U.S. patent application number 17/085233 was filed with the patent office on 2021-02-18 for super absorbent polymer and method for producing same.
This patent application is currently assigned to LG Chem, Ltd.. The applicant listed for this patent is LG Chem, Ltd.. Invention is credited to Chang Sun Han, Min Ho Hwang, Sang Gi Lee, Soo Jin Lee, Hye Mi Nam.
Application Number | 20210046449 17/085233 |
Document ID | / |
Family ID | 1000005181554 |
Filed Date | 2021-02-18 |
United States Patent
Application |
20210046449 |
Kind Code |
A1 |
Nam; Hye Mi ; et
al. |
February 18, 2021 |
Super Absorbent Polymer and Method for Producing Same
Abstract
The present invention relates to an olefin polymer and a method
for producing the same. The super absorbent polymer can exhibit
excellent absorbent properties even in a swollen state and thus
exhibit excellent anti-rewetting effects. Accordingly, when the
super absorbent polymer is used, it is possible to provide a
sanitary material such as a diaper or a sanitary napkin which can
give a smooth touch feeling even after the body fluid is
discharged.
Inventors: |
Nam; Hye Mi; (Daejeon,
KR) ; Lee; Sang Gi; (Daejeon, KR) ; Hwang; Min
Ho; (Daejeon, KR) ; Lee; Soo Jin; (Daejeon,
KR) ; Han; Chang Sun; (Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG Chem, Ltd. |
Seoul |
|
KR |
|
|
Assignee: |
LG Chem, Ltd.
Seoul
KR
|
Family ID: |
1000005181554 |
Appl. No.: |
17/085233 |
Filed: |
October 30, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16062778 |
Jun 15, 2018 |
10850260 |
|
|
PCT/KR2017/003966 |
Apr 12, 2017 |
|
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17085233 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 20/3021 20130101;
A61L 15/60 20130101; B01J 20/267 20130101; C08J 3/245 20130101;
A61L 15/24 20130101; C08F 220/06 20130101; B01J 20/3085 20130101;
C08J 3/243 20130101; C08J 2333/02 20130101 |
International
Class: |
B01J 20/26 20060101
B01J020/26; A61L 15/24 20060101 A61L015/24; A61L 15/60 20060101
A61L015/60; B01J 20/30 20060101 B01J020/30; C08F 220/06 20060101
C08F220/06; C08J 3/24 20060101 C08J003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 12, 2016 |
KR |
10-2016-0132254 |
Claims
1. A method for producing a super absorbent polymer comprising the
steps of: carrying out a crosslinking polymerization of a monomer
mixture including a water-soluble ethylenically unsaturated monomer
having at least partially neutralized acidic groups in the presence
of an internal crosslinking agent to form a hydrogel polymer;
drying, pulverizing and classifying the hydrogel polymer to form a
base polymer powder; and subjecting the surface of the base polymer
powder to a first crosslinking in the presence of an epoxy-based
surface crosslinking agent and then subjecting the surface of the
first cross-linked base polymer powder to a second crosslinking in
the presence of a non-epoxy-based surface crosslinking agent to
form a surface crosslinked layer, wherein a GBP change ratio of the
super absorbent polymer calculated by the following Formula 1 is
0.90 or less. 0.3 GBP - 0.3 AGBP 0.3 GBP [ Formula 1 ] ##EQU00007##
in Formula 1, 0.3 GBP is a gel bed permeability (GBP) under 0.3 psi
for a physiological saline solution of a super absorbent polymer,
0.3 AGBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution of a super absorbent polymer having a
particle size from 300 to 600 .mu.m after swelling and drying which
is obtained by a process of adding 5 g of the super absorbent
polymer to 125 g of distilled water, stirring the resultant for 1
minute, drying the swollen super absorbent polymer at 100.degree.
C. for 3 hours, and then classifying the dried polymer through a
U.S. standard 30 mesh screen and a U.S. standard 50 mesh
screen.
2. The method for producing a super absorbent polymer of claim 1,
wherein the epoxy-based surface crosslinking agent include at least
one polyglycidyl ether selected from the group consisting of
ethylene glycol diglycidyl ether, propylene glycol diglycidyl
ether, butanediol diglycidyl ether, hexanediol diglycidyl ether,
diethylene glycol diglycidyl ether, triethylene glycol diglycidyl
ether, polyethylene glycol diglycidyl ether, dipropylene glycol
diglycidyl ether, tripropylene glycol diglycidyl ether,
polypropylene glycol diglycidyl ether, and glycerol triglycidyl
ether.
3. The method for producing a super absorbent polymer of claim 1,
wherein, in the step of forming the surface crosslinked layer, the
surface of the base polymer powder is subjected to a first surface
crosslinking at a temperature of 120 to 160.degree. C. for 5 to 40
minutes using an epoxy-based surface crosslinking agent.
4. The method for producing a super absorbent polymer of claim 1,
wherein the non-epoxy-based surface crosslinking agent includes a
polyol, a carbonate-based compound or a mixture thereof.
5. The method for producing a super absorbent polymer of claim 4,
wherein the polyol includes at least one selected from the group
consisting of ethylene glycol, propylene glycol, 1,4-butanediol,
1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol,
2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol,
2-methyl-2,4-pentanediol, tripropylene glycol and glycerol.
6. The method for producing a super absorbent polymer of claim 4,
wherein the carbonate-based compound includes at least one selected
from the group consisting of ethylene carbonate and propylene
carbonate.
7. The method for producing a super absorbent polymer of claim 3,
wherein, in the step of forming the surface crosslinked layer, the
surface of the first surface cross-linked powder is subjected to a
second surface crosslinking at a temperature of 170 to 210.degree.
C. for 5 to 40 minutes using a non-epoxy-based surface crosslinking
agent.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a divisional of U.S. application Ser.
No. 16/062,778, filed Jun. 15, 2018 which is a national phase entry
under 35 U.S.C. .sctn. 371 of International Application No.
PCT/KR2017/003966, filed on Apr. 12, 2017, which claims the benefit
of Korean Patent Application No. 10-2016-0132254, filed on Oct. 12,
2016, the disclosures of which are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates to an olefin polymer having
remarkably improved anti-rewetting effects, and a method for
producing the same.
BACKGROUND ART
[0003] Super absorbent polymer (SAP) is a synthetic polymer
material capable of absorbing moisture from about 500 to about
1,000 times its own weight, and each manufacturer has denominated
it as different names such as SAM (Super Absorbency Material), AGM
(Absorbent Gel Material) or the like. Such super absorbent polymers
started to be practically applied in sanitary products, and now
they are widely used for preparation of various products, for
example, hygiene products such as paper diapers for children or
sanitary napkins, 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.
[0004] In most cases, these super absorbent polymers have been
widely used in the field of hygienic materials such as diapers or
sanitary napkins. For these applications, the super absorbent
polymer should exhibit a high moisture absorbency, it should not
release the absorbed water even in the external pressure, and
additionally it should well retain the shape even in a state where
the volume is expanded (swelled) by absorbing water, and thereby
exhibit excellent liquid permeability.
[0005] However, it is known that it is difficult to improve both a
centrifuge retention capacity (CRC), which is the physical property
showing the basic absorption capacity and the water retaining
capacity of the super absorbent polymer, and an absorbency under
load (AUL), which shows the properties of well retaining the
absorbed moisture even under the external pressure. This is
because, when the overall crosslinking density of the super
absorbent polymer is controlled to be low, the centrifuge retention
capacity can be relatively high, but the crosslinking structure may
be loose, the gel strength may be low and thus the absorbency under
load may be lowered. On the contrary, when controlling the
crosslink density to a high level to improve the absorbency under
load, it becomes difficult for moisture to be absorbed between
densely crosslinked structures, so that the basic centrifuge
retention capacity may be lowered. For the reasons described above,
there is a limitation in providing a super absorbent polymer having
improved centrifuge retention capacity and improved absorbency
under load together.
[0006] However, recently, as hygiene materials such as a diaper or
a sanitary napkin become thinner, super absorbent polymers are
required to have higher absorption performance. Among these,
improving both a centrifuge retention capacity and an absorbency
under load which are conflicting physical properties, improving a
liquid permeability, and so on, have become an important task.
[0007] In addition, pressure can be applied to hygiene materials
such as diapers or sanitary napkins due to the weight of the user.
In particular, when a super absorbent polymer applied to sanitary
materials such as diapers or sanitary napkins absorbs liquid and
then pressure is applied due to the weight of the user, a rewetting
phenomenon where some liquid absorbed in the super absorbent
polymer again leak out can occur. Accordingly, various attempts
have been made to improve the absorbency under load and the liquid
permeability in order to suppress such rewetting phenomenon.
However, concrete methods capable of effectively suppressing the
rewetting phenomenon have not been suggested.
DETAILED DESCRIPTION OF THE INVENTION
Technical Problem
[0008] It is one object of the present invention to provide a super
absorbent polymer capable of effectively suppressing the rewetting
phenomenon after absorbing a liquid and thus imparting a smooth
touch feeling.
[0009] It is another object of the present invention to provide a
method for producing the super absorbent polymer
Technical Solution
[0010] According to an embodiment of the invention, there is
provided a super absorbent polymer; comprising: a base polymer
powder including a cross-linked polymer in which a water-soluble
ethylenically unsaturated monomer having at least partially
neutralized acidic groups is cross-linked in the presence of an
internal crosslinking agent; and first and second surface
cross-linked layers that are further cross-linked from the
cross-linked polymer in the presence of a surface crosslinking
agent and are formed on the base polymer powder, wherein the first
and second surface cross-linked layers each include a cross-linked
structure derived from epoxy-based and non-epoxy-based surface
crosslinking agents, and having a GBP change ratio calculated by
the following Formula 1 of 0.90 or less.
0.3 GBP - 0.3 AGBP 0.3 GBP [ Formula 1 ] ##EQU00001##
[0011] in Formula 1.
[0012] 0.3 GBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution (0.9 wt % sodium chloride aqueous
solution) of a super absorbent polymer,
[0013] 0.3 AGBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution of a super absorbent polymer having a
particle size from 300 to 600 .mu.m after swelling and drying which
is obtained by a process of adding 5 g of the super absorbent
polymer to 125 g of distilled water, stirring the resultant for 1
minute, drying the swollen super absorbent polymer at 100.degree.
C. for 3 hours, and then classifying the dried polymer through a
U.S. standard 30 mesh screen and a U.S. standard 50 mesh
screen.
[0014] The super absorbent polymer may have a centrifuge retention
capacity (CRC) for a physiological saline solution of 30 to 40 g/g.
The super absorbent polymer may have an absorbency under load (AUL)
under 0.9 psi for a physiological saline solution of 19 to 25 g/g.
The super absorbent polymer may have a free swell gel bed
permeability (GBP) for a physiological saline solution of about 50
darcy to about 100 darcy.
