JP2025026271A - Manufacturing method of recycled water absorbent resin - Google Patents

Abstract

To provide a regenerated water-absorbing resin using a water-absorbing resin which is normally wasted, especially, a regenerated water-absorbing resin suitable for a water-retention material.SOLUTION: A manufacturing method of a regenerated water-absorbing resin includes: treating with an acid substance, a raw material water-absorbing resin containing a polymer having one or more kinds of salt selected from the group consisting of sodium salt, potassium salt, lithium salt, magnesium salt, calcium salt and iron salt of a carboxyl group; adding to the raw material water-absorbing resin treated with the acid substance, an amount of a substance containing at least one kind of element selected from the group consisting of a potassium element and a nitrogen element so that a replacement ratio of the carboxyl group is more than 5 mol% and less than 40 mol%; and replacing the carboxyl group existing in the raw material water-absorbing resin with the element.SELECTED DRAWING: None

Description

The present invention relates to a method for producing recycled water-absorbent resin. In particular, the present invention relates to a method for producing recycled water-absorbent resin, particularly water-retaining material, using water-absorbent resin contained in used absorbent articles or waste from the water-absorbent resin production process.

A super absorbent polymer (SAP) is a water-swellable, water-insoluble polymer gelling agent, and is used in various absorbent articles, such as disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), hygiene materials (hygienic products) such as pet sheets, soil water retention agents for agricultural and horticultural use, industrial water-stopping agents, etc. For example, Patent Document 1 describes a water retention material containing a water absorbent resin having at least one element selected from the group consisting of phosphorus, potassium, and nitrogen elements inside the water absorbent resin, which does not constitute the polymer skeleton of the resin.

JP 2021-10303 A

However, the water-retaining material in Patent Document 1 was not sufficient as a water-retaining material because the elements were washed away when the material absorbed water or when it rained.

On the other hand, the main applications of water-absorbent resins are sanitary materials such as disposable diapers and sanitary napkins. After a short period of use (at most about one day), these sanitary materials are discarded in large quantities as used absorbent articles and incinerated. On the other hand, in recent years, from the perspective of environmental protection, attempts have been made to recycle such used absorbent articles. For example, in addition to the aforementioned water-absorbent resin, sanitary materials contain raw materials such as pulp, nonwoven fabrics, and adhesives, and material recycling has been attempted in which these raw materials are separated from used sanitary materials and reused.

Therefore, the present invention has been made in consideration of the above circumstances, and aims to provide a technology for producing recycled water-absorbent resin, particularly recycled water-absorbent resin suitable for use as a water-retaining material, using water-absorbent resin that should otherwise be discarded, such as water-absorbent resin contained in used absorbent articles or water-absorbent resin discarded during the water-absorbent resin manufacturing process.

The present inventors conducted intensive research to solve the above problems. As a result, they discovered that the above problems can be solved by making potassium and nitrogen elements exist in a certain ratio in the skeleton of absorbent resins that should be discarded, such as absorbent resins contained in used absorbent articles and absorbent resins discarded during the process of manufacturing absorbent resins, and thus completed the present invention.

That is, the above-mentioned object can be achieved by a method for producing a regenerated water-absorbent resin, which comprises treating a raw water-absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, potassium salts, lithium salts, magnesium salts, calcium salts, and iron salts of carboxyl groups with an acidic substance, and adding a substance containing at least one element selected from the group consisting of potassium element and nitrogen element to the raw water-absorbent resin treated with the acidic substance in an amount such that the substitution rate of the carboxyl groups is more than 5 mol % and less than 40 mol %, thereby substituting the carboxyl groups present in the raw water-absorbent resin with the element.

The above object can also be achieved by a water-absorbent resin composition that contains a water-absorbent resin having carboxyl groups, and in which a portion of the carboxyl groups is substituted with at least one element selected from the group consisting of potassium and nitrogen at a substitution rate of 5 mol % to 100 mol %, and a fibrous material.

According to the present invention, it is possible to produce recycled water-absorbent resin, particularly recycled water-absorbent resin suitable for use as a water-retaining material, using water-absorbent resin that would otherwise be discarded, such as water-absorbent resin contained in used absorbent articles or water-absorbent resin discarded during the water-absorbent resin manufacturing process.

One aspect of the present invention relates to a method for producing a recycled water-absorbent resin, which comprises treating a raw water-absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, potassium salts, lithium salts, magnesium salts, calcium salts, and iron salts of carboxyl groups with an acidic substance (acid treatment step), and adding a substance containing at least one element selected from the group consisting of potassium element and nitrogen element to the raw water-absorbent resin treated with the acidic substance in an amount such that the substitution rate of the carboxyl groups is more than 5 mol% and less than 40 mol% to substitute the carboxyl groups present in the raw water-absorbent resin with the element (substitution step). With this configuration, even when using a water-absorbent resin that should be discarded, such as a water-absorbent resin contained in a used absorbent article or a water-absorbent resin discarded during a water-absorbent resin manufacturing process, a recycled water-absorbent resin, particularly a recycled water-absorbent resin suitable for a water-retaining material, can be produced.

Another aspect of the present invention relates to a water-absorbent resin composition comprising a water-absorbent resin having carboxyl groups, and a portion of the carboxyl groups being substituted with at least one element selected from the group consisting of potassium and nitrogen at a substitution rate of 5 mol % to 100 mol %, and a fibrous material. With this configuration, even when a water-absorbent resin substituted with a wider range of substituents is used, the water-absorbent resin composition can be suitably used as a water-retaining material by combining it with a fibrous material.

In this specification, "a polymer having at least one salt selected from the group consisting of sodium salts, potassium salts, lithium salts, magnesium salts, calcium salts, and iron salts of a carboxyl group" is also referred to simply as "raw polymer" or "raw polymer according to the present invention." "A raw absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, potassium salts, lithium salts, magnesium salts, calcium salts, and iron salts of a carboxyl group" is also referred to simply as "raw absorbent resin" or "raw absorbent resin according to the present invention." "A material containing at least one element selected from the group consisting of potassium and nitrogen elements" is also referred to simply as "K/N-containing material" or "K/N-containing material according to the present invention."

As mentioned above, used absorbent articles such as paper diapers and sanitary napkins are discarded and incinerated in large quantities. However, in recent years, from the viewpoint of environmental protection, attempts have been made to recycle such used absorbent articles, and water-retaining materials have been considered as one of the methods. In the water-absorbent resin constituting the water-retaining material described in Patent Document 1, one or more elements of phosphorus, potassium, and nitrogen are present inside the resin so as not to form a polymer skeleton. It is known that the elements are useful as fertilizers. However, it has been found that the water-retaining material of Patent Document 1 may not be effective as a fertilizer in some cases. The inventors of the present invention have conducted intensive studies on the above phenomenon and have speculated that this is because the elements are not incorporated into the polymer skeleton, and therefore the elements flow out of the water-retaining material together with water when absorbing water or during rainfall. Therefore, in order to suppress and prevent the outflow of the elements from the water-retaining material, it is considered that it is effective to incorporate potassium and nitrogen elements in an appropriate amount into the polymer skeleton. Further intensive studies have been conducted on the incorporation of the elements. As a result, it was considered effective to convert the salt of the carboxyl group of the polymer constituting the water-absorbing resin, which should be discarded, into a carboxyl group (-COOH) by acid treatment, and then to substitute potassium (-COOK) or a nitrogen element (e.g., -COO ^- _NH4 ⁺ ) and incorporate these elements into the polymer skeleton. However, it was found that sufficient plant growth (e.g., germination rate) cannot be obtained simply by incorporating the above elements into the polymer skeleton. The present inventors further investigated the above phenomenon and speculated that the substitution rate of the carboxyl group with the potassium element or the nitrogen element has a strong influence on plant growth (e.g., germination rate). Based on the above speculation, further investigation into the substitution rate was conducted, and it was found that plant growth (e.g., germination rate) can be improved by adjusting the substitution rate to more than 5 mol% and less than 40 mol%.

Note that the above is speculation and does not limit the technical scope of the present invention.

The following describes preferred embodiments of the present invention. Note that the present invention is not limited to the following embodiments, and can be modified in various ways within the scope of the claims. In addition, the embodiments described in this specification can be combined in any manner to form other embodiments.

In this specification, the term "(meth)acrylic" includes both acrylic and methacrylic. Thus, for example, the term "(meth)acrylic acid" includes both acrylic acid and methacrylic acid. Also, "~acid (salt)" means "~acid and/or its salt."

[1] Definition of Terms [1-1] Regenerated Water Absorbent Resin The "regenerated water absorbent resin" produced by the method of the present invention is a water-swellable water-insoluble polymer gelling agent, and refers to a conventional water absorbent resin having a water absorption capacity of 10 to 1000 times, although it is not particularly limited thereto. More specifically, the water absorbent resin before absorbing the liquid to be absorbed preferably satisfies the physical property of a deionized water CRC of 5 g/g or more as "water swelling". The definition of the deionized water CRC will be described later.

The recycled water-absorbing resin includes a polymer having a carboxyl group partially neutralized with potassium or nitrogen. Specific examples of recycled water-absorbing resins include polyacrylic acid (salt)-based resins, polysulfonic acid (salt)-based resins, maleic anhydride (salt)-based resins, polyacrylamide-based resins, polyvinyl alcohol-based resins, polyethylene oxide-based resins, polyaspartic acid (salt)-based resins, polyglutamic acid (salt)-based resins, polyalginic acid (salt)-based resins, starch-based resins, cellulose-based resins, (meth)acrylate crosslinked polymers, crosslinked saponified (meth)acrylate-vinyl acetate copolymers, starch-acrylate graft polymers and crosslinked products thereof.

In this specification, the recycled water-absorbent resin is not limited to an embodiment in which the total amount (100 mass%) is the water-absorbent resin alone, but may be a water-absorbent resin composition that contains additives, etc.

[1-2] Raw Water Absorbent Resin The "raw water absorbent resin" includes a polymer (raw polymer) having at least one salt selected from the group consisting of sodium salt, potassium salt, lithium salt, magnesium salt, calcium salt, and iron salt of a carboxyl group. Preferably, the raw water absorbent resin includes a polymer having a sodium salt and/or a lithium salt of a carboxyl group. More preferably, the raw water absorbent resin includes a polymer having a sodium salt of a carboxyl group.

In one embodiment of the present invention, the raw absorbent resin has a neutralization rate of 40 mol% or more. The raw absorbent resin preferably has a neutralization rate of 50 mol% or more and 100 mol% or less, more preferably 60 mol% or more and 90 mol% or less.

In one embodiment of the present invention, the CRC (before moisture absorption) of the raw absorbent resin is 20 g/g or more, 25 g/g or more, 27 g/g or more, or 30 g/g or more. In one embodiment of the present invention, the CRC (before moisture absorption) of the raw absorbent resin is 70 g/g or less, 50 g/g or less, or 45 g/g or less. The definition of CRC is as described in [1-5-1] below.

In one embodiment of the present invention, the AAP (0.3 psi) of the raw water absorbent resin is 10 g/g or more, 15 g/g or more, 20 g/g or more, 22 g/g or more, 24 g/g or more, or 25 g/g or more. In one embodiment of the present invention, the AAP (0.3 psi) of the raw water absorbent resin is 50 g/g or less, 45 g/g or less, 40 g/g or less, 35 g/g or less, or 30 g/g or less. The definition of AAP is as described in [1-5-3] below.

[1-3] Recovered water-absorbent resin The term "recovered water-absorbent resin" refers to water-absorbent resin that should be discarded. Specifically, there are water-absorbent resins contained in used absorbent articles (for example, water-absorbent resins that have absorbed body fluids such as urine and blood in used absorbent articles) and water-absorbent resins discarded during the water-absorbent resin manufacturing process (for example, fine powder or non-standard products of water-absorbent resins generated during the water-absorbent resin manufacturing process).

Among these, from the viewpoint of environmental protection, it is preferable to use a water-absorbent resin recovered (removed and collected) from used absorbent articles. That is, in one embodiment of the present invention, the raw water-absorbent resin is a recovered water-absorbent resin recovered from used absorbent articles or waste during the water-absorbent resin manufacturing process. In one embodiment of the present invention, the raw water-absorbent resin is a recovered water-absorbent resin recovered from used absorbent articles containing a polymer having a sodium salt, potassium salt, and/or lithium salt of a carboxyl group. In one embodiment of the present invention, the raw water-absorbent resin is a recovered water-absorbent resin recovered from used absorbent articles containing a polymer having a sodium salt of a carboxyl group.

Here, "used absorbent articles" refer to sanitary materials that have been used by consumers and have absorbed bodily fluids such as urine and blood. Examples of sanitary materials include paper diapers, sanitary napkins, and adult incontinence products (incontinence pads). Products similar to sanitary materials include pet sheets and pet diapers, and in the present invention, these animal urine treatment products are also included in the sanitary materials.

The recovered water-absorbent resin is not limited to an embodiment in which the total amount (100% by mass) is the water-absorbent resin alone, but may be a water-absorbent resin composition containing fibrous materials, nonwoven fabrics, adhesives, additives, etc. In addition, when the recovered water-absorbent resin is an embodiment of a water-absorbent resin contained in a used absorbent article, the recovered water-absorbent resin may be in a state of a hydrous gel in which water such as urine water has been absorbed. Therefore, the water-absorbent resin may include a water-absorbent resin in a state of a hydrous gel.

The mass of the absorbent resin and the recovered absorbent resin is a value converted into a solid content unless otherwise specified. For example, when the mass of the absorbent resin contained in the used paper diaper is unknown, the mass is calculated using the mass of the absorbent resin contained in the unused paper diaper and the general value of the water absorption ratio of the absorbent resin in the used paper diaper, or the mass of the absorbent resin is measured by assuming that the content of the absorbent resin (solid content equivalent value) relative to the total mass of the used paper diaper is 3% by mass or more and 9% by mass or less. Although the actual content of the absorbent resin may vary greatly depending on the usage state of the used paper diaper, a person skilled in the art can adjust the decomposition conditions as appropriate while checking the actual decomposition state.

[1-4] Absorbent Articles An "absorbent article" is an article used for absorbing water. More specifically, an "absorbent article" is an absorbent article comprising an absorbent body containing a water-absorbent resin and a fibrous material, a top sheet having liquid permeability, and a back sheet having liquid impermeability. The absorbent body is suitably manufactured by blending a water-absorbent resin with a fibrous material, or by sandwiching a water-absorbent resin between fibrous materials and molding the mixture into a film, a tube, a sheet, or the like. Examples of the fibrous material include hydrophilic fibers such as ground wood pulp, cotton linters, crosslinked cellulose fibers, rayon, cotton, wool, acetate, vinylon, and the like.

