CA2146073A1

CA2146073A1 - Water-swellable hydrophilic polymers

Info

Publication number: CA2146073A1
Application number: CA002146073A
Authority: CA
Inventors: Rudiger Funk; Fritz Engelhardt; Ulrich Riegel; Michael Wessling
Original assignee: Rudiger Funk; Fritz Engelhardt; Ulrich Riegel; Michael Wessling; Cassella Aktiengesellschaft
Current assignee: Sanofi Aventis Deutschland GmbH
Priority date: 1994-04-02
Filing date: 1995-03-31
Publication date: 1995-10-03
Also published as: EP0675142A1; JPH0841136A; EP0675142B1; DE59504943D1; US5594083A; US5684104A; DE4411536A1; ES2128603T3; MX9501618A

Abstract

ABSTRACT

The present invention relates to water-swellable hydrophilic polymers which can be prepared by free radical (co)polymerization of one or more hydrophilic monomers or grafting (co)polymerization of one or more hydrophilic monomers onto a suitable grafting base, characterized in that a free radical initiator which can form di- or poly-radicals is employed.

Description

~ 21~117~
Ref.3574 - 1 - ! `
Bu0129 Water-swellable hydrophilic polymers ~;

The present invention relates to water-swellable ~ `;
hydrophilic polymers which can be prepared by ~ree radical polymerization using free radical initiator~
which form di- or poly-radicals.
Water-swellable hydrophilic polymers, in particular crosslinked pol~mers and copolymers based on acrylic or methacrylic acid, acrylamidopropanesulphonic acid copolymers or graft polymers on qtarch or poly-alkylene oxides, have been known for a long time and ared~cribed, ~or example, in US 4,931,497, US 5,011,892, US 5,041,496, US 3,926,891 and the literature references cited therein. -They can a~sorb several times their weight of water or aqueous liquids, such as urine or blood, and are ;therefore employed as absorbents, in particular in hygiene articles such as nappies for babies and incon tinence pants for adults, and also tampons and the like.
Such water-swellable hydrophilic polymers are as ~` 20 a rule prepared by free radical polymerization in an ~-aqueous solution which contains the monomers and, if appropriate, grafting base~and orosslinking agent. The polymerization can be initiated b~ high-energy~radiation -~
;~ ~ and/or ahemically. Chemical initiators which are employed here are, for example, peroxide compounds, such as per~
oxodisulphates, hydrogen peroxide, benzoyl peroxide, tert.-butyl hydroperoxide or tert.-butyl perpivalate, azo initiators, such as 2,2'-azobis(isobutyronitrile) ~AIBN) or~ 2,2'~azobis(2-amidinopropane)~ dihydrochloride, or redox syst2ms, such a~, for example sodium peroxodisulphate/
30dium pyrosulphite or hydroge~ peroxide/hydroxylamine chloride. ~en~oin, bensil and derivatives thereof or -~
acetophenone derivatives can also be used a~ photo- ~
:initiat~r~ All these initiators have the common feature `~ -that they form mono-radicals which trigger off the pol~merizatio~
However, the products prepared in this manner ~ l`
: `., . `:;

~ ' .'.'' ~:
~;~

~ 21~6073 ; :::

- 2 - !
~ have various network defects which are due to unwa~ted secondary reactions during the polymerization and which adversely influence the properties of the products. For example, oligomers are formed which are not incorporated into the polymeric network and therefore can be extracted from the swollen ~ietwork and are thus inactive con-stituents. In addition, polymer chains which are bonded to the network on only one side are also inactiv~.
It ha~ now been found that the network defect~
mentioned can be avoided or largely avoided if compound~
which instead of forming only one free radical site per molecule, ~uch as the compounds previouily employed according to the prior art, orm two or more free radical ~ites per molecule are employed a~ the free radical initiators.
., . . : ,~
The present invention thuc relates to water~
~wellable hydrophilic polymer~ which can be prepared by free radical (co)polymerization of one o~ more hydrophilic monomeirs or gra~t (co)polymerization of one or more hydrophilic monomers onto a suitable grafting baset characterized in that a free radical initiator which forms two or more ~ree radical site~ per molecule is employed. ;~
~uitable hydrophilic monom~rs are, for example, acids which ara capable of polymerization, such as acrylic acid, methacrylic acid, caproic acid, vinyl- ;
sulphonic acid, vinylphosphonic acid, maleic acid, including the anhydride thereof, ~maric acid, itaconic acid, 2-acrylamido-2-methylpropanesulphonic acid and 30 their amides, hydroxyalkyl esters and ester~ and amides ; ~`
containing amino groupæ or ammioniumigroups. Water-solublei~
N-vinylamides or el~ diallyldLmethylammoniumi chloride ~ - are ~urthermore ~uitable.
Preferred hydrophilic monomers are compounds of 35 the general formula I ~ ~
:: ;' ~' ' ''. " '' :: ~

