Preparation method of superabsorbent polymer

Abstract

The present invention relates to a method for preparing a superabsorbent polymer, and according to the present invention, the surface crosslinking strength of the superabsorbent polymer is increased, thereby enabling the preparation of a superabsorbent polymer having little or no decrease in pressure absorption rate even after treatment with an anti-caking agent.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2020-0142300 filed on October 29, 2020, and Korean Patent Application No. 10-2021-0144931 filed on October 27, 2021, in the Korean Intellectual Property Office, the disclosures of which are hereby incorporated by reference in their entirety.

[0003] The present invention relates to a method for preparing superabsorbent polymers. Background Art

[0004] Superabsorbent polymer (SAP) is a synthetic polymer material that can absorb 500 to 1,000 times its own weight in water. Depending on the developer, it is variously referred to as superabsorbent material (SAM) or absorbent gelling material (AGM). SAPs were first commercialized as sanitary products and are currently used extensively as soil water retention materials, waterproofing materials for civil engineering and construction, seedling sheets, food preservatives, and hot compresses. They are primarily used in sanitary products such as disposable diapers and sanitary napkins.

[0005] With the worldwide trend towards thinner diapers, the proportion of pulp constituting the absorbent core of the diaper is decreasing, while the proportion of superabsorbent polymers is increasing.

[0006] As the proportion of superabsorbent polymer in diapers increases, the importance of SAP properties also increases. In particular, the importance of pressure absorption rate and pressure permeability, which affect the rewetting or absorption speed of the diaper under load, is increasing.

[0007] Because superabsorbent polymers (SAPs) absorb water, they tend to clump in high-humidity areas. To prevent this, anti-caking agents are added. Non-hygroscopic inorganic substances are generally used as anti-caking agents. Because inorganic substances prevent SAP particles from clumping together, they can also be used as additives to improve permeability.

[0008] However, inorganic substances increase surface friction during the swelling process of superabsorbent polymers and hinder swelling under pressure, thereby deteriorating the absorbency under pressure. Summary of the Invention

[0009] Technical issues

[0010] An object of the present invention is to provide a superabsorbent polymer having little or no decrease in absorbency under pressure even after treatment with an anti-caking agent by increasing the surface crosslinking strength of the superabsorbent polymer.

[0011] Technical Solution

[0012] According to one embodiment of the present invention, a method for preparing a superabsorbent polymer is provided, which includes the following steps: cross-linking and polymerizing a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups are neutralized in the presence of a polymerization initiator and an internal cross-linking agent to form a hydrogel polymer comprising a cross-linked polymer of the water-soluble ethylenically unsaturated monomer; drying, grinding, and classifying the hydrogel polymer to obtain a base resin powder; performing a first surface cross-linking of the base resin powder while raising the temperature to a first temperature in the presence of a first surface cross-linking agent; and performing a second surface cross-linking of the first surface cross-linked base resin powder at a second temperature in the presence of a second surface cross-linking agent to form superabsorbent polymer particles, wherein the first temperature is above 180°C, the second temperature is between 120°C and 150°C, and the second surface cross-linking agent includes an epoxy compound having a weight-average molecular weight greater than that of the first surface cross-linking agent and capable of forming covalent bonds at the second temperature.

[0013] According to another embodiment of the present invention, a superabsorbent polymer prepared by the above method is provided, which includes: a base resin powder and a surface cross-linked layer, wherein the base resin powder includes a cross-linked polymer of a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups is neutralized; the surface cross-linked layer is located on the base resin powder, wherein the surface cross-linked layer includes a first cross-linked polymer, wherein part of the cross-linked polymer is additionally cross-linked by a first surface cross-linking agent; a second cross-linked polymer, wherein the remaining cross-linked polymer is additionally cross-linked by a second surface cross-linking agent, and the second surface cross-linking agent includes an epoxy compound having a weight-average molecular weight greater than that of the first surface cross-linking agent and capable of forming covalent bonds at 120°C to 150°C.

[0014] The terms used herein are intended only to explain specific embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless explicitly stated or it is obvious from the context that this is not intended. As used herein, the terms "including," "equipped with," or "having" are intended to specify the presence of an implemented feature, quantity, step, structural element, or combination thereof, and 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.

[0015] Although various modifications may be made to the present invention and the present invention may have various forms, specific examples will be described and explained in detail below. However, it should be understood that these are not intended to limit the present invention to the specific disclosure, and that the present invention includes all modifications, equivalents, or alternatives thereof without departing from the spirit and technical scope of the present invention.

[0016] As used herein, the term "polymer" refers to the polymerized state of water-soluble ethylenically unsaturated monomers, and may include polymers in all water content ranges or particle size ranges. Among these polymers, those having a water content of about 40% by weight or more after polymerization and before drying may be referred to as hydrogel polymers.

[0017] Also, "base resin" refers to resin particles or powders obtained by drying and grinding a hydrogel polymer before undergoing additional treatments such as surface cross-linking, fine reorganization, re-drying, re-grinding, re-classification, and the like.

[0018] Furthermore, "superabsorbent polymer" refers to the polymer or base resin itself, or, depending on the context, includes polymers or base resins suitable for productization through additional processing (e.g., surface cross-linking, fine restructuring, drying, grinding, classification, etc.).

[0019] Hereinafter, a method for preparing a super absorbent polymer according to a specific embodiment of the present invention and a super absorbent polymer prepared thereby will be explained in detail.

[0020] Specifically, the method for preparing superabsorbent polymer according to one embodiment of the present invention comprises the following steps:

[0021] Cross-linking and polymerizing a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups are neutralized in the presence of a polymerization initiator and an internal cross-linking agent to form a hydrogel polymer comprising a cross-linked polymer of the water-soluble ethylenically unsaturated monomer (step 1);

[0022] drying, grinding, and classifying the hydrogel polymer to obtain a base resin powder (step 2);

[0023] performing first surface crosslinking of the base resin powder while raising the temperature to a first temperature in the presence of a first surface crosslinking agent (step 3); and

[0024] performing a second surface crosslinking of the first surface crosslinked base resin powder at a second temperature in the presence of a second surface crosslinking agent to form superabsorbent polymer particles (step 4),

[0025] The first temperature is above 180°C, the second temperature is between 120°C and 150°C, and

[0026] The second surface cross-linking agent includes an epoxy compound having a weight average molecular weight greater than that of the first surface cross-linking agent and capable of forming a covalent bond at a second temperature.

[0027] Generally speaking, the larger the surface area of a superabsorbent polymer, the faster it absorbs. However, due to limitations in production facilities, if the surface area of a superabsorbent polymer is wide, it may be difficult to fully apply the surface crosslinking agent, and the pressure absorption rate of the superabsorbent polymer finally prepared may be significantly deteriorated.

[0028] Therefore, in the present disclosure, when preparing a superabsorbent polymer, a first surface crosslinking reaction using a surface crosslinking agent with a low weight-average molecular weight is performed at a high temperature, allowing the surface crosslinking agent to deeply penetrate the surface of the superabsorbent polymer, thereby ensuring the superabsorbent polymer's pressure absorption rate. Subsequently, a second surface crosslinking reaction is performed using an epoxy-based surface crosslinking agent with a larger weight-average molecular weight than the first surface crosslinking agent and capable of forming covalent bonds within an optimal temperature range. This prevents or reduces the degradation of the pressure absorption rate even after anti-caking treatment. Furthermore, this preparation method is particularly effective for preparing superabsorbent polymers with a wide surface area and a high absorption rate.

[0029] Furthermore, although the degradation of the absorbency under pressure of the superabsorbent polymer caused by the anti-caking treatment can be reduced by a conventional preparation method that only performs the first surface crosslinking, in this case, the surface crosslinking reaction should proceed more, and therefore the centrifuge retention capacity (CRC) may be significantly degraded. Therefore, in the present disclosure, by using a surface crosslinking agent having a larger Mw than the first surface crosslinking agent and a fast reaction rate even at low temperatures, the unreacted surface after the first surface crosslinking is subjected to the second surface crosslinking, and the decrease in the absorbency under pressure can be greatly reduced to less than 5%, while the loss of the centrifuge retention capacity can be minimized.

[0030] In addition, in the second surface cross-linking process, a surface cross-linking agent with a fast reaction speed at low temperatures is used. Therefore, after the first surface cross-linking process is carried out at a high temperature of about 180°C, the second surface cross-linking can be carried out in the cooling process by the residual heat of the first surface cross-linking without a separate heating process.

[0031] Hereinafter, the method for preparing a super absorbent polymer according to one embodiment of the present invention will be explained in more detail according to steps.

[0032] (Step 1)

[0033] In the preparation method according to one embodiment of the present invention, step 1 is a step of preparing a hydrogel polymer.

[0034] Specifically, step 1 can be performed by mixing a water-soluble ethylenically unsaturated monomer having at least a portion of its acidic group neutralized, a polymerization initiator, and an internal crosslinking agent to prepare a monomer composition and polymerizing the mixture. For example, the monomer composition can be prepared by introducing a polymerization initiator and an internal crosslinking agent into a neutralized solution of a water-soluble ethylenically unsaturated monomer having an acidic group and mixing them.

[0035] The water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups are neutralized can be prepared by neutralizing the acidic groups of the water-soluble ethylenically unsaturated monomer using a neutralizing agent.

[0036] The water-soluble ethylenically unsaturated monomer may be any monomer commonly used to prepare superabsorbent polymers. As a non-limiting example, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following Chemical Formula 1:

[0037] [Chemical Formula 1]

[0038] R1-COOM

[0039] In Chemical Formula 1,

[0040] R1 is a C2-5 alkyl group having an unsaturated bond,

[0041] M is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

[0042] Preferably, the monomer may be one or more selected from the group consisting of methacrylic acid and its monovalent (alkali) metal salts, divalent metal salts, ammonium salts and organic amine salts.

[0043] The use of methacrylic acid or a salt thereof as a water-soluble ethylenically unsaturated monomer is advantageous because a superabsorbent polymer having improved absorption properties can be obtained. Furthermore, as monomers, one or more selected from the group consisting of maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acrylamide, N-substituted (meth)acrylates, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate or polyethylene glycol (meth)acrylate, (N,N)-dimethylaminoethyl (meth)acrylate or (N,N)-dimethylaminopropyl (meth)acrylamide can be used.

[0044] Furthermore, the concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition can be appropriately controlled, taking into account the polymerization time and reaction conditions, and is preferably 20% to 90% by weight, or 40% to 65% by weight. As described below, such a concentration range can be beneficial for controlling the grinding efficiency during polymer grinding while avoiding the need to remove unreacted monomers after polymerization using the gelation phenomenon generated during the polymerization reaction of the high-concentration aqueous solution. However, if the monomer concentration is too low, the yield of the superabsorbent polymer may be reduced. Conversely, if the monomer concentration is too high, some monomers may precipitate, or the grinding efficiency may be reduced during the grinding of the polymerized hydrogel polymer, leading to processing problems and potentially deteriorating the properties of the superabsorbent polymer.