[0015] Meanwhile, according to one embodiment of the present
invention, there is provided a method for producing a super
absorbent polymer comprising the steps of: carrying out a
crosslinking polymerization of a monomer mixture including a
water-soluble ethylenically unsaturated monomer having at least
partially neutralized acidic groups in the presence of an internal
crosslinking agent to form a hydrogel polymer; drying, pulverizing
and classifying the hydrogel polymer to form a base polymer powder;
and subjecting the surface of the base polymer powder to a first
crosslinking in the presence of an epoxy-based surface crosslinking
agent and then subjecting the surface of the first cross-linked
base polymer powder to a second crosslinking in the presence of a
non-epoxy-based surface crosslinking agent to form a surface
crosslinked layer, wherein a GBP change ratio of the super
absorbent polymer calculated by the following Formula 1 is 0.90 or
less.
0.3 GBP - 0.3 AGBP 0.3 GBP [ Formula 1 ] ##EQU00002##
[0016] in Formula 1,
[0017] 0.3 GBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution (0.9 wt % sodium chloride aqueous
solution) of a super absorbent polymer,
[0018] 0.3 AGBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution of a super absorbent polymer having a
particle size from 300 to 600 .mu.m after swelling and drying which
is obtained by a process of adding 5 g of the super absorbent
polymer to 125 g of distilled water, stirring the resultant for 1
minute, drying the swollen super absorbent polymer at 100.degree.
C. for 3 hours, and then classifying the dried polymer through a
U.S. standard 30 mesh screen and a U.S. standard 50 mesh
screen.
[0019] In the step of forming the surface crosslinked layer,
examples of the epoxy-based surface crosslinking agent may include
at least one polyglycidyl ether selected from the group consisting
of ethylene glycol diglycidyl ether, propylene glycol diglycidyl
ether, butanediol diglycidyl ether, hexanediol diglycidyl ether,
diethylene glycol diglycidyl ether, triethylene glycol diglycidyl
ether, polyethylene glycol diglycidyl ether, dipropylene glycol
diglycidyl ether, tripropylene glycol diglycidyl ether,
polypropylene glycol diglycidyl ether, and glycerol triglycidyl
ether.
[0020] Further, as the non-epoxy-based surface crosslinking agent,
a polyol, a carbonate-based compound or a mixture thereof may be
used. Among them, examples of the polyol may include at least one
selected from the group consisting of ethylene glycol, propylene
glycol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol,
1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol,
2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene
glycol and glycerol, and examples of the carbonate-based compound
may include at least one selected from the group consisting of
ethylene carbonate and propylene carbonate.
[0021] In the step of forming the surface crosslinked layer, the
surface of the base polymer powder may be subjected to a first
surface crosslinking at a temperature of 120 to 160.degree. C. for
5 to 40 minutes using an epoxy-based surface crosslinking agent.
The surface of the first surface cross-linked powder can be
subjected to a second surface crosslinking at a temperature of 170
to 210.degree. C. for 5 to 40 minutes using a non-epoxy-based
surface crosslinking agent.
[0022] Meanwhile, according to one embodiment of the present
invention, there is provided a super absorbent polymer; comprising:
a base polymer powder including a cross-linked polymer in which a
water-soluble ethylenically unsaturated monomer having at least
partially neutralized acidic groups is cross-linked in the presence
of an internal crosslinking agent; and first and second surface
cross-linked layers that are further cross-linked from the
cross-linked polymer in the presence of a surface crosslinking
agent and are formed on the base polymer powder, wherein the first
and second surface cross-linked layers each include a cross-linked
structure derived from surface crosslinking agents that are
different from each other; and having an absorbency under load
(AUL) under 0.9 psi for a physiological saline solution is 19 to 25
g/g, and a GBP change ratio calculated by the following Formula 1
is 0.90 or less.
0.3 GBP - 0.3 AGBP 0.3 GBP [ Formula 1 ] ##EQU00003##
[0023] in Formula 1,
[0024] 0.3 GBP is a gel bed permeability (GBP) under 0.3 psi for a
0.9 wt % sodium chloride aqueous solution of a super absorbent
polymer,
[0025] 0.3 AGBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution of a super absorbent polymer having a
particle size from 300 to 600 .mu.m after swelling and drying which
is obtained by a process of adding 5 g of the super absorbent
polymer to 125 g of distilled water, stirring the resultant for 1
minute, drying the swollen super absorbent polymer at 100.degree.
C. for 3 hours, and then classifying the dried polymer through a
U.S. standard 30 mesh screen and a U.S. standard 50 mesh
screen.
Advantageous Effects
[0026] The super absorbent polymer according to one embodiment of
the present invention can exhibit excellent absorbent properties
even in a swollen state and thus exhibit excellent anti-rewetting
effects. Accordingly, when the super absorbent polymer is used, it
is possible to provide a sanitary material such as a diaper or a
sanitary napkin which can give a smooth touch feeling even after
the body fluid is discharged.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIGS. 1 to 3 are schematic views of an exemplary apparatus
for measuring the gel bed permeability and the components provided
in the apparatus.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0028] Hereinafter, a super absorbent polymer according to a
specific embodiment of the present invention, a method for
producing the same, and the like will be described.
[0029] According to one embodiment of the invention, there is
provided a super absorbent polymer; comprising: a base polymer
powder including a cross-linked polymer in which a water-soluble
ethylenically unsaturated monomer having at least partially
neutralized acidic groups is cross-linked in the presence of an
internal crosslinking agent; and first and second surface
cross-linked layers that are further cross-linked from the
cross-linked polymer in the presence of a surface crosslinking
agent and are formed on the base polymer powder, wherein the first
and second surface cross-linked layers each include a cross-linked
structure derived from epoxy-based and non-epoxy-based surface
crosslinking agents, and having a GBP change ratio calculated by
the following Formula 1 is 0.90 or less.
0.3 GBP - 0.3 AGBP 0.3 GBP [ Formula 1 ] ##EQU00004##
[0030] in Formula 1,
[0031] 0.3 GBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution (0.9 wt % sodium chloride aqueous
solution) of a super absorbent polymer,
[0032] 0.3 AGBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution of a super absorbent polymer having a
particle size from 300 to 600 .mu.m after swelling and drying which
is obtained by a process of adding 5 g of the super absorbent
polymer to 125 g of distilled water, stirring the resultant for 1
minute, drying the swollen super absorbent polymer at 100.degree.
C. for 3 hours, and then classifying the dried polymer through a
U.S. standard 30 mesh screen and a U.S. standard 50 mesh
screen.
[0033] As a result of experiments, the present inventors have found
that when the super absorbent polymer is swollen, the surface
cross-linking is broken and different absorbent properties from
those of the super absorbent polymer before swelling is exhibited,
and thus the physical properties of the swollen super absorbent
polymer must be improved to improve the rewetting property of the
super absorbent polymer.
[0034] In particular, a super absorbent polymer having a GBP change
ratio calculated by Formula 1 of 0.90 or less can exhibit
anti-rewetting effects because the absorbent properties when
swollen are as good as the absorbent properties before being
swollen. For more specific method of measuring a GBP change ratio
change, reference may be made to Test Examples described later.
[0035] The GBP change ratio calculated by Formula 1 can be adjusted
to 0.85 or less or 0.80 or less for better anti-rewetting effects.
When the GBP change ratio is 0, it means that the swollen super
absorbent polymer exhibits the same physical properties as the
super absorbent polymer before being swollen, and thus the lower
limit of the GBP change ratio can be 0.
[0036] The super absorbent polymer having a small GBP change ratio
described above exhibits a high gel strength even in a swollen
state, so that it can exhibit well-balanced properties of a
centrifuge retention capacity, an absorbency under load and a
liquid permeability.
[0037] For example, the super absorbent polymer may have a
centrifuge retention capacity (CRC) for a physiological saline
solution of 30 g/g to 40 g/g, 30 g/g to 35 g/g or 30 g/g to 33 g/g.
The super absorbent polymer may have an absorbency under load (AUL)
under 0.9 psi for a physiological saline solution of 19 to 25 g/g,
19 g/g to 23 g/g or 19 g/g to 21 g/g. The super absorbent polymer
may have a free swell gel bed permeability (GBP) for a
physiological saline solution of 50 to 100 darcy, 50 to 80 darcy or
55 to 70 darcy.
[0038] The super absorbent polymer can exhibit the above-mentioned
CRC. AUL and GBP simultaneously. The super absorbent polymer
exhibiting such balanced absorption properties can rapidly absorb a
large amount of salt water at a high rate and then exhibit
excellent liquid permeability while well-retaining the absorbed
salt water even under the external pressure.
[0039] Consequently, the super absorbent polymer can exhibit
excellent gel strength even in the swollen state. Therefore, the
super absorbent polymer can effectively prevent the re-wetting
phenomenon in which the absorbed saline water leaks out again even
when the pressure is applied, thereby providing a sanitary material
such as a diaper or sanitary napkin capable of imparting smooth
touch feeling even after the body fluids are discharged.
[0040] On the other hand, in the present specification, psi is
mainly used in units of pressure. Since 1 psi is 6,894.73326 Pa
(N/m.sup.2), the pressure input in psi can be understood by
converting it to Pa which is SI unit.
[0041] The centrifuge retention capacity (CRC) for a physiological
saline solution can be measured according to EDANA recommended test
method No. WSP 241.2. More specifically, the centrifuge retention
capacity can be obtained in accordance with the following
Calculation Formula 1, after classifying super absorbent polymers
to prepare a super absorbent polymer having a particle diameter of
150 .mu.m to 850 .mu.m, and absorbing the same in physiological
saline solution for 30 minutes:
CRC(g/g)={[W.sub.2(g)-W.sub.1(g)]/W.sub.0(g)}-1 [Calculation
Formula 1]
[0042] in Calculation Formula 1,
[0043] W.sub.0(g) is an initial weight(g) of the super absorbent
polymer having a particle diameter of 150 to 850 .mu.m, W.sub.1(g)
is a weight of a nonwoven fabric-made empty bag not containing the
super absorbent polymer, measured after immersing the nonwoven
fabric-made empty bag in a 0.9 wt % aqueous sodium chloride aqueous
solution (physiological saline solution) for 30 minutes and then
dehydrating the same by using a centrifuge at 250 G for 3 minutes,
and W.sub.2(g) is a weight of the nonwoven fabric-made bag
including a super absorbent polymer, measured after soaking and
absorbing the super absorbent polymer having the particle diameter
of 150 .mu.m to 850 .mu.m in a physiological saline solution at
room temperature for 30 minutes, and then dehydrating the same by
using a centrifuge at 250 G for 3 minutes.
[0044] In addition, the absorbency under load (AUL) at 0.9 psi can
be measured according to EDANA recommended test method No. WSP
242.2. More specifically, the absorbency under load can be
calculated in accordance with the following Calculation Equation 2,
after absorbing the super absorbent polymer in a physiological
saline solution under a load of about 0.9 psi over 1 hour:
AUL(g/g)=[W.sub.4(g)-W.sub.3(g)]/W.sub.0(g) [Calculation Equation
2]
[0045] in Calculation Equation 2.
[0046] W.sub.0(g) is an initial weight(g) of the super absorbent
polymer, W.sub.3(g) is the total sum of a weight of the super
absorbent polymer and a weight of the device capable of providing a
load to the super absorbent polymer, and W.sub.4(g) is the total
sum of a weight of the super absorbent polymer and a weight of the
device capable of providing a load to the super absorbent polymer,
after absorbing a physiological saline solution to the super
absorbent polymer under a load (0.0.9 psi) for 1 hour.
