Patent Description:
Super absorbent polymer (SAP) is a synthetic polymer material capable of absorbing moisture from about <NUM> to about <NUM>,<NUM> times its own weight, and each manufacturer has denominated it as different names such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material) or the like. Such super absorbent polymers started to be practically applied in sanitary products, and now they are widely used for preparation of hygiene products such as paper diapers for children or sanitary napkins, 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 super absorbent polymers have been widely used in the field of hygienic materials such as diapers or sanitary napkins. In such hygienic materials, the super absorbent polymer is generally contained in a state of being spread in the pulp. In recent years, however, continuous efforts have been made to provide hygienic materials such as diapers having a thinner thickness. As a part of such efforts, the development of so-called pulpless diapers and the like in which the content of pulp is reduced or pulp is not used at all is being actively advanced.

As described above, in the case of hygienic materials in which the pulp content is reduced or the pulp is not used, a super absorbent polymer is contained at a relatively high ratio and these super absorbent polymer particles are inevitably contained in multiple layers in the hygienic materials. In order for the whole super absorbent polymer particles contained in the multiple layers to absorb liquid such as urine more efficiently, not only the super absorbent polymer needs to basically exhibit high absorption performance and absorption rate, but also it needs to exhibit more improved liquid permeability. That is, the super absorbent polymer should exhibit more improved liquid permeability, so the super absorbent polymer particles of the surface layer which first comes in contact with the liquid such as urine are absorbed and allow to absorb and pass the remaining liquid quickly. It becomes possible to absorb such remaining liquid effectively and quickly by the super absorbent polymer particles of the subsequent layer.

<CIT>, <CIT> and <CIT> disclose a method for producing a superabsorbent polymer, comprising the steps of crosslinking a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent, a foaming agent and a surfactant to form a hydrogel polymer containing a first crosslinked polymer, pulverizing the hydrogel polymer, drying and pulverizing the hydrogel polymer to form a base polymer powder, and heat treating and surface-crosslinking the base polymer powder in the presence of a surface crosslinking agent to form a superabsorbent polymer particle. These documents are silent regarding coarsely pulverizing the hydrogel polymer to prepare a hydrogel polymer having an average particle diameter of <NUM> to <NUM>.

<CIT> and <CIT> also disclose a method for producing a superabsorbent polymer, but are silent regarding the specific surfactant now used in the method of the present invention.

<CIT> discloses a method for producing a superabsorbent polymer using polysiloxane as surfactant.

<CIT> discloses a process for producing a superabsorbent polymer which may optionally contain a surfactant which is different from the surfactant used according to the present invention.

<CIT> discloses a method for producing a superabsorbent polymer utilizing, amongst a foaming agent, a mixture of a lipophilic surfactant and a polyethoxylated hydrophilic surfactant.

<CIT> discloses a method of preparing a superabsorbent polymer, wherein foaming agent and surfactant are utilized. The surfactant is preferably a polysiloxane with polyether side chains.

<CIT> discloses a method of preparing a superabsorbent polymer, but is silent regarding a simultaneous use of foaming agent and, especially, surfactant.

<CIT> dislcoses a method for producing a poly(meth)acrylic acid (salt)-based particulate water-absorbing agent. Surfactants may be added during the method, but no disclosure is given with regard to the specific surfactant as used according to the present invention.

<CIT> discloses a process for producing permeable water-absorbing polymer particles. Surfactants are disclosed, but the surfactant as used according to the present application is not mentioned.

Accordingly, recently, various attempts have been made to develop more improved super absorbent polymer, but these technical requirements are not sufficiently satisfied yet.

It is one object of the present invention to provide a method for producing a super absorbent polymer having excellent absorption performance.

In order to achieve the above objects, the present invention provides a method for producing a super absorbent polymer, comprising the steps of:
crosslinking a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent, a foaming agent and a surfactant to form a hydrogel polymer containing a first crosslinked polymer wherein the surfactant is a compound represented by the following Chemical Formula <NUM>:.

[Chemical Formula <NUM>]     R-SO<NUM>Na.

In order to improve the absorption performance of the super absorbent polymer, a method of increasing the surface area of the super absorbent polymer has been studied. In order to widen the surface area of the super absorbent polymer, a method of using a foaming agent during polymerization of a water-soluble ethylenically unsaturated monomer is known, but use of an excessive foaming agent may cause problems in distribution and storage because the gel strength of the super absorbent polymer is lowered or the density is lowered. As another method, there is a method of reducing the particle size through coarse pulverization of the hydrogel polymer, but when an excessive shearing force is applied during the coarse pulverization, there is a problem that the physical properties of the super absorbent polymer are deteriorated or the process of coarse pulverization is difficult.

Thus, according to the present invention, by using a surfactant together with a foaming agent during the polymerization of the water-soluble ethylenically unsaturated monomer as described later, it is possible to prevent the gel strength from being lowered and the density from being lowered by uniform forming. In addition, due to the surfactant during coarse pulverization of the hydrogel polymer, it is pulverized to be smaller than the particle size usually produced, thereby improving the absorption performance of the super absorbent polymer as a final product.

The water-soluble ethylenically unsaturated monomer constituting the first cross-linked polymer may be any monomer commonly used in the production of a super absorbent polymer. As a non-limiting example, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following Chemical Formula <NUM>:.

