Process for preparing superabsorbent polymers

Abstract

The present invention relates to a method of preparing a superabsorbent polymer. More particularly, the preparation method performs a polymerization reaction of a monomer in the presence of an aqueous dispersion of hydrophobic particles, thus enabling the preparation of a superabsorbent polymer having a high surface tension and an improved absorption rate without deterioration in absorption properties.

Description

TECHNICAL FIELD

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2020-0178430, filed December 18, 2020, and Korean Patent Application No. 10-2021-0180294, filed December 16, 2021, the disclosures of which are incorporated herein in their entireties by reference.

[0003] The present invention relates to a method of preparing a superabsorbent polymer. More particularly, the present invention relates to a method of preparing a superabsorbent polymer, which is capable of preparing a superabsorbent polymer having a high surface tension and an improved absorption rate without deteriorating the absorption properties, since the produced carbon dioxide gas bubbles can be effectively trapped by performing a polymerization reaction of monomers in the presence of an aqueous dispersion of hydrophobic particles. BACKGROUND

[0004] A superabsorbent polymer (SAP) is a synthetic polymer material that is capable of absorbing 500 to 1000 times its own weight of water. Various manufacturers have given it different names such as SAM (superabsorbent material), AGM (absorbent gel material), etc. Such superabsorbent polymers have been practically used in hygiene products at first, and now are widely used not only in hygiene products but also in gardening water-retaining soil products, civil engineering and construction water-stopping materials, seedling sheets, food distribution field preservatives, dressing materials, etc.

[0005] These superabsorbent polymers have been widely used in the field of hygiene materials such as paper diapers or sanitary napkins, etc. In such hygiene materials, superabsorbent polymers are generally contained in a state of being dispersed in paper pulp. However, in recent years, efforts have been made to provide hygiene materials such as paper diapers with a thinner thickness. As part of these efforts, development of so-called paper pulp-free diapers in which the content of paper pulp is reduced or paper pulp is not used at all, etc. is actively being pursued.

[0006] As described above, in the case of hygiene materials in which the content of paper pulp is reduced or paper pulp is not used, the proportion of superabsorbent polymers contained is relatively high, and the superabsorbent polymer particles are inevitably contained in multiple layers in the hygiene materials. In order for the superabsorbent polymer particles contained in the multiple layers to absorb a large amount of liquid such as urine more effectively as a whole, the superabsorbent polymer must exhibit high absorption performance as well as a fast absorption rate.

[0007] To prepare a superabsorbent polymer having an improved absorption rate, a method of increasing the specific surface area by forming pores using a blowing agent in a polymerization step is mainly employed. In particular, from the viewpoint of price and accessibility, a carbonate blowing agent is generally used, and when a polymerization step is performed in the presence of such a carbonate blowing agent, carbon dioxide bubbles are generated and the specific surface area in the crosslinked polymer increases. In addition, a bubble stabilizer is used to minimize the escape of the generated carbon dioxide bubbles from the crosslinked polymer network. However, the use of the bubble stabilizer causes a problem of deterioration of the general properties of the superabsorbent polymer.

[0008] Accordingly, there is a continuous demand for the development of a superabsorbent polymer having a fast absorption rate while maintaining a centrifuge retention capacity (CRC), which is a property indicating the basic absorbency and water retention capacity of the superabsorbent polymer, and an absorbent under pressure (AUP), which is a property capable of well retaining absorbed liquid even under external pressure. SUMMARY

[0009] TECHNICAL PROBLEM

[0010] Accordingly, the present application relates to a method of preparing a superabsorbent polymer, which is capable of preparing a superabsorbent polymer having a high surface tension and an improved absorption rate by effectively capturing bubbles generated by a blowing agent after adding an aqueous dispersion of hydrophobic particles in a monomer polymerization step.

[0011] TECHNICAL SOLUTION

[0012] To solve the above problem, a method of preparing a superabsorbent polymer is provided, the method including the steps of:

[0013] preparing a monomer composition comprising an acrylic monomer having at least partially neutralized acidic groups and an internal crosslinking agent (step 1);

[0014] preparing a hydrogel polymer by crosslinking polymerization of the monomer composition in the presence of an aqueous dispersion of hydrophobic particles and a carbonate blowing agent (step 2);

[0015] forming a powdered base resin by drying and pulverizing the hydrogel polymer (step 3); and

[0016] forming a surface crosslinked layer by further surface-crosslinking the surface of the base resin in the presence of a surface crosslinking agent (step 4),

[0017] wherein the aqueous dispersion of hydrophobic particles is a colloidal solution in which hydrophobic particles are dispersed via a surfactant, and

[0018] The hydrophobic particles contain a metal salt of a C7 to C24 fatty acid and have an average particle diameter of 1 to 100 μm.

[0019] Advantages

[0020] The method of producing a superabsorbent polymer according to the present application, when a hydrogel polymer is produced by cross-linking polymerization of a monomer in the presence of an aqueous dispersion of hydrophobic particles and a carbonate-based foaming agent, carbon dioxide generated can be effectively captured and the specific surface area of the superabsorbent polymer can be increased. Thus, the absorption rate of the produced superabsorbent polymer can be improved. In addition, since the use of the aqueous dispersion of hydrophobic particles makes it unnecessary to add a separate foam stabilizer, a decrease in the surface tension of the superabsorbent polymer caused by an ionic surfactant as a foam stabilizer does not occur, and thus the produced superabsorbent polymer can exhibit a high surface tension. DETAILED DESCRIPTION

[0021] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used herein, singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" "comprising," "includes" or "including" when used herein, specify the presence of stated features, steps, components or combinations thereof, but do not preclude the presence or addition of one or more other features, steps, components or combinations thereof.

[0022] Because the present application can be modified in various ways and has various forms, specific embodiments thereof will be shown and described in detail by way of examples. However, it is not intended to limit the present application to the specific forms disclosed, and it should be understood that the present application includes all modifications, equivalents, and substitutions within the idea and technological scope of the present application.

[0023] In addition, the terms used herein are merely for the purpose of describing particular embodiments, and are not intended to limit the present application. Singular expressions of the present application can include plural expressions, unless they are differently expressed in context.

[0024] The term "polymer" in the present application is in a state in which a water-soluble ethylenically unsaturated monomer is polymerized, and can include all ranges of water content, or all ranges of particle diameter. In the polymer, a polymer having a water content of about 40 wt% or more after polymerization and before drying can be referred to as a hydrogel polymer, and a particle in which the hydrogel polymer is pulverized and dried can be referred to as a cross-linked polymer.

[0025] In addition, the term "superabsorbent polymer particle" refers to a particulate material including a cross-linked polymer in which an acrylic monomer having an at least partially neutralized acidic group is polymerized and cross-linked by an internal cross-linking agent.

[0026] Furthermore, according to the context, the term "superabsorbent polymer" is used to encompass all materials: a crosslinked polymer in which an acrylic monomer having at least partially neutralized acidic groups is polymerized, or a base resin in the form of a powder composed of superabsorbent polymer particles in which the crosslinked polymer is pulverized, and the crosslinked polymer or the base resin which is further processed (e.g., surface crosslinking, fine particle recombination, drying, pulverization, fractionation, etc.) into a state suitable for commercialization. Accordingly, the term "superabsorbent polymer" can be interpreted to include various superabsorbent polymer particles.

