Method for producing water-absorbent resin powder

Abstract

[Problem] To provide a method for producing a water-absorbing resin powder having excellent water absorption speed. [Means for solving the problem] The method for producing a water-absorbing resin powder according to the present invention includes a polymerization step of polymerizing a monomer aqueous solution to obtain a water-containing gel-like crosslinked polymer; a gel pulverization step of pulverizing the water-containing gel-like crosslinked polymer using a gel pulverization device to obtain a particulate water-containing gel-like crosslinked polymer after the polymerization step; and a drying step of drying the particulate water-containing gel-like crosslinked polymer to obtain a dried product, the gel pulverization device having a feed inlet, a discharge outlet, and a main body having a plurality of rotary shafts, the rotary shafts each having a pulverization unit, the water-containing gel-like crosslinked polymer being continuously fed from the feed inlet in the gel pulverization step, the water-containing gel-like crosslinked polymer being continuously pulverized at 50°C or higher by the pulverization units, and the particulate water-containing gel-like crosslinked polymer being continuously taken out from the discharge outlet, the water-containing gel-like crosslinked polymer fed to the feed inlet having a polymerization rate of 90% by mass or more, and the particulate water-containing gel-like crosslinked polymer discharged from the discharge outlet having a mass average particle diameter d1 of 3 mm or less in terms of solid content.

Description

Technical Field

[0001] This invention relates to a method for manufacturing water-absorbing resin powder. Background Technology

[0002] Super Absorbent Polymer (SAP) is a water-swellable and water-insoluble polymer gelling agent commonly used in various fields such as absorbent products like diapers and sanitary napkins, water-retaining agents for agriculture, forestry, and horticulture, and industrial waterproofing agents.

[0003] Among the aforementioned water-absorbing resins, a large amount of monomers and hydrophilic polymers are used as raw materials. From the viewpoint of water absorption performance, polyacrylic acid (salt)-based water-absorbing resins that use acrylic acid and / or its salts as monomers are most commonly produced in industry.

[0004] With the increasing demand for high-performance absorbent resins, primarily used in diapers, various functional requirements (high physical properties) are being placed on these resins. Specifically, in addition to the basic properties of absorbency ratio under no pressure and absorbency ratio under pressure, absorbent resins are also required to exhibit various properties such as gel strength, water-soluble content, moisture content, absorption rate, liquid permeability, particle size distribution, urine resistance, antibacterial properties, damage resistance, powder flowability, deodorization, colorfastness, low dust, and low residual monomers. In particular, in applications such as diapers, with the trend towards thinner products, there is a desire to further improve absorption rate.

[0005] The commercial manufacturing method of the above-mentioned powdered or granular water-absorbing resin typically includes the following steps: a polymerization step; a gel pulverization (granulation) step performed after or during polymerization; a drying step of the granulated gel; a pulverization step of the dried material; a grading step of the pulverized material; a recovery step of the micropowder generated by pulverization and grading; and a surface crosslinking step of the graded water-absorbing resin powder.

[0006] One method for manufacturing water-absorbing resins proposed to date involves simultaneously performing a polymerization process and a gel pulverization process using a polymerization apparatus equipped with a pulverizing mechanism. In this method, while the liquid monomer undergoes a polymerization reaction, the resulting hydrogel is pulverized, and the finely granulated hydrogel is discharged from the polymerization apparatus. Specific examples include methods using an intermittent kneader and a continuous kneader, as shown in Patent Documents 1-3.

[0007] However, the size of the gel particles obtained by these devices is only a few millimeters to a few centimeters. In recent years, with the demand for further increases in water absorption rate, the gel is not sufficiently pulverized, requiring additional gel pulverization devices. Patent document 4 proposes a method using an intermittent kneader or a continuous kneader to wet pulverize to a size smaller than the gel particles that become the particle size of the water-absorbing resin product, but the device size becomes too large, making it impractical.

[0008] In addition, during the polymerization process, the highly adhesive hydrogels that react with monomers are gelled and crushed. As a result, the hydrogels tend to adhere to the components inside the device and react while still attached. This adhesion of the hydrogels can cause damage to the components or make cleaning during maintenance time-consuming.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 57-34101

[0012] Patent Document 2: Japanese Patent Application Publication No. 60-55002

[0013] Patent Document 3: International Publication No. 2001 / 038402 (Single Volume)

[0014] Patent Document 4: Japanese Patent Application Publication No. 05-112654 Summary of the Invention

[0015] In recent years, in order to obtain water-absorbing resins with excellent water absorption rates that are particularly sought after, it is necessary to pulverize hydrogels into particle sizes smaller than those previously achieved in the gel pulverization process. However, in the so-called kneading polymerization, which uses a mixer with multiple screws to simultaneously carry out the polymerization and gel pulverization processes, it is impossible to obtain hydrogels with the desired particle size.

[0016] Therefore, the object of the present invention is to provide a water-absorbing resin with excellent water absorption rate.

[0017] The inventors first discovered that, in cases where granular hydrogel-like crosslinked polymers (hereinafter also referred to as "granular hydrogels") are obtained using extruders (meat cutters) equipped with perforated plates, a multi-screw mixer (especially a twin-screw mixer) can be used to gel-pulverize the hydrogel-like crosslinked polymers, thereby continuously obtaining granular hydrogels. Furthermore, it was discovered that, by continuously pulverizing the hydrogel-like crosslinked polymers at temperatures above 50°C in this pulverizing unit, a water-absorbing resin powder with excellent water absorption rate can be obtained, thus completing the present invention.

[0018] That is, the present invention is a method for manufacturing a water-absorbing resin powder, comprising the following steps: a polymerization step of polymerizing an aqueous monomer solution to obtain a hydrogel-like crosslinked polymer; a gel pulverization step of pulverizing the hydrogel-like crosslinked polymer using a gel pulverization device after the polymerization step to obtain a granular hydrogel-like crosslinked polymer; and a drying step of drying the granular hydrogel-like crosslinked polymer to obtain a dried product. The gel pulverization device has an inlet, an outlet, and a main body with multiple rotating shafts, each of which has a pulverization unit. In the gel pulverization step, the hydrogel-like crosslinked polymer is continuously fed into the inlet, and the hydrogel-like crosslinked polymer is continuously pulverized at a temperature of 50°C or higher using the pulverization units. Granular hydrogel-like crosslinked polymer is continuously removed from the outlet. The polymerization rate of the hydrogel-like crosslinked polymer fed into the inlet is 90% by mass or higher, and the mass-average particle size d1 of the granular hydrogel-like crosslinked polymer discharged from the outlet is 3 mm or less. Attached Figure Description

[0019] Figure 1 This is a partially missing side view illustrating an example of a gel pulverizing apparatus used in the manufacturing method described in an embodiment of the present invention.

[0020] Figure 2 yes Figure 1 An enlarged view of the gel crushing device (view of the central part of the main body from above).

[0021] Figure 3 This is a flowchart illustrating a representative manufacturing process of water-absorbing resin. Detailed Implementation

[0022] The present invention will now be described in detail, but the scope of the invention is not limited to these descriptions. In addition to the examples described below, appropriate modifications may be made without departing from the spirit of the invention. Furthermore, the present invention is not limited to the embodiments described below, and various modifications may be made within the scope of the claims. Other embodiments obtained by appropriately combining the technical means disclosed for multiple embodiments are also included within the technical scope of the present invention.

[0023] [1] Definition of terminology

[0024] [1-1] "Water-absorbing resin"

[0025] In this invention, "water-absorbing resin" refers to a water-swellable and water-insoluble polymeric gelling agent that meets the following physical properties. Specifically, it refers to a polymeric gelling agent that, as a water-swellable agent, has a CRC (centrifuge capacity retention) of 5 g / g or more as specified in ERT441.2-02, and as a water-insoluble agent, has an Ext (water-soluble component) of 50% by mass or less as specified in ERT470.2-02.

[0026] The aforementioned water-absorbing resin can be designed to suit its intended use / purpose and is not particularly limited. A hydrophilic crosslinked polymer obtained by crosslinking unsaturated monomers with carboxyl groups is preferred. Furthermore, it is not limited to a form entirely of crosslinked polymer; as long as the aforementioned physical properties (CRC, Ext) meet the above-mentioned numerical ranges, it can be a composition containing additives, etc.

[0027] The "water-absorbing resin" in this invention may have undergone surface crosslinking (also known as post-crosslinking or secondary crosslinking) or may not have undergone surface crosslinking. It should be noted that in this specification, "water-absorbing resin powder" refers to powdered water-absorbing resin, preferably water-absorbing resin adjusted to a specified solid content (moisture content) and particle size (particle diameter). Furthermore, the water-absorbing resin powder obtained after completing the specified surface crosslinking treatment is also referred to as surface-crosslinked (post-crosslinked) water-absorbing resin powder and / or water-absorbing agent.

[0028] [1-2] "Poly(meth)acrylate (salt)"

[0029] In this invention, "poly(meth)acrylic acid (salt)" refers to poly(meth)acrylic acid and / or its salts, which is a crosslinked polymer that contains (meth)acrylic acid and / or its salts (hereinafter also referred to as "(meth)acrylic acid (salt)") as repeating units as the main component, and contains grafting components as optional components.

[0030] The aforementioned "main component" refers to the amount (content) of (meth)acrylic acid (salt) relative to the total monomers used in the polymerization (all monomers except crosslinking agents), preferably 50 mol% to 100 mol%, more preferably 70 mol% to 100 mol%, further preferably 90 mol% to 100 mol%, and particularly preferably substantially 100 mol%.

[0031] Here, "poly(meth)acrylate (salt)" may be unneutralized, preferably partially or completely neutralized, more preferably a monovalent salt, further preferably an alkali metal salt or ammonium salt, even more preferably an alkali metal salt, and particularly preferably a sodium salt.

[0032] [1-3] Definition of evaluation methods

[0033] "EDANA" is an abbreviation for the European Disposables and Nonwovens Associations. "ERT" is an abbreviation for EDANA Recommended Test Methods, a European standard that specifies methods for evaluating absorbent resins. In this invention, unless otherwise specified, the test methods already described in the original ERT (2002 revision) shall be performed according to those methods. For evaluation methods not described, the methods and conditions described in the examples shall be used for the tests.

[0034] [1-3-1] "CRC" (ERT441.2-02)

[0035] "CRC" is short for Centrifuge Retention Capacity, which refers to the water absorption ratio of a hydrophobic resin under no pressure (sometimes also called "water absorption ratio"). Specifically, it is calculated as follows: 0.2g of hydrophobic resin is placed in a non-woven bag, immersed in a significant excess of 0.9% sodium chloride aqueous solution for 30 minutes to allow free swelling, and then centrifuged (250G) for 3 minutes to control the water absorption; the water absorption ratio (unit: g / g) is then calculated. It should be noted that for hydrogels after polymerization and / or gel pulverization, 0.4g of hydrogel is used, the measurement time is changed to 24 hours, and solids content correction is performed to determine the CRC.

[0036] 〔1-3-2〕"Moisture Content"(ERT430.2-02)

[0037] "Moisture Content" refers to the moisture content defined by the loss on drying of the water-absorbing resin. Specifically, it is the value (unit: mass%) calculated from the loss on drying of 4.0g of water-absorbing resin at 105°C for 3 hours. It should be noted that in this invention, the loss on drying of 1.0g of the dried water-absorbing resin at 180°C for 3 hours is used to define the moisture content, while the loss on drying of 2.0g of the hydrogel before drying is used to define the moisture content.

[0038] [1-3-3] "PSD" (ERT420.2-02)

[0039] "PSD" is short for Particle Size Distribution, which refers to the particle size distribution of a hydroabsorbent resin as measured by sieving. It should be noted that the mass-average particle size (D50) and the logarithmic standard deviation (σζ) of the particle size distribution are determined using the same method as described in US Patent No. 7,638,570. It should be noted that in this invention, the particle size distribution (PSD) of the particulate hydrogel is defined by wet sieving using the method described later. Furthermore, the particle size (μm) converted from the solids content of the particulate hydrogel is defined based on the particle size (μm) of the particulate hydrogel and its solids content (mass%) using the calculation method described later.

[0040] [1-3-4] "AAP" (ERT442.2-02)

[0041] "AAP" is short for Absorption Against Pressure, which refers to the water absorption ratio of a water-absorbing resin under pressure. Specifically, it refers to the absorption ratio of 0.9g of water-absorbing resin at 2.06kPa (21g / cm³). 2 The water absorption ratio (in g / g) after swelling for 1 hour relative to a significantly excess of 0.9% sodium chloride aqueous solution under a load of 0.3 psi (0.3 psi). In this specification, it is defined as the water absorption ratio after changing the load condition to 4.83 kPa (equivalent to approximately 49 g / cm³). 2 The value was measured at approximately 0.7 psi.

[0042] [1-3-5] "Vortex"

[0043] In this instruction manual, "Vortex" refers to the water absorption rate of the superabsorbent resin, which is the time (in seconds) required for 2g of superabsorbent resin to absorb 50ml of 0.9% sodium chloride aqueous solution to a specified state.

[0044] [1-4] "Gel pulverization"

[0045] In this specification, "gel crushing" refers to the operation of applying shear or compressive forces to reduce the size and increase the surface area of ​​the hydrogel-like crosslinked polymer (hereinafter also referred to as "hydrogel") obtained in the polymerization process (preferably aqueous solution polymerization, unstirred aqueous solution polymerization (static aqueous solution polymerization), and particularly preferably belt polymerization) so as to facilitate drying.

[0046] Furthermore, the shape of the resulting hydrogel can vary depending on the type of polymerizer. For example, hydrogels obtained in static polymerization (especially belt polymerization) are in the form of sheets or blocks. Here, "sheet" refers to a polymer having a thickness along its plane, preferably 1 mm to 30 cm, particularly preferably 0.5 to 10 cm. Typically, sheet-like hydrogels are obtained through belt polymerization, roller polymerization, and batch film polymerization. The length and width of the sheet-like hydrogel are appropriately determined based on the size of the polymerization apparatus used. In the case of continuous polymerization (continuous belt polymerization or continuous roller polymerization), a sheet-like hydrogel with an endless length is obtained, and its width is the width of the belt or roller of the polymerization apparatus, preferably 0.1 to 10 m, more preferably 1 to 5 m. This endless sheet-like hydrogel can be appropriately cut along its length after polymerization for use. Additionally, block-like hydrogels are obtained through tank polymerization, etc. This block-like hydrogel can be appropriately broken into pieces of several centimeters to several meters square after polymerization. In the case of kneader polymerization, polymerization and gel pulverization are continuously performed in the same apparatus during the polymerization process, thus obtaining a hydrogel with a certain degree of fineness. However, the hydrogel particles obtained by kneader polymerization are not fined to the particle size level obtained by the manufacturing method described in this invention. Furthermore, to pulverize to the particle size level obtained in this invention by kneader polymerization, an excessively large apparatus is required, which is not practical as an industrial manufacturing method. Therefore, gel pulverization in this polymerization process is not included in the concept of "gel pulverization" in this invention. In addition, in this invention, the polymerization process is considered to be completed when the polymerization rate reaches the range described later. The operation of pulverizing the hydrogel with a certain degree of fineness obtained by kneader polymerization or other methods to the particle size level required by this invention is included in the concept of "gel pulverization" in this invention.

