Highly water-absorbent resin and method for producing same

Abstract

The present invention relates to a superabsorbent resin and a method for producing the same, and more particularly to a superabsorbent resin exhibiting excellent initial water absorption capacity and saturated water absorption capacity and a method for producing the same.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to Korean Patent Application No. 10-2022-0176831, filed December 16, 2022, Korean Patent Application No. 10-2022-0177310, filed December 16, 2022, and Korean Patent Application No. 10-2023-0183051, filed December 15, 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 superabsorbent resin and a method for producing the same, and more particularly to a superabsorbent resin exhibiting excellent initial water absorption capacity and saturated water absorption capacity and a method for producing the same. [Background technology]

[0003] Super absorbent polymer (SAP) is a synthetic polymeric substance that has the ability to absorb 500 to 1,000 times its own weight in water, and each developer has a different name for it, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Such super absorbent polymers first came into practical use as sanitary products, and are now widely used as soil rehydration agents for gardening, water-stopping materials for civil engineering and construction, seedling sheets, freshness-preserving agents in the food distribution field, and materials for packs.

[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 going a step further, the development of so-called pulpless diapers, which do not use pulp at all, has been actively promoted.

[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 resin is contained at a relatively high ratio, and the superabsorbent resin particles are inevitably contained in multiple layers in the sanitary material. In order for the overall superabsorbent resin particles contained in multiple layers to more efficiently absorb a large amount of liquid such as urine, the superabsorbent resin basically needs to exhibit not only high water absorption performance but also fast water absorption speed. Meanwhile, the most common method for improving such water absorption properties is to form a porous structure inside the superabsorbent resin to increase the surface area of ​​the superabsorbent resin, and 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 to form a porous structure in the base resin powder as crosslinking polymerization proceeds.

[0006] 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. For this reason, there is a continuing demand for the development of a technology that can improve the water absorption properties of a superabsorbent resin without using a foaming agent.

[0007] For this reason, in order to fundamentally solve these problems, there is a continuing demand for the development of a technology capable of producing a highly water-absorbent resin without generating fine powder. Summary of the Invention [Problem to be solved by the invention]

[0008] In order to solve such problems of the conventional technology, the present invention provides a superabsorbent resin that can realize excellent quality when the resin is applied to an actual product by adjusting the initial water absorption capacity and the saturated water absorption capacity, which are new parameters, to a specific ratio. [Means for solving the problem]

[0009] In order to solve the above problems, according to one embodiment of the present invention, A base resin powder comprising a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent; and the crosslinked polymer is additionally crosslinked via a surface crosslinking agent to form a surface crosslinked layer on the base resin powder, The initial water absorption capacity is 70 g / g or more, The saturated water absorption capacity is 430g / g or more. A superabsorbent polymer is provided.

[0010] According to another embodiment of the invention, A method for preparing the above-mentioned superabsorbent resin is provided, which specifically includes the following steps:

[0011] 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; ultrafinely grinding the polymer at a rotation speed of 500 rpm to 4,000 rpm; neutralizing at least a portion of the acidic groups of the polymer; drying and grinding the micronized and neutralized polymer to produce base resin particles; and forming a surface cross-linked layer on at least a portion of the surface of the base resin powder; Effect of the Invention

[0012] According to the superabsorbent resin of the present invention, by adjusting the initial water absorption capacity and the saturated water absorption capacity, which are new parameters, to a specific ratio, it is possible to provide a superabsorbent resin that can realize excellent quality when the resin is applied to an actual product.

[0013] In particular, when applied to sanitary materials such as diapers, it is possible to absorb discharged body fluids at a high speed and also to absorb a relatively large amount, thereby preventing problems such as the accumulation of body fluids inside the sanitary material or leakage to the outside.

[0014] That is, it is possible to provide a highly water-absorbent resin that, when applied to a product, can rapidly absorb body fluids and retain a large amount of body fluid without leaking out. [Brief description of the drawings]

[0015] [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

[0016] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present 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 possibility of the presence or addition of one or more other features, steps, components, or combinations thereof.

[0017] Although the present invention can be modified in various ways and has various forms, specific embodiments are exemplified and described in detail below. However, it is not intended to limit the present invention to the specific disclosed embodiments, and it should be understood that the present invention includes all modifications, equivalents, and alternatives falling within the spirit and technical scope of the present invention.

[0018] The terminology used herein is for the purpose of referring to particular embodiments only and is not intended to be limiting of the invention, and as used herein the singular forms "a," "an," and "the" also include the plural forms unless the language clearly dictates to the contrary.

[0019] 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.

[0020] In addition, the term "super absorbent polymer" refers to a crosslinked polymer or a powder-like base resin 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.

[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" is also used to distinguish between cutting the hydrogel polymer into small pieces on the order of millimeters to increase drying efficiency and grinding to micrometer or regular particle levels.

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

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

[0025] I. Super absorbent resin The hydrous gel polymer obtained by the polymerization reaction of acrylic acid monomers is commercially available as a powdered product, a superabsorbent resin, after undergoing 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 included in the monomer composition to form a porous structure in the base resin powder as the crosslinking polymerization progresses.

[0027] However, in conventional methods, it is difficult to form a sufficient surface area, and as a result, bodily fluids that cannot be absorbed during the time when urination actually occurs flow inside the sanitary material or leak to the outside, causing inconvenience to the user.

[0028] In order to solve the problems of the conventional techniques, the present inventors have confirmed that excellent quality can be achieved when the resin is applied to an actual product by adjusting the initial water absorption capacity and the saturated water absorption capacity, which are new parameters, to a specific ratio, and have completed the present invention.

[0029] According to one embodiment of the invention, A base resin powder comprising a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent; and the crosslinked polymer is additionally crosslinked via a surface crosslinking agent to form a surface crosslinked layer on the base resin powder, The initial water absorption capacity is 70 g / g or more, The saturated water absorption capacity is 430g / g or more. A superabsorbent polymer is provided.

[0030] The parameter of the initial absorbency rate (IAR) is a factor that influences the formation of a flow path for bodily fluids in the early stages when the resin is applied to sanitary materials such as diapers, and is an important factor for preventing problems such as discharged bodily fluids accumulating inside the sanitary materials or leaking out to the outside. Here, the initial stage refers to about 20 seconds after the discharged bodily fluids come into contact with the resin, and by forming a flow path for bodily fluids in the early stages, it is possible to realize excellent absorption efficiency for a long time thereafter, and the initial rapid absorption contributes to improving the wearing comfort of the product for the user.