[0027] In order to prepare a superabsorbent polymer having a rapid absorption rate, it is necessary to increase the specific surface area of the superabsorbent polymer particles. As a method of increasing the specific surface area of the superabsorbent polymer particles, a blowing agent is generally used. In order for the bubbles generated by the blowing agent to contribute to the increase in the surface area, they must be immediately trapped inside the polymer at the time of generation so as not to escape from the crosslinked polymer. For this purpose, a blowing agent and a bubble stabilizer are generally added together. Although anionic surfactants, which are mainly used as foam stabilizers, can effectively trap bubbles, there is a problem in that the surface tension of the superabsorbent polymer is reduced, thereby causing urine leakage in sanitary products in which the superabsorbent polymer is applied.

[0028] Accordingly, the present inventors have found that, when a superabsorbent polymer is prepared using an aqueous dispersion containing hydrophobic particles as a foam stabilizer instead of a conventional foam stabilizer, the bubbles generated by the blowing agent can be effectively trapped, and small and uniformly shaped bubbles can be uniformly distributed throughout the entire region of the crosslinked polymer, thereby completing the present invention. In addition, the superabsorbent polymer prepared in this way has the advantage that the specific surface area is increased due to the uniformly distributed bubbles, thereby increasing the absorption rate and having a high surface tension.

[0029] In particular, the method of preparing a superabsorbent polymer is characterized in that the hydrophobic particles are not used in the form of a powder, but are added to the monomer composition in the form of an aqueous dispersion. In other words, the hydrophobic particles are introduced into the monomer composition in the form of an "aqueous dispersion of hydrophobic particles", i.e., in the form of a colloidal solution in which the hydrophobic particles are stably dispersed without precipitation or agglomeration by a surfactant. The reason for this is that, when a polymerization process is performed by introducing the hydrophobic particles in the form of a powder into the monomer composition, the hydrophobic particles agglomerate and are not dispersed in the form of an aqueous solution in the monomer composition, so that the bubbles generated by the blowing agent cannot be effectively stabilized.

[0030] Further, the hydrophobic particles are stably dispersed in the aqueous dispersion by the surfactant without agglomeration between the particles. Specifically, the surfactant can form a double electric layer on the surface of the hydrophobic particles to induce an electrostatic repulsive force between the particles, which can stabilize the hydrophobic particles, or the surfactant can be adsorbed on the surface of the hydrophobic particles to induce a steric repulsive force between the particles, which can prevent the particles from agglomerating with each other. Thus, when the aqueous dispersion of the hydrophobic particles is used without the surfactant in the polymerization step, a phenomenon in which the hydrophobic particles agglomerate with each other or sink due to gravity can occur, and thus the dispersion of the hydrophobic particles cannot be stabilized. Accordingly, even when the aqueous dispersion of the hydrophobic particles without the surfactant is used together with the blowing agent in the polymerization step, the gas bubbles cannot be effectively trapped, and thus it is difficult to form pores having a uniform size in the superabsorbent polymer, and thus it is difficult to improve the absorption rate of the superabsorbent polymer.

[0031] In addition, the hydrophobic particles dispersed in the aqueous dispersion are characterized by having an average particle diameter of 1 μm to 100 μm. When the average particle diameter of the hydrophobic particles is less than 1 μm, it is difficult to effectively trap the generated gas bubbles, and thus it is difficult to form uniform pores. When the average particle diameter of the hydrophobic particles is greater than 100 μm, the viscosity of the monomer composition can excessively increase, which can cause a problem in polymerization stability, and it can be difficult to improve the absorption rate of the superabsorbent polymer due to a large pore size.

[0032] Hereinafter, each step of the method of preparing a superabsorbent polymer according to the specific embodiment of the present application will be described in more detail.

[0033] (Step 1)

[0034] In the preparation method according to one embodiment, Step 1 is a step of preparing a monomer composition including an acrylic monomer having an at least partially neutralized acidic group and an internal crosslinking agent.

[0035] The acrylic monomer is a compound represented by the following Chemical Formula 1:

[0036] [Chemical Formula 1]

[0037] R 1 -COOM 1

[0038] In Chemical Formula 1,

[0039] R 1 is a C2 to C5 hydrocarbon group having an unsaturated bond, and

[0040] M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0041] Preferably, the acrylic monomer can include at least one selected from acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts thereof.

[0042] Here, the acrylic monomer can be those having an acidic group that is at least partially neutralized. Preferably, an acrylic monomer that is partially neutralized with a basic substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, or the like can be used. The degree of neutralization of the acrylic monomer can be 40 to 95 mol%, 40 to 80 mol%, or 45 to 75 mol%. The range of the degree of neutralization can be adjusted depending on the final properties. Too high a degree of neutralization can cause the neutralized monomer to precipitate, and thus polymerization can not easily occur. In contrast, too low a degree of neutralization can not only reduce the absorbency of the polymer, but also impart properties to the polymer that are difficult to handle, such as those of an elastic rubber.

[0043] Further, the concentration of the acrylic monomer can be about 20 to 60% by weight, or about 40 to 50% by weight, based on the monomer composition including the superabsorbent polymer-containing raw material and the solvent, and is appropriately controlled in consideration of the polymerization time and the reaction conditions. When the concentration of the monomer is too low, the yield of the superabsorbent polymer is low and there can be a problem in terms of economic efficiency. In contrast, when the concentration is too high, some problems can occur in the process due to the fact that part of the monomer can be extracted out or the pulverization efficiency of the polymerized hydrogel polymer in the pulverization process can be reduced, and thus the physical properties of the superabsorbent polymer can be deteriorated.

[0044] Further, the term "internal crosslinking agent" used herein is different from the surface crosslinking agent that will be described below for crosslinking the surface of the superabsorbent polymer particles, and the internal crosslinking agent polymerizes by crosslinking the unsaturated bond of the water-soluble ethylenically unsaturated monomer. The crosslinking in the above-described step is performed both on the surface and the inside, but when the surface crosslinking process of the superabsorbent polymer particles described below is performed, the surface of the finally prepared superabsorbent polymer particles has a structure crosslinked by the surface crosslinking agent, and the inside of the particles has a structure crosslinked by the internal crosslinking agent.

[0045] As the internal crosslinking agent, any compound can be used as long as it allows the introduction of a crosslinking bond in the polymerization process of the acrylic monomer. Specifically, the internal crosslinking agent can be a crosslinking agent having one or more ethylenically unsaturated groups in addition to a functional group that can react with the water-soluble substituent of the acrylic monomer; or a crosslinking agent having two or more functional groups that can react with the water-soluble substituent of the monomer and / or the water-soluble substituent formed by the hydrolysis of the monomer.

[0046] For example, as the internal crosslinking agent, the following multifunctional crosslinking agents can be used alone or in combination of two or more, for example, N,N'-methylenebisacrylamide, trimethylpropane 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, butylene glycol 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, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol, or ethylene carbonate. However, the present application is not limited thereto.

[0047] Preferably, the internal crosslinking agent can be a polyalkylene glycol (meth)acrylate compound, for example, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, or polypropylene glycol (meth)acrylate. The above internal crosslinking agent is preferably used because it is easy to achieve foaming by the carbonic acid salt-based foaming agent described below.

