Process for the preparation of water absorptive composite material

Aqueous solution of alkali or ammonium acrylate is applied to a prefabricated fibrous substrate, and the acrylate is subjected to polymerization in two steps, namely the first step in which the acrylate is polymerized by means of a radical polymerization initiator and the second step which is subsequent to the first step and in which the unpolymerized acrylate is polymerized by means of irradiation with electromagnetic radiation or corpuscular ionizing radiation.

BACKGROUND OF THE INVENTION 
1. Field of the Art 
This invention relates to a process for preparing a water absorptive 
composite material comprising a water absorptive polymer and a 
prefabricated fibrous substrate. More particularly, this invention relates 
to a process for preparing a water absorptive composite material in which 
a highly water absorptive polymer is held on a prefabricated substrate, 
comprising applying an aqueous solution of an acrylic acid type monomer to 
a prefabricated fibrous substrate, polymerizing the acrylic acid type 
monomer by means of an aqueous radical polymerization initiator to obtain 
a precursor of the composite, which is further irradiated with 
electromagnetic radiation or corpuscular ionizing radiation. 
The water absorptive composite material obtained by the process according 
to this invention can be advantageously used in the production of a 
variety of water absorptive materials, because it is excellent in water 
absorption properties, has a high water absorption velocity and an 
extremely low content of unpolymerized monomers and the highly water 
absorptive polymer is held with good stability on the fibrous substrate. 
2. Prior Art 
Materials such as paper, pulp, nonwoven fabric, spongy urethane resins and 
the like have hitherto been used as water retentive materials for a 
variety of sanitary goods such as a sanitary napkin, paper diaper and the 
like and a variety of agricultural materials. However, these materials 
have a water absorption capacity of no more than 10-50 times their own 
weight, which will cause problems that an extensively increased bulk of 
the material is required for absorbing or retaining a large amount of 
water and that water is easily released from the material in which water 
has been absorbed on pressing it. 
There have recently been proposed a variety of highly water absorptive 
polymer materials in order to settle the aforementioned problems of the 
water absorptive materials of this kind. For instance, there have been 
proposed a graft polymer of starch (Japanese Patent Publication No. 
46199/78, etc.), a denaturated cellulose (Unexamined Published Japanese 
Patent Application No. 80376/75, etc.), a crosslinked water soluble 
polymer (Japanese Patent Publication No. 23462/68, etc.), a 
self-crosslinking polymer of an alkali metal salt of acrylic acid 
(Japanese Patent Publication No. 30710/79, etc.), and the like. 
However, these highly water absorptive polymer materials, while having a 
relatively high level of water absorption properties, are obtained as 
powder in most cases. Therefore, in order to use them for sanitary goods 
such as a sanitary napkin, paper diaper or the like, it is necessary to 
disperse them homogeneously on such substrates as tissue paper, nonwoven 
fabric, cotton or the like. However, the polymer powder having been 
dispersed in such a manner is difficult to be firmly held on the substrate 
and often agglomerate partially. Also, swollen gel after water absorption 
will easily move from the substrate without being held firmly on it. 
Therefore, if it is used for a paper diaper, for example, it will give the 
feeling of stiffness upon urination accompanied with the extremely 
uncomfortable feeling on wearing. Furthermore, in a process for obtaining 
an absorber by dispersing such a powdery polymer as described above on a 
substrate, the absorber will be very expensive because of complicated 
procedures for powder handling and of problems on processes for 
efficiently conducting uniform dispersion. 
As a method for dissolving these problems, there is disclosed a process for 
producing a water absorptive composite in which an aqueous solution of an 
acrylic acid type monomer is applied in a previously determined pattern to 
a prefabricated fibrous substrate to obtain a composite, which is then 
irradiated with electromagnetic radiation or corpuscular ionizing 
radiation to convert the acrylic acid type monomer into a highly water 
absorptive polymer (Unexamined Japanese Patent Publication No. 500546/82). 
According to this process, uniform dispersion and stable holding of the 
aforementioned powder on a substrate are considerably improved. However, 
since electromagnetic radiation or corpuscular ionizing radiation is 
employed for converting the monomer into the high water absorptive polymer 
in this process, the highly water absorptive polymer inherent to the 
specific monomer tends to be crosslinked excessively. As the result, the 
composite obtained will exhibit extremely poor properties as an absorber. 
Especially its water absorption capacity will be of a level of only half 
or less of that of the composite obtained by using the aforementioned 
highly water absorptive powdery polymer. 
More recently, Unexamined Published Japanese Patent Application No. 
149609/85 discloses a process for preparing a water absorptive composite 
material comprising previously impregnating a water absorptive organic 
material with an aqueous solution of an acrylic acid type monomer and 
adding thereto in a mist form a water soluble radical polymerization 
initiator, or, a water soluble radical polymerization initiator and a 
water soluble reducing agent to conduct polymerization. In this process, 
however, the water soluble polymerization initiator is added after the 
water absorptive organic material has been impregnated with the acrylic 
acid type monomer. Thus, although the polymerization initiator is added in 
a mist form, it is very difficult to completely polymerize the monomer 
because of occurrence of "uneven polymerization" and as the result the 
amount of the residual monomers is in a high level, which will cause 
problems on safety and lead to lowering of the properties of the resulting 
product, especially in respect of its water absorption capacity. 