.
-- 3 1 .
, R3 Rl :
(I) C H~
R ::
wherein is hydrogen, methyl or ethyl, R2 is the group ~CooR4, the sulphonyl group, the 5 phosphonyl group~ the phosphonyl group eisterified by :
; (Cl-C4)~ lkanol or a group of the formula ,..... :~

o 7H3 H~ C H 2--R 5 R3 is hydrogen, methyl, ethyl or the carboxyl group, R4 is hydrogen, amino or hydxoxy-(Cl-C4)-alkyl and R5 ls~ the ~:ulphonyl group, the phosphonyl ~roup or the .:. --:
carboxyl group.
Particularly preferred hydrophilic monomers are : acrylic àcid and methyacrylic:acid.
:Suitable grafting bases oan be of natural or : synthetic ~ori~in.~ ~Examples aro starch, cellulose or 15::~:ce~llulose~; derivakivas :and other polysaaohar~des and oligosaccharldes~ polyalkylens ~oxides~ in particular :~
polyethylene oxides~and~polypropylene:oxides, and hydro~
philic~polyesters~
Starah and~polyethylene oxide and polypropylene ..
0 oxides,.in~:particulax thosie described in US 4,9~1,497, ,`, i;
`US ~5,01:1,8i92~and:US~5~,;041,~96~, are~;pre~:erred.~:~he content~
of~these~paten~is i8~ also expres~ly a constituent of the ... ,.-prese~t disclo~sure. ;;~
ll aompountg~which ~orm two or more free radical ~.:; .`;
25 ; 9ites per molecule with or ~i hout the action of addi- `.:: .
tional acti~ator~ such as light, radiation, heat, ultra~ound~, redox agents and the like, can in principle ~:~ f be ~mployed as ~ree radical initiators. This means that -:;.. .~
::these frae radical initiators can contain:two, three or ~ ~

: ~ ".,'.' ;,~ '' f ~ ~

21~6~73 :;
, :, :
~ 23233-298 more groups whlch can form the radlcals. The free radlcal sltes . ~
can be formed here at the sarne time, but as a rule they are ~
: :, ;
formed at dlfferent tlmes, i.e. ln successlon. Compounds whlch contaln at least two hydroperoxide unlts, peroxide unlts or azo ~;
., unlts, for example, are sultable. ;~;

Cornpounds havlng two hydroperoxlde unlts are, ln ~
. . .
partlcular, dllsopropylbenzene dlhydroperoxlde (US 2,715,646) ; and 2,5-dlmethylhexane 2,5-dlhydroperoxlde. Sultable polyhydro-peroxldes can be obtalned, for example, by anodlc oxldatlon of ;~ 10 polycarboxyllc aclds, ln partlcular of polyacryllc acld and polymethacrylIc acld, ln the presence of oxygen (J. Pol. Scl.
Volume XXXIV, pages 287 to 307 (1959)).
Peroxlde unlts can be present, for example, as ~` ~ percarbonate or as perketal or perester unlts. ~xamples of such .
compounds are, in partlcular, dloxetane compounds and tert.-hutyl peresters, such as, for example, methyl acrylate/tert.-butyl peracrylate copolymers (J. Pol. Scl. Volume XXXIV, page . ..
; 301 (1959)). Polymerlc peroxy-esters furthermore can be obtalned -, ~
by reactlon of dicarboxyllc acld dlchlorldes wlth ~lshydro- ;~

2n peroxldes (~PA 461 7~7). ;~ ;~ , ,~ . , ~ Sultable compounds havlng several peroxide or hydro- ~;

;~ ~ peroxlde units and syntheses thereof furthermore are descrlbed ~ -in "The Chemlstry of Functlonal Groups, Peroxldes", edlted by S. Patal 1983, John Wlley & Sons Ltd., Chapter 13, by Ray ~; Ceresa.
. ., , ~, It ls preferable to employ free radlcal lnltlators contalnlng hydroperoxlde or peroxlde units together wlth reducing agents. Sultable reduclng agents are, for example, ',:
-' ."

~ ~.

