U.S. patent application number 13/319591 was filed with the patent office on 2012-03-15 for water absorbent storage layers.
This patent application is currently assigned to BASF SE. Invention is credited to Christophe Bauduin.
Application Number | 20120064792 13/319591 |
Document ID | / |
Family ID | 42246082 |
Filed Date | 2012-03-15 |
United States Patent
Application |
20120064792 |
Kind Code |
A1 |
Bauduin; Christophe |
March 15, 2012 |
Water Absorbent Storage Layers
Abstract
The present invention relates to improved water-absorbing
storage layers for use in hygiene articles, the water-absorbing
storage layers being essentially free of cellulose fibers.
Inventors: |
Bauduin; Christophe;
(Plankstadt, DE) |
Assignee: |
BASF SE
Ludwigshafen
DE
|
Family ID: |
42246082 |
Appl. No.: |
13/319591 |
Filed: |
May 17, 2010 |
PCT Filed: |
May 17, 2010 |
PCT NO: |
PCT/EP2010/056688 |
371 Date: |
November 9, 2011 |
Current U.S.
Class: |
442/417 |
Current CPC
Class: |
A61F 13/51401 20130101;
A61L 15/58 20130101; A61L 15/22 20130101; A61L 15/425 20130101;
A61L 15/60 20130101; Y10T 442/699 20150401; A61F 2013/53941
20130101; A61L 15/24 20130101; A61F 13/511 20130101; A61L 15/28
20130101; A61F 13/531 20130101; A61F 2013/530802 20130101; A61F
13/5323 20130101; A61F 13/539 20130101; A61L 15/26 20130101; A61F
2013/530131 20130101; A61F 13/53 20130101; A61F 2013/530007
20130101; A61F 2013/530481 20130101; A61L 15/42 20130101 |
Class at
Publication: |
442/417 |
International
Class: |
B32B 5/16 20060101
B32B005/16 |
Foreign Application Data
Date |
Code |
Application Number |
May 20, 2009 |
EP |
09160762.2 |
Claims
1. A water-absorbing storage layer consisting of a nonwoven
backsheet, water-absorbing polymer particles, and a liquid-pervious
topsheet, wherein the water-absorbing polymer particles are fixed
on the nonwoven backsheet.
2. The storage layer according to claim 1, wherein the
liquid-pervious topsheet has been adhesive bonded to the nonwoven
backsheet to form pockets.
3. The storage layer according to claim 2, wherein bridges between
the pockets are filled with further water-absorbing polymer
particles and said further water-absorbing polymer particles are
fixed to a second liquid-pervious topsheet by adhesive bonding to
the first liquid-pervious topsheet.
4. The storage layer according to claim 1, wherein the nonwoven
backsheet has fibers directed downward and the water-absorbing
polymer particles are present in the region of the fibers.
5. The storage layer according to claim 4, wherein the
liquid-pervious topsheet has been adhesive bonded to the
fibers.
6. The storage layer according to claim 1, wherein a
liquid-pervious matrix is present between the nonwoven backsheet
and the liquid-pervious topsheet.
7. The storage layer according to claim 6, wherein the
liquid-pervious matrix has been adhesive bonded to the nonwoven
backsheet and the liquid-pervious topsheet.
8. The storage layer according to claim 1, wherein the
water-absorbing polymer particles have a centrifuge retention
capacity of at least 15 g/g.
9. A hygiene article comprising a water-absorbing storage layer
according to claim 1.
Description
[0001] The present invention relates to improved water-absorbing
storage layers for use in hygiene articles, the water-absorbing
storage layers being essentially free of cellulose fibers.
[0002] The production of water-absorbing polymer particles and the
use thereof for producing hygiene articles is described, for
example, in the monograph "Modern Superabsorbent Polymer
Technology", F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998,
especially on pages 252 to 258. The water-absorbing polymer
particles are also referred to as superabsorbents.
[0003] The currently commercially available disposable diapers
consist typically of a liquid-pervious topsheet (A), a
liquid-impervious backsheet (B), a water-absorbing storage layer
(C) between layers (A) and (B), and an acquisition distribution
layer (D) between layers (A) and (C).
[0004] The water-absorbing storage layer consists typically of a
mixture of water-absorbing polymer particles and cellulose fibers,
the water-absorbing polymer particles being fixed by the cellulose
matrix.
[0005] In the last few years, there has been a trend toward ever
thinner disposable diapers. To produce ever thinner disposable
diapers, the proportion of cellulose fibers in the water-absorbing
storage layer has been lowered ever further. A disadvantage here is
that the cellulose matrix is made ever thinner as a result, and the
mobility of the water-absorbing polymer particles in the
water-absorbing storage layer increases.
[0006] Especially when water-absorbing storage layers essentially
free of cellulose fibers are desired, i.e. consist virtually
exclusively of water-absorbing polymer particles, there is the risk
that the water-absorbing polymer particles will slip within the
disposable diaper or even fall out of the disposable diaper
completely.
[0007] To solve this problem, novel water-absorbing storage layers
have been produced. For example, WO 97/17397 A1 describes a process
for producing water-absorbing foams. Use of such foams allows the
use of cellulose fibers to be dispensed with entirely.