[0047] [1-5] Others

[0048] In this specification, the range “X~Y” means “above X and below Y”. Furthermore, unless otherwise specified, “t (ton)” as a unit of mass means “Metric ton”, and “ppm” means “mass ppm” or “weight ppm”. Moreover, “mass” and “weight,” “parts by mass” and “parts by weight,” and “mass%” and “weight%” are considered synonyms. Additionally, “~acid (salt)” means “~acid and / or its salt,” and “(meth)acrylic acid” means “acrylic acid and / or methacrylic acid.”

[0049] [2] Method for manufacturing water-absorbing resin powder

[0050] The method for manufacturing the water-absorbing resin powder of the present invention includes a polymerization step, a gel pulverization step separately from the polymerization step, and a drying step. Preferably, the manufacturing method further includes a cooling step, a pulverization step of the dried material, a grading step, a surface crosslinking step, and a granulation step after surface crosslinking (see reference). Figure 3 In addition, it may include the preparation process of monomer aqueous solution, the addition process of various additives, the micron powder removal process and the micron powder recycling process, and the filling process. Furthermore, it may include various known processes depending on the purpose.

[0051] According to the manufacturing method of the present invention, a hydrogel-like crosslinked polymer with a polymerization rate of 90% by mass or more is gel-pulverized by using a mixer with multiple screws (especially a twin-screw mixer), and the particulate hydrogel with a mass-average particle size d1 of less than 3 mm is fed into a drying process, thereby obtaining an absorbent resin with excellent water absorption rate.

[0052] The following is a detailed explanation of each process.

[0053] [2-1] Preparation process of monomer aqueous solution

[0054] This step is preferably an optional step of preparing an aqueous solution (hereinafter referred to as "monomer aqueous solution") containing an unsaturated monomer with an acid group as the main component. It should be noted that a slurry of monomer can also be used within a range that does not reduce the water absorption performance of the resulting water-absorbing resin, but in this section, for convenience, the description focuses on the monomer aqueous solution.

[0055] In addition, the above-mentioned "main component" refers to the fact that the amount (content) of the unsaturated monomer containing acid groups is generally 50 mol% or more, preferably 70 mol% or more, and more preferably 90 mol% or more (up to 100 mol%) relative to the total amount of monomers (excluding internal crosslinking agents) supplied for the polymerization reaction of the water-absorbing resin.

[0056] (Unsaturated monomers containing acid groups)

[0057] The acid group specified in this invention is not particularly limited, and examples include carboxyl groups, sulfone groups, and phosphate groups. Examples of unsaturated monomers containing acid groups include (meth)acrylic acid, maleic acid (anhydride), itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluenesulfonic acid, vinyl toluenesulfonic acid, styrenesulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and (meth)acryloylphosphate 2-hydroxyethyl ester. From the viewpoint of water absorption performance, (meth)acrylic acid, maleic acid (anhydride), itaconic acid, and cinnamic acid are preferred, (meth)acrylic acid is more preferred, and acrylic acid is particularly preferred.

[0058] (Monomers other than unsaturated monomers containing acid groups)

[0059] As monomers other than unsaturated monomers containing acid groups, any compound that can polymerize to form a water-absorbing resin is acceptable. Examples include unsaturated monomers containing amide groups such as (meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, and N,N-dimethylaminopropyl(meth)acrylamide; unsaturated monomers containing amino groups such as N,N-dimethylaminoethyl(meth)acrylate and N,N-dimethylaminopropyl(meth)acrylate; unsaturated monomers containing mercapto groups; unsaturated monomers containing phenolic hydroxyl groups; and unsaturated monomers containing lactam groups such as N-vinylpyrrolidone.

[0060] (Neutralizing salt)

[0061] In this invention, a neutralized salt in which some or all of the acid groups in an unsaturated monomer containing acid groups are neutralized can be used. In this case, the salt of the unsaturated monomer containing acid groups is preferably a salt formed with a monovalent cation, more preferably at least one selected from alkali metal salts, ammonium salts and amine salts, further preferably an alkali metal salt, even more preferably at least one selected from sodium salts, lithium salts and potassium salts, and particularly preferably a sodium salt.

[0062] (Neutralizing agent)

[0063] The neutralizing agent used to neutralize the aforementioned unsaturated monomers containing acid groups is not particularly limited, and inorganic salts such as sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonium carbonate, as well as alkaline substances such as amine organic compounds containing amino or imino groups, can be appropriately selected and used. Two or more neutralizing agents selected from inorganic salts and alkaline substances can be used in combination. It should be noted that, unless otherwise specified, the monomers in this invention include the concept of neutralizing salts.

[0064] (Neutralization rate)

[0065] From the viewpoint of water absorption performance, the number of moles of the neutralizing salt relative to the total number of moles of the unsaturated monomer containing acid groups and its neutralizing salt (hereinafter referred to as "neutralization rate") is preferably 40 mol% or more, more preferably 40 mol% to 80 mol%, further preferably 45 mol% to 78 mol%, and particularly preferably 50 mol% to 75 mol%.

[0066] Methods for adjusting the above-mentioned neutralization rate include: mixing an unsaturated monomer containing an acid group with its neutralizing salt; adding a known neutralizing agent to an unsaturated monomer containing an acid group; and using a partially neutralizing salt of an unsaturated monomer containing an acid group (i.e., a mixture of an unsaturated monomer containing an acid group and its neutralizing salt) that has been pre-adjusted to a specified neutralization rate. These methods can also be combined.

[0067] The aforementioned adjustment of the neutralization rate can be performed before the polymerization reaction of the unsaturated monomers containing acid groups begins, during the polymerization reaction of the unsaturated monomers containing acid groups, or on the hydrogel-like crosslinked polymer obtained after the polymerization reaction of the unsaturated monomers containing acid groups ends. Furthermore, the neutralization rate can be adjusted at any stage—before, during, or after the polymerization reaction—or in multiple stages. It should be noted that in applications such as absorbent materials like diapers where there is potential direct contact with the human body, it is preferable to adjust the neutralization rate before and / or during the polymerization reaction, and more preferably before the polymerization reaction begins.

[0068] (Internal cross-linking agent)

[0069] In the manufacturing method of water-absorbing resin powder, an internal crosslinking agent is preferably used. This internal crosslinking agent is used to adjust the water absorption properties and gel strength of the resulting water-absorbing resin.

[0070] As an internal crosslinking agent, it is sufficient to have a total of two or more unsaturated bonds or reactive functional groups within one molecule. For example, examples of internal crosslinking agents having multiple polymerizable unsaturated groups (capable of copolymerizing with monomers) within the molecule include N,N-methylenebis(meth)acrylamide, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerol (meth)acrylate, glycerol acrylate methacrylate, ethylene oxide modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, and triallyl phosphate. Examples of internal crosslinking agents that have multiple reactive functional groups within the molecule (capable of reacting with functional groups (e.g., carboxyl groups) of monomers) include triallylamine, polyallyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, and polyethyleneimine. Additionally, examples of internal crosslinking agents that have polymerizable unsaturated groups and reactive functional groups within the molecule include glycidyl (meth)acrylate. Two or more of these can be used in combination.

[0071] Among these internal crosslinking agents, from the perspective of the effects of the present invention, compounds having multiple polymeric unsaturated groups within the molecule are preferred, compounds having (poly)alkylene structural units within the molecule are more preferred, compounds having polyethylene glycol structural units are even more preferred, and acrylate compounds having polyethylene glycol structural units are particularly preferred. The hydrogels obtained using these internal crosslinking agents have low adhesiveness. Drying this hydrogel with low adhesiveness reduces welding and aggregation during drying, and is therefore preferred.

[0072] The amount of the aforementioned internal crosslinking agent is appropriately set according to the type of monomer and the internal crosslinking agent. From the viewpoint of the gel strength of the resulting water-absorbing resin, it is preferably 0.001 mol% or more, more preferably 0.005 mol% or more, and even more preferably 0.01 mol% or more relative to the monomer. Furthermore, from the viewpoint of improving the water absorption performance of the water-absorbing resin, it is preferably 5 mol% or less, more preferably 2 mol% or less. It should be noted that under polymerization conditions where the self-crosslinking reaction of the monomer is effective, the aforementioned internal crosslinking agent may not be used.

[0073] (polymerization inhibitor)

[0074] For stability reasons, the monomers used in the polymerization preferably contain a small amount of polymerization inhibitor. A preferred polymerization inhibitor is p-methoxyphenol. The amount of polymerization inhibitor contained in the monomer (especially acrylic acid and its salts) is typically 1 ppm to 250 ppm, preferably 10 ppm to 160 ppm, and more preferably 20 ppm to 80 ppm.

[0075] (Other substances)

[0076] In the manufacturing method described in this invention, substances exemplified below (hereinafter referred to as "other substances") may also be added to the monomer aqueous solution within the scope of achieving the purpose of this invention.

[0077] Specific examples of other substances include chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphites; foaming agents such as carbonates, bicarbonates, azo compounds, and bubbles; chelating agents such as ethylenediaminetetra(methylenephosphonic acid) or its metal salts, ethylenediaminetetraacetic acid or its metal salts, and diethylenetriaminepentaacetic acid or its metal salts; and hydrophilic polymers such as polyacrylic acid (salts) and their cross-linked forms (e.g., water-absorbing resin powders to be reused), starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol. Other substances can be used alone or in combination of two or more.

[0078] There is no particular limitation on the amount of other substances used. In the case of micronized powder to be reused, it is 30% by mass or less relative to the monomer. In the case of other substances, it is preferably 10% by mass or less relative to the monomer, more preferably 0.001% to 5% by mass, and particularly preferably 0.01% to 1% by mass, based on the total concentration of other substances.

[0079] (Monomer concentration in aqueous solution)

[0080] In this process, from the viewpoint of the physical properties of the water-absorbing resin and productivity, the monomer concentration in the monomer aqueous solution (= total monomer amount / (total monomer amount + total polymerization solvent amount (usually water)) is preferably 10% to 90% by mass, more preferably 20% to 80% by mass, further preferably 30% to 70% by mass, and particularly preferably 40% to 60% by mass. Hereinafter, the monomer concentration is sometimes referred to as "monomer concentration".

[0081] (Polymerization initiator)

[0082] The polymerization initiator used in this invention is appropriately selected based on the polymerization morphology, etc., and is therefore not particularly limited. Examples include thermally decomposable polymerization initiators, photodecomposable polymerization initiators, or combinations thereof, or redox polymerization initiators obtained by combining them with a reducing agent that promotes the decomposition of the polymerization initiator. Specifically, one or more of the polymerization initiators disclosed in U.S. Patent No. 7,265,190 may be used. It should be noted that, from the viewpoint of the processability of the polymerization initiator and the physical properties of the water-absorbing resin, peroxides or azo compounds are preferred, peroxides are more preferred, and persulfates are even more preferred.

[0083] The amount of the polymerization initiator relative to the monomer is preferably 0.001 mol% to 1 mol%, more preferably 0.001 mol% to 0.5 mol%. Furthermore, when redox polymerization is performed as required, the amount of the reducing agent used in combination with the oxidant is preferably 0.0001 mol% to 0.02 mol% relative to the monomer.

[0084] (Dissolved oxygen content)

[0085] It should be noted that dissolved oxygen in the monomer aqueous solution before polymerization can be reduced by increasing the temperature or by replacing it with an inactive gas. For example, dissolved oxygen is preferably reduced to below 5 ppm, more preferably to below 3 ppm, and particularly preferably to below 1 ppm.

[0086] Alternatively, bubbles (especially the aforementioned inert gases) can be dispersed in the monomer aqueous solution. In this case, the polymerization reaction becomes foam polymerization.

[0087] [2-2] Polymerization process

[0088] This process involves polymerizing the aforementioned monomer aqueous solution to obtain a hydrogel-like crosslinked polymer. Preferably, the process involves obtaining a hydrogel, wherein the hydrogel is a crosslinked polymer with poly(meth)acrylate (salt) as the main component.

[0089] It should be noted that, in addition to the method of adding the aforementioned polymerization initiators to initiate the polymerization reaction, there are also methods such as irradiation with active energy rays such as radiation, electron beams, and ultraviolet rays. Furthermore, irradiation with active energy rays can be combined with the addition of polymerization initiators.

[0090] (Aggregation Form)

[0091] The polymerization process can be either batch or continuous aqueous solution polymerization. Alternatively, it can be belt polymerization or kneader polymerization. More preferably, continuous aqueous solution polymerization is used, although both continuous belt polymerization and continuous kneader polymerization are applicable. Specific polymerization methods include continuous belt polymerization (disclosed in U.S. Patent Nos. 4,893,999, 6,241,928, and 2005 / 215,734) and continuous kneader polymerization (disclosed in U.S. Patent Nos. 6,987,151 and 6,710,141). By employing these continuous aqueous solution polymerization methods, the production efficiency of the superabsorbent resin is improved.

[0092] Furthermore, as preferred forms of the aforementioned continuous aqueous solution polymerization, "high-temperature initiation polymerization" and "high-concentration polymerization" can be listed. "High-temperature initiation polymerization" refers to a form in which polymerization begins at a temperature preferably above 30°C, more preferably above 35°C, further preferably above 40°C, and particularly preferably above 50°C (upper limit being the boiling point). "High-concentration polymerization" refers to a form in which polymerization is carried out at a monomer concentration preferably above 30% by mass, more preferably above 35% by mass, further preferably above 40% by mass, and particularly preferably above 45% by mass (upper limit being the saturation concentration). These polymerization forms can also be used in combination.

[0093] (Polymerization rate of hydrogels)

[0094] The polymerization rate of the hydrogel-like crosslinked polymer obtained by the polymerization process is 90% by mass or more. Preferably, it is 95% by mass or more, more preferably 98% by mass or more, and particularly preferably 99% by mass or more. When gel pulverization is performed in a state with a low polymerization rate (i.e., a polymerization rate less than 90% by mass) (for example, when polymerization and gel pulverization are performed simultaneously, such as in kneading polymerization), a large number of unreacted monomers contained in the pulverized gel particles polymerize, causing the pulverized gel particles to adhere to each other, thus regenerating large-sized gel particles. Therefore, there are problems such as the need to pulverize the regenerated large-sized gel particles again during the polymerization process, increasing the energy required for pulverization and making the polymerization equipment too large. In addition, when the hydrogel with a low polymerization rate is pulverized after the polymerization process and then dried in a state containing a large number of unreacted monomers, a polymerization reaction occurs during drying, regenerating large-sized hydrogel particles from small-sized gel particles. Therefore, problems such as a decrease in the water absorption rate of the resulting water-absorbing resin and an increase in the particle size of the dried product occur. It should be noted that there is no specific upper limit to the polymerization rate; 100% by mass is ideal. However, high polymerization rates require long polymerization times and strict polymerization conditions, which can sometimes lead to a decrease in productivity and physical properties. An upper limit of 99.95% by mass, then 99.9% by mass, and typically around 99.8% by mass is sufficient. For example, the polymerization rate of hydrogel-like crosslinked polymers obtained through the polymerization process is 98–99.99% by mass.