[0031] By satisfying the initial water absorption capacity of 70 g / g or more, problems such as discharged body fluids accumulating inside the sanitary material or leaking out to the outside can be prevented. In particular, by simultaneously controlling the parameters of the initial water absorption capacity and the saturated water absorption capacity described later to satisfy specific ranges, it is possible to realize more excellent water absorption properties without deteriorating other physical properties.

[0032] The initial water absorption rate can be calculated by the following Equation 1.

[0033] [Formula 1] Initial water absorption capacity (g / g)={[W2(g)-W1(g)] / W0(g)}-1

[0034] In the above formula, W0(g) is the mass of the sample to be measured, W2 (g) is the mass of the bag measured after placing the sample to be measured in a nonwoven bag and immersing the bag in distilled water with a conductivity of 1 mS / cm or less for 20 seconds, W1 (g) is the mass of a nonwoven bag not containing a sample to be measured after immersing the bag in distilled water with a conductivity of 1 mS / cm or less for 20 seconds.

[0035] The method for measuring the initial water absorption capacity will be described in more detail in the experimental examples section below.

[0036] If the initial water absorption capacity is less than 70 g / g, the discharged body fluid may not be sufficiently absorbed, and may accumulate inside the sanitary material or flow out to the outside. Preferably, the initial water absorption capacity may be 70 g / g to 100 g / g or 75 g / g to 95 g / g.

[0037] The parameter of the free swell rate (FSR) means the maximum amount of excreted body fluid that can be retained when a resin is applied to sanitary products such as diapers, and is an important factor that determines the degree to which body fluid flows out and the wearing comfort of the product.

[0038] By satisfying the saturated water absorption capacity of 430 g / g or more, the excellent water absorption capacity prevents problems such as the discharged body fluid accumulating inside the sanitary material or leaking out. In particular, by simultaneously controlling the parameters of the saturated water absorption capacity and the initial water absorption capacity to satisfy specific ranges, it is possible to realize excellent water absorption properties without deteriorating other physical properties.

[0039] The saturated water absorption capacity can be calculated by the following formula 2.

[0040] [Formula 2] Saturated water absorption capacity (g / g)={[W4(g)-W3(g)] / W0(g)}-1

[0041] In the above formula, W0(g) is the mass of the sample to be measured, W4(g) is the mass of the bag measured after placing the sample to be measured in a nonwoven bag and immersing the bag in distilled water with a conductivity of 1 mS / cm or less for 30 minutes, W3 (g) is the mass of a nonwoven bag not containing a sample to be measured after it is immersed in distilled water with a conductivity of 1 mS / cm or less for 30 minutes.

[0042] The method for measuring the saturated water absorption capacity will be described in more detail in the experimental examples described later.

[0043] If the saturated water absorbency is less than 430 g / g, bodily fluids may leak out, significantly reducing the comfort of the wearer and requiring frequent replacement. Preferably, the saturated water absorbency is 430 g / g to 530 g / g, or 450 g / g to 500 g / g.

[0044] The parameters of the initial water absorption rate and the saturated water absorption capacity can be realized by adjusting the components / contents of the superabsorbent resin, the manufacturing process conditions of the superabsorbent resin, etc. More specifically, the superabsorbent resin is manufactured by a new process including an ultrafine pulverization process, which is not a conventional manufacturing method for the superabsorbent resin, and is suitable for controlling the initial water absorption rate and the saturated water absorption capacity within a specific range, which will be described in more detail in Section II. of the manufacturing method for the superabsorbent resin.

[0045] Each component of the highly water-absorbent resin will now be described in more detail.

[0046] The superabsorbent resin according to one embodiment of the present invention includes a base resin including a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent. The crosslinked polymer is preferably formed by polymerizing a monomer composition including components such as a monomer, an internal crosslinking agent, and a polymerization initiator.

[0047] Here, 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:

[0048] [Chemical formula 1] R-COOM'

[0049] 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.

[0050] 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.

[0051] 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 highly water-absorbent 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.

[0052] The water-soluble ethylenically unsaturated monomer has an acidic group. Meanwhile, in the production of the 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, but in the present invention, preferably, the acidic group is not neutralized during polymerization, but can be neutralized after the polymer is formed. More details on this will be described in the section on the production method of the superabsorbent resin.

[0053] The concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition can be appropriately adjusted in consideration of the polymerization time and reaction conditions, and is preferably about 20 to about 60% by weight, or about 20 to about 40% by weight.

[0054] 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 of introducing a crosslinking bond between the unsaturated bonds of the water-soluble ethylenically unsaturated monomer described above to form a polymer containing a crosslinked structure.

[0055] 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 new crosslinked structure by the surface crosslinking agent, and the inside of the superabsorbent resin particles may maintain the structure crosslinked by the internal crosslinking agent.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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 bond can be more stably maintained even during the neutralization process after the polymerization reaction described below.

[0061] 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.

[0062] 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.

[0063] In the monomer composition, such an 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 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 capacity. In particular, the above range is suitable for achieving the initial water absorption capacity and saturated water absorption capacity of the present invention within the desired range.

[0064] 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 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.

[0065] The polymer is obtained by polymerizing a monomer and an internal crosslinking agent in the presence of a polymerization initiator. The type of the polymerization initiator is not particularly limited. Preferably, the polymerization is carried out using a thermal polymerization method in a batch reactor, and thus a thermal polymerization initiator can be used as the polymerization initiator.

[0066] The thermal polymerization initiator may be one or more selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid. Specifically, examples of persulfate initiators include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), and ammonium persulfate ((NH4)2S2O8). Examples of azo initiators 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 diverse thermal polymerization initiators are described in detail in Principle of Polymerization by Odian (Wiley, 1981), p. 203, and are not limited to the above-mentioned examples.

[0067] 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 may be deteriorated, such as the water absorption capacity under pressure being reduced, which is not preferable.

[0068] Meanwhile, in one embodiment of the present invention, the polymerization can be initiated by adding the above-mentioned polymerization initiator and a reducing agent forming a redox couple to the monomer composition.

[0069] Specifically, the initiator and the reducing agent react with each other to form radicals when added to the polymer solution.

[0070] 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, and there is no need to increase the process temperature, making low-temperature polymerization possible, and the change in the physical properties of the polymer solution can be minimized.

[0071] The polymerization reaction using the oxidation-reduction reaction can be smoothly carried out at a temperature near 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] The monomer composition may further contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant, if necessary.

[0076] The monomer composition containing 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, that is, 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.

[0077] 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.