[0045] Furthermore, as the neutralizing agent, a basic material capable of neutralizing the acidic group, such as sodium hydroxide (or caustic soda), potassium hydroxide, ammonium hydroxide, etc., can be used.

[0046] There is no particular limitation on the amount of the neutralizing agent and the neutralization reaction conditions. The neutralization reaction can be carried out as follows: the degree of neutralization of the water-soluble ethylenically unsaturated monomer (i.e., the degree to which the acidic groups contained in the water-soluble ethylenically unsaturated monomer are neutralized by the neutralizing agent) can reach 50 mol% to 90 mol%, or 60 mol% to 85 mol%, or 65 mol% to 85 mol%, or 70 mol% to 80 mol%. Although the range of the degree of neutralization varies depending on the final performance, if the neutralization degree is too high, the neutralized monomer may precipitate, making it difficult to smoothly polymerize. Conversely, if the neutralization degree is too low, the absorptivity of the polymer may be significantly reduced, and the polymer may exhibit elastic rubber-like properties, making it difficult to handle.

[0047] Meanwhile, in the monomer composition, as a polymerization initiator, a thermal polymerization initiator or a photopolymerization initiator according to the polymerization method can be used. However, even in the case of photopolymerization, since a certain amount of heat is generated by UV irradiation, etc., and a certain degree of heat is generated as the exothermic polymerization reaction proceeds, a thermal polymerization initiator may be additionally included.

[0048] Photopolymerization initiator can be used without limiting the structure, as long as it is a compound that can form free radicals by light such as UV. As photopolymerization initiator, one or more of the group consisting of benzoin ether, dialkyl acetophenone, hydroxyalkyl ketone, glyoxylic acid phenyl ester, benzyl dimethyl ketal, acylphosphine and α-amino ketone can be used. Meanwhile, as the specific examples of acylphosphine, diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, phenyl bis (2,4,6-trimethylbenzoyl) phosphine oxide, (2,4,6-trimethylbenzoyl) phenyl phosphinate etc. can be mentioned. More photopolymerization initiators are recorded in " UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007) " page 115 of Reinhold Schwalm, and are not limited to the above examples.

[0049] The photopolymerization initiator may be included in an amount of 0.001 to 5 parts by weight or 0.005 to 4.5 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. If the concentration of the photopolymerization initiator is too low, the polymerization rate may be slow. If the concentration of the photopolymerization initiator is too high, the molecular weight of the superabsorbent polymer may be low, and the properties may become uneven.

[0050] Furthermore, as a thermal polymerization initiator, one or more selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specific examples of persulfate initiators include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), and the like. Specific examples of azo initiators include 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoyl azo)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), and the like. A variety of more thermal polymerization initiators are described in Odian's "Principle of Polymerization (Wiley, 1981)," page 203, and are not limited to the above examples.

[0051] The thermal polymerization initiator can be used in an amount of 0.01 to 5 parts by weight, or 0.1 to 4 parts by weight, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. If the concentration of the thermal polymerization initiator is too low, the polymerization rate may be slow, and a large amount of residual monomer may be extracted in the final product. Furthermore, if the concentration of the thermal polymerization initiator is too high, the polymer chains that make up the superabsorbent polymer network may become shorter, thereby increasing the content of water-soluble components and possibly reducing the pressure absorption rate of the superabsorbent polymer, thereby degrading the performance of the polymer.

[0052] Also, the monomer composition includes an internal cross-linking agent as a raw material of the superabsorbent polymer.

[0053] As the internal crosslinking agent, any compound can be used as long as they can introduce crosslinking during the polymerization of the water-soluble ethylenically unsaturated monomer. As non-limiting examples, as the internal crosslinking agent, a multifunctional crosslinking agent such as N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butylene diglycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol or ethylene carbonate can be used alone or in combination, but are not limited thereto.

[0054] Among them, considering the improved cross-linking properties of the superabsorbent polymer and the resulting excellent improvement in absorbency, polyethylene glycol diacrylate is preferably used, and more specifically, polyethylene glycol diacrylate having a weight average molecular weight of 300 g / mol to 800 g / mol or 400 g / mol to 800 g / mol can be used. The weight average molecular weight (Mw) of polyethylene glycol diacrylate can be measured by gel permeation chromatography. The specific measurement method and measurement conditions are as described below in the method for measuring the weight average molecular weight (Mw) of polycarboxylic acids, except that the Mw value is derived using a calibration curve of a polystyrene standard.

[0055] The internal crosslinking agent can be used in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. If the concentration of the internal crosslinking agent is too low, the absorption rate of the polymer may decrease, and the gel strength may be weak. Conversely, if the concentration of the internal crosslinking agent is too high, the absorption capacity of the superabsorbent polymer may decrease, making it less preferred as an absorbent. More specifically, the content of the internal crosslinking agent can be 0.1 to 5 parts by weight, or 0.2 to 3 parts by weight.

[0056] As used herein, the term "internal crosslinking agent" is used to distinguish it from the surface crosslinking agent used to crosslink the surface of the base resin, as described below. This agent functions to crosslink and polymerize the unsaturated bonds of the water-soluble ethylenically unsaturated monomers described above. In this step, crosslinking is performed without distinguishing between the surface and the interior. However, through the surface crosslinking process of the base resin described below, the surface of the superabsorbent polymer finally produced consists of a structure crosslinked by the surface crosslinking agent, while the interior consists of a structure crosslinked by the internal crosslinking agent.

[0057] Furthermore, the monomer composition may contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant as needed.

[0058] Furthermore, such a monomer composition can be prepared in the form of a solution obtained by dissolving the above raw materials in a solvent. The solvent can be used without limitation in structure, as long as it can dissolve or disperse the above raw materials. For example, one or more selected from the group consisting of water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide can be used alone or in combination.

[0059] The solvent may be the balance other than the above components based on the total content of the monomer composition.

[0060] Furthermore, the solid content of the monomer composition in a solution state, i.e., the concentrations of the monomer, internal crosslinking agent, and polymerization initiator, can be appropriately controlled taking into account the polymerization time and reaction conditions. For example, the solid content of the monomer composition can be 10% to 80% by weight, or 15% to 60% by weight, or 20% to 40% by weight. When the monomer composition has the above-mentioned solid content range, as described below, it is advantageous to control the grinding efficiency during polymer grinding while avoiding the need to remove unreacted monomers after polymerization using the gelation phenomenon generated during the polymerization reaction of the high-concentration aqueous solution.

[0061] Meanwhile, the method of thermal polymerization or photopolymerization of such a monomer composition to form a hydrogel polymer is not particularly limited in its constitution as long as it is a commonly used polymerization method.

[0062] Specifically, polymerization methods are mainly divided into thermal polymerization and photopolymerization according to the energy source. The polymerization can be carried out by either thermal polymerization or photopolymerization, or both can be carried out simultaneously. The order of thermal polymerization and photopolymerization is not particularly limited. Thermal polymerization can be carried out after photopolymerization, or photopolymerization can be carried out after thermal polymerization, or photopolymerization can be carried out simultaneously. Generally, thermal polymerization can be carried out in a reactor equipped with a stirring shaft, such as a kneader, and photopolymerization can be carried out in a reactor equipped with a movable conveyor belt. However, the above polymerization method is only an example, and the present invention is not limited thereto.

[0063] For example, the hydrogel polymer can be obtained by introducing the monomer composition into a reactor equipped with a stirring shaft, such as a kneader, and supplying hot air or heating the reactor to carry out thermal polymerization. The hydrogel polymer discharged from the reactor outlet can have a size ranging from several centimeters to several millimeters, depending on the shape of the stirring shaft provided in the reactor. Specifically, the size of the resulting hydrogel polymer can vary depending on, for example, the concentration and introduction rate of the monomer composition introduced, and typically, a hydrogel polymer having a particle size of 2 mm to 50 mm can be obtained.

[0064] Furthermore, if the photopolymerization of the monomer composition is performed in a reactor equipped with a movable conveyor as described above, the resulting hydrogel polymer can generally be a hydrogel polymer sheet having a belt width. While the thickness of the sheet can vary depending on the concentration and introduction rate of the monomer composition, it is preferred that the monomer composition be supplied to obtain a polymer sheet having a thickness of approximately 0.5 cm to 5 cm. If the polymer sheet obtained by supplying the monomer composition is too thin, production efficiency may be reduced. If the polymer sheet is thicker than 5 cm, polymerization may not occur uniformly throughout the entire thickness due to the excessive thickness.

[0065] The water content of the hydrogel polymer thus obtained can generally be 40% to 80% by weight. Throughout this specification, "water content" refers to the amount of water present in the total weight of the hydrogel polymer, and is the value obtained by subtracting the weight of the hydrogel polymer in its dry state from the weight of the hydrogel polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to evaporation of water from the polymer while drying the polymer at an elevated temperature using infrared heating. The temperature is raised from room temperature to approximately 180°C and then maintained at 180°C, with a total drying time of 20 minutes (including a 5-minute heating step).

[0066] According to one embodiment of the present invention, a coarse grinding process of the hydrogel polymer obtained above may be optionally further performed.

[0067] Among them, the grinding machine that can be used in the coarse grinding process is not limited in structure, but specifically, one selected from the group consisting of a vertical crusher, a turbine cutter, a turbine grinder, a rotary cutter, a cutter, a disc grinder, a pulverizer, a crusher, a shredder, and a disc cutter can be used, but is not limited thereto.

[0068] The coarse grinding can be performed to achieve a particle size of 2 mm to 20 mm for the hydrogel polymer. Due to the high water content of the hydrogel polymer, coarse grinding to a particle size of less than 2 mm is technically challenging and may cause agglomeration of the ground particles. Furthermore, if the coarse grinding is performed to a particle size greater than 20 mm, the efficiency of the subsequent drying step may not be significantly improved.

[0069] (Step 2)

[0070] Next, step 2 is a step of drying, grinding, and classifying the hydrogel polymer prepared in step 1 to prepare a base resin.

[0071] The drying process of the hydrogel polymer can be performed at 50°C to 250°C. If the drying temperature is lower than 50°C, the drying time may be too long, and the properties of the superabsorbent polymer produced may be deteriorated. If the drying temperature is higher than 250°C, only the polymer surface may be dried, and fine particles may be generated during the subsequent grinding process, and the properties of the superabsorbent polymer produced may be deteriorated. More preferably, the drying process can be performed at a temperature of 150°C to 200°C, and particularly preferably at a temperature of 160°C to 190°C. Meanwhile, considering processing efficiency, the drying process can be performed for 20 minutes to 15 hours, but the drying time is not limited thereto.

[0072] Furthermore, the drying method is not limited as long as it is a drying process generally used for hydrogel polymers. Specifically, drying can be performed by supplying hot air, infrared irradiation, microwave irradiation, ultraviolet irradiation, or the like.

[0073] After the drying step, the dried polymer may have a water content of 5 wt% or less, more specifically 0.1 wt% to 3 wt%.If the water content of the dried polymer is greater than 5 wt%, undried polymer may be obtained.

[0074] Subsequently, the dried polymer is ground and classified.

