 |
Description  |
|
|
This invention relates to an aqueous composition, a method of producing a
water absorbent polymer, a water absorbent polymer-coated article and a
method of producing the same. More particularly, this invention is
concerned, in one aspect, with an aqueous composition of an acrylic
polymer containing a peroxide radical initiator which is adapted to
decompose at a temperature of from about 40.degree. to about 180.degree.
C. and act on the polymer to crosslink the same. This composition can be
advantageously utilized to produce a highly water absorbent polymer, and a
highly water absorbent polymer-coated article. Therefore, in another
aspect, the present invention is concerned with an improved method of
producing an advantageous water absorbent polymer in which the
above-mentioned aqueous composition is heated, and, in further aspects,
concerned with a highly water absorbent polymer-coated article and with an
improved method of producing the same in which the aqueous composition is
applied to a substrate, and heated.
Basically, there are three methods known in the art to produce a water
absorbent polymer-coated or impregnated article such as a disposable
diaper, a sanitary napkin, a surgical pad, a surgical sheet, a paper towel
or the like. That is, according to method A, a powdery water absorbent
crosslinked polymer having acrylic acid monomeric units as a major
component, which can absorb water, urine, menstrual blood or the like in
an amount of several hundred times the weight of the polymer, is spread
over a substrate such as paper, non-woven fabric or the like, and steamed
to effect swelling of the polymer, followed by pressing and drying thereby
to cause the polymer to adhere to the substrate. In this connection, U.S.
Pat. Nos. 3,669,103 and 3,810,468 teach that a water soluble polymer
produced from a monomeric component such as acrylic acid and acrylamide
exhibits a high degree of water absorbency when crosslinking of the
polymer molecules is effected, and that crosslinking may be effected, for
example, by conducting copolymerization with a bifunctional monomer such
as N,N'-methylenebisacrylamide. According to method B, a substrate such as
fibrous cellulosic material is subjected to surface treatment such as
carboxymethylation to render the substrate water absorbent. According to
method C, a solution of a water soluble polymer is applied onto a
substrate, and the resulting water soluble polymer-applied substrate is
subjected to heat treatment in the presence of a crosslinking agent
generally during the drying step to insolubilize the water soluble
polymer, thereby imparting a water absorbency thereto.
With respect to method A, it has a drawback that due to the difference in
properties such as specific gravity and configuration between the acrylic
polymer and the fibrous substrate, the powder of the water absorbent
acrylic polymer is removed from the fibrous substrate and distributed
unevenly by the vibration occurring during the transportation of the
product, etc. Moreover, with respect to method A, there occurs a trouble
that a hydrogel formed as a result of water absorption of the water
absorbent polymer is separated from the fibrous substrate and comes to
direct contact with the skin of a person carrying the product to give an
uncomfortable feeling to the person. Moreover, the production of the water
absorbent crosslinked acrylic polymer to be employed in method A is
accompanied by following problem. To produce a crosslinked acrylic
polymer, according to the conventional method, a radical initiator is
added to an aqueous solution containing 40% by weight or more of acrylic
acid netralized with an alkali metal hydroxide and the resulting mixture
is heated. In that method, there rapidly occurs a polymerization reaction
while causing partial crosslinking, so that a partially self-crosslinked
acrylic polymer is produced. After the initiation of the polymerization
reaction, the viscosity of the aqueous solution increases and a gel is
formed with the progress of spontaneous crosslinking. While the
polymerization reaction is accelerated due to the elevation of the
temperature which is attributed to the heat of reaction, evaporation of
the steam out of the reaction system is hindered due to the increased
viscosity and the gelation of the mixture. As a result, removal of the
heat of reaction becomes difficult, and the reaction proceeds
uncontrollably with the formation of a popcorn-like material. Especially
when the reaction system becomes a gel, generally known "gel effect"
occurs to further increase the reaction rate, thereby causing controlling
of the polymerization reaction to be more difficult. To obviate such a
difficulty, there have been proposed various polymerization methods. For
example, it has been proposed to employ a redox system to initiate radical
polymerization since the above-mentioned spontaneous crosslinking can be
prevented by effecting polymerization under mild conditions at a low
temperature. For this method, a continuous polymerization process is
feasible. However, this redox system method is disadvantageous because in
this method the polymerization reaction requires a prolonged reaction time
and a complicated polymerization apparatus, and because in this method the
concentration of the aqueous reaction solution is to be kept low to ensure
controlling of the heat of polymerization. Therefore, in this method,
reduction of the manufacturing cost cannot be expected. Further, a reverse
phase suspension polymerization method has been proposed in which
polymerization is effected in a dispersion having fine particles of the
aqueous reaction solution dispersed in a hydrocarbon solvent. With respect
to this method, reference may be made to, for example, U.S. Pat. No.
