ABSORBENT MATERIAL

MATERIAU ABSORBANT

English Abstract

The invention provides a superabsorbent material which comprises a combination
of (1) a cationic superabsorbent in which from 20 to 100 % of the functional
groups are in basic form, and (2) a cationic exchanger in which from 50 to 100
% of the functional groups are in acid form. The combination is particularly
effective as a superabsorbent for electrolyte containing solutions such as
menses and urine.

French Abstract

L'invention porte sur un matériau superabsorbant fait d'une combinaison (1) d'un superabsorbant cationique dans lequel 20 à 100 % des groupes fonctionnels se présentent sous forme de bases, et (2) d'un échangeur cationique dans lequel 5 à 100 % des groupes fonctionnels se présentent sous forme d'acides. Ladite combinaison s'avère particulièrement efficace comme superabsorbant de solutions contenant un électrolyte telles que le flux menstruel et l'urine.

Claims

14
Claims
1. A superabsorbent material which comprises a combination
of
1) a cationic superabsorbent in which from 20 to 100% of
the functional groups are in basic form, and
2) a cation exchanger in which from 50 to 100% of the
functional groups are in acid form.
2. A superabsorbent material as claimed in claim 1 wherein
the cationic superabsorbent has from 50 to 100%, preferably
substantially 100% of the functional groups in basic form and
wherein the cation exchanger has substantially 100% of the
functional groups in acid form.
3. A superabsorbent material as claimed in claim 1 or 2
wherein the functional groups in the cationic superabsorbent
are primary, secondary or tertiary amines or quaternary
ammonium groups.
4. A superabsorbent material as claimed in claim 3 wherein
the functional groups are quaternary ammonium groups.
5. A superabsorbent as claimed in any of claims 1 to 4
wherein the functional groups are attached to a
polyacrylamide, polyvinyl alcohol, ethylene maleic anhydride
copolymer, polyvinylether, polyvinyl sulphonic acid,
polyacrylic acid, polyvinylpyrolidone, polyvinylmorpholine or
copolymers thereof, a starch or cellulose based polymers as
base polymer.
6. A superabsorbent material as claimed in claim 5 wherein
the starch or cellulose based polymer is hydroxypropyl
cellulose, carboxymethyl cellulose or an acrylic grafted
starch.
7. A superabsorbent as claimed in claim 5 or 6 wherein the

base polymer is a cross-linked polyacrylate or an isobutylene
maleic anhydride copolymer.
8. A superabsorbent as claimed in claim 7 wherein the base
polymer is a starch polyacrylate or a cross-linked
polyacrylate.
9. A superabsorbent material as claimed in any of claims
1 to 8 wherein the cationic superabsorbent is a
polysaccharide superabsorbent obtained by reacting a fibrous
polysaccharide with an excess of a quaternary ammonium
compound containing at least one group capable of reacting
with polysaccharide hydroxyl groups and having a degree of
substitution of 0.5 to 1.1.

16
10. A superabsorbent material as claimed in claim 9 wherein
the quaternary ammonium compound has the general formula
<IMG>
or
<IMG>
where n is an integer from 1 to 16; X is halogen; Z is an
anion such as halide or hydroxyl; and R, R1, R2 and R3, which
may be the same or different, are each hydrogen, alkyl,
hydroxyalkyl, alkenyl or aryl and R2 may additionally
represent a residue of formula
<IMG>
or
<IMG>
where p is an integer from 2 to 10 and n, R, R1, R3, X and Z
have the meanings already defined.
11. A superabsorbent material as claimed in any of claims
1 to 10 wherein the cationic superabsorbent is a fibrous
cationic polysaccharide having superabsorbent
characteristics, the polysaccharide being substituted by
quaternary ammonium groups and having a ds of at least 0.5
and the polysaccharide being cross-linked to a sufficient
extent that it remains insoluble in water.

