Technical Field
[0001] The present invention relates to a method of preparing a superabsorbent polymer
and a superabsorbent polymer prepared thereby and, more particularly, to a method of
preparing a superabsorbent polymer, which includes adding a water dispersion solution
containing particles having specific properties, and a superabsorbent polymer prepared
thereby.
Background Art
[0002] Superabsorbent polymers (SAPs) are synthetic polymers that are able to absorb
about 500 to 1000 times their own weight in water. Such superabsorbent polymers have
begun to be used in real-world applications for sanitary items, and are currently being
widely utilized not only in hygiene products, such as disposable baby diapers and the
like, but also in gardening soil repair agents, water stop materials for civil construction,
seeding sheets, freshness retaining agents in the field of food distribution, and
fomentation materials.
[0003] In the preparation of the superabsorbent polymer, water, which is a
polymerization medium, is used in various applications, including facilitating the
dispersion of the crosslinking solution during the surface crosslinking process, etc.
Also, residual moisture in the final product functions as an anti-static agent and a
plasticizer for resin, and plays a role in suppressing the generation of very small
superabsorbent polymer dust in the course of application and also preventing the
grinding of the superabsorbent polymer particles. Generally, however, when water is
added to the superabsorbent polymer, the surface stickiness of the polymer may be
increased by the water absorbed thereto, and irreversible agglomeration of the
superabsorbent polymer particles may take place. This increase in stickiness and
2
agglomeration may result in poor processability, imposing a burden on the preparation
and application processes, consequently increasing the particle size of the
superabsorbent polymer, deteriorating the properties thereof, and decreasing
productivity.
[0004] In this regard, Korean Patent Application Publication No. 2012-0081113
discloses a method of preparing an absorbent polymer containing water-insoluble
inorganic particles. However, this conventional technique is problematic because the
surface stickiness of the superabsorbent polymer increases with an increase in moisture
content on the surface thereof, undesirably incurring agglomeration, poor processability,
and low productivity, as mentioned above. Hence, there is required to develop
superabsorbent polymers, which may satisfy both high moisture content and high
processability.
[0005] In order to satisfy both high moisture content and high processability, porous
superhydrophobic particles in ultrafine powder form may be used. Porous
superhydrophobic particles in powder form have a very low bulk density of about 0.04
to 0.10 g/cm3, and thus the volume relative to the weight thereof is quite large,
undesirably increasing transport costs and the space necessary for storage. Furthermore,
it is not easy to add such particles in fixed amounts upon mass production and the
likelihood of scattering the particles in the air is high, undesirably deteriorating
workability and endangering the health of workers.
[0006] Accordingly, there is a need for superabsorbent polymers containing
microparticles that have both high moisture content and high processability and are
available in a liquid phase so as to facilitate the handling thereof, thereby increasing
economic efficiency and workability.
Disclosure
Technical Problem
[0007] The present invention has been made keeping in mind the above problems
encountered in the related art, and an object of the present invention is to provide a
method of preparing a superabsorbent polymer, which includes adding a water
dispersion solution containing microparticles having high transportability, storage
capability and workability, in order to produce a superabsorbent polymer that satisfies
3
both high moisture content and high processability, and a superabsorbent polymer
having improved properties prepared thereby.
Technical Solution
[0008] In order to accomplish the above object, the present invention provides a method
of preparing a superabsorbent polymer, comprising adding a hydrous gel polymer,
produced by polymerizing a monomer composition comprising a water-soluble ethylenic
unsaturated monomer and a polymerization initiator, with a water dispersion solution
containing particles having i) a BET specific surface area of 300 to 1500 m2/g and ii) a
porosity of 50% or more.
[0009] In addition, the present invention provides a superabsorbent polymer, prepared
by the above method.
Advantageous Effects
[0010] According to the present invention, the method of preparing a superabsorbent
polymer includes adding a water dispersion solution containing microparticles to the
superabsorbent polymer, thus obtaining the advantages of the use of microparticles, such
as high moisture content, high processability, attrition resistance, and anti-caking
properties, and overcoming the drawbacks of the use of microparticles, namely difficulty
in storage and transport, high scattering properties, and poor workability.
Description of Drawings
[0011] FIG. 1 illustrates the variation in particle size, ranging from 300 to 600 􀁐m, after
ball milling of the superabsorbent polymers of Comparative Example 1 and Examples 1
to 3; and
[0012] FIG. 2 illustrates the permeability of the superabsorbent polymers of
Comparative Example 1 and Examples 1 to 3, both before and after ball milling.
Best Mode
[0013] The present invention addresses a method of preparing a superabsorbent
polymer. The method of preparing a superabsorbent polymer according to the present
invention enables the production of a superabsorbent polymer having high moisture
content, high processability, and attrition resistance, using a water dispersion solution
4
containing microparticles, which are easy to store and transport and have good
workability.
[0014] Hereinafter, a detailed description will be given of the present invention.
[0015] The method of preparing the superabsorbent polymer according to the present
invention comprises adding a hydrous gel polymer, produced by polymerizing a
monomer composition comprising a water-soluble ethylenic unsaturated monomer and a
polymerization initiator, with a water dispersion solution containing particles. The
particles have either i) a BET specific surface area of 300 to 1500 m2/g or ii) a porosity
of 50% or more.
[0016] As used herein, the term "particles" may refer to porous or superhydrophobic
microparticles, and may be taken to have the same meaning as "microparticles".
[0017] In an embodiment of the present invention, the particles preferably have a
particle size ranging from 2 nm to 50 􀁐m, or have superhydrophobicity with a water
contact angle of 125􀁱 or more. More preferably, the particles have both the particle size
and the contact angle in the ranges as described above, but the present invention is not
limited thereto.
