Manufacturing method of superabsorbent resin

Abstract

The present invention relates to a method for producing a superabsorbent polymer, and more specifically, to a method for producing a superabsorbent polymer having excellent drying efficiency and excellent water absorption properties by controlling the initial pore formation rate of the polymer during the drying process to reduce the amount of wet matter generated.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0181773, filed December 22, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a method for producing a superabsorbent polymer, and more specifically, to a method for producing a superabsorbent polymer having excellent drying efficiency and excellent water absorption properties by controlling the initial pore formation rate of the polymer during the drying process to reduce the amount of wet matter generated. [Background technology]

[0003] Super absorbent polymer (SAP) is a synthetic polymer capable of absorbing 500 to 1,000 times its own weight in water, and different developers have given it different names, such as SAM (Super Absorbency Material) or AGM (Absorbent Gel Material). These super absorbent polymers first came into practical use as sanitary products, and are now widely used in a variety of applications, including as soil replenishers for horticulture, water-stopping materials for civil engineering and construction, seedling sheets, freshness-preserving agents in the food distribution industry, and adhesive patches.

[0004] Such superabsorbent polymers are widely used in the field of sanitary materials, such as diapers and sanitary napkins. In these sanitary materials, the superabsorbent polymer is generally contained in a dispersed state within the pulp. However, in recent years, efforts have been made to provide thinner sanitary materials, such as diapers. As part of these efforts, the pulp content has been reduced, or even a step further, the development of so-called pulpless diapers, which do not use pulp at all, has been actively pursued.

[0005] In this way, in the case of sanitary materials with a reduced pulp content or no pulp used, the superabsorbent polymer is contained at a relatively high ratio, and the superabsorbent polymer particles are inevitably contained in multiple layers within the sanitary material. In order for the overall superabsorbent polymer particles contained in such multiple layers to more efficiently absorb a large amount of liquid such as urine, the superabsorbent polymer basically needs to exhibit not only high water absorption performance but also a fast water absorption speed.

[0006] Generally, such superabsorbent polymers are manufactured by polymerizing monomers to produce a hydrogel polymer containing a large amount of water, drying the hydrogel polymer, and then pulverizing it into resin particles having a desired particle size. However, when the hydrogel polymer is dried and then pulverized, a large amount of fine powder is generated, which reduces the physical properties of the final superabsorbent polymer.

[0007] In order to reuse such fine powder, it is common to mix the fine powder with water to agglomerate it, produce a regranulated fine powder, and then add the regranulated fine powder produced through processes such as drying, pulverization, and classification. However, in this case, the water used increases the amount of energy consumed during the drying process, and increases the load on the equipment, which can reduce the productivity of the superabsorbent polymer production.

[0008] In addition, hydrogel polymers polymerized during the superabsorbent resin manufacturing process tend to aggregate with each other, and when they are manufactured into aggregated fine particles, the cohesive force increases, causing problems with 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, but the increased cohesive force of the hydrogel polymer significantly reduces the porosity within the drying layer, which in turn increases the hot air pressure difference, increasing the amount of undried crumbs and causing warping of the outer periphery of the drying layer, making it difficult to dry large quantities.

[0009] That is, if the wet rate becomes high or the dried product warps, it is difficult to achieve the desired physical properties of the product, resulting in defects. Therefore, there is a continuous demand for the development of a technology to improve drying efficiency without such problems. Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, the present invention provides a superabsorbent polymer that can reduce the amount of undried material generated by controlling the initial void formation rate of the polymer during the drying process, thereby achieving excellent drying efficiency and excellent water absorption properties. [Means for solving the problem]

[0011] In order to solve the above problem, according to one embodiment of the present invention, a step (step 1) of polymerizing a monomer composition including 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; neutralizing at least a portion of the acidic groups of the polymer (Step 2); atomizing the polymer in the presence of a surfactant (Step 3); and drying the neutralized and atomized polymer to produce base resin particles (Step 4); In step 4, the initial void formation rate of the polymer measured after 30 to 60% of the total drying time is 0.8 to 1.7% / min. A method for producing a highly water-absorbent resin is provided. [Effects of the Invention]

[0012] According to the method for producing a superabsorbent polymer of the present invention, the content of undried material generated during the drying process can be reduced by controlling the initial void formation rate of the polymer during the drying process, thereby making it possible to produce a superabsorbent polymer that can achieve the desired water absorption properties with excellent drying efficiency.

[0013] In addition, according to the method for producing a superabsorbent polymer of the present invention, by undergoing the steps of unneutralized polymerization, neutralization and atomization under specific conditions, particles in the form of aggregated fine particles are realized, and the surface area is increased, thereby significantly improving the water absorption rate and enabling the production of a superabsorbent polymer that exhibits excellent water absorption properties. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a flowchart showing a conventional method for producing a highly water-absorbent resin. DETAILED DESCRIPTION OF THE INVENTION

[0015] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise. It should be understood that the terms "comprise," "comprise," or "have" used in this specification are intended to specify the presence of embodied features, steps, components, or combinations thereof, and do not preclude the possibility of the presence or addition of one or more other features, steps, components, or combinations thereof.

[0016] Although the present invention can be embodied in various forms through various modifications, specific embodiments are described in detail below by way of example, but it should be understood that this is not intended to limit the present invention to the specific disclosed embodiments, and that the present invention encompasses all modifications, equivalents, and alternatives falling within the spirit and technical scope of the present invention.

[0017] Prior to this, the terminology used herein is for the purpose of referring to particular embodiments only and is not intended to limit the present invention, and the singular forms used herein also include the plural forms unless the language clearly dictates otherwise.

[0018] The term "polymer" or "polymeric" as used herein means a polymerized state of water-soluble ethylenically unsaturated monomers, and can encompass any range of moisture content or particle size.

[0019] Furthermore, depending on the context, the term "superabsorbent polymer" may refer to a crosslinked polymer or a powder-like base resin consisting of superabsorbent polymer particles obtained by pulverizing the crosslinked polymer, or may be used to encompass all of the crosslinked polymers or base resins that have been subjected to additional processes, such as drying, pulverization, classification, surface crosslinking, etc., to be in a state suitable for commercialization.

[0020] The term "fine powder" refers to superabsorbent resin particles having a particle size of less than 150 μm. The particle size of such resin particles can be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP220.3 method.

[0021] The term "chopping" refers to cutting the hydrogel polymer into small pieces on the order of millimeters to improve drying efficiency, and is used to distinguish it from grinding to micrometer or regular particle levels.

[0022] Furthermore, the term "micronizing" refers to the grinding of a hydrogel polymer into particles of several tens to several hundreds of micrometers, and is used to distinguish it from "chopping."

[0023] Hereinafter, the method for producing a superabsorbent polymer and the superabsorbent polymer will be described in more detail with reference to specific embodiments of the invention.

[0024] According to one embodiment of the invention, a step (step 1) of polymerizing a monomer composition including 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; neutralizing at least a portion of the acidic groups of the polymer (Step 2); atomizing the polymer in the presence of a surfactant (Step 3); and drying the neutralized and atomized polymer to produce base resin particles (Step 4); In step 4, the initial void formation rate of the polymer measured after 30 to 60% of the total drying time is 0.8 to 1.7% / min. A method for making a superabsorbent polymer is provided.

[0025] Typically, a hydrogel polymer obtained by a polymerization reaction of an acrylic acid-based monomer is subjected to processes such as drying, pulverization, classification, and surface cross-linking, and is then commercially available as a powdered product called a superabsorbent resin.

[0026] Among the manufacturing processes, the process of drying the hydrogel polymer typically involves applying hot air using a belt dryer equipped with a perforated plate. However, this process has the drawback of significantly reducing the porosity within the drying layer due to the increased cohesive force of the hydrogel polymer. This increases the hot air differential pressure, increasing the amount of undried crumbs (crumbs), causing warping at the outer edge of the drying layer, and making it difficult to dry large quantities, reducing process efficiency. Therefore, if excessive drying is performed to reduce the amount of undried crumbs, the moisture content of the base resin drops to approximately 1-2 wt%, increasing the generation of fine powder and reducing process efficiency due to energy consumption.

[0027] Therefore, if the moisture content of the base resin becomes too low due to excessive drying or if the occurrence of wet parts increases, it becomes difficult to achieve the desired physical properties of the product, resulting in defects. Therefore, the present inventors have conducted repeated research to solve this problem. As a result, the present inventors have confirmed that if the drying process is carried out using a hydrogel polymer polymerized in an unneutralized state and the void formation rate is controlled within a specific range in the early stages of drying, it is possible to produce a base resin with a relatively high moisture content while minimizing the occurrence of wet parts, and have completed the invention.