Cellulose-free hygiene articles can also be secured to suitable
nonwoven backsheets by fixing of water-absorbing polymer particles
by means of thermoplastic polymers, especially of hotmelt
adhesives, provided that these thermoplastic polymers are spun out
to form fine fibers. Such products are described, for example, in
US 2003/0181115, US 2004/0167486, US 2004/071363, US 2005/097025,
US 2007/156108, US 2008/0125735, EP 1 917 940 A1, EP 1 913 912 A1,
EP 1 913 913 A2, EP 1 913 914 A2, EP 1 911 425 A2, EP 1 911 426 A2,
EP 1 447 067 A1, EP 1 813 237 A2, EP 1 813 236 A2, EP 1 808 152 A2,
EP 1 447 066 A1. The production processes are disclosed in WO
2008/155722 A2, WO 2008/155702 A1, WO 2008/155711 A1, WO
2008/155710 A1, WO 2008/155701 A2, WO 2008/155699 A1. A
disadvantage is the relatively complex production process, since
the spinning of the adhesive fibers in the presence of
water-absorbing polymer particles is difficult and prone to
faults.
[0008] In addition, extensible cellulose-free hygiene articles are
known, and US 2006/0004336, US 2007/0135785, and US 2005/0137085
disclose production thereof by simultaneous fiber spinning of
suitable thermoplastic polymers and incorporation of
water-absorbing polymer particles. This process too is complex and
prone to faults.
[0009] It was an object of the present invention to provide
improved water-absorbing storage layers for hygiene articles,
especially disposable diapers. For the improved water-absorbing
storage layers, it should be possible to use the customary
water-absorbing polymer particles. Moreover, the improved
water-absorbing storage layers should be essentially free of
cellulose fibers, and the water-absorbing polymer particles in the
water-absorbing storage layer should neither slip nor fall out
either in the dry or moist state. In the context of this
application, "free of cellulose fibers" means that the cellulose
content in the inventive storage layer is preferably less than 30%
by weight, preferentially less than 20% by weight, more preferably
less than 10% by weight, most preferably less than 5% by weight.
Ideally, no cellulose at all is present.
[0010] The object is achieved by water-absorbing storage layers
consisting of a nonwoven backsheet, water-absorbing polymer
particles and a liquid-pervious topsheet, wherein the
water-absorbing polymer particles are fixed on the nonwoven
backsheet.
[0011] In one embodiment of the present invention, the
liquid-pervious topsheet is adhesive bonded to the nonwoven
backsheet to form pockets. For this purpose, customary adhesives
can be used. However, it is also possible that the liquid-pervious
topsheet and/or nonwoven backsheet is entirely or partly composed
of a thermoplastic polymer, and the liquid-pervious topsheet is
adhesive bonded to the nonwoven backsheet by partial melting.
Suitable nonwoven backsheets may consist of mixtures of
thermoplastic fibers (for example polyolefins, polyesters,
polyamides) and non-thermoplastic fibers (for example
cellulose).
[0012] The formation of pockets filled with water-absorbing polymer
particles imparts the form of a quilt to the water-absorbing
storage layer. The water-absorbing polymer particles are prevented
from slipping within the water-absorbing storage layer by the
pockets.
[0013] In a further preferred variant of this embodiment, the
depressions are partly filled with a liquid-conducting filler
material and the pockets are optionally also additionally covered
thereby. Useful filler materials for this purpose include
hydrophilic fibers alone (for example cellulose, viscose or rayon)
or in a mixture with other fibers (for example propylene or
cellulose acetate). The fibers may also be those which consist of
more than one component and which have a bi- or multilamellar or
hollow cross section. Such fibers typically conduct the liquid
better than simple smooth fibers.
[0014] Advantageously, the depressions formed in the
water-absorbing storage layer by virtue of the adhesive bonding of
the liquid-pervious topsheet to the nonwoven backsheet are filled
with further water-absorbing polymer particles and fixed to a
further liquid-pervious topsheet.
[0015] FIGS. 1a and 1b show cross sections, and FIG. 1c shows a
longitudinal section, of the inventive water-absorbing storage
layers of the first embodiment, the reference numerals having the
following meanings:
[0016] 1 nonwoven backsheet
[0017] 2 liquid-pervious topsheet
[0018] 3 water-absorbing polymer particles
[0019] 4 adhesive bond
[0020] 5 second liquid-pervious topsheet
[0021] 6 additional adhesive bond
[0022] 7 machine running direction.
[0023] In a second embodiment of the present invention, a nonwoven
substrate with preferably hydrophilic fibers protruding upward is
used. The water-absorbing polymer particles are fixed by the fibers
between the nonwoven backsheet and the liquid-pervious topsheet.
The liquid-pervious topsheet is preferably adhesive bonded to the
fibers of the nonwoven backsheet. The fibers protruding upward may
consist of all known polymers and mixtures thereof, but preference
is given to polyolefins, polyesters, polyurethanes, cellulose and
derivatives thereof, polyamides. The fibers may also be those which
consist of more than one component and which have a bi- or
multilamellar or hollow cross section.
