CA1304923C

CA1304923C - Absorbent structure comprising dispersed hydrogel particles

Info

Publication number: CA1304923C
Application number: CA000506551A
Authority: CA
Inventors: Elisabeth Hendrika Duenk; Walter Jakob Fisher
Original assignee: Procter and Gamble Co
Current assignee: Procter and Gamble Co
Priority date: 1985-04-15
Filing date: 1986-04-14
Publication date: 1992-07-14
Anticipated expiration: 2009-07-14
Also published as: MY102393A; JPS621453A; DK167953B1; EP0198683A2; DE3682808D1; FI861572A; EP0198683B2; GB8509591D0; KR860008033A; GB2175024A; EG18787A; MA20663A1; ES553950A0; AU602863B2; JPH0779965B2; EP0198683B1; IE57318B1; ES8706767A1; AU5612286A; DK171786D0

Abstract

ABSTRACT

Absorbent structures comprising a continuous matrix of hydrophilic fibers, having dispersed therein particles of a water-insoluble hydrogel, are disclosed.

The distribution of hydrogel particles is showed toward the back face of the structure, resulting in the back face half containing at least 60 % of the hydrogel material.

Description

1~04g23 ABSORBENT STRUCTURE COMPRISING
DISPERSED HYDROGEL PARTICLES

Technical ~ield This invention relates to absorbent structures which are used to absorb fluids, especially body fluids and wastes. The abeorbent structures herein are employed as the fluid absorbing member in products such as baby diapers, adult incontinence items, bandages, catamenials, bed pads, and the like.
n ~304923 Background of the Invention.

Water-insoluble hydrogels have been proposed for use in absorbent structures. A major drawback of the hydrogel materials is their relatively high cost, as compared to conventional absorbent materials like cotton and wocd pulp fibers. A second drawback is a phenomenon called gel-blocking: when exposed to a fluid, the outer layers of a hydrogel particle swell, thus preventing fluid to reach the inner layers of the particle other than by a relatively slow diffusion process. Because of this gel blocking, hydrogel materials have relatively poor absorption kinetics.

One general approach towards solving this problem is to provide a multilayer ~tructure, comprising one or more hydrogel containing layers and one or more wicking layers.

Examples of this approach are disclosed in:
US Patent No 4.364.992, issued December 21,1982 to Ito et al;
US Patent No. 4.102.340, issued July 25,1978 to Mesek et al; and in EP-PA 83306764.8, publication No 0 108 637.

A drawback of this approach is that the wicking layer does not have the excellent rewet properties of hydrogel material. Another drawback is the relatively high manufacturing cost of multilayer structures.

EP PA 8430 1578.5~ publication No, 0 122 042, discloses absorbent structures comprised of a hydrcphilic fibrous matrix having dispersed therein discrete particles of a water-insoluble hydrogel material. The structures are compressed to a relatively high density, so as to ensure good wicking.

Although this latter approach provides absorbent structures having a good performance in terms of rewet and absorbent capacity, there is still room for improvement. The high cost of hydrogel materials makes a further increase of the absorbent capacity per gram of material highly desirable.

It is therefore an object of the present invention to provide a hydrogel-comprising absorbent structure having an improved absorbent capacity per gram of hydrogel material used.
Summary of the Invention The present invention relates to absorbent structures having a top face and a back face, and comprising a continuous matrix of hydrophilic fibers having dispersed therein discrete particles of a water-insoluble hydrogel, characterized in that the back face half of the absorbent structure contains at least 60% of the water-insoluble hydrogel material.

Various aspects of the invention are as follows:

An absorbent structure having a top face, a back face, a top face half, and a back face half and comprising a continuous matrix of hydrophilic fibers having dispersed therein in both said top face half and said back face half discrete particles of a water-insoluble hydrogel, characterized in that the hydrogel content of said back face half of the absorbent structure contains at least 1.5 times the hydrogel content of said top face half of the absorbent structure so that at least 60% of the water-insoluble hydrogel material is contained within said back face half.

, i~

An absorbent structure having a top face and a back face and comprising a continuous matrix of hydrophilic fibers having dispersed therein discrete particles of a water-insoluble hydrogel, characterized in that the hydrogel concentration gradually increases from said top face to said back face so that the distribution of hydrogel particles is skewed towards said back face of the absorbent structure.

Detailed Description of the Invention This invention relates to absorbent structures comprising hydrogel particles dispersed in a continuous matrix of hydrophilic fibers. The basis of this invention is the discovery that the absorbent capacity of ~uch structures can be increased by preferentially placing the hydrogel particles near the face of the structure which is turned away from the point of fluid di~charge.

