Absorbent agent containing a surface-crosslinked (meth)acrylic acid (salt)-based superabsorbent resin and method for producing a (meth)acrylic acid (salt)-based superabsorbent resin

Abstract

To provide a water absorbing resin that has resistance to process damage, and also has high absorptivity while maintaining high resistance to process damage and a method for producing the same.SOLUTION: The present invention provides a water absorber comprising a surface-crosslinked (meth)acrylic acid (salt)-based water absorbing resin, satisfying all of the following conditions: (a) CRC>20 g / g, (b) pH of 4.0-5.4, (c) branch density of soluble matter of 0.03-0.70, and (d) the amount of soluble matter (16hr-Extr) of 5-25 mass%.

Description

[Technical Field]

[0001] The present invention relates to an absorbent containing a surface-crosslinked (meth)acrylic acid (salt)-based absorbent resin and a method for producing a (meth)acrylic acid (salt)-based absorbent resin. [Background technology]

[0002] Super absorbent polymers (SAPs) are water-swellable, water-insoluble polymeric gelling agents used in absorbent products such as disposable diapers, sanitary napkins, and adult incontinence products. While many monomers and hydrophilic polymers have been proposed as super absorbent polymers, polycarboxylic acid-based resins such as polyacrylic acid are widely used from the perspective of performance and cost.

[0003] Depending on the applications described above, attempts have been made to produce superabsorbent polymers under controlled conditions to achieve desired parameter properties. For example, to increase the aqueous absorption rate (AAP) and saline flow induction (SFC), a method is known in which a crosslinking agent having multiple functional groups that can react with reactive groups present in the superabsorbent polymer is used to form a crosslinked structure near the surface of the superabsorbent polymer, thereby increasing the surface crosslink density of the superabsorbent polymer (surface crosslinking) (for example, Non-Patent Document 1).

[0004] The manufactured superabsorbent resin can then be added to the assembly site of the absorbent article after the amount and method of addition have been designed according to the intended use, required performance, and absorption capacity of the absorbent article in which the superabsorbent resin will be used, thereby manufacturing the absorbent article.

[0005] As described above, the design and manufacture of absorbent articles by each absorbent article manufacturer are carried out using the physical property parameters of the incorporated water-absorbing resin as indicators according to the intended use, required performance, and absorption capacity. However, during the manufacturing process of the absorbent article, the water-absorbing resin is subjected to physical loads and impacts, so-called process damage. When a part of the optimal surface crosslinked layer of the water-absorbing resin is destroyed, even if the absorption performance (AAP and SFC) of the water-absorbing resin is enhanced, its performance will decline when it is put into the diaper, and the intended performance of the diaper will not be exhibited.

[0006] As a solution to this problem, methods such as making a surfactant as a lubricant present on the surface of the surface crosslinked water-absorbing resin particles (for example, Patent Document 1) and increasing the water content rate of the surface crosslinked water-absorbing resin particles (for example, Patent Document 2) have been proposed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] While the technologies proposed in Patent Documents 1 and 2 have some effect, the addition of surfactants has negative effects on the physical properties of absorbent materials due to a decrease in surface tension, and increasing the water content has negative effects on the physical properties due to an increase in the stickiness of the absorbent resin, thus requiring further improvement. Even if damage resistance is improved using conventional methods, a new problem arises: the performance of the absorbent material itself before it is added to the diaper deteriorates.

[0010] In view of the above circumstances, the problem that the invention aims to solve is to provide an absorbent resin that has process damage resistance and high absorption performance while maintaining high process damage resistance, and a method for producing the same. [Means for solving the problem]

[0011] The inventors diligently conducted research to solve the above problems. As a result, they have developed a water-absorbing agent containing a surface-crosslinked (meth)acrylic acid (salt)-based water-absorbing resin that satisfies all of the following conditions: (a) CRC > 20 g / g, (b) pH of 4.0 to 5.4, (c) branching density of soluble matter of 0.03 to 0.70, (d) soluble matter (16hr-Extr) The present invention was completed by providing a method for producing a (meth)acrylic acid (salt)-based superabsorbent resin, comprising the steps of: preparing an aqueous solution of (meth)acrylic acid (salt) in an amount of 5 to 25% by mass, and / or an aqueous solution of (meth)acrylic acid (salt)-based monomer; polymerizing the aqueous solution of (meth)acrylic acid (salt)-based monomer; and performing a surface crosslinking treatment after polymerization, wherein the neutralization rate of the (meth)acrylic acid (salt) is 30 mol% or more and less than 55 mol%, the concentration of the (meth)acrylic acid (salt)-based monomer in the aqueous monomer solution is 38 to 50% by mass, and the difference ΔT between the polymerization peak temperature and the polymerization start temperature in the polymerization step is 50°C or less. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a method for producing a water-absorbing agent and a water-absorbing resin that have process damage resistance and high absorption performance while maintaining high process damage resistance. [Modes for carrying out the invention]

[0013] [1] Definition of terms (Water-absorbent resin, resin powder (water-absorbent resin powder), surface-crosslinked resin (surface-crosslinked water-absorbent resin particles), water-absorbing agent) In this invention, "absorbent polymer" refers to a water-swellable polymer gelling agent that is insoluble in water. "Water-swellable" means that the CRC (absorption rate under no pressure) as defined by the EDANA method (WSP241.3(10)) is 5 g / g or more, and "water-insoluble" means that the Ext (soluble content) as defined by the EDANA method (WSP270.3(10)) is 50% by mass or less. In this invention, "absorbent polymer" may be a (meth)acrylic acid (salt)-based absorbent polymer. A (meth)acrylic acid (salt)-based absorbent polymer is an "absorbent polymer" that contains constituent units derived from (meth)acrylic acid and its salts. For example, an acrylic acid-based absorbent polymer having some acid groups that are neutralized (or some that are not neutralized) relative to the total number of acid groups is also included in the (meth)acrylic acid (salt)-based absorbent polymer.

[0014] The above-mentioned "absorbent resin" may be a (meth)acrylic acid (salt)-based absorbent resin, but it is not necessary for the entire amount (100% by mass) to be such a resin. It may also contain various other substances as long as it satisfies the above-mentioned performance (CRC, Ext).

[0015] Furthermore, the term "absorbent polymer" as used above may refer to either "a polymer that is crosslinked only internally (i.e., a polymer in which the crosslink density of the internal and surface is substantially the same)" or "a polymer that is crosslinked both internally and on the surface (i.e., a polymer in which the crosslink density of the surface is relatively higher than that of the internal crosslink density)." For convenience, in this specification, polymers that are crosslinked only internally are also referred to as "resin powder (absorbent polymer powder)," and polymers that are crosslinked both internally and on the surface are also referred to as "surface-crosslinked resin (surface-crosslinked absorbent polymer particles)."

[0016] In this invention, the "water absorbent" includes a water-absorbing resin. It also includes a water-absorbing resin that is ready for shipment as a final product. Therefore, when the "surface-crosslinked water-absorbing resin particles" are shipped as a final product, the "surface-crosslinked water-absorbing resin particles" become the "water absorbent." Furthermore, a composition obtained by mixing various substances with the "surface-crosslinked water-absorbing resin particles" can also be labeled as a "water absorbent" when it is shipped as a final product.

[0017] In the present invention, the "water absorbent containing a water-absorbing resin" preferably has a water-absorbing resin content of 50% to 100% by mass. This percentage can be 60% or more by mass, 70% or more by mass, 80% or more by mass, or 90% or more by mass. Furthermore, other components besides the water-absorbing resin, such as water or additives, may be included in amounts of 50% or less by mass, 40% or less by mass, 30% or less by mass, 20% or less by mass, 10% or less by mass, or 0.01% to 10% by mass.

[0018] ((meth)acrylic acid (salt)-based superabsorbent polymer) In this invention, "(meth)acrylic acid (salt)-based superabsorbent resin" refers to a superabsorbent resin made from (meth)acrylic acid (salt). In other words, a (meth)acrylic acid (salt)-based superabsorbent resin is a superabsorbent resin having structural units derived from (meth)acrylic acid (salt) in its polymer.

[0019] Specifically, (meth)acrylic acid (salt)-based superabsorbent resins are superabsorbent resins that contain 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, 97 mol% or more, or 99 mol% or more, or 100 mol% of (meth)acrylic acid (salt) relative to the total monomers involved in the polymerization reaction (excluding internal crosslinking agents).

[0020] (EDANA and WSP) "EDANA" is an abbreviation for European Disposables and Nonwovens Associations, and "WSP" is an abbreviation for World Strategic Partners. EDANA WSP is the European and American standard (almost a global standard) for methods of measuring the physical properties of superabsorbent polymers. In this invention, unless otherwise specified, the physical properties of superabsorbent polymers are measured in accordance with the original EDANA WSP document (revised in 2010 / publicly known document).

[0021] (others) In this specification, "X~Y" indicating a range means "X or greater and Y or less". Also, "t (ton)" as a unit of weight means "metric ton", and unless otherwise noted, "ppm" means "mass ppm". Furthermore, "weight" and "mass", "weight%" and "mass%", and "parts by weight" and "parts by mass" are treated as synonyms. Furthermore, "~acid (salt)" means "~acid and / or its salt", and "(meth)acrylic" means "acrylic and / or methacrylic". In addition, unless otherwise specified, physical properties are measured at room temperature (20~25°C) and relative humidity of 40~50%RH. Furthermore, "X or greater" and "X or greater" in the range "X~Y" can serve as the basis for the legality of correcting to "X or less". Also, "Y or less" and "Y or less" in the range "X~Y" can serve as the basis for the legality of correcting to "Y or greater".

[0022] [2] Water absorbent containing (meth)acrylic acid (salt) based water absorbent resin One aspect of the present invention is a water absorbent comprising a surface-crosslinked (meth)acrylic acid (salt)-based water absorbent resin, satisfying all of the following conditions: water absorbent: (a) CRC > 20 g / g, (b) pH 4.0 to 5.4, (c) branching density of soluble components 0.03 to 0.70, (d) soluble components (16hr-Extr) 5 to 25 mass%. With this configuration, it is possible to have process damage resistance and maintain high water absorption performance. Although the mechanism is not clear, it is thought that the flexibility of the (meth)acrylic acid (salt)-based water absorbent resin is controlled by pH, and the toughness of the three-dimensional crosslinked polymer structure is controlled by the amount of soluble components and the branching density of soluble components. Note that "surface-crosslinked (meth)acrylic acid (salt)-based water absorbent resin" is also simply referred to as "(meth)acrylic acid (salt)-based water absorbent resin".

[0023] (a) CRC In one embodiment, the CRC of an absorbent containing a (meth)acrylic acid (salt)-based absorbent resin is greater than 20 g / g. Setting the CRC to such a lower limit makes the absorbent suitable for use as an absorbent in absorbent articles such as disposable diapers and sanitary napkins. An absorbent containing a (meth)acrylic acid (salt)-based absorbent resin according to one embodiment of the present invention is excellent in that it can maintain a high CRC while having process damage resistance. In one embodiment, the CRC of an absorbent containing a (meth)acrylic acid (salt)-based absorbent resin is 21 g / g or more, 22 g / g or more, 23 g / g or more, 24 g / g or more, 25 g / g or more, or 26 g / g or more. In one embodiment, the CRC of an absorbent containing a (meth)acrylic acid (salt)-based absorbent resin is 45 g / g or less, more preferably 42 g / g or less, and even more preferably 40 g / g or less. The CRC can be controlled by the amount of internal crosslinking agent, etc.

[0024] (b) pH In one embodiment, the pH of the water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 4.0 to 5.4. (Meth)acrylic acid (salt)-based water absorbent resins with a pH in the above range exhibit excellent resistance to process damage. This is presumed to be because the water absorbent resin is highly flexible when the pH is within the above range, thus exhibiting excellent resistance to process damage. In one embodiment, the pH of the water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 5.3 or less, 5.2 or less, 5.1 or less, 5.0 or less, or 4.9 or less. In one embodiment, the pH of the water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 4.2 or higher, 4.4 or higher, 4.6 or higher, 4.7 or higher, or 4.8 or higher.

[0025] (c) Branching density of soluble matter In one embodiment, the branching density of the soluble components of the (meth)acrylic acid (salt)-based superabsorbent resin contained in the water absorbent is 0.03 to 0.70. Here, the soluble components are components (polymers) that are not incorporated into the (meth)acrylic acid (salt)-based superabsorbent resin (i.e., the three-dimensional crosslinked body of the (meth)acrylic acid (salt)-based polymer) and are soluble in water. Since the soluble components are not bound to the three-dimensional crosslinked body and internal crosslinking agent produced by the polymerization reaction of the superabsorbent resin, they can be extracted. Since the soluble components are influenced by the three-dimensional crosslinked polymer structure constituting the superabsorbent resin, measuring the branching density of the soluble components provides information that can estimate the three-dimensional crosslinked polymer structure. From this embodiment, it can be seen that the branching density of the soluble components is a factor that improves absorption performance while maintaining process damage resistance. It is important to keep the branching density of the soluble components within a predetermined range. In one embodiment, the branching density of the soluble component of the (meth)acrylic acid (salt)-based superabsorbent resin is 0.60 or less, 0.50 or less, 0.40 or less, 0.30 or less, or 0.26 or less. By lowering this value, a three-dimensional crosslinked polymer structure having a uniform acrylic acid (salt) chain with fewer branches is formed. That is, a polymer structure that is tough against impacts and strains such as process damage is formed, contributing to improved process damage resistance. In one embodiment, the branching density of the soluble component of the (meth)acrylic acid (salt)-based superabsorbent resin is 0.03 or more, 0.05 or more, 0.07 or more, 0.09 or more, 0.10 or more, 0.11 or more, or 0.13 or more. If the branching density of the soluble component is less than 0.03, it is undesirable because the balance of other physical properties such as CRC and AAP deteriorates. Also, process damage resistance may deteriorate.

[0026] (d) Soluble content (16hr-Extr) In one embodiment, the soluble content (16hr-Extr) of the (meth)acrylic acid (salt)-based superabsorbent resin contained in the water absorbent is 5 to 25% by mass. By setting the soluble content within a predetermined range, the toughness of the polymer structure can be improved, and process damage resistance and physical properties (especially AAP) can be improved. In one embodiment, the soluble content (16hr-Extr) of the (meth)acrylic acid (salt)-based superabsorbent resin contained in the water absorbent is 23.0% by mass or less, 20.0% by mass or less, 18.0% by mass or less, 15.0% by mass or less, 14.0% by mass or less, 13.0% by mass or less, 12.0% by mass or less, or 11.0% by mass or less. In one embodiment, the soluble content (16hr-Extr) of the (meth)acrylic acid (salt)-based superabsorbent resin is 5% by mass or more, 6% by mass or more, 7% by mass or more, 8% by mass or more, or 9% by mass or more.

[0027] (e) Molecular weight distribution of soluble matter Mw / Mn In one embodiment, the molecular weight distribution Mw / Mn of the soluble (meth)acrylic acid (salt)-based water-absorbing resin contained in the water-absorbing agent is 5.30 or less, or 5.00 or less. The molecular weight distribution Mw / Mn of the soluble portion shall be calculated to three decimal places and the value obtained by rounding to the third decimal place shall be adopted.

