Superabsorbent polymer and method for producing the same

Abstract

The present invention relates to a superabsorbent polymer which has excellent physical properties, such as centrifugal retention capacity, absorbency under pressure, absorption speed, 1-minute tap water absorption capacity, and rewetting characteristics, and which is used in sanitary materials such as diapers, exhibiting high absorption performance and excellent usability, and to a method for producing the same.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims the benefit of priority based on Korean Patent Application No. 10 - 2022 - 0159758 filed on November 24, 2022 and Korean Patent Application No. 10 - 2023 - 0150812 filed on November 3, 2023, and all the contents disclosed in the documents of the Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a superabsorbent resin having a high absorption rate, excellent centrifugal water retention ability and pressure absorption ability, and excellent characteristics of preventing rewetting and suppressing water leakage, and a method for producing the same.

Background Art

[0003] A superabsorbent polymer (SAP) is a synthetic polymer material having a function of absorbing about 500 to 1000 times its own weight of water, and is named differently such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) etc. for each development company. Such superabsorbent resins began to be commercialized for sanitary products, and currently are widely used in materials such as soil water retention agents for horticulture, water stop materials for civil engineering and construction, seedling - raising sheets, freshness - maintaining agents in the food distribution field, and wet compress materials.

[0004] Such superabsorbent resins are mainly widely used in the field of sanitary materials such as diapers and sanitary napkins. Inside the sanitary materials, it is common for the superabsorbent resin to be contained in a state of being diffused in pulp. However, recently, efforts have continued to provide sanitary materials such as thinner diapers, and as part of this, the development of so - called pulpless diapers with a reduced pulp content or even no pulp used at all has been actively promoted.

[0005] Superabsorbent polymers widely used in sanitary applications like these need to exhibit high absorption capacity and a fast absorption rate for moisture, prevent the absorbed moisture from escaping even under external pressure, and maintain their shape well even when swollen after absorbing water, exhibiting excellent permeability.

[0006] On the other hand, after a superabsorbent polymer applied to a sanitary material absorbs a liquid, if pressure is applied due to the user's weight, there is a possibility of rewetting (where some of the absorbed liquid seeps out again) and leakage (where urine leaks out). Therefore, there is a need to develop a superabsorbent polymer that suppresses rewetting and leakage.

[0007] However, absorption rate, centrifugal water retention capacity, and pressurized absorption capacity are mutually exclusive properties. When high-foaming polymerization is performed to increase the absorption rate, the strength of the polymer core weakens, leading to a decrease in centrifugal water retention capacity and pressurized absorption capacity. Furthermore, when increasing the crosslinking density of a superabsorbent polymer, pressurized absorption capacity can be improved, but centrifugal water retention capacity and absorption rate decrease. Thus, it is difficult to simultaneously improve properties such as absorption rate, centrifugal water retention capacity, and pressurized absorption capacity.

[0008] Therefore, there is a need for the development of superabsorbent polymers that excel in both pressurized absorption capacity, centrifugal separation water retention capacity, and absorption rate, as well as further improving the re-wetting prevention and leakage suppression properties of sanitary materials such as diapers. [Overview of the project] [Problems that the invention aims to solve]

[0009] The present invention aims to provide a superabsorbent polymer and a method for producing the same, which simultaneously satisfies the required physical properties of centrifugal separation water retention capacity, pressurized absorption capacity, absorption rate, and tap water absorption capacity per minute, and which also has re-wetting and leakage suppression properties. [Means for solving the problem]

[0010] Therefore, according to one embodiment of the present invention, i) The water retention capacity by centrifugal separation is 30 g / g or more, ii) The effective absorption capacity (EFFC, 0.3 psi) calculated by the following formula 1 is greater than 29.5 g / g, iii) The absorption rate by the vortex method is less than 37 seconds, iv) When 1 g of superabsorbent polymer is immersed in 2 L of tap water and swelled for 1 minute, the tap water absorption capacity per minute, defined as the weight of water absorbed by the superabsorbent polymer in 1 minute, exceeds 115 g. v) A superabsorbent polymer is provided in which, after immersing 4 g of the superabsorbent polymer in 100 g of a 0.9 wt% sodium chloride aqueous solution and swelling it for 120 minutes, a filter paper is placed on the superabsorbent polymer, and pressurized at 0.75 psi for 5 minutes, the re-wetting characteristic (re-wetting in pressurized salt water for 2 hours) is less than 3.7 g.

[0011] [Formula 1] Effective absorption capacity (EFFC, 0.3 psi) = {Centrifugal separation water retention capacity (CRC) + 0.3 psi pressurized absorption capacity (AUP)} / 2

[0012] Furthermore, according to one embodiment of the present invention, a method for producing the superabsorbent resin, A step of preparing a monomer composition comprising an acrylic acid monomer having at least a portion of neutralized acidic groups, an internal crosslinking agent, and a polymerization initiator, The steps include crosslinking the monomer composition to form a water-containing gel polymer, The steps include drying, pulverizing, and classifying the aforementioned water-containing gel polymer to form a base resin, The process includes a surface crosslinking step of 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, water, and an organic solvent having a boiling point of less than 90°C, The water / organic solvent weight ratio of the surface crosslinking solution is 1.0 or more and less than 3.0. A manufacturing method is provided in which the content of the organic solvent in the surface crosslinking solution is 24% by weight or more. [Effects of the Invention]

[0013] The superabsorbent polymer of the present invention is excellent in all physical properties, including centrifugal separation water retention capacity, pressurized absorption capacity, absorption rate, 1-minute tap water absorption capacity, and re-wetting. Therefore, it can be used in sanitary materials such as diapers and exhibits improved re-wetting and leakage suppression characteristics along with a fast absorption rate and high absorption capacity. [Modes for carrying out the invention]

[0014] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as “includes,” “equip,” or “have” are intended to specify the existence of implemented features, steps, components, or combinations thereof, and should not be understood to preemptively exclude the existence or possibility of adding one or more other features, steps, components, or combinations thereof.

[0015] The present invention can be modified in various ways and may take many forms; therefore, specific embodiments are illustrated and described in detail below. However, this should not be understood as limiting the present invention to any particular form of disclosure, but rather as including any modifications, equivalents, or substitutions that fall within the spirit and technical scope of the present invention.

[0016] The technical terms used herein are for the sole purpose of referring to specific examples and are not intended to limit the invention. The singular forms used herein also include the plural forms unless the text explicitly indicates otherwise.

[0017] The term "polymer" or "macromolecule" used in the specification of the present invention means a state in which an acrylic monomer, which is a water-soluble ethylenically unsaturated monomer, is polymerized, and can include all moisture content ranges or particle size ranges. Among the polymers, those in the state before drying after polymerization and having a water content (moisture content) of about 40% by weight or more can be referred to as hydrogel polymers, and particles obtained by pulverizing and drying such hydrogel polymers can be referred to as crosslinked polymers.

[0018] The term "superabsorbent resin particles" refers to substances on particles, including crosslinked polymers obtained by polymerizing acrylic monomer monomers containing acidic groups and at least a part of which acidic groups are neutralized and crosslinked by an internal crosslinking agent.

