Superabsorbent polymers and methods for their preparation

Abstract

The present disclosure relates to a superabsorbent polymer and a method for preparing the same. More particularly, the present disclosure relates to a superabsorbent polymer having a particle aspect ratio higher than a certain level and a large roughness, thereby exhibiting an improved absorption rate, and to a method for preparing the superabsorbent polymer.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2020-0178431 filed on December 18, 2020, and Korean Patent Application No. 10-2021-0180490 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 a superabsorbent polymer and a method for preparing the same. More particularly, the present disclosure relates to a superabsorbent polymer having excellent absorption rate and absorption performance and a high aspect ratio, and to a method for preparing the superabsorbent polymer. 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] In sanitary products such as pulp-free diapers with reduced or no pulp, superabsorbent polymers should not only act as absorbents 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 particle aspect ratio above a certain level and a large roughness, thereby exhibiting an improved absorption rate, and to a method for preparing the polymer.

[0011] Technical Solution

[0012] In order to solve the above problems, a superabsorbent polymer is provided, which comprises:

[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 cross-linked layer formed by further cross-linking the cross-linked polymer using a surface cross-linking agent on a base resin,

[0015] The superabsorbent polymer satisfies the following properties:

[0016] i) the average value of the aspect ratio (A / R), which is the ratio of the shortest diameter of the particles to the longest diameter of the particles measured for particles having a particle diameter of 300 μm to 600 μm, is 0.75 or more, and the average value of the convexity calculated by the following formula 1 is 0.9 or less, and

[0017] ii) Vortex time at 24.0°C is 60 seconds or less:

[0018] [Equation 1]

[0019] Convexity = convex hull perimeter / actual particle perimeter

[0020] In Equation 1,

[0021] When it is assumed that the image obtained by capturing the 3D image of the measured 3D particle as a 2D image is surrounded by an imaginary elastic band extending around the contour, the convex hull perimeter is the length of the elastic band, and

[0022] The actual particle circumference is the actual circumference of an image obtained by capturing a 3D image of the measured 3D particle as a 2D image.

[0023] In addition, a method for preparing a superabsorbent polymer is provided, the method comprising the following steps:

[0024] preparing a monomer composition comprising an acrylic monomer having at least partially neutralized acidic groups and an internal crosslinker (step 1);

[0025] preparing a hydrogel polymer by cross-linking and polymerizing a monomer composition in the presence of an aqueous dispersion of hydrophobic particles and a carbonate-based foaming agent (step 2);

[0026] forming a powdered base resin by drying and pulverizing the hydrogel polymer (step 3); and

[0027] After preparing a super absorbent polymer by further cross-linking the surface of the base resin in the presence of a surface cross-linking agent to form a surface cross-linked layer (step 4),

[0028] wherein the aqueous dispersion of hydrophobic particles is a colloidal solution in which hydrophobic particles having an average particle size of 0.2 μm to 50 μm are dispersed, and

[0029] The prepared superabsorbent polymer meets the following properties:

[0030] i) the average value of the aspect ratio (A / R), which is the ratio of the shortest diameter of the particles to the longest diameter of the particles measured for particles having a particle diameter of 300 μm to 600 μm, is 0.75 or more, and the average value of the convexity calculated by the following formula 1 is 0.9 or less, and

[0031] ii) Vortex time at 24.0°C is 60 seconds or less.

[0032] Beneficial effects

[0033] The superabsorbent polymer of the present invention has an average particle aspect ratio of 0.75 or greater, thereby having a shape close to a spherical shape, while having a large surface roughness, and is thus composed of particles with an average convexity of 0.9 or less. Therefore, an improved absorption rate can be achieved. In addition, the superabsorbent polymer can be prepared by introducing hydrophobic particles with a predetermined average particle size in the form of an aqueous dispersion, and the use of hydrophobic particles of a specific size dispersed in water can effectively capture the generated carbon dioxide. Therefore, small and uniform pores are formed on the surface of the superabsorbent polymer, which can increase the specific surface area, thereby improving the absorption rate of the prepared superabsorbent polymer. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a SEM image of the superabsorbent polymer prepared in Example 1.

[0035] Figure 2 is a SEM image of the superabsorbent polymer prepared in Comparative Example 1.

[0036] Figure 3 is a SEM image of the superabsorbent polymer prepared in Comparative Example 2.

[0037] Figure 4 Shown are the sample dispersion unit settings in the Malvern Panalytic morphologi 4.

[0038] Figure 5 Shown are the lighting settings in Malvern Panalytic's morphologi 4.

[0039] Figure 6 Shown are the optics selection settings in Malvern Panalytic's morphologi 4.

[0040] Figure 7 The scan area settings in Malvern Panalytic's morphologi 4 are shown.

[0041] Figure 8 Particle filtration settings in the Malvern Panalytic morphologi 4 are shown.

[0042] Figures 4 to 8 This is a computer screenshot and the technical information contained therein is as follows.

[0043] Figure 4 The Chinese meaning of the English text reflecting technical information in the computer screenshot:

[0044] Sample Dispersion Unit = Sample Dispersion Unit

[0045] Use SDU=Use SDU

[0046] Default Dispersion Settings=Default Dispersion Settings

[0047] Low Energy

[0048] High Energy=High Energy

[0049] Injection Pressure (bar) = Injection Pressure (bar)

[0050] Injection Time (ms) = Injection Time (ms)

[0051] Setting Time (sec) = Setting Time (sec)

[0052] Injection Volume Indicator

[0053] Figure 5 The Chinese meaning of the English text reflecting technical information in the computer screenshot:

[0054] Illumination Settings

[0055] Light Source

[0056] bottom light

[0057] top light

[0058] Bright field

[0059] Dark field (Manual light control only)

[0060] Polarizer=Polarizer

[0061] Light options

[0062] Automatic light calibration=Automatic light calibration

[0063] Calibration intensity

[0064] Intensity tolerance

[0065] Manual light control (advanced)

[0066] Light calibration over sample

[0067] Recommended for non-transparent substrates=Recommended for non-transparent substrates

[0068] Figure 6 The Chinese meaning of the English text reflecting technical information in the computer screenshot:

[0069] Optics Selection=Optics Selection

[0070] Select the optics based on your particle size range

[0071] Figure 7 The Chinese meaning of the English text reflecting technical information in the computer screenshot:

[0072] Scan Areas=Scan Areas

[0073] Select areas to analyse

[0074] Drag rectangles to define new areas or drag the edge of an existing area to change the size and / or position

[0075] Selected analysis area=Selected analysis area

[0076] Circular=Circular

[0077] Radius

[0078] Refine position before measurement=Refine position before measurement

[0079] Delete

[0080] Reset to default=Reset to default

[0081] Time estimate=Time estimate

[0082] Est.total time = Estimated total time (min)

[0083] Est.time this optic=Estimated time this optic

[0084] Figure 8 The Chinese meaning of the English text reflecting technical information in the computer screenshot:

[0085] Particle Filtering

[0086] Filters=Filters

[0087] Parameter=Parameter

[0088] Operator

[0089] Value

[0090] Solidity

[0091] Diameter=diameter

[0092] Add Constraint=Add constraint

[0093] Relative

[0094] Use filtering to remove contaminants from the analysis=Use filtering to remove contaminants from the analysis

[0095] Particles matching the filter criteria will be ignored. The particles are still stored and can be viewed using the particle view. DETAILED DESCRIPTION

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] In addition, the term "average diameter" of pores refers to the median of the longest diameter values of each pore in the plurality of pores included in the superabsorbent polymer. This is to make it less affected by abnormal values than a simple average value.