4a 23233-298 :

~ ,, :,' Fe2 , ascorblc acld, sulphlnic aclds, sulphltes and forma- .
mldlnesulphlnlc aclds and salts thereof.
Sultable compounds whlch contaln two or more azo unlts are, for exam~le, reactlon products of a) azodlcarboxylic acids with compounds whlch contain more :~.
than one oxlrane functlon. Di-, tri- to ollgo-compounds and .:-polymers can be obtalned ln thls manner, ;

, ;" ,~ - . .:

-: . . ~:
i: .,: :,: ~:.:
"~
.;.''. :'"`,' ~: , ,','-. ,... ,-, : :; ~

~ ::: ' ~', ,`'`. .~,, :.
; ~ . '', ""'.. ,`,;~:i, ., ' ,:': :, 2~46073 ;

-- depending on the oxirane compound used.
A preferred azodicarboxylic acid iq, in particu-lar, 4,4'-azobi~(4-~yanovaleric acid), which forms suitable free radical initiators, for example, with ethylene glycol diglycidyl ether or with polyglycerol polyglycidyl ethers.
b) Hydroxyl- and amino-functional azo compounds with compounds which contain more than one oxirane or isocyanate group.
Suitable azo compounds are, for example, 2,2'-azo-bis(N,N-dimethyleneisobutyramidine) or the aorres-ponding dihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis(2-methyl-N-(1,1-bis(hydroxy-methyl)-2-hydroxyethyl)propionamide), 2,2'-azobis(2-methyl-N-(1,1-bi~(hydroxymethyl)-ethyl)propionamide) or 2,2'-azobi~(2-methyl-N-(2-hydroxyethyl)-propion~mide), which form suitable free radical initiators, for example with the glycidyl ethers mentioned above under aj or with hexamethylene diisocyanate, tolylene diisocyanate or phenylene diisocyanate.
c) Azobisamides with aldehyde~. A suitable azobisamide is, in particular, 2,2'-azobi~(isobutyramide) dihydrate, which f orm8 suitable free radical initiators, for example, with formaldehyde or glyoxalO
d) ~20bisnitrileq with polyalcohols. In particular, reaction products of 2,2'-azobisisobutyronitrile with ethylene glycol, butane-1,4-diol or hexane-1,6-diol are preferred (Makromol. Chem. 178 2533 (1977)).
Suitable initiators ara, in addition, poly-functional photoinitiators, compounds obtained by ~reaction of Ce~ with polyfunctional alcohols,~ such as polyvinyl alcohol or cellulose (J. Po. Sci. Volume XXXI, page 242 ~t ~eq. (1958)) and ozonized starch (Chemistry and Engineering News, 37 27, 41 (1959)).
~he fre~ radical initiators mentionsd can be used by themselve~ or a~ any desired mixtures with one another for tha preparation of the hydrophilic polymers according to the in~ention.
They are prsfera~ly employed here in amounts of ' , '~:", 21~6073 ~0.001 to 20 % by weight, based on the total monomers.
0.05 to 3.0 % by weight is particularly preferred.
In a particular embodiment of the present inven-tion, free radical initiators in which the functions which form free radicals have different reactivities or axe activated by different mechanisms are used. Such initiators thus contain, for example, both azo and peroxide or hydroperoxide ~unctions, which are acti~ated in succes~ion in a predetermined manner and can thus be used, for examplal for the preparation of block polymers.
It may fuxthermore be of advantage to use initia-tors in which the functions which form free radicals lie at different spatial di~tance~ from one another in the molecule.
15The molecular weight of the initiakors whichjcan be used for the preparation of hydrophilic polymers according to the invention can of course vary within wide ~;; limits. The molecular weights are, in particular, in the range from 100 to 10,000,000.
20The hydrophilic polymers according to the inven-tion can also be prepared using suitable ~rosslinking agents, i.e. compounds having at lea3t two dou~le bonds, , ;, :.. :~ ..
; which can be polymerized into the polymer network.
he use of crosslinking agents is particularly 25 preferred if the free radical initiators u~ed form only ` ;-~
two~ree radical sites~per molecule and there~ore do not themselve3~have crosslinking properties. `
On the other~hand, free radical initiators which form three or more free radical sites per molecule them~
selve~ have cro linking properties, so that in these ases the~ crosslinking agent~ mentioned can ! alsol be;
dispensed with. Nevertheles~, the crosslinking agents i~
; mentioned can also be used in combination with initiators which for~ three or more free radical site~ per molecule.
; 35Suitable crosslin~ing agants are, in particular, methylenebisacrylamide and -methacrylamide, esters of unsaturated mono- or polycarboxylic acids with polyols, , , ~ ,;, ,~ ~
such as diacrylate or triacrylate, for example butanediol diacrylate or dLmethacrylate or ethylene glycol diacrylate ,., ~ ~, .......
,':. , ~..