[0019] The term "superabsorbent resin" means, depending on the context, a crosslinked polymer obtained by polymerizing an acrylic monomer containing acidic groups and at least a part of which acidic groups are neutralized, or a powder form base resin composed of particles obtained by pulverizing the crosslinked polymer, or includes all those made suitable for commercialization through additional processes such as surface crosslinking, micronized reprecipitation granulation, drying, pulverization, classification, etc. for the crosslinked polymer and the base resin. Therefore, the term "superabsorbent resin" can be interpreted as including a plurality of superabsorbent resin particles.

[0020] Superabsorbent resins used in sanitary materials such as diapers need to exhibit a high absorption capacity for moisture, excellent absorption capacity under pressure such that the absorbed moisture does not leak out under external pressure, and at the same time, a high absorption rate is required so that the liquid can be rapidly absorbed.

[0021] Therefore, the present invention provides a superabsorbent resin and a method for producing the same, both of which are excellent in various physical properties such as centrifugal water retention capacity, absorption capacity under pressure, absorption rate, 1-minute tap water absorption capacity, and rewetting characteristics.

[0022] Specifically, the superabsorbent resin according to one embodiment of the present invention satisfies all of the following physical properties i) to v), and exhibits excellent absorption rate and absorption capacity, re-wetting and leakage prevention performance.

[0023] i) The water retention capacity by centrifugal separation is 30 g / g or more, ii) The effective absorption capacity (EFFC, 0.3 psi) calculated by the following formula 1 is greater than 29.5 g / g, iii) The absorption rate by the vortex method is less than 37 seconds, iv) When 1 g of superabsorbent polymer is immersed in 2 L of tap water and swelled for 1 minute, the tap water absorption capacity per minute, defined as the weight of water absorbed by the superabsorbent polymer in 1 minute, exceeds 115 g. v) After immersing 4 g of superabsorbent polymer in 100 g of 0.9 wt% sodium chloride aqueous solution and swelling for 120 minutes, a filter paper is placed on the superabsorbent polymer and pressurized at 0.75 psi for 5 minutes, the re-wetting characteristic (re-wetting in pressurized salt water for 2 hours), defined as the weight of water that seeps out again from the superabsorbent polymer onto the filter paper, is less than 3.7 g.

[0024] [Formula 1] Effective absorption capacity (EFFC, 0.3 psi) = {Centrifugal separation water retention capacity (CRC) + 0.3 psi pressurized absorption capacity (AUP)} / 2

[0025] Generally, superabsorbent polymers with a fast absorption rate using the vortex method are produced by polymerizing monomers in the presence of an internal crosslinking agent and a foaming agent to form a porous structure. However, this method has the problem of resulting in a low crosslinking density, which reduces the water retention capacity and pressurized absorption capacity during centrifugal separation.

[0026] Consequently, to compensate for the decrease in internal crosslinking density due to the foam polymerization, conventional methods have been used to increase the surface crosslinking density. However, when the surface crosslinking density is higher than the internal crosslinking density, there is a problem in that the swelling capacity of the surface of the superabsorbent polymer decreases, resulting in a lower centrifugal separation water retention capacity. Furthermore, if the superabsorbent polymer swells excessively, the surface crosslinked portion may rupture, exposing the internal base resin to the outside, which can lead to re-wetting.

[0027] Therefore, the inventors conducted extensive research to provide a superabsorbent resin that simultaneously satisfies physical properties such as centrifugal water retention capacity, EFFC, absorption rate by vortex method, 1-minute tap water absorption capacity, and re-wetting. As a result, they confirmed that the above-mentioned required physical properties can be simultaneously satisfied by adjusting the internal crosslinking density and surface crosslinking density of the base resin, thus completing the present invention. In other words, because the difference between the internal crosslinking density and surface crosslinking density of the base resin of the superabsorbent resin of the present invention is small, the surface can expand well when absorbing water, thereby simultaneously improving centrifugal water retention capacity and pressurized absorption capacity, and also resulting in a high absorption rate.

[0028] The centrifugal separation water retention capacity (CRC) of the superabsorbent polymer may, for example, be 30 g / g or more, 31 g / g or more, or 32 g / g or more. The upper limit of the centrifugal separation water retention capacity is not theoretically limited, but for example, it may be 40 g / g or less, 38 g / g or less, or 35 g / g or less.

[0029] The effective absorption capacity (EFFC) is the arithmetic mean of the centrifugal water retention capacity (CRC) and the pressurized absorption capacity (AUP) at 0.3 psi. In one embodiment of the present invention, the superabsorbent polymer may have an EFFC exceeding 29.5 g / g, or exceeding 30 g / g, or exceeding 30.5 g / g, and being 40 g / g or less, or 38 g / g or less, or 35 g / g or less.

[0030] On the other hand, the pressure absorption capacity (AUP) of the superabsorbent polymer at 0.3 psi can specifically be 25 g / g or more, or 27 g / g or more, or 29 g / g or more, or 30 g / g or more, or 31 g / g or more, and satisfy the range of 40 g / g or less, or 38 g / g or less, or 35 g / g or less.

[0031] The absorption rate of the superabsorbent polymer by the vortex method satisfies less than 37 seconds and can satisfy 35 seconds or less, 32 seconds or less, 30 seconds or less, or 29 seconds or less. A faster absorption rate by the vortex method is preferable, and there is no lower limit, but it may be, for example, 10 seconds or more, 15 seconds or more, 20 seconds or more, or 25 seconds or more.

[0032] The water absorption capacity of the superabsorbent polymer per minute may be more than 115g, more than 125g, more than 130g, or 133g or more, and 200g or less, or 190g or less, or 180g or less.

[0033] The tap water used in the aforementioned 1-minute tap water absorption capacity measurement has an electrical conductivity of 170 to 180 μS / cm. Since the electrical conductivity of tap water greatly affects the measured physical properties, it is necessary to measure the physical properties using tap water with an equivalent level of electrical conductivity.

[0034] The superabsorbent polymer can satisfy the requirement that the re-wetting characteristic (re-wetting with pressurized salt water for 2 hours) is less than 3.7 g, defined as the weight of water that seeps out from the superabsorbent polymer to the filter paper after immersing 4 g of the superabsorbent polymer in 100 g of a 0.9 wt% sodium chloride aqueous solution for 120 minutes, swelling, placing a filter paper on the superabsorbent polymer, and pressurizing at 0.75 psi for 5 minutes. Preferably, the weight of the water may be 3.4 g or less, 3.2 g or less, 3.0 g or less, or 2.7 g or less. A smaller amount of re-wetting is preferable, and the weight of the water may theoretically be 0 g, or for example, 0.1 g or more, 0.2 g or more, or 0.3 g or more.

[0035] The methods for measuring the centrifugal separation water retention capacity, pressurized absorption capacity, absorption rate, 1-minute tap water absorption capacity, and re-wetting characteristics will be specifically described in the examples below.

[0036] As described above, the superabsorbent resin according to one embodiment of the present invention exhibits high centrifugal separation water retention capacity, EFFC, and 1 minute tap water absorption capacity, as well as a fast absorption rate, making it suitable for use in fields such as sanitary materials where a high absorption rate is required along with re-wetting and leak prevention properties.

