Link: Pharm/Biotech Resources
Title: Water-absorbing polymers with supramolecular
hollow molecules, method for producing them and use of the same
United States Patent: 6,958,429
Issued: October 25, 2005
Inventors: Bruhn; Christoph (Moers, DE); Herrmann; Edgar (Nettetal, DE); Issberner; Jörg (Krefeld, DE); Kersten; Dagmar (Meerbusch, DE); Mertens; Richard (Krefeld, DE); Werner; Georg (Toenisvorst, DE)
Assignee: Stockhausen GmbH & Co. KG (Krefeld, DE)
Appl. No.: 775185
Filed: February 11, 2004
Abstract
The invention relates to absorbent polymers based on optionally partially neutralised, monoethylenically unsaturated, acid group-carrying monomers. The surfaces of said polymers are re-cross-linked. The inventive polymers also have cyclodextrines and/or cyclodextrine derivatives which are covalently and/or ionically bonded and/or included therein.
Description of the Invention
The invention relates to absorbents, preferably for water and aqueous
liquids, which absorbents are based on polymers absorbing aqueous liquids,
wherein cyclodextrin or cyclodextrin derivatives have been incorporated
ionically, covalently and/or as a result of mechanical inclusion.
Commercially available superabsorbing polymers essentially are crosslinked
polyacrylic acids, crosslinked starch/acrylic acid graft copolymers,
crosslinked hydrolyzed starch/acrylonitrile graft copolymers, crosslinked
poly-(maleic anhydride-co-isobutylene), or mixtures of various of the
above-mentioned crosslinked polymers, wherein the carboxylic groups have
been subjected to partial neutralization with sodium and/or potassium ions.
Such polymers find use e.g. in hygiene articles capable of absorbing body
fluids such as urine or in materials for cable sheathings where they absorb
large amounts of aqueous liquids and body fluids such as urine or blood with
swelling and formation of hydrogels. Furthermore, the absorbed amount of
liquid must be retained under a pressure typical of use. During the further
technical development of superabsorbing polymers, the pattern of
requirements to be met by these products has changed significantly over the
years. To date, the development of superabsorbers has been forced
particularly with respect to the amount of absorbed liquid and pressure
stability.
Such crosslinked polymer products based on monomers containing acid groups
are obtained by using one or more primary crosslinkers and one or more
secondary crosslinkers and exhibit a combination of properties, namely, high
retention, high absorption under pressure, low solubles, and rapid
absorption of liquid, which has not been achieved so far. When used in
hygiene articles, these crosslinked polymer products have the advantage that
secreted fluids, once absorbed by the polymer product, can no longer contact
the skin. Thus, skin lesions such as diaper dermatitis can largely be
avoided. Such comfort can even be increased by absorbing malodorous
compounds.
According to Römpp Chemie Lexikon, the content of urine components is
subject to physiological fluctuations; also, particular substances are
secreted at concentrations varying within a daily period, so that more
precise data on the-urine composition invariably are related to the
so-called 24 hour urine which, in a healthy adult, contains e.g. urea
(average 20 g), uric acid (0.5 g), creatinine (1.2 g), ammonia (0.5 g),
amino acids (2 g), proteins (60 mg), reducing substances (0.5 g, about 70 mg
of which are D-glucose or urine sugar), citric acid (0.5 g) and other
organic acids, as well as certain vitamins (C, B12 etc.). The
following inorganic ions are present: Na+ (5.9 g), K+
(2.7 g), NH4+ (0.8 g), Ca2+ (0.5 g), Mg2+
(0.4 g); Cl- (8.9 g), PO43- (4.1 g), SO4-2
(2.4 g). The dry content is between 50 and 72 g. Inter alia,
alkylfurans, ketones, lactones, pyrrole, allyl isothiocyanate, and dimethyl
sulfone have been recognized as volatile components of urine. Most of the
volatile components are molecules having a molar mass below about 1000 g/mol
and a high vapor pressure.
Volatile components of urine have also been investigated by, inter alia, A.
