Use of glycidyl phosphonates as crosslinkers in the preparation of hydrogels

The present invention relates to the use of compounds of the general formula I ##STR1## in which R denotes alkyl, alkenyl or aryl, which can be optionally substituted, as crosslinkers in the preparation of hydrogels, and to the hydrogels thus prepared and their use.

The present invention relates to the use of glycidyl phosphonates as 
crosslinkers in the preparation of hydrogels, and to the hydrogels thus 
prepared and their use. 
For the preparation of hydrogels in aqueous solution, crosslinkers 
customarily employed are water-soluble compounds such as, for example, 
methylenebisacrylamide, bisacrylamidoacetic acid or alkenylphosphonic and 
phosphonic acid esters, but also poorly water-soluble compounds such as, 
for example, trimethylolpropane tri(meth)acrylate or tetraallyloxyethane. 
The object of the present invention is to make available novel, 
water-soluble compounds which act as crosslinkers, by whose use hydrogels 
having improved properties with respect to gel strength and water 
retention ability are obtained. 
This object is surprisingly achieved by the use of compounds of the general 
formula I 
##STR2## 
in which R denotes alkyl, alkenyl or aryl, which can be optionally 
substituted, as crosslinkers in the preparation of hydrogels. 
Preferably used compounds of the general formula I 
##STR3## 
are those in which R denotes (C.sub.1 -C.sub.6)-alkyl; (C.sub.3 
-C.sub.8)-cycloalkyl; a group of the general formula II 
##STR4## 
in which R.sup.1 and R.sup.2 independently of one another represent 
hydrogen or (C.sub.1 -C.sub.4)-alkyl; or a group of the general formula 
III 
##STR5## 
in which R.sup.3 represents hydrogen, halogen or (C.sub.1 -C.sub.4)-alkyl. 
Alkyl groups can be straight-chain or branched. (C.sub.1 -C.sub.6)-alkyl R 
in particular denotes methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, 
n-pentyl, i-pentyl, n-hexyl or i-hexyl, (C.sub.1 -C.sub.3)-alkyl, however, 
being particularly preferred. 
A particularly preferred (C.sub.3 -C.sub.8)-cycloalkyl is cyclohexyl. 
(C.sub.1 -C.sub.4)-alkyl R.sup.1 or R.sup.2 is particularly preferably 
methyl. R.sup.1 and R.sup.2, however, are very particularly preferably 
hydrogen. 
The radical R.sup.3 can be in the 2-, 3- or 4-position relative to the 
carbon-phosphorus bond. 
(C.sub.1 -C.sub.4)-alkyl R.sup.3 in particular denotes methyl, ethyl, 
n-propyl, i-propyl, n-butyl, i-butyl or tert-butyl. Halogen R.sup.3 in 
particular denotes fluorine, chlorine, bromine or iodine. 
R.sup.3 particularly preferably denotes methyl or chlorine, very 
particularly preferably hydrogen. 
The compounds of the general formula I can be prepared by known methods. 
Thus, for example, U.S. Pat. No. 2,856,369 describes the reaction of 
diallyl phosphonates with peracids to give the corresponding diglycidyl 
phosphonates of the general formula I, Doklady Akad. SSSR, 155 (1964) 1137 
describes the reaction of dichlorophosphonous acids with glycidol in the 
presence of base to give the corresponding diglycidyl phosphonites which, 
in turn, can be converted into compounds of the general formula I using 
oxidising agents such as N.sub.2 O.sub.4. 
Preferably, the compounds of the general formula I are prepared by reaction 
of the corresponding dichlorophosphonic acids with glycidol in the 
presence of base. The base is necessary for entraining the HCl formed in 
the reaction of dichlorophosphonic acid with glycidol, which leads to side 
reactions or secondary reactions. Zh. Obshch. Khim 116 (1984) 2404 
recommends NaH as base, but nitrogen-containing bases are more frequently 
used. These in particular include tertiary amines such as trimethylamine, 
triethylamine, tripropylamine or tributylamine. U.S. Pat. No. 2,856,369 
also recommends the use of pyridine. Trialkylamines are preferably 
employed, particularly preferably triethylamine. 
