Monomer compositions for preparing superabsorbent polymers contained in absorbent articles

Abstract

This invention relates to a monomer composition for preparing superabsorbent polymers. The superabsorbent polymer is then incorporated into an absorbent article. The monomer composition for preparing superabsorbent polymers of this invention has a fast polymerization rate and therefore can minimize bubble loss during foam polymerization, thus providing a superabsorbent polymer with excellent absorption rate. Furthermore, the superabsorbent polymer prepared from the monomer composition for preparing superabsorbent polymers has high gel strength and therefore maintains its shape well even after water absorption, thus exhibiting excellent absorption performance despite external pressure.

Description

Technical Field

[0001] The present invention relates to a monomer composition for preparing a superabsorbent polymer, and the superabsorbent polymer is included in absorbent articles.

Background Art

[0002] A superabsorbent polymer (SAP) is a synthetic polymer material that can absorb 500 to 1000 times its own weight of water. It has begun to be commercialized as an absorbent article. Currently, in addition to hygiene products such as baby diapers and sanitary napkins, it is widely used as a water retention material for soil, a water stop material for civil engineering and construction, a seedling raising sheet, a freshness maintaining material in the food circulation field, a wet compress material, and the like.

[0003] Generally, a superabsorbent polymer is prepared by neutralizing a water-soluble unsaturated ethylenic monomer with an alkali metal salt such as a sodium salt or a basic compound such as caustic soda, and then polymerizing a monomer composition added with a crosslinking agent, a polymerization initiator, and the like.

[0004] As a method for improving the absorption rate of a superabsorbent polymer, a method is known in which a foaming agent is added to perform polymerization while forming bubbles in the monomer composition, thereby forming a porous structure in the superabsorbent polymer to increase the surface area.

[0005] However, in the case of the conventional method for preparing a superabsorbent polymer using a foaming agent, the bubbles generated by the foaming agent disappear before the polymerization of the monomer, and thus sufficient pores cannot be formed in the superabsorbent polymer. Therefore, a method of using a bubble stabilizer together with the foaming agent to reduce the loss of bubbles has been proposed, but the reduction of the surface tension of the superabsorbent polymer by the bubble stabilizer has become a problem.

[0006] Therefore, there is a demand for the development of a superabsorbent polymer that exhibits a fast absorption rate without degrading properties such as surface tension.

Summary of the Invention

[0007] The object of the present invention is to provide a monomer composition for preparing superabsorbent polymers that can prepare superabsorbent polymers with a fast absorption rate and excellent gel strength, which are then incorporated into absorbent articles. [Means for solving the problem]

[0008] A monomer composition for preparing superabsorbent polymers for incorporation into absorbent articles, wherein the monomer composition is A monomer composition is provided, comprising an acrylic acid monomer having acidic groups, at least a portion of which are neutralized, a crosslinking agent, clay, a carbonate-based blowing agent, and a polymerization initiator. When a monomer composition for preparing superabsorbent polymers is irradiated with heat and / or light to carry out a polymerization reaction, the normalized gelation point represented by the following formula 1 is 0.01 to 0.1. [Formula 1] Normalized gelation point = gelation point (seconds) / total polymerization time (seconds) In Equation 1, the gelation point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for preparing superabsorbent polymers with respect to polymerization time and the loss modulus graph of the monomer composition for preparing superabsorbent polymers with respect to polymerization time.

[0009] Absorbent articles containing superabsorbent polymers produced by using monomer compositions may be diapers or pants.

[0010] The superabsorbent polymer may be incorporated into an absorbent core, which is included in an absorbent article.

[0011] The absorbent core can be manufactured by providing two nonwoven webs and incorporating a surface-crosslinked particulate absorbent resin between the two nonwoven webs.

[0012] The absorbent core between the two nonwoven webs may contain less than 20% by weight of cellulose fibers, preferably less than 10% by weight of cellulose fibers, and more preferably less than 5% by weight of cellulose fibers.

[0013] In the absorbent core, the superabsorbent polymer may be bonded and fixed between two nonwoven webs.

[0014] The absorbent article may include a top sheet, a back sheet, and an absorbent core between the top sheet and the back sheet.

[0015] Furthermore, a method is provided for preparing a monomer composition for superabsorbent polymer preparation, the method comprising the steps of mixing an acrylic acid monomer having acid groups, of which at least a portion of the acid groups are neutralized, a crosslinking agent, a carbonate-based blowing agent, and a polymerization initiator, and adding clay to the mixture and shear mixing at a stirring speed of 6,500 rpm or more.

[0016] Furthermore, a polymer prepared from a monomer composition for preparing superabsorbent polymers is provided, and when the water content is 40% to 80% by weight, the gel strength is 40,000 Pa to 60,000 Pa.

[0017] Furthermore, a superabsorbent polymer prepared from a monomer composition for superabsorbent polymer preparation is provided, having a performance index (PI) of 2.35 or higher, represented by the following formula 1, and a T20 of less than 190 seconds. [Formula 1] Performance index (PI)=(CRC / 27)+(AUP / 24)+(SFC / 30)-(T20 / 140) In formula 1, CRC is the centrifuge retention capacity (g / g) of a superabsorbent polymer after 30 seconds of centrifugation in a 0.9 wt% sodium chloride aqueous solution. AUP is the absorbency under 0.7 psi pressure (g / g) of the superabsorbent polymer against an aqueous solution of 0.9 wt% sodium chloride for 1 hour, SFC is the saline flow conductivity of an aqueous solution of 0.685 wt% sodium chloride, T20 is the time (seconds) it takes for 1 g of the superabsorbent polymer to absorb 20 g of an aqueous solution of an alcohol ethoxylate having 12 to 14 carbon atoms under 0.3 psi.

[0018] Also provided is a method for preparing a superabsorbent polymer, comprising: irradiating a monomer composition for preparing a superabsorbent polymer with heat and / or light to polymerize and prepare a hydrogel polymer; drying, pulverizing and classifying the hydrogel polymer to form a base resin; and forming a surface crosslinked layer on the surface of the base resin in the presence of a surface crosslinking solution containing a surface crosslinking agent and a solvent.

[0019] The superabsorbent polymer of the present invention is incorporated into an absorbent article after its production.

Advantages of the Invention

[0020] The monomer composition for preparing a superabsorbent polymer of the present invention has a high gelation rate after polymerization initiation and an excellent bubble stabilization effect, so that the loss of bubbles generated from a foaming agent can be minimized, and the superabsorbent polymer is incorporated into an absorbent article later. Therefore, the superabsorbent polymer prepared from the monomer composition for preparing a superabsorbent polymer of the present invention has a plurality of pores inside and can exhibit an excellent absorption rate.

[0021] In addition, the superabsorbent polymer prepared from the monomer composition for preparing a superabsorbent polymer of the present invention has a high gel strength, so that it can maintain a good form even after water absorption, and can exhibit excellent absorption performance despite external pressure.

Embodiments for Carrying Out the Invention

[0022] The language and terminology used herein are for illustrative purposes only and are not intended to limit the invention. Singular expressions include plural expressions unless the context expressly does not, or is otherwise obvious. Where used herein, terms such as “includes,” “equip,” or “have” are intended to specify the presence of implemented features, numbers, processes, components, or combinations thereof, and do not preclude the presence or possibility of adding one or more other features, numbers, processes, components, or combinations thereof.

[0023] The present invention can be modified in various ways and may take on various forms, but specific embodiments are described in detail below as examples. However, this is not intended to limit the invention to any particular disclosure, and should be understood to include all modifications, equivalents, or substitutions that fall within the spirit and scope of the invention.

[0024] As used herein, “clay” means particles of phyllosilicate minerals or aggregates of several such particles, and “clay dispersion” means a dispersion in which clay is dispersed in a solvent.

[0025] As used herein, (meth)acrylate is used to mean both acrylate and methacrylate.

[0026] As used herein, "base resin" or "base resin powder" refers to a polymer obtained by polymerizing, drying, and grinding a water-soluble ethylene-based unsaturated monomer into particles or powder, without surface modification or surface crosslinking.

[0027] The present invention will be described in detail below.