Zlatkis et al. (Anal. Chem. Vol. 45, 763ff.). It is also well-known that
consumption of asparagus results in an increase of the concentration of
organic sulfur-containing compounds in human urine (R. H. Waring,
Xenobiotika, Vol. 17, 1363ff.). In patients who are subject to specific
diets and generally, in patients who ingest specific medications, or in
elderly individuals with decreasing kidney function, the urine may include
malodorous substances. Patients suffering from urine incontinence have an
increased secretion of ureases which convert the urea contained in urine,
thereby liberating toxic ammonia. Also, a pathological change is well-known
which is referred to as fish smell syndrome. It results from an increased
secretion of quaternary ammonium compounds.
Previous approaches of achieving an odor reduction in incontinence products
are based on reducing the concentration of free ammonia. Basically, there
are two approaches to this end: preventing additional production of ammonia
from urea degradation by suitable urease inhibitors (A. Norberg et al.,
Gerontology, 1984, 30, 261ff.), or by protonating free ammonia and binding
thereof in the form of a carboxylate ammonium salt. This method is
disadvantageous in that essentially, merely ammonia and other
nitrogen-containing components can be controlled. Malodorous compounds
lacking basic groups, e.g. thiols, are still capable of entering the vapor
space.
It is well-known to those skilled in the art that certain hollow molecules,
also referred to as endohedral or concave molecules, are capable of
incorporating other, mostly smaller, so-called guest molecules, thereby
forming a host-guest complex. Such complex formation has an effect on the
chemical and physical properties of both guest and host molecule. These
hollow-forming molecules include the cyclodextrins.
Cyclodextrins are formed during starch degradation by Bacillus macerans
or Bacillus circulans under the action of cyclodextrin glycosyl
transferase. They are comprised of 6, 7, 8 or 9 glucose units α-1,4-linked
to form a ring (α-, β- or γ-cyclodextrins). They are capable of entrapping
hydrophobic guest molecules in varying amounts up to saturation ("molecular
encapsulation"), e.g. gases, alcohols or hydrocarbons. The use of
cyclodextrins as host molecule is reported comprehensively in the work of J.
Szejtli (Cyclodextrin Technology, Kluwer Academic Publishers, 1988).
Also, the production of polymers containing cyclodextrins is already known.
Thus, EP-A-0,483,380 obtains cyclodextrin-containing polymers by
copolymerizing cyclodextrins bearing aldehyde groups with polyvinyl alcohol.
Crosslinked, water-swellable, hydrophilic bead polymers made of
hydroxyalkylcyclodextrins and epichlorohydrin or polyepoxide type
crosslinkers are known from U.S. Pat. No. 5,360,899. These crosslinkers
involve a carcinogenic potential and therefore, such products cannot be used
in hygiene articles. These cyclodextrins immobilized by polymerization are
used as packing and separating material in chromatographic separation
columns.
Furthermore, water-swellable, hydrophilic bead polymers made of
cyclodextrins bearing glycidyl or methacrylate groups and optionally other
comonomers such as hydroxyethyl acrylate are known from U.S. Pat. No.
5,357,012. Likewise, these cyclodextrins immobilized by polymerization are
used as packing and separating material in chromatographic separation
columns.
DE-A-195 20 989 describes covalent binding of reactive cyclodextrin
derivatives having at least one nitrogen-containing heterocycle to polymers
bearing at least one nucleophilic group. Polymers linked to cyclodextrins
according to this method must have nucleophilic groups such as OH, NH, or SH
groups. Also, polymerizable cyclodextrin derivatives are mentioned which,
after suitable modification, are copolymerized with other monomers, e.g.
ethylenically unsaturated compounds. As noted in this publication, the
products according to the above-mentioned US patent specifications U.S. Pat.
No. 5,357,012 and U.S. Pat. No. 5,360,899 involve the drawback that
cyclodextrin incorporation is difficult to control in spatial terms and that
cyclodextrins fixed inside the polymers are no longer available for
utilization. The use of polymers, which include cyclodextrin derivatives, as
superabsorbing materials is not mentioned.
Inter alia, the use of cyclodextrins in hygiene products is known from
EP-A-806,195, WO 94/22501, and WO 94/22500. Therein, the cyclodextrins are
employed to absorb odors. In those cases where the cyclodextrins or
cyclodextrin complexes are not bound to the powdered absorbent, demixing
during storage or transportation of the hygiene articles may occur. As a
result, the effectiveness of the cyclodextrins as odor absorbent may be lost
due to demixing between absorbent and cyclodextrins.
