Patent Description:
Super Absorbent Polymer (SAP) is synthetic polymer material that can absorb moisture of about <NUM> to <NUM> times of the self weight, and is differently named as SAM(Super Absorbency Material), AGM(Absorbent Gel Material) and the like according to development companies. The super absorbent polymer began to be commercialized as sanitary goods, and currently, it is widely used as a soil water retention agent for horticulture, water stop material for civil engineering and construction, a sheet for raising seeding, a freshness maintaining agent in the field of food distribution, and fomentation material and the like, in addition to hygienic goods such as a disposable diaper for children.

The super absorbent polymer can be generally prepared in a powder type product by polymerizing monomers for polymer and drying and pulverizing it.

In the process of preparing super absorbent polymer, the step of polymerizing monomers is an important step for determining the physical properties of polymer. As the polymerization method, reverse phase suspension polymerization, thermal polymerization and photopolymerization and the like are known. Among them, as the photopolymerization method, a method of putting a monomer composition for polymer on a belt and irradiating from the top to polymerize the monomer composition is known.

However, according to the above polymerization method, the amount of irradiation may not be uniform according to the depth of the monomer composition, and thus, polymerization degree may become non-uniform according to the depth. For example, in case a monomer composition of a predetermined thickness is put on a belt, on the upper part of the monomer composition located closely to light source, polymerization progresses relatively excessively due to the large amount of light irradiation, while on the lower part of the monomer composition located far from the light source, the amount of light irradiation is insufficient and thus polymerization may not be completely conducted and non-polymerized components may be remained. In this case, the physical properties of super absorbent polymer may be degraded. To overcome this, quantity of light or light irradiation time may be varied according to the location of the monomer composition solution, which makes the process complicated and increases process time, thus lowering productivity.

<CIT> discloses a method for preparing superabsorbent polymer comprising the steps of: performing photopolymerization of a monomer composition comprising water soluble ethylene unsaturated monomers, a thermal polymerization initiator and a photopolymerization initiator at a temperature of <NUM> to <NUM>; performing a thermal polymerization and a photopolymerization at a temperature of <NUM> to <NUM> to form a hydrogel polymer; and drying the hydrogel polymer.

In order to overcome the problems of the prior art, it is an object of the present invention to provide a method for preparing super absorbent polymer having improved physical properties by comprising two polymerization steps.

In order to achieve the object, the present invention provides a method for preparing super absorbent polymer according to claim <NUM>.

According to the method for preparing super absorbent polymer of the present invention, super absorbent polymer that has a high absorption capacity and yet has a low water soluble component content can be prepared.

Hereinafter, the method for preparing super absorbent polymer according to one embodiment of the invention will be explained in detail.

The method for preparing super absorbent polymer comprises the steps of: performing photopolymerization of a monomer composition comprising water soluble ethylene unsaturated monomers, a thermal polymerization initiator and a photopolymerization initiator at a temperature of <NUM> to <NUM>; performing a thermal polymerization and a photopolymerization at a temperature of <NUM> to <NUM> to form a hydrogel polymer; and drying the hydrogel polymer, as the photopolymerization is progressed, the temperature of the monomer composition increases, and when the temperature of the monomer composition reaches <NUM>, thermal polymerization is additionally progressed by the action of the thermal initiator,.

In the method for preparing super absorbent polymer, the monomer composition, which is raw material of the super absorbent polymer, comprises water soluble ethylene unsaturated monomers and polymerization initiators.

As the water soluble ethylene unsaturated monomers, any monomers commonly used for the preparation of super absorbent polymer may be used without specific limitations. Specifically, at least one monomer selected from the group consisting of anionic monomers and salts thereof, non-ionic hydrophilic group-containing monomers, and amino group-containing unsaturated monomers and quaternarized compounds thereof may be used.

