Method of producing swellable, non-aging starch maleates, biologically degradable starch maleates as well as use

The aging of swellable starch maleates is reduced by reacting with one or more singly and/or multiply functional nucleophiles as in a Michael condensation reaction. Mixtures of mercaptoethanol and 1,2-bis-(2-mercaptoethoxy)-ethane or mixtures of sodium bisulfite and pentaerythrite-tetrakis-(2-mercaptoacetate) are preferred. The decrease of the retention capacity [SRV] in the reaction products after 100 days is preferably <10%. The products find use as biodegradable, non-aging superabsorbers.

CROSS REFERENCE TO RELATED APPLICATION 
This application claims priority from German Application No. 19702641.9, 
filed Jan. 25, 1997, the subject matter of which is incorporated herein by 
reference. 
FIELD OF THE INVENTION 
The invention relates to an absorption material which is based primarily on 
renewable raw materials, has an improved biological degradability in 
comparison to the polyacrylates primarily used currently as absorber 
material and which has improved aging resistance compared with starch 
maleates suggested for the same purpose. 
In particular, the invention relates to a method of producing swellable, 
non-aging starch materials which have a relatively high absorption 
capacity and which exhibit only a very slight tendency to gel blocking in 
comparison to other absorber materials with a polysaccharide base. 
"Non-aging" refers in this connection to the swelling properties and in 
particular not to the biological degradability. In addition, the invention 
discloses absorption material obtainable in accordance with the method of 
the invention as well as disclosing the use of the products. 
BACKGROUND AND PRIOR ART 
By far the most of the absorption materials currently in use, frequently 
also designated as superabsorbers, consist of slightly cross-linked 
polyacrylates and therefore only a small part, if any, is degradable (see 
e.g. Stegman et al., Waste Manage. Res. 11 (1993) 155). 
In addition to the pure polyacrylates there are also polyacrylates grafted 
onto stairch (DE-A 26 12 846). However, the starch content of these 
products (up to 25%) is low. In the case of higher starch contents a 
distinct deterioration of the absorption properties is observed. Due to 
the polyacrylate content the biological degradability of these products is 
also low. 
Likewise, up to approximately 25% of a polysaccharide which is 
water-soluble at least to a limited extent can be worked into a 
cross-linked polyacrylate superabsorber by introducing the polysaccharide 
into the reaction mixture during the polymerization of the acrylate (DE-A 
40 29 591, DE-A 40 29 592, DE-A-20 29 593). 
U.S. Pat. No. 5,079,354 describes an absorber material based on 
carboxymethyl starch, that is, a starch ether, which is produced by 
reacting starch with chloroacetic acid. In this process an equivalent 
amount of sodium chloride relative to the chloroacetic acid used is 
released, which is undesirable for ecological reasons. In addition, it is 
known, that etherified polysaccharides are only poorly biodegradable at 
high degrees of substitution (Mehltretter et al., J. Am. Oil Chem. Soc. 47 
(1970) 522). 
DE-A 0,603,837 describes the production of starch esters using organic acid 
anhydrides. To this end starch of diverse origins is allowed to react in a 
one-stage aqueous process with organic acid anhydrides of general formula 
I 
##STR1## 
in which R signifies alkyl, aryl, alkenyl, alkaryl or aralkyl with 1 to 7 
C atoms under certain conditions of pH, temperature and concentration. 
Starch esters enumerated by way of example in the specification of EP-A 
0,603,837 include starch acetate, starch propionate, starch butyrate, 
starch hexanoate, starch benzoate or also mixed starch 
acetates/propionates. In the examples of EP-A 0,603,837 the use of 
propionic acid anhydride, acetic anhydride and/or butyric acid anhydride 
is disclosed. The problems which otherwise resulted when using rather 
large amounts of anhydride such as e.g. the swelling or gelatinization of 
the starch and problems during separation of the starch esters from the 
reaction mixture are successfully avoided with the process described in 
EP-A 0,603,837. The product becomes hydrophobic as a result of the 
reaction and can therefore be filtered off in a simple manner. 
WO 93/01217 (PCT/EP 92/01553) teaches a method of producing starch esters 
for clinical, especially parenteral application. The starch esters 
according to WO 93/01217 are quite water-soluble, which is necessary for 
parenteral application. 
There have also already been attempts to create biodegradable 
superabsorbers. Thus, DE-A 42 06 857 teaches an absorption means 
consisting of a component based on special, renewable polysaccharide raw 
materials, of a special, water-swellable polymer, of a matrix material, of 
an ionic or covalent cross-linking agent and of a reactive addition. The 
component based on renewable polysaccharide raw materials comprises e.g. 
guar, carboxymethyl guar, xanthan gum, alginates, gum arabic, hydroxyethyl 
cellulose or hydroxypropyl cellulose, carboxymethyl cellulose and other 
cellulose derivatives, starch and starch derivatives such as carboxymethyl 
starch. It is furthermore known from DE-A 42 06 857 that the cited 
polymers can be modified by cross-linking in order to reduce their water 
solubility and to achieve better swelling properties. The cross-linking 
can take place in the entire polymer or only on the surface of the 
individual polymer particles. 