Possible Countermeasure 
Under these backgrounds, the present inventors have already proposed in 
Japanese Patent Application No. 193403/85 a method that an aqueous 
solution of an acrylic acid type monomer having a monomer concentration of 
25% by weight or more and either a water soluble radical polymerization 
initiator or a water soluble radical polymerization initiator and a water 
soluble reducing agent are previously mixed homogeneously and the mixture 
is applied in a mist form to a prefabricated fibrous substrate so that the 
resulting highly water absorptive polymer in the fibrous substrate will 
have a diameter in the range of 30-500 .mu.m, followed by polymerization; 
in Japanese Patent Application No. 202908/85 a method that an aqueous 
solution of an acrylic acid type monomer containing a small amount of a 
crosslinking agent and either a water soluble radical polymerization 
initiator or a water soluble radical polymerization initiator and a water 
soluble reducing agent are previously mixed homogeneously and the mixture 
is applied in a mist form to a prefabricated fibrous substrate so that the 
resulting highly water absorptive polymer in the fibrous substrate will 
have a diameter in the range of 30-500 .mu.m, followed by polymerization; 
in Japanese Patent Application No. 238421/85 a method that an aqueous 
solution of an acrylic acid type monomer containing a small amount of a 
crosslinking agent and an oxidizing radical polymerization initiator are 
previously mixed and the mixture is applied to a fibrous substrate, and 
then an amine or a reducing agent is added to conduct polymerization; and 
in Japanese Patent Application No. 238420/85 a method that an aqueous 
solution of an acrylic acid type monomer containing a small amount of a 
crosslinking agent and an amine or a reducing agent are mixed, followed by 
application to a fibrous substrate and then addition of an oxidizing 
radical polymerization initiator to conduct polymerization; and the like. 
It has been found according to these methods that polymerization proceeds 
very easily, "uneven polymerization" is appreciably reduced and a 
composite having a large water absorption capacity can be obtained. 
However, the water absorptive composite thus obtained is not always 
satisfactory in its water absorption velocity and unpolymerized monomers 
still remain in it, thus causing problems on use for sunitary goods such 
as a sanitary napkin, paper diaper and the like. 
SUMMARY OF THE INVENTION 
Object of the Invention 
This invention is an improvement of water absorptive composites described 
in Unexamined Japanese Patent Publication No. 500546/82 and Unexamined 
Published Japanese Patent Application No. 149609/85 and proposed by the 
present inventors in Japanese Patent Application Nos. 193403/85, 
202908/85, 238421/85 and 238420/85, providing a process for preparing very 
easily under a moderate condition a water absorptive composite material 
which is excellent in water absorption properties, especially in water 
absorption velocity and has an extremely reduced amount of unpolymerized 
monomers. 
The Invention 
The present inventors have conducted an intensive research in order to 
solved the aforementioned problems. As the result, they have found that a 
water absorptive composite material which is excellent in water absorption 
properties, especially in water absorption velocity, and has an extremely 
reduced amount of unpolymerized monomers and in which the highly water 
absorptive polymer is held with good stability on the fibrous substrate, 
can be obtained very easily at low cost by applying an aqueous solution of 
an acrylic acid type monomer to a prefabricated substrate to polymerize 
the acrylic acid type monomer and then irradiating electromagnetic 
radiation or corpuscular ionizing radiation, and finally reached this 
invention. 
Thus, the process for preparing the water absorptive composite material 
according to this invention is characterized by the combination of the 
following steps; 
(A) applying an aqueous solution of a polymerizable monomer comprising as a 
main component acrylic acid, of which 20% or more of the carboxyl groups 
have been neutralized to its alkali metal salt or ammonium salt, to a 
prefebricated fibrous substrate, 
(B) polymerizing the polymerizable monomers applied to said fibrous 
substrate by means of a water-soluble radical polymerization initiator to 
form a composite of a polymer derived from said polymerizable monomer and 
said fibrous substrate, and 
(C) irradiating said composite with electromagnetic radiation or 
corpuscular ionizing radiation to obtain a water absorptive composite of 
which the water absorption velocity is enhanced as compared with said 
composite. 
The process for preparing the water absorptive composite material of this 
invention is very advantageous in that most of the acrylic acid monomer 
applied to the prefabricated substrate are polymerized by means of a water 
soluble radical polymerization initiator to form a highly water absorptive 
polymer whereby the composite material obtained has an increased water 
absorption capacity, and that since said highly water absorptive polymer 
is subjected to irradiation with electromagnetic radiation or corpuscular 
ionizing radiation, the composite material obtained has a high water 
absorption velocity, an extremely reduced amount of unreacted monomers, 
and, the highly water absorptive polymer is held firmly on the fibrous 
substrate. Thus, a water absorptive composite material far excellent in 
properties as compared with those of the above mentioned prior art can be 
obtained easily and inexpensively. 
