U.S. patent application number 13/087237 was filed with the patent office on 2011-08-11 for polymerization method.
This patent application is currently assigned to BASF SE. Invention is credited to Andreas Daiss, Ronny De Kaey, Michael de Marco, Karl J. Possemiers, Dominicus van Esbroeck, Ieo Van Miert, Matthias Weismantel.
Application Number | 20110194984 13/087237 |
Document ID | / |
Family ID | 37402577 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110194984 |
Kind Code |
A1 |
Weismantel; Matthias ; et
al. |
August 11, 2011 |
Polymerization Method
Abstract
The invention relates to a process for continuously preparing
water-absorbing polymer particles by mixing a monomer solution with
a redox initiator consisting of at least one oxidizing agent and at
least one reducing agent and polymerizing it, wherein the monomer
solution is mixed with at least one oxidizing agent upstream of the
polymerization reactor and the at least one reducing agent is
metered into the polymerization reactor, and also to an apparatus
for performing the process.
Inventors: |
Weismantel; Matthias;
(Jossgrund-Oberndorf, DE) ; de Marco; Michael;
(Palo Alto, CA) ; Daiss; Andreas; (Deidesheim,
DE) ; van Esbroeck; Dominicus; (Nanjing, CN) ;
Possemiers; Karl J.; (Gravenwezel, BE) ; De Kaey;
Ronny; (Mortsel, BE) ; Van Miert; Ieo;
(Kapellen, BE) |
Assignee: |
BASF SE
Ludwigshafen
DE
|
Family ID: |
37402577 |
Appl. No.: |
13/087237 |
Filed: |
April 14, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12065322 |
Feb 29, 2008 |
7947794 |
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PCT/EP2006/065848 |
Aug 31, 2006 |
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13087237 |
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Current U.S.
Class: |
422/131 |
Current CPC
Class: |
B01J 19/2405 20130101;
B01J 2219/1943 20130101; B01J 2219/182 20130101; B01J 19/26
20130101; B01J 2219/00164 20130101; C08F 220/06 20130101; C08F 2/01
20130101; B01J 2219/1946 20130101 |
Class at
Publication: |
422/131 |
International
Class: |
B01J 19/00 20060101
B01J019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 7, 2005 |
DE |
10 2005 042 609.3 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. An apparatus for continuous polymerization, comprising i) a
polymerization reactor, ii) a first inlet to the polymerization
reactor iii) at least one inlet into the first inlet ii), iv) a
second inlet to the polymerization reactor and v) a device for
comminuting a polymer gel, where an extended axis of the first
inlet ii) intersects an extended axis of the second inlet iv) in
the polymerization reactor i).
12. The apparatus according to claim 11, wherein an inner surface
of the first inlet ii) between the polymerization reactor i) and
the inlet iii) at least partly has a contact angle for water of at
least 60.degree..
13. The apparatus according to claim 11, wherein a length of the
first inlet ii) between the polymerization reactor i) and the inlet
iii) is from 0.01 to 2 m.
14. The apparatus according to claim 11, wherein the first inlet
ii) is designed as a Venturi tube at a connection with the inlet
iii) and the inlet iii) opens into a narrowing zone of the Venturi
tube.
15. The apparatus according to claim 11, wherein the angle between
first inlet ii) and inlet iv) in flow direction is less than
90.degree..
Description
[0001] The present invention relates to a process for continuously
preparing water-absorbing polymer particles by mixing a monomer
solution with a redox initiator consisting of at least one
oxidizing agent and at least one reducing agent and polymerizing
it, wherein the monomer solution is mixed with at least one
oxidizing agent upstream of the polymerization reactor and the at
least one reducing agent is metered into the polymerization
reactor, and also to an apparatus for performing the process.
[0002] Further embodiments of the present invention can be taken
from the claims, the description and the examples. It is evident
that the features of the inventive subject matter which have been
mentioned above and will be explained below are usable not only in
the combination specified in each case but also in other
combinations without leaving the scope of the invention.
[0003] Water-absorbing polymers are especially polymers of
(co)polymerized hydrophilic monomers, graft (co)polymers of one or
more hydrophilic monomers on a suitable graft base, crosslinked
cellulose ethers or starch ethers, crosslinked
carboxymethylcellulose, partly crosslinked polyalkylene oxide or
natural products swellable in aqueous liquids, for example guar
derivatives, preference being given to water-absorbing polymers
based on partly neutralized acrylic acid. Such polymers are used as
products that absorb aqueous solutions to produce diapers, tampons,
sanitary napkins and other hygiene articles, but also as
water-retaining agents in market gardening.
