Method for preparing superabsorbent polymer

Abstract

The present invention relates to a method for preparing a superabsorbent polymer, and more particularly, to a method for preparing a superabsorbent polymer by combining a specific polymerization initiator and a chelating agent during a cross-linking polymerization step, thereby more efficiently preparing a superabsorbent polymer having both excellent absorbency and optimal gel strength.

Description

Technical field

[0001] Cross-reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2021-0015453, filed on February 3, 2021, and Korean Patent Application No. 10-2022-0012653, filed on January 27, 2022, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entirety.

[0003] The present invention relates to a method for preparing a superabsorbent polymer, and more particularly, to a method for preparing a superabsorbent polymer by combining a specific polymerization initiator and a chelating agent during a cross-linking polymerization step, thereby more efficiently preparing a superabsorbent polymer having both excellent absorption performance and optimal gel strength. [Background Technology]

[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 super absorbent material (SAM) or absorbent gel material (AGM). Superabsorbent polymers were first commercialized as sanitary products. In addition to children's diapers, they are currently used in a wide range of applications, including soil water retention materials, waterproofing materials for civil engineering and construction, sheets for seedling cultivation, and preservatives and hot compresses in the food distribution industry.

[0005] In most cases, such superabsorbent polymers are widely used in the field of sanitary products such as diapers or sanitary pads, and for this purpose, they should exhibit high absorption capacity for moisture, etc., and should exhibit excellent pressure absorbency that does not discharge moisture even under external pressure, as well as excellent permeability so that they can maintain their shape even when swollen due to water absorption.

[0006] Furthermore, when superabsorbent polymers are incorporated into sanitary products such as diapers, they should diffuse urine and other substances as widely as possible, even under the pressure of the wearer's body weight. This allows the superabsorbent polymer particles contained throughout the entire absorbent layer of the sanitary product to be utilized, further improving the product's absorption performance and absorption rate. Furthermore, this pressure-induced diffusion improves the diaper's rewetting properties, preventing urine and other substances from rewetting after absorption into the superabsorbent polymer. Furthermore, the diaper's leak-proof properties can be enhanced.

[0007] Previously, attempts have been made to diffuse urine and other liquids more widely by modifying the design of sanitary products such as diapers. For example, attempts have been made to improve the diffusibility of urine and other liquids by introducing an ADL (Acquisition Distribution Layer) or applying absorption channels to sanitary products. However, improving diffusibility through such design changes in sanitary products is not sufficient.

[0008] At the same time, in order to improve the absorption-related properties of superabsorbent polymers such as absorption capacity or absorption speed, various post-treatment processes such as surface cross-linking and foaming are carried out, or various additives are used, but in such processes, the cross-linking density of the polymer may be reduced, so that sufficient gel strength may not be achieved, and the additives may fall off, so when applied to the product, the user's wearing comfort may be reduced, such as the occurrence of a rash.

[0009] Therefore, there is a need for research on preparing superabsorbent polymers that not only maintain excellent absorbency but also achieve optimal gel strength. [Summary of the invention]

[0010]

Technical Issues

[0011] An object of the present invention is to provide a method for preparing a superabsorbent polymer, which can more efficiently prepare a superabsorbent polymer having both excellent absorption performance and optimal gel strength by combining a specific polymerization initiator and a chelating agent in the cross-linking polymerization step during the preparation process.

[0012]

Technical solution

[0013] In order to achieve the above object, a method for preparing a superabsorbent polymer is provided, which comprises the following steps:

[0014] performing cross-linking polymerization of a water-soluble ethylenically unsaturated monomer in which at least a portion of the acid groups are neutralized and an internal cross-linking agent in the presence of an azo-based polymerization initiator and diethylenetriaminepentaacetic acid to obtain a hydrogel polymer; and

[0015] The hydrogel polymer is dried, ground and classified to form a base resin powder,

[0016] Wherein, the weight ratio of the azo polymerization initiator to diethylenetriaminepentaacetic acid is 1:2 to 1:10.

[0017] Effects of the invention

[0018] According to the present invention, a method for preparing a superabsorbent polymer is provided, which can more efficiently prepare a superabsorbent polymer having both excellent absorption performance and optimal gel strength by utilizing a novel cross-linking structure to achieve an optimal cross-linking density. [Specific implementation method]

[0019] The terms used herein are for the purpose of explaining particular embodiments only and are not intended to limit the present invention.

[0020] Singular expressions include plural expressions unless explicitly stated or it is obvious from the context that such intention is not present. As used herein, the terms "including," "having," or "having" are intended to specify the presence of an implemented feature, quantity, step, constituent 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, constituent elements, or combinations thereof.

