(19) & (11) EP 1 622 655 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publication and mention (51) Int Cl.: of the grant of the patent: A61L 15/60 (2006.01) 24.08.2011 Bulletin 2011/34 (86) International application number: (21) Application number: 04760377.4 PCT/US2004/012707 (22) Date of filing: 23.04.2004 (87) International publication number: WO 2004/096304 (11.11.2004 Gazette 2004/46) (54) SUPERABSORBENT POLYMER WITH HIGH PERMEABILITY SUPERSAUGFÄHIGES POLYMER MIT HOHER PERMEABILITÄT POLYMERE SUPERABSORBANT PRESENTANT UNE PERMEABILITE ELEVEE (84) Designated Contracting States: (74) Representative: Lang, Arne et al AT BE BG CH CY CZ DE DK EE ES FI FR GB GR Evonik Degussa GmbH HU IE IT LI LU MC NL PL PT RO SE SI SK TR DG-IPM-PAT Bau 1042 / PB 15 (30) Priority: 25.04.2003 US 424195 Paul-Baumann-Strasse 1 45764 Marl (DE) (43) Date of publication of application: 08.02.2006 Bulletin 2006/06 (56) References cited: EP-A- 1 153 656 WO-A-00/50096 (73) Proprietor: Evonik Stockhausen, LLC WO-A-01/13841 WO-A-95/11932 Greensboro, NC 27406 (US) US-A- 4 587 308 US-A- 5 032 628 US-A- 5 409 771 US-A- 5 744 564 (72) Inventors: US-A- 6 124 391 US-A1- 2002 161 132 • SMITH, Scott, J. US-B1- 6 388 000 Greensboro, NC 27407 (US) • JOY, Mark, C. • PATENT ABSTRACTS OF JAPAN vol. 2003, no. Greensboro, NC 27410 (US) 02,5 February 2003 (2003-02-05) & JP 2002 302513 • HSU, Whei-Neen A (NIPPON SHOKUBAI CO LTD), 18 October 2002 Greensboro, NC 27410 (US) (2002-10-18) • FRANK, Markus 76532 Baden-Baden (DE) Note: Within nine months of the publication of the mention of the grant of the European patent in the European Patent Bulletin, any person may give notice to the European Patent Office of opposition to that patent, in accordance with the Implementing Regulations. Notice of opposition shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). EP 1 622 655 B1 Printed by Jouve, 75001 PARIS (FR) EP 1 622 655 B1 Description FIELD OF THE INVENTION 5 [0001] The invention relates to superabsorbent polymers which absorb water, aqueous liquids and blood wherein the superabsorbent polymers of the present invention have improved properties, in particular an improved relationship between gel bedpermeability and fluid retention includingachieving higher gel bedpermeability withoutthe disadvantages of low retention that are characteristic of higher gel strengths. The present invention also relates to preparation of these superabsorbent polymers and their use as absorbents in hygiene articles and in industrial fields. 10 BACKGROUND OF THE INVENTION [0002] Superabsorbent refers to a water- swellable, water-insoluble, organic or inorganic material capable of absorbing at least about 10 times its weight and up to about 30 times its weight in an aqueous solution containing 0.9 weight percent 15 sodium chloride solution in water. A superabsorbent polymer is a crosslinked polymer which is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure in accordance with the general definition of superabsorbent. [0003] The superabsorbent polymers that are currently commercially available are crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide 20 solution or potassium hydroxide solution. As a result of these characteristic properties, these polymers are chiefly used for incorporation into sanitary articles, such as babies’ diapers, incontinence products or sanitary towels. [0004] Superabsorbent polymers are described in several publications for example in Buchholz/Graham "Modern Superabsorbent Polymer Technology", Wiley-VCH 1998, pages 97-101; 174-175; 190-193; 208-209, US 5,669,894,EP 1 364 992 A1,US 2002/ 0128618 A1, US 5,032,628, US 6,124,391, WO 00/50096, EP 1153656, US 4,587,308, US 25 2002/161132, WO 01/13841 and WO95/11932. [0005] For fit, comfort and aesthetic reasons and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the high volume fluff fiber of these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more superabsorbent polymer content is being used in these sanitary articles. As a result of this, superabsorbent polymers must have increased 30 permeability characteristics while retaining other characteristics such as adequate absorption and retention. [0006] Permeability is a measure of the effective connectedness of a porous structure, be it a mat of fiber of a slab of foam or, in this case, crosslinked polymers and may be specified in terms of the void fraction and extent of connectedness of the superabsorbent polymer. Gel permeability is a property of the mass of particles as a whole and is related to particle size distribution, particle shape, the connectedness of the open pores, shear modulus and surface modification of the 35 swollen gel. In practical terms, the permeability of the superabsorbent polymer is a measure of how rapidly liquid flows