[0004] The preparation of the water-absorbing polymers is
described, for example, in the monograph "Modern Superabsorbent
Polymer Technology", F. L. Buchholz and A. T. Graham, Wiley-VCH,
1998, or in Ullmann's Encyclopedia of Industrial Chemistry, 6th
Edition, Volume 35, pages 73 to 103. The preferred preparation
process is solution or gel polymerization. In this technology, a
monomer solution is firstly prepared and is neutralized batchwise
and then transferred to a polymerization reactor, or initially
charged actually within the polymerization reactor. In the
batchwise or continuous process which follows, the reaction is
effected to give the polymer gel which, in the case of a stirred
polymerization, is already in comminuted form. The polymer gel is
subsequently dried, ground and sieved and then transferred to
further surface treatment.
[0005] A continuous polymerization process forms the basis, for
example, of WO-A-01/38402, in which the aqueous monomer solution,
together with the initiator and the inert gas, is fed continuously
to a mixing kneader with at least two axially parallel-rotating
shafts.
[0006] Continuous gel polymerizations are also known from
WO-A-03/004237, WO-A-03/022896 and WO-A-01/016197.
[0007] Frequently, the polymerizations are initiated by redox
initiators, asdosmihedin DE-A-199 55 861, EP-A-1 097 946 and
WO-A-03/022896.
[0008] According to DE-A-199 55 861, for example, sodium
peroxodisulfate, hydrogen peroxide and ascorbic acid are metered
into a continuous kneader together with the monomer solution via
separate lines.
[0009] EP-A-1 097 946 discloses the use of sodium persulfate,
hydrogen peroxide and ascorbic acid as redox initiators, the
initiator components being added to the monomer solution in
succession upstream of the belt reactor.
[0010] WO-A-03/022896 describes the use of sodium persulfate and
ascorbic acid as redox initiators. The monomer solution is premixed
with sodium persulfate and admixed with ascorbic acid immediately
upstream of the kneader (polymerization reactor).
[0011] It was an object of the present invention to provide an
improved process for preparing water-absorbing polymers, in which
the initiator system is mixed optimally into the monomer solution
and the risk of premature polymerization is reduced.
[0012] It was a further object of the present invention to provide
an improved polymerization process for preparing crosslinked
polymers, in which the monomer conversion should be at a maximum
and the proportion of uncrosslinked polymers at a minimum.
[0013] The object is achieved by a process for continuously
preparing water-absorbing polymer particles by mixing a monomer
solution with a redox initiator consisting of at least one
oxidizing agent and at least one reducing agent and polymerizing
it, which comprises mixing the monomer solution with at least one
oxidizing agent upstream of the polymerization reactor and metering
the at least one reducing agent into the polymerization
reactor.
[0014] The oxidizing agents used may be any compounds which
decompose into free radicals under the polymerization conditions,
for example peroxides, hydroperoxides, hydrogen peroxide and
persulfates. Preference is given to using water-soluble initiators.
In some cases, it is advantageous to use mixtures of different
initiators, for example mixtures of hydrogen peroxide and sodium
peroxodisulfate or potassium peroxodisulfate. Mixtures of hydrogen
peroxide and sodium peroxodisulfate can be used in any desired
ratio.
[0015] Suitable organic peroxides are, for example, acetylacetone
peroxide, methyl ethyl ketone peroxide, tert-butyl hydroperoxide,
cumene hydroperoxide, tert-amyl perpivalate, tert-butyl
perpivalate, tert-butyl perneohexanoate, tert-butyl perisobutyrate,
tert-butyl per-2-ethylhexanoate, tert-butyl perisoronanoate,
tert-butyl permaleate, tert-butyl perbenzoate, di(2-ethylhexyl)
peroxydicarbonate, dicyclohexyl peroxydicarbonate,
di(4-tert-butylcyclohexyl) peroxydicarbonate, dimyristyl
peroxydicarbonate, diacetyl peroxydicarbonate, allyl perester,
cumyl peroxyneodecanoate, tert-butyl per-3,5,5-trimethylhexanoate,
acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl
peroxide and tert-amyl perneodecanoate.
[0016] Preference is given to using at least two oxidizing agents.
The at least two oxidizing agents are preferably premixed, i.e.
metered into the monomer solution as a mixture.
