Superabsorbent polymers and their preparation methods

Abstract

This disclosure relates to a superabsorbent polymer and a method for preparing the same. More specifically, this disclosure relates to a superabsorbent polymer having reduced extractable content without degrading absorption performance, and a method for preparing the superabsorbent polymer by polymerization in the presence of a foaming agent and a carboxylic acid additive with a specific structure.

Description

Technical Field

[0001] Cross-references to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2020-0162890 filed with the Korean Intellectual Property Office on November 27, 2020, and Korean Patent Application No. 10-2021-0152460 filed with the Korean Intellectual Property Office on November 8, 2021, the disclosures of which are incorporated herein by reference in their entirety.

[0003] This disclosure relates to a superabsorbent polymer and a method for preparing the same. More specifically, this disclosure relates to a superabsorbent polymer having reduced extractable content without degrading absorption performance, and a method for preparing the superabsorbent polymer capable of producing the aforementioned superabsorbent polymer. Background Technology

[0004] Superabsorbent polymers (SAPs) are synthetic polymer materials capable of absorbing 500 to 1000 times their own weight in water. Various manufacturers have given them different names, such as SAM (superabsorbent material) and AGM (absorbent gel material). These superabsorbent polymers have begun to find practical applications in hygiene products, and are now widely used not only in hygiene products but also in water-retaining soil products for horticulture, waterproofing materials for civil engineering and construction, seedling sheets, preservatives in food distribution, and heat therapy materials.

[0005] These superabsorbent polymers have been widely used in the field of hygiene materials, such as diapers or sanitary napkins. In such hygiene materials, the superabsorbent polymer is typically contained in a state spread within the pulp. However, in recent years, there has been continuous effort to provide hygiene materials with thinner thicknesses, such as diapers. As part of these efforts, so-called pulp-free diapers, which reduce pulp content or use no pulp at all, are being actively developed.

[0006] As mentioned above, in the case of sanitary materials with reduced or no pulp content, the proportion of superabsorbent polymers is relatively high, and these superabsorbent polymer particles are inevitably contained in the sanitary material in a multi-layered form. In order for all the superabsorbent polymer particles contained in the multi-layered structure to more effectively absorb large amounts of liquid, such as urine, the superabsorbent polymer needs to have high absorbency and a rapid absorption rate. Furthermore, the superabsorbent polymer should not release the absorbed liquid even under external pressure, and additionally, it should maintain its shape well even when its volume expands (swells) due to the absorption of liquid, thus exhibiting excellent permeability.

[0007] Superabsorbent polymers include multiple hydrophilic portions on their surface to exhibit high absorption capacity for liquids, particularly water. Therefore, when exposed to air, superabsorbent polymer particles can aggregate and clump together by absorbing moisture from the air.

[0008] Therefore, in addition to improving centrifugal retention capacity (CRC) and pressure absorbability (AUP), there has been a need to develop superabsorbent polymers that have improved permeability without reducing the absorption rate and prevent clumping between superabsorbent polymer particles, where CRC and AUP are physical properties representing the basic absorption capacity and water retention capacity of the superabsorbent polymer. Summary of the Invention

[0009] Technical issues

[0010] Therefore, this disclosure relates to a method for preparing a superabsorbent polymer that has improved permeability and anti-caking efficiency while having a high absorption rate by polymerizing monomers in the presence of an encapsulating foaming agent, adding additives with a specific structure to the hydrogel polymer, and then coarsely pulverizing it.

[0011] Technical solution

[0012] To address this problem, according to one embodiment of this disclosure, a superabsorbent polymer is provided, the polymer comprising:

[0013] Superabsorbent polymer particles comprising a crosslinked polymer of a water-soluble olefinic unsaturated monomer having at least partially neutralized acidic groups and an internal crosslinking agent; and

[0014] Carboxylic acid additives

[0015] The carboxylic acid additive is at least one selected from the group consisting of carboxylic acids represented by the following chemical formula 1 and their salts, and

[0016] The superabsorbent polymer has the following physical properties:

[0017] 1) Based on the total weight of the superabsorbent polymer, the extractable contents, measured according to EDANA WSP 270.2 after swelling the superabsorbent polymer for 1 hour, are less than 4% by weight; and

[0018] 2) The BPI (Basic Polymer Index) calculated according to Equation 1 below is 31 or higher;

[0019] [Chemical Formula 1]

[0020]

[0021] In chemical formula 1,

[0022] A is a C5 to C21 alkyl group.

[0023] B1 is -OCO-, -COO-, or -COOCH(R1)COO-.

[0024] B2 is -CH2-, -CH2CH2-, -CH(R2)-, -CH=CH-, or -C≡C-.

[0025] R1 and R2 are each independently a C1 to C4 alkyl group.

[0026] n is an integer from 1 to 3, and

[0027] C is the carboxyl group.

[0028] [Equation 1]

[0029]

[0030] In Equation 1,

[0031] CRC is the centrifugation retention capacity measured according to EDANA WSP 241.3, and

[0032] ln(extractable content) is the natural logarithm of the extractable content.

[0033] In addition, a method for preparing a superabsorbent polymer is provided, comprising:

[0034] The step (step 1) involves the crosslinking polymerization of a water-soluble olefinic unsaturated monomer having at least partially neutralized acidic groups to form a hydrogel polymer in the presence of an internal crosslinking agent, an encapsulating foaming agent, a carboxylic acid additive, and a polymerization initiator.

[0035] The step of drying and pulverizing the hydrogel polymer (step 2);

[0036] The carboxylic acid additives mentioned therein are at least one selected from the group consisting of carboxylic acids represented by chemical formula 1 and their salts.

[0037] Beneficial effects

[0038] According to the superabsorbent polymer and its preparation method of the present invention, by including a carboxylic acid additive with a specific structure, the extractable content can be reduced without degrading the absorption performance of the superabsorbent polymer. Furthermore, this carboxylic acid additive can be added together with a foaming agent during the polymerization step to significantly reduce the surface tack of the hydrogel polymer. Therefore, polymer tearing is prevented during the hydrogel polymer shredding step, thereby reducing the extractable content of the final prepared superabsorbent polymer without degrading its absorption performance. Detailed Implementation

[0039] The terminology used herein is for illustrative purposes only and is not intended to limit the invention. The singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that, when used in this specification, the terms “comprising,” “having,” or “possessing” designate the presence of said features, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, steps, components, or combinations thereof.

[0040] Because the present invention can be modified in various ways and has various forms, its specific embodiments are shown by way of example and will be described in detail. However, this is not intended to limit the invention to the specific forms disclosed, and it should be understood that the invention includes all modifications, equivalents, and substitutions within the spirit and scope of the invention.

[0041] The preparation method of the superabsorbent polymer and the superabsorbent polymer will be described in more detail below according to specific embodiments of the present invention.

[0042] The terms used herein are for reference only in specific embodiments and are not intended to limit this disclosure unless explicitly stated otherwise. The singular expressions of this disclosure may include plural expressions unless they are expressed differently in the context.

[0043] The term "polymer" in this disclosure refers to the state in which water-soluble olefinically unsaturated monomers are polymerized, and can include all ranges of moisture content or particle size. Among these polymers, polymers having a moisture content of more than about 30% by weight after polymerization and before drying can be referred to as hydrogel polymers, and particles of hydrogel polymers that are pulverized and dried can be referred to as crosslinked polymers.

[0044] In addition, the term "superabsorbent polymer particles" refers to particulate materials containing cross-linked polymers, wherein water-soluble olefinic unsaturated monomers having at least partially neutralized acidic groups are polymerized and cross-linked by an internal cross-linking agent.

[0045] Furthermore, the term "superabsorbent polymer" is used to cover: all crosslinked polymers in which water-soluble olefinic unsaturated monomers having at least partially neutralized acidic groups are polymerized, or base resins in powder form consisting of superabsorbent polymer particles in which the crosslinked polymers are pulverized, and the crosslinked polymers or base resins are further processed, such as drying, pulverizing, grading, surface crosslinking, etc., to bring them to a commercially viable state, as applicable. Therefore, the term "superabsorbent polymer" can be interpreted as covering compositions comprising superabsorbent polymers, i.e., multiple superabsorbent polymer particles.

[0046] In addition, the term "ordinary superabsorbent polymer particles" refers to particles with a diameter of 150 μm to 850 μm in superabsorbent polymer particles.

[0047] In addition, the term "fine powder" refers to particles with a diameter of less than 150 μm in superabsorbent polymer particles.

[0048] In addition, the term "shredding" refers to cutting hydrogel polymers into small pieces to improve drying efficiency, and is used separately from grinding them into ordinary particle size.

[0049] Superabsorbent polymers are typically prepared by drying hydrogel polymers obtained from polymerizing monomers and then pulverizing them to the desired particle size. To facilitate drying and improve the efficiency of the pulverization process, a shredding or cutting step is performed before the drying process. However, because the surface of the typically prepared hydrogel polymer is viscous, it is difficult to shred it to obtain uniform sizes, resulting in reduced shredding efficiency and polymer tearing during the shredding step. In particular, foaming agents are used during polymerization to increase the absorption rate by increasing the specific surface area of ​​the superabsorbent polymer. In this case, the foaming agent does not adequately irradiate the interior of the monomer composition with light, and the viscosity of the hydrogel polymer increases due to some unpolymerized monomers, which becomes a further problem.

[0050] Furthermore, due to the surface adhesiveness of the hydrogel polymer, the polymer adheres to a portion of the polymerization reactor belt. As a result, the polymerization occurs unevenly due to the residue remaining on this portion of the polymerization reactor belt, which also leads to the problem of having to maintain the polymerization reactor more frequently.