[0048] In the monomer composition, the 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 water-soluble ethylenically unsaturated monomer. For example, the internal crosslinking agent can be used in an amount of 0.01 parts by weight or more, 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.15 parts by weight or more, and 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 parts by weight or less, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. When the internal crosslinking agent is used in too small an amount, crosslinking does not sufficiently occur, whereby it is difficult to achieve a strength higher than an appropriate strength, and when the internal crosslinking agent is used in too large an amount, the internal crosslinking density increases, and thus it can be difficult to achieve a water retention capacity at a desired level.

[0049] Further, the monomer composition can further include a polymerization initiator for initiating the polymerization of the monomers. The polymerization initiator is not particularly limited as long as it is generally used for the preparation of superabsorbent polymers.

[0050] Specifically, depending on the polymerization method, the polymerization initiator can be an initiator for thermal polymerization or an initiator for photopolymerization by ultraviolet radiation. However, even if a photopolymerization method is applied thereto, a certain amount of heat is generated due to ultraviolet irradiation or the like, and some heat is generated as the polymerization reaction, which is an exothermic reaction, proceeds. Therefore, the composition can additionally include a thermal polymerization initiator.

[0051] More specifically, any compound that can form a radical by light, for example, ultraviolet rays, can be used as a photopolymerization initiator without limitation.

[0052] For example, the photopolymerization initiator can be one or more compounds selected from the group consisting of benzoin ethers, dialkylbenzeneacetones, hydroxyalkyl ketones, phenylglyoxylic acids, benzyl dimethyl ketals, acyl phosphines, and a-amino ketones. Also, as a specific example of the acyl phosphine, a commercially available lucirin TPO, i.e., diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, can be used. Further photopolymerization initiators are well disclosed in "UV Coatings: Basics, Recent Developments and New Application (Elsevier, 2007)" written by Reinhold Schwalm, p. 115, without being limited thereto.

[0053] Based on the monomer composition, the photopolymerization initiator can be used at a concentration of about 0.01 wt% to about 1.0 wt%. When the concentration of the photopolymerization initiator is too low, the polymerization rate can be slowed, and when the concentration is too high, the molecular weight of the superabsorbent polymer can be lowered and the performance can be non-uniform.

[0054] Also, as the thermal polymerization initiator, one or more initiators selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specifically, sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), and the like can be used as examples of the persulfate initiator, and 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis(N,N-dimethylene)isobutylamidine dihydrochloride, 2-(carbamoylazo)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 4,4-azobis(4-cyanopentanoic acid), and the like can be used as examples of the azo initiator. Further kinds of thermal polymerization initiators are well disclosed in "Principle of Polymerization (Wiley, 1981)" written by Odian, p. 203, without being limited thereto.

[0055] Based on the monomer composition, the thermal polymerization initiator can be used at a concentration of about 0.001 wt% to about 0.5 wt%. When the concentration of the thermal polymerization initiator is too low, it is difficult for additional thermal polymerization to occur and the effect of adding the thermal polymerization initiator can be small. When the concentration of the thermal polymerization initiator is too high, the molecular weight of the superabsorbent polymer can be lowered and the performance can be non-uniform.

[0056] The amount of the polymerization initiator can be 2 parts by weight or less, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. When the concentration of the polymerization initiator is too low, the polymerization rate can be slowed, and a large amount of residual monomer can be extracted from the final product. In contrast, when the concentration of the polymerization initiator is higher than the above range, the network-forming polymer chains become short, so that the content of extractable components increases and the pressurized absorption rate decreases, thereby reducing the physical properties of the polymer.

[0057] If necessary, the monomer composition can further include additives such as a thickening agent, a plasticizer, a storage stabilizer, and an antioxidant.

[0058] In addition, the monomer composition containing the monomers can be, for example, in the form of a solution dissolved in a solvent such as water. The solid content of the monomer composition in the solution state, i.e., the concentrations of the monomers, the internal crosslinking agent, and the polymerization initiator, can be appropriately adjusted in consideration of the polymerization time and reaction conditions. For example, the solid content of the monomer composition can be 10 to 80% by weight, 15 to 60% by weight, or 30 to 50% by weight.

[0059] When the monomer composition has a solid content within the above range, it can be advantageous to control the pulverization efficiency in the polymer pulverization process to be described below, while eliminating the need to remove unreacted monomers after polymerization by taking advantage of the gel effect phenomenon that occurs in the polymerization process of a high-concentration aqueous solution.

[0060] At this time, any solvent capable of dissolving the above ingredients can be used without limitation. For example, the solvent can be at least one selected from the group consisting of 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, or a combination of two or more thereof.

[0061] (Step 2)

[0062] Subsequently, a step of preparing a hydrogel polymer by crosslinking and polymerizing the polymerized monomer composition in the presence of an aqueous dispersion of hydrophobic particles and a carbonate-based foaming agent is performed. In the above step, carbon dioxide bubbles are generated from the carbonate-based foaming agent, and the hydrophobic particles dispersed in water effectively capture these bubbles, thereby increasing the specific surface area of the prepared hydrogel polymer.

[0063] The hydrophobic particles include a metal salt of a C7 to C24 fatty acid. Here, the metal salt of a C7 to C24 fatty acid refers to a compound in which a metal cation is bonded to a terminal of an unsaturated or saturated fatty acid having a linear structure and having 7 to 24 carbon atoms in the molecule in place of a hydrogen ion of a carboxyl group, and can be a monovalent metal salt, or a polyvalent metal salt of two or more. At this time, when the hydrophobic particles are a metal salt of a fatty acid having less than 7 carbon atoms, the generated bubbles cannot be captured in the form of ionized particles in an aqueous solution. When the hydrophobic particles are a metal salt of a fatty acid having more than 24 carbon atoms, the chain of the fatty acid becomes long, which can result in difficulty in dispersion.

[0064] Specifically, when the metal salt of a fatty acid is a monovalent metal salt, it has a structure in which one fatty acid carboxylate anion is bonded to an alkali metal ion as a monovalent metal cation. In addition, when the metal salt of a fatty acid is a polyvalent metal salt of two or more, it has a structure in which as many fatty acid carboxylate anions as the valence of the metal cation are bonded to the metal cation.

[0065] In one embodiment, the hydrophobic particles can be a metal salt of a C12 to C20 saturated fatty acid. For example, the hydrophobic particles can be at least one saturated fatty acid metal salt selected from the group consisting of a metal salt of lauric acid containing 12 carbon atoms in the molecule; a metal salt of tridecanoic 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 nonadecanoic acid containing 19 carbon atoms in the molecule; and a metal salt of arachidic acid containing 20 carbon atoms in the molecule.

[0066] Preferably, the hydrophobic particles can be a metal salt of stearic acid. For example, the hydrophobic particles can be at least one stearic acid metal salt selected from the group consisting of calcium stearate, magnesium stearate, sodium stearate, zinc stearate, and potassium stearate.

[0067] Further, as described above, the hydrophobic particles dispersed in the aqueous dispersion are characterized in that the average particle diameter is 1 μm to 100 μm. For example, the average particle diameter of the hydrophobic particles can be 1 μm to 20 μm. For example, the average particle diameter (μm) of the hydrophobic particles can be 1 or more and 100 or less, 50 or less, 40 or less, 30 or less, or 20 or less.

