Patent Description:
A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture from <NUM> to <NUM> times its own weight. Various manufacturers have denominated it as different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since such superabsorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, sanitary pads, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice or the like.

In most cases, these superabsorbent polymers have been widely used in the field of hygienic materials such as diapers, sanitary pads, etc. For these applications, superabsorbent polymers are required to exhibit high absorbency with respect to water, etc., and not to release the absorbed water even under an external pressure. In addition, superabsorbent polymers are required to well retain the shape even in a state, in which the volume is expanded (swelled) by absorbing water, and to exhibit excellent liquid permeability.

Further, a pressure by a user's weight may be applied to hygiene materials such as diapers, sanitary pads, etc. In particular, when liquid is absorbed by the superabsorbent polymer used in hygiene materials such as diapers, sanitary pads, etc., and then a pressure by a user's weight is applied thereto, a rewetting phenomenon, in which some liquid absorbed into the superabsorbent polymer is re-exuded, and a urine leakage phenomenon may occur.

Accordingly, various attempts have been made to suppress such a rewetting phenomenon. However, concrete methods capable of effectively suppressing the rewetting phenomenon have not yet been suggested.

<CIT> discloses a water-absorbent resin obtained by reversed phase suspension polymerization of a water-soluble ethylenically unsaturated monomer crosslinked with ethylene glycol diglycidyl ether type internal-crosslinking agents.

<CIT> discloses a preparation method of a superabsorbent polymer comprising the steps of preparing a base resin comprising an acrylic-acid-based monomer and internal crosslinking agent pair comprising a first poly(meth)acrylate of a polyol compound and a second polyglycidyl ether of a polyol-based compound in weight ratio of <NUM>:<NUM> to <NUM>:<NUM>.

To solve the above problems of the prior art, there are provided a superabsorbent polymer capable of suppressing rewetting and urine leakage phenomena, and a preparation method thereof.

To achieve the above object, according to one aspect of the present invention, provided is a method of preparing a superabsorbent polymer, the method including the steps of:.

Further, according to another aspect of the present invention, provided is a superabsorbent polymer including:.

According to a superabsorbent polymer and a preparation method thereof of the present invention, it is possible to provide a superabsorbent polymer having excellent basic absorption properties while suppressing a rewetting phenomenon and a urine leakage phenomenon.

Hereinafter, a method of preparing a superabsorbent polymer according to one embodiment of the preset invention will be described in detail.

The method of preparing a superabsorbent polymer according to one embodiment of the preset invention may include the steps of:.

In the specification of the present invention, the "base resin" or "base resin powder" means a polymer in the form of particles or powder obtained by polymerizing a water-soluble ethylene-based unsaturated monomer, followed by drying and pulverizing. It refers to a polymer in a state in which the surface modification or surface crosslinking step described below is not performed.

A water-containing gel polymer obtained by the polymerization reaction of the acrylic acid-based monomer is subjected to drying, pulverizing, size-sorting, surface crosslinking, etc., and is marketed as a powdery superabsorbent polymer product.

In recent years, not only absorption properties of superabsorbent polymers, such as absorbency and liquid permeability, but also how dryness of the surface may be maintained in a situation where diapers are actually used may be an important measure for evaluating diaper characteristics.

It was found that the superabsorbent polymer obtained by the preparation method according to one embodiment of the present invention is superior in absorption properties such as water retention capacity, absorbency under pressure, and liquid permeability, may maintain a dry state even after being swollen with water, and may effectively prevent a rewetting phenomenon and a urine leakage phenomenon in which urine absorbed in the superabsorbent polymer is re-exuded, thereby completing the present invention.

In the method of preparing a superabsorbent polymer of the present invention, a monomer composition, as a raw material of the superabsorbent polymer, including an acrylic acid-based monomer having acidic groups which are at least partially neutralized, an internal crosslinking agent, and a polymerization initiator, is first prepared and polymerized to obtain a water-containing gel polymer, which is then dried, pulverized, and size-sorted to prepare a base resin.

This will be described in more detail below.

The monomer composition which is a raw material of the superabsorbent polymer includes an acrylic acid-based monomer having acidic groups which are at least partially neutralized and a polymerization initiator.

The acrylic acid-based monomer is a compound represented by the following Chemical Formula <NUM>:.

[Chemical Formula <NUM>]     R<NUM>-COOM<NUM>.

Preferably, the acrylic acid-based monomer includes one or more selected from the group consisting of acrylic acid, methacrylic acid, and a monovalent metal salt thereof, a divalent metal salt thereof, an ammonium salt thereof, and an organic amine salt thereof.

Herein, the acrylic acid-based monomers may be those having acidic groups which are at least partially neutralized. Preferably, the monomers may be those partially neutralized with an alkali substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc. In this regard, a degree of neutralization of the acrylic acid-based monomer may be <NUM> mol% to <NUM> mol%, <NUM> mol% to <NUM> mol%, or <NUM> mol% to <NUM> mol%. The degree of neutralization may vary depending on the final physical properties. An excessively high degree of neutralization causes the neutralized monomers to be precipitated, and thus polymerization may not readily occur, whereas an excessively low degree of neutralization not only greatly deteriorates absorbency of the polymer, but also endows the polymer with hard-to-handle properties, such as those of an elastic rubber.