"Absorbent articles" include both unused absorbent articles and used absorbent articles. Here, "used absorbent articles" particularly include used sanitary materials that have absorbed body fluids (absorbed liquids) such as urine and blood. Examples of the sanitary materials include sanitary materials (hygienic products) such as disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), and pet sheets.

[1-5] EDANA, WSP, and NWSP
"EDANA" is an abbreviation for European Disposables and Nonwovens Associations, and "WSP" is an abbreviation for World Strategic Partners. EDANA WSP is a European and American standard (almost a global standard) for measuring the physical properties of water-absorbent resins. In the present invention, unless otherwise specified, the physical properties of the water-absorbent resin are measured in accordance with the original EDANA WSP (revised in 2010/publicly known document).

"NWSP" stands for "Non-Woven Standard Procedures-Edition 2015."

[1-5-1] CRC
In this specification, the absorbency without load (CRC) is a value measured according to the following method.

(Measurement of Absorbency Capacity under No Load (CRC))
The "CRC" of a water-absorbent resin or a water-absorbent resin composition is an abbreviation for "Centrifuge Retention Capacity" and indicates the absorption capacity (unit: g/g) of a water-absorbent resin or a water-absorbent resin composition for a 0.90 mass % sodium chloride aqueous solution under no pressure for 30 minutes.

0.200 g of the water-absorbent resin or water-absorbent resin composition is uniformly placed in a bag (85 mm x 60 mm) made of nonwoven fabric (manufactured by Nangoku Pulp Kogyo Co., Ltd., product name: Heatlon Paper, model: GSP-22) and heat-sealed, and then immersed in a large excess (usually about 500 ml) of 0.90% by mass sodium chloride aqueous solution at room temperature. After 30 minutes, the bag is pulled out and drained for 3 minutes at a centrifugal force (250 G) described in Edana ABSORBENCY II 441.1-99 using a centrifuge (manufactured by Kokusan Co., Ltd., centrifuge: model H-122), and the mass W1 (g) of the bag is measured. In addition, the same operation is performed without using the water-absorbent resin or water-absorbent resin composition, and the mass W0 (g) at that time is measured. Then, from these W1 and W0, the CRC (centrifuge retention capacity) is calculated according to the following formula (1).
Formula (1):
CRC (g/g) = (W1-W0)/(mass of water-absorbent resin or water-absorbent resin composition)-1

[1-5-2] Water soluble content (16-hour value)
In this specification, the water-soluble content (16-hour value) is a value measured according to the following method. In this specification, the "water-soluble content (16-hour value)" is also simply referred to as the "water-soluble content" or "water-soluble content" or "soluble polymer content".

(Measurement of water soluble matter (16-hour value))
184.3 g of 0.90% by weight sodium chloride aqueous solution is weighed out into a 250 ml plastic container with a lid, 1.00 g of a water-absorbing resin or a water-absorbing resin composition is added to the aqueous solution, and the mixture is stirred for 16 hours to extract the water-soluble components in the resin. After stirring, the extract is filtered using a sheet of filter paper (Advantec Toyo Co., Ltd., product name: (JIS P 3801, No. 2), thickness 0.26 mm, retention particle size 5 μm), and 50.0 g of the resulting filtrate is used as the measurement liquid.

Next, the measurement solution is titrated with 0.1N NaOH aqueous solution until the pH becomes 10, and then titrated with 0.1N HCl aqueous solution until the pH becomes 2.7. The titration amounts at this time are calculated as [NaOH] ml and [HCl] ml, respectively.

The same procedure was also carried out using 184.3 g of 0.90% by mass sodium chloride aqueous solution without adding any water-absorbent resin or water-absorbent resin composition, and the blank titration amount ([bNaOH] ml, [bHCl] ml) was calculated.

When the water absorbent resin or the water absorbent resin composition is composed of a known amount of acrylic acid and a salt thereof, based on the average molecular weight of the monomer and the titration amount obtained by the above operation, the water-soluble content (unit: mass%) in the water absorbent resin or the water absorbent resin composition can be calculated by the following formula (2).
Formula (2):
Water soluble content (mass%)
= 0.1 x (average molecular weight of monomer) x 184.3 x 100 x ([HCl] - [bHCl]) / 1000 / 1.0 / 50.0

[1-5-3] AAP 0.3 (absorption capacity under pressure)
"AAP" in the water absorbent resin or water absorbent resin composition of the present invention is an abbreviation for "Absorption Against Pressure" and indicates the absorption capacity against pressure (unit: g/g) for a 0.90 mass% sodium chloride aqueous solution. In this specification, unless otherwise specified, "AAP0.3", "absorption capacity against pressure", and "AAP" are treated as synonyms.

AAP0.3 is measured according to NWSP 242.0.R2(15) except that the pressure condition is changed from 0.7 psi to 0.3 psi. Specifically, 0.9 g of the water absorbent resin or water absorbent resin composition is swollen under a pressure of 2.07 kPa (21 g/cm ² , 0.3 psi) for 1 hour using a large excess of 0.9% by mass sodium chloride aqueous solution, and then the AAP (absorbency under pressure) is measured. "NWSP" stands for "Non-Woven Standard Procedures-Edition 2015".

[1-5-4] Residual Monomer Amount 1.0 g of a water-absorbing resin or a water-absorbing resin composition is added to 200 ml of a 0.9% by mass aqueous sodium chloride solution, and stirred at 500 rpm for 1 hour using a 35 mm stirrer tip. The amount of monomer dissolved by this operation (unit: mass ppm) is measured using HPLC (high performance liquid chromatography).

[1-5-5] Deionized water CRC
In the measurement method of [1-5-1] CRC, i) the amount of the water absorbent resin or the water absorbent resin composition used is changed from 0.200 g to 0.020 g, and ii) a 0.9 mass % aqueous sodium chloride solution (physiological saline (abbreviated as saline)) is changed to deionized water, respectively, and the deionized water CRC of the water absorbent resin or the water absorbent resin composition is measured by carrying out the same operation.

[1-5-6] Molecular Weight of Water-Soluble Fraction of Water-Absorbent Resin or Water-Absorbent Resin Composition The molecular weight of the water-soluble fraction of the water-absorbent resin or water-absorbent resin composition is measured according to the following procedure.

(Preparation of measurement sample)
The sample is dissolved in the following solvent to obtain a solution with a concentration of 0.1% by mass. The obtained solution is then passed through a filter (GL Sciences: GL Chromatodisk, aqueous 25A, pore size 0.2 μm) to obtain a measurement sample. Solvent: An aqueous solution containing 60 mM sodium dihydrogen phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, and 400 ppm sodium azide (pH 6.35 to 6.38). Using this measurement sample, GPC measurement is performed under the following measurement conditions.

(GPC Measurement Conditions)
The measurement sample was subjected to GPC measurement using Viscotec TDAmax manufactured by Malvern Instruments. The measurement device is equipped with a size exclusion chromatograph, a refractive index detector, a light scattering detector, and a capillary viscometer. The measurement device and measurement conditions are as follows.
Pump/autosampler: Viscotec GPCmax (Malvern Instruments)
Guard column: OHpak SB-G (manufactured by Showa Denko K.K.)
Column: OHpak SB-806MHQ (Showa Denko K.K.), two columns connected in series. Detector: Viscotec TDAmax (Malvern Instruments).
Solvent: Aqueous solution containing 60 mM sodium dihydrogen phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, and 400 ppm sodium azide (pH 6.35 to 6.38)
Flow rate: 0.5 mL/min Injection volume: 100 μL.

The water used in GPC measurements is pure water from which impurities have been thoroughly removed. GPC measurements are also performed with a sufficient amount of solvent flowing through the measuring device and with a stable baseline for the detection value, particularly when there are no noise peaks in the light scattering detector.

The measurement device is calibrated using polyoxyethylene glycol [weight average molecular weight (Mw): 21966, molecular weight distribution (Mw/Mn): 1.0, differential refractive index (dn/dc): 0.132, solvent refractive index: 1.33] as a standard sample. The differential refractive index (dn/dc) of the measurement target is set to 0.12, and the solvent refractive index to 1.33. Data collection and analysis of refractive index, light scattering intensity, and viscosity are performed using Viscotek OmniSEC 4.7.0 (registered trademark) software.

After the measurements, the weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (Mw/Mn) are calculated using the refractive index (RI) and light scattering intensity (angle: 7°) (LALS), as well as the data obtained from the viscometer (DP).

[1-5-7] Fibrous Material The term "fibrous material" refers to a material having a shape that is at least twice, preferably at least 5 times, more preferably at least 10 times, and even more preferably at least 20 times longer than its thickness (width). Due to the characteristics of this shape, it often has a feature that the specific surface area is larger than that of a particulate material having the same mass. The fibrous material used in one embodiment of the present invention may be either hydrophilic or hydrophobic, or may be made of either a natural product or a synthetic polymer.

[2] Manufacturing method of regenerated water absorbent resin according to one embodiment of the first aspect In one embodiment of the present invention, the manufacturing method of the regenerated water absorbent resin of the present invention comprises treating a raw water absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, lithium salts, magnesium salts, calcium salts and iron salts of a carboxyl group with an acidic substance (acid treatment step); and adding a substance containing at least one element selected from the group consisting of potassium element and nitrogen element to the raw water absorbent resin treated with the acidic substance in an amount such that the substitution rate of the carboxyl group is more than 5 mol % and less than 40 mol %, thereby substituting the carboxyl group present in the raw water absorbent resin with the element (substitution step).

[2-1] Pretreatment process Before treating the raw absorbent resin with an acidic substance, other treatments such as a washing/sterilization process, a crushing process, a separation process, and a dehydration process may be performed (pretreatment process). The raw absorbent resin includes a polymer (raw polymer) having at least one salt selected from the group consisting of sodium salts, potassium salts, lithium salts, magnesium salts, calcium salts, and iron salts of a carboxyl group. The raw absorbent resin may be a recovered absorbent resin recovered from a used absorbent article or from waste during the process of producing the absorbent resin. In addition, when the raw absorbent resin is a recovered absorbent resin recovered from a used absorbent article, the raw absorbent resin may be a water absorbent resin previously separated from the used absorbent article, or may be a water absorbent resin that is still contained in the used absorbent article.

In particular, when the raw absorbent resin is a recovered absorbent resin recovered from used absorbent articles, it is preferable to carry out at least one of the washing/sterilization step, the crushing step, the separation step, and the dehydration step, and it is more preferable to carry out at least the dehydration step, and it is particularly preferable to carry out the washing/sterilization step, the crushing step, the separation step, and the dehydration step.

When the raw absorbent resin is a recovered absorbent resin recovered from used absorbent articles, it is preferable to perform a cleaning/sterilization step as a pretreatment step. The used absorbent articles can be cleaned and/or sterilized before the acid treatment step, or before the dehydration step (if a dehydration step is performed), or before the crushing step (if a crushing step is performed). The cleaning and/or sterilization may be performed by placing the used absorbent articles in a high-temperature and high-pressure environment, or by using a chemical agent to kill harmful bacteria contained in the used absorbent articles. When a chemical agent is used, the chemical agent is not particularly limited as long as the desired cleanliness can be obtained. Examples of the chemical agent include an aqueous solution in which a disinfectant such as sodium hypochlorite and chlorine dioxide is dissolved, ozone water, and electrolyzed water (acidic electrolyzed water). Here, when cleaning and/or sterilization is performed using the chemical agent, a solution (dispersion) containing the absorbent resin and the chemical agent can be obtained. In particular, when the cleaning and/or sterilization is performed using ozone water, it is preferable to perform the cleaning and/or sterilization under acidic conditions. Furthermore, in the above-mentioned cleaning and/or sterilization, it is preferable to carry out the cleaning and/or sterilization in a tank having an agitating blade and/or to circulate the solution to efficiently mix the absorbent article and the drug. After the above-mentioned cleaning and/or sterilization, the cleaned and/or sterilized absorbent article is separated from the liquid by a known method such as a filtration operation. The separated absorbent article can be subjected to an acid treatment step, a dehydration step (if a dehydration step is performed), or a crushing step (if a crushing step is performed).

When a crushing step is performed as a pretreatment step, the crushing step is a step of crushing the used absorbent articles or the recovered water-absorbent resin. The crushing step is performed by physical forces such as stirring, cutting, shredding, and crushing. The crushing step may be performed while the used absorbent articles or the recovered water-absorbent resin are immersed in water (aqueous solution). In addition, in order to facilitate separation of the water-absorbent resin in the separation step described below, the crushing step may be performed while stirring the used absorbent articles or the recovered water-absorbent resin.

When a separation process is performed as a pretreatment process, the separation process is a process of separating the crushed used absorbent articles or the recovered water-absorbent resin into each component material including the water-absorbent resin, and recovering at least the water-absorbent resin. In addition, in the separation process, each component material other than the water-absorbent resin may be recovered. The water-absorbent resin recovered in the separation process may be a water-absorbent resin in a hydrous gel state.

The method for separating the aforementioned constituent materials is not particularly limited, and examples thereof include a method for separating and recovering the constituent materials using a screen, and a method for separating and recovering the constituent materials from the crushed absorbent articles or recovered water-absorbent resin dispersed in water by utilizing the difference in specific gravity of each of the constituent materials. In addition, for example, it is also possible to crush the absorbent articles or recovered water-absorbent resin in water in the crushing step, prepare the crushed absorbent articles or recovered water-absorbent resin dispersed in water, and simultaneously carry out the separation step.

When the raw absorbent resin is a recovered absorbent resin recovered from used absorbent articles, it is preferable to carry out a dehydration step as a pretreatment step, and it is more preferable to carry out the dehydration step before carrying out the acid treatment. This allows the amount of water-soluble matter (16-hour value) and the amount of residual monomers to be significantly reduced.

In one embodiment of the present invention, the raw absorbent resin is a recovered absorbent resin recovered from a used absorbent article, which contains a polymer having a sodium salt and/or a lithium salt of a carboxyl group, and the raw absorbent resin is dehydrated (dehydration process) and then treated with the acidic substance (acid treatment process).

Here, the "dehydration process" refers to reducing the water content of the swollen water-absorbent resin. Specific examples of the method include, but are not limited to, a method of treating the water-absorbent resin with a polyvalent metal salt, a method of adding the swollen water-absorbent resin gel to a water-soluble hydrophilic organic solvent to dehydrate it, a method of shrinking the gel and releasing the absorbed liquid by changing the pH or temperature in the case of a water-absorbent resin whose swelling state changes depending on the pH or temperature change, a method of reducing the water content of the swollen water-absorbent resin gel by heating and drying, a method of contacting the water-absorbent resin gel with a gas such as heated steam, a method of reducing the water content of the water-absorbent resin gel by freeze-drying, and a method of contacting the water-absorbent resin gel with another water-absorbing agent and having the other water-absorbing agent absorb the absorbed liquid in the water-absorbent resin. Among these, from the viewpoints of being able to reduce the water content more efficiently, being simple and effective, the method of treating the water-absorbent resin with a polyvalent metal salt, the method of adding to a hydrophilic organic solvent for dehydration, and the method of changing the swelling state by changing the pH or temperature are preferred, and the method of treating the water-absorbent resin with a polyvalent metal salt and the method of adding to a hydrophilic organic solvent for dehydration are more preferred.

