WO2020059871A1

WO2020059871A1 - Production method of water absorbing resin including chelating agent

Info

Publication number: WO2020059871A1
Application number: PCT/JP2019/037091
Authority: WO
Inventors: 加奈子津留; 智嗣松本; 好希片田; 邦彦石▲崎▼
Original assignee: 株式会社日本触媒
Priority date: 2018-09-21
Filing date: 2019-09-20
Publication date: 2020-03-26
Also published as: CN112714770B; JPWO2020059871A1; KR20210058928A; KR102562511B1; JP7064614B2; CN112714770A

Abstract

According to the present invention, the addition amount of persulfate in a polymerization initiator from a polymerizing step before a drying step is reduced to the total amount of 0-0.04 mol% (with respect to a monomer during polymerization), and a gel particle size of a water-containing gel-like polymer before the drying step and a drying condition are controlled, thereby suppressing the decomposition of a chelating agent in the drying step.

Description

Method for producing water-absorbing resin containing chelating agent

The present invention relates to a method for producing a water absorbent resin containing a chelating agent.

A water-absorbent resin (SAP / Super Absorbent polymer) is a water-swellable, water-insoluble polymer gelling agent, and absorbent articles such as disposable diapers and sanitary napkins, as well as agricultural and horticultural water retention agents, and industrial water stoppages. As a material, it is mainly used for disposable purposes.

Such a water-absorbent resin is produced through a production step such as a polymerization step, a drying step, a removal step of an undried substance if necessary, a pulverization step, a classification step, a surface cross-linking step, and the like. With high performance, many functions (physical properties) are also required for a water absorbent resin. Specifically, the gel strength, water-soluble content, water absorption rate, water absorption capacity under pressure, liquid permeability, particle size distribution, urine resistance, antibacterial property, impact resistance (not limited to the mere water absorption capacity) Damage), powder fluidity, deodorant properties, coloring resistance (whiteness), low dust, and the like.

In order to improve the performance of the water-absorbent resin, as a method other than changing the monomer and the manufacturing process, a small amount of additives may be used in the water-absorbent resin, as various additives of the water-absorbent resin, Water-soluble or water-insoluble inorganic or organic powder, surfactant, plasticizer, water-soluble polymer, water-insoluble (thermoplastic) polymer, antibacterial agent, deodorant, reducing agent, antioxidant, organic acid, inorganic acid Additives such as anti-coloring agents and chelating agents are known.

Among the above-mentioned physical properties and various additives of the water-absorbent resin, in particular, polymerization stability, color tone stability of the water-absorbent resin (color stability when stored for a long time under high temperature and high humidity), urine resistance (gel) It is known to add a chelating agent (also called an ion-sequestering agent) to a water-absorbent resin for the purpose of improving deterioration prevention or the like.

For example, Patent Documents 1 to 4 disclose that a chelating agent is added to a monomer of a water-absorbing resin mainly for improving polymerization stability. Patent Literatures 5 to 11 mainly describe a process for polymerizing a water-absorbent resin, a drying process, a surface cross-linking process, a granulation process, a fine powder collection process, and the like, mainly for improving urine resistance of a water-absorbent resin (preventing urine deterioration). A method for producing a water-absorbing resin in which a chelating agent is added to a monomer or a polymer thereof in a production process is disclosed. Patent Documents 12 to 19 and 23 disclose a method for producing a water-absorbent resin in which a metal chelating agent is added in any of the production steps in order to suppress coloring (particularly coloring with time) of the water-absorbent resin.

In particular, Patent Literatures 12 and 14 describe that it is known that a chelating agent is preferably present inside a water-absorbing resin for preventing coloration. There is disclosed a method of adding a chelating agent to the inside of a water-absorbent resin by adding the chelating agent to a gel before drying. Patent Document 20 discloses a water-absorbing resin composition containing a chelating agent for improving the salt resistance of the water-absorbing resin. Patent Documents 21 and 23 disclose a method for producing a water-absorbent resin in which a metal chelating agent is added in any of the production steps in order to improve the urine resistance of the water-absorbent resin. Patent Document 22 discloses a method for producing a water-absorbent resin in which a chelating agent is added to a second-stage monomer in a two-stage polymerization in order to improve physical properties such as a water absorption capacity under pressure of the water-absorbent resin after reverse phase suspension polymerization. Is disclosed.

Here, the chelating agent used for the purpose of stabilizing polymerization in the production process and preventing drying deterioration in the above-mentioned Patent Documents 1 to 7 can also improve urine resistance when a water-absorbing resin is used if it remains in the final product. It works to prevent coloring.

WO93 / 005080 pamphlet International Publication No. 2011/120746 pamphlet Japanese Unexamined Patent Publication "JP-A-02-117903" Japanese Unexamined Patent Publication "JP-A-08-283318" Japanese Unexamined Patent Publication "JP-A-2000-38407" Japanese Unexamined Patent Publication "JP-A-2000-007790" Japanese Unexamined Patent Publication "JP-A-11-246677" Japanese Unexamined Patent Publication "JP-A-11-315148" Japanese Unexamined Patent Publication "JP-A-11-315147" European Patent Publication No. 0781146A WO 2015/053372 pamphlet WO 2008/09961 pamphlet WO 2003/059962 pamphlet WO2009 / 005114 pamphlet WO 2016/006134 pamphlet WO2018 / 029045 pamphlet International Publication No. 2011/040530 pamphlet WO 03/059961 pamphlet Japanese Unexamined Patent Publication "JP-A-05-086251" Japanese Unexamined Patent Publication "JP-A-59-230046" Japanese Unexamined Patent Publication "Japanese Patent Application Laid-Open No. 08-067821" International Publication No. 2007/004529 pamphlet WO 2014/054656 pamphlet

However, the present inventors further studied the method for producing a water-absorbent resin containing a chelating agent, and particularly when the chelating agent is added in the polymerization step and the drying step among the above-described production steps, the addition of the chelating agent in the final product A new problem has been found that it is difficult to obtain a sufficient effect commensurate with the amount. Then, as a cause of the problem, it was found that the amount of the chelating agent corresponding to the amount of the chelating agent added to the monomer or the polymer gel was not contained in the water absorbent resin of the final product.

In response to this problem, one embodiment of the present invention provides a chelating agent that can prevent the decomposition of the chelating agent in the process of manufacturing the water-absorbent resin and can improve the residual ratio of the chelating agent in the water-absorbent resin as a final product. It is a main object to provide a method for producing a water-absorbent resin containing the same.

Another object of one embodiment of the present invention is to provide a water-absorbent resin containing a chelating agent obtained by the above-described production method, which has good coloring resistance (whiteness).

The present inventors have investigated the cause thereof in order to solve the above-described problem that has been found this time, and found that in a step of drying a hydrogel polymer obtained by polymerizing a monomer, a step before the drying step was performed. It was found that the chelating agent that had been added to the sample was specifically reduced in the drying step. The inventors have found that the cause of the decrease in the chelating agent in the drying step is a polymerization initiator (particularly, persulfate) remaining in the hydrogel polymer.

In order to solve the above problems, the present invention controls the persulfate in the polymerization initiator at the time of polymerization or before drying (0.04 mol% or less), and further controls the gel particle size and drying conditions before drying. And suppressing the decomposition of the chelating agent in the drying step, and includes the inventions described in the following [1] to [20].

[1] A polymerization step of polymerizing a monomer aqueous solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer, and, if necessary, pulverizing the hydrogel polymer during and / or after the polymerization step A chelating agent having a water absorption capacity (CRC) of 15 g / g or more, comprising a gel pulverizing step and a drying step of drying the obtained particulate hydrogel polymer to obtain a particulate dry polymer. Wherein the persulfate used in the polymerization step is 0 to 0.04 mol% (based on the monomer at the time of polymerization) (provided that the persulfate is 0 mol%). In the case of (not used), another polymerization initiator is indispensably used), and a chelating agent is added to the monomer aqueous solution or the hydrogel polymer in a step prior to the drying step in a total of 10 ppm or more (relative to the polymerization time). Monomer or solid content of hydrogel polymer) Including a chelating agent, the weight-average particle diameter (D50) of the hydrated hydrogel polymer is 1 mm or less, and in the drying step, the drying time until the solid content is 80% by weight or more is 20 minutes or less. A method for producing a water absorbent resin.

[2] A polymerization step of polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer, and, if necessary, pulverizing the hydrogel polymer during and / or after the polymerization step A chelating agent having a water absorption capacity (CRC) of 15 g / g or more, comprising a gel pulverizing step and a drying step of drying the obtained particulate hydrogel polymer to obtain a particulate dry polymer. In the drying step, the chelating agent is 10 ppm or more (based on the solid content of the hydrogel polymer) and the persulfate is 0 to 0.04 mol% (based on the polymerization time). A chelating agent, by drying a particulate hydrogel polymer having a weight-average particle diameter (D50) of 1 mm or less containing a monomer) until the solid content becomes 80% by weight or more for a drying time of 20 minutes or less. A method for producing a water-absorbent resin containing the same. However, the drying time refers to a time until the solid content becomes 80% by weight or more.

[3] The method according to [1] or [2], wherein the total amount of the persulfate added from the polymerization step to before the drying step is 0 to 0.04 mol% (based on the monomer at the time of polymerization). Production method.

[4] The production method according to any one of [1] to [3], wherein in the drying step, hot-air drying at 150 to 200 ° C. is performed.

[5] The method according to any one of [1] to [4], wherein the chelating agent is at least one selected from the group consisting of an amino polycarboxylic acid chelating agent and an amino polyphosphoric acid chelating agent. The manufacturing method as described.

[6] The production method according to any one of [1] to [5], wherein the hydrogel polymer is formed into particles in the gel pulverizing step.

[7] The total amount of the chelating agent added in the steps before the drying step is 60 ppm to 1% (based on the solid content of the monomer at the time of polymerization or the solid content of the hydrogel polymer). The production method according to any one of [6].

[8] The production method according to any one of [1] to [7], wherein the polymerization is an aqueous solution polymerization.

[9] The production method according to any one of [1] to [8], wherein the polymerization is a foaming polymerization or a boiling polymerization, and the hydrogel polymer contains bubbles.

[10] The method according to any one of [1] to [9], wherein the polymerization is a high-temperature-initiated short-time polymerization of a polymerization initiation temperature of 30 ° C. or higher, a polymerization peak temperature of 80 to 130 ° C., and a polymerization time of 60 minutes or less. Production method.

[11] The production method according to any one of [1] to [10], further including a step of cross-linking the surface of the water-absorbent resin after the drying step.

[12] The production method according to any one of [1] to [11], further including a step of adding a chelating agent to the water-absorbent resin in the steps after the drying step.

[13] The production method according to any one of [1] to [12], wherein the hydrogel polymer before drying contains 0.1% by weight or more of a residual monomer.

[14] The monomer used in the polymerization step contains acrylic acid (salt), and the content of the acrylic acid (salt) is based on the total monomers (excluding the internal crosslinking agent) used in the polymerization step. The water-absorbing resin containing 50 to 100 mol% of the chelating agent has a residual amount (C1) of the chelating agent of 10 ppm or more, an L value of an initial color tone of 85 or more, and a YI value of 13 or less. The production method according to any one of [1] to [13], which is a polyacrylic acid (salt) -based water-absorbent resin.

[15] The water-absorbent resin containing a chelating agent has a residual amount (C1) of the chelating agent of 200 ppm or more, an L value of an initial color tone of 89 or more, and a YI value of 10 or less. The production method according to any one of [14].

[16] A polyacrylic acid (salt) -based water-absorbent resin having a chelating agent content (C2) of 200 ppm or more, an initial color tone L value of 89 or more, and a YI value of 10 or less.

[17] Water absorption capacity under no load (CRC) is 25 g / g or more, water absorption capacity under pressure (AAP (0.7 psi)) is 15 g / g or more, and water absorption capacity under pressure and water absorption capacity under no pressure The water-absorbent resin according to [16], wherein the ratio (AAP (0.7 psi) / CRC) is 0.5 or more.

[18] Acrylic acid (salt) is 50 to 100 mol% of the total monomer (excluding the internal cross-linking agent), 0.001 to 5 mol% of the internal cross-linking agent based on the monomer, and persulfate The water-absorbent resin according to [16] or [17], comprising a polyacrylic acid (salt) -based crosslinked polymer obtained from a monomer aqueous solution containing 0 to 0.04 mol% of a monomer.

[19] A hydrogel polymer obtained by polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator is gel-pulverized during and / or after polymerization, if necessary, to obtain a particulate hydrous polymer. A water-absorbent resin containing a chelating agent obtained by drying a gel polymer, wherein the chelating agent is added to the monomer aqueous solution or the hydrogel polymer in a step prior to the drying step in a total amount of 10 ppm or more (based on polymerization). The water-absorbent resin according to any one of [16] to [18], wherein a monomer at the time or a solid content of the hydrogel polymer is added.

[20] The water-absorbent resin according to any one of [16] to [19], which is in an irregularly crushed state.

[21] The water-absorbing resin according to any one of [16] to [20], wherein the residual ratio of the chelating agent is 50% or more.

[22] The water absorption according to any one of [16] to [21], including a chelating agent on the surface and inside, wherein the amount of the chelating agent present on the surface is larger than the amount of the chelating agent present inside. Resin.

According to one embodiment of the present invention, the decomposition of the chelating agent in the production process of the water-absorbing resin is prevented, the residual ratio of the chelating agent in the water-absorbing resin as the final product is improved, and the surface and the inside of the particles of the water-absorbing resin are improved. This has the effect that a chelating agent can be added.

Hereinafter, a method for producing a water-absorbing resin containing a chelating agent according to the present invention, and a water-absorbing resin containing a chelating agent obtained thereby will be described in detail, but the scope of the present invention is not limited to these descriptions. In addition, other than the following examples, the present invention can be appropriately changed and implemented without departing from the spirit of the present invention. Specifically, the present invention is not limited to the following embodiments, and various modifications are possible within the scope shown in the claims, and the technical means disclosed in the different embodiments are appropriately combined. The embodiments obtained by the above are also included in the technical scope of the present invention.

[1] Various definitions (1-1) "Water absorbent resin"
The “water-absorbent resin” in the present invention means a water-swellable, water-insoluble polymer gelling agent. Here, “water swelling” means that the CRC (absorption capacity under no pressure) specified by ERT442.2-02 is 5 [g / g] or more, and “water insoluble” means ERT470.2 It means that the Ext (water-soluble content) defined by −02 is 0 to 50% by weight.

The water-absorbent resin can be appropriately designed according to its use, and is not particularly limited, but is preferably a hydrophilic polymer obtained by polymerizing an unsaturated monomer having a carboxyl group. Further, the composition is not limited to a form in which the total amount (100% by weight) is a polymer, and may be a composition containing a surface-crosslinked resin, an additive, or the like as long as the above performance is maintained. According to the production method of the present invention, a particulate or powdery water-absorbent resin can be produced as a final product. In the present invention, the term "water-absorbent resin" refers to a water-absorbent resin before surface treatment or surface cross-linking, a water-absorbent resin after surface treatment or surface cross-linking, and a water absorbing resin having different shapes obtained in each step. Water-absorbent resin compositions containing additives such as a water-soluble resin (for example, sheet-like, fibrous, film-like, and gel-like shapes).

(1-2) "Polyacrylic acid (salt)"
The “polyacrylic acid (salt)” in the present invention includes acrylic acid and / or a salt thereof (hereinafter, sometimes referred to as acrylic acid (salt)) as a main component as a repeating unit, optionally including a graft component. Polymer. Specifically, of the total monomers (excluding the internal crosslinking agent) used for the polymerization, acrylic acid (salt) is essentially 50 to 100 mol%, preferably 70 to 100 mol%, more preferably 90 to 100 mol%. It refers to a polymer containing 100 mol%, particularly preferably substantially 100 mol%. When a polyacrylate is used as the polymer, a water-soluble salt is essential, a monovalent salt is preferable as a main component of the neutralized salt, an alkali metal salt or an ammonium salt is more preferable, and an alkali metal salt is further preferable. Preferably, the sodium salt is particularly preferred.

(1-3) “EDANA” and “ERT”
“EDANA” is an abbreviation of European Disposables and Nonwovens Assoiations, and “ERT” is an abbreviation of EDANA Recommended Test Metods, a method of measuring water-absorbent resin which is a European standard (almost a global standard). is there. In the present invention, the measurement is performed based on the original ERT (publicly known document: revised in 2002) unless otherwise specified.

(A) "CRC" (ERT441.2-02)
“CRC” is an abbreviation of Centrifuge Retention Capacity (centrifuge retention capacity), and means a non-pressurized water absorption capacity (hereinafter sometimes referred to as “water absorption capacity”). Specifically, 0.200 g of the water-absorbent resin in the non-woven fabric bag was freely swelled in a large excess of 0.9% by weight aqueous sodium chloride solution for 30 minutes, and then drained with a centrifuge. Magnification (unit: [g / g]). The CRC of the hydrogel polymer (hereinafter referred to as “gel CRC”) was measured by changing the sample to 0.4 g and the free swelling time to 24 hours.

(B) “AAP” (ERT442.2-02)
“AAP” is an abbreviation for Absorption Against Pressure, and means the water absorption capacity under pressure. Specifically, 0.900 g of a water-absorbing resin is swollen against a 0.9% by weight aqueous solution of sodium chloride for 1 hour under a load of 2.06 kPa (0.3 psi, 21 [g / cm ² ]). It is a water absorption capacity (unit: [g / g]) after the water absorption. In addition, in ERT442.2-02, although it is described as Absorption Under Pressure, it has substantially the same contents. In the present invention and examples, the measurement may be performed with the load condition changed to 4.83 kPa (0.7 psi, 49 [g / cm ² ]). In this case, AAP (0.7 psi) is used. Describe.

(C) "Ext" (ERT 470.2-02)
“Ext” is an abbreviation for Extractables and means a water-soluble component (amount of water-soluble component). Specifically, it is the amount of dissolved polymer (unit: wt%) after adding 1.000 g of the water-absorbing resin to 200 ml of 0.9 wt% aqueous sodium chloride solution and stirring for 16 hours. The measurement of the amount of the dissolved polymer is performed using pH titration. The water-soluble content of the hydrogel polymer (hereinafter, referred to as “gel Ext”) was measured by changing the sample to 5.0 g and the stirring time to 24 hours.

(D) "PSD" (ERT420.2-02)
“PSD” is an abbreviation of Particle Size Distribution, and means a particle size distribution measured by sieve classification. The weight average particle diameter (D50) and the particle diameter distribution width are described in “(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution” described in US Pat. No. 7,638,570. Measure in the same way. The standard sieve (mesh) used in the particle size measurement may be appropriately added depending on the particle size of the object. For example, a standard sieve having an opening of 710 μm, 600 μm, or the like may be added. The method for measuring the PSD of the hydrogel polymer will be described later.

(E) "Residual Monomers" (ERT410.2-02)
“Residual Monomers” means the amount of monomer (monomer) remaining in the water-absorbent resin (hereinafter, referred to as “residual monomer”). Specifically, 1.0 g of a water-absorbent resin was added to 200 ml of a 0.9% by weight aqueous sodium chloride solution, and the amount of dissolved monomer (unit: ppm) after stirring at 500 rpm for 1 hour using a 35 mm stirrer chip was determined. Say. The amount of the dissolved monomer is measured using HPLC (high performance liquid chromatography). The residual monomer of the hydrogel polymer was measured by changing the sample to 2 g and the stirring time to 3 hours, and converting the measured value to the weight of the hydrogel polymer per solid resin. (Unit: ppm).

(F) "Moisture Content" (ERT430.2-02)
"Moisture Content" means the water content of the water absorbent resin. Specifically, it is a value (unit:% by weight) calculated from the loss on drying when 1 g of the water absorbent resin is dried at 105 ° C. for 3 hours. In the present invention, the water content of the water-absorbent resin and the dried polymer was measured by changing the drying temperature to 180 ° C. The moisture content of the hydrogel polymer was measured by changing the sample to 2 g, the drying temperature to 180 ° C., and the drying time to 16 hours. Further, in the present invention, the value calculated by {100-water content (% by weight)} is referred to as "resin solid content", and can be applied to a water-absorbent resin, a dried polymer, and a hydrogel polymer.