[0024] FIG. 2 shows a cross section of the inventive
water-absorbing storage layers of the second embodiment, the
reference numerals having the following meanings:
[0025] 8 nonwoven backsheet
[0026] 9 liquid-pervious topsheet
[0027] 10 water-absorbing polymer particles
[0028] 11 fibers directed upward.
[0029] In a third embodiment of the present invention, a soft
matrix composed of a liquid-pervious material is applied to the
nonwoven backsheet, and the water-absorbing polymer particles are
introduced into the chambers of the matrix. The chambers of the
matrix are sealed with a liquid-pervious topsheet. The soft matrix
is preferably adhesive bonded to the nonwoven backsheet and the
liquid-pervious topsheet.
[0030] An advantage of this embodiment is that the matrix material
can be selected such that it additionally promotes liquid
distribution within the water-absorbing storage layer. Suitable for
this purpose are pressed hydrophilic fibers (for example of
cellulose, chemically precipitated cellulose or crosslinked
cellulose), or open-pore soft sponges. In the case of sponges,
hydrophilic types are preferred. The matrix material should have,
in the expanded state (unpressed), continuous pores with diameter
preferably of 0.001 to 2.0 mm, preferably of 0.01 to 1.0 mm, more
preferably of 0.03 to 0.5 mm, most preferably of 0.06 to 0.3
mm.
[0031] FIG. 3a shows a top view, and FIG. 3b shows cross sections,
of the inventive water-absorbing storage layers of the third
embodiment, the reference numerals having the following
meanings:
[0032] 12 nonwoven backsheet
[0033] 13 liquid-pervious topsheet
[0034] 14 water-absorbing polymer particles
[0035] 15 liquid-pervious matrix.
[0036] In all embodiments, in a further particularly preferred
variant, it is additionally possible to use a water-soluble
adhesive for dry fixing of the water-absorbing polymer particles.
The adhesive is applied, for example, to the nonwoven backsheet
before the application of the water-absorbing polymer particles.
The application can be effected, for example, in punctiform
fashion, over the whole area, or preferably in strips in or
transverse to or diagonally with respect to the machine running
direction. The water-soluble adhesive may consist, for example, of
polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol,
starch and starch derivatives, cellulose and cellulose derivatives,
or polyacrylic acid. Most preferably, the water-soluble adhesive
comprises at least one polyamine or consists thereof. Suitable
polyamines are polyvinylamines, polyethyleneimines,
polyallylamines. Particular preference is given to polyvinylamine.
On contact with moisture, the amine is released from the adhesive
and becomes attached to the swelling hydrogel, which additionally
causes a particular gel layer stability in the swollen state.
[0037] In preferred embodiments, a web of the nonwoven backsheet is
moved in machine direction, and strips or geometric patterns
comprising water-absorbing polymer particles are applied thereto.
In the second embodiment of the present invention, a continuous
surface may be obtained in this way. In all three embodiments,
however, any desired geometric forms and patterns are conceivable,
for example one which are arranged like cushions comprising
water-absorbing polymer particles in terms of area. The cushions or
the heaps of water-absorbing polymer particles applied may assume
any desired shape in terms of area, for example circles, ellipses,
rectangles, squares, triangles (viewed from above). Particular
preference is given to any desired polygons or mixtures of polygons
with which the two-dimensional surface can be covered without gaps.
Particular preference is also given to the application of one or
more continuous strips in machine running direction, the strips
running parallel to one another.
[0038] In the case of pockets, it is advantageous to fill them
loosely, in order that the water-absorbing polymer particles can
swell in a substantially unhindered manner. Optionally, however, an
elastic nonwoven can also be used as a topsheet or as a backsheet.
Such nonwovens are commercially available.
[0039] In all embodiments, the nonwoven backsheet is fixed on a
suitable machine by means of reduced pressure such that
water-absorbing polymer particles to be laid on can then be laid on
there by means of masks or similar means, such that these
water-absorbing polymer particles are held fixed from below by the
existing suction during processing. It is thus equally possible to
temporarily fix the other components.
[0040] A. Water-Absorbing Polymer Particles
[0041] The water-absorbing polymer particles are produced by
polymerizing a monomer solution or suspension and are typically
water-insoluble.
[0042] The monomers a) are preferably water-soluble, i.e. the
solubility in water at 23.degree. C. is typically at least 1 g/100
g of water, preferably at least 5 g/100 g of water, more preferably
at least 25 g/100 g of water, most preferably at least 35 g/100 g
of water.
[0043] Suitable monomers a) are, for example, ethylenically
unsaturated carboxylic acids, such as acrylic acid, methacrylic
acid and itaconic acid. Particularly preferred monomers are acrylic
acid and methacrylic acid. Very particular preference is given to
acrylic acid.
[0044] Further suitable monomers a) are, for example, ethylenically
unsaturated sulfonic acids, such as styrenesulfonic acid and
2-acrylamido-2-methylpropanesulfonic acid (AMPS).