The conversion of the polymers can take place with ionic cross-linking 
agents such as e.g. calcium compounds, aluminum compounds, zirconium and 
iron(III) compounds. Likewise, a conversion is possible with 
polyfunctional carboxylic acids such as citric acid, mucic acid, tartaric 
acid, malic acid, malonic acid, succinic acid, glutaric acid, adipic acid, 
with alcohols such as polyethylene glycols, glycerol, pentaerythrit, 
propane diols, sucrose, with carbonic acid esters such as glycoldiglycidyl 
ether, glycol di- or triglycidyl ether and epichlorohydrin, with acid 
anhydrides such as succinic acid anhydride and maleic acid anhydride, with 
aldehydes and multifunctional olefins such as bis-(acrylamido) acetic acid 
and methylene bisacrylamide. Of course, derivatives of the families cited 
can also be considered as well as heterofunctional compounds with 
different functional groups of the families cited above. 
Although the systems based on sodium carboxymethyl cellulose presented and 
documented with examples have quite favorable absorption properties in 
combination with sodium polyacrylate in various tests, no particulars 
about the biodegradability can be gathered from the publication. 
However, it is known that polyacrylates are substantially not biodegradable 
(R. Stegmann et al., Waste Water Res. 11(2) (1993) p. 155) and 
carboxymethyl cellulose, a polysaccharide ether, is only very poorly 
biodegradable (4.6% after 5 days; M. Seekamp, Textilveredlung, Vol. 25 
(1990) p. 125. 
An absorption material with a biodegradability which is considerably 
improved in comparison to the polyacrylates primarily used at the time is 
disclosed in EP-A 0,714,914, which discloses a swellable starch ester 
which consists of more than 50% by weight of non-water-soluble components 
and which has a retention capacity for 0.9% by weight aqueous NaCl 
solution of &gt;500% relative in each instance to the weight of the dry 
starch ester. The retention capacity is determined in that 0.1 g of the 
starch ester welded into a nylon bag with a mesh width of 52 .mu.m is 
allowed to swell for 30 min in 0.9% NaCl solution, the bag is subsequently 
centrifuged 5 min at 1400 rpm and then any resulting weight increase is 
gravimetrically determined. 
Although the absorption materials presented in EP-A 0,714,914 and based 
essentially on maleic acid anhydride have very good swelling properties 
and good biodegradability the starch maleates of EP-A 0,714,914 
unfortunately still have distinct disadvantages. 
A distinct decrease of the swelling properties occurs after a few weeks. 
After several weeks very poor swelling properties are obtained both in the 
measurement of the retention capacity (SRV) and in the determination of 
the absorption capacity with and without load (AFK5 and A20FK5). This 
aging has sharply hindered the commercial use of starch maleates as 
absorption materials in the past. 
SUMMARY OF THE INVENTION 
In view of the state of the art presented and discussed herein, one object 
of the invention is to indicate a method of producing swellable and 
non-aging starch maleales which can be readily carried out. A further 
object is to produce biodegradable starch maleates which can be obtained 
in accordance with the process that have an aging with respect to the 
swelling properties which is reduced. Another object of the invention is 
the use of swellable, biologically degradable starch maleates which are 
non-aging with respect to swelling capacity. 
It was found within the framework of the invention in a manner which could 
not have been readily foreseen that swellable starch maleates which are 
essentially non-aging as regards the swelling properties can be produced 
by reacting a swellable starch maleate with one or several mono- and/or 
multifunctional nucleophile(s) as in a Michael condensation reaction. 
In particular, material which can be obtained in this manner and is 
resistant to aging of the swelling properties is suitable for use as an 
absorption material for the absorption of water, aqueous solutions, 
dispersions and body fluids for hygiene and animal hygiene uses, 
especially in diapers and incontinence products as well as in packaging 
materials for meat and fish, as well as for the improvement of soil, for 
use in culture containers and as cable jacketings, and in which instances 
it is also biodegradable. 
The starch maleates to be reacted in accordance with the method of the 
invention are starch esters with a degree of substitution between 0.2 and 
2.0, where the degree of substitution indicates the number of substituents 
per glucose ring. In the case of the esters, pure maleates or mixed esters 
can be used, that is, in addition to ester groups derived from the maleic 
acid other ester groups are also present, in an amount up to 98%, 
preferably, however, only up to 50% and have preferably less than 50%, 
e.g. 25-49%. Nevertheless, even in the case of the latter compounds and 
even in the first instance, in which up to 98% other ester groups are 
present, the term starch maleates is used within the framework of the 
invention. Basically, however, pure maleates are likewise very well suited 
for the invention. 
The starch maleates are preferably obtained by reacting starch with maleic 
acid anhydride and, if necessary, with other acid anhydrides. The other 
anhydrides can be cyclic and/or open-chain anhydrides. However, according 
to the invention the sole reaction with maleic acid anhydride is by far 
preferred. 
The carboxylic acid anhydrides which are used in an especially advantageous 
manner together with maleic acid anhydride include, among others, acetic 
anhydride, propionic acid anhydride and/or succinic acid anhydride. 