[0028] Recently, however, attempts have been made to provide highly water-absorbent resins that exhibit an even faster water absorption rate.

[0029] The most common method for increasing the water absorption rate is to form a porous structure inside the superabsorbent resin to increase the surface area of ​​the superabsorbent resin. In order to increase the surface area of ​​the superabsorbent resin, a commonly used method is to include a foaming agent in the monomer composition and cause cross-linking polymerization to occur, thereby forming a porous structure in the base resin powder.

[0030] However, the use of a foaming agent has the disadvantages of reducing various physical properties of the superabsorbent resin, such as surface tension, liquid permeability, or volume density, and increasing the amount of fine powder generated. Therefore, there is a continuing demand for the development of a technology that can improve the water absorption rate of a superabsorbent resin without using a foaming agent.

[0031] Conventional superabsorbent polymers are manufactured by crosslinking a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer, drying the formed hydrogel polymer, and then pulverizing it to a desired particle size. To facilitate drying of the hydrogel polymer and increase the efficiency of the pulverization process, a chopping process is typically performed before the drying process to cut the hydrogel polymer into particles several millimeters in size. However, due to the stickiness of the hydrogel polymer, the hydrogel polymer cannot be pulverized to micro-sized particles during this chopping process, resulting in an aggregated gel. When this aggregated gel-like hydrogel polymer is dried, a plate-like dried product is formed. To pulverize this to micro-sized particles, a multi-stage pulverization process is required to reduce the polymer's stickiness, which creates a problem of generating a large amount of fine powder during this process.

[0032] Specifically, a flow chart of a conventional method for producing a superabsorbent polymer is shown in Figure 1. Referring to Figure 1, conventional superabsorbent polymers have been produced through the following steps:

[0033] (neutralizing) neutralizing at least a portion of the acidic groups of the water-soluble ethylenically unsaturated monomer; (Polymerization) A step of cross-linking and polymerizing a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group in the presence of an internal cross-linking agent and a polymerization initiator to form a hydrogel polymer; (chopping) chopping the hydrogel polymer; (Drying) drying the chopped hydrogel polymer; and (Crushing / Classification) Crushing the dried polymer and classifying it into normal particles and fine powder;

[0034] As described above, the chopped hydrogel polymer has an agglomerated gel shape with a size of approximately 1 cm to 10 cm. This chopped hydrogel polymer is stacked on a belt with a perforated bottom and dried by hot air supplied from the bottom or top. Because the polymer dried using this drying method exhibits a plate-like shape rather than a granular shape, the classification step after pulverization has typically been carried out by first coarsely pulverizing and classifying the polymer, and then finely pulverizing and classifying it again, so that the resulting particles are regular particles, i.e., particles with a diameter of 150 μm to 850 μm. In this manufacturing method, the amount of fine powder separated in the final classification step is large, about 20 wt% to about 30 wt% of the total weight of the final superabsorbent polymer. Therefore, the separated fine powder is mixed with an appropriate amount of water, re-granulated, and then reused by adding it to the chopping step or the step before drying.

[0035] However, when the re-granulated fine powder mixed with water for reuse is re-introduced into the crushing or drying process, problems have arisen, such as an increase in the load on the equipment and / or the amount of energy used, and the fine powder remaining unclassified has caused a deterioration in the physical properties of the superabsorbent polymer.

[0036] Therefore, the present inventors have recognized that the amount of fine powder generated in conventional manufacturing methods is significantly affected by the pulverization step, and have focused on the fact that the amount of fine powder generated during the manufacturing process can be significantly reduced by adding a surfactant and a neutralizing agent in the polymer pulverization step to post-neutralize the polymer, pulverizing it more finely than before, i.e., atomizing it, while simultaneously controlling aggregation to produce particles in the form of aggregates of fine particles.

[0037] Meanwhile, a method of adding a surfactant to reduce the stickiness of the hydrogel polymer during the chopping process has been proposed. However, when adding a surfactant during the chopping process, the high water content of the hydrogel polymer causes the surfactant to penetrate into the hydrogel polymer rather than being present at the interface of the hydrogel polymer, which results in the surfactant not being able to perform its function properly.

[0038] This is because chopped particles are formed into particles of several mm or several cm in size compared to the polymer before chopping, which can increase the surface area to some extent, but it is difficult to expect an effect that can effectively improve the water absorption rate. Therefore, in order to improve the water absorption rate, it is possible to increase the surface area by increasing the mechanical force and kneading during the chopping stage, but in this case, excessive aggregation occurs due to the stickiness inherent to polymers, and after chopping, drying, and pulverization, only the surface of the particles becomes an irregular, amorphous single particle, and excessive kneading or pulverization can actually increase the water-soluble components.

[0039] As a result of repeated research to solve this problem, it was found that, unlike conventional methods for producing superabsorbent polymers in which the acidic groups of a water-soluble ethylenically unsaturated monomer are neutralized before polymerization, polymerization is first carried out in a state in which the acidic groups are not neutralized to form a polymer, and the hydrogel polymer is then atomized in the presence of a surfactant, followed by neutralization of the acidic groups of the polymer; or, after neutralization of the acidic groups of the polymer to form a hydrogel polymer, the hydrogel polymer is then atomized in the presence of a surfactant, or, simultaneously with atomization, the acidic groups present in the polymer are neutralized. This allows a large amount of surfactant to be present on the surface of the polymer, thereby reducing the high viscosity of the polymer and preventing excessive aggregation of the polymer, and thereby fully fulfilling the role of adjusting the aggregation state to a desired level.

[0040] As a result, the polymer is prepared into secondary particles in the form of agglomerated primary particles, and the subsequent pulverization and drying processes are carried out under milder conditions, thereby significantly reducing the amount of fine powder generated during the process.

[0041] Furthermore, when a polymer is micronized in the presence of the surfactant, the hydrophobic functional groups contained in the surfactant impart hydrophobicity to the surfaces of the pulverized superabsorbent resin particles, thereby reducing the friction between the particles and increasing the apparent density of the superabsorbent resin, while the hydrophilic functional groups contained in the surfactant also bind to the superabsorbent resin particles, preventing a decrease in the surface tension of the resin. As a result, the superabsorbent resin produced by the above-mentioned production method can have a higher apparent density value while exhibiting the same level of surface tension as a resin that does not use a surfactant.

[0042] Furthermore, if polymerization is first carried out in an unneutralized state to form a polymer, and then the acidic groups present in the polymer are neutralized, it is possible to form a polymer with a longer chain, and the content of water-soluble components present in an uncrosslinked state due to incomplete crosslinking can be reduced.

[0043] The water-soluble component has the property of easily dissolving when the superabsorbent polymer comes into contact with a liquid, so if the content of the water-soluble component is high, most of the dissolved water-soluble component remains on the surface of the superabsorbent polymer, making the superabsorbent polymer sticky and reducing its liquid permeability. Therefore, in terms of liquid permeability, it is important to maintain the content of the water-soluble component low.

[0044] According to one embodiment of the present invention, the content of water-soluble components is reduced by carrying out polymerization in an unneutralized state, thereby improving the liquid permeability of the superabsorbent resin.

[0045] In addition, the superabsorbent resin produced according to one embodiment of the present invention may have a uniform particle size distribution, thereby providing a superabsorbent resin with excellent water absorption properties such as water retention capacity, water absorption capacity under pressure, rewet properties, and water absorption speed.

[0046] Hereinafter, each step of the method for producing a highly water-absorbent resin according to one embodiment will be described in more detail.

[0047] Stage 1: Polymerization Stage 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.

[0048] The step may include preparing a monomer composition by mixing the water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator, and polymerizing the monomer composition to form a polymer.

[0049] The water-soluble ethylenically unsaturated monomer may be any monomer commonly used in the production of superabsorbent resins. As a non-limiting example, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following Chemical Formula 1:

[0050] [Chemical formula 1] R-COOM'

[0051] In the above Chemical Formula 1, R is an alkyl group having 2 to 5 carbon atoms and containing an unsaturated bond, M' is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0052] Preferably, the monomer may be one or more selected from the group consisting of (meth)acrylic acid, and monovalent (alkali) metal salts, divalent metal salts, ammonium salts and organic amine salts of these acids.

[0053] In this way, when (meth)acrylic acid and / or its salt is used as the water-soluble ethylenically unsaturated monomer, it is advantageous to obtain a superabsorbent resin with improved water absorption. Other examples of the monomer that can be used include maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamido-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, and (N,N)-dimethylaminopropyl (meth)acrylamide.

