Method for preparing superabsorbent polymer

Abstract

The present invention relates to a method for preparing a superabsorbent polymer. More specifically, the present invention relates to a method for preparing a superabsorbent polymer that can achieve excellent drying efficiency and thus effectively suppress the warping of the resin.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the benefit of Korean Patent Application Nos. 10-2020-0169708, filed on December 7, 2020, and 10-2021-0173196, filed on December 6, 2021, the entire disclosures of which are incorporated herein by reference.

[0003] The present invention relates to a method for preparing a superabsorbent polymer. More specifically, the present invention relates to a method for preparing a superabsorbent polymer that can achieve excellent drying efficiency and thus effectively suppress warping of the resin.

Background Art

[0004] A superabsorbent polymer (SAP) is a synthetic polymer material that can absorb 500 to 1000 times its own weight of water, and is also differently named as a superabsorbent material (SAM), a water-absorbing gel material (AGM), etc. according to the developing company. The superabsorbent polymer began to be commercialized as a sanitary product and is currently widely used as a water retention material for soil, a water stop material for civil engineering, a sheet for raising seedlings, a preservative for the food circulation field, a hot compress material, etc.

[0005] Such superabsorbent polymers are mainly used in the field of sanitary products such as diapers and sanitary pads. Generally, in sanitary products, superabsorbent polymers are included in a state of being dispersed in pulp. However, recently, there have been continuous attempts to provide thinner sanitary products such as diapers, and as part of this, the development of sanitary products with reduced pulp content or pulp-free sanitary products is actively underway.

[0006] Therefore, in the case of sanitary products with reduced pulp content or pulp-free sanitary products, a relatively high ratio of superabsorbent polymers is included. Therefore, superabsorbent polymer particles are inevitably included in multiple layers of the sanitary product. The superabsorbent polymer should exhibit a fast absorption speed and a high absorption capacity so that all the superabsorbent polymer particles included in the multiple layers can effectively absorb a large amount of liquid, such as urine.

[0007] Meanwhile, such superabsorbent polymers are generally prepared by the following steps: polymerizing monomers to prepare a hydrogel polymer containing a large amount of water, drying the hydrogel polymer, and grinding it into resin particles having a desired particle size. However, in the case of performing the grinding process after drying the hydrogel polymer as described above, a large amount of fine powder may be generated, thereby deteriorating the properties of the finally prepared superabsorbent polymer.

[0008] Moreover, in order to reuse these fine powders, the fine powders are usually mixed with water and agglomerated to prepare a fine powder reconstitute, and then the fine powder reconstitute prepared through a drying / grinding / classifying process is introduced. However, due to the water used, the energy consumption during the drying process increases, and the equipment load increases, thereby reducing the productivity of the superabsorbent polymer.

[0009] In addition, the hydrogel polymers polymerized during the preparation of the superabsorbent polymer agglomerate with each other, and in the case of preparing the hydrogel polymers into agglomerated particles, the cohesive force may increase. Therefore, sufficient drying may not be achieved during the subsequent drying process. Specifically, the drying process is usually carried out by applying hot air in a belt dryer equipped with a perforated plate. However, due to the increase in the cohesive force of the hydrogel polymers, the porosity in the drying layer may be significantly reduced. Therefore, the hot air pressure difference may increase, warping may occur at the edges of the dried product, and it may be difficult to dry in large quantities.

[0010] That is, in the case where the undried rate increases or warping of the dried body occurs, it may be difficult to achieve the desired properties of the product and defects may be caused. Therefore, there is still a need to develop a technology that can increase the drying efficiency without these problems.

Summary of the Invention

[0011]

Technical Problem

[0012] Therefore, an object of the present invention is to provide a method for preparing a superabsorbent polymer. By finely cutting the polymerized hydrogel polymer using a specific additive and then drying it under specific conditions, the particle size can be easily controlled within the required range without generating a large amount of fine powder, and excellent drying efficiency is exhibited. Therefore, even when the drying amount increases, warping of the dried product can be effectively suppressed, thereby improving the properties of the finally prepared superabsorbent polymer.

[0013]

Technical Solution

[0014] To achieve the above object, the present invention provides a method for preparing a superabsorbent polymer, which includes the following steps:

[0015] Crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer in which at least part of the acid groups are neutralized in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer;

[0016] Mix a carboxylic acid additive with the hydrogel polymer and then finely cut it to prepare a mixture containing the finely cut hydrogel polymer; and

[0017] The mixture is dried under the condition of alternately introducing an up-flow air at 120°C to 210°C and a down-flow air at 120°C to 210°C into the mixture to form a base resin powder,

[0018] wherein the total introduction time of the down-flow air is longer than the total introduction time of the up-flow air.

[0019]

Beneficial effects

[0020] According to the method for preparing a superabsorbent polymer of the present invention, by mixing a polymerized hydrogel polymer with a carboxylic acid additive and then performing fine cutting, the particle size can be controlled within a desired range without agglomeration occurring between the hydrogel polymer particles. Therefore, the grinding process of subsequent coarse grinding and fine grinding after the drying process can be simplified, thereby significantly reducing the generation of fine powder during this process. And by controlling the drying conditions after fine cutting, excellent drying efficiency can be achieved. Therefore, even when the drying amount increases, warping of the dried product can be effectively suppressed. Finally, the superabsorbent polymer prepared thereby can achieve excellent properties.

Brief description of the drawings

[0021] Figure 1 Cross-sectional photographs of dried products prepared according to examples and comparative examples of the present invention are shown.

Detailed description of the invention

[0022] The terms used herein are only for explaining specific embodiments and are not intended to limit the present invention.

[0023] Singular expressions include their plural expressions, unless clearly stated or obvious from the context that this is not the intention. Singular expressions include their plural expressions. As used herein, terms such as "comprising", "having" or "including" are intended to specify the presence of implemented features, quantities, steps, structural elements or combinations thereof, and they are not intended to exclude the possibility of the presence or addition of one or more other features, quantities, steps, structural elements or combinations thereof.

[0024] Terms such as "first", "second", "third", etc. are used to explain various components, and these terms are only used to distinguish one component from other components.

[0025] Although various modifications can be made to the present invention and the present invention can have various forms, specific examples will be described and explained in detail below. However, it should be understood that these are not intended to limit the present invention to the specific disclosure, and without departing from the spirit and technical scope of the present invention, the present invention includes all its modifications, equivalents or substitutions.

[0026] As used herein, the term "polymer" refers to the polymerized state of water-soluble ethylenically unsaturated monomers, which may include those polymers in all ranges of water content or particle size. Among these polymers, those polymers having a water content of about 40% by weight or more before drying after polymerization may be referred to as hydrogel polymers, and the particles obtained by grinding and drying such hydrogel polymers may be referred to as crosslinked polymers.

[0027] Also, the term "superabsorbent polymer powder" refers to particulate matter comprising crosslinked polymers obtained by polymerizing water-soluble ethylenically unsaturated monomers in which at least part of the acid groups are neutralized and crosslinked by an internal crosslinking agent.

[0028] Also, the term "superabsorbent polymer" refers to crosslinked polymers formed by polymerizing water-soluble ethylenically unsaturated monomers in which at least part of the acid groups are neutralized, or base resin powders composed of superabsorbent polymer particles obtained by grinding crosslinked polymers, or crosslinked polymers or base resins suitable for production made by other processes such as surface crosslinking, fine particle reconstitution, drying, grinding, classification, etc.

