Method for producing superabsorbent polymer

Abstract

The present disclosure relates to a method for preparing a superabsorbent polymer. More specifically, the present invention relates to a method for preparing a superabsorbent polymer, which controls the initial pore formation rate of the polymer during drying, thereby reducing the content of undried debris generated, thereby producing a superabsorbent polymer having excellent drying efficiency and excellent absorption properties.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10 - 2022 - 01181773, filed with the Korean Intellectual Property Office on December 22, 2022, the disclosure of which is incorporated herein by reference in its entirety. Technical field

[0003] The present disclosure relates to a method for preparing a superabsorbent polymer. More specifically, the present invention relates to a method for preparing a superabsorbent polymer having excellent drying efficiency and excellent absorption properties, the method reducing the content of undried debris generated by controlling the initial pore formation rate during the drying process of the polymer. Background art

[0004] A superabsorbent polymer (SAP) is a synthetic polymer material capable of absorbing 500 to 1000 times its own weight of water. Various manufacturers have named it different names, such as SAM (superabsorbent material), AGM (absorbent gel material), etc. Such superabsorbent polymers began to be practically applied to sanitary products and are now widely used in water - retaining soil products for horticulture, water - stop materials for civil engineering and construction, sheets for raising seedlings, preservatives for the food distribution field, dressing materials, etc.

[0005] These superabsorbent polymers have been widely applied in the field of sanitary materials, such as diapers or sanitary napkins. In such sanitary materials, the superabsorbent polymer is usually included in a state dispersed in pulp. However, in recent years, efforts have been made to provide sanitary materials, such as diapers with a thinner thickness. As part of these efforts, the development of so - called pulp - free diapers, etc., in which the pulp content is reduced or no pulp is used at all, is being actively promoted.

[0006] In the case of the above - mentioned sanitary materials with reduced or no pulp content, the superabsorbent polymer is included at a relatively high ratio, such that the superabsorbent polymer particles are inevitably included in multiple layers in the sanitary material. In order for the entire superabsorbent polymer particles included in the multiple layers to more effectively absorb a large amount of liquid such as urine, the superabsorbent polymer basically needs to exhibit excellent absorption performance as well as a high absorption rate.

[0007] Meanwhile, the preparation of such a superabsorbent polymer is generally carried out by performing the following steps: a step of polymerizing monomers to prepare a hydrogel polymer containing a large amount of water; and a step of drying the hydrogel polymer and then crushing the hydrogel polymer into polymer particles having a desired particle size. However, when the hydrogel polymer is dried and then crushed as described above, a large amount of fine powder is generated, which has the problem of degrading the physical properties of the superabsorbent polymer to be prepared.

[0008] In order to reuse fine powder, the fine powder is usually coalesced by mixing it with water to produce a fine reconstitute, and then the fine reconstitute produced by processes such as drying / crushing / classifying is added. However, the water used here may increase the energy consumption during the drying process and increase the load on the equipment, thus reducing the productivity of manufacturing superabsorbent polymers.

[0009] In addition, the hydrogel polymer polymerized during the process of the superabsorbent polymer preparation method has the property of coalescing with each other, and when it is made into coalesced fine particles, the cohesive force increases, resulting in a problem of poor drying during the subsequent drying process. Specifically, this drying process is usually carried out by applying hot air in a belt dryer equipped with a perforated plate. As the cohesive force of the hydrogel polymer increases, the porosity in the drying layer significantly decreases, thereby increasing the pressure difference of the hot air and increasing the amount of undried crumbs. Therefore, warping occurs on the outside of the drying layer, making it difficult to dry in large quantities.

[0010] In other words, when the non-drying rate increases or when the dried crumbs warp, it is difficult to achieve the desired physical properties of the product, and defects may occur. Therefore, there is a continuous need to develop technologies that improve drying efficiency without these problems. Summary of the Invention

[0011] Technical Problem

[0012] Therefore, a superabsorbent polymer is provided that can achieve excellent drying efficiency and excellent absorption properties by controlling the initial porosity formation rate of the polymer during the drying process and reducing the content of undried crumbs generated.

[0013] Technical Solution

[0014] To solve the above problems, according to an embodiment of the present disclosure, a method for preparing a superabsorbent polymer is provided, the method comprising the following steps:

[0015] Polymerizing a monomer composition comprising a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized (step 1);

[0016] Neutralizing at least a part of the acidic groups of the polymer (step 2);

[0017] Micronizing the polymer in the presence of a surfactant (step 3); and

[0018] Drying the neutralized and micronized polymer to prepare base resin particles (step 4),

[0019] Among them, in step 4, the initial pore formation rate of the polymer measured after 30% to 60% of the total drying time is 0.8% / min to 1.7% / min.

[0020] Advantageous effects

[0021] According to the method for preparing a superabsorbent polymer of the present disclosure, by controlling the initial pore formation rate of the polymer during the drying process, thereby reducing the content of undried debris generated during the drying process, a superabsorbent polymer having excellent absorption properties and excellent drying efficiency can be provided.

[0022] In addition, according to the method for preparing a superabsorbent polymer of the present disclosure, particles having a coalesced fine particle morphology are achieved by performing polymerization, neutralization, and micronization in an unneutralized state under specific conditions. Therefore, the surface area increases, and thus a superabsorbent polymer that exhibits excellent absorption properties and has a significantly improved absorption rate can be provided. Description of the drawings

[0023] Figure 1 is a flow chart of a conventional method for preparing a superabsorbent polymer. Detailed description of the invention

[0024] The terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, the singular forms should also include the plural forms. It should be further understood that when the terms "comprising", "having", or "possessing" are used in this specification, the presence of the described features, steps, components, or combinations thereof is specified, but the presence or addition of one or more other features, steps, components, or combinations thereof is not excluded.

[0025] Since the present invention can be modified in various ways and has various forms, specific embodiments of the present invention are shown by way of example and will be described in detail herein. However, it is not intended to limit the present invention to the specific forms disclosed, and it should be understood that the present invention includes all modifications, equivalents, and substitutions within the spirit and scope of the present invention.

[0026] The terms used herein are only intended to refer to specific embodiments and are not intended to limit the present invention. In addition, as used herein, the singular forms also include the plural forms unless the phrase clearly indicates the contrary.

[0027] The term "polymer" in the present disclosure is in a state of polymerization of water-soluble ethylenically unsaturated monomers and may include all water content ranges or all particle size ranges.

[0028] In addition, the term "superabsorbent polymer" is used to include all crosslinked polymers or base resins in the form of powders consisting of superabsorbent polymer particles, where the crosslinked polymer is pulverized and the crosslinked polymer or base resin is further processed, such as dried, pulverized, classified, surface crosslinked, etc., to be in a state suitable for commercialization, depending on the context.

[0029] In addition, the term "fine powder" refers to particles in the superabsorbent polymer particles with a particle size less than 150 μm. The particle size of these polymer particles can be measured according to EDANA (European Disposables and Nonwovens Association) WSP 220.3.

[0030] In addition, the term "chopping" refers to cutting the hydrogel polymer into small pieces in millimeters to improve the drying efficiency, which is used separately from pulverizing to the micron or normal particle level.

[0031] In addition, the term "micronizing, micronization" refers to pulverizing the hydrogel polymer into a particle size of dozens to hundreds of microns, which is used separately from "chopping".

[0032] Hereinafter, the preparation method of the superabsorbent polymer and the superabsorbent polymer according to a specific embodiment will be described in more detail.

[0033] According to an embodiment of the present disclosure, a method for preparing a superabsorbent polymer is provided, the method comprising the following steps:

[0034] Polymerizing a monomer composition containing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized (step 1);

[0035] Neutralizing at least a part of the acidic groups of the polymer (step 2);

[0036] Micronizing the polymer in the presence of a surfactant (step 3); and

[0037] Drying the neutralized and micronized polymer to prepare base resin particles (step 4),

[0038] wherein in step 4, the initial pore formation rate of the polymer measured after 30% to 60% of the total drying time is 0.8% / min to 1.7% / min.

[0039] Generally, the hydrogel polymer obtained by polymerizing acrylic monomers undergoes processes such as drying, pulverizing, classifying, and surface crosslinking, and is sold as a superabsorbent polymer in powder form.

[0040] The drying process of the hydrogel polymer during the manufacturing process is usually carried out by applying hot air in a belt dryer equipped with a perforated plate. However, there is a problem that the porosity in the drying layer is significantly reduced due to the increased cohesion of the hydrogel polymer. In this case, the pressure difference of the hot air increases, resulting in an increase in the amount of undried debris. Therefore, warping occurs on the outside of the drying layer, making it difficult to dry in large quantities and resulting in a decrease in processing efficiency. Therefore, when over-drying to reduce the amount of undried debris, the water content of the base resin is reduced to about 1 wt% to 2 wt%, which not only increases the generation of fine powder but also reduces the process efficiency due to energy consumption.

[0041] When the water content of the base resin is excessively reduced due to over-drying or the generation of undried debris increases, it is difficult to achieve the desired physical properties of the product and defects may occur. Therefore, the present inventors have conducted repeated studies to solve this problem. As a result, the present inventors have confirmed that when using a hydrogel polymer polymerized in an un-neutralized state for the drying process and controlling the pore formation rate within a specific range at the start of drying, a base resin with a relatively high water content can be manufactured while minimizing the generation of undried debris, and thus completed the present invention.

