Note: Descriptions are shown in the official language in which they were submitted.
- CA 02319457 2000-08-02
1
Crosslinking of hydrogels using phosphoric esters
Description
The present invention relates to a process for the gel or surface
postcrosslinking of water-absorbing hydrogels using phosphoric
esters as crosslinkers, to polymers obtainable in this way and to
their use in hygiene articles and as packaging materials.
Hydrophilic highly swellable hydrogels are, in particular,
polymers composed of (co)polymerized hydrophilic monomers, or are
graft (co)polymers of one or more hydrophilic monomers on a
suitable graft base, crosslinked cellulose ethers or crosslinked
starch ethers, crosslinked carboxymethylcellulose, partially
crosslinked polyalkylene oxide, or natural products that are
swellable in aqueous liquids: guar derivatives, for example.
Hydrogels of this kind are used as products for absorbing aqueous
solutions in the production of diapers, tampons, sanitary towels
and other hygiene articles, and as water retainers in market
gardening.
To improve service properties such as diaper rewet and AUL
(absorbency under load), for example, hydrophilic highly
swellable hydrogels are generally subjected to surface or gel
postcrosslinking. This postcrosslinking is known to the person
skilled in the art and is preferably carried out in the aqueous
gel phase or as surface postcrosslinking of the milled and sieved
polymer particles.
Crosslinkers suitable for this purpose are compounds comprising
at least two groups which are able to form covalent bonds with
the carboxyl groups of the hydrophilic polymer. Examples of
suitable crosslinkers are diglycidyl or polyglycidyl compounds,
such as diglycidyl phosphonate, alkoxysilyl compounds,
polyaziridines, polyamines and polyamidoamines, and these
compounds can also be used in mixtures with one another (see for
example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438).
Polyamidoamines which are suitable as crosslinkers are described
in particular in EP-A-0 349 935.
A major disadvantage of these crosslinkers is their high
reactivity. Although this is desirable in terms of chemical
reaction, it carries with. it a relatively high toxicological
potential. In production operations, the processing of such
crosslinkers necessitates special protective measures in order to
meet the requirements of the governing safety provisions and
~
CA 02319457 2000-08-02
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workplace hygiene. Furthermore, the use of polymers modified in
this way in hygiene articles appears to be dubious.
Polyfunctional alcohols are also known as crosslinkers. For
example, EP-A-0 372 981, US-A-4 666 983 and US-A-5 385 983 teach
the use of hydrophilic polyalcohols and the use of polyhydroxy
surfactants. According to these documents the reaction is carried
out at temperatures of 120 - 250°C. The process has the
disadvantage that the esterification reaction which leads to
crosslinking is relatively slow even at such temperatures.
It is an object of the present invention to provide gel or
surface postcrosslinking equivalent to or superior to the prior
art by using relatively inert compounds capable of reacting with
carboxyl groups. This object is to be achieved with a very short
reaction time and a very low reaction temperature. Ideally, the
prevailing reaction conditions shall be the same as those
obtaining when highly reactive epoxides are used.
We have found that this object is achieved, surprisingly, by
esters of phosphoric acid with di- or polyols and amino alcohols.
The present invention accordingly provides a process for the
surface postcrosslinking of water-absorbing polymers, which
comprises (i) treating the polymer with a surface
postcrosslinking solution which includes a crosslinker comprising
one or more esters of phosphoric acid of the formula
OH OH
X-R-0-P=O (1) or O-P=O (2),
OH R-O
where X is OH or NH2 and R is C1-C12-alkylene,
dissolved in an inert solvent and (ii) postcrosslinking and
drying the moist product during or after the treatment by raising
the temperature.
The postcrosslinking temperature is preferably 50-250°C, in
particular between 50-200°C, specifically between 100-180°C.
In order to accelerate the reaction of the surface
postcrosslinking solution, an acidic catalyst can be added.
Catalysts which can be used in the process of the invention are
all inorganic acids, their corresponding anhydrides, and organic
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acids and their corresponding anhydrides. Examples are boric
acid, sulfuric acid, hydroiodic acid, phosphoric acid, tartaric
acid, acetic acid, and toluenesulfonic acid. Also suitable in
particular are their polymeric forms, anhydrides, and the acid
salts of the polybasic acids. Examples of these are boron oxide,
sulfur trioxide, diphosphorus pentoxide, and ammonium dihydrogen
phosphate.
