U.S. patent application number 10/028907 was filed with the patent office on 2002-09-12 for hydrogels.
This patent application is currently assigned to BASF Aktiengesellschaft. Invention is credited to Frenz, Volker, Herfert, Norbert, Hill, James, Majette, Thomas H., Riegel, Ulrich, Volz, William E..
Application Number | 20020128618 10/028907 |
Document ID | / |
Family ID | 7669160 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020128618 |
Kind Code |
A1 |
Frenz, Volker ; et
al. |
September 12, 2002 |
Hydrogels
Abstract
Described are water-insoluble water-swellable hydrogels which
have been coated with steric or electrostatic spacers and which
have 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, and preferably the following post-coating features: Centrifuge
Retention Capacity (CRC) of at least 24 g/g, Saline Flow
Conductivity (SFC) of at least 30.times.10.sup.-7 cm.sup.3s/g and
Free Swell Rate (FSR) of at least 0.15 g/g s and/or vortex Time of
not more than 160 s.
Inventors: |
Frenz, Volker;
(Mainz-Kostheim, DE) ; Herfert, Norbert;
(Altenstadt, DE) ; Riegel, Ulrich; (Frankfurt,
DE) ; Volz, William E.; (Blue Heron Pointe, VA)
; Majette, Thomas H.; (Schoolhouse Path, VA) ;
Hill, James; (Virginia Beach, VA) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
BASF Aktiengesellschaft
Ludwigshafen
DE
|
Family ID: |
7669160 |
Appl. No.: |
10/028907 |
Filed: |
December 28, 2001 |
Current U.S.
Class: |
604/368 |
Current CPC
Class: |
Y10T 428/31975 20150401;
Y10T 428/249953 20150401; Y10T 428/31978 20150401; A61L 15/60
20130101; B01J 20/26 20130101; Y10T 428/2998 20150115; Y10T
428/2991 20150115; Y10T 428/31971 20150401 |
Class at
Publication: |
604/368 |
International
Class: |
A61F 013/15; A61F
013/20 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 29, 2000 |
DE |
10065252.2 |
Claims
We claim:
1. 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.
2. Hydrogels as claimed in claim 1, characterized by the following
post-coating features: Centrifuge Retention Capacity (CRC) of at
least 24 g/g, Saline Flow Conductivity (SFC) of at least
30.times.10.sup.-7 cm.sup.3s/g and Free Swell Rte (FSR) of at least
0.15 g/g and/or Vortex Time of not more than 160 s.
3. Hydrogels as claimed in claim 1 or 2, wherein the steric spacers
are selected from bentonites, zeolites, active carbons and
silicas.
4. Hydrogels as claimed in claim 1 or 2, wherein the electrostatic
spacers are cationic polymers.
5. Hydrogels as claimed in claim 3, wherein the steric spacers are
applied to the surface of the hydrogel in an amount of from 0.05 to
5% by weight, based on the total weight of the coated
hydrogels.
6. A water-absorbent composition containing water-insoluble
water-swellable hydrogels as claimed in any of claims 1 to 5.
7. A water-absorbent composition as claimed in claim 6, wherein the
water-swellable hydrogels are 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.
8. The process for producing water-absorbent compositions as
claimed in claim 6 by preparing the water-swellable hydrogels,
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.
9. The use of water-absorbent compositions as claimed in either of
claims 6 and 7 for producing hygiene articles or other articles for
absorbing aqueous fluids.
10. Hygiene articles containing a water-absorbent composition as
claimed in either of claims 6 and 7 between a liquid-pervious
topsheet and a liquid-impervious backsheet.
11. Hygiene articles as claimed in claim 10 in the form of diapers,
sanitary napkins and incontinence products.
12. The 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 in any of
claims 1 to 5.
13. The method for determining water-absorbent compositions
possessing high permeability, capacity and swell rate by measuring
the Absorbency Under Load (AUL) and the gel strength of uncoated
hydrogels and determining the Centrifuge Retention Capacity (CRC),
Saline Flow Conductivity (SFC) and Free Swell Rate (FSR) of the
coated hydrogels for given water-absorbent compositions and
determining the water-absorbent compositions for which the
hydrogels exhibit the property spectrum mentioned in claim 1 or
2.
14. The use of water-insoluble water-swellable hydrogels as defined
in any of claims 1 to 5 in hygiene articles or other articles for
absorbing aqueous fluids to enhance the permeability, capacity and
swell rate.