To achieve improved adhesion on powdered absorbents, WO 94/22501 teaches
addition of polyethylene glycols or other linear polymers to cyclodextrin in
a "melt" or in solution and subsequent spraying on the powdered absorbent.
However, as is well-known to those skilled in the art, linear polymers have
a marked tendency to "thread" into the cyclodextrin cavity, which fact is
advantageously utilized in supramolecular chemistry in order to produce e.g.
rotaxans or catenanes (cf. the documents U.S. Pat. No. 5,538,655; G. Wenz,
Angew. Chem. 1994, 106, 851). Typically, the linear polymers have a
molecular weight (m.w.) of more than 200. Also, suitable polymers are e.g.
polyethylene glycol (PEG), polypropylene oxide (PEO) and polyethyleneimine.
Multiple cyclodextrins can be threaded on a linear polymer chain; Harada et
al. (J. Org. Chem. 58, 1993, 7524-28) report that 20 cyclodextrins can be
threaded on a polyethylene glycol having an average molecular weight of 2000
g/mol. Therefore, the process described in WO 94/22501 is particularly
disadvantageous, because the cyclodextrin cavities after such a polyethylene
glycol pretreatment are no longer quantitatively available for absorbing
malodorous compounds.
The invention therefore is based on the object of providing polymer products
capable of absorbing water or aqueous liquids, and capable of binding
malodorous organic compounds such as occurring e.g. in urine or other fluids
secreted from the body, and methods of producing same.
The polymer products should not involve the drawbacks of prior art and
enable a preferably uniform, marked reduction of gaseous, malodorous
compounds released during use. Moreover, a largely stable dispersion of the
deodorant component in the absorbent should be achieved, i.e., demixing
prior to and during use should be avoided as much as possible. In addition,
binding of the deodorant component should not be effected by using
carcinogenic or otherwise hazardous substances. Furthermore, the
effectiveness of the deodorant component in the absorbent should be
independent of its location, i.e., whether inside the polymer or at the
surface thereof.
According to the invention, said object is accomplished by providing
polymers based on crosslinked monomers bearing optionally partially
neutralized acid groups, which polymers have cyclodextrins and/or
derivatives thereof bound ionically and/or covalently and/or incorporated
therein.
As a result of the inventive binding to the preferably powdered polymer, the
cyclodextrin component can be extracted by the liquid to be absorbed to only
a lesser extent, or, in the dry state, undergoes demixing to only a lesser
extent. Despite the intimate linkage with the crosslinked absorber bearing
acid groups, the polymer according to the invention surprisingly shows
excellent absorption of odors which is even enhanced compared to unbound
cyclodextrin. In particular, the absorbent polymers exhibit high absorption
of odors even in those cases where the cyclodextrin is fixed inside the
absorber. This can be established by an effective reduction in the gas
concentration of malodorous substances.
Moreover, the polymer products of the invention are excellently suited for
incorporating active substances, and when used, these active substances can
optionally be released in a controlled fashion. By incorporation in the
absorbents of the invention, the stability of sensitive active substances is
markedly improved.
According to the invention, α,β,γ type cyclodextrins and derivatives thereof
are suitable. Above all, those derivatives are possible which permit
chemical linkage by ionic or covalent binding to the monomer bearing acid
groups or to the corresponding polymer. Covalent linkages preferably are via
C—C bonds as, for example, with cyclodextrin derivatives having
ethylenically unsaturated groups incorporated covalently in the polymer
chain already during polymerization of the monomers. For example, such
groups are (meth)acrylic, (meth)allyl and vinyl groups. According to the
invention, however, covalent linkage of the cyclodextrin component to the
polymer of ethylenically unsaturated monomers is also possible subsequent to
polymerization via ether, amide or ester groups.
Ionic binding of the cyclodextrin derivatives can be effected using anionic
or cationic groups, with cationic groups being preferred. Frequently, it is
advantageous when the cyclodextrin molecules have multiple substitutions
with ionic groups. Examples of anionic groups are carboxylate, sulfate and
sulfonate groups. Examples of cationic groups are quaternary grous
containing nitrogen.
