U.S. patent application number 12/501734 was filed with the patent office on 2009-10-29 for particulate water-absorbent polymer and production method thereof.
This patent application is currently assigned to NIPPON SHOKUBAI CO., LTD.. Invention is credited to Hiroyuki Ikeuchi, Makoto Nagasawa.
Application Number | 20090270538 12/501734 |
Document ID | / |
Family ID | 39644535 |
Filed Date | 2009-10-29 |
United States Patent
Application |
20090270538 |
Kind Code |
A1 |
Ikeuchi; Hiroyuki ; et
al. |
October 29, 2009 |
PARTICULATE WATER-ABSORBENT POLYMER AND PRODUCTION METHOD
THEREOF
Abstract
An object of the present invention is to provide a method for
producing a particulate water-absorbent polymer particle having
remarkably improved long-term color stability, without an adverse
effect (such as a delay in polymerization) on synthesis of the
particulate water-absorbent polymer. The method includes the steps
of polymerizing a monomer aqueous solution (B) containing (i) at
least one type of monomer (A) that is capable of forming a
particulate water-absorbent polymer by polymerization, (ii) at
least one type of crosslinking agent, (iii) at least one type of
polymerization initiator and (iv) an organophosphorus compound in
an amount of not less than 1 but not more than 100 ppm by mass with
respect to the monomer (A) so as to form a hydrogel polymer, and
drying the hydrogel polymer. The method enables to provide an
excellent particulate water-absorbent polymer that realizes (i)
long-term color stability and improvement in urine tolerance and
(ii) absorbing property, which are in such a trade-off
relationship. In this way, the above object is attained.
Inventors: |
Ikeuchi; Hiroyuki; (Hyogo,
JP) ; Nagasawa; Makoto; (Nara, JP) |
Correspondence
Address: |
TUROCY & WATSON, LLP
127 Public Square, 57th Floor, Key Tower
CLEVELAND
OH
44114
US
|
Assignee: |
NIPPON SHOKUBAI CO., LTD.
Osaka
JP
|
Family ID: |
39644535 |
Appl. No.: |
12/501734 |
Filed: |
July 13, 2009 |
Current U.S.
Class: |
524/115 |
Current CPC
Class: |
C08J 3/12 20130101; C08F
2/44 20130101; C08F 220/06 20130101; C08J 2300/14 20130101; C08F
222/1006 20130101; C08J 3/075 20130101 |
Class at
Publication: |
524/115 |
International
Class: |
C08K 5/49 20060101
C08K005/49 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2007 |
JP |
2007-014288 |
Claims
1. A method for producing a particulate water-absorbent polymer,
the method comprising: polymerizing a monomer aqueous solution (B)
containing (i) at least one type of monomer (A) that is capable of
forming a water-absorbent polymer by polymerization, (ii) at least
one type of crosslinking agent, (iii) at least one type of
polymerization initiator and (iv) an organic phosphorous compound
in an amount of not less than 1 ppm by mass but not more than 100
ppm by mass with respect to the monomer (A) so as to obtain a
hydrogel polymer; and thermally drying the hydrogel polymer.
2. A method for producing a particulate water-absorbent polymer,
the method comprising: polymerizing a monomer aqueous solution (B)
containing (i) at least one type of monomer (A) that is capable of
forming a water-absorbent polymer by polymerization, (ii) at least
one type of crosslinking agent and (iii) at least one type of
polymerization initiator so as to obtain a hydrogel polymer; and
thermally drying the hydrogel polymer, wherein the hydrogel polymer
before the drying contains an organic phosphorous compound, and a
drying temperature during the step of thermally drying is not less
than 150.degree. C. but not more than 250.degree. C.
3. The method according to claim 1, wherein the monomer solution
(B) contains the organic phosphorous compound in amount of not less
than 30 ppm by mass but not more than 100 ppm by mass with respect
to the monomer (A).
4. The method according to claim 1, wherein the at least one type
of monomer (A) contains not less than 70 mol % but not more than
100 mol % of acrylic acid and salt thereof, and the acrylic acid
and salt thereof contain not less than 1 mol % but not more than 50
mol % of acrylic acid and not less than 50 mol % but not more than
99 mol % of an alkali metal salt of acrylic acid.
5. The method according to claim 1, wherein the monomer aqueous
solution (B) contains iron in an amount of not less than 0.001 ppm
by mass but not more than 5 ppm by mass (based on Fe2O3) with
respect to the monomer (A).
6. The method according to claim 1, further comprising
surface-crosslinking the particulate water-absorbent polymer.
7. The method according to claim 6, wherein the step of
surface-crosslinking includes heating the particulate
water-absorbent polymer at a heating temperature of not less than
150.degree. C. but not more than 250.degree. C.
8. The method according to claim 1, further comprising adding the
organic phosphorous compound to the hydrogel polymer or to the
particulate water-absorbent polymer, after the polymerization.
9. The method according to claim 1, wherein moisture content of the
particulate water-absorbent polymer is not more than 5 mass %.
10. The method according to claim 1, wherein the hydrogel polymer
is thermally dried at a heating temperature of not less than
170.degree. C. but not more than 220.degree. C.
11. A particulate water-absorbent polymer obtainable through a
method as set forth in claim 1, the particulate water-absorbent
polymer exhibiting an L value (Lightness) of 70 or higher in
Hunter's Lab color system after 7-day exposure to atmosphere of
70.+-.1.degree. C. of temperature and 65.+-.1% of relative
humidity.
12. A particulate water-absorbent polymer containing an organic
phosphorous compound in an amount of not less than 30 ppm by mass
but not more than 500 ppm by mass with respect to the particulate
water-absorbent polymer, the particulate water-absorbent polymer
satisfying at least one of the followings (a) to (c): (a) a
particle of the particulate water-absorbent polymer after 7-day
exposure to atmosphere of 70.+-.1.degree. C. of temperature and
65.+-.1% of relative humidity exhibits an L value (Lightness) of 70
or higher in Hunter's Lab color system; (b) the particulate
water-absorbent polymer contains particles having a diameter of
less than 150 .mu.m in an amount of 0 or more mass % but not more
than 5 mass %; a mass median particle size (D50) is not less than
200 .mu.m but not more than 600 .mu.m; and a logarithmic standard
deviation (.sigma..zeta.) of a particle size distribution is not
less than 0.20 but not more than 0.40; and (c) an absorbency
against pressure (AAP) under pressure of 1.9 kPa or 4.8 kPa for
0.90 mass % sodium chloride aqueous solution for 60 minutes is at
least 20 (g/g).
13. The particulate water-absorbent polymer according to claim 12,
further comprising iron in an amount of not less than 0.001 ppm by
mass but not more than 5 ppm by mass (based on Fe2O3) with respect
to the particulate water-absorbent polymer.
14. The method according to claim 2, wherein the monomer solution
(B) contains the organic phosphorous compound in amount of not less
than 30 ppm by mass but not more than 100 ppm by mass with respect
to the monomer (A).
15. The method according to claim 2, wherein the at least one type
of monomer (A) contains not less than 70 mol % but not more than
100 mol % of acrylic acid and salt thereof, and the acrylic acid
and salt thereof contain not less than 1 mol % but not more than 50
mol % of acrylic acid and not less than 50 mol % but not more than
99 mol % of an alkali metal salt of acrylic acid.
16. The method according to claim 2, wherein the monomer aqueous
solution (B) contains iron in an amount of not less than 0.001 ppm
by mass but not more than 5 ppm by mass (based on Fe.sub.2O.sub.3)
with respect to the monomer (A).
17. The method according to claim 2, further comprising
surface-crosslinking the particulate water-absorbent polymer.
18. The method according to claim 17, wherein the step of
surface-crosslinking includes heating the particulate
water-absorbent polymer at a heating temperature of not less than
150.degree. C. but not more than 250.degree. C.
19. The method according to claim 2, further comprising adding the
organic phosphorous compound to the hydrogel polymer or to the
particulate water-absorbent polymer, after the polymerization.
20. The method according to claim 2, wherein moisture content of
the particulate water-absorbent polymer is not more than 5 mass
%.
21. The method according to claim 2, wherein the hydrogel polymer
is thermally dried at a heating temperature of not less than
170.degree. C. but not more than 220.degree. C.
22. A particulate water-absorbent polymer obtainable through a
method as set forth in claim 2, the particulate water-absorbent
polymer exhibiting an L value (Lightness) of 70 or higher in
Hunter's Lab color system after 7-day exposure to atmosphere of
70.+-.1.degree. C. of temperature and 65.+-.1% of relative
humidity.
Description
TECHNICAL FIELD
[0001] The present invention relates to a particulate
water-absorbent polymer having an excellent urine tolerance and
long-term color stability, and a method for producing the
particulate water-absorbent polymer.
[0002] More specifically, the present invention provides a method
for producing a particulate water-absorbent polymer having a
remarkably improved urine tolerance and remarkably improved
long-term color stability without an adverse effect (such as a
delay in polymerization) on synthesis of the water-absorbent
polymer, by allowing a slight amount of organic phosphorous
compound to coexist within a monomer aqueous solution.
[0003] The particulate water-absorbent polymer obtainable in the
present invention, the particulate water-absorbent polymer having
the improved urine tolerance and the long-term color stability, is
suitable for use in a sanitary material such as a disposable
diaper. The particulate water-absorbent polymer keeps its
clean-imaged white condition even over long-term storage under a
condition of high humidity and high temperature, when being used in
the sanitary material such as the disposable diaper.
BACKGROUND ART
[0004] Recently, a water-absorbent polymer having an excellent
absorption property has been developed and applied for various
purposes. For example, the water-absorbent polymer has been used in
a sanitary material such as a disposable diaper and a sanitary
napkin; a pet sheet; a water stop material; or the like.
[0005] As a raw material of such a water-absorbent polymer, a
variety of monomers and hydrophilic polymers are proposed. Among
those, a polyacrylic (polyacrylate) water-absorbent polymer
obtained by using acrylic acid and/or its salt as a monomer has
been mostly produced and used in view of cost performance. For
example, in a case of the sanitary material such as the disposable
diaper and the sanitary napkin, a powdered water-absorbent polymer
is mixed with white-colored pulp for use. In this case, in view of
consumer acceptability, the water-absorbent polymer is required to
be white for making a clean impression.
[0006] Generally, the water-absorbent polymer is in a form of white
powder when it is shipped. However, the water-absorbent polymer is
known to be colored (from white to yellow-brown) before it reaches
consumers in a form of the disposable diaper or the like, for
example if it is exposed under a condition of high humidity and
high temperature for long periods of time. Therefore, it is desired
to develop a water-absorbent polymer that excels in long-term color
stability even over long-term exposure under the condition of high
humidity and high temperature.
[0007] In order to prevent such a coloration, there have been known
such methods as: a method of polymerizing an acrylic acid monomer
and/or its salt with hydroxyperoxide and a reductant and then
treating the resultant polymer with a silane coupling agent (Patent
Literature 1); a method of adding an organophosphorus acid compound
or its salt to a water-absorbent polymer after polymerization
(Patent Literature 2); a method of controlling a total amount of
hydroquinone and benzoquinone contained in acrylic acid to 0.2 ppm
or less (Patent Literature 3); a method of adding an inorganic
reductant to a water-absorbent polymer (Patent Literatures 4 and
5); a method of adding organic carboxylic acid or its salt to a
water-absorbent polymer, and further adding an inorganic reducing
agent or the like to the water-absorbent polymer (Patent
Literatures 6 to 8); a production method in which polymerization is
carried out with use of tocopherol as a polymerization inhibitor
contained in acrylic acid (Patent Literature 9); and a production
method in which a metal chelator is added in producing a
water-absorbent polymer (Patent Literatures 10 and 11).
[0008] Among the metal chelators, a phosphorous chelator is used
not only for preventing the coloration, but also is proposed to be
used in other production methods, such as a production method of a
water-absorbent polymer in which the phosphorous chelator is used
as a polymerization stabilizer of a reversed phase suspension
polymerization (Patent Literature 12) and a production method in
which the phosphorous chelator is used as a gel stabilizer (Patent
Literature 13).
[0009] However, none of the above-described methods could make a
sufficient improvement in the coloration, and they rather have had
such problems as a property deterioration, a cost increase, and
depending on a compound to be used, a safety problem. Thus, (i) an
absorbing property and a urine tolerance and (ii) the long-term
color stability have conventionally been in such a trade-off
relationship.
[0010] Further, in a case where the water-absorbent polymer is
dried at a high temperature or surface-crosslinked at a high
temperature (for example, 150.degree. C. or more), the
water-absorbent polymer is often colored or thermally deteriorated
(for example, extractable content increases). Lowering a drying
temperature or a temperature at which the surface crosslinkage is
performed so as to avoid the above problems will be associated with
a decrease in productivity.
CITATION LIST
[0011] Patent Literature 1
[0012] Japanese Patent Application Publication, Tokukaihei, No.
4-331205 A
[0013] Patent Literature 2
[0014] Japanese Patent Application Publication, Tokukaihei, No.
5-86251 A
[0015] Patent Literature 3
[0016] U.S. Pat. No. 6,444,744
[0017] Patent Literature 4
[0018] Pamphlet of International Publication WO2000/55245
[0019] Patent Literature 5
[0020] United States Patent Application Publication No.