[0037] On the other hand, the superabsorbent resin is A step of preparing a monomer composition comprising an acrylic acid monomer having at least a portion of neutralized acidic groups, an internal crosslinking agent, and a polymerization initiator, The steps include crosslinking the monomer composition to form a water-containing gel polymer, The steps include drying, pulverizing, and classifying the aforementioned water-containing gel polymer to form a base resin, The process includes a surface crosslinking step in which a surface crosslinking layer is formed on the surface of the base resin in the presence of a surface crosslinking solution containing a surface crosslinking agent, water, and an organic solvent having a boiling point of less than 90°C. In this case, the surface crosslinking solution can be manufactured by a manufacturing method in which the weight ratio of water / organic solvent is 1.0 or more and less than 3.0, and the content of organic solvent in the surface crosslinking solution is 24% by weight or more.

[0038] The method for producing superabsorbent polymers will be explained in more detail step by step below, using specific examples of the invention.

[0039] (Step 1) In a manufacturing method according to one embodiment, step 1 is the step of preparing a monomer composition comprising an acrylic acid monomer having an acidic group, in which at least a portion of the acidic group is neutralized, an internal crosslinking agent, and a thermal polymerization initiator.

[0040] The aforementioned acrylic acid monomer is a compound represented by the following chemical formula 1.

[0041] [Chemical formula 1] R 1 -COOM 1

[0042] In the aforementioned chemical formula 1, R 1 This is an alkyl group having 2 to 5 carbon atoms and containing an unsaturated bond. M 1 This is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0043] Preferably, the acrylic acid 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.

[0044] Here, the acrylic acid monomer may have an acidic group, and at least a portion of the acidic group may be neutralized. Preferably, the monomer may be partially neutralized with an alkaline substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide. In this case, the degree of neutralization of the acrylic acid monomer may be 40 to 95 mol%, 40 to 80 mol%, or 45 to 75 mol%. The range of the degree of neutralization can be adjusted depending on the final physical properties. However, if the degree of neutralization is excessively high, the neutralized monomer may precipitate, making it difficult for polymerization to proceed smoothly. Conversely, if the degree of neutralization is excessively low, not only will the absorption capacity of the polymer decrease significantly, but it may also exhibit properties similar to an elastic rubber that is difficult to handle.

[0045] The concentration of the acrylic acid monomer may be about 20 to about 60% by weight, preferably about 40 to about 50% by weight, relative to the monomer composition containing the raw materials and solvent of the superabsorbent resin, and may be set to an appropriate concentration considering the polymerization time and reaction conditions. However, if the concentration of the monomer is too low, the yield of the superabsorbent resin will be low, which may cause economic problems. Conversely, if the concentration is too high, some of the monomer may precipitate, or the grinding efficiency of the polymerized water-containing gel polymer may be low, which may cause process problems and potentially degrade the physical properties of the superabsorbent resin.

[0046] Furthermore, the term "internal crosslinking agent" as used herein is used to distinguish it from a surface crosslinking agent used to crosslink the surface of superabsorbent polymer particles, as described later, and plays the role of crosslinking and polymerizing the unsaturated bonds of the aforementioned water-soluble ethylene-based unsaturated monomer. In the above step, crosslinking is performed without distinction between the surface and the interior, or if a surface crosslinking step for superabsorbent polymer particles, as described later, is performed, the surface of the particles of the final manufactured superabsorbent polymer has a structure crosslinked by the surface crosslinking agent, and the interior has a structure crosslinked by the internal crosslinking agent.

[0047] Any compound that enables the introduction of crosslinking bonds during the polymerization of the acrylic acid-based unsaturated monomer can be used as the internal crosslinking agent. Specifically, the internal crosslinking agent can be a crosslinking agent having one or more functional groups that can react with the water-soluble substituents of the acrylic acid-based unsaturated monomer, as well as one or more ethylenically unsaturated groups; or a crosslinking agent having two or more functional groups that can react with the water-soluble substituents of the monomer and / or water-soluble substituents formed by the hydrolysis of the monomer.

[0048] As a non-limiting example, the internal 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, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate, allyl(meth)acrylate Acrylate compounds such as acrylate; epoxy compounds such as ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, tripropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, polytetramethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol triglycidyl ether, diglycerol polyglycidyl ether, and polyglycerol polyglycidyl ether; triarylamines; propylene glycol; glycerin; or polyfunctional crosslinking agents such as ethylene carbonate may be used alone or in combination of two or more, but are not limited thereto.

[0049] In the monomer composition, such an internal crosslinking agent may be used in an amount of 0.01 to 5 parts by weight per 100 parts by weight of the acrylic acid monomer. For example, the internal crosslinking agent may be used in an amount 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 the water-soluble ethylene-based unsaturated monomer. If the content of the internal crosslinking agent is excessively low, crosslinking may not occur sufficiently, making it difficult to achieve a strength above an appropriate level. If the content of the internal crosslinking agent is excessively high, the internal crosslinking density may become high, making it difficult to achieve the desired water retention capacity.

[0050] Furthermore, the monomer composition further comprises a polymerization initiator for initiating the polymerization reaction of the monomer, and such polymerization initiator is not particularly limited as long as it is one that is commonly used in the production of superabsorbent polymers.

[0051] Specifically, the polymerization initiator is carried out by thermal polymerization or photopolymerization. However, even with photopolymerization, a certain amount of heat is generated by irradiation such as ultraviolet light, and a certain amount of heat is also generated by the progression of the polymerization reaction, which is an exothermic reaction. Therefore, both photopolymerization initiators and thermal polymerization initiators can be used.

[0052] In one embodiment, one or more initiators selected from the group consisting of persulfate compounds, azo compounds, hydrogen peroxide, and ascorbic acid may be used as the thermal polymerization initiator. 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 and 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride. Examples include dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 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 detail on page 203 of Odian's "Principle of Polymerization" (Wiley, 1981), and are not limited to the examples mentioned above.

[0053] More specifically, the thermal polymerization initiator may include the persulfate compound. For example, sodium persulfate (Na2S2O8) may be used as the thermal polymerization initiator.

[0054] The thermal polymerization initiator may be used in an amount of 0.05 to 0.25 parts by weight per 100 parts by weight of the acrylic acid monomer. If the thermal polymerization initiator is used in an amount of less than 0.05 parts by weight, the polymerization rate will be slow, and additional radical initiation reactions by reaction with the reducing agent will not occur sufficiently, so the residual monomer content will not decrease easily. If the thermal polymerization initiator is used in an amount exceeding 0.25 parts by weight, the water-soluble component content will increase, which may cause the superabsorbent polymer to re-wet.

[0055] Furthermore, the monomer composition may further contain a photopolymerization initiator. The photopolymerization initiator can be any compound that can form radicals when exposed to light such as ultraviolet light, and its composition is not limited.

[0056] As the photopolymerization initiator, for example, one or more selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkyl ketone, phenylgly oxylate, benzyl dimethyl ketal, acyl phosphine, and α-aminoketone may be used. On the other hand, as a specific example of acyl phosphine, commercial lucirin TPO, i.e., diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, may be used. A wider variety of photoinitiators are described in detail on page 115 of Reinhold Schwalm's book "UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)," and are not limited to the examples mentioned above.