Description
[0001] 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.
[0002] Hygiene articles such as infant diapers or sanitary napkins
have long been utilizing highly swellable hydrogels. This has
substantially reduced hygiene article bulk.
[0003] 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 barrier 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.
[0004] 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.
[0005] A possible way to provide improved transportation properties
and avoid gel blocking is to 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.
[0006] Another way to obtain improved gel permeability is surface
post-crosslinking, 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 body-weight 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 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.
[0007] 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 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.
[0008] A subsequent modification of the surface of the base
polymers (surface-postcrosslinked starting polymers) is known.
[0009] 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.
[0010] In the prior art, aluminum sulfate is used as sole
crosslinker or combined with other crosslinkers in surface
postcrosslinking.
[0011] 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.
[0012] 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.times.10.sup.-7 cm.sup.3s/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.,
there is a reciprocal relationship between retention and gel
permeability.
[0013] 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.
[0014] It is an object of the present invention to provide highly
swellable hydrogels or water-absorbent compositions having good
transportation properties and high permeability 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.
[0015] 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:
[0016] Absorbency Under Load (AUL) (0.7 psi) of at least 20
g/g,
[0017] Gel strength of at least 1 600 Pa.
[0018] The coated hydrogels additionally preferably have the
following features:
[0019] Centrifuge Retention Capacity (CRC) of at least 24 g/g,
[0020] Saline Flow Conductivity (SFC) of at least
30.times.10.sup.-7, preferably at least 60.times.10.sup.-7
cm.sup.3s/g and
[0021] Free Swell Rate (FSR) of at least 0.15 g/g and/or Vortex
Time of not more than 160 s.
[0022] 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.
[0023] The hydrogels of the invention and water-absorbent
compositions containing same are 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.
[0024] 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.
[0025] 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
gel strength of uncoated hydrogels and determining the centrifuge
retention capacity (CRC), Saline Flow Conductivity (SFC) and Free
Swell Rate (FSR) of the coated hydrogels for given water-absorbent
compositions and determining the water-absorbent compositions for
which the hydrogels exhibit the property spectrum mentioned
above.
[0026] The invention further provides for the use of hydrogels as
defined above in hygiene articles or other articles for absorbing
aqueous fluids to enhance the permeability, capacity and swell
rate.
[0027] 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 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.
[0028] The technique of adding steric or electrostatic spacers
makes it possible to produce hygiene 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 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.
[0029] The highly swellable polymer particles of the invention are
notable for high absorption 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 utilized.
[0030] The present invention relates to the production of novel
highly swellable hydrogels by
[0031] (1) preselecting highly swellable 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,
[0032] (2) aftertreating (coating) the surface of the base polymers
selected according to the above criteria with steric or
electrostatic spacers.
[0033] 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.
[0034] This accordingly generates hydrogels having the following
combinations of properties:
[0035] 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 not less
than 30 g/g, particularly preferably CRC not less than 32 g/g and
most preferably CRC not less than 35 g/g and
[0036] SFC not less than 30.times.10.sup.-7 cm.sup.3s/g, preferably
not less than 60.times.10.sup.-7 cm.sup.3s/g, preferably not less
than 80.times.10.sup.-7 cm.sup.3 s/g, more preferably not less than
100.times.10.sup.-7 cm.sup.3 s/g, even more preferably not less
than 120.times.10.sup.-7 cm.sup.3 s/g, especially preferably not
less than 150.times.10.sup.-7 cm.sup.3 s/g, very preferably not
less than 200.times.10.sup.-7 cm.sup.3 s/g, most preferably not
less than 300.times.10.sup.-7 cm.sup.3 s/g, and
[0037] 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 or
[0038] 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.
[0039] Water-Swellable Hydrogels with Spacers
[0040] 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 crosslinked polyalkylene oxide or natural products that
are swellable in aqueous fluids, for example guar derivatives,
alginates and carrageenans.
[0041] Suitable grafting bases can be of natural or synthetic
origin. Examples are starch, cellulose or cellulose derivatives and
also other polysaccharides and oligosacchardies, polyvinyl alcohol,
polyalkylene oxides, especially polyethylene oxides and
polypropylene oxides, polyamines, polyamides and also hydrophilic
polyesters. Suitable polyalkylene oxides have for example the
formula 1
[0042] where
[0043] R.sup.1 and R.sup.2 are independently hydrogen, alkyl,
alkenyl or aryl,
[0044] X is hydrogen or methyl and
[0045] n is an integer from 1 to 10 000.