4,093,776. In this method, it is possible to polymerize acrylic acid in a
high concentration aqueous solution. However, this method is
disadvantageous because in this method a large amount of a hydrocarbon
solvent must be used and a countermeasure against possible fire is
required, thereby inevitably leading to an increase in manufacturing cost.
Further, in this method, it is difficult to practice a continuous process.
With respect to method B, U.S. Pat. No. 3,005,456 discloses a method in
which carboxymethylation of a fibrous cellulose is effected to an extent
that solubilization of the cellulose does not occur. According to this
method, however, a fibrous product having a high degree of water
absorbency cannot be obtained. Further, this method is disadvantageous
from the economical point of view, because the carboxymethylation is
effected by reacting the cellulose with chloroacetic acid, which is an
expensive chemical, in a propanol solution. On the other hand, the method
as disclosed in Japanese patent application laid-open specification No.
51-144476/1976 in which absorbent polymer segments are grafted to a
fibrous carboxymethyl cellulose may produce a water absorbent fibrous
product improved in water absorbency, but inevitably leads to an increase
in cost when the method is practiced on a commercial scale because
expensive apparatuses and time-consuming operations are required.
With respect to method C, reference may be made to, for example, Japanese
patent application laid-open specification No. 50-82143/1975 (priority was
claimed on the basis of U.S. patent application Ser. No. 371,909 filed on
June 20, 1973), Japanese patent application laid-open specification No.
55-84304/1980 and Japanese patent application laid-open specification No.
58-84804/1983. As disclosed in Japanese patent application laid-open
specification No. 50-82143/1975, a water swellable film or coated article
is obtained by a method in which a film or coated article is produced from
an aqueous solution containing a polymeric electrolyte having carboxylate
groups such as sodium polyacrylate and a water soluble crosslinking agent
which is capable of reacting with carboxylate groups, and then the film or
coated article is heated thereby to effect crosslinking of the polymer. As
suitable crosslinking agents, there have been mentioned a polyhaloalkanol,
a haloepoxyalkane, a polyglycidyl ether, an amphoteric sulfonium salt, a
bisphenol-A-epichlorohydrine type epoxy resin and the like. Japanese
patent application laid-open specification No. 55-84304/1980 discloses
crosslinking of polymers which is effected by esterification reaction
between a polyfunctional alcohol and the carboxyl groups of a polymer.
These methods have the following drawback. That is, according to these
methods, crosslinking of the polymer is effected by the reactions between
the carboxyl or carboxylate groups of the polymer and the functional group
for example a hydroxyl group, of the crosslkinking agent. Such reaction
must generally be conducted at a high temperature for prolonged period of
time. This is disadvantageous from the viewpoint of avoiding the
decomposition of the polymer. This is also disadvantageous from the
viewpoint of production efficiency. Moreover, according to these methods,
it is difficult to attain a desirable degree of crosslinking. In the
method of Japanese patent application laid-open specification No.
58-84804/1983 the aqueous solution of the polymer has a high viscosity and
therefore is difficult to be uniformly permeated into the fibrous material
unless the aqueous solution is highly diluted to a solution having an
extremely low concentration. Moreover, the method is disadvantageous in
that it is difficult to control the crosslinking reaction and to attain
uniform crosslinking and density of crosslinking, thereby leading to
insufficient absorbency.