Although the improvement in the aging resistance can be achieved in 
accordance with the invention in the case of every swellable starch 
maleate, swellable starch maleates are reacted in a first preferred method 
variant according to a method in which starch or modified starch is 
reacted in a one-stage aqueous reaction with maleic acid anhydride or a 
mixture of maleic acid anhydride comprising carboxylic acid anhydrides at 
a pH of 7 to 11 and a temperature of 0 to 40.degree. C.; the pH is 
maintained in the desired range by the addition of aqueous alkali solution 
with a concentration of approximately 10 to 50% by weight. 
The pH is preferably maintained constant during the reaction of the 
anhydride or the anhydrides with the starch. The pH should be between 7 
and 11 during the reaction. A pH between 8 and 9 is preferred. The pH can 
be maintained constant in principle by the addition of any desired 
alkaline material. Alkali hydroxide and alkaline-earth hydroxides as well 
as the oxides and carbonates of alkali metals and/or of alkaline-earth 
metals are especially useful. The following are cited by way of example: 
Sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium 
hydroxide, magnesium hydroxide and sodium carbonate. In the production of 
products with a rather high degree of substitution an alkali solution with 
a rather high concentration, approximately 10 to 50%, is used in order to 
avoid an unnecessary dilution of the reaction medium. 
An advantageous second modification of the method of the invention provides 
that a swellable starch maleate is used which can be obtained according to 
a method in which starch or modified starch is reacted with maleic acid 
anhydride or with a mixture of maleic acid anhydride comprising carboxylic 
acid anhydrides in the presence of a base and optionally, also, a solvent, 
which modification is characterized in that the mixture is allowed to 
react at a temperature of 80.degree. to 180.degree. C. for 10 min to 10 h 
and that solvent is tolerated only up to a content of less than 100 parts 
relative to 100 parts starch. 
The use of sodium carbonate as the base is preferred for this embodiment of 
the method. 
In principle, any native, modified or substituted starch can be used as the 
starch basis of the starch maleate in accordance with the invention. Such 
starches have been isolated from any vegetable source and comprise e.g. 
potato starch, corn starch, wheat starch, waxy corn starch and starches 
with a high amylose content. Starch meal can also be used. Modified 
products based on one of the starches mentioned above such as e.g. 
acid-hydrolyzed starch, enzyme-hydrolyzed starch, dextrins and oxidized 
starch can also be used. Moreover, derivatized starches such as cationic 
starch, anionic starch, amphoteric starch or non-ionically modified starch 
such as, e.g., hydroxyethyl starch, can be used. The starches used can be 
granular or pre-gelatinized starch and the destruction of the granular 
structure can take place thermally, mechanically or chemically. 
The use of starch maleates based on starch soluble in cold water is 
especially advantageous for the invention. These are, in particular, 
pre-gelatinized or partially degraded starch, and include, among others, 
Aeromyl 115.TM. of the Sudstarke Company. 
According to the invention a swellable starch maleate is brought to 
reaction with one or several nucleophile(s) as in a Michael condensation 
reaction. An addition of the nucleophile to the double bond of the maleic 
acid in the starch maleate presumably occurs thereby. 
The nucleophiles which can be used for this in accordance with the 
invention include especially those of general formula I 
EQU H--X--R (I), 
in which 
X represents S or NH, and 
R represents hydrogen, a saturated or unsaturated alkyl group with 1 to 18 
carbon atoms, which alkyl group is branched or unbranched and can 
optionally comprise one or several oxygen atoms within the chain and can 
furthermore be optionally substituted by other functional groups such as 
e.g. carboxyl, hydroxyl, carbonyl, ester, amide, ether, sulfide, halogen, 
sulfonic acid, or an aryl group with 6-12 C atoms, which aryl group can be 
substituted up to four times with carboxyl, hydroxyl, carbonyl, ester, 
amide, sulfonic acid and/or halogen. 
Salts of the compounds of formula I can also be used with particular 
advantage, in which at least one hydrogen can be replaced by an alkali 
metal or an alkaline earth metal. A corresponding compound is, e.g., 
Na.sub.2 S. 
Sulfurous acid and its salts also are among the compounds which can be used 
in accordance with the invention. Sodium bisulfite and sodium hydrogen 
sulfite are especially preferred. 
Nucleophiles of general formula II 
EQU H--X--R--Y--H (II) 
can also be used in accordance with the invention in an especially 
favorable manner, in which 
X and R have the significance indicated for formula I and 
Y represents independently of X, S or NH. Salts can also be used from 
compounds of formula II as in the case of formula I compounds. 
In so far as optically active forms exist for the compounds of formulas I 
or II which can be used in accordance with the invention, they belong both 
individually as well as in a mixture, e.g. as enantiomers, diastereomers 
or racemates, to the invention. 
The nucleophiles of general formula I as well as of general formula II are 
added in the sense of a Michael-analogous addition to the double bond of 
the maleic acid structural elements present in the starch maleates. Amines 
and especially thiols are preferred in this connection. Especially in the 
case of the thiols, no additional catalysts are generally necessary in 
order to make possible a rapid addition to the double bonds. 
Based on these conditions, the method of the invention is characterized in 
another especially preferred embodiment in that a compound containing SH 
groups, such as a thiol, is used as nucleophile. 