[0068] Here, the average particle diameter of the hydrophobic particles refers to D50, and "particle diameter Dn" refers to the particle diameter at the n% point of the cumulative distribution of the number of particles according to particle diameter. In other words, D50 is the particle diameter at the 50% point of the cumulative distribution of the number of particles according to particle diameter, D90 is the particle diameter at the 90% point of the cumulative distribution of the number of particles according to particle diameter, and D10 is the particle diameter at the 10% point of the cumulative distribution of the number of particles according to particle diameter. Dn can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and introduced into a commercially available particle size measuring device (e.g., Microtrac S3500). Then, when the particles pass through a laser beam, the particle size distribution is obtained by measuring the difference in diffraction pattern according to particle diameter. In this measuring device, D10, D50, and D90 can be obtained by calculating the particle diameter at which the cumulative distribution of the number of particles according to particle diameter reaches 10%, 50%, and 90%.

[0069] Further, the content of the hydrophobic particles in the aqueous dispersion can be 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 too low or too high, the dispersion stability of the hydrophobic particles cannot be achieved, and problems such as inter-particle agglomeration or sinking due to gravity can occur.

[0070] Further, as the surfactant for dispersing the hydrophobic particles in the aqueous dispersion of the hydrophobic particles, a surfactant known in the art that is capable of stabilizing the dispersion of the hydrophobic particles can be used without limitation. For example, one or more surfactants selected from the group consisting of cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants can be used as the surfactant. Preferably, two or more surfactants can be used to stabilize the dispersion of the hydrophobic particles. More specifically, in consideration of the form of the hydrophobic particles (e.g., the form of a metal salt of a saturated fatty acid), a nonionic surfactant and an anionic surfactant can be used together in order to more effectively disperse the hydrophobic particles in water. For example, a nonionic surfactant having a long-chain hydrocarbon having 10 or more carbon atoms and an anionic surfactant based on a sulfate salt can be used together.

[0071] Examples of the cationic surfactant include dialkyldimethylammonium salts and alkylbenzylmethylammonium salts, examples of the anionic surfactant include alkyl polyoxyethylene sulfate, monoalkyl sulfate, alkylbenzenesulfonate, monoalkyl phosphate, sulfates having a functional group containing a long-chain hydrocarbon or sodium salts thereof (e.g., sodium lauryl sulfate, sodium dodecyl sulfate, or sodium laureth sulfate), examples of the amphoteric surfactant include alkylsulfobetaine and alkylcarboxybetaine, and examples of the nonionic surfactant include polyoxyethylene alkyl ether (e.g., polyethylene glycol, polyoxyalkylene alkylphenyl ether, polyoxyethylene arylphenyl ether), fatty acid ester (e.g., sorbitan monopalmitate, fatty acid sorbitan ester, glycerol monostearate), alkyl monoglyceryl ether, alkanolamide, and alkyl polyglycoside. However, the present application is not limited thereto.

[0072] Further, the pH of the aqueous dispersion of the hydrophobic particles can be 7 or more. When the pH of the aqueous dispersion of the hydrophobic particles is less than 7, it is acidic, and thus it is difficult to stabilize the hydrophobic particles as metal salts of fatty acids, which is not suitable.

[0073] Meanwhile, the amount of the hydrophobic particles is 0.01 to 0.5 parts by weight based on 100 parts by weight of the acrylic monomer. When the content of the hydrophobic particles is too low, the bubble stabilizing effect is not sufficient, and thus the absorption rate can be slowed down. When the content of the hydrophobic particles is too high, the amount of the surfactant for stabilizing the hydrophobic particles in the aqueous dispersion of the hydrophobic particles increases, so that the surface tension can be reduced. For example, the amount of the hydrophobic particles can be 0.01 parts by weight or more, 0.03 parts by weight or more, 0.05 parts by weight or more, or 0.08 parts by weight or more, and 0.5 parts by weight or less, 0.4 parts by weight or less, 0.3 parts by weight or less, or 0.2 parts by weight or less, based on 100 parts by weight of the acrylic monomer.

[0074] In addition, the carbonate-based foaming agent plays a role in increasing the surface area by forming pores in the hydrogel polymer through foaming during polymerization. For example, it can be at least one selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate.

[0075] The carbonate-based foaming agent can be used in an amount of 0.005 to 1 parts by weight based on 100 parts by weight of the acrylic monomer. When the content of the foaming agent is less than 0.005 parts by weight, the effect of using the foaming agent can not be apparent. When the content of the foaming agent exceeds 1 part by weight, the number of pores in the crosslinked polymer is too large, so that the gel strength of the superabsorbent polymer to be prepared is reduced, and the density is also reduced, which can cause problems in distribution and storage. For example, the carbonate-based foaming agent can be used in an amount of 0.01 parts by weight or more or 0.05 parts by weight or more and 0.5 parts by weight or less, 0.3 parts by weight or less, or 0.2 parts by weight or less, based on 100 parts by weight of the acrylic monomer.

[0076] Further, the carbonate-based foaming agent and the hydrophobic particles can be used in a weight ratio of 1:0.1 to 1:2. When the hydrophobic particles are used in a content that is too low compared to the carbonate-based foaming agent, it is difficult to effectively capture the generated bubbles. When the hydrophobic particles are used in a content that is too high compared to the foaming agent, various physical properties such as water retention capacity and absorption rate can be reduced. Specifically, the carbonate-based foaming agent and the hydrophobic particles can be used in a weight ratio of 1:0.4 or more, 1:0.6 or more, or 1:0.8 or more and 1:1.7 or less, 1:1.5 or less, or 1:1.2 or less. For example, the carbonate-based foaming agent and the hydrophobic particles can be used in a weight ratio of 1:1.

[0077] In addition, in steps 1 and 2, a surfactant such as an alkyl sulfate compound and a polyoxyethylene alkyl ether compound, which is generally used as a bubble stabilizer, can not be used. For example, in steps 1 and 2, a cationic surfactant such as a quaternary ammonium compound, for example, dodecyltrimethylammonium chloride or dodecyltrimethylammonium bromide; an anionic surfactant such as an alkyl sulfate compound, for example, sodium dodecyl sulfate, ammonium lauryl sulfate, sodium lauryl ether sulfate, or sodium myreth sulfate; or a nonionic surfactant such as an alkyl ether sulfate compound, for example, polyoxyethylene lauryl ether, can not be used. Thus, a problem of a reduction in the surface tension of the superabsorbent polymer caused by the use of a surfactant can be avoided.

[0078] Meanwhile, the polymerization of the monomer composition in the presence of the aqueous dispersion of such hydrophobic particles and carbonate-based foaming agent is not particularly limited as long as it is a commonly used polymerization method.

[0079] Specifically, depending on the energy source of polymerization, the polymerization method is mainly divided into thermal polymerization and photopolymerization. In the case of thermal polymerization, it is generally performed in a reactor equipped with a stirring shaft, such as a kneader. In the case of photopolymerization, it can be performed in a reactor equipped with a movable conveyor belt. However, the polymerization method is merely an example, and the present application is not limited thereto.

[0080] For example, in a reactor equipped with a stirring shaft such as a kneader, the hydrogel polymer obtained by the thermal polymerization by supplying hot air or heating the reactor can be discharged to the outlet of the reactor in the form of several centimeters to several millimeters depending on the shape of the stirring shaft provided in the reactor. Specifically, the size of the obtained hydrogel polymer can vary depending on the concentration and injection rate of the monomer composition to be injected, and generally a hydrogel polymer having a weight average particle diameter of 2 mm to 50 mm can be obtained.