[0051] Furthermore, in the case of superabsorbent polymers produced by polymer tearing during the shredding step, the polymer structure of some cross-linked polymers cannot be maintained after polymerization. Therefore, the extractable contents of the final superabsorbent polymer increase, while general physical properties, such as water retention capacity, may also decrease.

[0052] Therefore, the inventors completed this invention by confirming that when carboxylic acid additives with a specific structure are included, the extractable contents can be reduced without degrading the absorption performance of the superabsorbent polymer. Specifically, when the carboxylic acid additive is added together with the foaming agent during the polymerization step, the carboxylic acid additive is adsorbed onto the interior and / or surface of the polymerized hydrogel polymer without participating in the polymerization reaction, and the adsorbed carboxylic acid additive on the surface can impart hydrophobicity to the hydrogel polymer surface. Therefore, the surface tackiness of the hydrogel polymer can be significantly reduced, and tearing of the polymer during the chopping step can be prevented, thereby reducing the extractable contents of the final superabsorbent polymer without degrading its absorption performance.

[0053] Here, the carboxylic acid additive added to the hydrogel polymer is at least one compound selected from the group consisting of carboxylic acids represented by Formula 1 and their salts, and simultaneously possesses both hydrophobic and hydrophilic functional groups. Furthermore, the water-soluble olefinic unsaturated monomer contains acidic groups (-COOH) and / or neutralized acidic groups (-COO). - ), and due to acidic groups (-COOH) and / or neutralized acidic groups (-COO) - The additive does not participate in polymerization and remains, thus a large amount of hydrophilic portion exists on the surface of the hydrogel polymer prepared by polymerization. Therefore, the hydrophilic functional groups of the additive are adsorbed onto at least a portion of the hydrophilic portion present on the surface of the hydrogel polymer, and the polymer surface with the adsorbed additive becomes hydrophobic due to the hydrophobic functional groups located at the other end of the additive. Therefore, the surface of the hydrogel polymer can exhibit hydrophobicity, thereby reducing surface tack.

[0054] More specifically, in carboxylic acid additives, the hydrophobic functional group is a C5 to C21 alkyl group (A part), and the hydrophilic functional group is the C part, specifically a carboxyl group (COOH) or, in the case of salts, a carboxylate group (-COO). - The hydrophobic and hydrophilic functional groups are located at both ends of the additive. Specifically, carboxylic acid additives, in addition to the A and C portions at both ends, also include a (B1-B2) portion, and the (B1-B2) portion improves the adsorption performance on polymer surfaces; having only a partial C portion may be insufficient. Therefore, compared to compounds with an AC structure but lacking the (B1-B2) portion, additives with the chemical formula 1 exhibit superior adsorption performance on hydrophilic polymer surfaces.

[0055] Furthermore, compounds with an AC structure but without the (B1-B2) moiety do not exhibit sufficient hydrophilicity to dissolve in monomer compositions in water-based solvents, and therefore cannot be added to monomer compositions.

[0056] The superabsorbent polymer and its preparation method according to exemplary embodiments will be described in more detail below.

[0057] Superabsorbent polymers

[0058] According to one embodiment of the present disclosure, a superabsorbent polymer is provided, the polymer comprising:

[0059] Superabsorbent polymer particles comprising a crosslinked polymer of a water-soluble olefinic unsaturated monomer having at least partially neutralized acidic groups and an internal crosslinking agent; and

[0060] Carboxylic acid additives

[0061] The carboxylic acid additive is at least one selected from the group consisting of carboxylic acids represented by the following chemical formula 1 and their salts, and

[0062] The superabsorbent polymer has the following physical properties:

[0063] 1) Based on the total weight of the superabsorbent polymer, the extractable contents, measured according to EDANA WSP 270.2 after swelling the superabsorbent polymer for 1 hour, are less than 4% by weight; and

[0064] 2) The BPI (Basic Polymer Index) calculated according to Equation 1 is 31 or higher.

[0065] The superabsorbent polymer comprises a plurality of superabsorbent polymer particles, said superabsorbent polymer particles comprising a crosslinked polymer of a water-soluble olefinic unsaturated monomer having at least partially neutralized acidic groups and an internal crosslinking agent. Here, the crosslinked polymer is obtained by crosslinking polymerization of a water-soluble olefinic unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent, and may have a three-dimensional network structure, wherein the main chain formed by monomer polymerization is crosslinked by the internal crosslinking agent.

[0066] In other words, one embodiment of the superabsorbent polymer comprises a plurality of superabsorbent polymer particles, said superabsorbent polymer particles comprising a crosslinked polymer of a water-soluble olefinic unsaturated monomer having at least partially neutralized acidic groups and an internal crosslinking agent. When the crosslinked polymer has a three-dimensional network structure in which the main chain formed by monomer polymerization is crosslinked by an internal crosslinking agent, the water retention capacity and pressure absorption capacity, which are general physical properties of superabsorbent polymers, can be significantly improved compared to the case of having a two-dimensional linear structure that is not further crosslinked by an internal crosslinking agent.

[0067] In addition, superabsorbent polymers satisfy the following physical properties 1) and 2):

[0068] 1) Based on the total weight of the superabsorbent polymer, the extractable contents, measured according to EDANA WSP 270.2 after swelling the superabsorbent polymer for 1 hour, are less than 4% by weight; and

[0069] 2) The BPI (Basic Polymer Index) calculated according to Equation 1 is 31 or higher.

[0070] Here, the "extractable contents" of a superabsorbent polymer refer to the uncrosslinked polymeric form of the compound during the preparation of the superabsorbent polymer, and can arise during polymerization due to incomplete crosslinking (which may result in non-crosslinking), or due to the decomposition of the crosslinking agent or the breakage of the main molecular chain during chopping or drying. These extractable contents are problematic because they are easily eluted when the superabsorbent polymer absorbs liquid and swells. Therefore, when the extractable contents of a superabsorbent polymer exceed 4% by weight based on the total weight of the superabsorbent polymer, the superabsorbent polymer is eluted, leading to problems such as making the surface of the applied hygiene product sticky or causing skin irritation. More specifically, the extractable contents of a superabsorbent polymer can be less than 4% by weight, less than 3.8% by weight, less than 3.6% by weight, less than 3.5% by weight, or less than 3.4% by weight, based on the total weight of the superabsorbent polymer. Furthermore, since lower extractable contents are considered better, the lower limit is theoretically 0% by weight, but it can also be more than 1% by weight, more than 2% by weight, or more than 3% by weight.

[0071] Furthermore, when measured according to EDANA WSP 241.3, the centrifugal retention capacity (CRC) of the superabsorbent polymer can be above 30 g / g, above 32 g / g, above 33 g / g, or above 34 g / g, and below 40 g / g, below 38 g / g, or below 36 g / g.

[0072] Furthermore, according to Equation 1, the Basic Polymer Index (BPI) of the superabsorbent polymer is 31 or higher. When the BPI value is less than 31, even at the same level of water retention capacity, the extractable contents are high, and therefore, due to the weak network of cross-linked polymers in the superabsorbent polymer, there may be a problem of reduced strength. In addition, with increased rewetting, hygiene and safety are also unsatisfactory. More specifically, the BPI of the superabsorbent polymer can be 32 or higher, 33 or higher, 34 or higher, 35 or higher, or 36 or higher. Furthermore, since a higher BPI is considered better, there is no upper limit. For example, the upper limit could be below 45, below 43, below 40, or below 38.

[0073] Water-soluble olefinically unsaturated monomers can be any monomer commonly used in the preparation of superabsorbent polymers. As a non-limiting example, a water-soluble olefinically unsaturated monomer can be a compound represented by the following chemical formula 2:

[0074] [Chemical Formula 2]

[0075] R-COOM'

[0076] In chemical formula 2,

[0077] R is a C2 to C5 alkyl group with unsaturated bonds, and

[0078] M' is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0079] Preferably, the monomer may be at least one selected from the group consisting of (meth)acrylic acid and a monovalent (alkali) metal salt, a divalent metal salt, an ammonium salt, and an organic amine salt.

[0080] When (meth)acrylic acid and / or its salts are used as water-soluble olefinically unsaturated monomers, it is advantageous to obtain superabsorbent polymers with improved absorption properties. In addition, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acrylamide, N-substituted (meth)acrylates, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxy polyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, (N,N)-dimethylaminoethyl (meth)acrylate, or (N,N)-dimethylaminopropyl (meth)acrylamide, etc., can be used as monomers.

[0081] Here, the water-soluble olefinically unsaturated monomer may have acidic groups, at least a portion of which can be neutralized by a neutralizing agent. Specifically, in the step of mixing the water-soluble olefinically unsaturated monomer with acidic groups, the internal crosslinking agent, the polymerization initiator, and the neutralizing agent, at least a portion of the acidic groups of the water-soluble olefinically unsaturated monomer can be neutralized. In this case, alkaline substances capable of neutralizing acidic groups, such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide, can be used as neutralizing agents.

[0082] Furthermore, the degree of neutralization of water-soluble olefinic unsaturated monomers can be 50 to 90 mol%, 60 to 85 mol%, 65 to 85 mol%, or 65 to 75 mol%, where the degree of neutralization refers to the extent to which the acidic groups contained in the water-soluble olefinic unsaturated monomer are neutralized by the neutralizing agent. The range of the degree of neutralization can vary depending on the final physical properties. Excessive neutralization leads to precipitation of the neutralized monomers, thus making polymerization potentially difficult. Conversely, excessively low neutralization not only degrades the absorbency of the polymer but also imparts unmanageable properties, such as those of elastic rubber.

[0083] Furthermore, the term "internal crosslinking agent" used herein differs from the surface crosslinking agent used to crosslink the surface of the superabsorbent polymer particles described later. The internal crosslinking agent polymerizes the unsaturated bonds of water-soluble olefinic unsaturated monomers through crosslinking. While crosslinking occurs both on the surface and internally in the aforementioned steps, in the subsequent surface crosslinking step of the superabsorbent polymer particles, the final superabsorbent polymer particles have a structure on the surface crosslinked by the surface crosslinking agent, and a structure inside the particles crosslinked by the internal crosslinking agent.