[0046] R.sup.1 and R.sup.2 are each preferably hydrogen,
(C.sub.1-C.sub.4)-alkyl, (C.sub.2-C.sub.6)-alkenyl or phenyl.
[0047] Preferred hydrogel-forming polymers are crosslinked polymers
having acid groups which 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.
[0048] 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.
[0049] Examples of such monomers bearing acid groups are
monoethylenically unsaturated C.sub.3- to C.sub.25-carboxylic acids
or anhydrides such as acrylic acid, methacrylic acid, ethacrylic
acid, .alpha.-chloroacrylic acid, crotonic acid, maleic acid,
maleic anhydride, itaconic acid, citraconic acid, mesaconic acid,
glutaconic acid, aconitic acid and fumaric acid. It is also
possible to use monoethylenically unsaturated sulfonic or
phosphonic acids, for example vinylsulfonic acid, allylsulfonic
acid, sulfoethyl acrylate, sulfoethyl methacrylate, sulfopropyl
acrylate, sulfopropyl methacrylate, 2-hydroxy-3-acryloyloxypr-
opylsulfonic 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.
[0050] Preferred monomers are acrylic acid, methacrylic acid,
vinylsulfonic acid, acrylamidopropanesulfonic acid or mixtures
thereof, for example mixtures of acrylic and methacrylic acid,
mixtures of acrylic acid and acrylamidopropanesulfonic acid or
mixtures of acrylic acid and vinylsulfonic acid.
[0051] 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 nitriles
of monoethylenically unsaturated carboxylic acids, for example
acrylamide, methacrylamide and N-vinylformamide, N-vinylacetamide,
N-methyl-N-vinylacetamide, acrylonitrile and methacrylonitrile.
Examples of further suitable compounds are vinyl esters of
saturated C.sub.1- to C.sub.4-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 C.sub.3- to C.sub.6-carboxylic acids, for example
esters of monohydric C.sub.1- to C.sub.18-alcohols and acrylic
acid, methacrylic acid or maleic acid, monoesters of maleic 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 10 to 25 carbon atoms which have been reacted
with from 2 to 200 mol of ethylene oxide 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.sub.n) 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.
[0052] 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 50% by weight, preferably less than 20%
by weight.
[0053] 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.
[0054] Preference is given to crosslinked polymers of
monoethylenically unsaturated C.sub.3-C.sub.12-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.
[0055] 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 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 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 esterified with acrylic acid or methacrylic acid,
triallylamine, dialkyldiallylammonium halides such as
dimethyldiallylammonium chloride and diethyldiallylammonium
chloride, 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 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 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.
[0056] Possible crosslinkers also include compounds containing at
least one polymerizable ethylenically unsaturated group and at
least one further functional group. The functional 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, 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 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.
[0057] 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
tris[3-(1-aziridinyl)propionate], 1,6-hexamethylenediethyleneurea,
diphenylmethanebis-4,4'-N,N'-diethyleneu- rea, halo epoxy compounds
such as epichlorohydrin and a-methylepifluorohydrin,
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 products with epichlorohydrin, also polyquaternary amines
such as condensation products of dimethylamine with
epichlorohydrin, homo- and copolymers of diallyldimethylammonium
chloride and also homo- and copolymers of dimethylaminoethyl
(meth)acrylate which are optionally quaternized with, for example,
methyl chloride.
[0058] The crosslinkers are present in the reaction mixture for
example from 0.001 to 20%, preferably from 0.01 to 14%, by
weight.
[0059] 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 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
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, tert-butyl
perbenzoate, di(2-ethylhexyl) peroxydicarbonate, dicyclohexyl
peroxydicarbonate, di(4-tert-butylcyclohexyl) peroxydicarbonate,
dimyristyl peroxydicarbonate, diacetyl peroxydicarbonate, allyl
peresters, cumyl peroxyneodecanoate, tert-butyl
per-3,5,5-trimethylhexano- ate, acetylcyclohexylsulfonyl peroxide,
dilauryl peroxide, dibenzoyl peroxide and tert-amyl
perneodecanoate. Particularly suitable polymerization initiators
are water-soluble azo initiators, e.g.,
2,2'-azobis(2-amidinopropane) dihydrochloride,
2,2'-azobis(N,N'-dimethyle- ne)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 monomers to be polymerized.