[0029] Also, the term "crosslinked polymer" refers to those polymers obtained by crosslinking water-soluble ethylenically unsaturated monomers in the presence of an internal crosslinking agent, and the "base resin powder" refers to a material containing such crosslinked polymers.

[0030] A method for preparing a superabsorbent polymer according to an embodiment of the present invention includes the following steps: crosslinking and polymerizing water-soluble ethylenically unsaturated monomers in which at least part of the acid groups are neutralized in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer; mixing a carboxylic acid additive with the hydrogel polymer and then mincing to prepare a mixture containing minced hydrogel polymer; and drying the mixture while alternately introducing an upward air flow at 120°C to 210°C and a downward air flow at 120°C to 210°C to form a base resin powder, wherein the total introduction time of the downward air flow is longer than the total introduction time of the upward air flow.

[0031] In order to prepare superabsorbent polymer particles having a desired particle size during the preparation of superabsorbent polymers, a process of grinding (or mincing) and drying the polymerized hydrogel polymer is carried out. However, in the case of preparing the hydrogel polymer into particles, the cohesive force may increase, proper drying may not be achieved during the subsequent drying process, and the energy consumption of the drying process may increase. Also, re-agglomeration may occur. Therefore, in order to achieve the desired particle size, a long-time coarse grinding and fine grinding process is required after the drying process, and due to the additional grinding process, the amount of fine powder generated increases significantly.

[0032] Specifically, the drying process is usually carried out by applying hot air in a belt dryer equipped with a perforated plate. However, due to the increased cohesion of the hydrogel polymer, the porosity in the drying layer may be significantly reduced. As a result, the hot air pressure difference may increase, and warping may occur at the edges of the dried product. Also, in the case of an increased drying amount, the interior of the dried product may not be sufficiently dried, so the drying efficiency may be significantly reduced.

[0033] Therefore, the present inventors found that by finely cutting the hydrogel polymer in the presence of a specific carboxylic acid additive, the particle size can be controlled within a desired range without agglomeration occurring between the hydrogel particles. Thus, an additional grinding process over a long period after drying becomes unnecessary, significantly reducing the generation of fine powder. Moreover, they also found that by drying the finely cut mixture having reduced cohesion due to mixing with the additive under specific conditions, excellent drying efficiency can be achieved. Therefore, even when the drying amount increases, warping of the dried product can be effectively suppressed, and the present invention was completed.

[0034] Meanwhile, the carboxylic acid additive has both a hydrophobic functional group and a hydrophilic functional group. Since the water-soluble ethylenically unsaturated monomer contains an acid group (-COOH) and / or a neutralized acid group (-COO), a large amount of the hydrophilic portion due to the acid group (-COOH) and / or the neutralized acid group (-COO) that does not participate in the polymerization and remains exists on the surface of the hydrogel polymer prepared by polymerization. Therefore, when the hydrogel polymer is mixed with the additive, the hydrophilic functional group of the additive is adsorbed onto at least a part of the hydrophilic portion present on the surface of the hydrogel polymer, and the polymer surface adsorbed with the additive exhibits hydrophobicity due to the hydrophobic functional group located at the other end of the additive. Thus, agglomeration of the resin particles can be suppressed.

[0035] Hereinafter, the preparation method will be described in detail according to each step.

[0036] (Aggregation step)

[0037] The method for preparing a superabsorbent polymer according to an embodiment of the present invention includes crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer in which at least part of the acid groups are neutralized in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer.

[0038] Specifically, in this step, a monomer composition including a monomer mixture and a polymerization initiator is subjected to thermal polymerization or photopolymerization in the presence of an internal crosslinking agent to form a hydrogel polymer.

[0039] The water-soluble ethylenically unsaturated monomer can be any monomer commonly used in the preparation of superabsorbent polymers. As non-limiting examples, the water-soluble ethylenically unsaturated monomer can be a compound represented by the following Chemical Formula 2:

[0040] [Chemical Formula 2]

[0041] R1-COOM 1

[0042] In Chemical Formula 2,

[0043] R1 is a C2-5 alkyl group containing an unsaturated bond,

[0044] M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

[0045] Preferably, the monomer can be one or more selected from the group consisting of acrylic acid, methacrylic acid, and their monovalent metal salts, divalent metal salts, ammonium salts and organic amine salts. Advantageously, acrylic acid or its salt is used as the water-soluble ethylenically unsaturated monomer because a superabsorbent polymer with improved absorbency can be obtained. Additionally, as the monomer, one or more selected from the group consisting of anionic monomers such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid and their salts, nonionic hydrophilic group-containing monomers such as (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate or polyethylene glycol (meth)acrylate, and amino group-containing unsaturated monomers such as (meth)acrylic acid (N,N)-dimethylaminoethyl ester or (N,N)-dimethylaminopropyl (meth)acrylamide and their quaternized products can be used.

[0046] Among them, the water-soluble ethylenically unsaturated monomer has an acid group, and at least part of the acid group is partially neutralized using the neutralization solution of the present invention.

[0047] Among them, the degree of neutralization of the monomer can be 40 mol% to 95 mol%, or 40 mol% to 80 mol%, or 45 mol% to 75 mol%. Although the range of the degree of neutralization may vary depending on the final properties, if the degree of neutralization is too high, the neutralized monomer may precipitate, and thus it may be difficult to carry out polymerization smoothly. On the contrary, if the degree of neutralization is too low, the absorbency of the polymer may be significantly reduced, and it may exhibit properties similar to those of an intractable elastic rubber.

[0048] As used herein, the term "internal crosslinking agent" is used to distinguish from the surface crosslinking agent for crosslinking the surface of the base resin as described below, and it functions to crosslink the unsaturated bonds of the above water-soluble ethylenically unsaturated monomers for polymerization. In this step, crosslinking is carried out without distinguishing between the surface and the interior, but through the surface crosslinking process of the base resin as described below, the surface of the finally prepared superabsorbent polymer is composed of a structure crosslinked by the surface crosslinking agent, and the interior is composed of a structure crosslinked by the internal crosslinking agent.

[0049] As non-limiting examples of the internal crosslinking agent, polyfunctional crosslinking agents 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, butylene glycol di(meth)acrylate, butanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol or ethylene carbonate can be used alone or in combination, but the internal crosslinking agent is not limited thereto.

[0050] Based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer, the amount of the internal crosslinking agent can be from 0.01 part by weight to 5 parts by weight. For example, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer, the amount of the internal crosslinking agent can be 0.01 part by weight or more, 0.05 part by weight or more, 0.1 part by weight or 0.45 part 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 part by weight or less. If the content of the internal crosslinking agent is too small, crosslinking may not occur sufficiently, so it may be difficult to achieve strength above the optimal level. If the content of the internal crosslinking agent is too high, the internal crosslinking density may increase, so it may be difficult to achieve the required centrifuge retention capacity.

[0051] Moreover, in the monomer composition, a polymerization initiator commonly used for preparing superabsorbent polymers can be included. As the polymerization initiator, a thermal polymerization initiator or a photo-polymerization initiator can be used according to the polymerization method. However, even in the case of photo-polymerization, since a certain amount of heat is generated by UV irradiation, etc., and heat is generated to a certain extent as the exothermic polymerization reaction proceeds, a thermal polymerization initiator can also be included.

