Method for preparing superabsorbent polymer

Abstract

The present disclosure relates to a method for preparing a superabsorbent polymer, and more particularly, to a method for preparing a superabsorbent polymer by controlling the surface tension of a surface crosslinking solution within a specific range in a surface crosslinking step, according to the present invention, a superabsorbent polymer having excellent absorption properties, particularly optimal gel bed permeability (GBP), can be more efficiently prepared.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of Korean Patent Application Nos. 10 - 2022 - 0154686, filed on November 17, 2022, and 10 - 2023 - 0152074, filed on November 6, 2023, with the Korean Intellectual Property Office, the entire disclosures of which are incorporated herein by reference. Technical field

[0003] The present disclosure relates to a method for preparing a superabsorbent polymer, and more particularly, to a method for preparing a superabsorbent polymer, which can more effectively prepare a superabsorbent polymer having excellent absorption performance, particularly optimal gel bed permeability (GBP), by controlling the surface tension of a surface cross - linking solution within a specific range during a surface cross - linking step. Background art

[0004] A superabsorbent polymer (SAP) is a synthetic polymer material capable of absorbing 500 to 1000 times its own weight of water, and is also differently named as a superabsorbent material (SAM), an absorbent gel material (AGM), etc. according to the developing company. The superabsorbent polymer began to be commercialized as a hygiene product, and currently, it is widely used as a water - retaining material for soil, a water - stopping material for civil engineering and construction, a seedling - raising sheet, a fresh preservative in the food circulation field, a hot compress material, etc., as well as hygiene products such as baby diapers.

[0005] In most cases, such superabsorbent polymers are widely used in fields such as diapers or sanitary pads and other hygiene products, and for this purpose, it should exhibit a high water - absorption capacity, exhibit excellent absorbency under pressure so that the absorbed water does not escape even under external pressure, and appropriately maintain its shape even in a swollen state, thereby exhibiting excellent permeability.

[0006] In addition, when included in a hygiene product such as a diaper, the superabsorbent polymer should spread urine, etc. as widely as possible even under the pressure of the user's body weight. Therefore, the superabsorbent polymer particles included in the total area of the absorption layer of the hygiene product can be fully utilized to further improve the absorption performance and absorption speed of the hygiene product. In addition, due to this diffusion performance under pressure, it is possible to prevent urine, etc. from leaking out again once absorbed in the superabsorbent polymer, thereby further improving the re - wettability of the diaper while improving the leak - proof performance of the diaper.

[0007] Previously, attempts have been made to improve the spreading performance of urine and the like by changing the design of hygiene products such as diapers. For example, attempts have been made to improve the spreading characteristics of urine and the like by introducing an ADL (acquisition and distribution layer) into the hygiene product or applying an absorption channel. However, it is not sufficient to improve the spreading performance by changing the design of the hygiene product itself.

[0008] Meanwhile, various post-treatment methods such as surface crosslinking and foaming are carried out, or various additives are used to improve the absorbency or properties such as liquid permeability and absorption rate of the superabsorbent polymer. However, in such methods, the crosslinking density of the polymer is reduced, and thus sufficient gel strength cannot be achieved.

[0009] Therefore, it is necessary to study the preparation of a superabsorbent polymer that can achieve optimal liquid permeability and gel strength while maintaining excellent existing absorption performance. Summary of the Invention

[0010] [Technical Problem]

[0011] Therefore, the present disclosure relates to a method for preparing a superabsorbent polymer. More specifically, the object of the present invention is to provide a method for preparing a superabsorbent polymer, which can more effectively prepare a superabsorbent polymer with excellent absorption performance by controlling the surface tension of a surface crosslinking solution containing specific components in the surface crosslinking step.

[0012] [Technical Solution]

[0013] To achieve this object, a method for preparing a superabsorbent polymer is provided, the method comprising the following steps:

[0014] Forming a hydrogel polymer containing a crosslinked polymer, the crosslinked polymer being obtained by crosslinking polymerization of an acrylic monomer having at least partially neutralized acid groups in the presence of an internal crosslinking agent;

[0015] Drying, grinding and classifying the hydrogel polymer to form a base resin powder; and

[0016] Subjecting the surface of the base resin powder to surface crosslinking in the presence of a surface crosslinking solution, the surface crosslinking solution containing an epoxy-based surface crosslinking agent, a monohydric alcohol solvent and a hydrophobic dispersant, and having a surface tension of 36 mN / m to 50 mN / m.

[0017] [Technical Effect]

[0018] According to the method for preparing a superabsorbent polymer of the present invention, by controlling the surface tension of a surface crosslinking solution containing specific components in the surface crosslinking step, a superabsorbent polymer having excellent liquid permeability and optimal gel bed permeability (GBP) can be prepared more effectively. Detailed Description

[0019] The terms used herein are merely for explaining specific embodiments and are not intended to limit the present invention.

[0020] Singular expressions include their plural expressions unless the context clearly indicates otherwise. Throughout the specification, terms such as "comprising", "equipped with", or "having" are intended to indicate the presence of the features, quantities, steps, structural elements, or combinations thereof practiced, 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.

[0021] The terms "first", "second", "third", etc. are used to explain various structural elements, and they are only used to distinguish one structural element from other structural elements.

[0022] Although the present invention can be variously modified and the present invention can have various forms, specific examples will be described and explained in detail below. However, it should be understood that they are not intended to limit the present invention to the specific disclosure, and the present invention includes all its modifications, equivalents, or alternative ways without departing from the spirit and technical scope of the present invention.

