Manufacturing method of superabsorbent resin

Abstract

The present invention relates to a method for producing a superabsorbent polymer, and more particularly, to a method for producing a superabsorbent polymer with excellent absorption performance, particularly with appropriate gel bed permeability (GBP), by controlling the surface tension of a surface cross-linking solution within a specific range in a surface cross-linking step.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0154686 dated November 17, 2022 and Korean Patent Application No. 10-2023-0152074 dated November 6, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a method for producing a superabsorbent polymer, and more particularly, to a method for producing a superabsorbent polymer with excellent absorption performance, particularly with appropriate gel bed permeability (GBP), by controlling the surface tension of a surface cross-linking solution within a specific range in a surface cross-linking step. [Background technology]

[0003] Super absorbent polymer (SAP) is a synthetic polymer capable of absorbing 500 to 1,000 times its own weight in water, and different developers have given it different names, such as SAM (Super Absorbency Material) or AGM (Absorbent Gel Material). These super absorbent polymers first began to be used in sanitary products, and are now widely used in a variety of applications, including soil water retention agents in gardening, water-stopping materials in civil engineering and construction, seedling sheets, freshness-preserving agents in the food distribution industry, and compresses, in addition to sanitary products such as disposable baby diapers.

[0004] In most cases, such superabsorbent polymers are widely used in the field of sanitary materials such as diapers and sanitary napkins. For these applications, they must exhibit high absorbency against moisture, excellent absorbency under pressure so that absorbed moisture does not escape even when subjected to external pressure, and excellent permeability so that they maintain their shape well even when swollen due to absorption of water.

[0005] In addition, when the superabsorbent polymer is contained in a sanitary product such as a diaper, it is necessary to spread urine and other liquids as widely as possible even under pressure caused by the user's weight. This allows the superabsorbent polymer particles contained in the entire area of ​​the absorbent layer of the sanitary product to be fully utilized, further improving the absorption performance and absorption speed of the sanitary product. Furthermore, this property of spreading under pressure can further improve the rewetting properties of the diaper, which prevents urine and other liquids once absorbed by the superabsorbent polymer from leaking out again, and also improves the leakage prevention properties of the diaper.

[0006] Previously, attempts have been made to improve the properties of urine and other fluids by changing the design of sanitary products such as diapers. For example, attempts have been made to improve the properties of urine and other fluids by introducing an Acquisition Distribution Layer (ADL) into the sanitary products or by utilizing absorption channels. However, such design changes to the sanitary products themselves have not been sufficient to improve the properties of urine and other fluids.

[0007] Meanwhile, in order to improve the physical properties of superabsorbent resins, such as their absorbency, liquid permeability, and absorption rate, various post-treatment processes such as surface cross-linking and foaming are carried out or various additives are used. However, in the process of forming such absorbency, the cross-linking density of the resin decreases, making it difficult to achieve sufficient gel strength.

[0008] Therefore, there is a need for research into the production of highly water-absorbent polymers that can achieve appropriate liquid permeability and gel strength while maintaining the existing absorption properties. Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, the present invention relates to a method for producing a superabsorbent resin, and more specifically, to a method for more productively producing a superabsorbent resin having excellent absorption performance by controlling the surface tension of a surface cross-linking solution containing specific components in the surface cross-linking step. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention provides: forming a hydrogel polymer containing a crosslinked polymer obtained by crosslinking an acrylic acid monomer having at least a partially neutralized acid group in the presence of an internal crosslinking agent; drying, grinding and classifying the hydrogel polymer to form a base resin powder; cross-linking the surface of the base resin powder in the presence of a surface cross-linking solution containing an epoxy-based surface cross-linking agent, a monohydric alcohol-based solvent, and a hydrophobic dispersant, and having a surface tension of 36 mN / m to 50 mN / m; A method for producing a highly water-absorbent resin is provided. [Effects of the Invention]

[0011] According to the method for producing a superabsorbent polymer of the present invention, by controlling the surface tension of a surface cross-linking solution containing a specific component in the surface cross-linking step, a superabsorbent polymer having excellent liquid permeability and an appropriate gel bed permeability (GBP) can be produced more productively. DETAILED DESCRIPTION OF THE INVENTION

[0012] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention.

[0013] The singular includes the plural unless the context clearly dictates otherwise. As used herein, the terms "comprises," "comprises," "has," and the like are intended to specify the presence of embodied features, steps, components, or combinations thereof, but should be understood as not precluding the possible presence or addition of one or more other features, steps, components, or combinations thereof.

[0014] The terms first, second, third, etc. are used to describe various elements, and such terms are used only to distinguish one element from another.

[0015] Since the present invention can be modified in various ways and can have various forms, specific examples are exemplified and described in detail below, but it is not intended to limit the present invention to the specific disclosed forms, and it should be understood that the present invention includes any modifications, equivalents, or alternatives falling within the spirit and technical scope of the present invention.

[0016] The terms "polymer" or "macromolecule" as used herein refer to a state in which a water-soluble ethylenically unsaturated monomer is polymerized, and may encompass any range of water content or particle size. Among these polymers, a polymer having a water content (moisture content) of about 40% by weight or more in a state after polymerization and before drying may be referred to as a hydrogel polymer, and particles obtained by pulverizing and drying such a hydrogel polymer may be referred to as a crosslinked polymer.

[0017] Furthermore, the term "crosslinked polymer" as used herein means a crosslinked polymer of a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups have been neutralized, and the term "base resin powder" means a substance containing such a crosslinked polymer.

