Manufacturing method of superabsorbent resin

Abstract

The present invention relates to a method for producing a superabsorbent polymer, and more specifically to a method for producing a superabsorbent polymer in which the drying process conditions are controlled to achieve excellent drying efficiency and prevent warping of the dried product.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0183170, filed December 23, 2022, and Korean Patent Application No. 10-2023-0181011, filed 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 in which the drying process conditions are controlled to achieve excellent drying efficiency and prevent warping of the dried product. [Background technology]

[0003] Super absorbent polymers (SAPs) are synthetic polymers capable of absorbing 500 to 1,000 times their own weight in water, and each developer has their own name for them, such as SAM (Super Absorbency Material) or AGM (Absorbent Gel Material). These super absorbent polymers first came into practical use as sanitary products, and are now widely used in a variety of applications, including soil water retention agents for horticulture, water-stopping materials for civil engineering and construction, seedling sheets, freshness-preserving agents in the food distribution industry, and adhesive patches.

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

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

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

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

[0008] Furthermore, the hydrogel polymers polymerized during the superabsorbent resin manufacturing process have the property of coagulating with each other, and when they are manufactured into coagulated fine particles, the coagulation force increases, resulting in problems with the subsequent drying process. Specifically, this drying process is typically carried out by applying hot air in a belt dryer equipped with a perforated plate, but the increased coagulation force of the hydrogel polymers significantly reduces the porosity within the drying layer, which in turn increases the hot air pressure difference, causing warping of the outer periphery of the drying layer and making it difficult to dry large quantities.

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

[0010] Therefore, the present invention provides a method for producing a superabsorbent resin that minimizes warping of the dried product with excellent drying efficiency by controlling the drying process conditions. [Means for solving the problem]

[0011] In order to solve the above problem, according to one embodiment of the present invention, a step (step 1) of polymerizing a monomer composition including a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized; neutralizing at least a portion of the acidic groups of the polymer (Step 2); atomizing the polymer in the presence of a surfactant (Step 3); drying the neutralized and atomized polymer in a fixed-bed dryer to obtain a laminate of dried superabsorbent polymer particles (Step 4); and and (5) grinding the laminate of the dried superabsorbent resin particles to prepare superabsorbent resin particles. The step 4 includes a step of primary drying the neutralized and atomized polymer by introducing a downflow and a step of secondary drying by introducing an upflow, The implementation time of the primary drying step is 40 to 60% of the total implementation time of step 4; A method for producing a highly water-absorbent resin is provided. [Effects of the Invention]

[0012] According to the method for producing a superabsorbent polymer of the present invention, excellent drying efficiency can be achieved by controlling the drying process conditions, thereby making it possible to produce a superabsorbent polymer with minimized warping of the dried product.

[0013] According to the method for producing a superabsorbent polymer of the present invention, it is possible to produce a superabsorbent polymer that has a particle shape in which fine particles are aggregated, thereby increasing the surface area and significantly improving the water absorption rate, thereby exhibiting excellent water absorption properties.

[0014] In addition, by having a high molecular weight polymer, a uniform particle size distribution, and a low content of water-soluble components (EC), it is possible to provide a superabsorbent resin that is excellent in all of its water absorption properties, such as water retention capacity and water absorption capacity under pressure, liquid permeability, rewet properties, and water absorption speed. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a flowchart showing a conventional method for producing a highly water-absorbent resin. [Figure 2] 1 is a photograph showing a cross section of a dried body obtained in a drying step in the method for producing a superabsorbent resin of an example and a comparative example. [Figure 3] 1 is a drying curve graph showing the moisture content as a function of drying time in the drying step of the method for producing a superabsorbent resin of an example and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0016] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise. It should be understood that the terms "comprise," "comprise," or "have" used in this specification are intended to specify the presence of embodied features, steps, components, or combinations thereof, and do not preclude the possibility of the presence or addition of one or more other features, steps, components, or combinations thereof.

[0017] Although the present invention can be embodied in various forms through various modifications, specific embodiments are described in detail below by way of example, but it should be understood that this is not intended to limit the present invention to the specific disclosed embodiments, and that the present invention encompasses all modifications, equivalents, and alternatives falling within the spirit and technical scope of the present invention.

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

[0019] Furthermore, the term "polymer" or "macromolecule" as used herein means a polymerized state of a water-soluble ethylenically unsaturated monomer, and can encompass any range of moisture content or particle size.

[0020] Furthermore, depending on the context, the term "superabsorbent polymer" may refer to a crosslinked polymer or a powder-like base resin consisting of superabsorbent polymer particles obtained by pulverizing the crosslinked polymer, or may be used to encompass all of the crosslinked polymers or base resins that have been subjected to additional processes, such as drying, pulverization, classification, surface crosslinking, etc., to be in a state suitable for commercialization.

[0021] The term "fine powder" refers to superabsorbent resin particles having a particle size of less than 150 μm. The particle size of such resin particles can be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP220.3 method.

[0022] The term "chopping" refers to cutting the hydrogel polymer into small pieces on the order of millimeters to improve drying efficiency, and is used to distinguish it from grinding to micrometer or regular particle levels.

[0023] The term "micronizing" refers to the grinding of a hydrogel polymer into particles of several tens to several hundreds of micrometers, and is used to distinguish it from "chopping."

[0024] Hereinafter, the method for producing a superabsorbent polymer and the superabsorbent polymer will be described in more detail with reference to specific embodiments of the invention.

[0025] According to one embodiment of the invention, a step (step 1) of polymerizing a monomer composition including a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized; neutralizing at least a portion of the acidic groups of the polymer (Step 2); atomizing the polymer in the presence of a surfactant (Step 3); drying the neutralized and atomized polymer in a fixed-bed dryer to obtain a laminate of dried superabsorbent polymer particles (Step 4); and and (5) grinding the laminate of the dried superabsorbent resin particles to prepare superabsorbent resin particles. The step 4 includes a step of primary drying the neutralized and atomized polymer by introducing a downflow and a step of secondary drying by introducing an upflow, The implementation time of the primary drying step is 40 to 60% of the total implementation time of step 4; A method for making a superabsorbent polymer is provided.

[0026] Typically, a hydrogel polymer obtained by a polymerization reaction of an acrylic acid-based monomer is subjected to processes such as drying, pulverization, classification, and surface cross-linking, and is then commercially available as a powdered product called a superabsorbent resin.

[0027] When a fixed-bed drying process is applied to the above manufacturing process, the dried superabsorbent polymer is obtained in the form of a laminate, but the hydrogel polymer shrinks during the drying process. This causes variations in the degree of shrinkage across the width of the dried product depending on the drying conditions, which can lead to problems such as warping of the outer shell of the dried product. This warping of the outer shell of the dried product can cause unevenness in the pressure difference of the hot air, significantly reducing the drying efficiency and resulting in longer drying times, or insufficient drying of the interior of the dried product.

[0028] As a result of extensive research to solve this problem, the inventors have confirmed that excellent drying efficiency can be achieved by adjusting the drying process conditions to provide downward airflow at an initial stage and satisfying a specific ratio with respect to the total drying time, as described above, and have thus completed the present invention.

[0029] The present inventors have also found that by performing a step of atomizing the hydrogel polymer in the presence of a surfactant before the drying step and adjusting the direction of the hot air in the drying step, it is possible to effectively suppress the occurrence of warping in the dried laminate and achieve excellent drying efficiency even when a large amount of drying is performed, and have completed the present invention.

[0030] Recently, on the other hand, attempts have been made to provide highly water-absorbent resins that exhibit an even faster water absorption rate.

[0031] 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 included in the monomer composition, thereby forming a porous structure in the base resin powder as cross-linking polymerization progresses.

[0032] However, the use of a foaming agent has the disadvantages of reducing various physical properties of the superabsorbent resin, such as surface tension, liquid permeability, or volume density, and increasing the amount of fine powder generated. Therefore, there is a continuing demand for the development of a technology that can improve the water absorption rate of a superabsorbent resin without using a foaming agent.