[0075] The polymer powder obtained after grinding may have a particle size of 150 to 850 μm. As a grinder for grinding to such a particle size, specifically, a ball mill, pin mill, hammer mill, screw mill, roller mill, disc mill or micro-mill can be used, but it is not limited thereto.

[0076] Furthermore, in order to manage the properties of the superabsorbent polymer powder finally produced after the grinding step, a separate process of classifying the polymer powder obtained after grinding according to particle size can be performed. Preferably, the polymer powder having a particle size of 150 μm to 850 μm can be classified, and only the polymer powder having such a particle size can be subjected to the surface crosslinking step described below and commercialized.

[0077] The base resin powder obtained through the above-mentioned treatment is preferably prepared to have a particle size of 150 μm to 850 μm. More specifically, at least 95% by weight of the base resin powder may have a particle size of 150 μm to 850 μm, and the content of fine particles with a particle size of less than 150 μm may be less than 3% by weight. In this way, since the particle size distribution of the base resin powder is controlled within the preferred range, the superabsorbent polymer finally prepared can better exhibit the above-mentioned properties.

[0078] Unless otherwise specified herein, "particle size" or "particle size" can be measured by standard sieve analysis or laser diffraction, preferably standard sieve analysis, and "average particle size" or "weight-average particle size" can refer to the particle size (D50) that accounts for 50% of the weight percentage in the particle size distribution curve obtained by laser diffraction.

[0079] Meanwhile, fine particles below a certain particle size, i.e., less than about 150 μm, are called base resin fine particles, super absorbent polymer fine particles, SAP fine particles or fine particles (fine powder), and particles with a particle size of 150 μm to 850 μm are called regular particles.

[0080] Fine particles may be generated during the polymerization process, drying process, or grinding process of the dried polymer. However, if the fine particles are included in the final product, they may be difficult to handle and may cause gel blocking, thereby reducing performance. Therefore, it is preferable to restructure so that the fine particles are not included in the final polymer product, or the fine particles can be converted into conventional particles.

[0081] For example, a reconstitution process can be performed in which the fine particles are agglomerated into a conventional particle size. Typically, a reconstitution process is performed in which the fine particles are agglomerated in a wet state to increase agglomerate strength. The higher the moisture content of the fine particles, the greater the agglomerate strength of the fine particles. However, this may result in excessive reconstitution mass during the reconstitution process, leading to handling problems. If the moisture content is low, the reconstitution process may be easy to perform, but due to the low agglomerate strength, the reconstituted product may often be crushed into fine particles again (refined). Furthermore, the fine particle reconstituted product thus obtained may have degraded properties, such as centrifuge retention capacity (CRC) or absorbency under load (AUL), compared to conventional particles, resulting in degraded quality of the superabsorbent polymer.

[0082] Therefore, the preparation method according to one embodiment of the present invention may further include a step of adding water and additives to the fine particles and recombining to prepare a recombinant fine particle, and then mixing the recombinant fine particle with a base resin powder.

[0083] When preparing the fine particle recombinant, the fine particles, additives and water may be mixed by stirring at a speed of about 10 rpm to about 2000 rpm, about 100 rpm to about 1000 rpm or about 500 rpm to about 800 rpm using a mixing device or mixer capable of adding shear force.

[0084] Furthermore, after mixing, a drying process may be performed. Specifically, the drying process may be performed at about 120° C. to about 220° C., thereby forming a fine particle recombinant with improved agglomeration strength through covalent bonds, and the moisture content of the fine particle recombinant may be controlled to about 1 wt % to about 2 wt % within an appropriate time.

[0085] The drying process can be performed using conventional drying equipment, but according to one embodiment of the present invention, a hot air dryer, a paddle dryer, or a forced circulation dryer can be used. Furthermore, the temperature-raising device used for drying is not limited in structure. Specifically, usable heat sources include steam, electricity, ultraviolet light, infrared light, and the like, and a heating fluid can also be used.

[0086] The fine particle recombinant obtained by the step of preparing the fine particle recombinant has a high agglomeration strength, and therefore has a low re-crushing rate (ie, re-refining rate) after grinding.

[0087] The fine particle recombinant can be ground so that the particle size of the fine particle recombinant can be about 150 μm to about 850 μm. As a grinding machine for grinding to such a particle size, a pin mill, a hammer mill, a screw mill, a roller mill, a disc mill, or a micro-motion mill can be used, but is not limited thereto.

[0088] In order to manage the properties of the superabsorbent polymer powder finally produced after the grinding step, the reconstituted powder obtained after the grinding step is generally classified according to particle size. Preferably, the following step can be further performed: classification into reconstituted fine particles having a particle size of 150 μm or less (hereinafter referred to as "re-refined particles") and reconstituted conventional particles having a particle size of greater than 150 μm and less than 850 μm.

[0089] The recombinant conventional particles can be mixed with the base resin powder prepared above and then prepared into superabsorbent polymer through subsequent processing. The recombinant conventional particles can be mixed in an amount of 1 to 40 parts by weight based on 100 parts by weight of the base resin powder.

[0090] (Step 3)

[0091] Next, step 3 is a step of performing first surface crosslinking on the base resin powder or the base resin powder optionally containing recombinant conventional particles in the presence of a first surface crosslinking agent while being elevated to a first temperature.

[0092] Surface crosslinking increases the crosslink density around the surface of superabsorbent polymer particles. Typically, a surface crosslinking agent is applied to the surface of the superabsorbent polymer particles. Therefore, the reaction occurs at the surface of the superabsorbent polymer particles, thereby increasing the crosslinkability of the particle surface without substantially affecting the interior of the particles. Consequently, surface-crosslinked superabsorbent polymer particles have a higher crosslink density around the surface than within the particles.

[0093] In the preparation method according to one embodiment of the present invention, the surface crosslinking includes a first surface crosslinking step (step 3) performed while raising the temperature from an initial temperature to a first temperature, and a second surface crosslinking step (step 4) performed at a second temperature, wherein the second temperature is lower than the first temperature.

[0094] Compared with the existing surface cross-linking process in which the surface cross-linking reaction is carried out while maintaining a constant high temperature or while increasing the temperature, in the case where the first surface cross-linking step is carried out at a higher temperature and the second surface cross-linking step is carried out at a lower temperature than the first surface cross-linking step, the second surface cross-linking agent can be uniformly distributed on the surface of the superabsorbent polymer, and therefore, the surface cross-linking strength can be uniformly improved, thereby not only maintaining the performance of the superabsorbent polymer related to excellent absorption performance but also improving the anti-caking effect of the superabsorbent polymer.

[0095] Specifically, the first temperature in the first surface crosslinking step can be 180°C or higher, more specifically 180°C to 200°C, and the second temperature in the subsequent second surface crosslinking step can be 120°C to 150°C. As described above, by conducting the first surface crosslinking reaction at a high temperature of 180°C or higher, the surface of the base resin can be fully crosslinked, thereby exhibiting excellent pressure absorption rate. If the first temperature is lower than 180°C, the first surface crosslinking reaction may not fully occur, making it difficult to ensure the surface crosslinking strength. As a result, the 0.9AUL drop rate after anti-caking treatment may increase. Furthermore, if the second temperature is greater than 150°C, the solvent may evaporate rapidly before the second surface crosslinking agent is fully mixed with the first surface crosslinked base resin, and the second surface crosslinking agent may be locally adsorbed in the first surface crosslinked base resin. As a result, the surface reaction proceeds while the second surface crosslinking agent is locally applied, thereby reducing the effectiveness of the second surface crosslinking and deteriorating the absorption performance of the superabsorbent polymer produced, particularly CRC. Furthermore, if the second temperature is lower than 120°C, the second surface crosslinking reaction may not proceed sufficiently, and thus the 0.9AUL reduction rate of the superabsorbent polymer after the anti-blocking treatment may significantly increase. More specifically, the first temperature may be 180°C or higher, or 185°C or higher, and 200°C or lower, or 195°C or lower, or 190°C or lower, and the second temperature may be 120°C or higher, or 125°C or higher, or 130°C or higher, and 150°C or lower, or 145°C or lower, or 140°C or lower.

[0096] Furthermore, in the preparation method according to one embodiment, a compound capable of reacting with functional groups of the polymer and having a smaller weight-average molecular weight than the second surface crosslinker can be used as the first surface crosslinking agent for the first surface crosslinking. If the first surface crosslinking agent having a smaller weight-average molecular weight than the second surface crosslinker is subjected to a surface crosslinking reaction at a high temperature, the first surface crosslinker can penetrate deeply into the surface of the superabsorbent polymer, thereby ensuring the superabsorbent polymer's absorbency under pressure.

[0097] Specifically, as the first surface crosslinking agent, a compound having a weight average molecular weight satisfying the ratio of the weight average molecular weight of the second surface crosslinking agent to the weight average molecular weight of the first surface crosslinking agent is 1.8 or more, or 1.8 to 15 can be used.

[0098] More specifically, the weight average molecular weight of the first surface crosslinking agent may be 50 g / mol to 200 g / mol, more specifically, the weight average molecular weight is 50 g / mol or more, or 60 g / mol or more, or 70 g / mol or more, or 75 g / mol or more, and 200 g / mol or less, or 150 g / mol or less, or 120 g / mol or less, or 100 g / mol or less, or 90 g / mol or less, or 85 g / mol or less. The weight average molecular weight (Mw) of the first surface crosslinking agent can be measured using gel permeation chromatography. The specific measurement method and measurement conditions are as described below in the method for measuring the weight average molecular weight (Mw) of polycarboxylic acids, except that the Mw value is derived using a calibration curve of a polystyrene standard.

[0099] Also, the first surface cross-linking agent is different from the second surface cross-linking agent.

[0100] For example, the first surface crosslinking agent may be a non-epoxy compound, and the second surface crosslinking agent may be an epoxy compound having a weight average molecular weight greater than that of the first surface crosslinking agent and capable of forming a covalent bond at 120°C to 150°C.

[0101] Specifically, the first surface crosslinking agent may be a non-epoxy compound capable of forming a covalent bond at a temperature above 180° C. More specifically, the first surface crosslinking agent may include polyol compounds or alkylene carbonate compounds, one of which or a mixture thereof may be used.

[0102] Specific examples of polyol compounds include ethylene glycol, propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol. One of these or a mixture thereof can be used.

[0103] Specific examples of the alkylene carbonate compound include alkylene carbonates having 2 to 6 carbon atoms, such as ethylene carbonate and propylene carbonate, and any one of these or a mixture thereof can be used.

[0104] More specifically, the first surface crosslinking agent can be one or more selected from polyol compounds or alkylene carbonate compounds, which meet the above-mentioned molecular weight range conditions. Specifically, the ratio of the weight average molecular weight of the second surface crosslinking agent to the weight average molecular weight of the first surface crosslinking agent is 1.8 or greater, or 1.8 to 15. Furthermore, the first surface crosslinking agent can be one or more selected from polyol compounds or alkylene carbonate compounds having a weight average molecular weight of 50 g / mol to 200 g / mol.

