KR102562511B1 - Manufacturing method of absorbent polymer containing chelating agent - Google Patents

Abstract

The total addition amount of persulfate in the polymerization initiator from the polymerization step to the drying step was reduced to 0 to 0.04 mol% (to the monomer during polymerization), and further, the gel particle size and drying conditions of the hydrogel polymer before the drying step is controlled to suppress decomposition of the chelating agent in the drying process.

Description

Manufacturing method of absorbent polymer containing chelating agent

The present invention relates to a method for preparing a water absorbent polymer containing a chelating agent.

Super absorbent polymer (SAP/Super Absorbent Polymer) is a water-swellable, water-insoluble polymer gelling agent, and is mainly used for disposable purposes as absorbent articles such as paper diapers and sanitary napkins, as well as water retaining agents for agricultural and horticultural use, and industrial waterstop materials. .

Such an absorbent polymer is manufactured through a manufacturing process such as a polymerization process, a drying process, an undried material removal process if necessary, a crushing process, a classification process, and a surface crosslinking process. With the improvement of performance of paper diapers, which are the main use, the absorbent polymer is manufactured. Also, many functions (physical properties) are required. Specifically, it is not limited to the simple water absorption rate, but gel strength, water soluble content, absorption rate, water absorption rate under pressure, liquid permeability, particle size distribution, urine resistance, antibacterial property, impact resistance (damage resistance), powder fluidity, Performances such as deodorization, discoloration resistance (whiteness), and low dust may be mentioned.

In order to improve the performance of the water absorbent polymer, a small amount of additives may be used in the water absorbent polymer as a method other than changing the monomer or the manufacturing process, and as various additives for the water absorbent polymer, water-soluble or water-insoluble inorganic powder or organic Additives such as powders, surfactants, plasticizers, water-soluble polymers, water-insoluble (thermoplastic) polymers, antibacterial agents, deodorants, reducing agents, antioxidants, organic acids, inorganic acids, anti-coloring agents, and chelating agents are known.

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

For example, Patent Literatures 1 to 4 disclose that a chelating agent is added to the monomers of the water absorbent resin, mainly for the purpose of improving polymerization stability. In Patent Literatures 5 to 11, in manufacturing processes such as polymerization process/drying process/surface crosslinking process/granulation process/fine powder recovery process, monomers or their A method for preparing a water absorbent polymer in which a chelating agent is added to a polymer is disclosed. Patent Literatures 12 to 19 and 23 disclose a method for producing a water absorbent resin in which a metal chelating agent is added in any of the production steps in order to suppress coloration (particularly, coloration with time) of the water absorbent resin.

In particular, Patent Literatures 12 and 14 describe the fact that it is known that it is suitable to have a chelating agent inside the water absorbent polymer to prevent coloration, and adding the chelating agent to the monomer during polymerization or to the gel before drying results in a water absorbent polymer. A method of adding a chelating agent therein is disclosed. Patent Document 20 discloses a water absorbent resin composition containing a chelating agent for improving the salt resistance of the water absorbent polymer. Patent Literatures 21 and 23 disclose a method for producing a water absorbent polymer in which a metal chelating agent is added in any of the manufacturing steps to improve the urine resistance of the water absorbent polymer. Patent Literature 22 discloses a method for producing a water absorbent polymer in which a chelating agent is added to a monomer in the second stage of two-stage polymerization in order to improve physical properties such as water absorption capacity under pressure of the water absorbent polymer after reverse-phase suspension polymerization.

Here, the chelating agent used for the purpose of stabilizing polymerization in the manufacturing process or preventing deterioration from drying in Patent Documents 1 to 7 above, if remaining in the final product, improves urine resistance or prevents discoloration when the absorbent polymer is used.

International Publication No. 93/005080 pamphlet International Publication No. 2011/120746 pamphlet Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 02-117903" Japanese Laid-Open Patent Publication "Japanese Patent Laid-Open No. 08-283318" Japanese Laid-Open Patent Publication "Japanese Patent Laid-Open No. 2000-38407" Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 2000-007790" Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 11-246674" Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 11-315148" Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 11-315147" European Patent Publication No. 0781146A International Publication No. 2015/053372 pamphlet International Publication No. 2008/090961 pamphlet International Publication No. 2003/059962 pamphlet International Publication No. 2009/005114 pamphlet International Publication No. 2016/006134 pamphlet International Publication No. 2018/029045 pamphlet International Publication No. 2011/040530 pamphlet International Publication No. 03/059961 pamphlet Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 05-086251" Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 59-230046" Japanese Unexamined Patent Publication "Japanese Patent Laid-Open No. 08-067821" International Publication No. 2007/004529 pamphlet International Publication No. 2014/054656 pamphlet

However, as the present inventors have also studied a method for producing a water absorbent polymer containing a chelating agent, in the case where a chelating agent is added in a polymerization process or a drying process among the above manufacturing processes, in the final product, a method suitable for the addition amount of the chelating agent is found. A problem that a sufficient effect is difficult to obtain was newly discovered. Then, as a cause of the problem, it was found that the water absorbent resin of the final product did not contain a chelating agent in an amount commensurate with the amount of the chelating agent added to the monomer or polymer gel.

In relation to this problem, one aspect of the present invention is to prevent decomposition of the chelating agent in the water absorbent polymer manufacturing process, thereby improving the residual rate of the chelating agent in the water absorbent polymer as a final product, a water absorbent containing a chelating agent. A main object is to provide a method for producing resin.

Another aspect of the present invention, as a water absorbent resin containing a chelating agent obtained by the above production method, has a main object to provide a water absorbent resin exhibiting good color resistance (whiteness).

In order to solve the above problem discovered this time, the present inventors have investigated the cause, and in the process of drying the hydrogel polymer obtained by polymerizing the monomers, the chelating agent added before the drying process is specifically found to decrease And, it was found that the cause of the reduction of the chelating agent in the drying step was the polymerization initiator (especially persulfate) remaining in the hydrogel 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 to decompose the chelating agent in the drying process. It is characterized by suppressing, and includes the inventions described in [1] to [20] below.

[1] A polymerization step of polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer, a gel pulverization step of pulverizing the hydrogel polymer during and/or after the polymerization step, if necessary, and the obtained particulate function A method for producing a water absorbent polymer having a water absorption coefficient (CRC) of 15 g/g or more and containing a chelating agent, including a drying step of drying a gel polymer to obtain a dry polymer in particulate form, wherein the persulfate used in the polymerization step is 0 to 0.04 mol% (monomer at the time of polymerization) (however, when persulfate is 0 mol% (non-use), another polymerization initiator is necessarily used), and a chelating agent is added to the aqueous monomer solution in the process prior to the drying process Alternatively, a total of 10 ppm or more (monomers during polymerization or solid content of the hydrogel polymer) is added to the hydrogel polymer, and the weight average particle diameter (D50) of the particulate hydrogel polymer is 1 mm or less, and in the drying step , A method for producing a water absorbent polymer containing a chelating agent, wherein the drying time until the solid content reaches 80% by weight or more is 20 minutes or less.

[2] A polymerization step of polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer, a gel pulverization step of pulverizing the hydrogel polymer during and/or after the polymerization step, if necessary, and the obtained particulate function A method for producing a water absorbent resin having a water absorption coefficient (CRC) of 15 g/g or more and containing a chelating agent, including a drying step of drying a gel polymer to obtain a dry polymer in particulate form, wherein in the drying step, a chelating agent is A particulate hydrogel polymer having a weight average particle diameter (D50) of 1 mm or less, containing 10 ppm or more (to the solid content of the hydrogel polymer) and 0 to 0.04 mol% of persulfate (to the monomer at the time of polymerization), drying time 20 A method for producing a water-absorbent polymer containing a chelating agent, wherein drying is performed in a minute or less until the solid content is 80% by weight or more. However, the said drying time points out the time until solid content becomes 80 weight% or more.

[3] The production method according to [1] or [2], wherein the total addition amount of the persulfate from the polymerization step to the drying step is 0 to 0.04 mol% (monomers during polymerization).

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

[5] The production method according to any one of [1] to [4], wherein the chelating agent is at least one member selected from the group consisting of amino polyvalent carboxylic acid chelating agents and amino polyvalent phosphate chelating agents.

[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 preparation according to any one of [1] to [6], wherein the total amount of the chelating agent added in the step before the drying step is 60 ppm to 1% (solid content of monomers or large hydrogel polymer at the time of large polymerization) method.

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

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

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

[11] The manufacturing method according to any one of [1] to [10], further comprising a step of surface crosslinking 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 a step after the drying step.

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

[14] The monomers used in the polymerization step include acrylic acid (salt), and the content of the acrylic acid (salt) is 50 to 100 mol% based on the total monomers (excluding the internal crosslinking agent) used in the polymerization step. And, the water absorbent resin containing the chelating agent is a polyacrylic acid (salt)-based water absorbent resin having 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, [ 1] to the production method described in any one of [13].

[15] The absorbent polymer containing the chelating agent has a residual amount (C1) of chelating agent of 200 ppm or more, an L value of an initial color tone of 89 or more, and a YI value of 10 or less, any one of [1] to [14]. The manufacturing method described in one.

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

[17] Absorption capacity under pressure (CRC) is 25 g / g or more, absorption capacity under pressure (AAP (0.7 psi)) is 15 g / g or more, and the ratio of absorption capacity under pressure and absorption capacity under pressure (AAP (0.7psi)/CRC) is 0.5 or more, the water absorbent resin described in [16].

[18] Acrylic acid (salt) containing 50 to 100 mol% of the total monomers (excluding the internal crosslinking agent), 0.001 to 5 mol% of the internal crosslinking agent, and 0 to 0.04 mol% of persulfate based on the monomers The water-absorbent resin described in [16] or [17], which contains a polyacrylic acid (salt)-based crosslinked polymer obtained from an aqueous monomer solution.

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

[20] The water absorbent resin according to any one of [16] to [19], which is an irregular crushed phase.

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

[22] The water absorbent resin according to any one of [16] to [21], which contains a chelating agent on the surface and inside, and the amount of the chelating agent on the surface is greater than the amount of the chelating agent on the inside.

According to one aspect of the present invention, the decomposition of the chelating agent in the water-absorbent polymer manufacturing process is prevented, the residual rate of the chelating agent in the final product, the water-absorbent polymer is improved, and the chelating agent is blended on the surface and inside of the water-absorbent polymer particles exert what it can do.

Hereinafter, a method for producing a water absorbent polymer containing a chelating agent according to the present invention and a water absorbent polymer containing a chelating agent obtained thereby will be described in detail, but the scope of the present invention is not limited by these descriptions, and the following examples In addition, it can be changed and implemented as appropriate within a range that does not impair the gist of the present invention. Specifically, the present invention is not limited to each of the following embodiments, and various changes are possible within the scope indicated in the claims, and also for embodiments obtained by appropriately combining technical means disclosed in other embodiments, the present invention included in the technical scope.

[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 the CRC (water absorption capacity under no pressure) specified in ERT442.2-02 is 5 [g/g] or more, and "water insolubility" refers to Ext specified in ERT470.2-02. (Water soluble content) refers to 0 to 50% by weight.

The water absorbent resin can be appropriately designed depending on its use, and is not particularly limited, but is preferably a hydrophilic polymer obtained by polymerizing an unsaturated monomer having a carboxyl group. Moreover, it is not limited to the form in which the whole amount (100% by weight) is a polymer, and a composition containing a surface crosslinked resin or additives may be used 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 "absorbent resin" refers to a water absorbent resin before surface treatment or surface crosslinking, a water absorbent resin after surface treatment or surface crosslinking, and a water absorbent resin having a different shape obtained in each step (as a shape, For example, a sheet form, a fiber form, a film form, a gel form, etc.), a water absorbent resin composition containing additives, etc. are included.

(1-2) "Polyacrylic acid (salt)"

"Polyacrylic acid (salt)" in the present invention is a polymer containing, as a repeating unit, acrylic acid and/or a salt thereof (hereinafter sometimes referred to as acrylic acid (salt)) as a main component, optionally including a graft component. means Specifically, of the total monomers (excluding the internal crosslinking agent) used in the polymerization, essentially 50 to 100 mol% of acrylic acid (salt), preferably 70 to 100 mol%, more preferably 90 to 100 mol% , particularly preferably a polymer containing substantially 100% by mole. In addition, when polyacrylate is used as the polymer, it essentially contains a water-soluble salt, and as a main component of the neutralized salt, a monovalent salt is preferable, an alkali metal salt or an ammonium salt is more preferable, an alkali metal salt is still more preferable, and a sodium salt is particularly preferred.

(1-3) "EDANA" and "ERT"

"EDANA" is an abbreviation for European Disposables and Nonwovens Associations, and "ERT" is an abbreviation for Water Absorbent Resin Measurement Methods (EDANA Recommended Test Methods), which is a European standard (almost a global standard). In the present invention, unless otherwise specified, measurement is performed based on the original ERT (known document: revised in 2002).

(a) "CRC" (ERT441.2-02)

"CRC" is an abbreviation for Centrifuge Retention Capacity, and means absorption capacity under no pressure (hereinafter, also referred to as "absorption capacity"). Specifically, it is the water absorption ratio (unit: [g/g]) after freely swelling 0.200 g of the water-absorbent resin in a nonwoven fabric bag with a large excess of 0.9% by weight aqueous sodium chloride solution for 30 minutes and then dehydrating in a centrifugal separator. . In addition, the CRC (hereinafter referred to as "gel CRC") of the hydrogel polymer was measured by changing the sample to 0.4 g and the free swelling time to 24 hours, respectively.

(b) "AAP" (ERT442.2-02)

"AAP" is an abbreviation for Absorption Against Pressure, and means absorption capacity under pressure. Specifically, the water absorption factor (unit: [g/g) after swelling 0.900 g of the water absorbent polymer in a 0.9% by weight aqueous solution of sodium chloride for 1 hour under a load at 2.06 kPa (0.3 psi, 21 [g/cm]) ])am. Also, in ERT442.2-02, it is indicated as Absorption Under Pressure, but it is substantially the same. In addition, in the present invention and examples, there are cases in which the measurement is performed by changing the load condition to 4.83 kPa (0.7 psi, 49 [g/cm 2 ]), and in that case, it is described as AAP (0.7 psi).

(c) "Ext" (ERT470.2-02)

"Ext" is an abbreviation of Extractables, and means water-soluble content (amount of water-soluble component). Specifically, it is the amount of dissolved polymer (unit: wt%) after adding 1.000 g of water absorbent resin to 200 ml of 0.9 wt% sodium chloride aqueous solution and stirring for 16 hours. The amount of dissolved polymer is measured using pH titration. In addition, 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, respectively.

(d) "PSD" (ERT420.2-02)

"PSD" is an abbreviation for Particle Size Distribution, and means a particle size distribution measured by sieve classification. In addition, the weight average particle diameter (D50) and particle diameter distribution width are measured by the same method as "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution" described in US Patent No. 7638570. do. In addition, a standard sieve (eye size) used in particle size measurement may be appropriately added according to the particle size of the object. For example, a standard sieve with an eye size of 710 μm or 600 μm may be added. In addition, the method for measuring the PSD of the hydrogel polymer will be described later.

(e) 「Residual Monomers」(ERT410.2-02)

"Residual Monomers" means the amount of monomers (monomers) remaining in the water absorbent resin (hereinafter referred to as "residual monomers"). Specifically, it refers to the amount of dissolved monomer (unit: ppm) after adding 1.0 g of water absorbing resin to 200 ml of 0.9 wt% sodium chloride aqueous solution and stirring for 1 hour at 500 rpm using a 35 mm stirrer tip. The amount of dissolved monomer is measured using HPLC (high performance liquid chromatography). In addition, the residual monomer of the hydrogel polymer is measured by changing the sample to 2 g and the stirring time to 3 hours, respectively, and the obtained measured value is converted into the weight per resin solid content of the hydrogel polymer (unit: ppm). .

(f) 「Moisture Content」(ERT430.2-02)

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

(1-4) "liquid permeability"

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

"SFC (physiological saline flow inductivity)" refers to the liquid permeability of a 0.69% by weight aqueous sodium chloride solution to the absorbent polymer at a load of 2.07 kPa, and is measured according to the SFC test method disclosed in US Patent No. 5669894. In addition, "GBP" refers to the liquid permeability of a 0.69 wt% sodium chloride aqueous solution to the water absorbent polymer under load or free expansion, and is measured according to the GBP test method disclosed in International Publication No. 2005/016393.

