TITLE OFTHE INVENTION
PREPARATIONMETHOD OF SUPERABSORBENT POLYMER
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a preparation method of a superabsorbent
polymer, and specifically to a method that can improve process efficiency and the work
environment by minimizing fine powder generation and that can provide a
superabsorbent polymer having high water holding capacity and absorbing power under
pressure while having a low content of a water-soluble component.
(b) Description of the Related Art
A superabsorbent polymer (SAP) is a synthetic polymer material having a
function of absorbing about 500 to about 1000 times its weight of water, and it has been
differently called a superabsorbency material (SAM), an absorbent gel material (AGM),
and so on by developing enterprises. The superabsorbent polymer disclosed above
started to be commercialized for sanitary items, and is now being used widely as a water
combination soil for horticulture, a water-stop material for civil engineering and
construction, a nursery sheet, a freshness preservative in the food distribution field, a
poultice material, and the like in addition to the sanitary fittings like a paper diaper for a
child.
An inverse suspension polymerization method or an aqueous polymerization
method is known as a method of preparing a superabsorbent polymer. For example,
inverse suspension polymerization is disclosed in Japanese Patent Publication Nos.
Sho56-161408, Sho57-158209, Sho57-198714, and so on. As the aqueous
polymerization method, a thermal polymerization method of polymerizing a hydrous gel
phase polymer while fracturing and cooling the same in a kneader equipped with a
plurality of spindles, and a photo-polymerization method of exposing a highconcentrated
aqueous solution on a belt to UV rays and the like so as to carry out the
polymerization and dry it at the same time are known.
Meanwhile, a polymer powder obtained from the processes of polymerization,
pulverization, drying, and final milling is surface treated for obtaining a hydrous gel
phase polymer having excellent properties, and various modifications of the processes
have been attempted for increasing the effects of the steps of polymerization,
pulverization, and drying in order to obtain a hydrous gel phase polymer having
excellent properties.
SUMMARY OF THE INVENTION
It is an aspect of the present invention to provide a method of preparing a
superabsorbent polymer that can improve process efficiency and the work environment
by minimizing fine powder generation and that can provide the superabsorbent polymer
having high water holding capacity and absorbing power under pressure (AUP) while
having a low content of a water-soluble component.
The present invention provides a method of preparing a superabsorbent polymer
including the steps of: preparing a hydrous gel phase polymer by thermal polymerizing
or photo-polymerizing a monomer composition including a water-soluble ethylenebased
unsaturated monomer and a polymerization initiator; drying the hydrous gel phase
polymer; milling the dried polymer; classifying the milled hydrous gel phase polymer
into two or more grades by particle size; adding a surface cross-linking agent to each
hydrous gel phase polymer classified into two or more grades; and carrying out a
surface cross-linking reaction of the hydrous gel phase polymer to which the surface
cross-linking agent is added.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a drawing briefly showing the preparation method of the
superabsorbent polymer according to one embodiment of the present invention.
Fig. 2 is a drawing briefly showing the preparation method of the
superabsorbent polymer according to another embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Hereinafter, the method o f preparing the superabsorbent polymer according to
specific embodiments of the present invention is explained in more detail.
According to one embodiment of the present invention, a method o f preparing a
superabsorbent polymer is provided, including the steps of: preparing a hydrous gel
phase polymer by thermal polymerizing or photo-polymerizing a monomer composition
including a water-soluble ethylene-based unsaturated monomer and a polymerization
initiator; drying the hydrous gel phase polymer; milling the dried polymer; classifying
the milled hydrous gel phase polymer into two or more grades by particle size; adding a
surface cross-linking agent to each hydrous gel phase polymer classified into two or
more grades; and carrying out a surface cross-linking reaction of the hydrous gel phase
polymer to which the surface cross-linking agent is added.
From the results o f research o f the present inventors, it was recognized that the
surface cross-linking agent can be uniformly distributed throughout the polymer and the
surface cross-linking reaction can be uniformly and effectively fulfilled, and
accordingly, high water holding capacity and absorbing power under pressure can b e
secured while significantly lowering the content of water-soluble component, when the
surface cross-linking reaction is carried out by spraying a solution including the surface
cross-linking agent to each polymer of the two or more grades that are classified by
particle size.
Particularly, the method of preparing a superabsorbent polymer makes it
possible to form granules by cohering the fine powder having the particle size o f less
than 150 i, and accordingly not only can the amount of the fine powder generated in
the succeeding processes be apparently decreased but also the amount o f the fine
powder included in the final product can be minimized, by treating one group of
polymer having a particle size o f less than 150 m with the surface cross-linking agent
individually after classifying the polymer into one group having a particle size less than
15 0 m and one or more groups having a particle size of 150j¾m or more.
Heretofore, the surface cross-linking agent has been treated to the whole milled
o r classified polymer, and it has been usual for the polymer powder having a particle
size o f less than a certain value to not be used in the surface cross-linking reaction step
and it is recycled and reused in the polymerizing device or the monomer solution
composition, or other processes, because it is difficult for the polymer powder having
the particle size o f less than a certain value to be classified in the succeeding
classification process and there is a concern o f decreasing the properties o f the final
product or making the process conditions inferior. However, there was a problem that
the process for circulating such fine powder to a separate recycle process and reusing
the same incurs additional process costs and takes a lot o f additional time.
However, according to the method o f preparing a superabsorbent polymer o f
one embodiment o f the present invention, not only can the amount o f the fine powder
generated in the final manufacturing step be minimized without adding a process o f
separating and recycling the fine powder included in the milled polymer particles, but
also a superabsorbent resin having high water holding capacity and absorbing power
under pressure while largely reducing the content o f the water-soluble component can
be provided. Specifically, the content o f the fine powder having the particle size less
than 15 0 m included in the final superabsorbent polymer prepared according to said
method may be 1.2 weight% or less, and preferably 0.1 to 1.2 weight%.
The classification in the method o f preparing a superabsorbent polymer may be
carried out separating the particles into two grades having a particle size o f less than
15 0 m and having a particle size o f 15 0 m to 850 i; into three grades having a particle
size o f less than 150 m, having a particle size o f 150 n or more and less than 300 m,
and having a particle size o f 300j¾m to 850 i; or into four grades having a particle size
o f less than 150 i, having a particle size o f \ 5 or more and less than 300 m, having
a particle size o f 300 m or more and less than 600 m, and having a particle size o f
600 m to 85 0 m.