In detail, the anionic monomers may include (meth)acrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, <NUM>-acryloylethane sulfonic acid, <NUM>-methacryloylethane sulfonic acid, <NUM>-(meth)acryloylpropane sulfonic acid or <NUM>-(meth)acrylamide-<NUM>-methyl propane sulfonic acid; the non-ionic hydrophilic group-containing monomers may include (meth)acrylamide, N-substituted(meth)acrylate, <NUM>-hydroxyethyl(meth)acrylate, <NUM>-hydroxypropyl(meth)acrylate, methoxypolyethyleneglycol(meth)acrylate or polyethyleneglycol(meth)acrylate; and the amino group-containing unsaturated monomers may include (N,N)-dimethylaminoethyl(meth)acrylate or (N,N)-dimethylaminopropyl(meth)acrylamide.

More specifically, acrylic acid or salts thereof, for example, acrylic acid or an alkali metal salt thereof such as a sodium salt thereof and the like may be used, which enables preparation of super absorbent polymer having more excellent physical properties. In case the alkali metal salt of acrylic acid is used as the monomer, acrylic acid may be neutralized with a basic compound such as caustic soda(NaOH) before use.

The concentration of the water soluble ethylene unsaturated monomers may be about <NUM> to about <NUM> wt%, preferably about <NUM> to about <NUM> wt%, based on the monomer composition comprising the raw material of the super absorbent polymer and a solvent, and it may be appropriately controlled considering the polymerization time and reaction conditions, and the like. However, if the concentration of the monomer is too low, yield of super absorbent polymer may be decreased and there may be a problem in terms of economical feasibility, and if the concentration is too high, a part of the monomers may be precipitated or pulverization efficiency may be low when the polymerized hydrogel polymer is pulverized, thus causing process problems, and the physical properties of super absorbent polymer may be degraded.

Meanwhile, in the step of polymerizing a monomer composition to form a hydrogel polymer, the polymerization method is largely classified into thermal polymerization and photopolymerization according to polymerization energy source.

In case thermal polymerization is progressed, it may be commonly progressed in a reactor such as a kneader equipped with an agitation shaft, and in case photopolymerization is progressed, it may be progressed in a reactor equipped with a movable conveyor belt, and thus, the reactor may be varied according to the polymerization method.

For example, when hydrogel polymer is obtained by supplying hot wind to a reactor such as a kneader equipped with an agitation shaft or heating the reactor, the hydrogel polymer discharged to the outlet of the reactor may have a shape of several centimeters to several millimeters according to the shape of the agitation shaft of the reactor. Specifically, the size of the obtained hydrogel polymer may be varied according to the concentration of the introduced monomer composition and introduction speed, and the like, and commonly, hydrogel polymer having weight average particle diameter of <NUM> to <NUM> may be obtained.

And, in case photopolymerization is progressed in a reactor equipped with a movable conveyor belt, the obtained hydrogel polymer may commonly have a shape of a sheet having a width of the belt. Wherein, the thickness of the polymer sheet may be varied according to the concentration of the introduced monomer composition and the introduction speed, but it is preferable to supply a monomer composition so as to obtain a sheet shaped polymer having a thickness of about <NUM> to about <NUM>. If the monomer composition is supplied such that the thickness of the sheet shaped polymer may be too thin, production efficiency may be low, and if the thickness of the sheet shaped polymer is greater than <NUM>, the polymerization reaction may not be uniformly conducted over the whole thickness, due to the too thick thickness.

Meanwhile, according to one embodiment of the invention, both thermal polymerization and photopolymerizaiton are conducted in a reactor equipped with a conveyor belt.

The polymerization reaction for polymerizing ethylene unsaturated monomers is an exothermic reaction. Thus, even if the reaction begins at low temperature of the monomer composition, as the reaction progresses, the temperature of the monomer composition gradually increases to a high temperature state. Thus, in case thermal polymerization is progressed, in order to initiate a reaction, a thermal initiator for low temperature that can initiate a reaction even at low temperature, and a thermal initiator for high temperature that initiates a reaction at a high temperature state after the middle stage of the reaction are used in combination. However, in case thermal polymerization is progressed with two or more kinds of thermal initiators only, as the reaction progresses, it may be difficult to control the temperature and the decomposition speed of the thermal initiators in the monomer composition may be varied, and thus, the molecular weight distribution of the produced polymer may be non-uniform and wide, which may cause degradation of the physical properties of the final super absorbent polymer.