[0052] As the photopolymerization initiator, one or more compounds selected from the group consisting of benzoin ethers, dialkylacetophenones, hydroxyalkyl ketones, phenyl glyoxylates, benzyl dimethyl ketals, acylphosphines, and α-aminoketones can be used. Among them, as specific examples of the acylphosphine, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate, etc. can be cited. More various photopolymerization initiators are described in Reinhold Schwalm, "UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)", page 115, but are not limited to the above examples.

[0053] And, as the thermal polymerization initiator, one or more compounds selected from the group consisting of persulfate initiators, azo initiators, hydroperoxides, and ascorbic acid can be used. Specifically, as examples of the persulfate initiator, sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc. can be cited. And, as examples of the azo initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis(N,N-dimethyl) isobutyramidine dihydrochloride, 2-(carbamoyl) isobutyronitrile, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, and 4,4'-azobis(4-cyanovaleric acid), etc. can be cited. More various thermal initiators are described in "Principle of Polymerization (Wiley, 1981)", Odian, page 203, which can be used as a reference.

[0054] Based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer, the addition concentration of the polymerization initiator can be from 0.001 part by weight to 1 part by weight. Specifically, if the concentration of the polymerization initiator is too low, the polymerization rate may become slow, and a large amount of residual monomer may be extracted in the final product. And, if the concentration of the polymerization initiator is too high, the polymer chains constituting the superabsorbent polymer network may become short, so the water-soluble content may increase, and the pressure absorbency of the superabsorbent polymer may decrease, thereby deteriorating the properties of the polymer.

[0055] In addition, if necessary, the monomer composition may further contain additives such as thickeners, plasticizers, storage stabilizers, and antioxidants.

[0056] Furthermore, the foaming agent causes foaming during the polymerization process to form pores in the hydrogel polymer, thereby increasing the surface area. As the foaming agent, carbonates can be used. For example, sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, or magnesium carbonate can be used.

[0057] As a commercially available foaming agent that can be used, encapsulated foaming agent F-36D can be used, but it is not limited thereto.

[0058] Furthermore, based on the weight of the water-soluble ethylenically unsaturated monomer, it is preferably used in an amount of 1500 ppmw or less. If the amount of the foaming agent is greater than 1500 ppmw, the pores may increase excessively. Therefore, the gel strength of the superabsorbent polymer may decrease and the density may decrease, thereby causing problems in circulation and storage. Also, based on the weight of the water-soluble ethylenically unsaturated monomer, the amount of the foaming agent is preferably 500 ppmw or more or 1000 ppmw or more.

[0059] Furthermore, the surfactant causes uniform dispersion of the foaming agent, thereby preventing a decrease in gel strength and density due to uniform foaming. As the surfactant, an anionic surfactant can be preferably used. Specifically, a surfactant containing SO3 - anion represented by the following Chemical Formula 3 can be used.

[0060] [Chemical Formula 3]

[0061] R-SO3Na

[0062] In Chemical Formula 3,

[0063] R is an alkyl group having 8 to 16 carbon atoms.

[0064] Furthermore, based on the weight of the water-soluble ethylenically unsaturated monomer, it is preferably used in an amount of 300 ppmw or less. If the amount of the surfactant is greater than 300 ppmw, the content of the surfactant in the superabsorbent polymer may increase, which is not preferred. Also, based on the weight of the water-soluble ethylenically unsaturated monomer, the amount of the surfactant is preferably 100 ppmw or more or 150 ppmw or more.

[0065] Furthermore, such a monomer composition can be made into a solution form in which the above raw materials including water-soluble ethylenic monomers, polymerization initiators, and internal crosslinking agents are dissolved in a solvent.

[0066] Among them, the solvents that can be used are not limited as long as they can dissolve or disperse the above-mentioned raw materials. For example, one or more selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide can be used alone or in combination.

[0067] The formation of the hydrogel polymer by polymerization of the monomer composition can be carried out by common polymerization methods without particular limitation. As non-limiting examples, the polymerization methods are mainly classified into thermal polymerization and photopolymerization according to the energy source. Thermal polymerization can be carried out in a reactor equipped with a stirring shaft such as a kneader, and photopolymerization can be carried out in a reactor equipped with a movable conveyor belt.

[0068] For example, the hydrogel polymer can be obtained by introducing the above monomer composition into a reactor equipped with a stirring shaft such as a kneader and supplying hot air or heating the reactor to carry out thermal polymerization. Among them, the size of the hydrogel polymer discharged to the reactor outlet is several millimeters to several centimeters according to the shape of the stirring shaft equipped in the reactor. Specifically, the size of the obtained hydrogel polymer can vary according to the concentration and introduction rate of the introduced monomer composition, and generally, a hydrogel polymer with a particle size of 2 mm to 50 mm can be obtained.

[0069] Moreover, in the case where the photopolymerization of the monomer composition is carried out in a reactor equipped with a movable conveyor belt as described above, a hydrogel polymer in the form of a sheet can be obtained. Among them, the thickness of the sheet can vary according to the concentration and introduction rate of the introduced monomer composition. Preferably, the thickness is controlled to be 0.5 to 10 cm so that the entire sheet can be uniformly polymerized and the production speed can also be ensured.

[0070] The water content of the hydrogel polymer thus obtained can generally be 40 to 80% by weight. Throughout the specification, the "water content" is the content of water occupied based on the total weight of the hydrogel polymer, and it refers to the value obtained by subtracting the weight of the polymer in the dry state from the weight of the water gel polymer. Specifically, it is defined as the value calculated by measuring the weight loss caused by the evaporation of water in the polymer while drying by raising the temperature of the polymer by infrared heating. Among them, the temperature is raised from room temperature to about 180 °C and then maintained at 180 °C, and the total drying time is 20 minutes, including a 5-minute heating-up step.

[0071] (Fine cutting step)

[0072] Next, the method includes the steps of mixing a hydrogel polymer with a carboxylic acid additive and then mincing to prepare a mixture containing the minced hydrogel polymer.

[0073] In the case of mixing a hydrogel polymer having strong cohesion with the carboxylic acid additive of the present invention and then mincing, particles having a desired particle size can be prepared without agglomeration occurring between the crushed particles. Therefore, an additional grinding process after drying is unnecessary, and the generation of fine powder can be reduced. Also, by performing the subsequent drying process under specific conditions, excellent drying efficiency can be achieved, so that even when the drying amount increases, warping of the dried product can be effectively suppressed.

[0074] The carboxylic acid additive may be one or more selected from carboxylic acids represented by Chemical Formula 1 and metal salts thereof:

[0075] [Chemical Formula 1]

[0076]

[0077] In Chemical Formula 1,

[0078] A is a C5 to C21 alkyl group,

[0079] B1 is -OCO-, -COO-, or -COOCH(R1)COO-,

[0080] B2 is -CH2-, -CH2CH2-, -CH(R2)-, -CH=CH-, or -C≡C-,

[0081] wherein, R1 and R2 are each independently a C1 to C4 alkyl group, and

[0082] n is an integer from 1 to 3, and

[0083] C is a carboxyl group.