[0042] Meanwhile, in recent years, there have been continuous attempts to provide a superabsorbent polymer that exhibits a more improved absorption rate.

[0043] The most common method of increasing the absorption rate is to expand the surface area of the superabsorbent polymer by forming a porous structure in the superabsorbent polymer. To increase the surface area of the superabsorbent polymer, a method of forming a porous structure in the base resin powder by including a foaming agent in the monomer composition and performing crosslinking polymerization is usually selected.

[0044] However, the use of a foaming agent has the disadvantages of reducing the physical properties of the superabsorbent polymer (such as surface tension, permeability, or bulk density) and increasing the amount of fine powder generated. Therefore, there is a continuous demand for the development of a technology that can improve the absorption rate of the superabsorbent polymer without using a foaming agent.

[0045] Meanwhile, a superabsorbent polymer is usually prepared by crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer, then drying the formed hydrogel polymer, and subsequently crushing it to a desired particle size. At this time, a chopping process of cutting the hydrogel polymer into several millimeters of particles is usually carried out before the drying process to facilitate the drying of the hydrogel polymer and improve the efficiency of the crushing process. However, due to the viscosity of the hydrogel polymer during this chopping process, the hydrogel polymer cannot be crushed into micron-sized particles and becomes coalesced gel. When the hydrogel polymer in the form of coalesced gel is dried, a plate-like dried product is formed. In order to crush it into micron-sized particles, a multi-stage crushing process is required, so there is a problem of generating many fine particles during this process.

[0046] Specifically, Figure 1 is a flow chart of a conventional method for preparing a superabsorbent polymer. Referring to Figure 1 , a superabsorbent polymer is usually prepared by including the following steps.

[0047] (Neutralization) Neutralize at least a part of the acidic groups of the water-soluble ethylenically unsaturated monomer;

[0048] (Polymerization) In the presence of an internal crosslinking agent and a polymerization initiator, form a hydrogel polymer by crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups;

[0049] (Chopping) Chop the hydrogel polymer;

[0050] (Drying) Dry the chopped hydrogel polymer; and

[0051] (Crushing / Classification) Crush the dried polymer and then classify it into normal particles and fine powder;

[0052] As described above, the chopped hydrogel polymer has a coalesced gel shape with a size of about 1 cm to 10 cm. The chopped hydrogel polymer is laminated on a belt made of a perforated plate at the bottom and dried by hot air supplied from the bottom or the top. Since the polymer dried by the above drying method has a plate shape instead of a granular shape, the crushing and classification steps are carried out by coarse crushing, then classification, then fine crushing again, and then classification, so that the particles to be prepared become normal particles, that is, particles with a particle size of 150 μm to 850 μm. Since the amount of the fine powder separated in the final classification step is about 20% to about 30% based on the total weight of the final superabsorbent polymer, the separated fine powder is mixed with an appropriate amount of water, reconstituted, and then placed in the chopping step or the step before drying for reuse.

[0053] However, when the finely reconstituted product mixed with water is reinjected into the pulverization or drying process for reusing the fine powder, problems such as an increase in equipment load and / or energy consumption have occurred, and the remaining fine powder that has not been classified has deteriorated the physical properties of the superabsorbent polymer.

[0054] Therefore, the present inventors have found that the amount of fine powder generated in the conventional preparation method is largely affected by the pulverization process. In addition, it has been confirmed that, while controlling coalescence, by adding a surfactant and a neutralizing agent during the pulverization process of the polymer for post-neutralization and more finely pulverizing, i.e., micronizing, the amount of fine powder generated during the preparation process can be significantly reduced.

[0055] Meanwhile, a method of adding a surfactant has been proposed to reduce the adhesiveness of the hydrogel polymer during the chopping process. However, when a surfactant is added during the chopping process, due to the high water retention capacity of the hydrogel polymer, the surfactant penetrates into the interior of the hydrogel polymer rather than existing at the interface of the hydrogel polymer. Therefore, there is a problem that the surfactant cannot function properly.

[0056] The chopped particles are formed into particles with a size of several millimeters to several centimeters, so the surface area can be increased to a certain extent compared with the polymer before chopping, but it is difficult to expect an effective improvement in the absorption rate. Therefore, in order to improve the absorption rate, a method of increasing the surface area by kneading using the increased mechanical force during the chopping step can be considered. However, in this case, due to the viscosity of the polymer, excessive coalescence occurs, and after chopping, drying, and pulverization, only amorphous single particles with uneven particle surfaces are formed, and excessive kneading or pulverization may actually increase the extractable components.

[0057] After repeated studies to solve this problem, it was confirmed that when polymerization is first carried out in a state where the acidic groups are not neutralized to form a polymer, the hydrogel polymer is micronized in the presence of a surfactant, and then the acidic groups of the polymer are neutralized; when the acidic groups of the polymer are neutralized to form a hydrogel polymer, and then the hydrogel polymer is micronized in the presence of a surfactant; or when the acidic groups present in the polymer are neutralized while micronizing, different from the conventional preparation method of superabsorbent polymers in which polymerization is carried out in a state where the acidic groups of the water-soluble ethylenically unsaturated monomer are neutralized, a large amount of surfactant exists on the surface of the polymer, and it can sufficiently play the role of reducing the high adhesiveness of the polymer, preventing excessive coalescence of the polymer, and controlling the coalescence state to the desired level.

[0058] Therefore, by forming the polymer into secondary particles in which primary particles are coalesced, and then carrying out the pulverization and drying processes under milder conditions, the amount of fine powder generated during this process can be significantly reduced.

[0059] In addition, when the polymer is pulverized in the presence of a surfactant, the hydrophobic functional groups contained in the surfactant impart hydrophobicity to the surface of the pulverized superabsorbent polymer particles, thereby reducing the frictional force between the particles and increasing the bulk density of the superabsorbent polymer. In addition, the hydrophilic functional groups contained in the surfactant can also bind to the superabsorbent polymer particles, so that the surface tension of the polymer is not reduced. Therefore, the superabsorbent polymer prepared according to the above method can have a high bulk density while exhibiting a similar surface tension compared to a polymer without using a surfactant.

[0060] In addition, when polymerization is first carried out in an unneutralized state to form a polymer, and then the acidic groups present in the polymer are neutralized, a polymer with a longer chain can be formed and the crosslinking is incomplete, thereby reducing the content of extractable components present in a non-crosslinked state.

[0061] When the superabsorbent polymer comes into contact with a liquid, the extractable components have the property of being easily eluted. When the content of the extractable components is high, most of the eluted extractable components remain on the surface of the superabsorbent polymer and make the superabsorbent polymer sticky, thereby reducing the permeability. Therefore, it is important to keep the content of the extractable components low in terms of permeability.

[0062] According to one embodiment of the present disclosure, since the polymerization is carried out in an unneutralized state, the content of the extractable components is reduced, and thus the permeability of the superabsorbent polymer can be improved.

[0063] In addition, the superabsorbent polymer prepared according to one embodiment of the present disclosure can have a uniform particle size distribution, and thus can provide a superabsorbent polymer having excellent absorption properties (such as water retention capacity, absorption capacity under pressure, rewettability, and absorption rate).

[0064] Hereinafter, a method for preparing a superabsorbent polymer according to one embodiment will be described in more detail for each step.

[0065] Step 1: Aggregation step

[0066] First, a monomer composition containing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator is polymerized to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized.

[0067] The above steps may include the step of preparing a monomer composition by mixing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator, and the step of polymerizing the monomer composition to form a polymer.

[0068] The water-soluble ethylenically unsaturated monomer can be any monomer commonly used for preparing superabsorbent polymers. Specifically, the water-soluble ethylenically unsaturated monomer can be a compound represented by the following Chemical Formula 1:

[0069] [Chemical Formula 1]

[0070] R-COOM'

[0071] In Chemical Formula 1,

[0072] R is a C2 to C5 alkyl group having an unsaturated bond, and

[0073] M' is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0074] Preferably, the monomer can be at least one selected from the group consisting of (meth)acrylic acid and monovalent (base) metal salts, divalent metal salts, ammonium salts, and organic amine salts of the acid.

[0075] When (meth)acrylic acid and / or its salt is used as the water-soluble ethylenically unsaturated monomer, it is beneficial to obtain a superabsorbent polymer having improved absorption performance. Additionally, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, (N,N)-dimethylaminoethyl (meth)acrylate, (N,N)-dimethylaminopropyl (meth)acrylamide can be used as the monomer.

[0076] In this document, the water-soluble ethylenically unsaturated monomer has an acidic group. As described above, a hydrogel polymer is formed by crosslinking and polymerizing a monomer in which at least a part of the acidic groups are neutralized by a neutralizing agent. Specifically, in the step of mixing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, a polymerization initiator, and a neutralizing agent, at least a part of the acidic groups of the water-soluble ethylenically unsaturated monomer are neutralized.

