Method for producing superabsorbent polymer

Abstract

The present disclosure relates to a method for preparing a superabsorbent polymer. More specifically, the present invention relates to a method for preparing a superabsorbent polymer and a superabsorbent polymer, which exhibit excellent physical properties by performing pulverization and hydration processes under specific conditions to significantly reduce the amount of fine powder generation, and to a superabsorbent polymer.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2022-0175627, filed with the Korean Intellectual Property Office on December 15, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0003] The present disclosure relates to a method for preparing a superabsorbent polymer. More specifically, it relates to a method for preparing a superabsorbent polymer and a superabsorbent polymer that exhibit excellent physical properties by significantly reducing the amount of fine powder generated through a pulverization and hydration process under specific conditions. Background Art

[0004] A superabsorbent polymer (SAP) is a synthetic polymer material capable of absorbing 500 to 1000 times its own weight of water. Each manufacturer has named it different names, such as SAM (superabsorbent material) and AGM (absorbent gel material), etc. Such superabsorbent polymers were initially practically applied to sanitary products and now they are widely used in water-retaining soil products for horticulture, water-stopping materials for civil engineering and construction, sheets for raising seedlings, preservatives for the food circulation field, or materials for ointments, etc.

[0005] These superabsorbent polymers have been widely used in the field of sanitary materials such as diapers or sanitary napkins. Superabsorbent polymers are usually contained in such sanitary materials in a state dispersed in pulp. However, in recent years, there have been continuous efforts to provide thinner sanitary materials such as diapers. As part of such efforts, the development of so-called pulp-free diapers, etc., in which the pulp content is reduced or pulp is not used at all, is being actively carried out.

[0006] In the case of the above sanitary materials in which the pulp content is reduced or pulp is not used, the superabsorbent polymer is contained in a relatively high proportion, such that the superabsorbent polymer particles are inevitably contained in the sanitary material in a multi-layer form. In order for the entire superabsorbent polymer particles contained in a multi-layer form to more effectively absorb a large amount of liquid such as urine, the superabsorbent polymer basically needs to exhibit excellent absorption performance and a high absorption rate.

[0007] Meanwhile, such superabsorbent polymers are generally prepared by performing the following steps: a step of polymerizing monomers to prepare a hydrogel polymer containing a large amount of water; and a step of drying the hydrogel polymer and then pulverizing it into polymer particles having a desired particle size. However, when the hydrogel polymer is dried and then pulverized as described above, a large amount of fine powder is generated, which causes a problem of reducing the physical properties of the finally prepared superabsorbent polymer.

[0008] In addition, in order to reuse the fine powder, the fine powder is usually mixed with water for agglomeration to produce a fine re - assembly, and then the fine re - assembly is added to processes such as drying / pulverizing / classifying. However, the water used here may cause problems such as an increase in energy consumption during drying and an increase in the load on equipment, which may reduce the productivity of superabsorbent polymer production.

[0009] Therefore, in order to fundamentally solve these problems, there is a continuous need to develop a technology for preparing superabsorbent polymers without generating fine powder. SUMMARY OF THE INVENTION

[0010] [Technical Problem]

[0011] Therefore, there is provided a method for preparing a superabsorbent polymer and a superabsorbent polymer that exhibit excellent physical properties by significantly reducing the amount of fine powder generated through pulverization and hydration processes under specific conditions.

[0012] [Technical Solution]

[0013] To solve the above problems, there is provided a method for preparing a superabsorbent polymer, comprising the following steps:

[0014] Forming a hydrogel polymer containing a cross - linked polymer, the cross - linked polymer being obtained by cross - linking and polymerizing a water - soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal cross - linking agent (step 1);

[0015] Drying the hydrogel polymer to prepare a base resin (step 2);

[0016] Pulverizing the dried base resin once and classifying it into particles equal to or larger than a standard diameter and particles smaller than the standard diameter (step 3);

[0017] Obtaining a hydrated superabsorbent polymer by using an aqueous surface hydration solution for the classified base resin equal to or larger than the standard diameter (step 4); and

[0018] Pulverizing the hydrated superabsorbent polymer twice to obtain a superabsorbent polymer (step 5),

[0019] wherein the standard diameter is determined to be in the range of 300 μm or more and 850 μm or less.

[0020] [Advantageous Effects]

[0021] According to the method for preparing a superabsorbent polymer of the present disclosure, a superabsorbent polymer can be prepared that significantly reduces the amount of fine powder generated through pulverization and hydration processes under specific conditions, thereby exhibiting excellent physical properties. DETAILED DESCRIPTION

[0022] The terms used herein are for describing particular embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. It will be further understood that when used in this specification, the terms "comprising", "having" or "possessing" specify the presence of the stated features, steps, components or combinations thereof, but do not preclude the presence or addition of one or more other features, steps, components or combinations thereof.

[0023] Since the present invention can be modified in various ways and has various forms, specific embodiments thereof are shown by way of example and will be described in detail. However, it is not intended to limit the present invention to the specific forms disclosed, and it should be understood that the present invention includes all modifications, equivalents and substitutions within the concept and technical scope of the present invention.

[0024] Since the present invention can be modified in various ways and has various forms, specific embodiments thereof are shown by way of example and will be described in detail. However, it is not intended to limit the present invention to the specific forms disclosed, and it should be understood that the present invention includes all modifications, equivalents and substitutions within the concept and technical scope of the present invention.

[0025] Hereinafter, the method for preparing a superabsorbent polymer and the superabsorbent polymer of the present disclosure will be described in more detail.

[0026] The terms used herein are only intended to refer to particular embodiments and are not intended to limit the present invention. In addition, as used herein, the singular forms also include the plural forms unless the phrase clearly indicates the contrary.

[0027] The term "polymer" in the present disclosure is in a state of polymerization of water-soluble ethylenically unsaturated monomers and can include all moisture content ranges or all particle size ranges.

[0028] In addition, the term "superabsorbent polymer" is used to cover all crosslinked polymers or powders composed of superabsorbent polymer particles in which the crosslinked polymers are pulverized, and depending on the circumstances, the crosslinked polymers or the base resins are further processed, such as drying, pulverizing, classifying, surface crosslinking, etc., to make them in a state suitable for commercialization.

[0029] In addition, the term "fine powder" refers to particles having a particle size less than 150 μm in superabsorbent polymer particles. The particle size of these polymer particles can be measured according to EDANA (European Disposables and Nonwovens Association) WSP 220.3.

[0030] In addition, the term "chopping" refers to cutting the hydrogel polymer into small pieces of several millimeters to improve the drying efficiency, and is distinguished from pulverization that reaches the micron or normal particle level.

[0031] In addition, the term "micronization (micronized)" refers to pulverizing a hydrogel polymer to a particle size of several tens to several hundreds of micrometers, and is used separately from "chopping".

[0032] According to an embodiment of the present disclosure, a method for preparing a superabsorbent polymer is provided, which includes the following steps:

[0033] Forming a hydrogel polymer containing a crosslinked polymer, the crosslinked polymer being obtained by crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal crosslinking agent (step 1);

[0034] Drying the hydrogel polymer to prepare a base resin (step 2);

[0035] Pulverizing the dried base resin once and classifying it into particles equal to or larger than a standard diameter and particles smaller than the standard diameter (step 3);

[0036] Obtaining a hydrated superabsorbent polymer by using an aqueous surface hydration solution for the classified base resin equal to or larger than the standard diameter (step 4); and

[0037] Pulverizing the hydrated superabsorbent polymer a second time to obtain a superabsorbent polymer (step 5),

[0038] wherein the standard diameter is determined in the range of 300 μm or more and 850 μm or less.

[0039] Generally, such a superabsorbent polymer is prepared by performing the following steps: a step of polymerizing a monomer to prepare a hydrogel polymer containing a large amount of water; and a step of drying the hydrogel polymer and then pulverizing the hydrogel polymer into polymer particles having a desired particle size. However, when the hydrogel polymer is dried and then pulverized as described above, a large amount of fine powder is generated, which causes a problem of deterioration in the physical properties of the prepared superabsorbent polymer.

