Note: Descriptions are shown in the official language in which they were submitted.
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AHSORHENT 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
f
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
str~,:cture of ionisable functional groups. These groups are
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usually carboxyl groups, a high proportion of which are in
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 sE=ries 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 eacpansion
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 F>resence
of a significant concentration of electrolytes in the water
interferes with dissociation of the functional groups and
leads to the "salt poisoning" effect. Althouc_Th 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 superabsorbe:nts in
absorbing electrolyte containing liquids such as menaes 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 fibrou:a form.
Combining a superabsorbent with an ion exchanger atternpts to
alleviate the salt poisoning effect by using the ion
exchanger to reduce the salt content of the liquid. 7:'he 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
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3
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.
EP-A=0487975 relates to a cross-linked ampholytic
copolymer said to be highly absorbent to aqueous electrolyte
solutions and formed from an ampholytic ion pair monomer, a
co-monomer and a cross-linking agent. It is assumed that
when the ampholytic ion pair monomer is incorporated into the
polymer backbone the ion pairs act as ionic cross-links which
remain intact in deionised water but are broken in salt
solution. Accordingly the copolymer is sensitive to the
ionic strength of the solution in the sense that the
effective degree of cross-linking is reduced as the ionic
strength increases. Whilst this produces an absorbent whose
absorption capacity in deionised water and in salt solution
more closely approximate to one another, it does not
necessarily improve absorption in the presenee of salt as the
polymer is not able to desalt the ionic solution and thus
increase the absorption power.
EP-A-0161762 relates to a water swellable, water
insoluble polymer produced by inverse suspension
polymerisation of a diallylic ammonium salt monomer, an
acrylic monomer and a cross-linking agent . The product is an
acrylic acid polymer containing both cationic and anionic
groups in the chain which is intended for use as a
superabsorbent in salt form. It is claimed that the material
can absorb the same quantity of water irrespective of the
salt content of the water but absorption is at a low level
and the material does not show any significant improvement in
its water absorption in the presence of salt as compared to
' conventional superabsorbents. EP-A-0161763 relates to a
similar superabsorbent made by polymerising a diallyl
ammonium compound and a cross-linking agent by suspension
polymerisation.
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WO 9:2/20735 relates to a superabsorbent which is said
to be sub~stant:Lally tolerant to salt solutions and which
comprises a swellable hydrophobic polymer and an ionizable
surfactant:. The specification also discloses (but does not
claim) an. alternative embodiment which uses a cationic
' supezabso:-beat which exchanges C1' with OH' and an anionic
superabsorbent which exchanges Na' with H'. No working
examples of such a system are given and the superabsorbent
gels disclosed are generally aczylamide derivatives.
Aczylamidee derivatives include the amide bond which is
subject to hydrolysis at low alkaline pH !about pH e) with
release o:E toxi.c hydrolysis products. Hydrolysis problems
will be e:~cacerbated if the polymer is prepared and used in
base form. An ~~lkaline pH of about a may well arise in baby
urine if f;ermen~~ation of urea to ammonia takes place so that
tissue hyclrolys;is products would be liable to be formed from
acrylamides derivatives in contact with urine at this pH.
ZO An object ~~f an aspect of the present invention is to provide a
superabsorbent with improved performance in the presence of
electroly~:e, for example in the case of menses or urine.
The ~~resen~t invention provides a superabsorbenc material
ZS which com)~r~.saa a combination of
!I) an anioaic superabsorbent in which from 20 to 100
of the fu~action.al groups are in free acid form: and
30 (2) a cationic superabsorbent in which from 20 to 100~c
of the ftinctio;nal groups are in basic form, the cationic
superabso;rbent being based on a polysaccharide or a polymer
of units ~of a monomer of formula ( I )
3 5 ~Z=y Rl CFA=CHZ o .
H~;~ ~ N ~~Z XB I I )
Rz
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wo ~ms8i pcrms~isi39
wherein R1 -and Rz, which may be the same or different, are
each organic 'radicals which do not adversely affect the
properties of 'the polymer and X is an anion.
5 The anionic superabsorbent
preferably
has
50
to
100
and
more pre:cerably has substantially 100 of the functional
groups in free acid fona. The cationic superabsorbent
preferably has 50 to 100 and more preferably has
substantially ;LOO~C the functional groups in basic form.
of
As alreacly noted above, both anionic and cationic
superabsorbents have to have functional groups in salt form
before trey acr_ as superabsorbents. Coitanercially available
superabsc~rbent:3 are usually available in salt form. It has
15 now surF~risin~~ly been found according to the present
inventions chat a combination of an anionic superabsorbent in
free acid form with a cationic superabsorbeat as defined
above ix~ basic form is particularly effective as a
superabsc~rbeat 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
supera~bsc~rbent material according to the invention is
25 contactedl with an electrolyte containing solution as follows
(1) the anionic and the cationic superabsorbent are both
converted from a non-absorbing form into the salt forms in
which they act as superabsorbents; and
(2) conversion of the anionic and the cationic
30 superabsorbent into the salt forms has a de-ionising effect
on the solution .