[0017] Suitable reducing agents are, for example, ascorbic acid,
glucose, sorbose, the hydrogen sulfite, sulfite, thiosulfate,
hyposulfite, pyrosulfite or sulfide salts of ammonium or alkali
metals, or sodium hydroxymethylsulfoxylate. The reducing component
used in the redox initiator is preferably ascorbic acid or sodium
pyrosulfite.
[0018] In addition, it is also possible to use further initiators,
for example azo initiators.
[0019] The viscosity of the monomer solution at 15.degree. C. is
preferably from 5 to 200 mPas, more preferably from 10 to 100 mPas,
most preferably from 20 to 50 mPas, the viscosity being measured
with a Brookfield viscometer (spindle 2, 100 rpm).
[0020] The monomer concentration in the monomer solution is
preferably from 10 to 80% by weight, more preferably from 20 to 60%
by weight, most preferably from 30 to 50% by weight.
[0021] The monomer solution comprises at least one ethylenically
unsaturated monomer, preferably acrylic acid and/or salts thereof.
The proportion of acrylic acid and/or salts thereof in the total
amount of monomer is preferably at least 50 mol %, more preferably
at least 90 mol %, most preferably at least 95 mol %.
[0022] In a preferred embodiment of the present invention, the at
least one oxidizing agent is metered in via a Venturi tube.
[0023] A Venturi tube is a tube constriction of restricted length,
in which the pressure drop is converted substantially reversibly to
kinetic energy. To this end, the cross-sectional area F.sub.1 is
reduced to the cross section F.sub.2 over the distance L.sub.1
(narrowing zone), the cross-sectional area F.sub.2 is kept constant
over the distance L.sub.2 (constriction zone) and the
cross-sectional area F.sub.2 is widened again to the
cross-sectional area F.sub.1 over the distance L.sub.3 (diffuser).
The cross-sectional area F.sub.1 is greater than the
cross-sectional area F.sub.2 and the length L.sub.3 greater than
the length L.sub.1.
[0024] The oxidizing agent is preferably metered in the region of
the zone L.sub.1 or of the zone L.
[0025] FIG. 1 shows a typical Venturi tube, the reference symbols
being defined as follows:
A: monomer solution before metering of oxidizing agent B: oxidizing
agent feed C: monomer solution comprising oxidizing agent L.sub.1:
narrowing zone L.sub.2: constriction zone L.sub.3: diffuser
D.sub.1: diameter of the pipeline D.sub.2: diameter of the
constriction zone
[0026] The optimal design of a Venturi tube is known per se to
those skilled in the art. The Venturi tube is preferably designed
such that the pressure in the region of the zone L.sub.2 is less
than the ambient pressure (suction conveying) and/or that the flow
in the region of the zone L.sub.2 is turbulent, in which case the
Reynolds number should be at least 1000, preferably at least 2000,
more preferably at least 3000, most preferably at least 4000, and
typically less than 10 000 000.
[0027] The at least one oxidizing agent can be metered in via one
or more addition points.
[0028] For example, the oxidizing agent may be metered in via two,
three, four, five or six addition points, the addition points
preferably being arranged such that they have a common axis (for
two addition points) or form a symmetrical star (for at least three
addition points), and the axis or star is at right angles to the
flow direction of the monomer solution (multiple addition
points).
[0029] The division into a plurality of addition points brings
about more uniform mixing.
[0030] When a plurality of oxidizing agents are used, they may be
metered in separately or as a mixture, preferably as a mixture.
[0031] The residence time of the mixture between the addition point
of the at least one oxidizing agent and the polymerization reactor
is preferably less than 20 seconds, preferentially less than 15
seconds, more preferably less than 10 seconds, most preferably less
than 5 seconds. A very particularly advantageous residence time is
in the range from 0.1 to 1 second.
[0032] The at least one reducing agent is metered into the
polymerization reactor such that the reducing agent cannot come
into contact with the monomer solution until within the
polymerization reactor, preferably at least 5 cm, preferentially at
least 10 cm, more preferably at least 20 cm, most preferably at
least 50 cm, away from the point at which the monomer solution
enters the polymerization reactor.
[0033] In a preferred embodiment of the present invention, monomer
solution and reducing agent are metered into the polymerization
reactor such that the liquid streams mix, at the latest, at the
height which corresponds to the fill level of the polymerization
reactor.
[0034] FIG. 2 shows an example of this preferred embodiment, in
which the reference symbols are defined as follows:
A: inlet for monomer solution comprising oxidizing agent B: inlet
for reducing agent C: polymerization reactor
[0035] The polymerization tendency can be reduced when the
connection between oxidizing agent metering and polymerization
reactor at least partly, preferably at least to an extent of at
least 50% of the surface, more preferably as fully as possible in
construction terms, has a material surface which has a content
angle for water of at least 60.degree., preferably at least
90.degree., more preferably at least 100.degree..
