JP7064614B2 - Method for manufacturing a water-absorbent resin containing a chelating agent - Google Patents

Description

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, which is an absorbent article such as paper omelets and menstrual napkins, as well as agricultural and horticultural water-retaining agents and industrial water-stopping agents. As a material, it is mainly used for disposable purposes.

Such a water-absorbent resin is manufactured through a manufacturing process such as a polymerization step, a drying step, and if necessary, a undried substance removing step, a crushing step, a classification step, and a surface cross-linking step. As the performance increases, many functions (physical properties) are required for the water-absorbent resin. Specifically, it is not limited to a mere high water absorption ratio, but also has gel strength, water-soluble content, water absorption rate, water absorption ratio under pressure, liquid permeability, particle size distribution, urine resistance, antibacterial resistance, and impact resistance (antibacterial resistance). (Damage resistance), powder fluidity, deodorant property, color resistance (whiteness), low dust, etc.

In order to improve the performance of the water-absorbent resin, a small amount of additive may be used for the water-absorbent resin as a method other than changing the monomer or the manufacturing process. Water-soluble or water-insoluble inorganic or organic powders, surfactants, plastics, water-soluble polymers, water-insoluble (thermoplastic) polymers, antibacterial agents, deodorants, reducing agents, antioxidants, organic acids, inorganic acids , Anticoloring agents, chelating agents and other additives are known.

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

For example, Patent Documents 1 to 4 disclose that a chelating agent is added to a monomer of a water-absorbent resin mainly for improving polymerization stability. Patent Documents 5 to 11 describe the polymerization step / drying step / surface cross-linking step / granulation step / fine powder recovery step of the water-absorbent resin mainly for improving the urine resistance of the water-absorbent resin (preventing urine deterioration). A method for producing a water-absorbent resin in which a chelating agent is added to a monomer or a polymer thereof in the 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 one of the production steps in order to suppress coloring of the water-absorbent resin (particularly, coloration over time).

In particular, Patent Documents 12 and 14 describe that it is known that it is preferable to have a chelating agent inside the water-absorbent resin in order to prevent coloring, and the chelating agent can be used as a monomer at the time of polymerization. A method of adding a chelating agent to the inside of a water-absorbent resin by adding it to a gel before drying is disclosed. Patent Document 20 discloses a water-absorbent resin composition containing a chelating agent for improving the salt resistance of the water-absorbent 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 describes a method for producing a water-absorbent resin in which a chelating agent is added to a second-stage monomer of two-stage polymerization in order to improve physical properties such as the water absorption ratio under pressure of the water-absorbent resin after reverse-phase suspension polymerization. Is disclosed.

Here, if the chelating agent used for the purpose of stabilizing polymerization in the manufacturing process and preventing drying deterioration in the above Patent Documents 1 to 7 also remains in the final product, the urine resistance when using the water-absorbent resin can be improved. It works to prevent coloring.

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

In response to this problem, one aspect of the present invention is to provide a chelating agent that can prevent decomposition of the chelating agent in the manufacturing process of the water-absorbent resin and improve the residual rate of the chelating agent in the final product, the water-absorbent resin. The main purpose is to provide a method for producing a water-absorbent resin containing the mixture.

Further, one aspect of the present invention is mainly intended to provide a water-absorbent resin containing a chelating agent obtained by the above-mentioned production method and exhibiting good color resistance (whiteness).

In order to solve the above-mentioned problems found this time, the present inventors investigated the cause, and found that in the step of drying the hydrogel polymer obtained by polymerizing the monomer, before the drying step. It was found that the chelating agent added to the product was specifically reduced in the drying process. Then, it was found that the cause of the decrease in the chelating agent in the drying step was the polymerization initiator (particularly persulfate) remaining in the hydrogel-like polymer.

In order to solve the above problems, the present invention controls the persulfate in the polymerization initiator during polymerization or before drying (0.04 mol% or less), and further controls the gel particle size and drying conditions before drying. It is characterized by 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-like polymer, and if necessary, pulverizing the hydrogel-like polymer during and / or after the polymerization step. A chelating agent having a water absorption ratio (CRC) of 15 g / g or more and including a gel crushing step and a drying step of drying the obtained particulate hydrogel polymer to obtain a particulate dry polymer. In the method for producing a water-absorbent resin containing In the case of (not used), another polymerization initiator is indispensably used), and the chelating agent is added to the above-mentioned monomer aqueous solution or the above-mentioned hydrogel-like polymer in the step before the drying step in total of 10 ppm or more (single during polymerization). The solid content of the weight-containing gel polymer or the solid content of the water-containing gel polymer) is added to set the weight average particle diameter (D50) of the particulate water-containing gel polymer to 1 mm or less, and the solid content is 80% by weight or more in the drying step. A method for producing a water-absorbent resin containing a chelating agent, wherein the drying time is 20 minutes or less.

[2] A polymerization step of polymerizing a monomer aqueous solution containing a monomer and a polymerization initiator to obtain a hydrogel-like polymer, and if necessary, pulverizing the hydrogel-like polymer during and / or after the polymerization step. A chelating agent having a water absorption ratio (CRC) of 15 g / g or more and including a gel crushing 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 (solid content of the water-containing gel-like polymer) and the persulfate is 0 to 0.04 mol% (at the time of polymerization). A chelating agent that dries a particulate water-containing gel-like polymer having a weight average particle diameter (D50) of 1 mm or less, which contains a monomer), with a drying time of 20 minutes or less until the solid content becomes 80% by weight or more. A method for producing a water-absorbent resin containing. However, the drying time refers to the time until the solid content becomes 80% by weight or more.

[3] The amount of persulfate added from the polymerization step to before the drying step is 0 to 0.04 mol% (monomer at the time of polymerization) in total, according to [1] or [2]. 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 chelating agent is at least one selected from the group consisting of an amino polyvalent carboxylic acid chelating agent and an amino polyvalent phosphoric acid chelating agent, according to any one of [1] to [4]. The manufacturing method described.

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

[7] The total amount of the chelating agent added in the steps up to the drying step is 60 ppm to 1% (solid content of the monomer or the hydrogel-like polymer at the time of polymerization), [1] to The manufacturing 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 foam polymerization or boiling polymerization, and the hydrogel-like polymer contains bubbles.

[10] The item according to any one of [1] to [9], wherein the polymerization is a high-temperature start short-time polymerization having a polymerization start 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 comprising a surface cross-linking step of the water-absorbent resin after the drying step.

[12] The production method according to any one of [1] to [11], further comprising 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 the residual monomer.

[14] The monomer used in the polymerization step contains acrylic acid (salt), and the content of the acrylic acid (salt) is higher than that of the total monomer (excluding the internal cross-linking agent) used in the polymerization step. The water-absorbent 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 the 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 the chelating agent has a residual amount (C1) of the chelating agent of 200 ppm or more, an L value of the initial color tone of 89 or more, and a YI value of 10 or less, [1] to 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] The non-pressurized water absorption ratio (CRC) is 25 g / g or more, the pressurized water absorption ratio (AAP (0.7 psi)) is 15 g / g or more, and the pressurized water absorption ratio and the unpressurized water absorption ratio 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), the internal cross-linking agent is 0.001 to 5 mol% with respect to the monomer, and persulfate. The water-absorbent resin according to [16] or [17], which comprises a polyacrylic acid (salt) -based crosslinked polymer obtained from a monomer aqueous solution containing 0 to 0.04 mol% with respect to the monomer.

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

[20] The water-absorbent resin according to any one of [16] to [19], which is in the form of amorphous crushed material.

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

[22] The water absorption according to any one of [16] to [21], wherein the chelating agent is contained 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. Sex resin.

According to one aspect of the present invention, the decomposition of the chelating agent in the manufacturing process of the water-absorbent resin is prevented, the residual rate of the chelating agent in the final product, the water-absorbent resin, is improved, and the surface and the inside of the particles of the water-absorbent resin are covered. It has the effect of being able to blend a chelating agent.

Hereinafter, the method for producing a water-absorbent resin containing a chelating agent according to the present invention and the water-absorbent resin containing the chelating agent obtained thereby will be described in detail, but the scope of the present invention is not limited to these explanations. However, other than the following examples, the present invention may be appropriately modified and implemented as long as the gist of the present invention is not impaired. Specifically, the present invention is not limited to each of the following embodiments, and various modifications can be made within the scope of the claims, and the technical means disclosed in the different embodiments may be appropriately combined. The embodiments obtained 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. In addition, "water swelling property" means that CRC (water absorption ratio under no pressurization) defined by ERT442-2-02 is 5 [g / g] or more, and "water insoluble" means ERT470.2. Ext (water-soluble content) specified in -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 the form in which the total amount (100% by weight) is a polymer, and may be a composition containing a surface-crosslinked resin, additives, etc. within the range of maintaining the above performance. 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 water absorption having different shapes obtained in each step. It includes a water-absorbent resin composition containing a sex resin (for example, a sheet-like shape, a fibrous shape, a film-like shape, a gel-like shape, etc.), an additive, and the like.

(1-2) "Polyacrylic acid (salt)"
The "polyacrylic acid (salt)" in the present invention optionally contains a graft component, and has acrylic acid and / or a salt thereof (hereinafter, may be referred to as acrylic acid (salt)) as a main component as a repeating unit. Means a polymer to be used. Specifically, of the total monomers (excluding the internal cross-linking agent) used for the polymerization, acrylic acid (salt) is indispensably 50 to 100 mol%, preferably 70 to 100 mol%, and more preferably 90 to 90 to 100 mol%. A polymer containing 100 mol%, particularly preferably 100 mol%. When a polyacrylic acid salt is used as the polymer, a water-soluble salt is essential, a monovalent salt is preferable as the main component of the neutralizing salt, an alkali metal salt or an ammonium salt is more preferable, and an alkali metal salt is further preferable. Preferred, sodium salts are particularly preferred.

(1-3) "EDANA" and "ERT"
"EDANA" is an abbreviation for European Disposables and Nonwovens Assoiations, and "ERT" is an abbreviation for EDANA Recommended Test Metods, which is a European standard (almost the world standard). be. In the present invention, unless otherwise specified, the measurement is performed in accordance with the original ERT (publicly known document: revised in 2002).

(A) "CRC" (ERT441.2-02)
"CRC" is an abbreviation for Centrifuge Retention Capacity, and means a water absorption ratio under no pressure (hereinafter, may be referred to as a "water absorption ratio"). Specifically, 0.200 g of water-absorbent resin in a non-woven fabric bag is freely swollen with a large excess of 0.9 wt% sodium chloride aqueous solution for 30 minutes, and then drained with a centrifuge to absorb water. 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" (ERT442.2.2)
"AAP" is an abbreviation for Absorption Against Pressure and means the water absorption ratio under pressure. Specifically, 0.900 g of a water-absorbent resin is swollen with a load of 2.06 kPa (0.3 psi, 21 [g / cm ² ]) for 1 hour with respect to a 0.9 wt% sodium chloride aqueous solution. It is the water absorption ratio (unit; [g / g]) after making it. In ERT442-2-02, it is described as Absorption Under Pressure, but the contents are substantially the same. Further, in the present invention and the examples, the load condition may be changed to 4.83 kPa (0.7 psi, 49 [g / cm ² ]) for measurement, and in that case, AAP (0.7 psi) is used. Describe.

(C) "Ext" (ERT470.2-02)
"Ext" is an abbreviation for Extractables and means a water-soluble component (amount of water-soluble component). Specifically, the amount of the dissolved polymer (unit:% by weight) after adding 1.000 g of the water-absorbent resin to 200 ml of a 0.9 wt% sodium chloride aqueous solution and stirring for 16 hours. The amount of dissolved polymer is measured 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 for Particle Size Distribution and means a particle size distribution measured by sieving. The weight average particle diameter (D50) and the particle diameter distribution width are referred to as "(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. Further, the standard sieve (opening) used for particle size measurement may be added as appropriate 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 PSD of the hydrogel polymer will be described later.

(E) "Residual Monomers" (ERT410.2-02)
"Residual Monomers" means the amount of a 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 wt% sodium chloride aqueous solution, and the amount of the dissolved monomer (unit: ppm) after stirring at 500 rpm for 1 hour using a 35 mm stirrer chip was calculated. say. The amount of dissolved monomer is measured by 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, respectively, and the obtained measured value was converted into the weight per resin solid content of the hydrogel polymer. Value (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; weight%) calculated from the weight loss by 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 water 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, the value calculated by {100-moisture content (% by weight)} is defined as "resin solid content" in the present invention, and can be applied to water-absorbent resins, dry polymers and hydrogel-like polymers.

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

"SFC (Saline Flow Inducible)" refers to the liquid permeability of a 0.69 wt% sodium chloride aqueous solution to a water-absorbent resin at 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. Further, "GBP" refers to the liquid permeability of a 0.69 wt% sodium chloride aqueous solution to a water-absorbent resin under load or under free expansion, and conforms to the GBP test method disclosed in International Publication No. 2005/016393. Be measured.

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

(1-6) "Gel crushing"
The "gel crushing" in the present invention is an operation of applying shearing and compressive forces to reduce the size and increase the surface area for the purpose of facilitating the drying of the hydrogel-like polymer obtained in the polymerization step. Say that. By "gel pulverization", a particulate hydrogel polymer, particularly a particulate hydrogel polymer having a weight average particle diameter (D50) described later, is obtained.

In the case of unstirred aqueous solution polymerization (static aqueous solution polymerization, particularly belt polymerization), gel pulverization is performed after the polymerization, whereas in the case of kneader polymerization, polymerization and gel pulverization are continuously performed 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 later, and gel pulverization may be performed either during or after the polymerization. ..

(1-7) "Weight average molecular weight of water-soluble component"
The "water-soluble component weight average molecular weight" in the present invention refers to the weight average molecular weight of the component (water-soluble component) that dissolves when the water-absorbent resin is added to the water solvent by GPC (gel permeation chromatography). 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 (1-3) and (c) "Ext" above. The weight average molecular weight of the water-soluble component of the hydrogel polymer is 5.0 g for a sample having a particle size of 5 mm or less and further finely divided to 1 to 3 mm, and the stirring time is changed to 24 hours. Measurements were made.

(1-8) Others In the present specification, "XY" indicating a range means "X or more and Y or less". In addition, "mass" and "weight" are treated as synonyms, and "t (ton)", which is a unit of weight, means "Metric ton", and unless otherwise specified, "ppm". "" Means "weight ppm". Further, "-acid (salt)" means "-acid and / or a salt thereof" and "(meth) acrylic" means "acrylic and / or methacrylic".