[0018] The term "superabsorbent resin" refers to a crosslinked polymer obtained by polymerizing a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups have been neutralized, or a powder-like base resin powder consisting of superabsorbent resin particles obtained by pulverizing the crosslinked polymer, or is used to encompass all of the crosslinked polymers or base resins that have been subjected to additional processes, such as surface crosslinking, pulverization into fine powder, drying, pulverization, classification, etc., to be in a state suitable for commercialization.

[0019] (Method of manufacturing superabsorbent resin) According to one embodiment of the present invention, a method for producing a superabsorbent polymer includes the steps of: forming a hydrogel polymer including a crosslinked polymer by crosslinking an acrylic acid monomer having at least a partially neutralized acid group in the presence of an internal crosslinking agent; drying, grinding and classifying the hydrogel polymer to form a base resin powder; and cross-linking the surface of the base resin powder in the presence of a surface cross-linking solution containing an epoxy-based surface cross-linking agent, a monohydric alcohol-based solvent, and a hydrophobic dispersant, and having a surface tension of 36 mN / m to 50 mN / m.

[0020] In order to improve the basic absorbency of superabsorbent resins, various post-treatment processes such as surface cross-linking and foaming are carried out or various additives are used. However, these processes result in a decrease in the cross-linking density of the resin, making it difficult to achieve sufficient gel strength and also making it difficult to achieve the desired level of excellent liquid permeability.

[0021] Therefore, the present inventors have found that when the surface tension of a surface cross-linking solution containing a specific component is controlled in the surface cross-linking step, a superabsorbent resin having excellent liquid permeability and an improved absorption rate can be produced more productively without the above-mentioned problems, and have completed the present invention.

[0022] In particular, by using a combination of an epoxy-based surface crosslinking agent, a monohydric alcohol-based solvent, and a hydrophobic dispersant in the surface crosslinking solution during the surface crosslinking step, the surface tension of the solution can be adjusted within the aforementioned range, further slowing the diffusion rate of the surface crosslinking solution into the base resin and enabling the surface crosslinking solution to be uniformly applied to the surface of the base resin particles. As a result, the absorption properties of the final superabsorbent resin particles can be improved, particularly the gel bed permeability (GBP).

[0023] Hereinafter, each step of the method for preparing a superabsorbent polymer will be described in more detail according to a specific embodiment of the present invention.

[0024] (Polymerization step) First, a method for preparing a superabsorbent resin according to an embodiment of the present invention includes a step of forming a hydrogel polymer including a crosslinked polymer obtained by crosslinking an acrylic acid-based monomer having at least a partially neutralized acid group in the presence of an internal crosslinking agent.

[0025] The polymerization step is a step of forming a hydrogel polymer by photopolymerizing and / or thermally polymerizing a monomer composition containing an acrylic acid-based monomer having at least a partially neutralized acid group in the presence of an internal crosslinking agent.

[0026] The polymerization step may be carried out by cross-linking polymerization of a monomer composition containing components commonly used in the manufacture of superabsorbent resins in addition to the above-mentioned components.

[0027] First, a monomer mixture containing an acrylic acid-based monomer having at least a partially neutralized acid group is prepared in the presence of an internal crosslinking agent, and the monomer mixture may further contain a polymerization initiator for polymerization.

[0028] The acrylic acid-based monomer may be any monomer commonly used in the production of superabsorbent resins. As a non-limiting example, the acrylic acid-based monomer may be a compound represented by the following Chemical Formula 1:

[0029] [Chemical formula 1] R1-COOM 1

[0030] In the above Chemical Formula 1, R1 is an alkyl group having 2 to 5 carbon atoms and containing an unsaturated bond, M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0031] Preferably, the acrylic acid 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 an acrylic acid monomer is used in this manner, it is advantageous to obtain a superabsorbent resin with improved water absorption. Other examples of the monomer include anionic monomers such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and 2-(meth)acrylamido-2-methylpropanesulfonic acid, and salts thereof; nonionic hydrophilic monomers such as (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, and polyethylene glycol (meth)acrylate; and amino group-containing unsaturated monomers such as (N,N)-dimethylaminoethyl (meth)acrylate and (N,N)-dimethylaminopropyl (meth)acrylamide, and quaternized products thereof.

[0032] The acrylic acid monomer has an acidic group, and at least a portion of the acidic group is partially neutralized using a neutralizing solution. The neutralizing agent contained in the neutralizing solution may be a basic substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide that can neutralize the acidic group.

[0033] In this case, the degree of neutralization of the monomer may be 40 to 95 mol%, or 40 to 80 mol%, or 45 to 75 mol%. The range of the degree of neutralization may vary depending on the final properties, but if the degree of neutralization is too high, the neutralized monomer may precipitate and the polymerization may not proceed smoothly. Conversely, if the degree of neutralization is too low, the water absorption of the polymer may be significantly reduced and the polymer may exhibit properties similar to elastic rubber, which is difficult to handle.

[0034] The term "internal crosslinking agent" used herein is used to distinguish it from a "surface crosslinking agent" for crosslinking the surface of the base resin, and serves to crosslink and polymerize the unsaturated bonds of the acrylic acid-based monomer. The crosslinking in this step is performed without distinguishing between the surface and the interior, and the surface of the superabsorbent resin particles finally produced through the surface crosslinking process of the base resin described below has a structure crosslinked by the surface crosslinking agent, and the interior has a structure crosslinked by the internal crosslinking agent.

[0035] The internal crosslinking agent may be a polyfunctional component, and for example, one or more selected from the group consisting of 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, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerin, and ethylene carbonate may be used. Preferably, polyethylene glycol di(meth)acrylate and propylene glycol di(meth)acrylate can be used.