When treating the raw absorbent resin with a polyvalent metal salt, this can be done by adding the polyvalent metal salt to the absorbent resin. For example, a method can be used in which the raw absorbent resin is brought into contact with a solution containing a polyvalent metal salt. More specifically, a method can be used in which the raw absorbent resin is immersed in a solution containing a polyvalent metal salt. In this case, the contact (immersion) may be performed under stirring.

As the polyvalent metal salt, an alkaline earth metal salt, a transition metal salt, etc. can be used. As the alkaline earth metal salt, a water-soluble salt such as beryllium, magnesium, calcium, strontium, barium, etc. can be used. As the preferred alkaline earth metal salt, calcium chloride, calcium nitrate, magnesium chloride, magnesium nitrate, etc. can be mentioned, with calcium chloride being more preferred. As the transition metal salt, a water-soluble salt such as iron, cobalt, nickel, copper, etc. can be mentioned. As the transition metal salt, any inorganic acid salt, organic acid salt, complex, etc. can be used as long as it is incorporated into the raw water absorbent resin. Among them, from the viewpoint of cost, availability, etc., the transition metal salt is preferably an inorganic acid salt or an organic acid salt. As the inorganic acid salt, for example, iron salts such as iron chloride, iron sulfate, iron phosphate, iron nitrate, etc.; cobalt salts such as cobalt chloride, cobalt sulfate, cobalt phosphate, cobalt nitrate, etc.; nickel salts such as nickel chloride, nickel sulfate, etc.; copper salts such as copper chloride, copper sulfate, etc. can be mentioned. Examples of the organic acid salts include iron lactate, cobalt acetate, cobalt stearate, nickel acetate, and copper acetate. The polyvalent metal salts may be used alone or in combination of two or more. Of these, the polyvalent metal salt is preferably an alkaline earth metal salt, more preferably at least one selected from the group consisting of calcium chloride, calcium nitrate, magnesium chloride, and magnesium nitrate, and particularly preferably calcium chloride.

When the raw water absorbent resin is contacted with a solution containing a polyvalent metal salt, the solvent that can be used is not particularly limited as long as it can dissolve the polyvalent metal salt, and can be appropriately selected according to the type of polyvalent metal salt.Specific examples include water, lower alcohols such as methanol and ethanol, and ketones such as acetone and methyl ethyl ketone.In addition, the concentration of the polyvalent metal salt in the solution is not particularly limited.The concentration of the polyvalent metal salt in the solution is, for example, 0.5% by mass or more and 10.0% by mass or less, and preferably 1.0% by mass or more and 5.0% by mass or less.

The amount of polyvalent metal salt used may be determined based on the state of shrinkage of the absorbent resin after the dehydration operation, i.e., after treatment with the polyvalent metal salt. For example, the amount of polyvalent metal salt mixed (added) is preferably 50 parts by mass or more and 100 parts by mass or less, and more preferably 60 parts by mass or more and 90 parts by mass or less, per 100 parts by mass of the raw absorbent resin (solid content). With such an amount, the raw absorbent resin can be sufficiently dehydrated.

The conditions for the dehydration treatment using the polyvalent metal salt are not particularly limited as long as the conditions are sufficient for the polyvalent metal ions to be incorporated into the raw water absorbent resin. The dehydration treatment time is preferably 5 minutes or more and 3 hours or less, more preferably 5 minutes or more and 2 hours or less, and even more preferably 10 minutes or more and 1 hour or less. The dehydration treatment temperature is preferably 5°C or more and 80°C or less, more preferably 10°C or more and 70°C or less, and even more preferably 15°C or more and 60°C or less. Under such conditions, the raw water absorbent resin can be sufficiently dehydrated.

The dehydration process may be carried out once or repeatedly until the desired shrinkage state is achieved.

[2-2] Acid Treatment Step If necessary, after the pretreatment in the above [2-1], the raw water absorbent resin is treated with an acidic substance (acid treatment step). By this step, the salt of the carboxyl group (-COOX or -COO-Y-OOC-; X is sodium, potassium or lithium, and Y is an alkaline earth metal or a transition metal (e.g., magnesium, calcium, iron, etc.)) in the raw water absorbent resin is converted to a carboxyl group (-COOH).

The acid treatment method is not particularly limited as long as it can bring the raw absorbent resin into contact with an acidic substance. For example, methods that can be used include mixing the raw absorbent resin with an acidic substance, adding an acidic substance to the raw absorbent resin, adding the raw absorbent resin to an acidic substance, and immersing the raw absorbent resin in a solution containing an acidic substance.

The acidic substance is not particularly limited, and for example, inorganic acids and/or organic acids can be used. Examples of inorganic acids include sulfuric acid, hydrochloric acid, and nitric acid. Examples of the organic acid include organic substances having an acid group, such as a carboxyl group and a sulfo group. The acidic substance may be one selected from the inorganic acids and organic acids described above, or a mixture of two or more selected acidic substances. The acidic substance may be a combination of an inorganic acid and an organic acid. Specific examples of the organic acid include citric acid, tartaric acid, malic acid, succinic acid, oxalic acid, gluconic acid, pentanoic acid, butanoic acid, propionic acid, glycolic acid, acetic acid, formic acid, and sulfonic acid. Examples of the sulfonic acid include methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid. Of these, hydrochloric acid, citric acid, and sulfuric acid are preferred, and hydrochloric acid and citric acid are more preferred. The acid treatment process can also serve as a dehydration process. Therefore, when performing the acid treatment process, the dehydration process can be omitted.

In addition, when the acidic substance is used in the form of a solution, the solvent that can be used may be any solvent capable of dissolving the acidic substance, and examples of such solvents include water, lower alcohols such as methanol and ethanol, and ketones such as acetone and methyl ethyl ketone. The solvents may be used alone or in combination of two or more. The concentration of the acidic substance in the solution is not particularly limited, but is, for example, 0.5% by mass or more and 20% by mass or less, and preferably 1% by mass or more and 10% by mass or less.

The amount of the acidic substance to be added may be an amount that can sufficiently convert the salt of the carboxyl group in the raw absorbent resin to a carboxyl group (-COOH). For this reason, it is preferable that the amount of the acidic substance to be added is in excess of the salt of the carboxyl group in the raw absorbent resin. The amount of the acidic substance to be added is, for example, more than 1.0 mol and not more than 7.0 mol, preferably 1.1 mol to 5.0 mol, more preferably 1.2 mol to 4.0 mol, and even more preferably 1.2 mol to 3.5 mol, as the acid group of the acidic substance, per 1 mol of the salt of the carboxyl group in the raw absorbent resin. Note that, when the number (molar number) of the salt of the carboxyl group (-COOX or -COO-Y-OOC-; X is sodium or lithium, and Y is an alkaline earth metal or a transition metal (e.g., magnesium, calcium, iron, etc.)) in the raw absorbent resin is unknown, an amount exceeding the above-mentioned amount may be added. Alternatively, for example, the amount of the acidic substance to be added can be determined by neutralization titration.

The acid treatment conditions are appropriately selected in consideration of the conditions under which the salts of the carboxyl groups in the raw water absorbent resin can be sufficiently converted to carboxyl groups (-COOH). For example, the acid treatment temperature is preferably 5°C or higher and 100°C or lower, more preferably 10°C or higher and 90°C or lower, and even more preferably 15°C or higher and 80°C or lower. The acid treatment time is preferably 1 minute or higher and 2 hours or lower, more preferably 5 minutes or higher and 60 minutes or lower, and even more preferably 10 minutes or higher and 30 minutes or lower. When the acid treatment is performed by mixing the raw water absorbent resin with a solution containing an acidic substance, the acid treatment may be performed under stirring.

The acid treatment may be carried out once or repeatedly until all the salts of the carboxyl groups are converted to carboxyl groups.

The raw water absorbent resin treated with the acidic substance may remain in the dispersion liquid after the acid treatment (i.e., in the state of the acid treatment liquid). Alternatively, the raw water absorbent resin treated with the acidic substance may be in a state in which it has been removed from the dispersion liquid after the acid treatment. Here, the operation for removing the raw water absorbent resin from the acid treatment liquid is not particularly limited, and for example, a known method such as a filtration operation can be used. In one example, the dispersion liquid after the acid treatment is filtered through a wire mesh of 50 mesh or more and 200 mesh or less (e.g., 100 mesh), and is left as appropriate if necessary.

[2-3] Washing step The acid-treated raw water absorbent resin may be washed. In one embodiment of the present invention, the acid-treated raw water absorbent resin is washed with water to obtain a raw water absorbent resin washed with water. This step can remove impurities (e.g., acidic substances, polyvalent metal ions) present on the surface, inside, or gaps between particles of the acid-treated raw water absorbent resin, or reduce the content of such impurities, thereby improving the water absorption performance (e.g., deionized water CRC) of the final product, the regenerated water absorbent resin.

When performing the washing step, the washing operation is not particularly limited, and a known method can be used. For example, a method of immersing the acid-treated raw water absorbent resin in a washing liquid and stirring it, a method of spraying or showering the washing liquid onto the acid-treated raw water absorbent resin, or the like can be mentioned. The washing operation may be repeated. By repeating the washing operation, the washing effect is enhanced, and a larger amount of impurities (e.g., acidic substances, polyvalent metal ions) can be removed or the content thereof can be reduced.

The washing liquid used in the washing step may be water, saline, or seawater, and is preferably water. The type of water (washing water) that can be used may be deionized water, tap water, distilled water, or the like.

The cleaning conditions when performing the cleaning process are not particularly limited. For example, the cleaning temperature (liquid temperature of the cleaning liquid) is from room temperature to the boiling point of the cleaning liquid. Alternatively, high-temperature steam may be used. The cleaning temperature is preferably from 5°C to 100°C, more preferably from 10°C to 90°C, and even more preferably from 15°C to 80°C. When high-temperature cleaning water or high-temperature steam is used in the cleaning operation, not only the cleaning effect but also the sterilization effect can be expected. The cleaning time is, for example, from 1 minute to 60 minutes, and preferably from 5 minutes to 30 minutes.

The raw water absorbent resin after the washing treatment may remain in the washing liquid. Alternatively, the raw water absorbent resin after the washing treatment may be in a state of being removed from the washing liquid after the washing treatment is completed. Here, the operation for removing the raw water absorbent resin from the washing liquid is not particularly limited, and for example, a known method such as a filtration operation can be used. In one example, the dispersion after the washing treatment is completed is filtered through a wire mesh of 50 mesh or more and 200 mesh or less (for example, 100 mesh), and is left as appropriate if necessary.

[2-4] Substitution step (neutralization step)
In this step, a substance (K/N-containing substance) containing at least one element selected from the group consisting of potassium and nitrogen elements is added to the raw water absorbent resin treated in the acid treatment step [2-2] or the washing step [2-3] in such an amount that the substitution rate of the carboxyl groups is more than 5 mol % and less than 40 mol %. As a result, hydrogen atoms of carboxyl groups (-COOH) in a ratio of more than 5 mol % and less than 40 mol % of all carboxyl groups present in the raw water absorbent resin (raw polymer) after the acid treatment are substituted (neutralized) with potassium element (-COOK) or nitrogen element. With regard to the nitrogen element, it is preferable that the carboxyl group (-COOH) is converted (substituted) with an ammonium ion (-COO ^- NH ₄ ⁺ ) from the viewpoints that the carboxyl group is easily substituted and that the regenerated water absorbent resin is easily utilized by plants when used as a water retention material for plant growth.

Here, the amount of K/N-containing substance added is such that the substitution rate of the carboxyl group is more than 5 mol% and less than 40 mol%. When the amount of K/N-containing substance added is such that the substitution rate is 5 mol% or less, the resulting recycled water-absorbent resin has poor water absorption ability. Therefore, the resulting recycled water-absorbent resin is not suitable as a water-retaining material or water-absorbent article. In addition, the recycled water-absorbent resin does not have sufficient amounts of potassium and nitrogen elements. Therefore, the potassium source or nitrogen source is not sufficiently supplied to the plant, so that even if the recycled water-absorbent resin is used alone, the plant growth (particularly the germination rate) is not fully exhibited. On the other hand, when the amount of K/N-containing substance added is such that the substitution rate is 40 mol% or more, the plant growth (particularly the germination rate) is reduced, although the reason is unclear. Therefore, the resulting recycled water-absorbent resin is not suitable as a water-retaining material for plant growth. In particular, from the viewpoint of usefulness as a water-retaining material for plant growth (particularly further improvement of germination rate), the substitution rate of carboxyl groups is preferably 10 mol% or more and 39 mol% or less, more preferably 15 mol% or more and 35 mol% or less, and particularly preferably 15 mol% or more and 30 mol% or less. With such a substitution rate, plant growth (particularly germination rate) can be further improved while maintaining a moderate water-retaining capacity (e.g., deionized water CRC).

The substitution (neutralization) is carried out by contacting the raw absorbent resin treated in the acid treatment step [2-2] or the washing step [2-3] with a predetermined amount of K/N-containing substance. For example, the raw absorbent resin and the K/N-containing substance can be mixed, the K/N-containing substance or a solution containing the K/N-containing substance is added to the raw absorbent resin, and the raw absorbent resin is immersed in a solution containing the K/N-containing substance. Among these, the method of adding the K/N-containing substance or a solution containing the K/N-containing substance to the raw absorbent resin is preferred, and the method of adding and mixing the K/N-containing substance or a solution containing the K/N-containing substance to the raw absorbent resin is more preferred. In the method of adding the K/N-containing substance or a solution containing the K/N-containing substance to the raw absorbent resin, the addition method may be carried out stepwise or continuously. In addition, the addition method preferably includes addition in one go, addition by dropwise, and addition in portions.

The amount of K/N-containing substance added is an amount that results in the substitution rate of the carboxyl groups. If the number of carboxyl groups (-COOH) in the raw absorbent resin after the treatment in the acid treatment step [2-2] or the washing step [2-3] is unknown, the amount of K/N-containing substance added can be determined, for example, by neutralization titration of the absorbent resin.

