BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a preparation method of a superabsorbent
polymer, and specifically to a method of preparing a superabsorbent polymer of which
fine powder generation is small, and not only are water holding capacity and absorbing
power under pressure excellent, but also the content of a water soluble component is
low.
(b) Description of the Related Art
A superabsorbent polymer (SAP) is a synthetic polymer material having a
function of absorbing about 500 to about 1000 times its weight of water, and it has been
differently called a superabsorbency material (SAM), an absorbent gel material (AGM),
and so on by developing enterprises. The superabsorbent polymer disclosed above
started to be commercialized for sanitary items, and is now being used widely as a water
combination soil for horticulture, a water-stop material for civil engineering and
construction, a nursery sheet, a freshness preservative in the food distribution field, a
poultice material, and the like in addition to the sanitary fittings like a paper diaper for a
child.
An inverse suspension polymerization method or an aqueous polymerization
method is known as a method of preparing a superabsorbent polymer. For example,
inverse suspension polymerization is disclosed in Japanese Patent Publication Nos.
Sho56-161408, Sho57-158209, Sho57-198714, and so on. As the aqueous
polymerization method, a thermal polymerization method of polymerizing a hydrous gel
phase polymer while fracturing and cooling the same in a kneader equipped with a
plurality of spindles, and a photo-polymerization method of exposing a highconcentrated
aqueous solution on a belt to UV rays and the like so as to carry out the
polymerization and dry it at the same time are known.
Meanwhile, a polymer powder obtained from the processes of polymerization,
pulverization, drying, and final milling is surface treated for obtaining a hydrous gel
phase polymer having excellent properties, and various modifications of the processes
have been attempted for increasing the effects of the steps of polymerization,
pulverization, and drying in order to obtain a hydrous gel phase polymer having
excellent properties.
SUMMARY OF THE INVENTION
It is an aspect of the present invention to provide a method of preparing a
superabsorbent polymer of which fine powder generation is small, and not only are
water holding capacity and absorbing power under pressure excellent, but also the
content of a water soluble component is low.
The present invention provides a method of preparing a superabsorbent polymer,
including the steps of: preparing a hydrous gel phase polymer by thermal polymerizing
or photo-polymerizing of a monomer composition including a water-soluble ethylenebased
unsaturated monomer and a polymerization initiator; drying the hydrous gel phase
polymer; milling the dried polymer; adding a surface cross-linking agent to the milled
polymer; and elevating the temperature of the polymer including the surface crosslinking
agent at a speed of 3 °C/min to 15 °C/min, and carrying out a surface crosslinking
reaction at 100 °C to 250 °C.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Hereinafter, the method of preparing the superabsorbent polymer according to a
specific embodiment of the present invention is explained in more detail.
According to one embodiment of the present invention, a method of preparing a
superabsorbent polymer includes the steps of: preparing a hydrous gel phase polymer by
thermal polymerizing or photo-polymerizing a monomer composition including a watersoluble
ethylene-based unsaturated monomer and a polymerization initiator; drying the
hydrous gel phase polymer; milling the dried polymer; adding a surface cross-linking
agent to the milled polymer; and elevating the temperature of the polymer including the
surface cross-linking agent at a speed o f 3 C min to 15 °C/min, and carrying out a
surface cross-linking reaction at lOO C to 250 .
From the results o f research o f the present inventors, it was recognized that not
only can the efficiency o f the surface cross-linking process be improved and the
properties o f the superabsorbent polymer produced finally be enhanced, but also the
generation o f fine powder having a particle diameter less than 150/zni can be minimized
when the temperature increase for the surface cross-linking reaction is carried out at a
speed of 3 °C/min to 15 °C/min in the process of the surface cross-linking reaction of the
polymer that is carried out after the processes o f drying and milling the hydrous gel
phase polymer.
After the step o f adding the surface cross-linking agent to the milled polymer,
the temperature increase to the surface cross-linking reaction temperature may be
carried out at a speed o f 3 °C/min to 15 °C/min, preferably at 5 °C/min to 10 °C/min, and
more preferably at 6 °C/min to 8 °C/min.
When the speed of the temperature increase to the surface cross-linking reaction
temperature is too fast, the surface cross-linking agent may be unevenly dispersed in the
pulverized polymer, or the surface cross-linking depth may be shallow or the surface
cross-linking reaction may occur only at a part of the milled polymer because the
surface cross-linking agent may not sufficiently penetrate into the milled polymer, and
thus the properties o f the superabsorbent polymer finally prepared may be deteriorated.
Furthermore, when the speed o f the temperature increase to the surface cross-linking
reaction temperature is too slow and the temperature increase time is delayed a long
time, it shows a lower distribution than the desired water absorption magnification
because the penetration of the surface cross-linking agent becomes deep, the properties
may be deteriorated by excessive reaction, and the milled polymer may be fractured or
the ratio o f fine powder (for example, the powder having the particle diameter o f less
than \ 5 ) may increase excessively as the residence time in the reactor becomes
longer.
