U.S. patent application number 14/266367 was filed with the patent office on 2014-10-09 for method for producing water-absorbent polymer particles with a higher permeability by polymerizing droplets of a monomer solution.
This patent application is currently assigned to BASF SE. The applicant listed for this patent is BASF SE. Invention is credited to Stefan Blei, Rudiger Funk, Wilfried Heide, Annemarie Hillebrecht, Marco Kruger, Dennis Losch, Volker Seidl, Uwe Stueven, Matthias Weismantel.
Application Number | 20140302321 14/266367 |
Document ID | / |
Family ID | 38577537 |
Filed Date | 2014-10-09 |
United States Patent
Application |
20140302321 |
Kind Code |
A1 |
Stueven; Uwe ; et
al. |
October 9, 2014 |
METHOD FOR PRODUCING WATER-ABSORBENT POLYMER PARTICLES WITH A
HIGHER PERMEABILITY BY POLYMERIZING DROPLETS OF A MONOMER
SOLUTION
Abstract
A process for preparing water-absorbing polymer beads with high
permeability by polymerizing droplets of a monomer solution in a
gas phase surrounding the droplets, wherein a water-insoluble
inorganic salt is suspended in the monomer solution and the polymer
beads have a mean diameter of at least 150 .mu.m.
Inventors: |
Stueven; Uwe; (Bad Soden,
DE) ; Weismantel; Matthias; (Jossgrund-Oberndorf,
DE) ; Heide; Wilfried; (Freinsheim, DE) ;
Kruger; Marco; (Mannheim, DE) ; Seidl; Volker;
(Mannheim, DE) ; Blei; Stefan; (Mannheim, DE)
; Losch; Dennis; (Altrip, DE) ; Funk; Rudiger;
(Niedernhausen, DE) ; Hillebrecht; Annemarie;
(Kunzell, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BASF SE |
Ludwigshafen |
|
DE |
|
|
Assignee: |
BASF SE
Ludwigshafen
DE
|
Family ID: |
38577537 |
Appl. No.: |
14/266367 |
Filed: |
April 30, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12306800 |
Dec 29, 2008 |
8748000 |
|
|
PCT/EP2007/057141 |
Jul 12, 2007 |
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14266367 |
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Current U.S.
Class: |
428/402 ;
264/12 |
Current CPC
Class: |
C08F 220/06 20130101;
A61L 15/24 20130101; A61L 15/24 20130101; B01J 10/002 20130101;
C08F 2/10 20130101; C08F 2/44 20130101; Y10T 428/2982 20150115;
Y10T 428/2989 20150115; C08F 222/1006 20130101; C08F 2/00 20130101;
A61L 15/60 20130101; C08L 33/02 20130101 |
Class at
Publication: |
428/402 ;
264/12 |
International
Class: |
B01J 10/00 20060101
B01J010/00; C08F 220/06 20060101 C08F220/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 19, 2006 |
EP |
06117487.6 |
Claims
1. A process for preparing water-absorbing polymer beads comprising
polymerizing droplets of a monomer solution comprising a) at least
one ethylenically unsaturated monomer, b) at least one crosslinker,
c) at least one initiator, and d) water, in a gas phase surrounding
the droplets, wherein a water-insoluble inorganic salt is suspended
in the monomer solution and the polymer beads have a mean diameter
of at least 150 .mu.m.
2. The process according to claim 1, wherein the monomer a) has at
least one acid group.
3. The process according to claim 2, wherein the acid groups of the
monomer a) are at least partly neutralized.
4. The process according to claim 1, wherein monomer a) is acrylic
acid to an extent of at least 50 mol %.
5. The process according to claim 1, wherein the polymer beads have
a mean diameter of at least 200 .mu.m.
6. The process according to claim 1, wherein at least 90% by weight
of the polymer beads have a diameter of from 100 to 800 .mu.m.
7. The process according to claim 1, wherein a carrier gas flows
through a reaction chamber.
8. The process according to claim 7, wherein the carrier gas
leaving the reaction chamber is recycled at least partly after one
pass.
9. The process according to claim 7, wherein an oxygen content of
the carrier gas is from 0.001 to 0.15% by volume.
10. The process according to claim 1, wherein the polymer beads are
dried and/or postcrosslinked in at least one further process
step.
11. Water-absorbing polymer beads prepared according to the process
of claim 1.
12-21. (canceled)
22. A hygiene article comprising polymer beads prepared according
to the process of claim 1.
Description
[0001] The present invention relates to a process for preparing
water-absorbing polymer beads with high permeability by
polymerizing droplets of a monomer solution in a gas phase
surrounding the droplets, wherein a water-insoluble inorganic salt
is suspended in the monomer solution and the polymer beads have a
mean diameter of at least 150 .mu.m.
[0002] The preparation of water-absorbing polymer beads is
described in the monograph "Modern Superabsorbent Polymer
Technology", F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998,
pages 71 to 103.
[0003] Being products which absorb aqueous solutions,
water-absorbing polymers are used to produce diapers, tampons,
sanitary napkins and other hygiene articles, but also as
water-retaining agents in market gardening.
[0004] The properties of the water-absorbing polymers can be
adjusted via the degree of crosslinking. With increasing
crosslinking, the gel strength rises and the absorption capacity
falls. This means that the centrifuge retention capacity (CRC)
decreases with increasing absorbency under load (AUL) (at very high
degrees of crosslinking, the absorbency under load also decreases
again).
[0005] To improve the use properties, for example saline flow
conductivity (SFC) in the swollen gel bed in the diaper and
absorbency under load (AUL), water-absorbing polymer beads are
generally postcrosslinked. This increases only the degree of
crosslinking of the bead surface, which allows absorbency under
load (AUL) and centrifuge retention capacity (CRC) to be decoupled
at least partly. This postcrosslinking can be performed in aqueous
gel phase. However, preference is given to coating ground and
screened polymer beads (base polymer) with a postcrosslinker on the
surface, thermally postcrosslinking them and drying them.
Crosslinkers suitable for this purpose are compounds which comprise
at least two groups which can form covalent bonds with the
carboxylate groups of the hydrophilic polymer.
[0006] Spray polymerization combines the process steps of
polymerization and drying. In addition, the particle size is set
within certain limits by suitable process control.
[0007] The preparation of water-absorbing polymer beads by
polymerizing droplets of a monomer solution is described, for
example, in EP-A 0 348 180, WO 96/40427, U.S. Pat. No. 5,269,980,
DE-A 103 14 466, DE-A 103 40 253 and DE-A 10 2004 024 437, and also
the prior German applications 10 2005 002 412.2 and 10 2006 001
596.7. DE-A 10 2004 042 946, DE-A 10 2004 042 948 and DE-A 10 2004
042 955, and also the prior German application having the reference
number 10 2005 019 398.6, describe the preparation of thickeners by
spray polymerization.