[0105] The amount of this first surface crosslinking agent used can be 0.01 to 5 parts by weight based on 100 parts by weight of the base resin. If the amount of the first surface crosslinking agent used is less than 0.01 parts by weight, the effect of using the first surface crosslinking agent may not be significant, making it difficult to ensure the pressure absorption rate of the superabsorbent polymer. Moreover, if the amount used exceeds 5 parts by weight, surface crosslinking may be excessive, so that the performance of the superabsorbent polymer, especially the absorption performance, may be excessively deteriorated. More specifically, based on 100 parts by weight of the base resin, the amount of the first surface crosslinking agent used can be 0.01 parts by weight or more, or 0.1 parts by weight or more, or 0.2 parts by weight or more, or 0.4 parts by weight or more, or 1 part by weight or more, and 5 parts by weight or less, or 3 parts by weight or less, or 2 parts by weight or less, or 1.5 parts by weight or less.

[0106] Furthermore, in the first surface cross-linking step, an inorganic filler may be introduced together with the first surface cross-linking agent.

[0107] Examples of the inorganic filler include silica, fumed silica, clay, alumina, silica-alumina composite materials, titanium dioxide, zinc oxide, and silicates, and any one of these or a mixture thereof can be used. The inorganic filler content can be 0.01 to 0.5 parts by weight based on 100 parts by weight of the base resin. More specifically, the content can be 0.01 or more parts by weight, or 0.03 or more parts by weight, and 0.5 or less parts by weight, or 0.1 or less parts by weight, or 0.05 or less parts by weight.

[0108] Also, during the first surface cross-linking, one or more additives, such as an organic acid or a thickener, may optionally be further introduced.

[0109] The organic acid plays a role in promoting the cross-linking reaction.

[0110] Specifically, the organic acid may be carboxylic acids such as oxalic acid, acetic acid, lactic acid, citric acid, fumaric acid, tartaric acid, and maleic acid, and any one thereof or a mixture thereof may be used. Of these, oxalic acid is preferably used due to its excellent compatibility with the first surface crosslinking agent.

[0111] The organic acid may be introduced in an amount of 0.1 to 5 parts by weight, based on 100 parts by weight of the base resin powder, more specifically, in an amount of 0.1 part by weight or more, or 0.15 part by weight or more, and 5 parts by weight or less, or 3 parts by weight or less, or 1 part by weight or less, or 0.5 part by weight or less.

[0112] Moreover, when the first surface cross-linking reaction of the base resin powder is carried out in the presence of a thickener, even after grinding, performance degradation can be reduced to the greatest extent. Specifically, as a thickener, one or more selected from polysaccharides and hydroxy-containing polymers can be used. As a polysaccharide, a glue thickener and a cellulose thickener etc. can be used. As the specific example of the glue thickener, xanthan gum, gum arabic, karaya gum, tragacanth gum, ghatti gum, guar gum, locust bean gum and psyllium gum etc. can be enumerated, as the specific example of the cellulose thickener, hydroxypropyl methylcellulose, carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methylcellulose, hydroxymethyl propyl cellulose, hydroxyethyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose and methyl hydroxypropyl cellulose etc. can be enumerated. Simultaneously, as the specific example of the hydroxy-containing polymer, polyethylene glycol and polyvinyl alcohol etc. can be enumerated.

[0113] The first surface crosslinking agent and the base resin can be mixed by a commonly used mixing method. For example, the first surface crosslinking agent and the base resin can be placed in a reactor and mixed, or the first surface crosslinking agent can be sprayed into the base resin, or the base resin and the surface crosslinking agent can be continuously supplied to a continuously operated mixer and mixed.

[0114] Furthermore, when the first surface crosslinker is mixed with the base resin, the first surface crosslinker and optionally introduced components, specifically inorganic additives, organic acids, thickeners, etc., can be used while dissolved or dispersed in a solvent. Water, methanol, or a mixture thereof can be used as the solvent. Thus, when a solvent is added, the first surface crosslinker can be evenly distributed in the base resin. By controlling the amount of solvent introduced, uniform dispersion of the surface crosslinker can be induced, base resin agglomeration can be prevented, and the surface penetration depth of the crosslinker can be optimized. Specifically, the solvent can be used in an amount of 0.5 to 20 parts by weight, more specifically, 0.5 to 3 parts by weight, 5 to 6 parts by weight, and 10 to 8 parts by weight, or 7.5 to 100 parts by weight of the base resin. Furthermore, when the solvent comprises a mixture of water and methanol, the weight ratio of water to methanol can be 1:2 to 2:1, or 1:1.2 to 1.5:1.

[0115] The first surface crosslinking process using a first surface crosslinking agent can be performed in the presence of the first surface crosslinking agent while raising the temperature to a first temperature, specifically, 180°C or higher, more specifically, 180°C to 200°C. By performing the first surface crosslinking reaction at a high temperature, the surface of the base resin can be fully crosslinked, thereby exhibiting excellent pressure absorption. However, if the first temperature is lower than 180°C, the first surface crosslinking reaction may not fully occur, making it difficult to ensure surface crosslink strength. As a result, the 0.9AUL drop rate after anti-blocking treatment may increase. More specifically, the first temperature can be higher than 180°C, or higher than 185°C, and lower than 200°C, or lower than 195°C, or lower than 190°C.

[0116] The method for raising the temperature for the first surface crosslinking is not particularly limited. A heat medium may be provided, or a heat source may be directly supplied for heating. Examples of the heat medium include steam, hot air, and a temperature-elevated fluid such as hot oil. The temperature of the heat medium may be appropriately selected in consideration of the heat medium, the temperature-elevating device, and the target temperature. Examples of directly supplied heat sources include, but are not limited to, electric heating or gas heating.

[0117] Furthermore, the first surface crosslinking may further include a first holding step of maintaining the first temperature after the temperature is raised to the first temperature. Therefore, by further including the holding step, the surface crosslinking of the base resin can be sufficiently and uniformly achieved, thereby further improving the pressure absorption rate of the superabsorbent polymer.

[0118] Specifically, the first holding step may be performed for at least 50% of the total time for the first surface crosslinking, more specifically at least 50%, at least 55%, or at least 60%, and at most 80%, at most 70%, or at most 65%. The total time for the first surface crosslinking refers to the time from the start of heating to raise the temperature to the first temperature, specifically, from the time the mixture of the base resin and the first surface crosslinking agent is heated to the completion of the first surface crosslinking or the first surface crosslinking step.

[0119] (Step 4)

[0120] Next, step 4 is a step of subjecting the first surface-crosslinked base resin prepared in step 3 to a second surface crosslinking reaction at a second temperature in the presence of a second surface crosslinking agent to prepare a superabsorbent polymer.

[0121] As the second surface crosslinking agent, an epoxy compound having a weight average molecular weight larger than that of the first surface crosslinking agent can be used.

[0122] Specifically, the weight-average molecular weight of the second surface crosslinking agent may satisfy a ratio of the weight-average molecular weight of the second surface crosslinking agent to the weight-average molecular weight of the first surface crosslinking agent of 1.8 or more, or 1.95 or more, or 2 or more, or 2.2 or more. However, if the ratio of the weight-average molecular weight of the second surface crosslinking agent to the weight-average molecular weight of the first surface crosslinking agent is too high and outside the above range, the hydrophilicity may be significantly reduced, and thus the absorption performance of the superabsorbent polymer may be deteriorated. Therefore, the weight-average molecular weight of the second surface crosslinking agent may satisfy a ratio of the weight-average molecular weight of the second surface crosslinking agent to the weight-average molecular weight of the first surface crosslinking agent of 15 or less, or 10 or less, or 5 or less, or 3 or less.

[0123] When the ratio of the weight-average molecular weight of the second surface crosslinking agent to the weight-average molecular weight of the first surface crosslinking agent is within the above range, the second surface crosslinking agent can penetrate more slowly than the first surface crosslinking agent, and thus only the particle surfaces can be effectively crosslinked. As a result, the pressure absorption rate of the superabsorbent polymer can be further improved.

[0124] More specifically, the weight average molecular weight of the second surface crosslinking agent can be 100 g / mol to 600 g / mol, and even more specifically, 100 g / mol or more, or 120 g / mol or more, or 150 g / mol or more, or 170 g / mol or more, and 500 g / mol or less, or 300 g / mol or less, or 250 g / mol or less, or 220 g / mol or less. The weight average molecular weight (Mw) of the second surface crosslinking agent can be measured using gel permeation chromatography. The specific measurement method and measurement conditions are as described below in the measurement method for the weight average molecular weight (Mw) of polycarboxylic acids, except that the value of Mw is derived using a calibration curve of a polystyrene standard.

[0125] More specifically, in the preparation method, the weight average molecular weight of the first surface crosslinking agent may be 50 g / mol to 200 g / mol, more specifically, 50 g / mol or more, or 60 g / mol or more, or 70 g / mol or more, or 75 g / mol or more, and 200 g / mol or less, or 150 g / mol or less, or 120 g / mol or less, or 100 g / mol or less, or 90 g / mol or less, or 85 g / mol or less, and the weight average molecular weight of the second surface crosslinking agent may be 100 g / mol to 200 g / mol, more specifically, 50 g / mol or more, or 60 g / mol or more, or 70 g / mol or more, or 75 g / mol or more, and 200 g / mol or less, or 150 g / mol or less, or 120 g / mol or less, or 100 g / mol or less, or 90 g / mol or less, or 85 g / mol or less. The present invention relates to a surface cross-linking agent having an average molecular weight of at least 600 g / mol, more specifically, 100 g / mol or more, or 120 g / mol or more, or 150 g / mol or more, or 170 g / mol or more, and 500 g / mol or less, or 300 g / mol or less, or 250 g / mol or less, or 220 g / mol or less, provided that the ratio of the weight average molecular weight of the second surface cross-linking agent to the weight average molecular weight of the first surface cross-linking agent is 1.8 or more, or 1.95 or more, or 2 or more, or 2.2 or more, and 15 or less, or 10 or less, or 5 or less, or 3 or less.

[0126] Furthermore, since the second surface crosslinking reaction using the second surface crosslinking agent is carried out using the residual heat during the cooling process after the completion of the first surface crosslinking reaction, it is preferred to use a compound that can quickly form a covalent bond with an acidic group (specifically, a carboxylic acid group of a crosslinked polymer of a water-soluble ethylenically unsaturated monomer) within the reaction temperature range (specifically, 120°C to 150°C) as the second surface crosslinking agent. To this end, the following epoxy compound can be used as the second surface crosslinking agent: it has two or more epoxy functional groups that can crosslink with the crosslinked polymer present on the surface of the base resin powder, and has a boiling point of 150°C or above under normal pressure or atmospheric pressure conditions (specifically, under 1±0.2atm conditions). If the boiling point is lower than 150°C, the surface crosslinking agent may evaporate before the reaction due to the residual heat of the superabsorbent polymer, and thus the efficiency relative to the amount introduced may decrease. More specifically, the epoxy compound may have a boiling point of 150°C or more, 180°C or more, 200°C or more, 250°C or more, 260°C or more, or 265°C or more, and 500°C or less, 350°C or less, 300°C or less, or 280°C or less.