That is, in the preparation method o f the superabsorbent polymer of one
embodiment of the present invention, the speed of temperature increase to the surface
cross-linking reaction temperature directly influences the penetration depth of the
surface cross-linking agent or the surface cross-linking reaction depth.
In the present specification, "surface cross-linking reaction temperature" may
be defined as the average temperature of the reactant including the polymer and the
surface cross-linking agent included in the "effective volume of the reactor" for the
cross-linking reaction when 70 to 90% of the whole surface cross-linking reaction time
has passed, without particular limitation. Further, "effective volume of the reactor" or
"volume of the reactor" may be defined as the total volume of the reactant included in
the volume of the reactor.
The speed of temperature increase for the surface cross-linking reaction may is
represented by the following Formula 1.
[Formula 1]
Speed of temperature increase ( C/min) = (surface cross-linking reaction
temperature - temperature of polymer directly after surface cross-linking agent is
added) / time of temperature increase
Furthermore, the time of temperature increase in Formula 1 means "the time
spent on the temperature increase to the surface cross-linking reaction temperature"
directly after the surface cross-linking agent is added, and it may be represented by the
following Formula 2.
[Formula 2]
Time of temperature increase (min)
= (effective volume of reactor (m3) / [(feeding speed of polymer (Kg/min) /
apparent density of polymer (Kg/m3)]
Meanwhile, in the preparation method of the superabsorbent polymer, the
temperature for surface cross-linking the polymer to which the surface cross-linking
agent is added ("surface cross-linking reaction temperature") may be 100 to 250 °C,
preferably 120 to 220 °C, and more preferably 150 to 200 °C. By applying the surface
cross-linking reaction temperature of the above range, the surface cross-linking reaction
can be evenly carried out throughout the polymer to which the surface cross-linking
agent is added, and a part of the polymer being carbonized or not participating in the
reaction can be prevented.
Meanwhile, the monomer composition includes a polymerization initiator, and
it may include a photo-polymerization initiator in the case of a photo-polymerization
method or a thermal polymerization initiator in the case of a thermal polymerization
method. However, even in the case of the photo-polymerization method, a thermal
polymerization initiator may be additionally included because a certain amount of heat
is generated by irradiation of UV rays and the like and a certain amount of heat is
generated according to the progress of the exothermic polymerization reaction.
Specifically, at least one initiator selected from the group consisting of a
persulfate-based initiator, an azo-based initiator, hydrogen peroxide, and ascorbic acid
may be used as the thermal polymerization initiator. More specific examples of the
persulfate-based initiator include sodium persulfate (Na2S20 8), potassium persulfate
(K2S20 ), ammonium persulfate ((NH )2S20 8), and the like; and as examples of the azobased
initiator, 2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,Ndimethylene)
isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitril, 2,2-
azobis(2-[2-imidazolin-2-yl]propane) dihydrochloride, and 4,4-azobis-(4-cyanovaleric
acid) may be used. More various thermal polymerization initiators are well-disclosed
in "Principle of Polymerization" written by Odian, (Wiley, 1981), p203, however the
examples of the thermal polymerization initiator are not limited to or by these.
Meanwhile, at least one initiator selected from the group consisting of benzoin
ether, a dialkyl acetophenone, a hydroxyl alkylketone, phenyl glyoxylate, benzyl
dimethyl ketal, an acyl phosphine, and an a-aminoketone may be used as the photopolymerization
initiator. As the specific example of the acyl phosphine,
commercialized Lucirin® TPO, namely, 2,4,6-trimethyl-benzoyl-trimethyl phosphine
oxide, may be used. More various photo-polymerization initiators are well disclosed
in "UV Coatings: Basics, Recent Developments and New Applications" written by
Reinhold Schwalm, (Elsevier, 2007), pi 15, however the examples of the photopolymerization
initiator is not limited to or by these.
The "hydrous gel phase polymer" is a polymer obtained by polymerizing
certain monomers, and it means a gel-type polymer including a certain content of water.
In the method of preparing the superabsorbent polymer, the hydrous gel phase polymer
having a moisture content of 40 to 80 weight% which is prepared by thermal
polymerizing or photo-polymerizing the monomer composition including the watersoluble
ethylene-based unsaturated monomer and the polymerization initiator may be
used.
Throughout the present specification, "moisture content" means the content of
moisture in the weight of the whole hydrous gel phase polymer, and specifically it
means the value of the weight of the dried polymer subtracted from the weight of the
hydrous gel phase polymer. Furthermore, the moisture content may be defined as a
value calculated by measuring weight loss as water is evaporated from the polymer
during the drying process by elevating the temperature of the polymer through infrared
heating. At this time, the moisture content is measured by carrying out the drying
process with the drying condition of elevating the temperature from room temperature
to 180 °C and maintaining the temperature at 180 °C, wherein the total drying time is set
as 20 minutes including 5 minutes of the temperature increase step.
Furthermore, every monomer usually used to prepare the superabsorbent
polymer may be used as the water-soluble ethylene-based unsaturated monomer without
limitation. At least one selected from the group consisting of an anionic monomer and
a salt thereof, a nonionic hydrophilic monomer, and an amino group-containing
unsaturated monomer and the quaternary compound thereof may be used.