[0084] Specifically, the carboxylic acid additive is one or more selected from the group consisting of the carboxylic acid represented by Chemical Formula 1, the alkali metal salt of the carboxylic acid represented by Chemical Formula 1, and the alkaline earth metal salt of the carboxylic acid represented by Chemical Formula 1. More specifically, the carboxylic acid additive is one of the carboxylic acid represented by Chemical Formula 1, the alkali metal salt of the carboxylic acid represented by Chemical Formula 1, and the alkaline earth metal salt of the carboxylic acid represented by Chemical Formula 1.

[0085] In Chemical Formula 1, A is a hydrophobic moiety and can be a straight-chain or branched alkyl group having 5 to 21 carbon atoms. However, when A is a straight-chain alkyl group, agglomeration between the comminuted particles can be inhibited and dispersibility can be improved. Therefore, A is preferably a straight-chain alkyl group. If A is an alkyl group having less than 5 carbon atoms, due to the short chain length, effective control of agglomeration between the comminuted particles may not be possible. If A is an alkyl group having more than 21 carbon atoms, the fluidity of the additive may decrease. As a result, the additive may not be effectively mixed with the hydrogel polymer, and the unit cost of the composition may increase due to the increased cost of the additive.

[0086] Specifically, in Chemical Formula 1, A can be a straight-chain alkyl group having 5 to 21 carbon atoms, namely n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl, n-nonadecyl, n-eicosyl or n-henicosyl.

[0087] More specifically, it can be a straight-chain alkyl group having 6 to 18 carbon atoms. For example, A can be -C6H 13 、-C 11 H 23 、-C 12 H 25 、-C 17 H 35 or -C 18 H 37 .

[0088] Moreover, in Chemical Formula 1, the (B1 - B2) moiety functions to improve the adsorption ability to the polymer surface. The ability may be insufficient with only the C moiety. When the number of carbon atoms of B2 is 3 or more, the distance between the B1 moiety and the C moiety can increase, and thus the adsorption ability to the hydrogel polymer may decrease.

[0089] Among them, R1 and R2 can each independently be a straight-chain or branched C1 to 4 alkyl group. More specifically, R1 and R2 can each independently be methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl or tert-butyl. However, since the additive is adsorbed onto the superabsorbent polymer particles, it is advantageous that the molecular structure of the additive is not bulky. Thus, both R1 and R2 can be methyl.

[0090] Furthermore, in Chemical Formula 1, n can be 1, 2 or 3. More specifically, considering that the (B1 - B2) moiety enhances the adsorption ability of the C moiety and considering the molecular length for effectively adsorbing the carboxylic acid additive onto the hydrogel polymer, n representing the number of (B1 - B2) is preferably 1.

[0091] Specifically, in Chemical Formula 1, B1 can be or Among them, * is the binding site with adjacent atoms.

[0092] For example, B1 can be or

[0093] And, in Chemical Formula 1, B2 can be or Among them, * is the binding site with adjacent atoms. Among them, in order to improve the adsorption ability of the additive to the crosslinked polymer together with part C, B2 can preferably be or

[0094] And, in Chemical Formula 1, part C is a hydrophilic part and is a carboxyl group (COOH), but in the case where the carboxylic acid additive is a salt, C is a carboxylate group (COO - ).

[0095] That is to say, the carboxylic acid additive can be a compound represented by the following Chemical Formula 1a:

[0096] [Chemical Formula 1a]

[0097]

[0098] In Chemical Formula 1a,

[0099] M is H + , an alkali metal monovalent cation or an alkaline earth metal divalent cation,

[0100] When M is H + or an alkali metal monovalent cation, k is 1, and when M is an alkaline earth metal divalent cation, it is 2, and

[0101] A, B1, B2 and n are as defined in Chemical Formula 1.

[0102] More specifically, in the case where the carboxylic acid additive is an alkali metal salt of the carboxylic acid represented by Chemical Formula 1, the additive can be represented by the following Chemical Formula 1':

[0103] [Chemical Formula 1']

[0104]

[0105] In Chemical Formula 1',

[0106] M1 is an alkali metal, such as sodium or potassium, and

[0107] A, B1, B2 and n are as defined in Chemical Formula 1.

[0108] Further, when the carboxylic acid additive is an alkaline earth metal salt of the carboxylic acid represented by Chemical Formula 1, the additive can be represented by the following Chemical Formula 1":

[0109] [Chemical Formula 1"]

[0110]

[0111] In Chemical Formula 1", M2 is an alkaline earth metal such as calcium, and

[0112] A, B1, B2, and n are as defined in Chemical Formula 1.

[0113] For example, the carboxylic acid additive can be a carboxylic acid selected from the group consisting of the following compounds:

[0114]

[0115]

[0116] Alternatively, the carboxylic acid additive can be an alkali metal salt selected from the group consisting of the following:

[0117]

[0118]

[0119] Wherein,

[0120] Each M1' is independently an alkali metal.

[0121] Alternatively, the carboxylic acid additive can be an alkaline earth metal salt selected from the group consisting of the following:

[0122]

[0123]

[0124] Wherein,

[0125] Each M2' is independently an alkaline earth metal.

[0126] For example, the carboxylic acid additive can be one of the compounds represented by the following Chemical Formulas 1-1 to 1-7, but not limited thereto:

[0127]

[0128] Meanwhile, in the base resin powder prepared by drying a shredded mixture containing shredded hydrogel polymer in addition to the carboxylic acid additive, a compound formed by the decomposition of the ester bond of B1 during grinding and drying of the additive and the hydrogel polymer can also be contained.

[0129] Specifically, when the additive is a compound with n = 1 and B1 = -OCO-, the base resin powder may further contain an alcohol with the structure A-OH and a compound with the structure HOOC-B2-C.

[0130] Moreover, when the additive is a compound with n = 1 and B1 = -COO-, the base resin powder may further contain a carboxylic acid with the structure A-COOH and a compound with the structure HO-B2-C.

[0131] Furthermore, when the additive is a compound with n = 1 and B1 = -COOCH(R1)COO-, the base resin powder may further contain a carboxylic acid with the structure A-COOH and a compound with the structure HOCH(R1)COO-B2-C.

[0132] Thus, when the base resin powder further contains a compound formed by the decomposition of the ester bond in the additive molecule, the fluidity of the additive can be increased, thereby further preventing re-agglomeration after grinding.

[0133] Among them, based on the hydrogel polymer, the content of the carboxylic acid additive contained can be 100 ppmw to 5,000 ppmw. Preferably, the content of the additive contained can be 500 ppmw to 4,500 ppmw, or 1,000 ppmw to 4,000 ppmw, 1,500 ppmw to 4,000 ppmw, or 3,000 ppmw to 4,000 ppmw. If the content of the additive is too low, the agglomeration control effect of the additive may not be significant, so it may not be easy to grind to the required particle size, and due to excessive kneading during the fine cutting process, the water-soluble components may increase. If the additive content is too high, the carboxylic acid additive may remain on the particle surface, so the surface tension may decrease, and the initial water absorption rate of the superabsorbent polymer may decline.

[0134] The method of mixing such a carboxylic acid additive with the hydrogel polymer is not particularly limited as long as it can mix it uniformly with the hydrogel polymer.

[0135] For example, the additive can be mixed in the form of a solution dissolved in water. Among them, the solution and the hydrogel polymer can be put into a reactor for mixing, or the hydrogel polymer can be put into a mixer and the solution can be sprayed, or the solution and the hydrogel polymer can be continuously fed into a continuously operating mixer and mixed, etc.