[0018] In an embodiment of the present invention, the water dispersion solution may
comprise the particles, water, and an organic solvent, and the organic solvent may be at
least one selected from the group consisting of methanol, ethanol, isopropyl alcohol
(IPA), and acetone. Particularly useful is isopropyl alcohol (IPA).
[0019] Typically, a superabsorbent polymer has a hydrophilic surface, and irreversible
agglomeration thereof may occur due to capillary force, hydrogen bonding, or interparticular
van der Waals force, attributable to water present between the particles upon
drying after water absorption. Hence, water is essentially used in the course of
polymerization and surface crosslinking of the superabsorbent polymer, and thereby
agglomeration occurs, thus increasing internal load, ultimately incurring damage to the
system. Furthermore, since the agglomerated superabsorbent polymer has a large
particle size, which is unsuitable for use in practice, a disintegration process has to be
implemented so that the large particle size is suitably decreased. Also, strong force is
applied during the disintegration process, undesirably deteriorating the properties of the
superabsorbent polymer, attributable to attrition.
5
[0020] In order to solve these problems, attempts have been made to introduce a variety
of microparticles, which function to prevent direct agglomeration of the polymer
particles, to the surface of the superabsorbent polymer. In the case where the
microparticles are added in an excessive amount, agglomeration may be prevented, but
the absorption performance of the superabsorbent polymer may decrease.
[0021] To solve such problems, the microparticles introduced to the superabsorbent
polymer according to the present invention have a particle size ranging from 2 nm to 50 􀁐m. Also, the microparticles have a BET specific surface area of 300 to 1500 m2/g,
preferably 500 to 1500 m2/g, and more preferably 700 to 1500 m2/g. The microparticles
have superhydrophobicity with a water contact angle of 125􀁱 or more, preferably 140􀁱 or
more, and more preferably 145􀁱 or more. Furthermore, the microparticles have a
porosity of 50% or more, and preferably 90% or more. In the method of preparing the
superabsorbent polymer according to the present invention, the use of the microparticles
as described above may decrease the effect of water present on the surface of the
polymer, and furthermore, may remarkably reduce agglomeration. Even when a
relatively small amount of particles is used, permeability may be easily increased, and
high water content and maintenance thereof may be readily ensured.
[0022] In the present invention, the material for particles is not limited so long as it has
the above i) and ii) properties, and examples thereof may include, but are not limited to,
inorganic oxides, such as silica (SiO2), alumina, carbon, and titania (TiO2), inorganic
compounds, organic polymers, ion exchange resins, metals, metal salts, etc. Preferably
useful is silica (SiO2).
[0023] The particles are used in an amount of 1 to 25 parts by weight based on 100 parts
by weight of the mixture comprising water and organic solvent. When the amount of the
particles falls in the above range, dispersion is efficiently carried out, and there is no
gelling due to unstable dispersion upon long-term storage. Hence, an adjuvant for
preventing the gelling, such as an additive, a pH controller, a surfactant, or a stabilizer,
need not be used, and thus the inherent superhydrophobic and porous properties of the
particles may be maintained in the drying process. Accordingly, there is no need to
remove the adjuvant through pre-treatment, and the particles may be directly fed to the
process.
[0024] After the addition of the water dispersion solution, the mixing is preferably
6
performed at a rate of 200 to 3000 rpm. If the mixing rate is less than 200 rpm, a
sufficient mixing effect cannot be obtained. In contrast, if the mixing rate exceeds 3000
rpm, excessive grinding may occur.
[0025] After the addition of the water dispersion solution containing the particles to the
hydrous gel polymer, the mixing is preferably performed for a period of time ranging
from 10 sec to 3 min. If the mixing time is less than 10 sec, a sufficient mixing effect
cannot be obtained. On the other hand, if the mixing time exceeds 3 min, excessive
grinding may occur.
[0026] The method of preparing the superabsorbent polymer according to the present
invention includes polymerizing the monomer composition comprising the water-soluble
ethylenic unsaturated monomer and the polymerization initiator, thus preparing the
hydrous gel polymer.
[0027] In the preparation of the superabsorbent polymer according to the present
invention, the above polymer may be prepared by steps and methods typically used in
the art. Specifically, upon preparation of the superabsorbent polymer according to the
present invention, the monomer composition includes a polymerization initiator.
Depending on the polymerization method, when photopolymerization is performed, a
photopolymerization initiator is used, and when thermal polymerization is performed, a
thermal polymerization initiator is employed. Even when the photopolymerization is
conducted, a predetermined amount of heat is generated due to irradiation with UV light
and also through the polymerization, which is an exothermic reaction, and thus a thermal
polymerization initiator may be additionally used.
[0028] In the method of preparing the superabsorbent polymer according to the present
invention, the thermal polymerization initiator is not particularly limited, but preferably
includes at least one selected from the group consisting of a persulfate-based initiator, an
azo-based initiator, hydrogen peroxide, and ascorbic acid. Specifically, examples of the
persulfate-based initiator may include sodium persulfate (Na2S2O8), potassium persulfate
(K2S2O8), and ammonium persulfate ((NH4)2S2O8), and examples of the azo-based
initiator may include 2,2-azobis(2-amidinopropane)dihydrochloride, 2,2-azobis-(N,Ndimethylene)
isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutyronitrile, 2,2-
azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and 4,4-azobis-(4-cyanovaleric
acid).
