Process for the preparation of superabsorbent polymers

Abstract

The present disclosure relates to a method of producing a superabsorbent polymer capable of minimizing loss of bubbles during foaming polymerization. According to the production method of the present disclosure, a large number of small and uniform pores can be formed so that a superabsorbent polymer having a high surface area can be produced. Since the superabsorbent polymer exhibits excellent absorbency, it can be used for various products requiring high absorbency.

Description

Preparation method of superabsorbent polymer Technical Field

[0001] Cross-references to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2022-0016382, filed with the Korean Intellectual Property Office on February 8, 2022, and Korean Patent Application No. 10-2023-0016731, filed on February 8, 2023, the disclosures of which are incorporated herein by reference in their entirety.

[0003] This disclosure relates to a method for preparing superabsorbent polymers. Background Technology

[0004] Superabsorbent polymers (SAPs) are a class of synthetic polymer materials capable of absorbing 500 to 1000 times their own weight in water. Initially used in hygiene products, these SAPs are now widely used not only in hygiene products (such as disposable baby diapers), but also in horticultural soil products, waterproofing materials for civil engineering and construction, seedling sheets, food preservatives, and mud dressings.

[0005] These superabsorbent polymers need to exhibit rapid absorption rates. To improve the absorption rate of superabsorbent polymers, it is known to increase the surface area by adding a foaming agent during polymerization to form a porous structure.

[0006] However, in conventional methods for preparing superabsorbent polymers using foaming agents, there is a problem that carbon dioxide bubbles generated by the foaming agent are lost before the acrylic monomers polymerize, resulting in insufficient pores in the superabsorbent polymer. Therefore, a method has been proposed to reduce bubble loss by using a foam stabilizer and a foaming agent together. However, while increasing the amount of foam stabilizer to achieve sufficient bubble trapping has the advantage of more uniform and denser bubble size, it also reduces the surface tension of the prepared superabsorbent polymer due to the effect of the foam stabilizer, leading to a decrease in overall absorption performance (e.g., CRC, AUP, and permeability). Summary of the Invention

[0007] Technical issues

[0008] To address the aforementioned issues, a method for preparing a superabsorbent polymer that achieves excellent foam stabilization even with the use of small amounts of foam stabilizer is provided.

[0009] Technical solution

[0010] To achieve the above objective, a method for preparing a superabsorbent polymer is provided, the method comprising the following steps:

[0011] i) A mixture of an acrylic monomer having at least partially neutralized acidic groups; an internal crosslinking agent; and a polymerization initiator;

[0012] ii) Add a foam stabilizer to the mixture in i) and perform high-shear mixing at a Reynolds number of 10,000 or higher;

[0013] iii) Prepare monomer compositions by adding a foaming agent to the mixture of ii);

[0014] iv) Prepare hydrogel polymers by polymerizing monomer compositions; and

[0015] v) Drying, pulverizing, and classifying the hydrogel polymer.

[0016] High-shear mixing can be performed for 10 to 60 seconds.

[0017] High-shear mixing can be carried out at Reynolds numbers ranging from 10,000 to 20,000.

[0018] The content of foam stabilizer in the monomer composition can be above 10 ppm and less than 200 ppm.

[0019] The foaming agent content in the monomer composition can be from 100 ppm to 2000 ppm.

[0020] The foaming agent may be at least one selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate.

[0021] Foam stabilizers can be at least one selected from the group consisting of cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants.

[0022] The foaming agent can be sodium bicarbonate, and the foam stabilizer can be calcium stearate or sodium dodecyl sulfate.

[0023] The preparation method may further include step (vi) of forming a surface crosslinking layer by further crosslinking the surface of the superabsorbent polymer obtained in step v) in the presence of a surface crosslinking agent.

[0024] Beneficial effects

[0025] According to the preparation method disclosed herein, excellent foam stabilization can be achieved with a small amount of foam stabilizer, and a superabsorbent polymer with a large number of small and uniform pores can be obtained without deterioration of physical properties (e.g., surface tension). This superabsorbent polymer exhibits a significantly improved absorption rate due to its high surface area and possesses excellent absorption properties (e.g., water retention capacity and pressurized absorption rate), and therefore can be used in a variety of products requiring high absorption rates. Attached Figure Description

[0026] Figure 1 shows the sample dispersion unit settings in Malvern Panalytic's Morphologi 4.

[0027] Figure 2 shows the lighting settings in Malvern Panalytic's Morphologi 4.

[0028] Figure 3 shows the optics selection settings in Malvern Panalytic's Morphologi 4.

[0029] Figure 4 shows the scan area settings in Malvern Panalytical's Morphologi 4.

[0030] Figure 5 shows the particle filtration settings in Malvern Panalytical's Morphologi 4.

[0031] The meaning of the text in the computer screenshot in Figure 1:

[0032] Sample Dispersion Unit: Sample dispersion device

[0033] Use SDU: Use SDU

[0034] Default Dispersion Settings:

[0035] Low Energy

[0036] High Energy

[0037] Injection Pressure (bar): Injection pressure (bar)

[0038] Injection Time (ms): Injection time (ms)

[0039] Setting Time (sec): Set time (sec)

[0040] Injection Volume Indicator: Injection volume indicator

[0041] The meaning of the text in the computer screenshot in Figure 2:

[0042] Illumination Settings: Lighting Settings

[0043] Diascopic (bottom light): Transmitted light (light from the bottom)

[0044] Episcopic (top light): Reflected light from above.

[0045] Bright field: Bright field

[0046] Dark field (Manual light control only): Dark field (manual light control only)

[0047] Polarizer / DIC: Polarizing film / DIC

[0048] Light options: Light options

[0049] Automatic light calibration:

[0050] Calibration intensity: Calibration intensity

[0051] Intensity tolerance: strength tolerance

[0052] Manual light control (advanced): Manual light control (advanced)

[0053] Light calibration over sample: Light calibration over sample

[0054] Recommended for non-transparent substrates:

[0055] The meaning of the text in the computer screenshot in Figure 3:

[0056] Optics Selection: Optical Component Selection

[0057] Select the optics based on your particle size range.

[0058] The meaning of the text in the computer screenshot in Figure 4:

[0059] Scan Areas: Scan areas

[0060] Select areas to analyze: Select the areas to analyze.

[0061] Drag rectangles to define new areas or drag the edge of an existing area to change its size and / or position.

[0062] Selected analysis area: The selected analysis area

[0063] Circular: round

[0064] X pos. (mm): X position (mm)

[0065] Y pos.(mm):Y position(mm)

[0066] Radius(mm):radius(mm)

[0067] Refine position before measurement: refine the position before measurement

[0068] Area (mm) 2 Area (mm) 2 )

[0069] Delete: Remove

[0070] Reset to default: Resets the system to the default value.