(1-5) 「FSR」

"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 polymer absorbs 20 g of a 0.9% by weight sodium chloride aqueous solution.

(1-6) "Gel Grinding"

"Gel pulverization" in the present invention refers to an operation of reducing the size and increasing the surface area by applying shear and compressive force for the purpose of facilitating drying of the hydrogel polymer obtained in the polymerization step. By "gel pulverization", a particulate hydrogel polymer, particularly a particulate hydrogel polymer having a weight average particle diameter (D50) described later is obtained.

Further, in unstirred aqueous polymerization (stationary aqueous solution polymerization, particularly belt polymerization), gel pulverization is performed after polymerization, whereas in the case of kneader polymerization, polymerization and gel pulverization are performed continuously in the same apparatus. The weight average particle diameter (D50) of the particulate hydrogel polymer supplied to the drying step may be within the range described below, and gel grinding may be performed either during polymerization or after polymerization.

(1-7) "Weight average molecular weight of water-soluble content"

The "weight average molecular weight of the water-soluble component" in the present invention is a value measured by GPC (gel permeation chromatography) for the weight average molecular weight of the component (water-soluble component) dissolved when the water-absorbent resin is added to the water medium ( 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) (c) "Ext" above. In addition, the weight average molecular weight of the water-soluble component of the hydrogel polymer was measured by changing the stirring time to 24 hours and 5.0 g of a sample having a particle diameter of 5 mm or less and further refining to 1 to 3 mm.

(1-8) Others

In this specification, “X to Y” representing 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 noted, "ppm" is " weight ppm”. In addition, "~acid (salt)" means "~acid and/or its salt", and "(meth)acryl" means "acryl and/or methacryl".

[2] Method for producing a water absorbent polymer containing a chelating agent

The present invention relates to a polymerization step of obtaining a hydrogel polymer by polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator, a gel pulverization step of pulverizing the hydrogel polymer during and/or after the polymerization step, if necessary, and the obtained particulate function Methods 1 and 2 below for producing a water absorbent resin having a water absorption coefficient (CRC) of 15 g/g or more and containing a chelating agent, including a drying step of drying a gel polymer to obtain a dry polymer in particulate form. to provide.

(Method 1; stipulate persulfate in monomer/gel particle size/drying conditions)

The persulfate used in the polymerization process is 0 to 0.04 mol% (monomer during polymerization) (however, in the case of 0 mol% (non-use) of persulfate, another polymerization initiator must be used), prior to the drying process In the step, a chelating agent is added to the aqueous monomer solution or the hydrogel polymer in a total amount of 10 ppm or more (solid content of the monomer or hydrogel polymer at the time of polymerization), and the weight average particle diameter (D50) of the particulate hydrogel polymer is 1 mm. A method for producing a water absorbent polymer containing a chelating agent, wherein in the drying step, the drying time until the solid content reaches 80% by weight or more is set to 20 minutes or less.

(Method 2; Defining Persulfate/Gel Particle Size/Drying Conditions in Hydrogel Polymer)

In the drying step, the chelating agent is 10 ppm or more (solid content of hydrous gel polymer), and the persulfate is 0 to 0.04 mol% (monomer during polymerization), and the weight average particle size (D50) is 1 mm or less. A method for producing a water absorbent polymer containing a chelating agent, wherein a gel polymer is dried for a drying time of 20 minutes or less until the solid content is 80% by weight or more. However, the said drying time points out the time until solid content becomes 80 weight% or more.

Hereinafter, Method 1 and Method 2 will be described in detail.

(2-1) polymerization process

This step is a step of polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator to obtain a hydrogel polymer (hereinafter sometimes abbreviated as "hydrogel").

(monomer)

The water absorbent resin obtained in the present invention is a polyacrylic acid (salt) water absorbent resin that is usually polymerized in an aqueous solution using a monomer containing acrylic acid (salt) as a main component as its raw material (monomer).

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

In the water-containing gel polymer obtained by polymerization of the monomer aqueous solution, it is preferable that at least a part of the acid groups of the polymer are neutralized from the viewpoint of water absorption performance and residual monomer. Although this partially neutralized salt is not particularly limited, from the viewpoint of absorption performance, a monovalent salt selected from alkali metal salts, ammonium salts, and amine salts is preferable, and alkali metal salts are more preferable, and sodium salts, lithium salts, and potassium salts. Alkali metal salts are more preferred, and sodium salts are particularly preferred. Therefore, the basic substance used for neutralization is not particularly limited, but alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and lithium hydroxide, carbonate (hydrogen) salts such as sodium (hydrogen)carbonate and potassium carbonate (hydrogen)carbonate, etc. A monovalent basic substance is preferred, and sodium hydroxide is particularly preferred. When carbonate is used for neutralization, since carbonic acid gas is generated during neutralization, the carbonic acid gas can also be used as a foaming agent at the time of polymerization.

The neutralization can be performed in each form/state before polymerization, during polymerization, or after polymerization, for example, a hydrous gel obtained by polymerization of unneutralized or low-neutralized (eg, neutralization rate of 0 to 30 mol%) acrylic acid. The polymer may be neutralized, particularly neutralized simultaneously with gel pulverization, but it is preferable to neutralize the acrylic acid before polymerization from the viewpoint of improving productivity and physical properties. That is, it is preferable to use neutralized acrylic acid (a partially neutralized salt of acrylic acid) as a monomer.

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

In the present invention, when acrylic acid (salt) is used as a main component, a hydrophilic or hydrophobic unsaturated monomer other than acrylic acid (salt) (hereinafter referred to as "other monomers") may be used in combination. Examples of such other monomers include, but are not particularly limited to, methacrylic acid, (anhydride) maleic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acryloxyalkanesulfonic acid, and N-vinyl-2- Pyrrolidone, N-vinylacetamide, (meth)acrylamide, N-isopropyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, 2-hydroxyethyl (meth)acrylate, methoxy Polyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, stearyl acrylate, salts thereof, and the like are exemplified. When other monomers are used, the amount used is appropriately determined within a range that does not impair the water absorption performance of the resulting water absorbent polymer, and is not particularly limited, but is 0 to 50 mol% based on the total monomers (excluding the internal crosslinking agent). It is preferable, 0 to 30 mol% is more preferable, and 0 to 10 mol% is still more preferable.

(polymerization inhibitor)

The aqueous monomer solution may contain a polymerization inhibitor as necessary for the purpose of stabilization of polymerization and neutralization. When the monomer aqueous solution contains a polymerization inhibitor, the usage amount is typically 200 ppm or less, further 10 to 130 ppm, more preferably 20 to 100 ppm with respect to the monomers from the viewpoint of coloring and polymerization stability. Preferably, a methoxyphenol type polymerization inhibitor is used. More preferably, the polymerization inhibitor is p-methoxyphenol.

(Polymerization initiator)

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

The present invention (Method 1) reduces the persulfate in the polymerization initiator used in the polymerization process to 0 to 0.04 mol% (to the monomer during polymerization) (provided, that when the persulfate is 0 mol% (not used), other polymerization Initiator is essentially used) method. In addition, the present invention (method 2) controls the persulfate contained in the particulate hydrogel polymer to 0 to 0.04 mol% (to the monomer during polymerization), and further, the gel particle size and drying conditions of the hydrogel polymer before the drying step how to control Thus, the present invention suppresses decomposition of the chelating agent in the drying step.

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

When the added amount of persulfate exceeds 0.04 mol% with respect to the monomers during polymerization, in the subsequent drying step, the unreacted persulfate reacts with the chelating agent present in the hydrogel polymer and decomposes it. undesirable because it can

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

As the azo polymerization initiator used in the present invention, a water-soluble azo compound (e.g., 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.). Suitably, 2,2'-azobis(2-methylpropionamidine) dihydrochloride is used.

In addition, as the photopolymerization initiator used in the present invention, diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyldimethylketal, 4-(2-hydroxyethoxy)phenyl- acetophenone 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; benzophenone derivatives such as o-benzoylmethylbenzoate, 4-phenylbenzophenone, 4-benzoyl-4'-methyl-diphenyl sulfide, and (4-benzoylbenzyl)trimethylammonium chloride; thioxanthone-based compounds; acylphosphine oxide derivatives such as bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide and diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide; 2-hydroxymethylpropionitrile etc. are mentioned. Preferably, it is an acetophenone derivative, and among them, 1-hydroxycyclohexylphenyl ketone is preferably used.

The amount of the polymerization initiator containing persulfate (0 to 0.04 mol%) is preferably 0.0001 to 1 mol%, and more preferably 0.0005 to 0.5 mol%, in total with respect to the monomers at the time of polymerization. When the amount of the polymerization initiator used exceeds 1 mol%, control of the polymerization becomes difficult, and there is a risk that the color tone of the water absorbent resin obtained further deteriorates. In addition, when the usage amount of the polymerization initiator is less than 0.0001 mol%, there is concern about an increase in residual monomers.

In this invention, you may use together 2 or more types of polymerization initiators. Among them, persulfate salts are preferably used within the above range from the viewpoint of handling properties, physical properties of the water absorbent polymer, and the like. When a persulfate and another polymerization initiator are used together, it is preferable to use a second polymerization initiator selected from an azo polymerization initiator and a photopolymerization initiator as a polymerization initiator other than a persulfate from the viewpoint of the performance of the water absorbent polymer. The molar ratio of the persulfate to the polymerization initiator other than the persulfate (= azo polymerization initiator + photopolymerization initiator + other polymerization initiators) is 1/99 to 99/1, preferably 1/9 to 9/1, more preferably 2 /8 to 8/2, more preferably 3/7 to 7/3.

In addition, a reducing agent that promotes the decomposition of these polymerization initiators (particularly persulfates or peroxides) may be used in combination, and a redox initiator may be obtained by combining the two. Examples of such a reducing agent include (di)sulfurous acid (salt) such as sodium sulfite and sodium hydrogensulfite, reducing metal (salt) such as L-ascorbic acid (salt) and ferrous salt, amines, and the like.

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

(persulfate remaining in hydrogel)

In a method for producing a water absorbent polymer containing a chelating agent, a decrease in the chelating agent was discovered, and as a result of research on the reduction mechanism, it was found that the chelating agent was not substantially decomposed during polymerization and was decomposed during drying. It became. This is in addition to the experimental fact that the amount of the chelating agent contained in the hydrogel after polymerization is almost constant, even if the aqueous solution containing the chelating agent is heated in the presence of the monomer, the amount of the chelating agent does not decrease, but persulfate is added to the aqueous solution of the chelating agent. It can be confirmed by the experimental fact that the amount of chelating agent decreases when present.

Among the polymerization initiators used for polymerization, most (more than 80%) of persulfate remains in the hydrogel after polymerization under normal polymerization conditions due to its half-life. For this reason, the persulfate is decomposed by the high temperature at the time of drying, and radicals are generated in the hydrogel. However, since the monomer (remaining monomer) remaining in the hydrogel after polymerization is a small amount of 0.% to several%, it is assumed that the radicals of persulfate generated are difficult to react with the monomers and promote the decomposition of the chelating agent instead. do.

In the present invention, 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 ° C. half-life (τ) in 0.44 hours) is preferred, and sodium persulfate is more preferred. When sodium acrylate is used as a monomer of the water absorbent polymer, when a persulfate salt is used as a polymerization initiator within the above range, sodium persulfate is substantially obtained regardless of the type of salt. And, for sodium persulfate or potassium persulfate, the half-life (τ) is 2100 hours (30 ° C.), 499 hours (40 ° C.), 130 hours (50 ° C.), 36.5 hours (60 ° C.), 11.1 hours ( 70 ℃), 3.59 hours (80 ℃), 1.24 hours (90 ℃), 0.45 hours (100 ℃). Therefore, most (over 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 aqueous monomer solution further contains an internal crosslinking agent.

The internal crosslinking agent suitably used in the present invention is not particularly limited, and examples thereof include a polymerizable crosslinking agent that polymerizes with acrylic acid, a reactive crosslinking agent that reacts with a carboxyl group, and a crosslinking agent having these properties.

Examples of the polymerizable crosslinking agent include N,N'-methylenebisacrylamide, (poly)ethylene glycol di(meth)acrylate, (polyoxyethylene)trimethylolpropane tri(meth)acrylate, and poly(meth)acrylate. and compounds having at least two polymerizable double bonds in the molecule, such as allyloxyalkanes. Moreover, as said reactive crosslinking agent, For example, Polyglycidyl ether, such as ethylene glycol diglycidyl ether; polyhydric alcohols such as propanediol, glycerin, and sorbitol; multivalent metal compounds such as covalent bond crosslinking agents such as aluminum salts; Ion-bond crosslinking agents, such as these, are mentioned. Among these, from the viewpoint of water absorption performance, polymerizable crosslinking agents that polymerize with acrylic acid are more preferable, and acrylate-based, allyl-based, and acrylamide-based polymerizable crosslinking agents are particularly preferable. These internal crosslinking agents may be used individually by 1 type, or may use 2 or more types together. Further, when the polymerizable crosslinking agent and the covalent bond crosslinking agent are used together, the mixing ratio is preferably 10:1 to 1:10.

Further, the amount of the internal crosslinking agent used is preferably 0.001 to 5 mol%, more preferably 0.002 to 2 mol%, still more preferably 0.04 to 1 mol%, and 0.06 to 1 mol%, based on the monomers, from the viewpoint of physical properties. 0.5 mol% is particularly preferred, and 0.07 to 0.2 mol% is most preferred.

In one aspect of the present invention, the monomer aqueous solution contains 50 to 100 mol% of acrylic acid (salt), 0.001 to 5 mol% of the internal crosslinking agent, and a persulfate based on the total monomers (excluding the internal crosslinking agent). It is preferable to include 0 to 0.04 mol% with respect to the monomers. The water-absorbent resin containing the polyacrylic acid (salt)-based crosslinked polymer obtained from the monomer aqueous solution may exhibit a preferred numerical range as the L value and YI value of the initial color tone.

(other additives)

In one aspect of the present invention, further customary additives may be added to the aqueous monomer solution in order to improve the physical properties of the water absorbent resin obtained in the present invention.

Examples of such additives include water-soluble resins or water-absorbent resins such as starch, cellulose, polyvinyl alcohol (PVA), polyacrylic acid (salt), and polyethyleneimine; carbonates, various foaming agents that generate bubbles; Surfactant etc. are mentioned.

In addition to the above monomer aqueous solution, these additives may be added to a hydrogel polymer, dry polymer or water absorbent resin in any of the production steps of the present invention. 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% by weight, and particularly preferably 0 to 10% by weight, based on the monomers in the case of the water-soluble resin or water-absorbent resin. Preferably it is 0 to 3% by weight. On the other hand, in the case of the foaming agent or surfactant, 0 to 5% by weight is preferable, and 0 to 1% by weight is more preferable with respect to the monomers. In addition, a graft polymer or water absorbent resin composition is obtained by adding the water-soluble resin or water-absorbent resin, but these starch-acrylic acid polymers, PVA-acrylic acid polymers, etc. are also treated as water-absorbent resins in the present invention.

(chelating agent)

In the present invention, in the step prior to the drying step, a chelating agent is added to the aqueous monomer solution or the hydrogel polymer in a total amount of 10 ppm or more (solid content of the monomer or hydrogel polymer during polymerization). From the viewpoint of the physical properties of the water absorbent polymer, the total amount of the chelating agent added to the aqueous monomer solution or the hydrogel polymer (the solid content of the monomer or hydrogel polymer during polymerization), or the particulate hydrogel polymer provided in the drying step The content of the chelating agent (solids content of the gel polymer with water) is preferably 10 ppm or more, 40 ppm or more, 60 ppm or more, 100 ppm or more, 200 ppm or more, 250 ppm or more, 500 ppm or more, and 600 ppm or more in this order. In addition, from the viewpoint of the effect of the chelating agent (eg, prevention of discoloration, prevention of deterioration, etc.) and cost, the upper limit of the added amount or content of the chelating agent is 1% or less, 8000 ppm, based on the solid content of the monomer or hydrogel polymer at the time of polymerization. Below, it is preferable in order of 6000 ppm or less and 5000 ppm or less. In the present invention, any combination of the upper limit and lower limit of the added amount or content of the chelating agent is preferable. In addition, the above addition amount (ppm) of the chelating agent is the weight of the chelating agent at the time of addition (the solid content of the monomer or the large hydrogel polymer at the time of polymerization), and the salt exchange with the carboxy group of the monomer and polymer after mixing after addition Without consideration, the neutral salt type chelating agent is in the salt form, and the acid type chelating agent is in the acid form.