[0103] In order to produce superabsorbent polymers with a fast absorption rate, it is necessary to increase the specific surface area of superabsorbent polymer particles. Therefore, in order 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 particular, the size of the foam formed varies depending on how the foaming mechanism is controlled, thereby changing the shape of the final particles after crushing. In order to increase the specific surface area, it is advantageous to have a small aspect ratio at the same particle size. However, when the aspect ratio becomes less than a certain level, the absorption rate increases, but the balance of other properties (such as water retention capacity and pressure absorption rate) deteriorates. Therefore, it is necessary to implement a foaming mechanism that can increase the specific surface area of the particles while making the aspect ratio of the particles reach above a certain level.

[0104] Therefore, the present inventors have confirmed that when the particles of superabsorbent polymer are close to spherical, that is, have an aspect ratio of a certain level or higher, and at the same time, the roughness of the particle surface is increased by forming pores with an effective size on the particle surface to increase the specific surface area, the absorption rate and absorption performance of the superabsorbent polymer can be improved at the same time, thereby completing the present invention.

[0105] More specifically, as a result of efforts to quantify the shape of superabsorbent polymer particles, which influences absorption rate and absorption performance, the aspect ratio has been identified as a parameter for determining whether superabsorbent polymer particles have a spherical shape, while the convexity has been identified as a parameter for determining the surface roughness of superabsorbent polymer particles. Furthermore, it has been confirmed that when the aspect ratio has a value greater than or equal to a certain level and the convexity has a value less than or equal to a certain level, the superabsorbent polymer can exhibit a fast absorption rate and improve the balance between water retention capacity and pressure absorption rate.

[0106] This superabsorbent polymer can be produced by using an aqueous dispersion containing hydrophobic particles as a foam stabilizer instead of a conventional foam stabilizer. Specifically, when hydrophobic particles with an average particle size of 0.2 to 50 μm in the form of an aqueous dispersion are added during the monomer polymerization step, the hydrophobic particles dispersed in the water act as seeds that effectively capture bubbles, eliminating the need for a separate foam stabilizer. As a result, a superabsorbent polymer with a porous structure in which small, uniformly shaped pores are evenly distributed throughout the cross-linked polymer can be produced.

[0107] 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.

[0108] In addition, the hydrophobic particles can be stably dispersed in the aqueous dispersion by a dispersion stabilizer (such as a surfactant) to be described later, without agglomeration between the particles. Specifically, the dispersion stabilizer can form a double electric layer on the surface of the hydrophobic particles to generate an electrostatic repulsion force between the particles, which can stabilize the hydrophobic particles, or the surfactant can be adsorbed on the surface of the hydrophobic particles to generate a spatial repulsion force between the particles, which can prevent the particles from agglomerating with each other. On the other hand, when the dispersion stabilizer is not included in the aqueous dispersion of the hydrophobic particles, the phenomenon of the hydrophobic particles agglomerating or sinking due to gravity can be caused, so that the dispersion of the hydrophobic particles cannot be stabilized. Therefore, when the aqueous dispersion of the 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.

[0109] Meanwhile, each parameter representing the shape of super absorbent polymer particles according to an embodiment will be described below.

[0110] First, the aspect ratio is a parameter used to determine whether a particle has regular symmetry. It is the ratio of the shortest diameter of a particle to its longest diameter and can be calculated as "shortest diameter of the particle / longest diameter of the particle." In this case, the longest diameter of the particle refers to the major axis passing through the center of the particle, while the shortest diameter of the particle refers to the minor axis passing through the center of the particle and perpendicular to the major axis.

[0111] Thus, when the aspect ratio is close to 1, the particles can be considered to have regular symmetry, such as spheres or cubes, while when the aspect ratio deviates from 1, the particles can be considered to have different sizes along different axes, such as needles or ellipses.

[0112] At this time, the average value of the aspect ratio is measured after being spread on the stage in an arbitrary method by the vacuum in the measuring device, and is obtained as a statistical result by securing a sample number (n) of 200 or more and averaging them.

[0113] In addition, convexity is a parameter used to measure particle profile and surface roughness, and is calculated by the following equation1:

[0114] [Equation 1]

[0115] Convexity = convex hull perimeter / actual particle perimeter

[0116] In Equation 1,

[0117] When it is assumed that the image obtained by capturing the 3D image of the measured 3D particle as a 2D image is surrounded by an imaginary elastic band extending around the contour, the convex hull perimeter is the length of the elastic band, and

[0118] The actual particle circumference is the actual circumference of an image obtained by capturing a 3D image of the measured 3D particle as a 2D image.

[0119] Thus, the value of convexity ranges from 0 to 1. When the convexity is close to 1, the particle can be considered to have a very smooth profile, while when the convexity is closer to 0, the particle can be considered to have a rough or uneven profile.

[0120] At this time, the average value of the convexity is also measured after being spread on the platform in an arbitrary way by the vacuum in the measuring device in the same manner as the average value of the aspect ratio, and is obtained as a statistical result by ensuring a sample number (n) of 200 or more and averaging them.

[0121] These parameters can be measured using various commercial instruments that quantify and analyze particle morphology using particle-based image analysis. As an example, the parameters can be measured using the morphologi 4 manufactured by Malvern Panalytics, specifically by the following four steps, which will be described in more detail in the following experimental examples.

[0122] 1) Sample preparation: A sample was prepared by classifying superabsorbent polymer particles having a particle size of 300 to 600 μm using a Retsch classifier with an amplitude of 1.0 for 1 minute. At this time, the particle size of the superabsorbent polymer can be measured according to EDANA (European Disposables and Nonwovens Association) WSP 220.3.

[0123] 2) Image capture: The prepared sample was placed on the stage of the instrument and scanned at 2.5x magnification to capture images of individual particles.

