Note: Descriptions are shown in the official language in which they were submitted.
CA 02433044 2003-06-25
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2
HYDROGELS COATED WITH STERIC OR ELECTROSTATIC SPACERS
This invention relates to hydrogels, to water-absorbent compositions
containing same, to
processes for their production, to their use in hygiene articles and to
methods for
determining suitable water-absorbent compositions.
Hygiene articles such as infant diapers or sanitary napkins have long been
utilizing highly
swellable hydrogels. This has substantially reduced hygiene article bulk.
The current trend in diaper design is toward even thinner constructions having
a reduced
cellulose fiber content and an increased hydrogel content. The advantage of
thinner
constructions shows itself not only in improved wear comfort, but also in
reduced costs for
packaging and warehousing. The trend toward ever thinner diaper constructions
has
substantially changed the profile of properties required of the water-
swellable hydrophilic
polymers. The decisive property is now the ability of the hydrogel to conduct
and
distribute imbibed fluid. The greater amount of polymer per unit area in the
hygiene article
must not cause the swollen polymer to form a barner layer for subsequent fluid
(gel
blocking). Gel blocking occurs when fluid wets the surface of the highly
absorbent
hydrogel particles and the outer sheath swells. The result is the formation of
a barrier layer
which reduces diffusion of liquids into the particle interior and thus leads
to leakage. Good
gel permeability and thus good transportation properties ensures optimum
utilization of the
entire hygiene article.
The objective of higher use levels for highly swellable hydrogels has led
through targeted
adjustment of the degree of crosslinking in the starting polymer and
subsequent
postcrosslinking to an optimization of absorbency and gel strength. Improved
gel
permeability values can be generated only from a higher crosslink density in
the starting
polymer.. Higher crosslink densities, however, go hand in hand with reduced
absorption
capacity and a decrease in the swell rate in the polymer. The consequence is
that an
increase in the hydrogel content of the hygiene article necessitates the
incorporation of
additional layers to prevent leakage, which in turn leads to bulky hygiene
articles and is
contrary to the actual objective of manufacturing thinner hygiene products.
A possible way to provide improved transportation properties and avoid gel
blocking is to
3 0 shift the particle size spectrum to higher values. However, this leads to
a decrease in the
swell rate, since the surface area of the absorbent material is reduced. This
is undesirable.
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Another way to obtain improved gel permeability is surface postcrosslinking,
which
confers higher gel strength on the hydrogel body in the swollen state. Gels
having
insufficient strength are deformable by pressure, for example pressure due to
the
bodyweight of the wearer of the hygiene article, and so clog the pores in the
hydrogel/cellulose fiber absorbent and so prevent continued absorption of
fluid. Since, for
the above reasons, an increased crosslink density in the starting polymer is
out of the
question, surface postcrosslinking is an elegant Way to increase gel strength.
Surface
postcrosslinking increases the crosslink density in the shell of the hydrogel
particles, as a
to result of which Absorbency Under Load (AUL) by the base polymer thus
generated is
raised to a higher level. Whereas absorption capacity decreases in the
hydrogel shell, the
core of the hydrogel particles has an improved absorption capacity (compared
to the shell)
owing to the presence of mobile polymer chains, so that shell construction
ensures
improved fluid transmission.
However, high use levels of highly swellable hydrogels still give rise to the
phenomenon
of gel blocking. An important criterion must therefore be the ability to
conduct fluid in the
swollen state. Only good fluid conductance ensures full exploitations of the
actual
advantages of highly swellable hydrogels, namely their pronounced absorption
and
2o retention capacity for aqueous body fluids. However, it is important that
fluid conductance
take place in the intended use period of the hygiene article. And the full
absorption
capacity of the hydrogel should be utilized in the process. The ability of a
hydrogel to
conduct fluid is quantified in terms of the Saline Flow Conductivity (SFC).
SFC measures
the ability of the formed hydrogel layer to conduct fluid under a given
pressure. It is
believed that, at high use levels, hydrogel particles are in mutual contact in
the swollen
state to form a continuous absorption layer within which fluid distribution
takes place.
A subsequent modification of the surface of the base polymers (surface-
postcrosslinked
starting polymers) is known.
DE-A-3 523 617 relates to the addition of finely divided amorphous silicas to
dry hydrogel
powder following surface postcrosslinking with carboxyl-reactive crosslinker
substances.
In the prior art, aluminum sulfate is used as sole crosslinker or combined
with other
crosslinkers in surface postcrosslinking.
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WO 95/22356 relates to the modification of absorbent polymers with other
polymers to
improve the absorption properties. Preferred modifiers are polyamines and
polyimines.
However, the effects with regard to SFC are minimal according to Tables 1 and
2.
WO 95/26209 relates to absorbent structures having at Least one region
containing 60
100% of highly swellable hydrogel having an SFC of at Least 30 x 10-~ cm3s/g
and a PUP
0.7 psi of at least 23 g/g. It is exemplified that such highly swellable
hydrogels are
obtainable by surface postcrosslinking. As is evident from Tables 1 and 2,
this type of
treatment can provide an increased SFC only at the expense of a decreased gel
volume, i.e.,
to there is a reciprocal relationship between retention and gel permeability.
SFC increases with increasing particle size of the highly swellable hydrogel.
As particle
size increases, the surface area of the highly swellable hydrogel particles
decreases relative
to their volume, and this results in a decreased swell rate. It is therefore
possible to deduce
from the results of these experiments that swell rate too has a reciprocal
dependency on
SFC.
It is an object of the present invention to provide highly swellable hydrogels
or water-
absorbent compositions having good transportation properties and high
permeability
2o coupled with a high ultimate absorption capacity and a high swell rate when
used in
hygiene articles. Contrary to the prior art, where high absorption capacities
on the part of
the hydrogels, high liquid transportation performance and rapid swellability
are mutually
exclusive, the novel highly swellable hydrogels to be generated shall combine
the contrary
parameters. In addition, it shall be possible to produce thin hygiene articles
through high
use levels for the highly swellable hydrogels of the invention. Highly
swellable hydrogels
shall be generated for this purpose that simultaneously exhibit a high swell
or absorption
rate, a high gel permeability and a high retention. Given the excellent fluid
distribution
present, the high total capacity of the inventive highly swellable hydrogels
in the
absorption layer should be optimally utilizable.
We have found that this object is achieved according to the invention by water-
insoluble
water-swellable hydrogels coated with steric or electrostatic spacers,
characterized by the
following pre-coating features:
- Absorbency Under Load (AUL) (0.7 psi) of at least 20 g/g,
- Gel strength of at least 1 600 Pa.
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The coated hydrogels additionally preferably have the following features:
Centrifuge Retention Capacity (CRC) of at least 24 g/g,
B00/0606PC
s - Saline Flow Conductivity (SFC) of at least 30 x 10-x, preferably at least
60 x 10''
cm3s/g and
- Free Swell Rate (FSR) of at least 0.1 S g/g and/or Vortex Time of not more
than
160 s.
1 o The term "water-absorbent" relates to water and aqueous systems which may
contain
organic or inorganic compounds in solution, especially to body fluids such as
urine, blood
or fluids containing same.
The hydrogels of the invention and water-absorbent compositions containing
same are
~ 5 useful for producing hygiene articles or other articles for absorbing
aqueous fluids. The
invention consequently further relates to hygiene articles containing a water-
absorbent
composition according to the invention between a liquid-pervious topsheet and
a liquid-
impervious backsheet. The hygiene articles may be present in the form of
diapers, sanitary
napkins and incontinence products.
The invention also provides a method for improving the performance profile of
water-
absorbent compositions by enhancing the permeability, capacity and swell rate
of the
water-absorbent compositions by use of water-insoluble water-swellable
hydrogels as
defined above.