The weight percent of the chelating agent relative to the solid content of the monomer or hydrogel polymer can be converted into a molar ratio (mol%) to the monomer at the time of polymerization. In the present invention, the added amount or content of the chelating agent is preferably 0.0002 mol% or more, 0.001 mol% or more, 0.002 mol% or more, 0.005 mol% or more, and 0.01 mol% or more, in the order of the monomers at the time of polymerization. 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. Moreover, in this invention, any combination of the upper limit and the lower limit of the said molar ratio of a chelating agent is preferable. In addition, in order to improve the residual rate of the chelating agent, the lower the molar ratio of the persulfate to the chelating agent is, the more preferably, 50 times or less, 20 times or less, 10 times or less, 5 times or less, and 3 times or less in the order. , the lower limit is 0. In this way, the residual ratio of the chelating agent is improved, the content of the chelating agent inside the polymer in the obtained water absorbent polymer is increased, and the L value of the initial coloration of the water absorbent resin is increased and the YI value is lowered.

By adding a chelating agent to the monomers during polymerization or to the hydrogel before drying, the chelating agent can be uniformly blended into the polymer of the water absorbent polymer (particularly, the water absorbent polymer of the final product) after the drying process. By adding a chelating agent after the drying step and further in the surface crosslinking step or subsequent steps, the chelating agent is selectively incorporated into 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 also inside the polymer. This time, it was found that when a large number of chelating agents are used for polymerization, the initial coloration of the resulting water absorbent polymer deteriorates. However, in the present invention, there is no such problem, and the chelating agent can be uniformly blended into the water absorbent polymer.

When a chelating agent is added in the polymerization step described above, it can be added in the monomer aqueous solution like other additives.

In addition, when a chelating agent is added in the above-mentioned gel grinding step, it can also be added and kneaded to the hydrogel polymer like other additives, and gel grinding can also be carried out. Specifically, an aqueous solution containing a chelating agent can be supplied into the gel grinding device while the hydrogel polymer is staying in the gel grinding device. Alternatively, an aqueous solution containing a chelating agent may be added to the hydrogel polymer beforehand and introduced into the gel grinding device. You may combine the addition timing of these chelating agents suitably.

Examples of the chelating agent suitably used in the present invention include a polymeric or non-polymeric chelating agent, more preferably a non-polymeric chelating agent, and still more preferably a non-polymeric chelating agent with a molecular weight of 1000 or less.

Specific examples of the chelating agent used in the present invention include amino polyvalent carboxylic acids, organic polyvalent phosphoric acids (especially amino polyvalent phosphoric acids), inorganic polyvalent phosphoric acids, and tropolone derivatives. At least one chelating agent selected from the group consisting of these compounds is used in the present invention. In addition, the above-mentioned "polyvalent" refers to having a plurality of functional groups in one molecule, preferably having 2 to 30 functional groups, more preferably 3 to 20 functional groups, still more preferably 4 to 10 functional groups. point

Specifically, as the amino polyhydric carboxylic acid, iminodiacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetic acid, nitrilotripropionic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid (DTPA), Triethylenetetramine hexaacetic acid (TTHA), trans-1,2-diaminocyclohexanetetraacetic acid, N,N-bis(2-hydroxyethyl)glycine, diaminopropanoltetraacetic acid, ethylenediamine2propionic acid, N- Hydroxyethylethylenediaminetetraacetic acid, glycol etherdiaminetetraacetic acid, diaminopropanetetraacetic acid, N,N'-bis(2-hydroxybenzyl)ethylenediamine-N,N'-2acetic acid, 1,6-hexamethylene diamine-N,N,N',N'-tetraacetic acid and salts thereof; and the like.

Specifically, as the organic polyvalent phosphoric acid, nitriloacetic acid-di(methylenephosphinic acid), nitrilodiacetic acid-(methylenephosphinic acid), nitriloacetic acid-β-propionic acid-methylenephosphonic acid, nitrilotris(methylenephosphonic acid) ), 1-hydroxyethylidenediphosphonic acid, amino polyvalent phosphoric acid, and the like.

Specifically, as the amino polyhydric phosphoric acid, ethylenediamine-N,N'-di(methylenephosphinic acid), ethylenediaminetetra(methylenephosphinic acid), cyclohexanediaminetetra(methylenephosphonic acid), ethylenediamine-N,N '-Diacetic acid-N,N'-di(methylenephosphonic acid), ethylenediamine-N,N'-di(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), polymethylenediaminetetra(methylenephosphonic acid) , diethylenetriamine penta(methylenephosphonic acid), ethylenediamine tetramethylenephosphonic acid (EDTMP) and salts thereof, and the like.

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

Specific examples of the tropolone derivatives include tropolone, β-tyaplicine, and γ-tyaplicine.

Among these, an amino polyvalent carboxylic acid-based chelating agent and/or an amino polyvalent phosphate-based chelating agent are preferable. Specifically, as an amino polyhydric carboxylic acid-based chelating agent, specifically, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and triethylenetetraaminehexaacetic acid, and metal salts thereof such as sodium salt and potassium salt, etc. can be heard Moreover, as an amino polyvalent phosphoric acid type chelating agent, ethylenediamine tetramethylene phosphonic acid is mentioned specifically,.

In the present invention, in this way, a water absorbent resin containing a chelating agent uniformly inside the polymer is obtained. However, deterioration and coloring of the water absorbent resin are likely to occur on the surface of the particles. For this reason, in the present invention, by further adding a chelating agent to the water absorbent polymer in the step after the drying step, more chelating agents are added to the surface of the particles, that is, by adding the chelating agent multiple times, the surface and inside of the water absorbent polymer It is more preferable to form a concentration gradient such that a chelating agent is included and the amount of the chelating agent present on the surface is greater than the amount of the chelating agent present inside. The type and amount of the chelating agent added in the step after the drying step may be the same as or different from the type and amount of the chelating agent added in the step preceding the drying step, but are preferably the same type and amount as described above. In addition, the chelating agent may be added only to the monomer or hydrogel polymer, or the chelating agent may be added to the monomer or hydrogel polymer as a solution (particularly aqueous solution).

(polymerization method)

In the method for producing a water-absorbent polymer containing a chelating agent according to the present invention, the polymerization method may directly obtain a particulate hydrogel polymer by spray/droplet polymerization or reverse-phase suspension polymerization in a gas phase. From the viewpoints of liquidity (SFC) and absorption rate (FSR) and ease of polymerization control, etc., aqueous solution polymerization is employed.

In the aqueous solution polymerization, a water-containing gel polymer is obtained by tank type (silo type) or belt type non-agitation polymerization, and gel pulverization may be performed separately, or, like kneader polymerization, gel pulverization is performed in the middle of the polymerization step to obtain a particulate hydrogel polymer. Although it may be obtained, from the viewpoint of ease of polymerization control, kneader polymerization or belt polymerization is preferably employed. Further, from the viewpoint of productivity, preferably continuous aqueous solution polymerization, more preferably high-concentration continuous aqueous solution polymerization is employed. Further, stirring polymerization means polymerization of a hydrogel polymer (particularly, a hydrogel polymer having a polymerization rate of 10 mol% or more, furthermore 50 mol% or more) while stirring, particularly stirring, and subdividing. Further, before and after non-stirring polymerization, the monomer aqueous solution (polymerization rate of 0 to less than 10 mol%) may be suitably stirred.

As the continuous aqueous solution polymerization, for example, continuous kneader polymerization described in US Patent Nos. 6987171 and 6710141 and the like, and continuous belt described in US Patent Nos. 4893999 and 6241928 and US Patent Application Publication No. 2005/215734 and the like polymerization can be mentioned. Water-absorbent resins can be produced with high productivity by polymerization of these aqueous solutions.

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

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

The increase in the solid content of the hydrogel polymer relative to the solid content (total concentration of the monomer and the graft component) in the aqueous monomer solution [= (solid content [mass%] of the hydrous gel polymer) - (solid content [mass%] in the aqueous monomer solution)] is, 1% or more is preferable, 3% or more is more preferable, and 5% or more is still more preferable. The upper limit of the rising width is 15% or less from the viewpoint of easiness of polymerization control. Further, the solid content of the hydrogel polymer is preferably 40 to 75% by mass, more preferably 45 to 70% by mass, still more preferably 50 to 65% by mass. In this way, when a water-containing gel polymer having a high solid content is obtained by polymerization, decomposition of the chelating agent in the drying step is reduced, and the residual rate of the chelating agent is improved. However, when the solid content of the hydrogel polymer exceeds 75% by mass, the physical properties of the water-absorbent resin obtained may be deteriorated. In the present invention, an embodiment of foaming 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 polymer contains at least air bubbles, not only the absorption rate can be improved, but also the drying rate can be improved, and the chelating agent content (residual rate) after the drying process can be increased.

In addition, in the mechanism of the specific reduction of the chelating agent in the drying step found in the present invention, the reaction mechanism is not clear, but "radicals of persulfate react preferentially with monomers during polymerization, but in the hydrous gel phase after polymerization. Since the monomer remaining in the polymer is a small amount of several percent or less, and since the drying is performed at a higher temperature than during polymerization and the solid content is higher, in the drying step, the persulfate remaining after polymerization is preferential with the chelating agent. It is presumed that it responds with In the prior art, it was said that it is desirable that the residual monomer in the hydrogel polymer be 1000 ppm or less (US Patent No. 514906, US Patent No. 453323, European Patent No. 530438). However, in the present invention, it is not necessary to set the polymerization rate after polymerization to 100%, rather, the residual monomer in the hydrogel polymer before the drying step is 0.1% by mass or more, furthermore 0.5 to 10% by mass, 0.5 to 5% by mass, or 0.5% by mass. It is preferably contained in an amount of about 3% to 3% by weight. In order to make almost 100% of the chelating agent remain in the polymerization process, from the above estimation mechanism, the polymerization time is preferably shorter in order to sufficiently remain unreacted monomers at the end of the polymerization process. That is, the present invention is preferably applied to short-time polymerization in which the polymerization time is 60 minutes or less, 10 minutes or less, 5 minutes or less, or 3 minutes or less (the lower limit of the polymerization time is 1 second or more than 10 seconds).

Therefore, in order to achieve both persulfate reduction and remaining monomers, the polymerization conditions include a polymerization initiation temperature of 30° C. or more (and further the above range), a polymerization peak temperature of 80 to 130° C. (and the above range), and a polymerization time of 60 minutes or less (moreover, High-temperature initiation, short-time polymerization in the above range) is particularly preferred, and high-concentration, high-temperature initiation, short-time polymerization in which the monomer concentration is 35% by weight or more (and further in the above range) is most preferred.

The above high-concentration, high-temperature initiation short-time polymerization is disclosed in US Patent Nos. 6906159 and 7091253, but the polymerization method obtains a water-absorbent resin with high whiteness and is easy to produce on an industrial scale. Because it is, it is preferable.

Therefore, the polymerization method in the manufacturing method according to the present invention is preferably applied to large-scale manufacturing equipment with a large amount of production per line. Moreover, as said amount of production, 0.5 t/hr or more is preferable, 1 t/hr or more is more preferable, 5 t/hr or more is still more preferable, and 10 t/hr or more is especially preferable.

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

(2-2) Gel crushing process

This step is a step of subdividing the above-described hydrogel polymer during or after polymerization to obtain a particulate hydrogel polymer. In addition, this process is called "gel grinding" to distinguish from "grinding" in the following (2-4) pulverization process/classification process.

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

(Gel Grinder)

The gel grinding device used during polymerization or after polymerization used in this step is not particularly limited, and a batch type or continuous type double arm type kneader or the like, a gel grinder equipped with a plurality of rotary stirring blades, a single screw extruder, or a twin screw extruder , meat choppers, especially screw-type extruders, and the like.

Among them, a screw type extruder provided with a perforated plate at one end of the casing is preferable, and specifically, a screw type extruder disclosed in Japanese Patent Laid-Open No. 2000-63527 or WO2011/126079 is exemplified.

(gel crushing area)

In the present invention, the gel pulverization is performed during and/or after the polymerization process, and more preferably on the hydrogel polymer after the polymerization process. The state of "sufficient gelation" is referred to as a gel crushing step.

For example, in the case of employing kneader polymerization, the aqueous monomer solution changes to a hydrogel polymer as the polymerization time elapses. That is, the stirring region of the monomer aqueous solution at the start of polymerization, the stirring region of the hydrogel polymer having a certain viscosity and low degree of polymerization during polymerization, the gel crushing initiation region of part of the hydrogel polymer as polymerization proceeds, and the latter half of polymerization. Alternatively, the gel crushing region at the end stage is performed continuously. Therefore, in order to clearly distinguish between "stirring of the monomer aqueous solution" at the start of polymerization and "gel pulverization" at the final stage, judgment is made based on the state of "sufficient gelation".

The above "sufficient gelation" refers to a state in which the hydrogel polymer can be subdivided by applying a shear force after the polymerization temperature reaches the maximum (polymerization peak temperature). Alternatively, the polymerization rate (another name; conversion rate. The polymerization rate is calculated from the amount of polymer calculated from pH titration of the hydrogel polymer and the amount of remaining monomer) of the monomer in the aqueous monomer solution is preferably 90 mol% or more, more preferably After reaching 93 mol% or more, more preferably 95 mol% or more, and particularly preferably 97 mol% or more, it refers to a state in which the hydrogel polymer can be subdivided by applying a shear force. That is, in the gel pulverization step of the present invention, a hydrogel polymer having a monomer polymerization rate in the above range is gel pulverized. Further, in the case of a polymerization reaction that does not exhibit the polymerization peak temperature (for example, when polymerization always proceeds at a constant temperature or when the polymerization temperature continues to rise), with the polymerization rate of the monomer, "sufficient gelation" 」 is defined.

In addition, when the polymerization step is belt polymerization, the hydrogel polymer during and/or after the polymerization step before gel pulverization, preferably, the hydrogel polymer after the polymerization step is cut or coarsened to a size of about several 10 cm. can be crushed This operation makes it easier to fill the gel grinding device with the hydrogel polymer, and the gel grinding step can be carried out more smoothly. As the means for cutting or coarsely pulverizing, a means capable of cutting or coarsely pulverizing the hydrogel polymer so as not to break is preferable, and examples thereof include a guillotine cutter. In addition, the size and shape of the hydrogel polymer obtained by the above cutting or coarse pulverization is not particularly limited as long as it can be filled in a gel pulverizer.

(use of water)

In the gel grinding step of the present invention, gel grinding may be performed by adding water to the hydrogel polymer. In the present invention, "water" shall take any form of solid, liquid, or gas.

Regarding the addition of the water, there are no restrictions on the method or timing of addition, as long as water is supplied into the device while the hydrogel polymer is staying in the gel grinding device. Alternatively, water may be added to the hydrogel polymer beforehand and introduced into the gel grinding device. In addition, the said water is not limited to "water alone", You may add other additives (for example, surfactant, base for neutralization, a crosslinking agent, etc.) or solvent other than water. 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 substance.

In the present invention, the water can be used in any form of solid, liquid, or gas, but from the viewpoint of handleability, a liquid and/or gas is preferable. The supply amount of water is preferably 0 to 25 parts by weight, more preferably 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 in this order, based on 100 parts by weight of the hydrogel polymer. When the supply amount of the water exceeds 25 parts by weight, problems such as difficulty in controlling the particle size of the hydrogel polymer, reduction in the remaining amount of the chelating agent due to a long drying time, and generation of undried material during drying may occur. there is Further, from the viewpoint of drying efficiency, it is preferable that the supply amount of water in this step does not exceed the amount of the solvent evaporated in the polymerization step, particularly the evaporated water.