17
12. A superabsorbent material as claimed in any of claims
1 to 10 wherein the cationic superabsorbent is a water-swellable,
water-insoluble polymer comprising units derived
from a diallylic quaternary ammonium salt monomer,
cross-linked by a suitable polyfunctional vinyl compound,
characterised in that the polymer has been produced by
cationic polymerisation in an aqueous phase using a free
radical catalyst.
13. A superabsorbent material as claimed in any of claims
1 to 12 wherein the cation exchanger is a cation resin
containing functional groups in acid form.
14. A superabsorbent material as claimed in claim 13 wherein
the functional group is a carboxylic acid or sulphonic acid
group.
15. A superabsorbent material as claimed in any of claims
1 to 14 wherein the weight ratio of cationic superabsorbent
to cation exchanger is in the range of from 1:20 to 1:1.
16. A superabsorbent material as claimed in claim 15 wherein
the weight ratio of cationic superabsorbent to cationic
exchanger is from 1:3 to 1:1.
17. Use of a superabsorbent material as claimed in any of
claims 1 to 16 for the absorbtion of electrolyte containing
aqueous liquids.

Description

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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 hydrophilic 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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the salt form when 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. Although most
commercial superabsorbents are anionic, it is equally
possible to make cationic superabsorbents with the functional
groups being, for example, quaternary ammonium groups. Such
materials also need to be in salt form to act as
superabsorbents and their performance is also affected by 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 a 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

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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 the present invention is to provide a
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
(l) a cationic superabsorbent in which from 20 to 100%
of the functional groups are in basic form, and
(2~ a cation exchanger in which from 50 to 100% of the
functional groups are in acid form.
The cationic superabsorbent preferably has from 50 to
lO0~, more preferably substantially 100% of the functional
groups in basic form.
The cation exchanger preferably has substantially 100%
of the functional groups in acid form.
It has now surprisingly been found according to the
present invention that a combination of a cationic absorbent
in basic form with a cation exchange in acid 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:
(l) the cationic superabsorbent is converted from a non-
absorbing form into the salt form in which it acts as a
superabsorbent; and

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(2) conversion of the cationic superabsorbent into the salt
forms has a de-ionising effect on the solution which is
enhanced by the cation exchanger.
s The functional groups in cationic superabsorbents are
typically quaternary ammonium groups which are strong ion
exchangers. Thus when the cationic superabsorbent is
contacted with an electrolyte solution, for example a saline
solution, it swells and the OH- ions from the superabsorbent
are replaced in part with Cl- from solution and the pH of the
solution will become strongly basic. However the presence of
the cation exchange resin prevents the solution becoming
strongly basic by displacing the equilibrium reaction in
favour of the complete conversion of the cationic
superabsorbent into the salt from. In so doing the sodium
ions in solution are replaced by the cation exchange resin,
chloride ions in solution are replaced by the cationic
superabsorbent in base form thus causing substantial
desalification of the saline solution and in turn improved
absorbance of the superabsorbent.
This conversion of the anionic superabsorbent into the
salt form on contact with an electrolyte containing solution
and the effect of the cation exchanger in attaching sodium
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 cationic superabsorbent
in basic 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 electrolyte in solution
on the absorption capacity of a superabsorbent for that
solution is not linear in that absorption capacity does not

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decrease regularly with increasing salt content. Accordingly
over certain concentration ranges it is possible to bring
about a relatively iarge increase in absorption capacity by
effecting a relatively small reduction in salt content of the
solution.
The cationic superabsorbent can be any material having
superabsorbent properties in which the functional groups are
cationic. Generally the functional groups are attached to a
slightly cross-linked acrylic base polymer. For example, the
base polymer may be a polyacrylamide, polyvinyl alcohol,
ethylene maleic 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 maleic anhydride copolymers.
Particularly preferred base polymers are starch polyacrylates
and cross-linked polyacrylates.
Examples of suitable cationic functional groups include
quaternary ammonium groups or primary, secondary or tertiary
amines which should be present in base form. For cellulose
derivatives the degree of substitution (DS) of the derivative
with the functional group is defined as the number of
functional groups (generally quaternary ammonium 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 quaternary ammonium group per monomer unit of
polyacrylate. Preferred base polymers include
polysaccharides and polymers based on dimethyldiallyl
ammonium chloride.