Further, the polymer may be classified into five or more grades for inducing
uniform distribution o f the surface cross-linking agent added to the polymer particles as
occasion demands, and the ratio of specific surface area per the unit weight of the
polymer particle may be the criteria of the classified particle size.
When the number of classification grades increases, there is an advantage in
that the surface cross-linking agent added to the polymer particles can be distributed
more uniformly. Considering the uniformity of the added surface cross-linking agent
and the economics of the process, a suitable process may be selected from the two or
more classification grades.
For reference, Figs. 1 and 2 represent schematic drawings of post-treatment
processes including the final milling step in the method of preparing the superabsorbent
polymer according to one embodiment of the present invention. In Figs. 1 and 2, the
solid lines represent the transfer route of the polymer, and the broken lines represent the
additional transfer route of the recycled polymer. Specifically referring to Fig. 1, the
hydrous gel phase polymer provided from a polymer feeder 12 is milled to a particle
size of 150 to 850pm, for example in a milling device for a polymer 10, and is classified
in a first classifying device 20. The fine powder having a particle size of less than
150 h may not be classified separately, and only particles having a particle size larger
than 850 h may be classified and returned to the milling device, unlike prior techniques.
Meanwhile, the first classifying device 20 may classify the polymer into two or
more grades by particle size according to process design, and Fig. 1 represents an
example o f classifying the polymer into particles having a particle size of less than
300 m and particles having a particle size of 300 to 850 m. The particles having a
particle size of less than 300 m are transferred to a conveyor belt equipped with a
second surface cross-linking agent sprayer 34 in Fig. 1, and the surface cross-linking
agent is sprayed thereto. Further, particles having a particle size of 300 to 850 m are
transferred to the conveyor belt equipped with a first surface cross-linking agent sprayer
32 in Fig. 1, and the surface cross-linking agent is sprayed thereto. At this time, the
constituents of the first surface cross-lining agent and the second surface cross-lining
agent are may be same or different according to the case.
As disclosed above, the polymer particles including the fine powder
agglomerate together and form granules in the step of adding the surface cross-linking
agent or in the step of the succeeding surface cross-linking reaction, and the amount of
fine powder having a particle size of less than a certain value (for example, a particle
size of less than 150/ ) and the content of the same included in the final product can be
significantly reduced.
Meanwhile, the surface cross-linking reaction after the step of adding the
surface cross-linking agent may be carried out by feeding the hydrous gel phase
polymers classified into two or more grades to one surface cross-linking reactor 40, as
illustrated in Fig. 1. Furthermore, the surface cross-linking reaction may be carried out
by feeding the hydrous gel phase polymers including the surface cross-linking agent and
classified into two or more grades to each surface cross-linking reactor, as illustrated in
Fig. 2.
Specifically, Fig. 2 represents the processes of adding the surface cross-linking
agent to the hydrous gel phase polymers classified into two or more grades, feeding
each hydrous gel phase polymer to a first surface cross-linking reactor 42 and a second
surface cross-linking reactor 44, and carrying out the surface cross-linking reaction.
Meanwhile, "hydrous gel phase polymer" is a polymer obtained by
polymerizing certain monomers, and it means a gel-type polymer including a certain
content of water. In the method of preparing a superabsorbent polymer, a hydrous gel
phase polymer having a moisture content of 40 to 80 weight% that is prepared by
thermal polymerizing or photo-polymerizing the monomer composition including the
water-soluble ethylene-based unsaturated monomer and the polymerization initiator
may be used.
Throughout the present specification, "moisture content" means the content of
moisture in the weight of the entire hydrous gel phase polymer, and specifically it
means the value of the weight of the dried polymer subtracted from the weight of the
hydrous gel phase polymer. Furthermore, the moisture content may be defined as the
value calculated by measuring the weight loss as water is evaporated from the polymer
during a drying process by elevating the temperature of the polymer through infrared
heating. At this time, the moisture content is measured by carrying out the drying
process with the drying condition of elevating the temperature from room temperature
to 180 °C and maintaining the temperature at 180 °C, wherein the total drying time is set
as 20 minutes including 5 minutes of a temperature increase step.
As disclosed above, the hydrous gel phase polymer may be prepared by thermal
polymerizing or photo-polymerizing the monomer composition including the watersoluble
ethylene-based unsaturated monomer and the polymerization initiator.
Furthermore, any monomer usually used to prepare a superabsorbent polymer
may be used as the water-soluble ethylene-based unsaturated monomer without
limitation. At least one selected from the group consisting of an anionic monomer and
a salt thereof, a nonionic hydrophilic monomer, and an amino group containing
unsaturated monomer and the quaternary compound thereof may be used.
As the specific example of the water-soluble ethylene-based unsaturated
monomer, at least one selected from the group consisting of an anionic monomer such
as acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic
acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-
(meth)acryloylpropane sulfonic acid, and 2-(meth)acrylamide-2-methyl propane
sulfonic acid, and salts thereof; a nonionic hydrophilic monomer such as
(meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-
hydroxypropyl(meth)acrylate, methoxy polyethylene glycol (meth)acrylate, and
polyethylene glycol (meth)acrylate; and an amino group-containing unsaturated
monomer such as (N,N)-dimethylaminoethyl(meth)acrylate and (N,N)-
dimethylaminopropyl(meth)acrylate, and a quaternary compound thereof may be
preferably used.
Preferably, acrylic acid or a salt thereof may be used as the water-soluble
ethylene-based unsaturated monomer, and there is an advantage that a superabsorbent
polymer having improved water absorptivity can be obtained by using the acrylic acid
or the salt thereof as the monomer.
Meanwhile, the monomer composition may include a certain amount of the fine
powder generated during the processes of preparing the superabsorbent polymer, for
example the polymer or resin powder having a particle size of less than 150/im.
Such polymer or resin powder having a particle size of less than 150/ m may be
added to the monomer composition before the polymerization reaction is started, or in
the first stage, the middle stage, or the last stage after the polymerization reaction is
started. At this time, the amount of the polymer or resin powder that can be added
thereto is not particularly limited, however the amount is preferably 1 to 10 parts by
weight per 100 parts by weight of the monomer included in the monomer resin
composition for preventing property deterioration of the superabsorbent polymer finally
prepared.
