Manufacturing method of superabsorbent resin

Abstract

The present invention relates to a method for producing a superabsorbent polymer and the superabsorbent polymer, and more specifically, to a method for producing a superabsorbent polymer that exhibits excellent water absorption properties and significantly reduces the amount of fine powder generated.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0176831 dated December 16, 2022, Korean Patent Application No. 10-2022-0177310 dated December 16, 2022, and Korean Patent Application No. 10-2023-0181009 dated December 13, 2023, and all contents disclosed in the documents of said Korean patent applications 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 that exhibits excellent water absorption properties and significantly reduces the amount of fine powder generated. [Background technology]

[0003] Super absorbent polymer (SAP) is a synthetic polymeric material that can absorb 500 to 1,000 times its own weight in water, and different companies that develop it give 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 horticultural soil water retention materials, civil engineering and construction water stop materials, seedling sheets, freshness preserving agents in the food distribution industry, and as materials for poultices.

[0004] Such superabsorbent polymers are widely used in the field of sanitary materials, such as diapers and sanitary napkins. In the sanitary materials, the superabsorbent polymers are generally contained in a dispersed state in the pulp. However, in recent years, efforts have been made to provide sanitary materials, such as diapers, with thinner thicknesses, and as part of these efforts, the pulp content has been reduced or even no pulp is used at all, so-called pulpless diapers, have been actively developed.

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

[0006] Such superabsorbent resins are generally manufactured by polymerizing monomers to produce a hydrogel polymer containing a large amount of water, and then drying and pulverizing the hydrogel polymer 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 properties of the final superabsorbent resin, particularly its water absorption properties.

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

[0008] On the other hand, 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 method is generally adopted in which a foaming agent is added to the monomer composition and cross-linking polymerization is carried out to form a porous structure in the base resin powder.

[0009] However, the use of a foaming agent has the disadvantages of decreasing 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 development of a technology that can increase the water absorption rate of the superabsorbent resin without using a foaming agent.

[0010] Therefore, in order to fundamentally solve these problems, there is a continuing demand for the development of a technique capable of producing a highly water-absorbent resin without generating fine powder. Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, an object of the present invention is to provide a method for producing a superabsorbent resin which can significantly improve the water absorption rate by forming micropores in the polymer through an ultrafine grinding process to increase the surface area, significantly reduce the amount of fine powder generated during the process, and exhibit excellent water absorption properties. [Means for solving the problem]

[0012] 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; ultrafinely grinding the polymer at a rotation speed of 500 rpm to 4,000 rpm (step 2); neutralizing at least a portion of the acidic groups of the polymer (Step 3); drying the micronized and neutralized polymer to produce dry superabsorbent polymer particles (Step 4); and The dried superabsorbent resin particles are pulverized to produce superabsorbent resin particles (step 5). A method for making a superabsorbent polymer is provided. Effect of the Invention

[0013] According to the method for producing a superabsorbent polymer of the present invention, it is possible to produce a superabsorbent polymer that has excellent water absorption properties, since the surface area is increased by forming micropores in the hydrogel polymer through the ultrafine grinding process, and the water absorption speed is significantly improved and the amount of fine powder generated during the process is significantly reduced. [Brief description of the drawings]

[0014] [Figure 1] 1 is a flow chart relating to a method for producing a superabsorbent polymer according to an embodiment of the invention. [Diagram 2] 1 is a SEM image of a resin produced by a method for producing a superabsorbent resin according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the invention. A singular expression includes a plural expression unless the context clearly indicates otherwise. In this specification, the terms "include", "comprise", "have" and the like are intended to specify the presence of an implemented feature, step, component, or combination thereof, and should be understood not to preclude the presence or additional possibility of one or more other features, steps, components, or combinations thereof.

[0016] The present invention can be modified in various ways and can have various forms, and will be described in detail below by taking specific examples as examples. However, this is not intended to limit the present invention to the specific disclosed form, and it should be understood that the present invention includes all modifications, equivalents, and alternatives within the spirit and technical scope of the present invention.

[0017] 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 present invention.

[0018] Prior to this, the terminology used in this specification is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. Furthermore, the singular forms used herein include the plural forms unless the language clearly indicates otherwise.

[0019] 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; ultrafinely grinding the polymer at a rotation speed of 500 rpm to 4,000 rpm (step 2); neutralizing at least a portion of the acidic groups of the polymer (Step 3); drying the micronized and neutralized polymer to produce dry superabsorbent polymer particles (Step 4); and The dried superabsorbent resin particles are pulverized to produce superabsorbent resin particles (step 5). A method for making a superabsorbent polymer is provided.

[0020] The term "polymer" or "polymeric polymer" as used in the present specification means a polymerized state of water-soluble ethylenically unsaturated monomers, and can include any range of moisture content or particle size.

[0021] In addition, the term "super absorbent polymer" refers to a crosslinked polymer or a base resin in the form of a powder consisting of super absorbent polymer particles obtained by pulverizing the crosslinked polymer depending on the context, or is used to encompass all of the crosslinked polymer or base resin that have been subjected to additional processes such as drying, pulverization, classification, surface crosslinking, etc. to be made into a state suitable for commercialization.

[0022] The term "fine powder" refers to particles having a particle size of less than 150 μm in the superabsorbent resin particles. The particle size of such resin particles can be measured by the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3 method.

[0023] The term "chopping" refers to cutting the hydrogel polymer into small pieces on the order of millimeters to increase drying efficiency, and is used to distinguish from grinding to micrometer or normal particle levels.

[0024] Moreover, the term "micronizing" or "micronization" refers to pulverizing a hydrogel polymer to particles having a particle size of several tens to several hundreds of micrometers, and is used to distinguish it from "chopping."

[0025] The hydrogel polymer obtained by the polymerization reaction of acrylic acid monomers is commercially available as a powdered product, a superabsorbent resin, through processes such as drying, pulverization, classification, surface crosslinking, etc. Recently, attempts have been made to provide a superabsorbent resin that exhibits an improved water absorption rate.

[0026] 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 method is generally adopted in which a foaming agent is added to the monomer composition and cross-linking polymerization is carried out to form a porous structure in the base resin powder.

[0027] However, the use of a foaming agent has the disadvantages of decreasing 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 development of a technology that can increase the water absorption rate of the superabsorbent resin without using a foaming agent.

[0028] Meanwhile, conventional superabsorbent resins are manufactured by 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, drying the hydrogel polymer thus formed, and then pulverizing it to a desired particle size. In this case, a chopping process is usually performed before the drying process to cut the hydrogel polymer into particles of several millimeters in size in order to facilitate drying of the hydrogel polymer and to increase the efficiency of the pulverization process. However, in this chopping process, due to the stickiness of the hydrogel polymer, the hydrogel polymer is not pulverized to the micro-sized particle level, but is in an aggregated gel form. When the hydrogel polymer in the aggregated gel form is dried, a plate-shaped dried body is formed, and in order to pulverize it to the micro-sized particle level, a multi-stage pulverization process that serves to reduce the stickiness of the polymer must be performed, which causes a problem of generating a lot of fine powder during the process.

[0029] Specifically, conventional superabsorbent resins have been produced through the following steps:

[0030] (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;

[0031] As described above, the chopped hydrogel polymer has an aggregated gel shape of about 1 cm to 10 cm in size, and the chopped hydrogel polymer is stacked on a belt with a perforated plate at the bottom and dried by hot air supplied from the bottom or top. Since the polymer dried by the drying method is in a plate shape rather than a particle shape, the classification step after crushing has been performed by classifying the coarsely crushed particles so that the particles produced are normal particles, that is, particles having a particle size of 150 μm to 850 μm, and then classifying them again after fine crushing. Since the amount of fine powder separated in the final classification step in this manufacturing method is large, about 20 wt % to about 30 wt % of the total weight of the finally manufactured superabsorbent resin, the separated fine powder is mixed with an appropriate amount of water, regranulated, and then reused by feeding it to the chopping step or the step before drying.