[0054] Here, the water-soluble ethylenically unsaturated monomer has an acidic group. As described above, in conventional superabsorbent resin production, a water-containing gel polymer is formed by cross-linking a monomer in which at least a portion of the acidic groups have been neutralized with a neutralizing agent. Specifically, at least a portion of the acidic groups of the water-soluble ethylenically unsaturated monomer are neutralized in the step of mixing the water-soluble ethylenically unsaturated monomer having the acidic group, an internal cross-linking agent, a polymerization initiator, and a neutralizing agent.

[0055] However, according to one embodiment of the present invention, the water-soluble ethylenically unsaturated monomer is polymerized first in a state where the acid group is not neutralized to form a polymer.

[0056] A water-soluble ethylenically unsaturated monomer (e.g., acrylic acid) in which the acid group is not neutralized is in a liquid state at room temperature and has high miscibility with the solvent (water), so it exists in the form of a mixed solution in the monomer composition. However, a water-soluble ethylenically unsaturated monomer in which the acid group is neutralized is in a solid state at room temperature and has different solubility depending on the temperature of the solvent (water), with the solubility decreasing as the temperature decreases.

[0057] Such water-soluble ethylenically unsaturated monomers in which the acidic group is not neutralized have higher solubility or miscibility in a solvent (water) than monomers in which the acidic group is neutralized, and do not precipitate even at low temperatures, making them advantageous for long-term polymerization at low temperatures. As a result, long-term polymerization using the water-soluble ethylenically unsaturated monomers in which the acidic group is not neutralized can stably produce polymers with higher molecular weights and more uniform molecular weight distributions.

[0058] Furthermore, it is possible to form a polymer with a longer chain, and the effect of reducing the content of water-soluble components that exist in a state where polymerization and crosslinking are incomplete and not crosslinked can be achieved.

[0059] Furthermore, when the polymerization is first carried out in a state where the acidic groups of the monomers are not neutralized to form a polymer, and the polymer is then neutralized and then micronized in the presence of a surfactant, or when the acidic groups present in the polymer are neutralized in the presence of a surfactant, or when the acidic groups present in the polymer are neutralized simultaneously with micronization, the surfactant is present in a large amount on the surface of the polymer, and can sufficiently play a role in reducing the stickiness of the polymer.

[0060] The concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition can be appropriately adjusted in consideration of the polymerization time, reaction conditions, etc., and is preferably about 20 to about 60% by weight, or about 20 to about 40% by weight.

[0061] The term "internal crosslinking agent" used in this specification is a term used to distinguish it from a surface crosslinking agent for crosslinking the surface of the superabsorbent resin particles described later, and plays a role in introducing crosslinking bonds between unsaturated bonds of the water-soluble ethylenically unsaturated monomer described above to form a polymer containing a crosslinked structure.

[0062] The crosslinking in this step is carried out regardless of whether it is on the surface or inside. However, when the surface crosslinking process of the superabsorbent resin particles described below is carried out, the surface of the finally produced superabsorbent resin particles may include a structure newly crosslinked by the surface crosslinking agent, and the inside of the superabsorbent resin particles may maintain the structure crosslinked by the internal crosslinking agent.

[0063] According to an embodiment of the present invention, the internal crosslinking agent may include at least one of a multifunctional acrylate-based compound, a multifunctional allyl-based compound, and a multifunctional vinyl-based compound.

[0064] Non-limiting examples of the polyfunctional acrylate compound 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, butylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, pentaerythritol, Examples of the acrylate include dipentaerythritol 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, glycerin di(meth)acrylate, and glycerin tri(meth)acrylate, and these can be used alone or in combination of two or more.

[0065] Non-limiting examples of polyfunctional allyl compounds 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, butylene glycol 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, glycerin diallyl ether, and glycerin triallyl ether. These compounds may be used alone or in combination of two or more.

[0066] Non-limiting examples of polyfunctional vinyl compounds 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, butylene glycol 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, glycerin divinyl ether, and glycerin trivinyl ether, which can be used alone or in combination of two or more. Preferably, pentaerythritol triallyl ether can be used.

[0067] The above-mentioned polyfunctional allyl compounds or polyfunctional vinyl compounds can form a crosslinked structure during the polymerization process by bonding two or more unsaturated groups contained in the molecule with the unsaturated bond of the water-soluble ethylenically unsaturated monomer or the unsaturated bond of another internal crosslinking agent, respectively, and can maintain the crosslinked bond more stably even during the neutralization process after the above-mentioned polymerization reaction, unlike acrylate compounds containing an ester bond (-(C=O)O-) in the molecule.

[0068] This increases the gel strength of the produced superabsorbent resin, improves process stability during the extrusion process after polymerization, and minimizes the amount of water-soluble components.

[0069] The crosslinking polymerization of the water-soluble ethylenically unsaturated monomer in the presence of such an internal crosslinking agent may be carried out in the presence of a polymerization initiator, and, if necessary, a thickener, a plasticizer, a storage stabilizer, an antioxidant, etc.

[0070] In the monomer composition, the internal crosslinking agent can be used in an amount of 0.01 to 5 parts by weight per 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the internal crosslinking agent can be used in an amount of 0.01 part by weight or more, or 0.05 part by weight or more, or 0.1 part by weight or more and 5 parts by weight or less, or 3 parts by weight or less, or 2 parts by weight or less, or 1 part by weight or less, or 0.7 parts by weight or less per 100 parts by weight of the water-soluble ethylenically unsaturated monomer. If the content of the internal crosslinking agent is too low, crosslinking may not occur sufficiently, making it difficult to achieve an appropriate level of strength. If the content of the internal crosslinking agent is too high, the internal crosslink density may increase, making it difficult to achieve the desired water retention capacity.

[0071] The polymer formed using such an internal crosslinking agent has a three-dimensional network structure in which the main chain formed by polymerization of the water-soluble ethylenically unsaturated monomer is crosslinked by the internal crosslinking agent. When the polymer has such a three-dimensional network structure, the water retention capacity and water absorption capacity under pressure, which are various physical properties of the superabsorbent resin, can be significantly improved compared to when the polymer has a two-dimensional linear structure that is not additionally crosslinked by the internal crosslinking agent.

[0072] According to one embodiment of the present invention, the step of polymerizing the monomer composition to form a polymer is carried out in a batch type reactor.

[0073] In a typical method for producing a superabsorbent resin, the polymerization method is roughly divided into thermal polymerization and photopolymerization depending on the polymerization energy source. Generally, when thermal polymerization is carried out, it may be carried out in a reactor having a stirring shaft such as a kneader, and when photopolymerization is carried out, it may be carried out in a reactor equipped with a movable conveyor belt or in a vessel with a flat bottom.

[0074] On the other hand, when photopolymerization is carried out in a reactor equipped with a movable conveyor belt or a container with a flat bottom, the resulting hydrogel polymer is usually in the form of a sheet having the width of the belt. The thickness of the polymer sheet varies depending on the concentration and injection rate or amount of the monomer composition injected, but is usually about 0.5 to about 5 cm thick.

[0075] However, if the monomer composition is supplied so that the thickness of the sheet polymer is too thin, production efficiency is low, which is undesirable, and if the thickness of the sheet polymer is increased for productivity reasons, the polymerization reaction does not occur uniformly throughout the entire thickness, making it difficult to form a high-quality polymer.

[0076] In addition, in the polymerization in the reactor having a reactor agitator equipped with a conveyor belt, the polymerization is carried out continuously by feeding new monomer composition to the reactor while the polymerization result is moving, so polymers with different polymerization rates are mixed, which makes it difficult to carry out uniform polymerization throughout the monomer composition, and may result in deterioration of overall physical properties.

[0077] However, according to one embodiment of the present invention, polymerization is carried out in a stationary manner using a batch reactor, which reduces the risk of polymers with different polymerization rates being mixed together, thereby producing polymers with uniform quality.

[0078] In addition, the polymerization step is carried out in a batch reactor having a predetermined volume, and the polymerization reaction is carried out for a longer time, for example, 3 hours or more, than when polymerization is carried out in a continuous reactor equipped with a conveyor belt. Despite this long polymerization reaction time, since the polymerization is carried out on unneutralized water-soluble ethylenically unsaturated monomers, the monomers do not precipitate even when the polymerization is carried out for a long time, and therefore, it is advantageous for long-term polymerization.

[0079] Meanwhile, the polymerization in the batch reactor of the present invention utilizes a thermal polymerization method, and thus the polymerization initiator is a thermal polymerization initiator.