Preferred solvents for these reactions are diethyl ether, methyl tert-butyl 
ether, benzene, toluene or xylenes, but other inert solvents and mixtures 
of various solvents are also suitable. For industrial purposes, 
particularly suitable solvents are those such as methyl tert-butyl ether, 
tetrahydrofuran, toluene or xylenes and their mixtures; toluene is 
particularly preferably employed. 
The reactants and the required base are customarily employed in 
stoichiometric amounts, but excesses of base and/or glycidol may also be 
advantageous. Reasons for the preferred amounts used in each case can be 
of a process technology or application technology nature. Application 
technology reasons include, inter alia, purity criteria such as: colour, 
highest possible contents of active substance, lowest possible contents of 
by-products, lowest possible contents of starting compounds and lowest 
possible contents of hydrolysable and/or ionic chlorine. In the latter 
case, a lower excess, 1-20 mol %, preferably 1-5 mol %, of amine is 
frequently recommended. 
For the reaction, a mixture of glycidol and base in the solvent used is 
customarily introduced, and the dichlorophosphonic acid is added dropwise 
either in substance, or dissolved in one of the solvents described. Other 
procedures are also possible. For example, the amine and the 
dichlorophosphonic acid can be introduced and the glycidol added dropwise. 
Continuous procedures are also possible. To this end, for example, streams 
of glycidol and amine are brought together with a stream of the 
appropriate dichlorophosphonic acid and thus made to react. One or both 
streams then contains the required solvent which is necessary in order to 
avoid blockages of the pipelines by precipitating amine hydrochloride. 
After reaction is complete, the precipitated amine hydrochloride is 
customarily removed, for example by filtration or centrifugation. If a 
solvent-free product is required, the solvent is subsequently removed by 
distillation, optionally under reduced pressure. Further purification of 
the crude product thus obtained can be carried out by distillation under 
reduced pressure, either from the still, but preferably in a continuous 
manner by distillation via a thin film or short path evaporator. 
The present invention also relates to water-swellable hydrogels based on 
(co)polymerised hydrophilic monomers or based on natural hydrophilic 
polymers, characterised in that they are crosslinked with a compound of 
the general formula I. 
In the case of hydrogels based on (co)polymerised hydrophilic monomers, the 
compounds of the general formula I can even be added to the monomer 
mixture to be polymerised. However, the already polymerised so-called 
pre(co)polymers can also be crosslinked subsequently. Crosslinking is 
carried out by reaction of the epoxide groups of the compounds of the 
general formula I with the reactive groups, for example --COOH, --OH or 
--NHR, of the monomers or of the (co)polymers. 
The compounds of the general formula I are preferably employed in amounts 
of 0.05 to 10% by weight, relative to the total monomer weight, or the 
total polymer weight respectively. 
Suitable natural polymers which can be crosslinked with compounds of the 
general formula I to give hydrogels according to the invention can be 
employed either in the unpurified or in the purified form. 
Particularly suitable, in particular, are polysaccharides, such as, for 
example, guar, carboxymethylhydroxypropyl guar, starch, cellulose, 
hydroxyethylcellulose and alginates. 
Suitable copolymerisable hydrophilic monomers are in particular acrylic 
acid, methacrylic acid, crotonic acid, 
2-acrylamido-2-methylpropanesulphonic acid and -phosphonic acid, 
vinylphosphonic acid, vinylphosphonic acid half-esters, their salts, 
acrylamide, N-vinylamides or mixtures thereof. Acrylic acid and its salts 
are preferred. 