[0028] A monomer composition for preparing superabsorbent polymers to be later incorporated into absorbent articles, comprising an acrylic acid monomer having acid groups, with at least a portion of those acid groups being neutralized, a crosslinking agent, clay, a carbonate-based blowing agent, and a polymerization initiator, When a monomer composition for preparing superabsorbent polymers is irradiated with heat and / or light to allow a polymerization reaction to proceed, the normalized gelation point represented by the following formula 1 is 0.01 to 0.1. [Formula 1] Normalized gelation point = gelation point (seconds) / total polymerization time (seconds) In Equation 1, the gelation point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for preparing superabsorbent polymers with respect to polymerization time and the loss modulus graph of the monomer composition for preparing superabsorbent polymers with respect to polymerization time.

[0029] The normalized gelation point of the monomer composition for preparing superabsorbent polymers may be 0.1 or less, 0.09 or less, 0.08 or less, 0.06 or less, and 0.01 or more, or 0.02 or more. The method for measuring the gelation point will be specifically described in the examples below.

[0030] When forming a polymer from a monomer composition for preparing superabsorbent polymers, the liquid monomer composition transforms into a hydrogel polymer. Therefore, the polymerization rate can be estimated by determining the gelation point, which is the time it takes for the monomer composition to gel. At this time, the polymerization rate of the monomer composition for preparing superabsorbent polymers can be evaluated by comparing the "normalized gelation point," which eliminates factors affecting polymerization time such as the amount of monomer, the amount of initiator, the initial polymerization temperature, and / or other polymerization conditions.

[0031] The monomer composition for preparing superabsorbent polymers of the present invention exhibits a faster polymerization rate compared to conventional monomer compositions, thereby minimizing the loss of bubbles generated from the blowing agent. Consequently, the absorption rate of the prepared superabsorbent polymer can be improved without excessive use of the blowing agent.

[0032] However, if the normalized gelation point is less than 0.01 and the polymerization rate is too fast, gelation may occur before the monomer composition is introduced into the polymerization reactor, potentially leading to clogging of the introduction pipe. Furthermore, if the viscosity of the monomer composition increases rapidly up to the gelation point, and the gelation point is less than 0.01, mixing of the monomer composition may be difficult, making it challenging to form a uniform polymer. Additionally, if the normalized gelation point exceeds 0.1, the aforementioned effect of reducing bubble loss cannot be achieved, and therefore, the effect of improving the absorption rate of the superabsorbent polymer cannot be expected.

[0033] Furthermore, when the monomer composition for preparing superabsorbent polymers is subjected to a polymerization reaction by irradiation with heat and / or light, the rate of increase in gel strength (storage modulus) from the gelation point to the completion of polymerization (ΔG' / second, where ΔG' is the difference between the storage modulus at the completion of polymerization and the storage modulus at the gelation point, and seconds is the time from the gelation point to the completion of polymerization) may be 210 Pa / second or more.

[0034] Preferably, the rate of increase in gel strength may be 215 Pa / sec or more, or 220 Pa / sec or more, or 230 Pa / sec or more. On the other hand, there is no particular upper limit to the rate of increase in gel strength, but for example, it may be 400 Pa / sec or less, or 370 Pa / sec or less, or 350 Pa / sec or less. Thus, the monomer composition for preparing superabsorbent polymers of the present invention exhibits a fast polymerization rate and a high rate of increase in gel strength simultaneously, and therefore, it is possible to prepare polymers having excellent gel strength.

[0035] Thus, by improving the rate of increase in the gelation point and gel strength, a hydrogel polymer prepared from a monomer composition for preparing superabsorbent polymers obtained immediately after polymerization is complete may have a gel strength of 35,000 Pa or more, or 36,000 Pa or more, or 37,000 Pa or more, when the water content is 40-80% by weight or 40-60% by weight. On the other hand, there is no particular upper limit to the gel strength of the hydrogel polymer, but it can satisfy, for example, 60,000 Pa or less, or 58,000 Pa or less, or 55,000 Pa or less.

[0036] The monomer composition for preparing superabsorbent polymers comprises an acrylic acid monomer, an alkali metal salt or alkali compound capable of neutralizing a water-soluble ethylene-based unsaturated monomer, a crosslinking agent, clay, a carbonate-based blowing agent, and a polymerization initiator, and satisfies the aforementioned properties. Specifically, the monomer composition for preparing superabsorbent polymers of the present invention contains clay as an additive, and since such clay is uniformly dispersed within the monomer composition, it can exhibit the aforementioned properties.

[0037] Acrylic monomers are compounds represented by the following chemical formula 1, [Chemical formula 1] R 1 -COOM 1 R is an alkyl group with 2 to 5 carbon atoms that contains an unsaturated bond. M' is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0038] Preferably, the monomer comprises acrylic acid, methacrylic acid, and one or more selected from the group consisting of monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts thereof.

[0039] Here, the acrylic acid monomer has acidic groups, and at least a portion of these acidic groups may be neutralized. Preferably, the monomer can be partially neutralized with an alkaline substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide before use. In this case, the degree of neutralization of the acrylic acid monomer may be 40-95 mol%, 40-80 mol%, or 45-75 mol%. The range of the degree of neutralization can be controlled according to the final properties. However, if the degree of neutralization is too high, the neutralized monomer will be extracted, and therefore polymerization will not proceed smoothly. Conversely, if the degree of neutralization is too low, the adsorption capacity of the polymer will be significantly reduced, and the polymer may exhibit properties similar to elastic rubber, which are difficult to handle.

[0040] The concentration of the acrylic acid monomer may be about 20 to about 60% by weight, preferably about 40 to about 50% by weight, based on the monomer composition containing the raw materials and solvent for the superabsorbent polymer, and can be appropriately adjusted considering the polymerization time and reaction conditions. However, if the monomer concentration is too low, the yield of the superabsorbent polymer will be low, and therefore problems may arise in terms of economic feasibility. Conversely, if the concentration is too high, some of the monomer may be extracted, or the grinding efficiency of the hydrogel polymer may be low, and therefore process problems may occur, and the properties of the superabsorbent polymer may be reduced.

[0041] Any compound that can introduce crosslinking during the polymerization of acrylic acid monomers can be used as a crosslinking agent. Crosslinking agents are also called "internal crosslinking agents" to distinguish them from "surface crosslinking agents" for surface crosslinking of superabsorbent polymer particles. Crosslinking by a crosslinking agent contained in the monomer composition proceeds without surface or internal division. However, if a surface crosslinking process proceeds for superabsorbent polymer particles obtained after polymerization, drying, grinding, and classification of the monomer composition, the surface of the final prepared superabsorbent polymer particles may consist of a structure crosslinked by the surface crosslinking agent, and the interior may consist of a structure crosslinked by the internal crosslinking agent.

[0042] Specifically, as the crosslinking agent included in the monomer composition, a crosslinking agent can be used that has one or more functional groups that can react with water-soluble substituents of acrylic acid monomers and simultaneously has one or more ethylenically unsaturated groups, or a crosslinking agent that has two or more functional groups that can react with water-soluble substituents of monomers and / or water-soluble substituents formed by hydrolysis of monomers.

[0043] As non-restrictive examples, crosslinking agents include N,N'-methylenebisacrylamide, trimethylpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate Polyfunctional crosslinking agents such as phosphate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerin, or ethylene carbonate may be used alone or in combination of two or more, but are not limited thereto.

[0044] Preferably, polyalkylene glycol di(meth)acrylate compounds such as polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, or polypropylene glycol (meth)acrylate can be used as crosslinking agents. When such crosslinking agents are used, foaming by the carbonate-based blowing agent described later can be easily promoted.

[0045] In monomer compositions, such crosslinking agents may be used in an amount of 0.01 to 5 parts by weight per 100 parts by weight of acrylic acid monomer. For example, the crosslinking agent may be used in amounts of 0.01 parts by weight or more, 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.15 parts by weight or more, and 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, 1 part by weight or less, or 0.7 parts by weight or less per 100 parts by weight of acrylic acid monomer. If the crosslinking agent content is excessively low, crosslinking may not occur sufficiently, making it difficult to achieve strength exceeding an appropriate level. If the crosslinking agent content is excessively high, the internal crosslink density may become high, making it difficult to achieve the desired centrifugal separation water retention capacity.