[0040] Therefore, in order to reuse the fine powder, the fine powder is added to the hydrogel pulverization step, or the fine powder is mixed with water for agglomeration to produce a fine powder recombinant, and then added to processes such as drying / pulverizing / classifying. However, the water used here can increase the energy consumption during the drying process and increase the load on the equipment, thereby reducing the productivity of manufacturing the superabsorbent polymer.

[0041] Therefore, the present inventors have confirmed that by performing the pulverization and hydration processes under specific conditions, the amount of fine powder generated can be significantly reduced, thus completing the present invention.

[0042] Specifically, it was confirmed that by subjecting the dried hydrogel polymer to a first pulverization to have a relatively large diameter to reduce the generation of fine powder, and subjecting the large particles in the pulverized particles that are equal to or larger than the standard diameter to surface hydration and then performing a second pulverization, the generation of fine powder can be significantly reduced.

[0043] Hereinafter, the method for preparing the superabsorbent polymer will be described in more detail step by step according to the specific embodiments of the present invention.

[0044] (Polymerization)

[0045] First, the method for preparing the superabsorbent polymer according to the embodiments of the present disclosure includes: a step (step 1) of forming a hydrogel polymer by crosslinking polymerization of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent.

[0046] Specifically, this is a step of subjecting a monomer composition including a monomer mixture and a polymerization initiator to thermal polymerization or photopolymerization in the presence of an internal crosslinking agent to form a hydrogel polymer. In addition to the above components, the monomer composition may further include components commonly used for preparing superabsorbent polymers.

[0047] The water-soluble ethylenically unsaturated monomer may be any monomer commonly used for preparing superabsorbent polymers. Specifically, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following Chemical Formula 1:

[0048] [Chemical Formula 1]

[0049] R1-COOM 1

[0050] In Chemical Formula 1,

[0051] R1 is a C2 to C5 alkyl group having an unsaturated bond, and

[0052] M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0053] Preferably, the monomer may be at least one selected from the group consisting of acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts of these acids. When acrylic acid and / or its salts are used as the water-soluble ethylenically unsaturated monomer, it is advantageous for obtaining a superabsorbent polymer having improved absorption properties. Additionally, at least one selected from the group consisting of the following may be used: anionic monomers such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, or 2-(meth)acrylamide-2-methylpropane sulfonic acid and their salts; nonionic hydrophilic monomers such as (meth)acrylamide, N-substituted (meth)acrylates, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, or polyethylene glycol (meth)acrylate; and amino-containing unsaturated monomers such as (N,N)-dimethylaminoethyl (meth)acrylate or (N,N)-dimethylaminopropyl (meth)acrylamide, and their quaternary compounds.

[0054] The term "internal crosslinking agent" used herein is a term used to distinguish it from the "surface crosslinking agent" which is usually used to crosslink the surface of the base resin, and is used to crosslink and polymerize the unsaturated bonds of the above-mentioned water-soluble ethylenically unsaturated monomers. The crosslinking in the above steps is carried out both on the surface and inside, but when the surface crosslinking process of the base resin is carried out, the surface of the particles of the finally prepared superabsorbent polymer has a structure crosslinked by the surface crosslinking agent, and the inside of the particles has a structure crosslinked by the internal crosslinking agent.

[0055] As a non-limiting example, the internal crosslinking agent may be a polyfunctional crosslinking agent such as N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, 1,4-butanediol di(meth)acrylate, butyleneglycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol, or ethylene carbonate, and these may be used alone or in combination of two or more. Preferably, ethylene glycol diglycidyl ether may be used.

[0056] Based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer, the amount of the internal crosslinking agent used can be from 0.01 to 5 parts by weight. For example, the amount of the internal crosslinking agent used can be 0.01 parts by weight or more, 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.45 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. When the content of the internal crosslinking agent is too low, crosslinking cannot occur sufficiently, making it difficult to achieve a strength higher than an appropriate level. When the content of the internal crosslinking agent is too high, the internal crosslinking density increases, making it difficult to achieve the required water retention ability.

[0057] In the crosslinking polymerization step, a polymerization initiator commonly used for preparing a superabsorbent polymer can be included. For example, depending on the polymerization method, the polymerization initiator can be a thermal polymerization initiator or a photo-polymerization initiator. In particular, a thermal polymerization initiator can be used. However, even when applying a photo-polymerization method thereto, a certain amount of heat is generated due to UV irradiation or the like, and as the polymerization reaction (exothermic reaction) proceeds, some heat is generated. Therefore, a thermal polymerization initiator can be additionally included.

[0058] In addition, as the thermal polymerization initiator, one or more initiators selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specifically, sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc. can be used as examples of persulfate initiators; and 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethyl) isobutylamidine dihydrochloride, 2-(carbamoylazo)isobutylonitril, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), etc. can be used as examples of azo initiators. More various thermal polymerization initiators are well disclosed on page 203 of "Principle of Polymerization (Wiley, 1981)" written by Odian.

[0059] For example, the photoinitiator may be one or more compounds selected from the group consisting of benzoin ether, dialkylacetophenone, hydroxyalkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acylphosphine, and α-aminoketone. Meanwhile, specific examples of the acylphosphine include diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate, etc. More various photoinitiators are well disclosed on page 115 of "UV Coatings: Basics, Recent Developments and New Application (Elsevier, 2007)" written by Reinhold Schwalm, and the present disclosure is not limited thereto.

[0060] Based on the monomer composition containing a water-soluble ethylenically unsaturated monomer, the polymerization initiator can be used at a concentration of about 0.001 to 1% by weight. That is, when the concentration of the polymerization initiator is too low, the polymerization rate may become slow, and thus a large amount of residual monomer may be extracted from the final product, which is not preferred. On the contrary, when the concentration of the polymerization initiator is too high, the polymer chains forming the network may become short, and thus the physical properties of the polymer may be reduced, such as an increase in the content of the water-soluble component and a decrease in the pressure absorbency, which is not preferred.

[0061] In addition, if necessary, the monomer composition may further include additives such as a foaming agent, a surfactant, a thickener, a plasticizer, a storage stabilizer, and an antioxidant.

[0062] In addition, such a monomer composition can be prepared in the form of a solution in which raw materials such as the above-mentioned monomer, polymerization initiator, and internal crosslinking agent are dissolved in a solvent.

[0063] In the monomer composition containing an internal crosslinking agent and a water-soluble ethylenically unsaturated monomer having an acidic group, the concentration of the water-soluble ethylenically unsaturated monomer can be appropriately adjusted in consideration of the polymerization time and reaction conditions, and can be about 20% by weight to about 60% by weight, or about 20% by weight to about 40% by weight.

[0064] At this time, any solvent capable of dissolving the raw materials can be used without limitation, 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, N,N-dimethylacetamide, or a mixture thereof can be used.

[0065] The formation of a hydrogel polymer by polymerizing a monomer composition can be carried out by conventional polymerization methods, and the process is not particularly limited. As non-limiting examples, depending on the energy source for polymerization, the polymerization methods are roughly classified into thermal polymerization and photopolymerization. In the case of thermal polymerization, it is usually carried out in a reactor equipped with a stirring shaft (such as a kneader). In the case of photopolymerization, it can be carried out in a reactor equipped with a movable conveyor belt.

[0066] For example, a hydrogel polymer can be obtained by introducing a monomer composition into a reactor equipped with a stirring shaft (such as a kneader) and then supplying hot air or heating the reactor to carry out thermal polymerization. Depending on the shape of the stirring shaft provided in the reactor, the hydrogel polymer can be discharged to the reactor outlet in the form of particles having a size of several centimeters to several millimeters. Specifically, the shape of the obtained hydrogel polymer can vary according to the concentration and injection rate of the monomer mixture to be injected, and generally, a hydrogel polymer having a (weight-average) particle size of 2 to 50 mm can be obtained.