21~6073 ~::

, .
or dimethacrylate, trimethylolpropane triacrylate and vinyl methacrylate, and allyl compounds, such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid and vinylphosphonic acid derivatives, such as are described, for example, in EP-A 343 427. The content of EP-A 343 427 i~ al30 expre~sly a constituent of the present disclosure.
The crosslinking content i~ pre~erably 0 to 20 %
by weight, particularly preferably 0 to 3 % by weight, :~ based on the total monomer content.
Furthermoxe, khe hydrophilic polymers according to the invention can be post-crosslinked in the aqueous gel phase in a manner known per se or cros~linked on the sur~ace in the form of ground and sieved polymer ~: particles. Crosslinking agents which are suitable for :~ this are compounds which contain at least two groups : which can form covalent bonds with the carboxyl groups of the hydrophilic polymer. Suitable compounds are, for example, di- or polyglycidyl compounds, such as phosphonic acid diglycidyl ester, alkoxysilyl compounds, : polyaziridines, polyamines or polyi~midoamines, it also bein~ possible for the compounds mentioned to be used as mixtures with one another (see, for example, EP-A 83 022, : EP-A 543 303 and EP-A 530 438). Polyamidoamines which are suitable crosqlinking agent~ are described, in particu-lar, in EP-A 349 935. ~he content of the abovementioned patent appIications is expressly also a constituent o the present disclosure.
` I ` `i ~ The hydrophilic polymers according to the inven~
tion can be prepared by known polymerization proce~se~
Polymerization in aqueous olution by the process o~ 90 called geI polymerization i~ preferred. In this proce~s, 15 to 50 % stren~th by weight aqueous solutions o~ one or : more hydrophilic monomers and, if appropriate, a suitable grafting base are polymerized in the pre~ence o a free radical initiator which can form di- or poly radicals, prafarably without mechanical mixing and utilizing the - :
''` 21~6073 ,~ :, ~ Trommsdorff-Norrish effect (Bios Final Rep. 363.22;
Makromol. Chem. 1, 169 [1947)).
The polymerization reaction can be carried out in the temperature range between 0C and 130C, preferably between 10C and 100C, either under normal pressure or under increased pressure. As is customary, the poly-merization can also be carried out in an inert gas atmosphere, preferably under nitrogen.
~he quality properties o~ the polymers can be improved ~urther by subsequently heating the aqueous polymer gels in the temperature range from 50 to 130C, preferably 70 to 100C, for several hours.
~ he hydrophilic polymers according to the inven~
tion prepared by this route, which axe in the form of aqueous gels, can ~ obtained and employed in solid form by known drying processes, after mechanical comminution with æuitable appaxatuse~
The hydrophilic polymers according to the inven-tion have considerably higher molecular weights than the ~nown polymers o~ the prior art and have significant ad~antages compared with these. In particular, they have a high liquid-bonding capacity coupled with simul~
taneously high liquid retention values and a high mechanical strength of the swollen gel particles, with low extractable contents.
They are therefor~ outstandingly suitable as absorbents for water and aqueou~ liquids, such a8 urine or hlood, in hygiene articles such as nappies for babies and adults, bandages, tampons and the like. However, they can also be used as soil-improving agents in agriculture and hortiiculture, as moisture-binding agents in dable sheathing and for thickening aqueous waste products.