[0078] The polymer obtained by such a method can form a polymer having a high molecular weight and a uniform molecular weight distribution by polymerizing an unneutralized ethylenically unsaturated monomer, and the content of water-soluble components is reduced, so that the polymer is suitable for realizing the intended saturated water absorption capacity and initial water absorption capacity within an appropriate range.

[0079] 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.

[0080] 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, the pressure applied in the subsequent crushing step may increase, and therefore the polymer may be difficult to crush to a desired particle size.

[0081] Meanwhile, in the present specification, the term "moisture content" refers to the amount of moisture in the total polymer weight, which 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 by increasing the temperature of the crumb-state polymer by infrared heating. Here, 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.

[0082] The superabsorbent resin according to one embodiment of the present invention includes a base resin powder including a crosslinked polymer of the water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent; and a surface crosslinked layer formed on the base resin powder by additionally crosslinking the crosslinked polymer via a surface crosslinking agent.

[0083] The surface cross-linked layer may be formed on at least a part of the surface of the base resin powder, and may be formed by additional cross-linking of a cross-linked polymer contained in the base resin powder via a surface cross-linking agent.

[0084] As the surface cross-linking agent, surface cross-linking agents that have been used in the manufacture of conventional superabsorbent resins can be used without any particular restrictions.For example, the surface cross-linking agent can include one or more polyols selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol and glycerol; one or more carbonate-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; etc.

[0085] 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.

[0086] 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 to the above-mentioned range, a superabsorbent resin exhibiting excellent water absorption properties can be produced. In particular, the above-mentioned range is suitable for realizing the initial water absorption capacity and saturated water absorption capacity of the present invention within the target range.

[0087] The surface cross-linked layer may be formed by adding an inorganic substance to the surface cross-linking agent. That is, the surface of the base resin powder is additionally cross-linked in the presence of the surface cross-linking agent and the inorganic substance to form a surface cross-linked layer.

[0088] As such an inorganic substance, one or more inorganic substances selected from the group consisting of silica, clay, alumina, silica-alumina composite, titania, zinc oxide, and aluminum sulfate can be used. The inorganic substance can be used in powder form or liquid form, and can be used in particular in the form of alumina powder, silica-alumina powder, titania powder, or nanosilica solution. The inorganic substance can 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.

[0089] As described above, the superabsorbent resin including the base resin powder and the surface cross-linked layer formed on the base resin powder can absorb discharged body fluids at a high speed when applied to sanitary materials such as diapers by adjusting the initial water absorption capacity and the saturated water absorption capacity, which are the novel parameters of the present application, within specific ranges, and can also absorb a relatively large amount at the beginning, thereby preventing problems such as accumulation of body fluids inside the sanitary materials or leakage to the outside.

[0090] According to one embodiment of the invention, the superabsorbent polymer may have a water retention capacity (CRC) measured by EDANA method WSP241.3 of 35 g / g to 45 g / g, preferably 36 g / g or more or 36.5 g / g or more, or 43 g / g or less, 40 g / g or less, or 38 g / g or less. A specific method for measuring the water retention capacity will be described in more detail in the experimental examples described later.

[0091] According to one embodiment of the invention, the superabsorbent polymer may have a water absorbency under pressure (AUP) of 32 g / g to 40 g / g, preferably 32.5 g / g or more, 33 g / g or more, or 38 g / g or less, 37 g / g or less, or 35 g / g or less, as measured at 0.3 psi according to EDANA method WSP242.3. A specific method for measuring the water absorbency under pressure will be described in more detail in the experimental examples described later.

[0092] According to one embodiment of the invention, the superabsorbent polymer may have a Vortex water absorption rate of 25 seconds or less as measured by a Vortex method. The smaller the water absorption rate, the better, and the lower limit of the water absorption rate 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. A specific method for measuring the water absorption rate will be described in more detail in the experimental examples described below.

[0093] II. Method for producing superabsorbent resin 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, due to the stickiness of the hydrogel polymer in this chopping process, the hydrogel polymer cannot be pulverized to the micro-sized particle level and becomes an aggregated gel. When such an aggregated gel-like hydrogel polymer 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 this process.

[0094] Specifically, conventional superabsorbent resins have been produced through the following steps:

[0095] (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;

[0096] 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 has a plate shape rather than a granular shape, the classification step after pulverization has been performed by classifying the coarsely pulverized 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 the particles again after fine pulverization. Since the amount of fine powder separated at the final classification step in this manufacturing method is large, about 20% by weight to about 30% by weight based on 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 into the chopping step or the step before drying.

[0097] 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 polymer.

[0098] As a result of repeated research to solve this problem, it was found that, unlike the conventional superabsorbent resin manufacturing method, polymerization is first performed in a state where the acidic groups of the water-soluble ethylenically unsaturated monomer are neutralized, and a polymer is formed by first performing polymerization in a state where the acidic groups are not neutralized, and then the hydrogel polymer is ultrafinely ground by applying a high mechanical shear force, and then the acidic groups of the polymer are neutralized, or the acidic groups of the polymer are neutralized to form a hydrogel polymer, and then the hydrogel polymer is ultrafinely ground, or the acidic groups present in the polymer are neutralized simultaneously with ultrafine grinding, to form aggregated hydrogel particles having micropores. The hydrogel polymer manufactured by this process is manufactured into particles having stable micropores of 100 μm or less, and as the grinding and drying processes are subsequently performed under milder conditions, the amount of fine powder generated during the process can be significantly reduced. In addition, the ultrafine grinding process using the high-intensity mechanical shearing force can form micropores in the hydrogel polymer without using a separate foaming agent in the polymerization step, thereby significantly improving the water absorption rate. As a result, it was confirmed that the initial water absorption capacity and saturated water absorption capacity, which are the above-mentioned new parameters of the present invention, can be easily controlled within the desired range.

[0099] Meanwhile, the hydrogel ultrafine pulverization step may be preferably carried out in the presence of a surfactant. By using a surfactant in the ultrafine pulverization step, particle aggregation can be effectively controlled, thereby reducing the load on the equipment and further improving productivity.

[0100] In addition, if polymerization is first performed 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.

[0101] The water-soluble component has a property of being easily dissolved when the superabsorbent resin comes into contact with a liquid, so when the content of the water-soluble component is high, most of the dissolved water-soluble component remains on the surface of the superabsorbent resin, making the superabsorbent resin sticky and reducing the liquid permeability. Therefore, in terms of liquid permeability, it is important to maintain the content of the water-soluble component low.

[0102] According to one embodiment of the present invention, it has been confirmed that by carrying out polymerization in a non-neutralized state, the content of water-soluble components is reduced, and the liquid permeability of the superabsorbent resin can be improved, thereby making it easier to control the saturated water absorption capacity within a desired range.