[0046] The monomer composition also includes a polymerization initiator for initiating the polymerization reaction of the monomers. The polymerization initiator is not particularly limited, as long as it is one that is commonly used in the preparation of superabsorbent polymers.

[0047] Specifically, depending on the polymerization method, a thermal polymerization initiator, a redox partner initiator, or a photopolymerization initiator induced by UV irradiation can be used as the polymerization initiator. However, even in the case of photopolymerization, a certain amount of heat is generated by UV irradiation, and a certain amount of heat is generated as the exothermic polymerization reaction progresses, so a thermal polymerization initiator can also be included.

[0048] Photopolymerization initiators can be used without restriction on their composition, as long as they are compounds that can form radicals when exposed to light such as UV.

[0049] As photopolymerization initiators, for example, one or more compounds selected from the group consisting of benzoin ether, dialkylacetophenone, hydroxyalkyl ketone, phenylglyoxylate, benzyldimethyl ketal, acylphosphine, and α-aminoketone can be used. Among these, a specific example of acylphosphine is lucilin TPO, i.e., 2,4,6-trimethylbenzoyl-trimethylphosphine oxide. A wider variety of photoinitiators are described in Reinhold Schwalm, "UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)," p. 115, and are not limited to the examples given above.

[0050] The photopolymerization initiator may be included in the monomer composition at a concentration of about 0.001 to about 1.0% by weight. If the concentration of the photopolymerization initiator is too low, the polymerization rate may be slow, and if the concentration is too high, the molecular weight of the superabsorbent polymer may be low, and its properties may become non-uniform.

[0051] As a thermal polymerization initiator, one or more can be 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), while examples of azo initiators include 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutylamidine dihydrochloride, 2-(carbamoylazo)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, and 4,4-azobis-(4-cyanovaleric acid). A wider variety of thermal polymerization initiators are described in Odian, "Principle of Polymerization (Wiley, 1981)," p. 203, but the present invention is not limited thereto.

[0052] The thermal polymerization initiator may be included in the monomer composition at a content of approximately 0.001 to approximately 0.5% by weight. If the concentration of the thermal polymerization initiator is excessively low, little additional thermal polymerization may occur, and therefore the effect of adding the thermal polymerization initiator may be minimal. If the concentration of the thermal polymerization initiator is excessively high, the molecular weight of the superabsorbent polymer may become small, and its properties may become non-uniform.

[0053] A redox counterpart initiator is a substance that causes a redox reaction between itself and other elements in the monomer composition. For example, potassium metabisulfite and sodium persulfate, or ascorbic acid and hydrogen peroxide may be used as redox counterpart initiators.

[0054] Redox-versus-initiators are preferable to be introduced into the monomer composition immediately before polymerization, as they initiate polymerization before photopolymerization by generating radicals during the oxidation-reduction process. Redox-versus-initiators (i.e., oxidizing agents and reducing agents, respectively) can be used in an amount of about 0.001 to about 0.5% by weight relative to the monomer composition.

[0055] Clay is included in the monomer composition to improve the polymerization rate and enhance gel strength. As used herein, “clay” means clay particles having nanoscale, or aggregates of a plurality of such particles.

[0056] As the clay, either a swellable or non-swellable clay can be used. Swellable clays are layered organic materials with water absorption capacity, and can include montmorillonite, saponite, nontronite, laponite, byderite, hectorite, vermiculite, magadiite, and bentonite. Non-swellable clays can include kaolin, serpentine, and mica. One type of clay may be used alone, or two or more types of clay may be used in combination.

[0057] As clay, those with an average particle size of 0.025 μm or more, or 0.05 μm or more, or 0.1 μm or more, or 1 μm or more, and 10 μm or less, or 8 μm or less, or 5 μm or less, or 3 μm or less can be used. If the average particle size of the clay is less than 0.025 μm, the effect of improving the gel strength of the superabsorbent polymer may not be sufficiently obtained. If the average particle size of the clay exceeds 10 μm, it may be difficult to form a uniform aqueous dispersion due to the strong interlayer attraction of the clay, and therefore transparency may not be ensured after dispersion. The average particle size of the clay can be measured using a particle size analyzer that uses laser diffraction and dynamic light scattering.

[0058] The clay content may be 0.1 parts by weight or more, or 0.2 parts by weight or more, or 0.25 parts by weight or more, and 10 parts by weight or less, or 7 parts by weight or less, or 5 parts by weight or less, or 3 parts by weight or less, or 1 part by weight or less, per 100 parts by weight of acrylic acid monomer. If the clay content is less than 0.1 parts by weight per 100 parts by weight of acrylic acid monomer, the normalized gelation point range described above may not be met. Furthermore, if the clay content exceeds 10 parts by weight per 100 parts by weight of acrylic acid monomer, uniform dispersion within the monomer composition may be difficult, and the improvement in the absorption rate of the prepared superabsorbent polymer may not be obtained.

[0059] On the other hand, in order to satisfy the gelation point characteristics described above, the clay must be uniformly dispersed within the monomer composition.

[0060] In this regard, it is preferable to use clay that has been physically or chemically delaminated, or to delaminate the clay within the monomer composition.

[0061] Clay has a structure in which layers of silicate plates, approximately 1 nm thick, overlap each other due to strong van der Waals forces, with a distance of approximately 1 nm between each layer. Delaminated clay is clay in which the silicate layers have been physically or chemically delaminated, meaning that the distance between layers has spread to more than 1 nm. Delaminated clay exhibits a more stable dispersion in the composition.

[0062] For example, as the clay, a clay surface-modified with a polymer dispersant containing two or more functional groups selected from the group consisting of amine groups, carbonyl groups, and hydroxyl groups can be used. In such clay, the layered structure is exfoliated by the polymer dispersant, significantly improving the dispersion stability. As a result, even when included in a composition for preparing superabsorbent polymers containing alkali metal salts or basic compounds, the clay particles can be uniformly dispersed in the composition without aggregation or sedimentation.

[0063] In this specification, "two or more functional groups selected from the group consisting of amine groups, carbonyl groups, and hydroxyl groups" includes functional groups formed by combining two or more of the amine groups, carbonyl groups, and hydroxyl groups. Examples include amide groups containing both an amine group and a carbonyl group, and carboxyl groups containing both a carbonyl group and a hydroxyl group. For example, a "polymer dispersant containing an amine group and a carbonyl group" may contain an amine group and a carbonyl group respectively within its molecule, or it may contain an amide group that combines an amine group and a carbonyl group.

[0064] Polymer dispersants containing functional groups can bond to the clay surface through lone electrons of nitrogen and oxygen atoms, and in high-ion concentration compositions for preparing superabsorbent polymers, charges can be induced within the polymer by resonance. Due to these properties, polymer dispersants can improve the dispersibility of clay by exfoliating the layered structure of the clay. Furthermore, polymer dispersants do not induce discoloration or deterioration of the properties of superabsorbent polymers, and are therefore suitable for use in the preparation of superabsorbent polymers.

[0065] In this regard, the polymer dispersant may preferably contain an amine group and a carbonyl group, or a carbonyl group and a hydroxyl group. More preferably, the polymer dispersant may contain an amide group and / or a carboxyl group.

[0066] Specific examples of polymer dispersants include one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylamide, and polyacrylic acid. Preferably, polyvinylpyrrolidone can be used as the polymer dispersant.

[0067] The molecular weight of the polymer dispersant is not particularly limited, but for example, those with a weight-average molecular weight of 2,500 g / mol or more, or 5,000 g / mol or more, or 10,000 g / mol or more, and 200,000 g / mol or less, or 100,000 g / mol or less, or 40,000 g / mol or less can be used. If the weight-average molecular weight of the polymer dispersant is less than 2,500 g / mol, it may be difficult to ensure sufficient distance between silicate layers of clay when the polymer dispersant binds to the clay surface, and therefore problems may arise in terms of dispersibility and long-term stability. If the weight-average molecular weight of the polymer dispersant exceeds 200,000 g / mol, process difficulties may arise and it may not be effective in modifying the clay surface. Therefore, it is preferable to satisfy the above ranges.