It is noteworthy that the effect of hindering the aging of starch maleates 
in accordance with the invention takes place both in the case of the 
monofunctional structural elements of general formula I by blocking the 
double bond of the maleate group as well as in the case of the 
difunctional structural elements of general formula II by cross-linking, 
and therewith also blocking the double bond of the maleate group. 
The individual monofunctional structural elements which can be used within 
the framework of the method of the invention include, among others, amino 
acids (both .alpha.- and others) such as glycine, alanine, valine, 
leucine, isoleucine, serine, threonine, asparaginic acid, asparagine, 
glutamic acid, glutamine, proline, histidine, tryptophane, phenylalanine, 
tyrosine, methionine; linear and branched amines such as ethylamine, 
propylamine, butylamine, sec.-butylamine, tert.-butylamine, isobutylamine, 
pentylamine, isopentylamine, 2-aminopentane, hexylamine, heptylamine, 
2-heptylamine, octylamine, tert.-octylamine, nonylamine, decylamine, 
undecylamine, dodecylamine, ethanolamine, 3-amino-1-propanol, 
1-amino-2-propanol, alaninol, 2-amino-1-butanol, 4-amino-1-butanol, 
2-amino-2-methyl-1-propanol, 5-amino-1-pentanol, valinol, leucinol, 
tert.-leucinol, N-propylethylene diamine, 2-ethylamino-ethylamine; linear 
and branched thiols such as ethanethiol, 1-propanethiol, 2-propanethiol, 
1-butanethiol, sec.-butyl mercaptan, tert.-butyl mercaptan, isobutyl 
mercaptan, 1-pentanethiol, 1-hexanethiol, 1-heptanethiol, 1-octanethiol, 
1-nonanethiol, 1-decanethiol, 1-undecanethiol, 1-dodecanethiol, 
tert.-dodecanethiol, 1-tetradecanethiol, 1-hexadecanethiol, 
1-octadecanethiol, mercapto ethanol, 3-mercapto-1-propanol, 
4-mercapto-1-butanol, 3-mercapto-2-butanol, 6-mercapto-1-hexanol, 
8-mercapto-1-octanol, 10-mercapto-1-decanol, 11-mercapto-1-undecanol, 
mercaptoacetic acid and its salts and esters, 3-mercaptopropionic acid and 
its salts and esters, 11-mercaptoundecanoic acid and its salts and esters, 
mercaptoethane sulfonic acid and its salts, 3-mercapto-1-propane sulfonic 
acid and its salts, mercaptopyruvic acid and its salts, 2-diethylamino 
ethanethiol (-hydrochloride), 3-mercaptopropylmethyldimethoxysilane, 
3-mercaptopropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, 
N-methylmercaptoacetamide, mercaptophenol. 
The use of mercaptoethanol, 3-mercapto-1-propanol, mercaptoacetic acid, 
3-mercaptopropionic acid, mercaptoethane sulfonic acid and mercaptopropane 
sulfonic is especially preferred. 
The difunctional cross-linking molecules which can be used within the 
framework of the invention include, among others, amino acids with 
additional amino- or thiol groups such as cysteine, penicillamine, lysine, 
ornithine, arginine, linear or branched diamines such as diaminoethane, 
1,2-diamino-propane, 1,3-diaminopropane, 1,4-diaminobutane, 
1,2-diamino-2-methylpropane, 1,5-diaminopentane, 1,3-diaminopentane, 
1,3-diamino-2,2-dimethylpropane, 1,6-diaminohexane, 
1,5-diamino-2-methylpentane, 1,7-diaminoheptane, 1,8-diaminooctane, 
1,9-diaminononane, 1,10-diaminohexane, 1,12-diaminododecane, 
1,2-bis-(2-aminoethoxy)ethane, bis-(2-aminoethyl)ether; 
dithiols such as ethanedithiol, 1,3-propanedithiol, 1,4-butanedithiol, 
2,3-butanedithiol, 1,5-pentanedithiol, 1,6-hexanedithiol, 
1,7-heptanedithiol, 1,8-octanedithiol, 1,9-nonanedithiol, 
1,10-decanedithiol, 1,11-undecanedithiol, 1,12-docecanedithiol, 
1,16-hexadecanedithiol; 
aminothiols such as cysteamine, 2-mercapto-isobutylamine; 
bis-(2-mercaptoethyl)ether, bis-(2-mercaptoethyl)sulfide, 
1,2-bis-(2-mercaptoethoxy)-ethane (MEE), N-(2-mercaptopropionyl)-glycine, 
2,3-dimercapto-succinic acid, 2,3-dihydroxy-1,4-butanedithiol, 
2,3-dimercapto-1-propanol, 2,3-dimercapto-propane sulfonic acid and its 
salts, lipoic acid (reduced form), 
pentaerythritol-tetrakis-(3-mercapto-propionate), 
pentaerythritol-tetrakis-(2-mercaptoacetate), mixed esters of 
2-mercaptoacetic acid and of 3-mercaptoacetic acid with pentaerythritol, 
hydrogen sulfide, sodium sulfide, 1,3-dimercaptobenzene, o-benzol, 
thioresorcinol, 1,2-dimercaptobenzene, toluene-3,4-dithiol, oligosulfides 
with terminal thiol groups such as polymethylene tetrasulfide (Thiokol 
products of the Morton company), and oligo- and poly-peptides with at 
least two free amino groups and/or thiol groups. 
