Method for producing superabsorbent resin

Abstract

The present invention relates to a method for producing a superabsorbent polymer. More specifically, the method enables the production of a superabsorbent polymer that exhibits an improved water absorption rate without a decrease in water absorption properties when a monomer polymerization reaction is carried out in the presence of an aqueous dispersion of hydrophobic particles.

Description

Technical Field

[0001] Cross-reference to related applications This application claims the benefit of priority based on Korean Patent Application Nos. 10-2020-0178433 and 10-2021-0180293, filed on December 18, 2020, and all the contents disclosed in the documents of the Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a method for producing a superabsorbent resin. More specifically, when a polymerization reaction of a monomer is carried out in the presence of a hydrophobic particle aqueous dispersion, carbon dioxide bubbles generated can be effectively collected, and a superabsorbent resin having a high surface tension and an improved water absorption rate without a decrease in water absorption physical properties can be produced.

Background Art

[0003] A superabsorbent polymer (SAP) is a synthetic polymer material having a function of absorbing about 500 to 1,000 times its own weight of water, and is named with different names such as SAM (Super Absorbency Material) and AGM (Absorbent Gel Material) for each developing company. Since such a superabsorbent resin began to be put into practical use as a sanitary product, it is now widely used as a material for horticultural soil conditioner, civil engineering and building water stop material, seedling raising sheet, freshness retainer in the food distribution field, and wet tissue.

[0004] Such superabsorbent resins are mainly widely used in the field of sanitary materials such as diapers and sanitary napkins. In the sanitary material, the superabsorbent resin is generally contained in a state of being spread in pulp. However, recently, efforts have continued to provide sanitary materials such as thinner diapers, and as part of this, the development of so-called pulpless diapers in which the pulp content is reduced or the pulp is not used at all has been actively carried out.

[0005] Thus, in the case of a sanitary material in which the pulp content is reduced or no pulp is used, a relatively high proportion of superabsorbent resin is contained, and the superabsorbent resin particles are inevitably contained in multiple layers within the sanitary material. In order for the overall superabsorbent resin particles contained in multiple layers in this way to more efficiently absorb a large amount of liquid such as urine, the superabsorbent resin needs to exhibit not only basically high water absorption performance but also a fast water absorption rate.

[0006] For the production of such a superabsorbent resin with an improved water absorption rate, a method of increasing the specific surface area by generating pores using a foaming agent in the polymerization stage is mainly used. In particular, encapsulated foaming agents are usually used in terms of price and availability. When the polymerization stage is carried out in the presence of such an encapsulated foaming agent, the specific surface area increases within the crosslinked polymer while carbon dioxide bubbles are generated. Also, a bubble stabilizer is used to minimize the escape of the generated carbon dioxide bubbles outside the crosslinked polymer network, and problems have occurred in that the various physical properties of the superabsorbent resin are deteriorated due to the use of such a bubble stabilizer.

[0007] Therefore, there has been a continuous demand for the development of a superabsorbent resin that exhibits a fast water absorption rate while maintaining the water retention capacity (CRC), which is a physical property indicating the basic water absorption and water retention capacity of the superabsorbent resin, and the pressure absorption capacity (AUP), which indicates the property of well retaining the absorbed liquid even under external pressure.

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, the present invention relates to a method for producing a superabsorbent resin that can exhibit an improved water absorption rate by effectively collecting bubbles generated by a foaming agent by adding a hydrophobic particle aqueous dispersion in the monomer polymerization stage.

Means for Solving the Problems

[0009] In order to solve the above problems, the present invention Preparing a monomer composition containing an acrylic acid-based monomer having an acidic group and at least a part of the acidic group being neutralized and an internal crosslinking agent (Step 1); Crosslinking and polymerizing the monomer composition in the presence of a hydrophobic particle aqueous dispersion and an encapsulated foaming agent to produce a water-containing gel polymer (Step 2); Drying and pulverizing the water-containing gel polymer to form a base resin in powder form (Step 3); and In the presence of a surface crosslinking agent, additionally crosslinking the surface of the base resin to form a surface crosslinked layer (Step 4), The hydrophobic particle aqueous dispersion is a colloidal solution in which hydrophobic particles are dispersed by a surfactant, The hydrophobic particles contain a metal salt of a fatty acid having 7 to 24 carbon atoms, The encapsulated foaming agent has a structure including a core containing a hydrocarbon and a shell formed from a thermoplastic resin surrounding the core, and provides a method for producing a superabsorbent resin.

Advantages of the Invention

[0010] According to the method for producing a superabsorbent resin of the present invention, when crosslinking and polymerizing a monomer in the presence of a hydrophobic particle aqueous dispersion and an encapsulated foaming agent to produce a water-containing gel polymer, the generated carbon dioxide can be effectively collected and the specific surface area of the superabsorbent resin can be increased, whereby the water absorption rate of the generated superabsorbent resin can be increased.

Embodiments for Carrying Out the Invention

[0011] The terms used in this specification are merely used to explain exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as "including", "comprising", or "having" are intended to specify the presence of implemented features, steps, components, or combinations thereof, and should not be construed as precluding the presence or addition possibility of one or more other features, steps, components, or combinations thereof.

[0012] The present invention can be modified in various ways and can have various forms, and specific embodiments will be exemplified and described in detail below. However, this is not intended to limit the present invention to a specific disclosed form, and it should be understood to include all modifications, equivalents, and alternatives included in the spirit and technical scope of the present invention.

[0013] Also, the technical terms used in this specification are for referring to specific embodiments only and are not intended to limit the present invention. And the singular forms used herein include the plural forms as well, unless the context clearly indicates the contrary.

[0014] The term "polymer" or "macromolecule" used in the specification of the present invention means a state in which an acrylic acid monomer, which is a water-soluble ethylenic unsaturated monomer, is polymerized, and can include all moisture content ranges or particle size ranges. Among the polymers, those in the state before drying after polymerization and having a water content (moisture content) of about 40% by weight or more can be referred to as hydrogel polymers, and particles obtained by pulverizing and drying such hydrogel polymers can be referred to as crosslinked polymers.

[0015] Also, the term "superabsorbent resin particles" refers to particulate substances including crosslinked polymers in which acrylic acid monomers containing acidic groups and at least a part of the acidic groups are neutralized are polymerized and crosslinked by an internal crosslinking agent.

[0016] Also, the term "superabsorbent resin" means, depending on the context, a crosslinked polymer in which acrylic acid monomers containing acidic groups and at least a part of the acidic groups are neutralized are polymerized, or a base resin in powder form composed of superabsorbent resin particles obtained by pulverizing the crosslinked polymer, or all those that have been made suitable for commercialization through additional processes such as surface crosslinking, micropowder regranulation, drying, pulverization, classification, etc. for the crosslinked polymer or the base resin. Therefore, the term "superabsorbent resin" can be interpreted as including a plurality of superabsorbent resin particles.

[0017] In order to produce a superabsorbent resin having a high water absorption rate, the specific surface area within the superabsorbent resin particles must be increased. As a method for increasing the specific surface area of such superabsorbent resin particles, a foaming agent is usually used. In order for the bubbles generated by such a foaming agent to contribute to the increase in the specific surface area, the bubbles must be trapped inside the polymer as soon as they are generated so as not to escape outside the crosslinked polymer. For this reason, a foaming agent and a foam stabilizer are usually added together. In order for the anionic surfactant mainly used as the foam stabilizer to effectively trap the bubbles, an excessive amount of the foaming agent had to be used. However, when such an anionic surfactant is used in an excessive amount, there is a problem that the surface tension of the superabsorbent resin decreases, pores in the superabsorbent resin are not formed uniformly and evenly, and a large amount of fine powder is generated during production and transfer.

[0018] Therefore, when the present inventors produce a superabsorbent resin using an aqueous dispersion containing hydrophobic particles as a foam stabilizer in addition to a foam stabilizer such as a commonly used anionic surfactant, when bubbles are generated by the foaming agent, it plays the role of a seed to effectively generate and trap the bubbles, and it was confirmed that bubbles in a small and uniform form can be uniformly distributed throughout the crosslinked polymer, and thus the present invention was completed.

[0019] In particular, the method for producing the superabsorbent resin is characterized in that the hydrophobic particles are not used in powder form but are added to the monomer composition in a form dispersed in an aqueous dispersion. In other words, the hydrophobic particles are added to the monomer composition in the form of an "aqueous dispersion of hydrophobic particles", that is, in the form of a colloidal solution in which the hydrophobic particles are stably dispersed without precipitation or aggregation by a surfactant. This is because when the hydrophobic particles are added to the monomer composition in powder form and the polymerization step is carried out, the hydrophobic particles cannot be dispersed in the monomer composition in aqueous solution form and cannot effectively stabilize the bubbles generated by the foaming agent.