[0067] As another example, when the monomer composition is photopolymerized in a reactor equipped with a movable conveyor belt, a hydrogel polymer in the form of a sheet can be obtained. At this time, the thickness of the sheet can vary according to the concentration and injection rate of the monomer composition to be injected. In order to ensure the production rate while uniformly polymerizing the entire sheet, it is preferable to adjust the thickness to 0.5 to 10 cm.

[0068] According to an embodiment of the present disclosure, the crosslinking polymerization can be carried out at a temperature of 10°C to 140°C, preferably at a temperature of 70°C to 130°C or 70°C to 120°C. When carried out within the above temperature range, it is suitable for achieving an appropriate crosslinking density achieved by the chelating agent of the present disclosure.

[0069] The hydrogel polymer formed in this way can exhibit a water content of 30 to 80% by weight. At this time, the "water content" is the weight of water in the total weight of the hydrogel polymer, and it refers to the value obtained by subtracting the weight of the dried polymer from the weight of the hydrogel polymer. Specifically, the water content is defined as the value calculated by measuring the weight loss due to the evaporation of water from the polymer during the process of raising the temperature of the polymer used for drying by infrared heating. At this time, the drying conditions are maintained at about 180°C for about 40 minutes to measure the water content.

[0070] According to an embodiment of the present disclosure, before the polymerization step, a step of neutralizing at least some of the acidic groups of the water-soluble ethylenically unsaturated monomer having acidic groups may be included.

[0071] Specifically, the step of forming the hydrogel polymer (step 1) can be carried out by including the following steps: a step of neutralizing at least some of the acidic groups of the water-soluble ethylenically unsaturated monomer having acidic groups (step 1-1'); and a step of forming a hydrogel polymer by crosslinking polymerization of the water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent (step 1-2').

[0072] The neutralization step of the acidic groups is carried out by mixing with a neutralizing agent capable of neutralizing the acidic groups. Examples of the neutralizing agent include basic materials such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide, but are not limited thereto.

[0073] Here, the degree of neutralization of the acidic groups can be 40 to 95 mol%, 40 to 90 mol%, or 45 to 80 mol%. The range of the degree of neutralization can vary depending on the final physical properties. When the degree of neutralization is too high, the surface crosslinking reaction may not occur sufficiently, which may lead to a decrease in the absorption under pressure (AUP). On the contrary, when the degree of neutralization is too low, not only the absorbency of the polymer deteriorates, but also the polymer is given properties that are difficult to handle, such as the properties of an elastic rubber.

[0074] According to an embodiment of the present disclosure, a polymer is formed using a water-soluble ethylenically unsaturated monomer having unneutralized acidic groups, and then the polymer is neutralized. In this case, the polymerized polymer has acidic groups, and thus, a neutralization step may be included after the polymerization step. The neutralizing agent used in the neutralization step and the degree of neutralization of the acidic groups after neutralization may be the same as those described above.

[0075] Specifically, the step of forming the hydrogel polymer (step 1) can include a step of forming a polymer by crosslinking polymerization of a water-soluble ethylenically unsaturated monomer having acidic groups in the presence of an internal crosslinking agent (step 1-1); a step of micronizing the neutralization product (step 1-2); and a step of neutralizing at least some of the acidic groups of the polymer to form a hydrogel polymer (step 1-3). At the same time, in the neutralization step (1-3), a polymer agglomeration process is also carried out.

[0076] The water-soluble ethylenically unsaturated monomer in a state where the acidic groups are not neutralized is liquid at room temperature and has high miscibility with a solvent (water), and thus exists in the monomer composition in the form of a mixed solution. However, the water-soluble ethylenically unsaturated monomer in which the acidic groups are neutralized is solid at room temperature, has different solubilities depending on the temperature of the solvent (water), and has a lower solubility at a lower temperature.

[0077] Compared with monomers in which acidic groups are neutralized, water-soluble ethylenically unsaturated monomers with non-neutralized acidic groups have higher solubility or miscibility with a solvent (water). Therefore, they do not precipitate even at low temperatures, which is beneficial for long-term polymerization at low temperatures. Thus, by using water-soluble ethylenically unsaturated monomers with non-neutralized acidic groups for long-term polymerization, a polymer with a high molecular weight and a uniform molecular weight distribution can be stably formed.

[0078] In addition, long-chain polymers can be formed, thereby achieving the effect of reducing the content of extractable components present in a non-crosslinked state due to incomplete polymerization or crosslinking.

[0079] Meanwhile, when polymerization is first carried out in a state where the acidic groups of the monomers are not neutralized to form a polymer, and then micronization is carried out in the presence of a surfactant after neutralization; neutralization is carried out after micronization; or the acidic groups are neutralized during micronization, a large amount of surfactant can be present on the surface of the polymer, sufficiently playing the role of reducing the adhesiveness of the polymer.

[0080] According to an embodiment of the present disclosure, when the step of forming a hydrogel polymer (step 1) is carried out in such a manner that a neutralization step (step 1-3) is carried out after polymerization (step 1-1), the neutralization step and the micronization step (step 1-2) of the polymer can be carried out sequentially, simultaneously, or alternately. More preferably, the polymerization step (step 1-1), the micronization step (step 1-2) of the polymer, and the neutralization step (1-3) can be carried out sequentially. Here, agglomeration of the polymer can be carried out simultaneously during the neutralization step.

[0081] In other words, a mixture of the polymer and a surfactant (optionally used) is micronized and then neutralized by adding a neutralizing agent; the neutralizing agent is sprayed on the polymer to first neutralize the acidic groups of the polymer, and then the surfactant (optionally used) is added to the neutralized polymer to micronize the mixture mixed with the surfactant; or the polymer is neutralized and micronized by adding the neutralizing agent and the surfactant (optionally used) to the polymer simultaneously.

[0082] As described above, when the polymer is micronized in the presence of a surfactant, the hydrophobic functional groups contained in the surfactant are imparted to the surface of the superabsorbent polymer particles that are pulverized, thereby reducing the frictional force between the particles, and micronizing the superabsorbent polymer to reduce the deterioration of physical properties caused by interparticle friction.

[0083] Meanwhile, in order to uniformly neutralize the entire polymer, it may be desirable to leave a certain time interval between the addition of the neutralizing agent and the micronization process.

[0084] The surfactant used in the micronization process may be a compound represented by the following Chemical Formula 2 or a salt thereof, but the present disclosure is not limited thereto:

[0085] [Chemical Formula 2]

[0086]

[0087] In Chemical Formula 2,

[0088] A1, A2, and A3 are each independently a single bond, a carbonyl group, provided that at least one of these is a carbonyl group or wherein m1, m2, and m3 are each independently an integer from 1 to 8, each is connected to an adjacent oxygen atom, and -* is respectively connected to adjacent R1, R2, and R3,

[0089] R1, R2, and R3 are each independently hydrogen, a C6 to C18 straight-chain or branched alkyl group, or a C6 to C18 straight-chain or branched alkenyl group, and

[0090] n is an integer from 1 to 9.

[0091] The surfactant is mixed with the polymer and added so that the ultrafine pulverization step can be easily carried out without agglomeration.

[0092] The surfactant represented by Chemical Formula 2 is a nonionic surfactant and has excellent surface adsorption properties even when hydrogen-bonded to an unneutralized polymer, so it is suitable for achieving the desired agglomeration control effect. On the other hand, for anionic surfactants other than nonionic surfactants, when mixed with a polymer neutralized with a neutralizing agent such as NaOH or Na2SO4, it is adsorbed by the Na+ ions ionized at the carboxyl substituents of the polymer. When mixed with an unneutralized polymer, there is a problem that the adsorption efficiency of the polymer is relatively reduced due to competition with the anions of the carboxyl substituents of the polymer.