7
[0029] In the method of preparing the superabsorbent polymer according to the present
invention, the photopolymerization initiator is not particularly limited, but preferably
includes at least one selected from the group consisting of benzoin ether, dialkyl
acetophenone, hydroxyl alkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl
phosphine, and 􀁄-aminoketone. Specific examples of acyl phosphine may include
commercially available Lucirin TPO, namely 2,4,6-trimethyl-benzoyl-trimethyl
phosphine oxide.
[0030] In the method of preparing the superabsorbent polymer according to the present
invention, the water-soluble ethylenic unsaturated monomer is not particularly limited,
so long as it is a monomer typically used to synthesize a superabsorbent polymer, and
preferably includes any one or more selected from the group consisting of an anionic
monomer and salts thereof, a nonionic hydrophilic monomer, and an amino groupcontaining
unsaturated monomer and quaternary salts thereof. Particularly useful is at
least one selected from the group consisting of anionic monomers and salts thereof, such
as acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic
acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-
(meth)acryloylpropanesulfonic acid, and 2-(meth)acrylamide-2-methylpropane sulfonic
acid; nonionic hydrophilic monomers, such as (meth)acrylamide, N-substituted
(meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,
methoxypolyethyleneglycol (meth)acrylate, and polyethyleneglycol (meth)acrylate; and
amino group-containing unsaturated monomers and quaternary salts thereof, such as
(N,N)-dimethylaminoethyl (meth)acrylate, and (N,N)-dimethylaminopropyl
(meth)acrylamide. More preferably, acrylic acid or salts thereof are used. When acrylic
acid or salts thereof are used as the monomer, a superabsorbent polymer having high
absorbability may be advantageously obtained.
[0031] In the method of preparing the superabsorbent polymer according to the present
invention, the monomer composition may include a predetermined amount of a polymer
or resin powder having a particle size of less than 150 􀁐m, corresponding to dust of the
prepared superabsorbent polymer powder, in order to attain the effect of recycling.
Specifically, the polymer or resin powder having a particle size of less than 150 􀁐m may
be added before the initiation of the polymerization of the monomer composition, or in
the early, middle or late stages, after the initiation of polymerization. As such, the
8
amount thereof that is added is not limited, but is preferably set to 1 to 10 parts by
weight based on 100 parts by weight of the monomer contained in the monomer
composition, in order to prevent the properties of the final superabsorbent polymer from
deteriorating.
[0032] In the method of preparing the superabsorbent polymer according to the present
invention, the concentration of the water-soluble ethylenic unsaturated monomer of the
monomer composition may be appropriately determined in consideration of the
polymerization time and the reaction conditions, and is preferably set to 40 to 55 wt%.
If the concentration of the water-soluble ethylenic unsaturated monomer is less than 40
wt%, economic benefits are negated. In contrast, if the concentration thereof exceeds 55
wt%, the grinding efficiency of the hydrous gel polymer may decrease.
[0033] Whether the hydrous gel polymer is prepared from the monomer composition
using thermal polymerization or photopolymerization is not limited, so long as it is
typically useful. Specifically, polymerization methods are largely classified into thermal
polymerization and photopolymerization, depending on the polymerization energy
source. Typically, thermal polymerization is conducted using a reactor with a stirring
shaft, such as a kneader, and photopolymerization is implemented using a reactor with a
movable conveyor belt. However, the above polymerization methods are merely
illustrative, and the present invention is not limited to those polymerization methods.
[0034] For example, hot air is fed to a reactor with a stirring shaft, such as a kneader, or
the reactor is heated, so that thermal polymerization is carried out, yielding a hydrous gel
polymer, which is then discharged at a size ranging from ones of mm to ones of cm
through the outlet of the reactor, depending on the shape of the stirring shaft of the
reactor. Specifically, the size of the hydrous gel polymer may vary depending on the
concentration of the supplied monomer composition and the supply rate thereof, and
typically a hydrous gel polymer having a particle size of 2 to 50 mm may be obtained.
[0035] Also, when photopolymerization is carried out using a reactor with a movable
conveyor belt, a hydrous gel polymer in a sheet form having the width of the belt may
result. As such, the thickness of the polymer sheet may vary depending on the
concentration of the supplied monomer composition and the supply rate thereof, but the
monomer composition is preferably supplied so as to form a polymer sheet having a
thickness of 0.5 to 5 cm. In the case where the monomer composition is supplied to the
9
extent that a very thin polymer sheet is formed, production efficiency may undesirably
decrease. If the thickness of the polymer sheet is greater than 5 cm, polymerization may
not be uniformly carried out throughout the sheet, which is too thick.
[0036] The hydrous gel polymer thus obtained typically has a moisture content ranging
from 30 to 60 wt%. As used herein, the term "moisture content" refers to the amount of
moisture based on the total weight of the hydrous gel polymer, that is, a value obtained
by subtracting the weight of the dried polymer from the weight of the hydrous gel
polymer. (Specifically, it is defined as a value calculated by measuring the weight
reduction due to the evaporation of moisture from the polymer while the polymer is
dried at a high temperature via IR heating. As such, the drying is performed in such a
manner that the temperature is increased from room temperature to 180􀁱C and then
maintained at 180􀁱C, and the total drying time is set to 20 min, including 5 min
necessary for increasing the temperature.)
[0037] The hydrous gel polymer, obtained through the thermal polymerization or
photopolymerization, is dried, and is preferably dried at a drying temperature ranging
from 150 to 250􀁱C. As used herein, the term "drying temperature" refers to the
temperature of the heat medium used for the drying process or the temperature of the
drying reactor, including the heat medium and the polymer, in the drying process.