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According to one embodiment, the cationic superabsorbent
can be a polysaccharide superabsorbent obtained by reacting
a fibrous polysaccharide such as cellulose with an excess of
a quaternary ammonium compound containing at least one group
capable of reacting with polysaccharide hydroxyl groups and
having a degree of substitution of 0.5 to 1.1. The
quaternary ammonium compound may have the general formula:
Rl +
10CH2--CH (CHR) n N R2 z
OH R3
~1 +
CH2--5H ( CHR) r~. N R2 z_
O R3
where n is an integer from 1 to 16; X is halogen; Z is an
anion such as halide or hydroxyl; and R, Rl, R2 and R3, which
may be the same or different, are each hydrogen, alkyl,
hydroxyalkyl, alkenyl or aryl and R2 may additionally
represent a residue of formula
Rl +
( CH2 ) p I ( CHR ) n CH CH2 Z ~
R3 OH X
or _ _
Rl +
( CH2 ) p N ( CHR ) n
R3 o
where p is an integer from 2 to 10 and n, R, Rl, R3, X and Z
3s have the m~n;ngs already defined. Cationic polysaccharide
superabsorbents of this type are described in more detail in
WO92/19652.

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According to another embodiment the cationic
superabsorbent may be a cross-linked cellulose based
superabsorbent, in particular a fibrous cationic
polysaccharide having superabsorbent characteristics, the
polysaccharide being substituted by quaternary ammonium
groups and having a ds of at least 0.5 and the polysaccharide
being cross-linked to a sufficient extent that it remains
insoluble in water. Superabsorbents of this type are
described in more detail in our co-pending patent application
10 No........... (internal reference DR44).
According to a further embodiment the cationic
superabsorbent may be a water-swellable, water-insoluble
polymer comprising units derived from a diallylic quaternary
ammonium salt monomer, cross-linked by a suitable
polyfunctional vinyl compound, characterised in that the
polymer has been produced by radical polymerisation in an
aqueous phase using a free radical catalyst. Superabsorbents
of this type are described in more detail in our co-pending
20 patent application No.......... (internal reference DR43).
Ion exchange is the reversibie 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 phase. Ion exchange has been used
on an industrial basis since 1910 with the introduction of
water softening using natural and, later, synthetic zeolites.

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The introduction of syntheti~ 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
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 weak acid, cation-exchange resins are based
primarily on acrylic or methacrylic acid that has been
crosslinked with a disfunctional monomer - e.g. D~3
(divinylbenzene). Other weak acid resins have been made with
phenolic of phosphonic functional --~ps.
The weak acid resin has a high a C~ ~.e ~ cge~.
ion and, thus is easily regenerated w;~h s--c..g ac:ds. ..~e
property, however, limits the region in which salt spli~ lng
can occur to above pH 4.
The strong acid resins of commercial significance are
sulfonated copolymer of styrene and DVB, sulfonic acid,
sulfur trioxide, and chlorosulfonic acid have each been
utilized for sulfonation.
These materials are characterized by their ability to
exchange cations or split neutral salts and are useful across
the entire pH range.
The cation exchanger is preferably a cation resin

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containing functional groups in acid form. Suitable
functional groups include carboxylic or sulphonic acid
groups.
The following cationic exchange resins may be used in the
practise of the present invention:
Amberlite IR 120 which is a strong cation exchanger having
sulfonic acid functionality which is available in H+ form.
The total exchange capacity is 4.4 meq/g for the dry resin.
Amberlite IRC 76 which is a weak cation exchanger having
carboxylic functionality which is available in acid form.
The total exchange capacity is 10 meq/g for dry resin.
Dowex 50W YZ which is a strong cation exchanger which is
available in H+ form having sulfonic acid functionality. The
total exchange capacity is 5 meq/g for dry resin.
In general the weight ratio cationic superabsorbent to
cation exchanger is in the range 1:20 to l:l preferably 1:3
to 1:1 depending on molecular weight and ion exchange
capacity.
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