[2] Method for Producing Water Absorbent Resin Containing a Chelating Agent The present invention comprises a polymerization step of polymerizing a monomer aqueous solution containing a monomer and a polymerization initiator to obtain a hydrogel-like polymer, and, if necessary, a polymerization step. A gel crushing step of crushing the hydrogel-like polymer during and / or after the process, and a drying step of drying the obtained particulate water-containing gel-like polymer to obtain a particulate dry polymer are included. A method for producing a water-absorbent resin having a water absorption ratio (CRC) of 15 g / g or more and containing a chelating agent, the following methods 1 and 2 are provided.

(Method 1; Specify persulfate / gel particle size / drying conditions in the monomer)
If the persulfate used in the above polymerization step is 0 to 0.04 mol% (monomer for polymerization) (however, if the persulfate is 0 mol% (not used), another polymerization initiator In the step prior to the drying step, the chelating agent was added to the monomer aqueous solution or the hydrogel polymer in total of 10 ppm or more (solid content of the monomer or hydrogel polymer at the time of polymerization). ) Is added, 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 becomes 80% by weight or more is 20 minutes or less. , A method for producing a water-absorbent resin containing a chelating agent.

(Method 2: Specify persulfate / gel particle size / drying conditions in the hydrogel polymer)
In the above drying step, the weight average particle size (D50) containing 10 ppm or more of the chelating agent (solid content of the water-containing gel polymer) and 0 to 0.04 mol% of persulfate (monomer at the time of polymerization). A method for producing a water-absorbent resin containing a chelating agent, wherein a water-containing gel-like polymer having a particle size of 1 mm or less is dried with a drying time of 20 minutes or less 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.

Hereinafter, the above method 1 and the method 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-like polymer (hereinafter, may be abbreviated as “hydrogen gel”). It is a process.

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

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

Further, in the hydrogel-like polymer obtained by the polymerization of the above-mentioned monomer aqueous solution, it is preferable that at least a part of the acid groups of the polymer is neutralized from the viewpoint of water absorption performance and residual monomer. The partially neutralized salt is not particularly limited, but a monovalent salt selected from an alkali metal salt, an ammonium salt and an amine salt is preferable, an alkali metal salt is more preferable, and a sodium salt, a lithium salt and a potassium salt are preferable from the viewpoint of water absorption performance. Alkali metal salts selected from salts are more preferred, and sodium salts are particularly preferred. Therefore, the basic substance used for the above neutralization is not particularly limited, but is not particularly limited, but is an hydroxide of an alkali metal such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate (hydrogen), potassium carbonate (hydrogen) and the like. A monovalent basic substance such as a carbonate (hydrogen) salt of is preferable, and sodium hydroxide is particularly preferable. When a carbonate is used for neutralization, carbon dioxide gas is generated at the time of neutralization, so that the carbon dioxide gas can also be used as a foaming agent at the time of polymerization.

The above neutralization can be carried out in each form and state before, during or after the polymerization, and for example, an unneutralized or low-neutralized (for example, a neutralization rate of 0 to 30 mol%) acrylic acid. It is possible to neutralize the hydrogel polymer obtained by polymerizing, especially at the same time as crushing the gel, but from the viewpoint of improving productivity and physical properties, neutralize the acrylic acid before polymerization. It is preferable to do so. That is, it is preferable to use neutralized acrylic acid (partially neutralized salt of acrylic acid) as a monomer.

The neutralization rate in the above neutralization is not particularly limited, but the final water-absorbent resin is preferably 10 to 100 mol%, more preferably 30 to 95 mol%, still more preferably 45 to 90 mol%, and 60. -80 mol% is particularly preferred. The neutralization temperature is also not particularly limited, but is preferably 10 to 100 ° C, more preferably 30 to 90 ° C.

Further, 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 together. Such other monomers are not particularly limited, but are, for example, methacrylic acid, (anhydrous) maleic acid, 2- (meth) acrylamide-2-methylpropanesulfonic acid, (meth) acryloxialcansulfonic 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 thereof include acrylates, polyethylene glycol (meth) acrylates, stearyl acrylates, and salts thereof. When other monomers are used, the amount used is appropriately determined as long as the water absorption performance of the obtained water-absorbent resin is not impaired, and is not particularly limited, but with respect to the total monomer (excluding the internal cross-linking agent). Therefore, 0 to 50 mol% is preferable, 0 to 30 mol% is more preferable, and 0 to 10 mol% is further preferable.

(Polymerization inhibitor)
The monomer aqueous solution may contain a polymerization inhibitor, if necessary, 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, relative to the monomer from the viewpoint of coloring and polymerization stability. More preferably, it is 20 to 100 ppm. Preferably, a metosiphenol-based polymerization inhibitor is used. More preferably, the polymerization inhibitor is p-methoxyphenol.

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

The present invention (method 1) reduces the amount of persulfate in the polymerization initiator used in the polymerization step to 0 to 0.04 mol% (monomer at the time of polymerization) (however, 0 mol of persulfate). % (Not used), another polymerization initiator is indispensably used) method. Further, 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), and further before the drying step. This is a method for controlling the gel particle size and 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 and improving the physical properties, 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 addition amount of the persulfate in the above to 0 to 0.04 mol% (monomer at the time of polymerization) in total. The lower the amount of the persulfate added and the content of the persulfate in the hydrogel polymer, the more preferable, 0.04 mol% or less, 0.035 mol% or less, and 0.035 mol% or less with respect to 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 is the case where it is not used at the time of 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 0.001 mol% or more, from the viewpoint of reducing the residual monomer at the time of drying. It is desirable that the amount is 0.01 mol% or more and is contained in the above-mentioned monomer aqueous solution or the above-mentioned hydrogel-like polymer. In the present invention, any combination of the upper limit value and the lower limit value of the amount of persulfate added is preferable.

When the amount of persulfate added exceeds 0.04 mol% with respect to 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 (for example, sodium persulfate, potassium persulfate, ammonium persulfate, etc.) and a peroxide other than the persulfate (for example, hydrogen peroxide, t). -Butyl peroxide, methyl ethyl ketone peroxide, etc.), azo polymerization initiator, photopolymerization initiator may be used.

Examples of the azo polymerization initiator used in the present invention include water-soluble azo compounds (for example, 2,2'-azobis (2-methylpropionamidine) dihydrochloride, 2,2'-azobis [2- (2-imidazolin-). 2-yl) Propane] dihydrochloride, 2,2'-{azobis (2-methyl-N- [1,1'-bis (hydroxymethyl) -2-hydroxyethyl] propionamide)}, etc.). 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, 4- (2-hydroxyethoxy) phenyl-. Acetphenone derivatives such as (2-hydroxy) -2-propyl ketone and 1-hydroxycyclohexylphenyl ketone; benzoin alkyl ethers such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether and benzoin isobutyl ether; , 4-Phenylbenzophenone, 4-benzoyl-4'-methyl-diphenylsulfide, benzophenone derivatives such as (4-benzoylbenzyl) trimethylammonium chloride; thioxanthone-based compounds; bis (2,4,6-trimethyl) Examples thereof include acylphosphine oxide derivatives such as benzoyl) -phenylphosphine oxide and diphenyl (2,4,6-trimethylbenzoyl) -phosphinoxide; 2-hydroxymethylpropionnitrile and the like. Acetophenone derivatives are preferable, and 1-hydroxycyclohexylphenylketone is preferably used.

The amount of the polymerization initiator containing the persulfate (0 to 0.04 mol%) is preferably 0.0001 to 1 mol% in total, and 0.0005 to 0, based on the monomer at the time of polymerization. .5 mol% is more preferred. If the amount of the polymerization initiator used exceeds 1 mol%, it becomes difficult to control the polymerization, and the color tone of the obtained water-absorbent resin may deteriorate. Further, when the amount of the polymerization initiator used is less than 0.0001 mol%, there is a concern that the residual monomer may increase.

In the present invention, two or more kinds of polymerization initiators may be used in combination. Among them, persulfate is preferably used in the above range from the viewpoint of handleability, physical properties of the water-absorbent resin and the like. When a persulfate is used in combination with another polymerization initiator, 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-absorbent resin. Is preferably used. The molar ratio of the persulfate to the polymerization initiator other than the persulfate (= azo polymerization initiator + photopolymerization initiator + other polymerization initiator) is 1/99 to 99/1, preferably 1/9 to. It is 9/1, more preferably 2/8 to 8/2, and even more preferably 3/7 to 7/3.

Further, a reducing agent that promotes the decomposition of these polymerization initiators (particularly persulfate or peroxide) can be used in combination, and a redox-based initiator can be obtained by combining the two. Examples of such reducing agents include (heavy) sulfite (salt) such as sodium sulfite and sodium hydrogen sulfite, reducing metals (salts) such as L-ascorbic acid (salt) and ferrous salt, amines and the like. Can be mentioned.

Polymerization by irradiating the reaction system with active energy rays such as radiation, electron beam, and ultraviolet rays instead of using the above-mentioned polymerization initiator or in combination with the polymerization initiator in order to initiate the polymerization of the monomer aqueous solution. The reaction may be initiated.

(Persulfate remaining in hydrous gel)
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 mechanism of the decrease, the chelating agent is not substantially decomposed at the time of polymerization, and the chelating agent is decomposed at the time of drying. Was found. This is because, in addition to the experimental fact that the amount of the chelating agent contained in the hydrogel containing the polymerization is almost constant, the amount of the chelating agent is reduced even if 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.

Of the polymerization initiators used for polymerization, most (more than 80%) of the persulfate remains in the hydrogel after polymerization under normal polymerization conditions due to its half-life. Therefore, the persulfate is decomposed by the high temperature during drying, and radicals are generated in the hydrogel. However, the monomer (residual monomer) remaining in the hydrogel after polymerization is 0. Since the amount is as small as a few percent to a few percent, it is presumed that the generated radicals of the persulfate do not easily react with the monomer and instead promote the decomposition of the chelating agent.

In the present invention, the persulfate is sodium persulfate (half-life at 90 ° C. (τ); 1.24 hours), potassium persulfate (half-life at 90 ° C. (τ); 1.24 hours), ammonium persulfate (90). Half-life at ° C. (τ); 0.44 hours) is preferred, and sodium persulfate is more preferred. When sodium acrylate is used as the monomer of the water-absorbent resin, if persulfate is used as the polymerization initiator in the above range, it becomes substantially sodium persulfate regardless of the type of salt. The half-life (τ) of sodium persulfate or potassium persulfate is 2100 hours (30 ° C.), 499 hours (40 ° C.), 130 hours (50 ° C.), 36.5 hours (60 ° C.), 11 .1 hour (70 ° C), 3.59 hours (80 ° C), 1.24 hours (90 ° C), 0.45 hours (100 ° C). Therefore, most (more than 80%) of sodium persulfate or potassium persulfate remains in the hydrogel polymer before drying.

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

The internal cross-linking agent preferably used in the present invention is not particularly limited, but for example, a polymerizable cross-linking agent that polymerizes with acrylic acid, a reactive cross-linking agent that reacts with a carboxyl group, a cross-linking agent having these properties, and the like. Can be mentioned.

Examples of the polymerizable cross-linking agent include N, N'-methylenebisacrylamide, (poly) ethylene glycol di (meth) acrylate, (polyoxyethylene) trimethylolpropane tri (meth) acrylate, and poly (meth) allyloxy. Examples thereof include compounds having at least two polymerizable double bonds in the molecule, such as alkane. 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. Examples thereof include ion-binding cross-linking agents such as compounds. Among these, from the viewpoint of water absorption performance, a polymerizable cross-linking agent that polymerizes with acrylic acid is more preferable, and an acrylate-based, allyl-based, and acrylamide-based polymerizable cross-linking agent is particularly preferable. These internal cross-linking agents may be used alone or in combination of two or more. When the above-mentioned polymerizable cross-linking agent and the covalent cross-linking agent are used in combination, the mixing ratio thereof is preferably 10: 1 to 1:10.

The amount of the internal cross-linking agent used is preferably 0.001 to 5 mol%, more preferably 0.002 to 2 mol%, and 0.04 to 1 with respect to the monomer from the viewpoint of physical properties. More preferably, 0.06 to 0.5 mol% is particularly preferable, and 0.07 to 0.2 mol% is most preferable.

In one embodiment of the present invention, the aqueous monomer solution contains 50 to 100 mol% of total monomer (excluding the internal cross-linking agent) of acrylic acid (salt) and 0. It is preferable to contain 001 to 5 mol% and 0 to 0.04 mol% of the persulfate with respect to the monomer. The water-absorbent resin made of a 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 aspect of the present invention, in order to improve the physical properties of the water-absorbent resin obtained in the present invention, a further conventional additive may be added to the above-mentioned monomer aqueous solution.

Such additives include water-soluble resins or water-absorbent resins such as starch, cellulose, polyvinyl alcohol (PVA), polyacrylic acid (salt), polyethyleneimine; carbonates, various foaming agents that generate bubbles; surfactants. And so on.

In addition to the above-mentioned monomer aqueous solution, these additives can be added to a hydrogel-like polymer, a dry polymer, a water-absorbent resin, or the like in any of the production steps of the present invention. In the case of the water-soluble resin or the water-absorbent resin, the amount of these additives added is preferably 0 to 50% by weight, more preferably 0 to 20% by weight, still more preferably 0 to 10% with respect to the monomer. It is% by weight, particularly preferably 0 to 3% by weight. On the other hand, in the case of the foaming agent or the surfactant, 0 to 5% by weight is preferable, and 0 to 1% by weight is more preferable with respect to the monomer. A graft polymer or a water-absorbent resin composition can be obtained by adding the water-soluble resin or the water-absorbent resin, and these starch-acrylic acid polymers, PVA-acrylic acid polymers and the like are also water-absorbent resins in the present invention. Treat as.

(Chelating agent)
In the present invention, in the step prior to the drying step, the chelating agent is added to the above-mentioned monomer aqueous solution or the above-mentioned hydrogel-like polymer in a total amount of 10 ppm or more (monomer at the time of polymerization or anti-water-containing gel-like polymer). Solid content) Add. From the viewpoint of the physical properties of the water-absorbent resin, the total amount of the chelating agent added to the monomer aqueous 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 to be subjected to the drying step (solid content of the hydrogel polymer) 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 are preferable in this order. Further, from the viewpoint of the effect of the chelating agent (eg, prevention of coloring, deterioration, etc.) and cost, the upper limit of the addition amount or content of the chelating agent is the solid content of the monomer or the hydrogel polymer at the time of polymerization. On the other hand, 1% or less, 8000 ppm or less, 6000 ppm or less, and 5000 ppm or less are preferable in this order. In the present invention, any combination of the upper limit value and the lower limit value of the addition amount or the content of the chelating agent is preferable. The amount (ppm) of the chelating agent added is the weight of the chelating agent at the time of addition (the solid content of the monomer or the water-containing gel polymer at the time of polymerization), and is a single amount after mixing after the addition. Without considering the salt exchange with the carboxy group of the body and the polymer, the neutralized salt type chelating agent is shown as a salt type, and the acid type chelating agent is shown as a tangible amount at the time of addition as an acid type.

