Note: Descriptions are shown in the official language in which they were submitted.
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ABSORBENT MATERIAL
The present invention relates to an absorbent material,
more particularly a material of the type commonly referred to
as a "superabsorbent".
The substances currently termed "superabsorbents" are
typically slightly cross-linked hydrophillic polymers. The
polymers may differ in their chemical nature but they share
the property of being capable of absorbing and retaining even
under moderate pressure amounts of aqueous fluids equivalent
to many times their own weight. For example superabsorbents
can typically absorb up to 100 times their own weight or even
more of distilled water.
Superabsorbents have been suggested for use in many
different industrial applications where advantage can be
taken of their water absorbing and/or retaining properties
and examples include agriculture, the building industry, the
production of alkaline batteries and filters. However the
primary field of application for superabsorbents is in the
production of hygienic and/or sanitary products such as
disposable sanitary napkins and disposable diapers either for
children or for incontinent adults. In such hygienic and/or
sanitary products, superabsorbents are used, generally in
combination with cellulose fibres, to absorb body fluids such
as menses or urine. However, the absorbent capacity of
superabsorbents for body fluids is dramatically lower than
for deionised water. It is generally believed that this
effect results from the electrolyte content of body fluids
' and the effect is often referred to as "salt poisoning".
The water absorption and water retention characteristics
of superabsorbents are due to the presence in the polymer
structure of ionisable functional groups. These groups are
usually carboxyl groups, a high proportion of which are in
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.e salt form ~~.:en the polymer is dry but which undergo
dissociation and solvation upon contact with water. In the
dissociated state, the polymer chain will have a series of
functional groups attached to it which groups have the same
electric charge and thus repel one another. This leads to
expansion of the polymer structure which, in turn, permits
further absorption of water molecules although this expansion
is subject to the constraints provided by the cross-links in
the polymer structure . which must be sufficient to prevent
dissolution of the polymer. It is assumed that the presence
of a significant concentration of electrolytes in the water
interferes with dissociation of the functional groups and
leads to the "salt poisoning" effect.
. Attempts have been made to counteract the salt poisoning
effect and improve the performance of superabsorbents in
absorbing electrolyte containing liquids such as menses and
urine. Thus Japanese Patent Application_ OPI No. 57-45,057
discloses an absorbent which comprises a mixture of a
superabsorbent such as s cross-linked polyacrylate with an
ion exchange resin in powder or granular form. EP-A-0210756
relates to an absorbent structure comprising a superabsorbent
and an anion exchanger, optionally together with a cation
exchanger, wherein both ion exchangers are in fibrous form.
Combining a superabsorbent with an ion exchanger attempts to
alleviate the salt poisoning effect by using the ion
exchanger, generally as a combination of both an anion
exchanger and a cation exchanger, to reduce the salt content
of the liquid. The ion exchanger has no direct effect on the
performance of the superabsorbent and it may not be possible
to reduce the salt content sufficiently to have the desired
effect on the overall absorption capacity of the combination.
In addition, besides being expensive, the ion exchanger has
no absorbing effect itself and thus acts as a diluent to the
superabsorbent.
An object of an aspect of the present invention is to provide a
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superabsorbent with improved performance in the presence
of electrolyte, for example in the case of menses or
urine.
The present invention provides a superabsorbent
material which comprises a combination of
(1) an anionic superabsorbent in which from 20 to
1008 of the functional groups are in free acid form; and
(2) an anion exchanger in which from 20 to 100 of
the functional groups are in basic form.
In accordance with one embodiment of the present
invention, a superabsorbent material comprises a
combination of:
i) an anionic superabsorbent polymer in which from
to 100 of the functional groups of the polymer are in
free acid form; and
ii) an anion exchange resin in which from 20 to 100$
of the functional groups of the resin are in basic form;
20 wherein the superabsorbent material has improved
absorbent performance in the presence of polyelectrolyte,
relative to the anionic superabsorbent polymer alone.
The anionic superabsorbent preferably has from 50 to
100 and more preferably has substantially 100$ of the
functional groups in free acid form. The cationic
superabsorbent preferably has from 50 to 100 and more
preferably has substantially 100$ in basic form.