2006-0074160
[0021] Patent Literature 6
[0022] Japanese Patent Application Publication, Tokukai,
2000-327926 A
[0023] Patent Literature 7
[0024] Japanese Patent Application Publication, Tokukai, 2003-52742
A
[0025] Patent Literature 8
[0026] Japanese Patent Application Publication, Tokukai,
2005-186016 A
[0027] Patent Literature 9
[0028] Pamphlet of International Publication WO2003/53482
[0029] Patent Literature 10
[0030] United States Patent Application Publication No.
2005-0085604
[0031] Patent Literature 11
[0032] Japanese Patent Application Publication, Tokukai, No.
2003-206381 A
[0033] Patent Literature 12
[0034] Japanese Patent Application Publication, Tokukaihei, No.
2-117903 A
[0035] Patent Literature 13
[0036] Japanese Patent Application Publication, Tokukaihei,
1-275661 A
SUMMARY OF INVENTION
[0037] An object of the present invention is to provide (i) a safe
water-absorbent polymer being produced at low cost and having
excellent long-term color stability and (ii) a method of producing
water-absorbent polymer particles having the long-term color
stability.
[0038] Another object of the present invention is to provide a
method for efficiently producing a water-absorbent polymer having
excellent physical properties while preventing the water-absorbent
polymer from deteriorating or being colored by drying at a high
temperature or surface-crosslinking at a high temperature.
[0039] The inventors of the present invention diligently worked in
order to attain the object. As a result, the inventors of the
present invention found that a water-absorbent particle having the
improved urine tolerance and remarkably improved long-term color
stability is obtained at low cost without deterioration during
polymerization, by allowing in advance an organic phosphorous
compound to coexist in a monomer aqueous solution that is capable
of forming the water-absorbent polymer by polymerization,
polymerizing the monomer aqueous solution so as to obtain a
hydrogel polymer, and then drying the obtained hydrogel polymer.
Based on the finding, the inventors of the present invention have
accomplished the present invention.
[0040] The present invention particularly relates to a
water-absorbent polymer particle that excels in prevention of gel
deterioration, improvement in the urine tolerance, and the
long-term color stability. Further, the present invention provides
a method of using the organic phosphorous compound in a polymer for
preventing the water-absorbent polymer from deteriorating by heat
during a production process.
[0041] Specifically, the object of the present invention is
attained by means as set forth below.
[0042] (1) A method for producing a particulate water-absorbent
polymer, the method including: polymerizing a monomer aqueous
solution (B) containing (i) at least one type of monomer (A) that
is capable of forming a water-absorbent polymer by polymerization,
(ii) at least one type of crosslinking agent, (iii) at least one
type of polymerization initiator and (iv) an organic phosphorous
compound in an amount of not less than 1 ppm by mass but not more
than 100 ppm by mass with respect to the monomer (A) so as to
obtain a hydrogel polymer; and drying the hydrogel polymer.
[0043] (2) A method for producing a particulate water-absorbent
polymer, the method including: polymerizing a monomer aqueous
solution (B) containing (i) at least one type of monomer (A) that
is capable of forming a water-absorbent polymer by polymerization,
(ii) at least one type of crosslinking agent and (iii) at least one
type of polymerization initiator so as to obtain a hydrogel
polymer; and thermally drying the hydrogel polymer, wherein the
hydrogel polymer before the drying contains an organic phosphorous
compound, and a drying temperature during the step of thermally
drying is not less than 150.degree. C. but not more than
250.degree. C.
[0044] Since the methods (1) and (2) of producing the particulate
water-absorbent polymer employ the organic phosphorous compound in
the production process, it is possible to achieve extremely
favorable effects such that the obtained water-absorbent polymer
excels both in (i) the long-term color stability and the urine
tolerance and (ii) an absorbing property, and that it is possible
to prevent the water-absorbent polymer from deteriorating due to
breakage of main-chain caused by the thermally drying.
[0045] (3) The method according to (1) or (2), wherein the monomer
aqueous solution (B) contains the organic phosphorous compound in
an amount of not less than 30 ppm by mass but not more than 100 ppm
by mass with respect to the monomer (A).
[0046] (4) The method according to any one of (1) to (3), wherein
the at least one type of monomer (A) that is capable of forming a
particulate water-absorbent polymer by polymerization contains not
less than 70 mol % but not more than 100 mol % of acrylic acid and
salt thereof, and the acrylic acid and salt thereof contain not
less than 1 mol % but not more than 50 mol % of acrylic acid and
not less than 50 mol % but not more than 99 mol % of an alkali
metal salt of acrylic acid.
[0047] (5) The method according to any one of (1) to (4), wherein
the monomer aqueous solution (B) contains iron in an amount of not
less than 0.001 ppm by mass but not more than 5 ppm by mass (based
on Fe.sub.2O.sub.3) with respect to the monomer (A).
[0048] (6) The method according to any one of (1) to (5), further
including surface-crosslinking the particulate water-absorbent
polymer.
[0049] (7) The method according to any one of (1) to (6), wherein
the step of surface-crosslinking includes heating the particulate
water-absorbent polymer at a heating temperature of not less than
150.degree. C. but not more than 250.degree. C.
[0050] (8) The method according to any one of (1) to (7), further
including adding the organic phosphorous compound to the hydrogel
polymer after the polymerization or to the water-absorbent polymer
after the drying.
[0051] (9) The method according to any one of (1) to (8), wherein
moisture content of the particulate water-absorbent polymer is not
more than 5 mass %.
[0052] (10) The method according to any one of (1) to (9), wherein
the hydrogel polymer is thermally dried at a heating temperature of
not less than 170.degree. C. but not more than 220.degree. C.
[0053] (11) The particulate water-absorbent polymer according to
any one of (1) to (10), wherein the particulate water-absorbent
polymer after 7-day exposure to atmosphere of 70.+-.1.degree. C. of
temperature and 65.+-.1% of relative humidity exhibits an L value
(Lightness) of 70 or higher in Hunter's Lab color system.
[0054] (12) A particulate water-absorbent polymer containing an
organic phosphorous compound in an amount of not less than 30 ppm
by mass but not more than 500 ppm by mass with respect to the
particulate water-absorbent polymer, the particulate
water-absorbent polymer satisfying at least one of the followings
(a) to (c):
[0055] (a) a particle of the particulate water-absorbent polymer
after 7-day exposure to atmosphere of 70.+-.1.degree. C. of
temperature and 65.+-.1% of relative humidity exhibits an L value
(Lightness) of 70 or higher in Hunter's Lab color system;
[0056] (b) the particulate water-absorbent polymer contains
particles having a diameter of less than 150 .mu.m in an amount of
0 or more mass % but not more than 5 mass %; a mass median particle
size (D50) is not less than 200 .mu.m but not more than 600 .mu.m;
and a logarithmic standard deviation (.sigma..zeta.) of a particle
size distribution is not less than 0.20 but not more than 0.40;
and
[0057] (c) an absorbency against pressure (AAP) under pressure of
1.9 kPa or 4.8 kPa for 0.90 mass % sodium chloride aqueous solution
for 60 minutes is at least 20 (g/g).
[0058] (13) The particulate water-absorbent polymer according to
(12), further containing iron in amount of not less than 0.001 ppm
by mass but not more than 5 ppm by mass (based on Fe.sub.2O.sub.3)
with respect to the particulate water-absorbent polymer.
[0059] Since the method of the present invention employs a special
way of adding a characteristic additive, it is possible to minimize
a using amount of an additive (organic phosphorous compound) having
no positive effect on an absorbing ability. Accordingly, it is
possible to provide a method for obtaining a safe water-absorbent
polymer having the improved urine tolerance and the remarkably
improved long-term color stability at low cost without an adverse
effect and a loss of water absorbing characteristics during
polymerization.
[0060] Further, the method of the present invention provides a
particulate water-absorbent polymer that excels both in (i) the
long-term color stability and the urine tolerance and (ii) the
absorption property, which are in such a trade-off relationship. In
addition, the method of the present invention makes it possible to
prevent deterioration of a polymer and increase in extractable
content, which are caused by breakage of polymer main-chain due to
heat history during processes (e.g., drying process and
surface-crosslinking process) of producing the water-absorbent
polymer. Thus provided is a method for producing a water-absorbent
polymer having excellent physical properties in a productive way,
without deterioration and coloration due to drying at a high
temperature or surface-crosslinking at a high temperature.
[0061] Furthermore, the water-absorbent polymer with long-term
color stability that is obtained in the present invention can be
suitably used in a sanitary material such as a disposable diaper,
and keeps its clean-imaged white condition even over long-term
storage under a condition of high temperature and high humidity.
For example, in a case where a particle of the water-absorbent
polymer is exposed to an atmosphere of 70.+-.1.degree. C. of
temperature and 65.+-.1% of relative humidity for seven days, the
particle keeps its clean-imaged white condition (an L value
(Lightness) in Hunter's Lab color system is 70 or higher).
BRIEF DESCRIPTION OF DRAWINGS
[0062] FIG. 1
[0063] FIG. 1 is a cross-sectional view schematically illustrating
a device for measuring an absorbency against pressure (AAP).
EXPLANATION OF REFERENTIAL NUMERALS
[0064] 102 Particulate Water Absorbent
DESCRIPTION OF EMBODIMENTS
[0065] A particulate water-absorbent polymer according to the
present invention and a method for producing the water-absorbent
polymer are described as follows. Note that the particulate
water-absorbent polymer may be referred to as a water-absorbent
polymer particle, or may be referred to merely as a water-absorbent
polymer. Also, unless otherwise stated, a range of not less than A
but not more than B may be described as A to B. Further, parts by
mass is equivalent to weight by mass, mass % is equivalent to wt %,
and ppm by mass is equivalent to ppm by weight.
[0066] (1) Monomer (A)
[0067] A monomer (A) that is capable of forming a water-absorbent
polymer by polymerization that can be used in the present invention
may be selected from, for example, all the monomers disclosed in
the foregoing patents or patents described later. Examples of the
monomer (A) encompass: an unsaturated monomer containing an acid
group such as acrylic acid, methacrylic acid, maleic acid
(anhydride), fumaric acid, crotonic acid, itaconic acid, vinyl
sulphonic acid, 2-(meth)acrylamide-2-methylpropanesulphonic acid,
(meth)acryloxyalkane sulphonic acid; hydrophilic monomers such as
N-vinyl-2-pyrolidone, N-vinylacetamide, (meth)acrylamide,
N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide,
2-hydroxyethyl(meth)acrylate, methoxypolyethylene
glycol(meth)acrylate, polyethylene glycol(meth)acrylate; and salts
thereof.
[0068] The monomer is preferably an unsaturated monomer containing
an acid group, particularly preferably acrylic acid and/or its
salt. Most preferably used as a repeating unit of the polymer are
acrylic acid and its salt including 1 to 50 mol %, more preferably
1 to 40 mol % of acrylic acid and 50 to 99 mol %, more preferably
60 to 99 mol % of alkali metal salt of acrylic acid. That is, a
neutralization ratio of the acrylic acid serving as the repeating
unit of the polymer is 50 to 99 mol %, and preferably 60 to 99 mol
%. Neutralization may be carried out either by neutralizing the
monomer before polymerization or by neutralizing the polymer during
or after the polymerization. Alternatively, the neutralization may
be carried out in a combination of these neutralization methods.
Preferable example of the acrylic acid salt is alkali metal salt
such as sodium salt and potassium salt.
[0069] In the present invention, a total amount of the acrylic acid
and/or its salt used as the monomer (A) is preferably 10 to 100 mol
%, more preferably 70 to 100 mol %, particularly preferably 90 to
100 mol % with respect to a whole monomer amount (excluding a
crosslinking agent). If necessary, a monomer other than the
crosslinking agent may also be added in amount of 0 to 30 mol %,
more preferably 0 to 10 mol % (including or excluding 10 mol
%).
[0070] The acrylic acid preferably contains a specified amount of
polymerization inhibitor, which may preferably be a methoxyphenol,
and more preferably be p-methoxyphenol. The methoxyphenol is
contained in an amount of 10 to 200 ppm by mass, preferably 10 to
90 ppm by mass, and particularly preferably 20 to 90 ppm by mass,
with respect to the acrylic acid.
[0071] The acrylic acid of the present invention contains
impurities such as protoanemonin and/or furfural in an amount as
small as possible. The amount of the protoanemonin and/or furfural
contained in the acrylic acid is 0 to 20 ppm by mass, more
preferably 0 to 10 ppm by mass, further preferably 0 to 5 ppm by
mass, further more preferably 0 to 2 ppm by mass, and particularly
preferably 0 to 1 ppm by mass.