The process of the invention is preferably carried out by
spraying a solution of the surface postcrosslinker onto the dry
base polymer powder. Following spray application, the polymer
powder is dried thermally, it being possible for the crosslinking
reaction to take place either before or during drying. Preference
is given to the spray application of a solution of the
crosslinker in reaction mixers and spray mixers or in mixing and
drying systems such as, for example, Lodige mixers, ~BEPEX
mixers, ~NAUTA mixers, ~SHUGGI mixers or ~PROCESSALL. It is,
moreover, also possible to use fluidized-bed dryers. Drying can
take place in the mixer itself, by heating the outer casing, or
by blowing hot air in. Likewise suitable is a downstream dryer,
such as a shelf dryer, a rotary dryer or a heatable screw, for
example. Alternatively, azeotropic distillation, for example, can
be utilized as a drying technique. The residence time at the
preferred temperature in the reaction mixer or dryer is from 5 to
90 minutes, preferably less than 30 minutes and, with very
particular preference, less than 10 minutes.
As the inert solvent, preference is given to the use of water and
of mixtures of water with monohydric or polyhydric alcohols. It
is, however, also possible to use any organic solvent of
unlimited miscibility with water, such as certain esters and
ketones, for example, which are not themselves reactive under the
process conditions. Where an alcohol/water mixture is used, the
alcohol content of this solution is, for example, 10-90% by
weight, preferably 30-70% by weight, in particular 40-60% by
weight. Any alcohol of unlimited miscibility with water can be
used, as can mixtures of two or more alcohols (e.g., methanol +
glycerol + water). Particular preference is given to the use of
the following alcohols in aqueous solution: methanol, ethanol,
isopropanol, ethylene glycol and, with particular preference,
1,2-propanediol and also 1,3-propanediol. The surface
postcrosslinking solution is used in a ratio of 1-20% by weight,
based on the polymer mass. Particular preference is given to a
solution quantity of 2.5-15% by weight with respect to polymer.
The crosslinker itself is used in an amount of 0.01-1.0% by
weight, based on the polymer used.
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The water-absorbing polymer is preferably a polymeric acrylic
acid or a polyacrylate. This water-absorbing polymer can be
prepared in accordance with a method known from the literature.
Preference is given to polymers containing crosslinking
comonomers (0.001-10 mol%); very particular preference is given,
however, to polymers obtained by free-radical addition
polymerization using a polyfunctional ethylenically unsaturated
free-radical crosslinker which additionally carries at least one
free hydroxyl group (such as, for example, pentaerythritol
triallyl ether or trimethylolpropane diallyl ether).
The hydrophilic highly swellable hydrogels to be employed in the
process of the invention are, in particular, polymers composed of
(co)polymerized hydrophilic monomers, or are graft (co)polymers
of one or more hydrophilic monomers on a suitable graft base,
crosslinked cellulose ethers or crosslinked starch ethers, or
natural products which are swellable in aqueous liquids: guar
derivatives, for example. These hydrogels are known to the person
skilled in the art and are described, for example, in
US-A-4 286 082, DE-C-27 06 135, US-A-4 340 706, DE-C-37 13 601,
DE-C-28 40 010, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085,
DE-A-39 17 846, DE-A-38 07 289, DE-A-35 3 3337, DE-A-35 03 458,
DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261,
DE-A-35 11 086, DE-A-31 18 172, DE-A-30 28 043, DE-A-44 18 881,
EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A-0 312 952,
EP-A-0 205 874, EP-A-0 499 774, DE-A-26 12 846, DE-A-40 20 780,
EP-A-0 205 674, US-5 145 906, EP-A-0 530 438, EP-A-0 670 073,
US-Pr4 057 521, US-A-4 062 817, US-A-4 525 527, US-A-4 295 987,
US-A-5 011 892, US-A-4 076 663 or US-A-4 931 497. The content of
the abovementioned patent documents is expressly incorporated
into the present disclosure by reference.