[0008] It was an object of the present invention to provide a
process for preparing water-absorbing polymer beads with high
permeability, i.e. high saline flow conductivity through the
swollen gel bed.
[0009] The object is achieved by a process for preparing
water-absorbing polymer 10 beads by polymerizing droplets of a
monomer solution comprising
a) at least one water-soluble ethylenically unsaturated monomer, b)
at least one crosslinker, c) at least one initiator, d) water, in a
gas phase surrounding the droplets, wherein a water-insoluble
inorganic salt is suspended in the monomer solution and the polymer
beads have a mean diameter of at least 150 .mu.m.
[0010] The water-absorbing polymer beads obtainable by the process
according to the invention have a permeability (SFC) of typically
at least 5.times.10.sup.-7 cm.sup.3 s/g, preferably at least
15.times.10.sup.-7 cm.sup.3 s/g, preferably at least
35.times.10.sup.-7 cm.sup.3 s/g, more preferably at least
50.times.10.sup.-7 cm.sup.3 s/g, most preferably at least
120.times.10.sup.-7 cm.sup.3 s/g. The permeability (SFC) of the
water-absorbing polymer beads is typically less than
500.times.10.sup.-7 cm.sup.3 s/g.
[0011] The water-absorbing polymer beads obtainable by the process
according to the invention have a centrifuge retention capacity
(CRC) of typically at least 10 g/g, preferably at least 15 g/g,
preferentially at least 20 gig, more preferably at least 25 g/g,
most preferably at least 30 gig. The centrifuge retention capacity
(CRC) of the water-absorbing polymer beads is typically less than
50 g/g.
[0012] The water-insoluble inorganic salts can be added to the
monomer solution as a powder or as a dispersion in a suitable
dispersion medium, for example water.
[0013] Water-insoluble means a solubility of less than 2 g,
preferably of less than 0.1 g, more preferably of less than 0.01 g,
in 100 ml of water at 25.degree. C.
[0014] The water-insoluble inorganic salts have a mean particle
size of typically less than 400 .mu.m, preferably less than 100
.mu.m, preferentially less than 50 .mu.m, more preferably of less
than 10 .mu.m; very particular preference is given to the particle
size range of from 2 to 7 .mu.m.
[0015] Suitable water-insoluble inorganic salts are, for example,
calcium sulfate and calcium phosphate.
[0016] Preference is given to using calcium phosphate.
[0017] The amount of water-insoluble inorganic salt which is added
to the monomer solution is typically at least 0.1% by weight,
preferably at least 0.5% by weight, preferentially at least 1% by
weight, more preferably at least 2% by weight, most preferably at
least 5% by weight, based in each case on monomer a). Excessively
high amounts are difficult to disperse, i.e. a content of more than
25% by weight, based again on the monomer a), is therefore not
appropriate.
[0018] The mean diameter of the polymer beads is preferably at
least 200 .mu.m, more preferably from 250 to 600 .mu.m, very
particularly from 300 to 500 .mu.m, the bead diameter being
determinable by light scattering and meaning the volume-average
mean diameter. 90% of the polymer beads have a diameter of
preferably from 100 to 800 .mu.m, more preferably from 150 to 700
.mu.m, most preferably from 200 to 600 .mu.m.
[0019] The oxygen content of the gas phase is preferably from 0.001
to 0.15% by volume, more preferably from 0.002 to 0.1% by volume,
most preferably from 0.005 to 0.05% by volume.
[0020] In addition to oxygen, the gas phase comprises preferably
only inert gases, i.e. gases which do not intervene in the
polymerization under reaction conditions, for example nitrogen
and/or water vapor.
[0021] 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 50 g/100 g
of water, and preferably have at least one acid group each.
[0022] The concentration of the monomers a) in the monomer solution
is typically from 2 to 80% by weight, preferably from 5 to 70% by
weight, more preferably from 10 to 60% by weight.
[0023] Suitable monomers a) are, for example, ethylenically
unsaturated carboxylic acids such as acrylic acid, methacrylic
acid, maleic acid, fumaric acid and itaconic acid. Particularly
preferred monomers are acrylic acid and methacrylic acid. Very
particular preference is given to acrylic acid.
[0024] The preferred monomers a) have at least one acid group, the
acid groups preferably having been at least partly neutralized.
[0025] 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 % and most preferably at least 95 mol
%.
[0026] The acid groups of the monomers a) have typically been
neutralized partly, preferably to an extent of from 25 to 85 mol %,
preferentially to an extent of from 50 to 80 mol %, more preferably
to an extent of from 60 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 mixtures thereof. Instead of alkali
metal salts, it is also possible to use ammonium salts. Sodium and
potassium are particularly preferred alkali metals, but very
particular preference is given to sodium hydroxide, sodium
carbonate or sodium hydrogencarbonate and mixtures thereof.
Typically, the neutralization is achieved by mixing in the
neutralizing agent as an aqueous solution, as a melt or else
preferably as a solid. For example, sodium hydroxide may be present
with a water content significantly below 50% by weight as a waxy
mass with a melting point above 23.degree. C. In this case,
metering as piece material or a melt at elevated temperature is
possible.
[0027] The monomers a), especially acrylic acid, comprise
preferably up to 0.025% by weight of a hydroquinone monoether.
Preferred hydroquinone monoethers are hydroquinone monomethyl ether
(MEHQ) and/or tocopherols.
[0028] Tocopherol is understood to mean compounds of the following
formula
##STR00001##
where R.sup.1 is hydrogen or methyl, R.sup.2 is hydrogen or methyl,
R.sup.3 is hydrogen or methyl, and R.sup.4 is hydrogen or an acyl
radical having from 1 to 20 carbon atoms.
[0029] Preferred radicals for R.sup.4 are acetyl, ascorbyl,
succinyl, nicotinyl and other physiologically compatible carboxylic
acids. The carboxylic acids may be mono-, di- or tricarboxylic
acids.
[0030] Preference is given to alpha-tocopherol where
R.sup.1=R.sup.2=R.sup.3=methyl, in particular racemic
alpha-tocopherol. R.sup.1 is more preferably hydrogen or acetyl.
RRR-alpha-tocopherol is especially preferred.
[0031] The monomer solution comprises 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,
in particular around 50 ppm by weight, of hydroquinone monoether,
based In each case on acrylic acid, acrylic acid salts also being
considered as acrylic acid. For example, the monomer solution can
be prepared by using acrylic acid having an appropriate content of
hydroquinone monoether.