[0020] Furthermore, the hydrophobic particles are stably dispersed in the aqueous dispersion without inter-particle aggregation due to the surfactant. Specifically, the surfactant can form an electric double layer on the surface of the hydrophobic particles, inducing electrostatic repulsive forces between particles and stabilizing the hydrophobic particles, or it can adsorb to the surface of the hydrophobic particles and induce steric repulsive forces between particles, preventing aggregation between particles. Therefore, if the hydrophobic particle aqueous dispersion does not contain a surfactant, the hydrophobic particles will aggregate or sink due to gravity, preventing stable dispersion of the hydrophobic particles. Therefore, even if a surfactant-free hydrophobic particle aqueous dispersion is used with a blowing agent during the polymerization step, it will be difficult to effectively capture air bubbles and form uniformly sized pores in the superabsorbent polymer, which makes it difficult to improve the water absorption rate of the superabsorbent polymer.

[0021] Furthermore, when an epoxy compound is used as an internal crosslinking agent in the preparation of the superabsorbent resin, the gelation time is delayed compared to when an acrylate compound is used as an internal crosslinking agent. This means that bubbles generated by the blowing agent escape into the air before the crosslinked structure is formed, making it difficult to effectively capture the gas. Therefore, to increase the specific surface area of the superabsorbent resin prepared using an epoxy compound as an internal crosslinking agent, an excessive amount of blowing agent must be used. However, when the hydrophobic particle aqueous dispersion is used with a blowing agent, even with only a small amount of blowing agent, the hydrophobic particles act as seeds to effectively generate and capture bubbles, allowing the formation of a desired pore structure without the use of an excessive amount of blowing agent.

[0022] In addition, the superabsorbent resin thus produced has a water absorption rate (vortex time) at 24.0 °C of 35 seconds or less, and when 1 g of the superabsorbent resin is immersed in 2 L of tap water and swollen for 1 minute, the 1-minute tap water absorption capacity, which is defined as the weight of water absorbed by the superabsorbent resin in 1 minute, satisfies 115 g or more.

[0023] Hereinafter, the manufacturing method of the superabsorbent resin will be described in more detail for each step according to specific embodiments of the invention.

[0024] (Step 1) In the manufacturing method according to one embodiment, Step 1 is a step of preparing a monomer composition containing an acrylic acid-based monomer having an acidic group and at least a part of which is neutralized, and an internal crosslinking agent.

[0025] The acrylic acid-based monomer is a compound represented by the following Chemical Formula 1:

[0026] [Chemical Formula 1] R 1 -COOM 1

[0027] In the above Chemical Formula 1, R 1 is an alkyl group having 2 to 5 carbon atoms containing an unsaturated bond, M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0028] Preferably, the acrylic acid-based monomer contains one or more selected from the group consisting of acrylic acid, methacrylic acid, and their monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts.

[0029] Here, the acrylic acid monomer may have an acidic group and at least a part of the acidic group may be neutralized. Preferably, a monomer partially neutralized with an alkaline substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide can be used. At this time, the degree of neutralization of the acrylic acid monomer may be 40 to 95 mol%, or 40 to 80 mol%, or 45 to 75 mol%. The range of the degree of neutralization can be adjusted according to the final physical properties. However, if the degree of neutralization is excessively high, the neutralized monomer may precipitate and polymerization may not proceed smoothly. Conversely, if the degree of neutralization is excessively low, not only does the water absorption capacity of the polymer decrease significantly, but it may also exhibit properties such as an elastic rubber that is difficult to handle.

[0030] The concentration of the acrylic acid monomer may be about 20 to about 60% by weight, preferably about 40 to about 50% by weight, based on the monomer composition containing the raw material substances and solvent of the superabsorbent resin, and can be adjusted to an appropriate concentration considering the polymerization time and reaction conditions, etc. However, if the concentration of the monomer is excessively decreased, the yield of the superabsorbent resin may be low and economic problems may occur. Conversely, if the concentration is excessively increased, some of the monomer may precipitate or the pulverization efficiency may be low during the pulverization of the polymerized hydrogel polymer, etc., and problems may occur in the process, and the physical properties of the superabsorbent resin may deteriorate.

[0031] Also, the term "internal crosslinking agent" used in this specification is a term used to distinguish it from the surface crosslinking agent for crosslinking the surface of the superabsorbent resin particles described later, and plays a role in crosslinking and polymerizing the unsaturated bonds of the above-mentioned water-soluble ethylenic unsaturated monomers. The crosslinking at this stage is carried out without distinction between the surface and the inside. However, when the surface crosslinking process of the superabsorbent resin particles described later is carried out, the particle surface of the finally produced superabsorbent resin has a structure crosslinked by the surface crosslinking agent, and the inside has a structure crosslinked by the internal crosslinking agent.

[0032] As the internal crosslinking agent, any compound can be used as long as it enables the introduction of crosslinking bonds during the polymerization of the acrylic acid-based unsaturated monomer. Specifically, as the internal crosslinking agent, a crosslinking agent having one or more functional groups capable of reacting with the water-soluble substituent of the acrylic acid-based unsaturated monomer and having one or more ethylenically unsaturated groups; or a crosslinking agent having two or more functional groups capable of reacting with the water-soluble substituent of the monomer and / or the water-soluble substituent formed by hydrolysis of the monomer can be used.

[0033] As a non-limiting example, the internal crosslinking agent may be acrylate compounds such as N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate; epoxy compounds such as ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, tripropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, polytetramethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol triglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether; triarylamine; propylene glycol; glycerin; or ethylene carbonate. These polyfunctional crosslinking agents can be used alone or in combination of two or more, but are not limited thereto.

[0034] According to one embodiment, the epoxy compound can be used as the internal crosslinking agent. For example, a polyvalent epoxy compound having a valency of 2 or more, such as ethylene glycol diglycidyl ether, can be used as the internal crosslinking agent. In this case as well, foaming by the foaming agent is stably performed by the hydrophobic particles.

[0035] In the monomer composition, such an internal crosslinking agent can be used in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the acrylic acid-based monomer. For example, the internal crosslinking agent is 0.01 part by weight or more, 0.05 part by weight or more, 0.1 part by weight, or 0.15 part by weight or more based on 100 parts by weight of the water-soluble ethylene-based unsaturated monomer, and can be used in an amount of 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, 1 part by weight or less, or 0.7 part by weight or less. When the content of the upper internal crosslinking agent is excessively low, crosslinking may not occur sufficiently and it may be difficult to achieve a strength above an appropriate level. When the content of the upper internal crosslinking agent is excessively high, the internal crosslinking density increases and it may be difficult to achieve the desired water retention ability.

[0036] Further, the monomer composition may further contain a polymerization initiator for initiating the polymerization reaction of the monomer. The polymerization initiator is not particularly limited as long as it is generally used in the production of superabsorbent resins.

[0037] Specifically, as the polymerization initiator, a thermal polymerization initiator or a photopolymerization initiator by UV irradiation can be used depending on the polymerization method. However, even in the photopolymerization method, a certain amount of heat is generated by irradiation such as ultraviolet irradiation, and a certain amount of heat is generated by the progress of the polymerization reaction which is an exothermic reaction. Therefore, an additional thermal polymerization initiator may be included.

[0038] The photopolymerization initiator can be used without limitation of its structure as long as it is a compound capable of forming radicals by light such as ultraviolet rays.

[0039] As the photopolymerization initiator, for example, one or more selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, Benzyl Dimethyl Ketal, acyl phosphine, and α-aminoketone can be used. On the other hand, as a specific example of acyl phosphine, commercial lucirin TPO, that is, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide can be used. More diverse photoinitiators are well described in Reinhold Schwalm's book "UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)" p.115, and are not limited to the above examples.

[0040] The photopolymerization initiator may be contained at a concentration of about 0.01 to about 1.0% by weight based on the monomer composition. If the concentration of such a photopolymerization initiator is excessively low, the polymerization rate may become slow. If the concentration of the photopolymerization initiator is excessively high, the molecular weight of the superabsorbent resin may be small and the physical properties may become non-uniform.

[0041] In addition, as the thermal polymerization initiator, one or more selected from the group of initiators consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specifically, examples of persulfate initiators include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc. Examples of azo initiators include 2,2-azobis-(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), etc. More diverse thermal polymerization initiators are well described in 'Principle of Polymerization (Wiley, 1981)' by Odian, p. 203, and are not limited to the aforementioned examples.

[0042] The thermal polymerization initiator may be contained in a concentration of about 0.001 to about 0.5% by weight based on the monomer composition. When the concentration of such a thermal polymerization initiator is excessively low, additional thermal polymerization may hardly occur and the effect of adding the thermal polymerization initiator may be negligible. If the concentration of the thermal polymerization initiator is excessively high, the molecular weight of the superabsorbent resin may be small and the physical properties may become non-uniform.

[0043] Such a polymerization initiator can be used in an amount of 2 parts by weight or less based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. That is, when the concentration of the polymerization initiator is excessively low, the polymerization rate may become slow, and a large amount of residual monomer may be extracted in the final product, which is not preferable. Conversely, when the concentration of the polymerization initiator is higher than the above range, the physical properties of the resin may deteriorate, such as the polymer chains forming the network becoming short, the content of the water-soluble component increasing, and the pressure-absorbing ability decreasing, which is not preferable.