[0084] As an internal crosslinking agent, any compound can be used, as long as it allows the introduction of crosslinking bonds during the polymerization of the water-soluble olefinic unsaturated monomer. As a non-limiting example, the internal crosslinking agent can be a multifunctional crosslinking agent. For example, it can be an acrylate compound, such as N,N'-methylenebisacrylamide, trimethylpropane 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 tetra(meth)acrylate, etc. Alcohol pentaacrylate, glycerol tri(meth)acrylate, and pentaerythritol tetraacrylate; epoxy compounds, such as ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, tripropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, polybutylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol triglycidyl ether, diglycerol polyglycidyl ether, and polyglycerol polyglycidyl ether; triarylamine; propylene glycol; glycerol; or ethylene carbonate, and these may be used alone or in combination of two or more. However, this disclosure is not limited thereto.

[0085] According to one embodiment, epoxy compounds can be used as internal crosslinking agents. For example, ethylene glycol diglycidyl ether can be used as an internal crosslinking agent. In this case, unlike when only acrylate compounds such as polyethylene glycol di(meth)acrylate are used as internal crosslinking agents, the polymerization does not occur in situ, the reaction proceeds slowly, resulting in a low degree of crosslinking. Therefore, hydrogel polymers prepared using epoxy compounds as internal crosslinking agents have significantly higher surface tack compared to hydrogel polymers prepared using acrylate compounds such as polyethylene glycol di(meth)acrylate as internal crosslinking agents. Furthermore, the extractable contents of the superabsorbent polymer ultimately prepared from such polymers may be high. However, in the case of superabsorbent polymers containing the aforementioned carboxylic acid additives, even when using epoxy compounds as internal crosslinking agents, the surface tack of the polymer can be reduced.

[0086] In this case, acrylate compounds such as polyethylene glycol di(meth)acrylate can be used together with epoxy compounds as internal crosslinking agents.

[0087] Crosslinking polymerization of water-soluble olefinic unsaturated monomers in the presence of an internal crosslinking agent can be carried out through thermal polymerization, photopolymerization, or mixed polymerization with or without thickeners, plasticizers, preservation stabilizers, antioxidants, etc., in the presence of a polymerization initiator, but specific details will be described later.

[0088] In addition, the particle size of the superabsorbent polymer particles can be from about 150 to about 850 μm, and the particle size can be measured according to EDANA (European Disposable Products and Nonwovens Association) WSP 220.3.

[0089] Furthermore, the superabsorbent polymer includes carboxylic acid additives. As described above, by mixing and adding the additives to the hydrogel polymer, the hydrogel polymer can be easily pulverized without agglomeration. In this case, the carboxylic acid additive is at least one selected from the group consisting of carboxylic acids represented by Chemical Formula 1 and their metal salts. Specifically, the carboxylic acid additive is at least one selected from the group consisting of carboxylic acids represented by Chemical Formula 1, alkali metal salts of carboxylic acids represented by Chemical Formula 1, and alkaline earth metal salts of carboxylic acids represented by Chemical Formula 1. More specifically, the carboxylic acid additive is one of carboxylic acids represented by Chemical Formula 1, alkali metal salts of carboxylic acids represented by Chemical Formula 1, and alkaline earth metal salts of carboxylic acids represented by Chemical Formula 1.

[0090] In Formula 1, A is the hydrophobic moiety and can be a C5 to C21 straight-chain or branched alkyl group. However, having a straight-chain alkyl group is more advantageous in suppressing particle agglomeration and improving dispersibility. When A is an alkyl group with fewer than 5 carbon atoms, the chain is short, making it difficult to effectively control particle agglomeration. When A is an alkyl group with more than 21 carbon atoms, the flowability of the additive may decrease, thus carboxylic acid additives may not mix effectively with the hydrogel polymer, and the cost of the composition may increase due to the increased cost of the additive.

[0091] Specifically, in Formula 1, A can be a C5 to C21 straight-chain alkyl group, such as n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecanyl, n-hexadecyl, n-heptadecyl, n-octadecyl, n-nonadecanyl, n-icosanyl, or n-icosanyl.

[0092] More specifically, A can be a C6 to C18 straight-chain alkyl group. For example, A can be -C6H. 13 -C 11 H 23 -C 12 H25, -C 17 H 35 or -C 18 H 37 .

[0093] Furthermore, the (B1-B2) portion of Formula 1 enhances the adsorption performance on the polymer surface, which is insufficient when only the C portion is present. When the number of carbon atoms in B2 is 3 or more, the distance between the B1 and C portions increases, and the adsorption performance on the hydrogel polymer may deteriorate.

[0094] Here, R1 and R2 can each be independently C1 to C4 straight-chain or branched alkyl groups. More specifically, R1 and R2 can each be independently methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, or tert-butyl. Since the additive can be adsorbed onto the superabsorbent polymer particles, it is advantageous that the molecular structure of the additive is not large, therefore both R1 and R2 can be methyl.

[0095] Furthermore, n in Formula 1 can be 1, 2, or 3. More specifically, considering that the (B1-B2) portion is used to enhance the adsorption performance relative to the C portion and how long the molecular length is required for the carboxylic acid additive to be effectively adsorbed onto the hydrogel polymer, n, representing the amount of (B1-B2), is preferably 1.

[0096] Specifically, in chemical formula 1, B1 can be The asterisk (*) represents the bonding site with an adjacent atom.

[0097] For example, B1 could be

[0098] Furthermore, in chemical formula 1, B2 can be... The asterisk (*) represents the bonding site with an adjacent atom.

[0099] At this point, in order to improve the adsorption performance of the additive for the crosslinked polymer together with part C, B2 is preferably...

[0100]

[0101] Furthermore, in chemical formula 1, part C is the carboxyl group (COOH) as the hydrophilic part, and when the carboxylic acid additive is a salt, the hydrophilic part is the carboxylate group (COO). - ).

[0102] In other words, carboxylic acid additives can be compounds represented by the following chemical formula 1a:

[0103] [Chemical Formula 1a]

[0104]

[0105] In chemical formula 1a,

[0106] M is H + Monovalent cations of alkali metals or divalent cations of alkaline earth metals

[0107] If M is H + If it is a monovalent cation of an alkali metal, then k is 1; if it is a divalent cation of an alkaline earth metal, then k is 2.

[0108] The descriptions of A, B1, B2, and n are as defined in Chemical Formula 1.

[0109] More specifically, when the carboxylic acid additive is an alkali metal salt of a carboxylic acid represented by formula 1, the additive can be represented by the following formula 1':

[0110] [Chemical Formula 1']

[0111]

[0112] In chemical formula 1',

[0113] M1 is an alkali metal, such as sodium or potassium, and

[0114] The descriptions of A, B1, B2, and n are as defined in Chemical Formula 1.

[0115] Furthermore, when the carboxylic acid additive is an alkaline earth metal salt of a carboxylic acid represented by chemical formula 1, the additive can be represented by the following chemical formula 1":

[0116]

Chemical Formula 1"

[0117]

[0118] In chemical formula 1", M2 is an alkaline earth metal, such as calcium, and

[0119] The descriptions of A, B1, B2, and n are as defined in Chemical Formula 1.

[0120] For example, carboxylic acid additives can be any carboxylic acid selected from the group consisting of:

[0121]

[0122]

[0123] Alternatively, carboxylic acid additives may be any alkali metal salt selected from the following group:

[0124]

[0125]

[0126] In the above formula,

[0127] M1 is an alkali metal on its own.

[0128] Alternatively, the carboxylic acid additive can be any alkaline earth metal salt selected from the following group:

[0129]

[0130]

[0131] In the above formula,

[0132] M2 is an alkaline earth metal on its own.

[0133] For example, carboxylic acid additives can be any compound represented by the following chemical formulas 1-1 to 1-7, but are not limited to:

[0134]

[0135] In addition to carboxylic acid additives, superabsorbent polymers may also include compounds formed by decomposing the ester bonds of B1 during the drying process after the additives and the hydrogel polymer are pulverized.

[0136] Specifically, when the additive is a compound in which n is 1 and B1 is -OCO-, the superabsorbent polymer may also include alcohols having an A-OH structure and compounds having a HOOC-B2-C structure.

[0137] In addition, when the additive is a compound in which n is 1 and B1 is -COO-, the superabsorbent polymer may also include carboxylic acids having an A-COOH structure and compounds having a HO-B2-C structure.

[0138] In addition, when the additive is a compound in which n is 1 and B1 is -COOCH(R1)COO-, the superabsorbent polymer may also include carboxylic acids having an A-COOH structure and compounds having a HOCH(R1)COO-B2-C structure.

[0139] Since superabsorbent polymers also include compounds formed by breaking down ester bonds in additive molecules, they increase the fluidity of the additives and can further prevent re-agglomeration after pulverization.

[0140] In this case, the content of carboxylic acid additives can be from 0.01 to 10% by weight, based on the total weight of the superabsorbent polymer. Specifically, when 0.01 to 10 parts by weight of carboxylic acid additives are added to 100 parts by weight of water-soluble olefinic unsaturated monomers in the preparation of the crosslinked polymer, a superabsorbent polymer having the aforementioned extractable contents and BPI can be achieved. More specifically, when the content of additives in the superabsorbent polymer is too low, the effect of controlling agglomeration by additives is insufficient, and superabsorbent polymer particles that have not been pulverized to the desired particle size may be present. When the content of additives is too high, the water retention capacity and pressure absorption, which are general physical properties of superabsorbent polymers, may deteriorate.