[0032] However, when the regranulated fine powder mixed with water for reuse is reintroduced into the grinding 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 resin.

[0033] Therefore, the present inventors have realized that the amount of fine powder generated in the conventional production method is greatly affected by the pulverization process, and have realized that the amount of fine powder generated during the production process can be significantly reduced by adjusting the conditions of the polymer pulverization process.

[0034] On the other hand, 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 method is generally adopted in which a foaming agent is added to the monomer composition and cross-linking polymerization is carried out to form a porous structure in the base resin powder.

[0035] However, the use of a foaming agent reduces various physical properties of the superabsorbent resin, such as surface tension, liquid permeability, and volume density, and the pores formed by the foaming agent sharpen the shape of the particle surface, resulting in an increase in the amount of fine powder generated during processing. Therefore, there is a continuing demand for development of a technology that can improve the water absorption rate of the superabsorbent resin without using a foaming agent.

[0036] As a result of extensive research to solve this problem, it was found that, unlike conventional methods for producing superabsorbent resins, polymerization is first performed in a state where the acidic groups of the water-soluble ethylenically unsaturated monomer are neutralized, but the acidic groups are not neutralized to form a polymer, and then the hydrogel polymer is ultrafinely pulverized by applying a high-intensity mechanical shear force, and the acidic groups of the polymer are neutralized, or the acidic groups of the polymer are neutralized to form a hydrogel polymer, and the hydrogel polymer is ultrafinely pulverized, or the acidic groups present in the polymer are neutralized simultaneously with ultrafine pulverization to form aggregated hydrogel particles having micropores. The hydrogel polymer produced through this process is produced in the form of particles having stable micropores of 100 μm or less, and the subsequent grinding and drying processes are performed under milder conditions, thereby significantly reducing the amount of fine powder generated during the process. It was also found that the ultrafine pulverization process using the high-intensity mechanical shear force can form micropores in the hydrogel polymer without the need for a separate foaming agent in the polymerization step, thereby significantly improving the water absorption rate.

[0037] Meanwhile, the hydrogel ultrafine grinding step may be preferably carried out in the presence of a surfactant. By using a surfactant in the ultrafine grinding step, particle aggregation can be effectively controlled, thereby reducing the load on the equipment and further improving productivity.

[0038] In addition, 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, a polymer with longer chains can be formed, and the content of water-soluble components present in an uncrosslinked state due to incomplete crosslinking can be reduced.

[0039] The water-soluble components have the property of being easily dissolved when the superabsorbent resin comes into contact with a liquid, so if the content of the water-soluble components is high, most of the dissolved water-soluble components remain on the surface of the superabsorbent resin, making the superabsorbent resin sticky and reducing liquid permeability. Therefore, in terms of liquid permeability, it is important to maintain the content of the water-soluble components low.

[0040] According to one embodiment of the present invention, the content of water-soluble components is reduced by carrying out polymerization in a non-neutralized state, and therefore the liquid permeability of the highly water-absorbent resin can be improved.

[0041] In addition, the superabsorbent polymer prepared according to one embodiment of the present invention may have a uniform particle size distribution, and therefore, a superabsorbent polymer having excellent general water absorption properties such as water retention capacity and pressure absorption capacity, rewet properties, and water absorption speed may be provided.

[0042] Hereinafter, each step of the method for producing a highly water-absorbent resin according to one embodiment will be described in detail.

[0043] Manufacturing method of superabsorbent resin Phase 1: Polymerization Phase 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.

[0044] The steps 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.

[0045] The water-soluble ethylenically unsaturated monomer may be any monomer commonly used in the manufacture 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:

[0046] [Chemical formula 1] R-COOM'

[0047] 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.

[0048] 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.

[0049] 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 or 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, (N,N)-dimethylaminopropyl (meth)acrylamide, etc.

[0050] Here, the water-soluble ethylenically unsaturated monomer has an acidic group. As described above, in the conventional production of a superabsorbent resin, a monomer in which at least a part of the acidic group is neutralized with a neutralizing agent is cross-linked to form a polymer. Specifically, at least a part of the acidic group of the water-soluble ethylenically unsaturated monomer is 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.

[0051] However, according to one embodiment of the present invention, the water-soluble ethylenically unsaturated monomer is polymerized in a state where the acid group is not neutralized to form a polymer.

[0052] Water-soluble ethylenically unsaturated monomers (e.g., acrylic acid) in which the acid group is not neutralized are in a liquid state at room temperature and have high miscibility with the solvent (water) and exist in the form of a mixed solution in the monomer composition. However, water-soluble ethylenically unsaturated monomers in which the acid group is neutralized are in a solid state at room temperature and have different solubility depending on the temperature of the solvent (water), with the solubility decreasing as the temperature decreases.

[0053] As described above, the water-soluble ethylenically unsaturated monomer in which the acid group is not neutralized has a higher solubility or miscibility in a solvent (water) than a monomer in which the acid group is neutralized, and is not precipitated even at a low temperature, so it is advantageous for long-term polymerization at low temperatures. Therefore, by carrying out a long-term polymerization using the water-soluble ethylenically unsaturated monomer in which the acid group is not neutralized, a polymer having a higher molecular weight and a uniform molecular weight distribution can be stably formed.

[0054] In addition, since it is possible to form a polymer with a longer chain, it is possible to achieve the effect of reducing the content of water-soluble components that exist in an incompletely polymerized or incompletely crosslinked state.

[0055] In addition, when polymerization is first performed in a state where the acidic groups of the monomers are not neutralized to form a polymer, and the polymer is then micronized in the presence of a surfactant after neutralization, or the polymer is micronized in the presence of a surfactant and then neutralized, or the acidic groups present in the polymer are neutralized simultaneously with micronization, the surfactant can be present in a large amount on the surface of the polymer and can fully play a role in reducing the adhesion of the polymer.

[0056] 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 can be about 20 to about 60% by weight, or about 20 to about 40% by weight.

[0057] 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 a superabsorbent resin particle described later, and plays a role in forming a polymer including a crosslinked structure by introducing a crosslinking bond between the unsaturated bonds of the water-soluble ethylenically unsaturated monomer described above.

[0058] The crosslinking in this step is performed regardless of whether it is on the surface or inside. However, when the surface crosslinking process of the superabsorbent resin particles described below is performed, 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.

[0059] 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.

[0060] 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, and pentaerythritol. Examples of the acrylates include 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. These may be used alone or in combination of two or more.

[0061] 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 may be used alone or in combination of two or more.

[0062] 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.

[0063] 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 bonds of the water-soluble ethylenically unsaturated monomer or with the unsaturated bonds of other internal crosslinking agents, and unlike acrylate compounds containing an ester bond (-(C=O)O-) in the molecule, the crosslinked bonds can be more stably maintained even during the neutralization process after the above-mentioned polymerization reaction.

[0064] 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 matter.

[0065] The crosslinking polymerization of the water-soluble ethylenically unsaturated monomer in the presence of such an internal crosslinking agent can be carried out in the presence of a polymerization initiator, and optionally a thickener, a plasticizer, a storage stabilizer, an antioxidant, etc.

[0066] In the monomer composition, the internal crosslinking agent can be used in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the internal crosslinking agent can be used in an amount of 0.01 parts by weight or more, or 0.05 parts by weight or more, or 0.1 parts 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 based on 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 a strength above an appropriate level, and if the content of the internal crosslinking agent is too high, the internal crosslinking density may increase, making it difficult to achieve the desired water retention ability.

[0067] 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 a three-dimensional network structure, the water retention capacity and compression absorption capacity, which are general 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.

[0068] According to one embodiment of the present invention, the step of polymerizing the monomer composition to form a polymer may be carried out in a batch type reactor for one hour or more.

[0069] Polymerization methods in the production methods of conventional superabsorbent resins are largely divided into thermal polymerization and photopolymerization depending on the polymerization energy source. Thermal polymerization is generally carried out in a reactor having a stirring shaft such as a kneader, and photopolymerization is generally carried out in a vessel with a flat bottom.