[0045] Impurities can have a considerable influence on the
polymerization. The raw materials used should therefore have a
maximum purity. It is therefore often advantageous to specially
purify the monomers a). Suitable purification processes are
described, for example, in WO 2002/055469 A1, WO 2003/078378 A1 and
WO 2004/035514 A1. A suitable monomer a) is, for example, acrylic
acid purified according to WO 2004/035514 A1 comprising 99.8460% by
weight of acrylic acid, 0.0950% by weight of acetic acid, 0.0332%
by weight of water, 0.0203% by weight of propionic acid, 0.0001% by
weight of furfurals, 0.0001% by weight of maleic anhydride, 0.0003%
by weight of diacrylic acid and 0.0050% by weight of hydroquinone
monomethyl ether.
[0046] The proportion of acrylic acid and/or salts thereof in the
total amount of monomers a) is preferably at least 50 mol %, more
preferably at least 90 mol %, most preferably at least 95 mol
%.
[0047] The monomers a) typically comprise polymerization
inhibitors, preferably hydroquinone monoethers, as storage
stabilizers.
[0048] The monomer solution comprises preferably up to 250 ppm by
weight, preferably at most 130 ppm by weight, more preferably at
most 70 ppm by weight, preferably at least 10 ppm by weight, more
preferably at least 30 ppm by weight, especially around 50 ppm by
weight, of hydroquinone monoether, based in each case on the
unneutralized monomer a). For example, the monomer solution can be
prepared by using an ethylenically unsaturated monomer bearing acid
groups with an appropriate content of hydroquinone monoether.
[0049] Preferred hydroquinone monoethers are hydroquinone
monomethyl ether (MEHQ) and/or alpha-tocopherol (vitamin E).
[0050] Suitable crosslinkers b) are compounds having at least two
groups suitable for crosslinking. Such groups are, for example,
ethylenically unsaturated groups which can be polymerized
free-radically into the polymer chain, and functional groups which
can form covalent bonds with the acid groups of the monomer a). In
addition, polyvalent metal salts which can form coordinate bonds
with at least two acid groups of the monomer a) are also suitable
as crosslinkers b).
[0051] Crosslinkers b) are preferably compounds having at least two
polymerizable groups which can be polymerized free-radically into
the polymer network. Suitable crosslinkers b) are, for example,
ethylene glycol dimethacrylate, diethylene glycol diacrylate,
polyethylene glycol diacrylate, allyl methacrylate,
trimethylolpropane triacrylate, triallylamine, tetraallylammonium
chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di-
and triacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1,
EP 0 632 068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300
A1, WO 2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which,
as well as acrylate groups, comprise further ethylenically
unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55
401 A1, or crosslinker mixtures, as described, for example, in DE
195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/032962
A2.
[0052] Preferred crosslinkers b) are pentaerythrityl triallyl
ether, tetraalloxyethane, methylenebismethacrylamide, 15-tuply
ethoxylated trimethylolpropane triacrylate, polyethylene glycol
diacrylate, trimethylolpropane triacrylate and triallylamine.
[0053] Very particularly preferred crosslinkers b) are the
polyethoxylated and/or -propoxylated glycerols which have been
esterified with acrylic acid or methacrylic acid to give di- or
triacrylates, as described, for example, in WO 2003/104301 A1. Di-
and/or triacrylates of 3- to 10-tuply ethoxylated glycerol are
particularly advantageous. Very particular preference is given to
di- or triacrylates of 1- to 5-tuply ethoxylated and/or
propoxylated glycerol. Most preferred are the triacrylates of 3- to
5-tuply ethoxylated and/or propoxylated glycerol, especially the
triacrylate of 3-tuply ethoxylated glycerol.
[0054] The amount of crosslinker b) is preferably 0.05 to 1.5% by
weight, more preferably 0.1 to 1% by weight, most preferably 0.3 to
0.6% by weight, based in each case on monomer a). With rising
crosslinker content, the centrifuge retention capacity (CRC) falls
and the absorption under a pressure of 21.0 g/cm.sup.2 passes
through a maximum.
[0055] The initiators c) used may be all compounds which generate
free radicals under the polymerization conditions, for example
thermal initiators, redox initiators, photoinitiators. Suitable
redox initiators are sodium peroxodisulfate/ascorbic acid, hydrogen
peroxide/ascorbic acid, sodium peroxodisulfate/sodium bisulfite and
hydrogen peroxide/sodium bisulfite. Preference is given to using
mixtures of thermal initiators and redox initiators, such as sodium
peroxodisulfate/hydrogen peroxide/ascorbic acid. The reducing
component used is, however, preferably a mixture of the sodium salt
of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of
2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such
mixtures are obtainable as Bruggolite.RTM. FF6 and Bruggolite.RTM.
FF7 (Bruggemann Chemicals; Heilbronn; Germany).
[0056] Ethylenically unsaturated monomers d) copolymerizable with
the ethylenically unsaturated monomers a) bearing acid groups are,
for example, acrylamide, methacrylamide, hydroxyethyl acrylate,
hydroxyethyl methacrylate, dimethylaminoethyl methacrylate,
dimethylaminoethyl acrylate, dimethylaminopropyl acrylate,
diethylaminopropyl acrylate, dimethylaminoethyl methacrylate,
diethylaminoethyl methacrylate.
[0057] The water-soluble polymers e) used may be polyvinyl alcohol,
polyvinylpyrrolidone, starch, starch derivatives, modified
cellulose, such as methylcellulose or hydroxyethylcellulose,
gelatin, polyglycols or polyacrylic acids, preferably starch,
starch derivatives and modified cellulose.