[0081] Further, when the photopolymerization is performed in a reactor equipped with a movable conveyor belt as described above, generally a hydrogel polymer in the form of a sheet having the width of the conveyor belt can be obtained. At this time, the thickness of the polymer sheet can vary depending on the concentration and injection rate of the monomer composition to be injected, and it is preferable to supply the monomer composition so that the thickness of the polymer in the form of a sheet is about 0.5 cm to about 5 cm. When the degree of supply of the monomer composition is such that the thickness of the polymer sheet is too thin, the production efficiency can be low. When the thickness of the polymer sheet exceeds 5 cm, the polymerization reaction can not occur uniformly over the entire thickness due to the excessive thickness.

[0082] Generally, the moisture content of the hydrogel polymer obtained by the above-described method can be about 40% by weight to about 80% by weight. At this time, the "moisture content" in the present invention means the moisture content in the total weight of the polymer, and means the value after subtracting the weight of the dried polymer from the weight of the polymer. Specifically, the moisture content is defined as a value calculated by measuring the weight loss due to the evaporation of moisture from the polymer in the temperature rising process of drying the polymer by infrared heating. At this time, the drying conditions for determining the moisture content are to be heated to about 180°C and maintained at 180°C, and the total drying time is 20 minutes (including a 5-minute heating step).

[0083] (Step 3)

[0084] Subsequently, a step of drying and pulverizing the hydrogel polymer to form a powder-form base resin. If necessary, a coarse pulverization step can be further performed before drying to improve the efficiency of the drying step.

[0085] Here, the pulverizer used is not particularly limited. Specifically, it can include a vertical pulverizer, a turbo cutter, a turbo mill, a rotary cutting mill, a cutting mill, a disc mill, a pulverizing crusher, a crusher, a chopper, and a disc cutter, but is not limited thereto.

[0086] In the pulverization step, the polymer can be pulverized to a diameter of about 2 mm to about 10 mm. The hydrogel polymer is technically difficult to be pulverized to a diameter of less than 2 mm due to a high moisture content, and the pulverized particles can be adhered to each other. Meanwhile, when the polymer is pulverized to a diameter of more than 10 mm, the efficiency improvement effect in the subsequent drying step can not be significant.

[0087] The drying is performed on the polymer pulverized as described above, or immediately after polymerization without the pulverization step. The drying temperature in the drying step can be about 150°C to about 250°C. When the drying temperature is lower than 150°C, the drying time can become excessively long and the physical properties of the superabsorbent polymer finally formed can be reduced. When the drying temperature exceeds 250°C, only the surface of the polymer is excessively dried, fine powder can be generated in the subsequent pulverization process, and the physical properties of the final superabsorbent polymer can be reduced. Therefore, the drying can be preferably performed at a temperature of about 150°C to about 200°C, and more preferably at a temperature of about 160°C to about 180°C.

[0088] Meanwhile, the drying time can be about 20 minutes to about 90 minutes in consideration of the process efficiency, but is not limited thereto.

[0089] The drying method in the drying step can be any drying method commonly used in the drying process of the hydrogel polymer, and is not particularly limited. Specifically, the drying step can be performed by a method of providing hot air, infrared radiation, microwave radiation, ultraviolet radiation, etc. After the drying step, the moisture content of the polymer can be about 0.1 wt% to about 5 wt%.

[0090] Subsequently, a step of pulverizing the dried polymer obtained through the drying step is performed.

[0091] The base resin of the polymer powder obtained after the pulverization step can have a particle diameter of about 150 μm to about 850 μm. As a pulverizer for pulverization to the particle diameter, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a hand mill, etc. can be used, but the present application is not limited thereto.

[0092] In order to manage the physical properties of the superabsorbent polymer powder to be commercialized after the pulverization step, the base resin obtained after the pulverization is classified according to the particle diameter. Preferably, the polymer having a particle diameter of about 150 μm to about 850 μm is classified, and only the base resin having the particle diameter can be subjected to the surface cross-linking reaction step. In this case, the particle diameter can be measured according to EDANA (European Disposable and Nonwoven Association) WSP 220.3.

[0093] (Step 4)

[0094] Subsequently, a step of forming a surface crosslinked layer by further crosslinking the surface of the base resin in the presence of a surface crosslinking agent is performed. Through the above steps, a superabsorbent polymer is provided in which a surface crosslinked layer is formed on the surface of the base resin (more specifically, on at least a portion of the surface of each superabsorbent polymer particle).

[0095] Surface crosslinking is a step of increasing the crosslinking density in the vicinity of the surface of the superabsorbent polymer particle relative to the crosslinking density inside the particle. Typically, a surface crosslinking agent is applied on the surface of the superabsorbent polymer particle. Thus, a surface crosslinking reaction occurs on the surface of the superabsorbent polymer particle, which improves the crosslinking of the surface of the particle without substantially affecting the inside of the particle. Thus, the surface-crosslinked superabsorbent polymer particle has a higher crosslinking degree on the surface than inside.

[0096] As the surface crosslinking agent, any surface crosslinking agent conventionally used for preparing a superabsorbent polymer can be used without any particular limitation. Examples of the surface crosslinking agent can include at least one polyol 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; at least one carbonate compound selected from the group consisting of ethylene carbonate, propylene carbonate, and glycerol carbonate; an epoxy compound such as ethylene glycol diglycidyl ether; an oxazoline compound such as oxazolinone; a polyamine compound; an oxazoline compound; a mono-, di-, or poly-oxazolinone compound; a cyclic urea compound; and the like. Specifically, one or more, two or more, or three or more of the above-mentioned surface crosslinking agents can be used as the surface crosslinking agent. For example, ethylene carbonate can be used as the surface crosslinking agent.

[0097] In addition, the method of mixing the surface crosslinking agent with the base resin powder is not particularly limited. For example, a method of adding the surface crosslinking agent and the base resin powder into a reactor to mix them, a method of spraying the surface crosslinking agent onto the base resin powder, or a method of mixing the base resin powder and the surface crosslinking agent while continuously supplying them to a continuously operated mixer can be used.

[0098] When the surface crosslinking agent is added, water can be mixed together and added as a surface crosslinking solution. When water is added thereto, the advantage is that the surface crosslinking agent can be uniformly dispersed in the polymer. At this time, in order to uniformly disperse the surface crosslinking agent, prevent the agglomeration phenomenon of the polymer powder, and optimize the surface penetration depth of the surface crosslinking agent, the amount of water added can be appropriately controlled. For example, based on 100 parts by weight of the base resin, it can be preferable to add water in an amount of about 1 to about 10 parts by weight.

[0099] The surface crosslinking reaction is performed by heating the base resin to which the surface crosslinking solution comprising a surface crosslinking agent and a solvent is added at a temperature of about 100°C to about 150°C, preferably about 110°C to about 140°C for about 15 to about 80 minutes, preferably about 20 to about 70 minutes.

[0100] The heating means for the surface crosslinking reaction is not particularly limited. A heat medium can be provided thereto or a heat source can be directly provided thereto. At this time, the heat medium that can be used can be a heated fluid such as steam, hot air, hot oil, etc., but the present application is not limited thereto. In addition, the temperature of the heat medium provided thereto can be appropriately selected in consideration of the manner of the heat medium, the heating speed, and the target temperature of the heating. Meanwhile, an electric heater or a gas heater can be used as the heat source directly provided, but the present application is not limited thereto.