[0072] (2) Crosslinking Agent
[0073] The crosslinking agent that can be used in the present
invention is one or a combination of the following compounds listed
herein as examples: compounds containing at least two polymerizable
double bonds per molecule, such as N,N'-methylenebisacrylamide,
(poly)ethylene glycol di(meth)acrylate, (poly)propylene
glycoldi(meth)acrylate, (polyoxyethylene)
trimethylolpropanetri(meth)acrylate, trimethylolpropane
di(meth)acrylate, polyethylene
glycoldi(.beta.-acryloyloxypropionate),
trimethylolpropanetri(.beta.-acryloyloxypropionate), and
poly(meth)aryloxyalkanes; and a compound that can react with a
carboxyl group to form a covalent bond, such as polyglycidylether
(such as ethylene glycol diglycidylether), and polyol (such as
ethylene glycol, polyethylene glycol, glycerine, and sorbitol).
[0074] In a case where one or more types of the above crosslinking
agents are used, it is preferable that the compound having at least
two polymerizable double bonds per molecule be essentially
included, taking into consideration an absorbing property or the
like of the resultant water-absorbent polymer. In view of physical
properties, the crosslinking agent is added in an amount of 0.0001
to 5 mol %, preferably 0.005 to 2 mol %, and further preferably
0.008 to 1 mol %, with respect to the monomer (A) that is capable
of forming a water-absorbent polymer.
[0075] (3) Polymerization Initiator
[0076] A polymerization initiator used in the present invention is
selected as needed according to how the polymerization is carried
out. Examples of such polymerization initiator encompass: a
photodegradable type polymerization initiator, a
thermally-degradable type polymerization initiator, and a redox
polymerization initiator.
[0077] Examples of the photodegradable type polymerization
initiator encompass: a benzoin derivative, a benzyl derivative, an
acetophenone derivative, a benzophenone derivative, and an azo
compound.
[0078] Examples of the thermally-degradable type polymerization
initiator encompass: persulfate such as sodium persulfate,
potassium persulfate, ammonium persulfate; peroxide such as
hydrogen peroxide, t-butylperoxide, methyl ethyl ketone peroxide;
an azo compound such as an azo nitrile compound, an azo amidine
compound such as an cyclic azo amidine compound, an azo amide
compound, an alkyl azo compound,
2,2'-azobis(2-amidinopropane)dihydrochloride, and
2,2'-azobis[2-(2-imidazoline-2-yl) propane]dihydrochloride.
[0079] An example of the redox polymerization initiator is a system
including a combination of (i) the persulfate or peroxide and (ii)
a reducing compound such as L-ascorbic acid and acid sodium
sulfite. In the present invention, it is also a preferable
embodiment that the photodegradable type polymerization initiator
is used together with the thermally-degradable type polymerization
initiator.
[0080] An amount of the polymerization initiator used in the
present invention is 0.0001 to 1 mol %, preferably 0.001 to 0.5 mol
%, with respect to the monomer (A).
[0081] (A) Organic Phosphorous Compound
[0082] An organic phosphorous compound used in the present
invention is preferably an organic amino phosphoric acid having an
amino group. Such an organic amino phosphoric acid may be a
water-soluble organic amino phosphoric acid, and preferably a
water-soluble non-polymeric organic amino phosphoric acid. The
number of amino groups contained in one molecule of the organic
amino phosphoric acid is preferably 1 or more, more preferably 2 or
more, whereas the number of phosphate groups contained is
preferably 1 or more, more preferably 2 or more, particularly
preferably 3 or more. Generally, an upper limit of each of the
number of amino groups and the number of phosphate groups contained
is 100 or less, preferably 10 or less, particularly preferably 5 or
less. It should be noted that in this description, "water-soluble"
indicates that a compound is soluble in 100 g of water at
25.degree. C. by 0.1 g or more, preferably 1 g or more, and
particularly preferably 5 g or more. Further, a molecular weight
thereof is generally 50 to 5000, preferably 100 to 1000, more
preferably 200 to 500.
[0083] Examples of the organic phosphorous compound to be used
encompass: ethylenediamine-N,N'-di(methylenephosphinic acid),
ethylenediaminetetra(methylenephosphinic acid),
nitriloacetic-di(methylenephosphinic acid),
nitrilodiacetic-(methylenephosphinic acid),
nitriloacetic-.beta.-propionic acid-methylenephosphonic acid,
nitrilotris(methylenephosphonic acid),
cyclohexanediaminetetra(methylenephosphonic acid),
ethylenediamine-N,N'-diacetic acid-N,N'-di(methylenephosphonic
acid), ethylenediamine-N,N'-di(methylenephosphonic acid),
ethylenediaminetetra(methylenephosphonic acid), polymethylene
diaminetetra(methylenephosphonic acid),
diethylenetriaminepenta(methylenephosphonic acid),
1-hydroxyethylidenediphosphonic acid, and salts thereof. The most
preferred organic phosphorous compound in the present invention is
the ethylenediaminetetra(methylenephosphonic acid) or its salt.
Preferred as salts are alkali metal salts such as sodium salt and
potassium salt; ammonium salt; amine salt; or the like.
Particularly preferred as salts are sodium salt and potassium
salt.
[0084] Prior to the polymerization, the organic phosphorous
compound used in the present invention should be added, for the
polymerization, with at least one type of monomer (A) that is
capable of forming a water-absorbent polymer by polymerization.
This provides the effect attained by the present invention. In
order to preferably achieve the effect of the present invention,
the organic phosphorous compound is contained in an amount of
preferably 1 to 100 ppm by mass, more preferably 30 to 100 ppm by
mass, and further preferably 50 to 100 ppm by mass. The monomer (A)
is contained in a monomer aqueous solution (B).
[0085] If the amount of the phosphorous compound is less than 1 ppm
by mass, a urine tolerance may be reduced and long-term color
stability may become less effective. On the other hand, if the
amount of the phosphorous compound is more than 100 ppm by mass,
the polymerization may be inhibited or may become more difficult to
be controlled (e.g., polymerization may suddenly occur) and thereby
causes instability of physical properties of the resultant
water-absorbent polymer. Therefore, the amount of the phosphorous
compound more than 100 ppm by mass is not preferable as well. Since
the organic phosphorous compound is contained in the monomer
aqueous solution (B) in advance, the polymerization can be easily
controlled. This makes it possible to obtain a safe particulate
water-absorbent polymer that excels in the long-term color
stability at low cost only with a slight amount of addition of the
organic phosphorous compound. Furthermore, polymer chain
deterioration due to heat history during a production process such
as a later-described drying process can be prevented. This makes it
possible to obtain a water-absorbent polymer having excellent
characteristics.
[0086] In a case where a predetermined amount of organic
phosphorous compound is added to the monomer (A), the organic
phosphorous compound may be added in advance at a stage of
preparing the monomer (A) or the monomer aqueous solution (B),
before or after a polymerization initiator is added to the monomer
aqueous solution (B), or during the polymerization. The organic
phosphorous compound is added when a polymerization ratio is
preferably less than 80%, more preferably less than 50%.
[0087] The water-absorbent polymer may be prepared in such a manner
that the organic phosphorous compound is further added after the
monomer (A) containing the predetermined amount of organic
phosphorous compound is polymerized, for adjusting physical
properties of the water-absorbent polymer or for other
purposes.
[0088] The organic phosphorous compound may either be added to the
monomer (A) before or after the polymerization. That is, after the
polymerization, for example, the organic phosphorous compound may
be added to a hydrogel polymer, to a dried polymer after a drying
process, or may be added in a surface-crosslinking process. These
post-polymerization processes are described later.
[0089] In a case where the organic phosphorous compound is added in
the post-process after the polymerization, a total amount of the
organic phosphorous compounds with respect to the water-absorbent
polymer particle is less than 1000 ppm by mass, preferably not more
than 500 ppm by mass, more preferably 30 to 500 mass ppn,
particularly preferably 50 to 500 ppm by mass, and most preferably
50 to 300 ppm by mass.
[0090] The total amount of the organic phosphorous compounds
exceeding 1000 ppm by mass with respect to the water-absorbent
polymer particle is not preferable, because if so, surface tension
of liquid such as body fluid (e.g., blood and urine) and waste
fluid absorbed into the water-absorbent polymer becomes low. In the
case where the surface tension of the liquid (absorbed liquid) is
low, if for example a pressure such as body weight is applied on an
absorbing article such as a disposable diaper, urine may be
squeezed out, thereby failing to keep urine inside the absorbing
article.
[0091] (5) Hydroxycarboxylic Acid
[0092] A hydroxycarboxylic acid compound that can be used in the
present invention is carboxylic acid or its salt, which has a
hydroxyl group within its molecule. Examples of such
hydroxycarboxylic acid compound encompass: lactic acid, glycolic
acid, malic acid, glyceric acid, tartaric acid, citric acid,
isocitric acid, salicylic acid, mandelic acid, gallic acid,
mevalonic acid, chinic acid, shikimic acid, beta-hydroxypropionic
acid, and salts thereof. Among these, alpha-hydroxycarboxylic acids
are more preferably used in the present invention.
[0093] The alpha-hydroxycarboxylic acids are carboxylic acids
having a hydroxyl group bound to an alpha carbon in their
molecules. Preferred among those are non-polymeric
alpha-hydroxycarboxylic acids. In view of ease and effect of
addition, the non-polymeric alpha-hydroxycarboxylic acids have a
molecular weight of preferably 40 to 2000, more preferably 60 to
1000, and particularly preferably 100 to 500. The non-polymeric
alpha-hydroxycarboxylic acids are preferably soluble in water.
Examples of such alpha-hydroxycarboxylic acids encompass: lactic
acid and salt thereof, citric acid and salt thereof, malic acid and
salt thereof, isocitric acid and salt thereof, glyceric acid and
salt thereof, and poly alpha-hydroxy acrylic acid and salt
thereof.
[0094] By adding the hydroxycarboxylic acid compound (preferably an
alpha-hydroxycarboxylic acid) into a monomer aqueous solution (B)
so as to use the hydroxycarboxylic acid compound together with an
organic phosphorous compound, it is possible to further improve
long-term color stability.
[0095] In view of the long-term color stability, it is most
preferable that the hydroxycarboxylic acid compound be added to the
monomer aqueous solution (B) in advance. However, it is also
possible to add the hydroxycarboxylic acid compound at any stage of
the production process of the water-absorbent polymer, as long as
the hydroxycarboxylic acid compound is effective. The
hydroxycarboxylic acid compound may be added at any stage of the
production process of the water-absorbent polymer, such as during
polymerization, after the polymerization, when the water-absorbent
polymer is dried, when the water-absorbent polymer is pulverized,
before the surface-crosslinking, during the surface-crosslinking,
and after the surface-crosslinking. As described later, the
hydroxycarboxylic acid compound may also be added when a hydrogel
polymer is chipped.
[0096] In view of cost performance, a using amount of such
hydroxycarboxylic acid compounds is preferably 1 to 10000 ppm by
mass with respect to the monomer (A), or 1 to 10000 ppm by mass
with respect to solid content of the water-absorbent polymer.
[0097] (6) Polymerization Process of Monomer
[0098] The advantageous water-absorbent polymer obtainable in the
present invention is polyacrylate. For example, the water-absorbent
polymer is alkali metal salt of polyacrylic acid. Alkali metal salt
of polyacrylic acid is obtainable by polymerizing a monomer mixture
containing: a monomer (A) containing 1 to 50 mol % of acrylic acid,
50 to 99 mol % of acrylic acid alkali metal salt, and 0.005 to 2
mol % of crosslinking agent; 0.005 to 2 mol % of crosslinking
agent; and 1 to 100 ppm by mass, more preferably 30 to 100 ppm by
mass, and further preferably 50 to 100 ppm by mass, with respect to
the monomer (A), of an organic phosphorous compound.
[0099] At this point, an acrylic acid monomer aqueous solution is
neutralized prior to the polymerization. In another advantageous
embodiment, the acrylic acid is polymerized in the presence of the
organic phosphorous compound in an amount of 1 to 100 ppm by mass
with respect to the acrylic acid, and thereafter a hydrogel thus
obtained is neutralized with alkali metal salt, i.e.,
post-polymerization neutralization of the hydrogel is performed,
thereby obtaining the polyacrylate alkali metal salt.
[0100] In the present invention, a monomer aqueous solution (B) is
prepared by mixing the monomer (A), an internal
surface-crosslinking agent, and water with one another in amounts
within the ranges defined above. The monomer aqueous solution (B)
further contains the organic phosphorous compound in an amount
within the ranges defined above at a timing described above.
[0101] A monomer concentration of the monomer aqueous solution (B)
is 20 to 80 mass %, preferably 35 to 80 mass %, and further
preferably 40 to 70 mass %. The monomer aqueous solution (B) may
also be in a form of slurry, in which some solids are precipitated
out.
[0102] Further, a solvent other than the water may also be used
together if needed. The solvent used together with the water is not
limited to a particular kind. Also, a monomer compound or a
crosslinking agent compound may be used in the monomer (A) as
needed. For example, (i) approximately 0 to 30 mass % of polymers
such as a water-soluble polymer and a water-absorbent polymer and
(ii) approximately 0 to 1 wt % of chain transfer agent or the like
may be added.
[0103] The monomer aqueous solution (B) contains a slight amount of
iron for the sake of better polymerization control. An amount of
the iron is preferably 0.001 to 5 ppm by mass, and particularly
preferably 0.001 to 3 ppm by mass based on Fe.sub.2O.sub.3. If the
amount of iron is out of the ranges described above, the
polymerization becomes hard to be controlled due to a
polymerization delay or bumping.