[0058] Typically, an aqueous monomer solution is used. The water
content of the monomer solution is preferably from 40 to 75% by
weight, more preferably from 45 to 70% by weight, most preferably
from 50 to 65% by weight. It is also possible to use monomer
suspensions, i.e. monomer solutions with excess monomer a), for
example sodium acrylate. With rising water content, the energy
requirement in the subsequent drying rises, and, with falling water
content, the heat of polymerization can only be removed
inadequately.
[0059] For optimal action, the preferred polymerization inhibitors
require dissolved oxygen. The monomer solution can therefore be
freed of dissolved oxygen before the polymerization by
inertization, i.e. flowing an inert gas through, preferably
nitrogen or carbon dioxide. The oxygen content of the monomer
solution is preferably lowered before the polymerization to less
than 1 ppm by weight, more preferably to less than 0.5 ppm by
weight, most preferably to less than 0.1 ppm by weight.
[0060] Suitable reactors are, for example, kneading reactors or
belt reactors. In the kneader, the polymer gel formed in the
polymerization of an aqueous monomer solution or suspension is
comminuted continuously by, for example, contrarotatory stirrer
shafts, as described in WO 2001/038402 A1. Polymerization on a belt
is described, for example, in DE 38 25 366 A1 and U.S. Pat. No.
6,241,928. Polymerization in a belt reactor forms a polymer gel,
which has to be comminuted in a further process step, for example
in an extruder or kneader.
[0061] To improve the drying properties, the comminuted polymer gel
obtained by means of a kneader can additionally be extruded.
[0062] However, it is also possible to dropletize an aqueous
monomer solution and to polymerize the droplets obtained in a
heated carrier gas stream. This allows the process steps of
polymerization and drying to be combined, as described in WO
2008/040715 A2 and WO 2008/052971 A1.
[0063] The acid groups of the resulting polymer gels have typically
been partially neutralized. Neutralization is preferably carried
out at the monomer stage. This is typically done by mixing in the
neutralizing agent as an aqueous solution or preferably also as a
solid. The degree of neutralization is preferably from 25 to 95 mol
%, more preferably from 30 to 80 mol %, most preferably from 40 to
75 mol %, for which the customary neutralizing agents can be used,
preferably alkali metal hydroxides, alkali metal oxides, alkali
metal carbonates or alkali metal hydrogencarbonates and also
mixtures thereof. Instead of alkali metal salts, it is also
possible to use ammonium salts. Particularly preferred alkali
metals are sodium and potassium, but very particular preference is
given to sodium hydroxide, sodium carbonate or sodium
hydrogencarbonate and also mixtures thereof.
[0064] However, it is also possible to carry out neutralization
after the polymerization, at the stage of the polymer gel formed in
the polymerization. It is also possible to neutralize up to 40 mol
%, preferably 10 to 30 mol % and more preferably 15 to 25 mol % of
the acid groups before the polymerization by adding a portion of
the neutralizing agent actually to the monomer solution and setting
the desired final degree of neutralization only after the
polymerization, at the polymer gel stage. When the polymer gel is
neutralized at least partly after the polymerization, the polymer
gel is preferably comminuted mechanically, for example by means of
an extruder, in which case the neutralizing agent can be sprayed,
sprinkled or poured on and then carefully mixed in. To this end,
the gel mass obtained can be repeatedly extruded for
homogenization.
[0065] The polymer gel is then preferably dried with a belt drier
until the residual moisture content is preferably 0.5 to 15% by
weight, more preferably 1 to 10% by weight, most preferably 2 to 8%
by weight, the residual moisture content being determined by EDANA
(European Disposables and Nonwovens Association) recommended test
method No. WSP 230.2-05 "Moisture Content". In the case of too high
a residual moisture content, the dried polymer gel has too low a
glass transition temperature T.sub.g and can be processed further
only with difficulty. In the case of too low a residual moisture
content, the dried polymer gel is too brittle and, in the
subsequent comminution steps, undesirably large amounts of polymer
particles with an excessively low particle size are obtained
(fines). The solids content of the gel before the drying is
preferably from 25 to 90% by weight, more preferably from 35 to 70%
by weight, most preferably from 40 to 60% by weight. Optionally, it
is, however, also possible to use a fluidized bed drier or a paddle
drier for the drying operation.
[0066] Thereafter, the dried polymer gel is ground and classified,
and the apparatus used for grinding may typically be single- or
multistage roll mills, preferably two- or three-stage roll mills,
pin mills, hammer mills or vibratory mills.
[0067] The mean particle size of the polymer particles removed as
the product fraction is preferably at least 200 .mu.m, more
preferably from 250 to 600 .mu.m, very particularly from 300 to 500
.mu.m. The mean particle size of the product fraction may be
determined by means of EDANA (European Disposables and Nonwovens
Association) recommended test method No. WSP 220.2-05 "Particle
Size Distribution", where the proportions by mass of the screen
fractions are plotted in cumulated form and the mean particle size
is determined graphically. The mean particle size here is the value
of the mesh size which gives rise to a cumulative 50% by
weight.