[0044] The monomer composition may further contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant, if necessary.

[0045] And the monomer composition containing the monomer may be in a solution state dissolved in a solvent such as water, for example. The solid content in such a solution-state monomer composition, that is, the concentrations of the monomer, the internal cross-linking agent, and the polymerization initiator can be appropriately adjusted in consideration of the polymerization time, reaction conditions, and the like. For example, the solid content in the monomer composition may be 10 to 80% by weight, or 15 to 60% by weight, or 30 to 50% by weight.

[0046] When the monomer composition has a solid content in the above range, it may be advantageous for adjusting the grinding efficiency during the grinding of the polymer described below while eliminating the need to remove unreacted monomer using the gel effect phenomenon that appears in the polymerization reaction of the high-concentration aqueous solution.

[0047] The solvents that can be used at this time can be used without any limitation on their composition as long as they can dissolve the aforementioned components. For example, water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide, etc., can be used in combination by selecting one or more of them.

[0048] (Step 2) Next, in the presence of the hydrophobic particle aqueous dispersion and the encapsulated blowing agent, the monomer composition is cross-linked and polymerized to produce a water-containing gel polymer. At this stage, the capsules of the encapsulated blowing agent burst, and bubbles are generated by the hydrocarbon gas inside the capsules. Such bubbles can be effectively collected by the hydrophobic particles dispersed in water, increasing the specific surface area of the produced water-containing gel polymer.

[0049] The hydrophobic particles contain metal salts of fatty acids having 7 to 24 carbon atoms. Here, the metal salts of fatty acids having 7 to 24 carbon atoms refer to compounds in which the number of carbon atoms in the molecule is 7 to 24, and a metal cation is bonded in place of the hydrogen ion of the carboxyl group at the end of an unsaturated or saturated fatty acid having a linear structure. At this time, the metal salt may be a monovalent metal salt or a polyvalent metal salt of divalent or higher. At this time, when the hydrophobic particles are metal salts of fatty acids having less than 7 carbon atoms, they cannot collect the bubbles generated by ionization in the aqueous solution state. When the hydrophobic particles are metal salts of fatty acids having more than 24 carbon atoms, the fatty acid chain may be too long and difficult to disperse.

[0050] Specifically, when the metal salt of the fatty acid is a monovalent metal salt, it has a structure in which one fatty acid carboxylate anion is bonded to an alkali ion, which is a monovalent metal cation. Further, when the metal salt of the fatty acid is a polyvalent metal salt of divalent or higher, it has a structure in which the number of fatty acid carboxylate anions equal to the valence of the metal cation is bonded to the metal cation.

[0051] In one embodiment, the hydrophobic particles may be a metal salt of a saturated fatty acid having 12 to 20 carbon atoms. For example, the hydrophobic particles may be a metal salt of lauric acid containing 12 carbon atoms in the molecule; a metal salt of tridecylic acid containing 13 carbon atoms in the molecule; a metal salt of myristic acid containing 14 carbon atoms in the molecule; a metal salt of pentadecanoic acid containing 15 carbon atoms in the molecule; a metal salt of palmitic acid containing 16 carbon atoms in the molecule; a metal salt of margaric acid containing 17 carbon atoms in the molecule; a metal salt of stearic acid containing 18 carbon atoms in the molecule; a metal salt of nonadecylic acid containing 19 carbon atoms in the molecule; and a metal salt of arachidic acid containing 20 carbon atoms in the molecule, and may be one or more metal salts of saturated fatty acids selected from the group consisting of these.

[0052] Preferably, the hydrophobic particles may be a metal salt of stearic acid. For example, the hydrophobic particles may be one or more metal salts of stearic acid selected from the group consisting of calcium stearate, magnesium stearate, sodium stearate, zinc stearate, and potassium stearate.

[0053] In addition, the hydrophobic particles dispersed in the aqueous dispersion can have an average particle size of 0.5 μm to 20 μm as described above. When the hydrophobic particles have an average particle size of less than 0.5 μm, there is a problem that it is difficult to effectively collect the generated bubbles and uniform pores cannot be formed. When the hydrophobic particles have an average particle size exceeding 20 μm, the size of the formed pores may become excessively large and it may be difficult to improve the water absorption rate of the superabsorbent resin. Specifically, for example, the hydrophobic particles may have an average particle size (μm) of 0.5 or more, 1 or more, 2 or more, or 3 or more, while being 20 or less, 15 or less, or 10 or less.

[0054] Here, the average particle diameter of the hydrophobic particles means D50, and the "particle diameter Dn" means the particle diameter at the n% point of the cumulative particle number distribution according to the particle diameter. That is, D50 is the particle diameter at the 50% point of the cumulative particle number distribution according to the particle diameter, D90 is the particle diameter at the 90% point of the cumulative particle number distribution according to the particle diameter, and D10 is the particle diameter at the 10% point of the cumulative particle number distribution according to the particle diameter. The Dn can be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500). When the particles pass through the laser beam, the difference in the diffraction pattern due to the particle size is measured to calculate the particle size distribution. By calculating the particle diameters at the 10%, 50%, and 90% points of the cumulative particle number distribution according to the particle diameter in the measuring device, D10, D50, and D90 can be measured.

[0055] Also, the hydrophobic particles may be contained in the aqueous dispersion at 10 to 70% by weight based on the total weight of the aqueous dispersion. When the content of the hydrophobic particles in the hydrophobic aqueous dispersion is excessively low or excessively high, the hydrophobic particles may not be dispersion-stabilized, and problems such as aggregation of the particles with each other or sedimentation due to gravity may occur.

[0056] Furthermore, the surfactant that disperses the hydrophobic particles in the hydrophobic particle aqueous dispersion can be any surfactant known in the art to stabilize the dispersion of the hydrophobic particles. For example, the surfactant can be one or more surfactants selected from the group consisting of cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants. Preferably, two or more surfactants can be used to stabilize the dispersion of the hydrophobic particles. More specifically, considering the morphology of the hydrophobic particles, such as the metal salt form of saturated fatty acids, nonionic surfactants and anionic surfactants, such as nonionic surfactants bonded to long-chain hydrocarbons having 10 or more carbon atoms and sulfate-based anionic surfactants, can be used together to more effectively disperse the hydrophobic particles in water.

[0057] Examples of the cationic surfactant include dialkyldimethylammonium salts and alkylbenzylmethylammonium salts. Examples of the anionic surfactant include alkyl polyoxyethylene sulfates, monoalkyl sulfates, alkylbenzene sulfonates, monoalkyl phosphates, sulfates having a functional group containing a long chain hydrocarbon or its salt, such as sodium lauryl ether sulfate, ammonium lauryl sulfate, sodium dodecyl sulfate, sodium myreth sulfate, or sodium laureth sulfate. Examples of the amphoteric surfactant include alkyl sulfobetaines and alkyl carboxybetaines. Examples of the nonionic surfactant include polyoxyethylene alkyl ethers such as polyethylene glycol or polyoxyethylene lauryl ether, polyoxyalkylene alkylphenyl ethers, polyoxyethylene arylphenyl ethers, polysorbates, fatty acid sorbitan esters, or fatty acid esters such as glyceryl monostearate, alkyl monoglyceryl ethers, alkanolamides, and alkyl polyglucosides, but are not limited to these.

[0058] And the hydrophobic particle aqueous dispersion may have a pH of 7 or more. When the pH of the hydrophobic particle aqueous dispersion is less than 7, it becomes acidic, making it difficult to stabilize the hydrophobic particles, which are metal salts of fatty acids, and thus it is not suitable.

[0059] On the other hand, the hydrophobic particles can be used in an amount of 0.005 to 0.4 parts by weight based on 100 parts by weight of the acrylic acid-based monomer. When the content of the hydrophobic particles is excessively low, bubbles may not be sufficiently collected, resulting in a slow water absorption rate. When the content of the hydrophobic particles is excessively high, the amount of surfactant used to stabilize the hydrophobic particles in the hydrophobic particle aqueous dispersion may increase, potentially reducing the surface tension. For example, the hydrophobic particles can be used in an amount of 0.01 or more, 0.03 or more, 0.05 or more, or 0.1 or more, while being 0.35 parts by weight or less, 0.3 parts by weight or less, 0.25 parts by weight or less, or 0.2 parts by weight or less based on 100 parts by weight of the acrylic acid-based monomer.

[0060] Also, the encapsulated foaming agent refers to a thermally expandable microcapsule foaming agent having a core-shell structure, and has a core-shell structure including a core containing a hydrocarbon as described above and a shell made of a thermoplastic resin formed on the core. Specifically, the hydrocarbon constituting the core is a liquid hydrocarbon having a low boiling point and is easily vaporized by heat. Therefore, when heat is applied to the encapsulated foaming agent, the thermoplastic resin forming the shell softens, and at the same time, the liquid hydrocarbon in the core vaporizes, increasing the pressure inside the capsule and causing it to expand, thereby forming bubbles of a size larger than the original size.