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examples by tests carried out using saline solution (1~ NaCl)
and synthetic urine.
Pre~aration - Cationic Superabsorbent based on crosslinked
polydimethyl diallyl ammonium hydroxide
called Fai 7 OH.
Pre~aration of Fai 7 OH
133g of 60~ aqueous solution of dimethyl diallyl ammonium
chloride (DMAC available from fluka) were weighed into a
25Oml flask.
0.2g of bisacrylamide (BAC available from fluka~ were weighed
separately into a 5ml test tube and were dissolved in 2ml of
distilled water.
0.12g of ammonium persulfate (free radical initiator) were
dissolved in a 5ml test tube in 2ml of distilled water.
The monomer solution was disareated by vacuum using a vacuum
pump.
Thereafter under continuous stirring the crosslinker solution
and free radical initiator were added to the monomer
solution, the temperature was adjusted to 60~C by placing the
flask in a thermostatic bath for four hours.
The solid product formed was cut using a spatula and
transferred in 5 litre beaker containing 4 litres of
distilled water, after two hours the swelled gel was filtered
with a nonwoven tissue fabric filter. The gel was dried in
a ventilated oven at 60~C for 12 hours. 60g of dried polymer
was collected and called Fai 7 Cl. 20g of Fai 7 Cl was
placed in a 10 litre beaker and swelled by adding 4 litres of

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li
distilled water, under continuous stirring. When the polymer
had swelled (after 2 hours) 500 mi of 0.01 M NaOH solution
was added and after 30 minutes the gel was filtered using a
nonwoven fabric tissue filter. These operations
(alkalinization and filtration) were repeated until there
were no chloride ions in the washing waters (chloride ions
may be checked by AgNO3 reaction). At this point the gel was
washed with distilled water until there was no further
evidence of the basic reaction in the washing waters.
Thereafter the gel was dried in an ventilated oven at 60~C
for 12 hours. 10g of dried polymer was collected and was
called Fai 7 OH.
ExamPles
1. Comparative Tests of Liauid Absorbtion
A test was performed to demonstrate that the use of both a
cationic superabsorbent and a cation exchange resin may
improve the absorbing performances of the cationic
superabsorbent due to the desalting effect achieved by the
ion exchange mixture.
A 1~ NaCl solution (150ml) was placed in contact with 2.23g
of the cation exchange resin IR120 (H+) in a 250ml beaker for
2 hours under continuous stirring. The sodium ions in the
solution should be replaced by the H+ ions from the resin.
The solution was drawn up with a Pasteur pipette and dropped
into another 250ml beaker containing 0.11g of Fai 7 OH under
stirring; the addition is stopped when the gel swells no
more. At this point the gel is placed into a nonwoven tissue
"tea bag" small envelope and the absorbency after
centrifugation at 60 x g for 10 minutes was measured as
follows:
A = (Wwet - Wdry)/G

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12
= aDsorbe~.cy af e~ cent~ C gation ln g/g,
~wet = weight o~ envelope c_nta ning the wet AGM after
centrifugation in g,
s Wdry = weight of the envelope containing the dry AGM in
g,
G = weight of the AGM used in the test in g.
The test was also repeated using both Fai 9 OH- and Fai 9 Cl-
individually without the cation exchange resin.
Results are as follows:
Absorption g/g Absorption g/g
(Tea-bag) (Centrifuge)
Amount H20 1%NaCl H20 1%NaCl
(g)
Fai-7 OH- 0.11 351 55 300 42
Fai-7 Cl- 0.11 340 54 290 43
Fai-7 OH- 0.11 - 96.7 - 55
+ IR 120 (H~) + 2.23
The above results show that the cationic superabsorbent
in base form Fai-7 OH- and salt form (Fai-7 Cl-) shows
limited absorption in 1% NaCl solution as compared to
deionised water. However in combination with the cationic
~c~n~er in acid form IR120 (H') the material shows
significantly increased absorption.
It should also be noted that 1~ NaCl represents a
stringent test of the superabsorbent. Studies in the

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13
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.