SUPERABSORBENT POLYMER INCLUDING WATER-SOLUBLE SALT AND
METHOD OF PREPARING THE SAME
BACKGROUND OF THE INVENTION
1. Technical Field
[0001] The present invention relates to a superabsorbent polymer including a watersoluble
salt and a method of preparing the same and, more particularly, to a
superabsorbent polymer including a water-soluble salt and a method of preparing the
same, wherein the addition of a water-soluble metal salt in the polymerization process of
a superabsorbent polymer is very effective at decreasing the concentration of residual
monomer (RM).
2. Description of the Related Art
[0002] Superabsorbent polymers (SAPs) are synthetic polymers able to absorb water
about 500 ~ 1000 times their own weight. Such superabsorbent polymers have actually
begun to be used for sanitary items, and are being currently widely utilized in not only
hygiene products such as baby disposable diapers and so on, but also in gardening soil
repair agents, water stop agents for civil construction, seeding sheets, freshness retaining
agents in food distribution sectors, and fomentation materials. Compared to
conventional absorbent materials, superabsorbent polymers have an outstanding
absorption capacity and thus the market value thereof is increasing with the wider range
of applications thereof.
[0003] The absorption mechanism of the superabsorbent polymer is controlled by
osmotic pressure due to a difference in electric attraction represented by charges of a
polymer electrolyte, an affinity between water and a polymer electrolyte, molecular
expansion due to repulsive force between polymer electrolyte ions, and interactions of
expansion inhibition due to crosslinkages. Briefly, absorbability of the absorbent
polymer depends on the aforementioned affinity and molecular expansion, and the
absorption rate thereof is greatly affected by osmotic pressure of the absorbent polymer
itself.
2
[0004] In order to use the superabsorbent polymer as a hygiene material, the
superabsorbent polymer has to essentially possess a low concentration of RM. To lower
the concentration of RM, the use of acrylic acid having low dimer concentration is
fundamentally adopted, and other methods may also be proposed but are problematic
because other properties of the superabsorbent polymer may deteriorate or such
additional processing may be complicated.
[0005] Therefore, thorough research is ongoing into preparation of superabsorbent
polymers having high absorption rate and high absorption scale.
SUMMARY OF THE INVENTION
[0006] Accordingly, the present invention has been made keeping in mind the problems
encountered in the related art, and an object of the present invention is to provide a
superabsorbent polymer and a method of preparing the same, wherein adding a watersoluble
metal salt is further included in a conventional method of preparing a
superabsorbent polymer, thereby decreasing the concentration of RM.
[0007] In order to accomplish the above object, the present invention provides a method
of preparing a superabsorbent polymer, comprising reacting a water-soluble ethylenic
unsaturated monomer, a photoinitiator, a crosslinking agent, and a thermal
polymerization initiator, in the presence of a water-soluble metal salt.
[0008] Furthermore, the method of preparing the superabsorbent polymer comprises: a)
mixing the water-soluble ethylenic unsaturated monomer, the photoinitiator, and the
crosslinking agent; b) diluting a sodium hydroxide aqueous solution with a water-soluble
metal salt aqueous solution; c) neutralizing the mixture obtained in a) with the diluted
solution obtained in b); d) adding the mixture obtained in c) with the thermal
polymerization initiator, and then performing a radical polymerization reaction using
thermal polymerization or photopolymerization, thus forming a polymer sheet; and e)
adding the polymer sheet formed in d) with water, thus forming a hydrous gel polymer.
[0009] In addition, the present invention provides a superabsorbent polymer, prepared
by the above method.
3
[0010] According to the present invention, a superabsorbent polymer is advantageous
because the concentration of RM can be effectively lowered, compared to a conventional
superabsorbent polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graph illustrating changes in concentration of RM depending on the
kind and amount of metal sulfate of a superabsorbent polymer according to an
embodiment of the present invention;
[0012] FIGS. 2A to 2C are graphs illustrating changes in concentration of RM
depending on the kind and amount of metal sulfate; and
[0013] FIG. 3 is a graph illustrating a decrease in concentration of RM when Na2SO4 as
a metal sulfate is added in an excessive amount (15 wt%).
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0014] Hereinafter, a detailed description will be given of the present invention.
[0015] The present invention addresses a method of preparing a superabsorbent
polymer, comprising reacting a water-soluble ethylenic unsaturated monomer, a
photoinitiator, a crosslinking agent, and a thermal polymerization initiator, in the
presence of a water-soluble metal salt.
[0016] More specifically, the water-soluble metal salt preferably includes at least one
selected from the group consisting of a sulfate group, a nitrate group, a phosphate group,
a chloride group, a sulfite group, and a thiocyanate group. Particularly useful is a metal
sulfate.
[0017] The metal of the water-soluble metal salt preferably includes at least one selected
from the group consisting of sodium (Na), lithium (Li), or potassium (K).
[0018] According to the present invention, the method of preparing the superabsorbent
polymer comprises: a) mixing the water-soluble ethylenic unsaturated monomer, the
photoinitiator, and the crosslinking agent; b) diluting a sodium hydroxide aqueous
solution with a water-soluble metal salt aqueous solution; c) neutralizing the mixture
obtained in a) with the diluted solution obtained in b); d) adding the mixture obtained in
4
c) with the thermal polymerization initiator, and then performing a radical
polymerization reaction using thermal polymerization or photopolymerization, thus
forming a polymer sheet; and e) adding the polymer sheet formed in d) with water, thus
forming a hydrous gel polymer.
[0019] The method may further comprise, after e) forming the hydrous gel polymer, f)
drying and grinding the hydrous gel polymer, thus obtaining superabsorbent polymer
particles; and g) sorting the superabsorbent polymer particles depending on the particle
size, thus obtaining particles having a particle size of 150 ~ 850 􀁐m.
[0020] As used herein, the term “superabsorbent polymer particles” refers to particles
obtained by drying and grinding a hydrous gel polymer. More specifically, the hydrous
gel polymer is a material in solid jelly form with a size of 1 cm or more having water in
a large amount (50% or more) after completion of the polymerization. The hydrous gel
polymer is dried and ground in a powder phase, yielding superabsorbent polymer
particles. Thus, the hydrous gel polymer corresponds to a process intermediate.
[0021] In a) of the method of preparing the superabsorbent polymer according to the
present invention, the water-soluble ethylenic unsaturated monomer, the photoinitiator
and the crosslinking agent are mixed.
[0022] 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 any
one or more 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
5
(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
improved absorbability may be obtained advantageously. Also, 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 ~ 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%,
grinding efficiency of the hydrous gel polymer may decrease.