Example 1 1.0 g ~0.003571 mol) of 4,4'-azobis~4-cyano-valeric acid) was di~solved in 100 g of dimineralizedwater at 50C, 0.337 g (0.0019379 mol) of ethylene glycol diglycidyl ether was added and the mixture was le~t to stand at room temperature ~or 24 hours. 400 g of 21 ~ 60 73 _ - dimineralized water were initially introduced into a 1 1 ~ i glass polymerization flask, 70 g (0.83 mol) of Na bicar-bonate were suspended therein, and 200 g (2.77 mol) of -~
acrylic acid were added dropwise such that foaming over wa~ avoided. The monomer solution thereby cooled to about 10C. ~he previously prepared initiator solution was then transferred quantitatively to the reaction flask with the aid of 50 ml of dimineralized water as a rinsing agent, and the component~ were stirred homogeneously. The clear monomer solution was then left to stand under a CO2 atmosphere at room temperature for 14 hours~ without stirring and without a reaction baing detectable. It was then dilu~ed with 150 ml of dLmineralized water, rendered inert by passing in N2 and heated to an internal tempera-ture of 50C. When this temperature wa~ reached, thepol~merization reaction started Lmmediately and a high-vi6cosity paste wa~ formed, which was after-heated at 50C for 12 hours. A 0.1 % strength ~olution of the polymer in demineralized wa~er ~ba~ed on acrylic acid) had a relative vis-cosity of 28.7335, measured in an Ubbelohde capillary ~iscometer type Ic at 25C.
For comparison, the process wa3 repeated, b~t no ethylene glycol diglycidyl ether wa~ added to the initiator solution. A polymer o~ which a 0.1 % strength 25 solution in demineralized water (~ased on acrylic acid) had a relative visco~ity of 4.3345, measured in an Ubbelohde capillary viscometer type Ic at 25C, was ~ -obtained. -A ~ignificantly high molecular weight polymer, expressed by the relative viscosity, was thus obtained by ~sing the initiator which forms di-radicals than ~y using th~ initiator which forms mono-radicals.
The re~lllting gel according to the invention and the comparison gel were kne~ded with in each case 44.3 %
by weight of 50 % strength NaO~ (based on the acrylic acid) in a kneader until homogeneous, in each ca~e 0.5 %
by wei~ht (ba~ed on the acrylic acid) of methylphosphonic acid diglycidyl ester was then added, the mixture was homogenized at temperatures of 70 to 80C and the ,~

2~46073 :

composition, which was comminuted mechanically after .:-discharge from the kneader, wa3 dried in a stream of air at 180C. The product was ground and sieved (850/100 ~m).
The following performance data were obtained~
,.::., , . ~
Extractables CRC FSC AUL ~ :~
1 h 16 h ...
r%] [%] [g/g] [g/g] [g/g .,~
~el according to the invention 4.6 9.9 39 58 10.6 " ~, ,: , ~ ;:, ~ Comparison 7.3 16.2 34 54 8.2 :.... ..
~" ~ ., "., , "
CRC = Centrifuge Retention Capacity `~
FSC - Free Swell Capacity :...... 0.
10 AUL = Absorption Under ~oad [20 g/cm2] ;~
~ : , ............... , ~
Exa.mple 2 ~ .`
1 g (O.OQ3571 mol) of 4,.4'-azobis-4-cyanovaleric ~...... .-:
acid was dissolved in 20 ml of dimethylformamide, 0.31 g .~.. `.. ~.`-`~
0.00178 mol):of ethylene glycol diglycidyl ether was ` ~
:~ 15 added and the mixture was heated to 50C and kept at 50C ~....... . ~.
for 14 hours. .`,~.. :;.. `
60~ g of demineraliæed water were initially ~ introduced into a 1 1 polymerization flask, 200 g of :`;::~ acrylic acid were dissolved therein and this solution was heated to an internal temperature of 50C, while pa89ing ::: in N2. ~he previou~ly prepared initiator solution, warmed ;i ::;`
to 50C was then transferred quantitatively into the .`
reaction f~lask. After homogenization, the introduction of Na was stopped and the reaction ~olution wa~ left to ... `
25 ~tand~, without stirrin~. ~he polymerization reaction `
started immediately therea~ter, a high-vi3cosity paste being formed. The mixture wa~ ater-heated at 50C for .::. `..
~: 12 hours. A 0.1 % strength solution o~ the polymer in `.
damineralized water (based on acrylic acid) had a rela~
tivs vis~osity of 2.5902, measured in an Ubbelohde :~ capillary vi~cometer type Ic at 25C. ... ; ~."

;, ,~" 2~46073 ~' For comparison, the process was repeated, but no ethylene glycol diglycidyl ether was added to the initia-tor solution. A polymer whose 0.1 % strength solution in demineralized water (based on the acrylic acid) had a ,' 5 relative viscosity of 2.2894, measured in an Ubbelohde capillary viscometer type Ic at 25C, wa3 obtained. .: , The product according to the invention and the comparison product were subjected to post-crosslinking as described in ~xample 1. The following performance data were obtained: .:
, ` ''~,~, Extractables CRC FSC AUL , .. :~
1 h 16 h ~-:
~%] ~] ~g/g] [g/g] ~g/~
,, ,,,,~
Product accord- .' -ing to the 5.8 10.6 36 57 10.3 ' invention .'~
Comparison 8.1 16.9 33 54 8.3 '' 15 ExamPle 3 Example 2 was repeated, but only 0.155 g ~O.00089 mol) of ethylene glycol diglycidyl ether was employed. A polymer of which a O.1 % strength solution in , demineralized water (based 011 the acrylic acid) had a ' ,.
relati~e viscosity of 4.2890, measured in an Ubbelohde capillary viscometer type Ic at 25C, was obtained. :. ~.,:
The product was subjected to post cro~slinking as ~,,,,.' de~cribed in Example 1. The following performance data ."
were obtained~ "~"~