When supplying the said water as a liquid, 10-100 degreeC is preferable and, as for the temperature at the time of supply, 40-100 degreeC is more preferable. Further, 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 still more preferably 100 to 130°C. In addition, when supplying water as a gas, the manufacturing method is not particularly limited, and for example, a method using water vapor generated by heating a boiler, water is vibrated with ultrasonic waves, and gaseous water generated on the water surface is removed. How to use it, etc. In the present invention, when water is supplied as a gas, steam at a higher pressure than atmospheric pressure is preferred, and steam generated from a boiler is more preferred.

(use of additives)

As described above, it is preferable to add water to the hydrogel polymer and perform gel pulverization. However, it is also possible to add and knead other additives or neutralizers to the hydrogel polymer to perform gel pulverization. You can do it. Specifically, at the time of gel grinding, neutralization is performed by adding an aqueous solution containing the basic substance described in (2-1) above (eg, 10 to 50% by weight aqueous sodium hydroxide solution) (particularly in the range of the neutralization rate described above). ), or fine powder recycling may be performed by adding water absorbent resin fine powder (0.1 to 30% by weight (large resin solid content)). In addition, 0.001 to 3% by weight (large resin solid content) of a polymerization initiator may be added and mixed during gel grinding to reduce residual monomers.

(Physical properties of particulate hydrogel polymer after gel pulverization)

(a) particle size

In the present invention, it is essential to set the weight average particle diameter (D50) of the particulate hydrogel polymer to 1 mm or less, and gel grinding is performed so that the particles are finer than conventional gel grinding.

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

The weight average particle diameter (D50) (defined by sieve classification) of the particulate hydrogel polymer after gel grinding is 1 mm or less, 10 μm to 1 mm, 20 μm to 1 mm, 40 μm to 1 mm, 50 μm to 50 μm. It is more preferable in the order of 900 micrometers. Further, the upper limit of the weight average particle size 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 that order. In the present invention, any combination of the upper limit and lower limit of the weight average particle diameter is preferable. The gel particle diameter 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 a chelating effect suitable for 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, it becomes difficult to dry the entire gel layer. In addition, grinding after drying produces a large amount of fine powder, which makes it difficult to control the particle size and may deteriorate physical properties such as liquid permeability (SFC).

In addition, in order to gel pulverize or polymerize the weight average particle size of the hydrogel polymer to less than 10 μm, it is difficult to achieve only by ordinary gel pulverization or polymerization, and it is difficult to achieve by a separate special operation, for example, classification of the gel after pulverization (eg For example, Japanese Unexamined Patent Publication No. 6-107800, etc.) or particle size control during polymerization before gel pulverization (for example, a method for obtaining sharp particle size gel particles by reverse phase suspension polymerization; European Patent No. 0349240, etc. ) is required. Therefore, in addition to gel pulverization, the addition of a special method as described above may require a large amount of surfactant or organic solvent for polymerization or classification, decrease in productivity (increase in cost) or deterioration of physical properties (increase in residual monomer or fine powder). increase), etc., and there is a possibility that a new problem may occur. For this reason, it may be difficult to obtain a particulate hydrogel polymer having a weight average particle diameter of less than 10 µm.

(b) Gel CRC after gel crushing

In the present invention, the gel CRC of the particulate hydrogel polymer after gel grinding is preferably 10 to 35 g/g, more preferably 10 to 32 g/g, and even more preferably 15 to 30 g/g. Further, the gel CRC after gel grinding is preferably -1 to +3 g/g, more preferably 0.1 to 2 g/g, and even more preferably 0.3 to 1.5 g/g relative to the gel CRC before gel grinding. In addition, the gel CRC may be reduced by using a crosslinking agent or the like at the time of gel grinding, 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 grinding is preferably 0.1 to 20% by weight, more preferably 0.1 to 10% by weight, still more preferably 0.1 to 8% by weight, and 0.1 to 20% by weight. 5% by weight is particularly preferred. Further, the amount of increase in gel Ext of the particulate hydrogel polymer after gel grinding (the amount of increase relative to the gel Ext before gel grinding) is preferably 5% by weight or less, more preferably 4% by weight or less, and still more preferably 3% by weight or less. 2% by weight or less is particularly preferred, and 1% by weight or less is most preferred. The lower limit may be negative (for example, -3.0% by weight, furthermore -1.0% by weight), but is usually 0% by weight or more, preferably 0.1% by weight or more, more preferably 0.2% by weight or more, still more preferably. It is preferably 0.3% by weight or more. Specifically, it is preferable to gel crush until the gel Ext is increased to be within the arbitrary range of the above-mentioned upper limit and lower limit, such as preferably 0 to 5.0% by weight, more preferably 0.1 to 3.0% by weight, etc. In addition, the gel Ext may be reduced by using a crosslinking agent or the like at the time of gel grinding, but it is preferable to increase the gel Ext within the above range. Here, the significant digit of the increase in gel Ext is one decimal place, but for example, 5% by weight and 5.0% by weight are treated as synonyms.

(d) weight average molecular weight of the water-soluble component after gel grinding

In the present invention, the lower limit of the increase in the weight average molecular weight of the water-soluble component of the hydrogel polymer after gel grinding is preferably 10,000 Da or more, more preferably 20,000 Da or more, and still more preferably 30,000 Da or more. The upper limit is preferably 500,000 Da or less, more preferably 400,000 Da or less, still more preferably 250,000 Da or less, and particularly preferably 100,000 Da or less. For example, in the present invention, the increase in the weight average molecular weight of the water-soluble component of the particulate hydrogel polymer after gel grinding relative to the hydrogel polymer before gel grinding is preferably 10,000 to 500,000 Da, preferably 20,000 Da. to 400,000 Da, more preferably 30,000 to 250,000 Da, still more preferably 100,000 Da or less.

(e) Resin solid content after gel grinding

In the present invention, the resin solid content of the particulate hydrogel polymer after gel grinding is preferably 40 to 75% by mass, more preferably 45 to 70% by mass, still more preferably 50 to 75% by mass, from the viewpoint of physical properties. It is 65 mass %. By setting the resin solid content of the particulate hydrogel polymer after gel pulverization to the above range, it is easy to control the increase in CRC due to drying, the damage caused by drying is small (increase in water soluble content, etc.), and further, the decomposition of the chelating agent is prevented. This is preferable because it decreases and the residual rate is improved. In addition, the resin solid content after gel grinding can be appropriately controlled by the resin solid content before gel grinding, water added as needed, and water evaporation by heating during gel grinding.

(Number of measurement points)

In order to evaluate the physical properties of the hydrogel polymer before gel pulverization or the particulate hydrogel polymer after gel pulverization, it is necessary to perform sampling and measurement in a required amount and frequency from a manufacturing apparatus. In the present invention, evaluation is performed on the basis of the weight average molecular weight of the water-soluble component of the hydrogel polymer before gel grinding, but it is necessary to ensure that this value is sufficiently averaged. There, for example, in the case of continuous gel grinding using a continuous kneader or meat chopper, when the production amount of the water absorbent polymer is 1 to 20 t/hr or 1 to 10 t/hr, 2 points or more for every 100 kg of the hydrogel polymer, in total Sampling and measurement of at least 10 points or more may be performed, and in the case of batch type gel grinding (eg, batch type kneader), sampling and measurement of at least 10 points or more are performed from the batch sample to determine the physical properties of the particulate hydrogel polymer. you have to evaluate

(2-3) Drying process

This step is a step of drying the particulate hydrogel polymer pulverized to the specific particle diameter in the gel pulverization step to obtain a dried polymer. Hereinafter, a drying method preferably applied in the present invention will be described.

In the present invention, the particulate hydrogel containing the chelating agent is dried. In a method for producing a water absorbent polymer containing a chelating agent, it has been found that the chelating agent does not substantially decrease during polymerization or after the drying process is completed, and the chelating agent is decomposed during the drying process due to the remaining persulfate. Therefore, in order to incorporate the chelating agent into the water absorbent polymer, it is preferable to control the amount of persulfate remaining during the drying step to a low level. Further, in order to suppress decomposition of the chelating agent, it is preferable to perform rapid drying in the drying step. Therefore, in the present invention, the particle diameter of the hydrogel polymer is controlled to be small, specifically, the weight average particle diameter (D50) of the particulate hydrogel polymer is controlled to 1 mm or less, and the solid content is 80% by weight in a drying time of 20 minutes or less. It is particularly preferable to dry it until it becomes abnormal. However, the said drying time points out the time until solid content becomes 80 weight% or more. By the above method, the residual ratio of the chelating agent is improved, the amount of the chelating agent inside the polymer in the obtained water absorbent resin is increased, the L value of the initial coloring of the obtained water absorbent resin is increased, and the YI value is lowered.

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

Further, in method 2 according to the present invention, the chelating agent content of the particulate hydrogel polymer supplied in the drying step (vs. hydrogel polymer solid content) is 10 ppm or more, 40 ppm or more, 60 ppm or more, 100 ppm or more, or 200 ppm or more. , preferably in the order of 250 ppm or more, 500 ppm or more, and 600 ppm or more. Further, from the viewpoint of the effect of the chelating agent (eg, prevention of discoloration, prevention of deterioration, etc.) and cost, the upper limit of the content of the chelating agent is 1% or less, 8000 ppm or less, 6000 ppm or less, 5000 ppm or less, based on the solid content of the hydrogel polymer. preferred in order. In the present invention, any combination of the upper limit and the lower limit of the content of the chelating agent is preferable. In addition, the content of the chelating agent shows the weight ppm concentration as it is when added as a chelating agent without considering salt exchange with carboxy groups of monomers and polymers after mixing.

(drying device)

As the drying apparatus used in the drying step, a hot air transfer type dryer (hereinafter referred to as a "hot air dryer") is preferable from the viewpoint of drying speed. That is, as a drying format, hot air drying is preferable. Examples of the hot air dryer include hot air dryers such as ventilation belt (band) type, ventilation circuit type, ventilation vertical type, parallel flow belt (band) type, ventilation tunnel type, ventilation groove type stirring type, fluidized bed type, air flow type, and spray type. can In the present invention, a ventilation belt type hot air dryer is preferable from the viewpoint of physical property control.

In the case of using such a ventilation belt type hot air dryer, from the viewpoint of drying uniformity and physical properties such as liquid permeability, the wind direction of the hot air used in the dryer is perpendicular to the water-containing gel polymer layer stacked and stationary on the ventilation belt. It is preferable that it is a direction (for example, a combination of a vertical direction or an upward direction, a downward direction). In addition, the above-mentioned "vertical direction" means ventilation from above and below (from above to below, or from below to above the gel layer) with respect to the gel layer (particulate hydrogel polymer having a thickness of 10 to 300 mm and laminated on a punched metal or metal net). It refers to a state of doing, and is not limited to the strictly vertical direction as long as it ventilates in the vertical direction. For example, you may use hot air in an oblique direction, and in this case, it is within 30° with respect to the vertical direction, preferably within 20°, more preferably within 10°, still more preferably within 5°, particularly preferably Preferably, hot air of 0° is used.

Hereinafter, drying conditions and the like in the drying step of the present invention are described. By performing drying under the following drying conditions, the liquid permeability and water absorption rate of the water absorbent resin obtained by surface treatment of the dried polymer obtained 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. Further, the drying temperature is 200°C or less, preferably 190°C or less, and more preferably 180°C or less. In the present invention, any combination of the upper limit and the lower limit of the drying temperature is preferable. If the drying temperature is less than 80°C, it is not preferable because the drying time until a suitable resin solid content (moisture content) is obtained becomes long and the decomposition rate of the chelating agent increases. In addition, clogging may occur during the subsequent crushing process due to the formation of undried material. When the drying temperature exceeds 200°C, it is not preferable because the chelating agent is easily decomposed. In addition, the drying temperature refers to the temperature of the heating medium used for drying in the case of direct heating, the temperature of the hot air used for drying in the case of hot air drying, and the temperature of the heat transfer surface used for drying in the case of indirect heating. point

(drying time)

The drying time in the drying step of the present invention refers to the time until the solid content reaches 80% by weight or more, and is preferably 20 minutes or less, 18 minutes or less, 15 minutes or less, and 12 minutes or less in this order. The lower limit of the drying time is about 1 minute in consideration of the drying efficiency. In addition, it was found that decomposition of the chelating agent mainly occurs in the drying step until the solid content reaches 80% by weight. In addition, the total drying time in the present invention is preferably 60 minutes or less, more preferably 50 minutes or less, and 40 minutes or less in this order. If the drying time is short, undried material is produced and clogging may occur during the subsequent grinding process. If the total drying time exceeds 60 minutes, the decomposition rate of the chelating agent increases, which is not preferable. Further, in the present invention, even if water evaporation has not substantially started, that is, a step of heating and raising the temperature of the particulate hydrogel polymer in a dryer is also included in the drying step. The start of the drying time is the time when the hydrogel polymer is put into the dryer, and the total drying time is the time from putting the hydrogel polymer into the dryer until the hydrogel polymer is dried and taken out of the dryer.

In addition, the time until the water-containing gel polymer discharged from the polymerization process is introduced into the drying process and the temperature of the water-containing gel polymer, that is, the time for the water-containing gel polymer to move from the outlet of the polymerizer to the inlet of the dryer and when it moves to the inlet of the dryer The temperature of the hydrogel polymer is 80 ° C. or less for 30 minutes, 90 ° C. or less for 20 minutes, and 100 ° C. or less for 10 A combination of within a minute and within 5 minutes at 110°C is preferable. Further, at the laboratory level, the hydrogel polymer obtained in the polymerization step may be once cooled to 40° C. or lower, left for a long time, and then introduced into the drying step.

(resin solids)

The particulate hydrogel obtained in the gel pulverization step is dried in the above drying step to become a dried polymer. The resin solid content determined from the drying loss of the dry 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, still more preferably is 90 to 98% by weight.

(wind speed)

In the drying process of the present invention, the wind speed of the hot air in the ventilation dryer, particularly the belt type dryer, in the vertical direction (vertical direction) is preferably 0.8 to 2.5 m/s, and more preferably 1.0 to 2.0 m/s. desirable. By making the said wind velocity into the said range, it becomes easy to control the moisture content of the dry polymer obtained to a desired range. When the wind speed exceeds 2.5 m/s, problems such as the particulate hydrogel polymer flying up may occur during the drying period.

In addition, the control of the wind speed may be performed within a range that does not impair the effect of the present invention, for example, within a range of 70% or more of the drying time, preferably 90% or more, and more preferably 95% or more. In addition, the wind speed is expressed as an average flow speed of hot air passing in a vertical direction with respect to a horizontally moving band surface, taking a ventilation belt type dryer as an example. Therefore, the average flow rate of the hot air can be obtained by dividing the air volume blown to the ventilation belt dryer by the area of the ventilation belt.

(Fever stalls)

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

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

In the drying step of the present invention, when the particulate hydrogel polymer is dried, the particulate hydrogel polymer is continuously supplied in layers on a belt of a ventilated belt type dryer and dried with hot air. 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. Moreover, 10 m or less is preferable and, as for an upper limit, 5 m or less is more preferable. Moreover, 20 m or more is preferable and, as for the length of a belt, 40 m or more is more preferable. Moreover, 100 m or less is preferable and, as for an upper limit, 50 m or less is more preferable.

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

In addition, the moving speed of the particulate hydrogel polymer on the belt may be appropriately set according to the belt width, belt length, production amount, drying time, etc., but from the viewpoint of the load and durability of the belt drive device, 0.3 to 5 m/min is preferable. It is preferable, 0.5-2.5 m/min is more preferable, 0.5-2 m/min is still more preferable, and 0.7-1.5 m/min is especially preferable.

(2-4) Crushing process, classification process

The method for producing a water absorbent resin containing a chelating agent according to the present invention may further include a pulverization step and a classification step of pulverizing and classifying the dried polymer obtained in the above drying step. This step differs from the gel grinding step (2-2) in that the resin solid content during grinding, particularly the object to be crushed, is subjected to a drying step (preferably dried to the resin solid content). In addition, the water absorbent resin obtained after the pulverization step is also referred to as a pulverized product.