[0095] [2-3] Shredding process

[0096] The shredding process is an optional step that cuts or coarsely pulverizes the hydrogel-like crosslinked polymer to a size suitable for feeding into the gel pulverizing apparatus, after the polymerization step and before the gel pulverizing step. In particular, this shredding process is preferred when the polymerization step is belt polymerization and a sheet-like or block-like hydrogel is obtained. Therefore, in one embodiment of the invention, the hydrogel-like crosslinked polymer obtained after the polymerization step is sheet-like, and a shredding process is included before the gel pulverizing step to shred the sheet-like hydrogel-like crosslinked polymer. The unit for cutting or coarsely pulverizing the hydrogel in the shredding process is not particularly limited; a rotary cutter, roller cutter, guillotine cutter, etc., can be used. The size to be shredded is not particularly limited as long as it is within the range suitable for feeding into the gel pulverizing apparatus described later. The size of the shredded hydrogel is preferably 1 mm to 3 μm, more preferably 5 mm to 2.5 μm, and particularly preferably 1 cm to 2 μm. It should be noted that the shredding process may be omitted when achieving the objectives of the invention.

[0097] [2-4] Gel pulverization process

[0098] This process involves pulverizing and refining the hydrogel-like crosslinked polymer obtained through the polymerization process to obtain granular hydrogel-like crosslinked polymer (hereinafter referred to as "granular hydrogel"). To obtain a surface-crosslinked hydroabsorbent resin powder with the target shape and properties in high yield, the particle size of the granular hydrogel is adjusted to the preferred range described later. Furthermore, to obtain granular hydrogel with a specified particle size, this process may be performed two or more times.

[0099] (Gel pulverizing device)

[0100] In the manufacturing method described in this invention, such as Figure 1 , Figure 2 As shown, in the gel pulverization process following the polymerization process, a gel pulverization device is used, comprising an inlet, a main body with multiple internal rotating shafts, and an outlet. Each rotating shaft has a pulverization unit. In this gel pulverization device, the hydrogel-like crosslinked polymer continuously fed into the main body from the inlet is pulverized by the pulverization units of each rotating shaft at a temperature of 50°C or higher, and continuously extracted from the outlet in the form of granular hydrogel-like crosslinked polymer. Furthermore, in this invention, the main body refers to the body portion in which multiple rotating shafts and pulverization units are provided (…). Figure 1 The symbol 208 is also known as a barrel, trough, or casing.

[0101] The gel pulverizing apparatus used in the manufacturing method of this invention can be continuous, and can be vertical (with the hydrogel moving in a vertical direction), horizontal, or lateral (with the hydrogel moving in a horizontal or lateral direction). Furthermore, both vertical and horizontal gel pulverizing apparatuses can have an inclination of 0° to 90° relative to the horizontal direction. For example, in… Figure 1 In the case of the horizontal continuous pulverizing apparatus shown, an inclination may be appropriately provided as needed. The inclination from the inlet to the outlet (i.e., relative to the direction of travel of the hydrogel) can be downward or upward. Typically, the inclination angle is 0° to 10°, preferably 0° to 1°, and particularly preferably 0°.

[0102] It should be noted that in conventional manufacturing methods using extruders (meat cutters) for gel pulverization, the hydrogel is substantially pulverized near the die located at the extrusion port, with almost no gel pulverization occurring in the screw section involved in transporting the hydrogel. In contrast, the gel pulverizing apparatus (especially a mixer) used in the manufacturing method described in this invention has the following characteristic: the hydrogel being fed in is pulverized to a particle size of less than 3 mm by the pulverizing unit of the rotating shaft during the period until it reaches the discharge port.

[0103] In detail, in this gel pulverizing apparatus, the hydrogel fed into the inlet is pulverized to the target particle size during the period from input to output. Therefore, unlike conventional extruders (meat cutters), this gel pulverizing apparatus does not require extrusion from a mold; instead, granular hydrogel adjusted to the target particle size is removed from the output. In the manufacturing method described in this invention, by using this gel pulverizing apparatus, a water-absorbing resin with excellent water absorption rate is obtained.

[0104] From the viewpoint of continuously pulverizing gels at temperatures above 50°C, the gel pulverizing apparatus preferably includes a heating unit and / or a heat preservation unit. There are no particular limitations on the heating unit and / or heat preservation unit; however, from the viewpoint of preventing the adhesion and aggregation of hydrogels and particulate hydrogels, a heating unit based on direct heat conduction and / or indirect heat conduction is preferred. The direct heat conduction is based on convective heat conduction, and the indirect heat conduction is based on heat transfer from the heating surface of the gel pulverizing apparatus (the surface in contact with the hydrogel, the heat source portion) heated by the heat medium. More preferably, the heating unit is a ventilated heating type for direct heat conduction and an external wall heating type for indirect heat conduction.

[0105] From the viewpoint of reducing damage to the hydrogel, it is preferable to have a heating unit and / or a heat-insulating unit on the outer surface of the main body, and more preferably a heating unit. Examples of such heat-insulating units include covering part or all of the outer surface of the main body (preferably 50% or more, more preferably 80% or more, and particularly preferably the entire surface) with an insulating material. Examples of heating units include electric traces, steam traces, and jackets heated by a heat medium, all of which cover part or all of the outer surface of the main body (preferably 50% or more, more preferably 80% or more, and particularly preferably the entire surface). The particle size of the granular hydrogel sought in this invention is significantly smaller than in the past. Therefore, it is known that the adhesion and flowability of the hydrogel particles vary greatly due to temperature changes compared to situations envisioned within the scope of the prior art. As a result, research according to this invention clarifies that the energy required to pulverize the hydrogel and the aggregation of the pulverized gel particles vary significantly with temperature. The gel pulverizing apparatus, by incorporating the aforementioned heating unit and / or heat preservation unit, can perform the gel pulverizing process within a more favorable temperature range. Furthermore, it can prevent the quality deterioration of gel pulverization caused by temperature differences such as seasons and day / night cycles. Consequently, it can be smoothly induced to stable operation upon startup of the gel pulverizing apparatus.

[0106] The type of pulverizing unit in each rotating shaft is not particularly limited as long as the effects of the present invention can be achieved. For example, various shapes of disks can be listed as objects that have a shearing effect on hydrogels. Disks are sometimes referred to as chips, paddles, elements, kneading devices, rotors, etc. The shape of the disk is not particularly limited, and can be appropriately selected from circular, elliptical, or roughly triangular shapes. Different shaped disks can also be combined and used, and their arrangement can be appropriately adjusted from the viewpoint of the particle size of the target particulate hydrogel and the energy required for pulverization. In addition, as pulverizing units, arms, blades, scrapers, cut disks (CDs), etc., can be used in combination.

[0107] For example, when each rotating shaft has a circular or elliptical disc as a pulverizing unit, the ratio of the effective length L inside the main body to the maximum diameter D (Diameter; when using multiple discs of different diameters, this is the diameter of the largest disc) should be considered as L / D. This L / D is preferably 5 to 40, more preferably 6 to 30, and even more preferably 6.5 to 20. Furthermore, this effective length L refers to: Figure 1 As shown, the axial length (total length) from the inlet to the main body (bucket) including the outlet.

[0108] Furthermore, the distance (gap) between the disc and the main body (bucket) sometimes varies depending on the location. When the shortest distance between the outer periphery of the disc and the inner wall of the main body (bucket) is denoted as the minimum gap C, the minimum gap C relative to the maximum diameter D of the disc is preferably 20% or less, more preferably 15% or less, further preferably 10% or less, and particularly preferably 5% or less. If it is below the aforementioned upper limit, the shear force between the bucket and the disc becomes stronger during gel pulverization, resulting in better gel pulverization efficiency. Additionally, the minimum gap C relative to the maximum diameter D of the disc is preferably 0.2% or more, more preferably 0.5% or more, and further preferably 1% or more. If it is above the aforementioned lower limit, contact between the disc and the inner wall of the main body (bucket) is suppressed, and the ingress of metallic foreign matter due to wear is suppressed. A suitable form of the present invention is where the minimum gap C relative to the maximum diameter D of the disc is 0.2% to 20%.

[0109] As described below, in this gel pulverizing apparatus, the hydrogel is pulverized to a predetermined particle size by rotating multiple rotating shafts equipped with pulverizing units. The rotational speeds of these multiple rotating shafts can be uniform or non-uniform, and are appropriately set according to the apparatus, preferably in the range of 1 rpm to 1000 rpm, more preferably in the range of 3 rpm to 500 rpm, and even more preferably in the range of 5 rpm to 300 rpm. Furthermore, when the rotational speeds of the rotating shafts are different, the ratio of the rotational speed of the other rotating shafts to the rotational speed of one rotating shaft is typically in the range of 1 to 10, preferably in the range of 1 to 2.

[0110] Furthermore, when the multiple rotating shafts have discs as pulverizing units, the circumferential speed (V) of the discs, as defined by the following (Equation 3), can be uniform or non-uniform, and can be appropriately set according to the apparatus. Preferably, it is 0.05 m / s to 5 m / s, more preferably 0.1 m / s to 5 m / s, even more preferably 0.15 m / s to 3 m / s, and particularly preferably 0.2 m / s to 2 m / s. If it exceeds the above range, the shear force involved in the hydrogel becomes too large, resulting in deterioration of the physical properties of the pulverized hydrogel particles and excessive compaction, which is not preferred. Furthermore, if it is less than the above range, the throughput per unit time in the gel pulverization process decreases, which is also not preferred. Additionally, when the circumferential speeds of the discs on each rotating shaft are different, the ratio of the circumferential speed of other rotating shafts to the circumferential speed of one rotating shaft is typically in the range of 1 to 10, preferably in the range of 1 to 2.

[0111] Circular velocity (V) (m / s) = πD × n / 60…(Equation 3)

[0112] Here, in (Equation 3), V is the circumferential speed of the disk (unit: m / s), D is the maximum diameter of the disk (unit: m), and n is the rotational speed of the disk per unit time (unit: rpm).

[0113] Furthermore, the rotation directions of the multiple rotating shafts can be either unidirectional (rotating in the same direction) or antidirectional (rotating in opposite directions). In a unidirectional device, self-cleaning is expected, while in an antidirectional device, strong shearing force is expected. The rotation direction of each rotating shaft can be appropriately selected by combining it with the arrangement (disc pattern) of the aforementioned pulverizing units.

[0114] The gel pulverizing apparatus preferably has the function of supplying water and / or water vapor to the interior of the main body. By pulverizing the gel while simultaneously supplying water and / or water vapor (preferably water and water vapor), a water-absorbing resin powder with superior water absorption rate can be obtained. Therefore, according to one embodiment of the invention, water and / or water vapor are supplied to the interior of the main body during the gel pulverizing process. In another preferred embodiment, water and water vapor are supplied to the interior of the main body during the gel pulverizing process. As a means of supplying water and / or water vapor, the gel pulverizing apparatus may have multiple inlets. The location of the water and / or water vapor inlets is arbitrary, but it is preferably located on the side containing the hydrogel. Alternatively, water and water vapor can be supplied from different inlets separately.

[0115] When adding water vapor, there are no particular limitations; gases such as air, dry air, and nitrogen can be mixed with water vapor and added as a mixed gas. The pressure of the added water vapor is not particularly limited, but is preferably 0.2 to 0.8 MPa, more preferably 0.3 to 0.7 MPa. The temperature of the water and / or water vapor (including the mixed gas) is not particularly limited, but is preferably 50°C or higher, more preferably 60°C or higher, further preferably 70°C or higher, and particularly preferably 80°C or higher. From the viewpoint of suppressing excessive heating and drying of the hydrogel, it is preferably 200°C or lower, more preferably 170°C or lower, further preferably 150°C or lower, even more preferably 120°C or lower, and particularly preferably 100°C or lower. Preferably, the temperature of the water and / or water vapor supplied to the interior of the main body is 50 to 120°C. The temperature of the hydrogel and granular hydrogel in the gel pulverizing apparatus can also be adjusted by the temperature and amount of water and / or water vapor (including the mixed gas) used. In this case, water vapor and / or mixed gases act as a direct heat transfer medium, heating or holding the hydrogel and particulate hydrogel inside the matrix to a specified temperature. Additionally, additives such as gel-fluidizing agents, crosslinking agents, oxidizing agents, reducing agents, and polymerization initiators can be mixed into the added water and / or water vapor (including mixed gases).

[0116] The amount of water and / or water vapor supplied is preferably 0.1% to 50% by mass, more preferably 0.5% to 40% by mass, and even more preferably 1% to 30% by mass, relative to the mass of the solid component based on the hydrogel.

[0117] The gel pulverizing apparatus used in the manufacturing method of the present invention preferably has a heating unit and / or a heat preservation unit on the outer surface of the main body. A liquid heat medium such as warm water or oil can be introduced into a jacket or the like provided on the outer surface of the main body, or heated gas (hot air) can be introduced as the heat medium. These heat media function as indirect heat conduction media. From the viewpoint of heating efficiency and / or heat preservation efficiency of indirect heat conduction, the temperature of the heat medium is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, and particularly preferably 80°C or higher. On the other hand, from the viewpoint of suppressing excessive heating and drying of the hydrous gel, the temperature of the heat medium is preferably 200°C or lower, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 130°C or lower, and particularly preferably 110°C or lower. Warm water or water vapor is particularly preferred as the heat medium. Furthermore, the temperature of the heat medium can be a constant temperature or can be appropriately varied during the gel pulverization process.