[0070] Meanwhile, when the polymerization is performed in a continuous polymerization, for example, in a reactor having a reactor agitator equipped with a conveyor belt, the polymerization is performed in a continuous manner by feeding new monomer composition to the reactor while moving the polymerization product, so that polymers having different polymerization rates are mixed. Therefore, it is difficult to perform uniform polymerization throughout the monomer composition, and the overall physical properties may deteriorate.

[0071] However, according to one embodiment of the present invention, by performing the polymerization in a stationary manner using a batch reactor, there is little risk of polymers with different polymerization rates being mixed, and therefore a polymer with uniform quality can be obtained.

[0072] 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 than when the polymerization is carried out in a continuous reactor equipped with a conveyor belt, for example, 1 hour or more, 3 hours or more, or 6 hours or more. Preferably, the polymerization reaction is carried out for 1 hour to 24 hours, 1 hour to 18 hours, 1 hour to 12 hours, or 1 hour to 8 hours. Despite such a long polymerization reaction time, since the polymerization is carried out on a water-soluble ethylenically unsaturated monomer in an unneutralized state, the monomer is not precipitated much even if the polymerization is carried out for a long time, and therefore it is advantageous for carrying out the polymerization for a long time.

[0073] Meanwhile, the polymerization in the batch reactor of the present invention utilizes a thermal polymerization method, so that the polymerization initiator is a thermal polymerization initiator.

[0074] The thermal polymerization initiator may be at least one selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid. Specifically, examples of the persulfate initiator include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), and ammonium persulfate ((NH4)2S2O8). Examples of the azo initiator include 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, and the like. dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), etc. More various thermal polymerization initiators are disclosed in Principle of Polymerization by Odian (Wiley, 1981), page 203, and are not limited to the above-mentioned examples.

[0075] 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. In other words, if the concentration of the polymerization initiator is too low, the polymerization rate may be slowed down, and a large amount of residual monomer may be extracted in the final product, which is not preferable. On the other hand, if the concentration of the polymerization initiator is higher than the above range, the polymer chains forming the network become shorter, the content of water-soluble components increases, and the physical properties of the resin, such as the pressure absorption capacity, may decrease, which is not preferable.

[0076] Meanwhile, in one embodiment of the present invention, the polymerization can be initiated by adding the initiator and a reducing agent which forms a redox couple together.

[0077] Specifically, the initiator and the reducing agent react with each other to form radicals when added to the polymer solution.

[0078] 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 a small amount of initiator and reducing agent added. Since there is no need to increase the process temperature, low-temperature polymerization is possible, and changes in the physical properties of the polymer solution can be minimized.

[0079] The polymerization reaction using the oxidation-reduction reaction can be smoothly carried out at or below room temperature (25° C.) For example, the polymerization reaction can be carried out at a temperature of 5° C. to 25° C., or 5° C. to 20° C.

[0080] In one embodiment of the present invention, 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-sulfinoacteate.

[0081] For example, potassium persulfate can be used as the initiator and disodium 2-hydroxy-2-sulfinoacetate can be used as the reducing agent; ammonium persulfate can be used as the initiator and tetramethylethylenediamine can be used as the reducing agent; or sodium persulfate can be used as the initiator and sodium formaldehyde sulfoxylate can be used as the reducing agent.

[0082] 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 (Na2SO3), sodium metabisulfite (Na2S2O5); tetramethylethylenediamine (TMEDA); a mixture of iron (II) sulfate and EDTA (FeSO4 / EDTA); sodium formaldehyde sulfoxylate; disodium 2-hydroxy-2-sulfinoacteate; and disodium 2-hydroxy-2-sulfoacteate.

[0083] The monomer composition may further contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant, if necessary.

[0084] The monomer composition including the monomer may be in a solution state dissolved in a solvent such as water, and the solid content in the monomer composition in such a solution state, i.e., the concentration of the monomer, the internal crosslinking agent, and the 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% by weight, or 15 to 60% by weight, or 30 to 50% by weight.

[0085] In this case, the solvent that can be used is not limited in composition as long as it can dissolve the above-mentioned components. For example, one or more selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl 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.

[0086] The polymer obtained by this method can have a high molecular weight and a uniform molecular weight distribution as described above by polymerizing an unneutralized ethylenically unsaturated monomer, and the content of water-soluble components can be reduced.

[0087] The polymer may have a water content of 30 to 80% by weight. For example, the water content of the polymer may be 30% by weight or more, 45% by weight or more, or 50% by weight or more, and 80% by weight or less, 70% by weight or less, or 60% by weight or less.

[0088] If the moisture content of the polymer is too low, it may be difficult to secure an adequate surface area in the subsequent crushing step, and therefore the polymer may not be crushed effectively. If the moisture content of the polymer is too high, it may be difficult to crush the polymer to a desired particle size due to increased pressure in the subsequent crushing step.

[0089] Meanwhile, in the present specification, the term "moisture content" refers to the amount of moisture contained in the total weight of a polymer, and is 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 the evaporation of moisture in the polymer during the process of drying the polymer in a crumb state by increasing the temperature of the polymer by infrared heating. In this case, the drying conditions are a method of increasing the temperature from room temperature to about 180°C and then maintaining it at 180°C, and the total drying time is set to 40 minutes, including 5 minutes for the temperature increase step, and the moisture content is measured.

[0090] Stage 2: Ultrafine grinding stage Next, the step is a step of ultrafinely pulverizing the polymer at a rotation speed of 500 rpm to 4,000 rpm, which is a step in which the polymer is not chopped into millimeter size but is simultaneously chopped into tens to hundreds of micrometer size and aggregated. In this step, since a high mechanical shear force is applied, micropores of 100 μm or less are easily formed in the polymer, and therefore the surface roughness increases, and the total surface area of ​​the polymer is significantly increased due to the pores formed inside and outside the polymer particles. The micropores are formed in a more stable form than the pores formed using a foaming agent in the polymerization step, so the degree of fine powder generation due to the pores in the subsequent process can be significantly reduced. The highly water-absorbent resin particles produced in this step can have a significantly increased surface area and a significantly improved water absorption rate.

[0091] FIG. 2 shows an SEM image of the final superabsorbent polymer produced through the ultrafine grinding process in step 2 in the method for producing superabsorbent polymer according to one embodiment of the present invention, and it can be seen that micropores are uniformly formed inside and outside the polymer.

[0092] The ultrafine pulverization process is carried out at a rotation speed of 500 rpm to 4,000 rpm. If the rotation speed of the process is less than 500 rpm, it is difficult to form sufficient pores to the desired extent, making it difficult to expect a high water absorption rate and ensuring a desired level of productivity. If the rotation speed exceeds 4,000 rpm, the polymer chains may be damaged by excessive shear force, which may increase the water-soluble components and slightly decrease the various physical properties of the produced superabsorbent resin. Preferably, the ultrafine pulverization process can be carried out at a rotation speed of 1,500 rpm to 3,500 rpm or 2,000 rpm to 3,000 rpm. Within this range, it is easy to form the desired micropores without the above-mentioned problems.

[0093] According to one embodiment of the present invention, the ultrafine grinding step is carried out by applying high-intensity mechanical shear forces, and therefore, the step is carried out in a high-shear grinding device.

[0094] The high shear grinding device may include a body portion including a transfer space into which a polymer is transferred; a screw member rotatably installed within the transfer space to move the polymer; a drive motor for providing a rotational driving force to the screw member; a cutter member installed in the body portion to grind the polymer; and a porous plate having a number of holes formed therein and discharging the polymer ground by the cutter member to the outside of the body portion.

[0095] At this time, the rotation speed of the driving motor of the high shear grinding device is the same as that of the above-mentioned step 4.

[0096] The hole size of the perforated plate of the high shear grinding device may be 1 mm to 25 mm, or 5 mm to 20 mm, or 5 mm to 15 mm.

