[Technical Field]
5 Cross-reference to Related Application
The present application is based on, and claims priority from, Korean
Patent Application Nos. 10-2020-0166858 and 10-2021-0159519, filed on
December 2, 2020, and November 18, 2021, respectively, the disclosures of
which are hereby incorporated by reference herein in their entirety.
10
The present invention relates to a method of preparing a superabsorbent
polymer, in which an interpenetrating polymer network (IPN) is formed on the
surface of the superabsorbent polymer.
[Background Art]
15 A superabsorbent polymer (SAP) is a synthetic polymeric material
capable of absorbing moisture from 500 to 1000 times its own weight. Various
manufacturers have denominated it as different names, such as SAM (Super
Absorbency Material), AGM (Absorbent Gel Material), etc. Since such
superabsorbent polymers started to be practically applied in sanitary products,
20 now they have been widely used not only for hygiene products such as paper
diapers for children, etc., but also for water retaining soil products for gardening,
water stop materials for the civil engineering and construction, sheets for raising
seedling, fresh-keeping agents for food distribution fields, materials for poultice
or the like.
25
In most cases, these superabsorbent polymers have been widely used
in the field of hygienic materials such as diapers, sanitary pads, etc. In such
hygienic materials, the superabsorbent polymer is generally contained in a state
of being spread in the pulp. In recent years, however, continuous efforts have
30 been made to provide hygienic materials such as diapers having a thinner
thickness. As a part of such efforts, the development of so-called pulpless diapers
and the like in which the content of pulp is reduced or pulp is not used at all is
2
being actively advanced.
As described above, in the case of hygienic materials in which the
content of pulp is reduced or the pulp is not used, a superabsorbent polymer is
5 contained at a relatively high ratio and these superabsorbent polymer particles
are inevitably contained in multiple layers in the hygienic materials. In order for
the whole superabsorbent polymer particles contained in the multiple layers to
more efficiently absorb a liquid such as urine, etc., it is necessary for the
superabsorbent polymer to basically exhibit high absorption performance and
10 absorption rate. For this purpose, a method of crosslinking the surface of the
superabsorbent polymer is used.
The surface crosslinking of the superabsorbent polymer is to form a
surface-crosslinked layer by binding functional groups such as a carboxyl group,
15 etc. present on the surface of the superabsorbent polymer, and this surface
crosslinking may improve physical properties of the superabsorbent polymer.
However, when the surface-crosslinked layer is formed by this method, the
physical strength may increase, but the absorption rate of the superabsorbent
polymer may decrease. Therefore, there is a need for a method of forming the
20 surface-crosslinked layer without impairing the intrinsic physical properties of the
superabsorbent polymer.
[Disclosure]
[Technical Problem]
There is provided a method of preparing a superabsorbent polymer, in
25 which an interpenetrating polymer network (IPN) is formed on the surface of the
superabsorbent polymer during formation of a surface-crosslinked layer of the
superabsorbent polymer.
[Technical Solution]
In order to achieve the above object, there is provided a method of
30 preparing a superabsorbent polymer, the method including the following steps of:
1) carrying out a crosslinking polymerization of a water-soluble ethylenebased unsaturated monomer having acidic groups which are at least partially
3
neutralized, in the presence of an internal crosslinking agent, to form a watercontaining gel polymer including a first crosslinked polymer (step 1);
2) coarsely pulverizing the water-containing gel polymer (step 2);
3) drying, pulverizing, and size-sorting the water-containing gel polymer
5 to form a base polymer powder (step 3); and
4) reacting the base polymer powder with a surface crosslinking solution
to form an interpenetrating polymer network on the surface of the base polymer
powder (step 4),
wherein the surface crosslinking solution includes a monomer having an
10 amine group and a compound represented by the following Chemical Formula 1:
[Chemical Formula 1]
in Chemical Formula 1,
n is an integer of 1 to 10.
15
The method of preparing a superabsorbent polymer largely includes a
step of carrying out a polymerization of a water-soluble ethylene-based
unsaturated monomer to form a water-containing gel polymer, and a step of
pulverizing the water-containing gel polymer. Further, to improve various physical
20 properties of the superabsorbent polymer, a method of crosslinking the surface
of the prepared superabsorbent polymer is used.
Generally, the surface crosslinking of the superabsorbent polymer is
formed by binding functional groups with each other, such as hydroxyl groups,
25 etc., which are present on the surface of the superabsorbent polymer. Since the
functional groups present in the superabsorbent polymer are chemically reacted,
some physical properties of the superabsorbent polymer may deteriorate.