(1-4) "Liquid permeability"
“Liquid permeability” in the present invention refers to the fluidity of a liquid passing between particles of a swollen gel under a load or no load, and as a typical measurement method, SFC (Saline Flow Conductivity / physiology) is used. Saline flow inducibility) and GBP (Gel Bed Permeability / gel bed permeability).

"SFC (physiological saline flow conductivity)" refers to the permeability of a 0.69% by weight aqueous sodium chloride solution to a water-absorbent resin under a load of 2.07 kPa, and is an SFC test method disclosed in US Pat. No. 5,669,894. It is measured according to. "GBP" refers to the liquid permeability of a 0.69% by weight aqueous sodium chloride solution to a water-absorbent resin under load or free expansion, and is based on the GBP test method disclosed in WO 2005/016393. Measured.

(1-5) “FSR”
“FSR” in the present invention is an abbreviation of Free Swell Rate and means a water absorption rate (free swelling rate). Specifically, it is the speed (unit: [g / g / s]) when 1 g of the water-absorbent resin absorbs 20 g of the 0.9% by weight aqueous sodium chloride solution.

(1-6) “Gel pulverization”
In the present invention, "gel pulverization" refers to an operation of applying shear and compressive force to reduce the size and increase the surface area with the aim of facilitating drying of the hydrogel polymer obtained in the polymerization step. That means. By “gel pulverization”, a particulate hydrogel polymer, in particular, a particulate hydrogel polymer having a weight average particle diameter (D50) described below is obtained.

In addition, gel polymerization is performed after polymerization in non-stirring aqueous polymerization (static aqueous polymerization, particularly belt polymerization), whereas in kneader polymerization, polymerization and gel pulverization are performed continuously in the same apparatus. The weight average particle diameter (D50) of the particulate hydrogel polymer supplied to the drying step may be in the range described below, and gel pulverization may be performed during or after polymerization. .

(1-7) “Weight average molecular weight of water-soluble component”
The term “weight-average molecular weight of the water-soluble component” in the present invention refers to the weight-average molecular weight of a component (water-soluble component) that dissolves when a water-absorbent resin is added to an aqueous solvent, by GPC (gel permeation chromatography). It refers to the measured value (unit: daltons / hereinafter, abbreviated as [Da]). That is, it is the result of GPC measurement of the solution obtained by the measurement method described in the above (1-3) (c) “Ext”. The weight-average molecular weight of the water-soluble component of the hydrogel polymer was determined by changing the particle diameter to 5 mm or less, and further, to 5.0 g for a sample having a fine particle size of 1 to 3 mm, and changing the stirring time to 24 hours. A measurement was made.

(1-8) Others In this specification, “X to Y” indicating a range means “X or more and Y or less”. Also, “mass” and “weight” are treated as synonyms, and the unit of weight “t (ton)” means “metric ton (metric ton)”, and unless otherwise specified, “ppm”. "Means" ppm by weight ". Further, “to acid (salt)” means “to acid and / or salt thereof”, and “(meth) acryl” means “acryl and / or methacryl”.

[2] Method for producing water-absorbent resin containing chelating agent The present invention comprises a polymerization step of polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer, and, if necessary, a polymerization step. A gel pulverizing step of pulverizing the hydrogel polymer during and / or after, and a drying step of drying the obtained particulate hydrogel polymer to obtain a particulate dry polymer, The present invention provides a method for producing a water-absorbing resin having a water absorption capacity (CRC) of 15 g / g or more and containing a chelating agent, and the following methods 1 and 2.

(Method 1; persulfate in monomer / gel particle size / drying conditions)
The persulfate used in the above polymerization step is 0 to 0.04 mol% (based on the monomer at the time of polymerization) (however, when the persulfate is 0 mol% (unused), another polymerization initiator is used). ), And the chelating agent is added to the aqueous monomer solution or the hydrogel polymer in a step prior to the drying step in a total of 10 ppm or more (solid content of the monomer or the hydrogel polymer at the time of polymerization). ), The weight-average particle diameter (D50) of the particulate hydrogel polymer is 1 mm or less, and in the drying step, the drying time until the solid content is 80% by weight or more is 20 minutes or less. And a method for producing a water-absorbent resin containing a chelating agent.

(Method 2: Persulfate in hydrogel polymer / gel particle size / drying conditions are specified)
In the above drying step, the weight average particle diameter (D50) containing 10 ppm or more of the chelating agent (based on the solid content of the hydrogel polymer) and 0 to 0.04 mol% of the persulfate (based on the monomer at the time of polymerization). A) A method for producing a water-absorbent resin containing a chelating agent, wherein a particulate hydrogel polymer having a particle size of 1 mm or less is dried for a drying time of 20 minutes or less until the solid content becomes 80% by weight or more. However, the drying time refers to a time until the solid content becomes 80% by weight or more.

方法 Hereinafter, the above methods 1 and 2 will be described in detail.

(2-1) Polymerization Step In this step, a monomer aqueous solution containing a monomer and a polymerization initiator is polymerized to obtain a hydrogel polymer (hereinafter sometimes abbreviated as “hydrogel”). It is a process.

(Monomer)
The water-absorbent resin obtained by the present invention preferably uses a monomer containing acrylic acid (salt) as a main component as its raw material (monomer), and is usually polymerized in an aqueous solution state. (Salt) -based water-absorbing resin.

The concentration of the monomer (monomer) in the aqueous monomer solution is preferably 10 to 80% by weight, more preferably 20 to 80% by weight, still more preferably 30 to 70% by weight, and particularly preferably 40 to 60% by weight. .

From the viewpoint of water absorption performance and residual monomers, it is preferable that at least a part of the acid groups of the polymer is neutralized in the hydrogel polymer obtained by polymerization of the monomer aqueous solution. Such a partially neutralized salt is not particularly limited, but from the viewpoint of water absorption performance, a monovalent salt selected from alkali metal salts, ammonium salts, and amine salts is preferable, alkali metal salts are more preferable, and sodium salts, lithium salts, and potassium salts. Alkali metal salts selected from salts are more preferable, and sodium salts are particularly preferable. Accordingly, the basic substance used for the neutralization is not particularly limited, but includes hydroxides of alkali metals such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, sodium (hydrogen) carbonate, potassium (hydrogen) carbonate, and the like. Monovalent basic substances such as carbonic acid (hydrogen) salt are preferred, and sodium hydroxide is particularly preferred. When a carbonate is used for neutralization, carbon dioxide is generated at the time of neutralization, so that the carbon dioxide can be used as a foaming agent at the time of polymerization.

The above-mentioned neutralization can be performed in each form and state before, during or after the polymerization. For example, non-neutralized or low-neutralized (eg, a neutralization ratio of 0 to 30 mol%) acrylic acid Neutralization of the hydrogel polymer obtained by polymerizing the polymer can be carried out, particularly neutralization simultaneously with gel pulverization.However, from the viewpoint of improving productivity and physical properties, neutralization of acrylic acid before polymerization is carried out. It is preferred to do so. That is, it is preferable to use neutralized acrylic acid (partially neutralized salt of acrylic acid) as a monomer.

The neutralization ratio in the neutralization is not particularly limited, but is preferably from 10 to 100 mol%, more preferably from 30 to 95 mol%, further preferably from 45 to 90 mol%, as a final water-absorbing resin. Particularly preferred is ～ 80 mol%. Also, the neutralization temperature is not particularly limited, but is preferably from 10 to 100 ° C, more preferably from 30 to 90 ° C.

In the present invention, when acrylic acid (salt) is used as a main component, a hydrophilic or hydrophobic unsaturated monomer other than acrylic acid (salt) (hereinafter referred to as “other monomer”) ) May be used in combination. Such other monomers are not particularly limited, but include, for example, methacrylic acid, (anhydrous) maleic acid, 2- (meth) acrylamide-2-methylpropanesulfonic acid, (meth) acryloxyalkanesulfonic acid, N -Vinyl-2-pyrrolidone, N-vinylacetamide, (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, methoxypolyethylene glycol (meth) Examples include acrylate, polyethylene glycol (meth) acrylate, stearyl acrylate, and salts thereof. When other monomers are used, the amount used is appropriately determined within a range that does not impair the water absorbing performance of the obtained water-absorbing resin, and is not particularly limited, but is based on the total monomers (excluding the internal crosslinking agent). Thus, 0 to 50 mol% is preferable, 0 to 30 mol% is more preferable, and 0 to 10 mol% is further preferable.

(Polymerization inhibitor)
The aqueous monomer solution may optionally contain a polymerization inhibitor for the purpose of stabilizing polymerization and neutralization. When the aqueous monomer solution contains a polymerization inhibitor, the amount used is typically 200 ppm or less, more preferably 10 to 130 ppm, based on the monomer, from the viewpoints of coloring and polymerization stability. More preferably, it is 20 to 100 ppm. Preferably, a methoxyphenol-based polymerization inhibitor is used. More preferably, the polymerization inhibitor is p-methoxyphenol.

(Polymerization initiator)
In order to start the polymerization of the aqueous monomer solution, the aqueous monomer solution further contains a polymerization initiator. The polymerization initiator suitably used in the present invention is appropriately selected depending on the polymerization mode, and is not particularly limited, but is preferably a radical polymerization initiator.

In the present invention (method 1), the persulfate in the polymerization initiator used in the polymerization step is reduced to 0 to 0.04 mol% (based on the monomer at the time of polymerization) (however, the persulfate is 0 mol % (Unused), another polymerization initiator is used essentially). In the present invention (method 2), the persulfate contained in the particulate hydrogel polymer is controlled to 0 to 0.04 mol% (monomer at the time of polymerization), This is a method for controlling the gel particle diameter and the drying conditions of the hydrogel polymer. Thereby, the present invention suppresses the decomposition of the chelating agent in the drying step.

Further, in the present invention, for the purpose of reducing the residual monomer or improving the physical properties, etc., the persulfate can be added not only to the monomer but also to the hydrogel polymer, from the polymerization step to before the drying step. It is more preferable to control the total amount of the persulfate added to 0 to 0.04 mol% (based on the monomer at the time of polymerization). The addition amount of the persulfate and the content of the persulfate in the hydrogel polymer are preferably as low as possible, and are preferably 0.04 mol% or less, 0.035 mol% or less based on the monomer at the time of polymerization. It is preferable in the order of 0.03 mol% or less, 0.025 mol% or less, 0.02 mol% or less, and 0.015 mol% or less. The lower limit of the amount of persulfate added is 0 mol%, which means that it is not used during polymerization or is completely consumed before the drying step. However, since the persulfate is effective in reducing the residual monomer, the persulfate is 0.0001 mol% or more, preferably 0.001 mol% or more, more preferably, from the viewpoint of reducing the residual monomer during drying. It is desirable that the amount is contained in the monomer aqueous solution or the hydrogel polymer in an amount of 0.01 mol% or more. In the present invention, the upper and lower limits of the amount of persulfate to be added may be in any combination.

When the addition amount of the persulfate exceeds 0.04 mol% based on the monomer at the time of polymerization, unreacted persulfate coexists in the hydrogel polymer in the subsequent drying step. It is not preferable because it reacts with a chelating agent and can decompose it.

The radical polymerization initiator used in the present invention may be a persulfate (eg, sodium persulfate, potassium persulfate, ammonium persulfate, etc.), and a peroxide other than the persulfate (eg, hydrogen peroxide, t -Butyl peroxide, methyl ethyl ketone peroxide, etc.), an azo polymerization initiator, or a photopolymerization initiator.

Examples of the azo polymerization initiator used in the present invention include water-soluble azo compounds (eg, 2,2′-azobis (2-methylpropionamidine) dihydrochloride, 2,2′-azobis [2- (2-imidazoline- 2-yl) propane] dihydrochloride, 2,2 ′-{azobis (2-methyl-N- [1,1′-bis (hydroxymethyl) -2-hydroxyethyl] propionamide)} and the like. Preferably, 2,2'-azobis (2-methylpropionamidine) dihydrochloride is used.

The photopolymerization initiator used in the present invention includes diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyldimethylketal, and 4- (2-hydroxyethoxy) phenyl- Acetophenone derivatives such as (2-hydroxy) -2-propyl ketone and 1-hydroxycyclohexyl phenyl ketone; benzoin alkyl ethers such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether and benzoin isobutyl ether; methyl o-benzoyl benzoate Benzophenone derivatives such as, 4-phenylbenzophenone, 4-benzoyl-4'-methyl-diphenylsulfide, (4-benzoylbenzyl) trimethylammonium chloride; thioxanthone Compound, bis (2,4,6-trimethylbenzoyl) - phenyl phosphine oxide, diphenyl (2,4,6-trimethylbenzoyl) - acyl phosphine oxide derivatives such as phosphine oxide; 2-hydroxymethyl-propionitrile, and the like. Preferably, it is an acetophenone derivative, and more preferably, 1-hydroxycyclohexyl phenyl ketone is used.

The amount of the polymerization initiator containing a persulfate (0 to 0.04 mol%) is preferably 0.0001 to 1 mol% in total, preferably 0.0005 to 0 mol%, based on the monomers used in the polymerization. 0.5 mol% is more preferred. When the amount of the polymerization initiator exceeds 1 mol%, it is difficult to control the polymerization, and further, the color tone of the obtained water-absorbent resin may be deteriorated. When the amount of the polymerization initiator is less than 0.0001 mol%, there is a concern that the amount of the residual monomer may increase.

において In the present invention, two or more polymerization initiators may be used in combination. Among them, a persulfate is preferably used in the above range from the viewpoint of handleability, physical properties of the water-absorbing resin, and the like. When a persulfate and another polymerization initiator are used in combination, a second polymerization initiator selected from an azo polymerization initiator and a photopolymerization initiator as a polymerization initiator other than the persulfate from the viewpoint of the performance of the water-absorbing resin. It is preferable to use The molar ratio of the persulfate to the polymerization initiator other than the persulfate (= azo polymerization initiator + photopolymerization initiator + other polymerization initiator) is from 1/99 to 99/1, preferably from 1/9 to 99/1. The ratio is 9/1, more preferably 2/8 to 8/2, and still more preferably 3/7 to 7/3.

Furthermore, a redox initiator can be obtained by using together a reducing agent which promotes the decomposition of these polymerization initiators (particularly persulfate or peroxide) and combining them. Examples of such a reducing agent include (bis) sulfite (salt) such as sodium sulfite and sodium hydrogen sulfite, reducing metals (salt) such as L-ascorbic acid (salt) and ferrous salt, amines and the like. Is mentioned.

Instead of using the above polymerization initiator or in combination with the polymerization initiator to start the polymerization of the aqueous monomer solution, the polymerization is carried out by irradiating the reaction system with an active energy ray such as radiation, an electron beam, or an ultraviolet ray. The reaction may be started.

(Persulfate remaining in hydrogel)
In the method for producing a water-absorbent resin containing a chelating agent, a decrease in the chelating agent was found, and as a result of exploring the reduction mechanism, the chelating agent was not substantially decomposed during polymerization, and the chelating agent was decomposed when dried. Was found. This means that in addition to the experimental fact that the amount of chelating agent contained in the hydrogel after polymerization is almost constant, the amount of chelating agent decreases even when the aqueous solution containing the chelating agent is heated in the presence of the monomer. However, it can be confirmed by the experimental fact that the amount of the chelating agent decreases when the persulfate is present in the aqueous solution of the chelating agent.

(4) Among the polymerization initiators used in the polymerization, most of the persulfate remains in the hydrogel after polymerization under the ordinary polymerization conditions due to its half-life (more than 80%). For this reason, the persulfate is decomposed by the high temperature during drying, and radicals are generated in the hydrogel. However, the amount of monomer (residual monomer) remaining in the hydrogel after polymerization is 0.1%. Since the amount is as small as several% to several%, the generated radical of persulfate hardly reacts with the monomer, and is presumed to accelerate the decomposition of the chelating agent instead.

In the present invention, persulfate includes sodium persulfate (half-life (τ) at 90 ° C .; 1.24 hours), potassium persulfate (half-life (τ) at 90 ° C .; 1.24 hours), ammonium persulfate (90 C .; half-life (.tau.) At 0.44 hours), and sodium persulfate is more preferred. When sodium acrylate is used as the monomer of the water-absorbent resin, if persulfate is used in the above range as the polymerization initiator, it will be substantially sodium persulfate regardless of the type of salt. The half life (τ) of sodium or potassium persulfate is 2100 hours (30 ° C.), 499 hours (40 ° C.), 130 hours (50 ° C.), 36.5 hours (60 ° C.), 11 0.1 hour (70 ° C.), 3.59 hours (80 ° C.), 1.24 hours (90 ° C.), and 0.45 hours (100 ° C.). Therefore, sodium persulfate or potassium persulfate remains mostly (more than 80%) in the hydrogel polymer before drying.

(Internal crosslinking agent)
In one embodiment of the present invention, in order to improve the water absorbing performance of the water-absorbing resin obtained in the present invention, it is preferable that the aqueous monomer solution further contains an internal crosslinking agent.

The internal cross-linking agent suitably used in the present invention is not particularly limited, and includes, for example, a polymerizable cross-linking agent that polymerizes with acrylic acid, a reactive cross-linking agent that reacts with a carboxyl group, or a cross-linking agent having these properties. Is mentioned.

Examples of the polymerizable crosslinking agent include N, N′-methylenebisacrylamide, (poly) ethylene glycol di (meth) acrylate, (polyoxyethylene) trimethylolpropane tri (meth) acrylate, and poly (meth) allyloxy. Examples include compounds having at least two polymerizable double bonds in the molecule, such as alkanes. Examples of the reactive cross-linking agent include polyglycidyl ethers such as ethylene glycol diglycidyl ether; polyhydric alcohols such as propanediol, glycerin and sorbitol; covalent cross-linking agents such as; and polyvalent metals such as aluminum salts. And an ion-bonding crosslinking agent such as a compound. Among these, from the viewpoint of water absorption performance, a polymerizable crosslinker that polymerizes with acrylic acid is more preferable, and an acrylate, allyl, or acrylamide polymerizable crosslinker is particularly preferable. One of these internal crosslinking agents may be used alone, or two or more thereof may be used in combination. When the polymerizable crosslinking agent and the covalent crosslinking agent are used in combination, the mixing ratio is preferably from 10: 1 to 1:10.

From the viewpoint of physical properties, the amount of the internal crosslinking agent used is preferably 0.001 to 5 mol%, more preferably 0.002 to 2 mol%, and more preferably 0.04 to 1 mol%, based on the monomer. More preferably, it is 0.06 to 0.5 mol%, most preferably 0.07 to 0.2 mol%.

In one embodiment of the present invention, the aqueous monomer solution contains acrylic acid (salt) in an amount of 50 to 100 mol% of the total monomers (excluding the internal cross-linking agent), and the internal cross-linking agent in an amount of 0. It is preferable to contain 001 to 5 mol%, and 0 to 0.04 mol% of the persulfate based on the monomer. The water-absorbing resin composed of the polyacrylic acid (salt) -based crosslinked polymer obtained from the monomer aqueous solution can exhibit a preferable numerical range as the L value and the YI value of the initial color tone.

(Other additives)
In one embodiment of the present invention, a further conventional additive may be added to the aqueous monomer solution in order to improve the physical properties of the water-absorbent resin obtained in the present invention.

Examples of such additives include water-soluble resins or water-absorbing resins such as starch, cellulose, polyvinyl alcohol (PVA), polyacrylic acid (salt), and polyethyleneimine; carbonates, various foaming agents that generate air bubbles; surfactants And the like.