[0141] The content of additives in superabsorbent polymers can be determined by adding 1g of superabsorbent polymer to 1ml of distilled water, mixing thoroughly for 1 hour until swelling, filtering to extract only the solution fraction, performing HPLC analysis, and then analyzing the content of additives dissolved in the solution fraction.

[0142] More specifically, based on the total weight of the superabsorbent polymer, the content of carboxylic acid additives can be more than 0.01% by weight, more than 0.02% by weight, more than 0.05% by weight, more than 0.1% by weight, or more than 0.5% by weight, and less than 10% by weight, less than 8% by weight, less than 5% by weight, less than 3% by weight, less than 2% by weight, or less than 1% by weight.

[0143] Meanwhile, when the superabsorbent polymer does not further include a surface crosslinking layer, which will be described later, it may exclude additives other than multiple superabsorbent polymer particles, carboxylic acid additives, and hydrolysis products of additives generated during the preparation of the superabsorbent polymer due to additive hydrolysis.

[0144] Furthermore, specifically, the superabsorbent polymer of one embodiment may not include compounds having glucose units containing multiple hydroxyl groups in the molecule, such as microcrystalline cellulose. For example, when the superabsorbent polymer composition comprises microcrystalline cellulose with an average particle size of 1 to 10 μm, such as that represented by the following chemical formula 3, which is available from FMC... At pH-101, the aggregation between superabsorbent polymer particles may not be inhibited due to the presence of multiple hydroxyl groups, so the effects of the above additives may not be effectively demonstrated.

[0145] [Chemical Formula 3]

[0146]

[0147] Furthermore, one embodiment of the superabsorbent polymer may not include hydrophilic additives, such as polyethylene glycol, polypropylene glycol, poly(ethylene glycol)-poly(propylene glycol) copolymer, polyoxyethylene lauryl ether carboxylic acid, sodium polyoxyethylene lauryl ether carboxylic acid, lauryl sulfate, sodium lauryl sulfate, etc. Since these additives do not have the (B1-B2) portion of Formula 1 in their molecules, they cannot sufficiently adsorb onto the surface of the cross-linked polymer, thus failing to effectively inhibit the aggregation between superabsorbent polymer particles. Therefore, when the superabsorbent polymer contains hydrophilic additives as described above instead of carboxylic acid additives, the aggregation between particles cannot be inhibited after pulverizing the cross-linked polymer; therefore, the superabsorbent polymer contains a large amount of fine powder, has low water retention capacity, and low apparent density.

[0148] Furthermore, the superabsorbent polymer may also include a surface crosslinking layer, which is formed by further crosslinking the crosslinked polymer with a surface crosslinking agent on at least a portion of the surface of the superabsorbent polymer particles. This is to increase the surface crosslinking density of the superabsorbent polymer particles. As described above, when the superabsorbent polymer particles further include a surface crosslinking layer, they have a structure in which the crosslinking density on the outer side is higher than that on the inner side.

[0149] As a surface crosslinking agent, any surface crosslinking agent conventionally used in the preparation of superabsorbent polymers can be used without any particular limitation. Examples of surface crosslinking agents may include: at least one polyol selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; at least one carbonate compound selected from the group consisting of ethylene carbonate, propylene carbonate, and glycerol carbonate; epoxy compounds, such as ethylene glycol diglycidyl ether; oxazoline compounds, such as oxazolidinones; polyamine compounds; oxazoline compounds; mono-, di-, or poly-oxazolidinone compounds; cyclic urea compounds, etc.

[0150] Specifically, one or more, two or more, or three or more of the above-mentioned surface crosslinking agents can be used as surface crosslinking agents. For example, ethylene carbonate-propylene carbonate (ECPC), propylene glycol, and / or glycerol carbonate can be used.

[0151] Preparation method of superabsorbent polymer

[0152] Furthermore, according to another embodiment, a method for preparing a superabsorbent polymer is provided, comprising:

[0153] The step (step 1) involves the crosslinking polymerization of a water-soluble olefinic unsaturated monomer having at least partially neutralized acidic groups to form a hydrogel polymer in the presence of an internal crosslinking agent, a foaming agent, a carboxylic acid additive, and a polymerization initiator.

[0154] The step of drying and pulverizing the hydrogel polymer (step 2)

[0155] The carboxylic acid additives mentioned therein are at least one selected from the group consisting of carboxylic acids represented by chemical formula 1 and their salts.

[0156] (Step 1)

[0157] The above steps involve the crosslinking polymerization of water-soluble olefinic unsaturated monomers having at least partially neutralized acidic groups to form a hydrogel polymer in the presence of an internal crosslinking agent, a foaming agent, a carboxylic acid additive, and a polymerization initiator. This step may include preparing a monomer composition by mixing water-soluble olefinic unsaturated monomers, a foaming agent, a carboxylic acid additive, an internal crosslinking agent, and a polymerization initiator, and forming the hydrogel polymer by thermal polymerization or photopolymerization of the monomer composition. In this case, the description of each component can be found in the above description.

[0158] In the monomer composition, the amount of internal crosslinking agent used can be from 0.01 to 5 parts by weight based on 100 parts by weight of water-soluble olefinic unsaturated monomer. For example, the amount of internal crosslinking agent used can be more than 0.01 parts by weight, more than 0.05 parts by weight, or more than 0.1 parts by weight based on 100 parts by weight of water-soluble olefinic unsaturated monomer, and less than 5 parts by weight, less than 3 parts by weight, or less than 2.5 parts by weight. When too little internal crosslinking agent is used, crosslinking is insufficient, making it difficult to achieve a strength above the appropriate level; and when too much internal crosslinking agent is used, it may be difficult to achieve the required water retention capacity level.

[0159] Furthermore, the polymerization initiator can be appropriately selected depending on the polymerization method. In the case of thermal polymerization, a thermal polymerization initiator is used; in the case of photopolymerization, a photopolymerization initiator is used. Additionally, in the case of mixed polymerization (using both heat and light), both a thermal polymerization initiator and a photopolymerization initiator can be used simultaneously. However, even when photopolymerization is carried out, a certain amount of heat is generated due to ultraviolet irradiation, etc., and some heat is generated as the polymerization reaction proceeds and the exothermic reaction occurs. Therefore, the composition may additionally include a thermal polymerization initiator.

[0160] Here, any compound that can form free radicals through light, such as ultraviolet light, can be used as a photopolymerization initiator without limitation.

[0161] For example, photopolymerization initiators can be one or more compounds selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyalkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acylphosphine, and α-amino ketone. Specific examples of acylphosphine include diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ethyl(2,4,6-trimethylbenzoyl)phenylphosphine ester, etc. More various photopolymerization initiators are well disclosed on page 115 of Reinhold Schwalm's "UV Coatings: Basics, Recent Developments and New Applications (Elsevier, 2007)," and this disclosure is not limited thereto.

[0162] Furthermore, as a thermal polymerization initiator, one or more initiators selected from persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specifically, examples of persulfate initiators include sodium persulfate (Na2S2O8), potassium persulfate (Na2S2O8), and ammonium persulfate ((NH4)2S2O8); and examples of azo initiators include 2,2-azobis(2-amidinylpropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoyl)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2)-yl)propane] dihydrochloride, and 4,4-azobis-(4-cyanopentanoic acid). More details on various thermal polymerization initiators are well disclosed on page 203 of Odian’s “Principle of Polymerization” (Wiley, 1981), and this disclosure is not limited thereto.

[0163] Based on 100 parts by weight of water-soluble olefinic unsaturated monomer, the amount of polymerization initiator used can be less than 2 parts by weight. When the concentration of the polymerization initiator is too low, the polymerization rate slows down, and a large amount of residual monomer may be extracted from the final product. Conversely, when the concentration of the polymerization initiator is higher than the above range, the polymer chains forming the network become shorter, resulting in an increase in the content of extractable contents, a decrease in pressure absorption, and thus a reduction in the physical properties of the polymer.

[0164] If necessary, the monomer mixture may further include additives such as thickeners, plasticizers, preservation stabilizers, antioxidants, etc.

[0165] Additionally, the monomer composition containing the monomer can be, for example, a solution dissolved in a solvent such as water. The solids content of the monomer composition in solution, i.e., the concentrations of the monomer, internal crosslinking agent, and polymerization initiator, can be appropriately adjusted taking into account the polymerization time and reaction conditions. For example, the solids content of the monomer composition can be 10 to 80% by weight, 15 to 60% by weight, or 30 to 50% by weight.

[0166] When the monomer composition has a solids content within the range described above, the grinding efficiency during the grinding of the polymer, as described later, can be advantageously controlled, while the need to remove unreacted monomers after polymerization can be eliminated by using the gelation effect that occurs in the polymerization reaction of high-concentration aqueous solutions.

[0167] At this point, any solvent capable of dissolving the above components can be used without limitation. For example, the solvent may be at least one 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 pentyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide.

[0168] Furthermore, encapsulated foaming agents refer to thermally expandable microcapsule foaming agents with a core-shell structure. This core-shell structure has a core containing hydrocarbons and a shell formed of thermoplastic resin on the core. Specifically, the hydrocarbons constituting the core are low-boiling-point liquid hydrocarbons that readily vaporize upon heating. Therefore, when the encapsulated foaming agent is heated, the thermoplastic resin constituting the shell softens, while the liquid hydrocarbons in the core vaporize. Moreover, as the pressure inside the capsule increases, the encapsulated foaming agent expands, thus forming bubbles larger than their original size.

[0169] Therefore, encapsulated foaming agents produce hydrocarbon gases, unlike organic foaming agents that produce nitrogen gas through exothermic decomposition reactions between monomers involved in polymer production, and inorganic foaming agents that produce carbon dioxide gas by absorbing heat generated during polymer production.