[0068] The polymer dispersant may be used in an amount of more than 20 parts by weight and up to 200 parts by weight per 100 parts by weight of clay to ensure the dispersibility of the clay, preferably in amounts of 23 parts by weight or more, or 25 parts by weight or more, and 150 parts by weight or less, or 100 parts by weight or less.

[0069] If the polymer dispersant content is 20 parts by weight or less per 100 parts by weight of clay, the degree of surface modification of the clay may be insufficient, and the interlayer distance between clay particles may not be sufficiently increased. As a result, the dispersion stability of the clay may not be sufficient, and the clay particles may be prone to aggregation. Furthermore, if the polymer dispersant content exceeds 200 parts by weight per 100 parts by weight of clay, the polymer dispersant may reduce the property improvement effect that can be provided by the clay and may degrade the properties of the superabsorbent polymer.

[0070] Surface-modified clay with a polymer dispersant can be prepared by adding the clay and polymer dispersant to a solvent such as water, a lower alcohol such as ethanol, or a glycol, and stirring. Alternatively, surface-modified clay with a polymer dispersant can be added to a monomer composition for superabsorbent polymer preparation in the form of a dispersion in which the clay is dispersed in the solvent. As the solvent for preparing the clay dispersion, the same solvent used for the monomer for superabsorbent polymer preparation can be used, and the amount of solvent used can be appropriately adjusted considering the type and amount of clay and polymer dispersant, the intended use of the clay dispersion, etc.

[0071] For example, a dispersion containing clay surface-modified with a polymer dispersant may contain clay in amounts of 1 part by weight or more, 3 parts by weight or more, and 20 parts by weight or less, 10 parts by weight or less, or 8 parts by weight or less per 100 parts by weight of the dispersion.

[0072] On the other hand, when using clay that has not been surface-modified, the clay dispersion can be added to the monomer composition by shear mixing, or the clay can be added to the monomer composition and then shear-mixed. Such shear mixing allows for physical delamination of the clay, thereby enabling a more uniform and stable dispersion of the clay in the monomer composition.

[0073] For information regarding the solvent and clay content in a dispersion for preparing a clay dispersion containing unmodified clay, refer to the description of a dispersion containing surface-modified clay with a polymer dispersant.

[0074] Shear mixing can be carried out using equipment such as an in-line high-shear mixer, a high-shear batch mixer, or a homogenizer.

[0075] Shear mixing may be carried out at stirring speeds of 6,500 rpm or higher, 9,000 rpm or higher, or 12,000 rpm or higher. The upper limit of the stirring speed is not particularly limited, but may be, for example, 50,000 rpm or lower, 40,000 rpm or lower, or 30,000 rpm or lower.

[0076] By shear mixing at such a stirring speed for at least 10 seconds, preferably 20 seconds or 30 seconds or more, the clay can be physically delaminated. However, if the shear mixing time is too long, heat may be generated due to the shear force, so it is preferable to perform the shear mixing for 3 minutes or less, or 1 minute or less.

[0077] On the other hand, when clay is added to a monomer composition and then shear mixing is performed, the unmodified clay can be added to the monomer composition in the form of an aqueous dispersion in order to achieve a more uniform degree of dispersion. The clay aqueous dispersion can be prepared by mixing water and clay using the aforementioned shear mixing apparatus at a stirring speed of 6,500 rpm or more, or 9,000 rpm or more, or 12,000 rpm or more. There is no particular upper limit to the stirring speed during shear mixing, but it may be, for example, 50,000 rpm or less, or 40,000 rpm or less, or 30,000 rpm or less. The clay aqueous dispersion can be prepared by mixing at the aforementioned stirring speeds for 10 seconds or more, or 20 seconds or more, or 30 seconds or more, and 5 minutes or less, or 3 minutes or less.

[0078] In this case, in order to ensure uniform dispersion, the concentration of the clay aqueous dispersion, that is, the clay content in 100 parts by weight of the clay aqueous dispersion, may be appropriately 0.5 parts by weight or more, or 1.0 part by weight or more, or 2.0 parts by weight or more, and 5.0 parts by weight or less, or 4.0 parts by weight or less.

[0079] As mentioned above, by using clay whose surface has been modified with a polymer dispersant, or by physically delaminating unmodified clay, the clay can be uniformly dispersed within the monomer composition, and the prepared monomer composition can satisfy the gelation point characteristics described above.

[0080] On the other hand, whether or not the clay is "uniformly dispersed" within the monomer composition can be determined by whether or not layer separation occurs in the monomer composition after mixing the clay and letting it stand for one minute. That is, if layer separation does not occur even after letting the monomer composition stand for one minute, it can be predicted that the clay is uniformly dispersed within the monomer composition, and as a result, the composition can satisfy the gelation point characteristics described above.

[0081] Carbonate-based foaming agents foam during polymerization and then form pores in the hydrogel polymer to increase its surface area. For example, one or more selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium bicarbonate, magnesium bicarbonate, and magnesium carbonate can be used.

[0082] Carbonate-based blowing agents can be used in a content of 0.005 to 1 part by weight per 100 parts by weight of acrylic acid monomer. If the content of the blowing agent is less than 0.005 parts by weight, its function as a blowing agent may be minimal, and if the content of the blowing agent exceeds 1 part by weight, the gel strength of the prepared superabsorbent polymer may be low due to too many pores inside the crosslinked polymer, resulting in low density and potentially causing problems in distribution and storage. For example, carbonate-based blowing agents can be used in content of 0.01 parts by weight or more, 0.05 parts by weight or more, and 0.5 parts by weight or less, 0.3 parts by weight or less, or 0.2 parts by weight or less per 100 parts by weight of acrylic acid monomer.

[0083] Furthermore, when preparing the monomer composition, it is not necessary to use surfactants commonly used as foam stabilizers, such as alkyl sulfate ester compounds and polyoxyethylene alkyl ether compounds. For example, in steps 1 and 2, it is not necessary to use cationic surfactants such as quaternary ammonium compounds such as dodecyltrimethylammonium chloride and dodecyltrimethylammonium bromide, anionic surfactants such as alkyl sulfate compounds such as sodium dodecyl sulfate, ammonium lauryl sulfate, sodium lauryl ether sulfate, and sodium mireth sulfate, or nonionic surfactants such as alkyl ether sulfate compounds such as polyoxyethylene lauryl ether. This prevents a decrease in the surface tension of the superabsorbent polymer due to the use of surfactants.

[0084] The monomer composition may further contain additives such as thickeners, plasticizers, preservatives, and antioxidants, as needed.

[0085] Furthermore, 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 solution state, i.e., the concentrations of the monomer, crosslinking agent and polymerization initiator, can be appropriately adjusted considering the polymerization time and reaction conditions. For example, the solid content in the monomer composition may be 10 to 80% by weight, 15 to 60% by weight, or 30 to 50% by weight.

[0086] When the monomer composition has a solid content within the aforementioned range, the gelling effect that occurs during the polymerization reaction of a high-concentration aqueous solution eliminates the need to remove unreacted monomers after polymerization, and at the same time, it may be advantageous for controlling the grinding efficiency during the grinding of the polymer, as described later.

[0087] Here, the usable solvent is not limited as long as it can dissolve the aforementioned raw materials, and for example, 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, or mixtures thereof, can be used.

[0088] On the other hand, a method for preparing a superabsorbent polymer is provided, comprising the steps of: irradiating a monomer composition for preparing a superabsorbent polymer with heat and / or light to polymerize and prepare a hydrogel polymer; drying, pulverizing and classifying the hydrogel polymer to form a base resin; and forming a surface crosslinking layer on the surface of the base resin in the presence of a surface crosslinking solution containing a surface crosslinking agent and a solvent. This method further includes a step of incorporating the superabsorbent polymer into an absorbent article.

[0089] The method for polymerizing monomer compositions for preparing superabsorbent polymers is not particularly limited. Polymerization methods can be broadly divided into thermal polymerization and photopolymerization depending on the polymerization energy source. Typically, thermal polymerization is carried out in a reactor equipped with a stirring shaft such as a kneader, and photopolymerization can be carried out in a reactor equipped with a movable conveyor belt or a flat-bottomed vessel. However, the polymerization methods described above are merely examples, and the present invention is not limited thereto.

