Superabsorbent polymers, methods of making the same, and absorbent articles comprising the same

Abstract

This invention relates to superabsorbent polymers, methods for their preparation, and absorbent articles comprising the same. More specifically, the superabsorbent polymers are technologies that achieve excellent absorption durability when applied to practical products by simultaneously satisfying appropriate surface tension and excellent absorption properties.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0058944 filed on May 3, 2024 and Korean Patent Application No. 10-2025-0057387 filed on April 30, 2025, and all the contents disclosed in the respective Korean patent application documents are included as part of this specification.

[0003] This invention relates to superabsorbent polymers, methods for their preparation, and absorbent articles comprising the same. More specifically, the superabsorbent polymers are technologies that achieve excellent absorption durability when applied to practical products by simultaneously satisfying appropriate surface tension and excellent absorption properties. Background Technology

[0004] Superabsorbent polymers (SAPs) are synthetic polymer materials capable of absorbing approximately 500 to 1000 times their own weight in water. They are named differently by their developers, such as SAM (Super Absorbency Material) and AGM (Absorbent Gel Material). These superabsorbent polymers are beginning to be used in practical hygiene products and are currently widely used in applications such as soil conditioners for horticulture, waterproofing materials for civil engineering and construction, seedling sheets, preservatives in food distribution, and materials for mud dressings.

[0005] Such superabsorbent polymers are primarily used in hygiene products such as diapers or sanitary napkins. In hygiene products, superabsorbent polymers are typically contained within a pulp. However, there has been ongoing effort to provide hygiene products with thinner thicknesses, such as diapers, and as part of this, so-called pulp-free diapers, in which the pulp content is reduced or even completely eliminated, are being actively developed.

[0006] Therefore, in hygiene products where the pulp content is reduced or eliminated, superabsorbent polymers are included in a relatively high proportion, inevitably resulting in multi-layered superabsorbent polymer particles within the product. For these multi-layered superabsorbent polymer particles to more effectively absorb large amounts of fluid, such as urine, the superabsorbent polymer essentially needs to exhibit not only high absorbency but also a fast absorption rate.

[0007] In the case of such pulp-free hygiene products, under actual product use conditions, the swollen superabsorbent polymer may detach from the hygiene material inside the product. In this situation, the superabsorbent polymer tends to migrate to one side of the hygiene product, failing to absorb bodily fluids and causing leakage; or, if it cannot absorb bodily fluids quickly enough, the fluid previously absorbed by the superabsorbent polymer may be squeezed out again (rewetting). These problems can easily occur simultaneously and may lead to rashes in the user.

[0008] Therefore, there is a need to develop technologies that improve problems such as leakage or rewetting by providing excellent absorbency (the superabsorbent polymer does not detach from the product during actual use, such as hygiene products) and the rapid absorption time of hygiene products to body fluids (absorption time). Summary of the Invention

[0009] Technical issues

[0010] Therefore, the present invention provides a superabsorbent polymer, a method for preparing the same, and absorbent articles comprising the same, wherein the superabsorbent polymer can achieve excellent absorption durability under practical use conditions by simultaneously satisfying appropriate surface tension and excellent absorption properties.

[0011] Technical solution

[0012] To solve the above tasks,

[0013] This invention provides a polyacrylate-based superabsorbent polymer that satisfies the following i) to iv):

[0014] i) The surface tension measured by the hanging plate method at room temperature of 24±2℃ is 63 mN / m to 72 mN / m.

[0015] ii) The vortex absorption rate is 30 seconds or less.

[0016] iii) The centrifuge retention capacity (CRC) measured according to EDANA WSP 241.3 is 32 g / g or greater.

[0017] iv) The absorbency under pressure (AUP) at 0.9 psi, as measured by EDANA WSP 242.3, is 12 g / g or greater.

[0018] According to another embodiment of the present invention, a method for preparing a superabsorbent polymer is provided, comprising the following steps:

[0019] Hydrogel polymers are formed by polymerizing a monomer mixture comprising an acrylic acid-based monomer having at least partially neutralized acidic groups, an internal crosslinking agent, a surfactant having an HLB of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol, and an encapsulated foaming agent.

[0020] The base resin powder is formed by drying, pulverizing, and classifying the hydrogel polymer; and

[0021] A polyacrylate-based superabsorbent polymer in which a surface cross-linking layer is formed on the surface of the base resin powder is prepared by heat-treating the base resin powder in the presence of a surface cross-linking agent.

[0022] According to another embodiment of the present invention, an absorbent article comprising the above-mentioned superabsorbent polymer is provided, and specifically, a liquid-impermeable substrate, an absorbent layer and a liquid-permeable top sheet are sequentially laminated, wherein the absorbent layer comprises the above-mentioned superabsorbent polymer.

[0023] Beneficial effects

[0024] The superabsorbent polymer of the present invention exhibits excellent absorption durability when applied to actual products by simultaneously achieving excellent absorption properties and appropriate surface tension.

[0025] According to the method for preparing the superabsorbent polymer of the present invention, by using a combination of a low molecular weight surfactant with a specific HLB value and an encapsulating foaming agent in the polymerization step, a resin that achieves appropriate surface tension and excellent absorption properties can be prepared, and at the same time, the absorption durability of the absorbent articles using the resin is significantly improved. Detailed Implementation

[0026] The terminology used in this specification is for describing exemplary embodiments only and is not intended to limit the invention.

[0027] Unless the context clearly indicates otherwise, singular expressions include plural expressions. In this specification, terms such as “comprising / including,” “equipped with,” or “having” are intended to indicate the presence of the 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 or steps, components, or combinations thereof.

[0028] Terms such as first, second, and third are used to describe various components, and these terms are used only for the purpose of distinguishing one component from other components.

[0029] This invention can have various modifications and can take many forms; therefore, specific embodiments will be illustrated and described in detail below. However, this is not intended to limit the invention to the specific forms disclosed, but should be understood to include all modifications, equivalents, or alternatives contained within the spirit and scope of this invention.

[0030] As used in this specification, the terms "resin" or "polymer" refer to water-soluble olefinic unsaturated monomers in a polymerized state and cover a wide range of moisture contents and particle sizes. Among these polymers, those in a post-polymerization but pre-drying state with a moisture content of about 40% by weight or greater may be referred to as hydrogel polymers, and particles obtained by pulverizing and subsequently drying such hydrogel polymers may be referred to as crosslinked polymers.

[0031] Furthermore, the term "crosslinked polymer" refers to a crosslinked polymer that is crosslinked and polymerized in the presence of a water-soluble olefinic unsaturated monomer and an internal crosslinking agent, and "base resin" refers to a material containing such a crosslinked polymer, and "base resin powder or base resin particles" refers to their particulate form.

[0032] Furthermore, the term "superabsorbent polymer" is used, depending on the context, to cover: crosslinked polymers obtained by polymerizing water-soluble olefinic unsaturated monomers containing at least a portion of their acidic groups; base resins obtained by drying said crosslinked polymers; and those crosslinked polymers or base resins that have undergone additional processes, such as surface crosslinking, fine powder reassembly, drying, pulverizing, and grading, to bring them to a commercially viable state.

[0033] Furthermore, the term "superabsorbent polymer powder" refers to a particulate material containing a crosslinked polymer obtained by polymerizing a water-soluble olefinic unsaturated monomer containing at least a portion of its acidic groups and crosslinking it with an internal crosslinking agent.