through the mass of swollen particles. Low permeability indicates that liquid cannot flow readily through the superab- sorbent polymer, which is generally referred to gel blocking, and that any forced flow of liquid (such as a second application of urine during use of the diaper) must take an alternate path (e.g., diaper leakage). [0007] In particular, gel blocking is a well-known problem that may be associated with the use of superabsorbent 40 polymers in absorbent articles such as diapers. Gel blocking occurs when rapid expansion of the superabsorbent polymer particles around the point of entry of body fluid into an absorbent article causes a closing of the interstitial spaces and pores in the SAP-fluff matrix. Since the transport of liquid by diffusion through swollen hydrogel is much slower than transport through the interstitial spaces, a sealing effect occurs in the area of fluid entry. This effect is referred to as gel blocking. 45 [0008] Transportation of liquid through swollen superabsorbent polymer particles themselves follows the laws of dif- fusion and is a very slow process which plays no role in the distribution of the liquid in the use situation of the sanitary article. In superabsorbent polymers, which cannot maintain an open bed structure to effect capillary transportation because of a lack of gel stability, the separation of the particles from one another has been ensured by embedding the superabsorbent polymer into a fiber matrix. 50 [0009] In diaper constructions, for what is called the next generation, there is less fiber material, or potentially none at all, in the absorber layer to assist in transportation of the liquid or maintenance of an open, fluid permeable structure. The superabsorbent polymer of these next generation diaper constructions must have a sufficiently high stability in the swollen state, generally called gel strength, so the swollen gel has a sufficient amount of capillary spaces through which liquid can be transported. 55 [0010] To obtain a superabsorbent polymer with high gel strength, the degree of crosslinking of the polymer may be increased, which necessarily results in a reduction in the swellability and the retention capacity. To achieve the increased permeabilities needed in extremely thin, next generation articles with low fiber content, current art has taught to increase the amount of crosslinking in have higher gel strength achieved, typically having a shear modulus of greater than 9,500 2 EP 1 622 655 B1 dynes/cm2. However the absorption and retention values of the superabsorbent polymers are reduced to undesirably low levels. It is an important goal of the art of making superabsorbent polymers to develop a polymer having a high absorption and retention capacity for liquid in the after- surface crosslinking stage and increased permeability properties. It has been found that by using new surface modifications to the SAP particles, results of higher permeabilities without 5 very high gel strengths and the undesirable associated low absorption values are achieved. [0011] It is therefore an object of the present invention to provide an absorbing polymer composition that exhibits excellent properties such as capabilities of maintaining high liquid permeability and liquid retention even when the superabsorbent polymer is increased in percent by weight based on the absorbent structure. 10 SUMMARY OF THE INVENTION [0012] The present invention is directed to a superabsorbent polymer in the form of particles comprising a) from 55 to 99.9 wt. % of polymerizable unsaturated acid group containing monomers, polymerized in the presence 15 of; b) from 0.001 to 5.0 wt.% based on the weight of a) of internal crosslinking agent to obtain polymers wherein the polymerizable unsaturated acid group containing monomers are acrylic acid or methacrylic acid, the carboxyl groups of which are neutralized to the extend of 50-80 mol%; the internal cross linking agent being a di- or triacrylate ester of trimethylolpropane which is oxyalkylated or di(meth)acrylates or tri(meth)acrylates of ethylene glycol, 20 c) from 0.001 to 5.0 wt.% based on the weight of the dry polymer of surface crosslinking agent applied to the particle surface, wherein the crosslinking agent is an alkylene carbonate or an aqueous alcoholic solution thereof; wherein superabsorbent polymer particles having a particle size in the range of 400 to 600 microns have a gel bed permeability numeric value GBP according to the description, tested at 23 1 °C and 50 2 % relative humidity; of at 25 least [54000e-0.18x +75] x10-9cm2 where x is the numeric value of centrifuge retention capacity determined according to the description; and a shear modulus G’ less than 9,500 dynes/cm2 (950 Pa) determined with a pulse shearometer of Rank Brothers Ltd.