Superabsorbent polymer and preparation method thereof

Abstract

The present disclosure relates to superabsorbent polymers and methods for preparing the same. More specifically, the present disclosure relates to a superabsorbent polymer comprising a plurality of micropores, specifically an average of seven or more micropores per particle, having an average diameter of 100 μm or less and a maximum diameter of 300 μm or less, and thus exhibiting improved absorption rate and high surface tension, and also to methods for preparing the superabsorbent polymer.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Patent Application No. 10-2020-0178428 filed on December 18, 2020, and Patent Application No. 10-2021-0180575 filed on December 16, 2021, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entirety. Technical Field

[0003] The present disclosure relates to superabsorbent polymers and methods for preparing the same. More particularly, the present disclosure relates to superabsorbent polymers having a plurality of micropores and thus exhibiting improved absorption rate and high surface tension, and to methods for preparing the same. Background Art

[0004] Superabsorbent polymers (SAPs) are a class of synthetic polymer materials capable of absorbing 500 to 1000 times their weight in water. Manufacturers use different names for these materials, such as SAM (superabsorbent material) and AGM (absorbent gelling material). Initially used in hygiene products, these polymers are now widely used not only in hygiene products but also in water-retaining soils for gardening, waterproofing materials for civil engineering and construction, sheets for seedling cultivation, preservatives for food distribution, and dressings.

[0005] These superabsorbent polymers have been widely used in sanitary materials such as paper diapers and sanitary napkins. In these sanitary materials, the superabsorbent polymers are typically dispersed in pulp. However, in recent years, efforts have been underway to provide sanitary materials such as paper diapers with even thinner thicknesses. As part of these efforts, the development of so-called pulp-free diapers, which reduce the pulp content or use no pulp at all, is actively advancing.

[0006] Superabsorbent polymers used in sanitary products such as pulp-free diapers with reduced or no pulp should not only act as an absorbent for absorbing liquids such as urine, but also act as pulp. Therefore, they are required to exhibit fast absorption rates and excellent absorption properties.

[0007] To produce superabsorbent polymers with this improved absorption rate, methods for increasing the specific surface area by introducing a porous structure into the superabsorbent polymer are primarily used. Specifically, to increase the specific surface area of the superabsorbent polymer, a foaming agent can be used to generate bubbles, or a gas such as carbon dioxide, air, or nitrogen can be injected during the polymerization step. However, because bubbles are unstable in a neutralized solution, if a foam stabilizer is not used to capture these bubbles, the bubbles emerge from the neutralized solution, making it impossible to produce a superabsorbent polymer with a porous structure. Furthermore, excessive use of a foam stabilizer can deteriorate the general physical properties of the superabsorbent polymer.

[0008] Therefore, there is a continuing need to develop superabsorbent polymers having fast absorption rates while maintaining a basic absorbency. Summary of the Invention

[0009] Technical issues

[0010] Therefore, the present disclosure relates to a superabsorbent polymer having a developed porous structure including a large number of micropores and thus exhibiting an improved absorption rate and excellent physical properties such as high surface tension, and a method for preparing the same.

[0011] Technical Solution

[0012] To solve the above problems, a super absorbent polymer is provided, comprising:

[0013] a powdered base resin comprising a crosslinked polymer of an acrylic monomer having at least partially neutralized acidic groups and an internal crosslinking agent; and

[0014] A surface crosslinked layer formed by further crosslinking the crosslinked polymer using a surface crosslinking agent on a base resin,

[0015] wherein the superabsorbent polymer has an average of 7 or more pores per particle, the average diameter of the plurality of pores is 100 μm or less and the maximum diameter is 300 μm or less, and the number of particles having pores whose diameter is greater than or equal to the average diameter and less than or equal to the maximum diameter accounts for 10% to 50% of the total number of superabsorbent polymer particles, and

[0016] The superabsorbent polymer has a surface tension of 65 mN / m or more and a vortex time of 40 seconds or less at 24.0°C.

[0017] In addition, a method for preparing a superabsorbent polymer is provided, the method comprising the following steps:

[0018] preparing a monomer composition comprising: an acrylic monomer having at least partially neutralized acidic groups, a polymerization initiator, an internal crosslinking agent, and an aqueous dispersion of hydrophobic particles (step 1);

[0019] preparing a hydrogel polymer by cross-linking and polymerizing the monomer composition in the presence of a foaming agent or a bubble generating agent (step 2);

[0020] forming a powdered base resin by drying and pulverizing the hydrogel polymer (step 3); and

[0021] By further crosslinking the surface of the base resin in the presence of a surface crosslinking agent to form a surface crosslinked layer (step 4),

[0022] The aqueous dispersion of hydrophobic particles is a colloidal solution in which first hydrophobic particles and second hydrophobic particles are dispersed, and a ratio of an average particle size of the second hydrophobic particles to an average particle size of the first hydrophobic particles is 5 to 100.

[0023] Beneficial effects

[0024] The superabsorbent polymer of the present disclosure includes a large number of micropores having an average diameter of 100 μm or less and a maximum diameter of 300 μm or less, and in particular, can have a highly developed porous structure (in which a plurality of pores having very uniform diameters are formed). As a result, as the specific surface area of the superabsorbent polymer is greatly increased, an improved absorption rate can be exhibited.

[0025] This superabsorbent polymer can be produced by introducing two hydrophobic particles of different particle sizes into an aqueous dispersion during the polymerization step. The use of these hydrophobic particles results in a superabsorbent polymer with a highly developed porous structure and minimizes or eliminates the use of surfactants, typically used as foam stabilizers, thereby minimizing the degradation of the superabsorbent polymer's physical properties, such as surface tension.

[0026] Therefore, according to the present disclosure, it is possible to provide a superabsorbent polymer that is preferably used for pulp-free diapers or ultra-thin diapers by exhibiting excellent absorption rate and excellent other physical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a SEM image of the superabsorbent polymer prepared in Example 3.

[0028] Figure 2 is a SEM image of the superabsorbent polymer prepared in Comparative Example 2. DETAILED DESCRIPTION

[0029] The terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The singular form is also intended to include the plural form, unless the context clearly indicates otherwise. It should also be understood that when used in this specification, the terms "including", "having" or "having" specify the presence of the features, steps, components or their combination, but do not exclude the presence or addition of one or more other features, steps, components or their combination.

[0030] Since the present invention can be modified and has various forms, its specific embodiments will be shown and described in detail by way of example. However, it is not intended to limit the present invention to the specific form disclosed, and it should be understood that the present invention includes all modifications, equivalents and replacements within the scope of the concept and technology of the present invention.

[0031] In addition, the terms used refer only to specific embodiments and are not intended to limit the present invention. A singular expression of the present invention may include a plural expression unless they are expressed differently in the context.

[0032] The term "polymer" in the present invention refers to a state in which a water-soluble ethylenically unsaturated monomer is polymerized, and may include any range of moisture content or any range of particle size. Among the polymers, a polymer having a moisture content of about 40% by weight or more after polymerization and before drying may be referred to as a hydrogel polymer, and particles in which the hydrogel polymer has been pulverized and dried may be referred to as a cross-linked polymer.

[0033] Furthermore, the term "superabsorbent polymer particles" refers to particulate materials comprising a crosslinked polymer in which acrylic monomers having at least partially neutralized acidic groups are polymerized and crosslinked by an internal crosslinking agent.

[0034] Furthermore, the term "cross-linked polymer" refers to a polymer having a three-dimensional network structure in which main chains formed by polymerization of acrylic monomers are cross-linked by an internal cross-linking agent.

[0035] Furthermore, depending on the context, the term "superabsorbent polymer" is used to encompass all of the following materials: a cross-linked polymer in which an acrylic monomer having at least partially neutralized acidic groups is polymerized, or a base resin in powder form composed of superabsorbent polymer particles in which the cross-linked polymer is pulverized, as well as the cross-linked polymer or base resin that has been further processed (e.g., surface cross-linking, fine particle reconstitution, drying, pulverization, classification, etc.) to a state suitable for commercialization. Therefore, the term "superabsorbent polymer" can be interpreted as including a variety of superabsorbent polymer particles.

[0036] In addition, the term "average diameter" of pores refers to an average value of the longest diameter value of each pore among a plurality of pores included in the superabsorbent polymer.

[0037] In addition, the term "maximum diameter" of the pore refers to the maximum value among the longest diameter values of each of the plurality of pores included in the superabsorbent polymer.

[0038] In order to produce superabsorbent polymers with a fast absorption rate, it is necessary to increase the specific surface area of the superabsorbent polymer particles. Therefore, to produce superabsorbent polymers with a high specific surface area, methods have been used to form multiple pores in the superabsorbent polymer by inducing a foaming process during the manufacturing process, or methods of mechanically modifying the superabsorbent polymer. In the preparation methods of superabsorbent polymers with a foaming process, a surfactant-type foam stabilizer is used to trap the bubbles generated during the process inside the polymer and prevent them from escaping from the cross-linked polymer. However, when using such foam stabilizers, the pore size is large, the pore size range is wide, and the aspect ratio of the superabsorbent polymer particles is low. Therefore, there are problems such as the polymer being easily damaged by the process environment or the physical properties deteriorating under pressure due to uneven surface crosslinking efficiency. In addition, in the preparation methods of superabsorbent polymers with mechanical modification, excessive load is imposed on the equipment during the process, resulting in reduced productivity.