[0047] W.sub.0(g) described in Calculation Equations 1 and 2
corresponds to an initial weight(g) of the super absorbent polymer,
before absorbing a physiological saline solution to the super
absorbent polymer, and they may be the same or different from each
other.
[0048] The gel bed permeability (GBP) for a physiological saline
solution was measured in units of Darcy or cm.sup.2 according to
the following method described in Korean Patent Application No.
10-2014-7018005. One Darcy means that it permits a flow of 1 mm/s
of a fluid with viscosity of 1 cP under a pressure gradient of 1
atm/cm acting across an area of 1 cm.sup.2. Gel bed permeability
has the same unit as area, and 1 darcy is the same as
0.98692.times.10.sup.-12 m.sup.2 or 0.98692.times.10.sup.8
cm.sup.2.
[0049] More specifically, as used herein. GBP means a penetration
(or permeability) of a swollen gel layer (or bed) under conditions
referred to as 0 psi free swell state (a Gel Bed Permeability (GBP)
Under 0 psi Swell Pressure Test). 0.3 GBP means a penetration of
the swollen gel layer under a load of 0.3 psi, and 0.3 AGBP means
penetration of the swollen gel layer under a load of 0.3 psi for
the dried super absorbent polymer after swelling. The GBP can be
measured using the apparatus shown in FIGS. 1 to 3. 0.3 GBP and 0.3
AGBP can be measured by using a method of measuring the above GBP
except for applying a load of 0.3 psi during swelling of the super
absorbent polymer.
[0050] Specifically, referring to FIGS. 1-3, the test apparatus
assembly 528 in a device 500 for measuring GBP includes a sample
container 530 and a plunger 536. The plunger includes a shaft 538
having a cylinder hole bored down the longitudinal axis and a head
550 positioned at the bottom of the shaft. The shaft hole 562 has a
diameter of about 16 mm. The plunger head is attached to the shaft,
for example, by an adhesive. Twelve holes 544 are bored into the
radial axis of the shaft, and three holes positioned at every 90
degrees has a diameter of about 6.4 mm. The shaft 538 is machined
from a LEXAN rod or equivalent material, and has an outer diameter
of about 2.2 cm and an inner diameter of about 16 mm. The plunger
head 550 has seven inner holes 560 and fourteen outer holes 554,
all holes having a diameter of about 8.8 mm. Further, a hole of
about 16 mm is aligned with the shaft. The plunger head 550 is
machined from a LEXAN rod or equivalent material and has a height
of about 16 mm and a diameter sized such that it fits within the
cylinder 534 with minimum wall clearance but still moves freely.
The total length of the plunger head 550 and shaft 538 is about
8.25 cm, but can be machined at the top of the shaft to obtain the
desired size of the plunger 536. The plunger 536 includes a 100
mesh stainless steel cloth screen 564 that is biaxially stretched
to tautness and attached to the lower end of the plunger 536. The
screen is attached to the plunger head 550 using a suitable solvent
that causes the screen to be securely adhered to the plunger head
550. Care should be taken to avoid excess solvent moving into the
openings of the screen and reducing the open area for liquid flow
area. Acrylic solvent Weld-on 4 from IPS Corporation (having a
place of business in Gardena, Calif., USA) can be used
appropriately. The sample container 530 includes a cylinder 534 and
a 400 mesh stainless steel cloth screen 566 that is biaxially
stretched to tautness and attached to the lower end of the plunger
534. The screen is attached to the cylinder using a suitable
solvent that causes the screen to be securely adhered to the
cylinder. Care should be taken to avoid excess solvent moving into
the openings of the screen and reducing the open area for liquid
flow. Acrylic solvent Weld-on 4 from IPS Corporation (having a
place of business in Gardena, Calif., USA) can be used
appropriately. The gel particle sample (swollen super absorbent
polymer), indicated as 568 in FIG. 2, is supported on the screen
566 within the cylinder 534 during testing.
[0051] Cylinder 534 may be bored from a transparent LEXAN rod or
equivalent material, or it may be cut from LEXAN tubing or
equivalent material, and has an inner diameter of about 6 cm (for
example, a cross sectional area of about 28.27 cm.sup.2), a wall
thickness of about 0.5 cm and a height of about 7.95 cm. A step can
be formed by machining into the outer diameter of the cylinder 534
such that a region 534a having an outer diameter of 66 mm is
present at the bottom 31 mm of the cylinder 534. An O-ring 540
which fits the diameter of the region 534a may be placed on top of
the step.
[0052] The annular weight 548 has a counter-bored hole of about 2.2
cm in diameter and 1.3 cm deep so it slides freely onto the shaft
538. The annular weight also has a thru-bore 548a of about 16 mm.
The annular weight 548 may be made from stainless steel or from
other suitable material capable of corrosion resistance in a
physiological saline solution (0.9 wt % aqueous sodium chloride
solution). The combined weight of the plunger 536 and the annular
weight 548 is equal to about 596 g, which corresponds to a pressure
applied to the sample 568 of about 0.3 psi or about 20.7
dyne/cm.sup.2 (2.07 kPa), over a sample area of about 28.27
cm.sup.2.
[0053] When the test solution flows through the test apparatus
during testing of the GBP, the sample container 530 generally rests
on a weir 600. The purpose of the weir is to divert liquid that
overflows the top of the sample container 530, and diverts the
overflow liquid to a separate collection device 601. The weir can
be positioned above a scale 602 with a beaker 603 resting on it to
collect a physiological saline solution passing through the swollen
sample 568.
[0054] In order to perform the gel bed permeability test under
"free swell" conditions, the sample is swollen under no pressure in
the manner described in the following (i), and in order to perform
the gel bed permeability test under the condition of "pressure of
0.3 psi", the sample is swollen under pressure of 0.3 psi in the
manner described in the following (ii).
[0055] (i) The plunger 536 installed with the weight 548 is placed
in an empty sample container 530, and the height from the top of
the weight 548 to the bottom of the sample container 530 is
measured to an accuracy of 0.01 mm using an appropriate gauge. The
force to which the thickness gauge applies during the measurement
should be as low as possible, preferably less than about 0.74 N.
When using multiple test apparatus, it is important to keep each
empty sample container 530, plunger 536 and weight 548 and track of
which they are used.
[0056] Further, it is preferable that the base on which the sample
container 530 is placed is flat, and the surface of the weight 548
is parallel to the bottom surface of the sample container 530.
Then, a sample to be tested is prepared from the super absorbent
polymer for measuring GBP. As an example, a test sample is prepared
from a super absorbent polymer having a particle diameter of about
300 to about 600 .mu.m, which is passed through a US standard 30
mesh screen and retained on a US standard 50 mesh screen. About 2.0
g of a sample is placed in a sample container 530 and spread out
evenly on the bottom of the sample container. The container
containing 2.0 g of sample, without the plunger 536 and the weight
548 therein, is then submerged in the physiological saline solution
for about 60 minutes and allow the sample to swell under no load
condition.
[0057] (ii) The plunger 536 installed with the weight 548 is placed
in an empty sample container 530, and the height from the top of
the weight 548 to the bottom of the sample container 530 is
measured to an accuracy of 0.01 mm using an appropriate gauge. The
force to which the thickness gauge applies during the measurement
should be as low as possible, preferably less than about 0.74 N.
When using multiple test apparatus, it is important to keep each
empty sample container 530, plunger 536 and weight 548 and track of
which they are used.
[0058] Further, it is preferable that the base on which the sample
container 530 is placed is flat, and the surface of the weight 548
is parallel to the bottom surface of the sample container 530.
Then, a sample to be tested is prepared from the super absorbent
polymer for measuring GBP. As an example, a test sample is prepared
from a super absorbent polymer having a particle diameter of about
300 to about 600 .mu.m, which is passed through a US standard 30
mesh screen and retained on a US standard 50 mesh screen. About 2.0
g of a sample is placed in a sample container 530 and spread out
evenly on the bottom of the sample container. Then, the assembly of
plunger 536 and weight 548 is placed on a sample in the sample
container, and the sample container is then submerged in the
physiological saline solution for about 60 minutes and allow the
sample to swell under load of 0.3 psi.
[0059] In both cases (i) and (ii), the sample container 530 is
placed on the mesh located in a liquid reservoir so that the sample
container 530 is raised slightly above the bottom of the liquid
reservoir. As the mesh, those which do not affect the movement of
the physiological saline solution into the sample container 530 can
be used. As such mesh, part number 7308 from Eagle Supply and
Plastic (having a place of business in Appleton, Wis., USA) can be
used. During saturation, the height of the physiological saline
solution can be adjusted such that the surface within the sample
container is defined by the sample, rather than the physiological
saline solution.
[0060] At the end of this period, if the sample is swollen in the
manner described in (i) above, the assembly of the plunger 536 and
weight 548 is placed on the saturated sample 568 in the sample
container 530 and then the sample container 530, plunger 536,
weight 548 and sample 568 are removed from the solution. Meanwhile,
if the sample is swollen in the manner described in (ii) above, the
sample container 530, plunger 536, weight 548 and sample 568 are
removed from the solution.
[0061] Thereafter, before GBP measurement, the sample container
530, plunger 536, weight 548 and sample 568 are placed on a flat,
large grid non-deformable plate of uniform thickness for about 30
seconds. The plate will prevent liquid in the sample container from
being released onto a flat surface due to surface tension. The
plate has an overall dimension of 7.6 cm.times.7.6 cm, and each
grid has a dimension of 1.59 cm long.times.1.59 cm wide.times.1.12
cm deep. A suitable plate material is a parabolic diffuser panel,
catalogue number 1624K27, available from McMaster Carr Supply
Company (having a place of business in Chicago, Ill., USA), which
can then be cut to the proper dimensions.
[0062] Then, if the zero point has not changed from the initial
height measurement, the height from the top of the weight 548 to
the bottom of the sample container 530 is measured again by using
the same thickness gauge as previously used. The height measurement
should be made as soon as practicable after the thickness gauge is
installed. The height measurement of the empty assembly where the
plunger 536 and weight 548 are located in the empty sample
container 530 should be subtracted from the height measurement
obtained after saturating the sample 568. The resulting value is
the thickness, or height "H" of the saturated sample 568. Further,
if a plate is contained in the assembly containing the saturated
sample 568, the height including the plate should be measured even
when measuring the height of the empty assembly.
[0063] The GBP measurement is started by delivering a flow of a
physiological saline solution into the sample container 530
containing the saturated sample 568, the plunger 536 and the weight
548. The flow rate of physiological saline solution into the
container is adjusted to cause physiological saline solution to
overflow the top of the cylinder 534, thereby resulting in a
consistent head pressure equal to the height of the sample
container 530. The physiological saline solution may be added by
any suitable means that is sufficient to ensure a small, but
consistent amount of overflow from the top of the cylinder, such as
with a metering pump 604. The overflow liquid is diverted into a
separate collection device 601. The quantity of solution passing
through the sample 568 versus time is measured gravimetrically
using the scale 602 and beaker 603. Data points from the scale 602
are collected every second for at least sixty seconds once the
overflow has started. Data collection may be taken manually or with
data collection software. The flow rate (Q) passing through the
swollen sample 568 is determined in units of grams/second (g/s) by
a linear least-square fit of fluid passing through the sample 568
(in grams) versus time (in seconds).