[0124] 3) Image processing: For the captured images, the parameter values of each particle are measured, such as the 3D image of the 3D particle captured as a 2D image, CE diameter (circle equivalent diameter), shortest diameter, longest diameter, actual particle circumference, and convex hull circumference.

[0125] 4) Based on the analytical data of each particle, the distribution of each parameter of all particles contained in the sample is obtained.

[0126] 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.

[0127] Superabsorbent polymers

[0128] 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.

[0129] When measuring particles with a particle size of 300 μm to 600 μm, the average value of the aspect ratio (A / R) of the above-mentioned superabsorbent polymer particles is 0.75 or greater, and the average value of the convexity calculated by Equation 1 is 0.90 or less. When the average value of the aspect ratio of the particles with a particle size of 300 μm to 600 μm in the superabsorbent polymer is less than 0.75, the particles are not spherical, and thus the absorption rate is fast, but the balance between the water retention capacity and the pressurized absorbency expressed by the following Equations 2 and 3 may be reduced. When the average value of the convexity of the particles with a particle size of 300 μm to 600 μm in the superabsorbent polymer is greater than 0.9, the particles have no pore structure or have a smooth surface, and thus the absorption rate of the superabsorbent polymer may be reduced. Therefore, when the average value of the aspect ratio of particles having a particle size of 300 μm to 600 μm in the superabsorbent polymer is 0.75 or more and simultaneously satisfies the average value of the convexity of 0.90 or less, a superabsorbent polymer having an excellent balance between absorption rate and absorption performance can be provided.

[0130] More specifically, for example, when measuring particles having a particle size of 300 μm to 600 μm, the superabsorbent polymer may have an average particle aspect ratio (A / R) of 0.75 or greater, and 0.90 or less, 0.85 or less, or 0.80 or less. Furthermore, when measuring particles having a particle size of 300 μm to 600 μm, the superabsorbent polymer may have an average convexity of 0.7 or greater, 0.8 or greater, or 0.85 or greater, and 0.9 or less.

[0131] In addition, the superabsorbent polymer has a vortex time (absorption rate) of 60 seconds or less at 24.0°C. More specifically, the vortex time can be 50 seconds or less, 48 seconds or less, 46 seconds or less, or 45 seconds or less. Furthermore, the shorter the vortex time, the better the evaluation. The lower limit is theoretically 0 seconds, but it can be 10 seconds or more, 20 seconds or more, 30 seconds or more, or 35 seconds or more. The method for measuring the vortex time of the superabsorbent polymer will be described in more detail in the following experimental examples.

[0132] Meanwhile, the acrylic monomer is a compound represented by the following Chemical Formula 1:

[0133] [Chemical Formula 1]

[0134] R 1 -COOM 1

[0135] In Chemical Formula 1,

[0136] R 1 is a C2 to C5 hydrocarbon group having an unsaturated bond, and

[0137] M 1 It is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] For example, as the internal crosslinking agent, the following multifunctional crosslinking agents may be used alone or in combination of two or more, for example, N,N'-methylenebisacrylamide, trimethylpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol or ethylene carbonate. However, the present invention is not limited thereto.

[0144] Preferably, the internal crosslinking agent can be a polyalkylene glycol (meth) acrylate compound, such as polyethylene glycol (meth) acrylate, polyethylene glycol di(meth) acrylate, polypropylene glycol (meth) diacrylate or polypropylene glycol (meth) acrylate. It is preferred to use the above internal crosslinking agent because it can easily achieve foaming by using a carbonate foaming agent to be described below.

[0145] 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.

[0146] 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.

[0147] 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.

[0148] 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.

[0149] In addition, as the polyepoxy compound, ethylene glycol diglycidyl ether, glycidol, etc. can be used.

[0150] As the polyamine compound, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, polyamidepolyamine, and the like can be used.

[0151] Furthermore, as the halogenated epoxy compound, epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and the like can be used.

[0152] In addition, as the oxazoline-based compound, mono-, di- or polyoxazolidinone and the like can be used.

[0153] In addition, as the alkylene carbonate compound, ethylene carbonate, propylene carbonate, glycerol carbonate, etc. can be used.

[0154] 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.

[0155] In addition, the superabsorbent polymer may include hydrophobic particles. 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.

[0156] Therefore, the water contact angle of the hydrophobic particles is 50° or greater. More specifically, the water contact angle of the hydrophobic particles may be 70° or greater, 100° or greater, 120° or greater, or 150° or greater, and 175° or less.

[0157] In this case, the contact angle of hydrophobic particles can be measured as follows. First, a coating solution is prepared in which hydrophobic particles are dispersed in a dichloromethane solvent at a concentration of 5 wt%. Next, the coating solution is spin-coated onto a wafer with no surface roughness and dried at room temperature to remove the remaining solvent. The contact angle is then measured by dropping water onto the coating, and this is defined as the contact angle per hydrophobic particle.

[0158] In addition, the average particle size of the hydrophobic particles is 0.2 μm to 50 μm. When the average particle size of the hydrophobic particles is less than 0.2 μm, it is difficult to effectively capture bubbles generated during the manufacturing process, and thus there is a problem of not being able to form uniform pores. When the average particle size of the hydrophobic particles is greater than 50 μm, it may be difficult to increase the absorption rate of the superabsorbent polymer because the pore size formed is too large. Specifically, for example, the average particle size (μm) of the hydrophobic particles can be 0.5 or greater, 1 or greater, 2 or greater, or 4 or greater, and 40 or less, 35 or less, or 30 or less.

[0159] 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%.

[0160] The hydrophobic particles can be selected from hydrophobic silica, C7 to C 24 At least one member selected from the group consisting of metal salts of fatty acids and hydrophobic organic particles.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] 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.

[0167] In addition, the superabsorbent polymer may have a plurality of pores, and the average diameter of the plurality of pores may be 10 μm to 400 μm. When the average diameter of the pores of the superabsorbent polymer is too small or too large, the specific surface area of the superabsorbent polymer is insufficient, and it is difficult to expect an improvement in the absorption rate. In this case, the pore structure contained in the superabsorbent polymer can be varied depending on the type of hydrophobic particles dispersed in water to capture the gas foamed by the foaming agent, which will be described later.

[0168] Specifically, when the super absorbent polymer includes a metal salt of stearic acid such as calcium stearate, magnesium stearate, sodium stearate, and potassium stearate as the hydrophobic particles, the average diameter of the plurality of pores included in the super absorbent polymer may be 60 μm to 200 μm.

[0169] In addition, when the super absorbent polymer includes hydrophobic silica as the hydrophobic particles, the average diameter of the plurality of pores included in the super absorbent polymer may be 100 μm to 400 μm.

[0170] The average diameter of the pores can be achieved by preparing the superabsorbent polymer using an aqueous dispersion of hydrophobic particles so that pores having a uniform diameter (e.g., 10 to 400 μm, 50 to 300 μm, or 150 to 220 μm) are uniformly formed on the surface of the superabsorbent polymer.