[0071] Time estimate

[0072] Est.time this optic (mins): Estimated time (in minutes) for this optic.

[0073] Est.total time(mins): Estimated total time (minutes)

[0074] The meaning of the text in the computer screenshot in Figure 5:

[0075] Particle Filtering

[0076] Filters: Filters

[0077] Copy: Copy

[0078] Paste: to paste

[0079] Undo: to undo

[0080] Delete: Remove

[0081] Parameter: Parameter

[0082] Operator: Operator symbol

[0083] Value 1: Value 1

[0084] Value 2: Value 2

[0085] Solidity: robustness

[0086] CE Diameter (μm): CE diameter (μm)

[0087] Add Constraint: Add a constraint

[0088] Value: Value

[0089] Relative: Relationship

[0090] Use filtering to remove contaminants from the analysis.

[0091] Particles matching the filter criteria will be ignored. These particles are still stored and can be viewed using the particle view. Detailed Implementation

[0092] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that, when used in this disclosure, the terms “comprising,” “including,” or “having” specify the presence of the stated features, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, steps, components, or combinations thereof.

[0093] Because the present invention can be modified in various ways 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 invention to the specific forms disclosed, and it should be understood that the invention includes all modifications, equivalents, and substitutions within the concept and scope of the invention.

[0094] As used herein, “base resin” or “base resin powder” refers to a polymer prepared in granular or powder form by drying and pulverizing a polymer obtained by polymerizing acrylic monomers, and means a polymer in which no surface modification or surface crosslinking has been performed.

[0095] According to embodiments of this disclosure, a method for preparing a superabsorbent polymer includes the following steps:

[0096] i) A mixture of an acrylic monomer having at least partially neutralized acidic groups; an internal crosslinking agent; and a polymerization initiator;

[0097] ii) Add a foam stabilizer to the mixture in i) and perform high-shear mixing at a Reynolds number of 10,000 or higher;

[0098] iii) Prepare monomer compositions by adding a foaming agent to the mixture of ii);

[0099] iv) Preparation of hydrogel polymers by polymerizing monomer compositions; and

[0100] v) Drying, pulverizing, and classifying the hydrogel polymer.

[0101] When using foaming agents in polymerization to increase the surface area of ​​superabsorbent polymers, the use of foam stabilizers is known to prevent the loss of bubbles generated by the foaming agents. However, when large amounts of foam stabilizers are used to fully ensure the effect of foam stabilization, there is a problem of degradation of the physical properties of the superabsorbent polymer.

[0102] Therefore, in this disclosure, to achieve better foam stabilization with a small amount of foam stabilizer, a superabsorbent polymer is prepared by: firstly, adding the foam stabilizer to the monomer composition before adding the blowing agent, performing high-shear mixing, then adding the blowing agent, and subsequently polymerizing. According to this preparation method, gas is generated by the blowing agent while the foam stabilizer is uniformly dispersed in the composition, thus effectively preventing bubble loss even with a small amount of foam stabilizer. Furthermore, the generation of uniform and dense bubbles results in a large number of small and uniform pores, making it possible to manufacture a superabsorbent polymer with an excellent absorption rate.

[0103] Furthermore, the superabsorbent polymer particles prepared according to the above method have a near-spherical shape, i.e., a certain level of sphericity. In addition, appropriately sized pores increase the surface roughness of the particles and expand the specific surface area, thereby simultaneously improving both absorption rate and absorption performance.

[0104] The preparation method of the superabsorbent polymer disclosed herein will be described in more detail below.

[0105] First, i) the mixture is prepared by mixing an acrylic monomer having at least partially neutralized acidic groups, an internal crosslinking agent, and a polymerization initiator.

[0106] Acrylic monomers are compounds represented by the following chemical formula 1:

[0107] [Chemical Formula 1]

[0108] R 1 -COOM 1

[0109] In chemical formula 1,

[0110] R 1 It is a C2 to C5 alkyl group with unsaturated bonds, and

[0111] M 1 It can be a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0112] Preferably, the acrylic monomer may include at least one selected from acrylic acid, methacrylic acid and their monovalent metal salts, divalent metal salts, ammonium salts and organic amine salts.

[0113] Here, the acrylic monomers can be those having at least partially neutralized acidic groups. Preferably, acrylic monomers partially neutralized with alkaline substances such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide can be used. The degree of neutralization of the acrylic monomer can be 40 mol% to 95 mol%, 40 wt% to 80 wt%, or 45 mol% to 75 mol%. The range of neutralization can be adjusted according to the final properties. Too high a degree of neutralization will cause the neutralized monomer to precipitate, and therefore may not readily polymerize. Conversely, too low a degree of neutralization will not only reduce the polymer's absorbency but will also impart unmanageable properties to the polymer, such as those of elastic rubbers.

[0114] Furthermore, based on the monomer composition containing the raw materials and solvent of the superabsorbent polymer, the concentration of the acrylic monomer can be from about 20% to 60% by weight, or preferably from about 40% to 50% by weight, and appropriately controlled considering the polymerization time and reaction conditions. When the monomer concentration is too low, the yield of the superabsorbent polymer is low, and there may be economic efficiency problems. Conversely, when the concentration is too high, the following problems may occur in the process: some monomers may be extracted, or the pulverization efficiency of the polymerized hydrogel polymer in the pulverization process may be reduced, thus potentially degrading the physical properties of the superabsorbent polymer.

[0115] In the method for preparing the superabsorbent polymer disclosed herein, there are no particular limitations if the polymerization initiator used during polymerization is the same one commonly used for the preparation of superabsorbent polymers.

[0116] Specifically, depending on the polymerization method, the polymerization initiator can be an initiator for thermal polymerization or an initiator for photopolymerization via ultraviolet radiation. However, even when applying a photopolymerization method, a certain amount of heat is generated by UV radiation, etc., and some heat is also generated as the polymerization reaction (exothermic reaction) proceeds. Therefore, the composition may additionally contain a thermal polymerization initiator.

[0117] More specifically, any compound that can form free radicals through light (such as ultraviolet light) can be used as a photopolymerization initiator without limitation.

[0118] For example, the photopolymerization initiator can be one or more compounds selected from the group consisting of phenoxy ether, dialkyl acetophenone, hydroxyalkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acylphosphine, and α-amino ketone. Furthermore, as specific examples of acylphosphine, commercially available lucirin TPO (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide) or Irgacure 819 (bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide) can be used. A wider variety of photoinitiators are fully disclosed on page 115 of "UV Coatings: Basics, Recent Developments and New Application" by Reinhold Schwalm (Elsevier, 2007), but the invention is not limited thereto.

[0119] Based on 100 parts by weight of acrylic monomers, the content of photoinitiator can be between 0.001 parts by weight and 0.1 parts by weight. When the concentration of photoinitiator is too low, the polymerization rate may slow down, while when the concentration of photoinitiator is too high, the molecular weight of the superabsorbent polymer may decrease and the performance may be uneven.