Irradiation at the step (C) in this invention is known as a polymerization 
and/or grafting means. The effect of this invention of reducing the amount 
of unreacted monomers or holding firmly the high water absorptive polymer 
on the fibrous substrate by conducting the step (C) may be explained 
rather easily by the known function of such irradiation. However, the 
effect of this invention of enhancing the water absorption velocity by the 
irradiation should be considered unexpected from the known function of the 
irradiation. 
EMBODIMENT OF THE INVENTION Steps (A) and (B) 
Monomer 
The monomer used in this invention contains as a main component acrylic 
acid, of which 20% or more, preferably 50% or more of the carboxyl groups 
are neutralized into its alkali metal salt or an ammonium salt. If the 
partial neutralization degree is less than 20%, the water absorption 
capacity of the resulting polymer will be remarkably decreased. 
In this invention, a polymer having a higher water absorption capacity may 
be obtained by adding in addition to the aforementioned acrylic acid and 
its salts one or two of the monomers copolymerizable therewith selected 
from the group consisting of 2-acrylamide-2-methylpropanesulfonic acid, 
2-acryloylethanesulfonic acid, 2-acryloylpropanesulfonic acid, methacrylic 
acid and alkali metal salts or ammonium salts thereof, (meth)acrylamide, 
N,N-dimethyl (meth)acrylamide, 2-hydroxyethyl (meth)acrylamide, 
2-vinylpyridine, 2-hydroxyethyl (meth)acrylamide, 2-hydroxypropyl 
(meth)acrylate, polyethylene glycol mono(meth)acrylate, N,N'-methylene 
bis(meth)acrylamide and polyethylene glycol di(meth)acrylate. The term 
"(meth)acryl" herein used means acryl and methacryl. It is also possible 
to incorporate other monomers copolymerizable with acrylic acid and 
acrylic acid salts including water soluble monomers such as itaconic acid, 
maleic acid, fumaric acid, vinylsulfonic acid and alkali metal salts or 
ammonium salts thereof and in addition less water soluble monomers such as 
alkyl esters of acrylic acid, for example methyl acrylate, ethyl acrylate 
and the like, providing that "an aqueous solution of a polymerizable 
monomer" of this invention is formed. 
The "polymerizable monomer" of this invention comprises as a main component 
acrylic acid, of which 20% or more takes the salt form. Thus, the addition 
amount of the aforementioned copolymerizable monomer is usually less than 
50 mol %, preferably 20 mol % or less. 
For neutralization of the aforementioned acid monomers including acrylic 
acid may be used a hydroxide or bicarbonate of an alkali metal or ammonium 
hydroxide, preferably an alkali metal oxide, specifically sodium 
hydroxide, potassium hydroxide and lithium hydroxide. Sodium hydroxide or 
potassium hydroxide is preferred from the standpoint of commercial 
availability, price, safety and the like. 
In this invention, the polymerizable monomer comprising as a main component 
the aforementioned acrylic acid, of which 20% or more is present in its 
salt form, is applied in the form of an aqueous solution to a 
prefabricated fibrous substrate. Any concentration of the aqueous solution 
may be employed as far as it is suitable for the object. Specifically, it 
is preferably in the range of 30% by weight or more. 
This aqueous solution may contain a variety of substances providing that 
they are not apart from the object of this invention. As an example of 
such substances, there is mentioned a water soluble radical polymerization 
initiator (described in detail hereafter). The "aqueous solution" may be 
the one in which a small amount of a water soluble organic solvent is also 
present in solution, if desired. 
Prefabricated fibrous substrate 
A prefabricated substrate to which the aforementioned aqueous solution of 
the polymer is applied is specifically a substrate formed by loose 
fabrication of fiber such as a pad, a carded or air-laid web, tissue 
paper, a woven fabric like cotton gauze, knitted fabric or nonwoven 
fabric. The term "prefabricated" fibrous substrate herein used means the 
substrate which requires no web forming operation, though some operations 
such as cutting, bonding, shaping and the like may be required for 
incorporating the fibrous substrate into an article. 
In general, absorptive fibers including cellulose fibers such as wood pulp, 
rayon, cotton and the like and/or polyester fibers are preferably used as 
a main component for the fibrous substrate. Other kinds of fibers such as 
those of polyethylene, polypropylene, polystyrene, polyamide, polyvinyl 
alcohol, polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, 
polyurea, polyurethane, polyfluoroethylene, polyvinylidene cyanide and the 
like may be also incorporated into the prefabricated fibrous substrate. 
Application of an aqueous solution of the monomer and polymerization of the 
monomer (primary polymerization) 
In this invention, the aforementioned monomer solution is applied to the 
aforementioned prefabricated fibrous substrate, and the monomer is 
polymerized on the fibrous substrate by means of a water soluble radical 
polymerization initiator (primary polymerization). 
In order to apply the aqueous monomer solution to the prefabricated fibrous 
substrate, there may be used any means or manner suitable for the object 
as far as the monomer is uniformly dispersed and held on the fibrous 
substrate and can be subjected to polymerization. One of the typical means 
therefor is to impregnate the aqueous monomer solution into the fibrous 
substrate or to spray the aqueous monomer solution onto the fibrous 
substrate. 