[0077] However, according to an embodiment of the present disclosure, the polymerization is first carried out in a state where the acidic groups of the water-soluble ethylenically unsaturated monomer are not neutralized to form a polymer.

[0078] Water-soluble ethylenically unsaturated monomers (such as acrylic acid) in a state where the acidic groups are not neutralized are liquid at room temperature and have high miscibility with a solvent (water), and thus exist in the monomer composition in the form of a mixed solution. However, water-soluble ethylenically unsaturated monomers in which the acidic groups are neutralized are solid at room temperature, have different solubilities depending on the temperature of the solvent (water), and have a lower solubility at a lower temperature.

[0079] Water-soluble ethylenically unsaturated monomers in which the acidic groups are not neutralized have a higher solubility or miscibility with a solvent (water) than monomers in which the acidic groups are neutralized, so that they do not precipitate even at low temperatures, which is beneficial for long-term polymerization at low temperatures. Therefore, by using water-soluble ethylenically unsaturated monomers in which the acidic groups are not neutralized for long-term polymerization, a polymer having a higher molecular weight and a uniform molecular weight distribution can be stably formed.

[0080] In addition, long-chain polymers can be formed, thereby achieving the effect of reducing the content of extractable components present in a non-crosslinked state due to incomplete polymerization or crosslinking.

[0081] In addition, when polymerization is first carried out in a state where the acidic groups of the monomers are not neutralized to form a polymer, and then micronization is carried out in the presence of a surfactant after neutralization; when neutralization is carried out after micronization in the presence of a surfactant; or when the acidic groups present in the polymer are neutralized while micronizing, a large amount of surfactant may be present on the surface of the polymer to fully play the role of reducing the adhesiveness of the polymer.

[0082] Considering the polymerization time and reaction conditions, the concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition can be about 20% by weight to 60% by weight, or about 20% by weight to 40% by weight.

[0083] The term "internal crosslinking agent" used herein is a term used to distinguish it from a surface crosslinking agent that is usually used to crosslink the surface of superabsorbent polymer particles, and is used to form a polymer having a crosslinked structure by introducing crosslinks between the unsaturated bonds of the above-mentioned water-soluble ethylenically unsaturated monomers.

[0084] The crosslinking in the above steps is carried out both on the surface and inside, but when the surface crosslinking process of the superabsorbent polymer particles is carried out, the surface of the finally prepared superabsorbent polymer particles can have a structure newly crosslinked by the surface crosslinking agent, while the inside of the particles can maintain the structure crosslinked by the internal crosslinking agent.

[0085] According to an embodiment of the present disclosure, the internal crosslinking agent may include any one or more of polyfunctional acrylate compounds, polyfunctional allyl compounds, or polyfunctional vinyl compounds.

[0086] Non-limiting examples of polyfunctional acrylate compounds may include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, butanediol di(meth)acrylate, butanediol di(meth)acrylate, hexanediol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol di(meth)acrylate, dipentaerythritol tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerol di(meth)acrylate, and glycerol tri(meth)acrylate. These substances can be used alone or in combination of two or more types.

[0087] Non-limiting examples of polyfunctional allyl compounds may include ethylene glycol diallyl ether, diethylene glycol diallyl ether, triethylene glycol diallyl ether, tetraethylene glycol diallyl ether, polyethylene glycol diallyl ether, propylene glycol diallyl ether, tripropylene glycol diallyl ether, polypropylene glycol diallyl ether, butanediol diallyl ether, butanediol diallyl ether, hexanediol diallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, dipentaerythritol diallyl ether, dipentaerythritol triallyl ether, dipentaerythritol tetraallyl ether, dipentaerythritol pentaallyl ether, trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, glycerol diallyl ether, and glycerol triallyl ether. These substances can be used alone or in combination of two or more types.

[0088] Non-limiting examples of polyfunctional vinyl compounds may include ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, tetraethylene glycol divinyl ether, polyethylene glycol divinyl ether, propylene glycol divinyl ether, tripropylene glycol divinyl ether, polypropylene glycol divinyl ether, butanediol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, pentaerythritol divinyl ether, pentaerythritol trivinyl ether, pentaerythritol tetravinyl ether, dipentaerythritol divinyl ether, dipentaerythritol trivinyl ether, dipentaerythritol tetravinyl ether, dipentaerythritol pentavinyl ether, trimethylolpropane divinyl ether, trimethylolpropane trivinyl ether, glycerol divinyl ether, and glycerol trivinyl ether. These substances can be used alone or in combination of two or more types. Preferably, pentaerythritol triallyl ether can be used.

[0089] In the above-mentioned polyfunctional allyl compounds or polyfunctional vinyl compounds, two or more unsaturated groups contained in the molecule combine with the unsaturated bonds of the water-soluble ethylenically unsaturated monomer or the unsaturated bonds of another internal crosslinking agent, thereby forming a crosslinked structure during the polymerization process. In addition, different from acrylate compounds containing an ester bond (-(C=O)O-) in the molecule, even during the neutralization process after the above-mentioned polymerization reaction, the crosslinking can be maintained more stably.

[0090] Therefore, the gel strength of the final superabsorbent polymer can be improved, the process stability can be improved during the discharge process after polymerization, and the amount of extractable components can be minimized.

[0091] The crosslinking polymerization of the water-soluble ethylenically unsaturated monomer in the presence of the internal crosslinking agent can be carried out in the presence of a polymerization initiator (if necessary), a thickener, a plasticizer, a storage stabilizer, an antioxidant, etc.

[0092] Based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer in the monomer composition, the amount of the internal crosslinking agent can be 0.01 part by weight to 5 parts by weight. For example, the amount of the internal crosslinking agent can be 0.01 part by weight or more, 0.05 part by weight or more, or 0.1 part by weight or more, and 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, 1 part by weight or less, or 0.7 part by weight or less. When the content of the internal crosslinking agent is too low, crosslinking cannot occur sufficiently, making it difficult to achieve a strength higher than an appropriate level. When the content of the internal crosslinking agent is too high, the internal crosslinking density increases, making it difficult to achieve the required water retention capacity.

[0093] The polymer formed using this internal crosslinking agent has a three-dimensional network structure, in which the main chain formed by polymerizing the water-soluble ethylenically unsaturated monomer is crosslinked by the internal crosslinking agent. When the polymer has a three-dimensional network structure in this way, compared with a polymer having a two-dimensional linear structure without additional crosslinking by the internal crosslinking agent, the centrifugal retention capacity and the absorption rate under pressure, which are various physical properties of the superabsorbent polymer, can be significantly improved.

[0094] According to an embodiment of the present disclosure, the step of forming a polymer by polymerizing the monomer composition can be carried out in a batch reactor.

[0095] In the conventional method for preparing a superabsorbent polymer composition, according to the energy source of the polymerization, the polymerization method is mainly divided into thermal polymerization and photopolymerization. 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 or in a container having a flat bottom.

[0096] Meanwhile, when photopolymerization is carried out in a reactor equipped with a movable conveyor belt or in a container with a flat bottom, a hydrogel polymer in the form of a sheet having a bandwidth can generally be obtained. At this time, the thickness of the polymer sheet can vary depending on the concentration and injection rate or amount of the monomer composition to be injected, and can be from about 0.5 cm to about 5 cm.

[0097] When the monomer composition is supplied to such a degree that the thickness of the polymer sheet is too thin, the production efficiency may be low, which is undesirable. When the thickness of the polymer on the sheet is increased for productivity, the polymerization reaction cannot occur uniformly throughout the thickness, making it difficult to form a high-quality polymer.

[0098] In addition, in a reactor equipped with a conveyor belt and a stirring shaft, polymerization is carried out continuously by supplying a new monomer composition to the reactor while the polymerization product is moving. Therefore, polymers with different degrees of polymerization are mixed, and thus, it is difficult to achieve uniform polymerization throughout the monomer composition, and the overall physical properties may deteriorate.

[0099] However, according to an embodiment of the present disclosure, when polymerization is carried out in a fixed-bed type in a batch reactor, the risk of mixing polymers with different degrees of polymerization is small, and thus, a polymer with uniform quality can be obtained.

[0100] In addition, the polymerization step is carried out in a batch reactor having a predetermined volume, and the polymerization reaction takes a longer time than the polymerization in a continuous manner in a reactor equipped with a conveyor belt, for example, 3 hours or more. Although the polymerization time is long as described above, the monomer is not easily precipitated because the polymerization is carried out on an unneutralized water-soluble ethylenically unsaturated monomer. Therefore, it is advantageous to carry out the polymerization for a long time.

[0101] Meanwhile, since the polymerization in the batch reactor of the present disclosure uses a thermal polymerization method, a thermal polymerization initiator is used as the polymerization initiator.

[0102] In addition, one or more initiators selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used as thermal polymerization initiators. Specifically, sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc. can be used as examples of persulfate initiators; 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethyl) isobutylamidine dihydrochloride, 2-(carbamoylazo) 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 thermal polymerization initiators are fully disclosed in "Principle of Polymerization (Wiley, 1981)" written by Odian, page 203, and the present disclosure is not limited thereto.