[0120] Furthermore, at least one initiator selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used as the thermal polymerization initiator. Specifically, examples of persulfate initiators include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), and ammonium persulfate ((NH4)2S2O8); while examples of azo initiators include 2,2-azobis(2-amidinylpropane) dihydrochloride, 2,2-azobis(N,N-dimethylene)isobutylamidinyl dihydrochloride, 2-(carbamoylazo)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, and 4,4-azobis(4-cyanopentanoic acid). A wider variety of thermal polymerization initiators are fully disclosed on page 203 of Odian’s “Principle of Polymerization” (Wiley, 1981), but this disclosure is not limited thereto.

[0121] Based on 100 parts by weight of acrylic monomers, the content of thermal initiator can be between 0.01 parts by weight and 1 part by weight. When the content of thermal initiator is less than 0.01 parts by weight based on 100 parts by weight of acrylic monomers, the polymerization rate may slow down. When it exceeds 1 part by weight, the molecular weight of the prepared polymer will be very small, the gel sheet will become sticky, and therefore there may be productivity problems.

[0122] According to one embodiment of the present disclosure, the monomer composition comprises an internal crosslinking agent. The internal crosslinking agent is used to internally crosslink the polymer in which the acrylic monomer is polymerized, and is distinct from surface crosslinking agents used to crosslink the surface of the polymer.

[0123] For example, the internal crosslinking agent may be at least one selected from the group consisting of: N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, (meth)acrylate, propylene glycol di(meth)acrylate, polyethylene 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, and ethylene carbonate.

[0124] Based on 100 parts by weight of acrylic monomer, the content of internal crosslinking agent can be more than 0.01 parts by weight, more than 0.05 parts by weight, more than 0.1 parts by weight, or more than 0.5 parts by weight, and less than 2 parts by weight, less than 1.5 parts by weight, or less than 1 part by weight.

[0125] Simultaneously, any solvent capable of dissolving components contained in the monomer composition, such as acrylic monomers, internal crosslinking agents, polymerization initiators, foam stabilizers, blowing agents, and optional additives, can be used without limitation as a solvent in preparing the mixture and monomer composition in steps i) to iii). For example, the solvent can be at least one, or a combination of two or more, selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl pentyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide.

[0126] There are no particular limitations on the method for preparing the mixture in step i). For example, the mixture in step i) can be prepared by mixing acrylic monomers, an internal crosslinking agent, and a polymerization initiator, and adding an alkaline solvent containing a dissolved alkaline material (e.g., sodium hydroxide) to partially neutralize the acidic groups of the acrylic monomers. Alternatively, the mixture in step i) can be prepared by first neutralizing the acrylic monomers with an alkaline material, and then mixing them with the internal crosslinking agent and the polymerization initiator. However, this is merely an example, and there are no particular limitations on the order of addition of each raw material or the mixing method.

[0127] Subsequently, ii) foam stabilizer a is added to the mixture in i) and high-shear mixing is performed at a Reynolds number of 10,000 or higher to ensure that the foam stabilizer is uniformly dispersed in the mixture.

[0128] A foam stabilizer is a material that prevents bubbles generated by a foaming agent from escaping to the outside of the monomer composition and helps to generate bubbles more uniformly and densely. Materials commonly referred to in the art as foam stabilizers or surfactants can be used.

[0129] For example, one or more surfactants selected from the group consisting of cationic surfactants, anionic surfactants, amphoteric surfactants and nonionic surfactants can be used as foam stabilizers.

[0130] For example, examples of cationic surfactants include dialkyl dimethyl ammonium salts and alkyl benzyl methyl ammonium salts. Examples of anionic surfactants include fatty acid metal salts, alkyl polyoxyethylene sulfates, monoalkyl sulfates, alkylbenzene sulfonates, and monoalkyl phosphates. Examples of amphoteric surfactants include alkyl sulfobetaines and alkyl carboxybetaines. Examples of nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene arylphenyl ethers, fatty acid sorbitan esters, alkyl monoglycerides, alkanolamides, and alkyl glycosides. Furthermore, polyalkylene glycols, polyvinylimide, polyvinyl alcohol, polyacrylamide, and polyvinylpyrrolidone can be used as polymer surfactants.

[0131] In addition, hydrophobic particles can be used as foam stabilizers. These 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 selected from the following saturated fatty acids: a metal salt of lauric acid containing 12 carbon atoms; a metal salt of tridecanoic acid containing 13 carbon atoms; a metal salt of myristic acid containing 14 carbon atoms; a metal salt of pentadecanoic acid containing 15 carbon atoms; a metal salt of palmitic acid containing 16 carbon atoms; a metal salt of heptadecanoic acid containing 17 carbon atoms; a metal salt of stearic acid containing 18 carbon atoms; a metal salt of nonadecanoic acid containing 19 carbon atoms; and a metal salt of arachidic acid containing 20 carbon atoms. Among the metal salts of saturated fatty acids, the metal can be calcium, zinc, potassium, sodium, magnesium, etc.

[0132] For example, anionic surfactants can be used as foam stabilizers, preferably fatty acid metal salts or monoalkyl sulfates. Preferably, metal salts of saturated fatty acids having 12 to 20 carbon atoms (e.g., calcium stearate, sodium dodecyl sulfate, etc.) can be used as foam stabilizers.

[0133] Based on the total weight of the monomer composition to be prepared, the amount of foam stabilizer is preferably less than 200 ppm, less than 150 ppm, or less than 100 ppm, and more than 10 ppm or more than 20 ppm. Generally, to obtain sufficient foam stabilization, foam stabilizers of 200 ppm or more are used. However, in this disclosure, high-shear mixing is performed after adding the foam stabilizer at a Reynolds number of 10,000 or higher; therefore, excellent foam stabilization effects can be obtained even when using less than 200 ppm of foam stabilizer. If the content of the foam stabilizer is too low, below 10 ppm, the effect of using the foam stabilizer cannot be obtained; therefore, it is preferable to satisfy the above range.

[0134] After adding the foam stabilizer, high-shear mixing is performed to make the Reynolds number (Re) of the mixture greater than 10,000, 13,000, or 15,000.

[0135] The Reynolds number is the ratio of the inertial force to the viscous force of a fluid, as expressed by Equation 1 below.

[0136] [Equation 1]

[0137] Re=(ρx v s x L) / μ

[0138] In Equation 1, ρ is the density of the fluid, and v s is the average velocity of the fluid, L is the characteristic length, and μ is the viscosity of the fluid.