A concentration of the acrylic acid-based monomer may be about <NUM> wt% to about <NUM> wt%, preferably about <NUM> wt% to about <NUM> wt% with respect to the monomer composition including the raw material of the superabsorbent polymer and a solvent, and may be properly controlled in consideration of polymerization time, reaction conditions, etc. However, when the concentration of the monomer is excessively low, the yield of the superabsorbent polymer may become low and economical efficiency may be reduced. On the contrary, when the concentration of the monomer is excessively high, there is a process problem in that part of the monomers is precipitated, or pulverization efficiency is lowered upon pulverization of the polymerized water-containing gel polymer, and the physical properties of the superabsorbent polymer may be deteriorated.

In the method of preparing a superabsorbent polymer of the present invention, a polymerization initiator used upon polymerization is not particularly limited, as long as it is generally used in preparing superabsorbent polymers.

Specifically, the polymerization initiator may be a thermal polymerization initiator or a photo-polymerization initiator by UV radiation depending on the polymerization method. However, even in the case of the photo-polymerization method, a certain amount of heat may be generated by UV irradiation, etc., and a certain amount of heat is generated with exothermic polymerization reaction, and therefore, a thermal polymerization initiator may be further included.

As the photo-polymerization initiator, a compound capable of forming radicals by a light such as UV may be used without limitations in view of constitution.

For example, one or more selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine, and α-aminoketone may be used as the photo-polymerization initiator. Meanwhile, as the specific example of acyl phosphine, commercially available lucirin TPO, namely, <NUM>,<NUM>,<NUM>-trimethyl-benzoyl-trimethyl phosphine oxide may be used. More various photo-polymerization initiators are well disclosed in `<NPL>, however, they are not limited to the above described examples.

The photo-polymerization initiator may be included in an amount of about <NUM>% by weight to about <NUM>% by weight with respect to the monomer composition. If the concentration of the photo-polymerization initiator is too low, the polymerization rate may become low. If the concentration of the photo-polymerization initiator is too high, the molecular weight of the superabsorbent polymer may become low and its physical properties may not be uniform.

Further, one or more selected from the group consisting of persulfate-based initiators, azo-based initiators, hydrogen peroxide, and ascorbic acid may be used as the thermal polymerization initiator. Specific examples of the persulfate-based initiators may include sodium persulfate (Na<NUM>S<NUM>O<NUM>), potassium persulfate (K<NUM>S<NUM>O<NUM>), ammonium persulfate ((NH<NUM>)<NUM>S<NUM>O<NUM>) or the like. Examples of the azo-based initiators may include <NUM>,<NUM>-azobis(<NUM>-amidinopropane)dihydrochloride, <NUM>,<NUM>-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, <NUM>-(carbamoylazo)isobutyronitrile, <NUM>,<NUM>-azobis(<NUM>-[<NUM>-imidazolin-<NUM>-yl]propane)dihydrochloride, <NUM>,<NUM>-azobis-(<NUM>-cyanovaleric acid) or the like. More various thermal polymerization initiators are well-disclosed in `<NPL>, however, they are not limited to the above described examples.

According to one exemplary embodiment of the present invention, the monomer composition includes an internal crosslinking agent as a raw material of the superabsorbent polymer. The internal crosslinking agent is for cross-linking the interior of the polymerized polymer of the acrylic acid-based monomer, and is distinguished from a surface crosslinking agent for cross-linking the surface of the polymer.

In the present invention, epoxy-based crosslinking agents are included as the internal crosslinking agent, and a first epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more to less than <NUM>/eq and a second epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more are used at the same time.

As described, when the internal crosslinking agents having different epoxy equivalent weights are used at the same time, the two kinds of crosslinking agents form different network structures, and thus liquid permeability and rewetting property of the superabsorbent polymer may be greatly improved, as compared with use of a single crosslinking agent.

In other words, as two kinds of crosslinking agents are chemically bound to the main chain of the polymer, each part of the crosslinked polymer networks exhibits different flexibility, and therefore, when the produced superabsorbent polymer absorbs water, the degree of gel shrinkage against an external pressure and flow characteristics of water may vary. Due to this structure, the superabsorbent polymer may exhibit improved rewetting properties and liquid permeability.

In the method of preparing a superabsorbent polymer of the present invention, only the first and second epoxy crosslinking agents may be used as the internal crosslinking agent, or an internal crosslinking agent commonly used may be further used, in addition to the first and second epoxy crosslinking agents. However, to secure the effects of improving the liquid permeability and the rewetting property of the superabsorbent polymer, it is more preferable that only the first and second epoxy crosslinking agents are used.

As the first and second epoxy internal crosslinking agents, a crosslinking agent having two or more epoxy functional groups capable of reacting with the carboxylic acid and carboxylate of the acrylic acid-based monomer may be used.