The invention further provides a method for determining water-absorbent
compositions
possessing high permeability, capacity and swell rate by measuring the
absorbency under
load (AUL) and the geI strength of uncoated hydrogels and determining the
centrifuge
retention capacity (CRC), Saline Flow Conductivity (SFC) and Free Swell Rate
(FSR) of
3o the coated hydrogels for given water-absorbent compositions and determining
the water-
absorbent compositions for which the hydrogels exhibit the property spectrum
mentioned
above.
The invention further provides for the use of hydrogels as defined above in
hygiene articles
3s or other articles for absorbing aqueous fluids to enhance the permeability,
capacity and
swell rate.
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It was found that, surprisingly, the above object is achieved in full on using
base polymers
having an AUL (0.7 psi) of at least 20 g/g, preferably at least 22 g/g,
particularly
preferably at least 24 g/g, very preferably at least 26 g/g, and a gel
strength of at least
1 600 Pa, preferably at least ~1 800 Pa, particularly preferably at least 2
000 Pa, whose
s surface is subsequently coated with a steric (inert) or electrostatic
spacer. Base polymers
having these properties ensure that, under a restraining force, the spacer
effect is not offset
by excessively ready gel particle deformability.
The technique of adding steric or electrostatic spacers makes it possible to
produce hygiene
to articles having a high hydrogel content within the absorption layer. In
addition, hydrogels
with electrostatic spacers also possess improved binding to cellulose fibers,
since the latter
have a weak negative charge on the surface. This fact is particularly
advantageous, since it
enables said property profile of hydrogels with electrostatic spacers and
cellulose fibers to
produce an absorption layer without additional assistants to fix the hydrogel
within the
1 s fiber matrix. The binding to the cellulose fibers automatically effects
fixation of the
hydrogel material, so that there is no undesirable redistribution of the
hydrogel material,
for example to the surface of the absorbent core.
The highly swellable polymer particles of the invention are notable for high
absorption
2o capacities, improved liquid transportation performance and a higher swell
rate. For this
reason, the hygiene article can be made extremely thin. The increased level of
high
capacity highly swellable hydrogels of the invention provides enormous
absorption
performance, so that the leakage problem is circumvented as well. At the same
time, the
improved liquid distribution performance ensures that the high absorption
capacity is fully
25 utilized.
The present invention relates to the production of novel highly swellable
hydrogels by
(1) preselecting highly swellable base polymers having an AUL (0.7 psi) of at
Least 20
3o g/g, preferably at least 22 g/g, particularly preferably at least 24 g/g,
very
preferably at least 26 g/g, and a gel strength of at least 1 600 Pa,
preferably at least
1 800 Pa, particularly preferably at least 2 000 Pa,
(2) aftertreating (coating) the surface of the base polymers selected
according to the
35 above criteria with steric or electrostatic spacers.
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B00/0606PC
Coating these preselected hydrogels provides highly swellable hydrogels which,
contrary
to the prior art, combine a high swell or absorption rate with high gel
permeability and a
high retention.
This accordingly generates hydrogels having the following combinations of
properties:
- CRC not less than 24 g/g, preferably not less than 26 g/g, more preferably
not less
than 28 g/g, even more preferably nat less than 30 g/g, particularly
preferably CRC
not less than 32 g/g and most preferably CRC not less than 35 g/g
to and
- SFC not less than 30 x 10'' cm3s/g, preferably not less than 60 x 10-~
cm3s/g,
preferably not less than 80 x 10'~ cm3 s/g, more preferably not less than 100
x 10'~
cm3 s/g, even more preferably not less than 120 x 10'7 cm3 s/g, especially
preferably not less than 150 x 10-~ cm3 s/g, very preferably not less than 200
x 10-~
~ s cm3 s/g, most preferably not less than 300 x 10'~ cm3 s/g,
and
- Free Swell Rate not less than 0.15 g/gs, preferably not less than 0.20 g/gs,
more
preferably not less than 0.30 g/gs, even more preferably not less than 0.50
g/gs,
especially preferably not less than 0.70 g/gs, most preferably not less than
1.00 g/gs
20 or
Vortex Time not more than 160 s, preferably Vortex Time not more than 120 s,
more preferably Vortex Time not more than 90 s, particularly preferably Vortex
Time not more than 60 s, most preferably Vortex Time not more than 30 s.
25 Water-swellable hydrogels with spacers
Hydrogel-forming polymers are in particular polymers of (co)polymerized
hydrophilic
monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable
grafting
base, crosslinked cellulose or starch ethers, crosslinked
carboxymethylcellulose, partially
3o crosslinked polyalkylene oxide or natural products that are swellable in
aqueous fluids, for
example guar derivatives, alginates and carrageenans.
Suitable grafting bases can be of natural or synthetic origin. Examples are
starch, cellulose
or cellulose derivatives and also other polysaccharides and oligosacchardies,
polyvinyl
35 alcohol, polyalkylene oxides, especially polyethylene oxides and
polypropylene oxides,
polyamines, polyarnides and also hydrophilic polyesters. Suitable polyalkylene
oxides
have for example the formula
CA 02433044 2003-06-25
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X
R' - o - {cH2 - cH - o) n -R2
where
Rl and RZ are independently hydrogen, alkyl, alkenyl or aryl,
X is hydrogen or methyl and
n is an integer from 1 to 10 000.
B00/0606PC
Rt and RZ are each preferably hydrogen, (C,-C4)-alkyl, (C2-C6)-alkenyl or
phenyl.
Preferred hydrogel-forming polymers are crosslinked polymers having acid
groups which
1o are predominantly in the form of their salts, generally alkali metal or
ammonium salts.
Such polymers swell particularly strongly on contact with aqueous fluids to
form gels.
Preference is given to polymers which are obtained by crosslinking
polymerization or
copolymerization of acid-functional monoethylenically unsaturated monomers or
salts
thereof. It is further possible to (co)polymerize these monomers without
crosslinkers and to
crosslink subsequently.
Examples of such monomers bearing acid groups are monoethylenically
unsaturated C3- to
CZS-carboxylic acids or anhydrides such as acrylic acid, methacrylic acid,
ethacrylic acid,
2o a-chloroacrylic acid, crotonic acid, malefic acid, malefic anhydride,
itaconic acid, citraconic
acid, mesaconic acid, glutaconic acid, aconitic acid and fumaric acid. It is
also possible to
use monoethylenically unsaturated sulfonic or phosphoric acids, for example
vinylsulfonic
acid, allylsulfonic acid, sulfoethyl acrylate, sulfoethyl methacrylate,
sulfopropyl acrylate,
sulfopropyl methacrylate, 2-hydroxy-3-acryloyloxypropylsulfonic acid, 2-
hydroxy-3-
methacryloyloxypropylsulfonic acid, vinylphosphonic acid, allylphosphonic
acid,
styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid. The
monomers may
be used alone or mixed.
Preferred monomers are acrylic acid, methacrylic acid, vinylsulfonic acid,
3o acrylamidopropanesulfonic acid or mixhires thereof, for example mixtures of
acrylic and
rnethacrylic acid, mixtures of acrylic acid and acrylamidopropanesulfonic acid
or mixtures
of acrylic acid and vinylsulfonic acid.
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B00/0606PC
To optimize properties, it can be sensible to use additional monoethylenically
unsaturated
compounds which do not bear an acid group but are copolymerizable with the
monomers
bearing acid groups. Such compounds include for example the amides and
nitrites of
monoethylenically unsaturated carboxylic acids, for example acrylamide,
methacrylamide
s and N-vinylformamide, N-vinylacetamide, N-methyl-N-vinylacetamide,
acrylonitrile and
methacrylonitrile. Examples of further suitable compounds are vinyl esters of
saturated C,-
to C4-carboxylic acids such as vinyl formate, vinyl acetate or vinyl
propionate, alkyl vinyl
ethers having at least 2 carbon atoms in the alkyl group, for example ethyl
vinyl ether or
butyl vinyl ether, esters of monoethylenically unsaturated C3- to C6-
carboxylic acids, for
io example esters of monohydric CI- to C~g-alcohols and acrylic acid,
methacrylic acid or
malefic acid, monoesters of malefic acid, for example methyl hydrogen maleate,
N-
vinyllactams such as N-vinylpyrrolidone or N-vinylcaprolactam, acrylic and
methacrylic
esters of alkoxylated monohydric saturated alcohols, for example of alcohols
having from
to 25 carbon atoms which have been reacted with from 2 to 200 mot of ethylene
oxide
1s and/or propylene oxide per mole of alcohol, and also monoacrylic esters and
monomethacrylic esters of polyethylene glycol or polypropylene glycol, the
molar masses
(M") of the polyalkylene glycols being up to 2 000, for example. Further
suitable
monomers are styrene and alkyl-substituted styrenes such as ethylstyrene or
tert-
butylstyrene.