Ionic cyclodextrins can be produced by reacting cyclodextrin derivatives
with reactive compounds such as chloroacetic acid, sodium chloroacetate,
maleic acid, maleic anhydride, and succinic anhydride. In an aqueous
solution, these reaction products, e.g. carboxymethylcyclodextrin, carry a
negative charge in a basic medium due to the carboxylate group.
Cyclodextrin derivatives to be used according to the invention and having at
least one nitrogen-containing heterocycle can be produced according to the
teaching of DE-A-195 20 98, A1, the disclosure of which is hereby
incorporated by reference. In this way, cyclodextrin derivatives can be
obtained, which include another group active towards nucleophilic groups.
These derivatives can undergo direct reaction with polymers bearing
nucleophilic groups. Examples of nucleophilic groups are —OH, —NH or —SH
groups.
Other chemically modified cyclodextrins to be used according to the
invention can be obtained as described in A. P. Croft and R. A. Bartsch,
Tetrahedron Vol. 39, No. 9, pp. 1417-1473. They are obtained by reacting
nitrogen-containing compounds having at least one functional group capable
of reacting with the hydroxyl groups of the cyclodextrins to form ether,
ester or acetal groups.
Cationic cyclodextrins such as described in Ch. Roussel, A. Favrou, Journal
of Chromatography A, 704 (1995), 67-74, are particularly preferred. They are
obtained by reacting cyclodextrin with e.g.
N-(3-chloro-2-hydroxypropyl)-N,N,N-trimethylammonium chloride. The
cyclodextrins described in the above publication have a degree of
substitution of 0.2.
The ionic cyclodextrins including at least one nitrogen-containing aliphatic
residue, which can be used according to the invention, may also be produced
e.g. according to the methods described in U.S. Pat. Nos. 3,740,391;
4,153,585 and 4,638,058. The disclosure of the above-mentioned publications
is hereby incorporated by reference.
For example, N,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl
(meth)acrylate, N,N-dimethylaminoethyl(meth)acrylamide, and
N,N-dimethylaminopropyl(meth)acrylamide, or the quaternary derivatives
thereof obtained by reaction with alkyl halides may be mentioned as suitable
monomers. Preferably, N,N-dimethylaminoethyl acrylate (ADAME or ADAME-quat.)
and N,N-dimethylaminopropylacrylamide (DIMAPA or DIMAPA-quat.) are employed.
Here, the compound of formula I undergoes reaction:
wherein R1=H, CH3,
 | R2=C2-C4 alkylene group, |
| R3=H, C1-C4 alkyl group, |
| X=O, NH, |
| Y=Cl, SO4. |
The average degree of substitution (DS value) per anhydroglucose unit for
substituents containing nitrogen can be determined according to methods
known from literature using elemental analysis as described e.g. in U.S.
Pat. No. 5,134,127 and U.S. Pat. No. 3,453,257 for substituents containing
sulfur or nitrogen. When using the synthetic methods described in U.S. Pat.
Nos. 3,740,391 and 4,153,585, the DS value can be varied within wide limits.
3 hydroxyl groups per anhydroglucose unit of a cyclodextrin are capable of
undergoing further reaction. Therefore, the degree of substitution e.g. in
case of β-cyclodextrin can be between 0.05 and 3 at maximum. A degree of
substitution below 0.05 indicates that a mixture of non-modified
cyclodextrin and chemically modified cyclodextrin is present.
According to the invention, the degree of substitution (DS) of the
cyclodextrin derivatives is 0.005-2, preferably 0.05-1.5.
In addition to the above-mentioned groups required for binding to the
polymer, the cyclodextrins may also contain other substituents having no
reactivity towards the polymer. For example, these include reaction products
of cyclodextrins with alkylating agents, e.g. C1-C22
alkyl halides, e.g. methyl chloride, ethyl chloride, butyl chloride, butyl
bromide, benzyl chloride, lauryl chloride, stearyl chloride, or dimethyl
sulfate, or reaction products of cyclodextrins with alkylene oxides such as
ethylene oxide, propylene oxide, butylene oxide, or styrene oxide.