In most cases, fibrous products containing a water absorbent polymer are
used for the production of disposable articles. Hence, low cost is the
prime requirement for such absorbent fibrous products. However, the
efforts for providing low-cost water absorbent fibrous products have not
been satisfactorily successful.
As is apparent from the foregoing, all of the prior art methods and
products are advantageous in some points but disadvantageous in other
points.
Noting the advantages of method C as described above, the inventor has made
extensive and intensive studies to resolve the disadvantages of the
method. As a result, it has unexpectedly been found that an aqueous
composition comprising water, an acrylic polymer dissolved or swollen in
the water and a peroxide radical initiator substantially dissolved in the
water can be advantageously utilized to produce an advantageous water
absorbent crosslinked polymer and an advantageous water absorbent
polymer-coated article. Based on this unexpected finding, the present
invention has been completed.
It is, therefore, an object of the present invention to provide a novel
aqueous composition which can be advantageously utilized to produce a
water absorbent crosslinked polymer and a water absorbent polymer-coated
article.
It is another object of the present invention to provide an improved method
of producing a water absorbent crosslinked polymer which can absorb water,
urine, menstrual blood or the like in an amount of several hundred times
the weight of the polymer.
It is a further object of the present invention to provide an improved
method of producing a water absorbent polymer-coated article.
It is still a further object of the present invention to provide a novel
water absorbent polymer-coated article which is advantageously uniform
with respect to water absorption and which is safe to humans.
The foregoing and other objects, features and advantages of the present
invention will be apparent to those skilled in the art from the following
detailed description and appended claims.
In one aspect of the present invention, there is provided an aqueous
composition comprising water, a polymer dissolved or swollen in said
water, said polymer having at least 70% by weight, based on the total
weight of said polymer, of acrylic acid monomeric units, 60 to 90% of the
carboxyl groups of said monomeric units being in the form of an alkali
metal salt, and a peroxide radical initiator substantially dissolved in
said water, said initiator being adapted to decompose at a temperature of
from about 40.degree. to about 180.degree. C. and act on said polymer to
crosslink said polymer and being present in an amount sufficient to
crosslink said polymer, said water being present in an amount of at least
about 10% based on said composition.
In the present invention, the aqueous composition is prepared by adding a
water soluble peroxide radical initiator to an aqueous mixture comprised
of water and a polymer dissolved or swollen in the water. The polymer has
at least 70% by weight, based on the total weight of the polymer, of
acrylic acid monomeric units, 60 to 90% of the carboxyl groups of which
are in the form of an alkali metal salt. A polyacrylic acid salt is an
example of the suitable polymers. The other suitable polymers may be
prepared by copolymerizing acrylic acid with a comonomer as a minor
component such as methacrylic acid, maleic anhydride, fumaric acid,
acrylamide, and methacrylamide or by effecting graft copolymerization of
acrylic acid onto a water soluble polymer such as starch.
The above-defined aqueous mixture may be prepared according to customary
procedures. For example, it may be prepared as follows. First, a 10 to 40%
by weight aqueous monomeric component solution is prepared by dissolving a
monomeric component comprising at least 70% by weight of acrylic acid
based on the component in water and neutralizing 60 to 90% of the carboxyl
groups of the acrylic acid with an alkali metal hydroxide such as sodium
hydroxide and potassium hydroxide. Next, a radical initiator such as
ammonium persulfate and potassium persulfate is added to the aqueous
monomeric component solution in an amount of, generally, from 0.05 to 0.2%
by weight based on the monomeric component, and polymerization reaction is
effected to obtain an aqueous mixture. It is preferred that polymerization
reaction of the monomeric component be effected in an inert atmosphere
such as gaseous nitrogen. Polymerization reaction is generally effected at
a temperature of from 50.degree. to 100.degree. C. It is however possible
to conduct polymerization reaction at room temperature by employing a
redox initiator system in which a peroxide radical initiator is used in
combination with a reducing agent such as a sulfite salt which promotes
decomposition of the peroxide initiator. The polymerization reaction rate
and accordingly the polymerization reaction time depend on monomeric
component concentration, initiator concentration, reaction temperature and
other reaction conditions. Whether the resulting polymer is completely
dissolved in the water or obtained in a swollen form depends also on
monomeric component concentration, initiator concentration, reaction
temperature and other reaction conditions.