[0028] By measuring the molecular weight distribution Mw / Mn of the soluble components, information for estimating the three-dimensional crosslinked polymer structure can be obtained. From this embodiment, it can be seen that the molecular weight distribution Mw / Mn of the soluble components is a factor that improves absorption performance while maintaining process damage resistance. By setting this to a low value (5.30 or less, or 5.00 or less), a three-dimensional crosslinked polymer structure having acrylic acid (salt) chains with a uniform molecular weight distribution is formed. In other words, by creating a polymer structure that is tough against impacts and strains such as process damage, it is possible to achieve both high process damage resistance, a high AAP of 0.7 psi, and high SFC. In one embodiment, the molecular weight distribution Mw / Mn of the soluble components of the (meth)acrylic acid (salt)-based water-absorbing resin contained in the water absorbent is 4.60 or less, 4.30 or less, or 4.20 or less. In one embodiment, the molecular weight distribution Mw / Mn of the soluble components of the (meth)acrylic acid (salt)-based superabsorbent resin is 1.00 or higher, 1.50 or higher, 2.00 or higher, 2.50 or higher, 3.00 or higher, 3.50 or higher, or 4.00 or higher.

[0029] (f) Moisture content (105℃×3hr) In one embodiment, the water content (at 105°C for 3 hours) of a water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 3.0% by mass or less, 2.5% by mass or less, 2.0% by mass or less, 1.9% by mass or less, 1.8% by mass or less, 1.5% by mass or less, 1.3% by mass or less, 1.1% by mass or less, 0.9% by mass or less, 0.7% by mass or less, 0.5% by mass or less, or 0.4% by mass or less, from the viewpoint of the handling of the water absorbent and the process damage resistance of the powder, as described later. Furthermore, water absorbents with lower water content tend to show improved water absorption performance. In one embodiment, the water content (at 105°C for 3 hours) of the water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 0.2% by mass or more, greater than 0.2% by mass, 0.3% by mass or more, 0.5% by mass or more, 0.8% by mass or more, 1.2% by mass or more, 1.5% by mass or more, 1.8% by mass or more, 2.2% by mass or more, or 2.4% by mass or more. It is preferable to set the water content within the above range in order to achieve both improved handling properties of the water absorbent and improved process damage resistance of the powder.

[0030] (g) Residual monomer amount In one embodiment, the residual monomer amount of the water absorbent containing the (meth)acrylic acid (salt)-based water-absorbing resin is 300 ppm by mass or less, 250 ppm by mass or less, 200 ppm by mass or less, 195 ppm by mass or less, 185 ppm by mass or less, or 175 ppm by mass or less. By reducing the residual monomer amount, the acid odor when swollen is suppressed, and the irritation to the human skin and the like is reduced. Note that, although it is preferable that the residual monomer amount of the water absorbent containing the (meth)acrylic acid (salt)-based water-absorbing resin is as small as possible, realistically, it can be 30 ppm by mass or more, or 50 ppm by mass or more.

[0031] (h) AAP 0.7 psi<e In one embodiment, the AAP 0.7 psi of the water absorbent containing the (meth)acrylic acid (salt)-based water-absorbing resin is 20 g / g or more, more than 20 g / g, 21 g / g or more, 22 g / g or more, or 23 g / g or more. In one embodiment, the AAP 0.7 psi of the water absorbent containing the (meth)acrylic acid (salt)-based water-absorbing resin is 30 g / g or less, 28 g / g or less, 27 g / g or less, 26 g / g or less, 25 g / g or less, 24 g / g or less, 23 g / g or less, or 22 g / g or less. By setting the water absorption ratio under pressure within the above range, it becomes a water absorbent suitable for use as an absorber in absorbent articles such as paper diapers and sanitary napkins. The AAP can be controlled by known techniques, for example, by the crosslinking density of the surface crosslinked layer and the like.

[0032] (i) SFC In one embodiment, the SFC of the water absorbent containing the (meth)acrylic acid (salt)-based water-absorbing resin is 20×10 -7 cm 3 ·sec / g or more. In one embodiment, the SFC of the water absorbent containing the (meth)acrylic acid (salt)-based water-absorbing resin is 21×10 -7 cm 3 ·sec / g or more, 25×10 -7 cm 3 ·sec / g or more, 30×10 -7 cm 3 ·sec / g or more, 35×10 -7 cm 3·sec / g or more, 40×10 -7 cm 3 ·sec / g or more, 45×10 -7 cm 3 ·sec / g or more, 50×10 -7 cm 3 ·sec / g or more, 54×10 -7 cm 3 ·sec / g or more, 55×10 -7 cm 3 ·sec / g or more, 65×10 -7 cm 3 ·sec / g or more, 85×10 -7 cm 3 • sec / g or more, or 100 × 10 -7 cm 3 ·sec / g or more. In one embodiment, the SFC of a water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 300 × 10 -7 cm 3 ·sec / g or less, 250×10 -7 cm 3 ·Sec / g or less, 200×10 -7 cm 3 ·sec / g or less, 150×10 -7 cm 3 ·sec / g or less, 1200×10 -7 cm 3 ·sec / g or less, 110×10 -7 cm 3 ·sec / g or less, 90×10 -7 cm 3 ·sec / g or less, 80×10 -7 cm 3 ·sec / g or less, 75×10 -7 cm 3 ·sec / g or less, 70×10 -7 cm 3 ·sec / g or less, 65×10 -7 cm 3 • sec / g or less, or 62 × 10 -7 cm 3 The SFC (Surface Crosslinking) level is less than or equal to sec / g. The SFC can be increased by crosslinking the surface area with a surface crosslinking agent. The SFC can also be adjusted by the amount of surface crosslinking agent, etc.

[0033] (j) Particle size distribution and mass-average particle size (D50) In one embodiment, the water-absorbing agent (i.e., the (meth)acrylic acid (salt)-based water-absorbing resin contained in the water-absorbing agent) is particulate (powdered). The particulate water-absorbing agent (water-absorbing resin) may be a single particle or an aggregate of multiple particles, but the form of an aggregate of multiple particles is common. "Particulate" means having the form of particles, and a particle is a small granular solid or liquid body of measurable size (JIS Industrial Terminology Dictionary, 4th edition, p. 2002). The mass-average particle diameter (D50) of the water-absorbing agent (water-absorbing resin) is 320 μm or more, 330 μm or more, 340 μm or more, 350 μm or more, or 360 μm or more. In one embodiment, the mass-average particle size (D50) of the absorbent containing (meth)acrylic acid (salt)-based absorbent resin is 500 μm or less, 480 μm or less, 450 μm or less, 420 μm or less, 400 μm or less, or 380 μm or less. By setting the mass-average particle size (D50) of the absorbent containing (meth)acrylic acid (salt)-based absorbent resin within the above range, it is preferable to use the absorbent as an absorbent material in absorbent articles such as disposable diapers and sanitary napkins because it improves the feel of the absorbent article against the skin and further enhances the ability to diffuse absorbed liquids such as urine throughout the absorbent material.

[0034] In one embodiment, the water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin has a particle diameter of 150 μm or more, and the proportion of particles with a particle diameter of less than 850 μm is 90% by mass or more, 95% by mass or more, or 97% by mass or more. In another embodiment, the proportion of particles containing a (meth)acrylic acid (salt)-based water absorbent resin with a particle diameter of 600 μm or more, and less than 850 μm is less than 10% by mass, less than 9% by mass, less than 8% by mass, or less than 7% by mass (lower limit 0% by mass).

[0035] (k)ΔAAP0.7psi In one embodiment, before and after a water-absorbing agent damage test containing a (meth)acrylic acid (salt)-based water-absorbing resin, ΔAAP 0.7psi is 4.0% or less, 3.8% or less, 3.6% or less, 3.4% or less, 3.2% or less, 3.0% or less, 2.8% or less, 2.6% or less, 2.4% or less, 2.2% or less, or 2.0% or less. Here, ΔAAP 0.7psi refers to the AAP reduction rate in the water-absorbing agent damage test in the examples described later.

[0036] (l)ΔSFC In one embodiment, the ΔSFC before and after a water-absorbing agent damage test containing a (meth)acrylic acid (salt)-based water-absorbing resin is 25.0% or less, 23.0% or less, 20.0% or less, 18.0% or less, 15.0% or less, 13.0% or less, 11.0% or less, or 10.0% or less. Here, ΔSFC refers to the SFC reduction rate in the water-absorbing agent damage test in the examples described later.

[0037] (m) Handling of absorbent material To prevent moisture absorption and caking (clumping) of the absorbent material, it is stored and transported in bags made of highly hydrophobic material. Olefin materials are used as highly hydrophobic materials, such as polyethylene. When removing the absorbent material stored in these bags, it is preferable that the absorbent material (particulate or powdered) does not adhere to the bag due to electrostatic attraction. In that case, the absorbent material becomes easier to handle. To prevent the absorbent material from adhering to the bag, the interaction between the absorbent material and the polyolefin is reduced. One way to reduce this interaction is to control the water content of the absorbent material. The range of water content is as shown above. The reason why the water content makes the absorbent material easier to handle is not entirely clear, but it is assumed that the surface lubricity, hydrophilicity, and static electricity of the absorbent material (absorbent resin) are acting in combination. There are no particular limitations on the method for controlling the water content of the water absorbent, but in one embodiment, a method is to control the water content by adding water (for example, water only) to the surface-crosslinked resin (surface-crosslinked water absorbent resin particles). In another embodiment, a method is to control the amount of water evaporated from the surface crosslinking agent solution by making the heating conditions of the heat treatment process in surface crosslinking gentle (in one embodiment, 80-160°C for 20-120 minutes, or 90-150°C for 30-100 minutes).

[0038] (n) Charge amount (electrostatic charge amount) In one embodiment, the electrostatic charge of the water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 10.0 nC or less, 8.0 nC or less, 6.0 nC or less, 5.0 nC or less, 4.0 nC or less, 3.0 nC or less, 2.0 nC or less, 1.0 nC or less, or 0.5 nC or less. By adjusting the electrostatic charge to an appropriate value, the powder flowability can be improved. This suppresses electrostatic adsorption to bags (e.g., made of polyethylene) into which the (meth)acrylic acid (salt)-based water absorbent resin (water absorbent) can be filled, and to metals that cause friction as the powder flows, improving operability during filling and discharging of bags and reducing the risk of equipment blockage and operational trouble due to metal adhesion. The electrostatic charge is thought to be obtained through a complex mechanism of action involving a combination of the drying conditions of the water absorbent (water absorbent resin), the surface crosslinking agent, and the surface crosslinking conditions.

[0039] In one embodiment, the electrostatic charge of the water absorbent containing a (meth)acrylic acid (salt)-based water absorbent resin is 0.01 nC or more, or 0.05 nC or more.

[0040] (o) Particle shape In one embodiment, the (meth)acrylic acid (salt)-based superabsorbent resin contained in the water absorbent is in an irregularly shaped, crushed form. By making the (meth)acrylic acid (salt)-based superabsorbent resin irregularly shaped, the water absorption rate of the water absorbent containing the (meth)acrylic acid (salt)-based superabsorbent resin can be improved, and the immobilization of the water absorbent containing the (meth)acrylic acid (salt)-based superabsorbent resin in absorbent articles can also be facilitated. The irregularly shaped crushed form can be confirmed by observing the crushed cross-section using an electron microscope. The irregularly shaped crushed form can be controlled by gel crushing or crushing after drying.

[0041] [3] (meth)acrylic acid (salt)-based superabsorbent resin and method for producing the same One aspect of the present invention is a method for producing a (meth)acrylic acid (salt)-based superabsorbent resin, comprising the steps of: preparing an aqueous solution of (meth)acrylic acid (salt)-based monomer; polymerizing the aqueous solution of (meth)acrylic acid (salt)-based monomer; and performing a surface crosslinking treatment after polymerization, wherein the neutralization rate of the (meth)acrylic acid (salt) is 30 mol% or more and less than 55 mol%, the concentration of the (meth)acrylic acid (salt)-based monomer in the aqueous monomer solution is 38 to 50% by mass, and the difference ΔT between the polymerization peak temperature and the polymerization start temperature in the polymerization step is 50°C or less. With this configuration, process damage resistance is achieved, and high CRC can be maintained while simultaneously achieving high AAP and high SFC.

[0042] (Steps for preparing aqueous solutions of (meth)acrylic acid (salt) monomers) This step involves preparing an aqueous solution of (meth)acrylic acid (salt) monomers.

[0043] In one embodiment, the (meth)acrylic acid (salt) monomer aqueous solution is an aqueous solution containing (meth)acrylic acid (salt) as a monomer. The aqueous solution containing (meth)acrylic acid (salt) may also contain other monomers as long as it contains (meth)acrylic acid (salt). In one embodiment, the content of (meth)acrylic acid (salt) relative to the total monomers contained in the monomer aqueous solution (excluding internal crosslinking agents) is usually 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, 85 mol% or more, 90 mol% or more, 92 mol% or more, 94 mol% or more, 96 mol% or more, 98 mol% or more, or 99 mol% or more (upper limit is 100 mol%).

[0044] In one embodiment, the step of preparing an aqueous solution of (meth)acrylic acid (salt) monomer includes mixing the (meth)acrylic acid (salt) monomer with water, wherein the water is 30°C or higher, 31°C or higher, 35°C or higher, 40°C or higher, 45°C or higher, 50°C or higher, 55°C or higher, 60°C or higher, 65°C or higher, 70°C or higher, or 75°C or higher. By setting the lower limit of the water temperature to such values, the polymerization initiation temperature can be significantly increased (for example, to 50°C or higher). Furthermore, the intended effects of the present invention are efficiently achieved. In one embodiment, the step of preparing an aqueous solution of (meth)acrylic acid (salt) monomer includes mixing the (meth)acrylic acid (salt) monomer with water, wherein the water is 95°C or lower, 90°C or lower, 85°C or lower, 80°C or lower, 75°C or lower, 70°C or lower, 65°C or lower, or 60°C or lower.

[0045] ((meth)acrylic acid (salt)) (Meth)acrylic acid (salt) includes (meth)acrylic acid and its salts. In one embodiment, (meth)acrylic acid (salt) is obtained by neutralizing (meth)acrylic acid with a basic compound. Specific examples of basic compounds include alkali metal carbonates and bicarbonates, alkali metal hydroxides, ammonia, and organic amines. Among these, a strongly basic compound is selected from the viewpoint of the water absorption performance of the water absorbent. Therefore, alkali metal hydroxides such as sodium, potassium, and lithium are preferred, and sodium hydroxide is more preferred. From the viewpoint of ease of handling, the basic compound is preferably an aqueous solution (aqueous solution of basic salt). The (meth)acrylic acid may contain at least one component such as a polymerization inhibitor and impurities. Methoxyphenols are preferably used as the polymerization inhibitor, and p-methoxyphenols are more preferably used. The content (concentration) of the polymerization inhibitor in (meth)acrylic acid is preferably 200 ppm or less, 160 ppm or less, and 100 ppm or less from the viewpoint of the polymerizability of (meth)acrylic acid and the color tone of the water absorbent. The lower limit is, for example, 10 ppm or more, or 20 ppm or more. (Meth)acrylic acid may also contain the compounds described in U.S. Patent Application Publication No. 2008 / 0161512, in addition to the organic compounds such as acetic acid, propionic acid, and furfural as impurities.

[0046] (neutralization) In one embodiment, the timing of the neutralization of (meth)acrylic acid is not particularly limited and may be before polymerization, during polymerization, or after polymerization, and neutralization may be performed at multiple times or locations.

[0047] The neutralization rate of (meth)acrylic acid (salt) is 30 mol% or more and less than 55 mol% relative to the acid groups of the monomer, but in one embodiment, the neutralization rate of (meth)acrylic acid (salt) is 31 mol% or more, 32 mol% or more, 34 mol% or more, 35 mol% or more, 36 mol% or more, 37 mol% or more, or 38 mol% or more, relative to the acid groups of the monomer. In one embodiment, the neutralization rate of (meth)acrylic acid (salt) is 54 mol% or less, 52 mol% or less, 50 mol% or less, 49 mol% or less, or 48 mol% or less, relative to the acid groups of the monomer. Having such upper or lower limits allows the intended effects of the present invention to be efficiently achieved.