[0023] In the method of preparing the superabsorbent polymer according to the present
invention, the photoinitiator (photopolymerization initiator) is not particularly limited,
but preferably includes at least one selected from the group consisting of benzoin ether,
dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, benzyl dimethyl ketal,
acyl phosphine, and 􀁄-aminoketone. A specific example of the acyl phosphine may
include commercially available lucirin TPO, namely, 2,4,6-trimethyl-benzoyl-trimethyl
phosphine oxide, or commercially available Irgacure series may be used as the
photoinitiator able to form a thick polymer layer with relatively high permeability.
[0024] In the method of preparing the superabsorbent polymer according to the present
invention, the 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 crosslinking agent may include at least one selected from
the group consisting of a polyhydric alcohol compound; an acrylate-based 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.
[0025] 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-
6
butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,2-
cyclohexanedimethanol.
[0026] The acrylate-based compound may be exemplified by
poly(ethyleneglycol)diacrylate.
[0027] 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.
[0028] Examples of the haloepoxy compound may include epichlorohydrine,
epibromohydrine, and 􀁄-methylepichlorohydrine. 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 crosslinking process, the crosslinking
agent preferably includes at least one polyhydric alcohol compound, and more
preferably includes a polyhydric alcohol compound having 2 to 10 carbon atoms.
[0029] The amount of the crosslinking agent added to treat the surface of the polymer
particles may be properly determined depending on the kind of crosslinking agent or the
reaction conditions, and is set to 0.001 ~ 5 parts by weight, preferably 0.01 ~ 3 parts by
weight, and more preferably 0.05 ~ 2 parts by weight, based on 100 parts by weight of
the polymer. If the amount of the crosslinking agent is too small, a crosslinking reaction
seldom occurs. In contrast, if the amount thereof exceeds 5 parts by weight based on
100 parts by weight of the polymer, properties of the superabsorbent polymer may
deteriorate due to an excessive crosslinking reaction.
[0030] In b), the alkaline aqueous solution is diluted with the water-soluble metal salt
aqueous solution.
[0031] The water-soluble metal salt preferably includes at least one selected from the
group consisting of a sulfate group, a nitrate group, a phosphate group, a chloride group,
a sulfite group, and a thiocyanate group. Particularly useful is a metal sulfate.
[0032] The water-soluble metal salt is preferably used in an amount of 0.001 ~ 40.0
wt%, more preferably 2.0 ~ 20.0 wt%, and still more preferably 2.5 ~ 15.0 wt%, based
on the total weight of the water-soluble ethylenic unsaturated monomer. If the amount
7
of the water-soluble metal salt is less than 0.001 wt%, an effect of decreasing the
concentration of RM may become insignificant. In contrast, if the amount thereof
exceeds 40.0 wt%, the main component of the superabsorbent polymer may be
undesirably changed into a salt, but not the monomer. Taking into consideration the
mixing ratio of components, the maximum amount of the above component is
determined.
[0033] The water-soluble metal salt does not directly participate in an actual chemical
reaction and has an electrostatic influence on cation transfer. The water-soluble
ethylenic unsaturated monomer is mixed with a sodium hydroxide aqueous solution so
as to be partially neutralized, followed by radical polymerization. As such, the watersoluble
metal salt may be added to decrease electrical repulsion between anionic
monomers. The cation of the water-soluble metal salt is regarded as important, and the
repulsion between monomers may be decreased by virtue of a shielding effect due to the
cation, thus enabling efficient polymerization. Consequently, the superabsorbent
polymer prepared by the above method may be expected to exhibit an effect of
decreasing the concentration of RM.
[0034] In c), the mixture obtained in a) is introduced with the diluted solution obtained
in b) so as to be neutralized. As such, c) may be carried out at 30 ~ 50􀁱C.
[0035] In d), the mixture obtained in c) is further added with the thermal polymerization
initiator, after which radical polymerization using thermal polymerization or
photopolymerization may be implemented, thus forming a polymer sheet.
[0036] The superabsorbent polymer according to the present invention 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.
8
[0037] 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).
[0038] Also, the polymerization method is 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 method is merely illustrative, and
the present invention is not limited to such a polymerization method.
[0039] 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 to 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 ~ 50 mm may be obtained.
[0040] Also, when photopolymerization is carried out using a reactor with a movable
conveyor belt, a hydrous gel polymer in a sheet form with a belt width 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 obtain a polymer sheet having a thickness of
0.5 ~ 5 cm. In the case where the monomer composition is supplied to the extent that a
very thin polymer sheet is formed, production efficiency may decrease undesirably. If
the thickness of the polymer sheet is greater than 5 cm, polymerization may not be
uniformly carried out throughout the sheet that is too thick.
9
[0041] In an embodiment of the present invention, thermal polymerization or
photopolymerization in d) may be implemented by applying at least one heat source
selected from the group consisting of steam, electricity, UV light, and IR light. As such,
UV light may be applied at an intensity 1 ~ 20 mW/cm2.
[0042] In e), the polymer sheet formed in d) is added with water, yielding a hydrous gel
polymer.
[0043] The hydrous gel polymer thus obtained typically has a moisture content of 30 ~
60 wt%. As used herein, the term “moisture content” refers to an amount of moisture
based on the total weight of the hydrous gel polymer, namely, 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 a weight reduction due to
moisture evaporation from the polymer during drying the polymer at 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).
[0044] In f), the hydrous gel polymer is dried and ground, thus obtaining superabsorbent
polymer particles.
[0045] In the drying process, the drying temperature may be set to 150 ~ 250􀁱C. As
used herein, the term “drying temperature” refers to a temperature of a heat medium
supplied for the drying process or a temperature of a drying reactor including a heat
medium and a polymer in the drying process.
[0046] 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 thus
deteriorate. In contrast, if the drying temperature is higher than 250􀁱C, only the surface
of the polymer may be excessively dried, and thereby fine powder 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 ~ 250􀁱C,
and more preferably 160 ~ 200􀁱C.
[0047] The drying time is not limited, but may be set to 20 ~ 90 min taking into account
the process efficiency.
10
[0048] Also, the drying process is not limited so long as it is typically used to dry the
hydrous gel polymer. Specific examples thereof may include hot air supply, IR
irradiation, microwave irradiation, and UV irradiation. The polymer after the drying
process may have a moisture content of 0.1 ~ 10 wt%.