[0090] For example, a hydrogel polymer can be obtained by introducing a monomer composition into a reactor equipped with a stirring shaft, such as the kneader described above, and supplying hot air or heating the reactor to allow thermal polymerization to proceed. At this time, the hydrogel polymer discharged from the reactor outlet may be several centimeters to several millimeters in size, depending on the shape of the stirring shaft provided in the reactor. Specifically, the size of the obtained hydrogel polymer varies depending on the concentration and introduction rate of the monomer composition introduced, but typically, hydrogel polymers with a particle size of 2 to 50 mm can be obtained.

[0091] Furthermore, as mentioned above, when photopolymerization of a monomer composition is carried out in a reactor equipped with a movable conveyor belt, the resulting hydrogel polymer may typically be a hydrogel polymer sheet having the width of the belt. In this case, the thickness of the sheet may vary depending on the concentration and rate at which the monomer composition is introduced, but it is preferable to supply the monomer composition so that a polymer sheet with a thickness of about 0.5 to about 5 cm is obtained. If the monomer composition is supplied in such a way that the thickness of the polymer sheet may become excessively thin, the production efficiency may decrease, and if the thickness of the polymer sheet exceeds 5 cm, polymerization may not occur uniformly across the entire thickness due to the excessive thickness.

[0092] The water content of the hydrogel polymer obtained in this way is typically 40-80% by weight. In this specification, "water content" means the amount of water relative to the total weight of the hydrogel polymer, and is the value obtained by subtracting the weight of the dry polymer from the weight of the hydrogel polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to water evaporation in the polymer while drying it by raising the temperature of the polymer by infrared heating. Here, the temperature is raised from room temperature to about 180°C and then maintained at 180°C, and the total drying time is 20 minutes, including a 5 minute heating step.

[0093] Next, the hydrogel polymer is dried and classified to form a powdered base resin. To improve the efficiency of the drying process, a coarse grinding step may be performed before drying, if necessary.

[0094] Herein, the pulverizers that can be used are not limited in their configuration, and specifically, those selected from the group consisting of vertical pulverizers, turbo cutters, turbo grinders, rotary cutter mills, cutter mills, disc mills, shred crushers, crushers, choppers, and disc cutters can be used, but the present invention is not limited thereto.

[0095] In this process, grinding may be carried out so that the polymer particle size is approximately 2-10 mm. Grinding to a particle size of less than 2 mm is technically difficult due to the high water content of the hydrogel polymer and may cause aggregation between the ground particles. On the other hand, grinding to a particle size greater than 10 mm may only slightly improve the efficiency of the subsequent drying process.

[0096] As described above, the pulverized polymer, or the polymer immediately after polymerization that has not undergone the pulverization process, is dried. Here, the drying temperature in the drying process may be about 150 to about 250°C. If the drying temperature is less than 150°C, the drying time may be excessively long, which may reduce the specificity of the superabsorbent polymer finally prepared. If the drying temperature exceeds 250°C, only the surface of the polymer may be dried, and fine particles may be generated in the subsequent pulverization process, which may reduce the properties of the superabsorbent polymer finally prepared. Therefore, drying may be carried out at a temperature preferably of about 150 to about 200°C, more preferably of about 160 to about 180°C.

[0097] On the other hand, drying can proceed for approximately 20 to 90 minutes, taking process efficiency into consideration, but the drying time is not limited to this.

[0098] Furthermore, the drying method is not particularly limited, as long as it is a method commonly used for drying hydrogel polymers. Specifically, drying can be carried out by supplying hot air, infrared irradiation, microwave irradiation, or UV irradiation. The water content of the polymer after the drying process may be about 0.1 to about 5% by weight.

[0099] Next, the dried polymer obtained through the drying process is subjected to a grinding step.

[0100] The polymer powder obtained after the grinding process, i.e., the base resin, may have a particle size of approximately 150 to 850 μm. Specific grinders used to achieve this particle size include ball mills, pin mills, hammer mills, screw mills, roll mills, disc mills, or jog mills.

[0101] Furthermore, in order to control the properties of the superabsorbent polymer powder that is ultimately produced as a product after the grinding process, the base resin obtained after grinding is classified by particle size. Preferably, polymers with a particle size of about 150 to about 850 μm are classified, and only base resins with such particle sizes can be passed through the surface crosslinking reaction process. Such particle sizes can be measured by the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3 method.

[0102] Next, the surface of the base resin is further crosslinked in the presence of a surface crosslinking agent to form a surface crosslinked layer. Through this process, a superabsorbent polymer is provided in which a surface crosslinked layer is formed on the surface of the base resin, or more specifically, on at least a portion of the surface of each base resin particle.

[0103] Surface crosslinking is a process that increases the crosslinking density around the surface of superabsorbent polymer particles relative to the crosslinking density inside the particles. Generally, the surface crosslinking agent is applied to the surface of the superabsorbent polymer particles. Therefore, the reaction occurs on the surface of the superabsorbent polymer particles, which improves the crosslinkability of the particle surface without substantially affecting the inside of the particles. Thus, surface-crosslinked superabsorbent polymer particles have a higher crosslinking density near the surface than inside.

[0104] As surface crosslinking agents, those already used in the preparation of superabsorbent polymers can be used without any special restrictions. For example, surface crosslinking agents may include one or more polyols selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; one or more carbonate compounds selected from the group consisting of ethylene carbonate, propylene carbonate, and glycerol carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazolidinone compounds such as 2-oxazolidinone; polyamine compounds; oxazoline compounds; mono, di, or polyoxazolidinone compounds; or cyclic urea compounds. Specifically, one or more, two or more, or three or more of the aforementioned surface crosslinking agents can be used as the surface crosslinking agent. For example, ethylene carbonate can be used as the surface crosslinking agent.

[0105] The method of mixing the surface crosslinking agent with the base resin is not limited. The surface crosslinking agent and the base resin powder may be mixed in a reactor, the surface crosslinking agent may be sprayed onto the base resin powder, or the base resin and surface crosslinking agent may be continuously supplied to a continuously operating mixer and mixed.

[0106] When adding a surface crosslinking agent, it may be added as a surface crosslinking solution by mixing it with water. When water is added, the surface crosslinking agent can be uniformly dispersed within the polymer. In this case, it is preferable to add water in a ratio of about 1 to 10 parts by weight per 100 parts by weight of the base resin in order to induce uniform dispersion of the surface crosslinking agent, prevent aggregation of the polymer powder, and at the same time optimize the surface penetration depth of the surface crosslinking agent.

[0107] A surface crosslinking reaction can be carried out by heating a base resin to which a surface crosslinking solution containing a surface crosslinking agent and a solvent has been added, at a temperature of about 100 to about 150°C, preferably about 110 to about 140°C, for about 15 to about 80 minutes, preferably about 20 to about 70 minutes.

[0108] The means for raising the temperature for surface crosslinking is not particularly limited, and a heat transfer medium may be supplied, or a heat source may be supplied directly for heating. Here, suitable heat transfer mediums include heating fluids such as steam, hot air, and hot oil, and the temperature of the supplied heat transfer medium can be appropriately selected considering the heat transfer medium, the means for raising the temperature, and the target temperature. On the other hand, examples of directly supplied heat sources include electric heating and gas heating, but the present invention is not limited thereto.

[0109] The monomer composition for preparing superabsorbent polymers exhibits a rapid gelation rate after polymerization initiation, thereby minimizing the loss of bubbles generated from the foaming agent. Therefore, the monomer composition for preparing superabsorbent polymers can provide superabsorbent polymers that exhibit excellent absorption rates.

[0110] Specifically, a superabsorbent polymer prepared from a monomer composition for superabsorbent polymer preparation may have an index of performance (PI) of 2.35 or higher, represented by the following formula 1, and a T20 of less than 190 seconds. [Formula 1] Performance index (PI)=(CRC / 27)+(AUP / 24)+(SFC / 30)-(T20 / 140) In formula 1, CRC is the water retention capacity (g / g) of a superabsorbent polymer after 30 seconds of centrifugation in a 0.9 wt% sodium chloride aqueous solution. AUP is the absorption capacity (g / g) of a superabsorbent polymer under pressure for 1 hour in a 0.9 wt% sodium chloride aqueous solution. SFC is the saline conductivity (·10) of a 0.685 wt% sodium chloride aqueous solution. -7 cm 3 (seconds / g) T20 is the time (in seconds) it takes for 1 g of superabsorbent polymer to absorb 20 g of an aqueous solution of C12-C14 alcohol ethoxylate under a pressure of 0.3 psi.