[0103] Based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer, the amount of the polymerization initiator can be 2 parts by weight or less. When the concentration of the polymerization initiator is too low, the polymerization rate may become slow, and a large amount of residual monomer may be extracted from the final product. On the contrary, when the concentration of the polymerization initiator is higher than the above range, the polymer chains forming the network are shortened, resulting in an increase in the extractable content and a decrease in the absorption rate under pressure, thereby reducing the physical properties of the polymer.

[0104] Meanwhile, in one embodiment of the present disclosure, the polymerization can be initiated by adding a reducing agent to form a redox pair with the initiator.

[0105] Specifically, when the initiator and the reducing agent are added to the polymer solution, they react with each other to form free radicals.

[0106] Since the formed free radicals react with the monomer, and the redox reaction between the initiator and the reducing agent is very reactive, even a small amount of the initiator and the reducing agent added will initiate the polymerization, so it is not necessary to increase the process temperature. Therefore, low-temperature polymerization is possible, and the change in the physical properties of the polymer solution can be minimized.

[0107] Even at a temperature near room temperature (25°C) or below room temperature, the polymerization reaction using a redox reaction can occur smoothly. For example, the polymerization reaction can be carried out at a temperature of 5°C or higher and 25°C or lower, or 5°C or higher and 20°C or lower.

[0108] In one embodiment of the present disclosure, when a persulfate initiator is used as the initiator, the reducing agent may be at least one selected from the group consisting of sodium metabisulfite (Na2S2O5), tetramethylethylenediamine (TMEDA), a mixture of iron(II) sulfate and EDTA (FeSO4 / EDTA), sodium formaldehyde sulfoxylate, and disodium 2-hydroxy-2-sulfinoacetate.

[0109] In one example, potassium persulfate is used as the initiator and disodium 2-hydroxy-2-sulfinoacetate is used as the reducing agent; ammonium persulfate is used as the initiator and tetramethylethylenediamine is used as the reducing agent; or sodium persulfate is used as the initiator and sodium formaldehyde sulfoxylate is used as the reducing agent.

[0110] In another embodiment of the present disclosure, when a hydrogen peroxide oxidant is used as the initiator, the reducing agent may be at least one selected from the group consisting of ascorbic acid, sucrose, sodium sulfite (Na2SO3), sodium metabisulfite (Na2S2O5), tetramethylethylenediamine (TMEDA), a mixture of iron(II) sulfate and EDTA (FeSO4 / EDTA), sodium formaldehyde sulfoxylate, disodium 2-hydroxy-2-sulfinoacetate, and disodium 2-hydroxy-2-sulfinoacetate.

[0111] If desired, the monomer composition may further contain additives such as thickeners, plasticizers, storage stabilizers, and antioxidants.

[0112] In addition, the monomer composition containing monomers may, for example, be in the form of a solution dissolved in a solvent such as water. The solid content in the monomer composition in solution form, that is, the concentration of monomers, internal crosslinkers, and polymerization initiators, may be appropriately adjusted considering the polymerization time and reaction conditions. For example, the solid content in the monomer composition may be 10 wt% to 80 wt%, 15 wt% to 60 wt%, or 30 wt% to 50 wt%.

[0113] At this time, any solvent capable of dissolving the above components can be used without limitation. For example, the solvent may be at least one combination selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide.

[0114] The polymer obtained in this way can form a polymer having a high molecular weight and a uniform molecular weight distribution by polymerizing using unneutralized ethylenically unsaturated monomers, and the content of extractable components can be reduced.

[0115] The polymer obtained in this way is a hydrogel polymer, and its water content can be 30% to 80% by weight. For example, the water content of the polymer can be more than 30% by weight, more than 45% by weight, more than 50% by weight, and less than 80% by weight, less than 70% by weight or less than 60% by weight.

[0116] When the water content of the polymer is too low, it may not be possible to effectively pulverize the polymer because it is difficult to ensure an appropriate surface area in the subsequent pulverization step. When the water content of the polymer is too high, it may be difficult to pulverize the polymer into the desired particle size due to an increase in the pressure applied in the subsequent pulverization step.

[0117] Here, the "water content" in the present disclosure refers to the water content in the total weight of the polymer, which is the value obtained by subtracting the weight of the dried polymer from the weight of the polymer. Specifically, the water content is defined as the value calculated by measuring the weight loss caused by the evaporation of water from the polymer during the process of raising the temperature of the polymer in a fragmented state and drying it by infrared heating. At this time, the drying conditions for measuring the water content are as follows: raise the temperature to about 180 °C and maintain it at 180 °C, and the total drying time is 40 minutes, including a heating step of 5 minutes.

[0118] Step 2: Neutralization step and Step 3: Micronization step

[0119] Next, a step of neutralizing at least a part of the acidic groups of the polymer (step 2) is carried out.

[0120] In this article, a basic material capable of neutralizing acidic groups, such as sodium hydroxide, potassium hydroxide or ammonium hydroxide, can be used as a neutralizing agent.

[0121] In addition, the degree of neutralization (which refers to the degree to which the acidic groups contained in the polymer are neutralized by the neutralizing agent) can preferably be 50 mol% to 90 mol%, 60 mol% to 85 mol% or 65 mol% to 75 mol%. The range of the degree of neutralization can vary depending on the final physical properties. When the degree of neutralization is too high, the absorption rate of the superabsorbent polymer may decrease, and the concentration of carboxyl groups on the particle surface is too low, making it difficult to appropriately perform surface crosslinking in the subsequent process, resulting in a decrease in the absorption rate or liquid permeability under pressure. On the contrary, too low a degree of neutralization will not only reduce the absorption rate of the polymer but also make the polymer have properties that are difficult to handle, such as those of an elastic rubber.

[0122] Simultaneously with step 2, or before or after step 2, a step of micronizing the polymer in the presence of a surfactant (step 3) is carried out. That is, step 2 and step 3 can be carried out successively, alternately or simultaneously.

[0123] Step 3 is a step of micronizing the polymer in the presence of a surfactant, in which the hydrogel polymer is not chopped into millimeters but simultaneously cut into dozens to hundreds of microns and coalesced. That is to say, this is a step of preparing secondary coalesced particles, in which the primary particles cut into dozens to hundreds of microns coalesce by imparting appropriate adhesiveness to the hydrogel polymer. The water-containing superabsorbent polymer particles are the secondary coalesced particles prepared in this step, having a normal particle size distribution and a greatly increased surface area, so the absorption rate can be significantly improved.

[0124] As described above, after mixing the polymer and the surfactant, the polymer is micronized in the presence of the surfactant, so that water-containing superabsorbent polymer particles (=neutralized and micronized polymer) in the form of secondary coalesced particles can be prepared, in which the superabsorbent polymer particles and the surfactant are mixed and then cut and coalesced.

[0125] In this article, "water-containing superabsorbent polymer particles" refer to particles with a water content of about 30% by weight or more, and are prepared by cutting the polymer into particles and then coalescing without a drying process. Therefore, they can have a water content of 30% to 80% by weight, similar to the polymer.

[0126] According to an embodiment of the present disclosure, the surfactant may be a compound represented by the following Chemical Formula 2 or a salt thereof, but the present disclosure is not limited thereto:

[0127] [Chemical Formula 2]

[0128]

[0129] In Chemical Formula 2,

[0130] A1, A2 and A3 are each independently a single bond, a carbonyl group, provided that at least one of them is a carbonyl group or wherein m1, m2 and m3 are each independently an integer from 1 to 8, each connected to the adjacent oxygen atom, and respectively connected to the adjacent R1, R2 and R3,

[0131] R1, R2 and R3 are each independently hydrogen, a C6 to C18 straight-chain or branched-chain alkyl group or a C6 to C18 straight-chain or branched-chain alkenyl group, and

[0132] n is an integer from 1 to 9.

[0133] The surfactant is mixed with the polymer and added, so that the micronization step can be easily carried out without coalescence.

[0134] The surfactant represented by Chemical Formula 2 is a nonionic surfactant and has excellent surface adsorption properties even with an unneutralized polymer through hydrogen bonding, and thus is suitable for achieving a desired coalescence control effect. On the other hand, in the case of an anionic surfactant other than the nonionic surfactant, when mixed with a polymer neutralized with a neutralizing agent such as NaOH or Na2SO4, it is adsorbed through Na + ions ionized on the carboxyl substituents of the polymer. When mixed with an unneutralized polymer, there is a problem that the adsorption efficiency of the polymer is relatively reduced due to competition with the anions of the carboxyl substituents of the polymer.

[0135] Specifically, in the surfactant represented by Chemical Formula 2, the hydrophobic functional groups are the terminal functional groups of R1, R2, and R3 (if not hydrogen), and the hydrophilic functional groups include the glycerol-derived part in the chain and the terminal hydroxyl groups (when A n is a single bond and R n is hydrogen, n = 1 to 3). Herein, the glycerol-derived part and the terminal hydroxyl groups are hydrophilic functional groups for improving the adsorption properties on the polymer surface. Therefore, coalescence of the superabsorbent polymer particles can be effectively suppressed.