[0127] As the second surface cross-linking agent, specifically, one or more epoxy compounds can be used, for example, ethylene glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, propylene glycol diglycidyl ether, hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, tripropylene glycol diglycidyl ether, glycerol triglycidyl ether, polypropylene glycol diglycidyl ether or polyethylene glycol diglycidyl ether, etc.

[0128] Furthermore, as polyethylene glycol diglycidyl ether or polypropylene glycol diglycidyl ether, specifically, those having a weight average molecular weight of 200 g / mol to 600 g / mol can be used. The weight average molecular weight (Mw) of polyethylene glycol diglycidyl ether or polypropylene glycol diglycidyl ether can be measured by gel permeation chromatography. The specific measurement method and measurement conditions are as described below in the method for measuring the weight average molecular weight (Mw) of polycarboxylic acids, except that the Mw value is derived using a calibration curve of a polystyrene standard.

[0129] The amount of the second surface crosslinking agent used can be 0.001 to 1 part by weight based on 100 parts by weight of the first surface-crosslinked base resin. If the amount of the second surface crosslinking agent is less than 0.001 parts by weight, the second surface crosslinking may not be fully achieved, and thus the effect of improving the absorption performance of the superabsorbent polymer may not be significant. If it exceeds 1 part by weight, the surface crosslinking may be excessive, and thus the performance of the superabsorbent polymer, especially the dryness, may be deteriorated. More specifically, the amount of the second surface crosslinking agent used can be 0.001 parts by weight or more, or 0.003 parts by weight or more, or 0.005 parts by weight or more, and less than 1 part by weight, or less than 0.5 parts by weight, or less than 0.1 parts by weight, or less than 0.05 parts by weight, or less than 0.02 parts by weight, or less than 0.01 parts by weight, based on 100 parts by weight of the first surface-crosslinked base resin.

[0130] Also, during the second surface cross-linking, one or more selected from polycarboxylic acids or salts thereof may be further introduced optionally together with the second surface cross-linking agent.

[0131] The polycarboxylic acid or its salt acts as a lubricant, facilitating uniform application of the second surface crosslinking agent to the surface of the first surface-crosslinked base resin powder. Generally, among the properties of superabsorbent polymers, permeability is in a trade-off relationship with centrifuge retention capacity and pressurized absorbency. However, by incorporating a polycarboxylic acid or its salt, a superabsorbent polymer having not only excellent absorptive properties such as centrifuge retention capacity and pressurized absorbency but also improved permeability can be provided.

[0132] As long as they are used in the preparation of superabsorbent polymers, the polycarboxylic acid or its salt is not particularly limited and can be prepared by itself, or ACYMA-GK can be used. TM (manufactured by Aezis Ltd.) and other commercially available products.

[0133] Specifically, the polycarboxylic acid or a salt thereof may be a copolymer including one or more of a repeating unit represented by the following Chemical Formula 2-a or a repeating unit represented by the following Chemical Formula 2-b, or a salt thereof.

[0134] [Chemical Formula 2-a]

[0135]

[0136] [Chemical Formula 2-b]

[0137]

[0138] In Chemical Formulas 2-a and 2-b,

[0139] R 1 、R 2 and R 3 are each independently hydrogen or C1-6 alkyl,

[0140] RO is a C2-4 oxyalkylene group,

[0141] M 1 is hydrogen or a monovalent metal or non-metal ion,

[0142] X is -COO-, C1-5 alkoxy or C1-5 alkyldioxy,

[0143] m is an integer from 0 to 100,

[0144] n is an integer from 1 to 1000,

[0145] p is an integer of 1 to 150. When p is 2 or more, two or more repeated -RO- groups may be the same or different.

[0146] The polycarboxylic acid or its salt may be a copolymer or its salt comprising one or more repeating units of different structures selected from the repeating units represented by Chemical Formula 2-a; one or more repeating units of different structures selected from the repeating units represented by Chemical Formula 2-b; or a repeating unit represented by Chemical Formula 2-1 and a repeating unit represented by Chemical Formula 2-b.

[0147] Also, the salt of the polycarboxylic acid may be one or more selected from the group consisting of monovalent (alkali) metal salts, divalent metal salts, ammonium salts, and organic amine salts of the polycarboxylic acid.

[0148] More specifically, as the polycarboxylic acid or its salt, a random copolymer or its salt including repeating units derived from a hydrophilic monomer such as an alkoxypolyalkylene glycol mono(meth)acrylate-based monomer (as a representative example, methoxypolyethylene glycol monomethacrylate (MPEGMAA)) and (meth)acrylic acid or its ester-based monomer (as a representative example, (meth)acrylic acid, (meth)acrylate) can be advantageously used.

[0149] Furthermore, in order to better demonstrate the effect of adding a polycarboxylic acid or its salt, it is preferred that the weight average molecular weight of the polycarboxylic acid or its salt is 500 g / mol to 1,000,000 g / mol, more specifically, 500 g / mol or more, or 5,000 g / mol or more, or 10,000 g / mol or more, or 35,000 g / mol or more, or 40,000 g / mol or more, and 1,000,000 g / mol or less, or 800,000 g / mol or less, or 60,000 g / mol or less. If the molecular weight of the polycarboxylic acid or its salt is less than 500 g / mol, the lubricating effect may be deteriorated, and if it exceeds 1,000,000 g / mol, the water solubility may be deteriorated.

[0150] Meanwhile, the weight average molecular weight (Mw) of polycarboxylic acid or its salt can be determined using gel permeation chromatography. Specifically, as a gel permeation chromatography (GPC) device, the PL-GPC220 device manufactured by Waters was used, and the PLgel MIX-B column (300 mm long) manufactured by Polymer Laboratories was used. Wherein, the determination temperature was 160 ° C, with 1,2,4-trichlorobenzene as solvent, and the flow rate was 1 mL / minute. PL-SP260 (produced by Agilent Technologies) was used to dissolve 10 mg of polycarboxylic acid or its salt in 1,2,4-trichlorobenzene containing 0.0125% BHT, pretreated at 160 ° C for 10 hours, formulated to a concentration of 10 mg / 10 mL, and then fed in an amount of 200 μL. The calibration curve formed using polystyrene standards was used to derive the Mw value. The weight average molecular weight of the polystyrene standards is 2000 g / mol, 10000 g / mol, 30000 g / mol, 70000 g / mol, 200000 g / mol, 700000 g / mol, 2000000 g / mol, 4000000 g / mol, and 100000000 g / mol.

[0151] Based on 100 parts by weight of the first surface-crosslinked base resin, the amount of polycarboxylic acid or its salt introduced can be 0.01 parts by weight or more, or 0.03 parts by weight or more, or 0.035 parts by weight or more, and 0.1 parts by weight or less, or 0.08 parts by weight or less, or 0.075 parts by weight or less, or 0.05 parts by weight or less.

[0152] Furthermore, during the second surface crosslinking, an inorganic filler may be further introduced together with the second surface crosslinking agent. The inorganic filler is as described above and may be present in an amount of 0.01 to 0.5 parts by weight or 0.1 to 0.5 parts by weight based on 100 parts by weight of the first surface crosslinked base resin.

[0153] Furthermore, during the second surface crosslinking, a thickener may be further introduced together with the second surface crosslinking agent. The thickener is as described above and may be present in an amount of 0.01 to 0.5 parts by weight or 0.1 to 0.5 parts by weight based on 100 parts by weight of the first surface crosslinked base resin.

[0154] Furthermore, the method for mixing the second surface crosslinking agent with the first surface crosslinked base resin is not limited. For example, the second surface crosslinking agent and the first surface crosslinked base resin can be placed in a reactor and mixed, or the second surface crosslinking agent can be sprayed onto the base resin, or the base resin and the second surface crosslinking agent can be continuously supplied to a continuously operated mixer and mixed.

[0155] Furthermore, when the second surface crosslinking agent is mixed with the first surface crosslinked base resin, a solvent such as water or methanol may be further added to uniformly disperse the second surface crosslinking agent in the first surface crosslinked base resin. Furthermore, by controlling the amount of solvent added, uniform dispersion of the second surface crosslinking agent can be induced, agglomeration of the first surface crosslinked base resin can be prevented, and the surface crosslinking depth of the second surface crosslinking agent can be optimized. Specifically, the amount of solvent used may be 0.5 to 10 parts by weight based on 100 parts by weight of the first surface crosslinked base resin.

[0156] At the same time, a second surface crosslinking reaction using a second surface crosslinking agent is performed at a second temperature, specifically 120°C to 150°C.

[0157] If the second temperature is greater than 150°C, the solvent may evaporate rapidly before the second surface crosslinking agent is fully mixed with the first surface-crosslinked base resin, resulting in the second surface crosslinking agent being locally adsorbed on the first surface-crosslinked base resin. As a result, the surface reaction proceeds while the second surface crosslinking agent is locally applied, and the effectiveness of the second surface crosslinking may be degraded. Furthermore, if the second temperature is less than 120°C, the second surface crosslinking reaction may not proceed sufficiently, and the 0.9AUL reduction rate after the superabsorbent polymer anti-caking treatment may be significantly increased. More specifically, the second temperature may be greater than 120°C, or greater than 125°C, or greater than 130°C, and less than 150°C, or less than 145°C, or less than 140°C.

[0158] Also, the second surface cross-linking reaction may be performed under cooling conditions in which the temperature is lowered to within the above temperature range.

[0159] According to the preparation method of the present invention, the first surface crosslinking is performed at a high temperature, and then the second surface crosslinking is performed using the residual heat after the first surface crosslinking. Therefore, by controlling the introduction time of the second surface crosslinking agent and the cooling conditions, the residual heat after the first surface crosslinking reaction can be maximized to achieve the second surface crosslinking reaction. Furthermore, compared to performing the second surface crosslinking reaction while maintaining a constant temperature, the penetration rate of the second surface crosslinking agent into the surface of the superabsorbent polymer can be more easily controlled.

[0160] Specifically, the second surface crosslinking can be performed by introducing a second surface crosslinking agent when the temperature of the first surface crosslinked base resin is 140°C to 150°C, and then lowering the temperature to 120°C to 130°C.

[0161] If the second surface crosslinking is performed under the above conditions, the efficiency of the second surface crosslinking reaction can be further improved, thereby further reducing the 0.9AUL drop rate of the superabsorbent polymer after anti-caking treatment. If the temperature of the first surface crosslinked base resin powder is lower than 140°C when the second surface crosslinking agent is introduced, it may be difficult to maximize the efficiency of the second surface crosslinking reaction. Furthermore, if the temperature of the first surface crosslinked base resin powder is higher than 150°C when the second surface crosslinking agent is introduced, the solvent may evaporate rapidly before the second surface crosslinking agent is fully mixed with the first surface crosslinked base resin, and the second surface crosslinking agent may be locally adsorbed in the first surface crosslinked base resin. As a result, the surface reaction proceeds while the second surface crosslinking agent is locally applied, and the effectiveness of the second surface crosslinking may be degraded.