[0093] Specifically, in the surfactant represented by Chemical Formula 2, the hydrophobic functional groups are the terminal functional groups of R1, R2, and R3 (if not hydrogen), and the hydrophilic functional groups include the glycerol-derived part in the chain and the terminal hydroxyl group (when A n is a single bond and R n is hydrogen, n = 1 to 3). Here, the glycerol-derived part and the terminal hydroxyl group are hydrophilic functional groups and play a role in improving the adsorption properties to the polymer surface. Therefore, agglomeration of the superabsorbent polymer particles can be effectively suppressed.

[0094] In Chemical Formula 2, the hydrophobic functional groups of R1, R2, and R3 (if not hydrogen) are each independently a C6 to C18 straight-chain or branched-chain alkyl group or a C6 to C18 straight-chain or branched-chain alkenyl group. At this time, when R1, R2, and R3 (if not hydrogen) are alkyl or alkenyl groups having less than 6 carbon atoms, there is a problem that the chain length is too short to effectively control the agglomeration of the pulverized particles. When R1, R2, and R3 (if not hydrogen) are alkyl or alkenyl groups having more than 18 carbon atoms, there may be a problem that the mobility of the surfactant is reduced and it may not be effectively mixed with the polymer, and due to the increase in the cost of the surfactant, the unit price of the composition increases.

[0095] Preferably, R1, R2, and R3 are hydrogen, a C6 to C18 straight-chain or branched-chain alkyl group such as 2-methylhexyl, n-heptyl, 2-methylheptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, or n-octadecyl, or a C6 to C18 straight-chain or branched-chain alkenyl group such as 2-hexenyl, 2-heptenyl, 2-octenyl, 2-nonenyl, n-decenyl, 2-undecenyl, 2-dodecenyl, 2-tridecenyl, 2-tetradecenyl, 2-pentadecenyl, 2-hexadecenyl, 2-heptadecenyl, or 2-octadecenyl.

[0096] The surfactant may be selected from the compounds represented by the following Chemical Formulas 2-1 to 2-14:

[0097] [Chemical Formula 2-1]

[0098]

[0099] [Chemical Formula 2-2]

[0100]

[0101] [Chemical Formula 2-3]

[0102]

[0103] [Chemical Formula 2-4]

[0104]

[0105] [Chemical Formula 2-5]

[0106]

[0107] [Chemical Formula 2-6]

[0108]

[0109] [Chemical Formula 2-7]

[0110]

[0111] [Chemical formula 2-8]

[0112]

[0113] [Chemical formula 2-9]

[0114]

[0115] [Chemical formula 2-10]

[0116]

[0117] [Chemical formula 2-11]

[0118]

[0119] [Chemical formula 2-12]

[0120]

[0121] [Chemical formula 2-13]

[0122]

[0123] [Chemical formula 2-14]

[0124]

[0125] Meanwhile, depending on the productivity or the equipment load, based on 100 parts by weight of the polymer, the amount of the surfactant used can be 0.1 part by weight or less. When too much surfactant is used, the surface tension may decrease, and thus the overall physical properties of the final superabsorbent polymer may deteriorate. For example, based on 100 parts by weight of the polymer, its amount used can be 0.1 part by weight or less, preferably 0.05 part by weight or less, 0.005 part by weight or less, or 0.001 part by weight or less.

[0126] The method of mixing the surfactant with the polymer is not particularly limited and can be appropriately selected as long as it is a method capable of uniformly mixing the additive with the hydrogel polymer. Specifically, the surfactant can be dry-blended, dissolved in a solvent and then mixed in a solution state, or melted and then mixed.

[0127] For example, the surfactant can be mixed in the form of a solution dissolved in a solvent. At this time, all types of solvents including inorganic solvents and organic solvents can be used without limitation, but water is the most suitable considering the ease of the drying process and the cost of the solvent recovery system. In addition, methods such as adding the surfactant in solution form and the polymer to a reaction tank for mixing, spraying the solution after adding the polymer in a mixer, or continuously supplying the polymer and the solution to a continuously operating mixer for mixing can be used.

[0128] (Drying)

[0129] Subsequently, the method for preparing the superabsorbent polymer according to the embodiment of the present disclosure includes: a step of drying the hydrogel polymer formed after the polymerization step to obtain a base resin (step 2).

[0130] According to the embodiment of the present disclosure, when the above step of forming the hydrogel polymer (step 1) includes a step of neutralizing at least some acidic groups of the water-soluble ethylenically unsaturated monomer having acidic groups (step 1-1'); and a step of crosslinking and polymerizing the water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent to form a hydrogel polymer (step 1-2') (referred to as the pre-neutralized polymerization process), step 2 can be carried out as a process of drying the hydrogel polymer that has undergone the above steps.

[0131] If necessary, in order to improve the efficiency of the drying step, the hydrogel polymer can be further pulverized before drying.

[0132] At this time, the composition of the pulverizer used is not limited. Specifically, at least one pulverizer selected from the group consisting of a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutting mill, a cutting mill, a disk mill, a shredder, a crusher, a chopper, and a disk cutter can be used, but the present disclosure is not limited thereto.

[0133] Here, the hydrogel polymer can be pulverized so that the particle size is about 2 to about 10 mm. Due to the high water content of the hydrogel polymer, it is not technically easy to pulverize it to a particle size less than 2 mm, and agglomeration may occur between the pulverized particles. On the other hand, when pulverized to a particle size greater than 10 mm, the improvement in the efficiency of the subsequent drying step may not be significant.

[0134] Meanwhile, when the above step of forming the hydrogel polymer (step 1) includes a step of crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal crosslinking agent to form a polymer (step 1-1); and a step of forming a hydrogel polymer by neutralizing at least some of the acidic groups of the polymer (step 1-2) (a polymerization process called post-neutralization), agglomeration of particles can be inhibited without an additional pulverization process before drying, thereby achieving excellent drying efficiency.

[0135] The hydrogel polymer that has been pulverized as described above or immediately after polymerization without going through the pulverization step is dried. The drying temperature in the drying step can be about 150 to about 250 °C. When the drying temperature is lower than 150 °C, the drying time may become too long, and the physical properties of the finally formed superabsorbent polymer may be reduced. When the drying temperature exceeds 250 °C, only the surface of the polymer is over-dried, and fine powder may be generated during the subsequent pulverization process, and the physical properties of the finally formed superabsorbent polymer may be reduced. Therefore, drying is preferably carried out at a temperature of about 150 to about 200 °C, more preferably at a temperature of about 170 to about 190 °C. Drying can be carried out in multiple stages by changing the temperature within the above temperature range.

[0136] Meanwhile, considering process efficiency, the drying time can be 20 minutes to 90 minutes, but is not limited thereto.

[0137] The drying method in the drying step is not particularly limited as long as it is commonly used in the drying process of hydrogel polymers. Specifically, the drying step can be carried out by methods such as hot air supply, infrared irradiation, microwave irradiation, and ultraviolet irradiation. After the drying step, the moisture content of the polymer can be about 0.1 to about 10% by weight.

[0138] (Primary pulverization)

[0139] Subsequently, the method for preparing a superabsorbent polymer according to an embodiment of the present disclosure includes: a step of primary pulverization of the dried base resin and classifying it into particles equal to or larger than a standard diameter and particles smaller than the standard diameter (step 3). The step of pulverizing the dried polymer is referred to as 'primary pulverization' in this specification to distinguish it from the pulverization in step 4 described later (referred to as'secondary pulverization').

[0140] In primary pulverization, the hydrogel polymer is pulverized into particles with a relatively large diameter, thereby reducing the generation of initial fine powder and making the operation of secondary pulverization to reach the final diameter required by the present disclosure smooth. Therefore, the generation of fine powder in the finally manufactured product can be reduced.