[0048] The above-mentioned range of neutralization rates may be any of the neutralizations performed before, during, or after polymerization, but at least before polymerization is preferred. The same applies to the water-absorbing agent as the final product. Regarding neutralization conditions such as the neutralization apparatus, neutralization temperature, and residence time, the conditions described in International Publication No. 2009 / 123197 may be applied to the present invention, and all of their contents are incorporated into this specification by reference.

[0049] (Other monomers) In one embodiment, the aqueous solution of (meth)acrylic acid (salt) monomers may contain monomers other than (meth)acrylic acid (salt) (hereinafter referred to as "other monomers"). In one embodiment, specific examples of other monomers include the monomers (excluding acrylic acid (salt)) described in U.S. Patent Application Publication No. 2005 / 0215734. All of their contents are incorporated by reference into this specification. In one embodiment, other monomers may include water-soluble or hydrophobic unsaturated monomers.

[0050] (Concentration of monomers) The concentration of (meth)acrylic acid (salt) monomers in the monomer aqueous solution is 38 to 50% by mass. In one embodiment, the concentration of (meth)acrylic acid (salt) monomers in the monomer aqueous solution is the mass ratio of acrylate salts and acrylic acid to the monomer aqueous solution (even if the monomer aqueous solution contains other monomers besides (meth)acrylic acid (salt)). In one embodiment, the concentration of (meth)acrylic acid (salt) monomers in the monomer aqueous solution is 39% by mass or more, 40% by mass or more, 41% by mass or more, 42% by mass or more, 43% by mass or more, or 44% by mass or more. In one embodiment, the concentration of (meth)acrylic acid (salt) monomers in the monomer aqueous solution is 49% by mass or less, 48% by mass or less, 47% by mass or less, 46% by mass or less, 45% by mass or less, less than 45% by mass, or 44% by mass or less.

[0051] The concentration of the (meth)acrylic acid (salt) monomer can be calculated from the following formula (1).

[0052] Concentration (mass%) of (meth)acrylic acid (salt) monomer = [(mass of (meth)acrylic acid (salt) monomer) / (mass of monomer aqueous solution)] × 100 ... Equation (1).

[0053] (Internal crosslinking agent) In one embodiment, the aqueous solution of (meth)acrylic acid (salt) monomer contains an internal crosslinking agent. In one embodiment, the internal crosslinking agent is the internal crosslinking agent described in U.S. Patent No. 6,241,928, all of which are incorporated by reference into this specification. In one embodiment, the internal crosslinking agent has two or more polymerizable unsaturated groups. In one embodiment, the internal crosslinking agent has a polyhydric alcohol structure such as glycerol, trimethylolpropane, or (poly)alkylene glycol, and has two or more polymerizable unsaturated groups. It is also possible that the glycerol or trimethylolpropane has two or more polymerizable unsaturated groups after alkylene glycol has been added.

[0054] In one embodiment, the polymerizable unsaturated group is an allyl group or a (meth)acrylate group, etc. The n-number of the (poly)alkylene glycol structure is preferably 1 to 100, 6 to 50, 5 to 15, or 7 to 12.

[0055] In one embodiment, the internal crosslinking agent is (poly)alkylene glycol diacrylate or (poly)alkylene glycol triacrylate. In one embodiment, (poly)alkylene glycol diacrylate is (poly)ethylene glycol diacrylate.

[0056] In one embodiment, the amount of internal crosslinking agent used is preferably 0.0001 mol% to 10 mol%, more preferably 0.001 mol% to 5 mol%, even more preferably 0.01 mol% to 1 mol%, and even more preferably 0.01 to 0.1 mol%, relative to the total amount of monomer (excluding the internal crosslinking agent; in particular, the total amount of the (meth)acrylic acid (salt)). By using an amount within this range, a water absorbent with the desired water absorption performance can be obtained.

[0057] The above describes an embodiment in which the internal crosslinking agent is added in advance when preparing the monomer aqueous solution. In this embodiment, the crosslinking reaction is carried out simultaneously with the polymerization reaction. In another embodiment, the polymerization reaction can be started without adding the internal crosslinking agent to the monomer aqueous solution, and the internal crosslinking agent can be added during or after the polymerization reaction to carry out the crosslinking reaction. These methods can also be used in combination.

[0058] In one embodiment, the step of preparing the (meth)acrylic acid (salt) monomer aqueous solution includes mixing the monomer with water and a basic salt aqueous solution, and the monomer temperature is 15°C or higher, 20°C or higher, or 25°C or higher. In another embodiment, the step of preparing the (meth)acrylic acid (salt) monomer aqueous solution includes mixing the monomer with water and a basic salt aqueous solution, and the monomer temperature is 45°C or lower, 40°C or lower, or 35°C or lower.

[0059] In one embodiment, the step of preparing the (meth)acrylic acid (salt) monomer aqueous solution includes mixing the monomer, water, and a basic salt aqueous solution. In one embodiment, the temperature of the basic salt aqueous solution is 20°C or higher, 21°C or higher, 23°C or higher, 25°C or higher, 27°C or higher, or 29°C or higher. Having such a lower temperature limit allows the polymerization initiation temperature to be efficiently raised to 50°C or higher while also utilizing the heat of neutralization between the acid group in the monomer and the basic salt aqueous solution. In one embodiment, when the step of preparing the (meth)acrylic acid (salt) monomer aqueous solution includes mixing the monomer, water, and a basic salt aqueous solution, the temperature of the basic salt aqueous solution is 45°C or lower, 43°C or lower, 40°C or lower, or 30°C or lower.

[0060] (A process of polymerizing an aqueous solution of (meth)acrylic acid (salt) based monomers) This step involves polymerizing the aqueous solution of the (meth)acrylic acid (salt) monomer. This step yields a hydrated gel-like crosslinked polymer (hereinafter also referred to as "hydrated gel").

[0061] (Polymerization initiator) In this process, at least one polymerization initiator is used. In one embodiment, the polymerization initiator may be at least one of a thermal decomposition type polymerization initiator and a photodecomposition type polymerization initiator. In one embodiment, a redox polymerization initiator (e.g., L-ascorbic acid) used in combination with a reducing agent that promotes the decomposition of the polymerization initiator may also be used.

[0062] In one embodiment, the polymerization initiator is the polymerization initiator described in U.S. Patent No. 7,265,190, the entire contents of which are incorporated by reference into this specification. In one embodiment, the polymerization initiator is a peroxide or an azo compound. As peroxides, persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate are preferred. As azo compounds, 2,2'-azobis(2-methylpropionamidine) dihydrochloride is preferred.

[0063] In one embodiment, the amount of polymerization initiator used is preferably 0.001 mol% to 1 mol%, more preferably 0.001 mol% to 0.5 mol%, and even more preferably 0.01 mol% to 0.2 mol%, relative to the total monomer (excluding the internal crosslinking agent; in particular, the total amount of the (meth)acrylic acid (salt)). In another embodiment, the amount of reducing agent used is preferably 0.0001 mol% to 0.02 mol%, more preferably 0.0005 mol% to 0.015 mol%, relative to the total monomer (excluding the internal crosslinking agent; in particular, the total amount of the (meth)acrylic acid (salt)). Reducing the amount of polymerization initiator has the effect of reducing the amount of soluble matter. On the other hand, if the amount is too low, the polymerization reaction may become unstable, and the amount of residual monomer may increase.

[0064] In one embodiment, the polymerization may be initiated by irradiation with active energy rays such as radiation, electron beams, or ultraviolet rays. Alternatively, irradiation with active energy rays and a polymerization initiator may be used in combination.

[0065] (polymerization form) In one embodiment, examples of polymerization methods include aqueous solution polymerization, reversed-phase suspension polymerization, spray polymerization, droplet polymerization, bulk polymerization, and precipitation polymerization. Among these, aqueous solution polymerization or reversed-phase suspension polymerization is preferred from the viewpoint of ease of polymerization control and the water absorption performance of the water absorbent, aqueous solution polymerization is more preferred, and continuous aqueous solution polymerization is even more preferred.

[0066] In one embodiment, the reverse-phase suspension polymerization can be described in International Publication No. 2011 / 126079, Japanese Patent Publication No. 2009-509722, etc., and all of their contents are incorporated into this specification by reference.

[0067] Continuous aqueous solution polymerization, one embodiment of aqueous solution polymerization, can produce water absorbents with high productivity. Specific examples include continuous belt polymerization described in U.S. Patent No. 4,893,999, and continuous kneader polymerization described in U.S. Patent No. 6,987,151, all of which are incorporated by reference into this specification.

[0068] In one embodiment, the polymerization initiation temperature is set to 25°C or higher, 30°C or higher, 35°C or higher, 36°C or higher, 40°C or higher, 50°C or higher, 55°C or higher, 60°C or higher, 65°C or higher, 65°C or higher, 70°C or higher, 75°C or higher, 80°C or higher, greater than 80°C, 81°C or higher, 82°C or higher, 84°C or higher, 86°C or higher, or 87°C or higher. In one embodiment, the polymerization initiation temperature is set to less than the boiling point of the monomer aqueous solution, 98°C or lower, 96°C or lower, 94°C or lower, 92°C or lower, 90°C or lower, 88°C or lower, less than 88°C, 87°C or lower, 86°C or lower, less than 86°C, 85°C or lower, 70°C or lower, 60°C or lower, 50°C or lower, 40°C or lower, or 30°C or lower. The polymerization initiation temperature is the temperature at which, after adding a polymerization initiator to an aqueous solution of (meth)acrylic acid (salt) monomer, an increase in the temperature of the aqueous solution is confirmed, or an increase in the viscosity of the monomer aqueous solution is confirmed. More specifically, it may be the value measured by the method described in the examples. By significantly increasing the polymerization initiation temperature, the latent heat of vaporization of water evaporated by the heat of the polymerization reaction can be utilized, making it easier to control the temperature rise ΔT within the polymerization reaction system to a small value. It also becomes easier to reduce the amount of residual monomer.

[0069] In one embodiment, the polymerization peak temperature is set to 60°C or higher, 65°C or higher, 70°C or higher, 75°C or higher, 80°C or higher, 85°C or higher, 90°C or higher, 95°C or higher, 100°C or higher, 105°C or higher, 106°C or higher, 107°C or higher, or 112°C or higher. In another embodiment, the polymerization peak temperature is set to 130°C or lower, 125°C or lower, 120°C or lower, 117°C or lower, 115°C or lower, 114°C or lower, 113°C or lower, 110°C or lower, or 105°C or lower. The polymerization peak temperature is the temperature at which the temperature of the aqueous solution reaches its highest point during the polymerization of an aqueous solution of (meth)acrylic acid (salt) monomer from the start of polymerization to the end of polymerization. More specifically, it may be the value measured by the method described in the examples.

[0070] The difference ΔT between the polymerization peak temperature and the polymerization start temperature is 50°C or less, but in one embodiment, ΔT is 48°C or less, 46°C or less, 44°C or less, 43°C or less, 42°C or less, 40°C or less, 38°C or less, 36°C or less, 34°C or less, 32°C or less, 30°C or less, 28°C or less, 27°C or less, 26°C or less, less than 26°C, or 25°C or less. By significantly lowering ΔT, the amount of residual monomer can be reduced. In one embodiment, ΔT is 15°C or more, 17°C or more, 19°C or more, 21°C or more, 23°C or more, 24°C or more, or greater than 24°C.

[0071] In this way, by appropriately adjusting the polymerization start temperature, the polymerization peak temperature, and the difference between them, the intended effects of the present invention can be efficiently achieved.

[0072] (Method for controlling the polymerization initiation temperature) There are no restrictions on the method of controlling the polymerization initiation temperature, but methods include controlling the temperature of the monomer aqueous solution at which polymerization begins by adjusting the temperatures of the acrylic acid (salt) monomer aqueous solution, water, and basic salt aqueous solution being mixed, taking into account the heat of the neutralization reaction, or by allowing heat to dissipate as appropriate during stirring, or by heating or cooling the monomer aqueous solution from the outside after mixing. More specifically, the heat of neutralization and / or hydration generated when preparing the monomer aqueous solution is used to raise the temperature. Any excess or deficiency of the polymerization initiation temperature is controlled by adjusting the temperature of one or more of the (meth)acrylic acid (salt) monomer, water, basic salt, and monomer aqueous solution, but control by adjusting the temperature of the basic salt is preferred, and control by adjusting the temperature of the water is most preferred. It is preferable to keep the adjustment and heating of the (meth)acrylic acid (salt) monomer and / or monomer aqueous solution to the minimum necessary. Also, since the dimer of (meth)acrylic acid (salt) increases during the adjustment and heating period, it is preferable to keep the time as short as possible.

[0073] (Method for controlling the polymerization peak temperature) There are no restrictions on the method of controlling the polymerization peak temperature, but it can be controlled, for example, by appropriately adjusting the neutralization rate of (meth)acrylic acid (salt) and the concentration of (meth)acrylic acid (salt) monomers in the monomer aqueous solution. More specifically, the polymerization peak temperature rises as the concentration of (meth)acrylic acid (salt) monomers in the monomer aqueous solution increases. On the other hand, if the temperature inside the system exceeds 100°C during the polymerization reaction, the water in the monomer aqueous solution where gelation is progressing evaporates, and the heat of the polymerization reaction is taken away as latent heat of vaporization, which is effective in suppressing the polymerization peak temperature. Furthermore, by exhausting the steam (water vapor) that fills the polymerization reaction vessel, the evaporation of water from the monomer aqueous solution where gelation is progressing is promoted, and the polymerization peak temperature is further suppressed. The polymerization peak temperature can also be controlled by controlling the jacket temperature and refrigerant of the polymerization apparatus.

[0074] In one embodiment, the polymerization time is set to 60 minutes or less, 45 minutes or less, 30 minutes or less, 24 minutes or less, 20 minutes or less, 15 minutes or less, 10 minutes or less, 8 minutes or less, 6 minutes or less, or 4 minutes or less. Productivity can be increased by significantly reducing the polymerization time. In one embodiment, the polymerization time is set to 15 seconds or more, 30 seconds or more, 1 minute or more, 2 minutes or more, 10 minutes or more, or 20 minutes or more. Polymerization time is the time from the start of polymerization to the end of polymerization in the polymerization process of an aqueous (meth)acrylic acid (salt) monomer solution, and the end of polymerization is the time when the temperature of the aqueous monomer solution reaches the maximum temperature (polymerization peak temperature).

[0075] By appropriately adjusting the polymerization time in this way, the intended effects of the present invention can be efficiently achieved while also contributing to improved productivity.

[0076] In one embodiment, after polymerization is complete, the gelled monomer aqueous solution can be aged by maintaining its temperature at 50°C or higher, 55°C or higher, 60°C or higher, 65°C or higher, or 70°C or higher. In one embodiment, the holding time can be 1 minute or more, 3 minutes or more, 5 minutes or more, 8 minutes or more, or 10 minutes or more. In another embodiment, the holding time can be 60 minutes or less, 50 minutes or less, 40 minutes or less, or 30 minutes or less. This can further reduce the amount of residual monomer.

[0077] (Step to obtain particulate water-containing gel) In one embodiment, the method for producing a (meth)acrylic acid (salt)-based superabsorbent resin includes a step of gel-pulverizing the water-containing gel-like crosslinked polymer produced in the polymerization step to obtain particulate water-containing gel. This step may be referred to as "gel-pulverization" to distinguish it from the "pulverization" described later in the pulverization step.

[0078] In one embodiment, a gel grinder may be used for gel grinding. In one embodiment, the gel grinder may be a kneader, meat chopper, cutter mill, etc. When the polymerization method is kneader polymerization, the polymerization process and the gel grinding process are carried out simultaneously. Also, when a particulate water-containing gel is obtained in the polymerization process, such as inverse-phase suspension polymerization, spray polymerization, or droplet polymerization, the gel grinding process is considered to be carried out simultaneously with the polymerization process.