[0104] An example of a method for controlling the amount of iron
contained in the monomer aqueous solution (A) is an addition of a
water-soluble iron compound. For example, in a case where acrylic
acid is used as the monomer, the acrylic acid may be neutralized
within the ranges described above, with use of alkali metal salt
(such as sodium hydrate, potassium hydrate, sodium carbonate, and
sodium acid carbonate) containing a slight amount of iron.
[0105] A preferred condition or the like under which the
neutralization is carried out are exemplified in Pamphlet of
International Publication WO2006/522181, and are applied to the
present invention.
[0106] Generally, it is preferable that the polymerization be
carried out by aqueous polymerization or reversed phase suspension
polymerization in view of better performance and ease of the
polymerization control. Such polymerization may be carried out
under air atmosphere; however, it is preferable that the
polymerization be carried out under inert gaseous atmosphere (for
example, an amount of oxygen is not more than 1%) using an inner
gas of nitrogen, argon, or the like. It is also preferable that
dissolved oxygen be sufficiently replaced with the inert gas (for
example, an amount of the oxygen is less than 1 ppm) before the
polymerization of the monomeric component (monomer).
[0107] The reversed phase suspension polymerization is a
polymerization method in which a monomer aqueous solution is
suspended in a hydrophobic organic solvent. For example, the
reversed phase suspension polymerization is disclosed in U.S. Pat.
No. 4,093,776, No. 4,367,323, No. 4,446,261, No. 4,683,274, No.
4,880,886, No. 5,180,798, No. 5,210,159, No. 5,202,400, No.
5,244,735, No. 5,397,845, No. 5,408,006, No. 5,412,0237, No.
5,563,218, No. 5,807,916, No. 5,885,462, No. 5,998,553, United
States Patent Application Publication No. 2007-015887, and the
like.
[0108] The aqueous polymerization is a method in which the monomer
aqueous solution is polymerized without a dispersion solvent. For
example, the aqueous polymerization is disclosed in U.S. Pat. No.
4,625,001, No. 4,873,299, No. 4,286,082, No. 4,973,632, No.
4,985,518, No. 5,124,416, No. 5,250,640, No. 5,264,495, No.
5,145,906, No. 5,380,808, No. 4,769,427, No. 4,873,299, No.
6,455,600, No. 6,602,950, No. 6,710,141, and the like, and European
Patents No. 0811636, No. 0955086, No. 0922717, No. 1178059, and the
like. In performing the polymerization in the present invention, it
is also possible to use monomers, crosslinking agents,
polymerization initiators, and other additives disclosed in the
above-described Patent Literatures.
[0109] In view of physical properties, a polymerization method used
in the present invention is preferably the aqueous polymerization
or the reversed phase suspension polymerization, particularly
preferably the aqueous polymerization, and further preferably an
aqueous polymerization using belt polymerization (for example, U.S.
Pat. No. 4,857,610, No. 4,893,999, No. 6,174,978, and No.
6,911,499, and United States Patent Application Publications No.
2005-0215734 and No. 2006-0167198) or kneader polymerization (For
example, U.S. Pat. No. 6,710,141, No. 6,987,151, and No.
5,124,416), particularly preferably a continuous aqueous
polymerization. Preferred examples of a polymerization temperature
and a polymerization concentration are exemplified in U.S. Pat. No.
6,906,159 and No. 7,091,253.
[0110] Among the polymerization methods described above, the
reversed phase suspension polymerization is not
environmentally-friendly since a large amount of organic solvent is
used. Further, a surfactant used in the reversed phase suspension
polymerization may reduce surface tension. In view of these, the
aqueous polymerization is particularly preferable in the present
invention.
[0111] A hydrogel polymer obtained by the polymerization will
become a dried particle or the water-absorbent polymer of the
present invention through a subsequent process of drying.
[0112] (7) Process of Crushing Hydrogel Polymer
[0113] The hydrogel polymer obtained may be dried as it is;
however, if necessary, the hydrogel polymer is chipped by use of a
gel crusher or the like before being dried. The water-absorbent
polymer with long-term color stability of the present invention
(the water-absorbent polymer having long-term color stability) is
not limited to be in a particular shape, and may be in any shape
such as granules, powder, flakes, fibers, or the like. Therefore,
the polymer is chipped by a variety of methods. One example is a
method of crushing the polymer by chipping the polymer extruded
from a screw extruder with a porous die having pores of any shape.
In performing extrusion crushing, the aforementioned organic
phosphorous compound, the hydroxycarboxylic acid compound, or the
salts thereof in a form of aqueous solution may be added, so that a
urine tolerance can be improved and color degradation can be
further prevented.
[0114] (8) Drying Process
[0115] A drying temperature suitable in the present invention is
not particularly limited. However, a drying process is performed
for example at a temperature within a range of 50 to 300.degree. C.
(preferably performed under reduced pressure in a case of
100.degree. C. or lower), second preferably 100 to 250.degree. C.,
more preferably 150 to 250.degree. C., further preferably 160 to
250.degree. C., particularly preferably 170 to 250.degree. C., and
most preferably 170 to 220.degree. C. A drying period is preferably
approximately 1 to 120 minutes, further preferably 10 to 60
minutes, and particularly preferably 20 to 50 minutes. The present
invention realizes excellent physical properties and low coloration
degree even with thermal drying at a high temperature.
[0116] In a case of aqueous polymerization which is particularly
preferable in the present invention, the drying is carried out at a
temperature within a range of preferably 100 to 250.degree. C.,
more preferably 150 to 250.degree. C., further preferably 160 to
250.degree. C., particularly preferably 170 to 250.degree. C., and
most preferably 170 to 220.degree. C. The drying is carried out
particularly preferably with hot air.
[0117] A second production method according to the present
invention is a method including: polymerizing a monomer aqueous
solution (B) containing (i) at least one type of monomer (A) that
is capable of forming a water-absorbent polymer by polymerization,
(ii) at least one type of crosslinking agent and (iii) at least one
type of polymerization initiator so as to obtain a hydrogel
polymer; and thermally drying the hydrogel polymer, wherein the
hydrogel polymer before drying contains an organic phosphorous
compound, and the heated-air drying is performed at a temperature
within a range of 150 to 250.degree. C. In this method, the organic
phosphorous compound may be added during or after the
polymerization of the monomer aqueous solution (B).
[0118] In the present invention, use of the organic phosphorous
compound makes it possible to prevent a drying deterioration (e.g.,
breakage of polymer main chain due to the drying) even at a drying
temperature of as high as 100.degree. C. or more, particularly 150
to 250.degree. C. This makes it possible to improve productivity,
for example to shorten a drying period.
[0119] Examples of the applicable drying method encompass a variety
of methods such as thermal drying, hot-air drying, reduced-pressure
drying, fluidized-bed drying, infrared ray drying, microwave
drying, drum dryer drying, dehydration by azeotropy with a
hydrophobic organic solvent, and high-humidity drying with
high-temperature steam. Preferred among those is, for example,
drying by contacting the hydrogel polymer with gas whose dew-point
temperature is 40 to 100.degree. C., more preferably 50 to
90.degree. C.
[0120] The particulate water-absorbent polymer of the present
invention is adjusted to have moisture content described later,
through the above drying process.
[0121] (9) Pulverization, Classification, and Granulation
Processes
[0122] Dried particles or the water-absorbent polymer particles of
the present invention obtained by the drying may be subjected to a
process of pulverization, classification, granulation, or the like,
if needed, depending on its purpose. Methods of the pulverization,
classification, granulation, or the like are described in, for
example, Pamphlet of International Publication WO2004/69915.
[0123] (10) Surface-crosslinking Treatment Process
[0124] The dried particles and the water-absorbent polymer
particles of the present invention can be turned into a
water-absorbent polymer with long-term color stability that is more
suitable for a sanitary material, through a conventionally known
surface-crosslinking process. The surface-crosslinking is such that
a surface layer (shallow surface: generally, an area that is at a
distance of approximately several tens of .mu.m (including 10
.mu.m) from the surface layer of the water-absorbent polymer) of
the water-absorbent polymer is treated so that a crosslink density
on a part of the surface layer becomes higher. This can be
accomplished by radical crosslinking on the surface, surface
polymerization, crosslinking reaction with the surface-crosslinking
agent, or the like.
[0125] The surface-crosslinking treatment with use of a
surface-crosslinking agent more suitably applied in the present
invention is further described as follows.
[0126] Examples of the surface-crosslinking agent that can be used
in the present invention encompass a variety of organic or
inorganic crosslinking agents. However, in view of physical
properties and handlability, a crosslinking agent that is reactable
with a carboxyl group is preferably used. Examples of such a
crosslinking agent encompass: a polyalcohol compound; an epoxy
compound; a polyamine compound and a condensate of the polyamine
compound and a haloepoxy compound; a oxazoline compound; mono-,
di-, and poly-oxazolidinone compounds; polyvalent metal salt; and a
alkylene carbonate compound.
[0127] More specifically, examples of such a crosslinking agent
encompass compounds disclosed in U.S. Pat. No. 6,228,930, No.
6,071,976, No. 6,254,990, and the like. Such a crosslinking agent
may be, but not limited to, a polyalcohol compound such as mono-,
di-, tri-, tetra-, and polyethylene glycol, monopropylene glycol,
1,3-propanediol, dipropylene glycol,
2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin,
polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol,
1,5-pentanediol, 1,6-hexanediol, or 1,2-cyclohexandimethanol; an
epoxy compound such as ethylene glycol diglycidyl ether or
glycidol; a polyamine compound such as ethylenediamine, diethylene
triamine, triethylenetetramine, tetraethylenepentamine,
pentaethylenehexamine, polyethylenimine, or polyamide polyamine; a
haloepoxy compound such as epichlorohydrin, epibromohydrin, or
alpha-methylepichlorohydrin; a condensate of the polyamine compound
and the haloepoxy compound; an oxazolidinone compound such as
2-oxazolidinone; an alkylene carbonate compound such as ethylene
carbonate; an oxetane compound; and a cyclic urea compound such as
2-imidazolidinone.
[0128] A dehydration reaction crosslinking agent (a crosslinking
agent which causes dehydration reaction between a carboxyl group of
water-absorbent resin and a functional group of the crosslinking
agent) can attain the effect of the present invention remarkably.
For example, the polyalcohol compound, the oxazolidinone compound,
and the alkylenecarbonate compound can be used. Especially the
polyalcohol compound can be used.
[0129] A using amount of the surface-crosslinking agent is,
although depending on compounds to be used and a combination
thereof, preferably 0.001 parts by mass to 10 parts by mass, and
more preferably 0.01 parts by mass to 5 parts by mass, with respect
to 100 parts by mass of the water-absorbent polymer particles.
[0130] In the present invention, water may be used together with
the surface-crosslinking agent. In this case, a using amount of the
water is preferably 0.5 to 20 parts by mass, and more preferably
0.5 to 10 parts by mass, with respect to 100 parts by mass of the
water-absorbent polymer particles. Further, in the present
invention, not only the water but also a hydrophilic organic
solvent is usable. In this case, a using amount of the hydrophilic
organic solvent is 0 to 10 parts by mass, and preferably 0 to 5
parts by mass, with respect to 100 parts by mass of the
water-absorbent polymer particles. In mixing a crosslinking agent
solution into the water-absorbent polymer particles,
water-insoluble fine particles powder and/or a surfactant may be
added together, provided that they do not hinder the effect of the
present invention. A total amount of the water-insoluble fine
particles powder and the surfactant to be added is 0 to 10 mass %,
preferably 0 to 5 mass %, and further preferably 0 to 1 mass %.
Examples of the surfactant to be used and a using amount thereof
are exemplified in International Application No. WO2005JP1689
(International Filing Date: Feb. 2, 2005).
[0131] A mixing device for mixing the crosslinking solution may be
selected from a variety of mixing apparatuses. For example, a
high-speed stirring mixer is used, and particularly preferably a
high-speed stirring continuous mixer is used. Examples of such
mixers are Turbulizer (Hosokawa Micron Corporation, Japan) and
Loedige mixer (Gebruder Loedige Maschinenbau GmbH, Germany).
[0132] The water-absorbent polymer particles into which the
surface-crosslinking agent has been mixed are preferably heated for
facilitating the crosslinking reaction. The heat treatment heats
water-absorbent polymer particles at a temperature of preferably
100 to 250.degree. C., more preferably 150 to 250.degree. C.,
particularly preferably 160 to 250.degree. C., and most preferably
170 to 250.degree. C., for example for 1 to 180 minutes, further
preferably for 5 to 60 minutes.
[0133] The method of the present invention of producing the
particulate water-absorbent polymer containing an organic
phosphorous compound makes it possible to provide a particulate
water-absorbent polymer that can prevent polymer from thermal
deteriorating due to heating at a temperature within the same range
as that of the drying process.
[0134] Such surface-crosslinking methods are described in Patent
Literatures such as European Patents No. 0349240, No. 0605150, No.
0450923, No. 0812873, No. 0450924, and No. 0668080; Japanese Patent
Application Publications, Tokukaihei, No. 7-242709 and No.