[0136] In Chemical Formula 2, the hydrophobic functional groups of R1, R2, and R3 (if not hydrogen) are each independently a C6 to C18 straight-chain or branched alkyl group or a C6 to C18 straight-chain or branched alkenyl group. At this time, when R1, R2, and R3 (if not hydrogen) are alkyl groups or alkenyl groups having less than 6 carbon atoms, there is a problem that the chain length is too short to effectively control the coalescence of the pulverized particles. When R1, R2, and R3 (if not hydrogen) are alkyl groups or alkenyl groups having more than 18 carbon atoms, there may be a problem that the mobility of the surfactant is reduced, and it may not be effectively mixed with the polymer, and the unit price of the composition increases due to an increase in the cost of the surfactant.

[0137] Preferably, R1, R2, and R3 are hydrogen, a C6 to C18 straight-chain or branched alkyl group such as 2-methylhexyl, n-heptyl, 2-methylheptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, or n-octadecyl, or a C6 to C18 straight-chain or branched alkenyl group such as 2-hexenyl, 2-heptenyl, 2-octenyl, 2-nonenyl, n-decenyl, 2-undecenyl, 2-dodecenyl, 2-tridecenyl, 2-tetradecenyl, 2-pentadecenyl, 2-hexadecenyl, 2-heptadecenyl, or 2-octadecenyl.

[0138] The surfactant may be selected from the compounds represented by the following Chemical Formulas 2-1 to 2-14:

[0139] [Chemical Formula 2-1]

[0140]

[0141] [Chemical formula 2-2]

[0142]

[0143] [Chemical formula 2-3]

[0144]

[0145] [Chemical formula 2-4]

[0146]

[0147] [Chemical formula 2-5]

[0148]

[0149] [Chemical formula 2-6]

[0150]

[0151] [Chemical formula 2-7]

[0152]

[0153] [Chemical formula 2-8]

[0154]

[0155] [Chemical formula 2-9]

[0156]

[0157] [Chemical formula 2-10]

[0158]

[0159] [Chemical formula 2-11]

[0160]

[0161] [Chemical formula 2-12]

[0162]

[0163] [Chemical formula 2-13]

[0164]

[0165] [Chemical formula 2-14]

[0166]

[0167] Meanwhile, based on 100 parts by mass of the polymer, the amount of the surfactant can be about 3 parts by weight or less, preferably 2 parts by weight or less, 1 part by weight, 0.5 part by weight, 0.1 part by weight, 0.01 part by weight or less, or 0.001 part by weight or less. When too much surfactant is used, the surface tension may decrease, and thus the overall physical properties of the final superabsorbent polymer may deteriorate.

[0168] The method of mixing the surfactant with the polymer is not particularly limited and can be appropriately selected as long as it is a method capable of uniformly mixing the additive with the hydrogel polymer. Specifically, the surfactant can be dry-blended, dissolved in a solvent and then mixed in a solution state, or melted and then mixed.

[0169] For example, the surfactant can be mixed in the form of a solution dissolved in a solvent. At this time, all kinds of solvents including inorganic solvents and organic solvents can be used without limitation, but water is the most suitable in view of the ease of the drying process and the cost of the solvent recovery system. In addition, methods such as adding the surfactant and the polymer in the form of a solution to a reactor for mixing, spraying the solution after adding the polymer to a mixer, and continuously supplying the polymer and the solution to a continuously operating mixer for mixing can be used.

[0170] Meanwhile, according to one embodiment of the present disclosure, the step of neutralizing at least a part of the acidic groups of the polymer (step 2) and the step of micronizing the polymer in the presence of the surfactant (step 3) can be carried out successively, alternately or simultaneously. Meanwhile, the coalescence process of the polymer can be carried out simultaneously in the step of neutralizing the acidic groups.

[0171] In other words, a neutralizing agent can be added to the polymer to first neutralize the acidic groups, and then a surfactant can be added to the neutralized polymer to micronize the polymer mixed with the surfactant (carried out in the order of step 2 -> step 3). As an option, the polymer can be neutralized and micronized by simultaneously adding a neutralizing agent and a surfactant to the polymer (step 2 and step 3 are carried out simultaneously). As an option, the surfactant can be added first, and then the neutralizing agent can be added (carried out in the order of step 3 -> step 2). As an option, the neutralizing agent and the surfactant can be added alternately. As an option, the surfactant can be added first to micronize, then the neutralizing agent can be added to neutralize the product, and then additional surfactant can be added to the neutralized hydrogel polymer to further carry out the micronization process.

[0172] Meanwhile, in order to uniformly neutralize the entire polymer, it is advisable to leave a certain time interval between the addition of the neutralizing agent and the micronization process.

[0173] At least some or a large amount of surfactant may be present on the surface of the water-containing superabsorbent polymer particles.

[0174] In the present disclosure, "at least some or a substantial amount of surfactant is present on the surface of the water-containing superabsorbent polymer particles" means that at least some or a substantial amount of the surfactant is adsorbed or bound to the surface of the water-containing superabsorbent polymer particles. Specifically, the surfactant may be physically or chemically adsorbed on the surface of the superabsorbent polymer. More specifically, the hydrophilic functional groups of the surfactant may be physically adsorbed on the hydrophilic portion of the surface of the superabsorbent polymer by intermolecular forces such as dipole-dipole interactions. In this way, each hydrophilic portion of the surfactant is physically adsorbed on the surface of the superabsorbent polymer particles to surround the surface, and each hydrophobic portion in the surfactant is not adsorbed on the surface of the polymer particles, so each surfactant may be coated on the surface of the polymer particles in the form of a micelle structure. This is because the surfactant is not added during the polymerization of the water-soluble ethylenically unsaturated monomer, but is added in the micronization step after the polymer is formed. In this case, compared with the case where the surfactant is added during the polymerization and the surfactant is present inside the polymer, its function as a surfactant can be fully exerted, and at the same time, comminution and coalescence occur, so that particles with a large surface area in the form of coalesced fine particles can be obtained.

[0175] According to one embodiment of the present disclosure, the micronization step is carried out using a micronization device, for example, using a micronization device including a perforated plate having a plurality of holes with a diameter of 1 mm to 20 mm. When using a perforated plate having holes within the above range, it is more suitable for improving the efficiency of the drying process described later.

[0176] More specifically, the micronization device may include a main body (including a transfer space in which the polymer is transferred); a screw portion rotatably mounted inside the transfer space to move the polymer; a drive motor that provides a rotational driving force to the screw portion; a cutting portion mounted in the main body to comminute the polymer; and a perforated plate having a plurality of holes and capable of discharging the polymer comminuted by the cutting portion to the outside of the main body. Herein, the size of the holes provided in the perforated plate of the micronization device may be 1 mm to 20 mm, 5 mm to 15 mm, or 5 mm to 12 mm.

[0177] When the polymer mixed with the surfactant is micronized while controlling coalescence using the micronization device, a smaller particle size distribution is achieved, and then the drying and comminution processes can be carried out under milder conditions, thereby preventing the generation of fine particles and improving the physical properties of the superabsorbent polymer.

[0178] According to one embodiment of the present disclosure, the step of preparing the water-containing superabsorbent polymer particles by micronizing the polymer may be carried out one or more times, preferably 1 to 6 times, 1 to 4 times, or 1 to 3 times. This can be carried out using a plurality of micronizing devices or a single micronizing device including a plurality of perforated plates and / or a plurality of cutting parts. Alternatively, some micronizing devices may have a plurality of perforated plates and / or a plurality of cutting parts.

[0179] Step 4: Drying step

[0180] Next, a step of preparing the base resin particles by drying the neutralized and micronized polymer (or the water-containing superabsorbent resin particles) is carried out (step 4).

[0181] This step is a step of neutralizing at least a part of the acidic groups of the polymer and drying the moisture in the polymer obtained by micronizing the polymer in the presence of a surfactant.

[0182] Specifically, in the present disclosure, when using the hydrogel polymer polymerized in the unneutralized state as described above for the drying process, the pore formation rate is controlled within a specific range at the start of the drying process, thereby preparing the base resin particles having a relatively high water content while minimizing the generation of undried debris.

[0183] Therefore, in the drying step of step 4, when 30% to 60% of the total drying time has elapsed, the initial pore formation rate of the polymer satisfies 0.8% / minute to 1.7% / minute.

[0184] Herein, the initial pore formation rate is an index for determining the drying uniformity that can affect the formation of drying channels during the drying process. By adjusting the polymerization conditions, micronizing conditions, and drying conditions, the initial pore formation rate can be made to satisfy the above range. Herein, "initial" may mean when 30% to 60% of the total drying time has elapsed.

[0185] When the initial pore formation rate is less than 0.8% / minute, the drying channels may not be sufficiently fixed and non-uniform drying may occur. When it exceeds 1.7% / minute, heat exchange may not occur and hot air may leak. Therefore, the initial pore formation rate is preferably 1.0% / minute to 1.5% / minute, more preferably 1.0% / minute to 1.35% / minute.

[0186] The initial pore formation rate can be calculated by the following equation 1:

[0187] [Equation 1]

[0188] Initial pore formation rate (% / minute) = (R B 2 - R B1) / (t2 - t1)*100

[0189] In the above formula, R B refers to the pore volume ratio and is defined as the ratio of the pore volume (V B ) to the total volume (V).