[0171] On the other hand, when the superabsorbent polymer does not use these hydrophobic particles, uses a surfactant conventionally used as a foam stabilizer, or uses hydrophobic silica in powder form, non-uniform pores larger than the above range are formed, which is not preferred.

[0172] In this case, the average diameter of the pores in the superabsorbent polymer can be confirmed by observing the internal structure of the superabsorbent polymer particles to be measured using an electron microscope (SEM; magnification: X50 or X200). More specifically, after measuring the longest diameter of each particle contained in the superabsorbent polymer particles, the median of the measured longest diameters of the particles can be obtained as the average diameter. Here, it is preferred to obtain the average diameter after measuring the diameters of 300 or more pores in one superabsorbent polymer sample.

[0173] In addition, the superabsorbent polymer may have a centrifuge retention capacity (CRC) of 27 g / g or more as measured according to EDANA WSP 241.3, and an absorbency under pressure (AUP) of 23.5 g / g or more as measured according to EDANA WSP 242.3 at 0.7 psi.

[0174] More specifically, the superabsorbent polymer may have a centrifuge retention capacity (CRC) of 27 g / g or more or 27.5 g / g or more, and 34 g / g or less or 33 g / g or less, measured according to EDANA WSP 241.3.

[0175] Additionally, the superabsorbent polymer may have an Absorption Under Pressure (AUP) at 0.7 psi of 23.5 g / g or more or 24 g / g or more, and 28 g / g or less or 27 g / g or less, as measured according to EDANA WSP 242.3.

[0176] Meanwhile, the super absorbent polymer may include particles having a particle size of about 150 μm to about 850 μm in an amount of 90 wt % or more based on the total weight, and the particle size may be measured according to EDANA (European Disposables and Nonwovens Association) WSP 220.3.

[0177] Preparation method of superabsorbent polymer

[0178] Meanwhile, a superabsorbent polymer can be prepared by a method comprising the following steps: preparing a monomer composition comprising an acrylic monomer having an at least partially neutralized acidic group and an internal crosslinking agent (step 1); preparing a hydrogel polymer by crosslinking polymerization of the monomer composition in the presence of a first aqueous dispersion of hydrophobic particles, a second aqueous dispersion of hydrophobic particles, and a foaming agent or a bubble generator (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), wherein the aqueous dispersion of hydrophobic particles is a colloidal solution in which hydrophobic particles having an average particle size of 0.2 μm to 50 μm are dispersed, and the prepared superabsorbent polymer satisfies the above-mentioned aspect ratio, convexity, and absorption rate. For details on other descriptions of the hydrophobic particles, please refer to the above.

[0179] Hereinafter, each step of the method for preparing a super absorbent polymer according to one embodiment will be described in more detail.

[0180] (Step 1)

[0181] In the preparation method of the embodiment, step 1 is to prepare a monomer composition comprising an acrylic monomer having an at least partially neutralized acidic group and an internal crosslinking agent. For details on the acrylic monomer and the internal crosslinking agent, please refer to the above.

[0182] In the monomer composition, the internal crosslinking agent may 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 may be used in an amount of 0.01 to 5 parts by weight, 0.05 to 0.1 parts by weight, or 0.45 parts by weight, and less than 5 parts by weight, less than 3 parts by weight, less than 2 parts by weight, less than 1 part by weight, or less than 0.7 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. When the amount of the internal crosslinking agent is too small, crosslinking does not occur sufficiently, making it difficult to achieve a strength higher than the appropriate strength. When the amount of the internal crosslinking agent is too large, the internal crosslink density increases, making it difficult to achieve the desired level of water retention capacity.

[0183] 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.

[0184] 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.

[0185] More specifically, any compound that can form radicals by light such as ultraviolet rays can be used as the photopolymerization initiator without limitation.

[0186] 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.

[0187] 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.

[0188] 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.

[0189] 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.

[0190] 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.

[0191] If necessary, the monomer composition may further include additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant.

[0192] 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.

[0193] 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.

[0194] At this time, any solvent that can dissolve the above components can be used without limitation. For example, the solvent can be selected from at least one of water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate and N,N-dimethylacetamide, or a combination of two or more thereof.

[0195] (Step 2)

[0196] Subsequently, a step of preparing a hydrogel polymer is performed by cross-linking and polymerizing the polymerizable monomer composition in the presence of an aqueous dispersion of hydrophobic particles and a carbonate-based foaming agent. In this step, carbon dioxide bubbles are generated by the carbonate-based foaming agent, and the hydrophobic particles dispersed in water effectively capture these bubbles, thereby increasing the specific surface area of the prepared hydrogel polymer.

[0197] In addition, the hydrophobic particles may be included in the aqueous dispersion in an amount of 10% to 70% by weight, based on the total weight of the aqueous dispersion. When the content of hydrophobic particles in the hydrophobic aqueous dispersion is too low, the attraction between the particles decreases, thereby improving dispersion stability. However, this poses the problem of requiring large amounts of hydrophobic particles to be added during the preparation of the superabsorbent polymer due to low concentrations. When the content of hydrophobic particles in the hydrophobic aqueous dispersion is too high, dispersion stability may deteriorate due to agglomeration between the particles.

[0198] At the same time, the hydrophobic particles can be uniformly dispersed in the aqueous dispersion by a dispersion stabilizer (such as a surfactant or a polymer) around the particle surface. The surfactant can form an electric double layer on the surface of the hydrophobic particles to generate an electrostatic repulsion force 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 generate a spatial repulsion force between the particles, which can prevent the particles from agglomerating with each other, thereby improving the dispersion stability of the hydrophobic particles.

[0199] For example, one or more surfactants selected from the group consisting of a cationic surfactant, anionic surfactant, amphoteric surfactant and nonionic surfactant can be used as surfactant. Preferably, two or more surfactants can be used for the dispersion stabilization of hydrophobic particles. More specifically, when the metal salt comprising the above-mentioned fatty acid is used as the hydrophobic particle, in view of its form etc., a nonionic surfactant and anionic surfactant can be used together so that the hydrophobic particle is dispersed in water more effectively and stably. For example, a nonionic surfactant in combination with a long-chain hydrocarbon of 10 or more carbon atoms and an anionic surfactant based on sulfate or based on phosphate can be used together.