[0141] "Particles equal to or larger than the standard diameter" refers to particles having a relatively large particle size compared to particles after a typical pulverization process upon drying, and the standard diameter is determined within the range of more than 300 μm to less than 850 μm. When the standard diameter is determined within the above range, the initial fine powder generation amount can be significantly controlled. Preferably, the standard diameter can be determined within the range of more than 600 μm to less than 850 μm, and more preferably, the standard diameter can be 710 μm.

[0142] Generally, "fine powder" refers to particles having a particle size less than 150 μm. As described above, the primary pulverization process of the present disclosure pulverizes to a relatively large diameter, so the initial fine powder generation amount in the classified base resin is significantly reduced.

[0143] In step 3, primary pulverization can be performed such that particles equal to or larger than the standard diameter account for 40% by weight or more of the total pulverized particles. When particles equal to or larger than the standard diameter are included within the above range, by using the above particles to manufacture the final product through the hydration process described later, the fine powder generation amount in the final product can be effectively controlled. Preferably, primary pulverization can be performed such that particles equal to or larger than the standard diameter account for 45% by weight or more, 48% by weight or more, 50% by weight or more, 55% by weight or more, or 60% by weight or more of the total pulverized particles.

[0144] In addition, in step 3, primary pulverization can be performed such that particles equal to or larger than 10 mm account for less than 5% by weight of the total pulverized particles. Particles equal to or larger than 10 mm are particles that are not normally pulverized and may impose a load on the machine during the secondary pulverization process. Therefore, when pulverization is performed in the primary pulverization to include a trace amount of particles equal to or larger than 10 mm, the operation of the machine can be made smooth during the secondary pulverization, excessive pulverization can be reduced, and thus the fine powder generation amount can be reduced. Preferably, primary pulverization can be performed such that particles equal to or larger than 10 mm account for less than 4% by weight, less than 3% by weight, less than 2% by weight, less than 1% by weight, and most preferably about 0% by weight of the total pulverized particles.

[0145] The method of performing the primary pulverization process is not particularly limited and can be appropriately adjusted to meet the content range of the above particles.

[0146] Specifically, the primary pulverization process can be performed using a pulverizer having a perforated plate with a hole size of 5 mm to 20 mm, more preferably 8 mm to 12 mm. Here, the perforated plate is a perforated plate formed with a plurality of holes, and the diameter of the holes satisfies the above range. When the primary pulverization is performed under the above conditions, the surface hydration and secondary pulverization process of particles having a specific diameter described later can be easily performed, thereby significantly reducing the generation of fine powder during the manufacturing process of the final superabsorbent polymer.

[0147] Preferably, the grinder for primary comminution may be a rotary cutting mill including an orifice plate and a rotary screw. When using a rotary cutting mill, the comminution amount can be increased, and it is easier to comminute particles with a high moisture content.

[0148] (Hydration)

[0149] Subsequently, the method for preparing a superabsorbent polymer according to an embodiment of the present disclosure includes: a step (step 4) of obtaining a hydrated superabsorbent polymer by using an aqueous surface hydration solution for a base resin having a diameter equal to or greater than a standard diameter obtained after primary comminution and classification. By the hydration process, the amount of fines generated in the secondary comminution process described later can be significantly reduced.

[0150] Here, water can be used alone as the surface hydration solution.

[0151] Based on 100 parts by weight of the base resin, the usage amount of the surface hydration solution can be 0.1 to 10 parts by weight, preferably 0.5 to 7 parts by weight, or 1 to 5 parts by weight. When used within the above range, uniform surface hydration can be performed.

[0152] In addition, in addition to water, the surface hydration solution may further contain at least one additive selected from the group consisting of polyacrylic acid - polyethylene oxide comb polymers (GK), aluminum sulfate, polyethylene glycol, sodium dodecyl sulfate, glycerol monolaurate, and sodium stearoyl lactate. Preferably, polyacrylic acid - polyethylene oxide comb polymers (GK) and aluminum sulfate can be used. The use of the above additives improves the dispersibility of the surface hydration solution, thereby allowing more uniform surface hydration on the surface of the base resin particles.

[0153] Based on 100 parts by weight of the surface hydration solution, the usage amount of the additive can be 0.01 to 10 parts by weight, preferably 0.01 to 5 parts by weight, 0.05 to 3 parts by weight, or 0.1 to 1 part by weight. When used within the above range, uniform surface hydration can be performed with appropriate dispersibility.

[0154] The method of adding the surface hydration solution to the base resin is not particularly limited. For example, a method of adding the base resin and the surface hydration solution in a reactor for mixing, a method of spraying the surface hydration solution on the base resin, or a method of continuously supplying the base resin and the surface hydration solution to a continuously operating mixer while mixing them can be used.

[0155] Preferably, the hydration step can be carried out by adding the base resin and the surface hydration solution in a reactor and then stirring at 100 rpm to 1,000 rpm, preferably at 300 rpm to 800 rpm. When carried out under the above conditions, it is suitable for performing uniform surface hydration.

[0156] (Secondary pulverization)

[0157] Subsequently, a method for preparing a superabsorbent polymer according to an embodiment of the present disclosure includes: a step of secondary pulverization of the hydrated superabsorbent polymer to obtain a superabsorbent polymer (step 5).

[0158] The above hydration step is performed on a base resin having a diameter equal to or greater than a standard diameter, and the resulting hydrated superabsorbent polymer has particles having a diameter equal to or greater than the standard diameter. Therefore, the second pulverization step is performed so that the final superabsorbent polymer can have a desired diameter.

[0159] In some cases, a reclassification process may be further performed to exclude particles having a large diameter in the hydrated superabsorbent polymer that has been secondarily pulverized. For example, particles equal to or greater than 20 mm, preferably equal to or greater than 10 mm, may be reclassified and excluded. As described above, particles having too large a diameter are not dried and excluded so as not to increase the load during the pulverization process.

[0160] The secondary pulverization is performed to a smaller diameter than the primary pulverization. For the superabsorbent polymer that has already undergone the hydration step, even if pulverized to a smaller diameter, due to the appropriate moisture content, the amount of fine powder generated is significantly reduced.

[0161] In step 5, secondary pulverization may be performed such that particles having a diameter of 150 μm to 850 μm account for 90% by weight or more, preferably 95% by weight or more, of the total pulverized particles.

[0162] The method for performing the secondary pulverization process is not particularly limited and can be appropriately adjusted to meet the above particle content range.

[0163] Preferably, a pulverizer including a plurality of rollers having corrugations may be used to perform the secondary pulverization. The rollers may have 2 to 12 corrugations per about 1 cm, and the gaps between the plurality of rollers may each independently be about 0.1 mm to about 1 mm. The plurality of rollers of the pulverizer may rotate at different speeds and may be configured in multiple stages. More preferably, the pulverizer may be a two-stage roller mill. In the case of using a roller mill in multiple stages, according to the stage at which the pulverization is performed, the gap between the corresponding rollers becomes narrower, and it is easy to control the desired diameter by pulverization.

[0164] (Other steps)

[0165] According to an embodiment of the present disclosure, in order to control the physical properties of the superabsorbent polymer powder to be prepared as a final product, the superabsorbent polymer prepared after the secondary pulverization step may be classified according to diameter. Preferably, the superabsorbent polymer may be classified such that the particle size is 150 μm to 850 μm. As described above, the superabsorbent polymer manufactured by pulverizing and hydrating under specific conditions may include fine powder having a particle size of less than 150 μm in an amount of less than about 3% by weight, preferably less than about 1% by weight.

[0166] The method for preparing a superabsorbent polymer according to an embodiment of the present disclosure may further include: a step (step 6) of mixing the superabsorbent polymer pulverized secondarily in step 5 (or the superabsorbent polymer after an additional classification process) with particles having a diameter smaller than the standard diameter classified after primary pulverization in step 3.

[0167] The particles having a diameter smaller than the standard diameter classified after primary pulverization in step 3 are also pulverized into a relatively large diameter so as to contain fine powder having a particle size of less than 150 μm in an amount of less than about 3% by weight, preferably less than about 1% by weight. The superabsorbent polymer pulverized secondarily in step 5 may also contain fine powder having a particle size of less than 150 μm in an amount of less than about 3% by weight, preferably less than about 1% by weight.