[0136] The step of coarsely grinding the mixture of the carboxylic acid additive and the hydrogel polymer can be carried out in such a way that the hydrogel polymer is pushed to an outlet having pores with an average diameter of 1 mm to 5 mm.

[0137] Specifically, a meat grinder can be used, which includes a fine cutting module that contains a perforated plate with holes having an average diameter of 1 mm to 5 mm. Multiple perforated plates can be used. The grinding is carried out by pushing the hydrogel polymer mixed with the additive, so that it can be ground when passing through the holes of the perforated plate. Among them, in order to push the hydrogel polymer, an extruder can be used, for example, a single-screw or multi-screw type extruder can be used. The fine cutting process can be carried out at a discharge speed of 50 Hz to 8,000 Hz.

[0138] Among them, the average particle size (D50) of the finely cut hydrogel polymer is about 0.5 mm to 4 mm. In the case where the hydrogel polymer forms particles with such a small particle size, generally, due to the high moisture content in the hydrogel polymer, the particles may agglomerate with each other. However, by using the carboxylic acid additive of the present invention in combination, such a problem does not occur.

[0139] Meanwhile, throughout the specification, the average particle size "Dn" refers to the particle size or particle size at the n% point of the cumulative particle number distribution according to the particle size. That is, D50 represents the particle size at the 50% point of the cumulative particle number distribution according to the particle size, D90 represents the particle size at the 90% point of the cumulative particle number distribution according to the particle size, and D10 represents the particle size at the 10% point of the cumulative particle number distribution according to the particle size. Dn can be measured by methods such as laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size measuring device (such as Microtrac S3500). When the particles pass through the laser beam, the difference in the diffraction pattern according to the particle size is measured to obtain the particle size distribution. By calculating the particle sizes at the 10%, 50%, and 90% points of the cumulative particle number distribution according to the particle size, D10, D50, and D90 can be measured.

[0140] Meanwhile, at least a part of the carboxylic acid additive contained in the finely cut mixture can be present on the surface of the hydrogel polymer particles. The description that "at least a part of the additive is present on the surface of the hydrogel polymer" means that at least a part of the additive is adsorbed or bound to the surface of the hydrogel polymer particles. This is because the carboxylic acid additive is not introduced during the polymerization of the water-soluble ethylenically unsaturated monomer, but is introduced after the polymer is formed. Compared with the case where the additive is introduced during the polymerization and exists inside the polymer, the re-agglomeration between the hydrogel polymer particles can be suppressed.

[0141] (Drying step)

[0142] Next, the method includes the step of drying the mixture containing the finely cut hydrogel polymer to form a base resin powder under the condition of alternately introducing an upward air flow at 120°C to 210°C and a downward air flow at 120°C to 210°C into the mixture.

[0143] The upward air flow refers to introducing hot air from the lower part to the upper part into the object to be dried, while the downward air flow refers to introducing hot air from the upper part to the lower part into the object to be dried. That is, the upward air flow and the downward air flow refer to the directions of the hot air being opposite to each other. Although the angle of the direction can vary according to the conditions of the hot air dryer used, generally, the angle difference between the traveling directions of the upward air flow and the downward air flow is 180° ± 5°.

[0144] During drying, the number of introductions of the upward air flow and the downward air flow is not particularly limited, and they can be introduced alternately multiple times as long as, based on their respective total introduction times, the introduction time of the downward air flow is longer than the introduction time of the upward air flow. In the case of multiple introductions, the total introduction times of the upward air flow and the downward air flow are calculated as the sum.

[0145] Drying is carried out in a fixed-bed manner, which is a method of passing hot air from bottom to top through the material to be dried while fixing the material on a floor such as a perforated plate through which air can flow. Specifically, a breathable belt dryer equipped with a perforated plate is used. Specifically, it is carried out by introducing the finely cut mixture to be dried into the perforated plate. Therefore, the upward air flow refers to introducing from the bottom of the perforated plate to the top, while the downward air flow refers to introducing from the top of the finely cut mixture placed on the perforated plate to the bottom.

[0146] Among them, the total introduction time of the downward air flow is longer than the total introduction time of the upward air flow, thereby achieving excellent drying efficiency and suppressing resin warping. Even if hydrogel polymer particles are formed in the previous fine-cutting step, by using a carboxylic acid additive in combination, the object to be dried will not agglomerate. Moreover, by introducing the downward air flow with a longer introduction time, warping at the edges of the object to be dried can be effectively suppressed. In addition, even if the amount of the object to be dried increases, the target degree of drying efficiency can be achieved.

[0147] Meanwhile, in the case where the introduction time of the upward air flow is long, edge warping may occur. In particular, the inside may not be sufficiently dried, so the drying efficiency may be significantly reduced.

[0148] The hot air temperature in the drying step is 120°C to 210°C. If the temperature is lower than 120°C, it may be difficult to dry to the target degree. If it is higher than 210°C, polymer degradation may occur. Preferably, it can be carried out at 160°C to 180°C.

[0149] The total time of the drying step is not particularly limited as long as the introduction time of the downward air flow is longer than the introduction time of the upward air flow. However, for example, it can be carried out for 10 minutes to 20 hours, preferably 30 minutes to 40 minutes. The above range is preferred because the target degree of drying efficiency can be achieved.

[0150] Hot air drying is carried out by introducing hot air with a linear velocity of 0.5 m / s to 3.0 m / s, 0.7 m / s to 2.5 m / s, or 0.85 m / s to 2.0 m / s. When the above velocity ranges are satisfied, even the interior of the object to be dried can be uniformly dried. If the linear velocity is less than 0.5 m / s, the hot air may not reach the interior of the object to be dried. If it is greater than 3.0 m / s, the object to be dried may scatter in the dryer.

[0151] Preferably, based on the drying time, the total introduction time of the upward air flow can be 1% to 25%. When the introduction time of the upward air flow is controlled within the above range, even when a large amount of hydrogel polymer particles are included, uniform and excellent drying efficiency can be achieved without warping of the edge layer. Most preferably, based on the drying time, the total introduction time of the upward air flow is 1% to 12%.

[0152] Meanwhile, if the total introduction time of the upward air flow is less than 1%, the uniformity of the drying efficiency may be slightly deteriorated. If it is greater than 25%, warping of the drying layer may not be effectively suppressed, and thus the drying efficiency may be reduced.

[0153] In one embodiment of the present invention, the moisture content of the base resin powder prepared by the drying process under the above conditions is less than 8% by weight. The "moisture content" is the content of moisture based on the weight of the measured base resin powder, and it refers to the value calculated by subtracting the weight of the dried polymer from the weight of the base resin powder. Specifically, it is defined as the value calculated by measuring the weight loss caused by the evaporation of moisture in the base resin powder while heating the polymer to dry by infrared heating.

[0154] The moisture content is one of the indicators of the degree of drying efficiency, preferably 0.01% to 7% by weight, or 0.01% to 5% by weight. The method for measuring the moisture content of the base resin powder will be described in more detail in the experimental examples below.

[0155] (Grinding and classification step)

[0156] The preparation method according to one embodiment of the present invention may further include a step of grinding and classifying the dried base resin powder after the drying step.