[0049] 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 is 1 ~ 15 mm. Grinding the particle size of the polymer to less than 1 mm is
technically difficult due to high moisture content of the hydrous gel polymer, and also
the ground particles may agglomerate. In contrast, if the polymer is ground to a particle
size of greater than 15 mm, an effect of increasing the drying efficiency via the grinding
process may become insignificant.
[0050] In the simple grinding process before the drying process, any grinder may be
used without limitation. A 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.
[0051] When the grinding process is performed to increase the drying efficiency before
the drying process in this way, the polymer having 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. Specifically, the kind of usable additive 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.
[0052] In g), the superabsorbent polymer particles are sorted depending on the particle
size, thus obtaining particles having a particle size of 150 ~ 850 􀁐m.
11
[0053] The superabsorbent polymer particles resulting from the grinding process have a
particle size of 150 ~ 850 􀁐m. In the method of preparing the superabsorbent polymer
according to the present invention, a grinder used to obtain such a 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.
[0054] In addition, the present invention addresses a superabsorbent polymer prepared
by the preparation method as above. As measured by an EDANA measurement method,
such a superabsorbent polymer is advantageous because the concentration of RM is
lowered by the addition of the water-soluble metal salt, compared to when the watersoluble
metal salt is not added.
[0055] A better understanding of the present invention may be obtained via the
following examples that are set forth to illustrate, but are not to be construed as limiting
the scope of the present invention. The scope of the present invention is shown in the
claims, and also contains all modifications within the meaning and range equivalent to
the claims. Unless otherwise mentioned, “%” and “part” showing the amount in the
following examples and comparative examples refer to a mass basis.
[0056] Examples 1 to 7 and Comparative Example 1: Preparation of
superabsorbent polymer
[0057] Example 1
[0058] 192.7 g of an aqueous solution having 13.0 g (2.5 wt% based on acrylic acid) of
Na2SO4 dissolved therein was used to dilute 629.6 g of a 32 wt% sodium hydroxide
aqueous solution, after which acrylic acid, Irgacure 819 as a photoinitiator, and PEGDA
(Poly(ethyleneglycol)diacrylate) as a crosslinking agent were sequentially added using a
pump. The resulting mixture was gradually cooled using a cooler, placed in a plastic
beaker containing a sodium persulfate aqueous solution as a thermal initiator, poured
into a pre-stabilized UV irradiator, and then irradiated with UV light (at an intensity of
10 mW/cm2). The point of time when a foaming sound was heard was measured using a
stopwatch, and UV irradiation was stopped when the measurement time reached 1 min,
and the resulting product was allowed to stand for 2 min in the UV irradiator. The
polymerized sheet was snipped, uniformly mixed with 250 g of water so as to absorb
such water, and then cut into small pieces using a chopper. The small pieces thus cut
12
were evenly spread in an oven, dried and then ground. Then, a superabsorbent polymer
having a particle size of 150 ~ 850 􀁐m was obtained using a standard sieve.
[0059] Example 2
[0060] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 25.9 g (5.0
wt% based on acrylic acid) of Na2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0061] Example 3
[0062] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 77.7 g (15.0
wt% based on acrylic acid) of Na2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0063] Example 4
[0064] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 13.0 g (2.5
wt% based on acrylic acid) of Li2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0065] Example 5
[0066] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 25.9 g (5.0
wt% based on acrylic acid) of Li2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0067] Example 6
[0068] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 13.0 g (2.5
wt% based on acrylic acid) of K2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0069] Example 7
[0070] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 20.7 g (4.0
wt% based on acrylic acid) of K2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0071] Comparative Example 1
[0072] A superabsorbent polymer was prepared in the same manner as in Example 1,
with the exception that the water-soluble metal salt was not added.
[0073] The amounts of salts in Examples 1 to 7 and Comparative Example 1 are given
in Table 1 below.
[Table 1]
13
Salt
Amount of Salt
wt% based on acrylic acid Weight (g)
Ex. 1
Na2SO4
2.5 13.0
Ex. 2 5.0 25.9
Ex. 3 15.0 77.7
Ex. 4
Li2SO4
2.5 13.0
Ex. 5 5.0 25.9
Ex. 6
K2SO4
2.5 13.0
Ex. 7 4.0 20.7
C.Ex. 1 None - -
[0074] Test Example: Evaluation of properties of superabsorbent polymer
[0075] To evaluate the properties of the superabsorbent polymers of Examples 1 to 7
and Comparative Example 1, the following testing was performed.
[0076] Residual Monomer (RM)
[0077] In the superabsorbent polymers of Examples 1 to 7 and Comparative Example 1,
the concentration of RM was measured based on WSP 210.3 according to an EDANA
method. 1.000 g of a superabsorbent polymer sample having a particle size of 150 ~ 850 􀁐m and 200 g of 0.9% brine were placed in a 250 mL Erlenmeyer flask and stirred for 1
hr. Subsequently, the resulting mixture was filtered using filter paper, and the solution
was sampled and measured by HPLC.
[0078] Also, in the superabsorbent polymers of the examples and comparative example,
changes in the concentration of RM depending on the kind and amount of metal sulfate
were measured. The results are shown in Table 2 below and FIG. 1.
[Table 2]
Metal sulfate
RM (ppm)
Kind wt%
14
Ex. 1
Na2SO4
2.5 804
Ex. 2 5.0 850
Ex. 3 15.0 822
Ex. 4
Li2SO4
2.5 794
Ex. 5 5.0 676
Ex. 6
K2SO4
2.5 848
Ex. 7 4.0 853
C.Ex. 1 - - 977
[0079] The concentration values of RM in the above examples and comparative example
are average values of several tests.
[0080] As is apparent from Table 2 and FIG. 1, when the water-soluble metal salt, that
is, the metal sulfate, was added in the polymerization process of a superabsorbent
polymer, the concentration of RM was effectively decreased, compared to when the
metal sulfate was not added (Comparative Example 1).
[0081] More specifically, FIGS. 2A to 2C are graphs illustrating changes in the
concentration of RM depending on the kind and amount of metal sulfate. Based on the
average concentration values of RM, when the water-soluble metal salt, that is, the metal
sulfate was added in the same amount (mass) depending on the kind thereof, the salts
effective at decreasing the concentration of RM were represented by the sequence of
Li2SO4 > Na2SO4 > K2SO4, or were represented by the sequence of Li2SO4 > K2SO4 >
Na2SO4 in some cases. Although the extent of decrease in concentration of RM varied
depending on the amount of the salt, there were significant differences in the effects due
to the presence or absence of the metal sulfate as the water-soluble metal salt.