25Extractable~ CRC FSC AUL '~
1 h 16 h . - ' C~] ~%] ~g/g] 'g'g] ~g'g' --. ~
5.7 10.3 35 58 9.6 : .:

2146073 ; ~
- 12 ~
'.~ ':''~' ' '", 134.5 g of 50 % strength NaOH (degree of neutralization = 55 mol %) were stirred slowly ~nto a mixture of 344 g of demineralized water, 300 g of ice from demineralized water and 220 g o~ acrylic acid in a well-insulated polymerization flask while stirring and under adiabatic conditions. 1 g (0.45 % by weight) sf methylenebi~acrylamide was added, nitrogen was passed into the solution, while stirring, and the mixture wa~
brought to 5C. 0.19 g of 2,5-di~ethylhexane 2,5-dihydro-peroxide (product of PEROXID-CHEMIE Gmb9, Germany) and then 0.55 g of a 1 ~ strength a~ueous ascorbic acid ~olution were added, the mixture was stirred homogeneously, the stirrer was removed and the mixture was left to stand while passing in further N2. ~he reaction started after only a few minutes, and in the course thereof the temperature rose to a maximum of about 55C and a sliceable gel was formed. Thi~ was left to ~tand under the same conditions for about 6 hours and then comminuted mechanically, dried in a thin layer in a stream of air at 180C, ground and, if appropriate, sieved. A product was obtained which gave the performance data shown in Table 1.
For comparison, the abovementioned synthe is was repeated, with the difference that in tead of 2,5-di-methylhexane 2,5-dihydroperoxide, ammonium peroxodi-sulphate was employed. The performance data are likewise shown in Table 1.

Example 5 30 `` I Example 4, including the comparison, Iwas repeated, but instead of 0.45 % by weight, 0.87 ~ by weight o~ methylenebisacrylamlde was employed. The performance data can be ~een ~rom ~able 1.

Exam~le 6 Example 4, including the comparison, was repeated, but instead of 0.45 % by weight, 1.36 ~ by weight of methylenebisacrylamide was employed. The 21~6073 .

performance data are to be found in Table 1.

Example 7 The following were employed in accordance with Example 4:
220 g of ~cryllc acid neutralized with NaHCO3 to a degree of neutralization of 55 mol %.
0.3 % by weight, based on the acrylic acid, of trimethyl-olpropane triacryl~te as the crosslinking agent.
0.068 % by weight, based on the acxylic acid, o~ the reaction produ¢t of 2,2'-azobisisobutyronitrile with butane-1,4-diol (Pinner 3ynthesi~ analogou~ly to Example lb from Makromol. Chem. 178, 2533 (1977)~ as the free radical initiator.
For comparison, the synthesis wa5 repeated, but instead of the abovementioned free radical initiator, only the starting substances for its preparation were added. The performance data are to be found in Takle 1.

Example 8 a) Preparation of the free radical initiator:
2~8 g (1 mol) of 2,2'-azobii~-2-methyl-N-(2-hydroxyethyl)-propionamide were introduced slowly into a solution of 600 ml o~ anhydrous cyclohexane and 140 g ~; (l mol) o~ hexamethylene diisocyanate and 0.3 g of dibutyltin dilaurate, as the catalyst, while ~tirring, the reaction temperature being kept constant at 0 to 10C
by external cooling. The mixture wa~ sub~equently stirred at room temperature for 30 to 60 mlnutes, and the pre~
cipitate formed was then filtered off and freed from ` residual solvent under reduced pressure at a maximum of 40C, b) Preparation of the polymer according to the invention:
Th~ following were reacted in accordance with Example 4:
220 g o~ a mixture o~ acrylic acid and vinylphosphonia acid in a molar ratio of lQ0 : 1, neutralized with NaHCO3 to a degree of neutralization of 55 mol %.
0.3 % by weight, bai~ed on the monomers, of trimethylol-2 ~ 4 6 ~ 7 3 - 14 - ' propanetriacrylate a~ the crosslinking agent.
0.068 ~ by weight, based on the monomers, of the initi-ator prepared in accordance with a).
For comparison, the synthesis was repeated, but instead of the abovementioned free radical initiator, only the starting substances for its preparation were added~ ~he per~ormance data are to be found in Table 1.