[0200] For example, examples of cationic surfactants include dialkyldimethylammonium salts and alkylbenzylmethylammonium salts, examples of anionic surfactants include alkyl polyoxyethylene sulfates such as sodium fatty acid alcohol polyoxyethylene ether sulfate, monoalkyl sulfate, alkylbenzene sulfonate, monoalkyl phosphate such as sodium pyrophosphate or sodium phosphate trimer, and sulfates or sodium salts thereof having a functional group containing a long-chain hydrocarbon such as sodium lauryl ether sulfate, ammonium lauryl sulfate, sodium lauryl sulfate, sodium myristyl ether sulfate or sodium laureth sulfate, and examples of amphoteric surfactants include Alkyl sulfobetaine and alkyl carboxybetaine, examples of nonionic surfactants include styrene-maleic anhydride copolymers, polyoxyethylene-polyoxypropylene copolymers, polyoxyethylene-fatty acid ethers such as stearyl polyoxyethylene ether, polyoxyethylene alkyl ethers such as polyethylene glycol or polyoxyethylene lauryl ether, polyoxyalkylene alkylphenyl ethers, polyoxyethylene arylphenyl ethers, fatty acid esters such as polysorbate, fatty acid sorbitol esters, polyglycerol monolaurate, polyethylene glycol laurate or glycerol monostearate, alkyl monoglyceryl ethers, alkanolamides and alkyl polyglycosides. However, the present disclosure is not limited thereto.

[0201] In addition, as the polymer used as the dispersion stabilizer, polyalkylene glycol, polyethyleneimine, polyvinyl alcohol, polyacrylamide, or polyvinyl pyrrolidone can be used.

[0202] The hydrophobic particles are used in an amount of 0.001 to 5 parts by weight based on 100 parts by weight of the acrylic monomer. If the content of the aqueous dispersion of hydrophobic particles is too low, it is impossible to effectively capture generated bubbles. If the content of the aqueous dispersion of hydrophobic particles is too high, too many bubbles are captured, thereby reducing the bulk density. Specifically, the hydrophobic particles are used in an amount of 0.01 parts by weight or more, 0.05 parts by weight or more, or 0.07 parts by weight or more, and 3 parts by weight or less, 2 parts by weight or less, 1 part by weight or less, or 0.5 parts by weight or less, based on 100 parts by weight of the acrylic monomer.

[0203] In addition, the foaming agent may be a carbonate foaming agent. The carbonate foaming agent increases the surface area by foaming during the polymerization process to form pores in the hydrogel polymer. 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.

[0204] Based on 100 parts by weight of the acrylic monomer, the carbonate foaming agent can be used in an amount of 0.005 to 1 part by weight. When the content of the foaming agent is less than 0.005 parts by weight, the effect of using the foaming agent may not be significant. When the content of the foaming agent exceeds 1 part by weight, there are too many pores in the cross-linked polymer, which reduces the gel strength and density of the prepared superabsorbent polymer, which may cause problems in 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 0.01 parts by weight or more or 0.05 parts by weight or more, and 0.5 parts by weight or less, 0.3 parts by weight or less, or 0.2 parts by weight or less.

[0205] In addition, the carbonate foaming agent and the aqueous dispersion of hydrophobic particles can be used in a weight ratio of 1:0.1 to 1:4. When the aqueous dispersion of hydrophobic particles is used at too low a content compared to the carbonate foaming agent, it is difficult to effectively capture the bubbles generated. When the hydrophobic particles are used at too high a content compared to the foaming agent, various physical properties such as water retention capacity and absorption rate may be reduced. Specifically, the carbonate foaming agent and the hydrophobic particles can be used in a weight ratio of 1:0.4 or greater, 1:0.6 or greater or 1:0.8 or greater, and 1:1.7 or less, 1:1.5 or less or 1:1.2 or less. For example, the carbonate foaming agent and the hydrophobic particles can be used in a weight ratio of 1:1.

[0206] 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.

[0207] Meanwhile, the polymerization of the monomer composition in the presence of such an aqueous dispersion of hydrophobic particles and a carbonate-based foaming agent is not particularly limited as long as it is a commonly used polymerization method.

[0208] 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.

[0209] 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.

[0210] 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 5 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 5 cm, polymerization may not occur uniformly across the entire thickness due to the excessive thickness.

[0211] Typically, the hydrogel polymer obtained by the above method can have a moisture content of about 40% to about 80% by weight. In this context, "moisture content" refers to the moisture content of the total weight of the polymer, and is the value obtained by subtracting the weight of the dried polymer from the weight of the polymer. Specifically, the moisture content is defined as the value calculated by measuring the weight loss due to evaporation of water from the polymer during the temperature increase process of drying the polymer by infrared heating. The drying conditions for determining moisture content are: heating to and maintaining at approximately 180°C, and a total drying time of 20 minutes (including a 5-minute heating step).

[0212] (Step 3)

[0213] 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.

[0214] 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.

[0215] 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.

[0216] Drying is performed on the polymer pulverized as described above, or on the polymer immediately after polymerization in the absence of a pulverization step. The drying temperature in the drying step may be from about 150°C to about 250°C. When the drying temperature is lower than 150°C, the drying time may become too long and the physical properties of the superabsorbent polymer finally formed may be reduced. When the drying temperature exceeds 250°C, only the surface of the polymer is excessively dried, fine powder may be generated during the subsequent pulverization process, and the physical properties of the superabsorbent polymer may be reduced. Therefore, drying may be preferably performed at a temperature of from about 150°C to about 200°C, more preferably at a temperature of from about 160°C to about 180°C.

[0217] Meanwhile, considering process efficiency, the drying time may be about 20 minutes to about 90 minutes, but is not limited thereto.

[0218] 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.

[0219] Subsequently, a step of pulverizing the dried polymer obtained in the drying step is performed.

[0220] 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.

[0221] 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.

[0222] (Step 4)

[0223] 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.

[0224] 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.

[0225] 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.

[0226] 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.

[0227] 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.

[0228] 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.

[0229] 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.

[0230] 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.

[0231] 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.

[0232] When surface cross-linking conditions (particularly temperature increase 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.

[0233] 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.

[0234] After forming the surface cross-linked layer on the surface of the base resin as described above, an inorganic material may be further mixed.

[0235] 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.

[0236] 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.

[0237] 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.

[0238] The present invention will be described in more detail in the following examples. However, these examples are for illustrative purposes only, and the present invention is not limited to the following examples.

[0239] <Preparation Example>

[0240] Aqueous dispersions of hydrophobic particles used in the following examples were prepared as follows.

[0241] Preparation Example 1: Preparation of calcium stearate aqueous dispersion Ca-st (5)

[0242] First, 50 g of water containing two or more surfactants (including polyoxyethylene alkyl ether type nonionic surfactants and sulfate type anionic surfactants) was added to a high shear mixer and heated to 165 ° C., followed by the addition of 50 g of calcium stearate powder. Then, the mixture was stirred at 4000 rpm for 30 minutes under normal pressure to fully crush the calcium stearate, thereby obtaining an aqueous dispersion Ca-st (5) in which 50% by weight of calcium stearate with an average particle size of 5 μm was dispersed. At this time, the pH of the aqueous dispersion was 9.5. In addition, after preparing Ca-st (5), its average particle size (D50) was measured / calculated using a laser diffraction particle size measuring device (Microtrac S3500) as the particle size at 50% of the cumulative distribution of the number of particles.