[0103] In addition, the superabsorbent resin produced according to one embodiment of the present invention may have a uniform particle size distribution, and therefore, a superabsorbent resin having excellent water absorption properties such as water retention capacity and pressure water absorption capacity, rewet characteristics, and particularly excellent initial water absorption capacity may be provided.

[0104] Hereinafter, each step of the method for producing a highly water-absorbent resin according to one embodiment will be described in detail.

[0105] Phase 1: Polymerization Phase First, a monomer composition including a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent is polymerized to produce a base resin powder including a polymer in which the water-soluble ethylenically unsaturated monomer having an acidic group and the internal crosslinking agent are crosslinked and polymerized.

[0106] The step may include a step of 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 a step of polymerizing the monomer composition to form a polymer.

[0107] Here, the contents of each component described in the above item I. regarding the superabsorbent polymer are all equally applicable.

[0108] Meanwhile, 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, the internal cross-linking agent, the polymerization initiator, and the neutralizing agent.

[0109] 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.

[0110] 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.

[0111] 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 does not precipitate even at a low temperature, so it is advantageous for long-term polymerization at low temperatures. As a result, a polymer having a higher molecular weight and a uniform molecular weight distribution can be stably formed by performing long-term polymerization using the water-soluble ethylenically unsaturated monomer in which the acid group is not neutralized.

[0112] In addition, it is possible to form a polymer having a longer chain, and it is possible to achieve the effect of reducing the content of water-soluble components that exist in a non-crosslinked state due to incomplete polymerization or crosslinking, and this makes it suitable for realizing the saturated water absorption capacity, which is the novel parameter of the present invention described above, within a desired range.

[0113] Furthermore, when polymerization is first carried out in a state where the acidic groups of the monomers are not neutralized, to form a polymer, and then the polymer is neutralized and then micronized in the presence of a surfactant, 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.

[0114] 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.

[0115] In a typical method for producing a superabsorbent resin, the polymerization method is largely divided into thermal polymerization and photopolymerization depending on the polymerization energy source. In general, 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 vessel with a flat bottom.

[0116] 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 together, which makes it difficult to perform uniform polymerization throughout the monomer composition, and may result in deterioration of overall physical properties.

[0117] However, according to one embodiment of the present invention, as the polymerization proceeds in a batch reactor in a fixed bed mode, there is little risk of polymers with different polymerization rates being mixed together, and therefore a polymer with uniform quality can be obtained.

[0118] 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 a continuous polymerization is carried out in a reactor equipped with a conveyor belt, for example, 1 hour or more, 3 hours or more, or 6 hours or more. Despite such a long polymerization reaction time, since the polymerization is carried out on unneutralized water-soluble ethylenically unsaturated monomer, the monomer does not precipitate well even when the polymerization is carried out for a long time, and therefore it is advantageous for long-term polymerization.

[0119] On the other hand, the polymerization in the batch reactor of the present invention utilizes a thermal polymerization method, and the polymerization initiator is a thermal polymerization initiator, the components of which are as described above.

[0120] 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-intensity mechanical shear force is applied, micropores of 100 μm or less are easily formed in the polymer, which increases the surface roughness, 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 generation of fine powder due to the pores in the subsequent process can be significantly reduced. The highly water-absorbent resin particles produced in this step have a significantly increased surface area and can significantly improve the water absorption rate, and are particularly suitable for achieving the new parameters of initial water absorption capacity and saturated water absorption capacity within the desired range.

[0121] FIG. 2 is a SEM image of the polymer after the ultrafine grinding process in step 2 in the method for producing a superabsorbent resin according to one embodiment of the present invention. It can be seen that micropores are uniformly formed inside and outside the polymer.

[0122] 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 deteriorate the various physical properties of the produced superabsorbent resin. Preferably, the ultrafine pulverization process may 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.

[0123] According to one embodiment of the present invention, the ultrafine grinding step is carried out by applying high intensity mechanical shear forces, and for this reason, the step is carried out by a high shear grinding device.

[0124] 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 plurality of holes formed therein and discharging the polymer ground by the cutter member to the outside of the body portion.

[0125] 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.

[0126] The size of the holes provided in the perforated plate of the high shear crushing device may be 1 mm to 25 mm, or 5 mm to 20 mm, or 5 mm to 15 mm.

[0127] 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 steps 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.

[0128] According to one embodiment of the present invention, the step of ultrafine-pulverizing the polymer under specific conditions may be carried out one or more times, preferably 1 to 6 times, 1 to 4 times, or 1 to 3 times, which may be carried out using multiple grinding devices, or a single grinding device including multiple perforated plates and / or multiple cutter members, or some of the multiple grinding devices including multiple perforated plates and / or multiple cutter members.

[0129] According to one embodiment of the present invention, a surfactant may be added in the ultrafine grinding step, which effectively controls the aggregation between polymer particles and reduces the load on the equipment used in the grinding process, thereby further improving productivity.

[0130] Preferably, 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:

[0131] [ka]

[0132] In the above Chemical Formula 2, A1, A2 and A3 each independently represent a single bond, a carbonyl,

[0133] [ka]

[0134] where one or more of these is a carbonyl or

[0135] [ka]

[0136] wherein m1, m2, and m3 each independently represent an integer from 1 to 8;

[0137] [ka]

[0138] are each linked to adjacent oxygen atoms,

[0139] [ka]

[0140] are linked 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.

[0141] The surfactant is added so that it is mixed with the polymer and the micronizing step can be easily carried out without agglomeration phenomena.

[0142] 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, which makes it suitable for realizing the desired aggregation control effect. On the other hand, in the case of an anionic surfactant that is not a nonionic surfactant, when it is mixed with a polymer neutralized with a neutralizing agent such as NaOH or Na2SO4, the ionized Na is attached to the carboxyl group substituent of the polymer. + When 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.

[0143] 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 (When n is hydrogen, n=1 to 3) is further included, and 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. This makes it possible to effectively suppress the aggregation of the superabsorbent resin particles.

[0144] In the 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, when 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 when 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 cost of the composition may become high due to the increase in the cost of the surfactant.

[0145] Preferably, R1, R2, and R3 are hydrogen or, in the case of a linear 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 linear 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.

[0146] The surfactant may be selected from compounds represented by the following formulas 2-1 to 2-14:

[0147] [ka] [ka]

[0148] On the other hand, the amount of the surfactant used is not particularly limited, but can 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, 0.01 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 of the equipment. If the surfactant is used in an excessive amount, the surface tension value may be lowered, and various physical properties of the final superabsorbent resin may be deteriorated.