[0033] Conventional superabsorbent resins are manufactured by crosslinking a water-soluble ethylenically unsaturated monomer having at least a partially neutralized acidic group in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer, drying the formed hydrogel polymer, and then pulverizing it to a desired particle size. To facilitate drying of the hydrogel polymer and increase the efficiency of the pulverization process, a chopping process is typically performed before the drying process to cut the hydrogel polymer into particles several millimeters in size. However, due to the stickiness of the hydrogel polymer, the hydrogel polymer remains in an aggregated gel state rather than being pulverized to micro-sized particles. When this aggregated gel-like hydrogel polymer is dried, a plate-like dried body is formed. To pulverize this to micro-sized particles, multiple pulverization processes are required to reduce the stickiness of the polymer. This process generates a large amount of fine powder.

[0034] Specifically, a flow chart of a conventional method for producing a superabsorbent polymer is shown in Figure 1. Referring to Figure 1, a conventional superabsorbent polymer has been produced through the following steps:

[0035] (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 then classifying it into normal particles and fine powder;

[0036] As described above, the chopped hydrogel polymer has a gel-like aggregated shape with a size of approximately 1 cm to 10 cm. The chopped hydrogel polymer is stacked on a belt with a perforated bottom and dried by hot air supplied from the bottom or top. Because the polymer dried by this drying method exhibits a plate-like shape rather than a granular shape, the classification step after crushing has typically been carried out by first coarsely crushing and classifying the polymer to produce normal particles, i.e., particles with a diameter of 150 μm to 850 μm, followed by further fine crushing and classification. In this manufacturing method, the amount of fine powder separated in the final classification step is large, about 20 wt% to about 30 wt% of the total weight of the final superabsorbent polymer. Therefore, the separated fine powder is reused by mixing it with an appropriate amount of water, re-granulating the fine powder, and then adding it to the chopping step or the step before drying.

[0037] However, when the re-granulated fine powder mixed with water for reuse is re-introduced into the grinding or drying process, problems have arisen, such as an increase in the load on the equipment and / or energy consumption, and the fine powder remaining unclassified has caused a deterioration in the physical properties of the superabsorbent polymer.

[0038] Therefore, the present inventors recognized that the amount of fine powder generated in conventional manufacturing methods is largely influenced by the pulverization step, and focused on the fact that the amount of fine powder generated during the manufacturing process can be significantly reduced by adding a surfactant and a neutralizing agent in the polymer pulverization step to post-neutralize the polymer, pulverizing it more finely than before, i.e., atomizing it, while simultaneously controlling aggregation to produce particles in the form of aggregates of fine particles.

[0039] Meanwhile, a method of adding a surfactant to reduce the stickiness of the hydrogel polymer during the chopping process has been proposed. However, when adding a surfactant during the chopping process, the high water content of the hydrogel polymer causes the surfactant to penetrate into the hydrogel polymer rather than being present at the interface of the hydrogel polymer, which results in the surfactant not being able to perform its function properly.

[0040] This is because chopped particles are formed into particles of several mm or several cm in size compared to the polymer before chopping, which can increase the surface area to some extent, but it is difficult to expect an effect that can effectively improve the water absorption rate. Therefore, in order to improve the water absorption rate, one method to increase the surface area by increasing the mechanical force and kneading during the chopping step can be considered, but in this case, excessive aggregation occurs due to the stickiness unique to the polymer, and after chopping, drying, and pulverization, only the particle surface becomes an irregular, amorphous single particle, and excessive kneading or pulverization can actually increase the water-soluble components.

[0041] As a result of extensive research to solve this problem, it was found that, unlike conventional methods for producing superabsorbent polymers in which the acidic groups of a water-soluble ethylenically unsaturated monomer are neutralized before polymerization, polymerization is first carried out in a state in which the acidic groups are not neutralized to form a polymer, and the hydrogel polymer is atomized in the presence of a surfactant, and then the acidic groups of the polymer are neutralized; or, after forming a hydrogel polymer by neutralizing the acidic groups of the polymer, the hydrogel polymer is atomized in the presence of a surfactant; or, simultaneously with atomization, the acidic groups present in the polymer are neutralized; the surfactant is present in a large amount on the surface of the polymer, reducing the high viscosity of the polymer and preventing excessive aggregation of the polymer, and thereby fully fulfilling its role of adjusting the aggregation state to a desired level.

[0042] This allows the polymer to be prepared into secondary particles in the form of agglomerated primary particles, and the subsequent pulverization and drying processes can be carried out under milder conditions, thereby significantly reducing the amount of fine powder generated during the process.

[0043] Furthermore, when a polymer is micronized in the presence of the surfactant, the hydrophobic functional groups contained in the surfactant impart hydrophobicity to the surface of the pulverized superabsorbent resin particles, thereby reducing the friction between the particles and increasing the apparent density of the superabsorbent resin, while the hydrophilic functional groups contained in the surfactant also bind to the superabsorbent resin particles, preventing a decrease in the surface tension of the resin. As a result, the superabsorbent resin produced by the above-mentioned production method has a higher apparent density while exhibiting the same level of surface tension as a resin that does not use a surfactant.

[0044] Furthermore, if polymerization is first carried out in an unneutralized state to form a polymer, and then the acidic groups present in the polymer are neutralized, it is possible to form a polymer with a longer chain, and the content of water-soluble components present in an uncrosslinked state due to incomplete crosslinking can be reduced.

[0045] The water-soluble component has the property of easily dissolving when the superabsorbent polymer comes into contact with a liquid, so if the content of the water-soluble component is high, most of the dissolved water-soluble component remains on the surface of the superabsorbent polymer, making the superabsorbent polymer sticky and reducing its liquid permeability. Therefore, in terms of liquid permeability, it is important to maintain the content of the water-soluble component low.

[0046] According to one embodiment of the present invention, the content of water-soluble components is reduced by carrying out polymerization in an unneutralized state, thereby improving the liquid permeability of the superabsorbent resin.

[0047] In addition, the superabsorbent resin produced according to one embodiment of the present invention may have a uniform particle size distribution, thereby providing a superabsorbent resin with excellent water absorption properties such as water retention capacity, water absorption capacity under pressure, rewet properties, and water absorption speed.

[0048] Hereinafter, each step of the method for producing a highly water-absorbent resin according to one embodiment will be described in more detail.

[0049] (Method of manufacturing superabsorbent resin) Stage 1: Polymerization Stage First, a monomer composition containing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator is polymerized to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized.

[0050] The step may include preparing a monomer composition by mixing the water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator, and polymerizing the monomer composition to form a polymer.

[0051] The water-soluble ethylenically unsaturated monomer may be any monomer commonly used in the production of superabsorbent resins. As a non-limiting example, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following Chemical Formula 1:

[0052] [Chemical formula 1] R-COOM'

[0053] 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.

[0054] 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.

[0055] In this way, when (meth)acrylic acid and / or its salt is used as the water-soluble ethylenically unsaturated monomer, it is advantageous to obtain a superabsorbent resin with improved water absorption. Other examples of the monomer that can be used include maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, (N,N)-dimethylaminoethyl (meth)acrylate, and (N,N)-dimethylaminopropyl (meth)acrylamide.

[0056] 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 portion of the acidic groups have been neutralized with a neutralizing agent is cross-linked to form a hydrogel polymer. Specifically, at least a portion of the acidic groups of the water-soluble ethylenically unsaturated monomer are neutralized in the step of mixing the water-soluble ethylenically unsaturated monomer having the acidic group, an internal cross-linking agent, a polymerization initiator, and a neutralizing agent.

[0057] However, according to one embodiment of the present invention, the water-soluble ethylenically unsaturated monomer is polymerized first in a state where the acid group is not neutralized to form a polymer.

[0058] A water-soluble ethylenically unsaturated monomer (e.g., acrylic acid) in which the acid group is not neutralized is in a liquid state at room temperature and has high miscibility with the solvent (water), so it exists in the form of a mixed solution in the monomer composition. However, a water-soluble ethylenically unsaturated monomer in which the acid group is neutralized is in a solid state at room temperature and has different solubility depending on the temperature of the solvent (water), with the solubility decreasing as the temperature decreases.