Examples of hydrophilic monomers suitable for preparing these
hydrophilic highly swellable hydrogels are polymerizable acids,
such as acrylic acid, methacrylic acid, vinylsulfonic acid,
vinylphosphonic acid, malefic acid including its anhydride,
fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic
acid, 2-acrylamido-2-methylpropanephosphonic acid, and also their
amides, hydroxyalkyl esters and amino- or ammonium-containing
esters and amides. Also suitable, furthermore, are water-soluble
N-vinyl amides such as N-vinylformamide or else
diallyldimethylammonium chloride. Preferred hydrophilic monomers
are compounds of the formula
CA 02319457 2000-08-02
R3 R1
\ /
C=C (3)
/ \
H R2
5
in which
R1 hydrogen, methyl or ethyl,
R2 is -COOR4, a sulfonyl group, a phosphonyl group, a
(C1-C4)-alkanol-esterified phosphonyl group, or a group of the
formula .
CH3
0
s
~C~ ~C\ ,R 4
N ~ CHZ
H
CH3
in which
R3 is hydrogen, methyl, ethyl or a carboxyl group,
R4 is hydrogen, amino-(C1-C4)-alkyl, hydroxy-(C1-C4)-alkyl,
alkali metal or ammonium ion and
R5 is a sulfonyl group, a phosphonyl group, a carboxyl group or
the alkali metal or ammonium salts of these groups.
Examples of (C1-C4)-alkanols are methanol, ethanol, n-propanol,
isopropanol and n-butanol. Particularly preferred hydrophilic
monomers are acrylic and methacrylic acid and the alkali metal
ammonium salts of these acids, for example, sodium acrylate,
potassium acrylate or ammonium acrylate.
Suitable graft bases for hydrophilic hydrogels obtainable by
graft copolymerization of olefinically unsaturated acids or of
their alkali metal or ammonium salts maybe natural or synthetic
in origin. Examples are starch, cellulose and cellulose
derivatives, and also other polysaccharides and oligosaccharides,
polyalkylene oxides, especially polyethylene oxides and
polypropylene oxides, and hydrophilic polyesters.
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Suitable polyalkylene oxides have, for example, the formula
X
R6 - O - (CHy- CH - O)n - R7 (5)
in which
R6 and R7 independently of one another are hydrogen, alkyl,
alkenyl or aryl,
X is hydrogen or methyl, and
n is an integer from 1 to 10,000.
R6 and R7 are preferably hydrogen, (C1-C4)-alkyl, (C2-C6)-alkenyl
or phenyl. Particularly preferred hydrogels are polyacrylates,
polymethacrylates, and the graft copolymers described in
US-A-4 931 497, US-A-5 Oll 892 and US-A-5 041 496.
The hydrophilic highly swellable hydrogels are preferably in
crosslinked form; that is, they include compounds having at least
two double bonds which have been copolymerized into the polymer
network. Particularly suitable crosslinkers are
methylenebisacrylamide and methylenebismethacrylamide, esters of
unsaturated mono- or polycarboxylic acids with polyols, such as
diacrylate or triacrylate, examples being the diacrylates and
dimethacrylates of butanediol and of ethylene glycol, and
trimethylolpropane triacrylate, and also allyl compounds such as~~
allyl (meth)acrylate, triallyl cyanurate, diallyl maleate,
polyallyl esters, tetraallyloxyethane, triallylamine,
tetraallylethylenediamine, allyl esters of phosphoric acid, and
vinylphosphonic acid derivatives as described, for example, in
EP-A-0 343 427. In the process of the invention, however,
particular preference is given to hydrogels prepared using
polyallyl ethers as crosslinkers and by acidic homopolymerization
of acrylic acid. Suitable crosslinkers are pentaerythritol tri-
and tetraallyl ether, polyethylene glycol diallyl ether,
monoethylene glycol diallyl ether, glycerol di- and triallyl
ether, polyallyl ethers based on sorbitol, and alkoxylated
variants thereof.
The hydrophilic highly swellable hydrogels can be prepared by
conventional polymerization processes. Preference is given to
addition polymerization in aqueous solution by the process known
as gel polymerization. In this process from 15 to 50% strength by
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weight aqueous solutions of one or more hydrophilic monomers,
and, if desired, of a suitable graft base, are polymerized in the
presence of a free-radical initiator, preferably without
mechanical mixing, utilizing the Trommsdorff-Norrish effect
(Makromol. Chem. 1 (1947) 169).