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

Preferably, the above-described monomer may be at least one selected from the group consisting of acrylic acid, methacrylic acid, and a monovalent metal salt, a divalent metal salt, an ammonium salt, and an organic amine salt thereof. When acrylic acid or a salt thereof is used as the water-soluble ethylenically unsaturated monomer, it is advantageous in that a super absorbent polymer having improved absorption property can be obtained. In addition, as the monomer, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, <NUM>-acryloylethanesulfonic acid, <NUM>-methacryloylethanesulfonic acid, <NUM>-(meth)acryloylpropane sulfonic acid, or <NUM>-(meth)acrylamido-<NUM>-methylpropane sulfonic acid, (meth)acrylamide, N-substituted (meth)acrylate, <NUM>-hydroxyethyl(meth)acrylate, <NUM>-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol(meth)acrylate, polyethylene glycol (meth)acrylate, (N,N)-dimethylaminoethyl(meth)acrylate, (N,N)-dimethylaminopropyl(meth)acrylamide, and the like may be used.

Here, the water-soluble ethylenically unsaturated monomers have an acidic group, wherein at least a part of the acidic group is neutralized. Preferably, the monomers may be those partially neutralized with an alkali substance such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, or the like.

In this case, a degree of neutralization of the monomer may be <NUM> to <NUM> mol%, or <NUM> to <NUM> mol%, or <NUM> to <NUM> mol%. The range of 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 the absorbency of the polymer, but also endows the polymer with hard-to-handle properties, such as those of an elastic rubber.

The second cross-linked polymer is obtained by additionally crosslinking the surface of the base resin powder via a surface crosslinking agent. The surface crosslinking agent and the surface crosslinking method will be described later.

Meanwhile, the super absorbent polymer has a <NUM>-min gel-AUL at <NUM> psi of <NUM>/g or more. The <NUM>-min gel-AUL at <NUM> psi refers to the amount of a physiological saline solution (<NUM> wt% NaCl) which is absorbed by a super absorbent polymer under a load of <NUM> psi for <NUM> minutes after <NUM> of a super absorbent polymer is primarily swollen in <NUM> of physiological saline solution under no load. This means the ability of a super absorbent polymer to absorb a large amount of water continuously and rapidly under a load of <NUM> psi after the super absorbent polymer is primarily swollen under no load. The concrete measurement method thereof will be further specified in the following embodiments.

Preferably, the super absorbent polymer has a <NUM>-min gel-AUL at <NUM> psi of <NUM>/g or more, <NUM>/g or more, <NUM>/g or more, or <NUM>/g or more. In addition, the higher the value of the <NUM>-min gel-AUL, it is more excellent. Thus, there is no practical upper limit, but as an example, it is <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, or <NUM>/g or less.

Further, the super absorbent polymer has a <NUM>-min gel-AUL at <NUM> psi of <NUM>/g or more. The <NUM>-min gel-AUL at <NUM> psi is the same as the <NUM>-min gel-AUL at <NUM> psi previously described, but it is measured under load of <NUM> psi instead of <NUM> psi. In addition, the higher the value of the <NUM>-min gel-AUL, it is more excellent. Thus, there is no practical upper limit, but as an example, it is <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, or <NUM>/g or less.

Further, the super absorbent polymer has an AUL of <NUM>/g or more. The AUL means the amount of a saline solution absorbed under a load of <NUM> psi for <NUM> hour, which means the total amount of water which the superabsorbent resin can absorb. The concrete measurement method thereof will be further specified in the following embodiments.

Preferably, the AUL is <NUM>/g or more, <NUM>/g or more, <NUM>/g or more, <NUM>/g or more, or <NUM>/g or more. In addition, the higher the value of the AUL, it is more excellent. Thus, there is no practical upper limit, but as an example, it is <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, <NUM>/g or less, or <NUM>/g or less.

Further, the super absorbent polymer has a gel bed permeability (GBP) of <NUM> darcy or more. The GBP means a fluidity of water absorbed by the super absorbent polymer, which means the ability to rapidly transfer water absorbed by the super absorbent polymer to another super absorbent polymer. The concrete measurement method thereof will be further specified in the following embodiments.

Preferably, the GBP is <NUM> darcy or more, <NUM> darcy or more, or <NUM> darcy or more. Further, the upper limit of the GBP is <NUM> darcy or less, <NUM> darcy or less, or <NUM> darcy or less.

Further, the super absorbent polymer has a vortex time of <NUM> seconds or less as measured according to the measurement method of Vortex. The vortex time (absorption rate) means a time during which the vortex of the liquid disappears due to rapid absorption when the super absorbent polymer is added to the physiological saline solution and stirred. This can define a rapid water absorption capacity of the super absorbent polymer. The concrete measurement method thereof will be more specified in the following embodiments. Further, the lower limit of the vortex time is, for example, <NUM> seconds or more, <NUM> seconds or more, or <NUM> seconds or more.

Further, the super absorbent polymer has a centrifuge retention capacity (CRC) for a physiological saline solution (<NUM> wt% sodium chloride aqueous solution) for <NUM> minutes of <NUM>/g or more. The centrifuge retention capacity means the ability to retain water absorbed by the super absorbent polymer as it is. The concrete measurement method thereof will be further specified in the following embodiments.

Preferably, the centrifuge retention capacity is <NUM>/g or more, <NUM>/g or more, or <NUM>/g or more, and <NUM>/g or less, <NUM>/g or less, or <NUM>/g or less.

Further, preferably, the super absorbent polymer invention has an average particle diameter of <NUM> to <NUM>. Also preferably, in the super absorbent polymer, a super absorbent polymer having a particle diameter of <NUM> to <NUM> is contained in an amount of <NUM> to <NUM>% by weight. Further, preferably, in the super absorbent polymer, a super absorbent polymer having a particle diameter of <NUM> or less is contained in an amount of <NUM>% by weight or more.