In general the anionic superabsorbent does not behave
as an ion exchanger in the sense that contacting the material
35 alone 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
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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 cationic superabsorbent
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 both the anionic and the cationic
superabsorbent into the salt form on contact with an
electrolyte containing solution has a significant desalting
effect on the solution thereby improving the perfortr~ance of
the superabsorbent by alleviating the salt-poisoning effect.
In contrast with the use of an ion-exchange resin to desalt
the solution (see Japanese Patent Application CPI No.
57-45057 and EP-A-0210756 referred to above) the material
having the de-salting effect is the superabsorbent itself.
This allows a much greater de-salting effect to be achieved
and the material which brings about the de-salting effect
does not act as a diluent for the superabsorbent.
The anionic superabsorbent can be any material having
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
attached to a slightly cross-linked acrylic base polymer.
For example, the base polymer may be a polyacryl.amide,
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.
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WO 96/17681
Particularly preferred base polymers are starch poiyacrylaces
and cros:~-linked polyacrylates.
The functional groups will generally be carboxyl groups .
S
Man3r anionic superabsorbents are available commercially,
' for examF>le Dow*2090 (Dow), Favor X22 (Stockhausen), Sanwet
IM 1500 (Sanyo), Aqualon* ASV D3236 (Aqualon Company).
Commercially available anionic superabsorbents are generally
sold in a~alt form and need to be converted to the free acid
form for use according to the invention, for example, Favor
922 may be swelled in water, acidified with HC1 (O.olm),
washed with water to remove excess HCl and dried in an air
ventilated oven to obtain Favor 922 in acid form (FAVOR H) as
follows:
Pre~arati.o. f Favor H
10g of Favor 922 were placed in a 1 litre beaker, and swelled
with 500 »nl of distilled water under continuous stirring with
a magnetic stirrer. 250 ml of HC1Ø01 M were thereafter
added under continuous stirring, and after 30 minutes the gel
was file-eyed with a nonwoven fabric filter. The
acidifica~tioa and filtration steps were repeated until there
were no longer any sodium ions present in the wishing waters
( the sodium ion content may be determined by a potentiometric
method using a selective sodium sensitive electrode).
Fiaslly 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.
Alteernatively the anionic superabsorbent may be directly
synthesised in acid form by the radical polymerization o: the
acrylic acid monomer with a crosslinking agent, namely in the
same manner as commercially available superabsorbents are
synthes i::ed .
* Trade-dark
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The cationic superabsorbent can also be a material
formed from a polysaccharide based polymer as described above
for the anionic superabsorbent but with cationic functional
groups. Alternatively the cationic superabsorbent may be
based on a polymer of units of a monomer of formula (I):
CHz=CH Rl iH=CH2
H2C N CH2 ~ (I)
R2
wherein R1 and R2 which may be the same or differs:nt, are
each organic radicals which do not adversely affect the
properties of the polymer and X is a suitable anion.,
Preferably R1 and R2 are each independently an
optionally substituted saturated hydrocarbon group or aryl
group. For example the saturated hydrocarbon group may be an
alkyl group which may be straight or branched chain or
cyclic. The aryl group also includes arylalkyl groups.
Preferably the groups R1 and R2 have from 1 to 20 carbon
atoms, more preferably from 1 to 6 carbon atoms. The
saturated hydrocarbon groups or the aryl groups may be
substituted by one or more suitable substituents ~~elected
from carboxyl, ester, hydroxyl, ether, sulphate, sulphonate,
primary, secondary or tertiary amines or quaternary ammonium
groups. In this case of ester (-C02R) and ether (-G-R) the
R group is a hydrocarbon radical having from 1 to 20,
preferably from 1 to 6 carbon atoms, more preferably the R
group is methyl. In the case of aryl groups, suitable
substituents include saturated hydrocarbon groups as defined
above . The preferred groups for R1 and Rz are methyl groups .
X may be any suitable anion which may be inorganic or
organic. Suitable inorganic anions include hal:Lde (in
particular fluoride, chloride, bromide and iodide), nitrate,
phosphate, nitrite, carbonate, bicarbonate, borate, ~;ulphate
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9
and hydroxide. Suitable organic anions include carboxylate
such as acetate, citrate, salicilate and propionate.
Preferably the anion is a chloride or hydroxide ion.
a
Preferred monomers are diallyl dimethyl ammonium
chloride and dimethyl diallyl ammonium hydroxide.