[0118] More preferably, before adding the hydrogel into the gel pulverizing apparatus, the temperature of the interior (inner surface) of the main body is preferably heated to 50°C or higher, more preferably to 60°C or higher, even more preferably to 70°C or higher, and even more preferably to 80°C or higher. This reduces the adhesion of the hydrogel to the inner surface of the main body. Furthermore, the water absorption rate of the resulting water-absorbing resin powder is further increased. That is, in the manufacturing method described in this invention, the inner surface of the main body is preferably heated to the aforementioned temperature or higher before adding the hydrogel and when gel pulverizing begins. More preferably, the inner surface of the main body, the plurality of rotating shafts, and the outer surface of the pulverizing unit of each rotating shaft are preferably heated to the aforementioned temperature or higher. On the other hand, from the viewpoint of suppressing excessive heating and drying of the hydrogel, the heating temperature of the interior (inner surface) of the main body before adding the hydrogel into the gel pulverizing apparatus is preferably 200°C or lower, more preferably 170°C or lower, even more preferably 150°C or lower, even more preferably 130°C or lower, and particularly preferably 110°C or lower. For example, by circulating the heat medium within the jacket of the main body and maintaining it, the temperature inside the main body (inner surface) can be adjusted to a desired range. From the viewpoint of maintaining the temperature at 50°C or higher during the gel pulverization process, the temperature inside the main body (inner surface) is preferably maintained within the aforementioned range during the gel pulverization process.

[0119] Here, "continuously pulverizing the hydrogel-like cross-linked polymer at temperatures above 50°C" refers to: in Figure 1Within the interval shown in (A), in other words, from the inlet to the outlet, the hydrogel-like crosslinked polymer is continuously pulverized while maintaining its temperature at 50°C or higher. In other words, "continuously pulverizing the hydrogel-like crosslinked polymer at 50°C or higher using a pulverizing unit" means: "continuously pulverizing the hydrogel-like crosslinked polymer using a pulverizing unit while maintaining its temperature at 50°C or higher." For example, if the temperature T1 of the hydrogel-like crosslinked polymer introduced into the inlet of the gel pulverizing device is set to 50°C or higher, and the temperature of the heat medium in the jacket installed on the outside of the device body is set to 50°C or higher, then... Figure 1 Within the range of (A), the temperature of the hydrogel-like crosslinked polymer can be maintained at 50°C or higher, and the hydrogel-like crosslinked polymer can be continuously pulverized at 50°C or higher. Furthermore, it also includes the following cases: for example, even if the temperature T1 of the hydrogel-like crosslinked polymer introduced into the inlet of the gel pulverizing device is below 50°C, by supplying high-temperature water and / or steam to the inlet portion, or setting the temperature of the jacket heat medium of the main body of the device to a high temperature, the hydrogel-like crosslinked polymer can be rapidly heated, and the temperature in section (A) can be set above 50°C for continuous pulverization.

[0120] The temperature at which the hydrogel-like crosslinked polymer is continuously pulverized is 50°C or higher, preferably 60°C or higher, more preferably 70°C or higher, and even more preferably 80°C or higher.

[0121] There is no particular upper limit to the temperature for continuous pulverization of the hydrogel-like crosslinked polymer. From the viewpoint of suppressing excessive heating and drying of the hydrogel, it is preferably below 200°C, more preferably below 170°C, even more preferably below 150°C, even more preferably below 130°C, and particularly preferably below 110°C.

[0122] Furthermore, in the case of large gel pulverizing apparatuses, it is preferable to circulate the heat medium within multiple rotating shafts as a heating unit and / or a heat preservation unit. This shortens the time required to heat the interior of the main body when the gel pulverizing apparatus is started.

[0123] As long as the hydrogel is adjusted to the target particle size, the average residence time of the hydrogel in this gel pulverizing device is not particularly limited, but from the viewpoint of reducing mechanical damage to the hydrogel, it is preferably 30 seconds to 30 minutes. In this gel pulverizing device, the average residence time of the hydrogel is adjusted by the rotation speed of the rotating shaft and the feeding speed of the hydrogel.

[0124] In the gel pulverizing apparatus used in the manufacturing method of the present invention, it is most preferable not to use a mold (template). However, a mold can be provided at the discharge port as long as the effects of the present invention can be achieved. When a mold is used, the opening ratio of the mold is preferably 25% or more, more preferably 50% or more, further preferably 60% or more, even more preferably 70% or more, and particularly preferably 80% or more. There is no particular upper limit to the opening ratio. An opening ratio of 100% is synonymous with the case where no mold is used. In addition, a mold (template) refers to a plate having (multiple) through holes for discharging material from the interior of the body, which is provided near the discharge port of the pulverizing apparatus. Furthermore, the opening ratio refers to the ratio of the total planar area of ​​all through holes to the planar area of ​​the mold. The larger the opening ratio, the less likely it is to be blocked by material from the interior of the body, and the easier it is to be discharged. Therefore, the effects of the present invention become more significant.

[0125] Figure 1 and Figure 2 An example of a gel pulverizing apparatus 200 used in the manufacturing method of the present invention is shown. Figure 1 This is a partially incomplete side view of the gel pulverizing device 200. Figure 2 This is an enlarged view of the gel pulverizing device 200 (view of the central part of the main body from above). The following will use... Figure 1 and Figure 2 This will explain the basic structure and usage of the gel pulverizing device 200.

[0126] As shown in the figure, the gel pulverizing device 200 includes an inlet 204, a main body 208, two rotating shafts 206, an outlet 210, a drive unit 214, and a gas inlet 216. The main body 208 is also referred to as a container. Figure 1 Two rotating shafts 206 are arranged along the orthogonal direction of the paper. The rotating shafts 206 extend along the length of the main body 208. One end of the rotating shaft 206 passes through the main body 208 and is connected to the drive device 214. Although not shown, in this gel pulverizing device 200, the other end of the rotating shaft 206 is supported by a bearing behind the main body 208 in a freely rotatable manner. In other words, the rotating shaft 206 is in a form where both ends are held. However, the gel pulverizing device in the manufacturing method of this invention is not limited to this two-shaft support configuration; as long as the purpose of this invention is achieved, a so-called single-shaft support structure without a bearing behind the outlet 210 can be used. The inlet 204, gas inlet 216, and outlet 210 are respectively fixed to the main body 208 and communicate with the interior of the main body 208. Figure 1 The left-right direction in the figure represents the length direction of the main body 208 and is the axial direction of the rotation axis 206. Although not shown in the figure, the main body 208 has a jacket structure.

[0127] Figure 2A portion of the main body 208 of the gel pulverizing device 200 is shown. Figure 2 for Figure 1 An enlarged view of the gel pulverizing apparatus (viewed from above, showing the central part of the main body). As shown, in this gel pulverizing apparatus 200, two rotating shafts 206 are built into the main body 208. Pulverizing units 212 are respectively provided on the outer periphery of the two rotating shafts 206. That is, the pulverizing units 212 and the rotating shafts 206 are configured as separate entities. In this embodiment, the rotating shafts 206 have multiple discs as pulverizing units 212. Figure 2 The vertical direction is the width direction of the main body 208. Figure 2 The left and right directions are the length directions of the main body 208 and the axial direction of the rotation axis 206.

[0128] In a suitable embodiment of using the gel pulverizing apparatus 200 to perform the gel pulverizing process, the main body 208 is first heated by circulating a heat medium in a jacket (not shown). Subsequently, each rotating shaft 206 is rotated using a drive device 214 (e.g., a motor). As the rotating shafts 206 rotate, the rotating shafts 206 and the multiple discs that serve as pulverizing units 212 also rotate.

[0129] Next, the hydrogel is continuously fed into inlet 204. At this time, water and / or water vapor can be simultaneously introduced into inlet 204. Additionally, water vapor and / or water can be introduced into gas inlet 216. The hydrogel and the main body 208 are heated by the water and / or water vapor and held at a specified temperature.

[0130] The hydrogel injected into the main body 208 moves toward the discharge port 210.

[0131] The hydrogel comes into contact with the grinding unit 212 (i.e., multiple disks) within the main body 208. The hydrogel is granulated by the shearing action of the rotating disks. The hydrogel is pulverized by the shearing action of the grinding unit 212 and moves toward the discharge port 210. At the discharge port 210, the granular hydrogel adjusted to the specified particle size is removed.

[0132] The rotating shaft of the gel pulverizing apparatus has multiple discs. The discs can be the same or different in shape, but are preferably different. The combination of discs is adapted according to the physical properties of the hydrogel and the desired size of the pulverized gel, as described in, for example, patent document (Japanese Patent Application Publication No. 2005-35212).

[0133] Examples of gel pulverizing devices with this basic configuration include multi-screw mixers (kneaders) with twin or more screws. Specifically, two-screw, three-screw, four-screw, or eight-screw mixers can be cited. From a production efficiency point of view, this gel pulverizing device is suitable for continuous operation. Specifically, examples of gel pulverizing devices include the CKH type continuous mixer (HONDA MACHINERY WORKS CO.,LTD.), the TEX two-screw extruder (Japan Steel Works, Ltd.), the TEXαIII two-screw extruder (Japan Steel Works, Ltd.), the continuous kneader (CONTINUOUS KNEADER, DALTON Corporation), the KRC mixing reactor (KRC HYBRID REACTER, Kurimoto, Ltd.), the KRC kneader (KURIMOTO-READCO CONTINUOUS KNEADER, Kurimoto, Ltd.), the KEX extruder (KEX EXTRUDER, Kurimoto, Ltd.), the KEXD extruder (KEXD EXTRUDER, Kurimoto, Ltd.), the twin-arm kneader ruder (KNEADER-RUDER, MORIYAMA Corporation), and the TEX-SSG two-screw mixing extruder (ToshibaMachine). The invention includes various types of extruders, such as the TEX-CS (Toshiba Machine Co., Ltd.), TEX-SX (Toshiba Machine Co., Ltd.), TEX-DS (Toshiba Machine Co., Ltd.), TEX-A (Toshiba Machine Co., Ltd.), TEX-B (Toshiba Machine Co., Ltd.), TEX-BS (Toshiba Machine Co., Ltd.), and the WDR series (Teachnovel Co., Ltd.) four-screw and eight-screw extruders. Therefore, in a preferred embodiment of the invention, the gel pulverizing device is a continuous multi-screw extruder.

[0134] (Gel pulverizing energy)

[0135] Gel grinding energy (GGE) refers to the mechanical energy required per unit mass (unit mass of hydrogel) of a gel grinding device when gel grinding hydrogels, as described in International Patent Publication No. 2011 / 126079 (corresponding to U.S. Patent Application Publication No. 2013 / 026412 and U.S. Patent Application Publication No. 2016 / 332141), calculated according to the following formula (5) when the gel grinding device is driven by three-phase AC power.

[0136] Gel pulverization energy [J / g] = {3 1 / 2 ×Voltage×Current×Power Factor×Motor Efficiency} / {Mass of hydrous gel fed into the gel pulverizer within 1 second}…(Equation 5)

[0137] Here, the power factor and motor efficiency are inherent values ​​of the device, varying depending on the operating conditions of the gel pulverizer, and are typically between 0 and 1. When the gel pulverizer is driven by single-phase AC power, by using formula 3 above... 1 / 2 The calculation is changed to 1. In the above (Equation 5), the unit of voltage is [V], the unit of current is [A], and the unit of mass of hydrogel is [g]. The preferred gel pulverizing energy (GGE) used in this invention is 15 J / g or more, preferably 25 J / g or more, more preferably 40 J / g or more, further preferably 50 J / g or more, even more preferably 100 J / g or more, and most preferably 120 J / g or more. As an upper limit, it is acceptable as long as it is 200 J / g or less.

[0138] (Gel temperature)

[0139] From the viewpoint of continuously pulverizing the hydrogel-like crosslinked polymer at a temperature of 50°C or higher, the temperature T1 of the hydrogel-like crosslinked polymer (hereinafter also referred to as "gel temperature T1 at the inlet" or simply "gel temperature T1") of the polymer fed into the inlet of the pulverizing apparatus during the gel pulverizing process is preferably 50°C or higher. This gel temperature T1 is preferably measured using a thermometer installed at the inlet. From the viewpoint of preventing the hydrogels after pulverizing from adhering to each other, this gel temperature T1 is preferably 60°C or higher; from the viewpoint of further increasing the water absorption rate of the water-absorbing resin powder, it is more preferably 70°C or higher, and even more preferably 80°C or higher. From the viewpoint of suppressing excessive drying, the gel temperature T1 is preferably 130°C or lower, more preferably 110°C or lower, even more preferably 100°C or lower, and particularly preferably 90°C or lower. For the same reason, the gel temperature during pulverization is preferably 130°C or lower. It should be noted that, regarding the gel temperature T1, for the hydrogel-like crosslinked polymer fed into the gel crushing device, the hydrogel-like crosslinked polymer whose temperature rises due to the heat of polymerization can be kept warm or the obtained hydrogel-like crosslinked polymer can be heated, thereby adjusting it to the desired range.

[0140] From the viewpoint of suppressing the aggregation of hydrophilic gels after gel pulverization, the temperature T2 of the particulate hydrophilic gel crosslinked polymer discharged from the gel pulverization device (hereinafter referred to as "gel temperature T2 at the discharge port" or simply "gel temperature T2") is preferably 60°C to 140°C, more preferably 70°C to 130°C, further preferably 80°C to 125°C, 85°C to 120°C, particularly preferably 90°C to 115°C, and most preferably 100°C to 115°C. It is preferable to set the temperature T2 as this temperature range, and the temperature T1 as the aforementioned temperature range. The gel temperature T2 is preferably measured using a thermometer installed at the discharge port. It should be noted that the gel temperature T2 can be adjusted to a desired range by appropriately adjusting the set temperature of the heating unit and / or the heat preservation unit of the gel pulverization device, thereby adjusting the residence time of the hydrophilic gel crosslinked polymer inside the gel pulverization device.

[0141] From the viewpoint of further improving the water absorption rate of the absorbent resin powder, the gel temperature T2 at the outlet is preferably higher than the gel temperature T1 at the inlet. The difference ΔT = (T2 - T1) is preferably 5°C or more, more preferably 8°C or more, and even more preferably 10°C or more. Furthermore, the difference ΔT = (T2 - T1) is preferably 60°C or less, more preferably 50°C or less, even more preferably 40°C or less, and particularly preferably 35°C or less. It is preferable to set the difference ΔT = (T2 - T1) to this temperature range, and for temperatures T1 and T2 to be within the aforementioned temperature range. It should be noted that ΔT can be adjusted to the desired range as described above by adjusting T1 and T2 separately.

[0142] (Gel solids content)

[0143] In the gel pulverization process, the solid content of the hydrogel fed into the inlet of the gel pulverization apparatus (hereinafter referred to as the gel solid content) is determined by the measurement method described in the examples below. From the viewpoints of the degree of aggregation of the hydrogels after gel pulverization, the energy required for pulverization, the drying efficiency, and the absorption performance, the gel solid content is preferably 25% to 75% by mass, more preferably 30% to 70% by mass, even more preferably 35% to 65% by mass, and particularly preferably 40% to 60% by mass.

[0144] (Gel fluidizing agent)

[0145] In the manufacturing method described in this invention, it is preferable to add the gel flow agent before and / or during the gel pulverization process. This allows granular hydrogel containing the gel flow agent to be removed from the discharge port. The addition of the gel flow agent results in the following effects: inhibiting the strong adhesion or bonding of finely pulverized gel particles to each other, thus increasing the water absorption rate of the resulting superabsorbent resin. Furthermore, it reduces the load on the crushing steps in the pulverization and granulation processes following the drying process, thereby reducing the amount of fine powder generated. From the viewpoint that each particle of the resulting granular hydrogel uniformly contains the gel flow agent, addition during the gel pulverization process is more preferable, and addition simultaneously with the addition of the hydrogel is even more preferable.