[0061] Therefore, the encapsulated foaming agent generates a hydrocarbon gas and is distinguished from an organic foaming agent that generates nitrogen gas through an exothermic decomposition reaction between monomers participating in polymer formation and an inorganic foaming agent that absorbs heat generated during polymer formation and foams carbon dioxide gas.

[0062] Such encapsulated blowing agents may have different expansion characteristics depending on the components forming the core and shell and the weight and diameter of each component. By adjusting this, it is possible to expand to a desired size, and thereby adjust the porosity of the superabsorbent resin.

[0063] Specifically, the encapsulated blowing agent has a particle form with an average diameter (D0) before expansion of 5 to 30 μm. Making the encapsulated blowing agent have an average diameter of less than 5 μm is difficult in production, and when the average diameter of the encapsulated blowing agent exceeds 30 μm, the pore size may be excessively large, and it may be difficult to efficiently increase the surface area. Therefore, when the encapsulated blowing agent exhibits the above average diameter, it can be judged to be suitable for achieving an appropriate pore structure in the resin.

[0064] For example, the average diameter before expansion of the encapsulated blowing agent may be 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, or 10 μm or more, and may also be 30 μm or less, 25 μm or less, 20 μm or less, 17 μm or less, 16 μm or less, or 15 μm or less.

[0065] The average diameter (D0) before expansion of such an encapsulated blowing agent can be measured by measuring the diameter of each encapsulated blowing agent particle as the average Feret diameter through an optical microscope and then obtaining the average value of these.

[0066] At this time, the capsule thickness of the encapsulated blowing agent may be 2 to 15 μm.

[0067] In addition, the encapsulated blowing agent has a maximum expansion size in air of 20 to 190 μm. Here, the "maximum expansion size of the encapsulated blowing agent" means the diameter range of the top 10% by weight of the particles that have expanded the most after heat is applied to the encapsulated blowing agent. Making the maximum expansion size of the encapsulated blowing agent in air less than 20 μm is difficult in manufacturing, and when the maximum expansion size in air exceeds 190 μm, the pore size is excessively large, and it may be difficult to efficiently increase the surface area.

[0068] For example, the encapsulated blowing agent may have a maximum expansion size in air of 50 to 190 μm, or 70 to 190 μm, 75 to 190 μm, or 80 to 150 μm.

[0069] The maximum expansion size of such an encapsulated blowing agent in air can be determined by applying 0.2 g of the encapsulated blowing agent on a glass Petri dish, then leaving it on a hot plate preheated to 150 °C for 10 minutes, and then observing the expanded encapsulated blowing agent with an optical microscope and measuring the diameter of the top 10% by weight of the particles that have expanded the most through the optical microscope as the average Feret diameter.

[0070] And the encapsulated blowing agent has a maximum expansion ratio in air of 5 to 15 times. Here, the "maximum expansion ratio of the encapsulated blowing agent" means the ratio of the average diameter (D M ) of the top 10% by weight of the particles that have expanded the most after heat is applied to the average diameter (D0) measured before heat is applied to the encapsulated blowing agent. MIt means " / D0". When the maximum expansion ratio of the encapsulated foaming agent in air is less than 5 times, it is impossible to form an appropriate pore structure in the superabsorbent resin, and there is a problem that it is impossible to manufacture a superabsorbent resin with simultaneously improved water absorption capacity and water absorption rate even if it is used. Making the maximum expansion ratio of the encapsulated foaming agent in air exceed 15 times has a problem that it is difficult to manufacture when considering the average diameter before the above-mentioned expansion of the encapsulated foaming agent. Therefore, it can be judged that the encapsulated foaming agent having the maximum expansion ratio within the above-mentioned range is suitable for forming an appropriate pore structure in the superabsorbent resin.

[0071] For example, the maximum expansion ratio of the encapsulated foaming agent in air may be 5 times or more, 7 times or more, or 8 times or more, and may also be 15 times or less, 13 times or less, 11 times or less, or 10 times or less.

[0072] At this time, the average diameter (D0) measured before heating the encapsulated foaming agent can be measured as described above. Also, the average diameter (D M ) of the top 10% by weight of the particles that have expanded more after heating the encapsulated foaming agent can be measured by applying 0.2 g of the encapsulated foaming agent on a glass Petri dish, then leaving it on a hot plate preheated to 150 °C for 10 minutes, observing the expanded encapsulated foaming agent with an optical microscope, measuring the diameter of each of the top 10% by weight of the particles that have expanded more through the optical microscope as the average Feret diameter, and then obtaining the average value of these.

[0073] The expansion characteristics of the encapsulated foaming agent will be more specifically described in the examples below.

[0074] The reason for measuring the maximum expansion magnitude and maximum expansion ratio of the encapsulated blowing agent in air is to determine whether pores of a desired size are formed in a superabsorbent resin prepared using the encapsulated blowing agent. Specifically, the foaming form of the blowing agent can vary depending on the preparation conditions of the superabsorbent resin, making it difficult to define a single form. Therefore, by first foaming the encapsulated blowing agent in air and checking the expansion magnitude and expansion ratio, it can be determined whether the encapsulated blowing agent is suitable for forming desired pores.

[0075] The hydrocarbon constituting the core of the encapsulated blowing agent may be one or more selected from the group consisting of n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, iso-heptane, cycloheptane, n-octane, iso-octane, and cyclooctane. Among these, hydrocarbons having 3 to 5 carbon atoms (n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane) are suitable for forming pores of the aforementioned size, and iso-butane may be most suitable.

[0076] The thermoplastic resin constituting the shell of the encapsulated blowing agent may be a polymer formed from one or more monomers selected from the group consisting of (meth)acrylate compounds, (meth)acrylonitrile compounds, aromatic vinyl compounds, vinyl acetate compounds, and vinyl halide compounds. Among these, a copolymer of (meth)acrylate and (meth)acrylonitrile may be most suitable for forming pores of the aforementioned size.

[0077] The foaming initiation temperature (T start ) may be 60°C to 120°C, or 65°C to 120°C, or 70°C to 80°C, and the maximum foaming temperature (T max) may be 100°C to 160°C, or 105°C to 155°C, or 110°C to 120°C. When within the aforementioned range, foaming can easily occur in the subsequent thermal polymerization step or drying step, and a pore structure can be introduced into the polymer. Such foaming start temperature and maximum foaming temperature can be measured using a Thermomechanical Analyzer.

[0078] Also, the encapsulated blowing agent can be used in an amount of 0.005 to 1 part by weight based on 100 parts by weight of the acrylic monomer. When the content of the blowing agent is less than 0.005 part by weight, the role as a blowing agent may be insignificant. When the content of the blowing agent exceeds 1 part by weight, there may be too many pores in the crosslinked polymer, resulting in a decrease in the gel strength of the produced superabsorbent resin and a decrease in density, which may cause problems in distribution and storage. For example, the encapsulated blowing agent can be used in an amount of 0.01 part by weight or more, 0.03 part by weight or more, or 0.05 part by weight or more, while being 0.8 part by weight or less, 0.6 part by weight or less, or 0.5 part by weight or less based on 100 parts by weight of the acrylic monomer.

[0079] Also, such an encapsulated blowing agent and the hydrophobic particles can be used at a weight ratio of 1:0.1 to 1:3.5. When the hydrophobic particles are used at an excessively low content compared to the encapsulated blowing agent, it is difficult to effectively collect the generated bubbles. When used at an excessively high content compared to the blowing agent, various physical properties such as water retention capacity and water absorption rate may decline. For example, the hydrophobic particles can be used at a weight ratio of 0.1 times or more, 0.2 times or more, or 0.3 times or more, while being 3.5 times or less, 3.0 times or less, 2.5 times or less, 2.0 times or less, or 1.5 times or less the weight of the encapsulated blowing agent.

[0080] Also, in the step 2, a surfactant which is usually used as a bubble stabilizer can be further added together with the encapsulated foaming agent and the hydrophobic particle aqueous dispersion. Examples of surfactants that can be used at this time include cationic surfactants such as quaternary ammonium compounds such as dodecyltrimethylammonium chloride and dodecyltrimethylammonium bromide; anionic surfactants such as alkyl sulfate compounds such as sodium dodecyl sulfate, ammonium lauryl sulfate, sodium lauryl ether sulfate, or sodium myreth sulfate; or nonionic surfactants such as alkyl ether sulfate compounds such as polyoxyethylene lauryl ether, but are not limited thereto.

[0081] More specifically, in the step 2, one or more bubble stabilizers selected from the group consisting of an alkyl sulfate compound and a polyoxyethylene alkyl ether compound can be further added together with the encapsulated foaming agent and the hydrophobic particle aqueous dispersion. When the bubble stabilizer is additionally added, the dispersibility of the encapsulated foaming agent and the hydrophobic particle aqueous dispersion is improved, so that the generation and collection of bubbles occur uniformly, and the water absorption rate of the produced superabsorbent resin can be made faster.