[0157] As described above, since the base resin powder obtained by drying a mixture with reduced cohesive force under specific conditions has a significantly lower degree of re-agglomeration between particles, a base resin powder with a desired particle size can be easily obtained by grinding even with low grinding energy. That is, after the drying step, the grinding step can be reduced (coarse grinding can be omitted and only fine grinding can be carried out), and by the process of classifying the grinding product, a base resin powder with a desired particle size can be obtained.

[0158] According to one embodiment of the present invention, the undried rate of the base resin powder prepared by these processes is less than 15%.

[0159] The undried rate represents the drying degree of the fragmented powder in the base resin powder that is difficult to dry due to relatively large sizes, and is one of the indicators of drying efficiency.

[0160] Specifically, it refers to the mass fraction of the powder that has not been sieved through a 4-mesh sieve after classifying the base resin powder with a 4-mesh sieve for more than 10 minutes according to ASTM 311. The particles that have not been filtered after classifying with a 4-mesh sieve for more than 10 minutes refer to those particles that have not been sufficiently dried and re-agglomerated due to internal moisture. In fact, the moisture content of the powder that has not been sieved through a 4-mesh sieve after classifying for more than 10 minutes is 10% or more.

[0161] The lower the undried rate, the more excellent it is. Therefore, the lower limit of the undried rate is theoretically 0. It is preferably 0.1% to 13%, 0.1% to 10%, or 0.5% to 5%. The measurement method of the undried rate of the base resin powder will be described in more detail in the experimental examples later.

[0162] (Surface crosslinking step)

[0163] Next, the method for preparing a superabsorbent polymer according to one embodiment of the present invention further includes a step of heat-treating the base resin powder in the presence of a surface crosslinking agent to surface-crosslink at least a part of the surface of the base resin powder, thereby preparing a superabsorbent polymer.

[0164] The surface crosslinking step initiates a crosslinking reaction on the surface of the base resin powder 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 with an increased surface crosslinking density.

[0165] Specifically, a surface crosslinking layer can be formed by heat treatment in the presence of a surface crosslinking agent. This heat treatment process increases the surface crosslinking density, that is, the external crosslinking density, but does not change the internal crosslinking density. Therefore, the prepared superabsorbent polymer with a surface crosslinking layer has a structure in which the crosslinking density on the outside is higher than that on the inside.

[0166] 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.

[0167] Meanwhile, as the surface crosslinking agent contained in the surface crosslinking composition, those previously used for preparing superabsorbent polymers can be used without particular limitation. For example, the surface crosslinking agent may include: one or more polyhydric alcohols selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; one or more carbonate compounds selected from the group consisting of ethylene carbonate and propylene carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazoline compounds such as oxazolidinone; polyamine compounds; mono-, di-, or poly-oxazolidinone compounds; or cyclic urea compounds, etc. Preferably, the same substances as the above internal crosslinking agent can be used. For example, alkylene glycol diglycidyl ether compounds such as ethylene glycol diglycidyl ether can be used.

[0168] Based on 100 parts by weight of the base resin powder, the amount of the surface crosslinking agent can be 0.001 to 2 parts by weight. Preferably, its amount 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. By controlling the content range of the surface crosslinking agent within the above range, a superabsorbent polymer showing excellent properties such as absorption performance and permeability can be prepared.

[0169] Meanwhile, the surface crosslinking agent is added to the base resin powder in the form of a surface crosslinking composition containing the surface crosslinking agent, and the method of adding the surface crosslinking composition is not particularly limited. For example, the surface crosslinking composition and the base resin powder can be put into a reactor and mixed, or the surface crosslinking composition can be sprayed onto the base resin powder, or the surface crosslinking composition and the base resin powder can be continuously fed into a continuously operating mixer and mixed, etc.

[0170] Moreover, the surface crosslinking composition may further contain water and / or a hydrophilic organic solvent as a medium. Therefore, the surface crosslinking agent can be uniformly dispersed in the base resin powder. Among them, based on 100 parts by weight of the base resin powder, the contents of water and the hydrophilic organic solvent can be controlled to uniformly dissolve / disperse the surface crosslinking agent, prevent the agglomeration of the base resin, and optimize the surface penetration depth of the surface crosslinking agent.

[0171] 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 30 minutes or more. More specifically, the surface crosslinking can be carried out by heat treatment at the above maximum reaction temperature for 30 to 80 minutes or 40 to 70 minutes.

[0172] By satisfying these surface crosslinking process conditions (especially the heating conditions and the reaction at the highest reaction temperature), superabsorbent polymers with excellent properties such as pressure permeability can be prepared.

[0173] There is no particular limitation on the heating method for surface crosslinking. Heat medium can be supplied, or a heat source can be directly supplied for heating. Among them, as the heat medium that can be used, steam, hot air, heating fluids such as hot oil, etc. can be used, and the temperature of the supplied heat medium can be appropriately selected considering the heat medium, heating method, and target temperature. At the same time, as the directly supplied heat source, electric heating or gas heating, etc. can be cited, but it is not limited thereto.

[0174] At the same time, according to the method for preparing a superabsorbent polymer according to an embodiment of the present invention, various polyvalent metal salts such as aluminum salts such as aluminum sulfate can be further used during surface crosslinking to further improve properties such as permeability. Such polyvalent metal salts can be included in the surface crosslinked layer of the finally prepared superabsorbent polymer.

[0175] At the same time, the superabsorbent polymer prepared according to an embodiment of the present invention can have a particle size of 150 μm to 850 μm. More specifically, at least 95% by weight or more of the base resin powder and the superabsorbent polymer containing it have a particle size of 150 μm to 850 μm, the content of particles with a particle size of 300 μm to 600 μm can be 50% by weight or more, and the content of particles with a particle size less than 150 μm can be less than 3% by weight.

[0176] The superabsorbent polymer prepared according to the preparation method of the above-mentioned embodiment has excellent properties, and due to excellent drying efficiency, edge warping can be effectively suppressed, thereby improving the quality of the final product.

[0177] Specifically, the centrifuge retention capacity (CRC) of the superabsorbent polymer according to EDANA method WSP 241.2 can be 26 g / g or more, preferably 28 g / g or more, 30 g / g or more, and 50 g / g or less, 45 g / g or less, 40 g / g or less, 35 g / g or less. The specific measurement method of the centrifuge retention capacity will be described in detail in the experimental examples.

[0178] Specifically, the absorption under pressure (AUP) of the superabsorbent polymer at 0.3 psi according to EDANA method WSP 242.3 can be 24 g / g or more, preferably 26 g / g or more, 27 g / g or more, and 50 g / g or less, 40 g / g or less, 35 g / g or less, 30 g / g or less. The specific measurement method of the absorption under pressure will be described in detail in the experimental examples.

[0179] In the following, the functions and effects of the present invention will be described in detail through specific embodiments of the present invention. However, these embodiments are provided only as illustrations of the present invention, and the scope of the rights of the present invention is not determined by them.