The amount of cyclodextrin or derivatives thereof to be employed according
to the invention is 0.01-50 wt.-%, preferably 0.1-30 wt.-%, more preferably
0.5-10 wt.-%, relative to the total amount of polymer.
Well-known processes are possible for polymerizing the polymers of the
invention optionally having superabsorbent properties, e.g. bulk
polymerization, solution polymerization spray polymerization, inverse
emulsion polymerization, and inverse suspension polymerization.
Preferably, a solution polymerization is performed using water as solvent.
The solution polymerization may be conducted in a continuous or batchwise
fashion. The prior art includes a broad spectrum of possible variations with
respect to concentration conditions, temperatures, type and amount of
initiators and of secondary catalysts. Typical processes have been described
in the following patent specifications: U.S. Pat. No. 4,286,082; DE 27 06
135, U.S. Pat. No. 4,076,663, DE 35 03 458, DE 40 20 780, DE 42 44 548, DE
43 23 001, DE 43 33 056, DE 44 18 818 which hereby are incorporated as
disclosure of the process according to the invention.
Preferably, aliphatic, optionally substituted C2-C10,
preferably C2-C5 carboxylic acids or sulfonic acids,
such as acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid,
maleic acid, fumaric acid, itaconic acid, vinylacetic acid, vinylsulfonic
acid, methallylsulfonic acid, 2-acrylamido-2-methyl-1-propanesulfonic acid,
as well as the alkali and/or ammonium salts or mixtures thereof are possible
as ethylenically unsaturated monomers containing acid groups. It is
preferred to use acrylic acid and its alkali and/or ammonium salts and
mixtures thereof. Furthermore, it is also possible to use monomers being
hydrolyzed to form acid groups as late as subsequent to the polymerization,
e.g. the corresponding nitrile compounds.
In order to modify the polymer properties, up to 40 wt.-% of monomers other
than the monomers containing acid groups, which are soluble in the aqueous
polymerization batch, such as acrylamide, methacrylamide, acrylonitrile, (meth)allyl
alcohol ethoxylates, and mono(meth)acrylic acid esters of polyhydric
alcohols or ethoxylates can optionally be used.
Minor amounts of crosslinking monomers having more than one reactive group
in their molecules are copolymerized together with the above-mentioned
monomers, thereby forming partially crosslinked polymer products which are
no longer soluble in water but merely swellable. Bi- or multifunctional
monomers, e.g. methylenebisacryl- or -methacrylamide, or
ethylenebisacrylamide may be mentioned as crosslinking monomers, and also,
allyl compounds such as allyl (meth)acrylate, alkoxylated allyl (meth)acrylate
reacted preferably with from 1 to 30 mol of ethylene oxide units, triallyl
cyanurate, maleic acid diallyl ester, polyallyl esters, tetraallyloxyethane,
triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid or
phosphorous acid, and also, the N-methylol compounds of unsaturated amides
such as methacrylamide or acrylamide and the ethers derived therefrom, as
well as esters of polyols and alkoxylated polyols with unsaturated acids,
such as diacrylates or triacrylates, e.g. butanediol or ethylene glycol
diacrylate, polyglycol di(meth)acrylates, trimethylolpropane triacrylate, di-
and triacrylate esters of trimethylolpropane preferably oxyalkylated (ethoxylated)
with 1 to 30 mol alkylene oxide, acrylate and methacrylate esters of
glycerol and pentaerythritol, and of glycerol and pentaerythritol preferably
oxyethylated with 1 to 30 mol ethylene oxide. It is preferred to use
triallylamine, acrylates of polyhydric alcohols or alkoxylates thereof, and
methallyl alcohol acrylates or alkoxylates thereof. The ratio of
crosslinking monomers is from 0.01 to 3.0 wt.-%, preferably from 0.05 to 2.0
wt.-%, and more preferably from 0.05 to 1.5 wt.-%, relative to the total
weight of the monomers.
