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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a method for producing improved
water-absorbent resins.
Water-absorbent resins are used in the manufacture of sanitary napkins,
tampons, diapers, disposable dustcloths and other sanitary goods, and as
water-retaining agents in the fields of agriculture and horticulture. They
are also used for the purpose of coagulation of sludge, prevention of dew
condensation on building materials, dehydration of oil, and so on.
Known water-absorbent resins of this type include crosslinked
carboxymethylcellulose, partially crosslinked polyethylene oxide,
hydrolyzates of starch-acrylonitrile graft copolymers, partially
crosslinked polyacrylic acid salts, vinyl alcohol-acrylic acid salt
copolymers, and so on. However, each of them has its drawbacks, such as
unsatisfactory absorbency, poor gel strength in spite of high absorbency
(if attained), formation of sticky gel by water absorption, or slow rate
of water absorption.
It is known that the rate of water absorption can be increased by
increasing the density of crosslinking of a water-absorbent resin, thereby
lowering the water-absorbency thereof. However, this method is
undesirable, since the absorbency which is the most important
characteristic of the water-absorbent resin is reduced, because the
crosslinking density becomes too high.
Another known method of increasing the rate of water absorption of a
water-absorbent resin comprises admixing the water-absorbent resin with
water in the presence of a hydrophilic organic solvent such as a
monohydric lower alcohol to dissolve or disperse water in the alcohol,
whereby water is absorbed by the resin substantially uniformly,
crosslinking the resin with water substantially uniformly absorbed
therein, and then drying. In practicing this method, it is considered
preferable from the viewpoint of the characteristics of water-absorbent
resin to conduct the crosslinking in a state in which a large amount of
water is absorbed by the resin. However, in practice, the amount of water
is limited and, in addition, aggregation of resin particles which are in
the swollen state may occur even when the amount of water absorbed by the
resin is small, thus easily leading to lump formation. The method is thus
poor in workability, which renders the method less suited for commercial
purposes. Therefore, it is necessary, in practicing the method, to place
water-absorbent resin particles in a water-swollen state by adding a small
amount of water in the presence of a large amount of a hydrophilic organic
solvent, thereby preventing aggregation of resin particles, which
otherwise will occur, during crosslinking. If conducted in such a manner,
this method will encounter such problems as high production cost and low
productivity.
It is an object of the present invention to provide a modified
water-absorbent resin having a good water absorbency and a high water
absorption rate and capable of forming a gel having a high strength and
nonstickiness by water absorption.
A further object of the present invention is to provide a method for
producing the modified water-absorbent resin easily and inexpensively in a
good efficiency.
These and other objects of the present invention will become apparent from
the description hereinafter.
SUMMARY OF THE INVENTION
It has now been found that the above objects can be attained by using a
powder of an inert inorganic material in crosslinking water-absorbent
resins without need of any hydrophilic organic solvents which have been
used as essential components in the prior art.
In accordance with the present invention, there is provided a method for
producing an improved water-absorbent resin which comprises allowing a
water-absorbent resin to absorb a crosslinking agent and water in the
presence of a powder of an inert inorganic material, and heating the
resulting mixture with agitation, thereby effecting the crosslinking
reaction of said resin and removal of water, said resin containing units
of a monomer having a carboxyl group in the form of free acid or a salt as
a constituent component thereof.
DETAILED DESCRIPTION
The water-absorbent resins which can be modified by the method of the
present invention are not particularly limited, so long as they contains,
among the constituent components of a homopolymer or copolymer, a monomer
unit having a carboxyl group in the form of free acid or a salt. The
water-absorbent resins used in the present invention includes, for
instance, crosslinked acrylic or methacrylic acid polymers, crosslinked
polysaccharide-acrylic or methacrylic acid graft copolymers, crosslinked
acrylic or methacrylic acid-acrylamide-sulfonated acrylamide terepolymers,
and the alkali metal or alkaline earth metal salts thereof, e.g.