[0149] 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.

[0150] 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 operating mixer and mixing them.

[0151] 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 micronizing step of step 2 and the neutralization step of step 3 described above may be carried out sequentially, alternately or simultaneously.

[0152] That is, a neutralizer may be added to the polymer to neutralize the acidic groups first, and then a surfactant may be added to the neutralized polymer to ultrafinely grind the polymer mixed with the surfactant (performed in the order of step 3 → step 2), or a neutralizer and a surfactant may be added simultaneously to the polymer to neutralize and ultrafinely grind the polymer (performed in the order of steps 2 and 3 simultaneously). Alternatively, the surfactant may be added first, and the neutralizer may be added later (performed in the order of steps 2 → step 3). Alternatively, the neutralizer and the surfactant may be added alternately in a crossed manner. Alternatively, the surfactant may be added first and ultrafinely grinded, and then a neutralizer may be added to neutralize the neutralized hydrogel polymer, and an additional surfactant may be added to perform an additional ultrafine grinding process.

[0153] Here, when the neutralization step is carried out independently from the ultrafine grinding step of step 2, it may 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 plurality of holes may 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 may be about 50 rpm to 500 rpm, and the holes of the perforated plate may be about 3 mm to 25 mm, but are not limited thereto.

[0154] In this case, as the neutralizing agent, a basic substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc., which can neutralize the acidic group, can be used.

[0155] In addition, the degree of neutralization, which refers to the degree to which the acidic groups contained in the polymer are neutralized by the neutralizing agent, 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, making it difficult to perform surface crosslinking in the subsequent process, 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.

[0156] 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.

[0157] Stage 4: Drying stage Next, a step (Step 4) is performed in which the micronized and neutralized polymer is dried to produce a base resin powder.

[0158] This step is a step of drying water from a base resin powder obtained by neutralizing at least a part of the acid groups of a polymer and ultrafinely pulverizing the polymer.

[0159] In a typical method for producing a superabsorbent resin, the drying step is carried out so that the moisture content of the base resin powder becomes about 4 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.

[0160] The step 4 may be carried out by fixed-bed type drying, moving type drying, or a combination thereof.

[0161] According to one embodiment of the invention, said step 4 may be carried out in a fixed bed drying.

[0162] The fixed-bed drying method refers to a method in which the material to be dried is suspended on a bed such as a perforated iron plate through which air can pass, and hot air passes from bottom to top to dry the material.

[0163] Fixed-bed drying dries in a plate-like shape without particle flow, so it is difficult to achieve uniform drying with a simple hot air flow. Therefore, fixed-bed drying requires delicate adjustment of the hot air and temperature 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-like dried product during drying and to prevent 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% in all directions.

[0164] As an apparatus capable of drying by the fixed bed drying method, a belt type dryer or the like may be used, but is not limited thereto.

[0165] In the case of the fixed bed drying step, the drying process may be performed at a temperature of about 80°C to 200°C, and preferably 90°C to 190°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, such as exceeding 200°C, a superabsorbent resin having a moisture content lower than the desired moisture content is 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.

[0166] According to one embodiment of the invention, said step 4 may be carried out by fluidized drying.

[0167] The fluidized drying method refers to a method of drying while 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.

[0168] 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 may be used.

[0169] 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.

[0170] Phase 5: Crushing Phase The dried base resin powder is then milled.

[0171] Specifically, the pulverization step may be performed by pulverizing the dry base resin powder to have a particle size of normal particle level, that is, a particle size of 150 μm to 850 μm.

[0172] 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, and is not limited to the above-mentioned examples.

[0173] 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.

[0174] On the other hand, in the manufacturing method of the present invention, the ultrafine pulverization step can realize superabsorbent resin particles with a smaller particle size distribution than the conventional chopping step, and the moisture content after drying is maintained relatively high. Therefore, even if pulverization is performed under mild conditions with less pulverization force, a superabsorbent resin with a very high content of normal particle sizes of 150 μm to 850 μm can be formed, and the fine powder generation ratio can be greatly reduced.

[0175] 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. The particle size of such resin particles can be measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP220.3 method.

[0176] 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 has more than about 20% by weight to about 30% by weight of fine powder.

[0177] Additive dosing 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).

[0178] 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.

[0179] The additive introduction step may be carried out simultaneously with step 2, simultaneously with step 3, after step 2 and step 3, or at least one or more of these steps. The additive introduction step may be carried out multiple times as necessary, and may be carried out once or more in each step.

[0180] 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, the additive addition step may be carried out in a manner in which the additive is added simultaneously while the polymer is being ground.

[0181] 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 in the grinding step to be mixed with the polymer.

[0182] Classification stage Next, after the step of pulverizing the base resin powder (step 5), the method may further include a step of classifying the pulverized superabsorbent resin particles according to particle size.

[0183] 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 base resin particles in the presence of a surface cross-linking agent after pulverizing (step 5) and / or classifying the base resin powder, whereby the cross-linked polymer contained in the base resin powder is additionally cross-linked by the surface cross-linking agent, thereby forming a surface cross-linked layer on at least a part of the surface of the base resin powder.

[0184] The above-mentioned descriptions regarding the surface cross-linking agent are all equally applicable.

[0185] In addition, there is no limitation on the constitution of the method for mixing the surface crosslinking agent with the base resin powder. For example, a method of putting a composition containing the surface crosslinking agent and the base resin powder into a reaction tank and mixing them, a method of spraying the surface crosslinking agent into the composition, a method of continuously supplying the resin composition and the surface crosslinking agent to a continuously operating mixer and mixing them, etc. can be used.

[0186] When the surface crosslinking agent and the base resin powder are mixed, water and methanol may be added together. When water and methanol are added, there is an advantage that the surface crosslinking agent can be uniformly dispersed in the resin composition. At this time, the content of the added water and methanol can be appropriately adjusted to induce uniform dispersion of the surface crosslinking agent and prevent the resin composition from clumping, while optimizing the surface penetration depth of the crosslinking agent.

[0187] 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 surface cross-linking step conditions are satisfied, the surface of the highly water-absorbent resin particles is sufficiently cross-linked, and the water absorption capacity under pressure can be increased.

[0188] 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, hot oil, etc., 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 examples.

[0189] 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 base resin powder, the method may further include one or more steps of: 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.

[0190] In the hydration step, water or salt water can be used, which can control the amount of scum, etc. The amount of water used can be appropriately adjusted in consideration of the moisture content of the final product, and is preferably, but not limited to, 0.1 to 10% by weight, 0.5 to 8% by weight, or 1 to 5% by weight relative to the water absorbent resin.