[0039] The present inventors have discovered that, when two types of hydrophobic particles having different average particle sizes are used in the form of an aqueous dispersion instead of a conventional foam stabilizer, a superabsorbent polymer having a well-developed pore structure can be provided, wherein a plurality of pores are formed, specifically, an average of seven or more pores per superabsorbent polymer particle, each having an average diameter of 100 μm or less and a maximum diameter of 300 μm or less. Furthermore, in the superabsorbent polymer, the number of particles having pores having a diameter greater than or equal to the average diameter and less than or equal to the maximum diameter, that is, the number of particles having at least one pore having a relatively large diameter, accounts for 10% to 50% of the total number of superabsorbent polymer particles. This indicates that the number of particles having relatively large pore diameters is relatively small, and therefore, a large number of micropores having diameters close to or slightly smaller than the average diameter are very evenly distributed throughout the superabsorbent polymer particles. Consequently, the superabsorbent polymer can have a well-developed porous structure in which a large number of micropores having relatively small diameters are very evenly distributed.

[0040] It has been demonstrated that, due to this highly developed porous structure, superabsorbent polymers not only exhibit an ultra-high absorption rate but also possess high surface tension. Specifically, it has been demonstrated that when two hydrophobic particles having different particle sizes are introduced as an aqueous dispersion during the monomer polymerization step, the foam generated by the foaming agent or bubble generator is effectively trapped and stabilized, even when the input of a separate surfactant-type foam stabilizer is minimized or substantially eliminated. As a result, superabsorbent polymers having a well-developed porous structure, in which pores of small size and uniform shape are uniformly distributed throughout the cross-linked polymer, can be produced.

[0041] More specifically, in one embodiment of the superabsorbent polymer production method, the two types of hydrophobic particles are not used in powder form, but are each added to the monomer composition in the form of an "aqueous dispersion of hydrophobic particles." In other words, the hydrophobic particles are introduced into the monomer composition in the form of a colloidal solution, in which each hydrophobic particle is stably dispersed without precipitation or agglomeration. This is because, when the hydrophobic particles are introduced into the monomer composition in powder form for polymerization, the hydrophobic particles precipitate and cannot be dispersed in the monomer composition in the form of an aqueous solution, thereby failing to effectively stabilize the generated bubbles.

[0042] In addition, each hydrophobic particle is stably dispersed in the aqueous dispersion by a dispersion stabilizer, and there is no agglomeration between the particles. Specifically, the dispersion stabilizer can form a double electric layer on the surface of the hydrophobic particles to generate electrostatic repulsion between the particles, which can stabilize the hydrophobic particles, or the surfactant can be adsorbed on the surface of the hydrophobic particles to generate spatial repulsion between the particles, which can prevent the particles from agglomerating with each other. Therefore, when the dispersion stabilizer is not included in the aqueous dispersion of the hydrophobic particles, it will cause the hydrophobic particles to agglomerate or sink due to gravity, making the dispersion of the hydrophobic particles unable to be stabilized. Therefore, when the aqueous dispersion of hydrophobic particles without a dispersion stabilizer is used together with a foaming agent in the polymerization step, bubbles cannot be effectively captured, and therefore pores cannot be formed in the superabsorbent polymer, making it difficult to improve the absorption rate of the superabsorbent polymer. However, the type of dispersion stabilizer will be further described in the preparation method described later.

[0043] Furthermore, the average particle size of the first hydrophobic particles is less than 1 μm, while the average particle size of the second hydrophobic particles is greater than 1 μm. This is to reduce the interfacial energy of bubbles in the monomer composition when large and small particles are present together, thereby stabilizing small bubbles even when used in small amounts. Therefore, the superabsorbent polymer according to one embodiment exhibits a further improved absorption rate and high surface tension, and can therefore be highly preferably used in various sanitary products.

[0044] Hereinafter, a super absorbent polymer and a method of preparing the super absorbent polymer according to specific embodiments of the present disclosure will be described in more detail.

[0045] Superabsorbent polymers

[0046] A superabsorbent polymer according to one embodiment includes a powdered base resin comprising a crosslinked polymer of an acrylic monomer having at least partially neutralized acidic groups and an internal crosslinking agent; and a surface crosslinked layer formed by further crosslinking the crosslinked polymer using a surface crosslinking agent on the base resin.

[0047] In particular, the superabsorbent polymer is a porous superabsorbent polymer including a plurality of pores, wherein the average diameter of the pores is 100 μm or less and the maximum diameter is 300 μm or less. At this time, when the average diameter of the pores included in the superabsorbent polymer exceeds 100 μm or the maximum diameter exceeds 300 μm, the specific surface area of the superabsorbent polymer is insufficient, and therefore it is difficult to expect an improvement in the absorption rate. In addition, during this process, the surface of the superabsorbent polymer may be easily worn or broken, thereby deteriorating the physical properties and generating more fine powder. Therefore, compared with a porous superabsorbent polymer including a plurality of pores with an average diameter of 100 μm or less but a maximum diameter of more than 300 μm, the superabsorbent polymer according to the embodiment has a pore structure with uniformly distributed micropores, thereby showing a high absorption capacity while having a significantly improved absorption rate.

[0048] For example, the plurality of pores included in the super absorbent polymer may have an average diameter of 1 μm to 100 μm and a maximum diameter of 250 μm to 300 μm.

[0049] In addition, in the superabsorbent polymer, the content of pores satisfying the above-mentioned average diameter and maximum diameter may be an average of 7 or more, or 7 to 30 pores per superabsorbent particle.

[0050] Furthermore, one or more pores having a diameter equal to or greater than the average diameter and less than or equal to the maximum diameter, that is, one or more pores having a relatively large diameter among the plurality of pores, may be formed only in particles corresponding to 10% to 50% or 20% to 40% of the total number of superabsorbent polymer particles. This may mean that a large number of micropores having a diameter close to or slightly smaller than the average diameter are formed in a very uniform and narrow diameter distribution across the majority of superabsorbent polymer particles. As a result, the superabsorbent polymer may have a well-developed pore structure in which a large number of micropores having a very uniform diameter are formed across a large number of particles.

[0051] Due to this highly developed porous structure, the superabsorbent polymers described above can exhibit greatly improved absorption rates. In contrast, superabsorbent polymers foamed and prepared using conventional foaming agents or bubble generators without the use of an aqueous dispersion of hydrophobic particles have difficulty stabilizing bubbles. Consequently, they may not have a sufficiently developed porous structure, or other physical properties (such as the surface tension of the superabsorbent polymer) may be reduced, as the use of a large amount of foam stabilizer is unavoidable.

[0052] At the same time, the average diameter and maximum diameter of the pores in the above-mentioned superabsorbent polymer, the average number of pores per particle, and the proportion of particles with relatively large pores can be confirmed by observing the surface and / or internal images of the superabsorbent polymer particles to be measured using an electron microscope. More specifically, the average diameter can be obtained by measuring the longest diameter of each pore included in the superabsorbent polymer particles and then calculating the average of the longest diameters of the pores. In addition, the maximum diameter can be obtained by taking the maximum value. In addition, the average number of pores per particle can be obtained by calculating the number of pores whose longest diameter can be measured on an electron microscope. The proportion of particles whose pore diameter is greater than or equal to the average diameter and less than or equal to the maximum diameter can be calculated by measuring the number of pores whose longest diameter is greater than or equal to the average diameter and the number of particles with corresponding pores. In this case, it is preferred to select more than 300 pores for a superabsorbent polymer sample (for example, a superabsorbent polymer sample prepared by a single process), measure their diameters, and then obtain the average diameter and maximum diameter.

[0053] In addition, the acrylic monomer is a compound represented by the following Chemical Formula 1:

[0054] [Chemical Formula 1]

[0055] R 1 -COOM 1

[0056] In Chemical Formula 1,

[0057] R 1 is a C2 to C5 hydrocarbon group having an unsaturated bond, and

[0058] M 1 It is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

[0059] Preferably, the acrylic monomer may include at least one selected from acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts thereof.

[0060] Here, the acrylic monomers may be those having at least partially neutralized acidic groups. Preferably, acrylic monomers partially neutralized with alkaline substances such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc. may be used. The degree of neutralization of the acrylic monomers may be 40 mol% to 95 mol%, 40 mol% to 80 mol%, or 45 mol% to 75 mol%. The range of the degree of neutralization can be adjusted according to the final performance. Too high a degree of neutralization will cause the neutralized monomer to precipitate, and therefore polymerization may not easily occur. Conversely, too low a degree of neutralization will not only reduce the absorbency of the polymer, but also impart difficult-to-handle properties to the polymer, such as those of elastic rubber.

[0061] Furthermore, the concentration of the acrylic monomer can be approximately 20% to 60% by weight, or approximately 40% to 50% by weight, based on the monomer composition comprising the raw materials of the superabsorbent polymer and a solvent, and can be appropriately controlled in consideration of the polymerization time and reaction conditions. If the monomer concentration is too low, the yield of the superabsorbent polymer is low, and there may be problems with economic efficiency. Conversely, if the concentration is too high, some monomer may be extracted, or the pulverization efficiency of the polymerized hydrogel polymer may be reduced during the pulverization process, leading to process problems and possibly deterioration of the physical properties of the superabsorbent polymer.

[0062] In addition, the term "internal crosslinking agent" used herein is different from the surface crosslinking agent used to crosslink the surface of the superabsorbent polymer particles described below, and the internal crosslinking agent polymerizes the unsaturated bonds of the water-soluble ethylenically unsaturated monomer by crosslinking. The crosslinking in the above step is performed both on the surface and inside, but when the surface crosslinking process of the superabsorbent polymer particles described below is performed, the surface of the superabsorbent polymer particles finally prepared has a structure crosslinked by the surface crosslinking agent, while the inside of the particles has a structure crosslinked by the internal crosslinking agent.