The weight% of the chelating agent with respect to the solid content of the monomer or the water-containing gel polymer can be converted into a molar ratio (mol%) with respect to the monomer at the time of polymerization. In the present invention, the addition amount or 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 with respect to the monomer at the time of polymerization. , 0.01 mol% or more are preferable. The upper limit is preferably 0.2 mol% or less, 0.15 mol% or less, 0.12 mol% or less, and 0.1 mol% or less in this order. In the present invention, any combination of the upper limit value and the lower limit value of the molar ratio of the chelating agent is preferable. Further, in order to improve the residual ratio of the chelating agent, the lower the molar ratio of the persulfate to the chelating agent is, the more preferable, and the order is 50 times or less, 20 times or less, 10 times or less, 5 times or less, and 3 times or less. The lower limit is 0. By such a method, not only the residual rate of the chelating agent is improved and the content 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. It increases and the YI value decreases.

By adding a chelating agent to the monomer at the time of polymerization or the water-containing gel before drying, the chelating agent is uniformly blended inside the polymer of the water-absorbent resin (particularly the water-absorbent resin of the final product) after the drying step. be able to. By adding the chelating agent in the drying step, further in the surface cross-linking step or the subsequent steps, the chelating agent is selectively blended on the surface of the water-absorbent resin. 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, it was found that the initial coloring of the obtained water-absorbent resin deteriorates when many chelating agents are used for polymerization, but there is no such problem in the present invention, and the chelating agent is uniformly spread even inside the water-absorbent resin. Can be blended.

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

When a chelating agent is added in the above-mentioned gel crushing step, it can be added to and kneaded with a hydrogel-like polymer to crush the gel in the same manner as other additives. Specifically, while the hydrogel-like polymer stays in the gel crushing device, an aqueous solution containing a chelating agent can be supplied into the device. Further, an aqueous solution containing a chelating agent may be added in advance to the hydrogel polymer and charged into the gel crushing apparatus. The timing of addition of these chelating agents may be appropriately combined.

Preferred chelating agents in the present invention include polymer or non-polymeric chelating agents, more preferably non-polymeric chelating agents, and even more preferably non-polymeric chelating agents having a molecular weight of 1000 or less.

Specific examples of the chelating agent used in the present invention include amino polyvalent carboxylic acid, organic polyvalent phosphoric acid (particularly amino polyhydric phosphoric acid), inorganic polyhydric 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 above-mentioned "multivalent" means that one molecule has a plurality of functional groups, preferably 2 to 30, more preferably 3 to 20, and even more preferably 4 to 10. Refers to having a functional group.

Specific examples of the amino polyvalent carboxylic acid include imino 2 acetic acid, hydroxyethyl imino 2 acetic acid, nitrilo 3 acetic acid, nitrilo 3 propionic acid, ethylenediamine 4 acetic acid, diethylenetriamine 5 acetic acid (DTPA), and triethylene tetramine 6 acetic acid ( TTHA), trans-1,2-diaminocyclohexane 4 acetic acid, N, N-bis (2-hydroxyethyl) glycine, diaminopropanol 4 acetic acid, ethylenediamine 2 propionic acid, N-hydroxyethyl ethylenediamine 3 acetic acid, glycol ether diamine 4 acetic acid , Diaminopropane 4 acetic acid, N, N'-bis (2-hydroxybenzyl) ethylenediamine-N, N'-2 acetic acid, 1,6-hexamethylenediamine-N, N, N', N'-4 acetic acid and these. Salt and the like.

Specific examples of the organic polyvalent phosphoric acid include nitriloacetate-di (methylenephosphinic acid), nitrilogicacetic acid- (methylenephosphinic acid), nitriloacetate-β-propionic acid-methylenephosphonic acid, and nitrilotris (methylenephosphon). Acid), 1-hydroxyethylidene diphosphonic acid, aminopolyvalent phosphoric 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), and ethylenediamine-N, N. '-Diacetic acid-N, N'-di (methylenephosphonic acid), ethylenediamine-N, N'-di (methylenephosphonic acid), ethylenediaminetetra (methylenephosphonic acid), polymethylenediaminetetra (methylenephosphonic acid), diethylenetriamine Examples thereof include penta (methylenephosphonic acid), ethylenediaminetetramethylenephosphonic 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-thujaplicin, and γ-thujaplicin.

Among these, an amino polyvalent carboxylic acid chelating agent and / or an amino polyvalent phosphoric acid chelating agent are preferable. Specifically, examples of the aminopolyvalent carboxylic acid-based chelating agent include ethylenediamine 4acetic acid, diethylenetriamine5acetic acid and triethylenetetraamine6acetic acid, and metal salts thereof such as sodium salt and potassium salt. Can be mentioned. Specific examples of the amino polyvalent phosphoric acid-based chelating agent include ethylenediaminetetramethylenephosphonic acid.

In the present invention, a water-absorbent resin containing a chelating agent uniformly inside the polymer can be obtained by such a method. However, deterioration and coloring of the water-absorbent resin are likely to occur on the surface of the particles. Therefore, in the present invention, more chelating agent is blended on the particle surface by adding a chelating agent to the water-absorbent resin in the steps after the drying step, that is, water-absorbing by adding the chelating agent a plurality of times. It is more preferable to form a concentration gradient such that the chelating agent is contained on the surface and inside of the resin and the amount of the chelating agent present on the surface is larger than the amount of the chelating agent present inside. The type and amount of the chelating agent added in the steps after the drying step may be the same as or different from the type and amount of the chelating agent added in the step prior to the drying step. It is preferable that the type and amount of addition are added. Further, the chelating agent may be added to the monomer or the hydrogel-like polymer only with the chelate agent, or the chelate agent may be added to the monomer or the hydrogel-like polymer as 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 is to directly obtain a particulate water-containing gel polymer by spray / droplet polymerization in a gas phase or reverse phase suspension polymerization. However, aqueous polymerization is adopted from the viewpoint of the liquid permeability (SFC) and water absorption rate (FSR) of the obtained water-absorbent resin, the ease of polymerization control, and the like.

In the aqueous solution polymerization, a water-containing gel-like polymer may be obtained by tank-type (silo-type) or belt-type non-stirring polymerization, and gel crushing may be performed separately. This may be carried out to obtain a particulate water-containing gel polymer, but from the viewpoint of easiness of polymerization control, kneader polymerization or belt polymerization is preferably adopted. Further, from the viewpoint of productivity, continuous aqueous solution polymerization is preferably adopted, and high-concentration continuous aqueous solution polymerization is more preferably adopted. The stirring polymerization means that a water-containing gel-like polymer (particularly, a water-containing gel-like polymer having a polymerization rate of 10 mol% or more, further 50 mol% or more) is polymerized while stirring, particularly stirring and subdividing. Further, the monomer aqueous solution (polymerization rate of less than 0 to 10 mol%) may be appropriately stirred before and after the non-stirring polymerization.

As the continuous aqueous solution polymerization, for example, the continuous kneader polymerization described in US Pat. Nos. 6,987,171, 6710141, etc., and US Pat. No. 4,893999, No. 641,928, US Patent Application Publication No. 2005/215734, etc. Continuous belt polymerization of. By polymerizing these aqueous solutions, a water-absorbent resin can be produced with high productivity.

In the present invention, the decomposition of the chelating agent at the time of drying can be suppressed by increasing the solid content concentration of the hydrogel-like 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 further preferably 45% by weight or more (upper limit is the saturation concentration).

Further, since the decomposition (half-life) of the persulfate depends on the temperature, time and pH, when the persulfate is used in the present invention, high-temperature initiation polymerization is performed to further reduce the persulfate before the drying step. By setting it, the decomposition of the chelating agent at the time of drying can be suppressed. Specifically, in the high temperature start polymerization, the polymerization start temperature is preferably 30 ° C. or higher, more preferably 35 ° C. or higher, further preferably 40 ° C. or higher, and particularly preferably 50 ° C. or higher (upper limit is boiling point). The polymerization peak temperature is usually preferably 80 to 130 ° C., more preferably a boiling point to 120 ° C., which is high temperature polymerization. Further, the high-concentration / high-temperature start polymerization is a polymerization in which these are combined, and the monomer aqueous solution tends to be in a boiling state at the time of polymerization, and a solvent such as water evaporates, so that a water-containing gel-like polymer having a high solid content can be obtained. Be done.

Increase in solid content of hydrogel-like polymer with respect to solid content (total concentration of monomer and graft component) in aqueous monomer solution [= (solid content of hydrogel-like polymer [mass%])-( The solid content [mass%])] in the aqueous monomer solution is preferably 1% or more, more preferably 3% or more, still more preferably 5% or more. The upper limit of the increase width is 15% or less from the viewpoint of ease of polymerization control. The solid content of the hydrogel polymer is preferably 40 to 75% by mass, more preferably 45 to 70% by mass, and further preferably 50 to 65% by mass. When a hydrogel-like polymer having a high solid content is obtained by polymerization in this way, the decomposition of the chelating agent is reduced in the drying step, so that the residual rate of the chelating agent is improved. However, if the solid content of the hydrogel polymer exceeds 75% by mass, the physical properties of the obtained water-absorbent resin may deteriorate. In the present invention, a form of foam polymerization by introducing the above-mentioned foaming agent or air bubbles is also preferable. That is, the polymerization in the present invention is preferably foam polymerization or boiling polymerization. When the water-containing gel-like polymer contains at least bubbles, not only the water absorption rate can be improved, but also the drying rate can be improved, and the content rate (residual rate) of the chelating agent after the drying step can also be increased.

Furthermore, in the mechanism of specific reduction of the chelating agent in the drying step found in the present invention, although the reaction mechanism is not clear, "the radical of the persulfate is preferentially given to the monomer at the time of polymerization. Although it reacts, 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 is higher and the solid content ratio is higher during drying than during polymerization. Therefore, in the drying step, the persulfate remaining after the polymerization reacts preferentially with the chelating agent. " In the prior art, it was preferable that the residual monomer in the hydrogel polymer was 1000 ppm or less (US Pat. No. 51,906, US Pat. No. 4,533,323, European Patent No. 53438). However, in the present invention, it is not necessary to set the polymerization rate after polymerization to 100%, but rather, the residual monomer is 0.1% by mass or more in the hydrogel-like polymer before the drying step, and further 0.5 to 10% by mass. %, 0.5 to 5% by mass, and preferably 0.5 to 3% by mass. In order to leave almost 100% of the chelating agent in the polymerization step, it is preferable that the polymerization time is short so that the unreacted monomer remains sufficiently at the end of the polymerization step from the estimation mechanism. That is, the present invention is preferably applied to short-time polymerization with a polymerization time of 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, further 10 seconds or more). ..

Therefore, in order to reduce the amount of persulfate and to achieve both the residual monomer, the polymerization conditions are a polymerization start temperature of 30 ° C. or higher (further, the above range) and a polymerization peak temperature of 80 to 130 ° C. (further, the above range). High-temperature start short-time polymerization with a polymerization time of 60 minutes or less (further, the above range) is particularly preferable, and high-concentration, high-temperature start short-time polymerization having a monomer concentration of 35% by weight or more (further, the above range) is particularly preferable. Is the most preferable.

The above-mentioned high-concentration, high-temperature start short-time polymerization is disclosed in US Pat. Nos. 6,906,159, 7091253, etc., but the polymerization method can obtain a water-absorbent resin having a high whiteness, and is more industrial. It is preferable because it is easy to produce on a scale.

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

Although the above-mentioned polymerization can be carried out in an air atmosphere, it is preferable to carry out the polymerization in an inert gas atmosphere such as water vapor, nitrogen or argon (for example, an oxygen concentration of 1% by volume or less) from the viewpoint of preventing coloring. Further, it is preferable to polymerize after substituting (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 it is possible to provide a water-absorbent resin having higher physical characteristics and higher white color.

(2-2) Gel Crushing Step This step is a step of subdividing the above-mentioned water-containing gel-like polymer during or after polymerization to obtain a particulate water-containing gel-like polymer. In addition, this step is referred to as "gel crushing" to distinguish it from "crushing" in the following (2-4) crushing step / classification step.

In the present invention, a granular hydrogel-like polymer is obtained in the gel crushing step during and / or after the polymerization step, or directly in the polymerization step, but it is easy to control the target gel particle size. It is more preferable to obtain a particulate water-containing gel polymer in the gel pulverization step during and / or after the polymerization step.

(Gel crusher)
The gel crusher during or after the polymerization used in this step is not particularly limited, and is a gel crusher equipped with a plurality of rotary stirring blades such as a batch type or a continuous type double-armed kneader, or a single shaft. Examples thereof include an extruder, a twin-screw extruder, a meat chopper, particularly a screw type extruder and the like.

Among them, a screw type extruder in which a perforated plate is installed at one end of a casing is preferable, and specific examples thereof include a screw type extruder disclosed in Japanese Patent Application Laid-Open No. 2000-63527 and WO2011 / 126079. Be done.

(Gel crushing area)
In the present invention, the gel crushing is performed during and / or after the polymerization step, more preferably for the hydrogel-like polymer after the polymerization step, but the gel crushing is performed during the polymerization such as kneader polymerization. In the case of, the gel crushing step is performed when the monomer aqueous solution is "sufficiently gelled".

For example, when kneader polymerization is adopted, the monomer aqueous solution changes to a hydrogel-like polymer with the lapse of the polymerization time. That is, the stirring region of the monomer aqueous solution at the start of polymerization, the stirring region of the low polymerization degree water-containing gel-like polymer having a constant viscosity during the polymerization, and the gel of a part of the water-containing gel-like polymer as the polymerization progresses. The pulverization start region and the gel pulverization region in the latter half or the final stage of the polymerization are continuously performed. Therefore, in order to clearly distinguish between "stirring of the monomer aqueous solution" at the start of polymerization and "gel pulverization" at the end of the polymerization, the judgment is made based on the "sufficiently gelled" state.