[0032] The crosslinkers b) are compounds having at least two
free-radically polymerizable groups which can be polymerized by a
free-radical mechanism into the polymer network. Suitable
crosslinkers b) are, for example, ethylene glycol dimethacrylate,
diethylene glycol diacrylate, allyl methacrylate,
trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane,
as described in EP-A-0 530 438, di- and triacrylates, as described
in EP-A-0 547 847, EP-A-0 559 476, EP-A-0 632 068, WO 93/21237, WO
03/104299, WO 031104300, WO 03/104301 and in DE-A-103 31 450, mixed
acrylates which, as well as acrylate groups, comprise further
ethylenically unsaturated groups, as described in DE-A-103 314 56
and DE-A 103 55 401, or crosslinker mixtures, as described, for
example, in DE-A-195 43 368, DE-A-196 46 484, WO 90/15830 and WO
02/32962.
[0033] Suitable crosslinkers b) are in particular
N,N'-methylenebisacrylamide and N,N'-methylenebismethacrylamide,
esters of unsaturated mono- or polycarboxylic acids of polyols,
such as diacrylate or triacrylate, for example butanediol
diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate
or ethylene glycol dimethacrylate, and also trimethyloipropane
triacrylate and allyl compounds such as allyl(meth)acrylate,
triallyl cyanurate, diallyl maleate, polyallyl esters,
tetraallyloxyethane, triallylamine, tetraallylethylenediamine,
allyl esters of phosphoric acid and vinyiphosphonic acid
derivatives, as described, for example, in EP-A-0 343 427. Further
suitable crosslinkers b) are pentaerythritol diallyl ether,
pentaerythritol trialtyf ether and pentaerythritol tetraally/ether,
polyethylene glycol diallyl ether, ethylene glycol diallyl ether,
glycerol diallyl ether and glycerol Wallyl ether, polyallyl ethers
based on sorbitol, and ethoxylated variants thereof. In the process
according to the invention, it is possible to use di(meth)acrylates
of polyethylene glycols, the polyethylene glycol used having a
molecular weight between 300 and 1000.
[0034] However, particularly advantageous crosslinkers b) are di-
and triacrylates of 3- to 15-tuply ethoxylated glycerol, of 3- to
15-tuply ethoxylated trimethylolpropane, of 3- to 15-tuply
ethoxylated trimethylolethane, in particular di- and triacrylates
of 2- to 6-tuply ethoxylated glycerol or of 2- to 6-tuply
ethoxylated trimethylolpropane, of 3-tuply propoxylated glycerol or
of 3-tuply propoxylated trimethylolpropane, and also of 3-tuply
mixed ethoxylated or propoxylated glycerol or of 3-tuply mixed
ethoxylated or propoxylated trimethylolpropane, of 15-tuply
ethoxylated glycerol or of 15-tuply ethoxylated trimethylolpropane,
and also of 40-tuply ethoxylated glycerol, of 40-tuply ethoxylated
trimethylolethane or of 40-tuply ethoxylated
trimethylolpropane.
[0035] 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 03/104301. 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. These feature
particularly low residual contents (typically below 10 ppm by
weight) in the water-absorbing polymer, and the aqueous extracts of
the water-absorbing polymers thus produced have an almost unchanged
surface tension (typically at least 0.068 N/m) in comparison to
water at the same temperature.
[0036] The monomer solution comprises typically at least 0.2% by
weight, preferably at least 0.4% by weight, preferentially at least
0.6% by weight, more preferably at least 0.8% by weight and most
preferably at least 1.5% by weight, of crosslinker b), based in
each case on monomer a).
[0037] The initiators c) used may be all compounds which
disintegrate into free radicals under the polymerization
conditions, for example peroxides, hydroperoxides, hydrogen
peroxide, persulfates, azo compounds and redox initiators.
Preference is given to the use of water-soluble initiators. In some
cases, it is advantageous to use mixtures of various initiators,
for example mixtures of hydrogen peroxide and sodium or potassium
peroxodisulfate. Mixtures of hydrogen peroxide and sodium
peroxodisulfate can be used in any proportion.
[0038] Particularly preferred initiators c) are azo initiators such
as 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochioride and
2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride,
and photoinitiators such as 2-hydroxy-2-methylpropiophenone and
1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one,
redox initiators such as sodium persulfate/hydroxymethylsulfinic
acid, ammonium peroxodisulfate/hydroxymethylsulfinic acid, hydrogen
peroxide/hydroxymethylsulfinic acid, sodium persulfate/ascorbic
acid, ammonium peroxodisulfate/ascorbic acid and hydrogen
peroxide/ascorbic acid, photoinitiators such as
1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one and
mixtures thereof.
[0039] The initiators are used in customary amounts, for example in
amounts of from 0.001 to 5% by weight, preferably from 0.01 to 1%
by weight, based on the monomers a).
[0040] For optimal action, the preferred polymerization inhibitors
require dissolved oxygen. Therefore, the monomer solution can be
freed of dissolved oxygen before the polymerization by
inertization, i.e. flowing through with an inert gas, preferably
nitrogen. 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.
[0041] The polymerization inhibitors can also be removed by
absorption, for example on activated carbon.
[0042] For the polymerization in the gas phase, the monomer
solution can be dropletized.
[0043] The polymerization in the monomer solution droplets
preferably takes place in homogeneous phase, not taking account of
inhoniogeneities caused by the suspended inorganic salt. This means
that the monomer solution is homogeneous and that the monomer
solution remains homogeneous even during the polymerization. The
polymer may swell during the polymerization but not precipitate out
and form a second phase in the droplet. Otherwise, several polymer
nuclei would form in each droplet and form agglomerates of very
small primary particles during the drying. The aim of the process
according to the invention is the production of one primary
particle each per droplet. The monomers a) and the crosslinkers b)
are therefore to be selected such that the resulting polymer is
swellable in the aqueous phase of the droplet.
[0044] The process according to the invention is preferably
performed in the absence of hydrophobic inert solvents. Hydrophobic
inert solvents are virtually all water-immiscible liquids which do
not intervene in the polymerization, i.e. comprise no polymerizable
groups. Water-immiscible means that the solubility of the
hydrophobic solvents in water is less than 5 g/100 g, preferably
less than 1 g/100 g, more preferably less than 0.5 g/100 g.
[0045] The dropletization involves metering a monomer solution into
the gas phase to form droplets. The dropletization of the monomer
solution can be carried out, for example, by means of a dropletizer
plate.