~30A923 Absorbent structures intended for us~ in e.g., diapers or catamenials typically have a m~re or less planar configuration. For convenience, the face of the str w ture intended to be placed nearest to the body of the person using it will be referred to herein as the ~top face~.
m e opposing fa oe will be referred to as the ~back face~.
The absorbent structures herein comprise a continuous matrix of hydrophilic fibers having dispersed therein discrete particles of a water-insoluble hydrogel. The distribution of hydrogel particles is skewed bowards the back face of the structure. If the structures herein were to be aut into two halves along a plane substantially parallel to the top face and the back face, the resulting back face half would contain at least 60 % of the total am~unt of hydrogel material, whereas the top face half would contain not more than 40 % of the total amount.
Preerably, the back face half contains at least 70 ~ of the total anount of hydrogel, most preferably from 75 % to 100%. The hydrogel distribution in any given plane ~ubstantially parallel to the top face and back face can be hc~n~genous or nonhomogenous.

Any hydrogel distribution resulting in 60 % or more of the hydrogel to be dispersed in the back face half of the structure are suitable for the puposes of the present invention.

E~uu~ples of such hydrogel distributions i w lude ~tructures which, in he Z-direction (from top face to bottom face) comprise an area of, e.g., 1-3 mm of low hydrogel concentration, followed by a gradually increasing hydrogel concentration. Instead of a gradual increase of hydrogel co w entration, there can be a rather abrupt increase.

1:~04923 Highly preferred are structures having, imnediately adjacent to the back fa oe , a thin (e.g., 0.5-2 mm ) area of low (e.g., less than 10 %, preferably less than 5 ~) hydrogel concentration. Structures of this type mininize the risk of hydrogel particles ~erforating the backsheet material of articles (diapers, catamenials, etc) in which the structures are used. Moreover, this area of low hydrogel concentration acts as a wicking layer.

By ~hydrogel~ as used herein is meant an inorganic or organic compound capable of absorbing agueDus fluids and retaining them under moderate pressures. For good results, the hydrogels must be water insoluble. EKamples are inorganic materials such a~ silica gels and organic COmpOUndB such as cross-linked polymers. Cross-linking may be by covalent, ionic, vander Wbals, sr hydrogen bonding. Examples of polymers include polyacrylamides, polyvinyl alcohol, ethylene/maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, carboxymethyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine and the like. Okher suitable hydrogels are those disclosed in U.S. Patent 3,901,236, issued to Assarsson et al. August 26,1975, the disclosures of which are incorporated herein by reference. Particularly preferred polymers for use herein are hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, polyacrylates, and isobutylene/maleic anhydride copolymers, or mixtures thereof.

Prooesses for preparing hydrogels are disclosed ~n U.S. Patent 4,076,663, is~ued February 28,1978 Oo Fu~ayoshi Masuda et al.s in U.S. Patent 4,286,0~2, isRued ALgust 25,1981 to Tsuno Tsubakimoto et al.~ and further in V.S. Patents 3,734,876, 3,661,815, 3,670,731, 3,664,343, 37783,B71, ~nd Eelgian Patent 785,858.

As used herein ~Particles~ include p3rticles of any dhape, e.g. qpherital or somi-spherical, ~ubic, rod-like, polyhedral, etc.s but also shapes having a laxge greatest diwen~ion/~malle~t di~ension ratio, like needles, fl~kes snd fibers, are contemplated for used herein. ~y ~particle ~ize~ a8 used herein is meant the weight average of the smalle~t dimension of the indiv~dhal p~rticles.
Ccnglom~r~tes of hydrogel part$cle~ m~y ~l o be uJ d, prov~d~d the we$ght average ~ize of ~uch conglo~erates i~
within the l~mits set forth hereinbelow.

Although the absorbent ~tr w ture of the p~resent invention are e~pected to perform well w$th hydrogel particles h~ving a part$cles ~ize varying over a wide range, other cons$derat$on~ ~ay preclude the use of very small or very large particl . For reasons of indu~trial hygiene, tweight) aYerage particle æizes smaller than about 30 ~icrometer~ are 1e~B deæirable. Particles having a ~malle~t dimension larger than about 4 mm may c~u~e a ~eeling of grittines~ in the absorbent ~tructure, which i~
unde~irable from a consumer ætandpoint. Preferred for use herein are particles having an (weight) ~verage particle æize of fro~ aboue 50 micrometeræ to about 1 mm.