The first epoxy crosslinking agent may be used for overall internal crosslinking of the polymer which is obtained by polymerizing the acrylic acid-based monomers, and an epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more, or <NUM>/eq or more, and less than <NUM>/eq, or <NUM>/eq or less, and having two or more, preferably, two epoxy functional groups in the molecule may be used. When the epoxy equivalent weight of the first epoxy crosslinking agent is less than <NUM>/eq, there is a problem in that flexibility of the crosslinked polymer network may decrease and absorbency of the superabsorbent polymer may decrease. On the contrary, when the first epoxy crosslinking agent has the high epoxy equivalent weight of <NUM>/eq or more, there is a problem in that a uniform crosslinking structure may not be formed.

Specifically, the first epoxy crosslinking agent may be ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, or a combination thereof.

Preferably, the first epoxy crosslinking agent may be ethylene glycol diglycidyl ether or diethylene glycol diglycidyl ether having an epoxy equivalent weight of <NUM>/eq to <NUM>/eq.

As the second epoxy crosslinking agent, an epoxy crosslinking agent having a higher epoxy equivalent weight than the first epoxy crosslinking agent may be used in order to obtain a double crosslinking effect. Specifically, an epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more, <NUM>/eq or more, or <NUM>/eq or more, and <NUM>/eq or less, or <NUM>/eq or less may be used. When the epoxy equivalent weight of the second epoxy crosslinking agent is too high, the length of the crosslinked chain may be too long, leading to problems in gel strength, and thus it is preferable to satisfy the above range.

As the second epoxy crosslinking agent, a bifunctional epoxy crosslinking agent may be appropriately used. Specifically, one or more of poly(ethylene glycol) diglycidyl ethers having <NUM> to <NUM> ethylene glycol (-CH<NUM>CH<NUM>O-) repeating units may be used. Preferably, the second epoxy crosslinking agent may be poly(ethylene glycol) diglycidyl ether having <NUM> to <NUM> ethylene glycol repeating units. Preferably, the second epoxy crosslinking agent may be poly(ethylene glycol) diglycidyl ether having an epoxy equivalent weight of <NUM>/eq to <NUM>/eq and having <NUM> to <NUM> ethylene glycol repeating units.

The internal crosslinking agent may be included in an amount of <NUM> part by weight to <NUM> part by weight with respect to <NUM> parts by weight of the acrylic acid-based monomer, thereby crosslinking the polymerized polymer.

In this regard, the first epoxy crosslinking agent and the second epoxy crosslinking agent are included in an amount of <NUM> part by weight to <NUM> parts by weight with respect to <NUM> parts by weight of the acrylic acid-based monomer, respectively. Specifically, the first epoxy crosslinking agent may be included in an amount of <NUM> part by weight to <NUM> parts by weight with respect to <NUM> parts by weight of the acrylic acid-based monomer, and the second epoxy crosslinking agent may be included in an amount of <NUM> part by weight to <NUM> parts by weight with respect to <NUM> parts by weight of the acrylic acid-based monomer.

Meanwhile, , to secure an appropriate crosslinking degree of the polymer and flexibility and gel strength of the crosslinked polymer network, a weight ratio of first epoxy crosslinking agent : second epoxy crosslinking agent is <NUM>:<NUM> to <NUM>: <NUM>, or <NUM>:<NUM> to <NUM>: <NUM>.

In the preparation method of the present invention, the monomer composition of the superabsorbent polymer may further include an additive such as a thickener, a plasticizer, a preservation stabilizer, an antioxidant, etc., if necessary.

The raw materials such as the above-described acrylic acid-based monomer having acidic groups which are at least partially neutralized, photo-polymerization initiator, thermal polymerization initiator, internal crosslinking agent, and additive may be prepared in the form of a monomer composition solution in which the raw materials are dissolved in a solvent.

As the solvent to be applicable, any solvent may be used without limitations in view of the constitution as long as it is able to dissolve the above components, and for example, one or more selected from water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, <NUM>,<NUM>-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethylether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, and N,N-dimethylacetamide may be used in combination.

The solvent may be included at a residual quantity except for the above components with respect to the total weight of the monomer composition.

Meanwhile, the method of preparing the water-containing gel polymer by thermal polymerization or photo-polymerization of the monomer composition is not particularly limited, as long as it is a common polymerization method.

Specifically, the polymerization method is largely classified into the thermal polymerization and the photo-polymerization according to a polymerization energy source. The thermal polymerization may be commonly carried out in a reactor like a kneader equipped with agitating spindles whereas the photo-polymerization may be carried out in a reactor equipped with a movable conveyor belt. The above-described polymerization method is an example only, and the present invention is not limited to the above-described polymerization methods.