7-224304; U.S. Pat. No. 5,409,771, No. 5,597,873, No. 5,385,983,
No. 5,610,220, No. 5,633,316, No. 5,674,633, and No. 5,462,972; and
Pamphlets of International Publication WO99/42494, WO99/43720, and
WO99/42496, and the like. Surface-crosslinking methods disclosed in
the above Literatures are also applicable in the present
invention.
[0135] In the surface-crosslinking process, an aqueous liquid or
the like may further be added after termination of crosslinking
reaction, and before the granulation of the water-absorbent
polymer. For example, water-soluble polyvalent metal salt such as
an aluminum sulfate aqueous solution may be added. Such methods are
disclosed in Patent Literatures such as U.S. Pat. No. 5,369,148,
United States Patent Application Publication No. 2007/141338,
Pamphlets of International Publication WO2004/69915 522181,
WO2004/69293 522181, and the like, and are applicable in the
present invention.
[0136] The particulate water-absorbent polymer of the present
invention is adjusted to have moisture content described later,
through the surface-crosslinking process.
[0137] (11) Moisture Content Adjusting Process
[0138] In order to attain the object, the present invention is
arranged such that water content (moisture content) of its final
product is adjusted to not more than 5 mass %, preferably not more
than 3 mass %, more preferably 0 to 2 mass %, particularly
preferably 0 to 1 mass %, with respect to the water-absorbent
polymer, through heating processes such as the aforementioned
drying process and the surface-crosslinking process. The moisture
content is adjusted by performing heating at the aforementioned
high temperature for a predetermined period.
[0139] The water-absorbent polymer may further be subjected to
water content (moisture content) adjusting process, if necessary,
after the water content of the water-absorbent polymer is adjusted
to be within the range defined above.
[0140] If the water content of the water-absorbent polymer used in
the final product is out of the ranges defined above, coloring
reaction of the water-absorbent polymer is accelerated. This is not
preferable because long-term color stability of the present
invention will be lost.
[0141] (12) Other Processes
[0142] If necessary, the water-absorbent polymer may be subjected
to processes of such as pulverization, classification, granulation,
after the above processes. Methods of the pulverization,
classification, granulation, or the like are disclosed for example
in Pamphlet of International Publication WO2004/69915.
[0143] (13) Water-absorbent Polymer
[0144] The present invention provides a novel water-absorbent
polymer, which is produced by for example the methods described
above. Means for achieving a particle size and physical properties
is already described above; however, a method of producing the
water-absorbent polymer is not limited to those described
above.
[0145] The particulate water-absorbent polymer of the present
invention obtained as described above is such that 95% or more of
the particulate water-absorbent polymer advantageously has a
particle size of 10 to 10000 .mu.m, and more advantageously 100 to
1000 .mu.m.
[0146] The water-absorbent polymer obtainable in the present
invention is safe, excellent in cost performance, and suitably used
for a sanitary material such as a disposable diaper. The
water-absorbent polymer used in the sanitary material keeps a
clean-imaged white condition even over long-term storage under a
high humidity and a high temperature. Further, the water-absorbent
polymer obtainable in the present invention is a water-absorbent
polymer particle with long-term color stability, in which a
particle after 7-day exposure to atmosphere of 70.+-.1.degree. C.
of temperature and 65.+-.1% of relative humidity exhibits an L
value (Lightness) of at least 70 in Hunter's Lab color system
measured by a spectral calorimeter.
[0147] That is, the particulate water-absorbent polymer of the
present invention contains 30 to 500 ppm by mass of phosphorous
compound and satisfies at least one of the followings (a) to
(c):
(a) a particle of the particulate water-absorbent polymer after
7-day exposure to atmosphere of 70.+-.1.degree. C. of temperature
and 65.+-.1% of relative humidity exhibits an L value (Lightness)
of 70 or higher in Hunter's Lab color system; (b) the particulate
water-absorbent polymer contains particles having a diameter of
less than 150 .mu.m in an amount of 0 to 5 mass %; a mass median
particle size (D50) is 200 .mu.m to 600 .mu.m; and a logarithmic
standard deviation (.sigma..zeta.) of a particle size distribution
is 0.20 to 0.40; and (c) an absorbency against pressure (AAP) under
pressure of 1.9 kPa or 4.8 kPa for 0.90 mass % sodium chloride
aqueous solution for 60 minutes is at least 20 (g/g).
[0148] For attaining the object of the present invention, the
phosphorous compound is contained in an amount of preferably 50 to
300 ppm by mass, and more preferably in an amount of 70 to 200 ppm
by mass.
[0149] Characteristics of the particulate water-absorbent polymer
of the present invention are described as follows.
[0150] (a) Long-term Color Stability
[0151] The water-absorbent polymer obtainable in the present
invention is suitably used in a sanitary material such as a
disposable diaper, and keeps its remarkably white condition
associated with clean impression. That is, water-absorbent polymer
particles that have been exposed under atmosphere of
70.+-.1.degree. C. of temperature and 65.+-.1% of relative humidity
for 7 days present high level of whiteness, which is represented by
an L value as set forth below. The water-absorbent polymer
particles having high level of whiteness provide an excellent clean
impression when they are in practical use in an absorbing article.
Specifically, the particulate water absorbent, i.e., the
surface-crosslinked particulate water-absorbent polymer obtainable
in the above-described processes, exhibits, in Hunter's Lab color
system, the L value (Lightness) of at least 90, preferably 92 or
higher, and more preferably 95 or higher; a b value of at least 12
or lower, preferably 10 or lower, and more preferably 8 or lower;
and a YI value of at least 10 or lower, preferably 9 or lower, and
more preferably 8 or lower. The particulate water absorbent
satisfying the ranges defined above is in remarkably white
condition giving the clean impression. If the particulate water
absorbent does not satisfy the ranges defined above, a commercial
value of a diaper will be reduced due to coloration caused over
long periods of time when the particulate water absorbent is used
in the absorbing article such as the diaper.
[0152] (b) Particle Size
[0153] The water-absorbent polymer of the present invention is in a
particle shape, and preferably controlled into a specific particle
size. The particle size can be adjusted as needed by pulverization,
classification, granulation, fine powder collection, or the
like.
[0154] A mass median particle size (D50) of the water-absorbent
polymer is 200 to 600 .mu.m, preferably 250 to 550 .mu.m, more
preferably 200 to 500 .mu.m, and particularly preferably 350 to 450
.mu.m. An amount of particles having a particle size of less than
150 .mu.m is preferably as small as possible, and is generally
controlled to 0 to 5 mass %, preferably 0 to 3 mass %, and
particularly preferably 0 to 1 mass %. Further, an amount of
particles having a particle size of 850 .mu.m is preferably as
small as possible, and generally controlled to 0 to 5 mass %,
preferably 0 to 3 mass %, and particularly preferably 0 to 1 mass
%. A logarithmic standard deviation (.sigma..zeta.) of a particle
size distribution is preferably 0.20 to 0.40, more preferably 0.27
to 0.37, and further preferably 0.25 to 0.35. If the
water-absorbent polymer does not satisfy the ranges of the particle
size distribution defined above, the water-absorbent polymer cannot
be so suitable for use in an absorbing article such as disposable
diaper.
[0155] Furthermore, a bulk density (specified in JIS K-3362) is
controlled to preferably 0.40 to 0.90 g/ml, and more preferably
0.50 to 0.80 g/ml. Moreover, the particles having a particle size
of 600 to 150 .mu.m is preferably 60 to 100 mass %, more preferably
70 to 100 mass %, and further preferably 80 to 100 mass %, with
respect to a total amount of the particles.
[0156] (c) Absorbency Against Pressure (AAP)
[0157] In the present invention, a particulate water-absorbent
polymer is controlled so as to have an absorbency against pressure
(AAP) of preferably 20 (g/g) or more, and more preferably 25 (g/g)
or more, for example by the surface-crosslinking described above.
The AAP is measured under pressure of 1.9 kPa or 4.8 kPa for 0.9
mass % sodium chloride aqueous solution.
[0158] The absorbency against pressure (absorbing ability, AAP)
under pressure of 1.9 kPa or 4.8 kPa of less than 20 (g/g) is not
preferable, because if the particulate water-absorbent polymer
having the AAP defined above is used in, for example, a diaper, an
amount of liquid squeezed out from the diaper, i.e., Re-wet,
increases, and this may cause skin roughness on a baby.
[0159] (d) Absorbency without Pressure (Centrifuge Retention
Capacity, Gel Volume Saline/GVs)
[0160] In the present invention, the particulate water-absorbent
polymer is adjusted to have an absorbency without pressure (GVs)
for 0.9 mass % sodium chlorine aqueous solution of preferably not
less than 10 g/g, more preferably not less than 20 g/g, further
preferably not less than 25 g/g, and particularly preferably not
less than 30 g/g, for example by polymerization described above.
The GVs is preferably as high as possible and there is no upper
limit for the GVs; however, taking into consideration a balance
with other physical properties, the GVs is preferably 50 (g/g) or
less, more preferably 45 (g/g) or less, and further preferably 40
(g/g) or less.
[0161] The particulate water-absorbent polymer having the
absorbency without pressure (GVs) of less than 10 (g/g) is not
suitable for use in a sanitary material such as a diaper, because
such particulate water-absorbent polymer absorbs too small amount
of liquid to be used for the sanitary material. On the contrary,
the particulate water-absorbent polymer having the absorbency
without pressure (GVs) of more than 50 (g/g) is weak in gel
strength. With such a particulate water-absorbent polymer, it may
be impossible to obtain a water-absorbent that excels in liquid
permeability.
[0162] (e) Water Soluble Content (Extractable Content)
[0163] The water-absorbent polymer of the present invention is
produced so as to have water soluble content of preferably 0 to 35
mass %, more preferably not more than 25 mass %, further preferably
not more than 15 mass %, and particularly preferably not more than
10 mass %, for example by the polymerization described above. The
water-absorbent polymer having the water soluble content of more
than 35 mass. % may be weak in gel strength and inferior in liquid
permeability. In addition, absorption capacity (GVs, AAP, and the
like) of such water-absorbent polymer may decrease over time when
the water-absorbent polymer is used in a diaper for long periods of
time.
[0164] (f) Residual Monomer
[0165] The water-absorbent polymer of the present invention is
produced so as to have residual monomers in an amount within a
range of preferably 0 to 400 ppm by mass, more preferably 0 to 300
ppm by mass, and particularly preferably 0 to 200 ppm by mass, for
example by the polymerization described above.
[0166] (g) Deterioration-Induced Extractable Content
[0167] In the water-absorbent polymer of the present invention,
which is produced for example by the polymerization described
above, deterioration-induced extractable content (content that
would become extractable due to deterioration) is in an amount
within a range of 0 to 18 mass %, preferably 0 to 15 mass %, and
further preferably 0 to 12 mass %.
[0168] An amount of the deterioration-induced extractable content
is an index of long-term stability of the water-absorbent polymer.
When the water-absorbent polymer absorbs urine or the like and
turns into a gel form, the polymer is degraded by minute components
contained in the urine, and as a result, the polymer (gel) becomes
soluble due to deterioration over time. That is, a high
deterioration-induced extractable content indicates that the gel
more likely becomes soluble over time.
[0169] (14) Other Additives
[0170] In order to further add a variety of functions depending on
a desired purpose, the particulate water-absorbent polymer
according to the present invention may be produced by adding, to a
water-absorbent polymer (water-absorbent resin), organic acid, an
oxidant, and a reductant such as sulfite (bisulfite) salt; a
polyvalent metal compound exemplified in Pamphlets of International
Publication WO2004/69915, WO2004/113452, and WO2005/108472;
water-soluble inorganic and organic powder such as silica and metal
soap; a deodorizer; an antibacterial agent; polymer polyamine;
pulp; thermoplastic fiber; or the like, in an amount of 0 to 3 mass
%, and preferably 0 to 1 mass %.
[0171] (15) Use
[0172] The water-absorbent polymer of the present invention may be
used for any purpose; however, it is preferable to be used in an
absorbing article such as a disposable diaper, a sanitary napkin,
and an incontinence pad. In particular, it is preferable to be used
in a concentrated diaper (a diaper within which a large amount of
water-absorbent polymers (water-absorbent resin) are used), which
has conventionally had problems such as odor and coloration
attributed to raw materials of the water-absorbent polymer.
[0173] An absorbing article of the present invention includes the
water-absorbent polymer, an absorbent core obtainable by shaping
hydrophilic fiber into a sheet (if needed), a top sheet having
liquid permeability, and a back sheet having no liquid
permeability. The particulate water-absorbent polymer of the
present invention provides excellent performance especially in a
case where it is used in an upper layer part of the absorbent core
of the absorbing article.
[0174] The absorbent core without the hydrophilic fiber is made by
fixing the particulate water-absorbent polymer on a piece of paper
and/or nonwoven fabric. In a case where a fiber material (pulp) is
used, the absorbent core is made in such a manner that the
particulate water-absorbent polymer is sandwiched by the fiber
materials or blended with the fiber material. Examples of the fiber
material that can be used encompass: fractured wood pulp; cotton
linter and crosslinked cellulose fiber; rayon; cotton; wool;
acetate; vinylon; and the like. Especially preferred are those
air-laid.
