JP7591415B2 - Manufacturing method of water-absorbent resin - Google Patents

Description

The present invention relates to a method for producing a water-absorbent resin.

Super absorbent polymers (SAPs) are water-swellable, water-insoluble polymer gelling agents that are widely used in a variety of fields, including absorbent products such as disposable diapers and sanitary napkins, agricultural and horticultural water retention agents, and industrial water-stopping agents.

The above water-absorbent resins are made from many monomers and hydrophilic polymers as raw materials, but from the viewpoint of water-absorbing performance, the water-absorbent resins whose main component is a polymer composed of an acid group-containing unsaturated monomer are the most widely produced industrially.

The above water-absorbent resins are required to have various functions (higher physical properties) in line with the increasing performance of disposable diapers, which are the main application of the resins. Specifically, in addition to the basic physical properties of absorbency without pressure and absorbency under pressure, various physical properties are required of the water-absorbent resins, such as gel strength, water-soluble content, water content, water absorption speed, liquid permeability, particle size distribution, urine resistance, antibacterial properties, damage resistance, powder flowability, deodorizing properties, color resistance, low dust, and low residual monomer.

General methods for producing water-absorbent resins are roughly divided into aqueous solution polymerization and reversed-phase suspension polymerization. With reversed-phase suspension polymerization, water-absorbent resins in the form of pearls (spherical) or aggregates of spherical particles can be obtained. With reversed-phase suspension polymerization, droplets of an aqueous monomer solution are suspended or dispersed in a hydrophobic organic solvent, and polymerization is carried out. With reversed-phase suspension polymerization, an advantage is that it is possible to obtain powdered or particulate water-absorbent resins with controlled particle size and particle size distribution.

Patent Document 1 discloses a method for producing a water-absorbent resin in which a hydroperoxide is added to a polymer polymerized using a persulfate as a polymerization initiator, the water content is adjusted to 10-40% by weight, and crosslinking is performed with a crosslinking agent having two or more functional groups that can react with carboxyl groups. Specifically, after obtaining a polymer by reversed-phase suspension polymerization, the water content is adjusted by azeotropic dehydration, and then a crosslinking agent is added to carry out a crosslinking reaction. Patent Document 1 states that this production method produces a water-absorbent resin with excellent salt resistance and water absorption speed.

Japanese Unexamined Patent Publication No. 1205/1983

However, in Patent Document 1, the water absorption speed of the obtained water-absorbent resin is not sufficient, and there is still room for improvement. In addition, in order to make paper diapers thinner, there is a tendency to reduce bulky pulp and increase the amount and role of water-absorbent resin used, and for this reason, the physical properties of the water-absorbent resin, in particular the centrifugal retention capacity (CRC), are considered important.

The object of the present invention is to provide a method for producing a water-absorbent resin by reversed-phase suspension polymerization, which has a high initial water absorption rate and excellent water absorption capacity without pressure.

The present invention is a method for producing a water-absorbent resin, comprising a polymerization step of obtaining a hydrogel-like cross-linked polymer by reverse-phase suspension polymerization of a monomer composition containing an internal cross-linking agent and a monomer, and a drying step of obtaining a dried polymer by drying the hydrogel-like cross-linked polymer, in which the hydrogel-like cross-linked polymer subjected to the drying step contains 50 ppm by mass or more of hydrogen peroxide relative to the solid content mass of the hydrogel-like cross-linked polymer, and the hydrogel-like cross-linked polymer is heated during the drying step so that the maximum temperature reached by the cross-linked polymer exceeds 160°C, and/or the dried polymer is heated after the drying step so that the maximum temperature reached by the cross-linked polymer exceeds 160°C.

According to the method for producing the water-absorbent resin of the present invention, the water-absorbent resin obtained by the reversed-phase suspension polymerization method has a high water absorption capacity without pressure and also has a high initial water absorption rate.

FIG. 2 is a schematic diagram showing a part of a production process for a water-absorbent resin according to one embodiment of the present invention.

The present invention is described in detail below, but the scope of the present invention is not limited to these descriptions, and in addition to the examples given below, the present invention can be implemented with appropriate modifications within the scope that does not impair the spirit of the invention. Furthermore, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the claims. Other embodiments obtained by appropriately combining the technical means disclosed in each of the multiple embodiments are also included in the technical scope of the present invention.

[1] Definition of Terms [1-1] "Water-absorbent resin"
The term "water-absorbent resin" in the present invention refers to a water-swellable, water-insoluble polymer gelling agent that satisfies the following physical properties: That is, the water-swellable polymer gelling agent has a CRC (centrifuge retention capacity) of 5 g/g or more as specified in NWSP 241.0.R2(15) (NWSP will be described later) and a water-insoluble polymer gelling agent has an Ext (water-soluble content) of 50 mass% or less as specified in NWSP 270.0.R2(15).

The water-absorbent resin can be designed according to its application and purpose, and is not particularly limited, but is preferably a hydrophilic cross-linked polymer obtained by (internal) cross-linking polymerization of an unsaturated monomer having an acid group (preferably a carboxyl group). In addition, the water-absorbent resin is not limited to a form in which the entire amount is a cross-linked polymer, and may be a composition containing additives, etc.

The "water-absorbent resin" in the present invention may be surface-crosslinked (also called post-crosslinked or secondary crosslinked), or may not be surface-crosslinked. In the present invention, a water-absorbent resin that has undergone a predetermined surface-crosslinking process may be referred to separately as a surface-crosslinked water-absorbent resin.

[1-2] "Water absorbent"
The "water absorbing agent" in the present invention means an absorbing gelling agent for aqueous liquids, containing a water absorbing resin as a main component. The water absorbing agent according to the present invention is suitably used as a sanitary material for absorbing aqueous liquids. The water absorbing agent of the present invention preferably contains 60 to 100 mass%, 70 to 100 mass%, 80 to 100 mass%, or 90 to 100 mass% of the water absorbing resin, and may contain various additives described below as other components. In other words, a water absorbing resin composition in which these components are integrated also falls within the category of the water absorbing agent.

[1-3] Definition of Evaluation Method "NWSP" stands for "Non-Woven Standard Procedures-Edition 2015", which was jointly issued by EDANA (European Disposables and Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry) to unify the evaluation methods of nonwoven fabrics and their products in the United States and Europe, and indicates a standard measurement method for water-absorbent resins. Unless otherwise specified, in the present invention, the physical properties of the water-absorbent resin are measured in accordance with "Non-Woven Standard Procedures-Edition 2015". Regarding evaluation methods not described in the NWSP, measurements are performed according to the methods and conditions described in the Examples.

[1-4] Others In this specification, the range "X to Y" means "X or more and Y or less". In addition, unless otherwise noted, the unit of mass "t (ton)" means "Metricton", and "ppm" means "ppm by mass" or "ppm by weight". Furthermore, "mass" and "weight", "parts by mass" and "parts by weight", "% by mass" and "% by weight" are treated as synonyms. In addition, "acid (salt)" means "acid and/or its salt", and "(meth)acrylic" means "acrylic and/or methacrylic", respectively.

[2] Manufacturing method of water-absorbing agent The manufacturing method according to the present invention is a manufacturing method of a water-absorbing resin, comprising: a polymerization step of obtaining a hydrogel-like crosslinked polymer by reverse-phase suspension polymerization of a monomer composition containing an internal crosslinking agent and a monomer; and a drying step of obtaining a dried polymer by drying the hydrogel-like crosslinked polymer, wherein the hydrogel-like crosslinked polymer subjected to the drying contains 50 ppm by mass or more of hydrogen peroxide relative to the solid content mass of the hydrogel-like crosslinked polymer, and the hydrogel-like crosslinked polymer is heated during the drying step so that the maximum temperature reached by the polymer exceeds 160°C, and/or the dried polymer is heated after the drying step so that the maximum temperature reached by the polymer exceeds 160°C.

According to this manufacturing method, the absorbent resin obtained by the reversed-phase suspension polymerization method has a high water absorption capacity without pressure and also has a high initial water absorption rate.

In the reversed-phase suspension polymerization method, fine, roughly spherical primary particles aggregate to form secondary particles. In this manufacturing method, the polymer is crosslinked by an internal crosslinking agent. By having such a crosslinked structure, the particle structure of the primary particles constituting the hydrogel-like crosslinked polymer becomes stronger, and the adhesiveness of the particles decreases. Therefore, the fusion of the primary particles with each other is suppressed, and the roughness of the surface of the hydrogel can be suppressed. As a result, the decrease in the initial water absorption rate (the ratio of the initial (e.g., 5 minutes) water absorption rate to the equilibrium water absorption rate = reference water absorption swelling rate) is suppressed (the reference water absorption swelling rate is improved). In addition, the adhesiveness of the hydrogel is reduced, so that adhesion to the polymerization apparatus, etc. is reduced. On the other hand, when the crosslink density in the polymer particles due to internal crosslinking increases, the water absorption performance of the water absorbent resin, especially the water absorption rate without pressure, tends to decrease. This is thought to be because the swelling of the water absorbent resin is suppressed by the increase in crosslink density. In this manufacturing method, the hydrogel-like crosslinked polymer before drying contains hydrogen peroxide at a specific concentration (ppm). The hydrogel cross-linked polymer containing hydrogen peroxide is dried at a temperature exceeding 160°C, and/or after the drying process, the dried polymer is heated so that the maximum temperature reached exceeds 160°C, thereby efficiently improving the absorption capacity.

Therefore, according to this manufacturing method, in order to suppress fusion between particles in the hydrous gel state and to suppress surface roughness, the cross-linked structure inside the polymer is made dense, and after the drying process, the cross-linked structure becomes somewhat sparse, thereby improving the water absorption properties of the obtained water-absorbent resin, particularly the water absorption capacity without pressure. Furthermore, if this manufacturing method includes a surface cross-linking process, the water absorption capacity under pressure is also improved. This is thought to be because this manufacturing method can suppress roughness of the particle surface, so the surface cross-linking process is carried out efficiently, resulting in an improved water absorption capacity under pressure.

Note that the above assumptions do not limit the technical scope of the present invention in any way.

This manufacturing method includes, for example, (1) a step of preparing a monomer composition containing an internal crosslinking agent and a monomer, (2) a polymerization step of subjecting the monomer composition to reverse-phase suspension polymerization to obtain a hydrogel-like crosslinked polymer, and (4) a drying step, and may also include, as necessary, (3) a separation step and (5) a surface crosslinking step. The separation step is a step of obtaining a gel-like polymer by separating the hydrogel-like crosslinked polymer discharged from the reaction apparatus in the polymerization step from the organic solvent.

Hydrogen peroxide is added at any stage of the manufacturing process before drying the hydrogel cross-linked polymer.

In a preferred embodiment, the monomer composition contains 50 ppm or more of hydrogen peroxide based on the solids content.

In another preferred embodiment, hydrogen peroxide is added in an amount of 50 ppm or more based on the solid mass of the hydrous gel cross-linked polymer.

Specifically, hydrogen peroxide is preferably added to one or more of the manufacturing processes (1) to (3) below.