~304923 m e type of hydrophilic fibers is not critical for use in the present invention. Any type of hydrophilic fiber which is suitable for use in conventional absorbent products is also suitable for use in the absorbent structure of the present invention. Specific exa~ples include cellulose fibers, rayon, polyester fibers. Other examples of suitable hydrophilic fibers are hydrophilized hydrophobic fibers, like surfactant-treated or silica-treated thermoplastic fibers. Also, fibers which do not provide webs of sufficient absorbent cap~city to be useful in conventional absorbent structures, but which do provide good wicking properties, are suitable for use in the absorbent structures of the present invention. m is is so because, for the purposes of the present invention wicking properties of the fi~ers are far more imçortant than their absorbent capacity. For reasons of availabil$ty and cost, oe llulose fibers, in particular wood pulp fibers, ~re preferred.

Preferred are absorbent structures which contain from 3 to 40 %, by total dry weight of the structure, of the hydrogel material. Mbre preferred are structures containing from 8 % to 20 % of hydrogel material.

Since the fibrous matrix and the hydrogel particles are chemically different, the hydrogel distribution can be verified by chemical analysis, using well established chemical techniques.
Typically, repre&entative samples are taken from the top face half and and the back face half, respectively, of the absorbent structures. ffl e samples are then analyzed for hydrogel content. ~he hydrogel content of the back face half sample should be at least 1.5 times the hydrogel content of the sample taken from the top fa~ half of the structure.

1~04923 As an example, a structure containing 15 ~ hydrogel material (by total dry weight of the structure), 80 ~ of - which is disposed in the back face half of the structure, has a hydrogel content as follows:

back face half : 0.8 x 0.15 x 100% = 24%
. .
0.5 top face half : 0.2 x 0.15 x 100% s 6%
0.5 the back fa oe half containing four times as much hydrogel as the top face half.

Sin oe many of the commercially available hydrogel materials contain acid (carboxylic, sulfonic, etc) groups, an acid-base indicater can be used for a ~emi-quantitative determination of the hydrogel distribution.

For example, a crosslinked polyacrylate hydrogel has carboxy radicals as it functional groups. These boxy groups are predominantly in the salt ~e.g., sodium) form.
The acid-base indicator bronocresol purple (5', Sn dibrono - Q - cresolsulfone phtalein; yellow - pH 5.2 to 6.8 - purple) turns to its alkaline (purple) color when contacted with a p~lyacrylate hydrogel.

The distribution of a polycarboxylate hydrogel in an absorbent structure can be made visible as follows:
50 mg bro~ocresol purple is dissolved in 1 liter distilled water. ffl e p~ of the solution is adjusted to 5, using HCl. 400 ml of the yellow solution are p~ured evenly onto the absorbent structure to be tested.

g m e solution is allowed to penetrate for about two minutes. m e intensity of the purple color is a measure of the a unt of hydrogel present. m e hydrogel distribution can be studied by oomparing the color intensities at the top face and the back face of the absorbent structure.

In an alternate pro~edure, the structure is cut in the Z-direction (- from top face to back face) when still in the dry oondition. After application of the indicator solution, the hydrogel distribution in the Z-direction can be estimated.

m e assembly of baby diapers, catamenials and the like, using the herein-de æ ribed absorbent structures, employs various liquid perneable topsheet materials, liquid impermeable backsheet materials, leg band~, tape fasteners and the like, all of which are described in the patent literature. See U.S. Patents 3,848,594 (tape fasteners); 3,860,003 (diaper constructioh): 4,081,301 (attachment of leg elastics); 3,929,135, 4,041,951 and 4,342,314 (i~provements in topsheets) for thorough instructions regarding the assembly of diapers, catamenials, and the like.

The manufacture of absorbent mats, especially ~air-felt~, is well-known and involves using an air stream to conduct fibrous material (e.g. cellulose) into a concave template of any desired depth and shape. The depth of the template determines the thickness of the resulting mat. Mbisture or urine-impervious resins may be used to hold the mat in shape and the mat may be compressed to any desired Taber stiffnesS ; see U.S.
Patent No. 3,860,003, cited above.

INDUSI~UL~L APPLICATICN

m e following Example illustrates a preferred diaper msde according to the present invention. m e dimensions listed in the Example are for a diaper intended for use with a child in the 6 to 10 kilogram size range. These dimensions can be mcdified proportionately for diffenent size children, or for adult incontinence briefs, aooording to standard commercial practice. As is well-known in the art, such diapers will comprise: a backsheet (the sheet used outermost from the skin, and which is generally made from urine-inpenetrable plastic material to give the oontainment effect of plastic pants); an absorbent structure that will absorb and help oontain urine and/or fecal matters and, generally, a topsheet ~the sheet closest to the skin) that iB urine-permeable.