And, even in case photopolymerization is progressed, heat is generated to some degree as the polymerization reaction, which is also an exothermic reaction, is progressed, and thus, a method of progressing the polymerization with additionally using thermal initiator is known. However, according to the method, since thermal polymerization simultaneously occurs from the beginning at a common photopolymerization initiation temperature of <NUM>, the efficiency of the initiation of the photopolymerization may be decreased and it may be difficult to produce high molecular weight polymer.

Thus, in the preparation method of super absorbent polymer of the present invention, first, photopolymerization of the monomer composition is progressed at a temperature of <NUM> to <NUM>. As such, since photopolymerization is initiated at low temperature, the action of the thermal initiator is excluded and the photopolymerization reaction may be progressed at a constant speed. If the temperature is too lower than the above range, non-polymerized products may be generated, and if it is too high exceeding <NUM>, thermal polymerization may occur, which may not be preferable.

The light source that can be used in the photopolymerization step is not specifically limited, and UV light source known to cause photopolymerization may be used without specific limitations. For example, light with a wavelength of about <NUM> to about <NUM> may be used, and UV light source such as Xe lamp, mercury lamp, or metal halide lamp and the like may be used. And, the photopolymerization step may be conducted for about <NUM> seconds to about <NUM> minutes at the intensity of about <NUM> mw/cm<NUM> to about <NUM> kw/cm<NUM>. If the intensity of the light applied for the photopolymerization and the time are too small or short, the polymerization reaction may not sufficiently occur, and if the intensity and the time are too large or long, the quality of super absorbent polymer may be degraded.

As the photopolymerization is progressed, the temperature of the monomer composition increases, and when the temperature of the monomer composition reaches about <NUM>, thermal polymerization is additionally progressed by the action of the thermal initiator. The temperature of the second step during which the thermal polymerization and photopolymerization are progressed is <NUM> to <NUM>, or about <NUM> to about <NUM>. However, at this temperature, thermal polymerization may more predominantly occur than photopolymerization.

Additional heat source may be used so that the monomer composition may reach a suitable temperature. The heat source may be used without specific limitations, and for example, hot wind, microwave irradiation, near infrared ray irradiation and the like may be used.

As explained above, since both thermal polymerization and photopolymerization are conducted in the method of preparing super absorbent polymer, polymerization initiators used during polymerization includes both a thermal polymerization initiator and a photopolymerization initiator.

As the photopolymerization initiator, specific example of the acyl phosphine may include commercially available lucirin TPO, namely, <NUM>,<NUM>,<NUM>-trimethyl-benzoyl-trimethyl phosphine oxide. More various photoinitiators are well stated in "<NPL>, and are not limited to the above examples.

The photopolymerization initiator may be included in the concentration of about <NUM> to about <NUM> wt% based on the monomer composition. If the concentration of the photopolymerization initiator is too low, polymerization speed may become slow, and if the concentration of the photopolymerization initiator is too high, the molecular weight of high absorbent polymer may become small and the physical properties may become non-uniform.

And, as the thermal polymerization initiator, at least one selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide and ascorbic acid may be used. Specific examples of the persulfate 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>) and the like, and specific examples of the azo initiators may include <NUM>,<NUM>-azobis(<NUM>-amidinopropane) dihydrochloride, <NUM>,<NUM>-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, <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 initiator are well stated in "<NPL>, and are not limited to the above examples. Preferably, persulfate initiators such as 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 may be used.

The thermal polymerization may be included in the concentration of about <NUM> to about <NUM> wt% based on the monomer composition. If the concentration of the thermal polymerization initiator is too low, additional thermal polymerization may hardly occur and the effects resulting from the addition of the thermal polymerization initiator may be insignificant, and if the concentration of the thermal polymerization initiator is too high, the molecular weight of high super absorbent polymer may be small and the physical properties may become non-uniform.