The optional neutralization of the acidic monomers according to the
polymerization process of the invention can be performed in various ways. On
the one hand, according to the teaching of U.S. Pat. No. 4,654,039, the
polymerization may be conducted directly with the acidic monomers, with
neutralization being effected subsequently in the polymer gel. Preferably,
the acid groups of the monomers are already neutralized to 20-95%,
preferably 50-80% prior to polymerization, in which case they are present as
sodium and/or potassium and/or ammonium salts at the time polymerization is
begun. It is preferred to use those bases for neutralization which do not
adversely affect the subsequent polymerization. It is preferred to use
sodium or potassium hydroxide solution and/or ammonia, with sodium hydroxide
solution being particularly preferred; addition of sodium carbonate,
potassium carbonate or sodium bicarbonate may have an additional positive
effect as taught in U.S. Pat. Nos. 5,314,420 and 5,154,713. Before
initiating the polymerization in this adiabatic solution polymerization, the
partially neutralized monomer solution is cooled to a temperature of below
30° C., preferably below 20° C. The other polymerization processes comply
with the temperatures known from prior art as apparent from the literature
below.
The polymer products of the invention may optionally contain water-soluble
natural or synthetic polymers as a basis for grafting in amounts up to 30
wt.-%. Inter alia, these include partially or completely saponified
polyvinyl alcohols, starch or starch derivatives, cellulose or cellulose
derivatives, polyacrylic acids, polyglycols, or mixtures thereof. The
molecular weights of the polymers added as basis for grafting must be
adapted to the circumstances of the polymerization conditions. In the event
of an aqueous solution polymerization, for example, it may be necessary for
viscosity reasons to employ low to medium molecular weight polymers, whereas
this factor plays a minor role in a suspension polymerization.
In addition to polymers obtained by crosslinking polymerization of partially
neutralized acrylic acid, those are preferably used which are obtained by
employing starch or polyvinyl alcohol as graft basis.
The polymerization process of the invention can be initiated by various
conditions, e.g. by irradiating with radioactive, electromagnetic or
ultraviolet radiation, or by a redox reaction of two compounds, e.g. sodium
hydrogen sulfite with potassium persulfate, or ascorbic acid with hydrogen
peroxide. The thermally induced decomposition of a so-called free-radical
initiator such as azobisisobutyronitrile, sodium peroxodisulfate, t-butyl
hydroperoxide, or dibenzoyl peroxide is suitable as well. Furthermore, a
combination of some of the above-mentioned polymerization initiators is
possible.
Preferably, the polymer products of the invention are produced according to
two methods: According to the first method, the partially neutralized
acrylic acid is converted to a gel by means of free-radical polymerization
in aqueous solution and in the presence of crosslinkers and optional polymer
additives, which gel is subsequently crushed and dried until a powdered,
flowable state is reached, milled, and screened to the desired particle
size. The solution polymerization may be conducted in a continuous or
batchwise fashion. The patent literature includes a broad spectrum of
possible variations with respect to concentration conditions, temperatures,
type and amount of initiators, as well as a variety of secondary
crosslinking options. Typical processes have been described in the following
patent specifications: U.S. Pat. Nos. 4,076,663; 4,286,082; DE 27 06 135, DE
35 03 458, DE 35 44 770, DE 40 20 780, DE 42 44 548, DE 43 23 001, DE 43 33
056, DE 44 18 818, the disclosure of which is hereby incorporated by
reference.
The inverse suspension and emulsion polymerization process may also be used
to produce the polymer products of the invention. According to this process
variant, an aqueous, partially neutralized solution of acrylic acid is
dispersed in a hydrophobic organic solvent using protective colloids and/or
emulsifiers, and the polymerization is initiated using free-radical
initiators. The crosslinkers are either dissolved solved in the monomer
solution and pre-charged together with same or added separately and
optionally during polymerization. The optionally present polymeric grafting
bases are added via the monomer solution or by directly placing in the oil
phase. Subsequently, the water is removed azeotropically from the mixture,
and the polymer product is filtrated and optionally dried.
Using the process of subsequent surface crosslinking, the polymer products
according to the invention are improved in their pattern of properties,
particularly in their absorption of liquid under pressure, so that the
well-known phenomenon of "gel blocking" is suppressed, where slightly
swollen polymer particles adhere to each other, thereby impeding further
absorption of liquid and distribution of liquid in the absorbent articles.