These additives can be added to the hydrogel polymer, the dried polymer, the water-absorbing resin, or the like in any of the production steps of the present invention, in addition to the monomer aqueous solution. In the case of the water-soluble resin or the water-absorbing resin, the amount of these additives is preferably 0 to 50% by weight, more preferably 0 to 20% by weight, and still more preferably 0 to 10% by weight, based on the monomer. %, Particularly preferably 0 to 3% by weight. On the other hand, in the case of the foaming agent or the surfactant, it is preferably 0 to 5% by weight, more preferably 0 to 1% by weight, based on the monomer. The graft polymer or the water-absorbent resin composition can be obtained by adding the above-mentioned water-soluble resin or water-absorbent resin. These starch-acrylic acid polymer, PVA-acrylic acid polymer and the like are also used in the present invention. Treat as

(Chelating agent)
In the present invention, a chelating agent is added to the aqueous monomer solution or the hydrogel polymer in a step prior to the drying step in a total of 10 ppm or more (for the monomer at the time of polymerization or the hydrogel polymer). Solids). From the viewpoint of the physical properties of the water-absorbing resin, the total amount of the chelating agent added to the aqueous monomer solution or the hydrogel polymer (solid content of the monomer or the hydrogel polymer at the time of polymerization) ) Or the content of the chelating agent of the particulate hydrogel polymer (solid content of the hydrogel polymer) provided in the drying step is 10 ppm or more, 40 ppm or more, 60 ppm or more, 100 ppm or more. , 200 ppm or more, 250 ppm or more, 500 ppm or more, and 600 ppm or more in this order. In addition, from the viewpoints of the effects (eg, prevention of coloration and deterioration) of the chelating agent and the cost, the upper limit of the amount of the chelating agent or the upper limit of the content is determined by the solid content of the monomer or the hydrogel polymer at the time of polymerization. It is preferable that the content is 1% or less, 8000 ppm or less, 6000 ppm or less, and 5000 ppm or less. In the present invention, any combination of the upper limit and the lower limit of the added amount or the content of the chelating agent is preferable. The above-mentioned added amount (ppm) of the chelating agent is the weight of the chelating agent at the time of addition (to the monomer at the time of polymerization or the solid content of the hydrogel polymer), and is a single amount after mixing after addition. The salt exchange with the carboxy group of the polymer and the polymer is not taken into account, and the neutralized salt type chelating agent shows the amount of addition as a salt type, and the acid type chelating agent shows the actual addition amount as an acid type.

重量 The weight% of the chelating agent with respect to the solid content of the monomer or the hydrogel polymer can be converted into a molar ratio (mol%) to the monomer at the time of polymerization. In the present invention, the added amount or the content of the chelating agent is 0.0002 mol% or more, 0.001 mol% or more, 0.002 mol% or more, 0.005 mol% or more based on the monomer at the time of polymerization. , 0.01 mol% or more in this order. The upper limit is preferably in the order of 0.2 mol% or less, 0.15 mol% or less, 0.12 mol% or less, and 0.1 mol% or less. In the present invention, the upper and lower limits of the molar ratio of the chelating agent may be in any combination. Further, in order to improve the residual ratio of the chelating agent, the molar ratio of the persulfate to the chelating agent is preferably lower, more preferably 50 times or less, 20 times or less, 10 times or less, 5 times or less, 3 times or less, The lower limit is 0. By such a method, the residual ratio of the chelating agent is improved, and not only the content of the chelating agent in the polymer in the obtained water-absorbent resin is increased, but also the L value of the initial coloring of the obtained water-absorbent resin is increased. Increase and the YI value decreases.

By adding a chelating agent to the monomer at the time of polymerization or the hydrogel before drying, the chelating agent is uniformly blended into the polymer of the water-absorbing resin (particularly, the water-absorbing resin of the final product) after the drying step. be able to. The chelating agent is selectively added to the surface of the water-absorbent resin by adding the chelating agent after the drying step and further in the surface cross-linking step or subsequent steps. In the method of the present invention, the effect of the chelating agent can be further enhanced by blending the chelating agent inside the polymer. This time, when many chelating agents were used for polymerization, it was found that the initial coloring of the obtained water-absorbent resin was deteriorated, but in the present invention, there was no problem, and the chelating agent was uniformly dispersed inside the water-absorbent resin. Can be blended.

場合 When a chelating agent is added in the above-mentioned polymerization step, it can be added to the aqueous monomer solution as in the case of other additives.

In addition, when a chelating agent is added in the above-mentioned gel pulverizing step, similarly to the other additives, gel pulverization can be performed by adding and kneading to the hydrogel polymer. Specifically, an aqueous solution containing a chelating agent can be supplied into the gel crusher while the hydrogel polymer is retained in the gel crusher. Alternatively, an aqueous solution containing a chelating agent may be added in advance to the hydrogel polymer and charged into the gel pulverizer. The timing of adding these chelating agents may be appropriately combined.

キ As the chelating agent suitably used in the present invention, a high-molecular or non-polymer chelating agent, more preferably a non-polymer chelating agent, further preferably a non-polymer chelating agent having a molecular weight of 1,000 or less can be mentioned.

キ Specific examples of the chelating agent used in the present invention include amino polyvalent carboxylic acid, organic polyvalent phosphoric acid (particularly, amino polyvalent phosphoric acid), inorganic polyvalent phosphoric acid, and tropolone derivatives. At least one chelating agent selected from the group consisting of these compounds is used in the present invention. The term “polyvalent” refers to having a plurality of functional groups in one molecule, preferably 2 to 30, more preferably 3 to 20, and still more preferably 4 to 10. Refers to having a functional group.

As the amino polycarboxylic acid, specifically, imino diacetic acid, hydroxyethyl imino diacetic acid, nitrilotriacetic acid, nitrilotripropionic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid (DTPA), triethylenetetramine hexaacetic acid ( TTHA), trans -1,2-diaminocyclohexane tetraacetic acid, N, N-bis (2-hydroxyethyl) glycine, diaminopropanol tetraacetic acid, ethylenediamine-2-propionic acid, N-hydroxyethylethylenediamine triacetic acid, glycol ether diamine tetraacetic acid , Diaminopropanetetraacetic acid, N, N'-bis (2-hydroxybenzyl) ethylenediamine-N, N'-2acetic acid, 1,6-hexamethylenediamine-N, N, N ', N'-4acetic acid and these And the like.

Specific examples of the organic polyvalent phosphoric acid include nitriloacetic acid-di (methylenephosphinic acid), nitriloacetic acid- (methylenephosphinic acid), nitriloacetic acid-β-propionic acid-methylenephosphonic acid, and nitrilotris (methylenephosphonic acid). Acid), 1-hydroxyethylidene diphosphonic acid, amino polyphosphoric acid and the like.

Specific examples of the amino polyvalent phosphoric acid include ethylenediamine-N, N′-di (methylenephosphinic acid), ethylenediaminetetra (methylenephosphinic acid), cyclohexanediaminetetra (methylenephosphonic acid), ethylenediamine-N, N '-Diacetic acid-N, N'-di (methylene phosphonic acid), ethylenediamine-N, N'-di (methylenephosphonic acid), ethylenediaminetetra (methylenephosphonic acid), polymethylenediaminetetra (methylenephosphonic acid), diethylenetriamine Penta (methylene phosphonic acid), ethylenediaminetetramethylene phosphonic acid (EDTMP), and salts thereof.

Specific examples of the inorganic polyvalent phosphoric acid include pyrophosphoric acid, tripolyphosphoric acid, and salts thereof.

Specific examples of the tropolone derivative include tropolone, β-thiaprisin, γ-thiaprisin, and the like.

中でも Among these, an amino polycarboxylic acid chelating agent and / or an amino polyphosphoric acid chelating agent are preferable. Specifically, as the amino polycarboxylic acid chelating agent, specifically, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid and triethylenetetraaminehexaacetic acid, and metal salts thereof, for example, sodium salt, potassium salt and the like Is mentioned. Specific examples of the amino polyvalent phosphoric acid chelating agent include ethylenediaminetetramethylenephosphonic acid.

では According to the present invention, a water-absorbent resin containing a chelating agent uniformly in a polymer can be obtained by such a technique. However, the deterioration and coloring of the water-absorbing resin are easily performed on the particle surface. Therefore, in the present invention, by adding a chelating agent to the surface of the particles by adding a chelating agent to the water-absorbing resin in a step after the drying step, that is, by adding the chelating agent a plurality of times, the water absorbing It is more preferable to include a chelating agent on the surface and inside of the resin, and to form a concentration gradient such that the amount of the chelating agent present on the surface is greater than the amount of the chelating agent present inside. The type and amount of the chelating agent to be added in the steps after the drying step may be the same as or different from the type and the amount of the chelating agent to be added in the step before the drying step. It is preferable to use the following types and addition amounts. Further, the chelating agent may be added to the monomer or the hydrogel polymer only with the chelating agent, or may be added to the monomer or the hydrogel polymer in the form of a solution (particularly an aqueous solution). Good.

(Polymerization method)
In the method for producing a water-absorbent resin containing a chelating agent according to the present invention, the polymerization method comprises directly obtaining a particulate hydrogel polymer by spray / droplet polymerization or reverse phase suspension polymerization in the gas phase. From the viewpoint of the liquid permeability (SFC) and water absorption rate (FSR) of the obtained water-absorbent resin, ease of polymerization control, etc., aqueous solution polymerization is adopted.

In the aqueous solution polymerization, a hydrogel polymer may be obtained by a tank type (silo type) or belt type non-stirring polymerization, and may be separately subjected to gel pulverization. The reaction may be carried out to obtain a particulate hydrogel polymer, but from the viewpoint of easy control of polymerization, preferably, kneader polymerization or belt polymerization is employed. Further, from the viewpoint of productivity, continuous aqueous solution polymerization, more preferably, high-concentration continuous aqueous solution polymerization is employed. In addition, stirring polymerization means that a hydrogel polymer (especially, a hydrogel polymer having a polymerization rate of 10 mol% or more, more preferably 50 mol% or more) is polymerized while being stirred, particularly with stirring and fragmentation. Before and after the non-stirring polymerization, the aqueous monomer solution (polymerization rate: 0 to less than 10 mol%) may be appropriately stirred.

Examples of the continuous aqueous solution polymerization include continuous kneader polymerization described in U.S. Patent Nos. 6,987,171 and 6,710,141 and U.S. Patent Nos. 4,893,999 and 6,241,928, and U.S. Patent Application Publication No. 2005 / 215,734. Continuous belt polymerization. By such aqueous solution polymerization, a water-absorbent resin can be produced with high productivity.

In the present invention, the decomposition of the chelating agent during drying can be suppressed by increasing the solid content concentration of the hydrogel polymer by high concentration polymerization and shortening the drying step. Specifically, the monomer concentration (solid content) is preferably 35% by weight or more, more preferably 40% by weight or more, and even more preferably 45% by weight or more (the upper limit is the saturation concentration).

In addition, since the decomposition (half-life) of persulfate depends on temperature, time and pH, when persulfate is used in the present invention, high temperature initiation polymerization is performed to further reduce persulfate before the drying step. By doing so, the decomposition of the chelating agent during drying can be suppressed. Specifically, in the high temperature initiation polymerization, the polymerization initiation temperature is preferably 30 ° C. or higher, more preferably 35 ° C. or higher, still more preferably 40 ° C. or higher, and particularly preferably 50 ° C. or higher (the upper limit is the boiling point). The polymerization peak temperature is usually preferably from 80 to 130 ° C., more preferably from the boiling point of high-temperature polymerization to 120 ° C. In addition, high-concentration, high-temperature initiation polymerization is a combination of these, and the aqueous monomer solution tends to boil during the polymerization, and a solvent such as water evaporates, so that a high-solids hydrogel polymer is obtained. Can be

Increase in the solid content of the hydrogel polymer relative to the solid content (the total concentration of the monomer and the graft component) in the aqueous monomer solution [= (solid content of hydrogel polymer [% by mass]) − ( 1% or more is preferable, 3% or more is more preferable, and 5% or more is more preferable. The upper limit of the increase is not more than 15% from the viewpoint of easy control of polymerization. The solid content of the hydrogel polymer is preferably from 40 to 75% by mass, more preferably from 45 to 70% by mass, and still more preferably from 50 to 65% by mass. When a high solids hydrogel polymer is obtained by polymerization as described above, the decomposition of the chelating agent in the drying step is reduced, and the residual ratio of the chelating agent is improved. However, when the solid content of the hydrogel polymer exceeds 75% by mass, the physical properties of the obtained water-absorbing resin may be deteriorated. In the present invention, a mode in which foaming polymerization is carried out by further introducing the above-mentioned foaming agent and bubbles is also preferable. That is, the polymerization in the present invention is preferably foam polymerization or boiling polymerization. When the hydrogel polymer contains at least air bubbles, not only the water absorption rate can be improved but also the drying rate can be improved, and the content (residual rate) of the chelating agent after the drying step can be increased.

Furthermore, in the mechanism of the specific reduction of the chelating agent in the drying step found in the present invention, although the reaction mechanism is not clear, `` radicals of persulfate preferentially occur with monomers during polymerization. Reacts, but the amount of monomer (residual monomer) remaining in the hydrogel polymer after polymerization is as small as a few percent or less, and the temperature during drying is higher than that during polymerization and the proportion of solids is higher. Therefore, in the drying step, the persulfate remaining after the polymerization reacts preferentially with the chelating agent ". In the prior art, it was considered that the residual monomer in the hydrogel polymer was preferably 1000 ppm or less (US Pat. No. 514906, US Pat. No. 453323, European Patent 530438). However, in the present invention, it is not necessary to make the conversion after polymerization 100%. Rather, the residual monomer is 0.1% by mass or more, and more preferably 0.5 to 10% by mass, in the hydrogel polymer before the drying step. %, About 0.5 to 5% by weight, and about 0.5 to 3% by weight. In order to allow almost 100% of the chelating agent to remain in the polymerization step, it is preferable from the above estimation mechanism that the polymerization time be short in order to sufficiently leave unreacted monomer at the end of the polymerization step. That is, the present invention is preferably applied to short-time polymerization in which the polymerization time is 60 minutes or less, 10 minutes or less, 5 minutes or less, 3 minutes or less (the lower limit of the polymerization time is 1 second, and more preferably 10 seconds or more). .

Therefore, in order to reduce the persulfate salt and to keep the residual monomer, the polymerization conditions include a polymerization initiation temperature of 30 ° C. or higher (further in the above range) and a polymerization peak temperature of 80 to 130 ° C. Range), high-temperature short-time polymerization with a polymerization time of 60 minutes or less (and further in the above range) is particularly preferable, and high-concentration and high-temperature short-time polymerization in which the monomer concentration is 35% by weight or more (and further in the above range). Is most preferred.

The high-concentration, high-temperature-initiated short-time polymerization is disclosed in U.S. Pat. Nos. 6,906,159 and 7,091,253, and the like. This is preferable because production on a scale is easy.

Therefore, the polymerization method in the production method according to the present invention is preferably applied to a large-scale production apparatus having a large production volume per line. The production amount is preferably 0.5 t / hr or more, more preferably 1 t / hr or more, still more preferably 5 t / hr or more, and particularly preferably 10 t / hr or more.

Further, the above polymerization can be carried out in an air atmosphere, but is preferably carried out in an atmosphere of an inert gas such as steam, nitrogen or argon (for example, an oxygen concentration of 1% by volume or less) from the viewpoint of preventing coloration. Furthermore, it is preferable to carry out polymerization after replacing (degassing) the dissolved oxygen in the monomer or the solution containing the monomer with an inert gas (for example, less than 1 mg / L of oxygen). Even if such degassing is performed, the stability of the monomer is excellent, gelation before polymerization does not occur, and a water absorbing resin having higher physical properties and higher whiteness can be provided.

(2-2) Gel pulverizing step This step is a step of obtaining a particulate hydrogel polymer by fragmenting the hydrogel polymer during or after the polymerization described above. This step is referred to as “gel pulverization” in distinction from “pulverization” in the following (2-4) pulverization step / classification step.

In the present invention, a particulate hydrogel polymer is obtained in the gel pulverizing step during and / or after the polymerization step, or directly in the polymerization step. It is more preferable to obtain a particulate hydrogel polymer in the gel pulverizing step during and / or after the polymerization step.

(Gel crusher)
The gel crusher used during or after polymerization used in this step is not particularly limited, and a gel crusher equipped with a plurality of rotary stirring blades, such as a batch-type or continuous-type double-arm kneader, or a single-shaft crusher may be used. An extruder, a twin-screw extruder, a meat chopper, especially a screw type extruder and the like can be mentioned.

Among them, a screw type extruder in which a perforated plate is provided at one end of the casing is preferable, and specific examples include a screw type extruder disclosed in JP-A-2000-63527 and WO2011 / 126079. Can be

(Gel crushing area)
In the present invention, the gel pulverization is performed during and / or after the polymerization step, and more preferably is performed on the hydrogel polymer after the polymerization step. In this case, the state in which the aqueous monomer solution is "sufficiently gelled" is defined as a gel pulverizing step.

For example, when kneader polymerization is employed, the aqueous monomer solution changes into a hydrogel polymer as the polymerization time elapses. That is, the stirring region of the aqueous monomer solution at the start of the polymerization, the stirring region of the low-polymerized hydrogel polymer having a constant viscosity during the polymerization, the gel of a part of the hydrogel polymer as the polymerization proceeds. The pulverization start area and the gel pulverization area at the latter or late stage of polymerization are continuously performed. Therefore, in order to clearly distinguish between “stirring of the aqueous monomer solution” at the start of the polymerization and “gel pulverization” at the end, the judgment is made based on the state of “sufficiently gelled”.

The term “sufficiently gelled” refers to a state in which the hydrogel polymer can be finely divided by applying a shearing force after the time when the polymerization temperature reaches a maximum (polymerization peak temperature). Alternatively, the polymerization rate of the monomer in the monomer aqueous solution (also known as a conversion rate; the polymerization rate is preferably calculated from the amount of the polymer calculated from the pH titration of the hydrogel polymer and the amount of the remaining monomer) is preferable. A state where the hydrogel polymer can be finely divided by applying a shearing force after 90 mol% or more, more preferably 93 mol% or more, still more preferably 95 mol% or more, and particularly preferably 97 mol% or more. Means That is, in the gel pulverizing step of the present invention, a hydrogel polymer having a monomer polymerization rate within the above range is gel-pulverized. In the case of a polymerization reaction that does not show the above-mentioned polymerization peak temperature (for example, when the polymerization always proceeds at a constant temperature or when the polymerization temperature keeps increasing), “sufficient gelation” is performed with the polymerization rate of the monomer. Stipulate.

When the polymerization step is a belt polymerization, before the gel pulverization, a hydrogel polymer in the middle and / or after the polymerization step, preferably a hydrogel polymer after the polymerization step is reduced to about several tens cm. Can be cut or crushed to size. By this operation, it becomes easy to fill the hydrogel polymer into the gel pulverizer, and the gel pulverization step can be performed more smoothly. The means for cutting or crushing is preferably a means capable of cutting or crushing the hydrogel polymer without kneading, for example, a guillotine cutter or the like. The size and shape of the hydrogel polymer obtained by cutting or crushing are not particularly limited as long as they can be filled in a gel crusher.

(Use of water)
In the gel pulverizing step of the present invention, water can be added to the hydrogel polymer to carry out gel pulverization. Note that, in the present invention, “water” takes any form of solid, liquid, and gas.

There is no limitation on the method of addition and the timing of the addition of the water, and the water may be supplied into the gel crusher while the hydrated gel polymer remains in the gel crusher. Alternatively, water may be added in advance to the hydrogel polymer and charged into the gel pulverizer. Further, the water is not limited to “water alone”, and other additives (for example, a surfactant, a neutralizing base, a crosslinking agent, and the like) and a solvent other than water may be added. However, in this case, the content of water is preferably from 90 to 100% by weight, more preferably from 99 to 100% by weight, even more preferably substantially 100% by weight.