The present invention provides a method for producing a super absorbent polymer comprising the following steps:
crosslinking a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent, a foaming agent and a surfactant to form a hydrogel polymer containing a first crosslinked polymer (step <NUM>), wherein the surfactant is a compound represented by the following Chemical Formula <NUM>:.

Hereinafter, the above preparation method will be described in detail for each step.

Step <NUM> is a step of forming a hydrogel polymer which is a step of crosslinking an internal crosslinking agent, a foaming agent, a surfactant, and a monomer composition comprising a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups.

In this case, the water-soluble ethylenically unsaturated monomer is as described above. Further, the concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition may be appropriately adjusted in consideration of the polymerization time, the reaction conditions and the like, and it may be preferably <NUM> to <NUM>% by weight, or <NUM> to <NUM>% by weight. These concentration ranges may be advantageous for adjusting the pulverization efficiency during pulverization of the polymer described below, without needing to remove unreacted monomers after polymerization by using the phenomenon of gel effect occurring in the polymerization reaction of the highly concentrated aqueous solution. However, when the concentration of the monomer is excessively low, the yield of the super absorbent polymer can be lowered. Conversely, when the concentration of the monomer is excessively high, it may arise problems in the processes, for example, a part of the monomer may be precipitated, or the pulverization efficiency may be lowered during pulverization of the polymerized hydrogel polymer, etc., and the physical properties of the super absorbent polymer may be deteriorated.

Further, as the internal crosslinking agent, any compound can be used without particular limitation as long as it enables introduction of a crosslink bond upon polymerization of the water-soluble ethylenically unsaturated monomer. Non-limiting examples of the internal crosslinking agent may include multifunctional crosslinking agents, such as N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol(meth)acrylate, butanediol di(meth)acrylate, butylene glycol 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 pentacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerin, or ethylene carbonate, which may be used alone or in combination of two or more thereof, but are not limited thereto.

Such internal crosslinking agent may be added at a concentration of about <NUM> to <NUM>% by weight, based on the monomer composition. That is, if the concentration of the internal crosslinking agent is too low, the absorption rate of the polymer is lowered and the gel strength may become weak, which is undesirable. Conversely, if the concentration of the internal crosslinking agent is too high, the absorption capacity of the polymer is lowered and thereby is not preferred for an absorbent.

Further, in step <NUM>, a polymerization initiator generally used in the production of a super absorbent polymer can be included. As a non-limiting example, as the polymerization initiator, a thermal polymerization initiator, a photo-polymerization initiator or the like may be used depending on the polymerization method. In particular, the thermal polymerization initiator can be used. However, even in the case of the photo-polymerization method, a certain amount of heat is generated by ultraviolet irradiation or the like, and a certain amount of heat is generated in accordance with the progress of the polymerization reaction, which is an exothermic reaction, and thus, a thermal polymerization initiator may further be included.

As the thermal polymerization initiator, one or more compounds selected from the group consisting of a persulfate-based initiator, an azo-based initiator, hydrogen peroxide, and ascorbic acid may be used. Specific examples of the persulfate-based initiator 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>), and the like. In addition, examples of the azo-based initiator may include <NUM>,<NUM>-azobis(<NUM>-amidinopropane)dihydrochloride, <NUM>,<NUM>-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, <NUM>-(<NUM>-(carbamoylazo)isobutylonitril, <NUM>,<NUM>-azobis[<NUM>-(<NUM>-imidazolin-<NUM>-yl)propane]dihydrochloride, <NUM>,<NUM>-azobis-(<NUM>-cyanovaleric acid), and the like. More various thermal polymerization initiators are well disclosed in "Principle of Polymerization" written by Odian, (Wiley, <NUM>), p <NUM>.

The photo-polymerization initiator used herein may include, for example, one or more compounds selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine and α-aminoketone. Among them, as a specific example of the acylphosphine, a commonly used lucyrin TPO, that is, <NUM>,<NUM>,<NUM>-trimethyl-benzoyl-trimethyl phosphine oxide may be used. More various photo-polymerization initiators are well disclosed in "<NPL>.

The polymerization initiator may be added in a concentration of about <NUM> to <NUM>% by weight based on the monomer composition. That is, when the concentration of the polymerization initiator is too low, the polymerization rate may become slow and a large amount of residual monomer may be extracted in the final product, which is not preferable. Conversely, when the concentration of the polymerization initiator is higher than the above range, the polymer chains constituting the network become short, and thus the extractable content is increased and physical properties of the polymer may deteriorate such as a reduction in absorbency under load, which is not preferable.

Further, the monomer composition includes a foaming agent. The foaming agent acts to increase the surface area by causing foaming during polymerization to produce pores in the hydrogel polymer. As the foaming agent, a carbonate can be used. As an example, sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesiumbicarbonate or magnesium carbonate can be used.

Further, the foaming agent is preferably used in an amount of <NUM> ppmw or less based on the weight of the water-soluble ethylenically unsaturated monomer. When the amount of the foaming agent used is more than <NUM> ppmw, the pores become too large, the gel strength of the super absorbent polymer lowers and the density becomes low, which may cause problems in distribution and storage. Further, the foaming agent is preferably used in an amount of <NUM> ppmw or more, or <NUM> ppmw or more, based on the weight of the water-soluble ethylenically unsaturated monomer.