[0023] As used herein, the term "polymer" means a polymerized state of water-soluble ethylenically unsaturated monomers and may include polymers in all water content ranges or all particle size ranges. Among polymers, polymers that are not dried and have a water content of about 40% by weight or more may be referred to as hydrogel polymers, and ground and dried particles of such hydrogel polymers may be referred to as crosslinked polymers.

[0024] As used herein, the term "crosslinked polymer" refers to a polymer obtained by crosslinking polymerization of water-soluble ethylenically unsaturated monomers having at least partially neutralized acid groups, and "base resin powder" refers to a material containing such crosslinked polymers.

[0025] Furthermore, the term "superabsorbent polymer" is used to include crosslinked polymers polymerized from water-soluble ethylenically unsaturated monomers having at least partially neutralized acid groups, or base resin powders composed of ground particles of crosslinked polymers, or crosslinked polymers or base resins suitable for productization made by additional methods such as surface crosslinking, fine particle reassembly, drying, grinding, classification, etc.

[0026] Method for Preparing Superabsorbent Polymer

[0027] A method for preparing a superabsorbent polymer according to an embodiment of the present invention includes the following steps:

[0028] Forming a hydrogel polymer comprising a crosslinked polymer obtained by crosslinking polymerization of an acrylic monomer having at least partially neutralized acid groups in the presence of an internal crosslinking agent;

[0029] Drying, grinding and classifying the hydrogel polymer to form a base resin powder; and

[0030] Subjecting the surface of the base resin powder to surface crosslinking in the presence of a surface crosslinking solution comprising an epoxy-based surface crosslinking agent, a monohydric alcohol solvent and a hydrophobic dispersant, and having a surface tension of 36 mN / m to 50 mN / m.

[0031] To improve the basic absorbency of the superabsorbent polymer, various post-treatment methods such as surface crosslinking, foaming, etc. are carried out, or various additives are used. However, in such methods, the crosslinking density of the resin is reduced, so sufficient gel strength cannot be achieved, and it is difficult to achieve an excellent liquid permeability to the desired degree.

[0032] Therefore, the inventors of the present invention have found that if the surface tension of a surface crosslinking solution containing specific components is controlled in the surface crosslinking step, a superabsorbent polymer having excellent liquid permeability and further improved absorption rate can be more effectively prepared without the above problems, and the present invention has been completed.

[0033] In particular, in the surface crosslinking step, by using a combination of an epoxy-based surface crosslinking agent, a monohydric alcohol solvent and a hydrophobic dispersant as the surface crosslinking solution, the surface tension of the solution can be controlled within the above range, thereby further slowing down the diffusion rate of the surface crosslinking solution into the base resin, and the surface crosslinking solution can be uniformly applied to the surface of the base resin particles. Therefore, the absorption characteristics of the finally prepared superabsorbent polymer particles can be improved, and in particular, the gel bed permeability (GBP) can be further improved.

[0034] Hereinafter, a method for preparing a superabsorbent polymer according to a specific embodiment of the present invention will be described in detail according to the steps.

[0035] Polymerization step

[0036] First, a method for preparing a superabsorbent polymer according to an embodiment of the present invention includes a step of forming a hydrogel polymer comprising a crosslinked polymer obtained by crosslinking polymerization of an acrylic monomer having at least partially neutralized acid groups in the presence of an internal crosslinking agent.

[0037] In the polymerization step, a monomer composition containing an acrylic monomer having at least partially neutralized acid groups is subjected to photopolymerization and / or thermal polymerization in the presence of an internal crosslinking agent to form a hydrogel polymer.

[0038] In the polymerization step, crosslinked polymerization of a monomer composition containing components commonly used for preparing superabsorbent polymers and the above components can be carried out.

[0039] First, a monomer composition containing an acrylic monomer having at least partially neutralized acid groups and an internal crosslinking agent is prepared. In the monomer composition, a polymerization initiator for polymerization can be further included.

[0040] The acrylic monomer can be any monomer commonly used for preparing superabsorbent polymers. As a non-limiting example, the acrylic monomer can be a compound represented by the following Chemical Formula 1:

[0041] [Chemical Formula 1]

[0042] R1-COOM 1

[0043] In Chemical Formula 1,

[0044] R1 is a C2-5 alkyl group containing an unsaturated bond,

[0045] M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

[0046] Preferably, the acrylic monomer includes one or more selected from acrylic acid, methacrylic acid, their monovalent metal salts, divalent metal salts, ammonium salts and organic amine salts. Therefore, in the case of using an acrylic monomer, a superabsorbent polymer with improved absorption performance can be obtained. In addition, as the monomer, one or more selected from the following can be used: anionic monomers such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloyl propane sulfonic acid or 2-(meth)acrylamide-2-methyl propane sulfonic acid, and their salts; nonionic hydrophilic 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-containing unsaturated monomers such as (meth)acrylic acid (N,N)-dimethylaminoethyl ester or (meth)acrylic acid (N,N)-dimethylaminopropyl ester, and their quaternized products.

[0047] Among them, the acrylic monomer has an acid group, and at least a part of the acid group is neutralized with a neutralizing solution. As the neutralizing agent contained in the neutralizing solution, an alkaline substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc. can be used.

[0048] 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 can vary according to the final properties, if the degree of neutralization is too high, the neutralized monomer may precipitate, so it may be difficult to carry out polymerization smoothly. On the contrary, if the degree of neutralization is too low, the absorption capacity of the polymer may be significantly reduced, and it may exhibit elastic rubber-like properties that are difficult to handle.

