Process for the preparation of finely divided, water-swellable polysaccharide graft polymers

Finely divided polysaccharide graft polymers having a high absorption capacity for urine and water are prepared by semi-continuous reverse-phase suspension polymerization, in which an aqueous solution of an unsaturated carboxylic acid and an initiator is metered continuously into a suspension of a polysaccharide in an organic solvent and the polymer obtained is subsequently partial dehydrated and the crosslinked.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for the preparation of finely 
divided, porous and rapidly water swellable polysaccharide graft polymers. 
These polymers are prepared by semi-continuous reverse-phase suspension 
polymerization, partial dehydration and post-crosslinking. 
2. Description of the Background 
Water-absorbing polymers are in diverse use in the sanitary and hygiene 
sectors as water absorbents in paper napkins and paper towels, as tampons, 
patient undersheets and electrolyte thickeners in dry batteries, as 
moisture-retaining agents or water-storing agents in agriculture and as 
desiccants. 
Suitable polymers are polysaccharide derivatives to which are usually 
grafted water-soluble vinyl monomers. Such polymers include 
carboxymethylcellulose, hydrolysed starch/acrylonitrile graft polymers, 
acrylic acid/starch graft polymers or fully synthetic, weakly crosslinked 
polymers such as partially crosslinked polyacrylic acid salts and 
partially crosslinked polymaleic acid derivatives. 
A process for the preparation of resins having a high water absorption 
capacity is described in DE-C-26 12 846. In this process starch is also 
polymerized with acrylic acid and a crosslinking agent in aqueous or 
aqueous alcoholic solution. With solids contents of usually below 20%, 
however, rubber-like block gels are formed in this precipitation 
polymerization, from which the pulverulent end products are obtained only 
after drying and grinding. 
According to Japanese Patent 80/139 408, a graft polymer can be prepared 
which can subsequently be hydrolysed and crosslinked by polymerization of 
acrylonitrile in an aqueous medium in the presence of starch. A powder 
having a water-absorption capacity of 150 to 180 ml/g is obtained. 
A graft polymerization in suspension is taught in Japanese Patent 80/161 
813. In this process a mixture of n-hexane with sorbitan monostearate, 
starch, water, acrylic acid, sodium hydroxide solution and water-soluble 
initiator is first prepared, before the polymerization is initiated by 
heating. However, in this case the reaction product tends to agglomerate 
during the polymerization and is not obtained in finely divided form. 
According to DE-C-28 40 010 cationic, water-soluble polysaccharide graft 
polymers can be prepared by inverse suspension polymerization. In this 
process a polysaccharide is first suspended in the presence of a 
surface-active agent in a water-immiscible solvent. An aqueous monomer 
solution, which predominantly contains acrylamide and which can also 
contain acrylic acid in small amounts, is then added at room temperature. 
After addition of an initiator, the mixture is heated and polymerized. The 
solids content, based on the aqueous polymer mixture, is more than 50%. In 
this case no crosslinking agents are employed and the water-swellable 
gel-like polymers are not obtained. The batch-wise preparation process 
indicated leads, at the start of the polymerization, to temperature peaks 
which are difficult to control in the case of large batches. 
In EP-B-O 083 022 acrylic acid is polymerized in the presence of starch in 
aqueous solution. The products can then be crosslinked in an inert solvent 
in the presence of 0.01 to 1.3 parts of water per part of resin. The 
preparation of the starch graft polymers is effected at a solids content 
of only 20%. Moreover, these starch graft polymers--crosslinked and 
non-crosslinked--have only a low water-absorption capacity. 
DE-A-38 01 633 shows the preparation of polysaccharide graft polymers by 
inverse suspension polymerization, partial dehydration and crosslinking. 
The inverse suspension polymerization is, however, carried out in a single 
stage and discontinuously, by first adding together all reactants and then 
initiating the polymerization by heating in the presence of an initiator. 
With this process heat is instantaneously liberated at the start of the 
polymerization. In industrial production reactors a sufficiently rapid 
removal of the heat of polymerization can frequently be ensured only with 
difficulty. A need therefore continues to exist for a method of preparing 
polysaccharide graft polymers of good water absorptivity. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide a process 
for the preparation of finely divided polysaccharide graft polymers which 
provides products having a very good water-absorption capacity and which 
at the same time can be carried out well on pilot plant and production 
scales. In particular, it should be possible to reliably remove the heat 
of polymerization. 