[0034] Recently, in response to the demand for thinner hygiene products such as diapers or sanitary napkins, hygiene products with reduced or no pulp content have been actively developed. In such pulp-free hygiene products, under actual product use conditions, the swollen superabsorbent polymer tends to detach from the hygiene product. In this case, the superabsorbent polymer migrates to one side of the hygiene product and cannot absorb bodily fluids, causing leakage, or if urine is not quickly absorbed, urine already absorbed in the superabsorbent polymer may be squeezed out again (rewetting); both situations can easily lead to rashes in the user. Therefore, there is a need for the development of technologies that prevent the detachment of the superabsorbent polymer during actual use of hygiene products, thereby providing excellent absorbency and durability, and achieving rapid urine absorption time to improve problems such as leakage and rewetting.

[0035] Therefore, the inventors have determined that when a superabsorbent polymer simultaneously satisfies appropriate surface tension and the excellent absorption properties described below (i) to (iv), the superabsorbent polymer can exhibit excellent absorption durability under absorption conditions without such problems when applied to actual products.

[0036] Specifically, when the corresponding polymer is applied to actual absorbent articles, it is determined that the resin surface is hydrophobic to maximize the interaction with the adjacent adhesive in the article, so that the resin does not detach from the absorbent article under absorption conditions, thereby achieving excellent absorption durability, and thus completing the present invention.

[0037] Furthermore, the inventors have determined that by using a combination of a low molecular weight surfactant with a specific HLB value and an encapsulated foaming agent in the polymerization step to prepare such a resin, the absorption characteristics can be improved by utilizing a uniform foaming effect.

[0038] Superabsorbent polymers

[0039] According to one embodiment of the invention, the superabsorbent polymer is a polyacrylate-based superabsorbent polymer and satisfies the following i) to iv).

[0040] i) The surface tension measured by the hanging plate method at room temperature of 24±2℃ is 63 mN / m to 72 mN / m.

[0041] ii) The vortex absorption rate is 30 seconds or less.

[0042] iii) The centrifugation retention capacity (CRC) measured according to EDANA WSP 241.3 is 32 g / g or greater.

[0043] iv) The absorption rate (AUP) at 0.9 psi pressure, as measured by EDANA WSP 242.3, is 12 g / g or greater.

[0044] Superabsorbent polymers simultaneously satisfy physical properties i) to iv) (which are excellent absorption properties and appropriate surface tension), enabling them to exhibit excellent absorption durability under absorption conditions when applied to actual products.

[0045] The surface tension of superabsorbent polymers, according to the dipstick method, can be from 63 mN / m to 72 mN / m. By meeting this range, when applied to the final absorbent article, the polymer will not detach from the absorbent article under absorption conditions, thus achieving excellent absorption durability. If it is outside the above range, the absorption durability of the final product may be reduced.

[0046] The surface tension can preferably be from 64 mN / m to 71 mN / m, or from 64 mN / m to 70 mN / m. The specific method for measuring surface tension will be described in more detail in the experimental examples described later.

[0047] (ii) The vortex absorption rate of the superabsorbent polymer can be 30 seconds or less. If it is outside the above range, it may degrade the absorption performance of the final product itself and may also reduce the absorption durability under absorption conditions.

[0048] The absorption rate can preferably be 10 to 29 seconds, 10 to 28 seconds, 10 to 27 seconds, or 10 to 26 seconds. The method for measuring the absorption rate will be described in more detail in the experimental examples described later.

[0049] (iii) The centrifugal retention capacity (CRC) of the superabsorbent polymer, as measured according to EDANA WSP 241.3, can be 32 g / g or greater. If it is outside the above range, it may reduce the absorption performance of the final product itself and may also reduce the absorption durability under absorption conditions.

[0050] The centrifugation retention capacity can preferably be from 32 g / g to 38 g / g, 32 g / g to 36 g / g, or 33 g / g to 35 g / g. The specific measurement method will be described in more detail in the experimental examples described later.

[0051] (iv) The absorbance uptake (AUP) of superabsorbent polymers at 0.9 psi pressure, as measured according to EDANA WSP 242.3, can be 12 g / g or greater. If it falls outside this range, it may reduce the absorbency of the final product itself and may also reduce the absorption durability under the absorption conditions.

[0052] Preferably, the concentration can be from 13 g / g to 18 g / g, from 14 g / g to 16 g / g, or from 14 g / g to 16.5 g / g. Specific measurement methods will be described in more detail in the experimental examples to be described later.

[0053] According to one embodiment of the invention, the particle size of the superabsorbent polymer can be from 150 μm to 850 μm. More specifically, at least 95% or more of the superabsorbent polymer can have a particle size of 150 μm to 850 μm, which can contain 50% or more of particles with a particle size of 300 μm to 600 μm, and the fine powder with a particle size of less than 150 μm can be less than 3% by weight.

[0054] According to one embodiment of the invention, the polyacrylate-based superabsorbent polymer comprises: a base resin comprising a hydrogel polymer obtained by polymerizing a monomer mixture containing an acrylic acid-based monomer having at least partially neutralized acidic groups, an internal crosslinking agent, a surfactant having an HLB of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol, and an encapsulated foaming agent; and a surface crosslinking layer formed on the surface of the base resin.

[0055] In the preparation of the base resin, a low molecular weight surfactant having an HLB content of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol is used in combination with an encapsulating foaming agent. As a result, an optimal micelle structure is formed in the neutralized solution, thereby maximizing the foaming effect and achieving excellent retention capacity and absorption rate. In particular, due to the hydrophobic properties of the surfactant, the surface tension of the final superabsorbent polymer increases, thus preventing the superabsorbent polymer from detaching from the product when applied, and further improving absorption durability. Preferably, the surfactant is a non-polymer surfactant.

[0056] When the surfactant has an HLB content exceeding 10, the desired effect on improving absorbency durability is not significant. Furthermore, even with HLB values ​​between 1 and 10, polymeric surfactants with molecular weights exceeding 1,500 g / mol may, depending on the type of main chain and functional groups, degrade the fundamental absorption properties of the superabsorbent polymer. When applied to absorbent articles, they may reduce interactions with adjacent layers or even increase repulsive forces, thus hindering the achievement of the desired absorbency durability.

[0057] Preferably, the surfactant may have an HLB of 1.1 or greater, 1.3 or greater, 1.5 or greater, 2.0 or greater, or 3.0 or greater, or it may have an HLB of 9.8 or less, 9.7 or less, 9.5 or less, or 9.0 or less, for example, 1.1 to 9.8, 1.3 to 9.7, 1.5 to 9.5, 2.0 to 9.0, 3.0 to 8.0, or 3.0 to 7.0.

[0058] Components with known values ​​can be used as surfactants with specific HLB (Hydrophile-Lipophile Balance) values. HLB values ​​can typically be calculated based on the molecular structure and functional group composition, and can be calculated using methods such as the Griffin method, the Davies method, or methods determined experimentally based on actual emulsifying and solubilizing properties.

[0059] According to the Griffin method, it is calculated as HLB = (molecular weight of hydrophilic portion / total molecular weight) × 20, which is easily applied to nonionic surfactants.

[0060] According to the Davies method, it is calculated using the following formula, which is easily applied to ionic surfactants and complex surfactants.

[0061]

[0062] Here, m is the number of hydrophilic groups in the molecule, H is the coefficient of the second hydrophilic group, and n is the number of lipophilic groups in the molecule. The coefficient of the hydrophilic group follows a known value.

[0063] Preferably, the weight-average molecular weight of the surfactant can be from 200 g / mol to 1,200 g / mol, from 200 g / mol to 1,000 g / mol, from 200 g / mol to 800 g / mol, or from 200 g / mol to 700 g / mol, and it is preferable to achieve the desired effect within the above ranges.

[0064] For surfactants with a specific weight-average molecular weight, their structural features are well-defined, making it easy to identify the weight-average molecular weight components. However, if such structural features are not well-defined, in the case of a single-structure compound, the molecular structure can be determined by NMR (Nuclear Magnetic Resonance) and GC / MS (Gas Chromatography / Mass Spectrometry), and the weight-average molecular weight can be measured by analyzing individual constituent atoms. In the case of polymer compounds, it can be measured using the well-known gel permeation chromatography (GPC) method.