[0038] If the drying temperature is lower than 150􀁱C, the drying time may become
excessively long, and the properties of the final superabsorbent polymer may deteriorate.
In contrast, if the drying temperature is higher than 250􀁱C, only the surface of the
polymer may be excessively dried, and thereby dust may be generated in the subsequent
grinding process, and the properties of the final superabsorbent polymer may deteriorate.
The drying is preferably performed at a temperature of 150 to 250􀁱C, and more
preferably 160 to 200􀁱C.
[0039] The drying time is not limited, but may be set to 20 to 90 min taking into account
the processing efficiency.
[0040] Additionally, the drying process is not limited so long as it is typically used to
dry the hydrous gel polymer. Specific examples thereof may include the supply of hot
air, IR irradiation, microwave irradiation, and UV irradiation. The polymer after the
drying process may have a moisture content of 0.1 to 10 wt%.
10
[0041] Meanwhile, the method of preparing the superabsorbent polymer according to
the present invention may further comprise a simple grinding process before the drying
process, as necessary, in order to increase the drying efficiency. The simple grinding
process before the drying process is conducted so that the particle size of the hydrous gel
polymer ranges from 1 to 15 mm. Grinding the polymer until the particle size is less
than 1 mm is technically difficult due to the high moisture content of the hydrous gel
polymer, and furthermore, the ground particles may agglomerate. In contrast, if the
polymer is ground to a particle size larger than 15 mm, the effect of increasing the
drying efficiency via the grinding process may become insignificant.
[0042] In the simple grinding process before the drying process, any grinder may be
used without limitation. The specific example thereof may include, but is not limited to,
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.
[0043] When the grinding process is performed to increase the drying efficiency before
the drying process in this way, the polymer, which has high moisture content, may stick
to the surface of the grinder. Thus, in order to increase the grinding efficiency of the
hydrous gel polymer before the drying process, an additive able to prevent stickiness
upon grinding may be further used.
[0044] The specific kind of additive that may be found useful is not limited. Examples
thereof may include, but are not limited to, a powder agglomeration inhibitor, such as
steam, water, a surfactant, and inorganic powder such as clay or silica; a thermal
polymerization initiator, such as a persulfate-based initiator, an azo-based initiator,
hydrogen peroxide, and ascorbic acid; and a crosslinking agent, such as an epoxy-based
crosslinking agent, a diol-based crosslinking agent, a bifunctional or trifunctional or
higher polyfunctional acrylate, and a monofunctional compound having a hydroxyl
group.
[0045] After the drying process in the method of preparing the superabsorbent polymer
according to the present invention, the dried polymer is ground. Thereby, the resulting
polymer has a particle size ranging from 150 to 850 􀁐m.
11
[0046] In the method of preparing the superabsorbent polymer according to the present
invention, the grinder used to obtain this particle size may include, but is not limited to,
a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, or a jog mill.
[0047] In the method of preparing the superabsorbent polymer according to the present
invention, the ground hydrous gel polymer is added with a surface crosslinking agent, so
that a surface crosslinking reaction is carried out. The surface crosslinking reaction may
be performed either before or after the water dispersion solution containing particles is
added to the hydrous gel polymer.
[0048] In the method of preparing the superabsorbent polymer according to the present
invention, the surface crosslinking agent is not limited, so long as it is able to react with
the functional group of the polymer. In order to improve the properties of the
superabsorbent polymer, the surface crosslinking agent may include at least one selected
from the group consisting of a polyhydric alcohol compound, an epoxy compound, a
polyamine compound, a haloepoxy compound, a haloepoxy compound condensed
product, an oxazoline compound, a mono-, di- or poly-oxazolidinone compound, a
cyclic urea compound, a polyhydric metal salt, and an alkylene carbonate compound.
[0049] Specifically, the polyhydric alcohol compound may include at least one selected
from the group consisting of mono-, di-, tri-, tetra- or poly-ethylene glycol,
monopropylene glycol, 1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl-1,3-
pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,4-
butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,2-
cyclohexanedimethanol.
[0050] Examples of the epoxy compound may include ethylene glycol diglycidyl ether
and glycidol, and the polyamine compound may include at least one selected from the
group consisting of ethylene diamine, diethylene triamine, triethylene tetramine,
tetraethylene pentamine, pentaethylene hexamine, polyethyleneimine, and polyamide
polyamine.
[0051] Examples of the haloepoxy compound may include epichlorohydrin,
epibromohydrin, and 􀁄-methyl epichlorohydrin. The mono-, di- or poly-oxazolidinone
compound may be exemplified by 2-oxazolidinone. The alkylene carbonate compound
may include ethylene carbonate. These compounds may be used alone or in
combination. To increase the efficiency of the surface crosslinking process, the surface
12
crosslinking agent preferably includes at least one polyhydric alcohol compound, and
more preferably includes a polyhydric alcohol compound having 2 to 10 carbon atoms.
[0052] The amount of the surface crosslinking agent added to treat the surface of the
polymer particles may be appropriately determined depending on the kind of surface
crosslinking agent or the reaction conditions, and is set to 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,
based on 100 parts by weight of the polymer.
[0053] If the amount of the surface crosslinking agent is too small, the surface
crosslinking reaction does not readily occur. In contrast, if the amount thereof exceeds 5
parts by weight based on 100 parts by weight of the polymer, the properties of the
superabsorbent polymer may deteriorate due to excessive surface crosslinking reactions.
[0054] As such, adding the surface crosslinking agent to the polymer is not limited.