The cationic superabsorbents used according to the
present inventions are resistant to hydrolysis at, low
alkaline pH and thus are not subject to the problems with
release of toxic hydrolysis products referred to above in the
context of the acrylamide derivatives suggested by WO
92/20735. Examples of suitable cationic functional groups
include primary, secondary or tertiary amine groups or
quaternary ammonium groups which should be present in base
form. Preferably quaternary ammonium .groups are used.
Preferred base polymers include polysaccharides and polymers
based on dimethyldiallyl ammonium chloride.
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:
Ri +
~CHZ- CH - (CHR?n N R2 Z-
X OH R3
or
R1 +
CHZ - CH ( CHR ) n N R2 Z-
\ o i ~3
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io
where n is an integer from 1 to 15; X is halogen; Z is an
anion such as halide or hydroxyl; and R, R~, RZ and R3, which
may be the same or different, are each hydrogen, alkyl,
hydroxyalky~i, alkenyl or aryl and RZ may additionally
represent a~ residue of formula
Ru
I
CHZ ) p--- rT -- ( CHR ) n CF~F-~ CHZ Z
Rv OH X
or
R~~
CHZ ) ~- ~ ..- ( CFgt ) -- CH -~ Chi= Z
R-s ~0~
IS
where p is an integer froea 2 to 10 and n, R, Rl, R3, X and Z
have the meanings already defined. Cationic polysaccharide
superabsorf~ents of this type are described in more~detail in
W092/19652.,
According to another embodimeat the cationic
superabsor~~ent may be a crags-linked cellulose based
superabsor~~ent . :i.n particular a cationic polysaccharide , f or
example a fibrous polysaccharide, having superabsorbent
characteri:aics. the polysaccharide being substituted by
quaternary am~aonium 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.
According to a further embodiment the cationic
superabsorhent may be a water-swellable, water-insoluble
polymer cot~rorising units derived from a diallylic quaternary
ammonium salt monomer, cross-linked by a suitable
polyfuncti~~nal vinyl compound, characterised in that the
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11
polymer r~as-been produced by cationic polymerisation ~n an
aqueous phase using a free radical, catalyst .
Preferably the functional groups on anionic
superabsorbent are such that the superabsorbent is a weak
acid and those on the cationic superabsorbeat 'are such that
the superabsorbent is a strong bane.
In general the ratio of anionic to cationic
superabso:rbent is in the range 3:1 to 1:5 based on monomer
units, more preferably 2:1 to 1:2, each monomer unit having
one func~ional group therein. Most preferably the anionic
15 and cationic superabsorbents are used such that they have
equal exchange power so that pF~ extremes in the bodily fluids
absorbed are not reached and the aptimum desalting effect is
achieved. Cationic and anionic exchange power of the
superabso~:bent may be experimentally determined by, for
20 example, titration, or in the case of synthetic polymers by
a therotic:al calculation.
The absorbent material according to the'invention is
particularly suitable for use in applications where it is
25 desired tc~ absarb electrolyte containing aqueous liquids.
Examples ~cf such liquids include in particular menses and
urine and the absorbent material can be used as the filling
in catatae:nials and diapers generally in admixture with a
f ibzous ak~sorbent such as cellulose f luf f . For this purpose
30 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
35 aqueous liquids as is demonstrated below in the following
examples ~~y testy carried out using saline solution ( 1~ NaCl )
and synthca is urine.
and the polysaccharide being cross-
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m
Preo,-aratic~ - - Cationic Superabsorbent based on
Dimethyldiallylammonium chloride
SATI~?OLYMER IN' ACID FORM
S 219 grams of a 60~ aqueous solution of dimethyldiallyammonium
chloride (DMAC) available from Fluka were weighed into a
500m1 fla~~k. 0.4597 g of bisacrylamide (crosslinker agent)
were weighed separately into a 5 ml test tube and was
dissolved using 2 ml distilled water. 0.12 g of ammonium
~persulfate: lrad:ical initiator) were dissolved separately in
a 5 ml test tube in 2 ml distilled water. The air was
removed from the monomer solution by means of a vacuum pump.
Under continuous stirring, using a magnetic stirrer, the
crosslinke:r solution and the radical initiator solution were
added~to~t:he monomer solution, the temperature was adjusted
to 60°C by placing the flask in a thermostatic bath for four
hours. -
2o The solid. product formed was cut using a spatula and
transferred in a 5 litre beaker containing 4 litres of
distilled water, after two hours the swelled gel which had
formed wasp filtered by a nonwoven r_issue fabric filter. The
gel was dried in a ventilated oven at 60°C for 12 hours.
ZS l0og of a dried polymer called Fai.*9 C1' were collected.