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

The grinder that can be used in the crushing step is not particularly limited, but examples include vibrating mills, roll granulators, knuckle-type grinders, roll mills, high-speed rotary grinders (pin mills, hammer mills, screw mills), cylindrical mixers, and the like. can be heard Among these, it is preferable to use a multi-stage roll mill or a roll granulator from the viewpoint of particle size control.

In this classification step, a classification operation is performed so as to have the following particle sizes. When surface crosslinking is performed, it is preferable to perform the classification operation before the surface crosslinking step (first classification step), and also after surface crosslinking, a classification operation (second classification step) may be performed. In addition, although a 1st classification process is normally implemented after a grinding process, you may implement a classification process further before a grinding process. In addition, the weight average particle diameter (D50) and the like of the water-absorbent resin particles after classification are not particularly limited and can be appropriately adjusted according to the 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, still more preferably 300 to 500 μm. . The proportion of particles having a particle size of 850 to 150 µm is preferably 90% by weight or more, more preferably 95% by weight or more, and still more preferably 97% by weight or more. In addition, it is preferable that the water absorbent resin as a final product is also in particulate form, and the above particle size range is applied.

(2-5) Surface treatment process

The method for producing a water absorbent polymer 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 crosslinking step performed using a known surface crosslinking agent and a surface crosslinking method, and also includes other addition steps as necessary. In the present invention, it was found that the chelating agent does not decrease in the surface crosslinking or 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, preferably, as the surface crosslinking agent, a polyhydric alcohol compound, an epoxy compound, a polyhydric amine compound or a condensate with a halo-epoxy compound thereof, an oxazoline compound, a (mono, di or poly)oxazolidinone compound, an alkylene car A dehydration-reactive crosslinking agent containing a polyhydric alcohol compound, an alkylene carbonate compound, and an oxazolidinone compound that is a bonate compound and requires reaction at a particularly high temperature can be used. In the case of not using a dehydration-reactive crosslinking agent, more specifically, compounds exemplified in US Patent Nos. 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, 1,6- polyhydric alcohol compounds such as hexanediol and sorbitol; epoxy compounds such as ethylene glycol diglycidyl ether and glycidol; Alkylene carbonate compounds, such as ethylene carbonate; oxetane compounds; and cyclic urea compounds such as 2-imidazolidinone.

(solvent, etc.)

The amount of the surface crosslinking agent is appropriately determined to be preferably about 0.001 to 10 parts by weight, more preferably about 0.01 to 5 parts by weight, based on 100 parts by weight of the water-absorbent resin particles. In accordance with the surface cross-linking agent, water is preferably used. The amount of water used is preferably in the range of 0.5 to 20 parts by weight, and more preferably in the range of 0.5 to 10 parts by weight, based on 100 parts by weight of the water absorbent resin particles. When an inorganic surface crosslinking agent and an organic surface crosslinking agent are used in combination, each is preferably used in an amount of preferably 0.001 to 10 parts by weight, more preferably 0.01 to 5 parts by weight, based on 100 parts by weight of the water-absorbent resin particles.

In addition, 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. In addition, when the crosslinking agent solution is mixed with the water absorbent resin particles, the range that does not interfere with the effects of the present invention, for example, preferably 0 to 10 parts by weight, more preferably 0 to 5 parts by weight, still more preferably is 0 to 1 part by weight, and a water-insoluble fine particle powder or a surfactant may coexist. The surfactant used or its amount is exemplified in US Patent No. 7473739 and the like.

(mix)

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, it is preferable that it is 80-220 degreeC as a heating temperature. Moreover, it is preferable that heating time is 10 to 120 minutes.

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

(Other surface cross-linking methods)

As the surface crosslinking method used in the present invention, a surface crosslinking method using a radical polymerization initiator instead of the surface crosslinking agent (US Patent No. 4783510, International Publication No. 2006/062258) or a water absorbent resin You may use the surface crosslinking method (US 2005/048221, 2009/0239966, WO 2009/048160) which polymerizes a monomer on the surface of.

In the above surface crosslinking method, the radical polymerization initiator preferably used is persulfate, acrylic acid (salt) or the above-mentioned surface crosslinking agent as an optionally preferably used monomer, and water as a preferably used solvent, and these are water absorbent After being added to the surface of the resin, surface crosslinking is performed by performing crosslinking polymerization or a crosslinking reaction using a radical polymerization initiator on the surface of the water absorbing resin by active energy rays (especially ultraviolet rays) or heating.

(Ion-binding surface cross-linking agent)

In the present invention, an addition step of adding any one or more of a polyvalent metal salt, a cationic polymer, or inorganic fine particles may be further included simultaneously with or separately from the surface crosslinking step described above. That is, in addition to the above 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 organic surface cross-linking agent. Examples of the inorganic surface crosslinking agent used include salts (organic salts or inorganic salts) or hydroxides of a divalent or higher, preferably trivalent or tetravalent metal. Aluminum, zirconium, etc. are mentioned as a polyvalent metal which can be used, Specifically, aluminum lactate and aluminum sulfate are mentioned. It is preferably an aqueous solution containing aluminum sulfate.

(2-6) Other processes (differential powder recycling process, etc.)

In addition to the above steps, if necessary, an evaporation monomer recycling step, a granulation step, a fine powder removal step, a fine powder recycling step, etc. may be provided, and any part or In all, you may use the following additives as needed. That is, water-soluble or water-insoluble polymers, lubricants, deodorants, antibacterial agents, water, surfactants, water-insoluble fine particles, antioxidants, reducing agents, etc., relative to the water absorbent resin, are preferably 0 to 30% by weight, more preferably 0.01 to 10% by weight may be added and mixed. These additives can also be used as a surface treatment agent.

In addition, in the manufacturing method of the present invention, a fine powder recycling process may be included. The fine powder recycling step refers to separating 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 classification step, in the state as it is or after hydration. Thus, it refers to a process of recycling to a polymerization process or a drying process, and methods described in US Patent Application Publication No. 2006/247351 or US Patent No. 6228930 can be applied.

In addition, depending on the purpose, an oxidizing agent, an antioxidant, water, a polyvalent metal compound, a water-insoluble inorganic or organic powder such as silica or metal soap, a deodorant, an antibacterial agent, a polymeric polyamine, pulp or thermoplastic fiber, etc. are added in an amount of 0 to 3 weight in the water absorbent resin. %, preferably 0 to 1% by weight.

[3] Absorbent resin

The present invention provides a water absorbent resin containing a chelating agent obtained by the above-described production method according to the present invention. Hereinafter, various physical properties of the water absorbent polymer 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) cross-linked polymer water absorbent resin.

(Residual amount (C1), content (C2), residual rate of chelating agent in water absorbing resin)

The remaining amount (C1) of the chelating agent in the water absorbent polymer is the remaining amount (C1) of the neutral salt type chelating agent as a salt type and the acid type chelating agent as an acid type without considering the salt exchange with the carboxy group of the monomer and polymer after mixing. indicates the quantity. On the other hand, the chelating agent content (C2) of the water absorbent resin indicates the content as an acid type chelating agent. That is, when the chelating agent is an acid type, the concentration 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 as "the residual amount of the chelating agent (C1) x the molecular weight of the acid type/the molecular weight as it is at the time of addition of the chelating agent".

The residual ratio (%) of the chelating agent is calculated as "{(remaining amount of chelating agent (C1) [ppm])/(added amount of chelating agent [ppm])} × 100".

The remaining 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 or more, from the viewpoint of coloration prevention and deterioration prevention, etc. It is preferable in the order of above, 500 ppm or more, and 600 ppm or more. In addition, 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 or less from the viewpoint of the effect (coloration prevention, deterioration prevention, etc.) and cost of the chelating agent , 8000 ppm or less, 6000 ppm or less, preferably in the order of 5000 ppm or less.

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

In addition, the residual ratio of the chelating agent is the amount of chelating agent added [ppm] in the manufacturing process of the water absorbent polymer (solid content of the monomer or the large hydrogel polymer at the time of polymerization), and the remaining amount of the chelating agent in the water absorbent polymer. It is the ratio [%] of the amount (C1) [ppm]. In addition, the residual ratio of the chelating agent is determined from the ratio of the content of the chelating agent in the final product to the sum of the content of the chelating agent in the final product and the amount obtained by converting the amount (ppm) of decomposition products derived from the chelating agent into the content of the chelating agent. can be saved

(Initial color 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 an initial color tone (also referred to as "initial coloration") of at least 85, and preferably 89 or more, in the Hunter Lab colorimetric measurement by a spectroscopic colorimeter. It is preferable to represent 90 or more. In addition, although the upper limit of L value is 100 normally, if it is 85 in powder, the problem with color tone does not arise in products, such as a sanitary material.

Note that the initial color tone is the color tone after production of the water absorbent resin, but is generally considered to be the color tone measured before shipment from the factory. In addition, it is a value measured within 1 year after manufacture, if it is stored in the atmosphere of 30 degreeC or less, relative humidity 50% RH, for example.

In addition, the YI value (yellowness; yellowness index) of the initial color of the water absorbent resin according to the present invention is preferably 0 to 13, 0 to 10, 0 to 9, 0 to 7, 0 to 5, and 0 to 3 in the order. and almost no yellowing.

In one aspect of the present invention, a method for producing a water absorbent polymer containing a chelating agent according to the present invention described above, wherein the persulfate in the polymerization initiator used in the polymerization step is 0 to 0.04 mol% (to monomers during polymerization) (However, in the case of 0 mol% persulfate (non-use), other polymerization initiators are necessarily used), and in the process prior to the drying process, the chelating agent is added to the aqueous monomer solution or the hydrogel polymer in a total amount of 10 ppm or more (at the time of polymerization monomer or large hydrogel polymer solid content) is added, the weight average particle diameter (D50) of the particulate hydrogel polymer is set to 1 mm or less, and drying is performed until the solid content reaches 80% by weight or more in the drying step. By the manufacturing method in which the time is 20 minutes or less, as a final product, the residual amount (C1) of the chelating agent is 10 ppm or more, the L value of the initial color tone is 85 or more, and the YI value is 13 or less. there is.

In a further aspect of the present invention, a method for producing a water absorbent polymer containing the chelating agent according to the present invention described above, wherein the persulfate in the polymerization initiator used in the polymerization step is 0 to 0.015 mol% (for polymerization monomer) (however, if persulfate is 0 mol% (non-use), other polymerization initiators are necessarily used), and in the process prior to the drying process, the chelating agent is added to the aqueous monomer solution or the hydrogel polymer at least 200 ppm in total (large monomer at the time of polymerization or solid content of hydrogel polymer) is added, the weight average particle diameter (D50) of the particulate hydrogel polymer is set to 1 mm or less, and in the drying step, until the solid content reaches 80% by weight or more As a final product, a water-absorbent resin having a residual amount (C1) of a chelating agent of 200 ppm or more, an L value of an initial color tone of 89 or more, and a YI value of 10 or less is produced by a manufacturing method in which the drying time is 20 minutes or less. can do.

In a further 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 (and the above range), and the YI value is 10 or less (further the above range) range) of a polyacrylic acid (salt)-based water absorbent resin.

In one further aspect of the present invention, in the water absorbent resin, a chelating agent is included on the surface and inside, and the amount of the chelating agent present on the surface is greater than the amount of the chelating agent present therein. Provide a water absorbent resin do.

(Shape of water absorbing 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 irregularly crushed. In addition, the irregular crushed phase is a particle shape obtained by crushing a hydrogel polymer or a dry polymer and having an irregular shape.

(Particle diameter of water absorbing resin)

The weight average particle diameter (D50) of the water absorbent resin of the present invention is preferably 200 to 800 μm, more preferably 200 to 600 μm, and still 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 still more preferably 97% by weight or more. The water-absorbent resin of the present invention is easy to handle and exhibits water-absorbent performance in sanitary materials and the like because of the particle size described above.

(Absorption factor under no pressure of absorbent polymer)

In order to more remarkably express the effect of the chelating agent (eg, prevention of discoloration, prevention of deterioration, etc.), the water-absorbent resin according to the present invention preferably has a higher water absorption capacity under no pressure (CRC), preferably 15 g/day. g or more, more preferably 25 g/g or more, even 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, from the viewpoint of balance with other physical properties (eg, water absorption capacity under pressure (AAP)). less than g. CRC can be controlled by the crosslinking density during polymerization or surface crosslinking.

Also, when the CRC is less than 15 g/g, the crosslinking density of the water absorbent polymer is high, so that the effect of preventing deterioration by the chelating agent is difficult to appear. Further, it is not preferable to use the water absorbent resin for sanitary materials such as disposable diapers because the water absorption efficiency deteriorates.

(Absorption factor under pressure of absorbent polymer)

The water absorption capacity under pressure (AAP (0.7 psi)) of the water absorbent resin according to the present invention is controlled to preferably 15 g/g or more, more preferably 20 g/g or more, and 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, from the viewpoint of balance with other physical properties (eg, water absorption capacity under no pressure (CRC)). Preferably it is 33 g/g or less. AAP (0.7 psi) can be controlled by the crosslinking density at the time of surface crosslinking.

(Ratio of water absorption capacity under pressure and water absorption capacity under no pressure)

The ratio of water absorption capacity under pressure and water absorption capacity under no pressure (AAP (0.7psi)/CRC) of the water absorbent resin according to the present invention is preferably 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, 0.9 or more in the order. The upper limit of AAP (0.7 psi)/CRC is about 1.5 or less, 1.2 or less, or 1.0 or less. CRC removes pore water between gels after swelling in centrifugation, whereas AAP (0.7 psi) is an absorption capacity including pore water, so there are cases where AAP (0.7 psi) exceeds CRC, but the upper limit is usually within the above range. am.

[4] Use of water absorbent resin

The use of the absorbent resin obtained by the manufacturing method according to the present invention is not particularly limited, but is preferably used for absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads. The absorbent resin obtained by the manufacturing method according to the present invention is a high-concentration diaper (disposable diaper with a large amount of absorbent resin used per disposable diaper), which has been problematic in odor and coloration from raw materials, especially when used in the upper absorbent portion of the absorbent article. , it exhibits excellent performance. The ratio of the absorbent resin contained in the absorber of the disposable diaper is preferably 50% by weight or more, further 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] Difference from prior art

As described above, the present inventors have found that the chelating agent added in the step prior to the drying step specifically decreases in the step of drying the hydrogel polymer obtained by polymerizing the monomers. Then, the inventors of the present invention found that the cause of the reduction of the chelating agent in the drying step was the polymerization initiator (especially persulfate) remaining in the hydrogel 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 to decompose the chelating agent in the drying step. It is characterized by suppressing.

Patent Documents 1 to 23 described above describe the use of a chelating agent in the production process of a water absorbent resin. As a polymerization initiator for water absorbent resins, persulfates are the most common, and Patent Documents 1 to 23 also describe persulfates as polymerization initiators. However, Patent Literatures 1 to 23 do not describe at all about the decomposition of the chelating agent by persulfate, as well as the problems pointed out in the present application and means for solving the problems.

In addition, in the mechanism of the specific reduction of the chelating agent in the drying process found in the present invention, the reaction mechanism is not clear, but "radicals of persulfate preferentially react with monomers during polymerization, but the function after polymerization Since the monomer remaining in the gel (remaining monomer) is a small amount of several percent or less, and the drying process is performed at a higher temperature than during polymerization and the solid content is higher, the persulfate remaining after polymerization reacts with the chelating agent in the drying step.” It is estimated

Means for solving the problem will be described in more detail in an embodiment to be described later. In the present invention, the persulfate in the polymerization initiator is reduced to 0 to 0.04 mol% (monomer during polymerization) (0 mol% refers to the case where it is not used during polymerization or completely consumed before the drying step), Furthermore, since decomposition of the chelating agent in the drying process can be suppressed by controlling the gel particle size and drying conditions before drying, the problem can be solved without being limited to the examples described later.

Example

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

(a) weight average particle diameter (D50)

In accordance with WO2011/126079, the weight average particle diameter (D50) of the hydrogel polymer was measured by the following method.

That is, 20 g of a hydrogel polymer (solid content; α% by weight) at a temperature of 20 to 25 ° C., 20% by weight aqueous solution of sodium chloride containing 0.08% by weight Emal 20C (surfactant, manufactured by Gao Co., Ltd.) (hereinafter referred to as “Emal aqueous solution”) It was added in 500 g to form a dispersion, and stirred at 300 rpm for 60 minutes using a stirrer chip with a length of 50 mm and a diameter of 7 mm. use).