The following are especially preferred for the use as the nucleophile 
within the framework of the method of the invention: 
1,2-bis-(2-mercaptoethoxy)ethane) (MEE), 2,3-dimercaptosuccinic acid, 
cysteine, cysteamine, 2,3-dihydroxy-1,4-butane dithiol, 
pentaerythritol-tetrakis-(3-mercaptopropionate), 
pentaerythritol-tetrakis-(2-mercaptoacetate), bis-(2-mercaptoethyl)-ether, 
bis-(2-mercaptoethyl)-sulfide. 
An especially favorable effect can be achieved within the framework of the 
invention by combining a difunctional cross-linking agent (nucleophile of 
formula II) and a monofunctional structural element (nucleophile of 
formula I). The best swelling properties with the most extensive 
prevention of the aging of starch maleates are realized with the combined 
use of a monofunctional with a difunctional structural element. Thus, the 
method of the invention is characterized in a further, quite especially 
advantageous variation using a mixture of mercaptoethanol (ME) and 
1,2bis-(2-mercaptoethoxy)ethane (MEE) or a mixture of 
pentaerythrite-tetrakis-(2-mercaptoacetate) and sodium bisulfite. In 
particular, network make-up and aging stability can be combined with one 
another in a simple manner by the cross-linking of starch maleates with 
di- or oligothiols and the reaction of any residual maleate double bonds 
with thiols or sodium bisulfite. 
It is therefore also advantageous for the method of the invention to use 
between 0.001 and 0.5 equivalents of mercaptoethoxyethane (MEE) in 
combination with between 0.5 and 0.999 equivalents of mercaptoethanol (ME) 
as nucleophile, which equivalents refer to the amounts of maleic acid 
anhydride contained in the starch maleate. In this manner good swelling 
values can be adjusted as regards the SRV and the free swelling within 
this concentration of cross-linking agent and blocking agent. Excellent 
ranges are located between 0.001 and 0.03 eq MEE and 0.97 and 0.999 ME. 
It is furthermore advantageous for the method to use between 0.001 and 0.5 
equivalents of pentaerythrite-tetrakis-(2-mercaptoacetate) (PTMA) in 
combination with between 0.25 and 0.999 equivalents of sodium bisulfite as 
nucleophile, which equivalents refer to the amounts of maleic acid 
anhydride contained in the starch maleate. In this manner good swelling 
values can be adjusted as regards the SRV and the free swelling within 
this concentration of cross-linking agent and blocking agent. Excellent 
ranges are located between 0.001 and 0.03 equivalents of PTMA and 0.4 and 
0.9 equivalents of sodium bisulfite. 
Although the reaction of starch maleates with nucleophiles can be carried 
out within the framework of the invention with starch maleates of any 
origin, the method of the invention nevertheless demonstrates its special 
advantages in particular if the starch maleates are treated in a swollen 
or dissolved state with a nucleophile. For this reason the reaction can be 
carried out without isolation of intermediates, in particular following 
the actual synthesis reaction of the starch maleates. To this extent it is 
a preferred variation of the methold of the invention if the reaction is 
carried out with the one and/or several singly and/or multiply functional 
nucleophile(s) immediately following the synthesis of the starch maleate 
in a one-pot reaction at temperatures between room temperature and 
approximately 80.degree. C. over a time period of 10 min to 24 h, 
preferably between 10 min and 10 h. 
Network make-up and aging stability can be optimally combined with one 
another and monitored in particular by this extremely simple procedure 
within the framework of the invention by means of the cross-linking of the 
starch maleates preferably with dithiols and by the reaction of the 
residual maleate double bonds preferably with thiols. 
Degradable starch maleates obtainable in accordance with the method 
described herein are also subject matter of the invention. They are 
characterized by the exceedingly favorable aging behavior simultaneously 
with good swelling properties. In particular the biodegradable starch 
maleates of the invention are characterized in that a decrease of the SRV 
can be determined after 100 days of less than 20%. Furthermore, especially 
preferred starch maleates in accordance with the invention are those 
exhibiting a decrease of the SRV after 100 days of less than 10%. 
An especially advantageous procedure provides a reduction of the gel 
blocking of the superabsorber particles produced. As is generally known 
about superabsorber particles, slightly cross-linked products display a 
greater absorption of liquid than do strongly cross-linked ones; however, 
they also tend to a greater extent toward gel blocking, especially upon 
absorption under load. In order to prevent the gel blocking of slightly 
cross-linked gels, a starch maleate is reacted in accordance with the 
invention with a compound of formula I and/or formula II or with a mixture 
in such a manner that a slightly cross-linked gel is produced whose 
maleate double bonds are reacted only in part. After the drying and 
comminution the product is suspended in a solvent or solvent mixture in 
which a cross-linking agent of formula II or a mixture of compounds 
according to formula II and formula I is dissolved. According to this 
method an additional cross-linking takes place primarily on the surface of 
the particles. After this treatment the superabsorber gels display a 
distinctly lesser tendency toward gel blocking yet retain a significantly 
better absorption capacity than gels which are highly cross-linked 
throughout. 