[0097] In this way, when the polymer is ultrafinely pulverized using a high shear pulverizer, a smaller particle size distribution is achieved, and the subsequent drying and pulverization processes can be carried out under milder conditions, preventing the generation of fine powder. At the same time, appropriate micropores are formed on the surface of the polymer, thereby increasing the surface area and significantly improving the water absorption rate.

[0098] According to one embodiment of the present invention, the step of ultrafinely pulverizing the polymer under specific conditions can be carried out one or more times, preferably 1 to 6 times, 1 to 4 times, or 1 to 3 times, which can be carried out using multiple pulverizers, a single pulverizer comprising multiple perforated plates and / or multiple cutter members, or multiple pulverizers, some of which comprise multiple perforated plates and / or multiple cutter members.

[0099] According to one embodiment of the present invention, a surfactant may be added in the ultrafine grinding step, and therefore, the aggregation between polymer particles may be effectively controlled, thereby reducing the load on the equipment used in the grinding process and further improving productivity.

[0100] Preferably, the surfactant is a compound represented by the following Chemical Formula 2 or a salt thereof, but the present invention is not limited thereto.

[0101] [ka]

[0102] In the above Chemical Formula 2, A1, A2 and A3 each independently represent a single bond, a carbonyl,

[0103] [ka]

[0104] where one or more of these is carbonyl or

[0105] [ka]

[0106] wherein m1, m2, and m3 each independently represent an integer from 1 to 8;

[0107] [ka]

[0108] are each linked to an adjacent oxygen atom,

[0109] [ka]

[0110] are connected to adjacent R1, R2 and R3, respectively; R1, R2, and R3 are each independently hydrogen, linear or branched alkyl having 6 to 18 carbon atoms, or linear or branched alkenyl having 6 to 18 carbon atoms; n is an integer from 1 to 9.

[0111] The surfactant is added so that when mixed with the polymer the micronizing step can be easily carried out without agglomeration phenomena.

[0112] The surfactant represented by the formula 2 is a nonionic surfactant, and has excellent surface adsorption performance due to hydrogen bonding force even with unneutralized polymers, and is therefore suitable for achieving the desired aggregation control effect. On the other hand, in the case of an anionic surfactant, not a nonionic surfactant, when it is mixed with a polymer neutralized with a neutralizing agent such as NaOH or Na2SO4, the ionized Na in the carboxyl group substituent of the polymer is released. + When the compound is adsorbed via ions and mixed with an unneutralized polymer, there is a problem that the adsorption efficiency to the polymer is relatively reduced due to competition with the anions of the carboxyl group substituents of the polymer.

[0113] Specifically, in the surfactant represented by the above 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 is hydrogen, n=1 to 3), the glycerol-derived portion and the terminal hydroxyl group are hydrophilic functional groups that play a role in improving the adsorption performance to the polymer surface, and therefore, the aggregation of the superabsorbent resin particles can be effectively suppressed.

[0114] In Formula 2, the hydrophobic functional groups R1, R2, and R3 (when not hydrogen) are each independently a linear or branched alkyl having 6 to 18 carbon atoms or a linear or branched alkenyl having 6 to 18 carbon atoms. In this case, if the R1, R2, and R3 (when not hydrogen) are alkyl or alkenyl having less than 6 carbon atoms, there is a problem that the chain length is short and aggregation control of the milled particles is not effectively performed, and if the R1, R2, and R3 (when not hydrogen) are alkyl or alkenyl having more than 18 carbon atoms, there is 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 may be high due to the increase in the cost of the surfactant.

[0115] Preferably, R1, R2, and R3 are hydrogen, or when they are linear or branched alkyl having 6 to 18 carbon atoms, they may be 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 when they are linear or branched alkenyl having 6 to 18 carbon atoms, they may be 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.

[0116] The surfactant can be selected from compounds represented by the following Chemical Formula 2-1 to Chemical Formula 2-14.

[0117] [ka] [ka]

[0118] On the other hand, the amount of the surfactant used is not particularly limited, but may be 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, depending on the productivity or the load condition of the equipment. If the surfactant is used too much, the surface tension value may be lowered, and various physical properties of the final superabsorbent resin may be deteriorated.

[0119] 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 selected 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.

[0120] Among these, for example, the surfactant may be mixed in a state of solution dissolved in a solvent. In this case, any type of solvent may be used without limitation, whether it is an inorganic solvent or an organic solvent, but water is the most suitable in consideration of the ease of the drying process and the cost of the solvent recovery system. In addition, the solution may be mixed by putting the surfactant and the polymer in a reaction vessel, putting the polymer in a mixer and injecting the solution, or continuously supplying the polymer and the solution to a continuously operated mixer and mixing them.

[0121] Phase 3: Neutralization Phase According to one embodiment of the invention, a step (step 3) of neutralizing at least a portion of the acidic groups of the polymer is carried out, and the above-mentioned micronizing step (step 2) and the neutralization step (step 3) can be carried out sequentially, alternately, or simultaneously.

[0122] That is, a neutralizer is added to the polymer to neutralize the acidic groups first, and then a surfactant is added to the neutralized polymer and the polymer mixed with the surfactant is ultrafinely ground (performed in the order of step 3->step 2), or a neutralizer and a surfactant are added to the polymer at the same time to neutralize and ultrafinely ground the polymer (performed in the order of steps 2 and 3 at the same time). Alternatively, the surfactant can be added first and the neutralizer can be added later (performed in the order of steps 2->step 3). Alternatively, the neutralizer and the surfactant can be added alternately. Alternatively, the surfactant can be added first and ultrafinely ground, and then the neutralizer can be added to neutralize the neutralized hydrogel polymer, and an additional surfactant can be added to perform an ultrafinely ground process.

[0123] Here, when the neutralization step is carried out independently from the ultrafine grinding step of step 2, it can be carried out in a manner in which additives are added while the polymer is being ground. More specifically, a screw-type extruder including a perforated plate having a number of holes formed therein can be used. The screw-type extruder is a device in which grinding is carried out under milder conditions than the high-shear grinding device used in the above-mentioned ultrafine grinding step, and the rotation speed is about 50 rpm to 500 rpm, and the holes of the perforated plate can be about 3 mm to 25 mm, but are not limited thereto.

[0124] In this case, as the neutralizing agent, a basic substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc., capable of neutralizing the acidic group can be used.

[0125] In addition, the degree of neutralization, which indicates the degree of neutralization of the acidic groups contained in the polymer by the neutralizing agent, may be 50 to 90 mol%, or 60 to 85 mol%, or 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 resin may decrease, and the concentration of carboxyl groups on the particle surface may be too low to perform surface crosslinking well in the subsequent process, resulting in decreased water absorption properties or liquid permeability under pressure. On the other hand, if the degree of neutralization is too low, not only will the water absorption capacity of the polymer decrease significantly, but it may also exhibit properties similar to elastic rubber, which are difficult to handle.

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

[0127] Stage 4: Drying stage Next, a step (Step 4) is performed in which the micronized and neutralized polymer is dried to produce dry superabsorbent polymer particles.

[0128] This step is a step of drying water from superabsorbent resin particles, which are polymers obtained by neutralizing at least a portion of the acid groups of the polymer and ultrafinely pulverizing the polymer.

[0129] In a typical method for producing a superabsorbent polymer, the drying step is performed so that the moisture content of the superabsorbent polymer becomes about 1 to 20% by weight, about 4 to 15% by weight, or about 6 to 13% by weight, but the present invention is not limited thereto.

[0130] The step 4 can be carried out by fixed-bed type drying, moving type drying, or a combination thereof.

[0131] According to one embodiment of the invention, said step 4 can be carried out by drying in place.

[0132] The stationary drying method refers to a method in which the material to be dried is suspended at the bottom of a perforated iron plate through which air can pass, and hot air passes from bottom to top to dry the material.