[0059] Such water-soluble ethylenically unsaturated monomers in which the acidic group is not neutralized have higher solubility or miscibility in a solvent (water) than monomers in which the acidic group is neutralized, and do not precipitate even at low temperatures, making them advantageous for long-term polymerization at low temperatures. As a result, long-term polymerization using the water-soluble ethylenically unsaturated monomers in which the acidic group is not neutralized can stably produce polymers with higher molecular weights and more uniform molecular weight distributions.

[0060] Furthermore, it is possible to form a polymer with a longer chain, and the content of water-soluble components that exist in an uncrosslinked state due to incomplete polymerization or crosslinking can be reduced.

[0061] Furthermore, when the polymerization is first carried out in a state where the acidic groups of the monomers are not neutralized to form a polymer, and the polymer is then neutralized and then micronized in the presence of a surfactant, or when the acidic groups present in the polymer are neutralized in the presence of a surfactant, or when the acidic groups present in the polymer are neutralized simultaneously with micronization, the surfactant is present in a large amount on the surface of the polymer, and can sufficiently play a role in reducing the stickiness of the polymer.

[0062] The concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition can be appropriately adjusted in consideration of the polymerization time, reaction conditions, etc., and is preferably about 20 to about 60% by weight, or about 20 to about 40% by weight.

[0063] The term "internal cross-linking agent" used in this specification is a term used to distinguish it from a surface cross-linking agent for cross-linking the surface of the superabsorbent resin particles described later, and plays a role in introducing cross-linking bonds between unsaturated bonds of the water-soluble ethylenically unsaturated monomer described above to form a polymer containing a cross-linked structure.

[0064] The crosslinking in this step is carried out regardless of whether it is on the surface or inside. However, when the surface crosslinking process of the superabsorbent resin particles described below is carried out, the surface of the finally produced superabsorbent resin particles may include a structure newly crosslinked by the surface crosslinking agent, and the inside of the superabsorbent resin particles may maintain the structure crosslinked by the internal crosslinking agent.

[0065] 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.

[0066] Non-limiting examples of polyfunctional acrylate compounds 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 thereof 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, and these can be used alone or in combination of two or more.

[0067] Non-limiting examples of polyfunctional allyl compounds include ethylene glycol diallyl ether, diethylene glycol diallyl ether, triethylene glycol diallyl ether, tetraethylene glycol diallyl ether, polyethylene glycol diallyl ether, propylene glycol diallyl ether, tripropylene glycol diallyl ether, polypropylene glycol diallyl ether, butanediol diallyl ether, butylene glycol diallyl ether, hexanediol diallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, dipentaerythritol diallyl ether, dipentaerythritol triallyl ether, dipentaerythritol tetraallyl ether, dipentaerythritol pentaallyl ether, trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, glycerin diallyl ether, and glycerin triallyl ether. These compounds may be used alone or in combination of two or more.

[0068] 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. These compounds can be used alone or in combination. Pentaerythritol triallyl ether is preferably used.

[0069] The above-mentioned polyfunctional allyl compounds or polyfunctional vinyl compounds can form a crosslinked structure during the polymerization process by bonding two or more unsaturated groups contained in the molecule with the unsaturated bond of the water-soluble ethylenically unsaturated monomer or the unsaturated bond of another internal crosslinking agent, respectively, and can maintain the crosslinked bond more stably even during the neutralization process after the above-mentioned polymerization reaction, unlike acrylate compounds containing an ester bond (-(C=O)O-) in the molecule.

[0070] This increases the gel strength of the produced superabsorbent resin, improves process stability during the extrusion process after polymerization, and minimizes the amount of water-soluble components.

[0071] The crosslinking polymerization of the water-soluble ethylenically unsaturated monomer in the presence of such an internal crosslinking agent may be carried out in the presence of a polymerization initiator, and, if necessary, a thickener, a plasticizer, a storage stabilizer, an antioxidant, etc.

[0072] In the monomer composition, the internal crosslinking agent can be used in an amount of 0.01 to 5 parts by weight per 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the internal crosslinking agent can be used in an amount of 0.01 part by weight or more, or 0.05 part by weight or more, or 0.1 part by weight or more and 5 parts by weight or less, or 3 parts by weight or less, or 2 parts by weight or less, or 1 part by weight or less, or 0.7 parts by weight or less per 100 parts by weight of the water-soluble ethylenically unsaturated monomer. If the content of the internal crosslinking agent is too low, crosslinking may not occur sufficiently, making it difficult to achieve an appropriate level of strength. If the content of the internal crosslinking agent is too high, the internal crosslink density may increase, making it difficult to achieve the desired water retention capacity.

[0073] The polymer formed using such an internal crosslinking agent has a three-dimensional network structure in which the main chain formed by polymerization of the water-soluble ethylenically unsaturated monomer is crosslinked by the internal crosslinking agent. When the polymer has such a three-dimensional network structure, the water retention capacity and water absorption capacity under pressure, which are various physical properties of the superabsorbent resin, can be significantly improved compared to when the polymer has a two-dimensional linear structure that is not additionally crosslinked by the internal crosslinking agent.

[0074] According to one embodiment of the present invention, the step of polymerizing the monomer composition to form a polymer may be performed in a batch type reactor.

[0075] In a typical method for producing a superabsorbent resin, the polymerization method is roughly divided into thermal polymerization and photopolymerization depending on the polymerization energy source. Generally, when thermal polymerization is carried out, it may be carried out in a reactor having a stirring shaft such as a kneader, and when photopolymerization is carried out, it may be carried out in a reactor equipped with a movable conveyor belt or in a vessel with a flat bottom.

[0076] On the other hand, in most of these polymerization methods, the molecular weight of the polymer is not large due to the short polymerization reaction time (for example, 1 hour or less), and a polymer having a broad molecular weight distribution is formed.

[0077] On the other hand, when photopolymerization is carried out in a reactor equipped with a movable conveyor belt or a container with a flat bottom, the resulting hydrogel polymer is usually in the form of a sheet having the width of the belt. The thickness of the polymer sheet varies depending on the concentration and injection rate or amount of the monomer composition injected, but is usually about 0.5 to about 5 cm thick.

[0078] However, if the monomer composition is supplied to such an extent that the thickness of the sheet-like polymer is too thin, the production efficiency is low, which is undesirable, and if the thickness of the sheet-like polymer is increased for productivity reasons, the polymerization reaction does not occur uniformly throughout the entire thickness, making it difficult to form a high-quality polymer.

[0079] In addition, in the polymerization in the reactor having a reactor agitator equipped with a conveyor belt, the polymerization is carried out continuously by feeding new monomer composition to the reactor while the polymerization result is moving, so that polymers with different polymerization rates are mixed, which makes it difficult to carry out uniform polymerization throughout the monomer composition, and may result in deterioration of overall physical properties.

[0080] However, according to one embodiment of the present invention, polymerization is carried out in a batch reactor in a fixed bed mode, which reduces the risk of polymers with different polymerization rates being mixed together, thereby producing polymers with uniform quality.

[0081] In addition, the polymerization step is carried out in a batch reactor having a predetermined volume, and the polymerization reaction is carried out for a longer time, for example, 3 hours or more, than in the case of continuous polymerization in a reactor equipped with a conveyor belt. Despite this long polymerization reaction time, since the polymerization is carried out on unneutralized water-soluble ethylenically unsaturated monomers, the monomers do not precipitate even during long-term polymerization, and therefore, it is advantageous for long-term polymerization.

[0082] Meanwhile, the polymerization in the batch reactor of the present invention is carried out using a thermal polymerization method, and therefore, a thermal polymerization initiator is used as the polymerization initiator.

[0083] The thermal polymerization initiator may be one or more selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid. Specific examples of persulfate initiators include sodium persulfate (NaSO), potassium persulfate (KSO), and ammonium persulfate ((NHSO). Examples of azo initiators include 2,2-azobis(2-amidinopropane) dihydrochloride and 2,2-azobis(N,N-dimethylene)isobutyramidine dihydrochloride. dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), etc. A wide variety of thermal polymerization initiators are clearly described in Odian, "Principle of Polymerization" (Wiley, 1981), p. 203, and are not limited to the examples mentioned above.