(1) A step of preparing a monomer composition containing an internal crosslinking agent and a monomer (Mode 1);
(2) a dispersion step and/or a polymerization step (form 2);
(3) Any step from the separation step to the drying step (Mode 3)
The method of adding hydrogen peroxide is not particularly limited, but it is preferable to add hydrogen peroxide in the form of an aqueous solution in which hydrogen peroxide is dissolved, since it is easy to add hydrogen peroxide. The concentration of hydrogen peroxide in the aqueous solution is not particularly limited, but is usually about 1 to 40% by mass. As the water used in the hydrogen peroxide aqueous solution, for example, deionized water (ion-exchanged water), pure water, ultrapure water, distilled water, etc. can be used. In addition, the hydrogen peroxide aqueous solution may contain a small amount of a hydrophilic solvent. As the hydrophilic solvent, alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol can be exemplified. The solvent used in combination is used at, for example, 0 to 10% by mass relative to the water. The temperature at which hydrogen peroxide or the hydrogen peroxide solution is added is not particularly limited as long as the solution to be added maintains its fluidity, and may be, for example, in the range of -10 to 100°C, more preferably 0 to 30°C.

In the method for producing a water-absorbent resin according to the present invention, the polymerization method may be a reversed-phase suspension polymerization method in which a hydrogel-like crosslinked polymer is obtained by polymerizing droplets containing a monomer in a state where the monomer is dispersed or suspended in a liquid phase consisting of a hydrophobic organic solvent, and the polymerization method may be a batch method or a continuous method. In the batch method, a monomer composition is added or dropped into a hydrophobic organic solvent in a reaction apparatus and mixed to disperse or suspend the monomer composition, and then polymerization is performed to obtain a hydrogel-like crosslinked polymer. On the other hand, the continuous method is a method in which a monomer composition is continuously fed into a hydrophobic organic solvent in a reaction apparatus, dispersed or suspended, polymerized, and the hydrogel formed by the polymerization reaction and the hydrophobic organic solvent are continuously discharged from the reaction apparatus. A preferred embodiment of the present invention is continuous reversed-phase suspension polymerization, and more preferably liquid-phase droplet continuous polymerization in which a monomer composition is continuously dispersed in a hydrophobic organic solvent and polymerized. In the case of such a continuous production process, each step and each operation between steps can be continuously performed, which is preferable in that mass production is possible by long-term operation. In the production method of the water-absorbing resin according to the present invention, a separation step may be provided in which the hydrogel-like crosslinked polymer obtained in the polymerization step is separated from the hydrophobic organic solvent. In the continuous production process, it is preferable to recover the hydrophobic organic solvent separated from the hydrogel-like crosslinked polymer in the separation step and reuse it as the hydrophobic organic solvent in the polymerization step. By adopting such a circulatory production process, the amount of organic solvent used can be reduced, which is preferable in terms of production costs and waste liquid treatment. In addition, continuous polymerization is a form in which a monomer aqueous solution or a monomer composition containing a monomer aqueous solution is continuously suspended or dispersed as droplets in a hydrophobic organic solvent in a dispersion device, and the dispersion/suspension is continuously supplied to a reaction device, so it is clearly distinguished from a form in which dispersion and polymerization are performed in one device (batch operation, batch type). In addition, when the operation is performed continuously, the operation time is preferably 1 hour or more, more preferably 3 hours or more, and is usually 1 year or less.

The method for producing the water-absorbent resin according to the present invention includes an optional monomer composition preparation step; an optional dispersion step; a polymerization step; an optional separation step; and a drying step. After the drying step, the method may also include, optionally, a cooling step, a pulverization step, a classification step, a surface cross-linking step, a granulation step, a fine powder removal step, a granulation step, and a fine powder reuse step. The method may further include a transportation step, a storage step, a packaging step, a storage step, and the like.

Each process is described in detail below.

In the present invention, hydrogen peroxide may be added in any of the manufacturing steps before drying of the hydrogel crosslinked polymer, specifically, in any one or more of the following steps: (1) the preparation step of the monomer composition (form 1), (2) the dispersion step and/or polymerization step (form 2), and (3) any step from the separation step to the drying step (form 3).

The timing of adding hydrogen peroxide differs between forms 1, 2, and 3.

[2-1: Preparation of Monomer Composition]
This step is a step of preparing a composition containing a monomer constituting a polymer (hereinafter, also simply referred to as a monomer composition). The composition is preferably a composition containing an internal crosslinking agent and a monomer constituting a polymer. The composition is usually an aqueous solution containing a monomer and an internal crosslinking agent (hereinafter, also referred to as an "aqueous monomer solution"). The internal crosslinking agent is not limited to this form and may be added in the polymerization step.

The monomer used preferably contains an acid group-containing unsaturated monomer as a main component. Here, the above-mentioned "main component" means that the amount (content) of the acid group-containing unsaturated monomer used is usually 50 mol% or more, preferably 70 mol% or more, and more preferably 90 mol% or more (upper limit 100 mol%), based on the total monomers (excluding the internal crosslinking agent) used in the polymerization reaction of the water absorbent resin.

The acid group in the acid group-containing unsaturated monomer is not particularly limited, but examples thereof include a carboxyl group, a sulfone group, and a phosphoric acid group. Examples of the acid group-containing unsaturated monomer include (meth)acrylic acid, (anhydrous) maleic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloyloxyethanesulfonic acid, 2-(meth)acryloyloxypropanesulfonic acid, and 2-hydroxyethyl (meth)acryloyl phosphate. From the viewpoint of water absorption performance, preferred are (meth)acrylic acid, (anhydrous) maleic acid, itaconic acid, and cinnamic acid, more preferred are (meth)acrylic acid, and most preferred is acrylic acid.

It is preferable that a part or all of the acid groups contained in the acid group-containing unsaturated monomer contained in the monomer composition are neutralized. The salt of the acid group-containing unsaturated monomer is appropriately selected from salts with inorganic cations such as alkali metal salts, alkaline earth metal salts, and ammonium salts, and salts with basic substances such as amine-based organic compounds having amino groups or imino groups. Among them, salts with monovalent cations are more preferable, and at least one selected from alkali metal salts, ammonium salts, and amine salts are more preferable, and alkali metal salts are even more preferable. Among alkali metal salts, at least one selected from sodium salts, lithium salts, and potassium salts is even more preferable, and sodium salts are particularly preferable. The neutralization of the acid groups contained in the acid group-containing unsaturated monomer is not limited to the case where it is performed in this process, and may be performed at any stage of the production process.

The neutralizing agent used to neutralize the acid group-containing unsaturated monomer is not particularly limited, but may be a salt containing a cation that constitutes the salt of the acid group-containing insoluble monomer, and may be appropriately selected from inorganic salts such as sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonium carbonate, and basic substances such as amine-based organic compounds having amino groups or imino groups. Two or more basic substances may be used in combination as the neutralizing agent.

From the viewpoint of water absorption performance, the number of moles of the neutralization salt relative to the total number of moles of the acid group-containing unsaturated monomer and its neutralization salt (hereinafter referred to as the "neutralization rate") is adjusted to be preferably 40 mol% or more, more preferably 40 mol% to 80 mol%, even more preferably 45 mol% to 78 mol%, and particularly preferably 50 mol% to 75 mol%. The suitable range of the neutralization rate is also the same in the case of post-neutralization (neutralization in a hydrous gel-like cross-linked polymer).

The neutralization rate may be adjusted before the start of the polymerization reaction of the acid group-containing unsaturated monomer, during the polymerization reaction of the acid group-containing unsaturated monomer, or on the hydrogel-like crosslinked polymer obtained after the polymerization reaction of the acid group-containing unsaturated monomer (post-neutralization). The neutralization rate may be adjusted at one of the stages before the start of the polymerization reaction, during the polymerization reaction, or after the polymerization reaction, or may be adjusted in multiple stages. Furthermore, the neutralizing agent may be added in one step or in multiple stages (for example, two-stage neutralization). Two-stage neutralization involves adding the neutralizing agent in two stages.

The monomer composition of the present invention may contain monomers other than the acid group-containing unsaturated monomer.

The monomer other than the acid group-containing unsaturated monomer may be any compound that can be polymerized to become a water-absorbing resin. Examples include amide group-containing unsaturated monomers such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; amino group-containing unsaturated monomers such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide; mercapto group-containing unsaturated monomers; phenolic hydroxyl group-containing unsaturated monomers; and lactam group-containing unsaturated monomers such as N-vinylpyrrolidone.

The monomer concentration in the monomer composition of the present invention (=total monomer amount/composition amount, where the monomer includes monomers other than the acid group-containing unsaturated monomer) is preferably 10% by mass to 90% by mass, more preferably 20% by mass to 80% by mass, even more preferably 30% by mass to 70% by mass, and particularly preferably 40% by mass to 60% by mass, from the viewpoint of the physical properties and productivity of the water absorbent resin.

The monomer composition of the present invention contains an internal crosslinking agent. The internal crosslinking agent adjusts the water absorption performance and gel strength of the resulting water absorbent resin when it absorbs water.

The internal crosslinking agent may have a total of two or more unsaturated bonds or reactive functional groups in one molecule. For example, examples of internal crosslinking agents having multiple polymerizable unsaturated groups (copolymerizable with monomers) in the molecule include N,N-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin (meth)acrylate, glycerin acrylate methacrylate, ethylene oxide modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, trimethylolpropane di(meth)allyl ether, poly(meth)allyl ether of polyol having 2 to 10 carbon atoms, pentaerythritol triallyl ether, and tetraallyloxyethane. Examples of internal crosslinking agents having multiple reactive functional groups (which can react with functional groups of monomers (e.g., carboxy groups)) in the molecule include triallylamine, polyallyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, and polyethyleneimine (note that cyclic carbonates such as ethylene carbonate are crosslinking agents that further generate functional groups OH by reacting with carboxyl groups). Examples of internal crosslinking agents having polymerizable unsaturated groups and reactive functional groups in the molecule include glycidyl (meth)acrylate. Two or more of these may be used in combination.

Among these internal crosslinking agents, from the viewpoint of the effects of the present invention, a compound having multiple polymerizable unsaturated groups in the molecule is preferable, a compound having a (poly)alkylene oxide structure in the molecule is more preferable, a compound having a polyethylene glycol structure is even more preferable, an acrylate compound having a polyethylene glycol structure is even more preferable, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate is particularly preferable, and polyethylene glycol diacrylate is most preferable. In the hydrogel obtained by using these internal crosslinking agents, the effect that the water absorption capacity (particularly the water absorption capacity without pressure) is easily improved by drying is obtained.

The amount of the internal crosslinking agent used is appropriately set according to the type of monomer and internal crosslinking agent. From the viewpoint of increasing the gel strength of the obtained water-absorbent resin and preventing adhesion of the hydrogel to a polymerization apparatus, etc., the amount of the internal crosslinking agent used is preferably 0.005 mol% or more, more preferably 0.01 mol% or more, and even more preferably 0.02 mol% or more, based on the monomer. From the viewpoint of improving the water absorption performance of the water-absorbent resin, it is preferably 5 mol% or less, more preferably 2 mol% or less. If the amount of the internal crosslinking agent used is too small, the adhesion of the hydrogel-like crosslinked polymer after polymerization increases, and if the amount of the internal crosslinking agent used is too large, the absorption capacity of the obtained water-absorbent resin may decrease.

The water used in the monomer composition may be, for example, deionized water (ion-exchanged water), pure water, ultrapure water, distilled water, etc., but deionized water (ion-exchanged water) is preferred.

The monomer composition may contain a small amount of a hydrophilic solvent. Examples of hydrophilic solvents include alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol. The solvent used in combination is used in an amount of, for example, 0 to 10% by mass relative to water.