[0046] A dropletizer plate is a plate having at least one bore, the
liquid entering the bore from the top. The dropletizer plate or the
liquid can be oscillated, which generates a chain of ideally
monadisperse droplets at each bore on the underside of the
dropletizer plate.
[0047] The number and size of the bores is selected according to
the desired capacity and droplet size. The droplet diameter is
typically 1.9 times the diameter of the bore. What is important
here is that the liquid to be dropletized does not pass through the
bore too rapidly and the pressure drop over the bore is not too
great. Otherwise, the liquid is not dropletized, but rather the
liquid jet is broken up (sprayed) owing to the high kinetic energy.
The dropletizer is operated in the flow range of laminar jet
decomposition, i.e. the Reynolds number based on the throughput per
bore and the bore diameter is preferably less than 2000,
preferentially less than 1000, more preferably less than 500 and
most preferably less than 100. The pressure drop over the bore is
preferably less than 2.5 bar, more preferably less than 1.5 bar and
most preferably less than 1 bar. The dropletizer plate has
typically at least one bore, preferably at least 10, more
preferably at least 50 and typically up to 10 000 bores, preferably
up to 5000, more preferably up to 1000 bores, the bores typically
being distributed uniformly over the dropletizer plate, preferably
in so-called triangular pitch, i.e. three bores in each case form
the corners of an equilateral triangle.
[0048] The diameter of the bores is adjusted to the desired droplet
size.
[0049] It may be advantageous to place the dropletizer plate onto a
carrier plate, the carrier plate likewise having bores. In this
case, the bores of the carrier plate have a greater diameter than
the bores of the dropletizer plate and are arranged such that below
each bore of the dropletizer plate is disposed a concentric bore of
the carrier plate. This arrangement enables a rapid exchange of the
dropletizer plate, for example in order to generate droplets of
another size.
[0050] However, the dropletization can also be carried out by means
of pneumatic drawing dies, rotation, cutting of a jet or rapidly
actuable microvalve dies.
[0051] In a pneumatic drawing die, a liquid jet together with a gas
stream is accelerated through a hole diaphragm. The gas rate can be
used to influence the diameter of the liquid jet and hence the
droplet diameter.
[0052] In the case of dropletization by rotation, the liquid passes
through the orifices of a rotating disk. As a result of the
centrifugal force acting on the liquid, droplets of defined size
are torn off. Rotary dropletization is described, for example, in
DE-A 4308842 and U.S. Pat. No. 6,338,438.
[0053] The emerging liquid jet can also be cut into defined
segments by means of a rotating blade. Each segment then forms a
droplet.
[0054] In the case of use of microvalve dies, droplets with defined
liquid volume are generated directly.
[0055] The gas phase preferably flows as carrier gas through the
reaction chamber, The carrier gas can be conducted through the
reaction chamber in cocurrent or in countercurrent to the
free-falling droplets of the monomer solution, preferably in
cocurrent. After one pass, the carrier gas is preferably recycled
at least partly, preferably to an extent of at least 50%, more
preferably to an extent of at least 75%, into the reaction chamber
as cycle gas. Typically, a portion of the carrier gas is discharged
after each pass, preferably up to 10%, more preferably up to 3% and
most preferably up to 1%.
[0056] The polymerization is preferably carried out in a laminar
gas flow. A laminar gas flow is a gas flow in which the individual
layers of the flow do not mix but rather move in parallel. A
measure of the flow conditions is the Reynolds number (Re). Below a
critical Reynolds number (Re.sub.crit) of 2300, the gas flow is
laminar. The Reynolds number of the laminar gas flow is preferably
less than 2000, more preferably less than 1500 and most preferably
less than 1000. The lower limiting case of the laminar inert gas
flow is a standing inert gas atmosphere (Re=0), i.e. inert gas is
not fed in continuously.
[0057] The gas velocity is preferably adjusted such that the flow
in the reactor is directed, for example no convection currents
opposed to the general flow direction are present, and is, for
example, from 0.1 to 2 m/s, preferably from 0.5 to 1.8 m/s, more
preferably from 1 to 1.5 m/s.
[0058] The carrier gas is appropriately preheated to the reaction
temperature upstream of the reactor.
[0059] The reaction temperature in the thermally induced
polymerization is preferably from 70 to 250.degree. C., more
preferably from 100 to 220.degree. C. and most preferably from 120
to 200.degree. C.
[0060] The reaction can be carried out under elevated pressure or
under reduced pressure; preference is given to a reduced pressure
of up to 100 mbar relative to ambient pressure.
[0061] The reaction offgas, i.e. the carrier gas leaving the
reaction chamber, may, for example, be cooled in a heat exchanger.
This condenses water and unconverted monomer a). The reaction
offgas can then be reheated at least partly and recycled into the
reactor as cycle gas. A portion of the reaction offgas can be
discharged and replaced by fresh carrier gas, in which case water
and unconverted monomers a) present in the reaction offgas can be
removed and recycled.
[0062] Particular preference is given to a thermally integrated
system, i.e. a portion of the waste heat in the cooling of the
offgas is used to heat the cycle gas.
[0063] The reactors can be trace-heated. in this case, the trace
heating is adjusted such that the wall temperature is at least
5.degree. C. above the internal reactor temperature and
condensation on the reactor walls is reliably prevented.
[0064] The reaction product can be withdrawn from the reactor in a
customary manner, for example at the bottom by means of a conveying
screw, and, if appropriate, dried down to the desired residual
moisture content and to the desired residual monomer content.
[0065] Of course, the polymer beads can subsequently be
postcrosslinked for further improvement of the properties.
[0066] Suitable postcrosslinkers are compounds which comprise at
least two groups which can form covalent bonds with the carboxylate
groups of the hydrogel. Suitable compounds are, for example,
alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines,
di- or polyglycidyl compounds, as described in EP-A-0 083 022,
EP-A-0 543 303 and EP-A-0 937 736, di- or polyfunctional alcohols
as described in DE-C-33 14 019, DE-C-35 23 617 and EP-A-0 450 922,
or .beta.-hydroxyalkylamides, as described in DE-A-102 04 938 and
U.S. Pat. No. 6,239,230.
[0067] In addition, DE-A-40 20 780 describes cyclic carbonates,
DE-A-198 07 502 describes 2-oxazolidone and its derivatives such as
2-hydroxyethyl-2-oxazolidone, DE-A198 07 992 describes Ws- and
poly-2-oxazolidinones, DE-A-198 54 573 describes
2-oxotetrahydro-1,3-oxazine and its derivatives, DE-A-198 54 574
describes N-acyl-2-oxazolidones, DE-A-102 04 937 describes cyclic
ureas, DE-A-103 34 584 describes bicyclic amide acetals, EP-A-1 199
327 describes oxetanes and cyclic ureas, and WO 03/031482 describes
morpholine-2,3-dione and its derivatives, as suitable
postcrosslinkers.