The above-mentioned "sufficient gelation" refers to a state in which the hydrogel-like polymer can be subdivided by applying a shearing force after the time when the polymerization temperature reaches the maximum (polymerization peak temperature). Alternatively, the polymerization rate of the monomer in the aqueous monomer solution (also known as conversion rate. The polymerization rate is calculated from the amount of polymer calculated from the pH titration of the hydrogel polymer and the amount of residual monomer) is preferable. A state in which the hydrogel polymer can be subdivided by applying a shearing force after the time when the content is 90 mol% or more, more preferably 93 mol% or more, further preferably 95 mol% or more, and particularly preferably 97 mol% or more. It means that. That is, in the gel pulverization step of the present invention, the hydrogel-like polymer in which the polymerization rate of the monomer is in 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 rising, etc.), the polymerization rate of the above-mentioned monomer is used to perform "sufficient gelation". Prescribe.

When the polymerization step is belt polymerization, the hydrogel-like polymer during and / or after the polymerization step, preferably the water-containing gel-like polymer after the polymerization step, is about several tens of centimeters before gel pulverization. Can be cut or coarsely crushed to size. By this operation, it becomes easy to fill the gel crushing apparatus with the hydrogel-like polymer, and the gel crushing step can be carried out more smoothly. As the means for cutting or coarsely crushing, a means capable of cutting or coarsely crushing the hydrogel-like polymer without kneading is preferable, and examples thereof include a guillotine cutter and the like. Further, the size and shape of the hydrogel-like polymer obtained by the above-mentioned cutting or coarse crushing is not particularly limited as long as it can be filled in the gel crushing apparatus.

(Use of water)
In the gel crushing step of the present invention, water can be added to the hydrogel-like polymer to crush the gel. In the present invention, "water" takes any form of solid, liquid, or gas.

Regarding the addition of the above water, there is no limitation on the addition method and the addition timing, and water may be supplied into the apparatus while the hydrogel-like polymer stays in the gel crushing apparatus. Further, water may be added in advance to the hydrogel-like polymer and charged into the gel crushing apparatus. Further, the water is not limited to "water alone", and other additives (for example, surfactants, neutralizing bases, cross-linking agents, etc.) and solvents other than water may be added. However, in this case, the water content is preferably 90 to 100% by weight, more preferably 99 to 100% by weight, and even more preferably 100% by weight.

In the present invention, the water can be used in any form of solid, liquid or gas, but from the viewpoint of handleability, liquid and / or gas is preferable. The amount of water supplied is preferably 0 to 25 parts by weight, 0 to 15 parts by weight, 0 to 10 parts by weight, 0 to 4 parts by weight, and 0 to 2 parts by weight with respect to 100 parts by weight of the hydrogel polymer. Is more preferable in this order. When the amount of water supplied exceeds 25 parts by weight, it becomes difficult to control the particle size of the hydrogel polymer, the drying time becomes long and the residual amount of the chelating agent decreases, or the undried product is dried. There is a risk of problems such as generation. Further, from the viewpoint of drying efficiency, it is preferable that the amount of water supplied in this step does not exceed the amount of the solvent evaporated in the polymerization step, particularly the amount of evaporated water.

When the water is supplied as a liquid, the temperature at the time of supply is preferably 10 to 100 ° C, more preferably 40 to 100 ° C. When water is supplied as a gas, the temperature at the time of supply is preferably 100 to 220 ° C, more preferably 100 to 160 ° C, and even more preferably 100 to 130 ° C. When water is supplied as a gas, the preparation method is not particularly limited. For example, a method using steam generated by heating a boiler, a method of vibrating water with an ultrasonic wave, and a gaseous state generated from the water surface. The method of using the water of the above is mentioned. In the present invention, when water is supplied as a gas, steam having a pressure higher than that of 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-like polymer and pulverize the gel. However, in addition to water, other additives, neutralizers and the like are added to and kneaded with the hydrogel polymer to pulverize the gel. The water-absorbent resin thus obtained may be modified. Specifically, at the time of gel pulverization, an aqueous solution containing the basic substance described in (2-1) above (for example, a 10 to 50% by weight sodium hydroxide aqueous solution) is added for neutralization (particularly the above-mentioned neutralization). It may be (within the range of the ratio), or water-absorbent resin fine powder (0.1 to 30% by weight (relative to resin solid content)) may be added to recycle the fine powder. Further, the polymerization initiator may be added and mixed in an amount of 0.001 to 3% by weight (relative to the resin solid content) at the time of gel pulverization to reduce the residual monomer.

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

The hydrogel polymer obtained in the above polymerization step is pulverized into particles by using a gel crusher (kneader, meat chopper, screw type extruder, etc.) to which the above-mentioned 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) (specified 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. Further, 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 in this order. The lower limit of the weight average particle diameter is more preferably 100 μm or more and 200 μm or more in this order. In the present invention, any combination of the upper limit value and the lower limit value of the weight average particle diameter is preferable. The gel particle size can be measured by the method described in WO2011 / 126079.

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

In order to gel crush or polymerize the water-containing gel polymer to a weight average particle size of less than 10 μm, it is difficult to achieve it only by a normal gel crushing operation or a polymerization operation. Gel classification (for example, Japanese Patent Application Laid-Open No. 6-107800) and particle size control during polymerization before gel pulverization (for example, a method for obtaining gel particles having a sharp particle size by reverse phase suspension polymerization; European Patent No. 1 0349240 etc.) is required. Therefore, in addition to gel crushing, the addition of the special method as described above requires a large amount of surfactant or organic solvent for polymerization and classification, and reduces productivity (cost increase) and deterioration of physical properties (deterioration of physical properties). It may lead to an increase in residual monomers and an increase in fine powder), which may cause new problems. Therefore, 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 crushing
In the present invention, the gel CRC of the particulate hydrogel polymer after gel pulverization is preferably 10 to 35 g / g, more preferably 10 to 32 g / g, still more preferably 15 to 30 g / g. The gel CRC after gel crushing is preferably -1 to +3 g / g, more preferably 0.1 to 2 g / g, and 0.3 to 1.5 g with respect to the gel CRC before gel crushing. / G is more preferred. The gel CRC may be reduced by using a cross-linking agent or the like during gel pulverization, but it is preferable to increase the gel CRC within the above range.

(C) Gel Ext after gel crushing
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 more preferably 0.1 to 8% by weight. More preferably, 0.1 to 5% by weight is particularly preferable. Further, the gel ext increase amount (increase amount with respect to the gel ext before gel crushing) of the particulate hydrogel polymer after gel crushing is preferably 5% by weight or less, more preferably 4% by weight or less, and 3% by weight. The following is more preferable, 2% by weight or less is particularly preferable, and 1% by weight or less is most preferable. The lower limit may be negative (for example, −3.0% by weight, further −1.0% by weight), but is usually 0% by weight or more, preferably 0.1% by weight or more, and more preferably 0% by weight. It is 2% by weight or more, more preferably 0.3% by weight or more. Specifically, the gel Ext is increased so as to be within an arbitrary range of the above-mentioned upper limit value and lower limit value, such as preferably 0 to 5.0% by weight, more preferably 0.1 to 3.0% by weight, and the like. The gel may be crushed until it is crushed. The gel Ext may be decreased by using a cross-linking agent or the like during gel pulverization, but it is preferable to increase the gel Ext within the above range. Here, the significant figure of the gel Ext increase amount is one digit after the decimal point, but for example, 5% by weight and 5.0% by weight are treated as synonyms.

(D) Weight average molecular weight of water-soluble content after gel crushing In the present invention, the lower limit of the increase in the weight average molecular weight of the water-soluble content of the hydrogel-like polymer after gel crushing is 10,000 Da. The above is preferable, 20,000 Da or more is more preferable, and 30,000 Da or more is further preferable. 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 amount of increase in the weight average molecular weight of the water-soluble component of the particulate hydrogel polymer after gel pulverization with respect 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 crushing In the present invention, the resin solid content of the particulate hydrogel polymer after gel crushing is preferably 40 to 75% by mass, more preferably. It is 45 to 70% by mass, more preferably 50 to 65% by mass. By setting the resin solid content of the particulate hydrogel polymer after gel crushing within the above range, it is easy to control the increase in CRC due to drying, and the damage due to drying (increase in water-soluble content, etc.) is small. Further, it is preferable because the decomposition of the chelating agent is reduced and the residual rate is improved. The resin solid content after gel pulverization can be appropriately controlled by the resin solid content before gel pulverization, water added if necessary, and water evaporation due to heating during gel pulverization.

(Number of measurement points)
In order to evaluate the physical properties of the hydrogel polymer before gel crushing or the particulate hydrogel polymer after gel pulverization, it is necessary to perform sampling and measurement from a manufacturing apparatus at a required amount and frequency. In the present invention, the evaluation is performed based on the weight average molecular weight of the water-soluble component of the hydrogel-like polymer before gel pulverization, but it is necessary to make this value 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 crushing with a continuous kneader, a meat chopper, or the like, two or more points per 100 kg of the hydrogel polymer. A total of at least 10 points may be sampled and measured, and in the case of batch gel grinding (for example, batch kneader), at least 10 points or more may be sampled and measured from the batch sample to contain particulate water. The physical properties of the gel polymer may be evaluated.

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

In the present invention, the particulate hydrogel containing the chelating agent is dried. It has been found that in the method for producing a water-absorbent resin containing a chelating agent, the chelating agent does not substantially decrease at the time of polymerization or after the completion of the drying step, and the chelating agent is decomposed by the remaining persulfate during the drying step. rice field. Therefore, in order to contain the chelating agent inside the water-absorbent resin, it is preferable to control the amount of the persulfate remaining in the drying step to a low level. Further, in order to suppress the decomposition of the chelating agent, rapid drying in the drying step is preferable. Therefore, in the present invention, the particle size of the hydrogel-like polymer is small, specifically, the weight average particle size (D50) of the particle-like hydrogel-like polymer is controlled to 1 mm or less, and the drying time is 20 minutes or less. Therefore, it is particularly preferable to dry the mixture until the solid content reaches 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 rate of the chelating agent is improved, 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 value decreases.

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 with respect 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 is the case where it is not used at the time of 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 0.001 mol% or more, from the viewpoint of reducing the residual monomer at the time of drying. It is desirable that the amount is 0.01 mol% or more and is contained in the above-mentioned monomer aqueous solution or the above-mentioned hydrogel-like polymer. By the above method, the residual rate of the chelating agent is improved, 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 value decreases. In the present invention, any combination of the upper limit value and the lower limit value of the amount of persulfate added is preferable. The content of persulfate in the hydrogel polymer can be measured by the method described in WO2007 / 116778.

Further, in the method 2 according to the present invention, the content of the chelating agent of the particulate hydrogel polymer subjected to the drying step (solid content of the hydrogel polymer) is 10 ppm or more, and is 40 ppm. As mentioned above, 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 are preferable in this order. Further, from the viewpoint of the effect 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 with respect to the solid content of the hydrogel polymer. , 6000 ppm or less, preferably 5000 ppm or less, in that order. In the present invention, any combination of the upper limit value and the lower limit value of the chelating agent content is preferable. Further, the content of the chelating agent indicates the actual weight ppm concentration when added as a chelating agent without considering salt exchange between the monomer after mixing and the carboxy group of the polymer.

(Drying device)
As the drying apparatus 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 the drying speed. That is, hot air drying is preferable as the drying type. The hot air dryer includes a ventilation belt (band) type, a ventilation circuit type, a ventilation vertical 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, etc. A hot air dryer can be mentioned. In the present invention, a ventilation belt type hot air dryer is preferable from the viewpoint of controlling physical properties.

When the ventilation belt type hot air dryer is used, the wind direction of the hot air used in the dryer is water-containing laminated on the ventilation belt and allowed to stand, from the viewpoint of physical properties such as drying uniformity and liquid permeability. It is preferable that the direction is perpendicular to the gel-like polymer layer (for example, the combined use in the vertical direction, or the upward direction or the downward direction). The above "vertical direction" is above and below (from the top to the bottom of the gel layer) with respect to the gel layer (a particulate water-containing gel-like polymer having a thickness of 10 to 300 mm laminated on a punching metal or a metal mesh). , Or the state of ventilation from the bottom to the top of the gel layer), and is not limited to the strict vertical direction as long as the gel layer is ventilated in the vertical direction. For example, hot air in an oblique direction may be used, in which case it is within 30 ° with respect to the vertical direction, preferably within 20 °, more preferably within 10 °, still more preferably within 5 °, and particularly preferably within 0 °. ° hot air is used.

Hereinafter, the drying conditions and the like in the drying step of the present invention will be described. By performing drying under the following drying conditions, the liquid permeability and water absorption rate of the water-absorbent resin obtained by surface-treating the obtained dried polymer can be improved.

(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. The drying temperature is 200 ° C. or lower, preferably 190 ° C. or lower, and more preferably 180 ° C. or lower. In the present invention, any combination of the upper limit value and the lower limit value of the drying temperature is preferable. If the drying temperature is less than 80 ° C., the drying time until a suitable resin solid content (moisture content) is obtained becomes long, and the decomposition rate of the chelating agent increases, which is not preferable. In addition, undried matter is produced, and clogging may occur during the subsequent pulverization step. If 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 heat medium used for drying in the case of direct heating, refers to the temperature of the 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 20 minutes or less, preferably 18 minutes or less, 15 minutes or less, and 12 minutes or less in that order. The lower limit of the drying time is about 1 minute in consideration of the drying efficiency. It was also found that the decomposition of the chelating agent mainly occurs in the drying stage until the solid 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 produced, which may cause clogging during the subsequent grinding step. If the total drying time exceeds 60 minutes, the decomposition rate of the chelating agent increases, which is not preferable. In the present invention, even if the evaporation of water has not substantially started, that is, the step of heating the particulate water-containing gel polymer in a dryer to raise the temperature is also included in the drying step. Will be. The start of the drying time is when the hydrogel-like polymer is placed in the dryer, and the total drying time is the total drying time after the water-containing gel-like polymer is placed in the dryer, and then the hydrogel-like polymer is dried and dried. It is the time until it is taken out of the machine.

Further, the time until the hydrogel-like polymer discharged from the polymerization step is introduced into the drying step, and the temperature of the hydrogel-like polymer, that is, the hydrogel-like polymer from the polymer outlet to the dryer inlet. The moving time and the temperature of the hydrogel polymer when moved to the inlet of the dryer are specifically 30 ° C. or lower from the viewpoint of preventing coloration of the water-absorbent resin and improving the residual rate of the chelating agent. The combination of within 1 minute, within 20 minutes at 90 ° C. or lower, within 10 minutes at 100 ° C. or lower, and within 5 minutes at 110 ° C. is preferable. At the laboratory level, the hydrogel-like polymer obtained in the polymerization step can be once cooled to 40 ° C. or lower, left to stand for a long time, and then introduced into the drying step.

