Water-absorbent agent powders and manufacturing method of the same

A manufacturing method of water-absorbent agent powders of the present invention is a method of a reducing an amount of a residue of an epoxy crosslinking agent remaining therein by adding a nucleophilic reagent in a form of powder to surface region crosslinked water-absorbent resin powders having a carboxyl group under an applied heat in which the residue of the crosslinking agent remains. Since the method permits an amount of the residue of the crosslinking agent to be reduced by adding a nucleophilic reagent to the heated water-absorbent resin powders, the water-absorbent agent powders exhibiting well-balanced properties which are mutually negatively correlated from one another, i.e., high absorbency under pressure, a reduced amount of a residue of the epoxy crosslinking agent and a high absorbing rate compared with the conventional surface region crosslinked water-absorbent resin powders can be achieved. Such water-absorbent agent powders are suitably used in sanitary materials such as disposable diaper, sanitary napkins, etc.

FIELD OF THE INVENTION 
The present invention relates to water-absorbent agent powders and a 
manufacturing method of the same, and more particularly to water-absorbent 
agent powders befitting sanitary materials, which manifest as high an 
absorbency under pressure as without pressure, excel in safety as evinced 
by the absence of a residue of a cross-linking agent in the resin, while 
manifesting a high absorbing rate, and also relates to a manufacturing 
method of the same. 
BACKGROUND OF THE INVENTION 
In recent years, a water-absorbent resin is widely used in sanitary goods 
such as disposable diapers, sanitary napkins, etc., for the purpose of 
absorbing and holding body fluid such as urine, blood, etc., to prevent 
clothes from being contaminated. 
Examples of the conventionally known water-absorbent resins may be but are 
not limited to: 
a partially neutralized crosslinked polymer of polyacrylic acid (Japanese 
Unexamined Patent Publication No. 84304/1980 (Tokukaisho 55-84304), 
Japanese Unexamined Patent Publication No. 108407/1980 (Tokukaisho 
55-108407) and Japanese Unexamined Patent Publication No. 133413/1980 
(Tokukaisho 55-133413)); 
a hydrolyzed graft polymer of starch-acrylonitrile (Japanese Examined 
Patent Publication No. 43995/1971 (Tokukosho 46-43995)); 
a neutralized graft polymer of starch-acrylic acid (Japanese Unexamined 
Patent Publication No. 125468/1976 (Tokukaisho 51-125468)); 
a saponified copolymer of vinyl acetate-acrylic ester (Japanese Unexamined 
Patent Publication No. 14689/1977 (Tokukaisho 52-14689)); 
cross-linked carboxymethyl cellulose (U.S. Pat. No. 4,650,716 and U.S. Pat. 
No. 4,689,408); 
a hydrolyzed copolymer of acrylonitrile or of acrylamide, or a cross-linked 
copolymer of both (Japanese Unexamined Patent Publication No. 15959/1978 
(Tokukaisho 53-15959)); 
a crosslinked polymer of cationic monomer (Japanese Unexamined Patent 
Publication No. 154709/1983 (Tokukaisho 58-154709) and Japanese Unexamined 
Patent Publication No. 154710/1983 (Tokukaisho 58-154710)); 
cross-linked isobutylene-maleic anhydrous copolymers (U.S. Pat. No. 
4,389,513); and 
cross-linked copolymers of 2-acryl-amide-2-methylpropanesulfonic acid with 
acrylic acid (EP068189). 
The properties which the water-absorbent resins are desired to possess 
include, for example, high absorbency and high absorbing rate to be 
manifested on contact with aqueous liquids, liquid permeability, high 
strength exhibited by the gel swollen with liquid, and ability to aspirate 
water from the substrate impregnated with aqueous liquid, less residue of 
monomer (U.S. Pat. No. 4,794,166). 
These properties are not necessarily correlated positively to one another, 
and a problem arises, for example, in that such properties as the liquid 
permeability, the gel strength, and the absorbing rate are lowered in 
proportion as the absorbency is heightened. 
As a means to improve the various water-absorbent properties of the 
water-absorbent resin in finely balanced levels, the technique of 
cross-linking the surface regions of the water-absorbent resin has been 
known. Various methods have been proposed concerning the technique. 
For example, a method using polyhydric alcohols (Japanese Unexamined Patent 
Application No. 108233/1983 (Tokukaisho 58-108233), and Japanese 
Unexamined Patent Application No. 16903/1986 (Tokukaisho 61-16903)), a 
method using polyglycidyl compounds, poly aziridine compounds, polyamine 
compounds, and polyisocyanate compounds (Japanese Unexamined Patent 
Application No. 189103/1984 (Tokukaisho 59-189103) and (U.S. Pat. No. 
4,666,893)), methods using glyoxal (Japanese Unexamined Patent Application 
No. 117393/1977 (Tokukaisho 52-117393)) , methods using polyvalent metals 
(Japanese Unexamined Patent Application No. 136588/1976 (Tokukaisho 
51-136588), Japanese Unexamined Patent Application No. 257235/1986 
(Tokukaisho 61-257235) and Japanese Unexamined Patent Application No. 
7745/1987 (Tokukaisho 62-7745)), methods using a silane coupling agent 
(Japanese Unexamined Patent Application No. 211305/1986 (Tokukaisho 
61-211305), Japanese Unexamined Patent Application No. 252212/1986 
(Tokukaisho 61-252212) and Japanese Unexamined Patent Application No. 
264006/1986 (Tokukaisho 61-264006)), a method using a monoepoxy compound 
(Japanese Unexamined Patent Application No. 87638/1992 (Tokukaihei 
4-87638)), a method using a polymeric compound having an epoxy group (U.S. 
Pat. No. 4,758,617), a method using an epoxy compound and a hydroxy 
compound (Japanese Unexamined Patent Application No. 132103/1990 
(Tokukaihei 2-132103)), and a method using an alkylene carbonate 
(DE-4020780) have been known. 
Other than the described methods, methods requiring the presence of an 
inorganic inactive powder (Japanese Unexamined Patent Application No. 
163956/1985 (Tokukaisho 60-163956) and Japanese Unexamined Patent 
Application No. 255814/1985 (Tokukaisho 60-255814)), a method requiring 
the presence of a dihydric alcohol (Japanese Unexamined Patent Application 
No. 292004/1989 (Tokukaihei 1-292004)), a method requiring the presence of 
water and an ether compound (Japanese Unexamined Patent Application No. 
153903/1990 (Tokukaihei 2-153903)), a method requiring the presence of a 
water-soluble polymer (Japanese Unexamined Patent Application No. 
126730/1991 (Tokukaihei 3-126730)), and a method requiring the presence of 
the alkylene oxide additive of a monohydric alcohol, an organic acid salt, 
lactam, etc. (EP-0555692, and U.S. Pat. No. 5,322,896), a method of mixing 
a surface crosslinking agent with heated water-absorbent resin in which 
not less than 90 percent of particles have a size of not less than 250 
.mu.m (Japanese Unexamined Patent Application No. 224204/1995 (Tokukaihei 
7-224204)) and a method of mixing a reducing agent with a surface 
crosslinking agent (U.S. Pat. No. 5,382,610) have been known. 
Also, to attain improved properties of surface region crosslinked 
particles, a method of granulating the particles with an aqueous solution 
(U.S. Pat. No. 5,369,148), and a method of adding a cationic polymer 
having a molecular weight of not less than 2,000 in order to be fixed with 
the fiber material (U.S. Pat. No. 5,3282,610) have been known. 
Further, EP-668080 published on Aug. 23, 1995 discloses a method in which a 
surface crosslinkage is carried out by adding organic acid/inorganic 
acid/polyamino acid. 
The described methods permit some improvements in a balance of various 
properties of the water-absorbent resin, yet further improvements are 
needed to reach a desirable level. This has led to the need for further 
developments to attain improved properties of the water-absorbent resin. 
In recent years, such a water-absorbent resin that excels in basic 
water-absorbent properties under pressure, especially in absorbency under 
pressure, while maintaining as high absorbency without pressure as the 
conventional water-absorbent resin has been strongly demanded. 
Specifically, the demand for the water-absorbent resin which excels in 
absorbency under high pressure (for example, under load of 50 g/cm.sup.2) 
and exhibits high absorbency even under heavy load has been increasing. 
Here, a heavy load is defined to be a load incurred when not only a baby 
weighing around 10 Kg but also an adult person uses a sanitary material 
including a water-absorbent resin. 
To meet the described needs, an improvement in crosslinkage to be applied 
to the surface regions of the water-absorbent resin are essential. To be 
specific, in order to meet the demands, it is required to increase the 
degree of crosslinkage of the surface regions. To do so, an amount of use 
of the surface crosslinking agent is increased or in order to uniformly 
crosslink only the surface regions, an amount of water or solvent to be 
added with the crosslinking agent is reduced. 
However, in such cases, the crosslinking agent is likely to remain on the 
surface of the resin. Such problems do not occur when adopting a 
crosslinking agent that has low reactivity and excels in safety such as 
polyhydric alcohol, etc. 
However, in the case of adopting the crosslinking agent of high reactivity 
such as epoxy compound, etc., it is likely that surface regions are 
crosslinked immediately, and excellent properties are likely to be 
obtained. On the other hand, however, the surface crosslinking agent 
itself is acrid to skin. Thus, when a large amount of the crosslinking 
agent is contained in the water-absorbent resin, a new problem is raised 
in its safety when applying it to sanitary materials. Namely, in the 
conventional water-absorbent resins, an epoxy compound remains in an order 
of from several tens to 1,000 ppm. 
In order to reduce an amount of a residue in the resin of the crosslinking 
agent for crosslinking the surface regions, the surface regions of the 
water containing gel-like resin at a specific water content ranging from 
10 to 30 percent, a method of further adding a predetermined amount of 
water during the process is known (Japanese Unexamined Patent Application 
No. 195705/1991 (Tokukaihei 3-195705)). 
Such method necessitates complicated processes, and the surface 
crosslinking agent is penetrated to the inside of the particles because of 
its high water content. As a result, the absorbency under high pressure as 
well as without pressure is lowered to an insufficient level, and a 
significant effect of reducing the amount of a residue of the crosslinking 
agent cannot be expected. Namely, the inventors of the present invention 
have found that these properties are negatively correlated to one another. 
In order to meet a demand for thinner diapers of higher performances, there 
is a tendency of reducing a fiber material such as pulp from the 
water-absorbent material and increasing an amount of the water-absorbent 
resin, i.e., increasing the density of the water-absorbent resin. However, 
when the density is increase in a diaper, as the water-absorbent resin has 
a lower absorbing rate as compared with pulp, it is required to improve 
the absorbing rate of the water-absorbent resin. 
In order to ensure high absorbing rate of the water-absorbent resin, it is 
required to increase a surface area of the water-absorbent resin. However, 
if the absorbing rate is increased merely by reducing a particle diameter, 
liquid permeability is lowered. To avoid such problem, the surface area is 
increased without reducing the particle diameter, for example, by 
pulverizing the water-absorbent resin to be crushed in an irregular shape, 
or foaming the water-absorbent resin. 
Further, a method of crosslinking the surface of foamed and porous 
water-absorbent resin to improve the absorbing rate and the absorbency 
under pressure (U.S. Pat. No. 5,399,591) is also known. 
However, in the case of increasing the absorbency under high pressure by 
carrying out a surface crosslinkage of the foamed porous water-absorbent 
resin, due to its wide surface area, it is required to add the surface 
crosslinking agent in a still greater amount, and it is difficult to 
uniformly add the surface crosslinking agent to particles. As a result, it 
has been found that the amount of a residue of the surface crosslinking 
agent is increased. Namely, it has been found that an improvement in 
absorbing rate, which has been strongly demanded among the properties of 
the water-absorbent resin, and a reduction in amount of a residue of the 
surface crosslinking agent are negatively correlated to one another. 
In order to obtain absorbency under high pressure, in general, the amount 
of the surface crosslinking agent is increased from the conventional 
method as described above. However, in such case, as the crosslinking 
density of the surface regions is too high, the absorbing rate is reduced 
on the contrary. 
Namely, it is well known in the art that the absorbing rate is increased by 
the surface crosslinkage. However, it has been found that when surface 
crosslinkage is carried out to sufficiently increase the absorbency under 
high pressure (for example 50 g/cm.sup.2) to be durable even against 
heavier weight, the absorbing rate defined in the present invention is 
lowered compared with the water-absorbent resin before the surface 
crosslinkage. Namely, it has been found that an improvement in absorbency 
under high pressure and an improvement in absorbing rate may be negatively 
correlated to one another. 
As described, the inventors of the present invention have raised new 
problems that: (1) an improvement in absorbency under high pressure 
durable for heavy weight and a reduction in an amount of a residue of the 
epoxy crosslinking agent may be negatively correlated, (2) an improvement 
in absorbing rate by the area/weight ratio and a reduction in an amount of 
a residue of the epoxy crosslinking agent may be negatively correlated and 
(3) an improvement in absorbency under high pressure, and an improvement 
in absorbing rate may be negatively correlated. 
Accordingly, an object of the present invention is to provide new absorbent 
agent powders which exhibit an improvement in absorbency under high 
pressure, a reduction in an amount of a residue of the epoxy crosslinking 
agent and an improvement in absorbing rate, and the manufacturing method 
of the same. 