[0036] The contact angle is a measure of the wetting behavior and
can be measured by customary methods, preferably according to DIN
53900.
[0037] Suitable materials with corresponding wetting behavior are
polyethylene, polypropylene, polyester, polyamide,
polytetrafluorethylene, polyvinyl chloride, epoxy resins and
silicone resins. Very particular preference is given to
polypropylene.
[0038] Preference is given to mixing a preneutralized monomer
solution with the at least one crosslinker, inertizing the mixture,
mixing the inertized mixture with at least one oxidizing agent and
polymerizing.
[0039] The water-absorbing polymers are obtained, for example, by
polymerization of a monomer solution comprising [0040] a) at least
one ethylenically unsaturated acid-functional monomer, [0041] b) at
least one crosslinker, [0042] c) if appropriate one or more
ethylenically and/or allylically unsaturated monomers
copolymerizable with the monomer a), and [0043] d) if appropriate
one or more water-soluble polymers onto which the monomers a),
[0044] b) and if appropriate c) can be at least partly grafted.
[0045] Suitable monomers a) are, for example, ethylenically
unsaturated carboxylic acids, such as acrylic acid, methacrylic
acid, maleic acid, fumaric acid and itaconic acid, or derivatives
thereof, such as acrylamide, methacrylamide, acrylic esters and
methacrylic esters. Particularly preferred monomers are acrylic
acid and methacrylic acid. Very particular preference is given to
acrylic acid.
[0046] 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.
[0047] Tocopherol refers to 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.
[0048] Preferred R.sup.4 radicals are acetyl, ascorbyl, succinyl,
nicotinyl and other physiologically tolerable carboxylic acids. The
carboxylic acids may be mono-, di- or tricarboxylic acids.
[0049] Preference is given to alpha-tocopherol where
R.sup.1.dbd.R.sup.2.dbd.R.sup.3=methyl, especially racemic
alpha-tocopherol. R.sup.1 is more preferably hydrogen or acetyl.
Especially preferred is RRR-alpha-tocopherol.
[0050] The monomer solution comprises preferably not more than 130
ppm by weight, more preferably not more than 70 ppm by weight,
preferably not less than 10 ppm by weight, more preferably not less
than 30 ppm by weight and especially about 50 ppm by weight of
hydroquinone monoether, based in each case on acrylic acid, with
acrylic acid salts being counted as acrylic acid. For example, the
monomer solution can be prepared using acrylic acid having an
appropriate hydroquinone monoether content.
[0051] The crosslinkers b) are compounds having at least two
polymerizable groups which can be free-radically polymerized 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-A-93/21237, WO-A-03/104299, WO-A-03/104300,
WO-A-03/104301 and DE-A-103 31 450, mixed acrylates which, as well
as acrylate groups, comprise further ethylenically unsaturated
groups, as described in DE-A-103 31 456 and WO-A-04/013064, or
crosslinker mixtures as described, for example, in DE-A-195 43 368,
DE-A-196 46 484, WO-A-90/15830 and WO-A-02/32962.
[0052] Suitable crosslinkers b) include 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,
ethylene glycol dimethacrylate and also trimethylolpropane
triacrylate and allyl compounds, such as allyl (meth)acrylate,
triallyl cyanurate, diallyl maleate, polyallyl esters,
tetraallyloxyethane, triallylamine, tetraallylethylenediamine,
allyl esters of phosphoric acid and also vinylphosphonic acid
derivatives as described, for example, in EP-A-0 343 427. Suitable
crosslinkers b) further include pentaerythritol diallyl ether,
pentaerythritol triallyl ether, pentaerythritol tetraallyl ether,
polyethylene glycol diallyl ether, ethylene glycol diallyl ether,
glycerol diallyl ether, glycerol triallyl ether, polyallyl ethers
based on sorbitol, and also ethoxylated variants thereof. In the
process of 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.
[0053] 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, especially 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, of 3-tuply mixed ethoxylated
or propoxylated trimethylolpropane, of 15-tuply ethoxylated
glycerol or of 15-tuply ethoxylated trimethylolpropane, and of
40-tuply ethoxylated glycerol, of 40-tuply ethoxylated
trimethylolethane and also of 40-tuply ethoxylated
trimethylolpropane.