According to one embodiment of the invention, the monomer composition may further comprise an internal crosslinker as the raw material of super absorbent polymer. As the internal crosslinker, a crosslinker that has at least one functional group capable of reacting with the water soluble substituent of the water soluble ethylene unsaturated monomer and yet has at least one ethylene unsaturated group; or a crosslinker that has at least two functional groups capable of reacting with the water soluble substituent of the monomer and/or the water soluble substituent formed by the hydrolysis of the monomers may be used.

Specific examples of the internal crosslinker may include C8-C12 bisacrylamide, bismethacrylamide, poly(meth)acrylate of C2-C10 polyol or poly(meth)allylether of C2-C10 and the like, and more specifically, at least one selected from the group consisting of N,N'-methylenebis(meth)acrylate, ethyleneoxy(meth)acrylate, polyethyleneoxy(meth)acrylate, propyleneoxy(meth)acrylate, glycerin diacrylate, glycerin triacrylate, trimethylol triacrylate, triallylamine, triarylcyanurate, triallylisocyanate, polyethyleneglycol, diethyleneglycol and propyleneglycol may be used.

The internal crosslinker may be included at the concentration of about <NUM> to about <NUM> wt% based on the monomer composition, and it may crosslink the polymerized polymer.

In the preparation method of the present invention, the monomer composition of super absorbent polymer may further comprise additives such as a thickener, a plasticizer, a preservation stabilizer, an antioxidant and the like, as necessary.

The above explained raw materials of ethylene unsaturated monomers, photopolymerization initiator, thermal polymerization initiator, internal crosslinker and additives may be prepared in the form of a monomer composition solution dissolved in a solvent.

Wherein, the solvent that can be used is not specifically limited as long as it can dissolve the above explained components, and for example, at least one selected from the group consisting of ethanol, ethyleneglycol, diethyleneglycol, triethyleneglycol, <NUM>,<NUM>-butanediol, propyleneglycol, ethyleneglycolmonobutylether, propyleneglycolmonomethylether, propyleneglycolmonomethyletheracetate, methylethylketone, acetone, methylamylketone, cyclohexanone, cyclopentanone, diethyleneglycolmonomethylether, diethyleneglycolethylether, toluene, xylene, butyrolactone, carbitol, methylcellosolveacetate and N,N-dimethylacetamide and the like may be used in combination.

The solvent may be included in the remaining contents other than the above explained components, based on the total content of the monomer composition.

The hydrogel polymer obtained by the method may commonly have a moisture content of about <NUM> to about <NUM> wt%. Meanwhile, as used herein, the term "moisture content" refers to the content of moisture occupied based on total weight of the hydrogel polymer, and is a value calculated by subtracting the weight of dried polymer from the weight of hydrogel polymer. Specifically, it is defined by the value calculated by measuring weight loss according to moisture evaporation during polymerization in the process of raising the temperature of the polymer and drying it through infrared heating. Wherein, while the temperature is raised to about <NUM> and then maintained at <NUM>, the total drying time is set to <NUM> minutes including temperature raising step of <NUM> minutes, and the moisture content is measured.

Next, the obtained hydrogel polymer is dried.

Before the drying step, if necessary, a step of coarse-pulverizing may be further conducted so as to increase the efficiency of the drying step.

The construction of a pulverizer is not limited, but specifically, the pulverizer 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 thereto.

The pulverization step may be conducted such that the particle diameter of the hydrogel polymer may become about <NUM> to about <NUM>.

Pulverization to a particle diameter less than <NUM> is not technically easy due to high moisture content of the hydrogel polymer, and it may cause aggregation between pulverized particles. Meanwhile, if pulverization is conducted to a particle diameter greater than <NUM>, the effect for increasing the efficiency of the subsequent drying step may be insignificant.