[0170] Encapsulated foaming agents can have different expansion characteristics depending on the components that make up the core and shell, as well as the weight and diameter of each component. Therefore, the porosity of superabsorbent polymers can be controlled by adjusting the encapsulated foaming agent to expand to the desired size.

[0171] Specifically, the encapsulated foaming agent before expansion has a particle shape with an average particle size (D0) of 5 to 30 μm. It is difficult to manufacture encapsulated foaming agents with an average diameter less than 5 μm. When the average diameter of the encapsulated foaming agent exceeds 30 μm, it may be difficult to effectively increase the surface area due to the excessively large pore size. Therefore, when the encapsulated foaming agent has the aforementioned average particle size, it can be determined that the encapsulated foaming agent is suitable for achieving an appropriate pore structure in the resin.

[0172] For example, the average diameter of the encapsulated foaming agent before expansion can be 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, 10 μm or more, and less than 30 μm, less than 25 μm, less than 20 μm, less than 17 μm, less than 16 μm or less.

[0173] The average particle size (D0) of the encapsulated foaming agent before expansion can be determined by measuring the diameter of the encapsulated foaming agent particles with an optical microscope as the average Ferrette particle size and calculating its average value.

[0174] In this case, the capsule thickness of the encapsulated foaming agent can be 2 to 15 μm.

[0175] Furthermore, the maximum expansion size of encapsulated foaming agents in air ranges from 20 to 190 μm. Here, "maximum expansion size of encapsulated foaming agents" refers to the diameter range of the top 10% by weight of highly foamed particles after heating the encapsulated foaming agent. It is difficult to manufacture encapsulated foaming agents with a maximum expansion size in air of less than 20 μm, and when the maximum expansion size in air exceeds 190 μm, it may be difficult to effectively increase the surface area due to the large pore size.

[0176] For example, the maximum expansion size of encapsulated foaming agents in air can be 50 to 190 μm, 70 to 190 μm, 75 to 190 μm, or 80 to 150 μm.

[0177] The maximum expansion size of the encapsulated foaming agent in air can be determined by applying 0.2 g of the encapsulated foaming agent to a glass petri dish, placing it on a hot plate preheated to 150°C for 10 minutes, and then observing the expanded encapsulated foaming agent using an optical microscope. The average Ferrette diameter can then be obtained by measuring the diameter of the top 10% by weight of the highly expanded particles using an optical microscope.

[0178] Furthermore, encapsulated foaming agents exhibit a maximum expansion rate of 5 to 15 times in air. Here, "maximum expansion rate of encapsulated foaming agents" refers to the average diameter (D) of the first 10% by weight of the highly expanded particles after heating. M The ratio of the diameter of the encapsulated foaming agent to the average diameter (D0) measured before heating (D) M / D0). When the maximum expansion ratio of the encapsulated foaming agent in air is less than 5 times, it cannot form a suitable pore structure in the superabsorbent polymer, thus preventing the manufacture of superabsorbent polymers with improved absorption capacity and absorption rate. Considering the average diameter of the encapsulated foaming agent before expansion, it is difficult to manufacture encapsulated foaming agents with a maximum expansion ratio in air exceeding 15 times. Therefore, it can be determined that encapsulated foaming agents with a maximum expansion ratio within the above range are suitable for forming pore structures suitable for superabsorbent polymers.

[0179] For example, the maximum expansion rate of encapsulated foaming agents in air can be more than 5 times, more than 7 times, or more than 8 times, and less than 15 times, less than 13 times, less than 11 times, or less than 10 times.

[0180] At this point, the average diameter (D0) of the encapsulated foaming agent measured before heating can be measured as described above. Furthermore, the average diameter (D0) of the top 10% by weight of the highly expanded particles after heating... M The expansion of the encapsulated foaming agent can be determined by applying 0.2 g of the encapsulated foaming agent to a glass petri dish and placing it on a hot plate preheated to 150°C for 10 minutes, then observing the expanded encapsulated foaming agent under an optical microscope. The average Freret diameter can then be obtained by measuring the diameter of each of the first 10% by weight of the particles using an optical microscope and then averaging these values.

[0181] The swelling properties of encapsulated foaming agents can be further illustrated in the examples described later.

[0182] The reason for measuring the maximum expansion size and maximum expansion rate of the encapsulated foaming agent in air is to determine whether pores of the desired size are formed in the superabsorbent polymer prepared using the encapsulated foaming agent. Specifically, the foam shape can vary depending on the preparation conditions of the superabsorbent polymer, making it difficult to determine the foam shape. Therefore, the expansion size and expansion rate are first determined by foaming the encapsulated foaming agent in air and confirming whether the encapsulated foaming agent is suitable for forming the desired pores.

[0183] Furthermore, the hydrocarbon constituting the core of the encapsulated foaming agent can be at least one selected from the group consisting of n-propane, n-butane, isobutane, cyclobutane, n-pentane, isopentane, cyclopentane, n-hexane, isohexane, cyclohexane, n-heptane, isohexane, cycloheptane, n-octane, isooctane, and cyclooctane. C3 to C5 hydrocarbons (n-propane, n-butane, isobutane, cyclobutane, n-pentane, isopentane, cyclopentane) are suitable for forming pores with the aforementioned dimensions, and isobutane is most suitable.

[0184] Furthermore, the thermoplastic resin constituting the shell of the encapsulated foaming agent can be a polymer formed from at least one monomer selected from the group consisting of (meth)acrylate compounds, (meth)acrylonitrile compounds, aromatic vinyl compounds, vinyl acetate compounds, and halogenated vinyl compounds. Among these, copolymers of (meth)acrylates and (meth)acrylonitrile may be most suitable for forming pores with the aforementioned dimensions.

[0185] In addition, the foaming start temperature (T) of encapsulated foaming agents 开始 The foaming temperature can be 60℃ to 120℃, 65℃ to 120℃, or 70℃ to 80℃, with a maximum foaming temperature (T). max The temperature range can be 100°C to 160°C, 105°C to 155°C, or 110°C to 120°C. Within these ranges, foaming is more likely to occur during subsequent thermal polymerization or drying processes, thereby introducing a porous structure into the polymer. The foaming initiation temperature and the maximum foaming temperature can be measured using a thermomechanical analyzer.

[0186] Furthermore, the amount of encapsulating foaming agent used can be 0.05 to 1 part by weight relative to 100 parts by weight of water-soluble olefinic unsaturated monomer. When the content of the foaming agent is less than 0.05 parts by weight, the effect of adding the foaming agent may be insignificant. When the content of the foaming agent exceeds 1 part by weight, there are too many pores in the crosslinked polymer, so the extractable contents and BPI value may decrease due to the increased viscosity of the hydrogel polymer. In addition, the gel strength and density of the superabsorbent polymer to be prepared decrease, which may lead to distribution and storage problems. For example, based on 100 parts by weight of water-soluble olefinic unsaturated monomer, the amount of encapsulating foaming agent used can be more than 0.05 parts by weight, more than 0.07 parts by weight, more than 0.09 parts by weight, or more than 0.1 parts by weight, and less than 0.8 parts by weight, less than 0.5 parts by weight, less than 0.3 parts by weight, or less than 0.2 parts by weight.

[0187] Furthermore, surfactants commonly used as foam stabilizers can be added together with encapsulating blowing agents. For example, foam stabilizers may include at least one compound selected from the group consisting of alkyl sulfate compounds and polyoxyethylene alkyl ether compounds. Examples of alkyl sulfate compounds include sodium dodecyl sulfate, ammonium lauryl sulfate, sodium lauryl ether sulfate, sodium myristyl ether sulfate, etc. Examples of polyoxyethylene alkyl ether compounds include polyoxyethylene lauryl ether. Here, the alkyl sulfate compound is anionic surfactant, and the polyoxyethylene alkyl ether compound is nonionic surfactant.

[0188] Here, encapsulating foaming agents and foam stabilizers can be used in a weight ratio of 1:0.01 to 1:0.5.

[0189] Furthermore, carboxylic acid additives can be dry-mixed, dissolved in a solvent and then mixed, or melted and then mixed. For example, the additives can be mixed in the form of a solution dissolved in a solvent. In this case, any type of surface viscosity index or organic solvent can be used without limitation, but water is the preferred solvent considering ease of drying and the cost of solvent recovery systems.

[0190] Furthermore, based on 100 parts by weight of the water-soluble olefinically unsaturated monomer, the amount of carboxylic acid additive used can be from 0.01 to 10 parts by weight. In this case, when the amount of carboxylic acid additive used is less than 0.01 parts by weight, it is insufficient to reduce the surface tack of the hydrogel polymer. When the amount of carboxylic acid additive used exceeds 10 parts by weight, unreacted substances may be generated due to foaming. More specifically, based on 100 parts by weight of the water-soluble olefinically unsaturated monomer, the amount of carboxylic acid additive used is preferably 0.05 parts by weight or more, or 0.1 parts by weight or more, and less than 2 parts by weight, less than 1 part by weight, less than 0.5 parts by weight, or less than 0.4 parts by weight, to minimize the surface tack of the hydrogel polymer and not reduce the absorption performance of the superabsorbent polymer.

[0191] Simultaneously, cross-linking polymerization of water-soluble olefinic unsaturated monomers with at least partially neutralized acidic groups can be carried out without any particular limitations, as long as the hydrogel polymer can be formed by thermal polymerization, photopolymerization, or hybrid polymerization.

[0192] Specifically, based on the energy source used in polymerization, polymerization methods are mainly divided into thermal polymerization and photopolymerization. In the case of thermal polymerization, it is typically carried out in a reactor equipped with a stirring shaft, such as a kneader. In the case of photopolymerization, it is typically carried out in a reactor equipped with a movable conveyor belt, or in a flat-bottomed container. However, the above-described polymerization methods are merely examples, and the present invention is not limited thereto.

