Note: Descriptions are shown in the official language in which they were submitted.
' CA 02319455 2000-08-02
Postcrosslinking of hydrogels using boric esters
Description
The present invention relates to a process for the gel or surface
postcrosslinking of water-absorbing hydrogels using boric esters
of polyhydric alcohols, to the water-absorbing polymers
obtainable in this way and to their use in hygiene articles and
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
CA 02319455 2000-08-02
2
meet the requirements of the governing safety provisions and
workplace hygiene. Furthermore, the use of polymers modified in
this way in hygiene articles appears to be objectionable.
40
5 Polyfunctional alcohols are also known crosslinkers. For example,
EP-A-0 372 981, US-4 666 983 and US-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
10 the esterification reaction which leads to crosslinking is
relatively slow even at such temperatures.
The object was therefore, using compounds which are relatively
slow to react yet are reactive with carboxyl groups, to achieve
15 just as good if not better gel or surface postcrosslinking
compared with the prior art. This object was to be achieved with
a very short reaction time and a very low reaction temperature.
Ideally, the prevailing reaction conditions should be the same as
those obtaining when highly reactive epoxides are used.
It has surprisingly now been found that esters of boric acid with
polyhydric alcohols are highly suitable surface postcrosslinking
agents. These esters are readily synthesizable by reacting boric
acid or boron oxide with alcohol.
The invention provides a process for the surface postcrosslinking
of water-absorbing polymers by treating the polymers with a
surface postcrosslinking solution, the polymers being
postcrosslinked and dried by means of an increase in temperature
during or after the treatment, wherein the crosslinker comprises
an ester of boric acid with a dihydric or polyhydric alcohol in
solution in an inert solvent.
A boric ester is a compound of the formula B(OR)3. Boric esters
are formed, for example, in the reaction of boric anhydride B203
with alcohols, accompanied by formation of boric acid, as
follows:
B203 + 3 ROH -+ B(OR)3 + H3B03
or in the case of a higher alcohol excess, in accordance with
B203 + 6 ROH -+ 2 B ( OR ) 3 + 3 H20
CA 02319455 2000-08-02
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or by the reaction of boric acid with alcohols, which is
accompanied by elimination of water during the esterification
reaction, in accordance with
B(OH)3 + 3 ROH ~ B(OR)3 + 3 H20
Higher esters of boric acid can be obtained, for example, by
transesterification reactions:
B(OR1)3 + 3 RZOH --~ B(ORZ)3 + 3 R10H,
the lower-boiling alcohol R10H being separated from the mixture by
distillation.
The boric esters used in the process of the invention for surface
postcrosslinking are esters of difunctional or polyfunctional
alcohols. In the reaction of boric acid or boric anhydride or in
the transesterification reaction with bifunctional or
polyfunctional alcohols, cyclic compounds or polyesters may also
be formed. Considering the reaction of ethylene glycol (R1=H in
the formulae la-ld) or 1,2-propanediol (R1=CH3 in the formulae
la-ld), the following boric esters may be formed:
With a stoichiometric deficit of alcohol, a partially esterified
boric acid is formed preferentially first of all, e.g.
O
--B-OH ( la)
O
R1
or else the corresponding anhydride of these compounds:
R1
O
/ B-O-~ ( lb )
O
R1
Complete esterification leads preferentially to the following
products:
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R1
O
~ H
r--B CH2- C~~~ ( lc )
°O
R1
R1
along with a smaller amount of cyclic compounds and polyesters
having the following repeating unit
O H
B~ ~CHZ-C~ (ld)
R1
' °J
Using difunctional or polyfunctional alcohols other than ethylene
glycol or 1,2-propanediol, the analogous boric esters are formed.
The radical R1 is hydrogen or an alkyl group having preferably 1
to 12, especially 1 to 6, carbon atoms.
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
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
CA 02319455 2000-08-02
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
5 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.
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
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6
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 33 337, 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-A-4 057 521, 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, alkali metal
salts and ammonium salts of monomers containing acid groups, and
also their amides, hydroxyalkyl esters and amino alkyl- 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
R3 R1
\ /
C=C (2)
/ \
R2
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
45
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7
CH3
O
C ~C\ ,R5 (3),
~ 'N ~ CH2
H
CH3
in which
R3 is hydrogen, methyl, ethyl or a carboxyl group,
R4 is hydrogen, alkali metal ion or ammonium ion,
amino-(C1-C4)-alkyl or hydroxy-(C1-C4)-alkyl, and
RS 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 sodium,
potassium and ammonium salts of these acids. If desired, these
acids may also be in partly neutralized form.
Suitable graft bases for hydrophilic hydrogels obtainable by
graft copolymerization of olefinically unsaturated acids may be
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.
Suitable polyalkylene oxides have, for example, the formula
X
R6 - 0- (CHy- CH- O)n- R~ (4)
in which
R6 and R~ independently of one another are hydrogen, alkyl,
alkenyl or acyl,
X is hydrogen or methyl, and
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8
n is an integer from 1 to 10,000.