Example 9 a) Prep8ration of the free radical initiator A solution (slightly cloudy) of 23.6 g (0.1 mol) of 2,2'-azobis(2~methylpropionamide) dihydrate in 500 g of water was brought to a weakly basic p~ of 7.5 with dilute iodium car~onate 901ution . 14 . 5 g ( 0 - 1 mol) of 40 % strength aqueous glyoxal were then added and the reaction solution was heated to 40C and stirred at this tempierature for 6 hours. After being cooled to c 20C, the reaction solution was employed directly for the polymerization experiments.
b~ Preparation of the polymer according to the invention:
The following were reacted in accordance with ~xample 4~
220 g of acrylic acid which, after the polymexization, was neutralized with NaOH to a degree of neutralization of 70 mol %.
0.45 % by weight, ba~ed on the acrylic acid, of tetra allyloxyethane as the crosRlinking agent.
0.091 % by weightl based on the i~orylic acid, of the nitiator prepaxsd in accordance with a).
For comparison, the synthe~is was repeated, but ~instead o~ the abovementioned free radical initiator, only the starting substances for its preparatio~ were ; added. The performance data are to be found in Table 1.

Example 10 The following were reacted in accordance with Example 4:
632 g of 2-acrylamido-2-methylpropanesulphonic acid, neutralized with ~a~CO3 to a degree of neutralization of .", ~, .

2 1 ~ 6 0 73 ~

70 mol %.
0.1 % by weight, based on the monomer, of tetraallyloxy-ethane as the crosslinking agent 0.046 % by weight, based on the monomer, of the initiator 5 prepared according to Example 9a). -:
~ or ~omparison ~ the synthesis was repeated, but instead of the abovementioned initiator, only the start-ing sub~tances for its preparation were added. The performance data are to be found in Table 1. ~ -Exa~ple_11 The following were reacted in accordance with Example 4: :
220 g of acrylic acid, neutralized with NaHCO3 to a degree of neutralization o~ 55 mol %.
:: 15 0.3 % by weight, based on the acrylic acid, o~ trimethyl-olpropane triacrylate. :~
:~ 0.022 ~ by weight, based on the acrylic acid, o~ 2,5- :
: dimethylhexane 2,5-dihydroperoxide and 0.025 % by weight, : based on the acrylic acid, of the free radical initiator employed in Example 7.
For comparison, the synthesis was repeated, but instead of the abovementioned initiator mlxture, only a mixture of 2,2'-azobisisobutyronitrile and butane-1,4-diol was added. The perormance data are to be found in -. 25 Table 1.

Exam~le 12 .
a3 Preparation of the free radical initiator~
Apparatus: doubla-walled glass beaker electrolysis cell -' ;6 ' 1 i ~aving a ground glass adaptor on the side, Teflon stopper with bore~ for electrodes, gas inlet tube and thermo~
meter; Pt shee~ ~lectrodes on holder; cryostat; galvano~
stat with current lead~, meaæuring equipment and the ~: lik~
0.35 q of NaO~ (0.0086 mol) was added to 150 g of an aqueou3 solution which contained 8.3 % (12.45 g, ; -:~:
0.173 molar equivalent of COOH) of polyacrylic acid (M~
about 200,000), and the mixture was transferred to the .'''' ~:' ~ 21~1~073 '''~`-''''''"';

-- 1 6 -- , I .... ..
- electrolysis cell and temperature-controlled at 10C with the aid of a cryostat. A constant stream of O2 was now passed into the solution via the gas inlet tube, onto the ' ~,':
lower end of which was fused a glass frit. Electrolysis . '~
5 was carried out under a current of 150 mA, while stir~
rin~, up to a charge throughput of 1800 C, the internal temperature being kept at 10C and the electrolyte being ''.~', flushed constantly with oxygen. The electrolysate wa~
employed in this form directly for the polymerization 10 experiment . , b) Preparation of the polymer according to the invention: "~
The ~ollowing were reacted in accordance with '','~."j' Example 4: .:,,, .:
220 g of acrylic acid which, after the polymeri2ation, ":'';,.
15 was neutralized with NaOH to a degree of neutralization of 6 8 mol % . ' ;.: '.:'- ; ', 5.8 % by weight, based on the acrylic acid, o~ the ,~
initiator prepared according to a). .~
The performance data are to be found in Table 1. ,.':.;-,':.'' For comparison, the synthesis wa~ repeated, the .''~
solution mentioned under a) being employed as the initi~
ator without electrolysis. No polymerization took place under these con~itioni3 (Compari30n 1).
The comparison was repeated, but O.09 % by 25 weight, baqed on the acrylic acid, of a~nonium peroxo~
disulphate additionally being added. A water-soluble, '::~.;;';.'~
non-swellable polymer was formed under these conditions -~,~'.`
(Comparison 23 Example 13 30 ' Tlie synthesis according to Example 12 'was repeated, 0.1 % by weight, based on the acrylic acid, of methylenebisa~rylamide additionally being added. '- ,~
: For compa:rision, the 6ynthe~iis was repeated, but :
the solution mentioYled under a), without electrolysis, 35 and O.09 % by wei~ht, based on the acrylic acid, of ~, ....
anmlonium peroxodisulphate w0re employed as the initiator.
The performance data are to be found in Table 1. ~,~