Meanwhile, the concentration of the water-soluble ethylene-based unsaturated
monomer in the monomer composition may be suitably determined by considering the
polymerization time and the reaction conditions, and it may preferably be 40 to 55
weight%. When the concentration of the water-soluble ethylene-based unsaturated
monomer is less than 40 weight%, it is disadvantageous in the aspect of economic
feasibility, and when the concentration is larger than 55 weight%, the milling efficiency
in the milling process of the polymerized hydrous gel phase polymer may become low.
Further, a general method may be used without limitation if the method can
prepare a hydrous gel phase polymer from such monomer composition by thermal
polymerization or photo-polymerization. Specifically, the polymerization method is
largely classified into the thermal polymerization and the photo-polymerization
according to the polymerization energy source, and the thermal polymerization may be
carried out in a reactor like a kneader equipped with agitating spindles and the photopolymerization
may be carried out in a reactor equipped with a movable conveyor belt,
however the polymerization methods disclosed above are just examples and the present
invention is not limited to or by said methods.
For example, the hydrous gel phase polymer obtained from the thermal
polymerization in a reactor like a kneader equipped with the agitating spindles
disclosed above by providing hot air thereto or heating the reactor may have the size of
centimeters or millimeters when it is discharged from the outlet of the reactor,
according to the type of agitating spindles equipped in the reactor. Specifically, the
size of the obtained hydrous gel phase polymer can be variously shown according to
the concentration of the monomer composition fed thereto, the feeding speed, and the
like, and the hydrous gel phase polymer of which the weight average particle diameter
is 2 to 50 mm can be generally obtained.
Furthermore, in the case of the photo-polymerization carried out with a reactor
equipped with a movable conveyor belt disclosed above, the obtained hydrous gel phase
polymer may be a sheet-type hydrous gel phase polymer having the same width as the
belt. At this time, the thickness of the polymer sheet may vary according to the
concentration of the monomer composition fed thereto and the feeding speed, and it is
preferable to provide the monomer composition so that a sheet-type hydrous gel phase
polymer having a width of 0.5 to 5cm is obtained. It is not preferable for the monomer
composition to be fed so that the thickness of the sheet-type polymer becomes too thin
because the production efficiency is low, and when the thickness of the sheet type
polymer is larger than 5cm, the polymerization reaction may not occur evenly
throughout the whole thickness due to its excessive thickness.
Meanwhile, the monomer composition includes a polymerization initiator, and
it may include a photo-polymerization initiator in the case of a photo-polymerization
method or a thermal polymerization initiator in the case of a thermal polymerization
method, according to the case. However, even in the case of the photo-polymerization
method, a thermal polymerization initiator may be additionally included because a
certain amount of heat is generated by irradiation of UV rays and the like and a certain
amount of heat is generated according to the progress of the exothermic polymerization
reaction.
Specifically, at least one initiator selected from the group consisting of a
persulfate-based initiator, an azo-based initiator, hydrogen peroxide, and ascorbic acid
may be used as the thermal polymerization initiator. More specific examples of the
persulfate-based initiator include sodium persulfate (Na2S20 ), potassium persulfate
( 2S20 ), ammonium persulfate ((NH4 2S208), and the like; and as examples of the
azo-based initiator, 2,2-azobis(2-amidinopropane) dihydrochloride, 2.2-azobis-(N,Ndimethylene)
isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-
azobis(2-[2-imidazolin-2-yl]propane) dihydrochloride, and 4,4-azobis-(4-cyanovaleric
acid) may be used. More various thermal polymerization initiators are well-disclosed
in "Principle of Polymerization" written by Odian, (Wiley, 1981), p203, however the
examples of the thermal polymerization initiator are not limited to or by these.
Meanwhile, at least one initiator selected from the group consisting of benzoin
ether, a dialkyl acetophenone, a hydroxyl alkylketone, phenyl glyoxylate, benzyl
dimethyl ketal, an acyl phosphine, and an a-aminoketone may be used as the photopolymerization
initiator. As the specific example of the acyl phosphine,
commercialized Lucirin® TPO, namely, 2,4,6-trimethyl-benzoyl-trimethyl phosphine
oxide, may be used. More various photo-polymerization initiators are well disclosed
in "UV Coatings: Basics, Recent Developments and New Applications" written by
Reinhold Schwalm, (Elsevier, 2007), pi 15, however the examples of the photopolymerization
initiator is not limited to or by these.
Meanwhile, the hydrous gel phase polymer obtained according to the thermal
polymerization or the photo-polymerization disclosed above passes through a drying
step, and it may further pass through a pulverizing step before the drying step for raising
the efficiency of the drying step, as occasion demands.
A general pulverizing device may be used in the pulverizing step before the
drying step without limitation if the device can be used for pulverizing the hydrous gel
phase resin, for example, any one or more devices 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 may be used.
In the pulverizing step before the drying step, the hydrous gel phase polymer
obtained according to the thermal polymerization or the photo polymerization disclosed
above may be pulverized so that the weight average particle diameter becomes 1mm to
15mm. When the weight average particle diameter is less than 1mm, the particles may
agglomerate and it is technically not easy to pulverize the hydrous gel phase polymer to
be less than 1mm due to its high moisture content. Furthermore, when the weight
average particle diameter is larger than 15mm, the increasing effect of the efficiency of
the succeeding drying step may be insignificant.
Meanwhile, in the pulverizing step before the drying step, the hydrous gel phase
polymer may stick to the surface of the pulverizing device because it has a high
moisture content. Accordingly, a certain additive may be added to the hydrous gel
phase polymer for raising the efficiency of the pulverizing step before the drying step.
The kind of the usable additive is not particularly limited, and for example, it
may be an anti-agglomeration agent for the fine powder such as steam, water, a
surfactant, an inorganic powder such as clay or silica, and the like; a thermal
polymerization initiator such as a persulfate-based initiator, an azo-based initiator,
hydrogen peroxide, ascorbic acid, and the like; or a cross-linking agent such as an
epoxy-based cross-linking agent, a diol-based cross-linking agent, a cross-linking agent
including 2-functional or poly-functional (3 or more -functional) acrylate, a monofunctional
compound including a hydroxyl group.
When the hydrous gel phase polymer obtained according to the thermal
polymerization or the photo-polymerization or the hydrous gel phase polymer
pulverized in the pulverizing step before the drying step passes through the drying step,
the drying temperature of the drying step may be 150 °C to 250 °C. Said "drying
temperature" may mean the temperature of the heating medium provided thereto for
drying, or the temperature of the drying reactor including the heating medium and the
polymer during the drying process.