Examples of K/N-containing substances include potassium-containing substances such as potassium hydroxide, potassium sulfate, potassium chloride, potassium carbonate, potassium hydrogen carbonate, potassium nitrate, and potassium chloride; and nitrogen-containing substances such as ammonium carbonate, ammonium chloride, ammonium nitrate, ammonium sulfate, and diammonium hydrogen phosphate. Of these, potassium hydroxide, ammonium carbonate, and potassium carbonate are preferred, with potassium hydroxide and potassium carbonate being particularly preferred, from the viewpoints that the carboxyl groups are easily replaced and that the recycled water-absorbent resin is easily utilized by plants when used as a water-retaining material for plant growth.

When the K/N-containing substance is used in the form of a solution, the solvent that can be used may be any solvent capable of dissolving the K/N-containing substance, such as water, lower alcohols such as methanol and ethanol, and ketones such as acetone and methyl ethyl ketone. The solvents may be used alone or in combination of two or more. The concentration of the K/N-containing substance in the solution is not particularly limited, but is, for example, 5% by mass to 60% by mass, and preferably 10% by mass to 50% by mass. When the K/N-containing substance is added directly to the raw water absorbent resin, the form of the K/N-containing substance is not particularly limited, and may be powdery, granular, or the like.

The replacement treatment conditions are appropriately selected in consideration of the conditions under which the carboxyl groups in the raw absorbent resin can be replaced at a predetermined replacement rate. For example, the replacement treatment temperature is preferably 5°C or more and 100°C or less, more preferably 10°C or more and 90°C or less, and even more preferably 15°C or more and 80°C or less. The replacement treatment time is preferably 5 minutes or more and 2 hours or less, more preferably 10 minutes or more and 150 minutes or less, and even more preferably 20 minutes or more and 60 minutes or less. The raw absorbent resin and the K/N-containing substance may be mixed under stirring.

The replacement process may be performed once or repeatedly until a desired replacement rate is achieved.

[2-5] Other Steps In addition to the steps described above, the production method according to the present invention may further include a drying step, a pulverizing step, a classification step, a surface cross-linking step, a sizing step, and the like, as necessary.

[2-5-1] Drying step This step is a step of obtaining a dried polymer by drying the regenerated water absorbent resin to a desired resin solid content. The resin solid content is determined from the loss on drying (the mass change when 1 g of the water absorbent resin is heated at 180 ° C. for 3 hours), and is preferably 80% by mass or more, more preferably 85% by mass to 99% by mass, even more preferably 90% by mass to 98% by mass, and particularly preferably 92% by mass to 97% by mass. The drying method is not particularly limited, but examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, and high humidity drying using high-temperature water vapor. The drying temperature (the temperature of the hot air in the case of hot air drying) is, for example, about 20 ° C. to 250 ° C. In addition, the drying conditions other than the drying temperature, such as the wind speed of the hot air and the drying time, may be appropriately set according to the moisture content and total mass of the water absorbent resin to be dried and the target 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 as drying conditions.

[2-5-2] Pulverization Step This step is a step of pulverizing (and classifying, if necessary) the regenerated water-absorbent resin or the dried product obtained in the above drying step.

The pulverizer (disintegrator) is not particularly limited, and examples include vibration mills, roll granulators, knuckle-type pulverizers, roll mills, high-speed rotary pulverizers (pin mills, hammer mills, screw mills), cylindrical mixers, etc. Roll granulators (Matsubo Co., Ltd.), granulators (Kurimoto Iron Works Co., Ltd.), and Randel mills (Tokuju Co., Ltd.), etc., can be used because they cause less damage to the dried material.

[2-5-3] Classification step A classification step may be performed after the pulverization. In this case, the classification step is a step of removing coarse particles and fine powder from the pulverized product obtained above using a classifier. As the classifier, a vibrating or rocking type sieve classifier using a sieve mesh is used.

[2-5-4] Surface cross-linking step This step is a step of providing a part with a higher cross-linking density on the surface layer (particularly, a part several tens of μm from the surface of the water-absorbent resin precursor) of the regenerated water-absorbent resin or the water-absorbent resin obtained through any of the above steps (hereinafter, also simply referred to as "water-absorbent resin precursor"), and may be composed of a mixing step, a heat treatment step, and a cooling step (optional). In the surface cross-linking step, a surface-cross-linked water-absorbent resin is obtained by radical cross-linking, surface polymerization, cross-linking reaction with a surface cross-linking agent, etc. on the surface of the water-absorbent resin precursor.

(Surface Cross-Linking Agent)
The surface crosslinking agent used in one embodiment of the present invention is not particularly limited, but includes organic or inorganic surface crosslinking agents. Among them, from the viewpoint of the physical properties of the water-absorbent resin, the handling property of the surface crosslinking agent, etc., organic surface crosslinking agents that react with carboxyl groups are preferred. For example, one or more surface crosslinking agents disclosed in U.S. Patent No. 7,183,456 can be mentioned. More specifically, polyhydric alcohol compounds, epoxy compounds, haloepoxy compounds, polyamine compounds or their condensates with haloepoxy compounds, oxazoline compounds, oxazolidinone compounds, polyvalent metal compounds, alkylene carbonate compounds, cyclic urea compounds, etc. can be mentioned.

Specifically, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, tetramethylolpropane ... Polyhydric alcohol compounds such as taerythritol and sorbitol; epoxy compounds such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol polyglycidyl ether, glycidol, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, trimethylolpropane polyglycidyl ether, neopentyl glycol diglycidyl ether, and 1,6-hexanediol diglycidyl ether; polyamine compounds such as pentanetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, and inorganic or organic salts thereof; polyisocyanate compounds such as 2,4-tolylene diisocyanate, hexamethylene diisocyanate; aziridine compounds such as polyaziridine; polyoxazoline compounds such as 1,2-ethylenebisoxazoline, bisoxazoline, and polyoxazoline; carbonic acid derivatives such as urea, thiourea, guanidine, dicyandiamide, and 2-oxazolidinone; 1,3-dioxolane-2-one (ethylene carbonate), 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one alkylene carbonate compounds such as 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, and 4,6-dimethyl-1,3-dioxan-2-one; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin, and polyvalent amine adducts thereof; oxetane compounds; silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; polyvalent metal compounds such as hydroxides, chlorides, sulfates, nitrates, or carbonates of zinc, calcium, magnesium, aluminum, iron, and zirconium; and the like. Two or more of these may be used in combination. Among the above surface cross-linking agents, one or more selected from polyhydric alcohol compounds, polyvalent metal compounds, epoxy compounds, oxazoline compounds, and alkylene carbonate compounds are preferred.

The amount of the surface cross-linking agent used (the total amount used when multiple agents are used) is preferably 0.01 parts by mass or more and 10 parts by mass or less, more preferably 1 part by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin precursor. The surface cross-linking agent is preferably added as an aqueous solution, and in this case, the amount of water used is preferably 0.5 parts by mass or more and 20 parts by mass or less, more preferably 1 part by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the water-absorbent resin precursor. 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, relative to 100 parts by mass of the water-absorbent resin precursor.

(Mixing process)
This step is a step of mixing the water-absorbent resin precursor with the surface crosslinking agent. The method of mixing the surface crosslinking agent is not particularly limited, but for example, a method of preparing a surface crosslinking agent solution in advance and mixing the liquid with the water-absorbent resin precursor is preferred. In this case, the surface crosslinking agent solution is mixed with the water-absorbent resin precursor by more preferably spraying or dropping, and more preferably 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.

(Heat treatment process)
This step is a step in which heat is applied to the mixture discharged from the mixing step to cause a crosslinking reaction on the surface of the water-absorbent resin precursor.

The device for carrying out the crosslinking reaction is not particularly limited, but preferably includes a paddle dryer. The reaction temperature in the crosslinking reaction is appropriately set according to the type of surface crosslinking agent used, and is preferably 50°C or more and 300°C or less, more preferably 100°C or more and 200°C or less. The reaction time in the crosslinking reaction is appropriately set according to the moisture content of the object of the heat treatment process, the type of surface crosslinking agent, the thermal efficiency of the heating device, etc., and it is preferable to heat until the moisture content becomes 10 mass% or less. For example, the reaction time in the crosslinking reaction is, for example, 10 minutes or more and 120 minutes or less, and preferably 30 minutes or more and 90 minutes or less.

(Cooling process)
This step is an optional step that is installed 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 obtained in the heat treatment step is forcibly cooled, as necessary, in the cooling step, preferably to 40°C or more and 80°C or less, more preferably to 50°C or more and 70°C or less.

After the surface cross-linking step, a sizing step may be further carried out. In addition to the above, the method according to the present invention may include at least one step selected from the steps of adding various additives, removing fine powder, recycling fine powder (recovering fine powder), and filling. Furthermore, various known steps may be included depending on the purpose. The fine powder removed in the removing fine powder step may be used as recovered water-absorbent resin.

[2-5-5] Particle Size Adjustment Step This step is a step for adjusting the particle size of the surface cross-linked water absorbent resin. By this particle size adjustment step, a water absorbent resin in which the particle size or particle size distribution is more actively controlled can be obtained.

The sizing process preferably includes a crushing step and/or a classification step. The crushing step is a step in which the particulate matter that has been loosely aggregated through the surface cross-linking process is broken down by a crusher to adjust the particle size. The classification step is a step in which coarse particles and fine powder are removed from the surface cross-linked particulate matter or the crushed matter thereof using a classifier.

The crusher is not particularly limited, and examples thereof include a vibration mill, a roll granulator, a knuckle type crusher, a roll mill, a high-speed rotary crusher (pin mill, hammer mill, screw mill), a cylindrical mixer, etc. A crusher that causes little damage to the surface-crosslinked water-absorbent resin is preferable, and specific examples thereof include a roll granulator (Matsubo Co., Ltd.), a granulator (Kurimoto Iron Works Co., Ltd.), and a Randel Mill (Tokuju Co., Ltd.). As a classifier, a vibrating or rocking sieve classifier using a sieve mesh is used. It is possible to omit the crushing step in this process.

[3] Physical Properties of Regenerated Water Absorbent Resin In the regenerated water absorbent resin obtained by the production method according to the present invention, more than 5% but less than 40% of the total number of carboxyl groups (the total number of -COOH and -COOZ (Z = K or N-containing group)) are substituted with potassium element (-COOK) or nitrogen element (preferably ammonium ion, -COO ^- NH ₄ ⁺ ).

When the recycled water-absorbent resin obtained by the manufacturing method according to the present invention is used as a water-retaining material, it is desirable that it satisfy at least one of the following conditions [3-1], [3-2], and [3-3], more preferably at least two of them, and most preferably all of them.

[3-1] Water soluble matter (16-hour value)
The regenerated water absorbent resin produced by the method according to the present invention preferably has a small water-soluble content (16-hour value). In one embodiment, the water-soluble content (16-hour value) of the regenerated water absorbent resin produced by the method according to the present invention or the water retention agent containing the water absorbent resin is 20% by mass or less. In one embodiment, the water-soluble content (16-hour value) of the regenerated water absorbent resin or the water retention agent containing the water absorbent resin is less than 20.0% by mass, 19.0% by mass or less, 18.0% by mass or less, less than 17.0% by mass, 16.0% by mass or less, 15.0% by mass or less, 14.0% by mass or less, 13.0% by mass or less, or 12.0% by mass or less (lower limit: 0% by mass). By setting the water-soluble content at such a level, when the regenerated water absorbent resin is used in a sanitary material, the toughness of the polymer structure can be improved, and the process damage resistance and water absorption properties (especially AAP 0.3) can be improved. In addition, when used as a water retention material for plant growth, it is possible to further suppress and prevent changes in water absorption properties caused by external environments such as soil.

[3-2] Residual Monomer Amount The regenerated water absorbent resin produced by the method according to the present invention has a small amount of residual monomer. In one embodiment, the regenerated water absorbent resin produced by the method according to the present invention or the water retention agent containing the water absorbent resin has a residual monomer amount of 100 mass ppm or less. In one embodiment, the regenerated water absorbent resin or the water retention agent containing the water absorbent resin has a residual monomer amount of 75 mass ppm or less, 50 mass ppm or less, 40 mass ppm or less, 35 mass ppm or less, 30 mass ppm or less, 20 mass ppm or less, less than 15 mass ppm, or 10 mass ppm or less (lower limit: 0 mass ppm). By setting the residual monomer amount in this manner, low skin irritation can be expected when the regenerated water absorbent resin is used in sanitary materials. In addition, when used as a water retention material for plant growth, a water absorbent resin composition with even greater safety can be provided.

[3-3] Deionized water CRC (absorption capacity without pressure)
The absorbency without pressure (deionized water CRC) of the regenerated water absorbent resin produced by the method according to the present invention is usually 5 g/g or more, preferably 50 g/g or more, more preferably 60 g/g or more, even more preferably more than 70 g/g, and particularly preferably 80 g/g or more. There is no particular limit to the upper limit, and a higher deionized water CRC is preferable, but from the viewpoint of balance with other physical properties, it is preferably 400 g/g or less, more preferably 350 g/g or less, and even more preferably 320 g/g or less. By setting the absorbency without pressure (deionized water CRC) at such a level, sufficient water retention can be ensured when used as a water retention material for plant growth.

[4] A method for producing a water-absorbent resin according to an embodiment of the second aspect The method produces a recycled water-absorbent resin. The recycled water-absorbent resin thus obtained can be used as a raw material for a new water-absorbent resin.

That is, in one embodiment of the second aspect of the present invention, a method is provided in which a recycled water-absorbent resin is produced using the method according to the present invention, and the recycled water-absorbent resin is used as part of a raw material to produce a water-absorbent resin.

In one aspect of the second embodiment, the method for producing a water-absorbent resin is a method for producing a regenerated water-absorbent resin in which hydrogen atoms of carboxyl groups present in a raw water-absorbent resin (raw polymer skeleton) are replaced with at least one of potassium element and nitrogen element (nitrogen-containing group, preferably ammonium ion) according to the method according to one embodiment of the first aspect, and then using the regenerated water-absorbent resin as a part of the raw material to newly produce a water-absorbent resin. The "new production of a water-absorbent resin" means the production of a conventional water-absorbent resin using the monomers constituting the water-absorbent resin as raw materials. The process for producing a water-absorbent resin is any one of a process for preparing an aqueous monomer solution, a polymerization process, a hydrogel crushing process, a drying process, and a surface crosslinking process. These processes are not particularly limited and are known. For example, see International Publication No. 2009/123197, U.S. Patent Application Publication No. 2008/0194863, U.S. Patent Application Publication No. 2005/0215734, U.S. Patent No. 7,265,190, U.S. Patent No. 4,893,999, U.S. Patent No. 6,241,928, U.S. Patent Application Publication No. 2005/215734, and the like.
No. 6,987,151, U.S. Pat. No. 6,710,141, WO 2011/126079, WO 2006/100300, WO 2011/025012, WO 2011/025013, WO 2011/111657, etc., or the methods described above can be used with appropriate modifications.