[0049] As used herein, the term "internal crosslinking agent" is used to distinguish from the surface crosslinking agent used to crosslink the surface of the base resin, and it is used to crosslink the unsaturated bonds of the above-mentioned acrylic monomer for polymerization. In this step, the crosslinking is carried out without distinguishing between the surface and the interior. In the surface crosslinking process of the base resin described below, the surface of the finally prepared superabsorbent polymer particles is composed of a structure crosslinked by a surface crosslinking agent, and the interior of the superabsorbent polymer particles is composed of a structure crosslinked by an internal crosslinking agent.

[0050] As the internal crosslinking agent, a polyfunctional component can be used. For example, one or more selected from 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 butanediol di(meth)acrylate diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol, and ethylene carbonate can be used. Preferably, polyethylene glycol di(meth)acrylate or propylene glycol di(meth)acrylate can be used.

[0051] Based on the weight of the acrylic monomer, the internal crosslinking agent can be used in an amount of 100 ppmw to 10,000 ppmw. When the internal crosslinking agent is included within the above content range, sufficient crosslinking can be achieved, thereby achieving a strength higher than the optimal level, and due to the introduction of the optimal crosslinking structure, sufficient water retention capacity can be achieved. Preferably, its content can be 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 5,000 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, and thus it may be difficult to achieve a strength higher than the optimal level, and if the content of the internal crosslinking agent is too high, the internal crosslinking density may increase, and thus it may be difficult to achieve the desired water retention capacity.

[0052] As the polymerization initiator, a thermal polymerization initiator or a photoinitiator can be used according to the polymerization method. However, even when photopolymerization is carried out, since a certain amount of heat is generated by ultraviolet irradiation or the like, and a certain degree of heat is generated according to the progress of the exothermic polymerization reaction, a thermal polymerization initiator can be additionally included.

[0053] Among them, as the photoinitiator, for example, one or more selected from the group consisting of benzoin ethers, dialkyl acetophenones, hydroxyalkyl ketones, phenylglyoxylates, benzyl dimethyl ketals, acylphosphines, and α-aminoketones can be used. Among them, as a specific example of the acylphosphine, commercially available lucirin TPO, i.e., diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, can be used. More various photoinitiators are described on page 115 of Reinhold Schwalm “UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)”, and are not limited to the above examples.

[0054] As a thermal polymerization initiator, one or more selected from persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specific examples of persulfate initiators can include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc., and specific examples of azo initiators can include 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethyl) isobutylamidine dihydrochloride, 2-(carbamoylazo) isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), etc. More various thermal initiators are described in Odian "Principle of Polymerization (Wiley, 1981)", page 203, but the thermal initiator is not limited thereto.

[0055] Based on the weight of the acrylic monomer, such a polymerization initiator can be used in a content of 10 ppmw to 10000 ppmw. Preferably, its content can be 10 ppmw or more, 30 ppmw or more, 50 ppmw or more, and 10000 ppmw or less, 5000 ppmw or less, or 3000 ppmw or less, or 30 ppmw to 5000 ppmw, 50 ppmw to 3000 ppmw, or 80 ppmw to 2500 ppmw. If the concentration of the polymerization initiator is too low, the polymerization rate may become slow, and a large amount of residual monomer can be extracted in the final product. On the contrary, if the concentration of the polymerization initiator is too high, the polymer chains constituting the network may become short. Therefore, the extractable content may increase and the absorption under pressure may decrease, and the performance of the resin may deteriorate. In the case where a photoinitiator and a thermal polymerization initiator are used together, the content of the polymerization initiator refers to the mixed content.

[0056] In addition, in the monomer composition, additives such as a foaming agent, a surfactant, a thickener, a plasticizer, an antiseptic stabilizer, an antioxidant, etc. can be further included as needed.

[0057] The foaming agent is used to induce foaming during polymerization 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 bicarbonate, magnesium bicarbonate, or magnesium carbonate can be used.

[0058] Based on the weight of the water-soluble ethylenically unsaturated monomer, it is preferable to use the foaming agent in an amount of 1500 ppmw or less. If the amount of the foaming agent used is greater than 1500 ppmw, too many pores may be formed, and thus, the gel strength of the superabsorbent polymer may decrease and the density may decrease, resulting in logistics and storage problems. In addition, based on the weight of the water-soluble ethylenically unsaturated monomer, it is preferable to use the foaming agent in an amount of 500 ppmw or more or 1000 ppmw or more.

[0059] The surfactant causes uniform dispersion and uniform foaming of the foaming agent, thereby preventing a decrease in gel strength or density during foaming. As the surfactant, an anionic surfactant can be preferably used. Specifically, as the surfactant, a compound containing SO3 represented by the following Chemical Formula 2 can be used. - Anionic compound.

[0060] [Chemical Formula 2]

[0061] R-SO3Na

[0062] In Chemical Formula 2,

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

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

[0065] In addition, such a monomer composition can be prepared in the form of a solution in which the above raw materials including monomers, internal crosslinking agents, foaming agents, initiators, etc. are dissolved in a solvent.

[0066] Among them, the solvent that can be used is not limited as long as it can dissolve the above components. For example, as the solvent, water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, N,N-dimethylacetamide or a mixture thereof can be used.

[0067] The step of forming the hydrogel polymer by polymerization of the monomer composition can be carried out by a conventional polymerization method, and this method is not particularly limited. As a non-limiting example, it can be carried out in a reactor equipped with a movable conveyor belt.

[0068] Specifically, in the case of photopolymerizing a monomer composition in a reactor equipped with a movable conveyor belt, a hydrogel polymer sheet is obtained. Among them, the thickness of the sheet can vary according to the concentration and injection rate of the monomer composition, and in order to induce uniform polymerization of the entire sheet and ensure the production speed, it is generally preferred to control the thickness to be 0.5 cm to 10 cm.