For example, the water-containing gel polymer may be obtained by performing thermal polymerization while providing hot air to the above-described reactor like a kneader equipped with the agitating spindles or heating the reactor. The water-containing gel polymer may have a size of centimeters or millimeters when it is discharged from an outlet of the reactor, according to the type of agitating spindles equipped in the reactor. Specifically, the size of the obtained water-containing gel polymer may vary depending on a concentration of the monomer composition fed thereto, a feeding speed or the like, and the water-containing gel polymer having a weight average particle size of <NUM> to <NUM> may be generally obtained.

Further, as described above, when the photo-polymerization is carried out in a reactor equipped with a movable conveyor belt, the obtained water-containing gel polymer may be usually a sheet-like water-containing gel polymer having a width of the belt. In this case, the thickness of the polymer sheet may vary depending on the concentration of the monomer composition fed thereto and the feeding speed. Usually, it is preferable to supply the monomer composition so that a sheet-like polymer having a thickness of about <NUM> to about <NUM> may be obtained. When the monomer composition is supplied to such an extent that the thickness of the sheet-like polymer becomes too thin, it is undesirable because the production efficiency is low, and when the thickness of the sheet-like polymer is more than <NUM>, the polymerization reaction may not evenly occur over the entire thickness because of the excessive thickness.

The water-containing gel polymer obtained by the above-mentioned method may have a water content of about <NUM>% by weight to about <NUM>% by weight. Meanwhile, the "water content" as used herein means a weight occupied by water with respect to the total weight of the water-containing gel polymer, which may be a value obtained by subtracting the weight of the dried polymer from the weight of the water-containing gel polymer. Specifically, the water content may be defined as a value calculated by measuring the weight loss due to evaporation of moisture in the polymer during the process of drying by raising the temperature of the polymer through infrared heating. At this time, the water content is measured under the drying conditions determined as follows: the drying temperature is increased from room temperature to about <NUM> and then the temperature is maintained at <NUM>, and the total drying time is set to <NUM> minutes, including <NUM> minutes for the temperature rising step.

Next, the step of drying the obtained water-containing gel polymer is performed.

Herein, to increase the drying efficiency, a coarse pulverizing step may be further performed before the drying step, if necessary.

In this regard, a pulverizer used here is not limited by its configuration, and specifically, it may include any one selected from the group consisting of a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter, but is not limited to the above-described examples.

In this regard, the pulverization step may be carried out so that the particle diameter of the water-containing gel polymer becomes about <NUM> to about <NUM>.

Pulverizing of the water-containing gel polymer into a particle diameter of less than <NUM> is not technically easy due to its high water content, and an agglomeration phenomenon between the pulverized particles may occur. Meanwhile, when the polymer is pulverized into a particle diameter of greater than <NUM>, the effect of increasing the efficiency in the subsequent drying step may be insignificant.

The water-containing gel polymer pulverized as above or the water-containing gel polymer immediately after polymerization without the pulverizing step is subjected to a drying step. In this regard, a drying temperature of the drying step may be about <NUM> to about <NUM>. When the drying temperature is lower than <NUM>, the drying time becomes too long and the physical properties of the final superabsorbent polymer may be deteriorated. When the drying temperature is higher than <NUM>, only the polymer surface is excessively dried, and thus fine powder may be generated during the subsequent pulverization process and the physical properties of the superabsorbent polymer finally formed may be deteriorated. Therefore, the drying may be preferably performed at a temperature of about <NUM> to about <NUM>, and more preferably at a temperature of about <NUM> to about <NUM>.

Meanwhile, the drying step may be carried out for about <NUM> minutes to about <NUM> minutes, in consideration of the process efficiency, but is not limited thereto.

In the drying step, any drying method may be selected and used without limitation in view of constitution, as long as it is commonly used in the process of drying the water-containing gel polymer. Specifically, the drying step may be carried out by a method such as hot air supply, infrared irradiation, microwave irradiation or ultraviolet irradiation. When the drying step as above is finished, the water content of the polymer may be about <NUM>% by weight to about <NUM>% by weight.

Next, the step of pulverizing the dried polymer obtained through the drying step is performed.

The polymer powder obtained through the pulverizing step may have a particle diameter of about <NUM> to about <NUM>. Specific examples of a pulverizer that may be used to achieve the above particle diameter may include a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, etc., but the present invention is not limited to the above-described examples.

In order to manage the physical properties of the superabsorbent polymer powder that is finally commercialized after the pulverization step, a separate process of size-sorting the polymer powders obtained after the pulverization according to the particle size may be carried out. The polymer powders may be size-sorted to have a constant weight ratio according to the particle size.

Next, the dried and pulverized polymer, i.e., the base resin is mixed with a surface crosslinking agent.

In the general method of preparing a superabsorbent polymer, a surface crosslinking solution containing a surface crosslinking agent is mixed with a dried and pulverized polymer, that is, a base resin, and then the mixture is heated to carry out a surface crosslinking reaction of the pulverized polymer.

The surface crosslinking step is a step of inducing a crosslinking reaction on the surface of the pulverized polymer in the presence of a surface crosslinking agent to form a superabsorbent polymer having improved physical properties. Through the surface crosslinking, a surface crosslinked layer (surface modified layer) is formed on the surface of the pulverized polymer particles.