When the drying temperature is lower than 150 °C, there is a concern that the
drying time becomes excessively long or the properties of the superabsorbent polymer
finally formed may be deteriorated, and when the drying temperature is higher than
250 °C, only the surface of the polymer is dried, and thus there is a concern that fine
powder may be generated and the properties of the superabsorbent polymer finally
formed may be deteriorated, The drying temperature may preferably be 150 °C to 200 °C,
and more preferably 160 °C to 180 °C.
The time for the drying step may be suitably controlled, considering the amount
or the properties of the superabsorbent polymer being prepared, the size of the reactor,
and so on, and the drying step may be carried out for 20 to 90 minutes, considering the
process efficiency.
Furthermore, any generally known method or device may be used in the drying
step without limitation if it can be used for drying the hydrous gel phase polymer, and
for example, the drying step may be carried out by a method of supplying hot air,
irradiating infrared rays, irradiating microwaves, irradiating ultraviolet rays, and the like.
When the drying step disclosed above is finished, the moisture content of the hydrous
gel phase polymer may be 0.1 to 10 weight%.
Meanwhile, the polymer obtained from the drying step may pass through a
certain milling step. The details of a milling device that can be used in the milling
step are not particularly limited, and for example, a pin mill, a hammer mill, a screw
mill, a roll mill, a disc mill, a jog mill, and the like may be used. The polymer
powder obtained from the milling step may have a weight average particle diameter of
150 to 850 im.
Meanwhile, s classifying step for obtaining the milled polymer of which the
weight average particle diameter is 150 to 850/im may be additionally carried out before
the step of milling the polymer obtained from the drying step and adding the surface
cross-linking agent thereto. The properties of the superabsorbent polymer powder
finally manufactured can be properly controlled through such classifying step, and only
a polymer powder having a weight average particle diameter of 150 to 850 im obtained
in the classifying step is selectively applied to the surface cross-linking reaction and
finally manufactured. In the classification step, a conventional device or method using
vibration may be used without limitation, and for example, a device for separating
particles of a specific size by using a fluidized bed and a cyclone may be used.
Meanwhile, a surface cross-linking agent may be added for the surface crosslinking
of the milled polymer powder. The surface cross-linking agent is not
particularly limited as long as it is a compound that can react to the functional group of
the milled polymer, and for preferable examples, there are a polyhydric alcohol
compound, an epoxy compound, a polyamine compound, a haloepoxy compound, a
condensation product of the haloepoxy compound, an oxazoline compound, a mono-,
di-, or polyoxazolidinone compound, a cyclic urea compound, a polyvalent metal salt,
an alkylene carbonate compound, and a mixture of two or more of said compounds.
More specifically, as examples of the polyhydric alcohol compound, there is a
mono-, di-, tri-, tetra-, or polyethylene glycol, monopropylene glycol, 1,3-propanediol,
dipropylene glycol, 2,3,4-trimethyl-l,3-pentanediol, polypropylene glycol, glycerol,
polyglycerol, 2-butene-l,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-
hexanediol, 1,2-cyclohexane dimethanol, or a mixture of two or more of said
compounds.
As examples of the epoxy compound, ethylene glycol diglycidyl ether and
glycidol may be used, and as the polyamine compound, there is ethylene diamine,
diethylene triamine, tnethylene triamine, tetraethylene pentamine, pentaethylene
hexamine, polyethylene amine, polyamide polyamine, or a mixture of two or more of
said compounds.
Meanwhile, epichlorohydrin, epibromohydrin, and a-methylephichlorohydrin
may be used as the haloepoxy compound. 2-oxazolidinone may be used as the mono-,
di-, or polyoxazolidinone compound. Ethylene carbonate may be used as the alkylene
carbonate compound.
In order to raise the efficiency of the surface cross-linking reaction process, it
is preferable to use one or more polyhydric alcohol compounds among said crosslinking
agent, and it is more preferable to use a C2-Ci0 polyhydric alcohol compound.
The amount of the surface cross-linking agent used may be suitably controlled
according to the kind of surface cross-linking agent, the characteristics of the milled
polymer, or the surface cross-linking reaction conditions, and the amount may be 0.001
to 5 parts by weight, preferably 0.01 to 3 parts by weight, and more preferably 0.05 to
2 parts by weight per 100 parts by weight of the milled polymer. When the amount of
the surface cross-linking agent used is too small, the surface cross- linking reaction may
not occur practically, and when the surface cross-linking agent is used excessively, the
absorptivity and the properties of the final product may be decreased due to excessive
surface cross-linking reaction.
The method of adding the surface cross-linking agent to the milled polymer is
not particularly limited, and for example, a method of feeding the surface cross-linking
agent and the milled polymer powder to the reactor and mixing the same, spraying the
surface cross-linking agent to the polymer powder, or mixing the milled polymer
powder and the cross-linking agent while continuously feeding the same to a mixer
being continuously operated may be used.
At this time, the surface cross-linking agent may be added by mixing it with
additional water. When water is added to the surface cross-linking agent, the surface
cross-linking agent can be evenly dispersed in the polymer. The amount of water
added thereto may be 0.5 to 10 parts by weight per 100 parts by weight of the milled
polymer for the purpose of inducing uniform dispersion of the surface cross-linking
agent, preventing agglomeration of the polymer powder, and optimizing the surface
penetrating depth of the cross-linking agent at the same time.
Meanwhile, the surface temperature of the milled polymer may be 20 to 90 °C,
and preferably 50 to 80 °C, in the step of adding the surface cross-linking agent. When
the surface temperature of the milled polymer is maintained in the above range, the
surface temperature can be elevated to the surface cross-linking reaction temperature
within 1 minute to 60 minutes, and thus a proper surface cross-linking reaction can
occur and appropriate properties of the final product can be secured. In order for the
milled polymer to have the temperature of said range, the succeeding process may be
continuously carried out after the drying step that is carried out at a relatively high
temperature, the later process time is reduced, or the polymer is heated separately when
it is difficult for the process time to be reduced.
Furthermore, in addition to the method of maintaining or controlling the surface
temperature of the milled polymer in a proper range, the temperature of the surface
cross-linking agent itself added thereto may be controlled to be 5 to 90 °C, preferably
10 °C to 60 , and more preferably 20 to 40 °C. When the temperature of the surface
cross-linking agent is lower than 5 °C, the effect of reducing the temperature increase
speed influencing the surface cross-linking reaction according to the temperature
increase may be insignificant, and when the temperature of the surface cross-linking
agent is higher than 90 °C, uniform mixing of the surface treating agent may be
disturbed. Throughout the present specification, the surface cross-linking reaction
temperature is defined as the total temperature of the surface cross-linking agent added
for the cross-linking reaction and the polymer.