DISCLOSURE OF INVENTION 
Earnest researches have been made to accomplish the above object. As a 
result, the inventors of the present invention have found that by 
processing with a specific compound, a water-absorbent resin having a 
carboxyl group whose surface regions are denatured by the crosslinking 
agent having an epoxy group and in which the crosslinking agent remains as 
a residue, water-absorbent agent powders which exhibit high absorbing rate 
and a reduced amount of a residue of the crosslinking agent while 
maintaining various water-absorbent properties such as high absorbency 
under high pressure, etc., with can be obtained. 
The manufacturing method of the water-absorbent agent powders of the 
present invention is characterized in that a nucleophilic reagent is added 
to heated water-absorbent resin powders having a carboxyl group in a form 
of a powder, whose surface regions are crosslinked by a crosslinking agent 
having an epoxy group, in which the crosslinking agent remaining as a 
residue is reduced. 
Another manufacturing method of the water-absorbent agent powders of the 
present invention is characterized in that water-absorbent resin powders 
having a carboxyl group, in which surface regions are crosslinked by a 
crosslinking agent having an epoxy group and in which the crosslinking 
agent remains therein is washed so as to reduce the amount of the residue 
of the crosslinking agent. 
A still another manufacturing method of water-absorbent agent powders of 
the present invention is characterized as comprising the step of: 
adding at least one member selected from the group consisting of a 
water-soluble surface active agent and a water-soluble polymer to dried 
water-absorbent resin powders of an irregular crushed shape having a 
carboxyl group, whose surface regions are crosslinked, in a sufficient 
amount for increasing an absorbing rate (g/g/sec) of the water-absorbent 
resin powders defined based on 28 times swelling time with artificial 
urine above an absorbing rate of the surface crosslinked water-absorbent 
resin powders, said water-absorbent resin powders having an absorbency 
under pressure based on the physiologic saline solution under load of 50 
g/cm.sup.2 increased to at least 20 g/g. 
Yet still another method of manufacturing water-absorbent resin powders of 
the present invention is characterized by including the step of adding 
water to at least partially porous water-absorbent resin powders having a 
carboxyl group, and the water-absorbent resin powders being surface 
crosslinked by a crosslinking agent having an epoxy group and containing a 
residual surface crosslinking agent having the epoxy group so as to have 
an absorbency under pressure of not less than 20 g/g based on a 
physiologic saline solution under load of 50 g/cm.sup.2, whereby an amount 
of the residue of the crosslinking agent of the mixture is reduced. 
The water-absorbent agent powders of the present invention are at least 
partially porous water-absorbent resin powders, in which surface regions 
of the water-absorbent resin powders are crosslinked by a crosslinking 
agent having an epoxy group, and an amount of the residue of the 
crosslinking agent is not more than 2 ppm. 
The following will describe the present invention in detail. 
The water-absorbent resin of the present invention is the water-absorbent 
resin powders having a carboxyl group in which surface regions are 
crosslinked by a crosslinking agent having an epoxy group, and the 
crosslinking agent remains therein. 
It is preferable that the water-absorbent resin powders have an absorbency 
under pressure based on a physiologic saline solution under load of 50 
g/cm.sup.2 of not less than 20 g/g and more preferably not less than 25 
g/g. 
From the viewpoint of absorbing rate, the water-absorbent resin powders of 
an irregular crushed shape having a large specific surface area, and 
preferably water-absorbent resin powders in which at least a part of the 
particles is porous, having a specific surface area based on the particles 
in size ranging from around 300 to 600 .mu.m before the surface 
crosslinkage being less than 0.025 m.sup.2 /g. It is preferable that the 
water content of the water-absorbent resin powders is less than 10 
percent, more preferably less than 5 percent. 
For example, such water-absorbent resin powders can be obtained by 
crosslinking the surface regions of a hydrophilic cross-linked polymer as 
a resin precursor by the crosslinking agent having an epoxy group under a 
specific condition. However, the water-absorbent resin powders having a 
carboxyl group in which the crosslinking agent does not remain in the 
resin shows low absorbency under high pressure, and such resin is not 
suited for the present invention. 
In the present invention, the amount of a residue of the crosslinking agent 
having an epoxy group in the water-absorbent resin powders is required to 
be not less than a predetermined amount in order to increase the 
absorbency under high pressure. It is preferable that the predetermined 
value is above 2 ppm, more preferably not less than 5 ppm and still more 
preferably not less than 10 ppm. 
The upper limit of the amount of a residue of the crosslinking agent having 
an epoxy group is not particularly limited in the present invention. 
However, it is not efficient to have a residue in an excessive amount of 
the crosslinking agent because more than a certain level of the absorbency 
under pressure would not be obtained, and a longer time and a larger 
amount of the nucleophilic reagent would be required for reducing the 
amount of the residue of the crosslinking agent. In consideration of the 
above, it is preferable that an upper limit amount of the residue is not 
more than 2,000 ppm, more preferably not more than 1,000 ppm, and still 
more preferably not more than 500 ppm. Namely, in order to obtain high 
grade properties and a significant reduction in the amount of a residue of 
the surface crosslinking agent, it is preferable that the amount of a 
residue of the crosslinking agent is in a range of from 2 ppm to 2,000 
ppm, more preferably from 3 to 1,000 ppm and still more preferably from 4 
to 500 ppm. 
Hereinafter, a water-absorbent polymer which is not surface crosslinked is 
defined to be the hydrophilic crosslinked polymer or the resin precursor, 
a hydrophilic crosslinked polymer or the surface crosslinked resin 
precursor which is surface crosslinked is defined to be water-absorbent 
resin, and further the water-absorbent resin to which the process of the 
present invention is applied is defined to be water-absorbent agent 
powders. 
The water-absorbent resin powders of the present invention is obtained by 
mixing an aqueous solution in an amount of from 0.005 part by weight to 2 
parts by weight of a crosslinking agent having an epoxy group, more 
preferably from 0.02 part by weight to 1.5 parts by weight, still more 
preferably from 0.06 part by weight to 1 part by weight, and from 0.1 part 
by weight to 10 parts by weight of water based on 100 parts by weight of 
the resin precursor before surface regions are crosslinked although a 
suitable amount of the aqueous solution differs depending on a surface 
area of the resin precursor, i.e., the grain size and shape of the resin 
precursor powder and presence or absence of foams. 
By controlling the respective amounts of the crosslinking agent having an 
epoxy group and the aqueous solution with respect to the resin precursor 
in a specific range, the resulting water-absorbent resin powders show high 
absorbency under high pressure in which a residue of the crosslinking 
agent having an epoxy group remains, which are suited for use in the 
present invention. 
Under the described control, the water-absorbent resin powders of the 
present invention show high absorbency under high pressure and contain a 
residue of the crosslinking agent having an epoxy group and have a 
carboxyl group. 
The water-absorbent resin powders become the water-absorbent agent powders 
of the present invention having high absorbency under high pressure, and 
less residue of the cross-linking agent having an epoxy group, by applying 
the process of the present invention. 
In the present invention, in order to improve properties and reduce an 
amount of a residue of the crosslinking agent, for the aqueous solution 
containing the crosslinking agent, a combined use of organic hydrophilic 
solvent and water is adopted as the aqueous solution containing a 
crosslinking agent. Examples of such hydrophilic organic solvent may be 
but are not limited to: lower alcohols, such as methyl alcohol, ethyl 
alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl 
alcohol, and t-butyl alcohol; 
ketones, such as acetone; 
ethers, such as dioxane, tetrahydrofuran, alkoxypolyethylene glycol; 
amides, such as N,N-dimethylformamide; 
sulfoxides, such as dimethylsulfoxide; etc. 
In the present invention, the amount of use of such hydrophilic organic 
solvent is in a range of from 0 to 10 parts by weight, preferably less 
than 5 parts by weight with respect to 100 parts by weight of the solid 
portion of the resin precursor. 
The resin precursor to be used in the present invention is not particularly 
limited, and any resin precursor having a carboxyl group may be used. For 
example, a hydrophilic crosslinked polymer which forms substantially 
water-insoluble hydrogel swollen with a large amount of water, preferably 
swollen with from 10 to 100 times by weight of a physiologic saline 
solution without pressure may be used. The crosslinked polymer is such as 
a partially neutralized polyacrylic acid salt crosslinked polymer, a graft 
polymer of starch-acrylic acid, crosslinked carboxymethyl cellulose, etc. 
The substantially water-insoluble hydrogel is defined to be those having a 
solubility of the absorbent resin in excessive amount of pure water, i.e., 
the content of extractables is not more than 50 percent by weight, 
preferably not more than 20 percent by weight and still more preferably 
not more than 10 percent by weight. 
For the resin precursor in the present invention, a hydrophilic crosslinked 
polymer resulting from polymerizing a hydrophilic aqueous monomer 
containing acrylic acid and/or a salt thereof as main components in a 
presence of the crosslinking agent or grafted main chain is suitably and 
typically used. 
Examples of the salt of acrylic acid may be but are not limited to: alkali 
metal salt of acrylic acid, ammonium salt of acrylic acid, amine salt of 
acrylic acid, etc. Among them, alkali metal salt is preferable, and sodium 
salt is still more preferable. 
For the unit structure of the polymer, that having from 40 to 100 mole 
percent, preferably from 50 to 95 mole percent and still more preferably 
from 60 to 90 mole percent neutralized carboxyl group obtained from 
acrylic acid is preferable. The neutralization may be carried out with 
respect to the monomer before polymerization, or hydrogel polymer during 
or after polymerization. 
When the water-absorbent resin is obtained by polymerizing a hydrophilic 
monomer having acrylic acid and/or a salt thereof as a main component 
thereof, for example, with a combined use of acrylic acid and or salts 
thereof, other monomers may be copolymerized if necessary. The method of 
manufacturing acrylate salt suited for use in the polymerization of the 
hydrophilic crosslinked polymer is exemplified in U.S. Pat. No. 5,338,819 
and EP Patent No. 0574260. 
Examples of the monomer usable for the copolymerization, other than acrylic 
acid, may be but are not limited to: 
anionically unsaturated monomers such as methacrylic acid, maleic acid, 
.beta.-acryloyloxy propionic acid, vinyl sulfonic acid, styrene sulfonic 
acid, 2-(meth)acrylamide-2-methyl propanesulfonic acid, 2-(meth)acryloyl 
ethanesulfonic acid, and 2-(meth)acryloyl propanesulfonic acid and salts 
thereof; 
nonionic hydrophilic group-containing unsaturated monomers such as 
acrylamide, methacrylamide, N-ethyl(meth)acrylamide, 
N-n-propyl(meth)acrylate, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth) 
acrylamide, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, 
methoxypolyethylene glycol (meth)acrylate, polyethyleneglycol 
mono(meth)acrylate, vinylpyridine, N-vinyl pyrrolidone, 
N-acryloylpiperidine, and N-acryloyl pyrrolidine; and 
cationically unsaturated monomers such as N,N-dimethylaminoethyl 
(meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethyl 
aminopropyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and 
quaternary salts thereof. 
It is also permitted to use hydrophobic unsaturated monomer such as 
isobutylene, stearyl (meth)acrylate, etc., as long as the water-absorbent 
properties are not adversely affected. The amount of other monomer is 
generally in the range of from 0 to 50 mol percent, preferably from 0 to 
30 mol percent, most preferably from 0 to 10 mol percent, based on the 
amount of all the component monomers for the copolymerization. 
As the water-absorbent resin for use in this invention, although it has a 
cross-linking structure, the water-absorbent resin obtained by the 
copolymerization or the reaction using an inner crosslinking agent having 
not less than two polymerizing unsaturated groups or not less than two 
reacting groups or both of polymerizing unsaturated group and reactive 
group is more preferable than the water-absorbent resin of the 
self-crosslinkable type which has no use for the crosslinking agent. 
Examples of the inner cross-linking agent may be but are not limited to: 
N,N'-methylene-bis (meth)acrylamide, (poly) ethyleneglycol di (meth) 
acrylate, (poly) propylene glycoldi(meth)acrylate, trimethylol-propane 
tri(meth)acrylate, trimethylolpropane di(meth) acrylate, glycerol 
tri(meth)acrylate, glycerol acrylate methacrylate, ethyleneoxidemodified 
trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, 
dipentaerythritol hexa (meth)acrylate, triallyl cyanurate, triallyl 
isocyanurate, triallyl phosphate, triallyl amine, 
poly(meth)allyloxyalkane, (poly)ethyleneglycol diglycidyl ether, glycerol 
diglycidyl ether, ethylene glycol, polyethyleneglycol, propyleneglycol, 
glycerol, pentaerythritol, ethylenediamine, polyethyleneimine, and 
glycidyl (meth)acrylate. These inner cross-linking agents, when necessary, 
may be used in the form of a combination of two or more members. 
From the viewpoint of the water-absorbent properties of the produced 
water-absorbent resin, it is particularly preferable to use essentially a 
compound having not less than two polymerizable unsaturated groups as an 
inner cross-linking agent. The amount of the inner cross-linking agent to 
be used is preferably in the range of from 0.005 to 2 mole percent, more 
preferably from 0.01 to 1 mole percent to the monomers. 
The monomers to be polymerized may incorporate therein a hydrophilic 
polymeric compound such as starch or cellulose, starch derivatives or 
cellulose derivatives, polyvinyl alcohol, polyacrylic acid (polyacrylate 
salt) or crosslinked polyacrylic acid (polyacrylate salt), a chain 
transfer agent such as hypophosphorous acid (salt), surfactants, and 
foaming agents such as carbonates, etc. 