[0180] [Example]

[0181] <Preparation of Superabsorbent Polymer>

[0182] Example 1

[0183] (Step 1)

[0184] In a 3 L glass container equipped with a stirrer and a thermometer, 100 g (1.388 mol) of acrylic acid, 0.001 g of polyethylene glycol diacrylate (Mn = 508), 0.24 g of ethylene glycol diglycidyl ether as an internal cross-linking agent, 0.008 g of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide photoinitiator, 0.125 g of sodium persulfate thermal initiator, 123.5 g of 32% caustic soda solution, 0.1 g of encapsulated blowing agent (F-36D), and 0.02 g of aqueous sodium dodecyl sulfate (SDS) solution were mixed with 66.0 g of water at room temperature (25 ± 1 °C) to prepare a monomer composition with a total solids content of 42.0 wt% (neutralization degree of acrylic acid: 70 mol%).

[0185] Then, the monomer composition was fed onto a conveyor belt 10 cm wide, 2 m long, and rotating at 50 cm / min at a rate of 500 mL / min to 2000 mL / min. And, while feeding the monomer composition, UV was irradiated at an intensity of 10 mW / cm 2 for 60 seconds for polymerization to obtain a hydrogel polymer sheet with a water content of 50 wt%.

[0186] (Step 2)

[0187] Next, an aqueous solution containing sodium stearoyl-2-lactate (Almax-6900, Ilshin Wells) represented by the following Chemical Formula A-1 (0.4 parts by weight of carboxylic acid additive based on 100 parts by weight of the hydrogel polymer) was applied to the hydrogel polymer sheet obtained by the polymerization reaction and mixed, and then the hydrogel polymer sheet was finely cut using a meat grinder with a pore size of 3 mm to prepare a mixture containing finely cut hydrogel particles. The average particle size (D50) of the finely cut hydrogel particles was about 3.66 mm. The water content of the hydrogel polymer contained in the finely cut mixture was 46 wt%:

[0188] [Chemical Formula A-1]

[0189]

[0190] (Step 3)

[0191] Then, 1,000 g of the finely cut mixture was introduced into a belt dryer equipped with a 200 mm × 200 mm × 1 mm perforated plate capable of transmitting air volume up and down. On top of the perforated plate, the finely cut mixture was placed and dried by first introducing an upward air flow at 160 °C with a linear velocity of 0.93 m / s from the lower part to the upper part of the perforated plate for 8 minutes and 18 seconds, and then introducing a downward air flow at 160 °C with a velocity of 0.93 m / s for 24 minutes and 42 seconds, thereby preparing the base resin powder.

[0192] (Step 4)

[0193] Next, the base resin powder was classified using an ASTM standard sieve to obtain a base resin powder with a particle size of 150 μm to 850 μm.

[0194] Specifically, 100 parts by weight of the base resin powder was mixed with a surface crosslinking composition (4.8 parts by weight of water, 5.0 parts by weight of methanol, 0.03 parts by weight of ethylene glycol diglycidyl ether, 0.01 parts by weight of silica Aerosil 200), and a surface crosslinking reaction was carried out at 140 °C for 30 minutes. After surface crosslinking, the superabsorbent polymer was classified using an ASTM standard sieve to have a particle size of 150 μm to 850 μm.

[0195] Examples 1 to 6 and Comparative Examples 1 to 5

[0196] A superabsorbent polymer containing the base resin powder was prepared by the same method as in Example 1, except that the conditions of the finely cut step and the drying step were changed as shown in Table 1 below.

[0197]

Table 1

[0198]

[0199] <Experimental Example 1: Evaluation of the base resin powder>

[0200] For the base resin powder prepared in the examples and comparative examples, the properties were evaluated as follows, and the results are shown in Table 2.

[0201] (1) Evaluation of undried rate

[0202] For the base resin powder after drying and before surface crosslinking prepared in the examples and comparative examples, the base resin powder sample (A1) was classified using an ASTM E11 4-mesh sieve for 10 minutes to obtain a powder sample (A2) that was not filtered by the sieve, and the ratio of the weight A'2 (g) of the unfiltered powder sample to the initial weight A'1 (g) of the base resin powder sample (A1) was measured.

[0203] [Mathematical formula 1]

[0204] Un-dried rate (wt%) = {A'2 (g) / A'1 (g)} * 100

[0205] (2) Evaluation of moisture content

[0206] For the base resin powders prepared in the Examples and Comparative Examples after drying and before surface crosslinking, samples with particle sizes ranging from 150 μm to 850 μm were taken from the base resin powders, and the weight loss during drying with the evaporation of moisture in the base resin powders was measured.

[0207] Specifically, a sample of the base resin powder with an initial weight of H0 (g) was maintained at 180 °C for 40 minutes by infrared heating, then the weight H1 (g) was measured, and the moisture content was calculated according to the following Mathematical Formula 2.

[0208] [Mathematical Formula 2]

[0209] Moisture content (wt%) = {[H0 (g) - H1 (g)] / H0 (g)} * 100

[0210] (3) Warping evaluation

[0211] For the base resin powders prepared in the Examples and Comparative Examples after drying and before surface crosslinking, the warpage degree of the dried products was evaluated according to the following evaluation criteria, and the results are shown in Table 2 below. And, cross-sectional photographs of the dried products are as Figure 1 shown. Figure 1 (a) is a cross-sectional photograph of the dried product of Example 2, and (b) is a cross-sectional photograph of the dried product of Comparative Example 6.

[0212] <Evaluation Criteria for Warpage>

[0213] ○: When the edge rises by more than 10% based on the thickness of the central part of the dried product.

[0214] △: When the edge rises by more than 5% and less than 10% based on the thickness of the central part of the dried product.

[0215] ×: When the edge rises by less than 5% based on the thickness of the central part of the dried product.

[0216] <Experimental Example 2: Evaluation of Superabsorbent Polymer>

[0217] For the surface-crosslinked superabsorbent polymers prepared in the Examples and Comparative Examples, the properties were evaluated as follows, and the results are shown in Table 2.

[0218] Unless otherwise specified, all property evaluations are carried out under constant temperature and humidity (23 ± 1 °C, relative humidity 50 ± 10 %), and physiological saline or brine refers to an aqueous solution of 0.9 wt% sodium chloride (NaCl).

[0219] (1) Centrifugal Retention Capacity (CRC)

[0220] For each resin composition, the centrifugal retention capacity according to non-pressurized absorbency is measured in accordance with the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP241.3.

[0221] Specifically, each superabsorbent polymer obtained in the examples and comparative examples is classified using a #30 - 50 sieve to obtain a classified resin. Resin W0 (g) (about 0.2 g) is evenly placed in a small packet made of non-woven fabric and the packet is sealed, and then it is immersed in physiological saline (0.9 wt%) at room temperature. After 30 minutes, the packet is dehydrated using a centrifuge at 250 G for 3 minutes, and the weight W2 (g) of the packet is measured. And after performing the same operation without using the resin, the mass W1 (g) at this time is measured.

[0222] Using the obtained weights, CRC (g / g) is calculated according to the following mathematical formula 3.

[0223] [Mathematical formula 3]

[0224] CRC (g / g) = {[W2 (g) - W1 (g)] / W0 (g)}... 3

[0225] (3) Pressurized Absorbency (AUP)

[0226] For each superabsorbent polymer of the examples and comparative examples, the 0.3 psi pressurized absorbency is measured according to the EDANA method WSP 242.3.

[0227] First, for the measurement of pressurized absorbency, the classified resin used in the CRC measurement is used.