While material~ used in the assembly of disposable diapers and the like are well-known, the following may be mentioned, solely by way of exeu~ple.

Backsheet : m e ~acksheet can oomprise a urine-impervious polymer sheet, for example polyethylene or polypropylene, that is thin enough to be flexible. A
polyethylene sheet 0.01-2 ~ thick is typical.

ToPsheet : The diaper topsheet can ccmprise any loosely-woven or nonwoven cloth or scrim,type material that is urine-porous and comfortable to the skin. A
nonwoven sheet cx~l~rising polypropylene fibres is typical.

Elastic Members : m e elastic used to provide leg gathers in the diapers can cowprise elastic bands or threads, or elastic adhesive applied as a band or ribbon.
One or more elastics can be applied longitud~nally along both sides of the diaper, and laid-down either on the topsheet, on the backsheet, or sandwiched between said sheets. In a typical mode, the elastic is pre-stretched, then glued to the diaper using an elastic adhesive, all in well-kn~wn fashion. (see U.S. Patent 4 081 301).

Fasteners : ffl e diapers can be fastened by any .
convenient means, such as pins, snaps and the like.
Typical fasteners comprise adhesive tapes, especially tapes in the ~Y~ configuration described in the patent literature.

Assembly Means : ffl e diapers herein can be assembled by any convenient bond~ng means, such as heat-sealing, ultrasonic sealing, and the llke. Typic~lly, urine-stable ~dhesives are used to assemble disposable ~iapers.

Assembly Method~ and APparatus : The apparatus used to form the diapers herein oonstitutes no part of the present invention. Indeed, such articles may be assembled by hand.

EW~D~LE I

In a oonventional air-laying method, absorbent structures are formed by air-canveying a mass of dry woad pulp fibers ontD a rotating wire mesh drum. A pressure gradient is maintained over the wire mesh to ensure proper dep~sition of the fluff.

m e structures of the present invention can be made by metering hydrogel particles into the wood pulp fiber stream. Metering can be done by means of e.g., a weighing belt or a screw pump.

13049;~

Feedinq of the hydrogel particles onto the laydown - drum can be by gravity or forced air feed.

In either case, a nozzle is used to spread the hydrogel particles over the crotch-width of the absorbent structure.

Since the lay-down drum rotates while wood pulp fibers are being deposited onto it, the position of the hydrogel feed nozzle with respect to the drum and the wood pulp infeed largely determines the distribution of the hydrogel particles in the str w ture.

The face of the str w ture which is in contact with the drum will beoome the back face. Seen from a vantage point from which the drum appears to have a clock-wise rotation,the left hand part of the wood pulp stream reaches the drum first. By pl æ lng the hydrogel in a way as to ensure predomlnant mixlng with this left hand part of the wood pulp ~tream, the desired hydroqel distri~utio~
is obtained.

Absorbent structures were made, comprised of wood pulp fibers (~airfelt~) and particulate crosslinked polyacrylate hydrogel. m e structures had a target weight of 43.58 ~, 14 % of which was hydroqel, and 85.1 % of which was airfelt. The structures were calendered to a thickness of about 6.5 mm.

The hydrogel distribution was determined, using the bromocresol purple indicator method described hereinabove.

Structures made with gravity feed of the hydrogel had the following distribution (Z-direction, going from top face to back face).

Top 1-2 mm : hydrogel/airfelt ratio 0/100 - 10/90 next 4-5 mm : hydrogel/airfelt ratio 20/80 - 50/50 back face 0.5 mm : hydrogel/airfelt ratio O/100 - 10/90 Based on the above estimates, the back face half of the structure was calculated to contain 67 ~ of the total amount of hydrogel material. Having a thin area of low hydrogel concentration imnediately adjacent to the ~ack face, this str w ture is an example of the highly preferred structures herein.

Str w tures made with for oe d air feed had the following hydrogel distribution (Z~direction, from top faoe to back face).

Top 4-5 mm: hydrogel/airfelt ratio O/100-5/95 Back 1.5 - 2.5 mm : hydrogel/airfelt ratio 50/50.

Based on these estimates, the back face half of the structure was calculated to oontain at least 85 % of the total a unt of hydrogel material.

8Oth str w tures, when incorporated in an otherwise conventional diaper, had improved leakage performance and wicking performance as compared to homogenous hydrogel/airfelt blend str w tures containing the same amounts of hydrogel and airfelt.