[0063] As the internal crosslinking agent, any compound can be used as long as it allows the introduction of crosslinking bonds during 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 reactive with a water-soluble substituent of the acrylic monomer; or a crosslinking agent having two or more functional groups reactive with a water-soluble substituent of the monomer and / or a water-soluble substituent formed by hydrolysis of the monomer.

[0064] For example, as the internal crosslinking agent, a multifunctional crosslinking agent can be used alone or in combination of two or more thereof. Specifically, examples of the internal crosslinking agent include acrylate compounds such as 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, 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, Ester, dipentaerythritol pentaacrylate, glyceryl tri(meth)acrylate and 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 and polyglycerol polyglycidyl ether; triarylamine; propylene glycol; glycerol and ethylene carbonate, but the present disclosure is not limited thereto.

[0065] The cross-linking polymerization of the water-soluble ethylenically unsaturated monomer in the presence of an internal cross-linking agent can be carried out by thermal polymerization, photopolymerization or mixed polymerization in the presence of a polymerization initiator with or without a thickener, a plasticizer, a storage stabilizer, an antioxidant, etc., but the specific details will be described later.

[0066] In addition, the superabsorbent polymer further includes a surface crosslinked layer formed by further crosslinking the crosslinked polymer contained in the base polymer on at least a portion of the surface of the base resin using a surface crosslinking agent. This is to increase the surface crosslink density of the superabsorbent polymer. When the superabsorbent polymer further includes the surface crosslinked layer as described above, it has a structure in which the crosslink density on the outer side is higher than that on the inner side.

[0067] As the surface crosslinking agent, any surface crosslinking agent conventionally used for preparing superabsorbent polymers can be used without particular limitation. For example, the surface crosslinking agent may include at least one selected from the group consisting of polyol-based compounds, polyepoxide-based compounds, polyamine compounds, halogenated epoxy compounds, condensation products of halogenated epoxy compounds, oxazoline-based compounds, and alkylene carbonate-based compounds.

[0068] Specifically, as the polyol-based compound, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol or polyethylene glycol, monopropylene glycol, 1,3-propylene glycol, dipropylene glycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, etc. can be used.

[0069] In addition, as the polyepoxy compound, ethylene glycol diglycidyl ether, glycidol, etc. can be used.

[0070] As the polyamine compound, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, polyamidepolyamine, and the like can be used.

[0071] Furthermore, as the halogenated epoxy compound, epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and the like can be used.

[0072] In addition, as the oxazoline-based compound, mono-, di- or polyoxazolidinone and the like can be used.

[0073] In addition, as the alkylene carbonate compound, ethylene carbonate, propylene carbonate, glycerol carbonate, etc. can be used.

[0074] More specifically, the surface crosslinking agents described above may be used alone or in combination with one another. For example, an alkylene carbonate compound such as ethylene carbonate may be used as the surface crosslinking agent.

[0075] In addition, the superabsorbent polymer may include first hydrophobic particles and second hydrophobic particles, wherein the first hydrophobic particles have an average particle size of less than 1 μm, and the second hydrophobic particles have an average particle size of 1 μm or more.

[0076] Herein, hydrophobic particles refer to water-insoluble particles having a water contact angle of 50° or greater or being insoluble in water. Particles having a water contact angle of less than 50° and water-soluble particles can be dissolved in the monomer composition in the form of an aqueous solution, making it difficult to capture bubbles generated during the polymerization process. On the other hand, the hydrophobic particles are located at the interface between the hydrophobic bubbles (such as carbon dioxide) in the neutralization solution and the neutralization solution, thereby effectively capturing the bubbles and then stabilizing them.

[0077] Therefore, the water contact angle of each of the first and second hydrophobic particles is 50° or greater. More specifically, the water contact angle of each of the first and second hydrophobic particles may be 70° or greater, 100° or greater, 120° or greater, or 150° or greater, and 175° or less.

[0078] In this case, the contact angles of the first and second hydrophobic particles can be measured as follows. First, a coating solution is prepared in which each hydrophobic particle is dispersed in a dichloromethane solvent at a concentration of 5 wt%. Next, the coating solution is spin-coated on a wafer without surface roughness and dried at room temperature to remove the remaining solvent. The contact angle is then measured by dropping water on the coating, and this is defined as the contact angle of each hydrophobic particle.

[0079] Furthermore, the average particle size of the first hydrophobic particles is less than 1 μm, specifically, 10 nm or more and less than 1 μm. More specifically, the average particle size of the first hydrophobic particles can be 10 nm or more, 50 nm or more, or 100 nm or more, and 800 nm or less, 600 nm or less, 400 nm or less, or 300 nm or less.

[0080] The second hydrophobic particles have an average particle size of 1 μm or more, specifically 1 μm to 100 μm. More specifically, the second hydrophobic particles have an average particle size of 2 μm or more, or 3 μm or more, and 50 μm or less, 30 μm or less, 20 μm or less, 10 μm or less, or 7 μm or less.

[0081] Herein, the average particle size of the hydrophobic particles refers to D50, and "particle size Dn" refers to the particle size at the n% point of the cumulative distribution of the number of particles according to the particle size. In other words, D50 is the particle size at the 50% point of the cumulative distribution of the number of particles according to the particle size, D90 is the particle size at the 90% point of the cumulative distribution of the number of particles according to the particle size, and D10 is the particle size at the 10% point of the cumulative distribution of the number of particles according to the particle size. 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 (such as Microtrac S3500). The particle size distribution is then obtained by measuring the difference in the diffraction pattern according to the particle size when the particles pass through the laser beam. In the measuring device, D10, D50 and D90 can be obtained by calculating the particle size at the points where the cumulative distribution of the number of particles reaches 10%, 50% and 90%.

[0082] In addition, the ratio of the average particle size of the second hydrophobic particles to the average particle size of the first hydrophobic particles may be 5 to 100, more specifically, 10 to 50. When the second hydrophobic particles have an excessively small average particle size compared to the first hydrophobic particles, or when the second hydrophobic particles have an excessively large average particle size compared to the first hydrophobic particles, it is difficult to capture small-sized bubbles, and thus the bubble size may increase.

[0083] In addition, the first hydrophobic particles and the second hydrophobic particles can be included in a weight ratio of 1:5 to 1:50. When the second hydrophobic particles are included in an amount too small compared to the first hydrophobic particles, it may be difficult to capture and retain the gas in the monomer composition. When the second hydrophobic particles are included in an amount too large compared to the first hydrophobic particles, the size of the pores may increase. For example, the amount of the second hydrophobic particles can be more than 6 times, more than 8 times, or more than 10 times, and less than 40 times, less than 30 times, or less than 20 times the weight of the first hydrophobic particles.

[0084] In addition, the first hydrophobic particles and the second hydrophobic particles are each independently selected from the group consisting of hydrophobic silica, metal salts of C7 to C24 fatty acids, and hydrophobic organic particles.

[0085] Specifically, the first hydrophobic particles and the second hydrophobic particles are both hydrophobic silica; one of the first hydrophobic particles and the second hydrophobic particles is hydrophobic silica and the other is a metal salt of C7 to C24 fatty acid; or the first hydrophobic particles and the second hydrophobic particles are both metal salts of C7 to C24 fatty acid.

[0086] Herein, hydrophobic silica is a general term for silica having a water contact angle of 50° or greater due to a small content of silanol (—SiOH) on its surface, and hydrophobic silica known in the art may be used without limitation.

[0087] In addition, C7 to C 24 Metal salts of fatty acids are compounds in which a metal cation, rather than the hydrogen ion of a carboxyl group, is bonded to the terminal end of an unsaturated or saturated fatty acid having a linear structure and 7 to 24 carbon atoms in the molecule. These can be monovalent metal salts or divalent or higher-valent polyvalent metal salts. When the hydrophobic particles are metal salts of fatty acids with fewer than 7 carbon atoms, it is impossible to capture bubbles generated in the form of particles by ionization in an aqueous solution. When the hydrophobic particles are metal salts of fatty acids with more than 24 carbon atoms, the fatty acid chain becomes longer, which may make dispersion difficult.

[0088] 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 ion (monovalent metal cation). In addition, when the metal salt of a fatty acid is a divalent or higher-valent polyvalent metal salt, 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.

[0089] In one embodiment, the hydrophobic particles can be metal salts of C12 to C20 saturated fatty acids. For example, the hydrophobic particles can be at least one metal salt of a saturated fatty acid selected from the group consisting of a metal salt of lauric acid having 12 carbon atoms in the molecule; a metal salt of tridecanoic acid having 13 carbon atoms in the molecule; a metal salt of myristic acid having 14 carbon atoms in the molecule; a metal salt of pentadecanoic acid having 15 carbon atoms in the molecule; a metal salt of palmitic acid having 16 carbon atoms in the molecule; a metal salt of heptadecanoic acid having 17 carbon atoms in the molecule; a metal salt of stearic acid having 18 carbon atoms in the molecule; a metal salt of nonadecanoic acid having 19 carbon atoms in the molecule; and a metal salt of arachidic acid having 20 carbon atoms in the molecule.

[0090] Preferably, the metal salt of fatty acid can be a metal salt of stearic acid. For example, it can be at least one metal salt of stearic acid selected from the group consisting of calcium stearate, magnesium stearate, sodium stearate, zinc stearate and potassium stearate.