[0079] The operating conditions for gel grinding in one embodiment are those described in International Publication No. 2011 / 126079 and International Publication No. 2016 / 204302, and all of those contents are incorporated into this specification by reference.

[0080] In one embodiment, the method of controlling the particle size of the particulate water-containing gel is not limited, but for example, when using a screw extrusion type gel grinder such as a meat chopper, it can be controlled by adjusting the rotation speed, the diameter of the holes in the perforated plate (die), the number of holes, the thickness, the shape of the holes, etc.

[0081] (Particle size of particulate water-containing gel) The particle size of particulate water-containing gel can also be controlled by classification, formulation, etc. In one embodiment, the particle size of particulate water-containing gel is controlled by gel grinding. In one embodiment, the mass-average particle diameter (D50) is 300 μm to 2500 μm, 400 μm to 2000 μm, and 500 μm to 1800 μm. Having a particle size within this range allows for more uniform shear compressive force on the particulate water-containing gel. Furthermore, in the subsequent drying process, the drying rate between the surface and interior of the particulate water-containing gel can be made more uniform, thereby improving the overall physical properties of the water-absorbing agent. Also, having a particle size within this range allows for appropriate drying in the subsequent drying process, further improving the overall physical properties of the water-absorbing agent.

[0082] In one embodiment, the gel solid content of the particulate water-containing gel is 40% by mass or more, 42% by mass or more, or 44% by mass or more. In another embodiment, the gel solid content of the particulate water-containing gel is 60% by mass or less, 55% by mass or less, or 53% by mass or less.

[0083] (Process for obtaining dry polymer) In one embodiment, the method for producing a (meth)acrylic acid (salt)-based superabsorbent resin includes a step of drying the particulate water-containing gel to obtain a dried polymer. In this step, the resin can be dried until it reaches a desired range of resin solids to obtain a dried polymer. The above resin solids are a value calculated from the loss on drying (the change in mass when 1 g of the sample is dried at 180°C for 3 hours). In one embodiment, the above resin solids are 80% by mass or more, 85-99% by mass, 90-98% by mass, or 92-97% by mass.

[0084] In one embodiment, drying methods include heating drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with hydrophobic organic solvents, and high-humidity drying using high-temperature steam. Among these, from the viewpoint of drying efficiency, hot air drying is preferred, and hot air drying using a continuous ventilation belt is more preferred.

[0085] In one embodiment, the drying temperature is 120°C to 250°C, 130°C to 230°C, or 140°C to 210°C, from the viewpoint of drying efficiency and the color tone of the absorbent. In one embodiment, the drying time is 5 minutes or more, 10 minutes or more, 20 minutes or more, or 25 minutes or more. In one embodiment, the drying time is 3 hours or less, 2 hours or less, 1.5 hours or less, or 1 hour or less.

[0086] In one embodiment, the drying temperature is usually defined by the temperature of the heat transfer medium (for example, the temperature of the hot air in the case of hot air drying), but in the case of drying that cannot be defined by the temperature of the heat transfer medium, such as microwave drying, the drying temperature is defined by the temperature of the particulate water-containing gel. Furthermore, the drying temperature may be constant, but it may be changed as appropriate during the drying process. Regarding drying conditions other than the drying temperature and drying time, they may be set as appropriate according to the water content of the particulate water-containing gel, the amount supplied to the drying process, and the target resin solid content. When hot air drying using a continuous ventilation belt is selected, the drying conditions described in International Publication No. 2006 / 100300, etc., may be applied to the present invention.

[0087] (Process for obtaining resin powder (water-absorbing resin powder)) In one embodiment, a method for producing a (meth)acrylic acid (salt)-based superabsorbent resin includes a step of crushing and / or classifying the dried polymer to obtain resin powder. Here, resin powder (superabsorbent resin powder) refers to the superabsorbent resin before surface crosslinking is applied.

[0088] In one embodiment, the pulverizer used for grinding is not limited, but examples of pulverizers include high-speed rotary pulverizers such as roll mills, hammer mills, screw mills, and pin mills, as well as vibratory mills, knuckle-type pulverizers, and cylindrical mixers. Multiple pulverizers can also be used in combination. Examples of classification methods include sieve classification using JIS standard sieves (JIS Z 8801-1(2000)) and airflow classification.

[0089] In one embodiment, the dry polymer is pulverized and / or classified at 20°C or higher, 30°C or higher, 40°C or higher, or 50°C or higher. In another embodiment, the dry polymer is pulverized and / or classified at 120°C or lower, 110°C or lower, 100°C or lower, or 90°C or lower. Pulverizing and / or classifying at 50°C or higher can suppress or prevent the increase in tackiness of the water-absorbing resin due to condensation, especially in the environment of continuous commercial operation. In this embodiment, all pulverization and classification were performed after cooling to 25°C.

[0090] (Physical properties of resin powder (water-absorbing resin powder)) In one embodiment, the CRC of the water-absorbent polymer powder is 24 g / g or more, 25 g / g or more, 26 g / g or more, 27 g / g or more, 28 g / g or more, 29 g / g or more, or 30 g / g. Since surface crosslinking tends to decrease the CRC, if the CRC of the water-absorbent polymer powder (which is the target of surface crosslinking) is 24 g / g or more, the CRC of the final product will also be above a certain value. In one embodiment, the CRC of the water-absorbent polymer powder is 45 g / g or less, more preferably 42 g / g or less, and even more preferably 40 g / g or less. The CRC of the water-absorbent polymer powder may be controlled by the amount of internal crosslinking agent during polymerization, monomer concentration, polymerization start temperature, polymerization peak temperature, the difference ΔT between the polymerization peak temperature and the polymerization start temperature, drying temperature, etc.

[0091] In one embodiment, the mass-average particle size (D50) of the water-absorbent resin powder is 300 μm or more, 310 μm or more, 320 μm or more, 330 μm or more, 340 μm or more, or 350 μm or more. In another embodiment, the mass-average particle size (D50) of the water-absorbent resin powder is 500 μm or less, 490 μm or less, or 480 μm or less. The mass-average particle size (D50) of the water-absorbent resin powder can be controlled by the particle size of the water-containing gel, pulverization after drying, classification, etc.

[0092] In one embodiment, the water-absorbent resin powder preferably satisfies at least one of the following physical properties, and more preferably satisfies all of them: CRC of 25 g / g or more, and mass-average particle size (D50) of 320 to 500 μm.

[0093] (Step to obtain surface-crosslinked resin (surface-crosslinked water-absorbing resin particles)) In one embodiment, a method for producing a (meth)acrylic acid (salt)-based superabsorbent resin includes a step of surface crosslinking the resin powder to obtain a surface-crosslinked resin. This step is a step of providing a portion with a higher crosslinking density on the surface layer of the superabsorbent resin powder. In one embodiment, the surface crosslinking step includes a mixing step and a heat treatment step, and optionally includes a cooling step, etc. In the surface crosslinking step, radical crosslinking, surface polymerization, crosslinking reactions with a surface crosslinking agent, etc. occur on the surface of the superabsorbent resin powder, and a surface-crosslinked superabsorbent resin (hereinafter also referred to as "surface-crosslinked superabsorbent resin particles") is obtained.

[0094] (Mixing process) This process involves mixing a solution containing a surface crosslinking agent (hereinafter referred to as "surface crosslinking agent solution") with water-absorbent resin powder in a mixing device.

[0095] (Surface crosslinking agent) In one embodiment, the surface crosslinking agent is the surface crosslinking agent described in U.S. Patent No. 7,183,456, all of which are incorporated by reference into this specification.

[0096] In one embodiment, the surface crosslinking agent is preferably an organic compound having two or more functional groups that react with carboxyl groups and form a covalent bond. In one embodiment, suitable surface crosslinking agents include ethylene glycol, propylene glycol, glycerin, pentaerythritol, sorbitol, ethylene carbonate, propylene carbonate, diethylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentadiol, 2,4-pentadiol, 1,2-hexanediol, 1,6-hexanediol, 2,5-hexanediol, trimethylolpropane, diethanolamine, 4,5-dimethyl-1,3-dioxolan-2-one, triethylene glycol, tetraethylene glycol, and dipropylene glycol. Examples include tripropylene glycol, triethanolamine, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, ethylenediamine, diethylenetriamine, triethylenetetramine, 2,4-tolylene diisocyanate, hexamethylene diisocyanate, epichlorohydrin, epibromohydrin, etc. Any two or more of these can be combined.

[0097] In one embodiment, the amount of surface crosslinking agent used (total amount if multiple types are used) is 0.01 to 10 parts by mass, 0.01 to 5 parts by mass, or 0.01 to 2.5 parts by mass per 100 parts by mass of water-absorbent resin powder. By using the surface crosslinking agent within this range, an optimal crosslinking structure can be formed on the surface layer of the water-absorbent resin powder, resulting in a water-absorbent agent with high physical properties.

[0098] In one embodiment, the surface crosslinking agent added to the water-absorbent resin powder may be in the form of an aqueous solution. In one embodiment, the amount of water is 0.1 to 20 parts by mass, 0.3 to 15 parts by mass, or 0.5 to 10 parts by mass per 100 parts by mass of water-absorbent resin powder. By keeping the amount of water used within this range, the handling of the surface crosslinking agent solution is improved, and the surface crosslinking agent can be uniformly mixed with the water-absorbent resin powder. In one embodiment, the surface crosslinking agent solution may contain a hydrophilic (water-soluble) organic solvent. In one embodiment, the amount of hydrophilic organic solvent used is 10 parts by mass or less, 5 parts by mass or less, or 1 part by mass or less per 100 parts by mass of water-absorbent resin powder. In one embodiment, specific examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, and isopropyl alcohol; ketones such as acetone; ethers such as dioxane; amides such as N,N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; polyhydric alcohols such as ethylene glycol; and the like.

[0099] (Mixing method, mixing conditions) In one embodiment, (i) a surface crosslinking agent may be added to the water-absorbent resin powder, and water may be added after the addition of the surface crosslinking agent; (ii) a portion of the surface crosslinking agent may be added, and then the remaining surface crosslinking agent and water may be mixed and added; (iii) a mixed solution of water and a water-soluble organic solvent may be added after the addition of the surface crosslinking agent; and (iv) a pre-prepared surface crosslinking agent solution may be added and mixed. Preferably, (iv) the surface crosslinking agent solution is prepared in advance, and the solution is sprayed or dropped onto the water-absorbent resin powder to mix the water-absorbent resin powder and the surface crosslinking agent solution. In one embodiment, the mixing device used for the above mixing has the torque necessary to uniformly and reliably mix the water-absorbent resin powder and the surface crosslinking agent. In one embodiment, the mixing device is a high-speed stirring type mixer such as a Redigge mixer, turbulizer, or vertical mixer, and among these, a high-speed stirring type continuous mixer is preferred.

[0100] In one embodiment, the temperature of the water-absorbing resin powder when mixed with the surface crosslinking agent is 20°C or higher, or 35°C or higher. In one embodiment, the temperature of the water-absorbing resin powder when mixed with the surface crosslinking agent is 85°C or lower, 80°C or lower, 70°C or lower, or 60°C or lower. In one embodiment, the mixing time is 1 second to 1 hour, or 5 seconds to 10 minutes.

[0101] (Heat treatment process) This process involves applying heat to a mixture of a surface crosslinking agent and a water-absorbent resin powder to cause a crosslinking reaction on the surface of the water-absorbent resin powder.

[0102] (Heat treatment method, heat treatment conditions) The heat treatment may be carried out with the mixture in a static state or in a fluid state using power such as stirring. The latter is preferred because it allows the entire mixture to be heated uniformly in a short time.

[0103] In one embodiment, the heat treatment apparatus may include a paddle dryer, a multi-fin processor, a tawdar dryer, and the like.

[0104] In one embodiment, the heating temperature is 80°C or higher, 90°C or higher, 100°C or higher, 120°C or higher, 130°C or higher, 150°C or higher, 160°C or higher, 170°C or higher, or 180°C or higher. In one embodiment, the upper limit of the heating time is 2 hours or less, 1 hour or less, 50 minutes or less, 40 minutes or less, 30 minutes or less, 20 minutes or less, 18 minutes or less, 16 minutes or less, or 14 minutes or less. In the present invention, the neutralization rate of (meth)acrylic acid (salt) is less than 55 mol%. Therefore, the reaction rate of surface crosslinking is fast, and for example, if the heating temperature is 160°C or higher, sufficient surface crosslinking can be performed within 30 minutes. Also, if the heating temperature is 160°C or higher, the process proceeds in a direction that balances water absorption performance and process damage resistance. In one embodiment, the heating temperature is 250°C or lower, 230°C or lower, 210°C or lower, 200°C or lower, or 190°C or lower. In one embodiment, the lower limit of the heating time is 5 minutes or more, or 7 minutes or more.

[0105] (cooling process) This step is an optional step that may be added after the heat treatment step as needed. This step involves forcibly cooling the high-temperature surface-crosslinked water-absorbent resin particles that have completed the heat treatment step to a predetermined temperature, thereby quickly terminating the surface crosslinking reaction.

[0106] (Cooling method, cooling conditions) The above-mentioned surface-crosslinked water-absorbent resin particles may be cooled in a static state or in a fluid state using power such as stirring, but cooling under stirring is preferable because it allows for uniform cooling of the entire surface-crosslinked water-absorbent resin particle system in a short time.

[0107] In one embodiment, the cooling device may be a paddle dryer, a multi-fin processor, a tower dryer, etc. These cooling devices can also be the same specifications as the heat treatment equipment used in the heat treatment process. This is because they can be used as cooling devices by changing the heat transfer medium of the heat treatment equipment to a refrigerant. In one embodiment, the temperature of the surface-crosslinked water-absorbing resin particles after cooling can be appropriately set according to the heating temperature in the heat treatment process, the water absorption performance of the water-absorbing agent, etc., but is typically 40°C to 100°C, 40°C to 90°C, 50°C to 80°C, or 50°C to 70°C.

[0108] In one embodiment, the moisture content (at 105°C for 3 hours) of the surface-crosslinked resin (surface-crosslinked water-absorbing resin particles) is controlled to be between 0.2 and 3.0% by mass. Within this range, deterioration of powder flowability due to static electricity is suppressed. This suppresses electrostatic adsorption to bags (e.g., made of polyethylene) into which the particles can be filled, and to metals that cause friction as the powder flows, improving operability during filling and discharging of bags, and reducing the risk of equipment blockage and operational problems due to metal adhesion.

[0109] (Additive addition process) This process involves adding additives to the resin powder (water-absorbing resin powder) used in the surface crosslinking process described above, or to the surface-crosslinked water-absorbing resin particles (surface-crosslinked resin) obtained through the surface crosslinking process described above.

[0110] In one embodiment, the additive is at least one compound selected from water and / or water vapor, polyvalent metal salts, cationic polymers, inorganic fine particles, chelating agents, inorganic reducing agents, surfactants, deodorants, and antibacterial agents. Two or more types of additives may be used in combination as needed. The amount of the additive added is set appropriately depending on the selected compound. In one embodiment, the mixing of the resin powder (water-absorbing resin powder) subjected to the surface crosslinking step, and the surface-crosslinked water-absorbing resin particles (surface-crosslinked resin) obtained through the surface crosslinking step, with the additive is the same as the mixing method and mixing conditions used in the surface crosslinking agent addition step.