[0190] t1 is the time when drying starts. For example, it can refer to the point where the neutralized and micronized polymer (or water-containing superabsorbent polymer particles) is added to the dryer or the heating point after addition. When calculating, t1 can be 0.

[0191] t2 is the time when 30% to 60% of the total drying time has elapsed. For example, it can refer to the time at any point when 30% to 60% of the total drying time has elapsed. Specifically, it can refer to the point when 30% to 60% has elapsed after the neutralized and micronized polymer (or water-containing superabsorbent polymer particles) is added to the dryer, or the point when 30% to 60% has elapsed after the heating process after addition.

[0192] R B 1 and R B 2 respectively refer to the R values at t1 and t2 B values.

[0193] The initial pore formation rate can be measured using XRM (manufacturer: ZEISS, model name: Xradia 620Versa), and the specific measurement method will be described in the experimental examples described later.

[0194] Preferably, the drying step of step 4 can be carried out using a fixed-bed type belt drying device. The difference between the fixed-bed type method and the moving type method lies in whether there is movement of the material during the drying process.

[0195] Fixed-bed type drying refers to a method in which the material to be dried is fixed on a bottom plate, such as a porous iron plate through which air can pass, and hot air passes through the material from bottom to top for drying. Moving type drying refers to a method in which the material to be dried is dried while being mechanically agitated. At this time, the direction in which the hot air passes through the material can be the same as or different from the circulation direction of the material. As an option, the material can be circulated in the dryer and dried by passing a heat transfer fluid through a separate pipe outside the dryer.

[0196] In the case of the drying process using a fixed-bed type belt dryer, a superabsorbent polymer is obtained in the form of a laminate, and there is a problem that the porosity in the dried laminate is significantly reduced due to an increase in the cohesive force of the hydrogel polymer. As a result, the pressure difference of the hot air increases, the content of undried material in the form of debris increases, and warping occurs on the outside of the dried product. Therefore, the present inventors have found that even when using a fixed-bed type belt dryer, drying while controlling the initial pore formation rate within the above range can produce a superabsorbent polymer having an appropriate water content without reducing the drying efficiency of the laminate or causing external warping.

[0197] The drying step of Step 4 can be carried out using hot air at 70°C to 250°C. The polymer polymerized, neutralized, and micronized according to the present disclosure can achieve excellent drying efficiency even under relatively mild conditions. In addition, the above initial pore formation rate can be satisfied within the above range of hot air. Preferably, the temperature of the hot air can be 75°C to 200°C.

[0198] The drying step of Step 4 can be carried out for 20 minutes to 90 minutes. The polymer polymerized, neutralized, and micronized according to the present disclosure can achieve excellent drying efficiency in a relatively short drying time even under relatively mild conditions. In addition, the above initial pore formation rate can be satisfied within the above range of hot air. Preferably, the drying can be carried out for 25 minutes to 60 minutes.

[0199] According to one embodiment of the present disclosure, the water content of the superabsorbent polymer obtained after drying can satisfy 3% by weight to 10% by weight, and the amount of undried material generated in the form of debris is significantly reduced while achieving a relatively high water content.

[0200] In the case of a superabsorbent polymer dried using a conventional fixed-bed type belt dryer, the water content of the polymer after drying is usually about 1% by weight to 2% by weight. However, the polymer polymerized, neutralized, and micronized according to the present disclosure is preferred because it can achieve excellent drying efficiency and a relatively high water content even when drying is carried out for a relatively short drying time under relatively mild conditions.

[0201] Crushing step

[0202] According to one embodiment of the present disclosure, a step of preparing superabsorbent polymer particles by pulverizing the superabsorbent polymer particles obtained by carrying out Step 4 can be carried out.

[0203] Specifically, the pulverizing step can be carried out by pulverizing the dried superabsorbent polymer particles to have a normal particle size, that is, a particle size of 150 μm to 850 μm.

[0204] The pulverizer for this purpose can be a vertical pulverizer, turbo cutter, turbo grinder, rotary cutter mill, cutter mill, disc mill, shred crusher, crusher, chopper or disc cutter, but the present disclosure is not limited thereto.

[0205] Alternatively, a pin mill, hammer mill, screw mill, roll mill, disc mill or jog mill can be used as the pulverizer, but the present disclosure is not limited thereto.

[0206] Meanwhile, in the micronization step of the preparation method of the present disclosure, superabsorbent polymer particles with a smaller particle size distribution than in the conventional chopping step can be provided, and when mobile drying is carried out, the water content after drying remains at a relatively high level of more than 10% by weight. Therefore, even when pulverizing with a smaller force under mild conditions, superabsorbent polymers with a very high content of normal particles having a particle size of 150 μm to 850 μm can be formed, thus greatly reducing the generation of fine powder.

[0207] Based on the total weight, the superabsorbent polymer particles prepared as described above may contain 80% by weight or more, 85% by weight or more, 89% by weight or more, 90% by weight or more, 92% by weight or more, 93% by weight or more, 94% by weight or more, or 95% by weight or more of superabsorbent polymer particles (normal particles) having a particle size of 150 μm to 850 μm. The particle size of these polymer particles can be measured according to EDANA (European Disposables and Nonwovens Association) WSP 220.3.

[0208] In addition, based on the total weight, the superabsorbent polymer particles may contain about 20% by weight or less, about 18% by weight or less, about 15% by weight or less, about 13% by weight or less, about 12% by weight or less, about 11% by weight or less, about 10% by weight or less, about 9% by weight or less, about 8% by weight or less, or about 5% by weight or less of fine powder having a particle size of less than 150 μm. This is in contrast to superabsorbent polymers prepared according to conventional preparation methods, which have a fine powder content of greater than about 20% to about 30%.

[0209] Classification step

[0210] According to one embodiment of the present disclosure, after pulverizing the superabsorbent polymer particles, it may further include a step of classifying the pulverized superabsorbent polymer particles according to particle size.

[0211] Preferably, particles having a particle size of 150 μm to about 850 μm are classified, and only particles having such a particle size can be commercialized after the surface crosslinking reaction. More specifically, the classified superabsorbent polymer particles have a particle size of 150 μm to 850 μm, may contain more than 50% by weight of particles having a particle size of 300 μm to 600 μm, and less than 3% by weight of fine powder having a particle size of less than 150 μm.

[0212] Surface crosslinking step

[0213] According to one embodiment of the present disclosure, after pulverizing and / or classifying the superabsorbent polymer particles, it may further include a step of forming a surface crosslinked layer on at least a part of the surface of the superabsorbent polymer particles in the presence of a surface crosslinking agent.

[0214] Through the above steps, the crosslinked polymer contained in the superabsorbent polymer particles can be further crosslinked by the surface crosslinking agent, thereby forming a surface crosslinked layer on at least a part of the surface of the superabsorbent polymer particles.

[0215] As the surface crosslinking agent, any crosslinking agent conventionally used in the preparation of superabsorbent polymers can be used without any particular limitation. Examples of the surface crosslinking agent may include at least one polyol selected from the group consisting of ethylene glycol, propylene glycol, 1,3 - propanediol, 1,4 - butanediol, 1,6 - hexanediol, 1,2 - hexanediol, 1,3 - hexanediol, 2 - methyl - 1,3 - propanediol, 2,5 - hexanediol, 2 - methyl - 1,3 - pentanediol, 2 - methyl - 2,4 - pentanediol, tripropylene glycol, and glycerol; at least one carbonate compound selected from the group consisting of ethylene carbonate, propylene carbonate, and glycerol carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazoline compounds such as oxazolidinone; polyamine compounds; oxazoline compounds; mono -, di - or poly - oxazolidinone compounds; cyclic urea compounds; and the like.

[0216] Specifically, one or more, two or more, or three or more of the above - mentioned surface crosslinking agents can be used as the surface crosslinking agent. For example, ethylene carbonate - propylene carbonate (ECPC), propylene glycol, and / or glycerol carbonate can be used.

[0217] Based on 100 parts by weight of superabsorbent polymer particles, the amount of the surface crosslinking agent can be from 0.001 part by weight to 5 parts by weight. For example, the amount can be more than 0.005 part by weight, more than 0.01 part by weight or more than 0.05 part by weight, and less than 5 parts by weight, less than 4 parts by weight or less than 3 parts by weight. When the content range of the surface crosslinking agent is adjusted within the above range, superabsorbent polymers exhibiting excellent various physical properties can be prepared.

[0218] In addition, the step of forming the surface crosslinked layer can be carried out by adding an inorganic material in addition to the surface crosslinking agent. That is to say, the step of forming the surface crosslinked layer by further crosslinking the surface of the superabsorbent polymer particles in the presence of the surface crosslinking agent and the inorganic material can be carried out.

[0219] As the inorganic material, at least one inorganic material selected from the group consisting of silica, clay, alumina, silica-alumina composite, titanium dioxide, zinc oxide and aluminum sulfate can be used. The inorganic material can be used in powder form or liquid form. In particular, alumina powder, silica-alumina powder, titanium dioxide powder or nano-silica solution can be used. In addition, based on 100 parts by weight of superabsorbent polymer particles, the amount of the inorganic material can be from about 0.001 part by weight to about 1 part by weight.