[0084] The polymerization initiator can be used in an amount of 2 parts by weight or less per 100 parts by weight of the water-soluble ethylenically unsaturated monomer. That is, if the concentration of the polymerization initiator is too low, the polymerization rate may be slowed and a large amount of residual monomer may be extracted into the final product, which is undesirable. Conversely, if the concentration of the polymerization initiator is higher than the above range, the polymer chains forming the network may become shorter, the content of water-soluble components may increase, and the physical properties of the resin may be degraded, such as a decrease in water absorption capacity under pressure, which is undesirable.

[0085] Meanwhile, in one embodiment of the present invention, polymerization can be initiated by adding a reducing agent that forms a redox couple with the initiator.

[0086] Specifically, when the initiator and the reducing agent are added to the polymer solution, they react with each other to form radicals.

[0087] The formed radicals react with the monomer, and the oxidation-reduction reaction between the initiator and the reducing agent is highly reactive, so polymerization can be initiated even with only a small amount of initiator and reducing agent added, eliminating the need to increase the process temperature, allowing low-temperature polymerization and minimizing changes in the physical properties of the polymer solution.

[0088] The polymerization reaction using the oxidation-reduction reaction can occur smoothly at or below room temperature (25°C). For example, the polymerization reaction may be carried out at a temperature of 5°C to 25°C or 5°C to 20°C.

[0089] In one embodiment of the present invention, when a persulfate initiator is used as the initiator, the reducing agent can be at least one selected from the group consisting of sodium metabisulfite (NaSO); tetramethylethylenediamine (TMEDA); a mixture of iron (II) sulfate and EDTA (FeSO / EDTA); sodium formaldehyde sulfoxylate; and disodium 2-hydroxy-2-sulfinoacetate.

[0090] For example, potassium persulfate can be used as the initiator and disodium 2-hydroxy-2-sulfinoacetate as the reducing agent; ammonium persulfate can be used as the initiator and tetramethylethylenediamine as the reducing agent; or sodium persulfate can be used as the initiator and sodium formaldehyde sulfoxylate as the reducing agent.

[0091] In another embodiment of the present invention, when a hydrogen peroxide-based initiator is used as the initiator, the reducing agent may be one or more selected from the group consisting of ascorbic acid; sucrose; sodium sulfite (NaSO), sodium metabisulfite (NaSO); tetramethylethylenediamine (TMEDA); a mixture of iron (II) sulfate and EDTA (FeSO / EDTA); sodium formaldehyde sulfoxylate; disodium 2-hydroxy-2-sulfinoacteate; and disodium 2-hydroxy-2-sulfoacetate.

[0092] The monomer composition may further contain additives such as a thickener, a plasticizer, a storage stabilizer, an antioxidant, a surfactant, etc., as needed. Here, the additives may be any components commonly used in the art without any particular limitations, as long as they do not impair the effects of the invention. The surfactant may also be the surfactant used in Step 3 below (the compound represented by Chemical Formula 2 or a salt thereof).

[0093] The monomer composition containing the monomer may be in a solution state, for example, dissolved in a solvent such as water, and the solid content in such a solution state of the monomer composition, i.e., the concentrations of the monomer, internal crosslinking agent, and polymerization initiator, can be appropriately adjusted in consideration of the polymerization time, reaction conditions, etc. For example, the solid content in the monomer composition may be 10 to 80 wt %, 15 to 60 wt %, or 30 to 50 wt %.

[0094] In this case, the solvent that can be used is not limited in composition as long as it can dissolve the above-mentioned components, and 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.

[0095] The polymer obtained by this method can form a polymer having a high molecular weight and a uniform molecular weight distribution by polymerizing an unneutralized ethylenically unsaturated monomer, as described above, and can reduce the content of water-soluble components.

[0096] The polymer obtained by such a method may be in a hydrogel polymer state with 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, or 70% by weight or less.

[0097] If the water content of the polymer is too low, it may be difficult to secure an adequate surface area in the subsequent pulverization step, and the polymer may not be effectively pulverized. If the water content of the polymer is too high, the pressure applied in the subsequent pulverization step may increase, making it difficult to pulverize the polymer to a desired particle size.

[0098] Throughout this specification, the term "moisture content" refers to the amount of water relative to the total weight of the polymer, calculated by subtracting the weight of the polymer in a dry state from the weight of the polymer. Specifically, it is defined as a value calculated by measuring the weight loss due to evaporation of water in the polymer during the drying process of raising the temperature of a crumb-state polymer using infrared heating. The drying conditions are to raise the temperature from room temperature to about 180°C and then maintain it at 180°C, and the total drying time is set to 40 minutes, including a 5-minute temperature rise phase, and the moisture content is measured.

[0099] Stage 2: Neutralization Stage and Stage 3: Atomization Stage Next, a step (step 2) is carried out in which at least some of the acid groups of the polymer are neutralized.

[0100] In this case, the neutralizing agent may be a basic substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide, which can neutralize the acidic group.

[0101] Furthermore, the degree of neutralization, which refers to the degree to which the acidic groups contained in the polymer are neutralized by the neutralizing agent, may be 50 to 90 mol%, 60 to 85 mol%, 65 to 85 mol%, or 65 to 75 mol%. The range of the degree of neutralization may vary depending on the final physical properties, but if the degree of neutralization is too high, the water absorption capacity of the superabsorbent polymer may be reduced, and the concentration of carboxyl groups on the particle surface may be too low, making it difficult to properly perform surface cross-linking in subsequent processes, resulting in reduced water absorption properties or liquid permeability under pressure. Conversely, if the degree of neutralization is too low, not only will the water absorption capacity of the polymer be significantly reduced, but it may also exhibit properties similar to elastic rubber, which is difficult to handle.

[0102] Simultaneously with, before or after step 2, a step of atomizing the polymer in the presence of a surfactant is carried out (step 3).

[0103] This step involves atomizing the polymer in the presence of a surfactant, not by chopping the polymer into millimeter-sized particles, but by simultaneously chopping and aggregating the polymer into particles of tens to hundreds of micrometers. That is, this step involves imparting appropriate adhesiveness to the polymer to produce secondary agglomerated particles formed by agglomeration of primary particles chopped into particles of tens to hundreds of micrometers. The hydrous superabsorbent resin particles, which are secondary agglomerated particles, produced in this step have a normal particle size distribution and a significantly increased surface area, thereby significantly improving the water absorption rate.

[0104] After mixing the polymer and surfactant in this manner, the polymer is atomized in the presence of the surfactant to produce hydrous superabsorbent resin particles (=neutralized and atomized polymer) in the form of secondary agglomerated particles formed by chopping and agglomerating the superabsorbent resin particles and surfactant in a mixed state.

[0105] Here, "water-containing superabsorbent resin particles" are particles with a water content (moisture content) of approximately 30% by weight or more, and are formed by chopping and agglomerating a polymer into particles without a drying process, so they can have a moisture content of 30 to 80% by weight, the same as the polymer.

[0106] According to one embodiment of the present invention, the surfactant may be a compound represented by the following Chemical Formula 2 or a salt thereof, but the present invention is not limited thereto:

[0107] [ka]

[0108] In the above Chemical Formula 2, A1, A2 and A3 each independently represent a single bond, a carbonyl,

[0109] [ka]

[0110] and one or more of these is a carbonyl or

[0111] [ka]

[0112] wherein m1, m2, and m3 each independently represent an integer of 1 to 8;

[0113] [ka]

[0114] are each linked to adjacent oxygen atoms,

[0115] [ka]

[0116] are linked to adjacent R1, R2 and R3, respectively; R1, R2, and R3 are each independently hydrogen, a linear or branched alkyl group having 6 to 18 carbon atoms, or a linear or branched alkenyl group having 6 to 18 carbon atoms; n is an integer of 1 to 9.

[0117] The surfactant is added so that the atomization step can be easily carried out without agglomeration when mixed with the polymer.