において In the present invention, the water can be used in any form of solid, liquid, and gas, but liquid and / or gas is preferable from the viewpoint of handleability. The amount of water to be supplied is preferably 0 to 25 parts by weight, preferably 0 to 15 parts by weight, 0 to 10 parts by weight, 0 to 4 parts by weight, 0 to 2 parts by weight based on 100 parts by weight of the hydrogel polymer. Are more preferable. When the supply amount of water is more than 25 parts by weight, it becomes difficult to control the particle size of the hydrogel polymer, the drying time becomes longer, the remaining amount of the chelating agent decreases, May be generated. From the viewpoint of drying efficiency, it is preferable that the supply amount of water in this step does not exceed the amount of the solvent evaporated in the polymerization step, particularly, the amount of evaporated water.

供給する When the water is supplied as a liquid, the temperature during the supply is preferably 10 to 100 ° C, more preferably 40 to 100 ° C. When water is supplied as a gas, the temperature during the supply is preferably 100 to 220 ° C., more preferably 100 to 160 ° C., and further preferably 100 to 130 ° C. When water is supplied as a gas, the preparation method is not particularly limited. For example, a method using water vapor generated by heating a boiler, a method in which water is vibrated by ultrasonic waves, and a gas state generated from the water surface is used. And the like utilizing water. In the present invention, when water is supplied as a gas, steam having a pressure higher than the atmospheric pressure is preferable, and steam generated in a boiler is more preferable.

(Use of additives)
As described above, it is preferable to add water to the hydrogel polymer and pulverize the gel.However, in addition to water, other additives and a neutralizing agent are added to the hydrogel polymer and kneaded. The water absorbent resin thus obtained may be modified. Specifically, at the time of gel pulverization, an aqueous solution containing a basic substance described in the above (2-1) (for example, a 10 to 50% by weight aqueous sodium hydroxide solution) is added to neutralize (particularly the neutralization described above). %), Or fine powder may be recycled by adding fine water-absorbent resin powder (0.1 to 30% by weight (based on resin solid content)). Further, 0.001 to 3% by weight (based on resin solids) of a polymerization initiator may be added and mixed during gel pulverization to reduce residual monomers.

(Physical properties of particulate hydrogel polymer after gel pulverization)
(A) Particle Size In the present invention, it is essential that the weight-average particle size (D50) of the above-mentioned particulate hydrogel polymer is 1 mm or less, and gel pulverization is performed so that the particles are finer than conventional gel pulverization. I do.

The hydrogel polymer obtained in the above polymerization step is pulverized into particles by using a gel pulverizer (kneader, meat chopper, screw type extruder, etc.) to which the above-described gel pulverization of the present invention is applied. . The gel particle size can be controlled by classification, preparation, etc., but preferably the gel particle size is controlled by the gel pulverization of the present invention.

The weight-average particle diameter (D50) (defined by sieve classification) of the particulate hydrogel polymer after gel pulverization is 1 mm or less, and is 10 μm to 1 mm, 20 μm to 1 mm, 40 μm to 1 mm, 50 μm to 900 μm. More preferred in order. In addition, the upper limit of the weight average particle diameter is more preferably 800 μm or less, 700 μm or less, and 600 μm or less. The lower limit of the weight average particle diameter is more preferably 100 μm or more and 200 μm or more. In the present invention, the upper limit and the lower limit of the weight average particle diameter may be in any combination. The gel particle diameter can be measured by the method described in WO2011 / 126079.

(4) When the weight average particle size exceeds 1 mm, the amount of the chelating agent tends to decrease in the subsequent drying step, and the chelating effect corresponding to the amount of the chelating agent cannot be obtained. On the other hand, when the weight average particle diameter is less than 10 μm, it becomes difficult to dry the entire gel layer. In addition, pulverization after drying generates a large amount of fine powder, which makes it difficult not only to control the particle size but also to deteriorate physical properties such as liquid permeability (SFC).

In addition, it is difficult to achieve gel crushing or polymerization of the hydrogel polymer to a weight-average particle diameter of less than 10 μm by ordinary gel crushing operation or polymerization operation alone. Separate special operations, for example, after crushing (For example, JP-A-6-107800) and particle size control during polymerization before gel pulverization (for example, a method of obtaining gel particles of sharp particle size in reverse phase suspension polymerization; European Patent No. No. 0349240). Therefore, in addition to the gel pulverization, the addition of the special method as described above requires a large amount of a surfactant or an organic solvent for polymerization and classification, decreases productivity (increases costs) and deteriorates physical properties ( (Residual monomer increase and fine powder increase), which may cause new problems. For this reason, it may be difficult to obtain a particulate hydrogel polymer having a weight average particle diameter of less than 10 μm.

(B) Gel CRC after gel pulverization
In the present invention, the gel CRC of the particulate hydrogel polymer after gel pulverization is preferably from 10 to 35 g / g, more preferably from 10 to 32 g / g, even more preferably from 15 to 30 g / g. The gel CRC after gel pulverization is preferably -1 to +3 g / g, more preferably 0.1 to 2 g / g, and more preferably 0.3 to 1.5 g, relative to the gel CRC before gel pulverization. / G is more preferred. In addition, the gel CRC may be reduced by using a cross-linking agent at the time of gel pulverization, but it is preferable to increase the gel CRC within the above range.

(C) Gel Ext after gel pulverization
In the present invention, the gel Ext of the particulate hydrogel polymer after gel pulverization is preferably 0.1 to 20% by weight, more preferably 0.1 to 10% by weight, and 0.1 to 8% by weight. More preferred is 0.1 to 5% by weight. Further, the increase amount of the gel Ext of the particulate hydrogel polymer after gel pulverization (the increase amount relative to the gel Ext before gel pulverization) is preferably 5% by weight or less, more preferably 4% by weight or less, and 3% by weight. The following is more preferred, the content is particularly preferably 2% by weight or less, and most preferably 1% by weight or less. The lower limit may be minus (for example, -3.0% by weight, or even -1.0% by weight), but is usually 0% by weight or more, preferably 0.1% by weight or more, and more preferably 0.1% by weight or more. It is at least 2% by weight, more preferably at least 0.3% by weight. Specifically, the gel Ext is increased so as to fall within the arbitrary range of the above-mentioned upper limit and lower limit, such as preferably 0 to 5.0% by weight, more preferably 0.1 to 3.0% by weight. The gel may be crushed until it is done. The gel Ext may be reduced by using a cross-linking agent at the time of gel pulverization, but it is preferable to increase the gel Ext within the above range. Here, the effective number of the increase amount of the gel Ext is one digit after the decimal point. For example, 5% by weight and 5.0% by weight are treated as synonyms.

(D) Weight average molecular weight of water-soluble component after pulverization of gel In the present invention, the lower limit of the weight-average molecular weight of the water-soluble component of the hydrogel polymer after gel pulverization is 10,000 Da. Or more, more preferably 20,000 Da or more, and even more preferably 30,000 Da or more. Further, the upper limit is preferably 500,000 Da or less, more preferably 400,000 Da or less, further preferably 250,000 Da or less, and particularly preferably 100,000 Da or less. For example, in the present invention, the increase in the weight average molecular weight of the water-soluble portion of the particulate hydrogel polymer after gel pulverization relative to the hydrogel polymer before gel pulverization is 10,000 to 500,000 Da. It is preferably 20,000 to 400,000 Da, more preferably 30,000 to 250,000 Da, and further preferably 100,000 Da or less.

(E) Resin solid content after gel pulverization In the present invention, the resin solid content of the particulate hydrogel polymer after gel pulverization is preferably from 40 to 75% by mass, more preferably from the viewpoint of physical properties. The content is 45 to 70% by mass, and more preferably 50 to 65% by mass. By setting the resin solid content of the particulate hydrogel polymer after gel pulverization within the above range, the increase in CRC due to drying is easy to control, and the damage due to drying (such as an increase in water-soluble components) is reduced. Further, it is preferable because the decomposition of the chelating agent is reduced and the residual ratio is improved. The resin solid content after gel pulverization can be appropriately controlled by the resin solid content before gel pulverization, water to be added as necessary, and further, evaporation of water due to heating during gel pulverization.

(Number of measurement points)
In order to evaluate the physical properties of the hydrogel polymer before the gel pulverization or the particulate hydrogel polymer after the gel pulverization, it is necessary to perform sampling and measurement at a required amount and frequency from the production apparatus. In the present invention, the hydrogel polymer before gel pulverization is evaluated based on the weight-average molecular weight of the water-soluble component, but it is necessary that this value be a sufficiently averaged value. . Therefore, for example, when the production amount of the water-absorbent resin is 1 to 20 t / hr or 1 to 10 t / hr by continuous gel pulverization using a continuous kneader or meat chopper, two or more points are obtained for every 100 kg of the hydrogel polymer. At least 10 points or more may be sampled and measured in total, and in the case of batch-type gel pulverization (for example, a batch-type kneader), at least 10 points or more are sampled and measured from a batch sample to obtain particulate water-containing powder. The physical properties of the gel polymer may be evaluated.

(2-3) Drying Step This step is a step of drying the particulate hydrogel polymer pulverized to the specific particle size in the gel pulverizing step to obtain a dry polymer. Hereinafter, a drying method preferably applied in the present invention will be described.

In the present invention, the particulate hydrogel containing the chelating agent is dried. In the method for producing a water-absorbent resin containing a chelating agent, it has been found that the chelating agent does not substantially decrease at the time of polymerization or after completion of the drying step, and the chelating agent is decomposed during the drying step by remaining persulfate. Was. Therefore, in order to contain a chelating agent inside the water-absorbing resin, it is preferable to control the amount of persulfate remaining in the drying step to be low. Further, in order to suppress the decomposition of the chelating agent, it is preferable to perform rapid drying in the drying step. Therefore, in the present invention, the particle diameter of the hydrogel polymer is reduced, specifically, the weight average particle diameter (D50) of the particulate hydrogel polymer is controlled to 1 mm or less, and the drying time is 20 minutes or less. It is particularly preferable to dry until the solid content becomes 80% by weight or more. However, the drying time refers to the time until the solid content becomes 80% by weight or more. By the above method, the residual ratio of the chelating agent is improved, and not only the amount of the chelating agent inside the polymer in the obtained water-absorbent resin is increased, but also the L value of the initial coloring of the obtained water-absorbent resin is increased. , YI values decrease.

In the method 2 according to the present invention, the content of the persulfate in the particulate hydrogel polymer subjected to the drying step is specifically 0.04 to the monomer at the time of polymerization. It is preferable in the order of mol% or less, 0.035 mol% or less, 0.03 mol% or less, 0.025 mol% or less, 0.02 mol% or less, and 0.015 mol% or less. The lower limit of the amount of persulfate added is 0 mol%, which means that it is not used during polymerization or is completely consumed before the drying step. However, since the persulfate is effective in reducing the residual monomer, the persulfate is 0.0001 mol% or more, preferably 0.001 mol% or more, more preferably, from the viewpoint of reducing the residual monomer during drying. It is desirable that the amount is contained in the monomer aqueous solution or the hydrogel polymer in an amount of 0.01 mol% or more. By the above method, the residual ratio of the chelating agent is improved, and not only the amount of the chelating agent inside the polymer in the obtained water-absorbent resin is increased, but also the L value of the initial coloring of the obtained water-absorbent resin is increased. , YI values decrease. In the present invention, the upper and lower limits of the amount of persulfate to be added may be in any combination. The persulfate content in the hydrogel polymer can be measured by the method described in WO2007 / 116778.

In the method 2 according to the present invention, the content of the chelating agent of the particulate hydrogel polymer (based on the solid content of the hydrogel polymer) provided in the drying step is 10 ppm or more, and 40 ppm or more. The above is preferred in the order of at least 60 ppm, at least 100 ppm, at least 200 ppm, at least 250 ppm, at least 500 ppm, and at least 600 ppm. In addition, from the viewpoint of the effects of the chelating agent (eg, prevention of coloring, prevention of deterioration, etc.) and cost, the upper limit of the content of the chelating agent is 1% or less and 8000 ppm or less based on the solid content of the hydrogel polymer. 6000 ppm or less and 5000 ppm or less are preferred. In the present invention, the upper limit and the lower limit of the content of the chelating agent may be in any combination. In addition, the content of the above chelating agent indicates the apparent weight ppm concentration when added as a chelating agent without considering salt exchange with the carboxy group of the monomer and the polymer after mixing.

(Drying device)
As a drying device used in the drying step, a hot air heat transfer type dryer (hereinafter, referred to as “hot air dryer”) is preferable from the viewpoint of a drying speed. That is, hot air drying is preferable as the drying method. Examples of the hot air dryer include a ventilation belt (band) type, a ventilation circuit type, a vertical ventilation type, a parallel flow belt (band) type, a ventilation tunnel type, a ventilation groove type stirring type, a fluidized bed type, an air flow type, a spray type and the like. A hot air dryer is mentioned. In the present invention, a ventilation belt type hot air dryer is preferable from the viewpoint of physical property control.

When using such a ventilation belt type hot air dryer, from the viewpoint of physical properties such as uniformity of drying and liquid permeability, the wind direction of the hot air used in the dryer is a water-containing layer which is laminated on the ventilation belt and allowed to stand still. It is preferable that the direction is perpendicular to the gel polymer layer (for example, both in the vertical direction or in the upward and downward directions). The “vertical direction” refers to the vertical direction (from the top to the bottom of the gel layer) with respect to the gel layer (a particulate hydrogel polymer having a thickness of 10 to 300 mm laminated on a punching metal or metal net). Or, a state in which air is passed from the bottom to the top of the gel layer), and is not limited to a strictly vertical direction as long as air is passed in the vertical direction. For example, hot air in an oblique direction may be used. In this case, it is within 30 ° with respect to the vertical direction, preferably within 20 °, more preferably within 10 °, even more preferably within 5 °, and particularly preferably 0 °. ° hot air is used.

乾燥 Hereinafter, drying conditions and the like in the drying step of the present invention will be described. By performing drying under the following drying conditions, it is possible to improve the liquid permeability and the water absorption rate of the water-absorbent resin whose surface is treated with the obtained dried polymer.

(Drying temperature)
The drying temperature in the drying step of the present invention is 80 ° C. or higher, preferably 100 ° C. or higher, more preferably 120 ° C. or higher, and particularly preferably 150 ° C. or higher. Further, the drying temperature is 200 ° C. or lower, preferably 190 ° C. or lower, more preferably 180 ° C. or lower. In the present invention, any combination of the upper limit and the lower limit of the drying temperature is preferable. If the drying temperature is lower than 80 ° C., the drying time until a suitable resin solid content (moisture content) is obtained is long, and the decomposition rate of the chelating agent is undesirably increased. In addition, undried matter may be generated, and clogging may occur in a subsequent pulverizing step. When the drying temperature exceeds 200 ° C., the chelating agent is easily decomposed, which is not preferable. The drying temperature refers to the temperature of the heating medium used for drying in the case of direct heating, the temperature of hot air used for drying in the case of hot air drying, and the heat transfer surface used for drying in the case of indirect heating. Refers to the temperature of

(Drying time)
The drying time in the drying step of the present invention refers to the time until the solid content becomes 80% by weight or more, and is preferably 20 minutes or less, more preferably 18 minutes or less, 15 minutes or less, and 12 minutes or less. The lower limit of the drying time is about 1 minute in consideration of the drying efficiency. It has also been found that the decomposition of the chelating agent occurs mainly in the drying stage until the solids content reaches 80% by weight. Further, the total drying time in the present invention is preferably 60 minutes or less, more preferably 50 minutes or less, and more preferably 40 minutes or less. If the drying time is short, undried matter is generated, and clogging may occur during the subsequent pulverization step. If the total drying time exceeds 60 minutes, the rate of decomposition of the chelating agent is undesirably increased. In the present invention, even if the evaporation of water has not substantially started, that is, the step of heating the particulate hydrogel polymer in a dryer to raise the temperature is also included in the drying step. Shall be taken. The start of the drying time is when the hydrogel polymer is placed in the dryer, and the total drying time is after the hydrogel polymer is placed in the dryer, and the hydrogel polymer is dried and dried. It is the time until it is taken out of the machine.

Further, the time until the hydrogel polymer discharged from the polymerization step is introduced into the drying step, and the temperature of the hydrogel polymer, that is, the hydrogel polymer is from the outlet of the polymerizer to the inlet of the dryer. From the viewpoint of preventing the coloring of the water-absorbent resin and improving the residual ratio of the chelating agent, the temperature for moving and the temperature of the hydrogel polymer at the time of moving to the dryer entrance are specifically 30 ° C. or lower. It is preferable to use a combination of not more than 90 minutes, not more than 20 minutes at 90 ° C, not more than 10 minutes at not more than 100 ° C, and not more than 5 minutes at 110 ° C. At the laboratory level, the hydrogel polymer obtained in the polymerization step can be once cooled to 40 ° C. or lower, left for a long time, and then introduced into the drying step.

(Resin solids)
The particulate hydrogel obtained in the above-mentioned gel pulverizing step is dried in the above-mentioned drying step to be a dried polymer. The resin solid content determined from the loss on drying of the dried polymer (measured by heating 1 g of powder or particles at 180 ° C. for 3 hours) is preferably 80% by weight or more, more preferably 85 to 99% by weight, More preferably, it is 90 to 98% by weight.

(wind speed)
In the drying step of the present invention, the velocity of the hot air in the through-air dryer, particularly the belt-type dryer, is preferably 0.8 to 2.5 m / s in the vertical direction (vertical direction), and is 1.0 to 2.5 m / s. ２．０2.0 m / s is more preferable. By setting the above-mentioned wind speed in the above range, it is easy to control the water content of the obtained dried polymer to a desired range. If the wind speed exceeds 2.5 m / s, problems such as the soaking of the particulate hydrogel polymer during the drying period may occur.

The wind speed may be controlled within a range that does not impair the effects of the present invention. For example, the wind speed may be controlled within a range of 70% or more of the drying time, preferably 90% or more, and more preferably 95% or more. In addition, the above-mentioned wind speed is represented by an average flow velocity of hot air passing in a direction perpendicular to a horizontally moving band surface, taking a ventilation belt type dryer as an example. Therefore, the average flow velocity of the hot air may be obtained by dividing the amount of air blown to the ventilation belt dryer by the area of the ventilation belt.

(Dew point of hot air)
In the drying step of the present invention, the hot air used in the ventilation belt type dryer preferably contains at least water vapor and has a dew point of 30 to 100 ° C, more preferably 30 to 80 ° C. By controlling the dew point of hot air and more preferably the gel particle diameter in the above range, the amount of residual monomer can be reduced, and further, a decrease in the bulk specific gravity of the dried polymer can be prevented. The dew point is a value at the time when the water content of the particulate hydrogel polymer is at least 10% by weight or more, preferably 20% by weight or more.

Further, in the drying step of the present invention, from the viewpoint of residual monomers, water absorption performance, prevention of coloring, and the like, the dew point near the dryer outlet (or at the end of drying; for example, 50% or more of the drying time) is closer to the dryer inlet. Preferably, the dew point is high (or early in the drying; for example, before 50% of the drying time). Specifically, it is preferable to contact hot air having a dew point of preferably 10 to 50 ° C., more preferably 15 to 40 ° C., with the particulate hydrogel polymer. By controlling the dew point within the above range, a decrease in the bulk specific gravity of the dried polymer can be prevented.

In the drying step of the present invention, when drying the particulate hydrogel polymer, the particulate hydrogel polymer is continuously supplied in a layer form on a belt of a ventilation belt type dryer. , Hot air dried. The width of the belt of the ventilation belt type dryer used at this time is not particularly limited, but is preferably 0.5 m or more, more preferably 1 m or more. Further, the upper limit is preferably 10 m or less, more preferably 5 m or less. Further, the length of the belt is preferably 20 m or more, and more preferably 40 m or more. The upper limit is preferably 100 m or less, more preferably 50 m or less.

The layer length (thickness of the gel layer) of the particulate hydrogel polymer on the belt is preferably 10 to 300 mm, more preferably 50 to 200 mm, and more preferably 80 to 150 mm from the viewpoint of solving the problems of the present invention. Is more preferable, and 90 to 110 mm is particularly preferable.

Further, the moving speed of the particulate hydrogel polymer on the belt may be appropriately set according to the belt width, the belt length, the production amount, the drying time, etc., but from the viewpoint of the load of the belt driving device, durability and the like. From this, it is preferably 0.3 to 5 m / min, more preferably 0.5 to 2.5 m / min, further preferably 0.5 to 2 m / min, and particularly preferably 0.7 to 1.5 m / min.