[0139] In this disclosure, the characteristic length refers to the diameter of the container in which mixing takes place in step ii). The viscosity of the fluid (i.e., the mixture with added foam stabilizer) is the viscosity at a high shear mixing temperature (e.g., 45°C to 50°C), and can be measured using a viscometer (e.g., a Brookfield LVDV-IPrime viscometer) at spindle number S63 and a rotation speed of 1000 rpm. The density of the fluid can be calculated from the weight and volume of the mixture.

[0140] In this disclosure, after adding a foam stabilizer, the mixture is subjected to high-shear mixing at a Reynolds number of 10,000 or higher to ensure uniform dispersion of the foam stabilizer in the mixture, thereby achieving sufficient foam stabilization even with a small amount of foam stabilizer. The Reynolds number of the mixture during high-shear mixing can be calculated using the equation described above. More simply, as described in Chemical Engineering and Processing 57-58 (2012) 25-41, measuring the power consumption of the high-shear mixer can predict when the Reynolds number will reach 10,000. That is, when power consumption is measured while increasing the rpm of the high-shear mixer, the Reynolds number is above 10,000 when the power consumption no longer increases beyond a certain rpm.

[0141] Meanwhile, when the Reynolds number is above 4000, the flow can generally be considered turbulent. However, even with turbulence, when the Reynolds number is below 10000, the foam stabilizer cannot be sufficiently dispersed in the mixture, thus failing to achieve the desired effect. Therefore, during high-shear mixing, the Reynolds number is set above 10000, allowing the foam stabilizer to mix under fully turbulent conditions.

[0142] Meanwhile, higher Reynolds numbers during the mixing period result in better evaluation. Therefore, theoretically there is no upper limit, but it can be, for example, below 20,000.

[0143] The high-shear mixing time can be appropriately adjusted according to the composition of the mixture, for example, to be more than 10 seconds or more than 20 seconds, and less than 60 seconds, 50 seconds, or 40 seconds. When the high-shear mixing time is less than 10 seconds, sufficient mixing cannot be achieved, while when the mixing time is too long, exceeding 60 seconds, there may be a problem of too much air being incorporated into the neutralization solution, which may slow down the polymerization rate.

[0144] High-shear mixing can be performed using commercially available high-shear mixers, such as in-line high-shear mixers, high-shear batch mixers, or homogenizers.

[0145] Subsequently, iii) a final monomer composition is prepared by adding a foaming agent to the mixture of ii) after high-shear mixing.

[0146] As a blowing agent, inorganic blowing agents commonly used in the preparation of superabsorbent polymers can be used without restriction; for example, carbonate blowing agents can be used.

[0147] For example, carbonate foaming agents may be at least one selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium bicarbonate, magnesium bicarbonate, and magnesium carbonate, but this disclosure is not limited thereto.

[0148] Based on the total weight of the final monomer composition to be prepared, the amount of blowing agent can be above 100 ppm, above 300 ppm, or above 400 ppm, and below 2000 ppm, below 1500 ppm, or below 1000 ppm. When the content of blowing agent is too low, the prepared superabsorbent polymer may not have sufficient porosity due to insufficient bubbles during polymerization. When the content of blowing agent is too high, the porosity of the superabsorbent polymer may be too high, which may lead to a decrease in mechanical strength.

[0149] When adding a foaming agent, a magnetic stirrer or similar device can be used to stir the agent to ensure it is evenly dispersed. The stirring speed can be, for example, above 50 rpm, above 100 rpm, or above 150 rpm, and below 500 rpm, below 400 rpm, or below 300 rpm. The stirring time can be, for example, from 1 to 30 seconds, or from 5 to 20 seconds.

[0150] If necessary, the monomer composition may also include additives such as thickeners, plasticizers, storage stabilizers, and antioxidants. The additive may be appropriately added in at least one of steps i) to iii), preferably when the foam stabilizer is added in step i) or ii).

[0151] Subsequently, iv) hydrogel polymers are formed by thermal polymerization or photopolymerization of the monomer composition.

[0152] In the case of thermal polymerization, it is typically carried out in a reactor equipped with a stirring shaft, such as a kneader. Furthermore, thermal polymerization can be carried out at temperatures above about 80°C and below about 110°C, so that the internal crosslinking agent is not thermally decomposed. There are no particular limitations on the heating method for the above-mentioned polymerization temperature; heating can be achieved by supplying a heat medium to the reactor or by directly supplying a heat source. The available heat medium can be a heated fluid, such as steam, hot air, hot oil, etc., but the present invention is not limited to these. Moreover, the temperature of the heat medium supplied can be appropriately selected considering the type of heat medium, the heating rate, and the target heating temperature. Simultaneously, an electric heater or a gas heater can be used as a directly provided heat source, but the present disclosure is not limited to these. When thermal polymerization is carried out using a reactor equipped with a stirring shaft, a hydrogel polymer with a size of several centimeters to several millimeters can be obtained depending on the shape of the stirring shaft installed in the reactor. Specifically, the shape of the obtained hydrogel polymer can vary depending on the concentration of the monomer composition to be injected and the injection rate.

[0153] Meanwhile, in the case of photopolymerization, it can be carried out in a reactor equipped with a movable conveyor belt. However, the above-described polymerization method is merely an example, and this disclosure is not limited thereto. When polymerization is carried out in a reactor as described above, a hydrogel polymer in sheet form with a strip width can generally be obtained. The thickness of the hydrogel polymer sheet can vary depending on the concentration and injection rate of the monomer composition to be injected, and it is preferable to supply the monomer composition such that the thickness of the polymer sheet is from about 0.5 cm to about 5 cm. When the monomer composition is supplied to such a degree that the polymer sheet is too thin, the production efficiency may be low. When the thickness of the polymer sheet exceeds 5 cm, the polymerization reaction may not occur uniformly across the entire thickness due to the excessive thickness.

[0154] The polymerization temperature of the monomer composition is not particularly limited, for example, it can be from 80°C to 120°C, preferably from 90°C to 110°C. Furthermore, the polymerization time of the monomer composition is not particularly limited, and can be adjusted to approximately 30 seconds to 60 minutes.

[0155] Typically, the moisture content of hydrogel polymers can be from about 40% to about 80% by weight. Here, "moisture content" in this invention refers to the moisture content in the total weight of the polymer, which is the value obtained by subtracting the weight of the dried polymer from the total weight of the polymer. Specifically, the moisture content is defined as the value calculated by measuring the weight loss caused by moisture evaporation in the polymer during the drying process of increasing the polymer temperature by infrared heating. The drying conditions for measuring the moisture content are: heating to about 180°C and maintaining at 180°C for a total drying time of 20 minutes (including a 5-minute heating step).

[0156] Subsequently, the hydrogel polymer is dried, pulverized, and classified (v).

[0157] If necessary, further coarse grinding can be performed before drying to improve the efficiency of the drying step.