[0021] The terms “first,” “second,” “third,” etc. are used to explain various constituent elements, and these terms are used only to distinguish one constituent element from other constituent elements.

[0022] Although various modifications can be made to the present invention and the present invention can have various forms, specific examples will be described and explained in detail below. However, it should be understood that these are not intended to limit the present invention to the specific disclosures, and that the present invention includes all modifications, equivalents, or replacements thereof without departing from the spirit and technical scope of the present invention.

[0023] As used herein, the term "polymer" refers to the polymerized state of a water-soluble ethylenically unsaturated monomer, and may include polymers having any range of moisture content or particle size. Among these polymers, those having a moisture content of about 40% by weight or more after polymerization but before drying may be referred to as hydrogel polymers, and particles obtained by grinding and drying such hydrogel polymers may be referred to as cross-linked polymers.

[0024] Also, the term "crosslinked polymer" refers to those polymers obtained by crosslinking polymerization of water-soluble ethylenically unsaturated monomers in which at least part of the acid groups are neutralized, and "base resin powder" refers to a material containing such a crosslinked polymer.

[0025] Furthermore, the term "superabsorbent polymer" refers to a cross-linked polymer obtained by polymerization of water-soluble ethylenically unsaturated monomers in which at least a portion of the acid groups are neutralized, or a base resin powder composed of superabsorbent polymer particles obtained by grinding a cross-linked polymer, or a cross-linked polymer or base resin suitable for production prepared by other processes including, for example, surface cross-linking, fine particle reconstitution, drying, grinding, classification, etc.

[0026] (Method for preparing superabsorbent polymer)

[0027] A method for preparing a superabsorbent polymer according to one embodiment of the present invention includes the following steps: performing crosslinking polymerization of a water-soluble ethylenically unsaturated monomer in which at least a portion of the acid group is neutralized and an internal crosslinking agent in the presence of an azo-based polymerization initiator and diethylenetriaminepentaacetic acid satisfying a specific content ratio to obtain a hydrogel polymer; and drying, grinding, and classifying the hydrogel polymer to form a base resin powder.

[0028] In order to improve the basic absorption properties of superabsorbent polymers, various post-treatment processes such as surface cross-linking and foaming are carried out, or various additives are used. However, in these processes, the cross-linking density of the polymer is reduced, so sufficient gel strength cannot be achieved, and the additives fall off. As a result, when applied to products, the user's wearing comfort is reduced, and for example, a rash may occur.

[0029] Therefore, the present inventors have found that, when a specific polymerization initiator and a chelating agent are used in combination in the cross-linking polymerization step, a novel cross-linking structure can be uniformly formed in the polymer without the above-mentioned problems, so that a superabsorbent polymer having both excellent absorbency and optimal gel strength can be prepared more efficiently, thereby completing the present invention.

[0030] Hereinafter, the method for preparing superabsorbent polymer according to a specific embodiment of the present invention will be described in detail according to the steps.

[0031] (Cross-linking polymerization step)

[0032] First, according to one embodiment of the present invention, a method for preparing a superabsorbent polymer includes the following steps: in the presence of an azo polymerization initiator and diethylenetriaminepentaacetic acid satisfying a specific content ratio, cross-linking polymerization is performed on a water-soluble ethylenically unsaturated monomer in which at least a portion of the acid group is neutralized and an internal cross-linking agent to obtain a hydrogel polymer.

[0033] The cross-linking polymerization step may be performed by cross-linking a monomer composition containing components generally used for preparing super absorbent polymers in addition to the above components.

[0034] First, cross-linking polymerization is carried out in the presence of an azo-type polymerization initiator. In addition, a diethylenetriaminepentaacetic acid (DTPA) chelating agent is used in combination at a specific content ratio to introduce a novel cross-linking structure during the polymerization process, while achieving excellent absorbency and high gel strength of the base resin powder, thereby significantly improving the properties of the finally prepared superabsorbent polymer.

[0035] Specifically, azo polymerization initiators, which initiate the crosslinking reaction of monomers, have superior free radical stability compared to persulfate polymerization initiators and tend to form polymers with long chain structures. In particular, when used in combination with diethylenetriaminepentaacetic acid, they effectively prevent metal-induced chain transfer, resulting in the formation of long, uniform polymer chains. This allows for the simultaneous achievement of excellent absorbency and gel strength when used in sanitary products, for example.

[0036] Diethylenetriaminepentaacetic acid (DTPA) is a chelating agent used in combination with the above-mentioned azo polymerization initiator to introduce a new cross-linking structure into the polymer during the polymerization process, thereby simultaneously achieving excellent absorbency and high gel strength of the base resin powder, thereby significantly improving the properties of the finally prepared superabsorbent polymer.