(2-4) Pulverizing Step and Classifying Step The method for producing a water-absorbent resin containing a chelating agent according to the present invention further includes a pulverizing step and a classifying step of pulverizing and classifying the dried polymer obtained in the drying step. May be. This step is different from the above-mentioned (2-2) gel pulverizing step in that the resin solid content at the time of pulverization, particularly the point that the object to be pulverized has undergone a drying step (preferably, the resin solid content is dried). Further, the water-absorbing resin obtained after the pulverizing step may be referred to as a pulverized product.

The dried polymer obtained in the drying step can be used as it is as a water-absorbing resin, but it is preferable to control the particle size to a specific particle size in order to improve the physical properties in the surface treatment step described below, particularly in the surface crosslinking step. . The particle size control is not limited to the main pulverization step and the classification step, but can be appropriately performed in the polymerization step, the fine powder recovery step, the granulation step, and the like.

The pulverizer that can be used in the pulverization step is not particularly limited. For example, a vibration mill, a roll granulator, a knuckle-type pulverizer, a roll mill, a high-speed rotary pulverizer (pin mill, hammer mill, screw mill), cylindrical A mixer etc. can be mentioned. Among them, it is preferable to use a multi-stage roll mill or roll granulator from the viewpoint of particle size control.

In this classification step, the classification operation is performed so as to have the following particle size. In the case where the surface cross-linking is performed, the classification operation is preferably performed before the surface cross-linking step (first classification step), and further after the surface cross-linking. A classification operation (second classification step) may also be performed. In addition, the first classification step is usually performed after the pulverizing step, but may be further performed before the pulverizing step. The weight-average particle size (D50) of the water-absorbent resin particles after classification is not particularly limited, and can be appropriately adjusted according to the application. For example, when used as a sanitary material, the weight-average particle size (D50) of the classified water-absorbent resin particles is preferably 200 to 800 μm, more preferably 200 to 600 μm, and further preferably 300 to 500 μm. Further, the proportion of particles having a particle diameter of 850 to 150 μm is preferably 90% by weight or more, more preferably 95% by weight or more, and even more preferably 97% by weight or more. Note that the water-absorbent resin as the final product is also preferably in the form of particles, and the above-described range of the particle diameter is applied.

(2-5) Surface Treatment Step The method for producing a water-absorbent resin containing a chelating agent according to the present invention preferably further includes a surface treatment step for controlling physical properties. The surface treatment step includes a surface cross-linking step performed using a known surface cross-linking agent and a surface cross-linking method, and further includes other addition steps as necessary. In the present invention, it has been found that the chelating agent does not decrease in the surface crosslinking or heating step.

(Surface cross-linking agent)
As the surface cross-linking agent that can be used in the present invention, various organic or inorganic surface cross-linking agents can be exemplified, and preferably, an organic surface cross-linking agent can be used. In terms of physical properties, the surface crosslinking agent is preferably a polyhydric alcohol compound, an epoxy compound, a polyvalent amine compound or a condensate thereof with a haloepoxy compound, an oxazoline compound, a (mono, di, or poly) oxazolidinone compound, or an alkylene carbonate compound. In particular, a dehydration-reactive cross-linking agent comprising a polyhydric alcohol compound, an alkylene carbonate compound, and an oxazolidinone compound, which requires a reaction at a high temperature, can be used. When the dehydration-reactive cross-linking agent is not used, more specifically, compounds exemplified in U.S. Pat. Nos. 6,228,930, 6,071,976 and 6,254,990 can be exemplified. Such compounds include, for example, mono, di, tri, tetra or propylene glycol, 1,3-propanediol, glycerin, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, Polyhydric alcohol compounds such as 6-hexanediol and sorbitol; epoxy compounds such as ethylene glycol diglycidyl ether and glycidol; alkylene carbonate compounds such as ethylene carbonate; oxetane compounds; and cyclic urea compounds such as 2-imidazolidinone. No.

(Solvent, etc.)
The amount of the surface cross-linking agent to be used is appropriately determined, preferably in the range of about 0.001 to 10 parts by weight, more preferably about 0.01 to 5 parts by weight, based on 100 parts by weight of the water absorbent resin particles. Water is preferably used according to the surface crosslinking agent. The amount of water used is preferably 0.5 to 20 parts by weight, more preferably 0.5 to 10 parts by weight, based on 100 parts by weight of the water absorbent resin particles. When the inorganic surface cross-linking agent and the organic surface cross-linking agent are used in combination, the amount is preferably 0.001 to 10 parts by weight, more preferably 0.01 to 5 parts by weight, based on 100 parts by weight of the water absorbent resin particles. Is done.

At this time, a hydrophilic organic solvent may be used, and the amount of the organic solvent is preferably from 0 to 10 parts by weight, more preferably from 0 to 5 parts by weight, based on 100 parts by weight of the water-absorbing resin particles. It is. When mixing the crosslinking agent solution with the water-absorbent resin particles, the effect of the present invention is not impaired, for example, preferably 0 to 10 parts by weight, more preferably 0 to 5 parts by weight, and still more preferably 0 to 1 part by weight. In parts by weight, a water-insoluble fine particle powder and a surfactant may coexist. The surfactant used and the amount used are exemplified in U.S. Pat. No. 7,473,739.

(mixture)
When the surface crosslinking agent solution is mixed with the water-absorbing resin particles, the water-absorbing resin particles swell due to water or the like in the surface crosslinking agent solution. The swollen water-absorbent resin particles are dried by heating. At this time, the heating temperature is preferably from 80 to 220 ° C. The heating time is preferably from 10 to 120 minutes.

縦 In addition, a vertical or horizontal high-speed stirring type mixer is preferably used for mixing the surface crosslinking agent. The rotation speed of the mixer is preferably 100 to 10000 rpm, and more preferably 300 to 2000 rpm. Further, the residence time is preferably within 180 seconds, more preferably 0.1 to 60 seconds, further preferably 1 to 30 seconds.

(Other surface cross-linking methods)
As the surface cross-linking method used in the present invention, a surface cross-linking method using a radical polymerization initiator (US Pat. No. 4,783,510, WO 2006/062258) instead of the surface cross-linking using the surface cross-linking agent, and water absorption A surface cross-linking method of polymerizing a monomer on the surface of a conductive resin (US Patent Application Publication Nos. 2005/048221, 2009/0239966, and International Publication No. 2009/048160) may be used.

In the above surface cross-linking method, the radical polymerization initiator preferably used is persulfate, and arbitrarily preferably used monomers include acrylic acid (salt) and the above-described surface cross-linking agent, and water is preferably used as a solvent. After these are added to the surface of the water-absorbent resin, surface crosslinking is performed by performing a cross-linking polymerization or a cross-linking reaction with a radical polymerization initiator on the surface of the water-absorbent resin by active energy rays (especially ultraviolet rays) or heating. Will be

(Ion-binding surface cross-linking agent)
The present invention may further include an addition step of adding at least one of a polyvalent metal salt, a cationic polymer, and inorganic fine particles simultaneously with or separately from the above-described surface crosslinking step. That is, in addition to the organic surface cross-linking agent, an inorganic surface cross-linking agent may be used or used in combination to improve the liquid permeability and the water absorption rate. The inorganic surface crosslinking agent can be used simultaneously with or separately from the organic surface crosslinking agent. Examples of the inorganic surface cross-linking agent to be used include salts (organic salts or inorganic salts) or hydroxides of divalent or higher, preferably trivalent or tetravalent polyvalent metals. Examples of the polyvalent metal that can be used include aluminum and zirconium, and specifically, aluminum lactate and aluminum sulfate. Preferably, it is an aqueous solution containing aluminum sulfate.

(2-6) Other processes (fine powder recycling process, etc.)
In addition to the above steps, if necessary, a recycling step of the evaporated monomer, a granulation step, a fine powder removing step, a fine powder recycling step, and the like may be provided. The following additives may be used, if necessary, for any or all of them. That is, a water-soluble or water-insoluble polymer, a lubricant, a deodorant, an antibacterial agent, water, a surfactant, water-insoluble fine particles, an antioxidant, a reducing agent, and the like are preferably added to the water-absorbing resin in an amount of 0 to 30. % By weight, more preferably 0.01 to 10% by weight. These additives can also be used as a surface treatment agent.

製造 Further, the production method of the present invention can include a fine powder recycling step. The fine powder recycling process is a process in which fine powder (especially fine powder containing 70% by weight or more of powder having a particle diameter of 150 μm or less) generated in a drying step and, if necessary, a crushing step and a classifying step is separated and is used as it is or in water. It refers to a step of recycle to a polymerization step and a drying step, and a method described in U.S. Patent Application Publication No. 2006/247351 or U.S. Patent No. 6,228,930 can be applied.

Furthermore, depending on the purpose, oxidizing agents, antioxidants, water, polyvalent metal compounds, water-insoluble inorganic or organic powders such as silica and metal soap, deodorants, antibacterial agents, high-molecular polyamines, pulp and thermoplastic fibers And the like may be added to the water-absorbent resin in an amount of 0 to 3% by weight, preferably 0 to 1% by weight.

[3] Water-absorbing resin The present invention provides a water-absorbing resin containing a chelating agent, which is obtained by the above-described production method according to the present invention. Hereinafter, various physical properties of the water-absorbing resin containing the chelating agent according to the present invention will be described.

水性 The water-absorbing resin containing a chelating agent obtained by the production method according to the present invention is preferably a polyacrylic acid (salt) -based crosslinked polymer water-absorbing resin.

(Remaining amount (C1), content (C2), remaining ratio of chelating agent of water-absorbing resin)
The residual amount (C1) of the chelating agent of the water-absorbing resin is determined by taking into account the salt exchange with the carboxy group of the monomer and polymer after mixing, the neutralized salt type chelating agent as a salt type, and the acid type chelating agent. The agent shows the remaining amount as it is when added as an acid form. On the other hand, the content (C2) of the chelating agent of the water-absorbing resin indicates the content as an acid-type chelating agent. That is, when the chelating agent is in the acid form, the amount is the same as the remaining amount (C1) of the chelating agent, and when the chelating agent is in the salt form, the concentration is calculated as the acid form of the same chelating agent. Specifically, the content (C2) of the chelating agent is calculated by “the remaining amount of the chelating agent (C1) × the molecular weight of the acid form / the molecular weight at the time of adding the chelating agent”.

The remaining rate (%) of the chelating agent is calculated by “{(remaining amount of chelating agent (C1) [ppm]) / (adding amount of chelating agent [ppm])} × 100”.

The residual amount (C1) and content (C2) of the chelating agent in the water-absorbent resin according to the present invention are 10 ppm or more, 40 ppm or more, 60 ppm or more, 100 ppm or more, 200 ppm or more, 250 ppm from the viewpoint of prevention of coloring and deterioration prevention. More preferably, the order is 500 ppm or more and 600 ppm or more. Further, the upper limit of the remaining amount (C1) and the content (C2) of the chelating agent in the water-absorbing resin according to the present invention is 1% by weight from the viewpoint of the effect (prevention of coloring, deterioration prevention, etc.) and cost of the chelating agent. Hereinafter, 8000 ppm or less, 6000 ppm or less, and 5000 ppm or less are preferable.

In the water-absorbent resin according to the present invention, the residual ratio of the chelating agent is preferably 50% or more, and more preferably 60% or more.

The residual ratio of the chelating agent is defined as the amount of the chelating agent added to the water-absorbent resin in the production process [ppm] (based on the solid content of the monomer at the time of polymerization or the solid content of the hydrogel polymer). It is the ratio [%] of the remaining amount (C1) [ppm] of the chelating agent. Further, the residual rate of the chelating agent is defined as the chelating agent content of the final product with respect to the sum of the content of the chelating agent in the final product and the amount obtained by converting the amount (ppm) of the decomposition product derived from the chelating agent into the content of the chelating agent. It may be determined from the ratio of the contents of the agent.

(Initial color tone of water absorbent resin)
The water-absorbing resin according to the present invention is a water-absorbing resin that can be suitably used for sanitary materials such as disposable diapers, and is preferably a white powder. The water-absorbent resin according to the present invention has an L value (Lightness: lightness index) of an initial color tone (also referred to as “initial coloring”) of at least 85, and more preferably 89 or more, in a Hunter Lab color system measurement using a spectral colorimeter. , Preferably 90 or more. The upper limit of the L value is usually 100, but if the value is 85 for powder, no problem due to the color tone occurs in products such as sanitary materials.

Note that the initial color tone is a color tone after the production of the water-absorbent resin, but is generally a color tone measured before shipment from the factory. Further, for example, in the case of storage in an atmosphere of 30 ° C. or lower and a relative humidity of 50% RH, the value is measured within one year after production.

The water-absorbent resin according to the present invention has an initial color tone YI value (yellowness; yellowness index) of 0 to 13, 0 to 10, 0 to 9, 0 to 7, 0 to 5, and 0 to 3 in this order. Preferably, there is almost no yellowing.

In one embodiment of the present invention, there is provided a method for producing a water-absorbent resin containing the chelating agent according to the present invention, wherein the persulfate in the polymerization initiator used in the polymerization step is 0 to 0.04 mol%. (In the case of 0 mol% of persulfate (not used), another polymerization initiator is indispensable), and the chelating agent is used in the step before the drying step. A total of 10 ppm or more (solid content of the monomer at the time of polymerization or the solid content of the hydrogel polymer) is added to the aqueous polymer solution or the hydrogel polymer, and the weight average particle diameter of the particulate hydrogel polymer is increased. (D50) is set to 1 mm or less, and in the drying step, the remaining amount of the chelating agent (C1) is 10 ppm as a final product by a manufacturing method in which the drying time until the solid content becomes 80% by weight or more is 20 minutes or less. The L value of the initial color tone is 85 A top, it is possible to produce a water-absorbent resin YI value is 13 or less.

In a further aspect of the present invention, there is provided a method for producing a water-absorbent resin containing a chelating agent according to the present invention as described above, wherein the persulfate in the polymerization initiator used in the polymerization step is 0 to 0.015 mol. % (Based on the monomer at the time of polymerization) (however, in the case of 0 mol% of persulfate (not used), another polymerization initiator is indispensably used). A total of 200 ppm or more (solid content of the monomer at the time of polymerization or the solid content of the hydrogel polymer) was added to the aqueous monomer solution or the hydrogel polymer, and the weight average particle of the particulate hydrogel polymer was added. The diameter (D50) is set to 1 mm or less, and in the drying step, the remaining amount of the chelating agent (C1) is determined as a final product by a manufacturing method in which the drying time until the solid content becomes 80% by weight or more is 20 minutes or less. 200ppm or more, initial color L value is not less 89 or more, YI value can be produced water-absorbent resin is 10 or less.

In a further embodiment of the present invention, the content (C2) of the chelating agent is 200 ppm or more (further in the above range), the L value of the initial color tone is 89 or more (further in the above range), and the YI value is 10 or less. (Furthermore, the present invention provides a polyacrylic acid (salt) -based water-absorbent resin having the above range).

According to a further aspect of the present invention, there is provided a water-absorbent resin, wherein the water-absorbent resin contains a chelating agent on the surface and inside, and the amount of the chelating agent present on the surface is larger than the amount of the chelating agent present inside. I do.

(Shape of water absorbent resin)
The shape of the water-absorbent resin of the present invention is not particularly limited as long as it can be handled as a powder, but is preferably an irregular crushed shape. The amorphous crushed shape is a particle shape having an irregular shape obtained by crushing a hydrogel polymer or a dried polymer.

(Particle size of water absorbent resin)
The weight average particle size (D50) of the water absorbent resin of the present invention is preferably from 200 to 800 μm, more preferably from 200 to 600 μm, and still more preferably from 300 to 500 μm. Further, the proportion of particles having a particle diameter of 850 to 150 μm is preferably 90% by weight or more, more preferably 95% by weight or more, and even more preferably 97% by weight or more. The water-absorbent resin of the present invention having the above-mentioned particle size is easy to handle, and easily exhibits water-absorbing performance with sanitary materials and the like.

(Water absorption ratio of water absorbent resin under no pressure)
In order to more remarkably exhibit the effects of the chelating agent (eg, prevention of coloration, prevention of deterioration, etc.), the water absorption capacity under no pressure (CRC) of the water-absorbent resin according to the present invention is preferably higher, more preferably 15 g / g. g or more, more preferably 25 g / g or more, further preferably 30 g / g or more, and particularly preferably 33 g / g or more. The upper limit of the CRC is preferably 60 g / g or less, more preferably 50 g / g or less, and still more preferably 45 g / g or less, from the viewpoint of balance with other physical properties (for example, absorbency against pressure (AAP)). is there. CRC can be controlled by the crosslinking density at the time of polymerization or surface crosslinking.

If the CRC is less than 15 g / g, the cross-linking density of the water-absorbent resin is high, so that the effect of preventing deterioration by the chelating agent is difficult to appear. In addition, when the water-absorbing resin is used as a sanitary material such as a disposable diaper, the water-absorbing efficiency deteriorates, which is not preferable.

(Water absorption under pressure of water absorbent resin)
The water absorption capacity under pressure (AAP (0.7 psi)) of the water-absorbent resin according to the present invention is controlled to preferably 15 g / g or more, more preferably 20 g / g or more, and further preferably 23 g / g or more. The upper limit of AAP (0.7 psi) is preferably 40 g / g or less, more preferably 35 g / g or less, and still more preferably from the viewpoint of balance with other physical properties (for example, water absorption capacity under no pressure (CRC)). Is 33 g / g or less. AAP (0.7 psi) can be controlled by the crosslinking density at the time of surface crosslinking.

(Ratio between water absorption capacity under pressure and water absorption capacity under no pressure)
The ratio of the water absorption capacity under pressure to the water absorption capacity under no pressure (AAP (0.7 psi) / CRC) of the water-absorbent resin according to the present invention is 0.5 or more, 0.6 or more, 0.7 or more, and 0. 8 or more, preferably 0.9 or more. The upper limit of AAP (0.7 psi) / CRC is about 1.5 or less, 1.2 or less, and about 1.0 or less. While CRC removes interstitial water between gels after swelling by centrifugation, AAP (0.7 psi) has a water absorption capacity that includes interstitial water, so AAP (0.7 psi) may exceed CRC. However, the upper limit is usually within the above range.

[4] Use of Water Absorbent Resin The use of the water absorbent resin obtained by the production method according to the present invention is not particularly limited, but it is preferably used for absorbent articles such as paper diapers, sanitary napkins, incontinence pads, and the like. The water-absorbent resin obtained by the production method according to the present invention is a high-concentration diaper (a paper diaper that uses a large amount of the water-absorbent resin per paper diaper), in which odors and coloring derived from raw materials have been a problem. Particularly, when used in the upper layer portion of the absorbent article, excellent performance is exhibited. The proportion of the water-absorbing resin contained in the absorbent body of the disposable diaper is preferably at least 50% by weight, more preferably at least 60% by weight, at least 70% by weight, at least 80% by weight, and at least 90% by weight.

[5] Difference from conventional art As described above, the present inventors have found that a chelating agent added in a step prior to a drying step in a drying step of a hydrogel polymer obtained by polymerizing a monomer. Was found to decrease specifically. The present inventors have found that the cause of the decrease in the chelating agent in the drying step is a polymerization initiator (particularly, persulfate) remaining in the hydrogel polymer. In order to solve the problem, the present invention controls persulfate in a polymerization initiator at the time of polymerization or before drying (0.04 mol% or less), and further controls gel particle diameter and drying conditions before drying. Of the chelating agent in the drying step.

特許 The above-mentioned Patent Documents 1 to 23 disclose the use of a chelating agent in the process of producing a water absorbent resin. Persulfates are the most widely used polymerization initiators for water-absorbing resins, and Patent Documents 1 to 23 also disclose persulfates as polymerization initiators. However, in Patent Documents 1 to 23 described above, there is no description about the problems pointed out in the present application and means for solving the problems, let alone decomposition of the chelating agent by persulfate.