For polymerizing the monomer which has been dispersed uniformly on the 
fibrous substrate as described above by means of a water soluble radical 
polymerization initiator, any method can be used as far as it is suitable 
for the object. Typical methods include a method wherein a radical 
polymerization initiator has previously been added in the aqueous monomer 
solution and is decomposed on the fibrous substrate, a method wherein a 
radical polymerization initiator is applied uniformly in the form of a 
separate solution from the aqueous monomer solution to the fibrous 
substrate, to which the aqueous monomer solution has been applied, by 
spraying or the like and is decomposed on the fibrous substrate and a 
method wherein a radical polymerization initiator is applied uniformly in 
the form of a separate solution from the aqueous monomer solution to the 
fibrous substrate and then the aqueous monomer solution is uniformly 
applied thereto by spraying, coating or the like. 
The water soluble radical polymerization initiator used in this invention 
is one well known in the art of polymer chemistry. There may be mentioned 
specifically inorganic or organic peroxides such as persulfates (ammonium 
salts, alkali metal salts, particularly potassium salts, or the like), 
hydrogen peroxide, ditert-butyl peroxide, acetyl peroxide and the like. In 
addition to these peroxides, it is also possible to use such a radical 
polymerization initiator as an azo compound or the like, for example 
2,2'-azobis (2-amidinopropane) dihydrochloride, providing that water 
solubility in a certain level can be obtained. 
The polymerization is initiated by the decomposition of the radical 
polymerization initiator. Well known as a conventional means for 
decomposing the initiator is heating (As is often the case, when the 
initiator is contacted with the monomer the reaction mixture has already 
been raised at the decomposition temperature and thus the polymerization 
is initiated only by adding the polymerization initiator to the monomer 
without heating. This case is involved herein in the category of the 
decomposition by heating). Promotion of the decomposition of the 
polymerization initiator by means of a chemical substance is also well 
known in the art. When the polymerization initiator is a peroxide, a 
promoter of the decomposition thereof is a reducing compound (which is 
water soluble in this invention) such as an acidic sulfite, ascorbic acid 
and an amine for a persulfate, and a polymerization initiator comprising a 
combination of a peroxide and a reducing compound is well known in the art 
of polymer chemistry as "redox initiator". Thus, the term "polymerization 
initiator" herein used also involves initiator combined with such 
decomposition promoting substances, particularly redox initiators. 
The polymerization of the monomer comprising as a main component acrylic 
acid, of which 20% or more is in the salt form, should give in principle a 
non-crosslinking water soluble polymer as far as a diethylenic monomer 
such as N,N'-methylene bis(meth)acrylamide is not concomitantly. However, 
it has been practically known that crosslinking usually occurs between 
acrylic acids (or its salts) or the polymers thereof or/and between those 
and the fibrous substrate. Accordingly, the polyacrylic acid (salt) 
produced in this step may be considered as highly water absorptive polymer 
rather than water soluble polymer. 
In addition, the polymerization by means of the water soluble radical 
polymerization initiator should be substantially aqueous solution 
polymerization. Accordingly, the step (B) should be conducted while 
avoiding the excessively dry state. 
The amount of the monomer applied to the fibrous substrate during the step 
(A) is in a proportion of 1-10,000 parts by weight, preferably 10-1,000 
parts by weight per 100 parts by weight of the fibrous substrate. The 
monomer thus applied should be polymerized in the step (B) to an extent of 
50% or more, preferably 80% or more. Rate of polymerization ordinarily 
reaches up to 80-95% in the step (B). 
Some of the embodiments of the steps (A) and (B) are illustrated as 
follows: 
(1) A method that an aqueous solution of an acrylic acid type monomer 
having a monomer concentration of 25% by weight or more and a water 
soluble radical polymerization initiator are previously mixed 
homogeneously and the mixture is applied in a mist form to a prefabricated 
fibrous substrate so that the resulting highly water absorptive polymer in 
the fibrous substrate will have a diameter in the range of 30-500 .mu.m, 
followed by the polymerization by heating if the polymerization initiator 
used is not a redox type (see Japanese Patent Application No. 193403/85); 
(2) A method that an aqueous solution of an acrylic acid type monomer 
containing a small amount of a crosslinking agent and a water soluble 
radical polymerization initiator are previously mixed homogeneously and 
the mixture is applied in a mist form to a prefabricated fibrous substrate 
so that the resulting highly water absorptive polymer in the fibrous 
substrate will have a diameter in the range of 30-500 .mu.m, followed by 
polymerization by heating if the polymerization initiator used is not a 
redox type (see Japanese Patent Application No. 202908/85); 
(3) A method that an aqueous solution of an acrylic acid type monomer 
containing a small amount of a crosslinking agent and an oxidizing radical 
polymerization initiator are previously mixed, the mixture is applied to a 
fibrous substrate and an amine or a reducing agent is added to form a 
redox system thereby initiating polymerization (see Japanese Patent 
Application No. 238421/85); 
(4) A method that an aqueous solution of an acrylic acid type monomer 
containing a small amount of a crosslinking agent and an amine or a 
reducing agent are mixed, followed by application to a fibrous substrate 
and then addition of an oxidizing radical polymerization initiator to form 
a redox system thereby initiating polymerization (see Japanese Patent 
Application No. 238420/85); and 
(5) A method that an aqueous solution of an acrylic acid type monomer is 
previously impregnated into a fibrous substrate and then a water soluble 
radical polymerization initiator is added in a mist form, followed by 
polymerization by heating if the polymerization initiator used is not a 
redox type (see Japanese Patent Application No. 149609/85). 