[0191] After the water addition step, a maturation step may be further carried out.

[0192] 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 absorption by the water absorbent resin. The aging step may 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.

[0193] The additives added in the post-treatment step may be surfactants, inorganic salts, liquid permeability improvers, anti-caking agents, flowability improvers, and antioxidants, but the present invention is not limited thereto.

[0194] By selectively carrying out the cooling step, the hydration step, and the post-treatment step, it is possible to control the generation of scum, improve the moisture content of the final superabsorbent polymer, and produce a higher quality superabsorbent polymer product.

[0195] The functions and effects of the present invention will be described in more detail below through specific examples of the present invention, however, these examples are merely presented as examples of the present invention and do not define the scope of the invention.

[0196] <Examples and Comparative Examples> 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 3,380g of water were stirred and mixed, and reacted while maintaining the temperature at 5℃. Nitrogen was introduced into the glass vessel containing the mixture at 1,000cc / min for 1 hour to replace the atmosphere under nitrogen conditions. Next, 0.2g of 30wt% hydrogen peroxide aqueous solution, 20g of 1.6% ascorbic acid aqueous solution, and 44g of 1% 2,2'-azobis-(2-amidinopropane) dihydrochloric acid aqueous solution were added as polymerization initiators, and 10g of 0.01% iron sulfate aqueous solution was added as a reducing agent 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.

[0197] (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. Thereafter, the polymer was extruded through a perforated plate having 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 crusher to perform an ultrafine crushing process.

[0198] 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-type extruder installed inside a cylindrical grinder to carry out an additional grinding process. At each stage of the screw-type extruder, 340 g of 32% NaOH aqueous solution (stage 3: neutralization stage) was added to neutralize some of the acidic groups of the polymer, and then 100 g of fine powder (additional additive addition stage) and 35.5 g of 5% Na2SO4 aqueous solution (additional additive addition stage) were added to produce hydrous superabsorbent resin particles (= ultrafinely ground and neutralized polymer).

[0199] (Step 4: Drying step) 1,000 g of the water-containing superabsorbent resin particles were placed in a ventilated belt dryer containing a perforated plate capable of shifting 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 resin was about 10%, and then hot air at 100°C was blown from bottom to top for 15 minutes to uniformly dry the polymer.

[0200] (Step 5: Crushing and classification process) The dried product was pulverized in a pulverizer (GRAN-U-LIZER™, MPE) and then classified using a standard mesh sieve according to ASTM standards to obtain a base resin powder with a size of 150 to 850 μm.

[0201] (Step 6: Surface cross-linking process) Next, 6 g of a surface cross-linking agent aqueous solution containing 0.08 g of ethylene glycol diglycidyl ether (EJ-1030J) and 0.3 g of propylene glycol was sprayed per 100 g of the base resin powder, and the mixture was stirred at room temperature so that the surface cross-linking liquid was uniformly distributed on the superabsorbent resin powder. Next, the base 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 base 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.

[0202] After the surface cross-linking step, the resin was classified using a standard mesh sieve according to ASTM standards to produce a highly water-absorbent resin having a particle size of 150 μm to 850 μm.

[0203] Example 2 A superabsorbent resin was prepared in the same manner as in Example 1, except that in the ultrafine pulverization step of Step 2 of Example 1, the rotation speed of the high-speed rotary chopper was adjusted to 3,000 rpm.

[0204] Example 3 A superabsorbent resin was produced in the same manner as in Example 1, except that in the ultrafine pulverization step of Step 2 of Example 1, the rotation speed of the high-speed rotary chopper was adjusted to 1,500 rpm.

[0205] Example 4 A superabsorbent resin was produced in the same manner as in Example 1, except that in the surface crosslinking step of Step 6 of Example 1, 6 g of a surface crosslinking agent aqueous solution containing 0.06 g of ethylene glycol diglycidyl ether (EJ-1030J) and 0.4 g of propylene glycol was sprayed per 100 g of base resin powder.

[0206] Comparative Example 1 (Step 1': Polymer production step - pre-neutralization foaming polymerization) A monomer composition was prepared by mixing 495g of acrylic acid, 1.35g of ethylene glycol diglycidyl ether as an internal crosslinking agent, 19.2g of 1% IGAGURE819 as a photopolymerization initiator, 0.8g of capsule-type foaming agent F-36D as a foaming agent, 0.1g of sodium dodecyl sulfate as a foaming stabilizer, and 206.1.5g 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 630.1g of 31% by weight sodium hydroxide aqueous 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 more 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, 41.2g of 2wt% sodium persulfate aqueous solution and 21.2g of a mixture of 0.6g of sodium bicarbonate dissolved in 1wt% sodium dodecyl sulfate aqueous solution were added. The mixed solution was poured into a Vat-shaped tray (15cm wide x 15cm long) installed in a square polymerization vessel preheated to 80°C with a light irradiation device attached on top, and light irradiation was started. After about 15 seconds of light irradiation, 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 3 minutes to obtain a sheet-shaped hydrogel polymer.

[0207] (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 250 rpm, and the hole size of the perforated plate was 10 mm.

[0208] (Step 3': Drying Step) 1,000 g of the pulverized polymer was placed in a ventilation belt dryer including a perforated plate capable of shifting the airflow up and down. The polymer was uniformly dried by blowing 180°C hot air from bottom to top for 15 minutes, and then from top to bottom for 15 minutes so that the moisture content of the dried product was about 2%, to produce a dried base resin powder.

[0209] (Step 4': Crushing and classification process) The dried base resin powder was pulverized with a pulverizer (GRAN-U-LIZER™, MPE) and then classified with a standard mesh sieve according to ASTM standards to obtain a highly water-absorbent resin powder with a size of 150 to 850 μm.

[0210] (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 allow the surface cross-linking reaction to proceed. 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.

[0211] After the surface cross-linking step, the resin was classified using a standard mesh sieve according to ASTM standards to produce a highly water-absorbent resin having a particle size of 150 μm to 850 μm.

[0212] Comparative Example 2 (Step 1-1: Polymerization step) 74 g of acrylic acid was added as a water-soluble ethylenically unsaturated monomer to a 300 mL beaker. Then, while cooling from the outside, 97.4 g of 31.5 wt% aqueous sodium hydroxide solution and 40 g of water were dropped into the beaker to neutralize the acrylic acid monomer to a degree of neutralization of 75 mol%. Then, 0.044 g of hydroxyethyl cellulose as a thickener, 0.037 g of potassium persulfate and 0.092 g of V-50 (2,2'-azobis(2-methylpropionamidine) dihydrochloride) as polymerization initiators, and 0.02 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to produce a first monomer aqueous solution.