[0101] After forming the surface crosslinking layer on the surface of the base resin as described above, the inorganic material can be further mixed.

[0102] The inorganic material can be, for example, at least one selected from the group consisting of silica, clay, alumina, silica-alumina composite, and titanium dioxide, and is preferably silica.

[0103] The inorganic material can be used in an amount of 0.01 parts by weight or more, 0.05 parts by weight or more, or 0.1 parts by weight or more, and 5 parts by weight or less, 3 parts by weight or less, or 1 part by weight or less, based on 100 parts by weight of the superabsorbent polymer.

[0104] Meanwhile, in order to control the properties of the superabsorbent polymer finally commercialized, a step of classifying the superabsorbent polymer obtained after the surface crosslinking step according to the particle diameter can be further performed. Preferably, the polymer having a particle diameter of about 150 μm to about 850 μm is classified, and then only the superabsorbent polymer having the particle diameter can be used as the final product.

[0105] The superabsorbent polymer obtained by the above preparation method can satisfy the following physical properties by achieving a balance between the absorption rate (spiral time) and the absorption properties.

[0106] 1) a spiral time (absorption rate) of 55 seconds or less at 24.0°C;

[0107] 2) a surface tension of 65 mN / m or more;

[0108] 3) a centrifuge retention capacity (CRC) of 25 g / g to 30 g / g measured according to EDANA WSP 241.3, and

[0109] 4) a pressure absorption rate (AUP) of 20 g / g to 26 g / g at 0.7 psi measured according to EDANA WSP 242.3.

[0110] More specifically, the superabsorbent polymer prepared by the above method can have a vortex time (absorption rate) of 50 seconds or less, 48 seconds or less, 46 seconds or less, or 45 seconds or less at 24°C by the vortex method. Further, since the lower the vortex time is the better, the theoretical lower limit is 0 seconds, but it can also be 10 seconds or more, 20 seconds or more, 30 seconds or more, or 35 seconds or more. The measurement method of the vortex time of the superabsorbent polymer will be described in more detail in the Experimental Examples below.

[0111] Further, the surface tension of the superabsorbent polymer can be 65 mN / m or more, 66 mN / m or more, or 68 mN / m or more, and 72 mN / m or less. The method of measuring the surface tension of the superabsorbent polymer will be described in more detail in the Experimental Examples below.

[0112] Further, the centrifuge retention capacity (CRC) of the superabsorbent polymer can be 25 g / g or more, 26 g / g or more, or 27 g / g or more, and 30 g / g or less or 29.5 g / g or less, measured according to EDANA WSP 241.3.

[0113] Further, the absorbency under pressure (AUP) of the superabsorbent polymer at 0.7 psi can be 20 g / g or more, 22 g / g or more, or 23 g / g or more, and 26 g / g or less or 25.5 g / g or less, measured according to EDANA WSP 242.3.

[0114] The present application will be described in more detail in the following examples. However, these examples are for illustrative purposes only, and the scope of the present application is not limited by the following examples.

[0115] <Preparation Example>

[0116] The aqueous dispersion of the hydrophobic particles used in the following examples was prepared as follows.

[0117] Preparation Example 1: Preparation of calcium stearate aqueous dispersion Ca-st(5)

[0118] First, 50 g of water containing two or more surfactants including a sulfate type anionic surfactant and a fatty acid ester type nonionic surfactant was added to a high shear mixer, heated to 165°C, and then 50 g of calcium stearate powder was added. Then, the calcium stearate was sufficiently pulverized by stirring at 4000 rpm for 30 minutes under normal pressure to obtain an aqueous dispersion Ca-st(5) in which 50% by weight of calcium stearate having an average particle diameter of 5 μm was dispersed. At this time, the pH of the aqueous dispersion was 9.5. Further, after the preparation of Ca-st(5), the average particle diameter (D50) thereof was measured / calculated as the particle diameter at 50% of the cumulative number distribution of particles using a laser diffraction particle diameter measuring device (Microtrac S3500).

[0119] Preparation Example 2: Preparation of calcium stearate aqueous dispersion Ca-st(20)

[0120] An aqueous dispersion Ca-st(20) in which 50% by weight of calcium stearate having an average particle diameter of 20 μm was dispersed was obtained in the same manner as in Preparation Example 1, except that the stirring conditions were changed. At this time, the pH of the aqueous dispersion was 9.5, and the average particle diameter of Ca-st(20) was measured / calculated in the same manner as in Preparation Example 1.

[0121] Preparation Example 3: Preparation of magnesium stearate aqueous dispersion Mg-st(1)

[0122] First, 50 g of water containing two or more surfactants including a sulfate type anionic surfactant and a fatty acid ester type nonionic surfactant was added to a high shear mixer, heated to 150°C, and then 10 g of magnesium stearate powder was added. Then, the magnesium stearate was sufficiently pulverized by stirring at 4000 rpm for 30 minutes under normal pressure to obtain an aqueous dispersion Mg-st(1) in which 17% by weight of magnesium stearate having an average particle diameter of 1 μm was dispersed. At this time, the pH of the aqueous dispersion was 9, and the average particle diameter of Mg-st(1) was measured / calculated in the same manner as in Preparation Example 1.

[0123] Preparation Example 4: Preparation of magnesium stearate aqueous dispersion Mg-st(5)

[0124] An aqueous dispersion Mg-st(5) in which 17% by weight of magnesium stearate having an average particle diameter of 5 μm was dispersed was obtained in the same manner as in Preparation Example 3, except that the stirring conditions were changed. At this time, the pH of the aqueous dispersion was 9, and the average particle diameter of Mg-st(5) was measured / calculated in the same manner as in Preparation Example 1.

[0125] Preparation Example 5: Preparation of sodium stearate aqueous dispersion Na-st(5)

[0126] First, 50 g of water containing two or more surfactants including a sulfate type anionic surfactant and a fatty acid ester type nonionic surfactant was added to a high shear mixer, and 50 g of sodium stearate powder was added thereto at room temperature. Then, it was stirred at 2000 rpm for 5 minutes under normal pressure to sufficiently pulverize the sodium stearate, and an aqueous dispersion Na-st(5) in which 50% by weight of sodium stearate having an average particle diameter of 5 μm was dispersed was obtained. At this time, the pH of the aqueous dispersion was 9, and the average particle diameter of Na-st(5) was measured / calculated in the same manner as in Preparation Example 1.

[0127] Preparation Example 6: Preparation of an aqueous potassium stearate dispersion K-st(5)

[0128] First, 50 g of water containing two or more surfactants including a sulfate type anionic surfactant and a fatty acid ester type nonionic surfactant was added to a high shear mixer, and 50 g of sodium stearate powder was added thereto at room temperature. Then, it was stirred at 2000 rpm for 5 minutes under normal pressure to sufficiently pulverize the sodium stearate, and an aqueous dispersion Na-st(5) in which 50% by weight of sodium stearate having an average particle diameter of 5 μm was dispersed was obtained. At this time, the pH of the aqueous dispersion was 9, and the average particle diameter of Na-st(5) was measured / calculated in the same manner as in Preparation Example 1.