Other Substances The following substances (hereinafter referred to as "other substances") may be added to the monomer composition within the scope of the object of the present invention.

Specific examples of other substances include polymerization inhibitors; chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphites; foaming agents such as carbonates, bicarbonates, azo compounds, and bubbles; chelating agents such as ethylenediaminetetra(methylenephosphinic acid) and its metal salts, ethylenediaminetetraacetic acid and its metal salts, and diethylenetriaminepentaacetic acid and its metal salts; hydrophilic polymers such as polyacrylic acid (salts) and their crosslinked products (for example, fine powders made of water-absorbent resins), starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol. The other substances may be used alone or in combination of two or more.

The amount of other substances used is not particularly limited, but the total concentration of other substances is preferably 10% by mass or less, more preferably 0.001% by mass to 5% by mass, and particularly preferably 0.01% by mass to 1% by mass, relative to the monomer.

The present invention is characterized in that the hydrogel crosslinked polymer to be subjected to the drying step contains 50 ppm or more of hydrogen peroxide relative to the solid content, and is heated at a temperature exceeding 160°C (preferably 170 to 220°C) during and/or after the drying step. Here, the method for obtaining a hydrogel crosslinked polymer containing 50 ppm or more of hydrogen peroxide relative to the solid content may be either (1) a method of preparation before the polymerization reaction, or (2) a method of preparation after the polymerization reaction and before the drying step, or a combination of (1) and (2).

Specific methods for adjusting the hydrogen peroxide content in the hydrous gel cross-linked polymer include the following (1) and (2).

Method for adjusting the hydrogen peroxide content before the polymerization reaction (1);
In the monomer composition preparation step, dispersion step, or polymerization step, a predetermined amount of hydrogen peroxide is added before the polymerization reaction, and polymerization conditions (e.g., polymerization time, polymerization temperature, suppression of the use of hydrogen peroxide decomposition promoters such as ultraviolet rays and reducing agents, co-use of hydrogen peroxide stabilizers, etc.) are selected such that hydrogen peroxide remains at 50 ppm or more after the polymerization reaction. In other words, the hydrogen peroxide can be added in the monomer composition preparation step (form 1) or in the dispersion step/polymerization step (form 2). The preferred polymerization conditions are not particularly limited as long as 50 ppm or more of hydrogen peroxide remains in the hydrogel crosslinked polymer after polymerization, but a method of short-time polymerization is preferred.

A method for adjusting the content of hydrogen peroxide after the polymerization reaction and before the drying step (2);
Hydrogen peroxide is added to the hydrogel crosslinked polymer after the polymerization reaction (including hydrogen peroxide optionally used before the polymerization reaction) to adjust the hydrogen peroxide content to 50 ppm or more. Specifically, hydrogen peroxide may be added after the polymerization reaction is completed and before the drying step, and examples of such hydrogen peroxide include the polymerization step (form 2) and any step from the separation step to the drying step (form 3). For example, in the polymerization step (form 2), hydrogen peroxide can be added to the hydrogel crosslinked polymer discharged from the polymerization apparatus after polymerization. When hydrogen peroxide is added in any step from the separation step to the drying step (form 3), hydrogen peroxide may be added, for example, before, during, or after the separation step, and before the drying step, so that hydrogen peroxide is 50 ppm or more. In addition, hydrogen peroxide may be added to the transport step, storage step, aging step, and each of these steps that are optionally provided after the polymerization step and before the drying step, so that hydrogen peroxide is 50 ppm or more.

(Form 1)
In the embodiment 1, hydrogen peroxide is added in the step of preparing the monomer composition. That is, a composition containing hydrogen peroxide and an acid group-containing unsaturated monomer (preferably together with an internal crosslinking agent) is obtained, and the acid group-containing unsaturated monomer is polymerized in the next step using the composition.

The order of addition of the monomer, internal crosslinking agent, and hydrogen peroxide is not particularly limited, and may be any of the following: adding hydrogen peroxide to an aqueous solution containing an acid group-containing unsaturated monomer and an internal crosslinking agent; adding an acid group-containing unsaturated monomer, an internal crosslinking agent, and hydrogen peroxide to water at the same time; adding an acid group-containing unsaturated monomer and an internal crosslinking agent to an aqueous hydrogen peroxide solution.

In the case of adding hydrogen peroxide to the monomer composition in the first embodiment, the amount of hydrogen peroxide added is preferably 50 ppm or more relative to the monomer composition (solid content), although it depends on the polymerization conditions (polymerization time, polymerization temperature, etc.), considering the improvement of the physical properties of the resulting water-absorbent resin, particularly the water absorption capacity without pressure. That is, in a preferred embodiment of the present invention, a hydrogel-like crosslinked polymer is obtained by inverse suspension polymerization of a monomer composition containing 50 ppm or more of hydrogen peroxide relative to the solid content. The polymerization conditions are appropriately selected to obtain the desired hydrogen peroxide concentration as described above, and the amount of hydrogen peroxide remaining in the hydrogel-like crosslinked polymer after polymerization (ppm) may be measured. In addition, when hydrogen peroxide is contained in the composition in form 1, the amount of hydrogen peroxide added relative to the monomer composition (solid content) is preferably 50 ppm (0.005% by mass) to 10,000 ppm (1.0% by mass), and more preferably 100 ppm (0.01% by mass) to 5,000 ppm (0.5% by mass). In addition, form 1 may be combined with any one or more of forms 2 to 3 other than form 1, in which case the amount of hydrogen peroxide added is appropriately adjusted so that the amount of hydrogen peroxide is 50 ppm or more relative to the solid content mass of the hydrous gel cross-linked polymer.

[2-2: Dispersion process/polymerization process]
[2-2-1. Dispersion process]
The dispersion step is a step of dispersing or suspending droplets containing a monomer in a hydrophobic organic solvent. In the following, when the term "dispersion" is used, it is intended to include suspension. Specifically, the monomer composition is added to a hydrophobic organic solvent, and then mixed and stirred to disperse the monomer composition. For example, stirring blades (propeller blades, paddle blades, anchor blades, turbine blades, Pfaudler blades, A stirring device equipped with a stirring blade (such as a ribbon blade) may be used. When using a stirring device with such a stirring blade, the dispersion droplet size can be adjusted by the type, blade diameter, rotation speed, etc. of the stirring blade. This is particularly suitable for use in batch reverse phase suspension polymerization. A dispersion can be obtained by the methods described in International Publication Nos. 2009/025235 and 2013/018571. In the dispersion step, a monomer solution (or a monomer composition) and a hydrophobic organic solvent are separately supplied to a dispersion device, and droplets containing the monomer dispersed in the hydrophobic organic solvent are formed. This is the process. Here, the term "separately" supplying the monomer solution (or monomer composition) and the hydrophobic organic solvent to the dispersing device means that the monomer solution (or monomer composition) and the hydrophobic organic solvent are This does not mean that the mixture is "separately" fed to the dispersing device, but that the monomer solution (or monomer composition) and the hydrophobic organic solvent are "separately" fed to the dispersing device independently. It is.

Dispersion devices used in the dispersion process include, but are not limited to, spray nozzles, high-speed rotary shear type agitators (rotary mixer type, turbo mixer type, disk type, double cylinder type, etc.), cylindrical nozzles such as needles, orifice plates with many holes directly formed in the plate, spray nozzles, centrifugal atomizers such as rotating wheels, etc.

The materials used in this process are described below.

"Hydrophobic organic solvent"
A preferred hydrophobic organic solvent is at least one organic solvent selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. Specific examples include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, and decalin; aromatic hydrocarbons such as benzene, toluene, and xylene; and halogenated hydrocarbons such as chlorobenzene, bromobenzene, carbon tetrachloride, and 1,2-dichloroethane. Among these, n-hexane, n-heptane, and cyclohexane are preferred from the viewpoints of availability and quality stability. It is also possible to use a mixed solvent of two or more kinds.

In the present invention, as long as the object of the present invention is not hindered, a dispersing aid such as a surfactant or a polymer additive may be added to the hydrophobic organic solvent as necessary. The type of dispersing aid is appropriately selected depending on the combination of the hydrophobic organic solvent and the monomer used, and examples of dispersing aids that can be used include the following surfactants and polymer additives.

Specific examples of the surfactant include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylarylformaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. Two or more of these may be used in combination. Polymerizable surfactants having polymerizability may also be used. Specific examples of the polymerizable surfactant include compounds having the following structure.

In the formula, ^R1 and ^R2 are each independently hydrogen, methyl or ethyl, and n is an integer of 3 to 20. Among the above surfactants, fatty acid esters such as sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, and polyethylene glycol fatty acid esters are preferred, and among these, sucrose fatty acid esters are preferred.

Specific examples of the above polymer additives include maleic anhydride modified polyethylene, maleic anhydride modified polypropylene, maleic anhydride modified ethylene-propylene copolymer, maleic anhydride modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, ethylhydroxyethyl cellulose, etc. Among these, from the viewpoint of dispersion stability of the monomer aqueous solution, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer are preferred. Among these, two or more may be used in combination. These polymer additives may also be used in combination with the above-mentioned surfactant. Among these, it is preferred to use a polymer additive, and it is more preferred to use a maleic anhydride-modified ethylene-propylene copolymer. In another preferred embodiment, the polymer additive is used alone without using a surfactant.

The amount of the dispersion aid used is appropriately set depending on the polymerization form, the monomer composition, the type of hydrophobic organic solvent, etc. Specifically, the concentration of the dispersion aid in the hydrophobic organic solvent is preferably 0.0001 to 2% by mass, and more preferably 0.0005 to 1% by mass.

[2-2-2. Polymerization process]
The polymerization step is a step in which the droplet-like monomer composition obtained in the dispersion step is polymerized to obtain a hydrogel-like crosslinked polymer (hydrogel).

(Reaction Apparatus)
The reaction apparatus used in the polymerization step may be the same as the dispersing apparatus used in the dispersion step, or may be a different apparatus. In the case of batch-type reversed-phase suspension polymerization, the apparatus used in the dispersion step can be used as the reaction apparatus as is, which is preferable in terms of workability. When the reaction apparatus is a different apparatus from the dispersing apparatus, the suspension/dispersion of the monomer composition obtained in the dispersion step is supplied to the reaction apparatus.

In addition, the shape of the reactor in which the polymerization reaction is carried out is not particularly limited, and a known reactor can be used. As described above, a stirring device that can be suitably used in the dispersion step can also be suitably used in the polymerization reaction. In the case of a continuous production method, the shape is preferably such that the monomer (composition) can undergo polymerization reaction while moving as a droplet-like dispersed phase in the hydrophobic organic solvent that is the continuous phase formed in the reactor. Examples of such a reactor include a reactor in which a tubular reaction tube is arranged vertically, horizontally, or spirally. In this embodiment, the monomer (composition) is supplied into the hydrophobic organic solvent moving in the reaction section, so that the droplets of the monomer composition do not stagnate but move together with the hydrophobic organic solvent. This suppresses contact between monomer reactants with different polymerization rates. Furthermore, according to this production method, it is possible to increase the amount of crosslinking agent contained in the monomer composition that constitutes the droplets, so that the adhesion of the hydrogel obtained by the polymerization reaction is reduced, and fusion of the droplets with each other and adhesion to the reactor are suppressed. This reduces the risk of the hydrous gel melting together to form coarse gels, or the surface of the hydrous gel becoming uneven. It also reduces the risk of clumping due to adhesion and accumulation on reaction equipment, etc., and any problems resulting from these.