[0168] More preferably, after the mixing step of step 6, based on the total amount of the superabsorbent polymer, the amount of the fine powder having a particle size of less than 150 μm may be less than about 3% by weight, preferably less than about 1% by weight.

[0169] Meanwhile, after the mixing in step 6, an additional classification process may be optionally performed, and superabsorbent polymer particles having a particle size of 150 μm to 850 μm may be recovered using a classification sieve.

[0170] (Surface crosslinking step)

[0171] The method for preparing a superabsorbent polymer according to an embodiment of the present disclosure may further include: a step (step 7) of heat-treating the superabsorbent polymer in the presence of a surface crosslinking agent to crosslink the surface of the superabsorbent polymer. Here, the superabsorbent polymer to be surface-crosslinked may be the superabsorbent polymer pulverized secondarily in step 5, or the superabsorbent polymer after the mixing step of step 6. Additionally, the object of surface crosslinking may be a polymer that has undergone an optional additional classification step.

[0172] The surface crosslinking step induces a crosslinking reaction on the surface of the superabsorbent polymer in the presence of a surface crosslinking agent, and the unsaturated bonds of the water-soluble ethylenically unsaturated monomers remaining uncrosslinked on the surface are crosslinked by the surface crosslinking agent, thereby forming a superabsorbent polymer having a high surface crosslinking density.

[0173] Specifically, the surface crosslinked layer can be formed by a heat treatment process in the presence of a surface crosslinking agent. The heat treatment process increases the surface crosslinking density, i.e., the external crosslinking density, but does not change the internal crosslinking density. Therefore, the prepared superabsorbent polymer with a surface crosslinked layer has a structure in which the external crosslinking density is higher than the internal crosslinking density.

[0174] In the surface crosslinking step, a surface crosslinking composition containing an alcohol solvent and water in addition to the surface crosslinking agent can be used.

[0175] As the surface crosslinking agent contained in the surface crosslinking composition, any crosslinking agent conventionally used for preparing superabsorbent polymers can be used without any particular limitation. Examples of the surface crosslinking agent can include at least one polyol 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; at least one carbonate compound selected from the group consisting of ethylene carbonate and propylene carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazoline compounds such as oxazolidinone; polyamine compounds; oxazoline compounds; mono -, di - or poly - oxazolidinone compounds; and cyclic urea compounds, etc. Preferably, the surface crosslinking agent can be the same as the internal crosslinking agent. For example, a diglycidyl ether compound of an alkylene glycol such as ethylene glycol diglycidyl ether can be used.

[0176] Based on 100 parts by weight of the superabsorbent polymer, the amount of the surface crosslinking agent used can be from 0.001 to 2 parts by weight. Preferably, its amount used can be 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.02 parts by weight or more, and 0.5 parts by weight or less, or 0.3 parts by weight or less. When the content range of the surface crosslinking agent is adjusted within the above range, a superabsorbent polymer exhibiting excellent absorption performance, permeability, and other various physical properties can be prepared.

[0177] Meanwhile, the surface crosslinking agent is added to the superabsorbent polymer particles in the form of the surface crosslinking composition containing it, and the method of adding the surface crosslinking composition is not particularly limited. For example, a method of adding the surface crosslinking composition and the base resin powder to a reactor for mixing, a method of spraying the surface crosslinking composition onto the base resin powder, or a method of mixing the base resin powder and the surface crosslinking composition and continuously feeding them to a continuously operating mixer can be used.

[0178] In addition, the surface crosslinking composition may further contain water and / or hydrophilic organic solvents as a medium. Thereby, it has the advantage that the surface crosslinking agent can be uniformly dispersed on the base resin powder. At this time, for the purpose of inducing uniform dissolution / dispersion of the surface crosslinking agent, preventing agglomeration of the base resin powder, and optimizing the surface penetration depth of the surface crosslinking agent, the addition amounts of water and hydrophilic organic solvents based on 100 parts by weight of the base resin powder can be appropriately controlled.

[0179] The surface crosslinking step can be carried out by heat treatment at a temperature of 110°C to 200°C or 110°C to 150°C for more than 30 minutes. More specifically, the surface crosslinking reaction can be carried out by heat treatment at the maximum reaction temperature of the above-mentioned temperature for 30 to 80 minutes or 40 to 70 minutes.

[0180] When these surface crosslinking process conditions (especially, the heating-up conditions and the reaction conditions at the maximum reaction temperature) are satisfied, a superabsorbent polymer that appropriately meets physical properties such as better pressure permeability can be manufactured.

[0181] The heating means for the surface crosslinking reaction is not particularly limited. Heat medium can be supplied to it or heat source can be directly supplied to it. At this time, the available heat medium can be a heated fluid, such as steam, hot air, and hot oil, etc., but the present invention is not limited thereto. In addition, the temperature of the heat medium supplied to it can be appropriately selected in consideration of the means of the heat medium, the heating rate, and the target temperature to be heated. At the same time, an electric heater or a gas heater can be used as the directly supplied heat source, but the present disclosure is not limited thereto.

[0182] Meanwhile, in the method for preparing a superabsorbent polymer according to an embodiment of the present disclosure, aluminum salts such as aluminum sulfate salts and various other polyvalent metal salts can be further used during surface crosslinking to further improve permeability. Such polyvalent metal salts can be included in the surface crosslinking layer of the finally prepared superabsorbent polymer.

[0183] Hereinafter, the functions and effects of the present invention will be described in more detail through specific examples. However, these examples are for illustrative purposes only, and the present invention is not intended to be limited by these examples.

[0184] <Example>

[0185] Example 1

[0186] (Step 1 - Polymerization)

[0187] In a 3 L glass container equipped with a stirrer and a thermometer, 100 g of acrylic acid, 130 g of a 31.5 wt% caustic soda (NaOH) solution, 0.15 g of polyethylene glycol diacrylate as an internal crosslinking agent, 0.2 g of sodium persulfate as a thermal polymerization initiator, 0.01 g of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator, and 45.0 g of water were mixed at room temperature (25 ± 1 °C) to prepare a monomer composition. Thereafter, the monomer composition was placed in a square reactor with a width of 30 cm and a height of 30 cm, and subjected to a polymerization reaction for 60 seconds by irradiating ultraviolet light with an intensity of 10 mW / cm 2 to obtain a hydrogel polymer.

[0188] (Step 2 - Drying)

[0189] The polymerized hydrogel polymer was cut into pieces with a width of 5 cm and a height of 5 cm, and then chopped using a spiral cutter (meat grinder) with a hole size of 10 mm.

[0190] The chopped hydrogel polymer was placed in a fluidized bed dryer and dried at 180 °C for 30 minutes to obtain a base resin.

[0191] (Step 3 - Primary Crushing and Classification)

[0192] The dried base resin was added to a cutting mill with a screen size of 8 mm and subjected to primary crushing at a rotational speed of 2000 rpm for the rotary blades. The crushed base resin particles were classified using ASTM standard sieves into particles (A) equal to or larger than the standard diameter (710 μm) and particles (B) smaller than the standard diameter. At this time, particles (A) equal to or larger than the standard diameter (710 μm) accounted for 64 wt% of the total crushed particles. Additionally, in the primary crushed particles, particles equal to or larger than 10 mm accounted for 0 wt% of the total crushed particles.

[0193] (Step 4 - Hydration)

[0194] 100 g of the particles (A) equal to or larger than the standard diameter (710 μm) obtained after primary crushing and classification were added to a high-speed mixer and stirred at 300 rpm. While stirring, 5 g of a surface hydration solution (0.1% GK aqueous solution) was added and further stirred for 30 seconds. Thus, the base resin was surface-hydrated to obtain a hydrated superabsorbent polymer.