[0129] <Example>

[0130] Example 1

[0131] (Step 1) In a 3L glass container equipped with a stirrer and a thermometer, 450 g of acrylic acid, 3 g of PEGDA 400 (polyethylene glycol diacrylate 400) as an internal crosslinking agent, and 0.04 g of diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide as a photoinitiator were added and dissolved. Then, 580 g of a 31.5% sodium hydroxide solution was added to prepare a monomer composition (degree of neutralization: 70 mol%; solid content: 41% by weight).

[0132] (Step 2) The aqueous calcium stearate dispersion Ca-st(5) was added to the monomer composition such that 0.1 g of calcium stearate was added based on 100 g of acrylic acid, and 0.1 g of sodium bicarbonate (SBC) as a carbonate-based blowing agent was added based on 100 g of acrylic acid. 1000 g of the prepared aqueous solution was put into a stainless steel container having a width of 250 mm, a length of 250 mm, and a height of 30 mm, and ultraviolet polymerization was performed by irradiating ultraviolet rays for 90 seconds (irradiation amount: 10 mW / cm 2 ) to thereby obtain a hydrogel polymer. After the obtained hydrogel polymer was coarsely pulverized to a size of 2 mm x 2 mm, the moisture content was measured to be 40.1%.

[0133] (Step 3) Subsequently, the sheet-type hydrogel polymer was cut to a size of about 5 cm x 5 cm, and then pulverized in a meat grinder to obtain hydrogel particle chips having a size of 1 mm to 10 mm. Thereafter, the chips were dried in an oven capable of changing the up-and-down wind direction. Thereafter, hot air of 180°C or more was blown from the bottom to the top for 15 minutes, and then from the top to the bottom for 15 minutes, to uniformly dry the same, and the moisture content of the dried chips was set to 1% or less. After drying, pulverization was performed using a pulverizer, and then fractionation was performed to produce base resin having a diameter of 150 μm to 850 μm.

[0134] (Step 4) 6 parts by weight of a surface crosslinking aqueous solution containing 3 parts by weight of ethylene carbonate was sprayed to 100 parts by weight of the base resin powder prepared, and stirring was performed at room temperature to uniformly distribute the surface crosslinking solution on the base resin powder. Then, the base resin powder mixed with the surface crosslinking solution was placed in a surface crosslinking reactor, and surface crosslinking reaction was performed.

[0135] In the surface crosslinking reactor, it was confirmed that the base resin powder was gradually heated from an initial temperature of about 80°C, and reached a maximum reaction temperature of 190°C after 30 minutes. After reaching the maximum reaction temperature, the reaction was continued for 15 minutes, and a finally prepared superabsorbent polymer sample was taken out. After the surface crosslinking process, the superabsorbent polymer of Example 1 having a particle size of 150 μm to 850 μm was prepared by fractionation using an ASTM standard screen.

[0136] Example 2

[0137] The superabsorbent polymer was prepared in the same manner as in Example 1, except that a magnesium stearate aqueous dispersion Mg-st(5) was added instead of the calcium stearate aqueous dispersion Ca-st(5) of Example 1 as the aqueous dispersion of hydrophobic particles, so that 0.1 g of magnesium stearate was added based on 100 g of acrylic acid.

[0138] Example 3

[0139] The superabsorbent polymer was prepared in the same manner as in Example 1, except that a sodium stearate aqueous dispersion Na-st(5) was added instead of the calcium stearate aqueous dispersion Ca-st(5) of Example 1 as the aqueous dispersion of hydrophobic particles, so that 0.1 g of sodium stearate was added based on 100 g of acrylic acid.

[0140] Example 4

[0141] The superabsorbent polymer was prepared in the same manner as in Example 1, except that a potassium stearate aqueous dispersion K-st(5) was added instead of the calcium stearate aqueous dispersion Ca-st(5) of Example 1 as the aqueous dispersion of hydrophobic particles, so that 0.1 g of potassium stearate was added based on 100 g of acrylic acid.

[0142] Example 5

[0143] The superabsorbent polymer was prepared in the same manner as in Example 1, except that a calcium stearate aqueous dispersion Ca-st(20) was added instead of the calcium stearate aqueous dispersion Ca-st(5) in Example 1 as the aqueous dispersion of hydrophobic particles.

[0144] Example 6

[0145] The superabsorbent polymer was prepared in the same manner as in Example 1, except that a magnesium stearate aqueous dispersion Mg-st(1) was added instead of the calcium stearate aqueous dispersion Ca-st(5) in Example 1 as the aqueous dispersion of hydrophobic particles, so that 0.1 g of magnesium stearate was added based on 100 g of acrylic acid.

[0146] Comparative Example 1

[0147] The superabsorbent polymer was prepared in the same manner as in Example 1, except that the aqueous dispersion of hydrophobic particles in Example 1 was not used.

[0148] Comparative Example 2

[0149] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.02 g of a 25 wt% sodium dodecyl sulfate solution (SDS, manufactured by Sigma Aldrich) was added instead of the calcium stearate aqueous dispersion Ca-st(5) in Example 1.

[0150] Comparative Example 3

[0151] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.02 g of dodecyltrimethylammonium chloride (DTAC) was added instead of the calcium stearate aqueous dispersion Ca-st(5) in Example 1.

[0152] Comparative Example 4

[0153] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.2 g of SBC and 0.05 g of a 25 wt% sodium dodecyl sulfate solution (SDS) were added instead of the calcium stearate aqueous dispersion Ca-st(5) in Example 1.

[0154] Comparative Example 5

[0155] The superabsorbent polymer was prepared in the same manner as in Example 1, except that the aqueous dispersion of hydrophobic particles and sodium bicarbonate (SBC) in Example 1 were not used.

[0156] Comparative Example 6

[0157] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.1 g of calcium stearate in powder form having an average particle diameter of 5 μm was used instead of Ca-st(5) in the form of an aqueous dispersion of hydrophobic particles in Example 1, based on 100 g of acrylic acid. However, it was confirmed that the calcium stearate in powder form could not be dispersed in the monomer composition and agglomerated in the neutralization solution.

[0158] Experimental Examples

[0159] The physical properties of the superabsorbent polymers prepared in the examples and comparative examples were evaluated in the following manner, and are shown in Table 1 below. Unless otherwise specified, all procedures were performed in a constant temperature and humidity chamber (23 ± 0.5°C, relative humidity 45 ± 0.5%). To prevent measurement errors, the average of three measurements was taken as the measurement data. In addition, the physiological saline or saline used for evaluating the following physical properties refers to a 0.9 wt% sodium chloride (NaCl) aqueous solution.

[0160] (1) Centrifuge Retention Capacity (CRC)

[0161] The centrifuge retention capacity was measured according to EDANA WSP 241.3 by measuring the absorbency ratio of each polymer under non-loaded conditions.

[0162] Specifically, after inserting W0 (g, about 0.2 g) of the superabsorbent polymer uniformly into a nonwoven fabric envelope and sealing it, it was immersed in saline (0.9 wt%) at room temperature. After 30 minutes, the envelope was centrifuged at 250 G for 3 minutes to drain, and the envelope weight W2 (g) was measured. In addition, the same operation was performed without using the resin, and the weight of the envelope W1 (g) was measured thereafter. Then, the CRC (g / g) was calculated using the obtained weight values according to the following Equation 1.