[0054] Very particularly preferred crosslinkers b) are
polyethoxylated and/or -propoxylated glycerols which have been
esterified with acrylic acid or methacrylic acid to di- or
triacrylates, as described, for example, in WO-A-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. The triacrylates of 3- to 5-tuply
ethoxylated and/or propoxylated glycerol are most preferred. These
are notable for particularly low residual levels (typically below
10 ppm by weight) in the water-absorbing polymer and the aqueous
extracts of the water-absorbing polymers produced therewith have an
almost unchanged surface tension (typically not less than 0.068
N/m) compared with water at the same temperature.
[0055] The amount of crosslinker b) is preferably from 0.01 to 1%
by weight, more preferably from 0.05 to 0.5% by weight, most
preferably from 0.1 to 0.3% by weight, based in each case on the
monomer a).
[0056] Examples of ethylenically unsaturated monomers c) which are
copolymerizable with the monomers a) are acrylamide,
rnethacrylamide, crotonamide, dimethylaminoethyl methacrylate,
dimethylaminoethyl acrylate, dimethylaminopropyl acrylate,
diethylaminopropyl acrylate, dimethylaminobutyl acrylate,
dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate,
dimethylaminoneopentyl acrylate and dimethylaminoneopentyl
methacrylate.
[0057] Useful water-soluble polymers d) include polyvinyl alcohol,
polyvinylpyrrolidone, starch, starch derivatives, polyglycols or
polyacrylic acids, preferably polyvinyl alcohol and starch.
[0058] For optimal action, the preferred polymerization inhibitors
require dissolved oxygen. Typically, the monomer solutions are
substantially freed of oxygen before the polymerization
(inertization), for example by means of flowing an inert gas,
preferably nitrogen, through them. This distinctly weakens the
action of the polymerization inhibitors. The oxygen content of the
monomer solution is preferably lowered to less than 1 ppm by weight
and more preferably to less than 0.5 ppm by weight before the
polymerization.
[0059] The preparation of a suitable base polymer and also further
suitable hydrophilic ethylenically unsaturated monomers d) are
described in DE-A-199 41 423, EP-A-0 686 650, WO-A-01/45758 and
WO-A-03/104300.
[0060] Water-absorbing polymers are typically obtained by addition
polymerization of an aqueous monomer solution and, if desired,
subsequent comminution of the hydrogel, Suitable preparation
methods are described in the literature. Water-absorbing polymers
are obtainable, for example, by [0061] gel polymerization in the
batch process or tubular reactor and subsequent comminution in meat
grinder, extruder or kneader (EP-A-0 445 619, DE-A-19 846 413)
[0062] addition polymerization in kneader with continuous
comminution by contrarotatory stirring shafts for example
(WO-A-01/38402) [0063] addition polymerization on belt and
subsequent comminution in meat grinder, extruder or kneader
(DE-A-38 25 366, U.S. Pat. No. 6,241,928) [0064] emulsion
polymerization, which produces bead polymers having a relatively
narrow gel size distribution (EP-A-0 457 660) [0065] in situ
addition polymerization of a woven fabric layer which, usually in a
continuous operation, has previously been sprayed with aqueous
monomer solution and subsequently been subjected to a
photopolymerization (WO-A-02/94328, WO-A-02/94329).
[0066] The reaction is preferably carried out n a kneader, as
described, for example, in WO-A-01/38402, or on a belt reactor, as
described, for example, in EP-A-0 955 086,
[0067] Neutralization can also be carried out partly after the
polymerization, at the hydrogel stage. It is therefore also
possible to neutralize up to 40 mol %, preferably from 10 to 30 mol
% and more preferably from 15 to 25 mol % of the acid groups before
the polymerization by adding a portion of the neutralizing agent to
the monomer solution and setting the desired final degree of
neutralization only after the polymerization, at the hydrogel
stage. The monomer solution can be neutralized by mixing in the
neutralizing agent. The hydrogel may be comminuted mechanically,
for example by means of a meat grinder, 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 ground in the meat grinder for homogenization.
Neutralization of the monomer solution to the final degree of
neutralization is preferred.