[0068] The process according to the invention enables the
preparation of water-absorbing polymer beads having a high
permeability (SFC) and a high centrifuge retention capacity (CRC).
For this combination of properties, an additional postcrosslinking
step has to date been absolutely necessary.
[0069] The present invention further provides water-absorbing
polymer beads which are obtainable by the process according to the
invention.
[0070] The inventive water-absorbing polymer beads have a content
of hydrophobic solvent of typically less than 0.005% by weight,
preferably less than 0.002% by weight, more preferably less than
0.001% by weight and most preferably less than 0.0005% by weight.
The content of hydrophobic solvent can be determined by gas
chromatography, for example by means of the headspace
technique.
[0071] Polymer beads which have been obtained by reverse suspension
polymerization still comprise typically approx. 0.01% by weight of
the hydrophobic solvent used as the reaction medium.
[0072] The inventive water-absorbing polymer beads have a
surfactant content of typically less than 1% by weight, preferably
less than 0.5% by weight, more preferably less than 0.1% by weight
and most preferably less than 0.05% by weight.
[0073] Polymer beads which have been obtained by reverse suspension
polymerization still comprise typically at least 1% by weight of
the surfactant used to stabilize the suspension.
[0074] The inventive water-absorbing polymer beads are
approximately round, i.e. the polymer beads have a mean sphericity
of typically at least 0.84, preferably at least 0.86, more
preferably at least 0.88 and most preferably at least 0.9. The
sphericity (SPHT) is defined as
SPHT = 4 .pi. A U 2 ##EQU00001##
where A is the cross-sectional area and U is the cross-sectional
circumference of the polymer beads. The mean sphericity is the
volume-average sphericity.
[0075] The mean sphericity can be determined, for example, with the
Carnsizer.RTM. image analysis system (Retsch Technolgy GmbH;
Germany):
[0076] For the measurement, the product is introduced through a
funnel and conveyed to the falling shaft with a metering channel.
While the particles fall past a light wall, they are recorded
selectively by a camera. The images recorded are evaluated by the
software in accordance with the parameters selected.
[0077] To characterize the roundness, the parameters designated as
sphericity in the program are employed. The parameters reported are
the mean volume-weighted sphericities, the volume of the particles
being determined via the equivalent diameter xcmin. To determine
the equivalent diameter xcee, the longest chord diameter for a
total of 32 different spatial directions is measured in each case.
The equivalent diameter xcmin, is the shortest of these 32 chord
diameters. The equivalent diameter xcmin, corresponds to the mesh
size of a screen that the particle can just pass through. To record
the particles, the so-called CCD-zoom camera (CAM-Z) is used. To
control the metering channel, a surface coverage fraction of 0.5%
is predefined.
[0078] Polymer beads with relatively low sphericity are obtained by
reverse suspension polymerization when the polymer beads are
agglomerated during or after the polymerization.
[0079] The water-absorbing polymer beads prepared by customary
solution polymerization (gel polymerization) are ground and
classified after drying to obtain irregular polymer beads. The mean
sphericity of these polymer beads is between approx. 0.72 and
approx. 0.78.
[0080] The present invention further relates to the use of the
crosslinked water-absorbing polymer beads mentioned above in
hygiene articles. For example, the hygiene article may be
constructed as follows: [0081] (A) an upper liquid-pervious cover
[0082] (B) a lower liquid-impervious layer [0083] (C) a core
disposed between (A) and (B), comprising [0084] from 10 to 100% by
weight of the inventive water-absorbing polymer beads from 0 to 90%
by weight of hydrophilic fiber material [0085] preferably from 30
to 100% by weight of the inventive water-absorbing polymer beads,
from 0 to 70% by weight of hydrophilic fiber material more
preferably from 50 to 100% by weight of the inventive
water-absorbing polymer beads, from 0 to 60% by weight of
hydrophilic fiber material, [0086] especially preferably from 70 to
100% by weight of the inventive water-absorbing polymer beads, from
0 to 30% by weight of hydrophilic fiber material, and most
preferably from 90 to 100% by weight of the inventive
water-absorbing polymer beads, from 0 to 10% by weight of
hydrophilic fiber material, [0087] (D) if appropriate a tissue
layer disposed immediately above and below the core (C) and [0088]
(E) if appropriate an absorption layer disposed between (A) and
(C).
[0089] Hygiene articles are understood to mean, for example,
incontinence pads and incontinence briefs for adults or diapers for
babies.
[0090] The liquid-pervious cover (A) is the layer which is in
direct contact with the skin. The material for this consists of
customary synthetic or semisynthetic fibers or films of polyester,
polyolefins, rayon or natural fibers such as cotton. In the case of
nonwoven materials, the fibers should generally be bound by binders
such as poiyacrylates. Preferred materials are polyester, rayon and
blends thereof, polyethylene and polypropylene. Examples of
liquid-pervious layers are described in WO 99/57355, EP-A 1 023
883.
[0091] The liquid-impervious layer (B) consists generally of a film
of polyethylene or polypropylene,
[0092] In addition to the inventive water-absorbing polymer beads,
the core (C) comprises hydrophilic fiber material Hydrophilic is
understood to mean that aqueous liquids spread rapidly over the
fibers. Usually, the fiber material is cellulose, modified
cellulose, rayon, polyester such as polyethylene terephthalate.
Particular preference is given to cellulose fibers such as chemical
pulp. The fibers generally have a diameter of from 1 to 200 .mu.m,
preferably from 10 to 100 .mu.m. In addition, the fibers have a
minimum length of 1 mm.
[0093] The structure and the shape of diapers is common knowledge
and is described, for example, in WO 95/26209 page 66 line 34 to
page 69 line 11, DE-A 196 04 601, EP-A 0 316 518 and EP-A 0 202
127. Diapers and other hygiene articles are also described in
general terms in WO 00/65084, especially on pages 6 to 15, WO
00/65348, especially on pages 4 to 17, WO 00/35502, especially
pages 3 to 9, DE-A 197 37 434 and WO 98/08439. Hygiene articles for
feminine hygiene are described in the following references. The
inventive water-absorbing polymer beads can be used there.