The polymerization reaction can be conducted in the temperature
range between 0°C and 150°C, preferably between 10°C and
100°C,
either at atmospheric pressure or under an increased or reduced
pressure. The polymerization may also be performed in an inert
gas atmosphere, preferably under nitrogen.
The polymerization can be initiated using high-energy
electromagnetic radiation or by the customary chemical
polymerization initiators. Examples of these are organic
peroxides, such as benzoyl peroxide, tert-butyl hydroperoxide,
methyl ethyl ketone peroxide and cumene hydroperoxide, azo
compounds, such as azodiisobutyronitrile, and inorganic peroxo
compounds, such as (NH4)2Sy08, KZS208 or H202. They can if desired
be used in combination with reducing agents such as sodium
hydrogen sulfite or iron(II) sulfate, or redox systems. Redox
systems include a reducing component, which is generally an
aliphatic or aromatic sulfinic acid, such as benzenesulfinic acid
or toluenesulfinic acid or derivatives of these acids, such as
Mannich adducts of sulfinic acid, aldehydes and amino compounds,
as described in DE-C-13 O1 566.
The qualities of the polymers can be improved further by
continuing to heat the polymer gels for a number of hours within
the temperature range from 50 to 130°C, preferably from 70 to
100°C .
The resultant gels are neutralized to the extent of 0-100 mol%
based on monomer employed, preferably 25-100 mol% and with
particular preference 50-85 mol%, it being possible to use the
customary neutralizing agents, preferably alkali metal hydroxides
or alkali metal oxides, and with particular preference sodium
hydroxide, sodium carbonate or sodium bicarbonate. Neutralization
is usually effected by mixing in the neutralizing agent as an
aqueous solution or else, preferably, as a solid. For this
purpose the gel is mechanically comminuted, by means of a miNCer
for example, and the neutralizing agent is sprayed on, scattered
over or poured on, and then carefully mixed in. To effect
homogenization the resultant gel mass may be passed through the
mincer again a number of times.
CA 02319457 2000-08-02
The neutralized gel mass is then dried with a belt dryer or roll
dryer until the residual moisture content is less than 10% by
weight, preferably below 5% by weight. The dried hydrogel is then
ground and sieved, the usual grinding apparatus being roll mills,
pin mills or vibrator mills. The preferred particle size of the
sieved hydrogel lies in the range 45-1000 Eun, with particular
preference 45-850 Eun and with very particular preference
200-850 Eun.
According to the invention, acrylate polymers are crosslinked
using esters of di- or polyols and aminoalcohols. The esters of
the polyols are described by the formula
OH
HO-R-O-P=O (6)
OH
and the esterification of aminoalcohols results in compounds of
the formula
OH
HyN - R - O - P = O (7).
OH
The use of polyols may also lead to the formation of cyclic
esters of the general formula
OH
O-P-O (2),
R - O
R3 Ri
\ /
C=C (2)
/ \
H R2
In the formulae (2), (6) and (7), R is C1- to C12-alkylene,
preferably.C2- to C6-alkylene, especially -CH2-CHZ- or
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- CH -CHZ -
CH3
These compounds may be obtained not only by esterification of the
free phosphoric acid with the alcohols, by transesterification
reactions or by the reaction of the diols, polyols or
aminoalcohols with phosphorus pentoxide or phosphorus
oxychloride.
In order to ascertain the quality of surface postcrosslinking the
dried hydrogel is then tested using the test methods known from
the prior art and described below:
Methods:
1) Centrifuge retention capacity (CRC):
This method measures the free swellability of the hydrogel in a
teabag. Approximately 0.200 g of dry hydrogel is sealed into a
teabag (format: 60 mm x 60 mm, Dexter 1234T paper) and soaked for
minutes in 0.9% strength by weight sodium chloride solution.
The teabag is then spun for 3 minutes in a customary commercial
25 spindryer (Bauknecht WS 130, 1400 rpm, basket diameter 230 mm).