After stirring, the center of a JIS standard sieve (diameter 21 cm, sieve opening size: 8 mm / 4 mm / 2 mm / 1 mm / 0.60 mm / 0.30 mm / 0.15 mm / 0.075 mm) installed on the rotating disk. Then, the dispersion liquid was introduced. After rinsing the entire hydrous gel polymer on the sieve using 100 g of the aqueous emulsion solution, 6000 g of the aqueous emulsion solution was poured from the top, while rotating the sieve by hand (20 rpm), showering from a height of 30 cm (piercing 72 holes, liquid volume: 6.0 [L/min]) was thoroughly injected so that the injection number range (50 cm 2 ) spread over the entire sieve, and the hydrogel polymer was classified. The water-containing gel polymer on the sieve of the first stage classified was dried for about 2 minutes and then weighed. Sieves in the second stage and beyond were also classified in the same manner, and the hydrogel polymer remaining on each sieve after draining water was weighed.

From the weight of the hydrogel polymer remaining on each sieve, the weight% ratio was calculated from the following formula (1). The size of the opening of the sieve after draining was plotted according to the following equation (2), and the particle size distribution of the hydrogel polymer was plotted on logarithmic probability paper. From this graph, the particle diameter corresponding to the residual percentage of 50% by weight was read as the weight average particle diameter (D50) of the hydrogel polymer.

X[%]=(w/W)×100... Equation (1)

R(α) [mm] = (20/w) ^1/3 x r... Equation (2)

here,

X; Weight% [%] of hydrogel remaining on each sieve after classification and drying

w; Weight of each hydrogel remaining on each sieve after classification and drying [g]

W; Total weight of hydrogel remaining on each sieve after classification and drying [g]

R(α); Sieve opening size when converted to hydrogel of solid content α% by weight [mm]

r; Size of the sieve [mm] in which the hydrogel swollen in a 20% by weight aqueous solution of sodium chloride was classified

am.

(b) Residual amount (C1) and residual ratio of chelating agent, content of chelating agent (C2)

By the method described in Patent Document 11 (Pamphlet of International Publication No. 2015/053372), the remaining amount (C1) of the chelating agent in the water absorbent resin was extracted and analyzed.

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

Thereafter, the gel swollen with the water-absorbent resin was filtered using a filter paper (No. 2/retention particle size specified in JIS P 3801; 5 µm/manufactured by Toyo Roshi Co., Ltd.).

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

The content of the chelating agent in the particulate water-absorbent polymer was determined by considering the dilution rate of the particulate water-absorbent polymer in physiological saline using a calibration curve obtained by measuring a standard monomer solution at a known concentration as an external standard. In addition, the HPLC measurement conditions were appropriately changed depending on the type of chelating agent. Specifically, for diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), according to the following measurement condition 1, ethylenediaminetetra(methylenephosphonic acid) (EDTMP) Regarding each, quantification was performed according to measurement condition 2 below.

Measurement condition 1:

<eluent>; A mixed solution of 0.3 ml of 0.4 mol/L aqueous solution of alum, 450 ml of 0.1 N aqueous solution of potassium hydroxide, 3 ml of 0.4 mol/L aqueous solution of tetran-butylammonium hydroxide, 3 ml of sulfuric acid, 1.5 ml of ethylene glycol 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℃

<flow rate>; 1ml/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 Co., Ltd.)

<column temperature>; 40℃

<flow rate>; 1ml/min

<detector>; RI

Since the content of the chelating agent is affected by the moisture content, in the present invention, the value obtained by correcting the moisture content is a value converted per 100 parts by weight of the solid content of the water absorbent resin.

When the added chelating agent is an anionic type, for convenience, it is considered that the salt of the added chelating agent is not salt-exchanged and is present even in the water absorbent resin.

The content (C2) of the chelating agent is the same as the remaining amount (C1) of the chelating agent when the chelating agent is acid type. When the chelating agent is a salt type, the concentration is calculated as the acid type of the same chelating agent. That is, the content of the chelating agent (C2) was calculated as "the residual amount of the chelating agent (C1) x the molecular weight of the acid type/the molecular weight as it is at the time of addition of the chelating agent".

The residual rate (%) of the chelating agent was calculated as "{(remaining amount of chelating agent [ppm])/(added amount of chelating agent [ppm])} × 100".

(c) Evaluation of coloration of absorbent polymer (surface color evaluation)

Color evaluation of the water absorbent resin was performed using a spectroscopic color difference meter SZ-Σ80COLOR MEASURING SYSTEM manufactured by Nippon Denshoku Kogyo Co., Ltd. As the setting conditions for the measurement, reflection measurement is selected, an attached powder/paste sample stand with an inner diameter of 30 mm and a height of 12 mm is used, the standard round white plate for powder/paste No. 2 is used as a standard, and a 30φ transmission pipe is used. has been used About 5 g of water absorbing resin was filled in the prepared sample stand. This charging was a state in which about 60% of the beach sample stand was charged. Under the conditions of room temperature (20 to 25 ° C.) and humidity of 50 RH%, the L value (lightness: brightness index) and YI value (yellowness index; yellowness index) in the Hunter Lab colorimetric system of the surface are measured with the spectroscopic color difference meter. did. This value is referred to as "initial coloring".

Subsequently, about 5 g of water absorbing resin was filled in the paste sample stand, and a thermo-hygrostat (manufactured by Espek Co., Ltd., product name: small environmental tester, model) adjusted to an atmosphere of 70 ± 1 ° C. and 65 ± 1% relative humidity SH-641) was exposed for 7 days to a paste sample table filled with an absorbent resin. This exposure is a 7-day coloring acceleration test. After exposure, the L value (lightness) and YI value (yellowness) in the Hunter Lab color system of the surface were measured with the above-mentioned spectroscopic color difference meter. This measured value is referred to as "coloration over time (70±1°C, 65±1% relative humidity, 7 days)". The higher the L value, the better, and the smaller the YI value, the lower the coloration and the closer to true white.

(d) Amount of persulfate relative to solid content in hydrogel

By the method described in WO2007/116778, the amount of persulfate relative to the solid content in the hydrogel was measured.

Specifically, in a lidded polypropylene container having a capacity of 260 ml, 3 g of a hydrogel as a sample and 100 g of a 5% by weight aqueous sodium chloride solution (if the gel swells and cannot be stirred, the salt concentration or the amount of the aqueous solution is appropriately adjusted) was added, shielded from light at room temperature, and stirred at 500 rpm using a 25 mm stirrer coated with Teflon (registered trademark). After 2 hours had elapsed, the solution was taken out and passed through a filter (manufactured by GL Sciences, GL Chromatography Disc, water system 25A, pore size: 0.45 µm). 5.00 g of the solution was placed in a glass sample bottle (capacity 50 ml, diameter 35 mm, height about 80 mm) with a screw cap. Thereafter, 0.50 g of a 44% by weight potassium iodide aqueous solution was immediately added, and the mixture was stirred at room temperature while shielding from light. After 1 hour, the solution was transferred to a plastic 1 cm cell, and the absorbance (measurement wavelength: 350 nm) was measured using a spectrophotometer (Hitachi Recio Beam Spectrophotometer Model U-1100) (5% by weight). The absorbance of an aqueous solution (blank) obtained by adding 0.50 g of a 44% by weight potassium iodide aqueous solution to 5 g of an aqueous sodium chloride solution was set to 0). In addition, 5% by weight aqueous sodium chloride solutions containing 0 ppm (without addition), 5 ppm, 10 ppm, 15 ppm, and 20 ppm of persulfate, respectively, were prepared, absorbance was determined by the above operation, and a calibration curve was prepared. Then, the amount (ppm) of persulfate in the hydrogel of the sample was calculated from the absorbance and calibration curve of the obtained sample. In addition, the amount (mol%) of persulfate was determined by calculation.

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

(e) Solid content of absorbent polymer

In the water absorbent resin, the proportion occupied by components that do not volatilize at 180°C is expressed as solid content. The relationship between solid content and moisture content is as follows;

Solid content (mass %) = 100 - moisture content (mass %)

The method for measuring the solid content is as follows.

About 1 g of water-absorbent resin was weighed (mass W1) in an aluminum cup (mass W0) with a bottom diameter of about 5 cm, left to stand in a windless dryer at 180° C. for 3 hours, and dried. The mass (W2) of the "aluminum cup + water absorbent resin" after drying was measured, and the following formula was used.

Solid content (% by mass) = ((W2-W0) / W1) × 100

The solid content was obtained by

In addition, the block-shaped dry polymer was measured after acquiring 5 samples from various positions and crushing so that the particle diameter would be 5 mm or less, and the average value was adopted.

(f) solid content of hydrogel polymer

The solid content of the particulate water-containing gel polymer was measured in the same manner as the method for measuring the "solid content of water absorbing resin". However, the amount of the hydrogel polymer was changed to about 2 g, the drying temperature to 180°C, and the drying time to 24 hours.

(g) CRC (ERT441.2-02)

"CRC" is an abbreviation for Centrifuge Retention Capacity, and means absorption capacity under no pressure (hereinafter, also referred to as "absorption capacity"). Specifically, after freely swelling 0.200 g of the water-absorbent resin in a nonwoven fabric bag with a large excess of 0.9% by weight aqueous sodium chloride solution for 30 minutes, and then dehydrating in a centrifugal separator, the water absorption ratio (unit: [g / g]) Measured.

(h) AAP (0.7psi) (ERT442.2-02)

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

(i) Residual monomer (ERT410.2-02)

1.0 g of water absorbing resin was added to 200 ml of 0.9% by weight sodium chloride aqueous solution, stirred at 500 rpm for 1 hour using a 35 mm stirrer chip, and the amount of dissolved monomer (unit: ppm) was measured by HPLC (high performance liquid chromatography). Measured. In addition, 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 measured value obtained was converted into the weight per resin solid content of the hydrogel polymer (unit: ppm). .

[Example 1]

A water absorbent resin was prepared under the conditions of chelating agent (DTPA) = 1000 ppm, persulfate (NaPS) = 0.04 mol%, and gel particle diameter = 958 µm (about 0.9 mm).

(polymerization process)

In acrylic acid (288.24 g), polyethylene glycol diacrylate (number average molecular weight; 523) (1.67 g, 0.08 mol% relative to the acrylic acid) as an internal crosslinking agent, and 10% by weight diethylenetriamine pentaacetic acid (DTPA) as a chelating agent Solution (A) to which 5 sodium aqueous solution (3.52 g, 1000 ppm based on monomer) was added, and a solution obtained by diluting 48.5% by weight aqueous sodium hydroxide solution (240.82 g) with deionized water (281.58 g) adjusted to 50 ° C. ( B) was prepared in a container made of polypropylene with a capacity of 1.5 L and an inner diameter of 80 mm, respectively.

A solution (C) was prepared by adding and mixing the solution (B) while stirring the solution (A) using a magnetic stirring bar.

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

Thereafter, stirring of the solution (C) was continued, and when the temperature of the solution (C) reached 95° C., a 10% by weight aqueous solution of sodium persulfate (NaPS; 3.81 g) was added to the solution (C) as a polymerization initiator. , 0.04 mol% relative to the above acrylic acid, and a molar ratio of persulfate to chelating agent; 2.3) were added, and stirred for about 3 seconds to obtain an aqueous monomer solution (1). The neutralization rate of the monomer aqueous solution (1) was 73% by mole, and the monomer concentration was 43% by weight.

Next, the aqueous monomer solution (1) was introduced into a stainless steel bat-shaped vessel into an air open system. In addition, the stainless steel bat-shaped container has a bottom surface size of 250 mm × 250 mm, an upper surface size of 640 mm × 640 mm, a height of 50 mm, a central cross section of a trapezoidal shape, and a silicone sheet attached to the inner surface. was In addition, the stainless steel bat-shaped container was placed on a hot plate (NEO HOTPLATE H1-1000, manufactured by Iuchi Seido Co., Ltd.) heated to 100°C, and preheated.

The polymerization reaction started about 5 seconds after the monomer aqueous solution (1) was introduced into the stainless steel bat-shaped vessel.

The polymerization reaction proceeds while the aqueous monomer solution (1) expands and foams upwards in all directions while generating steam, and then the obtained water-containing gel polymer (1) reaches a size slightly larger than the bottom surface of the stainless steel bat-shaped container. contracted and ended. In addition, the polymerization reaction (expansion and contraction) ended within about 1 minute, but after that, the hydrogel polymer (1) was maintained in a stainless steel bat-shaped container for 3 minutes. By the polymerization reaction (equilibrium polymerization), a water-containing gel polymer (1) containing air cells was obtained.

(Gel crushing process)

Next, after dividing the hydrous gel polymer (1) obtained in the polymerization reaction into 16 parts, a tabletop meat chopper (MEAT-CHOPPER TYPE: 12VR-400KSOX, manufactured by Iizuka Kogyo Co., Ltd.) having a perforated plate with a hole diameter of 11 mm was prepared. , gel pulverization was performed while adding 50 g/min of deionized water adjusted to about 100° C. simultaneously with the introduction of the hydrogel polymer. The obtained gel was further put into a table-top meat chopper and subjected to gel pulverization for the second time to obtain a particulate hydrogel polymer (1). In the second and subsequent gel grinding, deionized water was not added.

The particulate hydrogel polymer (1) obtained by the gel pulverization had a weight average particle diameter (D50) of 958 µm. In addition, the remaining monomer of the particulate hydrogel polymer (1) was 1.1% by weight.

(drying process)

Then, the particulate water-containing gel polymer (1) was spread on a metal net having an opening size of 300 μm (50 mesh), and placed in a hot air dryer.

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

(crushing/classification process)

Subsequently, the dried polymer (1) was put into a roll mill (WML type roll mill, manufactured by Yugen Kai Shainokuchi Kiken Co., Ltd.) and pulverized, and then two types of JIS standard sieves with opening sizes of 850 µm and 150 µm were used. and classified to obtain a water-absorbent resin (1) in irregular crushed form.

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

(Surface Crosslinking Process)

Then, 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 alcohol (1 part by weight) By adding a surface crosslinking agent solution (1) (5.05 parts by weight) containing (5.05 parts by weight) to the irregularly crushed water-absorbent resin (1) (100 parts by weight) and mixing until uniform, the moistened mixture (1) got it Subsequently, the humidified mixture (1) was uniformly placed in a stainless steel container (width of about 22 cm, depth of about 28 cm, height of about 5 cm), heat treatment was performed at 180 ° C. for 40 minutes, and the surface crosslinked water-absorbent resin ( 1) was obtained.

Thereafter, the surface crosslinked water absorbent polymer (1) was passed through a JIS standard sieve having an opening size of 850 µm to obtain a water absorbent polymer (1) as a final product. The resulting water absorbent polymer (1) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 2]

Reducing the addition amount of the polymerization initiator in Example 1 (NaPS = 0.04 mol% → 0.015 mol%);

In the above Example 1, except that the addition amount of the polymerization initiator (10% by weight aqueous solution of sodium persulfate) in the polymerization step was 1.43 g (0.015 mol% relative to acrylic acid), in the same manner as in Example 1, the water absorbent resin ( 2) was prepared.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (2) as a final product. The resulting water absorbent polymer (2), which is the final product, was analyzed. The production 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 coloring with the passage of time (YI value with the passage of time - initial YI value) was calculated. The calculation results are shown in Table 3 below.

[Example 3]

Change the gel particle diameter of Example 1 (about 0.9 mm → about 0.5 mm);

In the above Example 1, in order to change the gel particle size, after the second gel grinding in the gel grinding step, the obtained gel was further put into a tabletop meat chopper and the third gel grinding was performed. In the same manner as in 1, a water absorbent resin (3) was produced.

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). In addition, the irregularly crushed water absorbent resin (3) had a CRC of 28.3 g/g.