The hydrogel pulverized as described above, or the hydrogel immediately after polymerization without pulverization is dried. Wherein, the drying temperature of the drying step may be about <NUM> to about <NUM>. If the drying temperature is less than <NUM>, drying time may become too long and the physical properties of the finally formed super absorbent polymer may be degraded, and if the drying temperature is greater than <NUM>, only the surface of the polymer may be dried to generate fine particles in the subsequent pulverization process, and the physical properties of the finally formed super absorbent polymer may be degraded. Thus, the drying may be preferably conducted at a temperature of about <NUM> to about <NUM>, more preferably at a temperature of about <NUM> to about <NUM>.

Meanwhile, considering the process efficiency and the like, the drying time may be about <NUM> minutes to about <NUM> minutes, but is not limited thereto.

The drying method of the drying step is not specifically limited as long as it is commonly used as a drying process of hydrogel polymer. Specifically, the drying step may be progressed by hot wind, UV irradiation, microwave irradiation, or infrared irradiation and the like. The moisture content of the polymer after progressing the drying step may be about <NUM> to about <NUM> wt%.

Next, the dried polymer obtained by the drying step is pulverized.

The polymer powder obtained after the pulverization may have a particle diameter of about <NUM> to about <NUM>µm. As the pulverizer used for pulverization to such particle diameter, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill or a jog mill and the like may be used, but is not limited thereto.

And, in order to manage the physical properties of supper absorbent polymer powder finally made into products after the pulverization, a step of classifying the polymer powder obtained after the pulverization according to the particle diameter may be further conducted. Preferably, polymer having a particle diameter of about <NUM> to about <NUM>µm is classified, and only the polymer powder having such particle diameter is crosslinked to make it into a product.

Next, a surface crosslinker is added to the pulverized polymer and a surface crosslinking reaction is progressed.

The surface crosslinking is a step of increasing the crosslinking density near the surface of super absorbent polymer particles with regard to the crosslinking density inside of the particles. In general, the surface crosslinker is coated on the surface of super absorbent polymer particles. Thus, this reaction occurs on the surface of super absorbent polymer particles, which improves crosslinking capacity on the surface of the particles without substantially influencing on the inside of the particles. Thus, the surface-crosslinked super absorbent polymer particles have higher crosslinking density near the surface than inside of the particles.

Wherein, as the surface crosslinker, compounds that can react with the functional group of polymer may be used without specific limitation on the construction.

Preferably, to improve the properties of the produced super absorbent polymer, as the surface crosslinker, at least one selected from the group consisting of polyhydric alcohol compounds; epoxy compounds; polyamine compounds; polyamine compounds; haloepoxy compounds; condensation products of haloepoxy compounds; oxazoline compounds; mono-, di- or polyoxazolidinone compounds; cyclic urea compounds; multivalent metal salts; and alkylene carbonate compounds may be used.

Specifically, as the polyhydric alcohol compounds, at least one selected from the group consisting of mono-, di-, tri-, tetra- or polyethyleneglycol, monopropyleneglycol, <NUM>,<NUM>-propanediol, dipropyleneglycol, <NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>-pentanediol, polypropyleneglycol, glycerol, polyglycerol, <NUM>-butene-<NUM>,<NUM>-diol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-butanediol, <NUM>,<NUM>-pentanediol, <NUM>,<NUM>-hexanediol, and <NUM>,<NUM>-cyclohexanedimethanol may be used.

As the epoxy compounds, at least one selected from the group consisting of ethyleneglycol diglycidyl ether and glycidol and the like may be used, and, as the polyamine compounds, at least one selected from the group consisting of ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine and polyamidepolyamine may be used.

As the haloepoxy compounds, epichlorohydrin, epibromohydrin and α-methylepichlorohydrin may be used, and as the mono-, di- or polyoxazolidinone compounds, <NUM>-oxazolidinone and the like may be used.

And, as the alkylene carbonate compounds, ethylene carbonate and the like may be used. They may be used alone or in combinations. Meanwhile, in order to increase the efficiency of the surface crosslinking process, it is preferable to include at least one polyhydric alcohol compounds in the surface crosslinker, and more preferably, a C2-<NUM> polyhydric alcohol compound may be used.