[0064] Using the data thus obtained, the gel bed permeability can
be confirmed by calculating the GBP (cd) according to the following
Calculation Equation 3.
K=[Q*H*.mu.]/[A*.rho.*P] [Calculation Equation 3]
[0065] in Calculation Equation 3,
[0066] K is a gel bed permeability (cm.sup.2),
[0067] Q is a flow rate (g/sec)
[0068] H is a height of swollen sample (cm),
[0069] .mu. is a liquid viscosity (poise) (about one cP for the
test solution used with this Test),
[0070] A is a cross-sectional area for liquid flow (28.27 cm.sup.2
for the sample container used with this Test),
[0071] .rho. is a liquid density (g/cm.sup.3)(about one g/cm.sup.3,
for the test solution used with this Test), and
[0072] P is a hydrostatic pressure (dyne/cm.sup.2) (normally about
7,797 dynes/cm.sup.2).
[0073] The hydrostatic pressure is calculated from the equation
P=.rho.*g*h, where .rho. is a liquid density (g/cm.sup.3), g is a
gravitational acceleration (nominally 981 cm/sec.sup.2), and h is a
fluid height (for example, 7.95 cm for the GBP Test described
herein).
[0074] Meanwhile, according to another embodiment of the present
invention, there is provided a method for producing a super
absorbent polymer having a GBP change ratio calculated by Formula 1
of 0.90 or less.
[0075] Specifically, the method for producing a super absorbent
polymer comprises the steps of: carrying out a crosslinking
polymerization of a monomer mixture including a water-soluble
ethylenically unsaturated monomer having at least partially
neutralized acidic groups in the presence of an internal
crosslinking agent to form a hydrogel polymer; drying, pulverizing
and classifying the hydrogel polymer to form a base polymer powder;
and subjecting the surface of the base polymer powder to a first
crosslinking in the presence of an epoxy-based surface crosslinking
agent and then subjecting the surface of the first cross-linked
base polymer powder to a second crosslinking in the presence of a
non-epoxy-based surface crosslinking agent to form a surface
crosslinked layer.
[0076] The water-soluble ethylenically unsaturated monomer may
include at least one selected from the group consisting of anionic
monomers such as (meth)acrylic acid, maleic acid, maleic anhydride,
fumaric acid, crotonic acid, itaconic acid, sorbic acid,
vinylphosphonic acid, vinylsulfonic acid, allylsulfonic acid,
2-(meth)acryloylethanesulfonic acid,
2-(meth)acryloyloxyethanesulfonic acid,
2-(meth)acryloylpropanesulfonic acid or
2-(meth)acrylamido-2-methylpropanesulfonic acid, and their salts;
non-ionic, hydrophilic group-containing monomers of
(meth)acrylamide. N-substituted (meth)acrylamide,
2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,
methoxypolyethylene glycol(meth)acrylate or polyethylene glycol
(meth)acrylate; and amino group-containing unsaturated monomers of
(N,N)-dimethylaminoethyl(meth)acrylate or
(N,N)-dimethylaminopropyl(meth)acrylamide, and their quaternary
product.
[0077] In particular, the water-soluble ethylenically unsaturated
monomer may be composed of a monomer (a salt of an anionic monomer)
in which at least a part thereof is neutralized with an acidic
group contained in the anionic monomer.
[0078] More specifically, acrylic acid or a salt thereof can be
used as the water-soluble ethylenically unsaturated monomer, and in
the case where acrylic acid is used, it can be used after
neutralizing at least a part thereof. By using such monomer, it
becomes possible to prepare a super absorbent polymer having more
excellent physical properties. For example, when an alkali metal
salt of acrylic acid is used as the water-soluble ethylenically
unsaturated monomer, acrylic acid may be used by neutralizing it
with a neutralizing agent such as sodium hydroxide (NaOH). At this
time, the degree of neutralization of the acrylic acid can be
adjusted to about 50 to 95 mol %, or about 60 to 85 mol %. Within
this range, it is possible to provide a super absorbent polymer
having excellent centrifuge retention capacity without fear of
precipitation during neutralization.
[0079] In the monomer mixture containing the water-soluble
ethylenically unsaturated monomer, the concentration of the
water-soluble ethylenically unsaturated monomer may be about 20% to
about 60% by weight, or about 25% to about 50% by weight based on
the total amount of the monomer mixture including respective raw
materials, polymerization initiator, solvent and the like described
below, which may be appropriately adjusted in consideration of
polymerization time, the reaction conditions and the like. However,
if the concentration of the monomer is excessively low, the yield
of the super absorbent polymer can be lowered and thus economic
problems may arise. On the other hand, if the concentration is
excessively high, it may arise problems in the processes, for
example, a part of the monomer may be precipitated, or the
pulverization efficiency may be lowered during pulverization of the
polymerized hydrogel polymer, etc., and the physical properties of
the super absorbent polymer may be deteriorated.
[0080] The internal crosslinking agent is included in the monomer
mixture in order to carry out a cross-linking polymerization of the
water-soluble ethylenically unsaturated monomer. The internal
crosslinking agent is composed of a compound containing two or more
crosslinkable functional groups in the molecule. The internal
crosslinking agent may include a carbon-carbon double bond in the
crosslinkable functional group for smooth cross-linking
polymerization reaction of the above-mentioned water-soluble
ethylenically unsaturated monomer. More specific examples of these
internal crosslinking agents include at least one selected from the
group consisting of polyethylene glycol diacrylate (PEGDA),
glycerine diacrylate, glycerin triacrylate, unmodified or
ethoxylated trimethylolpropane triacrylate (TMPTA), hexanediol
diacrylate, and triethylene glycol diacrylate.
[0081] The internal crosslinking agent can be contained at a
concentration of about 0.01 to about 5% by weight with respect to
the monomer mixture, thereby forming a cross-linked polymer
exhibiting high absorption rate while having excellent centrifuge
retention capacity and absorbency under load.
[0082] In addition, the monomer mixture may further include a
polymerization initiator that is generally used in the production
of a super absorbent polymer.
[0083] Specifically, the polymerization initiator can be
appropriately selected depending on the polymerization method. When
a thermal polymerization method is carried out, a thermal
polymerization initiator is used. When a photo-polymerization
method is carried out, a photo-polymerization initiator is used.
When a hybrid polymerization method (a method using both thermal
and photo) is used, both a thermal polymerization initiator and a
photo-polymerization initiator can be used. However, even in the
case of the photo-polymerization method, a certain amount of heat
is generated by light irradiation such as ultraviolet irradiation
or the like, and a certain amount of heat is generated in
accordance with the progress of the polymerization reaction, which
is an exothermic reaction, and thus, a thermal polymerization
initiator may be further used.
[0084] The photo-polymerization initiator that can be used is not
particularly limited by its constitution as long as it is a
compound capable of forming a radical by light such as ultraviolet
rays.
[0085] The photo-polymerization initiator used herein may include,
for example, at least one selected from the group consisting of
benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl
glyoxylate, benzyl dimethyl ketal, acyl phosphine and
.alpha.-aminoketone. Meanwhile, specific examples of the
acylphosphine include diphenyl(2,4,6-trimethylbenzoyl)phosphine
oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide,
ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate, and the like. More
various photo-polymerization initiators are well disclosed in "UV
Coatings: Basics, Recent Developments and New Application" written
by Reinhold Schwalm, (Elsevier, 2007), p 115, the content of which
is incorporated herein by reference.
[0086] The photo-polymerization initiator may be included in a
concentration of about 0.0001 to about 2.0% by weight with respect
to the monomer mixture. When the concentration of the
photo-polymerization initiator is too low, the polymerization rate
may become slow, and when the concentration of the
photo-polymerization initiator is too high, the molecular weight of
the super absorbent polymer may be small and the physical
properties may become uneven.
[0087] Further, as the thermal polymerization initiator, at least
one selected from the group consisting of persulfate-based
initiator, azo-based initiator, hydrogen peroxide and ascorbic acid
can be used. Specifically, examples of the persulfate-based
initiators include sodium persulfate (Na.sub.2S.sub.2O.sub.8),
potassium persulfate (K.sub.2S.sub.2O.sub.8), ammonium persulfate
((NH.sub.4).sub.2S.sub.2O.sub.8) and the like, and examples of the
azo-based initiator include
2,2-azobis(2-amidinopropane)dihydrochloride,
2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride,
2-(carbamoylazo)isobutylonitril,
2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride,
4,4-azobis-(4-cyanovaleric acid) and the like. More various thermal
polymerization initiators are well disclosed in "Principle of
Polymerization" written by Odian, (Wiley, 1981), p 203, the content
of which is incorporated herein by reference.
[0088] The thermal polymerization initiator may be included at a
concentration of about 0.001 to about 2.0% by weight with respect
to the monomer mixture. If the concentration of such a thermal
polymerization initiator is too low, additional thermal
polymerization hardly occurs and the effect due to the addition of
the thermal polymerization initiator may be insignificant. If the
concentration of the thermal polymerization initiator is
excessively high, the molecular weight of the super absorbent
polymer may be small and the physical properties may become
uneven.
[0089] The monomer mixture may further include a foaming agent in
order to produce the super absorbent polymer having a porous
structure. As such foaming agent, carbonate can be used. As the
carbonate, at least one selected from the group consisting of
magnesium carbonate, calcium carbonate, sodium bicarbonate, sodium
carbonate, potassium bicarbonate and potassium carbonate can be
used.
[0090] The foaming agent may be used in an amount ranging from
about 0.1% to about 0.3% by weight based on the total weight of the
monomer mixture, and thus a polymer exhibiting a gel strength while
having a porous structure can be provided.
[0091] In addition, the monomer mixture may further include a
surfactant in order to induce a stable bubble generation. As such a
surfactant, at least one surfactant selected among anionic
surfactants, nonionic surfactants, cationic surfactants and
amphoteric surfactants can be used.