[0111] The measurement methods for CRC, AUP, SFC, and T20 will be specifically described in the examples below.

[0112] Preferably, the PI of the superabsorbent polymer may be 2.37 or higher, 2.40 or higher, or 2.41 or higher, and 4.0 or lower, or 3.0 or lower. Also, the T20 of the superabsorbent polymer may be 180 seconds or less, 160 seconds or less, 140 seconds or less, 130 seconds or less, 125 seconds or less, and 100 seconds or more.

[0113] Furthermore, the superabsorbent polymer prepared from the monomer composition for superabsorbent polymer preparation of the present invention has high gel strength and maintains its shape well even after water absorption, and therefore can exhibit excellent absorption performance despite external pressure.

[0114] Absorbent articles The superabsorbent polymers of this disclosure are incorporated into absorbent articles, such as absorbent cores contained within absorbent articles.

[0115] "Absorbent articles" refer to devices that absorb and contain bodily excrement, specifically urine and other water-containing liquids, and more specifically, devices that are placed in contact with or close to the wearer's body to absorb and contain various types of excrement released from the body. Absorbent articles may include diapers (for infants and toddlers, and for adult incontinence) and pants (for infants and toddlers, and for adult incontinence). As used herein, the term "excrement" includes, but is not limited to, urine, blood, vaginal discharge, sweat, and feces. Preferred absorbent articles of the present invention are disposable absorbent articles, more preferably disposable diapers, disposable pants, and disposable absorbent inserts.

[0116] As used herein, “absorbent core” refers to a structure positioned between the top sheet and back sheet of an absorbent article, which absorbs and contains the liquid that the absorbent article receives.

[0117] "Disposable" is used in its ordinary sense to mean an item that is disposed of or discarded after a limited number of uses over various periods, for example, fewer than 10 times, fewer than 5 times, or fewer than 2 times. When disposable absorbent items are diapers, pants, absorbent inserts, sanitary napkins, sanitary pads, or wet wipes for personal hygiene purposes, disposable absorbent items are, in most cases, intended to be disposed of after a single use. Used and disposed of absorbent items may or may not be recycled thereafter. As used herein, the terms "absorbent items," "pants," and "diapers" also always refer to disposable absorbent items, disposable pants (and disposable absorbent pants), and disposable diapers.

[0118] "Diapers" and "pants" refer to absorbent articles commonly worn by infants, toddlers and incontinent individuals around the lower torso so as to surround the wearer's waist and legs, and in particular, adapted to receive and contain urine and feces. In pants, as used herein, the longitudinal edges of a first waist region and the longitudinal edges of a second waist region are attached to each other to pre-form waist and leg openings. Pants are positioned on the wearer by inserting the wearer's legs into the leg openings and sliding the absorbent article of the pants into place around the wearer's lower torso. Pants may be pre-formed by any preferred method, including, but not limited to, reattachable and / or non-reattachable joints (e.g., sews, welds, adhesives, tacks, fasteners, etc.) to join parts of the absorbent article together. Pants may be pre-formed at any position along the outer circumference of the article (e.g., side fastening, front waist fastening). In a diaper, the waist opening and leg openings are formed only when the diaper is applied to the wearer by attaching the longitudinal edges of the first waist region and the second waist region to each other (removably) on both sides by a suitable fastening system.

[0119] Each absorbent article may include a top sheet, a back sheet, an absorbent core, and optionally a gain / distribution system. The absorbent core is placed between the back sheet and the top sheet, and the optional gain / distribution system is typically placed between the absorbent core and the top sheet.

[0120] The superabsorbent polymers of this disclosure may be incorporated into an absorbent core of an absorbent article. The absorbent core may or may not contain other absorbent materials, such as non-crosslinked cellulose fibers (pulp fibers), in addition to the superabsorbent polymer. The absorbent core may contain at least 60% by mass, at least 75% by mass, at least 85% by mass, at least 95% by mass, or at least 98% by mass, or 100% by mass of the superabsorbent polymers disclosed herein.

[0121] Diapers or pants may also include elastic leg cuffs and barrier leg cuffs, in particular, to provide improved containment of liquids and other bodily waste within the leg opening area. Typically, each of the leg cuffs and barrier cuffs includes one or more elastic strings.

[0122] "Female care absorbent products" are personal care products used by women during menstruation to absorb and retain substances from menstruation, vaginal discharge, and other bodily functions related to the vulva. Examples of female care absorbent products include panty liners and sanitary napkins.

[0123] The following are preferred embodiments to aid in a good understanding of the present invention. However, these embodiments are merely illustrative of the present invention, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the present invention and the technical idea, and that such changes and modifications fall within the scope of the appended claims. [Examples]

[0124] Comparative Example 1 (1) Preparation of monomer compositions for superabsorbent polymers In a 3L glass container equipped with a stirrer and thermometer, 450g of acrylic acid, 3g of PEGDA400 (polyethylene glycol diacrylate 400) internal crosslinking agent, and 0.04g of diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide photoinitiator were dissolved, and then 580g of 31.5% sodium hydroxide solution was added to prepare a monomer mixture (degree of neutralization: 70 mol%, solids content: 41 wt%).

[0125] A monomer composition for preparing superabsorbent polymers was prepared by adding sodium bicarbonate (SBC) as a carbonate-based blowing agent to a monomer mixture at a ratio of 0.1 parts by weight per 100 parts by weight of acrylic acid.

[0126] (2) Preparation of superabsorbent polymers A portion (86.6 g) of the monomer composition for preparing the superabsorbent polymer prepared in (1) was placed in a rheometer (TA Instruments, ARES) equipped with a UV accessory, and UV polymerization was carried out for 180 seconds by irradiation with UV light. Specifically, the polymerization temperature was maintained at 60°C and UV light was irradiated for 60 seconds (irradiation dose: 100 mW / cm²). 2 The reaction was then allowed to continue for another 120 seconds.

[0127] Here, the storage modulus (G') and loss modulus (G") of the monomer composition were measured from the polymerization start time and plotted against the polymerization time. The water content of the hydrogel polymer obtained after the polymerization reaction was measured and confirmed to be 48%.

[0128] The remaining monomer composition for preparing the superabsorbent polymer that was not put into the rheometer was placed in a polymerization chamber where the temperature was controlled to 60°C and a UV light source was present above. It was then irradiated with UV light for 60 seconds, and the reaction was allowed to proceed for an additional 120 seconds to obtain a hydrogel polymer. The prepared hydrogel polymer was cut into pieces approximately 5cm x 5cm in size, then placed in a meat chopper and crushed to obtain hydrogel crumbs ranging from 1mm to 10mm in size. The crumbs were then dried in an oven with vertical airflow. The crumbs were uniformly dried by flowing hot air at over 180°C from bottom to top for 15 minutes, and then from top to bottom for another 15 minutes, until the moisture content of the dried material was less than 1%. After drying, the material was crushed in a pulverizer, classified, and selected to obtain base resins ranging from 150 to 850 μm in size.

[0129] A surface crosslinking solution containing 1.5 parts by weight of ethylene carbonate and 6 parts by weight of water per 100 parts by weight of base resin was sprayed onto 100 parts by weight of the prepared base resin, and the mixture was stirred and mixed at room temperature so that the surface crosslinking solution was uniformly distributed on the surface of the base resin powder. Next, the base resin powder mixed with the surface crosslinking solution was placed in a surface crosslinking reactor and the surface crosslinking reaction was carried out.

[0130] In the surface crosslinking reactor, the temperature of the base resin powder was observed to gradually rise from the initial temperature of approximately 80°C, and the reactor was operated to reach a maximum reaction temperature of 190°C after 30 minutes. After reaching the maximum reaction temperature, the reaction was allowed to continue for an additional 15 minutes, after which a sample of the final prepared superabsorbent polymer was obtained. After the surface crosslinking process, the sample was classified using an ASTM standard sieve to prepare superabsorbent polymers with particle sizes ranging from 150 μm to 850 μm.