References on feminine hygiene: WO 95/24173: Absorption Article for
Controlling Odour, WO 91/11977: Body Fluid Odour Control, EP-A 0
389 023: Absorbent Sanitary Articles, WO 94/25077: Odour Control
Material, WO 97/01317: Absorbent Hygienic Article, WO 99118905,
EP-A 0 834 297, U.S. Pat. No. 5,762,644, U.S. Pat. No. 5,895,381,
WO 98157609, WO 00/65083, WO 00/69485, WO 00/69484, WO 00/69481,
U.S. Pat. No. 6,123,693, EP-A 1 104 666, WO 01/24755, WO 01/00115,
EP-A 0 105 373, WO 01/41692, EP-A 1 074 233. Tampons are described
in the following documents: WO 98/48753, WO 98/41179, WO 97109022,
WO 98/46182, WO 98/46181, WO 01/43679, WO 01/43680, WO 00/61052,
EP-A 1 108 408, WO 01/33962, DE-A 100 20 662, WO 01/01910, WO
01/01908, WO 01/01909, WO 01/01906, WO 01/01905, WO 01/24729.
Incontinence articles are described in the following documents:
Disposable Absorbent Article for Incontinent individuals: EP-A 0
311 344 description pages 3 to 9, Disposable Absorbent Article:
EP-A 0 850 623, Absorbent Article: WO 95/26207, Absorbent Article:
EP-A 0 894 502, Dry Laid Fibrous Structure: EP-A 0 850 616, WO
98/22063, WO 97/49365, EP-A 0 903134, EP-A 0 887 060, EP-A 0 887
059, EP-A 0 887 058, EP-A 0 887 057, EP-A 0 887 056, EP-A 0 931
530, WO 99/25284, WO 98/48753. Feminine hygiene articles and
incontinence articles are described in the following documents:
Catamenial Device: WO 93/22998 description pages 26 to 33,
Absorbent Members for Body Fluids: WO 95/26209 description pages 36
to 69, Disposable Absorbent Article: WO 98/20916 description pages
13 to 24, Improved Composite Absorbent Structures: EP-A 0 306 262
description pages 3 to 14, Body Waste Absorbent Article: WO
99/45973. These references are hereby incorporated explicitly into
the disclosure of the invention.
[0094] In addition to the above-described inventive water-absorbing
polymer beads, constructions which comprise the inventive
water-absorbing polymer beads or to which they are fixed may be
present in the absorbent composition according to the present
invention. A suitable construction is any which can accommodate the
inventive water-absorbing polymer beads and which can additionally
be integrated into the absorption layer. A multitude of such
compositions is already known. A construction for the incorporation
of the inventive water-absorbing polymer beads may, for example, be
a fiber matrix which consists of a cellulose fiber blend (airlaid
web, wet laid web) or of synthetic polymer fibers (meitbiown web,
spunbonded web), or else of a fiber blend composed of cellulose
fibers and synthetic fibers. Possible fiber materials are described
in detail in the chapter which follows. The process of an airlaid
web is outlined, for example, in the patent application WO
98/28478. In addition, open-pore foams or the like can serve for
the incorporation of water-absorbing polymer beads.
[0095] Alternatively, such a construction can be formed by fusing
two individual layers to form one chamber or, better, a multitude
of chambers which comprise the inventive water-absorbing polymer
beads. Such a chamber system is outlined in detail in the patent
application EP-A 0 615 736 page 7 lines 26 ff.
[0096] In this case, at least one of the two layers should be
water-pervious. The second layer may be either water-pervious or
water-impervious. The layer material used may be tissues or other
fabric, closed or open-cell foams, perforated films, elastomers or
fabrics made from fiber material. When the absorbent composition
consists of a construction of layers, the layer material should
have a pore structure whose pore dimensions are small enough to
retain the inventive water-absorbing polymer beads. The above
examples of the construction of the absorbent composition also
include laminates of at least two layers, between which the
inventive water-absorbing polymer beads are incorporated and
fixed.
[0097] Generally, it is possible to fix hydrogel particles within
the absorbent core to improve the so-called dry and wet integrity.
Dry and wet integrity is understood to mean the capability of
incorporating water-absorbing polymer beads into the absorbent
composition such that they withstand external forces both in the
wet and in the dry state and there are no dislocations or leakage
of highly swellable polymer. Forces are understood to mean in
particular mechanical stresses, as occur while moving around when
wearing, the hygiene article, or else the weight stress on the
hygiene article in the case of incontinence in particular. For the
fixing, there are a multitude of possibilities which are known to
the person skilled in the art. Examples such as fixing by heat
treatment, addition of adhesives, thermoplastics, binder materials
are noted in the patent application WO 95/26209 page 37 line 36 to
page 41 line 14. Said passage is thus part of this invention.
Methods for increasing the wet strength are also found in the
patent application WO 00/36216.
[0098] In addition, the absorbent composition may also consist of a
carrier material, for example a polymer film, on which the
water-absorbing polymer beads are fixed. The fixing may be
undertaken either on one side or on both sides. The carrier
material may be water-pervious or water-impervious.
[0099] In above constructions of the absorbent composition, the
inventive water-absorbing polymer beads are incorporated with a
proportion by weight of from 10 to 100% by weight, preferably from
30 to 100% by weight, more preferably from 50 to 100% by weight,
especially preferably from 70 to 100% by weight, and most
preferably from 90 to 100% by weight, based on the total weight of
the construction and of the water-absorbing polymer beads.
[0100] The structure of the present inventive absorbent composition
is based on various fiber materials, which are used as a fiber
network or matrices. The present invention includes both fibers of
natural origin (modified or unmodified) and synthetic fibers.
[0101] A detailed overview of examples of fibers which can be used
in the present invention is given by the patent application WO
95/26209 page 28 line 9 to page 36 line 8. Said passage is thus
part of this invention.
[0102] Examples of cellulose fibers include cellulose fibers which
are customarily used in absorption products, such as fluff pulp and
cellulose of the cotton type. The materials (soft- or hardwoods),
production processes such as chemical pulp, semichemical pulp,
chemothermomechanical pulp (CTMP) and bleaching processes are not
particularly restricted. For example, natural cellulose fibers such
as cotton, flax, silk, wool, jute, ethylcellulose and cellulose
acetate are used.