(Resin solid content)
The particulate hydrogel obtained in the gel crushing step is dried in the drying step described above to obtain a dry polymer. The resin solid content obtained from the drying weight loss 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 wind speed of hot air in the airflow dryer, particularly the belt type dryer, is preferably 0.8 to 2.5 m / s in the vertical direction (vertical direction), preferably 1.0. ~ 2.0 m / s is more preferable. By setting the wind speed in the above range, it becomes easy to control the water content of the obtained dry polymer in a desired range. If the wind speed exceeds 2.5 m / s, problems such as the soaring of particulate water-containing gel-like polymer during the drying period may occur.

The wind speed may be controlled within a range that does not impair the effect of the present invention. For example, the wind speed may be controlled within a range of 70% or more, preferably 90% or more, and more preferably 95% or more of the drying time. Further, the wind speed is expressed by the average flow velocity of hot air passing in the direction perpendicular to the band surface that moves horizontally, taking a ventilation belt type dryer as an example. Therefore, the average flow velocity of 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 steam and has a dew point of 30 to 100 ° C, more preferably 30 to 80 ° C. By controlling the dew point of the hot air and more preferably the gel particle diameter within the above range, the residual monomer can be reduced, and further, the decrease in the bulk specific density of the dried polymer can be prevented. The dew point is a value at a time when the water content of the particulate water-containing gel 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 monomer, water absorption performance, color prevention, etc., from the dew point near the dryer outlet (or at the end of drying; for example, after 50% of the drying time), near the dryer inlet. It is preferable that the dew point is high (or in the early stage of drying; for example, before 50% of the drying time). Specifically, it is preferable to bring hot air having a dew point preferably 10 to 50 ° C., more preferably 15 to 40 ° C., into contact with the particulate hydrogel polymer. By controlling the dew point within the above range, it is possible to prevent a decrease in the bulk specific density of the dried polymer.

In the drying step of the present invention, when the particulate hydrogel polymer is dried, the particulate hydrogel polymer is continuously supplied on the belt of the ventilation belt type dryer in a layered manner. , 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, and more preferably 1 m or more. 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, 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 problem 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 drive device, durability, etc. Therefore, 0.3 to 5 m / min is preferable, 0.5 to 2.5 m / min is more preferable, 0.5 to 2 m / min is further preferable, and 0.7 to 1.5 m / min is particularly preferable.

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

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

The crusher that can be used in the crushing process is not particularly limited, but is, for example, a vibration mill, a roll granulator, a knuckle type crusher, a roll mill, a high-speed rotary crusher (pin mill, hammer mill, screw mill), and a cylindrical shape. A mixer and the like can be mentioned. Among these, from the viewpoint of particle size control, it is preferable to use a multi-stage roll mill or roll granulator.

In this classification step, the classification operation is performed so as to have the following particle size, but when 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. Also, a classification operation (second classification step) may be carried out. Further, although the first classification step is usually carried out after the crushing step, a further classification step may be carried out before the crushing step. The weight average particle diameter (D50) of the water-absorbent resin particles after classification is not particularly limited, and can be appropriately adjusted according to the intended use. For example, when used as a sanitary material, the weight average particle diameter (D50) of the water-absorbent resin particles after classification is preferably 200 to 800 μm, more preferably 200 to 600 μm, and even more preferably 300 to 500 μm. 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 further preferably 97% by weight or more. The water-absorbent resin as a final product is also preferably in the form of particles, and the above range of 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 by 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 cross-linking and the heating process thereof.

(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, but an organic surface cross-linking agent can be preferably used. In terms of physical properties, the surface cross-linking agent is preferably a polyhydric alcohol compound, an epoxy compound, a polyvalent amine compound or a condensate with a haloepoxy compound thereof, an oxazoline compound, a (mono, di, or poly) oxazolidinone compound, or an alkylene carbonate compound. Therefore, a dehydration-reactive cross-linking agent composed of a polyhydric alcohol compound, an alkylene carbonate compound, and an oxazolidinone compound, which requires a reaction at a particularly high temperature, can be used. When the dehydration-reactive cross-linking agent is not used, more specifically, the compounds exemplified in US Pat. Nos. 6,228,930, 6071976, 6254990 and the like can be mentioned. Examples of the compound include mono, di, tri, tetra or propylene glycol, 1,3-propanediol, glycerin, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1, 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 cabonate; oxetane compounds; cyclic urea compounds such as 2-imidazolidinone Can be mentioned.

(Solvent, etc.)
The amount of the surface cross-linking agent used is appropriately determined to be about 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. Water is preferably used in combination with the surface cross-linking agent. The amount of water used is preferably in the range of 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. Even when the inorganic surface cross-linking agent and the organic surface cross-linking agent are used in combination, the combined use is preferably 0.001 to 10 parts by weight, more preferably 0.01 to 5 parts by weight, respectively, with respect to 100 parts by weight of the water-absorbent resin particles. Will be done.

Further, at this time, a hydrophilic organic solvent may be used, and the amount thereof is preferably in the range of 0 to 10 parts by weight, more preferably 0 to 5 parts by weight, based on 100 parts by weight of the water-absorbent resin particles. Is. Further, when mixing the cross-linking agent solution with the water-absorbent resin particles, a range that does not interfere with the effect of the present invention, for example, preferably 0 to 10 parts by weight, more preferably 0 to 5 parts by weight, still more preferably 0-1. Water-insoluble fine particle powder or a surfactant may coexist in parts by weight. The surfactant used and the amount used thereof are exemplified in US Pat. No. 7,473,739 and the like.

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

Further, for mixing the surface cross-linking agent, a vertical or horizontal high-speed rotary stirring type mixer is preferably used. The rotation speed of the mixer is preferably 100 to 10000 rpm, preferably 300 to 2000 rpm. The residence time is preferably 180 seconds or less, more preferably 0.1 to 60 seconds, still more 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 Patent No. 4783510, International Publication No. 2006/062258) instead of the surface cross-linking using the above-mentioned surface cross-linking agent, and water absorption. A surface cross-linking method for polymerizing a monomer on the surface of a sex resin (US Application Publication No. 2005/048221, 2009/0239966, International Publication No. 2009/048160) may be used.

In the above surface cross-linking method, the radical polymerization initiator preferably used is persulfate, and optionally, the monomer preferably used includes acrylic acid (salt) and the above-mentioned surface cross-linking agent, and water is preferably used as the solvent. After these are added to the surface of the water-absorbent resin, surface cross-linking is carried out by performing 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 (particularly ultraviolet rays) or heating. Will be.

(Ionic-bonding surface cross-linking agent)
The present invention may further include an addition step of adding any one or more of the polyvalent metal salt, the cationic polymer or the inorganic fine particles at the same time as or separately from the above-mentioned surface cross-linking 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 liquid permeability, water absorption rate and the like. The inorganic surface cross-linking agent can be used simultaneously with or separately from the above-mentioned organic surface cross-linking agent. Examples of the inorganic surface cross-linking agent used include salts (organic salts or inorganic salts) or hydroxides of divalent or higher, preferably trivalent or tetravalent polyvalent metals. Examples of the multivalent metal that can be used include aluminum and zirconium, and specific examples thereof include aluminum lactate and aluminum sulfate. An aqueous solution containing aluminum sulfate is preferable.

(2-6) Other processes (fine powder recycling process, etc.)
In addition to the above steps, if necessary, an evaporative monomer recycling step, a granulation step, a fine powder removing step, a fine powder recycling step, etc. may be provided, and each of the above steps is for the purpose of stabilizing the color tone over time and preventing gel deterioration. The following additives may be used for any part or all of the above, if necessary. That is, a water-soluble or water-insoluble polymer, a lubricant, a deodorant, an antibacterial agent, water, a surfactant, a water-insoluble fine particle, an antioxidant, a reducing agent and the like are preferably applied to the water-absorbent resin from 0 to 30. A% by weight, more preferably 0.01 to 10% by weight, can be added and mixed. These additives can also be used as surface treatment agents.

In addition, the production method of the present invention can include a fine powder recycling step. The fine powder recycling step is the fine powder (particularly fine powder containing 70% by weight or more of powder having a particle diameter of 150 μm or less) generated in the drying step and, if necessary, the crushing step and the classification step, and then as it is or in water. In addition, it refers to a step of recycling into a polymerization step and a drying step, and the methods described in US Patent Application Publication No. 2006/247351 and US Patent No. 6228930 can be applied.

Furthermore, depending on the purpose, oxidants, antioxidants, water, polyvalent metal compounds, water-insoluble inorganic or organic powders such as silica and metal soap, deodorants, antibacterial agents, polymer polyamines, pulps and thermoplastic fibers Etc. 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-absorbent resin The present invention provides a water-absorbent resin containing a chelating agent, which is obtained by the above-mentioned production method according to the present invention. Hereinafter, various physical properties of the water-absorbent resin containing the chelating agent according to the present invention will be described.

The water-absorbent 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-absorbent resin.

(Residual amount (C1), content (C2), residual rate of chelating agent of water-absorbent resin)
The residual amount (C1) of the chelating agent of the water-absorbent resin does not consider the salt exchange between the monomer after mixing and the carboxy group of the polymer, and the neutralized salt-type chelating agent is salt-type and acid-type chelate. The agent shows the residual amount in the form when it is added as an acid type. On the other hand, the content (C2) of the chelating agent of the water-absorbent resin represents the content as an acid-type chelating agent. That is, when the chelating agent is an acid type, it is the same as the residual amount (C1) of the chelating agent, and when the chelating agent is a salt type, the concentration is calculated as the acid type of the same chelating agent. Specifically, the content of the chelating agent (C2) is calculated by "residual amount of chelating agent (C1) x acid type molecular weight / molecular weight as it is when the chelating agent is added".

The residual rate (%) of the chelating agent is calculated by "{(residual amount of chelating agent (C1) [ppm]) / (addition amount of chelating agent [ppm])} x 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 color prevention and deterioration prevention. As mentioned above, 500 ppm or more and 600 ppm or more are preferable in this order. Further, the upper limit of the residual amount (C1) and the content (C2) of the chelating agent in the water-absorbent resin according to the present invention is 1% by weight from the viewpoint of the effect (coloring prevention, deterioration prevention, etc.) and cost of the chelating agent. Hereinafter, it is preferable in the order of 8000 ppm or less, 6000 ppm or less, and 5000 ppm or less.

Further, in the water-absorbent resin according to the present invention, the residual rate of the chelating agent is preferably 50% or more, more preferably 60% or more.

The residual ratio of the chelating agent is the water-absorbent resin with respect to the amount of the chelating agent added [ppm] (solid content of the monomer or the water-containing gel-like polymer at the time of polymerization) in the manufacturing process of the water-absorbent resin. The ratio [%] of the residual amount (C1) [ppm] of the chelating agent. The residual rate of the chelating agent is the chelating of the final product with respect to the sum of the content of the chelating agent in the final product and the amount of the decomposition product derived from the chelating agent (ppm) converted into the content of the chelating agent. It may be obtained from the ratio of the content of the agent.

(Initial color tone of water-absorbent resin)
The water-absorbent resin according to the present invention is a water-absorbent 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 at least 85, and further 89 or more, in the initial color tone (also referred to as “initial coloring”) in the Munsell Lab color system measurement by a spectroscopic color difference meter. , It is preferable to show 90 or more. The upper limit of the L value is usually 100, but if it is 85 for powder, no problem due to color tone occurs in products such as sanitary materials.

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

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

In one aspect of the present invention, in the above-mentioned method for producing a water-absorbent resin containing a chelating agent according to the present invention, the amount of persulfate in the polymerization initiator used in the above-mentioned polymerization step is 0 to 0.04 mol%. (Polymer at the time of polymerization) (However, in the case of 0 mol% persulfate (not used), another polymerization initiator is indispensably used), and the chelating agent is used in the step before the drying step. A total of 10 ppm or more (monomer at the time of polymerization or solid content of the water-containing gel-like polymer) is added to the aqueous solution of the weight or the water-containing gel-like polymer, and the weight average particle diameter of the particulate water-containing gel-like polymer is added. In the above-mentioned drying step, the residual amount (C1) of the chelating agent is 10 ppm as a final product by a production method in which (D50) is 1 mm or less and the drying time until the solid content becomes 80% by weight or more is 20 minutes or less. With the above, it is possible to produce a water-absorbent resin having an initial color tone L value of 85 or more and a YI value of 13 or less.

Further, in one aspect of the present invention, in the above-mentioned method for producing a water-absorbent resin containing a chelating agent according to the present invention, the amount of persulfate in the polymerization initiator used in the above-mentioned polymerization step is 0 to 0.015 mol. % (Polymer for polymerization) (However, in the case of 0 mol% persulfate (not used), another polymerization initiator is indispensably used), and the chelating agent is used in the steps prior to the drying step. A total of 200 ppm or more (solid content of the monomer or the water-containing gel-like polymer at the time of polymerization) is added to the monomer aqueous solution or the water-containing gel-like polymer, and the weight average particles of the particulate water-containing gel-like polymer are added. By the production method in which the diameter (D50) is 1 mm or less and the drying time until the solid content becomes 80% by weight or more is 20 minutes or less in the above drying step, the residual amount (C1) of the chelating agent is reduced as a final product. It is possible to produce a water-absorbent resin having an L value of 200 ppm or more, an initial color tone of 89 or more, and a YI value of 10 or less.

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

Further, in one aspect of the present invention, 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. 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 it is preferably in an amorphous crushed form. The amorphous crushed form is a particle shape whose shape is not constant, which is obtained by crushing a hydrogel-like polymer or a dry polymer.

(Particle diameter of water-absorbent resin)
The weight average particle size (D50) of the water-absorbent resin of the present invention is preferably 200 to 800 μm, more preferably 200 to 600 μm, and even more preferably 300 to 500 μm. 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 further preferably 97% by weight or more. Since the water-absorbent resin of the present invention has the above particle size, it is easy to handle and easily exhibits water-absorbing performance as a sanitary material or the like.

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

When the CRC is less than 15 g / g, the crosslink density of the water-absorbent resin is high, so that the effect of preventing deterioration by the chelating agent is unlikely to appear. Further, it is not preferable to use the water-absorbent resin as a sanitary material such as a disposable diaper because the water-absorbing efficiency deteriorates.