[0175] An effect of the present invention is provided in a case
where a contained amount (core concentration) of the
water-absorbent polymer in the absorbent core of the absorbing
article is 30 to 100 mass %, preferably 40 to 100 mass %, more
preferably 50 to 100 mass %, further preferably 60 to 100 mass %,
particularly preferably 70 to 100 mass %, and most preferably 75 to
95 mass %. For example, in a case where the particulate
water-absorbent polymer of the present invention is used especially
in the upper layer part of the absorbent core in the amount
described above, the absorbent core becomes excellent in liquid
permeability (liquid permeability under pressure) and in
diffuseness of absorption liquid such as urine. Accordingly, the
absorption liquid is efficiently distributed on the absorbing
article such as the disposable diaper, and thereby an absorbing
amount in the entire absorbing article is improved. As a result, it
is possible to provide an absorbing article including an absorbent
core that keeps its hygienic white condition.
[0176] The absorbent core is preferably compression-molded into a
density of not less than 0.06 g/cc but not more than 0.50 g/cc, and
a basis weight of not less than 0.01 g/cm.sup.2 but not more than
0.20 g/cm.sup.2. Further, if a thickness of the absorbent core is
not more than 30 mm, and preferably not more than 20 mm, it is
possible to provide an absorbing article that is suitably
applicable to a thinner disposable diaper.
EXAMPLES
[0177] The present invention is further described through the
following Examples. However, it is not intended to limit the
present invention to the following Examples. Properties (a) to (g)
described in Claims and Embodiment of the present invention were
worked out through the following measurement methods. Note that the
following measurement methods are for a particulate water-absorbent
polymer; however, other forms of water-absorbent polymers such as a
water absorbent that is described as a surface-crosslinked
water-absorbent polymer in Examples and a particulate
water-absorbent polymer are also measured through the measurement
methods. Further, the term "weight" is equivalent to "mass".
[0178] (1) Evaluation of Polymerizability
[0179] On the basis of a polymerization condition (especially
polymerization period) in Production Examples, polymerizability was
evaluated by measuring a time from an initiation of the
polymerization to a termination of the polymerization reaction (a
point at which the polymerization reaction reaches a peak
temperature).
Same as Production Example: YES
[0180] Different from Production Example: NO
[0181] (2) Absorbency without Load (Gel Volume Saline/GVs)
[0182] 0.2 g of the particulate water-absorbent polymer was evenly
contained in a bag (60.times.60 mm) made of a nonwoven fabric and
was sealed. Then, the bag was soaked in 100 g of 0.9 mass % sodium
chloride aqueous solution (physiological saline) at 25
(.+-.3).degree. C., and was withdrawn 30 minutes later. By use of a
centrifugal separator, the bag was drained for three minutes at
centrifugal force 250 G, and a weight W1 of the bag was measured.
Further, the same operation was performed without using the
particulate water absorbent, and a weight W2 was measured. Then,
from the weights W1 and W2, absorbency was calculated through the
following Mathematical Formula 1.
GVs=(W1-W2)/0.2-1 Formula 1
[0183] (3) Water Soluble Polymer Content and Extractable Content
Ratio
[0184] Note that water soluble polymer content hereinafter may be
referred to as an amount of extractable content. A water soluble
polymer may also be referred to as an extractable content. An
extractable content ratio is a ratio (mass %) of the extractable
content with respect to the water-absorbent polymer.
[0185] Into a 250 ml plastic container having a cover, 184.3 g of
0.90 mass % sodium chlorate aqueous solution was measured and
pored. Into the solution, 1.00 g of the water-absorbent polymer was
added, and the plastic container containing the solution and the
water-absorbent polymer was stirred for 16 hours, thereby
extracting the extractable content from resin. An extract solution
obtained was filtered through a piece of filter paper (ADVANTEC
toyo kaisha, Ltd.; product name: JIS P3801, No. 2, thickness: 0.26
mm, diameter of retainable particles: 5 .mu.m), thereby obtaining a
filtrate. Then, 50.0 g of the filtrate was measured so as to be
used as a measurement solution.
[0186] First, only 184.3 g of the physiological saline (0.90 mass %
sodium chlorine aqueous solution) was titrated by using a 0.1N NaOH
solution until pH of the saline reached pH10. Thereafter, the
saline was titrated by using a 0.1N HCl solution until pH of the
saline reached pH2.7. In this way, blank titration amounts
([bNaOH]ml and [bHCl]ml) were measured. The same operation was
performed with respect to the measurement solution, thereby
measuring titration amounts ([NaOH]ml and [HCl]ml). For example in
a case of a particulate water absorbent including a known amount of
acrylic acid and its salt, the extractable content ratio of the
water-absorbent polymer can be calculated in accordance with the
following Mathematical Formula 2, from an average molecular weight
of monomers of the water absorbent and the titration amounts
obtained by the foregoing operation. A main component of the
extracted extractable content is the extracted water-soluble
polymer. In a case where the average molecular weight of the
monomer is unknown, the average molecular weight of the monomer can
be calculated by using a neutralization ratio worked out by
titration. The neutralization ratio is calculated through the
following Mathematical Formula 3.
Extractable Content Ratio(mass %)=0.1.times.(average molecular
weight of
monomer).times.184.3.times.100.times.([HCl]-[bHCl])/1000/1.0/50.0
Formula 2
Neutralization Ratio(mol
%)=(1-([NaOH]-[bNaOH])/([HCl]-[bHCl])).times.100 Formula 3
[0187] It should be noted that in a case of polymer with high
moisture content, e.g., a hydrogel crosslinked polymer, the
extractable content ratio can be measured by calculating solid
content of the water-absorbent polymer from the moisture content,
and then using a given amount of the hydrogel crosslinked
polymer.
[0188] (4) Residual Monomer
[0189] A residual monomer (residual acrylic acid and salt thereof)
of the water-absorbent polymer after drying was analyzed in terms
of ppm by mass (with respect to the particulate water absorbent) of
the residual monomer of a particulate water absorbent, by
performing a UV analysis via a liquid chromatography on a filtrate
that has been separately prepared through two hours of agitation in
the above (2). The residual monomer of a hydrogel polymer before
the drying was calculated by: stirring a minced hydrogel polymer
containing approximately 500 mg of resin solid contents for 16
hours; performing the UV analysis via the liquid chromatography on
thus obtained filtrate; and then adjusting the solid content.
[0190] (5) Coloring Evaluation with Respect to Water-Absorbent
Polymer (Hunter's Lab Color System/L Value)
[0191] Coloring of the water-absorbent polymer was evaluated by
using a spectral calorimeter SZ-.SIGMA.80 COLOR MEASURING SYSTEM
(NIPPON DENSHOKU). A reflection measurement was selected as a
preset condition of measurement, and an accessory powder-paste
sample table having internal diameter of 30 mm and height of 12 mm
was used. Further, a powder-paste standard rounded white plate No.
2, and 30.PHI. floodlight pipe were used as a standard. About 5 g
of particulate water absorbent was provided in the built-in sample
table (so as to occupy about 60% of the built-in sample table).
Then, an L value (Lightness: lightness index) of a surface was
measured by the spectral colorimeter at room temperature (from 20
to 25.degree. C.) and humidity of 50 RH %. This value indicates
"lightness index before exposure".
[0192] At the same time, it is possible to measure other values
such as an a value and a b value (chromaticity), a YI (Yellowness
Index), and a WB (White Balance) by performing the same method with
use of the same apparatus. As the WB becomes higher and the YI, a
value, and b value become lower, the water-absorbent polymer
becomes less colored and becomes closer to substantial white.
[0193] Next, about 5 g of the particulate water absorbent was
placed in the paste sample table, and the paste sample table
containing the particulate water absorbent was exposed for 7 days
in a thermo-hygrostat (TABAI ESPEC CORPORATION, PLATINOUS LUCIFFER,
PL-2G) in which temperature had been adjusted to 70.+-.1.degree. C.
and relative humidity had been adjusted to 65.+-.1%. The exposure
was a test for promoting coloring for 7 day. After the exposure,
the L value (Lightness) of the surface was measured by the spectral
colorimeter. The value thus measured indicates "L value (Lightness)
of the particle in Hunter's Lab color system after 7-day exposure
in atmosphere at 70.+-.1.degree. C. and 65.+-.1% of relative
humidity". The a value, b value, and YI value were also measured
under the same condition.
[0194] (6) Absorbency Against Pressure (AAP)
[0195] With reference to U.S. Pat. No. 6,228,930, No. 6,071,976,
and No. 6,254,990, absorbency against pressure (under load) of the
particulate water absorbent for physiological saline was measured.
According to the method described in the United States Patents
cited above, a weight of the physiological saline absorbed in 0.9 g
of particulate water absorbent under a predetermined load (1.9 kPa
or 4.8 kPa) over 60 minutes was calculated from a weight measured
by using a balance. The same was performed without using the
particulate water absorbent so as to calculate a weight of a
physiological saline 11 absorbed in a material other than the
particulate water absorbent, such as a filter paper 7, from a
weight measured by using a balance 1. Thus measured weight
represents a blank value. Next, the absorbency against pressure
under pressure of 1.9 kPa and 4.8 kPa (g/g) were calculated by
dividing a weight of the physiological saline actually absorbed in
the particulate water absorbent (calculated by subtracting the
blank value from the measured weight of the physiological saline
absorbed) by the weight of the particulate water absorbent (0.9
g).
[0196] The measuring method is specifically described as follows.
On a bottom of a plastic supporting cylinder 100 having a 60 mm
internal diameter, a stainless metal net 101 of 400 mesh (mesh size
of 38 .mu.m) was fusion-bonded. Then, 0.900 g of a particulate
water absorbent 102 was evenly dispersed on the stainless metal net
101. Subsequently, a piston 103 and a load 104, which had been so
adjusted as to evenly apply a 4.8 kPa (0.7 psi) load onto the
particulate water absorbent, were placed in this order on the
particulate water absorbent. External diameters of the piston 103
and the load 104 were slightly smaller than 60 mm which was the
internal diameter of the supporting cylinder 100, so that there was
no gap between the piston and the supporting cylinder, and upward
and downward movements of the piston 103 and the load 104 would not
be hampered. Then, a weight W3 (g) of the entire measuring
apparatus was measured.
[0197] In a case of measuring the absorbency against pressure under
pressure of 1.9 kPa (0.3 psi), the above procedure is performed
with use of a 1.9 kPa load instead of the 4.8 kPa load.
[0198] Inside a petri dish 105 having a 150 mm diameter, a glass
filter 106 (Sougo Rikagaku Glass Seisakusho Co., Ltd.; diameter of
fine pores: 100 .mu.m to 120 .mu.m) having a 90 mm diameter was
placed. Thereafter, physiological saline 108 (20.degree. C. to
25.degree. C.) was added until it reached a level of an upper
surface of the glass filter. Then, a piece of filter paper 107
(Advantec Toyo Kaisha, Ltd.; product name: JIS P3801, No. 2;
thickness: 0.26 mm; diameter of retained particles: 5 .mu.m) having
a 90 mm diameter was placed thereon so that an entire surface of
the filter paper 107 was wetted. An excess of the physiological
saline 108 was removed.
[0199] A set of the measuring apparatus was placed on the wet
filter paper. Then, the water absorbent was made to absorb the
solution under the load. One hour later, the set of the measuring
apparatus was lifted, and a weight W4 (g) thereof was measured.
From the weights W3 and W4, the absorbency against pressure (g/g)
was calculated according to the following Mathematical Formula
4.
AAP=(W4-W3)/0.9 Formula 4
[0200] (7) Deterioration-Induced Extractable Content Ratio
[0201] A 35 mm stirrer chip was added to a 250 ml plastic container
with a cover. Then, 200.0 g of 0.90 mass % sodium chlorine aqueous
solution containing 0.05% L-ascorbic acid was measured and pored
into the plastic container, and thereafter, 1.00 g of the
particulate water absorbent which had been classified to 600/300
.mu.m was added. The plastic container was stopped tightly with an
internal cover and an external cover.
[0202] The plastic container was allowed to stand for two hours
inside a thermostat in which temperature had been adjusted to
60.+-.2.degree. C. Two hours later, the plastic container was
removed from the thermostat, and then content of the plastic
container was stirred with use of a stirrer (approximately 150 rpm)
for an hour so as to extract extractable content of the particulate
water absorbent. The extracted solution containing the extractable
content was filtered through a piece of filter paper (Advantec Toyo
Kaisha, Ltd.; product name: JIS P3801, No. 2; thickness: 0.26 mm;
diameter of retained particles: 5 .mu.m), and 50.0 g of the
filtrate obtained was used as a measuring solution. Thereafter, the
same procedure as the measurement of extractable content ratio
(neutralizing titration) was performed so as to calculate the
extractable content ratio. Thus obtained extractable content ratio
(mass %) is a deterioration-induced extractable content ratio.