R6 and R~ are preferably hydrogen, (C1-C4)-alkyl, (CZ-C6)-alkenyl
or phenyl. Particularly preferred hydrogels are polyacrylates,
polymethacrylates, and the graft copolymers described in US-A-4
931 497, US-A-5 011 892 and US-A-5 041 496.
The hydrophilic highly soluble 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 methylene-bismethacrylamide, 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, tetraallyoxyethane, 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% by weight
strength 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 05C and 1505C, preferably between 105C and 1005C,
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.
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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)2S208, K2S20s or Hz02. 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.
20
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 hydrogen carbonate.
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.
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 mm, with particular
preference 45-850 mm and with very particular preference
200-850 mm.
The invention further provides a water-absorbing polymer
obtainable by the process described above.
CA 02319455 2000-08-02
The invention additionally provides for the use of the products
produced by the process of the invention in hygiene articles,
packaging materials, and nonwovens.
5 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 are sealed into a
teabag (format: 60 mm x 60 mm, Dexter 1234T paper) and soaked for
30 minutes in 0.9% strength by weight sodium chloride solution.
The teabag is then spun for 3 minutes in a customary commercial
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
(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 (2068.5 Pa (0.3 psi) / 3447.5 Pa
(0.5 psi) / 4826.5 Pa (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 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 with the swollen
gel is removed from the filter plate and the apparatus is
reweighed following removal of the weight.
AMENDED SKEET
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The absorbency under load (AUL) is calculated as follows:
AUL [g/gl = ( ~ - 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 consists of measuring cylinder and cover plate.
Examples
Preparation of boric esters
Boric ester 1
A three-necked flask with stirrer, internal thermometer and
reflux condenser is charged with 9 mol of ethylene glycol, and
1 mol of boric anhydride is added slowly to this initial charge.
The solution is stirred at 80~C for 2 hours. Subsequently,
unreacted ethylene glycol and water are separated off by
distillation under reduced pressure. On cooling, a white waxlike
substance is formed.
Boric ester 2
A three-necked flask with stirrer, internal thermometer and
reflux condenser is charged with 4 mol of propanediol (1,2), and
1 mol of boric acid is added to this initial charge. The mixture
is heated to boiling and the water is distilled off under
atmospheric pressure. Subsequently, the excess 1,2-propanediol is
distilled off under reduced pressure. Again, a white waxlike
substance is obtained which solidifies on cooling.
These boric esters are used in accordance with the invention for
crosslinking superabsorbent polymers. The examples below
illustrate the crosslinking action of the boric esters.
Example 1
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
CA 02319455 2000-08-02
12
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.
This base polymer (1 kg) is spray-coated with the surface
postcrosslinking solution in a Lodige plowshare mixer in a
two-stage process.
Stage 1: First of all, a solution of the boric ester 1 (0.5% by
weight based on base polymer) in ethylene glycol as solvent (5%
by weight based on base polymer) is applied by spraying.
Stage 2: Subsequently, the temperature of the heating jacket is
increased linearly from 50~C to 200~C. As soon as the product
temperature has reached 80-90~C, an additional 5% by weight of
water (based on base polymer) is sprayed on. The process is over
after about 30 minutes, and the hydrogel is sieved again in order
to remove lumps and can then be used, for example, as a
water-absorbing polymer in diapers. The values measured for CRC
and AUL are indicated in the table.
Example 2
A base polymer prepared in accordance with Example 1 is sprayed
with crosslinker solution in a blaring laboratory mixer. The
composition of the solution is such that the following dosage,
based on base polymer employed, is achieved: 0.5% by weight boric
ester 1, 4.5% by weight propylene glycol, and 4.5% by weight
water. The moist polymer is then dried at 175~C for 60 minutes.
The table indicates the values measured for CRC and AUL.
Example 3
A base polymer prepared in accordance with Example 1 is sprayed
with crosslinker solution in a blaring laboratory mixer. The
composition of the solution is such that the following dosage,
based on base polymer employed, is achieved: 0.5% by weight boric
ester 2, 4.5% by weight propylene glycol, and 4.5% by weight
water.. The moist polymer is then dried at 175~C for 60 minutes.
The table indicates the typical properties of the polymer.
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Example 4
A base polymer prepared in accordance with Example 1 is sprayed
with crosslinker solution in a Telschig laboratory mixer. The
composition of the solution is such that the following dosage,
based on base polymer employed, is achieved: 0.5% by weight boric
ester 2, 7% by weight methanol, and 3% by weight water. The moist
polymer is then dried at 150~C for 60 minutes. The table indicates
the typical properties of this water-absorbing polymer.
Table
Polymer from CRC AUL 2068.5 AUL 3447.5 AUL 4826.5
[g/g] Pa (0.3 psi) Pa (0.5 psi) Pa (0.7 psi)
[g/gl [g/gl [g/gl
Example 1 35 35 26 18
Example 2 37 34 20 14
Example 3 33 32 28 23
Example 4 29 30 27 25
Comparison 42 10 9 9
base polymer
30
40
AMENDED SHEET