[0065] Surfactants can be used without any particular restrictions, provided they meet the above-mentioned HLB and molecular weight ranges. For example, at least one of the following can be used: calcium stearate (calcium stearate, Mw 607, HLB 5.0), glyceryl monolaurate (glyceryl monolaurate, Mw 274.4, HLB 5.0), polyoxyethylene (10) lauryl ether (polyoxyethylene (10) lauryl ether, Mw 626.86, HLB 9.7), sucrose dioleate (sucrose dioleate, Mw 871.2, HLB 7.1), propylene glycol monostearate (propylene glycol monostearate, Mw 342.56, HLB 3.4), and ethylene glycol distearate (ethylene glycol distearate, Mw 594.99, HLB 1.5).

[0066] The surfactant may be included in an amount from 100 ppmw to 10,000 ppmw relative to the total content of acrylic acid-based monomers, and preferably, it may be 200 ppmw or more, 300 ppmw or more, or 400 ppmw or more, or it may be 8,000 ppmw or less, 5,000 ppmw or less, 3,000 ppmw or less, or 1,000 ppmw or less, and it may be 200 ppmw to 8,000 ppmw, 300 ppmw to 5,000 ppmw, or 400 ppmw to 3,000 ppmw, or 400 ppmw to 1,000 ppmw. Preferably, it is used within the above content range to achieve the above-mentioned improved absorption properties.

[0067] Encapsulated blowing agents are components that increase the surface area of ​​a polymer by forming pores within the polymer during the polymerization stage of the base resin. By combining the encapsulated blowing agent with a low molecular weight surfactant having an HLB value of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol, the foaming effect is maximized through the formation of an optimal micelle structure in the neutralized solution, thereby achieving excellent water retention capacity and absorption rate. Furthermore, the combination with the aforementioned surfactant helps improve the absorption durability of the final product.

[0068] On the other hand, when using conventionally employed carbonate-based blowing agents, it is difficult to control foaming performance, and therefore it may be difficult to achieve the desired levels of surface tension improvement and absorption properties. The encapsulated blowing agent of this invention expands and subsequently contracts within the polymerization temperature range, thereby promoting controlled foaming performance and the formation of desired pores. Furthermore, a synergistic effect can be achieved when used in combination with low molecular weight surfactants having specific HLB values.

[0069] Encapsulated foaming agents can have a structure comprising a core containing hydrocarbons and a shell surrounding the core and formed of a thermoplastic resin.

[0070] In the encapsulated foaming agent, the hydrocarbon can be selected from one or more of the following: n-propane, n-butane, isobutane, cyclobutane, n-pentane, isopentane, cyclopentane, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, cycloheptane, n-octane, isooctane, and cyclooctane; and the thermoplastic resin can be a polymer formed from at least one monomer selected from the following: (meth)acrylate, (meth)acrylonitrile, aromatic vinyl monomers, vinyl acetate, ethylene halide, and vinylidene halide. Commercially available products include F36D from MATSUMOTO and MS140DS from Dongjin Semichem.

[0071] The encapsulated blowing agent can be included in an amount from 10 ppmw to 10,000 ppmw relative to the total content of acrylic-based monomers, and preferably, 500 ppmw or more, 1,000 ppmw or more, 1,500 ppmw or more, 1,800 ppmw or more, or 2,000 ppmw or more, or it can be 8,000 ppmw or less, 5,000 ppmw or less, or 3,000 ppmw or less, and it can be 500 ppmw to 8,000 ppmw, 1,000 ppmw to 8,000 ppmw, 1,500 ppmw to 5,000 ppmw, or 2,000 ppmw to 3,000 ppmw. Preferably, it is used within the above content range to achieve the above-mentioned improved absorption properties.

[0072] Meanwhile, the surfactant and the encapsulating foaming agent can be used in a weight ratio of 1:1 to 1:50, preferably 1:1 to 1:40, 1:1 to 1:30, 1:1 to 1:20, or 1:1 to 1:10, and this is preferred because the above synergistic effect can be further enhanced by using them in combination within the above content range.

[0073] The term "internal crosslinking agent" as used in this specification is used to distinguish it from "surface crosslinking agent," which is used to polymerize the unsaturated bonds of the acrylic-based monomers by crosslinking them. Crosslinking in this step is carried out without distinction between the surface and the interior; however, through the surface crosslinking process of the base resin described later, the final superabsorbent polymer particles are composed of a surface crosslinked structure via a surface crosslinking agent and an interior crosslinked structure via an internal crosslinking agent.

[0074] As an internal crosslinking agent, a multifunctional component can be used, for example, at least one selected from the following: N,N'-methylenebisacrylamide, allyl methacrylate, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol, glycerol, and ethylene carbonate. Preferably, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, and allyl (meth)acrylate can be used.

[0075] The internal crosslinking agent can be used in amounts from 100 ppmw to 10,000 ppmw, based on the weight of the water-soluble olefinic unsaturated monomer. By including it in the above range, a certain level or greater strength can be achieved through sufficient crosslinking, and sufficient retention capacity can be achieved by introducing an appropriate crosslinking structure. Preferably, it can be included in amounts of 200 ppmw or more, 500 ppmw or more, or 1,000 ppmw or more, and 8,000 ppmw or less, 5,000 ppmw or less, 3,000 ppmw or less, or 2,000 ppmw or less, and 200 ppmw to 5,000 ppmw, 300 ppmw to 3,000 ppmw, or 1,000 ppmw to 2,000 ppmw. If the content of the internal crosslinking agent is too low, crosslinking cannot occur sufficiently, making it difficult to achieve a certain level or greater strength, and significantly reducing drying efficiency. If the content of the internal crosslinking agent is too high, the internal crosslinking density increases, making it difficult to achieve the desired retention capacity.

[0076] In the following text, the contents of the base resin and the other components of the surface crosslinking layer will be described in more detail in the preparation method of the superabsorbent polymer described later.

[0077] Preparation method of superabsorbent polymer

[0078] A method for preparing a superabsorbent polymer according to one embodiment of the present invention comprises the following steps: forming a hydrogel polymer by polymerizing a monomer mixture comprising an acrylic acid-based monomer having at least partially neutralized acidic groups, an internal crosslinking agent, a surfactant having an HLB of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol, and an encapsulating foaming agent; forming a base resin powder by drying, pulverizing, and classifying the hydrogel polymer; and preparing a polyacrylate-based superabsorbent polymer wherein a surface crosslinking layer is formed on the surface of the base resin powder by heat-treating the base resin powder in the presence of a surface crosslinking agent.

[0079] Therefore, the prepared superabsorbent polymer can satisfy the above-mentioned appropriate surface tension and excellent absorption properties (the following properties (i) to (iv)).

[0080] The preparation method of the superabsorbent polymer according to an embodiment of the present invention will be described in detail below for each step.

[0081] (Aggregation step)

[0082] A method for preparing a superabsorbent polymer according to one embodiment of the present invention includes the following steps: forming a hydrogel polymer by polymerizing a monomer mixture comprising an acrylic acid-based monomer having at least partially neutralized acidic groups, an internal crosslinking agent, a surfactant having an HLB of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol, and an encapsulating foaming agent.

[0083] Hereinafter, the contents described in the superabsorbent polymer section also apply to internal crosslinking agents, surfactants having HLB of 1 to 10 and weight-average molecular weights of 200 g / mol to 1,500 g / mol, and encapsulated foaming agents.