[0091] In addition, the hydrophobic organic particles may be at least one selected from the group consisting of ethylene polymers, propylene polymers, styrene polymers, butadiene polymers, styrene-butadiene copolymers, alkyl acrylate polymers, alkyl methacrylate polymers, alkyl acrylate-acrylonitrile copolymers, acrylonitrile-butadiene copolymers, acrylonitrile-butadiene-styrene copolymers, acrylonitrile-alkyl acrylate-styrene copolymers, alkyl methacrylate-butadiene-styrene copolymers, and alkyl acrylate-alkyl methacrylate copolymers.

[0092] At the same time, the superabsorbent polymer may include particles having a particle size of about 150 μm to about 850 μm, accounting for 90% by weight or more or 90% to 100% by weight relative to the total weight. The particle size can be measured according to EDANA (European Disposables and Nonwovens Association) WSP 220.3. For superabsorbent polymer particles having a particle size of 150 μm to about 850 μm, the average diameter of the pores, the longest diameter, the average number of pores per particle, and the proportion of particles having pores of a predetermined diameter, etc. can be measured and calculated.

[0093] In addition, the superabsorbent polymer may have a vortex time (absorption rate) of 40 seconds or less, 39 seconds or less, or 38 seconds or less at 24.0°C. The shorter the vortex time, the better the evaluation. The lower limit is theoretically 0 seconds, but it may be 5 seconds or more, 10 seconds or more, or 20 seconds or more. The method of measuring the vortex time of the superabsorbent polymer will be described in more detail in the following experimental examples.

[0094] In addition, the surface tension of the superabsorbent polymer may be 65 mN / m or more, 66 mN / m or more, or 68 mN / m or more, and 72 mN / m or less or 71 mN / m or less. The method of measuring the surface tension of the superabsorbent polymer will be described in more detail in the following experimental examples.

[0095] In addition, the superabsorbent polymer may have a centrifuge retention capacity (CRC) of greater than 27 g / g, as measured according to EDANA WSP 241.3, and an absorbency under pressure (AUP) of greater than 20 g / g at 0.7 psi, as measured according to EDANA WSP 242.3. More specifically, the superabsorbent polymer may have a centrifuge retention capacity (CRC) of 27.5 g / g or greater, or 28 g / g or greater, and 34 g / g or less, or 33 g / g or less, as measured according to EDANA WSP 241.3. In addition, the superabsorbent polymer may have an absorbency under pressure (AUP) of 21 g / g or greater, 22 g / g or greater, or 23 g / g or greater, and 28 g / g or less, or 27 g / g or less, at 0.7 psi, as measured according to EDANA WSP 242.3.

[0096] Preparation method of superabsorbent polymer

[0097] Meanwhile, the superabsorbent polymer can be prepared by a method comprising the following steps: preparing a monomer composition comprising an aqueous dispersion of an acrylic monomer having an at least partially neutralized acidic group, a polymerization initiator, an internal crosslinking agent, and hydrophobic particles (step 1); preparing a hydrogel polymer by crosslinking polymerization of the monomer composition in the presence of a foaming agent or a bubble generating agent (step 2); forming a powdered base resin by drying and pulverizing the hydrogel polymer (step 3); and forming a surface crosslinked layer by further crosslinking the surface of the base resin in the presence of a surface crosslinking agent (step 4).

[0098] Herein, the aqueous dispersion of hydrophobic particles is a colloidal solution in which first hydrophobic particles and second hydrophobic particles are dispersed, and a ratio of an average particle size of the second hydrophobic particles to an average particle size of the first hydrophobic particles is 5 to 100.

[0099] At this time, the aqueous dispersion of hydrophobic particles may be in the form of an aqueous dispersion including first hydrophobic particles and second hydrophobic particles together, or may be in the form of a mixture of a first aqueous dispersion of hydrophobic particles in which the first hydrophobic particles are dispersed and a second aqueous dispersion of hydrophobic particles in which the second hydrophobic particles are dispersed.

[0100] In addition, the average particle size of the first hydrophobic particles is less than 1 μm, and the average particle size of the second hydrophobic particles is greater than 1 μm. For details on other descriptions of the first hydrophobic particles and the second hydrophobic particles, please refer to the above.

[0101] Hereinafter, each step of the method for preparing a super absorbent polymer according to one embodiment will be described in more detail.

[0102] (Step 1)

[0103] The above steps are used to prepare a monomer composition comprising an acrylic monomer having at least partially neutralized acidic groups, a polymerization initiator, an internal crosslinking agent, and an aqueous dispersion of hydrophobic particles. For detailed information on the acrylic monomer and the internal crosslinking agent, please refer to the above text.

[0104] The first and second hydrophobic particles may be included in the aqueous dispersion in an amount of 0.01% to 60% 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, the attraction between the particles is reduced, thereby improving dispersion stability. However, due to the low concentration, a large amount of hydrophobic particles must be added when preparing the superabsorbent polymer. When the content of the hydrophobic particles in the hydrophobic aqueous dispersion is too high, the dispersion stability may deteriorate due to agglomeration between the particles. However, it is appropriate to have the content of the hydrophobic particles in the aqueous dispersion as high as possible within a range that ensures dispersion stability. This is because the higher the content of the hydrophobic particles in the aqueous dispersion, the better the bubble capture ability, and thus the superabsorbent polymer can exhibit a more developed porous structure and an improved absorption rate.

[0105] At the same time, the first hydrophobic particles and the second hydrophobic particles can be uniformly dispersed in the aqueous dispersion by a dispersion stabilizer (e.g., a surfactant or a polymer) around the surface of the particles. The surfactant can form a double electric layer on the surface of the hydrophobic particles to induce electrostatic repulsion between the particles, which can stabilize the hydrophobic particles to improve the dispersion stability of the hydrophobic particles. In addition, the polymer can be adsorbed on the surface of the hydrophobic particles to induce steric repulsion between the particles, which can prevent the particles from agglomerating with each other, thereby improving the dispersion stability of the hydrophobic particles.

[0106] 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 for the dispersion stabilization of hydrophobic particles. More specifically, considering the form of the hydrophobic particles, for example, the form of a metal salt of a saturated fatty acid, a nonionic surfactant and an anionic surfactant can be used together to more effectively and stably disperse the hydrophobic particles in water. For example, a nonionic surfactant in combination with a long-chain hydrocarbon having more than 10 carbon atoms and an anionic surfactant based on sulfate can be used together.

[0107] For example, examples of cationic surfactants include dialkyldimethylammonium salts and alkylbenzylmethylammonium salts, examples of anionic surfactants include alkyl polyoxyethylene sulfates, monoalkyl sulfates, alkylbenzene sulfonates, monoalkyl phosphates, and sulfates or sodium salts thereof having a functional group containing a long-chain hydrocarbon, such as sodium lauryl sulfate, sodium dodecyl sulfate, or sodium lauryl ether sulfate, examples of amphoteric surfactants include alkyl sulfobetaine and alkyl carboxybetaine, examples of nonionic surfactants include polyoxyethylene alkyl ethers, such as polyethylene glycol, polyoxyalkylene alkylphenyl ether, polyoxyethylene arylphenyl ether, fatty acid esters such as fatty acid sorbitol esters, glycerol monostearate, alkyl monoglyceryl ether, alkanolamide, and alkyl polyglycosides. However, the present disclosure is not limited thereto.

[0108] In addition, as the polymer used as the dispersion stabilizer, polyalkylene glycol, polyethyleneimine, polyvinyl alcohol, polyacrylamide, or polyvinyl pyrrolidone can be used.

[0109] At the same time, based on 100 parts by weight of acrylic monomer, each of the first hydrophobic particles and the second hydrophobic particles can be used in an amount of 0.005 to 1 part by weight. Specifically, based on 100 parts by weight of acrylic monomer, the first hydrophobic particles can be used in an amount of 0.005 to 0.007 parts by weight, and less than 0.25 parts by weight, less than 0.1 parts by weight, or less than 0.05 parts by weight. Based on 100 parts by weight of acrylic monomer, the second hydrophobic particles can be used in an amount of 0.025 to 0.035 parts by weight, and less than 0.75 parts by weight, less than 0.5 parts by weight, or less than 0.25 parts by weight. In this case, as described above, the first hydrophobic particles and the second hydrophobic particles are used in a weight ratio of 1:5 to 1:50.

[0110] In addition, the total weight of the first and second hydrophobic particles can be 0.01 to 2 parts by weight based on 100 parts by weight of the acrylic monomer. If the total weight of the hydrophobic particles is too low, they may not function as a foam stabilizer. If the total weight of the aqueous dispersion of hydrophobic particles is too high, the surface tension of the superabsorbent polymer may decrease, and the fluidity of the hydrophobic particles may increase.

[0111] 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 to 5 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer, and 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 parts by weight or less. When too little internal crosslinking agent is used, crosslinking does not occur sufficiently, and thus it may be difficult to achieve a strength higher than an appropriate level. When too much internal crosslinking agent is used, the internal crosslink density increases, and thus it may be difficult to achieve a desired level of water retention capacity.

[0112] In addition, the monomer composition may further include a polymerization initiator for initiating a polymerization reaction of the monomers. The polymerization initiator is not particularly limited as long as it is generally used in the preparation of superabsorbent polymers.

[0113] Specifically, depending on the polymerization method, the polymerization initiator may be an initiator for thermal polymerization or an initiator for photopolymerization by ultraviolet radiation. However, even when photopolymerization is applied, a certain amount of heat is generated due to ultraviolet irradiation, etc., and some heat is generated as the polymerization reaction, which is an exothermic reaction, proceeds. Therefore, the composition may further include a thermal polymerization initiator.