The content of the surface crosslinker may be appropriately selected according to the kind of the surface crosslinker or reaction conditions, but commonly, based on <NUM> parts by weight of the polymer, about <NUM> to about <NUM> parts by weight, preferably about <NUM> to about <NUM> parts by weight, more preferably about <NUM> to about <NUM> parts by weight of the surface crosslinker may be used.

If the content of the surface crosslinker is too small, surface crosslinking may hardly occur, and if it is greater than <NUM> parts by weight based on <NUM> parts by weight of the polymer, absorption capacity and physical properties may be lowered due to the excessive surface crosslinking reaction.

The method of adding the surface crosslinker to the polymer is not specifically limited. The surface crosslinker and polymer powder may be put in a reactor and mixed, the surface crosslinker may be sprayed on polymer powder, or the surface crosslinker and polymer may be continuously supplied to a continuously operated mixer and mixed, and the like.

When the surface crosslinker is added, water may be additionally mixed together. In case water is added, the surface crosslinker may be evenly dispersed on the polymer. Wherein, the content of added water may be preferably about <NUM> to about <NUM> parts by weight based on <NUM> parts by weight of the polymer, so as to induce even dispersion of the surface crosslinker, prevent the agglomeration of polymer powder, and optimize the surface penetration depth of the crosslinker.

By heating polymer particles to which the surface crosslinker is added at a temperature of about <NUM> to about <NUM>, preferably about <NUM> to about <NUM> for about <NUM> to about <NUM> minutes, preferably about <NUM> to about <NUM> minutes, a surface crosslinking reaction and drying may be simultaneously conducted. If the crosslinking temperature is less than <NUM>, surface crosslinking may not occur, and if it is greater than <NUM>, foreign substance and odor may be generated due to carbonization, and due to the excessive reaction, the physical properties may be degraded and stable process operation conditions cannot be secured. And, if the crosslinking reaction time is too short as less than <NUM> minutes, sufficient crosslinking may not be achieved, and if it is greater than <NUM> minutes, due to excessive surface crosslinking, polymer particles may be damaged and the physical properties may be degraded.

A temperature raising means for surface crosslinking is not specifically limited. Heating medium may be supplied or heat source may be directly supplied and heated. Wherein, the kind of heating medium that can be used may include steam, hot wind, temperature-raised fluid such as hot oil, and the like, but is not limited thereto, and the temperature of the supplied heating medium may be appropriately selected considering the means of heating medium, temperature-raising speed, and target temperature. Meanwhile, the directly supplied heat source may include electric heating, gas heating, but is not limited to the above examples.

Super absorbent polymer obtained according to the preparation method of the present invention may exhibit high centrifuge retention capacity(water holding capacity) and high absorption capacity under pressure, and low water soluble component content.

The super absorbent polymer prepared according to the preparation method of the present invention exhibits centrifuge retention capacity of <NUM>/g to <NUM>/g as measured according to EDANA method WSP <NUM>, and absorption capacity under pressure of <NUM>/g to <NUM>/g as measured according to EDANA method WSP <NUM>, thus exhibiting excellent centrifuge retention capacity and absorption capacity under pressure. And, it exhibits low water soluble component content of <NUM> to <NUM> wt%.

Hereinafter, the present invention will be explained with the following examples. However, these examples are only to illustrate the invention, and the scope of the invention is not limited thereto.

<NUM> of acrylic acid monomers, <NUM> of caustic soda(NaOH) and <NUM> of water were mixed, and to the mixture, <NUM> of diphenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)-phosphine oxide as a photopolymerization initiator, <NUM> of sodium persulfate as a thermal polymerization initiator and <NUM> of polyethyleneglycol diacrylate as a crosslinker were added to prepare a monomer composition.

The temperature of the monomer composition was maintained at <NUM> using a thermostat, the composition was injected into a rotary belt in the form of a <NUM>-axis silicon belt, and UV was irradiated thereto at the intensity of 10mW for <NUM> seconds using mercury UV lamp light source. And, when thermal polymerization was conducted after photopolymerization, the internal temperature of the reactor was maintained at <NUM>, and hot wind and insulation device was installed so that thermal polymerization can smoothly occur.