[0057] The photopolymerization initiator may be used in an amount of 0.001 to 0.010 parts by weight per 100 parts by weight of the acrylic acid monomer. If the concentration of the photopolymerization initiator is excessively low, the polymerization rate may be slowed, and if the concentration of the photopolymerization initiator is excessively high, the molecular weight of the superabsorbent resin may be small, and the physical properties may become non-uniform.

[0058] The monomer composition may further contain, if necessary, reducing agents such as thickeners, plasticizers, preservatives, and antioxidants.

[0059] 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 such a monomer composition in solution state, that is, the concentrations of the monomer, internal 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.

[0060] When the monomer composition has a solid content within the range described above, it is advantageous for adjusting the grinding efficiency during the grinding of the polymer, as described later, while eliminating the need to remove unreacted monomers after polymerization using the gel effect phenomenon that occurs in the polymerization reaction of a high-concentration aqueous solution.

[0061] In this case, the solvent used can be any solvent capable of dissolving the aforementioned components, and its composition is not limited. For example, one or more solvents selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide may be used in combination.

[0062] (Step 2) Next, the monomer composition is crosslinked and polymerized to produce a water-containing gel polymer.

[0063] The polymerization of the monomer composition is not particularly limited in its composition, as long as it is carried out by a commonly used polymerization method.

[0064] Specifically, polymerization methods are broadly divided into thermal polymerization and photopolymerization depending on the polymerization energy source. When thermal polymerization is performed, it may be carried out in a reactor with a stirring shaft such as a kneader, and when photopolymerization is performed, it may be carried out in a reactor equipped with a movable conveyor belt. However, the polymerization methods described above are just examples, and the present invention is not limited to the polymerization methods described above.

[0065] As an example, the hydrated gel polymer obtained by thermal polymerization by supplying hot air to a reactor such as a kneader equipped with a stirring shaft, as described above, may be several centimeters to several millimeters in size when discharged from the reactor outlet, depending on the configuration of the stirring shaft in the reactor. Specifically, the size of the obtained hydrated gel polymer can be obtained in various ways depending on the concentration and injection rate of the monomer composition injected, but typically a hydrated gel polymer with a weight-average particle size of 2 to 50 mm is obtained.

[0066] Furthermore, when photopolymerization is performed in a reactor equipped with a movable conveyor belt as described above, the form of the water-containing gel polymer that is usually obtained may be a water-containing gel polymer on a sheet having the width of the belt. In this case, the thickness of the polymer sheet varies depending on the concentration and injection rate of the monomer composition being injected, or it is preferable to supply the monomer composition so that a polymer on a sheet having a thickness of usually about 0.5 to about 5 cm is obtained. If the monomer composition is supplied to such an extent that the thickness of the polymer on the sheet is excessively thin, the production efficiency is low and undesirable, and if the thickness of the polymer on the sheet exceeds 5 cm, the polymerization reaction may not occur uniformly across the entire thickness due to the excessive thickness.

[0067] On the other hand, in order to improve the absorption rate of the superabsorbent polymer, a blowing agent that can introduce a porous structure into the superabsorbent polymer during the crosslinking polymerization of the monomer composition may be used. Such a blowing agent can be any compound known in the art that can generate foam in the polymerization step, but for example, a carbonate-based blowing agent or an encapsulated blowing agent may be used.

[0068] As an example, one or more carbonate-based foaming agents may be selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate.

[0069] Specifically, the carbonate-based blowing agent may be used in an amount of 0.005 to 1 part by weight per 100 parts by weight of the acrylic acid monomer. If the content of the blowing agent is less than 0.005 parts by weight, its role as a blowing agent may be negligible, and if the content of the blowing agent exceeds 1 part by weight, the number of pores in the crosslinked polymer may be excessively large, which may reduce the gel strength and density of the superabsorbent polymer produced, potentially causing problems in distribution and storage.

[0070] Furthermore, a heat-expandable microcapsule blowing agent having a core-shell structure may be used as the encapsulated blowing agent. More specifically, the encapsulated blowing agent has a core-shell structure comprising a core containing hydrocarbons and a shell made of a thermoplastic resin formed on the core. Specifically, the hydrocarbons constituting the core readily vaporize into liquid hydrocarbons with low boiling points when heated. Therefore, when heat is applied to the encapsulated blowing agent, the thermoplastic resin forming the shell softens and the liquid hydrocarbons in the core vaporize, causing the pressure inside the capsule to increase and expand, thereby forming bubbles larger than the existing size.

[0071] The hydrocarbon constituting the core of the encapsulated foaming agent may be one or more selected from the group consisting of n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, iso-heptane, cycloheptane, n-octane, iso-octane, and cyclooctane. Among these, hydrocarbons having 3 to 5 carbon atoms (n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane) are suitable for forming pores of the aforementioned size, and iso-butane is the most suitable.

[0072] Furthermore, the thermoplastic resin constituting the shell of the encapsulated foaming agent may be a polymer formed from one or more monomers selected from the group consisting of (meth)acrylate compounds, (meth)acrylonitrile compounds, aromatic vinyl compounds, vinyl acetate compounds, and vinyl halogenated compounds. Among these, a copolymer of (meth)acrylate and (meth)acrylonitrile is considered the most suitable for forming pores of the aforementioned size.

[0073] Furthermore, such encapsulated foaming agents may have a structure comprising a core containing hydrocarbons and a shell formed of thermoplastic resin surrounding the core, with an average diameter of 5 to 30 μm before expansion and a maximum expansion ratio of 5 to 15 times in air.

[0074] The encapsulated blowing agent may be used in an amount of 0.005 to 1 part by weight per 100 parts by weight of the acrylic acid monomer. If the amount of the blowing agent is less than 0.005 parts by weight, its role as a blowing agent may be negligible, and if the amount of the blowing agent exceeds 1 part by weight, the number of pores in the crosslinked polymer may be excessively large, which may reduce the gel strength and density of the superabsorbent polymer produced, potentially causing problems in distribution and storage.

[0075] Furthermore, in step 2, a surfactant commonly used as a foam stabilizer may be added along with the carbonate-based foaming agent. Examples of such surfactants include alkyl sulfate compounds and polyoxyethylene alkyl ether compounds. Examples of alkyl sulfate compounds include sodium dodecyl sulfate, ammonium lauryl sulfate, sodium lauryl ether sulfate, or sodium myreth sulfate, while an example of a polyoxyethylene alkyl ether compound is polyoxyethylene lauryl ether. In this case, the alkyl sulfate compound is an anionic surfactant, and the polyoxyethylene alkyl ether compound is a nonionic surfactant.

[0076] The typical water content of a water-containing gel polymer obtained by this method may be about 40 to about 80% by weight. On the other hand, throughout this specification, "water content" refers to the amount of water content relative to the total weight of the polymer, and means the value obtained by subtracting the weight of the polymer in a dry state from the weight of the polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to water evaporation in the polymer during the drying process in which the temperature of the polymer is raised by infrared heating. In this case, the drying conditions are set to raise the temperature to about 180°C at room temperature and then maintain it at 180°C, and the total drying time is set to 20 minutes, including a 5-minute temperature rise step, and the water content is measured.