[0068] The proportion of particles with a particle size of at least
150 .mu.m is preferably at least 90% by weight, more preferably at
least 95% by weight, most preferably at least 98% by weight.
[0069] Polymer particles with too small a particle size lower the
saline flow conductivity or gel bed permeability (SFC or GBP). The
proportion of excessively small polymer particles (fines) should
therefore be small.
[0070] Excessively small polymer particles are therefore typically
removed and recycled into the process. This is preferably done
before, during or immediately after the polymerization, i.e. before
the drying of the polymer gel. The excessively small polymer
particles can be moistened with water and/or aqueous surfactant
before or during the recycling.
[0071] It is also possible in later process steps to remove
excessively small polymer particles, for example after the surface
postcrosslinking or another coating step. In this case, the
excessively small polymer particles recycled are surface
postcrosslinked or coated in another way, for example with fumed
silica.
[0072] When a kneading reactor is used for polymerization, the
excessively small polymer particles are preferably added during the
last third of the polymerization.
[0073] When the excessively small polymer particles are added at a
very early stage, for example actually to the monomer solution,
this lowers the centrifuge retention capacity (CRC) of the
resulting water-absorbing polymer particles. However, this can be
compensated, for example, by adjusting the amount of crosslinker b)
used.
[0074] When the excessively small polymer particles are added at a
very late stage, for example not until in an apparatus connected
downstream of the polymerization reactor, for example to an
extruder, the excessively small polymer particles can be
incorporated into the resulting polymer gel only with difficulty.
Insufficiently incorporated, excessively small polymer particles
are, however, detached again from the dried polymer gel during the
grinding, are therefore removed again in the course of
classification and increase the amount of excessively small polymer
particles to be recycled.
[0075] The proportion of particles having a particle size of at
most 850 .mu.m is preferably at least 90% by weight, more
preferably at least 95% by weight, most preferably at least 98% by
weight.
[0076] The proportion of particles having a particle size of at
most 600 .mu.m is preferably at least 90% by weight, more
preferably at least 95% by weight, most preferably at least 98% by
weight.
[0077] Polymer particles with too great a particle size lower the
swell rate. The proportion of excessively large polymer particles
should therefore likewise be small.
[0078] Excessively large polymer particles are therefore typically
removed and recycled into the grinding of the dried polymer
gel.
[0079] To further improve the properties, the polymer particles can
be surface postcrosslinked. Suitable surface postcrosslinkers are
compounds which comprise groups which can form covalent bonds with
at least two carboxylate groups of the polymer particles. Suitable
compounds are, for example, polyfunctional amines, polyfunctional
amido amines, polyfunctional epoxides, as described in EP 0 083 022
A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional
alcohols, as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0
450 922 A2, or .beta.-hydroxyalkylamides, as described in DE 102 04
938 A1 and U.S. Pat. No. 6,239,230.
[0080] Additionally described as suitable surface postcrosslinkers
are cyclic carbonates in DE 40 20 780 C1, 2-oxazolidone and its
derivatives, such as 2-hydroxyethyl-2-oxazolidone in DE 198 07 502
A1, bis- and poly-2-oxazolidinones in DE 198 07 992 01,
2-oxotetrahydro-1,3-oxazine and its derivatives in DE 198 54 573
A1, N-acyl-2-oxazolidones in DE 198 54 574 A1, cyclic ureas in DE
102 04 937 A1, bicyclic amide acetals in DE 103 34 584 A1, oxetanes
and cyclic ureas in EP 1 199 327 A2 and morpholine-2,3-dione and
its derivatives in WO 2003/031482 A1.
[0081] Preferred surface postcrosslinkers are ethylene carbonate,
ethylene glycol diglycidyl ether, reaction products of polyamides
with epichlorohydrin, and mixtures of propylene glycol and
1,4-butanediol.
[0082] Very particularly preferred surface postcrosslinkers are
2-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and
1,3-propanediol.
[0083] In addition, it is also possible to use surface
postcrosslinkers which comprise additional polymerizable
ethylenically unsaturated groups, as described in DE 37 13 601
A1.
[0084] The amount of surface postcrosslinkers is preferably 0.001
to 2% by weight, more preferably 0.02 to 1% by weight, most
preferably 0.05 to 0.2% by weight, based in each case on the
polymer particles.
[0085] In a preferred embodiment of the present invention,
polyvalent cations are applied to the particle surface in addition
to the surface postcrosslinkers before, during or after the surface
postcrosslinking.
[0086] The polyvalent cations usable in the process according to
the invention are, for example, divalent cations such as the
cations of zinc, magnesium, calcium, iron and strontium, trivalent
cations such as the cations of aluminum, iron, chromium, rare
earths and manganese, tetravalent cations such as the cations of
titanium and zirconium. Possible counterions are chloride, bromide,
sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate,
phosphate, hydrogenphosphate, dihydrogenphosphate and carboxylate,
such as acetate, tartate, citrate and lactate. Aluminum sulfate,
basic aluminum acetate and aluminum lactate are preferred. Apart
from metal salts, it is also possible to use polyamines as
polyvalent cations.