A water soluble peroxide radical initiator adapted to decompose at a
temperature of from about 40.degree. to about 180.degree. C. and act on
the polymer to crosslink the same is added to the above-described aqueous
mixture in an amount sufficient to crosslink the polymer to obtain an
aqueous composition of the present invention.
The use of an organic peroxide is known in the plastics and rubber
industries to effect radical crosslinking of polyethylene,
poly(ethylene-co-vinyl acetate), poly(ethylene-co-propylene) rubber or the
like. Such radical crosslinking is effective only for limited kinds of
thermoplastic polymers and is not effective for butyl rubber or the like.
To perform radical crosslinking of the thermoplastic polymers, an oil
soluble organic peroxide is generally employed. Formation of crosslinking
proceeds with the progress of decomposition of the peroxide, and is
completed within a short period of several minutes. A water soluble
peroxide radical initiator lacks compatibility with these polymers and
hence cannot be used to effect radical crosslinking thereof. As far as the
inventor's knowledge extends, there is no report indicating the use of a
water soluble peroxide radical initiator for effecting radical
crosslinking of polymers.
The inventor has conceived of preparation of a wet absorbent polymer by
effecting radical crosslinking with respect to an aqueous composition of
an acrylic polymer to which a water soluble peroxide radical initiator has
been added, has studied and has made the following unexpected finding. A
detailed study has been made of the aqueous compositions of a polyacrylic
acid, 75% of the carboxyl groups of which have been neutralized with
caustic soda, containing potassium persulfate in an amount of 2% by weight
based on the polymer, which compositions have been caused to have various
water contents by effecting concentration under reduced pressure at a
temperature at which decomposition of the persulfate substantially does
not occur. From the study, it has unexpectedly been found that radical
crosslinking of the polymer in the aqueous composition scarcely occurs in
the absence of water or at a water content of less than about 10% by
weight in the aqueous composition, and that it is preferred for radical
crosslinking of the polymer in the aqueous composition that water be
present in an amount of greater than 20%, especially 30% by weight but
less than 60%, especially 50% by weight based on the composition. This
suggests that water functions as a plasticizer, and that in the absence of
water or at a water content as low as less than about 10% by weight,
polymer molecules tend to be frozen, thereby causing the crosslinking
reaction to be hindered.
With respect to polymerization, in the presence of a peroxide radical
initiator, of an aqueous solution of a monomeric component comprising at
least 70% by weight of acrylic acid, 60 to 90% of the carboxyl groups of
which have been neutralized with an alkali metal hydroxide, it may be
assumed that when the initiator is added in an amount exceeding that to be
consumed in the polymerization reaction and polymerization is effected, a
portion of the initiator added remains undecomposed so that the same
composition as that of the present invention is obtained. That is, it may
be assumed that the composition of the present inveniton is not different
from the above-defined composition, because both of them are to comprise
water, the same kind of polymer and the same kind of initiator. However,
this assumption is wrong for the following reason. When polymerization of
such a monomeric component is effected using a peroxide radical initiator
in an amount exceeding that to be consumed in the polymerization reaction,
spontaneous crosslinking and gelation of the reaction mixture occur,
thereby causing control and continuation of the polymerization reaction to
be difficult. At the same time, the quality of the resulting polymer is
caused to be poor. These are especially apparent when the monomeric
component concentration of the aqueous solution is at a high level.
Therefore, polymerization of the monomeric component using an initiator in
an amount exceeding that to be consumed in the polymerization reaction
does not lead to the aqueous composition of the present invention.