[0114] More specifically, any compound that can form radicals by light such as ultraviolet rays can be used as the photopolymerization initiator without limitation.

[0115] For example, the photopolymerization initiator can be one or more compounds selected from benzoin ether, dialkyl acetophenone, hydroxyalkyl ketone, phenylglyoxylic acid, benzyl dimethyl ketal, acylphosphine and α-aminoketone. In addition, as a specific example of acylphosphine, commercially available lucirin TPO, i.e. diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, can be used. More photopolymerization initiators are well disclosed on page 115 of "UV Coatings: Basics, Recent Developments and New Applications (Elsevier, 2007)" written by Reinhold Schwalm, but the present invention is not limited thereto.

[0116] The photopolymerization initiator may be used in a concentration of about 0.01 wt % to about 1.0 wt % based on the monomer composition. When the concentration of the photopolymerization initiator is too low, the polymerization rate may be slow, while when the concentration is too high, the molecular weight of the superabsorbent polymer may be low and the properties may be uneven.

[0117] In addition, as thermal polymerization initiators, one or more initiators selected from persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specifically, sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc. can be used as examples of persulfate initiators; while 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis(N,N-dimethylene)isobutylamidine dihydrochloride, 2-(carbamoyl azo)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 4,4-azobis(4-cyanovaleric acid), etc. can be used as examples of azo initiators. More types of thermal polymerization initiators are well disclosed in "Principles of Polymerization" written by Odian (Wiley, 1981), page 203, but the present invention is not limited thereto.

[0118] The thermal polymerization initiator may be used at a concentration of about 0.001% to about 0.5% by weight based on the monomer composition. When the concentration of the thermal polymerization initiator is too low, additional thermal polymerization is difficult to occur and the effect of adding the thermal polymerization initiator may be small. When the concentration of the thermal polymerization initiator is too high, the molecular weight of the superabsorbent polymer may become low and the properties may be uneven.

[0119] The amount of the polymerization initiator used may 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 may slow down, and a large amount of residual monomer may be extracted from the final product. Conversely, when the concentration of the polymerization initiator is higher than the above range, the polymer chains forming the network become shorter, resulting in an increase in the content of extractable components and a decrease in the pressure absorption rate, thereby reducing the physical properties of the polymer.

[0120] If necessary, the monomer composition may further include additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant.

[0121] Alternatively, the monomer composition containing the monomers may be 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, internal crosslinking agent, and polymerization initiator, may be appropriately adjusted in consideration of polymerization time and reaction conditions. For example, the solid content of the monomer composition may be 10% to 80% by weight, 15% to 60% by weight, or 30% to 50% by weight.

[0122] When the monomer composition has a solid content within the above range, it may 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 utilizing the gel effect phenomenon that occurs during the polymerization process of a high-concentration aqueous solution.

[0123] At this time, any solvent that can dissolve the above components can be used without limitation. For example, the solvent can be at least one selected from 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.

[0124] (Step 2)

[0125] Subsequently, a step of preparing a hydrogel polymer by cross-linking and polymerizing the monomer composition in the presence of a foaming agent or a bubble generator is performed. In this step, the foaming agent or bubble generator generates bubbles, which are effectively captured by the two types of hydrophobic particles dispersed in water, thereby increasing the specific surface area of the prepared hydrogel polymer.

[0126] Meanwhile, the foaming agent may be a carbonate foaming agent. The carbonate foaming agent is used to increase the surface area by forming pores in the hydrogel polymer by foaming during the polymerization process. For example, it may be at least one selected from the group consisting of sodium bicarbonate (sodium monobicarbonate), sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate.

[0127] The carbonate foaming agent can be used in an amount of 50 ppmw to 1 part by weight based on 100 parts by weight of the acrylic monomer. When the foaming agent content is less than 50 ppmw, the effect of using the foaming agent may not be significant. When the foaming agent content exceeds 1 part by weight, excessive pores are present in the cross-linked polymer, resulting in reduced gel strength and density of the prepared superabsorbent polymer, which may cause problems with distribution and storage. For example, based on 100 parts by weight of the acrylic monomer, the carbonate foaming agent can be used in an amount of 100 ppmw to 0.8 part by weight or less, or 0.7 part by weight or less.

[0128] In addition, carbonate foaming agent and (the first hydrophobic particle and the second hydrophobic particle) can be used in a weight ratio of 1:0.01 to 1:10. When used with too low a content compared to the aqueous dispersion of carbonate foaming agent hydrophobic particles, it is difficult to effectively capture the bubbles produced, thereby increasing pore size. When used with too high a content compared to the foaming agent, the problems of increased mobility, reduced bulk density and reduced surface tension of the final product may occur. For example, the gross weight of the first hydrophobic particle and the second hydrophobic particle can be more than 0.05 times, more than 0.1 times or more than 0.5 times the weight of the carbonate foaming agent, and less than 5 times, less than 3 times or less than 2 times.

[0129] At the same time, a bubble generator can be used instead of a foaming agent, and any microbubble generator used to foam a monomer composition in the preparation of a superabsorbent polymer can be used without any particular restrictions. The example of this microbubble generator is a tubular flow path in which a plurality of protruding pins are installed with a predetermined supply rate, such as 50 (L / min) to 1500 (L / min) through the monomer composition, and the monomer composition is collided with the protruding pin and foamed. The example of this microbubble generator is disclosed in Korean Patent Publication No. 2020-0128969, and can also be obtained and applied to the commercial products used in the following examples.

[0130] Furthermore, surfactants commonly used as foam stabilizers, such as alkyl sulfate-based compounds and polyoxyethylene alkyl ether-based compounds, may not be used in steps 1 and 2. For example, anionic surfactants, such as alkyl sulfate-based compounds, such as sodium lauryl sulfate, ammonium lauryl sulfate, sodium lauryl ether sulfate, or sodium myristyl ether sulfate; or nonionic surfactants, such as alkyl ether sulfate-based compounds, such as polyoxyethylene lauryl ether, may not be used in steps 1 and 2. This prevents the problem of reduced surface tension of the superabsorbent polymer caused by the use of surfactants.

[0131] Meanwhile, the polymerization of the monomer composition in the presence of such an aqueous dispersion of hydrophobic particles is not particularly limited as long as it is a commonly used polymerization method.

[0132] Specifically, polymerization methods are mainly divided into thermal polymerization and photopolymerization according to the energy source of polymerization. In the case of thermal polymerization, it is usually carried out in a reactor equipped with a stirring shaft, such as a kneader. In the case of photopolymerization, it can be carried out in a reactor equipped with a movable conveyor belt. However, the polymerization method is only an example, and the present invention is not limited thereto.

[0133] For example, in a reactor equipped with a stirring shaft, such as a kneader, the hydrogel polymer obtained by thermal polymerization by supplying hot air or heating the reactor can be discharged to the reactor outlet in a size 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 of the monomer composition to be injected and the injection rate, and generally, a hydrogel polymer having a weight-average particle size of 2 mm to 50 mm can be obtained.

[0134] Furthermore, when photopolymerization is performed in a reactor equipped with a movable conveyor belt as described above, a hydrogel polymer sheet having the width of the conveyor belt can generally be obtained. The thickness of the polymer sheet can vary depending on the concentration and injection rate of the monomer composition to be injected, and preferably the monomer composition is supplied so that the polymer sheet has a thickness of about 0.5 cm to about 10 cm. If the monomer composition is supplied at such a level that the polymer sheet is too thin, production efficiency may be low. If the thickness of the polymer sheet exceeds 10 cm, polymerization may not occur uniformly across the entire thickness due to the excessive thickness.

[0135] (Step 3)

[0136] Subsequently, the hydrogel polymer is dried and pulverized to form a powdered base resin. If necessary, a coarse pulverization step may be further performed before drying to improve the efficiency of the drying step.

[0137] Here, the pulverizer used is not particularly limited. Specifically, it can include a pulverizer selected from a vertical pulverizer, a turbine cutter, a turbine grinder, a rotary cutting mill, a cutting mill, a disc mill, a pulverizing crusher, a crusher, a shredder and a disc cutter, but is not limited thereto.

[0138] The hydrogel polymer can be pulverized to obtain a particle size of 0.01 mm to 50 mm, or 0.01 mm to 30 mm. Specifically, to improve drying efficiency, the hydrogel polymer is preferably pulverized into particles of 50 mm or less. However, excessive pulverization may result in agglomeration of particles, so the hydrogel polymer is preferably pulverized into particles of 0.01 mm or greater.

[0139] In addition, since the gel grinding of the hydrogel polymer is carried out in a state with a relatively low moisture content, the hydrogel polymer may adhere to the surface of the gel grinding machine. In order to minimize this phenomenon, steam, water, surfactants, anti-agglomeration agents (such as clay, silica, etc.), persulfate-based initiators, azo-based initiators, hydrogen peroxide, thermal polymerization initiators, epoxy resin-based crosslinkers, glycol-based crosslinkers, crosslinkers containing difunctional, trifunctional or higher-functional multifunctional acrylates, monofunctional crosslinkers containing hydroxyl groups, etc. can be added to the hydrogel polymer as necessary.

[0140] After the gel pulverization described above, the hydrogel polymer can be dried. Drying can be performed at a temperature of 120°C to 250°C, preferably 140°C to 200°C, and more preferably 150°C to 200°C. In this case, the drying temperature can be defined as the temperature of the heating medium supplied for drying or the temperature inside the drying reactor, including the heating medium and polymer, during the drying process. When the drying time is prolonged due to low drying temperatures, process efficiency decreases. To prevent this, the drying temperature is preferably 120°C or higher. In addition, when the drying temperature is too high, the surface of the hydrogel polymer is excessively dried, which may increase the generation of fine powder in the subsequent pulverization step, thereby reducing the physical properties of the final polymer. To prevent this, the drying temperature is preferably 250°C or lower.