[0162] Furthermore, by controlling the cooling rate in the second surface cross-linking reaction under the above cooling conditions, the efficiency of the surface cross-linking reaction can be improved, thereby further improving the performance of the prepared superabsorbent polymer.

[0163] Specifically, the second surface crosslinking reaction can be carried out by introducing a second surface crosslinking agent when the temperature of the first surface crosslinked base resin powder is 140°C to 150°C, and then reducing the temperature to 120°C to 130°C at a cooling rate of -1.5°C / min to -2.0°C / min. In the cooling rate, "-" means that the reaction is carried out while the temperature is reduced.

[0164] Furthermore, if the cooling rate during the second surface crosslinking process is less than -1.5°C / min, it may be difficult to ensure sufficient surface crosslink strength due to rapid evaporation of the solvent in the surface crosslinking agent and the resulting low penetration depth of the surface crosslinking agent. Furthermore, if the cooling rate is greater than -2.0°C / min, the rapid drop in temperature may slow the reaction rate of the surface crosslinking agent, resulting in insufficient reaction and potential deterioration of the surface crosslink strength.

[0165] Furthermore, cooling during the second surface cross-linking reaction can be achieved by natural cooling, which allows the temperature of the reaction system to naturally decrease to room temperature after the first surface cross-linking reaction is completed, or by conventional cooling methods, such as circulation of cold air, cold water, or cooling oil. For example, cooling can be achieved by introducing the second surface cross-linking agent, then placing the reactor or liquid reservoir including the reaction system in an oil circulation tank, and circulating low-temperature oil.

[0166] Meanwhile, step 4 may further include a process of classifying the base resin cross-linked on the second surface.

[0167] By classifying the base resin cross-linked on the second surface according to the particle size, the performance of the superabsorbent polymer powder finally produced can be controlled. It is appropriate to prepare the superabsorbent polymer obtained by the grinding and classification process and make it have a particle size of about 150μm to 850μm. More specifically, at least about 95% by weight of the base resin cross-linked on the second surface can have a particle size of about 150μm to 850μm, and the content of fine particles with a particle size of less than about 150μm can be less than about 3% by weight. Therefore, by controlling the particle size distribution of the superabsorbent polymer within a preferred range, the superabsorbent polymer finally prepared can exhibit excellent absorption performance. Therefore, in the classification step, polymers with a particle size of about 150μm to about 850μm can be classified and commercialized.

[0168] The superabsorbent polymer prepared by the above-mentioned preparation method of the present invention includes: a base resin powder, which includes a crosslinked polymer of a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups are neutralized; and a surface crosslinked layer, which is located on the base resin powder, wherein the surface crosslinked layer includes a first crosslinked polymer, wherein a portion of the crosslinked polymer is additionally crosslinked by a first surface crosslinking agent; and a second crosslinked polymer, wherein the remaining crosslinked polymer is additionally crosslinked by a second surface crosslinking agent. The first and second surface crosslinking agents are as described above.

[0169] More specifically, in the surface cross-linked layer, the first cross-linked polymer is located adjacent to the base resin powder, and the second cross-linked polymer is located outside the superabsorbent polymer. This increases the cross-linking density toward the outside of the superabsorbent polymer, thereby enhancing the surface strength of the superabsorbent polymer and reducing the decrease in AUL caused by the anti-caking treatment.

[0170] Specifically, the centrifuge retention capacity (CRC) of the superabsorbent polymer for a saline solution (0.9 wt% sodium chloride aqueous solution) for 30 minutes, as measured by EDANA method 442.0-96, can be 30 g / g or more, more specifically 32 g / g or more. The higher the value, the better, so the upper limit is essentially unlimited, but for example, it can be 50 g / g or less, or 40 g / g or less.

[0171] Furthermore, the 0.9AUL reduction rate of the superabsorbent polymer calculated by the following mathematical formula 1 is 0.05 or less, more specifically 0.03 or less. A smaller 0.9AUL reduction rate is superior, and thus, there is essentially no lower limit, but for example, it can be 0.001 or more, 0.01 or more, or 0.02 or more.

[0172] [Mathematical formula 1]

[0173] 0.9AUL drop rate = [(AB) / A]

[0174] In Mathematical Formula 1,

[0175] A is the 0.9AUL of the superabsorbent polymer before anti-blocking treatment and is the absorbency under pressure measured according to EDANA method 442.0-96 after the superabsorbent polymer before anti-blocking treatment has been swollen under a load of 0.9 psi for 1 hour,

[0176] B is the 0.9AUL of the superabsorbent polymer after anti-caking treatment, which is the pressure absorption rate measured as follows: 100 parts by weight of superabsorbent polymer and 0.0005 to 0.001 parts by weight, more specifically 0.001 parts by weight of fumed silica (anti-caking agent) are dry-mixed at 200 rpm for 30 seconds to 1 minute, more specifically, 1 minute, and the anti-caking treatment is then performed. Then, according to EDANA method 442.0-96, the superabsorbent polymer after anti-caking treatment is swollen under a load of 0.9 psi for 1 hour and then measured.

[0177] Specifically, the 0.9AUL of the superabsorbent polymer before the anti-caking treatment may be 15 to 30, and the 0.9AUL after the anti-caking treatment may be 15 to 25.

[0178] Specifically, the 0.9AUL of the superabsorbent polymer before the anti-caking treatment may be 15-30, or 18-23, and the 0.9AUL after the anti-caking treatment may be 15-25, or 17-22.

[0179] Therefore, the superabsorbent polymer can be very preferably used for various sanitary products, such as adult diapers, and in particular, it can be effectively used for sanitary products with reduced pulp content. The sanitary products may include disposable absorbent products, preferably diapers, and the diapers may be children's diapers or adult diapers.

[0180] Beneficial effects

[0181] According to the preparation method of the present invention, by increasing the surface cross-linking strength of the superabsorbent polymer, a superabsorbent polymer having little or no decrease in pressure absorption rate even after anti-caking treatment can be prepared. DETAILED DESCRIPTION

[0182] Hereinafter, the functions and effects of the present invention will be described in detail by way of specific examples of the present invention. However, these examples are provided only as illustrations of the present invention, and the scope of the rights of the present invention is not limited thereto.

[0183] <Preparation of base resin>

[0184] Preparation Example

[0185] 100 parts by weight of acrylic acid, 126.8 parts by weight of 31.5% caustic soda (NaOH), 46 parts by weight of water, and the following components were mixed to prepare a monomer composition.

[0186] -Internal crosslinking agent: polyethylene glycol diacrylate (PEGDA; Mw = 400) 0.2 parts by weight (2000 ppmw);

[0187] -Polymerization initiator: 0.008 parts by weight (80 ppmw) of bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (photopolymerization initiator) and 0.12 parts by weight (1200 ppmw) of sodium persulfate (thermal polymerization initiator),

[0188] The composition was introduced into the feeding section of the polymerization reactor which consisted of a continuously moving conveyor belt and irradiated with ultraviolet light for 1.5 minutes (about 2 mW / cm 2 ) to carry out a polymerization reaction, thereby obtaining a hydrogel polymer as a product.

[0189] The hydrogel polymer was cut with a cutter. Subsequently, the hydrogel polymer was dried in a hot air dryer at 190° C. for 40 minutes, and the dried hydrogel polymer was ground with a grinder. Subsequently, the polymer was classified into particle sizes of 150 μm to 850 μm using a sieve to obtain a base resin powder.

[0190] The CRC value of the obtained base resin powder was about 54 g / g.

[0191] <Preparation of Superabsorbent Polymer>

[0192] Example 1

[0193] Based on 100 parts by weight of the base resin powder prepared in Preparation Example, 4 parts by weight of water, 3 parts by weight of methanol, 1.5 parts by weight of ethylene carbonate (weight average molecular weight = 88.06 g / mol) as a first surface crosslinking agent, and 0.05 parts by weight of fumed silica were mixed to prepare a first surface crosslinking solution.

[0194] The first surface crosslinking solution was added to 100 parts by weight of the base resin powder prepared in the preparation example and thoroughly mixed using a high-speed mixer at about 200 rpm for less than 10 seconds. The resulting mixture was introduced into a reactor, heated to about 180° C. over about 20 minutes, and then maintained at the elevated temperature for 30 minutes to perform the first surface crosslinking (first surface crosslinking, total reaction time 50 minutes).

[0195] After the first surface crosslinking was completed, when the temperature of the reaction system dropped to 150° C., a second surface crosslinking solution prepared as follows was introduced: based on 100 parts by weight of the first surface crosslinked base resin, 3 parts by weight of water, 0.02 parts by weight of ethylene glycol diglycidyl ether (weight average molecular weight = 174.19 g / mol, boiling point: 266.8° C. (at 1 atm)) as a second surface crosslinking agent, and 0.1 parts by weight of a 50 wt% polycarboxylic acid aqueous solution (ACYMA-GK TM , manufactured by Aezis Ltd.) was mixed. And, a high-speed mixer was used to stir at a speed of about 200 rpm for less than 10 seconds while being fully mixed.

[0196] The resulting mixture was placed in a stirrer and stirred without heating while undergoing a second surface crosslinking step. During the stirring step, the reservoir containing the mixture was placed in an oil circulation tank, where the oil circulated while the temperature was lowered, resulting in a cooling rate of -1.5°C / minute from 150°C to 120°C (second surface crosslinking, total reaction time: 20 minutes).

[0197] After the surface cross-linking is completed, the obtained resin powder is put into a grinder and ground, and classified with a sieve to obtain a super absorbent polymer with a particle size of 150 μm to 850 μm.

[0198] Example 2

[0199] Based on 100 parts by weight of the base resin powder prepared in the preparation example, 3 parts by weight of water, 3.5 parts by weight of methanol, 1 part by weight of ethylene carbonate (weight average molecular weight = 88.06 g / mol) as a first surface crosslinking agent, and 0.03 parts by weight of fumed silica were mixed to prepare a first surface crosslinking solution.

[0200] The first surface crosslinking solution was added to 100 parts by weight of the base resin powder prepared in the preparation example and thoroughly mixed while stirring at about 200 rpm for less than 10 seconds using a high-speed mixer. The resulting mixture was introduced into a reactor, heated to about 180° C. over about 20 minutes, and then maintained at the elevated temperature for 30 minutes to perform the first surface crosslinking (first surface crosslinking, total reaction time 50 minutes).

[0201] Then, when the temperature of the reaction system dropped to 150° C., a second surface crosslinking solution prepared as follows was introduced: based on 100 parts by weight of the first surface crosslinked base resin, 3 parts by weight of water, 0.005 parts by weight of 1,4-butanediol diglycidyl ether (weight average molecular weight = 202.25 g / mol, boiling point: 266° C.) as an epoxy-based second surface crosslinking agent, and 0.07 parts by weight of a 50% by weight polycarboxylic acid aqueous solution (ACYMA-GK TM , manufactured by Aezis Ltd.) was mixed. And, a high-speed mixer was used to stir at a speed of about 200 rpm for less than 10 seconds while being fully mixed.