As a specific example of the water-soluble ethylene-based unsaturated
monomer, at least one selected from the group consisting of an anionic monomer such
as acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic
acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-
(meth)acryloylpropane sulfonic acid, and 2-(meth)acrylamide-2-methyl propane
sulfonic acid, and salts thereof; a nonionic hydrophilic monomer such as
(meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-
hydroxypropyl(meth)acrylate, methoxy polyethylene glycol (meth)acrylate, and
polyethylene glycol (meth)acrylate; and an amino group-containing unsaturated
monomer such as (N,N)-dimethylaminoethyl(meth)acrylate and (N,N)-
dimethylaminopropyl(meth)acrylate, and a quaternary compound thereof may be
preferably used.
Preferably, acrylic acid or a salt thereof may be used as the water-soluble
ethylene-based unsaturated monomer, and there is an advantage that a superabsorbent
polymer having improved water absorptivity can be obtained by using the acrylic acid
or the salt thereof as the monomer.
Meanwhile, the concentration of the water-soluble ethylene-based unsaturated
monomer in the monomer composition may be suitably determined by considering the
polymerization time and the reaction conditions, and it may preferably be 40 to 55
weight%. When the concentration of the water-soluble ethylene-based unsaturated
monomer is less than 40 weight%, it is disadvantageous in the aspect of economic
feasibility, and when the concentration is larger than 55 weight%, the milling efficiency
in the milling process of the polymerized hydrous gel phase polymer may become low.
Meanwhile, a general method may be used without limitation if the method can
prepare a hydrous gel phase polymer from such monomer composition by thermal
polymerization or photo-polymerization. Specifically, the polymerization method is
largely classified into the thermal polymerization and the photo-polymerization
according to the polymerization energy source, and the thermal polymerization may be
carried out in a reactor like a kneader equipped with agitating spindles and the photopolymerization
may be carried out in a reactor equipped with a movable conveyor belt,
however the polymerization methods disclosed above are just examples and the present
invention is not limited to or by said methods.
For example, the hydrous gel phase polymer obtained from the thermal
polymerization in a reactor like a kneader equipped with the agitating spindles disclosed
above by providing hot air thereto or heating the reactor may have the size of
centimeters or millimeters when it is discharged from the outlet of the reactor, according
to the type of agitating spindles equipped in the reactor. Specifically, the size of the
obtained hydrous gel phase polymer can be variously shown according to the
concentration of the monomer composition fed thereto, the feeding speed, and the like,
and the hydrous gel phase polymer of which the weight average particle diameter is 2 to
50 mm can be generally obtained.
Furthermore, in the case of the photo-polymerization carried out with a reactor
equipped with a movable conveyor belt disclosed above, the obtained hydrous gel phase
polymer may be a sheet-type hydrous gel phase polymer having the same width as the
belt. At this time, the thickness of the polymer sheet may vary according to the
concentration of the monomer composition fed thereto and the feeding speed, and it is
preferable to provide the monomer composition so that a sheet-type hydrous gel phase
polymer having a width of 0.5 to 5cm is obtained. It is not preferable for the monomer
composition to be fed so that the thickness of the sheet-type polymer becomes too thin
because the production efficiency is low, and when the thickness of the sheet type
polymer is larger than 5cm, the polymerization reaction may not occur evenly
throughout the whole thickness due to its excessive thickness.
Meanwhile, the hydrous gel phase polymer obtained according to the thermal
polymerization or the photo-polymerization disclosed above passes through a drying
step, and it may further pass through a pulverizing step before the drying step for raising
the efficiency of the drying step, as occasion demands.
A general pulverizing device may be used in the pulverizing step before the
drying step without limitation if the device can be used for pulverizing the hydrous gel
phase resin, and for example, any one or more devices selected from the group
consisting of a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a
cutter mill, a disc mill, a shred crusher, a crusher, a chopper, and a disc cutter may be
used.
In the pulverizing step before the drying step, the hydrous gel phase polymer
obtained according to the thermal polymerization or the photo-polymerization disclosed
above may be pulverized so that the weight average particle diameter becomes 2mm to
10mm. When the weight average particle diameter is less than 2mm, the particles may
agglomerate and it is technically not easy to pulverize the hydrous gel phase polymer to
be less than 2mm due to its high moisture content. Furthermore, when the weight
average particle diameter is larger than 10mm, the increasing effect of the efficiency o f
the succeeding drying step may be insignificant.
Meanwhile, in the pulverizing step before the drying step, the hydrous gel phase
polymer may stick to the surface of the pulverizing device because it has a high
moisture content. Accordingly, a certain additive may be added to the hydrous gel
phase polymer for raising the efficiency of the pulverizing step before the drying step.