[5] Manufacturing method of water-retaining material according to one embodiment of the third aspect The regenerated water-absorbent resin manufactured by the method has a moderate water-retaining capacity (e.g., deionized water CRC). In addition, the regenerated water-absorbent resin manufactured by the method has a moderate potassium source or nitrogen source inside the polymer skeleton. Therefore, the regenerated water-absorbent resin has excellent plant growth (particularly germination rate). In addition, the regenerated water-absorbent resin manufactured by the method retains the potassium source and nitrogen source inside the polymer skeleton without flowing out even after water absorption or rainfall. Therefore, the regenerated water-absorbent resin obtained in this manner is suitable for use as a water-retaining material (particularly a water-retaining material for plant growth).

That is, in one aspect of the third embodiment, a method for producing a water-retaining material (particularly a water-retaining material for plant cultivation) is provided, which comprises producing a recycled water-absorbent resin using the method according to the present invention.

As described above, the recycled water-absorbent resin produced by the method has a suitable water-retaining capacity (e.g., deionized water CRC) and fertilizer source (potassium source, nitrogen source). Therefore, the recycled water-absorbent resin produced by the method according to the present invention can be used as it is as a water-retaining material (particularly a water-retaining material for plant cultivation). Alternatively, the recycled water-absorbent resin produced by the method according to the present invention can be used as a water-retaining material in combination with other additives (e.g., soil, leaf mold, phosphorus source).

That is, in another embodiment of the third aspect, a water retention material (particularly a water retention material for plant cultivation) is provided that contains or is composed of the recycled water absorbent resin produced using the method according to the present invention.

[6] Recycled water-absorbent resin according to one embodiment of the fourth aspect In one embodiment of the fourth aspect, there is provided a water-absorbent resin recycled from a used absorbent article or a waste from a water-absorbent resin production process, the water-absorbent resin comprising a polymer having a carboxyl group and in which a part of the carboxyl group is substituted with at least one element selected from the group consisting of potassium element and nitrogen element.

In this embodiment, the polymer is preferably substituted with potassium at a substitution rate of more than 5 mol% and less than 40 mol% (preferably 10 mol% to 39 mol%, more preferably 15 mol% to 35 mol%, particularly preferably 15 mol% to 30 mol%). The polymer is preferably substituted with nitrogen at a substitution rate of more than 5 mol% and less than 40 mol%. The polymer preferably has an ammonium salt (-COONH ₄ ) of the carboxyl group at a substitution rate of more than 5 mol% and less than 40 mol%. A polymer having such a substitution rate can further improve plant growth (particularly germination rate) while ensuring a moderate water retention capacity (e.g., deionized water CRC). Therefore, the recycled water-absorbent resin can be suitably used as a water retention material for plant growth.

[7] Water Retention Material According to an Embodiment of the Fifth Aspect The recycled water absorbent resin according to an embodiment of the fourth aspect can be suitably used as a water retention material for plant growth.

In one embodiment of the fifth aspect, a water retention material is provided that includes the recycled water absorbent resin according to the present invention or is composed of the recycled water absorbent resin.

The water-retaining material according to this embodiment gels when water is supplied (irrigated), and then retains water inside. Therefore, the water-retaining material according to this embodiment can function as a water supply source for the plant body during the growth of the plant body. The water-retaining material according to this embodiment supports the nutrient sources of the plant body, such as a potassium source and a nitrogen source, and slowly releases the nutrient sources. Therefore, the water-retaining material according to this embodiment has excellent germination and rooting rates when sowing, and continues to slowly release the nutrient sources for a long period of time, so that it can continue to promote the growth and development of plants. In addition, the water-retaining material according to this embodiment can be directly cultivated with the swelling gel. The water-retaining material according to this embodiment can also be expected to improve soil quality and promote greening by applying it to soil or sandy soil. More specifically, the water-retaining material according to this embodiment can be used as a water-retaining carrier for rice field cultivation, open-field cultivation, water-saving cultivation, greening construction, etc., and can be a water-retaining carrier for plants that has a high water absorption rate, is easy to handle, and does not inhibit plant growth. The water-retaining material according to this embodiment can be used as an alternative soil, and can ensure excellent germination and rooting rates without the use of other plant-growing carriers. In addition, because the plant can be rooted in the water-retaining material, the plant can be supported or held. The water-retaining material according to this embodiment can be in the form of granules, and in this case, it shows excellent powder fluidity. For this reason, it can be a water-retaining material for plant cultivation (e.g., hydroculture) that is excellent in workability and handling.

Therefore, the present invention provides a water-retaining material for plant growth that has extremely excellent irrigation efficiency and is used for soil improvement and greening.

Depending on the application, the water-retaining material according to this embodiment may further contain other additives such as deodorants, antibacterial agents, pest and animal repellents, agricultural chemicals (insecticides, fungicides, herbicides, etc.), plant stimulants, plant life-extending agents, plant hormones, minerals, pigments, dyes, thickeners, adhesives, salts, pH adjusters, kaolin soil, clay, soil, etc. The content of the other additives in this case may be appropriately selected depending on the application, but may be, for example, more than 0% by mass and not more than 30% by mass, preferably 3% by mass or more and not more than 10% by mass, relative to the water-retaining material.

The water-retentive material according to this embodiment may be used, for example, by being spread on fields or may be gelled and used as a water-retentive material in seedbeds, etc. Alternatively, the water-retentive material according to this embodiment may be mixed into artificial ground soil, etc., for use in dry areas such as deserts and sand dunes, median strips and side strips of roads, for street trees, for indoor ornamental purposes, and for rooftop greening of buildings, etc. Alternatively, the water-retentive material according to this embodiment may be incorporated into formed seedlings, etc., and transplanted together with plants. There are no limitations on the type or condition of the target seeds or plants, and it may be used, for example, for seed germination, seedling raising, growth of leafy vegetables, fruit vegetables, root vegetables, flowers, etc., and replanting of mature trees, etc.

[8] Water absorbent resin composition according to one embodiment of the sixth aspect In one embodiment of the present invention, the water absorbent resin composition of the present invention includes a water absorbent resin having a carboxyl group, and a part of the carboxyl group being substituted with at least one element selected from the group consisting of potassium element and nitrogen element at a substitution rate of 5 mol % or more and 100 mol % or less, and a fibrous material.

In the above [2], from the viewpoint of plant growth (particularly germination rate), it was necessary to set the carboxyl group substitution rate to less than 40 mol%. On the other hand, from the viewpoint of higher water absorption capacity, a higher carboxyl group substitution rate is preferable. For this reason, the inventors have conducted extensive research into measures for improving plant growth (particularly germination rate) even when the carboxyl group substitution rate is increased. As a result, they have found that good plant growth can be achieved even when the carboxyl group substitution rate is increased by combining with a fibrous material. This is presumably due to the following reasons. Note that the present invention is not limited by the following presumptions.

The water-absorbent resin composition according to this embodiment further contains fibrous material in addition to the (regenerated) water-absorbent resin. A water-absorbent resin with a high substitution rate (neutralization rate) of carboxyl groups can hold a large amount of water (high water absorption per unit mass), but the water absorption speed is not so fast. Although the fibrous material has a low water absorption per unit mass compared to the water-absorbent resin (about 1/10 of the water-absorbent resin), it has excellent short-term water absorption (high water absorption speed). When water is sprayed onto a water-absorbent resin composition that combines these, the fibrous material quickly captures the sprayed water and transfers the water captured by the fibrous material to the water-absorbent resin. Therefore, according to the water-absorbent resin composition according to this embodiment, the fibrous material can capture the sprayed water that could not be captured by the water-absorbent resin alone from the viewpoint of water absorption speed, so that the amount of water captured can be greater than that of the water-absorbent resin alone. The presence of water is very important for the growth of plants. Therefore, a water-absorbent resin composition that combines a water-absorbent resin with a fibrous material can efficiently capture sprayed water without adversely affecting the water-absorption properties of the water-absorbent resin, improving plant growth.

[8-1] Water-absorbent resin In this embodiment, the water-absorbent resin has a carboxyl group, and a part of the carboxyl group is substituted with at least one element selected from the group consisting of potassium and nitrogen at a substitution rate of 5 mol% to 100 mol%. From the viewpoint of further improving water absorption ability and plant growth (particularly germination rate) and better balance between them, the substitution rate of the carboxyl group is preferably 10 mol% to less than 100 mol%, more preferably 15 mol% to 90 mol%, and particularly preferably 15 mol% to 80 mol%. With such a substitution rate, plant growth (particularly germination rate) can be further improved while maintaining a moderate water retention capacity (e.g., deionized water CRC).

[8-2] Fibrous Material The fibrous material is not particularly limited, and can be one that is normally used for the desired application (for example, sanitary materials (hygienic products) such as paper diapers, sanitary napkins, incontinence products for adults (incontinence pads), and pet sheets, soil water retention agents for agricultural and horticultural use, and industrial water-stopping agents). Specifically, wood pulp fibers such as mechanical pulp, chemical pulp, semi-chemical pulp, and dissolving pulp from wood, and artificial cellulose fibers such as rayon and acetate can be used. Synthetic polymer fibers such as polyolefin, polyester, polyacrylonitrile, and nylon can also be used. A mixture of multiple types of fibers described above can also be used.

The content of the fibrous material in the water absorbent resin composition is, for example, 0.01 parts by mass or more, preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably more than 0.1 parts by mass, and particularly preferably 0.5 parts by mass or more, relative to 100 parts by mass of the water absorbent resin composition. If it is within the above range, the fibrous material can capture the sprayed water sufficiently and efficiently. In addition, the content of the fibrous material in the water absorbent resin composition is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, even more preferably 20 parts by mass or less, and particularly preferably 15 parts by mass or less, relative to 100 parts by mass of the water absorbent resin composition. If it is within the above range, a sufficient amount of water absorbent resin is present, so that a sufficient amount of water can be captured (sufficient deionized water CRC can be ensured).

In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.01 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.01 parts by mass or more and 25 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.01 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.01 parts by mass or more and 15 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.05 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.05 parts by mass or more and 25 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.05 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.05 parts by mass or more and 15 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.1 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.1 parts by mass or more and 25 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.1 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.1 parts by mass or more and 15 parts by mass or less with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is more than 0.1 parts by mass and not more than 30 parts by mass with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is more than 0.1 parts by mass and not more than 25 parts by mass with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is more than 0.1 parts by mass and not more than 20 parts by mass with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is more than 0.1 parts by mass and not more than 15 parts by mass with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.5 parts by mass or more and not more than 30 parts by mass with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.5 parts by mass or more and not more than 25 parts by mass with respect to 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.5 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the water absorbent resin composition. In one embodiment of the present invention, the content of the fibrous material in the water absorbent resin composition is 0.5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the water absorbent resin composition.

The measurement of the content of the fibrous material in the water-absorbent resin composition is not particularly limited, and a known method can be used. Specifically, when the water-absorbent resin composition is composed of a water-absorbent resin and a fibrous material, and the fibrous material is pulp,
i) A method in which water and cobalt chloride are added to a mixture (water-absorbent resin composition) of a water-absorbent resin and a fibrous material to selectively color the water-absorbent resin, the mixture is shaken in a separating funnel, and the water-absorbent resin and the fibrous material are separated and recovered by utilizing the difference in sedimentation speed between the water-absorbent resin and the fibrous material, the recovered materials are each dried, and the masses of the water-absorbent resin and the fibrous material are measured;
ii) a method in which water and a pulp-decomposing enzyme such as cellulase are added to a mixture of a water-absorbent resin and a fibrous material (a water-absorbent resin composition), and only the pulp is solubilized, and the remaining water-absorbent resin is quantified by filtration or the like, and the amount of the reduced amount is regarded as the amount of pulp; and iii) a method in which water and a solubilizing agent for solubilizing the water-absorbent resin (e.g., permanganate, hydrogen peroxide-cobalt salt, hydrogen peroxide-copper salt; Journal of the National Environmental Research Association, Vol. 36, No. 1 (2011) P. 51-58) are added to a mixture of a water-absorbent resin and a fibrous material (a water-absorbent resin composition), and only the water-absorbent resin is solubilized, and the remaining pulp is quantified by filtration can be used.

The amount of fibrous matter measured by the above methods i) to iii) is substantially the same. In this specification, the amount measured by the above method iii) is defined as the amount of fibrous matter in the water absorbent resin composition.

When the raw material water absorbent resin contains fibrous material, the amount of fibrous material contained in the water absorbent resin is quantified by the above method, and the total amount including the amount of fibrous material added later is regarded as the amount of fibrous material contained in the water absorbent resin composition.

The size of the fibrous material is not particularly limited. The length of the fibrous material (number average fiber length) is, for example, 0.1 mm or more and 20 mm or less, preferably 0.2 mm or more and 15 mm or less, more preferably 0.3 mm or more and 10 mm or less, and particularly preferably 0.4 mm or more and 5.0 mm or less. With such a fiber length, the fibrous material can be mixed more homogeneously with the water-absorbent resin.

In this specification, the length of the fibrous material (number average fiber length) is a value measured according to the following method. The fibrous material was photographed using a scanning electron microscope (Keyence Corporation: VF-9800) at a magnification of 5 to 100 times. From the photographed image, 50 points in the sample were randomly selected and the fiber length of each was measured, and the average value was calculated, and this average value was taken as the length of the fibrous material (number average fiber length).

[8-3] Other components The water-absorbent resin composition according to the present embodiment essentially contains a water-absorbent resin and a fibrous material, but may further contain other components in addition to the above. Preferably, the water-absorbent resin composition is substantially composed of a water-absorbent resin and a fibrous material (the total content of the water-absorbent resin and the fibrous material in the water-absorbent resin composition is more than 90 mass%, preferably more than 95 mass%). Particularly preferably, the water-absorbent resin composition is composed of a water-absorbent resin and a fibrous material (the total content of the water-absorbent resin and the fibrous material in the water-absorbent resin composition is 100 mass%).

[9] Physical Properties of Water Absorbent Resin Composition The water absorbent resin composition according to the present invention includes a water absorbent resin having a carboxyl group, and a part of the carboxyl group being substituted with at least one element selected from the group consisting of potassium element and nitrogen element at a substitution rate of 5 mol % or more and 100 mol % or less, and a fibrous material.

When the water-absorbent resin composition according to the present invention is used as a water-retaining material, it is desirable to satisfy at least one of the following [9-1], [9-2], and [9-3], more preferably at least two, and most preferably all of them.