[0082] As illustrated in FIG. 3, when the water-soluble metal salt, that is, the metal
sulfate Na2SO4 was added in an excessive amount (15 wt%), the concentration of RM
was uniformly decreased in a negative (-) slope, compared to when the metal sulfate was
not added (Comparative Example 1). Thereby, the effect of the metal sulfate on
decreasing the concentration of RM was superior despite the excessive use thereof.
15
[0083] Therefore, the superabsorbent polymer according to the present invention can be
effectively decreased in the concentration of RM by the addition of a water-soluble
metal salt, compared to a conventional superabsorbent polymer.
16

WHAT IS CLAIMED IS:
1. A method of preparing a superabsorbent polymer, comprising reacting a
water-soluble ethylenic unsaturated monomer, a photoinitiator, a crosslinking agent, and
a thermal polymerization initiator, in the presence of a water-soluble metal salt.
2. The method of claim 1, wherein the water-soluble metal salt comprises at least
one selected from the group consisting of a sulfate group, a nitrate group, a phosphate
group, a chloride group, a sulfite group, and a thiocyanate group.
3. The method of claim 1, wherein a metal of the water-soluble metal salt
comprises at least one selected from the group consisting of sodium (Na), lithium (Li),
and potassium (K).
4. The method of claim 1, wherein the water-soluble metal salt is contained in an
amount of 0.001 ~ 40.0 wt%, based on a total weight of the water-soluble ethylenic
unsaturated monomer.
5. The method of claim 1, comprising:
a) mixing the water-soluble ethylenic unsaturated monomer, the photoinitiator,
and the crosslinking agent;
b) diluting an alkaline aqueous solution with a water-soluble metal salt aqueous
solution;
c) neutralizing a mixture obtained in a) with a diluted solution obtained in b);
d) adding a mixture obtained in c) with the thermal polymerization initiator, and
then performing a radical polymerization reaction using thermal polymerization or
photopolymerization, thus forming a polymer sheet; and
e) adding the polymer sheet formed in d) with water, thus forming a hydrous gel
polymer.
6. The method of claim 5, further comprising, after e) forming the hydrous gel
polymer,
17
f) drying and grinding the hydrous gel polymer, thus obtaining superabsorbent
polymer particles; and
g) sorting the superabsorbent polymer particles depending on a particle size, thus
obtaining particles having a particle size of 150 ~ 850 􀁐m.
7. The method of claim 1, wherein the water-soluble ethylenic unsaturated
monomer comprises at least one selected from the group consisting of an anionic
monomer and salts thereof, including 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; a nonionic hydrophilic monomer,
including (meth)acrylamide, N-substituted (meth)acrylate, 2-
hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,
methoxypolyethyleneglycol (meth)acrylate, and polyethyleneglycol (meth)acrylate; and
an amino group-containing unsaturated monomer and quaternary salts thereof, including
(N,N)-dimethylaminoethyl (meth)acrylate, and (N,N)-dimethylaminopropyl
(meth)acrylamide.
8. The method of claim 1, wherein the photoinitiator comprises at least one
selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl
alkylketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine, and 􀁄-
aminoketone.
9. The method of claim 1, wherein the crosslinking agent comprises at least one
selected from the group consisting of a polyhydric alcohol compound; an acrylate-based
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.
10. The method of claim 5, wherein c) is performed at 30 ~ 50􀁱C.
18
11. The method of claim 1, wherein the thermal polymerization initiator
comprises at least one selected from the group consisting of a persulfate-based initiator,
including sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), and ammonium
persulfate ((NH4)2S2O8); an azo-based initiator, including 2,2-azobis(2-
amidinopropane)dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine
dihydrochloride, 2-(carbamoylazo)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-
yl)propane]dihydrochloride, and 4,4-azobis-(4-cyanovaleric acid); hydrogen peroxide;
and ascorbic acid.
12. The method of claim 5, wherein the thermal polymerization or
photopolymerization in d) is performed by irradiation with at least one heat source
selected from the group consisting of steam, electricity, UV light, and IR light.
13. The method of claim 12, wherein the irradiation with UV light is performed
by applying UV light at an intensity of 1 ~ 20 mW/cm2.
14. A superabsorbent polymer, prepared by the method of claim 1.
19
ABSTRACT
Disclosed are a superabsorbent polymer including a water-soluble salt and a
method of preparing the same, wherein the addition of a water-soluble metal salt in the
polymerization process of a superabsorbent polymer is very effective at decreasing the
concentration of residual monomer.
20
FIG. 1
FIG. 2A
21
FIG. 2B
FIG. 2C
22
FIG. 3

[Translation of KR 10-2015-0081378]
KOREAN INTELLECTUAL
PROPERTY OFFICE
This is to certify that the following application annexed hereto is a true copy
from the records of the Korean Intellectual Property Office.
Application No. : 10-2015-0081378
Filing Date : June 9, 2015
Applicant : LG CHEM, LTD.
June 11, 2015
COMMISSIONER(SEAL)
[Application Data Sheet]
􀬸Document Name􀬹 Document for Application
􀬸Right Classification􀬹 Patent
􀫺Applicant􀫻
􀫺Name􀫻 LG CHEM, LTD.
􀬸Applicant Code􀬹 1-2001-013456-3
􀬸Agent􀬹
􀫺Name􀫻 LEE, Myong Ku
􀬸Agent Code􀬹 9-2009-001103-9
􀬸Registration Number of General Power of Attorney􀬹2013-047909-5
􀬸Title􀬹 Super Absorbent Polymer Resin Containing Water-
Soluble Salt And Method Of Preparing The Same
􀬸Inventor􀬹
􀬸Name􀬹 KIM, Young-Sam
􀬸Address􀬹 LG Chem Research Park, 188, Munji-ro, Yuseong-gu
Daejeon, Republic Of Korea
􀬸Nationality􀬹 US
􀬸Inventor􀬹
􀬸Name􀬹 HONG, Yeon-Woo
􀬸Resident Registration No.􀬹 870721-2XXXXXX
􀬸Zip Code􀬹 305-738
􀬸Address􀬹 LG Chem Research Park, 188, Munji-ro, Yuseong-gu
Daejeon
􀬸Nationality􀬹 KR
􀬸Inventor􀬹
􀬸Name􀬹 LEE, Hyun-Jin
􀬸Resident Registration No.􀬹 811203-2XXXXXX
􀬸Zip Code􀬹 305-738
􀬸Address􀬹 LG Chem Research Park, 188, Munji-ro, Yuseong-gu
Daejeon
􀬸Nationality􀬹 KR
􀬸Priority claimed􀬹
􀬸Country􀬹 KR
􀬸Application No.􀬹 10-2014-0076594
􀬸Application Date􀬹 2014.06.23
􀬸Proof Documents􀬹 Not attached
􀬸Requesting Examination􀬹 Filed
We submit the document to commissioner as above.