In addition, the monomer composition includes a surfactant. The surfactant allows to induce uniform dispersion of the foaming agent to perform a uniform foaming when foaming, thereby preventing the gel strength from being lowered or the density being lowered. Moreover, in step <NUM>, which will be described later, pulverization is performed to be a size smaller than the particle size usually produced due to the surfactant during coarse pulverization of the hydrogel polymer, thereby improving the absorption performance of the super absorbent resin as the final product.

As the surfactant, a compound represented by the following Chemical Formula <NUM> is used.

in Chemical Formula <NUM>,
R is an alkyl having <NUM> to <NUM> carbon atoms.

Further, the surfactant is preferably used in an amount of <NUM> ppmw or less based on the weight of the water-soluble ethylenically unsaturated monomer. When the amount of the surfactant used exceeds <NUM> ppmw, the content of the surfactant in the super absorbent polymer increases, which is not preferable. Further, the surfactant is preferably used in an amount of <NUM> ppmw or more, or <NUM> ppmw or more, based on the weight of the water-soluble ethylenically unsaturated monomer.

In addition, the monomer composition may further include additives such as a thickener, a plasticizer, a preservation stabilizer, an antioxidant, etc., if necessary.

Further, such a monomer composition can be prepared in the form of a solution in which a raw material such as the above-mentioned monomer is dissolved in a solvent. In this case, any usable solvent can be used without limitation in the constitution as long as it can dissolve the above-mentioned raw material. Examples of the solvent may include 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, N,N-dimethylacetamide, or a mixture thereof.

Further, the formation of the hydrogel polymer through polymerization of the monomer composition may be performed by a general polymerization method, and the process is not particularly limited. As a non-limiting example, the polymerization method are largely classified into a thermal polymerization and a photo-polymerization according to the type of the polymerization energy source, and the thermal polymerization may be carried out in a reactor like a kneader equipped with agitating spindles and the photo-polymerization may be carried out in a reactor equipped with a movable conveyor belt.

As an example, the monomer composition is injected into a reactor like a kneader equipped with the agitating spindles, and thermal polymerization is performed by providing hot air thereto or heating the reactor, thereby obtaining the hydrogel polymer. In this case, the hydrogel polymer, which is discharged from the outlet of the reactor according to the type of agitating spindles equipped in the reactor, may be obtained as particles with a size of centimeters or millimeters. Specifically, the hydrogel polymer may be obtained in various forms according to the concentration of the monomer composition injected thereto, the injection speed, or the like, and the hydrogel polymer having a (weight average) particle diameter of <NUM> to <NUM> may be generally obtained.

As another example, when the photo-polymerization of the monomer composition is performed in a reactor equipped with a movable conveyor belt, a sheet-shaped hydrogel polymer may be obtained. In this case, the thickness of the sheet may vary depending on the concentration of the monomer composition injected thereto and the injection speed, and the polymer sheet is preferably controlled to have typically a thickness of <NUM> to <NUM> in order to secure the production speed or the like while enabling a uniform polymerization of the entire sheet.

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

Step <NUM> is a step of coarsely pulverizing the hydrogel polymer prepared in step <NUM> to prepare a hydrogel polymer having a small average particle diameter.

In particular, as described above, in the present invention, as a surfactant is used in the production of the hydrogel polymer, the hydrogel polymer is pulverized into a particle diameter of <NUM> to <NUM> which is smaller than the particle diameter which is usually produced. In order to pulverize the hydrogel polymer into the above-mentioned particle diameter as in the present invention without using a surfactant, an excessive shearing force is required, which is difficult in the process, and the physical properties of the super absorbent polymer are deteriorated. However, as the present invention uses a surfactant, pulverization can be made into a smaller particle diameter, and thereby, the surface area of the super absorbent polymer is widened and excellent absorption capacity can be exhibited.

A pulverizing machine used herein may include, but its configuration is not limited to, for example, 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. However, it is not limited to the above-described examples.

Further, for the efficiency of the coarse pulverization, the coarse pulverization can be performed plural times depending on the size of the particle diameter. For example, the hydrogel polymer is subjected to the first coarse pulverization into an average particle of about <NUM>, which can be again subjected to the second coarse pulverization into an average particle of about <NUM>, followed by the third coarse pulverization into the above-mentioned average particle.

Step <NUM> is a step of drying and pulverizing the hydrogel polymer prepared in step <NUM> to prepare a surface crosslinking described later.

The drying temperature may be <NUM> to <NUM>. When the drying temperature is less than <NUM>, it is likely that the drying time becomes too long or the physical properties of the super absorbent polymer finally formed is deteriorated. When the drying temperature is higher than <NUM>, only the surface of the polymer is excessively dried, and thus fine powder may be generated and the physical properties of the super absorbent polymer finally formed may be deteriorated. The drying may be preferably carried out at a temperature of <NUM> to <NUM>, and more preferably at a temperature of <NUM> to <NUM>. Meanwhile, the drying time may be <NUM> minutes to <NUM> hours, in consideration of the process efficiency and the like, but it is not limited thereto.

In the drying step, any drying method may be selected and used without limitation in the constitution if it is a method commonly used in the relevant art. 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 <NUM> to <NUM>% by weight.

Next, a step of pulverizing the dried polymer obtained through such a drying step is carried out.

The polymer powder obtained through the pulverizing step may have a particle diameter of <NUM> to <NUM>. Specific examples of a pulverizing device that can be used to pulverize into the above particle diameter may include a ball mill, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill or the like, but it is not limited to the above-described examples.