[0036] The internal crosslinking agent can be used in an amount of 100 ppmw to 10,000 ppmw based on the weight of the acrylic acid-based monomer. When included within this content range, sufficient crosslinking is possible, achieving strength above an appropriate level, and sufficient water retention capacity can be achieved by introducing an appropriate crosslinked structure. Preferably, the internal crosslinking agent is included in an amount of 100 ppmw or more, 200 ppmw or more, 300 ppmw or more, or 600 ppmw or more, and 10,000 ppmw or less, 9,000 ppmw or less, 7,000 ppmw or less, or 200 ppmw to 9,000 ppmw, 300 ppmw to 7,000 ppmw, or 600 ppmw to 5,000 ppmw. If the content of the internal crosslinking agent is too low, crosslinking may not occur sufficiently, making it difficult to achieve strength above an appropriate level. If the content of the internal crosslinking agent is too high, the internal crosslink density may increase, making it difficult to achieve the desired water retention capacity.

[0037] The polymerization initiator may be a thermal polymerization initiator or a photopolymerization initiator depending on the polymerization method, but a thermal polymerization initiator may also be included since a certain amount of heat is generated by UV irradiation and the polymerization reaction, which is an exothermic reaction, also generates a certain amount of heat in the photopolymerization method.

[0038] The photopolymerization initiator may be one or more compounds selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine, and α-aminoketone. Specific examples of acyl phosphine include commercially available lucirin TPO, i.e., 2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide, and diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide. More details about various photopolymerization initiators are disclosed on page 115 of "UV Coatings: Basics, Recent Developments and New Applications" by Reinhold Schwalm (Elsevier, 2007), which can be referenced.

[0039] The thermal polymerization initiator may be one or more compounds selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid. Specific examples of persulfate initiators include sodium persulfate (NaSO), potassium persulfate (KSO), and ammonium persulfate ((NHSO). Furthermore, examples of azo initiators include 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and 4,4-azobis-(4-cyanovaleric acid). Examples of thermal polymerization initiators include methyl methyl acrylate, ...

[0040] The polymerization initiator may be used in an amount of 10 ppmw to 10,000 ppmw based on the weight of the acrylic acid-based monomer. Preferably, the polymerization initiator may be present in an amount of 10 ppmw or more, 30 ppmw or more, or 50 ppmw or more, and 10,000 ppmw or less, 5,000 ppmw or less, or 3,000 ppmw or less, such as 30 ppmw to 5,000 ppmw, 50 ppmw to 3,000 ppmw, or 80 ppmw to 2,500 ppmw. An excessively low concentration of the polymerization initiator is undesirable because it can slow the polymerization rate and result in large amounts of residual monomer being extracted into the final product. Conversely, an excessively high concentration of the polymerization initiator is undesirable because it can shorten the polymer chains forming the network, increase the content of water-soluble components, and reduce the physical properties of the resin, such as reduced absorbency under pressure. When both a photopolymerization initiator and a thermal polymerization initiator are used, the content of the polymerization initiator refers to the combined content.

[0041] In addition, the monomer composition may further contain additives such as a foaming agent, a surfactant, a thickener, a plasticizer, a storage stabilizer, and an antioxidant, if necessary.

[0042] The foaming agent increases the surface area by forming pores in the hydrogel polymer through foaming during polymerization. The foaming agent may be a carbonate, such as sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, or magnesium carbonate.

[0043] The blowing agent is preferably used in an amount of 1500 ppmw or less based on the weight of the water-soluble ethylenically unsaturated monomer. If the amount of blowing agent used exceeds 1500 ppmw, the pores become too large, reducing the gel strength of the superabsorbent resin and decreasing the density, which may cause problems in distribution and storage. The blowing agent is preferably used in an amount of 500 ppmw or more or 1000 ppmw or more based on the weight of the water-soluble ethylenically unsaturated monomer.

[0044] The surfactant induces uniform dispersion of the foaming agent, enabling uniform foaming during foaming and preventing a decrease in gel strength or density. An anionic surfactant is preferably used as the surfactant. Specifically, the surfactant is SO3 - The compound contains an anion and can be represented by the following chemical formula 2.

[0045] [Chemical formula 2] R-SO3Na

[0046] In the above Chemical Formula 2, R is alkyl having 8 to 16 carbon atoms.

[0047] The surfactant is preferably used in an amount of 300 ppmw or less based on the weight of the acrylic acid-based monomer. If the amount of the surfactant used exceeds 300 ppmw, the surfactant content in the superabsorbent resin increases, which is undesirable. The surfactant is preferably used in an amount of 100 ppmw or more or 150 ppmw or more based on the weight of the water-soluble ethylenically unsaturated monomer.

[0048] The monomer composition may be prepared in the form of a solution in which raw materials such as the above-mentioned monomer, internal crosslinking agent, foaming agent, initiator, etc. are dissolved in a solvent.

[0049] In this case, any solvent can be used without limitation as long as it can dissolve the raw materials described above. For example, the solvent may be water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl 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.

[0050] The step of forming the hydrogel polymer by polymerizing the monomer composition may be carried out by a conventional polymerization method, and the process is not particularly limited. As a non-limiting example, the step may be carried out in a reactor equipped with a movable conveyor belt.

[0051] Specifically, when the monomer composition is photopolymerized in a reactor equipped with a movable conveyor belt, a sheet-like hydrogel polymer can be obtained. The thickness of the sheet can vary depending on the concentration and injection rate of the monomer composition, but is preferably adjusted to a thickness of 0.5 to 10 cm to ensure uniform polymerization throughout the sheet while maintaining a stable production rate.