Accordingly, these and other objects of the present invention as 
hereinafter will become more readily apparent can be attained in a method 
for the preparation of finely divided, porous and rapidly water-swellable 
polysaccharide graft polymer by adding an aqueous solution, which contains 
60 to 95 parts of a 50 to 100 per cent neutralized olefinically 
unsaturated carboxylic acid, 0 to 50 parts of other olefinically 
unsaturated monomers, 0 to 2 parts of crosslinking agent and 0.005 to 5 
parts of polymerization initiator, continuously at 40.degree. to 
100.degree. C. over 0.5 to 5 hours to a suspension of 5 to 40 parts of 
polysaccharide and 0 to 2 parts of polymerization initiator in a nonpolar 
organic solvent which contains a dispersing agent mixture consisting of 
(a) 50 to 100 per cent by weight of nonionic surfactant having a 
hydrophilic/lipophilic balance of 0.5 to 10 and 
(b) 0 to 50 per cent by weight of nonionic surfactant having a 
hydrophilic/lipophilic balance of 10.5 to 20 and thereby conducting 
semi-continuous reverse phase polymerization; 
partially dehydrating the polymer product obtained in the polymerization 
step; and then 
cross-linking the partially dehydrated polymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It has been surprisingly found that a premixing of the reactants is not 
necessary for a virtually complete reaction of polysaccharide and 
unsaturated carboxylic acid and that it is possible to meter in the 
unsaturated carboxylic acid only during the graft polymerization. 
Suitable polysaccharides which can be used in the process include starches, 
starch derivatives and cellulose derivatives. Natural starches obtained 
from potatoes, maize, wheat, rice and tapioca roots and also wax maize or 
high amylose starch and their derivatives, such as, for example, starch 
ethers and starch esters, can be employed. Starches which are of low 
viscosity on boiling and which usually slightly degrade upon hydrolysis or 
oxidation are particularly suitable. Starches having a viscosity of 20 to 
25,000 mPa.s, measured on a 10% paste at 20.degree. C, are preferred; 
viscosities of 40 to 500 mPa.s being particularly preferred. 
Preferably, 10 to 25 parts of starch are suspended in the organic solvent. 
The solvents employed for the organic phase of the reaction include 
hydrocarbons having 6 to 12 C atoms. Aliphatic or alicyclic hydrocarbons 
such as cyclohexane, n-hexane, C.sub.8 -isoparaffins or industrial benzine 
fractions, such as normal benzine, ligroin, test benzine or solvent 
naphtha, having an aromatic content of up to 20% and a boiling point in 
the range from 50.degree. to 200.degree. C. can be used. 
Lipophilic sorbitan esters such as, for example, sorbitan monolaurate, 
sorbitan monopalmitate and sorbitan monooleate, are preferably employed in 
the reaction as the nonionic surfactant having an HLB value of 0.5 to 10, 
which should be at least partially soluble in the organic solvent. In 
addition, polyether esters, such as polyethylene glycol(200) monooleate, 
polyethylene glycol(200) monolaurate and polyethylene glycol(300) oleate 
are also very suitable. 
The co-use of a predominantly water-soluble, nonionic dispersing agent 
having an HLB value of 10.5 to 20 is advantageous. Substances of this type 
are, for example, water-soluble polyethylene glycols having a molecular 
weight of 200 to 20,000, in particular of 400 to 5,000, and also 
polyethylene glycol ethers obtained from an aliphatic monohydric alcohol 
having 6 to 20 C atoms and a polyethylene glycol having 3 to 30, in 
particular 4 to 20, ethylene oxide units. Commercially available C.sub.12 
fatty alcohol polyglycol ethers having 7 to 19 ethylene oxide units and an 
HLB value of 13 to 18 are also suitable. Polyoxyethylene sorbitan fatty 
acid esters, such as, for example, polyoxyethylene sorbitan monolaurate 
and polyoxyethylene sorbitan monooleate are also suitable. 
In a preferred embodiment, the dispersing agent mixture consists of 50 to 
90 per cent by weight of nonionic surfactant having an HLB value of 4 to 
10 and of 10 to 50 per cent by weight of nonionic surfactant having an HLB 
value of 12 to 18. 