[0158] At this time, there are no restrictions on the configuration of the pulverizer used. Specifically, at least one pulverizer selected from the group consisting of vertical pulverizer, turbine cutter, turbine mill, rotary cutting mill, cutting mill, disc mill, crusher, shredder, and disc cutter can be used, but this disclosure is not limited thereto.

[0159] In the pulverization step, the hydrogel polymer can be pulverized to a particle size of about 2 mm to about 10 mm.

[0160] Due to the high moisture content of hydrogel polymers, pulverizing them into particles smaller than 2 mm is technically difficult, and agglomeration may occur between the pulverized particles. On the other hand, when pulverizing to a particle size larger than 10 mm, the effect on increasing the efficiency of subsequent drying steps may not be significant.

[0161] The hydrogel polymer, which has been pulverized according to the above method, is dried, or dried immediately after polymerization without a pulverization step. The drying temperature can be from about 150°C to about 250°C. When the drying temperature is below 150°C, the drying time may become too long, and the physical properties of the resulting superabsorbent polymer may decrease. When the drying temperature exceeds 250°C, only the polymer surface is over-dried, which may produce fine powder in the subsequent pulverization process, and the physical properties of the final superabsorbent polymer may decrease. Therefore, drying is preferably carried out at a temperature of about 150°C to about 200°C, and more preferably at a temperature of about 160°C to about 180°C.

[0162] Meanwhile, considering process efficiency, the drying time can be from about 20 minutes to about 90 minutes, but is not limited to this.

[0163] There are no particular restrictions as long as the drying method used in the drying process of hydrogel polymers is one commonly used in the drying process. Specifically, the drying step can be carried out by providing hot air, infrared radiation, microwave radiation, ultraviolet radiation, etc. After the drying step, the moisture content of the polymer can be from about 0.1% by weight to about 10% by weight.

[0164] Subsequently, a step is performed to pulverize the dried polymer obtained through the drying step.

[0165] The polymer powder obtained after the pulverization step can have a particle size of about 150 μm to about 850 μm. As a pulverizer for pulverizing to said particle size, a pin mill, hammer mill, screw mill, roller mill, disc mill, hand-cranked mill, etc. can be used, but the present invention is not limited thereto.

[0166] In addition, a separate process can be performed to classify the polymer powder obtained after the pulverization step according to the particle size, and the polymer powder can be classified according to the particle size at a predetermined weight ratio.

[0167] For example, dried polymers can be pulverized and classified as follows: First, the dried polymer is pulverized using a pulverizer (e.g., a cutting mill), and then classified using a classifier (e.g., a sieve shaker). The particles obtained after classification, with a mesh size of 20 or larger (particles with a diameter of 841 μm or larger), are then fed back into the pulverizer for a second pulverization, and then classified again. This pulverization and classification method is described in more detail in the following examples. However, the pulverization and classification methods in the preparation methods of this disclosure are not limited to the above examples, and various pulverization and classification methods used in the field of this disclosure can be applied.

[0168] In addition, in order to form a surface crosslinking layer on the superabsorbent polymer (i.e., the base resin) that has been dried, pulverized and graded, a further step vi) of crosslinking the surface of the base resin in the presence of a surface crosslinking agent was performed.

[0169] Specifically, a surface crosslinking agent is mixed with a base resin, and then the mixture is heated to carry out a surface crosslinking reaction on the pulverized polymer.

[0170] The surface crosslinking step is a process in which a crosslinking reaction is induced on the surface of a base resin in the presence of a surface crosslinking agent, thereby forming a superabsorbent polymer with improved physical properties. Through surface crosslinking, a surface crosslinked layer (surface modified layer) is formed on the surface of the base resin.

[0171] A surface crosslinking agent is applied to the surface of the superabsorbent polymer particles. Therefore, the surface crosslinking reaction occurs on the surface of the superabsorbent polymer particles, which improves the degree of surface crosslinking without substantially affecting the interior of the particles. Consequently, the surface-crosslinked superabsorbent polymer particles exhibit a higher degree of crosslinking at the surface than at the interior.

[0172] Simultaneously, compounds capable of reacting with the functional groups of the polymer are used as surface crosslinking agents. For example, polyols, polyepoxides, polyamines, halogenated epoxides, condensation products of halogenated epoxides, oxazoline compounds, or alkylene carbonates can be used.

[0173] Specifically, as a polyol compound, at least one of the following can be used: diethylene glycol, triethylene glycol, tetraethylene glycol or polyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-buten-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,2-cyclohexanediol, etc.

[0174] In addition, as a polyepoxide compound, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, or glycidyl ether can be used. As a polyamine compound, at least one selected from ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, and polyamide polyamine can be used.

[0175] Furthermore, epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin can be used as halogenated epoxy compounds. Meanwhile, 2-azolidinone can be used as monoazolidinone, diazolidinone, or polyazolidinone compounds.

[0176] In addition, ethylene carbonate and the like can be used as alkylene carbonate compounds.

[0177] Surface crosslinking agents can be used alone or in combination with each other.

[0178] The amount of surface crosslinking agent added can be appropriately selected according to the type of surface crosslinking agent added or the reaction conditions, and based on 100 parts by weight of base resin, it can be from about 0.001 parts by weight to about 5 parts by weight, preferably from about 0.01 parts by weight to about 3 parts by weight, more preferably from about 0.05 parts by weight to about 3 parts by weight.

[0179] When the content of the surface crosslinking agent is too low, it is difficult for the surface crosslinking reaction to occur. When the content of the surface crosslinking agent exceeds 5 parts by weight per 100 parts by weight of polymer, the absorption performance (such as water retention capacity) may be degraded due to excessive surface crosslinking reaction.

[0180] Furthermore, there are no particular limitations on the method of adding the surface crosslinking agent to the base resin powder. For example, methods such as adding the surface crosslinking agent and the base resin powder into a reactor for mixing, spraying the surface crosslinking agent onto the base resin powder, or mixing the base resin powder and the surface crosslinking agent while continuously feeding them into a continuously operating mixer can be used.

[0181] When adding a surface crosslinking agent, water can be added separately in the form of a surface crosslinking solution. The advantage of adding water is that the surface crosslinking agent can be uniformly dispersed in the polymer. At this point, the amount of water added can be appropriately controlled to ensure uniform dispersion of the surface crosslinking agent, prevent agglomeration of the polymer powder, and optimize the surface penetration depth of the surface crosslinking agent. 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.

[0182] Meanwhile, the base resin is surface modified by heating the mixture of the base resin and the surface crosslinking solution to increase the temperature.

[0183] Depending on the type of surface crosslinking agent, surface modification can be carried out under known conditions, for example, at a temperature of 100°C to 200°C for 20 to 60 minutes. In a more specific embodiment, when the surface crosslinking agent is a polyvalent epoxy compound, it can be carried out by heating at a temperature of about 120°C to about 180°C, or about 120°C to about 150°C for about 10 to about 50 minutes, or about 15 to about 40 minutes. When the surface modification temperature is below 100°C or the reaction time is too short, the surface crosslinking reaction cannot occur normally, and the light transmittance may decrease. When the temperature exceeds 200°C or the reaction time is too long, the water retention capacity may decrease.