[0082] At this time, the encapsulated foaming agent and the foam stabilizer can be used at a weight ratio of 1:0.01 to 1:0.5. However, when the foam stabilizer is used in an excessive amount, the foam stabilizer may coat the surface of the base resin or the superabsorbent resin, resulting in a decrease in processability and a decrease in the water absorption-related physical properties of the superabsorbent resin, which is not suitable. For example, the foam stabilizer can be used at a weight ratio of 0.015 times or more, 0.02 times or more, or 0.05 times or more with respect to the weight of the encapsulated foaming agent, while being 0.4 times or less, 0.35 times or less, or 0.3 times or less.

[0083] On the other hand, the polymerization of the monomer composition in the presence of such a hydrophobic particle aqueous dispersion and the encapsulated foaming agent is not particularly limited in configuration as long as it is a commonly used polymerization method.

[0084] Specifically, the polymerization method is roughly divided into thermal polymerization and photopolymerization depending on the polymerization energy source. Usually, when performing thermal polymerization, it can be carried out in a reactor having a stirring shaft such as a kneader, and when performing photopolymerization, it can be carried out in a reactor equipped with a movable conveyor belt. However, the above-mentioned polymerization method is only an example, and the present invention is not limited to the above-mentioned polymerization method.

[0085] As an example, the hydrogel polymer obtained by performing thermal polymerization by supplying hot air to a reactor such as a kneader equipped with a stirring shaft as described above or heating the reactor depends on the form of the stirring shaft provided in the reactor. The hydrogel polymer discharged from the reactor outlet may be in the form of several centimeters to several millimeters. Specifically, the size of the obtained hydrogel polymer is variously shown depending on the concentration and injection rate of the monomer composition to be injected. Usually, a hydrogel polymer having a weight average particle diameter of 2 to 50 mm is obtained.

[0086] Also, when performing photopolymerization in a reactor equipped with a conveyer belt movable as described above, usually, the form of the obtained hydrogel polymer may be a sheet-like hydrogel polymer having the width of the belt. At this time, although the thickness of the polymer sheet varies depending on the concentration and injection rate of the monomer composition to be injected, usually, it is preferable to supply the monomer composition so that a sheet-like polymer having a thickness of about 0.5 to about 5 cm can be obtained. When the monomer composition is supplied to such an extent that the thickness of the sheet-like polymer is excessively thin, the production efficiency is low and not preferable. When the thickness of the sheet-like polymer exceeds 5 cm, the polymerization reaction may not occur uniformly over the entire thickness due to the excessively thick thickness.

[0087] At this time, the normal water content rate of the hydrogel polymer obtained by such a method may be about 40 to about 80% by weight. On the other hand, throughout this specification, the "water content rate" means the content of water in the total polymer weight, and means a value obtained by subtracting the weight of the polymer in the dry state from the weight of the polymer. Specifically, it is defined as a value calculated by measuring the weight reduction due to water evaporation in the polymer during the process of drying by raising the temperature of the polymer through infrared heating. At this time, the drying conditions are such that the temperature is raised from room temperature to about 180°C and then maintained at 180°C. The total drying time is set to 20 minutes including 5 minutes in the temperature rising stage to measure the water content rate.

[0088] (Step 3) Next, the step of drying and pulverizing the hydrogel polymer to form a base resin in powder form is performed. If necessary, a step of rough pulverization can be further performed before drying to enhance the efficiency of the drying step.

[0089] At this time, the pulverizer used has no configuration limitations. Specifically, it can include any one selected from a group of pulverizing devices consisting of a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter, but is not limited to the aforementioned examples.

[0090] At this time, the pulverizing stage can pulverize the polymer so that the particle size of the polymer becomes about 2 to about 10 mm.

[0091] Pulverizing to a particle size less than 2 mm is not technically easy due to the high water content of the water-containing gel polymer, and there may also be a phenomenon of aggregation between the pulverized particles. On the other hand, when pulverizing to a particle size exceeding 10 mm, the effect of increasing the efficiency of the subsequent drying stage is negligible.

[0092] Drying is performed on the polymer immediately after polymerization that has been pulverized as described above or has not undergone the pulverizing stage. At this time, the drying temperature in the drying stage may be about 150 to about 250 °C. If the drying temperature is less than 150 °C, the drying time may become excessively long and the physical properties of the finally formed superabsorbent resin may deteriorate. If the drying temperature exceeds 250 °C, only the surface of the polymer may be excessively dried, and fine powder may be generated in the subsequent pulverizing process, and the physical properties of the finally formed superabsorbent resin may deteriorate. Therefore, preferably, the drying can be performed at a temperature of about 150 to about 200 °C, and more preferably at a temperature of about 160 to about 180 °C.

[0093] On the other hand, in the case of the drying time, it can be performed for about 20 to about 90 minutes in consideration of process efficiency and the like, but is not limited thereto.

[0094] The drying method in the drying stage can be selected and used without limitation as long as it is commonly used in the drying process of the water-containing gel polymer. Specifically, the drying stage can be carried out by methods such as hot air supply, infrared irradiation, extremely high ultra-short wave irradiation, or ultraviolet irradiation. The water content of the polymer after such a drying stage may be about 5 to about 10% by weight.

[0095] Next, a step of pulverizing the dried polymer obtained through such a drying stage is performed.

[0096] The base resin, which is the polymer powder obtained after the pulverization step, may have a particle size of about 150 to about 850 μm. Specifically, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, etc. can be used as the pulverizer used to pulverize to such a particle size, but the present invention is not limited to the above examples.

[0097] And after such a pulverization step, in order to control the physical properties of the superabsorbent resin powder to be finally made into a product, the base resin obtained after pulverization is classified by particle size. Preferably, the polymer having a particle size of about 150 to about 850 μm is classified, and only the base resin having such a particle size may be subjected to a surface crosslinking reaction step. Such a particle size can be measured by the method of European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3.

[0098] (Step 4) Next, a step of additionally crosslinking the surface of the base resin in the presence of a surface crosslinking agent to form a surface crosslinked layer is performed. Through the above step, a superabsorbent resin in which a surface crosslinked layer is formed on the surface of the base resin, more specifically, at least a part of the surface of each base resin particle, is produced.

[0099] Surface crosslinking is a step of increasing the crosslinking density near the surface of the superabsorbent polymer particles in relation to the crosslinking density inside the particles. Generally, the surface crosslinking agent is applied to the surface of the superabsorbent resin particles. Therefore, this reaction occurs on the surface of the superabsorbent resin particles, which substantially does not affect the inside of the particles while improving the crosslinkability on the surface of the particles. Therefore, the surface-crosslinked superabsorbent resin particles have a higher crosslinking degree near the surface than inside.

[0100] As the surface crosslinking agent, all surface crosslinking agents used in the production of existing superabsorbent resins can be used without any particular limitation. For example, the surface crosslinking agent is one or more polyols selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; one or more carbonate compounds selected from the group consisting of ethylene carbonate, propylene carbonate, and glycerol carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazoline compounds such as oxazolidinone; polyamine compounds; mono-, di- or polyoxazolidinone compounds; or cyclic urea compounds; and the like can be included.

[0101] According to one embodiment, the surface crosslinking agent can be the same as the internal crosslinking agent. For example, the surface crosslinking agent can include a divalent or higher polyvalent epoxy compound, for example, ethylene glycol diglycidyl ether. Or, only ethylene glycol diglycidyl ether can be used alone as the surface crosslinking agent. The ethylene glycol diglycidyl ether has high water solubility, has two epoxy groups, and can easily undergo a surface reaction even at a relatively low temperature.

[0102] The content of the surface crosslinking agent can be appropriately selected according to the type of the surface crosslinking agent specifically added and the reaction conditions. However, based on 100 parts by weight of the base resin, about 0.001 to about 5 parts by weight, preferably about 0.01 to about 3 parts by weight, and more preferably about 0.05 to about 2 parts by weight can be used. If the content of the surface crosslinking agent is excessively low, the surface crosslinking reaction hardly occurs. When it exceeds 5 parts by weight based on 100 parts by weight of the base resin, a decrease in water absorption properties such as water retention ability may occur due to the progress of excessive surface crosslinking reaction.

[0103] There is no limitation on the configuration of the method for mixing the surface crosslinking agent with the base resin. The surface crosslinking agent and the base resin powder can be put into a reaction tank and mixed, or the surface crosslinking agent can be sprayed onto the base resin powder, or a method of continuously supplying and mixing the base resin and the surface crosslinking agent to a continuously operating mixer can be used.

[0104] When adding the surface crosslinking agent, water can be mixed together and added in the form of a surface crosslinking solution. When adding water, there is an advantage that the surface crosslinking agent can be uniformly dispersed in the polymer. At this time, the content of the added water is preferably added in a ratio of about 1 to about 10 parts by weight based on 100 parts by weight of the base resin for the purpose of inducing uniform dispersion of the surface crosslinking agent, preventing the polymer powder from solidifying, and optimizing the surface penetration depth of the surface crosslinking agent.

[0105] According to an embodiment of the present invention, in addition to water, it contains propylene glycol or propylene carbonate as a solvent. Also, it does not contain an alcohol-based solvent such as methanol.