Further, in order to control the physical properties of the super absorbent polymer powder finally commercialized after the pulverization step, a separate step of classifying the polymer powder obtained after the pulverization depending on the particle diameter may be undergone. Preferably, a polymer having a particle diameter of <NUM> to <NUM> is classified and only the polymer powder having such a particle diameter is subjected to the surface crosslinking reaction described later and finally commercialized.

Sep <NUM> is a step of crosslinking the surface of the base resin polymer prepared in step <NUM>, which is a step of heat-treating and surface-crosslinking the base polymer powder in the presence of a surface crosslinking solution containing a surface crosslinking agent to form a super absorbent polymer particle.

Here, the kind of the surface crosslinking agent contained in the surface crosslinking solution is not particularly limited. As a non-limiting example, the surface crosslinking agent may be at least one compound selected from the group consisting of ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, ethylene carbonate, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, propanediol, dipropylene glycol, polypropylene glycol, glycerin, polyglycerin, butanediol, heptanediol, hexanediol trimethylol propane, pentaerythritol, sorbitol, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, iron hydroxide, calcium chloride, magnesium chloride, aluminum chloride, and iron chloride.

In this case, the content of the surface crosslinking agent may be properly controlled according to the type of the surface crosslinking agent or reaction conditions, and preferably, the content may be controlled to <NUM> to <NUM> parts by weight based on <NUM> parts by weight of the base polymer. If the content of the surface crosslinking agent is too low, surface modification may not be properly performed to deteriorate physical properties of the final super absorbent polymer. On the contrary, if the surface crosslinking agent is excessively used, excessive surface crosslinking reaction may occur, leading to deterioration in absorption capability of the super absorbent polymer, which is not preferable.

In addition, the surface crosslinking solution may further include at least one solvent selected from the group consisting of 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. The solvent may be included in an amount of <NUM> to <NUM> parts by weight based on <NUM> parts by weight of the base polymer.

In addition, the surface crosslinking solution may further include a thickener. If the surface of the base polymer powder is further crosslinked in the presence of the thickener, it is possible to minimize the deterioration of the physical properties even after the pulverization. Specifically, as the thickener, at least one selected from a polysaccharide and a hydroxy-containing polymer may be used. The polysaccharide may be a gum type thickener, a cellulose type thickener and the like. Specific examples of the gum type thickener include xanthan gum, arabic gum, karaya gum, tragacanth gum, ghatti gum, guar gum, locust bean gum, and psyllium seed gum. Specific examples of the cellulose type thickener include hydroxypropylmethyl cellulose, carboxymethyl cellulose, methylcellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethylmethyl cellulose, hydroxymethylpropyl cellulose, hydroxyethylhydroxypropyl cellulose, ethylhydroxyethyl cellulose, and methylhydroxypropyl cellulose. Meanwhile, specific examples of the hydroxy-containing polymer include polyethylene glycol, polyvinyl alcohol and the like.

Meanwhile, in order to perform the surface crosslinking, a method of placing the surface crosslinking solution and the base polymer into a reaction tank and mixing them, a method of spraying a surface crosslinking solution onto the base polymer, a method in which the base polymer and the surface crosslinking solution are continuously supplied in a continuously operating mixer and mixed, or the like can be used.

In addition, the surface crosslinking may be carried out at a temperature of <NUM> to <NUM>, and may be continuously performed after the drying and pulverizing step proceeding at a relatively high temperature. At this time, the surface crosslinking reaction may be carried out for <NUM> to <NUM> minutes, or <NUM> to <NUM> minutes, or <NUM> to <NUM> minutes. That is, in order to prevent a reduction in physical properties due to damages of the polymer particles by excessive reaction while inducing the minimal surface crosslinking reaction, the surface modification step may be performed under the above-described conditions.

As described above, the super absorbent polymer according to the present invention exhibits excellent absorption performance and is preferably used for hygienic materials such as diapers, and thus can exhibit excellent performance.

Hereinafter, preferred examples are provided for better understanding of the invention.

A solution (solution A) in which <NUM> of <NUM>% IRGACURE <NUM> initiator (<NUM> ppmw based on the monomer composition) diluted with acrylic acid and <NUM> of <NUM>% polyethylene glycol diacrylate (PEGDA, Mw=<NUM>) diluted with acrylic acid were mixed was prepared. Then, a solution (solution B) in which <NUM> of <NUM>% allyl methacrylate diluted with acrylic acid was mixed was prepared.

<NUM> of acrylic acid, the solution A and the solution B were injected into a <NUM> glass reactor surrounded by a jacket through which a heat medium pre-cooled at <NUM> was circulated. Then, <NUM> of <NUM>% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about <NUM> or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about <NUM> mol%. <NUM> of <NUM>% sodium dodecylsulfate solution (solution D-<NUM>) diluted with water was prepared as a surfactant. Further, <NUM> of <NUM>% sodium bicarbonate solution (solution D-<NUM>) diluted with water and <NUM> of <NUM>% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about <NUM>, the solutions D-<NUM>, D-<NUM> and E previously prepared were injected into the mixed solution and mixed.

Then, the mixed solution prepared in step <NUM> was poured in a Vat-type tray (<NUM> in width × <NUM> in length) installed in a square polymerizer which had a light irradiation device installed at the top and whose inside was preheated to <NUM>. Subsequently, the mixed solution was irradiated with light. It was confirmed that a gel was formed on the surface after about <NUM> seconds from light irradiation, and it was confirmed that polymerization reaction occurred simultaneously with foaming after about <NUM> seconds from light irradiation. Then, the polymerization reaction was performed for additional <NUM> minutes, and the polymerized sheet was taken out and cut into a size of <NUM>×<NUM>. Then, the cut sheet was subjected to a chopping process using a meat chopper to prepare crumbs. The average particle diameter of the prepared crumbs was <NUM>.