The polymerisation can be carried out in the homogeneous phase, for example 
in aqueous solution, as a so-called gel polymerisation. A further 
possibility for the synthesis of the hydrogels according to the invention 
is offered by precipitation polymerisation from organic solvents, such as, 
for example, alcohols, preferably tert-butanol, or hydrocarbons such as 
hexane or cyclohexane. 
The polymerisation can be initiated by free-radical formers such as, for 
example, organic or inorganic peroxides and azo compounds. Examples are 
benzoyl peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, 
(NH.sub.4).sub.2 S.sub.2 O.sub.8, K.sub.2 S.sub.2 O.sub.8, H.sub.2 S.sub.2 
O.sub.8, H.sub.2 O.sub.2 or azodiisobutyronitrile. Redox systems are also 
outstandingly suitable as polymerisation initiators. 
Finally, polymerisation can also be initiated by energy-rich radiation. 
If the compound of the general formula I is only added to a non-crosslinked 
pre(co)polymer, this is as a rule carried out before drying by homogeneous 
mixing, for example by kneading an aqueous polymer gel in a kneader. 
Spraying dilute solution onto a polymer powder is also possible; a 
subsequent temperature treatment in this case is not absolutely necessary, 
but accelerates the crosslinking reaction. 
The hydrogels according to the invention are outstandingly suitable as 
absorbents for aqueous fluids, for the formulation of cosmetic 
preparations, as strengtheners and/or binders of fibrous surface 
structures containing reactive groups and as drilling muds and cement 
slurries in oil production. 
For use as so-called "super absorbing polymers" (SAP) for use in hygiene 
articles, for example nappies, tampons or sanitary towels, hydrogels 
according to the invention based on acrylic acid are particularly 
suitable, it being possible for these to be partially present as the 
alkali metal or ammonium salt. Neutralisation can be carried out either 
before or after polymerisation. 
Hydrogels according to the invention based on polysaccharides are also 
outstandingly suitable for use as SAP. 
As the compounds of the general formula I are completely soluble both in 
water and in organic solvents, water-swellable hydrogels according to the 
invention having a more homogeneous network than compounds of the prior 
art can be obtained. As a result, the hydrogels also have high gel 
strength in addition to high absorption capacity. 
By subsequent crosslinking with compounds of the general formula I of 
already previously crosslinked polymers, which are intended for use as 
SAP, their performance can be substantially improved with respect to 
absorption under pressure. 
A further advantage of the compounds of the general formula I is their 
biological degradability.

EXAMPLE 1 
Synthesis of Diglycidyl Methanephosphonate (IV) 
940 g of toluene are cooled to -5.degree. C. and 148 g (2.0 mol) of 
glycidol and 213 g (2.1 mol) of triethylamine are subsequently added. 133 
g (1.0 mol) of methanedichlorophosphonic acid dissolved in 60 g of toluene 
are added dropwise with vigorous stirring, in the course of 1 to 2 hours, 
at an internal temperature between -5.degree. and 0.degree. C. The mixture 
is subsequently stirred at 0.degree. C. for 15 hours and the triethylamine 
hydrochloride formed is then filtered off with suction. It is washed with 
toluene and the filtrate is freed from toluene by distillation in vacuo. 
The residue is distilled via a short path evaporator at 1 mbar and a bath 
temperature of 175.degree. C. 198 g of diglycidyl methanephosphonate are 
obtained, corresponding to a yield of 95%. The boiling point is determined 
by means of a distillation experiment: 114.degree.-115.degree. C./0.1 
mbar. The product is obtained as a diastereomer mixture. 
C.sub.7 H.sub.13 O.sub.5 P (208.2): 
calc. 40.38% C, 6.29% H, 14.88% P. 
found: 40.4% C, 6.1% H, 14.7% P. 