[0118] The surfactant represented by the formula 2 is a nonionic surfactant, and has excellent surface adsorption performance due to hydrogen bonding with unneutralized polymers, making it suitable for achieving the desired aggregation control effect. On the other hand, in the case of anionic surfactants that are not nonionic surfactants, when mixed with polymers neutralized with a neutralizing agent such as NaOH or Na2SO4, ionized Na is attached to the carboxyl group substituent of the polymer. +When adsorption occurs via ions and the compound is mixed with an unneutralized polymer, there is a problem that the adsorption efficiency to the polymer decreases relatively due to competition with the anions of the carboxyl group substituents of the polymer.

[0119] Specifically, in the surfactant represented by the chemical formula 2, the hydrophobic functional groups are the terminal functional groups R1, R2, and R3 (if they are not hydrogen), and the hydrophilic functional groups are the glycerol-derived portion in the chain and the terminal hydroxyl group (A n is a single bond and at the same time R n (When n is hydrogen, n=1 to 3) The glycerol-derived portion and the terminal hydroxyl group are hydrophilic functional groups that improve the adsorption performance on the polymer surface, thereby effectively suppressing the aggregation of superabsorbent resin particles.

[0120] In Formula 2, the hydrophobic functional groups R1, R2, and R3 (when not hydrogen) are each independently a straight-chain or branched-chain alkyl having 6 to 18 carbon atoms, or a straight-chain or branched-chain alkenyl having 6 to 18 carbon atoms. If the R1, R2, and R3 (when not hydrogen) are alkyl or alkenyl having less than 6 carbon atoms, the chain length is too short, which can cause problems such as ineffective aggregation control of the milled particles. If the R1, R2, and R3 (when not hydrogen) are alkyl or alkenyl having more than 18 carbon atoms, the mobility of the surfactant can decrease, preventing effective mixing with the polymer, and the increased cost of the surfactant can result in a high unit price for the composition.

[0121] Preferably, R1, R2, and R3 are hydrogen or, in the case of a straight-chain or branched alkyl having 6 to 18 carbon atoms, 2-methylhexyl, n-heptyl, 2-methylheptyl, n-octyl, n-nonyl, n-decanyl, n-undecanyl, n-dodecanyl, n-tridecanyl, n-tetradecanyl, n-pentadecanyl, n-hexadecanyl, n-heptadecanyl, or n-octadecanyl, or, in the case of a straight-chain or branched alkenyl having 6 to 18 carbon atoms, 2-hexenyl, 2-heptenyl, 2-octenyl, 2-nonenyl, n-dekenyl, 2-undekenyl, 2-dodekenyl, 2-tridekenyl, 2-tetradekenyl, 2-pentadekenyl, 2-hexadekenyl, 2-heptadekenyl, or 2-octadekenyl.

[0122] The surfactant may be selected from compounds represented by the following formulas 2-1 to 2-14:

[0123] [ka] [ka]

[0124] The surfactant can be used in an amount of 0.01 to 10 parts by weight per 100 parts by weight of the polymer. If the surfactant is used in an amount that is too small, it may not be uniformly adsorbed onto the surface of the polymer, resulting in re-aggregation of particles after pulverization. If the surfactant is used in an amount that is too large, the properties of the final superabsorbent resin may be reduced. For example, the surfactant can be used in an amount of 0.01 part by weight or more, 0.015 parts by weight or more, or 0.1 part by weight or more and 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, or 1 part by weight or less per 100 parts by weight of the polymer.

[0125] The method of mixing such a surfactant with a polymer is not particularly limited as long as it can be uniformly mixed with the polymer, and can be appropriately adopted and used. Specifically, the surfactant can be mixed in a dry state, dissolved in a solvent and then mixed in a solution state, or melted and then mixed.

[0126] For example, the surfactant may be mixed in a solvent in the form of a solution. Any type of solvent, including inorganic and organic solvents, may be used. However, considering the ease of the drying process and the cost of the solvent recovery system, water is the most suitable. The solution may be prepared by mixing the surfactant and polymer in a reaction vessel, by adding the polymer to a mixer and injecting the solution, or by continuously supplying the polymer and solution to a continuously operating mixer and mixing them.

[0127] Meanwhile, according to one embodiment of the present invention, the step of neutralizing at least a portion of the acidic groups of the polymer (Step 2) and the step of atomizing the polymer in the presence of a surfactant (Step 3) may be performed sequentially, alternately, or simultaneously.

[0128] That is, a neutralizing agent may be added to the polymer to neutralize the acidic groups first, and then a surfactant may be added to the neutralized polymer and the surfactant-mixed polymer may be atomized (performed in the order of step 2 → step 3), or a neutralizing agent and surfactant may be added to the polymer simultaneously to neutralize and atomize the polymer (performed in the order of step 2 → step 2). Alternatively, the surfactant may be added first and the neutralizing agent may be added later (performed in the order of step 3 → step 2). Alternatively, the neutralizing agent and surfactant may be added alternately. Alternatively, the surfactant may be added first to atomize the polymer, and then a neutralizing agent may be added to neutralize the neutralized hydrogel polymer, and an additional surfactant may be added to perform an additional atomization process.

[0129] On the other hand, in order to ensure uniform neutralization of the entire polymer, it is preferable to leave a certain time lag between the addition of the neutralizing agent and the atomization step.

[0130] At least a portion or a substantial amount of the surfactant may be present on the surface of the hydrated superabsorbent resin particles.

[0131] Here, the presence of the surfactant on the surface of the hydrated superabsorbent resin particles means that at least a portion or a substantial amount of the surfactant is adsorbed or bound to the surface of the hydrated superabsorbent resin particles. Specifically, the surfactant may be physically or chemically adsorbed to the surface of the superabsorbent resin. More specifically, the hydrophilic functional groups of the surfactant may be physically adsorbed to the hydrophilic portion of the superabsorbent resin surface by intermolecular forces such as dipole-dipole interaction. In this way, the hydrophilic portion of the surfactant may physically adsorb to the surface of the superabsorbent resin particles, surrounding the surface, while the hydrophobic portion of the surfactant may not adsorb to the surface of the resin particles, and the surfactant may coat the resin particles in the form of a type of micelle structure. This is because the surfactant is added not during the polymerization process of the water-soluble ethylenically unsaturated monomer but at the micronization stage after the polymer is formed. This allows the surfactant to faithfully fulfill its role as compared to when the surfactant is added during the polymerization process and exists inside the polymer, and particles with a large surface area can be obtained in the form of aggregates of fine particles due to simultaneous pulverization and aggregation.

[0132] According to one embodiment of the present invention, the step of atomizing the polymer to prepare the water-containing superabsorbent resin particles may be performed two or more times.

[0133] According to one embodiment of the present invention, the atomization step is performed by an atomization device, which may include a body having a transfer space into which the polymer is transferred, a screw member rotatably installed within the transfer space to move the polymer, a drive motor providing rotational driving force to the screw member, a cutter member installed in the body to pulverize the polymer, and a perforated plate having a plurality of holes formed therein and discharging the polymer pulverized by the cutter member to the outside of the body. In this case, the hole size of the perforated plate of the atomization device may be 1 mm to 20 mm, 5 mm to 15 mm, or 5 mm to 12 mm.

[0134] In this way, when the polymer mixed with the surfactant is atomized using an atomizer while controlling aggregation, a smaller particle size distribution is achieved, and the subsequent drying and pulverization steps can be carried out under milder conditions, thereby preventing the generation of fine powder and improving the physical properties of the superabsorbent resin.

[0135] Stage 4: Drying Next, the neutralized and atomized polymer is dried in a fixed-bed dryer to obtain a laminate of dried superabsorbent resin particles (Step 4).

[0136] This step involves drying the water content of the hydrated superabsorbent polymer particles, which are polymers obtained by neutralizing at least some of the acid groups of the polymer and atomizing the polymer in the presence of a surfactant.

[0137] Here, the drying step is carried out using a fixed-bed type dryer, which is distinguished from a moving type dryer in that the material moves during drying.

[0138] Fixed-bed drying refers to a method in which hot air passes over the material to be dried while it is suspended on a bed such as a perforated iron plate through which air can pass. Fluidized-bed drying refers to a method in which the material is dried while being mechanically stirred. The direction in which the hot air passes over the material may be the same as or different from the direction in which the material circulates. Alternatively, the material may be circulated inside the dryer, and a heat transfer fluid (heat medium flow) may be passed through a separate pipe outside the dryer to dry the material.