[0084] The polymerization step is a step of forming a hydrogel polymer by thermal polymerization or photopolymerization of a monomer composition comprising: a monomer mixture containing an acrylic acid-based monomer having an acidic group, and a surfactant, a foaming agent, an internal crosslinking agent and / or a polymerization initiator.

[0085] First, the acrylic acid-based monomer can be any monomer commonly used in the preparation of superabsorbent polymers. As a non-limiting example, the acrylic acid-based monomer can be a compound represented by the following chemical formula 1:

[0086] [Chemical Formula 1]

[0087]

[0088] In chemical formula 1,

[0089] R1 is a hydrocarbon group containing 2 to 5 carbon atoms with unsaturated bonds, and

[0090] M 1 It can be a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0091] Preferably, the monomer can be selected from at least one of the following: acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts of these acids. In this way, it is advantageous to use acrylic acid or its salts as water-soluble olefinically unsaturated monomers because superabsorbent polymers with improved absorbency can be obtained. In addition, as monomers, at least one of the following can be used: anionic monomers and their salts, such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid; nonionic monomers containing hydrophilic groups, such as (meth)acrylamide, N-substituted (meth)acrylates, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxy polyethylene glycol (meth)acrylate or polyethylene glycol (meth)acrylate; and amino-containing unsaturated monomers and their quaternized products, such as (N,N)-dimethylaminoethyl (meth)acrylate or (N,N)-dimethylaminopropyl (meth)acrylamide.

[0092] Here, the acrylic acid-based monomer has acidic groups, and at least a portion of these acidic groups are neutralized. Preferably, this can be done by partially neutralizing the monomer with an alkaline substance, and examples of usable neutralizing agents include alkaline substances such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide.

[0093] At this point, the degree of neutralization of the monomer can be 40 mol% to 95 mol%, or 40 mol% to 80 mol%, or 45 mol% to 75 mol%. The range of neutralization can be varied depending on the final physical properties, but if the neutralization is too high, the neutralized monomers may precipitate, making polymerization difficult to proceed smoothly. Conversely, if the neutralization is too low, the absorbability of the polymer may be significantly reduced, and it may exhibit unmanageable properties such as elastic rubber.

[0094] A method for preparing a superabsorbent polymer according to one embodiment of the present invention uses a combination of a surfactant having a specific HLB range and weight-average molecular weight with an encapsulating foaming agent in the polymerization step.

[0095] In the polymerization step, by using an encapsulated foaming agent in combination with a low molecular weight surfactant having an HLB of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol, the foaming effect is maximized due to the formation of an optimal micelle structure in the neutralized solution, thereby achieving excellent water retention capacity and absorption rate. In particular, by increasing the surface tension of the final superabsorbent polymer via hydrophobicity, it does not detach from the product when applied, thus further improving absorption durability. Preferably, the surfactant is a non-polymer surfactant.

[0096] On the other hand, when using commonly used carbonate-based blowing agents, it is difficult to control the foaming performance, and therefore it may be difficult to achieve the desired level of surface tension improvement and absorption properties. In contrast, the encapsulated blowing agent of the present invention expands and then contracts within the polymerization temperature range, making it easy to control the foaming performance, promoting the formation of pores as expected, and enabling a synergistic effect when used in combination with the aforementioned low molecular weight surfactants having specific HLB values.

[0097] In addition, the monomer composition may also contain additives such as polymerization initiators, thickeners, plasticizers, preservation stabilizers and antioxidants, as needed.

[0098] As polymerization initiators, thermal polymerization initiators or photopolymerization initiators can be used depending on the polymerization method. However, even in photopolymerization, a certain amount of heat is generated by ultraviolet irradiation, etc., and in addition, a certain amount of heat is generated as the polymerization reaction (exothermic reaction) proceeds, so a thermal polymerization initiator may also be included.

[0099] Here, as a photopolymerization initiator, one or more compounds selected from the following can be used, for example: benzoin ether, dialkyl acetophenone, hydroxyalkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acylphosphine, and α-aminoketone. As a specific example of an acylphosphine, commercially available lucirin TPO, namely 2,4,6-trimethylbenzoyl-trimethylphosphine oxide, or phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, can be used. More different photopolymerization initiators are disclosed on page 115 of Reinhold Schwalm's "UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)" and may be referenced therein.

[0100] As thermal polymerization initiators, compounds selected from one or more of the following can be used: persulfate-based initiators, azo-based initiators, hydrogen peroxide, and ascorbic acid. Specifically, examples of persulfate-based initiators include sodium persulfate (Na₂S₂O₈), potassium persulfate (K₂S₂O₈), and ammonium persulfate ((NH₄)₂S₂O₈). Furthermore, examples of azo-based initiators include 2,2-azobis(2-amidinylpropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoylazo)-isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, and 4,4-azobis(4-cyanopentanoic acid). Various other thermal polymerization initiators are disclosed on page 203 of Odian’s book “Principle of Polymerization” (Wiley, 1981), which may be consulted.

[0101] Based on the weight of the acrylic acid-based monomer, the polymerization initiator can be used in amounts from 10 ppmw to 10,000 ppmw. When present within the above content range, sufficient strength levels can be achieved through adequate crosslinking, and sufficient absorption properties can be achieved by introducing appropriate crosslinking structures. Preferably, the polymerization initiator can be present in amounts of 50 ppmw or more, 80 ppmw or more, 100 ppmw or more, or 500 ppmw or more, and 10,000 ppmw or less, 9,000 ppmw or less, 7,000 ppmw or less, or 5,000 ppmw or less. More specifically, it can be included in amounts from 50 ppmw to 5,000 ppmw, 80 ppmw to 5,000 ppmw, or 80 ppmw to 3,000 ppmw. These amounts refer to the total amount when a photopolymerization initiator and a thermal polymerization initiator are used in combination.

[0102] When the concentration of the polymerization initiator is too low, the polymerization rate may decrease, and a large amount of residual monomer may dissolve from the final product, which is undesirable. Conversely, when the concentration of the polymerization initiator is too high, the polymer chains forming the network become shorter, leading to an increase in the content of water-soluble components and a decrease in the absorption rate under pressure, thus deteriorating the physical properties of the resin, which is also undesirable.

[0103] Furthermore, such monomer compositions can be prepared in the form of a solution in which raw materials such as the aforementioned acrylic-based monomers, internal crosslinking agents, surfactants, foaming agents, and polymerization initiators are dissolved in a solvent.

[0104] In this case, any solvent capable of dissolving the aforementioned raw materials can be used, and there are no restrictions on its composition. For example, the solvent may be 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 pentyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, N,N-dimethylacetamide, or mixtures thereof.

[0105] The step of forming a hydrogel polymer by polymerizing a monomer composition can be carried out by conventional polymerization methods, and the process is not particularly limited. As a non-limiting example, polymerization methods can be broadly classified into thermal polymerization and photopolymerization based on the type of energy source for polymerization. In the case of thermal polymerization, it can be carried out in a reactor with a stirring shaft, such as a kneader, while in the case of photopolymerization, it can be carried out in a reactor equipped with a movable conveyor belt.

[0106] For example, hydrogel polymers can be obtained by feeding a monomer composition into a reactor, such as a kneader, equipped with a stirring shaft, and thermally polymerizing it by supplying hot air to the reactor or by heating the reactor. The hydrogel polymer discharged from the reactor outlet can then be recovered as particles ranging from a few millimeters to a few centimeters, depending on the shape of the stirring shaft within the reactor. Specifically, the resulting hydrogel polymer can take various forms depending on the concentration of the monomer composition and the injection rate, and typically yields hydrogel polymers with a (weight-average) particle size of 2 mm to 50 mm.