Cationic t~~lvmer in basic form
20 g Fai !~ C1 polymer were placed in a l0 litre beaker and
30 swelled ur.~der continuous stirring by adding 4 L of distilled
water. Alter the polymer had swelled 500 ml of 0.01 M NaOK
solution mere added and after 30 minutes the gel was ~=ltered
using a r.onwoven fabric tissue filter. These operations
(alkalini::ation and filtering) were repeated until there were
35 no chloride ions in the washing waters (chloride ions may be
checked b5r AgNO~ reaction).
* Trade-mark
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At this point the gel was washed with distilled water until
no further evidence of the basic reaction was found in the
washing waters. The geI was dried in an air ventilated oven
at 60°C for 12 hours, 12 g of polymer were collected and it
was called Fai 9 OH.
Examples
Preparation - Anionic polvmer in acid form
10 g of superabsorbent polymer Favor 922 (available from
Stockhausen) were placed in a 2 litre beaker, and swelled
with 500 ml of distilled water under continuous stirring
(magnetic stirrer) for 1 hour.
500 ml of 0.01 M HC1 was added and stirred continuously for
1 hour.
The gel was filtered in a nonwoven fabric tissue filter, the
step of acidification and filtering of the gel containing
solution was repeated until the disappearance of sodium ions
from the washing waters (sodium ion content of the solution
can be measured by potentiometric method using a sodium
sensitive electrode).
Finally the gel was washed with distilled water until the
washing waters were neutral; the gel was dried in a
ventilated oven for 10 hours at 70°C to give 5.5 g of a dried
product which was called Favor H*.
2. Comparative Tests of Liquid Absorption
S
The test is to demonstrate that the use of both an anionic
AGM in acid form and a cationic AGM in base form, when in
contact with an aqueous saline solution, act as anionic and
cationic ion exchange resins and cause deionization of the
solution. The AGMs are converted in the salt form with
improved absorbency due to the reduced salt content of the
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14
solution.
0.2 g of Favor H (0.2 x 1000/72 = 2.78 mmoles) and 0.4 g of
Fai 9 OH (0.4 x 1000/143 - 2.80 mmoles) are weighed into a_
250 ml beaker. Under continuous stirring NaCl 1% solution is
dropped irito the beaker, the addition is stopped when the gel
formed is unable to absorb further solution. A minimum time
of two hours is allowed to elapse.
The gel is transferred in a tea-bag type envelope and is
suspended for 10 min to remove unabsorbed water aftE:r which
the envelope is weighed. Absorbency is measured as follows:
A = (Wwet - Wdry) / (G1 + G2 )
where:
A - absorbency in g/g
Wwet - weight of the envelope containing the wet AGMs in
g
Wdry = weight of the envelope containing the dry AGMs in
g
G1 - weight of the dry anionic AGM in g
G2 - weight of the dry cationic AGM in g
Absorbency after centrifugation ("retention") is measured by
placing the tea-bag envelope in a centrifuge for 10 m~.n at 60
x g after which the envelope is weighed.
Retention is measured as follows:
R = ( W' wet - Wdry ) / ( G1 + G2 )
where:
R - absorbency after centrifugation at 60 x g in g/g
W'wet= weight of the envelope containing the wet AGM
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after centrifugation in g
Wdry. G1 and G2 are as defined above.
Each of samples A to D were put into a saline solution (1%)
S or solution of synthetic urine and into deionized water.
Sample E was tested only in saline/synthetic urine.
Results are as follows:
water Retention g/g
Deionised Water 1% NaCl Solution
A- FAVOR ( H'" ) 3 0 3
B-FAVOR (Na*) 400 40
C-Fai 9 (OH) 300 45
D-Fai 9 (Cl-) 290 44
E-1/3 FAVOR (H+)
+ - 56
2/3 Fai (OH-) 1)
1 ) 1 part by weight Favor H+ is mixed with . two parts by
weight Fai 9 OH- in order to obtain an equimolar mixture of
the two polymers.
The above results show that the anionic superabsorbents
in acid form (FAVOR H+) shows very little absorption by
itself in 1% NaCl solution. However in combination with the
cationic superabsorbent in base form lFai 9 OH) , the material
shows significantly increased absorption over either FAVOR
Na+ or Fai 9 C1-.
r It should be noted that the theoretical retention to be
expected of 1/3 FAVOR H+ + 2/3 Fai 9 OH is about 31 g/g
whereas the theoretical retention of 1/3 FAVOR Na+ + 2/3 Fai
9 Cl' is about 43 g/g. The actual measured amount of 56 g/g
for 1/3 FAVOR H+ + 2/3 Fai 9 OH is equivalent to the result
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16
to be expected of 1/3 FAVOR Na'' + 2/3 Fai 9 C1- in 0.4% NaCl
and 0.4% NaCl corresponds to the desalting effect that would
be obtained by treating 1% NaCl with the mixture of FAVOR H''
+ Fai 9 OH.
It should also 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 re~~resents
the maximum likely to the encountered in practice.
r