[0220] In addition, the method of mixing the surface crosslinking agent and the superabsorbent polymer is not particularly limited. For example, a method of adding the surface crosslinking agent and the superabsorbent polymer composition to a reactor for mixing, a method of spraying the surface crosslinking agent onto the superabsorbent polymer composition, or a method of continuously supplying the superabsorbent polymer composition and the surface crosslinking agent to a continuously operating mixer while mixing them can be used.

[0221] When mixing the surface crosslinking agent and the superabsorbent polymer composition, water and methanol can be further mixed therewith. When water and methanol are added thereto, there is an advantage that the surface crosslinking agent can be uniformly dispersed in the superabsorbent polymer composition. At this time, in order to induce the uniform dispersion of the surface crosslinking agent, prevent the coalescence phenomenon of the superabsorbent polymer composition, and optimize the surface penetration depth of the crosslinking agent, the amounts of water and methanol to be added can be appropriately controlled.

[0222] The surface crosslinking process can be carried out at a temperature of about 80 °C to about 250 °C. More specifically, the surface crosslinking process can be carried out at a temperature of about 100 °C to about 220 °C or about 120 °C to about 200 °C for about 20 minutes to about 2 hours or about 40 minutes to about 80 minutes. When the above surface crosslinking process conditions are satisfied, the surface of the superabsorbent polymer particles is sufficiently crosslinked to increase the absorption rate under pressure.

[0223] The heating means for the surface crosslinking reaction is not particularly limited. A heat medium can be provided thereto or a heat source can be directly provided thereto. At this time, the available 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 thereto can be appropriately selected in consideration of the manner of the heat medium, the heating rate, and the target temperature to be heated. Meanwhile, an electric heater or a gas heater can be used as the directly provided heat source, but the present disclosure is not limited thereto.

[0224] According to one embodiment of the present disclosure, after forming a surface crosslinking layer on at least a part of the surface of the superabsorbent polymer particles, at least one of a cooling step of cooling the superabsorbent polymer particles having the surface crosslinking layer formed thereon, a hydration step of adding water to the superabsorbent polymer particles having the surface crosslinking layer formed thereon, and a post-treatment step of adding an additive to the superabsorbent polymer particles having the surface crosslinking layer formed thereon can be further performed. At this time, the cooling step, the hydration step, and the post-treatment step can be performed successively or simultaneously.

[0225] The additives added in the post-treatment step may include a permeability enhancer, an anti-caking agent, a fluidity enhancer, an antioxidant, etc., but the present disclosure is not limited thereto.

[0226] When the cooling step, the hydration step, and the post-treatment step are optionally performed, the water content of the final superabsorbent polymer can be improved, and a high-quality superabsorbent polymer product can be manufactured.

[0227] The superabsorbent polymer prepared by the above method has a very small content of undried material in the form of debris during the drying process, and effectively controls the occurrence of warping of the dried material, thereby achieving excellent absorption properties.

[0228] Specifically, the superabsorbent polymer prepared by the above method can have a fast absorption rate and a low content of fine powder, and at the same time has a water retention capacity (CRC) (a general absorption property), similar to or superior to the superabsorbent polymer prepared by a conventional method.

[0229] Hereinafter, the functions and effects of the present invention will be described in more detail through specific examples. However, these examples are for illustrative purposes only, and the present invention is not limited by these examples.

[0230] Examples

[0231] Example 1

[0232] (Polymerization)

[0233] In a 5L glass container equipped with a stirrer and a thermometer, 1494 g of acrylic acid, 4.5 g of pentaerythritol triallyl ether (PETTAE) as an internal crosslinking agent, and 3392 g of water were mixed while stirring. At this time, nitrogen gas was added to the mixture at a rate of 1000 cc / min for 1 hour while maintaining the reaction temperature at 5°C. Then, 19.92 g of a 35% aqueous hydrogen peroxide solution, 22.4 g of a 1.6% aqueous ascorbic acid solution, and 44.8 g of a 1% aqueous 2,2'-azobis(2-methylpropionamidine) dihydrochloride solution were added as polymerization initiators, and 22.4 g of a 0.01% aqueous ferrous sulfate solution was added and mixed as a reducing agent. After the polymerization reaction started in the mixture and the temperature of the polymer reached 85°C, the temperature was maintained at 90 ± 2°C for about 6 hours to prepare the polymer.

[0234] (Neutralization and micronization)

[0235] Glycerol monolaurate (GML) was dissolved in water at 60°C or higher and added to 1000 g of the obtained polymer in the form of an aqueous solution at a rate of 0.01 g per 1000 g of the polymer as a surfactant. After that, it was pushed through a perforated plate with multiple 10 mm holes at a rotational speed of 3000 rpm using a high-speed rotating blade cutter (F-150 / manufactured by Karl Schnell) installed in a cylindrical grinder. Using a screw extruder installed in the cylindrical grinder, the collected crushed gel was passed through a perforated plate with 6 mm holes twice and through a perforated plate with 4 mm holes once at a rotational speed of 500 rpm. At each stage of the screw extruder, 237.8 g of a 50% aqueous NaOH solution, 60 g of fine powder, and 75.5 g of a 10% aqueous Na2SO4 solution were added per 1 kg of debris to neutralize a part of the acidic groups.

[0236] (Drying)

[0237] 736 g of the neutralized and micronized hydrogel polymer was dried in a dryer capable of changing the wind direction up and down. Specifically, it was dried by flowing hot air at 200°C from bottom to top for 2.5 minutes, flowing hot air at 100°C from bottom to top for 7.5 minutes and from top to bottom for 15 minutes, and then flowing hot air at 75°C from top to top for 3 minutes (a total of 28 minutes). For XRM measurement, dry chips were used for measurement at this stage. Here, the initial pore formation rate measured after 16 minutes (57% of the total drying time) in the dryer was 1.34% / minute.

[0238] (Crushing and classification)

[0239] The dried product is pulverized using a pulverizer (GRAN-U-LIZERTM, MPE), and then classified using ASTM standard sieves to obtain a superabsorbent polymer containing base resin particles having a particle size of 150 μm to 850 μm.

[0240] Example 2

[0241] A superabsorbent polymer containing base resin particles is prepared in the same manner as in Example 1, except that drying is carried out by flowing hot air at 120 °C from bottom to top for 20 minutes and from top to bottom for 20 minutes (a total of 40 minutes).

[0242] Here, the initial pore formation rate measured after placing in the dryer for 16 minutes (40% of the total drying time) is 1.11% / minute.

[0243] Example 3

[0244] A superabsorbent polymer containing base resin particles is prepared in the same manner as in Example 1, except that the amount of the internal crosslinking agent pentaerythritol triallyl ether (PETTAE) is 3.5 g instead of 4.5 g.

[0245] Here, the initial pore formation rate measured after placing in the dryer for 16 minutes (57% of the total drying time) is 1.13% / minute.

[0246] Example 4

[0247] A superabsorbent polymer containing base resin particles is prepared in the same manner as in Example 1, except that the neutralization and micronization process is changed from passing through a perforated plate with 6 mm holes twice and a perforated plate with 4 mm holes once to passing through a perforated plate with 6 mm holes twice and a perforated plate with 8 mm holes once.

[0248] Here, the initial pore formation rate measured after placing in the dryer for 16 minutes (57% of the total drying time) is 1.01% / minute.

[0249] Comparative Example 1: Other grade foaming process

[0250] 1) Polymerization

[0251] In a 3 L glass container equipped with a stirrer and a thermometer, 430 g of acrylic acid, 1100 ppm (based on acrylic acid) of sodium bicarbonate as a foaming agent, 6000 ppm (based on acrylic acid) of propylene glycol di(meth)acrylate as an internal crosslinking agent, 100 ppm (based on acrylic acid) of acylphosphine as a photopolymerization initiator, 2000 ppm (based on acrylic acid) of sodium persulfate as a thermal polymerization initiator, and 600 g of 30% NaOH aqueous solution were mixed at room temperature (25 ± 1 °C) to prepare a monomer composition with a total solid content of 40 wt% (neutralization degree of acrylic acid: 70 mol%).

[0252] After that, 400 ppm (based on acrylic acid) of sodium metabisulfite as a reducing agent was added to the monomer composition. Additionally, while supplying the monomer composition, polymerization was carried out by light irradiation for 1 minute.

[0253] 2) Gel comminution and drying

[0254] The hydrogel polymer was coarsely comminuted with a chopper having a pore size of 10 mm and dried with hot air at about 200 °C for 28 minutes. Here, the initial pore formation rate measured after placing in the dryer for 16 minutes (57% of the total drying time) was 0.5% / minute.

[0255] 3) Comminution and classification

[0256] The polymer dried in step 4 was comminuted with a pulverizer and then classified using ASTM standard sieves to obtain base resin particles with a particle size of 150 μm to 850 μm.