[0111] (Water and / or water vapor) When water and / or water vapor are used as additives, in one embodiment, from the viewpoint of suppressing static charge of the resulting water absorbent (or water-absorbing resin), the amount of water added is 0-10% by mass, 0.05-7.5% by mass, 0.1-5% by mass, or 0.2-2.5% by mass relative to the water-absorbing resin. In one embodiment, when mixing the water-absorbing resin and water, a hydrophilic organic solvent may be used as a mixed solvent. In one embodiment, the hydrophilic organic solvent is a lower alcohol such as methyl alcohol, ethyl alcohol, or isopropyl alcohol; a ketone such as acetone; an ether such as dioxane; an amide such as N,N-dimethylformamide; a sulfoxide such as dimethyl sulfoxide; or a polyhydric alcohol such as ethylene glycol. In one embodiment, the amount of hydrophilic organic solvent added is 0-500% by mass, 1-400% by mass, 5-300% by mass, or 10-200% by mass relative to the water. In one embodiment, after mixing the water-absorbing resin with water and / or an aqueous solution of a hydrophilic organic solvent, the surface of the water-absorbing agent (or water-absorbing resin) is dried at 50-120°C, 55-110°C, or 60-100°C to dry the hydrophilic organic solvent and to volatilize it.

[0112] (Polyvalent metal salts) When using a polyvalent metal salt as an additive, the polyvalent metal cations of the polyvalent metal salt are preferably 2 or higher, more preferably 2 to 4, and even more preferably 3 or 4. Examples of usable polyvalent metals include aluminum and zirconium. Therefore, examples of polyvalent metal salts include aluminum lactate, zirconium lactate, aluminum sulfate, and zirconium sulfate. From the viewpoint of improving SFC, aluminum lactate or aluminum sulfate is more preferred, and aluminum sulfate is even more preferred. For the above polyvalent metal salt, the amount of metal cations in the polyvalent metal salt should be 3.6 × 10⁶ per 1 g of water-absorbent resin powder or surface-crosslinked water-absorbent resin particles. -5 Less than a mole, 1.4 × 10⁻⁶ -5 Less than a mole, or 1.0 × 10⁻⁶ -5 It is added in a quantity less than one mole.

[0113] (cationic polymer) When a cationic polymer is used as an additive, examples of cationic polymers include those described in U.S. Patent No. 7,098,284, all of which are incorporated by reference into this specification. Among these, vinylamine polymers are more preferred from the viewpoint of improving SFC. Furthermore, the weight-average molecular weight of the cationic polymer is preferably 5,000 to 1,000,000. In one embodiment, the cationic polymer is added in an amount of less than 2.5 parts by mass, less than 2.0 parts by mass, or less than 1.0 part by mass per 100 parts by mass of water-absorbent resin powder or surface-crosslinked water-absorbent resin particles.

[0114] (Inorganic fine particles) When inorganic fine particles are used as an additive, examples of inorganic fine particles include substances described in U.S. Patent No. 7,638,570, all of which are incorporated by reference into this specification. In one embodiment, from the viewpoint of improving SFC, the inorganic fine particles are silicon dioxide. In one embodiment, the average particle size of the inorganic fine particles is 10 to 100 nm. In one embodiment, the amount of inorganic fine particles added is less than 2.0 parts by mass, less than 1.5 parts by mass, less than 1.2 parts by mass, or less than 1.0 part by mass per 100 parts by mass of the water-absorbing resin.

[0115] (Other processes) In one embodiment, the manufacturing method of the present invention includes at least one of the following: a granulation step, a sizing step, a fine powder removal step, a fine powder recovery step, a fine powder reuse step, a step of adding other additives, an iron removal step, etc. In one embodiment, the manufacturing method of the present invention includes at least one step selected from a transportation step, a storage step, a packaging step, a storage step, etc.

[0116] In one embodiment, the sizing step includes a step of classifying and removing fine powder after the surface crosslinking step, and a step of classifying and grinding when the surface crosslinked water-absorbing resin particles aggregate and exceed the desired size.

[0117] In one embodiment, the fine powder reuse step includes adding the fine powder as is, or after converting it into a large water-containing gel in a granulation step, to one of the steps in the manufacturing process of the water-absorbent resin.

[0118] In one embodiment, the step of adding other additives refers to the step of adding chelating agents, inorganic reducing agents, organic reducing agents, hydroxycarboxylic acid compounds, surfactants, compounds containing phosphorus atoms, oxidizing agents, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, thermoplastic fibers, and other additives in order to impart various functions to the water-absorbent resin. These other additives can be mixed simultaneously with or separately from the above-mentioned surface crosslinking agent and / or additives.

[0119] In one embodiment, the other additive is a chelating agent. In one embodiment, the chelating agent is an aminopolycarboxylic acid or an aminopolyphosphate. In one embodiment, these chelating agents may be added to or contained in an aqueous solution of (meth)acrylic acid (salt) monomer or a water-absorbent resin in amounts of 0.001% to 2% by mass, 0.01% to 1.5% by mass, 0.1% to 1.3% by mass, or 0.5% to 1.2% by mass.

[0120] In one embodiment, the chelating agent is a chelating agent with high ion-sealing ability and chelating ability for Fe and Cu. Specifically, a chelating agent with a stability constant of 10 or more, preferably 20 or more, for Fe ions is more preferred, an aminopolycarboxylic acid and its salt or an aminopolyphosphate and its salt is even more preferred, and an aminopolycarboxylic acid and its salt having three or more carboxyl groups is particularly preferred.

[0121] Furthermore, some of the acid groups contained in the amino polycarboxylic acid salt may be neutralized, or all of the acid groups may be neutralized. Specifically, these amino polycarboxylic acids include diethylenetriaminepentaacetic acid, triethylenetetraaminehexaacetic acid, cyclohexane-1,2-diaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, ethylene glycol diethyl etherdiaminetetraacetic acid, ethylenediaminetetrapropionacetic acid, N-alkyl-N'-carboxymethyl aspartic acid, N-alkenyl-N'-carboxymethyl aspartic acid, and alkali metal salts, alkaline earth metal salts, ammonium salts, or amine salts thereof.

[0122] Furthermore, in the case of aminopolyhydric phosphates, some of the acid groups contained may be neutralized, or all of the acid groups may be neutralized. These polyphosphates specifically include ethylenediamine-N,N'-di(methylenephosphinic acid), ethylenediaminetetra(methylenephosphinic acid), nitriloacetate-di(methylenephosphinic acid), nitrilodiacetate-(methylenephosphinic acid), nitriloacetate-β-propionic acid-methylenephosphonic acid, nitrilotris(methylenephosphonic acid), cyclohexanediaminetetra(methylenephosphonic acid), ethylenediamine-N,N'-diacetate-N,N'-di(methylenephosphonic acid), ethylenediamine-N,N'-di(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), polymethylenediaminetetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid), 1-hydroxyethylidenediphosphonic acid, and their salts.

[0123] One or more chelating agents selected from these groups are used. Among them, diethylenetriaminepentaacetic acid, triethylenetetraaminehexaacetic acid, N-hydroxyethylethylenediaminetriacetic acid and their salts are preferred.

[0124] In the present invention, the amount of chelating agent, particularly aminopolycarboxylic acid, used is in trace amounts, usually 0.00001 to 10 parts by mass, preferably 0.0001 to 1 part by mass, per 100 parts by mass of water-absorbent resin powder or surface-crosslinked water-absorbent resin particles.

[0125] In one embodiment, other additives include surfactants. Examples of surfactants include substances described in International Publication No. 2005 / 075070, all of which are incorporated by reference into this specification. In one embodiment, nonionic surfactants are used from the viewpoint of reducing process damage and suppressing static charge during the manufacturing process of the resulting water absorbent (or water absorbent resin). In one embodiment, the amount of surfactant added is 0 to 1% by mass, 0.05 ppm to 5000 ppm by mass, 0.1 to 2500 ppm by mass, or 1 to 1000 ppm by mass relative to the water absorbent resin.

[0126] In one embodiment, the other additive is a deodorant. Examples of deodorants include substances described in International Publication No. 2009 / 048145, all of which are incorporated by reference into this specification. In one embodiment, the deodorant is a bamboo extract or a tea extract. In one embodiment, the amount of deodorant added is 0 ppm to 5% by mass, 0.2 ppm to 4% by mass, or 0.5 ppm to 3% by mass.

[0127] In this invention, by specifying a high monomer concentration and a low neutralization rate of monomers within a specific range, and by reducing the temperature difference ΔT within the polymerization reaction system, it becomes easier to obtain acrylic acid (salt)-based superabsorbent resins with a specific range for both the branching density of the soluble component and the soluble component. In particular, the formation of soluble polymer chains with fewer branches is an unexpected effect. As a result, it becomes possible to obtain surface-crosslinked superabsorbent resin particles with high process damage resistance. Therefore, by using this embodiment, it is possible to provide surface-crosslinked resin particles with excellent process damage resistance. Furthermore, by specifying a high monomer concentration and a low neutralization rate of monomers within a specific range, and by reducing the temperature difference ΔT within the polymerization reaction system, it is possible to obtain a superabsorbent resin powder with a low dispersion degree Mw / Mn of the soluble polymer.

[0128] [4] Uses of absorbent materials containing (meth)acrylic acid (salt) based water-absorbing resins (absorbents) In one embodiment, (meth)acrylic acid (salt)-based superabsorbent resin is used as an absorbent. The absorbent is preferably used as the absorbent body of absorbent articles such as disposable diapers and sanitary napkins, and more preferably as the absorbent body of absorbent articles in which a large amount is used per absorbent article. The term "absorbent body" refers to the absorbent being molded into a sheet, fiber, tube, or the like.

[0129] The absorbent material described above can also be used in combination with absorbent materials such as pulp fibers, adhesives, nonwoven fabrics, etc., when forming absorbent materials such as pulp fibers, in addition to the water absorbent material according to the present invention. In this case, the amount of water absorbent material in the absorbent material (hereinafter referred to as "core concentration") 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.

[0130] By keeping the core concentration within the above range, the thickness of the absorbent material can be reduced, which is preferable as it alleviates discomfort for the wearer. Furthermore, the absorbent material using the absorbent agent according to the present invention is resistant to process damage that may be unintentionally applied to the absorbent agent during the diaper manufacturing process, and therefore has excellent absorption and diffusion capabilities for bodily fluids such as urine and blood, enabling efficient liquid distribution. As a result, the amount of absorbent material absorbed under load can be increased, which is preferable. In addition, the problem of discrepancies in the parameter properties of the absorbent resin before and after it is added to the absorbent article is improved, and the intended performance of the diaper is achieved.

[0131] [5] Absorbent articles In one embodiment, the absorbent article includes the absorbent material described above. Examples of absorbent articles include sanitary products such as disposable diapers and sanitary napkins. When the absorbent article is, for example, a disposable diaper, the diaper is manufactured by sandwiching an absorbent material containing the absorbent agent of the present invention between a liquid-permeable top sheet located on the side that touches the person's skin when worn and a liquid-impermeable back sheet located on the outside when worn. The disposable diaper is further provided with components known to those skilled in the art, such as adhesive tape for securing the diaper after it has been worn. [Examples]

[0132] The present invention will be described more specifically below with reference to examples and comparative examples. However, the present invention is not limited to these examples and comparative examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention. Unless otherwise noted, the electrical equipment used for measuring the various physical properties uses a 200V or 100V / 60Hz power supply. Unless otherwise noted, the various physical properties were measured under room temperature (20℃~25℃) and relative humidity of 50±5%RH. Also, for convenience, "liter" may be written as "l" or "L", and "mass%" and "weight%" may be written as "wt%". In the following measurement methods, the arithmetic mean value obtained from three measurements is used.

[0133] <Measurement method> [Polymerization start temperature, polymerization peak temperature (maximum temperature reached)] A Keyence NR-1000 PC card data acquisition system was used to measure the polymerization temperature with a calibrated thermocouple placed in the center of the polymerization system, with a sampling period of 0.1 seconds. The polymerization start temperature and peak temperature (highest temperature reached) were read from the resulting temperature-time chart.

[0134] [CRC (absorption ratio under no pressure)] The CRC (absorption ratio under no pressure) was measured according to the EDANA method (WSP241.3(10)). Specifically, 0.2 g of the sample to be measured (absorbent polymer) (i.e., absorbent polymer powder, absorbent agent (surface cross-linked absorbent polymer particles), etc.) was placed in a nonwoven fabric bag and immersed in a large excess of 0.9 mass% sodium chloride aqueous solution for 30 minutes to allow the sample to swell freely. After dehydration using a centrifuge (centrifugal force: 250 G), the CRC (absorption ratio under no pressure) (unit: g / g) was measured.

[0135] [AAP (Absorption Ratio under Pressure)] The AAP (absorption ratio under pressure) was measured according to the EDANA method (WSP242.3(10)). Specifically, a large excess of 0.9 mass% sodium chloride aqueous solution was used, and 0.9 g of the sample to be measured (absorbent polymer) (i.e., absorbent polymer powder, absorbent agent (surface cross-linked absorbent polymer particles), etc.) was subjected to 4.83 kPa (49 g / cm³) for 1 hour. 2 After swelling under a pressure of 0.7 psi, the AAP (absorption rate under pressure) (unit: g / g) was measured.

[0136] [SFC (Saline Flow Inducing)] SFC (Saline Flow Inducing) (Unit: ×10) -7 cm 3 The values ​​(·sec / g) were measured in accordance with the measurement method described in U.S. Patent No. 5669894.

[0137] [Moisture content] The moisture content was measured according to the EDANA method (WSP230.3(10)). Specifically, 4.0 g of the sample (absorbent polymer, i.e., absorbent polymer powder, absorbent agent (surface-crosslinked absorbent polymer particles, etc.)) was left in an oven at 105°C for 3 hours, and the moisture content (unit: mass%) was measured from the loss of drying.

[0138] [pH] pH was measured according to the EDANA method (WSP200.3(10)). Specifically, 0.5 g of the sample to be measured (absorbent polymer, i.e., absorbent polymer powder, absorbent agent (surface-crosslinked absorbent polymer particles), etc.) was dispersed in 100 ml of a 0.9 mass% sodium chloride aqueous solution, which had been pre-adjusted to pH 6.0, while stirring for 10 minutes, and then the pH (unit: dimensionless) was measured using a pH electrode.

[0139] [Amount of residual monomers] The amount of residual monomer was measured according to the EDANA method (WSP210.3(10)). Specifically, 1.0 g of the substance to be measured (absorbent polymer) (i.e., absorbent polymer powder, water absorbent agent (surface cross-linked absorbent polymer particles), etc.) was dispersed in 200 ml of 0.9 mass% sodium chloride aqueous solution while stirring for 1 hour, and the amount of residual monomer (unit: mass ppm) was measured by analyzing the filtrate by HPLC.

[0140] [Soluble content, molecular weight distribution of soluble content, branching density of soluble content] The soluble content (soluble content (16hr-Extr)) was measured according to the EDANA method (WSP270.3(10)). However, the extraction time for the soluble content was extended from 1 hour to 16 hours. Specifically, 1.0 g of the substance to be measured (absorbent polymer) (i.e., absorbent polymer powder, water absorbent agent (surface cross-linked absorbent polymer particles), etc.) was dispersed in 200 ml of 0.9 mass% sodium chloride aqueous solution while stirring for 16 hours, and then the filtrate was collected and the amount of soluble content (unit: mass%) was measured by potentiometric titration.

[0141] The molecular weight distribution (Mw / Mn) of the soluble portion was calculated by dividing the weight-average molecular weight (Mw) by the number-average molecular weight (Mn). The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were measured by gel permeation chromatography (GPC) after passing the filtrate obtained from the above measurement of soluble portion through a filter (GL Chromatodisk, aqueous system 25A, pore size 0.2 μm, manufactured by GL Sciences).

[0142] (GPC measurement conditions) Measurements were performed using the TDA302 (registered trademark) manufactured by Viscotec. The apparatus consisted of a size exclusion chromatography system, a refractive index detector, a light scattering detector, and a capillary viscometer. The measurement apparatus and conditions were as follows. The filtrate obtained from the soluble content measurement was subjected to GPC analysis.