[0106] The propylene glycol or propylene carbonate does not participate in the surface crosslinking reaction and serves as a solvent. As a result, the effect that the surface crosslinking liquid is gradually absorbed by the base resin and the coating is uniformly applied can be achieved. Usually, in the case of an alcohol-based solvent such as methanol used as the solvent of the surface crosslinking solution, there is a disadvantage of inducing an unpleasant odor. However, according to an embodiment of the present invention, by including propylene glycol or propylene carbonate as the solvent of the surface crosslinking solution and eliminating the alcohol-based solvent, the above-described effect can be achieved without a specific odor.

[0107] In addition, when propylene glycol or propylene carbonate is included as a solvent instead of the alcohol-based solvent, it was confirmed that the rewetting phenomenon can be improved while maintaining both the water retention ability and the pressure water absorption ability at a high level.

[0108] At this time, the content of the added propylene glycol or propylene carbonate is about 0.1 part by weight or more, or about 0.2 part by weight or more, or about 0.3 part by weight or more, and about 5 parts by weight or less, or about 4 parts by weight or less, or about 3 parts by weight or less with respect to 100 parts by weight of the base resin, for the purpose of inducing uniform dispersion of the surface crosslinking agent and preventing the polymer powder from solidifying, and at the same time optimizing the surface penetration depth of the surface crosslinking agent. It is preferably added.

[0109] The surface crosslinking bonding reaction is carried out by heating the base resin added with the surface crosslinking solution containing the surface crosslinking agent and the solvent at a temperature of about 100 to about 150 ° C, preferably about 110 to about 140 ° C for about 15 to about 80 minutes, preferably about 20 to about 70 minutes. When the crosslinking reaction temperature is less than 100 ° C, the surface crosslinking reaction may not occur sufficiently. When it exceeds 150 ° C, the propylene glycol or propylene carbonate contained as a solvent may participate in the surface crosslinking reaction and an additional surface crosslinking reaction by these compounds may occur. When the surface crosslinking reaction by propylene glycol or propylene carbonate is carried out in this way, the surface crosslinking density may increase and the water absorption ability may be significantly reduced.

[0110] The temperature raising means for the surface crosslinking reaction is not particularly limited. Heating can be performed by supplying a heat medium or directly supplying a heat source. In this case, the type of heat medium that can be used may be a heated fluid such as steam, hot air, or hot oil, but the present invention is not limited to these. The temperature of the heat medium to be supplied may be appropriately selected in consideration of the heat medium means, the temperature raising rate, and the target temperature. Meanwhile, examples of a heat source that is directly supplied include electric heating and gas heating, but the present invention is not limited to these examples.

[0111] After forming the surface cross-linked layer on the surface of the base resin as described above, an inorganic material may be further mixed.

[0112] The inorganic material may be, for example, one or more selected from the group consisting of silica, clay, alumina, silica-alumina composites, and titania, and preferably silica.

[0113] The inorganic substance can be used in an amount of 0.01 parts by weight or more, or 0.05 parts by weight or more, or 0.1 parts by weight or more, but not more than 5 parts by weight, or not more than 3 parts by weight, or not more than 1 part by weight, based on 100 parts by weight of the superabsorbent resin.

[0114] Meanwhile, in order to control the physical properties of the superabsorbent polymer to be manufactured as a final product, a step of classifying the superabsorbent polymer obtained after the surface cross-linking reaction step by particle size can be additionally carried out. Preferably, polymers having a particle size of about 150 to about 850 μm are classified, and only the superabsorbent polymer having such particle size can be used as the final product.

[0115] The superabsorbent resin obtained by the above-mentioned production method has a balanced water absorption rate and water absorption properties, and can satisfy the following properties.

[0116] 1) The water absorption rate (vortex time) at 24.0 °C is 35 seconds or less; and 2) When 1 g of the superabsorbent resin is immersed in 2 L of tap water and swollen for 1 minute, the 1-minute tap water absorption capacity, defined as the weight of water absorbed by the superabsorbent resin in 1 minute, is 115 g or more. 3) The centrifugal water retention capacity (CRC) measured by the method of EDANA method WSP241.3 is 28 - 35 g / g; 4) The 0.7 psi pressure water absorption capacity (AUP) measured by the method of EDANA method WSP242.3 is 16 - 22 g / g; and 5) The effective water absorption capacity (EFFC) calculated by the following formula 1 is 22 g / g or more:

[0117] [Formula 1] Effective water absorption capacity (EFFC) = {Water retention capacity (CRC) + 0.7 psi pressure water absorption capacity (AUP)} / 2.

[0118] In the above formula 1, The water retention capacity (CRC) means the centrifugal water retention capacity (CRC) of the superabsorbent resin measured by the method of EDANA method WSP241.3, The 0.7 psi pressure water absorption capacity (AUP) means the 0.7 psi pressure water absorption capacity (AUP) of the superabsorbent resin measured by the method of EDANA method WSP242.3.

[0119] More specifically, the superabsorbent resin produced by the above production method may have a water absorption rate (vortex time) at 24.0 °C of 35 seconds or less, 34 seconds or less, or 33 seconds or less. Also, the smaller the value of the water absorption rate, the better, and the lower limit of the water absorption rate is theoretically 0 seconds, but as an example, it may be 10 seconds or more, or 15 seconds or more, 18 seconds or more, or 20 seconds or more. At this time, the method for measuring the water absorption rate of the superabsorbent resin will be described more specifically in the following examples.

[0120] Furthermore, the superabsorbent polymer may have a one-minute tap water absorption capacity, defined as the weight of water absorbed by the superabsorbent polymer in one minute when 1 g of the superabsorbent polymer is immersed in 2 L of tap water and allowed to swell for one minute, of 115 g or more, 120 g or more, or 125 g or more, but 200 g or less, 190 g or less, or 180 g or less.

[0121] Additionally, the superabsorbent polymer may have a centrifuge retention capacity (CRC) measured by EDANA method WSP241.3 of 29 g / g or more, or 30 g / g or more, and 34 g / g or less, or 33 g / g or less.

[0122] The superabsorbent polymer may also have an absorbency under pressure (AUP) of 18 g / g or more, or 19 g / g or more, at 0.7 psi, measured by the EDANA method WSP242.3, and 22 g / g or less, or 20 g / g or less.

[0123] Furthermore, the superabsorbent polymer may have an effective water absorption capacity (EFFC) calculated by the above formula 1 of 23 g / g or more, or 24 g / g or more, and 28 g / g or less, 27 g / g or less, or 26 g / g or less.

[0124] The present invention will be described in more detail with reference to the following examples, but the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention. [Example]

[0125] <Production example> The aqueous dispersions of hydrophobic particles and the encapsulated foaming agents used in the following examples were prepared in the following manner.

[0126] Production Example 1: Production of calcium stearate aqueous dispersion (Ca-st) First, 50 g of water containing two or more surfactants (including a polyoxyethylene alkyl ether nonionic surfactant and a sulfate anionic surfactant) was placed in a high shear mixer and heated to 165°C. Then, 50 g of calcium stearate powder was added. Next, it was stirred at 4000 rpm for 30 minutes at normal pressure so that the calcium stearate was sufficiently pulverized, and an aqueous dispersion Ca-st in which 50 wt% of calcium stearate having an average particle size of 5 μm was dispersed was obtained. At this time, the pH of the aqueous dispersion was 9.5. Also, after the production of the Ca-st, its average particle size (D50) was measured / calculated at the particle size at the 50% point of the particle number cumulative distribution using a laser diffraction particle size analyzer (Microtrac S3500).

[0127] Production Example 2: Production of zinc stearate aqueous dispersion Zn-st First, 70 g of water containing two or more surfactants (including a polyoxyethylene alkyl ether nonionic surfactant and a sulfate anionic surfactant) was placed in a high shear mixer and heated to 140°C. Then, 30 g of zinc stearate powder was added. Next, it was stirred at 4000 rpm for 30 minutes at normal pressure so that the zinc stearate was sufficiently pulverized, and an aqueous dispersion Zn-st in which 30 wt% of zinc stearate having an average particle size of 1 μm was dispersed was obtained. At this time, the pH of the aqueous dispersion was 9.5, and the average particle size of Zn-st was measured / calculated in the same manner as in Production Example 1.

[0128] Production Example 3: Preparation of encapsulated blowing agent As the encapsulated blowing agent used in the examples, F-36D manufactured by Matsumoto Co., Ltd., in which the core is iso-butane and the shell is composed of a copolymer of acrylate and acrylonitrile, was prepared. At this time, the foaming start temperature (T start ) of F-36D is 70°C to 80°C, and the maximum foaming temperature (T max ) is 110°C to 120°C.

[0129] The diameter of each encapsulated blowing agent was measured as the average Feret diameter through an optical microscope. Then, the average value of the diameters of the encapsulated blowing agents was obtained and defined as the average diameter of the encapsulated blowing agent.