[0133] In stationary drying, since the particles are dried in a plate shape without any flow of particles, it is difficult to perform uniform drying with a simple hot air flow. Therefore, in stationary drying, delicate adjustment of the hot air and temperature is required to obtain a dried product with a uniform high moisture content. In the present invention, the hot air is changed from below to above to prevent warping of the plate-shaped dried product during drying and leakage of hot air. In addition, the drying temperature is adjusted by section so that the upper, middle and lower layers inside the dried product can be dried uniformly with a moisture content deviation of less than 5%.

[0134] The device capable of drying by the stationary drying method may be a belt dryer, but is not limited thereto.

[0135] In the case of the stationary drying step, the drying process may be performed at a temperature of about 80° C. to 200° C., and preferably 90° C. to 200° C., 100° C. to 200° C., or 100° C. to 180° C. If the drying temperature is less than 80° C., the drying time may be excessively long, and if the drying temperature is excessively high, exceeding 200° C., a superabsorbent resin having a lower moisture content than the desired moisture content may be obtained. Meanwhile, the drying temperature may refer to the temperature of the hot air used or the internal temperature of the equipment during the drying process.

[0136] According to one embodiment of the invention, step 4 can be carried out by fluidized drying.

[0137] The fluidized drying method refers to a method of drying by mechanically stirring the material during drying. At this time, the direction in which the hot air passes through 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.

[0138] As an apparatus capable of drying by such a fluidized drying method, a horizontal-type mixer, a rotary kiln, a paddle dryer, a steam tube dryer, or a commonly used fluidized dryer can be used.

[0139] In the case of the fluidized drying step, the drying process may be performed at a temperature of about 100° C. to 300° C., and preferably 120° C. to 280° C. or 150° C. to 250° C. If the drying temperature is too low, less than 100° C., the drying time may be too long, and if the drying temperature is too high, more than 300° C., the polymer chains of the superabsorbent resin may be damaged, resulting in a decrease in various physical properties, and the superabsorbent resin may have a moisture content lower than the desired moisture content.

[0140] Phase 5: Crushing Phase Next, the dried superabsorbent resin particles are pulverized to produce superabsorbent resin particles.

[0141] Specifically, the pulverization step can be performed by pulverizing the dried superabsorbent resin particles to have a particle size on the normal particle level, that is, a particle size of 150 μm to 850 μm.

[0142] The crusher 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-mentioned examples.

[0143] Alternatively, the grinding machine may be a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, or the like, but is not limited to the above examples.

[0144] Meanwhile, in the manufacturing method of the present invention, it is possible to realize superabsorbent resin particles with a smaller particle size distribution in the ultrafine pulverization stage than in the conventional chopping stage, and the moisture content after drying is maintained relatively high. Therefore, even if pulverization is performed under mild conditions with less pulverization 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 fine powder generation ratio can be greatly reduced.

[0145] The superabsorbent resin particles prepared as described above may contain superabsorbent resin particles having a particle size of 150 μm to 850 μm, i.e., normal particles, at a ratio of 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 based on the total weight. The particle size of such resin particles may be measured according to the EDANA WSP 220.3 method of the European Disposables and Nonwovens Association (EDANA) standard.

[0146] In addition, 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 the case where a superabsorbent resin is produced by a conventional production method, which contains more than about 20% by weight to about 30% by weight of fine powder.

[0147] Additive addition stage Meanwhile, according to one embodiment of the present invention, the method may further include a step of adding an additive to the ultrafinely ground and neutralized polymer before the drying step (step 4).

[0148] The additive introduction step is a step for improving physical properties by using additional additives within a range that does not impair the intended effect, and the type of the additive is not particularly limited, and examples thereof include a polymerization initiator for removing residual monomers, a liquid permeability improver for improving water absorption properties, a fine powder for recirculating generated fine powder, an anti-caking agent, a flowability improver, an antioxidant, a neutralizing agent, a surfactant, and the like, but are not limited thereto.

[0149] The additive introduction step can be carried out simultaneously with step 2, simultaneously with step 3, after step 2 and step 3, or at least one of these steps. The additive introduction step can be carried out multiple times as necessary, and can be carried out once or more in each step.

[0150] When the additive addition step is carried out independently from steps 2 and 3, i.e., after steps 2 and 3 and before step 4, it can be carried out in a manner in which the additives are added simultaneously while the polymer is being ground.

[0151] The grinding step may be the same as the grinding step in step 5 described above, and the additive may be added once or multiple times during the grinding step to be mixed with the polymer.

[0152] Classification stage Next, after the step of pulverizing the superabsorbent resin particles (step 5), the method may further include a step of classifying the pulverized superabsorbent resin particles according to particle size.

[0153] Surface cross-linking stage In addition, the method may further include a step of forming a surface cross-linked layer on at least a part of the surface of the superabsorbent resin particles in the presence of a surface cross-linking agent after crushing (step 5) and / or classifying the superabsorbent resin particles. Through this step, the cross-linked polymer contained in the superabsorbent resin particles may be additionally cross-linked through the medium of the surface cross-linking agent, so that a surface cross-linked layer may be formed on at least a part of the surface of the superabsorbent resin particles.

[0154] The surface cross-linking agent may be any surface cross-linking agent that has been used in the manufacture of superabsorbent resins without any particular limitation. For example, the surface cross-linking agent may 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-based 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.

[0155] 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.

[0156] 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, or 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-mentioned range, a superabsorbent resin exhibiting excellent general water absorption properties can be produced.

[0157] In addition, the step of forming the surface cross-linked layer can be performed by adding an inorganic substance to the surface cross-linking agent, that is, the step of forming the surface cross-linked layer can be performed by additionally cross-linking the surface of the superabsorbent resin particles in the presence of the surface cross-linking agent and the inorganic substance.

[0158] 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, and may be used in the form of 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 highly water-absorbent resin particles.

[0159] In addition, there is no limitation on the constitution of the method for mixing the surface cross-linking agent with the superabsorbent resin composition. For example, a method of mixing the surface cross-linking agent and the superabsorbent resin composition in a reaction tank, a method of spraying the surface cross-linking agent to the superabsorbent resin composition, a method of continuously supplying the superabsorbent resin composition and the surface cross-linking agent to a continuously operated mixer, and the like can be used.

[0160] When the surface cross-linking agent and the superabsorbent polymer composition are mixed, water and methanol may be added. When water and methanol are added, there is an advantage that the surface cross-linking agent can be uniformly dispersed in the superabsorbent polymer composition. At this time, the content of the added water and methanol can be appropriately adjusted to induce uniform dispersion of the surface cross-linking agent and prevent the superabsorbent polymer composition from clumping, while optimizing the surface penetration depth of the cross-linking agent.

[0161] The surface cross-linking step can be carried out at a temperature of about 80° C. to about 250° C. More specifically, the surface cross-linking step 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-mentioned surface cross-linking step conditions are satisfied, the surface of the highly water-absorbent resin particles can be sufficiently cross-linked to increase the pressure absorption capacity.

[0162] 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. At this time, the type of heat medium that can be used can be a heated fluid such as steam, hot air, or hot oil, but is not limited thereto, and the temperature of the heat medium to be supplied can be appropriately selected in consideration of the means of the heat medium, the heating rate, and the target temperature of the heat raising. On the other hand, examples of the heat source to be directly supplied include heating by electricity and heating by gas, but are not limited to the above-mentioned examples.

[0163] Post-processing stage According to one embodiment of the present invention, after the step of forming a surface cross-linked layer on at least a part of the surface of the superabsorbent resin particles, the method may further include at least one 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. At this time, the cooling step, the hydration step, and the post-treatment step may be performed sequentially or simultaneously.

[0164] In the hydration step, water or salt water can be used, thereby controlling the amount of residues, etc. The amount of water used can be appropriately adjusted in consideration of the moisture content of the final product, etc., and is preferably 0.1 to 10% by weight, 0.5 to 8% by weight, or 1 to 5% by weight relative to the water absorbent resin, but is not limited thereto.