Specifically, the surface crosslinking agent and the polymer powder may be placed in a
reaction bath and mixed, the surface crosslinking agent may be sprayed onto the polymer
powder, or the polymer and the crosslinking agent may be continuously supplied and
mixed using a reaction bath such as a mixer that operates continuously.
[0055] The temperature of the polymer itself may be 20 to 90􀁱C when the surface
crosslinking agent is added, so that the temperature is increased to the reaction
temperature within 1 to 60 min to perform surface crosslinking in the presence of the
surface crosslinking agent. To show the temperature of the polymer itself as above,
processes after the drying process, which is carried out at relatively high temperature,
are continuously performed, and the processing time may be shortened. Alternatively,
the polymer may be heated separately when it is difficult to shorten the processing time.
[0056] In the method of preparing the superabsorbent polymer according to the present
invention, the surface crosslinking agent that is added to the polymer may be heated in
order to increase the temperature to the reaction temperature within 1 to 60 min to
perform surface crosslinking in the presence of the surface crosslinking agent.
[0057] Meanwhile, in the method of preparing the superabsorbent polymer according to
the present invention, when the surface crosslinking reaction is carried out after the
temperature is increased to the reaction temperature within 1 to 60 min so as to prepare
for surface crosslinking, the efficiency of the surface crosslinking process may be
increased. Ultimately, the residual monomer content of the final superabsorbent
13
polymer may be minimized, and a superabsorbent polymer having superior properties
may be attained. As such, the temperature of the added surface crosslinking agent is
adjusted within the range from 5 to 60􀁱C, and preferably 10 to 40􀁱C. If the temperature
of the surface crosslinking agent is lower than 5􀁱C, the effect of decreasing the rate of
heating to the surface crosslinking reaction temperature by heating the surface
crosslinking agent may become insignificant. On the other hand, if the temperature of
the surface crosslinking agent is higher than 60􀁱C, the surface crosslinking agent may
not be uniformly dispersed in the polymer. As used herein, the surface crosslinking
reaction temperature may be defined as the total temperature of the polymer and the
surface crosslinking agent that is added for the crosslinking reaction.
[0058] The heating member for the surface crosslinking reaction is not limited.
Specifically, a heat medium may be supplied, or direct heating may be conducted using
electricity, but the present invention is not limited thereto. Specific examples of the heat
source may include steam, electricity, UV light, and IR light. Additionally, a heated
thermal fluid may be used.
[0059] In the method of preparing the superabsorbent polymer according to the present
invention, after heating for the crosslinking reaction, the crosslinking reaction is carried
out for a period ranging from 1 to 120 min, preferably 5 to 40 min, and more preferably
10 to 20 min. If the crosslinking reaction time is shorter than 1 min, the crosslinking
reaction may not sufficiently occur. In contrast, if the crosslinking reaction time is
longer than 60 min, the properties of the superabsorbent polymer may deteriorate due to
the excessive surface crosslinking reaction, and the polymer may be subjected to
attrition due to long-term residence in the reactor.
[0060] The superabsorbent polymer, produced by reacting the hydrous gel polymer with
the surface crosslinking agent, may be additionally ground. The superabsorbent polymer
thus ground has a particle size ranging from 150 to 850 􀁐m. The grinder used to obtain
this particle size may include, but is not limited to, a pin mill, a hammer mill, a screw
mill, a roll mill, a disc mill, or a jog mill.
[0061] In addition, the present invention addresses a superabsorbent polymer prepared
by the above preparation method.
[0062] A better understanding of the present invention may be obtained via the
following non-limited examples, which are set forth to illustrate, but are not to be
14
construed as limiting the scope of the present invention. The scope of the present
invention is given by the claims, and also contains all modifications within the meaning
and range equivalent to the claims. Unless otherwise mentioned, "%" and "part",
indicating amounts in the following examples and comparative examples, are given on a
mass basis.
[0063] Examples
[0064] Preparation Example: Preparation of hydrous gel polymer
[0065] 100 g of acrylic acid, 0.3 g of polyethyleneglycol diacrylate as a crosslinking
agent, 0.033 g of diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide as an initiator, 38.9
g of sodium hydroxide (NaOH), and 103.9 g of water were mixed, thus preparing a
monomer mixture. The monomer mixture was then placed on a continuously moving
conveyor belt and irradiated with UV light (at 2 mW/cm2) so that UV polymerization
was carried out for 2 min, thus obtaining a hydrous gel polymer.
[0066] Preparation Example: Preparation of superabsorbent polymer
[0067] The hydrous gel polymer thus obtained was cut to a size of 5 x 5 mm, dried in a
hot air oven at 170􀁱C for 2 hr, ground using a pin mill, and then sorted using a sieve,
thereby obtaining a superabsorbent polymer having a particle size of 150 to 850 􀁐m.
[0068] Thereafter, the superabsorbent polymer was surface-crosslinked using 3.5%
ethyleneglycol diglycidyl ether, reacted at 120􀁱C for 1 hr, ground, and then sorted using
a sieve, yielding a surface-treated superabsorbent polymer having a particle size of 150
to 850 􀁐m.
[0069] Examples: Preparation of superabsorbent polymer including water
dispersion solution containing microparticles
[0070] [Example 1]
[0071] 250 g of the superabsorbent polymer obtained in Preparation Example as
described above was placed in a stirrer and stirred at 1000 rpm for 60 sec. Thereafter,
the superabsorbent polymer was added with 7.075 g of a microparticle dispersion
solution and then mixed for 60 sec. The resulting mixture was aged for 30 min and then
sorted using a sieve, thus obtaining a superabsorbent polymer having a particle size of
150 to 850 􀁐m.