[0077] (Step 3) Next, the water-containing gel polymer is dried, pulverized, and classified to form a base resin.

[0078] On the other hand, before drying the water-containing gel polymer, a step of coarsely grinding the water-containing gel polymer may be further performed. The type of grinder used for grinding is not particularly limited and may include any one selected from the group of grinding equipment consisting of a vertical pulverizer, turbo cutter, turbo grinder, rotary cutter mill, cutter mill, disc mill, shred crusher, crusher, chopper, and disc cutter.

[0079] Coarse grinding may be performed so that the particle size of the water-containing gel polymer is approximately 1 mm to approximately 10 mm. Grinding to a particle size of less than 1 mm is technically difficult due to the high water content of the water-containing gel polymer, and there is also a possibility that the ground particles will aggregate with each other. On the other hand, grinding to a particle size of more than 10 mm has little effect on increasing the efficiency of the subsequent drying step.

[0080] The drying temperature in the drying step 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 physical properties of the final superabsorbent polymer. If the drying temperature exceeds 250°C, only the polymer surface may be excessively dried, which may generate fine powder in the subsequent grinding step, and may reduce the physical properties of the final superabsorbent polymer. Therefore, preferably, the drying may be carried out at a temperature of about 150 to about 200°C, and more preferably at a temperature of about 170 to about 200°C.

[0081] On the other hand, the drying time may be approximately 20 to 90 minutes, taking into consideration process efficiency, but it is not limited to this.

[0082] The drying method in the drying step can be selected and used without limitation, as long as it is one that is commonly used in the drying process of water-containing gel polymers. Specifically, the drying step may be carried out by methods such as supplying hot air, infrared irradiation, ultra-high frequency irradiation, or ultraviolet irradiation. The water content of the polymer after such a drying step may be about 5 to about 10% by weight.

[0083] Next, the dried polymer obtained through this drying step is pulverized. At this time, the pulverization may be carried out to obtain a powder containing particles with a particle size of approximately 150 to approximately 850 μm. The pulverizer used to pulverize to such a particle size may be a pin mill, hammer mill, screw mill, roll mill, disc mill, or jog mill, but is not limited to the examples described above.

[0084] After this grinding step, the base resin obtained after grinding is classified by particle size in order to control the physical properties of the superabsorbent polymer powder to be finalized as a product. Preferably, polymers with a particle size of about 150 to about 850 μm are classified, and only base resins with such particle size may undergo the surface crosslinking reaction step. At this time, the particle size may be measured by the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3 method.

[0085] (Step 4) Next, a surface crosslinking step is performed in which a surface crosslinking layer is formed on the surface of the base resin in the presence of a surface crosslinking solution containing a surface crosslinking agent, water, and an organic solvent with a boiling point of less than 90°C.

[0086] Through the above steps, a superabsorbent resin is produced in which a surface crosslinking layer is formed on at least a portion of the surface of the base resin, more specifically, on at least a portion of the surface of each base resin particle.

[0087] The aforementioned surface crosslinking is a step that increases the crosslinking density near the surface of the base resin particles, in relation to the crosslinking density inside the particles. Generally, the surface crosslinking agent is applied to the surface of the superabsorbent resin particles. Therefore, this reaction occurs on the surface of the base resin particles, which improves the crosslinking properties on the particle surface without substantially affecting the inside of the particles. Thus, surface-crosslinked superabsorbent resin particles have a higher degree of crosslinking near the surface than inside.

[0088] As the surface crosslinking agent, an epoxy-based surface crosslinking agent containing two or more epoxy groups in the molecule may be used. For example, as the surface crosslinking agent, one or more selected from the group consisting of ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether (ethylene glycol repeating unit number 2 to 22), propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether (propylene glycol repeating unit number 2 to 66), glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether (glycerol repeating unit number 2 to 3), and sorbitol polyglycidyl ether may be used.

[0089] According to one embodiment, the surface crosslinking agent may be the same as the internal crosslinking agent. For example, the surface crosslinking agent may contain ethylene glycol diglycidyl ether. The ethylene glycol diglycidyl ether has high water solubility, has two epoxy groups, and can easily undergo surface reactions even at relatively low temperatures.

[0090] The content of the surface crosslinking agent may be appropriately selected depending on the type of surface crosslinking agent added and the reaction conditions, but it may be about 0.001 to about 5 parts by weight, preferably about 0.01 to about 3 parts by weight, and more preferably about 0.03 to about 2 parts by weight per 100 parts by weight of the base resin. If the content of the surface crosslinking agent is too low, the surface crosslinking reaction will hardly occur, and if it exceeds 5 parts by weight per 100 parts by weight of the base resin, excessive surface crosslinking reaction may occur, potentially leading to a decrease in absorption properties such as water retention capacity.

[0091] In this invention, the surface crosslinking agent is applied to the base resin in the form of a surface crosslinking solution dissolved in a solvent. Adding the surface crosslinking agent in this solution form is preferable because it allows the surface crosslinking agent to be evenly dispersed by the base resin.

[0092] In particular, the present invention uses water and an organic solvent with a boiling point of less than 90°C as the solvent for the surface crosslinking solution, and satisfies a water / organic solvent weight ratio of 1.0 or more and less than 3.0, thereby increasing the coating efficiency of the surface crosslinking solution and producing a superabsorbent resin that simultaneously satisfies the physical properties i) to v) described above.

[0093] As described above, when an organic solvent with a boiling point of less than 90°C (hereinafter referred to as "organic solvent") is used as a cosolvent along with water, the organic solvent with a lower boiling point than water is evaporated first after the surface crosslinking solution is applied to the surface of the base resin. Subsequently, both water and the surface crosslinking agent can penetrate deeply into the base resin, forming a deeper and more uniform surface crosslinked layer.

[0094] As the organic solvent, for example, one or more selected from the group consisting of methanol, ethanol, isopropanol, acetone, and ethyl acetate may be used.

[0095] Preferably, the organic solvent may be ethanol and / or isopropanol, and more preferably isopropanol.

[0096] Isopropanol is preferable because it has low toxicity and odor, and surface crosslinking solutions containing isopropanol can be absorbed into the base resin surface at an appropriate rate. In other words, if the surface crosslinking solution is absorbed too rapidly into the base resin surface, it is difficult to apply the solution evenly. However, when using isopropanol, the surface crosslinking solution is absorbed into the base resin at an appropriate rate, allowing for more even application to the base resin surface.

[0097] In this case, by ensuring that the weight ratio of water / organic solvent in the surface crosslinking solution is 1.0 or more and less than 3.0, and that the content of organic solvent in the total weight of the surface crosslinking solution is 24% by weight or more, the coating efficiency of the surface crosslinking solution can be increased, thereby enabling the production of a superabsorbent resin that simultaneously satisfies the physical properties i) to v) described above.