[0243] Preparation Example 2: Preparation of magnesium stearate aqueous dispersion Mg-st(5)

[0244] First, 50 g of water containing two or more surfactants (including a sulfate-type anionic surfactant and a fatty acid ester-type nonionic surfactant) was added to a high shear mixer and heated to 150° C., followed by the addition of 50 g of magnesium stearate powder. The mixture was then stirred at 4000 rpm for 30 minutes under normal pressure to fully pulverize the magnesium stearate, thereby obtaining an aqueous dispersion Mg-st(5) in which 50% by weight of magnesium stearate having an average particle size of 5 μm was dispersed. The pH of the aqueous dispersion was 9 at this point, and the average particle size of Mg-st(5) was measured and calculated in the same manner as in Preparation Example 1.

[0245] Preparation Example 3: Preparation of sodium stearate aqueous dispersion Na-st (5)

[0246] First, 50 g of water containing two or more surfactants (including a sulfate-type anionic surfactant and a fatty acid ester-type nonionic surfactant) was added to a high-shear mixer, and 50 g of sodium stearate powder was added thereto at room temperature. The mixture was then stirred at 2000 rpm for 5 minutes under normal pressure to fully pulverize the sodium stearate, thereby obtaining an aqueous dispersion Na-st(5) in which 50% by weight of sodium stearate having an average particle size of 5 μm was dispersed. At this time, the pH of the aqueous dispersion was 9, and the average particle size of Na-st(5) was measured / calculated in the same manner as in Preparation Example 1.

[0247] Preparation Example 4: Preparation of potassium stearate aqueous dispersion K-st (5)

[0248] First, 50 g of water containing two or more surfactants (including a sulfate-type anionic surfactant and a fatty acid ester-type nonionic surfactant) was added to a high shear mixer and heated to 100° C., and then 20 g of potassium stearate powder was added thereto at room temperature. The mixture was then stirred at 4000 rpm for 30 minutes under normal pressure to fully pulverize the potassium stearate, thereby obtaining an aqueous dispersion K-st (5) in which 29% by weight of potassium stearate having an average particle size of 5 μm was dispersed. At this time, the pH of the aqueous dispersion was 9, and the average particle size of K-st (5) was measured / calculated in the same manner as in Preparation Example 1.

[0249] Preparation Example 5: Preparation of an aqueous dispersion of hydrophobic silica

[0250] First, 100 g of water was added to a high shear mixer, and then hydrophobic silica having an average particle size of 10 μm and a water contact angle of 130° was slowly added thereto while stirring at 5000 rpm to disperse it in an amount of 2% by weight based on the total weight of the final aqueous dispersion. When the silica was completely added, the mixture was stirred at 8000 rpm for 30 minutes at a temperature of 45°C. At this time, 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.

[0251] Preparation Example 6: Preparation of an aqueous dispersion of hydrophobic silica

[0252] An aqueous dispersion of hydrophobic silica was obtained in the same manner as in Preparation Example 5, except that hydrophobic silica having an average particle size of 20 μm and a water contact angle of 130° was added so as to be dispersed in an amount of 2% by weight based on the total weight of the final aqueous dispersion. The average particle size of the hydrophobic silica was measured / calculated in the same manner as in Preparation Example 1.

[0253] Preparation Example 7: Preparation of an aqueous dispersion of hydrophobic silica

[0254] An aqueous dispersion of hydrophobic silica was obtained in the same manner as in Preparation Example 5, except that hydrophobic silica having an average particle size of 30 μm and a water contact angle of 130° was added so as to be dispersed in an amount of 2% by weight based on the total weight of the final aqueous dispersion. The average particle size of the hydrophobic silica was measured / calculated in the same manner as in Preparation Example 1.

[0255] <Example>

[0256] Example 1

[0257] (Step 1) In a 3 L glass container equipped with a stirrer and a thermometer, 100 g of acrylic acid, 0.5 g of PEGDA 400 (polyethylene glycol diacrylate 400) as an internal crosslinker, and 0.01 g of diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide as a photoinitiator were added and dissolved. Then, 890 g of a 22% sodium hydroxide solution was added to prepare a monomer composition.

[0258] (Step 2) The aqueous dispersion of calcium stearate Ca-st (5) prepared in Preparation Example 1 was added to the monomer composition so that 0.1 g of calcium stearate was added to 100 g of acrylic acid, and 0.1 g of sodium bicarbonate (SBC) as a carbonate foaming agent was added thereto. Thereafter, the monomer composition was supplied to a conveyor belt at a rate of 500 mL / min to 2000 mL / min, wherein the belt was 10 cm wide and 2 m long and rotated at a rate of 50 cm / min. Furthermore, while the monomer composition was being supplied, an irradiation intensity of 10 mW / cm 2 The polymer was subjected to ultraviolet light for 60 seconds, thereby obtaining a sheet-like hydrogel polymer having a water content of 55% by weight.

[0259] (Step 3) The sheet-shaped hydrogel polymer was then cut into approximately 5 cm x 5 cm pieces and pulverized in a meat grinder to obtain hydrogel particle crumbs ranging in size from 1 mm to 10 mm. The crumbs were then dried in an oven with variable airflow. Hot air at 180°C or higher was then blown upward for 15 minutes and then downward for another 15 minutes to uniformly dry the crumbs. The moisture content of the dried crumbs was set to 2% or less. After drying, the crumbs were pulverized in a grinder and then classified to produce base resin particles with diameters ranging from 150 μm to 850 μm.

[0260] 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.

[0261] 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.

[0262] Example 2

[0263] A superabsorbent polymer was prepared in the same manner as in Example 1, except that the magnesium stearate aqueous dispersion Mg-st(5) prepared in Preparation Example 2 was added instead of the calcium stearate aqueous dispersion Ca-st(5) as the aqueous dispersion of hydrophobic particles in Example 1, so that 0.1 g of magnesium stearate was added based on 100 g of acrylic acid.

[0264] Example 3

[0265] A superabsorbent polymer was prepared in the same manner as in Example 1, except that the magnesium stearate aqueous dispersion Na-st(5) prepared in Preparation Example 3 was added instead of the calcium stearate aqueous dispersion Ca-st(5) as the aqueous dispersion of hydrophobic particles in Example 1, so that 0.1 g of sodium stearate was added based on 100 g of acrylic acid.