crosslinked products of acrylic acid (or its salt) homopolymer, acrylic
acid (or its salt)-methacrylic acid (or its salt) copolymers and
starch-acrylic acid (or its salt) graft copolymers; crosslinked
polysaccharide-alkyl acrylate or methacrylate graft copolymer
hydrolyzates, crosslinked polysaccharide-acrylonitrile graft copolymer
hydrolyzates, and crosslinked polysaccharide-acrylamide copolymer
hydrolyzates, e.g. crosslinked products of hydrolyzed starch-ethyl
acrylate graft copolymer, hydrolyzed starch-methyl methacrylate graft
copolymer, hydrolyzed starch-acrylonitrile graft copolymer and hydrolyzed
starch-acrylamide graft copolymer; crosslinked alkyl acrylate or
methacrylate-vinyl acetate copolymer hydrolyzates, e.g. crosslinked
products of hydrolyzed ethyl methacrylate-vinyl acetate copolymer and
hydrolyzed methyl acrylate-vinyl acetate copolymer; crosslinked
starch-acrylonitrile-acrylamide-2-methylpropanesulfonic acid graft
copolymer hydrolyzates; crosslinked starch-acrylonitrile-vinylsulfonic
acid graft copolymer hydrolyzates; crosslinked sodium
carboxymethylcellulose, and the like. These may be used alone or in
admixture thereof.
Preferred among the above-mentioned water-absorbent resins are crosslinked
acrylic or methacrylic acid polymers, crosslinked polysaccharide-acrylic
or methacrylic acid graft copolymers, crosslinked acrylic or methacrylic
acid-acrylamide-sulfonated acrylamide terepolymers, and alkali metal or
alkaline earth metal salts of these polymers.
The water-absorbent resins are employed in the form of particles. So long
as they are in the form of particles such as powder or granule, the
particle size and shape therof are not particularly limited. Generally,
however, a particle size of about 10 to 600 meshes is preferred.
The inert inorganic material used in powder form in the present invention
includes, for instance, silicon dioxide powder, hydrated aluminum oxide
powder, hydrated titanium oxide powder, anhydrides of these metal oxides,
and powders of materials containing these metal oxide hydrates or
anhydrides as main components. They may be used alone or in admixture
thereof. The crystal system of the inorganic material is not critical. For
instance, in case of aluminum oxide powder, the .alpha., .beta. and
.gamma. forms can be used equally. Also, in case of titanium oxide, it may
be TiO, Ti.sub.2 O.sub.3 or TiO.sub.2. Furthermore, in case of hydrate
powders, the degree of hydration is not critical. Thus, for instance,
Al.sub.2 O.sub.3.H.sub.2 O powder, Al.sub.2 O.sub.3.2H.sub.2 O powder and
Al.sub.2 O.sub.3.3H.sub.2 O powder are equally usable as aluminum oxide
hydrate powder, and the titanium dioxide hydrate powder may be
TiO.sub.2.H.sub.2 O powder or TiO.sub.2. 2H.sub.2 O powder. Examples of
the above-mentioned powders of materials containing the metal oxide
hydrates or anhydrides as main components are, for instance, those mainly
containing hydrated silicon dioxide and/or anhydrous silicon dioxide
(hereinafter referred to as "fine silica") such as colloidal silica, white
carbon and ultrafine silica, those mainly containing hydrated and/or
anhydrous aluminum oxide, such as plate hydrated alumina and fibrous
hydrated alumina, and those mainly containing hydrated and/or anhydrous
titanium oxide of rutile or anatase type. Among these inert inorganic
powders preferred are fine silica, titanium dioxide powder and alumina
powder. The inorganic powder preferably has an average particle size of
0.001 to 10 .mu.m, especially 0.005 to 1 .mu.m. In any case, it is
preferable that the inorganic powder has a property of improving the
dispersibility of the water-absorbent resin particles which are in the
swollen state by water absorption, namely improving the flowability
thereof.
The crosslinking agent used in the present invention are those having two
or more functional groups capable of reacting with a functional group
present in the water-absorbent resin to be modified such as carboxyl group
or its salt form, hydroxyl group, sulfo group, amino group or the like.
Any of such crosslinking agents can be used without any particular
limitations. Such crosslinking agents includes, for instance, diglycidyl
ether compounds, polyvalent metal salts, haloepoxy compounds, aldehyde
compounds, isocyanate compounds, and the like.