[0139] When a fixed-bed drying process is applied, the dried superabsorbent polymer is obtained in the form of a laminate, but the increased cohesive force of the hydrogel polymer significantly reduces the porosity within the laminate during drying, which increases the hot air pressure difference and causes warping of the outer shell of the dried product. Also, when there is a large amount of material to be dried, there is a problem that the drying efficiency is significantly reduced, such as the interior of the dried product not being sufficiently dried.

[0140] In the present invention, downward air is introduced at an early stage and is applied at a specific ratio relative to the total drying time, thereby effectively suppressing warping of the dried laminate and achieving excellent drying efficiency even when the drying amount is large.

[0141] More specifically, the drying step is performed by changing the direction of hot air between a primary drying step using downward airflow and a secondary drying step using upward airflow. The primary drying step is performed for 40 to 60% of the total drying time, as described below.

[0142] The downflow means that hot air is blown downward onto the object to be dried, and the upflow means that hot air is blown upward onto the object to be dried. That is, the upflow and downflow mean that the hot air flows in opposite directions. The angle between the directions can be changed depending on the conditions of the hot air dryer used, but the difference in the angle between the upflow and downflow directions is usually considered to be 180°±5°.

[0143] When the drying process is performed under these conditions, the constant rate drying period can be quickly switched to a falling-rate drying period, i.e., the falling-rate drying period can be relatively increased, thereby achieving excellent drying efficiency. Increasing the falling-rate drying period in this way allows the upper, middle, and lower layers of the superabsorbent polymer laminate to be dried to a uniform degree. Furthermore, drying using an initial downward airflow can minimize the occurrence of warping in the dried product.

[0144] FIG. 3 is a drying curve graph showing the moisture content as a function of drying time in the drying step of the superabsorbent polymer manufacturing method of the Examples and Comparative Examples in the experimental examples of the present invention described below. As can be seen from the graph, when the drying conditions of the present invention are met, the constant-rate drying section is switched to the falling-rate drying section relatively quickly, and the falling-rate drying section increases.

[0145] According to one embodiment of the invention, the fixed bed dryer used in the drying step (step 4) may be a belt dryer.

[0146] Specifically, the neutralized and atomized polymer to be dried is fed into a perforated plate of a fixed-bed dryer, and thus, downward wind means that the polymer placed on the perforated plate is fed from above to below, and upward wind means that the polymer is fed from below to above the perforated plate.

[0147] As described above, by performing the primary drying with downflow and the secondary drying with upflow, the resin laminate is prevented from warping and the hot air passes uniformly through the resin laminate, improving the drying efficiency and achieving a uniform drying degree for the upper, middle, and lower layers of the dried superabsorbent polymer laminate.

[0148] In particular, by carrying out the primary drying step using downward airflow for 40% to 60% of the total drying time, a uniform degree of drying can be achieved without under-drying of the superabsorbent polymer laminate. If the primary drying step using downward airflow is carried out for less than 40%, the upper layer may not be dried properly and the lower layer may warp. If it is carried out for more than 60%, the lower layer may not be dried properly and a uniform degree of drying cannot be achieved. Preferably, the primary drying step using downward airflow can be carried out for 40% to 50% of the total drying time.

[0149] The drying step (Step 4) may be carried out at an initial drying temperature of preferably 180°C to 230°C, followed by a temperature reduction to 100°C to 120°C, or more preferably at an initial drying temperature of 200°C to 220°C, followed by a temperature reduction to 100°C to 110°C. Drying at this temperature range is preferred in that the superabsorbent polymer laminate can be dried without warping, thereby improving drying efficiency and achieving uniform drying of the upper, middle, and lower layers of the dried superabsorbent polymer laminate. On the other hand, if the drying step is carried out under heated conditions, the drying time can be shortened, but it may be somewhat difficult to produce a dried product with a relatively high moisture content. Heat drying may prevent the middle layer of the superabsorbent polymer laminate from drying sufficiently to the desired degree, resulting in a slight decrease in the uniformity of the dried superabsorbent polymer laminate.

[0150] The duration of the drying step (Step 4) can be appropriately controlled depending on the desired drying amount and the capacity of the dryer used under the above-mentioned temperature and wind direction conditions.

[0151] However, assuming that the same amount of dried resin is formed to the same laminate height, excellent drying efficiency can be achieved under conditions that are relatively short compared to conventional methods, and in particular, the upper layer, middle layer, and lower layer of the resin laminate can all be dried to a uniform degree, and warping can also be effectively suppressed.

[0152] The average moisture content of the dried superabsorbent resin particle laminate obtained by the drying step (step 4) may be 4% by weight to 9% by weight, and preferably 4.5% by weight to 8.5% by weight. Here, the "average moisture content" of the laminate refers to the arithmetic average value of the moisture content of the resin in each layer measured by dividing the cross section of the laminate sample into three layers of uniform height, namely, the upper layer, the middle layer, and the lower layer, and the moisture content of the resin in each layer measured three times. The same measurement object was measured three times and the arithmetic average value was calculated.

[0153] The "moisture content" refers to the amount of water contained in the resin powder to be measured, and is calculated by subtracting the weight of the dry polymer from the weight of the resin powder. Specifically, the moisture content is calculated by measuring the weight loss due to evaporation of water from the resin powder during the drying process by increasing the temperature of the polymer using infrared heating. This will be explained in more detail in the experimental examples below.

[0154] Meanwhile, the standard deviation of the moisture content at each stacking position (top, middle, bottom) of the dried superabsorbent resin particle laminate obtained by the drying step (step 4) may be 0.1 to 1.5, and preferably 0.5 to 1.5.

[0155] In other words, by adjusting the drying conditions, the drying efficiency can be improved, and the upper, middle, and lower layers of the dried superabsorbent polymer laminate can all be dried to a uniform degree, and the occurrence of warping can also be effectively suppressed.

[0156] Stage 5: Crushing Stage Next, the laminate of the dried superabsorbent resin particles is pulverized to produce superabsorbent resin particles.

[0157] Specifically, the pulverization step may be performed by pulverizing the layered body of dried superabsorbent resin particles to have a particle size of normal particle level, that is, a particle size of 150 μm to 850 μm.

[0158] The pulverizer used for this purpose may be, specifically, a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, or a disc cutter, but is not limited to the above examples.

[0159] Alternatively, the crusher 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.

[0160] On the other hand, in the manufacturing method of the present invention, it is possible to realize superabsorbent resin particles with a smaller particle size distribution in the atomization step than in the conventional chopping step, and when fluidized (moving-type) drying is performed, the moisture content after drying is maintained relatively high at 10% by weight or more. Therefore, even if the 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 rate of fine powder generation can be significantly reduced.

[0161] The superabsorbent resin particles prepared as described above may contain 80% by weight or more, 85% by weight or more, 89% by weight or more, 90% by weight or more, 92% by weight or more, 93% by weight or more, 94% by weight or more, or 95% by weight or more of superabsorbent resin particles having a particle size of 150 μm to 850 μm, i.e., normal particles, based on the total weight. The particle size of such resin particles can be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP220.3 method.

[0162] Furthermore, the superabsorbent polymer particles may contain fine powder having a particle size of less than 150 μm in an amount of 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, based on the total weight of the superabsorbent polymer particles. This is in contrast to the fine powder content of more than about 20% by weight to about 30% by weight when superabsorbent polymer particles are produced by conventional manufacturing methods.

[0163] Additional Stages After the step of pulverizing the superabsorbent resin particles, the method may further include a step of classifying the pulverized superabsorbent resin particles according to particle size.

[0164] In addition, the method may further include a step of forming a surface cross-linked layer on at least a portion of the surface of the superabsorbent resin particles in the presence of a surface cross-linking agent after pulverizing and / or classifying the superabsorbent resin particles, whereby the cross-linked polymer contained in the superabsorbent resin particles is additionally cross-linked by the surface cross-linking agent, thereby forming a surface cross-linked layer on at least a portion of the surface of the superabsorbent resin particles.