[0072] The microparticle dispersion solution was prepared by mixing 1.5 g of a solution
comprising superhydrophobic microparticulate silica Aerogel (AeroZelTM, JIOS),
15
isopropyl alcohol and water at a ratio of 1:4.5:4.5, with 5.545 g of water. As such,
isopropyl alcohol, which is the organic solvent, was used as a dispersion aid to disperse
the particles.
[0073] The silica Aerogel had a particle size of 30 nm, a BET specific surface area of
500 m2/g, a water contact angle of 150􀁱, and a porosity of 95%.
[0074] The particle size of the Aerogel was measured through Laser Diffraction using
HELOS (Helium-Neon Laser Optical System) based on ISO 13320. The specific
surface area thereof was measured using a BET meter (Micromeritics 3Flex). The
porosity thereof was determined from the tap density (􀁕t) and the true density (􀁕s) of
Equation 1 below.
[Equation 1]
Porosity (%) = (1 - 􀁕t/􀁕s)*100
[0075] In order to measure the true density, a pycnometer (Accupyc II 1340) was used,
and the tap density was measured using a volumeter (Engelsmann Model STAV II).
[0076] The water contact angle was measured using a contact angle analyzer (KRUSS
DSA100), and was specifically determined in a manner in which double-sided tape was
attached to a flat glass plate, microparticles were applied in a monolayer thereon, and
then 5 􀁐L of ultrapure water was placed in the form of drop on the monolayer, and the
angle between the water drop and the glass plate was measured four times and averaged.
[0077] [Example 2]
[0078] A microparticle dispersion solution was prepared by mixing 2.5 g of a solution
comprising microparticulate silica Aerogel (AeroZelTM, JIOS), isopropyl alcohol and
water at a ratio of 1:4.5:4.5, with 5.125 g of water, and then added in an amount of 7.625
g to a superabsorbent polymer.
[0079] The superabsorbent polymer was obtained in the same manner as in Example 1,
with the exception that the amount of the microparticle dispersion solution was changed.
[0080] [Example 3]
[0081] A microparticle dispersion solution was prepared by mixing 2.5 g of a solution
comprising superhydrophobic microparticulate silica Aerogel (AeroZelTM, JIOS),
isopropyl alcohol and water at a ratio of 1:4.5:4.5, with 11.375 g of water, and then
added in an amount of 13.875 g to a superabsorbent polymer.
[0082] The superabsorbent polymer was obtained in the same manner as in Example 1,
16
with the exception that the amount of the microparticle dispersion solution was changed.
[0083] [Comparative Example 1]
[0084] A superabsorbent polymer was obtained in the same manner (stirring for 120 sec
without microparticles) as in Example 1, with the exception that the microparticle
dispersion solution was not used.
[0085] [Comparative Example 2]
[0086] 250 g of the superabsorbent polymer prepared in Preparation Example as
described above and 0.15 g of microparticulate silica Aerogel (AeroZelTM, JIOS) were
placed in a stirrer and stirred at 1000 rpm for 60 sec. The stirred mixture was added
with 6.25 g of water and further stirred for 60 sec. Thereafter, the resulting mixture was
sorted using a sieve, yielding a superabsorbent polymer having a particle size of 150 to
850 􀁐m.
[0087] [Comparative Example 3]
[0088] A superabsorbent polymer was obtained in the same manner as in Comparative
Example 2, with the exception that 0.25 g of microparticulate silica Aerogel (AeroZelTM,
JIOS) was used.
[0089] [Comparative Example 4]
[0090] A superabsorbent polymer was obtained in the same manner as in Comparative
Example 2, with the exception that 0.25 g of microparticulate silica Aerogel (AeroZelTM,
JIOS) and 12.5 g of water were used.
[0091] The features of preparation methods of Examples 1 to 3 and Comparative
Examples 1 to 4 are summarized in Table 1 below.
[Table 1]
Form of added
microparticles
Amount of microparticle
dispersion solution (g)
Amount of
microparticles (g)
Total amount of
added water (g)
Ex. 1 Dispersion solution 7.075 0.15 6.25
Ex. 2 Dispersion solution 7.625 0.25 6.25
Ex. 3 Dispersion solution 13.875 0.25 12.5
C.Ex. 1 - - - -
C.Ex. 2 Powder - 0.15 6.25
C.Ex. 3 Powder - 0.25 6.25
C.Ex. 4 Powder - 0.25 12.5
[0092] Test Examples: Evaluation of properties
[0093] In order to evaluate the properties of the superabsorbent polymers of Examples 1
17
to 3 and Comparative Examples 1 to 4, the following tests were performed.
[0094] Before the following tests, the superabsorbent polymers of Examples 1 to 3 and
Comparative Examples 1 to 4 were subjected to ball milling. 20 g of the superabsorbent
polymer and ceramic balls having a diameter of 2.5 cm were placed in a 1 L ceramic
bottle, and were then subjected to milling through rotating at 300 rpm for 15 min.
Thereafter, the particle size was determined through the method of Test Example 4, and
the superabsorbent polymers before and after ball milling were tested in Test Examples
1 to 3.
[0095] Test Example 1: Centrifugal retention capacity (CRC)
[0096] The superabsorbent polymers of Examples 1 to 3 and Comparative Examples 1
to 4 were measured for CRC before and after ball milling. Specifically, 0.2 g of a
sample having a particle size of 300 to 600 􀁐m, of the prepared superabsorbent polymer,
was placed in a teabag and then immersed in a 0.9% saline solution for 30 min.