Typical examples of the above-mentioned glycidyl ether compounds for, for
instance, ethylene or polyethylene glycol diglycidyl ether, propylene or
or polypropylene glycol diglycidyl ether and glycerol or polyglycerol
diglycidyl ether. Among them the most preferred is ethylene glycol
diglycidyl ether.
As the above-mentioned polyvalent metal salts, there are mentioned
compounds capable of forming crosslinkages by ionic reaction with the
functional group such as carboxyl group present in the water-absorbent
resin. Typical examples thereof are halides, sulfates and nitrates of
bivalent metals (e.g. magnesium, calcium, barium, zinc) or trivalent
metals (e.g. aluminum, iron) and, more particularly, magnesium sulfate,
aluminum sulfate, ferric chloride, calcium chloride, magnesium chloride,
aluminum chloride, poly(aluminum chloride), ferric nitrate, calcium
nitrate and aluminum nitrate.
Typical examples of the above-mentioned haloepoxy compounds are, for
instance, epichlorohydrin, epibromohydrin and
.alpha.-methylepichlorohydrin. Typical examples of the aldehyde compounds
are, for instance, glutaraldehyde and glyoxal. Typical examples of the
isocyanate compounds are, for instance, 2,4-tolylene diisocyanate and
hexamethylene diisocyanate.
The crosslinking agents may be used alone or in admixture thereof. It is
preferable that an adequate crosslinking agent is selected depending on
the kind of the water-absorbent resin to be modified. The purpose of their
use is to provide again the water-absorbent resin with a crosslinking
structure, thereby improving the properties of the water-absorbent resin
to be modified. Among the crosslinking agents mentioned above, diglycidyl
ether compounds, polyvalent metal salts and haloepoxy compounds are
particularly suited for this purpose.
In the present invention, a mixture of water-absorbent resin particles,
water, a powder of an inorganic material and a crosslinking agent is
agitated at an elevated temperature to cause the crosslinking of the
resin, while distilling away water. Water and the crosslinking agent are
absorbed by the resin, and the resin particles are in the semi-swollen
state upon agitation. There are used, per 100 parts by weight of a
water-absorbent resin to be modified, 0.1 to 30 parts by weight,
preferably 1 to 20 parts by weight, of an inert inorganic material powder,
and 5 to 65 parts by weight, preferably 10 to 50 parts by weight, of
water.
When the amount of the inert inorganic powder is less than 0.1 part by
weight per 100 parts by weight of the water-absorbent resin, agitation of
the resin in the semi-swollen state easily results in aggregation of the
resin particles, leading to failure in uniform progress of the
crosslinking reaction or making the crosslinking reaction itself difficult
to progress. Also, even if the inorganic material is used in an amount
exceeding 30 parts by weight, an additional effect is scarcely obtained,
and rather a tendency to decrease the absorbency per unit weight of the
water-absorbent resin may appear.
When water is used in an amount of less than 5 parts by weight per 100
parts by weight of the water-absorbent resin, the water-absorbent resin
modified by further crosslinking is still insatisfactory in strength and
stickiness of the gel formed by water absorption. When the amount of water
is more than 65 parts by weight, aggregation of the resin particles in the
semi-swollen state take place, leading to lump formation, and accordingly
the crosslinking reaction does not uniformly proceed. When water is used
in an amount of 5 to 65 parts by weight, preferably 10 to 50 parts by
weight, there can be obtained a modified water-absorbent resin which has a
good absorbency and is high both in rate of water absorption and in gel
strength and does not become sticky upon water absorption. Moreover, the
method of the invention does not require the use of any hydrophilic
organic solvent as in the prior art, and a lump formation due to
aggregation of swollen resin particles can be prevented by the presence of
the inert inorganic powder alone, thus the reaction system is made
homogeneous and the crosslinking reaction can be easily carried out in the
state that agitation is quite possible. Furthermore, as stated above, no
organic solvent is used at all in the present invention and, therefore,
the volumetric efficiency in production of water-absorbent resin (yield
per unit volume of apparatus) can be increased to a great extent. In
addition, steps for the organic solvent recovery and purification are not
required and this can contribute to reducing the cost of water-absorbent
resin production.