[0165] The surface cross-linking agent may be any surface cross-linking agent that has been conventionally used in the production of superabsorbent resins, without any particular limitation. For example, the surface cross-linking agent 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 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.

[0166] Specifically, one or more, two or more, or three or more of the above-mentioned surface cross-linking agents can be used as the surface cross-linking agent. For example, ethylene carbonate-propylene carbonate (ECPC), propylene glycol, and / or glycerol carbonate can be used.

[0167] Such a surface cross-linking agent can be used in an amount of about 0.001 to about 5 parts by weight relative to 100 parts by weight of the superabsorbent resin particles. For example, the surface cross-linking agent can be used in an amount of 0.005 parts by weight or more, or 0.01 parts by weight or more, or 0.05 parts by weight or more and 5 parts by weight or less, or 4 parts by weight or less, or 3 parts by weight or less relative to 100 parts by weight of the superabsorbent resin particles. By adjusting the content range of the surface cross-linking agent within the above range, a superabsorbent resin exhibiting excellent water absorption properties can be produced.

[0168] In addition, the step of forming the surface cross-linked layer may be performed by adding an inorganic substance to the surface cross-linking agent, i.e., the surface of the superabsorbent resin particles may be additionally cross-linked in the presence of the surface cross-linking agent and the inorganic substance to form the surface cross-linked layer.

[0169] The inorganic substance may be one or more selected from the group consisting of silica, clay, alumina, silica-alumina composite, titania, zinc oxide, and aluminum sulfate. The inorganic substance may be used in powder or liquid form, particularly alumina powder, silica-alumina powder, titania powder, or nanosilica solution. The inorganic substance may be used in an amount of about 0.001 to about 1 part by weight per 100 parts by weight of the superabsorbent resin particles.

[0170] Furthermore, there is no limitation on the configuration of the method for mixing the surface cross-linking agent with the superabsorbent resin composition. For example, a method in which the surface cross-linking agent and the superabsorbent resin composition are mixed in a reaction tank, a method in which the surface cross-linking agent is sprayed onto the superabsorbent resin composition, a method in which the superabsorbent resin composition and the surface cross-linking agent are continuously supplied to a continuously operating mixer and mixed therein, etc. can be used.

[0171] When the surface cross-linking agent and the superabsorbent polymer composition are mixed, water and methanol may be added together. The addition of water and methanol has the advantage of enabling the surface cross-linking agent to be uniformly dispersed in the superabsorbent polymer composition. The amounts of water and methanol added can be appropriately adjusted to induce uniform dispersion of the surface cross-linking agent, prevent clumping of the superabsorbent polymer composition, and optimize the surface penetration depth of the cross-linking agent.

[0172] The surface cross-linking step may be carried out at a temperature of about 80°C to about 250°C. More specifically, the surface cross-linking step may be carried out at a temperature of about 100°C to about 220°C, or about 120°C to about 200°C, for about 20 minutes to about 2 hours, or about 40 minutes to about 80 minutes. When the above-mentioned conditions for the surface cross-linking step are satisfied, the surfaces of the superabsorbent resin particles are sufficiently cross-linked, thereby increasing the water absorption capacity under pressure.

[0173] The temperature raising means for the surface crosslinking reaction is not particularly limited. Heating can be achieved by supplying a heat medium or directly supplying a heat source. At this time, the type of heat medium that can be used may be a heated fluid such as steam, hot air, or hot oil, but is not limited thereto. The temperature of the heat medium to be supplied can be appropriately selected in consideration of the heat medium means, the temperature raising rate, and the target temperature. On the other hand, examples of a heat source that can be directly supplied include, but are not limited to, electric heating and gas heating methods.

[0174] According to one embodiment of the present invention, after the step of forming a surface cross-linked layer on at least a portion of the surface of the superabsorbent resin particles, the method may further include one or more of the following steps: a cooling step of cooling the superabsorbent resin particles having the surface cross-linked layer formed thereon, a hydration step of adding water to the superabsorbent resin particles having the surface cross-linked layer formed thereon, and a post-treatment step of adding an additive to the superabsorbent resin particles having the surface cross-linked layer formed thereon. In this case, the cooling step, the hydration step, and the post-treatment step may be performed sequentially or simultaneously.

[0175] The additives added in the post-treatment step may be, but are not limited to, a liquid permeability improver, an anti-caking agent, a flowability improver, an antioxidant, and the like.

[0176] By selectively carrying out the cooling step, the hydration step, and the post-treatment step, the moisture content of the final superabsorbent polymer can be improved, and a higher quality superabsorbent polymer product can be produced.

[0177] (Super absorbent resin) On the other hand, according to one embodiment of the present invention, a highly water-absorbent resin can be provided by the above-mentioned manufacturing method.

[0178] The highly water-absorbent resin has excellent properties and excellent drying efficiency, which effectively prevents warping of the outer shell, thereby improving the quality of the final product.

[0179] The superabsorbent resin may have a particle size of 150 to 850 μm. More specifically, at least 95% by weight of the base resin powder and the superabsorbent resin containing the base resin powder may have a particle size of 150 to 850 μm, and may contain 50% by weight or more of particles having a particle size of 300 to 600 μm, with less than 3% by weight of fine powder having a particle size of less than 150 μm.

[0180] The functions and effects of the present invention will be described in more detail below through specific examples of the present invention, but these examples are presented only as examples of the present invention and do not define the scope of the invention.

[0181] <Example> Example 1 (Step 1: Polymer production step) In a 10L glass vessel equipped with a stirrer and thermometer, 1500g of acrylic acid, 5.0g of pentaerythritol triallyl ether as an internal crosslinker, and 3406g of water were mixed and stirred while maintaining the temperature at 5°C. Nitrogen was introduced into the glass vessel containing the mixture at 1000cc / min for 1 hour to flush the atmosphere with nitrogen. Next, 20.0g of 0.3% aqueous hydrogen peroxide solution, 22.5g of 1% aqueous ascorbic acid solution, and 45.0g of 2% aqueous 2,2'-azobis-(2-amidinopropane)dihydrochloric acid solution were added as polymerization initiators, and 22.5g of 0.01% aqueous iron sulfate solution was added as a reducing agent to initiate polymerization. After the mixture reached a temperature of 85°C, the mixture was polymerized at 90±2°C for approximately 6 hours to obtain a polymer.

[0182] (Steps 2 and 3: Neutralization and Atomization Steps) 5000 g of the polymer was added to a Micronizer (F200, Karl Schnell) equipped with a perforated plate containing 10 mm holes, rotating at 1500 rpm, to atomize it into primary particles with diameters of several tens to several hundreds of micrometers. 299 g of a 0.45 wt% aqueous solution of glycerol monolaurate (GML) was added to prevent excessive aggregation.

[0183] The micronized polymer was then added to a meat chopper, a screw-type chopper equipped with a perforated plate containing multiple 6mm holes, while rotating at 500 rpm to produce secondary agglomerated particles. This process was repeated three times. During the first pass, 1,252 g of 50% NaOH aqueous solution was added to neutralize some of the acidic groups of the polymer. During the second pass, 157.8 g of 10% Na2SO4 aqueous solution was added to neutralize some of the acidic groups of the polymer. During the third pass, the mixture was passed without adding any additives to produce hydrated superabsorbent resin particles.

[0184] (Step 4: Drying step) 1,472 g of the hydrated superabsorbent resin particles were placed in a belt dryer equipped with a 200 mm wide, 200 mm long, and 1 mm perforated plate capable of vertical airflow transfer. The hydrated superabsorbent resin particles were positioned above the perforated plate, and a downward airflow from above the perforated plate was first introduced for 20 minutes, followed by an upward airflow from below the perforated plate for 30 minutes. Simultaneously, the temperature of the hot air from the dryer was maintained at 200°C for 5 minutes, then reduced to 100°C and maintained for 45 minutes, yielding a laminate of dried superabsorbent resin particles.

[0185] FIG. 3 shows a drying curve graph obtained by measuring the moisture content as a function of the drying time in the drying step.

[0186] (Step 5: Crushing and Classification Step) The layered body of the dried highly water-absorbent resin particles was pulverized using a two-stage roll mill (GRAN-U-LIZER™, MPE) to give particles having a particle size of 150 μm to 850 μm.