Agent LEE, Myong Ku (seal)
􀫺Fees􀫻
􀫺Basic Filing Fee􀫻 0 pages 46,000 Won
􀫺Additional Filing Fee􀫻 33 pages 0 Won
􀫺Priority Claim Fee􀫻 1 Case 18,000 Won
􀫺Requesting Examination􀫻 15 Claims 803,000 Won
􀫺Total Amounts􀫻 867,000 Won
1
SUPERABSORBENT POLYMER INCLUDING WATER-SOLUBLE SALT AND
METHOD OF PREPARING THE SAME
BACKGROUND OF THE INVENTION
1. Technical Field
[0001] The present invention relates to a superabsorbent polymer including a watersoluble
salt and a method of preparing the same and, more particularly, to a
superabsorbent polymer including a water-soluble salt and a method of preparing the
same, wherein the addition of a water-soluble metal salt in the polymerization process of
a superabsorbent polymer is very effective at decreasing the concentration of residual
monomer (RM).
2. Description of the Related Art
[0002] Superabsorbent polymers (SAPs) are synthetic polymers able to absorb water
about 500 ~ 1000 times their own weight. Such superabsorbent polymers have actually
begun to be used for sanitary items, and are being currently widely utilized in not only
hygiene products such as baby disposable diapers and so on, but also in gardening soil
repair agents, water stop agents for civil construction, seeding sheets, freshness retaining
agents in food distribution sectors, and fomentation materials. Compared to
conventional absorbent materials, superabsorbent polymers have an outstanding
absorption capacity and thus the market value thereof is increasing with the wider range
of applications thereof.
[0003] The absorption mechanism of the superabsorbent polymer is controlled by
osmotic pressure due to a difference in electric attraction represented by charges of a
polymer electrolyte, an affinity between water and a polymer electrolyte, molecular
expansion due to repulsive force between polymer electrolyte ions, and interactions of
expansion inhibition due to crosslinkages. Briefly, absorbability of the absorbent
polymer depends on the aforementioned affinity and molecular expansion, and the
absorption rate thereof is greatly affected by osmotic pressure of the absorbent polymer
itself.
2
[0004] In order to use the superabsorbent polymer as a hygiene material, the
superabsorbent polymer has to essentially possess a low concentration of RM. To lower
the concentration of RM, the use of acrylic acid having low dimer concentration is
fundamentally adopted, and other methods may also be proposed but are problematic
because other properties of the superabsorbent polymer may deteriorate or such
additional processing may be complicated.
[0005] Therefore, thorough research is ongoing into preparation of superabsorbent
polymers having high absorption rate and high absorption scale.
SUMMARY OF THE INVENTION
[0006] Accordingly, the present invention has been made keeping in mind the problems
encountered in the related art, and an object of the present invention is to provide a
superabsorbent polymer and a method of preparing the same, wherein adding a watersoluble
metal salt is further included in a conventional method of preparing a
superabsorbent polymer, thereby decreasing the concentration of RM.
[0007] In order to accomplish the above object, the present invention provides a method
of preparing a superabsorbent polymer, comprising reacting a water-soluble ethylenic
unsaturated monomer, a photoinitiator, a crosslinking agent, and a thermal
polymerization initiator, in the presence of a water-soluble metal salt.
[0008] Furthermore, the method of preparing the superabsorbent polymer comprises: a)
mixing the water-soluble ethylenic unsaturated monomer, the photoinitiator, and the
crosslinking agent; b) diluting a sodium hydroxide aqueous solution with a water-soluble
metal salt aqueous solution; c) neutralizing the mixture obtained in a) with the diluted
solution obtained in b); d) adding the mixture obtained in c) with the thermal
polymerization initiator, and then performing a radical polymerization reaction using
thermal polymerization or photopolymerization, thus forming a polymer sheet; and e)
adding the polymer sheet formed in d) with water, thus forming a hydrous gel polymer.
[0009] In addition, the present invention provides a superabsorbent polymer, prepared
by the above method.
3
[0010] According to the present invention, a superabsorbent polymer is advantageous
because the concentration of RM can be effectively lowered, compared to a conventional
superabsorbent polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graph illustrating changes in concentration of RM depending on the
kind and amount of metal sulfate of a superabsorbent polymer according to an
embodiment of the present invention;
[0012] FIG. 2A is a graph illustrating changes in concentration of RM depending on the
kind and amount of Na2SO4, FIG. 2B is a graph illustrating changes in concentration of
RM depending on the kind and amount of Li2SO4, FIG. 2C is a graph illustrating
changes in concentration of RM depending on the kind and amount of K2SO4; and
[0013] FIG. 3 is a graph illustrating a decrease in concentration of RM when Na2SO4 as
a metal sulfate is added in an excessive amount (15 wt%).
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0014] Hereinafter, a detailed description will be given of the present invention.
[0015] The present invention addresses a method of preparing a superabsorbent
polymer, comprising reacting a water-soluble ethylenic unsaturated monomer, a
photoinitiator, a crosslinking agent, and a thermal polymerization initiator, in the
presence of a water-soluble metal salt.
[0016] More specifically, the water-soluble metal salt preferably includes at least one
selected from the group consisting of a sulfate group, a nitrate group, a phosphate group,
a chloride group, a sulfite group, and a thiocyanate group. Particularly useful is a metal
sulfate.
[0017] The metal of the water-soluble metal salt preferably includes at least one selected
from the group consisting of sodium (Na), lithium (Li), potassium (K), aluminum (Al),
zirconium (Zr), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese
(Mn), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), silver (Ag), platinum (Pt), and gold
(Au). Particularly useful is sodium (Na), lithium (Li), or potassium (K).
4
[0018] According to the present invention, the method of preparing the superabsorbent
polymer comprises: a) mixing the water-soluble ethylenic unsaturated monomer, the
photoinitiator, and the crosslinking agent; b) diluting a sodium hydroxide aqueous
solution with a water-soluble metal salt aqueous solution; c) neutralizing the mixture
obtained in a) with the diluted solution obtained in b); d) adding the mixture obtained in
c) with the thermal polymerization initiator, and then performing a radical
polymerization reaction using thermal polymerization or photopolymerization, thus
forming a polymer sheet; and e) adding the polymer sheet formed in d) with water, thus
forming a hydrous gel polymer.