The proportion of the dispersing agent mixture is 1 to 10 per cent by 
weight, preferably 2 to 5 per cent by weight, based on the weight of the 
olefinically unsaturated carboxylic acid. 
The olefinically unsaturated carboxylic acids have 3 to 10 carbon atoms. 
Examples of these acids include acrylic acid, methacrylic acid, crotonic 
acid, tiglic acid and angelic acid. Acrylic acid and methacrylic acid are 
preferably employed. The acids can be neutralized or partially neutralized 
with alkali metal hydroxide or ammonium hydroxide solutions. Sodium 
hydroxide solution is preferably employed. The aqueous solutions of the 
unsaturated carboxylic acids usually have a solids content in the range 
from 20 to 45%. Preferably, 75 to 90 parts of unsaturated carboxylic acid 
are employed. 
In addition to the unsaturated carboxylic acids, other, olefinically 
unsaturated monomers such as acrylamide, methacrylamide, the Na salt of 
2-acrylamide-2-methylpropanesulfonic acid, 2-methacryloylethanesulfonic 
acid, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 
N,N-dimethylaminoethyl acrylate and N,N-dimethylaminoethyl methacrylate 
and its quaternary ammonium salts in the form of an aqueous solution 
thereof can be used for the polymerization. 
The aqueous solution which is added for the polymerization of the 
polysaccharide suspension initially introduced can also contain completely 
or predominantly water soluble crosslinking agents. Suitable crosslinking 
agents include vinyl compounds such as N,N-methylene-bis-acrylamide, 
butanediol 1,4-di(meth)acrylate, ethanediol di(meth)acrylate, 
diallylmaleinate, glycidyl(meth)acrylate, allyl methacrylate, polyethylene 
glycol(450) dimethacrylate, and polyepoxides such as, for example, 
ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, 
glycerol triglycidyl ether, diglycerol tetraglycidyl ether, and the like. 
Preferably, however, only a post-crosslinking reaction is carried out and 
no crosslinking agents are added to the aqueous solution in the 
polymerization step. 
Customary polymerization initiators are used for the graft polymerization 
reaction. Suitable initiators include, for example, ammonium 
peroxodisulfate, sodium peroxodisulfate and potassium peroxodisulfate and 
the corresponding peroxomonosulfates, dibenzoyl peroxide, dilauroyl 
peroxide, di-2-ethylhexyl peroxodicarbonate, dicyclohexyl 
peroxodicarbonate, tert.-butyl perpivalate, tert.-butyl perbenzoate, 
tert.-butyl permaleinate, tert.-butyl hydroperoxide, di-tert.-butyl 
peroxide, hydrogen peroxide and also redox catalysts, suitable reducing 
components being ascorbic acid, sodium methylsulfinate, disodium sulfite 
and sodium hydrogen sulfite. Azo initiators such as 
azo-bis-isobutyronitrile, 2,2-azo-bis-(2-amidinopropane) dihydrochloride, 
2,2'-azo-bis-(4-cyanopentanecarboxylic acid) and 
2-carbamoylazoisobutyronitrile are also suitable. 
The initiators can be added to the polysaccharide suspension in the 
solution of the unsaturated carboxylic acid. However, they can also be 
metered in as a separate aqueous solution. It is also possible initially 
to introduce a portion of the initiator in the organic phase and to add 
another portion with the aqueous solution of the unsaturated carboxylic 
acid. Preferably, 0.03 to 0.5 part of initiator is initially introduced 
into the organic phase and 0.05 to 1.5 parts of initiator are metered in 
an aqueous solution. 
The potassium peroxodisulfate and ammonium peroxodisulfate are preferably 
employed. 
Polysaccharide suspensions and aqueous solutions can, in addition, contain 
customary auxiliaries and additives, such as anti-foams and complexing 
agents. Thus, for example, nitrilotriacetate, ethylenediamine tetraacetate 
and diethylenetriamine pentaacetate can be added in order to complex 
traces of iron. 
The polymerization is preferably carried out at 50.degree. to 75.degree. C. 
In the case of the reverse-phase suspension polymerization an aqueous 
polymer mixture is obtained which preferably has a solids content of 30 to 
50%, based on the sum of polymer and water. 