[0080] The thermal polymerization initiator may be one or more selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid. Specific examples of persulfate initiators include sodium persulfate (NaSO), potassium persulfate (KSO), and ammonium persulfate ((NHSO). Examples of azo initiators include 2,2-azobis(2-amidinopropane) dihydrochloride and 2,2-azobis(N,N-dimethylene)isobutyramidine dihydrochloride. dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), etc. A wide variety of thermal polymerization initiators are clearly described in Odian, "Principle of Polymerization" (Wiley, 1981), p. 203, and are not limited to the examples mentioned above.

[0081] The polymerization initiator can be used in an amount of 2 parts by weight or less per 100 parts by weight of the water-soluble ethylenically unsaturated monomer. That is, if the concentration of the polymerization initiator is too low, the polymerization rate may be slowed and a large amount of residual monomer may be extracted into the final product, which is undesirable. Conversely, if the concentration of the polymerization initiator is higher than the above range, the polymer chains forming the network may become shorter, the content of water-soluble components may increase, and the physical properties of the resin may be degraded, such as a decrease in water absorption capacity under pressure, which is undesirable.

[0082] Meanwhile, in one embodiment of the present invention, polymerization can be initiated by adding a reducing agent that forms a redox couple with the initiator.

[0083] Specifically, when the initiator and the reducing agent are added to the polymer solution, they react with each other to form radicals.

[0084] The formed radicals react with the monomer, and the oxidation-reduction reaction between the initiator and the reducing agent is highly reactive, so polymerization can be initiated even with only a small amount of initiator and reducing agent added, eliminating the need to increase the process temperature, allowing low-temperature polymerization and minimizing changes in the physical properties of the polymer solution.

[0085] The polymerization reaction using the oxidation-reduction reaction can occur smoothly at or below room temperature (25° C.). For example, the polymerization reaction may be carried out at a temperature of 5° C. to 25° C., or 5° C. to 20° C.

[0086] In one embodiment of the present invention, when a persulfate initiator is used as the initiator, the reducing agent can be at least one selected from the group consisting of sodium metabisulfite (NaSO); tetramethylethylenediamine (TMEDA); a mixture of iron (II) sulfate and EDTA (FeSO / EDTA); sodium formaldehyde sulfoxylate; and disodium 2-hydroxy-2-sulfinoacetate.

[0087] For example, potassium persulfate can be used as the initiator and disodium 2-hydroxy-2-sulfinoacetate as the reducing agent; ammonium persulfate can be used as the initiator and tetramethylethylenediamine as the reducing agent; or sodium persulfate can be used as the initiator and sodium formaldehyde sulfoxylate as the reducing agent.

[0088] In another embodiment of the present invention, when a hydrogen peroxide-based initiator is used as the initiator, the reducing agent may be one or more selected from the group consisting of ascorbic acid; sucrose; sodium sulfite (NaSO); sodium metabisulfite (NaSO); tetramethylethylenediamine (TMEDA); a mixture of iron (II) sulfate and EDTA (FeSO / EDTA); sodium formaldehyde sulfoxylate; disodium 2-hydroxy-2-sulfinoacetate; and disodium 2-hydroxy-2-sulfoacetate.

[0089] The monomer composition may further contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant, if necessary.

[0090] The monomer composition containing the monomer may be in a solution state, for example, dissolved in a solvent such as water, and the solid content in such a solution state of the monomer composition, i.e., the concentrations of the monomer, internal crosslinking agent, and polymerization initiator, can be appropriately adjusted in consideration of the polymerization time, reaction conditions, etc. For example, the solid content in the monomer composition may be 10 to 80 wt %, 15 to 60 wt %, or 30 to 50 wt %.

[0091] The solvent that can be used in this case is not limited in composition as long as it can dissolve the above-mentioned components, and for example, one or more solvents 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 can be used in combination.

[0092] The polymer obtained by this method can form a polymer having a high molecular weight and a uniform molecular weight distribution by polymerizing an unneutralized ethylenically unsaturated monomer, as described above, and can reduce the content of water-soluble components.

[0093] The polymer obtained by such a method may have a water content of 30 to 80% by weight in the state of a hydrogel polymer. For example, the water content of the polymer may be 30% by weight or more, or 45% by weight or more, or 50% by weight or more, and 80% by weight or less, or 70% by weight or less, or 60% by weight or less.

[0094] If the water content of the polymer is too low, it may be difficult to secure an adequate surface area in the subsequent pulverization step, and the polymer may not be effectively pulverized. If the water content of the polymer is too high, the pressure applied in the subsequent pulverization step may increase, making it difficult to pulverize the polymer to a desired particle size.

[0095] Throughout this specification, the term "moisture content" refers to the amount of water relative to the total weight of the polymer, calculated by subtracting the weight of the polymer in a dry state from the weight of the polymer. Specifically, it is defined as a value calculated by measuring the weight loss due to evaporation of water in the polymer during the drying process of raising the temperature of the polymer in a crumb state using infrared heating. The drying conditions are to raise the temperature from room temperature to about 180°C and then maintain it at 180°C, and the total drying time is set to 40 minutes, including 5 minutes for the temperature rise step, and the moisture content is measured.

[0096] Stage 2: Neutralization Stage and Stage 3: Atomization Stage Next, a step (step 2) is carried out in which at least some of the acid groups of the polymer are neutralized.

[0097] In this case, the neutralizing agent may be a basic substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide, which can neutralize the acidic group.

[0098] Furthermore, the degree of neutralization, which refers to the degree to which the acidic groups contained in the polymer are neutralized by the neutralizing agent, may be 50 to 90 mol%, 60 to 85 mol%, 65 to 85 mol%, or 65 to 75 mol%. The range of the degree of neutralization may vary depending on the final physical properties, but if the degree of neutralization is too high, the water absorption capacity of the superabsorbent polymer may be reduced, and the concentration of carboxyl groups on the particle surface may be too low, making it difficult to perform surface cross-linking in subsequent processes, resulting in reduced water absorption properties or liquid permeability under pressure. Conversely, if the degree of neutralization is too low, the polymer may not only exhibit significantly reduced water absorption capacity but also exhibit properties similar to elastic rubber, which makes it difficult to handle.

[0099] Simultaneously with, before or after step 2, a step of atomizing the polymer in the presence of a surfactant (step 3) is carried out. That is, steps 2 and 3 may be carried out sequentially, alternately or simultaneously.

[0100] Step 3 is a step of atomizing the polymer in the presence of a surfactant, in which the polymer is not chopped into millimeter-sized particles but is simultaneously chopped and aggregated into particles of tens to hundreds of micrometers. That is, this step involves imparting appropriate adhesiveness to the polymer to produce secondary aggregate particles formed by aggregation of primary particles chopped into particles of tens to hundreds of micrometers. The hydrous superabsorbent resin particles, which are secondary aggregate particles produced in this step, have a normal particle size distribution and a significantly increased surface area, resulting in a significantly improved water absorption rate.

[0101] After mixing the polymer and surfactant in this manner, the polymer is atomized in the presence of the surfactant to produce hydrous superabsorbent resin particles (=neutralized and atomized polymer) in the form of secondary agglomerated particles obtained by chopping and agglomerating the superabsorbent resin particles and surfactant in a mixed state.

[0102] Here, "water-containing superabsorbent resin particles" are particles with a water content (moisture content) of approximately 30% by weight or more, and are formed by chopping and agglomerating a polymer into particles without a drying process, so they can have a moisture content of 30 to 80% by weight, similar to the polymer.

[0103] According to one embodiment of the present invention, the surfactant may be a compound represented by the following Chemical Formula 2 or a salt thereof, but the present invention is not limited thereto:

[0104] [ka]

[0105] In the above Chemical Formula 2, A1, A2 and A3 each independently represent a single bond, a carbonyl,

[0106] [ka]

[0107] and one or more of these is a carbonyl or

[0108] [ka]

[0109] wherein m1, m2, and m3 each independently represent an integer of 1 to 8;

[0110] [ka]

[0111] are each linked to adjacent oxygen atoms,

[0112] [ka]

[0113] are linked to adjacent R1, R2 and R3, respectively; R1, R2, and R3 are each independently hydrogen, a linear or branched alkyl group having 6 to 18 carbon atoms, or a linear or branched alkenyl group having 6 to 18 carbon atoms; n is an integer of 1 to 9.

[0114] The surfactant is added so that the atomization step can be easily carried out without agglomeration when mixed with the polymer.

[0115] The surfactant represented by Chemical Formula 2 is a nonionic surfactant and has excellent surface adsorption performance due to hydrogen bonding even with unneutralized polymers, making it suitable for achieving the desired aggregation control effect. On the other hand, in the case of an anionic surfactant that is not a nonionic surfactant, when it is mixed with a polymer neutralized with a neutralizing agent such as NaOH or Na2SO4, the ionized Na is attached to the carboxyl group substituent of the polymer. +When adsorption occurs via ions and the compound is mixed with an unneutralized polymer, there is a problem that the adsorption efficiency relative to the polymer decreases due to competition with the anions of the carboxyl group substituents of the polymer.