As already noted above, anionic superabsorbents have
to have functional groups in salt form before they act as
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superabsorbents. Commercially available superabsorbents
are usually available in salt form. It has now
surprisingly been found according to the present
invention that a combination of an anionic superabsorbent
in free acid form with an anion exchanger in basic form
is particularly effective as a superabsorbent in the case
of electrolyte containing solutions, for example menses
and urine.
Whilst not wishing to be bound by any particular
theory, it is believed that there is a two fold effect
when the superabsorbent material according to the
invention is contacted with an electrolyte containing
solution as follows:
(1) the anionic superabsorbent is converted from a non-
absorbing form into the salt forms in which it acts as a
superabsorbent; and
(2) conversion of the anionic superabsorbent into the
salt form has a de-ionising effect on the solution which
is
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enhanced by the anion exchanger. '
In general the anionic superabsorbent does not behave
as an ion exchanger in the sense that contacting the material
in acid form with an electrolyte containing solution does not
result in conversion to the salt form. The functional groups
in anionic superabsorbents are typically carboxyl groups
which act as a weak acid which does not dissociate when
placed, for example, in a sodium chloride solution. However,
presence of the anion exchanger has the effect of attaching
chloride ions from sodium chloride solution, thereby
displacing the equilibrium in favour of conversion of the
anionic superabsorbent into the salt form.
This conversion of the anionic superabsorbent into the
salt form on contact with an electrolyte containing solution
and the effect of the anion exchanger in attaching chloride
ions has a significant desalting effect on the solution
thereby improving the performance of the superabsorbent by
alleviating the salt-poisoning effect. In contrast with the
use of an ion-exchange resin to desalt the solution in
combination with a superabsorbent which is already in salt
form (see Japanese Patent Application OPI No. 57-45057 and
EP-A-0210756 referred to above), the superabsorbent in acid
form also has a de-salting effect on the solution. This
allows a much greater de-salting effect to be achieved than
by use of ion exchanger and superabsorbent in salt form. It
should be noted that the effect of the electrolyte in
solution on the absorbtion capacity of a superabsorbent for
that solution is not linear in that absorption capacity does
not decrease regularly with increasing salt content.
Accordingly over certain concentration ranges it is possible
to bring about a relatively large increase in absorption
capacity by effecting a relatively small reduction in salt
content of the solution.
The anionic superabsorbent can be any material having
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superabsorbent properties in which the functional groups are
anionic, namely sulphonic groups, sulphate groups, phosphate
groups or carboxyl groups. Preferably the functional groups
are carboxyl groups. Generally the functional groups are
5 attached to a slightly cross-linked acrylic base polymer.
For example, the base polymer may be a polyacrylamide,
polyvinyl alcohol, ethylene malefic anhydride copolymer,
polyvinylether, polyvinyl sulphonic acid, polyacrylic acid,
polyvinylpyrrolidone and polyvinylmorpholine. Copolymers of
these monomers can also be used. Starch and cellulose based
polymers can also be used including hydroxypropyl cellulose,
carboxymethyl cellulose and acrylic grafted starches.
Particular base polymers include cross-linked polyacrylates,
hydrolysed acrylonitrile grafted starch, starch
polyacrylates, and isobutylene malefic anhydride copolymers,
Particularly preferred base polymers are starch polyacrylates
and cross-linked polyacrylates.
The functional groups will generally be carboxyl groups .
For cellulose derivatives the degree of substitution
(DS) of the derivative with the functional group is defined
as the number of functional groups (generally carboxyl
groups) per anhydroglucose units of cellulose. The DS is
generally from 0.1 to 1.5. In an analogous manner the DS for
synthetic polymers may be defined as the number of functional
groups per monomer or comonomer unit . The DS is generally 1,
for example 1 carboxyl group per monomer unit of
polyacrylate.
Many anionic superabsorbents are available commercially,
for example Favor 922 (Stockhausen), Sanwet IM 1500 (Sanyo),
AQU D3236 (Aqualon Company (Hercules)) or DOW 2090. (DOW).