As the water soluble peroxide radical initiator to be employed in the
present invention, there may be mentioned, for example, persulfate salts
such as ammonium persulfate, sodium persulfate and potassium persulfate,
inorganic compounds such as hydrogen peroxide and organic compounds such
as acetic acid peroxide, succinic acid peroxide and t-butyl peroxymaleic
acid. From the viewpoints of the decomposition temperature, crosslinking
efficiency and cost of the initiator, ammonium persulfate and potassium
persulfate are most preferred. The water soluble peroxide radical
initiator is incorporated in the aqueous composition of the present
invention in an amount sufficient to crosslink the polymer and in such an
amount that the water substantially dissolves the initiator therein. The
radical initiator may be incorporated in the aqueous composition of the
present invention in an amount of generally from 0.01 to 10% by weight,
preferably from 0.1 to 5% by weight, more preferably from 0.5 to 3% by
weight based on the polymer. As mentioned above, the radical initiator is
substantially dissolved in the aqueous composition of the present
invention. The terminology "substantially dissolved" as used herein means
that generally the initiator is completely dissolved in the aqueous
composition but it may occur that a portion, for example at most about
10%, remains undissolved. When an excess amount of a radical initiator is
added so that a large portion of the initiator remains undissolved, the
ultimate products become disadvantageously non-uniform with respect to
crosslinking and hence water absorbing properties. The radical initiator
may be added in the form of powder or an aqueous solution. When the
polymer is in a swollen form, uniform dispersion of the initiator may be
attained by a kneader or other mechanical means.
The aqueous composition of the present invention can be advantageously
utilized to produce a water absorbent crosslinked polymer. Accordingly, in
another aspect of the present invention, there is provided a method of
producing a water absorbent crosslinked polymer, which comprises the steps
of:
(1) providing an aqueous mixture comprised of water and a polymer dissolved
or swollen in said water, said polymer having at least 70% by weight,
based on the total weight of said polymer, of acrylic acid monomeric
units, 60 to 90% of the carboxyl groups of said monomeric units being in
the form of an alkali metal salt;
(2) adding a water soluble peroxide radical initiator, said initiator being
adapted to decompose at a temperature of from about 40.degree. to about
180.degree. C. and act on said polymer to crosslink said polymer, to said
mixture in an amount sufficient to crosslink said polymer and in such an
amount that said water substantially dissolves said initiator thereby to
obtain an aqueous composition; and
(3) heating said aqueous composition at a temperature of from about
40.degree. to about 180.degree. C.
Moreover, the aqueous composition of the present invention can be
advantageously utilized to produce a water absorbent polymer-coated
article. Accordingly, in a further aspect of the present invention, there
is provided a method of producing a water absorbent polymer-coated
article, which comprises the steps of:
(1) providing an aqueous mixture comprised of water and a polymer dissolved
or swollen in said water, said polymer having at least 70% by weight,
based on the total weight of said polymer, of acrylic acid monomeric
units, 60 to 90% of the carboxyl groups of said monomeric units being in
the form of an alkali metal salt;
(2) adding a water soluble peroxide radical initiator, said initiator being
adapted to decompose at a temperature of from about 40.degree. to about
180.degree. C. and act on said polymer to crosslink said polymer, to said
mixture in an amount sufficient to crosslink said polymer and in such an
amount that said water substantially dissolves said initiator therein
thereby to obtain an aqueous composition;
(3) coating or impregnating a substrate with said aqueous composition; and
(4) heating the resulting coated or impregnated substrate at a temperature
of from about 40.degree. to about 180.degree. C.
As mentioned hereinbefore, radical crosslinking of the acrylic polymer by a
water soluble peroxide radical initiator scarcely occurs in the absence of
water or at a water content as low as less than about 10% by weight, and
water is needed for effective crosslinking of the polymer. When the amount
of water in the aqueous mixture is less than such an amount as will cause
the aqueous composition to have a water content of at least about 10% by
weight, water is added together with a peroxide radical initiator to the
aqueous mixture so that the resulting aqueous composition has a water
content of at least about 10% by weight. It has been found that radical
crosslinking of the acrylic polymer by a water soluble peroxide radical
initiator more effectively occurs when water is present in an amount of at
least about 20%, especially from about 20% to about 60%, more especially
from about 30% to about 50% based on the composition.