[0146] The amount of gel flow agent added can be appropriately set according to the solid content of the hydrogel or granular hydrogel and the type of gel flow agent. The amount added relative to the solid content of the hydrogel is preferably 0.001% to 5% by mass, more preferably 0.01% to 3% by mass, further preferably 0.02% to 2% by mass, and particularly preferably 0.03% to 1% by mass.

[0147] Examples of gel flow agents include anionic, cationic, nonionic, and amphoteric surfactants; as well as their low-molecular-weight or high-molecular-weight surfactants, high-molecular-weight lubricants, etc. Among these, surfactants are preferred.

[0148] (surfactant)

[0149] Specifically, the surfactants used as gel flow agents include (1) nonionic surfactants such as sucrose fatty acid esters, polyglycerol fatty acid esters, dehydrated sorbitol fatty acid esters, polyoxyethylene dehydrated sorbitol fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropylene alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl glucoamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkylallyl ethers; and (2) octyl dimethylamine. Alkyl dimethylaminoacetic acid betaine, lauryl dimethylaminoacetic acid betaine, myristyl dimethylaminoacetic acid betaine, stearyl dimethylaminoacetic acid betaine, and other alkyl dimethylaminoacetic acid betaine; alkyl amamidopropyl betaine, coconut oil fatty acid amamidopropyl betaine, palm kernel oil fatty acid amamidopropyl betaine, and other alkyl amamidopropyl betaine; alkyl hydroxysulfonyl betaine, and other alkyl hydroxysulfonyl betaine; alkyl carboxymethyl hydroxyethyl imidazoline betaine, and other alkyl carboxymethyl hydroxyethyl imidazoline betaine, and other amphoteric surfactants; (3) monosodium laurylaminodiacetic acid, potassium laurylaminodiacetic acid, sodium myristylaminodiacetic acid, and other alkyl aminodiacetic acid monoalkali metal salts, and other anionic surfactants; (4) cationic surfactants such as long-chain alkyl dimethylaminoethyl quaternary ammonium salts, etc. Among these, two or more can be used in combination. From the viewpoint of further improving the water absorption rate of the water-absorbing resin powder, an amphoteric surfactant is preferred, and alkyl dimethyl aminoacetic acid betaine is more preferred.

[0150] (Polymer lubricant)

[0151] In the manufacturing method described in this invention, within the scope of achieving the purpose of this invention, the following exemplified polymeric lubricants can be added to the above-mentioned monomer aqueous solution or hydrogel.

[0152] Specifically, examples of the aforementioned polymeric lubricants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose, polyepoxides such as polyethylene glycol, and polysiloxanes modified with side chains and / or terminal polyethers. Their molecular weights (weight-average molecular weights) are preferably selected in the range of 2 million to 2 million, more preferably in the range of 4 million to 1 million. Two or more of these can be used in combination.

[0153] Furthermore, these polymeric lubricants and the aforementioned surfactants can be used in combination as gel flow agents. When using a combination of surfactants and polymeric lubricants, the total amount added is appropriately set according to the polymerization form, the composition of the monomer aqueous solution, and the water content of the hydrogel. When added to the monomer aqueous solution, the amount is set in terms of concentration relative to the monomer component; when added to the hydrogel, the amount is set in terms of concentration relative to its solid component; and when added to both, the amount is set in the total of the above.

[0154] The total amount of surfactant and polymeric lubricant added is preferably 5% by mass or less, more preferably 3% by mass or less, and more preferably 0.001% by mass or more, particularly preferably 0.01% by mass or more, relative to the solid component containing hydrogel.

[0155] (Surface tension)

[0156] The type and amount of gel flow agent can be appropriately adjusted considering factors such as the inhibition of aggregation of particulate hydrogels during the gel pulverization and drying processes. Based on factors such as the reflux rate of the resulting absorbent resin powder in actual use, a type and amount of gel flow agent that does not excessively reduce the surface tension of the absorbent resin in the final product is preferred. For example, the type and amount of gel flow agent are selected such that the surface tension of the absorbent resin (the surface tension of the absorbent resin dispersion in physiological saline) is preferably 55 mN / m or more, more preferably 60 mN / m or more, and even more preferably 65 mN / m or more. This surface tension is measured using the method described in WO2015 / 129917. Amphoteric surfactants can be cited as examples of gel flow agents that can keep the surface tension within this range.

[0157] (Other additives)

[0158] During the gel pulverization process or during the transition from the gel pulverization process to the subsequent drying process, the surface crosslinking agent described in [2-7-1] or other additives described in [2-10] may be added.

[0159] (Particle size of granular hydrogels)

[0160] In the manufacturing method described in this invention, from the viewpoint of the absorption rate of the manufactured hydrophilic resin powder, the mass-average particle size d1, calculated from the solid component of the granular hydrophilic gel crosslinked polymer discharged from the outlet of the gel pulverizer, is 3 mm or less. If d1 exceeds 3 mm, hydrophilic resin powder cannot be obtained because it cannot be supplied to subsequent processes (Experimental Example 5 described later). d1 is preferably 1 μm to 3 mm, more preferably 10 μm to 3 mm, further preferably 30 μm to 2 mm, even more preferably 50 μm to 1 mm, and particularly preferably 100 μm to 200 μm. The mass-average particle size d1, calculated from the solid component of the granular hydrophilic gel crosslinked polymer discharged from the outlet of the gel pulverizer, can be controlled based on, for example, the temperature of the hydrophilic gel crosslinked polymer during pulverization (controlled by, for example, the temperature inside the pulverizer body, the temperature of the water and / or water vapor supplied to the body), the minimum gap of the gel pulverizer relative to the maximum diameter of the disc, the feeding rate of the hydrophilic gel, the rotational speed of the rotating shaft of the gel pulverizer, and the gel breaking energy (GGE). It should be noted that the mass-average particle size d1 of the solid component of the granular hydrogel is specified according to the physical property determination methods (g) and (h) described later.

[0161] Furthermore, regarding the particle size distribution of the particulate hydrogel, the content of substances with a particle size of less than 150 μm, calculated based on the solid content, is preferably 10% by mass or more, more preferably 25% by mass or more, and even more preferably 40% by mass or more. Additionally, regarding the particle size distribution of the particulate hydrogel, the content of substances with a particle size of less than 850 μm, calculated based on the solid content, is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and up to a maximum of 100% by mass. The logarithmic standard deviation (σζ) of the particle size distribution is 0.2 to 1.0, more preferably 0.2 to 0.8, and even more preferably 0.2 to 0.7.

[0162] (Solid content of granular hydrogels)

[0163] The solid content of the granular hydrogel discharged from the outlet of the gel pulverizer is preferably 25% to 75% by mass, more preferably 30% to 70% by mass, even more preferably 35% to 65% by mass, and particularly preferably 40% to 60% by mass. By feeding the granular hydrogel with a solid content in the above range into the drying process, a granular dried product with high CRC and minimal damage caused by drying (such as an increase in water-soluble components) is obtained.

[0164] (Polymerization rate of granular hydrogels)

[0165] The polymerization rate of the granular hydrogel discharged from the gel pulverizing apparatus is within the range of the polymerization rate before it was fed into the gel pulverizing apparatus, and further polymerization can be carried out in the gel pulverizing process. The degree of polymerization is appropriately adjusted by the heating and residence time in the gel pulverizing apparatus, the residual amount of polymerization initiator in the polymerized hydrogel, and the optional amount of post-addition of polymerization initiator. The polymerization rate after the gel pulverizing process is specified using the same physical property determination method as before gel pulverization. The polymerization rate of the granular hydrogel after gel pulverization is 90% by mass or more, preferably 95% by mass or more, more preferably 98 to 99.99% by mass, and ideally 100%. Granular hydrogels with polymerization rates within the above range can avoid aggregation and adhesion during drying.

[0166] [2-5] Drying process

[0167] This process involves drying a particulate hydrogel crosslinked polymer, preferably a particulate hydrogel crosslinked polymer containing a gel flow agent, to a desired solids content to obtain a dried product. The "solids content" refers to the value calculated from the drying loss (the change in mass when 1.0 g of the sample is dried at 180°C for 3 hours).

[0168] The solid content of the dried product obtained after the drying process is preferably 80% by mass or more, more preferably 85% to 99.8% by mass, further preferably 90% to 99.7% by mass, even more preferably 92% to 99.5% by mass, particularly preferably 96% to 99.5% by mass, and extremely preferably 98% to 99.5% by mass. If the solid content after drying is too high, not only will a longer drying time be required, but deterioration of physical properties and discoloration may also occur. In addition, if the solid content after drying is low, the productivity in the granulation process described later may decrease, and the water absorption ratio (CRC) may decrease. When the surface crosslinking process described later is performed after the drying process, the physical properties are further improved by drying to the above-mentioned solid content, which is therefore preferred. It should be noted that the moisture content of the dried product (=100-solid content) is calculated based on the above-mentioned solid content.

[0169] The drying method used in the drying process of this invention is not particularly limited, and static drying, stirring drying, fluidized bed drying, etc., can be appropriately used. In addition, various drying methods such as heating drying, hot air drying, reduced pressure drying, infrared drying, microwave drying, drum dryer drying, azeotropic dehydration drying with hydrophobic organic solvents, and high-humidity drying using high-temperature steam can be used.

[0170] (Drying device)

[0171] The drying equipment used in the drying process is not particularly limited, and one or more types of dryers, such as thermally conductive dryers, radiant thermally conductive dryers, hot air thermally conductive dryers, and induction heating dryers, can be appropriately selected. It can be intermittent or continuous. Furthermore, it can be directly heated or indirectly heated. Examples of thermally conductive dryers include, for example, ventilated belt dryers, ventilated circuit dryers, ventilated vertical dryers, parallel flow belt dryers, ventilated tunnel dryers, ventilated stirred dryers, ventilated rotary dryers, fluidized bed dryers, and airflow dryers.

[0172] The heating unit is not particularly limited. From the viewpoint of drying efficiency and reducing heat damage to the absorbent resin, the heating unit for the granular hydrogel is preferably a heating unit based on direct heat conduction and / or indirect heat conduction. The direct heat conduction is based on convective heat conduction, and the indirect heat conduction is based on heat transfer from the heating surface (the surface in contact with the granular hydrogel, the heat source portion) of the dryer heated by the heat medium. More preferably, the heating unit is a ventilated heating type for direct heat conduction and an external wall heating type or a tubular heating type for indirect heat conduction.

[0173] In the drying process, gas can be introduced into the dryer. There are no particular limitations on the gas; examples include air, drying gas, nitrogen, water vapor, and mixtures thereof. The gas acts as a carrier gas, promoting drying by expelling the water vapor generated during drying outside the dryer. Furthermore, when a heating gas is used, the gas also acts as a heat medium, further promoting drying. Nitrogen, water vapor, and mixtures thereof with air are preferred. When using a mixture containing water vapor (hereinafter also referred to as a high-humidity mixture), the interior of the dryer is in a low-oxygen state, suppressing oxidation and deterioration during drying. As a result, improved performance and reduced coloring of the absorbent resin can be achieved. Furthermore, the direction of gas movement relative to the direction of movement of the particulate hydrogel being dried can be co-current or convective, and they can also be mixed.

[0174] The drying conditions are appropriately selected based on the type of drying equipment and the solid content of the granular hydrogel. The drying temperature is preferably 100°C to 300°C, more preferably 150°C to 250°C, further preferably 160°C to 220°C, and particularly preferably 170°C to 200°C. Below these ranges, the drying time becomes excessively long, which is uneconomical. Above these ranges, the physical properties of the absorbent resin deteriorate and significant discoloration occurs, which is not preferred. Furthermore, the drying time is preferably 1 minute to 10 hours, more preferably 5 minutes to 2 hours, further preferably 10 minutes to 120 minutes, and particularly preferably 20 minutes to 60 minutes. Below these ranges, the drying temperature needs to be excessively increased, resulting in deterioration of the absorbent resin's physical properties and significant discoloration, which is not preferred. Above these ranges, the dryer becomes excessively large or the throughput decreases, which is uneconomical.

[0175] [2-6] Crushing and grading processes

[0176] This process involves pulverizing and / or classifying the dried material obtained through the above-described drying process, preferably to obtain a water-absorbing resin powder of a specific particle size. It should be noted that this differs from the gel pulverization process described in (2-4) in that the material being pulverized has undergone a drying process.

[0177] This process is performed before and / or after the surface crosslinking process in [2-7], preferably before the surface crosslinking process in [2-7], and can be performed at least twice before and after the surface crosslinking process in [2-7].

[0178] Examples of high-speed rotary pulverizers such as roller mills, hammer mills, screw mills, and pin mills, as well as vibratory mills, knuckle pulverizers, and barrel agitators, can be used in combination as needed.

[0179] (granularity)

[0180] From the perspective of water absorption rate and water absorption ratio under pressure, the weight average particle size (D50) of the water-absorbing resin powder before surface crosslinking is preferably 200 μm or more, more preferably 200 μm to 600 μm, even more preferably 250 μm to 550 μm, and particularly preferably 300 μm to 500 μm.

[0181] In addition, the lower the content of particles with a particle size of less than 150 μm as specified by standard sieve classification, the better. It is preferably 0 to 5% by weight, more preferably 0 to 3% by weight, and even more preferably 0 to 2% by weight, relative to the total amount of water-absorbing resin powder.

[0182] Furthermore, the fewer coarse particles with a particle size of 850 μm or more as specified by standard sieve grading, the better. From the viewpoint of water absorption rate, the percentage is preferably 0 to 5% by weight, more preferably 0 to 3% by weight, and even more preferably 0 to 1% by weight relative to the total water-absorbing resin powder.

[0183] In addition, considering factors such as water absorption rate and water absorption ratio under pressure, the proportion of particles with a particle size of 150 μm or more and less than 850 μm is preferably 90% by weight or more, more preferably 95% by weight or more, even more preferably 98% by weight or more, and particularly preferably 99% by weight or more (up to 100% by weight).

[0184] [2-7] Surface crosslinking process

[0185] This step involves adding a surface crosslinking agent to the water-absorbing resin powder obtained through the crushing and grading processes [2-6], causing it to undergo a crosslinking reaction. This step is also known as the post-crosslinking step. In the manufacturing method of this invention, in this step, a surface crosslinking agent is added to the water-absorbing resin powder, followed by heat treatment to induce a crosslinking reaction. This step includes a surface crosslinking agent addition step and a heat treatment step, and a cooling step may be added after the heat treatment step as needed.