The reactor may also be equipped with a temperature control means so that the inside of the reactor can be heated or cooled from the outside, if necessary.

The polymerization reaction can be carried out by adding a polymerization initiator. The polymerization initiator may be contained in the monomer composition containing the monomer (and the internal crosslinking agent). From the viewpoint of workability, it is preferable to add a thermally decomposable polymerization initiator and/or a photodecomposable polymerization initiator to the monomer composition in the polymerization process. In addition, when the monomer composition contains a polymerization initiator, gelation or an increase in viscosity of the monomer aqueous solution may occur, so it is preferable to add the polymerization initiator immediately before dispersing/suspending the monomer composition in a hydrophobic organic solvent, to cool the monomer aqueous solution and mix it with the polymerization initiator at a temperature lower than room temperature (20°C or lower, preferably around 0°C), or to subject the monomer aqueous solution and the polymerization initiator to the dispersion process while line mixing. The polymerization initiator is appropriately selected depending on the polymerization form, etc., so there is no particular limit, but examples include a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, or a combination of these, or a redox-based polymerization initiator combined with a reducing agent that promotes decomposition of the polymerization initiator. Specifically, one or more of the polymerization initiators disclosed in U.S. Patent No. 7,265,190 are used. From the viewpoint of the ease of handling of the polymerization initiator and the physical properties of the water-absorbing resin, preferably a peroxide or an azo compound, more preferably a peroxide, and even more preferably a persulfate such as sodium persulfate, ammonium persulfate, or potassium persulfate is used. Hydrogen peroxide is known as an oxidizing agent for polymerization initiators, particularly redox-based polymerization initiators, but in the present embodiment, in consideration of the polymerization efficiency, it is preferable to use other thermally decomposable polymerization initiators in combination with hydrogen peroxide, and particularly, it is preferable to use hydrogen peroxide in combination with a persulfate.

In this embodiment, hydrogen peroxide can be used as an oxidizing agent for the redox polymerization initiator, and can be used in combination with a reducing compound such as L-ascorbic acid or sodium hydrogen sulfite. In the present invention, as described above, a suitable form is one in which a sufficient amount of hydrogen peroxide remains in the hydrogel crosslinked polymer after the polymerization step in the drying step, and the hydrogel crosslinked polymer to be dried contains 50 ppm or more of hydrogen peroxide relative to its solid mass. That is, when using a redox polymerization initiator that uses a reducing agent and an oxidizing agent in combination, it is preferable to add an excess amount of hydrogen peroxide relative to the amount of the reducing compound added so that a sufficient amount of hydrogen peroxide remains in the hydrogel crosslinked polymer after the polymerization step in the drying step. The amount of hydrogen peroxide added (ppm) is preferably 10 times or more by mole relative to the reducing compound, more preferably 100 times or more by mole, and even more preferably 1000 times or more by mole. More preferably, the polymerization is performed without using a reducing compound such as L-ascorbic acid or sodium hydrogen sulfite, which is also a decomposing agent for hydrogen peroxide. The amount of the reducing compound added is preferably 10 ppm or less, more preferably 1 ppm or less, even more preferably 0.1 ppm or less, and most preferably substantially 0 ppm (absent), relative to the solid mass of the hydrogel crosslinked polymer. In this embodiment, a particularly preferred embodiment is one in which the reducing compound is in the above range (particularly absent), and the polymerization initiator consists only of hydrogen peroxide and persulfate.

The amount of the thermally decomposable polymerization initiator or photodecomposable polymerization initiator other than hydrogen peroxide used is preferably 0.001 mol% to 1 mol%, more preferably 0.001 mol% to 0.5 mol%, even more preferably 0.005 to 0.5 mol%, and even more preferably 0.01 to 0.5 mol%, based on the monomer.

In addition to the above-mentioned method of carrying out a polymerization reaction by adding a polymerization initiator, there is also a method of irradiating with active energy rays such as radiation, electron beams, and ultraviolet rays. In addition, irradiation with active energy rays may be used in combination with the addition of a polymerization initiator.

The polymerization time may be appropriately determined depending on the type of monomer and polymerization initiator, the polymerization temperature, etc., but it is preferable to keep the polymerization time short, from the viewpoint of productivity and in view of the influence on the remaining amount of hydrogen peroxide to be added, and specifically, it is preferably within 2 hours, more preferably within 1 hour, even more preferably within 30 minutes, even more preferably within 20 minutes, and particularly preferably within 10 minutes. The polymerization time refers to the time during which the polymerization reaction of the monomer is possible in the reaction system (due to the decomposition of the polymerization initiator, etc.), and specifically refers to the time during which the reaction system is heated or kept at a temperature sufficient to decompose the thermally decomposable polymerization initiator. In the case of a continuous manufacturing method, the polymerization time refers to the time during which droplets containing the monomer and the polymerization initiator or hydrogel particles, which are the polymerization reaction product thereof, are present in the reaction vessel. The lower limit of the polymerization time is not particularly limited as long as the polymerization proceeds sufficiently, but is preferably 10 seconds or more, more preferably 20 seconds or more, and even more preferably 30 seconds or more.

The polymerization temperature, which is the reaction temperature in the polymerization process, may be set appropriately depending on the type and amount of polymerization initiator used, but is preferably 20°C to 100°C, and more preferably 40°C to 90°C. Polymerization temperatures higher than 100°C are not preferred because a rapid polymerization reaction occurs. The polymerization temperature is the temperature of the hydrophobic organic solvent, which is the dispersion medium (hereinafter referred to as "Td").

In the polymerization process, since the monomer (aqueous solution) is dispersed in the hydrophobic organic solvent in the form of droplets, the temperature of the aqueous monomer solution rises quickly due to heat transfer from the hydrophobic organic solvent. If the polymerization initiator contained in the droplets is a thermally decomposable polymerization initiator, the thermally decomposable polymerization initiator decomposes as the temperature rises, generating radicals. The generated radicals then initiate a polymerization reaction, and as the polymerization reaction progresses, a hydrous gel is formed.

In the case of a continuous production method, the formed hydrogel moves inside the reactor with the moving continuous phase and is discharged from the reactor together with the hydrophobic organic solvent that constitutes the continuous phase.

When the monomer aqueous solution contains a thermally decomposable polymerization initiator, the Td is preferably 70°C or higher, more preferably 75°C or higher, and even more preferably 80°C or higher, from the viewpoint of polymerization rate. There is no particular upper limit for Td, but from the viewpoint of safety, it is appropriately selected within a range that does not exceed the boiling point of the hydrophobic organic solvent.

In the manufacturing method of the present invention, from the viewpoint of obtaining an appropriate aggregate particle size, multi-stage polymerization may be performed. Specifically, after the end of the first polymerization step, the above-mentioned aqueous monomer solution is added while stirring the reaction solution to carry out the polymerization reaction.

A hydrous gel-like polymer is obtained by polymerization of an acid group-containing unsaturated monomer. In addition, because the polymerization is carried out using an internal crosslinking agent, a hydrous gel-like crosslinked polymer is obtained.

The solid content of the hydrogel (hereinafter referred to as gel solid content) is preferably 25% by mass or more, more preferably 25% by mass to 75% by mass, even more preferably 25% by mass to 65% by mass, even more preferably 25% by mass to less than 60% by mass, and particularly preferably 40% by mass to less than 60% by mass. The gel solid content is measured by the method described in the examples below.

(Form 2)
In form 2, hydrogen peroxide is added in the dispersion step and/or polymerization step. Here, the timing of adding hydrogen peroxide is not particularly limited, but specifically, hydrogen peroxide is added to a hydrophobic organic solvent; hydrogen peroxide is added during polymerization of an aqueous monomer solution containing an acid group-containing unsaturated monomer; hydrogen peroxide is added to a hydrogel obtained by polymerizing an aqueous monomer solution containing an acid group-containing unsaturated monomer; and the like. The amount of hydrogen peroxide added relative to the monomer composition (solid content) is preferably 50 ppm (0.005% by mass) to 10,000 ppm (1.0% by mass), more preferably 100 ppm (0.01% by mass) to 5,000 ppm (0.5% by mass). In addition, form 1 may be combined with at least one of, for example, form 1 and form 3, in which case the amount of hydrogen peroxide added is appropriately adjusted to be 50 ppm or more of hydrogen peroxide relative to the solid content mass of the hydrogel crosslinked polymer.

[2-3. Separation process]
This step is a step of separating the hydrogel-like crosslinked polymer obtained by polymerization in the reaction apparatus in the above polymerization step from the organic solvent.

In the present invention, the type and structure of the separation device are not particularly limited, but known methods such as filtration, sedimentation, centrifugation, and squeezing can be used. Alternatively, the mixture may be heated under normal or reduced pressure using a stirring device having stirring blades used in the polymerization step, and separated from the hydrophobic organic solvent by azeotropic dehydration and/or solvent distillation. In batch-type reversed-phase suspension polymerization, azeotropic dehydration and/or solvent distillation are preferably performed before the drying step (of the present invention). During solvent separation by azeotropic dehydration and/or solvent distillation, a surface crosslinking agent described later may be added, and surface crosslinking may be performed simultaneously with azeotropic dehydration and/or solvent distillation.

(Form 3)
In form 3, hydrogen peroxide is added in any of the steps from the separation step to the drying step. Specifically, there are forms in which hydrogen peroxide is added to the hydrogel and/or the organic solvent during the separation step; forms in which hydrogen peroxide is added to the hydrogel put into the drying device after the separation step and before drying and heating; and forms in which hydrogen peroxide is added to the hydrogel in a sizing step optionally provided between the separation step and the drying step. The amount of hydrogen peroxide added to the hydrogel-like crosslinked polymer is appropriately set in consideration of the amount of hydrogen peroxide remaining before drying, and is, for example, preferably 50 ppm (0.005% by mass) to 10,000 ppm (1.0% by mass), more preferably 100 ppm (0.01% by mass) to 5,000 ppm (0.5% by mass) relative to 100 parts by mass (solid content) of the hydrogel-like crosslinked polymer. In addition, form 1 may be combined with at least one of form 1 and form 3 other than form 1. In this case, the amount of hydrogen peroxide added is appropriately adjusted so that the amount of hydrogen peroxide is 50 ppm or more relative to the solid content mass of the hydrous gel-like crosslinked polymer.

The polymerization rate of the hydrogel is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more, from the viewpoint of suppressing aggregation during drying of the hydrogel-like crosslinked polymer and reducing residual monomers in the obtained water-absorbent resin. If a drying step is performed using a hydrogel with a low polymerization rate containing a large amount of unreacted monomers, a polymerization reaction may proceed during drying, and large-particle-sized gel particles may be regenerated or by-produced from small-particle-sized gel particles. Large-particle-sized gel particles cause problems such as a decrease in the water absorption rate of the obtained water-absorbent resin, large particles in the dried product, and fine powder generation due to re-grinding to the desired product particle size. In order to avoid this problem, it is preferable to set the polymerization rate of the hydrogel within the above range.

There is no particular upper limit to the polymerization rate of the hydrogel, with 100% by mass being ideal. However, a long polymerization time and strict polymerization conditions are required to achieve a high polymerization rate, which can lead to reduced productivity and physical properties, so an upper limit of 99.95% by mass, or even 99.9% by mass, and usually around 99.8% by mass is sufficient. A typical upper limit is 98 to 99.99% by mass.