After the polymerization is complete, a partial dehydration is carried out, 
the residual water content preferably being adjusted to 5 to 30%, based on 
the sum of polymer and water. The partial dehydration is generally carried 
out at 50.degree. to 100.degree. C. by azeotropic distillation, during 
which vacuum can also be applied. Conventional dehydration equipment, with 
which the organic phase is recycled, can also be used. 
After the partial dehydration, preferably 0.005 to 5 per cent by weight of 
crosslinking agent, based on the graft polymer, are added. Preferably, the 
content is 0.05 to 0.5 per cent by weight, epoxides being preferred. 
Suitable compounds include, inter alia, polyglycidyl ethers such as, for 
example, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl 
ether, glycerol triglycidyl ether and diglycerol tetraglycidyl ether. In 
addition, polyaldehydes, such as glyoxal or haloepoxy compounds such as 
epichlorohydrin, can also be used. These crosslinking agents are 
appropriately added in aqueous or organic solution. The post-crosslinking 
takes place by heating to 50.degree. to 100.degree. C., heating preferably 
being to 60.degree. to 80.degree. C. The crosslinking reaction is complete 
after 0.5 to 4 hours. 
After the crosslinking, the starch graft polymers are obtained in the form 
of pulverulent, porous grains which are composed of extremely finely 
divided primary particles and have good flow properties. The grains can 
easily be separated from the continuous organic phase, for example by 
filtration or centrifugation. Subsequently, they can be dried by 
conventional processes, for example under vacuum or using a fluidized bed, 
tumble or paddle drier, to give a pulverulent product. The filtrate can be 
re-used in the subsequent polymerization batch. Solvent and water can also 
be separated from the polymer powder by distillation. 
The polymerization leads to a uniformly fine product having a narrow 
particle size distribution and a high absorption capacity for water and 
body fluids. 
The products having particle sizes of below 2 mm are understood to be 
finely divided in the sense of this invention, and more than 85 per cent 
by weight of the products should have particle sizes of below 1,000 .mu.m. 
The formation of coarse particle agglomerates and caking is very slight. 
The products are particularly suitable for incorporation in 
cellulose-containing, absorbent hygiene articles, such as disposable 
napkins, sanitary towels, dishcloths and patient undersheets. They can be 
used as desiccants, as swelling agents in sealing compositions, as 
thickening agents and as water-storing agents and as moisture-retaining 
agents in agriculture. 
Since the unsaturated monomers metered in are polymerized immediately, the 
evolution of heat during the polymerization can be monitored and 
controlled very well. In the case of continuous addition, heat is 
generated continually and can be readily removed. Temperature peaks do not 
occur. The semi-continuous reverse-phase suspension polymerization process 
according to the invention can therefore be carried out without any great 
difficulties even on a production scale. 
The process is generally carried out by dispersing the polysaccharide in 
the organic solvent, with stirring, in a stirred vessel and heating the 
dispersion to the desired polymerization temperature. An aqueous solution 
containing the unsaturated carboxylic acid and the polymerization 
initiator is then metered in, a polymerization with grafting taking place. 
After the polymerization, partial dehydration is carried out by azeotropic 
distillation on a water separator. A post-crosslinking is then carried out 
and the polymer can then be separated as a finely divided product. 
Having now generally described this invention, a further understanding can 
be obtained by reference to certain specific examples which are provided 
herein for purposes of illustration only and are not intended to be 
limiting unless otherwise specified. 
FLUID RETENTIVITY 
In a 100 ml centrifuge flask, 70 ml of distilled water are added to 0.050 g 
of polymer, or 70 ml of synthetic urine which is a mixture of 3,883 g of 
distilled water, 33.2 g of NaCl, 4.0 g of MgSO.sub.4 .multidot.7H.sub.2 O, 
2.4 g of CaCl.sub.2 and 77.6 g of urea is added to 0.500 g of polymer, and 
the polymer is swollen for one hour, with gentle stirring. The gel phase 
is then repeated from the sol phase for 0.5 hour at 4,500 rpm by 
centrifugation and weighed. 