[0060] 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, 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
3.times.10.sup.-6 to 1 mol % may be used for the reducing component
of the redox catalyst system and from 0.001 to 5.0 mol % for the
oxidizing component of the redox catalyst, for example.
[0061] When the polymerization is initiated using high energy
radiation, the initiator used is customarily a photoinitiator.
Photoinitiators include for example x-splitters, H-abstracting
systems or else azides. Examples of such initiators are
benzophenone derivatives such as 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:
[0062] 2-(N,N-dimethylamino)ethyl 4-azidocinnamate,
2-(N,N-dimethylamino)ethyl 4-azidonaphthyl ketone,
2-(N,N-dimethylamino)ethyl 4-azidobenzoate, 5-azido-1-naphthyl
2'-(N,N-dimethylamino)ethyl sulfone,
N-(.sup.4-sulfonylazidophenyl)maleim- ide,
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-azidobenzylidene)-- 4-methylcyclohexanone.
Photoinitiators, if used, are customarily used in amounts of from
0.01 to 5% of the weight of the monomers to be polymerized.
[0063] 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.degree.
C.
[0064] Suitable functional groups were already mentioned above,
i.e., hydroxyl, amino, epoxy, isocyante, ester, amido and aziridino
groups, as well examples of such crosslinkers.
[0065] 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 used.
[0066] 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
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.
[0067] 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.
Graham, Wiley-VCH, 1998.
[0068] 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.
[0069] The polymerization reaction may be carried out at from 0 to
150.degree. C., preferably at from 10 to 100.degree. C., not only
at atmospheric pressure but also at superatmospheric or reduced
pressure. As is customary, the polymerization may also be conducted
in a protective gas atmosphere, preferably under nitrogen.
[0070] By subsequently heating the polymer gels at from 50 to
130.degree. C., preferably at from 70 to 100.degree. C., for
several hours, the performance characteristics of the polymers can
be further improved.
[0071] Preference is given to hydrogel-forming polymers which have
been surface postcrosslinked. Surface postcrosslinking may be
carried out in a conventional manner using dried, ground and
classified polymer particles.
[0072] 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-propanol or acetone.
[0073] Suitable surface postcrosslinkers include for example:
[0074] di- or polyglycidyl compounds such as diglycidyl
phosphonates or ethylene glycol diglycidyl ether, bischlorohydrin
ethers of polyalkylene glycols,
[0075] alkoxysilyl compounds,
[0076] polyaziridines, aziridine compounds based on polyethers or
substituted hydrocarbons, for example bis-N-aziridinomethane,
[0077] polyols such as ethylene glycol, 1,2-propanediol,
1,4-butanediol, glycerol, methyltriglycol, polyethylene glycols
having an average molecular weight M.sub.w 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 carbonate or propylene carbonate,
[0078] carbonic acid derivatives such as urea, thiourea, guanidine,
dicyandiamide, 2-oxazolidinone and its derivatives, bisoxazoline,
polyoxazolines, di- and polyisocyanates,
[0079] di- and poly-N-methylol compounds such as, for example,
methylenebis(N-methylolmethacrylamide) or melamine-formaldehyde
resins,
[0080] compounds having two or more blocked isocyanate groups such
as, for example, trimethylhexamethylene diisocyanate blocked with
2,2,3,6-tetramethylpiperidin-4-one.
[0081] If necessary, acidic catalysts may be added, for example
p-toluenesulfonic acid, phosphoric acid, boric acid or ammonium
dihydrogenphosphate.
[0082] Particularly suitable surface postcrosslinkers are di- or
polyglycidyl compounds such as ethylene glycol diglycidyl ether,
the reaction products of polyamidoamines with epichlorohydrin and
2-oxazolidinone.
[0083] 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 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.degree. C.,
preferably 80-190.degree. C., particularly preferably at from 100
to 160.degree. 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 drying may also take place in the
mixer itself, by heating the jacket or by blowing in a preheated
carrier gas.
[0084] Steric Spacers
[0085] Useful steric spacers include inert materials (powders) for
example silicates having a band, 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 precipitated silicas and pyrogenic
silicas according to their method of preparation. Both variants are
commercially available under the name AEROSIL.RTM. (pyrogenic
silicas) or Silica FK, Sipernat.RTM., Wessalon.RTM. (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 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 constitution of the primary particles. Pyrogenic silica
(for example AEROSIL.RTM.) is available in particle size fractions
of from 7 to 40 nm.