These monomers without acid groups may also be used in mixture with other
monomers,
for example mixtures of vinyl acetate and 2-hydroxyethyl acrylate in any
proportion. These
monomers without acid groups are added to the reaction mixture in amounts
within the
range from 0 to SO% by weight, preferably less than 20% by weight.
2s
Preference is given to crosslinked polymers of monoethylenically unsaturated
monomers
which bear acid groups and which are optionally converted into their alkali
metal or
ammonium salts before or after polymerization and 0-40% by weight, based on
their total
weight, of monoethylenically unsaturated monomers which do not bear acid
groups.
Preference is given to crosslinked polymers of monoethylenically unsaturated
C3-Ci2-
carboxylic acids and/or their alkali metal or ammonium salts. Preference is
given in
particular to crosslinked polyacrylic acids, 25-100% of whose acid groups are
present as
alkali metal or ammonium salts.
3s
Possible crosslinkers include compounds containing at least two ethylenically
unsaturated
double bonds. Examples of compounds of this type are N,N'-
methylenebisacrylamide,
polyethylene glycol diacrylates and polyethylene glycol dimethacrylates each
derived from
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polyethylene glycols having a molecular weight of from 106 to 8 500,
preferably from 400
to 2 000, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate,
ethylene
glycol diacrylate, ethylene glycol dimethacrylate, propylene glycol
diacrylate, propylene
glycol dimethacrylate, butanediol diacrylate, butanediol dimethacrylate,
hexanediol
s diacrylate, hexanediol dimethacrylate, allyl methacrylate, diacrylates and
dimethacrylates
of block copolymers of ethylene oxide and propylene oxide, polyhydric
alcohols, such as
glycerol or pentaerythritol, doubly or more highly esterif ed with acrylic
acid or
methacrylic acid, triallylamine, dialkyldiallylammonium halides such as
dimethyldiallylammonium chloride and diethyldiallylammonium chloride,
1 o tetraallylethylenediamine, divinylbenzene, diallyl phthalate, polyethylene
glycol divinyl
ethers of polyethylene glycols having a molecular weight of from 106 to 4 000,
trimethylolpropane diallyl ether, butanediol divinyl ether, pentaerythritol
triallyl ether,
reaction products of 1 mol of ethylene glycol diglycidyl ether or polyethylene
glycol
diglycidyl ether with 2 mol of pentaerythritol triallyl ether or allyl
alcohol, and/or
1 s divinylethyleneurea. Preference is given to using water-soluble
crosslinkers, for example
N,N'-methylenebisacrylamide, polyethylene glycol diacrylates and polyethylene
glycol
dimethacrylates derived from addition products of from 2 to 400 mol of
ethylene oxide
with 1 mol of a diol or polyol, vinyl ethers of addition products of from 2 to
400 mol of
ethylene oxide with 1 mol of a diol or polyol, ethylene glycol diacrylate,
ethylene glycol
2o dimethacrylate or triacrylates and trimethacrylates of addition products of
from 6 to 20 mol
of ethylene oxide with 1 mol of glycerol, pentaerythritol triallyl ether
and/or divinylurea.
Possible crosslinkers also include compounds containing at least one
polymerizable
ethylenically unsaturated group and at least one further functional group. The
functional
2s group of these crosslinkers has to be capable of reacting with the
functional groups,
essentially the acid groups, of the monomers. Suitable functional groups
include for
example hydroxyl, amino, epoxy and aziridino groups. Useful are for example
hydroxyalkyl esters of the abovementioned monoethylenically unsaturated
carboxylic
acids, e.g., 2-hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl
acrylate,
3o hydroxyethyl methacrylate, hydroxypropyl methacrylate and hydroxybutyl
methacrylate,
allylpiperidinium bromide, N-vinylimidazoles, for example N-vinylimidazole, 1-
vinyl-
2-methylimidazole and N-vinylimidazolines such as N-vinylimidazoline, 1-vinyl-
2-
methylimidazoline, 1-vinyl-2-ethylimidazoline or 1-vinyl-2-propylimidazoline,
which can
be used in the form of the free bases, in quaternized form or as salt in the
polymerization. It
35 is also possible to use dialkylaminoalkyl acrylates and dialkylaminoalkyl
methacrylates
such as dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate,
diethylaminoethyl
acrylate and diethylaminoethyl methacrylate. The basic esters are preferably
used in
quaternized form or as salt. It is also possible to use glycidyl
(meth)acrylate, for example.
s CA 02433044 2003-06-25
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B00/0606PC
Useful crosslinkers further include compounds containing at least two
functional groups
capable of reacting with the functional groups, essentially the acid groups,
of the
monomers. Suitable functional groups were already mentioned above, i.e.,
hydroxyl,
amino, epoxy, isocyanate, ester, amido and aziridino groups. Examples of such
crosslinkers are ethylene glycol, diethylene glycol, triethylene glycol,
tetraethylene glycol,
polyethylene glycol, glycerol, polyglycerol, triethanolamine, propylene
glycol,
polypropylene glycol, block copolymers of ethylene oxide and propylene oxide,
ethanolamine, sorbitan fatty acid esters, ethoxylated sorbitan fatty acid
esters,
trimethylolpropane, pentaerythritol, 1,3-butanediol, 1,4-butanediol, polyvinyl
alcohol,
sorbitol, starch, polyglycidyl ethers such as ethylene glycol diglycidyl
ether, polyethylene
glycol diglycidyl ether, glycerol diglycidyl ether, glycerol polyglycidyl
ether, diglycerol
polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitol polyglycidyl
ether,
pentaerythritol polyglycidyl ether, propylene glycol diglycidyl ether and
polypropylene
glycol diglycidyl ether, polyaziridine compounds such as 2,2-
bishydroxymethylbutanol
Iris[3-(1-aziridinyl)propionate], 1,6-hexamethylenediethyleneurea,
diphenylmethanebis-
4,4'-N,N'-diethyleneurea, halo epoxy compounds such as epichlorohydrin and a-
methyl-
epifluorohydrin, polyisocyanates such as 2,4-toluylene diisocyanate and
hexamethylene
diisocyanate, alkylene carbonates such as 1,3-dioxolan-2-one and 4-methyl-1,3-
dioxolan-
2-one, also bisoxazolines and oxazolidones, polyamidoamines and also their
reaction
2o products with epichlorohydrin, also polyquaternary amines such as
condensation products
of dimethylamine with epichlorohydrin, homo- and copolymers of diallyl-
dimethylammonium chloride and also homo- and copolymers of dimethylaminoethyl
(meth)acrylate which are optionally quaternized with, for example, methyl
chloride.
The crosslinkers are present in the reaction mixture for example from 0.001 to
20%,
preferably from 0.01 to 14%, by weight.