[0069] The hydrogel polymer thus obtained usually has a moisture content of 40% to 80% by weight. Among them, the "moisture content" is the content occupied by moisture 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 hydrogel polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to the evaporation of moisture in the polymer while raising the temperature of the polymer to dryness 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 can be set to 20 minutes, including a 5-minute heating-up step.

[0070] Drying, grinding, and classification steps

[0071] Next, a method for preparing a superabsorbent polymer according to an embodiment of the present invention includes the steps of drying, grinding, and classifying the prepared hydrogel polymer to form a base resin powder.

[0072] Specifically, dry the hydrogel polymer obtained above. To improve the efficiency of the drying step, if necessary, a rough grinding (chopping) step of the hydrogel polymer can be carried out before drying.

[0073] Among them, the grinder used is not limited, and specifically, it can include any one selected from the group consisting of a vertical grinder, a turbo cutter, a turbo mill, a rotary cutting mill, a cutting mill, a disk mill, a crushing crusher, a crusher, a chopper, and a disk cutter, but is not limited thereto.

[0074] Among them, a rough grinding step can be carried out so that the particle size of the hydrogel polymer can become about 2 mm to about 10 mm. Due to the high moisture content of the hydrogel polymer, it is technically not easy to grind to a particle size less than 2 mm, and it may cause aggregation between the ground particles. At the same time, if the grinding is carried out to a particle size greater than 10 mm, the effect of improving the efficiency of the subsequent drying step may not be significant.

[0075] The hydrogel that has been coarsely ground as described above, or the hydrogel that has just been polymerized without being coarsely ground, is dried. Among them, the drying temperature in the drying step can be about 150 °C to about 250 °C. If the drying temperature is lower than 150 °C, the drying time may be too long, and the properties of the finally formed superabsorbent polymer may deteriorate. And if the drying temperature is greater than 250 °C, only the surface of the polymer may be dried. Therefore, fine particles may be generated during the subsequent grinding process, and the properties of the finally formed superabsorbent polymer may deteriorate. Therefore, preferably, the drying can be carried out at a temperature of about 150 °C to about 200 °C, more preferably about 150 °C to about 190 °C.

[0076] Regarding the drying time, considering process efficiency and the like, the drying can be carried out for about 20 to about 90 minutes, but the drying time is not limited to this.

[0077] Meanwhile, the drying step can be carried out by a multi-stage method within the above temperature range.

[0078] The drying method in the drying step is not limited as long as it is generally used as the drying process for hydrogel polymers. Specifically, the drying can be carried out by supplying hot air, infrared irradiation, microwave irradiation, or ultraviolet irradiation. Supplying hot air can be carried out using an oven capable of transferring the air volume up and down.

[0079] The dried polymer can have a moisture content of about 0.1 wt% to about 10 wt%.

[0080] Next, the dried polymer obtained through the drying step is ground.

[0081] The polymer powder obtained after the grinding step can have a particle size of 150 μm to 850 μm. As the grinder used to grind to such a particle size, specifically, a needle mill, a hammer mill, a screw mill, a roll mill, a disk mill, a slow mill, etc. can be used, but it is not limited to this.

[0082] In addition, in order to manage the properties of the finally obtained superabsorbent polymer powder after the grinding step, the polymer powder obtained after grinding is classified according to the particle size. Preferably, the polymer with a particle size of about 150 μm to about 850 μm is classified, and only the polymer powder with such a particle size can be subjected to the surface crosslinking reaction step and productized. More specifically, the classified base resin powder can have a particle size of 150 μm to 850 μm, and can contain particles with a particle size of 300 μm to 600 μm in a content of more than 50 wt%, and the content of fine powder with a particle size less than 150 μm can be less than 3 wt%.

[0083] Meanwhile, during the aggregation, drying, and grinding steps, fines (fine powder) with a particle size less than 150 μm can be generated, and the fines with a particle size less than 150 μm classified in the above classification step can be subjected to a fines recombination process and prepared into normal particles with a size above 150 μm, so as to be reused as the base resin powder.

[0084] For the fines reassembly process, methods commonly applied in the art can be used. For example, a fines wet aggregation process can be carried out. Specifically, the classified fines are mixed with water for aggregation, and then the water is dried by a secondary drying method to prepare normal particles. During the recombination process, additives such as water-soluble polymers can be selectively used to enhance the aggregation strength of the particles.

[0085] Surface crosslinking step

[0086] Meanwhile, after performing the above classification step to prepare the base resin powder, a step of surface crosslinking and heat treatment of the base resin powder in the presence of a surface crosslinking agent can be further carried out.

[0087] According to an embodiment of the present invention, a surface crosslinking solution with a surface tension controlled within a specific range is used for the surface crosslinking step. Specifically, by satisfying that the surface tension of the surface crosslinking solution is 36 mN / m to 50 mN / m, a superabsorbent polymer with excellent liquid permeability can be prepared.

[0088] More specifically, by using a combination of an epoxy-based surface crosslinking agent, a monohydric alcohol solvent, and a hydrophobic dispersant as the surface crosslinking solution, the surface tension of the solution can be controlled within the above range. Compared with the surface tension of the commonly used surface crosslinking solution, this range is a relatively low surface tension (36 mN / m to 50 mN / m), and the diffusion rate of the surface crosslinking solution into the base resin is further slowed down. Therefore, the surface crosslinking solution can be uniformly applied to the surface of the base resin particles, and the absorption performance of the finally prepared superabsorbent polymer particles can be improved. In particular, the gel bed permeability (GBP) can be further improved.