[0052] The water content of the hydrogel polymer obtained by this method may typically be 40% to 80% by weight. Throughout this specification, the term "water content" refers to the water content relative to the total weight of the hydrogel polymer, and refers to the weight of the hydrogel polymer minus the weight of the polymer in a dry state. Specifically, it is defined as the value calculated by measuring the weight loss due to evaporation of water in the polymer during the drying process by increasing the temperature of the polymer using infrared heating. The drying conditions are as follows: the temperature is increased from room temperature to 180°C and then maintained at 180°C, and the total drying time is set to 20 minutes, including a 5-minute temperature increase step, and the water content is measured.

[0053] (Drying, grinding and classification steps) Next, the method for producing a superabsorbent resin according to an embodiment of the present invention further includes the steps of drying, pulverizing, and classifying the produced hydrogel polymer to form a base resin powder.

[0054] Specifically, the obtained hydrogel polymer is dried, and if necessary, the hydrogel polymer may be coarsely pulverized (chopped) before drying to increase the efficiency of the drying step.

[0055] In this case, the crusher used is not limited in configuration, and specifically may include any one selected from the group of crushing equipment consisting of a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter, but is not limited to the above examples.

[0056] In this case, the coarse grinding step can grind the hydrogel polymer to a particle size of 2 mm to 10 mm. Grinding to a particle size of less than 2 mm is technically difficult due to the high water content of the hydrogel polymer, and the ground particles may aggregate together. On the other hand, grinding to a particle size of more than 10 mm may result in little effect in increasing the efficiency of the subsequent drying step.

[0057] The hydrogel polymer immediately after polymerization, either coarsely pulverized as described above or not subjected to the coarse pulverization step, is dried. The drying temperature in the drying step may be 150 to 250°C. If the drying temperature is less than 150°C, the drying time may be too long, which may result in a deterioration in the physical properties of the final superabsorbent polymer. If the drying temperature is more than 250°C, only the polymer surface may be excessively dried, which may result in the generation of fine powder in the subsequent pulverization step, which may result in a deterioration in the physical properties of the final superabsorbent polymer. Therefore, the drying may be preferably performed at a temperature of 150 to 200°C, more preferably at a temperature of 150 to 190°C.

[0058] The drying time may be 20 to 90 minutes, taking into consideration process efficiency, but is not limited thereto.

[0059] Meanwhile, the drying step may be carried out in multiple steps within the above-mentioned temperature range.

[0060] The drying method for the drying step can be selected and used without any limitations as long as it is a method commonly used in the drying process of hydrogel polymers. Specifically, the drying step may be performed by a method such as supplying hot air, irradiating with infrared rays, irradiating with ultrashort waves, or irradiating with ultraviolet rays. In the case of supplying hot air, the drying step may be performed by using an oven whose air direction can be changed up and down.

[0061] After such a drying step, the polymer may have a moisture content of from about 0.1 to about 10% by weight.

[0062] Next, the dried polymer obtained through such a drying step is subjected to a step of pulverizing.

[0063] The polymer powder obtained after the pulverization step may have a particle size of 150 to 850 μm. Specific examples of the pulverizer used to pulverize to such a particle size include, but are not limited to, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, or a jog mill.

[0064] In order to control the physical properties of the superabsorbent resin powder produced as a final product after the pulverization step, the polymer powder obtained after pulverization may be classified according to particle size. Preferably, polymers having particle sizes of 150 μm to 850 μm are classified, and only the polymer powders having such particle sizes are subjected to a surface cross-linking reaction step to produce a final product. More specifically, the classified base resin powder may have a particle size of 150 μm to 850 μm, and may contain 50 wt % or more particles having a particle size of 300 μm to 600 μm, and may contain less than 3 wt % fine powder having a particle size of less than 150 μm.

[0065] Meanwhile, fines having a particle size of less than 150 μm may be generated during the polymerization, drying, and pulverization steps. The fines having a particle size of less than 150 μm classified in the classification step can be reassembled to normal particles having a particle size of 150 μm or more and reused as the base resin powder.

[0066] The reassembly process of the fine powder may be carried out by a method commonly used in the art, for example, by agglomerating the fine powder in a wet state. Specifically, the classified fine powder may be mixed with water to agglomerate it, and then the water may be dried in a re-drying process to produce normal particles. In the reassembly process, an additive such as a water-soluble polymer may be optionally used to improve the agglomeration strength of the particles. The surface cross-linking solution is contained in an amount of 1 to 10 parts by weight based on 100 parts by weight of the base resin powder.

[0067] (Surface cross-linking step) Meanwhile, after the base resin powder is produced through the above-mentioned classification step, the method may further include a step of surface cross-linking the base resin powder while heat treating it in the presence of a surface cross-linking agent.

[0068] According to one embodiment of the present invention, the surface cross-linking step is carried out using a surface cross-linking solution whose surface tension is controlled within a specific range. Specifically, when the surface tension of the surface cross-linking solution is 36 mN / m to 50 mN / m, a superabsorbent resin having excellent liquid permeability can be produced.

[0069] More specifically, the surface cross-linking solution is prepared by combining an epoxy surface cross-linking agent, a monohydric alcohol solvent, and a hydrophobic dispersant, thereby adjusting the surface tension of the solution to the aforementioned range. This is a relatively low surface tension (36 mN / m to 50 mN / m) compared to the surface tension of commonly used surface cross-linking solutions, which further slows the diffusion rate of the surface cross-linking solution into the base resin. As a result, the surface cross-linking solution can be uniformly applied to the surface of the base resin particles, improving the absorption properties of the final superabsorbent resin particles, and in particular, further improving the gel bed permeability (GBP).