The polymerization rate of the hydrogel can be measured as follows. After sampling the hydrogel, 1.00 g of the hydrogel is immediately put into 1000 g of ion-exchanged water at room temperature (the polymerization reaction essentially stops at this point), and the mixture is stirred at 300 rpm for 2 hours, followed by filtration to remove insoluble matter. The amount of monomer extracted in the filtrate obtained by the above operation is measured using a liquid chromatograph. When the amount of the monomer is the residual monomer amount m (g), the polymerization rate C (mass%) of the hydrogel is calculated according to the following (Equation 1), and the residual monomer amount R (mass%) of the hydrogel is calculated according to the following (Equation 2). It is preferable to measure the polymerization rate immediately after sampling the hydrogel, but if it takes time from sampling to measurement, it is necessary to perform a polymerization termination operation by forced cooling (contact with dry ice, liquid nitrogen, ice water, etc.).

C (mass%) = 100 x {1-m/(α x M/100)} ... (Formula 1)
R (mass%) = 100-C... (Formula 2)
In the formula (1), M is the mass (g) of the hydrogel, α is the solid content (mass%) of the hydrogel, and in the formula (2), C is the polymerization rate (mass%) of the hydrogel. ) means

The CRC (centrifuge retention capacity) of the hydrogel is preferably 5 g/g to 80 g/g, more preferably 10 g/g to 50 g/g, even more preferably 15 g/g to 45 g/g, and particularly preferably 20 g/g to 40 g/g, calculated as solid content. Note that the solid content calculation means that the physical properties such as CRC are measured for the hydrogel, and then converted into the physical properties per unit of the water-absorbent resin solid content in the hydrogel (for example, for a hydrogel with a water content of 50% (solid content of 50%), the physical properties are converted into twice the measured values for the hydrogel). Specifically, the measurements are performed by the method described in the examples below.

The solid content of the hydrogel (hereinafter, gel solid content) is preferably 25% by mass or more. From the viewpoints of suppressing aggregation of hydrogel particles after gel crushing, the energy required for crushing, drying efficiency, and absorption performance, the gel solid content is more preferably 25% by mass to 75% by mass, even more preferably 25% by mass to 65% by mass, even more preferably 25% by mass to less than 60% by mass, and particularly preferably 40% by mass to less than 60% by mass. The gel solid content is measured by the method described in the examples below.

The shape of the water-absorbent resin in the obtained hydrogel is usually spherical. "Spherical" is a concept including shapes other than a perfect sphere (e.g., approximately spherical), and means particles with a ratio of the average major axis to the average minor axis (also called "sphericity") of preferably 1.0 to 3.0. The average major axis and the average minor axis of the particles are measured based on an image taken with a microscope. The hydrogel may be formed as an aggregate of minute spherical gels, or may be obtained as a mixture of minute spherical gels and the aggregates of the spherical gels.

When the hydrogel is an aggregate of spherical gels, the particle size of each spherical gel constituting the aggregate is referred to as the primary particle size. The average primary particle size of the hydrogel to be dried is not particularly limited, but from the viewpoint of the water absorption performance of the resulting water-absorbent resin, it is preferably 1000 μm or less, more preferably 5 to 1000 μm, even more preferably 10 to 800 μm, and particularly preferably 10 to 200 μm. The value measured by the following method is used.

In this manufacturing method, the hydrogen peroxide content in the hydrous gel subjected to drying is 50 ppm or more relative to the mass of the hydrous gel solids. When the hydrogen peroxide content is 50 ppm or more, the water absorption capacity without load (CRC) of the obtained water absorbent resin is significantly improved.

From the viewpoint of improving the water absorption capacity without pressure, the hydrogen peroxide content in the hydrogel to be dried is preferably more than 100 ppm, more preferably 150 ppm or more. In addition, from the viewpoint of suppressing an excessive increase in the soluble content, the hydrogen peroxide content (converted into solid content) in the hydrogel to be dried is preferably 50,000 ppm or less, more preferably 1,0000 ppm or less, and even more preferably 5000 ppm or less.

[2-4] Drying step This step is a step of drying the hydrogel obtained above to a desired resin solid content to obtain a dried polymer. The resin solid content is determined from the loss on drying (the change in mass when 1 g of the water-absorbing resin is heated at 180° C. for 3 hours), and is preferably 80% by mass or more, more preferably 85 to 99% by mass, even more preferably more than 90% by mass and 99% by mass or less, even more preferably more than 90% by mass and 98% by mass or less, and particularly preferably 92 to 97% by mass.

Methods for drying the hydrous gel are not particularly limited, but examples include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, azeotropic dehydration with a hydrophobic organic solvent, and high humidity drying using high-temperature water vapor. Among these, hot air drying is preferred from the viewpoint of drying efficiency.

The set temperature of the dryer used for drying is, in order of preference, more than 160°C, more than 160°C and not more than 250°C, 170 to 220°C, and 170 to 200°C. By controlling the set temperature in this manner, it becomes easier to set the maximum temperature of the hydrogel crosslinked polymer during the drying process to a desired temperature, and deterioration of the water-absorbent resin is suppressed. That is, in a preferred embodiment of the present invention, the hydrogel crosslinked polymer is dried in a dryer set at a temperature exceeding 160°C.
Hot air drying methods include a method of drying in a stationary state, a method of drying in an agitated state, a method of drying in a vibrated state, a method of drying in a fluidized state, and a method of drying with an air flow. Among these, from the viewpoint of efficiency, agitation drying or stationary drying is preferred, agitation drying using a rotating device or ventilation band drying is more preferred, and hot air drying using continuous ventilation band drying (continuous ventilation band drying) is particularly preferred. When hot air drying is performed, the hot air temperature is, in order of preference, more than 160°C, more than 160°C and 250°C or less, 170 to 220°C, and 170 to 200°C. By controlling the hot air temperature in this way, it is easy to set the maximum temperature of the hydrogel-like crosslinked polymer during the drying process to a desired temperature, and deterioration of the water-absorbent resin is suppressed.

Another suitable form is agitation drying using an agitation drying device. This agitation drying is a type of drying operation by heat transfer, and can be performed on the material to be dried, preferably continuously, using an indirect heating method. Therefore, it has the advantage of high drying efficiency. The agitation method and form of the agitation drying device are not particularly limited, and it is sufficient that the contents in the drying device are agitated by agitation means such as an arm, blade, or paddle, or a rotating cylinder. In other words, the above-mentioned agitation drying device includes a container rotation type dryer in which the container itself that contains the contents rotates, vibrates, or swings; a mechanical agitation type dryer that agitates the contents with a rotating shaft equipped with an agitation blade such as an arm, blade, or paddle; a floating agitation type dryer that suspends the contents with a gas such as air; a flow path division type dryer that divides the flow path by gravity and a branch plate; a high-speed shear type dryer; an impact type dryer, etc. Among them, it is preferable to use a container rotation type dryer because it causes less mechanical damage. For a description of specific devices, etc., refer to WO 2018/092863 and WO 2018/092864. When using an indirect heating type drying device as described above, the material to be dried can be heated by heating a jacket or heating tube provided in the drying device with a heat medium, and the temperature of the heat medium is preferably, in order of more than 160°C, more than 160°C and not more than 250°C, 170 to 220°C, or 170 to 200°C.

In the present invention, during or after the drying process, the hydrous gel cross-linked polymer or the dried polymer is heated so that its maximum temperature (hereinafter also simply referred to as the maximum temperature) exceeds 160°C.

In one embodiment, the maximum temperature of the hydrous gel-like cross-linked polymer in the drying process is a temperature exceeding 160°C (form A). If the maximum temperature of the hydrous gel-like cross-linked polymer in the drying process is 160°C or less, the water-absorbent resin obtained will have a low water-absorption capacity without pressure. In addition, there is a risk that a large amount of hydrogen peroxide will remain in the obtained water-absorbent resin. This makes it necessary to reduce the amount of internal cross-linking agent used to obtain a water-absorbent resin with a high water-absorption capacity without pressure, which causes problems such as adhesion of the hydrous gel to the device. The maximum temperature of the water-absorbent resin during drying is preferably more than 160°C and 250°C or less, more preferably 170 to 220°C, and most preferably 170 to 200°C. If the heat medium temperature or the maximum temperature is 250°C or less, deterioration of the water-absorbent resin during drying is prevented. The maximum temperature of the water-absorbent resin during drying is measured by a contact thermometer installed in the drying device. Examples of contact thermometers include thermocouples, platinum thermometers, and bimetal thermometers, particularly thermocouples (e.g., K-wire sheathed thermocouples). Typically, the drying temperature is measured at the center of the material layer (particulate hydrogel or granular dried material) (e.g., at a position about 5 cm deep when the material is 10 cm thick).

The drying conditions other than the above drying temperature, such as the hot air speed and drying time, can be set appropriately depending on the water content and total weight of the (particulate) hydrogel to be dried and the desired resin solid content. When band drying is performed, the conditions described in International Publication Nos. 2006/100300, 2011/025012, 2011/025013, 2011/111657, etc. are appropriately applied.

The drying time is preferably 10 to 120 minutes, more preferably 20 to 60 minutes, from the viewpoint of the water absorption capacity (CRC) and residual monomers.

In this drying step, a drying aid can be added. The drying aid is added to maintain fluidity during the next step of stirring and drying, and examples of the drying aid include surfactants and polymer lubricants. Polymer lubricants and surfactants can be used in combination as a drying aid.

Specific examples of surfactants used in drying aids include: (1) sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. (2) nonionic surfactants such as capryldimethylaminoacetic acid betaine, lauryldimethylaminoacetic acid betaine, myristyldimethylaminoacetic acid betaine, stearyldimethylaminoacetic acid betaine, and other alkyl dialkylaminoacetic acid betaines; alkyl amidopropyl betaines such as lauric acid amidopropyl betaine, coconut oil fatty acid amidopropyl betaine, palm kernel oil fatty acid amidopropyl betaine, alkyl hydroxysulfobetaine such as lauryl hydroxysulfobetaine, and alkyl carboxymethyl hydroxyethyl imidazolinium betaine such as 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine; (3) anionic surfactants such as monoalkali metal alkylaminodiacetate such as monosodium laurylaminodiacetate, potassium laurylaminodiacetate, and sodium myristylaminodiacetate; and (4) cationic surfactants such as long-chain alkyl dimethylaminoethyl quaternary salts. Two or more of these may be used in combination.

Specific examples of polymer lubricants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, ethylhydroxyethyl cellulose, polyalkylene oxide such as polyethylene glycol, etc. The molecular weight (weight average molecular weight) of these is preferably selected appropriately in the range of 2 to 2 million, more preferably 4 to 1 million. Two or more of these may be used in combination.

The total amount of surfactant and polymer lubricant added is preferably 1.0% by mass or less, more preferably 0.5% by mass or less, and is preferably 0.01% by mass or more, and particularly preferably 0.05% by mass or more, based on the solid content of the gel fed into the drying device.

The absorbency (CRC) of the water-absorbent resin of the dried polymer obtained in the drying process is preferably 40 g/g or more. When the absorbency (CRC) of the water-absorbent resin after the drying process is 40 g/g or more, it becomes easy to adjust the absorption performance by surface cross-linking, etc., and it is possible to obtain a water-absorbent resin after surface cross-linking and a water-absorbing agent as a final product with excellent absorbency. The absorbency (CRC) after the drying process is more preferably 45 g/g or more, and even more preferably 50 g/g or more.