Generally, surface crosslinking agents are applied on the surface of the superabsorbent polymer particles, so that surface crosslinking reactions occur on the surface of the superabsorbent polymer particles, which improves crosslinkability on the surface of the particles without substantially affecting the interior of the particles. Therefore, the surface crosslinked superabsorbent polymer particles have a higher degree of crosslinking near the surface than in the interior.

Meanwhile, the surface crosslinking agent is a compound capable of reacting with functional groups of the polymer. Epoxy compounds, are used.

To further improve absorbency without deteriorating the rewetting property of the superabsorbent polymer, epoxy-based surface crosslinking agents are used.

Examples of the epoxy-based surface crosslinking agent satisfying the above conditions may include ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, triethyleneglycol diglycidyl ether, tetraethyleneglycol diglycidyl ether, glycerin polyglycidyl ether, sorbitol polyglycidyl ether, etc..

The amount of the epoxy-based surface crosslinking agent to be added is <NUM> parts by weight or more, or <NUM> part by weight or more, or <NUM> parts by weight or more, and <NUM> parts by weight or less, or <NUM> part by weight or less, or <NUM> parts by weight or less with respect to <NUM> parts by weight of the base resin.

When the amount of the epoxy-based surface crosslinking agent is too small, crosslinking density of the surface crosslinked layer is too low, and absorption properties such as absorbency under pressure, liquid permeability becomes low, and when the amount thereof is too large, the rewetting property may deteriorate due to excessive surface crosslinking reaction.

When the epoxy-based surface crosslinking agent is added, it is additionally mixed with water, and then added in the form of a surface crosslinking solution. When water is added, it is advantageous in that the surface crosslinking agent may be evenly dispersed in the polymer. At this time, the amount of water to be added may be preferably about <NUM> part by weight to about <NUM> parts by weight with respect to <NUM> parts by weight of the polymer in order to induce uniform dispersion of the surface crosslinking agent, to prevent agglomeration of the polymer powder, and at the same time, to optimize the surface penetration depth of the surface crosslinking agent.

Meanwhile, in addition to the above-described surface crosslinking agent, a multivalent metal salt, for example, an aluminum salt, more specifically, one or more selected from the group consisting of sulfates, potassium salts, ammonium salts, sodium salts, and hydrochloride salts of aluminum may be further included.

As the multivalent metal salt is additionally used, the liquid permeability of the superabsorbent polymer prepared by the method of one embodiment may be further improved. The multivalent metal salt may be added, together with the surface crosslinking agent, to the surface crosslinking solution, and may be used in an amount of <NUM> part by weight to <NUM> parts by weight with respect to <NUM> parts by weight of the base resin.

Meanwhile, before raising the temperature in order to perform surface crosslinking reaction by mixing the base resin with the surface crosslinking agent, the base resin is optionally mixed with a hydrophobic material, thereby further enhancing the rewetting property. The surface crosslinking efficiency may also be improved, thereby further enhancing absorption rate and liquid permeability, as compared with a resin of using no hydrophobic material.

As the hydrophobic material, a material having HLB meeting the lower limit of <NUM> or more, or <NUM> or more, or <NUM> or more, and the upper limit of <NUM> or less, or <NUM> or less, or <NUM> or less may be used. Further, since the hydrophobic material must be melted during the surface crosslinking reaction and must be placed in the surface-modified layer of the base resin, a material having a melting point lower than the surface crosslinking reaction temperature may be used.

Examples of the applicable hydrophobic material may include glyceryl stearate, glycol stearate, magnesium stearate, glyceryl laurate, sorbitan stearate, sorbitan trioleate, PEG-<NUM> dilaurate, etc. Preferably, glyceryl stearate or glyceryl laurate may be used, but the present invention is not limited thereto.

The hydrophobic material is distributed in the surface-modified layer of the surface of the base resin to prevent agglomeration or aggregation between the swelled resin particles due to the increased pressure when the superabsorbent polymer swells by absorbing a liquid, and the hydrophobic material provides the surface with hydrophobicity, thereby further facilitating liquid permeation and diffusion. Therefore, the hydrophobic material may contribute to improving the rewetting property of the superabsorbent polymer.

The hydrophobic material may be mixed in an amount of about <NUM> parts by weight or more, or about <NUM> parts by weight or more, or about <NUM> parts by weight or more, and about <NUM> parts by weight or less, or about <NUM> parts by weight or less, or about <NUM> parts by weight or less with respect to <NUM> parts by weight of the base resin. If the amount of the hydrophobic material is as too small as less than <NUM> parts by weight, it may be insufficient to improve rewetting property. If the amount of the hydrophobic material is as too large as more than <NUM> parts by weight, the base resin and the hydrophobic material may be separated from each other, and thus there is a problem in that the effect of improving the rewetting property may not be obtained or it may act as an impurity. In this point of view, the hydrophobic material may be preferably used in the above range of part by weight.