The compounds for addition to these monomers are disclosed in U.S. Pat. No. 
4,076,663, U.S. Pat. No. 4,286,082, U.S. Pat. No. 4,320,040, U.S. Pat. No. 
4,833,222, U.S. Pat. No. 5,118,719, U.S. Pat. No. 5,149,750, U.S. Pat. No. 
5,154,713, U.S. Pat. No. 5,264,495, EP No. 03729831, and EP No. 0496594. 
In the polymerization of the monomer for the production of the 
water-absorbent resin for use in the present invention, though bulk 
polymerization or precipitation polymerization is available, it is 
preferable to prepare the monomer in the form of an aqueous solution and 
subject the aqueous solution to a solution polymerization or 
reversed-phase suspension polymerization from the viewpoint of the quality 
of product and the ease of control of polymerization. Examples of the 
solution polymerization may be but are not limited to: a casting 
polymerization to be carried out within a mold, a thin layer 
polymerization to be carried out by placing thinly on a belt conveyer, a 
polymerization to be carried out by dividing the resulting hydrogel 
polymer into fine pieces, etc. 
When carrying out the solution polymerization, the concentration of the 
aqueous solution is in the range of from 10 to 70 percent by weight, 
preferably from 20 percent by weight to a saturated concentration. 
Further, both continuous-type and batch-type polymerizations can be 
adopted for such solution polymerization, and under any of reduced 
pressure, applied pressure, or atmospheric pressure. In general, it is 
further preferable that the polymerization is carried out in an inactive 
air flow such as nitrogen, helium, argon, carbon dioxide gas, etc. 
When carrying out the polymerization, for example, known polymerization 
methods such as a polymerization using a radical polymerization initiator, 
an irradiation polymerization, an electron beam polymerization, an 
ultraviolet ray polymerization by a photosensitizer, etc., may be adopted; 
however, in order to carry out a polymerization quantitatively and 
completely, it is preferable to adopt a polymerization by a radical 
polymerization initiator. Examples of the radical polymerization are: a 
reverse-phase suspension polymerization (U.S. Pat. Nos. 4,093,776, 
4,367,323, 4,446,261, 4,683,274, 4,973,632); and solution polymerization 
(U.S. Pat. Nos. 4,552,938, 4,625,001, 4,654,393, 4,703,067, 4,873,299, 
4,985,514, 5,124,416, 5,250,640). 
To initiate a polymerization, for example, a radical polymerization 
initiator such as potassium persulfate, ammonium persulfate, sodium 
persulfate, t-butylhydroperoxide, hydrogenperoxide, 
2,2'-azobis(2-amidinopropane)dihydrochloride, etc., or an active energy 
ray, such as an ultraviolet ray, an electron beam, etc., may be used. 
However, it is preferable to adopt the radical polymerization initiator. 
In the case of employing an oxidative radical polymerization initiator, a 
redox polymerization may be carried out with a combined use of a reducing 
agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, 
formamidine sulphinic acid, L-ascorbic acid (salt), etc. It is permitted 
to use more than one kind of such polymerization initiators and the 
reducing agents, and the amount of use thereof is preferably in the range 
of from 0.001 to 2 mole percent, more preferably in the range of from 0.01 
to 0.5 mole percent to the monomers. 
In the case of polymerizing a monomer component by the solution 
polymerization, it is preferable to dry the resulting gel-like polymer by 
a known drying method such as a hot-air drying method, a drying method 
under specific moisture (U.S. Pat. No. 4,920,202), a microwave drying 
method (U.S. Pat. No. 5,075,344), drying method under reduced pressure, a 
drying method using a drum dryer, or an azeotropic dehydration in a 
hydrophobic organic solution. 
The drying temperature preferably ranges from 70 to 300.degree. C., more 
preferably from 100 to 250.degree. C., and more preferably from 150 to 
200.degree. C. Prior to applying the drying treatment, water-absorbent 
resin fine powders may be recycled into a gel-like polymer, and ground by 
the method disclosed in U.S. Pat. No. 5,064,582 and U.S. Pat. No. 
5,478,879, or the polymer in the form of gel may be cut into fine pieces 
by the method disclosed in U.S. Pat. No. 5,275,773, or a supply of the 
polymer in the form of gel to the dryer may be controlled by the method 
disclosed in U.S. Pat. No. 5,229,487. 
The resulting hydrophilic crosslinked polymer in the present invention as a 
resin precursor from the described polymerization preferably has an 
irregular crushed shape, spherical, fiber, or sheet. In the case of having 
substantially spherical shape, the polymer is preferably obtained by a 
reverse phase suspension. However, to maximize the effects of the present 
invention, i.e., to obtain excellent water-absorbent agent powders which 
show high absorbency both under pressure and without pressure, excel in 
safety as evinced by the absence of a residue of a cross-linking agent, it 
is preferable to use spherical or irregular crushed shape as a raw 
material. 
From the viewpoint of high absorbing rate evinced by a high specific 
surface area, and fixability to pulp, it is preferable to use a 
hydrophilic crosslinked polymer of irregular crushed shape resulting 
essentially from carrying out a solution polymerization and subsequent 
pulverization, and more preferably at least a partially porous hydrophilic 
crosslinked polymer of irregular crushed shape as the resin precursor. 
Further, whether the porosity is of an open cell or closed cell is not 
specified. 
In the present invention, at least partially porous particle is defined 
such that existence and absence of pores in a plurality of particles is 
observable in a from 30 to 100 times enlarged electron micrograph. In the 
present invention, not less than 2 percent, preferably not less than 5 
percent and still more preferably not less than 10 percent particles are 
porous among all particles. Further, from the viewpoint of high absorbing 
rate, the crosslinked polymer particles having a BET specific surface area 
of not less than 0.025 m.sup.2 /g, preferably not less than 0.03 m.sup.2 
/g and more preferably not less than 0.04 m.sup.2 /g based on particles 
having a size ranging from 300 to 600 .mu.m may be adopted. The 
manufacturing method of the present invention has an advantageous feature 
in that even when adopting irregular crushed shape water-absorbent resin, 
porous absorbent resin or fine recycled absorbent resin powders in which a 
crosslinking agent is likely to remain as a residue because of its high 
specific surface area, the water-absorbent agent powders without a residue 
of the crosslinking agent or with a small amount of a residue can be 
obtained. 
In the present invention, in the case of using the foamed porous 
hydrophilic polymer as the resin precursor, although a method of boiling 
and then foaming the hydrogel polymer when carrying out a polymerization 
or drying may be used, it is preferable to use the foaming agent when 
manufacturing the resin precursor. Examples of the foaming agent to be 
used in the present invention may be but are not limited to: an inactive 
gas such as nitrogen, various organic solvents such as methyl alcohol, 
cyclohexane, etc.; 
carbonates such as sodium (hydrogen)carbonate, ammonium 
(hydrogen)carbonate, potassium (hydrogen)carbonate, magnesium carbonate, 
carbon dioxide, ethylene carbonate, etc.; 
water-soluble azo compounds such as 2,2'-azobis(2-methylpropionamizine) 
dihydrochloride, 2,2'-azobis(2-(2-imidazoline-2-il)propane) 
dihydrochloride, 2,2'-azobis [2-methyl-N-(2-hydroxyethyl)-propionamide], 
etc.; and 
water uniformly dispersed azo compounds such as 
2,2'-azobis(2-methylpropioneamizine)diacrylate, etc. 
Among these foaming agent, a water soluble or water dispersable azo 
compound, or carbonate is preferable, and further from the view point of 
controlling foams, a water-soluble polymer or a surfactant may be used in 
combination. 
Although a suitable amount of such foaming agent, water soluble polymer or 
surfactant varies, it is normally based on the total amount of the monomer 
component, not more than 200 percent by weight, preferably not more than 
100 percent by weight in the case of carbonates; not more than 5 percent 
by weight in the case of the azo compound, preferably not more than 1 
percent by weight; in the case of the water soluble polymer, not more than 
10 percent by weight, and in the case of the surfactant not more than 2 
percent by weight and more preferably not more than 1 percent by weight. 
As to the particle size of the resin precursor, those having an average 
particle diameter ranging from 200 .mu.m to 600 .mu.m, and not more than 
10 percent by weight of the resin including particles having a diameter of 
less than 150 .mu.m are the most preferable. In the case of adopting the 
particles having an average diameter of less than 200 .mu.m, a sufficient 
improvement in absorbency under pressure may be difficult to be achieved. 
On the other hand, when adopting particles having an average diameter of 
above 600 .mu.m, the absorbing rate would be low, and thus, a long time 
would be required for reaching a saturated amount of absorption. On the 
other hand, when the resin having a particle diameter of less than 150 
.mu.m exceeds 10 percent by weight, an amount of a residue of the 
crosslinking agent is difficult to be reduced. 
The resin precursor to be used in the present invention has water content 
ranging from 1 to below 50 percent, preferably ranging from 1 to below 20 
percent and still more preferably less than 10 percent. It is yet still 
more preferable that such resin precursor is in the form of powder. If the 
water content is high, the crosslinking agent having an epoxy group is 
penetrated into the inside of the resin precursor, and a reduction in an 
amount of a residue of the crosslinking agent having an epoxy group can be 
expected. On the other hand, however, the absorbency is lowered, and 
improvement in water-absorbent properties under high pressure cannot be 
expected. 
In the present invention, the resin precursor having a carboxyl group 
obtained in the described manner is heat-treated by adding a crosslinking 
agent having an epoxy group as an essential component and surface regions 
of the resin precursor are crosslinked to obtain the water-absorbent resin 
powders, in which a residue of the crosslinking agent having an epoxy 
group remains is used as a starting material. 
In the case where the water-absorbent resin powders which are crosslinked 
by a crosslinking agent having an epoxy group do not contain the residue 
of the crosslinking agent, if such water-absorbent resin powders are used 
as a starting material, in general, the problem is raised in that the 
water-absorbent agent powders resulting from the water-absorbent resin 
powders do not show sufficient absorbency both without pressure and under 
high pressure probably because a uniforms crosslinkage only on the 
particle surface layer of the water-absorbent resin are not obtained. For 
the reason set forth above, it is not preferable to adopt such 
water-absorbent resin powders as a starting material. In the present 
invention, it is further preferable that the crosslinking agent having an 
epoxy group is added to the resin precursor in the form of an aqueous 
solution and then heated. Based on 100 parts by weight of the resin 
precursor prior to be crosslinked at its surface, the crosslinking agent 
having an epoxy group is used in an amount ranging from 0.005 to 2 parts 
by weight, preferably ranging from 0.02 to 1.5 parts by weight, and still 
more preferably ranging from 0.06 to 1 part by weight, while the aqueous 
solution including the crosslinking agent is used preferably in an amount 
ranging from 0.1 to 10 parts by weight. 
Here, the aqueous solution is water or a mixture of water and hydrophilic 
organic solvent. 
After adding the aqueous solution, the heating temperature preferably 
ranges from 50 to 230.degree. C., and more preferably from 100 to 
200.degree. C. It is further preferable that the heat-treated absorbent 
resin powders contain a solid portion of above 90 percent, more preferably 
not less than 95 percent, and still more preferably not less than 98 
percent. 
When respective amounts of the crosslinking agent and the aqueous solution 
to be added deviate from the described range, improvement in 
water-absorbent properties such as absorbency under high pressure may not 
be obtained, or an amount of a residue of the crosslinking agent having an 
epoxy group may not be reduced even after carrying out the process. 
The crosslinking agent having an epoxy group of the present invention is a 
compound reactive to a plurality of carboxyl groups in the resin 
precursor, which has at least one epoxy group in a molecule. 
Such compound may be, but is not limited to: 
glycidyl ethers such as ethylene glycol diglycidyl ether, polyethylene 
diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl 
ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, 
polypropylene glycol diglycidyl ether, etc.; 
glycidyl compounds such as glycidol, 
.gamma.-glycidoxypropyltrimethoxysilane, etc.; 
epihalohydrins such as epichlorohydrin, epibromohydrin, etc.; 
phosphonic acid glycidyl ethers such as methyl phosphonic acid diglycidyl 
ether, n-propyl phosphonic acid diglycidyl ether, etc.; 
cyclic epoxy compounds such as 3,4-epoxycyclohexane carboxylic 
acid-3',4'-epoxycyclohexyl ester (product name: Celoxide R 2021, DAICEL 
Chemical Industries LTD), and the like. 
Among all, from the viewpoint of water-absorbent properties of the 
water-absorbent agent powders, the polyglycidyl compounds are preferable, 
and polyglycidyl ethers such as ethylene glycol diglycidyl ether is still 
more preferable as the crosslinking agent having an epoxy group. 