[0107] Furthermore, as another example, sheet-like hydrogel polymers can be obtained when the monomer composition is photopolymerized in a reactor equipped with a movable conveyor belt. In this case, the thickness of the sheet can vary depending on the concentration and injection rate of the injected monomer composition, but it is generally preferred to control it to a thickness of 0.5 cm to 10 cm to ensure that the entire sheet can polymerize uniformly while achieving production speed, etc.

[0108] The typical moisture content of the hydrogel polymer obtained by this method can be from 40% to 80% by weight. Throughout this specification, "moisture content" refers to the amount of water relative to the total weight of the hydrogel polymer, and is the value obtained by subtracting the weight of the polymer in its dried state from the weight of the hydrogel polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to evaporation of moisture in the polymer during the process of raising the polymer's temperature by infrared heating and drying it. The drying conditions are as follows: raising the temperature from room temperature to 180°C and then maintaining it at 180°C, with a total drying time set to 20 minutes, including a 5-minute heating phase, and measuring the moisture content during this process.

[0109] The typical moisture content of the hydrogel polymer obtained by the above method can be from 40% to 80% by weight. Here, "moisture content" refers to the amount of water relative to the total weight of the hydrogel polymer, and is the value obtained by subtracting the weight of the dried polymer from the weight of the hydrogel polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to moisture evaporation during the drying process of raising the polymer temperature by infrared heating. In this case, the drying conditions used to measure the moisture content involve raising the temperature from room temperature to approximately 180°C and holding it for approximately 40 minutes.

[0110] (Drying, pulverizing and grading steps)

[0111] Next, it includes the step of forming a base resin powder by drying, pulverizing and classifying the hydrogel polymer.

[0112] Specifically, the obtained hydrogel polymer is dried. If necessary, to improve the efficiency of the drying step, a further step of coarsely pulverizing the hydrogel polymer can be performed before the drying step.

[0113] At this time, there are no restrictions on the configuration of the pulverizer used, but specifically, it may include any of the following groups of pulverizing equipment: vertical pulverizer, turbine cutter, turbine mill, rotary cutter mill, cuttermill, disc mill, fragment crusher, crusher, chopper and disc cutter, but it is not limited to the above examples.

[0114] At this point, the coarse grinding step can reduce the particle size of the hydrogel polymer to 2 mm to 10 mm. Due to the high moisture content of the hydrogel polymer, grinding it to a particle size of less than 2 mm is technically difficult, and agglomeration may also occur between the ground particles. On the other hand, when the particle size exceeds 10 mm, the effect of improving the efficiency of the subsequent drying step may not be significant.

[0115] Immediately after polymerization, the hydrogel polymer, whether coarsely pulverized as described above or without undergoing a coarse pulverization step, is dried. The drying temperature can be between 150°C and 250°C. When the drying temperature is below 150°C, the drying time becomes too long, potentially degrading the physical properties of the final superabsorbent polymer. Conversely, when the drying temperature exceeds 250°C, only the polymer surface is over-dried, which may cause fine powder to form during subsequent pulverization, again potentially degrading the physical properties of the final superabsorbent polymer. Therefore, drying is preferably carried out at a temperature between 150°C and 200°C, more preferably between 170°C and 195°C.

[0116] Meanwhile, considering factors such as process efficiency, the drying time can range from 20 to 90 minutes, but is not limited to this.

[0117] The drying method for the drying step can be selected and used without limitation on its configuration, as long as it is generally used in the drying process of hydrogel polymers. Specifically, the drying step can be carried out by methods such as hot air supply, infrared irradiation, microwave irradiation, or ultraviolet irradiation. After such a drying step, the moisture content of the polymer can be from about 0.1% by weight to about 10% by weight.

[0118] Next, the dried polymer obtained through this drying step is pulverized.

[0119] The particle size of the powder obtained after the pulverization step can be from 150 μm to 850 μm. Specifically, a pin mill, hammer mill, spiral mill, roller mill, disc mill, or jog mill can be used as a pulverizer for pulverizing to such a particle size, but it is not limited to the above examples.

[0120] Furthermore, in order to control the physical properties of the superabsorbent polymer particles that are ultimately commercialized after such a pulverization step, a separate process can be performed to classify the polymer particles obtained after pulverization according to their particle size. Preferably, the particles are classified into particles with a particle size of 150 μm to 850 μm, and only polymer particles with such a particle size can be commercialized after undergoing the surface crosslinking reaction step.

[0121] More specifically, the graded base resin powder has a particle size of 150 μm to 850 μm and may contain 50% by weight or more of particles with a particle size of 300 μm to 600 μm.

[0122] Furthermore, the centrifugal retention capacity (CRC) of the base resin, as measured according to EDANA WSP 241.3, can be 42 g / g or greater. Preferably, it can be 42 g / g to 45 g / g, 42 g / g to 44.5 g / g, or 42.5 g / g to 44.5 g / g. The specific measurement method will be described in more detail in the experimental examples described later.

[0123] The vortex absorption rate of the base resin can be 50 seconds or less. Preferably, it can be 48 seconds or less, or 48 seconds or less. The lower limit of the vortex absorption rate can be 10 seconds or more, or 15 seconds or more. The specific measurement method will be described in more detail in the experimental examples described later.

[0124] (Surface crosslinking step)

[0125] Next, a method for preparing a superabsorbent polymer according to an embodiment of the present invention includes the following steps: preparing a polyacrylate-based superabsorbent polymer wherein a surface crosslinking layer is formed on the surface of the base resin powder by heat-treating a base resin powder in the presence of a surface crosslinking agent.

[0126] The surface crosslinking step is to induce a crosslinking reaction on the surface of the base resin in the presence of a surface crosslinking agent, and to crosslink the unsaturated bonds of the uncrosslinked acrylic-based monomers that remain on the surface through the surface crosslinking agent, thereby forming a polyacrylate-based superabsorbent polymer with increased surface crosslinking density.

[0127] Specifically, a surface crosslinking layer can be formed on at least a portion of the surface of the base resin powder by performing a heat treatment process in the presence of a surface crosslinking agent. According to the heat treatment process, the external crosslinking density increases due to the increased surface crosslinking density, while the internal crosslinking density remains unchanged. Therefore, the prepared superabsorbent polymer in which the surface crosslinking layer is formed has a structure in which the external crosslinking density is higher than the internal crosslinking density.

[0128] In the surface crosslinking step, a surface crosslinking agent composition containing an alcohol-based solvent and water in addition to the surface crosslinking agent can be used.

[0129] On the other hand, any crosslinking agent component commonly used in the preparation of superabsorbent polymers can be used as the surface crosslinking agent in the composition without any particular limitation. For example, the surface crosslinking agent may include: at least one polyol selected from ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; at least one carbonate-based compound selected from ethylene carbonate and propylene carbonate; and an epoxy compound such as ethylene glycol diglycidyl ether. Azoline compounds, for example azole ketones; polyamine compounds; Azoline compounds; monozoline compounds azole ketone compounds, di azole ketone compounds or more Zolpidemone compounds; or cyclic urea compounds; etc. Preferably, the same crosslinking agent as the aforementioned internal crosslinking agent can be used, for example, compounds based on alkylene glycol diglycidyl ethers, such as propylene glycol diglycidyl ether and ethylene glycol diglycidyl ether.

[0130] Based on 100 parts by weight of the base resin powder, such a surface crosslinking agent can be used in amounts from 0.001 parts by weight to 2 parts by weight. Preferably, it can be used in amounts of 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.02 parts by weight or more, as well as 0.5 parts by weight or less, or 0.3 parts by weight or less. By controlling the content of the surface crosslinking agent within the above-mentioned range, superabsorbent polymers exhibiting various physical properties such as excellent absorption performance and liquid permeability can be prepared.