A particularly preferred anionic superabsorbent is FAVOR 922
(Stockhausen). Commercially available anionic
superabsorbents are generally sold in salt form and need to
be converted to the free acid form for use according to the
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invention, for example by the following method:
Preparation of Favor H
lOg of Favor 922 were placed in a 1 litre beaker, and
swelled with 500m1 of distilled water, under continuous
stirring with a magnetic stirrer and a magnetic bar. 250m1
of HC1 0.01 M were added under continuous stirring, and after
30 minutes the gel was filtered with a nonwoven fabric
filter. The acidification and filtration steps were repeated
until there were no further sodium ions in the washing waters
(the sodium ion content may be determined by a potentometric
method using a selective sodium sensitive electrode).
Finally the gel was washed with distilled water to remove the
excess acid and the gel was dried in an air ventilated oven
at 60°C for 10 hours. The dried polymer obtained was called
Favor H.
Ion exchange is the reversible interchange of ions
between a solid and liquid in which there is no permanent
change in the structure of the solid, which is the ion-
exchange material.
Ion exchange occurs in a variety of substances - e.g.
silicates, phosphates, fluorides, humus, cellulose, wool,
proteins, alumina, resins, lignin, cells, glass, barium
sulphate, and silver chloride.
However, they are used for ion exchange materials that
depend on properties other than the interchange of ions
between liquid and solid phases.. Ion exchange has been used
on an industrial basis since 1910 with the introduction of
water softening using natural and, later, synthetic zeolites .
The introduction of synthetic organic ion exchange
resins in 1935 resulted from the synthesis of phenolic
condensation products containing either sulfonic or amine
groups which could be used for the reversible exchange of
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cations or anions.
Inorganic ion exchange materials include both the
naturally occurring materials such as the mineral zeolites
(e.g. cliptonite) the green sands and clay (e.g. the
montmorillonite group), and synthetic products such as the
gel zeolites, the hydrous oxides of polyvalent metals and the
insoluble salts of polybaric acids with polyvalent metals.
Synthetic organic products include cation and anion ion
exchange resins both of strong and weak type.
The ability of the weak base resins to sorb acids
depends on their own basicity and the pH of the acid
involved.
A variety of base strengths are obtained depending on
the nature of the amine functionality. Primary, secondary
and tertiary amine functionality, or mixtures of them, can be
put into various structures ranging from epichlorohydrin
amine condensates and acrylic polymers, to styrene-devinyl
benzene (DVB) copolymers.
These resins are capable of sorbing strong acids in good
capacity but are limited by kinetics.
Strong base, anion exchange resins especially those
based on styrene-DVB copolymer are classed as type I and II.
Type I is a quaternarized amine product made by the reaction
of trimethylamine with the copolymer after chloromethylation
with chloromethyl methyl ether (CMME).
The type I functional group is the most strongly basic
functional group available and has the greatest affinity for
the weak acids that commonly are removing during a water
demineralization process (e.g. silic acid and carbonic acid) .
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Type II functionality is obtained by the reaction 'of the
styrene-DVB copolymer with dimethylethanolamine. This
quaternary amine has lower basicity than that of the type I
resin, yet it is enough to remove the weak acid anions for
most applications.
Quaternary amine functionality has been introduced into
pyridinic and acrylate polymers with limited~commercial
application.
The anion exchanger is preferably an anion exchange
resin containing functional groups in basic form. Suitable
functional groups include amine groups, i.e. primary,
secondary and tertiary amine groups and quaternary ammonium
~ groups.
Anion exchange resins which are commercially available and
may be used in the present invention are:
TM
8mberlite IRA 400 - This is a strong anion exchanger having
quaternary ammonium functionality which is available in the
chloride form. For use in the present invention it is
necessary to convert it to OH- form, for example by NaOH
treatment in a chromatographic column and washing with
distilled water. The total exchange capacity is 3.8 meq/g of
dry resin.
8mberlite IRA 69 - This a weak basic anion exchanger having
tertiary amine functionality which is available in the free
base form. The total exchange capacity is 5.6 meq
(milliequivalents/g of dry resin). Amberlite ion exchangers
are a trade mark of Rohn.
ION exchancer tvfle IIi from Merck - This is a strong anion
exchanger resin, the exchange capacity is about 5 meq/g.
ION exchanger tyre iI form Merck - This is a weak anion
exchange resin, the exchange capacity is about 5 meq/g.