[0089] If the surface tension of the surface crosslinking solution is less than 36 mN / m, the surface crosslinking solution containing an epoxy-based surface crosslinking agent may be difficult to penetrate into the base resin powder, and thus it may be difficult to achieve a uniform surface crosslinking to the desired degree. On the other hand, if the surface tension is greater than 50 mN / m, the surface crosslinking solution may penetrate excessively into the base resin powder, and as a result, the surface crosslinking density may increase and the water retention capacity may decrease. Preferably, the surface tension of the surface crosslinking solution may be 36 mN / m or more, 37 mN / m or more, 38 mN / m or more, and 49 mN / m or less, 48.5 mN / m or less. Within the above range, a superabsorbent polymer having excellent absorption performance can be prepared without the above problems. The surface tension of the surface crosslinking solution can be controlled according to the types and contents of the specific components of the solution (i.e., the epoxy-based crosslinking agent, the monohydric alcohol solvent, and the hydrophobic dispersant), but is not limited thereto.

[0090] Meanwhile, the surface tension can be measured at room temperature of 23 ± 2 °C according to the Wilhelmy plate method. The specific measurement method of the surface tension will be described in detail in the experimental examples later.

[0091] The surface crosslinking step induces a crosslinking reaction on the surface of the base resin powder in the presence of a surface crosslinking agent, where the unsaturated bonds of the non-crosslinked acrylic monomers remaining on the surface are crosslinked by the surface crosslinking agent, thereby forming a superabsorbent polymer with an increased surface crosslinking density.

[0092] Specifically, a surface crosslinking layer can be formed through a heat treatment process in the presence of a surface crosslinking agent, and the heat treatment process increases the surface crosslinking density, i.e., the external crosslinking density, without changing the internal crosslinking density. Therefore, the prepared superabsorbent polymer with a surface crosslinking layer can have a higher external crosslinking density than the internal one.

[0093] The surface crosslinking solution used in the surface crosslinking step contains an epoxy-based surface crosslinking agent.

[0094] The epoxy-based crosslinking agent can easily crosslink with the ester groups present on the surface of the base resin at a relatively low temperature (about 100 °C to 140 °C), and thus can further improve the properties under pressure, such as 0.9 AUL, GBP, etc. In addition, it is used in combination with a monohydric alcohol solvent and a hydrophobic dispersant in the surface crosslinking solution to be able to control the surface tension within the target range.

[0095] The epoxy crosslinking agent may be a compound containing at least one epoxy group, preferably more than 2 epoxy groups in the molecule, and it may be a compound containing an ether structure and an epoxy group in the molecule. As the epoxy crosslinking agent, for example, one or more selected from 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.

[0096] Meanwhile, in the case of using a compound such as epoxyalkyl phosphate as a crosslinking agent, although it contains an epoxy group, it may have an adverse effect on the absorption performance of the absorbent according to the concentration of the phosphate ester, and compared with the epoxy crosslinking agent according to the present invention, it does not satisfy the target surface tension (36 mN / m to 50 mN / m) under the same water retention capacity.

[0097] Preferably, the epoxy crosslinking agent is not an epoxyalkyl phosphate compound.

[0098] Based on 100 parts by weight of the base resin powder, the epoxy crosslinking agent can be used in an amount of 0.01 to 10 parts by weight. Preferably, it can be used in a content of 0.05 parts by weight or more, 0.1 parts by weight or more, and 8 parts by weight or less, 5 parts by weight or less, 1 part by weight or less. By controlling the content range of the epoxy crosslinking agent within the above range, a superabsorbent polymer exhibiting excellent absorption performance and liquid permeability can be prepared.

[0099] The surface crosslinking solution for the surface crosslinking step contains a monohydric alcohol solvent as the solvent.

[0100] The monohydric alcohol solvent relatively reduces the surface tension of the surface crosslinking solution to slow down the diffusion of the surface crosslinking solution, so that the surface crosslinking solution can be more evenly applied on the surface of the base resin powder. Therefore, it can effectively improve the gel bed permeability (GBP) in the properties under pressure, and can control the surface tension within the required range together with the epoxy crosslinking agent and the hydrophobic dispersant in the surface crosslinking solution.

[0101] As examples of the monohydric alcohol solvent, one or more selected from methanol, ethanol, n-propanol, isopropanol, and butanol can be used, and preferably ethanol and isopropanol can be used.

[0102] In addition to the monohydric alcohol solvent, the surface crosslinking solution may further contain another solvent, and for example, it may also contain water and / or a hydrophilic organic solvent, and preferably, water can be used together with the monohydric alcohol solvent.

[0103] 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 crosslinking solution. Preferably, it may be used in an amount of more than 1.5 parts by weight, more than 2 parts by weight and less than 4.5 parts by weight, less than 4 parts by weight, less than 3 parts by weight. By controlling the content range of the monohydric alcohol solvent within the above range, the surface tension of the surface crosslinking solution can be controlled within a desired range, thereby preparing a superabsorbent polymer exhibiting excellent absorption performance, liquid permeability, etc.

[0104] The surface crosslinking solution for the surface crosslinking step contains a hydrophobic dispersant.

[0105] The hydrophobic dispersant relatively reduces the surface tension of the surface crosslinking solution to slow down the diffusion of the surface crosslinking solution, thereby enabling the surface crosslinking solution to be more uniformly applied on the surface of the base resin powder. Therefore, it can effectively improve the gel bed permeability (GBP) in the properties under pressure, and can control the surface tension within the required range together with the epoxy crosslinking agent and the monohydric alcohol solvent in the surface crosslinking solution.