[0131] On the other hand, the surface crosslinking agent is added to the base resin powder in the form of a surface crosslinking agent composition containing it, and there are no particular limitations on the method of adding the surface crosslinking agent composition. For example, methods such as placing both the surface crosslinking agent composition and the base resin powder into a reaction vessel and mixing them, spraying the surface crosslinking agent composition onto the base resin powder, or continuously supplying the base resin powder and the surface crosslinking agent composition into a continuously operating mixer and mixing them can be used.

[0132] Furthermore, the surface crosslinking agent composition may also contain water and / or a hydrophilic organic solvent as a medium. This provides the advantage of uniformly dispersing the surface crosslinking agent on the base resin. In this case, to induce uniform dissolution / dispersion of the surface crosslinking agent and prevent agglomeration of the base resin, and simultaneously optimize the surface penetration depth of the surface crosslinking agent, the content of water and hydrophilic organic solvent can be adjusted by regulating the addition ratio relative to 100 parts by weight of the base resin powder.

[0133] The surface crosslinking step can be performed by heat treatment at a temperature of 110°C to 200°C, or 110°C to 150°C, for 30 minutes or longer. More specifically, surface crosslinking can be performed by heat treatment at the highest reaction temperature for 30 minutes to 80 minutes, or 40 minutes to 70 minutes, where the above temperature is the highest reaction temperature.

[0134] By satisfying such surface crosslinking process conditions (especially heating conditions and reaction conditions at the highest reaction temperature), superabsorbent polymers that appropriately satisfy physical properties such as better pressurized liquid permeability can be prepared.

[0135] There are no particular limitations on the heating method used for surface crosslinking reactions. Heating can be achieved by supplying a heat medium or by directly supplying a heat source. As for the type of heat medium available, heated fluids such as steam, hot air, or hot oil can be used, but it is not limited to these. Furthermore, the temperature of the supplied heat medium can be appropriately selected considering the type of heat medium, the heating rate, and the target temperature. On the other hand, as a directly supplied heat source, heating methods using electricity or gas can be mentioned, but it is not limited to the examples above.

[0136] On the other hand, in a method for preparing a superabsorbent polymer according to one embodiment of the present invention, various polyvalent metal salts, such as aluminum salts like aluminum sulfate, can be further used during surface crosslinking to further improve liquid permeability, etc. Such polyvalent metal salts can be included on the surface crosslinked layer of the finally prepared superabsorbent polymer.

[0137] absorbent products

[0138] According to another embodiment of the invention, a superabsorbent polymer is used in absorbent articles to achieve excellent absorption durability.

[0139] In the case of absorbent articles, components commonly used in the art can be used, and various conventionally applied laminated structures can be applied without particular limitation. For example, in the case of absorbent articles, samples can be prepared by sequentially laminating a liquid-impermeable substrate, an absorbent layer containing a superabsorbent polymer, and a liquid-permeable top sheet.

[0140] The liquid-impermeable substrate is a material in which liquid is prevented from penetrating, and materials commonly used in the art can be used without particular limitation. For example, nonwoven fabrics prepared by spinning olefin resins such as polyethylene or polypropylene can be used. The method of preparing the nonwoven fabric is not particularly limited, and nonwoven fabrics prepared by processing methods such as air-laid, thermal bonding, hydroentangling, spunbond, melt-blown, and stitch-bonded methods can be used without limitation. Furthermore, a breathable and waterproof membrane may be additionally included as needed.

[0141] The absorbent layer may include at least one resin layer containing the superabsorbent polymer described above and at least one nonwoven fabric.

[0142] In this case, the preparation of the resin layer containing the superabsorbent polymer can be carried out by methods commonly used in the art, for example, by uniformly applying the superabsorbent polymer to the nonwoven fabric layer. For example, a uniform absorbent layer can be prepared by continuously supplying the superabsorbent polymer to the nonwoven fabric layer using a roller feeder.

[0143] According to one embodiment of the invention, the absorbent layer may be a structure in which an upper nonwoven fabric, a first resin layer, a middle nonwoven fabric, a second resin layer, and a lower nonwoven fabric are sequentially laminated. The layers can be laminated using methods commonly used in the art, for example, by using an adhesive to laminate the layers. Furthermore, an acquisition distribution layer (ADL) may be additionally laminated between the absorbent layer and the liquid-permeable topsheet to improve diffusion.

[0144] In this context, the nonwoven fabric can be a hydrophilic nonwoven fabric made from synthetic or natural fibers such as polypropylene, polyethylene, polyester, polyethylene terephthalate, rayon, cotton, or degreased cotton. There are no particular limitations on the method of preparing the nonwoven fabric, and there are no restrictions on the nonwoven fabrics prepared by processing methods such as air-laid, thermal bonding, hydroentangling, spunbond, meltblown, and stitch-bonded methods.

[0145] The liquid-permeable topsheet is made of a hydrophilic material that allows for rapid liquid permeation, and can be made of any material commonly used in the art without particular limitation. Furthermore, materials that do not irritate the user's skin can be suitably selected. For example, it can be a hydrophilic nonwoven fabric, a porous membrane such as a porous polyethylene membrane, or a foamed membrane such as polyurethane foam; one or more of these can be used in layers. More specifically, the hydrophilic nonwoven fabric can be made from synthetic or natural fibers such as polypropylene, polyethylene, polyester, polyethylene terephthalate, rayon, cotton, or degreased cotton. There are no particular limitations on the method of manufacturing the nonwoven fabric, and nonwoven fabrics produced by processing methods such as air-laid, thermal bonding, hydroentangling, spunbond, melt-blown, and stitch-bonded can be used without limitation.

[0146] As described above, by using a combination of surfactants with HLB values ​​of 1 to 10 and weight-average molecular weights of 200 g / mol to 1,500 g / mol with a foaming agent, the superabsorbent polymer maximizes the foaming effect by forming an optimal micelle structure in the neutralized solution, thereby achieving excellent retention capacity and absorption rate. In particular, due to its hydrophobicity, the surface tension of the superabsorbent polymer ultimately increases, further improving absorption durability by preventing the superabsorbent polymer from detaching from the product. Specifically, this superabsorbent polymer exhibits excellent interaction with the binder layer and provides excellent absorption durability under absorption conditions.

[0147] Superabsorbent polymers possess excellent absorption durability, which indicates the degree to which the superabsorbent polymer detaches from the absorbent article when it is subjected to physical impact under brine absorption conditions.

[0148] Specifically, the absorption durability, measured by the following method, is 1,500 cycles or more, preferably 2,000 cycles or more, 3,000 cycles or more, or may be 2,000 to 3,000 cycles.

[0149] The method for measuring absorption durability is as follows: 70 ml of a 0.9% by weight sodium chloride (NaCl) aqueous solution containing the dye at 23±1°C was injected into a sample comprising an absorbent containing an absorbent resin, and allowed to absorb for 5 minutes. The impact test was repeated, in which the sample was lifted vertically 2 cm from the ground and then allowed to fall freely. The number of times half of the area in the sample that absorbed the dye was detached was measured. Further details will be described in the experimental examples section, which will be described later.

[0150] When superabsorbent polymers are prepared into absorbent articles, they can absorb brine for a short time without deterioration of absorbency, even when brine is cumulatively injected. Specifically, the absorbent articles have excellent physical properties of cumulative absorption time (first absorption time / second absorption time / third absorption time) for brine, and when brine is injected three times at 20-minute intervals for absorption, the total absorption time is 100 seconds or less, preferably 95 seconds or less, 90 seconds or less, 60 to 100 seconds, or 60 to 90 seconds.

[0151] The cumulative absorption time can be measured as follows: a sample of an absorbent article including an absorbent layer containing a superabsorbent polymer is placed in a symmetrical U-shaped frame with a 70° inclination (the bottom of the U-shaped frame is a concave arc with a radius of 5 cm and a central angle of 90°, and the sides extending to both sides from the end of the arc have a 70° inclination).