(Water absorption ratio under pressure of water-absorbent resin)
The water absorption ratio under pressure (AAP (0.7 psi)) of the water-absorbent resin according to the present invention is preferably controlled to 15 g / g or more, more preferably 20 g / g or more, still more 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, still more preferably, from the viewpoint of balance with other physical properties (for example, unpressurized water absorption ratio (CRC)). Is 33 g / g or less. AAP (0.7 psi) can be controlled by the cross-linking density at the time of surface cross-linking.

(Ratio of water absorption ratio under pressure and water absorption ratio under no pressure)
The ratio (AAP (0.7psi) / CRC) between the water absorption ratio under pressure and the water absorption ratio under no pressure of the water-absorbent resin according to the present invention is 0.5 or more, 0.6 or more, 0.7 or more, 0. It is preferable in the order of 8 or more and 0.9 or more. The upper limit of AAP (0.7 psi) / CRC is about 1.5 or less, 1.2 or less, and 1.0 or less. While CRC removes the crevice water between gels after swelling by centrifugation, AAP (0.7 psi) has a water absorption magnification including crevice water, so there are cases where AAP (0.7 psi) exceeds CRC. However, the upper limit is usually in the above range.

[4] Uses 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 is preferably used for absorbent articles such as disposable diapers, menstrual napkins, and incontinence pads. The water-absorbent resin obtained by the production method according to the present invention includes high-concentration diapers (paper diapers that use a large amount of water-absorbent resin per sheet of paper diapers), which have had problems such as odor and coloring derived from raw materials. In particular, it exhibits excellent performance when used in the upper layer of the absorbent body of the above-mentioned absorbent article. The proportion of the water-absorbent resin contained in the absorbent body of the disposable diaper is preferably 50% by weight or more, more preferably 60% by weight or more, 70% by weight or more, 80% by weight or more, and 90% by weight or more in this order.

[5] Differences from the prior art As described above, in the drying step of the hydrogel polymer obtained by polymerizing the monomer, the present inventors added the chelating agent in the step prior to the drying step. Was found to decrease specifically. Then, the present inventors have found that the cause of the decrease in the chelating agent in the drying step is the polymerization initiator (particularly persulfate) remaining in the hydrogel-like polymer. In order to solve the problem, the present invention controls the persulfate in the polymerization initiator during polymerization or before drying (0.04 mol% or less), and further controls the gel particle size and drying conditions before drying. It is characterized by suppressing the decomposition of the chelating agent in the drying process.

The above-mentioned Patent Documents 1 to 23 describe the use of a chelating agent in the manufacturing process of a water-absorbent resin. Persulfate is the most versatile as a polymerization initiator of a water-absorbent resin, and Patent Documents 1 to 23 also describe persulfate as a polymerization initiator. However, the above-mentioned Patent Documents 1 to 23 do not describe the problems pointed out in the present application and the means for solving the problems, let alone the decomposition of the chelating agent by the 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, "the radical of the persulfate is preferentially given to the monomer at the time of polymerization. Although it reacts, the amount of monomer (residual monomer) remaining in the hydrogel after polymerization is as small as a few percent or less, and because the temperature is higher and the solid content is higher during drying than during polymerization, polymerization is carried out in the drying step. The residual persulfate later reacts with the chelating agent. "

The means for solving the problem will be described in more detail in Examples described later. In the present invention, 0 to 0.04 mol% (monomer at the time of polymerization) (0 mol%) of the persulfate in the polymerization initiator is not used at the time of polymerization or is completely consumed before the drying step. By controlling the gel particle size and drying conditions before drying, the decomposition of the chelating agent in the drying step can be suppressed, so that the present invention is not limited to the examples described later. The problem can be solved.

Unless otherwise specified, the claims of the present invention and the various physical properties described in the examples were determined according to the EDANA method and the following measurement method under the conditions of room temperature (20 to 25 ° C.) and humidity of 50 RH%. Further, the electrical equipment presented in Examples and Comparative Examples used a 200 V or 100 V, 60 Hz power supply. For convenience, "liter" may be referred to as "L" and "weight%" may be referred to as "wt%".

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

That is, 20 g of a hydrogel polymer (solid content; α% by weight) having a temperature of 20 to 25 ° C. and a 20% by weight sodium chloride aqueous solution containing 0.08% by weight Emar 20C (surfactant, manufactured by Kao Co., Ltd.) ( Hereinafter, it was added to 500 g (referred to as "Emar aqueous solution") to prepare a dispersion liquid, and stirred at 300 rpm for 60 minutes using a stirrer chip having a length of 50 mm and a diameter of 7 mm (made of a cylindrical polypropylene having a height of 21 cm and a diameter of 8 cm). 1.14L container is used).

After stirring, the center of the 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) installed on the turntable. The above dispersion was put into the portion. After washing out the total hydrogel polymer on the sieve using 100 g of the Emar aqueous solution, shower (72 holes) from a height of 30 cm while rotating the sieve by hand (20 rpm) with 6000 g of the Emar aqueous solution from the top. , Liquid volume; 6.0 [L / min]) was poured evenly so that the water injection range (50 cm ² ) was spread over the entire sieve, and the hydrogel polymer was classified. The water-containing gel-like polymer on the first-stage sieve that had been classified was drained for about 2 minutes and then weighed. The second and subsequent sieves were also classified by the same operation, and the hydrogel polymer remaining on each sieve after draining was weighed.

From the weight of the hydrogel polymer remaining on each sieve, the weight% ratio was calculated from the following formula (1). The mesh size of the sieve after draining was plotted on logarithmic probability paper according to the following formula (2). From this graph, the particle size corresponding to the residual percentage of 50% by weight was read as the weight average particle size (D50) of the hydrogel polymer.
X [%] = (w / W) × 100 ・ ・ ・ Equation (1)
R (α) [mm] = (20 / w) ^1/3 × r ・ ・ ・ Equation (2)
here,
X; Weight% of hydrogel remaining on each sieve after classification and draining [%]
w; Weight of each hydrogel remaining on each sieve after classification and draining [g]
W; Total weight of hydrogel remaining on each sieve after classification and draining [g]
R (α); Sieve opening when converted to a hydrogel containing α% by weight of solid content [mm]
r; Sieve opening in which the hydrogel swollen in a 20 wt% sodium chloride aqueous solution is classified [mm]
Is.

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

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

Then, the swollen gel of the water-absorbent resin was filtered using a filter paper (No. 2 / reserved particle size specified in JIS P 3801; 5 μm / manufactured by Toyo Filter Paper Co., Ltd.).

Then, the obtained filtrate is passed through an HPLC sample pretreatment filter (chromatographic disk 25A / aqueous type, pore size; 0.45 μm / manufactured by Kurashiki Spinning Co., Ltd.), and the filtrate is used by high performance liquid chromatography (HPLC). The content of the chelating agent in it was measured.

For the content of the chelating agent in the particulate water-absorbent resin, the dilution rate of the particulate water-absorbent resin with respect to physiological saline is taken into consideration using the calibration curve obtained by measuring a monomer standard solution having a known concentration as an external standard. I asked for it. The HPLC measurement conditions were appropriately changed depending on the type of chelating agent. Specifically, the following measurement condition 1 is followed for diethylenetriamine5 acetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), and nitrilo3acetic acid (NTA), and the following measurement condition 2 is for ethylenediaminetetra (methylenephosphonic acid) (EDTMP). According to each, quantification was performed.

Measurement condition 1:
<Eluent>; 0.4 mol / L myoban aqueous solution 0.3 ml, 0.1 N potassium hydroxide aqueous solution 450 ml, 0.4 mol / L tetra n-butylammonium hydroxide aqueous solution 3 ml, sulfuric acid 3 ml, ethylene glycol Mixed solution of 1.5 ml and 2550 ml of ion-exchanged water <column>; LichroCART 250-4 Superspher 100 RP-18e (4 μm) (manufactured by 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 sulfuric acid aqueous 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, it is a value corrected for the water content and is a value converted per 100 parts by weight of the water-absorbent resin solid content.

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

The content of the chelating agent (C2) is the same as the residual amount of the chelating agent (C1) when the chelating agent is in the acid type. If the chelating agent is of the salt type, calculate the concentration as the acid type of the same chelating agent. That is, the content of the chelating agent (C2) was calculated by "residual amount of the chelating agent (C1) x acid type molecular weight / molecular weight as it is when the chelating agent is added".

The residual rate (%) of the chelating agent was calculated by "{(residual amount of chelating agent [ppm]) / (addition amount of chelating agent [ppm])} x 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-Σ80COLOR MEASURING SYSTEM manufactured by Nippon Denshoku Industries Co., Ltd. As the setting condition for the measurement, reflection measurement is selected, and the attached powder / paste sample table having an inner diameter of 30 mm and a height of 12 mm is used, and the standard round white plate No. for powder / paste is used as a standard. 2 was used and a 30Φ floodlight pipe was used. The provided sample table was filled with about 5 g of water-absorbent resin. This filling was in a state of filling about 60% of the provided sample table. Under the conditions of room temperature (20 to 25 ° C.) and humidity of 50 RH%, the L value (Lightness: lightness index) and YI value (yellowness; yellowness index) in the hunter Lab color system on the surface of the spectroscopic color difference meter are used. 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% (manufactured by Espec Co., Ltd., product name: small environmental tester). , Form SH-641) was filled with a water-absorbent resin, and the paste sample table was exposed for 7 days. This exposure is a 7-day color promotion test. After the exposure, the L value (Lightness) and the YI value (yellowness) in the surface Hunter Lab color system were measured by the spectroscopic color difference meter. This measured value is defined as "coloring over time (70 ± 1 ° C., relative humidity 65 ± 1%, 7 days)". The higher the L value, the better, and the smaller the YI value, the lower the coloring and the closer to the substantially white color.

(D) Amount of persulfate relative to solids in hydrogel The amount of persulfate relative to solids in hydrogel was measured by the method described in WO2007 / 116778.

Specifically, in a polypropylene container with a lid having a capacity of 260 ml, 3 g of a hydrogel as a sample and 100 g of a 5 wt% sodium chloride aqueous solution (if the gel swells and cannot be stirred, the salt concentration or the amount of the aqueous solution is appropriately adjusted. ), Shaded at room temperature, and stirred at 500 rpm using a 25 mm stirrer coated with Teflon (registered trademark). After 2 hours, the solution was taken out and passed through a filter (GL Chromatography Disc, water system 25A, pore size 0.45 μm) manufactured by GL Sciences. 5.00 g of the solution was placed in a glass sample bottle with a screw cap (capacity 50 ml, diameter 35 mm, height about 80 mm). Immediately after that, 0.50 g of a 44 wt% potassium iodide aqueous solution was added, and the mixture was stirred at room temperature with light shielding. 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 wt% chloride). The absorbance of the aqueous solution (blank) obtained by adding 0.50 g of a 44 wt% potassium iodide aqueous solution to 5 g of the sodium aqueous solution was set to 0). Further, a 5 wt% sodium chloride aqueous solution containing 0 ppm (without addition) of persulfate, 5 ppm, 10 ppm, 15 ppm, and 20 ppm, respectively, was prepared, and the absorbance was determined by the above operation to prepare a calibration curve. Then, the amount of persulfate (ppm) in the hydrogel of the sample was calculated from the absorbance and the calibration curve of the obtained sample. Furthermore, the amount of persulfate (mol%) was determined by calculation.

As for the water-absorbent resin after drying, the amount of persulfate can be measured by the same method. Further, the measurement limit is appropriately determined depending on the amount of polymer, sensitivity and the like. In the case of the water-containing gel and the water-absorbent resin of the present invention, the detection limit is usually 0.5 ppm, and the value below the detection limit is expressed as N.D. (Non-Detactable).

(E) Solid content of water-absorbent resin The proportion of components that do not volatilize at 180 ° C. in the water-absorbent resin is expressed as solid content. The relationship between solid content and water content is as follows;
Solid content (% by mass) = 100-moisture content (% by mass)
The method for measuring the solid content is as follows.

About 1 g of the water-absorbent resin was weighed in an aluminum cup (mass W0) having a bottom surface diameter of about 5 cm (mass W1), and allowed to stand in a windless dryer at 180 ° C. for 3 hours to dry. The mass (W2) of the "aluminum cup + water-absorbent resin" after drying is measured, and the following formula solid content (mass%) = ((W2-W0) / W1) × 100
The solid content was determined by.

For the block-shaped dry polymer, 5 samples were obtained from various positions, crushed so that the particle size was 5 mm or less, and then measured, and the average value was adopted.

(F) Solid content of the water-containing gel-like polymer The solid content of the particulate water-containing gel-like polymer was measured by the same method as the above-mentioned method for measuring the “solid content of the water-absorbent resin”. However, the amount of the hydrogel-like 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 for Centrifuge Retention Capacity, and means a water absorption ratio under no pressure (hereinafter, may be referred to as a "water absorption ratio"). Specifically, 0.200 g of water-absorbent resin in a non-woven fabric bag is freely swollen in a large excess of 0.9 wt% sodium chloride aqueous solution for 30 minutes, and then drained with a centrifuge to absorb water. The magnification (unit; [g / g]) was measured.

(H) AAP (0.7 psi) (ERT442.2.2)
"AAP" is an abbreviation for Absorption Against Pressure and means the water absorption ratio under pressure. AAP (0.7 psi) is a load of 0.900 g of a water-absorbent resin against a 0.9 wt% sodium chloride aqueous solution at 4.83 kPa (0.7 psi, 49 [g / cm ² ]) for 1 hour. The water absorption ratio (unit: [g / g]) after swelling underneath was measured.

(I) Remaining monomer (ERT410.2-02)
The amount of dissolved monomer (unit: ppm) after adding 1.0 g of water-absorbent resin to 200 ml of a 0.9 wt% sodium chloride aqueous solution and stirring at 500 rpm for 1 hour using a 35 mm stirrer chip is determined by HPLC (high performance liquid chromatography). Chromatography) was used for measurement. The residual monomer of the hydrogel polymer was measured by changing the sample to 2 g and the stirring time to 3 hours, respectively, and the obtained measured value was converted into the weight per resin solid content of the hydrogel polymer. The value (unit: ppm) was used.

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

(Polymerization process)
Acrylic acid (288.24 g), polyethylene glycol diacrylate (number average molecular weight; 523) (1.67 g, 0.08 mol% with respect to the above acrylic acid) as an internal cross-linking agent, and a chelating agent 10. A solution (A) to which a weight% diethylenetriamine 5 acetate (DTPA) / 5 sodium aqueous solution (3.52 g, 1000 ppm with respect to a monomer) was added and a 48.5% by weight sodium hydroxide aqueous solution (240.82 g) were added. A solution (B) diluted with deionized water (281.58 g) whose temperature was 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 (C) was prepared by adding and mixing the solution (B) while stirring the solution (A) using a magnetic stirrer.