Meanwhile, various means for temperature increase may be used in the step of
elevating the temperature of the polymer to which the surface cross-linking agent is
added for the surface cross-linking reaction, and for example, the temperature increase
may be carried out by providing a heating medium or by directly providing a heat
source.
As a specific example of the heating medium, a hot fluid such as steam, hot air,
hot oil, and the like may be used, however the specific example is not limited to these.
The temperature of the heating medium may be properly controlled by considering the
means of the heating medium, the temperature increase speed, and the target
temperature. As an example of the heat source provided directly, an electric heater or
a gas heater may be used, but it is not limited to these. However, it is preferable that
the heating medium has a temperature of 100 °C or more and that the heat source
provides thermal energy at a temperature of 100 °C or more, considering that the surface
cross-linking reaction temperature disclosed above is 100 to 250 °C.
The surface cross-linking reaction may be carried out within a certain time,
considering the cross-linking reaction temperature, the characteristics and the amount of
the reactants, or the reaction conditions, and the cross-linking reaction may be carried
out for 1 minute to 120 minutes, preferably for 1 minute to 60 minutes, and more
preferably for 10 minutes to 50 minutes, after the temperature increase to the crosslinking
reaction temperature is completed. When the reaction time is shorter than 1
minute, the cross-linking reaction cannot be sufficiently obtained, and when the crosslinking
time is longer than 20 minutes, property deterioration may occur because the
polymer particles are damaged by excessive surface cross-linking reaction.
According to the method of preparing the superabsorbent polymer disclosed
above, the process efficiency and the work environment can be improved by minimizing
fine powder generation, and the superabsorbent polymer having high water holding
capacity and absorbing power under pressure (AUP) while having a low content of
water-soluble component can be provided.
The problems with generating a lot of fine powder in the preparation method of
superabsorbent polymer are that the work environment becomes poor, and the efficiency
of the process and the quality of the superabsorbent polymer finally manufactured are
decreased by the fine powder. However, according to the method of preparing
superabsorbent polymer disclosed above, the process efficiency and the work
environment can be improved and the property and the quality of the final product can
be enhanced because the amount of the fine powder generated in the preparing process
can be significantly reduced.
Furthermore, the superabsorbent polymer prepared by the preparation method
shows a very low content of monomer residue of 0.05 parts by weight per 100 parts by
weight of the high absorbent polymer, and thus it is possible to realize high stability
when the final product is applied in practice and a final product of excellent quality can
be provided. Furthermore, the superabsorbent polymer provided according to the
preparation method may have water holding capacity measured according to the
EDANA WSP 241.2 method of 30g/g to 60g/g, and water-soluble component measured
according to the EDANA WSP 270.2 method of 15 weight% or less.
As disclosed above, since the preparation method of the present invention does
not need the additional process for recycling the fine powder polymer, the process
efficiency can be improved and the superabsorbent polymer having excellent properties
can be prepared.
Hereinafter, the function and effect of the invention are explained in more detail
through concrete examples of the present invention. However, the following examples
are only for illustrating the present invention, and the scope of the present invention is
not determined to or by them.
PREPARATION EXAMPLES: Preparation of Polymer Powder>
Preparation Example 1: Preparation of Polymer Powder (without recycling
fine powder)
A monomer composition of which the monomer concentration was 50 weight%
was prepared by mixing lOOg of acrylic acid, O.lg of polyethylene glycol diacrylate as a
cross-linking agent, 38. g of caustic soda (NaOH), and 103.9g of water.
Subsequently, the monomer composition was fed to a polymerizing reactor with
a continuously rotating kneader through a feed section, and then l g of a 1% hydrogen
peroxide solution and l g o f a 2% ascorbic aqueous solution were introduced thereto as
the polymerization initiator and mixed with the monomer.
The polymerization was started 1 minute after mixing the polymerization
initiator, and the polymerization reaction was carried out for 15 minutes. At this time,
the internal temperature of the reactor was 99 °C. The polymerized hydrous gel phase
polymer was transferred to a cutter and cut to a diameter of 0.2cm. The moisture
content of the cut hydrous gel phase polymer was 50%.
Thereafter, the polymer was obtained by drying the polymer discharged from
the cutter with a hot air dryer of 180 °C for 1 hour and milling the same with a pin mill.
The obtained polymer was classified into a polymer having a particle size o f less than
150 m and a polymer having a particle size of 150/L to 850 h by using a sieve.
The polymer powder classified in like way showed water holding capacity o f
45g/g and a content of water-soluble component of 13%.
Preparation Example 2 : Preparation of Polymer Powder (while recycling
fine powder)
Monomer Composition 1 of which the monomer concentration was 50 weight%
was prepared by mixing lOOg of acrylic acid, O.lg of polyethylene glycol diacrylate as a
cross-linking agent, 38.9g of caustic soda (NaOH), and 103.9g of water.
Monomer Composition 2 was prepared by feeding the fine powder having a
particle size of less than 15 0 m prepared in Preparation Example 1 to Monomer
Composition 1 at a ratio of 14 parts by weight per 100 parts by weight of the monomer
(acrylate formed from acrylic acid and NaOH).
Subsequently, Monomer Composition 2 in which the fine powder was dispersed
was fed to a polymerizing reactor of a continuously rotating kneader through a feed
section, and then l g of a 1% hydrogen peroxide solution and l g of a 2% ascorbic
aqueous solution were introduced thereto as the polymerization initiator and mixed with
the monomer.
The polymerization was started 1 minute after mixing the polymerization
initiator, and the polymerization reaction was carried out for 15 minutes. At this time,
the internal temperature of the reactor was 99 °C. The polymerized hydrous gel phase
polymer was transferred to a cutter and cut to a diameter of 0.2cm. The moisture
content of the polymer was 49%.
There after, the polymer was obtained by drying the polymer discharged from
the cutter in a hot air dryer at 180 °C for 1 hour and milling the same with a pin mill.
The obtained polymer was classified into a polymer having a particle size of less than
150 m and a polymer having a particle size of 150 m to 850 m by using a sieve.