The obtained hydrogel type polymer was cut to an average particle diameter of <NUM> with a cutter, dried in a hot air dryer at <NUM> for <NUM> minutes, and then, repulverized using a rotary mixer, and classified into those having a particle size of <NUM> to <NUM>µm using an apparatus for measuring particle size distribution to prepare base polymer.

A surface treatment solution comprising <NUM> of water, <NUM> of methanol, <NUM> of <NUM>,<NUM>-propanediol was sprayed to <NUM> of the base polymer and evenly dispersed on the surface of the particles, and then, reacted at <NUM> for <NUM> minutes to obtain surface-treated super absorbent polymer.

Super absorbent polymer was prepared by the same method as Example <NUM>, except that the initial temperature of the monomer composition was maintained at <NUM> using a thermostat.

Super absorbent polymer was prepared by the same method as Example <NUM>, except that <NUM> of diphenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)-phosphine oxide was used.

Super absorbent polymer was prepared by the same method as Example <NUM>, except that <NUM> of lucirin TPO was used instead of diphenyl(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)-phosphine oxide.

Super absorbent polymer was prepared by the same method as Example <NUM>, except that <NUM> of sodium persulfate was used.

Super absorbent polymer was prepared by the same method as Example <NUM>, except that the initial temperature of the monomer composition was maintained at <NUM>.

Super absorbent polymer was prepared by the same method as Example <NUM>, except that the internal temperature of the reactor was maintained at <NUM>.

Centrifuge retention capacity was measured according to EDANA method WSP <NUM>. <NUM> of super absorbent polymer that was classified into <NUM>~<NUM> mesh was put in a teabag, soaked in a <NUM>% brine solution for <NUM> minutes, and then, water was removed in a centrifuge set to <NUM> for <NUM> minutes, and it was weighed to measure the amount of water held by super absorbent polymer, thereby measuring the water holding capacity.

Absorption capacity under pressure was measured according to EDANA method WSP <NUM>. Specifically, <NUM> of super absorbent polymer of <NUM> to <NUM>µm was evenly distributed in a cylinder prescribed in the EDANA method, and pressurized under pressure of <NUM>/cm<NUM> with a piston and weights, and then, absorption capacity under pressure was calculated as the amount of absorbing a <NUM>% brine solution for <NUM> hour.

Water soluble component content was measured according to EDANA method <NUM>. Specifically, <NUM> of super absorbent polymer was put in a <NUM> Erlenmeyer flask, and then, eluted in <NUM> of a <NUM>% brine solution for <NUM> hours. The gel part of the eluted solution was filtered with a filter paper(No.<NUM>), only the part dissolved in the <NUM>% brine solution was taken and the content was analyzed, and thus, water soluble component content was measured by calculating the weight ratio of the eluted super absorbent polymer to the weight of super absorbent polymer before elution.

The physical property values of Examples and Comparative Examples measured according to the above method are shown in the following Table <NUM>.

Claim 1:
A method for preparing super absorbent polymer comprising the steps of:
performing photopolymerization of a monomer composition comprising water soluble ethylene unsaturated monomers, a thermal polymerization initiator and a photopolymerization initiator at a temperature of <NUM> to <NUM>;
performing a thermal polymerization and a photopolymerization at a temperature of <NUM> to <NUM> to form a hydrogel polymer; and
drying the hydrogel polymer,
as the photopolymerization is progressed, the temperature of the monomer composition increases, and when the temperature of the monomer composition reaches <NUM>, thermal polymerization is additionally progressed by the action of the thermal initiator,
wherein the photopolymerization initiator includes acyl phosphine,
wherein the super absorbent polymer has a centrifuge retention capacity of <NUM> to <NUM>/g, an absorption capacity under pressure of <NUM> to <NUM>/g, and a water soluble component content of <NUM> to 13wt%, wherein the centrifuge retention capacity is measured according to EDANA method WSP <NUM>, the absorption capacity under pressure is measured according to EDANA method WSP <NUM>, and the water soluble component content is measured according to EDANA method <NUM>.