A method of mixing the hydrophobic material may be appropriately adopted without particular limitation, as long as it is able to evenly mix the hydrophobic material with the base resin.

For example, the hydrophobic material may be dry-mixed before mixing the base resin with the surface crosslinking solution containing the epoxy-based surface crosslinking agents, or the hydrophobic material may be dispersed, together with the surface crosslinking agents, in the surface crosslinking solution, and then mixed with the base resin. Alternatively, separately from the surface crosslinking solution, the hydrophobic material may be heated to a melting point or higher, and mixed in a solution state.

Next, the step of surface-modifying the base resin may be performed by raising the temperature of the mixture of the base resin and the epoxy-based surface crosslinking agents by heating.

The surface modification step may be performed by heating at a temperature of about <NUM> to about <NUM>, preferably about <NUM> to about <NUM> for about <NUM> minutes to about <NUM> minutes, preferably about <NUM> minutes to about <NUM> minutes. If the crosslinking reaction temperature is lower than <NUM> or the reaction time is too short, the surface crosslinking reaction does not properly occur and thus permeability may be reduced, and if the crosslinking reaction temperature is higher than <NUM> or the reaction time is too long, there is a problem in that water retention capacity may be reduced.

A means for raising the temperature for surface modification reaction is not particularly limited. Heating may be performed by providing a heating medium or by directly providing a heat source. In this regard, the kind of the applicable heating medium may include a hot fluid such as steam, hot air, hot oil or the like, but the present invention is not limited thereto. The temperature of the heating medium to be provided may be properly controlled in consideration of the means of the heating medium, the heating rate, and the target temperature. Meanwhile, as the heat source to be directly provided, an electric heater or a gas heater may be used, but the present invention is not limited to these examples.

By the above surface modification step, a surface crosslinked structure formed by reacting the functional groups of epoxy-based surface crosslinking agents with the functional groups of the base resin is formed on the surface of the base resin. Inside this surface-crosslinked structure, a surface-modified layer in which the above-described hydrophobic material is uniformly distributed may be formed.

Therefore, as the base resin has the double crosslinking structure due to the two kinds of epoxy-based internal crosslinking agents having the different epoxy equivalent weights, the superabsorbent polymer prepared by the preparation method of the present invention may have improved rewetting property and initial absorption rate without deterioration in the physical properties such as water retention capacity and absorbency under pressure.

According to another embodiment of the present invention, provided is a superabsorbent polymer including: a base resin including a crosslinked polymer in which an acrylic acid-based monomer having acidic groups which are at least partially neutralized is crosslinked in the presence of an internal crosslinking agent including at the same time a first epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more to less than <NUM>/eq, and a second epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more, ; wherein the first epoxy crosslinking agent and the second epoxy crosslinking agent are included in an amount of <NUM> part by weight to <NUM> parts by weight, respectively, with respect to <NUM> parts by weight of the acrylic acid-based monomer and a surface crosslinked layer which is formed on the particle surface of the base resin and is prepared by further crosslinking the crosslinked polymer via an epoxy-based surface crosslinking agent, wherein the epoxy-based surface crosslinking agent is added in an amount of <NUM> parts by weight or more to <NUM> parts by weight or less with respect to <NUM> parts by weight of the base resin, and wherein a weight ratio of first epoxy crosslinking agent : second epoxy crosslinking agent is <NUM>:<NUM> to <NUM>:<NUM>.

Detailed descriptions of a specific method of preparing the superabsorbent polymer and physical properties thereof are the same as those described in the method of preparing a superabsorbent polymer.

The superabsorbent polymer may have centrifuge retention capacity (CRC) in the range of about <NUM>/g or more, about <NUM>/g or more, or about <NUM>/g or more, and about <NUM>/g or less, about <NUM>/g or less, or about <NUM>/g or less, as measured in accordance with the EDANA method WSP <NUM>.

Further, the superabsorbent polymer may have liquid permeability (unit; sec) of about <NUM> seconds or less, or about <NUM> seconds or less, as measured according to the following Equation <NUM>. As the value of the liquid permeability is smaller, it means more excellent liquid permeability. Therefore, the theoretical lower limit is <NUM> second, but it may be, for example, about <NUM> seconds or more, or about <NUM> seconds or more, or about <NUM> seconds or more. <MAT> in Equation <NUM>, T1 represents a time taken for a liquid level to decrease from <NUM> to <NUM>, when <NUM>±<NUM> of a size-sorted superabsorbent polymer sample (<NUM> ~ <NUM>) is put in a chromatography column, brine is applied thereto at a volume of <NUM>, and then left for <NUM> minutes, and B represents a time taken for a liquid level in the brine-filled chromatography column to decrease from <NUM> to <NUM>.

Further, the superabsorbent polymer may exhibit more improved rewetting property while exhibiting excellent absorption properties.