[0070] If the surface tension of the surface cross-linking solution is less than 36 mN / m, the surface cross-linking solution containing the epoxy-based surface cross-linking agent does not penetrate into the base resin powder, making it difficult to achieve the desired level of uniform surface cross-linking. If the surface tension is greater than 50 mN / m, the surface cross-linking solution may penetrate into the base resin powder excessively, resulting in an increased surface cross-linking density and reduced water retention. Preferably, the surface tension of the surface cross-linking solution may be 36 mN / m or more, 37 mN / m or more, 38 mN / m or more, or 49 mN / m or less, 48.5 mN / m or less. Within these ranges, a superabsorbent resin with excellent absorption properties can be produced without the aforementioned problems. The surface tension of the surface cross-linking solution may be adjusted depending on the type and content of the specific components of the solution, namely, the epoxy-based cross-linking agent, the monohydric alcohol-based solvent, and the hydrophobic dispersant, but is not limited thereto.

[0071] Meanwhile, the surface tension can be measured by the Wilhelmy plate method at room temperature of 23±2° C. A specific method for measuring the surface tension will be described in more detail in the experimental examples below.

[0072] The surface cross-linking step induces a cross-linking reaction on the surface of the base resin powder in the presence of a surface cross-linking agent, and the unsaturated bonds of the acrylic acid monomer that remain on the surface without being cross-linked are cross-linked by the surface cross-linking agent, thereby forming a superabsorbent resin with a high surface cross-linking density.

[0073] Specifically, a surface cross-linked layer can be formed by a heat treatment process in the presence of a surface cross-linking agent. The heat treatment process increases the surface cross-linking density, i.e., the external cross-linking density, but does not change the internal cross-linking density. Therefore, the produced superabsorbent resin having a surface cross-linked layer has a structure in which the external cross-linking density is higher than the internal cross-linking density.

[0074] The surface cross-linking solution used in the surface cross-linking step contains an epoxy-based surface cross-linking agent.

[0075] The epoxy-based crosslinking agent can easily undergo a crosslinking reaction with the ester groups present on the surface of the base resin powder at a relatively low temperature (about 100°C to 140°C), thereby further improving pressure properties such as 0.9 AUL and GBP. In addition, it is used in combination with a monohydric alcohol-based solvent and a hydrophobic dispersant in the surface crosslinking solution, allowing the surface tension to be controlled within the desired range.

[0076] The epoxy crosslinking agent is a compound containing at least one epoxy group, preferably two or more epoxy groups, in the molecule, and may be a compound containing an ether structure in the molecule together with the epoxy group. As the epoxy crosslinking agent, for example, one or more selected from the group consisting of ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, glycerol triglycidyl ether, polyethylene glycol diglycidyl ether, and propylene glycol diglycidyl ether can be used, and preferably ethylene glycol diglycidyl ether or polyethylene glycol diglycidyl ether can be used.

[0077] On the other hand, even if the crosslinking agent is a compound containing an epoxy group, in the case of a compound such as an epoxy alkyl phosphate, the concentration of the phosphate may adversely affect the absorption properties of the absorbent when forming the crosslink density of the polymer, and compared to the epoxy-based crosslinking agent of the present invention, the surface tension will deviate from the desired level (36 mN / m to 50 mN / m) under the same water retention property standard.

[0078] Preferably, the epoxy-based crosslinker is not an epoxy alkyl phosphate compound.

[0079] The epoxy-based crosslinking agent can be used in an amount of 0.01 to 10 parts by weight per 100 parts by weight of the base resin powder. Preferably, the amount can be 0.05 to 0.1 parts by weight or 8 to 5 to 1 part by weight. By adjusting the amount of the epoxy-based crosslinking agent within the above range, a superabsorbent resin can be produced that exhibits excellent properties such as excellent absorption capacity and liquid permeability.

[0080] The surface cross-linking solution used in the surface cross-linking step contains a monohydric alcohol solvent as a solvent.

[0081] The monohydric alcohol-based solvent relatively reduces the surface tension of the surface cross-linking solution, slowing down the diffusion of the surface cross-linking solution and enabling the surface cross-linking solution to be more uniformly applied to the surface of the base resin powder. To this end, the monohydric alcohol-based solvent can effectively improve the gel bed permeability (GBP) among the pressure properties, and can control the surface tension within a desired range together with the epoxy-based cross-linking agent and hydrophobic dispersant in the surface cross-linking solution.

[0082] Examples of the monohydric alcohol solvent include one or more selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, and butanol, and preferably, ethanol and isopropanol can be used.

[0083] The surface cross-linking solution may further contain an additional solvent other than the monohydric alcohol-based solvent, for example, may further contain water and / or a hydrophilic organic solvent, and preferably, water may be used together with the monohydric alcohol-based solvent.

[0084] The monohydric alcohol solvent may be included in an amount of 1 to 5 parts by weight based on the total content of the surface cross-linking solution. Preferably, the monohydric alcohol solvent can be used in an amount of 1.5 parts by weight or more, 2 parts by weight or more, or 4.5 parts by weight or less, 4 parts by weight or less, or 3 parts by weight or less. By adjusting the content range of the monohydric alcohol solvent within the above-mentioned range, the surface tension of the surface cross-linking solution can be adjusted to a desired range, and a superabsorbent resin exhibiting various physical properties such as excellent absorption performance and liquid permeability can be produced.

[0085] The surface cross-linking solution used in the surface cross-linking step contains a hydrophobic dispersant.

[0086] The hydrophobic dispersant relatively reduces the surface tension of the surface cross-linking solution, slowing down the diffusion of the surface cross-linking solution and enabling the surface cross-linking solution to be more uniformly applied to the surface of the base resin powder. To this end, the hydrophobic dispersant can effectively improve the gel bed permeability (GBP) among the pressure properties, and can control the surface tension within a desired range together with the epoxy-based cross-linking agent and the monohydric alcohol-based solvent in the surface cross-linking solution.