[9-1] Water soluble matter (16-hour value)
The water absorbent resin composition according to the present invention preferably has a small water-soluble content (16-hour value). In one embodiment, the water-soluble content (16-hour value) of the water absorbent resin composition according to the present invention or the water retention agent containing the water absorbent resin composition is less than 17% by mass. In one embodiment, the water-soluble content (16-hour value) of the water absorbent resin composition or the water retention agent containing the water absorbent resin composition is 16.0% by mass or less, 15.0% by mass or less, 14.0% by mass or less, 13.0% by mass or less, 12.0% by mass or less, or 11.0% by mass or less (lower limit: 0% by mass). By setting the water-soluble content at such a level, when the water absorbent resin composition is used for sanitary materials, the toughness of the polymer structure can be improved, and the process damage resistance and water absorption properties (particularly AAP 0.3) can be improved. In addition, when used as a water retention material for plant growth, changes in water absorption properties due to external environments such as soil can be further suppressed and prevented.

[9-2] Residual Monomer Amount The water absorbent resin composition according to the present invention has a small residual monomer amount. In one embodiment, the residual monomer amount of the water absorbent resin composition according to the present invention or the water retention agent containing the water absorbent resin composition is 100 mass ppm or less. In one embodiment, the residual monomer amount of the water absorbent resin composition or the water retention agent containing the water absorbent resin composition is 80 mass ppm or less, 70 mass ppm or less, 60 mass ppm or less, 50 mass ppm or less, 40 mass ppm or less, 30 mass ppm or less, or less than 30 mass ppm (lower limit: 0 mass ppm). By setting the residual monomer amount in this range, low skin irritation can be expected when the water absorbent resin composition is used for sanitary materials. In addition, when used as a water retention material for plant growth, a water absorbent resin composition with even better safety can be provided.

[9-3] Deionized water CRC (absorption capacity without pressure)
The absorbency without pressure (deionized water CRC) of the water absorbent resin composition according to the present invention is usually 5 g/g or more, preferably 50 g/g or more, more preferably 60 g/g or more, even more preferably more than 70 g/g, and particularly preferably 80 g/g or more. There is no particular limit to the upper limit, and a higher deionized water CRC is preferable, but from the viewpoint of balance with other physical properties, it is preferably 500 g/g or less, more preferably 450 g/g or less, and even more preferably 400 g/g or less. By setting the absorbency without pressure (deionized water CRC) at such a level, sufficient water retention can be ensured when used as a water retention material for plant growth.

[10] Manufacturing method of water absorbent resin composition according to one embodiment of the seventh aspect The water absorbent resin composition according to this embodiment can be manufactured in the same manner as the above [2], except that the substitution rate of the carboxyl group is within the above range. That is, in one embodiment of the present invention, the manufacturing method of the water absorbent resin composition of the present invention comprises treating a raw water absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salt, lithium salt, magnesium salt, calcium salt and iron salt of a carboxyl group with an acidic substance (acid treatment step); adding a substance containing at least one element selected from the group consisting of potassium element and nitrogen element to the raw water absorbent resin treated with the acidic substance in an amount such that the substitution rate of the carboxyl group is 5 mol% or more and 100 mol% or less, and substituting the carboxyl group present in the raw water absorbent resin with the element (substitution step).

[10-1] Pretreatment step Before treating the raw water absorbent resin with an acidic substance, other treatments such as a washing/sterilization step, a crushing step, a separation step, and a dehydration step may be performed (pretreatment step). This step is the same as the above [2-1].

[10-2] Acid Treatment Step If necessary, after the pretreatment in the above [10-1], the raw water absorbent resin is treated with an acidic substance (acid treatment step). By this step, the salt of the carboxyl group (-COOX or -COO-Y-OOC-; X is sodium, potassium or lithium, and Y is an alkaline earth metal or a transition metal (e.g., magnesium, calcium, iron, etc.)) in the raw water absorbent resin is converted to a carboxyl group (-COOH). This step is the same as the above [2-2].

[10-3] Washing step The acid-treated raw water absorbent resin may be washed. In one embodiment of the present invention, the acid-treated raw water absorbent resin is washed with water to obtain a raw water absorbent resin washed with water. This step can remove or reduce the content of impurities (e.g., acidic substances, polyvalent metal ions) present on the surface, inside, or gaps between particles of the acid-treated raw water absorbent resin, and improve the water absorption performance (e.g., deionized water CRC) of the final product, the regenerated water absorbent resin. This step is the same as the above [2-3].

[10-4] Substitution step (neutralization step)
In this step, a substance (K/N-containing substance) containing at least one element selected from the group consisting of potassium and nitrogen elements is added to the raw water absorbent resin treated in the acid treatment step [10-2] or the washing step [10-3] in such an amount that the substitution rate of the carboxyl groups is 5 mol % or more and 100 mol % or less. As a result, of all the carboxyl groups present in the raw water absorbent resin (raw polymer) after the acid treatment, hydrogen atoms of carboxyl groups (-COOH) at a ratio of 5 mol % or more and 100 mol % or less are substituted (neutralized) with potassium elements (-COOK) or nitrogen elements. With regard to the nitrogen element, it is preferable that the carboxyl group (-COOH) is converted (substituted) into an ammonium ion (-COO ^- NH ₄ ⁺ ) from the viewpoints that the carboxyl group is easily substituted and that the regenerated water absorbent resin is easily utilized by plants when used as a water retention material for plant growth.

The amount of K/N-containing substance added is such that the substitution rate of the carboxyl groups is 5 mol% or more and 100 mol% or less, and the preferred amount is such that the substitution rate of the carboxyl groups is the ratio described in [8-1] above. This process is the same as [2-4] above, except that the amount of K/N-containing substance added is such that the substitution rate of the carboxyl groups is the ratio described above.

[10-5] Other Steps The production method according to the present invention may further include, in addition to the steps described above, a drying step, a pulverizing step, a classification step, a surface cross-linking step, a sizing step, and the like, as necessary.

[10-5-1] Drying step This step is a step of obtaining a dried polymer by drying the water-absorbent resin to a desired resin solid content. This step is the same as the above [2-5-1].

[10-5-2] Grinding step This step is a step of grinding (and classifying, if necessary) the water-absorbent resin or the dried product obtained in the drying step. This step is the same as the above [2-5-2].

[10-5-3] Classification step After pulverization, a classification step may be carried out. This step is the same as [2-5-3] above.

[10-5-4] Surface cross-linking step This step is a step of providing a part with a higher cross-linking density on the surface layer (particularly a part several tens of μm from the surface of the water-absorbent resin precursor) of the water-absorbent resin or the water-absorbent resin obtained through any of the steps described above (hereinafter, also simply referred to as "water-absorbent resin precursor"), and may be composed of a mixing step, a heat treatment step, and a cooling step (optional). In the surface cross-linking step, a surface-cross-linked water-absorbent resin is obtained by radical cross-linking, surface polymerization, cross-linking reaction with a surface cross-linking agent, etc. on the surface of the water-absorbent resin precursor. This step is the same as the above [2-5-4].

[10-5-5] Particle Size Adjustment Step This step is a step for adjusting the particle size of the surface cross-linked water absorbent resin. This particle size adjustment step provides a water absorbent resin in which the particle size or particle size distribution is more actively controlled. This step is the same as the above [2-5-5].

As described above, the water-absorbent resin contained in the water-absorbent resin composition is the same as that described in [2] above, except for the substitution rate of the carboxyl group.

[10-6] Fibrous Material Addition Step In this embodiment, the location of the fibrous material is not particularly limited. The fibrous material may be added before, during, or after any of the steps of the above [10-1] pretreatment step, the above [10-2] acid treatment step, the above [10-3] washing step, the above [10-4] substitution step (neutralization step), or the above [10-5] other steps (the above [10-5-1] drying step, the above [10-5-2] pulverization step, the above [10-5-3] classification step, the above [10-5-4] surface crosslinking step, or the above [10-5-5] sizing step). Among these, from the viewpoint of suppressing loss of fibrous material during the operation, it is preferable to add the fibrous material before the above [10-5] other steps. Alternatively, in the case of using a water absorbent resin that does not substantially contain fibrous impurities (content of fibrous material = less than 0.01 mass%, preferably less than 0.005 mass%), from the viewpoints of further improving workability, higher accuracy of the amount of fibrous material added, and a balance between these, it is preferable to add the fibrous material before the above-mentioned [10-2] acid treatment step, or after the above-mentioned [10-4] substitution step (neutralization step); after the above-mentioned [10-1] pretreatment step, before the above-mentioned [10-2] acid treatment step, after the above-mentioned [10-5-3] classification step, It is more preferable to add the fibrous material before the surface cross-linking step [10-5-4], or after the surface cross-linking step [10-5-4] and before the sizing step [10-5-5], it is even more preferable to add the fibrous material after the classification step [10-5-3] and before the surface cross-linking step [10-5-4], or after the surface cross-linking step [10-5-4] and before the sizing step [10-5-5], it is particularly preferable to add the fibrous material after the surface cross-linking step [10-5-4] and before the sizing step [10-5-5]. Also, when using a water-absorbent resin recovered from used paper diapers, it is preferable to add the fibrous material before the acid treatment step [10-2].

In this process, the amount of fibrous material added is preferably the amount described in [8-2] Fibrous material above.

[11] Water-retaining material according to one embodiment of the eighth aspect The water-absorbent resin composition according to one embodiment of the sixth aspect, or the water-absorbent resin composition produced by the method according to one embodiment of the seventh aspect, can be suitably used as a water-retaining material for plant growth.

In one embodiment of the eighth aspect, a water-retaining material is provided that includes the water-absorbent resin composition of the present invention or is composed of the water-absorbent resin composition.

In this embodiment, in the water-retaining material according to one embodiment of the fifth aspect of [7] above, the recycled water-absorbent resin can be read as the water-absorbent resin composition according to the present invention, and a similar water-retaining material can be obtained.

Although the embodiments of the present invention have been described in detail, it is clear that the same are illustrative and exemplary and are not limiting, and the scope of the present invention should be interpreted by the appended claims.

The present invention includes the following aspects and configurations:

1. A raw water absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, potassium salts, lithium salts, magnesium salts, calcium salts, and iron salts of a carboxyl group is treated with an acidic substance;
a method for producing a regenerated water absorbent resin, comprising: adding a substance containing at least one element selected from the group consisting of potassium element and nitrogen element to the raw water absorbent resin treated with the acidic substance in such an amount that the substitution rate of the carboxyl groups is more than 5 mol % and less than 40 mol %, thereby substituting the carboxyl groups present in the raw water absorbent resin with the element;
2. The method according to 1. above, wherein the raw water absorbent resin is a recovered water absorbent resin recovered from used absorbent articles or waste during a water absorbent resin manufacturing process;
3. The raw water absorbent resin is a recovered water absorbent resin recovered from a used absorbent article, which contains a polymer having a sodium salt, a potassium salt, and/or a lithium salt of a carboxyl group,
The method according to 1. or 2., wherein the raw water absorbent resin is dehydrated and then treated with the acidic substance;
4. A method for producing a water-absorbing resin, comprising producing a recycled water-absorbing resin by using the method according to any one of 1. to 3. above, and producing a water-absorbing resin by using the recycled water-absorbing resin as a part of a raw material;
5. A method for producing a water-retaining material, comprising producing a recycled water-absorbent resin by using any one of the methods described in 1 to 3 above;
6. A water-absorbent resin recycled from a used absorbent article or waste from a water-absorbent resin manufacturing process, comprising a polymer having carboxyl groups and in which a part of the carboxyl groups is substituted with at least one element selected from the group consisting of potassium and nitrogen;
7. The recycled water absorbent resin according to 6. above, wherein the carboxyl group of the polymer is substituted with potassium element at a substitution rate of more than 5 mol % and less than 40 mol %;
8. The recycled water absorbent resin according to 6. or 7., wherein the polymer has the carboxyl group substituted with nitrogen at a substitution rate of more than 5 mol % and less than 40 mol %;
9. The recycled water absorbent resin according to any one of 6. to 8., wherein the polymer has an ammonium salt (—COONH ₄ ) of a carboxyl group at a substitution rate of more than 5 mol % and less than 40 mol %;
10. The recycled water absorbent resin according to any one of 6. to 9., wherein the water-soluble content is 20 mass% or less;
11. The recycled water absorbent resin according to any one of 6. to 10., wherein the amount of residual monomer is 100 ppm by mass or less;
12. A water retention material comprising or made of the recycled water absorbent resin according to any one of 6. to 11.
13. A water-absorbent resin composition comprising a water-absorbent resin having carboxyl groups, and a part of the carboxyl groups being substituted with at least one element selected from the group consisting of potassium and nitrogen at a substitution rate of 5 mol % to 100 mol %, and a fibrous material;
14. The water-absorbent resin composition according to 13., wherein the fibrous material is contained in an amount of 0.01 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the water-absorbent resin composition;
15. A water-retaining material comprising or consisting of the water-absorbent resin composition according to 13 or 14 above;
16. A raw water absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, lithium salts, magnesium salts, calcium salts, and iron salts of a carboxyl group is treated with an acidic substance;
15. A method for producing a water absorbent resin composition according to any one of 13. to 15. above, comprising adding a substance containing at least one element selected from the group consisting of potassium element and nitrogen element to the raw water absorbent resin treated with the acidic substance in such an amount that a substitution rate of the carboxyl group is 5 mol % or more and 100 mol % or less, thereby substituting the carboxyl group present in the raw water absorbent resin with the element;
17. The method according to 16. above, wherein the raw water absorbent resin is a recovered water absorbent resin recovered from used absorbent articles or waste during a water absorbent resin manufacturing process.

The effects of the present invention will be explained using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention. In the following examples, unless otherwise specified, operations were performed at room temperature (25°C). Furthermore, unless otherwise specified, "%" and "parts" mean "% by mass" and "parts by mass", respectively.

[Production Example a]
A reaction vessel was charged with 244.9 g of acrylic acid, 0.71 g (0.040 mol % relative to the carboxyl group-containing unsaturated monomer) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 1.83 g of 1.0 mass % trisodium diethylenetriaminepentaacetate (DTPA.3Na), 99.5 g of 48.5 mass % aqueous sodium hydroxide solution, and 387.5 g of deionized water, and these were mixed to prepare an aqueous monomer solution (a').

Next, the monomer aqueous solution (a') was cooled while stirring. When the liquid temperature reached 40.0°C, 96.7 g of 48.5% by mass aqueous sodium hydroxide solution adjusted to 40°C was added and mixed to prepare the monomer aqueous solution (a). At this time, the temperature of the monomer aqueous solution (a) rose to 78.0°C due to the heat of neutralization in the second stage. Immediately after starting to add the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved and eventually became a transparent homogeneous solution.

