) Amount added mol % 0.020 0.030 0.030 0.030 0.020 0.020 relative to acrylic acid Chelating agent species 1 (sodium salt of diethylenetriaminepentaacetic acid) Amount added ppm — — — — 428 856 relative to monomer solid content (as acid type) Chelating agent species 2 (sodium salt of ethylenediamine tetramethylene phosphonic acid) Amount added ppm 799 1597 2396 2396 399 799 relative to monomer solid content (as acid type) 48.5% by mass g 959 1009 959 959 1009 959 aqueous sodium hydroxide solution Deionized water g 1463 1354 1463 1463 1354 1463 (Gel-crushing step) Inorganic reducing Sodium Sodium Sodium Sodium Sodium Sodium agent species sulfite hydrogen sulfite hydrogen sulfite sulfite sulfite sulfite Amount added ppm 5000 5000 10000 1000 5000 10000 relative to solid content of hydrogel Concentration % by 10 10 20 2.0 10 20 of aqueous mass inorganic reducing agent

TABLE-US-00010 TABLE 10 Example 39 40 41 (Step for preparing an aqueous monomer solution ) Acrylic acid g 1142 1142 1142 Internal crosslinking agent (10% by mass aqueous polyethylene glycol diacrylate solution) Amount added relative to acrylic mol % 0.020 0.020 0.020 acid Chelating agent species 1 (sodium salt of diethylenetriaminepentaacetic acid) Amount added relative to monomer ppm 1712 1712 1712 solid content (as acid type) Chelating agent species 2 (sodium salt of ethylenediamine tetramethylene phosphonic acid) Amount added relative to monomer ppm 1597 1597 1597 solid content (as acid type) 48.5% by mass aqueous sodium g 959 959 959 hydroxide solution Deionized water g 1463 1463 1463 (Gel-crushing step) Inorganic reducing agent species Sodium sulfite Sodium Sodium hydrogen hydrogen sulfite sulfite Amount added relative to solid ppm 1000 5000 10000 content of hydrogel Concentration of aqueous inorganic % by 2.0 10 20 reducing agent mass

TABLE-US-00011 TABLE 11 Example Example Example Example Example 21 22 23 24 25 CRC [g/g] 45 44 42 35 39 Residual [ppm] 315 365 443 342 471 monomer Content [ppm] 341 402 421 482 1028 of chelating agent C7 Residual [%] 43 51 53 61 65 ratio of chelating agent Content [ppm] N.D. 44 112 629 1077 of reducing agent Initial 12.0 11.0 9.9 7.7 6.6 color tone Y1 C1/C2 [—] 1.0 1.0 1.0 1.0 1.0

TABLE-US-00012 TABLE 12 Example Example Example Example Example 26 27 28 29 30 CRC [g/g] 38 40 45 44 45 Residual [ppm] 450 519 264 352 437 monomer Content [ppm] 1672 1431 321 369 393 of chelating agent C7 Residual [%] 71 61 44 50 54 ratio of chelating agent Content [ppm] 5214 40 N.D. 39 139 of reducing agent Initial 1.1 11.0 14.0 13.5 13.0 color tone Y1 C1/C2 [—] 1.0 1.0 1.0 1.0 1.0

TABLE-US-00013 TABLE 13 Example Example Example Example Example 31 32 33 34 35 CRC [g/g] 42 35 39 40 41 Residual [ppm] 309 138 146 273 109 monomer Content [ppm] 408 442 412 934 1900 of chelating agent C7 Residual [%] 56 60 56 64 86 ratio of chelating agent Content [ppm] 950 1546 1432 1410 4958 of reducing agent Initial 12.0 11.5 11.5 11.5 9.0 color tone Y1 C1/C2 [—] 1.0 1.0 1.0 1.0 1.0

TABLE-US-00014 TABLE 14 Exam- Exam- Exam- Exam- Exam- Exam- ple ple ple ple ple ple 36 37 38 39 40 41 CRC [g/g] 42 42 44 46 45 44 Residual [ppm] 453 155 99 504 387 101 monomer Content [ppm] 1455 415 1219 2411 2594 2732 of chelating agent C7 Residual [%] 66 54 80 79 85 90 ratio of chelating agent Content [ppm] 8 1269 5967 12 1916 5403 of reducing agent Initial 13.5 11.5 9.0 13.5 11.5 9.0 color tone Y1 C1/C2 [—] 1.0 1.0 1.0 1.0 1.0 1.0

[0275] [Discussion]

[0276] The same trend was observed in Examples 21 to 41 as in Examples 1 to 20.

Example 42

[0277] To 100 parts by mass of the particulate water-absorbing agent (22) in Example 22, 1 part by mass of 10% by mass aqueous sodium sulfite solution was added and mixed using a spatula until the mixture became homogeneous, then uniformly placed in a stainless-steel container (width: about 22 cm, depth: about 28 cm, and height: about 5 cm) and heated at 80° C. for 30 minutes. Subsequently, the final product, a particulate water-absorbing agent (42) was obtained by passing through a JIS standard sieve with a mesh size of 850 μm. The chelating agent ratio was 1.0 as in the case of the particulate water-absorbing agent (22). The initial color tone YI of the particulate water-absorbing agent (42) was 11.1, the residual monomer was 345 ppm, and the concentration of the reducing agent was 980 ppm.

Example 43

[0278] To 100 parts by mass of the particulate water-absorbing agent (42) in Example 42, 0.3 parts by mass of hydrotalcite (product name: DHT-6, manufactured by Kyowa Chemical Industry Co., Ltd., Mg.sub.6A.sub.12 (OH).sub.16CO.sub.3.4H.sub.2O [in general formula (1), x=0.25, m=0.50], volume-average particle size: 0.5 μm) was mixed to obtain the final product, a particulate water-absorbing agent (43). The mixing was carried out by placing 30 g of the water-absorbing resin together with hydrotalcite in a mayonnaise bottle with a capacity of 225 mL and mixing at 800 (cycle/min) (CPM) for 3 minutes by vibration of a paint shaker (Product No. 488 test disperser, manufactured by Toyo Seiki Seisakusho, Ltd.). The initial color tone YI of the particulate water-absorbing agent (43) was 9.5.

Example 44

[0279] The final product, a particulate water-absorbing agent (44), was obtained by carrying out the same procedure as in Example 43, except that 0.3 parts by mass of AEROSIL™ 200, a fumed silica, was added in place of 0.3 parts by weight of hydrotalcite in Example 43. The initial color tone YI of the particulate water-absorbing agent (44) was 10.6.

[0280] [Discussion]

[0281] The initial color tone YI of the particulate water-absorbing agent (43) with the addition of hydrotalcite having a volume-average particle size of 0.5 μm was improved compared to that of the particulate water-absorbing agent (42) and particulate water-absorbing agent (44).

[0282] The present application is based on Japanese Patent Application No. 2019-204870, filed on Nov. 12, 2019, the entire disclosure of which is incorporated herein by reference.