In this secondary crosslinking, the carboxyl groups of the polymer molecules
are crosslinked at the surface of the polymer particles at elevated
temperature using crosslinking agents. Inter alia, methods of secondary
crosslinking have been described in the following publications: DE 40 20
780, EP 317,106 and WO 94/9043. According to the invention, all those
surface crosslinking agents known to a person skilled in the art from U.S.
Pat. No. 5,314,420, page 8, lines 3-45, may be employed advantageously in
combination with a crosslinker used during polymerization or a combination
of crosslinkers. As a rule, these compounds contain at least two functional
groups capable of reacting with carboxylic acid or carboxyl groups. Alcohol,
amine, aldehyde, and carbonate groups are preferred and also, crosslinker
molecules having multiple different functions are employed. Preferably,
polyols, polyamines, polyaminoalcohols, and alkylene carbonates are used.
Preferably, one of the following crosslinking agents is used: ethylene
glycol, diethylene glycol, triethylene glycol, polyethylene glycol,
glycerol, polyglycerol, propylene glycol, diethanolamine, triethanolamine,
polypropylene glycol, block copolymers of ethylene oxide and propylene
oxide, sorbitan fatty acid esters, ethoxylated sorbitan fatty acid esters,
trimethylolpropane, ethoxylated trimethylolpropane, pentaerythritol,
ethoxylated pentaerythritol, polyvinyl alcohol, sorbitol, ethylene
carbonate, propylene carbonate. It is particularly preferred to use polyols
and ethylene carbonate as surface crosslinking agents. The crosslinking
agent is employed in an amount of from 0.01 to 30 wt.-%, preferably 0.1-10
wt.-%, relative to the polymer to be crosslinked.
Following polymerization, the polymer product is dried, milled, screened for
the respective grain fraction favorable in application-technical terms, and
subsequently subjected to surface crosslinking. In some cases, however, it
has proven beneficial to add the surface secondary crosslinkers at an early
stage prior to drying the polymer gel or prior to crushing the partially or
predominantly dried polymer. Secondary crosslinking to be performed
according to the invention has been described in U.S. Pat. No. 4,666,983 and
DE 40 20 780 which hereby are incorporated by reference. Advantageously, the
secondary crosslinker frequently is added in the form of a solution in
water, organic solvents or mixtures thereof, particularly in those cases
where low amounts of secondary crosslinking agent are used. Suitable mixing
apparatus for applying the secondary crosslinking agent are, e.g.,
Patterson-Kelley mixers, DRAIS turbulence mixers, Lödige mixers, Ruberg
mixers, screw mixers, pan mixers, and fluid-bed mixers, as well as
continuously operated vertical mixers wherein the powder is mixed at a rapid
frequency using rotating knives (Schugi mixer). Once the surface crosslinker
has been mixed with the crosslinked polymer, heating to temperatures of from
60 to 250° C., preferably from 135 to 200° C., and more preferably from 150
to 185° C. is effected in order to perform the surface crosslinking
reaction. The time period of the heat treatment is limited by the risk of
destroying the desired pattern of properties of the superabsorbent polymer
product as a result of heat damage.
Depending on the type of use, various screening fractions are employed for
processing the polymer products as superabsorbers, e.g. between 100 and 1000
μm and preferably between 150 and 850 μm for diapers. In general, this grain
fraction is produced by milling and screening prior to and/or subsequent to
secondary crosslinking.
According to the process of the invention, the cyclodextrins or derivatives
thereof are employed as substance or dissolved in a solvent. A preferred
solvent is water, but mixtures of water and organic solvents such as ethyl
alcohol, acetone are also used.
The addition of the cyclodextrin component can be effected at various
process stages in the production of the polymer products according to the
invention. The amount of cyclodextrins or derivatives thereof is 0.01-50
wt.-%, preferably 0.1-30 wt.-%, and more preferably 0.5-10 wt.-%, relative
to the amount of polymer product.
Thus, addition to the monomer solution is possible, where the cyclodextrin
or its derivative is added directly to the aqueous monomer solution prior to
the polymerization thereof. In case the polymer product of the invention is
produced by suspension polymerization, it is also possible to pre-charge all
or part of the cyclodextrin in the oil phase and meter the monomer solution
thereto. Where only a part of the cyclodextrin is pre-charged, the remainder
can be introduced via the monomer solution.