[0202] The resulting mixture was introduced into a stirrer and stirred without heating while undergoing a second surface crosslinking step. During the stirring step, the reservoir containing the mixture was placed in an oil circulation tank, and the oil was circulated while the temperature was lowered, thereby reducing the reaction temperature from 150°C to 120°C at a cooling rate of -1.5°C / minute (second surface crosslinking, total reaction time: 20 minutes).

[0203] After the surface cross-linking is completed, the obtained resin powder is put into a grinder and ground, and classified with a sieve to obtain a super absorbent polymer with a particle size of 150 μm to 850 μm.

[0204] Example 3

[0205] Based on 100 parts by weight of the base resin powder prepared in the preparation example, 4 parts by weight of water, 3.5 parts by weight of methanol, 0.2 parts by weight of 1,3-propylene glycol (weight average molecular weight = 79.09 g / mol) as a first surface crosslinking agent, 0.15 parts by weight of oxalic acid, and 0.01 parts by weight of fumed silica were mixed to prepare a first surface crosslinking solution.

[0206] The first surface crosslinking solution was added to 100 parts by weight of the base resin powder prepared in the preparation example and thoroughly mixed while stirring at about 200 rpm for less than 10 seconds using a high-speed mixer. The resulting mixture was introduced into a reactor, heated to about 190° C. over about 20 minutes, and then maintained at the elevated temperature and reacted for 20 minutes, thereby performing the first surface crosslinking (first surface crosslinking, total reaction time 40 minutes).

[0207] Then, when the temperature of the reaction system dropped to 150° C., a second surface crosslinking solution prepared as follows was introduced: based on 100 parts by weight of the first surface crosslinked base resin, 3 parts by weight of water, 0.005 parts by weight of ethylene glycol diglycidyl ether (weight average molecular weight = 174.19 g / mol, boiling point: 266.8° C. (at 1 atm)) as a second surface crosslinking agent, and 0.15 parts by weight of a 50 wt% polycarboxylic acid aqueous solution (ACYMA-GK TM , manufactured by Aezis Ltd.) was mixed. And, a high-speed mixer was used to stir at a speed of about 200 rpm for less than 10 seconds while being fully mixed.

[0208] The resulting mixture was introduced into a stirrer and stirred without heating while undergoing a second surface crosslinking step. During the stirring step, the reservoir containing the mixture was placed in an oil circulation tank, and the oil was circulated while the temperature was lowered, thereby reducing the reaction temperature from 150°C to 130°C at a cooling rate of -2.0°C / minute (second surface crosslinking, total reaction time: 10 minutes).

[0209] After the surface cross-linking is completed, the obtained resin powder is put into a grinder and ground, and classified with a sieve to obtain a super absorbent polymer with a particle size of 150 μm to 850 μm.

[0210] Example 4

[0211] Based on 100 parts by weight of the base resin powder prepared in the preparation example, 4 parts by weight of water, 3.5 parts by weight of methanol, 0.4 parts by weight of 1,3-propylene glycol (weight average molecular weight = 79.09 g / mol) as a first surface crosslinking agent, 0.23 parts by weight of oxalic acid, and 0.04 parts by weight of fumed silica were mixed to prepare a first surface crosslinking solution.

[0212] The first surface crosslinking solution was added to 100 parts by weight of the base resin powder prepared in the preparation example and thoroughly mixed while stirring at about 200 rpm for less than 10 seconds using a high-speed mixer. The resulting mixture was introduced into a reactor, heated to about 185°C over 20 minutes, and then maintained at the elevated temperature for 25 minutes to perform the first surface crosslinking (first surface crosslinking, total reaction time 45 minutes).

[0213] Then, when the temperature of the reaction system dropped to 140° C., a second surface crosslinking solution prepared as follows was introduced: based on 100 parts by weight of the first surface crosslinked base resin, 3 parts by weight of water, 0.01 parts by weight of ethylene glycol diglycidyl ether (weight average molecular weight = 174.19 g / mol, boiling point: 266.8° C. (at 1 atm)) as a second surface crosslinking agent, and 0.15 parts by weight of a 50 wt% polycarboxylic acid aqueous solution (ACYMA-GK TM, manufactured by Aezis Ltd.) was mixed. And, a high-speed mixer was used to stir at a speed of about 200 rpm for less than 10 seconds while being fully mixed.

[0214] The resulting mixture was introduced into a stirrer and stirred without heating while undergoing a second surface crosslinking step. During the stirring step, the reservoir containing the mixture was placed in an oil circulation tank, which circulated the oil while lowering the temperature, thereby reducing the reaction temperature from 140°C to 120°C at a cooling rate of -1.5°C / minute (second surface crosslinking, total reaction time: 13 minutes).

[0215] After the surface cross-linking is completed, the obtained resin powder is put into a grinder and ground, and classified with a sieve to obtain a super absorbent polymer with a particle size of 150 μm to 850 μm.

[0216] Comparative Example 1

[0217] A superabsorbent polymer was prepared by the same method as in Example 1, except that only the first surface crosslinking in Example 1 was performed.

[0218] Comparative Example 2

[0219] The superabsorbent polymer was prepared by the same method as in Example 2, except that only the first surface crosslinking in Example 2 was performed.

[0220] Comparative Example 3

[0221] A superabsorbent polymer was prepared in the same manner as in Comparative Example 1, except that the content of ethylene carbonate in the first surface cross-linking solution in Comparative Example 1 was increased to 1.8 parts by weight.

[0222] Comparative Example 4

[0223] Based on 100 parts by weight of the base resin powder prepared in the preparation example, 3 parts by weight of water, 0.02 parts by weight of ethylene glycol diglycidyl ether (weight average molecular weight = 174.19 g / mol, boiling point: 266.8° C. (at 1 atm)) as an epoxy-based surface crosslinking agent, and 0.1 parts by weight of a 50% by weight polycarboxylic acid aqueous solution (ACYMA-GK TM , manufactured by Aezis Ltd.) were mixed to prepare a first surface cross-linking solution.

[0224] The first surface crosslinking solution was added to 100 parts by weight of the base resin powder prepared in the preparation example and thoroughly mixed while stirring at a speed of about 200 rpm for less than 10 seconds using a high-speed mixer. The resulting mixture was introduced into a reactor and reacted at about 130° C. for 20 minutes to perform the first surface crosslinking (first surface crosslinking, total reaction time 20 minutes).

[0225] A second surface crosslinking solution prepared by mixing 4 parts by weight of water, 3 parts by weight of methanol, 1.5 parts by weight of ethylene carbonate (weight average molecular weight = 88.06 g / mol), and 0.05 parts by weight of fumed silica, based on 100 parts by weight of the first surface crosslinked base resin, was introduced into 100 parts by weight of the first surface crosslinked base resin. The mixture was stirred at a speed of about 200 rpm for less than 10 seconds using a high-speed mixer while being thoroughly mixed.

[0226] The resulting mixture was introduced into a stirrer, heated to about 180° C. over 20 minutes, and then reacted for 30 minutes while maintaining the heated temperature, thereby performing second surface crosslinking (second surface crosslinking, total reaction time 50 minutes).

[0227] After the surface cross-linking is completed, the obtained resin powder is put into a grinder and ground, and classified with a sieve to obtain a super absorbent polymer with a particle size of 150 μm to 850 μm.

[0228] Comparative Example 5

[0229] The superabsorbent polymer was prepared by the same method as in Example 1, except that after the first surface crosslinking was completed, the second surface crosslinking solution was introduced when the temperature of the reaction system dropped to 110°C, and the second surface crosslinking was carried out while the reaction temperature was lowered from 110°C to 80°C at a cooling rate of -1.5°C / min.

[0230] Comparative Example 6

[0231] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 1,3-butadiene diepoxide (weight average molecular weight: 86.01 g / mol) was used instead of ethylene glycol diglycidyl ether as the epoxy-based second surface crosslinking agent when preparing the second surface crosslinking solution.

[0232] Comparative Example 7

[0233] A superabsorbent polymer was prepared using the same method as in Example 2, except that after the first surface crosslinking, a second surface crosslinking solution prepared by mixing 3 parts by weight of water, 0.03 parts by weight of PEG600 (boiling point 200°C (at 1 atm)), and 0.1 parts by weight of a 50% polycarboxylic acid aqueous solution was introduced into 100 parts by weight of the first surface crosslinked base resin. Furthermore, the second surface crosslinking reaction was carried out at 180°C for a total reaction time of 50 minutes.

[0234] Comparative Example 8

[0235] The superabsorbent polymer was prepared by the same method as in Example 1, except that 0.05 parts by weight of aluminum sulfate (Mw: 342.15 g / mol) was used instead of ethylene glycol diglycidyl ether as the epoxy second surface crosslinking agent when preparing the second surface crosslinking solution.

[0236] Experimental example

[0237] For the superabsorbent polymers obtained in Examples and Comparative Examples, absorption-related properties were measured as follows.

[0238] (1) Centrifugal retention capacity (CRC)

[0239] The centrifuge retention capacity (CRC) was determined according to EDANA method WSP 241.3.

[0240] Specifically, a superabsorbent polymer having a particle size of 300 μm to 600 μm, passing through a U.S. standard 30 mesh sieve and retained on a U.S. standard 50 mesh sieve was prepared. Furthermore, W0 (g, approximately 0.2 g) of superabsorbent polymer having a particle size of 300 μm to 600 μm was evenly placed in an envelope made of non-woven fabric and the envelope was sealed. Furthermore, the envelope was immersed in a 0.9 wt% saline solution. After 30 minutes, the envelope was drained at 250G using a centrifuge for 3 minutes, and the weight W4 (g) of the envelope was measured. At the same time, the same operation was performed using an empty envelope without superabsorbent polymer, and the weight W3 (g) at this time was measured.

[0241] Using the weight thus obtained, the centrifuge retention capacity was determined by the following Mathematical Formula 2.

[0242] [Mathematical formula 2]

[0243] CRC(g / g)={[W4(g)-W3(g)] / W0(g)}-1

[0244] In Mathematical Formula 2,

[0245] W0 (g) is the initial weight (g) of superabsorbent polymer with a particle size of 300 μm to 600 μm,

[0246] W3 (g) is the weight of the device without superabsorbent polymer, measured after emptying the device using a centrifuge at 250G for 3 minutes.

[0247] W4 (g) is the weight of a device including a superabsorbent polymer, which is measured by immersing a superabsorbent polymer having a particle size of 300 μm to 600 μm in a 0.9 wt % saline solution at room temperature for 30 minutes for absorption, and then emptying it using a centrifuge at 250 G for 3 minutes.

[0248] (2) Absorption under load (AUL)

[0249] The super absorbent polymers prepared in Examples and Comparative Examples were subjected to an anti-blocking treatment, and then the 0.9AUL of each super absorbent polymer was measured, and the decrease rate of the pressurized absorbency after the anti-blocking treatment was calculated based on the results.

[0250] Specifically, 100 parts by weight of superabsorbent polymer and 0.001 parts by weight of fumed silica anti-caking agent (Evonik, 200) was placed in a polyethylene (PE) bottle, and then dry-mixed at a speed of 200 rpm for 30 seconds using a vortex mixer (IKA MS3), thereby performing anti-caking treatment on the superabsorbent polymer.