[0037] Specifically, when diethylenetriamine pentaacetic acid is added together with an azo polymerization initiator during the polymerization process, the five -COOH structures can effectively prevent chain transfer of metal ions during the polymerization process, thereby easily forming a uniform long-chain structure compared to other chelating agents. In particular, since it is used together during the polymerization process, it does not dissociate.

[0038] Specifically, the azo-based polymerization initiator and diethylenetriaminepentaacetic acid are included in a weight ratio of 1:2 to 1:10. By including them within the above content range, a desired novel cross-linked structure is formed, thereby achieving both excellent absorbency and high gel strength of the base resin powder, thereby significantly improving the properties of the finally prepared superabsorbent polymer.

[0039] Meanwhile, if the weight ratio of the azo polymerization initiator to diethylenetriaminepentaacetic acid is outside the above range and the content of diethylenetriamine is less than 2 parts by weight based on 1 part by weight of the azo polymerization initiator, it may be difficult to achieve the desired uniform cross-linked structure. For example, if the content of the azo polymerization initiator is relatively too large, the chain length in the polymer may become short, and thus, it may be difficult to achieve the desired structure. Furthermore, if the content of diethylenetriamine is greater than 10 parts by weight based on 1 part by weight of the azo polymerization initiator, additional side reactions may occur, and thus, the absorbency may be deteriorated.

[0040] Preferably, the azo-based polymerization initiator and diethylenetriaminepentaacetic acid may be included in a weight ratio of 1: 3 to 1: 7. When included in this weight ratio, the synergistic effect of the combination can be maximized.

[0041] Specific examples of the azo polymerization initiator include one or more selected from the group consisting of 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 and 4,4-azobis(4-cyanovaleric acid). Preferably, 2,2-azobis(2-amidinopropane) dihydrochloride can be used.

[0042] The content of the azo-based polymerization initiator is not particularly limited, but may be 500 to 1,500 ppmw, preferably 600 to 900 ppmw or 700 to 850 ppmw, based on the weight of the water-soluble ethylenically unsaturated monomer. This is a very small amount compared to conventional polymerization initiators, and in the present disclosure, it is used in combination with the above-mentioned diethylenetriaminepentaacetic acid, so even a very small amount can achieve the intended effect.

[0043] The content of diethylenetriaminepentaacetic acid is not particularly limited, but can be 1,000 to 10,000 ppmw, preferably 3,000 to 9,000 ppmw or 3,500 to 7,500 ppmw, based on the weight of the water-soluble ethylenically unsaturated monomer. It is used in combination with an azo initiator within the above content range to achieve the desired effect.

[0044] 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:

[0045] [Chemical Formula 1]

[0046] R1-COOM 1

[0047] In Chemical Formula 1,

[0048] R1 is a C2-5 alkyl group containing an unsaturated bond,

[0049] M 1 It is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

[0050] Preferably, the monomer may be one or more selected from the group consisting of acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts thereof. Advantageously, acrylic acid or a salt thereof is used as the water-soluble ethylenically unsaturated monomer because a superabsorbent polymer having improved absorbency can be obtained. In addition, as the monomer, one or more monomers selected from the group consisting of anionic monomers such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid and their salts, nonionic hydrophilic group-containing monomers such as (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate or polyethylene glycol (meth)acrylate, and amino group-containing unsaturated monomers such as (N,N)-dimethylaminoethyl (meth)acrylate or (N,N)-dimethylaminopropyl (meth)acrylamide and their quaternized products can be used.

[0051] The water-soluble ethylenically unsaturated monomer has an acid group, and at least part of the acid groups are partially neutralized using the neutralization solution of the present invention. Preferably, a monomer partially neutralized with an alkaline substance such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide can be used.

[0052] The degree of neutralization of the monomers may be 40 mol% to 95 mol%, or 40 mol% to 80 mol%, or 45 mol% to 75 mol%. Although the range of the degree of neutralization may vary depending on the final properties, if the neutralization degree is too high, the neutralized monomers may precipitate, making it difficult to smoothly polymerize. Conversely, if the neutralization degree is too low, the absorbency of the polymer may be significantly reduced, and the polymer may exhibit elastic, rubber-like properties that are difficult to handle.

[0053] In the super absorbent polymer of one embodiment, the hydrogel polymer may be a polymer formed by cross-linking polymerization of monomers in the presence of one or more internal cross-linking agents selected from the group consisting of C8-12 bis(meth)acrylamide, C2-10 polyol poly(meth)acrylate, and C2-10 polyol poly(meth)allyl ether.