Accordingly, in the present invention, instead of the functional groups
30 such as hydroxyl groups, etc. present on the surface of the superabsorbent
polymer, a new polymer chain is introduced as an interpenetrating polymer
4
network (IPN) on the surface of the superabsorbent polymer to form a surfacecrosslinked layer. Therefore, the present invention is characterized in that the
surface-crosslinked layer is formed without deteriorating the intrinsic physical
properties of the superabsorbent polymer, and consequently, various physical
5 properties of the superabsorbent polymer are improved.
Hereinafter, each step of the present invention will be described in detail.
(Step 1)
10 The step 1 is a step of preparing a water-containing gel polymer, and
specifically, a step of forming a water-containing gel polymer by carrying out a
crosslinking polymerization of a monomer composition including a water-soluble
ethylene-based unsaturated monomer having acidic groups which are at least
partially neutralized.
15
The water-soluble ethylene-based unsaturated monomer may be any
monomer commonly used in the preparation of superabsorbent polymers.
Specifically, the water-soluble ethylene-based unsaturated monomer may be a
compound represented by the following Chemical Formula 2:
20 [Chemical Formula 2]
R1
-COOM1
in Chemical Formula 2,
R1
is an alkyl group containing an unsaturated bond and having 2 to 5
carbon atoms, and
M1 25 is a hydrogen atom, a monovalent or divalent metal, an ammonium
group, or an organic amine salt.
Preferably, the water-soluble ethylene-based unsaturated monomer
may include one or more selected from the group consisting of acrylic acid,
30 methacrylic acid, and a monovalent metal salt thereof, a divalent metal salt
thereof, an ammonium salt thereof, and an organic amine salt thereof. As
described, when acrylic acid or a salt thereof is used as the water-soluble
5
ethylene-based unsaturated monomer, it is advantageous in terms of obtaining a
superabsorbent polymer having improved absorbency. In addition, the monomer
may include one or more selected from the group consisting of an anionic
monomer such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-
5 acryloyl ethane sulfonic acid, 2-methacryloyl ethane sulfonic acid, 2-
(meth)acryloyl propane sulfonic acid, or 2-(meth)acrylamide-2-methylpropane
sulfonic acid, and a salt thereof; a nonionic hydrophilic monomer such as
(meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate,
2-hydroxypropyl(meth)acrylate, methoxypolyethyleneglycol(meth)acrylate, or
10 polyethyleneglycol(meth)acrylate; and an unsaturated monomer containing an
amino group such as (N,N)-dimethylaminoethyl(meth)acrylate or (N,N)-
dimethylaminopropyl(meth)acrylamide, and a quaternary compound thereof.
The water-soluble ethylene-based unsaturated monomer may have an
15 acidic group, in which at least a part of the acidic group is neutralized. Preferably,
those partially neutralized with an alkaline substance such as sodium hydroxide,
potassium hydroxide, ammonium hydroxide or the like may be used as the
monomer.
20 In this regard, the degree of neutralization of the monomer may be 40
mol% to 95 mol%, or 40 mol% to 80 mol%, or 45 mol% to 75 mol%. The range of
the degree of neutralization may vary depending on the final physical properties.
An excessively high degree of neutralization causes the neutralized monomers
to be precipitated, and thus polymerization may not readily occur. On the contrary,
25 an excessively low degree of neutralization not only deteriorates the absorbency
of the polymer but also endows the polymer with hard-to-handle properties, like
elastic rubber.
The monomer composition may include a polymerization initiator
30 commonly used in the preparation of superabsorbent polymers.
As the polymerization initiator, a thermal polymerization initiator, a
6
photopolymerization initiator or the like may be used depending on the
polymerization method. However, even in the case of the photopolymerization
method, a certain amount of heat is generated by ultraviolet irradiation, etc., and
a certain amount of heat is generated in accordance with the progression of the
5 polymerization reaction, which is an exothermic reaction, and therefore, a thermal
polymerization initiator may be further included.
The photopolymerization initiator may include, for example, one or more
compounds selected from the group consisting of benzoin ether, dialkyl
10 acetophenone, hydroxyl alkyl ketone, phenyl glyoxylate, benzyl dimethyl ketal,
acyl phosphine, and α-aminoketone. Among them, as a specific example of the
acylphosphine, a commercially available lucirin TPO, i.e., 2,4,6-trimethylbenzoyl-trimethyl phosphine oxide may be used. More various
photopolymerization initiators are well disclosed in “UV Coatings: Basics, Recent
15 Developments and New Application(Elsevier, 2007)” written by Reinhold
Schwalm, p 115, the content of which is incorporated herein by reference.