[0257] Comparative Example 2: No foaming process for other grades

[0258] 1) Polymerization

[0259] In a 3 L glass container equipped with a stirrer and a thermometer, 450 g of acrylic acid, 0.036 g of IRGACURE 819 as an initiator, and 1.17 g of PEGDA as an internal crosslinking agent were added, stirred, and mixed with 554.9 g of 31.5% NaOH aqueous solution and 509.7 g of water to obtain a 70% neutralized monomer composition. After that, the monomer composition was cooled to 40 °C, mixed with 50.63 g of 1.78% SPS solution, and polymerization was carried out by light irradiation for 1 minute.

[0260] 2) Gel comminution and drying

[0261] After that, the hydrogel polymer was coarsely comminuted with a chopper having a pore size of 10 mm and dried with hot air at about 197 °C for 28 minutes. Here, the initial pore formation rate measured after placing in the dryer for 16 minutes (57% of the total drying time) was 0.33% / minute.

[0262] Comparative Example 3: Non-foaming process of other grades - Different gel pulverization conditions

[0263] The procedure was the same as in Comparative Example 2, except that a 16-mm hole was used instead of a 10-mm hole for gel pulverization.

[0264] Here, the initial pore formation rate measured after 16 minutes (57% of the total drying time) in the dryer was 0.56% / minute.

[0265] Experimental Example 1. Control of the drying process

[0266] (1) Measurement of the degree of pore formation

[0267] Dry chips during the drying process in the manufacturing processes of the Examples and Comparative Examples were used as follows. The degree of pore formation was measured using XRM (manufacturer: ZEISS, model name: Xradia 620Versa), and the pore formation rate was calculated according to Equation 1. The results are shown in Table 1.

[0268] (1) The 3D-reconstructed XRM cross-sectional 2D image was converted into a binary image using Otsu's thresholding technique to distinguish the background and sap (superabsorbent polymer) particles.

[0269] (2) 3D rendering was performed by stacking the binary cross-sectional 2D images.

[0270] (3) The surface area [1] and volume of the 3D-rendered sap particles were measured.

[0271] (4) Surface area: Calculated using the method disclosed in Reference [1; Lehmann, Gaetan and David Legland. Efficient N-Dimensional surface estimation using Crofton formula and run-length encoding, The Insight Journal, 2012. https: / / insight-journal.org / browse / publication / 852].[[]END]]

[0272] (5) Volume: The number of voxels in the actual 3D binary image was counted.

[0273] (6) Internal pores: Measured using an algorithm that calculates closed pores considering 3D image connectivity [Reference 2; Soille, P., Morphological Image Analysis: Principles and Applications, Springer-Verlag, 1999, pages 173 - 174].

[0274] [Equation 1]

[0275] Initial pore formation rate (% / min) = (R B 2 - R B 1) / (t2 - t1) * 100

[0276] In the above formula, R B refers to the pore volume ratio and is defined as the ratio of the pore volume (V B ) to the total volume (V),

[0277] t1 is the time when drying starts,

[0278] t2 is the time when 30% to 60% of the total drying time has elapsed, and

[0279] R B 1 and R B 2 refer to the R values at t1 and t2 respectively. B values.

[0280] (2) Measurement of drying efficiency

[0281] The dried plate - like laminates obtained in step 4 of the examples and comparative examples after the drying process (before surface cross - linking) were cut into 10×10 cm to obtain samples. The samples were divided into three parts based on the cross - section (based on the height of the laminate) to obtain the upper layer, middle layer, and lower layer respectively. Samples were taken from the superabsorbent polymer of each layer, and their initial weight H0 (g) and the weight H1 (g) after being held at 180 °C for 40 minutes by infrared heating were measured. Then, the water content was calculated according to Equation 2 below.

[0282] [Equation 2]

[0283] Water content (wt%) = {[H0 (g) - H1 (g)] / H0 (g)} * 100

[0284] The above - mentioned measurement was repeated 5 times to obtain the average value and standard deviation.

[0285]

Table 1

[0286]

[0287] In the case of Examples 1 to 4 that satisfy the initial pore formation rate of the present disclosure, it was confirmed that due to excellent drying efficiency, high water content was achieved simultaneously, and the change in water content with the stacking position was small. In the case of Comparative Examples where the initial pore formation rate was not within the scope of the present disclosure, although the drying efficiency itself may be excellent, the water content was only about 1 wt% to 2 wt%, and the physical properties of the final superabsorbent polymer were significantly lower than those of the Examples, as seen in Experimental Example 2 described later.

[0288] Experimental Example 2. Evaluation of Physical Properties of Absorbent Polymer

[0289] The physical properties of the superabsorbent polymers containing a base resin (hereinafter referred to as BR) prepared in the Examples and Comparative Examples were evaluated as follows and listed in Table 2 below.

[0290] Unless otherwise specified, all procedures were carried out in a thermo-hygrostat chamber (23 ± 1 °C, relative humidity of 50 ± 10%), and physiological saline or brine refers to an aqueous solution of 0.9 wt% sodium chloride (NaCl).

[0291] (1) Centrifugal Retention Capacity (CRC, g / g)

[0292] The centrifugal retention capacity of the BR prepared in the Examples and Comparative Examples in terms of absorption ratio under unloaded conditions was measured according to EDANA (European Disposables and Nonwovens Association) WSP 241.3.

[0293] Specifically, after inserting W0 (g) (about 0.2 g) of the BR prepared in one of the Examples and Comparative Examples uniformly into a nonwoven fabric envelope and sealing it, it was immersed in physiological saline (an aqueous solution of 0.9 wt% sodium chloride). After 30 minutes, the envelope was centrifuged at 250G for 3 minutes to dehydrate, and the mass W2 (g) of the envelope was measured. In addition, after performing the same operation without using the polymer, the mass W1 (g) of the envelope was measured.

[0294] Then, the CRC (g / g) was calculated using the obtained masses according to Equation 3 below.

[0295] [Equation 3]

[0296] CRC (g / g) = {[W2 (g) - W1 (g)] / W0 (g)} – 1

[0297] (2) Vortex Time

[0298] The vortex time was measured in seconds according to the method described in International Patent Publication No. 1987 - 00320.

[0299] Specifically, 2 g of BR was added to 50 mL of saline at 23°C to 24°C, and a magnetic bar (8 mm in diameter and 30 mm in length) was stirred at 600 rpm to measure the time (in seconds) taken until the vortex disappeared.

[0300]

Table 2

[0301] Category CRC (g / g) Vortex time (seconds) Example 1 46.1 26 Example 2 45.8 27 Example 3 50.2 26 Example 4 45.7 25 Comparative Example 1 38.3 56 Comparative Example 2 51.5 82 Comparative Example 3 50.2 90

[0302] As can be seen from the experimental data in Table 2, by controlling the initial pore formation rate within a specific range during the drying process, the embodiments of the present disclosure can achieve excellent absorption properties, especially a significantly improved absorption rate and excellent drying efficiency.

[0303] In the case where the comparative example deviates from the initial pore formation rate of the present disclosure, it is confirmed that the absorption rate is significantly reduced compared to the example.

Claims

1. A method for preparing a superabsorbent polymer, the method comprising the following steps: Polymerizing a monomer composition comprising a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized (step 1); Neutralizing at least a part of the acidic groups of the polymer (step 2); Micronizing the polymer in the presence of a surfactant (step 3); And Drying the neutralized and micronized polymer to prepare base resin particles (step 4), wherein, in step 4, the initial pore formation rate of the polymer measured after 30% to 60% of the total drying time is 0.8% / min to 1.7% / min.

2. The method for preparing a superabsorbent polymer according to claim 1, wherein, The initial pore formation rate is calculated by the following equation 1: [Equation 1] Initial pore formation rate (% / min) = (R B 2 - R B 1) / (t1 - t0) * 100 In the above formula, R B refers to the pore volume ratio and is defined as the ratio of the pore volume (V B ) to the total volume (V). t1 is the time when drying starts, t2 is the time when 30% to 60% of the total drying time has elapsed, and R B 1 and R B 2 respectively refer to the R values at t1 and t2 B values.

3. The method for preparing a superabsorbent polymer according to claim 1, wherein, Steps 2 and 3 are carried out successively, alternately or simultaneously.

4. The method for preparing a superabsorbent polymer according to claim 1, wherein, The micronization step of step 3 is carried out using a micronization device comprising a perforated plate having a plurality of holes with a diameter of 1 mm to 20 mm.

5. The method for preparing a superabsorbent polymer according to claim 1, wherein, The drying step of step 4 is carried out using a fixed-bed type belt dryer.

6. The method for preparing a superabsorbent polymer according to claim 1, wherein, The drying step of step 4 is carried out using hot air at 70°C to 250°C.

7. The method for preparing a superabsorbent polymer according to claim 1, wherein, The drying step of step 4 is carried out for 20 minutes to 90 minutes.

8. The method for preparing a superabsorbent polymer according to claim 1, wherein, The water content of the base resin particles obtained by carrying out step 4 is 3 wt% to 10 wt%.

9. The method for preparing a superabsorbent polymer according to claim 1, wherein, The method further comprises the steps of crushing and classifying the base resin particles.

10. The method for preparing a superabsorbent polymer according to claim 1 or claim 9, wherein, The method further comprises the step of forming a surface crosslinked layer on at least a part of the surface of the base resin particles.

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