Next, 12.1 g of 4.0% by mass sodium persulfate aqueous solution was added to the stirred monomer aqueous solution (a), and the mixture was immediately poured into a stainless steel bat-shaped container (bottom surface 340 x 340 mm, height 25 mm, inner surface; Teflon (registered trademark) coating) in an open-air system. The time from the start of the second stage neutralization to pouring the monomer aqueous solution (a) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate until the surface temperature reached 40°C. The polymerization reaction started 60 seconds after the monomer aqueous solution (a) was poured into the bat-shaped container. The polymerization reaction proceeded by expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, the hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel") (a) was taken out. The series of operations was carried out in an open-air system.

The hydrogel (a) obtained by the polymerization reaction was cut into strips and crushed with a meat chopper having a die diameter of 7.5 mm. The hydrogel (a) was then spread on a 50-mesh wire net and dried with hot air at 190°C for 60 minutes. The gel was then crushed using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an irregularly crushed water-absorbent resin precursor (a).

100 parts by mass of the obtained water-absorbent resin precursor (a) was homogeneously mixed with a surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain water-absorbent resin (a). The obtained water-absorbent resin (a) had a CRC of 35.9 g/g, an AAP0.3 of 29.4 g/g, and a water-soluble content (16-hour value) of 17.1 mass%. The neutralization rate of the obtained water-absorbent resin (a) was about 70 mol%.

Next, 3,000 parts by mass of artificial urine (urea 1.9% by mass, sodium chloride 0.80% by mass, magnesium chloride hexahydrate 0.10% by mass, calcium chloride dihydrate 0.10% by mass, ion-exchanged water 97.1% by mass) was added to 100 parts by mass of the water-absorbent resin (a), mixed, and left at room temperature for 12 hours or more. This resulted in the preparation of a simulated used water-absorbent resin (a) as a raw water-absorbent resin.

[Production Example b]
99.0 parts by mass of the water absorbent resin (a) obtained in Production Example a and 1.0 part by mass of pulp fibers (pulp fibers extracted from the absorbent part of "GOON (registered trademark)" manufactured by Daio Paper Corporation, which is a commercially available paper diaper, number average fiber length 2.5 mm) (fibrous material) were thoroughly mixed in a container equipped with a stirring blade to obtain a water absorbent resin composition (1).

3,000 parts by mass of artificial urine (urea 1.9% by mass, sodium chloride 0.80% by mass, magnesium chloride hexahydrate 0.10% by mass, calcium chloride dihydrate 0.10% by mass, ion-exchanged water 97.1% by mass) was added to 100 parts by mass of this water-absorbent resin composition (1), and the mixture was left at room temperature for 12 hours or more to prepare a simulated used water-absorbent resin composition (b).

[Production Example c]
90.0 parts by mass of the water absorbent resin (a) obtained in Production Example a and 10.0 parts by mass of pulp fibers (pulp fibers extracted from the absorbent part of "GOOON (registered trademark)" manufactured by Daio Paper Corporation, which is a commercially available paper diaper; number average fiber length: 2.5 mm) (fibrous material) were thoroughly mixed in a container equipped with a stirring blade to obtain a water absorbent resin composition (2).

3,000 parts by mass of artificial urine (urea 1.9% by mass, sodium chloride 0.80% by mass, magnesium chloride hexahydrate 0.10% by mass, calcium chloride dihydrate 0.10% by mass, ion-exchanged water 97.1% by mass) was added to 100 parts by mass of this water-absorbent resin composition (2), and the mixture was left at room temperature for 12 hours or more to prepare a simulated used water-absorbent resin composition (c).

[Production Example d]
78.0 parts by mass of the water absorbent resin (a) obtained in Production Example a and 22.0 parts by mass of pulp fibers (pulp fibers extracted from the absorbent portion of a commercially available disposable diaper ("GOON (registered trademark)" manufactured by Daio Paper Co., Ltd., number average fiber length 2.5 mm) (fibrous material) were thoroughly mixed in a container equipped with a stirring blade to obtain a water absorbent resin composition (3).

3,000 parts by mass of artificial urine (urea 1.9% by mass, sodium chloride 0.80% by mass, magnesium chloride hexahydrate 0.10% by mass, calcium chloride dihydrate 0.10% by mass, ion-exchanged water 97.1% by mass) was added to 100 parts by mass of this water-absorbent resin composition (3), and the mixture was left at room temperature for 12 hours or more to prepare a simulated used water-absorbent resin composition (d).

[Example 1]
100 parts by mass of the simulated used water absorbent resin (a) (solid content 3.23% by mass) obtained in Production Example a and 242 parts by mass of a 3.0% by mass aqueous citric acid solution (corresponding to about 3.1 moles of carboxyl groups of citric acid per mole of carboxyl groups of the water absorbent resin) were added to a reaction vessel equipped with a stirring blade, and the mixture was stirred for 15 minutes (acid treatment). The acid treatment converted all sodium salts (-COONa) of carboxyl groups present in the simulated used water absorbent resin (a) to carboxyl groups (-COOH). Furthermore, the acid treatment caused the simulated used water absorbent resin (a) to release more water than in the state before the acid treatment, and the resin was in a shrunk state (the acid treatment with the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes (water washing treatment).

Next, the reaction vessel content after the water washing treatment was filtered through a 100-mesh stainless steel wire net, and the filtered matter was returned to the reaction vessel again, and 1.54 parts by mass of a 20% by mass aqueous potassium hydroxide solution (corresponding to a substitution rate of 15 mol%) was dropped into the filtered matter while stirring (substitution treatment). After the completion of the dropping, the mixture was left to stand for 30 minutes to obtain a neutralized water-containing gel-like water-absorbent resin (regenerated water-absorbent resin (1)).

The neutralized water-absorbent resin in the form of a hydrous gel (regenerated water-absorbent resin (1)) was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours, then pulverized in a vibration mill and classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain an irregularly pulverized water-absorbent resin precursor (1).

100 parts by mass of the obtained water-absorbent resin precursor (1) was homogeneously mixed with a surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, it was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain recycled water-absorbent resin (1). The physical properties of recycled water-absorbent resin (1) are shown in Table 1.

[Example 2]
The same operation as in Example 1 was performed except that the amount of the 20% by mass aqueous potassium hydroxide solution used in the substitution treatment was changed from 1.54 parts by mass to 3.59 parts by mass (corresponding to a substitution rate of 35 mol%) in Example 1, to obtain a recycled water absorbent resin (2). The physical properties of the recycled water absorbent resin (2) are shown in Table 1.

[Example 3]
The same operation as in Example 1 was carried out except that the replacement treatment was carried out by adding 0.26 parts by mass of ammonium carbonate powder (corresponding to a replacement rate of 15 mol%) to the filtered product, to obtain a recycled water absorbent resin (3). The physical properties of the recycled water absorbent resin (3) are shown in Table 1.

[Example 4]
The same operation as in Example 3 was performed to obtain a recycled water absorbent resin (4), except that the amount of ammonium carbonate powder used in the substitution treatment was changed from 0.26 parts by mass to 0.62 parts by mass (corresponding to a substitution rate of 35 mol%) in Example 3. The physical properties of the recycled water absorbent resin (4) are shown in Table 1.

[Example 5]
A simulated used water absorbent resin (a) was produced in the same manner as in Production Example a.

100 parts by mass of this simulated used water absorbent resin (a) (solid content 3.23% by mass) and 120 parts by mass of a 1.7% by mass calcium chloride aqueous solution were added to a reaction vessel equipped with an agitator blade and stirred for 15 minutes (dehydration treatment). As a result of the dehydration treatment, the simulated used water absorbent resin (a) released more water than it had before the dehydration treatment and was in a shrunk state.

After the dehydration treatment, the contents of the reaction vessel were filtered through a 100-mesh stainless steel wire screen, and the filtered material was returned to the reaction vessel. 134 parts by mass of a 1.0% by mass aqueous hydrochloric acid solution (equivalent to approximately 1.0 mole per mole of -COONa groups) was added thereto, and the mixture was stirred for 15 minutes (acid treatment).

Next, the content of the reaction vessel after the acid treatment is filtered through a 100 mesh stainless steel wire mesh, and the obtained unneutralized filtrate is returned to the reaction vessel again. While stirring the filtrate,
3.59 parts by mass of a 20% by mass aqueous solution of potassium hydroxide (corresponding to a substitution rate of 35 mol%) was dropped thereto (substitution treatment). After the completion of the dropping, the mixture was left to stand for 30 minutes to obtain a neutralized water-absorbent resin in a hydrous gel state (regenerated water-absorbent resin (5)).

The neutralized water-absorbent resin in the form of a hydrous gel (regenerated water-absorbent resin (5)) was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours, then pulverized in a vibration mill and classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain an irregularly pulverized water-absorbent resin precursor (5).

100 parts by mass of the obtained water-absorbent resin precursor (5) was homogeneously mixed with a surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, it was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain recycled water-absorbent resin (5). The physical properties of recycled water-absorbent resin (5) are shown in Table 1.

[Example 6]
100 parts by mass of the simulated used water absorbent resin composition (b) (containing 3.19 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution (corresponding to about 3.1 moles of carboxyl groups of citric acid per mole of carboxyl groups of the water absorbent resin) were added to a reaction vessel equipped with a stirring blade and an acid treatment was performed by stirring for 15 minutes. The simulated used water absorbent resin composition (b) after the acid treatment released more water than before the treatment and was in a shrunk state (the acid treatment using the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel again. While stirring the filtered material, 1.53 parts by mass of a 20% by mass aqueous potassium hydroxide solution (corresponding to a substitution rate of 15 mol%) was dripped (substitution treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a neutralized water-containing absorbent resin composition (regenerated water-absorbent resin composition) was obtained.

The neutralized water-absorbent resin composition (regenerated water-absorbent resin composition) containing water was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a water-absorbent resin composition precursor (6) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

100 parts by mass of the obtained water absorbent resin composition precursor (6) was uniformly mixed with a surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water-absorbent resin composition (6). The physical properties of the recycled water-absorbent resin composition (6) are shown in Table 1.

[Example 7]
In a reaction vessel equipped with a stirring device equipped with a stirring blade, 100 parts by mass of the simulated used water absorbent resin composition (b) (containing 3.19 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution were added, and the mixture was stirred for 15 minutes to perform an acid treatment. The simulated used water absorbent resin composition (b) after the acid treatment released more water than in the state before the treatment and was in a shrunk state (the acid treatment with the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel again. While stirring the filtered material, 3.56 parts by mass of a 20% by mass aqueous potassium hydroxide solution (equivalent to a substitution rate of 35 mol%) was dripped (substitution treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a hydrated neutralized water absorbent resin composition (regenerated water absorbent resin composition) was obtained.

The neutralized water-absorbent resin composition (regenerated water-absorbent resin composition) containing water was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a water-absorbent resin composition precursor (7) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

100 parts by mass of the obtained water absorbent resin composition precursor (7) was uniformly mixed with a surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water-absorbent resin composition (7). The physical properties of the recycled water-absorbent resin composition (7) are shown in Table 1.

[Example 8]
In a reaction vessel equipped with a stirring device equipped with a stirring blade, 100 parts by mass of the simulated used water absorbent resin composition (b) (containing 3.19 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution were added, and the mixture was stirred for 15 minutes to perform an acid treatment. The simulated used water absorbent resin composition (b) after the acid treatment released more water than in the state before the treatment and was in a shrunk state (the acid treatment using the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel again. While stirring the filtered material, 7.22 parts by mass of a 20% by mass aqueous potassium hydroxide solution (corresponding to a substitution rate of 70 mol%) was dripped (substitution treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a neutralized water-containing absorbent resin composition (regenerated water-absorbent resin composition) was obtained.

The neutralized water-absorbent resin composition (regenerated water-absorbent resin composition) containing water was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a water-absorbent resin composition precursor (8) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

100 parts by mass of the obtained water absorbent resin composition precursor (8) was uniformly mixed with a surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water-absorbent resin composition (8). The physical properties of the recycled water-absorbent resin composition (8) are shown in Table 1.

[Example 9]
In a reaction vessel equipped with a stirring device equipped with a stirring blade, 100 parts by mass of the simulated used water absorbent resin composition (b) (containing 3.19 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution were added, and the mixture was stirred for 15 minutes to perform an acid treatment. The simulated used water absorbent resin composition (b) after the acid treatment released more water than in the state before the treatment and was in a shrunk state (the acid treatment using the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 32.3 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel again. While stirring the filtered material, 1.24 parts by mass of ammonium carbonate powder (corresponding to a substitution rate of 70 mol%) was dripped (substitution treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a neutralized water-containing absorbent resin composition (regenerated water-absorbent resin composition) was obtained.

The neutralized water-absorbent resin composition (regenerated water-absorbent resin composition) containing water was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a water-absorbent resin composition precursor (9) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

100 parts by mass of the obtained water absorbent resin composition precursor (9) was uniformly mixed with a surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water-absorbent resin composition (9). The physical properties of the recycled water-absorbent resin composition (9) are shown in Table 1.

[Example 10]
100 parts by mass of the simulated used water absorbent resin composition (c) (containing 2.90 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution (corresponding to about 3.4 moles of carboxyl groups of citric acid per mole of carboxyl groups of the water absorbent resin) were added to a reaction vessel equipped with a stirring blade and an agitator, and the mixture was stirred for 15 minutes to perform an acid treatment. The simulated used water absorbent resin composition (c) after the acid treatment released more water than before the treatment and was in a shrunk state (the acid treatment using the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel. While stirring the filtered material, 6.56 parts by mass of a 20% by mass aqueous potassium hydroxide solution (corresponding to a substitution rate of 70 mol%) was dripped (substitution treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a neutralized water-containing absorbent resin composition (regenerated water-absorbent resin composition) was obtained.

The neutralized water-absorbent resin composition (regenerated water-absorbent resin composition) containing water was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm to obtain a water-absorbent resin composition precursor (10) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

100 parts by mass of the obtained water absorbent resin composition precursor (10) was uniformly mixed with a surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water-absorbent resin composition (10). The physical properties of the recycled water-absorbent resin composition (10) are shown in Table 1.

[Example 11]
100 parts by mass of the simulated used water absorbent resin (a) (3.19 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution (equivalent to approximately 3.1 moles of carboxyl groups of citric acid per mole of carboxyl groups of the water absorbent resin) were added to a reaction vessel equipped with a stirring blade and an acid treatment was performed by stirring for 15 minutes. The simulated used water absorbent resin (a) after the acid treatment released more water than before the treatment and was in a shrunk state (the acid treatment with the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel. While stirring the filtered material, 7.22 parts by mass of a 20% by mass potassium hydroxide aqueous solution (corresponding to a replacement rate of 70 mol%) was dripped (replacement treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a hydrated neutralized water-absorbent resin (regenerated water-absorbent resin) was obtained.