Thereafter, dehydration was performed for 3 min by centrifugal force equivalent to 250
G (gravity), and the amount of absorbed saline solution was measured.
[0097] Test Example 2: Absorption under pressure (AUP)
[0098] The superabsorbent polymers of Examples 1 to 3 and Comparative Examples 1
to 4 were measured for AUP before and after ball milling. Specifically, 0.9 g of a
sample having a particle size of 300 to 600 􀁐m, of the prepared superabsorbent polymer,
was placed in a cylinder according to the EDANA method, and a pressure of 0.7 psi was
applied using a piston and a weight. Thereafter, the amount of 0.9% saline solution
absorbed for 60 min was measured.
[0099] Test Example 3: Permeability
[00100] The superabsorbent polymers of Examples 1 to 3 and Comparative
Examples 1 to 4 were measured for permeability before and after ball milling.
[00101] In order to prevent the generation of bubbles between a cock and a glass
filter in the lower portion of a chromatography column, about 10 mL of water was added
in the opposite direction into the column, and the column was washed two or three times
with saline and then filled with at least 40 mL of 0.9% saline. A piston was placed in
the chromatography column, the lower valve was opened, and the period of time (B: sec)
required for the liquid surface to move from 40 mL to 20 mL was recorded, thus
completing blank testing. 0.2 g of a sample having a particle size ranging from 300 to
18
600 􀁐m, of the prepared superabsorbent polymer, was placed and then saline was added.
The total amount of saline that resulted was 50 mL. The sample was allowed to stand
for 30 min so that the superabsorbent polymer was sufficiently swollen. The piston (0.3
psi) with a weight was placed in the chromatography column and then allowed to stand
for 1 min. The cock at the bottom of the chromatography column was opened, and the
period of time (T1: sec) required for the liquid surface to move from 40 mL to 20 mL
was recorded. The permeability was represented as follows.
Permeability = T1 - B
[00102] The resultant permeabilities of the superabsorbent polymers of Examples
1 to 3 and Comparative Example 1 are shown in FIG. 2. The superabsorbent polymers
of Examples 1 to 3 exhibited permeability superior to that of Comparative Example 1.
[00103] Test Example 4: Particle size of superabsorbent polymer
[00104] The superabsorbent polymers of Examples 1 to 3 and Comparative
Examples 1 to 4 were measured for particle size. The particle size of the superabsorbent
polymer was measured using the EDANA method WSP 240.3. 100 g of the
superabsorbent polymer was sorted at 850 􀁐m, 600 􀁐m, 300 􀁐m, and 150 􀁐m using a
mesh from Pan, and vibrated for 10 min under conditions of an amplitude of 1.44 mm
and a vibration frequency of 50 Hz, and the amount remaining on each sieve was
determined.
[00105] The results of the particle size variations of the superabsorbent polymers
of Examples 1 to 3 and Comparative Example 1 are shown in FIG. 1. The
superabsorbent polymers of Examples 1 to 3 had low particle size variation after ball
milling, compared to Comparative Example 1.
[00106] The results of measurement of CRC, AUP and permeability of the
superabsorbent polymers of Examples 1 to 3 and Comparative Examples 1 to 4 before
and after ball milling are shown in Table 2 below.
[Table 2]
Ball milling CRC (g/g) AUP (g/g) Permeability (sec)
Ex.1
Before 33 23 434
After 34 20 553
Ex.2
Before 33 23 440
After 34 22 572
Ex.3 Before 32 23 315
19
After 33 21 387
C.Ex.1
Before 34 25 649
After 35 22 1277
C.Ex.2
Before 35 22 373
After 35 22 585
C.Ex.3
Before 35 23 361
After 35 22 460
C.Ex.4
Before 34 23 228
After 34 20 299
[00107] The moisture content and the dust generation after ball milling of the
superabsorbent polymers of Examples 1 to 3 and Comparative Examples 1 to 4 are
shown in Table 3 below.
[Table 3]
Moisture content (%) Dust generation (%) after ball milling
Ex. 1 2.7 11.9
Ex. 2 2.5 7.8
Ex. 3 4.4 7.5
C.Ex. 1 0.6 13.6
C.Ex. 2 2.6 8.4
C.Ex. 3 2.6 7.1
C.Ex. 4 4.6 6.7
[00108] The results of measurement of the particle size distribution of Test
Example 4 are given in Table 4 below.
[Table 4]
Particle size distribution
150 􀁐m or less 150 to 300 􀁐m 300 to 600 􀁐m 600 to 850 􀁐m 850 􀁐m or more
Ex. 1 1.69 24.22 42.70 31.30 0.09
Ex. 2 1.19 17.58 43.85 37.34 0.04
Ex. 3 1.27 19.86 43.02 35.74 0.11
C.Ex. 1 2.4818 20.4425 41.3137 35.6723 0.0897
C.Ex. 2 0.28 11.89 49.77 37.94 0.12
C.Ex. 3 0.79 16.13 49.23 33.83 0.02
C.Ex. 4 1.46 18.60 48.26 31.67 0.01
[00109] As is apparent from the above results, the superabsorbent polymers, the
surface of which was introduced with the microparticle dispersion solution, were
improved in attrition resistance and properties, as in the case where the microparticles
20
were added in powder form.
[00110] Typically, a superabsorbent polymer is surface-crosslinked in a manner in
which a surface crosslinking agent is dissolved in water and is then mixed with the
superabsorbent polymer, thereby inducing uniform distribution and penetration on the
surface of the superabsorbent polymer. As such, the use of water may increase the
stickiness of the surface of the superabsorbent polymer, undesirably causing
agglomeration. Grinding the agglomerated superabsorbent polymer requires a strong
force, undesirably damaging the superabsorbent polymer.