[0092] Specific examples of the anionic surfactant include fatty
acid salts such as mixed fatty acid sodium soap, semi-hardened milk
fatty acid sodium soap, sodium stearate soap, potassium oleate
soap, castor oil potassium soap or the like; alkylsulfuric acid
ester salts such as sodium dodecylsulfate, higher alcohol sodium
sulfate, sodium laurylsulfate and triethanolamine laurylsulfate;
alkylbenzenesulfonic acid salts such as sodium
dodecylbenzenesulfonate; alkyl naphthalenesulfonic acid salts such
as sodium alkylnaphthalenesulfonate; alkylsulfosuccinate salts such
as sodium dialkylsulfosuccinate; alkyl diphenyl ether disulfonate
salts such as sodium alkyl diphenyl ether disulfonate; alkyl
phosphates such as potassium alkyl phosphate; polyoxyethylene alkyl
(or alkylallyl) sulfuric acid ester salts such as polyoxyethylene
lauryl ether sodium sulfate, polyoxyethylene alkyl ether sodium
sulfate, triethanolamine polyoxyethylene alkyl ether sulfate,
sodium polyoxyethylene alkyl phenyl ether sulfate or the like;
special reactive anionic surfactants; special carboxylic acid type
surfactant; naphthalenesulfonic acid-formaldehyde condensate such
as sodium salt of .beta.-naphthalenesulfonic acid-formaldehyde
condensate, sodium salt of special aromatic sulfonic
acid-formaldehyde condensate or the like; special polycarboxylic
acid-based polymer surfactant; polyoxyethylene alkyl phosphates,
and the like. As specific examples of the nonionic surfactant
include polyoxyethylene alkyl ethers such as polyoxyethylene lauryl
ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether,
polyoxyethylene oleyl ether, polyoxyethylene higher alcohol ether
or the like; polyoxyethylene alkyl aryl ethers such as
polyoxyethylene nonylphenyl ether; polyoxyethylene derivatives;
sorbitan fatty acid esters such as sorbitan monolaurate, sorbitan
monopalmitate, sorbitan monostearate, sorbitan tristearate,
sorbitan monooleate, sorbitan trioleate, sorbitan sesquioleate,
sorbitan distearate or the like; polyoxyethylene sorbitan fatty
acid esters such as polyoxyethylene sorbitan monolaurate,
polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan
monopalmitate, polyoxyethylene sorbitan monostearate,
polyoxyethylene sorbitan tristearate, polyoxyethylene sorbitan
monooleate, polyoxyethylene sorbitan trioleate, or the like;
polyoxyethylene sorbitol fatty acid ester such as tetraoleic acid
polyoxyethylene sorbitol; glycerin fatty acid esters such as
glycerol monostearate, glycerol monooleate, self-emulsifying
glycerol monostearate, or the like; polyoxyethylene fatty acid
esters such as polyethylene glycol monolaurate, polyethylene glycol
monostearate, polyethylene glycol distearate, polyethylene glycol
monooleate, or the like; polyoxyethylene alkylamine;
polyoxyethylene hardened castor oil; alkyl alkanolamides, and the
like. As specific examples of the cationic surfactant and the
amphoteric surfactant include alkylamine salts such as coconut
amine acetate, stearylamine acetate, or the like; quaternary
ammonium salts such as lauryl trimethyl ammonium chloride, stearyl
trimethyl ammonium chloride, cetyltrimethylammonium chloride,
distearyldimethylammonium chloride, alkylbenzyldimethylammonium
chloride, or the like; alkyl betaine such as lauryl betaine,
stearyl betaine, lauryl carboxymethyl hydroxyethyl imidazolinium
betaine, or the like; amine oxide such as lauryl dimethylamine
oxide, and the like.
[0093] The surfactant may be used in an amount of about 0.001 to
0.1% by weight based on the monomer mixture, which can induce a
stable bubble generation of the foaming agent and induce so that
the bubbles are stably dispersed in the monomer mixture for a long
period of time.
[0094] The monomer mixture may further include additives such as a
thickener, a plasticizer, a preservation stabilizer, an
antioxidant, etc., if necessary.
[0095] The raw materials such as the water-soluble ethylenically
unsaturated monomer, the polymerization initiator, the internal
crosslinking agent and the additives may be prepared in the form of
the monomer mixture solution which is dissolved in a solvent.
[0096] The solvent that can be used is not limited by its
constitution as long as it can dissolve the above-described
components. For example, one or more solvents selected from the
group consisting of water, ethanol, ethyleneglycol,
diethyleneglycol, triethyleneglycol, 1,4-butanediol,
propyleneglycol, ethyleneglycol monobutylether, propyleneglycol
monomethylether, propyleneglycol monomethylether acetate,
methylethylketone, acetone, methylamylketone, cyclohexanone,
cyclopentanone, diethyleneglycol monomethylether, diethyleneglycol
ethylether, toluene, xylene, butylolactone, carbitol,
methylcellosolve acetate, and N,N-dimethyl acetamide, and so on may
be used alone or in combination with each other.
[0097] The solvent may be included in a residual amount of
excluding the above-described components from the total weight of
the monomer mixture.
[0098] Meanwhile, the method for forming a hydrogel polymer by the
thermal polymerization or photopolymerization of such a monomer
composition is not particularly limited by its constitution as long
as it is a polymerization method commonly used in the art.
[0099] Specifically, the polymerization process may be largely
classified into a thermal polymerization and a photo-polymerization
depending on a polymerization energy source. Usually, in the case
of the thermal polymerization, it may be carried out in a reactor
like a kneader equipped with stirring spindles. At this time, the
polymerization temperature of the monomer mixture can be adjusted
to about 30 to 110.degree. C. to form a hydrogel polymer having an
appropriate cross-linking structure. Means for achieving the
polymerization temperature within the above-described range is not
particularly limited, and the heating can be carried out by
providing a heating medium or directly providing a heating source.
The type of heat medium that can be used here includes a heated
fluid such as steam, hot air, hot oil, etc., but it is not limited
thereto. Further, the temperature of the heating medium to be
provided can be appropriately selected in consideration of the
means of the heating medium, the temperature raising speed, and the
temperature raising target temperature. Meanwhile, as a heat source
to be provided directly, there may be mentioned a heating method
using electricity or a heating method using gas, but is not limited
to the above example.
[0100] Meanwhile, in the case of the photo-polymerization, it may
be carried out in a reactor equipped with a movable conveyor belt.
However, the above-described polymerization method is an example
only, and the present invention is not limited thereto.
[0101] For example, when the thermal polymerization is carried out
by providing hot air to a reactor like a kneader equipped with the
agitating spindles, or heating the reactor, the hydrogel polymer
discharged from the outlet of the reactor can be obtained. The
hydrogel polymer thus obtained can be in the form of several
centimeters or several millimeters depending on the type of
agitating spindles equipped in the reactor. Specifically, the size
of the obtained hydrogel polymer may vary depending on the
concentration of the monomer mixture to be injected thereto, the
injection speed, or the like.
[0102] Further, as described above, when the photo-polymerization
is carried out in a reactor equipped with a movable conveyor belt,
the obtained hydrogel polymer may be usually a sheet-like hydrogel
polymer having a width of the belt. In this case, the thickness of
the polymer sheet may vary depending on the concentration of the
monomer mixture to be injected thereto and the injection speed, but
usually, it is preferable to supply the monomer mixture so that a
sheet-like polymer having a thickness of about 0.5 to about 10 cm
can be obtained. If the monomer mixture is supplied to such an
extent that the thickness of the sheet-like polymer is too thin, it
is undesirable because the production efficiency is low, and if the
thickness of the sheet-like polymer is more than 10 cm, the
polymerization reaction cannot be uniformly carried out over the
entire thickness.
[0103] The polymerization time of the monomer mixture is not
particularly limited, and can be adjusted to about 30 seconds to 60
minutes.
[0104] The hydrogel polymer obtained by the above-mentioned method
may have a water content of about 30 to about 80% by weight.
Meanwhile, the "water content" as used herein means a weight
occupied by moisture with respect to a total amount of the hydrogel
polymer, which may be the value obtained by subtracting the weight
of the dried polymer from the weight of the hydrogel polymer.
Specifically, the water content can be defined as a value
calculated by measuring the weight loss due to evaporation of
moisture in the polymer in the process of drying by raising the
temperature of the polymer through infrared heating. At this time,
the water content is measured under the drying conditions
determined as follows: the drying temperature is increased from
room temperature to about 180.degree. C. and then the temperature
is maintained at 180.degree. C., and the total drying time is set
to 40 minutes, including 5 minutes for the temperature rising
step.
[0105] After the monomers are polymerized into cross-linked
polymer, the base polymer powder can be obtained through steps such
as drying, pulverization, classification, and the like, and through
the steps such as pulverization and classification, the base
polymer powder and the super absorbent polymer obtained therefrom
are suitably produced and provided so as to have a particle
diameter of about 150 to 850 .mu.m. More specifically, at least
about 95% by weight or more of the base polymer powder and the
super absorbent polymer obtained therefrom has a particle diameter
of about 150 .mu.m to 850 .mu.m and a fine powder having a particle
diameter of less than about 150 .mu.m can contained in an amount of
less than about 3% by weight.
[0106] As described above, as the particle diameter distribution of
the base polymer powder and the super absorbent polymer is adjusted
to the preferable range, the super absorbent polymer finally
produced can exhibit excellent absorbent properties.
[0107] On the other hand, the method of drying, pulverization and
classification will be described in more detail below.
[0108] First, when drying the hydrogel polymer, a coarsely
pulverizing step may be further carried out before drying in order
to increase the efficiency of the drying step, if necessary.
[0109] A pulverizing machine is not limited by its configuration,
and specifically, it may include 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. However, it is
not limited to the above-described examples.
[0110] In this case, the coarsely pulverizing step may be carried
out so that the particle diameter of the hydrogel polymer becomes
about 0.2 mm to about 15 mm.
[0111] Pulverizing the hydrogel polymer into a particle diameter of
less than 0.2 mm is technically not easy due to its high moisture
content, and agglomeration phenomenon between the pulverized
particles may occur. Meanwhile, if the polymer is pulverized into a
particle diameter of greater than 15 mm, the effect of increasing
the efficiency in the subsequent drying step may be
insignificant.
[0112] The hydrogel polymer coarsely pulverized as above or the
hydrogel polymer immediately after polymerization without the
coarsely pulverizing step is subjected to a drying step. In this
case, the drying temperature of the drying step may be about
50.degree. C. to about 250.degree. C.
[0113] When the drying temperature is less than 50.degree. C., it
is likely that the drying time becomes too long or the physical
properties of the super absorbent polymer finally formed is
deteriorated, and when the drying temperature is higher than
250.degree. C., only the surface of the polymer is excessively
dried, and thus it is likely that fine powder is generated during
the subsequent pulverizing step, and the physical properties of the
super absorbent polymer finally formed is deteriorated.
[0114] Meanwhile, the drying time may be about 20 minutes to about
15 hours, in consideration of the process efficiency and the like,
but it is not limited thereto.
[0115] In the drying step, the drying method may also be selected
and used without being limited by its constitution if it is a
method generally used for drying the hydrogel polymer.
Specifically, the drying step may be carried out by a method such
as hot air supply, infrared irradiation, microwave irradiation or
ultraviolet irradiation. After the drying step as above is carried
out, the moisture content of the polymer may be about 0.1% to about
10% by weight.
[0116] Subsequently, the dried polymer obtained through the drying
step is subjected to a pulverization step.
[0117] The polymer powder obtained through the pulverizing step may
have a particle diameter of about 150 .mu.m to about 850 .mu.m.
Specific examples of a pulverizing device that can be used to
achieve the above particle diameter may include a pin mill, a
hammer mill, a screw mill, a roll mill, a disc mill, a jog mill or
the like, but the present invention is not limited thereto.
[0118] Also, in order to control the physical properties of the
super absorbent polymer powder finally commercialized after the
pulverization step, a separate step of classifying the polymer
powder obtained after the pulverization depending on the particle
diameter may be undergone. Preferably, a polymer having a particle
diameter of about 150 .mu.m to about 850 .mu.m is classified and
only the polymer powder having such a particle diameter is
subjected to the surface crosslinking reaction and finally
commercialized. Since the particle diameter distribution of the
base polymer powder obtained through such a process has already
been described above, a further detailed description thereof will
be omitted.