[0019] The method may further comprise, after e) forming the hydrous gel polymer, f)
drying and grinding the hydrous gel polymer, thus obtaining superabsorbent polymer
particles; and g) sorting the superabsorbent polymer particles depending on the particle
size, thus obtaining particles having a particle size of 150 ~ 850 􀁐m.
[0020] As used herein, the term “superabsorbent polymer particles” refers to particles
obtained by drying and grinding a hydrous gel polymer. More specifically, the hydrous
gel polymer is a material in solid jelly form with a size of 1 cm or more having water in
a large amount (50% or more) after completion of the polymerization. The hydrous gel
polymer is dried and ground in a powder phase, yielding superabsorbent polymer
particles. Thus, the hydrous gel polymer corresponds to a process intermediate.
[0021] In a) of the method of preparing the superabsorbent polymer according to the
present invention, the water-soluble ethylenic unsaturated monomer, the photoinitiator
and the crosslinking agent are mixed.
[0022] 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 any
one or more 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
5
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
improved absorbability may be obtained advantageously. Also, 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 ~ 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%,
grinding efficiency of the hydrous gel polymer may decrease.
[0023] In the method of preparing the superabsorbent polymer according to the present
invention, the photoinitiator (photopolymerization initiator) is not particularly limited,
but preferably includes at least one selected from the group consisting of benzoin ether,
dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, benzyl dimethyl ketal,
acyl phosphine, and 􀁄-aminoketone. A specific example of the acyl phosphine may
include commercially available lucirin TPO, namely, 2,4,6-trimethyl-benzoyl-trimethyl
phosphine oxide, or commercially available Irgacure series may be used as the
photoinitiator able to form a thick polymer layer with relatively high permeability.
[0024] In the method of preparing the superabsorbent polymer according to the present
invention, the 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 crosslinking agent may include at least one selected from
the group consisting of a polyhydric alcohol compound; an acrylate-based 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.
6
[0025] 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.
[0026] The acrylate-based compound may be exemplified by
poly(ethyleneglycol)diacrylate.
[0027] 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.
[0028] Examples of the haloepoxy compound may include epichlorohydrine,
epibromohydrine, and 􀁄-methylepichlorohydrine. 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 crosslinking process, the crosslinking
agent preferably includes at least one polyhydric alcohol compound, and more
preferably includes a polyhydric alcohol compound having 2 to 10 carbon atoms.
[0029] The amount of the crosslinking agent added to treat the surface of the polymer
particles may be properly determined depending on the kind of crosslinking agent or the
reaction conditions, and is set to 0.001 ~ 5 parts by weight, preferably 0.01 ~ 3 parts by
weight, and more preferably 0.05 ~ 2 parts by weight, based on 100 parts by weight of
the polymer. If the amount of the crosslinking agent is too small, a crosslinking reaction
seldom occurs. In contrast, if the amount thereof exceeds 5 parts by weight based on
100 parts by weight of the polymer, properties of the superabsorbent polymer may
deteriorate due to an excessive crosslinking reaction.
[0030] In b), the alkaline aqueous solution is diluted with the water-soluble metal salt
aqueous solution.
7
[0031] The alkaline aqueous solution is preferably a sodium hydroxide (NaOH) aqueous
solution or a potassium hydroxide (KOH) aqueous solution. More preferably useful is a
sodium hydroxide (NaOH) aqueous solution.
[0032] The water-soluble metal salt preferably includes at least one selected from the
group consisting of a sulfate group, a nitrate group, a phosphate group, a chloride group,
a sulfite group, and a thiocyanate group. Particularly useful is a metal sulfate.
[0033] The metal of the water-soluble metal salt preferably includes at least one selected
from the group consisting of sodium (Na), lithium (Li), potassium (K), aluminum (Al),
zirconium (Zr), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese
(Mn), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), silver (Ag), platinum (Pt), and gold
(Au). Particularly useful is sodium (Na), lithium (Li), or potassium (K).
[0034] The water-soluble metal salt is preferably used in an amount of 0.001 ~ 40.0
wt%, more preferably 2.0 ~ 20.0 wt%, and still more preferably 2.5 ~ 15.0 wt%, based
on the total weight of the water-soluble ethylenic unsaturated monomer. If the amount
of the water-soluble metal salt is less than 0.001 wt%, an effect of decreasing the
concentration of RM may become insignificant. In contrast, if the amount thereof
exceeds 40.0 wt%, the main component of the superabsorbent polymer may be
undesirably changed into a salt, but not the monomer. Taking into consideration the
mixing ratio of components, the maximum amount of the above component is
determined.
[0035] The water-soluble metal salt does not directly participate in an actual chemical
reaction and has an electrostatic influence on cation transfer. The water-soluble
ethylenic unsaturated monomer is mixed with a sodium hydroxide aqueous solution so
as to be partially neutralized, followed by radical polymerization. As such, the watersoluble
metal salt may be added to decrease electrical repulsion between anionic
monomers. The cation of the water-soluble metal salt is regarded as important, and the
repulsion between monomers may be decreased by virtue of a shielding effect due to the
cation, thus enabling efficient polymerization. Consequently, the superabsorbent
polymer prepared by the above method may be expected to exhibit an effect of
decreasing the concentration of RM.
[0036] In c), the mixture obtained in a) is introduced with the diluted solution obtained
in b) so as to be neutralized. As such, c) may be carried out at 30 ~ 50􀁱C.
8
[0037] In d), the mixture obtained in c) is further added with the thermal polymerization
initiator, after which radical polymerization using thermal polymerization or
photopolymerization may be implemented, thus forming a polymer sheet.
[0038] The superabsorbent polymer according to the present invention 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.
[0039] 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).
[0040] Also, the polymerization method is 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 method is merely illustrative, and
the present invention is not limited to such a polymerization method.
[0041] 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 to a size ranging from ones of mm to ones of cm
9
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 ~ 50 mm may be obtained.
[0042] Also, when photopolymerization is carried out using a reactor with a movable
conveyor belt, a hydrous gel polymer in a sheet form with a belt width 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 obtain a polymer sheet having a thickness of
0.5 ~ 5 cm. In the case where the monomer composition is supplied to the extent that a
very thin polymer sheet is formed, production efficiency may decrease undesirably. If
the thickness of the polymer sheet is greater than 5 cm, polymerization may not be
uniformly carried out throughout the sheet that is too thick.
[0043] In an embodiment of the present invention, thermal polymerization or
photopolymerization in d) may be implemented by applying at least one heat source
selected from the group consisting of steam, electricity, UV light, and IR light. As such,
UV light may be applied at an intensity 1 ~ 20 mW/cm2.