The amount of the crosslinking agent varies depending on the kinds of
crosslinking agent and water-absorbent resin to be modified, the amount of
water, the kind and amount of the inert inorganic powder, the intended
purpose of the water-absorbent resin and other factors. Generally, the
crosslinking agent is used in an amount of about 0.005 to 5.0%, preferably
0.01 to 1.0%, based on the water-absorbent resin used. Generally, the use
of a smaller amount of the crosslinking agent than 0.005% produces little
modification effects, and when the amount is greater than 5%, the degree
of crosslinking tends to become so high that the absorbency is decreased.
The modified water-absorbent resin of the present invention is produced,
for example, by admixing a water-absorbent resin to be modified with a
powder of an inert inorganic material, then adding an aqueous solution of
a crosslinking agent with stirring or, alternatively, adding a
crosslinking agent and water separately with stirring, raising the
temperature of the reaction system to a predetermined level to conduct the
crosslinking reaction, and continuing the reaction, while removing the
added water from the system under ordinary pressure or reduced pressure,
to give the desired water-absorbent resin.
Another method of producing the modified water-absorbent resin comprises
admixing a water-absorbent resin to be modified with a powder of an
inorganic material, heating the mixture to a predetermined temperature,
adding thereto an aqueous solution of a crosslinking agent (or a
crosslinking agent and water separately) with stirring, and then
maintaining the mixture at a predetermined temperature with stirring to
thereby effect the crosslinking reaction and drying.
In the methods as mentioned above, a manner of adding the crosslinking
agent and water is not particularly limited. Any manners are adoptable so
long as predetermined amounts of the crosslinking agent and water can be
substantially uniformly added to the water-absorbent resin particles. From
the industrial standpoint, the so-called showering method and spraying
method are preferred.
A manner of stirring conducted during the addition of crosslinking agent
and water to the resin particles or during the subsequent crosslinking
reaction is not particularly limited. Any manners which can achieve
substantially uniform mixing of these components are adoptable. For
example, stirrers, pneumatic stirrers, kneaders and pipeline mixers, with
various types and shapes of stirring blades, can be used.
The temperature condition suited for smoothly conducting the crosslinking
reaction varies depending on the kind of crosslinking agent used, the kind
and amount of inert inorganic powder, the intended purpose of the modified
water-absorbent resin and other factors, and accordingly cannot be
specifically given. It is generally preferred, however, to carry out the
reaction within the temperature range of 40.degree. to 150.degree. C.
The modified water-absorbent resin obtained by the method of the invention
has a high absorbency and can absorb water at a high absorption rate.
Also, it gives a gel which is unsticky and has high gel strength.
Furthermore, in accordance with the present invention, the
water-absorbent, resin modified as mentioned above can be produced easily
and efficiently.
The present invention is more specifically described and explained by means
of the following Examples. It is to be understood that the present
invention is not limited to the Examples, and various changes and
modifications may be made in the invention without departing from the
spirit and scope thereof.
EXAMPLE 1
A 300 ml three-necked separable flask was charged with 100 g of a powder of
a crosslinked polyacrylic acid potassium salt (commercially available
under the trade mark "Arasorb" made by Arakawa Kagaku Kogyo Kabushiki
Kaisha) and 3 g of finely divided silica (commercially available under the
trade mark "Aerosil 200" made by Nippon Aerosil Kabushiki Kaisha; 0.012
.mu.m in average particle size). They were stirred well using a stirrer,
and a solution of 0.20 g of ethylene glycol diglycidyl ether dissolved in
25 g of water was added gradually to the flask, while stirring until a
uniform dispersion was obtained. The resultant mixture was then heated at
about 120.degree. C. for about 1 hour to crosslink the polymer, while
distilling away water. Thereafter, the residual water was distilled away
under reduced pressure (about 30 mmHg) for about 10 minutes to give 95 g
of a modified water-absorbent resin.
The obtained water-absorbent resin was subjected to measurement of water
absorbency, water absorption rate, gel strength and gel stickiness by the
procedures mentioned below.