Then, the crumbs prepared in step <NUM> were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at <NUM> from the bottom to the top for <NUM> minutes and from the top to the bottom for <NUM> minutes, so that the dried product had a water content of about <NUM>% or less. The dried crumbs were pulverized using a pulverizer and classified to obtain a base polymer having a particle diameter of <NUM> to <NUM>. The base polymer thus prepared had a centrifuge retention capacity of <NUM>/g. The centrifuge retention capacity was measured according to Experimental Example described below.

Thereafter, <NUM> of the base polymer prepared in step <NUM> was mixed with a crosslinking agent solution obtained by mixing <NUM> of water, <NUM> of ethylene carbonate, and <NUM> of Aerosil <NUM> (Evonik), and then surface crosslinking reaction was carried out at <NUM> for <NUM> minutes. The resultant product was pulverized and sieved to obtain a surface-crosslinked super absorbent polymer having a particle diameter of <NUM> to <NUM>. <NUM> of Aerosil <NUM> was dry-added to the obtained super absorbent polymer and mixed in a dry state to prepare a super absorbent polymer.

A mixed solution was prepared in the same manner as in step <NUM> of Example <NUM>, except that <NUM> of the solution B (solution in which <NUM> of <NUM>% allyl methacrylate diluted with acrylic acid was mixed) was used and <NUM> of the solution D-<NUM> (<NUM>% sodium bicarbonate solution diluted with water) was used.

Thereafter, <NUM> of the base polymer prepared in step <NUM> was mixed with a crosslinking agent solution obtained by mixing <NUM> of water, <NUM> of ethylene carbonate, <NUM> of propylene carbonate and <NUM> of Aerosil <NUM> (Evonik), and then surface crosslinking reaction was carried out at <NUM> for <NUM> minutes. The resultant product was pulverized and sieved to obtain a surface-crosslinked super absorbent polymer having a particle diameter of <NUM> to <NUM>. <NUM> of alumina powder (Alu <NUM>, Evonik) was dry-added to the obtained super absorbent polymer and mixed in a dry state to prepare a super absorbent polymer.

A mixed solution was prepared in the same manner as in step <NUM> of Example <NUM>, except that <NUM> of the solution D-<NUM> (<NUM>% sodium dodecylsulfate solution diluted with water) was used.

Thereafter, <NUM> of the base polymer prepared in step <NUM> was mixed with a crosslinking agent solution obtained by mixing <NUM> of water, <NUM> of ethylene carbonate and <NUM> of alumina powder (Alu <NUM>, Evonik), and then surface crosslinking reaction was carried out at <NUM> for <NUM> minutes. The resultant product was pulverized and sieved to obtain a surface-crosslinked super absorbent polymer having a particle diameter of <NUM> to <NUM>. <NUM> of alumina powder (Alu <NUM>, Evonik) was dry-added to the obtained super absorbent polymer and mixed in a dry state to prepare a super absorbent polymer.

A solution (solution A) in which <NUM> of <NUM>% IRGACURE <NUM> initiator (<NUM> ppmw based on the monomer composition) diluted with acrylic acid and <NUM> of <NUM>% polyethylene glycol diacrylate (PEGDA, Mw=<NUM>) diluted with acrylic acid were mixed was prepared. Then, <NUM> of a solution (solution B) of <NUM>% trimethylolpropane triacrylate containing <NUM> mol % of ethylene oxide (Ethoxylated-TMPTA, TMP(EO)9TA, M-<NUM> manufactured by Miwon Specialty Chemical Co. ) diluted with acrylic acid was prepared.

<NUM> of acrylic acid, the solution A and the solution B were injected into a <NUM> glass reactor surrounded by a jacket through which a heat medium pre-cooled at <NUM> was circulated. Then, <NUM> of <NUM>% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about <NUM> or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about <NUM> mol%. <NUM> of <NUM>% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about <NUM>, the solution E previously prepared was injected into the mixed solution and mixed.

Then, the polymerization and chopping processes were carried out in the same manner as in Example <NUM>. The average particle diameter of the prepared crumbs was <NUM>. Then, the drying and pulverizing processes were carried out in the same manner as in Example <NUM>. The base polymer thus prepared had a centrifuge retention capacity of <NUM>/g. The centrifuge retention capacity was measured according to Experimental Example described below. Then, the surface crosslinking and after-treatment were carried out in the same manner as in Example <NUM> to obtain a super absorbent polymer.

A solution (solution A) in which <NUM> of <NUM>% IRGACURE <NUM> initiator (<NUM> ppmw based on the monomer composition) diluted with acrylic acid and <NUM> of <NUM>% polyethylene glycol diacrylate (PEGDA, Mw=<NUM>) diluted with acrylic acid were mixed was prepared.

<NUM> of acrylic acid and the solution A were injected into a <NUM> glass reactor surrounded by a jacket through which a heat medium pre-cooled at <NUM> was circulated. Then, <NUM> of <NUM>% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about <NUM> or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about <NUM> mol%. <NUM> of <NUM>% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about <NUM>, the solution E previously prepared was injected into the mixed solution and mixed.