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (3) as a final product. The resulting water absorbent polymer (3), which is the final product, was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 4]

Change the type of polymerization initiator in Example 1 (NaPS → azo polymerization initiator);

In Example 1, the polymerization initiator was an azo polymerization initiator, 10% by weight of 2,2'-azobis (2-methylpropionamidine) dihydrochloride aqueous solution (trade name: Wako Pure Chemical V-50) 4.34 g A water absorbent resin (4) was produced in the same manner as in Example 1 except for changing to (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. In addition, the irregularly crushed water absorbent resin (4) had a CRC of 30.0/g.

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (4) as a final product. The resulting water absorbent polymer (4), which is the final product, was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 5]

Changed the chelating agent of Example 1 (DTPA→EDTMP);

In the above Example 1, except that the chelating agent was changed to 10% by weight ethylenediaminetetramethylenephosphonic acid (EDTMP)/penta sodium aqueous solution (3.52 g, 1000 ppm with respect to the monomer), the same procedure as in Example 1 was performed, and water absorption Resin (5) was prepared.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (5) as a final product. The resulting water absorbent polymer (5), which is the final product, was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 6]

Reducing the addition amount of the polymerization initiator in Example 5 (NaPS = 0.04 mol% → 0.015 mol%);

In the above Example 5, except that the addition amount of the polymerization initiator (10% by weight aqueous solution of sodium persulfate) in the polymerization step was 1.43 g (0.015 mol% with respect to acrylic acid), in the same manner as in Example 5, the water absorbent resin ( 6) was prepared.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (6) as a final product. The resulting water absorbent polymer (6) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 7]

Change the type of polymerization initiator in Example 5 (persulfate → azo polymerization initiator);

In the above Example 5, the polymerization initiator was 4.34 g of 10% by weight 2,2'-azobis (2-methylpropionamidine) dihydrochloride aqueous solution (V-50) as an azo polymerization initiator (0.04% based on acrylic acid). A water absorbent resin (7) was prepared in the same manner as in Example 5, except for changing to mol %).

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (7) as a final product. The resulting water absorbent polymer (7), which is the final product, was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 8]

Change the polymerization initiator of Example 5 (persulfate → UV polymerization in UV polymerization initiator);

In the above Example 5, the polymerization initiator (10% by weight aqueous solution of sodium persulfate) in the polymerization step was changed to a UV polymerization initiator, and the addition amount was 0.04 mol%, and UV polymerization was carried out in the same manner as in Example 5. , the water absorbent resin (8) was prepared.

Hereinafter, the polymerization process of performing UV polymerization is demonstrated.

(polymerization process)

Acrylic acid (285.30 g), polyethylene glycol diacrylate (number average molecular weight; 523) as an internal crosslinking agent (0.08 mol% with respect to the acrylic acid), 10% by weight ethylenediaminetetramethylenephosphonic acid (EDTMP) as a chelating agent, 5 sodium aqueous solution (3.52 g, 1000 ppm based on monomers) and 3.27 g of 10% by weight IRGACURE (registered trademark; manufactured by BASF: 1-hydroxycyclohexylphenylketone) 184 acrylic acid solution as a UV polymerization initiator (0.04 mol% based on the acrylic acid) A solution (A) to which was added and a solution (B) obtained by diluting a 48.5% by weight aqueous solution of sodium hydroxide (240.82 g) with deionized water (285.06 g) adjusted to 50 ° C. Each was prepared in a container made of propylene.

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

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

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

In addition, the stainless steel bat-shaped container has a bottom surface size of 250 mm × 250 mm, an upper surface size of 640 mm × 640 mm, a height of 50 mm, a central cross section of a trapezoidal shape, and a silicone sheet attached to the inner surface. was In addition, the stainless steel bat-shaped container was placed on a hot plate (NEO HOTPLATE H1-1000, manufactured by Iuchi Seido Co., Ltd.) heated to 100°C, and preheated.

The polymerization reaction started about 7 seconds after the monomer aqueous solution (8) was introduced into the stainless steel bat-shaped container.

The polymerization (stationary aqueous solution polymerization) reaction proceeded while generating steam. Polymerization reached peak temperature within about 1 minute (polymerization peak temperature: 102° C.). After 3 minutes, irradiation of ultraviolet rays was stopped, and a water-containing gel polymer (8) containing air bubbles was obtained.

(After gel crushing process)

Steps after the gel crushing step were performed in the same manner as in Example 1. In this way, the water absorbent resin (8) was produced.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (8) as a final product. The resulting water absorbent polymer (8), which is the final product, was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 1]

Change the addition amount of polymerization initiator in Example 1 (NaPS = 0.04 mol% → 0.05 mol%);

In Example 1, the comparative water-absorbent resin was carried out in the same manner as in Example 1, except that the addition amount of the polymerization initiator (10% by weight aqueous solution of sodium persulfate) in the polymerization step was 4.8 g (0.05 mol% with respect to acrylic acid). (1) was prepared.

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 comparative water absorbent polymer (1) in irregular crushed form had a CRC of 28.8 g/g.

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a comparative water absorbent polymer (1) as a final product. The obtained comparative water absorbent resin (1) was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 2]

Change the gel particle size of Example 1 (about 0.9 mm → 5 mm or more);

In the above Example 1, in order to change the gel particle size, comparative water-absorbent resin (2) was produced in the same manner as in Example 1, except that gel grinding was performed once in the gel grinding step.

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 comparative water absorbent resin (2) in irregular crushed form had a CRC of 27.1 g/g.

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a comparative water absorbent polymer (2) as a final product. The obtained comparative water absorbent resin (2) was analyzed. The production 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 coloring with the passage of time (YI value with the passage of time - initial YI value) was calculated. The calculation results are shown in Table 3 below.

[Comparative Example 3]

Change the gel particle size of Example 5 (about 0.9 mm → 5 mm or more);

In the above Example 5, in order to change the gel particle size, a comparative water absorbent polymer (3) was produced in the same manner as in Example 5, except that gel grinding was performed once in the gel grinding step.

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 comparative water absorbent resin (3) of irregularly crushed shape had a CRC of 27.2 g/g.

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

[Example 9]

Change the addition amount of the chelating agent in Example 1 (DTPA = 1000 ppm → 300 ppm);

A water absorbent polymer (9) was produced in the same manner as in Example 1, except that the amount of DTPA pentasodium added was 300 ppm (molar ratio of persulfate to chelating agent: 7.7) relative to the monomer. did.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (9) as a final product. The resulting water absorbent polymer (9), which is the final product, was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 10]

Change the addition amount of the chelating agent in Example 1 (DTPA = 1000ppm → 50ppm);

Water absorbent polymer (10) was prepared in the same manner as in Example 1, except that the amount of DTPA pentasodium added was 50 ppm (molar ratio of persulfate to chelating agent: 46.3) relative to the monomer. did.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (10) as a final product. The resulting water absorbent polymer (10) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 11]

Changing the addition amount of the chelating agent in Example 1 (DTPA = 1000 ppm → 50 ppm), and using two types of polymerization initiators together;

In the above Example 1, while the addition amount of DTPA pentasodium was 50 ppm with respect to the monomer, the addition amount of the polymerization initiator (10% by weight aqueous solution of sodium persulfate) in the polymerization step was 1.43 g (0.015 mol with respect to acrylic acid). %), and further, as an azo polymerization initiator, 2.71 g (0.025 mol% based on acrylic acid) of 10% by weight 2,2'-azobis(2-methylpropionamidine) dihydrochloride aqueous solution (V-50) was added. A water-absorbent resin (11) was produced in the same manner as in Example 1 except that they were used together.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (11) as a final product. The resulting water absorbent polymer (11) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 12]

Changing the drying temperature and drying time of Example 1 (160 ° C → 120 ° C, 30 minutes → 120 minutes);

In the above Example 1, except that the drying temperature and drying time of the drying step were changed to 120 ° C. for 120 minutes (120 ° C. hot air for 120 minutes), the water absorbent polymer 12 was prepared in the same manner as in Example 1. manufactured

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (12) as a final product. The resulting water absorbent polymer (12) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 13]

Changing the drying temperature and gel particle diameter of Example 1 (160 ° C. → 120 ° C., about 0.9 mm → about 0.5 mm);

In the above Example 1, in order to change the gel particle size, after the second gel grinding in the gel grinding step, the obtained gel was further put into a tabletop meat chopper to perform the third gel grinding, and the drying step A water absorbent resin 13 was manufactured in the same manner as in Example 1, except that the drying temperature was changed to 120°C (120°C hot air for 30 minutes).

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (13) as a final product. The resulting water absorbent resin (13) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 4]

Changing the drying temperature and drying time of Example 1 and the gel particle size (160°C→120°C, 30 minutes→180 minutes, about 0.9 mm→5 mm or more);

In the above Example 1, in order to change the gel particle size, the gel grinding in the gel grinding step was performed once, and the drying temperature and drying time in the drying step were 120 ° C. for 180 minutes (120 ° C. hot air for 180 minutes) A comparative water absorbent polymer (4) was prepared in the same manner as in Example 1 except for changing to ventilation).

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. In addition, the irregularly crushed comparative water absorbent resin (4) had a CRC of 24.4 g/g and a solid content of 91.5%.

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

[Comparative Example 5]

Changing the drying temperature and drying time of Example 1 and the gel particle size (160°C→150°C, 30 minutes→180 minutes, about 0.9 mm→1.2 to 1.6 cm);

In the above Example 1, in order to change the gel particle size, the hydrogel polymer was cut using scissors as the gel crusher in the gel crushing step, and a comparative hydrogel polymer having a size of 1.2 to 1.6 cm on one side was obtained. Then, a comparative water absorbent polymer (5) was produced in the same manner as in Example 1, except that the drying temperature and drying time in the drying step were changed to 150°C for 180 minutes (150°C hot air for 180 minutes).

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 comparative water-absorbent resin (5) in the irregularly crushed form had a CRC of 31.6 g/g.

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

[Example 14]

Changed the chelating agent of Example 1 (DTPA→EDTA);

In the above Example 1, except that the chelating agent was changed to 10% by weight ethylenediaminetetraacetic acid (EDTA) tetrasodium aqueous solution (3.52 g, 1000 ppm relative to the monomer), in the same manner as in Example 1, the water absorbent resin ( 14) was prepared.

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

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (14) as a final product. The resulting water absorbent polymer (14) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 15]

Reducing the addition amount of the polymerization initiator in Example 14 (NaPS = 0.04 mol% → 0.015 mol%);

A water absorbent polymer ( 15) was prepared.

The particulate hydrogel polymer 15 obtained in the gel crushing step had a weight average particle diameter (D50) of 930 μm. In addition, the irregularly crushed water absorbent resin 15 had a CRC of 29.5 g/g.

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (15) as a final product. The resulting water absorbent polymer (15) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Example 16]

Changed the chelating agent of Example 1 (DTPA→NTA);

In Example 1, except that the chelating agent was changed to a 10% by weight nitrilotriacetic acid (NTA)/trisodium aqueous solution (3.52 g, 1000 ppm relative to the monomer), in the same manner as in Example 1, the water absorbent resin ( 16) was prepared.

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 irregularly crushed water absorbent resin 16 had a CRC of 29.1 g/g.

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain a water absorbent resin (16) as a final product. The resulting water absorbent polymer (16) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 6]

Change the addition amount of the polymerization initiator in Example 14 (NaPS = 0.04 mol% → 0.05 mol%);

In the above Example 14, except that the addition amount of the polymerization initiator (10% by weight aqueous solution of sodium persulfate) in the polymerization step was 4.8 g (0.05 mol% with respect to acrylic acid), the same procedure was carried out as in Example 14, and comparative water absorbent polymer (6) was prepared.

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 comparative water absorbent resin (6) of irregularly crushed shape had a CRC of 29.5 g/g.

Subsequently, surface crosslinking was performed in the same manner as in Example 1 to obtain comparative water absorbent polymer (6) as a final product. The obtained comparative water absorbent resin (6) was analyzed. The production 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 simultaneously.

(Polymerization and gel crushing process)

425.2 g of acrylic acid, 4499.5 g of 37% by weight sodium acrylate aqueous solution, 513.65 g of pure water, and polyethylene glycol di After adding 11.1 g of acrylate (molecular weight 523) to make a reaction solution, it was degassed for 20 minutes in a nitrogen gas atmosphere. Subsequently, 16.5 g of a 10% by weight aqueous solution of sodium persulfate (0.03% by mole of major monomer) and 23.6 g of an aqueous solution of 0.1% by weight of L-ascorbic acid were separately added while stirring the reaction solution. After about 3 minutes, Polymerization was started, and polymerization was performed at 25 to 95°C while disintegrating the resulting hydrogel crosslinked polymer (17).

After 70 minutes from the initiation of polymerization, 10.5 g of 20% by weight diethylenetriaminepentaacetate trisodium (1000 ppm chelating agent as major monomer) was added, and after 100 minutes from the initiation of polymerization, the hydrogel crosslinked polymer (17) was added to the reactor. was extracted from In addition, the obtained particulate hydrogel polymer (17) had a weight average particle diameter (D50) of 479 µm.

(Drying/grinding/classification process)

The fine-grained hydrogel crosslinked polymer (17) was spread on a metal mesh having an opening size of 300 μm (50 mesh) and placed in a hot air dryer. Thereafter, the particulate hydrogel polymer (17) was dried by blowing hot air at 170°C for 30 minutes to obtain a dried particulate polymer (17). Subsequently, the dry polymer 17 was put into a roll mill (WML type roll mill, manufactured by Yugenkaisha Inokuchi Kiken Co., Ltd.) and pulverized, and then two types of JIS standard sieves with opening sizes of 850 μm and 150 μm were used. and classified to obtain a water-absorbent resin (17) in irregular crushed form. In addition, the irregularly crushed water absorbent resin 17 had a CRC of 31.8 g/g.

(Surface Crosslinking Process)

Next, a surface crosslinking agent solution (1) having the same composition as in Example 1 (5.05 parts by weight) was mixed with 100 parts by weight of the irregularly crushed water absorbent resin (17) to obtain a moist mixture (17). Subsequently, heat treatment was performed at 180°C for 40 minutes in the same manner as in Example 1 to obtain a surface crosslinked water absorbent resin (17). Thereafter, the surface crosslinked water absorbent polymer 17 was passed through a JIS standard sieve having an opening size of 850 µm to obtain a water absorbent polymer 17 as a final product. The resulting water absorbent polymer (17) as the final product was analyzed. The production conditions are shown in Table 1 below, and the analysis results are shown in Table 2 below.

[Comparative Example 7]

Change the addition amount of the polymerization initiator in Example 17 (NaPS = 0.05 mol% → 0.03 mol%);

In the polymerization step of Example 17, except that the addition amount of the polymerization initiator, 10% by weight aqueous sodium persulfate solution, was increased from 16.5 g (0.03 mol% of the main monomer) to 28.3 g (0.05 mol% of the main monomer), Polymerization was carried out in the same manner as in Example 17. As in Example 17, polymerization started after about 3 minutes, and polymerization was performed at 25 to 95°C while disintegrating the resulting comparative hydrogel crosslinked polymer (7). Hereafter, after 70 minutes from the initiation of polymerization, 1000 ppm of a chelating agent was added in the same manner as in Example 17, and after 100 minutes from the initiation of polymerization, the comparative hydrogel crosslinked polymer (7) (weight average particle size (D50) of 494 μm) was removed from the reactor. took out

Next, after drying in the same manner as in Example 17 to obtain a particulate comparative dry polymer (7), pulverization and classification were performed to obtain an irregularly crushed comparative water absorbent resin (7) of 850 to 150 µm.

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

[Comparative Example 8]

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

[Comparative Example 9]

Comparative water absorbent resin (9) was obtained according to the method described in Example 2-23 of International Publication No. 2014/054656 pamphlet (Patent Document 23). The obtained final product, comparative water absorbent resin (9), was analyzed. Manufacturing conditions and analysis results are shown in Tables 1 to 3 below.

[Comparative Example 10]

Comparative water absorbent resin (10) was obtained according to the method described in Examples 2-24 of International Publication No. 2014/054656 pamphlet (Patent Document 23). The obtained final product, Comparative Water Absorbent Resin (10), was analyzed. Manufacturing conditions and analysis results are shown in Tables 1, 2 and 4 below.