To produce a water absorbent crosslinked polymer, the aqueous composition
is heated at a temperature of about 40.degree. to about 180.degree. C.
When an aqueous composition having a high water content, for example 90%
is employed, it is preferred that the composition be concentrated by
heating, under reduced pressure, at a temperature, for example, less than
about 40.degree. C., at which decomposition of the initiator substantially
does not occur, to arrive at a suitable water content of, for example 20%
to 60%, thereby enabling effective crosslinking to occur. To attain
effective radical crosslinking promptly, it is preferred that use be made
of an aqueous mixture comprised of water and a polymer dissolved or
swollen in the water, which polymer has at least 70% by weight, based on
the total weight of the polymer, of acrylic acid monomeric units, 60 to
90% of the carboxyl groups of which are in the form of an alkali metal
salt, and which water is contained in the aqueous mixture in an amount
such as will cause the aqueous composition to have a water content of at
least about 20%, especially from about 20% to about 60%.
On the other hand, to produce a water absorbent polymer-coated article, a
substrate is coated or impregnated with an aqueous composition, followed
by heating at a temperature of from 40.degree. to 180.degree. C. To
facilitate the coating or impregnation, it is desired that the aqueous
composition have a low viscosity, for example about 50 to about 50,000 cps
at 25.degree. C. as measured using a Brookfield viscometer. The aqueous
composition having a viscosity of about 50 to about 50,000 cps may be
prepared by using a high watercontent aqueous mixture, for example, an
aqueous mixture comprised of water and a polymer dissolved or swollen in
the water, which polymer has at least 70% by weight, based on the total
weight of the polymer, of acrylic acid monomeric units, 60 to 90% of the
carboxyl groups of which are in the form of an alkali metal salt, and
which water is present in an amount of more than about 60%, especially
more than about 90% based on the mixture. Further, the aqueous composition
having a viscosity of about 50 to about 50,000 cps may also be prepared by
adding a volatile organic solvent to the aqueous mixture of the acrylic
polymer, which solvent is capable of forming a mixed solvent with water.
As a suitable volatile, water soluble solvent, there may be mentioned, for
example, an alcohol such as methanol and ethanol and a ketone compound
such as acetone and methyl ethyl ketone. Of the above-mentioned solvents,
methanol is most preferred. To attain a substantial decrease in the
viscosity of the aqueous composition, it is preferred that the solvent be
present in an amount of from 0.1 to 5 in terms of the volume ratio of the
solvent to water. When an aqueous composition having a water content as
high as more than 60% by weight based on the composition, it is generally
preferred that the aqueous composition be concentrated by heating, under
reduced pressure, at a temperature, for example, less than about
40.degree. C., at which decomposition of the initiator substantially does
not occur, to arrive at a suitable water content of, for example 20 to
60%, thereby enabling effective crosslinking to occur.
The kind of the substrate to be coated or impregnated with the aqueous
composition of the present invention is not critical, as far as it does
not contain any substance which adversely affects formation of a
crosslinked polymer coat layer, such as an inorganic salt and a radical
inhibitor. The substrate may be porous or non-porous. However, a porous
substrate generally finds more applications. As a suitable substrate,
there may be mentioned a cotton-like material, paper, non-woven fabric,
fabric, felt, mat or the like which is composed of natural fibers such as
those of pulp, cotton, rayon and wool or systhetic fibers such as those of
a polyolefin, a polyester, a polyamide (nylon) and a polyacrylonitrile.
When a porous substrate is employed, coating can be effected deep inside
the substrate. Penetration of the aqueous composition can be freely
regulated by varying the viscosity and polymer concentration of the
aqueous composition. A non-porous, smooth surface substrate can also be
used. For example, it can be used for the purpose of dew condensation
prevention. The substrate may be coated or impregnated with the aqueous
composition of the present invention by means of a sprayer, rolls, a slit
extrusion-type coating machine or the like.