[0186] [2-7-1] Surface crosslinking agent addition process

[0187] This process involves preparing a water-absorbing resin powder containing a surface crosslinking agent, which is then supplied to the surface crosslinking process, by mixing the aforementioned water-absorbing resin powder with a surface crosslinking agent.

[0188] (Surface crosslinking agent)

[0189] As the aforementioned surface crosslinking agent, a surface crosslinking agent capable of reacting with multiple functional groups (preferably multiple carboxyl groups) of the water-absorbing resin can be used. Preferably, a surface crosslinking agent capable of forming covalent or ionic bonds, and thus capable of forming covalent bonds, can be used. Specifically, examples include 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, glycerol, polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanediol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, ethylene oxide-propylene oxide block copolymer, pentaerythritol, sorbitol, and other polyol compounds; ethylene glycol... Epoxy compounds including diglycidyl alcohol ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol polyglycidyl ether, glycidyl, sorbitol polyglycidyl ether, pentaerythritol polyglycidyl ether, trimethylolpropane polyglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, etc.; polyamine compounds including ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, etc., and their inorganic or organic salts; 2,4-toluene diisocyanate. Polyisocyanate compounds such as cyanate esters and hexamethylene diisocyanate; aziridine compounds such as polyaziridine; polyoxazoline compounds such as 1,2-ethylene bisoxazoline, bisoxazoline, and polyoxazoline; carbonate derivatives such as urea, thiourea, guanidine, dicyandiamide, and 2-oxazolineone; ethylene carbonate (1,3-dioxolane-2-one), 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, and 4-hydroxymethyl-1,3-dioxolane-2-one. Hydrocarbonyl ester compounds such as oxopentane-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, and 1,3-dioxopentane-2-one; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepicochlorohydrin, and their polyamine adducts; oxacyclobutane compounds; silane coupling agents such as γ-epoxypropoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; and polyvalent metal compounds such as hydroxides, chlorides, sulfates, nitrates, or carbonates of zinc, calcium, magnesium, aluminum, iron, zirconium, etc. Two or more of these can be used in combination. Among the above-mentioned surface crosslinking agents, one or more are preferably selected from polyvalent metal ions, epoxy compounds, oxazoline compounds, and hydrocarbon ester compounds.

[0190] (Surface crosslinking agent solution)

[0191] The amount of the aforementioned surface crosslinking agent added relative to the solid content of the water-absorbing resin is preferably 5% by mass or less, more preferably 3% by mass or less, further preferably 2% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0.1% by mass or less. Furthermore, as a lower limit, it is preferably 0.001% by mass or more, more preferably 0.01% by mass or more.

[0192] The surface crosslinking agent described above can be added in its original form, but for ease of addition, it is preferred to add it as a solution obtained by dissolving it in water or an organic solvent. The concentration of this surface crosslinking agent solution is preferably 1% by mass or more, more preferably 2% by mass or more. The total amount of solvent selected from water and organic solvents is preferably 0 to 10% by mass relative to the solid content of the water-absorbing resin, more preferably 0.1% to 8% by mass, and even more preferably 0.5% to 5% by mass. When water and organic solvents are used in combination, water is preferably the main component.

[0193] When added in the form of an aqueous solution, the concentration of the aqueous solution can be adjusted according to the water content of the water-absorbing resin powder at the moment of contact with the surface crosslinking agent, which is therefore preferable. If a solution is not formed due to the low solubility of the surface crosslinking agent relative to water, it is preferable to add a suitable hydrophilic solvent such as an alcohol to prepare a homogeneous solution.

[0194] [2-6-2] Heat treatment process

[0195] This process involves heating water-absorbing resin powder containing a surface crosslinking agent to obtain a dried product with surface crosslinking.

[0196] (Surface crosslinking temperature)

[0197] In this process, a water-absorbing agent is obtained by heating a water-absorbing resin powder containing a surface crosslinking agent to above 100°C. The preferred maximum temperature varies depending on the type of surface crosslinking agent, ranging from 100°C to 250°C, more preferably from 120°C to 230°C, and even more preferably from 150°C to 210°C.

[0198] (time)

[0199] The heat treatment time can be appropriately set according to the moisture content of the granular dried material, the type of surface crosslinking agent, and the thermal efficiency of the heating device. As a general reference, heating until the moisture content is below 10% by mass is sufficient, and the time ranges from 10 minutes to 120 minutes, preferably from 30 minutes to 90 minutes.

[0200] (Heating mode)

[0201] There are no particular limitations on the heating device used in the surface crosslinking process. From the viewpoint of minimizing uneven heating, it is suitable to use a heating device with a stirring mechanism that is based on solid-to-solid heat conduction.

[0202] [2-7] Cooling process

[0203] Preferably, after the aforementioned drying or surface cross-linking process and before the subsequent granulation process, a cooling process is included to forcibly cool the dried material or the surface-cross-linked dried material and adjust it to a desired temperature. The cooling process can be performed using existing, known cooling methods. Furthermore, the cooling temperature can be appropriately adjusted.

[0204] [2-8] Granulation process

[0205] This process involves adjusting the particle size of the surface-crosslinked dried material. Through this granulation process, a water-absorbing resin powder with more actively controlled particle size or particle size distribution is obtained.

[0206] The granulation process preferably includes a crushing step and / or a grading step. The crushing step is a step of breaking down and adjusting the particle size of the slowly aggregated granular dried material that has undergone the surface treatment process using a crusher. The grading step is a step of removing coarse particles and fine powder from the surface-crosslinked granular dried material or its crushed form using a classifier.

[0207] The crusher is not particularly limited, and examples include vibratory mills, roller granulators, hook-claw crushers, roller mills, high-speed rotary crushers (pin mills, hammer mills, screw mills), and barrel agitators. Crushers that cause minimal damage to dried materials or surface-crosslinked dried materials are preferred. Specifically, examples include roller granulators (MATSUBO CORPORATION), granulators (Kurimoto, Ltd.), and circular mills (TOKUJU CORPORATION). As a classifier, vibrating or shaking sieve classifiers using screens are used.

[0208] [2-9] Micro powder recycling process

[0209] "Micronized powder reuse process" refers to a process in which micronized powder removed by the grading step is directly supplied to any process or supplied to any process after granulation. Preferably, it is a process that reuses micronized powder or micronized powder granules before the drying process. Examples of processes before the drying process include monomer aqueous solutions before polymerization, hydrogels during polymerization, pulverization processes of hydrogels after polymerization, and drying processes of granular hydrogels. Micronized powder can be added directly to these processes, or it can be added after swelling, gelling, and / or granulating the micronized powder with water. Additionally, water, crosslinking agents, binders other than water (e.g., water-soluble polymers, thermoplastic resins), polymerization initiators, reducing agents, chelating agents, and anti-coloring agents can be added together with the micronized powder.

[0210] The optimal amount of micronized powder recovered is set appropriately based on the target particle size.

[0211] [2-10] Other processes

[0212] In addition to the above-mentioned processes, the manufacturing method of the present invention may further include, as needed, a crushing process, a grading process, a rewetting process, a granulation process, a transportation process, a storage process, a packaging process, and a preservation process.

[0213] (Other additives)

[0214] In addition to the surface crosslinking agents and gel flow agents that can be used as optional additives, other known components such as polymer powders (e.g., starch such as cassava acetate starch), inorganic microparticles, anti-dust agents, dried water-absorbing resins (micropowder), liquid permeability improvers, and reducing agents (e.g., sodium sulfite) can also be added before or after drying.

[0215] [3] Physical properties of the water-absorbing resin powder as an article

[0216] Regarding the absorbent resin powder (especially the surface-crosslinked absorbent resin powder is also referred to as an absorbent agent) obtained by the manufacturing method described in this invention, when using the absorbent resin powder or absorbent agent in absorbent articles, especially in diapers, it is ideal that at least one, preferably two, more preferably three, and even more preferably all of the physical properties shown in (3-1) to (3-7) below are controlled within the desired range. If all the physical properties do not meet the following ranges, the effects of this invention cannot be fully obtained, especially if the amount of absorbent agent used in each diaper is high, and so-called high-concentration diapers may not be able to fully exert their performance.

[0217] [3-1] CRC (Centrifuge Retention Capacity)

[0218] The CRC (centrifuge retention capacity) of the water-absorbing resin powder (water absorbent) of the present invention is generally 5 g / g or more, preferably 15 g / g or more, and more preferably 25 g / g or more. There is no particular limitation on the upper limit, but a higher CRC is preferred. From the viewpoint of balancing with other physical properties, it is preferably 70 g / g or less, more preferably 50 g / g or less, and even more preferably 40 g / g or less.

[0219] When the CRC (Chemical Reabsorption Rate) is less than 5 g / g, the absorbency is low, making it unsuitable as an absorbent material for absorbent products such as diapers. Furthermore, when the CRC exceeds 70 g / g, the absorption rate of bodily fluids such as urine and blood decreases, making it unsuitable for use in high-absorbency diapers. It should be noted that the CRC can be controlled by changing the type and amount of internal cross-linking agents and surface cross-linking agents.

[0220] [3-2] Moisture content and solid content

[0221] The surface-crosslinked water-absorbing resin powder (water absorbent) preferably has a moisture content of more than 0% by mass and less than 20% by mass, more preferably 1% to 15% by mass, even more preferably 2% to 13% by mass, and particularly preferably 2% to 10% by mass. By setting the moisture content within the above range, a water absorbent with excellent powder properties (e.g., flowability, handling, damage resistance, etc.) is obtained. Furthermore, the solid content of the surface-crosslinked water-absorbing resin powder (water absorbent) is preferably 80% by mass or more, more preferably 85% to 99% by mass, even more preferably 87% to 98% by mass, and particularly preferably 90% to 98% by mass.

[0222] [3-3] Particle size

[0223] The weight-average particle size d3 (D50) of the water-absorbing resin powder (water absorbent) is preferably 200 μm or more, more preferably 200 μm to 600 μm, even more preferably 250 μm to 550 μm, and particularly preferably 300 μm to 500 μm. Furthermore, the proportion of particles with a particle size less than 150 μm is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less. Additionally, the proportion of water-absorbing resin particles with a particle size greater than 850 μm is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less. This water absorbent preferably contains 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, and particularly preferably 99% by mass or more of particles with a particle size of 150 μm to 850 μm. Ideally, it contains 100% by mass. The logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 to 0.50, more preferably 0.25 to 0.40, and even more preferably 0.27 to 0.35.

[0224] [3-4] AAP (Amount Absorption Ratio under Pressure)

[0225] The AAP (average absorbency under pressure) of the water-absorbing resin powder (water absorbent) is preferably 15 g / g or more, more preferably 20 g / g or more, further preferably 23 g / g or more, particularly preferably 24 g / g or more, and most preferably 25 g / g or more. There is no particular limitation on the upper limit, but it is preferably 30 g / g or less.

[0226] When the AAP level is less than 15 g / g, the liquid re-flow rate (sometimes called "Re-Wet") increases when pressure is applied to the absorbent, making it unsuitable as an absorbent for absorbent products such as diapers. It should be noted that the AAP level can be controlled by adjusting the particle size and changing the surface crosslinking agent.

[0227] [3-5] Vortex (water absorption rate)

[0228] The Vortex (water absorption rate) of the water-absorbing resin powder (water absorbent) is preferably 65 seconds or less, more preferably 60 seconds or less, further preferably 50 seconds or less, even more preferably 40 seconds or less, particularly preferably 30 seconds or less, and most preferably 25 seconds or less. There is no particular limitation on the lower limit, but it is preferably 5 seconds or more, more preferably 10 seconds or more.

[0229] By setting the Vortex to the aforementioned range, a specified amount of liquid can be absorbed in a short time. When used in absorbent materials such as diapers, users experience less skin dampness, reducing discomfort and leakage.

[0230] [4] Uses of water-absorbing resin powder (water absorbent)

[0231] The uses of absorbent resin powder (absorbent agent) are not particularly limited, but its preferred applications include absorbent bodies for absorbent products such as diapers, sanitary napkins, and incontinence pads. In particular, it can be used as an absorbent body for high-absorbency diapers. Furthermore, the absorbent agent has excellent absorption time and controlled particle size distribution; therefore, significant effects can be expected when used in the upper layer of the aforementioned absorbent bodies.

[0232] Alternatively, absorbent materials such as pulp fibers can be used as raw materials for the absorbent, together with the absorbent agent. In this case, the content (core concentration) of the absorbent agent in the absorbent is preferably 30% to 100% by mass, more preferably 40% to 100% by mass, even more preferably 50% to 100% by mass, even more preferably 60% to 100% by mass, particularly preferably 70% to 100% by mass, and most preferably 75% to 95% by mass.

[0233] By setting the core concentration to the aforementioned range, when the absorbent is used in the upper part of an absorbent article, the absorbent article can be maintained in a clean, white state. Furthermore, the absorbent exhibits excellent diffusion properties with bodily fluids such as urine and blood; therefore, it is estimated that its absorption capacity is increased through effective liquid distribution.

[0234] Example

[0235] The present invention will be described in more detail by way of the following experimental examples, but the present invention is not limited to these descriptions. Experimental examples obtained by appropriately combining the technical means disclosed in each experimental example are also included within the scope of the present invention.

[0236] It should be noted that the term "hygroscopic resin" as used below refers to granular dried material that has undergone a drying process, granular dried material that has undergone surface cross-linking, or hygroscopic resin powder, as well as hygroscopic resin powder that has undergone surface cross-linking. "Hydrogel" refers to hydrogel-like cross-linked polymer or granular hydrogel-like cross-linked polymer that has not undergone a drying process.

[0237] In addition, unless otherwise specified, the electrical equipment used in the experimental examples (including the equipment for measuring the physical properties of water-absorbing resins) used a 60Hz power supply of 200V or 100V. Furthermore, unless otherwise specified, the physical properties of the water-absorbing resins and hydrogels described below were measured at room temperature (20℃~25℃) and a relative humidity of 50%RH±10%.

[0238] Additionally, for convenience, "liter" is sometimes expressed as "l" or "L", and "mass%" or "weight%" is expressed as "wt%". When determining trace components, the limit of detection is sometimes expressed as ND (Non Detected).

[0239] [Methods for determining physical properties]

[0240] (a) CRC (Centrifuge Retention Capacity) of water-absorbing resin powder

[0241] The CRC (centrifuge hold-up capacity) of the absorbent resin powder was determined according to the EDANA method (ERT441.2-02).

[0242] (b) Moisture content and solid content of the water-absorbing resin powder

[0243] The moisture content of the superabsorbent resin powder was determined according to the EDANA method (ERT430.2-02). It should be noted that during the determination, the sample mass was changed to 1.0 g, and the drying temperature was changed to 180℃. The moisture content and solid content of the superabsorbent resin powder were calculated based on the loss on drying after 3 hours.