The kind of the usable additive is not particularly limited, and for example, it
may be an anti-agglomeration agent for the fine powder such as steam, water, a
surfactant, an inorganic powder such as clay or silica, and the like; a thermal
polymerization initiator such as a persulfate-based initiator, an azo-based initiator,
hydrogen peroxide, ascorbic acid, and the like; or a cross-linking agent such as an
epoxy-based cross-linking agent, a diol-based cross-linking agent, a cross-linking agent
including 2-functional or poly- functional (3 or more -functional) acrylate, or a monofunctional
compound including a hydroxyl group.
When the hydrous gel phase polymer obtained according to the thermal
polymerization or the photo-polymerization or the hydrous gel phase polymer
pulverized in the pulverizing step before the drying step passes through the drying step,
the drying temperature of the drying step may be 150 °C to 250 C. Said "drying
temperature" may mean the temperature of the heating medium provided thereto for
drying, or the temperature of the drying reactor including the heating medium and the
polymer during the drying process.
When the drying temperature is lower than 150 °C, there is a concern that the
drying time becomes excessively long or the properties of the superabsorbent polymer
finally formed may be deteriorated, and when the drying temperature is higher than
250 °C, only the surface of the polymer is dried, and thus there is a concern that fine
powder may be generated and the properties of the superabsorbent polymer finally
formed may be deteriorated, The drying temperature may preferably be 150 °C to 200 °C,
and more preferably 160 °C to 180 °C.
The time for the drying step may be suitably controlled considering the amount
or the properties of the superabsorbent polymer being prepared, the size o f the reactor,
and so on, and the drying step may be carried out for 20 to 90 minutes, considering the
process efficiency.
Furthermore, any generally known method or device may be used in the drying
step without limitation if it can be used for drying the hydrous gel phase polymer, and
for example, the drying step may be carried out by a method of supplying hot air,
irradiating infrared rays, irradiating microwaves, irradiating ultraviolet rays, and the like.
When the drying step disclosed above is finished, the moisture content o f the
hydrous gel phase polymer may be 0.1 to 10 weight%.
Meanwhile, the polymer obtained from the drying step may pass through a
certain milling step. The details of a milling device that can be used in the milling step
are not particularly limited, and for example, a pin mill, a hammer mill, a screw mill, a
roll mill, a disc mill, a jog mill, and the like may be used. The polymer powder
obtained from the milling step may have a weight average particle diameter o f 150 to
Meanwhile, s classifying step for obtaining the milled polymer o f which the
weight average particle diameter is 150 to 85 0 m may be additionally carried out before
the step o f milling the polymer obtained from the drying step and adding the surface
cross-linking agent thereto. The properties of the superabsorbent polymer powder
finally manufactured can be properly controlled through such classifying step, and only
a polymer powder having a weight average particle diameter of 150 to 85 0 m obtained
in the classifying step is selectively applied to the surface cross-linking reaction and
finally manufactured.
A vibration classifier may be used in the classifying step, however it is not
limited to or by this. The classifier may be a quadrangular or circular classifier, and a
cyclone using a fluidized bed may be used.
Meanwhile, a surface cross-linking agent may be added for the surface crosslinking
of the milled polymer powder. The surface cross-linking agent is not
particularly limited as long as it is a compound that can react to the functional group of
the milled polymer, and for preferable examples, there are a polyhydric alcohol
compound, an epoxy compound, a poly amine compound, a haloepoxy compound, a
condensation product of the haloepoxy compound, an oxazoline compound, a mono-,
di-, or polyoxazolidinone compound, a cyclic urea compound, a polyvalent metal salt,
an alkylene carbonate compound, and a mixture of two or more of said compounds.
More specifically, as examples of the polyhydric alcohol compound, there is a
mono-, di-, tri-, tetra-, or polyethylene glycol, monopropylene glycol, 1,3 -propanediol,
dipropylene glycol, 2,3,4-trimethyl-l,3-pentanediol, polypropylene glycol, glycerol,
polyglycerol, 2-butene-l,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-
hexanediol, 1,2-cyclohexane dimethanol, or a mixture of two or more of said
compounds.
As examples of the epoxy compound, ethylene glycol diglycidyl ether and
glycidol may be used, and as the polyamine compound, there is ethylene diamine,
diethylene triamine, triethylene triamine, tetraethylene pentamine, pentaethylene
hexamine, polyethylene amine, polyamide polyamine, or a mixture of two or more of
said compounds.
Meanwhile, epichlorohydrin, epibromohydrin, and a-methylephichlorohydrin
may be used as the haloepoxy compound. 2-oxazolidinone may be used as the mono-,
di,- or polyoxazolidinone compound. Ethylene carbonate may be used as the alkylene
carbonate compound.
In order to raise the efficiency of the surface cross-linking reaction process, it is
preferable to use one or more polyhydric alcohol compounds among said cross-linking
agent, and it is more preferable to use a C2-C10 polyhydric alcohol compound.