Heating of the aqueous composition or the aqueous composition-coated or
impregnated substrate to effect crosslinking of the acrylic polymer is
conducted at a temperature at which the water soluble peroxide radical
initiator is decomposed, which temperature is generally in the range of
from about 40.degree. to about 180.degree. C. It is preferred that the
above-mentioned heating be conducted at a temperature at which the half
life of the decomposition of the peroxide radical initiator is in the
range of from about 60 to about 20 seconds. This temperature is, for
example, in the range of about 120.degree. to about 130.degree. C. with
respect to persulfate salts. In this respect, reference may be made to I.
M. Kolthoff and I. K. Miller : J. Am. Chem. Soc., 73, 3056 (1951). It is
apparent that these temperatures are also suitable for evaporating water
thereby to dry the product. Hence, persulfate salts are suitable radical
initiators in the present invention. In this connection, it is to be noted
that when an ordinary dryer is employed, the temperature of the aqueous
composition is about 5.degree. to 20.degree. C. lower than the temperature
of the dryer due to evaporation of water. When a substrate of fibers
coated or impregnated with the aqueous composition is heated, rapid
evaporation of water occurs. Hence, to effect cross-linking promptly while
maintaining an appropriate water content for effecting crosslinking of the
polymer, it is preferred that heating be conducted by steam heated to
about 130.degree. to about 150.degree. C. The period of time during which
heating is continued depends on the kind and amount of the initiator,
temperature of the heating, etc. However, it is generally in the range of
from about 1 to about 10 minutes.
In the present invention, radical crosslinking uniformly occurs between the
polymer molecules. Uniformity of radical crosslinking can be assessed by
the water absorbing and swelling properties of the product and the feeling
upon touch on the hydrogel formed as a result of the water absorption of
the product. The degree of crosslinking of the polymer can be freely
regulated by varying the amount of the initiator, the amount of water and
other conditions.
The water absorbent polymer-coated article according to the present
invention is excellent in uniformity with respect to water absorption and
bonding strength between the substrate and the hydrogel as compared with
the conventional products produced by spreading a polymer powder. It can
be safely and advantageously utilized as a disposable diaper, sanitary
napkin, surgical pad, surgical sheet, paper towel or the like.
Applications of the article according to the present invention are also
found in a wall material for dew condensation prevention, a water
absorbent non-woven fabric and an improved absorbent cotton. In the
present invention, a porous inorganic material such as pumice can also be
employed as a substrate, and the product can be utilized as a water
absorbent, water retaining material. For example, it may be blended with
soil to give a water retaining effect suited for growth of plants.
The aqueous composition of the present invention, besides the
above-mentioned advantages, has also the following interesting property.
When the aqueous composition of the present invention is applied to an
article comprising a substrate and, spread thereover, a powder of an
acrylic polymer, a coat layer is formed so as to act as a strong binder
between the powder and the substrate. With respect to this product,
separation of a hydrogel formed by water absorption can be advantageously
avoided.
The present invention will be illustrated in more detail with reference to
the following Examples, which should not be construed to be limiting the
scope of the present invention. The parts and percents are by weight
unless otherwise indicated.
EXAMPLE 1
To a solution made by dissolving 32 parts of sodium hydroxide (purity: 93%)
in 144 parts of water were added 72 parts of acrylic acid and 0.1 part of
potassium persulfate to prepare an aqueous solution. The solution had an
acrylic acid salt concentration of 35%, 75% of the entire amount of the
acrylic acid had been converted to its sodium salt. The obtained aqueous
solution was stirred in nitrogen atmosphere while maintaining the
temperature of the solution within a range of 50.degree. to 55.degree. C.
to effect polymerization. The polymerization was continued for 3 hours to
obtain a highly viscous aqueous polymer solution.
An aliquot of the solution was collected and evaporated to dryness under
reduced pressure to obtain a polymer. The obtained polymer was weighed to
determine the concentration of the aqueous polymer solution. As a result,
the solution was found to have a polymer concentration of 35%. This
indicated that the polymerization had proceeded almost completely.