[2-5] Crushing step, classification step This step is a step of pulverizing the dried polymer obtained in the above drying step as necessary (crushing step), and adjusting the particle size to a predetermined range (classification step) to obtain a water absorbent resin powder (a powdered water absorbent resin before surface crosslinking is conveniently referred to as a "water absorbent resin powder").

The equipment used in the grinding process includes, for example, high-speed rotary grinders such as roll mills, hammer mills, screw mills, and pin mills, vibration mills, knuckle-type grinders, and cylindrical mixers, which are used in combination as necessary.

The method of adjusting the particle size in the classification process is not particularly limited, but examples include sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)) and air flow classification. It is also possible to remove non-standard products (e.g., fine powder) classified in the classification process, and the non-standard products may be recycled during the manufacturing process, for example, by granulation.

The water-absorbent resin powder obtained in the above process (water-absorbent resin powder before the surface cross-linking process, so-called base polymer) has a mass average particle diameter (D50) of preferably 200 to 600 μm, more preferably 200 to 550 μm, and even more preferably 250 to 500 μm. In addition, the proportion of particles having a particle diameter of 45 μm or more and less than 150 μm (particles that pass through a sieve with a mesh size of 150 μm on a JIS standard sieve (JIS Z8801-1 (2000)) and do not pass through a sieve with a mesh size of 45 μm) is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. The proportion of particles having a particle diameter of 850 μm or more (particles that do not pass through a sieve with a mesh size of 850 μm on a JIS standard sieve (JIS Z8801-1 (2000)) is preferably 5% by mass or less, more preferably 3% by mass or less. In addition, the lower limit of the proportion of these particles is preferably as low as possible in any case, and 0% by mass is desirable, but it may be about 0.1% by mass. Furthermore, the logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 to 0.50, more preferably 0.25 to 0.40, and even more preferably 0.27 to 0.35. These particle sizes are measured using standard sieves in accordance with the measurement methods disclosed in U.S. Patent No. 7,638,570 and NWSP 220.0. R2 (15).

The above-mentioned particle size is applied not only to the water-absorbent resin after surface cross-linking (hereinafter, for convenience, may be referred to as "water-absorbent resin particles"), but also to the water-absorbing agent as a final product. Therefore, in the water-absorbent resin particles, it is preferable to perform a surface cross-linking treatment (surface cross-linking process) so as to maintain the particle size in the above range, and the particle size may be adjusted by providing a sizing process (pulverization, classification process) after the surface cross-linking process.

[0223] [0224] [0225] [0226] [0227] [0228] [0229] [0230] [0231] [0229] [0232] [0229] [0233] [0226] [0227] [0228] [0229] [0234] [0229] [0235] [0236] [0237] [0238] [0239] [0240] [0241] [0225] [0239] [0242] [0239] This step is a step of providing a portion with higher crosslink density on a surface layer (a portion several tens of µm from the surface of the water absorbent resin powder) of the water absorbent resin powder, and is composed of a mixing step, a heat treatment step, and a cooling step (optional).

In the surface crosslinking step, a water-absorbent resin (water-absorbent resin particles) that has been surface-crosslinked by radical crosslinking or surface polymerization on the surface of the water-absorbent resin powder, a crosslinking reaction with a surface crosslinking agent, or the like is obtained. In terms of water absorption performance such as water absorption capacity under load, it is preferable to perform surface crosslinking. That is, in a preferred embodiment, the hydrogel-like crosslinked polymer and/or the dried polymer are further surface-crosslinked. The surface crosslinking step may be performed simultaneously with the drying step of [2-4], with reference to the descriptions in International Publication Nos. 2018/092863 and 2014/038324. As described above, in the separation step of [2-3], the surface crosslinking agent may be mixed during azeotropic dehydration of the organic solvent, and heated simultaneously with azeotropic dehydration to perform surface crosslinking, or a surface crosslinking agent may be added after dehydration and heated to perform surface crosslinking, or a surface crosslinking agent may be mixed during evaporation and distillation of the solvent and heated simultaneously to perform surface crosslinking.

[2-6-1: Mixing process]
This step is a step of mixing a water-absorbent resin powder or a hydrous gel-like crosslinked polymer with a surface crosslinking agent. The method of mixing the surface crosslinking agent is not particularly limited, but it is preferable to prepare a surface crosslinking agent solution in advance. The liquid is then preferably sprayed or dropped onto the water-absorbent resin powder or the hydrogel-like crosslinked polymer, and more preferably mixed by spraying.

The device for performing the mixing is not particularly limited, but is preferably a high-speed stirring mixer, more preferably a high-speed stirring continuous mixer.

The surface cross-linking agent is not particularly limited, but may be an organic or inorganic surface cross-linking agent. Among them, from the viewpoint of the physical properties of the water-absorbent resin and the handling of the surface cross-linking agent, an organic surface cross-linking agent that reacts with a carboxyl group is preferred. For example, one or more types of surface cross-linking agents disclosed in U.S. Patent No. 7,183,456 may be mentioned. More specifically, polyhydric alcohol compounds, epoxy compounds, haloepoxy compounds, polyvalent amine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, oxazolidinone compounds, polyvalent metal salts, alkylene carbonate compounds, cyclic urea compounds, etc. may be mentioned.

Specific examples of organic surface crosslinking agents include polyalcohol compounds such as (di-, tri-, tetra-, poly)ethylene glycol, (di-, poly)propylene glycol, 1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, (poly)glycerin, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, trimethylolpropane, di- or triethanolamine, pentaerythritol, and sorbitol; epoxy compounds such as (poly)ethylene glycol diglycidyl ether, (di-, poly)glycerol polyglycidyl ether, and glycidol; 2-oxazolidone, Oxazoline compounds such as N-hydroxyethyl-2-oxazolidone and 1,2-ethylenebisoxazoline; alkylene carbonate compounds such as 1,3-dioxolan-2-one (ethylene carbonate), 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; and the like.

In addition to the above organic surface cross-linking agents, polyvalent cationic polymers such as polyamine polymers or water-soluble polyvalent metal cation-containing compounds may be used in combination as ion-bonding surface cross-linking agents.

The amount of the surface cross-linking agent used (the total amount used when a plurality of agents are used) is preferably 0.01 to 10 parts by mass, more preferably 0.01 to 5 parts by mass, based on 100 parts by mass of the water-absorbent resin powder or the water-containing gel-like cross-linked polymer (solid content). The surface cross-linking agent is preferably added as an aqueous solution, and in this case, the amount of water used is preferably 0.1 to 20 parts by mass, more preferably 0.5 to 10 parts by mass, based on 100 parts by mass of the water-absorbent resin powder or the water-containing gel-like cross-linked polymer (solid content). Furthermore, when a hydrophilic organic solvent is used as necessary, the amount used is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, based on 100 parts by mass of the water-absorbent resin powder.

As described above, a water-soluble polyvalent metal cation-containing compound may be added in the surface cross-linking step.

[2-6-2: Heat treatment process]
This step is a step of applying heat to the mixture discharged from the above-mentioned mixing step to cause a crosslinking reaction on the surface of the water absorbent resin powder. This step may be carried out simultaneously with the drying step, or may be carried out after the drying step. Also, the water absorbent resin powder may be heat-treated in this step without mixing a surface crosslinking agent in the above-mentioned mixing step.

The device for carrying out the crosslinking reaction is not particularly limited. When the crosslinking reaction is carried out in the drying step, the device for carrying out the crosslinking reaction is a drying device, and when the water absorbent resin powder after drying is subjected to heat treatment, a disk dryer or a paddle dryer is preferably used. The reaction temperature in the crosslinking reaction is appropriately set depending on the type of the surface crosslinking agent used, and is preferably 50 to 300°C, more preferably 100 to 200°C. In addition, as described above, during the azeotropic dehydration in the separation step of [2-3], heat treatment may be carried out simultaneously with the azeotropic dehydration in the coexistence of the surface crosslinking agent.

In the present invention, as described above, the hydrogel cross-linked polymer or the dried polymer is heated during or after the drying process so that the maximum temperature reached exceeds 160°C. If the maximum temperature reached by the hydrogel cross-linked polymer during the drying process does not exceed 160°C, heating may be performed in this heat treatment process or a subsequent process so that the maximum temperature reached by the dried polymer exceeds 160°C (form B). Furthermore, in a preferred embodiment, in the surface cross-linking process after the drying process, heating is performed so that the maximum temperature reached by the dried polymer exceeds 160°C. If the drying temperature is 160°C or lower, the water absorption capacity of the obtained water absorbent resin without pressure is low. In addition, there is a risk that a large amount of hydrogen peroxide will remain in the obtained water absorbent resin. This makes it necessary to reduce the amount of internal cross-linking agent used in order to obtain a water absorbent resin with a high water absorption capacity without pressure, which causes problems such as adhesion of the hydrogel to the device. The maximum temperature reached is preferably more than 160°C and not more than 250°C, and more preferably 170 to 220°C. The maximum temperature is measured by a contact thermometer installed in the heating device that performs the heating. Examples of contact thermometers include thermocouples, platinum temperature detectors, bimetal thermometers, and in particular thermocouples (e.g., K-wire sheathed thermocouples).

[2-6-3: Cooling process]
This step is an optional step which is performed as necessary after the heat treatment step.

The cooling device is not particularly limited, but is preferably a device with the same specifications as the device used in the heat treatment step, and more preferably a paddle dryer. This is because it can be used as a cooling device by changing the heat medium to a refrigerant. The water-absorbent resin particles obtained in the heat treatment step are forcibly cooled, as necessary, in the cooling step, preferably to 40 to 80°C, more preferably to 50 to 70°C.

[2-7] Particle Sizing Step This step is a step of adjusting the particle size of the surface crosslinked (particulate) dried polymer. By carrying out the particle sizing step after the surface crosslinking step, a water absorbent resin powder in which the particle size or particle size distribution is highly controlled can be obtained.

Preferably, the sizing step includes a grinding step and/or a classification step. The grinding step and classification step are the same as those described in the above-mentioned drying step and classification step sections.

From the viewpoint of water absorption performance, the particle size of the water absorbent resin powder obtained through the granulation process is within the above range.

[2-8] Step of Adding Other Additives This step is an optional step carried out for the purpose of imparting various additional functions to the water-absorbent resin and improving the water-absorbing performance.

Examples of other additives include chelating agents, organic reducing agents, inorganic fine particles, polyvalent metal salts, inorganic reducing agents, oxidizing agents, hydroxycarboxylic acid compounds, surfactants, compounds having phosphorus atoms, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, thermoplastic fibers, etc. In addition, liquid permeability is one example of the water absorption performance imparted to the water absorbent resin, and additives for improving the liquid permeability include the following polyvalent metal salts, cationic polymers, and inorganic fine particles, and it is preferable to use at least one of these. The amount of these to be added is preferably less than 2.0 parts by mass, more preferably less than 1.5 parts by mass, and even more preferably less than 1.0 part by mass, relative to 100 parts by mass of the water absorbent resin powder.

[2-9] Other Steps In addition to the steps described above, the production method according to the present invention may further include a cooling step, a rewetting step, a pulverizing step, a classification step, a fine powder granulation step, a transporting step, a storing step, a packaging step, a storage step, and the like, as necessary.