[0086] Silica under the tradenames of Silica FK, Sipernat.RTM. and
Wessalon.RTM. can be obtained as a powder of particle size fraction
5-100 .mu.m and a specific surface area of 50-450 m.sup.2/g.
[0087] For use as steric spacer, the particle size of the inert
powders is preferably at least 1 .mu.m, more preferably at least 4
.mu.m, particularly preferably at least 20 .mu.m, very preferably
at least 50 .mu.m. The use of precipitated silicas is particularly
preferred.
[0088] The handling of inert silica grades is generally
physiologically safe. This permits unreserved use of materials of
this kind in a hygiene article.
[0089] 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.
[0090] Electrostatic Spacers
[0091] Cationic components may be added as electrostatic
spacers.
[0092] It is generally possible to add cationic polymers for the
purpose of electrostatic repulsion. This is accomplished for
example with polyethyleneimines, polyvinylamines, polyamines such
as polyalkylenepolyamines, cationic derivatives of polyacrylamides,
polyethyleneimines, polyquaternary amines, for example,
condensation products of hexamethylenediamine, dimethylamine and
epichlorohydrin, condensation products of dimethylamine and
epichlorohydrin, copolymers of hydroxyethylcellulose and
diallyldimethylammonium chloride, copolymers of acrylamide and
.beta.-methacryloxyethyltrimethylammonium 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
methacrylate. Polyquaternary amines are available in a wide
molecular weight range. 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 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,
tetraethylenepentamine, pentaethylenehexamine and
polyethyleneimines and also polyamines having molar masses of up to
4 000 000 in each case.
[0093] 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
Mg.sup.2+, Ca.sup.2+, Al.sup.3+, Sc.sup.3+, Ti.sup.4+,
Mn.sup.2+Fe.sup.2+/3+, Co.sup.2+, Ni.sup.2+, Cu.sup.+/2+,
Zn.sup.2+, Y.sup.3+, Zr.sup.4+, Ag.sup.+, La.sup.3+, Ce.sup.4+,
Hf.sup.4+ and Au.sup.+/3+, preferred metal cations are Mg.sup.2+,
Ca.sup.2+, Al.sup.3+, Ti.sup.4+, Zr.sup.4+ and La.sup.3+ and
particularly preferred metal cations are Al.sup.3+, Ti.sup.4+ and
Zr.sup.4+. The metal cations may be used not only alone 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 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.
[0094] 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 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.degree. 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 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 treatment conditions are chosen in
such a way that it is only the polyamine coating layer which is
crosslinked, but not the polycarboxylate underneath.
[0095] 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.
[0096] The hydrogels mentioned are notable for high absorbency for
water and aqueous solutions and therefore are preferentially used
as absorbents in hygiene articles.
[0097] 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.
[0098] The invention also provides a process for producing
water-absorbent compositions by
[0099] preparing the water-swellable hydrogels,
[0100] optionally coating the hydrogels with a steric or
electrostatic spacer and
[0101] 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.
[0102] 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.
[0103] Description of Test Methods
[0104] Centrifuge Retention Capacity (CRC)
[0105] This method measures the free swellability of the hydrogel
in a teabag. 0.2000.+-.0.0050 g of dried hydrogel (particle size
fraction 106-850 .mu.m) is sealed into a teabag 60.times.85 mm in
size. The teabag is then soaked for 30 minutes in an excess of 0.9%
by eight sodium chloride solution (at least 0.83 l of sodium
chloride solution/l g of polymer powder). The teabag is then
centrifuged for three minutes at 250 g. The amount of liquid is
determined by weighing the centrifuged teabag.
[0106] Absorbency Under Load (AUL) 0.7 psi
[0107] 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 .mu.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
W.sub.0. 0.900.+-.0.005 g of hydrogel-forming polymer (particle
size distribution: 150-800 .mu.m) is then weighed into the
Plexiglas cylinder and 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.sub.a. 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 .mu.m in pore size
(S&S 589 Schwarzband from Schleicher & Schull) 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 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 W.sub.b.