In the present invention, the described crosslinking agent having an epoxy 
group may be used in combination with another crosslinking agent that is 
reactive with a carboxyl group. Such crosslinking agent may be the 
below-listed known crosslinking agents for use in crosslinking the surface 
regions of the compound. Examples of such surface crosslinking agent may 
be, but are not limited to: 
polyhydroxy alcohol compounds such as ethylene glycol, diethylene glycol, 
propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene 
glycol; propylene glycol, 1,3-propanediol, dipropylene glycol, 
2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, 
polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,5-pentanediol, 
1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylol 
propane, diethanolamine, triethanolamine, polyoxypropylene, 
oxyethylene-oxypropylene block copolymer, pentaerythritol, sorbitol, etc.; 
polyamine compounds such as ethylenediamine, diethylenetriamine, 
triethylenetetramine, tetraethylene pentamine, pentaethylenehexamine, 
polyamidepolyamine, polyethyleneimine, etc., and condensation products of 
these polyamine compounds and haloepoxy compounds; 
polyisocyanate compounds such as 2,4-tolylene diisocyanate, hexamethylene 
diisocyanate, etc.; 
polyoxazoline compounds such as 1,2-ethylene bisoxazoline, etc.; 
a silane coupling agent such as .gamma.-glycidoxypropyltrimethoxysilane, 
.gamma.-aminopropyltrimethoxy silane, etc.; 
alkylene carbonate compounds such as 1,3-dioxolane-2-one, 
4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 
4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 
4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 
4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, 
1,3-dioxopane-2-one, etc.; and 
polyvalent metallic compounds such as hydroxides and chlorides of metals 
such as zinc, calcium, magnesium, aluminum, iron, zirconium, etc. Only one 
kind of the above-listed crosslinking agent may be adopted, or two or more 
kinds thereof may be suitably mixed and adopted. 
The water-absorbent resin powders having a carboxyl group containing a 
residue of a crosslinking agent having an epoxy group, obtained by the 
described method are formed into water-absorbent agent powders of the 
present invention by reducing the amount of the residue by the method 1): 
adding a nucleophilic reagent to water-absorbent resin powders in a form 
of powder under an applied heat or the method 2): washing water-absorbent 
resin powders. By carrying out at least either one of the post processes 
1) and 2), desirable water-absorbent agent powders of the present 
invention, that show desirable absorbing characteristics while reducing 
the amount of the residue of the crosslinking agent can be achieved 
efficiently at low cost. 
First, the first method of manufacturing water-absorbent agent powders 
characterized in that an amount of the residue of the crosslinking agent 
is reduced by adding a nucleophilic reagent to heated water-absorbent 
resin powders having a carboxyl group, in which surface regions are 
crosslinked by a crosslinking agent having an epoxy group, and a residue 
of the crosslinking agent is contained, will be explained in detail. 
In the present invention, it is an essential condition that the 
water-absorbent resin powders to be processed are heated. In the case of 
adopting the water-absorbent resin powders at room temperature or cooled 
off at below room temperature without heat, it is extremely difficult to 
supply the water-absorbent resin powders stably and continuously, and thus 
it is difficult to industrially manufacture the water-absorbent agent 
powders in practical applications or to ensure quality water-absorbent 
agent powders. Sufficient effects of reducing the amount of a residue of 
the crosslinking agent cannot be expected also because the absorbing rate 
or adsorbing rate of the nucleophilic reagent into the water-absorbent 
resin powders is too low. Such problems are not very obvious when 
manufacturing the water-absorbent powders in a small amount, i.e., for 
example, in an experimental level. However, with an increase in production 
of the water-absorbent agent powders to the industrial production level, 
the inventors of the present invention have faced the described problems, 
and have coped with the problems to find the solution. Inventors of the 
present invention have succeeded in solving such problems in a simple and 
efficient manner, i.e., by heating beforehand the water-absorbent resin 
powders to be processed. 
To be specific, it is an essential condition that the heating temperature 
of the water-absorbent resin powders is above room temperature. Further, 
in order to solve the described problems, the heating temperature is 
required to be not less than 30.degree. C., preferably not less than 
35.degree. C., and more preferably not less than 40.degree. C. 
The water-absorbent resin powders can be supplied stably and continuously 
by setting the heating temperature to above room temperature, preferably 
not less than 30.degree. C., and more preferably not less than 35.degree. 
C. It should be noted here that if the temperature of the water-absorbent 
resin powders is too high, the absorbing rate or the adsorbing rate of the 
nucleophilic reagent into the water-absorbent resin powders becomes too 
high, which generates a non-uniform mixture, or an amount of the residue 
of the crosslinking agent may be reduced in a smaller amount. 
In consideration of the above, the upper limit of the temperature of the 
water-absorbent resin powders is normally at below 200.degree. C., 
preferably at below 100.degree. C., and more preferably at below 
80.degree. C. The upper limit temperature below 65.degree. C. is still 
more preferable. 
The water-absorbent resin powders are heated beforehand, i.e., an essential 
condition of the present invention, by externally applying heat to the 
water-absorbent resin powders to a predetermined temperature before the 
nucleophilic reagent is added, for example, by a dielectric heater, a 
contact-type heater, a hot air heater, etc. It is also permitted to adjust 
the heating temperature to a predetermined temperature or the temperature 
at which the water-absorbent resin powders is kept after being heated, 
i.e., after being polymerized, dried, pulverized, surface-crosslinked, 
etc. 
In the present invention, a nucleophilic reagent is added to the heated 
water-absorbent resin powders having a carboxyl group, which contain the 
residue of the crosslinking agent having an epoxy group. 
In the present invention, the powder form suggests a state where the 
water-absorbent resin powders do not form a block by contacting each other 
and can be ground with ease even after the nucleophilic reagent is added 
without being formed into a gel by swelling, and the powdered state 
thereof is maintained even after the treatment. 
When processing with the nucleophilic reagent, in the case that the 
water-absorbent resin powders are not in a powdered form, the amount of 
the residue of the epoxy crosslinking agent cannot be reduced to a 
sufficient level, or the absorbing rate or absorbency under pressure may 
be lowered after the nucleophilic reagent treatment. Thus, the object of 
the present invention, i.e., to reduce the amount of the residue of the 
crosslinking agent, to improve the absorbency under high pressure and the 
absorbing rate may not be fully satisfied in the above case. 
A suitable nucleophilic reagent for the present invention may be, but is 
not limited to: nucleophilic reagents having a nucleophilic atom of a 
carbon or oxygen, nitrogen compounds having a nucleophilic atom of 
nitrogen, halogen compounds having a nucleophilic atom of halogen, sulfur 
compounds having a nucleophilic atom of sulfur, phosphorous compounds 
having a nucleophilic atom of phosphorous, a nucleophilic reagent having a 
nucleophilic point of a hydrogen group, a nucleophilic reagent having a 
nucleophilic point of a carboxyl group, etc. 
The nucleophilic reagent having a nucleophilic atom of carbon or oxygen may 
be acetals, acetoacetates, alcoholates, acetonitriles, acetylenes, acid 
anhydrides, water, alcohols, inorganic hydroxides, aldehydes, organic 
peroxides such as organic hydroperoxides, hydroxymethyl urea, carbon 
dioxide, carboxylic acids, cyanoacetates, olefins such as cyclopentadiene, 
ketones, malonic acids, phenols, etc. 
Nitrogen compounds having a nucleophilic atom of nitrogen may be, but are 
not limited to: nitrates of alkaline-earth such as barium nitrate, amides, 
primary amine compounds, secondary amine compounds, tertiary amine 
compounds, polyamine compounds, ammonia, ammonium carbonate, azides, 
cyanamides, (iso)cyanates, ethyleneimine, hydrazine compounds, lactam 
compounds, futalimide, sulfonamides, pyridine, nicotine amide, urea, 
thiourea, etc. 
Halogen compounds having a nucleophilic atom of halogen may be acyl halides 
such as acetyl chloride, etc., alkyl halides, antimony trihalogen, bismuth 
halide, boron halides such as boron trioxide, etc., carbamoyl halides such 
as carbamoyl chloride, etc., chlorosilane compounds, etc. 
Sulfur compounds having a nucleophilic atom of sulfur may be but are not 
limited to: aminothiols, carbon disulfide, ethylene sulfide, hydrogen 
sulfide, sulfurous acid (sulfite), hydrogen sulfurous acid (hydrogen 
sulfide), thiosulfate (thiosulfide), etc. Phosphorous compounds having a 
nucleophilic atom of phosphorous may be phosphate. 
Among the nucleophilic reagents having a nucleophilic atom of oxygen, 
those, in which a nucleophilic point can be a hydrogen group, may be but 
are not limited to: water, propylene glycol, sodium hydroxide, potassium 
hydroxide, polyethylene glycol, butyl alcohol, alkoxy(poly)ethylene 
glycol, etc. Examples of the nucleophilic reagent in which the carboxyl 
group may be a nucleophilic point may be, but are not limited to lactate, 
citrate, propionate, etc. 
In the present invention, it is preferable to adopt the neutral or basic 
nucleophilic reagent having a pH of not less than 5. The nucleophilic 
reagent may be solid, liquid or gas at room temperature. In order to 
achieve the object of the present invention, the nucleophilic reagent that 
is liquid at room temperature is preferable. Further, in view of 
properties, as a large amount of the residue of the nucleophilic reagent 
may damage the absorbency, volatile liquid that can be removed with ease 
after the treatment is preferable. It is also preferable that the 
nucleophilic reagent has a boiling point of not less than 60.degree. C., 
more preferably not less than 100.degree. C. On the other hand, it is 
preferable that the upper limit of the boiling point of the nucleophilic 
reagent is not more than 150.degree. C. It is further preferable that the 
nucleophilic reagent includes water as an essential component in view of 
not only reducing the amount of a residue of a crosslinking agent but also 
improving the absorbing rate. 
For example, in the case where liquid such as water is used for the 
nucleophilic reagent, the amount of use of the liquid is normally in a 
range of from 1 to 30 percent by weight with respect to water-absorbent 
resin powders, preferably in a range of from 2 to 20 percent by weight, 
more preferably in a range of from 3 to 10 percent by weight, and still 
more preferably in a range of from 4 to 8 percent by weight. In the case 
where the amount of use of water is above the range, sufficient effects of 
reducing the amount of a residue of a crosslinking agent for the amount of 
use cannot be obtained. Moreover, the absorbency under load or the 
absorbing rate may be lowered. Additionally, when adding water with 
respect to the water-absorbent resin powders, water may be added in a form 
of mist, moisture, steam, etc. 
In the present invention, a liquid nucleophilic reagent, preferably 
volatile liquid, more preferably water, and at least one kind of a 
nucleophilic reagent in which a nitrogen and/or sulfur atom may be a 
nucleophilic point is used in combination with the liquid nucleophilic 
reagent from the view of absorbing characteristics such as reducing the 
amount of a residue of a crosslinking agent having an epoxy group, 
improving the absorbing rate of the water-absorbent agent powders, etc. 
The nucleophilic reagent is absorbed by the water-absorbent resin powders, 
and preferably, it is absorbed or adsorbed by the water-absorbent resin 
powders from the surface of reducing an amount of a residue of the 
crosslinking agent. When adding the nucleophilic reagent, it is further 
preferable that at least one member selected from the group consisting of 
water-soluble surface active agent and the water-soluble polymer be added 
simultaneously or separately in order to prevent a reduction in absorbing 
rate by the surface crosslinkage in the absorbent resin powders, and thus 
the polymer shows a still improved absorbing rate. 
Further, for the nucleophilic reagent to be used together with water, from 
the view of safety and effect, amines, ammonia, ammonium carbonate, 
sulfurous acid (sulfite), hydrogen sulfurous acid (hydrogen sulfide), 
thiosulfate (thiosulfide), urine, thiourine, etc., is preferable, and 
polyamine and/or hydrogen sulfurous acid (hydrogen sulfide) is the most 
preferable. 
Such compound may be, but is not limited to sodium bisulfite, potassium 
bisulfite, ammonium bisulfite, polyallyl amine, poly(diallyl amine), 
poly(N-alkyl allyl amine), poly(alkyldiallyl amine), a copolymer of 
monoallyl amine and diallyl amine, a copolymer of monoallyl amine and 
N-alkylallyl amine, a copolymer of monoallyl amine and dialkyl diallyl 
ammonium salt, a copolymer of diallyl amine and dialkyl diallyl ammonium 
salt, straight chain polyethylene imine, branched chain polyethylene 
imine, polyethylene polyamine, polypropylene polyamine, polyamide 
polyamine, polyether polyamine, polyvinyl amine, polyamide 
polyamine-epichlorohydrine resin, etc. 
In the present invention, the amount of use of the nucleophilic reagent of 
the volatile liquid which is preferably used in combination with water 
differs depending on the amount of residual nucleophilic reagent including 
an epoxy group for use in crosslinking the surface of the water-absorbent 
resin. However, it is normally in a range of from 0.005 to 10 parts by 
weight, preferably in a range of from 0.01 to 5 parts by weight, and still 
more preferably in a range of from 0.1 to 3 parts based on 100 parts by 
weight of a solid portion of the water-absorbent resin powders. 
It is not preferable to use the nucleophilic in an amount of more than 10 
parts by weight in an economical aspect. Moreover, this causes an excess 
amount in achieving the desirable effects of reducing the amount of a 
residue of a crosslinking agent, or it may cause the absorbency under 
pressure to be lowered. On the other hand, if the nucleophilic reagent is 
used in an amount less than 0.005 parts by weight, a sufficient 
improvement in absorbency under high pressure or absorbing rate may not be 
ensured, thereby presenting the problem that the desirable effects of 
reducing the amount of a residue of the crosslinking agent cannot be 
obtained. In consideration of the above, it is more preferable that the 
amount of use of the above nucleophilic reagent is in a range of from 0.02 
to 2 parts by weight. 