[3. Properties of the obtained water-absorbent resin]
The present invention also provides a water-absorbing resin obtained by the manufacturing method of the above embodiment. The physical properties of the water-absorbing resin are preferably as follows. The preferred ranges of the physical properties of the water-absorbing agent are the same as the preferred ranges of each physical property (CRC, etc.) of the water-absorbing resin described below.

[3-1] CRC (water absorption capacity without pressure, centrifuge holding capacity)
"CRC" is an abbreviation for Centrifuge Retention Capacity, and means the water absorption capacity of a water absorbent resin under no pressure (sometimes referred to as "water absorption capacity"). In the above description, this refers to the 30-minute CRC in the examples below.

The CRC of the water-absorbent resin is usually 5 g/g or more, and is preferably 15 g/g or more, more preferably 25 g/g or more, 30 g/g or more, 35 g/g or more, 38 g/g or more, 40 g/g or more, and further 45 g/g or more, 50 g/g or more. There is no particular limit to the upper limit, and a higher CRC is preferable, but from the viewpoint of balance with other physical properties, it is preferably 70 g/g or less, more preferably 65 g/g or less. The present invention can be suitably applied to the production of water-absorbent resins with a high water-absorbency capacity (CRC) before surface crosslinking of 30 g/g or more, 35 g/g or more, 40 g/g or more, and even 45 g/g or more, 50 g/g or more, and 55 g/g or more, and water-absorbent resins with high water-absorbency capacity that were previously difficult to produce due to the adhesiveness of hydrous gels can be easily produced.

If the CRC is less than 5 g/g, the absorption capacity is low and the resin is not suitable for use as an absorbent in absorbent articles such as disposable diapers. If the CRC is more than 70 g/g, the rate at which the resin absorbs body fluids such as urine and blood decreases, making the resin not suitable for use in disposable diapers with high water absorption rates. This manufacturing method makes it possible to obtain a water-absorbent resin with a high CRC.

[3-2] AAP (absorbency under pressure)
The AAP (absorption capacity under pressure) of the water-absorbent resin is preferably 15 g/g or more, more preferably 20 g/g or more, even more preferably 23 g/g or more, and particularly preferably 24 g/g or more. The upper limit is not particularly limited, but is preferably 40 g/g or less.

If the AAP of the absorbent resin is less than 15 g/g, the amount of liquid that returns when pressure is applied is large, making it unsuitable as an absorbent for absorbent articles such as disposable diapers. The AAP can be controlled by adjusting the particle size or changing the surface cross-linking agent.

[3-3] Residual Amount of Hydrogen Peroxide The residual amount of hydrogen peroxide in the water absorbent resin is preferably 10 ppm or less, more preferably 1 ppm or less, and most preferably 0 ppm (below the detection limit). By keeping the residual amount of added hydrogen peroxide below the above upper limit, the influence on the water absorption properties of the water absorbent resin is small, and safety is excellent.

[3-4] Standard water absorption swelling rate (=5 min CRC/30 min CRC)
The standard water absorption and swelling rate is an index for comparing the water absorption capacity under pressure at the initial stage (5 minutes) and at a substantially equilibrium state (30 minutes). The closer it is to 1, the faster the initial water absorption rate is.

The standard water absorption swelling rate of the water absorbent resin is preferably 0.95 or more, and more preferably 0.98 or more.

[4] Uses of the water-absorbent resin The uses of the water-absorbent resin are not particularly limited, but preferably include absorbent applications in absorbent articles such as paper diapers (for infants and adults), sanitary napkins, incontinence pads, etc. In particular, it can be used as an absorbent for high-concentration paper diapers. Examples of other absorbent articles include soil water retention agents, seedling sheets, seed coating materials, condensation prevention sheets, drip absorbents, freshness-preserving materials, disposable hand warmers, cooling bandanas, ice packs, medical waste liquid solidifying agents, residual soil solidifying materials, water damage prevention waste liquid gelling agents, water-absorbing sandbags, disaster-use portable toilets, compresses, thickeners for cosmetics, water-stopping materials for electric and electronic material communication cables, gasket packing, sustained-release agents for fertilizers, various sustained-release agents (space disinfectants, air fresheners, etc.), pet sheets, cat litter, wound protection dressing materials, condensation prevention building materials, oil moisture removers, paints, adhesives, anti-blocking agents, light diffusing agents, matting agents, additives for decorative panels, additives for artificial marble, additives for toners, and other resin additives.

In addition, as the raw material of the absorbent body, an absorbent material such as pulp fiber can be used together with the water-absorbent resin. In this case, the content (core concentration) of the water-absorbent resin in the absorbent body is preferably 30% by mass to 100% by mass, more preferably 40% by mass to 100% by mass, even more preferably 50% by mass to 100% by mass, even more preferably 60% by mass to 100% by mass, particularly preferably 70% by mass to 100% by mass, and most preferably 75% by mass to 95% by mass.

By setting the core concentration within the above range, when the absorbent is used in the upper layer of an absorbent article, the absorbent article can be maintained in a clean white state. Furthermore, since the absorbent has excellent diffusibility of body fluids such as urine and blood, efficient liquid distribution is achieved, and an improvement in the absorption amount can be expected.

The present invention will be explained in more detail with reference to the following experimental examples. However, the present invention should not be interpreted as being limited to these explanations, and experimental examples obtained by appropriately combining the technical means disclosed in each experimental example are also included in the scope of the present invention.

In the following, "water-absorbent resin" refers to a granular dried material that has been subjected to a drying process, a surface-crosslinked granular dried material or water-absorbent resin powder, and a surface-crosslinked water-absorbent resin powder, and "water-containing gel" refers to a water-containing gel-like crosslinked polymer that has not been subjected to a drying process or a particulate water-containing gel-like crosslinked polymer.

In addition, unless otherwise noted, the electrical equipment used in the experimental examples (including equipment for measuring the physical properties of the water-absorbent resin) used a 60 Hz, 200 V or 100 V power supply. In addition, unless otherwise noted, the physical properties of the water-absorbent resin and hydrous gel described below were measured under conditions of room temperature (20°C to 25°C) and a relative humidity of 50% RH ±10%.

For convenience, "liter" may be written as "l" or "L", and "mass%" or "weight%" as "wt%". When measuring trace components, amounts below the detection limit may be written as N.D (Non Detected).

(Measurement method)
(a) Water absorption capacity without pressure (CRC, 30 minutes CRC) (NWSP 241.0.R2 (15))
The water absorption capacity without pressure (CRC, 30-minute CRC) was measured according to NWSP 241.0. R2 (15). That is, 0.200 g (mass W0 (g)) of the water-absorbent resin was weighed, uniformly placed in a nonwoven bag (60 x 85 mm) and heat-sealed, and then immersed in 500 mL of a 0.90% by mass aqueous sodium chloride solution adjusted to 23 ± 2 ° C. After 30 minutes, the bag was pulled up and drained at 250 G for 3 minutes using a centrifuge (Kokusan Co., Ltd. Centrifuge: Model H-122). Thereafter, the mass (W1 (g)) of the bag was measured. The same operation was performed without adding the water-absorbent resin, and the mass (W2 (g)) of the bag at that time was measured. The water absorption capacity without load (CRC) was calculated from the obtained W0 (g), W1 (g), and W2 (g) according to the following (Equation 1).

The 5-minute CRC was calculated in the same manner as above, except that the immersion time was 5 minutes.

The hydrogel CRC (gel CRC) was determined by the same procedure as above, except that 0.8 g of hydrogel-like cross-linked polymer was used as the sample and the free swelling time was set to 24 hours. In addition, the resin solid content of the hydrogel-like cross-linked polymer was measured separately, the mass of the water-absorbent resin in the above 0.8 g of hydrogel-like cross-linked polymer was determined, and the gel CRC was calculated according to the following formula (2). Each sample was measured five times, and the average value was used.

Here,
msi: mass (g) of the hydrogel crosslinked polymer before measurement
mb: Mass (g) of blank (nonwoven fabric only) after free swelling and draining
mwi: Mass (g) of the hydrogel crosslinked polymer after free swelling and draining
Wn: solid content (mass%) of the hydrogel crosslinked polymer
It is.

(b) AAP (absorbency under pressure) (NWSP 242.0.R2(15))
"AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption capacity of a water absorbent resin under pressure, which was measured in accordance with NWSP 242.0. R2 (15). Specifically, it means the water absorption capacity (unit: g/g) after 0.9 g of a water absorbent resin is swollen under a load of 4.83 kPa (about 49 g/cm ² , equivalent to about 0.7 psi) for 1 hour in a large excess of a 0.9 wt % aqueous sodium chloride solution.

(c) Particle size distribution of the water absorbent resin, mass average particle size (D50) and logarithmic standard deviation (σζ)
The particle size of the water-absorbent resin (PSD (Particle Size Distribution), particle size distribution of the water-absorbent resin measured by sieve classification) and the logarithmic standard deviation (σζ) of the particle size distribution were measured according to the measurement method disclosed in U.S. Patent Application Publication No. 2006/204755.

(d) Solid content and moisture content Approximately 1 g of water-absorbent resin (water-absorbing agent) was weighed out (mass W9 (g)) into an aluminum cup (mass W8 (g)) with a bottom diameter of approximately 5 cm, and the cup was left to stand for 3 hours in a windless dryer at 180°C to dry. The total mass (W10 (g)) of the aluminum cup and water-absorbent resin (water-absorbing agent) after drying was measured, and the solid content was calculated using the following formula 4. The moisture content was calculated using the following formula 5.

(e) Water content and solid content of hydrous gel The water content of the hydrous gel before drying was measured in the above (c) with 2.0 g of hydrous gel and 24 hours of drying time. After 2.00 g of water-absorbent resin (hydrous gel) was placed in an aluminum cup with a bottom diameter of 50 mm, the total mass W1 (g) of the sample (water-absorbent resin and aluminum cup) was accurately weighed. Next, the sample was placed in an oven set to an atmospheric temperature of 180°C. After 24 hours, the sample was removed from the oven and the total mass W2 (g) was accurately weighed. When the mass of the water-absorbent resin (hydrous gel) used in this measurement was M (g), the water content (100-α) (mass%) of the water-absorbent resin (hydrous gel) was calculated according to the following (Equation 6). Note that α is the solid content (mass%) of the water-absorbent resin (hydrous gel).
(100-α) (mass%) = {(W1-W2)/M}×100 (Formula 6)
(f) Amount of hydrogen peroxide relative to the solid content in the hydrogel 2 g of hydrogel, 12 g of 0.9% by mass sodium chloride aqueous solution, and 8 g of methanol (if the gel swells and cannot be stirred, the salt concentration or the amount of the aqueous solution or methanol is appropriately adjusted) were placed in a glass sample bottle with a screw cap (volume 50 ml, diameter 35 mm, height approximately 80 mm), and stirred at 300 rpm at room temperature in the dark using a 25 mm stirrer coated with Teflon (registered trademark). After 4 hours, the solution was taken out and passed through a filter paper (manufactured by ADVANTEC, No. 2). 10.0 g of the solution was placed in a glass sample bottle with a screw cap (volume 20 ml, diameter 25 mm, height approximately 50 mm).