[0187] The pulverized product was subjected to a classification sieve to selectively collect only highly water-absorbent resin particles having particle sizes of 150 μm to 850 μm.

[0188] (Step 6: Surface cross-linking step) Next, 3 g of water, 4 g of methanol, 0.1 g of ethylene glycol diglycidyl ether (EJ-1030S), 0.1 g of propylene glycol, and 0.2 g of aluminum sulfate were added to 100 g of the obtained superabsorbent resin particles to prepare a surface cross-linking liquid, which was mixed for 1 minute, and the surface cross-linking reaction was carried out at 140°C for 40 minutes to obtain a surface cross-linked superabsorbent resin.

[0189] Examples 2 to 6 and Comparative Examples 1 to 6 A superabsorbent resin was obtained in the same manner as in Example 1, except that the process conditions for the drying step were changed as shown in Table 1 below.

[0190] For Comparative Example 2, a drying curve graph showing the moisture content as a function of the drying time in the drying step is shown in FIG.

[0191] In addition, in the case of Comparative Examples 2 to 6, the moisture content was high after the drying step, making it difficult to pulverize into particles, and the subsequent steps were not carried out.

[0192] [Table 1]

[0193] <Experimental Example> Experimental example 1: Drying characteristics evaluation (1) Measurement of moisture content before drying In the examples and comparative examples, a sample was obtained from the hydrated superabsorbent polymer obtained in step 3 before the drying step, and its initial weight H0 (g) was measured after maintaining it at 180°C for 40 minutes by infrared heating, and the weight H1 (g) was measured. The moisture content was calculated using the following formula, and the results are shown in Table 2.

[0194] [Formula 1] Moisture content (wt%)={[H0(g)-H1(g)] / H0(g)}×100

[0195] (2) Measurement of moisture content after drying In the Examples and Comparative Examples, samples were taken by cutting a plate-shaped dried laminate of dried superabsorbent resin particles obtained in Step 4 (before surface cross-linking) after the drying process into 10 x 10 cm pieces. The sample was divided into three equal parts based on the cross section (based on the height of the laminate), and each was divided into an upper layer, a middle layer, and a lower layer. Samples were taken from the superabsorbent resin in each layer, and the moisture content of each layer was measured in the same manner as described above. The standard deviation between these samples was calculated, and the results are shown in Table 2.

[0196] (3) Degree of warping In the examples and comparative examples, the dried superabsorbent resin particles obtained in step 4 (before surface cross-linking) after the drying process were cut into a 10 x 10 cm plate-shaped dried laminate to obtain samples. The degree of warping of the cross section of each sample was evaluated according to the following evaluation criteria, and the results are shown in Table 2 below.

[0197] Furthermore, cross-sectional photographs of the dried product after the drying step are shown in Figure 2. Figure 2(a) is a cross-sectional photograph of the dried laminate after the drying step in Example 1, and Figure 2(b) is a cross-sectional photograph of the dried laminate after the drying step in Comparative Example 4.

[0198] <Evaluation criteria for warpage> ○: When the outer part of the dried product has risen by 10% or more compared to the thickness of the center part. △: When the outer part of the dried product has risen by 5% to 10% of the thickness of the center part. X: When the outer shell of the dried product has risen by less than 5% compared to the thickness of the center of the dried product.

[0199] [Table 2]

[0200] In Examples 1 to 6, which satisfied the drying conditions of the present invention, it was confirmed that the drying efficiency was excellent and the properties of the final superabsorbent resin were also excellent.

[0201] In Comparative Example 1, the upward wind was minimized during the initial drying period, but some warping of the dried product occurred. Therefore, drying was performed at a lower temperature, which resulted in a decrease in drying efficiency, and it was confirmed that there was a slight difference in the moisture content of each layer in the dried product.

[0202] In addition, in the case of Comparative Example 2, the upward wind was minimized in the early stages of drying, and the dried product did not warp. However, although the upper and middle layers had a low moisture content, they were not dried sufficiently, and the lower layer was not dried at all, resulting in a significant difference in moisture content between the layers of the dried product.

[0203] Experimental Example 2: Evaluation of the physical properties of superabsorbent resin In the above examples and comparative examples, the superabsorbent resin obtained in step 6 after the final surface cross-linking process was evaluated for physical properties by the following methods, and the properties are shown in Table 3 below.

[0204] Unless otherwise specified, all of the following physical property evaluations were carried out at constant temperature and humidity (23±1°C, relative humidity 50±10%), and saline or salt water refers to a 0.9 wt% aqueous sodium chloride (NaCl) solution.

[0205] [Table 3]

[0206] In Examples 1 to 6, which satisfied the drying conditions of the present invention, it was confirmed that the drying efficiency was excellent and the properties of the final superabsorbent resin were also excellent.

[0207] In the case of Comparative Examples 2 to 6, the moisture content was high after drying, making it difficult to pulverize into particles, and therefore additional steps were not performed, and no additional experiments were carried out.

Claims

1. Step 1: carrying out polymerization on a monomer composition including a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized; neutralizing at least a portion of the acidic groups of the polymer (Step 2); atomizing the polymer in the presence of a surfactant (Step 3); drying the neutralized and atomized polymer in a fixed-bed dryer to obtain a laminate of dried superabsorbent polymer particles (Step 4); and and (5) grinding the laminate of the dried superabsorbent resin particles to produce superabsorbent resin particles. The step 4 includes a step of primary drying the neutralized and atomized polymer by introducing a downflow and a step of secondary drying by introducing an upflow, The implementation time of the primary drying step is 40 to 60% of the total implementation time of step 4; A method for producing superabsorbent resin.

2. The step 4 is carried out at an initial drying temperature of 180°C to 230°C, followed by reducing the temperature to 100°C to 120°C. A method for producing the highly water-absorbent resin according to claim 1.

3. The average moisture content of the dried superabsorbent resin particle laminate obtained by performing step 4 is 4% by weight to 9% by weight. A method for producing the highly water-absorbent resin according to claim 1.

4. The standard deviation of the moisture content of the dried superabsorbent resin particle laminate obtained by performing step 4 is 0.1 to 1.5 at each layering position. A method for producing the highly water-absorbent resin according to claim 1.

5. The step of forming the polymer (step 1) is carried out in a batch type reactor. A method for producing the highly water-absorbent resin according to claim 1.

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

7. The atomization step (Step 3) is performed by an atomization device, which comprises: a body portion including a transfer space into which the polymer is transferred; a screw member rotatably disposed within the transfer space for moving the polymer; a drive motor for providing rotational drive to said screw member; a cutter member provided in the body portion for pulverizing the polymer; and The polymer pulverized by the cutter member is discharged to the outside of the body portion, and the perforated plate has a plurality of holes formed therein. A method for producing the highly water-absorbent resin according to claim 1.

8. At least a portion of the surfactant of step 3 is present on the surface of the polymer. A method for producing the highly water-absorbent resin according to claim 1.

9. 2. The method for producing a superabsorbent polymer according to claim 1, wherein the surfactant in step 3 comprises a compound represented by the following Formula 2 or a salt thereof: 【Chemistry 1】 In the above Chemical Formula 2, A １ , A ２ and A ３ are each independently a single bond, a carbonyl, 【Chemistry 2】 wherein one or more of these is a carbonyl or 【Transformation 3】 wherein m1, m2, and m3 each independently represent an integer from 1 to 8; 【Chemistry 4】 are each linked to adjacent oxygen atoms, 【Transformation 5】 is the adjacent R １ , R ２ and R ３ are connected to R １ , R ２ and R ３ are each independently hydrogen, a linear or branched alkyl having 6 to 18 carbon atoms, or a linear or branched alkenyl having 6 to 18 carbon atoms; n is an integer from 1 to 9.

10. The method further comprises the step of classifying the superabsorbent resin particles according to particle size. A method for producing the highly water-absorbent resin according to claim 1.

11. The method further comprises forming a surface cross-linked layer on at least a portion of the surface of the superabsorbent resin particles. A method for producing the highly water-absorbent resin according to claim 1 or 10.

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