[0116] Specifically, in the surfactant represented by the chemical formula 2, the hydrophobic functional groups are the terminal functional groups R1, R2, and R3 (if they are not hydrogen), and the hydrophilic functional groups are the glycerol-derived portion in the chain and the terminal hydroxyl group (A n is a single bond and at the same time R n (When n is hydrogen, n=1 to 3) The glycerol-derived portion and the terminal hydroxyl group are hydrophilic functional groups that improve the adsorption performance on the polymer surface, thereby effectively suppressing the aggregation of superabsorbent resin particles.

[0117] In Formula 2, the hydrophobic functional groups R1, R2, and R3 (when not hydrogen) are each independently a straight-chain or branched-chain alkyl having 6 to 18 carbon atoms, or a straight-chain or branched-chain alkenyl having 6 to 18 carbon atoms. If the R1, R2, and R3 (when not hydrogen) are alkyl or alkenyl having less than 6 carbon atoms, the chain length is too short, which can cause problems such as ineffective aggregation control of the milled particles. If the R1, R2, and R3 (when not hydrogen) are alkyl or alkenyl having more than 18 carbon atoms, the mobility of the surfactant can decrease, preventing effective mixing with the polymer, and the increased cost of the surfactant can result in a high unit price for the composition.

[0118] Preferably, R1, R2, and R3 are hydrogen or, in the case of a straight-chain or branched alkyl having 6 to 18 carbon atoms, 2-methylhexyl, n-heptyl, 2-methylheptyl, n-octyl, n-nonyl, n-decanyl, n-undecanyl, n-dodecanyl, n-tridecanyl, n-tetradecanyl, n-pentadecanyl, n-hexadecanyl, n-heptadecanyl, or n-octadecanyl, or, in the case of a straight-chain or branched alkenyl having 6 to 18 carbon atoms, 2-hexenyl, 2-heptenyl, 2-octenyl, 2-nonenyl, n-dekenyl, 2-undekenyl, 2-dodekenyl, 2-tridekenyl, 2-tetradekenyl, 2-pentadekenyl, 2-hexadekenyl, 2-heptadekenyl, or 2-octadekenyl.

[0119] The surfactant may be selected from compounds represented by the following formulas 2-1 to 2-14:

[0120] [ka] [ka]

[0121] The surfactant can be used in an amount of about 3 parts by weight or less, preferably 2 parts by weight or less, 1 part by weight or less, 0.5 parts by weight or less, 0.1 parts by weight or less, 0.01 parts by weight or less, or 0.001 parts by weight or less, based on 100 parts by weight of the polymer. If the surfactant is used in an excessive amount, the surface tension value may decrease, which may result in deterioration of various physical properties of the final superabsorbent polymer.

[0122] The method of mixing such a surfactant with a polymer is not particularly limited as long as it can be uniformly mixed with the polymer, and can be appropriately adopted and used. Specifically, the surfactant can be mixed in a dry state, dissolved in a solvent and then mixed in a solution state, or melted and then mixed.

[0123] For example, the surfactant may be mixed in a solvent in the form of a solution. Any type of solvent, including inorganic and organic solvents, may be used. However, considering the ease of the drying process and the cost of the solvent recovery system, water is the most suitable. The solution may be prepared by mixing the surfactant and polymer in a reaction vessel, by adding the polymer to a mixer and injecting the solution, or by continuously supplying the polymer and solution to a continuously operating mixer and mixing them.

[0124] According to one embodiment of the present invention, the step of neutralizing at least a portion of the acidic groups of the polymer (Step 2) and the step of atomizing the polymer in the presence of a surfactant (Step 3) may be performed sequentially, alternately, or simultaneously. The step of neutralizing the acidic groups may be performed simultaneously with a polymer coagulation process.

[0125] That is, a neutralizing agent may be added to the polymer to neutralize the acidic groups first, and then a surfactant may be added to the neutralized polymer and the surfactant-mixed polymer may be atomized (performed in the order of step 2 → step 3), or a neutralizing agent and surfactant may be added to the polymer simultaneously to neutralize and atomize the polymer (performed in the order of step 2 → step 2). Alternatively, the surfactant may be added first and the neutralizing agent may be added later (performed in the order of step 3 → step 2). Alternatively, the neutralizing agent and surfactant may be added alternately. Alternatively, the surfactant may be added first to atomize the polymer, and then a neutralizing agent may be added to neutralize the neutralized hydrogel polymer, and an additional surfactant may be added to perform an additional atomization process.

[0126] On the other hand, in order to ensure uniform neutralization of the entire polymer, it is preferable to leave a certain time lag between the addition of the neutralizing agent and the atomization step.

[0127] At least a portion or a substantial amount of the surfactant can be present on the surface of the hydrated superabsorbent resin particles.

[0128] Here, the presence of the surfactant on the surface of the hydrated superabsorbent resin particles means that at least a portion or a substantial amount of the surfactant is adsorbed or bound to the surface of the hydrated superabsorbent resin particles. Specifically, the surfactant may be physically or chemically adsorbed to the surface of the superabsorbent resin. More specifically, the hydrophilic functional groups of the surfactant may be physically adsorbed to the hydrophilic portion of the superabsorbent resin surface by intermolecular forces such as dipole-dipole interaction. In this way, the hydrophilic portion of the surfactant may physically adsorb to the surface of the superabsorbent resin particles, surrounding the surface, while the hydrophobic portion of the surfactant may not adsorb to the surface of the resin particles, and the surfactant may coat the resin particles in the form of a type of micelle structure. This is because the surfactant is added not during the polymerization process of the water-soluble ethylenically unsaturated monomer but at the micronization stage after the polymer is formed. This allows the surfactant to faithfully fulfill its role as compared to when the surfactant is added during the polymerization process and exists inside the polymer, and particles with a large surface area can be obtained in the form of aggregated fine particles due to simultaneous pulverization and aggregation.

[0129] According to one embodiment of the present invention, the atomization step may be performed using an atomization device, for example, an atomization device including a perforated plate having a number of holes with a diameter of 1 mm to 20 mm. When a perforated plate having holes in this range is used, it is more suitable for improving the efficiency of the drying process described below.

[0130] More specifically, the atomization device may include a body having a transfer space into which the polymer is transferred, a screw member rotatably installed within the transfer space to move the polymer, a drive motor to provide rotational driving force to the screw member, a cutter member installed in the body to pulverize the polymer, and a perforated plate having a plurality of holes formed therein and discharging the polymer pulverized by the cutter member to the outside of the body. The hole size of the perforated plate of the atomization device may be 1 mm to 20 mm, 5 mm to 15 mm, or 5 mm to 12 mm.

[0131] In this way, when the polymer mixed with the surfactant is atomized using an atomizer while controlling aggregation, a smaller particle size distribution is achieved, and the subsequent drying and pulverization steps can be carried out under milder conditions, thereby preventing the generation of fine powder and improving the physical properties of the superabsorbent resin.

[0132] According to one embodiment of the present invention, the step of producing hydrated superabsorbent resin particles by atomizing the polymer may be performed one or more times, preferably 1 to 6 times, 1 to 4 times, or 1 to 3 times. This may be performed using multiple atomizers, a single atomizer including multiple perforated plates and / or multiple cutter members, or multiple atomizers, some of which may include multiple perforated plates and / or multiple cutter members.

[0133] Stage 4: Drying Next, the neutralized and atomized polymer (or hydrated superabsorbent resin particles) is dried to produce base resin particles (Step 4).

[0134] In this step, at least a portion of the acid groups of the polymer are neutralized, and the polymer is atomized in the presence of a surfactant, followed by drying the water content of the resulting polymer.

[0135] Specifically, in the present invention, a hydrogel polymer polymerized in an unneutralized state as described above is used, and the void formation rate is controlled within a specific range in the initial stage of the drying process to produce base resin particles having a relatively high moisture content while minimizing the occurrence of wet particles.

[0136] Therefore, in the drying step of step 4, the initial void formation rate of the polymer satisfies 0.8 to 1.7% / min when 30 to 60% of the total drying time has elapsed.

[0137] Here, the initial void formation rate is an index that can determine the drying uniformity by affecting the formation of drying channels during the drying process. The initial void formation rate can be achieved to satisfy the above-mentioned range by adjusting the polymerization conditions, atomization conditions, and drying conditions. Here, "initial" can mean the point when 30 to 60% of the total drying time has elapsed.