[0054] As internal crosslinking agent, any compound capable of introducing crosslinks during the polymerization of the water-soluble ethylenically unsaturated monomers may be used. As non-limiting examples, multifunctional crosslinking agents such as N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol 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 may be used alone or in combination of two or more, but are not limited thereto. Among them, ethylene glycol diglycidyl ether may be preferably used.

[0055] In the monomer composition, the internal crosslinking agent may be used in an amount of 500 ppmw to 1,500 ppmw based on the weight of the water-soluble ethylenically unsaturated monomer. This allows for control of the degree of internal crosslinking of the hydrogel polymer and the base resin powder, thereby optimizing the absorbency and permeability of the superabsorbent polymer. However, if the internal crosslinking agent content is too high, the basic absorption properties of the superabsorbent polymer may be degraded.

[0056] Furthermore, the monomer composition may contain additives such as a photopolymerization initiator, a foaming agent, a surfactant, a thickener, a plasticizer, a storage stabilizer, and an antioxidant.

[0057] As the photopolymerization initiator, one or more compounds selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyalkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acylphosphine, and α-aminoketone can be used. Specific examples of acylphosphine include diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, phenyl di (2,4,6-trimethylbenzoyl) phosphine oxide, and ethyl (2,4,6-trimethylbenzoyl) phenyl phosphinate. A variety of photopolymerization initiators are described in Reinhold Schwalm, “UV Coatings: Basics, Recent Developments and New Applications (Elsevier 2007)”, page 115, but are not limited to the above examples.

[0058] The photopolymerization initiator may be included in an amount of 10 ppmw to 500 ppmw, preferably 50 ppmw to 300 ppmw or 50 ppmw to 100 ppmw, based on the weight of the water-soluble ethylenically unsaturated monomer. If the concentration of the photopolymerization initiator is too low, the polymerization rate may slow, and a large amount of residual monomer may be extracted in the final product. Conversely, if the concentration of the photopolymerization initiator is higher than the above range, the polymer chains constituting the network may shorten, thereby deteriorating the properties of the polymer.

[0059] Also, the monomer composition may be prepared in the form of a solution in which the above-mentioned raw materials such as the monomer, polymerization initiator, chelating agent, and internal cross-linking agent are dissolved in a solvent.

[0060] Among them, the solvent that can be used is not limited, as long as it can dissolve or disperse the above-mentioned raw materials. For example, one or more selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate and N,N-dimethylacetamide can be used alone or in combination.

[0061] Furthermore, the formation of the hydrogel polymer by polymerization of the monomer composition can be carried out by a conventional polymerization method without particular limitation. As non-limiting examples, polymerization methods are mainly divided into thermal polymerization and photopolymerization based on the energy source. Thermal polymerization can be carried out in a reactor equipped with a stirring shaft, such as a kneader, and photopolymerization can be carried out in a reactor equipped with a movable conveyor belt.

[0062] For example, the hydrogel polymer can be obtained by introducing the monomer composition into a reactor equipped with a stirring shaft, such as a mixer, and supplying hot air or heating the reactor to perform thermal polymerization. The size of the hydrogel polymer discharged from the reactor outlet can range from several millimeters to several centimeters 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 introduced monomer composition, and typically, a hydrogel polymer having a particle size of 2 mm to 50 mm can be obtained.

[0063] Furthermore, when the photopolymerization of the monomer composition is carried out in a reactor equipped with a movable conveyor as described above, a hydrogel polymer can be obtained in the form of a sheet. The thickness of the sheet can be varied depending on the concentration and introduction rate of the introduced monomer composition, but is preferably controlled to be between 0.5 and 5 cm to ensure uniform polymerization throughout the sheet while maintaining production speed.

[0064] The moisture content of the hydrogel polymer thus obtained can generally be 40 to 80% by weight. Throughout this specification, "moisture content" refers to the amount of water contained in the total weight of the hydrogel polymer and is the value obtained by subtracting the weight of the hydrogel polymer in a dry state from the weight of the hydrogel polymer. Specifically, it is defined as the value calculated by measuring the weight loss caused by evaporation of moisture from the polymer while drying it by increasing its temperature through infrared heating. The temperature is raised from room temperature to approximately 180°C and then maintained for approximately 40 minutes.

[0065] (Drying, grinding and classification steps)

[0066] Next, the method for preparing a super absorbent polymer according to one embodiment of the present invention includes the steps of drying, grinding, and classifying the prepared hydrogel polymer to form a base resin powder.

[0067] Specifically, the step of drying the obtained hydrogel polymer is performed. If necessary, in order to improve the efficiency of the drying step, a step of coarsely grinding (mincing) the hydrogel polymer may be further performed before drying.