[0103] Suitable synthetic fibers are produced from polyvinyl
chloride, polyvinyl fluoride, polytetrafluoroethylene,
polyvinylidene chloride, poiyacrylic compounds such as ORLON.RTM.,
polyvinyl acetate, polyethyl vinyl acetate, soluble or insoluble
polyvinyl alcohol. Examples of synthetic fibers include
thermoplastic polyolefin fibers, such as polyethylene fibers
(PULPEX.RTM.), polypropylene fibers and polyethylene-polypropylene
bicomponent fibers, polyester fibers, such as polyethylene
terephthalate fibers (DACRON.RTM. or KODEL.RTM.), copolyesters,
polyvinyl acetate, polyethyl vinyl acetate, polyvinyl chloride,
polyvinylidene chloride, polyacrylics, polyamides, copolyamides,
polystyrene and copolymers of the aforementioned polymers and also
bicomponent fibers composed of polyethylene
terephthalate-polyethylene-isophthalate copolymer, polyethyl vinyl
acetate/polypropylene, polyethylene/polyester,
polypropylene/polyester, copolyesteripolyester, polyamide fibers
(nylon), polyurethane fibers, polystyrene fibers and
polyacrylonitrile fibers. Preference is given to polyolefin fibers,
polyester fibers and their bicomponent fibers. Preference is
further given to thermally adhesive bicomponent fibers composed of
polyolefin of the core-sheath type and side-by-side type on account
of their excellent dimensional stability following fluid
absorption.
[0104] The synthetic fibers mentioned are preferably used in
combination with thermoplastic fibers. In the course of the heat
treatment, the latter migrate to some extent into the matrix of the
fiber material present and so constitute bond sites and renewed
stiffening elements on cooling. In addition, the addition of
thermoplastic fibers means that there is an increase in the present
pore dimensions after the heat treatment has taken place. This
makes it possible, by continuous metered addition of thermoplastic
fibers during the formation of the absorbent layer, to continuously
increase the fraction of thermoplastic fibers in the direction of
the topsheet, which results in a similarly continuous increase in
the pore sizes. Thermoplastic fibers can be formed from a multitude
of thermoplastic polymers which have a melting point of less than
190.degree. C., preferably in the range from 75.degree. C. to
175.degree. C. These temperatures are too low for damage to the
cellulose fibers to be likely.
[0105] Lengths and diameters of the above-described synthetic
fibers are not particularly restricted, and generally any fiber
from 1 to 200 mm in length and from 0.1 to 100 denier (gram per
9000 meters) in diameter may preferably be used. Preferred
thermoplastic fibers are from 3 to 50 mm in length, particularly
preferred thermoplastic fibers are from 6 to 12 mm in length. The
preferred diameter for the thermoplastic fibers is in the range
from 1.4 to 10 decitex, and the range from 1.7 to 3.3 decitex (gram
per 10 000 meters) is particularly preferred. The form of the
fibers may vary; examples include woven types, narrow cylindrical
types, cut/split yarn types, staple fiber types and continuous
filament fiber types.
[0106] The fibers in the absorbent composition of the present
invention may be hydrophilic, hydrophobic or a combination of the
two. According to the definition of Robert F. Gould in the 1964
American Chemical Society publication "Contact angle, wettability
and adhesion", a fiber is referred to as hydrophilic when the
contact angle between the liquid and the fiber (or the fiber
surface) is less than 90.degree. or when the liquid tends to spread
spontaneously on the same surface. The two processes are generally
coexistent, Conversely, a fiber is termed hydrophobic when a
contact angle of greater than 90.degree. is formed and no spreading
is observed.
[0107] Preference is-given to using hydrophilic fiber. material.
Particular preference is given to using fiber material which is
weakly hydrophilic on the body side and at its most hydrophilic in
the region surrounding the water-absorbing polymer beads. In the
manufacturing process, the use of layers having different
hydrophilicities creates a gradient which channels impinging fluid
to the hydrogel, where it is ultimately absorbed.
[0108] Suitable hydrophilic fibers for use in the absorbent
composition of the present invention include for example cellulose
fibers, modified cellulose fibers, rayon, polyester fibers, for
example polyethylene terephthalate (DACRON.RTM.), and hydrophilic
nylon (HYDROFIL.RTM.). Suitable hydrophilic fibers may also be
obtained by hydrophilizing hydrophobic fibers, for example the
treatment of thermoplastic fibers obtained from polyolefins (e.g.
polyethylene or polypropylene, polyamides, polystyrenes,
polyurethanes, etc.) with surfactants or silica. However, for
reasons of cost and ease of availability, cellulose fibers are
preferred.
[0109] The inventive water-absorbing polymer beads are embedded
into the fiber material described. This can be done in various
ways, for example by using the hydrogel material and the fibers
together to create an absorbent layer in the form of a matrix, or
by incorporating water-absorbing polymer beads into fiber blend
layers, where they are ultimately fixed, whether by means of
adhesive or lamination of the layers.
[0110] The fluid-acquiring and -distributing fiber matrix may
comprise synthetic fiber or cellulose fiber or a mixture of
synthetic fiber and cellulose fiber, in which case the blend ratio
may vary from (100 to 0) synthetic fiber: (0 to 100) cellulose
fiber. The cellulose fibers used may additionally have been
chemically stiffened to increase the dimensional stability of the
hygiene article.
[0111] The chemical stiffening of cellulose fibers may be provided
in different ways. One way of achieving fiber stiffening is by
adding suitable coatings to the fiber material. Such additives
include, for example, polyamide-epichiorohydrin coatings
(Kymene.RTM. 557 H, Hercules, Inc. Wilmington, Del., USA),
polyacryiamide coatings (described in U.S. Pat. No. 3,556,932 or as
the Perez.RTM. 631 NC commercial product from American Cyanamid
Co., Stamford, Conn., USA), melamine-formaldehyde coatings and
polyethyleneimine coatings.
[0112] Cellulose fibers can also be chemically stiffened by
chemical reaction. For example, suitable crosslinker substances can
be added to bring about crosslinking which takes place within the
fiber. Suitable crosslinker substances are typical substances which
are used to crosslink monomers. They include, but are not limited
to, C.sub.2-C.sub.8-dialdehydes, C.sub.2-C.sub.8-monoaldehydes
having acid functionality and in particular
C.sub.2-C.sub.9-polycarboxylic acids. Specific substances from this
group are, for example, glutaraldehyde, glyoxal, glyoxylic acid,
formaldehyde and citric add. These substances react with at least
two hydroxyl groups within any one cellulose chain or between two
adjacent cellulose chains within any one cellulose fiber. The
crosslinking stiffens the fibers, to which greater dimensional
stability is imparted as a result of this treatment. In addition to
their hydrophilic character, these fibers exhibit uniform
combinations of stiffening and elasticity, This physical property
makes it possible to retain the capillary structure even under
simultaneous contact with fluid and compressive forces and to
prevent premature collapse.
[0113] Chemically crosslinked cellulose fibers are known and
described in WO 91/11162, U.S. Pat. No. 3,224,926, U.S. Pat. No.
3,440,135, U.S. Pat. No. 3,932,209, U.S. Pat. No. 4,035,147, U.S.