[0143] Pump / Autosampler: Viscotec GPCmax Guard column: OHpak SB-G (manufactured by Showa Denko Corporation) Columns: Two OHpak SB-806MHQ columns (manufactured by Showa Denko Corporation) connected in series are used. Detector: Viscotec TDA302 (system temperature maintained at 30°C) Solvent: 60 mM sodium disodium phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, 400 ppm sodium azide aqueous solution (pH 6.35~6.38) Flow rate: 0.5mL / min Injection volume: 100μL.

[0144] The pure water used for this measurement must be thoroughly purified to remove impurities. Furthermore, the measurement should be performed with a sufficient amount of solvent flowing through the apparatus, ensuring a stable baseline for the detected values. In particular, the measurement should be performed without any noise peaks in the light scattering detector.

[0145] Instrument calibration was performed using polyoxyethylene glycol (weight-average molecular weight (Mw) 21966, molecular weight distribution (Mw / Mn = 1.00), differential refractive index (dn / dc) = 0.132, solvent refractive index 1.33) as a standard sample.

[0146] When the object to be measured (absorbent polymer) (i.e., absorbent polymer powder, absorbent agent (surface-crosslinked absorbent polymer particles), etc.) is obtained by polymerizing monomers containing 99 mol% or more of acrylic acid and / or its salts, the differential refractive index (dn / dc) of the polymer to be analyzed (soluble portion of the absorbent polymer) was set to 0.12 and the solvent refractive index to 1.33 for measurement. Furthermore, when the object to be measured (absorbent polymer) (i.e., absorbent polymer powder, absorbent agent (surface-crosslinked absorbent polymer particles), etc.) is copolymerized in an amount of more than 1 mol% other than acrylic acid and / or its salts, the differential refractive index (dn / dc) in the above solvent specific to that polymer can be measured and that value can be used.

[0147] After reviewing the measurement results chart, if the peaks in the light scattering intensity measurement results contained a lot of noise, the measurement was repeated.

[0148] Refractive index, light scattering intensity, and viscosity data were collected and analyzed using Viscotek OmniSEC 4.6.2 (registered trademark) software. From the refractive index (RI), light scattering intensity (angle 7°) LALS, and viscometer (DP) data, the weight-average molecular weight (Mw), number-average molecular weight (Mn), molecular weight distribution (Mw / Mn), and branching density (Branch Freq.) were calculated. For the measurement of branching density, the following values ​​and calculation methods were applied to the Branching Parameters of the software, and there were no upper or lower limits on the molecular weight used for calculation.

[0149] MH Exponent(a): 0.8741 MH Intercept(logK):-4.4152 g / mol Structure Factor: 0.75 See material:100000 Branching Calculation:Random,polydisperse(tri-functional).

[0150] [Particle size distribution and mass-average particle size (D50)] The particle size distribution and mass-average particle size (D50) (unit: μm) were measured in accordance with the measurement method described in "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution" in U.S. Patent No. 7638570. However, the sieves used were JIS standard sieves with mesh sizes of 850 μm, 710 μm, 600 μm, 500 μm, 300 μm, 150 μm, and 45 μm, and a receiving tray (THE IIDA TESTING SIEVE: diameter 8 cm). Specifically, 10.0 g of water-absorbing agent was placed in the sieve under room temperature (approximately 23°C) and 50% RH humidity conditions. Classification was performed for 5 minutes using a vibrating classifier (IIDA SIEVE SHAKER, TYPE: ES-65, SER. No. 0501). After that, the mass of water-absorbing resins such as water-absorbing agent remaining on the sieve was measured, and the percentage R of residue on the sieve relative to the total amount of water-absorbing resins such as water-absorbing agent was calculated as the particle size distribution. The proportion of particles with a particle size of 150 μm or more and less than 850 μm was determined as the sum of the residue percentages on the sieves for mesh sizes of 710 μm, 600 μm, 500 μm, 300 μm, and 150 μm. The proportion of particles with a particle size of 600 μm or more and less than 850 μm was determined as the sum of the residue percentages on the sieves for mesh sizes of 710 μm and 600 μm. Furthermore, by plotting the sieve residue percentage R on logarithmic probability paper, the particle size corresponding to R = 50 mass% was read as the mass-average particle diameter (D50, unit: μm).

[0151] [Process damage resistance evaluation] Process damage resistance was evaluated by comparing the physical properties of the absorbent obtained using the method described later with those of the absorbent after damage. Specifically, the reduction rate of absorption ratio under pressure (AAP reduction rate, unit: %) and the reduction rate of saline flow induction (SFC reduction rate, unit: %) were calculated, as expressed by the following formulas.

[0152] AAP reduction rate (%) = ((AAP before damage) - (AAP after damage)) / (AAP before damage) × 100 ... Equation (1) SFC decrease rate (%) = ((SFC before damage) - (SFC after damage)) / (SFC before damage) × 100 ... Equation (2).

[0153] [Handling of absorbent material] 150g of absorbent material was placed in a polyethylene bag (180mm x 270mm, 0.03mm thick) and stored for 24 hours. Approximately 1g of absorbent material was then taken from the bag using a stainless steel spatula. Immediately before taking the sample, the polyethylene bag was inflated with air to homogenize the particle size distribution of the absorbent material to be taken, and then the entire bag was shaken 10 times to mix the absorbent material inside the bag. Subsequently, the state of the absorbent material on the stainless steel spatula when taking the sample was evaluated according to the following criteria.

[0154] Evaluation criteria; A: The absorbent material (particulate or powdered) is not attracted to the surface of the polyethylene bag from the spatula, making it easy to remove.

[0155] B: Some of the water-absorbing agent (particulate and powdered) was attracted to the surface of the polyethylene bag from the spatula and adsorbed onto it. However, there were no particular problems with the actual workability.

[0156] [Electrostatic charge] The electrostatic charge was measured using an electrostatic charge measuring instrument (manufactured by Kasuga Electric Co., Ltd.) consisting of a Faraday gauge body KQ-1400 and a coulomb meter NK1001. The measurement environment was 23-24°C and 40-50% RH. The person performing the measurement wore insulating rubber gloves to prevent charge transfer between the person and the sample and the equipment during handling. 20.0 g of water-absorbing agent for measuring electrostatic charge was placed in the included stainless steel cup with a lid, the lid was closed, and the cup was grounded with an earth wire. The stainless steel cup was then shaken 100 times to create friction in the water-absorbing agent. After shaking, the lid was opened and the water-absorbing agent was placed in the charge measurement container of the Faraday gauge body. The absolute value of the charge measured by the coulomb meter was defined as the electrostatic charge (unit: nC).

[0157] [Example 1] An aqueous solution was prepared by adding 374 parts by mass of acrylic acid at 30°C, 1.36 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 2.15 parts by mass of 1.0% by mass diethylenetriaminepentaacetic acid trisodium aqueous solution, and 403 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was preheated to 30°C.

[0158] Next, while stirring the aqueous solution, 212 parts by mass of a 48% by mass sodium hydroxide aqueous solution at 24°C was added to the aqueous solution over approximately 30 seconds while exposed to the atmosphere, and the monomer aqueous solution was prepared by mixing. Due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomer aqueous solution rose to approximately 80°C. Throughout all examples and comparative examples, the above method for controlling the polymerization initiation temperature was implemented as needed to design and control the temperature of the monomer aqueous solution.

[0159] Subsequently, 7.79 parts by mass of an 8% by mass sodium persulfate aqueous solution was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction solution with a monomer concentration (mass ratio of acrylic acid and sodium acrylate in the monomer aqueous solution) of 43% by mass, a neutralization rate of 49 mol%, and polyethylene glycol diacrylate of 0.05 mol%.

[0160] Next, the reaction solution was poured into a stainless steel tray-shaped container under open atmospheric conditions. The tray-shaped container had a base of 200 mm x 260 mm, a top of 460 mm x 560 mm, and a height of 140 mm. The cross-section of the central part was trapezoidal, and a silicone sheet was attached to the inner surface. The tray-shaped container was preheated by placing it on a hot plate heated to 50°C before pouring in the reaction solution.

[0161] After the above reaction solution was poured into the vat-shaped container, the polymerization reaction began within 1 minute. During the polymerization reaction, the reaction solution expanded upward and in all directions, generating water vapor, and proceeded with foaming before contracting to a size slightly larger than the bottom of the vat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately 3 minutes. This polymerization reaction yielded a water-containing gel-like crosslinked polymer. The polymerization start temperature was 80°C, the polymerization peak temperature was 106°C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 26°C.

[0162] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0163] Next, the particulate water-containing gel was spread onto a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (1).

[0164] The superabsorbent resin powder (1) obtained by the above series of operations had a CRC of 32.1 g / g and a soluble content of 15.8% by mass.

[0165] Next, 4.9 parts by mass of a surface crosslinking agent solution consisting of 1.2 parts by mass of ethylene carbonate, 2.7 parts by mass of deionized water, and 1.0 part by mass of ethanol was spray-added to 100 parts by mass of the water-absorbent resin powder (1) and mixed. After heat treatment at 190°C for 20 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain surface-crosslinked water-absorbent material (1). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0166] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (1) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibration using a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (1') after damage. Tables 1 and 2 show the various physical properties of the water absorbent (1) and the water absorbent (1') after damage. The test disperser manufactured by Toyo Seiki Co., Ltd. is the one described in Patent No. 3688418, and all of its contents are incorporated into this specification by reference. The vibration of the Toyo Seiki test disperser was performed under fixed conditions (100 V / 60 Hz with a vibration speed of 750 c.pm).

[0167] [Example 2] An aqueous solution was prepared by adding 383 parts by mass of acrylic acid at 30°C, 1.11 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 2.15 parts by mass of a 1.0% by mass aqueous solution of diethylenetriaminepentaacetic acid trisodium, and 427 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was preheated to 53°C.

[0168] Next, while stirring the aqueous solution, 177 parts by mass of a 48% by mass sodium hydroxide aqueous solution at 40°C was added to the aqueous solution over approximately 30 seconds while open to the atmosphere, and mixed to prepare a monomer aqueous solution. Due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomer aqueous solution rose to approximately 85°C.

[0169] Subsequently, 9.31 parts by mass of an 8% by mass aqueous solution of sodium persulfate was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction solution with a monomer concentration of 43% by mass, a neutralization rate of 40 mol%, and polyethylene glycol diacrylate of 0.04 mol%.

[0170] Next, the reaction solution was poured into a stainless steel tray-shaped container under open atmospheric conditions. The tray-shaped container had a base of 200 mm x 260 mm, a top of 460 mm x 560 mm, and a height of 140 mm. The cross-section of the central part was trapezoidal, and a silicone sheet was attached to the inner surface. The tray-shaped container was preheated by placing it on a hot plate heated to 50°C before pouring in the reaction solution.

[0171] After the above reaction solution was poured into the vat-shaped container, the polymerization reaction began within 1 minute. During the polymerization reaction, the reaction solution expanded upward and in all directions, generating water vapor, and proceeded with foaming before contracting to a size slightly larger than the bottom of the vat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately 3 minutes. This polymerization reaction yielded a water-containing gel-like crosslinked polymer. The polymerization start temperature was 85°C, the polymerization peak temperature was 110°C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 25°C.

[0172] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0173] Next, the particulate water-containing gel was spread onto a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (2).

[0174] The superabsorbent resin powder (2) obtained by the above series of operations had a CRC of 25.9 g / g and a soluble content of 12.9% by mass.

[0175] Next, 3.2 parts by mass of a surface crosslinking agent solution consisting of 0.5 parts by mass of triethylene glycol, 0.2 parts by mass of 1,6-hexanediol, and 2.5 parts by mass of deionized water was spray-added to 100 parts by mass of the water-absorbent resin powder (2) and mixed. After heat treatment at 190°C for 13 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain the surface-crosslinked water-absorbent agent (2). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0176] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (2) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (2') after damage. Tables 1 and 2 show the various physical properties of the water absorbent (2) and the water absorbent (2') after damage.

[0177] [Example 3] An aqueous solution was prepared by adding 383 parts by mass of acrylic acid at 30°C, 0.83 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 2.15 parts by mass of 1.0% by mass diethylenetriaminepentaacetic acid trisodium aqueous solution, and 427 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was preheated to 53°C.

[0178] Next, while stirring the aqueous solution, 177 parts by mass of a 48% by mass sodium hydroxide aqueous solution at 40°C was added to the aqueous solution over approximately 30 seconds while open to the atmosphere, and mixed to prepare a monomer aqueous solution. Due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomer aqueous solution rose to approximately 85°C.

[0179] Subsequently, 9.31 parts by mass of an 8% by mass sodium persulfate aqueous solution was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction solution with a monomer concentration of 43% by mass, a neutralization rate of 40 mol%, and polyethylene glycol diacrylate of 0.03 mol%.

[0180] Next, the reaction solution was poured into a stainless steel tray-shaped container under open atmospheric conditions. The tray-shaped container had a base of 200 mm x 260 mm, a top of 460 mm x 560 mm, and a height of 140 mm. The cross-section of the central part was trapezoidal, and a silicone sheet was attached to the inner surface. The tray-shaped container was preheated by placing it on a hot plate heated to 50°C before pouring in the reaction solution.

[0181] After the above reaction solution was poured into the vat-shaped container, the polymerization reaction began within 1 minute. During the polymerization reaction, the reaction solution expanded upward and in all directions, generating water vapor, and proceeded with foaming before contracting to a size slightly larger than the bottom of the vat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately 3 minutes. This polymerization reaction yielded a water-containing gel-like crosslinked polymer. The polymerization start temperature was 85°C, the polymerization peak temperature was 110°C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 25°C.

[0182] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0183] Next, the particulate water-containing gel was spread onto a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (3).

[0184] The superabsorbent resin powder (3) obtained by the above series of operations had a CRC of 28.2 g / g and a soluble content of 16.1% by mass.

[0185] Next, 3.7 parts by mass of a surface crosslinking agent solution consisting of 0.4 parts by mass of ethylene carbonate, 0.6 parts by mass of propylene glycol, and 2.7 parts by mass of deionized water was spray-added to 100 parts by mass of the water-absorbent resin powder (3) and mixed. After heat treatment at 190°C for 13 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain the surface-crosslinked water-absorbent agent (3). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0186] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (3) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (3') after damage. The physical properties of the water absorbent (3) and the water absorbent (3') after damage are shown in Tables 1 and 2.

[0187] [Example 4] An aqueous solution was prepared by adding 389 parts by mass of acrylic acid at 30°C, 0.70 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 2.15 parts by mass of a 1.0% by mass aqueous solution of diethylenetriaminepentaacetic acid trisodium, and 442 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was preheated to 67°C.

[0188] Next, while stirring the aqueous solution, 157 parts by mass of a 48% by mass sodium hydroxide aqueous solution at 40°C was added to the aqueous solution over approximately 30 seconds while open to the atmosphere, and the monomer aqueous solution was prepared by mixing. Due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomer aqueous solution rose to approximately 86°C.

[0189] Subsequently, 9.43 parts by mass of an 8% by mass aqueous solution of sodium persulfate was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction solution with a monomer concentration of 43% by mass, a neutralization rate of 35 mol%, and polyethylene glycol diacrylate of 0.025 mol%.

[0190] Next, the reaction solution was poured into a stainless steel tray-shaped container under open atmospheric conditions. The tray-shaped container had a base of 200 mm x 260 mm, a top of 460 mm x 560 mm, and a height of 140 mm. The cross-section of the central part was trapezoidal, and a silicone sheet was attached to the inner surface. The tray-shaped container was preheated by placing it on a hot plate heated to 50°C before pouring in the reaction solution.