[0131] Example 1 In a 3L glass container equipped with a stirrer and thermometer, 450g of acrylic acid, 3g of PEGDA400 (polyethylene glycol diacrylate 400) internal crosslinking agent, and 0.04g of diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide photoinitiator were dissolved, and then 580g of 31.5% sodium hydroxide solution was added to prepare a monomer mixture (degree of neutralization: 70 mol%, solids content: 41 wt%).

[0132] A monomer mixture was mixed with an aqueous dispersion of kaolin (average particle size 0.6 μm) at a ratio of 0.25 parts by weight of kaolin per 100 parts by weight of acrylic acid. The mixture was then homogenized using an IKA Ultra-Turrax at 6500 rpm for 3 minutes to disperse the mixture. The aqueous dispersion of kaolin was prepared by mixing 96 g of water and 4 g of kaolin using an IKA Ultra-Turrax at 6500 rpm for 30 seconds.

[0133] Subsequently, 0.1 parts by weight of sodium bicarbonate (SBC) was added to 100 parts by weight of acrylic acid and mixed. Immediately before the start of polymerization, 0.12 g of hydrogen peroxide (H2O2), 0.04 g of ascorbic acid, and 2 g of sodium persulfate were added to prepare a monomer composition for superabsorbent polymer preparation.

[0134] Using this monomer composition, a superabsorbent polymer was prepared in the same manner as in Comparative Example 1 (2).

[0135] Example 2 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Example 1, except that kaolin (average particle size 0.6 μm) was added at a rate of 0.5 parts by weight per 100 parts by weight of acrylic acid.

[0136] Example 3 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Example 1, except that kaolin (average particle size 0.6 μm) was added at a rate of 1 part by weight per 100 parts by weight of acrylic acid.

[0137] Example 4 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Example 1, except that bentonite (average particle size 0.8 μm) was added at a rate of 0.25 parts by weight per 100 parts by weight of acrylic acid.

[0138] Example 5 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Example 1, except that bentonite (average particle size 0.8 μm) was added at a rate of 0.5 parts by weight per 100 parts by weight of acrylic acid.

[0139] Example 6 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Example 1, except that bentonite (average particle size 0.8 μm) was added at a rate of 1 part by weight per 100 parts by weight of acrylic acid.

[0140] Comparative Example 2 A superabsorbent polymer was prepared in the same manner as in Comparative Example 1, except that the polymerization temperature was set to 50°C.

[0141] Comparative Example 3 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Comparative Example 1, except that 0.12 g of hydrogen peroxide (H2O2) and 0.04 g of ascorbic acid were added during the preparation of the monomer composition.

[0142] Comparative Example 4 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Comparative Example 1, except that sodium bicarbonate (SBC) was added as a blowing agent at a content of 0.2 parts by weight per 100 parts by weight of acrylic acid during the preparation of the monomer composition.

[0143] Comparative Example 5 A monomer composition for preparing a superabsorbent polymer and a superabsorbent polymer were prepared in the same manner as in Comparative Example 1, except that sodium bicarbonate (SBC) was added as a blowing agent at a content of 0.3 parts by weight per 100 parts by weight of acrylic acid during the preparation of the monomer composition.

[0144] Experimental Example 1: Characterization of monomer compositions for superabsorbent polymer preparation From the G' and G'' plots obtained during polymerization of each monomer composition for preparing superabsorbent polymers in the examples and comparative examples, the gelation point (GP), normalized GP, G' at GP, the final G' of the polymer after polymerization is complete, and the rate of increase of G' from the gelation point to the completion of polymerization (ΔG' / sec) were calculated, and the results are shown in Table 1 below.

[0145] The gelation point is the time (in seconds) at the intersection of the G' plot and the G'' plot, and the normalized gelation point was calculated according to Equation 1 below. [Formula 1] Normalized gelation point = gelation point (seconds) / total polymerization time (seconds)

[0146] Experimental Example 2: Characterization of Superabsorbent Polymers (1) Vortex time of the base resin The eddy times for each superabsorbent polymer in the examples and comparative examples were measured as follows. Superabsorbent polymers classified using a sieve of #30 to #50 were used for measuring the eddy times. (1) First, 50 mL of 0.9% saline solution was placed into a 100 mL flat-bottom beaker using a 100 mL graduated cylinder. (2) Next, the beaker was placed in the center of the magnetic stirrer, and then a circular magnetic rod (8 mm in diameter, 30 mm in length) was placed in the beaker. (3) Subsequently, the agitator was operated so that the magnetic rod was stirred at 600 rpm, and the lowest part of the vortex generated by the stirring was in contact with the magnetic bar. (4) After confirming that the temperature of the salt water in the beaker reached 24.0°C, 2 ± 0.01 g of the superabsorbent polymer sample was added, and a stopwatch was started at the same time to measure the time it took for the vortex to disappear and the liquid surface to become completely horizontal. This time was defined as the vortex time.

[0147] (2) Centrifugal water retention capacity (CRC) The centrifugal water retention capacity (CRC) was measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.3. Superabsorbent polymer W0 (g, approximately 0.2g) was uniformly placed in a nonwoven fabric bag, sealed, and then immersed in a 0.9 wt% sodium chloride aqueous solution (physiological saline). After 30 minutes, the liquid was drained from the bag using a centrifuge at 250G for 3 minutes, and the weight of the bag W2 (g) was measured. The same procedure was performed without the superabsorbent polymer, and the weight W1 (g) was measured. Using these weights, the CRC (g / g) was calculated using formula 2 below to confirm the centrifugal water retention capacity. [Formula 2] CRC(g / g)={[W2(g)-W1(g)-W0(g)] / W0(g)}

[0148] (3) Absorption under pressure (AUP) The absorbent capacity (AUP) under pressure was measured for the superabsorbent polymers prepared in the examples and comparative examples according to the European Disposables and Nonwovens Association standard EDANA WSP 242.3-10.

[0149] First, a 400-mesh stainless steel wire mesh was placed at the bottom of a plastic cylinder with an inner diameter of 60 mm. Then, W0 (g, approximately 0.90 g) of each superabsorbent polymer obtained in the examples and comparative examples was uniformly sprayed onto the wire mesh at a temperature of 23 ± 2°C and a humidity of 45%. A piston capable of uniformly applying a load of 4.83 kPa (0.7 psi) was added, with an outer diameter slightly smaller than 60 mm, and no gap between it and the inner wall of the cylinder to prevent interference with its vertical movement. At this time, the weight W3 (g) of the apparatus was measured.

[0150] A glass filter with a diameter of 125 mm and a thickness of 5 mm was placed in a Petri dish with a diameter of 150 mm, and a 0.90 wt% sodium chloride saline solution was poured in up to the same height as the top of the glass filter. The aforementioned apparatus was then placed on the glass filter, and the liquid was aspirated under load for 1 hour. After 1 hour, the measuring apparatus was lifted, and its weight W4 (g) was measured.

[0151] Using the measured weight, the AUP (g / g) was calculated using formula 3 below to confirm the absorption capacity under pressure. [Formula 3] AUP(g / g) = [W4(g) - W3(g)] / W0(g) In formula 3, W0(g) is the initial weight (g) of the superabsorbent polymer. W3(g) is the sum of the weight of the superabsorbent polymer and the weight of the device that can apply a load to the superabsorbent polymer. W4(g) is the sum of the weight of the superabsorbent polymer after it has absorbed physiological saline solution under load (0.7 psi) for 1 hour, and the weight of the apparatus capable of applying the load to the superabsorbent polymer.

[0152] (3) Saltwater conductivity (SFC) The measurements and calculations were performed according to the method described in columns 54-59 of U.S. Patent Registration No. 5562646.

[0153] (4) T20 9 g of sodium chloride and 0.1 g of Lorodac (main component: linear alcohol ethoxylate with 12-14 carbon atoms, CAS #68439-50-9) were dissolved in 1 L of distilled water to form an aqueous solution. The time it took for 1 g of superabsorbent polymer to absorb 20 g of the aqueous solution was calculated and measured under a load of 0.3 psi. The specific method for measuring T20 is described in detail on pages 13-18 of European Patent No. 2535027(A1).