[0251] Also, the absorbency under load (AUL) of the superabsorbent polymer for saline solution was measured before / after the anti-caking treatment according to EDANA method 442.0-96.

[0252] Specifically, a 400-mesh sieve was installed at the bottom of a plastic cylinder with an inner diameter of 25 mm. Furthermore, at room temperature and 50% humidity, W0 (g, approximately 0.16 g) of superabsorbent polymer, whose pressurized absorbency was to be measured, was evenly sprinkled onto the sieve. A piston capable of uniformly applying a 6.3 kPa (0.9 psi) load was then placed on the superabsorbent polymer. The piston had an outer diameter slightly smaller than 25 mm, ensuring no gap with the inner wall of the cylinder and without hindering its up-and-down movement. The weight of the device, W5 (g), was then measured. A 90 mm diameter, 5 mm thick glass filter was then placed in a 150 mm diameter Petri dish, and a 0.9 wt% saline solution was poured into the dish. The saline solution was injected so that the water level was level with the upper side of the glass filter. A sheet of filter paper with a diameter of 90 mm was placed on the glass filter. The device was then placed on the glass filter, allowing the superabsorbent polymer in the device to swell under the load. After 1 hour, the weight W6 (g) of the device including the swollen superabsorbent polymer was measured.

[0253] The pressure absorption rate was calculated using the measured weight according to the following mathematical formula 3.

[0254] [Mathematical formula 3]

[0255] AUL(g / g)=[W6(g)-W5(g)] / W0(g)

[0256] In Mathematical Formula 3,

[0257] W0 (g) is the initial weight of the superabsorbent polymer (g),

[0258] W5 (g) is the sum of the weight of the superabsorbent polymer and the weight of the device capable of applying a load to the superabsorbent polymer,

[0259] W6(g) is the sum of the weight of the superabsorbent polymer and the weight of a device capable of applying a load to the superabsorbent polymer after the saline solution is absorbed into the superabsorbent polymer under a load of 0.9 psi for 1 hour.

[0260] Using the 0.9AUL before and after the anti-caking treatment measured above, the 0.9AUL reduction rate was calculated according to the following mathematical formula 1:

[0261] [Mathematical formula 1]

[0262] 0.9AUL drop rate = [(AB) / A]

[0263] In Mathematical Formula 1,

[0264] A is the 0.9AUL of the superabsorbent polymer before anti-blocking treatment, and is the absorbency under pressure measured according to EDANA method 442.0-96 after the superabsorbent polymer before anti-blocking treatment has been swollen under a load of 0.9 psi for 1 hour, and

[0265] B is the 0.9AUL of the superabsorbent polymer after anti-caking treatment, which is the pressure absorption rate measured as follows: 100 parts by weight of superabsorbent polymer and 0.0005 to 0.001 parts by weight of fumed silica as an anti-caking agent are dry-mixed at 200 rpm for 30 seconds to 1 minute to perform anti-caking treatment. Then, according to EDANA method 442.0-96, the superabsorbent polymer after anti-caking treatment is swollen under a load of 0.9 psi for 1 hour and then measured.

[0266] The measurement results are shown in Table 1 below.

[0267] [Table 1]

[0268]

[0269] The experimental results show that the superabsorbent polymers of Examples 1 to 4 exhibit excellent CRC compared with the comparative examples, and even after the anti-caking treatment, the 0.9 AUL drop rate is significantly reduced.

[0270] In contrast, in the case of Comparative Examples 1 and 2 in which only the first surface crosslinking was performed, the 0.9AUL reduction rate after the anti-caking treatment was as high as 0.15 or more, and in the case of Comparative Example 3 in which only the first surface crosslinking was performed but the content of the surface crosslinking agent was increased, the 0.9AUL reduction rate was lower than that of Comparative Examples 1 and 2, but was significantly increased compared with the examples.

[0271] Furthermore, in Comparative Example 4, which did not meet the first and second surface crosslinking conditions of the present invention, the 0.9 AUL reduction rate was even greater than those of Comparative Examples 1-3, which only underwent the first surface crosslinking. In Comparative Example 4, despite two surface crosslinking steps, the use of a surface crosslinking agent with a high weight-average molecular weight during the first surface crosslinking prevented the agent from penetrating deeply into the surface of the superabsorbent polymer. Since a surface crosslinking agent with a low weight-average molecular weight was subsequently used for the second surface crosslinking, the surface crosslinking strength was not sufficiently maintained compared to Comparative Examples 1-3.

[0272] Furthermore, in Comparative Example 5, where the temperature during the second surface crosslinking was too low, the second surface crosslinking reaction did not proceed sufficiently, resulting in a large decrease in the 0.9AUL after the anti-blocking treatment. Furthermore, in Comparative Example 6, where the weight-average molecular weight requirements for the first and second surface crosslinking agents were not met, the decrease in the 0.9AUL after the anti-blocking treatment was lower than that of the other comparative examples, but was greater than that of the examples, and exhibited deteriorated CRC performance.

[0273] Also, in the case of Comparative Example 7, in which PEG600 that forms a covalent bond in a temperature range greater than 150° C. is used as the second surface cross-linking agent, the 0.9AUL decrease rate after the anti-blocking treatment is large.

[0274] Furthermore, in the case of Comparative Example 8 using aluminum sulfate as the second surface crosslinking agent, the 0.9AUL reduction rate was lower than that of Comparative Example 1. However, compared with Example 1 using a different second surface crosslinking agent under the same conditions, sufficient surface crosslinking strength could not be ensured, and thus the 0.9AUL reduction rate after the anti-caking treatment was large.

Claims

1. A method for preparing a superabsorbent polymer, comprising the following steps: cross-linking and polymerizing a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups are neutralized in the presence of a polymerization initiator and an internal cross-linking agent to form a hydrogel polymer comprising a cross-linked polymer of the water-soluble ethylenically unsaturated monomer; drying, grinding, and classifying the hydrogel polymer to obtain a base resin powder; performing first surface crosslinking of the base resin powder while raising the temperature to a first temperature in the presence of a first surface crosslinking agent; and In the presence of a second surface crosslinking agent, the first surface crosslinked base resin powder is subjected to a second surface crosslinking at a second temperature to form superabsorbent polymer particles. Wherein, the first temperature is above 180°C, the second temperature is between 120°C and 150°C, and The second surface crosslinking agent includes an epoxy compound having a weight average molecular weight greater than that of the first surface crosslinking agent and capable of forming a covalent bond at the second temperature.

2. The method for preparing a superabsorbent polymer according to claim 1, wherein: The ratio of the weight average molecular weight of the second surface crosslinking agent to the weight average molecular weight of the first surface crosslinking agent is 1.8 or more.

3. The method for preparing a superabsorbent polymer according to claim 1, wherein: The first surface cross-linking agent includes one or more compounds selected from the group consisting of polyols and alkylene carbonates.

4. The method for preparing a superabsorbent polymer according to claim 1, wherein: The first surface crosslinking agent includes one or more selected from the group consisting of ethylene carbonate, propylene carbonate, ethylene glycol, propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol and glycerol.

5. The method for preparing a superabsorbent polymer according to claim 1, wherein: During the first surface cross-linking, one or more selected from inorganic fillers and organic acids are further introduced.

6. The method for preparing a superabsorbent polymer according to claim 5, wherein: The inorganic filler includes one or more selected from the group consisting of silica, fumed silica, clay, alumina, silica-alumina composite material, titanium dioxide, zinc oxide and silicate.

7. The method for preparing a superabsorbent polymer according to claim 5, wherein: The organic acid includes one or more selected from the group consisting of oxalic acid, acetic acid, lactic acid, citric acid, fumaric acid, tartaric acid and maleic acid.

8. The method for preparing a superabsorbent polymer according to claim 1, wherein: The second surface crosslinking agent includes an epoxy compound having two or more epoxy functional groups in a molecule and a boiling point of 150° C. or higher under atmospheric pressure.

9. The method for preparing a superabsorbent polymer according to claim 1, wherein: The second surface crosslinking agent includes one or more epoxy compounds selected from the group consisting of ethylene glycol diglycidyl ether, 1,4-butanediol diglycidyl ether, propylene glycol diglycidyl ether, hexanediol diglycidyl ether, diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, tripropylene glycol diglycidyl ether, glycerol triglycidyl ether, polypropylene glycol diglycidyl ether and polyethylene glycol diglycidyl ether.

10. The method for preparing a superabsorbent polymer according to claim 1, wherein: During the second surface cross-linking, a polycarboxylic acid or a salt thereof is further introduced.

11. The method for preparing a superabsorbent polymer according to claim 1, wherein: The first surface cross-linking includes a first maintaining step of maintaining the first temperature after the temperature is raised to the first temperature. The time for performing the first holding step is 50% or more of the total time for performing the first surface cross-linking.

12. The method for preparing a superabsorbent polymer according to claim 1, wherein: The first temperature is 180°C to 200°C.

13. The method for preparing a superabsorbent polymer according to claim 1, wherein: During the second surface cross-linking, the second surface cross-linking agent is introduced when the temperature of the first surface cross-linked base resin powder is 140° C. to 150° C., and the second surface cross-linking is performed while cooling to a temperature of 120° C. to 130° C. at a cooling rate of -1.5° C. / min to -2.0° C. / min.

14. A superabsorbent polymer comprising: a base resin powder comprising a crosslinked polymer of a water-soluble ethylenically unsaturated monomer in which at least a portion of the acidic groups is neutralized; and a surface crosslinked layer disposed on the base resin powder. in, The surface cross-linked layer comprises: a first cross-linked polymer, wherein a portion of the cross-linked polymer is additionally cross-linked by a first surface cross-linking agent; and a second cross-linked polymer, wherein the remaining cross-linked polymer is additionally cross-linked by a second surface cross-linking agent, The second surface crosslinking agent includes an epoxy compound having a weight average molecular weight greater than that of the first surface crosslinking agent and capable of forming a covalent bond at 120-150° C., and The super absorbent polymer has a 0.9AUL reduction rate calculated by the following mathematical formula 1 of 0.05 or less: 【Mathematical formula 1】 0.9AUL drop rate = [(AB) / A] In Mathematical Formula 1, A is the 0.9AUL of the superabsorbent polymer before anti-blocking treatment and is the absorbency under pressure measured after swelling the superabsorbent polymer before anti-blocking treatment under a pressure of 0.9 psi for 1 hour according to EDANA method 442.0-96; B is the 0.9AUL of the anti-caking treated superabsorbent polymer, and is the absorbency under pressure measured after anti-caking treatment by dry-mixing 100 parts by weight of the superabsorbent polymer and 0.0005 to 0.001 parts by weight of a fumed silica anti-caking agent at 200 rpm for 30 seconds to 1 minute, and then swelling the anti-caking treated superabsorbent polymer under a pressure of 0.9 psi for 1 hour according to EDANA method 442.0-96.

15. The superabsorbent polymer according to claim 14, wherein The superabsorbent polymer has a centrifuge retention capacity of 30 g / g or greater as measured by EDANA method WSP 241.3.

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