Pat. No. 4,822,453, U.S. Pat. No. 4,888,093, U.S. Pat. No.
4,898,642 and U.S. Pat. No. 5,137,537. The chemical crosslinking
brings about stiffening of the fiber material, which is ultimately
reflected in improved dimensional stability for the hygiene article
as a whole. The individual layers are joined together by methods
known to the person skilled in the art, for example melting by heat
treatment, addition of hot-melt adhesives, latex binders, etc.
[0114] Examples of processes to obtain an absorbent composition
which comprises, for example, a carrier material to which
water-absorbing polymer beads are fixed on one or both sides are
known and included by the invention but not limited thereto.
[0115] Examples of processes to obtain an absorbent composition
comprising for example water-absorbing polymer beads (c) embedded
into a fiber material blend of synthetic fibers (a) and cellulose
fibers (b), the blend ratio varying from (100 to 0) synthetic
fiber: (0 to 100) cellulose fiber, include (1) a process where (a),
(b) and (c) are mixed together at one and the same time, (2) a
process where a mixture of (a) and (b) is mixed into (c), (3) a
process where a mixture of (b) and (c) is mixed with (a), (4) a
process where a mixture of (a) and (c) is mixed into (b), (5) a
process where (b) and (c) are mixed and (a) is continuously metered
in, (6) a process where (a) and (c) are mixed and (b) is
continuously metered in, and (7) a process where (b) and (c) are
mixed separately into (a). Of these examples, processes (1) and (5)
are preferred. The apparatus used in this process is not
particularly restricted and any customary apparatus known to the
person skilled in the art can be used. The absorbent composition
obtained in this way can optionally be subjected to a heat
treatment, so as to result in an absorption layer having excellent
dimensional stability in the moist state. The heat treatment
process is not particularly restricted. Examples include heat
treatment by feeding hot air or infrared irradiation. The
temperature of the heat treatment is in the range from 60.degree.
C. to 230.degree. C., preferably from 100.degree. C. to 200.degree.
C., particularly preferably from 100.degree. C. to 180.degree.
C.
[0116] The duration of the heat treatment depends on the type of
synthetic fiber, its amount and the hygiene article production
rate. Generally the duration of the heat treatment is in the range
from 0.5 second to 3 minutes, preferably from 1 second to 1
minute.
[0117] The absorbent composition is generally provided for example
with a liquid-pervious topsheet and a liquid-impervious backsheet.
Furthermore, leg cuffs and adhesive tabs are attached to finalize
the hygiene article. The materials and types of pervious topsheet
and impervious backsheet and of the leg cuffs and adhesive tabs are
known to the person skilled in the art and are not particularly
restricted. Examples thereof can be found in WO 95/26209.
[0118] The water-absorbing polymer beads are tested by means of the
test methods described below.
Methods:
[0119] The measurements should, unless stated otherwise, be carried
out at an ambient temperature of 23.+-.2.degree. C. and a relative
atmospheric humidity of 50.+-.10%. The water-absorbing polymers are
mixed thoroughly before the measurement.
Saline Flow Conductivity (SFC)
[0120] The saline flow conductivity of a swollen gel layer under
pressure load of 0.3 psi (2070 Pa) is, as described in EP-A-0 640
330, determined as the gel layer permeability of a swollen gel
layer of superabsorbent polymer, although the apparatus described
on page 19 and in FIG. 8 in the aforementioned patent application
was modified to the effect that the glass frit (40) is no longer
used, the plunger (39) consists of the same polymer material as the
cylinder (37) and now comprises 21 bores of equal size distributed
uniformly over the entire contact surface. The procedure and the
evaluation of the measurement remains unchanged from EP-A-0 640
330. The flow rate is recorded automatically.
[0121] The saline flow conductivity (SFC) is calculated as
follows:
SFC [cm.sup.3s/g]=(Fg(t=0).times.L0)/(d.times.A.times.WP),
where Fg(t=0) is the flow rate of NaCl solution in g/s, which is
obtained by means of a linear regression analysis of the Fg(t) data
of the flow determinations by extrapolation to t=0, L0 is the
thickness of the gel layer in cm, d is the density of the NaCl
solution in g/cm.sup.3, A is the surface area of the gel layer in
cm.sup.2 and WP is the hydrostatic pressure over the gel layer in
dyn/cm.sup.2.
Centrifuge Retention Capacity (CRC)
[0122] The centrifuge retention capacity of the water-absorbing
polymer beads is determined by the EDANA (European Disposables and
Nonwovens Association) recommended test method No. 441.2-02
"Centrifuge Retention Capacity".
[0123] The values reported for the centrifuge retention capacity
are based on the anhydrous water-absorbing polymer beads, i.e. the
values measured were corrected according to the water content of
the water-absorbing polymer beads before the measurement. The water
content of the water-absorbing polymer beads is determined by the
EDANA (European Disposables and Nonwovens Association) recommended
test method No. 430.2-02 "Moisture content".
[0124] The EDANA test methods are obtainable, for example, from the
European Disposables and Nonwovens Association, Avenue Eugene
Plasky 167, B-1030 Brussels, Belgium.
EXAMPLES
[0125] 14.6 kg of sodium acrylate (37.5% by weight solution in
water) and 1.4 kg of acrylic acid were mixed with 22.4 g of
15-tuply ethoxylated trimethylolpropane triacrylate. The solution
was dropletized into a heated dropletization tower filled with
nitrogen atmosphere (180.degree. C., height 12 m, width 2 m, gas
velocity 0.1 m/s in cocurrent). The metering rate was 16 kg/h. The
dropletizer plate had 37 bores of 170 .mu.m. The diameter of the
dropletizer plate was 65 mm. The initiator and calcium phosphate
were metered individually into the monomer solution via a Venturi
mixer just upstream of the dropletizer, The initiator used was a
15% by weight solution of
2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride in water.
Calcium phosphate was used in the form of a 30% by weight aqueous
solution (calcium phosphate dispersion type TCP 130 from Rhodia).
The metering rate of the initiator solution was 0.224 kg/h. The gas
exit temperature from the dropletization tower was 130.degree. C.
The mean particle diameter of the resulting polymer beads was 270
.mu.m.
[0126] The resulting water-absorbing polymer beads were then
analyzed. The results are summarized in Table 1:
TABLE-US-00001 TABLE 1 Influence of the inorganic salt Calcium
phosphate CRC SFC Example content*) [g/g] [10.sup.-7 cm.sup.3s/g] 1
none 41.6 1 2 0.5% by wt. 40.6 7 3 3% by wt. 39.0 19 4 5% by wt.
37.9 35 *)based on acrylic acid
* * * * *