In the present invention, in the case of adopting water as the nucleophilic 
reagent, other than water, hydrophilic organic solvent may be used in 
combination. A presence or absence of nucleophilicity of the hydrophilic 
organic solvent does not matter. 
Such hydrophilic organic solvent may be, but is not limited to: 
lower alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, 
isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol; 
ketones, such as acetone; 
ethers, such as dioxane, alkoxy(poly)ethylene glycol, tetrahydrofuran, 
etc.,; 
amides, such as N,N-dimethylformamide, etc.; and 
sulfoxides, such as dimethylsulfoxide, etc. 
An amount of use of the hydrophilic organic solvent varies depending on the 
kind and particle size of the water-absorbent resin powders; however, a 
preferred range with respect to 100 parts by weight of the solid content 
of the water-absorbent resin powders is normally in a range of from 0 to 
10 parts by weight, preferably less than 5 parts by weight. 
In the present invention, water-absorbent resin powders having a carboxyl 
group, in which a residue of a crosslinking agent having an epoxy group 
remains can be formed into water-absorbent agent powders of the present 
invention which have a significantly reduced amount of a residue of the 
crosslinking agent by adding the described nucleophilic reagent and, 
desirably, have an improved absorbing rate. 
Further, processes of the present invention are preferably carried out in 
such a manner that a reaction between the residue of the crosslinking 
agent having an epoxy group with a nucleophilic reagent does not affect 
the absorbing characteristics of the water-absorbent agent powders. The 
method of carrying out such processes is not particularly limited as long 
as the powdered nucleophilic reagent can be uniformly added to the 
water-absorbent resin, and, for example, the following method may be 
adopted: 
1) The water-absorbent resin powders are reacted with the nucleophilic 
reagent on contact therewith in a form of gas; 
2) The nucleophilic reagent is mixed with the water-absorbent resin powders 
to be reacted therewith; 
3) A solution containing the nucleophilic reagent is mixed with the 
water-absorbent resin powders to be reacted therewith; or 
4) Water-absorbent resin powders are brought into contact with the solution 
containing the nucleophilic reagent to be reacted therewith, etc. 
It is further preferable that the reaction is completed in a short period 
of time so that the absorbing characteristics of the water-absorbent agent 
powders are not adversely affected by applying an auxiliary treatment such 
as heating, or adding a catalyst, etc., if necessary. 
When processing the water-absorbent resin powders with the nucleophilic 
reagent, the following devices are preferably adopted: a fluidized bed 
mixer, a cylindrical mixer, a screw mixer, a turbulizer, Nauta mixer, a 
V-shaped mixer, a ribbon type mixer, a twin arm type kneader, an airborne 
mixer, a rotary disk mixer, a roll mixer, an airborne dryer, a shelf-type 
rotary dryer, a paddle dryer, a rotary disk dryer, a fluidized bed dryer, 
a belt-type dryer, paddle dryer, a rotary disk dryer, a belt-type dryer, a 
Nauta heater, an infra red ray dryer, a microwave dryer, etc. 
In order to achieve the above object, it is preferable to apply a heat 
treatment in a presence of a nucleophilic reagent, and more preferably to 
dry it after the heat treatment. Heating time and temperature are 
determined appropriately in consideration of the kind and amount of the 
nucleophilic reagent, and an amount of reduction of the residue of the 
crosslinking agent. However, sufficient effects cannot be achieved if an 
application time is too short. In the case of adopting water, a heat 
treatment is preferably applied for more than 5 minutes, normally, from 6 
to 1,000 minutes, preferably from 10 minutes to 600 minutes, and more 
preferably from 20 to 300 minutes. 
In the present invention, it is preferable to apply a heat treatment to 
water-absorbent resin powders by setting the temperature of the material 
for the water-absorbent resin powders so that at least a part of a liquid 
nucleophilic agent contact with the water-absorbent resin powders in a 
vapor state steam. Here, the temperature of the material for the 
water-absorbent resin powders is set to not more than 150.degree. C., more 
preferably not more than 100.degree. C. Further, in the case where a 
drying treatment is applied simultaneously with a heat application 
process, or separately from the heat application process, the content of 
the final water-absorbent resin powders is not less than 90 percent by 
weight to the water-absorbent agent powders, and more preferably not less 
than 95 percent by weight, to ensure desirable characteristics. 
In the first method (process with the nucleophilic reagent) of the present 
invention, in the case of adopting water for the nucleophilic reagent, it 
is preferable to apply a heat treatment to a mixture resulting from adding 
water to the water-absorbent resin powders having a carboxyl group in a 
powdered form. Surface regions of the water-absorbent resin powders are 
crosslinked by a crosslinking agent having an epoxy group and include a 
residue of the crosslinking agent. The water-absorbent resin powders 
manifest improved absorbency under pressure of at least 20 g/g with 
respect to a physiologic saline solution under load of 50 g/cm.sup.2 by 
the crosslinkage, a part of the particles thereof being foamed. 
Other than the above, an amount of a residue of the crosslinking agent can 
be reduced by leaving the water-absorbent resin powders at room 
temperature for not less than 10 days. In this case, sufficient effects of 
reducing the amount of a residue of the crosslinking agent cannot be 
expected in a short period of time, and it is required to leave the 
water-absorbent resin powders having water absorbed therein or added 
thereto, for not less than ten days, preferably not less than 20 days, 
more preferably not less than 30 days. Here, the desirable amount of water 
to be added is as described earlier, and it may be preferable to add such 
water partially as moisture, etc., continuously. 
In the present invention, if a large amount of the nucleophilic reagent is 
added all at the same time, the problem may be raised in that the 
absorbency under pressure is reduced. Thus, in order to reduce the amount 
of a residue of the crosslinking agent effectively, it is preferable to 
divide the nucleophilic reagent to be repetitively added little by little. 
In the present invention, it is preferable that in the water-absorbent 
agent powders processed with the nucleophilic reagent, the amount of the 
crosslinking agent having an epoxy group is reduced to not more than 2 
ppm, more preferably to not more than a lower detectable limit 
(hereinafter referred to as ND). It is further preferable that the 
absorbency under pressure of the water-absorbent agent powders is kept at 
not less than 20 g/g, more preferably at not less than 25 g/g, while 
manifesting high absorbing rate. The described three beneficial properties 
can be accomplished by the method of the present invention. 
Next, the second method will be explained which is characterized in that 
surface regions of the water-absorbent resin powders are crosslinked by 
the crosslinking agent having an epoxy group, and an amount of a residue 
of the crosslinking agent is reduced by washing the water-absorbent resin 
powders having a carboxyl group, in which a residue of the crosslinking 
agent having the epoxy group remains. 
The washing treatment suggests the process of making (i) the 
water-absorbent resin powders having a carboxyl group in which the 
crosslinking agent having an epoxy group remains, in contact with a 
washing agent in gas, solid, or liquid that can remove the crosslinking 
agent, and (ii) subsequently separating the washing agent from the 
water-absorbent resin powders. 
In this case, a washing treatment is carried out by a method of separating 
a mixed solution from the water-absorbent resin powders preferably after 
contacting the water-absorbent resin in contact with the liquid, more 
preferably after contacting it with an organic solvent, more preferably by 
a method of separating the mixed solution from water-absorbent resin 
powders after contacting the powders with a mixed solution composed of 
water and hydrophilic organic solvent. 
For the organic solvent, those of a low boiling point, i.e., less than 
150.degree. C., preferably less than 100.degree. C. are preferable. 
Although it is permitted to use a hydrophobic organic solvent such as 
cyclohexane, etc., the hydrophilic organic solvent is preferable in terms 
of efficiency such a as lower ketone such as acetone; etc. For the 
hydrophilic solvent to be mixed with water, lower alcohol, etc., such as 
methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, t-butyl 
alcohol, etc., is preferable. 
It is further preferable that the mixed ratio of water and hydrophilic 
organic solvent is selected such that the water-absorbent resin powders 
are not swollen with the mixed solution. In this case, the mixed ratio 
differs depending on a chemical composition of the water-absorbent resin 
powders. However, the percent by weight of the mixed ratio can be 
confirmed by a preliminary test with ease. 
In most cases, the ratio (percent by weight) of water and hydrophilic 
organic solvent is water: hydrophilic organic solvent=1.about.50: 
99.about.50. The amount of use of the washing solution is determined based 
on the amount of a residue of a crosslinking agent in the water-absorbent 
resin powders and the washing solution. It is typically used in an amount 
of from 50 to 2,000 parts by weight, more preferably from 100 to 1,000 
parts by weight to 100 parts by weight of the water-absorbent resin 
powder. 
In the present invention, when making the mixed solution of water and 
hydrophilic organic solvent, the contact is carried out by the continuous 
or non-continuous batch process. 
For example, when adopting the method in which the water-absorbent resin 
agent powders are made in a mixed solution composed of water and 
hydrophilic organic solvent, the water-absorbent agent powders are made by 
following process: the water-absorbent resin powders are contacted with 
the mixed solution due to stirring if necessary, and subsequently, the 
water-absorbent resin powders are washed by separating them from the mixed 
solution by decantation or suction by filtering. In the case of 
continuously carrying out a washing treatment, the flow of the 
water-absorbent resin powders and the washing solution can be in both the 
same direction and the countercurrent direction. However, in terms of 
washing effect, the countercurrent direction is preferable. In the batch 
system, the number of times a washing treatment is carried out by batch 
process is not particularly limited. 
In each of the described methods, a washing treatment is typically carried 
out in a range of from 15 seconds to 2 hours, preferably from 30 seconds 
to 60 minutes, and more preferably from 1 minute to 30 minutes. A wide 
range is applicable for the temperature of the liquid and powders when 
washing. However, it is preferable that the washing temperature is above 
room temperature, more preferable from 30 to 100.degree. C., more 
preferably from 40 to 80.degree. C., and still more preferably from 40 to 
60.degree. C. The described washing treatment may be carried out under an 
applied pressure, reduced pressure or normal pressure. However, it is 
typically carried out under normal pressure. The water-absorbent resin 
powders after carrying out the washing treatment may be dried, if 
necessary. In the water-absorbent agent powders, the amount of a residue 
of the cross-linking agent having an epoxy group is preferably not more 
than 2 ppm, more preferably not more than ND. It is further preferable 
that the absorbency under pressure of the water-absorbent agent powders is 
not less than 20 g/g, more preferably not less than 25 g/g. These 
properties are achieved by the method of the present invention. 
In the described first method (treatment with a nucleophilic reagent) and 
the second method (washing treatment), in order to prevent a drop in 
absorbing rate by the surface crosslinkage and improve the absorbing rate, 
at least one of the water-soluble polymer and the surfactant, preferably, 
the water-soluble surfactant is used together with the nucleophilic 
reagent and the washing agent. To be specific, such material may be added 
separately from the nucleophilic reagent and the washing agent, more 
preferably with the water-soluble surfactant with the nucleophilic reagent 
and the washing agent. Here, it is more preferable that water be added 
when adding these additives. 
The present invention provides a method of manufacturing water-absorbent 
agent powders made from water-absorbent resin powders, which are surface 
crosslinked and dried water-absorbent resin powders to which at least one 
member selected from the group consisting of the water-soluble surfactant 
and the water-soluble polymer are added in a sufficient amount to increase 
the absorbing rate of water-absorbent resin powders, the absorbing rate 
being defined as 28 times swelling time with an artificial urine. 
The above water-absorbent resin powder has an irregular crushed shape whose 
absorbency under load of 50 g/cm.sup.2 is increased to at least 20 g/g 
based on the physiologic saline solution by the surface crosslinkage. It 
is preferable that the water-absorbing rate be heightened by not less than 
0.02 (g/g/sec), more preferably before adding, more preferably by not less 
than 0.05 (g/g/sec), and more preferably by not less than 0.1 (g/g/sec) 
compared with the water-absorbing rate before adding the above material. 
As described, it has been found that when carrying out a surface 
crosslinkage with a crosslinking agent having an epoxy group, in order to 
increase the absorbency under high pressure that is durable under a heavy 
weight, specifically, when the absorbency under pressure with respect to 
the physiologic saline solution under an applied load of 50 g/cm.sup.2 is 
increased to not less than 20 g/g, the absorbing rate (g/g/sec) based on 
28 times swelling time with artificial urine may be dropped on the 
contrary. 
However, like the present invention, it is preferable to add the 
water-soluble surfactant or the water-soluble polymer, and is more 
preferable to further add water since the water-absorbent agent powders in 
which a drop in absorbing rate by the surface crosslinking is suppressed 
can be obtained while maintaining the absorbency under high pressure 
compared with the state where the process of the present invention has not 
been applied. For the crosslinking agent to be adopted for the surface 
crosslinkage, the crosslinking agent having an epoxy group as the surface 
crosslinking agent is especially preferable. However, other crosslinking 
agents may be used. 
For the surfactant to be adopted in the present invention, a nonionic or 
anionic surfactant, preferably nonionic surfactant having HLB 
(Hydrophile-Lipophile Balance) of not less than 7, more preferably not 
less than 9, and still more preferably not less than 11 may be used. 