The polymerization is initiated in the generally customary manner, by means of
an
initiator. But the polymerization may also be initiated by electron beams
acting on the
3o polymerizable aqueous mixture. However, the polymerization may also be
initiated in the
absence of initiators of the abovementioned kind, by the action of high energy
radiation in
the presence of photoinitiators. Useful polymerization initiators include all
compounds
which decompose into free radicals under the polymerization conditions, for
example
peroxides, hydroperoxides, hydrogen peroxides, persulfates, azo compounds and
redox
catalysts. The use of water-soluble initiators is preferred. In some cases it
is advantageous
to use mixtures of different polymerization initiators, for example mixtures
of hydrogen
peroxide and sodium peroxodisulfate or potassium peroxodisulfate. Mixtures of
hydrogen
peroxide and sodium peroxodisulfate may be used in any proportion. Examples of
suitable
CA 02433044 2003-06-25
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' -11-
organic peroxides are acetylacetone peroxide, methyl ethyl ketone peroxide,
tert-butyl
hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl
perpivalate, tert-
butyl perneohexanoate, tert-butyl perisobutyrate, tert-butyl per-2-
ethylhexanoate, tert-butyl
perisononanoate, tert-butyl permaleate, tent-butyl perbenzoate, di(2-
ethylhexyl)
s peroxydicarbonate, dicyclohexyl peroxydicarbonate, di(4-tert-
butylcyclohexyl) peroxy-
dicarbonate, dimyristyl peroxydicarbonate, diacetyl peroxydicarbonate, allyl
peresters,
cumyl peroxyneodecanoate, tent-butyl per-3,5,5-trimethylhexanoate,
acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxide and
tert-amyl
perneodecanoate. Particularly suitable polymerization initiators are water-
soluble azo
to initiators, e.g., 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-
azobis-
(N,N'-dimethylene)isobutyramidine dihydrochloride, 2-
(carbamoylazo)isobutyronitrile,
2,2'-azobis[2-(2'-imidazolin-2-yl)propane) dihydrochloride and 4,4'-azobis(4-
cyanovaleric
acid). The polymerization initiators mentioned are used in customary amounts,
for example
in amounts of from 0.01 to 5%, preferably from 0.05 to 2.0%, by weight, based
on the
15 monomers to be polymerized.
Useful initiators also include redox catalysts. In redox catalysts, the
oxidizing component
is at least one of the above-specified per compounds and the reducing
component is for
example ascorbic acid, glucose, sorbose, ammonium or alkali metal bisulfite,
sulfite,
2o thiosulfate, hyposulfite, pyrosulfite or sulfide, or a metal salt, such as
iron(II) ions or
sodium hydroxymethylsulfoxylate. The reducing component in the redox catalyst
is
preferably ascorbic acid or sodium sulfite. Based on the amount of monomers
used in the
polymerization, from 3x10-6 to 1 mol% may be used for the reducing component
of the
redox catalyst system and from 0.001 to 5.0 mol% fox the oxidizing component
of the
25 redox catalyst, for example.
When the polymerization is initiated using high energy radiation, the
initiator used is
customarily a photoinitiator. Photoinitiators include for example a-splitters,
H-abstracting
systems or else azides. Examples of such initiators are benzophenone
derivatives such as
30 Michler's ketone, phenanthrene derivatives, fluorene derivatives,
anthraquinone
derivatives, thioxanthone derivatives, coumarin derivatives, benzoin ethers
and derivatives
thereof, azo compounds such as the abovementioned free-radical formers,
substituted
hexaarylbisimidazoles or acylphosphine oxides. Examples of azides are:
2-(N,N-dimethylamino)ethyl 4-azidocinnamate, 2-(N,N-dirnethylamino)ethyl 4
35 azidonaphthyl ketone, 2-(N,N-dimethylamino)ethyl 4-azidobenzoate, 5-azido-1-
naphthyl
2'-(N,N-dimethylamino)ethyl sulfone, N-(4-sulfonylazido-phenyl)maleimide, N-
acetyl
4-sulfonylazidoaniline, 4-sulfonylazidoaniline, 4-azidoaniline, 4-
azidophenacyl bromide,
p-azidobenzoic acid, 2,6-bis(p-azidobenzylidene)cyclohexanone and 2,6-bis(p
CA 02433044 2003-06-25
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B00/0606PC
azidobenzylidene)-4-methyl-cyclohexanone. Photoinitiators, if used, are
customarily used
in amounts of from 0.01 to 5% of the weight of the monomers to be polymerized.
The subsequent crosslinking stage comprises polymers which were prepared by
polymerization of the abovementioned monoethylenically unsaturated acids and
optionally
monoethylenically unsaturated comonomers and which have a molecular weight of
more
than 5 000, preferably more than 50 000, being reacted with compounds having
at least
two groups which are reactive toward acid groups. This reaction can take place
at room
temperature or else at elevated temperatures of up to 220°C.
Suitable functional groups were already mentioned above, i.e., hydroxyl,
amino, epoxy,
isocyante, ester, amido and aziridino groups, as well examples of such
crosslinkers.
Crosslinkers are added to the acid-functional polymers or salts in amounts of
from 0.5 to
25% by weight, preferably from 1 to 15% by weight, based on the amount of
polymer
is used.
Crosslinked polymers are preferably used in fully neutralized form. However,
neutralization may also be partial only. The degree of neutralization is
preferably within
the range from 25 to 100%, especially within the range from 50 to 100%. Useful
2o neutralizing agents include alkali metal bases or ammonia/amines.
Preference is given to
the use of aqueous sodium hydroxide solution or aqueous potassium hydroxide
solution.
However, neutralization may also be effected using sodium carbonate, sodium
bicarbonate,
potassium carbonate or potassium bicarbonate or other carbonates or
bicarbonates or
ammonia. Moreover primary, secondary and tertiary amines may be used.
Industrial processes useful for making these products include all processes
which are
customarily used to make superabsorbents, as described for example in Chapter
3 of
"Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Crraham,
Wiley-
VCH, 1998.
Polymerization in aqueous solution is preferably conducted as a gel
polymerization. It
involves 10-70% strength by weight aqueous solutions of the monomers and
optionally of
a suitable grafting base being polymerized in the presence of a free-radical
initiator by
utilizing the Trommsdorff Norrish effect.
'lhe polymerization reaction may be carried out at from 0 to 150°C,
preferably at from 10
to 100°C, not only at atmospheric pressure but also at superatmospheric
or reduced
CA 02433044 2003-06-25
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B00/0606PC
pressure. As is customary, the polymerization may also be conducted in a
protective gas
atmosphere, preferably under nitrogen.
By subsequently heating the polymer gels at from 50 to 130°C,
preferably at from 70 to
100°C, for several hours, the performance characteristics of the
polymers can be further
improved.
Preference is given to hydrogel-forming polymers which have been surface
postcrosslinked. Surface postcrosslinking may be corned out in a conventional
manner
using dried, ground and classified polymer particles.
To effect surface postcrosslinking, compounds capable of reacting with the
functional
groups of the polymers by crosslinking are applied to the surface of the
hydrogel particles,
preferably in the form of an aqueous solution. The aqueous solution may
contain water-
miscible organic solvents. Suitable solvents are alcohols such as methanol,
ethanol, i-
t 5 propanol or acetone.
Suitable surface postcrosslinkers include for example:
- di- or polyglycidyl compounds such as diglycidyl phosphonates or ethylene
glycol
2o diglycidyl ether, bischlorohydrin ethers of polyalkylene glycols,
- alkoxysilyl compounds,
polyaziridines, aziridine compounds based on polyethers or substituted
hydrocarbons, for example bis-N-aziridinomethane,
- polyols such as ethylene glycol, 1,2-propanediol, 1,4-butanediol, glycerol,
methyltriglycol, polyethylene glycols having an average molecular weight MW of
200 - 10 000, di- and polyglycerol, pentaerythritol, sorbitol, the ethoxylates
of these
polyols and their esters with carboxylic acids or carbonic acid such as
ethylene
3o carbonate or propylene carbonate,
- carbonic acid derivatives such as urea, thiourea, guanidine, dicyandiamide,
2-
oxazolidinone and its derivatives, bisoxazoline, polyoxazolines, di- and
polyisocyanates,
- di- and poly-N-methylol compounds such as, for example, methylenebis(N-
methylolmethacrylamide) or melamine-formaldehyde resins,
~ CA 02433044 2003-06-25
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- compounds having two or more blocked isocyanate groups such as, for example,
trimethylhexamethylene diisocyanate blocked with 2,2,3,6-tetramethylpiperidin-
4-
one.
s If necessary, acidic catalysts may be added, for example p-toluenesulfonic
acid, phosphoric
acid, boric acid or ammonium dihydrogenphosphate.