[0108] AUL is calculated by the following equation:
AUL 0.7 psi[g/g]=[W.sub.b-W.sub.a]/[W.sub.a-W.sub.0]
[0109] Free Swell Rate (FSR)
[0110] 1.00 g (W.sub.H) 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.times.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 KCl, 2.0
g of Na.sub.2SO.sub.4, 0.85 g of NH.sub.4H.sub.2PO.sub.4, 0.15 g of
(NH.sub.4).sub.2HPO.sub.4, 0.19 g of CaCl.sub.2, and 0.23 g of
MgCl.sub.2 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
FSR=W.sub.U/(W.sub.H.times.t.sub.A)
[0111] Saline Flow Conductivity (SFC)
[0112] The test method for determining SFC is described in WO
95/262/9.
[0113] Vortex Time
[0114] 50 ml of 0.9% by weight NaCl 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 become flat.
[0115] Gel Strength
[0116] 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.
[0117] Sample Preparation:
[0118] The measurements are carried out on hydrogel particles of
the sieve fraction 300-400 .mu.m which had previously been
preswelled for 1 hour in 0.9% by weight NaCl solution in a ratio of
1:60. To prepare the samples to be measured, the NaCl solution 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 carried out, this preparative method has
to be complied with exactly, or the rheological measurement would
be impaired and the measured results distorted.
[0119] Measurement Procedure:
[0120] The gel strength is determined using the Carrimed CS
rheometer via the oscillation mode using a plate-45 plate geometry
(diameter 6 cm). To avoid the slip effect, sand-blasted 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.
[0121] The reported measurements are number averages of 3 series of
determinations.
[0122] The examples which follow illustrate the invention.
EXAMPLES
Inventive Example 1
[0123] A 10 l 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 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.degree. 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 .sub.90.degree. 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
.mu.m. 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.degree. C. for 60 minutes. The
herein described product has the following properties:
[0124] CRC=28.4 g/g
[0125] AUL 0.7 psi=25.1 g/g
[0126] Gel strength=2 350 Pa
[0127] SFC=35.times.10.sup.-7 cm.sup.3s/g
Inventive Example 2
[0128] A 30 l 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 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.degree. 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.degree. 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.degree.
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.degree. C., ground and classified to a
particle size distribution of 100-850 .mu.m. 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.degree. C. for 60
minutes. The following properties were measured:
[0129] CRC=32.3 g/g
[0130] AUL 0.7 psi=26.4 g/g
[0131] Gel strength=1 975 Pa
[0132] SFC=25.times.10.sup.-7 cm.sup.3s/g
Inventive Example 3
[0133] A WERNER & PFLEIDERER laboratory kneader having a
working capacity of 2 l 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.degree. 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 urge a
preheated jacket heating circuit on bypass at 75.degree. 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 T.sub.max, 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.degree. C., ground
and classified to a particle size distribution of 100-850 .mu.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.degree. C. for 70 minutes. The following properties were
measured:
[0134] CRC=26.3 g/g
[0135] AUL 0.7 psi=23.8 g/g
[0136] Gel strength=2 680 Pa
[0137] SFC=50.times.10.sup.-7 cm.sup.3s/g
Inventive Example 4
[0138] 1 000 g of the polymer of inventive example 1 were plowshare
mixed for 15 minutes with 10 g of Sipernat D 17 (hydrophobic
precipitated silica, commercial product of Degussa AG, average
particle size 10 .mu.m). The product thus coated has the following
properties:
[0139] CRC=28.9 g/g
[0140] SFC=115.times.10.sup.-7 cm.sup.3s/g
[0141] Vortex Time=80 s
Inventive Example 5
[0142] 1 000 g of the polymer of inventive 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
(Al2(So.sub.4).sub.3) in a plowshare mixer and subsequently
supplementarily mixed therein for 30 minutes. The product thus
obtained has the following properties:
[0143] CRC=27.2 g/g
[0144] SFC=160.times.10.sup.-7 cm.sup.3s/g
[0145] Free Swell Rate=0.25 g/gs
Inventive Example 6
[0146] 1 000 g of the polymer of inventive example 1 were plowshare
ixed for 15 minutes with 8 g of Sipernat 22 (hydrophilic
precipitated silica, commercial product of Degussa AG, average
article size 100 .mu.m). The product thus coated has the following