EXAMPLE 2 
Synthesis of Diglycidyl Propanephosphonate (V) 
1000 g of toluene are cooled to -5.degree. C. and 148 g (2.0 mol) of 
glycidol and 213 g (2.1 mol) of triethylamine are subsequently added. 161 
g (1 mol) of propanedichlorophosphonic acid (technical isomer mixture, 
consisting of about 95% n-propyl and about 5% isopropyl isomers) are added 
dropwise with vigorous stirring, in the course of 1 to 2 hours, at an 
internal temperature between -5.degree. and 0.degree. C. The mixture is 
subsequently stirred at 0.degree. C. for 15 hours and the triethylamine 
hydrochloride formed is then filtered off with suction. It is washed with 
toluene and the filtrate is freed from toluene by distillation in vacuo. 
The residue is distilled via a short path evaporator at 1 mbar and a bath 
temperature of 185.degree. C. 227 g of diglycidyl propanephosphonate are 
obtained, corresponding to a yield of 95%. The boiling point is determined 
by a distillation experiment: 115.degree.-124 .degree. C./0.04 mbar. The 
product is obtained as a diastereomer mixture. 
C.sub.9 H.sub.17 O.sub.5 P (236.2): 
calc.: 45.77% C, 7.25% H, 13.1% P. 
found: 45.5% C, 7.3% H, 13.0% P. 
EXAMPLE 3 
Synthesis of Diglycidyl Vinylphosphonate (VI) 
1000 g of toluene are cooled to -5.degree. C. and 148 g (2.0 mol) of 
glycidol and 213 g (2.1 mol) of triethylamine are subsequently added. 145 
g (1.0 mol) of vinyldichlorophosphonic acid are added dropwise with 
vigorous stirring, in the course of 1-2 hours, at an internal temperature 
between -5.degree. and 0.degree. C. The mixture is subsequently stirred at 
0.degree. C. for 15 hours and the triethylamine hydrochloride formed is 
then filtered off with suction. It is washed with toluene and the filtrate 
is freed from toluene by distillation in vacuo. The residue is distilled 
via a short path evaporator at 0.1 mbar and a bath temperature of 
150.degree. C. 203 g of diglycidyl vinylphosphonate are obtained, 
corresponding to a yield of 92%. The product is obtined as a diastereomer 
mixture. 
C.sub.8 H.sub.13 O.sub.5 P (220.2): 
calc.: 43.64% C, 5.05% H, 14.07% P. 
found: 43.7% C, 6.0% H, 13.9% P. 
EXAMPLE 4 
Synthesis of Diglycidyl Benzenephosphonate (VII) 
1000 g of toluene are cooled to -5.degree. C. and 148 g (2.0 mol) of 
glycidol and 213 g (2.1 mol) of triethylamine are subsequently added. 195 
g (1.0 mol) of benzenedichlorophosphonic acid are added dropwise with 
vigorous stirring, in the course of 1-2 hours, at an internal temperature 
between -5.degree. and 0.degree. C. The mixture is subsequently stirred at 
0.degree. C. for 15 hours and the triethylamine hydrochloride formed is 
then filtered off with suction. It is washed with toluene and the filtrate 
is freed from toluene by distillation in vacuo. The residue is distilled 
via a short path evaporator at 0.15 mbar and a bath temperature of 
200.degree. C. 251 g of diglycidyl benzenephosphonate are obtained, 
corresponding to a yield of 93%. The boiling point is determined by a 
distillation experiment: 170.degree.-175.degree. C./0.1 mbar. 
C.sub.12 H.sub.15 O.sub.5 P (270.2): 
calc.: 53.34% C, 5.60% H, 11.46% P. 
found: 51.1% C, 5.6% H, 11.3% P. 