[0098] If the water / organic solvent weight ratio is 3.0 or higher, the surface crosslinking solution cannot be uniformly applied to the base resin. Consequently, in this case, the degree of surface crosslinking of each base resin particle will differ from one another, making it impossible to achieve uniform absorption properties. Furthermore, excessive debris, which is aggregates formed when the base resin solidifies with a particle size exceeding 850 μm during the surface crosslinking process, will be generated. This can hinder the smooth operation of the process and potentially lead to productivity problems.

[0099] Therefore, preferably, the weight ratio of water / organic solvent in the surface crosslinking solution is 2.9 or less, or 2.8 or less, or 2.7 or less, and may also be 1.2 or more, or 1.7 or more, or 2.3 or more. If the proportion of water in the solvent of the surface crosslinking solution is excessively low, a thin surface crosslinked layer may be formed, and cracks may occur on the surface when the superabsorbent resin swells. Also, if the proportion of organic solvent in the solvent of the surface crosslinking solution is excessively high, a large amount of organic solvent may remain in the superabsorbent resin, which is undesirable.

[0100] Furthermore, the content of the organic solvent in the total weight of the surface crosslinking solution may be 24% by weight or more, or 25% by weight or more, or 25.9% by weight or more, and 40% by weight or less, or 35% by weight or less, or 30% by weight or less. If the content of the organic solvent is less than 24% by weight, the coating efficiency of the surface crosslinking solution may decrease, and if it exceeds 40% by weight, organic solvents may remain in the manufactured superabsorbent resin, so it is preferable to satisfy the above range.

[0101] On the other hand, the surface crosslinking solution may further contain additives to further improve the coating efficiency of the solution. For example, the additive may be one or more glycol compounds selected from the group consisting of propylene glycol, dipropylene glycol, tripropylene glycol, and glycerin; and / or a polycarboxylic acid copolymer. Preferably, propylene glycol and a polycarboxylic acid copolymer may be used as the additive. Propylene glycol and polycarboxylic acid copolymers are non-toxic and do not produce an unpleasant odor after production, so they can be used more preferably.

[0102] The additive may be used in an amount of 15% by weight or less, or 12% by weight or less, of the total weight of the surface crosslinking solution, and in an amount of 1% by weight or more, or 2% by weight or more, or 3% by weight or more.

[0103] Specifically, glycol compounds may be used in the surface crosslinking solution in an amount of 1% or more by weight, or 2% or more by weight, or 3% or more by weight, and 10% or less by weight, or 9% or less by weight. In addition, polycarboxylic acid copolymers may be used in the surface crosslinking solution in an amount of 0.05% or more by weight, or 0.1% or more by weight, and 1% or less by weight, or 0.5% or less by weight.

[0104] Furthermore, it is preferable that the surface crosslinking solution has a surface crosslinking agent represented by the following formula 3 with a density of 1.0 or more and less than 17.0.

[0105] [Formula 3] Density of surface crosslinking agent = (Weight of surface crosslinking agent) / (Weight of water) × 1,000

[0106] If the density of the surface crosslinking agent is less than 1.0, the amount of surface crosslinking agent may be insufficient, making it difficult to achieve an appropriate degree of surface crosslinking. Furthermore, if the density of the surface crosslinking agent is 17.0 or higher, the crosslinking density is excessively high, which may cause cracks to form on the surface-crosslinked surface when the superabsorbent polymer swells to its maximum extent, exposing the internal base resin. Therefore, it is preferable that the density of the surface crosslinking agent satisfies the aforementioned range, and more preferably satisfies 2.0 or higher, or 3.0 or higher, or 4.0 or higher, and 15.0 or lower, or 12.0 or lower, or 9.0 or lower, or 6.0 or lower.

[0107] On the other hand, the surface crosslinking solution may further contain one or more polyvalent metal salts, such as aluminum salts, more specifically, one or more selected from the group consisting of aluminum sulfate, potassium salt, ammonium salt, sodium salt, and hydrochloride salt.

[0108] When such polyvalent metal salts are used further, the permeability of the superabsorbent resin can be further improved. For example, the polyvalent metal salt may be used in an amount of 0.01 to 4 parts by weight per 100 parts by weight of the base resin.

[0109] The means for raising the temperature for the surface crosslinking reaction are not particularly limited. Heating can be performed by supplying a heat transfer medium or by directly supplying a heat source. In this case, the type of heat transfer medium that can be used may be steam, hot air, hot oil, or other heated fluids, but the present invention is not limited to these, and the temperature of the supplied heat transfer medium can be appropriately selected considering the type of heat transfer medium, the heating rate, and the target temperature. On the other hand, as a directly supplied heat source, there are heating methods using electricity or gas, but the present invention is not limited to the examples described above.

[0110] The surface crosslinking reaction may be carried out under atmospheric pressure (760 ± 50 torr), and the reaction temperature may be 100°C or higher, or 110°C or higher, or 120°C or higher, and less than 160°C, or 150°C or lower, or 140°C or lower, or 130°C or lower. If the reaction temperature is below 100°C, the reaction may not proceed smoothly, and if it is above 160°C, the glycol-based compound used to improve the coating efficiency of the surface crosslinking solution may undergo a crosslinking reaction, making it impossible to achieve the desired effect.

[0111] As described above, when the surface crosslinking step is performed using a surface crosslinking solution that satisfies the weight ratio of water / organic solvent, the surface crosslinking solution is evenly applied to the base resin at an appropriate concentration, preventing the base resins from agglomerating with each other and allowing the surfaces of each base resin to be crosslinked to a uniform degree. As a result, the amount of waste generated after the surface crosslinking step may be 4.0% by weight or less, or 3.0% by weight or less, or 2.8% by weight or less.

[0112] On the other hand, as mentioned above, inorganic materials can be further mixed in after forming a surface crosslinking layer on the surface of the base resin.

[0113] The inorganic material may be one or more selected from the group consisting of silica, clay, alumina, silica-alumina composite material, and titania, and preferably silica may be used.

[0114] Such inorganic substances may be used in an amount of 0.01 parts by weight or more, or 0.05 parts by weight or more, or 0.1 parts by weight or more, and 5 parts by weight or less, or 3 parts by weight or less, or 1 part by weight or less, per 100 parts by weight of the superabsorbent resin.

[0115] On the other hand, in order to control the physical properties of the superabsorbent polymer used in the final product, an additional step may be performed to classify the superabsorbent polymer obtained after the surface crosslinking reaction step according to its particle size. Preferably, polymers with a particle size of about 150 to about 850 μm are classified so that only superabsorbent polymers with such particle sizes can be used in the final product.

[0116] Through the manufacturing method described above, a superabsorbent polymer with a small difference between internal crosslinking density and surface crosslinking density can be provided. Such a superabsorbent polymer expands well on the surface when absorbing moisture, thereby simultaneously improving centrifugal water retention capacity and pressurized absorption capacity, and consequently providing a superabsorbent polymer with a high absorption rate. As a result, the superabsorbent polymer can simultaneously satisfy the aforementioned physical properties of i) centrifugal water retention capacity, ii) pressurized absorption capacity, iii) absorption rate, iv) tap water absorption capacity in one minute, and v) re-wetting characteristics.

[0117] The present invention will be described in more detail by the following examples. However, the following examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples.