[0068] The grinder used is not limited, but specifically, one selected from the group consisting of a vertical mill, a turbine cutter, a turbine grinder, a rotary cutting mill, a cutting mill, a disc mill, a shredder, a crusher, a shredder, and a disc cutter can be used, but is not limited thereto.

[0069] The shredding step may be performed to reduce the particle size of the hydrogel polymer to 2 mm to 10 mm. Due to the high moisture content of the hydrogel polymer, grinding to a particle size of less than 2 mm is technically difficult, and agglomeration may occur between the ground particles. Furthermore, if the particle size is greater than 10 mm, the efficiency of the subsequent drying step may not be significantly improved.

[0070] The hydrogel polymer, which has been shredded as described above or has not been shredded directly after polymerization, is then dried. The drying temperature may be between 150°C and 250°C. If the drying temperature is lower than 150°C, the properties of the resulting superabsorbent polymer may deteriorate. If the drying temperature is higher than 250°C, only the polymer surface may be dried, potentially producing fine particles during the subsequent grinding process, and the properties of the resulting superabsorbent polymer may be deteriorated. Therefore, drying is preferably performed at a temperature between 150°C and 200°C, more preferably between 150°C and 190°C.

[0071] In consideration of processing efficiency, drying may be performed for 20 minutes to 90 minutes, but the drying time is not limited thereto.

[0072] Meanwhile, the drying step may be performed through a multi-stage process within the above temperature range.

[0073] The drying method is not limited as long as it is commonly used in the drying process of hydrogel polymers. Specifically, the drying step can be carried out by supplying hot air, irradiating with infrared rays, irradiating with microwaves, or irradiating with ultraviolet rays. Hot air supply can be carried out using an oven capable of transferring air volume vertically.

[0074] The polymer after such a drying step may have a moisture content of about 0.1 wt % to about 10 wt %.

[0075] Next, a step of grinding the dried polymer is performed.

[0076] The polymer powder obtained after the grinding step may have a particle size of 150 to 850 μm. As a grinder for grinding to such a particle size, specifically, a pin mill, hammer mill, spiral mill, roller mill, disc mill, or inching mill can be used, but is not limited thereto.

[0077] Furthermore, to manage the properties of the superabsorbent polymer ultimately produced after such a grinding step, the polymer powder obtained after grinding can be classified according to particle size. Preferably, polymer having a particle size of approximately 150 μm to approximately 850 μm can be classified, and only polymer powder having such a particle size can be subjected to the surface crosslinking step and production. More specifically, the classified base resin powder can have a particle size of 150 μm to 850 μm and can contain 50% by weight or more of particles having a particle size of 300 μm to 600 μm, and less than 3% by weight of fine powder having a particle size of less than 150 μm.

[0078] The base resin powder prepared by the above method may have a centrifuge retention capacity (CRC) of 45.0 g / g or greater, preferably 45.0 g / g to 48.0 g / g, or 46.0 g / g to 47 g / g, as measured according to EDANA method WSP 241.2. The specific method for measuring the centrifuge retention capacity will be described in detail in the experimental examples below.

[0079] The base resin powder prepared by the above method may have a gel strength of 0.30 N or greater, as measured by immersing 2.5 g of the base resin powder in 50 g of an ascorbic acid saline solution and allowing it to swell in a 40° C. oven for 24 hours, followed by measuring the swollen base resin powder using a tensile and compression tester. The ascorbic acid saline solution refers to an aqueous solution of 0.9 wt % NaCl and 0.005 wt % ascorbic acid.

[0080] The gel strength is an indicator of the resistance to degradation of the L-ascorbic acid component contained in urine and body fluids. The specific method for measuring the gel strength will be described in detail in the experimental examples below.

[0081] Preferably, the gel strength may be greater than 0.30 N, more preferably 0.30 N to 0.50 N. The specific method for measuring the gel strength will be described in detail in the following experimental examples.

[0082] (Surface cross-linking step)

[0083] Meanwhile, after the base resin powder is prepared through the above-mentioned classification process, a step of cross-linking the surface of the base resin powder while heat-treating in the presence of a surface cross-linking agent may be further performed.

[0084] The surface crosslinking step initiates a crosslinking reaction on the surface of the base resin powder in the presence of a surface crosslinking agent, and the unsaturated bonds of the water-soluble ethylenically unsaturated monomer remaining on the surface without crosslinking are crosslinked by the surface crosslinking agent, thereby forming a superabsorbent polymer with an increased surface crosslinking density.