[0119] On the other hand, after the step of forming the base
polymer powder described above, the surface of the base polymer
powder can be subjected a double crosslinking to form the first and
second surface cross-linked layers, whereby a super absorbent
polymer having GBP change ratio of 0.90 or less can be
provided.
[0120] Specifically, in the step of forming the surface
cross-linked layer, the surface of the base polymer powder can be
further subjected to a first crosslinking using an epoxy-based
surface crosslinking agent. Consequently, a first surface
cross-linked layer containing a cross-linked structure derived from
an epoxy-based surface crosslinking agent is formed on the base
polymer powder.
[0121] In this case, as the surface crosslinking agent,
polyglycidyl ether may be used. More specifically, at least one
selected from the group consisting of ethylene glycol diglycidyl
ether, propylene glycol diglycidyl ether, butanediol diglycidyl
ether, hexanediol diglycidyl ether, diethyleneglycol diglycidyl
ether, triethylene glycol diglycidyl ether, polyethylene glycol
diglycidyl ether, dipropylene glycol diglycidyl ether, tripropylene
glycol diglycidyl ether, polypropylene glycol diglycidyl ether and
glycerol triglycidyl ether may be used.
[0122] Such an epoxy-based surface crosslinking agent may be used
in an amount of about 0.01 to 3% by weight based on the total
weight of the base polymer powder. It is possible to provide a
super absorbent polymer exhibiting excellent physical properties
even after being swollen by adjusting the content range of the
epoxy-based surface crosslinking agent within the above-mentioned
range.
[0123] In the first surface crosslinking step, the first surface
crosslinking reaction can be carried out by further adding one or
more inorganic materials selected from the group consisting of
silica, clay, alumina, silica-alumina composite material, titania,
zinc oxide and aluminum sulfate in addition to the epoxy-based
surface crosslinking agent. The inorganic material can be used in
the form of powder or liquid, and in particular, it can be used as
alumina powder, silica-alumina powder, titania powder or nanosilica
solution. In addition, the inorganic material can be used in an
amount of about 0.05% to about 2% by weight based on the total
weight of the base polymer powder.
[0124] Moreover, in the first surface crosslinking step, as the
surface crosslinking proceeds by adding a polyvalent metal cation
in place of the inorganic material or together with the inorganic
material, the surface crosslinking structure of the super absorbent
polymer can be further optimized. This is presumably because such a
metal cation can further reduce the crosslinking distance by
forming a chelate with the carboxyl group (COOH) of the super
absorbent polymer.
[0125] The method of adding the surface crosslinking agent, and
optionally an inorganic substance and/or a polyvalent metal cation
to the base polymer powder are not limited by its constitution. For
example, a method of adding surface crosslinking agent and a base
polymer powder to a reaction tank and mixing them, or a method of
spraying a surface crosslinking agent or the like to the base
polymer powder, or a method of adding a base polymer powder and a
surface crosslinking agent to a continuously operated mixer and
mixing them, or the like, may be used.
[0126] When the surface crosslinking agent is added, water and
methanol can be additionally mixed together and added. When water
and methanol are added, there is an advantage that the surface
crosslinking agent can be uniformly dispersed in the base polymer
powder. At this time, the amount of water and methanol added can be
appropriately adjusted in order to induce a more uniform dispersion
of the epoxy-based crosslinking agent, prevent the aggregation
phenomenon of the polymer powders, and further optimize the depth
of penetration of the epoxy-based surface crosslinking agent.
[0127] The first surface crosslinking reaction may be performed by
heating the base polymer powder to which the epoxy-based surface
crosslinking agent is added at a temperature of about 120.degree.
C. to 160.degree. C., about 130.degree. C. to 150.degree. C. or
about 140.degree. C. for about 5 to 40 minutes, about 10 to 30
minutes or about 20 minutes. In particular, in order to produce a
super absorbent polymer that more suitably fulfills physical
properties according to one embodiment, the conditions of the first
surface crosslinking step can adjust the maximum reaction
temperature to about 120.degree. C. to 160.degree. C. The retention
time at the maximum reaction temperature can be adjusted under the
conditions of about 5 to 40 minutes, or about 10 to 30 minutes or
about 20 minutes. In addition, from a temperature at the beginning
of the first reaction, for example, a temperature of about
80.degree. C. or more, the temperature raising time until reaching
the maximum reaction temperature can be controlled to be about 10
minutes or more, or about 10 minutes or more and 1 hour or
less.
[0128] Meanwhile, in the step of forming the surface cross-linked
layer, the surface of the first surface cross-linked powder
obtained through the first surface cross-linking step is subjected
to a second crosslinking using a non-epoxy based surface
crosslinking agent. Consequently, a first surface cross-linked
layer containing a crosslinking structure derived from an
epoxy-based surface crosslinking agent and a second surface
cross-linked layer containing a crosslinking structure derived from
a non-epoxy based surface crosslinking agent are formed on the base
polymer powder.
[0129] In the step of forming the surface cross-linked layer, as
the first surface crosslinking step using an epoxy based surface
crosslinking agent and the second surface crosslinking step using a
non-epoxy based surface crosslinking agent are sequentially
performed, the second surface crosslinking layer may be formed on
the first surface crosslinking layer.
[0130] Due to such double surface crosslinking step, it is possible
to provide a super absorbent polymer having a small GBP change
ratio calculated by Formula 1.
[0131] As the non-epoxy based surface crosslinking agent, a polyol,
a carbonate compound or a mixture thereof may be used. More
specifically, the polyol may be at least one selected from the
group consisting of ethylene glycol, propylene glycol,
1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol,
2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol,
2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol. The
carbonate-based compound may be at least one selected from the
group consisting of ethylene carbonate and propylene carbonate.
Among them, a carbonate-based compound can be used to exhibit more
excellent gel strength at the time of swelling.
[0132] Such non-epoxy based surface crosslinking agent may be used
in an amount of about 0.1 to 3% by weight based on the total weight
of the first surface cross-linked powder. It is possible to provide
a super absorbent polymer exhibiting excellent physical properties
even after being swollen by adjusting the content range of the
non-epoxy based surface crosslinking agent within the above
range.
[0133] In the second surface crosslinking step, an inorganic
material, a polyvalent metal cation, water, methanol and the like
can be further used together with the non-epoxy based surface cross
linking agent, similarly to the first surface crosslinking step. As
described in the first surface crosslinking step, a non-epoxy based
surface crosslinking agent may be added to the first surface
cross-linked powder. Specific details related thereto have been
described above, and so they are omitted here.
[0134] The second surface crosslinking reaction may be performed by
heating the first surface cross-linked powder to which the
non-epoxy based surface crosslinking agent is added at a
temperature of about 170.degree. C. to 210.degree. C., about
180.degree. C. to 200.degree. C. or about 190.degree. C. for about
5 to 40 minutes, about 10 to 30 minutes or about 20 minutes. In
particular, in order to produce a super absorbent polymer that more
suitably fulfills physical properties according to one embodiment,
the conditions of the second surface crosslinking step can adjust
the maximum reaction temperature to about 170.degree. C. to
210.degree. C. The retention time at the maximum reaction
temperature can be adjusted under the conditions of about 5 to 40
minutes, or about 10 to 30 minutes or about 20 minutes. In
addition, from a temperature at the beginning of the first
reaction, for example, a temperature of about 80.degree. C. or
more, the temperature raising time until reaching the maximum
reaction temperature can be controlled to be about 10 minutes or
more, or about 10 minutes or more and 1 hour or less.
[0135] The temperature raising means for the first and second
surface crosslinking reactions is not particularly limited. The
heating can be carried out by providing a heating medium or
directly providing a heating source. The type of heat medium that
can be used here includes a heated fluid such as steam, hot air,
hot oil, etc., but it is not limited thereto. Further, the
temperature of the heating medium to be provided can be
appropriately selected in consideration of the means of the heating
medium, the temperature raising speed, and the temperature raising
target temperature. Meanwhile, a heat source to be provided
directly may include a heating method using electricity or a
heating method using gas, but is not limited thereto.
[0136] Meanwhile, according to another embodiment of the present
invention, there is provided a super absorbent polymer; comprising:
a base polymer powder including a cross-linked polymer in which a
water-soluble ethylenically unsaturated monomer having at least
partially neutralized acidic groups is cross-linked in the presence
of an internal crosslinking agent; and first and second surface
cross-linked layers that are further cross-linked from the
cross-linked polymer in the presence of a surface crosslinking
agent and are formed on the base polymer powder, wherein the first
and second surface cross-linked layers each include a cross-linked
structure derived from surface crosslinking agents that are
different from each other; and having an absorbency under load
(AUL) under 0.9 psi for a physiological saline solution is 19 to 25
g/g, and a GBP change ratio calculated by the following Formula 1
is 0.90 or less.
0.3 GBP - 0.3 AGBP 0.3 GBP [ Formula 1 ] ##EQU00005##
[0137] in Formula 1,
[0138] 0.3 GBP is a gel bed permeability (GBP) under 0.3 psi for a
0.9 wt % sodium chloride aqueous solution of a super absorbent
polymer,
[0139] 0.3 AGBP is a gel bed permeability (GBP) under 0.3 psi for a
physiological saline solution of a super absorbent polymer having a
particle size from 300 to 600 .mu.m after swelling and drying which
is obtained by a process of adding 5 g of the super absorbent
polymer to 125 g of distilled water, stirring the resultant for 1
minute, drying the swollen super absorbent polymer at 100.degree.
C. for 3 hours, and then classifying the dried polymer through a
U.S. standard 30 mesh screen and a U.S. standard 50 mesh
screen.
[0140] In the super absorbent polymer according to another
embodiment of the present invention, the first and second surface
cross-linked layers include a cross-linked structure derived from
surface crosslinking agents that are different from each other, and
the AUL satisfies 19 to 25 g/g, and thus the above-mentioned
effects can be exhibited, even if the first and second surface
cross-linked layers do not include the cross-linked structure
derived from epoxy-based and non-epoxy-based surface crosslinking
agents, as in the super absorbent polymer according to one
embodiment. Therefore, the super absorbent polymer according to
another embodiment can be produced as described above except for
the above limitations.
[0141] Hereinafter, the function and effect of the present
invention will be described in more detail by way of specific
examples. It is to be understood, however, that these examples are
provided for illustrative purposes only and the scope of the
invention is not limited in any way.
Example 1: Preparation of Super Absorbent Polymer
[0142] To a glass reactor were added 500 g of acrylic acid, 39 g of
polyethylene glycol diacrylate (PEGDA, weight average molecular
weight: 400), 2.1 g of allyl methacrylate and 20 g of IRGACURE 819.
Then, 809.5 g of a 24 wt % caustic soda solution was slowly added
dropwise to the glass reactor.
[0143] It was waited that, during dropwise addition of the caustic
soda solution, the temperature of the mixed solution was increased
to about 72.degree. C. or higher due to neutralization heat, and
then the mixed solution was cooled. The degree of neutralization of
acrylic acid in the mixed solution thus obtained was about 70 mol
%.