[0044] In e), the polymer sheet formed in d) is added with water, yielding a hydrous gel
polymer.
[0045] The hydrous gel polymer thus obtained typically has a moisture content of 30 ~
60 wt%. As used herein, the term “moisture content” refers to an amount of moisture
based on the total weight of the hydrous gel polymer, namely, 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 a weight reduction due to
moisture evaporation from the polymer during drying the polymer at 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).
[0046] In f), the hydrous gel polymer is dried and ground, thus obtaining superabsorbent
polymer particles.
10
[0047] In the drying process, the drying temperature may be set to 150 ~ 250􀁱C. As
used herein, the term “drying temperature” refers to a temperature of a heat medium
supplied for the drying process or a temperature of a drying reactor including a heat
medium and a polymer in the drying process.
[0048] 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 thus
deteriorate. In contrast, if the drying temperature is higher than 250􀁱C, only the surface
of the polymer may be excessively dried, and thereby fine powder 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 ~ 250􀁱C,
and more preferably 160 ~ 200􀁱C.
[0049] The drying time is not limited, but may be set to 20 ~ 90 min taking into account
the process efficiency.
[0050] Also, the drying process is not limited so long as it is typically used to dry the
hydrous gel polymer. Specific examples thereof may include hot air supply, IR
irradiation, microwave irradiation, and UV irradiation. The polymer after the drying
process may have a moisture content of 0.1 ~ 10 wt%.
[0051] 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 is 1 ~ 15 mm. Grinding the particle size of the polymer to less than 1 mm is
technically difficult due to high moisture content of the hydrous gel polymer, and also
the ground particles may agglomerate. In contrast, if the polymer is ground to a particle
size of greater than 15 mm, an effect of increasing the drying efficiency via the grinding
process may become insignificant.
[0052] In the simple grinding process before the drying process, any grinder may be
used without limitation. A 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.
11
[0053] When the grinding process is performed to increase the drying efficiency before
the drying process in this way, the polymer having 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. Specifically, the kind of usable additive 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.
[0054] In g), the superabsorbent polymer particles are sorted depending on the particle
size, thus obtaining particles having a particle size of 150 ~ 850 􀁐m.
[0055] The superabsorbent polymer particles resulting from the grinding process have a
particle size of 150 ~ 850 􀁐m. In the method of preparing the superabsorbent polymer
according to the present invention, a grinder used to obtain such a 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.
[0056] In addition, the present invention addresses a superabsorbent polymer prepared
by the preparation method as above. As measured by an EDANA measurement method,
such a superabsorbent polymer is advantageous because the concentration of RM is
lowered by the addition of the water-soluble metal salt, compared to when the watersoluble
metal salt is not added.
[0057] A better understanding of the present invention may be obtained via the
following examples that are set forth to illustrate, but are not to be construed as limiting
the scope of the present invention. The scope of the present invention is shown in the
claims, and also contains all modifications within the meaning and range equivalent to
the claims. Unless otherwise mentioned, “%” and “part” showing the amount in the
following examples and comparative examples refer to a mass basis.
[0058] Examples 1 to 7 and Comparative Example 1: Preparation of
superabsorbent polymer
12
[0059] Example 1
[0060] 192.7 g of an aqueous solution having 13.0 g (2.5 wt% based on acrylic acid) of
Na2SO4 dissolved therein was used to dilute 629.6 g of a 32 wt% sodium hydroxide
aqueous solution, after which acrylic acid, Irgacure 819 as a photoinitiator, and PEGDA
(Poly(ethyleneglycol)diacrylate) as a crosslinking agent were sequentially added using a
pump. The resulting mixture was gradually cooled using a cooler, placed in a plastic
beaker containing a sodium persulfate aqueous solution as a thermal initiator, poured
into a pre-stabilized UV irradiator, and then irradiated with UV light (at an intensity of
10 mW/cm2). The point of time when a foaming sound was heard was measured using a
stopwatch, and UV irradiation was stopped when the measurement time reached 1 min,
and the resulting product was allowed to stand for 2 min in the UV irradiator. The
polymerized sheet was snipped, uniformly mixed with 250 g of water so as to absorb
such water, and then cut into small pieces using a chopper. The small pieces thus cut
were evenly spread in an oven, dried and then ground. Then, a superabsorbent polymer
having a particle size of 150 ~ 850 􀁐m was obtained using a standard sieve.
[0061] Example 2
[0062] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 25.9 g (5.0
wt% based on acrylic acid) of Na2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0063] Example 3
[0064] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 77.7 g (15.0
wt% based on acrylic acid) of Na2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0065] Example 4
[0066] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 13.0 g (2.5
wt% based on acrylic acid) of Li2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0067] Example 5
[0068] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 25.9 g (5.0
wt% based on acrylic acid) of Li2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0069] Example 6
13
[0070] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 13.0 g (2.5
wt% based on acrylic acid) of K2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0071] Example 7
[0072] In 192.7 g of water used to dilute sodium hydroxide in Example 1, 20.7 g (4.0
wt% based on acrylic acid) of K2SO4 was dissolved, after which the same procedures as
in Example 1 were performed, yielding a superabsorbent polymer.
[0073] Comparative Example 1
[0074] A superabsorbent polymer was prepared in the same manner as in Example 1,
with the exception that the water-soluble metal salt was not added.
[0075] The amounts of salts in Examples 1 to 7 and Comparative Example 1 are given
in Table 1 below.
[Table 1]
Salt
Amount of Salt
wt% based on acrylic acid Weight (g)
Ex. 1
Na2SO4
2.5 13.0
Ex. 2 5.0 25.9
Ex. 3 15.0 77.7
Ex. 4
Li2SO4
2.5 13.0
Ex. 5 5.0 25.9
Ex. 6
K2SO4
2.5 13.0
Ex. 7 4.0 20.7
C.Ex. 1 None - -
[0076] Test Example: Evaluation of properties of superabsorbent polymer
[0077] To evaluate the properties of the superabsorbent polymers of Examples 1 to 7
and Comparative Example 1, the following testing was performed.
[0078] Residual Monomer (RM)
14
[0079] In the superabsorbent polymers of Examples 1 to 7 and Comparative Example 1,
the concentration of RM was measured based on WSP 210.3 according to an EDANA
method. 1.000 g of a superabsorbent polymer sample having a particle size of 150 ~ 850 􀁐m and 200 g of 0.9% brine were placed in a 250 mL Erlenmeyer flask and stirred for 1
hr. Subsequently, the resulting mixture was filtered using filter paper, and the solution
was sampled and measured by HPLC.