[0228] Specifically, a 400-mesh stainless steel wire mesh is installed at the bottom of a plastic cylinder with an inner diameter of 25 mm. At room temperature and 50 % humidity, W0 (g) (0.16 g) of the superabsorbent polymer composition is evenly spread on the wire mesh, and a piston with an outer diameter slightly smaller than 25 mm that can further apply a 0.3 psi load is placed on it, such that there is no gap between the piston and the inner wall of the cylinder and the up and down movement is unobstructed. At this time, the weight W3 (g) of the measuring device is measured.

[0229] Place a glass filter with a diameter of 90 mm and a thickness of 5 mm in a petri dish with a diameter of 150 mm. Pour physiological saline composed of 0.9% by weight of sodium chloride to the same level as the upper surface of the glass filter. Place a filter paper with a diameter of 90 mm on it. Place the above measurement device on the filter paper and absorb the liquid under load for 1 hour. After 1 hour, lift the measurement device and measure the weight W4 (g).

[0230] Using the obtained weight, calculate the pressure absorbency (g / g) according to the following mathematical formula 4.

[0231] [Mathematical formula 4]

[0232] AUP (g / g) = [W4 (g) – W3 (g)] / W0 (g)

[0233]

Table 2

[0234]

[0235] As confirmed by the data in Table 2, in the examples of the preparation method according to the present invention, by introducing a specific additive during the mincing process of the hydrogel polymer and controlling the drying process conditions, the agglomeration of the hydrogel polymer was inhibited. Therefore, the base resin powder exhibited excellent drying efficiency, and even when the drying amount increased during the drying process, the warping of the dried product could be effectively inhibited. And it was confirmed that the superabsorbent polymer prepared thereby achieved excellent absorbency.

[0236] Meanwhile, in the case of not using the additive of the present invention, due to the re-agglomeration of the hydrogel particles, it was difficult to control the particle size range. And in the case of mincing the hydrogel polymer in the same manner as in the examples using a 3 mm meat grinder, the re-agglomeration rate of the particles was high, so the operation of the meat grinder stopped, making the experiment difficult to carry out.

[0237] Therefore, it was confirmed that in Comparative Examples 1 to 3, without using the additive of the present invention and using a 16 mm meat grinder for mincing, it was difficult to inhibit the warping of the dried product. And it was confirmed that compared with Examples 5 and 6 where the drying amount increased, in Comparative Examples 1 to 3, the drying amount (feed introduction amount) increased and the drying efficiency decreased sharply.

[0238] In Comparative Example 4, without using the additive of the present invention and using an 8 mm meat grinder for mincing, the re-agglomeration rate was high and a large amount of debris was formed during the drying process. Therefore, it could be confirmed that the CRC performance of the finally prepared superabsorbent polymer was significantly reduced compared with the examples.

[0239] In Comparative Examples 5 and 6 where the additive of the present invention was used but the drying process conditions of the present invention were not satisfied, the drying efficiency decreased, and warping of the dried product occurred. It can be seen from this that the CRC performance of the finally prepared superabsorbent polymer was significantly reduced compared with that of the examples.

[0240] At the same time, due to the moisture content of the base resin, some comparative examples achieved CRC values similar to those of the examples. However, in these cases, due to the re-agglomeration of the hydrogel, it was difficult to control the particle size, and the drying efficiency decreased as the drying amount (feed introduction amount) increased.

Claims

1. A method for preparing a superabsorbent polymer, the method comprising the following steps: Crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer in which at least a part of the acid groups are neutralized in the presence of an internal crosslinking agent and a polymerization initiator to form a hydrogel polymer; Mixing a carboxylic acid additive with the hydrogel polymer and then finely cutting to prepare a mixture containing the finely cut hydrogel polymer; and Drying the mixture while alternately introducing an upward air flow at 120 °C to 210 °C and a downward air flow at 120 °C to 210 °C into the mixture to form a base resin powder, wherein the total introduction time of the downward air flow is longer than the total introduction time of the upward air flow, wherein the carboxylic acid additive is one or more selected from the group consisting of carboxylic acids and their salts represented by the following chemical formula: [Chemical formula 1] In Chemical formula 1, A is a C5 to 21 alkyl group, B1 is -OCO-, -COO- or -COOCH(R1)COO-, B2 is -CH2-, -CH2CH2-, -CH(R2)-, -CH=CH- or -C≡C-, wherein R1 and R2 are each independently a C1 to 4 alkyl group, and n is an integer from 1 to 3, and C is a carboxyl group.

2. The method for preparing a superabsorbent polymer according to claim 1, wherein, The average particle size D50 of the finely cut hydrogel polymer is 0.5 mm to 4 mm.

3. The method for preparing a superabsorbent polymer according to claim 1, wherein, In Chemical Formula 1, A is -C6H 13 , -C 11 H 23 , -C 12 H 25 , -C 17 H 35 or -C 18 H 37 .

4. The method for preparing a superabsorbent polymer according to claim 1, wherein, In Chemical formula 1, B1 is or wherein * is the binding site to an adjacent atom.

5. The method for preparing a superabsorbent polymer according to claim 1, wherein, In Chemical formula 1, B2 is or wherein * is the binding site to an adjacent atom.

6. The method for preparing a superabsorbent polymer according to claim 1, wherein, The carboxylic acid additive is one or more selected from the group consisting of the carboxylic acid represented by Chemical formula 1, its alkali metal salts and its alkaline earth metal salts.

7. The method for preparing a superabsorbent polymer according to claim 1, wherein, The carboxylic acid additive is one of the compounds represented by the following Chemical formulas 1-1 to 1-7:

8. The method for preparing a superabsorbent polymer according to claim 1, wherein, Based on the hydrogel polymer, the mixing amount of the carboxylic acid additive is 100 ppmw to 5,000 ppmw.

9. The method for preparing a superabsorbent polymer according to claim 1, wherein The drying is carried out with the total introduction time of the upward air flow being 1% to 25% of the total drying time.

10. The method for preparing a superabsorbent polymer according to claim 1, wherein, The drying is carried out by a fixed bed type.

11. The method for preparing a superabsorbent polymer according to claim 1, wherein, The finely cutting is carried out in such a way as to push the hydrogel polymer to an outlet having holes with an average diameter of 1 mm to 5 mm.

12. The method for preparing a superabsorbent polymer according to claim 1, wherein, The undried rate is less than 14% by weight, and the undried rate is defined as the mass fraction of the powder that has not been sieved through after being classified with a 4-mesh sieve for more than 10 minutes according to ASTM E11.

13. The method for preparing a superabsorbent polymer according to claim 1, wherein, The moisture content of the base resin powder is less than 8% by weight.

14. The method for preparing a superabsorbent polymer according to claim 1, further comprising the steps of grinding and classifying the base resin powder.

15. The method for preparing a superabsorbent polymer according to claim 1 or claim 14, further comprising the step of heat-treating the base resin powder to perform surface crosslinking on at least a part of the surface of the base resin powder to prepare a superabsorbent polymer.

16. The method for preparing a superabsorbent polymer according to claim 15, wherein, The centrifugal retention capacity (CRC) of the superabsorbent polymer according to the EDANA method WSP 241.2 is 26 g / g or more.

17. The method for preparing a superabsorbent polymer according to claim 15, wherein, The absorbency under pressure (AUP) of the superabsorbent polymer at 0.3 psi according to the EDANA method WSP 242.3 is 24 g / g or more.

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