Step (C) 
Irradiation (secondary polymerization) 
The composite comprising the highly water absorptive polymer obtained as 
above and the prefabricated fibrous substrate is then irradiated with 
electromagnetic radiation or corpuscular ionizing radiation. 
As the radiation in this case are used high energy radiation such as 
accelerated electron or gamma rays. Dose to be irradiated varies depending 
on the amount of the unreacted monomer or the water content in the 
aforementioned composite and ranges from 0.01 to 100 Mrad, preferably 
0.1-50 Mrad. If the dose exceeds 100 Mrad, water absorption capacity is 
extremely reduced. If it is less than 0.01 Mrad, it is difficult to obtain 
a composite which has water absorption capacity, especially high water 
absorption velocity, and has an extremely reduced amount of unpolymerized 
monomers. 
As the water content of the composite in this case is adopted a content in 
a proportion of 40 parts by weight or less, preferably 10 parts by weight 
or less per 1 part by weight of the fibrous substrate. If the water 
content exceeds 40 parts by weight, it is not preferred because of little 
effect of enhancing the water absorption velocity and adverse effect on 
reducing unreacted monomers. 
The irradiation with the high energy radiation on the aforementioned 
composite according to this invention may be conducted under vacuum, in 
the presence of an inorganic gas such as nitrogen, argon, helium or the 
like, and preferably in air. When the irradiation is conducted in air, the 
advantages intended by the invention, namely, high water absorption 
capacity, especially high water absorption velocity, and the extremely 
reduced amount of unreacted monomers, are obtained most easily. 
The temperature on irradiation is not limited, and the object of 
irradiation can be satisfactorily achieved at room temperature. 
There might be a case that unreacted monomers still remain in a small 
amount even if the step (C) of this invention has been conducted. If it is 
desired to reduce the amount of such residual monomers, it can be achieved 
by a suitable treatment such as heating or the like.

EXAMPLES 
EXAMPLE 1 
In a 100 cc conical flask, 13.1 g of sodium hydroxide (purity: 95% by 
weight) was placed and neutralized by slowly adding 30 g of acrylic acid 
under ice cooling. The aqueous solution exhibited a neutralization degree 
of about 75% and a monomer concentration of about 45% by weight. 
As a radical polymerization initiator, 0.05 g of potassium persulfate was 
added to and dissolved in the aqueous solution, and deaeration was 
conducted using N.sub.2. 
Separately, 0.1569 g of a polyester nonwoven fabric was provided, and the 
whole surface of the nonwoven fabric was coated and impregnated with the 
above mentioned monomer solution. The weight of the monomer thus 
impregnated was 6.2 times the weight of the nonwoven fabric. The nonwoven 
fabric was placed in a constant temperature reaction bath which had 
preliminarily been deaerated with N.sub.2 and heated to 90.degree. C. 
Polymerization started immediately and a composite in which a highly water 
absorptive polymer comprising a partially neutralized self-crosslinked 
sodium polyacrylate was firmly held on the polyester nonwoven fabric was 
obtained. 
Next, the composite was adjusted to a water content of 20% by weight and 
was irradiated with electron beam in a dose of 20 Mrad under the 
atmosphere of air by means of an electron beam generating apparatus 
provided with an accelerator (DYNAMITRON, Radiation Dynamics, Inc., 
Melville, N.Y., U.S.A.) to obtain a water absorptive composite material. 
The properties of the water absorptive composite material is shown below 
(as in the following Examples). 
EXAMPLE 2 
In a 100 cc conical flask, 30 g of acrylic acid was placed and 9.3 g of 
pure water was added to and mixed with it. The mixture was neutralized by 
slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice 
cooling and maintained at a temperature of 50.degree. C. The aqueous 
solution exhibited a neutralization degree of about 75% and a monomer 
concentration of about 74% by weight. 
Separately, as a radical polymerization initiator, 0.05 g of potassium 
persulfate was dissolved in 1 g of water and the aqueous solution was 
coated on the whole surface of 0.0985 g of a rayon nonwoven fabric. 
Thereafter the monomer solution as the raw material was rapidly coated on 
the whole surfaces of the nonwoven fabric, the fabric was placed in a 
reaction bath which had been preliminarily deaerated with N.sub.2 and 
maintained at a temperature of 90.degree. C. The amount of the monomer 
thus impregnated was 10 times the weight of the nonwoven fabric. 
Polymerization started immediately and a composite in which a highly water 
absorptive polymer comprising a partially neutralized self-crosslinked 
potassium polyacrylate was firmly held on the rayon nonwoven fabric was 
obtained. 
Next, the composite was adjusted to a water content of 20% by weight and 
was irradiated with electron beam in a dose of 20 Mrad under the 
atmosphere of air by means of an electron beam generating apparatus 
provided with an accelerator (DYNAMITRON) to obtain a water absorptive 
composite material. 