[0165] After the water addition step, a maturation step may be further carried out.

[0166] When salt water is used in the hydration step, the solution absorption rate is relatively low due to the conductivity of the salt water, and the salt water is uniformly dispersed in the aging step, enabling uniform water absorption by the water absorbent resin. The aging step can be performed by a commonly used method without any particular limitation, for example, at 100°C or less, 80°C or less, preferably 50°C or less, using a rotary stirrer, for 10 minutes to 1 hour.

[0167] The additives added in the post-treatment step may be surfactants, inorganic salts, liquid permeability improvers, anti-caking agents, flow improvers, and antioxidants, but the present invention is not limited thereto.

[0168] By selectively carrying out the cooling step, the hydration step, and the post-treatment step, it is possible to control the generation of residues and improve the moisture content of the final superabsorbent polymer, thereby producing a higher quality superabsorbent polymer product.

[0169] Super water absorbent resin The superabsorbent polymer produced by the above-mentioned production method has a high water absorption rate and a low fine powder content, and its general water absorption properties, such as water retention capacity (CRC) and absorbent capacity under pressure (AUP), can be at the same level or higher than those of superabsorbent polymers produced by conventional methods.

[0170] In addition, the particle size distribution can be narrowed to provide a uniform particle size distribution, and the water-soluble component (EC) content can be reduced to provide a superabsorbent resin with excellent liquid permeability and rewet properties.

[0171] The superabsorbent polymer according to one embodiment may have a vortex time of 30 seconds or less, or 28 seconds or less, or 27 seconds or less, or 26 seconds or less, or 25 seconds or less, or 24 seconds or less, or 23 seconds or less, or 22 seconds or less, or 21 seconds or less, or 20 seconds or less, or 19 seconds or less, or 18 seconds or less, as measured by a vortex method. The smaller the value of the vortex time, the better the vortex time. The lower limit of the vortex time is theoretically 0 seconds, but may be, for example, about 5 seconds or more, about 10 seconds or more, or about 12 seconds or more.

[0172] The functions and effects of the present invention will be described in more detail below with reference to specific examples of the present invention. However, these examples are presented only as examples of the present invention and do not define the scope of the invention.

[0173] <Example> Example 1 (Step 1: Polymer production step) In a 5L glass vessel equipped with a stirrer and a thermometer, 1404g of acrylic acid, 4.5g of pentaerythritol triallyl ether (PETTAE) as an internal crosslinking agent, and 3387g of water were stirred and mixed, and the temperature was maintained at 5℃ to react. Nitrogen was introduced into the glass vessel containing the mixture at 1000cc / min for 1 hour to replace the nitrogen condition. Next, 0.2g of 30wt% hydrogen peroxide solution, 20g of 1.6% ascorbic acid solution, and 44g of 1% 2,2'-azobis-(2-amidinopropane) dihydrochloric acid solution were added as polymerization initiators, and 10g of 0.01% iron sulfate solution was added at the same time to start polymerization. After the temperature of the mixture reached 85℃, the mixture was polymerized at 90±2℃ for about 6 hours to obtain a polymer.

[0174] (Steps 2 and 3: Ultrafine grinding, neutralization and addition of additional additives) Glycerol monolaurate (GML) was dissolved in water at 60°C to a ratio of 0.01 g per 1,000 g of the hydrogel polymer and added in the form of an aqueous solution to 1,000 g of the polymer obtained in step 1. Then, the polymer was extruded through a perforated plate with a plurality of 10 mm holes at a rotation speed of 2,500 rpm using a high-speed rotating fine cutter (F-150 / Karl Schnell) installed inside a cylindrical grinder to perform an ultrafine grinding process.

[0175] The recovered hydrogel polymer was then extruded three times through a perforated plate with multiple 10 mm holes at a rotation speed of 250 rpm using a screw extruder installed inside a cylindrical grinder to carry out an additional grinding process. 340 g of 32% NaOH aqueous solution was added to each stage of the screw extruder (stage 3: neutralization stage) to neutralize some of the acidic groups of the polymer, and then 100 g of fine powder and 35.5 g of 5% Na2SO4 aqueous solution were added as additional additives to produce hydrous superabsorbent resin particles (= ultrafinely ground and neutralized polymer).

[0176] (Step 4: Drying step) 1,000 g of the water-containing superabsorbent polymer particles were placed in a ventilated belt dryer containing a perforated plate capable of shifting the airflow up and down. Hot air at 200°C and 100°C was blown from top to bottom for 5 minutes and 10 minutes, respectively, so that the water content of the dried superabsorbent polymer was about 10%, and then hot air at 100°C was blown from bottom to top for 15 minutes to uniformly dry the polymer, producing a dried superabsorbent polymer.

[0177] (Step 5: Crushing and classification process) The dried superabsorbent resin was pulverized in a pulverizer (GRAN-U-LIZER™, MPE) and then classified using a standard sieve according to ASTM standards to obtain a superabsorbent resin powder having a size of 150 to 850 μm.

[0178] (Surface crosslinking process) Next, 6 g of a surface cross-linking agent solution containing 0.08 g of ethylene glycol diglycidyl ether (EJ-1030J) and 0.3 g of propylene glycol per 100 g of the superabsorbent resin was sprayed and stirred at room temperature to uniformly distribute the surface cross-linking liquid on the superabsorbent resin powder. Next, the superabsorbent resin powder mixed with the surface cross-linking liquid was placed in a surface cross-linking reactor to carry out a surface cross-linking reaction. In the surface cross-linking reactor, the superabsorbent resin powder was subjected to a surface cross-linking reaction at about 140° C. for 40 minutes to obtain a surface cross-linked superabsorbent resin.

[0179] After the surface cross-linking step, the resin was classified using a standard sieve according to ASTM standards to produce a highly water-absorbent resin having a particle size of 150 μm to 850 μm.

[0180] Example 2 In the drying step of step 4 of Example 1, 1,000 g of water-containing superabsorbent resin particles was put into a rotary kiln dryer, which is a fluidized dryer, and the internal temperature of the dryer was maintained at 210°C, and drying was performed for 35 minutes. A superabsorbent resin was produced in the same manner as in Example 1, except that.

[0181] Example 3 A superabsorbent resin was prepared in the same manner as in Example 1, except that in the polymerization step of Step 1 of Example 1, 3.5 g of pentaerythritol triallyl ether (PETTAE) and 1.0 g of polyether polyol triacrylate were used as internal crosslinking agents.

[0182] Example 4 A superabsorbent resin was prepared in the same manner as in Example 1, except that 3.5 g of pentaerythritol triallyl ether (PETTAE) and 1.0 g of ethylene glycol diglycidyl ether (EJ-1030J) were used as internal crosslinking agents in the polymerization step of Step 1 of Example 1.

[0183] Example 5 A superabsorbent resin was produced in the same manner as in Example 1, except that the step of adding additional additives was changed to adding 10 g each of a 1% aqueous sodium persulfate (Na2S2O8) solution and a 1% aqueous sodium metabisulfite (Na2S2O5) solution after the neutralization step in Step 3 of Example 1.

[0184] Example 6 A highly water-absorbent resin was produced in the same manner as in Example 1, except that in the ultrafine pulverization step of Step 2, the rotation speed of the high-speed rotary chopper was adjusted to 3,400 rpm.

[0185] Example 7 A superabsorbent resin was produced in the same manner as in Example 1, except that the surfactant GML was not used in the ultrafine pulverization step of Step 2 of Example 1.