The amount of use of the surfactant is preferably in a range of from 0.001 
to 2 parts by weight, more preferably in a range of from 0.01 to 1 part by 
weight, more preferably in a range of from 0.02 to 0.5 parts by weight 
based on 100 parts by weight of water-absorbent resin powders. If the 
surfactant is used in an amount less than 0.01 parts by weight, a 
sufficient adding effect cannot be obtained. On the other hand, when the 
surfactant is added in an amount greater than 2 parts by weight, a 
sufficient effect of improving the absorbing rate for the amount of 
surfactant to be added cannot be obtained. Moreover, the absorbency under 
pressure may be dropped on the contrary, i.e., undesirably reduced. 
Examples of the water-soluble polymer may be but are not limited to: a 
water soluble polymer such as starch, methylcellulose, 
carboxymethylcellulose, hydroxyethylcellulose, polyalkyleneoxide, 
polyacrylic acid, polyacrylate salt, etc. It is preferable that 
water-soluble polymer be a nonionic or anionic water-soluble polymer. 
Examples of the anionic surfactant to be used in the described surfactant 
may be but are not limited to: sodium oleate, fatty acid salt such as 
potassium castor oil, sodium laurylsulfate, alkylsulfate such as ammonium 
lauryl sulate, etc., alkyl benzene sulfonate such as sodiumdodecylbenzene 
sulfonate, etc., alkyl naphthalene sulfonate, dialkyl sulfosuccinate, 
alkyl phosphate, naphtalenesulfonic acid formalin condensate, 
polyoxyethylene alkyl sulfate, etc. 
Examples of the nonionic surfactant to be used in the described surfactant 
may be but are not limited to: polyoxyethylenealkylether, 
polyoxyethylenealkylphenol ether, polyoxyethylene fatty acid ester, 
polyoxyethylene sorbitan fatty acid ester, polyoxy ethylenealkylamine, 
fatty acid ester, oxyethylene-oxypropylene block polymer, etc. 
Examples of the cationic surfactant to be adopted in the surfactant may be 
but are not limited to: alkyl amine salt such as lauryl amine acetate, 
stearylamine acetate, etc., quatenary ammonium salt such as lauryl 
trimethylammonium chloride, stearyltrimethylammonium chloride, etc. 
For the amphoteric surfactant to be adopted as the described surfactant, 
lauryl dimethylamineoxide, etc., may be used. 
As described, with regard to the first method and the second method, in 
order to achieve the object of the present invention, the present 
invention provides a method of manufacturing water-absorbent resin powders 
being dried having a carboxyl group, in which to water-absorbent resin 
powders of irregular crushed type in which the surface thereof is 
crosslinked, having an absorbency under pressure with respect to a 
physiologic saline solution under load of 50 g/cm.sup.2 is increased to at 
least 20 g/g, at least one member selected from the group consisting of a 
water-soluble surfactant and a water-soluble polymer is added in a 
sufficient amount exceeding an absorbing rate of the water-absorbent resin 
powders after the surface crosslinking treatment, the absorbing rate being 
defined by 28 times swelling time with artificial urine. 
The manufacturing method of the water-absorbent agent powders of the 
present invention enables new water-absorbent agent powders to be 
manufactured. To be specific, the present invention provides well-balanced 
properties which are mutually contradictory and cannot be obtained by the 
conventional methods, i.e., absorbency under pressure, absorbing rate, an 
amount of a residue of the epoxy crosslinking agent. 
The water-absorbent agent powders resulting from the present invention are 
at least partially porous and exhibit a reduced amount of a residue of the 
crosslinking agent of from 1/several tens to 1/several hundreds times of 
that of the convention water-absorbent agent powders in spite of a large 
surface area. 
Namely, the water-absorbent agent powders of the present invention are at 
least partially porous and surface regions are crosslinked by a 
crosslinking agent having an epoxy group, and an amount of a residue of 
the crosslinking agent is reduced to not more than 2 ppm, and more 
particularly to ND. 
It is preferable that the water-absorbent agent powders exhibit an 
absorbency under load of 50 g/cm.sup.2 based on a physiologic saline 
solution of not less than 20 g/m.sup.2, more particularly not less than 25 
g/m.sup.2, while exhibiting an absorbency without pressure of not less 
than 35 g/g. 
The water-absorbent agent powders have a BET specific surface area of resin 
powders (resin precursor) of not less than 0.025 m.sup.2 /g based on the 
resin powders having a particle diameter size ranging from 300 to 600 
.mu.m, more preferably not less than 0.03 m.sup.2 /g, more preferably not 
less than 0.04 m.sup.2 /g, and an absorbing rate defined herein of not 
less than 0.4 (g/g/sec) and preferably not less than 0.7 (g/g/sec). 
The manufacturing method of the present invention enables water-absorbent 
agent powders to be manufactured efficiently, which exhibit excellent 
water-absorbent agent properties such as significantly reduced amount of a 
residue of a crosslinking agent by the nucleophilic reagent treatment 
while ensuring a sufficient absorbency under high pressure safely, 
befitting sanitary materials. 
In the present invention, it is permitted for functions to add additives to 
the water-absorbent agent powders resulting from the present invention, 
such as a deodorizer, perfume, inorganic powders, foaming agent, pigment, 
dye, hydrophilic short fiber, plasticizer, binder, surface active agent, 
fertilizer, etc. 
Such compounds are disclosed, for example, in U.S. Pat. Nos. 4,179,367, 
4,190,563, 4,500,670, 4,693,713, 4,812,486, 4,863,989, 4,929,717, 
4,959,060, 4,972,019, 5,078,992, 5,229,488, EP Patent No. 0009977, EP 
Patent No. 0493011, etc. 
It is further permitted to further granulate or mold the water-absorbent 
agent powders of the present invention. The granulating method is, for 
example, disclosed in U.S. Pat. No. 4,734,478, EP Patent No. 0450922, EP 
Patent No. 480031, etc.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
In order that the invention may be more readily understood, the following 
non-limiting examples and comparative examples are given. 
The present invention will be described in detail by way of examples and 
comparative examples. However, the present invention is not limited to the 
disclosure below. 
The following method of various properties of the water-absorbent agent 
powders is measured by the following method. 
(a) Absorbency Without Pressure 
A sample, 0.2 grams of the water-absorbent agent powders was uniformly 
placed into a tea-bag like pouch (40 mm.times.150 mm) made of nonwoven 
fabric, immersed in an aqueous 0.9 percent physiological saline solution 
for 60 minutes. After leaving it for 60 minutes, the bag was taken out. 
The pouch was subjected to hydro-extraction for a predetermined period of 
time, and then weighed (w.sub.1), while the tea bag-like pouch empty of 
the sample as a blank was processed in the same manner and then weighed 
(W.sub.0). The absorbency without pressure (g/g) was calculated from the 
weights W.sub.1 and W.sub.0 in accordance with the following equation: 
EQU Absorbency without pressure (g/g)=(Weight W.sub.1 (g)-Weight W.sub.0 
(g))/Weight of Water-Absorbent Agent Powders 0.2 (g). 
(b) Absorbency Under High Pressure 
The measuring device to be used in measuring the absorbency under pressure 
will be explained in reference to FIG. 1. 
As shown in FIG. 1, the measuring device includes a balance 1, a container 
2 of a predetermined capacity placed on the balance 1, an air-intake pipe 
3, a conduit 4, a glass filter 6, and a measuring section 5 placed on the 
glass filter 6. 
The container 2 has an opening 2a on the top and an opening 2b on the side, 
and the air-intake pipe 3 is inserted through the opening 2a while the 
conduit 4 is fixed to the opening 2b. Further, a predetermined amount of 
the physiological saline solution 12 is poured in the container 2. 
The lower end portion of the air-intake pipe 3 is dipped into the 
physiological saline solution 12. The air-intake pipe 3 is provided for 
keeping the pressure within the container 2 substantially at atmospheric 
pressure. The glass filter 6 has a diameter of, for example, 55 mm, and is 
mounted to a filter section 8 so as to close the upper end of the opening 
of the filter section 8. 
The opening 2b on the side and the lower end opening of the filter section 
8 is connected through the conduit 4 so that the inside of the container 2 
and the inside of the filter section 8 communicate with each other through 
the conduit 4. The relative position and the height of the glass filter 6 
to the container 2 are fixed. The measuring section 5 includes a paper 
filter 7, a supporting cylinder 9, a metal gauze 10 affixed to the bottom 
of the supporting cylinder 9, and a cylindrical weight 11. 
The supporting cylinder 9 has the same inner diameter as the upper end 
opening of the filter section 8. The weight 11 is provided so as to be 
freely slidable in the axial direction of the supporting cylinder 9. 
In the measuring section 5, the paper filter 7, and the supporting cylinder 
9 (that is, the metal gauze 10) are sequentially placed on the glass 
filter 6 in this order, and the weight 11 is placed on the metal gauze 10 
inside the supporting cylinder 9. 
The metal gauze 10 is made of stainless steel to have a 400-mesh (the size 
of each mesh: 38 .mu.m). The height position of the upper surface of the 
metal gauze 10, i.e., a contact surface between the upper surface of the 
metal gauze 10 and the water-absorbent agent powders 15 is set equivalent 
to the height of the lower end surface 3a of the air-intake pipe 3. 
A predetermined amount of the water-absorbent agent powders is uniformly 
scattered on the metal gauze 10. The weight 11 is adjusted in such a 
manner to apply a load of 50 g/cm.sup.2 evenly to the metal gauze 10, that 
is, the water-absorbent agent powders 15. 
The absorbency under pressure of the water-absorbent agent powders was 
measured by using the above-arranged measuring device in the manner 
described below. 
To begin with, preparatory operations are carried out, that is, a 
predetermined amount of physiological saline solution 12 was poured into 
the container 2, and the air-intake pipe 3 was inserted into the container 
2. Then, the paper filter 7 was placed onto the glass filter 6. In the 
meantime, 0.9 g of water-absorbent agent powders were uniformly scattered 
inside the supporting cylinder, i.e., on the metal gauze 10, and the 
weight 11 was placed on the water-absorbent agent powders 15. 
The metal gauze 10, that is, the supporting cylinder 9 having the 
water-absorbent agent powders and the weight 11 inside, is placed on the 
glass filter 6 in such a manner that the supporting cylinder 9 is coaxial 
to the glass filter 6. 
Then, the weight of the physiological saline solution 12, which has been 
absorbed by the water-absorbent agent powders for 60 minutes since the 
supporting cylinder 9 was placed on the paper filter 7 was measured by the 
balance 1. 
Then, in the same procedures but without the water-absorbent agent powders 
15, the weight of the physiological saline solution 12 that was absorbed 
by the paper filter 7, etc., was calculated using the balance 1 as a blank 
value. 
The absorbency (g/g) under high pressure with respect to the 
water-absorbent agent powders 15 was obtained by deducting the blank 
value, and the weight of the physiological saline solution 12 that is 
actually absorbed by the water-absorbent agent powders 15 divided by the 
initial weight (0.9 g) of the water-absorbent agent powders. 
(c) Residual Amount of Crosslinking Agent Having an Epoxy Group in the 
Water-absorbent Resin Powders or the Water-absorbent Agent Powders 
A sample, 2.0 g of the water-absorbent agent powders was added to 100 ml 
beaker, and 2 ml of composite solution with a ratio of methyl 
alcohol/water of 2/1 percent by weight was added to the beaker, and then 
the beaker was closed with a tap, and was left for one hour. Then, 5 ml of 
methyl alcohol was added to the beaker and the content in the beaker was 
filtered off, and 1.0 g of the filtered solution was placed in an eggplant 
type flask, and 0.05 ml of 12 wt % of nicotine amide solution was added to 
the flask. 
The air cooling tube was mounted to the eggplant type flask, and was heated 
for 30 minutes in a boiled water bath, and then the reactant solution in 
the flask was filtered by the paper filter. Next, after the filtered 
solution was concentrated, the additive of nicotine amide--crosslinking 
agent in the concentrated solution was analyzed by UV absorption using a 
high performance liquid chromatography. 
On the other hand, in place of the water-absorbent agent powders, a 
predetermined amount of the crosslinking agent was added, and the line of 
the detected amount was determined to be an external standard, and an 
amount of a residue of the surface crosslinking agent (ppm) in the 
water-absorbent agent powders were calculated in consideration of the 
dilution rate of the filtered solution. 
(d) Absorbing Rate (Swell Rate) 
0.358 g of water-absorbent agent powders (having a particle size ranging 
from 300 to 850 .mu.m) were dispersed in a glass test tube (with height of 
126 mm) with an inner diameter of around 14.1 mm. Next, 10.0 g of 
artificial urine set to 25.degree. C. was poured still at once from the 
top center. After 0.358 g of absorbent powders absorbed all of the urine 
by sight, a time required for forming a 28 times swollen gel (g/g) was 
measured in seconds, and the absorbing rate (g/g/sec) was obtained by 
dividing 28 times (g/g) with seconds. The larger the value of the 
absorbing rate becomes, the faster the water-absorbent agent powders 
absorb the urine. The artificial urine used in the present invention is a 
solution in which 0.2 percent by weight of sodium sulfate, 0.2 percent by 
weight of potassium chloride, 0.05 percent by weight of magnesium chloride 
hexahydrate, 0.025 percent by weight of calcium chloride dixahydrate, 
0.085 percent by weight of ammonium dihydrogenphosphate, and 0.015 percent 
by weight of diammonium hydrogenphosphate were dissolved in water. 