Particularly suitable surface postcrosslinkers are di- or polyglycidyl
compounds such as
ethylene glycol diglycidyl ether, the reaction products of polyamidoamines
with
1o epichlorohydrin and 2-oxazolidinone.
The crosslinker solution is preferably applied by spraying with a solution of
the crosslinker
in conventional reaction mixers or mixing and drying equipment such as
Patterson-Kelly
mixers, DRAIS turbulence mixers, Lodige mixers, screw mixers, plate mixers,
fluidized
15 bed mixers and Schugi Mix. The spraying of the crosslinker solution may be
followed by a
heat treatment step, preferably in a downstream dryer, at from 80 to
230°C, preferably 80-
190°C, particularly preferably at from 100 to 160°C, for from 5
minutes to 6 hours,
preferably from 10 minutes to 2 hours, particularly preferably from 10 minutes
to 1 hour,
during which not only cracking products but also solvent fractions can be
removed. But the
2o drying may also take place in the mixer itself, by heating the jacket or by
blowing in a
preheated carrier gas.
Steric spacers
Useful steric spacers include inert materials (powders) for example silicates
having a band,
25 chain or sheet structure (montmorillonite, kaolinite, talc), zeolites,
active carbons or silicas.
Inorganic inert spacers further include for example magnesium carbonate,
calcium
carbonate, barium sulfate, aluminum oxide, titanium dioxide and iron(II)
oxide. Organic
inert spacers include for example polyalkyl methacrylates or thermoplastics
such as for
example polyvinyl chloride. Preference is given to using silicas, which divide
into
30 precipitated silicas and pyrogenic silicas according to their method of
preparation. Both
variants are commercially available under the name AEROSIL~ (pyrogenic
silicas) or
Silica FK, Sipernat~, Wessalon~ (precipitated silica). The surface of the
silica particles
bears siloxane and silanol groups. There are more of the siloxane groups. They
are the
reason for the substantially inert character of this synthetic silica.
Specific types of silica
35 are available for different applications. For instance, silane may be added
to chemically
modify the silica surface so that the originally hydrophilic silica is
transformed into
hydrophobic variants. Some silica grades are available as mixed oxides, for
example in a
blend with aluminum oxide. The spacer function can be controlled according to
the surface
d
CA 02433044 2003-06-25 Bp0/0606PC
-15-
constitution of the primary particles. Pyrogenic silica (for example AEROSIL~)
is
available in particle size fractions of from 7 to 40 nrn.
Silica under the tradenames of Silica FK, Sipernat~ and Wessalon~ can be
obtained as a
powder of particle size fraction 5-100 pm and a specific surface area of 50-
450 m2/g.
Fox use as steric spacer, the particle size of the inert powders is preferably
at least I pm,
more preferably at least 4 p,m, particularly preferably at least 20 Vim, very
preferably at
least 50 pm. The use of precipitated silicas is particularly preferred.
l0 The handling of inert silica grades is generally physiologically safe. This
permits
unreserved use of materials of this kind in a hygiene article.
The base polymers coated with inert spacer material may be produced by
applying the inert
spacers in an aqueous or water-miscible medium or else by applying the inert
spacers in
powder form to pulverulent base polymer material. The aqueous or water-
miscible media
are preferably applied by spraying onto dry polymer powder. In a particularly
preferred
version of the production process, pure powder/powder blends are produced from
pulverulent inert spacer material and base polymer. The inert spacer material
is applied to
the surface of the base polymer in a proportion of 0.05 to 5% by weight,
preferably from
0.1 to 1.5% by weight, particularly preferably from 0.3 to 1 % by weight,
based on the total
weight of the coated hydrogel.
Electrostatic spacers
Cationic components may be added as electrostatic spacers.
It is generally possible to add cationic polymers for the purpose of
electrostatic repulsion.
This is accomplished for example with polyethyleneirnines, polyvinylamines,
polyamines
such as polyalkylenepolyarnines, cationic derivatives of polyacrylamides,
polyethyleneimines, polyquaternary amines, for example, condensation products
of
hexamethylenediamine, dimethylamine and epichlorohydrin, condensation products
of
3o dimethylamine and epichlorohydrin, copolymers of hydroxyethylcellulose and
diallyldimethylammonium chloride, copolymers of acrylamide and (3-methacryloxy-
ethyltrimethylammonium chloride, hydroxycellulose reacted with epichlorohydrin
and then
quaternized with trimethylamine, homopolymers of diallyldimethylammonium
chloride or
addition products of epichlorohydrin with amidoamines. Polyquaternary amines
may
further be synthesized by reaction of dimethyl sulfate with polymers, such as
polyethyleneimines, copolymers of vinylpyrrolidone and dimethylaminoethyl
methacrylate
or copolymers of ethyl methacrylate and diethylaminoethyl rnethacrylate.
Polyquatemary
amines are available in a wide molecular weight range.
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Booio6o6PC
Electrostatic spacers are also generated by applying a crosslinked, cationic
sheath, either
by means of reagents capable of forming a network with themselves, for example
addition
products of epichlorohydrin with polyamidoamines, or by applying cationic
polymers
capable of reacting with an added crosslinker, for example polyamines or
polyimines
s combined with polyepoxides, multifunctional esters, multifunctional acids or
multifunctional (meth)acrylates. It is also possible to use any
multifunctional amines
having primary or secondary amino groups, for example polyethyleneimine,
polyallylamine, polylysine, preferably polyvinylamine. Further examples of
polyamines
are ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylene-
pentamine,
to pentaethylenehexamine and polyethyleneimines and also poly-amines having
molar masses
of up to 4 000 000 in each case.
Electrostatic spacers may also be applied by adding solutions of divalent or
more highly
valent metal salt solutions. Examples of divalent or more highly valent metal
cations are
15 Mg2+, Ca2+, A13+, Sc3+, Ti4+, Mn2+, Fe2+r~+, Co2+~ Ni2+, Cu+~+, Zn2+, Y3+,
Zr4+, Ag+, La3+,
Ce4+, Hf4+ and Au+~3+, preferred metal cations are Mg2+, Ca2+, A13+, Ti4+,
Zr4+ and La3+ and
particularly preferred metal cations are Al3+, Ti4+ and Zr4+. The metal
cations may be used
not only atone but also mixed with each other. Of the metal cations mentioned,
all salts are
suitable that possess adequate solubility in the solvent to be used. Of
particular suitability
2o are metal salts with weakly complexing anions such as for example chloride,
nitrate and
sulfate. Useful solvents for the metal salts include water, alcohols, DMF,
DMSO and also
mixtures thereof. Particular preference is given to water and water-alcohol
mixtures, for
example water-methanol or water-1,2-propanediol.
25 In the production process, the electrostatic spacers may be applied like
the inert spacers by
application in an aqueous or water-miscible medium. This is the preferred
production
process in the case of the addition of metal salts. Cationic polymers are
applied to
pulverulent base polymer material by applying an aqueous solution or in a
water-miscible
solvent, optionally also as dispersion, or else by application in powder form.
The aqueous
3o or water-miscible media are preferably applied by spraying onto dry polymer
powder. The
polymer powder may optionally be subsequently dried, in which case the coated
base
polymers are exclusively dried at temperatures of not more than 100°C.