[0244] (c) Mass-average particle size (d3) of water-absorbing resin powder

[0245] The mass-average particle size (d3) of the absorbent resin powder was determined according to the method described in columns 27 and 28 of U.S. Patent No. 7,638,570.

[0246] (d) Polymerization rate of hydrogels

[0247] Add 1.00 g of the sampled hydrogel to 1000 g of ion-exchanged water at room temperature (at this point, the polymerization reaction is essentially stopped), stir at 300 rpm for 2 hours, and then remove insoluble components by filtration. Use liquid chromatography to determine the amount of monomer extracted from the filtrate obtained through the above operation. Let this amount of monomer be the residual monomer amount m (g), and calculate the polymerization rate C (mass %) of the hydrogel according to the following formula (Equation 8). Furthermore, it is preferable to determine the polymerization rate immediately after sampling the hydrogel. If the time from sampling to measurement is long, forced cooling (contact with dry ice, liquid nitrogen, ice water, etc.) is required to stop the polymerization operation.

[0248] C (mass%) = 100 × {1 - m / (M × α / 100)} … (Equation 8)

[0249] In Equation 8, M refers to the mass (g) of the hydrogel and α refers to the solid content (mass%) of the hydrogel.

[0250] (e) Solid content of hydrogels

[0251] The water content of the hydrogel was determined according to the EDANA method (ERT430.2-02). It should be noted that during the determination, the sample mass was changed to 2.0 g, the drying temperature was changed to 180 °C, and the drying time was changed to 24 hours. Specifically, 2.0 g of the hydrogel was added to an aluminum cup with a bottom diameter of 50 mm, and the total mass W1 (g) of the sample (hydrogel and aluminum cup) was accurately weighed. Then, the sample was placed in an oven with an atmosphere temperature set to 180 °C. After 24 hours, the sample was removed from the oven, and the total mass W2 (g) was accurately weighed. Taking the mass of the hydrogel used in this determination as M (g), the solid content α (mass%) of the hydrogel was calculated according to the following formula (Equation 10).

[0252] α (mass%) = 100 - {(W1-W2) / M} × 100… (Equation 10).

[0253] (f) Particle size of granular hydrogels

[0254] The mass-average particle size (D50) of the particulate hydrogel was determined according to the method described in WO2016 / 204302.

[0255] That is, 20g of hydrogel (solid content α (mass%)) at a temperature of 20-25°C is added to 1000g of a 20% sodium chloride aqueous solution (hereinafter referred to as "EMAL aqueous solution") containing 0.08% EMAL 20C (surfactant, manufactured by Kao Corporation) to prepare a dispersion. The dispersion is stirred at 300rpm for 16 hours using a rotor blade with a length of 50mm and a diameter of 7mm (using a cylindrical polypropylene container with a height of 21cm and a diameter of 8cm, approximately 1.14L).

[0256] After stirring, the above dispersion was added to the center of a JIS standard sieve (inner diameter: 20cm, mesh size: 8mm / 4mm / 2mm / 1mm / 0.60mm / 0.30mm / 0.15mm / 0.075mm) placed on a rotating disc. After washing all the hydrogel onto the sieve with 100g of EMAL aqueous solution, 6000g of EMAL aqueous solution was used. While rotating the sieve by hand (20rpm), a sprayer (72 holes, flow rate: 6.0L / min) was used from a height of 30cm above the top, spraying water over a 50cm area. 2 Water is evenly injected throughout the entire sieve, and this operation is repeated four times to classify the hydrogel. The hydrogel on the first-stage sieve is drained of water for approximately 2 minutes and then weighed. The same process is repeated for the second and subsequent sieves, and the hydrogel remaining on each sieve is weighed after draining the water. It should be noted that the type of sieve used should be adjusted according to the particle size of the hydrogel. For example, if the hydrogel particles are small and sieves with a mesh size of 0.15mm or 0.075mm become clogged, a larger diameter JIS standard sieve (30cm in diameter, with mesh sizes of 0.15mm and 0.075mm) should be used.

[0257] Based on the mass of the hydrogel remaining on each sieve, the proportion (mass%) of all hydrogels is calculated using the following formula (11). After water removal, the mesh size of the sieves is plotted on logarithmic probability paper according to the following formula (12). The cumulative %R on the plotted sieves, corresponding to 50% by mass, is defined as the mass-average particle size (D50) of the hydrogel with a solid content α (mass%).

[0258] X(%)=(w / W)×100 … (Equation 11)

[0259] R(α)(mm)=(20 / W) 1 / 3 ×r … (Equation 12)

[0260] Additionally, here...

[0261] X: Mass percentage (%) of hydrogel remaining on each sieve after grading and water control.

[0262] w: The mass (g) of the hydrogel remaining on each sieve after grading and water control.

[0263] W: Total mass (g) of hydrogel remaining on each sieve after grading and water control.

[0264] R(α): Mesh size (mm) of the sieve when converted to a hydrogel with a solid content percentage α (mass%).

[0265] r: The mesh size (mm) of a grading sieve after the hydrogel swells in a 20% by mass sodium chloride aqueous solution.

[0266] (g) Mass-average particle size (d1) calculated from the solid components of granular hydrogels.

[0267] According to WO2016 / 204302, based on the solid content percentage α (mass%) of the granular hydrogel obtained in (e) above and the mass-average particle size (D50) of the hydrogel with the solid content percentage α (mass%) obtained in (f) above, the particle size converted from the solid content of the granular hydrogel (mass-average particle size obtained by converting the solid content into the dried granular hydrogel) d1 is calculated according to the following (Equation 14).

[0268] SolidD50(d1)=GelD50×(α / 100) 1 / 3 … (Equation 14)

[0269] Additionally, here...

[0270] GelD50: The mass-average particle size (μm) of particulate hydrogels with a solid content percentage α (mass%).

[0271] α: Solid content (mass%) of granular hydrogel

[0272] SolidD50(d1): Mass-average particle size (μm) of the solid component in hydrogel.

[0273] (h) Vortex (water absorption time) of water-absorbing resin powder

[0274] The Vortex (water absorption time) of the water-absorbing resin powder was determined according to the following steps. First, 0.02 parts by weight of edible blue No. 1 (Brilliant Blue), a food additive, was added to 1000 parts by weight of pre-prepared physiological saline (0.9% sodium chloride aqueous solution), and the liquid temperature was adjusted to 30°C.

[0275] Next, 50 ml of the above-mentioned physiological saline was measured into a 100 ml beaker. While stirring with a rotor blade of 40 mm in length and 8 mm in diameter at 600 rpm, 2.0 g of water-absorbing resin powder was added. The time from the addition of the water-absorbing resin powder to the point where the water-absorbing resin powder absorbs the physiological saline and covers the rotor blade was defined as Vortex (water absorption time) (unit: seconds), and was measured.

[0276] (i) AAP (Absorbency Ratio under Pressure) of water-absorbing resin powder

[0277] The AAP (water uptake ratio under pressure) of the water-absorbing resin powder was determined according to the EDANA method (ERT442.2-02). It should be noted that the loading condition was changed to 4.83 kPa (0.7 psi) during the determination.

[0278] (j) Average stay time

[0279] The average residence time (in seconds) of the hydrogel in the gel pulverizer is determined using the following method.

[0280] First, in Manufacturing Example 1 described later, 1% by mass of Blue No. 1 was added to the monomer aqueous solution, and polymerization was carried out in the same manner to produce a blue-colored hydrogel. Next, using an uncolored hydrogel, it was fed into a gel pulverizer at a predetermined feeding rate and allowed to operate stably. Without changing the feeding rate of the hydrogel, the blue-colored hydrogel was fed for 2 ± 5 seconds, and then the uncolored hydrogel was fed at the same rate to continue gel pulverization. The moment the blue hydrogel was first fed was designated as 0 seconds, and granular hydrogel samples were taken from the gel pulverizer every 5 seconds.

[0281] 15g of the sampled hydrogel was placed into a zippered polyethylene bag (size A, manufactured in Japan, 70mm long, 50mm wide, 0.04mm thick). A 15kg weight with a square base of 80mm x 80mm was placed on the bag for 5 seconds to form a sheet. Care was taken to avoid trapping air inside the bag. Next, using a spectrophotometer SZ-Σ80COLORMEASURING SYSTEM (manufactured by Nippon Denshoku Kogyo Co., Ltd.), under reflectance measurement / standard white board No. 1 / 30φ floodlight conditions, the b-value of the resulting sheet sample was measured. Five measurements were performed for each sample, and the average value was calculated. During each measurement, the sample shape was adjusted by placing the weight. The b-value was also calculated for the hydrogel sampled every 5 seconds. The more blue the hydrogel, the smaller the b-value (less than 0 and with a large absolute value). The sampling time of the most blue (smallest b-value) hydrogel was set as the average residence time (minutes). It should be noted that when performing multiple gel pulverization processes, the average residence time of each process is measured and the sum of these is taken as the average residence time (minutes).

[0282] [Manufacturing Example 1]

[0283] Prepare a monomer aqueous solution comprising 300 parts by weight of acrylic acid, 100 parts by weight of 48% sodium hydroxide aqueous solution, 0.61 parts by weight of polyethylene glycol diacrylate (average n number is 9), 16.4 parts by weight of 0.1% trisodium diethylenetriaminepentaacetate aqueous solution, and 273.2 parts by weight of deionized water.

[0284] Next, the monomer aqueous solution, with its temperature adjusted to 38°C, was continuously supplied using a metering pump, and then 150.6 parts by mass of a 48% sodium hydroxide aqueous solution was continuously mixed via pipeline mixing. Meanwhile, the liquid temperature of the monomer aqueous solution rose to 87°C due to the heat of neutralization.

[0285] Subsequently, after continuously mixing 14.6 parts by mass of a 4% sodium persulfate aqueous solution via pipeline mixing, the monomer aqueous solution was continuously supplied to a continuous polymerizer having a planar polymerization belt with baffles at both ends at a thickness of 10 mm. Polymerization was then carried out continuously for 3 minutes to obtain a strip-shaped (sheet-shaped) hydrogel crosslinked polymer (1a). The obtained strip-shaped hydrogel (1a) was cut according to the processing speed and feeding interval in the gel pulverizer described later to obtain short strip-shaped hydrogels (1b) with a width of several cm. For example, when the processing speed of the gel pulverizer was set to 0.64 kg / min and the short strip-shaped hydrogels were fed at 2.5-second intervals, the mass of each short strip-shaped hydrogel was set to 0.0267 kg. Furthermore, the polymerization rate of the short strip-shaped hydrogel (1b) was 98.5% by mass, and the solid content was 53% by mass.

[0286] [Experimental Example 1]

[0287] <Gel Pulverization>

[0288] As a gel pulverizing device, a twin-screw mixer with a main body (bucket) is used, wherein the main body has two rotating shafts that rotate in the same direction to pulverize short strip-shaped hydrogels (1b). Each rotating shaft is provided with a circular plate-shaped disk that mainly serves as a pulverizing unit. The bucket has a jacketed structure and a gas inlet that penetrates the jacket and introduces water vapor into the main body.

[0289] First, a 60°C heat transfer medium is circulated within the jacket to maintain the internal temperature of the main body (bucket) at 60°C. Then, the rotation speed is set to 40 rpm, and a short strip of hydrogel (1b) heated to 60°C is fed into the inlet of a twin-screw mixer at a rate of 0.64 kg / min. Simultaneously, 60°C water and hydrogel (1b) are supplied from the inlet, and then 0.6 MPa of water vapor is supplied from the gas inlet. The amount of water supplied at 60°C is 11.8% by mass relative to the solid content of the short strip of hydrogel (1b). The amount of water vapor supplied at 0.6 MPa is 9.7% by mass relative to the solid content of the short strip of hydrogel (1b). The diameter D of the discs used in the gel pulverization is as described in Table 1, and the minimum gap between the bucket and the disc is 6 mm (15% of the disc diameter D). The gel pulverization conditions are shown in Table 1. The characteristics of the granular hydrogel (A) obtained from the pulverization are shown in Table 2. It should be noted that the GGE during gel pulverization was 19 J / g.

[0290] <Drying / Surface Treatment>

[0291] The obtained granular hydrogel (A) was dried using a hot air dryer. The dryer consisted of a cage (30cm x 20cm at the bottom) made of a 1.2mm mesh metal mesh. 500g of the granular hydrogel (A) was spread approximately evenly on the bottom surface of the cage, and hot air at 190°C was blown from below for 30 minutes to obtain the dried product. The cooled dried product was then fed to a roller mill for pulverization, and classified using JIS standard sieves with mesh sizes of 850μm and 150μm. The fraction that passed through the 850μm sieve but not the 150μm sieve was collected to obtain the hydroabsorbent resin powder (AP1).

[0292] Next, 100 parts by weight of the water-absorbing resin powder (AP1) were sprayed with a surface crosslinking agent solution containing 0.025 parts by weight of ethylene glycol diglycidyl ether, 0.3 parts by weight of ethylene carbonate, 0.5 parts by weight of propylene glycol, and 2.0 parts by weight of deionized water, and mixed. This mixture was then heat-treated at 200°C for 35 minutes to obtain surface-crosslinked water-absorbing resin powder (AP2). The physical properties of the surface-crosslinked water-absorbing resin powder (AP2) are shown in Table 3.

[0293] [Experimental Example 2]

[0294] The minimum gap between the bucket and the pan was changed to 2 mm (4.16% of the pan diameter D), and otherwise the same procedure was followed as in Experimental Example 1 to obtain granular hydrogel (B). The pan configuration pattern was set to be the same as in Experimental Example 1, but the GGE during gel pulverization was 41 J / g. The gel pulverization conditions are shown in Table 1. The properties of the granular hydrogel (B) obtained by pulverization are shown in Table 2.

[0295] For the above-mentioned particulate hydrogel (B), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (BP1) and surface-crosslinked water-absorbing resin powder (BP2). The physical properties are shown in Table 3.

[0296] [Experiment Example 3]

[0297] The heating temperature of the short strip hydrogel (1b) (the gel temperature T1 introduced into the inlet of the gel pulverizer) was changed to 80°C. A 10% (w / w) aqueous solution of lauryl dimethylaminoacetic acid betaine was simultaneously supplied from the inlet along with the short strip hydrogel (1b). The temperature of the supplied water was changed to 90°C, the rotational speed of the rotating shaft was changed to 100 rpm, the temperature of the jacket's heat medium was changed to 105°C (i.e., the internal temperature of the main body was maintained at 105°C), and the minimum gap between the bucket and the pan was changed to 1 mm (2% of the pan diameter D). Otherwise, the operation was the same as in Experimental Example 1, resulting in a granular hydrogel (C). It should be noted that the amount of lauryl dimethylaminoacetic acid betaine supplied, based on solids, was 0.15% (w / w) relative to the solids content of the short strip hydrogel (1b). The gel pulverization conditions are shown in Table 1. The characteristics of the pulverized granular hydrogel (C) are shown in Table 2. It should be noted that the GGE during gel pulverization was 125 J / g.