~`' 21~6073 ::

, - 17 - I .
Table 1 : :
, ~ , Performance data of the polymer~ according to Examples 4 to 13.
.
Example Extractables CRC FSC AUL .
1 h 16 h t%] ~%] ~/g~ ~g~g] ~g/g]
:: :
4 4.3 7.4 38 54 9.4 4 Comparison 8.1 19.2 35 52 9 1.5 4.5 28 47 21.4 5 Comparison 6.6 9.1 29 45 15.4 ~; 6 1 2.9 22 40 25.3 : 6 Comparison 4.3 7.5 21 39 20.9 : 7 5.1 8.2 43 63 g 7 Comparison 9.9 20 39 60 8.1 ` 8 8~1 12.4 35 70 8.5 .- ~. .
~ 8 Compa~i~on 12 19.3 29 63 8.4 ~ . -9 ~ ~ ~.3 8.1 31 55 11.4 ; 9 Compari~on 7.5 9.7 26 4~ 9.9 ~ ;
10 ~ 9.9 I3.1 31 54 ~ 8.4 ;~
lO Comparison11.2 16~2 27 50 8.1 ~ 4.5 8.2 ~4 58 10 :~ 20 11 Compari~on 6.2 9.1 39 51 8~8 :~
12 7.6 16~3 26 : 5~ 8.3 -.``~`
12 Comparison 1 no polymarization 12 Comparison 2 waker-soluble polymer : ~
; 13~ 5.6 14.4 28 57 9.7 " . `.
: 25 13 Comparison 7.3 16.9 25 56 8.1 `~
: , ,,' .

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Claims

1. A water-swellable hydrophilic polymer preparable by free radical (co)polymerization of one or more hydrophilic monomers or grafting (co)polymerization of one or more hydrophilic monomers onto a grafting base, in the presence of a free radical initiator which forms two or more free radical sites per molecule.

2. A water-swellable hydrophilic polymer according to claim 1, wherein the hydrophilic monomer is a compound of the general formula I:

(I) wherein R1 is hydrogen, methyl or ethyl, R2 is the group -COOR4, the sulphonyl group, the phosphonyl group, the phosphonyl group esterified by (C1-C4)-alkanol or a group of the formula:

R3 is hydrogen, methyl, ethyl or the carboxyl group, R4 is hydrogen, amino or hydroxy-(C1-C4)-alkyl and R5 is the sulphonyl group, the phosphonyl group or the carboxyl group.

3. A water-swellable hydrophilic polymer according to claim 1, wherein said grafting base is starch, a polyethyl-eneoxide or a polypropyleneoxide.

4. A water-swellable hydrophilic polymer according to any one of claims 1 to 3, wherein said free radical initiator is a compound which contains at least two hydroperoxide units, peroxide units or azo units.

5. A water-swellable hydrophilic polymer according to any one of claims 1 to 3, wherein said free radical initiator comprises diisopropylbenzene dihydroperoxide or 2,5-dimethyl-hexane 2,5-dihydroperoxide.

6. A water-swellable hydrophilic polymer according to any one of claims 1 to 3, wherein said free radical initiator comprises reaction product of 4,4'-azobis(4-cyanovaleric acid) with ethylene glycol diglycidyl ether.

7. A water-swellable hydrophilic polymer according to any one of claims 1 to 3, preparable in the presence of a cross-linking agent.

8. A process for preparing a water-swellable hydrophilic polymer according to any one of claims 1 to 3, which process comprises polymerizing a 15 to 50 % strength by weight aqueous solution of one or more hydrophilic monomers and, if required, a grafting base by gel polymerization in the presence of a free radical initiator which can form di- or poly-radicals.

9. Use of a water-swellable hydrophilic polymer according to any one of claims 1 to 3 as absorbents for water and aqueous liquids in hygiene articles.