[0195] (Step 5 - Secondary Crushing)

[0196] The hydrated superabsorbent polymer was placed in a two-roll mill with roll gaps of 0.3 m and 0.15 mm respectively, and subjected to secondary crushing to obtain a superabsorbent polymer (C).

[0197] (Step 6 - Mixing)

[0198] Perform the step (Step 6) of mixing the particles (B) smaller than the standard diameter classified in Step 3 with the superabsorbent polymer (C) secondary pulverized in Step 5.

[0199] (Step 7 - Surface Crosslinking)

[0200] Use ASTM standard sieves to further additionally classify the superabsorbent polymer mixed in Step 6 to have a particle size of 150 μm to 850 μm. Mix 100 g of the additionally classified polymer with a surface crosslinking solution containing 5.0 g of water, 0.1 g of propylene glycol, 1.0 g of ethylene carbonate, 1.0 g of propylene carbonate, and 1.0 g of a 23% aqueous solution of aluminum sulfate for 2 minutes, and perform a surface crosslinking reaction at 180 °C for 40 minutes. After the surface crosslinking reaction, use ASTM standard sieves to classify the superabsorbent polymer again to have a particle size of 150 μm to 850 μm.

[0201] Example 2

[0202] Prepare the superabsorbent polymer in the same manner as in Example 1, except that the amount of the surface hydration solution in Step 4 of Example 1 is changed to 4 g.

[0203] Example 3

[0204] Prepare the superabsorbent polymer in the same manner as in Example 1, except that the amount of the surface hydration solution in Step 4 of Example 1 is changed to 3 g.

[0205] Example 4

[0206] Prepare the superabsorbent polymer in the same manner as in Example 1, except that in Step 4 of Example 1, 5 g of water is used instead of the GK aqueous solution as the surface hydration solution.

[0207] Example 5

[0208] Prepare the superabsorbent polymer in the same manner as in Example 1, except that in Step 4 of Example 1, a 0.2% GK aqueous solution is used as the surface hydration solution.

[0209] Example 6

[0210] Prepare the superabsorbent polymer in the same manner as in Example 1, except that in Step 4 of Example 1, a 0.3% GK aqueous solution is used as the surface hydration solution.

[0211] Example 7

[0212] The superabsorbent polymer was prepared in the same manner as in Example 1, except that in Step 4 of Example 1, 5 g of a 2% aqueous aluminum sulfate solution was used as the surface hydration solution.

[0213] Example 8

[0214] The superabsorbent polymer was prepared in the same manner as in Example 1, except that in Step 4 of Example 1, 5 g of a 0.1% aqueous GK solution + 2% aqueous aluminum sulfate solution was used as the surface hydration solution.

[0215] Example 9

[0216] The superabsorbent polymer was prepared in the same manner as in Example 1, except that in Step 3 of Example 1, pulverization was carried out such that particles (A) having a diameter equal to or larger than the standard diameter (710 μm) accounted for 48% by weight of the total pulverized particles.

[0217] Example 10

[0218] The superabsorbent polymer was prepared in the same manner as in Example 1, except that in Step 3 of Example 1, pulverization was carried out such that particles (A) having a diameter equal to or larger than the standard diameter (710 μm) accounted for 75% by weight of the total pulverized particles.

[0219] Example 11

[0220] Steps 3 to 7 were carried out in the same manner as in Example 1, and the polymerization in Step 1 and the drying in Step 2 of Example 1 were changed as follows to prepare the superabsorbent polymer.

[0221] (Step 1-1 - Polymerization)

[0222] In a 10 L glass container equipped with a stirrer and a thermometer, 1500 g of acrylic acid, 6.3 g of pentaerythritol triallyl ether as an internal crosslinking agent, and 3387 g of water were stirred and mixed, and stirring was carried out while maintaining the temperature at 5°C. The glass container containing the mixture was replaced with a nitrogen atmosphere by flowing nitrogen at 1000 cc / min for 1 hour. Thereafter, 20.1 g of a 0.3% aqueous hydrogen peroxide solution, 22.5 g of a 1% aqueous ascorbic acid solution, and 45.0 g of a 2% aqueous solution of 2,2'-azobis-(2-amidinopropane) dihydrochloride were added as polymerization initiators, and at the same time, 22.3 g of a 0.01% aqueous iron sulfate solution was added as a reducing agent to initiate polymerization. After the temperature of the mixture reached 85°C, polymerization was carried out at 90 ± 2°C for about 3 hours to obtain a polymer.

[0223] (Step 1-3: Micronization; Step 1-2: Neutralization (agglomeration))

[0224] While rotating a micronizer (F200, Karl Schnell) equipped with a perforated plate having multiple holes with a hole size of 10 mm at 1500 rpm, 5000 g of the obtained polymer was added and micronized into primary particles with a particle size of dozens to hundreds of microns. At this time, 90 g of a 1.5 w% aqueous solution of glycerol monostearate (GML) was added to prevent excessive agglomeration.

[0225] Then, while rotating a meat grinder (a screw cutter equipped with a perforated plate having multiple holes with a hole size of 6 mm) at 500 rpm, the micronized polymer was added thereto to prepare secondary agglomerated particles. Here, the process of preparing the secondary agglomerated particles was repeated three times. At this time, 1904 g of a 50% aqueous NaOH solution was added in the first pass to neutralize some acidic groups of the polymer. 18.8 g of a 15% aqueous Na2SO3 solution was added in the second pass to reduce some of the polymer. In the third pass, a neutralized and micronized hydrogel polymer was prepared by passing it through without adding any additives.

[0226] (Step 2: Drying)

[0227] The polymerized hydrogel polymer was cut into pieces with a width of 5 cm and a height of 5 cm, and then chopped using a screw cutter (meat grinder) with a hole size of 10 mm.

[0228] The chopped hydrogel polymer was placed in a flash dryer and dried at 120 °C for 40 minutes to obtain a base resin.

[0229] Thereafter, Steps 3 to 7 were carried out in the same manner as in Example 1 to prepare a superabsorbent polymer.

[0230] Comparative Example 1

[0231] A superabsorbent polymer was prepared in the same manner as in Example 1, except that the hydration in Step 4 was not carried out; and in the secondary pulverization in Step 5 of Example 1, pulverization was carried out using a two-stage roll mill with roll gaps of 0.3 mm and 0.2 mm. That is, Steps 1 to 3 were carried out in the same manner as in Example 1.

[0232] Comparative Example 2

[0233] (Preparation of fine powder recombinant)

[0234] First, fine particles were obtained by obtaining particles smaller than 150 μm separated by classification in each step of Comparative Example 1 (obtained by classification in Steps 3 and 7).

[0235] While putting 100 g of the obtained fine particles into a high-speed mixer and stirring at 500 rpm, 100 g of water was added thereto and mixed for 10 seconds to prepare a fine powder recombinant.

[0236] (Recycling of fine powder recombinant)

[0237] A hydrogel polymer was prepared in the same manner as in Step 1 (=Example) of Comparative Example 1.

[0238] Thereafter, 1000 g of the hydrogel polymer obtained by cutting the polymerized hydrogel polymer into pieces with a width of 5 cm and a height of 5 cm was mixed with 100 g of the previously obtained fine powder recombinant, and the mixture was chopped using a screw chopper (meat grinder) with a pore size of 10 mm.

[0239] The chopped hydrogel polymer was put into a fluidized bed dryer and dried at 180 °C for 30 minutes to obtain a base resin. Thereafter, the subsequent process was carried out in the same manner as in Comparative Example 1 to prepare a superabsorbent polymer.

[0240] Comparative Example 3

[0241] A superabsorbent polymer was prepared in the same manner as in Example 11, except that the hydration in Step 4 was not carried out; and in the secondary pulverization in Step 5 of Example 11, pulverization was carried out using a two-stage roll mill with roll gaps of 0.3 mm and 0.2 mm. That is, Steps 1 to 3 were carried out in the same manner as in Example 11.