[0141] Meanwhile, considering process efficiency, the drying time may be about 20 minutes to about 90 minutes, but is not limited thereto.

[0142] The drying method in the drying step is not particularly limited, as long as it is a drying method commonly used in drying hydrogel polymers. Specifically, the drying step can be performed by applying hot air, infrared radiation, microwave radiation, ultraviolet radiation, or the like. After the drying step, the moisture content of the polymer can be from about 5% to about 10% by weight.

[0143] Subsequently, a step of pulverizing the dried polymer obtained in the drying step is performed.

[0144] The base resin as the polymer powder obtained after the pulverization step may have a particle size of about 150 μm to about 850 μm. As a pulverizer for pulverizing to such a particle size, a pin mill, a hammer mill, a screw mill, a roller mill, a disc mill, a hand mill, etc. may be used, but the present invention is not limited thereto.

[0145] 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 particle size. Preferably, polymers with a particle size of about 150 μm to about 850 μm are classified, and only base resins with such particle sizes are subjected to the surface crosslinking reaction step.

[0146] (Step 4)

[0147] Meanwhile, after the base resin powder is prepared by the classification method, the base resin powder is heat-treated in the presence of a surface crosslinking agent, and superabsorbent polymer particles can be formed by surface crosslinking. Surface crosslinking in the presence of the surface crosslinking agent induces a crosslinking reaction on the surface of the base resin powder, and a surface modified layer (surface crosslinked layer) can be formed on the surface of the base resin powder by surface crosslinking.

[0148] The content of the surface crosslinking agent can be appropriately selected according to the type of surface crosslinking agent added or the reaction conditions, and can be used in an amount of about 0.001 to about 5 parts by weight based on 100 parts by weight of the base resin. When the content of the surface crosslinking agent is too low, the surface modification may not be properly performed, and the physical properties of the final polymer may deteriorate. Conversely, when an excessive amount of surface crosslinking agent is used, the basic absorbency of the polymer may be significantly deteriorated due to excessive surface crosslinking reaction, which is not preferred.

[0149] In addition, the method for mixing the surface crosslinking agent with the base resin is not particularly limited. For example, a method of adding the surface crosslinking agent and the base resin to a reactor and mixing them, a method of spraying the surface crosslinking agent on 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.

[0150] When adding a surface crosslinking agent, water can be mixed together and added in the form of a surface crosslinking solution. When water is added thereto, the advantage is that the surface crosslinking agent can be evenly dispersed in the polymer. At this time, in order to make the surface crosslinking agent uniformly dispersed, prevent the agglomeration 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, water can preferably be added in an amount of about 1 to about 10 parts by weight.

[0151] In addition, the surface crosslinking step may be performed by further using at least one selected from the group consisting of polyvalent metal salts such as aluminum salts, more specifically, sulfates, potassium salts, ammonium salts, sodium salts and aluminum hydrochlorides, in addition to the surface crosslinking agent.

[0152] The use of a polyvalent metal salt can further improve the permeability of the superabsorbent polymer prepared by the method of the embodiment. The polyvalent metal salt can be added to the surface crosslinking solution together with the surface crosslinking agent and can be used in an amount of 0.01 to 4 parts by weight based on 100 parts by weight of the base resin powder.

[0153] At the same time, a surface crosslinking solution containing water and / or a hydrophilic organic solvent (e.g., an alcohol-based polar organic solvent such as methanol) can be used as a liquid medium together with the above-mentioned surface crosslinking agent to carry out the surface crosslinking process. At this time, the amount of water and hydrophilic organic solvent added based on 100 parts by weight of the base resin powder can be appropriately controlled to induce uniform dispersion of the surface crosslinking agent, prevent agglomeration of the base resin powder, and optimize the surface penetration depth of the surface crosslinking agent.

[0154] The method for adding the surface crosslinking solution to the base resin powder is not particularly limited. For example, a method of adding the surface crosslinking solution and the base resin powder in a reactor for mixing, a method of spraying the surface crosslinking solution onto the base resin powder, or a method of mixing the base resin powder and the surface crosslinking solution while continuously supplying them to a continuously operated mixer can be used.

[0155] Specifically, surface crosslinking can be performed by heating the base resin powder to which the surface crosslinking solution is added to increase its temperature from an initial temperature of 20°C to 130°C to a maximum temperature of 140°C to 200°C over 10 to 30 minutes, and then performing a heat treatment while maintaining the maximum temperature for 5 to 60 minutes. More specifically, surface crosslinking can be performed by heat treatment while maintaining a maximum temperature of 140°C to 200°C, or 170°C to 195°C, for 5 to 60 minutes, or 10 to 50 minutes.

[0156] When surface cross-linking conditions (particularly temperature elevation conditions and reaction conditions at the maximum reaction temperature) are satisfied, a superabsorbent polymer that appropriately satisfies the physical properties of the embodiment can be more efficiently produced.

[0157] The heating means for the surface cross-linking reaction is not particularly limited. A heat medium can be provided or a heat source can be provided directly. In this case, the heat medium can be a heated fluid such as steam, hot air, hot oil, etc., but the present invention is not limited thereto. In addition, the temperature of the heat medium provided can be appropriately selected taking into account the type of heat medium, the heating rate, and the target temperature of the heating. At the same time, an electric heater or a gas heater can be used as a directly provided heat source, but the present invention is not limited thereto.

[0158] After forming the surface cross-linked layer on the surface of the base resin as described above, an inorganic material may be further mixed.

[0159] The inorganic material may be, for example, at least one selected from silica, clay, alumina, a silica-alumina composite, and titania, and is preferably silica.

[0160] The inorganic material may 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.

[0161] The superabsorbent polymer obtained according to the above-mentioned preparation method maintains excellent absorption properties such as water retention capacity and pressure absorption rate, and satisfies improved absorption rate, etc. Therefore, it can meet various physical properties of the embodiment and can be suitably used as sanitary products such as diapers, in particular, ultra-thin sanitary products with reduced pulp content.

[0162] Example

[0163] <Preparation Example>

[0164] Aqueous dispersions of hydrophobic particles used in the following examples were prepared as follows.

[0165] Preparation Example 1: Preparation of Calcium Stearate Aqueous Dispersion (Ca-st)

[0166] First, 50g of water containing two or more surfactants (including polyoxyethylene alkyl ether type nonionic surfactants and sulfate type anionic surfactants) is added to a high shear mixer and heated to 165°C. 50g of calcium stearate powder is subsequently added. The mixture is then stirred at 4000rpm for 30 minutes under normal pressure to fully crush the calcium stearate to obtain an aqueous dispersion of calcium stearate having an average particle size of 5 μm and 50% by weight dispersed therein. At this point, the pH of the aqueous dispersion is 9.5. In addition, the average particle size (D50) of calcium stearate is measured / calculated as the particle size at 50% of the cumulative distribution of the number of particles using a laser diffraction particle size measuring device (Microtrac S3500).

[0167] Preparation Example 2: Preparation of aqueous zinc stearate dispersion (Zn-st)

[0168] First, 50g of water containing two or more surfactants (including polyoxyethylene alkyl ether type and fatty acid ester type nonionic surfactants and sulfate type anionic surfactants) is added to a high shear mixer and heated to 140 ° C, followed by the addition of 50g of zinc stearate powder. Then, the mixture was stirred at 4000rpm for 30 minutes under normal pressure so that zinc stearate can be fully crushed to obtain an aqueous dispersion Zn-st of zinc stearate having an average particle size of 0.1 μm and disperseed therein 50% by weight. At this point, the pH of the aqueous dispersion was 9.5. The average particle size (D50) of zinc stearate was measured / calculated using the same method as in Preparation Example 1.

[0169] Preparation Example 3: Preparation of an aqueous dispersion of hydrophobic silica

[0170] First, 100 g of water was added to a high shear mixer, and then hydrophobic silica having an average particle size of 0.3 μm and a water contact angle of 130° and hydrophobic silica having an average particle size of 3 μm and a water contact angle of 130° were slowly added thereto while stirring at 5000 rpm, so that they were dispersed in an amount of 0.2 wt % and 2 wt % relative to the total weight of the final aqueous dispersion, respectively. When the silica was completely added, the mixture was stirred at 8000 rpm for 30 minutes at a temperature of 45°C. At this point, the pH of the aqueous dispersion was 9, and the average particle size (D50) of the hydrophobic silica was measured / calculated in the same manner as in Preparation Example 1.

[0171] Preparation Example 4: Preparation of an aqueous dispersion of hydrophobic silica

[0172] An aqueous dispersion of hydrophobic silica was obtained in the same manner as in Preparation Example 3, except that hydrophobic silica having an average particle size of 0.3 μm and a water contact angle of 130° and hydrophobic silica having an average particle size of 3 μm and a water contact angle of 130° were added so as to be dispersed in amounts of 0.2 wt % and 4 wt %, respectively, relative to the total weight of the final aqueous dispersion.

[0173] Preparation Example 5: Preparation of an aqueous dispersion of hydrophobic silica

[0174] An aqueous dispersion of hydrophobic silica was obtained in the same manner as in Preparation Example 3, except that only hydrophobic silica having an average particle size of 0.3 μm and a water contact angle of 130° was added to be dispersed in an amount of 0.2 wt % relative to the total weight of the final aqueous dispersion.