[0138] If the initial void formation rate is less than 0.8% / min, the drying flow path may not be sufficiently secured, resulting in non-uniform drying, and if it exceeds 1.7% / min, heat exchange may not be performed, resulting in hot air leakage. Therefore, the initial void formation rate may be preferably 1.0 to 1.5% / min, and more preferably 1.0 to 1.35% / min.

[0139] The initial void formation rate may be calculated using the following Equation 1:

[0140] [Formula 1] Initial void formation rate (% / min)=(R B 2-R B 1) / (t2-t1)×100

[0141] In the above formula, R B means the ratio of void volume to the total bulk volume (V) B ) is defined as the ratio t1 is the time when drying begins, and can mean, for example, the time when the neutralized and micronized polymer (or water-containing superabsorbent polymer particles) are put into a dryer or the time when heating is started after putting them into the dryer, and can mean 0 in calculations.

[0142] t2 is the time when 30 to 60% of the total drying time has elapsed, and may be, for example, the time at any one of the points when 30 to 60% of the total drying time has elapsed. Specifically, it may mean the time when 30 to 60% has elapsed after the neutralized and atomized polymer (or water-containing superabsorbent resin particles) are introduced into the dryer, or the time when 30 to 60% has elapsed after the heating step is carried out after introduction.

[0143] R B 1 and R B 2 are R at time t1 and t2, respectively. B means the value.

[0144] The initial void formation rate can be measured using an XRM (manufacturer: ZEISS, model name: Xradia 620 Versa), and a specific measurement method therefor will be described in the experimental examples section below.

[0145] Preferably, the drying step of step 4 may be carried out using a fixed-bed belt dryer. This is distinguished from moving-type drying by the presence or absence of material movement during drying in the case of a fixed-bed type method.

[0146] Stationary drying refers to a method in which the material to be dried is suspended on a perforated iron plate or other floor through which air can pass, and hot air passes over the material from bottom to top. Fluidized-bed drying, on the other hand, refers to a method in which the material is dried while being mechanically stirred. The direction in which the hot air passes over the material may be the same as or different from the direction in which the material circulates. Alternatively, the material may be circulated inside the dryer, and a heat transfer fluid (heat medium flow) may be passed through a separate pipe outside the dryer to dry the material.

[0147] In the drying process using a stationary belt dryer, the superabsorbent polymer is obtained in the form of a laminate, but the increased cohesive force of the hydrogel polymer significantly reduces the void ratio within the dried laminate, which increases the hot air differential pressure, increasing the content of undried crumbs and causing warping of the outer shell of the dried product. Therefore, the inventors have confirmed that by drying while controlling the initial void formation rate within the aforementioned range, it is possible to produce a superabsorbent polymer with an appropriate moisture content without reducing the drying efficiency of the laminate or causing warping of the outer shell, even when using a stationary belt dryer.

[0148] The drying step of step 4 may be performed using hot air at 70°C to 250°C. As described above, in the case of the polymer polymerized, neutralized, and atomized according to the present invention, excellent drying efficiency can be achieved even under relatively mild conditions. Furthermore, the above-mentioned initial void formation rate can be satisfied within the above-mentioned hot air temperature range. Preferably, the hot air temperature may be 75°C to 200°C.

[0149] The drying step of step 4 may be carried out for 20 to 90 minutes. As described above, in the case of the polymer polymerized, neutralized, and atomized according to the present invention, excellent drying efficiency can be achieved even when the drying is carried out under relatively mild conditions and for a relatively short drying time. Furthermore, the above-mentioned hot air range can satisfy the above-mentioned initial void formation rate. Preferably, the drying is carried out for 25 to 60 minutes.

[0150] According to one embodiment of the present invention, the moisture content of the superabsorbent resin obtained after drying can satisfy 3% to 10% by weight, and the amount of undried material in the form of crumbs can be significantly reduced while simultaneously achieving a relatively high moisture content.

[0151] In the case of a superabsorbent resin that is dried using a conventional stationary belt dryer, the moisture content of the resin after drying is generally about 1 to 2% by weight. However, in the case of a polymer that has been polymerized, neutralized, and atomized according to the present invention, excellent drying efficiency and a relatively high moisture content can be achieved even under relatively mild conditions and for a relatively short drying time, which is preferable.

[0152] Crushing stage According to one embodiment of the present invention, a step of crushing the superabsorbent resin particles obtained by carrying out step 4 to prepare superabsorbent resin particles may be carried out.

[0153] Specifically, the pulverization step may be performed by pulverizing the dried superabsorbent resin particles to have a particle size of normal particle level, that is, a particle size of 150 μm to 850 μm.

[0154] The pulverizer used for this purpose may be, specifically, a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, or a disc cutter, but is not limited to the above examples.

[0155] Alternatively, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, or the like may be used as the pulverizer, but the pulverizer is not limited to the above examples.

[0156] On the other hand, in the manufacturing method of the present invention, it is possible to realize superabsorbent resin particles with a smaller particle size distribution in the atomization step than in the conventional chopping step, and when fluidized (moving-type) drying is performed, the moisture content after drying is maintained relatively high at 10% by weight or more. Therefore, even if the grinding is performed under mild conditions with less grinding force, it is possible to form a superabsorbent resin with a very high content of normal particle sizes of 150 μm to 850 μm, and the rate of fine powder generation can be significantly reduced.

[0157] The superabsorbent resin 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 resin particles having a particle size of 150 μm to 850 μm, i.e., normal particles, based on the total weight. The particle size of such resin particles can be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP220.3 method.

[0158] Furthermore, the superabsorbent resin particles may contain about 20% by weight or less, or about 18% by weight or less, or about 15% by weight or less, or about 13% by weight or less, or about 12% by weight or less, or about 11% by weight or less, or about 10% by weight or less, or about 9% by weight or less, or 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 based on the total weight. This is in contrast to superabsorbent resins produced by conventional manufacturing methods, which contain more than about 20% by weight to about 30% by weight of fine powder.

[0159] Classification stage According to one embodiment of the present invention, after the step of pulverizing the superabsorbent resin particles, the method may further include a step of classifying the pulverized superabsorbent resin particles according to particle size.

[0160] Preferably, particles having a particle size of 150 to 850 μm are classified, and only the particles having such a particle size are subjected to a surface cross-linking reaction step to be manufactured into a product. More specifically, the classified superabsorbent resin particles have a particle size of 150 to 850 μm, and may contain 50% by weight or more of particles having a particle size of 300 to 600 μm, and may contain less than 3% by weight of fine powder having a particle size of less than 150 μm.

[0161] Surface cross-linking stage According to one embodiment of the present invention, after pulverizing and / or classifying the superabsorbent resin particles, the method may further include a step of forming a surface cross-linked layer on at least a portion of the surface of the superabsorbent resin particles in the presence of a surface cross-linking agent.

[0162] Through this step, the cross-linked polymer contained in the superabsorbent resin particles is additionally cross-linked by the surface cross-linking agent, so that a surface cross-linked layer can be formed on at least a portion of the surface of the superabsorbent resin particles.

[0163] The surface cross-linking agent can be any surface cross-linking agent that has been conventionally used in the production of superabsorbent resins without any particular limitation. For example, the surface cross-linking agent can include one or more polyols 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; one or more carbonate compounds 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; mono-, di-, or polyoxazolidinone compounds; or cyclic urea compounds.

[0164] Specifically, one or more, two or more, or three or more of the above-mentioned surface cross-linking agents can be used as the surface cross-linking agent. For example, ethylene carbonate-propylene carbonate (ECPC), propylene glycol, and / or glycerol carbonate can be used.

[0165] Such a surface cross-linking agent can be used in an amount of about 0.001 to about 5 parts by weight relative to 100 parts by weight of the superabsorbent resin particles. For example, the surface cross-linking agent can be used in an amount of 0.005 parts by weight or more, or 0.01 parts by weight or more, or 0.05 parts by weight or more and 5 parts by weight or less, or 4 parts by weight or less, or 3 parts by weight or less relative to 100 parts by weight of the superabsorbent resin particles. By adjusting the content range of the surface cross-linking agent within the above range, a superabsorbent resin exhibiting excellent water absorption properties can be produced.

[0166] In addition, the step of forming the surface cross-linked layer may be performed by adding an inorganic substance to the surface cross-linking agent, i.e., the surface of the superabsorbent resin particles may be additionally cross-linked in the presence of the surface cross-linking agent and the inorganic substance to form the surface cross-linked layer.