[0068] The neutralized hydrogel is then dried with a belt or drum
dryer until the residual moisture content is preferably below 15%
by weight and especially below 10% by weight, the water content
being determined by EDANA (European Disposables and Nonwovens
Association) recommended test method No. 430.2-02 "Moisture
content". If desired, drying can also be carried out using a
fluidized bed dryer or a heated plowshare mixer. To obtain
particularly white products, it is advantageous to dry this gel
while ensuring rapid removal of the evaporating water. To this end,
the dryer temperature must be optimized, the air feed and removal
has to be controlled, and sufficient venting must be ensured in
each case. The higher the solids context of the gel, the simpler
the drying, by its nature, and the whiter the product. The solids
content of the gel before the drying is therefore preferably
between 30% and 80% by weight. It is particularly advantageous to
vent the dryer with nitrogen or another nonoxidizing inert gas. If
desired, however, it is possible simply just to lower the partial
pressure of the oxygen during the drying in order to prevent
oxidative yellowing processes. In general, though, adequate venting
and removal of the water vapor also still lead to an acceptable
product. A very short drying time is generally advantageous with
regard to color and product quality.
[0069] The dried hydrogel is preferably ground and sieved, useful
grinding apparatus typically including roll mills, pin mills or
swing mills. The particle size of the sieved, dry hydrogel is
preferably below 1000 .mu.m, more preferably below 900 .mu.m and
most preferably below 800 .mu.m, and preferably above 100 .mu.m,
more preferably above 150 .mu.m and most preferably above 200
.mu.m.
[0070] Very particular preference is given to a particle size
(sieve cut) of from 106 to 850 .mu.m.
[0071] The particle size is determined according to EDANA (European
Disposables and Nonwovens Association) recommended test method No.
420.2-02 "Particle size distribution".
[0072] The base polymers are then preferably surface
postcrosslinked. Postcrosslinkers suitable for this purpose are
compounds comprising two or more groups capable of forming 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-543 303 and EP-A-937 736, di- or
polyfunctional alcohols, as described in DE-C-33 14 019, DE-C-35 23
617 and EP-A-450 922, or .beta.-hydroxyalkylamides, as described in
DE-A-102 04 938 and U.S. Pat. No. 6,239,230.
[0073] In addition, DE-A-40 20 780 describes cyclic carbonates,
DE-A-198 07 502 992-oxazolidone and its derivatives, such as
2-hydroxyethyl-2-oxazolidone, DE-A-198 07 992 bis- and
poly-2-oxazolidinones. DE-A-198 54 573 2-oxotetrahydro-1,3-oxazine
and its derivatives, DE-A-198 54 574 N-acyl-2-oxazolidones,
DE-A-102 04 937 cyclic ureas. DE-A-103 34 584 bicyclic amide
acetals. EP-A-1 199 327 oxetanes and cyclic ureas and
WO-A-03/031482 morpholine-2,3-dione and its derivatives, as
suitable surface postcrosslinkers.
[0074] The postcrosslinking is typically carried out in such a way
that a solution of the surface postcrosslinker is sprayed onto the
hydrogel or onto the dry base polymer powder. After the spraying,
the polymer powder is dried thermally, and the crosslinking
reaction may take place either before or during drying.
[0075] The spraying with a solution of the crosslinker will
preferably be carried out in mixers having moving mixing
implements, such as screw mixers, paddle mixers, disk mixers,
plowshare mixers and shovel mixers. Particular preference is given
to vertical mixers and very particular preference to plowshare
mixers and shovel mixers. Suitable mixers are, for example,
Lodige.RTM. mixers, Bepex.RTM. mixers, Nauta.RTM. mixers,
Processall.RTM. mixers and Schugi.RTM. mixers.
[0076] The thermal drying is preferably carried out in contact
dryers, more preferably shovel dryers and most preferably disk
dryers. Suitable dryers are, for example, Bepex.RTM. dryers and
Nara.RTM. dryers. It is also possible to use fluidized bed
dryers.
[0077] The drying can be effected in the mixer itself, by heating
the jacket or blowing in warm air. It is equally possible to use a
downstream dryer, for example a tray dryer, a rotary tube oven or a
heatable screw. It is also possible, for example, to utilize an
azeotropic distillation as a drying process.
[0078] Preferred drying temperatures are in the range from 50 to
250.degree. C., preferably in the range from 50 to 200.degree. C.
and more preferably in the range from 50 to 150.degree. C. The
preferred residence time at this temperature in the reaction mixer
or dryer is below 30 minutes and more preferably below 10
minutes.
[0079] The process according to the invention enables the
economically viable continuous preparation of water-absorbing
polymer particles. The polymerization process is relatively
reliable. The proportion of unconverted monomer and of
uncrosslinked polymers is particularly low.