[0184] There are no particular limitations on the heating method for the surface crosslinking reaction. A heat medium or a heat source can be provided directly. The heat medium can be a heated fluid, such as steam, hot air, or hot oil, but the invention is not limited to this. Furthermore, the temperature of the heat medium provided can be appropriately selected considering the type of heat medium, the heating rate, and the target heating temperature. Simultaneously, an electric heater or a gas heater can be used as a directly installed heat source, but this disclosure is not limited to this.

[0185] After surface modification, an additional step can be performed to classify the obtained superabsorbent polymer powder according to particle size.

[0186] According to the preparation method described above, excellent foam stabilization can be achieved even with a small amount of foam stabilizer, thus yielding a superabsorbent polymer with a large number of small and uniform pores. Therefore, the superabsorbent polymer exhibits excellent absorption rate, and its physical properties (e.g., surface tension) do not deteriorate.

[0187] For example, the superabsorbent polymer prepared according to the preparation method can have a vortex time (absorption rate) of less than 45 seconds, less than 42 seconds, less than 38 seconds, or less than 35 seconds. Since a shorter vortex time is considered better, the lower limit is theoretically 0 seconds, but it can be more than 5 seconds, more than 10 seconds, or more than 12 seconds. The method for measuring the absorption rate is described in more detail in the following examples.

[0188] In addition, because the particles have a near-spherical shape and contain small and uniform pores, the superabsorbent polymer prepared according to the above preparation method exhibits a high specific surface area, thereby exhibiting excellent absorption rate and absorption performance.

[0189] Specifically, when measuring particles with a diameter of 300 μm to 600 μm, the average sphericity of the superabsorbent polymer prepared according to the above preparation method is calculated to be 0.75 or higher according to Equation 2, while the average convexity is calculated to be 0.9 or lower according to Equation 3.

[0190] [Equation 2]

[0191] Circularity = Equivalent circumference / Actual particle circumference

[0192] [Equation 3]

[0193] Convexity = Perimeter of convex shell / Actual perimeter of particle

[0194] In equations 2 and 3

[0195] The actual particle perimeter is the actual perimeter of the projected image obtained by capturing a 3D image of the measured 3D particle as a 2D image.

[0196] When it is assumed that there exists a circle with an area equal to the area of ​​a projected image obtained by capturing a 3D image of the measured 3D particle as a 2D image, the equivalent circumference of that circle is the circumference of the corresponding particle, and

[0197] When it is assumed that the projected 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 circumference of the convex shell is the length of the elastic band.

[0198] Circularity is a parameter used to determine how close a particle is to a perfect circle, and is calculated as the ratio of the equivalent circumference of a circle to the actual circumference of the particle (Equation 2 above).

[0199] Therefore, the closer the roundness is to 1, the closer the 2D shape of the particle is to a circle. The further the roundness is from 1, the further the 2D shape of the particle is from a perfect circle. In terms of aspect ratio, the value can be 1 for both circles and squares, but in terms of roundness, a value of 1 can only be obtained when the 2D shape of the particle is a perfect circle.

[0200] At this point, the average roundness is measured after the sample is spread on the platform in any way through a vacuum in the measuring device, and the statistical result is obtained by ensuring a sample number (n) of 200 or more and taking their average.

[0201] In addition, convexity is a parameter used to measure particle profile and surface roughness, and is calculated using Equation 3.

[0202] Therefore, the value of convexity ranges from 0 to 1. When the convexity is close to 1, the particles can be considered to have a very smooth profile, while when the convexity is closer to 0, the particles can be considered to have a rough or uneven profile.

[0203] At this point, the average convexity is also measured after being spread on the platform in an arbitrary manner by the vacuum in the measuring device, in the same way as the average roundness, and statistical results are obtained by ensuring a sample number (n) of 200 or more and taking their average.

[0204] Meanwhile, roundness and convexity can be measured using various commercial instruments that quantify and analyze particle morphology using particle-based image analysis. As an example, these parameters can be measured using the Malvern Panalytical Morphologi 4, specifically through the following four steps, which will be described in more detail in the experimental examples below.

[0205] 1) Sample preparation: Using a particle classifier (e.g., a shaking sieve manufactured by Retsch), the superabsorbent polymer is classified for 10 minutes at an amplitude of 1.0 to prepare samples with a particle size of 300 μm to 600 μm. At this point, the particle size of the superabsorbent polymer can be measured according to EDANA (European Association for Disposable and Nonwoven Products) WSP 220.3.

[0206] 2) Image capture: The prepared sample is placed on the instrument platform and scanned at 2.5x magnification to capture images of individual particles.

[0207] 3) Image processing: For the captured image, measure the parameter values ​​of each particle, such as the projection image of the 3D particle captured as a 2D image, the equivalent diameter of the circle, the shortest diameter, the longest diameter, the actual particle circumference, the equivalent circumference of the circle, and the circumference of the convex shell.

[0208] 4) Based on the analysis data of each particle, the distribution of each parameter of all particles contained in the sample is obtained.

[0209] When the average sphericity of particles with a size of 300 μm to 600 μm in a superabsorbent polymer is less than 0.75, the particles are not spherical, resulting in a rapid absorption rate, but the balance between water retention capacity and pressurized absorption rate may deteriorate. When the average convexity of particles with a size of 300 μm to 600 μm in a superabsorbent polymer is greater than 0.9, the pore structure in the particles is underdeveloped or the particles have smooth surfaces, thus the absorption rate of the superabsorbent polymer may decrease. Therefore, a superabsorbent polymer with an average sphericity of particles with a size of 300 μm to 600 μm greater than 0.75 and an average convexity less than 0.90 can provide a superabsorbent polymer with an excellent balance between absorption rate and absorption performance.

[0210] More specifically, the average sphericity of the superabsorbent polymer particles with a particle size of 300 μm to 600 μm prepared according to the embodiments of this disclosure is 0.75 or higher, and 0.90, 0.85 or lower, or 0.83 or lower. Furthermore, for particles with a particle size of 300 μm to 600 μm, the average convexity is 0.7 or higher, 0.8 or higher, or 0.85 or higher, and 0.9 or lower.

[0211] In addition, the superabsorbent polymer meets the above-mentioned ranges of roundness and convexity, and the vortex time is less than 45 seconds, less than 42 seconds, less than 38 seconds or less than 35 seconds, and more than 0 seconds, more than 5 seconds, more than 10 seconds or more than 12 seconds.

[0212] The present invention will be described in more detail below through the following preferred embodiments, but these embodiments are for illustrative purposes only. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope and spirit of the invention. Therefore, it is clear that these changes and modifications are within the scope of the invention.