[0144] When the temperature of the mixed solution was cooled to
about 45.degree. C. 2.14 g of sodium bicarbonate and 0.27 g of
sodium dodecyl sulfate were added to the mixed solution, and light
was irradiated for 1 minute to perform light polymerization.
[0145] Then, the polymer obtained through the polymerization
reaction was passed through a hole having a diameter of 11 mm using
a meat chopper to produce a crump.
[0146] Then, the crumps were dried in an oven capable of shifting
airflow upward and downward. The crumps were uniformly dried by
flowing hot air at 180.degree. C. from the bottom to the top for 15
minutes and again from the top to the bottom for 15 minutes, so
that a water content of the dried crump became 2% or less.
[0147] The dried crump was pulverized using a pulverizing device
and classified to obtain a base polymer having a size of 150 to 850
.mu.m.
[0148] On the other hand, 0.04 g of ethylene glycol diglycidyl
ether (DENACOL EX 810), 3 g of water and 3 g of methanol were mixed
to prepare a first surface cross-linked solution, 0.2 g of ethylene
carbonate, 3 g of water and 3 g of methanol to prepare a second
surface crosslinking liquid.
[0149] Subsequently, the first surface crosslinking liquid was
sprayed onto 100 g of the prepared base polymer powder and mixed
with stirring at room temperature so that the first surface
crosslinking liquid was evenly distributed on the base polymer
powder. Then, the base polymer powder mixed with the first surface
crosslinking liquid was added to the surface cross-linking reactor
and the surface cross-linking reaction proceeded. In the surface
crosslinking reactor, it was confirmed that the base polymer powder
was gradually heated at an initial temperature near 80.degree. C.
Then, the surface crosslinking reactor was operated so as to reach
the maximum reaction temperature of 140.degree. C. after 30 minutes
from the initial temperature. After reaching the maximum reaction
temperature, additional reaction was carried out for 20 minutes to
obtain a first surface cross-linked powder.
[0150] Then, the second surface crosslinking liquid was sprayed
onto the first surface crosslinking powder and mixed with stirring
at room temperature so that the second surface crosslinking liquid
was distributed evenly on the first surface cross-linked powder.
Then, the first surface cross-linked powder mixed with the second
surface crosslinking liquid was added to the surface crosslinking
reactor, and the surface crosslinking reaction proceeded. In the
surface crosslinking reactor, it was confirmed that the first
surface cross-linked powder was gradually heated at an initial
temperature near 80.degree. C. Then, the surface crosslinking
reactor was operated so as to reach the maximum reaction
temperature of 190.degree. C. after 30 minutes from the initial
temperature. After reaching the maximum reaction temperature,
additional reaction was carried out for 20 minutes to obtain a
second surface cross-linked powder. Then, such super absorbent
polymer was pulverized and classified into a standard mesh
according to ASTM standard to obtain a super absorbent polymer
having a particle diameter of 150 to 850 .mu.m.
Example 2: Production of Super Absorbent Polymer
[0151] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that a solution prepared
by mixing 1 g of ethylene carbonate and 4.3 g of water was used as
the second surface crosslinking liquid in Example 1.
Example 3: Production of Super Absorbent Polymer
[0152] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that a solution prepared
by mixing 1 g of propylene carbonate and 4.3 g of water was used as
the second surface crosslinking liquid in Example 1.
Example 4: Production of Super Absorbent Polymer
[0153] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that a solution prepared
by mixing 1 g of ethylene carbonate, 1 g of propylene carbonate and
4.3 g of water was used as the second surface crosslinking liquid
in Example 1.
Example 5: Production of Super Absorbent Polymer
[0154] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that a solution prepared
by mixing 0.4 g of ethylene carbonate, 3 g of water and 3 g of
methanol was used as the second surface crosslinking liquid in
Example 1.
Comparative Example 1: Production of Super Absorbent Polymer
[0155] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that 0.04 g of ethylene
carbonate, 3 g of water and 3 g of methanol were used as the first
surface crosslinking liquid, and 0.2 g of glycerol, 3 g of water
and 3 g of methanol were used as a second surface crosslinking
liquid in Example 1.
Comparative Example 2: Production of Super Absorbent Polymer
[0156] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that a solution prepared
by mixing 0.04 g of ethylene glycol diacrylate, 3 g of water and 3
g of methanol was used as the first surface crosslinking liquid in
Example 1.
Comparative Example 3: Production of Super Absorbent Polymer
[0157] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that the surface of the
base polymer powder was cross-linked once under the condition of
the second surface crosslinking step, by using a solution prepared
by mixing 0.04 g of ethylene glycol diglycidyl ether (DENACOL EX
810), 0.5 g of ethylene glycol, 3 g of water and 3 g of methanol,
instead of the first and second surface crosslinking liquids in
Example 1.
Comparative Example 4: Production of Super Absorbent Polymer
[0158] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that the second surface
crosslinking was performed without spraying the second surface
crosslinking liquid onto the first surface cross-linked powder in
Example 1.
Comparative Example 5: Production of Super Absorbent Polymer
[0159] A surface cross-linked super absorbent polymer was produced
in the same manner as in Example 1, except that the first
crosslinking was performed by spraying the second surface
crosslinking liquid at the spraying time point of the first surface
crosslinking liquid, and then the second surface crosslinking was
performed without further spraying the second surface crosslinking
liquid at the spraying time point of the first surface
cross-linking liquid.
Test Example 1: Evaluation of Properties of Super Absorbent
Resin
[0160] Properties of the super absorbent polymer produced according
to Examples and Comparative Examples were evaluated by the
following method, and the results are shown in Table 1 below.
[0161] (1) Centrifuge Retention Capacity (CRC)
[0162] The centrifuge retention capacity (CRC) for a physiological
saline solution was measured for each super absorbent polymer
produced according to Examples and Comparative Examples according
to EDANA recommended test method No. WSP 241.2.
[0163] Specifically, a super absorbent polymer having a particle
diameter of 150 to 850 .mu.m which was passed through a U.S.
standard 20 mesh screen and retained on a U.S. standard 100 mesh
screen was prepared from a super absorbent polymer for measuring
the centrifuge retention capacity.
[0164] Then, the super absorbent polymer W.sub.0 (g, about 0.2 g)
having a particle diameter of 150 to 850 .mu.m was uniformly placed
into a nonwoven fabric-made bag, followed by sealing. Then, the bag
was immersed in 0.9% by weight of physiological saline solution at
room temperature. After 30 minutes, the bag was dehydrated at 250 G
for 3 minutes with a centrifuge, and the weight W.sub.2(g) of the
bag was then measured. Meanwhile, after carrying out the same
procedure using an empty bag not containing a super absorbent
polymer, the resultant weight W.sub.1(g) was measured.
[0165] Using the respective weights thus obtained, a centrifuge
retention capacity was confirmed according to the following
Calculation Formula 1:
CRC(g/g)={[W.sub.2(g)-W.sub.1(g)]/W.sub.0(g)}-1 [Calculation
Formula 1]
[0166] in Calculation Formula 1,
[0167] W.sub.0(g) is an initial weight(g) of the sample having a
particle diameter of 150 to 850 .mu.m,
[0168] W.sub.1(g) is a weight of a nonwoven fabric-made empty bag
not containing the sample, measured after immersing the nonwoven
fabric-made empty bag in a physiological saline solution for 30
minutes and then dehydrating the same by using a centrifuge at 250
G for 3 minutes, and
[0169] W.sub.2(g) is a weight of the nonwoven fabric-made bag
including a sample, measured after soaking and absorbing the
nonwoven fabric-made bag in a physiological saline solution at room
temperature for 30 minutes, and then dehydrating the same by using
a centrifuge at 250 G for 3 minutes.
[0170] (2) Absorbency Under Load (AUL)
[0171] The absorbency under load (AUL) at 0.9 psi for a
physiological saline solution of the super absorbent polymer was
measured according to EDANA recommended test method No. WSP
242.2.
[0172] Specifically, a 400 mesh stainless steel net was installed
in the bottom of a plastic cylinder having an inner diameter of 25
mm. W.sub.0 (g, 0.16 g) of a super absorbent polymer for measuring
the absorbency under load were uniformly scattered on the screen
under conditions of room temperature and relative humidity of 50%.
Then, a piston which could provide a load of 6.3 kPa (0.9 psi)
uniformly was put thereon. At this time, the piston used was
designed so that the outer diameter was slightly smaller than 25 mm
and thus it could move freely up and down without any gap with the
inner wall of the cylinder. Then, the weight W.sub.3(g) of the
device prepared in this way was measured.
[0173] After putting a glass filter having a diameter of 90 mm and
a thickness of 5 mm in a Petri dish having the diameter of 150 mm,
0.90% by weight of a sodium chloride aqueous solution
(physiological saline solution) was poured in the Petri dish. At
this time, the physiological saline solution was poured until the
surface level became equal to the upper surface of the glass
filter. Then, a sheet of filter paper having a diameter of 90 mm
was put on the glass filter.
[0174] Subsequently, the prepared device was placed on the filter
paper so that the super absorbent polymer in the device was swelled
by a physiological saline solution under load. After one hour, the
weight W.sub.4(g) of the device containing the swollen super
absorbent polymer was measured.
[0175] Using the weight thus measured, the absorbency under load
was calculated according to the following Calculation Equation
2.
AUL(g/g)=[W.sub.4(g)-W.sub.3(g)]/W.sub.0(g) [Calculation Equation
2]
[0176] in Calculation Equation 2,
[0177] W.sub.0(g) is an initial weight(g) of the super absorbent
polymer, W.sub.3(g) is the total sum of a weight of the super
absorbent polymer and a weight of the device capable of providing a
load to the super absorbent polymer, and W.sub.4(g) is the total
sum of a weight of the super absorbent polymer and a weight of the
device capable of providing a load to the super absorbent polymer,
after absorbing a physiological saline solution to the super
absorbent polymer under a load (0.9 psi) for 1 hour.
[0178] (3) Gel Bed Permeability (GBP)
[0179] The gel bed permeability (GBP) for a physiological saline
solution of the super absorbent polymer was measured according to
the following method described in Korean Patent Application No.
10-2014-7018005.
[0180] Specifically, the apparatus shown in FIGS. 1 to 3 was used
to measure the free swell GBP. First, the plunger 536 installed
with the weight 548 was placed in an empty sample container 530,
and the height from the top of the weight 548 to the bottom of the
sample container 530 was measured to an accuracy of 0.01 mm using
an appropriate gauge. The force to which the thickness gauge
applied during the measurement was adjusted to less than about 0.74
N.
[0181] Meanwhile, a super absorbent polymer having a particle
diameter of 300 to 600 .mu.m was obtained by selectively
classifying a super absorbent polymer which was passed through a US
standard 30 mesh screen and retained on a US standard 50 mesh
screen.
[0182] About 2.0 g of the super absorbent polymer classified in
this way was placed in the sample container 530 and spread out
evenly on the bottom of the sample container. Then, the container
not containing the plunger 536 and the weight 548 therein, was
submerged in a 0.9 wt % sodium chloride aqueous solution
(physiological saline solution) for about 60 minutes and allowed
the super absorbent polymer to swell under no load condition. At
this time, the sample container 530 was placed on the mesh located
in a liquid reservoir so that the sample container 530 was raised
slightly above the bottom of the liquid reservoir. As the mesh,
those which did not affect the movement of the physiological saline
solution into the sample container 530 were used. During
saturation, the height of the physiological saline solution could
be adjusted such that the surface within the sample container was
defined by the swollen super absorbent polymer, rather than the
physiological saline solution.