[0191] After the above reaction solution was poured into the above-mentioned vat-shaped container, the polymerization reaction began within 1 minute. During the polymerization reaction, the reaction solution expanded upward and in all directions, generating water vapor, and proceeded with the polymerization reaction while foaming, before contracting to a size slightly larger than the bottom surface of the vat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately 3 minutes. This polymerization reaction yielded a water-containing gel-like crosslinked polymer. The polymerization start temperature was 86°C, the polymerization peak temperature was 110°C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 24°C.

[0192] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0193] Next, the particulate water-containing gel was spread on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (4).

[0194] The superabsorbent resin powder (4) obtained by the above series of operations had a CRC of 26.7 g / g and a soluble content of 16.1% by mass.

[0195] Next, 3.7 parts by mass of a surface crosslinking agent solution consisting of 0.4 parts by mass of ethylene carbonate, 0.6 parts by mass of propylene glycol, and 2.7 parts by mass of deionized water was spray-added to 100 parts by mass of the water-absorbent resin powder (4) and mixed. After heat treatment at 190°C for 10 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain a surface-crosslinked water-absorbent agent (4). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0196] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (4) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (4') after damage. The physical properties of the water absorbent (4) and the water absorbent (4') after damage are shown in Tables 1 and 2.

[0197] [Example 5] An aqueous solution was prepared by adding 394 parts by mass of acrylic acid at 30°C, 0.43 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 2.15 parts by mass of a 1.0% by mass aqueous solution of diethylenetriaminepentaacetic acid trisodium, and 457 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was preheated to 78°C.

[0198] Next, while stirring the above aqueous solution, 137 parts by mass of a 48% by mass sodium hydroxide aqueous solution at 40°C was added to the aqueous solution over approximately 30 seconds while open to the atmosphere, and the monomer aqueous solution was prepared by mixing. Due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomer aqueous solution rose to approximately 88°C.

[0199] Subsequently, 9.57 parts by mass of an 8% by mass aqueous solution of sodium persulfate was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction solution with a monomer concentration of 43% by mass, a neutralization rate of 30 mol%, and polyethylene glycol diacrylate of 0.015 mol%.

[0200] Next, the reaction solution was poured into a stainless steel tray-shaped container under open atmospheric conditions. The tray-shaped container had a base of 200 mm x 260 mm, a top of 460 mm x 560 mm, and a height of 140 mm. The cross-section of the central part was trapezoidal, and a silicone sheet was attached to the inner surface. The tray-shaped container was preheated by placing it on a hot plate heated to 50°C before pouring in the reaction solution.

[0201] After the above reaction solution was poured into the above-mentioned vat-shaped container, the polymerization reaction began within 1 minute. During the polymerization reaction, the reaction solution expanded upward and in all directions, generating water vapor, and proceeded with the polymerization reaction while foaming, before contracting to a size slightly larger than the bottom surface of the vat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately 3 minutes. This polymerization reaction yielded a water-containing gel-like crosslinked polymer. The polymerization start temperature was 88°C, the polymerization peak temperature was 114°C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 26°C.

[0202] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0203] Next, the particulate water-containing gel was spread on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (5).

[0204] The superabsorbent resin powder (5) obtained by the above series of operations had a CRC of 26.9 g / g and a soluble content of 18.0% by mass.

[0205] Next, 6.1 parts by mass of a surface crosslinking agent solution consisting of 0.1 parts by mass of diethylene glycol diglycidyl ether, 1.0 part by mass of propylene glycol, 3.0 parts by mass of deionized water, and 2.0 parts by mass of ethanol was spray-added to 100 parts by mass of the water-absorbing resin powder (5) and mixed. After heat treatment at 150°C for 20 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 850 μm to obtain a surface-crosslinked water-absorbing agent (5). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0206] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (5) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (5') after damage. The physical properties of the water absorbent (5) and the water absorbent (5') after damage are shown in Tables 1 and 2.

[0207] [Example 6] The pulverized dry polymer obtained in Example 3 was classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an amorphous crushed water-absorbent resin powder (6).

[0208] Next, 6.1 parts by mass of a surface crosslinking agent solution consisting of 0.1 parts by mass of diethylene glycol diglycidyl ether, 1.0 part by mass of propylene glycol, 3.0 parts by mass of deionized water, and 2.0 parts by mass of ethanol was spray-added to 100 parts by mass of the water-absorbent resin powder (6) and mixed. After heat treatment at 150°C for 15 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 850 μm to obtain a surface-crosslinked water-absorbent agent (6). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0209] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (6) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (6') after damage. The physical properties of the water absorbent (6) and the water absorbent (6') after damage are shown in Tables 1 and 2.

[0210] [Example 7] An aqueous solution was prepared by adding 400 parts by mass of acrylic acid at 30°C, 1.31 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 2.30 parts by mass of 1.0% by mass diethylenetriaminepentaacetic acid trisodium aqueous solution, and 361 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was preheated to 30°C.

[0211] Next, while stirring the aqueous solution, 227 parts by mass of a 48% by mass sodium hydroxide aqueous solution at 24°C was added to the aqueous solution over approximately 30 seconds while open to the atmosphere, and the monomer aqueous solution was prepared by mixing. Due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomer aqueous solution rose to approximately 80°C.

[0212] Subsequently, 8.33 parts by mass of an 8% by mass aqueous solution of sodium persulfate was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction solution with a monomer concentration of 46% by mass, a neutralization rate of 49 mol%, and polyethylene glycol diacrylate of 0.045 mol%.

[0213] Next, the reaction solution was poured into a stainless steel tray-shaped container under open atmospheric conditions. The tray-shaped container had a base of 200 mm x 260 mm, a top of 460 mm x 560 mm, and a height of 140 mm. The cross-section of the central part was trapezoidal, and a silicone sheet was attached to the inner surface. The tray-shaped container was preheated by placing it on a hot plate heated to 50°C before pouring in the reaction solution.

[0214] After the above reaction solution was poured into the above-mentioned vat-shaped container, the polymerization reaction began within 1 minute. During the polymerization reaction, the reaction solution expanded upward and in all directions, generating water vapor, and proceeded with the polymerization reaction while foaming, before contracting to a size slightly larger than the bottom surface of the vat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately 3 minutes. This polymerization reaction yielded a water-containing gel-like crosslinked polymer. The polymerization start temperature was 80°C, the polymerization peak temperature was 115°C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 35°C.

[0215] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0216] Next, the particulate water-containing gel was spread on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (7).

[0217] The superabsorbent resin powder (7) obtained by the above series of operations had a CRC of 32.4 g / g and a soluble content of 16.3% by mass.

[0218] Next, 6.1 parts by mass of a surface crosslinking agent solution consisting of 0.1 parts by mass of diethylene glycol diglycidyl ether, 1.0 part by mass of triethylene glycol, 3.0 parts by mass of deionized water, and 2.0 parts by mass of ethanol was spray-added to 100 parts by mass of the water-absorbent resin powder (7) and mixed. The mixture was then heat-treated at 150°C for 30 minutes, and the mixture was passed through a JIS standard sieve with a mesh size of 850 μm to obtain a surface-crosslinked water-absorbent agent (7). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0219] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (7) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (7') after damage. The physical properties of the water absorbent (7) and the water absorbent (7') after damage are shown in Tables 1 and 2.

[0220] [Example 8] An aqueous solution was prepared by adding 187 parts by mass of acrylic acid at 30°C, 0.68 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 1.08 parts by mass of a 1.0% by mass aqueous solution of diethylenetriaminepentaacetic acid trisodium, and 201 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was at 25°C.

[0221] Next, while stirring the aqueous solution, 106 parts by mass of a 48% by mass sodium hydroxide aqueous solution at 24°C was added to the aqueous solution over approximately 30 seconds while open to the atmosphere, and mixed to prepare a monomer aqueous solution with a monomer concentration of 43% by mass, a neutralization rate of 49 mol%, and polyethylene glycol diacrylate of 0.05 mol%.

[0222] The obtained monomer aqueous solution was poured into a stainless steel tray measuring 320 mm in length, 220 mm in width, and 50 mm in height, with its inner surface coated with Teflon®. At this time, the thickness of the monomer aqueous solution was approximately 5 mm. The top of the stainless steel tray was sealed with a polyethylene film equipped with a nitrogen inlet, exhaust port, and polymerization initiator inlet, and then immersed in a water bath. Nitrogen gas was introduced into the aqueous solution for 20 minutes while adjusting the temperature of the monomer aqueous solution to 28°C to remove dissolved oxygen from the liquid.

[0223] Subsequently, 3.89 parts by mass of an 8% by mass aqueous solution of sodium persulfate and 1.30 parts by mass of 1% by mass L-ascorbic acid were added as redox polymerization initiators and thoroughly mixed with a magnetic stirrer. Polymerization began 3 minutes after the addition of the polymerization initiators. The polymerization temperature was controlled by alternately immersing a stainless steel tray in a 12°C water bath and a 70°C water bath. After 20 minutes, the polymerization peak temperature was 71°C, so it was aged for another 5 minutes by immersion in a 70°C water bath to obtain a hydrated gel-like crosslinked polymer.

[0224] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0225] Next, the particulate water-containing gel was spread on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (8).

[0226] The superabsorbent resin powder (8) obtained by the above series of operations had a CRC of 30.0 g / g and a soluble content of 11.7% by mass.

[0227] Next, 4.9 parts by mass of a surface crosslinking agent solution consisting of 1.2 parts by mass of ethylene carbonate, 2.7 parts by mass of deionized water, and 1.0 part by mass of ethanol was spray-added to 100 parts by mass of the water-absorbent resin powder (8) and mixed. After heat treatment at 150°C for 30 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 850 μm to obtain a surface-crosslinked water-absorbent agent (8). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0228] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (8) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (8') after damage. The physical properties of the water absorbent (8) and the water absorbent (8') after damage are shown in Tables 1 and 2.

[0229] [Example 9] The particulate water-containing gel obtained in Example 2 was spread on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 150°C through it for 90 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm to obtain an amorphous crushed water-absorbing resin powder (9).

[0230] The superabsorbent resin powder (9) obtained by the above series of operations had a CRC of 34.7 g / g and a soluble content of 21.6% by mass.

[0231] Next, 5.1 parts by mass of a surface crosslinking agent solution consisting of 0.1 parts by mass of diethylene glycol diglycidyl ether, 1.5 parts by mass of propylene glycol, and 3.5 parts by mass of deionized water was spray-added to 100 parts by mass of the water-absorbent resin powder (9) and mixed. After that, the mixture was heat-treated at 100°C for 60 minutes, and then passed through a JIS standard sieve with a mesh size of 850 μm to obtain a surface-crosslinked water-absorbent agent (9). The aggregates remaining on the sieve were also crushed and allowed to pass through.

[0232] To evaluate the resistance to process damage, 30 g of the obtained water absorbent (9) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the water absorbent (9') after damage. The physical properties of the water absorbent (9) and the water absorbent (9') after damage are shown in Tables 1 and 2.

[0233] [Comparative Example 1] (Equivalent to Example 4 in Japanese Patent Publication No. 11-263850) 5022 parts by mass of an aqueous solution of sodium acrylate with a neutralization rate of 40 mol% (monomer concentration in the aqueous solution: 30% by mass) was dissolved with 0.05 mol% trimethylolpropane triacrylate as an internal crosslinking agent. After degassing with nitrogen gas for 30 minutes, the monomer aqueous solution was supplied to a reactor consisting of a jacketed stainless steel double-arm type kneader with two sigma-shaped blades and a lid, with an internal volume of 10 L. Nitrogen purging of the reaction system was continued while maintaining a temperature of 42°C. Next, while rotating the blades, 22.38 parts by mass of a 10% by mass aqueous solution of sodium persulfate and 9.32 parts by mass of 1% by mass L-ascorbic acid were added as redox polymerization initiators. Polymerization started after about 1 minute, and the polymerization peak temperature was 91°C (ΔT: 49°C) after about 11 minutes, so maturation was continued with further stirring. The resulting hydrated gel polymer was granulated to about 5 mm to 1 mm or less, and the hydrated gel polymer was removed 40 minutes after the start of polymerization.

[0234] Next, the particulate water-containing gel was spread onto a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 150°C through it for 2 hours to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using a JIS standard sieve with a mesh size of 850 μm to obtain an amorphous crushed comparative water-absorbing resin powder (1).

[0235] The comparative superabsorbent polymer powder (1) obtained by the above series of operations had a CRC of 31.5 g / g and a soluble content of 10.3% by mass.

[0236] Next, 7.1 parts by mass of a surface crosslinking agent solution consisting of 0.1 parts by mass of ethylene glycol diglycidyl ether, 1.0 part by mass of propylene glycol, 3.0 parts by mass of deionized water, and 3.0 parts by mass of isopropyl alcohol was spray-added to 100 parts by mass of the comparative water-absorbing resin powder (1) and mixed. After heat treatment at 150°C for 20 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 850 μm to obtain the surface-crosslinked comparative water-absorbing agent (1). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0237] To evaluate the resistance to process damage, 30 g of the obtained comparative water absorbent (1) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain comparative water absorbent (1') after damage. The physical properties of comparative water absorbent (1) and comparative water absorbent (1') after damage are shown in Tables 1 and 2.

[0238] [Comparative Example 2] (Equivalent to Example 5 of Japanese Patent Publication No. 11-263850) 5295 parts by mass of an aqueous solution of sodium acrylate with a neutralization rate of 50 mol% (monomer concentration in the aqueous solution: 37% by mass) was dissolved with 0.05 mol% trimethylolpropane triacrylate as an internal crosslinking agent. After degassing with nitrogen gas for 30 minutes, the monomer aqueous solution was supplied to a reactor consisting of a jacketed stainless steel double-arm kneader with two sigma-shaped blades and a lid, with an internal volume of 10 L. Nitrogen purging of the reaction system was continued while maintaining a temperature of 27°C. Next, while rotating the blades, 28.33 parts by mass of a 10% by mass aqueous solution of sodium persulfate and 11.80 parts by mass of 1% by mass L-ascorbic acid were added as redox polymerization initiators. Polymerization started after about 1 minute, and the polymerization peak temperature was 95°C (ΔT: 68°C) after about 11 minutes, so maturation was continued with further stirring. The resulting hydrated gel polymer was granulated to about 5 mm to 1 mm or less, and the hydrated gel polymer was removed 40 minutes after the start of polymerization. Next, the particulate water-containing gel was spread and placed on a wire mesh with a mesh size of 300 μm (50 mesh) and put into a hot air dryer. Then, hot air at 150 °C was passed through for 2 hours to dry the particulate water-containing gel, and a dried polymer was obtained. Subsequently, the dried polymer was put into a roll mill (WML type roll crusher / manufactured by Inoguchi Giken Co., Ltd.) and crushed, and then classified using a JIS standard sieve with a mesh size of 850 μm to obtain an amorphous crushed comparative water-absorbent resin powder (2).

[0239] The comparative water-absorbent resin powder (2) obtained by the above series of operations had a CRC of 30.6 g / g and a soluble content of 5.4% by mass.

[0240] Next, 7.1 parts by mass of a surface crosslinking agent solution composed of 0.1 part by mass of ethylene glycol diglycidyl ether, 1.0 part by mass of propylene glycol, 3.0 parts by mass of deionized water, and 3.0 parts by mass of isopropyl alcohol was spray-added and mixed while adding to 100 parts by mass of the above comparative water-absorbent resin powder (2), and heat-treated at 150 °C for 20 minutes, and then passed through a JIS standard sieve with a mesh size of 850 μm to obtain a surface-crosslinked comparative water absorbent (2). In addition, the aggregates remaining on the sieve were also crushed and all passed through.