[0152] Next, 50 ml of a 0.9% by weight sodium chloride (NaCl) aqueous solution at 35±1°C was injected into the sample, and the time for complete absorption of the brine was measured. The time for complete absorption was visually assessed as the point at which the brine disappeared from the top layer of the absorbent material. Subsequently, the cumulative absorption time was measured three times at 20-minute intervals from the start of the brine injection. The absorption times for each of the first, second, and third rounds could independently be less than 40 seconds, less than 38 seconds, or less than 36 seconds, and could be 10 seconds or longer, or 15 seconds or longer. Simultaneously, their sum could meet the above range (100 seconds or less). Further details will be described in the experimental examples section later.

[0153] In the following, the operation and effects of the invention will be described in more detail through specific embodiments. However, these embodiments are presented merely as examples of the invention, and the scope of the invention is not defined thereto.

[0154] [Example]

[0155] <Preparation of Superabsorbent Polymers>

[0156] Example 1

[0157] (Step 1: Aggregation)

[0158] In a 2L glass container equipped with a stirrer and thermometer, add 100 g of acrylic acid, 0.2 g of ethylene glycol diglycidyl ether as an internal crosslinking agent (2,000 ppmw relative to 100 parts by weight of acrylic acid), 0.008 g of phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide as a photopolymerization initiator (80 ppmw relative to 100 parts by weight of acrylic acid), and 0.2 g of sodium persulfate (SPS) as a thermal polymerization initiator (2,000 ppmw relative to 100 parts by weight of acrylic acid) and dissolve them. Then mix with 128 g of 31.5% caustic soda solution. Calcium stearate (HLB 5 / Mw 607, 400 ppmw relative to 100 parts by weight of acrylic acid) as a surfactant was dispersed in 63.5 g of water, and 0.25 g of F36D (2,000 ppmw relative to 100 parts by weight of acrylic acid) as a foaming agent was mixed with it to prepare a monomer aqueous solution (neutralization degree: 75 mol%; solids content: 43 wt%).

[0159] When the temperature of the monomer aqueous solution rises to 40°C due to the heat of neutralization, ultraviolet light (irradiance: 10 mV / cm) is used. 2 The mixture was irradiated for 1 minute to perform UV polymerization, thereby obtaining a hydrogel polymer sheet.

[0160] The obtained hydrogel polymer sheet was passed through a shredder with a 16 mm aperture to prepare pulverized material (fragments).

[0161] (Step 2: Drying, pulverizing and grading)

[0162] Next, the pulverized material (fragments) is dried in an oven that allows airflow to move up and down. The drying is carried out in multiple stages, and specifically, an airflow oven is used, allowing hot air at 185°C to flow from bottom to top for 15 minutes and from top to bottom for 15 minutes to ensure uniform drying, and the moisture content of the dried body is set to 2% or less after drying.

[0163] The dried hydrogel polymer was pulverized using a pulverizer and graded using a standard sieve according to ASTM standards to obtain a base resin powder with a particle size of 150 μm to 850 μm.

[0164] (Step 3: Surface cross-linking)

[0165] First, a surface crosslinking composition comprising 3.5 parts by weight of water, 3.0 parts by weight of propylene glycol and 0.15 parts by weight of ethylene glycol diglycidyl ether (EGDGE) was prepared (based on 100 parts by weight of base resin powder).

[0166] Subsequently, 100 parts by weight of base resin powder and 6.62 parts by weight of the prepared surface crosslinking composition were uniformly mixed, and the surface crosslinking mixture was supplied to a surface crosslinking reactor to carry out the surface crosslinking reaction of the base resin particles at 120°C for 30 minutes, thereby preparing the final superabsorbent polymer.

[0167] Examples 2 to 7 and Comparative Examples 1 to 8

[0168] The superabsorbent polymer was prepared in the same manner as in Example 1, except that the components and amounts shown in Table 1 below were used in the polymerization step.

[0169] [Table 1]

[0170]

[0171] [Experimental Example 1: Evaluation of the physical properties of superabsorbent polymers] The physical properties of the base resin in step 2 and the final superabsorbent polymers prepared in the examples and comparative examples were evaluated in the following manner, and the results are shown in Table 2.

[0172] (1) Centrifugation retention capacity (CRC)

[0173] For each of the base resins prepared in step 2 of the Examples and Comparative Examples and the final superabsorbent polymer, those with a particle size of 150 μm to 850 μm were taken, and the centrifugal retention capacity (CRC) of the absorbent capacity under no-load was measured according to the European Disposable and Nonwovens Association (EDANA) standard EDANA WSP 241.3.

[0174] Specifically, resins graded using a #30 to #50 sieve were obtained from the base resin and superabsorbent polymer obtained through the examples and comparative examples, respectively. Approximately 0.2 g of this resin was evenly placed into a nonwoven fabric bag and sealed. The bag was then immersed in physiological saline (0.9% by weight) at room temperature. After 30 minutes, the water was removed from the bag using a centrifuge at 250 G for 3 minutes, and the weight of the bag was measured. Furthermore, the same operation was performed without using resin, and then the mass was measured. Using the obtained masses, calculate according to the following Equation 1. .

[0175] [Equation 1]

[0176]

[0177] (2) Absorption rate under pressure (AUP)

[0178] For the superabsorbent polymers prepared in the examples and comparative examples, particles with a diameter of 150 μm to 850 μm were taken, and the absorbance (AUP) at 0.9 psi pressure was measured according to the European Disposable Products and Nonwovens Association (EDANA) standard EDANA method WSP 242.3.

[0179] First, install a 400-mesh stainless steel wire mesh at the bottom of a plastic cylinder with an inner diameter of 60 mm. Under room temperature and 50% humidity conditions, [the following steps are taken]. The superabsorbent polymer is uniformly distributed on a wire mesh, and a piston capable of uniformly applying a further load of 0.9 psi is made with an outer diameter slightly less than 60 mm, with no gap between it and the inner wall of the cylinder, and unimpeded in its upward and downward movement. The weight of the device is then measured. .

[0180] A 90 mm diameter, 5 mm thick glass filter was placed in a 150 mm diameter petri dish, and physiological saline solution consisting of 0.9 wt% sodium chloride was brought to the same level as the top surface of the glass filter. A 90 mm diameter filter paper was placed on top of the filter. The measuring device was placed on the filter paper and allowed to absorb the liquid under load for 1 hour. After 1 hour, the measuring device was lifted and its weight was measured. Using the obtained masses, calculate the absorption rate (g / g) under pressure according to the following Equation 2.

[0181] [Equation 2]

[0182]

[0183] (3) Vortex absorption rate (seconds)

[0184] For each of the base resin prepared in step 2 and the final superabsorbent polymers in the examples and comparative examples, measurements were taken in seconds according to the method described in International Patent Publication No. 1987-003208.

[0185] First, for each of the base resin and the final superabsorbent polymer, particles with a diameter of 150 μm to 850 μm were taken, and 2 g of resin sample was placed in 50 mL of physiological saline (24.4 ± 0.2 °C) and stirred at 600 rpm to measure the time (in seconds) until the vortex disappeared.

[0186] (4) Surface tension (mN / m)

[0187] For the superabsorbent polymers prepared in the examples and comparative examples, the surface tension was measured according to the dipstick method.

[0188] First, 1 g of the superabsorbent polymer was placed in 200 ml of brine and stirred at 500 rpm for 5 minutes at room temperature (24 ± 2 °C). After 5 minutes, the solution was filtered through filter paper to obtain the filtered solution, and then a process tensiometer (manufactured by KRUSS) was used to measure the force at the end of the plate after the filtrate to be measured contacts the surface at the end of the plate and then separates from the solution.