The results are shown in Table 1 with the result of observation of the
state of the water-absorbent resin during the crosslinking reaction.
[Water absorbency]
To a 200 ml beaker were added 150 g of deionized water and 0.12 g of the
water-absorbent resin obtained in accordance with the present invention.
After allowing to stand for 30 minutes, the resin was filtered off through
a 200-mesh wire net, and the effluent water was weighed and the absorbency
was calculated according to the following equation.
##EQU1##
[Rate of water absorption]
In a 100 ml beaker were placed 50 g of physiological saline water (0.9% by
weight aqueous solution of sodium chloride) and a stirring bar. While
stirring at 600 r.p.m. on a magnetic stirrer, 2.0 g of a water-absorbent
resin was added, whereby gelation took place due to water absorption and
swelling, leading to decrease in fluidity and disappearance of the eddy
around the center of stirring. The time from the addition of the resin to
the disappearance of the eddy was measured and shown as an index for the
rate of water absorption.
[Gel strength]
A gel was formed by mixing 60 g of physiological saline water with 2.0 g of
a water-absorbent resin (this gel being hereinafter referred to as
"30-fold gel"), and the hardness of the gel was measured using
Neocurdometer made by Iio Denki Kabushiki Kaisha. The hardness means the
elastic force at breakage of the gel.
[Stickiness of gel]
In general, there is a tendency that materials which show a breaking force
do not reveal a consistency, whereas materials which show a consistency do
not reveal a breaking force. Therefore, the breaking force or consistency
of the 30-fold gel was measured by the Neocurdometer, and the stickiness
of the gel was estimated in terms of the measured value. The breaking
force as used herein means a force required to break or rupture the
elastic body against the limit of elastic force, and the consistency as
used herein means an apparent viscosity acting in the form of frictional
force in opposition to gel flow.
EXAMPLES 2 TO 13 AND COMPARATIVE EXAMPLES 2 AND 5
Modified water-absorbent resins were prepared in the same manner as in
Example 1 except that the reaction systems shown in Table 1 were employed,
and the physical properties thereof was estimated.
The results are shown in Table 1.
COMPARATIVE EXAMPLE 1
In a 1 liter three-necked separable flask was placed 100 g of a crosslinked
polyacrylic acid potassium salt (commercially available under the trade
mark "Arasorb" made by Arakawa Kagaku Kogyo Kabushiki Kaisha), and 200 g
of methanol was added to the flask and stirred thoroughly by a stirrer to
disperse the polymer particles. To the flask was added dropwise a solution
of 0.20 g of ethylene glycol diglycidyl ether dissolved in a mixture of
100 g of methanol and 30 g of water to give a slurry. The slurry was
heated with stirring to conduct crosslinking of the polymer, while
distilling away methanol and water. The residual methanol and water were
then distilled away under reduced pressure (30 mmHg) for 30 minutes to
give 90 g of a modified water-absorbent resin.
The obtained modified water-absorbent resin was estimated in the same
manner as in Example 1. The results are shown in Table 1 with the state of
the resin observed during the crosslinking reaction.
COMPARATIVE EXAMPLES 3, 4 AND 6
The procedures of Example 1 were repeated except that the reaction system
shown in Table 1 were employed, but no desired products were obtained due
to occurrence of blocking of the polymer particles during the addition of
water or the crosslinking reaction.