EXAMPLE 5 
4780 g of demineralised water are introduced into a polyethylene vessel 
which is well insulated by foamed synthetic material and has a capacity of 
10 l, 1696 g of sodium bicarbonate are suspended therein and 1994 g of 
acrylic acid are slowly metered in in such a way that frothing over of the 
reaction solution is avoided, the latter being cooled to a temperature of 
about 5.degree.-3.degree. C. 6 g of the compound V (R=C.sub.3 H.sub.7), 
prepared according to Example 2, and 10 g of a sodium 
diisooctylsulphosuccinate (Rewopol V 2133 from REWO, Steinau) are then 
added. The initiators, a redox system consisting of 2.2 g of 
2,2'-azobisamidinopropane dihydrochloride, dissolved in 20 g of 
demineralised water, 4 g of potassium peroxodisulphate, dissolved in 150 g 
of demineralised water and 0.4 g of ascorbic acid, dissolved in 20 g of 
demineralised water are added successively at a temperature of 4.degree. 
C. and the mixture is stirred. The reaction solution is then allowed to 
stand without stirring, a solid gel being formed by commencing 
polymerisation, in the course of which the temperature rises to about 
89.degree. C. This gel is then mechanically comminuted, dried at 
temperatures above 80.degree. C. and ground. 
The product described was incorporated in a conventional manner into a baby 
nappy and is in this case distinguished by a particularly good liquid 
retention. 
EXAMPLE 6 
The procedure is completely analogous to Example 5, only 6.0 g of the 
compound VI (R=CH.dbd.CH.sub.2), prepared according to Example 3, are now 
employed. The product resulting in this case is also outstandingly 
suitable for use in baby nappies and is distinguished by good liquid 
retention. 
EXAMPLE 7 
1287 g of demineralised water cooled to 15.degree. C. are introduced under 
adiabatic conditions into a 1.5 l cylindrical wide-necked reaction flask 
and 255 g of acrylic acid and 1.28 g of tetraallyloxyethane are dissolved 
therein. Nitrogen is introduced into the monomer solution (about 2 l/min. 
for about 20 min.) in order to reduce the oxygen content. 7.7 g of a 10% 
strength aqueous solution of 2,2'-azobis(2-amidinopropane) dihydrochloride 
are added at a content of 1.5 ppm of O.sub.2, 2.6 g of a 1% strength 
H.sub.2 O.sub.2 solution are added after further introduction of N.sub.2 
and at an O.sub.2 content of 1.3 ppm and finally 6.4 g of a 0.1% strength 
ascorbic acid solution are added at an O.sub.2 content of 1.0 ppm. Owing 
to commencing polymerisation, in the course of which the temperature rises 
to about 65.degree. C., a solid gel is formed which is then mechanically 
comminuted. 400 g of the comminuted gel are treated with 56.5 g of 50% 
strength sodium hydroxide solution (degree of neutralisation of the 
acrylic acid 74 mol %), thoroughly kneaded twice, treated with 25 g of a 
1% strength aqueous solution of the compound V (R=C.sub.3 H.sub.7), 
prepared according to Example 2, again kneaded twice, then dried at 
temperatures over 150.degree. C. in a thin layer, ground and sieved. 
A product is obtained characterised essentially, inter alia, by the 
following physical data, all measured in 0.9% NaCl. Extractable contents 
(1 h value)=2.1%, absorption under pressure (20 g/cm.sup.2)=29.8 g/g. 
EXAMPLE 8 
0.23 g of the compound IV (R=CH.sub.3), prepared according to Example 1, 
dissolved in 40 g of water is added to 300 g of a comminuted 30% strength 
polymer gel, neutralised to 73 mol %, having a monomer composition of 
99.7% by weight of acrylic acid and 0.3% by weight of triallylamine, 
prepared analogously to Example 5, and the mixture is kneaded until 
homogeneous, comminuted, dried at 180.degree. C. to a residual moisture of 
3%, ground and sieved. 
EXAMPLE 9 
The procedure is completely analogous to Example 8, only 0.46 g of the 
compound IV (R=CH.sub.3), prepared according to Example 1, is employed. 
EXAMPLE 10 
The procedure is completely analogous to Example 8, only 0.23 g of the 
compound V (R=C.sub.3 H.sub.7), prepared according to Example 2, is 
employed. 