[0087] The amount of polyvalent cation used is, for example, 0.001
to 1.5% by weight, preferably 0.005 to 1% by weight, more
preferably 0.02 to 0.8% by weight, based in each case on the
polymer particles.
[0088] The surface postcrosslinking is typically performed in such
a way that a solution of the surface postcrosslinker is sprayed
onto the dried polymer particles. After the spraying, the polymer
particles coated with surface postcrosslinker are dried thermally,
and the surface postcrosslinking reaction can take place either
before or during the drying.
[0089] The spraying of a solution of the surface postcrosslinker is
preferably performed in mixers with moving mixing tools, such as
screw mixers, disk mixers and paddle mixers. Particular preference
is given to horizontal mixers such as paddle mixers, very
particular preference to vertical mixers. The distinction between
horizontal mixers and vertical mixers is made by the position of
the mixing shaft, i.e. horizontal mixers have a horizontally
mounted mixing shaft and vertical mixers a vertically mounted
mixing shaft. Suitable mixers are, for example, horizontal
Pflugschar.RTM. plowshare mixers (Gebr. Lodige Maschinenbau GmbH;
Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa
Micron BV; Doetinchem; the Netherlands), Processall Mixmill mixers
(Processall Incorporated; Cincinnati; US) and Schugi Flexomix.RTM.
(Hosokawa Micron BV; Doetinchem; the Netherlands). However, it is
also possible to spray on the surface postcrosslinker solution in a
fluidized bed.
[0090] The surface postcrosslinkers are typically used in the form
of an aqueous solution. The content of nonaqueous solvent and/or
total amount of solvent can be used to adjust the penetration depth
of the surface postcrosslinker into the polymer particles.
[0091] When exclusively water is used as the solvent, a surfactant
is advantageously added. This improves the wetting performance and
reduces the tendency to form lumps. However, preference is given to
using solvent mixtures, for example isopropanol/water,
1,3-propanediol/water and propylene glycol/water, where the mixing
ratio is preferably from 20:80 to 40:60.
[0092] The thermal drying is preferably carried out in contact
driers, more preferably paddle driers, most preferably disk driers.
Suitable driers are, for example, Hosokawa Bepex.RTM. horizontal
paddle driers (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa
Bepex.RTM. disk driers (Hosokawa Micron GmbH; Leingarten; Germany)
and Nara paddle driers (NARA Machinery Europe; Frechen; Germany).
Moreover, it is also possible to use fluidized bed driers.
[0093] The drying can be effected in the mixer itself, by heating
the jacket or blowing in warm air. Equally suitable is a downstream
drier, for example a shelf drier, a rotary tube oven or a heatable
screw. It is particularly advantageous to mix and dry in a
fluidized bed drier. Preferred drying temperatures are in the range
from 100 to 250.degree. C., preferably 120 to 220.degree. C., more
preferably 130 to 210.degree. C., most preferably 150 to
200.degree. C. The preferred residence time at this temperature in
the reaction mixer or drier is preferably at least 10 minutes, more
preferably at least 20 minutes, most preferably at least 30
minutes, and typically at most 60 minutes.
[0094] Subsequently, the surface postcrosslinked polymer particles
can be classified again, excessively small and/or excessively large
polymer particles being removed and recycled into the process.
[0095] To further improve the properties, the surface
postcrosslinked polymer particles can be coated or
remoisturized.
[0096] The optional remoisturizing is carried out preferably at 30
to 80.degree. C., more preferably at 35 to 70.degree. C. and most
preferably at 40 to 60.degree. C. At excessively low temperatures,
the water-absorbing polymer particles tend to form lumps, and, at
higher temperatures, water already evaporates noticeably. The
amount of water used for remoisturizing is preferably from 1 to 10%
by weight, more preferably from 2 to 8% by weight and most
preferably from 3 to 5% by weight. The remoisturizing increases the
mechanical stability of the polymer particles and reduces their
tendency to static charging.
[0097] Suitable coatings for improving the swell rate and the
saline flow conductivity or gel bed permeability (SFC or GBP) are,
for example, inorganic inert substances, such as water-insoluble
metal salts, organic polymers, cationic polymers and di- or
polyvalent metal cations. Suitable coatings for dust binding are,
for example, polyols. Suitable coatings for counteracting the
undesired caking tendency of the polymer particles are, for
example, fumed silica, such as Aerosil.RTM. 200, and surfactants,
such as Span.RTM. 20.
[0098] The water-absorbing polymer particles produced by the
process according to the invention have a moisture content of
preferably 0 to 15% by weight, more preferably 0.2 to 10% by
weight, most preferably 0.5 to 8% by weight, the moisture content
being determined by EDANA (European Disposables and Nonwovens
Association) recommended test method No. WSP 230.2-05 "Moisture
Content".
[0099] The water-absorbing polymer particles produced by the
process according to the invention have a centrifuge retention
capacity (CRC) of typically at least 15 g/g, preferably at least 20
g/g, preferentially at least 22 g/g, more preferably at least 24
g/g, most preferably at least 26 g/g. The centrifuge retention
capacity (CRC) of the water-absorbing polymer particles is
typically less than 60 g/g. The centrifuge retention capacity (CRC)
is determined by EDANA (European Disposables and Nonwovens
Association) recommended test method No. WSP 241.2-05 "Centrifuge
Retention Capacity".