TABLE 1
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Reaction system (g)
Water-absorbent resin
Inorganic powder
Crosslinking agent
Solvent
__________________________________________________________________________
Ex. 1 Arasorb
(100) Aerosil 200
(3)
EGDG (0.2)
water (25)
Ex. 2 Resin A
(100) " (3)
" (0.2)
" (25)
Ex. 3 Resin B
(100) " (3)
" (0.2)
" (25)
Ex. 4 Arasorb
(100) " (3)
" (0.2)
" (10)
Ex. 5 " (100) " (3)
" (0.2)
" (50)
Ex. 6 " (100) " (3)
" (0.05)
" (25)
Ex. 7 " (100) " (3)
" (1.0)
" (25)
Ex. 8 " (100) " (1)
" (0.2)
" (25)
Ex. 9 " (100) " (10)
" (0.2)
" (25)
Ex. 10
" (100) TiO.sub.2
(3)
" (0.2)
" (25)
Ex. 11
" (100) Al.sub.2 O.sub.3
(3)
" (0.2)
" (25)
Ex. 12
" (100) Aerosil 200
(3)
Epichlorohydrin
(0.3)
" (25)
Ex. 13
" (100) " (3)
MgCl.sub.2
(0.6)
" (25)
Com. Ex. 1
" (100 -- EGDG (0.2)
methanol
(300)
water (30)
Com. Ex. 2
" (100) Aerosil 200
(3)
" (0.2)
methanol
(25)
Com. Ex. 3
" (100) -- " (0.2)
water (25)
Com. Ex. 4
" (100) Aerosil 200
(20)
" (0.2)
" (120)
Com. Ex. 5
" (100) " (3)
" (0.2)
" (3)
Com. Ex. 6
" (100) " (0.5)
" (0.2)
" (25)
__________________________________________________________________________
Physical properties of gel
Yield
State during
Water Rate of water
Hardness
Breaking forth
Consistency
(g) reaction
absorbency
absorption (sec.)
(dyne/cm.sup.2)
(dyne/cm.sup.2)
(dyne/cm.sup.3)
__________________________________________________________________________
Ex. 1 95 good 330 0.6 38.32 .times. 10.sup.3
12.94 .times. 10.sup.4
--
Ex. 2 96 " 360 0.8 30.77 .times. 10.sup.3
9.76 .times. 10.sup.4
--
Ex. 3 96 " 310 0.5 41.27 .times. 10.sup.3
14.21 .times. 10.sup.
--
Ex. 4 96 " 460 0.7 28.61 .times. 10.sup.3
10.22 .times. 10.sup.4
--
Ex. 5 96 " 290 0.4 43.29 .times. 10.sup.3
15.30 .times. 10.sup.4
--
Ex. 6 95 " 620 0.9 25.20 .times. 10.sup.3
8.24 .times. 10.sup.4
--
Ex. 7 96 " 190 0.4 44.02 .times. 10.sup.3
15.89 .times. 10.sup.4
--
Ex. 8 94 " 360 0.6 31.73 .times. 10.sup.3
10.12 .times. 10.sup.4
--
Ex. 9 103 " 300 0.5 37.31 .times. 10.sup.3
14.03 .times. 10.sup.4
--
Ex. 10
95 " 410 0.8 30.08 .times. 10.sup.3
10.00 .times. 10.sup.4
--
Ex. 11
95 " 370 0.6 33.72 .times. 10.sup.3
11.71 .times. 10.sup.4
--
Ex. 12
95 " 460 0.7 28.91 .times. 10.sup.3
8.76 .times. 10.sup.4
--
Ex. 13
95 " 280 0.4 39.11 .times. 10.sup.3
13.19 .times. 10.sup.4
--
Com. Ex. 1
90 " 350 0.5 36.37 .times. 10.sup.3
12.74 .times. 10.sup.4
--
Com. Ex. 2
93 " 760 0.6 0.62 .times. 10.sup.3
-- 4.8 .times. 10.sup.4
Com. Ex. 3
-- blocking
-- -- -- -- --
Com. Ex. 4
-- blocking
-- -- -- -- --
Com. Ex. 5
92 good 690 0.7 7.53 .times. 10.sup.3
-- 5.02 .times. 10.sup.4
Com. Ex. 6
-- blocking
-- -- -- -- --
__________________________________________________________________________
(Notes)
Arasorb: crosslinked polyacrylic acid potassium salt made by Arakawa
Kagaku Kogyo Kabushiki Kaisha
Resin A: commercially available crosslinked starchsodium acrylate graft
copolymer
Resin B: crosslinked acrylamide/potassium acrylate/potassium
2acrylamide-2-methylpropanesulfonate terpolymer in a molar ratio of 3/4/3
Aerosil 200: finely divided silica made by Nippon Aerosil Kabushiki Kaish
EGDG: ethylene glycol diglycidyl ether
In addition to the ingredients used in the Examples, other ingredients can
be used in the Examples as set forth in the specification to obtain
substantially the same results.
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