[0087] Examples of the hydrophobic dispersant include sodium dodecyl sulfate, poly(dimethyldiallylammonium chloride), water-dispersible silica, calcium stearate, di(C 12-20 ) One or more selected from the group consisting of alkyldimethylammonium salts and polyethylene glycols can be used, and preferably, sodium dodecyl sulfate and water-dispersed silica can be used.

[0088] The hydrophobic dispersant may be included in an amount of 0.0001 to 1 part by weight based on the total content of the surface cross-linking solution. Preferably, the amount is 0.0002 to 0.003 parts by weight, or 0.5 to 0.45 parts by weight. By adjusting the content range of the hydrophobic dispersant within the above-mentioned range, the surface tension of the surface cross-linking solution can be adjusted to the desired range, thereby producing a superabsorbent resin exhibiting various properties such as excellent absorption performance and liquid permeability. For reference, when water-dispersed silica is used as the hydrophobic dispersant, the content is based on solid silica.

[0089] The surface cross-linking solution may further include an additive in addition to the above-mentioned components. Specifically, the additive included in the surface cross-linking solution may increase the mixing efficiency of the base resin particles and the surface cross-linking solution, thereby allowing the surface cross-linking solution to be applied more uniformly and improving the GBP properties among the pressure properties.

[0090] Specific examples of the additional additive include one or more selected from the group consisting of methylene glycol, ethylene glycol, propylene glycol, and butylene glycol, and more preferably, propylene glycol can be used.

[0091] The additional additive may be included in an amount of 0.1 to 5 parts by weight, preferably 0.5 to 3 parts by weight, 1 to 3 parts by weight, or 1.5 to 2.5 parts by weight, based on the total content of the surface cross-linking solution. When the additional additive is included in the amount within the above content range, it can improve the coatability of the surface cross-linking solution and thereby further improve the GBP properties among the pressure properties.

[0092] On the other hand, the surface crosslinking agent is added to a base resin powder in the state of a surface crosslinking agent composition containing the same, and a method for adding such a surface crosslinking agent composition is not particularly limited in its constitution. For example, a method in which the surface crosslinking agent composition and the base resin powder are placed in a reaction tank and mixed, a method in which water of the surface crosslinking agent composition is sprayed onto the base resin powder, a method in which the base resin powder and the surface crosslinking agent composition are continuously supplied to a continuously operated mixer and mixed, etc. can be used.

[0093] The surface cross-linking step can be performed by heat treatment for 30 minutes or more at a temperature of 110°C to 200°C, or 110°C to 150°C. More specifically, the surface cross-linking reaction can be performed by heat treatment at the maximum reaction temperature for 30 to 80 minutes, or 40 to 70 minutes, using the aforementioned temperature as the maximum reaction temperature.

[0094] By satisfying such surface cross-linking process conditions (particularly, temperature rising conditions and reaction conditions at the maximum reaction temperature), it is possible to produce a highly water-absorbent resin that appropriately satisfies physical properties such as superior pressurized liquid permeability.

[0095] The temperature raising means for the surface cross-linking reaction is not particularly limited. Heating can be performed by supplying a heat medium or directly supplying a heat source. In this case, the type of heat medium that can be used may be a heated fluid such as steam, hot air, or hot oil, but is not limited thereto, and the temperature of the heat medium to be supplied can be appropriately selected taking into consideration the means of the heat medium, the rate of temperature rise, and the target temperature of temperature rise. On the other hand, examples of a heat source that is directly supplied include heating through electricity and heating through gas, but are not limited to the above-mentioned examples.

[0096] Meanwhile, in the method for producing a superabsorbent resin according to an embodiment of the present invention, aluminum salts such as aluminum sulfate and various other polyvalent metal salts may be further used during surface cross-linking to further improve liquid permeability, etc. Such polyvalent metal salts may be contained on the surface cross-linked layer of the final superabsorbent resin.

[0097] (Super absorbent resin) According to one embodiment of the present invention, there is provided a superabsorbent polymer manufactured by the above-described method for manufacturing a superabsorbent polymer. The superabsorbent polymer manufactured by the above-described method for manufacturing a superabsorbent polymer of one embodiment has an appropriate crosslinking density, and can achieve excellent absorption performance and liquid permeability, particularly excellent gel bed permeability (GBP).

[0098] The superabsorbent polymer may have a gel bed permeability (GBP) of 50 darcy or more, more preferably 53 darcy or more, 55 darcy or more, or 110 darcy or less, 105 darcy or less. A specific method for measuring the gel bed permeability is the same as the method described in Korean Patent Application No. 10-2014-7018005, and will be described in more detail in the experimental examples below.

[0099] The functions and effects of the present invention will be described in more detail below through specific examples of the present invention, but these examples are presented only as examples of the present invention and do not define the scope of the invention.

[0100] [Example] Example 1 The superabsorbent resin was produced using a continuous production system consisting of a polymerization process, a hydrogel crushing process, a drying process, a crushing process, a classification process, a surface cross-linking process, a cooling process, a classification process, and a transport process connecting the individual processes.

[0101] A monomer solution was prepared by mixing 100 parts by weight of acrylic acid with 0.43 parts by weight of polyethylene glycol diacrylate (weight average molecular weight: ~500 g / mol) as an internal crosslinker and 0.01 parts by weight of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator. Next, while continuously feeding the monomer solution to a metering pump, 175 parts by weight of a 31.5 wt% aqueous sodium hydroxide solution was continuously line-mixed to prepare a monomer aqueous solution. A monomer mixture was also prepared by continuously line-mixing 1.5 parts by weight of a 0.3 wt% aqueous sodium persulfate solution, 0.5 parts by weight of an aqueous solution containing 0.1 wt% sodium bicarbonate as a foaming agent, and 0.25 parts by weight of an aqueous solution containing 0.05 wt% calcium stearate as a surfactant.