[0213] Thereafter, 0.736 g of a stearic acid ester (manufactured by Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) was dissolved as a surfactant in 300 g of n-heptane to obtain a surfactant solution. The surfactant solution was poured into a separable flask, and the first monomer aqueous solution was poured in while stirring at a stirrer speed of 200 rpm. After replacing the inside of the system with nitrogen, the flask was heated to 70°C and subjected to reverse phase suspension polymerization for 60 minutes to produce a first mixture containing a hydrous gel polymer and water.

[0214] (Step 1-2: Additional Polymerization Step) Next, 103 g of acrylic acid was added as a water-soluble ethylenically unsaturated monomer to a separate beaker with an internal volume of 300 mL. Then, while cooling from the outside, 136 g of 31.5 mass% sodium hydroxide aqueous solution and 35 g of water were dropped into the beaker to neutralize the acrylic acid monomer to a neutralization degree of 75 mol%. Then, 0.052 g of potassium persulfate as a water-soluble radical polymerization initiator, 0.129 g of V-50 (2,2'-azobis(2-methylpropionamidine) dihydrochloride) as a thermal polymerization initiator, and 0.088 g (0.057 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent were added and dissolved to prepare a second monomer aqueous solution.

[0215] Next, while stirring at a stirrer speed of 300 rpm, the separable flask containing the first mixture was cooled to 25°C, and the second monomer composition was added thereto. Then, the system was replaced with nitrogen for 30 minutes, and the flask was heated to 70°C and subjected to reverse phase suspension polymerization for 30 minutes to produce a 1-2 mixture containing a hydrous gel polymer and water.

[0216] (Step 2: Azeotropic Distillation Step) Thereafter, the temperature was raised to 125° C., and 241 g of water was removed from the system by azeotropic distillation of n-heptane and water while refluxing n-heptane, thereby producing a second mixture containing a base resin powder.

[0217] (Step 3: Surface cross-linking step) Thereafter, the temperature of the second mixture was lowered to 80°C, and a surface cross-linking solution containing 4.3 g of water and 0.088 g of Ethylene Glycol Diglycidyl Ether EJ-1030 was added to the flask, and the surface cross-linking reaction was carried out for 2 hours to prepare a third mixture containing superabsorbent resin particles.

[0218] (Step 4: Decantation Step) Next, the third mixture was left at room temperature (24±1° C.) for 10 minutes, and then 290 g of the supernatant was removed. The amount of the removed 290 g of the supernatant was 97 wt % of the total weight of the solvent contained in the third mixture.

[0219] The mixture from which the supernatant was removed was dried in a dryer at 125° C. for 10 minutes to obtain a highly water-absorbent resin.

[0220] Comparative Example 3 The same procedure as in Example 1 was used, but the micronizing step in step 2 was omitted.

[0221] Comparative Example 4 (Step 1: Polymerization Step) A monomer solution was prepared by mixing 100 parts by weight of acrylic acid with 0.6 parts by weight of polyethylene glycol diacrylate (weight average molecular weight: 500 g / mol) as an internal crosslinking agent and 0.01 parts by weight of IRGACURE819 as a photoinitiator. Next, the monomer solution was continuously supplied by a metering pump while 140 parts by weight of 31% by weight aqueous sodium hydroxide solution was continuously line mixed 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.

[0222] When the temperature was cooled to 40° C., solid sodium bicarbonate, which was a foaming agent, was added to the aqueous monomer solution, and at the same time, 6 parts by weight of a 2% by weight aqueous sodium persulfate solution was added.

[0223] 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-like hydrogel polymer.

[0224] (Step 2: Coarse grinding step) The sheet-shaped hydrogel polymer obtained in step 1 was confirmed to have a temperature between 70 and 90° C. 200 ml of water at room temperature was sprayed onto the hydrogel polymer.

[0225] The hydrogel polymer was cut into pieces of 3 cm x 3 cm in size, and then crushed by extruding the hydrogel through a perforated plate having a plurality of holes using a screw-type extruder mounted inside a cylindrical crusher.

[0226] (Step 3: Drying step) Next, the hydrogel polymer pulverized in step 2 was dried in a dryer capable of shifting airflow up and down. Hot air at 180°C was blown from bottom to top for 15 minutes, and then again from top to bottom for 15 minutes, so that the moisture content of the dried powder was about 2% or less, and the hydrogel polymer was uniformly dried.

[0227] (Step 4: Crushing Step) The polymer dried in step 3 was pulverized in a pulverizer and then classified to obtain a base resin powder having a size of 150 to 850 μm.

[0228] (Step 5: Surface cross-linking step) Thereafter, 6 g of a surface crosslinking agent aqueous solution containing 3 parts by weight of ethylene carbonate was sprayed onto 100 parts by weight of the prepared base resin powder, and the mixture was stirred at room temperature to uniformly distribute the surface crosslinking liquid on the base resin powder. Next, the base resin powder mixed with the surface crosslinking liquid was placed in a surface crosslinking reactor to carry out a surface crosslinking reaction.

[0229] In the surface crosslinking reactor, the base resin powder was gradually heated from an initial temperature of about 80°C, and was operated so as to reach a maximum reaction temperature of 190°C after 30 minutes. After reaching the maximum reaction temperature, the reaction was continued for an additional 15 minutes, and then a final superabsorbent resin sample was taken. After the surface crosslinking process, the superabsorbent resin was classified using a standard mesh sieve according to the ASTM standard to produce a superabsorbent resin having a particle size of 150μm to 850μm.

[0230] <Experimental Example> The properties of the superabsorbent resins prepared in the above Examples and Comparative Examples were evaluated by the following methods and are shown in Table 1 below.

[0231] 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.

[0232] Moreover, unless otherwise stated, the physical property evaluation of the surface-crosslinked final superabsorbent resin was carried out on a resin having a particle size of 150 μm to 850 μm classified using an ASTM standard sieve.

[0233] (1) Initial absorption rate (IAR, g / g) The initial water absorption capacity of the superabsorbent resins of the above Examples and Comparative Examples was measured by the following method.

[0234] Specifically, the superabsorbent resin W0(g) (about 1 g) obtained in each of the examples and comparative examples was evenly placed in a 20×30 cm nonwoven bag, sealed, and then immersed in 1 L of distilled water (conductivity <1 mS / cm, A222 ORION A series, Thermo Scientific) at 24° C. After 20 seconds, the bag was removed and fixed for 5 minutes to drain the water, and the mass W2(g) of the bag was measured. In addition, the same operation was performed without using the resin, and the mass W1(g) at that time was measured.