[0080] Also, in the superabsorbent polymers of the examples and comparative example,
changes in the concentration of RM depending on the kind and amount of metal sulfate
were measured. The results are shown in Table 2 below and FIG. 1.
[Table 2]
Metal sulfate
RM (ppm)
Kind wt%
Ex. 1
Na2SO4
2.5 804
Ex. 2 5.0 850
Ex. 3 15.0 822
Ex. 4
Li2SO4
2.5 794
Ex. 5 5.0 676
Ex. 6
K2SO4
2.5 848
Ex. 7 4.0 853
C.Ex. 1 - - 977
[0081] The concentration values of RM in the above examples and comparative example
are average values of several tests.
[0082] As is apparent from Table 2 and FIG. 1, when the water-soluble metal salt, that
is, the metal sulfate, was added in the polymerization process of a superabsorbent
polymer, the concentration of RM was effectively decreased, compared to when the
metal sulfate was not added (Comparative Example 1).
[0083] More specifically, FIGS. 2A to 2C are graphs illustrating changes in the
concentration of RM depending on the kind and amount of metal sulfate. Based on the
15
average concentration values of RM, when the water-soluble metal salt, that is, the metal
sulfate was added in the same amount (mass) depending on the kind thereof, the salts
effective at decreasing the concentration of RM were represented by the sequence of
Li2SO4 > Na2SO4 > K2SO4, or were represented by the sequence of Li2SO4 > K2SO4 >
Na2SO4 in some cases. Although the extent of decrease in concentration of RM varied
depending on the amount of the salt, there were significant differences in the effects due
to the presence or absence of the metal sulfate as the water-soluble metal salt.
[0084] As illustrated in FIG. 3, when the water-soluble metal salt, that is, the metal
sulfate Na2SO4 was added in an excessive amount (15 wt%), the concentration of RM
was uniformly decreased in a negative (-) slope, compared to when the metal sulfate was
not added (Comparative Example 1). Thereby, the effect of the metal sulfate on
decreasing the concentration of RM was superior despite the excessive use thereof.
[0085] Therefore, the superabsorbent polymer according to the present invention can be
effectively decreased in the concentration of RM by the addition of a water-soluble
metal salt, compared to a conventional superabsorbent polymer.
16
WHAT IS CLAIMED IS:
1. A method of preparing a superabsorbent polymer, comprising reacting a
water-soluble ethylenic unsaturated monomer, a photoinitiator, a crosslinking agent, and
a thermal polymerization initiator, in the presence of a water-soluble metal salt.
2. The method of claim 1, wherein the water-soluble metal salt comprises at least
one selected from the group consisting of a sulfate group, a nitrate group, a phosphate
group, a chloride group, a sulfite group, and a thiocyanate group.
3. The method of claim 1, wherein a metal of the water-soluble metal salt
comprises at least one selected from the group consisting of sodium (Na), lithium (Li),
potassium (K), aluminum (Al), zirconium (Zr), scandium (Sc), titanium (Ti), vanadium
(V), chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn),
silver (Ag), platinum (Pt), and gold (Au).
4. The method of claim 1, wherein the water-soluble metal salt is contained in an
amount of 0.001 ~ 40.0 wt%, based on a total weight of the water-soluble ethylenic
unsaturated monomer.
5. The method of claim 1, comprising:
a) mixing the water-soluble ethylenic unsaturated monomer, the photoinitiator,
and the crosslinking agent;
b) diluting an alkaline aqueous solution with a water-soluble metal salt aqueous
solution;
c) neutralizing a mixture obtained in a) with a diluted solution obtained in b);
d) adding a mixture obtained in c) with the thermal polymerization initiator, and
then performing a radical polymerization reaction using thermal polymerization or
photopolymerization, thus forming a polymer sheet; and
e) adding the polymer sheet formed in d) with water, thus forming a hydrous gel
polymer.
17
6. The method of claim 5, further comprising, after e) forming the hydrous gel
polymer,
f) drying and grinding the hydrous gel polymer, thus obtaining superabsorbent
polymer particles; and
g) sorting the superabsorbent polymer particles depending on a particle size, thus
obtaining particles having a particle size of 150 ~ 850 􀁐m.
7. The method of claim 5, wherein the alkaline aqueous solution is a sodium
hydroxide (NaOH) aqueous solution or a potassium hydroxide (KOH) aqueous solution.
8. The method of claim 1, wherein the water-soluble ethylenic unsaturated
monomer comprises at least one selected from the group consisting of an anionic
monomer and salts thereof, including 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; a nonionic hydrophilic monomer,
including (meth)acrylamide, N-substituted (meth)acrylate, 2-
hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,
methoxypolyethyleneglycol (meth)acrylate, and polyethyleneglycol (meth)acrylate; and
an amino group-containing unsaturated monomer and quaternary salts thereof, including
(N,N)-dimethylaminoethyl (meth)acrylate, and (N,N)-dimethylaminopropyl
(meth)acrylamide.
9. The method of claim 1, wherein the photoinitiator comprises at least one
selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl
alkylketone, phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine, and 􀁄-
aminoketone.
10. The method of claim 1, wherein the crosslinking agent comprises at least one
selected from the group consisting of a polyhydric alcohol compound; an acrylate-based
compound; an epoxy compound; a polyamine compound; a haloepoxy compound; a
haloepoxy compound condensed product; an oxazoline compound; a mono-, di- or poly18
oxazolidinone compound; a cyclic urea compound; a polyhydric metal salt; and an
alkylene carbonate compound.
11. The method of claim 5, wherein c) is performed at 30 ~ 50􀁱C.
12. The method of claim 1, wherein the thermal polymerization initiator
comprises at least one selected from the group consisting of a persulfate-based initiator,
including sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), and ammonium
persulfate ((NH4)2S2O8); an azo-based initiator, including 2,2-azobis(2-
amidinopropane)dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine
dihydrochloride, 2-(carbamoylazo)isobutyronitrile, 2,2-azobis[2-(2-imidazolin-2-
yl)propane]dihydrochloride, and 4,4-azobis-(4-cyanovaleric acid); hydrogen peroxide;
and ascorbic acid.
13. The method of claim 5, wherein the thermal polymerization or
photopolymerization in d) is performed by irradiation with at least one heat source
selected from the group consisting of steam, electricity, UV light, and IR light.
14. The method of claim 13, wherein the irradiation with UV light is performed
by applying UV light at an intensity of 1 ~ 20 mW/cm2.
15. A superabsorbent polymer, prepared by the method of claim 1.