The polymer powder classified in this way showed water holding capacity o f
40g/g and a content of water-soluble component of 18%.
Preparation Example 3: Preparation of Polymer Powder (while recycling
fine powder)
A monomer composition of which the monomer concentration was 50 weight%
was prepared by mixing lOOg of acrylic acid, O.lg of polyethylene glycol diacrylate as a
cross-linking agent, 38.9g of caustic soda (NaOH), and 103.9g of water.
Subsequently, the monomer composition was fed to a polymerizing reactor with
a continuously rotating kneader through a feed section, and then 1g of a 1% hydrogen
peroxide solution and l g of a 2% ascorbic aqueous solution were introduced thereto as
the polymerization initiator and mixed with the monomer.
The polymerization was started 1 minute after mixing the polymerization
initiator, and the polymer (fine powder) having a particle size of less than 150 LP
prepared in Preparation Example l was fed into the kneader reactor at a ratio of 14 parts
by weight per 100 parts by weight of the monomer (acrylate formed from acrylic acid
and NaOH) 5 minutes after starting the polymerization and mixed, and then the
polymerization was further carried out for 10 minutes. At this time, the internal
temperature of the reactor was 99 °C . The polymerized hydrous gel phase polymer was
transferred to a cutter and cut to a diameter of 0.2cm. The moisture content of the cut
hydrous gel phase polymer was 48%.
Thereafter, the polymer was obtained by drying the discharged polymer with a
hot air dryer of 180 °C for 1 hour and milling the same with a pin mill. The obtained
polymer was classified into a polymer having a particle size of less than \ 5 bΐίί and a
polymer having a particle size o f 150 m to 850 m by using a sieve.
The polymer powder classified in this way showed water holding capacity o f
43g/g and a content o f water-soluble component of 15%.

Example 1: Preparation of Superabsorbent Polymer (the surface crosslinking
reaction is carried out in one surface cross-linking reactor after the surface
cross-linking agent is sprayed to each polymer classified into two grades)
Among 100 parts by weight of the polymer classified according to Preparation
Example 1, polymer A classified into the particle size of less than 150j¾m was 14 parts
by weight and polymer B classified into the particle size of 150 to 850 m was 86 parts
by weight. The solution including 1.0 part by weight of 1,3-propanediol and 1.0 part
by weight of water per 100 parts by weight of the polymer was sprayed to and mixed
with each polymer A and B classified by particle size.
The polymers A and B to which the surface cross-linking agent (1,3-
propanediol) and water were sprayed were inserted into one surface cross-linking
reactor of which the temperature was increased by heated oil and electricity together,
and the surface cross-linking reaction was carried out at 180 °C for 20 minutes. The
superabsorbent polymer powder was obtained by passing through an additional cooling
step after the surface cross-linking reaction.
Example 2 ; Preparation of Superabsorbent Polymer (the surface crosslinking
reactions are carried out in two surface cross-linking reactors after the
surface cross-linking agent is sprayed to each polymer classified into two grades)
The superabsorbent polymer powder was obtained substantially according to
the same method as in Example 1, except that each polymer A and B classified into the
particle size of less than 150j¾m and the particle size of 150 to 850 m according to
Preparation Example 1 was separately inserted into two surface cross-linking reactors of
which the temperature was increased by heated oil and electricity and the surface crosslinking
reaction was carried out.
Example 3 ; Preparation of Superabsorbent Polymer (the surface crosslinking
reactions are carried out in one surface cross-linking reactor after the
surface cross-linking agent is sprayed to each polymer classified into three grades)
The polymer prepared according to Preparation Example 1 was further
classified, and polymer A having the particle size of less than 150 m, polymer B having
a particle size of 150 h or more and less than 300 m, and polymer C having a particle
size of 300 im to 850 m were prepared.
Among 100 parts by weight of total polymer, polymer A classified into the
particle size less than 150j¾m was 14 parts by weight, polymer B classified into the
particle size of 150 m or more and less than 300 h was 16 parts by weight, and
polymer C classified into the particle size of 300 m to 850 was 70 parts by weight.
The solution including 1.0 part by weight of 1,3-propanediol and 1.0 part by
weight of water was sprayed to and mixed with 100 parts by weight o f each polymer A,
B, and C classified by particle size.
The polymers A, B, and C to which the surface cross-linking agent (1,3-
propanediol) and water were sprayed were inserted into one surface cross-linking
reactor of which the temperature was increased by heated oil and electricity together,
and the surface cross-linking reaction was carried out at 180 °C for 20 minutes.
The superabsorbent polymer powder was obtained by passing through an
additional cooling step after the surface cross-linking reaction.
Example 4: Preparation of Superabsorbent Polymer (the surface crosslinking
reactions are carried out in one surface cross-linking reactor after the
surface cross-linking agent is sprayed to each polymer classified into three grades)
The polymer prepared according to Preparation Example 2 was further
classified, and polymer A having a particle size of less than 150j«m, polymer B having a
particle size of 150 m or more and less than 300 i, and polymer C having a particle
size of 300 m to 85 0 m were prepared.
Among 100 parts by weight of total polymer, polymer A classified into the
particle size of less than 150 im was 14 parts by weight, polymer B classified into the
particle size of 150 ih or more and less than 300 m was 16 parts by weight, and
polymer C classified into the particle size o f 300 to 850 hi was 70 parts by weight.
The solution including 1.0 part by weight o f 1,3-propanediol and 1 part by
weight of water was sprayed to and mixed with 100 parts by weight of each polymer A,
B, and C classified by particle size.
The polymers A, B, and C to which the surface cross-linking agent (1,3-
propanediol) and water were sprayed were inserted into one surface cross-linking
reactor of which the temperature was increased by heated oil and electricity together,
and the surface cross-linking reaction was carried out at 180 °C for 20 minutes.
The superabsorbent polymer powder was obtained by passing through an
additional cooling step after the surface cross-linking reaction.
Example 5 : Preparation of Superabsorbent Polymer (the surface crosslinking
reactions are carried out in one surface cross-Unking reactor after the
surface cross-Unking agent is sprayed to each polymer classified into three grades)
The polymer prepared according to Preparation Example 3 was further
classified, and polymer A having a particle size of less than 150 p , polymer B having a
particle size of 150/ or more and less than 300/im, and polymer C having a particle
size of 300j¾m to 850 i were prepared.