[0203] (8) Moisture Content
[0204] Firstly, 1.00 g of the water-absorbent polymer was measured
and poured into an aluminum cup having a bottom surface of about 50
mm in diameter, and a total weight W5 (g) of the water-absorbent
polymer and the aluminum cup was measured. Then, the aluminum cup
was left to stand for 3 hours in an oven at 180.degree. C. so that
the water-absorbent polymer was dried. Three hours later, the
aluminum cup containing the water-absorbent polymer was removed
from the oven and cooled down to a room temperature, and
thereafter, a sum of weight W6 (g) of the aluminum cup and the
water-absorbent polymer after drying was measured. The solid
content was calculated according to the following Mathematical
Formula 5.
Moisture Content(mass %)=((W5-W6)/(weight(g) of water-absorbent
resin).times.100) Formula 5
[0205] (9) Iron (Fe) Content of Water-absorbent Polymer
[0206] 1.000 g of the particulate water-absorbent polymer was
measured and pored into a platinum crucible. The platinum crucible
was heated with use of an electrical furnace (YAMATO SCIENTIFIC
CO., LTD, Muffle Furnace FO300) so that the particulate
water-absorbent polymer contained in the platinum crucible was
ashen.
[0207] After the platinum crucible was removed from the electrical
furnace, approximately 5 ml of nitric acid aqueous solution (an
aqueous solution prepared by mixing special grade nitric acid of
Wako Pure Chemical Industries, Ltd. and an ion-exchange water in a
ratio of 1:1) was added so as to dissolve the ash. Thereafter, the
ion-exchange water was further added to obtain approximately 15 ml
of an aqueous solution of the ash.
[0208] The same procedure was followed by using a platinum crucible
without the particulate water-absorbent polymer so as to obtain a
blank.
[0209] Fe content of the aqueous solution obtained in the above
procedure was measured with ICP emission spectrometry described in
JISK1200-6. The ICP emission spectrometry apparatus used here was
ULTIMA, which is a product of HORIBA, Ltd.
Production Example 1
[0210] In a reactor formed by attaching a cover to a double-arm
type stainless kneader having a capacity of 10 liters and equipped
with two sigma type blades and a jacket, a reaction liquid was
obtained by dissolving 0.10 mol % (with respect to a total monomer)
of polyethylene glycol diacrylate to 5500 g of sodium acrylate
aqueous solution (monomer concentration was 37.7 wt %, iron content
(based on Fe.sub.2O.sub.3) was 0.1 ppm by mass, p-methoxyphenol
content was 50 ppm by mass, protoanemonin and/or furfural content
were N.D. (not detectable)) having a neutralization ratio of 75 mol
%. In the polyethylene glycol diacrylate serving as an internal
crosslinking agent, an average addition molar number n of
ethyleneoxide was 8.2. Next, a monomer solution was added to the
sigma type double-arm kneader while being maintained at 35.degree.
C., and thereafter, nitrogen gas was blown into the kneader so as
to replace inside air with nitrogen so that oxygen dissolved in a
system was not more than 1 ppm by mass. Subsequently, 35.8 g of 10
wt % sodium persulfate aqueous solution and 1.49 g of 1 wt %
L-ascorbic acid aqueous solution were added to the reaction liquid
while being stirred. About 20 seconds later, the monomer solution
reached 35.5.degree. C., and then polymerization started.
[0211] The polymerization was carried out while a gel generated by
the polymerization was being stirred. After 14 minutes from
polymerization initiation, a polymerization temperature reached its
peak temperature of 95.degree. C. After 44 minutes from the
polymerization initiation, a hydrogel crosslinked polymer (1) was
removed. Amount of extractable content of the hydrogel crosslinked
polymer (1) was 1.1%. Thus obtained hydrogel crosslinked polymer
(1) had been minced into a diameter of approximately not more than
5 mm. The minced hydrogel crosslinked polymer (1) was spread on a
metal net of 20 mesh (a mesh opening size is 850 .mu.m), and then
hot-air dried at 180.degree. C. for 45 minutes. The dried hydrogel
crosslinked polymer (1) was pulverized with a roller mill, and then
classified with a JIS standard sieves having a mesh opening size of
850 .mu.m and 150 .mu.m so as to obtain a water-absorbent polymer
(a) in which a mass median particle size was 300 .mu.m,
.sigma..zeta.=0.35, and particles having a diameter of less than
150 .mu.m accounted for 2% of a total amount of the particles. The
water-absorbent polymer (a) had a b value of 5.2 before exposure,
and a b value of 16.1 after the exposure. Other physical properties
of the water-absorbent polymer (a) are described in Table 1.
Production Example 2
[0212] The water-absorbent polymer was produced with use of an
apparatus illustrated in FIG. 3 of United States Patent Application
Publication No. 2004/0092688.
[0213] With use of an apparatus illustrated in FIG. 1 of
aforementioned United States Patent Application Publication No.
2004/0092688, a monomer solution 20 was prepared by adjusting (i)
48.5 wt % aqueous sodium hydroxide (iron content (based on
Fe.sub.2O.sub.3) was 0.5 ppm by mass) so that a flow rate thereof
was 5.12 g/s, (ii) acrylic acid (p-methoxyphenol content was 80 ppm
by mass, protoanemonin content and/or furfural content were N.D.
(not detectable)) so that the flow rate thereof was 6.10 g/s, (iii)
30 wt % polyethylene glycol diacrylate aqueous solution (I) so that
the flow rate thereof was 0.15 g/s, (iv) a solution (II) obtained
by mixing 50.0 parts by mass of 1.0 wt %
2-hydroxymethyl-2-methylpropiophenone acrylic acid solution and
50.0 parts by mass of 0.5 wt % trisodium
diethylenetriamine-pentaacetic acid aqueous solution so that the
flow rate thereof is 0.16 g/s and (v) water so that the flow rate
thereof was 4.81 g/s. A temperature of the monomer solution 20 was
stable and approximately 95.degree. C.
[0214] In the polyethylene glycol diacrylate serving as an internal
crosslinking agent, an average addition molar number n of
ethyleneoxide was 8.2.
[0215] The monomer solution 20 was stirred by use of a static mixer
formed by inserting an element (length: 18.6 mm, diameter: 6 mm)
that was twisted 540 degrees into a pipe (diameter: 6 mm), and
thereafter, 2 wt % sodium persulfate aqueous solution, which was a
polymerization initiator, was added at a flow rate of 0.151 g/s at
a point approximately 3 cm downstream of an end point of the
element, thereby obtaining a mixture liquid 40. A stirring Reynolds
number at this point was calculated to be 2280 (.rho.=1160,
.mu.=0.001). The mixture liquid 40 was supplied to a belt
polymerization apparatus 70 so as to be continuously polymerized,
thereby obtaining a band of hydrogel polymer. The belt
polymerization apparatus 70 includes: a fluoropolymer coated
endless belt (length: 3.8 m, width: 60 cm); a UV lamp located above
the endless belt; and an induction pipe located in the middle of
the belt polymerization apparatus 70, wherein a bottom surface and
surrounding area thereof of the endless belt were heated up to and
maintained at approximately 100.degree. C., and the induction pipe
collects distilled water. A length of the pipe from a point of
addition of the polymerization initiator to a discharge hole toward
a polymerization apparatus was 30 cm. The band of hydrogel polymer
whose surface temperature was approximately 70.degree. C. was
continuously crushed with use of a meat chopper, hot-air dried at
180.degree. C., and then pulverized with use of a roller mill.
Thereafter, the pulverized product was further classified and
blended by use of JIS standard sieves having a mesh opening size of
850 .mu.m and 150 .mu.m, thereby obtaining a water-absorbent
polymer (b) in which a mass median particle size was 310 .mu.m,
.sigma..zeta.=0.36, and particles having a diameter of less than
150 .mu.m accounted for 2% of a total amount of the particles.
Physical properties of the water-absorbent polymer (b) obtained are
described in Table 1.
Example 1
[0216] In a reactor formed by attaching a cover to a double-arm
type stainless kneader having a capacity of 10 liters and equipped
with two sigma type blades and a jacket, a reaction liquid was
obtained by dissolving 0.10 mol % (with respect to a total monomer)
of polyethylene glycol diacrylate to 5500 g of sodium acrylate
aqueous solution (monomer concentration was 37.7 wt %, iron content
(based on Fe.sub.2O.sub.3) was 0.1 ppm by mass, p-methoxyphenol
content was 50 ppm by mass, protoanemonin and/or furfural content
were N.D. (not detectable)) having a neutralization ratio of 75 mol
%. In the polyethylene glycol diacrylate serving as an internal
crosslinking agent, an average addition molar number n of
ethyleneoxide was 8.2. Next, a monomer solution was added to the
sigma type double-arm kneader while being maintained at 35.degree.
C., and thereafter, nitrogen gas was blown into the kneader so as
to replace inside air with nitrogen so that oxygen dissolved in a
system was not more than 1 ppm by mass. Subsequently, 35.8 g of 10
wt % sodium persulfate aqueous solution, 1.49 g of 1 wt %
L-ascorbic acid aqueous solution, and 50 ppm by mass (with respect
to a total monomer) of ethylenediaminetetra (methylene phosphonic
acid) were added to the reaction liquid while being stirred. About
20 seconds later, the monomer solution reached 35.5.degree. C., and
then polymerization started.
[0217] The polymerization was carried out while a gel generated by
the polymerization was being crushed. After 14 minutes from
polymerization initiation, a polymerization temperature reached its
peak temperature of 96.degree. C. After 44 minutes from the
polymerization initiation, a hydrogel crosslinked polymer (2) was
removed. Amount of extractable content of the hydrogel crosslinked
polymer (2) was 7.9 mass %.
[0218] The obtained hydrogel crosslinked polymer (2) had been
minced into a diameter of approximately not more than 5 mm. The
minced hydrogel crosslinked polymer (2) was spread on a metal net
of 20 mesh (a mesh opening size is 850 .mu.m), and then hot-air
dried at 180.degree. C. for 45 minutes. The dried hydrogel
crosslinked polymer (2) was pulverized with a roller mill, and then
classified with JIS standard sieves having a mesh opening size of
850 .mu.m and 150 .mu.m so as to obtain a water-absorbent polymer
(1) in which a mass median particle size was 305 .mu.m,
.sigma..zeta.=0.35, and particles having a diameter of less than
150 .mu.m accounted for 2% of a total amount of the particles.
Physical properties of the water-absorbent polymer (1) are
described in Table 1.
Example 2
[0219] Example 2 was performed in the same manner as in Example 1,
except that an additive amount of the ethylenediaminetetra
(methylene phosphonic acid) was 100 ppm by mass. The polymerization
progressed in substantially the same manner as Production Example 1
and Example 1, that is, a polymerization temperature reached its
peak temperature of 98.degree. C. after 14.5 minutes from
polymerization initiation. Physical properties of a water-absorbent
polymer (2) obtained are described in Table 1.
Example 3
[0220] In a reactor formed by attaching a cover to a double-arm
type stainless kneader having a capacity of 10 liters and equipped
with two sigma type blades and a jacket, a reaction liquid was
obtained by dissolving 0.10 mol % (with respect to a total monomer)
of polyethylene glycol diacrylate, 100 ppm by mass of
ethylenediaminetetra (methylene phosphonic acid), and 100 ppm by
mass of malic acid to 5500 g of sodium acrylate aqueous solution
(monomer concentration was 37.7 wt %, iron content (based on
Fe.sub.2O.sub.3) is 0.01 ppm by mass, p-methoxyphenol content was
70 ppm by mass, protoanemonin and/or furfural content were N.D.
(not detectable)) having a neutralization ratio of 75 mol %. In the
polyethylene glycol diacrylate serving as an internal crosslinking
agent, an average addition molar number n of ethyleneoxide was 8.2.
Next, a monomer solution was added to the sigma type double-arm
kneader while being maintained at 22.degree. C., and thereafter,
nitrogen gas was blown into the kneader so as to replace inside air
with nitrogen so that oxygen dissolved in a system was not more
than 1 ppm by mass. Subsequently, 35.8 g of 10 wt % sodium
persulfate aqueous solution and 1.49 g of 1 wt % L-ascorbic acid
aqueous solution were added to the reaction liquid while being
stirred. About 20 seconds later, the monomer solution reached
35.5.degree. C., and then polymerization started.
[0221] The polymerization was carried out while a gel generated by
the polymerization was being crushed. After 14.5 minutes from
polymerization initiation, a polymerization temperature reached its
peak temperature of 98.degree. C. After 44 minutes from the
polymerization initiation, a hydrogel crosslinked polymer was
removed.
[0222] The obtained hydrogel crosslinked polymer had been minced
into a diameter of approximately not more than 5 mm. The minced
hydrogel crosslinked polymer was spread on a metal net of 20 mesh
(a mesh opening size was 850 .mu.m), and then hot-air dried at
180.degree. C. for 45 minutes. The dried hydrogel crosslinked
polymer was pulverized with a roller mill, and then classified with
JIS standard sieves having a mesh opening size of 850 .mu.m and 150
.mu.m so as to obtain a water-absorbent polymer (3) in which a mass
median particle size was 300 .mu.m, .sigma..zeta.=0.35, and
particles having a diameter of less than 150 .mu.m accounted for 2%
of a total amount of the particles. Physical properties of the
water-absorbent polymer (3) are described in Table 1.