[0085] Specifically, a surface cross-linked layer can be formed by heat treatment in the presence of a surface cross-linking agent. The heat treatment process increases the surface cross-linking density, i.e., the outer cross-linking density, but does not change the inner cross-linking density. Therefore, the superabsorbent polymer with a surface cross-linked layer has a structure in which the cross-linking density on the outside is higher than that on the inside.

[0086] In the surface cross-linking step, a surface cross-linking composition containing an alcohol solvent and water in addition to the surface cross-linking agent may be used.

[0087] Meanwhile, as the surface crosslinking agent contained in the surface crosslinking composition, those previously used for preparing super absorbent polymers can be used without particular limitation. For example, the surface crosslinking agent may include: one or more polyols selected from the group consisting of 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 or more carbonate compounds selected from the group consisting of ethylene carbonate and propylene carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazoline compounds such as oxazolidinone; polyamine compounds; mono-, di- or poly-oxazolidinone compounds; or cyclic urea compounds. Preferably, the same substances as the above-mentioned internal cross-linking agents can be used, for example, alkylene glycol diglycidyl ether compounds such as ethylene glycol diglycidyl ether can be used.

[0088] The surface crosslinking agent may be used in an amount of 0.001 to 2 parts by weight based on 100 parts by weight of the base resin powder. Preferably, the amount may be 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.02 parts by weight or more, and 0.5 parts by weight or less, or 0.3 parts by weight or less. By controlling the content of the surface crosslinking agent within the above range, a superabsorbent polymer exhibiting excellent properties such as absorption performance and permeability can be prepared.

[0089] At the same time, the surface crosslinking agent is added to the base resin powder in the form of a surface crosslinking composition containing the surface crosslinking agent. The method of adding the surface crosslinking composition is not particularly limited. For example, the surface crosslinking composition and the base resin powder can be placed in a reactor and mixed, or the surface crosslinking composition can be sprayed onto the base resin powder, or the surface crosslinking composition and the base resin powder can be continuously added to a continuously operating mixer and mixed.

[0090] Furthermore, the surface crosslinking composition may further comprise water and / or a hydrophilic organic solvent as a medium. Thus, the surface crosslinking agent can be uniformly dispersed in the base resin powder. The content of water and the hydrophilic organic solvent, based on 100 parts by weight of the base resin powder, can be controlled to uniformly dissolve / disperse the surface crosslinking agent, prevent base resin agglomeration, and optimize the surface penetration depth of the surface crosslinking agent.

[0091] The surface crosslinking step can be performed by heat treatment at a temperature of 110 to 200° C. or 110 to 150° C. for more than 30 minutes. More specifically, the surface crosslinking can be performed by heat treatment at the above maximum reaction temperature for 30 to 80 minutes or 40 to 70 minutes.

[0092] By satisfying these surface cross-linking process conditions (particularly the elevated temperature conditions and the reaction at the maximum reaction temperature), a superabsorbent polymer having excellent properties such as pressure permeability can be prepared.

[0093] The method for heating the surface crosslinking is not particularly limited. A heat medium may be supplied, or a heat source may be directly supplied for heating. Possible heat media include steam, hot air, and heating fluids such as hot oil. The temperature of the supplied heat medium can be appropriately selected based on the heat medium, the heating method, and the target temperature. Directly supplied heat sources include, but are not limited to, electric heating and gas heating.

[0094] Meanwhile, according to the method for preparing a superabsorbent polymer according to one embodiment of the present invention, various multivalent metal salts, such as aluminum salts such as aluminum sulfate, can be further used during the surface crosslinking process to further improve permeability, etc. Such multivalent metal salts can be included in the surface crosslinked layer of the superabsorbent polymer finally prepared.

[0095] (Superabsorbent polymer)

[0096] According to one embodiment of the present invention, a super absorbent polymer prepared by the above-mentioned super absorbent polymer preparation method is provided. The super absorbent polymer prepared by the above-mentioned super absorbent polymer preparation method can achieve excellent absorbency and optimal gel strength, and has an optimal crosslinking density.

[0097] Hereinafter, the functions and effects of the present invention will be described in detail through specific embodiments of the present invention. However, these embodiments are provided only as an illustration of the present invention, and the scope of the rights of the present invention is not determined by them.