EXAMPLE 11 
The procedure is completely analogous to Example 8, only 0.46 g of the 
compound V (R=C.sub.3 H.sub.7), prepared according to Example 2, is 
employed. 
EXAMPLE 12 
The procedure is completely analogous to Example 8, only 1.00 g of the 
compound VII (R=phenyl), prepared according to Example 4, is employed. 
The resulting products of Examples 8 to 12 are characterised by the 
following data summarised in Table I: 
TABLE I 
______________________________________ 
Extractable Absorption 
contents under pressure 
Gel 
16 h value (20 g/cm.sup.2) 
strength* 
Example (%) (g/g) (Pa) 
______________________________________ 
Starting 11.8 8.4 510 
polymer 
untreated 
8 9.4 11.0 600 
9 8.6 22.6 3250 
10 8.4 22.2 3100 
11 8.3 28.0 3500 
12 8.1 29.6 3700 
______________________________________ 
All values are measured in 0.9% strength NaCl. 
* = measured after a swelling time of 24 hours in 4.75% strength 
concentration 
EXAMPLE 13 
0.2% by weight of the compound V (R=C.sub.3 H.sub.7), prepared according to 
Example 2, in 10% strength aqueous solution in a PETTERSON & KELLY mixer 
is sprayed onto a commercial, partly neutralised, crosslinked polyacrylic 
acid warmed to 45.degree. C. for use as a super-absorber in baby nappies 
and the mixture is mixed for 10 min. After cooling to room temperature, 
the following improved values, listed in Table II, are found in comparison 
to the starting material: 
TABLE II 
______________________________________ 
Extractable 
Absorption 
contents under pressure 
Gel 
16 h value 
(20 g/cm.sup.2) 
strength* 
(%) (g/g) (Pa) 
______________________________________ 
Starting 7.1 8.3 1500 
material 
employed 
Example 13 
6.1 27.4 3100 
______________________________________ 
All values are measured in 0.9% strength NaCl. 
*measured after a swelling time of 24 hours in 2.5% strength concentratio 
 
EXAMPLE 14 
100 g of high molecular weight, non-crosslinked polyacrylic acid having a 
degree of neutralisation of 53 mol % in the form of a comminuted gel, 
prepared analogously to Example 5 without crosslinker is kneaded until 
homogeneous with 120 g of guar flour and 100 g of 0.15% aqueous solution 
of the compound IV, prepared according to Example 1, and the mixture is 
comminuted, dried in a stream of air at 180.degree. C. for 15 min., ground 
and sieved. A water-swellable product having a water absorption capacity 
of several times its own weight is obtained. 
In Examples 15 to 21, listed in Table III, the preparation of 
water-swellable products having good absorption ability by crosslinking of 
polymers of varying origin with compounds according to the invention is 
described, which is carried out by making the polymers into a paste in 
water, treating with the crosslinker, kneading until homogeneous, drying 
in a stream of air at 180.degree. C., grinding and sieving. 
TABLE III 
______________________________________ 
Compound according 
Charge 
to the invention, 
in % 
prepared according 
by 
Example 
Polymer to Ex. weight 
______________________________________ 
15 Guar flour, type 6382 
IV 1 0.5 
16 Commercial IV 1 0.5 
hydroxyethylcellulose 
17 Commercial VII 4 0.7 
hydroxyethylcellulose 
18 Commercial IV 1 0.5 
carboxymethylhy- 
droxypropyl guar 
19 Commercial V 1 0.5 
cationically modified 
guar 
20 Commercial IV 1 0.5 
Na alginate 
21 RHODIGEL.sup.1) 
IV 1 0.5 
______________________________________ 
.sup.1) : Polymer from RHONE POULENC 
EXAMPLE 22 
600 ml of hexane are introduced into a 1 l glass polymerisation flask 
provided with a stirrer, thermometer and reflux condenser, and 98.9 g of 
acrylic acid and 1.1 g of the compound VI (R=CH.dbd.CH.sub.2), prepared 
according to Example 3, are dissolved therein. While passing in a gentle 
stream of N.sub.2, the flask is heated to 68.degree. C. by means of an 
electrically heated water bath, whereupon the addition of 1.0 g of 
dilauryl peroxide is carried out. After the polymerisation commences, 
reflux clearly occurs and the polymer formed flocculates. The mixture is 
subsequently stirred under reflux for 3 hours, and the polymer is then 
filtered off with suction and dried to constant weight in a drying oven. 