[0106] As an example of the hydrophobic dispersant, one or more selected from sodium dodecyl sulfate, poly(dimethyldiallylammonium chloride), water-dispersed silica, calcium stearate, di(C 12-20 )alkyldimethylammonium salts, and polyethylene glycol can be used, and preferably, sodium dodecyl sulfate and water-dispersed silica can be used.

[0107] Based on the total content of the surface crosslinking solution, the hydrophobic dispersant may be used in an amount of 0.0001 to 1 part by weight. Preferably, it may be used in an amount of more than 0.0002 part by weight, 0.003 part by weight, and less than 0.5 part by weight, less than 0.45 part by weight. By controlling the content range of the hydrophobic dispersant within the above range, the surface tension of the surface crosslinking solution can be controlled within a desired range, thereby preparing a superabsorbent polymer exhibiting excellent absorption performance, liquid permeability, etc. For reference, in the case where water-dispersed silica is used as the hydrophobic dispersant, the content is based on solid silica.

[0108] In addition to the above components, the surface crosslinking solution may further contain additional additives. Specifically, the additional additives contained in the surface crosslinking solution can increase the mixing efficiency of the base resin particles and the surface crosslinking solution to more uniformly apply the surface crosslinking solution, thereby improving the GBP in the properties under pressure.

[0109] As specific examples of the additional additives, one or more selected from the group consisting of methylene glycol, ethylene glycol, propylene glycol, and butylene glycol can be mentioned, and more preferably, propylene glycol can be used.

[0110] Based on the total content of the surface crosslinking solution, additional additives can be included in an amount of 0.1 to 5 parts by weight, and preferably, they can be used in an amount of 0.5 to 3 parts by weight, 1 to 3 parts by weight, 1.5 to 2.5 parts by weight, and when included within the above content range, the applicability of the surface crosslinking solution can be increased to further improve the GBP in the properties under pressure.

[0111] Meanwhile, the surface crosslinking agent is added to the base resin powder in the state of the surface crosslinking composition containing it, 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 base resin powder and the surface crosslinking composition can be continuously fed into a continuously operating mixer and mixed, etc.

[0112] 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 longer. More specifically, the above temperature can be set as the maximum reaction temperature, and the surface crosslinking reaction can be carried out by heat treatment at the maximum reaction temperature for 30 to 80 minutes or 40 to 70 minutes.

[0113] By satisfying such surface crosslinking process conditions (particularly, the temperature increase condition and the reaction at the maximum reaction temperature), a superabsorbent polymer that appropriately meets excellent properties such as permeability under pressure can be prepared.

[0114] The method of increasing the temperature for the surface crosslinking reaction is not particularly limited. A heat transfer medium can be provided, or a heat source can be directly provided for heating. Among them, as the types of heat transfer media that can be used, heating fluids such as steam, hot air, and hot oil can be used, but the present invention is not limited thereto, and the temperature of the supplied heat transfer medium can be appropriately selected considering the device of the heat transfer medium, the temperature increase device, and the target temperature. Meanwhile, as the directly supplied heat source, electric heating and gas heating can be mentioned, but the present invention is not limited thereto.

[0115] Meanwhile, in 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 permeability, etc. Such polyvalent metal salts can be included on the surface crosslinking layer of the finally prepared superabsorbent polymer.

[0116] Superabsorbent polymer

[0117] According to an embodiment of the present invention, there is provided a superabsorbent polymer prepared by a method for preparing a superabsorbent polymer. The superabsorbent polymer prepared by the method for preparing a superabsorbent polymer according to the above embodiment can achieve excellent absorption performance and liquid permeability, particularly excellent gel bed permeability (GBP), and has an optimal crosslinking density.

[0118] The gel bed permeability (GBP) of the superabsorbent polymer can be 50 darcy or more, more preferably 53 darcy or more, 55 darcy or more, and 110 darcy or less, 105 darcy or less. The specific measurement method of the gel bed permeability is as described in Korean Patent Application No. 10-2014-7018005, and will be explained in more detail in the experimental examples below.

[0119] Hereinafter, the functions and effects of the present invention will be explained in more detail through specific examples of the present invention. However, these examples are given only as illustrations of the present invention, and the scope of the rights of the present invention is not determined thereby.

[0120] Examples

[0121] Example 1

[0122] As a device for preparing a superabsorbent polymer, a continuous manufacturing device composed of a polymerization step, a hydrogel grinding step, a drying step, a grinding step, a classification step, a surface crosslinking step, a cooling step, a classification step, and a transfer step connecting each step is used.

[0123] 100 parts by weight of acrylic acid, 0.43 parts by weight of polyethylene glycol diacrylate (weight average molecular weight: about 500 g / mol) as an internal crosslinking agent, and 0.01 parts by weight of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator are mixed to prepare a monomer solution. Subsequently, while continuously feeding the monomer solution with a metering pump, 175 parts by weight of a 31.5 wt% aqueous sodium hydroxide solution is mixed online to prepare an aqueous monomer solution. In addition, 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% of sodium bicarbonate as a foaming agent, and 0.25 parts by weight of an aqueous solution containing 0.05 wt% of calcium stearate as a surfactant are continuously mixed online to prepare a monomer mixture.

[0124] Through this transfer, the aqueous monomer solution is injected into a polymerization reactor composed of a moving conveyor belt and irradiated with ultraviolet rays (dose: 2 mW / cm 2 ) by an ultraviolet irradiation device, thereby performing ultraviolet polymerization for 2 minutes to prepare a hydrogel polymer.