In the mechanism of the specific reduction of the chelating agent in the drying step found in the present invention, although the reaction mechanism is not clear, `` radicals of persulfate preferentially form monomers during polymerization. It reacts, but the monomer (residual monomer) remaining in the hydrogel after polymerization is as small as several percent or less, and the temperature is higher and the solid content is higher during the drying than during the polymerization. The remaining persulfate later reacts with the chelating agent. "

手段 Means for solving the problem will be described in more detail in embodiments described later. In the present invention, the persulfate in the polymerization initiator is 0 to 0.04 mol% (based on the monomer at the time of polymerization) (0 mol% is not used during the polymerization or is completely consumed before the drying step. By decomposing the chelating agent in the drying step by controlling the gel particle diameter and the drying conditions before drying, so that the present invention is not limited to the examples described below. The problem can be solved.

諸 Unless otherwise specified, various physical properties described in the claims and examples of the present invention were determined at room temperature (20 to 25 ° C) and a humidity of 50 RH% according to the EDANA method and the following measurement methods. Furthermore, the electric devices presented in the examples and comparative examples used a power supply of 200 V or 100 V, 60 Hz. For convenience, "liter" may be referred to as "L" and "wt%" may be referred to as "wt%".

(A) Weight average particle size (D50)
According to WO2011 / 126079, the weight average particle diameter (D50) of the hydrogel polymer was measured by the following method.

That is, 20 g of a hydrogel polymer (solid content: α wt%) at a temperature of 20 to 25 ° C. is mixed with a 20 wt% aqueous sodium chloride solution containing 0.08 wt% emal 20C (surfactant, manufactured by Kao Corporation) (Hereinafter referred to as "emar aqueous solution") was added to 500 g to form a dispersion, and the mixture was stirred at 300 rpm for 60 minutes using a stirrer chip having a length of 50 mm and a diameter of 7 mm (a cylinder made of polypropylene having a height of 21 cm and a diameter of 8 cm). 1.14 L container is used).

After completion of the stirring, the center of a JIS standard sieve (diameter: 21 cm, sieve opening: 8 mm / 4 mm / 2 mm / 1 mm / 0.60 mm / 0.30 mm / 0.15 mm / 0.075 mm) placed on a rotating disk The above-mentioned dispersion liquid was added to the part. After washing out all the hydrogel polymer on the sieve using 100 g of the Emal aqueous solution, 6000 g of the Emal aqueous solution was showered from the top with a height of 30 cm (72 holes) while rotating the sieve by hand (20 rpm). , Liquid volume: 6.0 [L / min]), and the mixture was poured evenly so that a water injection range (50 cm ² ) was distributed over the entire sieve, and the hydrogel polymer was classified. The classified hydrogel polymer on the first-stage sieve was drained for about 2 minutes and weighed. The second and subsequent sieves were classified in the same manner, and the water-containing gel polymer remaining on each sieve after draining was weighed.

From the weight of the hydrogel polymer remaining on each sieve, the percentage by weight was calculated from the following formula (1). According to the following formula (2), the particle size distribution of the hydrogel polymer was plotted on log probability paper according to the following formula (2). From this graph, the particle diameter corresponding to the residual percentage of 50% by weight was read as the weight average particle diameter (D50) of the hydrogel polymer.
X [%] = (w / W) × 100 (1)
R (α) [mm] = (20 / w) ^1/3 × r (2)
here,
X:% by weight of hydrogel remaining on each sieve after classification and drainage [%]
w: Weight of each hydrogel remaining on each sieve after classification and drainage [g]
W: Total weight of hydrogel remaining on each sieve after classification and drainage [g]
R (α); sieve opening when converted to a hydrogel having a solid content of α wt% [mm]
r: Mesh size of the sieve from which the hydrogel swelled in a 20% by weight aqueous sodium chloride solution was classified [mm]
It is.

(B) Residual amount of chelating agent (C1) and residual ratio, content of chelating agent (C2)
The remaining amount (C1) of the chelating agent in the water-absorbent resin was extracted and analyzed by the method described in Patent Document 11 (International Publication No. WO 2015/053372).

Specifically, 1 g of the particulate water-absorbing resin is added to 100 g of physiological saline (a 0.9% by weight aqueous solution of sodium chloride), and the mixture is stirred at room temperature for 1 hour (stirring speed: 500 ± 50 rpm). The chelating agent was extracted into physiological saline.

(5) Thereafter, the swollen gel of the water-absorbent resin was filtered using filter paper (retained particle size specified by JIS P # 3801; 5 μm / manufactured by Toyo Filter Paper Co., Ltd.).

Next, the obtained filtrate was passed through a filter for HPLC sample pretreatment (Chromatodisk 25A / water-based type, pore size: 0.45 μm / manufactured by Kurashiki Spinning Co., Ltd.), and the filtrate was subjected to high-performance liquid chromatography (HPLC). The content of the chelating agent therein was measured.

The content of the chelating agent in the particulate water-absorbent resin is determined by taking into account the dilution ratio of the particulate water-absorbent resin to physiological saline using the calibration curve obtained by measuring a standard solution of known concentration as an external standard. I asked. The HPLC measurement conditions were appropriately changed depending on the type of the chelating agent. Specifically, the following measurement conditions 1 were used for diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA), and the following measurement conditions 2 were used for ethylenediaminetetra (methylenephosphonic acid) (EDTMP). Quantification was performed according to the above.

Measurement condition 1:
<Eluent>; 0.3 ml of 0.4 mol / L alum aqueous solution, 450 ml of 0.1 N potassium hydroxide aqueous solution, 3 ml of 0.4 mol / L aqueous solution of tetra-n-butylammonium hydroxide, 3 ml of sulfuric acid, ethylene glycol Mixed solution of 1.5 ml and ion-exchanged water 2550 ml <Column>; LichroCART 250-4 Superspher 100 RP-18e (4 μm) (Merck Co., Ltd.)
<Column temperature>; 23 ± 2 ° C
<Flow rate>; 1 ml / min
<Detector>; UV, wavelength 258 nm.

Measurement condition 2:
<Eluent>; 0.003 mol / L aqueous sulfuric acid solution <Column>; Shodex IC NI-424 (manufactured by Showa Denko KK)
<Column temperature>; 40 ° C
<Flow rate>; 1 ml / min
<Detector>; RI
Since the content of the chelating agent is affected by the water content, in the present invention, the content is a value corrected for the water content, and is a value converted per 100 parts by weight of the water-absorbent resin solids.

When the chelating agent to be added is of an anionic type, for convenience, it is considered that the salt of the added chelating agent is present in the water-absorbent resin without salt exchange.

含有 The content (C2) of the chelating agent is the same as the remaining amount of the chelating agent (C1) when the chelating agent is of the acid type. If the chelating agent is in salt form, the concentration is calculated as the acid form of the same chelating agent. That is, the content (C2) of the chelating agent was calculated by “the remaining amount of the chelating agent (C1) × the molecular weight of the acid form / the apparent molecular weight when the chelating agent was added”.

The remaining ratio (%) of the chelating agent was calculated by “{(remaining amount of chelating agent [ppm]) / (addition amount of chelating agent [ppm])} × 100”.

(C) Coloring evaluation of water-absorbent resin (surface color evaluation)
The coloring of the water-absorbent resin was evaluated using a spectroscopic color difference meter SZ- # 80COLOR MEASURING SYSTEM manufactured by Nippon Denshoku Industries Co., Ltd. As the measurement setting conditions, reflection measurement was selected, an attached powder / paste sample table having an inner diameter of 30 mm and a height of 12 mm was used, and a standard round white plate No. for powder / paste was used as a standard. 2 was used and a 30Φ floodlight pipe was used. The provided sample stage was filled with about 5 g of the water-absorbing resin. In this filling, about 60% of the provided sample stage was filled. Under the conditions of room temperature (20 to 25 ° C.) and a humidity of 50 RH%, the L value (Lightness: lightness index) and the YI value (yellowness; yellowness index) in the Hunter Lab color system on the surface are measured by the above-mentioned spectroscopic colorimeter. Was measured. This value is referred to as “initial coloring”.

Subsequently, the paste sample table was filled with about 5 g of a water-absorbent resin and adjusted to an atmosphere of 70 ± 1 ° C. and a relative humidity of 65 ± 1% (product name: small environmental tester manufactured by Espec Corporation). , Type SH-641) filled with a water-absorbent resin was exposed for 7 days. This exposure is a 7-day color acceleration test. After the exposure, the L value (Lightness) and the YI value (yellowness) in the Hunter Lab color system on the surface were measured by the above-mentioned spectral colorimeter. This measured value is referred to as “coloring with time (70 ± 1 ° C., relative humidity 65 ± 1%, 7 days)”. The higher the L value, the better, and the lower the YI value, the lower the coloration and the closer to substantially white.

(D) Amount of persulfate based on solid content in hydrogel The amount of persulfate based on solid content in the hydrogel was measured by the method described in WO2007 / 116778.

Specifically, 3 g of a hydrogel and 100 g of a 5% by weight aqueous solution of sodium chloride (if the gel swells and cannot be stirred) are placed in a 260 ml-capacity polypropylene container with a lid, and the salt concentration or the amount of the aqueous solution is appropriately adjusted. ) Was placed under light shielding at room temperature, and the mixture was stirred at 500 rpm using a 25 mm stirrer coated with Teflon (registered trademark). After a lapse of 2 hours, the solution was taken out and passed through a filter (GL Science, GL chromatodisk, aqueous 25A, pore size 0.45 μm). 5.00 g of the solution was put into a glass sample bottle with a screw cap (capacity 50 ml, diameter 35 mm, height about 80 mm). Then, immediately, 0.50 g of a 44% by weight aqueous solution of potassium iodide was added, and the mixture was stirred at room temperature while shielding light. After 1 hour, the solution was transferred to a plastic 1 cm cell, and the absorbance (measurement wavelength; 350 nm) was measured using a spectrophotometer (Hitachi Ratio Beam Spectrophotometer U-1100) (5% by weight chloride). The absorbance of an aqueous solution (blank) obtained by adding 0.50 g of a 44% by weight aqueous solution of potassium iodide to 5 g of an aqueous sodium solution was set to 0). In addition, 5 wt% aqueous sodium chloride solutions containing 0 ppm (not added), 5 ppm, 10 ppm, 15 ppm, and 20 ppm of persulfate were prepared, and the absorbance was determined by the above operation to prepare a calibration curve. Then, the amount (ppm) of persulfate in the hydrogel of the sample was calculated from the absorbance of the obtained sample and the calibration curve. Further, the amount (mol%) of the persulfate was determined by calculation.

In addition, the amount of persulfate can be measured in the same manner for the water-absorbent resin after drying. The measurement limit is appropriately determined depending on the amount of polymer, sensitivity, and the like. In the case of the hydrogel and the water-absorbent resin of the present invention, the detection limit is usually 0.5 ppm, and below the detection limit is expressed as ND (Non-Detactable).

(E) Solid Content of Water Absorbent Resin In the water absorbent resin, the ratio occupied by components that do not volatilize at 180 ° C. is represented as solid content. The relationship between solids and moisture content is as follows:
Solid content (% by mass) = 100-water content (% by mass)
The method for measuring the solid content is as follows.

About 1 g of the water-absorbing resin was weighed and placed in an aluminum cup (mass W0) having a bottom diameter of about 5 cm (mass W1), and left standing in a windless drier at 180 ° C. for 3 hours to dry. The mass (W2) of the “aluminum cup + water-absorbent resin” after drying was measured, and the solid content (% by mass) = ((W2−W0) / W1) × 100
To determine the solids content.

ブロック In addition, as for the block-shaped dry polymer, five samples were obtained from various positions, crushed so that the particle diameter became 5 mm or less, measured, and the average value was adopted.

(F) Solid content of hydrogel polymer The solid content of the particulate hydrogel polymer was measured in the same manner as in the above-mentioned method of measuring “solid content of water-absorbent resin”. However, the amount of the hydrogel polymer was changed to about 2 g, the drying temperature was changed to 180 ° C., and the drying time was changed to 24 hours.

(G) CRC (ERT441.2-02)
“CRC” is an abbreviation of Centrifuge Retention Capacity (centrifuge retention capacity), and means a non-pressurized water absorption capacity (hereinafter sometimes referred to as “water absorption capacity”). Specifically, 0.200 g of the water-absorbent resin in the non-woven fabric bag was freely swelled in a large excess of 0.9% by weight aqueous sodium chloride solution for 30 minutes, and then drained with a centrifuge. Magnification (unit: [g / g]) was measured.

(H) AAP (0.7 psi) (ERT442.2-02)
“AAP” is an abbreviation for Absorption Against Pressure, and means the water absorption capacity under pressure. AAP (0.7 psi) is a method in which 0.900 g of a water-absorbing resin is applied to a 0.9% by weight aqueous sodium chloride solution for 1 hour at 4.83 kPa (0.7 psi, 49 [g / cm ² ]). The water absorption capacity (unit: [g / g]) after swelling was measured.

(I) Residual monomer (ERT410.2-02)
1.0 g of a water-absorbing resin was added to 200 ml of a 0.9% by weight aqueous sodium chloride solution, and stirred for 1 hour at 500 rpm using a 35 mm stirrer chip. (Chromatography). The residual monomer of the hydrogel polymer was measured by changing the sample to 2 g and the stirring time to 3 hours, and converting the measured value to the weight of the hydrogel polymer per solid resin. (Unit: ppm).

[Example 1]
A water-absorbing resin was produced under the conditions of a chelating agent (DTPA) = 1000 ppm, persulfate (NaPS) = 0.04 mol%, and a gel particle size = 958 µm (about 0.9 mm).

(Polymerization step)
To acrylic acid (288.24 g), polyethylene glycol diacrylate (number average molecular weight; 523) (1.67 g, 0.08 mol% based on the above acrylic acid) as an internal crosslinking agent, and 10 as a chelating agent A solution (A) to which a 5% by weight aqueous solution of diethylenetriaminepentaacetic acid (DTPA) · 5 sodium (3.52 g, 1000 ppm based on the monomer) was added, and a 48.5% by weight aqueous solution of sodium hydroxide (240.82 g) were added. The solution (B) diluted with deionized water (281.58 g) adjusted to 50 ° C. was prepared in a polypropylene container having a capacity of 1.5 L and an inner diameter of 80 mm.

The solution (B) was added and mixed while stirring the solution (A) using a magnetic stirrer to prepare a solution (C).

(4) The temperature of the solution (C) rose to 101.7 ° C. due to the heat of neutralization and the heat of dissolution generated during the mixing.

Thereafter, stirring of the solution (C) was continued, and when the temperature of the solution (C) reached 95 ° C., a 10% by weight aqueous solution of sodium persulfate (NaPS; 3%) was added to the solution (C) as a polymerization initiator. 0.81 g, 0.04 mol% based on the acrylic acid, and a molar ratio of persulfate to the chelating agent; 2.3) were added, and the mixture was stirred for about 3 seconds to obtain a monomer aqueous solution (1). The neutralization ratio of the aqueous monomer solution (1) was 73 mol%, and the monomer concentration was 43% by weight.

(5) Next, the monomer aqueous solution (1) was poured into a stainless steel vat-type container in an atmosphere open system. The stainless steel bat type container is a container having a bottom surface size of 250 mm × 250 mm, an upper surface size of 640 mm × 640 mm, a height of 50 mm, a trapezoidal center cross section, and a silicone sheet attached to the inner surface. there were. The stainless steel bat type container was placed on a hot plate (NEO HOTPLATE H1-1000, manufactured by Iuchi Seieido Co., Ltd.) heated to 100 ° C. and preheated.

(4) After the monomer aqueous solution (1) was poured into the stainless steel vat-type container, a polymerization reaction started about 5 seconds later.

In the polymerization reaction, after the aqueous monomer solution (1) expands and foams in all directions upward while generating water vapor, the polymerization reaction proceeds to a size slightly larger than the bottom surface of the stainless steel vat container. The hydrogel polymer (1) thus shrunk was completed. The polymerization reaction (expansion and shrinkage) was completed within about 1 minute, but after that, the hydrogel polymer (1) was held in the stainless steel bat type container for 3 minutes. By the polymerization reaction (boiling polymerization), a hydrogel polymer (1) containing bubbles was obtained.

(Gel crushing process)
Next, after dividing the hydrogel polymer (1) obtained by the above polymerization reaction into 16 equal parts, a tabletop meat chopper (MEAT-CHOPER TYPE: 12VR-400KSOX, Iizuka Kogyo Co., Ltd.) having a perforated plate having a hole diameter of 11 mm. The gel was pulverized while adding 50 g / min of deionized water adjusted to about 100 ° C. simultaneously with the introduction of the hydrogel polymer. The obtained gel was further charged into a tabletop meat chopper and subjected to a second gel pulverization to obtain a particulate hydrogel polymer (1). Note that, in the second and subsequent gel pulverizations, no deionized water was added.

粒子 The particulate hydrogel polymer (1) obtained by the above gel pulverization had a weight average particle size (D50) of 958 μm. The residual monomer of the particulate hydrogel polymer (1) was 1.1% by weight.

(Drying process)
Next, the particulate hydrogel polymer (1) was spread and placed on a wire mesh having a mesh size of 300 μm (50 mesh) and placed in a hot-air dryer.

(4) Thereafter, the particulate hydrogel polymer (1) was dried by passing hot air at 160 ° C. for 30 minutes to obtain a particulate dried polymer (1).

(Pulverization / classification process)
Subsequently, the dried polymer (1) was put into a roll mill (WML-type roll grinder, manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and thereafter, using two kinds of JIS standard sieves having openings of 850 μm and 150 μm. By classifying, an amorphous crushed water-absorbent resin (1) was obtained.

不 The amorphous crushed water-absorbent resin (1) obtained by the above series of operations had a CRC of 28.9 g / g.

(Surface crosslinking process)
Next, ethylene glycol diglycidyl ether (trade name: Denacol EX-810, manufactured by Nagase ChemteX Corporation, 0.05 parts by weight), propylene glycol (1 part by weight), deionized water (3.0 parts by weight), and isopropyl A surface crosslinking agent solution (1) (5.05 parts by weight) composed of alcohol (1 part by weight) is added to the irregularly crushed water-absorbent resin (1) (100 parts by weight), and mixed until uniform. By doing so, a humidified mixture (1) was obtained. Subsequently, the humidified mixture (1) was uniformly placed in a stainless steel container (about 22 cm in width, about 28 cm in depth, about 5 cm in height), and was heat-treated at 180 ° C. for 40 minutes to obtain a surface-crosslinked water-absorbent resin ( 1) was obtained.

(5) Thereafter, the surface-crosslinked water-absorbent resin (1) was passed through a JIS standard sieve having openings of 850 μm to obtain a water-absorbent resin (1) as a final product. The water-absorbent resin (1) as the final product obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 2]
The amount of the polymerization initiator added in Example 1 was reduced (NaPS = 0.04 mol% → 0.015 mol%);
Example 1 Example 1 was repeated except that the amount of the polymerization initiator (10% by weight aqueous sodium persulfate solution) in the polymerization step was changed to 1.43 g (0.015 mol% based on acrylic acid). In the same manner as in the above, water-absorbent resin (2) was produced.

The particulate hydrogel polymer (2) obtained in the gel pulverizing step had a weight average particle size (D50) of 942 μm. The irregularly crushed water-absorbent resin (2) had a CRC of 28.8 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (2) as a final product. The resulting water-absorbent resin (2) as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below. Further, a difference between the YI value of the initial coloring and the YI value of the temporal coloring (the temporal YI value−the initial YI value) was calculated. The calculation results are shown in Table 3 below.

[Example 3]
Changing the gel particle diameter of Example 1 (from about 0.9 mm to about 0.5 mm);
In the above Example 1, except that after the second gel pulverization in the gel pulverization step, the obtained gel was further charged into a tabletop meat chopper to perform the third gel pulverization in order to change the gel particle diameter. In the same manner as in Example 1, a water absorbent resin (3) was produced.