100 g of a white powder are obtained, which can be employed as an acidic 
thickener in cosmetic preparations. 
EXAMPLE 23 
600 ml of tert-butanol are introduced into a 1 l glass polymerisation flask 
provided with a stirrer, thermometer, reflux condenser, gas inlet tube and 
electrically heated water bath, and 0.1 g of the compound VII (R=phenyl), 
prepared according to Example 4, and 65 g of 
acrylamido-2-methyl-propanesulphonic acid (AMPS) are suspended therein 
with stirring. About 5.5 g of ammonia gas are then passed in via the gas 
inlet tube, a slightly turbid solution being formed. The pH of this 
solution must be &gt;7. 15 g of acrylamide and 20 g of 
N-vinyl-N-methylacetamide are then added and the solution is heated to a 
temperature of 50.degree. C. while passing in a gentle stream of N.sub.2. 
1.0 g of azodiisobutyronitrile is added as initiator and the stirring 
speed is restricted to 60-80 rpm. After about 10 min., the polymerisation 
commences, which can be ascertained from a flocculation of the polymer and 
a temperature rise. In the course of about 20 min., a thick paste is 
formed and the temperature rises to about 75.degree. C. After reaching the 
maximum temperature, the mixture is subsequently stirred at 80.degree. C. 
for 2 hours, and the polymer is filtered off with suction and dried to 
constant weight in a vacuum drying oven at 60.degree. C. 105 g of white 
powder having a bulk density of about 0.2 kg/l are obtained, which is 
outstandingly suitable as an additive in drilling muds and cement slurries 
in natural gas and oil exploration. 
EXAMPLE 24 
A 10% strength aqueous copolymer solution having the monomer composition 
90% by weight of acrylic acid and 10% by weight of vinylphosphonic acid, 
partially neutralised with NaOH to a pH of 5.5-6.0 and treated with 1.0% 
by weight (relative to polyacrylic acid) of the compound V, prepared 
according to Example 2, is suitable for use as a binder/strengthener of 
non-wovens, combined with the advantage of better absorptive power. For 
this purpose, it is sprayed uniformly onto both side of an absorbent pad 
of cellulose fluff (about 6.times.20.times.1.5 cm/b.times.l.times.h) in 
such a way that the absorbent pad is loaded with 1% of the polymer solid, 
relative to the dry weight of the absorbent pad. After storage at room 
temperature in air for 24 hours or a corres-ponding shorter residence time 
at higher temperatures, the absorbent pads thus treated were examined for 
strength and absorptive capacity against corresponding untreated absorbent 
pads. The strength was tested by exposing the absorbent pads to a defined 
stream of air in a special vortexing vessel. The components separated from 
the absorbent pad by the vortexing were sucked off through a screen of 
defined mesh width. The content of undestroyed pad material not sucked off 
and remaining in front of the screen was then determined in % relative to 
the starting weight. 
The suction capacity was determined as follows: the absorbent pad, lying 
flat on a screen, was immersed in 0.9% strength NaCl solution for one 
minute. The screen was then taken out and allowed to drip for one minute. 
For this purpose, the experimental arrangement was inclined at about 
45.degree. C. The weight increase per gram of absorbent pad was 
calculated. 
It was possible to determine an improvement of about 20% with respect to 
strength for the treated absorbent pads in comparison with the untreated 
pads, and of about 10% with respect to absorbent capacity.