[0167] The inorganic substance may be one or more selected from the group consisting of silica, clay, alumina, silica-alumina composite, titania, zinc oxide, and aluminum sulfate. The inorganic substance may be used in powder or liquid form, particularly alumina powder, silica-alumina powder, titania powder, or nanosilica solution. The inorganic substance may be used in an amount of about 0.001 to about 1 part by weight per 100 parts by weight of the superabsorbent resin particles.

[0168] Furthermore, there is no limitation on the configuration of the method for mixing the surface cross-linking agent with the superabsorbent resin composition. For example, a method in which the surface cross-linking agent and the superabsorbent resin composition are mixed in a reaction tank, a method in which the surface cross-linking agent is sprayed onto the superabsorbent resin composition, a method in which the superabsorbent resin composition and the surface cross-linking agent are continuously supplied to a continuously operating mixer and mixed therein, etc. can be used.

[0169] When the surface cross-linking agent and the superabsorbent polymer composition are mixed, water and methanol may be added. The addition of water and methanol has the advantage of enabling the surface cross-linking agent to be uniformly dispersed in the superabsorbent polymer composition. The amounts of water and methanol added can be appropriately adjusted to induce uniform dispersion of the surface cross-linking agent, prevent clumping of the superabsorbent polymer composition, and optimize the surface penetration depth of the cross-linking agent.

[0170] The surface cross-linking step may be carried out at a temperature of about 80°C to about 250°C. More specifically, the surface cross-linking step may 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-mentioned conditions for the surface cross-linking step are satisfied, the surfaces of the superabsorbent resin particles are sufficiently cross-linked, thereby increasing the water absorption capacity under pressure.

[0171] The temperature raising means for the surface crosslinking reaction is not particularly limited. Heating can be performed by supplying a heat medium or directly supplying a heat source. In this case, the type of heat medium that can be used may be a heated fluid such as steam, hot air, or hot oil, but is not limited thereto. The temperature of the heat medium to be supplied may be appropriately selected in consideration of the means of the heat medium, the rate of temperature rise, and the target temperature of temperature rise. Meanwhile, examples of a heat source that can be directly supplied include, but are not limited to, electric heating and gas heating methods.

[0172] According to one embodiment of the present invention, after the step of forming a surface cross-linked layer on at least a portion of the surface of the superabsorbent resin particles, the method may further include one or more of the following steps: a cooling step of cooling the superabsorbent resin particles having the surface cross-linked layer formed thereon, a hydration step of adding water to the superabsorbent resin particles having the surface cross-linked layer formed thereon, and a post-treatment step of adding an additive to the superabsorbent resin particles having the surface cross-linked layer formed thereon. In this case, the cooling step, the hydration step, and the post-treatment step may be performed sequentially or simultaneously.

[0173] The additives added in the post-treatment step may be, but are not limited to, a liquid permeability improver, an anti-caking agent, a flowability improver, an antioxidant, and the like.

[0174] By selectively carrying out the cooling step, the hydration step, and the post-treatment step, the moisture content of the final superabsorbent polymer can be improved, and a higher quality superabsorbent polymer product can be produced. The superabsorbent polymer prepared by the above method has a significantly low content of crumb-shaped undried material during the drying process, and the occurrence of warping of the dried material is effectively controlled, thereby achieving excellent water absorption properties.

[0175] Specifically, the superabsorbent polymer produced by the above-described method has a high water absorption rate, a low fine powder content, and water retention capacity (CRC), which is a water absorption property, equivalent to or higher than that of superabsorbent polymers produced by conventional methods.

[0176] The functions and effects of the present invention will be described in more detail below through specific examples of the present invention, but these examples are presented only as examples of the present invention and do not define the scope of the invention.

[0177] <Example> Example 1 (Polymerization stage) In a 5L glass vessel equipped with a stirrer and thermometer, 1494g of acrylic acid, 4.5g of pentaerythritol triallyl ether (PETTAE) as an internal crosslinker, and 3392g of water were mixed with stirring. The reaction temperature was maintained at 5°C, and nitrogen was introduced into the mixture at 1000cc / min for 1 hour. Next, 19.92g of 35% aqueous hydrogen peroxide solution, 22.4g of 1.6% aqueous ascorbic acid solution, and 44.8g of 1% aqueous 2,2'-azobisamidinopropane dihydrochloride solution were added as polymerization initiators, and 22.4g of 0.01% aqueous ferrous sulfate solution was added as a reducing agent and mixed. After the polymerization reaction initiated and the temperature of the polymer reached 85°C, the temperature was maintained at 90±2°C for approximately 6 hours to produce the polymer.

[0178] (Neutralization and atomization stage) To 1,000 g of the polymer produced in the polymerization step, a surfactant, glycerol monolaurate (GML), was dissolved in water at 60°C or higher at a ratio of 0.01 g per 1,000 g of polymer. The resulting solution was then extruded at 3,000 rpm through a perforated plate with 10 mm holes using a high-speed rotary shredder (F-150 / Karl Schnell) mounted inside a cylindrical grinder. The recovered crushed gel was then extruded twice through a perforated plate with 6 mm holes and once through a perforated plate with 4 mm holes using a screw extruder mounted inside the cylindrical grinder at 500 rpm. To neutralize some of the acidic groups, 237.8 g of 50% NaOH aqueous solution, 60 g of fine powder, and 75.5 g of 10% NaSO aqueous solution were added per 1 kg of crumb to each stage of the screw extruder.

[0179] (Drying stage) 736 g of the neutralized and atomized hydrogel polymer was dried in a dryer capable of rotating airflow from top to bottom. 200°C hot air was blown from the top for 2.5 minutes, followed by 100°C hot air from the top for 7.5 minutes and then from the bottom for 15 minutes, followed by 75°C hot air from the bottom for 3 minutes (total drying time: 28 minutes). XRM measurements were performed using dried chips at this stage. Here, the initial void formation rate measured 16 minutes after being placed in the dryer (57% of the total drying time) was 1.34% / min.

[0180] (Crushing and classification stage) The dried product was pulverized in a pulverizer (GRAN-U-LIZER™, MPE) and then classified using a standard ASTM mesh sieve to obtain a highly water-absorbent resin containing base resin particles of 150 to 850 μm in size.

[0181] Example 2 A superabsorbent resin containing base resin particles was obtained in the same process as in Example 1, except that in the drying stage of Example 1, hot air at 120°C was applied from above for 20 minutes and from below for 20 minutes to dry (total drying process: 40 minutes).

[0182] Here, the initial void formation rate measured 16 minutes after being placed in the drying device (40% of the total drying time) was 1.11% / min.

[0183] Example 3 A superabsorbent resin containing base resin particles was obtained in the same manner as in Example 1, except that the internal crosslinking agent, pentaerythritol triallyl ether (PETTAE), was changed from 4.5 g to 3.5 g in the polymerization step of Example 1.

[0184] Here, the initial void formation rate measured 16 minutes after being placed in the drying device (57% of the total drying time) was 1.13% / min.

[0185] Example 4 A superabsorbent resin containing base resin particles was obtained in the same manner as in Example 1, except that in the neutralization and pulverization process, a screw extruder was used, and the mixture was passed through a perforated plate with 6 mm holes twice and a perforated plate with 4 mm holes once, but instead of this, the mixture was passed through a perforated plate with 6 mm holes twice and a perforated plate with 8 mm holes once.

[0186] Here, the initial void formation rate measured 16 minutes after being placed in the drying device (57% of the total drying time) was 1.01% / min.

[0187] Comparative Example 1: Foaming process of other grades 1) Polymerization stage In a 3L glass vessel equipped with a stirrer and a thermometer, 430g of acrylic acid, 1,100ppm (relative to acrylic acid) of sodium bicarbonate as a blowing agent, 6,000ppm (relative to acrylic acid) of propylene glycol di(meth)acrylate as an internal crosslinking agent, 100ppm (relative to acrylic acid) of acylphosphine as a photopolymerization initiator, 2,000ppm (relative to acrylic acid) of sodium persulfate as a thermal polymerization initiator, and 600g of 30% NaOH aqueous solution were mixed at room temperature (25±1°C) to prepare a monomer composition with a total solid content of 40wt% (neutralization degree of acrylic acid: 70mol%).

[0188] Thereafter, 400 ppm (relative to acrylic acid) of sodium metabisulfate was added as a reducing agent to the monomer composition, and the monomer composition was simultaneously supplied and irradiated with light for 1 minute to allow polymerization to proceed.

[0189] 2) Gel crushing and drying process The hydrogel polymer was coarsely crushed using a chopper with a hole size of 10 mm and dried for 28 minutes using hot air at about 200°C. The initial void formation rate measured 16 minutes after being placed in the drying device (57% of the total drying time) was 0.5% / min.