粒子 The particulate hydrogel polymer (3) obtained in the gel pulverizing step had a weight average particle size (D50) of 514 μm (about 0.5 mm). Further, the amorphous crushed water-absorbent resin (3) had a CRC of 28.3 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (3) as a final product. The water-absorbent resin (3) obtained as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 4]
Changing the type of polymerization initiator in Example 1 (NaPS → azo polymerization initiator);
3. In Example 1, the polymerization initiator was changed to an aqueous solution of 10% by weight of 2,2′-azobis (2-methylpropionamidine) dihydrochloride (trade name; Wako Pure Chemical V-50), which is an azo polymerization initiator. A water-absorbent resin (4) was produced in the same manner as in Example 1, except that the amount was changed to 34 g (0.04 mol% based on acrylic acid).

The particulate hydrogel polymer (4) obtained in the gel pulverizing step had a weight average particle size (D50) of 971 μm. In addition, the amorphous crushed water-absorbent resin (4) had a CRC of 30.0 / g.

Next, the surface was crosslinked in the same manner as in Example 1 to obtain a water absorbent resin (4) as a final product. The resulting water-absorbent resin (4) as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 5]
Changing the chelating agent of Example 1 (DTPA → EDTMP);
Same as Example 1 except that the chelating agent was changed to a 10% by weight aqueous solution of ethylenediaminetetramethylenephosphonic acid (EDTMP) · 5 sodium (3.52 g, 1000 ppm based on monomer). Thus, a water-absorbing resin (5) was produced.

粒子 The particulate hydrogel polymer (5) obtained in the gel pulverizing step had a weight average particle size (D50) of 964 μm (about 0.9 mm). In addition, the amorphous crushed water-absorbent resin (5) had a CRC of 28.5 g / g.

Next, the surface was crosslinked in the same manner as in Example 1 to obtain a water absorbent resin (5) as a final product. The resulting water-absorbent resin (5) as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 6]
The amount of the polymerization initiator added in Example 5 was reduced (NaPS = 0.04 mol% → 0.015 mol%);
Example 5 Example 5 was repeated except that the amount of the polymerization initiator (10% by weight aqueous sodium persulfate solution) in the polymerization step was changed to 1.43 g (0.015 mol% based on acrylic acid). In the same manner as in the above, water-absorbent resin (6) was produced.

The particulate hydrogel polymer (6) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 923 μm. In addition, the amorphous crushed water-absorbent resin (6) had a CRC of 29.3 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (6) as a final product. The resulting water-absorbent resin (6) as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 7]
Changing the type of polymerization initiator of Example 5 (persulfate → azo polymerization initiator);
In Example 5, 4.34 g of a 10% by weight aqueous solution of 2,2′-azobis (2-methylpropionamidine) dihydrochloride (V-50), which is an azo polymerization initiator, was used in Example 5 (based on acrylic acid). Water-absorbent resin (7) was produced in the same manner as in Example 5, except that the water-absorbent resin (7) was changed to 0.04 mol%).

粒子 The particulate hydrogel polymer (7) obtained in the gel pulverizing step had a weight average particle size (D50) of 933 μm. In addition, the amorphous crushed water-absorbent resin (7) had a CRC of 29.1 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (7) as a final product. The resulting water-absorbent resin (7) as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

Example 8
Changing the polymerization initiator of Example 5 (persulfate → UV polymerization with UV polymerization initiator);
The procedure of Example 5 was repeated except that the polymerization initiator (10% by weight aqueous sodium persulfate solution) in the polymerization step was changed to a UV polymerization initiator, and the amount added was 0.04 mol% to perform UV polymerization. In the same manner as in Example 5, a water absorbent resin (8) was produced.

重合 Hereinafter, the polymerization step of performing UV polymerization will be described.

(Polymerization step)
To acrylic acid (285.30 g), polyethylene glycol diacrylate (number average molecular weight; 523) (0.08 mol% based on the above acrylic acid) as an internal crosslinking agent, and 10% by weight ethylenediaminetetramethylene phosphone as a chelating agent Acid (EDTMP) · 5 sodium aqueous solution (3.52 g, 1000 ppm based on monomer) and 10% by weight of IRGACURE (registered trademark; manufactured by BASF: 1-hydroxycyclohexyl phenyl ketone) as a UV polymerization initiator 184 A solution (A) to which 3.27 g of an acrylic acid solution (0.04 mol% based on the above acrylic acid) was added, and a 48.5% by weight aqueous sodium hydroxide solution (240.82 g) were adjusted to 50 ° C. A solution (B) diluted with deionized water (285.06 g) was mixed with a polyp having a capacity of 1.5 L and an inner diameter of 80 mm. They were prepared pyrene-made container.

(4) While stirring the solution (A) using a magnetic stirrer, the solution (B) was added and mixed to prepare a solution (C), which was used as a monomer aqueous solution (8).

(4) The temperature of the aqueous monomer solution (8) rose to 101.9 ° C. due to heat of neutralization and heat of dissolution generated during the mixing process. The neutralization ratio of the aqueous monomer solution (8) was 73 mol%, and the monomer concentration was 43% by weight.

Next, when the temperature of the aqueous monomer solution (8) reaches 95 ° C., the aqueous monomer solution (8) is poured into the stainless steel vat-shaped container, and at the same time, a height of 600 mm from the bottom surface of the stainless steel vat-shaped container. The monomer aqueous solution (8) was irradiated with ultraviolet rays by an ultraviolet irradiation device (Toskure # 401; Model name: HC-04131-B; Lamp: H400L / 2; manufactured by Harrison Toshiba Lighting Co., Ltd.) installed at the position of (1).

The stainless steel bat type container is a container having a bottom surface size of 250 mm × 250 mm, an upper surface size of 640 mm × 640 mm, a height of 50 mm, a trapezoidal center cross section, and a silicone sheet attached to the inner surface. there were. The stainless steel bat type container was placed on a hot plate (NEO HOTPLATE H1-1000, manufactured by Iuchi Seieido Co., Ltd.) heated to 100 ° C. and preheated.

{Circle around (7)} After the monomer aqueous solution (8) was poured into the stainless steel vat type container, a polymerization reaction started about 7 seconds later.

The polymerization (static solution polymerization) proceeded while generating steam. The polymerization reached a peak temperature within about 1 minute (polymerization peak temperature: 102 ° C.). After 3 minutes, the irradiation of ultraviolet rays was stopped to obtain a hydrogel polymer (8) containing bubbles.

(After the gel crushing process)
The steps after the gel pulverizing step were performed in the same manner as in Example 1. Thus, a water absorbent resin (8) was produced.

粒子 The particulate hydrogel polymer (8) obtained in the gel pulverizing step had a weight average particle size (D50) of 917 μm. Further, the amorphous crushed water-absorbent resin (8) had a CRC of 28.0 g / g.

Next, the surface was crosslinked in the same manner as in Example 1 to obtain a water absorbent resin (8) as a final product. The resulting water-absorbent resin (8) as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 1]
The amount of the polymerization initiator added in Example 1 was changed (NaPS = 0.04 mol% → 0.05 mol%);
Example 1 Example 1 was repeated except that the amount of the polymerization initiator (10% by weight aqueous sodium persulfate solution) in the polymerization step was changed to 4.8 g (0.05 mol% based on acrylic acid). Comparative Water Absorbent Resin (1) was produced in the same manner as in Example 1.

粒子 The particulate comparative hydrogel polymer (1) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 900 µm. In addition, the irregularly crushed comparative water-absorbent resin (1) had a CRC of 28.8 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a comparative water-absorbent resin (1) as a final product. The comparative water absorbent resin (1) obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 2]
Changing the gel particle diameter of Example 1 (about 0.9 mm → 5 mm or more);
A comparative water-absorbent resin (2) was produced in the same manner as in Example 1 except that the gel pulverizing step was performed only once in order to change the gel particle diameter.

粒子 The particulate hydrogel polymer (2) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 5000 μm (5 mm) or more. The amorphous crushed comparative water absorbent resin (2) had a CRC of 27.1 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a comparative water-absorbent resin (2) as a final product. The comparative water absorbent resin (2) obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below. Further, a difference between the YI value of the initial coloring and the YI value of the temporal coloring (the temporal YI value−the initial YI value) was calculated. The calculation results are shown in Table 3 below.

[Comparative Example 3]
Changing the gel particle diameter of Example 5 (from about 0.9 mm to 5 mm or more);
A comparative water-absorbent resin (3) was produced in the same manner as in Example 5 except that the gel pulverizing step was performed once in order to change the gel particle diameter.

粒子 The particulate hydrogel polymer (3) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 5000 μm (5 mm) or more. The amorphous crushed comparative water-absorbent resin (3) had a CRC of 27.2 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a comparative water-absorbent resin (3) as a final product. The comparative water absorbent resin (3) obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 9]
The amount of the chelating agent added in Example 1 was changed (DTPA = 1000 ppm → 300 ppm);
In the same manner as in Example 1 except that the amount of DTPA / 5 sodium added was 300 ppm based on the monomer (the molar ratio of persulfate to the chelating agent: 7.7). And a water-absorbing resin (9).

The particulate hydrogel polymer (9) obtained in the gel pulverizing step had a weight average particle size (D50) of 922 μm. In addition, the amorphous crushed water-absorbent resin (9) had a CRC of 29.8 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (9) as a final product. The water-absorbent resin (9) as the final product obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 10]
The amount of the chelating agent added in Example 1 was changed (DTPA = 1000 ppm → 50 ppm);
In the above-mentioned Example 1, DTPA-5 sodium was added in the same manner as in Example 1 except that the addition amount of the monomer was 50 ppm relative to the monomer (the molar ratio of the persulfate to the chelating agent; 46.3). And a water-absorbing resin (10).

The particulate hydrogel polymer (10) obtained in the gel pulverizing step had a weight average particle size (D50) of 950 μm. In addition, the amorphous crushed water-absorbent resin (10) had a CRC of 30.3 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (10) as a final product. The water-absorbent resin (10) as the final product obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 11]
The addition amount of the chelating agent of Example 1 was changed (DTPA = 1000 ppm → 50 ppm), and two kinds of polymerization initiators were used in combination;
In Example 1 described above, the addition amount of DTPA · 5 sodium was set to 50 ppm with respect to the monomer, and the addition amount of the polymerization initiator (10% by weight aqueous sodium persulfate solution) was 1.43 g ( 0.015 mol% with respect to acrylic acid), and 2.71 g of an aqueous solution of 2,2'-azobis (2-methylpropionamidine) dihydrochloride (V-50) (10% by weight) as an azo polymerization initiator (acrylic). Water-absorbent resin (11) was produced in the same manner as in Example 1 except that 0.025 mol% of the acid was used in combination.

The particulate hydrogel polymer (11) obtained in the gel pulverizing step had a weight average particle size (D50) of 919 μm. Further, the amorphous crushed water-absorbent resin (11) had a CRC of 30.1 g / g.

Next, the surface was crosslinked in the same manner as in Example 1 to obtain a water absorbent resin (11) as a final product. The resulting water-absorbent resin (11) as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 12]
Changing the drying temperature and drying time in Example 1 (160 ° C. → 120 ° C., 30 minutes → 120 minutes);
In the above Example 1, except that the drying temperature and the drying time in the drying step were changed to 120 ° C. for 120 minutes (hot air of 120 ° C. was passed for 120 minutes), the water absorbing resin (12 ) Manufactured.

The particulate hydrogel polymer (12) obtained in the gel pulverizing step had a weight average particle size (D50) of 937 μm. The irregularly crushed water-absorbent resin (12) had a CRC of 26.3 g / g and a solid content of 94.3%.

Next, the surface was crosslinked in the same manner as in Example 1 to obtain a water absorbent resin (12) as a final product. The water-absorbent resin (12) as the final product obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

Example 13
Changing the drying temperature and gel particle diameter of Example 1 (160 ° C. → 120 ° C., about 0.9 mm → about 0.5 mm);
In Example 1 above, in order to change the gel particle diameter, after the second gel pulverization in the gel pulverization step, the obtained gel was further charged into a tabletop meat chopper to perform the third gel pulverization, In addition, a water-absorbent resin (13) was produced in the same manner as in Example 1 except that the drying temperature in the drying step was changed to 120 ° C. (hot air of 120 ° C. was passed for 30 minutes).

粒子 The particulate hydrogel polymer (13) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 510 μm (about 0.5 mm). In addition, the amorphous crushed water-absorbent resin (13) had a CRC of 26.4 g / g and a solid content of 93.4%.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (13) as a final product. The water-absorbent resin (13) obtained as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 4]
Changing the drying temperature and drying time of Example 1 and the gel particle diameter (160 ° C. → 120 ° C., 30 minutes → 180 minutes, about 0.9 mm → 5 mm or more);
In Example 1 described above, in order to change the gel particle diameter, the gel pulverization in the gel pulverization step was performed once, and the drying temperature and the drying time in the drying step were set to 120 ° C. for 180 minutes (hot air of 120 ° C.). A comparative water-absorbent resin (4) was produced in the same manner as in Example 1, except that the air flow was changed to 180 minutes.

粒子 The particulate hydrogel polymer (4) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 5000 μm (5 mm) or more. In addition, the amorphous crushed comparative water-absorbent resin (4) had a CRC of 24.4 g / g and a solid content of 91.5%.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a comparative water-absorbent resin (4) as a final product. The comparative water absorbent resin (4) obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 5]
Changing the drying temperature and drying time of Example 1 and the gel particle size (160 ° C. → 150 ° C., 30 minutes → 180 minutes, about 0.9 mm → 1.2 to 1.6 cm);
In Example 1, in order to change the gel particle diameter, the hydrogel polymer was cut using scissors as the gel pulverization in the gel pulverization step, and the size of each side was 1.2 to 1.6 cm. It was a hydrogel polymer. Then, a comparative water-absorbent resin (5) was produced in the same manner as in Example 1 except that the drying temperature and the drying time in the drying step were changed to 150 ° C. for 180 minutes (hot air at 150 ° C. was passed for 180 minutes). did.

比較 The particulate hydrogel polymer (5) obtained in the gel pulverizing step had a weight average particle size (D50) of 1.2 to 1.6 cm. In addition, the comparative crushed amorphous water-absorbent resin (5) had a CRC of 31.6 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a comparative water-absorbent resin (5) as a final product. The obtained comparative water absorbent resin (5) was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 14]
Changing the chelating agent of Example 1 (DTPA → EDTA);
In the same manner as in Example 1, except that the chelating agent was changed to a 10% by weight aqueous solution of ethylenediaminetetraacetic acid (EDTA) · 4 sodium (3.52 g, 1000 ppm with respect to the monomer). And a water-absorbing resin (14).

粒子 The particulate hydrogel polymer (14) obtained in the gel pulverizing step had a weight average particle size (D50) of 955 μm (about 0.9 mm). Further, the amorphous crushed water-absorbent resin (14) had a CRC of 29.2 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a water absorbent resin (14) as a final product. The water-absorbent resin (14) as the final product obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 15]
The amount of the polymerization initiator added in Example 14 was reduced (NaPS = 0.04 mol% → 0.015 mol%);
Example 14 Example 14 was repeated except that the addition amount of the polymerization initiator (10% by weight aqueous sodium persulfate solution) in the polymerization step was changed to 1.43 g (0.015 mol% based on acrylic acid). In the same manner as in the above, water-absorbent resin (15) was produced.

The particulate hydrogel polymer (15) obtained in the gel pulverizing step had a weight average particle size (D50) of 930 μm. Further, the amorphous crushed water-absorbent resin (15) had a CRC of 29.5 g / g.

Next, the surface was crosslinked in the same manner as in Example 1 to obtain a water absorbent resin (15) as a final product. The water-absorbent resin (15) obtained as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 16]
Changing the chelating agent of Example 1 (DTPA → NTA);
In the same manner as in Example 1, except that the chelating agent was changed to a 10% by weight nitrilotriacetic acid (NTA) · trisodium aqueous solution (3.52 g, 1000 ppm with respect to the monomer). And a water-absorbing resin (16).

粒子 The particulate hydrogel polymer (16) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 943 μm (about 0.9 mm). In addition, the amorphous crushed water-absorbent resin (16) had a CRC of 29.1 g / g.

Next, the surface was crosslinked in the same manner as in Example 1 to obtain a water absorbent resin (16) as a final product. The water-absorbent resin (16) obtained as the final product was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 6]
The amount of the polymerization initiator added in Example 14 was changed (NaPS = 0.04 mol% → 0.05 mol%);
Example 14 Example 14 was repeated except that the amount of the polymerization initiator (10% by weight aqueous solution of sodium persulfate) in the polymerization step was changed to 4.8 g (0.05 mol% based on acrylic acid). In the same manner as in the above, a comparative water absorbent resin (6) was produced.

The particulate comparative hydrogel polymer (6) obtained in the gel pulverizing step had a weight average particle diameter (D50) of 929 µm. In addition, the amorphous crushed comparative water-absorbent resin (6) had a CRC of 29.5 g / g.

Next, surface cross-linking was performed in the same manner as in Example 1 to obtain a comparative water-absorbent resin (6) as a final product. The comparative water absorbent resin (6) obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 17]
Hereinafter, in Example 17 and Comparative Example 7, the polymerization step and the gel pulverization step were performed simultaneously.

(Polymerization and gel crushing process)
A reactor formed by attaching a lid to a double-armed jacketed stainless steel kneader having two sigma-type blades having an inner volume of 10 L, 425.2 g of acrylic acid, 4499.5 g of a 37% by weight aqueous solution of sodium acrylate, and pure After 513.65 g of water and 11.1 g of polyethylene glycol diacrylate (molecular weight: 523) were added to prepare a reaction solution, the mixture was degassed for 20 minutes in a nitrogen gas atmosphere. Subsequently, 16.5 g of a 10% by weight aqueous solution of sodium persulfate (0.03 mol% with respect to the monomer) and 23.6 g of a 0.1% by weight aqueous solution of L-ascorbic acid were separately stirred, and the reaction solution was stirred. After about 3 minutes, polymerization started, and polymerization was carried out at 25 to 95 ° C. while crushing the formed hydrogel crosslinked polymer (17).

70 minutes after the start of the polymerization, 10.5 g of 20% by weight of trisodium diethylenetriaminepentaacetate (1000 ppm of the chelating agent based on the monomer) was added, and after 100 minutes from the start of the polymerization, the hydrogel crosslinked polymer (17) Was removed from the reactor. In addition, the obtained particulate hydrogel polymer (17) had a weight average particle size (D50) of 479 μm.

(Drying / crushing / classifying process)
The finely divided hydrogel crosslinked polymer (17) was spread on a metal mesh having a mesh size of 300 μm (50 mesh) and placed in a hot air drier. Then, the particulate hydrogel polymer (17) was dried by passing hot air at 170 ° C. for 30 minutes to obtain a particulate dry polymer (17). Subsequently, the dried polymer (17) was put into a roll mill (WML type roll pulverizer, manufactured by Inoguchi Giken Co., Ltd.) and pulverized. By classifying, an amorphous crushed water-absorbent resin (17) was obtained. In addition, the amorphous crushed water-absorbent resin (17) had a CRC of 31.8 g / g.

(Surface crosslinking process)
Next, 100 parts by weight of the irregularly crushed water-absorbing resin (17) was mixed with the surface cross-linking agent solution (1) (5.05 parts by weight) having the same composition as in Example 1 to obtain the humidified mixture ( 17) was obtained. Then, it heat-processed at 180 degreeC for 40 minutes like Example 1, and obtained the water absorbent resin (17) surface-crosslinked. Thereafter, the surface-crosslinked water-absorbent resin (17) was passed through a JIS standard sieve having openings of 850 μm to obtain a water-absorbent resin (17) as a final product. The water-absorbent resin (17) as the final product obtained was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 7]
The amount of the polymerization initiator added in Example 17 was changed (NaPS = 0.05 mol% → 0.03 mol%);
In the polymerization step of Example 17, the amount of the 10% by weight aqueous solution of sodium persulfate, which is a polymerization initiator, was changed from 16.5 g (0.03 mol% based on monomer) to 28.3 g (based on monomer). Polymerization was carried out in the same manner as in Example 17 except that the amount was increased to 0.05 mol%). As in Example 17, polymerization started after about 3 minutes, and the resulting comparative hydrogel crosslinked polymer (7) was polymerized at 25 to 95 ° C. while crushing. After 70 minutes from the start of the polymerization, a chelating agent was added at 1000 ppm in the same manner as in Example 17, and after 100 minutes from the start of the polymerization, the comparative hydrogel crosslinked polymer (7) (the weight average particle diameter (D50) was 494 μm) ) Was removed from the reactor.