[0102] Through this transfer, the aqueous monomer solution was introduced into a polymerization reactor consisting of a moving conveyor belt, and then irradiated with ultraviolet light through a UV irradiation device (irradiation dose: 2 mW / cm 2 ) and UV polymerization was carried out for 2 minutes to produce a hydrogel polymer.

[0103] The hydrogel was cut into pieces with an average size of about 300 mm or less, and then put into a crusher (equipped with a perforated plate containing a plurality of holes with a diameter of 10 mm) and crushed.

[0104] Next, the crushed hydrogel was dried in a dryer with an adjustable airflow direction, and the hydrogel was uniformly dried by passing hot air at 180°C so that the moisture content of the dried powder was about 2% or less.

[0105] The dried resin was crushed in a crusher and then classified to obtain a base resin having a size of 150 to 850 μm.

[0106] Thereafter, 10.28 g of a surface crosslinking solution (containing 6 g of water, 2 g of isopropanol, 0.15 g of ethylene glycol diglycidyl ether as a surface crosslinking agent, 0.07 g of water-dispersed silica (STO) as a hydrophobic dispersant, 2 g of propyl glycol and 0.06 g of silicon dioxide (Aerosil 200) as other additives) was sprayed onto 100 parts by weight of the prepared base resin powder, and the mixture was stirred at room temperature to mix so that the surface crosslinking solution was evenly distributed on the base resin powder.

[0107] Next, the base resin powder mixed with the surface cross-linking solution was placed in a surface cross-linking reactor to carry out a surface cross-linking reaction.

[0108] In this surface cross-linking reactor, the base resin powder was further reacted at 140°C for 40 minutes. After surface cross-linking, 0.08 parts by weight of silicon dioxide (Aerosil 200) was added to 40 parts by weight of the resin powder and dried, after which a sample of the final superabsorbent resin was taken. After the surface cross-linking process, the superabsorbent resin of Example 1 was produced with a particle size of 150µm to 850µm by sieving using a standard ASTM sieve.

[0109] (Examples 2 to 9 and Comparative Examples 1 to 4) A superabsorbent resin was prepared in the same manner as in Example 1, except that the components and contents used in the surface cross-linking step were as shown in Table 1 below.

[0110] [Table 1]

[0111] [Experimental Example 1. Evaluation of the physical properties of the surface cross-linking solution] The physical properties of the surface cross-linking solutions used in the above Examples and Comparative Examples were evaluated by the following methods, and the results are shown in Table 2.

[0112] Unless otherwise stated, all of the following physical property evaluations were carried out at room temperature (25±1°C).

[0113] (1)Surface tension (mN / m) The surface tension of the surface crosslinking solutions used in Examples and Comparative Examples was measured by the "plate method (the Wilhelmy plate method)," which is a method for measuring static surface tension. Specifically, the surface tension means the maximum tension value until the solution to be measured hits the surface of the plate tip, and was measured using a Process Tensiometer (KRUSS), and the results are shown in Table 1.

[0114] (2) Evaluation of water absorption of surface cross-linking solution (absorption rate, min) In the examples and comparative examples, the degree of absorption of the surface cross-linking solution into the base resin powder before surface cross-linking in the production step was measured.

[0115] Specifically, 2 g of base resin powder was added to a beaker containing 50 g of the surface cross-linking solution, and the time required for the resin to become completely saturated was measured. The results are shown in Table 1.

[0116] For reference, the physical property refers to the degree of application of the base resin surface cross-linking solution, and the longer the absorption, the greater the application property on the base resin surface, which indirectly indicates excellent pressure resistance.

[0117] [Experimental Example 2. Evaluation of the physical properties of superabsorbent resin] The properties of the superabsorbent resins produced in the above Examples and Comparative Examples were evaluated by the following methods, and the results are shown in Table 2.

[0118] The evaluation of the following physical properties, except for the absorption rate (Vortex) evaluation, was carried out at room temperature (23±1°C) and relative humidity (45±1%), and saline or salt water refers to a 0.9 wt% aqueous sodium chloride (NaCl) solution.

[0119] (1) Centrifuge Retention Capacity (CRC, Centrifuge Retention Capacity, g / g) Samples having particle sizes of 150 to 850 μm were taken from the superabsorbent resin powders prepared in the Examples and Comparative Examples, and the centrifuge retention capacity (CRC) was measured based on the no-load absorption capacity according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.2.

[0120] Specifically, the sample W0 (g) (approximately 0.2 g) was evenly placed in a nonwoven fabric envelope, sealed, and then immersed in physiological saline (0.9 wt%) at room temperature. After 30 minutes, the envelope was centrifuged at 250 g for 3 minutes to remove water, and the mass of the envelope W2 (g) was measured. The same procedure was repeated without the sample, and the mass W1 (g) was then measured. The CRC (g / g) was calculated using the obtained masses according to the following formula 1.

[0121] [Formula 1] CRC(g / g)={[W2(g)-W1(g)] / W0(g)}-1

[0122] (2) Absorbency Under Pressure (AUP, g / g) The absorbency of each superabsorbent resin under pressure of 0.9 psi was measured using the EDANA method NWSP 242.0.R2. The resin fraction used in the CRC measurement was used for the absorbency measurement under pressure.