[0175] <Example>

[0176] Example 1

[0177] A monomer solution was prepared by mixing 100 parts by weight of acrylic acid, 0.27 parts by weight of ethylene glycol diglycidyl ether, and 0.1 parts by weight of diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide as a photoinitiator. Subsequently, while the monomer solution was continuously supplied with a metering pump, 160 parts by weight of a 24% by weight aqueous sodium hydroxide solution was continuously added, followed by 6 parts by weight of a 4% by weight aqueous sodium persulfate solution, 5 parts by weight of a 4% by weight aqueous sodium bicarbonate solution, and 5 parts by weight of an aqueous dispersion of hydrophobic silica prepared in Preparation Example 3 (containing 0.2% by weight of hydrophobic silica with an average particle size of 0.3 μm and 2% by weight of hydrophobic silica with an average particle size of 3 μm) in a high-speed line manner to form a monomer composition. The monomer composition was introduced into a polymerization reactor equipped with a moving conveyor belt by transfer and UV polymerization was performed by irradiating ultraviolet rays using a UV irradiation device for 3 minutes to prepare a sheet-like hydrogel polymer.

[0178] The hydrogel polymer was cut into pieces with an average size of approximately 300 mm or less and placed in a grinder (equipped with a porous plate containing multiple holes with a diameter of 10 mm) to cut the hydrogel. The pulverized hydrogel was then dried in an oven with variable airflow. The hydrogel was uniformly dried by flowing 180°C hot air, resulting in a water content of approximately 2% or less after drying. The dried polymer was pulverized in a grinder and then classified to produce a base resin with a diameter of 150 to 850 μm.

[0179] Next, 6 parts by weight of a surface crosslinking aqueous solution containing 3 parts by weight of ethylene carbonate was sprayed onto 100 parts by weight of the prepared base resin powder and stirred at room temperature to uniformly distribute the surface crosslinking solution throughout the base resin powder. The base resin powder mixed with the surface crosslinking solution was then placed in a surface crosslinking reactor to undergo a surface crosslinking reaction.

[0180] In this surface crosslinking reactor, the base resin powder was gradually heated from an initial temperature of approximately 80°C, reaching a maximum reaction temperature of 190°C after 30 minutes. After reaching the maximum reaction temperature, the reaction was continued for an additional 15 minutes, and a sample of the final superabsorbent polymer was removed. Following the surface crosslinking process, the superabsorbent polymer of Example 1 was prepared by classification using an ASTM standard sieve to obtain a particle size of 150 to 850 μm.

[0181] Example 2

[0182] A superabsorbent polymer was prepared in the same manner as in Example 1, except that a bubble generator (OB-750S, manufactured by O2 bubbles) was used instead of a carbonate foaming agent to generate bubbles with a size of 10 μm to 300 μm for 5 minutes and injected into the monomer composition solution in Example 1; and 5 parts by weight of an aqueous dispersion of hydrophobic silica prepared in Preparation Example 4 (containing 0.2% by weight of hydrophobic silica with an average particle size of 0.3 μm and 4% by weight of silica with an average particle size of 3 μm) was used instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3.

[0183] Example 3

[0184] A superabsorbent polymer was prepared in the same manner as in Example 1, except that the aqueous dispersion of hydrophobic silica prepared in Preparation Example 5 and the aqueous dispersion of calcium stearate prepared in Preparation Example 1 were used instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3, thereby adding 0.01 parts by weight of hydrophobic silica and 0.1 parts by weight of calcium stearate based on 100 g of acrylic acid.

[0185] Example 4

[0186] A superabsorbent polymer was prepared in the same manner as in Example 1, except that the aqueous dispersion of zinc stearate prepared in Preparation Example 2 and the aqueous dispersion of calcium stearate prepared in Preparation Example 1 were used instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3 in Example 1, thereby adding 0.01 parts by weight of zinc stearate and 0.1 parts by weight of calcium stearate based on 100 g of acrylic acid.

[0187] Comparative Example 1

[0188] A superabsorbent polymer was prepared in the same manner as in Example 1, except that neither a carbonate-based foaming agent nor an aqueous dispersion of hydrophobic particles was used in Example 1; the hydrogel was cut into an average size of about 300 mm or less and placed in a pulverizer (provided with a porous plate including a plurality of holes with a diameter of 8 mm), and then pulverized under various conditions.

[0189] Comparative Example 2

[0190] A super absorbent polymer was prepared in the same manner as in Example 1, except that 1 part by weight of a 3 wt % sodium dodecyl sulfate (SDS) solution was added instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3.

[0191] Comparative Example 3

[0192] A superabsorbent polymer was prepared in the same manner as in Example 1, except that 7 parts by weight of a 3 wt % aqueous dispersion of hydrophilic silica (Klebosol 20, manufactured by Merck) was added instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3.

[0193] Comparative Example 4

[0194] A superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.15 parts by weight of hydrophobic fumed silica in powder form ( DM-30S, manufactured by Tokuyama) was used instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3.

[0195] Comparative Example 5

[0196] A superabsorbent polymer was prepared in the same manner as in Example 1, except that the aqueous dispersion of hydrophobic silica prepared in Preparation Example 5 (in which only hydrophobic silica having an average particle size of 0.3 μm was dispersed) was added instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3. At this time, the added weight of the hydrophobic silica of Comparative Example 5 was the same as the total weight of the two types of hydrophobic particles of Example 1.

[0197] Comparative Example 6

[0198] A superabsorbent polymer was prepared in the same manner as in Example 1, except that only the aqueous dispersion of calcium stearate prepared in Preparation Example 1 was added instead of the aqueous dispersion of hydrophobic silica prepared in Preparation Example 3. At this time, the added weight of calcium stearate of Comparative Example 6 was the same as the total weight of the two types of hydrophobic particles of Example 1.

[0199] Experimental Example 1: Measurement of pore size and particle count of superabsorbent polymer

[0200] To confirm the pore structure of the superabsorbent polymers prepared in the Examples and Comparative Examples, a scanning electron microscope (SEM) was used to capture images of the surface and interior of the superabsorbent polymer particles. The measured images were then used to calculate the number of pores with a measurable maximum diameter for each superabsorbent polymer particle. The average and maximum diameters of the pores were calculated based on the maximum diameter of each of the more than 300 pores measured. Furthermore, the average number of pores per particle was calculated based on the number of pores with a measured maximum diameter. Furthermore, these pores were classified into those with a maximum diameter greater than or equal to the average diameter and those less than or equal to the average diameter, and the number of pores with a diameter greater than or equal to the average diameter and the number of particles forming these pores were measured. From this, the proportion of particles with pores greater than or equal to the average diameter and less than or equal to the maximum diameter was calculated.

[0201] At this time, the images of Example 3 and Comparative Example 2 in the measured SEM images are shown in FIG. Figure 1 and 2 middle.

[0202] refer to Figure 1 and 2 Unlike the superabsorbent polymer of Comparative Example 2, the superabsorbent polymer of Example 3 was found to have uniformly formed numerous pores having an average diameter of 1 to 100 μm and a maximum diameter of 280 to 300 μm. Furthermore, the superabsorbent polymer of Example 3 was found to have an average of 7 or more pores per particle, for example, 7 to 30 pores per particle. Furthermore, in the superabsorbent polymer of Example 3, the number of particles having pores having a diameter greater than or equal to the average diameter and less than or equal to the maximum diameter accounted for approximately 20% to 40% of the total number of superabsorbent polymer particles.

[0203] In addition, although not shown in the present disclosure, it was confirmed that the superabsorbent polymers of Comparative Examples 3 and 4 did not form pores on the surface.

[0204] Experimental Example 2: Measurement of physical properties of superabsorbent polymers

[0205] The physical properties of the superabsorbent polymers prepared in the Examples and Comparative Examples were evaluated as follows 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 used as the measured data. The physiological saline or saline solution used to evaluate the following physical properties refers to a 0.9 wt% sodium chloride (NaCl) aqueous solution.

[0206] (1) Centrifugal retention capacity (CRC)

[0207] The centrifugal retention capacity (absorption ratio) under non-load conditions was measured according to EDANA (European Disposables and Nonwovens Association) WSP 241.3. After a superabsorbent polymer of W0 (g, about 0.2g) was uniformly inserted into a nonwoven envelope and sealed, it was immersed in saline (0.9 wt % sodium chloride aqueous solution) at room temperature. After 30 minutes, the envelope was centrifuged at 250G for 3 minutes to drain, and the weight W2 (g) of the envelope was measured. In addition, after performing the same operation without using superabsorbent polymer, the weight W1 (g) of the envelope was measured. The weight value obtained was then used to calculate CRC (g / g) according to the following equation 1.

[0208] [Equation 1]

[0209] CRC(g / g)={[W2(g)-W1(g)-W0(g)] / W0(g)}

[0210] (2) Absorption under pressure (AUP)

[0211] Absorbency under pressure (AUP) was measured according to EDANA (European Disposables and Nonwovens Association) WSP 242.3.

[0212] First, a 400-mesh stainless steel sieve was mounted on the bottom of a plastic cylinder with an inner diameter of 60 mm. At a temperature of 23 ± 2°C and a relative humidity of 45%, W0 (g, 0.90 g) of the superabsorbent polymer prepared in Examples 1 to 6 and Comparative Examples 1 to 4 was evenly dispersed on the sieve. A piston capable of uniformly applying a load of 4.83 kPa (0.7 psi) was then placed on top. The piston had an outer diameter slightly smaller than 60 mm, leaving no gap with the inner wall of the cylinder, and the cylinder's fixture was inching without interruption. At this point, the weight of the device was measured, W3 (g).