[0213] Example

[0214] Example 1

[0215] In a 3L glass container equipped with a stirrer and thermometer, 100g of acrylic acid, 0.5g of PEGDA 400 (polyethylene glycol diacrylate 400) as an internal crosslinking agent, and 0.01g of diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide as a photoinitiator were added and dissolved. Then, 890g of a 22% sodium hydroxide solution was added to prepare an aqueous monomer solution.

[0216] Sodium dodecyl sulfate (SDS) was added as a foam stabilizer to the monomer aqueous solution, and high-shear mixing was performed for 30 seconds using a linear high-shear mixer (Silverson, L5M-A) at a Reynolds number of 10,000. During high-shear mixing, the viscosity of the mixture at 45°C was confirmed to be 10 cP. Then, high-shear mixing was stopped, and solid sodium bicarbonate (SBC) was added as a foaming agent. The final monomer composition was prepared by stirring at 250 rpm for 5 seconds using a magnetic stirrer (IKA, C-MAG Hs7). In the above, based on the total amount of the final monomer composition, the amount of SDS added was 20 ppm, and the amount of SBC added was 1000 ppm.

[0217] The monomer composition was poured into a Vat-shaped tray (15cm wide × 15cm long) installed in a square polymerizer, the top of which was equipped with a light irradiation device and preheated to 80°C. Light irradiation was then applied to initiate polymerization. After 60 seconds of light irradiation, a further reaction was carried out for 120 seconds to obtain a sheet-like hydrogel polymer.

[0218] 150g of water was evenly sprayed onto 1500g of hydrogel polymer for lubrication, and then pulverized using a shredder with a 10mm perforated plate. The pulverized hydrogel polymer was dried in a dryer capable of alternating airflow direction. The hydrogel polymer was uniformly dried by blowing hot air at 180°C from bottom to top for 15 minutes, followed by blowing from top to bottom for 15 minutes, resulting in a dried powder with a moisture content of approximately 2% or less.

[0219] Next, the base resin was pulverized once using Fritsch's Pulverisette 19, which is equipped with a 12mm sieve. Then, the pulverized base resin was classified for 10 minutes at an amplitude of 1.0 using a Retsch shaking sieve. Subsequently, only particles larger than 20 mesh (particle size greater than 841 μm) were pulverized again using a Pulverisette 19 with a 1mm sieve. The pulverized superabsorbent polymer was then further classified for 10 minutes at an amplitude of 1.0 using a shaking sieve. The particles obtained from the first pulverization and classification were combined with those obtained from the second pulverization and classification to form the base resin (particle size from 150 μm to 850 μm).

[0220] Then, 6 parts by weight of a surface crosslinking aqueous solution containing 3 parts by weight of ethylene carbonate were sprayed onto 100 parts by weight of the obtained base resin powder, and stirred at room temperature to ensure that the surface crosslinking solution was uniformly distributed on the base resin powder. The base resin powder mixed with the surface crosslinking solution was then placed in a surface crosslinking reactor for a surface crosslinking reaction.

[0221] In this surface crosslinking reactor, the base resin powder was gradually heated from an initial temperature of approximately 80°C, and the reaction reached a maximum reaction temperature of 190°C after 30 minutes. After reaching the maximum reaction temperature, the reaction was continued for another 15 minutes, and the final superabsorbent polymer sample was then removed. Following the surface crosslinking process, the superabsorbent polymer of Example 1 with a particle size ranging from 150 μm to 850 μm was prepared by grading using ASTM standard sieves.

[0222] Example 2

[0223] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 100 ppm of calcium stearate (Ca-st) with a particle size of 5 μm was added to replace SDS as a foam stabilizer based on the total amount of the final monomer composition.

[0224] Example 3

[0225] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 500 ppm of SBC was added based on the total amount of the final monomer composition.

[0226] Example 4

[0227] The superabsorbent polymer was prepared in the same manner as in Example 2, except that 400 ppm of SBC was added based on the total amount of the final monomer composition.

[0228] Comparative Example 1

[0229] Based on the total amount of the final monomer composition, 1000 ppm of SDS and 200 ppm of SBC were simultaneously added to a monomer aqueous solution prepared in the same manner as in Example 1, and stirred at 250 rpm for 30 seconds, rather than high-shear mixing, to prepare the monomer composition. Subsequently, polymerization, drying, pulverizing, grading, and surface crosslinking processes were performed in the same manner as in Example 1 to prepare the superabsorbent polymer.

[0230] Comparative Example 2

[0231] The superabsorbent polymer was prepared in the same manner as in Comparative Example 1, except that 1000 ppm of calcium stearate (Ca-st) with a particle size of 5 μm was added to replace SDS as a foam stabilizer based on the total amount of the final monomer composition.

[0232] Comparative Example 3

[0233] The superabsorbent polymer was prepared in the same manner as in Example 1, except that SBC and SDS were added simultaneously and then subjected to high-shear mixing.

[0234] Comparative Example 4

[0235] The superabsorbent polymer was prepared in the same manner as in Example 2, except that SBC and Ca-st were added simultaneously and then subjected to high-shear mixing.

[0236] Comparative Example 5

[0237] The superabsorbent polymer was prepared in the same manner as in Example 1, except that after adding SDS, it was stirred at 250 rpm for 30 seconds using a magnetic stirrer instead of high-shear mixing.

[0238] Comparative Example 6

[0239] The superabsorbent polymer was prepared in the same manner as in Example 1, except that after adding SDS, it was mixed at a high shear rate of 5000 for 30 seconds.

[0240] [Experimental Example]

[0241] The physical properties of the superabsorbent polymers prepared in the examples and comparative examples were measured using the following methods, and the results are summarized in Table 1.

[0242] (1) Vortex time (absorption rate, seconds)

[0243] The vortex time was measured according to the Japanese standard method (JIS K 7224). Specifically, 50 mL of 24°C brine (0.9 wt% sodium chloride aqueous solution) and a magnetic rod (8 mm in diameter and 31.8 mm in length) were placed in a 100 mL beaker and stirred at 600 rpm. 2.0 g of superabsorbent polymer was added to the stirred brine, and the time taken until the vortex disappeared was measured in seconds to calculate the vortex time.

[0244] (2) Centrifugation Retention Capacity (CRC)

[0245] The centrifugal retention capacity of each polymer was measured according to EDANA WSP 241.3.

[0246] After uniformly inserting W0 (g, approximately 0.2 g) of superabsorbent polymer into a nonwoven cover and sealing it, the cover was immersed in brine (0.9 wt%) at room temperature. After 30 minutes, the cover was centrifuged at 250 G for 3 minutes to drain, and the weight W2 (g) of the cover was measured. Furthermore, the same procedure was performed without using the polymer, and the weight W1 (g) of the bag was measured. The CRC (g / g) was then calculated using the obtained weight values ​​according to Equation 2 below.