[0130] Also, in order to confirm the expansion characteristics of the encapsulated blowing agent, after applying 0.2 g of the prepared encapsulated blowing agent onto a glass Petri dish, it was left on a hot plate preheated to 150 °C for 10 minutes. The encapsulated blowing agent gradually expanded due to heat, and this was observed with an optical microscope to measure the maximum expansion ratio and the maximum expansion size of the encapsulated blowing agent in the air.

[0131] The diameter of the top 10% by weight of the particles that expanded more after heating the encapsulated blowing agent was measured in order of the particles that expanded more, and defined as the maximum expansion size. The ratio (D M ) of the average diameter (D M ) of the top 10% by weight of the particles that expanded more after heating to the average diameter (D0) measured before heating the encapsulated blowing agent was obtained and defined as the maximum expansion ratio.

[0132] The average diameter of the prepared encapsulated blowing agent before expansion was 13 μm, the maximum expansion ratio in the air was about 9 times, and the maximum expansion size was about 80 - 150 μm.

[0133] <Example> Example 1 (Step 1) 100 g of acrylic acid, 0.001 g of PEGDA400 (polyethylene glycol diacrylate 400) which is an internal crosslinking agent, 0.27 g of ethylene glycol diglycidyl ether (EJ1030s), 0.008 g of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide (I-819), and 0.2 g of thermal initiator sodium persulfate were added to a 3 L glass container equipped with a stirrer and a thermometer and dissolved. Then, 188 g of a 22% sodium hydroxide solution was added to produce a monomer composition (neutralization degree: 75 mol%; solid content: 41 wt%).

[0134] (Step 2) 0.18 g of calcium stearate aqueous dispersion Ca-st prepared in Preparation Example 1 was added to the monomer composition so that 0.09 g of calcium stearate was added per 100 g of acrylic acid, and 0.06 g of encapsulated foaming agent (F-36D) was also added. Then, the monomer composition was supplied at a rate of 500 to 2000 mL / min onto a conveyor belt having a width of 10 cm and a length of 2 m, rotating at a speed of 50 cm / min. Simultaneously with the supply of the monomer composition, a 10 mW / cm 2 The polymerization reaction was carried out for 60 seconds by irradiating the film with ultraviolet light having an intensity of 1000 kJ / cm 2 for 60 seconds, thereby obtaining a sheet-shaped hydrogel polymer having a water content of 55% by weight.

[0135] (Step 3) Next, the sheet-form hydrogel polymer was cut into pieces approximately 5 cm x 5 cm in size and then placed in a meat chopper to crush the polymer, yielding hydrogel particle crumbs with sizes ranging from 1 mm to 10 mm. The crumbs were then dried in an oven capable of rotating airflow from top to bottom. Hot air at 185°C or higher was blown from bottom to top for 15 minutes, then from top to bottom for another 15 minutes, ensuring uniform drying, until the moisture content of the dried product was 2% or less. After drying, the crumbs were crushed in a crusher and classified to select particles ranging from 150 to 850 μm in size, to prepare the base resin.

[0136] (Step 4) A surface cross-linking solution was prepared by mixing 4 g of water, 1.0 g of propylene glycol, 0.08 g of ethylene glycol diglycidyl ether, and 0.05 g of a polycarboxylic acid copolymer (copolymer of methoxypolyethylene glycol monomethacrylate and methacrylic acid, Mw = 40,000) with respect to 100 g of the base resin. The obtained 100 g of the base resin powder was sprayed with the surface cross-linking solution and mixed, and this was placed in a container composed of a stirrer and a double jacket and subjected to a surface cross-linking reaction at 140 °C for 35 minutes. Thereafter, the surface-treated powder was classified with a standard mesh sieve according to ASTM standards to obtain a superabsorbent resin powder having a particle size of 150 to 850 μm. Thereafter, 0.1 g of fumed silica (AEROSIL (registered trademark) 200) was additionally mixed with respect to 100 g of the obtained resin powder.

[0137] Example 2 A superabsorbent resin was produced in the same manner as in Example 1, except that 0.12 g of the foaming agent (F-36D) encapsulated in Example 1 was added.

[0138] Example 3 A superabsorbent resin was produced in the same manner as in Example 1, except that 0.3 g of the foaming agent (F-36D) encapsulated in Example 1 was added.

[0139] Example 4 A superabsorbent resin was produced in the same manner as in Example 1, except that 0.12 g of the foaming agent (F-36D) encapsulated in Example 1 was added, and 0.3 g of the zinc stearate aqueous dispersion Zn-st prepared in Production Example 2 was added so that 0.09 g of zinc stearate was added to 100 g of acrylic acid instead of 0.18 g of Ca-st as the hydrophobic particle aqueous dispersion.

[0140] Example 5 In Step 2 of Example 2, a superabsorbent resin was produced in the same manner as in Example 2, except that 0.03 g (0.0084 g of SDS) of an aqueous solution of 28% sodium dodecyl sulfate (SDS) (ELOTANTE TM SL130, manufactured by LG Life Sciences, Ltd.) as a foam stabilizer was further added to the monomer composition together with the hydrophobic particle aqueous dispersion and the encapsulated foaming agent.

[0141] Example 6 In Step 2 of Example 3, a superabsorbent resin was produced in the same manner as in Example 3, except that 0.03 g (0.0084 g of SDS) of an aqueous solution of 28% sodium dodecyl sulfate (SDS) (ELOTANTE TM SL130, manufactured by LG Life Sciences, Ltd.) as a foam stabilizer was further added to the monomer composition together with the hydrophobic particle aqueous dispersion and the encapsulated foaming agent.

[0142] Example 7 In Step 2 of Example 2, a superabsorbent resin was produced in the same manner as in Example 2, except that 0.03 g of polyoxyethylene lauryl ether (LE-6, manufactured by Hannong Chemical Co., Ltd.) as a foam stabilizer was further added to the monomer composition together with the hydrophobic particle aqueous dispersion and the encapsulated foaming agent.

[0143] Example 8 In Step 2 of Example 3, a superabsorbent resin was produced in the same manner as in Example 3, except that 0.03 g of polyoxyethylene lauryl ether (LE-6, manufactured by Hannong Chemical Co., Ltd.) as a foam stabilizer was further added to the monomer composition together with the hydrophobic particle aqueous dispersion and the encapsulated foaming agent sodium bicarbonate.

[0144] Comparative Example 1 A superabsorbent resin was produced in the same manner as in Example 1, except that the hydrophobic particle aqueous dispersion was not used in Example 1.

[0145] Comparative Example 2 A superabsorbent resin was produced in the same manner as in Example 2, except that the hydrophobic particle aqueous dispersion was not used in Example 2.

[0146] Comparative Example 3 A superabsorbent resin was produced in the same manner as in Example 3, except that the hydrophobic particle aqueous dispersion was not used in Example 3.

[0147] Comparative Example 4 A superabsorbent resin was produced in the same manner as in Example 5, except that the hydrophobic particle aqueous dispersion was not used in Example 5.

[0148] Comparative Example 5 A superabsorbent resin was produced in the same manner as in Example 7, except that the hydrophobic particle aqueous dispersion was not used in Example 7.

[0149] Comparative Example 6 A superabsorbent resin was produced in the same manner as in Example 1, except that calcium stearate (manufactured by Tokuyama Kagaku Co., Ltd.) having an average particle size of 5 μm in powder form was used at 0.09 g per 100 g of acrylic acid instead of the hydrophobic particle aqueous dispersion form of Ca-st. However, the calcium stearate in powder form did not disperse in the monomer composition and remained aggregated, floating on the neutralized solution, and a superabsorbent resin with a uniform pore structure distribution was not produced.

[0150] Experimental Example: Physical Property Measurement of Superabsorbent Resin For the superabsorbent resins produced in the above Examples and Comparative Examples, the physical properties were evaluated by the following methods and shown in Table 1 below. Unless otherwise noted, all the following physical property evaluations were carried out in a thermostatic and humid chamber (23 ± 0.5 °C, relative humidity 45 ± 0.5 %), and the measured data was the average value of three measurements to prevent measurement errors. Also, the physiological saline or brine used in the following physical property evaluations means a 0.9 wt% aqueous sodium chloride (NaCl) solution.

[0151] (1) Centrifuge Retention Capacity (CRC) The water retention capacity based on the water absorption ratio of each resin under no load was measured according to EDANA WSP241.3.

[0152] Specifically, after uniformly placing a superabsorbent resin W0 (g) (about 0.2 g) in an envelope made of non-woven fabric and sealing it, it was immersed in physiological saline (0.9% by weight) at room temperature. After 30 minutes, moisture was removed from the envelope for 3 minutes under the condition of 250G using a centrifuge, and the mass W2 (g) of the envelope was measured. Also, after performing the same operation without using the resin, the mass W1 (g) at that time was measured. Using the obtained masses, CRC (g / g) was calculated by the following formula.

[0153] [Equation 1] CRC (g / g) = {[W2 (g) - W1 (g)] / W0 (g)} - 1

[0154] (2) Absorption Under Pressure (AUP) The absorption under pressure of each resin at 0.7 psi was measured according to the EDANA method WSP242.3.