[0190] 3) Grinding and classification process The polymer dried in step 4 was pulverized in a pulverizer and then classified using a standard ASTM mesh sieve to obtain a base resin powder with a size of 150 to 850 μm.

[0191] Comparative Example 2: Non-foaming process of other grades 1) Polymerization process 450g of acrylic acid, 0.036g of IRGACURE 819 as an initiator, and 1.17g of PEGDA as an internal crosslinker were added to a 3L glass container equipped with a stirrer and thermometer, and then stirred. It was then mixed with 554.9g of 31.5% NaOH aqueous solution and 509.7g of water to obtain a 70% neutralized monomer composition. The monomer composition was then cooled to 40°C and mixed with 50.63g of 1.78% SPS solution, followed by irradiation with light for 1 minute to allow polymerization to proceed.

[0192] 2) Gel crushing and drying process The hydrogel polymer was coarsely crushed using a chopper with a hole size of 10 mm and dried for 28 minutes using hot air at about 197°C. The initial void formation rate measured 16 minutes after being placed in the dryer (57% of the total drying time) was 0.33% / min.

[0193] Comparative Example 3: Non-foaming process of other grades - Differences in gel crushing conditions The procedure was the same as in Comparative Example 2, except that 16 mm holes were used instead of 10 mm holes in the gel crushing step.

[0194] Here, the initial void formation rate measured 16 minutes after being placed in the drying device (57% of the total drying time) was 0.56% / min.

[0195] <Experimental Example 1 - Control of the drying process> (1) Measurement of the degree of void formation In the manufacturing processes of the Examples and Comparative Examples, the dried chips were subjected to the drying process. Therefore, the degree of void formation was measured using an XRM (manufacturer: ZEISS, model name: Xradia 620 Versa) according to the following procedure, and then the void formation rate was calculated using Equation 1. The results are shown in Table 1.

[0196] (1) The 3D reconstructed XRM cross-sectional 2D image is converted into a binarized image using Otsu's thresholding method to distinguish between the background and sap particles. (2) The binarized cross-sectional 2D images are stacked and rendered in 3D. (3) Measure the surface area [1] and volume of 3D rendered SAP particles. (4) Surface area: Calculate using the method disclosed in the paper [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.] (5) Volume: Count the number of voxels in the actual 3D binary image. (6) Internal porosity: Measured using an algorithm that calculates closed pores by taking into account the connectivity of the three-dimensional image [ref2; Soille, P., Morphological Image Analysis: Principles and Applications, Springer-Verlag, 1999, pp. 173-174.]

[0197] [Formula 1] Void formation rate (% / min)=(R B 2-R B 1) / (t2-t1)×100

[0198] In the above formula, R B means the ratio of void volume to the total bulk volume (V) B ) is defined as the ratio t1 is the time when drying begins, t2 is the time when 30-60% of the total drying time has elapsed, R B 1 and R B 2 are R at time t1 and t2, respectively. B means the value.

[0199] (2) Measuring the efficiency of the drying process In the Examples and Comparative Examples, the dried laminate plate obtained in Step 4 after the drying process (before surface crosslinking) was cut into 10 x 10 cm samples. The sample was divided into three equal parts based on the cross section (based on the height of the laminate), and each was divided into an upper layer, a middle layer, and a lower layer. A sample was taken from the superabsorbent resin of each layer, and its initial weight H0 (g) was measured after maintaining it at 180°C for 40 minutes by infrared heating, and its weight H1 (g) was measured. The moisture content was then calculated using the following Equation 2, and the results are shown in Table 1.

[0200] [Formula 2] Moisture content (wt%)={[H0(g)-H1(g)] / H0(g)}×100

[0201] The measurement was repeated five times, and the average value and standard deviation were calculated.

[0202] [Table 1]

[0203] In Examples 1 to 4, which satisfy the initial void formation rate of the present invention, it was confirmed that high moisture content was achieved while at the same time excellent drying efficiency and moisture content deviation at each stacking position was small. In the comparative example, where the initial void formation rate exceeds the range of the present invention, the drying process efficiency itself was excellent, but the moisture content was only about 1 to 2 wt%, and as can be confirmed in Experimental Example 2 described below, the physical properties of the final superabsorbent polymer were significantly lower than those of the examples.

[0204] <Experimental Example 2 - Evaluation of the physical properties of superabsorbent resin> The properties of the superabsorbent resins (hereinafter referred to as BR) containing the base resins prepared in the Examples and Comparative Examples were evaluated by the following methods and are shown in Table 2 below.

[0205] Unless otherwise specified, all of the following physical property evaluations were carried out at constant temperature and humidity (23±1°C, relative humidity 50±10%), and saline or salt water refers to a 0.9 wt% aqueous sodium chloride (NaCl) solution.

[0206] (1) Centrifuge Retention Capacity (CRC, Centrifuge Retention Capacity, g / g) The water retention capacity of the BRs of the Examples and Comparative Examples was measured in terms of the water absorption capacity under no load in accordance with the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP241.3.

[0207] Specifically, W0 (g) (approximately 0.2 g) of BR obtained in each of the Examples and Comparative Examples was uniformly placed in a nonwoven fabric envelope, sealed, and then immersed in physiological saline (0.9 wt%) at room temperature. After 30 minutes, the envelope was centrifuged at 250 G for 3 minutes to remove water, and the mass of the envelope, W2 (g), was measured. The same procedure was repeated without using the resin, and the mass, W1 (g), was then measured.

[0208] Using the obtained masses, the CRC (g / g) was calculated according to the following formula 3.

[0209] [Formula 3] CRC(g / g)={[W2(g)-W1(g)] / W0(g)}-1

[0210] (2) Water absorption rate (Vortex time) The water absorption rate (vortex time) was measured in seconds according to the method described in WO 1987 / 003208.

[0211] Specifically, 2 g of BR was placed in 50 mL of saline at 23 to 24°C, and the time it took for the vortex to disappear after stirring with a magnetic bar (8 mm diameter, 30 mm length) at 600 rpm was measured in seconds.

[0212] [Table 2]

[0213] As can be seen from the experimental data in Table 2, in the case of the examples of the present invention, by controlling the initial void formation rate during the drying process within a specific range, it was confirmed that excellent drying efficiency and excellent water absorption properties, in particular, a significantly improved water absorption rate, could be achieved.

[0214] It was confirmed that in the case of the comparative example, which has an initial void formation rate exceeding that of the present invention, the water absorption rate characteristic is significantly lower than that of the examples.

Claims

1. Step 1: carrying out polymerization on a monomer composition including 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; neutralizing at least a portion of the acidic groups of the polymer (Step 2); atomizing the polymer in the presence of a surfactant (Step 3); and drying the neutralized and atomized polymer to produce base resin particles (Step 4); In step 4, the initial void formation rate of the polymer measured after 30 to 60% of the total drying time is 0.8 to 1.7% / min. A method for producing superabsorbent resin.

2. The initial void formation rate is calculated by the following equation 1: A method for producing the highly water-absorbent polymer according to claim 1: [Formula 1] Initial void formation rate (% / min) = (R) Ｂ 2-R Ｂ 1) / (t1-t0)×100 In the above formula, R Ｂ means the ratio of void volume, and is the void volume (V) to the total bulk volume (V Ｂ ) is defined as the ratio of t1 is the time when drying begins, t2 is the time when 30 to 60% of the total drying time has elapsed, R Ｂ 1 and R Ｂ 2 are R at time t1 and t2, respectively. Ｂ means the value.

3. Steps 2 and 3 are performed sequentially, alternately, or simultaneously; A method for producing the highly water-absorbent resin according to claim 1.

4. The atomization step of step 3 is This is carried out using an atomization device including a perforated plate having a large number of holes with a diameter of 1 mm to 20 mm. A method for producing the highly water-absorbent resin according to claim 1.

5. The drying step of step 4 is carried out using a stationary belt dryer. A method for producing the highly water-absorbent resin according to claim 1.

6. The drying step of step 4 is carried out using hot air at 70°C to 250°C. A method for producing the highly water-absorbent resin according to claim 1.

7. The drying step of step 4 is carried out for 20 to 90 minutes. A method for producing the highly water-absorbent resin according to claim 1.

8. The moisture content of the base resin particles obtained by carrying out step 4 is 3 to 10% by weight. A method for producing the highly water-absorbent resin according to claim 1.

9. further comprising the steps of grinding and classifying the base resin particles. A method for producing the highly water-absorbent resin according to claim 1.

10. Further comprising forming a surface cross-linked layer on at least a portion of the surface of the base resin particles. A method for producing the highly water-absorbent resin according to claim 1 or 9.

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