[00111] The superabsorbent polymers prepared by the addition of the
microparticle water dispersion solution in Examples 1 to 3 exhibited similar particle size
distribution to the superabsorbent polymer of Comparative Example 1, despite the
presence of 2.5% and 5.0% of water. This is because the agglomeration due to water
was reduced thanks to the effect of the added microparticles.
[00112] The results of dust generation after ball milling are shown in Table 3. In
Comparative Example 1 and Examples 1 to 3, the dust generation was reduced by the
addition of the microparticle dispersion solution. The surface of the superabsorbent
polymer was coated with microparticles and 2.5% or 5% of water was positioned
therein, thus increasing attrition resistance, thereby reducing the generation of dust.
Furthermore, the dust generation was reduced with an increase in the amounts of the
microparticles and water.
[00113] Examples 1, 2 and 3 using the microparticle dispersion solution were
compared with Comparative Examples 2, 3 and 4. Comparative Examples 2 to 4 pertain
to the superabsorbent polymers, obtained by stirring the microparticles in powder form,
together with superabsorbent polymer, and then adding water thereto. When the
microparticles were added in the form of a dispersion solution by being dispersed in
water, similar attrition resistance and properties (Tables 2 and 3) and particle size
distribution (Table 4) were manifested. Accordingly, even when the microparticles,
which are in dispersion solution form, are added to the superabsorbent polymer, the
same properties are confirmed to be exhibited.

CLAIMS
1. A method of preparing a superabsorbent polymer, comprising:
adding a hydrous gel polymer, produced by polymerizing a monomer
composition comprising a water-soluble ethylenic unsaturated monomer and a
polymerization initiator, with a water dispersion solution containing particles having i) a
BET specific surface area of 300 to 1500 m2/g and ii) a porosity of 50% or more.
2. The method of claim 1, wherein the particles have a particle size ranging from
2 nm to 50 􀁐m.
3. The method of claim 1, wherein the particles have superhydrophobicity with a
water contact angle of 125􀁱 or more.
4. The method of claim 1, wherein the particles have a particle size ranging from
2 nm to 50 􀁐m and superhydrophobicity with a water contact angle of 125􀁱 or more.
5. The method of claim 1, wherein the water dispersion solution comprises the
particles, water, and an organic solvent.
6. The method of claim 5, wherein the organic solvent is at least one selected
from the group consisting of methanol, ethanol, isopropyl alcohol (IPA), and acetone.
7. The method of claim 5, wherein the organic solvent is isopropyl alcohol (IPA).
8. The method of claim 1, wherein the particles are at least one selected from the
group consisting of silica (SiO2), alumina, carbon, and titania (TiO2).
9. The method of claim 1, wherein the particles are silica (SiO2).
10. The method of claim 1, wherein the particles have a BET specific surface
area of 500 to 1500 m2/g.
22
11. The method of claim 1, wherein the particles have a BET specific surface
area of 700 to 1500 m2/g.
12. The method of claim 3, wherein the particles have superhydrophobicity with
a water contact angle of 140􀁱 or more.
13. The method of claim 3, wherein the particles have superhydrophobicity with
a water contact angle of 145􀁱 or more.
14. The method of claim 1, wherein the particles have a porosity of 90% or more.
15. The method of claim 1, wherein the particles are used in an amount of 1 to 25
parts by weight based on 100 parts by weight of the water and organic solvent.
16. The method of claim 1, further comprising drying the hydrous gel polymer
produced by the polymerizing.
17. The method of claim 16, further comprising grinding the dried hydrous gel
polymer, after the drying.
18. The method of claim 17, further comprising adding a surface crosslinking
agent to the ground hydrous gel polymer so that surface crosslinking is carried out and
then adding the water dispersion solution containing particles.
19. The method of claim 17, further comprising adding the water dispersion
solution containing particles to the ground hydrous gel polymer and then adding a
surface crosslinking agent so that surface crosslinking is carried out.
20. The method of claim 16, further comprising grinding the hydrous gel
polymer to a particle size of 1 to 15 mm, before the drying.
23
21. The method of claim 16, wherein the drying is performed at a temperature
ranging from 150 to 250􀁱C.
22. The method of claim 18 or 19, wherein the surface crosslinking agent is any
one or more selected from the group consisting of a polyhydric alcohol compound, an
epoxy compound, a polyamine compound, a haloepoxy compound, a haloepoxy
compound condensed product, an oxazoline compound, a mono-, di- or polyoxazolidinone
compound, a cyclic urea compound, a polyhydric metal salt, and an
alkylene carbonate compound.
23. The method of claim 18 or 19, wherein the surface crosslinking agent is
added in an amount of 0.001 to 5 parts by weight based on 100 parts by weight of the
ground polymer.
24. The method of claim 18 or 19, wherein the adding the surface crosslinking
agent is performed under a condition that a surface temperature of the polymer is 60 to
90􀁱C.
25. The method of claim 18 or 19, wherein the surface crosslinking agent has a
temperature ranging from 5 to 40􀁱C.
26. The method of claim 18 or 19, wherein the surface crosslinking is carried out
for 10 to 120 min.
27. The method of claim 18 or 19, wherein the surface crosslinking is carried out
through heating by applying any one or more selected from the group of heat sources
including steam, electricity, UV light, and IR light.
28. The method of claim 18 or 19, further comprising grinding the
superabsorbent polymer to a particle size ranging from 150 to 850 􀁐m, after the surface
crosslinking.
24