[0155] Specifically, a stainless steel 400-mesh wire mesh was attached to the bottom of a plastic cylinder with an inner diameter of 60 mm. Under the conditions of room temperature and 50% humidity, a superabsorbent resin W0 (g) (0.90 g) was uniformly spread on the wire mesh, and a piston that could uniformly apply an additional load of 0.3 psi was made slightly smaller than 60 mm in outer diameter so that there was no gap with the inner wall of the cylinder and the up and down movement was not obstructed. At this time, the weight W3 (g) of the device was measured.

[0156] A glass filter with a diameter of 90 mm and a thickness of 5 mm was placed inside a Petri dish with a diameter of 150 mm, and physiological saline composed of 0.9% by weight sodium chloride was made to be at the same level as the upper surface of the glass filter. One piece of filter paper with a diameter of 90 mm was placed on it. The measuring device was placed on the filter paper, and the liquid was absorbed under load for 1 hour. After 1 hour, the measuring device was lifted, and its weight W4 (g) was measured.

[0157] The obtained masses were used to calculate the water absorption capacity under pressure (g / g) according to the following formula.

[0158] [Formula 2] AUP(g / g) = [W4(g) - W3(g)] / W0(g)

[0159] (3) Water absorption rate (Vortex time) The water absorption rate (vortex time) of the superabsorbent resins of the Examples and Comparative Examples was measured by the following method.

[0160] (i) First, 50 mL of 0.9% saline was poured into a 100 mL beaker with a flat bottom using a 100 mL measuring cylinder. (ii) Next, the beaker was placed in the center of a magnetic stirrer, and a magnetic bar (diameter 8 mm, length 30 mm) was placed in the beaker. (iii) Then, the agitator was operated so that the magnetic bar was agitating at 600 rpm, and the lowest part of the vortex generated by the agitation was brought into contact with the magnetic bar. (iv) After confirming that the temperature of the saltwater in the beaker had reached 24.0°C, 2±0.01 g of the superabsorbent polymer sample was added while simultaneously starting a stopwatch, and the time until the vortex disappeared and the liquid surface became completely horizontal was measured in seconds, and this was taken as the water absorption rate.

[0161] (4) 1-minute tap water absorption capacity The one-minute tap water absorbency of the superabsorbent resins of the Examples and Comparative Examples was measured by the following method.

[0162] (i) First, 1 g of a superabsorbent resin was placed in a tea bag measuring 15 cm in width and 30 cm in length, and the tea bag was immersed in 2 L of tap water and allowed to swell for 1 minute. (ii) Then, the tea bag containing the swollen superabsorbent polymer was lifted out of the tap water, and after one minute had passed, the weights of the tea bag and the superabsorbent polymer were measured, and the weight of the empty tea bag was subtracted from the weight, which was then used as the one-minute tap water absorption capacity.

[0163] In this case, the tap water used had an electrical conductivity of 170 to 180 μS / cm when measured using Orion Star A222 (manufacturer: Thermo Scientific).

[0164] (5) Effective Water Absorption Capacity (EFFC) Based on the centrifuge retention capacity (CRC) and 0.7 psi pressure absorption capacity (AUP) measured above, the effective water absorption capacity (EFFC) was calculated according to Equation 1 above.

[0165] [Table 1]

[0166] Referring to Table 1 above, it can be seen that the superabsorbent resins of the Examples prepared using the hydrophobic particle aqueous dispersion in the polymerization step exhibit a faster water absorption rate and improved 1-minute tap water absorption capacity without a decrease in effective water absorption capacity compared to the superabsorbent resins of the Comparative Examples, which used the same amount of encapsulated blowing agent but did not use the hydrophobic particle aqueous dispersion or only a cell stabilizer. In particular, it can be seen that the superabsorbent resin of Example 2 not only exhibits a significantly improved water absorption rate and 1-minute tap water absorption capacity, but also has improved effective water absorption capacity compared to the superabsorbent resin of Comparative Example 6, which used the same amount of encapsulated blowing agent but hydrophobic particles in powder form.

[0167] In addition, it has been confirmed that the water absorption rate of the superabsorbent resin can be further improved when a bubble stabilizer is used together with the encapsulated blowing agent and the aqueous dispersion of hydrophobic particles during the polymerization step.

[0168] Therefore, when crosslinking and polymerizing a monomer in the presence of a hydrophobic particle aqueous dispersion, it can be seen that it is possible to effectively collect the gas generated by the foaming agent and produce a superabsorbent resin with a significantly increased water absorption rate.

Claims

1. Preparing a monomer composition comprising an acrylic acid-based monomer having an acidic group and at least a part of the acidic group neutralized and an internal crosslinking agent (Step 1); In the presence of a hydrophobic particle aqueous dispersion and an encapsulated foaming agent, crosslinking and polymerizing the monomer composition to produce a water-containing gel polymer (Step 2); Drying and pulverizing the water-containing gel polymer to form a base resin in powder form (Step 3); and In the presence of a surface crosslinking agent, additionally crosslinking the surface of the base resin to form a surface crosslinked layer (Step 4), The hydrophobic particle aqueous dispersion is a colloidal solution in which hydrophobic particles are dispersed by a surfactant, The hydrophobic particles contain a metal salt of a fatty acid having 7 to 24 carbon atoms, The encapsulated foaming agent has a structure including a core containing a hydrocarbon and a shell formed from a thermoplastic resin surrounding the core, The encapsulated foaming agent and the hydrophobic particles are used in a weight ratio of 1:0.1 to 1:3.5, A method for producing a superabsorbent resin, The superabsorbent resin satisfies the following 1) to 3): A method for producing a superabsorbent resin: 1) The water absorption rate (vortex time) at 24.0 °C is 35 seconds or less; 2) When 1 g of the superabsorbent resin is immersed in 2 L of tap water and swollen for 1 minute, the 1-minute tap water absorption capacity defined as the weight of water absorbed by the superabsorbent resin in 1 minute is 115 g or more; and 3) The effective water absorption capacity (EFFC) calculated by the following formula 1 is 22 g / g or more, [Formula 1] Effective water absorption capacity (EFFC) = {Water retention capacity (CRC) + 0.7 psi Pressurized water absorption capacity (AUP)} / 2 In the above formula 1, The water retention capacity (CRC) means the centrifugal water retention capacity (CRC) of the superabsorbent resin measured by the method of EDANA method WSP241.3, The 0.7 psi pressurized water absorption capacity (AUP) means the 0.7 psi pressurized water absorption capacity (AUP) of the superabsorbent resin measured by the method of EDANA method WSP242.

3.

2. The hydrophobic particles are one or more metal salts of stearic acid selected from the group consisting of calcium stearate, magnesium stearate, sodium stearate, zinc stearate, and potassium stearate. The method for producing a superabsorbent resin according to Claim 1.

3. The hydrophobic particles have an average particle size of 0.5 μm to 20 μm. The method for producing a superabsorbent resin according to Claim 1 or 2.

4. The hydrophobic particles are used in an amount of 0.005 to 0.4 parts by weight based on 100 parts by weight of the acrylic acid-based monomer, and the method for producing a superabsorbent resin according to any one of claims 1 to 3.

5. The surfactant includes a nonionic surfactant and an anionic surfactant, and the method for producing a superabsorbent resin according to any one of claims 1 to 4.

6. The encapsulated foaming agent has an average diameter before expansion of 5 to 30 μm and a maximum expansion ratio in air of 5 to 15 times, and the method for producing a superabsorbent resin according to any one of claims 1 to 5.

7. The encapsulated foaming agent has a maximum expansion size in air of 20 to 190 μm, and the method for producing a superabsorbent resin according to any one of claims 1 to 6.

8. The hydrocarbon is one or more selected from the group consisting of n - propane, n - butane, iso - butane, cyclobutane, n - pentane, iso - pentane, cyclopentane, n - hexane, iso - hexane, cyclohexane, n - heptane, iso - heptane, cycloheptane, n - octane, iso - octane, and cyclooctane, and the method for producing a superabsorbent resin according to any one of claims 1 to 7.

9. The thermoplastic resin is a polymer formed from one or more monomers selected from the group consisting of (meth)acrylate - based compounds, (meth)acrylonitrile - based compounds, aromatic vinyl compounds, vinyl acetate compounds, and vinyl halide compounds, and the method for producing a superabsorbent resin according to any one of claims 1 to 8.

10. In the step 2, one or more foam stabilizers selected from the group consisting of an alkyl sulfate - based compound and a polyoxyethylene alkyl ether - based compound are further added together with the encapsulated foaming agent, and the method for producing a superabsorbent resin according to any one of claims 1 to 9.

11. The encapsulated foaming agent and the foam stabilizer are used in a weight ratio of 1:0.01 to 1:0.5, and the method for producing a superabsorbent resin according to claim 10.

12. The surface cross - linking agent uses the same one as the internal cross - linking agent, and the method for producing a superabsorbent resin according to any one of claims 1 to 11.

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