Organic solvents miscible with water are suitable as solvents for the 
subsequent cross-linking in accordance with the invention. Alcohols such 
as methanol, ethanol, and i-propanol and their mixtures with water in a 
ratio of 50:50 to 99:1 are preferred. Mixtures of ethanol with water in a 
ratio of 70:30 to 99:1 are especially preferable. 
The invention also includes a process for using the specified starch 
maleates in an amount of 100 parts by weight together with 0.7 to 70 parts 
by weight of an antiblocking agent based on natural or synthetic, 
preferably hydrophilic fibers or materials with a large surface as 
suberabsorbers. The use of starch maleate as superabsorber together with 1 
to 5 parts by weight silicic acid or cellulose fibers as antiblocking 
agent is preferred. The starch ester of the invention finds further use as 
an absorption agent for the absorption of water, aqueous solutions, 
dispersions and body fluids in hygiene and animal hygiene, especially in 
diapers, tampons and incontinence products as well as in packaging 
materials for meat and fish, as absorption material for the absorption of 
water and aqueous solutions in culture containers and for soil improvement 
or as absorption material for the absorption of water and aqueous 
solutions in cable jacketings. 
In order to further improve the behavior under pressure loading in 
non-aging starch maleates in accordance with the invention it is 
furthermore especially advantageous to subsequently cross-link the 
particle surface of the starch maleates. This can prevent a gel blocking 
with retention of good values in the SRV and AFK5.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is explained in detail in the following with reference made 
to exemplary embodiments. 
I) Methods 
A) Determination of the Substitution Yield via the Consumption of NaOH 
During Synthesis 
In the reaction of starch with maleic acid anhydride (MSA) in aqueous 
solution one mole NaOH is used per mole maleic acid anhydride. If no 
addition of the maleic acid anhydride to the starch takes place but rather 
a hydrolysis to disodium maleate then 2 moles NaOH are used per mole MSA 
with the pH being maintained constant. If no non-reacted anhydride is 
present any more at the end of the reaction the degree of substitution can 
be calculated according to 
##EQU1## 
B) Method of Determining the Saline Retention Capacity (SRV) 
A tea bag test was carried out to determine the retention capacity. To this 
end 0.10 g of the test substance is weighed into a bag of nylon fabric 
with a mesh width of 52 .mu.m. The welded nylon bag is placed into a 0.9% 
solution of NaCl and the test material allowed to swell for 30 minutes. 
The bag is subsequently removed and centrifuged 5 minutes at 1400 rpm in a 
centrifuge tray with sieve plate. The absorption of liquid is determined 
gravimetrically and converted to 1 g of the substance to be tested. The 
value obtained in this manner is designated as the retention capacity 
(abbreviated as SRV for "saline retention value"). 
C) Method for Determining the Absorption Capacity with and without Load 
(AFK5, A20FK5) 
5 parts precipitated silicic acid (FK500 LS, Degussa) are mixed with 95 
parts of the substance to be examined. This mixture is designated below as 
the "test substance". 
0.5 g test substance are placed onto a G3 sintered-glass-fit filter 3 cm in 
diameter. The filter is connected via a hose to a burette filled with 0.9% 
solution of NaCl. The amount of liquid which the test substance soaks up 
in 10 min is determined. During the duration of the test the burette is 
refilled in such a manner that the meniscus is always at the height of the 
glass-fit filter. The value determined in this manner is converted to 1 g 
test substance and designated with AFK5. 
In order to determine the absorption capacity under load (A20FK5) a weight, 
exerting a pressure of 20 g/cm.sup.2 is additionally placed on the test 
substance in the embodiment described above. The value obtained in this 
manner is likewise converted to 1 g test substance. 
II) EXAMPLES AND REFERENCE EXAMPLES 
Example 1 
Synthesis of Starch Maleate 
50 g (=0.278 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 9.5%) are 
dissolved in 400 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 27.2 g (=0.278 mol) solid 
maleic acid anhydride is added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which the reaction mixture is brought to room temperature 
(postreaction). The degree of substitution (DS(NaOH)) is 0.88. 
Example 2 
Synthesis of Starch Maleate 
50 g (=0.278 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 9.5%) are 
dissolved in 400 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 40.91 g (=0.417 mol) solid 
maleic acid anhydride is added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which the reaction mixture is brought to room temperature 
(postreaction). The degree of substitution (DS(NaOH)) is 1.15. 
Examples 3-8 
Immediately after the one-hour postreaction of the starch maleate of 
example 1 the mixture is heated to 60.degree. C. and a mixture of 
cross-linking agent consisting of 1,2-bis-(2-mercaptoethoxy-ethane) (MEE) 
and/or mercaptoethanol (ME) is added to the reaction mixture in the 
amounts indicated in Table 1. The pH during the addition is between 7 and 
8. The mixture is homogenized and the agitator mechanism cut off. After 
approximately 1 minute gel formation occurs. The mixture is allowed to 
postreact 3 h at 60.degree. C. and the reaction mixture is cooled down 
overnight. The water is evaporated using Rotavapor at 80.degree. C. in a 
vacuum and the product is dried overnight in a vacuum drying oven until 
constancy of weight is obtained. The starch maleate obtained in this 
manner is comminuted and ground. The absorption capacity (AFK5; A20FK5) 
and the retention capacity (SRV) of the product obtained in this manner 
are determined. The measurements are repeated after storage (see Table 1). 