Next, after drying in the same manner as in Example 17 to obtain a comparative dry polymer (7) in the form of particles, it is pulverized and classified to obtain a comparative crushed amorphous water-absorbent resin (7) having a size of 850 to 150 μm. Obtained.

比較 Surface-crosslinked comparative water-absorbent resin (7) was obtained by subjecting the amorphous crushed comparative water-absorbent resin (7) to surface cross-linking in the same manner as in Example 17. Next, by passing through a 850 μm JIS standard sieve as in Example 17, a comparative water-absorbent resin (7) as a final product was obtained. The resulting final product, comparative water absorbent resin (7), was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 8]
A comparative water-absorbent resin (8) was obtained according to the method described in Example 1-8 of International Publication No. 2011/040530 (Patent Document 17). The resulting final product, comparative water-absorbent resin (8), was analyzed. The analysis results are shown in Table 3 below. The manufacturing conditions and analysis results are shown in Tables 1 to 3 below.

[Comparative Example 9]
Comparative water-absorbent resin (9) was obtained according to the method described in Example 2-23 of International Publication WO 2014/054656 (Patent Document 23). The resulting final product, comparative water-absorbent resin (9), was analyzed. The manufacturing conditions and analysis results are shown in Tables 1 to 3 below.

[Comparative Example 10]
A comparative water-absorbent resin (10) was obtained according to the method described in Example 2-24 of International Publication WO2014 / 054656 (Patent Document 23). The resulting final product, comparative water-absorbent resin (10), was analyzed. The production conditions and analysis results are shown in Tables 1, 2, and 4 below.

Although not shown in Table 1, the amount of the chelating agent in the hydrogel after the polymerization step in each of the Examples and Comparative Examples, and the amount of the chelating agent in the hydrogel after the gel pulverizing step are as follows: 97% to almost 100% of the amount of 1000 ppm added to the monomer remained. Therefore, substantially no decrease in the chelating agent was observed in the polymerization step or the gel pulverizing step.

Further, after passing hot air of 160 ° C. for 30 minutes, the amount of the chelating agent in the water-absorbent resin or the comparative water-absorbent resin (before the surface cross-linking step) obtained by pulverization and the temperature of 180 ° C. The heat treatment was performed for 40 minutes, and the amount of the chelating agent in the surface-crosslinked water-absorbent resin or the comparative water-absorbent resin was compared, but no difference was found between the two. Therefore, even when the surface crosslinking step was performed at 180 ° C., no decrease in the chelating agent was observed.

(Summary)
Tables 1 to 4 show the following.

(1) Comparison between Examples 1 and 2 and Comparative Example 1 in which the chelating agent (DTPA) and the gel particle diameter are the same and the amount of the polymerization initiator (persulfate; NaPS) is changed, and the chelate According to the results of Examples 5 and 6 in which the agent (EDTMP) and the gel particle diameter were the same and the amount of the polymerization initiator (persulfate; NaPS) was changed, the amount of persulfate (NaPS) was 0.1%. It can be seen that the content of the chelating agent in the water-absorbent resin after the drying step (and further in the final product) is improved to 50% or more by setting the content to 04 mol% or less. Further, it can be seen that the production method according to the present invention increases the content of the chelating agent, increases the L value of the initial coloring of the obtained water-absorbent resin, and decreases the YI value.

(2) The results of Examples 1 and 4, wherein the chelating agent (DTPA), the gel particle diameter, and the amount of the polymerization initiator (0.04 mol%) were the same, and the type of the polymerization initiator was changed, and , The chelating agent (EDTMP), the gel particle diameter, and the addition amount of the polymerization initiator (0.04 mol%) were the same, and the type of the polymerization initiator was changed. From the results of Examples 5, 7, and 8, Compared with persulfate (NaPS), the azo polymerization initiator and UV polymerization initiator significantly improve the residual amount of the chelating agent in the water-absorbing resin after the drying step (and even the final product) to nearly 100%. You can see that Furthermore, in such a polymerization initiator, it can be seen that, in addition to the increase in the content of the chelating agent, the L value of the initial coloring of the obtained water-absorbent resin increases and the YI value decreases (in Examples 7 and 8, L value = 91, YI value = 2).

(3) Comparison between Examples 1 and 3 and Comparative Example 2 in which the chelating agent (DTPA) and the polymerization initiator were the same and the gel particle diameter was changed, and the chelating agent (EDTMP) and the polymerization initiator were the same From the result of comparison between Example 5 and Comparative Example 3 in which the gel particle diameter was changed, the chelating agent in the water-absorbent resin after the drying step (and further of the final product) was obtained by making the gel particle diameter smaller. It can be seen that the residual amount of is significantly improved. Furthermore, it can be seen that the production method according to the present invention increases the content of the chelating agent and also increases the L value of the initial coloring of the obtained water-absorbent resin (the same chelating agent improves the L value by one point). ). Also, from the comparison results of Comparative Example 2 and Comparative Example 5 in which the amount of the polymerization initiator added (0.04 mol%), the amount of the chelating agent added, and the gel particle diameter were the same, the solid content when dried was 80%. It can be seen that as the time for achieving the above becomes shorter, the residual ratio of the chelating agent is improved, and the L value of the initial coloring is increased. In the production method according to the present invention, it can be seen that the L value increases as the gel particle diameter is smaller and the time to achieve a solid content of 80% upon drying is faster.

(4) Example 1 (1000 ppm based on monomer; 0.0173 mol, in which the polymerization initiator and the amount added (0.04 mol%) were the same and the amount added of the chelating agent (DTPA) was changed. %), Example 9 (300 ppm based on the monomer; 0.0052 mol%), and Example 10 (50 ppm based on the monomer; 0.0009 mol%) show that the persulfate for the chelating agent was used. It can be seen that as the molar ratio increases from 2.3 to 7.7 and further to 46.3, the residual amount of the chelating agent in the water absorbent resin of the final product decreases from 50% to 39% and further to 30%. .

(5) From the results of Examples 10 and 11 that the addition amount of the polymerization initiator (0.04 mol%) and the addition amount of the chelating agent (50 ppm) are the same, the case where the addition amount of the polymerization initiator is the same When the persulfate and the azo polymerization initiator are used in combination, the residual amount of the chelating agent in the water-absorbing resin of the final product is improved from 15% to 30%, compared to using only the persulfate (NaPS). You can see that. Further, it can be seen that, with such a polymerization initiator, the content of the chelating agent increases and the YI value of the initial coloring of the obtained water-absorbent resin decreases (YI = 11 decreases to YI = 6).

(6) Example 2 (0.015 mol%; 942 μm) in which the chelating agent (DTPA) and its addition amount (0.0173 mol%) were the same, and the addition amount of the polymerization initiator and the gel particle diameter were changed. From the results of comparison with Comparative Example 2 (0.04 mol%; ≧ 5000 μm), the molar ratio of the persulfate to the chelating agent and the gel particle diameter were reduced, so that the water absorption resin in the final product was reduced. It can be seen that the residual amount of the chelating agent is remarkably improved, and the YI values of the initial coloring and the coloring with the lapse of time are reduced, and in particular, the resistance to the coloring with the lapse of time is improved.

(7) From the results of comparison of Example 2, Comparative Example 2 and Comparative Example 9 with the same amount of chelating agent (DTPA) and different amounts of persulfate in the polymerization initiator, the addition amount of the polymerization initiator was set to 0.1. It can be seen that when the content is controlled to not more than 04 mol%, the residual ratio of the chelating agent is increased, the L value of the initial coloring and the coloring over time is increased, and the YI value is decreased.

比較 Comparative Example 8 has a smaller amount of the chelating agent added during the polymerization and a larger amount of the persulfate as the polymerization initiator than Example 2. Therefore, it can be seen that the residual ratio of the chelating agent decreases and the initial coloring (L value, YI value) deteriorates.

(8) From the result of comparison between Examples 10 and 11 and Comparative Example 10 in which the amount of the chelating agent added in the polymerization step is the same, the amount of persulfate (NaPS) is controlled to 0.04 mol% or less. By doing so, it is understood that the L value of the initial coloring increases and the YI value decreases.

[Example 18]
To 100 parts by weight of the water-absorbent resin (1) obtained in Example 1, 1 part by weight of a 1% by weight DTPA · 5 sodium aqueous solution was added, and the inside thereof contained 501 ppm of a chelating agent, and further contained 100 ppm of a chelating agent on the surface. Water-absorbing resin (18) was obtained. Almost all of the chelating agent added later was contained near the surface of the water absorbent resin (18), and the content of the chelating agent in the water absorbent resin (18) was increased by about 100 ppm. The CRC and AAP (0.7 psi) of the water absorbent resin (18) were almost the same as those of the water absorbent resin (1) (decrease corresponding to 1% of added water).

[Examples 19 to 21]
In the same manner as in Example 18, for each of the water-absorbent resins (2) to (4) obtained in Examples 2 to 4, 1% by weight of a 1% by weight DTPA / 5 sodium aqueous solution was added to 100 parts by weight of the water-absorbent resin. Was added to obtain water-absorbing resins (19) to (21). Each of the water-absorbent resins (19) to (21) contained the same amount of the chelating agent as the water-absorbent resins (2) to (4), respectively, and further contained 100 ppm of the chelating agent on the surface. Almost all of the chelating agent added later is contained in the vicinity of the surface of the water-absorbing resins (19) to (21), and the content of the chelating agent in the water-absorbing resins (19) to (21) is It increased by about 100 ppm. The CRC and AAP (0.7 psi) of the water absorbent resins (19) to (21) were almost the same as those of the water absorbent resins (2) to (4) (decrease corresponding to 1% of added water).

(Reference Example 1)
The amount and amount of the chelating agent in each hydrogel obtained in the above polymerization step of Example 1 (0.04 mol% of persulfate) and the above polymerization step of Comparative Example 1 (0.05% of persulfate). The amount of sulfate was measured. As a result, about 82% of the persulfate remained in each of the hydrogels, and almost 100% of the chelating agent remained. Therefore, it was found that the amount of persulfate before drying (0.04 mol% or less, further 0.035 mol% or less) has an important effect on the remaining amount of the chelating agent.

(Reference Example 2)
The remaining amount of the chelating agent of Example 1 (0.04 mol% of persulfate) after the drying step and after the surface crosslinking step was measured. As a result, it was found that the chelating agent was substantially reduced only in the drying step.

(Reference Example 3)
In order to confirm the heat resistance of the chelating agent (DTPA) itself, the chelating agent (DTPA) was heated under the drying conditions in the drying step of Example 1. However, substantially no decrease in the chelating agent was observed.

(Reference Example 4)
In order to confirm the heat resistance of the chelating agent (DTPA) itself, an aqueous solution containing only the chelating agent (DTPA) was heated at 80 ° C. However, substantially no decrease in the chelating agent was observed.

(Reference Example 5)
In the aqueous solution containing only the chelating agent (DTPA) prepared in Reference Example 4, the same molar ratio as the chelating agent as in Example 1 (molar ratio of persulfate to the chelating agent; 2.3) was used. Persulfate was added and heated at 80 ° C. As a result, the residual ratio of the chelating agent was 13%, and it was confirmed that the chelating agent was reduced by the persulfate. Further, the aqueous solution was slightly colored brown.

(Reference Example 6)
In the aqueous solution containing only the chelating agent (DTPA) prepared in Reference Example 4, the same molar ratio as the chelating agent as in Example 8 (molar ratio of UV polymerization initiator to chelating agent; 2.3) was used. , A photopolymerization initiator was added, and UV irradiation was performed at 80 ° C. As a result, the residual ratio of the chelating agent was 98%, and no substantial decrease in the chelating agent was observed.

(Reference Example 7)
DTPA and persulfate were added in a 0.1 to 1% by weight aqueous solution of sodium acrylate so that the molar ratio of persulfate to chelating agent was 2.3, and the mixture was heated at 80 ° C. As a result, the residual ratio of the chelating agent was improved as the concentration of sodium acrylate was increased.

(Summary)
From the results of Reference Examples 1 to 6, it was found that in the drying step, the persulfate remaining in the hydrogel specifically decomposed the chelating agent. Therefore, it was found that the present invention can solve the problem. From the results of Reference Example 7, it can be seen that the presence of a certain amount or more of the residual monomer contributes to the residual ratio of the chelating agent after drying. This mechanism is presumed to be due to the fact that persulfate reacts more easily with the residual monomer than the chelating agent.

And, as shown in the above-mentioned Examples and Tables 1 to 4, the water-absorbent resin obtained by the production method of the present invention shows a high chelating agent residual ratio with respect to the chelating agent added before the drying step, A sufficient amount of the chelating agent remained in the water absorbent resin as the final product. Therefore, a chelating agent can be blended on the surface and inside of the particles of the water-absorbing resin, and due to the coloring resistance of the chelating agent, the water-absorbing resin as a final product exhibited a low YI value and exhibited good whiteness.

The water-absorbent resin produced by the production method of the present invention is useful for sanitary articles such as disposable diapers, sanitary napkins, and medical blood retention agents. In addition, pet urine absorbents, urine gelling agents for portable toilets and freshness preserving agents for fruits and vegetables, drip absorbents for meat and seafood, cooling agents, disposable warmers, gelling agents for batteries, water retaining agents for plants and soil, etc. It can also be used in various applications such as anti-condensation agents, water blocking agents and packing agents, and artificial snow.

Claims

A polymerization step of polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer,
A gel pulverizing step of pulverizing the hydrogel polymer during and / or after the polymerization step, if necessary;
A drying step of drying the obtained particulate hydrogel polymer to obtain a particulate dry polymer,
A method for producing a water-absorbent resin having a water absorption capacity (CRC) of 15 g / g or more and containing a chelating agent, comprising:
The persulfate used in the above polymerization step is 0 to 0.04 mol% (based on the monomer at the time of polymerization) (however, when the persulfate is 0 mol% (unused), another polymerization initiator is used). Is required),
In a step prior to the drying step, a chelating agent is added to the aqueous monomer solution or the hydrogel polymer in a total amount of 10 ppm or more (solid content of the monomer at the time of polymerization or hydrogel polymer),
The weight average particle diameter (D50) of the particulate hydrogel polymer is 1 mm or less,
In the above drying step, a method for producing a water-absorbent resin containing a chelating agent, wherein the drying time until the solid content becomes 80% by weight or more is 20 minutes or less.
A polymerization step of polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer,
A gel pulverizing step of pulverizing the hydrogel polymer during and / or after the polymerization step, if necessary;
A drying step of drying the obtained particulate hydrogel polymer to obtain a particulate dry polymer,
A method for producing a water-absorbent resin having a water absorption capacity (CRC) of 15 g / g or more and containing a chelating agent, comprising:
In the drying step, a weight average particle diameter (D50) containing 10 ppm or more of the chelating agent (based on the solid content of the hydrogel polymer) and 0 to 0.04 mol% of the persulfate (based on the monomer at the time of polymerization) is used. A) producing a water-absorbent resin containing a chelating agent, wherein the particulate hydrogel polymer having a particle size of 1 mm or less is dried for a drying time of 20 minutes or less until the solid content becomes 80% by weight or more.
However, the drying time refers to a time until the solid content becomes 80% by weight or more.
(3) The production method according to (1) or (2), wherein the total amount of the persulfate added from the polymerization step to before the drying step is 0 to 0.04 mol% (based on the monomer at the time of polymerization).
(4) The production method according to any one of (1) to (3), wherein in the drying step, hot air drying at 150 to 200 ° C. is performed.
(5) The method according to any one of (1) to (4), wherein the chelating agent is at least one selected from the group consisting of an amino polycarboxylic acid chelating agent and an amino polyphosphoric acid chelating agent.
(6) The production method according to any one of (1) to (5), wherein the hydrogel polymer is formed into particles in the gel pulverizing step.
The method according to any one of claims 1 to 6, wherein a total amount of the chelating agent added in the steps before the drying step is 60 ppm to 1% (solid content of the monomer at the time of polymerization or solid content of the hydrogel polymer). The production method according to claim 1.
(8) The production method according to any one of (1) to (7), wherein the polymerization is aqueous solution polymerization.
The production method according to any one of claims 1 to 8, wherein the polymerization is foaming polymerization or boiling polymerization, and the hydrogel polymer contains bubbles.
(10) The production method according to any one of (1) to (9), wherein the polymerization is a high-temperature-initiated short-time polymerization in which the polymerization initiation temperature is 30 ° C. or higher, the polymerization peak temperature is 80 to 130 ° C., and the polymerization time is 60 minutes or less.
(11) The production method according to any one of (1) to (10), further comprising a step of cross-linking the surface of the water-absorbent resin after the drying step.
The method according to any one of claims 1 to 11, further comprising a step of adding a chelating agent to the water-absorbent resin in the steps after the drying step.
The process according to any one of claims 1 to 12, wherein the hydrogel polymer before drying contains 0.1% by weight or more of residual monomers.
The monomer used in the polymerization step contains acrylic acid (salt), and the content of the acrylic acid (salt) is 50 to 100 with respect to the total monomers (excluding the internal crosslinking agent) used in the polymerization step. Mole%,
The water-absorbent resin containing the chelating agent has a polyacrylic acid (salt) -based water absorption in which the remaining amount (C1) of the chelating agent is 10 ppm or more, the L value of the initial color tone is 85 or more, and the YI value is 13 or less. 14. The production method according to claim 1, which is a conductive resin.
The water-absorbing resin containing a chelating agent according to any one of claims 1 to 14, wherein the remaining amount (C1) of the chelating agent is 200 ppm or more, the L value of the initial color tone is 89 or more, and the YI value is 10 or less. The production method according to claim 1.
(4) A polyacrylic acid (salt) -based water-absorbent resin having a chelating agent content (C2) of 200 ppm or more, an initial color tone L value of 89 or more, and a YI value of 10 or less.
The water absorption capacity under pressure (CRC) is 25 g / g or more, the water absorption capacity under pressure (AAP (0.7 psi)) is 15 g / g or more, and the ratio of the water absorption capacity under pressure to the water absorption capacity under no pressure ( The water-absorbent resin according to claim 16, wherein AAP (0.7 psi) / CRC is 0.5 or more.
Acrylic acid (salt) is 50-100 mol% of the total monomer (excluding the internal crosslinking agent),
0.001 to 5 mol% of the internal crosslinking agent based on the monomer, and
The water-absorbent resin according to claim 16 or 17, comprising a polyacrylic acid (salt) -based crosslinked polymer obtained from a monomer aqueous solution containing 0 to 0.04 mol% of a persulfate based on the monomer.
The hydrogel polymer obtained by polymerizing the monomer aqueous solution containing the monomer and the polymerization initiator is subjected to gel pulverization during and / or after the polymerization, if necessary, and the resulting particulate hydrogel weight is obtained. A water-absorbing resin containing a chelating agent obtained by drying the coalescence,
In a step prior to the drying step, a total of 10 ppm or more (solid content of the monomer at the time of polymerization or the solid content of the hydrogel polymer) of the chelating agent is added to the aqueous monomer solution or the hydrogel polymer. Item 19. The water-absorbent resin according to any one of Items 16 to 18.
水性 The water-absorbent resin according to any one of claims 16 to 19, which is in an irregular crushed shape.
The water-absorbent resin according to any one of claims 16 to 20, wherein the residual ratio of the chelating agent is 50% or more.
The water-absorbent resin according to any one of claims 16 to 21, comprising a chelating agent on the surface and inside, wherein the amount of the chelating agent present on the surface is larger than the amount of the chelating agent present inside.