[0193] For example, hydrogel polymers can be obtained by supplying hot air or heating the reactor to a stirring-axial reactor, such as a kneader, for thermal polymerization. Depending on the shape of the stirring shaft equipped in the reactor, the resulting hydrogel polymers can have dimensions ranging from a few centimeters to a few millimeters. Specifically, the size of the obtained hydrogel polymers can vary depending on the concentration and injection rate of the monomer composition injected into them, and hydrogel polymers with a weight-average particle size of 2 to 50 mm can be obtained.

[0194] Furthermore, as mentioned above, when photopolymerization is carried out in a reactor equipped with a movable conveyor belt or in a container with a flat bottom, the resulting hydrogel polymer is typically a sheet-like hydrogel polymer with a strip width. In this case, the thickness of the polymer sheet can vary depending on the concentration of the monomer composition to be injected, the injection rate, or the injection volume, but generally, it is preferable to supply the monomer composition such that a sheet-like polymer with a thickness of about 0.5 to about 5 cm can be obtained. When the monomer mixture is supplied in a way that makes the sheet-like polymer too thin, production efficiency is low, which is undesirable. When the thickness of the sheet-like polymer is greater than 5 cm, the polymerization reaction cannot proceed uniformly across the entire thickness due to the excessive thickness.

[0195] At this point, the moisture content of the resulting hydrogel polymer can be between 30% and 70% by weight. For example, the moisture content of the hydrogel polymer can be 35% or more by weight, 40% or more by weight, 45% or more by weight, or 50% or more by weight, and less than 70% by weight, less than 65% by weight, or less than 60% by weight. When the moisture content of the hydrogel polymer is too low, it is difficult to ensure an appropriate surface area in the subsequent pulverization step, so pulverization may be ineffective. When the moisture content of the hydrogel polymer is too high, the pressure it experiences in the subsequent pulverization step increases, so pulverization may be difficult to achieve the desired particle size.

[0196] Meanwhile, the "moisture content" in this specification refers to the water content in the total weight of the hydrogel polymer, which is the value obtained by subtracting the weight of the dried polymer from the weight of the hydrogel polymer. Specifically, the moisture content is defined as the value calculated based on the weight loss due to moisture evaporation from the polymer during the temperature rise process of the granular polymer used for drying by infrared heating. The drying conditions for determining the moisture content are as follows: heating to approximately 180°C and maintaining at 180°C for a total drying time of 40 minutes, including a 5-minute heating step.

[0197] Meanwhile, the surface tackiness index (25°C) of the hydrogel polymer, measured according to steps 1) to 4) below, can be less than 200g:

[0198] 1) Two samples were prepared, in which the hydrogel polymer with the bottom surface (the surface that contacts the reaction vessel when preparing the hydrogel polymer) was cut into dimensions of 2.5 cm wide × 2.5 cm long × 2 cm thick;

[0199] 2) Fix the two prepared samples on the upper and lower supports of the texture analyzer respectively, wherein the two samples are fixed such that the bottom surface of the sample protrudes 1 mm from each support.

[0200] 3) By bringing the upper / lower supports of the two fixed samples close together, reducing the distance between the supports to 1 mm, and then holding them for 5 seconds, the bottom surfaces of the two samples are adhered; and

[0201] 4) When the parts of the two samples attached to each other are detached by pulling the upper support, the maximum force (g) is measured and used as the surface viscosity index.

[0202] In this context, the "bottom surface" of the hydrogel polymer refers to the surface that, during polymerization, is in contact with the reaction vessel but does not directly receive the light source used for polymerization. Furthermore, since this bottom surface is typically viscous, it can be used to measure the viscosity index as described above.

[0203] This quantitative analysis of the surface viscosity index allows for the quantification of the viscosity of hydrogel polymers, thus revealing the relationship between the surface viscosity of hydrogel polymers and the extractable contents. More specifically, the surface viscosity index of hydrogel polymers can be below 200g, below 180g, below 150g, or below 50g. Furthermore, since a lower surface viscosity index is generally considered better, the theoretical lower limit is 0g, but it can also be above 5g, above 10g, or above 20g.

[0204] (Step 2)

[0205] The above steps involve drying the prepared hydrogel polymer and then pulverizing it to a normal particle size to prepare the final superabsorbent polymer.

[0206] First, before drying the hydrogel polymer, a step can be performed to chop or cut the hydrogel polymer using a shredder.

[0207] At this time, there are no particular restrictions on the type of pulverizer used; any one of the following groups may be selected: vertical pulverizer, turbine cutter, turbine mill, rotary cutting pulverizer, milling mill, disc mill, fragment crusher, crusher, shredder, and disc cutter. However, this disclosure is not limited thereto.

[0208] The chopping process can be performed using a chopper, more specifically, a meat slicer. The meat slicer can include a chopping module with one or more perforated plates, each plate having multiple fine holes of a specific size through which the hydrogel polymer can pass. Furthermore, the aperture of the fine holes in the perforated plate can be from 3 mm to 16 mm. In other words, it can be seen that by pushing the hydrogel polymer mixed with the additive through the perforated plate, the hydrogel polymer is pulverized while passing through the fine holes of the perforated plate. The hydrogel polymer can then be extruded using an extruder. For example, a single-screw or multi-screw extruder can be used.

[0209] Subsequently, the shredded hydrogel polymer is dried to remove moisture. Specifically, the shredded hydrogel polymer can be dried such that the moisture content of each of the plurality of superabsorbent polymer particles included in the prepared superabsorbent polymer is less than about 10% by weight, specifically from about 0.01% to about 10% by weight.

[0210] Here, the drying temperature in the drying step can be from about 150 to about 250°C. When the drying temperature is below 150°C, the drying time may become too long, and the physical properties of the resulting superabsorbent polymer may decrease. When the drying temperature exceeds 250°C, only the polymer surface is over-dried, which may produce fine powder during subsequent pulverization, and the physical properties of the superabsorbent polymer may ultimately decrease. Therefore, drying can preferably be carried out at a temperature of about 150 to about 200°C, and more preferably at a temperature of about 160 to about 180°C.

[0211] Meanwhile, considering processing efficiency, the drying time can be from about 20 minutes to about 90 minutes, but is not limited to this.

[0212] There are no particular limitations on the drying method used in the drying step, as long as it is commonly used in the drying process of hydrogel polymers. Specifically, the drying step can be carried out by methods such as hot air supply, infrared irradiation, microwave irradiation, and ultraviolet irradiation. After the drying step, the moisture content of the polymer can be from about 5% to about 10% by weight.

[0213] Subsequently, the dried polymer obtained through the drying step is pulverized.

[0214] The base resin used as the polymer powder obtained after the pulverization step can have a particle size of about 150 to about 850 μm. As a pulverizer for pulverizing to such a particle size, a pin mill, hammer mill, spiral mill, roller mill, disc mill, jog mill, etc. can be used, but the present invention is not limited thereto.

[0215] To manage the physical properties of the superabsorbent polymer powder to be commercialized after the pulverization step, the base resin obtained after pulverization is classified according to particle size. Preferably, polymers with particle sizes from about 150 to about 850 μm are classified, and the surface crosslinking reaction step can be performed only on base resins with this particle size. In this case, the particle size can be measured according to EDANA (European Disposable Products and Nonwovens Association) WSP 220.3.

[0216] (Surface crosslinking step)

[0217] Subsequently, if necessary, the process may further include the step of forming a surface crosslinking layer on at least a portion of the surface of the superabsorbent polymer particles in the presence of a surface crosslinking agent. Through the above steps, the crosslinked polymer contained in the superabsorbent polymer particles can be further crosslinked with the surface crosslinking agent, thereby forming a surface crosslinking layer on at least a portion of the surface of the superabsorbent polymer particles.

[0218] As a surface crosslinking agent, any surface crosslinking agent conventionally used in the preparation of superabsorbent polymers can be used without any particular limitation. Examples of surface crosslinking agents may include: at least one polyol selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; at least one carbonate compound selected from the group consisting of ethylene carbonate, propylene carbonate, and glycerol carbonate; epoxy compounds, such as ethylene glycol diglycidyl ether; oxazoline compounds, such as oxazolidinones; polyamine compounds; mono-, di-, or poly-oxazolidinone compounds; cyclic urea compounds, etc.

[0219] Specifically, one or more, or two or more, or three or more of the above-mentioned surface crosslinking agents can be used as surface crosslinking agents. For example, ethylene carbonate-propylene carbonate (ECPC), propylene glycol, and / or glycerol carbonate can be used.

[0220] Based on 100 parts by weight of superabsorbent polymer particles, the amount of surface crosslinking agent used can be from about 0.001 to about 5 parts by weight. For example, based on 100 parts by weight of superabsorbent polymer particles, the amount of surface crosslinking agent used can be 0.005 parts or more, 0.01 parts or more, or 0.05 parts or more, and less than 5 parts by weight, less than 4 parts by weight, or less than 3 parts by weight. By adjusting the content of the surface crosslinking agent within the above range, a superabsorbent polymer with excellent absorption properties can be prepared.

[0221] Furthermore, the step of forming the surface crosslinking layer can be performed by adding an inorganic material to the surface crosslinking agent. That is, the surface of the superabsorbent polymer particles can be further crosslinked in the presence of the surface crosslinking agent and the inorganic material to form a surface crosslinking layer.

[0222] As an inorganic material, at least one inorganic material selected from the group consisting of silica, clay, alumina, silica-alumina composites, titanium dioxide, zinc oxide, and aluminum sulfate can be used. The inorganic material can be used in powder or liquid form, particularly alumina powder, silica-alumina powder, titanium dioxide powder, or nano-silica solution. Furthermore, the amount of inorganic material used can be from about 0.001 to about 1 part by weight based on 100 parts by weight of superabsorbent polymer particles.