roperties:
[0147] CRC=29.5 g/g
[0148] SFC=x 10.sup.-7 cm.sup.3s/g
[0149] Free Swell Rate=0.56 g/gs
Inventive Example 7
[0150] 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 .mu.m). The product thus coated has the following
properties:
[0151] CRC=32.6 g/g
[0152] SFC=95.times.10.sup.-7 cm.sup.3s/g
[0153] Vortex Time=58 s
Inventive Example 8
[0154] 1 000 g of the polymer of inventive example 2 were sprayed
with a solution consisting of 40 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:
[0155] CRC=35.4 g/g
[0156] SFC=90.times.10.sup.-7 cm.sup.3s/g
[0157] Vortex Time=45
Inventive Example 9
[0158] 1 000 g of the polymer of inventive example 3 were plowshare
mixed for 15 minutes with 10 g of Sipernat D 17. The product thus
coated has the following properties:
[0159] CRC=25.8 g/g
[0160] SFC=210.times.10.sup.-7 cm.sup.3s/g
[0161] Vortex Time=105 s
Inventive Example 10
[0162] 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.degree. C. for 1 hour, the
product has the following properties:
[0163] CRC=25.6 g/g
[0164] SFC=330.times.10.sup.-7 cm.sup.3s/g
[0165] Vortex Time=20 s
Comparative Example 1
[0166] A 10 l 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'-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.degree. 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 90.degree. 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 .mu.m. 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.degree. C. for 60 minutes. The herein described
product has the following properties:
[0167] CRC=36.2 g/g
[0168] AUL 0.7 psi=25.9 g/g
[0169] Gel strength=1 580 Pa
[0170] SFC=8.times.10.sup.-7 cm.sup.3s/g
Comparative Example 2
[0171] WERNER & PFLEIDERER laboratory kneader having a working
capacity of 2 l 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.degree. 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%, 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.degree. 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 T.sub.max, 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.degree. C., ground and classified to a particle
size distribution of 100-850 .mu.m. The following properties were
measured:
[0172] CRC=29.4 g/g
[0173] AUL 0.7 psi=15.8 g/g
[0174] Gel strength=1 920 Pa
[0175] SFC=5.times.10.sup.-7 cm.sup.3s/g
Comparative Example 3
[0176] 1 000 g of the polymer of comparative example 1 were
plowshare mixed for 15 minutes with 10 g of Sipernat D 17
(hydrophobic precipitated silica, commercial product of Degussa AG,
average particle size 10 .mu.m). The product thus coated has the
following properties:
[0177] CRC=35.8 g/g
[0178] SFC=11.times.10.sup.-7 cm.sup.3s/g
[0179] Vortex Time=85 s
Comparative Example 4
[0180] 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
(Al2(SO.sub.4).sub.3) in a plowshare mixer and subsequently
supplementarily mixed therein for 30 minutes. The product thus
obtained has the following properties:
[0181] CRC=34.6 g/g
[0182] SFC=9.times.10.sup.-7 cm.sup.3s/g
[0183] Free Swell Rate=0.48 g/gs
Comparative Example 5
[0184] 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:
[0185] CRC=29.7 g/g
[0186] SFC=6.times.10.sup.-7 cm.sup.3s/g
[0187] Vortex Time=78 s
Comparative Example 6
[0188] 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.degree. C. for 1 hour, the
product has the following properties:
[0189] CRC=27.8 g/g
[0190] SFC=15.times.10.sup.-7 cm.sup.3s/g
[0191] Vortex Time=35 s
Comparative Example 7
[0192] A 30 l 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.degree. 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.degree. 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.degree.
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.degree. C., ground and classified to a
particle size distribution of 100-850 .mu.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.degree. C. for 60
minutes. The following properties were measured:
[0193] CRC 31.8 g/g
[0194] SFC=25.times.10.sup.-7 cm.sup.3s/g
[0195] Vortex Time=65 s
Comparative Example 8
[0196] A 30 l 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.degree. 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.degree. 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.degree.
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.degree. C., ground and classified to a
particle size distribution of 100-850 .mu.m. 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 Al.sub.2(SO.sub.4).sub.3, 30 g of demineralized
water and 30 g of 1,2-propanediol in a plowshare mixer and
subsequently heat treated at 150.degree. C. for 60 minutes. The
following properties were measured:
[0197] CRC=31.5 g/g
[0198] SFC=24.times.10.sup.-7 cm.sup.3s/g
[0199] Vortex Time 73 s
* * * * *