##EQU1## 
ABSORBENCY 
A 0.05 g amount of polymer, when distilled water is used or 0.100 g of 
polymer, when synthetic urine is used, is scattered on a glass frit (type 
G3, diameter 3 cm) which is connected to a fluid-filled burette, the level 
of which is adjusted to that of the glass frit. The amount of fluid 
absorbed is measured on the burette after 0.5 and 5 minutes. 
##EQU2## 
The yield in g indicated in the examples always relates to products which 
have been dried for 24 hours at 50.degree. C. in a vacuum drying cabinet 
(15 mbar vacuum) to a residual moisture content of 7 per cent by weight. 
The viscosities indicated relate to 10 per cent aqueous pastes at 
20.degree. C. 
EXAMPLE 1 
A 2,400 ml amount of cyclohexane is initially introduced into a 4 1 glass 
reactor, fitted with a stirrer, a nitrogen inlet, a metering device and a 
water separator, and heated to 68.degree. C. 
______________________________________ 
75 g of natural maize starch, 
4 g of sorbitan monolaurate (SPANR .RTM. 20 from 
Atlas, Wilmington, Del., USA), 
0.25 g of potassium peroxodisulfate in 30 ml of water 
and 
50 mg of ethylenediamine tetraacetate (Na salt) 
______________________________________ 
are dispersed therein, with stirring (500 rpm). Oxygen is then driven off 
with nitrogen, after which a monomer solution consisting of 
______________________________________ 
312 g of acrylic acid, 
480 g of 25% NaOH, 
10 g of sorbitan monolaurate, 
0.1 g of trimethylolpropane triacrylate and 
0.5 g of ammonium peroxodisulfate in 20 ml of water 
______________________________________ 
is metered in over the course of 45 minutes. The mixture is stirred for a 
further half-hour. 280 ml of water are then removed by azeotropic 
distillation at 75.degree. C., after which 0.3 g of ethylene glycol 
diglycidyl ether in 5 ml of water is added at 70.degree. C. The mixture is 
stirred for a further 2 hours, after which a pulverulent product is 
removed by filtration. 
Yield: 417 g of product. 
The particle size distribution determined by sieve analysis and also the 
absorption characteristics can be seen from Table 1. Polarographic 
recordings show that 16% of the starch fraction is homogeneously 
incorporated in the end product. 
EXAMPLE 2 
In the apparatus of Example 1, 44 g of starch, which has a low viscosity on 
boiling and which has a viscosity of 127 mPa.s (AMISOL.RTM. 05515 from 
Cerestar, D-4150 Krefeld), 
______________________________________ 
12 g of sorbitan monolaurate, 
4 g of polyethylene glycol having a molar 
mass of 1 550 (POLYDIOL 1550 from 
Huls AG, D-4370 Marl 1), 
50 mg of ethylenediamine tetraacetate and 
0.3 g of ammonium peroxodisulfate in 12 ml of water 
______________________________________ 
are dispersed in 1,800 ml of cyclohexane, with stirring, at 68.degree. C. 
An aqueous solution consisting of 
______________________________________ 
258 g of acrylic acid, 
400 g of 25% NaOH and 
0.5 g of ammonium peroxodisulfate in 20 ml of water 
______________________________________ 
is then metered in continuously over the course of 35 minutes, with 
stirring. 
A 2 g amount of pyrogenic silica (AEROSIL.RTM. 2000 C from Degussa AG, 
D-6000 Frankfurt) are added and 233 ml of water are then removed by 
azeotropic distillation. 
For post-crosslinking, 0.12 g of ethylene glycol diglycidyl ether, 
dissolved in 5 ml of a water/methyl ethyl ketone mixture (1:1), is added 
at 70.degree. C., after which the mixture is stirred for a further 2 
hours. 
Yield: 364 g of product. 
A residual monomer content of 31 ppm of unconverted acrylate is determined 
in the powder. Caking on the stirrer and the wall of the vessel amounts to 
21 g. 
EXAMPLE 3 
In the apparatus from Example 1, 
______________________________________ 
100 g of starch of low viscosity on boiling from 
Example 2, 
14 g of sorbitan monolaurate, 
4 g of polyethylene glycol having a molar mass of 
1,550, 
50 mg of ethylenediamine tetraacetate, 
0.5 g of potassium peroxodisulfate in 20 ml of water 
and 
200 g of water 
______________________________________ 
are dispersed in 2,200 ml of cyclohexane, with stirring (400 rpm), and the 
dispersion is heated to the reflux temperature (68.degree. C.). After 
flushing with nitrogen, an aqueous solution, prepared from 
______________________________________ 
312 g of acrylic acid, 
480 g of 25% NaOH and 
0.5 g of ammonium peroxodisulfate in 20 ml of water, 
______________________________________ 
is metered in over the course of 30 minutes. 