Higher
temperatures would lead to the formation of covalent bonds between the
polyamine
component and the polycarboxylate, which should be avoided under any
circumstances in
3s order that the additional crosslinking brought about as a result may not
excessively lower
the capacity of the product. For this reason, there is preferably no heat
treatment step
involved when coating with polyamines. When an additional crosslinker is used,
the heat
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treatment conditions are chosen in such a way that it is only the polyamine
coating layer
which is crosslinked, but not the polycarboxylate underneath.
The cationic spacers are applied to the surface of the base polymer in a
proportion of from
0.05 to 5% by weight, preferably from 0.1 to 1.5% by weight, particularly
preferably from
0.1 to 1 % by weight, based on the total weight of the coated hydrogel.
The hydrogels mentioned are notable for high absorbency for water and aqueous
solutions
and therefore are preferentially used as absorbents in hygiene articles.
to
The water-swellable hydrogels may be present in conjunction with a box
material for the
hydrogels, preferably embedded as particles in a polymer fiber matrix or an
open-celled
polymer foam, fixed on a sheetlike base material or present as particles in
chambers
formed from a base material.
The invention also provides a process for producing water-absorbent
compositions by
- preparing the water-swellable hydrogels,
- optionally coating the hydrogels with a steric or electrostatic spacer and
- introducing the hydrogels into a polymer fiber matrix or an open-celled
polymer
foam or into chambers formed from a base material or fixing on a sheetlike
base
material.
The hygiene articles producible from the water-absorbent compositions of the
invention
are known per se and have been described. They are preferably diapers,
sanitary napkins
and incontinence products such as incontinence liners. The construction of
such products is
known.
Description of test methods
3o Centrifuge Retention Capacity (CRC)
This method measures the free swellability of the hydrogel in a teabag. 0.2000
~ 0.0050 g
of dried hydrogel (particle size fraction I06-850 Vim) is sealed into a teabag
60 x 85 mm in
size. The teabag is then soaked for 30 minutes in an excess of 0.9% by weight
sodium
chloride solution (at least 0.83 1 of sodium chloride solution/1 g of polymer
powder). The
3s teabag is then centrifuged for three minutes at 250 g. The amount of liquid
is determined
by weighing the centrifuged teabag.
CA 02433044 2003-06-25
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Absorbency Under Load (AUL) 0.7 psi
B00/0606PC
The measuring cell for determining AUL 0.7 psi is a Plexiglas cylinder 60 mm
in internal
diameter and 50 mm in height. Adhesively attached to its underside is a
stainless steel
sieve bottom having a mesh size of 36 ~,m. The measuring cell further includes
a plastic
plate having a diameter of 59 mm and a weight which can be placed in the
measuring cell
together with the plastic plate. The weight of the plastic plate and of the
weight totals 1345
g. AUL 0.7 psi is determined by measuring the weight of the empty Plexiglas
cylinder and
of the plastic plate and recorded as Wo. 0.900 t 0.005 g of hydrogel-forming
polymer
(particle size distribution: 150 - 800 ~,m) is then weighed into the Plexiglas
cylinder and
1o distributed very uniformly over the stainless steel sieve. The plastic
plate is then carefully
placed in the Plexiglas cylinder, the entire unit is weighed and the weight is
recorded as
W$. The weight is then placed on the plastic plate in the Plexiglas cylinder.
A ceramic filter
plate 120 mm in diameter and 0 in porosity is then placed in the middle of a
Petri dish 200
mm in diameter and 30 mm in height and sufficient 0.9% by weight sodium
chloride
solution is introduced for the surface of the liquid to be level with the
filter plate surface
without the surface of the filter plate being wetted. A round filter paper 90
mm in diameter
and < 20 ~m in pore size (S&S 589 Schwarzband from Schleicher & Schiill) is
subsequently placed on the ceramic plate. The Plexiglas cylinder containing
hydrogel-
forming polymer is then placed with plastic plate and weight on top of the
filter paper and
2o left there for 60 minutes. At the end of this period, the complete unit is
removed from the
filter paper in the Petri dish and subsequently the weight is removed from the
Plexiglas
cylinder. The Plexiglas cylinder containing swollen hydrogel is weighed
together with the
plastic plate and the weight recorded as Wb.
AUL is calculated by the following equation:
AUL 0.7 psi [g/g] - ~Wb-W$] / [W$ Wo]
Free Swell Rate (FSR)
1.00 g (WH) of hydrogel is uniformly spread out on the bottom of a plastic
weighing boat
having a round bottom of about 6 cm. The plastic weighing boat is round and
about 6 cm
in diameter at the bottom, about 2.5 cm deep and about 7.5 cm x 7.5 cm square
at the top.
A funnel is then used to add 20 g (WU) of a synthetic urine solution
preparable by
dissolving 2.0 g of KCI, 2.0 g of Na2S04, 0.85 g of NH4HZPOø, 0.15 g of
(NH4)ZHP04,
0.19 g of CaCl2, and 0.23 g of MgCl2 in 1 liter of distilled water to the
center of the
weighing boat. The time for the hydrogel to absorb all of the fluid, as
indicated by the
absence of pooled fluid, is recorded and noted as tA. The Free Swell Rate then
computes
from
CA 02433044 2003-06-25
Boo/o6o6Pc
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FSR = WU/(WH x tA)
Saline Flow Conductivity (SFC)
The test method for determining SFC is described in WO 95/262/9.
Vortex Time
50 ml of 0.9% by weight NaCI solution are measured into a 100 ml beaker. While
the
saline solution is being stirred with a magnetic stirrer at 600 rpm 2.00 g of
hydrogel is
poured in quickly in such a way that clumping is avoided. The time in seconds
is taken for
the vortex created by the stirring to close and for the surface of the saline
solution to
1o become flat.
Gel strength
The rheological studies to determine the gel strength were carried out on a
CSL 100
controlled stress rheometer from Carrimed. All measurements are carried out at
room
temperature.
Sample preparation: The measurements are carned out on hydrogel particles of
the sieve
fraction 300-400 ~m which had previously been preswelled for 1 hour in 0.9% by
weight
NaCI solution in a ratio of 1:60. To prepare the samples to be measured, the
NaCI solution
2o is initially charged to 100 ml beakers and the dry hydrogel particles are
gradually added
with (magnetic) stirring so that there is no clumping. The stirring bar is
subsequently
removed, and the beaker is sealed with a film and set aside for 1 hour in that
state at room
temperature for swelling. To ensure the same conditions prior to the
measurement being
carned out, this preparative method has to be complied with exactly, or the
rheological
measurement would be impaired and the measured results distorted.
Measurement procedure: The gel strength is determined using the Carrimed CS
rheometer
via the oscillation mode using a plate-plate geometry (diameter 6 cm). To
avoid the slip
effect, sandblasted plate systems are used for this purpose. The sample is
placed on the
baseplate and the ramp is slowly lifted to enable the gap to be closed slowly.
The
measuring gap measures 1 mm and has to be absolutely completely filled with
sample
material. Gel strength is the modulus of elasticity of the hydrogel preswollen
as defined
and is measured similarly to the modulus of elasticity in the linearly
viscoelastic region of
the sample, which is determined in a preliminary test on the same sample. To
subsequently
determine the gel strength, a torque sweep is carried out within the linearly
viscoelastic
region at a constant frequency (1 Hz) in the oscillation mode. Given elastic
behavior, the
measuring curve obtained is a straight line which quantifies the gel strength
as a material
constant of the elastic solid.
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The reported measurements are number averages of 3 series of determinations.
The examples which follow illustrate the invention.
Examples
Inventive Example 1:
A 10 1 capacity polyethylene vessel thoroughly insulated by foamed plastic
material is
charged with 3 600 g of deionized water and 1 400 g of acrylic acid, followed
by 4.0 g of
tetraallyloxyethane and 5.0 g of allyl methacrylate. The initiators,
consisting of 2.2 g of
to 2,2'-azobisamidinopropane dihydrochloride (dissolved in 20 g of deionized
water), 4 g of
potassium peroxodisulfate (dissolved in 150 g of deionized water) and 0.4 g of
ascorbic
acid (dissolved in 20 g of deionized water) are successively added and stirred
in at 4°C.