[0080] The present invention further provides an apparatus for
carrying out the process according to the invention, comprising
i) a polymerization reactor, ii) a first inlet to the
polymerization reactor i), iii) at least one inlet into the first
inlet ii), iv) a second inlet to the polymerization reactor i) and
v) a device for comminuting a polymer gel, where the extended axis
of the first inlet ii) intersects the extended axis of the second
inlet iv) in the polymerization reactor i).
[0081] The polymerization reactor is preferably a continuous
kneader or a belt reactor.
[0082] In a kneader, the polymer gel which forms is comminuted by
the stirrer shafts actually within the reactor. In a belt reactor,
the polymer gel which forms has to be comminuted after leaving the
belt reactor, for example in a kneader, an extruder or similar
units.
[0083] Advantageously, the inner surface of the inlet ii) between
polymerization reactor i) and inlet iii) at least partly has a
contact angle for water of at least 60.degree., preferably at least
90.degree., more preferably at least 100.degree..
[0084] The contact angle is a measure of the wetting behavior and
can be determined by customary methods, preferably according to DIN
53900.
[0085] Suitable materials with corresponding wetting behavior are
polyethylene, polypropylene, polyester, polyamide,
polytetrafluoroethylene, polyvinyl chloride, epoxy resins and
silicone resins. Very particular preference is given to
polypropylene.
[0086] The length of the inlet ii) between polymerization reactor
i) and inlet iii) is preferably from 0.01 to 2 m, most preferably
from 0.05 to 1 m, most preferably from 0.1 to 0.5 m.
[0087] The cross-sectional area of the inlet ii) is preferably from
1 to 200 cm.sup.2, more preferably from 5 to 100 cm.sup.2, most
preferably from 10 to 50 cm.sup.2. The inlet ii) preferably has a
circular cross section.
[0088] Preferably at least two inlets iii) are present, more
preferably two, three, four, five or six inlets iii), the inlets
iii) preferably being arranged such that they have a common axis
(for two inlets) or form a symmetrical star (for at least three
inlets), and the axis or star is at right angles to the flow
direction of the monomer solution (multiple addition points)
[0089] Particularly advantageously, two, three or four multiple
addition points are arranged in succession.
[0090] For example, at least eight inlets iii) may be present, in
which case four inlets iii) in each case in a cross shape open into
the monomer line, the at least 2 groups of four inlets iii) being
arranged in succession and offset relative to one another.
[0091] In a preferred embodiment, the inlets iii) are arranged such
that the angle between the inlet ii) and the inlet iii) in flow
direction is less than 90.degree.. The angle is preferably from 10
to 80.degree., more preferably from 20 to 70', most preferably from
30 to 60.degree..
[0092] In a preferred embodiment of the present invention, the
first inlet ii) is designed as a Venturi tube at the opening of the
inlet iii).
[0093] The apparatus is preferably free of dead spaces and the
surfaces should have minimum roughness.
[0094] Dead spaces are sections of the apparatus in which the
average residence time is increased in the course of operation as
intended.
[0095] The inventive apparatus is outstandingly suitable for redox
polymerization.
Methods:
[0096] The measurements should, unless stated otherwise, be carried
out at an ambient temperature of 23.+-.2.degree. C. and relative
humidity of 50.+-.10%. The water-absorbing polymers are mixed
thoroughly before the measurement.
Residual Monomers
[0097] The level of residual monomer of the water-absorbing polymer
particles is determined in accordance with EDANA (European
Disposables and Nonwovens Association) recommended test method No.
410.2-02 "Residual monomers" by means of HPLC,
Extractables
[0098] The content of extractables in the water-absorbing polymer
particles is determined in accordance with EDANA (European
Disposables and Nonwovens Association) recommended test method No.
470.2-02 "Extractables".
[0099] The EDANA test methods are obtainable, for example, from the
European Disposables and Nonwovens Association, Avenue Eugene
Plasky 157, B-1030 Brussels, Belgium.
EXAMPLES
Example 1
[0100] Continuous mixing of water, 50% by weight sodium hydroxide
solution and acrylic acid prepared a 38.8% by weight acrylic
acid/sodium acrylate solution with a degree of neutralization of
71.3 mol %. As the components were mixed, the monomer solution was
cooled to a temperature of 29.degree. C. continuously by a heat
exchanger.
[0101] The polyethylenically unsaturated crosslinker used was
polyethylene glycol diacryates (diacrylate of a polyethylene glycol
having a mean molar mass of 400 g/mol). The amount used was 2 kg
per t of monomer solution.