Table 1 shows the changes in the SRV since experience has shown that the 
aging effects occur most distinctly there (see the reference examples 
too). 
The cross-linking with MEE/ME can also be carried out at room temperature. 
The time until gel formation is then extended to 20-45 minutes. 
Postreaction at room temperature overnight and workup are carried out in 
the same manner. 
TABLE 1 
______________________________________ 
Survey of the synthesized products. The indications of amount in 
the column headed "Explanation" correspond to molar equivalents 
(eg) relative to maleic acid anhydride. The drying took place at 
80.degree. C. 
SRV (g/g) 
SRV (g/g) 
Deviation 
Ex. Explanation initial value after (x) days (%) 
______________________________________ 
3 starch maleate, 
5.6 5.6 (128) 
0 
0.5 eq MEE 
4 starch maleate, 5.3 5.4 (128) +2 
0.25 eq MEE 
0.5 eq ME 
5 starch maleate, 7.9 9.5 (122) -5 
0.1 eq MEE 
0.8 eq ME 
6 starch maleate, 15.9 14.1 (127) -11 
0.03 eq MEE 
7 starch maleate, 12.5 12.4 (115) -1 
0.01 eq MEE 
0.99 eq ME 
8 starch maleate, 13.3 12.9 (115) -3 
0.005 eq MEE 
0.98 eq ME 
______________________________________ 
Example 9 
Synthesis of a Mixed Starch Ester 
50 g (=0.278 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 9.5%) are 
dissolved in 400 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. A mixture of 1.088 g (0.011 
mol) solid maleic acid anhydride and of 26.71 g (0.2668 mol) solid 
succinic acid anhydride is added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which time the reaction mixture is brought to room temperature 
(postreaction). The degree of substitution (DS(NaOH)) is 0.88. 
Immediately after the one-hour postreaction time of the starch maleate the 
mixture is heated to 60.degree. C. and 0.506 g (0.00278 mol) of the 
cross-linking agent 1,2-bis-(2-mercaptoethoxy-ethane) (MEE) is added to 
the reaction mixture. The pH during the addition is between 7 and 8. The 
mixture is homogenized and the agitator mechanism cut off. Gel formation 
occurs after approximately 25 minutes. The mixture is allowed to postreact 
3 h at 60.degree. C. and the reaction mixture is cooled off overnight. The 
water is evaporated using a Rotavapor at 45.degree. C. in a vacuum and the 
product is dried overnight in a vacuum drying oven until constancy of 
weight is obtained. The starch maleate obtained in this manner is 
comminuted and ground. 
SRV=23.4 g/g 
Example 10 
The product of example 9 is mixed with 5% of precipitated silicic acid 
(FK500LS, Degussa AG). 
AFK5=10.2 ml/g; A20FK5=3.4 ml/g 
Example 11 
100 g (=0.568 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 7.9%) are 
dissolved in 800 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 55.7 g (=0.568 mol) solid 
maleic acid anhydride is added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which the reaction mixture is brought to room temperature 
(postreaction). The degree of substitution (DS(NaOH)) is 0.86. 48.6 g 
sodium bisulfite in 100 ml water and 3.11 g 
1,2-bis-(2-mercaptoethoxy-ethane) (MEE) in 25 ml ethanol are subsequently 
added. The mixture is homogenized and the agitator mechanism cut off. The 
mixture is allowed to postreact overnight. The water is evaporated using a 
Rotavapor at 80.degree. C. in a vacuum and the product dried overnight in 
a vacuum drying oven until constancy of weight is obtained. The starch 
maleate obtained in this manner is comminuted and ground. 
SRV=17.4 g/g; AFK5=20.6 ml/g; A20FK5=3.9 ml/g 
SRV after 126 days=16.0 g/g (-8%) 
Example 12 
100 g (=0.568 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 7.9%) are 
dissolved in 800 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 27.8 g (=0.284 mol) solid 
maleic acid anhydride is added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which the reaction mixture is brought to room temperature 
(postreaction). The degree of substitution (DS(NaOH)) is 0.48. 24.3 g 
sodium bisulfite in 100 ml water and 3.69 g 
pentaerythrite-tetrakis-(2-mercaptoacetate) (PTMA) in 25 ml THF are 
subsequently added. The mixture is homogenized and the agitator mechanism 
cut off. The mixture is allowed to postreact overnight. The water is 
evaporated using a Rotavapor at 80.degree. C. in a vacuum and the product 
dried overnight in a vacuum drying oven until constancy of weight is 
obtained. The starch maleate obtained in this manner is comminuted and 
ground. 
SRV=14.2 g/g; AFK5=21.3 ml/g; A20FK5=6.4 ml/g 
SVR after 106 days=12.6 g/g (-11%) 
Example 13 
100 g (=0.568 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 7.9%) is 
dissolved is 800 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 27.8 g (=0.284 mol) solid 
maleic acid anhydride are added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which the reaction mixture is brought to room temperature 
(postreaction). The degree of substitution (DS(NaOH)) is 0.45. 24.3 g 
sodium bisulfite in 100 ml water are subsequently added. The reaction 
mixture is heated to 40.degree. C. and 1.84 g 
pentaerythrite-tetrakis-(2-mercaptoacetate) (PTMA) in 25 ml THF are added. 