[0098] [Example]

[0099] Example 1

[0100] (Polymerization Step) In a 3 L glass container equipped with a stirrer and a thermometer, 500 g of acrylic acid, 1,500 ppmw (based on 100 parts by weight of acrylic acid) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 80 ppmw (based on 100 parts by weight of acrylic acid) of diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide as a photoinitiator were added and dissolved. Then, 627 g of a 31.5% sodium hydroxide solution was added to prepare an aqueous solution of a water-soluble unsaturated monomer (neutralization degree: 70 mol%; solid content: 44.9 wt%). After the temperature of the aqueous solution of the water-soluble unsaturated monomer increased to 40° C. due to the heat of neutralization, the mixed solution was placed in a container containing 750 ppmw (based on 100 parts by weight of acrylic acid) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a thermal polymerization initiator and 5,000 ppmw of diethylenetriaminepentaacetic acid (DTPA) as a chelating agent. The mixture was then irradiated with ultraviolet light (irradiation dose: 10 mV / cm 2 ) was irradiated with ultraviolet light for 1 minute to perform ultraviolet polymerization, thereby obtaining a hydrogel polymer sheet.

[0101] (Chopping step) The obtained hydrogel polymer sheet was passed through a chopper having a pore size of 16 mm to prepare crumbs.

[0102] (Drying Step) Next, the chips were dried in an oven capable of vertically conveying airflow. Drying was performed in multiple stages, specifically using an airflow oven at 150°C for 5 minutes, 150°C for 8 minutes, 170°C for 5 minutes, 175°C for 5 minutes, 180°C for 5 minutes, and 160°C for 5 minutes. This drying process yielded a base resin powder.

[0103] Examples 2 to 4 and Comparative Examples 1 to 7

[0104] A super absorbent polymer was prepared by the same method as in Example 1, except that the components and contents shown in Table 1 below were used in the polymerization step.

[0105] Comparative Example 8

[0106] (Polymerization Step) In a 3 L glass container equipped with a stirrer and a thermometer, 500 g of acrylic acid, 1,500 ppmw (based on 100 parts by weight of acrylic acid) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 80 ppmw (based on 100 parts by weight of acrylic acid) of diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide as a photoinitiator were added and dissolved. Then, 627 g of a 31.5% sodium hydroxide solution was added to prepare an aqueous solution of a water-soluble unsaturated monomer (neutralization degree: 70 mol%; solid content: 44.9 wt%). After the temperature of the aqueous solution of the water-soluble unsaturated monomer increased to 40° C. due to the heat of neutralization, the mixed solution was placed in a container containing 750 ppmw (based on 100 parts by weight of acrylic acid) of 2,2'-azobis(2-amidinopropane) dihydrochloride as a thermal polymerization initiator, and then irradiated with ultraviolet light (irradiation dose: 10 mV / cm 2 ) was irradiated with ultraviolet light for 1 minute to perform ultraviolet polymerization, thereby obtaining a hydrogel polymer sheet.

[0107] (Chopping step) The obtained hydrogel polymer sheet and 3,000 ppmw of diethylenetriaminepentaacetic acid (DTPA), a chelating agent, were introduced into a chopper having a pore size of 16 mm to prepare crumbs.

[0108] (Drying Step) Next, the chips were dried in an oven capable of vertically conveying airflow. Drying was performed in multiple stages, specifically using an airflow oven at 150°C for 5 minutes, 150°C for 8 minutes, 170°C for 5 minutes, 175°C for 5 minutes, 180°C for 5 minutes, and 160°C for 5 minutes. This drying process yielded a base resin powder.

[0109]

Table 1

[0110]

[0111] [Experimental example]

[0112] For the superabsorbent polymer compositions prepared in Examples and Comparative Examples, properties were evaluated as follows, and the results are shown in Table 2 below.

[0113] Unless otherwise specified, the following property evaluations were performed at room temperature (25±1° C.), saline solution refers to an aqueous solution of 0.9 wt % sodium chloride (NaCl), and ascorbic acid saline solution refers to an aqueous solution of 0.9 wt % sodium chloride (NaCl) and 0.005 wt % ascorbic acid.

[0114] (1) Centrifugal retention capacity (CRC, g / g)

[0115] Among the base resin powders prepared according to Examples and Comparative Examples, samples having a particle size of 150 to 850 μm were taken and centrifuge retention capacity (CRC) according to unpressurized absorbency was measured according to European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.2.

[0116] Specifically, a W0 (g) sample (about 0.2g) was evenly placed in a small bag made of non-woven fabric and sealed, and then immersed in a saline solution (0.9 wt%) at room temperature. After 30 minutes, it was dehydrated for 3 minutes using a centrifuge at 250g, and the weight W2 (g) of the small bag was measured. In addition, after the same operation was performed without using the sample, the weight W1 (g) at this time was measured. Using the obtained weight, CRC (g / g) was calculated according to the following mathematical formula 1.