Among 100 parts by weight of total polymer, polymer A classified into the
particle size of less than 150 h was 14 parts by weight, polymer B classified into the
particle size of 150j¾m or more and less than 300 m was 16 parts by weight, and
polymer C classified into the particle size of 300 ih to 850j¾m was 70 parts by weight.
The solution including 1.0 part by weight of 1,3-propanediol and 1 part by
weight of water was sprayed to and mixed with 100 parts by weight of each polymer A,
B, and C classified by particle size.
The polymers A, B, and C to which the surface cross-linking agent (1,3-
propanediol) and water were sprayed were inserted to one surface cross-linking reactor
of which the temperature was increased by heated oil and electricity together, and the
surface cross-linking reaction was carried out at 180 °C for 20 minutes.
The superabsorbent polymer powder was obtained by passing through an
additional cooling step after the surface cross-linking reaction.
COMPARATIVE EXAMPLES>
Comparative Example 1: Preparation of Superabsorbent Polymer
The superabsorbent polymer powder was obtained by carrying out the surface
cross-linking process substantially according to the same method as in Example 1,
except that the polymer having a particle size of less than 150 ih (14 weight% o f total
polymer) and the polymer having a particle size of 150 to 850 m (86 weight% of total
polymer) prepared according Preparation Example 1 were mixed together and the
surface cross-linking reaction was carried out by spraying the solution including the
surface cross-linking agent to the mixed polymer.
Comparative Example 2 : Preparation of Superabsorbent Polymer
The superabsorbent polymer powder was obtained by carrying out the surface
cross-linking process substantially according to the same method as in Example 1,
except that the surface cross-linking reaction was carried out by spraying the solution
including the surface cross-linking agent only to the polymer having the particle size o f
150 to 850j¾m (86 weight% o f total polymer) prepared according Preparation Example 2.
Comparative Example 3 : Preparation of Superabsorbent Polymer
The superabsorbent polymer powder was obtained by carrying out the surface
cross-linking process substantially according to the same method as in Example 1,
except that the polymer having a particle size of less than 150 pi (14 weight% of total
polymer) and the polymer having a particle size of 150 to 850/im (86 weight% of total
polymer) prepared according Preparation Example 2 were mixed together and the
surface cross-linking reaction was carried out by spraying the solution including the
surface cross- linking agent to the mixed polymer.
Comparative Example 4: Preparation of Superabsorbent Polymer
The superabsorbent polymer powder was obtained by carrying out the surface
cross-linking process substantially according to the same method as in Example 1,
except that the surface cross-linking reaction was carried out by spraying the solution
including the surface cross-linking agent only to the polymer having a particle size of
150 to 850 im (86 weight% of total polymer) prepared according Preparation Example 3.
Comparative Example 5: Preparation of Superabsorbent Polymer
The superabsorbent polymer powder was obtained by carrying out the surface
cross-linking process substantially according to the same method as in Example 1,
except that the polymer having a particle size of less than 150/ini (14 weight% of total
polymer) and the polymer having a particle size of 150 to 850//ni (86 weight% of total
polymer) prepared according Preparation Example 3 were mixed together and the
surface cross-linking reaction was carried out by spraying the solution including the
surface cross-linking agent to all of the mixed polymer.
EXPERIMENTAL EXAMPLES: Evaluation on Properties of
Superabsorbent Polymer>
Experimental Example 1: Measurement of Water Holding Capacity of
Polymer
The water holding capacity of the superabsorbent polymers obtained in the
examples and comparative examples was measured according to the EDANA WSP
241.2 method.
Specifically, after inserting 0.2g of the specimen having the particle size of 850
to 150 m among the superabsorbent polymer powders of the examples and comparative
examples in a tea bag and soaking the same in a 0.9% salt water solution for 30 minutes,
the water holding capacity was measured by eliminating water from the specimen for 3
minutes by using a centrifugal separator set to 250G and weighing the specimen so as to
determine the amount of water held in the superabsorbent polymer.
Experimental Example 2: Measurement of Content of Water-soluble
Component of Superabsorbent Polymer
The content of the water-soluble component of the superabsorbent polymers
obtained in the examples and comparative examples was measured according to the
EDANA WSP 270.2 method.
Specifically, after inserting 1.0g of the specimen having the particle size of 850
to \ 5 [ among the superabsorbent polymer powders of the examples and comparative
examples in 200g of a 0.9% salt water solution and soaking the same while stirring at
500rpm for 16 hours, the aqueous solution was filtered with a filter paper. The
solution filtered in this way primarily titrated to pH 10.0 with a 0.1N caustic soda
solution and then it was counter-titrated to pH 2.7 with a 0. hydrogen chloride
solution, and the amount o f polymer material not cross-linked was calculated from the
amount needed for neutralization.
Experimental Example 3: Measurement of Absorbing Power Under
Pressure of Superabsorbent Polymer
The absorbing power under pressure o f the superabsorbent polymers obtained
in the examples and comparative examples was measured according to the EDANA
WSP 242.2 method. Specifically, after uniformly distributing 0.9g of the specimen
having the particle size of 850 to 150j¾m among the superabsorbent polymer powders o f
the examples and comparative examples in a cylinder regulated in the EDANA method
and pressing the specimen with a pressure of 21g/cm2 by using a piston and a weight,
the absorbing power under pressure was calculated as the amount o f the 0.9% salt water
solution that was absorbed in the specimen for 1 hour.
Experimental Example 4: Measurement of Content of Fine Powder in
Final SAP powder
The weight ratio of the powder having the particle size of less than \ 50
among the superabsorbent polymer prepared by above method was measured. The
content was measured as a ratio o f the powder remaining in the upper part of sieves by
shaking the sieves having a mesh size of 850j¾m, 600 i, 300 m and 150/zm with a
frequency o f 1.0mm for 10 minutes according to the EDANA WSP 220.0 method.
The results o f Experimental Examples 1 to 4 are listed in the following Table 1.
[Table 1] Results of Experimental Examples 1 to 4
Example 1 36.4 8.6 25.2 0.7
Example 2 36. 1 8.9 25.6 1.1
Example 3 36.0 8.0 26.3 0.8
Example 4 34.2 15.3 24.8 0.7
Example 5 35.2 13.7 25.3 0.9
Comparative 37.6 12.4 23.4 1.6
Example 1
Comparative 34.2 15.4 23.6 0.8
Example 2
Comparative 34.8 16.2 2 1.7 1.4
Example 3
Comparative 34.4 13.9 23.3 0.7
Example 4
Comparative 35. 1 14.2 22.8 1.7
Example 5
From the results disclosed above, it is recognized that when the surface crosslinking
reaction is carried out by spraying the solution including the surface crosslinking
agent to each group of polymer having different particle sizes, the surface crosslinking
agent can be uniformly distributed to the whole polymer and the surface crosslinking
can be uniformly and effectively carried out, and thus high water holding
capacity and absorbing power under pressure can be secured while lowering the content
of the water-soluble component.