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Preferred anion exchange resins include Duolite A-102-
OH, (Dia-prosim, France) which is a strong anionic exchange
resin having quaternary ammonium functionality. The ion
exchange capacity is 1.3 meq/ml. Other suitable anion
exchange resins can be found in the product ranges of
manufacturers such as Rohn and Merck.
In general the weight ratio of anionic superabsorbent
to anionic exchanger is in the range 1:20 to 1:1 depending on
molecular weight and ion exchange capacity, preferably the
weight ratio is 1:2 to 1:4
The absorbent material according to the invention is
particularly suitable for use in applications where it is
. desired to absorb electrolyte containing aqueous liquids.
Examples of such liquids include in particular menses and
urine and the absorbent material can be used as the filling
in catamenials and diapers generally in admixture with a
fibrous absorbent such as cellulose fluff. For this purpose
the absorbent according to the invention can be present as
granules or fibres.
The absorbent materials according to the invention show
particularly good absorption of electrolyte containing
aqueous liquids as is demonstrated below in the following
examples by teats carried out using saline solution ( 1~ NaCl )
and synthetic urine.
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Examples
1. Preparation of Favor H+:
5 lOg of Favor 922 were placed in a 1 litre beaker, and swelled
with 500m1 of distilled water, under continuous stirring with
a magnetic stirrer and a magnetic bar. 250m1 of HCl 0.01 M
were added under continuous stirring, and after 30 minutes
the gel was filtered with a nonwoven fabric filter. The
10 acidification and filtration steps were repeated until there
were no further sodium ions in the washing waters (the sodium
ion content may be determined by a potentometric method using
a selective sodium sensitive electrode). Finally the gel was
washed with distilled water to remove the excess acid and the
gel was dried in an air ventilated oven at 60°C for 10 hours.
The dried polymer obtained was called Favor H.
2. Comparative tests of Liquid Absorption
The test was performed to show that, when in contact with an
aqueous saline solution, an anion exchange resin in basic
form together with an anionic superabsorbent in acid form act
as an anion and cationic exchange mixture and thus
deionization of the saline solution occurs. The anionic
superabsorbent is then converted to the salt form and thus
has improved absorbency due to the low salt content of the
solution.
1% NaCl solution (150m1) was placed in contact with the anion
exchange resin A102 OH (3.9g), in a 250m1 beaker for 2 hours
under continuous stirring. This step allows the chloride
ions from the solution to be replaced by the hydroxide ions
from the resin. The solution was then drawn up by a Pasteur
pipette and transferred into another 2501 beaker containing
0.25g of Favor H being stirred. The addition of solution was
stopped when the gel did not swell any further. Thereafter
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the gel was placed into a nonwove~- tissue tea bag type
envelope, which had one edge which was not sealed, and the
absorbency after centrifugation at 60 x g for 10 minutes was
measured as follows:
A = (Wwet - Wdry) /G
where:
A - absorbency after centrifugation in g/g
wwet - weight of envelope containing the wet AGM after
centrifugation in g
Wdry = weight of the envelope containing the dry AGM in
g
G - weight of the AGM used in the test in g.
Results are as follows:
Amount Water Retention g/g
(g) . Deionised 1% NaCl
Water Solution
(A) FAVOR (H*) 0.25 30
3
(B) FAVOR (Na*) 0.25 400
40
(C) ANION EXCHANGE 3.9 -
0.29
RESIN (A-102-OH)
(D) FAVOR (H*) 0.25 - 100
+ A-102-OH + 3.9
NOTE: Results relate to 25 ml of 1% NaCl solution.
The above results show that the anionic superabsorbent
in acid form (FAVOR H*) shows very little absorption by
itself in 1% NaCl solution. FAVOR Na* shows some absorbtion
but much less than for deionised water. The anion exchange
resin has essentially no absorption. However, in combination
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with the anion exchanger in base form (A-102-OH), FAVOR (H+)
shows significantly increased absorption over FAVOR Na+.
It should be noted that 1% NaCl represents a stringent
test of the superabsorbent. Studies in the literature show
that the salt content of urine varies depending on a number
of factors but 1% by weight represents the maximum likely to
the encountered in practice.