[0235] Using the obtained masses, the IAR (g / g) was calculated according to the following formula 1.

[0236] [Formula 1] IAR(g / g)={[W2(g)-W1(g)] / W0(g)}-1

[0237] The measurement was repeated five times, and the average value and standard deviation were calculated.

[0238] (2) Saturated water absorption rate (FSR, Free swell rate, g / g) The initial water absorption capacity of the superabsorbent resins of the above Examples and Comparative Examples was measured by the following method.

[0239] Specifically, the superabsorbent resin W0(g) (about 1 g) obtained in each of the examples and comparative examples was evenly placed in a 20 x 30 cm nonwoven bag, sealed, and then immersed in 1 L of distilled water (conductivity < 1 mS / cm, A222 ORION A series, Thermo Scientific) at 24 ° C. After 30 minutes, the bag was taken out and fixed for 10 minutes to drain the water, and the mass W4 (g) of the bag was measured. In addition, the same operation was performed without using the resin, and the mass W3 (g) at that time was measured.

[0240] Using the obtained masses, the FSR (g / g) was calculated according to the following formula 2.

[0241] [Formula 2] FSR(g / g)={[W4(g)-W3(g)] / W0(g)}-1

[0242] The measurement was repeated five times, and the average value and standard deviation were calculated.

[0243] (3) Centrifuge Retention Capacity (CRC, Centrifuge Retention Capacity, g / g) The water retention capacity of the superabsorbent resins of the above Examples and Comparative Examples in terms of the water absorption capacity under no load was measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP241.3.

[0244] Specifically, the superabsorbent resin W0(g) (about 0.2g) obtained in each of the examples and comparative examples was evenly placed in a nonwoven bag, sealed, and then immersed in physiological saline (0.9% by weight) at room temperature. After 30 minutes, the bag was drained for 3 minutes at 250G using a centrifuge, and the mass W6(g) of the bag was measured. The same procedure was performed without using the resin, and the mass W5(g) at that time was measured.

[0245] Using the obtained masses, the CRC (g / g) was calculated according to the following formula 3.

[0246] [Formula 3] CRC(g / g)={[W6(g)-W5(g)] / W0(g)}-1

[0247] The measurement was repeated five times, and the average value and standard deviation were calculated.

[0248] (4) Absorbency under Pressure (AUP, g / g) The water absorption capacity of the superabsorbent resins of the above Examples and Comparative Examples at a pressure of 0.3 psi was measured by EDANA method WSP242.3.

[0249] Specifically, a stainless steel wire mesh of 400 mesh was attached to the bottom of a plastic cylinder with an inner diameter of 25 mm. Under conditions of room temperature and humidity of 50%, a superabsorbent resin W0(g) (0.9 g) was evenly spread on the wire mesh, and a piston capable of applying a load of 0.3 psi evenly on the wire mesh was made to be slightly smaller than the outer diameter of 25 mm, with no gap between it and the inner wall of the cylinder, and without impeding its up and down movement. At this time, the weight W7(g) of the device was measured.

[0250] 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 solution consisting 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 W8 (g) was measured.

[0251] Using the obtained masses, the water absorption capacity under pressure (g / g) was calculated according to the following formula 4.

[0252] [Formula 4] AUP(g / g)=[W8(g)-W7(g)] / W0(g)

[0253] The measurement was repeated five times, and the average value and standard deviation were calculated.

[0254] (5) Water absorption rate (Vortex time, sec) The vortex time was measured in seconds according to the method described in WO 1987 / 003208.

[0255] 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.

[0256] [Table 1]

[0257] As can be seen from Table 1, in the present invention, by controlling the initial water absorption capacity and the saturated water absorption capacity within a specific range, it was confirmed that the water absorption property can be significantly improved, and in particular, the water absorption speed can be improved.

Claims

1. A base resin powder comprising a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group and an internal crosslinking agent; and the crosslinked polymer is additionally crosslinked via a surface crosslinking agent to form a surface crosslinked layer on the base resin powder, The initial water absorption capacity is 70 g / g or more, The saturated water absorption capacity is 430 g / g or more. Super absorbent resin.

2. The initial water absorption capacity is calculated by the following formula 1: The highly water-absorbent polymer according to claim 1: [Formula 1] Initial water absorption ratio (g / g) = {[W ２ (g) - W １ (g)] / W ０ (g)}-1 In the above formula, W ０ (g) is the mass of the sample to be measured, W ２ (g) is the mass of a nonwoven bag containing a sample to be measured and immersing the bag in distilled water having a conductivity of 1 mS / cm or less for 20 seconds, W １ (g) is the mass of a nonwoven bag not containing a sample to be measured after immersing the bag in distilled water having a conductivity of 1 mS / cm or less for 20 seconds.

3. The initial water absorption capacity is 70 g / g to 100 g / g. The highly water-absorbent resin according to claim 1.

4. The saturated water absorption capacity is calculated by the following formula 2: The highly water-absorbent polymer according to claim 1: [Formula 2] Saturated water absorption ratio (g / g) = {[W ４ (g) - W ３ (g)] / W ０ (g)}-1 In the above formula, W ０ (g) is the mass of the sample to be measured, W ４ (g) is the mass of a nonwoven bag containing a sample to be measured and immersing the bag in distilled water having a conductivity of 1 mS / cm or less for 30 minutes, W ３ (g) is the mass of a nonwoven bag not containing a sample to be measured after immersing the bag in distilled water having a conductivity of 1 mS / cm or less for 30 minutes.

5. The saturated water absorption capacity is 430 g / g to 530 g / g. The highly water-absorbent resin according to claim 1.

6. The superabsorbent resin has a water retention capacity (CRC) of 35 g / g to 45 g / g as measured by EDANA method WSP241.

3. The highly water-absorbent resin according to claim 1.

7. The superabsorbent resin has an absorbency under pressure (AUP) of 32 g / g to 40 g / g, as measured at 0.3 psi according to EDANA method WSP242.

3. The highly water-absorbent resin according to claim 1.

8. The superabsorbent resin has a water absorption speed of 25 seconds or less when measured by a vortex method. The highly water-absorbent resin according to claim 1.

9. A method for producing the highly water-absorbent resin according to claim 1, A step of 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; Micronizing the polymer at a rotation speed of 500 rpm to 4,000 rpm; neutralizing at least a portion of the acidic groups of the polymer; drying and grinding the micronized and neutralized polymer to produce base resin particles; and forming a surface cross-linked layer on at least a portion of the surface of the base resin powder; A method for producing a superabsorbent resin.

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

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