<NUM> of acrylic acid and the solution A were injected into a <NUM> glass reactor surrounded by a jacket through which a heat medium pre-cooled at <NUM> was circulated. Then, <NUM> of <NUM>% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about <NUM> or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about <NUM> mol%. <NUM> of <NUM>% Sugar ester (S-<NUM>) (solution D-<NUM>) diluted with acrylic acid was prepared as a surfactant. Further, <NUM> of <NUM>% sodium bicarbonate solution (solution D-<NUM>) diluted with water and <NUM> of <NUM>% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about <NUM>, the solutions D-<NUM>, D-<NUM> and E previously prepared were injected into the mixed solution and mixed.

<NUM> of acrylic acid and the solution A were injected into a <NUM> glass reactor surrounded by a jacket through which a heat medium pre-cooled at <NUM> was circulated. Then, <NUM> of <NUM>% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about <NUM> or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about <NUM> mol%. <NUM> of <NUM>% Sugar ester (S-<NUM>) (solution D-<NUM>) diluted with acrylic acid was prepared as a surfactant. Further, <NUM> of <NUM>% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about <NUM>, the solutions D-<NUM> and E previously prepared were injected into the mixed solution and mixed.

For comparison, a product (product name: IM-<NUM>), produced and sold commercially by Sandia, was used as Comparative Example <NUM>.

The physical properties of the super absorbent polymer prepared in Examples and Comparative Examples were evaluated by the following methods, and the results are shown in Table <NUM> below.

The absorbency under load (AUL) at <NUM> psi for a physiological saline solution was measured for each super absorbent polymer prepared in Examples and Comparative Examples according to EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP <NUM>. Among the super absorbent polymers to be measured, super absorbent polymers which was passed through a US standard <NUM> mesh screen and retained on a US standard <NUM> mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of <NUM> to <NUM>, and the AUL thereof was measured.

Specifically, a <NUM> mesh metal made of stainless steel was installed at the bottom of a plastic cylinder having an inner diameter of <NUM>. W<NUM>(g, about <NUM>) of a super absorbent polymer for measuring the absorbency under load were uniformly scattered on the screen at room temperature and relative humidity of <NUM>%. Then, a piston capable of uniformly providing a load of <NUM> kPa (<NUM> psi) was put thereon. At this time, the piston used was designed so that the outer diameter was slightly smaller than <NUM> and thus it could move freely up and down without any gap with the inner wall of the cylinder. Then, the weight W<NUM>(g) of the device thus prepared was measured.

A glass filter having a diameter of <NUM> and a thickness of <NUM> was put inside a Petri dish having the diameter of <NUM>, and then <NUM> wt% of a physiological saline solution was poured in the Petri dish. At this time, the physiological saline solution was poured until the surface level became equal to the upper surface of the glass filter. Then, a sheet of filter paper having a diameter of <NUM> was put on the glass filter.

Subsequently, the prepared device was placed on the filter paper so that the super absorbent polymer in the device was swelled by a physiological saline solution under load. After one hour, the weight W<NUM>(g) of the device containing the swollen super absorbent polymer was measured.

Using the weight thus measured, the absorbency under load was calculated according to the following Mathematical Formula <NUM>. <MAT> in Mathematical Formula <NUM>,
W<NUM>(g) is an initial weight (g) of the super absorbent polymer, W<NUM>(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, and W<NUM>(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, after absorbing a physiological saline solution to the super absorbent polymer under a load (<NUM> psi) for <NUM> hour.

The Gel-AUL at <NUM> psi was measured using the same device as that used in the '(<NUM>) Absorbency under load (AUL)'. Among the super absorbent polymers to be measured, super absorbent polymers which was passed through a US standard <NUM> mesh screen and retained on a US standard <NUM> mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of <NUM> to <NUM>, and the Gel-AUL thereof was measured.

Specifically, the resin W<NUM> (g, <NUM>) obtained in Examples and Comparative Examples was put into an AUL kit used in the '(<NUM>) Absorbency under load (AUL)', and a piston capable of providing a load of <NUM> psi was put thereon. Then, the weight W<NUM>(g) of the device thus prepared was measured. Subsequently, the piston was removed, and the super absorbent polymer was immersed in <NUM> of physiological saline solution under no load to perform a primary swelling. Then, the physiological saline solution was absorbed under a load of <NUM> psi for <NUM> minutes and then subjected to a vacuum desorption under a vacuum pressure of <NUM> psi for <NUM> seconds to remove a physiological saline solution existing between gels. The weight W<NUM> (g) of AUL kit including the physiological saline solution wholly containing gel therein was measured.

Using the respective weights thus obtained, the <NUM>-min Gel-AUL at <NUM> psi was calculated according to the following Mathematical Formula <NUM>. <MAT> in Mathematical Formula <NUM>,.

In addition, the gel-AUL at <NUM> psi was measured in the same manner as described above, except that a load of <NUM> psi was changed to <NUM> psi.

The centrifuge retention capacity(CRC) by water absorption capacity under a non-loading condition was measured for the super absorbent polymers of Examples and Comparative Examples in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP <NUM>. Among the super absorbent polymers to be measured, super absorbent polymers which was passed through a US standard <NUM> mesh screen and retained on a US standard <NUM> mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of <NUM> to <NUM>, and the CRC thereof was measured.

Specifically, W<NUM> (g, about <NUM>) of the super absorbent polymers of Examples and Comparative Examples were uniformly put in a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed in a physiological saline solution composed of <NUM> wt% aqueous sodium chloride solution at room temperature. After <NUM> minutes, water was removed from the bag by centrifugation at <NUM> for <NUM> minutes, and the weight W<NUM>(g) of the bag was then measured. Further, the same procedure was carried out without using the super absorbent polymer, and then the resultant weight W<NUM>(g) was measured.