[0163] [Equation 1]

[0164] CRC (g / g) = {[W2 (g) - W1 (g)] / W0 (g)} - 1

[0165] (2) Absorption Under Pressure (AUP)

[0166] The absorbency of each polymer under a pressure of 0.7 psi was measured according to EDANA WSP 242.3.

[0167] Specifically, a 400-mesh stainless steel screen was installed in a plastic cylindrical bottom having an inner diameter of 60 mm. At room temperature and 50% humidity, W0(g, 0.90 g) of superabsorbent polymer was uniformly scattered on the screen. Thereafter, a piston capable of uniformly providing a load of 0.7 psi was placed thereon. Here, the outer diameter of the piston was slightly smaller than 60 mm, and there was no gap with the cylindrical inner wall, and the displacement of the cylinder was not interrupted. At this time, the weight W3(g) of the measuring device was measured.

[0168] Subsequently, a glass filter having a diameter of 90 mm and a thickness of 5 mm was placed in a petri dish having a diameter of 150 mm, and physiological saline (0.9 wt% sodium chloride) was poured into the petri dish. At this time, the physiological saline was poured until the liquid level of the physiological saline was level with the upper surface of the glass filter. A filter paper having a diameter of 90 mm was placed thereon. After the measuring device was placed on the filter paper, the liquid was absorbed under a load for 1 hour. After 1 hour, the measuring device was lifted, and the weight W4(g) was measured.

[0169] Then, the pressurized absorption rate (g / g) was calculated using the obtained weight values according to the following equation.

[0170] [Equation 2]

[0171] AUP (g / g) = [W4(g) - W3(g)] / W0(g)

[0172] (3) Vortex time (absorption rate)

[0173] The vortex time (absorption rate) of the superabsorbent polymer of the examples and comparative examples was measured in the following manner.

[0174] ① First, 50 mL of 0.9% physiological saline was added to a 100 mL beaker using a 100 mL mass cylinder.

[0175] ② Next, the beaker was placed in the center of a magnetic stirrer, and then a circular magnetic bar (diameter 30 mm) was placed in the beaker.

[0176] ③ Then, the stirrer was operated so that the magnetic bar was stirred at 600 rpm, and the lowermost part of the vortex generated by the stirring reached the top of the magnetic bar.

[0177] ④ After confirming that the temperature of the physiological saline in the beaker reached 24.0℃, 2±0.01 g of a superabsorbent polymer sample was added while a stopwatch was timed. Then, the time taken for the vortex to disappear and the liquid surface to become completely level was measured in seconds, and taken as the vortex time.

[0178] (4) Surface tension (S / T)

[0179] The surface tension of the superabsorbent polymer of the examples and comparative examples was measured as follows.

[0180] ① First, 40 g of normal saline consisting of 0.9 wt% sodium chloride was put into a 50 mL beaker and stirred at a speed of 350 rpm for 3 minutes.

[0181] ② Subsequently, 0.5 g of superabsorbent polymer was added to the stirred solution, stirred for another 3 minutes, and then allowed to stand for 2 minutes to allow the swollen superabsorbent polymer to settle to the bottom.

[0182] ③ Thereafter, the supernatant (solution immediately below the surface) was pipetted and transferred to another clean cup, and the surface tension was measured using a surface tensiometer (Force Tensiometer-K11 / K100, manufactured by Kruss).

[0183] [Table 1]

[0184]

[0185] As shown in Table 1, the superabsorbent polymer of the example in which the polymerization of monomers was performed in the presence of an aqueous dispersion of hydrophobic particles was confirmed to exhibit a fast absorption rate (vortex time) and a high surface tension, and the absorption performance was not deteriorated, compared to the superabsorbent polymers of the comparative examples.

[0186] Specifically, the superabsorbent polymer of the example was confirmed to exhibit an absorption rate (vortex time) equivalent to that of the superabsorbent polymers of Comparative Examples 2 to 4, and a surface tension significantly higher than that of Comparative Examples 2 to 4, in which the superabsorbent polymers of Comparative Examples 2 to 4 used sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium chloride (DTAC), which are mainly used as a conventional foam stabilizer.

[0187] In addition, it can be seen that the absorption rate (vortex time) of the superabsorbent polymer of Example 1 was significantly improved, compared to the superabsorbent polymer of Comparative Example 1, which did not use a foam stabilizer, and the superabsorbent polymer of Comparative Example 6, which contained hydrophobic particles in a powder form.

[0188] Accordingly, it was confirmed that when a hydrogel polymer is prepared by crosslinking polymerization of monomers in the presence of an aqueous dispersion of hydrophobic particles, the specific surface area of the superabsorbent polymer can be increased by effectively capturing carbon dioxide generated by a blowing agent without reducing the surface tension.

Claims

1. A method for preparing a superabsorbent polymer, the method comprising the following steps: Step 1: Prepare a monomer composition comprising an acrylic monomer having at least partially neutralized acidic groups and an internal crosslinking agent; Step 2: Prepare a hydrogel polymer by crosslinking and polymerizing the monomer composition in the presence of an aqueous dispersion of hydrophobic particles and a carbonate foaming agent; Step 3: Form a powdered base resin by drying and pulverizing the hydrogel polymer; and Step 4: A surface crosslinking layer is formed by further crosslinking the surface of the base resin in the presence of a surface crosslinking agent. The aqueous dispersion of the hydrophobic particles is a colloidal solution in which the hydrophobic particles are dispersed by a surfactant. The hydrophobic particles contain a metal salt of a C7 to C24 fatty acid, and have an average particle diameter D 50 from 1 μm to 100 μm, Wherein, based on 100 parts by weight of the acrylic monomer, the amount of the hydrophobic particles is from 0.01 parts by weight to 0.5 parts by weight, and the amount of the carbonate foaming agent is from 0.005 parts by weight to 1 part by weight.

2. The method for preparing the superabsorbent polymer as described in claim 1, wherein The hydrophobic particles are at least one metal stearate salt selected from the group consisting of calcium stearate, magnesium stearate, sodium stearate, zinc stearate, and potassium stearate.

3. The method for preparing the superabsorbent polymer as described in claim 1, wherein The average particle diameter D of the hydrophobic particles is preferably 1 μm to 20 μm. 50 is 1 μm to 20 μm.

4. The method for preparing the superabsorbent polymer as described in claim 1, wherein The carbonate foaming agent is at least one selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate.

5. The method for preparing the superabsorbent polymer as described in claim 1, wherein The carbonate foaming agent is used in a weight ratio of 1:0.1 to 1:2 with the hydrophobic particles.

6. The method for preparing the superabsorbent polymer as described in claim 1, wherein The surfactants include nonionic surfactants and anionic surfactants.

7. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The superabsorbent polymer has a vortex time of less than 55 seconds at 24°C.

8. The method for preparing the superabsorbent polymer as described in claim 1, wherein The surface tension of the superabsorbent polymer is above 65 mN / m.

9. The method for preparing the superabsorbent polymer as described in claim 1, wherein, When measured according to EDANA WSP 241.3, the centrifugal retention capacity of the superabsorbent polymer is 25 g / g to 30 g / g.

10. The method for preparing the superabsorbent polymer as described in claim 1, wherein According to EDANA WSP 242.3 measurements, the superabsorbent polymer has a pressure absorption rate of 20 g / g to 26 g / g at 0.7 psi.

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