The reaction solution is then left to stand without stirnng. The ensuing
polymerization, in
the course of which the temperature rises up to about 90°C, produces a
firm gel. This is
subsequently subjected to mechanical comminution and adjusted to pH 6.0 with
50% by
weight aqueous sodium hydroxide solution. The gel is then dried, ground and
classified to
a particle size distribution of 100 - 850 lCm. 1 kg of this dried hydrogel is
sprayed with a
solution consisting of 60 g of demineralized water, 40 g of i-propanol and 1.0
g of ethylene
glycol diglycidyl ether in a plowshare mixer and subsequently heat treated at
140°C for 60
2o minutes. The herein described product has the following properties:
CRC - 28.4 g/g
AUL 0.7 psi - 25.1 g/g
Gel strength - 2 350 Pa
SFC - 35 x 10-~ cm3s/g
Inventive Egamule 2:
A 30 1 capacity polyethylene vessel thoroughly insulated by foamed plastic
material is
charged with 14 340g of demineralized water and 42 g of sorbitol triallyl
ether. 3 700 g of
sodium bicarbonate are suspended in this initial charge and S 990 g of acrylic
acid are
3o gradually added at a rate such that overfoaming of the reaction solution is
avoided; the
reaction solution cools down to about 3-5°C. The initiators, 6.0 g of
2,2'-
azobisamidinopropane dihydrochloride (dissolved in 60 g of demineralized
water), 12 g of
potassium peroxodisulfate (dissolved in 450 g of demineralized water) and also
1.2 g of
ascorbic acid (dissolved in 50 g of demineralized water) are successively
added and
thoroughly stirred in at 4°C. The reaction solution is then left to
stand without stirring. The
ensuing polymerization, in the course of which the temperature rises up to
about 85°C,
produces a gel. This gel is subsequently transferred into a kneader and
adjusted to a pH of
6.2 by addition of 50% by weight aqueous sodium hydroxide solution. The
comminuted
CA 02433044 2003-06-25
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B00/0606PC
gel is then dried in an airstream at 170°C, ground and classified to a
particle size
distribution of 100 - 850 pm. 1 kg of this product is sprayed with a solution
of 2 g of
RETEN 204 LS (polyamidoamine-epichlorohydrin adduct from Hercules), 30 g of
demineralized water and 30 g of 1,2-propanediol in a plowshare mixer and
subsequently
heat treated at 150°C for 60 minutes. The following properties were
measured:
CRC - 32.3 g/g
AUL 0.7 psi - 26.4 g/g
Gel strength - 1 975 Pa
SFC - 25 x 10'' cm3s/g
Inventive Eacample 3:
A WERNER & PFLEIDERER laboratory kneader having a working capacity of 2 1 is
evacuated to 980 mbar absolute by means of a vacuum pump and a previously
separately
prepared monomer solution which has been cooled to about 25°C and
inertized by passing
nitrogen into it is sucked into the kneader. The monomer solution has the
following
composition: 825.5 g of deionized water, 431 g of acrylic acid, 335 g of NaOH
50%, 4.5 g
of ethoxylated trimethylolpropane triacrylate (SR 9035 oligomer from SARTOMER)
and
1.5 g of pentaerythritol triallyl ether (P-30 from Daiso). To improve the
inertization, the
kneader is evacuated and subsequently refilled with nitrogen. This operation
is repeated
three times. A solution of 1.2 g of sodium persulfate (dissolved in 6.8 g of
deionized water)
is then sucked in, followed after a further 30 seconds by a further solution
consisting of
0.024 g of ascorbic acid dissolved in 4.8 g of deionized water. After a
nitrogen purge a
preheated jacket heating circuit on bypass at 75°C is switched over to
the kneader jacket
and the stirrer speed increased to 96 rpm. Following the onset of
polymerization and the
reaching of Tm~, the jacket heating circuit is switched back to bypass, and
the batch is
supplementarily polymerized for 15 minutes without heating/cooling,
subsequently cooled
and discharged. The resultant gel particles are dried at above 100°C,
ground and classified
to a particle size distribution of 100 - 850 ~.m. 500 g of this product are
sprayed with a
solution of 2 g of 2-oxazolidinone, 25 g of deionized water and 10 g of 1,2-
propanediol in
a plowshare mixer and subsequently heat treated at 185°C for 70
minutes. The following
properties were measured:
CRC - 26.3 g/g
AUL 0.7 psi - 23.8 g/g
Gel strength - 2 680 Pa
SFC - 50 x 10-~ cm3s/g
Inventive Example 4:
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1 000 g of the polymer of inventive example 1 were plowshare mixed for 15
minutes with
g of Sipernat D 17 (hydrophobic precipitated silica, commercial product of
Degussa
AG, average particle size 10 pm). The product thus coated has the following
properties:
CRC - 28.9 g/g
5 SFC - 115 x 10-~ cm3s/g
Vortex Time - 80 s
Inventive Example 5:
1 000 g of the polymer of inventive example 1 were sprayed with 50 g of a
solution
1o consisting of 90 parts by weight of deionized water and 10 parts by weight
of aluminum
sulfate (A12(S04)3) in a plowshare mixer and subsequently supplementarily
mixed therein
for 30 minutes. The product thus obtained has the following properties:
CRC - 27.2 g/g
SFC - 160 x 10-~ cm3s/g
Free Swell Rate - 0.25 g/gs
Inventive Example 6:
1 000 g of the polymer of inventive example 1 were plowshare mixed for 15
minutes with
8 g of Sipernat 22 (hydrophilic precipitated silica, commercial product of
Degussa AG,
2o average particle size 100 pm). The product thus coated has the following
properties:
CRC - 29.5 g/g
SFC - 100 x 10-~ cm3s/g
Free Swell Rate - 0.56 g/gs
Inventive Example 7:
1 000 g of the polymer of inventive example 2 were plowshare mixed for 15
minutes with
10 g of Kieselsaure FK 320 (hydrophilic precipitated silica, commercial
product of
Degussa AG, average particle size 15 pm). The product thus coated has the
following
properties:
3o CRC - 32.6 g/g
SFC - 95 x 10'' cm3s/g
Vortex Time - 58 s
Inventive Example 8:
1 000 g of the polymer of inventive example 2 were sprayed with a solution
consisting of
g of deionized water, 20 g of Polymin G 100 solution and 0.5 g of SPAN 20 in a
plowshare mixer and subsequently supplementarily mixed therein for 20 minutes.