In addition, although not shown in Table 1, the amount of chelating agent in the hydrogel after the polymerization step and the amount of the chelating agent in the hydrogel after the gel pulverization step in each Example and each Comparative Example were 97% of the amount added at 1000 ppm with respect to the monomers. to nearly 100% remained. Therefore, substantially no reduction of the chelating agent was observed in the polymerization process or gel milling process.

In addition, the amount of the chelating agent in the water-absorbent resin obtained by blowing hot air at 160 ° C. for 30 minutes and then pulverizing (before the surface cross-linking step) or in the comparative water-absorbent resin, heat treatment at 180 ° C. for 40 minutes in the surface cross-linking step, and the surface The amount of the chelating agent in the crosslinked water absorbent resin or in the comparative water absorbent resin was compared, but no difference occurred between the two. Therefore, even if the surface crosslinking step was performed at 180°C, no reduction of the chelating agent was observed.

(organize)

From Tables 1 to 4, the following can be seen.

(1) Comparison between Examples 1 and 2 and Comparative Example 1 in which the chelating agent (DTPA) and gel particle size are the same, and the addition amount of the polymerization initiator (persulfate; NaPS) is changed, and the chelating agent (EDTMP) and gel From the results of Examples 5 and 6, in which the particle size was the same and the addition amount of the polymerization initiator (persulfate; NaPS) was changed, by setting the amount of persulfate (NaPS) to 0.04 mol% or less, the final product after the drying step ) It can be seen that the residual amount of the chelating agent in the water absorbent polymer is improved to 50% or more. In addition, 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 resulting water absorbent resin, and decreases the YI value.

(2) Results of Examples 1 and 4 in which the chelating agent (DTPA), the gel particle size and the addition amount (0.04 mol%) of the polymerization initiator were the same and the type of polymerization initiator was changed, and the chelating agent (EDTMP) and the gel particle size And from the results of Examples 5, 7, and 8, in which the addition amount (0.04 mol%) of the polymerization initiator was the same and the type of polymerization initiator was changed, compared to persulfate (NaPS), in the azo polymerization initiator and the UV polymerization initiator, It can be seen that the remaining amount of the chelating agent in the water absorbent polymer (in the final product) after the drying process is remarkably improved to nearly 100%. In addition, in such a polymerization initiator, it is found that the content of the chelating agent increases, the L value of the initial coloration of the obtained water absorbent resin increases, and the YI value decreases (L value = 91 in Examples 7 and 8). , YI value = 2).

(3) Contrast between Examples 1 and 3 and Comparative Example 2 in which the chelating agent (DTPA) and the polymerization initiator were the same and the gel particle size was changed, and the chelating agent (EDTMP) and the polymerization initiator were the same and the gel particle size was changed , From the results of the comparison between Example 5 and Comparative Example 3, it can be seen that the residual amount of the chelating agent in the water absorbent resin after the drying step (and in the final product) is remarkably improved by making the gel particle size smaller. In addition, it can be seen that the L value of the initial coloration of the obtained water absorbent resin is increased by the production method according to the present invention, in addition to the increase in the content of the chelating agent (the L value is improved by 1 point with the same chelating agent). In addition, from the comparison results of Comparative Example 2 and Comparative Example 5, in which the addition amount of the polymerization initiator (0.04 mol%), the addition amount of the chelating agent, and the same gel particle size, the faster the time to achieve the solid content of 80% during drying, the faster the chelating agent. It can be seen that the residual ratio of is improved and the L value of initial coloring is increased. In the production method according to the present invention, it is understood that the L value increases as the gel particle size is smaller and the time for achieving 80% solid content in drying is faster.

(4) Example 1 (1000 ppm with respect to monomer; 0.0173 mol%) and Example 9 (300 ppm with respect to monomer ; It can be seen that the remaining amount of the chelating agent is reduced from 50% to 39% and further to 30%.

(5) From the results of Examples 10 and 11, in which the addition amount of the polymerization initiator (0.04 mol%) and the addition amount of the chelating agent (50 ppm) are the same, when the addition amount of the polymerization initiator is the same, compared to using only persulfate (NaPS) , it can be seen that when the persulfate and the azo polymerization initiator are used in combination, the remaining amount of the chelating agent in the water absorbent resin of the final product is improved from 15% to 30%. In addition, it can be seen that the YI value of the initial coloring of the obtained water absorbent resin decreases (YI = 11 decreases to YI = 6) in addition to the increase in the content of the chelating agent in such a polymerization initiator.

(6) Example 2 (0.015 mol%; 942 μm) and Comparative Example 2 (0.04 mol%), in which the chelating agent (DTPA) and its addition amount (0.0173 mol%) were the same, and the polymerization initiator addition amount and gel particle size were changed. ≥ 5000 µm), the residual amount of the chelating agent in the water-absorbent polymer of the final product is remarkably improved by reducing the molar ratio of persulfate to the chelating agent and the gel particle size, and further, the initial coloring and coloring over time It can be seen that the YI value is lowered and, in particular, the resistance to discoloration with time is improved.

(7) From the results of the comparison 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 persulfate in the polymerization initiator is different, by controlling the addition amount of the polymerization initiator to 0.04 mol% or less, chelate It can be seen that the residual ratio of the agent increases, the L value of the initial coloration and coloration with time increases, and the YI value decreases.

In Comparative Example 8, the addition amount of the chelating agent during polymerization is smaller than that of Example 2, and the addition amount of the persulfate salt of the polymerization initiator is large. Therefore, it can be seen that the residual ratio of the chelating agent decreases and the initial coloration (L value, YI value) deteriorates.

(8) From the results of comparison between Examples 10 and 11 and Comparative Example 10 where the amount of the chelating agent added in the polymerization step was the same, by controlling the amount of persulfate (NaPS) to 0.04 mol% or less, L of the initial coloring It can be seen that the value increases and the YI value decreases.

[Example 18]

To 100 parts by weight of the water absorbent resin (1) obtained in Example 1, 1 part by weight of a 1% by weight DTPA 5 sodium aqueous solution was added, 501 ppm of a chelating agent was added, and 100 ppm of a chelating agent was added to the surface. Resin (18) was obtained. Almost the entire amount of the chelating agent added later was contained near the surface of the water absorbent resin 18, and the content of the chelating agent in the water absorbent resin 18 increased by about 100 ppm. The CRC and AAP (0.7 psi) of the water absorbent polymer (18) were almost the same as those of the water absorbent polymer (1) (decrease corresponding to 1% of additives).

[Examples 19 to 21]

In the same manner as in Example 18, to each of the water absorbent resins (2) to (4) obtained in Examples 2 to 4, 1 part by weight of a 1% by weight DTPA/penta sodium aqueous solution was added to 100 parts by weight of the water absorbent resin, and the water absorption Resins (19) to (21) were obtained. Each of the water absorbent polymers (19) to (21) contained the same amount of chelating agent as the water absorbent polymers (2) to (4) inside, and further contained 100 ppm of the chelating agent on the surface. Almost all of the chelating agent added later was contained near the surface of the water absorbent resins (19) to (21), and the content of the chelating agent in the water absorbent resins (19) to (21) increased by about 100 ppm, respectively. The CRC and AAP (0.7 psi) of the absorbent polymers (19) to (21) were almost the same as those of the absorbent polymers (2) to (4) (decrease corresponding to 1% of additives).

(Reference Example 1)

The amounts of chelating agent and persulfate in each hydrogel obtained in the polymerization step of Example 1 (0.04 mol% persulfate) and the polymerization step of Comparative Example 1 (0.05% persulfate) were measured. As a result, about 82% of the persulfate remained in either hydrogel, and almost 100% of the chelating agent remained. Therefore, it can be seen that the amount of persulfate before drying (0.04 mol% or less, furthermore 0.035 mol% or less) has a significant effect on the remaining amount of the chelating agent.

(Reference example 2)

The remaining amounts of the chelating agent in Example 1 (persulfate: 0.04 mol%) after the above drying step and after the surface crosslinking step were measured. As a result, it can be seen that the chelating agent is substantially reduced only by the drying step.

(Reference example 3)

In order to confirm the heat resistance of the chelating agent (DTPA) itself, the chelating agent (DTPA) was heated under the drying conditions in the drying step of Example 1. However, no substantial reduction of the chelating agent was seen.

(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, no substantial reduction of the chelating agent was seen.

(Reference Example 5)

To an aqueous solution containing only the chelating agent (DTPA) prepared in Reference Example 4, a persulfate was added so that the molar ratio to the chelating agent was the same as in Example 1 (molar ratio of persulfate to chelating agent: 2.3), and heated to 80°C. heated in As a result, the residual rate of the chelating agent was 13%, and it was confirmed that the chelating agent was reduced in persulfate. Additionally, the aqueous solution was slightly colored brown.

(Reference Example 6)

To the aqueous solution containing only the chelating agent (DTPA) prepared in Reference Example 4, a photopolymerization initiator was added so as to have the same molar ratio (molar ratio of UV polymerization initiator to chelating agent; 2.3) with respect to the chelating agent as in Example 8, and UV irradiation was performed at °C. As a result, the residual rate of the chelating agent was 98%, and the reduction of the chelating agent was not seen substantially.

(Reference example 7)

DTPA and a persulfate were added in a 0.1 to 1% by weight aqueous solution of sodium acrylate so that the molar ratio of the persulfate to the chelating agent was 2.3, 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.

(organize)

From the results of Reference Examples 1 to 6, it can be seen that the persulfate salt remaining in the hydrogel specifically decomposes the chelating agent in the drying step. Therefore, it turns out that the subject can be solved by this invention. 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. This mechanism is presumed to be because the persulfate is more likely to react with the remaining monomer than the chelating agent.

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

The absorbent resin produced by the production method of the present invention is useful for sanitary products such as paper diapers, sanitary napkins, and medical blood-retaining agents. In addition, pet urine absorbents, urine gelling agents for portable toilets and freshness retainers for fruits and vegetables, drip absorbents for meat and fish and shellfish, coolants, disposable Cairo, battery gelling agents, moisture retaining agents for plants and soil, condensation inhibitors, It can also be used for various purposes such as waterstopping agents, packing agents, and artificial snow.

Claims (26)

A polymerization step of obtaining a hydrogel polymer by polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator;
Drying step of drying the particulate hydrogel polymer to obtain a particulate dry polymer
A method for producing a water absorbent polymer having a water absorption coefficient (CRC) of 15 g/g or more and containing a chelating agent, comprising:
The persulfate used in the polymerization process is 0 to 0.04 mol% (monomer during polymerization) (provided, that in the case of 0 mol% persulfate (non-use), another polymerization initiator is necessarily used),
In a step prior to the drying step, a total of 10 ppm or more of a chelating agent is added to the aqueous monomer solution or the hydrogel polymer (solid content of the monomer or hydrogel polymer at the time of large polymerization),
The weight average particle diameter (D50) of the particulate hydrogel polymer is 1 mm or less,
In the drying step, the drying time until the solid content is 80% by weight or more is 20 minutes or less, a method for producing a water absorbent polymer containing a chelating agent. A polymerization step of obtaining a hydrogel polymer by polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator;
Drying step of drying the particulate hydrogel polymer to obtain a particulate dry polymer
A method for producing a water absorbent polymer having a water absorption coefficient (CRC) of 15 g/g or more and containing a chelating agent, comprising:
In the drying step, a particle phase having a weight average particle size (D50) of 1 mm or less, containing 10 ppm or more of a chelating agent (for the solid content of the hydrous gel polymer) and 0 to 0.04 mol% of a persulfate (for the monomer at the time of polymerization) A method for producing a water absorbent polymer containing a chelating agent, wherein a water-containing gel polymer is dried for a drying time of 20 minutes or less until the solid content is 80% by weight or more.
However, the said drying time points out the time until solid content becomes 80 weight% or more. The manufacturing method according to claim 1 or 2, comprising a gel pulverization step of pulverizing the hydrogel polymer during and/or after the polymerization step. The production method according to claim 1 or 2, wherein the molar ratio of the persulfate to the chelating agent is 20 times or less. The production method according to claim 1 or 2, wherein the total addition amount of the persulfate from the polymerization step to the drying step is 0 to 0.04 mol% (to monomers during polymerization). The manufacturing method according to claim 1 or 2, wherein hot air drying at 150 to 200°C is performed in the drying step. The production method according to claim 1 or 2, wherein the chelating agent is at least one selected from the group consisting of an amino polycarboxylic acid chelating agent and an amino polyvalent phosphate chelating agent. The manufacturing method according to claim 3, wherein the hydrogel polymer is made into particles in the gel pulverization step. The production method according to claim 1 or 2, wherein the total amount of the chelating agent added in the step before the drying step is 60 ppm to 1% (solid content of the monomer or hydrogel polymer at the time of polymerization). The production method according to claim 1 or 2, wherein the polymerization is aqueous solution polymerization. The production method according to claim 1 or 2, wherein the polymerization is foaming polymerization or boiling polymerization, and the hydrogel polymer contains cells. The production method according to claim 1 or 2, wherein the polymerization is high-temperature initiation, short-time polymerization at 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 manufacturing method according to claim 1 or 2, further comprising a surface crosslinking step of water absorbent resin after the drying step. The manufacturing method according to claim 1 or 2, further comprising a step of adding a chelating agent to the water absorbent resin in a step after the drying step. The production method according to claim 1 or 2, wherein the hydrogel polymer before drying contains 0.1% by weight or more of the residual monomer. The method according to claim 1 or 2, wherein the monomer used in the polymerization step contains acrylic acid (salt), and the content of the acrylic acid (salt) is based on the total monomers (excluding the internal crosslinking agent) used in the polymerization step. 50 to 100 mol%, relative to
The water-absorbent resin containing the chelating agent is a polyacrylic acid (salt)-based water-absorbent resin having a residual amount (C1) of the chelating agent of 10 ppm or more, an L value of an initial color tone of 85 or more, and a YI value of 13 or less. Manufacturing method. The method according to claim 1 or 2, wherein the water absorbent polymer containing the chelating agent has a residual amount (C1) of chelating agent of 200 ppm or more, an L value of an initial color tone of 89 or more, and a YI value of 10 or less. . A polyacrylic acid (salt)-based water absorbent resin having a chelating agent content (C2) of 200 ppm or more, an L value of an initial color tone of 89 or more, and a YI value of 10 or less. 19. The method of claim 18, wherein the no-pressure absorption capacity (CRC) is 25 g/g or more, the pressure absorption capacity (AAP (0.7 psi)) is 15 g/g or more, and the ratio of the absorption capacity under pressure and the absorption capacity under no pressure is A water absorbent polymer having a ratio (AAP (0.7 psi)/CRC) of 0.5 or more. The method according to claim 18 or 19, wherein the acrylic acid (salt) is 50 to 100 mol% of the total monomers (excluding the internal crosslinking agent),
0.001 to 5 mol% of an internal crosslinking agent based on the monomer, and
A water absorbent resin comprising a polyacrylic acid (salt)-based crosslinked polymer obtained from an aqueous monomer solution containing 0 to 0.04 mol% persulfate based on the monomer. The water-absorbent resin according to claim 18 or 19, containing a chelating agent obtained by drying a particulate hydrogel polymer obtained by polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator,
A water absorbent polymer in which a total of 10 ppm or more of a chelating agent is added to the monomer aqueous solution or the hydrogel polymer in a step prior to the drying step (solid content of the monomer or hydrogel polymer at the time of large polymerization). The chelate according to claim 18 or 19, which is obtained by gel-pulverizing a hydrogel polymer obtained by polymerizing an aqueous monomer solution containing a monomer and a polymerization initiator during and/or after polymerization, and drying the obtained particulate hydrogel polymer. As a water-absorbent resin containing an agent,
A water absorbent polymer in which a total of 10 ppm or more of a chelating agent is added to the monomer aqueous solution or the hydrogel polymer in a step prior to the drying step (solid content of the monomer or hydrogel polymer at the time of large polymerization). The water absorbent polymer according to claim 21, wherein the molar ratio of the persulfate to the chelating agent is 20 times or less. The water absorbent resin according to claim 18 or 19, which is an irregular crushed phase. The water absorbent polymer according to claim 18 or 19, wherein the residual ratio of the chelating agent is 50% or more. The absorbent polymer according to claim 18 or 19, which contains a chelating agent on the surface and inside, and the amount of the chelating agent on the surface is greater than the amount of the chelating agent on the inside.