[0117] [Mathematical formula 1]

[0118] CRC(g / g)={[W2(g)-W1(g)] / W0(g)}……1

[0119] (2) Gel strength (N)

[0120] In the base resin powder prepared according to the examples and comparative examples, a sample with a particle size of 150 μm to 850 μm was taken, 2.5 g of the superabsorbent polymer was immersed in 50 g of a saline solution of ascorbic acid, and swelled in an oven at 40° C. for 24 hours, and then the gel strength of the swollen base resin powder was measured using a tensile and compression tester.

[0121] Specifically, the swollen base resin powder was measured using a tensile and compression tester, a digital dynamometer FGP-2, wherein the peak value (N) of the force applied to the tip when the tip penetrated was measured 3 times under the following conditions, and the arithmetic mean was determined as the gel strength (unit: N).

[0122] Tip size: end diameter 10±0.1mm

[0123] Beaker size: 50±0.1mm

[0124] Penetration speed: 500±0.5mm / min

[0125]

Table 2

[0126] As shown in Table 2, it was confirmed that the combined use of an azo polymerization initiator and diethylenetriaminepentaacetic acid in the polymerization step achieves an optimal crosslink density in the polymer, thereby simultaneously achieving excellent absorbency and gel strength in the base resin powder. Consequently, the properties of the resulting superabsorbent polymer can be significantly improved.

[0127] In the comparative examples where DTPA was not used or a sulfate-based polymerization initiator was used instead of the azo-based polymerization initiator, the gel strength was significantly reduced compared to the examples. It was confirmed that it was difficult to simultaneously achieve the desired gel strength and centrifugal retention capacity when other chelating agents were used instead of DTPA.

[0128] In the case of Comparative Examples 6 and 7 where the content ratio of the azo-based polymerization initiator to diethylenetriaminepentaacetic acid was outside the range of the present invention, it was difficult to form a uniform long chain structure, and thus it was difficult to simultaneously achieve excellent absorbency and gel strength.

[0129] In the case of Comparative Example 8 in which diethylenetriaminepentaacetic acid was contained in the chopping step after polymerization rather than in the polymerization step, the gel strength was significantly lowered compared with the Examples.

Claims

1. A method for preparing a superabsorbent polymer, comprising the following steps: performing crosslinking polymerization of a water-soluble ethylenically unsaturated monomer in which at least a portion of the acid group is neutralized and an internal crosslinking agent in the presence of a photopolymerization initiator, an azo-based polymerization initiator, and diethylenetriaminepentaacetic acid to obtain a hydrogel polymer; and The hydrogel polymer is dried, ground and classified to form a base resin powder, Wherein, the weight ratio of the azo polymerization initiator to the diethylenetriaminepentaacetic acid is 1:2 to 1:10, wherein the photopolymerization initiator is one or more selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyalkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acylphosphine and α-amino ketone, Wherein, the content of the photopolymerization initiator is 10 ppmw to 500 ppmw based on the weight of the water-soluble ethylenically unsaturated monomer.

2. The method for preparing a superabsorbent polymer according to claim 1, wherein: The weight ratio of the azo polymerization initiator to the diethylenetriaminepentaacetic acid is 1:3 to 1:

7.

3. The method for preparing a superabsorbent polymer according to claim 1, wherein: The azo-based polymerization initiator is included in an amount of 500 ppmw to 1,500 ppmw based on the weight of the water-soluble ethylenically unsaturated monomer.

4. The method for preparing a superabsorbent polymer according to claim 1, wherein: The diethylenetriaminepentaacetic acid is included in an amount of about 1,000 ppmw to about 5,000 ppmw based on the weight of the water-soluble ethylenically unsaturated monomer.

5. The method for preparing a superabsorbent polymer according to claim 1, wherein: The azo polymerization initiator is one or more selected from the group consisting of 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 and 4,4-azobis(4-cyanovaleric acid).

6. The method for preparing a superabsorbent polymer according to claim 1, wherein: The internal crosslinking agent is one or more selected from the group consisting of N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerol tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerol and ethylene carbonate.

7. The method for preparing a superabsorbent polymer according to claim 1, wherein: The internal cross-linking agent is included in an amount of 500 ppmw to 1,500 ppmw based on the weight of the water-soluble ethylenically unsaturated monomer.

8. The method for preparing a superabsorbent polymer according to claim 1, wherein: The base resin powder has a centrifuge retention capacity (CRC) measured according to EDENA method WSP 241.2 of 45.0 g / g or more.

9. The method for preparing a superabsorbent polymer according to claim 1, wherein: The base resin powder has a gel strength of 0.30 N or more as measured using a tensile and compression tester, the gel strength being obtained by immersing 2.5 g of the base resin powder in 50 g of a saline solution of ascorbic acid and allowing it to swell in an oven at 40° C. for 24 hours, and then measuring the swollen base resin powder.

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