(e) Specific Surface Area 
The area/weight ratio was obtained in the following manner. The powdered 
resin precursor classified into from 300 to 600 .mu.m by a gauze of JIS 
standard was deaerated at 150.degree. C. for 40 minutes, and the specific 
surface area was measured by the BET (Brunauer-Emmett-Teller) absorption 
method based on krypton gas while cooling with liquid nitrogen. 
(f) Water-Soluble Component 
After 0.5 g of water-absorbent resin powders are stirred for 16 hours with 
1 liter deionized water, the swollen gel was removed with the paper 
filter. Then, the amount of polyanion obtained from the water-soluble 
polymer in the filtered solution was measured by the titrimetric colloid 
determination. By the measuring method, the water-soluble polymer, i.e., 
extractables eluted from the water-absorbent resin powders was measured in 
the water-absorbent resin powders. 
(g) Solid Portion 
1.000 g of water-absorbent resin powders were placed in an aluminum cup, 
and was dried at 180.degree. C. for 3 hours in an oven with no wind. Then, 
the solid portion was measured based on its loss in weight on drying. 
REFERENCE EXAMPLE 1 
In 5,500 g (with monomer density of 37 percent) monomer solution of 75 mole 
percent neutralized acrylic acid sodium salt, 1.77 g (0.05 mole percent) 
of N,N'-methylene-bis acrylamide (inner crosslinking agent) was dissolved 
to obtain a mixed solution. After the mixed solution was deaerated for 30 
minutes with nitrogen gas, it was supplied into a reactor of a covered 
kneader having an inner volume of 10 liters and equipped with two sigma 
vanes. The kneader is twin arm type equipped with a jacket and is made 
from stainless steel. While maintaining the mixed solution at 30.degree. 
C., it was further replaced with nitrogen. 
Next, with respect to the mixed solution, 2.40 g of sodium persulfate and 
0.12 g of L-ascorbic acid were added to the mixed solution while agitating 
the mixed solution by the vanes. Then, a polymerization was started in one 
minute. After an elapse of time of 16 minutes, the peak of the temperature 
in the mixed solution, i.e., the polymerization reaction, reached 
83.degree. C. 
The hydrogel polymer resulting from the polymerization reaction was 
substantially transparent without foam, and was subdivided into a diameter 
of around 5 mm. Then, after carrying out a polymerization reaction with 
further stirring for 60 minutes, the hydrogel polymer was taken out. The 
resulting finely divided hydrogel polymer was placed on a metal gauze of 
300 .mu.m (50 mesh) and dried under hot air at 150.degree. C. for 90 
minutes. Then, the resulting dried polymer was pulverized by a roll mill 
and further classified by a metal gauze of 850 .mu.m mesh to obtain a 
resin precursor (A). The resulting resin precursor (A) had an irregular 
shape, and an average particle diameter of 360 .mu.m, the resin precursor 
(A) having a content of resin having a particle diameter of less than 150 
.mu.m of 5 percent by weight and a water content of 6 percent by weight. 
The water-soluble component in the resin precursor (A) was less than 10 
percent. To the resin precursor (A), foams are not observed by an electric 
microscope. The BET specific surface area of the particles with a diameter 
of from 300 to 600 .mu.m in the resin precursor (A) was 0.018 m.sup.2 /g. 
An electron micrograph of the non-porous particles of the irregular shape 
(with a diameter of from 300 to 600 .mu.m) in the resin precursor (A) is 
shown in FIG. 2. The properties thereof are summarized in Table 1. 
TABLE 1 
__________________________________________________________________________ 
TEMP OF RESIDUAL 
POWDERS ABSORBENCY 
CROSS- 
WATER- DURING UNDER LINKING 
ABSORBING 
ABSORBENT 
THE ABSORBENCY 
PRESSURE 
AGENT RATE 
AGENT PROCESS 
(g/g) (g/g) (ppm) (g/g/sec) 
__________________________________________________________________________ 
RESIN -- 44 11 (NOT 0.31 
PRECURSOR USED) 
(A) 
RESIN -- 52 9 (NOT 0.30 
PRECURSOR USED) 
(B) 
RESIN -- 45 8 (NOT 0.72 
PRECURSOR USED) 
(C) 
RESIN -- 44 8 (NOT 0.65 
PRECURSOR USED) 
(D) 
WATER- -- 40 22 130 0.22 
ABSORBENT 
RESIN (1) 
WATER- -- 43 26 40 0.28 
ABSORBENT 
RESIN (2) 
WATER- -- 38 24 70 0.70 
ABSORBENT 
RESIN (3) 
WATER- -- 37 23 60 0.65 
ABSORBENT 
RESIN (4) 
WATER- -- 30 15 3 0.30 
ABSORBENT 
RESIN (5) 
__________________________________________________________________________ 
REFERENCE EXAMPLE 2 
As a monomer to be used in polymerization, to 5,500 g of a monomer solution 
of acrylate sodium salt that was 75 mole percent neutralized sodium salt 
acrylate (with a monomer density of 33 percent), 4.9 g (0.045 mole 
percent) of polyethylene glycol diacrylate (n=8) was dissolved as an inner 
crosslinking agent. 
After the mixed solution was replaced with nitrogen, it was polymerized by 
leaving at rest with a thickness of around 5 cm in the following manner. 
2.40 g of sodium persulfate and 0.12 g of L-ascorbic acid were added as a 
polymerization initiator, and further 4 g of 2,2'-azobis(2-amidinopropane) 
dihydrochloride which serves both as a foaming agent and a polymerization 
initiator was added to be uniformly dissolved therein. After an elapse of 
time of 1 minute, a polymerization was started. The mixed solution has a 
peak temperature reached to 70.degree. C. in the polymerization reaction. 
The resulting hydrogel polymer from the polymerization reaction was porous 
gel-like polymer containing foam with a diameter ranging from 1 to 2 mm. 
The hydrogel polymer was pulverized by a meat chopper, and was placed on a 
metal gauze of 300 .mu.m and dried under hot air at 160.degree. C. for 60 
minutes. Then, the resulting dried polymer was pulverized by a roll mill 
and further classified by a metal gauze of 850 .mu.m mesh to obtain a 
resin precursor (B). The resulting resin precursor (B) had an irregular 
shape, and an average particle diameter of 330 .mu.m, the resin precursor 
(B) having a content a resin having a particle diameter of less than 150 
.mu.m of 8 percent by weight and a water content of 6 percent by weight. 
The water-soluble component in the resin precursor (B) was less than 10 
percent. The BET specific surface area of particles with a diameter 
ranging from 300 to 600 .mu.m in the resin precursor (B) was 0.025 m.sup.2 
/g. The resin precursor (B) was observed with the electron microscope and 
was found to be porous in which foams are partially formed in the 
particles. An electron micrograph of the partially porous particles of the 
irregular shape (with a diameter of from 300 to 600 .mu.m) in the resin 
precursor (B) is shown in FIG. 2. The properties thereof are summarized in 
Table 1. 
REFERENCE EXAMPLE 3 
While keeping the temperature of the solution at 20.degree. C., to 36 parts 
of 10 percent concentration of 2,2'-azobis(2-methylpropionamizine) 
dihydrochloride solution, 6.7 parts of 37 percent solution of sodium 
acrylate were added with stirring at 1200 rpm to form a mixed solution. 
After an elapse of time of several seconds, the mixed solution was turbid, 
thereby obtaining a white solid portion in a form of fine particles with 
an average diameter of 10 .mu.m. 
By filtering off the white turbid solution, around 2.2 parts of white solid 
portion of fine particles with an average particle diameter of 10 .mu.m 
were isolated, and were rinsed off with water to be purified. The solid 
portion was confirmed with UV absorption (365 nm) showing a particular azo 
group, and based on the results of element analysis, it was found that the 
resulting solid was 2,2'-azobis(2-methylpropionamizine) diacrylate having 
a uniform water dispersibility. 
In the polymerization device to be used in Example 2, as a monomer for use 
in polymerization, to 5500 g (with a monomer content of 38 percent) of 75 
mole percent neutralized acrylate sodium salt, 3.49 g (0.05 mole percent) 
trimethylolpropane triacrylate was dissolved as an inner crosslinking 
agent, and 4 g of 2,2'-azobis(2-methylpropionamizine) diacrylate complex 
was uniformly dispersed as the foaming agent. Then, sodium persulfate and 
L-ascorbic acid were added in the same manner as Reference 2. 
In the resulting hydrogel polymer, foam (bubbles) with a diameter of not 
more than 100 .mu.m was uniformly contained. The hydrogel polymer is a 
porous gel-like polymer which is white in color due to the resulting foam 
(bubbles). The hydrogel polymer was cut into pieces of from 5 to 10 mm, 
and was placed on a metal gauze with 300 .mu.m (50 mesh), and was dried 
for 60 minutes at 150.degree. C. The dried product was pulverized by a 
roll mill and further classified by a metal gauze of 850 .mu.m mesh, 
thereby obtaining a resin precursor (C). 
The resin precursor (C) has an irregular crushed shape with an average 
particle diameter of 300 .mu.m, and a ratio of the resin having a particle 
diameter of less than 150 .mu.m of 8 percent by weight and a water content 
of 6 percent by weight. 
The water-soluble content of the resin precursor (C) was less than 10 
percent. In the resin precursor (C), the BET specific surface area with a 
particle of from 300 to 600 .mu.m was 0.04 m.sup.2 /g. The resin precursor 
(C) was observed by the electron microscope, and was found to be porous in 
which foam (bubbles) is uniformly formed in the particle. An electron 
micrograph which shows the particle structure of the porous irregular 
crushed type (with a diameter of from 300 to 600 .mu.m) shown in FIG. 4. 
The properties thereof are summarized in Table 1. 
REFERENCE EXAMPLE 4 
Reference example 3 was repeated except that the foaming agent was changed 
to 50 g of sodium carbonate, and 2 g of polyoxyethylene sorbitan 
monostearate and 10 g of hydroxy ethyl cellulose were used as an auxiliary 
material of the foaming agent with respect to the monomer solution to 
uniformly disperse the foaming agent to the monomer solution. 
The monomer solution was polymerized in the same manner as Example 3. The 
resulting hydrogel polymer was porous gel-like polymer which was white in 
color as foam (bubbles) with a particle diameter of not more than 100 
.mu.m were uniformly formed. The hydrogel polymer was cut into pieces of 
from around 5 to 10 mm, and was dried, pulverized, and was classified like 
Example 3, and the resin precursor (D) was obtained. 
The resin precursor (D) had an irregular shape having a content of 
particles with an average particle diameter of 360 .mu.m and a content of 
the particles of particle diameter of less than 150 .mu.m of 8 percent by 
weight and water content of 6 percent by weight. The water-soluble 
component of the resin precursor (D) was less than 10 percent. In the 
resin precursor (D), the BET specific surface area of particles with a 
particle diameter of from 300 to 600 .mu.m was 0.03 m.sup.2 /g. The resin 
precursor (D) was observed with the electron microscope and was found to 
be porous in which foams are uniformly formed entirely. The properties of 
the resin precursor (D) are summarized in Table 1. 
COMATIVE EXAMPLE 1 
To 100 parts of the non-porous resin precursor (A) resulting from Reference 
Example 1, 0.1 parts of ethylene glycol diglycidyl ether as a crosslinking 
agent with an epoxy group and 4 parts of water were mixed, and a heat 
treatment was applied to the mixture for 40 minutes at 120.degree. C., 
thereby obtaining the water-absorbent resin powders (1). The properties 
thereof were summarized in Table 1. 
COMATIVE EXAMPLE 2 
To 100 parts of the resin precursor (B) resulting from Reference Example 2, 
which was partially porous, 0.1 part of ethylene glycol diglycidyl ether 
as a crosslinking agent with an epoxy group, 4 parts of water and 0.5 
parts of isopropyl alcohol were mixed, and a heat treatment was applied to 
the mixture for 30 minutes at 120.degree. C., thereby obtaining the 
water-absorbent resin powders (2). The properties thereof are summarized 
in Table 1. 
COMATIVE EXAMPLE 3 
To 100 parts of the resin precursor (C) resulting from Reference Example 3, 
which was homogeneously porous, a solution of a crosslinking agent 
composed of 0.15 part of ethylene glycol diglycidyl ether as a 
crosslinking agent with an epoxy group, 4 parts of water and 1 part of 
ethyl alcohol were mixed, and a heat treatment was applied to the mixture 
for 30 minutes at 180.degree. C., thereby obtaining the water-absorbent 
resin powders (3). The properties thereof are summarized in Table 1. 
COMATIVE EXAMPLE 4 
To 100 parts of the resin precursor (D) resulting from Reference Example 4, 
which was homogeneously porous, a solution of a crosslinking agent 
composed of 0.15 part of ethylene glycol diglycidyl ether having an epoxy 
group, 4 parts of water and 1 part of ethyl alcohol were mixed, and the 
mixture was heated for 40 minutes at 120.degree. C., thereby obtaining the 
water-absorbent resin powders (4). The properties thereof are summarized 
in Table 1. 
COMATIVE EXAMPLE 5 
In order to reduce the amount of a residue of the crosslinking agent, the 
amount of the crosslinking agent having an epoxy group was reduced, and an 
amount of water was increased on the contrary. Namely, to 100 parts of 
non-porous resin precursor (A) resulting from reference example 1, a 
solution of the crosslinking agent composed of 0.01 parts of ethylene 
glycol diglycidyl ether and 40 parts of water, and 10 parts of isopropanol 
was mixed. Then, a heat treatment was applied to the resulting mixture at 
120.degree. C. for 40 minutes, thereby obtaining the water-absorbent resin 
powders (5). As summarized in Table 1, the resulting water-soluble resin 
powders (5) were inferior in their properties, especially absorbency under 
high pressure when the amount of a residue of crosslinking agent after the 
surface-crosslinkage was small as shown in Table 1. 