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

After that, stirring of the solution (C) was continued, and when the temperature of the solution (C) reached 95 ° C., a 10 wt% sodium persulfate aqueous solution (NaPS; 3) was added to the solution (C) as a polymerization initiator. 81 g, 0.04 mol% with respect to the acrylic acid, and the molar ratio of persulfate to the chelating agent; 2.3) were added, and the mixture was stirred for about 3 seconds to prepare a monomer aqueous solution (1). The neutralization rate of the aqueous monomer solution (1) was 73 mol%, and the monomer concentration was 43% by weight.

Next, the above-mentioned monomer aqueous solution (1) was poured into a stainless steel vat-shaped container in an open system to the atmosphere. The stainless steel bat-shaped container has a bottom surface size of 250 mm x 250 mm, a top surface size of 640 mm x 640 mm, a height of 50 mm, a trapezoidal central cross section, and a silicone sheet attached to the inner surface. there were. The stainless steel vat-shaped container was placed on a hot plate (NEO HOTPLATE H1-1000, manufactured by Inuchi Seieidou Co., Ltd.) heated to 100 ° C. and preheated.

After pouring the monomer aqueous solution (1) into the stainless steel vat-shaped container, the polymerization reaction started about 5 seconds later.

The polymerization reaction proceeded while the monomer aqueous solution (1) expanded and foamed upward while generating water vapor, and then the size was slightly larger than the bottom surface of the stainless steel bat-shaped container. The water-containing gel-like polymer (1) shrank and ended. The polymerization reaction (expansion and contraction) was completed within about 1 minute, and then the hydrogel polymer (1) was held in a stainless steel vat-shaped container for 3 minutes. The polymerization reaction (boiling polymerization) gave a hydrogel polymer (1) containing bubbles.

(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-CHOPPER TYPE: 12VR-400KSOX) having a porous plate having a pore diameter of 11 mm, Iizuka Kogyo Co., Ltd. The gel was pulverized while adding 50 g / min of deionized water whose temperature was adjusted to about 100 ° C. at the same time as the addition of the hydrogel polymer. The obtained gel was further put into a tabletop meat chopper and pulverized for the second time to obtain a particulate water-containing gel polymer (1). In the second and subsequent gel pulverizations, deionized water was not added.

The particulate hydrogel polymer (1) obtained by the above gel pulverization had a weight average particle diameter (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 out on a wire mesh having an opening of 300 μm (50 mesh) and placed in a hot air dryer.

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

(Crushing / classification process)
Subsequently, the dried polymer (1) was put into a roll mill (WML type roll crusher, manufactured by Inoguchi Giken Co., Ltd.) and crushed, and then using two types of JIS standard sieves with a mesh size of 850 μm and 150 μm. By classifying, an amorphous crushed water-absorbent resin (1) was obtained.

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

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

Then, the surface-crosslinked water-absorbent resin (1) was passed through a JIS standard sieve having an opening of 850 μm to obtain a final product, the water-absorbent resin (1). The obtained final product, the water-absorbent resin (1), 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 except that the amount of the polymerization initiator (10 wt% sodium persulfate aqueous solution) added in the polymerization step was 1.43 g (0.015 mol% with respect to acrylic acid) in Example 1 above. The water-absorbent resin (2) was produced in the same manner as above.

The particulate hydrogel polymer (2) obtained in the gel pulverization step had a weight average particle diameter (D50) of 942 μm. In addition, the amorphous crushed water-absorbent resin (2) had a CRC of 28.8 g / g.

Then, the surface was crosslinked in the same manner as in Example 1 to obtain the final product, the water-absorbent resin (2). The obtained final product, the water-absorbent resin (2), was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below. In addition, the difference between the YI value of the initial coloring and the YI value of the temporal coloring (YI value over time-initial YI value) was calculated. The calculation results are shown in Table 3 below.

[Example 3]
The gel particle size of Example 1 was changed (about 0.9 mm → about 0.5 mm);
In Example 1 above, in order to change the gel particle size, after the second gel crushing in the gel crushing step, the obtained gel was further put into a tabletop meat chopper to perform the third gel crushing. Made a water-absorbent resin (3) in the same manner as in Example 1.

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

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

[Example 4]
The type of the polymerization initiator of Example 1 was changed (NaPS → azo polymerization initiator);
4. In Example 1 above, the polymerization initiator is a 10 wt% 2,2'-azobis (2-methylpropionamidine) dihydrochloride aqueous solution (trade name; Wako Pure Chemical Industries, Ltd. V-50), which is an azo polymerization initiator. The 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% with respect to acrylic acid).

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

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

[Example 5]
The chelating agent of Example 1 was changed (DTPA → EDTMP);
Same as Example 1 except that the chelating agent was changed to 10 wt% ethylenediaminetetramethylenephosphonic acid (EDTMP) / 5 sodium aqueous solution (3.52 g, 1000 ppm with respect to the monomer) in Example 1 above. The water-absorbent resin (5) was produced.

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain the final product, the water-absorbent resin (5). The obtained final product, the 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 6]
The amount of the polymerization initiator added in Example 5 was reduced (NaPS = 0.04 mol% → 0.015 mol%);
Example 5 except that the amount of the polymerization initiator (10 wt% sodium persulfate aqueous solution) added in the polymerization step was 1.43 g (0.015 mol% with respect to acrylic acid) in Example 5 above. The water-absorbent resin (6) was produced in the same manner as above.

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

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

[Example 7]
The type of the polymerization initiator of Example 5 was changed (persulfate → azo polymerization initiator);
In Example 5 above, the polymerization initiator is 4.34 g (against acrylic acid) of a 10 wt% 2,2'-azobis (2-methylpropionamidine) dihydrochloride aqueous solution (V-50) which is an azo polymerization initiator. The water-absorbent resin (7) was produced in the same manner as in Example 5 except that it was changed to 0.04 mol%).

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, a water-absorbent resin (7). The obtained final product, the 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.

[Example 8]
The polymerization initiator of Example 5 was changed (persulfate → UV polymerization with UV polymerization initiator);
In Example 5, the polymerization initiator (10 wt% sodium persulfate aqueous solution) in the polymerization step was changed to a UV polymerization initiator, and UV polymerization was carried out with the addition amount being 0.04 mol%. The water-absorbent resin (8) was produced in the same manner as in Example 5.

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

(Polymerization process)
Acrylic acid (285.30 g), polyethylene glycol diacrylate (number average molecular weight; 523) (0.08 mol% with respect to the above acrylic acid) as an internal cross-linking agent, and 10 wt% ethylenediaminetetramethylenephosphone as a chelating agent. Acrylic acid (EDTMP), 5 sodium aqueous solution (3.52 g, 1000 ppm per monomer) and 10 wt% IRGACURE (registered trademark; BASF: 1-hydroxycyclohexylphenylketone) 184, which is a UV polymerization initiator. A solution (A) containing 3.27 g of an acrylic acid solution (0.04 mol% with respect to the above acrylic acid) and a 48.5 wt% sodium hydroxide aqueous solution (240.82 g) were adjusted to 50 ° C. The solution (B) diluted with deionized water (285.06 g) was prepared in a polypropylene container having a capacity of 1.5 L and an inner diameter of 80 mm, respectively.

The solution (C) was prepared by adding and mixing the solution (B) while stirring the solution (A) using a magnetic stirrer to prepare a monomer aqueous solution (8).

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

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

The stainless steel bat-shaped container has a bottom surface size of 250 mm x 250 mm, a top surface size of 640 mm x 640 mm, a height of 50 mm, a trapezoidal central cross section, and a silicone sheet attached to the inner surface. there were. The stainless steel vat-shaped container was placed on a hot plate (NEO HOTPLATE H1-1000, manufactured by Inuchi Seieidou Co., Ltd.) heated to 100 ° C. and preheated.

After pouring the aqueous monomer solution (8) into the stainless steel vat-shaped container, the polymerization reaction started about 7 seconds later.

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

(After the gel crushing process)
The steps after the gel crushing step were carried out in the same manner as in Example 1. As a result, the water-absorbent resin (8) was produced.

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a water-absorbent resin (8) as a final product. The obtained final product, the water-absorbent resin (8), 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 except that the amount of the polymerization initiator (10 wt% sodium persulfate aqueous solution) added in the polymerization step was 4.8 g (0.05 mol% with respect to acrylic acid) in Example 1 above. In the same manner as above, the comparative water-absorbent resin (1) was produced.

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

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

[Comparative Example 2]
The gel particle size of Example 1 was changed (about 0.9 mm → 5 mm or more);
In Example 1, the comparative water-absorbent resin (2) was produced in the same manner as in Example 1 except that the gel crushing in the gel crushing step was performed once in order to change the gel particle size.

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, the comparative water-absorbent resin (2). The obtained comparative water-absorbent resin (2) was analyzed. The manufacturing conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below. In addition, the difference between the YI value of the initial coloring and the YI value of the temporal coloring (YI value over time-initial YI value) was calculated. The calculation results are shown in Table 3 below.

[Comparative Example 3]
The gel particle size of Example 5 was changed (about 0.9 mm → 5 mm or more);
In Example 5, the comparative water-absorbent resin (3) was produced in the same manner as in Example 5, except that the gel crushing in the gel crushing step was performed once in order to change the gel particle size.

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, the comparative water-absorbent resin (3). The obtained comparative water-absorbent resin (3) 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 Example 1 above, the same as in Example 1 except that the amount of DTPA / 5 sodium added was 300 ppm with respect to the monomer (molar ratio of persulfate to chelating agent; 7.7). , A water-absorbent resin (9) was produced.

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, a water-absorbent resin (9). The obtained final product, the water-absorbent resin (9), 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 Example 1 above, the same as in Example 1 except that the amount of DTPA / 5 sodium added was 50 ppm with respect to the monomer (molar ratio of persulfate to chelating agent; 46.3). , A water-absorbent resin (10) was produced.

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

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

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

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

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

[Example 12]
The drying temperature and drying time of Example 1 were changed (160 ° C → 120 ° C, 30 minutes → 120 minutes);
In the above-mentioned Example 1, the water-absorbent resin (12) was the same as in Example 1 except that the drying temperature and the drying time in the drying step were changed to 120 ° C. and 120 minutes (aeration of hot air at 120 ° C. for 120 minutes). ) Was manufactured.

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

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

[Example 13]
The drying temperature and gel particle size of Example 1 were changed (160 ° C → 120 ° C, about 0.9 mm → about 0.5 mm);
In Example 1 above, in order to change the gel particle size, after the second gel crushing in the gel crushing step, the obtained gel was further put into a tabletop meat chopper to perform the third gel crushing. Further, the 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 at 120 ° C. was aerated for 30 minutes).

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

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

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

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

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

[Comparative Example 5]
The drying temperature and drying time of Example 1 and the gel particle size were changed (160 ° C → 150 ° C, 30 minutes → 180 minutes, about 0.9 mm → 1.2 to 1.6 cm);
In Example 1 above, in order to change the gel particle size, the hydrogel polymer was cut using scissors as gel crushing in the gel crushing step, and the size of one side was 1.2 to 1.6 cm. A hydrogel-like polymer was used. Then, the 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. and 180 minutes (hot air at 150 ° C. was aerated for 180 minutes). bottom.

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, the comparative water-absorbent resin (5). 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]
The chelating agent of Example 1 was changed (DTPA → EDTA);
In Example 1 above, the same as in Example 1 except that the chelating agent was changed to 10 wt% ethylenediaminetetraacetic acid (EDTA) / 4 sodium aqueous solution (3.52 g, 1000 ppm with respect to the monomer). , A water-absorbent resin (14) was produced.

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

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, a water-absorbent resin (14). The obtained final product, the water-absorbent resin (14), 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 except that the amount of the polymerization initiator (10 wt% sodium persulfate aqueous solution) added in the polymerization step was 1.43 g (0.015 mol% with respect to acrylic acid) in Example 14 above. The water-absorbent resin (15) was produced in the same manner as above.

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

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

[Example 16]
The chelating agent of Example 1 was changed (DTPA → NTA);
In Example 1 above, the same as in Example 1 except that the chelating agent was changed to a 10 wt% nitrilo 3 acetic acid (NTA) / 3 sodium aqueous solution (3.52 g, 1000 ppm with respect to the monomer). , A water-absorbent resin (16) was produced.

The particulate hydrogel polymer (16) obtained in the gel pulverization 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.

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, a water-absorbent resin (16). The obtained final product, the water-absorbent resin (16), 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 except that the amount of the polymerization initiator (10 wt% sodium persulfate aqueous solution) added in the polymerization step was 4.8 g (0.05 mol% with respect to acrylic acid) in Example 14 above. The comparative water-absorbent resin (6) was produced in the same manner as above.

The particulate comparative hydrogel polymer (6) obtained in the gel pulverization 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.

Then, the surface was crosslinked in the same manner as in Example 1 to obtain a final product, the comparative water-absorbent resin (6). The obtained comparative water-absorbent resin (6) 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 crushing step were carried out at the same time.

(Polymerization and gel crushing process)
425.2 g of acrylic acid, 4499.5 g of 37 wt% sodium acrylate aqueous solution, pure After adding 513.65 g of water and 11.1 g of polyethylene glycol diacrylate (molecular weight 523) to prepare a reaction solution, the mixture was degassed in a nitrogen gas atmosphere for 20 minutes. Subsequently, 16.5 g of a 10 wt% sodium persulfate aqueous solution (0.03 mol% with respect to the monomer) and 23.6 g of a 0.1 wt% L-ascorbic acid aqueous solution were separately added, and the reaction solution was stirred. When the mixture was added while the mixture was added, the polymerization started after about 3 minutes, and the produced aqueous gel-like crosslinked polymer (17) was polymerized at 25 to 95 ° C. while being crushed.

After 70 minutes from the start of polymerization, 10.5 g of 20 wt% diethylenetriamine 5 sodium acetate (1000 ppm of chelating agent with a monomer) was added, and 100 minutes after the start of polymerization, a hydrogel crosslinked polymer (17) was added. Was removed from the reactor. The obtained particulate hydrogel polymer (17) had a weight average particle diameter (D50) of 479 μm.