EXAMPLE 3 
In a 100 cc conical flask, 13.1 g of sodium hydroxide (purity: 95% by 
weight) was placed and dissolved in 39.0 g of pure water under ice 
cooling. The aqueous solution was neutralized by slowly adding 30 g of 
acrylic acid under ice cooling. The aqueous solution exhibited a 
neutralization degree of about 75% and a monomer concentration of about 
45% by weight. 0.005 g of N,N'-methylene bisacrylamide as a crosslinking 
agent and 0.1 g of 2,2'-azobis(2-amidinopropane) dihydrochloride as a 
radical polymerization initiator were dissolved in the aqueous monomer 
solution, and deaeration was conducted with N.sub.2. 
Separately, 0.1505 g of a polyester nonwoven fabric was provided, and the 
whole surface of the nonwoven fabric was coated and impregnated with the 
above mentioned raw material. The amount of the monomer thus impregnated 
was 7.5 times the weight of the nonwoven fabric. The nonwoven fabric was 
placed in a constant-temperature reaction bath which had preliminarily 
been deaerated with N.sub.2 and heated to 90.degree. C. Polymerization 
started immediately and a composite in which a highly water absorptive 
polymer comprising a partially neutralized sodium acrylate crosslinked 
with N,N'-methylene bisacrylamide was firmly held on the polyester 
nonwoven fabric was obtained. 
Next, the composite was adjusted to a water content of 18% by weight and 
was irradiated with electron beam in a dose of 20 Mrad under the 
atmosphere of air by means of an electron beam generating apparatus 
provided with an accelerator (DYNAMITRON) to obtain a water absorptive 
composite material. 
EXAMPLE 4 
In a 100 cc conical flask, 30 g of acrylic acid was placed and 9.3 g of 
pure water was added to and mixed with it. The mixture was neutralized by 
slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice 
cooling and maintained at a temperature of 70.degree. C. The aqueous 
solution exhibited a neutralization degree of about 75% and a monomer 
concentration of about 74% by weight. 
Separately, as a radical polymerization initiator, 0.2 g of potassium 
persulfate was dissolved in 3 g of water. 
0.5869 g of a polyester nonwoven fabric was provided and maintained at a 
temperature of about 70.degree. C. in a constant temperature bath. The 
aqueous radical polymerization initiator solution was mixed with the 
aqueous monomer solution mentioned above, and the mixture was immediately 
sprayed through a spraying nozzle onto the above mentioned nonwoven 
fabric. Polymerization started immediately and a composite in which a 
highly water absorptive polymer comprising a partially neutralized 
self-crosslinked potassium polyacrylate was firmly held on the rayon 
nonwoven fabric was obtained. The amount of the monomer thus coated was 12 
times the weight of the nonwoven fabric, and the highly water absorptive 
polymer had a particle diameter in the range of 100-300 .mu.m. 
Next, the composite was adjusted to a water content of 20% by weight and 
was irradiated with electron beam in a dose of 20 Mrad under the 
atmosphere of air by means of an electron beam generating apparatus 
provided with an accelerator (DYNAMITRON) to obtain a water absorptive 
composite material. 
EXAMPLE 5 
In a 100 cc conical flask, 26.9 g of 25% aqueous ammonia was placed and 
neutralized by slowly adding 30 g of acrylic acid under ice cooling and 
heated to a temperature of 70.degree. C. The aqueous solution exhibited a 
neutralization degree of about 95% and a monomer concentration of about 
65% by weight. 
Separately, 0.2 g of potassium persulfate as a radical polymerization 
initiator was dissolved in 3 g of water. 
0.4695 g of a polyester nonwoven fabric was provided and maintained at a 
temperature of about 70.degree. C. in a constant temperature bath. The 
aqueous radical polymerization initiator solution was mixed with the 
aqueous monomer solution mentioned above, and the mixture was immediately 
sprayed through a spraying nozzle onto the above mentioned nonwoven 
fabric. Polymerization started immediately and a composite in which a 
highly water absorptive polymer comprising a partially neutralized 
self-crosslinked ammonium polyacrylate was firmly held on the rayon 
nonwoven fabric was obtained. The amount of the monomer thus coated was 8 
times the weight of the nonwoven fabric, and the highly water absorptive 
polymer had a particle diameter in the range of 100-250 .mu.m. 
Next, the composite was adjusted to a water content of 20% by weight and 
was irradiated with electron beam in a dose of 20 Mrad under the 
atmosphere of air by means of an electron beam generating apparatus 
provided with an accelerator (DYNAMITRON) to obtain a water absorptive 
composite material. 
EXAMPLE 6 
In a 100 cc conical flask, 13.1 g of sodium hydroxide (purity: 95% by 
weight) was placed and dissolved in 39.0 g of pure water under ice 
cooling. The aqueous solution was neutralized by slowly adding 30 g of 
acrylic acid under ice cooling. The aqueous solution exhibited a 
neutralization degree of about 75% and a monomer concentration of about 
45% by weight. 