Example 4
[0223] A water-absorbent polymer was produced with use of an
apparatus used in Production Example 2. The water-absorbent polymer
was prepared in the same manner as in Production Example 1, except
that a flow rate of 48.5 wt % aqueous sodium hydroxide was 5.12
g/s, flow rate of acrylic acid was 6.10 g/s, flow rate of 30 wt %
polyethylene glycol diacrylate (in which an average addition molar
number n of ethyleneoxide was 8.2) aqueous solution (I) was 0.15
g/s, flow rate of a solution (III) obtained by mixing 33.4 parts by
mass of 1.0 wt % 2-hydroxymethyl-2-methylpropiophenone acrylic acid
solution and 66.6 parts by mass of 0.5 wt % ethylenediaminetetra
(methylenephosphonic acid) aqueous solution was 0.23 g/s, and flow
rate of water was 4.7 g/s.
[0224] The water-absorbent polymer obtained was a water-absorbent
polymer (4) in which a mass median particle size was 310 .mu.m,
.sigma..zeta.=0.36, and particles having a diameter of less than
150 .mu.m accounted for 2% of a total amount of the particles.
Physical properties of the water-absorbent polymer (4) are
described in Table 1.
Example 5
[0225] Example 5 was performed in the same manner as in Example 4,
except that an amount of the solution (III) obtained by mixing 21.1
parts by mass of 1.0 wt % 2-hydroxymethyl-2-methylpropiophenone
acrylic acid solution and 78.9 parts by mass of 0.5 wt %
ethylenediaminetetra (methylene phosphonic acid) aqueous solution
was 0.38 g/s.
[0226] A water-absorbent polymer obtained was a water-absorbent
polymer (5) in which a mass median particle size was 305 .mu.m,
.sigma..zeta.=0.36, and particles having a diameter of less than
150 .mu.m accounted for 2% of a total amount of the particles.
[0227] Physical properties of the water-absorbent polymer (5) are
described in Table 1.
Example 6
[0228] A surface-crosslinking agent including 0.4 parts by mass of
1,4-butanediol, 0.6 parts by mass of propylene glycol, and 3.0
parts by mass of ion exchange water was sprayed and mixed into 100
parts by mass of the water-absorbent polymer (1) obtained in
Example 1. The water-absorbent polymer (1) was then heat-treated at
210.degree. C. for 40 minutes, thereby obtaining a water absorbent
(1). The water absorbent (1) had a mass median particle size of 390
.mu.m, .sigma..zeta.=0.35, and particles having a diameter of less
than 150 .mu.m accounted for 1% of a total amount of the particles.
A b value of the water absorbent (1) before exposure was 5.1, and a
b value after the exposure was 8.5. Other physical properties of
the water absorbent (1) are described in Table 1. It should be
noted that AAP in Table 1 is an absorbency against pressure under
pressure of 4.8 kPa for 0.90 mass % sodium chloride aqueous
solution for 60 minutes. The same applies to the other Examples and
Comparative Examples.
Example 7
[0229] Example 7 was performed in the same manner as in Added
Example 5, except that a water-absorbent polymer (5) was used
instead of the water-absorbent polymer (1) of Example 6, so as to
obtain a water absorbent (2). The water absorbent (2) had a mass
median particle size of 400 .mu.m, .sigma..zeta.=0.36, and
particles having a diameter of less than 150 .mu.m accounted for 1%
of a total amount of the particles. Physical properties of the
water absorbent (2) are described in Table 1.
Example 8
[0230] 4 parts by mass of 1.0 wt % ethylenediaminetetra (methylene
phosphonic acid) aqueous solution was sprayed and mixed into 100
parts by mass of the water-absorbent polymer (4) obtained in
Example 4. The water-absorbent polymer (4) was then hot-air dried
at 60.degree. C. for 1 hour, thereby obtaining a water-absorbent
polymer (6).
[0231] A surface-crosslinking agent including 0.4 parts by mass of
1,4-butanediol, 0.6 parts by mass of propylene glycol, and 3.0
parts by mass of ion exchange water was sprayed and mixed into 100
parts by mass of the obtained water-absorbent polymer (6). The
water-absorbent polymer (6) was then heat-treated at 210.degree. C.
for 40 minutes, thereby obtaining a water absorbent (3). The water
absorbent (3) had a mass median particle size of 380 .mu.m,
.sigma..zeta.=0.35, and particles having a diameter of less than
150 .mu.m accounted for 1% of a total amount of the particles.
Physical properties of the obtained water absorbent (6) and the
water absorbent (3) are described in Table 1.
Comparative Example 1
[0232] In a reactor formed by attaching a cover to a double-arm
type stainless kneader having a capacity of 10 liters and equipped
with two sigma type blades and a jacket, a reaction liquid was
obtained by dissolving 0.10 mol % (with respect to a total monomer)
of polyethylene glycol diacrylate to 5500 g of sodium acrylate
aqueous solution (monomer concentration was 37.7 wt %, monomer
concentration was 37.7 wt %, iron content (based on
Fe.sub.2O.sub.3) was 0.1 ppm by mass, p-methoxyphenol content was
50 ppm by mass, protoanemonin and/or furfural content was N.D. (not
detectable)) having a neutralization ratio of 75 mol %. In the
polyethylene glycol diacrylate serving as an internal crosslinking
agent, an average addition molar number n of ethyleneoxide was 8.2.
Next, a monomer solution was added to the sigma type double-arm
kneader while being maintained at 35.degree. C., and thereafter,
nitrogen gas was blown into the kneader so as to replace inside air
with nitrogen so that oxygen dissolved in a system was not more
than 1 ppm by mass. Subsequently, 35.8 g of 10 wt % sodium
persulfate aqueous solution, 1.49 g of 1 wt % L-ascorbic acid
aqueous solution, and 500 ppm by mass (with respect to a total
monomer) of ethylenediaminetetra (methylene phosphonic acid) were
added to the reaction liquid while being stirred. Polymerization
did not start even after 5 minutes from the addition.
Comparative Example 2
[0233] Comparative Example 2 was performed in the same manner as in
Example 4, except that (i) a solution (IV) obtained by mixing 9.1
parts by mass of 1.0 wt % 2-hydroxymethyl-2-methylpropiophenone
acrylic acid solution and 90.9 parts by mass of 0.5 wt %
ethylenediaminetetra (methylene phosphonic acid) aqueous solution
whose flow rate was adjusted to 0.83 g/s was used instead of the
solution (III) obtained by mixing 33.4 parts by mass of 1.0 wt %
2-hydroxymethyl-2-methylpropiophenone acrylic acid solution and
66.6 parts by mass of 0.5 wt % ethylenediaminetetra (methylene
phosphonic acid) aqueous solution, and (ii) flow rate of water was
adjusted to 4.7 g/s. However, a sudden polymerization occurred and
a polymerization condition was unstable. The water-absorbent
polymer obtained was a comparative water-absorbent polymer (C-2) in
which a mass median particle size was 310 .mu.m,
.sigma..zeta.=0.36, and particles having a diameter of less than
150 .mu.m accounted for 2% of a total amount of the particles.
Physical properties of the obtained comparative water-absorbent
polymer (C-2) are described in Table 1.
Comparative Example 3
[0234] A surface-crosslinking agent including 0.4 parts by mass of
1,4-butanediol, 0.6 parts by mass of propylene glycol, and 3.0
parts by mass of ion exchange water was sprayed and mixed into 100
parts by mass of the comparative water-absorbent polymer (C-2)
obtained in Comparative Example 2. The comparative water-absorbent
polymer (C-2) was then heat-treated at 210.degree. C. for 40
minutes, thereby obtaining a comparative water absorbent (C-3).
Physical properties of the comparative water absorbent (C-3)
obtained are described in Table 1.
Comparative Example 4
[0235] 0.1 mass % of 1-hydroxyethylidene 1,1-di-phosphonic acid was
sprayed and mixed into the water-absorbent polymer (a) obtained in
Production Example 1. The water-absorbent polymer (a) was then
dried at 100.degree. C. for an hour, thereby obtaining a
comparative water-absorbent polymer (C-4). An amount of the
1-hydroxyethylidene 1,1-di-phosphonic acid added to the
water-absorbent polymer was 0.5 mass %. The b value of the
comparative water-absorbent polymer (C-4) before exposure was 9.1,
whereas the b value after the exposure was 10.1.
TABLE-US-00001 TABLE 1 DETERIORATION- EXTRACTABLE INDUCED GVs AAP
CONTENT EXTRACTABLE POLYMERIZABILITY (g/g) (g/g) (mass %) CONTENT
(mass %) PDN. EX. 1 WATER-ABSORBENT POLYMER(a) YES 33 -- 5.8 --
PDN. EX. 2 WATER-ABSORBENT POLYMER(b) YES 33 -- 6.1 -- EX. 1
WATER-ABSORBENT POLYMER(1) YES 33 -- 9.0 -- EX. 2 WATER-ABSORBENT
POLYMER(2) YES 33 -- 10.0 -- EX. 3 WATER-ABSORBENT POLYMER(3) YES
34 -- 10.5 -- EX. 4 WATER-ABSORBENT POLYMER(4) YES 32 -- 7.2 -- EX.
5 WATER-ABSORBENT POLYMER(5) YES 35 -- 8.3 -- EX. 6
WATERABSORBENT(1) -- 27 23 -- 12.0 EX. 7 WATERABSORBENT(2) -- 28 25
-- 11.5 EX. 8 WATER-ABSORBENT POLYMER(6) YES 32 -- 7.2 --
WATERABSORBENT(3) -- 27 23 -- 9.9 COM. EX. 1 -- NO -- -- -- -- (NOT
POLYMERIZED) COM. EX. 2 COM. WATER-ABSORBENT NO** 37 -- 16.4 --
POLYMER(C-2) COM. EX. 3 COM. WATERABSORBENT(C-3) -- 32 15 -- 19.5
COM. EX. 4 COM. WATER-ABSORBENT -- 29 7 4.8 -- POLYMER(C-4) L VALUE
(Lightness) IN HUNTER'S RESIDUAL LAB COLORSYSTEM MOISTURE CONTENT
MONOMER BEFORE BEING AFTER BEING (mass %) (ppm by mass) EXPOSED
EXPOSED* PDN. EX. 1 WATER-ABSORBENT POLYMER(a) 5.0 170 91 63 PDN.
EX. 2 WATER-ABSORBENT POLYMER(b) 5.2 500 91 69 EX. 1
WATER-ABSORBENT POLYMER(1) 4.2 200 92 75 EX. 2 WATER-ABSORBENT
POLYMER(2) 4.8 210 91 81 EX. 3 WATER-ABSORBENT POLYMER(3) 4.4 200
92 81 EX. 4 WATER-ABSORBENT POLYMER(4) 4.1 400 90 80 EX. 5
WATER-ABSORBENT POLYMER(5) 4.0 520 91 82 EX. 6 WATERABSORBENT(1)
1.0 -- 92 80 EX. 7 WATERABSORBENT(2) 1.2 -- 90 80 EX. 8
WATER-ABSORBENT POLYMER(6) 4.5 400 90 82 WATERABSORBENT(3) 1.3 --
90 82 COM. EX. 1 -- -- -- -- -- COM. EX. 2 COM. WATER-ABSORBENT 5.5
910 91 80 POLYMER(C-2) COM. EX. 3 COM. WATERABSORBENT(C-3) 1.2 --
91 80 COM. EX. 4 COM. WATER-ABSORBENT 6.7 200 90 71 POLYMER(C-4)
Abbreviation: PDN. stands for PRODUCTION COM. stands for
COMPARATIVE EX. stands for EXAMPLE *After 7-day exposure to
atmosphere of 70 .+-. 1.degree. C. of temperature and 65 .+-. 1% of
relative humidity. **Polymerization proceeds too fast and its
condition is unstable.
[0236] As shown in Table 1, the water-absorbent polymer or the
water absorbent according to the present invention exhibits such a
high L value of 75 or higher after being exposed. This proves that
the water-absorbent polymer or the water absorbent according to the
present invention well achieves white condition of particles, which
gives clean impression. It can also be said that, from the fact
that the white condition was achieved even with a drying
temperature during the production process of as high as 180.degree.
C. (150.degree. C. or higher), deterioration caused by breakage of
main-chain in the polymer was prevented. Further, the
water-absorbent polymer or the water absorbent according to the
present invention exhibits preferred values of GVs. Furthermore,
the water absorbents (1) to (3) have deterioration-induced
extractable content in an amount of 12.0 or less; that is, a
possibility of a gel becoming extractable is extremely low.
[0237] The present invention makes it possible to obtain an
extremely high-quality water-absorbent polymer and water absorbent
that improve in long-term color stability and in urine tolerance
while maintaining an absorbing property. It can be said that this
proves that the present invention is beneficial.
INDUSTRIAL APPLICABILITY
[0238] The present invention provides an excellent particulate
water-absorbent polymer that achieves (i) long-term color stability
and urine tolerance and (ii) absorbing property, which are in such
a trade-off relationship. The present invention is applicable in a
variety of fields such as sanitary materials such as disposable
diapers and sanitary napkins; pet sheets; water stop materials; and
the like.
* * * * *