[0100] The water-absorbing polymer particles produced by the
process according to the invention have an absorption under a
pressure of 49.2 g/cm.sup.2 of typically at least 15 g/g,
preferably at least 20 g/g, preferentially at least 22 g/g, more
preferably at least 24 g/g, most preferably at least 26 g/g. The
absorption under a pressure of 49.2 g/cm.sup.2 of the
water-absorbing polymer particles is typically less than 35 g/g.
The absorption under a pressure of 49.2 g/cm.sup.2 is determined
analogously to EDANA (European Disposables and Nonwovens
Association) recommended test method No. WSP 242.2-05 "Absorption
under Pressure", except that a pressure of 49.2 g/cm.sup.2 is
established instead of a pressure of 21.0 g/cm.sup.2.
[0101] B. Hygiene Articles
[0102] The hygiene articles, especially disposable diapers, consist
of [0103] (A) an upper liquid-pervious layer, [0104] (B) a lower
liquid-impervious layer, [0105] (C) a water-absorbing storage layer
(core) between layer (A) and layer (B), and [0106] (D) optionally
an acquisition distribution layer between layer (A) and layer
(C).
[0107] The upper liquid-pervious layer (A) is the layer which has
direct contact with the skin.
[0108] The material for this consists of customary synthetic or
semisynthetic fibers, such as polyesters, polyolefins and rayon, or
of customary natural fibers, such as cotton. In the case of
nonwoven materials, the fibers should generally be bonded by
binders such as polyacrylates. Preferred materials are polyester,
rayon, polyethylene and polypropylene. Examples of liquid-pervious
layers are described, for example, in WO 99/57355 A1 and EP 1 023
883 A2.
[0109] The lower liquid-impervious layer (B) consists typically of
a polyethylene or polypropylene film. However, it may also consist
of any other film-forming polymer, for example of polyester,
polyamide, especially biodegradable polyester.
[0110] The inventive water-absorbing storage layers are essentially
free of cellulose fibers or have a proportion of cellulose fibers
of preferably less than 30% by weight, preferentially less than 20%
by weight, more preferably less than 10% by weight, most preferably
less than 5% by weight. The water-absorbing polymer particles
usable are not subject to any restriction. Preference is given,
however, to using water-absorbing polymer particles with a saline
flow conductivity (SFC) of 50 to 150.times.10.sup.-7 cm.sup.3 s/g,
the saline flow conductivity (SFC) being determinable by the method
described in WO 2008/092843 A1 (page 30, lines 16 to 36).
[0111] It is likewise possible to use water-absorbing polymer
particles with a gel bed permeability (GBP) of 10 to 100 darcies.
In a particular embodiment, water-absorbing polymer particles with
a gel bed permeability (GBP) of 100 to 1000 darcies are used. The
gel bed permeability (GBP) is determined to US 2005/0256757.
[0112] It is additionally advantageous to use water-absorbing
polymer particles with a centrifuged retention capacity (CRC) of at
least 33 g/g and an absorption under pressure of 49.2
g/cm.sup.2(AUL0.7 psi) of at least 12 g/g.
[0113] It is additionally advantageous when the absorption rate of
the water-absorbing polymer particles for aqueous body fluids is
adjusted optimally to the particular demands in the water-absorbing
storage layer. To determine the absorption rate, preference is
given to using the vortex test described in the literature, for
example in the monograph "Modern Superabsorbent Polymer
Technology". F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, on
pages 156 and 157. The vortex times of the water-absorbing polymer
particles should be less than 120 seconds, preferably less than 80
seconds, preferentially less than 50 seconds, more preferably less
than 40 seconds, most preferably less than 20 seconds.
[0114] The acquisition distribution layer (D) consists typically of
cellulose fibers, modified cellulose or synthetic fibers, and has
the task of rapidly absorbing aqueous liquids, for example urine,
and passing them on to the water-absorbing layer (C).
[0115] For the acquisition distribution layer (D), preferably
modified, more preferably chemically modified, most preferably
chemically stiffened, cellulose fibers are used. Suitable agents
for chemical stiffening are cationically modified starches,
polyamide-epichlorohydrin resins, polyacrylamides,
urea-formaldehyde resins, melamine-formaldehyde resins and
polyethyleneimine resins.
[0116] The stiffening can also be effected by modifying the
chemical structure, for example by crosslinking. The crosslinkers
can crosslink the polymer chains by formation of covalent bonds.
Suitable crosslinkers are, for example, C.sub.2- to
C.sub.8-dialdehydes, C.sub.2- to C.sub.8-monoaldehydes with a
carboxylic acid group and C.sub.2- to C.sub.8-dicarboxylic
acids.
[0117] According to the present invention, improved water-absorbing
storage layers are obtained, as are hygiene articles which comprise
them. The separation of liquid storage and liquid conduction can
firstly significantly lower material consumption, especially of
fibers, for production of the storage layers; secondly, thin and
soft hygiene articles are obtained, which have outstanding
integrity when dry and in use, since the water-absorbing polymer
particles can be fixed significantly more efficiently.
* * * * *