The amount of the surface cross-linking agent used may be suitably controlled
according to the kind of surface cross-linking agent, the characteristics of the milled
polymer, or the surface cross-linking reaction conditions, and the amount may be 0.001
to 5 parts by weight, preferably 0.01 to 3 parts by weight, and more preferably 0.05 to 2
parts by weight per 100 parts by weight of the milled polymer. When the amount of
the surface cross-linking agent used is too small, the surface cross-linking reaction may
not occur practically, and when the surface cross-linking agent is used excessively, the
absorptivity and the properties of the final product may be decreased due to excessive
surface cross-linking reaction.
The method of adding the surface cross-linking agent to the milled polymer is
not particularly limited, and for example, a method of feeding the surface cross-linking
agent and the milled polymer powder to the reactor and mixing the same, spraying the
surface cross-linking agent to the polymer powder, or mixing the milled polymer
powder and the cross-linking agent while continuously feeding the same to a mixer
being continuously operated may be used.
At this time, the surface cross-linking agent may be added by mixing it with
additional water. When water is added to the surface cross-linking agent, the surface
cross-linking agent can be evenly dispersed in the polymer. The amount of water
added thereto may be 0.5 to 10 parts by weight per 100 parts by weight of the milled
polymer for the purpose of inducing uniform dispersion of the surface cross-linking
agent, preventing agglomeration of the polymer powder, and optimizing the surface
penetrating depth of the cross-linking agent at the same time.
Meanwhile, the surface temperature of the milled polymer may be 20 to 90 °C in
the step of adding the surface cross-linking agent. The temperature of the milled
polymer may be elevated to the surface cross-linking reaction temperature at a speed of
3 °C/min to 15 °C/min so that proper surface cross-linking reaction occurs, and the
surface temperature of the milled polymer may be 20 to 90 °C, and preferably 20 to 60 °C,
for securing proper properties o f the final product. In order for the milled polymer to
have the temperature of said range, the succeeding process may be continuously carried
out after the drying step which is carried out at a relatively high temperature, the latter
process time is reduced, or the polymer is heated separately when it is difficult to reduce
the process time.
Furthermore, in addition to the method of maintaining or controlling the surface
temperature of the milled polymer in a proper range, the temperature of the surface
cross-linking agent itself added thereto may be controlled to be 10 to 90 °C, and
preferably 20 °C to 60 °C. By heating the surface cross- linking agent or controlling the
temperature of the same, the temperature can be elevated with a speed of 3 °C/min to
15 °C/min to the surface cross-linking reaction temperature, the surface cross- linking
reaction can be properly carried out, and the proper properties of the final product can
be obtained. When the temperature of the surface cross-linking agent is lower than
IOC the properties of the superabsorbent polymer may be deteriorated because the
absorption speed of the surface cross-linking agent is late and the penetration depth of
the surface cross-linking is shallow, and the effect of shortening the temperature
increase speed influences the surface cross-linking reaction according to the temperature
increase may be insignificant. Furthermore, when the temperature of the surface crosslinking
agent is higher than 90 °C, the penetrating speed of the surface cross-linking
agent may be excessively rapid or uniform mixing of the surface treating agent may be
disturbed.
Meanwhile, various means for temperature increase may be used in the step of
elevating the temperature of the polymer to which the surface cross-linking agent is
added to the surface cross-linking reaction temperature, and for example, the
temperature increase may be carried out by providing a heating medium or by directly
providing a heat source.
As a specific example of the heating medium, a hot fluid such as steam, hot air,
hot oil, and the like may be used, however the specific example is not limited to these.
The temperature of the heating medium may be properly controlled by considering the
means of the heating medium, the temperature increase speed, and the target
temperature. As an example of the heat source provided directly, an electric heater or
a gas heater may be used, but it is not limited to these. However, it is preferable that
the heating medium has a temperature of 100 °C or more and that the heat source
provides thermal energy at a temperature of 100 °C or more, considering that the surface
cross-linking reaction temperature disclosed above is 100 to 250 °C.
The surface cross-linking reaction may be carried out within a certain time,
considering the cross-linking reaction temperature, the characteristics and the amount of
the reactants, or the reaction conditions, and the cross-linking reaction may be carried
out for 1 minute to 120 minutes, preferably for 1 minute to 60 minutes, and more
preferably for 10 minutes to 50 minutes, after the temperature increase to the crosslinking
reaction temperature is completed. When the reaction time is shorter than 1
minute, the cross-linking reaction cannot be sufficiently obtained, and when the crosslinking
time is longer than 120 minutes, property deterioration may occur because the
polymer particles are damaged by excessive surface cross-linking reaction.
According to the method of preparing the superabsorbent polymer disclosed
above, the superabsorbent polymer o f which a small amount of fine powder is generated,
and not only are the water holding capacity and the absorbing power under pressure
excellent, but a low the content of the water soluble component can be provided.
The problems with generating a lot of fine powder in the preparation method o f
superabsorbent polymer are that the work environment becomes poor, and the efficiency
of the process and the quality o f the superabsorbent polymer finally manufactured are
decreased by the fine powder. However, according to the method of preparing the
superabsorbent polymer disclosed above, it is possible to reduce the amount of fine
powder generated in the preparation process, and particularly, it is possible to control
the proportion of the superabsorbent polymer having a particle diameter o f less than
15 0 m to be less than 6 weight% after the surface cross-linking reaction. Accordingly,
the method of preparing the superabsorbent polymer disclosed above can improve the
process efficiency and the work environment by reducing the fine powder generation
and can enhance the properties and the quality o f the final product.