[0118] <Examples> Example 1 (Step 1) In a 3L glass container equipped with a stirrer and thermometer, 100g of acrylic acid, 0.001g of the internal crosslinking agent PEGDA400 (polyethylene glycol diacrylate, Mw=400), 0.18g of ethylene glycol diglycidyl ether (EJ1030s), 0.008g of the photopolymerization initiator diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide (I-819), and 0.12g of the thermal polymerization initiator sodium persulfate (SPS) were added and dissolved. Then, 188g of a 22% sodium hydroxide solution was added to produce the monomer composition (degree of neutralization: 75 mol%; solid content: 43 wt%).

[0119] (Step 2) 0.02 g of a 28% sodium dodecyl sulfate (SDS) aqueous solution and 0.18 g of encapsulated foaming agent F-36D (manufactured by Matsumoto) were added to the monomer composition. The monomer composition was then supplied at a rate of 500-2000 mL / min onto a conveyor belt that was 10 cm wide and 2 m long and rotating at a speed of 50 cm / min. Along with the supply of the monomer composition, a flow rate of 10 mW / cm² was generated. 2 A polymerization reaction was carried out for 60 seconds by irradiating the polymer with ultraviolet light of a certain intensity to obtain a sheet-type hydrated gel polymer with a water content of 55% by weight.

[0120] (Step 3) Next, 0.032 parts by weight of sodium metabisulfite (SMBS; Na2S2O5, manufactured by Fukko Sangyo Co., Ltd.) was dissolved in 54 g of water with 100 parts by weight of the acrylic acid monomer. The prepared sodium metabisulfite aqueous solution was then sprayed onto a hydrated gel polymer cut to a size of approximately 5 cm x 5 cm. This was then placed in a meat chopper and finely chopped to obtain hydrated gel particle powder (crumb) having a size of 1 mm to 10 mm.

[0121] Subsequently, the crumb was dried in an oven with adjustable airflow direction. Hot air at over 190°C was blown from below to above for 15 minutes, and then from above to below for another 15 minutes to ensure uniform drying, so that the moisture content of the dried material was 2% or less. After drying, the material was crushed in a pulverizer, then classified to select particles between 150 and 850 μm in size to prepare the base resin (BR).

[0122] (Step 4) A surface crosslinking solution was prepared by mixing 7 g of water with 100 g of the base resin as a basis; 3 g of isopropanol (IPA) as an organic solvent; 0.035 g of ethylene glycol diglycidyl ether (EJ1030s) as a surface crosslinking agent; and 1.0 g of propylene glycol (PG), 0.05 g of polycarboxylic acid copolymer (methoxypolyethylene glycol monomethacrylate and methacrylic acid copolymer, GK, Mw=40,000), and 0.04 g of aluminum sulfate (Al-S) as additives.

[0123] The surface crosslinking solution was sprayed onto 100 g of the base resin obtained above and mixed. This mixture was then placed in a container equipped with a stirrer and a double jacket, and a surface crosslinking reaction was carried out at 125°C for 35 minutes. Subsequently, the surface-treated powder was classified using an ASTM standard sieve to obtain a superabsorbent resin powder having particle sizes of 150 to 850 μm. Then, 0.15 g of fumed silica (AEROSIL® 200) was added and mixed to 100 parts by weight of the obtained resin powder.

[0124] Examples 2, 4, Comparative Example 1, Comparative Example 4, and Comparative Example 5 In Step 4, a superabsorbent polymer was produced in the same manner as in Example 1, except that the water, isopropanol (IPA), propylene glycol (PG), and ethylene glycol diglycidyl ether (EJ1030s) contained in the surface crosslinking solution were used in the amounts shown in Table 1 below.

[0125] Example 3, Comparative Example 2, and Comparative Example 3 The base resin was prepared in the same manner as in steps 1 to 3 of Example 1, except that 0.16 g of ethylene glycol diglycidyl ether (EJ1030s) was used in the preparation of the monomer composition in step 1.

[0126] Subsequently, a superabsorbent polymer was produced in the same manner as in step 4 of Example 1, except that the water, isopropanol (IPA), propylene glycol (PG), and ethylene glycol diglycidyl ether (EJ1030s) contained in the surface crosslinking solution were used in the amounts shown in Table 1 below.

[0127] In Table 1 below, the content of each component is expressed as the weight (g) per 100g of base resin.

[0128] [Table 1]

[0129] [Table 2]

[0130] Experimental example: Measurement of the physical properties of superabsorbent polymers The physical properties of the base resins and superabsorbent polymers produced in the above examples and comparative examples were evaluated using the following methods and are shown in Tables 2 and 3 below. Table 2 shows the physical properties of each base resin, and Table 3 shows the physical properties of the superabsorbent polymers obtained after the surface treatment step. The properties of the superabsorbent polymers were measured after production without further classification. For the base resins, the centrifugal separation water retention capacity (CRC) and absorption rate (Vortex time) were measured using the classified portion (particle size 300-425 μm) classified using a #40-50 sieve.

[0131] Unless otherwise stated, the following physical property evaluations were conducted entirely in a constant temperature and humidity chamber (23±0.5℃, relative humidity 45±0.5%), and the average of three measurements was used as the measurement data to prevent measurement errors. In addition, the physiological saline or salt water used in the following physical property evaluations refers to a 0.9 wt% sodium chloride (NaCl) aqueous solution.

[0132] (1)Centrifuge Retention Capacity (CRC) The water retention capacity of each resin, based on its absorption ratio under no load, was measured using EDANAWSP 241.3.

[0133] Specifically, superabsorbent polymer W0 (g) (approximately 0.2g) was uniformly placed in a nonwoven fabric envelope and sealed. The envelope was then immersed in physiological saline solution at room temperature. After 30 minutes, the envelope was centrifuged under 250G conditions for 3 minutes to remove moisture, and the mass of the envelope W2 (g) was measured. The same procedure was then performed without the polymer, and the mass W1 (g) was measured. Using the obtained masses, the CRC (g / g) was calculated using the following formula.

[0134] [Formula 2] CRC(g / g)={[W2(g)-W1(g)] / W0(g)}-1

[0135] (2) Absorbance under pressure (AUP) The pressure absorption capacity of each resin at 0.3 psi was measured using the EDANA method WSP 242.3.

[0136] Specifically, a 400-mesh stainless steel mesh was attached to the bottom of a plastic cylinder with an inner diameter of 60 mm. Superabsorbent polymer W3 (g) (0.90 g) was uniformly spread on the mesh under normal temperature and 50% humidity conditions, and a piston capable of uniformly applying a load of 0.3 psi was placed on top. The piston was slightly smaller than the 60 mm outer diameter, ensuring no gap between it and the inner wall of the cylinder, and allowing for unrestricted vertical movement. At this time, the weight W4 (g) of the device was measured.

[0137] A glass filter with a diameter of 90 mm and a thickness of 5 mm was placed inside a 150 mm diameter petroleum dish, and physiological saline solution was poured in until it was level with the top surface of the glass filter. A sheet of filter paper with a diameter of 90 mm was placed on top of that. The measuring device was placed on top of 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 W5 (g) was measured.

[0138] Using the obtained masses, the pressurized absorption capacity (g / g) was calculated using the following formula.