EXAMPLE 1 
100 parts of the water-absorbent resin powders (1) resulting from 
comparative example 1, while keeping the temperature of the powders at 
50.degree. C., were mixed with 5 parts of 30 percent polyethylene imine 
solution (epomine P-1000 available from Nippon Shokubai, Co Ltd.) as a 
nucleophilic reagent to be absorbed therein and the mixture was heated at 
40.degree. C. for 30 minutes in a form of powders, and then the product 
sized by passing a metal gauze of 20 mesh (850 .mu.m) was taken out, thus 
obtaining the water-absorbent agent powders (1) of this invention. The 
resulting water-absorbent agent powders (1) had an absorbency without 
pressure of 38 g/g and an absorbency under an applied high pressure of 21 
g/g and the amount of residual crosslinking agent (ethylene glycol 
diglycidyl ether) of 1 ppm. The respective properties thereof are 
summarized in Table 2. 
TABLE 2 
__________________________________________________________________________ 
TEMP OF 
POWDERS RESIDUAL 
WATER- DURING ABSORBENCY 
CROSS- 
ABSORBENT 
THE UNDER LINKING 
ABSORBING 
AGENT PROCESS 
ABSORBENCY 
PRESSURE 
AGENT RATE 
POWDERS (.degree. C.) 
(g/g) (g/g) (ppm) (g/g/sec) 
__________________________________________________________________________ 
POWDERS (1) 
50 38 21 1 0.22 
POWDERS (2) 
50 41 22 ND 0.24 
POWDERS (3) 
40 42 21 ND 0.22 
POWDERS (4) 
60 43 26 6 0.31 
POWDERS (5) 
60 43 26 2 0.32 
POWDERS (6) 
60 43 25 1 0.31 
POWDERS (7) 
70 37 23 7 0.66 
POWDERS (8) 
40 35 21 10 0.65 
POWDERS (9) 
40 32 20 7 0.60 
POWDERS (10) 
40 43 26 4 0.40 
POWDERS (11) 
90 43 24 2 0.30 
POWDERS (12) 
130 43 23 4 0.29 
POWDERS (13) 
45 38 24 5 0.74 
POWDERS (14) 
45 37 23 ND 0.84 
POWDERS (15) 
45 37 23 ND 0.88 
POWDERS (16) 
45 37 23 2 0.70 
POWDERS (17) 
30 35 22 8 0.71 
COMATIVE 
20 43 21 15 0.30 
POWDERS (6) 
COMATIVE 
40 44 10 1 0.25 
POWDERS (7) 
__________________________________________________________________________ 
powder: waterabsorbent agent powder 
In Table 2, ND indicates below detectable limit. 
EXAMPLE 2 
Example 1 was repeated except that 5 parts of 30 percent sodium bisulfite 
solution was used as a nucleophilic reagent in place of the 30 percent 
polyethylene imine solution to obtain water-absorbent agent powders (2). 
The water-absorbent agent powders (2) showed absorbency without pressure 
of 41 g/g, and an absorbency under high pressure of 22 g/g, in which the 
residual ethyleneglycol diglycidyl ether was not detected. The properties 
thereof were summarized in Table 1. 
EXAMPLE 3 
100 g of the water-absorbent resin powders (1) resulting from comparative 
example 1 was washed by contacting with 1,000 cc mixed solution of water 
and ethyl alcohol with a weight ratio of 50:50 with stirring for 30 
minutes while keeping the temperature of the powders at 40.degree. C., and 
thereafter the resulting mixture was filtered off and was dried under 
reduced pressure for 24 hours at 50.degree. C., thereby obtaining the 
water-absorbent agent powders (3) of the present invention. The resulting 
water-absorbent agent powders (3) showed absorbency without pressure of 42 
g/g and the absorbency under high pressure of 21 g/g, and the residual 
ethyleneglycol diglycidyl ether was not detected. The result of respective 
properties are shown in Table 2. 
EXAMPLE 4 
To 100 parts of water-absorbent resin powders (2) resulting from 
comparative example 2 while maintaining its temperature at 60.degree. C., 
3 parts of water was added as the nucleophilic reagent to be absorbed 
therein. Thereafter, the resulting mixture was dried at 80.degree. C. for 
1 hour, and was passed through a gauze of JIS standard of 850 .mu.m, 
thereby obtaining the water-absorbent agent powders (4). The properties of 
the water-absorbent agent powders (4) are shown in Table 2. 
EXAMPLE 5 
Example 4 was repeated except that 2 parts by weight of water was further 
added, i.e., the total amount of water to be added as the nucleophilic 
reagent was 5 parts, thereby obtaining water-absorbent agent powders (5). 
The water-absorbent agent powders (5) showed properties summarized in 
Table 2. 
EXAMPLE 6 
Example 4 was repeated except that 10 parts of water were added as the 
nucleophilic reagent in total, thereby obtaining water-absorbent agent 
powders (6). The water-absorbent agent powders (6) showed properties 
summarized in Table 2. 
EXAMPLE 7 
To 100 parts by weight of water-absorbent resin powders (3) resulting from 
comparative example 3, while keeping the temperature of the powders at 
70.degree. C. in a form of powder, 3 parts of diethanol amine 
(nucleophilic reagent) were added to be absorbed therein. The resulting 
mixture was heated for 1 hour at 60.degree. C. in a form of powder, and 
then the mixture was passed through a metal gauze of 850 .mu.m, thereby 
obtaining the water-absorbent agent powders (7). The resulting 
water-absorbent agent powders (7) showed properties summarized in Table 2. 
EXAMPLE 8 
To 100 parts by weight of water-absorbent resin powders (4) resulting from 
comparative example 4, while keeping the temperature of the powders at 
40.degree. C., 5 parts of propylene glycol (nucleophilic reagent) were 
added therein. The resulting mixture was heated in a form of powder for 1 
hour at 80.degree. C., and then the mixture was passed through a metal 
gauze of 850 .mu.m, thereby obtaining the water-absorbent agent powders 
(8). The resulting water-absorbent agent powders (8) showed properties 
summarized in Table 2. 
EXAMPLE 9 
Example 8 was repeated except that an amount of propylene glycol 
(nucleophilic reagent) was altered to 15 parts, thereby obtaining 
water-absorbent agent powders (9). The resulting water-absorbent agent 
powders had properties shown in Table 2. 
EXAMPLE 10 
Example 4 was repeated except that 0.1 part of nonionic surfactant agent 
polyoxyethylene sorbitan monostearate (HLB=14.9) and 5 parts of water 
(nucleophilic reagent) were added, and subsequently, a heat treatment was 
applied for 1 hour at 60.degree. C. while maintaining the solid portion, 
and then dried, thus obtaining a water-absorbent agent powders (10). The 
resulting water-absorbent agent powders (10) had properties shown in Table 
2. 
COMATIVE EXAMPLE 6 
Example 6 was repeated except that the temperature of the water-absorbent 
resin powders (2) to which the nucleophilic reagent was to be added was 
changed from 60.degree. C. (Example 6) to 20.degree. C. to obtain 
comparative water-absorbent agent powders (6). In this comparative 
example, the nucleophilic reagent was continuously mixed with the 
water-absorbent resin powders, and a cohered lump was formed in the resin 
to be supplied gradually, resulting in non-uniform mixture. The properties 
of the final product of the comparative water-absorbent agent powders (6) 
are shown in Table 2. Compared with the water-absorbent agent powders (6) 
resulting from example 6, the comparative water-absorbent agent powders 
(6) showed apparently a larger amount of agglomeration, and inferior 
properties. 
COMATIVE EXAMPLE 7 
Example 6 in which processes were carried out in form of powder was 
repeated except that the amount of additive (nucleophilic reagent) was set 
to 100 parts. Then, in the same manner as Example 6, water-absorbent resin 
powders (2) were processed in a gel state, and were passed through by a 
gauze of 850 .mu.m after applying a heat treatment, thereby obtaining 
comparative water-absorbent agent powders (7). The resulting comparative 
water-absorbent agent powders (7) had properties shown in Table 2. The 
comparative water-absorbent agent powders (7) showed substantially the 
same effect of reducing the amount of a residue of the crosslinking agent; 
however, the absorbency thereof under pressure was greatly lowered. 
EXAMPLE 11 
Example 6 was repeated except that the temperature of the water-absorbent 
resin powders (2) to which the nucleophilic reagent was to be added was 
changed from 60.degree. C. (Example 6) to 90.degree. C. to obtain 
water-absorbent agent powders (11). The mixability of the nucleophilic 
reagent to the water-absorbent resin powders was significantly superior to 
that of comparative Example 5 at 20.degree. C. but slightly inferior to 
that of example 6 in which the process was carried out at 60.degree. C. 
The resulting water-absorbent agent powders (11) were slightly inferior in 
its absorbency under high pressure and the effect of reducing the residual 
crosslinking agent to those of example 6. 
EXAMPLE 12 
Example 6 was repeated except that the temperature of the water-absorbent 
resin powders to which the nucleophilic reagent was to be added was 
changed from 60.degree. C. (Example 6) to 130.degree. C. to obtain 
water-absorbent agent powders (12). The mixability of the nucleophilic 
reagent to the water-absorbent resin powders was significantly superior to 
that of comparative example 5, but slightly inferior to that of example 11 
in which the process was carried out at 90.degree. C. The resulting 
water-absorbent agent powders (12) had properties shown in Table 2. The 
water-absorbent agent powders (12) were slightly inferior in its 
absorbency under high pressure and the effect of reducing the. residual 
crosslinking agent to those of example 6. 
EXAMPLE 13 
To 100 parts by weight of water-absorbent resin powders (3) resulting from 
comparative example 3, while keeping the temperature of the powders at 
45.degree. C., 8 parts of water (nucleophilic reagent) were added to be 
absorbed therein. Then, while keeping the solid portion, the mixture was 
further heated for 1 hour at 60.degree. C. in a form of powder, and then 
dried, and was passed through a metal gauze of 850 .mu.m, thereby 
obtaining the water-absorbent agent powders (13). The respective 
properties of the water-absorbent agent powders (13) were shown in Table 
2. 
EXAMPLE 14 
Example 13 was repeated except that 0.1 part of polyoxyethylene sorbitan 
monostearate (HLB=14.9) as nonionic surfactant and 8 parts of water as 
nucleophilic reagent were added as nucleophilic reagent, further 1 part of 
triethanol amine was used, thereby obtaining water-absorbent agent powders 
(14). The resulting water-absorbent agent powders (14) showed properties 
summarized in Table 2. 
EXAMPLE 15 
Example 13 was repeated except that 0.1 parts of polyoxyethylene sorbitan 
monostearate (HLB=14.9) as a nonionic surfactant and 8 parts of water as a 
nucleophilic reagent were added. Further, 3 parts of urea were used, 
thereby obtaining water-absorbent agent powders (15). The resulting 
water-absorbent agent powders (15) had properties summarized in Table 2. 
EXAMPLE 16 
Example 13 was repeated except that in addition to 8 parts of water as a 
nucleophilic reagent, 2 parts of sodium hydroxide were used, and further 
0.1 part of sodium polyoxyethylene lauryl ether sulfonate as a surfactant 
were added, thereby obtaining water-absorbent agent powders (16). The 
resulting water-absorbent agent powders (16) showed properties summarized 
in Table 2. 
EXAMPLE 17 
To 100 parts of water-absorbent resin powders (3) obtained from comparative 
example 3 maintained at 30.degree. C., 8 parts of water were absorbed 
under temperature of 30.degree. C. and humidity of 90% RH. Next, the 
resulting mixture was sealed and left in a form of powder for 40 days 
without an application of heat, thereby obtaining the water-absorbent 
agent powders (17). The water-absorbent agent powders (17) showed 
properties summarized in Table 2. 
INDUSTRIAL APPLICATIONS OF THE PRESENT INVENTION 
The manufacturing method of water-absorbent agent powders of the present 
invention in which to conventional surface crosslinked water-absorbent 
resin powders in a form of powder under an applied heat, a nucleophilic 
reagent is added to reduce an amount of a residue of a crosslinking agent, 
offers water-absorbent agent powders which show as high absorbency under 
high pressure as without pressure without having a residue of a 
crosslinking agent containing a highly reactive epoxy group in the 
water-absorbent agent powders with ease and stably. The water-absorbent 
agent powders of the present invention manifest a high absorbing rate, 
liquid permeability and are not likely to be moved or separated from a 
pulp. Having the described beneficial properties, the water-absorbent 
agent powders of the present invention are suited for use in especially 
sanitary materials such as disposable diapers, sanitary napkins, etc. 
The water-absorbent agent powders of the present invention show as high 
absorbency under high pressure as without pressure without having a 
residue of a crosslinking agent of a highly reactive epoxy group, high 
absorbing rate, high permeability of liquid, and are not likely to be 
shifted or fallen off from pulp. Such water-absorbent agent powders are 
suited for use in sanitary material such as disposable diaper, sanitary 
napkins, etc.