[0102] After the crosslinker, sodium peroxodisulfate was metered
into the monomer solution. The amount used per t of monomer
solution was 3.1 kg of 15% by weight aqueous sodium
peroxodisulfate.
[0103] The throughput of the monomer solution was 18 t/h.
[0104] The monomer solution, hydrogen peroxide and ascorbic acid
were metered continuously into a List Contiknet reactor (from List,
Arisdorf, Switzerland). The pressure in the reactor was increased
by 10 mbar relative to the environment. The amount of 0.25% by
weight hydrogen peroxide used was 1.0 kg per t of monomer solution.
The amount of 1% by weight ascorbic acid used was 1.1 kg per t of
monomer solution.
[0105] Upstream of the feed, the reaction solution was degassed
with nitrogen and had a temperature of 23.5.degree. C. at the feed.
The reactor was operated with a rotational speed of the shafts of
38 rpm. The residence time of the reaction mixture in the reactor
was 15 minutes.
[0106] The polymerization proceeded without disruption.
[0107] After polymerization had ended and gel comminution, the
polymer gel was placed onto a belt dryer. During the drying, a
pressure reduced by 5 mbar relative to ambient pressure was set.
The precomminuted polymer gel was placed onto the belt dryer with a
layer thickness of 10 cm and dried with warm air (175.degree. C.).
The residence time in the belt dryer was 37 minutes.
[0108] The resulting polymer powder was ground and sieved (100 to
800 .mu.m). The polymer powder was analyzed. The results are
summarized in Table 1.
Example 2
[0109] The procedure of Example 1 was repeated. Sodium
peroxodisulfate and hydrogen peroxide were metered separately into
the monomer solution upstream of the polymerization reactor.
[0110] The polymerization proceeded without disruption.
[0111] The polymer powder was analyzed. The results are compiled in
Table 1.
Example 3
[0112] The procedure of Example 1 was repeated. Sodium
peroxodisulfate, hydrogen peroxide and ascorbic acid were metered
separately into the monomer solution via a Venturi tube at the
reactor inlet.
[0113] To meter in the crosslinker, a 684 cm-long Venturi tube was
used (FIG. 1), in which the pipeline narrowed from a diameter of 5
cm to 2.5 cm over a distance of 6.2 cm (zone L.sub.1), retained the
diameter of 2.5 cm over a distance of 20.3 cm (zone L.sub.2) and
widened again from a diameter of 2.5 cm to 5 cm over a distance of
41.9 cm (zone L.sub.3).
[0114] Sodium peroxodisulfate, hydrogen peroxide and ascorbic acid
were metered into the constriction zone of the Venturi tube in
succession via two addition points opposite one another in each
case.
[0115] Occasionally, the monomer solution polymerized actually
within the Venturi tube.
[0116] The polymer powder was analyzed. The results are compiled in
Table 1.
Example 4
[0117] The procedure of Example 3 was repeated. Sodium
peroxodisulfate and hydrogen peroxide were premixed.
[0118] Occasionally, the monomer solution polymerized actually
within the Venturi tube.
[0119] The polymer powder was analyzed. The results are compiled in
Table 1.
Example 5
[0120] The procedure of Example 3 was repeated. Sodium
peroxodisulfate and hydrogen peroxide were premixed. Ascorbic acid
was metered into the polymerization reactor in parallel to the
monomer solution.
[0121] The polymerization proceeded without disruption.
[0122] The polymer powder was analyzed. The results are compiled in
Table 1.
TABLE-US-00001 TABLE 1 Metering sequence of the initiators Example
NaPS H.sub.2O.sub.2 ACA Extractables Residual monomer 1 A B B 18.4%
by wt. 0.041% by wt. 2 A A B 15.9% by wt. 0.036% by wt. 3*.sup.) D
D D 11.0% by wt. 0.029% by wt. 4*.sup.) C C D 10.5% by wt. 0.025%
by wt. 5 C C B 10.6% by wt. 0.026% by wt. NaPS: sodium
peroxodisulfate H.sub.2O.sub.2: hydrogen peroxide ACA: ascorbic
acid *.sup.)polymerization A: separate metering into monomer
solution B: separate metering into reactor C: joint metering into
Venturi tube D: separate metering into Venturi tube
[0123] The results show that polymers having a low level of
residual monomer and extractables are obtained when the oxidizing
agent is mixed in upstream of the polymerization reactor and the
reducing agent is not added until within the polymerization
reactor. Irrespective of this, the content of residual monomer and
extractables is reduced when a Venturi tube is used as a mixing
unit.
* * * * *