0.1 g of N,N'-methylene bisacrylamide as a crosslinking agent was added and 
dissolved in the aqueous monomer solution, and the mixture was heated to 
50.degree. C. 0.2 g of potassium persulfate as a radical polymerization 
initiator was also dissolved in the above mentioned mixture. 
Separately, 0.1598 g of a polyester nonwoven fabric was provided, and the 
whole surface of the nonwoven fabric was coated and impregnated with the 
above mentioned raw material, and the coated nonwoven fabric was 
maintained at a temperature of about 50.degree. C. in a constant 
temperature bath. The amount of the monomer thus impregnated was 7.0 times 
the weight of the nonwoven fabric. 
Next, an aqueous solution of 5% sodium hydrogen sulfite as a reducing agent 
was sprayed on the whole surface of the above mentioned nonwoven fabric. 
Polymerization started immediately and a composite in which a highly water 
absorptive polymer comprising a partially neutralized sodium acrylate 
crosslinked with N,N'-methylene bisacrylamide was firmly held on the 
polyester nonwoven fabric was obtained. 
Next, the composite was adjusted to a water content of 20% by weight and 
was irradiated with electron beam in a dose of 20 Mrad under the 
atmosphere of air by means of an electron beam generating apparatus 
provided with an accelerator (DYNAMITRON) to obtain a water absorptive 
composite material. 
EXAMPLE 7 
In a 100 cc conical flask, 30 g of acrylic acid was placed and 16.9 g of 
pure water was added to and mixed with it. The mixture was neutralized by 
slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice 
cooling. The aqueous solution exhibited a neutralization degree of about 
75% and a monomer concentration of about 65% by weight. 
0.1 g of N,N'-methylene bisacrylamide as a crosslinking agent was added to 
and dissolved in the above mentioned monomer solution, and the mixture was 
heated to 40.degree. C. 0.4 g of 31% aqueous hydrogen peroxide as a 
radical polymerization initiator was dissolved in the mixture. 
0.1869 g of a polyester nonwoven fabric was provided, and the whole surface 
of the nonwoven fabric was coated and impregnated with the aforementioned 
raw material, and the nonwoven fabric thus treated was maintained at a 
temperature of 40.degree. C. in a constant temperature bath. The amount of 
the monomer thus impregnated was 5.8 times the weight of the nonwoven 
fabric. 
Next, an aqueous solution of 5% L-ascorbic acid was sprayed through a 
spraying nozzle onto the whole surface of the above mentioned nonwoven 
fabric. Polymerization started immediately and a composite in which a 
highly water absorptive polymer comprising a partially neutralized 
potassium polyacrylate crosslinked with N,N'-methylene bisacrylamide was 
firmly held on the polyester nonwoven fabric was obtained. 
Next, the composite (having a water content of about 19% by weight) was 
irradiated with electron beam in a dose of 20 Mrad under the atmosphere of 
air by means of an electron beam generating apparatus provided with an 
accelerator (DYNAMITRON) to obtain a water absorptive composite material. 
EXAMPLE 8 
In a 100 cc conical flask, 30 g of acrylic acid was placed and 16.9 g of 
pure water was added to and mixed with it. The mixture was neutralized by 
slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice 
cooling. The aqueous solution exhibited a neutralization degree of about 
75% and a monomer concentration of about 65% by weight. 
0.1 g of N,N'-methylene bisacrylamide as a crosslinking agent was added to 
and dissolved in the above mentioned monomer solution, and the mixture was 
heated to 30.degree. C. 0.2 g of L-ascorbic acid as a radical 
polymerization initiator was dissolved in the mixture. 
0.2582 g of a polyester nonwoven fabric was provided, and the whole surface 
of the nonwoven fabric was coated and impregnated with the aforementioned 
raw material, and the nonwoven fabric thus treated was maintained at a 
temperature of 30.degree. C. in a constant temperature bath. The amount of 
the monomer thus impregnated was 6.2 times the weight of the nonwoven 
fabric. 
Next, 10% aqueous hydrogen peroxide was sprayed through a spraying nozzle 
onto the whole surface of the above mentioned nonwoven fabric. 
Polymerization started immediately and a composite in which a highly water 
absorptive polymer comprising a partially neutralized potassium 
polyacrylate crosslinked with N,N'-methylene bisacrylamide was firmly held 
on the polyester nonwoven fabric was obtained. 
Next, the composite (having a water content of aoubt 25% by weight) was 
irradiated with electron beam in a dose of 20 Mrad under the atmosphere of 
air by means of an electron beam generating apparatus provided with an 
accelerator (DYNAMITRON) to obtain a water absorptive composite material. 
EXAMPLE 9 
A water absorptive composite material was obtained in the same manner as in 
Example 1 except that the mixture of 28 g of acrylic acid and 2 g of 
methacrylic acid was used in place of the acrylic acid in Example 1. 
EXAMPLE 10 
A water absorptive composite material was obtained in the same manner as in 
Example 3 except that the mixture of 30 g of acrylic acid and 3.5 g of 
2-hydroxyethyl methacrylate was used in place of the acrylic acid in 
Example 3. 