Specifically, the superabsorbent polymer of the examples can significantly
reduce the content of the water-soluble component and can secure water holding
capacity and absorbing power under pressure that are equal or superior to the
superabsorbent polymer of the comparative examples. Particularly, in the examples,
the fine powders having the particle size of less than 50 m agglomerate together and
form granules, and the amount of the fine powder generated in the process and the
amount of the fine powder included in the final product were reduced by treating the
one group of particles having the particle size of less than 150/ II with the surface crosslinking
agent individually.
The water holding capacity of the superabsorbent polymer relates to the
evaluation of the moisture absorbing performance, that is, it relates to basic performance
of the superabsorbent polymer, and the water-soluble component relates to the content
of the component that is soluble in water in the superabsorbent polymer, for example, to
the content of a low molecular weight polymer component.
Generally, the property of the superabsorbent polymer can be evaluated as
being superior when the water holding capacity and the absorbing power under pressure
are higher, and when the superabsorbent polymer is applied to consumer goods like a
diaper, there is less displeasure of a user due to wetness, and the property of the
superabsorbent polymer can be evaluated as being superior when the amount of the
water-soluble component is lower. However, it is generally known that the higher the
water holding capacity, the higher the content of the water-soluble component, and there
have been difficulties in improving overall properties of the superabsorbent polymer.
However, it is recognized that the superabsorbent polymer prepared in
Examples 1 to 3 can maintain the low content of the water-soluble component while
having high water holding capacity and absorbing power under pressure, and the
content of the fine powder included in the final product is low. Furthermore, in the
case of surface treating the superabsorbent polymers prepared in Examples 4 and 5 after
classifying the same by particle size, they show superior properties to the case without
classifying the same.
[Explanation of Numbers]
10: Milling device for polymer
12; Polymer feeder
20: First classifying device
32: First surface cross-linking agent sprayer
34: Second surface cross-linking agent sprayer
40: Surface cross-linking reactor
42: First surface cross-linking reactor
44: Second surface cross-linking reactor
50: Second classifying device

WHAT IS CLAIMED IS:
1. A method o f preparing a superabsorbent polymer, including the steps of:
preparing a hydrous gel phase polymer by thermal polymerizing or photopolymerizing
a monomer composition including a water-soluble ethylene-based
unsaturated monomer and a polymerization initiator;
drying the hydrous gel phase polymer;
milling the dried polymer;
classifying the milled hydrous gel phase polymer into two or more grades by
particle size;
adding a surface cross- linking agent to each hydrous gel phase polymer
classified into two or more grades; and
carrying out a surface cross-linking reaction of the hydrous gel phase polymer
to which the surface cross-linking agent is added.
2. The method o f preparing a superabsorbent polymer according to Claim 1,
wherein the milling step of the dried polymer is carried out so that the particle size of
the milled polymer is 150 to 850
3. The method o f preparing a superabsorbent polymer according to Claim 1,
wherein the classifying step is carried out into two grades of the particles having a
particle size of less than 150 i and the particles having a particle size o f 150 m to
850 .
4. The method of preparing superabsorbent polymer according to Claim 1,
wherein the classifying step is carried out into three grades of particles having a particle
size of less than 150j¾m, particles having a particle size of 15 0 m or more and less than
300 i, and particles having a particle size o f 3 0 0 m to 850 m.
5. The method o f preparing superabsorbent polymer according to Claim 1,
wherein the classifying step is carried out into four grades o f particles having a particle
size o f less than 150/ , particles having a particle size o f 150/^m or more and less than
30 0 m, particles having a particle size o f 300/ m or more and less than 600//m, and the
particles having a particle size o f 600 ni to 850/ini.
6. The method o f preparing a superabsorbent polymer according to Claim 1,
further including the step o f feeding the hydrous gel phase polymer classified into two
or more grades to a surface cross-linking reactor, after the step o f adding the surface
cross-linking agent.
7. The method o f preparing a superabsorbent polymer according to Claim 1,
further including the step o f classifying the polymer into the particles having a particle
size o f 150 to 85 0 m, after the step o f surface cross-linking reaction.
8. The method o f preparing a superabsorbent polymer according to Claim 1,
further including the step o f pulverizing the hydrous gel phase polymer to have the
particle size o f 1 to 15mm, before the drying step of the hydrous gel phase polymer.
9. The method o f preparing a superabsorbent polymer according to Claim 1,
wherein the drying step o f the hydrous gel phase polymer is carried out at a temperature
o f 150 °C to 250 °C.
10. The method o f preparing a superabsorbent polymer according to Claim 1,
wherein the surface cross-linking agent is at least one selected from the group consisting
o f a polyhydric alcohol compound; an epoxy compound; a polyamine compound; a
haloepoxy compound; a condensation product o f the haloepoxy compound; an
oxazoline compound; a mono-, di-, or polyoxazolidinone compound; a cyclic urea
compound; a polyvalent metal salt; and an alkylene carbonate compound.
11. The method of preparing a superabsorbent polymer according to Claim 1,
wherein 0.001 to 5 parts by weight of the surface cross-linking agent is added to 100
parts by weight of the milled polymer.
12. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the surface temperature of the milled polymer is 20 to 90 °C in the step of
adding the surface cross-linking agent.
13. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the temperature of the added surface cross-linking agent is 10 to 60 °C.
14. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the surface cross-linking reaction is carried out for 1 minute to 1 0 minutes.
15. The method of preparing a superabsorbent polymer according to Claim 1,
wherein a temperature increase for the surface cross-linking reaction is carried out by
providing one or more heat sources selected from the group consisting of steam,
electricity, ultraviolet rays, and infrared rays.
16. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the water holding capacity measured according to EDANA WSP 241 .2 method
is 30g/g to 40 g/g.
17. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the amount of water-soluble component of the superabsorbent polymer
measured according to the EDANA WSP 270.2 method is 13 weight% or less.
18. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the absorbing power under pressure measured by the EDANA WSP 242.2
method is 23g/g or more.