The mixture is homogenized and the agitator mechanism cut off. The mixture 
is allowed to postreact 2 h at 40.degree. C. and overnight at room 
temperature. The product is dried at 50.degree. C. in a circulating-air 
drying oven. The starch maleate obtained in this manner is comminuted and 
ground. 
SRV=17.5 g/g; AFK5=19.2 ml/g; A20FK5=3.8 ml/g 
SRV after 118 days=15.7 g/g (-10%) 
Example 14 
100 g (=0.568 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 7.9%) is 
dissolved in 800 ml water. The pH is adjusted with 3 N NaOH to 8 and 
maintained constant during the reaction. 27.8 g (=0.284 mol) solid maleic 
acid anhydride are added over a period of 2 h at a reaction temperature of 
0.degree. C. The mixture is agitated for a further hour during which the 
reaction mixture is brought to room temperature (postreaction). The degree 
of substitution (DS(NaOH)) is 0.45. 21.3 g sodium bisulfite in 100 ml 
water are subsequently added. The reaction mixture is heated to 60.degree. 
C. and 1.84 g pentaerythrite-tetrakis-(2-mercaptoacetate) (PTMA) in 25 ml 
THF are added. The mixture is homogenized and the agitator mechanism cut 
off. The mixture is allowed to postreact 2 h at 40.degree. C. and 
overnight at room temperature. The product is dried at 50.degree. C. in a 
circulating-air drying oven. The starch maleate obtained in this manner is 
comminuted and ground. 
SRV=15.8 g/g; AFK5=20.8 ml/g; A20FK5=4.1 ml/g 
SRV after 112 days=14.6 g/g (-8%) 
Example 15 
100 g (=0.568 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 7.9%) is 
dissolved in 800 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 27.8 g (=0.284 mol) solid 
maleic acid anhydride are added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which the reaction mixture is brought to room temperature 
(postreaction). The degree of substitution (DS(NaOH)) is 0.45. 
Subsequently 24.3 g sodium bisulfite in 100 ml water and 0.92 g 
pentaerythrite-tetrakis-(2-mercaptoacetate) (PTMA) in 25 ml THF is 
subsequently added. The mixture is homogenized and the agitator mechanism 
cut off. The mixture is allowed to postreact overnight. The water is drawn 
off on a Rotavapor at 80.degree. C. in a vacuum and the product dried 
overnight in a vacuum drying oven until constancy of weight is obtained. 
The starch maleate obtained in this manner is comminuted and ground. 
SRV=20.6 g/g; AFK5=21.2 ml/g; A20FK5=4.2 ml/g 
SRV after 706 days=78.0 g/f (-13%) 
Reference Example 1 
The reaction solution of example 1 is rotated in and subsequently dried in 
a vacuum drying oven. The product is ground and again dried 30 min at 
100.degree. C. in a vacuum drying oven. 
SRV=14.3 g/g 
Reference Example 2 
The product of reference example 1 is mixed with 5% of a precipitated 
silicic acid (FK500LS, Degussa AG). 
AFK5=21.1 ml/g; A20FK5=7.6 ml/g 
Reference Example 3 
50 g (=0.278 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 9.5%) is 
dissolved in 400 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 27.27 g (=0.278 mol) solid 
maleic acid anhydride are added over a period of 2 h at a reaction 
temperature of 0.degree. C. The mixture is agitated for a further hour 
during which the reaction mixture is brought to room temperature. The 
mixture is subsequently allowed to postreact another 2 h at 60.degree. C. 
The degree of substitution (DS(NaOH)) is 0.76. The reaction solution is 
rotated in and subsequently dried in a vacuum drying oven. The product is 
ground and reground for 30 min at 100.degree. C. in the vacuum drying 
oven. 
SRV=12.6 g/g 
Reference Example 4 
50 g (=0.278 mol) Aeromyl 115 (physically modified starch, soluble in cold 
water, of the Sudstarke company; residual moisture=approximately 9.5%) is 
dissolved in 400 ml water. The pH is adjusted with 3 N NaOH to a pH of 8 
and maintained constant during the reaction. 13.6 mg benzoquinone are 
added. 27.27 g (=0.278 mol) solid maleic acid anhydride is added over a 
period of 2 h at a reaction temperature of 0.degree. C. The mixture is 
agitated for a further hour during which the reaction mixture is brought 
to room temperature (postreaction). The degree of substitution (DS(NaOH)) 
is 0.83. The reaction solution is rotated in and subsequently dried in a 
vacuum drying oven. The product is ground and reground for 30 min at 
100.degree. C. in the vacuum drying oven. 
SRV=19.4 g/g 
Aging of the substances of the reference examples 
Measurements of the products of the reference examples were repeated after 
storage (see Table 2). 
TABLE 3 
______________________________________ 
SRV SRV after 
Deviation 
Ref. Example initial value (x) days (%) 
______________________________________ 
1 14.3 10.7 (84) 
-25 
3 12.6 9.7 (84) -23 
4 19.4 10.0 (84) -48 
______________________________________