Further, in the case of the superabsorbent polymer prepared by the preparation
method o f the superabsorbent polymer disclosed above, it is recognized that the water
holding capacity measured according to the EDANA WSP 241.2 method is 33g/g or
more, and the absorbing power under pressure measured by the EDANA WSP 242.2
method is 22g/g or more, and thus its water holding capacity and absorbing power under
pressure are superior. Furthermore, such a superabsorbent polymer shows a very low
water-soluble component measured according to the EDANA WSP 270.2 method of 15
weight% or less, and it is expected to be used in industrial fields related to
superabsorbent polymer preparation.
According to the present invention, the method of preparing the superabsorbent
polymer of which the fine powder generation is small, and not only are the water
holding capacity and the absorbing power under pressure excellent, but also the content
o f the water soluble component is low, and the superabsorbent polymer prepared by the
same can be provided.
Hereinafter, the function and effect of the invention are explained in more detail
through concrete examples of the present invention. However, the following examples
are only for illustrating the present invention, and the scope of the present invention is
not determined to or by them.
PREPARATION EXAMPLE 1: Preparation of Polymer Powder>
A monomer composition of which the monomer concentration was 50 weight%
was prepared by mixing lOOg of acrylic acid, O.lg of polyethylene glycol diacrylate as a
cross-linking agent, 38. g of caustic soda (NaOH), and 103.9g of water.
Then, the monomer composition was provided on a continuously moving
conveyor belt through a monomer feed section and lOg of 3% sodium persulfate
aqueous solution was mixed thereto as a polymerization initiator. The polymerization
was started 1 minute after mixing the polymerization initiator, and the polymerization
reaction was carried out for 4 minutes in the reactor of which the internal temperature
was 99 °C. The hydrous gel phase polymer (moisture content was 50%) polymerized in
this way was transferred to a cutter and pulverized so that the weight average particle
diameter of the hydrous gel phase polymer was 2mm.
The pulverized and discharged hydrous gel phase polymer was then dried in a
hot air dryer of 180 °C for 1 hour. Subsequently, the dried polymer was milled with a
pin mill, and classified by using a sieve so as to obtain the milled polymer having a
particle diameter of 150 to 850
The polymer powder prepared in this way showed water holding capacity o f
1g/g and the content of the water soluble component of 16%.
[EXAMPLES]
Example 1: Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol as a surface cross- linking agent and 1.0
part by weight o f water were sprayed to and mixed with 100 parts by weight o f the
polymer powder prepared according to Preparation Example 1. The temperature of the
polymer to which the surface cross-linking agent was added was 30 °C after the mixing
process.
The mixed polymer was inserted into a reactor of which the temperature was
increased by an oil heater and an electric heater. At this time, the surface cross-linking
reaction temperature was 180 °C, the speed of the temperature increase was 3 °C/min,
and the surface cross-linking reaction time was 20 minutes after the temperature was
elevated to 180 °C.
Example 2 ; Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol as a surface cross-linking agent and 1.0
part by weight of water were sprayed to and mixed with 100 parts by weight of the
polymer powder prepared according to Preparation Example 1. The temperature of the
polymer to which the surface cross-linking agent was added was 30 °C after the mixing
process.
The mixed polymer was inserted into a reactor of which the temperature was
increased by an oil heater and an electric heater. At this time, the surface cross-linking
reaction temperature was 180 °C, the speed of the temperature increase was 5 °C/min,
and the surface cross-linking reaction time was 20 minutes after the temperature was
elevated to 180 °C.
Example 3 ; Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol and 1.0 part by weight of water were
sprayed to and mixed with 100 parts by weight of the polymer powder prepared
according to Preparation Example 1. The temperature of the polymer to which the
surface cross-linking agent was added was 30 °C after the mixing process.
The mixed polymer was inserted into a reactor of which the temperature was
increased by an oil heater and an electricity heater. At this time, the surface crosslinking
reaction temperature was 180 °C, the speed of the temperature increase was
10 °C/min, and the surface cross-linking reaction time was 20 minutes after the
temperature was elevated to 180 °C.
Example 4 : Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol and 1.0 part by weight of water were
sprayed to and mixed with 100 parts by weight of the polymer powder prepared
according to Preparation Example 1. The temperature of the polymer to which the
surface cross-linking agent was added was 30 °C after the mixing process.
The mixed polymer was inserted into a reactor of which the temperature was
increased by an oil heater and an electric heater. At this time, the surface cross-linking
reaction temperature was 200 °C, the speed of the temperature increase was 7 °C/min,
and the surface cross- linking reaction time was 10 minutes after the temperature was
elevated to 200 °C.
Example 5 ; Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol and 1.0 part by weight of water were
sprayed to and mixed with 100 parts by weight of the polymer powder prepared
according to Preparation Example 1. The temperature of the polymer to which the
surface cross-linking agent was added was 60 °C after the mixing process.