[0298] For the above-mentioned particulate hydrogel (C), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (CP1) and surface-crosslinked water-absorbing resin powder (CP2). The physical properties are shown in Table 3.

[0299] [Experiment Example 4]

[0300] The heating temperature (gel temperature T1 introduced into the inlet of the gel pulverizer) of the short strip hydrogel (1b) was changed to 70°C, water and steam were not supplied, the rotation speed of the rotating shaft was changed to 100 rpm, the temperature of the jacket's heat medium was changed to 80°C (i.e., the temperature inside the main body was maintained at 80°C), and the minimum gap between the bucket and the disc was changed to 1 mm (2% of the disc diameter D). Otherwise, the same procedure as in Experimental Example 1 was followed to obtain granular hydrogel (D). The gel pulverization conditions are shown in Table 1. The characteristics of the pulverized granular hydrogel (D) are shown in Table 2. It should be noted that the GGE during gel pulverization was 54 J / g.

[0301] For the above-mentioned particulate hydrogel (D), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (DP1) and surface-crosslinked water-absorbing resin powder (DP2). The physical properties are shown in Table 3.

[0302] [Experiment Example 5]

[0303] The heating temperature of the short strip hydrogel (1b) (the gel temperature T1 introduced into the inlet of the gel pulverizer) was changed to 80°C, the speed of the short strip hydrogel (1b) was changed to 0.45 kg / min, the temperature of the supplied water was changed to 90°C, the minimum gap between the bucket and the pan was changed to 1 mm (2% of the pan diameter D), the rotational speed of the rotating shaft was changed to 70 rpm, and the temperature of the heat medium in the jacket was changed to 105°C. Otherwise, the operation was the same as in Experimental Example 1, resulting in a granular hydrogel (E). The gel pulverization conditions are shown in Table 1. The characteristics of the pulverized granular hydrogel (E) are shown in Table 2. It should be noted that the GGE during gel pulverization was 10 J / g.

[0304] When the above-mentioned particulate hydrogel (E) was dried using the same procedure as in Experimental Example 1, the drying of the coarse gels of about 10 mm in the particulate hydrogel (E) became insufficient, and no dried material was formed that could be supplied to the subsequent roller mill pulverization process.

[0305] [Experiment Example 6]

[0306] The temperature of the short strip of hydrogel (1b) was changed to room temperature (20°C), water and water vapor were not supplied, and the temperature of the jacket's heat medium was changed to room temperature (20°C) (i.e., the internal temperature of the main body was maintained at room temperature (20°C)). Otherwise, the same operation as in Experimental Example 4 was performed. As a result, the device was stopped due to overload. After stopping, when the container was opened to check the contents, the hydrogel (F) had solidified into a cake shape and could not be supplied to subsequent manufacturing processes.

[0307] [Experiment Example 7]

[0308] Water at 90°C was supplied simultaneously to the short strip of hydrogel (1b) through the inlet, and the procedure was otherwise the same as in Experimental Example 4, to obtain granular hydrogel (G). The pattern of the disc was the same as in Experimental Example 4, but the GGE during gel pulverization was 48 J / g. The gel pulverization conditions are shown in Table 1. The properties of the granular hydrogel (G) obtained by pulverization are shown in Table 2.

[0309] For the above-mentioned particulate hydrogel (G), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (GP1) and surface-crosslinked water-absorbing resin powder (GP2). The physical properties are shown in Table 3.

[0310] [Experiment Example 8]

[0311] Without supplying 0.6 MPa water vapor, the procedure was the same as in Experimental Example 3, yielding a granular hydrogel (H). The disk pattern was the same as in Experimental Example 3, but the GGE during gel pulverization was 157 J / g. The gel pulverization conditions are shown in Table 1. The properties of the pulverized granular hydrogel (H) are shown in Table 2.

[0312] For the above-mentioned particulate hydrogel (H), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (HP1) and surface-crosslinked water-absorbing resin powder (HP2). The physical properties are shown in Table 3.

[0313] [Experiment Example 9]

[0314] The jacket's heat transfer medium temperature was changed to 60°C (i.e., the internal temperature of the main body was maintained at 60°C), and otherwise the same procedure as in Experimental Example 3 was followed to obtain granular hydrogel (I). The disk pattern was the same as in Experimental Example 3, but the GGE during gel pulverization was 135 J / g. The gel pulverization conditions are shown in Table 1. The properties of the pulverized granular hydrogel (I) are shown in Table 2.

[0315] For the above-mentioned particulate hydrogel (I), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (IP1) and surface-crosslinked water-absorbing resin powder (IP2). The physical properties are shown in Table 3.

[0316] [Experiment Example 10]

[0317] A 10% by mass aqueous solution of polyethylene glycol 2000 (manufactured by Tokyo Chemical Industry Co., Ltd., with a weight-average molecular weight of 2000) was simultaneously supplied to the short strip hydrogel (1b) through the inlet. Otherwise, the procedure was the same as in Experimental Example 2, yielding a granular hydrogel (J). It should be noted that the amount of polyethylene glycol 2000 supplied, based on solids, was 0.8% by mass relative to the solids content of the short strip hydrogel (1b). The gel pulverization conditions are shown in Table 1. The characteristics of the pulverized granular hydrogel (J) are shown in Table 2. It should be noted that the GGE during gel pulverization was 38 J / g.

[0318] For the above-mentioned particulate hydrogel (J), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (JP1) and surface-crosslinked water-absorbing resin powder (JP2). The physical properties are shown in Table 3.

[0319] [Experiment Example 11]

[0320] A 10% by mass aqueous solution of KF-354L (Shin-Etsu Chemical Industry Co., Ltd., a side-chain polyether-modified polysiloxane) was supplied simultaneously with the short strip hydrogel (1b) through the inlet. Otherwise, the procedure was the same as in Example 2, yielding a granular hydrogel (K). It should be noted that the amount of KF-354L supplied, based on solids, was 0.05% by mass relative to the solids content of the short strip hydrogel (1b). The gel pulverization conditions are shown in Table 1. The characteristics of the pulverized granular hydrogel (K) are shown in Table 2. It should be noted that the GGE during gel pulverization was 36 J / g.

[0321] For the above-mentioned particulate hydrogel (K), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain hydroabsorbent resin powder (KP1) and surface-crosslinked hydroabsorbent resin powder (KP2). The physical properties are shown in Table 3.

[0322] [Experimental Example 12]

[0323] The heating temperature of the short strip hydrogel (1b) (the gel temperature T1 at the inlet of the gel pulverizer) was changed to 80°C, the processing speed of the short strip hydrogel (1b) was changed to 0.36 kg / min, and cassava acetate starch BK-V (manufactured by Tokai Starch Co., Ltd.) powder was simultaneously supplied from the inlet along with the short strip hydrogel (1b). The rotational speed of the rotating shaft was set to 50 rpm, the water supply was set to 52.2% by mass relative to the solid content of the short strip hydrogel (1b), and the jacket heat medium temperature was changed to 90°C (i.e., the internal temperature of the main body was maintained at 90°C). Otherwise, the same procedure as in Experimental Example 7 was followed to obtain granular hydrogel (L). It should be noted that the supply amount of cassava acetate starch BK-V, based on solid content, was 25% by mass relative to the solid content of the short strip hydrogel (1b). The gel pulverization conditions are shown in Table 1. The characteristics of the pulverized granular hydrogel (L) are shown in Table 2. It should be noted that the GGE during gel pulverization was 30 J / g.

[0324] For the above-mentioned particulate hydrogel (L), the same drying / surface treatment procedure as in Experimental Example 1 was performed to obtain water-absorbing resin powder (LP1) and surface-crosslinked water-absorbing resin powder (LP2). The physical properties are shown in Table 3.

[0325] In Table 1, gel temperature T1 represents the gel temperature at the inlet of the gel pulverizer, and gel temperature T2 represents the gel temperature at the outlet of the gel pulverizer. In Table 2, d1 represents the mass-average particle size converted from the solid composition of the granular hydrogel. In Table 3, d3 represents the mass-average particle size of the water-absorbing resin powder. It should be noted that in all embodiments except Experimental Example 6, the hydrogel-like crosslinked polymer was continuously pulverized at a temperature above 50°C using the pulverizing unit. Furthermore, in all embodiments except Experimental Example 6, the interior of the main body was heated to a temperature above 50°C before the hydrogel was introduced.

[0326] [Table 1]

[0327] (Table 1) Conditions for gel pulverization

[0328]

[0329] [Table 2]

[0330] (Table 2) Properties of granular hydrogels

[0331]

[0332] [Table 3]

[0333] (Table 3) Characteristics of surface-crosslinked water-absorbing resin powder

[0334]

[0335] In the manufacturing method described in this invention, a mixer with multiple screws is used to gel-pulverize the hydrogel-like crosslinked polymer obtained in the polymerization process, resulting in a gel-pulverized product suitable for subsequent processes. Furthermore, the resulting water-absorbing resin powder also exhibits excellent absorption rates (Experimental Examples 1-4, 7-12).

[0336] The comparison between Experiment 1 and 2 confirms that by reducing the gap between the bucket and the plate, the particle size of the granular hydrogel is reduced, thereby increasing the water absorption rate of the water-absorbing resin powder.

[0337] Based on the comparison between Experiments 4 and 7, it can be confirmed that by simultaneously supplying water into the body and pulverizing the gel, the particle size of the granular hydrogel increases, but the water absorption rate of the water-absorbing resin powder increases.

[0338] Based on the comparison of Experiments 3, 8 and 9, it can be confirmed that by simultaneously supplying water vapor into the body and performing gel pulverization at high temperature (above 100°C), the water absorption rate of the water-absorbing resin powder is increased.

[0339] Based on the comparison of Experiments 2, 10 and 11, it can be confirmed that by adding a gel flow agent at the same time as the hydrogel and then crushing the gel, the particle size of the granular hydrogel increases, but the water absorption rate of the water-absorbing resin powder increases.

[0340] Industrial availability

[0341] The absorbent resin powder obtained by this invention is suitable for use as an absorbent in hygiene products such as diapers.

[0342] This application is based on Japanese Patent Application No. 2020-161054, filed on September 25, 2020, the disclosure of which is incorporated herein by reference in its entirety.

[0343] Explanation of reference numerals in the attached figures

[0344] 200… Gel pulverizer

[0345] 204… Input Port

[0346] 206… Rotation axis

[0347] 208…Main body (bucket)

[0348] 210…outlet

[0349] 212… Crushing Unit

[0350] 214…Driver

[0351] 216…Gas Inlet

Claims

1. A method for manufacturing a water-absorbing resin powder, comprising the following steps: The polymerization process of polymerizing monomer aqueous solution to obtain hydrogel-like crosslinked polymer; Following the polymerization step, the hydrogel-like crosslinked polymer is pulverized using a gel pulverizer to obtain granular hydrogel-like crosslinked polymer; and The particulate hydrogel-like crosslinked polymer is dried to obtain a dried product. The gel pulverizing device has an inlet, an outlet, and a main body with multiple internal rotating shafts, each of which has a pulverizing unit. The gel pulverizing device is a continuous multi-screw mixer. In the gel pulverization process, the hydrogel-like crosslinked polymer is continuously fed into the inlet, and the pulverization unit continuously pulverizes the hydrogel-like crosslinked polymer at a temperature above 50°C. Particulate hydrogel-like crosslinked polymer is then continuously removed from the outlet. The forward direction of the hydrogel-like crosslinked polymer is left-right, which is the length direction of the main body of the gel pulverizing device and the axial direction of the rotation axis. The hydrogel introduced into the main body is pulverized and moves towards the discharge port. The polymerization rate of the hydrogel-like crosslinked polymer added to the inlet is 90% or more by mass. The mass-average particle size d1 of the particulate hydrogel-like crosslinked polymer discharged from the outlet is less than 3 mm.

2. The manufacturing method according to claim 1, wherein, The gel temperature T1 of the hydrogel-like crosslinked polymer introduced into the inlet is above 50°C.

3. The manufacturing method according to claim 1 or 2, wherein, The gel pulverizing device has a heating and / or heat preservation unit.

4. The manufacturing method according to claim 1 or 2, wherein, The gel temperature T2 at the outlet of the gel pulverizer is higher than the gel temperature T1 at the inlet.

5. The manufacturing method according to claim 1 or 2, wherein, The inlet is located near one end of the rotating shaft, and the outlet is located near the other end of the rotating shaft.

6. The manufacturing method according to claim 1 or 2, wherein, The outlet is located near the rear of the main body.

7. The manufacturing method according to claim 1 or 2, wherein, The monomer aqueous solution contains unsaturated monomers with acid groups as the main component.

8. The manufacturing method according to claim 1 or 2, wherein, The pulverizing unit of the rotating shaft is selected from a group consisting of a disc, small pieces, blades, components, kneaders, and a rotor.

9. The manufacturing method according to claim 1 or 2, wherein, Each of the rotating shafts has a pulverizing unit that is a disc, and the minimum gap C of the gel pulverizing device is 0.2 to 20% of the maximum diameter D of the disc.

10. The manufacturing method according to claim 1 or 2, wherein, Each of the rotating shafts has a crushing unit that is a disc, and the ratio of the effective length L inside the main body to the maximum diameter D of the disc, L / D, is 5 to 40.

11. The manufacturing method according to claim 1 or 2, wherein, The gel temperature T2 at the outlet of the gel pulverizer is 60–140°C.

12. The manufacturing method according to claim 1 or 2, wherein, Before the hydrogel-like crosslinked polymer is introduced from the inlet, the interior of the body is heated to above 50°C.

13. The manufacturing method according to claim 1 or 2, wherein, The hydrogel-like crosslinked polymer obtained after the polymerization process is in sheet form. Before the gel crushing process, a shredding process is also included to shred the sheet-like hydrogel-like crosslinked polymer.

14. The manufacturing method according to claim 1 or 2, wherein, In the gel pulverization process, water and / or water vapor are supplied to the interior of the body.

15. The manufacturing method according to claim 14, wherein, The temperature of the water and / or water vapor supplied to the interior of the main body is 50–120°C.

16. The manufacturing method according to claim 14, wherein, The pressure of the water vapor supplied to the interior of the main body is 0.2 to 0.8 MPa.

17. The manufacturing method according to claim 1 or 2, wherein, The solid content of the hydrogel-like crosslinked polymer fed into the inlet is 25-75% by mass.

18. The manufacturing method according to claim 1 or 2, wherein, The solid content of the particulate hydrogel-like cross-linked polymer discharged from the outlet is 25-75% by mass.

19. The manufacturing method according to claim 1 or 2, wherein, The hydrogel-like crosslinked polymer is a crosslinked polymer with poly(meth)acrylate (salt) as the main component.

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