EXAMPLE 11 
A water absorptive composite material was obtained in the same manner as in 
Example 7 except that the mixture of 30 g of acrylic acid and 3.5 g of 
acrylamide was used in place of the acrylic acid in Example 7. 
EXAMPLE 12 
A water absorptive composite material was obtained in the same manner as in 
Example 7 except that the mixture of 30 g of acrylic acid and 5 g of 
2-acrylamide-2-methylpropanesulfonic acid was used in place of the acrylic 
acid in Example 7. 
Comparative Examples 1-8 
The precursors obtained in Examples 1-8, that is the composites before the 
electron beam irradiation are herein regarded as the composites in 
Comparative Examples 1-8, respectively. 
COMATIVE EXAMPLE 9 
In a 100 cc conical flask, 30 g of acrylic acid was placed and 16.9 g of 
pure water was added to and mixed with it. The mixture was neutralized by 
slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice 
cooling. The aqueous solution exhibited a neutralization degree of about 
75% and a monomer concentration of about 65% by weight. 
0.3852 g of a polyester nonwoven fabric was provided, and the whole surface 
of the nonwoven fabric was coated and impregnated with the above mentioned 
raw material. The amount of the monomer impregnated was 7.5 times the 
weight of the nonwoven fabric. 
Next, the nonwoven fabric having been impregnated with the aqueous solution 
of the partially neutralized potassium acrylate monomer was irradiated 
with electron beam at a dose of 20 Mrad by means of an electron beam 
generating apparatus equipped with an accelerator (DYNAMITRON). 
Polymerization started immediately and a composite in which a highly water 
absorptive polymer comprising a partially neutralized self-crosslinked 
potassium polyacrylate was firmly held on the polyester nonwoven fabric 
was obtained. 
For the water absorptive composite materials obtained in Examples and the 
composites obtained in Comparative Examples, the following tests were 
carried out to evaluate physiological saline absorption capacity, water 
absorption velocity and unpolymerized monomer concentration. The results 
are shown in Table 1. 
A. Physiological saline absorption capacity 
About 0.5 g of the composite or water absorptive composite material and 
about 200 g of a saline solution having a concentration of 0.9% by weight 
were precisely weighed, respectively and charged in a 300 ml beaker. The 
beaker was left standing for about 4 hours to swell the polymer 
satisfactorily with the solution. The beaker content was filtered through 
a 100-mesh sieve, and the amount of the filtrate is weighed and the 
physiological saline absorption capacity is calculated according to the 
following equation: 
##EQU1## 
B. Water absorption velocity 
About 200 g of a saline solution having a concentration of 0.9% by weight 
was weighed and charged in a 300 ml beaker. Sebsequently, about 0.5 g of 
the composite or water absorptive composite material was weighed and added 
to the above mentioned solution. After 5 minutes, the beaker content was 
filtered through a 100 mesh sieve. The amount of the filtrate was weighed 
and the physiological saline absorption capacity was calculated according 
to the equation described in A, which was regarded as water absorption 
velocity. 
C. Unpolymerized monomer concentration 
0.5 g of the composite or water absorptive composite material was precisely 
weighed and added to 1 liter of ion exchanged water in a 2 liter beaker to 
swell sufficiently with stirring for about 10 hours. The swollen polymer 
gel was filtered through a 200-mesh sieve and the filtrate was analyzed by 
a high speed liquid chromatography. 
Separately, standard monomer solutions having determined concentrations 
were prepared to make a calibration curve, and the absolute monomer 
concentration of the filtrate was determined with consideration for the 
degree of dilution (1/2000). The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Physiological 
saline Water Unpolymerized 
absorption absorption monomer 
capacity velocity concentration 
Example No. 
(g/g) (g/g) (ppm by wt.) 
______________________________________ 
Example 
1 42.5 35.8 235 
2 43.2 32.1 365 
3 37.5 30.8 285 
4 68.2 49.5 354 
5 71.3 55.8 421 
6 51.3 40.8 568 
7 53.2 45.5 515 
8 49.5 41.2 681 
9 45.0 38.2 315 
10 48.0 41.5 355 
11 75.4 52.3 681 
12 55.3 40.5 585 
Comp. Example 
1 39.5 12.1 3586 
2 42.1 10.1 25865 
3 35.2 8.6 5255 
4 67.8 31.5 5682 
5 67.6 25.5 6821 
6 41.3 8.2 15681 
7 51.2 11.2 13482 
8 48.8 9.3 15255 
9 15.2 9.5 895 
______________________________________ 
The water absorptive composite material obtained by the process of this 
invention, as apparent from the results shown in Table 1, has high water 
absorption capacity, epecially very high water absorption velocity, and 
has an extremely low content of unpolymerized monomers and thus possessing 
very high safety, as compared with those in prior art. Further, the 
composite material handles easily because of its sheet form as compared 
with conventional powdery water absorptive resins, so that they can be 
used advantageously for the production of a variety of sanitary goods such 
as a sanitary napkin, paper diaper and the like. 
The water absorptive composite material according to this invention, taking 
advantage of its excellent water absorption capacity and easy handling, 
can be also used for the production of a variety of materials for 
gardening and agriculture such as a soil conditioner and a water retaining 
agent which have recently attracted public attention.