[0125] The hydrogel is cut into an average size of about 300 mm or less, and then injected into a grinder (equipped with a perforated plate having a plurality of holes, each hole having a diameter of 10 mm) and ground.

[0126] Subsequently, the ground hydrogel is dried in a dryer capable of transferring the air volume up and down. Hot air at 180 °C is made to flow so that the moisture content of the dried debris becomes about 2% or less, thereby drying the hydrogel uniformly.

[0127] The dried resin is ground with a grinder and then classified to obtain a base resin of 150 μm to 850 μm.

[0128] Thereafter, 10.28 g of a surface crosslinking solution (containing 6 g of water, 2 g of isopropyl alcohol, 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 propylene glycol as other additives, and 0.06 g of silica (Aerosil 200)) is sprayed onto 100 parts by weight of the prepared base resin powder, and they are stirred and mixed at room temperature so that the surface crosslinking solution is uniformly distributed on the base resin powder.

[0129] Subsequently, the base resin powder mixed with the surface crosslinking solution is put into a surface crosslinking reactor and subjected to a surface crosslinking reaction.

[0130] In the surface crosslinking reactor, the base resin powder is additionally reacted at 140 °C for 40 minutes. After surface crosslinking, 0.08 part by weight of silica (Aerosil 200) is added to 40 parts by weight of the base resin, and then the finally prepared superabsorbent polymer sample is taken out. After the surface crosslinking process, it is classified with a standard sieve of ASTM standard to prepare a superabsorbent polymer of Example 1 having a particle size of 150 μm to 850 μm.

[0131] Examples 2 to 9 and Comparative Examples 1 to 4

[0132] A superabsorbent polymer is prepared by the same method as in Example 1 except that the components and contents described in Table 1 below are used in the surface crosslinking step.

[0133] [Table 1]

[0134]

[0135] Experimental Example 1. Evaluation of the properties of the surface crosslinking solution

[0136] For the surface crosslinking solutions used in the examples and comparative examples, the performance was evaluated as follows, and the results are shown in Table 2.

[0137] Unless otherwise specified, the following performance evaluations are carried out at room temperature (25 ± 1 °C).

[0138] (1) Surface tension (mN / m)

[0139] The "plate method (Wilhelmy plate method)" is a method for measuring static surface tension, and this method is used to measure the surface tension of each surface crosslinking solution used in the examples and comparative examples. Specifically, it refers to the maximum tension value until the surface of the solution to be measured reaches the tip of the plate, and it is measured using a process tensiometer (KRUSS company), and the results are shown in Table 1.

[0140] (2) Evaluation of the adsorption performance of the surface crosslinking solution (adsorption time, min)

[0141] For the base resin powder before surface crosslinking in the preparation steps of the examples and comparative examples, the absorbance of the surface crosslinking solution is measured.

[0142] Specifically, 2 g of the base resin powder is injected into a beaker containing 50 g of the surface crosslinking solution, and the time required for the corresponding resin to be completely saturated is measured, and the results are shown in Table 1.

[0143] As a reference, this property can refer to the degree of application of the surface crosslinking solution on the surface of the base resin, and as the absorption is delayed, the application on the surface of the base resin increases, which indirectly indicates excellent properties under pressure.

[0144] Experimental Example 2. Evaluation of the properties of superabsorbent polymers

[0145] For each superabsorbent polymer prepared in the examples and comparative examples, the performance is measured as follows, and the results are shown in Table 2.

[0146] The following property evaluations except for the vortex evaluation are carried out at room temperature (23 ± 1 °C) and relative humidity (45 ± 1 %), and physiological saline or brine means an aqueous solution of 0.9 wt% sodium chloride (NaCl).

[0147] (1) Centrifugal retention capacity (CRC, g / g)

[0148] In the superabsorbent polymer powders prepared in the examples and comparative examples, samples with a particle size of 150 μm to 850 μm are taken, and the centrifugal retention capacity (CRC) according to the absorption ratio under no load is measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.2.

[0149] Specifically, put the sample W0 (g) (about 0.2 g) evenly into an envelope made of non-woven fabric, seal the envelope, and then soak it in physiological saline (0.9 wt%) at room temperature. After 30 minutes, drain the envelope for 3 minutes at 250 G using a centrifuge, and measure the mass W2 (g) of the envelope. In addition, perform the same operation without using the corresponding sample, and then measure the mass W1 (g) at this time. Using the obtained masses, calculate CRC (g / g) according to the following formula.

[0150] [Formula 1]

[0151] CRC (g / g) = {[W2 (g) - W1 (g)] / W0 (g)} - 1

[0152] (2) Absorbency Under Pressure (AUP, g / g)

[0153] For each superabsorbent polymer prepared in the examples and comparative examples, measure the absorbency under a pressure of 0.9 psi according to the EDANA method NWSP242.0.R2. To measure the absorbency under pressure, use the classified resin for CRC measurement.

[0154] Specifically, at the bottom of a plastic cylinder with an inner diameter of 25 mm, install a 400-mesh wire mesh made of stainless steel. Under the conditions of room temperature (23 ± 1 °C) and relative humidity (45 ± 1%), evenly spray W0 (g) (0.16 g) of the absorbent polymer on the wire mesh, and place a piston with an outer diameter slightly smaller than 25 mm and capable of further evenly delivering a load of 0.9 psi on it, such that there is no gap between the piston and the inner wall of the cylinder and the up and down movement of the piston is not hindered. At this time, measure the weight W3 (g) of the measuring device.