More specifically, the superabsorbent polymer may have the rewetting property (long-term tap water rewetting under pressure) of <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less, the rewetting property defined by the weight of water that soaks out from the superabsorbent polymer to a filter paper, when <NUM> of the superabsorbent polymer is immersed in <NUM> of tap water and allowed to swell for <NUM> hours, and then the swollen superabsorbent polymer is left on the filter paper under a pressure of <NUM> psi for <NUM> minute. As the weight of the water is smaller, it means more excellent rewetting property. Therefore, the theoretical lower limit is <NUM>, but it may be, for example, <NUM> or more, <NUM> or more, or <NUM> or more.

The tap water used in the evaluation of the rewetting property has conductivity of <NUM>/cm to <NUM>/cm. Since the conductivity of tap water greatly influences the properties to be measured, it is necessary to measure the physical properties such as rewetting property by using tap water having conductivity equivalent thereto.

As described above, the superabsorbent polymer of the present invention may have excellent absorbency and may suppress rewetting and urine leakage phenomena even at the time of absorbing a large amount of urine.

The present invention will be described in more detail with reference to the following Examples. However, the following Examples are only for illustrating the present invention, and the description of the present invention is not limited by the following Examples.

<NUM> of acrylic acid, <NUM> (<NUM> parts by weight with respect to <NUM> parts by weight of acrylic acid) of ethylene glycol diglycidyl ether (EGDGE, epoxy equivalent weight of <NUM>/eq), <NUM> (<NUM> part by weight with respect to <NUM> parts by weight of acrylic acid) of poly(ethylene glycol) diglycidyl ether having <NUM> ethylene glycol repeating units (PEGDGE, epoxy equivalent weight of <NUM>/eq), and <NUM> of diphenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)-phosphine oxide were added to a <NUM>-L glass container equipped with a stirrer, a nitrogen feeder, and a thermometer, and dissolved. Then, <NUM> of <NUM>% sodium hydroxide solution was added thereto and nitrogen was continuously injected to prepare a water-soluble unsaturated monomer aqueous solution. The water-soluble unsaturated monomer aqueous solution was cooled to <NUM>.

<NUM> of this aqueous solution was fed to a stainless steel container having a width of <NUM>, a length of <NUM>, and a height of <NUM>, and UV polymerization was performed by UV radiation (exposure dose: <NUM> mV/cm<NUM>) for <NUM> seconds to obtain a water-containing gel polymer. The water-containing gel polymer thus obtained was pulverized to a size of <NUM>*<NUM>, and then the obtained gel-type resin was spread as thick as about <NUM> on a stainless wire gauze having a hole size of <NUM> and dried in a hot air oven at <NUM>° C for <NUM> minutes. The dry polymer thus obtained was pulverized with a pulverizer, and then size-sorted through an ASTM standard sieve to obtain a base resin having a particle size of <NUM> to <NUM>.

A surface crosslinking solution containing <NUM> parts by weight of water, <NUM> parts by weight of ethylene glycol diglycidyl ether, <NUM> parts by weight of aluminum sulfate, and <NUM> parts by weight of glyceryl stearate (HLB <NUM>) was sprayed onto <NUM> parts by weight of the base resin, and mixed with each other. This mixture was put in a container equipped with a stirrer and a double jacket, and a surface crosslinking reaction was performed at <NUM> for <NUM> minutes. Thereafter, the surface-treated powder was size-sorted through an ASTM standard sieve to obtain a superabsorbent polymer powder having a particle size of <NUM> to <NUM>.

A superabsorbent polymer powder was obtained in the same manner as in Example <NUM>, except that <NUM> parts by weight of ethylene glycol diglycidyl ether and <NUM> parts by weight of poly(ethylene glycol) diglycidyl ether having <NUM> ethylene glycol repeating units were used with respect to <NUM> parts by weight of acrylic acid in step (<NUM>).

A superabsorbent polymer powder was obtained in the same manner as in Example <NUM>, except that <NUM> parts by weight of ethylene glycol diglycidyl ether and <NUM> part by weight of poly(ethylene glycol) diglycidyl ether having <NUM> ethylene glycol repeating units were used with respect to <NUM> parts by weight of acrylic acid in step (<NUM>).

A superabsorbent polymer powder was obtained in the same manner as in Example <NUM>, except that poly(ethylene glycol) diglycidyl ether (epoxy equivalent weight of <NUM>/eq) having <NUM> ethylene glycol repeating units was used instead of poly(ethylene glycol) diglycidyl ether having <NUM> ethylene glycol repeating units in step (<NUM>).

A superabsorbent polymer powder was obtained in the same manner as in Example <NUM>, except that diethylene glycol diglycidyl ether (DGDGE, epoxy equivalent weight of <NUM>/eq) was used instead of ethylene glycol diglycidyl ether in step (<NUM>).

A superabsorbent polymer powder was obtained in the same manner as in Example <NUM>, except that, as the internal crosslinking agent, poly(ethylene glycol) diglycidyl ether having <NUM> ethylene glycol repeating units was not included, and only ethylene glycol diglycidyl ether was used in an amount of <NUM> parts by weight with respect to <NUM> parts by weight of acrylic acid in step (<NUM>).