[0242] <Experimental Example>

[0243] Experimental Example 1 - Evaluation of Physical Properties of Particles

[0244] The superabsorbent polymers prepared in each of the examples and comparative examples were classified using a classifier with ASTM standards of 4000 μm, 2000 μm, 1400 μm, 850 μm, 710 μm, 600 μm, 300 μm, 150 μm, and pan and vibrating at an amplitude of 1.5 mm / g for 10 minutes. The weight of the particles on each sieve was measured, and then the average diameter and the amount of fines generated were measured as follows.

[0245] (1) Average diameter (μm)

[0246] The average diameter of the classified superabsorbent polymer was measured according to Equation 1 below, and the results are shown in Table 1.

[0247] [Equation 1]

[0248]

[0249] (2) Amount of fines generated

[0250] The amount of fines of the classified superabsorbent polymer was measured according to Equation 2 below, and the results are shown in Table 1.

[0251] [Equation 2]

[0252] Amount of fines (%) = (weight of particles smaller than 150 μm) / (weight of particles larger than 150 μm and smaller than 850 μm) * 100

[0253] Experimental Example 2 - Evaluation of Physical Properties of Superabsorbent Polymer

[0254] The physical properties of the superabsorbent polymers prepared in each of the Examples and Comparative Examples were evaluated as follows and are shown in Table 1 below.

[0255] Unless otherwise specified, all procedures were carried out in a thermo-hygrostat chamber (23 ± 1 °C, relative humidity of 50 ± 10%), and physiological saline or brine refers to an aqueous solution of 0.9 wt% sodium chloride (NaCl).

[0256] (1) Centrifugal Retention Capacity (CRC, g / g)

[0257] The centrifugal retention capacity of the superabsorbent polymers prepared in each of the Examples and Comparative Examples was measured according to EDANA WSP 241.3 (European Disposables and Nonwovens Association).

[0258] After uniformly inserting W0 (g, approximately 0.2 g) of the superabsorbent polymers prepared in each of the Examples and Comparative Examples into a non-woven bag and sealing it, it was immersed in brine (0.9 wt%) at room temperature. After 30 minutes, the bag was centrifuged at 250 G for 3 minutes to drain, and the weight W2 (g) of the bag was measured. In addition, after performing the same operation without using the superabsorbent polymer, the weight W1 (g) of the bag was measured.

[0259] Then, the CRC (g / g) was calculated according to the following Equation 3 using the obtained weight values.

[0260] [Equation 3]

[0261] CRC (g / g) = {[W2 (g) - W1 (g)] / W0 (g)} - 1

[0262] (2) Absorbency Under Pressure (AUP, g / g)

[0263] The absorbency under pressure of the superabsorbent polymers prepared in each of the Examples and Comparative Examples at 0.3 psi was measured according to EDANA WSP 242.3.

[0264] Specifically, a 400-mesh stainless steel screen is installed in the plastic cylindrical bottom with an inner diameter of 25 mm. At room temperature and 50% humidity, W0 (g, 0.9 g) of the superabsorbent polymer is evenly dispersed on the screen. Thereafter, a piston capable of uniformly providing a 0.3 psi load is placed thereon. Here, the outer diameter of the piston is slightly less than 25 mm, there is no gap with the inner wall of the cylinder, and the up-and-down movement (jig-jog) of the cylinder is not interrupted. At this time, the weight W3 (g) of the measuring device is measured.

[0265] Subsequently, a glass filter with a diameter of 90 mm and a thickness of 5 mm is placed in a petri dish with a diameter of 150 mm, and brine composed of 0.9 wt% sodium chloride is poured into the petri dish. At this time, brine is poured until the surface level of the brine is equal to the upper surface of the glass filter. A filter paper with a diameter of 90 mm is placed thereon. After placing the measuring device on the filter paper, the liquid is absorbed under load for 1 hour. After 1 hour, the measuring device is lifted and the weight W4 (g) is measured.

[0266] Then, the pressure absorption (g / g) is calculated according to the following Equation 4 by using the obtained weight values.

[0267] [Equation 4]

[0268] AUP (g / g) = [W4 (g) - W3 (g)] / W0 (g)

[0269] The above measurements are repeated 5 times, and the average value and standard deviation are obtained.

[0270] (3) Vortex time (seconds)

[0271] The vortex time is measured in seconds according to the method described in International Patent Publication No. 1987-003208.

[0272] Specifically, 2 g of the superabsorbent polymer is added to 50 mL of brine at 23 to 24 °C, and a magnetic bar (diameter 8 mm, length 30 mm) is stirred at 600 rpm, and the time required for the vortex to disappear is measured in seconds.

[0273] [Table 1]

[0274]

[0275] Referring to Table 1, in the examples, it was confirmed that by performing the pulverization and hydration processes under specific conditions, the amount of fine powder generated was significantly reduced, thereby achieving excellent absorption performance. In the case of the comparative examples where the pulverization and hydration processes were not carried out according to the present disclosure, it was confirmed that the amount of fine powder generated was significantly increased compared to the examples, and thus physical properties such as the vortex time were reduced.

Claims

1. A method for preparing a superabsorbent polymer, comprising the following steps: Step 1: Form a hydrogel polymer containing a crosslinked polymer, which is obtained by crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal crosslinking agent; Step 2: Dry the hydrogel polymer to prepare a base resin; Step 3: Subject the dried base resin to primary comminution and classify it into particles equal to or larger than a standard diameter and particles smaller than the standard diameter; Step 4: Obtain a hydrated superabsorbent polymer by using an aqueous surface hydration solution for the classified base resin equal to or larger than the standard diameter; and Step 5: Subject the hydrated superabsorbent polymer to secondary comminution to obtain a superabsorbent polymer, wherein the standard diameter is determined within a range of more than 300 μm to less than 850 μm.

2. The method for preparing a superabsorbent polymer according to claim 1, Among them, wherein in Step 3, the primary comminution is carried out such that the particles equal to or larger than the standard diameter account for 40% by weight or more of the total comminuted particles.

3. The method for preparing a superabsorbent polymer according to claim 1, Among them, wherein in Step 3, the primary comminution is carried out such that the particles equal to or larger than 10 mm account for less than 5% by weight of the total comminuted particles.

4. The method for preparing a superabsorbent polymer according to claim 1, Among them, wherein the standard diameter is determined within a range of more than 600 μm to less than 850 μm.

5. The method for preparing a superabsorbent polymer according to claim 1, Among them, wherein in Step 4, based on 100 parts by weight of the base resin, the amount of the surface hydration solution used is 0.1 part by weight to 10 parts by weight.

6. The method for preparing a superabsorbent polymer according to claim 1, Among them, wherein in Step 4, the surface hydration solution further contains an additive selected from the group consisting of polyacrylic acid-polyethylene oxide comb polymer (GK), aluminum sulfate, polyethylene glycol, sodium dodecyl sulfate, glycerol monolaurate, and sodium stearoyl lactate.

7. The method for preparing a superabsorbent polymer according to claim 6, Among them, wherein based on 100 parts by weight of the surface hydration solution, the content of the additive is 0.01 part by weight to 10 parts by weight.

8. The method for preparing a superabsorbent polymer according to claim 1, Among them, wherein Step 4 is carried out by stirring at 100 rpm to 1000 rpm.

9. The method for preparing a superabsorbent polymer according to claim 1, Among them, wherein in Step 5, the secondary comminution is carried out such that the particles having a diameter of 150 μm to 850 μm account for 90% by weight or more of the total comminuted particles.

10. The method for preparing a superabsorbent polymer according to claim 1, further comprising Step 6 of mixing the particles smaller than the standard diameter classified in Step 3 with the superabsorbent polymer obtained by secondary comminution in Step 5.

11. The method for preparing a superabsorbent polymer according to claim 1 or claim 10, further comprising Step 7 of subjecting the superabsorbent polymer to heat treatment in the presence of a surface crosslinking agent to crosslink the surface of the superabsorbent polymer.

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