[0247] CRC(g / g)={[W2(g)-W1(g)] / W0(g)}–1

[0248] (3) Absorption rate at 0.7 psi (0.7 AUP)

[0249] The absorbance of each polymer at 0.7 psi was measured according to EDANA WSP 242.3.

[0250] A 400-mesh stainless steel sieve was installed at 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 uniformly dispersed onto the sieve. A piston capable of uniformly providing a 0.7 psi load was then placed on top. In this study, the piston's outer diameter was slightly less than 60 mm, with no gap between it and the inner wall of the cylinder, and the cylinder's movement was uninterrupted. At this point, the weight of the measuring device was W3 (g).

[0251] Subsequently, a 90 mm diameter, 5 mm thick glass filter was placed in a 150 mm diameter petri dish, and a brine solution consisting of 0.9 wt% sodium chloride was poured into the dish. The brine was poured until its surface level was equal to the top surface of the glass filter. A 90 mm diameter filter paper was then placed on top. The measuring device was placed on the filter paper and allowed to absorb the liquid under load for 1 hour. After 1 hour, the measuring device was lifted and its weight W4 (g) was measured.

[0252] Then, the pressurized absorption rate (g / g) is calculated using the obtained weight value according to the following equation.

[0253] AUP(g / g)=[W5(g)–W4(g)] / W3(g)

[0254] (4) Surface tension (S / T)

[0255] The surface tension of the superabsorbent polymers in the following examples and comparative examples was measured.

[0256] ① First, place 40g of brine consisting of 0.9% sodium chloride in a 50mL beaker and stir at 350rpm for 3 minutes.

[0257] ② Subsequently, 0.5g of superabsorbent polymer was added to the stirred solution, stirred for 3 minutes, and then allowed to stand for 2 minutes to allow the swollen superabsorbent polymer to precipitate at the bottom.

[0258] ③ Next, use a pipette to extract the supernatant (the solution directly below the surface) and transfer it to another clean cup. Use a surface tension meter (Force Tensiometer-K11 / K100, manufactured by Kruss) to measure the surface tension.

[0259] (5) Determine the shape parameters of superabsorbent polymer particles

[0260] The roundness and convexity of the superabsorbent polymers of the examples and comparative examples were measured using Morphologi 4 manufactured by Malvern Panalytics by the following method.

[0261] ① Sample preparation: The superabsorbent polymer was fractionated for 10 minutes at an amplitude of 1.0 using a Retsch shaker to prepare 1g of sample with individual particle sizes ranging from 300μm to 600μm. Figure 1 shows the sample dispersion unit settings at this time.

[0262] ② Image capture: Place the prepared sample on the instrument's stage and scan it at 2.5x magnification to capture the projected image of a single particle. The illumination and optical selection settings are shown in Figures 2 and 3, respectively.

[0263] ③ Image Processing: For the captured image, the parameter values ​​of each particle are measured, such as the projected image of the 3D particle as a 2D image, the equivalent diameter of the circle, the shortest diameter, the longest diameter, the actual particle circumference, the equivalent circumference of the circle, and the circumference of the convex shell. At this time, the scan area settings and particle filter settings are displayed in Figures 4 and 5, respectively.

[0264] ④ Based on the analysis data of each particle, the distribution of each parameter of all particles contained in the sample is obtained.

[0265] Calculate the roundness and convexity using Equations 2 and 3 respectively.

[0266] [Equation 2]

[0267] Circularity = Equivalent circumference / Actual particle circumference

[0268] [Equation 3]

[0269] Convexity = Perimeter of convex shell / Actual perimeter of particle

[0270] In equations 2 and 3

[0271] The actual particle perimeter is the actual perimeter of the projected image obtained by capturing a 3D image of the measured 3D particle as a 2D image.

[0272] When it is assumed that there exists a circle with an area equal to the area of ​​a projected image obtained by capturing a 3D image of the measured 3D particle as a 2D image, the equivalent circumference of that circle is the circumference of the corresponding particle, and

[0273] When it is assumed that the projected 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 circumference of the convex shell is the length of the elastic band.

[0274] [Table 1]

[0275]

[0276] Referring to Table 1, the superabsorbent polymers of Examples 1 to 4, prepared by first adding a foam stabilizer to the monomer mixture, performing high-shear mixing, and then adding a foaming agent, exhibited excellent foam stabilization effects even with the use of small amounts of foam stabilizer in the manufacturing process. They also demonstrated excellent absorption performance without reducing surface tension, particularly showing a significantly improved absorption rate. Furthermore, it was confirmed that the superabsorbent polymers of Examples 1 to 4 have a near-spherical shape with an average sphericity of 0.75 or higher, and a large surface roughness with an average convexity of 0.9 or lower.

[0277] However, it can be confirmed by Comparative Examples 1 to 6 that the above-mentioned effects cannot be achieved if the preparation method of this disclosure is not met, for example, if foam stabilizer and foaming agent are added simultaneously, if high-shear mixing is not performed after adding foam stabilizer, or if the Reynolds number is less than 10,000 during high-shear mixing.

Claims

1. A method for preparing a superabsorbent polymer, the method comprising the following steps: i) Acrylic monomers having at least partially neutralized acidic groups; internal crosslinking agents; Mix with polymerization initiator; ii) Add a foam stabilizer to the mixture of i) and mix under high shear at a Reynolds number of 10,000 or higher for 10 to 60 seconds; iii) Prepare a monomer composition by adding a foaming agent to the mixture after the high shear mixing of ii), wherein the content of the foam stabilizer in the monomer composition is 10 ppm or more and less than 200 ppm, and the content of the foaming agent in the monomer composition is 100 ppm to 2000 ppm; iv) Prepare a hydrogel polymer by polymerizing the monomer composition; and v) Dry, pulverize, and classify the hydrogel polymer.

2. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The high-shear mixing is performed for 10 to 50 seconds.

3. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The high-shear mixing is carried out at Reynolds numbers of 10,000 to 20,000.

4. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The content of the foam stabilizer in the monomer composition is more than 20 ppm and less than 200 ppm.

5. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The content of the foaming agent in the monomer composition is from 100 ppm to 1500 ppm.

6. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The 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.

7. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The foam stabilizer is selected from at least one of the group consisting of cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants.

8. The method for preparing the superabsorbent polymer as described in claim 1, wherein, The foaming agent is sodium bicarbonate, and the foam stabilizer is calcium stearate or sodium dodecyl sulfate.

9. The method for preparing the superabsorbent polymer according to claim 1, the method further comprising step (vi): forming a surface crosslinking layer by further crosslinking the surface of the superabsorbent polymer obtained in step (v) in the presence of a surface crosslinking agent.

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