The amount of liquid absorbed is determined by weighing the
centrifuged teabag. The absorption capacity of the teabag itself
is taken into account by determination of a blank value (teabag
without hydrogel), which is deducted from the weighing result
30 (teabag with swollen hydrogel).
Retention CRC [g/g] _ (weighing result teabag - blank value -
initial weight of hydrogel) . initial weight of hydrogel
2) Absorbency under load (0.3 / 0.5 / 0.7 psi):
For the absorbency under load, 0.900 g of dry hydrogel is
distributed uniformly on the screen base of a measuring cell. The
measuring cell consists of a Plexiglas cylinder (height = 50 mm,
diameter = 60 mm) whose base is formed by sticking on a screen of
steel mesh (mesh size 36 microns, or 400 mesh).
A cover plate is placed over the uniformly distributed hydrogel
and loaded with an appropriate weight. The cell is then placed on
a filter paper (S&S 589 black band, diameter = 90 mm) lying on a
porous glass filter plate, this filter plate itself lying in a
Petri dish (height = 30 mm, diameter = 200 mm) which contains
CA 02319457 2000-08-02
0.9% strength by weight sodium chloride solution so that the
liquid level at the beginning of the experiment is level with the
top edge of the glass frit. The hydrogel is then left to absorb
the salt solution for 60 minutes. Subsequently, the complete cell
5 with the swollen gel is removed from the filter plate and the
apparatus is reweighed following removal of the weight.
The absorbency under load (AUL) is calculated as follows:
10 AUL (g/gj = ( Wb - Wa ) / Ws
where Wb is the mass of the apparatus + gel after swelling,
Wa is the mass of the apparatus + initial weight of gel before
swelling, and
Ws is the initial weight of dry hydrogel.
The apparatus is measuring cylinder + cover plate.
Examples
Preparation of crosslinkers 1 to 3
Crosslinker 1:
2-Aminoethyl dihydrogenphosphate
In a three-neck flask with stirrer, internal thermometer, reflux
condenser and gas inlet tube, 1 mol of aminoethanol is dissolved
in 2 mol of water and POC13 is gradually added dropwise with
cooling: The resulting hydrochloric acid is neutralized with
ammonia and subsequently stirred at room temperature for 24
hours. The water is then distilled off and the ester prepared is
recrystallized from water / alcohol at low temperature to provide
a brownish compound having a melting point of 240°C (with
decomposition). The compound is moderately soluble in water and
water/alcohol mixtures and is slow to hydrolyze.
Crosslinker 2:
2-Hydroxypropyl or 1-hydroxy-2-propyl dihydrogenphosphate or
mixture of the two compounds
Propanediol is reacted with POC13 in chloroform in a similar
manner to the preparation of crosslinker 1 to provide a
hygroscopic brownish compound which is but slow to hydrolyze, yet
is readily soluble in water and water/alcohol mixtures.
CA 02319457 2000-08-02
IZ
Crosslinker 3:
Mono- and diphosphoric esters or mixture of the two compounds.
A three-neck flask with stirrer, internal thermometer and
condenser is charged with 3 mol of 2-hydroxyethyl methacrylate
and 1 mol of phosphorus pentoxide is added in such a way with
cooling that 40°C is not exceeded. This is followed by stirring at
room temperature for 1 hour to provide a brownish substance which
is paraffinic below 20°C, yet is readily soluble in water and
water/alcohol mixtures. The clear viscous compound is dissolved
in water and free-radically polymerized at elevated temperature
under nitrogen to form a low molecular weight polymer.
Inventive examples:
The inventive examples illustrate the effect of the surface
postcrosslinking on the superabsorbent polymers. As will be known
to those skilled in the art, this postcrosslinking can be
determined by measuring the centrifuge retention capacity (CRC)
and the absorbency under load (AUL). This surface crosslinking
causes the CRC to decrease typically by 5-10 g/g, while the AUL
0.7 psi increases by about 10 and the AUL 0.3 psi by more than
20 g/g.