[0183] At the end of this period, the assembly of the plunger 536
and weight 548 was placed on the swollen super absorbent polymer
568 in the sample container 530 and then the sample container 530,
plunger 536, weight 548 and swollen super absorbent polymer 568
were removed from the solution. Thereafter, before GBP measurement,
the sample container 530, plunger 536, weight 548 and swollen super
absorbent polymer 568 were placed on a flat, large grid
non-deformable plate of uniform thickness for about 30 seconds. The
height from the top of the weight 548 to the bottom of the sample
container 530 was measured again by using the same thickness gauge
as previously used. Then, the height measurement value of the
device in which the plunger 536 equipped with the weight 548 was
placed in the empty sample container 530 was subtracted from the
height measurement value of the device including the swollen super
absorbent polymer 568, thereby obtaining the thickness or height
"H" of the swollen super absorbent polymer.
[0184] For the GBP measurement, 0.9 wt % physiological saline
solution was flowed into the sample container 530 containing the
swollen super absorbent polymer 568, the plunger 536 and the weight
548. The flow rate of a physiological saline solution into the
container was adjusted to cause the physiological saline solution
to overflow the top of the cylinder 534, thereby resulting in a
consistent head pressure equal to the height of the sample
container 530. Then, the quantity of solution passing through the
swollen super absorbent polymer 568 versus time was measured
gravimetrically using the scale 602 and beaker 603. Data points
from the scale 602 were collected every second for at least sixty
seconds once the overflow has started. The flow rate (Q) passing
through the swollen super absorbent polymer 568 was determined in
units of grams/second (g/s) by a linear least-square fit of fluid
passing through the sample 568 (in grams) versus time (in
seconds).
[0185] Using the data thus obtained, the GBP (cd) was calculated
according to the following Calculation Equation 3.
K=[Q.times.H.times..mu.]/[A.times..rho..times.P] [Calculation
Equation 3]
[0186] in Calculation Equation 3,
[0187] K is a gel bed permeability (cm.sup.2),
[0188] Q is a flow rate (g/sec)
[0189] H is a height of swollen super absorbent polymer (cm),
[0190] .mu. is a liquid viscosity (poise) (about 1 cP for the test
solution used with this Test),
[0191] A is a cross-sectional area for liquid flow (28.27 cm.sup.2
for the sample container used with this Test),
[0192] .rho. is a liquid density (g/cm.sup.3)(about 1 g/cm.sup.3,
for the physiological saline solution used with this Test), and
[0193] P is a hydrostatic pressure (dynes/cm.sup.2) (normally about
7,797 dyne/cm.sup.2).
[0194] The hydrostatic pressure was calculated from the equation
P=.rho.*g*h, where .rho. is a liquid density (g/cm.sup.3), g is a
gravitational acceleration (nominally 981 cm/sec.sup.2), and h is a
fluid height (for example, 7.95 cm for the GBP Test described
herein)
[0195] At least two samples were tested and the results were
averaged to determine the free swell GBP of the super absorbent
polymer, and the unit was converted to darcy (1
darcy=0.98692.times.10.sup.-8 cm.sup.2).
[0196] (4) Gel Bed Permeability at 0.3 Psi (0.3 GBP)
[0197] The initial gel bed permeability (GBP) under 0.3 psi (0.3
GBP) of the super absorbent polymers produced according to Examples
and Comparative Examples, and the gel bed permeability (GBP) under
0.3 psi (0.3 AGBP) measured after swelling the super absorbent
polymer with distilled water and then drying the swollen polymer
were measured, and the GBP change ratio was calculated according to
the following Formula 1. The results are shown in Table 1
below.
0.3 GBP - 0.3 AGBP 0.3 GBP [ Formula 1 ] ##EQU00006##
[0198] Specifically, 5 g of the super absorbent polymer was added
to 125 g of distilled water and stirred for 1 minute to swell the
super absorbent resin. Then, the swollen super absorbent polymer
was taken out and spread out evenly on a mesh screen to prevent the
polymer from sticking, and then dried in an oven at 100.degree. C.
for 3 hours.
[0199] In order to measure the gel bed permeability at 0.3 psi of
the super absorbent polymer after swelling and drying, a super
absorbent polymer passed through a US standard 30 mesh screen and
retained on a US standard 50 mesh screen was selectively classified
to obtained a super absorbent polymer having a particle diameter of
300 to 600 .mu.m.
[0200] Then, the gel bed permeability (GBP) under 0.3 psi (0.3 GBP)
for a physiological saline solution of the super absorbent polymers
produced according to Examples and Comparative Examples, and the
super absorbent polymers swollen and dried according to the above
description was measured.
[0201] Specifically, the gel bed permeability (GBP) under 0.3 psi
was measured in the same manner as in (3) method of measuring the
free swell gel bed permeability, except for that, in the
above-mentioned (3) method of measuring the free swell gel bed
permeability, about 2.0 g of the classified super absorbent polymer
was placed in the sample container 530 and spread out evenly on the
bottom of the sample container, the assembly of the plunger 536 and
weight 548 was placed on the swollen super absorbent polymer in the
sample container and then the sample container is submerged in the
physiological saline solution for about 60 minutes and allow the
super absorbent polymer to swell under load of 0.3 psi.
TABLE-US-00001 TABLE 1 Free 0.3 0.3 GBP CRC AUL swell GBP GBP AGBP
change [g/g] [g/g] [darcy] [darcy] [darcy] ratio Example 1 31.1
20.5 62 2.5 0.3 0.88 Example 2 31.4 20.0 66 1.9 0.4 0.79 Example 3
30.9 20.6 65 3.5 0.7 0.80 Example 4 30.0 21.0 55 5.0 0.9 0.82
Example 5 31.8 19.3 56 4.5 0.8 0.82 Comparative 30.6 19.5 45 3.5
0.0 1.00 Example 1 Comparative 29.0 20.7 47 4.2 0.2 0.95 Example 2
Comparative 30.0 20.4 35 5.0 0.2 0.96 Example 3 Comparative 30.9
20.5 48 3.0 0.1 0.97 Example 4 Comparative 31.5 19.3 40 0.5 0.0
1.00 Example 5
[0202] Referring to Table 1, it was confirmed that the super
absorbent polymers of Examples 1 to 5, in which the first surface
crosslinking was performed with the epoxy-based surface
crosslinking agent and then the second surface crosslinking was
performed with the non-epoxy based surface crosslinking agent
according to an embodiment of the present invention exhibited
well-balanced properties of a centrifuge retention capacity, an
absorbency under load and a liquid permeability.
[0203] Meanwhile, it was confirmed that the super absorbent
polymers of Comparative Examples 1 and 2 in which an epoxy-based
surface crosslinking agent was not used in the first surface
crosslinking step, the super absorbent polymer of Comparative
Examples 3 in which the surface crosslinking step was performed
only once by using two types of surface crosslinking agents
simultaneously, and the super absorbent polymers of Comparative
Examples 4 and 5 in which the surface crosslinking step was
performed twice by using one type of surface crosslinking agent,
exhibited a high GBP change ratio and thus did not exhibit a
balanced absorption performance.
Test Example 2: Evaluation of Properties of Diaper
[0204] In order to confirm that the rewetting properties is better
as the GBP change ratio GBP is smaller, a diaper sample was
prepared using the super absorbent polymers produced according to
Examples 1 to 5 in which the GBP change ratio was low and
Comparative Examples 4 and 5 in which the GBP change ratio was
high, and their re-wetting properties were evaluated and the
results are shown in Table 2 below.
[0205] (1) Production of Diaper Sample
[0206] The initial gel bed permeability (GBP) under 0.3 psi (0.3
GBP) of the super absorbent polymers produced according to Examples
and Comparative
[0207] The super absorbent polymers were classified so that
particles having 600 to 850 .mu.m (classified using US standard 20
and 30 mesh screen), about 300 to 600 .mu.m (classified using US
standard 30 and 50 mesh screen) and about 90 to 300 .mu.m
(classified using US standard 50 and 170 mesh screen) had a weight
ratio of 10:70:20.
[0208] Using the super absorbent polymers thus classified, the core
of the diaper was composed of 70% by weight of super absorbent
polymer and 30% by weight of fluff, and an acquisition distribution
layer (ADL) and an ADL (acquisition distribution layer) and a top
cover were laminated was laminated on the top of the core.
[0209] (2) Rewetting Properties of Diaper
[0210] The rewetting properties of the diaper were evaluated
according to the method developed by Kimberly-Clark, capable of
confirming the rewetting properties under no load or under
load.
[0211] Specifically, in order to evaluate the rewetting properties
under load, 85 mL of 0.9 wt % sodium chloride aqueous solution
(physiological saline solution) was injected into the diaper. After
15 minutes, the weight was placed on the diaper and 85 mL of
physiological saline solution was injected again while applying a
load of 0.42 psi. After 15 minutes, the weight placed on the diaper
was temporarily removed, and then the paper was placed on the
diaper, the weight was again placed on the paper, and the paper was
interposed between the diaper and the weight. After 2 minutes, the
amount of the salt water leaking out by paper from the diaper was
measured, and the rewetting amount (g/g) was calculated by the
Calculation Equation 4.
Rewetting Amount (g)=W.sub.6(g)-W.sub.5(g) [Calculation Equation
4]
[0212] in Calculation Equation 4,
[0213] W.sub.5(g) is an initial weight of the paper, W.sub.6(g) is
a weight of the paper that has absorbed a liquid leaking out from
the diaper under a load (0.42 psi) for 2 minutes after a
physiological saline solution was injected into the diaper under no
load and under load.
TABLE-US-00002 TABLE 2 Example Example Example Example Example
Comparative Comparative 1 2 3 4 5 Example 4 Example 5 GBP 0.88 0.79
0.80 0.82 0.82 0.97 1.00 change ratio Rewetting 4.34 3.18 4.80 4.64
5.00 5.88 5.59 amount [g]
[0214] Referring to Table 2, it was confirmed that when the super
absorbent polymers of Examples having a low GBP change ratio was
used, more excellent rewetting properties were obtained compared
with the case of using the super absorbent polymers of Comparative
Examples having a high GBP change ratio.
EXPLANATION OF SIGN
[0215] 500: GBP measuring device [0216] 528: Test apparatus
assembly [0217] 530: Sample container [0218] 534: Cylinder [0219]
534a: Region having an outer diameter of 66 mm [0220] 536: Plunger
[0221] 538: Shift [0222] 540: O-ring [0223] 544, 554, 560: hole
[0224] 548: Annular weight [0225] 548a: Thru-bore [0226] 550:
Plunger head [0227] 562: Shaft hole [0228] 564: 100 mesh stainless
steel cross screen [0229] 566: 400 mesh stainless steel cross
screen [0230] 568: Sample [0231] 600: Weir [0232] 601: Collection
device [0233] 602: Scale [0234] 603: Beaker [0235] 604: Gauge
pump
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