The mixed polymer was inserted into a reactor of which the temperature was
increased by an oil heater and an electric heater. At this time, the surface cross-linking
reaction temperature was 180 °C, the speed of the temperature increase was 3 °C/min,
and the surface cross-linking reaction time was 20 minutes after the temperature was
elevated to 180 °C.
[COMPARATIVE EXAMPLES]
Comparative Example 1: Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol and 1.0 part by weight of water were
sprayed to and mixed with 100 parts by weight of the polymer powder prepared
according to Preparation Example 1. The temperature of the polymer to which the
surface cross-linking agent was added was 30 °C after the mixing process.
The mixed polymer was inserted into a reactor of which the temperature was
increased by an oil heater and an electric heater. At this time, the surface cross-linking
reaction temperature was 180 °C, the speed of the temperature increase was 2 °C/min,
and the surface cross-linking reaction time was 20 minutes after the temperature was
elevated to 180 C.
Comparative Example 2 : Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol and 1.0 part by weight of water were
sprayed to and mixed with 100 parts by weight of the polymer powder prepared
according to Preparation Example 1. The temperature of the polymer to which the
surface cross-linking agent was added was 30 °C after the mixing process.
The mixed polymer was inserted to a reactor of which the temperature was
increased by an oil heater and an electric heater. At this time, the surface cross-linking
reaction temperature was 180 °C, the speed of the temperature increase was 20 °C/min,
and the surface cross-linking reaction time was 20 minutes after the temperature was
elevated to 180 °C.
Comparative Example 3 ; Preparation of Superabsorbent Polymer
1.0 part by weight of 1,3-propanediol and 1.0 part by weight of water were
sprayed to and mixed with 100 parts by weight of the polymer powder prepared
according to Preparation Example 1. The temperature of the polymer to which the
surface cross-linking agent was added was 60 °C after the mixing process.
The mixed polymer was inserted into a reactor of which the temperature was
increased by an oil heater and an electric heater. At this time, the surface cross-linking
reaction temperature was 180 °C, the speed of the temperature increase was 20 °C/min,
and the surface cross-linking reaction time was 20 minutes after the temperature was
elevated to 180 °C.
The surface cross-linking reaction conditions of the examples and comparative
examples are listed in the following Table 1.
For reference, "surface cross-linking reaction temperature" in the examples and
the comparative examples was disclosed as the average value of the temperatures at the
time of passing 70%, at the time of passing 80%, and at the time of passing 90% of the
total surface cross-linking reaction time of 100% after the surface cross-linking reaction
was started.
[Table 1]
EXPERIMENTAL EXAMPLES: Evaluation of Properties of
Superabsorbent Polymer
Experimental Example 1: Particle Size Distribution
The particle size distribution of the product after the surface cross-linking
reaction was measured by shaking the same for 10 minutes by using sieves having the
mesh distribution of 20, 30, 50, 80, and 100 pan, and by using a shaker for measuring
particle size distribution.
The weight of the polymer existing on each mesh was represented as a weight
ratio to the total weight.
Experimental Example 2: Water Holding Capacity
The water holding capacity was measured for the particles classified by the
method disclosed above. The measurement of the water holding capacity followed the
EDANA WSP 241.2 method. After inserting 0.2 g of the specimen classified by
30-50 mesh in a tea bag and soaking the same in a 0.9% salt water solution for 30
minutes, the water holding capacity was measured by eliminating water from the
specimen for 3 minutes by using a centrifugal separator set to 250G and weighing the
specimen so as to determine the amount of water held in the superabsorbent polymer.
Experimental Example 3 : Water Soluble Component
The water soluble component was measured for the particles classified by the
method disclosed above. The measurement of the water holding capacity followed the
EDANA WSP 270.2 method. After inserting 1.0 g of the specimen classified by
30~50 mesh in 200g of a 0.9% salt water solution and soaking the same while stirring at
500 rpm for 16 hours, the aqueous solution was filtered with a filter paper. The
solution filtered in this way was primarily titrated to pH 10.0 with a 0.1N caustic soda
solution and was then counter-titrated to pH 2.7 with a 0.1N hydrogen chloride solution,
and the amount of polymer material not cross-linked was calculated from the amount
needed for the neutralization.
Experimental Example 4: Absorbing Power Under Pressure
The absorbing power under pressure was analyzed by using a measuring device
regulated in the EDANA WSP 242.2 method and a particle size distribution of
150~850/im.
The properties of superabsorbent polymers prepared according to the examples
and the comparative examples are listed in the following Table 2.
Comparative 1.9 33.9 16.6 22.9
Example 3
From the results disclosed above, it is recognized that there is a tendency for the
amount of fine powder of less than 15 0 ni to increase and the water soluble component
to increase when the temperature increase speed to the surface cross-linking reaction
temperature is slow. Further, when the temperature increase speed to the surface
cross-linking reaction temperature is fast, a decrease o f absorbing power under pressure
(AUP) is shown, which seems to come from an insufficient time for the surface crosslinking
agent to be distributed in and to penetrate into the inside and the surface of the
polymer.