Example 1
Base polymer
In a 40 1 plastic bucket, 6.9 kg of pure acrylic acid are diluted
with 23 kg of water. 45 g of pentaerythritol triallyl ether are
added with stirring to this solution, and the sealed bucket is
rendered inert by passing nitrogen through it. The polymerization
is then initiated by adding about 400 mg of hydrogen peroxide and
200 mg of ascorbic acid. After the end of the reaction the gel is
mechanically comminuted and sodium hydroxide solution is added in
an amount sufficient to achieve a degree of neutralization of
75 mol%, based on the acrylic acid employed. The neutralized gel
is then dried on a roll dryer, ground with a pin mill and,
finally, isolated by sieving. This is the base polymer used in
the subsequent examples. CRC, AUL 20 and AUL 40 are reported in
the table.
CA 02319457 2000-08-02
12
Example 2
The base polymer prepared in Example 1 is sprayed with
crosslinker soluton in a blaring laboratory mixer. The composition
of the solution is such that the following dosage, based on the
base polymer used, is obtained:
0.40% by weight of crosslinker 1, 5% by weight of propylene
glycol and 5% by weight of water as solvent. The moist polymer is
then divided into two batches (a) and (b), which are each dried
at 175°C. The drying time is 60 min for batch (a) and 90 min for
batch (b).
The crosslinking effect is not attributable to the propylene
glycol, since surface postcrosslinking is obtained even in purely
aqueous systems if the use level is increased. CRC and AUL are
reported in the table.
Example 3:
The base polymer prepared in Example 1 is sprayed with
crosslinker soluton in a blaring laboratory mixer. The composition
of the solution is such that the following dosage, based on the
base polymer used, is obtained:
3.0% by weight of crosslinker 1 and 10% by weight of water. The
moist polymer is then dried at 175°C
(a) for 60 min and
(b) for 90 min.
The use of catalysts shortens the reaction time for the surface
postcrosslinking and hence the residence time in the reactor.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 4
The base polymer prepared in Example 1 is sprayed with
crosslinker soluton in a blaring laboratory mixer. The composition
of the solution is such that the following dosage, based on the
base polymer used, is obtained:
1.5% by weight of crosslinker 2, 10% by weight of water and 0.2%
of boric acid as catalyst. The moist polymer is then dried at
175°C
' . CA 02319457 2000-08-02
13
(a) for 30 min and
(b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 5
The base polymer prepared in Example 1 is sprayed with
crosslinker soluton in a blaring laboratory mixer. The composition
of the solution is such that the following dosage, based on the
base polymer used, is obtained:
5.0% by weight of crosslinker 3, 12% by weight of water and 0.2%
by weight of ammonium dihydrogenphosphate. The moist polymer is
then dried at 175°C
(a) for 30 min and
(b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 6
The base polymer prepared in Example 1 is sprayed with
crosslinker soluton in a blaring laboratory mixer. The composition
of the solution is such that the following dosage, based on the
base polymer used, is obtained:
0.20% by weight of crosslinker 2, 5% by weight of methanol, 5% by
weight of water and 0.2% of ammonium dihydrogenphosphate. The
moist polymer is then dried at 175°C
(a) for 30 min and
(b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 7
The base polymer prepared in Example 1 is sprayed with
crosslinker soluton in a blaring laboratory mixer. The composition
of the solution is such that the following dosage, based on the
base polymer used, is obtained:
CA 02319457 2000-08-02
14
0.50% by weight of crosslinker 2, 5% by weight of 1,2-propanediol
and 5% by weight of water. The moist polymer is then dried at
175°C
(a) for 30 min and
(b) for 60 min.
CRC, AUL 20 and AUL 40 are reported in the table.
Example 8
The base polymer prepared in Example 1 is sprayed with
crosslinker soluton in a blaring laboratory mixer. The composition
of the solution is such that the following dosage, based on the
base polymer used, is obtained:
0.20% by weight of crosslinker 3, 6% by weight of 1,2-propanediol
and 6% by weight of water. The moist polymer is then dried at
175°C
(a) for 30 min and
(b) for 60 min.
(c) For comparison, the base polymer is sprayed in similar
fashion with 6% by weight of 1,2-propanediol and 6% by weight
of water. The moist polymer is then dried at 175°C for
60 min. This comparative example thus does not employ a
crosslinker. CRC, AUL 20 and AUL 40 are reported in the
table.
40
CA 02319457 2000-08-02
15
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