As the thermal polymerization initiator, one or more compounds
selected from the group consisting of a persulfate-based initiator, an azo-based
20 initiator, hydrogen peroxide, and ascorbic acid may be used. Specific examples
of the persulfate-based initiator may include sodium persulfate (Na2S2O8),
potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc. In
addition, examples of the azo-based initiator may include 2,2-azobis(2-
amidinopropane)dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine
25 dihydrochloride, 2-(carbamoylazo)isobutylonitrile, 2,2-azobis[2-(2-imidazolin-2-
yl)propane]dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), etc. More various
thermal polymerization initiators are well disclosed in “Principle of
Polymerization(Wiley, 1981)” written by Odian, p 203, the content of which is
incorporated herein by reference.
30
The polymerization initiator may be included at a concentration of about
0.001% by weight to about 1% by weight, or 0.005% by weight to about 0.1% by
7
weight, based on the monomer composition. In other words, when the
concentration of the polymerization initiator is too low, the polymerization rate
may become slow and a large amount of residual monomer may be extracted in
the final product, which is not preferred. On the contrary, when the concentration
5 of the polymerization initiator is too high, polymer chains constituting the network
become short, and thus the content of water-soluble components is increased
and physical properties of the polymer may deteriorate, such as a reduction in
absorbency under pressure, which is not preferred.
10 Meanwhile, the polymerization of the monomer composition may be
carried out in the presence of a crosslinking agent (“internal crosslinking agent”)
in order to improve physical properties of the polymer by polymerization of the
water-soluble ethylene-based unsaturated monomer. The crosslinking agent is
used for internal crosslinking of the water-containing gel polymer, and may be
15 used, separately from “a surface crosslinking agent” described below.
The internal crosslinking agent may be any compound, as long as it
enables introduction of crosslinking during the polymerization of the water-soluble
ethylene-based unsaturated monomer. As non-limiting examples of the internal
20 crosslinking agent, multifunctional crosslinking agents such as N,N’-
methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol
di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol
di(meth)acrylate, polypropylene glycol di(meth)acrylate, butanediol
di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol
25 di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate,
tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate,
dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol
tetraacrylate, triallylamine, allyl(meth)acrylate, ethylene glycol diglycidyl ether,
propylene glycol, glycerin, or ethylene carbonate may be used alone or in
30 combination of two or more thereof.

[Claim 1]
A method of preparing a superabsorbent polymer, the method
comprising the steps of:
5 1) carrying out a crosslinking polymerization of a water-soluble ethylenebased unsaturated monomer having acidic groups which are at least partially
neutralized, in the presence of an internal crosslinking agent, to form a watercontaining gel polymer including a first crosslinked polymer (step 1);
2) coarsely pulverizing the water-containing gel polymer (step 2);
10 3) drying, pulverizing, and size-sorting the water-containing gel polymer
to form a base polymer powder (step 3); and
4) reacting the base polymer powder with a surface crosslinking solution
to form an interpenetrating polymer network on the surface of the base polymer
powder (step 4),
15 wherein the surface crosslinking solution includes a monomer having an
amine group and a compound represented by the following Chemical Formula 1:
[Chemical Formula 1]
in Chemical Formula 1,
20 n is an integer of 1 to 10.
[Claim 2]
The method of claim 1, wherein
a solvent of the surface crosslinking solution is water.
25
[Claim 3]
The method of claim 1, wherein
pH of the surface crosslinking solution is 1 to 3.
30 [Claim 4]
The method of claim 1, wherein
22
the monomer having an amine group is chitosan or cyclodextrin.
[Claim 5]
The method of claim 1, wherein
5 a concentration of the monomer having an amine group in the surface
crosslinking solution is 1 N to 10 N.
[Claim 6]
The method of claim 1, wherein
10 a concentration of the compound represented by Chemical Formula 1 in
the surface crosslinking solution is 1 N to 10 N.
[Claim 7]
The method of claim 1, wherein
15 a weight ratio of the chitosan and the compound represented by
Chemical Formula 1 in the surface crosslinking solution is 1:1 to 5:1.
[Claim 8]
The method of claim 1, wherein
20 n of Chemical Formula 1 is an integer of 2 to 4.
[Claim 9]
The method of claim 1, wherein
the compound represented by Chemical Formula 1 is glutaraldehyde.
25
[Claim 10]
The method of claim 1, wherein
the step 4 is carried out at a temperature of 20°C to 90°C.
30 [Claim 11]
The method of claim 1, wherein
CRC of the superabsorbent polymer is 30 g/g to 40 g/g.
23
[Claim 12]
The method of claim 1, wherein
a gel strength of the superabsorbent polymer is 1.50 N to 3.00 N.