[0186] Comparative Example 1 (Step 1': Polymer production step - pre-neutralization foaming polymerization) A monomer composition was prepared by mixing 982g of acrylic acid, 2.25g of ethylene glycol diglycidyl ether as an internal crosslinking agent, 0.25g of diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide as a photopolymerization initiator, 1.8g of capsule-type foaming agent F-36D as a foaming agent, 0.22g of sodium dodecyl sulfate as a foaming stabilizer, and 445.5g of water in a 5L glass container equipped with a stirrer and a thermometer. Next, the monomer solution was continuously supplied by a metering pump, and 1230.1g of 31 wt% aqueous sodium hydroxide solution was continuously line mixed at the same time to prepare an aqueous monomer solution. At this time, it was confirmed that the temperature of the aqueous monomer solution had risen to about 72°C or higher due to the heat of neutralization, and then the temperature was allowed to cool to 40°C. When the temperature was cooled to 40°C, 71.2g of 2 wt% aqueous sodium persulfate solution was added. The solution was poured into a Vat-shaped tray (15 cm wide x 15 cm long) installed in a square polymerization vessel with a light irradiation device attached on top and the inside preheated to 80° C., and light irradiation was started. After about 15 seconds of light irradiation, a gel was generated from the surface, and after about 30 seconds, it was confirmed that a polymerization reaction occurred simultaneously with foaming, and the reaction was continued for an additional 3 minutes to obtain a sheet-shaped hydrogel polymer.

[0187] (Step 2': Crushing step) The hydrogel polymer prepared in step 1 was cut into pieces 5 cm wide and 5 cm long, and the hydrogel was crushed using a screw-type chopper (meat chopper) equipped with a perforated plate containing a plurality of holes. At this time, the rotation speed of the screw-type chopper was 160 rpm, and the hole size of the perforated plate was 16 mm.

[0188] (Step 3': Drying Step) 1,000 g of the water-containing superabsorbent polymer particles were placed in a ventilated belt dryer including a perforated plate capable of shifting the airflow up and down. Hot air at 180° C. was blown from bottom to top for 20 minutes, and then again from top to bottom for 20 minutes, so that the water content of the dried superabsorbent polymer was about 3%, to uniformly dry the polymer and produce a dried superabsorbent polymer.

[0189] (Step 4': Crushing and classification process) The dried superabsorbent resin was pulverized in a pulverizer (GRAN-U-LIZER™, MPE) and then classified using a standard sieve according to ASTM standards to obtain a superabsorbent resin powder having a size of 150 to 850 μm.

[0190] (Step 5': Surface cross-linking process) Next, 6 g of a surface cross-linking agent solution containing 0.08 g of ethylene glycol diglycidyl ether (EJ-1030J) and 0.3 g of propylene glycol per 100 g of the superabsorbent resin was sprayed and stirred at room temperature to uniformly distribute the surface cross-linking liquid on the superabsorbent resin powder. Next, the superabsorbent resin powder mixed with the surface cross-linking liquid was placed in a surface cross-linking reactor to carry out a surface cross-linking reaction. In the surface cross-linking reactor, the superabsorbent resin powder was subjected to a surface cross-linking reaction at about 140° C. for 40 minutes to obtain a surface cross-linked superabsorbent resin.

[0191] After the surface cross-linking step, the resin was classified using a standard sieve according to ASTM standards to produce a highly water-absorbent resin having a particle size of 150 μm to 850 μm.

[0192] Comparative Example 2 A superabsorbent resin was produced in the same manner as in Example 1, except that in the ultrafine pulverization step of Step 2, the rotation speed of the high-speed rotary chopper was adjusted to 4,500 rpm.

[0193] Comparative Example 3 A highly water-absorbent resin was prepared in the same manner as in Example 1, except that in the ultrafine pulverization step of Step 2, the rotation speed of the high-speed rotary chopper was adjusted to 500 rpm.

[0194] Comparative Example 4 (Step 1': Polymer production step - pre-neutralization foaming polymerization) A monomer aqueous solution composition was prepared by mixing 100g of acrylic acid, 0.6g of polyethylene glycol diacrylate (PEGDA, Mn=523) as a crosslinking agent, 0.008g of bis(2,4,6-trimethylbenzoyl)-phenylphosphinoxide as a photoinitiator, 0.10g of sodium persulfate (SPS) as a thermal initiator, 0.30g of sodium bicarbonate (SBC) as a foaming agent, 0.03g of sodium dodecyl sulfate (SDS) as a surfactant, 123.3g of 31.5% caustic soda (NaOH), and 38.53g of water. When the aqueous monomer solution composition was cooled to a final temperature of 40°C, the aqueous monomer solution composition was placed in a tray, and then irradiated with ultraviolet light (irradiation dose: 10mW / cm2) using a UV irradiation device while maintaining a polymerization atmosphere temperature of 80°C, and UV polymerization was carried out for 1 to 3 minutes to produce a hydrogel polymer sheet.

[0195] (Step 2': Ultrafine grinding step) Glycerol monolaurate (GML) was dissolved in water at 60°C to a ratio of 0.001 g per 100 g of the hydrogel polymer obtained in step 1 and added in the form of an aqueous solution. Then, the polymer was extruded through a perforated plate with a plurality of 10 mm holes at a rotation speed of 2,500 rpm using a high-speed rotating fine cutter (F-150 / Karl Schnell) installed inside a cylindrical grinder to perform an ultrafine grinding process.

[0196] Thereafter, the recovered hydrous gel polymer was subjected to an additional pulverization process by extruding it three times through a porous plate having a plurality of 10 mm holes at a rotation speed of 250 rpm using a screw-type extruder mounted inside a cylindrical pulverizer, thereby producing hydrous superabsorbent resin particles.

[0197] (Step 3': Drying Step) 100 g of the water-containing superabsorbent polymer particles were placed in a ventilated belt dryer containing a perforated plate capable of shifting the airflow up and down. Hot air at 200°C and 100°C was blown from top to bottom for 5 and 10 minutes, respectively, so that the water content of the dried superabsorbent polymer was about 10%, and then hot air at 100°C was blown from bottom to top for 15 minutes to uniformly dry the polymer, producing a dried superabsorbent polymer.

[0198] (Step 4': Crushing and classification process) The dried superabsorbent resin was pulverized in a pulverizer (GRAN-U-LIZER™, MPE) and then classified using a standard sieve according to ASTM standards to obtain a superabsorbent resin powder having a size of 150 to 850 μm.

[0199] (Step 5': Surface cross-linking process) Next, 6 g of a surface cross-linking agent solution containing 0.08 g of ethylene glycol diglycidyl ether (EJ-1030J) and 0.3 g of propylene glycol per 100 g of the superabsorbent resin was sprayed and stirred at room temperature to uniformly distribute the surface cross-linking liquid on the superabsorbent resin powder. Next, the superabsorbent resin powder mixed with the surface cross-linking liquid was placed in a surface cross-linking reactor to carry out a surface cross-linking reaction. In the surface cross-linking reactor, the superabsorbent resin powder was subjected to a surface cross-linking reaction at about 140° C. for 40 minutes to obtain a surface cross-linked superabsorbent resin.

[0200] After the surface cross-linking step, the resin was classified using a standard sieve according to ASTM standards to produce a highly water-absorbent resin having a particle size of 150 μm to 850 μm.

[0201] Comparative Example 5 A superabsorbent resin was prepared in the same manner as in Comparative Example 4, except that in the ultrafine pulverization step of Step 2', the rotation speed of the high-speed rotary chopper was adjusted to 1,500 rpm.

[0202] <Experimental Example> The properties of the superabsorbent resins prepared in the above Examples and Comparative Examples were evaluated as follows and are shown in Table 1 below.

[0203] Unless otherwise specified, all of the following physical property evaluations were performed at constant temperature and humidity (23±1°C, relative humidity 50±10%), and saline or salt water refers to a 0.9 wt% aqueous solution of sodium chloride (NaCl).

[0204] In addition, the tap water used in the rewetting property evaluation had an electrical conductivity of 170 to 180 μS / cm when measured using Orion Star A222 (Thermo Scientific).

[0205] Moreover, unless otherwise specified, the physical property evaluation of the final surface-crosslinked superabsorbent resin was carried out on a resin having a particle size of 150 μm to 850 μm classified using an ASTM standard sieve.