A superabsorbent polymer powder was obtained in the same manner as in Example <NUM>, except that, as the internal crosslinking agent, poly(ethylene glycol) diglycidyl ether having <NUM> ethylene glycol repeating units was not included, and only diethylene glycol diglycidyl ether (DGDGE, epoxy equivalent weight of <NUM>/eq) was used in an amount of <NUM> parts by weight with respect to <NUM> parts by weight of acrylic acid in step (<NUM>).

A superabsorbent polymer powder was obtained in the same manner as in Example <NUM>, except that, as the internal crosslinking agent, ethylene glycol diglycidyl ether was not included, and only poly(ethylene glycol) diglycidyl ether having <NUM> ethylene glycol repeating units was used in an amount of <NUM> parts by weight with respect to <NUM> parts by weight of acrylic acid in step (<NUM>).

Physical properties were evaluated for the superabsorbent polymers prepared in Examples and Comparative Examples by the following methods.

Unless otherwise indicated, the following physical properties were all evaluated at constant temperature and humidity (<NUM> ± <NUM>, relative humidity of <NUM> ± <NUM>%), and physiological saline or brine means a <NUM> wt% sodium chloride (NaCl) aqueous solution.

Further, tap water used in the following evaluation of the rewetting property was tap water having a conductivity of <NUM>/cm to <NUM>/cm, as measured using Orion Star A222 (company: Thermo Scientific).

The centrifuge retention capacity by absorption capacity under no load was measured for each polymer in accordance with EDANA WSP <NUM>.

In detail, after uniformly introducing W<NUM>(g) (about <NUM>) of the superabsorbent polymer in a nonwoven fabric-made bag and sealing the same, it was immersed in physiological saline (<NUM> wt%) at room temperature. After <NUM> minutes, the bag was dehydrated by using a centrifuge at <NUM> for <NUM> minutes, and then the weight W<NUM>(g) of the bag was measured. Further, after carrying out the same operation without using the polymer, the weight W<NUM>(g) of the bag was measured. CRC (g/g) was calculated using each obtained weight according to the following Equation: <MAT>.

Lines were marked on the liquid levels of <NUM> and <NUM> in a chromatography column (F20 mm) with a piston. Thereafter, in order to prevent bubbles between a glass filter and a cock at the bottom of the chromatography column, water was injected upward and filled up to about <NUM>, and the column was washed <NUM>-<NUM> times with brine, and <NUM>% brine was filled up to <NUM> or more. The piston was placed in the chromatography column, and the lower valve was opened to record the time (B) at which the liquid level decreased from the marked line of <NUM> to the marked line of <NUM>.

<NUM> of brine was left in the chromatography column, <NUM> ± <NUM> of the size-sorted superabsorbent polymer sample (<NUM> to <NUM>) was added thereto, and brine was added up to <NUM>, and left for <NUM> minutes. Thereafter, a piston with a weight (<NUM> psi = <NUM>) was placed in the chromatography column, and left for <NUM> minute. Then, the lower valve of the chromatography column was opened to record the time (T1) at which the liquid level decreased from the marked line of <NUM> to the marked line of <NUM>. The time (unit: second) of T <NUM> - B was calculated.

The values of the physical properties of Examples and Comparative Examples are described in Table <NUM> below.

Referring to Table <NUM>, it was confirmed that Examples <NUM> to <NUM> exhibited excellent liquid permeability and rewetting property while having excellent water retention capacity. In contrast, Comparative Examples <NUM>, <NUM>, and <NUM>, in which only one of the first and second epoxy-based crosslinking agents was used as the internal crosslinking agent, exhibited remarkably poor liquid permeability and rewetting property, as compared with Examples <NUM> to <NUM>, in which the internal crosslinking agents were used in the equal amount. In addition, Comparative Example <NUM>, in which one kind of the internal crosslinking agents was used, but the content of the crosslinking agent was increased, showed slightly improved liquid permeability, but showed poor water retention capacity and rewetting property.

Claim 1:
A method of preparing a superabsorbent polymer, the method comprising the steps of:
preparing a base resin in which an acrylic acid-based monomer having acidic groups which are at least partially neutralized and an internal crosslinking agent are crosslinked; and
heating the base resin in the presence of an epoxy-based surface crosslinking agent to carry out surface modification of the base resin, wherein the epoxy-based surface crosslinking agent is added in an amount of <NUM> parts by weight or more to <NUM> parts by weight or less with respect to <NUM> parts by weight of the base resin,
wherein the internal crosslinking agent includes at the same time a first epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more to less than <NUM>/eq, and a second epoxy crosslinking agent having an epoxy equivalent weight of <NUM>/eq or more, wherein the first epoxy crosslinking agent and the second epoxy crosslinking agent are included in an amount of <NUM> part by weight to <NUM> parts by weight, respectively, with respect to <NUM> parts by weight of the acrylic acid-based monomer, and
wherein a weight ratio of first epoxy crosslinking agent : second epoxy crosslinking agent is <NUM>: <NUM> to <NUM>:<NUM>.