The
product thus obtained has the following properties:
CA 02433044 2003-06-25
B00/0606PC
- 23 -
CRC - 3 5.4 g/g
SFC - 90 x 10'' cm3s/g
Vortex Time - 45 s
Inventive Example 9:
1 000 g of the polymer of inventive example 3 were plowshare mixed for 15
minutes with
g of Sipernat D 17. The product thus coated has the following properties:
CRC - 25.8 g/g
SFC - 210 x 10'' cm3s/g
l0 Vortex Time - 105 s
Inventive Egamule 10:
1 000 g of the polymer of inventive example 3 were sprayed with a solution
consisting of
50 g of deionized water, 10 g of polyvinylamine (K 88), 0.1 g of ethylene
glycol diglycidyl
ether and 0.5 g of SPAN 20 in a plowshare mixer and subsequently
supplementarily mixed
therein for 20 minutes. After heat treatment in a laboratory drying cabinet at
80°C for 1
hour, the product has the following properties:
CRC - 25.6 g/g
SFC - 330 x 10'' cm3s/g
Vortex Time - 20 s
Comparative Example 1:
A 10 1 capacity polyethylene vessel thoroughly insulated by foamed plastic
material is
charged with 3 600 g of deionized water and 1 400 g of acrylic acid, followed
by 14 g of
tetraallylammonium chloride. The initiators, consisting of 2.2 g of 2,2'-
azobisamidino
propane dihydrochloride (dissolved in 20 g of deionized water), 4 g of
potassium
peroxodisulfate (dissolved in 150 g of deionized water) and 0.4 g of ascorbic
acid
(dissolved in 20 g of deionized water) are successively added and stirred in
at 4°C. The
reaction solution is then left to stand without stirring. The ensuing
polymerization, in the
3o course of which the temperature rises up to about 90°C, produces a
firm gel. This is
subsequently subjected to mechanical comminution and adjusted to pH 6.0 with
50% by
weight aqueous sodium hydroxide solution. The gel is then dried, ground and
classified to
a particle size distribution of 100 - 850 Vim. 1 kg of this dried hydrogel is
sprayed with a
solution consisting of 40 g of demineralized water, 40 g of i-propanol and 0.5
g of ethylene
glycol diglycidyl ether in a plowshare mixer and subsequently heat treated at
140°C for 60
minutes. The herein described product has the following properties:
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CRC - 36.2 g/g
AUL 0.7 psi - 25.9 g/g
Gel strength - 1 580 Pa
SFC - 8 x 10'' cm3s/g
Comparative Example 2:
A WERNER & PFLEIDERER laboratory kneader having a working capacity of 2 1 is
evacuated to 980 mbar absolute by means of a vacuum pump and a previously
separately
prepared monomer solution which has been cooled to about 25°C and
inertized by passing
1o nitrogen into it is sucked into the kneader. The monomer solution has the
following
composition: 825.5 g of deionized water, 431 g of acrylic acid, 335 g of NaOH
50%, 3.0 g
of methylenebisacrylamide. To improve the inertization, the kneader is
evacuated and
subsequently refilled with nitrogen. This operation is repeated three times. A
solution of
1.2 g of sodium persulfate (dissolved in 6.8 g of deionized water) is then
sucked in,
followed after a further 30 seconds by a further solution consisting of 0.024
g of ascorbic
acid dissolved in 4.8 g of deionized water. After a nitrogen purge a preheated
jacket
heating circuit on bypass at 75°C is switched over to the kneader
jacket and the stirrer
speed increased to 96 rpm. Following the onset of polymerization and the
reaching of Tma,;,
the jacket heating circuit is switched back to bypass, and the batch is
supplementarily
2o polymerized for 15 minutes without heating/cooling, subsequently cooled and
discharged.
The resultant gel particles are dried at above 100°C, ground and
classified to a particle size
distribution of 100 - 850 ~tm. The following properties were measured:
CRC - 29.4 g/g
AUL 0.7 psi - 15.8 g/g
Gel strength - 1 920 Pa
SFC - 5 x 10'' cm3s/g
Comparative Example 3:
1 000 g of the polymer of comparative example 1 were plowshare mixed for 15
minutes
3o with 10 g of Sipernat D 17 (hydrophobic precipitated silica, commercial
product of
Degussa AG, average particle size 10 ~,m). The product thus coated has the
following
properties:
CRC - 35.8 g/g
SFC - 11 x 10-' cm3s/g
Vortex Time - 85 s
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Comparative Egamule 4:
1 000 g of the polymer of comparative example 1 were sprayed with 50 g of a
solution
consisting of 90 parts by weight of deionized water and 10 parts by weight of
aluminum
sulfate (A12(S04)3) in a plowshare mixer and subsequently supplementarily
mixed therein
for 30 minutes. The product thus obtained has the following properties:
CRC - 34.6 g/g
SFC - 9 x 10-~ cm3s/g
Free Swell Rate - 0.48 g/gs
1o Comparative Ezample 5:
1 000 g of the polymer of comparative example 2 were plowshare mixed for 15
minutes
with 10 g of Sipernat D 17. The product thus coated has the following
properties:
CRC - 29.7 g/g
SFC - 6 x 10-~ cm3s/g
Vortex Time - 78 s
Comparative Ezamule 6:
1 000 g of the polymer of comparative example 2 were sprayed with a solution
consisting
of 50 g of deionized water, 10 g of polyvinylamine (K 88), 0.1 g of ethylene
glycol
diglycidyl ether and 0.5 g of SPAN 20 in a plowshare mixer and subsequently
supplementarily mixed therein for 20 minutes. After heat treatment in a
laboratory drying
cabinet at 80°C for 1 hour, the product has the following properties:
CRC - 27.8 g/g
SFC - 15 x 10-' cm3s/g
Vortex Time - 35 s
Comuarative Eacamule 7:
A 30 1 capacity polyethylene vessel thoroughly insulated by foamed plastic
material is
charged with 14 340 g of demineralized water and 42 g of sorbitol triallyl
ether. 3 700 g of
3o sodium bicarbonate are suspended in this initial charge and 5 990 g of
acrylic acid are
gradually added at a rate such that overfoaming of the reaction solution is
avoided; the
reaction solution cools down to about 3-5°C. The initiators, 6.0 g of
2,2'-
azobisamidinopropane dihydrochloride (dissolved in 60 g of demineralized
water), 12 g of
potassium peroxodisulfate (dissolved in 450 g of demineralized water) and also
1.2 g of
ascorbic acid (dissolved in 50 g of demineralized water) are successively
added and
thoroughly stirred in at 4°C. The reaction solution is then left to
stand without stirring. The
ensuing polymerization, in the course of which the temperature rises up to
about 85°C,
produces a gel. This gel is subsequently transferred into a kneader and
adjusted to a pH of
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6.2 by addition of 50% by weight aqueous sodium hydroxide solution. The
comminuted
gel is then dried in an airstream at 170°C, ground and classified to a
particle size
distribution of 100 - 850 ~,m and homogeneously mixed with 1.0% by weight of
Aerosil
200 (pyrogenic silica, commercial product of Degussa AG, average primary
particle size
12 nm). 1 kg of this product is sprayed with a solution of 2 g of RETEN 204 LS
(polyamidoamine-epichlorohydrin adduct from Hercules), 30 g of demineralized
water and
30 g of 1,2-propanediol in a plowshare mixer and subsequently heat treated at
150°C for 60
minutes. The following properties were measured:
CRC - 31.8 g/g
l0 SFC - 25 x 10'' cm3s/g
Vortex Time - 65 s
Comparative Example 8:
A 30 1 capacity polyethylene vessel thoroughly insulated by foamed plastic
material is
charged with 14 340 g of demineralized water and 42 g of sorbitol triallyl
ether. 3 700 g of
sodium bicarbonate are suspended in this initial charge and 5 990 g of acrylic
acid are
gradually added at a rate such that overfoaming of the reaction solution is
avoided; the
reaction solution cools down to about 3-5°C. The initiators, 6.0 g of
2,2'
azobisamidinopropane dihydrochloride (dissolved in 60 g of demineralized
water), 12 g of
2o potassium peroxodisulfate (dissolved in 450 g of demineralized water) and
also 1.2 g of
ascorbic acid (dissolved in 50 g of demineralized water) are successively
added and
thoroughly stirred in at 4°C. The reaction solution is then left to
stand without stirring. The
ensuing polymerization, in the course of which the temperature rises up to
about 85°C,
produces a gel. This gel is subsequently transferred into a kneader and
adjusted to a pH of
6.2 by addition of 50% by weight aqueous sodium hydroxide solution. The
comminuted
gel is then dried in an airstream at 170°C, ground and classified to a
particle size
distribution of 100 - 850 Vim. 1 kg of this product is sprayed with a solution
of 2 g of
RETEN 204 LS (polyamidoamine-epichlorohydrin adduct from Hercules), 5 g of
aluminum sulfate Alz(S04)3, 30 g of demineralized water and 30 g of 1,2-
propanediol in a
3o plowshare mixer and subsequently heat treated at 150°C for 60
minutes. The following
properties were measured:
CRC - 31.5 g/g
SFC - 24 x 10-~ cm3s/g
Vortex Time - 73 s