[0154] (5) Performance index (PI) CRC (g / g), AUP (g / g), SFC (·10) were measured using the method described above. -7 cm 3 The figure of merit (PI, unitless) was calculated using formula 1 below, with (seconds / g) and T20 (seconds). [Formula 1] Performance index (PI)=(CRC / 27)+(AUP / 24)+(SFC / 30)-(T20 / 140)

[0155] [Table 1] * parts by weight relative to 100 parts by weight of acrylic acid

[0156] [Table 2]

[0157] The dimensions and values ​​disclosed herein should not be understood as being strictly limited to the exact numerical values ​​listed. Instead, unless otherwise specified, each such dimension is intended to mean both the listed value and the functionally equivalent range encompassing that value. For example, a dimension disclosed as "40 mm" is intended to mean "approximately 40 mm."

[0158] All documents cited herein, including cross-referenced or related patents or applications, are incorporated herein in their entirety by reference unless expressly excluded or otherwise limited. No citation of any document shall be deemed prior art to any invention disclosed or claimed herein, nor shall it be deemed to teach, suggest or disclose any such invention, either alone or in combination with any one or more other references. Furthermore, in the event of any conflict between any meaning or definition of a term in this document and any meaning or definition of the same term in a document incorporated by reference, the meaning or definition given to the term in this document shall prevail.

[0159] While specific embodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. Therefore, it is intended that all such changes and modifications within the scope of the invention be covered in the appended claims.

Claims

1. A method for producing an absorbent article comprising a superabsorbent polymer, wherein the superabsorbent polymer is prepared by using a monomer composition, and the monomer composition is The material comprises an acrylic acid monomer having an acid group, of which at least a portion of the acid group is neutralized, a crosslinking agent, clay, a carbonate-based blowing agent, and a polymerization initiator. When the monomer composition for preparing the superabsorbent polymer is irradiated with heat and / or light to allow the polymerization reaction to proceed, the normalized gelation point represented by the following formula 1 is 0.01 to 0.

1. [Formula 1] Normalized gelation point = gelation point (seconds) / total polymerization time (seconds) A method in which, in formula 1, the gelation point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for preparing the superabsorbent polymer and the loss modulus graph of the monomer composition for preparing the superabsorbent polymer.

2. The method according to claim 1, wherein when the monomer composition for preparing the superabsorbent polymer is irradiated with heat and / or light to allow the polymerization reaction to proceed, the rate of increase in gel strength from the gelation point to the completion of polymerization is 210 Pa / second or more.

3. The method according to claim 1 or 2, wherein the clay is included in the monomer composition in an amount of 0.1 to 5 parts by weight per 100 parts by weight of the acrylic acid monomer.

4. The method according to any one of claims 1 to 3, wherein the average particle size of the clay in the monomer composition for preparing the superabsorbent polymer is 0.025 μm to 10 μm.

5. The method according to any one of claims 1 to 4, wherein the surface of the clay in the monomer composition for preparing the superabsorbent polymer is modified with a polymer dispersant containing two or more functional groups selected from the group consisting of amine groups, carbonyl groups, and hydroxyl groups.

6. The method according to any one of claims 1 to 5, wherein the polymer dispersant in the monomer composition for preparing the superabsorbent polymer is one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylamide, and polyacrylic acid.

7. The method according to any one of claims 1 to 6, further comprising the step of incorporating the superabsorbent polymer into the absorbent article.

8. A method for producing an absorbent article, the method comprising the step of producing a monomer composition for preparing a superabsorbent polymer, the monomer composition is The material comprises an acrylic acid monomer having an acid group, of which at least a portion of the acid group is neutralized, a crosslinking agent, clay, a carbonate-based blowing agent, and a polymerization initiator. When the monomer composition for preparing the superabsorbent polymer is irradiated with heat and / or light to allow the polymerization reaction to proceed, the normalized gelation point represented by the following formula 1 is 0.01 to 0.

1. [Formula 1] Normalized gelation point = gelation point (seconds) / total polymerization time (seconds) In formula 1, the gelation point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for preparing the superabsorbent polymer and the loss modulus graph of the monomer composition for preparing the superabsorbent polymer, and the method is as follows: A step of mixing the acrylic acid monomer having acid groups and having at least a portion of the acid groups neutralized, the crosslinking agent, the carbonate blowing agent, and the polymerization initiator, The process includes adding the clay to the mixture and shearing it at a stirring speed of 6500 rpm or more. The method comprises a further step of incorporating the superabsorbent polymer prepared with the monomer composition into an absorbent article.

9. A method for producing an absorbent article according to claim 8, wherein the polymer is prepared from the monomer composition for preparing the superabsorbent polymer, and the polymer has a gel strength of 35,000 Pa to 60,000 Pa when its water content is 40% to 80% by weight.

10. An absorbent article comprising the superabsorbent polymer prepared from the monomer composition according to any one of claims 1 to 7, The figure of performance (PI) of the superabsorbent polymer, as shown in the calculation formula 1 below, is 2.35 or higher, and T20 is less than 190 seconds. [Formula 1] Performance index (PI) = (CRC / 27) + (AUP / 24) + (SFC / 30) - (T20 / 140) In the above calculation formula 1, CRC is the water retention capacity (g / g) of a superabsorbent polymer after 30 seconds of centrifugation in a 0.9 wt% sodium chloride aqueous solution. AUP is the absorption capacity (g / g) of a superabsorbent polymer in a 0.9 wt% sodium chloride aqueous solution at 0.7 psi pressure for 1 hour. The SFC of the superabsorbent polymer is measured by the saline conductivity (·10) of a 0.685 wt% sodium chloride aqueous solution. －７ cm ３ (seconds / g) An absorbent article in which T20 is the time (in seconds) it takes for 1 g of the superabsorbent polymer to absorb 20 g of an aqueous solution of a carbon-12 to carbon-14 alcohol ethoxylate under a pressure of 0.3 psi.

11. A method for producing an absorbent article, wherein the absorbent article comprises a superabsorbent polymer, and the method is A step of preparing a hydrogel polymer by irradiating a monomer composition for preparing a superabsorbent polymer with heat and / or light to polymerize it, The monomer composition, The material comprises an acrylic acid monomer having an acid group, of which at least a portion of the acid group is neutralized, a crosslinking agent, clay, a carbonate-based blowing agent, and a polymerization initiator. When the monomer composition for preparing the superabsorbent polymer is irradiated with heat and / or light to allow the polymerization reaction to proceed, the normalized gelation point represented by the following formula 1 is 0.01 to 0.

1. [Formula 1] Normalized gelation point = gelation point (seconds) / total polymerization time (seconds) In the above formula 1, the gelation point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for preparing the superabsorbent polymer and the loss modulus graph of the monomer composition for preparing the superabsorbent polymer, and the process is as follows: The process involves drying, pulverizing, and classifying the hydrogel polymer to form a base resin. The process includes forming a surface crosslinked layer on the surface of the base resin in the presence of a surface crosslinking solution containing a surface crosslinking agent and a solvent, The method further comprises the step of incorporating the superabsorbent polymer into an absorbent article.

12. The method according to claim 11, wherein the absorbent article is a diaper or pants.

13. The method according to claim 11 or 12, wherein the superabsorbent polymer is incorporated into an absorbent core, and the absorbent core is included in the absorbent article.

14. The method according to claim 13, wherein the absorbent core is manufactured by providing two nonwoven webs and incorporating the surface crosslinked particulate water-absorbing resin between the two nonwoven webs.

15. The method according to claim 14, wherein the absorbent core between the two nonwoven webs comprises less than 20% by weight of cellulose fibers, preferably less than 10% by weight of cellulose fibers, and more preferably less than 5% by weight of cellulose fibers.

16. The method according to claim 14 or 15, further comprising the step of adhering and fixing the superabsorbent polymer between the two nonwoven webs.

17. The aforementioned method, Further processes for providing the top sheet, Further processes for providing the backsheet, The method according to any one of claims 13 to 16, further comprising the step of providing the absorbent core between the top sheet and the back sheet.

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