A 300 ml amount of water is removed by azeotropic distillation. After 
adding a mixture of 0.2 g of sorbitan triglycidyl ether (DENACOL.RTM. 614 
B from Nagase Chemicals Ltd., Osaka, Japan) and 0.1 g of ethylene glycol 
diglycidyl ether in 5 ml of water, the reaction mixture is stirred for a 
further 2 hours at 70.degree. C. 
Yield: 505 g of finely divided polymer powder. 
______________________________________ 
Caking on vessel wall and stirrer: 
23 g 
Residual acrylate content: 
18 ppm 
______________________________________ 
EXAMPLE 4 
In the apparatus from Example 1, 
______________________________________ 
100 g of starch of low viscosity on boiling 
from Example 2, 
12 g of sorbitan monolaurate, 
4 g of polyethylene glycol having a molar mass of 
1,550, 
50 mg of ethylenediamine tetraacetate and 
0.4 g of ammonium peroxodisulfate in 5 ml of water 
______________________________________ 
are dispersed in 1,800 ml of cyclohexane at room temperature. After 
flushing with nitrogen, the dispersion is heated to 68.degree. C. and an 
aqueous solution consisting of 
______________________________________ 
118 g of acrylic acid, 
290 g of 25% NaOH and 
0.4 g of ammonium peroxodisulfate in 16 ml of water 
______________________________________ 
is then added dropwise over the course of 35 minutes. A 170 ml amount of 
water is removed by azeotropic distillation. After adding 0.1 g of 
ethylene glycol diglycidyl ether in 5 ml of water, the reaction mixture is 
stirred for a further 2 hours at 70.degree. C. 
Yield: 260 g of finely divided polymer powder. 
______________________________________ 
Caking on vessel wall and stirrer: 
19 g 
Residual acrylate content: 
38 ppm 
______________________________________ 
If Example 4 is carried out as a one-step, discontinuous inverse suspension 
polymerization, in which all reactants are mixed at room temperature and 
heated, the polymerization starts at 65.degree. C. The temperature rapidly 
rises to 80.degree. C. Instantaneous cooling is necessary in order to 
control the polymerization. A lumpy product is obtained, which even after 
grinding has only a low water-absorption capacity. 
The characteristics of the products from Examples 1 to 4 are shown in Table 
1 below. 
TABLE 1 
__________________________________________________________________________ 
Absorption 
Starch Sieve analysis 
capacity (g/g) 
content Contents in % by weight 
Syn- 
Absorbency (g/g) 
% by 500- thetic 
Water Synthetic urine 
Example 
weight 
&gt;800 .mu. 
800 .mu. 
&lt;500 .mu. 
Water 
urine 
0.5 min 
5 min 
0.5 min 
5 min 
__________________________________________________________________________ 
1 16 12 60 28 320 36 42 110 19 22 
2 12 3.5 44.5 
52 348 36 52 108 17 23 
3 20 4.6 85 10.4 338 34 52 114 17 23 
4 39 0.5 18 81.5 280 32 40 104 15 20 
__________________________________________________________________________ 
EXAMPLES 5 TO 10 
The procedure followed is as described in Example 3. However, different 
polysaccharides are employed. The absorption capacity of the resulting 
products for synthetic urine can be seen from Table 2. 
TABLE 2 
______________________________________ 
Example 
5 6 7 8 9 10 
______________________________________ 
Poly- Dextrin Maize starch 
Maize starch 
saccharide 
M.sub.w = 
degraded by 
degraded by 
52,000 oxidation acid 
Viscosity 
-- 45 370 1,500 4,400 19,000 
(mPa.s) 
Carboxyl 
-- 0.41 0.22 -- -- -- 
(%) 
Absorption 
55 44 36 38 34 30 
capacity 
synth. urine 
(g/g) 
______________________________________ 
Having now fully described the invention, it will be apparent to one of 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth herein.