[0213] Subsequently, a glass filter with a diameter of 125 mm and a thickness of 5 mm was placed in a culture dish with a diameter of 150 mm, and salt water (0.9 wt% sodium chloride) was poured into the culture dish. At this time, the salt water was poured until the surface level of the salt water was equal to the upper surface of the glass filter. A 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 the weight W4 (g) was measured.

[0214] Then, AUP (g / g) was calculated according to the following Equation 2 using the obtained weight value.

[0215] [Equation 2]

[0216] AUP(g / g)=[W4(g)–W3(g)] / W0(g)

[0217] In equation 2,

[0218] W0 (g) is the initial weight of the superabsorbent polymer (g),

[0219] W3 (g) is the sum of the weight of the superabsorbent polymer and the weight of the means for providing a load for the superabsorbent polymer, and

[0220] W4 (g) is the sum of the weight of the superabsorbent polymer and the weight of the device providing the load to the superabsorbent polymer after the superabsorbent polymer has been allowed to absorb saline under a load (0.7 psi) for 1 hour.

[0221] (3) Vortex time (absorption rate)

[0222] The vortex time (absorption rate) of the superabsorbent polymers of Examples and Comparative Examples was measured in the following manner.

[0223] ① First, use a 100mL graduated cylinder to add 50mL of 0.9% saline into a 100mL flat-bottom beaker.

[0224] ② Next, place the beaker in the center of the magnetic stirrer and then place a round magnetic bar (30 mm in diameter) into the beaker.

[0225] ③ Then, the stirrer was operated so that the magnetic bar stirred at 600 rpm and the lowermost portion of the vortex generated by the stirring reached the top of the magnetic bar.

[0226] ④ After confirming that the temperature of the physiological saline in the beaker has reached 24.0°C, add 2 ± 0.01g of the superabsorbent polymer sample while starting a stopwatch. Then, measure the time in seconds it takes for the vortex to disappear and the liquid surface to become completely horizontal, and use this as the vortex time.

[0227] (4) Surface tension (S / T)

[0228] The surface tension of the superabsorbent polymers of Examples and Comparative Examples was measured as follows.

[0229] ① First, 40 g of saline solution composed of 0.9 wt% sodium chloride was placed in a 50 mL beaker, a round magnetic bar (30 mm in diameter) was placed therein, and stirred at 350 rpm for 3 minutes.

[0230] ② Subsequently, 0.5 g of super absorbent polymer was added to the stirred solution, stirred for another 3 minutes, and then allowed to stand for 2 minutes to allow the swollen super absorbent polymer to settle to the bottom.

[0231] ③ Thereafter, the supernatant (the solution just below the surface) was extracted with a pipette and transferred to another clean cup, and the surface tension was measured using a surface tensiometer (Force Tensiometer-K11 / K100, manufactured by Kruss).

[0232]

Table 1

[0233]

[0234] As shown in Table 1, unlike the superabsorbent polymers of the comparative examples, the superabsorbent polymers of the examples, in which the polymerization reaction of the monomers was carried out in the presence of an aqueous dispersion containing two types of hydrophobic particles, demonstrated a pore structure comprising an average of seven or more pores per particle, with the average pore diameter being 100 μm or less and the maximum diameter being 300 μm or less. Furthermore, the proportion of particles having pores with a diameter greater than or equal to the average diameter was approximately 10% to 50%, and micropores with a diameter close to the average diameter were demonstrated to be very uniformly formed throughout the majority of the remaining particles.

[0235] In addition, compared with the superabsorbent polymer of the comparative example, it was confirmed that the superabsorbent polymer of the example showed surface tension above a certain level and a fast absorption rate without deterioration of absorption performance.

[0236] Specifically, the super absorbent polymers of Examples exhibited significantly improved absorption rates compared to Comparative Example 1, which did not use a foaming agent and a foam stabilizer. Furthermore, it can be seen that the super absorbent polymers of Examples had faster absorption rates and higher surface tension than Comparative Example 2, which used sodium lauryl sulfate (SDS), which is primarily used as a conventional foam stabilizer.

[0237] In addition, compared with the superabsorbent polymer of Comparative Example 5 using only an aqueous dispersion of hydrophobic silica having an average particle size of less than 1 μm and the superabsorbent polymer of Comparative Example 6 using only an aqueous dispersion of hydrophobic silica having an average particle size of greater than 1 μm, it was confirmed that the superabsorbent polymer of the Examples had a developed porous structure in which a large number of micropores having uniform diameters were formed, thereby exhibiting a rapid absorption rate.

[0238] In particular, in the superabsorbent polymer of Comparative Example 3 using an aqueous dispersion of hydrophilic silica and the superabsorbent polymer of Comparative Example 4 using hydrophobic silica in powder form, it was confirmed that the absorption rate could not be improved due to failure to form a pore structure on the surface.

Claims

1. A superabsorbent polymer comprising: a powdered base resin comprising a crosslinked polymer of an acrylic monomer having at least partially neutralized acidic groups and an internal crosslinking agent; and a surface crosslinked layer formed by further crosslinking the crosslinked polymer using a surface crosslinking agent on the powdered base resin, in, The superabsorbent polymer comprises an aqueous dispersion of a first hydrophobic particle and an aqueous dispersion of a second hydrophobic particle, The average particle size of the first hydrophobic particles is less than 1 μm, and the average particle size of the second hydrophobic particles is greater than 1 μm. The ratio of the average particle size of the second hydrophobic particles to the average particle size of the first hydrophobic particles is 5 to 100, and The first hydrophobic particles and the second hydrophobic particles are contained in a weight ratio of 1:5 to 1:50, wherein each superabsorbent polymer particle has an average of more than 7 pores, the average diameter of the pores is 100 μm or less and the maximum diameter is 300 μm or less, and the number of particles with pores having a diameter greater than or equal to the average diameter and less than or equal to the maximum diameter accounts for 10% to 50% of the total number of superabsorbent polymer particles, and The super absorbent polymer has a surface tension of 65 mN / m or more and a vortex time at 24.0° C. of 40 seconds or less.

2. The superabsorbent polymer according to claim 1, in, The first hydrophobic particles and the second hydrophobic particles are each independently selected from the group consisting of hydrophobic silica, metal salts of C7 to C24 fatty acids, and hydrophobic organic particles.

3. The superabsorbent polymer according to claim 2, in, The metal salt of fatty acid is at least one metal stearate selected from the group consisting of calcium stearate, magnesium stearate, sodium stearate, zinc stearate and potassium stearate.

4. The superabsorbent polymer according to claim 1, in, The superabsorbent polymer has an average of 7 to 30 pores per particle.

5. The superabsorbent polymer according to claim 1, in, The surface cross-linking agent includes at least one selected from the group consisting of a polyol-based compound, a polyepoxy compound, a polyamine compound, a halogenated epoxy compound, a condensation product of a halogenated epoxy compound, an oxazoline-based compound, and an alkylene carbonate-based compound.

6. The superabsorbent polymer according to claim 1, in, The superabsorbent polymer has a centrifuge retention capacity (CRC) greater than 27 g / g as measured according to EDANA WSP 241.3 and an absorbency under pressure (AUP) greater than 20 g / g at 0.7 psi as measured according to EDANA WSP 242.

3.

7. A method for preparing a superabsorbent polymer, comprising the steps of: Step 1: preparing a monomer composition comprising: an acrylic monomer having at least partially neutralized acidic groups, a polymerization initiator, an internal crosslinking agent, and an aqueous dispersion of hydrophobic particles; Step 2: preparing a hydrogel polymer by cross-linking and polymerizing the monomer composition in the presence of a foaming agent or a bubble generating agent; Step 3: Drying and pulverizing the hydrogel polymer to form a powdered base resin; and Step 4: forming a surface crosslinked layer 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 first hydrophobic particles and second hydrophobic particles are dispersed, and the ratio of the average particle size of the second hydrophobic particles to the average particle size of the first hydrophobic particles is 5 to 100, the average particle size of the first hydrophobic particles is less than 1 μm, and the average particle size of the second hydrophobic particles is greater than 1 μm, and the first hydrophobic particles and the second hydrophobic particles are contained in a weight ratio of 1:5 to 1:

50.

8. The method for preparing a superabsorbent polymer according to claim 7, in, Each of the first and second hydrophobic particles is used in an amount of 50 ppmw or more and 1 part by weight or less, based on 100 parts by weight of the acrylic monomer.

9. The method for preparing a superabsorbent polymer according to claim 7, in, The total weight of the first hydrophobic particles and the second hydrophobic particles is 0.01 to 2 parts by weight based on 100 parts by weight of the acrylic monomer.

10. The method for preparing a superabsorbent polymer according to claim 7, in, The first and second hydrophobic particles are dispersed in an aqueous dispersion of the hydrophobic particles in the presence of at least a nonionic surfactant and an anionic surfactant.

11. The method for preparing a superabsorbent polymer according to claim 7, in, The surface cross-linking agent includes at least one selected from the group consisting of a polyol-based compound, a polyepoxy compound, a polyamine compound, a halogenated epoxy compound, a condensation product of a halogenated epoxy compound, an oxazoline-based compound, and an alkylene carbonate-based compound.

12. The method for preparing a superabsorbent polymer according to claim 7, in, The surface crosslinking is performed by increasing the temperature from an initial temperature of 20 to 130° C. to a maximum temperature of 140 to 200° C. over 10 to 30 minutes, and then performing heat treatment while maintaining the maximum temperature for 5 to 60 minutes.

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