(Drying / crushing / classification process)
The finely divided hydrogel crosslinked polymer (17) was spread on a wire mesh having an opening of 300 μm (50 mesh) and placed in a hot air dryer. Then, the particulate hydrogel polymer (17) was dried by aerating 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 crusher, manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then using two types of JIS standard sieves having an opening of 850 μm and 150 μm. By classifying, an amorphous crushed water-absorbent resin (17) was obtained. The amorphous crushed water-absorbent resin (17) had a CRC of 31.8 g / g.

(Surface cross-linking process)
Next, a humidifying mixture (1) (5.05 parts by weight) having the same composition as that of Example 1 was mixed with 100 parts by weight of the amorphous crushed water-absorbent resin (17). 17) was obtained. Subsequently, it was heat-treated at 180 ° C. for 40 minutes in the same manner as in Example 1 to obtain a surface-crosslinked water-absorbent resin (17). Then, the surface-crosslinked water-absorbent resin (17) was passed through a JIS standard sieve having an opening of 850 μm to obtain a final product, the water-absorbent resin (17). The obtained final product, the water-absorbent resin (17), 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 sodium persulfate aqueous solution as the polymerization initiator added was 16.5 g (0.03 mol% with respect to the monomer) to 28.3 g (with respect to the monomer). Polymerization was carried out in the same manner as in Example 17 except that the amount was increased to 0.05 mol%). Polymerization started after about 3 minutes in the same manner as in Example 17, and the produced comparative hydrogel-like crosslinked polymer (7) was polymerized at 25 to 95 ° C. while being crushed. Hereinafter, 70 minutes after the start of the polymerization, 1000 ppm of a chelating agent was added in the same manner as in Example 17, and 100 minutes after the start of the polymerization, the comparative hydrogel-like crosslinked polymer (7) (weight average particle size (D50) was 494 μm. ) Was removed from the reactor.

Then, it was dried in the same manner as in Example 17 to obtain a particulate comparative dry polymer (7), and then pulverized and classified to obtain an amorphous crushed comparative water-absorbent resin (7) having a size of 850 to 150 μm. Obtained.

The amorphous crushed comparative water-absorbent resin (7) was surface-crosslinked in the same manner as in Example 17 to obtain a surface-crosslinked comparative water-absorbent resin (7). Then, by passing through a JIS standard sieve of 850 μm in the same manner as in Example 17, the final product, the comparative water-absorbent resin (7), was obtained. The obtained final product, the 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 Pamphlet (Patent Document 17). The obtained final product, the 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]
A comparative water-absorbent resin (9) was obtained according to the method described in Example 2-23 of International Publication No. 2014/054656 Pamphlet (Patent Document 23). The obtained final product, the 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 No. 2014/054656 Pamphlet (Patent Document 23). The obtained final product, the comparative water-absorbent resin (10), was analyzed. The manufacturing 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 water-containing gel after the polymerization step of each Example and each comparative example and the amount of the chelating agent in the water-containing gel after the gel crushing step are shown. 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 and the gel pulverization step.

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

(summary)
The following can be seen from Tables 1 to 4.

(1) Comparison between Examples 1 and 2 and Comparative Example 1 in which the chelating agent (DTPA) and the gel particle size are the same and the amount of the polymerization initiator (persulfate; NaPS) added is changed, and chelating. From the results of Examples 5 and 6 in which the agent (EDTMP) and the gel particle size were the same and the amount of the polymerization initiator (persulfate; NaPS) was changed, the amount of persulfate (NaPS) was 0. It can be seen that when the content is 04 mol% or less, the residual amount of the chelating agent in the water-absorbent resin (furthermore, the final product) after the drying step is improved to 50% or more. 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 in which the chelating agent (DTPA), the gel particle size, and the amount of the polymerization initiator added (0.04 mol%) were the same and the type of the polymerization initiator was changed, and , The chelating agent (EDTMP), the gel particle size, and the amount of the polymerization initiator added (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 to persulfate (NaPS), with azo polymerization initiators and UV polymerization initiators, the residual amount of chelating agent in the water-absorbent resin (and the final product) after the drying process is significantly improved to nearly 100%. I know that I will do it. Further, it can be seen that in the polymerization initiator, the content of the chelating agent is increased, the L value of the initial coloring of the obtained water-absorbent resin is increased, and the YI value is decreased (in Examples 7 and 8). L value = 91, YI value = 2).

(3) The chelating agent (DTPA) and the polymerization initiator are the same, the gel particle size is changed, the comparison between Examples 1 and 3 and Comparative Example 2, and the chelating agent (EDTMP) and the polymerization initiator are the same. From the results of comparison between Example 5 and Comparative Example 3 in which the gel particle size was changed, the chelating agent in the water-absorbent resin after the drying step (and the final product) was made smaller by making the gel particle size smaller. It can be seen that the residual amount of is significantly improved. Further, 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 L value is improved by 1 point with the same chelating agent). ). Further, from the results of comparison between 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 size were the same, the solid content at the time of drying was 80%. It can be seen that the faster the achievement time, the better the residual rate of the chelating agent and the higher the L value of the initial coloring. It can be seen that in the production method according to the present invention, the smaller the gel particle size and the faster the achievement time of 80% solid content during drying, the higher the L value.

(4) Example 1 (1000 ppm with respect to the monomer; 0.0173 mol) in which the polymerization initiator and the addition amount thereof (0.04 mol%) were the same and the addition amount of the chelating agent (DTPA) was changed. %), Example 9 (300 ppm with respect to the monomer; 0.0052 mol%), and Example 10 (50 ppm with respect to the monomer; 0.009 mol%). It can be seen that 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% as the molar ratio of the above increases from 2.3 to 7.7 and further increases to 46.3. ..

(5) From the results of Examples 10 and 11, where the amount of the polymerization initiator added (0.04 mol%) and the amount of the chelating agent added (50 ppm) are the same, the amount of the polymerization initiator added is the same. , The residual amount of the chelating agent in the water-absorbent resin of the final product is improved from 15% to 30% by using the persulfate and the azo polymerization initiator in combination rather than using only the persulfate (NaPS). You can see that. Further, it can be seen that the polymerization initiator increases the content of the chelating agent and lowers the YI value of the initial coloring of the obtained water-absorbent resin (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 size 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 size were reduced to reduce the gel particle size in the water-absorbent resin of the final product. It can be seen that the residual amount of the chelating agent is significantly improved, the YI values of the initial coloring and the aging coloring are lowered, and the resistance to the aging coloring is particularly improved.

(7) From the comparison results of Example 2, Comparative Example 2 and Comparative Example 9 in which the amount of the chelating agent (DTPA) is the same and the amount of the persulfate in the polymerization initiator is different, the addition amount of the polymerization initiator is 0. It can be seen that by controlling the content to 04 mol% or less, the residual rate of the chelating agent is increased, the L value of the initial coloring and the aging coloring is increased, and the YI value is decreased.

Further, in Comparative Example 8, the amount of the chelating agent added at the time of polymerization was smaller than that of Example 2, and the amount of the persulfate added as the polymerization initiator was large. Therefore, it can be seen that the residual rate of the chelating agent decreases and the initial coloring (L value, YI value) deteriorates.

(8) From the results of comparison between Examples 10 and 11 in which the amount of the chelating agent added in the polymerization step is the same and Comparative Example 10, the amount of persulfate (NaPS) is controlled to 0.04 mol% or less. It can be seen 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 wt% DTPA / 5 sodium aqueous solution was added, and 501 ppm of a chelating agent was contained inside, and 100 ppm of a chelating agent was further contained on the surface. A water-absorbent resin (18) was obtained. Almost the entire amount of the chelating agent added later was contained in the vicinity of the surface of the water-absorbent resin (18), and the content of the chelating agent of the water-absorbent resin (18) 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) (a 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 1% by weight DTPA / 5 sodium aqueous solution per 100 parts by weight of the water-absorbent resin. Parts were added to obtain water-absorbent resins (19) to (21). Each of the water-absorbent resins (19) to (21) contained the same amount of chelating agent as the water-absorbent resins (2) to (4) inside, and further contained 100 ppm of the chelating agent on the surface. Almost the entire amount of the chelating agent added later is contained in the vicinity of the surface of the water-absorbent resins (19) to (21), and the content of the chelating agent of the water-absorbent resins (19) to (21) is, respectively. 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) (a decrease corresponding to 1% of added water).

(Reference example 1)
The amount and excess of the chelating agent in each hydrogel obtained in the above polymerization step of Example 1 (persulfate 0.04 mol%) and the above polymerization step of Comparative Example 1 (persulfate 0.05%). The amount of sulfate was measured. As a result, about 82% of the persulfate remained in each water-containing gel, 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 residual amount of the chelating agent.

(Reference example 2)
The residual amount of the chelating agent after the drying step and the surface cross-linking step of Example 1 (persulfate 0.04 mol%) 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 above drying step of Example 1. However, there was virtually no reduction in chelating agents.

(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, there was virtually no reduction in chelating agents.

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

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

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

(summary)
From the results of Reference Example 1 to Reference Example 6, it was found that the persulfate remaining in the hydrous gel specifically decomposes the chelating agent in the drying step. 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 rate of the chelating agent after drying. It is presumed that this mechanism is because the persulfate is more likely to react with the residual monomer than the chelating agent.

Then, 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 showed a high residual rate of the chelating agent with respect to the chelating agent added before the drying step. A sufficient amount of chelating agent remained in the final product, the water-absorbent resin. Therefore, a chelating agent can be blended on the surface and inside of the particles of the water-absorbent resin, and the water-absorbent resin, which is a final product, shows a low YI value and a good whiteness due to the coloring resistance of the chelating agent.

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

Claims (22)

A polymerization step of polymerizing a monomer aqueous solution containing a monomer and a polymerization initiator to obtain a hydrogel-like polymer, and a polymerization step.
If necessary, a gel pulverization step of pulverizing the hydrogel-like polymer during and / or after the polymerization step, and
A drying step of drying the obtained particulate water-containing gel polymer to obtain a particulate dry polymer, and
A method for producing a water-absorbent resin having a water absorption ratio (CRC) of 15 g / g or more and containing a chelating agent.
If the persulfate used in the above polymerization step is 0 to 0.04 mol% (monomer at the time of polymerization) (however, if the persulfate is 0 mol% (not used), another polymerization initiator Mandatory use),
In the step prior to the drying step, a chelating agent is added to the monomer aqueous solution or the hydrogel polymer in total of 10 ppm or more (the 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 set to 1 mm or less.
In the 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 a monomer aqueous solution containing a monomer and a polymerization initiator to obtain a hydrogel-like polymer, and a polymerization step.
If necessary, a gel pulverization step of pulverizing the hydrogel-like polymer during and / or after the polymerization step, and
A drying step of drying the obtained particulate water-containing gel polymer to obtain a particulate dry polymer, and
A method for producing a water-absorbent resin having a water absorption ratio (CRC) of 15 g / g or more and containing a chelating agent.
In the above drying step, the weight average particle size (D50) containing 10 ppm or more of the chelating agent (solid content of the water-containing gel polymer) and 0 to 0.04 mol% of persulfate (monomer at the time of polymerization). A method for producing a water-absorbent resin containing a chelating agent, wherein a water-containing gel-like polymer having a particle size of 1 mm or less is dried with a drying time of 20 minutes or less 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. The production method according to claim 1 or 2, wherein the total amount of the persulfate added from the polymerization step to the drying step is 0 to 0.04 mol% (monomer at the time of polymerization). The production method according to any one of claims 1 to 3, wherein in the drying step, hot air drying at 150 to 200 ° C. is performed. The production method according to any one of claims 1 to 4, wherein the chelating agent is at least one selected from the group consisting of an amino polyvalent carboxylic acid chelating agent and an amino polyvalent phosphoric acid chelating agent. The production method according to any one of claims 1 to 5, wherein the water-containing gel polymer is made into particles in the gel pulverization step. Any of claims 1 to 6, wherein the total amount of the chelating agent added in the steps before the drying step is 60 ppm to 1% (solid content of the monomer or the hydrogel-like polymer at the time of polymerization). The manufacturing method according to item 1. The production method according to any one of claims 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 foam polymerization or boiling polymerization, and the hydrogel-like polymer contains bubbles. The production method according to any one of claims 1 to 9, wherein the polymerization is a high-temperature start short-time polymerization having a polymerization start temperature of 30 ° C. or higher, a polymerization peak temperature of 80 to 130 ° C., and a polymerization time of 60 minutes or less. The production method according to any one of claims 1 to 10, further comprising a surface cross-linking step of the water-absorbent resin after the drying step. The production 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 production method according to any one of claims 1 to 12, wherein the hydrogel polymer before drying contains 0.1% by weight or more of the residual monomer. 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 monomer (excluding the internal cross-linking agent) used in the polymerization step. Mol%
The water-absorbent resin containing the chelating agent has a residual amount (C1) of the chelating agent of 10 ppm or more, an L value of the initial color tone of 85 or more, and a YI value of 13 or less. The production method according to any one of claims 1 to 13, which is a sex resin. Any of claims 1 to 14 of the water-absorbent resin containing the chelating agent, wherein the residual 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 manufacturing method according to one item. 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 unpressurized water absorption ratio (CRC) is 25 g / g or more, the pressurized water absorption ratio (AAP (0.7 psi)) is 15 g / g or more, and the ratio of the pressurized water absorption ratio to the unpressurized water absorption ratio ( The water-absorbent resin according to claim 16, wherein the AAP (0.7 psi) / CRC) is 0.5 or more. Acrylic acid (salt) 50-100 mol% of total monomer (excluding internal cross-linking agent),
The internal cross-linking agent is 0.001 to 5 mol% with respect to the monomer, and
The water-absorbent resin according to claim 16 or 17, which comprises a polyacrylic acid (salt) -based crosslinked polymer obtained from a monomer aqueous solution containing 0 to 0.04 mol% of persulfate with respect to the monomer. A hydrogel-like polymer obtained by polymerizing a monomer aqueous solution containing a monomer and a polymerization initiator is gel-pulverized during and / or after polymerization as necessary, and the obtained particulate hydrogel-like weight is obtained. A water-absorbent resin containing a chelating agent obtained by drying the coalescence.
A claim for adding a total of 10 ppm or more (solid content of the monomer or the hydrogel polymer at the time of polymerization) to the monomer aqueous solution or the hydrogel polymer in the step prior to the drying step. Item 6. The water-absorbent resin according to any one of Items 16 to 18. The water-absorbent resin according to any one of claims 16 to 19, which is in the form of amorphous crushed material. The water-absorbent resin according to any one of claims 16 to 20, wherein the residual rate of the chelating agent is 50% or more. The water-absorbent resin according to any one of claims 16 to 21, which 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.