Using the respective weights thus obtained, CRC (g/g) was calculated according to the following Mathematical Formula <NUM>. <MAT> in Mathematical Formula <NUM>,.

The free swell Gel Bed Permeability (GBP) for a physiological saline solution was measured for each super absorbent polymer prepared in Examples and Comparative Examples according to the following method described in <CIT>.

Specifically, the apparatus shown in FIGS. <NUM> to <NUM> was used to measure the free swell GBP. First, the plunger <NUM> installed with the weight <NUM> was placed in an empty sample container <NUM>, and the height from the top of the weight <NUM> to the bottom of the sample container <NUM> was measured to an accuracy of <NUM> using an appropriate gauge. The force to which the thickness gauge applied during the measurement was adjusted to less than about <NUM> N.

Meanwhile, among the super absorbent polymers for measuring GBP, super absorbent polymers which was passed through a US standard <NUM> mesh screen and retained on a US standard <NUM> mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of <NUM> to <NUM>.

About <NUM> of the super absorbent polymer classified in this way was placed in the sample container <NUM> and spread out evenly on the bottom of the sample container. Then, the container not containing the plunger <NUM> and the weight <NUM> therein, was submerged in <NUM> wt % physiological saline solution for about <NUM> minutes and allowed the super absorbent polymer to swell under no load condition. At this time, the sample container <NUM> was placed on the mesh located in a liquid reservoir so that the sample container <NUM> was raised slightly above the bottom of the liquid reservoir. As the mesh, those which did not affect the movement of the physiological saline solution into the sample container <NUM> were used. During saturation, the height of the physiological saline solution was allowed to be adjusted such that the surface within the sample container was defined by the swollen super absorbent polymer, rather than the physiological saline solution.

At the end of this period, the assembly of the plunger <NUM> and weight <NUM> was placed on the swollen super absorbent polymer <NUM> in the sample container <NUM> and then the sample container <NUM>, plunger <NUM>, weight <NUM> and swollen super absorbent polymer <NUM> were removed from the solution. Thereafter, before GBP measurement, the sample container <NUM>, plunger <NUM>, weight <NUM> and swollen super absorbent polymer <NUM> were placed on a flat, large grid non-deformable plate of uniform thickness for about <NUM> seconds. The height from the top of the weight <NUM> to the bottom of the sample container <NUM> was measured again by using the same thickness gauge as previously used. Then, the height measurement value of the device in which the plunger <NUM> equipped with the weight <NUM> was placed in the empty sample container <NUM> was subtracted from the height measurement value of the device including the swollen super absorbent polymer <NUM>, thereby obtaining the thickness or height "H" of the swollen super absorbent polymer.

For the GBP measurement, <NUM> wt % physiological saline solution was flowed into the sample container <NUM> containing the swollen super absorbent polymer <NUM>, the plunger <NUM> and the weight <NUM>. The flow rate of a physiological saline solution into the sample container <NUM> was adjusted to cause the physiological saline solution to overflow the top of the cylinder <NUM>, thereby resulting in a consistent head pressure equal to the height of the sample container <NUM>. Then, the quantity of solution passing through the swollen super absorbent polymer <NUM> versus time was measured gravimetrically using the scale <NUM> and beaker <NUM>. Data points from the scale <NUM> were collected every second for at least sixty seconds once the overflow has started. The flow rate (Q) passing through the swollen super absorbent polymer <NUM> was determined in units of grams/second (g/s) by a linear least-square fit of fluid passing through the sample <NUM> (in grams) versus time (in seconds).

Using the data thus obtained, the GBP (cm<NUM>) was calculated according to the following Mathematical Formula <NUM>. <MAT> in Mathematical Formula <NUM>.

The hydrostatic pressure was calculated from P=ρ×g×h, where ρ is a liquid density (g/cm<NUM>), g is a gravitational acceleration (nominally <NUM>/sec<NUM>), and h is a fluid height (for example, <NUM> for the GBP Test described herein).

At least two samples were tested and the results were averaged to determine the free swell GBP of the super absorbent polymer, and the unit was converted to darcy (<NUM> darcy=<NUM>×<NUM>-<NUM> UW) and shown in Table <NUM> below.

The absorption rate of the super absorbent polymers prepared in Examples and Comparative Examples was measured in second unit according to the method described in International Publication <CIT>.

Specifically, the absorption rate (vortex time) was calculated by a process in which <NUM> of the super absorbent polymer was added to <NUM> of physiological saline solution at <NUM> to <NUM>, and stirred at <NUM> rpm by a magnetic stirring bar (diameter <NUM>, length <NUM>), and the time required for the vortex to disappear was determined in second unit.

The above measurement results are shown in Table <NUM> below.

Claim 1:
A method for producing a super absorbent polymer, comprising the steps of:
crosslinking a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent, a foaming agent and a surfactant to form a hydrogel polymer containing a first crosslinked polymer wherein the surfactant is a compound represented by the following Chemical Formula <NUM>:

        [Chemical Formula <NUM>]     R-SO<NUM>Na

in Chemical Formula <NUM>,
R is an alkyl having <NUM> to <NUM> carbon atoms;
coarsely pulverizing the hydrogel polymer to prepare a hydrogel polymer having an average particle diameter of <NUM> to <NUM>;
drying and pulverizing the hydrogel polymer to form a base polymer power; and
heat-treating and surface-crosslinking the base polymer powder in the presence of a surface crosslinking agent to form a super absorbent polymer particle.