[0241] To evaluate the process damage resistance, 30 g of the obtained comparative water absorbent (2) and 10 g of glass beads (one with a diameter of about 6 mm and a mass of about 0.3 g) were put into a glass bottle with a capacity of 225 mL, and the impact of the glass beads was applied for 20 minutes by the vibration of a paint shaker (a test disperser manufactured by Toyo Seiki) to obtain a comparative water absorbent (2') after damage. The physical properties of the comparative water absorbent (2) and the comparative water absorbent (2') after damage are shown in Table 1 and Table 2.

[0242] [Comparative Example 3] Into a 2 L polypropylene container, 354 parts by mass of acrylic acid, 1.80 parts by mass of polyethylene glycol diacrylate (average number of polyethylene glycol units (average n number); 9), 2.15 parts by mass of a 1.0 mass% aqueous solution of diethylenetriaminepentaacetic acid trisodium salt, and 341 parts by mass of deionized water were charged and mixed to prepare an aqueous solution. The deionized water was preheated to 40 °C.

[0243] Subsequently, while stirring the above aqueous solution, 287 parts by mass of a 48 mass% aqueous sodium hydroxide solution was added to the aqueous solution over about 30 seconds in an open atmosphere to prepare a monomer aqueous solution. In the process of the above mixing, due to the heat of neutralization and heat of dissolution generated, the temperature of the monomer aqueous solution rose to about 84 °C.

[0244] Thereafter, 8.60 parts by mass of an 8 mass% aqueous sodium persulfate solution was added as a polymerization initiator and stirred for about 5 seconds to obtain a reaction solution with a monomer concentration of 43 mass%, a neutralization rate of 70 mol%, and 0.07 mol% of polyethylene glycol diacrylate.

[0245] Next, the above reaction solution was poured into a stainless steel bat-shaped container in an open atmosphere. The bat-shaped container had a bottom size of 200 mm × 260 mm, a top size of 460 mm × 560 mm, a height of 140 mm, a trapezoidal cross-section at the center, and a silicone sheet attached to the inner surface. Also, the bat-shaped container was placed on a hot plate heated to 50 °C and preheated before pouring the reaction solution.

[0246] After the above reaction solution was poured into the bat-shaped container, the polymerization reaction started within 1 minute. By the polymerization reaction, the reaction solution expanded in all directions upward while generating water vapor, and the polymerization reaction proceeded while foaming, and then shrank to a size slightly larger than the bottom surface of the bat-shaped container. The polymerization reaction (expansion, contraction) ended within about 3 minutes. By the polymerization reaction, a water-containing gel-like crosslinked polymer was obtained. The polymerization start temperature was 84 °C, the polymerization peak temperature was 106 °C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 22 °C.

[0247] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0248] Next, the particulate water-containing gel was spread onto a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an amorphous crushed comparative water-absorbing resin powder (3).

[0249] The comparative superabsorbent polymer powder (3) obtained by the above series of operations had a CRC of 33.2 g / g and a soluble content of 8.3% by mass.

[0250] Next, 3.7 parts by mass of a surface crosslinking agent solution consisting of 0.4 parts by mass of ethylene carbonate, 0.6 parts by mass of propylene glycol, 3.0 parts by mass of deionized water, and 2.0 parts by mass of ethanol was spray-added to 100 parts by mass of the comparative water-absorbing resin powder (3) and mixed. After heat treatment at 190°C for 30 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 850 μm to obtain the surface-crosslinked comparative water-absorbing agent (3). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0251] To evaluate the resistance to process damage, 30 g of the obtained comparative absorbent (3) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the absorbent (3') after damage. The physical properties of the comparative absorbent (3) and the comparative absorbent (3') after damage are shown in Tables 1 and 2.

[0252] [Comparative Example 4] An aqueous solution was prepared by adding 396 parts by mass of acrylic acid, 0.35 parts by mass of polyethylene glycol diacrylate (average polyethylene glycol unit count (average n); 9), 2.15 parts by mass of 1.0% by mass diethylenetriaminepentaacetic acid trisodium aqueous solution, and 458 parts by mass of deionized water to a 2L polypropylene container and mixing them. The deionized water was preheated to 40°C.

[0253] Next, while stirring the aqueous solution, 128 parts by mass of a 48% by mass sodium hydroxide aqueous solution was added to the aqueous solution under open air for approximately 30 seconds and mixed to prepare a monomer aqueous solution. Due to the heat of neutralization and heat of dissolution generated during the mixing process, the temperature of the monomer aqueous solution rose to approximately 88°C.

[0254] Subsequently, 9.62 parts by mass of an 8% by mass aqueous solution of sodium persulfate was added as a polymerization initiator, and the mixture was stirred for about 5 seconds to obtain a reaction solution with a monomer concentration of 43% by mass, a neutralization rate of 28 mol%, and polyethylene glycol diacrylate of 0.012 mol%.

[0255] Next, the reaction solution was poured into a stainless steel tray-shaped container under open atmospheric conditions. The tray-shaped container had a base of 200 mm x 260 mm, a top of 460 mm x 560 mm, and a height of 140 mm. The cross-section of the central part was trapezoidal, and a silicone sheet was attached to the inner surface. The tray-shaped container was preheated by placing it on a hot plate heated to 50°C before pouring in the reaction solution.

[0256] After the above reaction solution was poured into the above-mentioned vat-shaped container, the polymerization reaction began within 1 minute. During the polymerization reaction, the reaction solution expanded upward and in all directions, generating water vapor, and proceeded with the polymerization reaction while foaming, before contracting to a size slightly larger than the bottom surface of the vat-shaped container. The polymerization reaction (expansion and contraction) was completed within approximately 3 minutes. This polymerization reaction yielded a water-containing gel-like crosslinked polymer. The polymerization start temperature was 88°C, the polymerization peak temperature was 114°C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 26°C.

[0257] Next, the above-mentioned water-containing gel was cut into appropriate sizes and then supplied to a screw extruder (HL-G22SN / manufactured by Remacom Co., Ltd.) having a die with a plate hole diameter of 8.0 mm and a four-blade knife to pulverize the gel and obtain particulate water-containing gel.

[0258] Next, the particulate water-containing gel was spread on a wire mesh with a mesh size of 300 μm (50 mesh) and placed in a hot air dryer. The particulate water-containing gel was then dried by passing hot air at 190°C through it for 30 minutes to obtain a dried polymer. Subsequently, the dried polymer was put into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized, and then classified using two types of JIS standard sieves with mesh sizes of 710 μm and 150 μm to obtain an amorphous crushed comparative water-absorbing resin powder (4).

[0259] The comparative superabsorbent polymer powder (4) obtained by the above series of operations had a CRC of 27.3 g / g and a soluble content of 18.9% by mass.

[0260] Next, 4.5 parts by mass of a surface crosslinking agent solution consisting of 1.0 part by mass of ethylene carbonate, 3.0 parts by mass of deionized water, and 0.5 parts by mass of isopropyl alcohol was spray-added to 100 parts by mass of the comparative water-absorbing resin powder (4) and mixed. After heating at 190°C for 10 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 710 μm to obtain the surface-crosslinked comparative water-absorbing agent (4). Aggregates remaining on the sieve were also crushed and allowed to pass through.

[0261] To evaluate the resistance to process damage, 30 g of the obtained comparative absorbent (4) and 10 g of glass marbles (each marble approximately 6 mm in diameter and 0.3 g in mass) were placed together in a 225 mL glass bottle. The marbles were then subjected to impact for 20 minutes by vibrating a paint shaker (a test disperser manufactured by Toyo Seiki Co., Ltd.) to obtain the absorbent (4') after damage. The physical properties of the comparative absorbent (4) and the comparative absorbent (4') after damage are shown in Tables 1 and 2.

[0262] [Comparative Example 5] (Surface crosslinking was performed on Example 1 of Japanese Patent Publication No. 1-318022) Into a 2 L polypropylene container, 342.9 parts by mass of acrylic acid, 0.095 parts by mass of N,N'-methylenebisacrylamide, and 57.5 parts by mass of deionized water were charged and mixed to prepare an aqueous solution.

[0263] Subsequently, while stirring the above aqueous solution, 176.6 parts by mass of a 48.5 mass% sodium hydroxide aqueous solution was added to the aqueous solution over about 10 seconds under an open atmosphere and mixed to prepare a monomer aqueous solution. During the mixing process, due to the heat of neutralization and heat of dissolution generated, the temperature of the monomer aqueous solution rose to 104 °C and then gradually decreased by continuing stirring.

[0264] Thereafter, 22.84 parts by mass of a 5 mass% 2,2'-azobis(2-amidinopropane) dihydrochloride aqueous solution was added as a polymerization initiator and stirred for about 10 seconds to obtain a reaction solution with a monomer concentration of 65 mass%, a neutralization rate of 45 mol%, and 0.013 mol% of N,N'-methylenebisacrylamide.

[0265] Next, the above reaction solution at 71 °C was poured into a stainless steel bat-shaped container under an open atmosphere. The bat-shaped container had a bottom size of 200 mm × 260 mm, a top size of 460 mm × 560 mm, and a height of 140 mm, with a trapezoidal cross-section at the center part and a silicone sheet attached to the inner surface. Also, the bat-shaped container was placed on a hot plate heated to 75 °C and preheated before pouring the reaction solution.

[0266] After pouring the above reaction solution into the bat-shaped container, the polymerization reaction started in about 20 seconds. Due to the polymerization reaction, the reaction solution expanded while vigorously generating water vapor, foamed, and the polymerization reaction proceeded, and then shrank to a size smaller than the bottom surface of the bat-shaped container. The polymerization reaction (expansion, shrinkage) ended within about 4 minutes and was further aged on the bat for 4 minutes. By the polymerization reaction, a water-containing gel-like crosslinked polymer was obtained. The polymerization start temperature was 71 °C, the polymerization peak temperature was 104 °C, and the difference ΔT between the polymerization start temperature and the polymerization peak temperature was 33 °C.

[0267] The obtained polymer was almost dry, but it was dried under reduced pressure at 80°C for 3 hours. Subsequently, the dried polymer was fed into a roll mill (WML type roll pulverizer / manufactured by Inoguchi Giken Co., Ltd.) and pulverized. Then, it was classified using two types of JIS standard sieves with mesh sizes of 850 μm and 150 μm to obtain an amorphous crushed comparative water-absorbing resin powder (5).

[0268] The comparative superabsorbent polymer powder (5) obtained by the above series of operations had a CRC of 29.0 g / g and a soluble content of 36.3% by mass.

[0269] Next, 3.9 parts by mass of a surface crosslinking agent solution consisting of 1.2 parts by mass of ethylene carbonate, 2.7 parts by mass of deionized water, and 1.0 part by mass of ethanol was spray-added to 100 parts by mass of the comparative water-absorbing resin powder (5) and mixed. After heating at 150°C for 30 minutes, the mixture was passed through a JIS standard sieve with a mesh size of 850 μm to obtain the surface-crosslinked comparative water-absorbing agent (5). Aggregates remaining on the sieve were also crushed and allowed to pass through. Table 1 shows the various physical properties of the comparative water-absorbing agent (5). Compared to the examples, AAP and SFC were inferior, and in particular, the performance of SFC did not manifest. Because AAP and SFC were significantly inferior, the physical properties after damage were not evaluated.

[0270] [Table 1]

[0271] [Table 2]

[0272] [Examples 10 and 11] Fifty parts by mass of the water absorbent (2) from Example 2 were placed in a 250 ml container and humidified by spraying 0.25 parts by mass or 0.50 parts by mass of deionized water while stirring with a three-way motor. The mixture was then left to stand in a 60°C oven for 1 hour, and then passed through a JIS standard sieve with a mesh size of 710 μm to obtain water absorbent (10) and water absorbent (11), respectively. The electrostatic charge of the obtained water absorbent (10) and water absorbent (11), as well as water absorbent (2) before the addition of deionized water, was measured and is shown in Table 3. Furthermore, the electrostatic charge of water absorbent (9), which was prepared by performing the surface crosslinking process at a low temperature to maintain the highest possible water content, is also shown.

[0273] [Table 3]

[0274] [Table 4]

Claims

1. A water absorbent comprising a (meth)acrylic acid (salt)-based water absorbent resin, The (meth)acrylic acid (salt)-based superabsorbent resin is a polymer in which a superabsorbent resin powder, which is crosslinked only internally, is surface-crosslinked. The following absorbent must meet all of the following criteria: (a) CRC>20g / g, (b) pH is 4.0 to 5.4 (c) Branching density of soluble components: 0.03 to 0.70 (d) Soluble content (16hr-Extr) 5-25% by mass.

2. The water absorbent according to claim 1, satisfying the following conditions: (e) The molecular weight distribution of the soluble components is Mw / Mn ≤ 5.

00.

3. The water absorbent according to claim 1 or 2, which satisfies the following: (f) The moisture content (at 105°C × 3hr) is 0.2 to 3.0% by mass.

4. The water absorbent according to any one of claims 1 to 3, satisfying the following conditions: (g) The amount of residual monomer is ≤ 200 ppm by mass.

5. The water absorbent according to any one of claims 1 to 4, satisfying at least one of the following: (h) AAP0.7psi>20g / g (i)SFC>20×10 －７ cm ３ ・sec / g。

6. A step of preparing an aqueous solution of a (meth)acrylic acid (salt) monomer containing an internal crosslinking agent, A step of polymerizing the (meth)acrylic acid (salt) monomer aqueous solution, A step of pulverizing the water-containing gel-like crosslinked polymer produced in the polymerization step to obtain particulate water-containing gel, A step of drying the particulate water-containing gel to obtain a dry polymer, A step of crushing and / or classifying the dry polymer to obtain a water-absorbent resin powder, and The process involves surface crosslinking the water-absorbing resin powder, A method for producing a water absorbent comprising a surface-crosslinked (meth)acrylic acid (salt)-based water absorbent resin having, In the step of preparing the monomer aqueous solution, the neutralization rate of the (meth)acrylic acid (salt) is adjusted to 30 mol% or more and less than 55 mol%, and the concentration of the (meth)acrylic acid (salt) monomer in the monomer aqueous solution is adjusted to 38 to 50% by mass. A method for producing a water-absorbing agent containing a surface-crosslinked (meth)acrylic acid (salt)-based water-absorbing resin, wherein the difference ΔT between the polymerization peak temperature and the polymerization start temperature is 50°C or less in the polymerization step.

7. The step of preparing the (meth)acrylic acid (salt) monomer aqueous solution comprises mixing the (meth)acrylic acid (salt) monomer with water. The manufacturing method according to claim 6, wherein the temperature of the water is 30°C or higher.

8. The step of preparing the (meth)acrylic acid (salt) monomer aqueous solution comprises mixing the (meth)acrylic acid (salt) monomer, water, and a basic salt aqueous solution. The manufacturing method according to claim 6 or 7, wherein the temperature of the basic salt aqueous solution is 20°C or higher.

9. The manufacturing method according to any one of claims 6 to 8, wherein the aqueous solution of the (meth)acrylic acid (salt) monomer contains 0.01 to 0.1 mol% of an internal crosslinking agent relative to the total amount of the (meth)acrylic acid (salt).

10. The manufacturing method according to any one of claims 6 to 9, wherein the polymerization initiation temperature is 50°C or higher.

11. The manufacturing method according to any one of claims 6 to 10, wherein the polymerization peak temperature is 100°C or higher.

12. The manufacturing method according to any one of claims 6 to 11, wherein the heating temperature in the surface crosslinking step is 160°C or higher and the heating time is 30 minutes or less.

13. The manufacturing method according to any one of claims 6 to 12, wherein the water content (at 105°C × 3hr) of the (meth)acrylic acid (salt)-based water-absorbing resin is controlled to be 0.2 to 3.0% by mass.

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