To 20 parts of the aqueous polymer solution was added 0.14 part of
potassium persulfate, which corresponds to 2% of the polymer, to obtain a
homogeneous solution. The obtained solution was put into a vessel to form
a layer of the solution having a thickness of about 5 mm. The vessel was
put in a dryer maintained at a temperature of 130.degree. C. and dried for
5 hours to obtain a solid product in the form of a sheet. The solid
product was pulverized into a powdery product. The powdery product was
sifted with a sieve to obtain a powdery product having a particle diameter
of 48 to 200 mesh (Tyler).
0.5 g of the obtained powdery product was separately put in 1000 ml of pure
water and 100 ml of 0.9% saline solution (physiological saline solution),
and 30 minutes later the formed hydrogel was filtered with a 80-mesh metal
sieve and subjected to weighing. As a result, it was found that the
absorbency of the polymer was 45 g per g of the polymer for 0.9 g saline
solution and 490 g per g of the polymer for pure water.
The hydrogel was not sticky and had a comfortable touch.
EXAMPLE 2
The aqueous polymer solution prepared in Example 1 was heated and
concentrated to obtain a gel-like composition having a concentration of
48%. The concentration was determined by drying the composition and
weighing the resulting dried solid.
To 20 parts of this composition was added 0.08 part of ammonium persulfate,
which corresponds to 0.83% of the polymer and the resulting mixture was
kneaded in a kneader to dissolve the ammonium persulfate homogeneously
into the composition. The obtained mixture was formed into a sheet having
a thickness of about 3 mm, put in a dryer maintained at a temperature of
130.degree. C. and dried for 3 hours to obtain a solid polymer. The
polymer was pulverized into a powdery product. The powdery product was
sifted with a sieve to obtain a powdery product having a particle diameter
48 to 200 mesh (Tyler). The absorbency of the powdery product was measured
by the method as described in Example 1. As a result, it was found that
the absorbency of the powdery product was 51 g/g for 0.9% saline solution
and 540 g/g for pure water.
EXAMPLE 3
To a solution made by dissolving 30 parts of sodium hydroxide in 192 parts
of water were added 64.8 parts of acrylic acid and 7.1 parts of acrylamide
to obtain an aqueous solution. To the aqueous solution was added 0.1 part
of potassium persulfate to prepare an aqueous solution. The solution had a
concentration of 30%. 78% of the entire amount of the acrylic acid had
been converted to its sodium salt. The solution was stirred in nitrogen
atmosphere while maintaining the temperature within a range of 50.degree.
to 55.degree. C. to effect polymerization. The polymerization was
continued for 3 hours to obtain a highly viscous aqueous polymer solution.
The aqueous polymer solution was heated to concentrate the solution. As a
result, a gel-like composition having a concentration of 50% was obtained.
To 20 parts of the composition was added 0.08 part of potassium
persulfate, which corresponds to 0.8% of the polymer. The obtained mixture
was kneaded in a kneader to dissolve potassium persulfate homogeneously
into the composition. The mixture was subjected to the same treatment as
described in Example 1 to obtain a dried powdery product having a particle
diameter of 48 to 200 mesh (Tyler). The absorbency of the obtained powdery
product was measured by the method described in Example 1. As a result, it
was found that the absorbency of the powdery product per g of the polymer
was 42 g for 0.9% saline solution and 430 g for pure water.
EXAMPLE 4
To a solution made by dissolving 33.5 parts of sodium hydroxide (purity:
93%) in 113 parts of water were added 72 parts of acrylic acid to obtain
an aqueous solution. To the aqueous solution was added 0.1 part of
potassium persulfate to prepare an aqueous solution. The solution had a
concentration of 40%. 78% of the entire acrylic acid had been converted to
its sodium salt. The obtained solution was poured into a vessel to form a
layer of the solution having a thickness of about 2 cm and heated in
nitrogen atmosphere. The polymerization reaction was initiated when the
temperature of the solution reached about 50.degree. C. The polymerization
reaction was continued for about 10 min with vigorous generation of steam.
A portion of the resulting product was collected, dried and subjected to
weighing. From the weight of the dried polymer, the water content of the
| | |