Immediately after that, 0.30 g of 2N aqueous sulfuric acid solution and 0.1 g of 30% titanium (IV) sulfate solution (Wako Pure Chemical Industries, Ltd.) were added and stirred at room temperature in the dark. After 1 minute, the solution was transferred to a plastic 1 cm cell and the absorbance (measurement wavelength: 410 nm) was measured using a spectrophotometer (Hitachi Ratio Beam Spectrophotometer U-5100) (the absorbance of 0.30 g of 2N aqueous sulfuric acid solution and 0.1 g of 30% titanium (IV) sulfate solution added to 3 g of 0.9% by mass sodium chloride solution and 2 g of methanol was set to 0). The amount of hydrogen peroxide (ppm) in the hydrogel was calculated from the absorbance thus obtained.

The calibration curve was also created from the absorbance measured by preparing mixtures of 60% 0.9% sodium chloride aqueous solution and 40% methanol, each containing 0.0001%, 0.0005%, 0.0010%, 0.0025%, and 0.0050% hydrogen peroxide by mass, and carrying out the above procedure.

If the formula for the calibration curve is hydrogen peroxide amount (ppm) = a x (absorbance) (a is a constant), the amount of hydrogen peroxide (ppm) relative to the solid content in the hydrous gel is expressed by the following formula.

(g) Method for measuring the average primary particle size of hydrous gel The hydrous gel is photographed with an optical microscope (KH-3000, manufactured by Hirox Co., Ltd.), and the major axis of the primary particles is measured from the obtained image. This measurement is performed for 50 primary particles, and the arithmetic average value is regarded as the average primary particle size of the hydrous gel.

[Example 1]
A series of steps was carried out according to the production process shown in FIG. 1 to prepare a hydrous gel (1), and then the obtained hydrous gel (1) was dried to produce a water-absorbent resin (1).

First, n-heptane (density: 0.68 g/ml) was charged as an organic solvent into the dispersion device 12, the polymerization device 14, the separation device 16, and the pipes connecting them. Next, the liquid delivery pump 18 was operated to start circulating the organic solvent at a flow rate of 300 ml/min. The entire amount of the organic solvent was charged into the polymerization device 14 via the dispersion device 12. In addition, the heat exchanger 20 was operated to heat the circulating organic solvent to a temperature of 90°C. Next, maleic anhydride-modified polyethylene (acid value: 60 mg KOH/g) was mixed with n-heptane as a dispersion aid, and dissolved by heating to 90°C to prepare a 0.030 wt% dispersion aid solution (1). Next, the dispersion aid solution (1) obtained by the above operation was added to n-heptane flowing through the pipe 33 at a flow rate of 50 ml/min for 30 minutes via the pipe 43. The content of maleic anhydride-modified polyethylene relative to the total amount of organic solvent before the start of polymerization was 0.005% by weight.

Acrylic acid, 48.5% by weight of sodium hydroxide aqueous solution, and ion-exchanged water were mixed, and further polyethylene glycol diacrylate (average degree of polymerization: 9) and diethylenetriamine pentacetate pentasodium were added to prepare a monomer aqueous solution (1). Separately, sodium persulfate and ion-exchanged water were mixed to prepare a 6% by weight of sodium persulfate aqueous solution (1). Next, the monomer aqueous solution (1) obtained by the above operation was supplied to the mixing device 10 via pipe 41, and the sodium persulfate aqueous solution (1) was supplied to the mixing device 10 via pipe 42 to prepare a monomer composition (1). The monomer concentration of the monomer composition (1) was 43% by weight, and the neutralization rate was 75 mol%. In addition, the internal crosslinking agent, polyethylene glycol diacrylate, was 0.020 mol% relative to the monomer, the chelating agent, diethylenetriaminepentaacetic acid pentasodium, was 200 ppm relative to the monomer, and the polymerization initiator, sodium persulfate, was 0.1 g/mol (0.05 mol%) relative to the monomer.

The mixture of the organic solvent and the dispersion aid was fed to the pipe 35 of the dispersion device 12, which is a double-cylinder high-speed rotary shear type agitator, at a flow rate of 300 mL. The rotor of the dispersion device 12 was rotated at a rotation speed of 7,200 rpm, and then the monomer composition (1) was fed to the pipe 31 of the dispersion device at a flow rate of 40 ml/min (47.2 g/min). The supplied monomer composition (1) was dispersed in the organic solvent in the form of fine droplets by the dispersion device.

The dispersion liquid in which the monomer composition was dispersed in the form of fine droplets was supplied to the polymerization apparatus 14. The polymerization apparatus used was a vertically arranged tube made of PFA (perfluoroalkoxyalkane) (inner diameter: 25 mm, total length: 10 m).

The droplets of the monomer composition (1) changed into tiny spherical hydrogels (1) as the polymerization reaction progressed while falling through the polymerization apparatus filled with the organic solvent, which was the continuous phase.

The hydrogel (1) obtained by the above series of operations was continuously discharged from the polymerization apparatus 14 together with the organic solvent, and the discharged hydrogel (1) and organic solvent were continuously supplied as they were to the separation apparatus 16. In the separation apparatus, the hydrogel (1) and the organic solvent were separated. The organic solvent separated in the separation apparatus was supplied to the heat exchanger 20 via the pipe 32, the liquid delivery pump 18, and the pipe 33, and the temperature was adjusted in the heat exchanger 20 so that the set temperature (organic solvent temperature) was 90°C, and then supplied to the dispersion apparatus 12 and the polymerization apparatus 14 via the pipes 34 and 35 while maintaining the temperature at 70°C or higher.

The hydrogel (1) obtained by the above procedure had the shape of tiny spherical hydrogel particles (average primary particle size: 60 μm).

The hydrous gel (1) (solid content 44% by mass) discharged from the separation device 16 was continuously supplied as it was to the indirect heating type stirring and drying device 22 via the pipe 36, and a previously prepared ethanol solution of polyethylene glycol 400 (PEG 400) (concentration 20% by mass) and a 1% by mass aqueous hydrogen peroxide solution were added via the pipe 44. The amount of the PEG 400 ethanol solution relative to the hydrous gel (1) was 2.5% by mass, and the amount of the aqueous hydrogen peroxide solution was 1.3% by mass (300 ppm by mass relative to the hydrous gel solid content). Next, the heat medium temperature of the drying device was adjusted to 180°C, and the hydrous gel (1) was continuously dried while being mixed with PEG 400 to obtain a particulate dried polymer (1). The temperature of the dried material just before the discharge port of the drying device was 179°C using a K-wire sheathed thermocouple. The dried polymer (1) obtained from the pipe 37 was crushed by a crusher, and then sieved with a metal sieve (JIS standard sieve) having mesh sizes of 850 μm and 150 μm to obtain a water-absorbent resin powder (1).

The 5-minute and 30-minute CRCs of the water absorbent resin powder ( 1 ) were 65 g/g and 66 g/g, respectively, the reference water absorption swelling rate (=5-minute CRC/30-minute CRC) was 0.98, and hydrogen peroxide was not detected.

[Comparative Example 1]
A water absorbent resin powder (2) was obtained in the same manner as in Example 1, except that the temperature of the heat medium in the drying device was changed to 150°C.

The 5-minute and 30-minute CRCs of water-absorbent resin powder (2) were 35 g/g and 37 g/g, respectively, and the reference water absorption and swelling rate was 0.96.

[Comparative Example 2]
In Example 1, the amount of polyethylene glycol diacrylate used for preparing the monomer composition was 0.004 mol% relative to the monomer, and the hydrogel discharged from the separation device was supplied to the drying device without adding an aqueous hydrogen peroxide solution, in the same manner as in Example 1, to obtain a comparative water absorbent resin (3).

The 5-minute and 30-minute CRCs of the comparative water-absorbent resin powder (3) were 57 g/g and 67 g/g, respectively, and the reference water absorption and swelling rate was 0.85.

[Example 2]
A surface crosslinking agent solution consisting of 0.025 parts by weight of ethylene glycol diglycidyl ether, 1.0 parts by weight of propylene glycol, and 3.0 parts by weight of pure water was sprayed with a spray nozzle to 100 parts by weight of the water absorbent resin powder (1) obtained in Example 1, and was mixed uniformly using a mixer. Next, the mixture was heat-treated for 40 minutes in a heat treatment machine with an atmospheric temperature adjusted to 195°C ± 2°C to obtain a water absorbent resin (1a). The 5-minute and 30-minute CRC of the water absorbent resin (1a) were 39 g/g and 39 g/g, respectively, and the standard water absorption swelling rate was 1.0. The AAP was 24 g/g.

[Comparative Example 3]
A comparative water absorbent resin (3b) was obtained in the same manner as in Example 2, except that the comparative water absorbent resin powder (3) obtained in Comparative Example 2 was used instead of the water absorbent resin powder (1) in Example 2. The 5-minute and 30-minute CRCs of the water absorbent resin (1a) were 36 g/g and 41 g/g, respectively, and the standard water absorption swelling rate was 0.88. The AAP was 21 g/g.

As shown above, the water-absorbent resin obtained by the manufacturing method of the embodiment had a high CRC and a high water absorption rate. In addition, the surface-crosslinked water-absorbent resin obtained by the manufacturing method of the embodiment had a high AAP.

10 mixing device,
12 Dispersion device,
14 reaction apparatus,
16 Separation device,
18 Liquid delivery pump,
20 heat exchanger,
22 Drying device.

Claims (11)

a polymerization step of obtaining a hydrogel-like crosslinked polymer by inverse suspension polymerization of a monomer composition containing an internal crosslinking agent and a monomer;
A drying step of drying the hydrogel crosslinked polymer to obtain a dried polymer,
The hydrogel crosslinked polymer to be dried contains hydrogen peroxide in an amount of 50 ppm by mass or more relative to the solid content mass of the hydrogel crosslinked polymer,
A method for producing a water-absorbent resin, wherein the hydrogel-like crosslinked polymer is heated during the drying step so that the maximum temperature reached exceeds 160°C, and/or the dried polymer is heated after the drying step so that the maximum temperature reached exceeds 160°C. The method according to claim 1, wherein the average primary particle size of the hydrogel crosslinked polymer is 1000 μm or less. 3. The method according to claim 1 , wherein hydrogen peroxide is added to the hydrogel crosslinked polymer in an amount of 50 ppm or more based on the solid content by mass of the hydrogel crosslinked polymer. The method according to any one of claims 1 to 3, wherein the internal crosslinking agent is a compound having a plurality of polymerizable unsaturated groups in the molecule. The method according to any one of claims 1 to 4, wherein the hydrogel cross-linked polymer and/or the dry polymer are further surface-cross-linked. The method according to any one of claims 1 to 5, wherein the hydrogel cross-linked polymer contains more than 100 ppm of hydrogen peroxide relative to its solid mass. The method according to any one of claims 1 to 6, wherein the amount of the internal crosslinking agent used is 0.005 mol% or more relative to the amount of the monomer used. The manufacturing method according to any one of claims 1 to 7, wherein a thermally decomposable polymerization initiator and/or a photodecomposable polymerization initiator is added to the monomer composition in the polymerization step. The method according to any one of claims 1 to 8, wherein the hydrogel cross-linked polymer is dried in a dryer set at a temperature exceeding 160°C. The manufacturing method according to any one of claims 1 to 9, wherein the absorbency (CRC) of the water-absorbent resin is 38 g/g or more. The method according to any one of claims 1 to 10, wherein the internal cross-linking agent has a polyethylene glycol structure.