[0266] Example 4

[0267] A superabsorbent polymer was prepared in the same manner as in Example 1, except that the magnesium stearate aqueous dispersion K-st(5) prepared in Preparation Example 4 was added instead of the calcium stearate aqueous dispersion Ca-st(5) as the aqueous dispersion of hydrophobic particles in Example 1, so that 0.1 g of potassium stearate was added based on 100 g of acrylic acid.

[0268] Example 5

[0269] 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 was added instead of the aqueous dispersion of calcium stearate Ca-st(5) as the aqueous dispersion of hydrophobic particles in Example 1, so that 0.1 g of hydrophobic silica having an average particle size of 10 μm was added based on 100 g of acrylic acid.

[0270] Example 6

[0271] 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 6 was added instead of the aqueous dispersion of calcium stearate Ca-st (5) as the aqueous dispersion of hydrophobic particles in Example 1, so that 0.1 g of hydrophobic silica having an average particle size of 20 μm was added based on 100 g of acrylic acid.

[0272] Example 7

[0273] 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 7 was added instead of the aqueous dispersion of calcium stearate Ca-st (5) as the aqueous dispersion of hydrophobic particles in Example 1, so that 0.1 g of hydrophobic silica having an average particle size of 30 μm was added based on 100 g of acrylic acid.

[0274] Comparative Example 1

[0275] A super absorbent polymer was prepared in the same manner as in Example 1, except that neither the aqueous dispersion of hydrophobic particles nor the carbonate-based foaming agent was used in Example 1.

[0276] Comparative Example 2

[0277] A superabsorbent polymer was prepared in the same manner as in Example 1, except that in Example 1, the aqueous dispersion of hydrophobic particles was not used.

[0278] Comparative Example 3

[0279] A super absorbent polymer was prepared in the same manner as in Example 1, except that 0.02 g of a 25 wt% sodium lauryl sulfate solution (SDS, manufactured by Sigma Aldrich) was added instead of the aqueous dispersion of calcium stearate Cat-st (5) in Example 1.

[0280] Comparative Example 4

[0281] A superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.1 g of calcium stearate in powder form having an average particle size of 5 μm was used based on 100 g of acrylic acid instead of the aqueous dispersion form of hydrophobic particles of Ca-st (5) in Example 1. However, it was confirmed that calcium stearate in powder form could not be dispersed in the monomer composition and floated on the neutralization solution.

[0282] Experimental Example 1: Measuring the Pore Size of Superabsorbent Polymer

[0283] To confirm the pore structure of the superabsorbent polymers prepared in Examples and Comparative Examples, internal images of the superabsorbent polymer particles were taken at a magnification of 20 to 24 times using a scanning electron microscope (SEM, product name: JCM-6000, manufacturer: JEOL). Specifically, the pore size of the superabsorbent polymer was measured as follows: Figures 1 to 3 The images of Example 1, Comparative Example 1 and Comparative Example 2 are shown in FIG.

[0284] 1) First, the superabsorbent polymer of Example was classified at an amplitude of 1.0 for 1 minute into individual particles having a particle size of 600 μm to 710 μm without damaging the particles using a particle classifier of Retsch, thereby obtaining a 2 g sample.

[0285] 2) Afterwards, the prepared sample particles were randomly arranged and placed on the SEM stage.

[0286] 3) Subsequently, the sample particles, randomly arranged on a SEM stage, were fixed with carbon tape, and the pore size formed on the surface of the superabsorbent particles was measured at a magnification of 20 to 24 times. In this case, an average of 1,000 or more superabsorbent polymer particles was selected, and the pore size of 300 or more particles in which pores were clearly visible was measured. Herein, the criterion for "clearly visible pores" is as follows: assuming that the bubbles are formed into spherical shapes, they are evaluated as being most clearly visible when they have a nearly hemispherical shape on the surface of the superabsorbent polymer particles through the pulverization process.

[0287] 4) Subsequently, the pore sizes of 300 or more particles finally measured were obtained to statistically calculate the average diameter as the median, and the results are shown in Table 1. In this case, the interquartile range was used to examine the pore size distribution to exclude measurement errors of the measurer and outliers.

[0288] refer to Figures 1 to 3 , unlike the superabsorbent polymers of Comparative Examples 2 and 3, it was confirmed that a plurality of pores having a diameter of 150 μm to 220 μm were uniformly formed in the superabsorbent polymer of Example 1.

[0289] Experimental Example 2: Measurement of physical properties of superabsorbent polymers

[0290] The physical properties of the superabsorbent polymers prepared in Examples and Comparative Examples were evaluated in the following manner and are shown in Table 1 below.

[0291] Unless otherwise stated, all evaluations were performed at room temperature (25° C.), and physiological saline or saline refers to a 0.9 wt % sodium chloride (NaCl) aqueous solution.

[0292] In addition, the electrical conductivity of tap water used for evaluation of the following physical properties was 170 μS / cm to 180 μS / cm when measured using Orion Star A222 (manufactured by Thermo Scientific).

[0293] (1) Centrifugal retention capacity (CRC)

[0294] The centrifuge retention capacity was measured according to EDANA WSP 241.3 by the absorbance ratio of each polymer under unloaded conditions.

[0295] Specifically, W0 (g, about 0.2g) of superabsorbent polymer was evenly inserted into a nonwoven envelope and sealed, and then immersed in saline (0.9% by weight) at room temperature. After 30 minutes, the envelope was centrifuged at 250G for 3 minutes to drain, and the envelope weight W2 (g) was weighed. In addition, the same operation was performed without using the resin, and the weight W1 (g) of the envelope was then measured. Then, the obtained weight value was used to calculate CRC (g / g) according to the following equation 2.

[0296] [Equation 2]

[0297] CRC(g / g)={[W2(g)-W1(g)] / W0(g)}–2

[0298] (2) Absorption under pressure (AUP)

[0299] The absorbency under 0.7 psi pressure of each polymer was measured according to EDANA WSP 242.3.

[0300] Specifically, a 400-mesh stainless steel screen was installed in the bottom of a plastic cylinder with an inner diameter of 60 mm. At room temperature and 50% humidity, W0 (g, 0.90 g) of superabsorbent polymer was evenly spread on the screen. A piston, capable of applying a uniform 0.7 psi load, was then placed on top. The piston's outer diameter was slightly less than 60 mm, ensuring no gap between it and the inner wall of the cylinder, allowing for uninterrupted movement. At this point, the device's weight, W3 (g), was measured.

[0301] Subsequently, a glass filter with a diameter of 90 mm and a thickness of 5 mm was placed in a culture dish with a diameter of 150 mm, and physiological saline (0.9 wt% sodium chloride) was poured into the culture dish. At this time, the physiological saline was injected until the liquid level of the physiological saline was level with the upper surface of the glass filter. A piece of filter paper with a diameter of 90 mm was placed on it. After placing a measuring device on the filter paper, the liquid was absorbed under load for 1 hour. After 1 hour, the measuring device was lifted and the weight W4 (g) was measured.