[0223] Furthermore, there are no particular limitations on the method of mixing the surface crosslinking agent with the superabsorbent polymer composition. For example, methods such as adding the surface crosslinking agent and the superabsorbent polymer composition to a reactor for mixing, spraying the surface crosslinking agent onto the superabsorbent polymer composition, or mixing the superabsorbent polymer composition and the surface crosslinking agent while continuously supplying them to a continuously operating mixer can be used.

[0224] When mixing the surface crosslinking agent and the superabsorbent polymer composition, water and methanol can be further mixed with them. Adding water and methanol has the advantage that the surface crosslinking agent can be uniformly dispersed in the superabsorbent polymer composition. At this point, the amount of water and methanol to be added can be appropriately controlled to induce uniform dispersion of the surface crosslinking agent, prevent aggregation of the superabsorbent polymer composition, and optimize the surface penetration depth of the surface crosslinking agent.

[0225] The surface crosslinking process can be carried out at a temperature of about 80°C to about 250°C. More specifically, the surface crosslinking process can be carried out at a temperature of about 100°C to about 220°C, or about 120°C to about 200°C, for about 20 minutes to about 2 hours, or about 40 minutes to about 80 minutes. When the above surface crosslinking conditions are met, the surface of the superabsorbent polymer particles is fully crosslinked, thereby increasing pressure absorption.

[0226] There are no particular limitations on the heating method used for the surface crosslinking reaction. A heat medium or a direct heat source can be provided. The available heat medium can be a heating fluid such as steam, hot air, or hot oil, but the invention is not limited to these. Furthermore, the temperature of the heat medium supplied can be appropriately selected considering the type of heat medium, the heating rate, and the target heating temperature. Simultaneously, an electric heater or a gas heater can be used as a direct heat source, but the invention is not limited to these.

[0227] The invention will be described in more detail below with reference to embodiments. However, these embodiments are for illustrative purposes only, and the invention is not limited thereto.

[0228] <Example>

[0229] Preparation of encapsulated foaming agents

[0230] As the encapsulating foaming agent used in the examples, F-36D manufactured by Matsumoto was prepared, which has an isobutane core and a shell of a copolymer of acrylate and acrylonitrile. The foaming start temperature of F-36D at this time (T...) 开始 The maximum foaming temperature is 70℃ to 80℃. max The temperature range is 110℃ to 120℃.

[0231] The diameter of each encapsulated foaming agent was measured using an optical microscope as the average Ferrette diameter. Then, the average diameter of the encapsulated foaming agents was obtained and defined as the average diameter of the encapsulated foaming agents.

[0232] Furthermore, to confirm the expansion characteristics of the encapsulated foaming agent, 0.2 g of the prepared encapsulated foaming agent was applied to a glass petri dish and then placed on a hot plate preheated to 150°C for 10 minutes. The encapsulated foaming agent expanded slowly upon heating, and it was observed using an optical microscope to determine the maximum expansion rate and maximum expansion size of the encapsulated foaming agent in air.

[0233] The diameter of the first 10% by weight of highly expanded particles after heating the encapsulated foaming agent is defined as the maximum expansion size, and the average diameter (D) of the first 10% by weight of highly expanded particles after heating is defined as the maximum expansion size. M The ratio of the average diameter (D0) measured before heating the encapsulated foaming agent to the average diameter (D) is... M / D0) is defined as the maximum expansion rate.

[0234] The prepared encapsulated foaming agent has an average diameter of 13 μm before expansion, a maximum expansion ratio of about 9 times in air, and a maximum expansion size of about 80 to 150 μm.

[0235] Example 1

[0236] (Step 1)

[0237] 100 g (1.388 mol) of acrylic acid, 0.001 g of polyethylene glycol diacrylate (Mn = 508) as an internal crosslinking agent, 0.24 g of ethylene glycol diglycidyl ether, 0.008 g of diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide as a photopolymerization initiator, 0.2 g of sodium persulfate as a thermal polymerization initiator, 123.5 g of 32% caustic soda solution, 0.1 g of encapsulating foaming agent (F-36D), 0.4 g of sodium stearoyl-2-lactic acid (Almax-690, manufactured by Ilshin Wells) represented by the following chemical formulas 1-6, and 0.09 g of sodium dodecyl sulfate (SDS) aqueous solution were mixed with 59.0 g of water at room temperature in a 3 L glass container equipped with a stirrer and thermometer to prepare a monomer composition with a total solids content of 45 wt% (degree of neutralization of acrylic acid: 70 mol%). At this point, dissolve sodium stearoyl-2-lactic acid in water and then add it.

[0238] [Chemical Formulas 1-6]

[0239]

[0240] Subsequently, the monomer composition is fed onto a conveyor belt at a rate of 500 to 2000 mL / min, wherein the conveyor belt, which is 10 cm wide and 2 m long, rotates at a speed of 50 cm / min. Simultaneously, the irradiation intensity is 10 mW / cm². 2 The ultraviolet light was used to carry out a polymerization reaction for 60 seconds to obtain a sheet-like hydrogel polymer with a water content of 50% by weight.

[0241] (Step 2)

[0242] Subsequently, a meat slicer was used to coarsely pulverize the hydrogel polymer obtained through the polymerization reaction, thereby giving the hydrogel polymer a particle size of 300 to 5000 μm. The meat slicer used included a perforated plate with multiple fine slits of 3 mm in diameter.

[0243] The pulverized product was then dried in a convection oven with adjustable airflow direction by circulating hot air at 180°C for 43 minutes to obtain the base resin in powder form. The resulting product was then graded using ASTM standard sieves to prepare superabsorbent polymers with particle sizes ranging from 150 to 850 μm.

[0244] Example 2

[0245] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.15 g of encapsulating foaming agent (F-36D) was used in Example 1.

[0246] Example 3

[0247] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.2 g of encapsulating foaming agent (F-36D) was used in Example 1.

[0248] Example 4

[0249] The superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.1 g of sodium stearoyl-2-lactic acid, represented by chemical formulas 1-6, was used in Example 1.

[0250] Example 5

[0251] The superabsorbent polymer was prepared in the same manner as in Example 1, except that monolaurate maleate, represented by the following chemical formula 1-1, was used instead of sodium stearoyl-2-lactic acid. Here, monolaurate maleate, represented by the following chemical formula 1-1, was prepared by mixing maleic anhydride and 1-dodecanool in a 1:1 molar ratio and then reacting at 60°C for 3 hours.

[0252] [Chemical Formula 1-1]

[0253]

[0254] Example 6

[0255] The superabsorbent polymer was prepared in the same manner as in Example 1, except that monostearic acid ester represented by the following chemical formulas 1-4 was used instead of sodium stearoyl-2-lactic acid. Here, monostearic acid ester represented by the following chemical formulas 1-4 was prepared by mixing maleic anhydride and stearyl alcohol in a 1:1 molar ratio and then reacting at 80°C for 3 hours.

[0256] [Chemical Formulas 1-4]

[0257]

[0258] Example 7

[0259] The superabsorbent polymer was prepared in the same manner as in Example 1, except that monolaurate succinate, represented by the following chemical formulas 1-5, was used instead of sodium stearoyl-2-lactic acid. Here, monolaurate succinate, represented by the following chemical formulas 1-5, was prepared by mixing succinic anhydride and 1-dodecanool in a 1:1 molar ratio and then reacting at 110°C for 3 hours.

[0260] [Chemical Formulas 1-5]

[0261]

[0262] Comparative Example 1

[0263] The superabsorbent polymer was prepared in the same manner as in Example 1, except that sodium stearoyl-2-lactic acid was not used in Example 1.

[0264] Comparative Example 2

[0265] The superabsorbent polymer was prepared in the same manner as in Example 1, except that sodium stearoyl-2-lactic acid was not used in Example 1, and 0.15 g of encapsulating foaming agent (F-36D) was used.

[0266] Comparative Example 3

[0267] The superabsorbent polymer was prepared in the same manner as in Example 1, except that sodium stearoyl-2-lactic acid was not used in Example 1, and 0.2 g of encapsulating foaming agent (F-36D) was used.

[0268] Comparative Example 4

[0269] The superabsorbent polymer was prepared in the same manner as in Example 1, except that sodium stearate was used instead of sodium stearoyl-2-lactic acid. However, it is insoluble in water and immiscible with the monomer composition, therefore the superabsorbent polymer could not be prepared in the same manner as in Example 1.

[0270] Experimental Example 1

[0271] The centrifugal retention capacity (CRC), pressure absorbance (AUP), eddy current time, permeability, and anti-caking efficiency of the superabsorbent polymers prepared in the Examples and Comparative Examples were evaluated as follows, and the results are shown in Table 1 below. Unless otherwise stated, all procedures were performed in a constant temperature and humidity chamber (23±2℃, relative humidity 45±10%). To prevent measurement errors, the average of three measurements was taken as the measurement data. Additionally, the physiological saline or saline solution used to evaluate the following physical properties refers to a 0.9% by weight aqueous solution of sodium chloride (NaCl).

[0272] (1) Centrifugal Retention Capacity (CRC)

[0273] Centrifugal retention capacity of the absorbance of each polymer composition under unloaded conditions was measured according to EDANA (European Disposable Products and Nonwovens Association) WSP 241.3 method.

[0274] Specifically, the polymer compositions prepared in the examples and comparative examples were passed through a sieve of #30-50 to obtain the polymer compositions. After uniformly inserting W0 (g, approximately 0.2g) of the polymer composition into a nonwoven sealing bag and sealing it, the bag was immersed in brine (0.9% by weight) at room temperature. After 30 minutes, the sealing bag was centrifuged at 250G for 3 minutes to dehydrate, and the weight W2 (g) of the sealing bag was measured. Furthermore, after performing the same operation without using resin, the weight W1 (g) of the sealing bag was measured.