[0155] 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, and fill the petri dish with physiological saline composed of 0.9 wt% sodium chloride to the same level as the upper surface of the glass filter. Place a piece of filter paper with a diameter of 90 mm on it. Place the above-mentioned measuring device on the filter paper and absorb the liquid under load for 1 hour. After 1 hour, pick up the measuring device and measure the weight W4 (g).

[0156] Using the obtained masses, calculate the absorbency under pressure (g / g) according to the following Formula 2.

[0157] [Formula 2]

[0158] AUP (g / g) = [W4 (g) - W3 (g)] / W0 (g)

[0159] (3) Gel Bed Permeability (GBP, Darcy)

[0160] For each of the superabsorbent polymers prepared in the examples and comparative examples, the free-swelling gel bed permeability to physiological saline was measured, and the results are shown in Table 2.

[0161] The gel bed permeability was measured by the method (using the same apparatus) described in Korean Patent Application No. 10-2014-7018005 and calculated according to the following Mathematical Formula 3.

[0162] [Mathematical Formula 3]

[0163] K = [Q × H × Mu] / [A × Rho × P]

[0164] In Mathematical Formula 3,

[0165] K is the permeability (cm 2 ), and Q is the flow rate (g / sec),

[0166] H is the height of the sample (cm), Mu is the liquid viscosity (poise) (about 1 cps for the test solution used in the test),

[0167] A is the cross-sectional area of the liquid flow (cm 2 ),

[0168] Rho is the liquid density (of the test solution used in the relevant test) (g / cm 3 ), and P is the hydrostatic pressure (dynes / cm 2 ) (usually about 3923 dynes / cm 2 ).

[0169] The hydrostatic pressure was calculated by the following Mathematical Formula 3-1.

[0170] [Mathematical Formula 3-1]

[0171] P = Rho × g × h

[0172] In Mathematical Formula 3-1,

[0173] Rho is the liquid density (g / cm 3 ),

[0174] g is the acceleration due to gravity, usually 981 cm / sec 2 ,

[0175] h is the liquid height (e.g., in the case of the permeation test described herein, 7.8 cm)

[0176] (4) Vortex time (sec)

[0177] For each superabsorbent polymer prepared in the examples and comparative examples, 2 ± 0.0001 g of the superabsorbent polymer (based on the unclassified resin) was added to 50 mL of physiological saline (24.4 ± 0.2 °C), stirred at 600 rpm, and the time taken until the vortex disappeared was measured.

[0178] [Table 2]

[0179] CRC (g / g) 0.9 AUL (g / g) GBP (Darcy) Vortex (sec) Example 1 31.4 19.0 78 38 Example 2 31.4 19.2 78 39 Example 3 31.4 18.7 69 37 Example 4 31.3 18.9 76 35 Example 5 31.2 18.7 68 35 Example 6 31.2 18.7 69 35 Example 7 31.1 18.7 69 36 Comparative Example 1 29.5 17.8 38 34 Comparative Example 2 31.5 15.5 24 32 Comparative Example 3 29.5 17.8 38 34 Comparative Example 4 31.1 18.4 49 32

[0180] From the data in Table 2, it can be confirmed that the superabsorbent polymer obtained by the surface crosslinking step using the surface crosslinking solution of the present invention exhibits excellent absorption characteristics. In particular, it can be confirmed that excellent vortex time is exhibited in the examples, and at the same time, the gel bed permeability is significantly improved.

Claims

1. A method for preparing a superabsorbent polymer, the method comprising the following steps: Forming a hydrogel polymer comprising a crosslinked polymer, the crosslinked polymer being obtained by crosslinking polymerization of an acrylic monomer having at least partially neutralized acid groups in the presence of an internal crosslinking agent; Drying, grinding and classifying the hydrogel polymer to form a base resin powder; And Subjecting the surface of the base resin powder to surface crosslinking in the presence of a surface crosslinking solution, the surface crosslinking solution comprising an epoxy-based surface crosslinking agent, a monohydric alcohol solvent and a hydrophobic dispersant, and having a surface tension of 36 mN / m to 50 mN / m.

2. The method according to claim 1, wherein, The monohydric alcohol solvent is one or more selected from the group consisting of methanol, ethanol, n-propanol, isopropanol and butanol.

3. The method according to claim 1, wherein, Based on the total content of the surface crosslinking solution, the content of the monohydric alcohol solvent comprised is 1 to 5 parts by weight.

4. The method according to claim 1, wherein The hydrophobic dispersant is one or more selected from sodium dodecyl sulfate, poly(dimethyldiallylammonium chloride), water-dispersed silica, calcium stearate, di(C 12-20 )alkyldimethylammonium salt, and polyethylene glycol.

5. The method according to claim 1, wherein Based on the total content of the surface crosslinking solution, the content of the hydrophobic dispersant comprised is 0.0001 to 1 part by weight.

6. The method according to claim 1, wherein The epoxy-based surface crosslinking 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.

7. The method according to claim 1, wherein Based on the total content of the base resin powder, the content of the epoxy-based crosslinking agent comprised is 0.01 to 10 parts by weight.

8. The method according to claim 1, wherein The surface crosslinking solution further comprises one or more additional additives selected from the group consisting of methylene glycol, ethylene glycol, propylene glycol and butanediol.

9. The method according to claim 8, wherein, Based on the total content of the surface crosslinking solution, the content of the additional additive comprised is 0.1 to 5 parts by weight.

10. The method according to claim 1, wherein, Based on 100 parts by weight of the base resin powder, the content of the surface crosslinking solution comprised is 1 to 10 parts by weight.

11. The method according to claim 1, wherein The superabsorbent polymer has a gel bed permeability (GBP) of more than 50 darcies.

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