[0302] Then, the pressure absorption rate (g / g) was calculated according to the following Equation 3 by using the obtained weight value.

[0303] [Equation 3]

[0304] AUP(g / g)=[W4(g)–W3(g)] / W0(g)–3

[0305] (3) Vortex time (absorption rate)

[0306] The vortex time (absorption rate) of the superabsorbent polymers of Examples and Comparative Examples was measured in the following manner.

[0307] ① First, use a 100mL graduated cylinder to add 50mL of 0.9% saline into a 100mL flat-bottom beaker.

[0308] 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.

[0309] 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.

[0310] 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.

[0311] (4) Determination of shape parameters of superabsorbent polymer particles

[0312] The aspect ratio and convexity of the superabsorbent polymers of Examples and Comparative Examples were measured by the following methods using morphologi 4 manufactured by Malvern Panalytics.

[0313] ① Sample preparation: First, using a particle classifier of Retsch, the superabsorbent polymer was classified at an amplitude of 1.0 for 1 minute into individual particles having a particle size of 300 μm to 600 μm without damaging the particles, thereby obtaining 1 g of a sample. Figure 4 The sample dispersion unit setting values at this time are shown.

[0314] ② Image capture: Place the prepared sample on the table of the instrument and scan it at a magnification of 2.5 times to capture the image of a single particle. At this time, the illumination setting value and the optical selection setting value are as follows: Figure 5 and 6 shown.

[0315] ③ Image processing: For the captured image, the parameter values of each particle are measured, such as the image captured as a 2D image of the 3D particle, the CE diameter (circle equivalent diameter), the shortest diameter, the longest diameter, the actual particle circumference, and the convex hull circumference. At this time, the scanning area setting value and the particle filter setting value are respectively displayed on Figure 7 and 8 middle.

[0316] ④ Based on the analytical data of each particle, the distribution of each parameter of all particles contained in the sample is obtained.

[0317]

Table 1

[0318]

[0319] As shown in Table 1 above, it can be seen that unlike the superabsorbent polymer of the comparative example, the superabsorbent polymer of the example, in which the monomer polymerization reaction was carried out in the presence of an aqueous dispersion of hydrophobic particles, contained particles having an average aspect ratio of 0.75 or greater and an average convexity of 0.90 or less, and at the same time had a structure in which small and uniform pores were distributed. Therefore, it was confirmed that the superabsorbent polymer of the example exhibited improved absorption rate and significantly improved absorption rate compared to the superabsorbent polymer of the comparative example.

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 cross-linked layer formed by further cross-linking the cross-linked polymer using a surface cross-linking agent on the base resin, in, The super absorbent polymer comprises hydrophobic particles having an average particle size of 0.2 μm to 50 μm, and the super absorbent polymer satisfies the following properties: i) the average value of the aspect ratio (A / R), which is the ratio of the shortest diameter to the longest diameter of particles measured for particles having a particle diameter of 300 μm to 600 μm, is 0.75 or more, and the average value of the convexity calculated by the following formula 1 is 0.9 or less, and ii) Vortex time at 24.0°C is 60 seconds or less: [Equation 1] Convexity = convex hull perimeter / actual particle perimeter In equation 1, When it is assumed that the image obtained by capturing the 3D image of the measured 3D particle as a 2D image is surrounded by an imaginary elastic band extending around the contour, the convex hull perimeter is the length of the elastic band, and The actual particle circumference is the actual circumference of an image obtained by capturing a 3D image of the measured 3D particle as a 2D image.

2. The superabsorbent polymer according to claim 1, in, The superabsorbent polymer has a centrifuge retention capacity (CRC) of 27 g / g or more as measured according to EDANA WSP 241.3 and an absorbency under pressure (AUP) of 23.5 g / g or more as measured according to EDANA WSP 242.

3.

3. The superabsorbent polymer according to claim 1, in, The super absorbent polymer has a plurality of pores, and an average diameter of the plurality of pores is 10 μm to 400 μm.

4. The superabsorbent polymer according to claim 1, in, The hydrophobic particles are selected from hydrophobic silica, C7 to C 24 At least one member selected from the group consisting of metal salts of fatty acids and hydrophobic organic particles.

5. The superabsorbent polymer according to claim 4, in, C7 to C 24 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.

6. 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 and an internal crosslinking agent; Step 2: preparing a hydrogel polymer by cross-linking polymerization of the monomer composition in the presence of an aqueous dispersion of hydrophobic particles and a carbonate-based foaming agent; Step 3: forming a powdered base resin by drying and pulverizing the hydrogel polymer; and Step 4: preparing a super absorbent polymer by further cross-linking the surface of the base resin in the presence of a surface cross-linking agent to form a surface cross-linked layer, wherein the aqueous dispersion of hydrophobic particles is a colloidal solution in which hydrophobic particles having an average particle size of 0.2 μm to 50 μm are dispersed, and The superabsorbent polymer satisfies the following properties: i) the average value of the aspect ratio (A / R), which is the ratio of the shortest diameter to the longest diameter of particles measured for particles having a particle diameter of 300 μm to 600 μm, is 0.75 or more, and the average value of the convexity calculated by the following formula 1 is 0.9 or less, and ii) Vortex time at 24.0°C is 60 seconds or less: [Equation 1] Convexity = convex hull perimeter / actual particle perimeter In equation 1, When it is assumed that the image obtained by capturing the 3D image of the measured 3D particle as a 2D image is surrounded by an imaginary elastic band extending around the contour, the convex hull perimeter is the length of the elastic band, and The actual particle circumference is the actual circumference of an image obtained by capturing a 3D image of the measured 3D particle as a 2D image.

7. The method for preparing a superabsorbent polymer according to claim 6, in, The hydrophobic particles are used in an amount of 0.001 to 5 parts by weight based on 100 parts by weight of the acrylic monomer.

8. The method for preparing superabsorbent polymer according to claim 6, in, The carbonate foaming agent is at least one selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate and magnesium carbonate.

9. The method for preparing a superabsorbent polymer according to claim 6, in, The carbonate foaming agent and the hydrophobic particles are used in a weight ratio of 1:0.1 to 1:

4.

10. The method for preparing superabsorbent polymer according to claim 6, in, The aqueous dispersion of hydrophobic particles is wherein C7 to C 24 The metal salts of fatty acids are dispersed as colloidal solutions of hydrophobic particles in the presence of surfactants.

11. The method for preparing a superabsorbent polymer according to claim 10, in, The surfactant includes a nonionic surfactant and an anionic surfactant.

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