[0275] Then, CRC(g / g) is calculated using the obtained weight value according to the following Equation 2.

[0276] [Equation 2]

[0277] CRC(g / g)={[W2(g)-W1(g)] / W0(g)}–1

[0278] (2) Extractable contents (E / C)

[0279] The graded (#40-50) superabsorbent polymer particles from the superabsorbent polymers of the examples and comparative examples were used as samples. The samples were swollen for 1 hour, and then the extractable contents were measured according to the EDANA WSP 270.2 method.

[0280] Specifically, 1.0 g of superabsorbent polymer (ASTM standard mesh #40-50 classification) was placed in 200 g of 0.9 wt% NaCl solution and allowed to swell freely for 1 hour while stirring at 500 rpm. The aqueous solution was then filtered through filter paper. The filtered solution was first titrated to pH 10.0 with 0.1 N caustic soda solution, and then back-tied to pH 2.7 with 0.1 N hydrogen chloride solution. Using the amount required for neutralization, the non-crosslinked polymer material was calculated and measured as the extractable contents.

[0281] (3) BPI (Basic Polymer Index)

[0282] The BPI values ​​of the superabsorbent polymers in the examples and comparative examples were obtained using the CRC and extractable contents obtained above, according to Equation 1 below. In this case, the obtained values ​​were rounded to two decimal places.

[0283] [Equation 1]

[0284]

[0285] (4) Surface viscosity index (quantitative analysis of surface viscosity)

[0286] The surface viscosity index (25°C) of the hydrogel polymer prepared in step 1 of the superabsorbent polymers of the Examples and Comparative Examples was measured according to steps 1) to 4) below. Here, a superabsorbent polymer was measured three times with a separate sample, and the average value was obtained.

[0287] 1) Two samples were prepared, in which the hydrogel polymer with the bottom surface (the surface in contact with the reaction vessel when preparing the hydrogel polymer) was cut into dimensions of 2.5 cm wide × 2.5 cm long × 2 cm thick.

[0288] 2) Fix the two prepared samples onto the upper and lower supports of the texture analyzer, respectively. Here, the two samples are fixed such that the bottom surface of the sample protrudes 1 mm from each support.

[0289] 3) By bringing the upper / lower supports of the two fixed samples close together, making the distance between the supports 1 mm, and then holding them for 5 seconds, the bottom surfaces of the two samples are attached.

[0290] 4) Measure the maximum force when the parts of the two samples attached to the upper support are detached by pulling, and use it as the surface viscosity index.

[0291] (5) Surface viscosity analysis (qualitative analysis of surface viscosity)

[0292] When the hydrogel polymer prepared in step 1 of the superabsorbent polymers in the examples and comparative examples is touched by hand, if it is not sticky, it is marked as "X", if it is slightly sticky, it is marked as "△", and if it is quite sticky, it is marked as "O".

[0293] Table 1

[0294]

[0295] 1) Based on the number of parts by weight of 100 parts by weight of acrylic acid

[0296] 2) Based on the number of parts by weight of acrylic acid per 100 parts by weight

[0297] Referring to Table 1 above, compared with the superabsorbent polymers of the comparative examples, the superabsorbent polymers of the embodiments including carboxylic acids or their salts represented by Chemical Formula 1 significantly reduced the viscosity of the hydrogel polymer during the manufacturing process, and thus exhibited an extractable content of less than 4% by weight and a BPI (Basic Polymer Index) value of 31 or more based on the total weight of the superabsorbent polymer.

Claims

1. A superabsorbent polymer, the polymer comprising: superabsorbent polymer particles including a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acid group that is at least partially neutralized, an internal crosslinking agent, and an encapsulated blowing agent; and a carboxylic acid additive, wherein the carboxylic acid additive is used in an amount of 0.01 to 10 parts by weight with respect to 100 parts by weight of the superabsorbent polymer, wherein the encapsulated blowing agent is used in an amount of 0.05 to 1 parts by weight with respect to 100 parts by weight of the water-soluble ethylenically unsaturated monomer, wherein the carboxylic acid additive is at least one selected from the group consisting of carboxylic acids represented by the following Chemical Formula 1 and salts thereof, and the superabsorbent polymer satisfies the following physical properties: 1) extractable content measured according to EDANA WSP 270.2 after swelling the superabsorbent polymer for 1 hour is 4% by weight or less based on the total weight of the superabsorbent polymer; and 2) a base polymer index BPI calculated according to the following Equation 1 is 31 or more; [Chemical Formula 1] in Chemical Formula 1, A is a C5 to C21 alkyl group, B1 is -OCO-, -COO- or -COOCH(R1)COO-, B2 is -CH2-, -CH2CH2-, -CH(R2)-, -CH=CH- or -C≡C-, wherein R1 and R2 are each independently a C1 to C4 alkyl group, n is an integer of 1 to 3, and C is a carboxyl group, [Equation 1] in Equation 1, CRC is a centrifuge retention capacity measured according to EDANA WSP 241.3, and ln (extractable content) is a natural logarithm value of the extractable content.

2. The superabsorbent polymer of claim 1, wherein in Chemical Formula 1, A is -C6H 13 , -C 11 H 23 , -C 12 H 25 , -C 17 H 35 or -C 18 H 37 .

3. The superabsorbent polymer of claim 1, wherein in Chemical Formula 1, B1 is wherein * is a bonding site with an adjacent atom.

4. The superabsorbent polymer of claim 1, wherein in Chemical Formula 1, B2 is wherein * is a bonding site with an adjacent atom.

5. The superabsorbent polymer of claim 1, wherein the carboxylic acid additive is at least one selected from the group consisting of carboxylic acids represented by Chemical Formula 1, alkali metal salts thereof and alkaline earth metal salts thereof.

6. The superabsorbent polymer of claim 1, wherein the carboxylic acid additive is any one of compounds represented by the following Chemical Formulas 1-1 to 1-7:

7. The superabsorbent polymer of claim 1, further comprising a surface crosslinked layer formed by further crosslinking the crosslinked polymer using a surface crosslinking agent on at least a part of the surface of the superabsorbent polymer particles.

8. A method of preparing the superabsorbent polymer of claim 1, the method comprising: Step 1: forming a hydrogel polymer by crosslinking polymerization of a water-soluble ethylenically unsaturated monomer having an acid group that is at least partially neutralized in the presence of an internal crosslinking agent, an encapsulated blowing agent, a carboxylic acid additive and a polymerization initiator; and Step 2: drying and pulverizing the hydrogel polymer; wherein the carboxylic acid additive is at least one selected from the group consisting of carboxylic acids and salts thereof represented by the following Chemical Formula 1: [Chemical Formula 1] In Chemical Formula 1, A is C5 to C21 alkyl, B1 is -OCO-, -COO- or -COOCH(R1)COO-, B2 is -CH2-, -CH2CH2-, -CH(R2)-, -CH=CH- or -C≡C-, wherein R1 and R2 are each independently C1 to C4 alkyl, n is an integer of 1 to 3, and C is a carboxyl group.

9. The preparation method of superabsorbent polymers according to claim 8, wherein the encapsulated blowing agent has a structure comprising a core and a shell, the core comprising a hydrocarbon, and the shell being formed of a thermoplastic resin surrounding the core.

10. The preparation method of superabsorbent polymers according to claim 9, wherein the hydrocarbon is at least one selected from the group consisting of n-propane, n-butane, isobutane, cyclobutane, n-pentane, isopentane, cyclopentane, n-hexane, isohexane, cyclohexane, n-heptane, isoheptane, cycloheptane, n-octane, isooctane and cyclooctane, and the thermoplastic resin is a polymer formed of at least one monomer selected from the group consisting of (meth)acrylate compounds, (meth)acrylonitrile compounds, aromatic vinyl compounds, vinyl acetate compounds and halogenated vinyl compounds.

11. The preparation method of superabsorbent polymers according to claim 8, wherein the encapsulated blowing agent has an average diameter of 5 to 30 μm before expansion, and a maximum expansion rate in air of 5 to 15 times.

12. The preparation method of superabsorbent polymers according to claim 8, wherein the encapsulated blowing agent is used in an amount of 0.05 to 1 parts by weight, relative to 100 parts by weight of the water-soluble ethylenically unsaturated monomer.

13. The preparation method of superabsorbent polymers according to claim 8, wherein the carboxylic acid additive is used in an amount of 0.01 to 10 parts by weight, relative to 100 parts by weight of the water-soluble ethylenically unsaturated monomer.

14. The preparation method of superabsorbent polymers according to claim 8, wherein, the surface tackiness index of the hydrogel polymer measured at 25°C according to the following 1) to 4) is 200 g or less: 1) two test samples are prepared in which a hydrogel polymer having a bottom surface is cut into a size of 2.5 cm in width x 2.5 cm in length x 2 cm in thickness, wherein the bottom surface is a surface which is in contact with a reaction vessel at the time of preparing the hydrogel polymer; 2) the two test samples prepared are respectively fixed to upper and lower supports of a texture analyzer, wherein the two test samples are fixed so that the bottom surface of the test sample protrudes 1 mm from each support; 3) the upper / lower supports on which the two samples are fixed are brought close to each other so that the distance between the supports becomes 1 mm, and then they are held for 5 seconds, thereby attaching the bottom surface of the two test samples; and 4) when the attached portions of the two test samples are detached by pulling the upper support, the maximum force is measured in g, and this is taken as the surface tackiness index.

15. The preparation method of superabsorbent polymers according to claim 8, It further comprises the step of forming a surface crosslinked layer on at least a portion of the surface of the prepared superabsorbent polymer particles in the presence of a surface crosslinker. It further comprises the step of forming a surface crosslinked layer on at least a portion of the surface of the prepared superabsorbent polymer particles in the presence of a surface crosslinker.

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