Meanwhile, it can be recognized that the superabsorbent polymers prepared
according to the examples have high water holding capacity and absorbing power under
pressure (AUP), and low content of water soluble components at the same time, in
comparison with the superabsorbent polymers according to the comparative examples.
The water holding capacity and the absorbing power under pressure of the
superabsorbent polymer relate to the evaluation on the moisture absorbing performance,
they relate to basic performance o f the superabsorbent polymer, and the water soluble
component relates to the content o f the component that is soluble in water in the
superabsorbent polymer, for example, to the content of a low molecular weight polymer
component.
Generally, the properties of the superabsorbent polymer can be evaluated as
being superior as the water holding capacity and the absorbing power under pressure are
higher, and when the superabsorbent polymer is applied to a consumer good like a
diaper, there is less displeasure of a user due to wetness, and the property of the
superabsorbent polymer can be evaluated as being superior as the amount of the water
soluble component is lower. However, it is generally known that the higher the water
holding capacity, the higher the content o f the water soluble component, and there have
been difficulties in improving overall properties of the superabsorbent polymer.
However, it is recognized that the superabsorbent polymer prepared in
Examples 1 to 5 can maintain the low content of the water soluble component while
having high water holding capacity and absorbing power under pressure. In addition,
since the superabsorbent polymers according to the examples realize superior
characteristics disclosed above and the amount of the fine powder (particle diameter less
than 150/zni) generated after the surface cross-linking reaction is very small, specifically
the ratio of the superabsorbent polymer having the particle diameter of less than 150 hi
after the surface cross-linking reaction can be controlled to be less than 6 weight%, the
process efficiency and the work environment can be improved, and the properties and
the quality of the final product can be improved.

WHAT IS CLAIMED IS:
1. A method of preparing a superabsorbent polymer, including the steps of:
preparing a hydrous gel phase polymer by thermal polymerizing or photopolymerizing
a monomer composition including a water-soluble ethylene-based
unsaturated monomer and a polymerization initiator;
drying the hydrous gel phase polymer;
milling the dried polymer;
adding a surface cross-linking agent to the milled polymer; and
elevating the temperature of the polymer including the surface cross-linking
agent at a speed of 3 °C/min to 15 °C/min, and carrying out a surface cross-linking
reaction at 100 °C to 250 °C.
2. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the moisture content of the thermal polymerized or photo-polymerized hydrous
gel phase polymer is 40 to 80 weight%.
3. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the moisture content of the dried polymer is 0.1 to 10 weight%.
4. The method of preparing a superabsorbent polymer according to Claim 1,
further including the step of pulverizing the hydrous gel phase polymer so that the
weight average particle diameter is 2 to 10mm, before the drying step of the hydrous gel
phase polymer.
5. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the drying step of the hydrous gel phase polymer is carried out at a temperature
of 150 °C to 250 °C.
6. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the milling step of the dried polymer is carried out so that the weight average
particle diameter of the milled polymer is 150 to 850 / n.
7. The method of preparing a superabsorbent polymer according to Claim 1,
further including the step of classifying the milled polymer so that the weight average
particle diameter is 150 to 850 i, before the step o f adding the surface cross-linking
agent to the milled polymer.
8. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the surface cross-linking agent is at least one selected from the group consisting
of a polyhydric alcohol compound, an epoxy compound, a polyamine compound, a
haloepoxy compound, a condensation product of the haloepoxy compound, an oxazoline
compound; a mono-, di-, or polyoxazolidinone compound, a cyclic urea compound, a
polyvalent metal salt, and an alkylene carbonate compound.
9. The method of preparing a superabsorbent polymer according to Claim 1,
wherein 0.001 to 5 parts by weight o f the surface cross-linking agent is added to 100
parts by weight of the milled polymer.
10. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the surface temperature of the milled polymer is 20 to 90 °C in the step o f
adding the surface cross-linking agent.
11. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the temperature of the added surface cross-linking agent is 10 to 90 °C.
12. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the surface cross-linking reaction is carried out for 1 minute to 120 minutes.
13. The method of preparing a superabsorbent polymer according to Claim 1,
wherein the temperature increase for the surface cross-linking reaction is carried out by
providing a heating medium o r by directly providing a heat source.
14. The method o f preparing a superabsorbent polymer according to Claim 1,
wherein the ratio o f the superabsorbent polymer having the particle diameter o f less
than 150j¾m is less than 6 weight% after the surface cross-linking reaction.
15. The method o f preparing a superabsorbent polymer according to Claim 1,
wherein the water holding capacity measured according to the EDANA WSP 241.2
method is 33g/g or more, and the absorbing power under pressure measured by the
EDANA WSP 242.2 method is 22g/g or more.
16. The method o f preparing a superabsorbent polymer according to Claim 1,
wherein the water-soluble component o f the superabsorbent polymer measured
according to the EDANA WSP 270.2 method is 15 weight% or less.