[0123] Specifically, a 400-mesh stainless steel iron net was attached to the bottom of a plastic cylinder with an inner diameter of 25 mm. Under conditions of room temperature (23±1°C) and relative humidity (45±1%), water-absorbent resin W0 (g) (0.16 g) was evenly spread on the iron net, and a piston capable of uniformly applying a load of 0.9 psi was placed on top of it. The piston had an outer diameter slightly smaller than 25 mm, and there was no gap between it and the inner wall of the cylinder, ensuring unhindered up-and-down movement. At this point, the weight of the device, W3 (g), was measured.

[0124] A glass filter with a diameter of 90 mm and a thickness of 5 mm was placed inside a 150 mm diameter petroleum dish, and physiological saline solution composed of 0.9 wt% sodium chloride was poured so that it was flush with the top surface of the glass filter. A sheet of filter paper with a diameter of 90 mm was placed on top of it. The measuring device was placed on top of the filter paper and allowed to absorb the liquid under load for one hour. After one hour, the measuring device was lifted and its weight W4 (g) was measured.

[0125] Using the obtained masses, the absorbency under pressure (g / g) was calculated according to the following formula 2.

[0126] [Formula 2] AUP(g / g) = [W4(g) - W3(g)] / W0(g)

[0127] (3) Gel Bed Permeability (GBP, darcy) The free swelling gel bed permeability of the superabsorbent resins prepared in the examples and comparative examples to physiological saline was measured, and the results are shown in Table 2 below.

[0128] The gel bed permeability was measured according to the method (using the same device) described in Korean Patent Application No. 10-2014-7018005, using the following Equation 3:

[0129] [Formula 3] K = [Q × H × Mu] / [A × Rho × P]

[0130] In the above formula 3, K is the transmittance (cm 2 ) and Q is the flow rate (g / velocity) H is the height of the sample (cm), Mu is the liquid viscosity (poise) (approximately 1 cps in the test solution used in the test), A is the cross-sectional area for liquid flow (cm 2 ) and Rho is the liquid density (g / cm 3 ) (for the test solution used in the relevant test), and P is the hydrostatic pressure (dynes / cm 2 ) (usually about 3,923 dynes / cm 2 )

[0131] The hydrostatic pressure is calculated using the following formula 3-1.

[0132] [Formula 3-1] P=Rho×g×h

[0133] In the above formula 3-1, Rho is the liquid density (g / cm 3 ) and g is the acceleration of gravity, typically 981 cm / sec 2 and h is the fluid height (e.g., 7.8 cm for the permeability tests described herein).

[0134] (4) Absorption speed (Vortex time, sec) For the superabsorbent resins produced in the Examples and Comparative Examples, 2±0.0001 g of superabsorbent resin (based on unclassified resin) was added to 50 mL of physiological saline (24.4±0.2°C), stirred at 600 rpm, and the time until the vortex disappeared was measured in seconds.

[0135] [Table 2]

[0136] As can be seen from the data in Table 2, the superabsorbent resins that were subjected to a surface cross-linking step using the surface cross-linking solution of the present invention exhibited excellent absorption properties. In particular, the examples exhibited excellent absorption speed and significantly improved gel bed permeability.

Claims

1. forming a hydrogel polymer containing a crosslinked polymer obtained by crosslinking an acrylic acid monomer having at least a partially neutralized acid group in the presence of an internal crosslinking agent; drying, grinding and classifying the hydrogel polymer to form a base resin powder; cross-linking the surface of the base resin powder in the presence of a surface cross-linking solution containing an epoxy-based surface cross-linking agent, a monohydric alcohol-based solvent, and a hydrophobic dispersant, and having a surface tension of 36 mN / m to 50 mN / m; A method for producing superabsorbent resin.

2. The monohydric alcohol solvent is at least one selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, and butanol. A method for producing the highly water-absorbent resin according to claim 1.

3. The monohydric alcohol solvent is contained in an amount of 1 to 5 parts by weight based on the total content of the surface crosslinking solution. A method for producing the highly water-absorbent resin according to claim 1.

4. The hydrophobic dispersant may be sodium dodecyl sulfate, poly(dimethyldiallylammonium chloride), water-dispersible silica, calcium stearate, di(C １２－２０ ) one or more selected from the group consisting of alkyldimethylammonium salts and polyethylene glycols; A method for producing the highly water-absorbent resin according to claim 1.

5. The hydrophobic dispersant is contained in an amount of 0.0001 to 1 part by weight based on the total content of the surface cross-linking solution. A method for producing the highly water-absorbent resin according to claim 1.

6. the epoxy-based surface cross-linking agent is one or more selected from the group consisting of ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, glycerol triglycidyl ether, polyethylene glycol diglycidyl ether, and propylene glycol diglycidyl ether; A method for producing the highly water-absorbent resin according to claim 1.

7. The epoxy-based crosslinking agent is included in an amount of 0.01 to 10 parts by weight based on the total content of the base resin powder. A method for producing the highly water-absorbent resin according to claim 1.

8. The surface cross-linking solution further comprises one or more additional additives selected from the group consisting of methylene glycol, ethylene glycol, propylene glycol, and butylene glycol; A method for producing the highly water-absorbent resin according to claim 1.

9. The additional additive is included in an amount of 0.1 to 5 parts by weight based on the total content of the surface cross-linking solution. A method for producing the highly water-absorbent resin according to claim 8.

10. The surface cross-linking solution is contained in an amount of 1 to 10 parts by weight based on 100 parts by weight of the base resin powder. A method for producing the highly water-absorbent resin according to claim 1.

11. The superabsorbent resin has a gel bed permeability (GBP) of 50 darcy or more. A method for producing the highly water-absorbent resin according to claim 1.

Images