[0139] [Formula 3] AUP(g / g)=[W5(g)-W4(g)] / W3(g)

[0140] (3) Effective absorption capacity (EFFC, 0.3 psi) The effective absorption capacity (EFFC, 0.3psi) of each superabsorbent polymer was calculated using Equation 1 below, based on the CRC and 0.3psi AUP calculated above.

[0141] [Formula 1] Effective absorption capacity (EFFC) = {Water retention capacity (CRC) + 0.3 psi pressurized absorption capacity (AUP)} / 2

[0142] (4) Absorption rate (Vortex time) The absorption rate (vortex time) of the superabsorbent polymers in the above examples and comparative examples was measured according to the Japanese standard method (JIS K 7224) using the method described below.

[0143] (i) First, 50 mL of physiological saline was added to a 100 mL beaker with a flat bottom using a 100 mL mass cylinder. (ii) Next, the beaker was placed in the center of the magnetic stirrer, and a magnetic bar (8 mm in diameter, 30 mm in length) was placed inside the beaker. (iii) Subsequently, the agitator was operated so that the magnetic bar agitated at 600 rpm, so that the lowest part of the vortex generated by the agitation struck the magnetic bar. (iv) After confirming that the temperature of the salt water in the beaker reached 24.0°C, 2 ± 0.01 g of superabsorbent polymer sample (particle size 300 to 600 μm) was added while simultaneously operating a stopwatch. The time taken in seconds until the vortex disappeared and the liquid surface became completely horizontal was measured and defined as the absorption rate.

[0144] (5) Re-wet with pressurized saline solution for 2 hours (i) 4 g of superabsorbent polymer was evenly distributed in a 130 mm diameter petroleum dish, 100 g of physiological saline was poured in, and the dish was covered and allowed to swell for 120 minutes. Separately, the weight W6 (g) of 20 sheets of filter paper (manufacturer: whatman, catalog No. 1004-110, pore size 20~25 μm, diameter 110 mm) was measured. (ii) After 120 minutes had elapsed, the 20 sheets of filter paper were stacked on top of the superabsorbent resin and pressurized for 5 minutes with a weight measuring 110 mm in diameter and weighing 5 kg to apply a load of 0.75 psi. (iii) After 5 minutes, the weight W7 (g) of the filter paper that had absorbed the saline solution was measured. (iv) Using the obtained masses, the amount of physiological saline absorbed into the filter paper (re-wetting amount, g) was calculated using the following formula.

[0145] [Formula 4] Rewetting amount (g)=W7(g)-W6(g)

[0146] (6) Tap water free absorption capacity in 1 minute The 1-minute tap water absorption capacity of the superabsorbent polymers in the examples and comparative examples was measured by the following method.

[0147] (i) First, 1 g of superabsorbent polymer was placed in a tea bag measuring 15 cm wide and 30 cm long, and the tea bag was immersed in 2 L of tap water and allowed to swell for 1 minute. (ii) After that, the tea bag containing the swollen superabsorbent polymer was lifted out of the tap water and after 1 minute had passed, the weight of both the tea bag and the superabsorbent polymer was measured, and the value obtained by subtracting the weight of the empty tea bag was defined as the 1-minute tap water absorption capacity.

[0148] In this study, tap water was used that had an electrical conductivity of 170 to 180 μS / cm when measured using an Orion Star A222 (manufactured by Thermo Scientific).

[0149] [Table 3]

[0150] [Table 4]

[0151] Referring to Table 4 above, it can be confirmed that the superabsorbent polymers of Examples 1 to 4 exhibit a fast absorption rate, excellent centrifugal separation water retention capacity and pressurized absorption capacity, improved re-wetting characteristics due to low re-wetting amount, and excellent 1-minute tap water absorption capacity.

[0152] Superabsorbent polymers, which exhibit excellent centrifugal separation water retention capacity, absorption rate, EFFC (Efficient Fluid Capacity), and 1-minute tap water absorption capacity, can rapidly absorb liquids, and the absorbed liquid does not seep out or leak even under pressure. Therefore, they can be used in diapers and other applications, providing excellent absorbency and comfort.

Claims

1. i) The water retention capacity by centrifugal separation is 32 g / g to 40 g / g, ii) The effective absorption capacity (EFFC, 0.3 psi) calculated by the following formula 1 is 30 g / g to 40 g / g, iii) The absorption rate by the vortex method is less than 20 seconds to 37 seconds, iv) When 1 g of superabsorbent polymer is immersed in 2 L of tap water and swelled for 1 minute, the tap water absorption capacity per minute, defined as the weight of water absorbed by the superabsorbent polymer in 1 minute, exceeds 115 g. v) A superabsorbent polymer having a re-wetting property (re-wetting in pressurized salt water for 2 hours) of 3.4 g or less, defined as the weight of water that seeps out again from the superabsorbent polymer to the filter paper after immersing 4 g of the superabsorbent polymer in 100 g of a 0.9 wt% sodium chloride aqueous solution for 120 minutes and swelling, then placing filter paper on the superabsorbent polymer and pressurizing at 0.75 psi for 5 minutes. [Formula 1] Effective absorption capacity (EFFC, 0.3 psi) = {Centrifugal water retention capacity (CRC) + 0.3 psi pressurized absorption capacity (AUP)} / 2

2. A step of preparing a monomer composition comprising an acrylic acid monomer having at least a portion of neutralized acidic groups, an internal crosslinking agent, and a polymerization initiator, The steps include crosslinking the monomer composition to form a water-containing gel polymer, The steps include drying, pulverizing, and classifying the aforementioned water-containing gel polymer to form a base resin, The process includes a surface crosslinking step of 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, water, and an organic solvent having a boiling point of less than 90°C, The water / organic solvent weight ratio of the surface crosslinking solution is 1.2 or more and less than 3.

0. The method for producing a superabsorbent resin according to claim 1, wherein the content of the organic solvent in the surface crosslinking solution is 24% by weight or more.

3. The method for producing a superabsorbent resin according to claim 2, wherein the content of the organic solvent in the surface crosslinking solution is 25% by weight to 30% by weight.

4. The method for producing a superabsorbent resin according to claim 2 or 3, wherein the surface crosslinking solution has a surface crosslinking agent represented by the following formula 3 with a density of 1.0 or more and less than 17.

0. [Formula 3] Density of surface crosslinking agent = (weight of surface crosslinking agent) / (weight of water) × 1,000

5. The method for producing a superabsorbent resin according to claim 2, wherein the organic solvent is one or more selected from the group consisting of methanol, ethanol, isopropanol, acetone, and ethyl acetate.

6. The method for producing a superabsorbent resin according to claim 2, wherein, after the surface crosslinking step, the amount of debris generated when the base resin solidifies with a particle size exceeding 850 μm is less than 4.0% by weight.

7. The method for producing a superabsorbent resin according to claim 2, further comprising the step of adding and mixing one or more inorganic substances selected from the group consisting of silica, clay, alumina, silica-alumina composite material, and titania to the superabsorbent resin after the surface crosslinking step.

8. The method for producing a superabsorbent resin according to claim 7, wherein the inorganic substance is added in an amount of 0.01 to 5 parts by weight per 100 parts by weight of the superabsorbent resin.

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