[0206] (1)Centrifuge Retention Capacity (CRC) The water retention capacity of the superabsorbent resins of the Examples and Comparative Examples was measured in terms of the water absorption capacity under no load according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP241.3.

[0207] Specifically, the superabsorbent resin W0(g) (approximately 0.2g) obtained in each of the examples and comparative examples was evenly placed in a nonwoven envelope, sealed, and then immersed in physiological saline (0.9% by weight) at room temperature. After 30 minutes, the envelope was dewatered for 3 minutes at 250G using a centrifuge, and the mass W2(g) of the envelope was measured. The same procedure was also performed without using the resin, and the mass W1(g) was measured.

[0208] The obtained masses were used to calculate the CRC (g / g) according to the following formula 1.

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

[0210] (2) Absorbency under Pressure (AUP) The absorbency at a pressure of 0.3 psi of the highly absorbent resins of the above Examples and Comparative Examples was measured by EDANA method WSP242.3.

[0211] Specifically, a stainless steel iron net with 400 mesh was attached to the bottom of a plastic cylinder with an inner diameter of 25 mm. The superabsorbent resin W0(g) (0.9 g) was evenly spread on the iron net under conditions of room temperature and 50% humidity, and a piston capable of evenly applying a load of 0.3 psi on the net was slightly smaller than the outer diameter of 25 mm, had no gap with the inner wall of the cylinder, and was not hindered by its up and down movement. At this time, the weight W3(g) of the device was measured.

[0212] A glass filter with a diameter of 90 mm and a thickness of 5 mm was placed inside a petroleum dish with a diameter of 150 mm, and physiological saline composed of 0.9% by weight of sodium chloride was placed at the same level as the upper surface of the glass filter. A sheet of filter paper with a diameter of 90 mm was placed on top of the glass filter. The measuring device was placed on the filter paper, and the liquid was absorbed under a load for one hour. After one hour, the measuring device was lifted and its weight W4 (g) was measured.

[0213] The obtained masses were used to calculate the absorbency under pressure (g / g) according to the following formula 2.

[0214] [Formula 2] AUP(g / g)=[W4(g)-W3(g)] / W0(g)

[0215] The measurement was repeated five times, and the average value and standard deviation were calculated.

[0216] (3) Water absorption rate (Vortex time) The vortex time was measured in seconds according to the method described in WO 1987 / 003208.

[0217] Specifically, 2 g of superabsorbent resin was placed in 50 mL of saline solution at 23°C to 24°C, and the solution was stirred at 600 rpm with a magnetic bar (diameter 8 mm, length 30 mm), and the time until the vortex disappeared was measured in seconds.

[0218] (4) Long-term rewetting with pressurized tap water (6-hour rewet) (i) 4 g of superabsorbent resin was evenly spread on a Petri dish having a diameter of 13 cm, and distributed evenly using a spatula. 200 g of tap water was poured into the dish to allow it to swell. (ii) The superabsorbent resin that had been swollen for 6 hours was placed on 20 sheets of filter paper (Whatman, catalog No. 1004-110, pore size 20-25 μm, diameter 11 cm) with a diameter of 11 cm, and pressure was applied to the 11 cm diameter with a 5 kg weight (0.75 psi) for 1 minute. (iii) After applying pressure for 1 minute, the amount of tap water (unit: g) that had permeated the filter paper was measured.

[0219] (5) Water-soluble components (EC, Extractable Contents) After swelling 2 g of the superabsorbent resin for 1 hour using the EDANA method WSP 270.3, the water soluble component (1h EC) was measured.

[0220] (6) Fine powder content The base resin (BR) powders of the Examples and Comparative Examples were classified using standard sieves having size scales of 850 μm (#20), 600 μm (#30), 300 μm (#50), and 150 μm (#100) according to ASTM standards, and the weight of fine particles having a particle size of less than 150 μm was measured and expressed as a percentage based on the total weight of the base resin powder.

[0221] [Table 1]

[0222] As shown in Table 1, it was confirmed that the superabsorbent resin of the examples produced by the ultrafine pulverization process under specific conditions according to the production method of the present invention had a lower fine powder content than the comparative examples, and at the same time, had excellent water absorption properties (especially water absorption speed).

[0223] On the other hand, in the case of Comparative Example 1, it was confirmed that the water retention capacity of the base resin was significantly reduced and the amount of fine powder generation increased significantly because a large amount of chemical foaming agent was used without performing the ultrafine grinding process under high shear conditions. As a result, the rewet property was significantly reduced due to the low water retention capacity and high content of water-soluble components in the final product.

[0224] In the case of Comparative Example 2, the polymer chain structure of the resin was damaged due to an increase in shear force caused by an excessively high rpm. In this case, the improvement in water absorption speed was negligible, and the damage to the polymer chain structure increased the amount of water-soluble components, resulting in a significant decrease in rewet of the final product.

[0225] In the case of Comparative Example 3, it was confirmed that the water absorption rate decreased because sufficient air bubbles were not formed inside the polymer due to the reduced shear force caused by the low rpm.

[0226] In the case of Comparative Examples 4 and 5, a neutralization process was carried out at the polymerization stage prior to ultrafine pulverization, and the polymer itself was hard, making it difficult to carry out the ultrafine pulverization process. As a result, it was confirmed that the water absorption properties were reduced and the amount of fine powder generated was significantly increased compared to the Examples.

Claims

1. A step 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 (step 1); Micronizing the polymer at a rotation speed of 500 rpm to 4,000 rpm (Step 2); neutralizing at least a portion of the acidic groups of the polymer (Step 3); drying the micronized and neutralized polymer to produce dry superabsorbent polymer particles (Step 4); and and (5) grinding the dried superabsorbent resin particles to produce superabsorbent resin particles. A method for producing a superabsorbent resin.

2. Step 1 is carried out in a batch type reactor for at least 1 hour. A method for producing the highly water-absorbent resin according to claim 1.

3. The step 2 is carried out at a rotation speed of 1,500 rpm to 3,500 rpm; A method for producing the highly water-absorbent resin according to claim 1.

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

5. The step 3 is carried out by a high shear grinding device, the high shear grinding device comprising: a body portion including a transfer space into which the polymer is transferred; a screw member rotatably mounted within the conveying space to move the polymer; a drive motor for providing rotational driving force to said screw member; A cutter member disposed in the body portion for pulverizing the polymer; and The polymer crushed by the cutter member is discharged to the outside of the body portion, and the porous plate having a plurality of holes formed therein is included. A method for producing the highly water-absorbent resin according to claim 1.

6. The size of the holes formed in the perforated plate is 1 mm to 25 mm. A method for producing the highly water-absorbent resin according to claim 5.

7. The step 4 is carried out by fixed-bed type drying, moving type drying, or a combination thereof; A method for producing the highly water-absorbent resin according to claim 1.

8. When step 4 is performed by drying in place, The drying is carried out at 80°C to 200°C. A method for producing the highly water-absorbent resin according to claim 7.

9. When step 4 is carried out by fluidized drying, The drying is carried out at 100°C to 300°C. A method for producing the highly water-absorbent resin according to claim 7.

10. The moisture content of the dried superabsorbent resin particles obtained by carrying out step 4 is 1% by weight to 20% by weight. A method for producing the highly water-absorbent resin according to claim 1.

11. Before the step 4, the method further includes an additive introduction step of mixing the ultrafinely pulverized and neutralized polymer with any one of additional additives consisting of a polymerization initiator, a neutralizing agent, a surfactant, a fine powder, a liquid permeability improver, an anti-caking agent, and an antioxidant; A method for producing the highly water-absorbent resin according to claim 1.

12. The additive introduction step includes: Simultaneous with step 2, simultaneous with step 3, after step 3, or at least one of these steps; A method for producing the highly water-absorbent resin according to claim 11.

13. The method further includes forming a surface cross-linked layer on at least a portion of the surface of the highly absorbent resin particles. A method for producing the highly water-absorbent resin according to claim 1 or 11.

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