[0189] [Table 2]

[0190]

[0191] As can be seen from Table 2, the superabsorbent polymer of the present invention, through the use of a combination of a surfactant with an HLB of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol and a foaming agent, maximizes the foaming effect due to the formation of an optimal micelle structure in the neutralized solution, thereby improving excellent retention capacity and absorption rate. In particular, it was determined that absorption durability can be further improved because the superabsorbent polymer does not detach from the product when applied, due to the increased surface tension of the final superabsorbent polymer caused by hydrophobicity. In comparative examples using polymer surfactants that do not meet the HLB range or have large weight-average molecular weight values, the absorption characteristics are lower than those of the examples, and the surface tension is significantly reduced, thus determining that durability may be significantly degraded when applied to actual products.

[0192] [Experimental Example 2: Evaluation of the Physical Properties of Absorbent Products]

[0193] First, using the superabsorbent polymers prepared in the examples and comparative examples, absorbent articles were prepared according to the following method.

[0194] The liquid-impermeable film, the absorbent layer containing superabsorbent polymer, and the liquid-permeable top film were each cut into 47 cm pieces. 19 cm thick, and these layers were sequentially laminated to prepare a superabsorbent polymer sample. The absorbent layer was prepared by sequentially laminating an upper nonwoven fabric, a first resin layer, a middle nonwoven fabric, a second resin layer, and a lower nonwoven fabric using a styrene-based adhesive. An ADL (collection and distribution layer) was additionally laminated between the absorbent layer and the top sheet to improve diffusion. 6.5 g of the superabsorbent polymer prepared in the examples and comparative examples was applied to each of the first and second resin layers by feeding with a roller feeder.

[0195] The physical properties of the prepared absorbent product samples were evaluated in the following manner, and the results are shown in Table 3.

[0196] (1) Absorption durability (count)

[0197] 70 ml of a 0.9% by weight sodium chloride (NaCl) aqueous solution at 23±1℃ was injected into the sample (47 cm²) including an absorbent layer containing 13 g of superabsorbent polymer. The sample was placed in a solution of sodium chloride (19 cm) and allowed to absorb for 5 minutes. For clearer measurement, dye was added to the sodium chloride solution and the stained saline solution was injected at a point 7.5 cm above the center of the sample; the area in which the saline solution was absorbed was then calculated.

[0198] Next, an impact test was conducted by repeatedly raising the sample containing the salt water vertically 2 cm above the ground and then letting it fall freely; the number of drops required until half of the dye-absorbing area on the sample was removed was measured, and the results are shown in Table 3.

[0199] (2) Cumulative absorption time (first absorption time / second absorption time / third absorption time, seconds)

[0200] The sample (47 cm) will be used as an absorbent layer containing 13 g of superabsorbent polymer. An absorbent article (19 cm) is placed in a symmetrical U-shaped frame with a 70° inclination (the bottom of the U-shaped frame is a concave arc with a radius of 5 cm and a central angle of 90°, and the sides extending from the end of the arc to both sides have a 70° inclination).

[0201] Next, 50 ml of a 0.9% by weight sodium chloride (NaCl) aqueous solution at 35±1℃ was injected into the sample, and the time for complete absorption of the brine was measured. The time for complete absorption of the brine was visually assessed as the time when the brine disappeared from the top layer of the absorbent material. Subsequently, the cumulative absorption time was measured by repeating the process three times at 20-minute intervals from the point where the brine was first injected, and the results are shown in Table 3 below.

[0202] [Table 3]

[0203]

[0204] As shown in Table 3, absorbent articles using superabsorbent polymers that satisfy the four physical properties of the present invention exhibit excellent cumulative absorption rates under practical use conditions. In particular, due to appropriate hydrophobicity, the surface tension of the resin increases, thereby enabling excellent absorption durability under practical use conditions.

Claims

1. A polyacrylate-based superabsorbent polymer satisfying i) to iv) below: i) a surface tension of 63 mN / m to 72 mN / m measured according to the Loop Test at room temperature of 24 ± 2°C, ii) a vortex absorption rate of 30 seconds or less, iii) a centrifuge retention capacity (CRC) of 32 g / g or more measured according to EDANA WSP 241.3, iv) an absorbency under pressure (AUP) of 12 g / g or more at 0.9 psi measured according to EDANA WSP 242.

3.

2. The superabsorbent polymer according to claim 1, wherein the polyacrylate-based superabsorbent polymer comprises: a base resin comprising a hydrogel polymer obtained by polymerizing a monomer mixture comprising an acrylic acid-based monomer having at least partially neutralized acidic groups, an internal crosslinking agent, a surfactant having an HLB of 1 to 10 and a weight average molecular weight of 200 g / mol to 1,500 g / mol, and an encapsulated blowing agent; and a surface crosslinking layer formed on the surface of the base resin powder.

3. The superabsorbent polymer according to claim 1, wherein the surfactant includes at least one selected from the group consisting of calcium stearate, glycerol monolaurate, polyoxyethylene (10) lauryl ether, sucrose dioleate, propylene glycol monostearate, and ethylene glycol distearate.

4. The superabsorbent polymer according to claim 1, wherein the surfactant is included in an amount of 10 ppmw to 10,000 ppmw based on the total amount of the acrylic acid-based monomer.

5. The superabsorbent polymer according to claim 1, wherein the encapsulated blowing agent has a structure comprising: a core comprising a hydrocarbon; and a shell surrounding the core and formed of a thermoplastic resin.

6. The superabsorbent polymer according to claim 5, wherein the hydrocarbon includes at least one selected from the group consisting of n-propane, n-butane, isobutane, cyclobutane, n-pentane, isopentane, cyclopentane, n-hexane, isohexane, cyclohexane, n-heptane, isohexane, cycloheptane, n-octane, isooctane, and cyclooctane, and the thermoplastic resin includes a polymer formed of at least one monomer selected from the group consisting of (meth)acrylate, (meth)acrylonitrile, aromatic vinyl monomer, vinyl acetate, haloethylene, and partial haloethylene.

7. The superabsorbent polymer according to claim 1, wherein the encapsulated blowing agent is included in an amount of 10 ppmw to 10,000 ppmw based on the total content of the acrylic acid-based monomer.

8. The superabsorbent polymer according to claim 1, wherein the surfactant and the encapsulated blowing agent are included in a weight ratio of 1:1 to 1:

50.

9. The superabsorbent polymer according to claim 1, The internal crosslinking agent comprises at least one selected from the following: N,N'-methylenebisacrylamide, allyl methacrylate, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol, glycerol, and ethylene carbonate.

10. The superabsorbent polymer according to claim 1, The internal crosslinking agent is included in an amount from 100 ppmw to 10,000 ppmw, based on the total content of the acrylic acid-based monomers.

11. A method for preparing a superabsorbent polymer, comprising the following steps: Hydrogel polymers are formed by polymerizing a monomer mixture comprising an acrylic acid-based monomer having at least partially neutralized acidic groups, an internal crosslinking agent, a surfactant having an HLB of 1 to 10 and a weight-average molecular weight of 200 g / mol to 1,500 g / mol, and an encapsulated foaming agent. The base resin powder is formed by drying, pulverizing, and classifying the hydrogel polymer; as well as A polyacrylate-based superabsorbent polymer with a surface cross-linked layer formed on the surface of the base resin powder is prepared by heat-treating the base resin powder in the presence of a surface cross-linking agent.

12. An absorbent article comprising: A sequentially laminated liquid-impermeable film, an absorbent layer, and a liquid-permeable top film. The absorbent layer comprises the superabsorbent polymer according to claim 1.

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