The neutralized water-absorbent resin (regenerated water-absorbent resin) was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm, to obtain a water-absorbent resin precursor (11) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

99.0 parts by mass of the obtained water-absorbent resin precursor (11) and 1.0 part by mass of pulp fiber (pulp fiber extracted from the absorbent part of a commercially available disposable diaper ("Goon (registered trademark)" manufactured by Daio Paper Co., Ltd.) (fibrous material) were thoroughly mixed in a container equipped with a stirring blade to obtain a water-absorbent resin composition precursor (11).

100 parts by mass of the obtained water absorbent resin composition precursor (11) was uniformly mixed with a surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water-absorbent resin composition (11). The physical properties of the recycled water-absorbent resin composition (11) are shown in Table 1.

[Example 12]
In a reaction vessel equipped with a stirring device equipped with a stirring blade, 100 parts by mass of the simulated used water absorbent resin (a) (3.19 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution were added, and the mixture was stirred for 15 minutes to perform an acid treatment. The simulated used water absorbent resin (a) after the acid treatment released more water than in the state before the treatment and was in a shrunk state (the acid treatment with the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel. While stirring the filtered material, 7.22 parts by mass of a 20% by mass potassium hydroxide aqueous solution (corresponding to a replacement rate of 70 mol%) was dripped (replacement treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a hydrated neutralized water-absorbent resin (regenerated water-absorbent resin) was obtained.

The neutralized water-absorbent resin (regenerated water-absorbent resin) was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm, to obtain a water-absorbent resin precursor (12) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

100 parts by mass of the obtained water-absorbent resin precursor (12) was uniformly mixed with a surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

The mixture was then cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain recycled water-absorbent resin (12).

99.0 parts by mass of the recycled water absorbent resin (12) and 1.0 part by mass of pulp fiber (fibrous material) (pulp fiber extracted from the absorbent part of a commercially available disposable diaper (GOOON (registered trademark) manufactured by Daio Paper Co., Ltd.)) were thoroughly mixed in a container equipped with a stirring blade to obtain a recycled water absorbent resin composition (12). The physical properties of the recycled water absorbent resin composition (12) are shown in Table 1.

[Example 13]
100 parts by mass of the simulated used water absorbent resin composition (d) (containing 2.52 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution (corresponding to about 3.9 moles of carboxyl groups of citric acid per mole of carboxyl groups of the water absorbent resin) were added to a reaction vessel equipped with a stirring blade and an agitator, and the mixture was stirred for 15 minutes to perform an acid treatment. The simulated used water absorbent resin composition (d) after the acid treatment released more water than before the treatment and was in a shrunk state (the acid treatment using the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered material was returned to the reaction vessel. While stirring the filtered material, 5.69 parts by mass of a 20% by mass aqueous potassium hydroxide solution (corresponding to a neutralization rate of 70 mol%) was dripped (replacement treatment). After the dripping was completed and the mixture was allowed to stand for 30 minutes, a neutralized water-containing absorbent resin composition (regenerated water-absorbent resin composition) was obtained.

The neutralized water-absorbent resin composition (regenerated water-absorbent resin composition) containing water was placed in a vacuum dryer adjusted to 60°C and dried for 18 hours. The dried product was then pulverized using a vibration mill and further classified using JIS standard sieves with openings of 850 μm and 106 μm to obtain a water-absorbent resin composition precursor (13) with an average particle size of 360 μm that passed through the sieve with openings of 850 μm and remained on the sieve with openings of 106 μm.

100 parts by mass of the obtained water absorbent resin composition precursor (13) was uniformly mixed with a surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a recycled water-absorbent resin composition (13). The obtained recycled water-absorbent resin composition (13) contained a large amount of fibrous matter, and therefore had poor fluidity and was somewhat difficult to handle compared to the recycled water-absorbent resin compositions (1) to (12).

[Comparative Example 1]
The same operation as in Example 1 was performed except that the amount of the 20% by mass aqueous potassium hydroxide solution used in the substitution treatment was changed from 1.54 parts by mass to 7.19 parts by mass (corresponding to a substitution rate of 70 mol%) in Example 1, to obtain a recycled water absorbent resin (C1). The physical properties of the recycled water absorbent resin (C1) are shown in Table 1.

[Comparative Example 2]
A recycled water absorbent resin (C2) was obtained by the same operation as in Example 3, except that the amount of ammonium carbonate powder used in the substitution treatment was changed from 0.26 parts by mass to 1.24 parts by mass (corresponding to a substitution rate of 70 mol%) in Example 3. The physical properties of the recycled water absorbent resin (C2) are shown in Table 1.

[Comparative Example 3]
The water-absorbing resin (a) obtained in Production Example a was directly used as recycled water-absorbing resin (C3). The physical properties of the recycled water-absorbing resin (C3) are shown in Table 1.

[Comparative Example 4]
A recycled water absorbent resin (C4) was obtained by the same procedure as in Example 1, except that the replacement treatment was not performed. The physical properties of the recycled water absorbent resin (C4) are shown in Table 1.

[Comparative Example 5]
99.0 parts by mass of the water absorbent resin (a) (sodium hydroxide neutralization rate: 70 mol%) produced in Production Example a and 1.0 part by mass of pulp fiber (pulp fiber extracted from an absorbent part of a commercially available paper diaper ("GOOON (registered trademark)" manufactured by Daio Paper Co., Ltd.) (fibrous material) were thoroughly mixed in a reaction vessel equipped with a stirrer to obtain a comparative water absorbent resin composition (C5).

[Comparative Example 6]
In a reaction vessel equipped with a stirring device equipped with a stirring blade, 100 parts by mass of the simulated used water absorbent resin composition (b) (containing 3.19 parts by mass of water absorbent resin solid content) and 242 parts by mass of a 3.0% by mass aqueous citric acid solution were added, and the mixture was stirred for 15 minutes to perform an acid treatment. The simulated used water absorbent resin composition (b) after the acid treatment released more water than in the state before the treatment and was in a shrunk state (the acid treatment using the aqueous citric acid solution also served as a dehydration treatment).

The contents of the reaction vessel after the acid treatment were filtered through a 100 mesh (150 μm mesh) stainless steel wire mesh, and the filtered material was returned to the reaction vessel, to which 323 parts by mass of deionized water was added and stirred for 15 minutes.

Then, the contents in the reaction vessel after the water washing treatment were filtered through a 100-mesh stainless steel wire mesh, and the filtered matter was placed in a vacuum dryer adjusted to 60°C without carrying out a replacement treatment, and dried for 18 hours. The dried matter was then pulverized using a vibration mill, and further classified using JIS standard sieves with mesh sizes of 850 μm and 106 μm, to obtain a comparative water-absorbent resin composition precursor (C6) with an average particle size of 360 μm that passed through the 850 μm sieve and remained on the 106 μm sieve.

100 parts by mass of the obtained comparative water absorbent resin composition precursor (C6) was uniformly mixed with a surface crosslinking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether (EGDGE), 1.35 parts by mass of propylene glycol, and 3.15 parts by weight of deionized water, and the mixture was heated at 100°C for 45 minutes.

Then, the mixture was cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a comparative recycled water-absorbent resin composition (C6). The physical properties of the comparative recycled water-absorbent resin composition (C6) are shown in Table 1.

The recycled water-absorbent resins (1) to (5) and (C1) to (C4) were evaluated for plant growth according to the following method. The results are shown in Table 1. In Table 1, the cases where no treatment was performed are marked with "-".

The recycled water-absorbent resin compositions (6) to (13) and (C5) to (C6) were evaluated for plant growth according to the following method. The results are shown in Table 2. In Table 2, the cases where no treatment was performed are marked with "-".

[Plant growth test (germination test)]
90 g of deionized water was added to 10 g of each recycled water absorbent resin or recycled water absorbent resin composition, and the recycled water absorbent resin or recycled water absorbent resin composition was allowed to stand for 1 hour to fully swell. Then, 35 g of the swollen recycled water absorbent resin or recycled water absorbent resin composition (hereinafter referred to as "swollen gel") was weighed out and placed in a plastic container with a capacity of 120 ml, and the bottom of the container was tapped to remove air.

Next, 10 seeds of radish sprouts were sown on the swollen gel, the plastic container was loosely covered with a lid, and the container was left at room temperature in the dark. After 2 days, the germination state of the radish sprouts was visually observed, and the germination rate (%) was calculated from the number of seeds that germinated.

(summary)
Table 1 shows the water-soluble content, residual monomer content, and deionized water CRC of the obtained recycled water absorbent resin, as well as the results of a plant growth test (germination rate) using the recycled water absorbent resin as a water retention material.

In the plant growth test, good results (germination rate) were obtained in Examples 1 to 5, but poorer results were obtained in Comparative Examples 1 to 4. These results show that when the replacement rate of potassium or nitrogen elements for the carboxyl groups of the water-absorbent resin is within an appropriate range, plant growth is good. For this reason, the recycled water-absorbent resin and recycled water-absorbent resin of the present invention are expected to be useful as water-retaining materials for plant growth.

The amount of water-soluble matter and the amount of residual monomer were significantly reduced in Examples 1 to 5 and Comparative Examples 1, 2, and 4, compared to the water-absorbent resin (a) that had not been subjected to the substitution treatment. This result is believed to be due to the fact that when the water contained in the simulated used water-absorbent resin (a) was released by the acid treatment (or the acid treatment and the dehydration treatment), the water-soluble matter and the residual monomer were simultaneously released from the water-absorbent resin. In addition, all of the deionized water CRCs showed values of 80 g/g or more, except for Comparative Example 4, in which the substitution treatment was not performed during the regeneration treatment.

From these results, it can be said that the recycled water-absorbent resin of the present invention is excellent in water absorbency and plant growth properties, has low water-soluble matter and residual monomer content, and is a water-absorbent resin that takes safety for human skin into consideration.

Table 2 also shows the results of measuring the water-soluble content, residual monomer content, and deionized water CRC of the recycled water-absorbent resin composition, as well as the results of a plant growth test (germination rate) using the water-absorbent resin composition as a water-retaining material.

In the plant growth test, good results (germination rate) were obtained in Examples 6 to 13, but poorer results were obtained in Comparative Examples 5 and 6. These results and the results in Table 1 show that by combining fibrous materials, good plant growth can be achieved with a water-absorbent resin in which the carboxyl groups are substituted with potassium or nitrogen elements at a wider range of substitution rates. For this reason, the water-absorbent resin composition according to the present invention is also expected to be useful as a water-retaining material for plant growth.

The amount of water-soluble matter and the amount of residual monomers were significantly reduced in Examples 6 to 13 and Comparative Example 6, compared to the water-absorbent resin composition of Comparative Example 5, which was not subjected to a regeneration treatment. This is thought to be because the amount of water-soluble matter and the amount of residual monomers were released from the water-absorbent resin when the water-absorbent resin in the simulated used water-absorbent resin composition released moisture due to the acid treatment and dehydration treatment carried out in the regeneration treatment process. In addition, the deionized water CRC showed a high value of 80 g/g or more in all cases except Comparative Example 6, which was not subjected to a replacement treatment during the regeneration treatment.

From these results, it can be said that the water-absorbent resin composition of the present invention is excellent in water absorbency and plant growth properties, has low water-soluble matter and residual monomer content, and is a composition that takes safety to human skin into consideration.

Claims (17)

A raw water absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, potassium salts, lithium salts, magnesium salts, calcium salts, and iron salts of a carboxyl group is treated with an acidic substance;
a method for producing a regenerated water absorbent resin, comprising: adding a substance containing at least one element selected from the group consisting of potassium and nitrogen elements to the raw water absorbent resin treated with the acidic substance in an amount such that a substitution rate of the carboxyl groups is more than 5 mol % and less than 40 mol %, thereby substituting the carboxyl groups present in the raw water absorbent resin with the element. The method according to claim 1, wherein the raw absorbent resin is a recovered absorbent resin recovered from used absorbent articles or waste from a process for producing the absorbent resin. The raw water absorbent resin is a recovered water absorbent resin recovered from a used absorbent article, the recovered water absorbent resin comprising a polymer having a sodium salt, a potassium salt, and/or a lithium salt of a carboxyl group,
2. The method according to claim 1, wherein the raw water absorbent resin is dehydrated and then treated with the acidic substance. A method for producing a water-absorbent resin, comprising producing a recycled water-absorbent resin using the method according to any one of claims 1 to 3, and producing a water-absorbent resin using the recycled water-absorbent resin as part of a raw material. A method for producing a water-retaining material, comprising producing a recycled water-absorbent resin using the method according to any one of claims 1 to 3. A water-absorbent resin recycled from used absorbent articles or waste from the water-absorbent resin manufacturing process, comprising a polymer having carboxyl groups and in which a portion of the carboxyl groups is substituted with at least one element selected from the group consisting of potassium and nitrogen elements. The recycled water absorbent resin according to claim 6, wherein the carboxyl groups of the polymer are substituted with potassium at a substitution rate of more than 5 mol % and less than 40 mol %. The recycled water absorbent resin according to claim 6, wherein the carboxyl groups of the polymer are substituted with nitrogen elements at a substitution rate of more than 5 mol % and less than 40 mol %. 7. The recycled water absorbent resin according to claim 6, wherein the polymer has an ammonium salt ( _{--COONH.sub.4} ) of a carboxyl group with a substitution rate of more than 5 mol % and less than 40 mol %. The recycled water-absorbent resin according to claim 6, wherein the water-soluble content is 20% by mass or less. The recycled water absorbent resin according to claim 6, wherein the amount of residual monomer is 100 ppm by mass or less. A water-retaining material comprising or made of the recycled water-absorbent resin according to any one of claims 6 to 11. A water-absorbent resin composition comprising a water-absorbent resin having carboxyl groups, and a portion of the carboxyl groups being substituted with at least one element selected from the group consisting of potassium and nitrogen at a substitution rate of 5 mol % to 100 mol %, and a fibrous material. The water-absorbent resin composition according to claim 13, wherein the fibrous material is contained in a ratio of 0.01 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the water-absorbent resin composition. A water-retaining material comprising or composed of the water-absorbent resin composition according to claim 13 or 14. A raw water absorbent resin containing a polymer having at least one salt selected from the group consisting of sodium salts, lithium salts, magnesium salts, calcium salts, and iron salts of a carboxyl group is treated with an acidic substance;
14. The method for producing a water absorbent resin composition according to claim 13, comprising adding a substance containing at least one element selected from the group consisting of potassium element and nitrogen element to the raw water absorbent resin treated with the acidic substance in such an amount that a substitution rate of the carboxyl group is 5 mol % or more and 100 mol % or less, thereby substituting the carboxyl group present in the raw water absorbent resin with the element. The method according to claim 16, wherein the raw absorbent resin is a recovered absorbent resin recovered from used absorbent articles or waste generated during the process of manufacturing the absorbent resin.