It is also possible to apply the cyclodextrin component onto a non-dried
polymer gel, where the cyclodextrin or its derivative as substance or
dissolved in water and/or an organic solvent is applied onto the crushed
polymer gel, preferably by spraying and mixing.
However, it is also possible to dry and crush the polymer gel initially, and
subsequently apply the cyclodextrin or its derivative as substance or
dissolved in water and/or an organic solvent onto the powder. The resulting
product immediately can be processed further or dried to remove solvents.
The cyclodextrin component may also be added onto the crushed and dried
absorbent material during surface crosslinking of the polymer product.
Suitable mixing apparatus for applying the crosslinking agent and the
cyclodextrin component are e.g. Patterson-Kelley mixers, DRAIS turbulence
mixers, Lödige mixers, Ruberg mixers, screw mixers, pan mixers, and
fluid-bed mixers, as well as continuously operated vertical mixers wherein
the powder is mixed at a rapid frequency using rotating knives (Schugi
mixer).
Also, the cyclodextrin component can be applied onto the crushed, already
surface-crosslinked polymer product. In this process variant, according to
the invention, preferably ionically modified cyclodextrins as substance or
dissolved in water and/or an organic solvent are sprayed onto the preferably
powdered polymer, followed by evaporating the solvent.
According to the process of the invention, the cyclodextrin component may
also be introduced at various stages of the production process, so as to
optionally optimize its effect. In this way it is possible, for example, to
polymerize a non-modified cyclodextrin together with the monomer solution
and fix an ionically modified cyclodextrin on the surface of the polymer
during surface crosslinking.
It is also possible to bind the cyclodextrin component to the polymer in an
additional surface crosslinking.
Using the methods according to the invention, final products are obtained
wherein the cyclodextrin or its derivative is incorporated in the synthetic
polymer in such a way that the amount of cyclodextrin extractable with water
is significantly less than the amount actually contained in the final
product. In the products according to the invention, the extractable
percentage of cyclodextrins is below 85% of the amount present in the
product, preferably 60%, and more preferably 45%.
Owing to their excellent absorptive capacity, the polymer products of the
invention are suitable as absorbents which, compared to powdered absorbents
including no cyclodextrin or derivative thereof, exhibit improved absorption
of malodorous compounds.
The polymers according to the invention find use e.g. in hygiene articles
capable of absorbing body fluids such as urine, or in the packaging sector,
e.g. meat and fish products, where they absorb large amounts of aqueous
liquids and body fluids such as urine or blood, with swelling and formation
of hydrogels. The polymer products of the invention are incorporated
directly as powders in constructions for absorbing liquids, or previously
fixed in foamed or non-foamed sheet materials. For example, such
constructions for absorbing liquids are diapers for babies, incontinence
articles or absorbent inserts in packaging units for foodstuffs.
Moreover, the absorbents of the invention were found to be excellently
suited for incorporating active substances. The stability of sensitive
active substances, e.g. with respect to oxidative degradation, is
substantially improved as a result of incorporation in the absorbents of the
invention.
Furthermore, the polymers according to the invention find use in plant
breeding and in pest control in agriculture. In plant breeding, the polymers
in the vicinity of plant roots provide for sufficient supply of liquid and
previously incorporated nutrients and are capable of storing and releasing
same over a prolonged period of time.
In pest control, the polymers can incorporate single active substances or a
combination of multiple active substances which in use are released in a
controlled fashion in terms of time and amount.
Production and properties of the polymer products according to the invention
will be illustrated in the following Examples which also comprise the
production of ionic cyclodextrins used according to the invention.
Claim 1 of 31 Claims
1. A sheet material comprising absorptive polymer particles comprising one
or more polymers comprising polymerized units of one or more
monoethylenically unsaturated monomers having at least one acid group,
wherein the surface of the absorptive polymer particles is secondary
surface crosslinked, and
wherein the polymer is at least partially neutralized and has one or more
cyclodextrins, cyclodextrin derivatives, or both cyclodextrins and
cyclodextrin derivatives, bonded covalently thereto, bonded ionically
thereto, incorporated therein or a combination thereof.
____________________________________________
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about this patent, please go directly to the U.S.
Patent and Trademark Office Web site to access the full
patent.
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