EX~MPLE II

A diaper is assembled from the following materials 1. Backsheet ; 0~025-0.070 mm polyethylene: width at top and bottom 33 cm; notched inwardly on both sides to a width-at-center of 28.5 cm; length 50.2 cm.

2. Topsheet : spun-bonded polypropylene, basis weight 21.5g/m2-24.5g/m2; width at top and bottom 33 cm;
notched inwardly on both sides to a width-at-center of 28.5 cm; length 50.2 cm.

3. Absorbent structure: prepared in the manner of E~uu~ple I;, Taber range 7 - 9.5; 8.4 mm thick, calendered; wid~h at top and bottom 28.6 cmS notched inwardly at both sides to a width-at-center of 10.2 cm; length 44.5 cm.

4. Elastics : four individual rubber strips (2 per side)5 width 4.77 ~m; length 370 mm; thickness 0.178 mm (all the foregoing dimensions being in the relaxed state.) The diaper of E~mqple II is assembled by overlaying the topsheet on the top ace of said absorbent structure;
overlaying the backsheet over the back face of ~aid absorbent structure~ and gluing the assembly together with urine-stable adhesive.

m e elastic bands ~designated inner~ and ~outer, corresponding to the bands closest to, and farthest from, the core, respectively) are stretched to ca. 50.2 cm and positioned between the topsheet:backsheet along each longitudinal side (2-bands per side) of the core. m e inner bands along each side are positioned ca. 55 mm from the narrowest width of the core (measured from the inner edge of the elastic band~. m e bands are affixed with glue.

The tape functions are affixed to the assembled diaper, which is then ready for use.

Claims

1. An absorbent structure having a top face, a back face, a top face half, and a back face half and comprising a continuous matrix of hydrophilic fibers having dispersed therein in both said top face half and said back face half discrete particles of a water-insoluble hydrogel, characterized in that the hydrogel content of said back face half of the absorbent structure contains at least 1.5 times the hydrogel content of said top face half of the absorbent structure so that at least 60% of the water-insoluble hydrogel material is contained within said back face half.

2. An absorbent structure according to Claim 1 comprising from 3% to 40%, by dry weight of the structure, of water-insoluble hydrogel.

3. An absorbent structure according to Claim 1 or 2 wherein the water-insoluble hydrogel is selected from the group consisting of hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, polyacrylates, copolymers of isobutylene and maleic anhydride, and mixtures thereof.

4. An absorbent structure according to Claim 1 or 2 wherein the hydrogel particles have an average particle size of from 30 micrometer to 4 mm.

5. An absorbent structure according to Claim 1 or 2 wherein the hydrophilic fibers are cellulosic fibers, preferably wood pulp fibers.

6. An absorbent structure according to Claim 1 or 2 wherein the back face half contains at least 70% of the water-insoluble hydrogel material.

7. An absorbent structure according to Claim 1 or 2 comprising from 8% to 20%, by weight of the structure, of water-insoluble hydrogel.

8. An absorbent structure according to Claim 1 having a thin area of low hydrogel concentration immediately adjacent to the back face.

9. An absorbent structure according to Claim 1 wherein the hydrogel content of said back face half of the absorbent structure contains at least 4 times the hydrogel content of said top face half of the absorbent structure so that at least 80% of the water-insoluble hydrogel material is contained within said back face half.

10. An absorbent structure according to Claim 9 wherein said back face half contains at least 85% of the water-insoluble hydrogel material.

11. An absorbent structure according to Claim 10 wherein said top face half contains a hydrogel/airfelt ratio of less than about 5/95.

12. An absorbent structure according to Claim 10 comprising from 3% to 40%, by dry weight of the structure, of water-insoluble hydrogel.

13. An absorbent structure according to Claim 12 wherein the water-insoluble hydrogel is selected from the group consisting of hydrolized acrylonitrile grafted starch, acrylic acid grafted starch, polyacrylates, copolymers of isobutylene and maleic anhydride, and mixtures thereof.

14. An absorbent structure according to Claim 13 wherein the hydrophilic fibers are cellulosic fibers, preferably wood pulp fibers.

15. An absorbent structure according to Claim 14 wherein the hydrogel particles have an average particle size of from 30 micrometer to 4 mm.

16. An absorbent structure according to Claim 15 comprising from 8% to 20%, by weight of the structure, of water-insoluble hydrogel.

17. An absorbent structure according to Claim 16 wherein said hydrogel distribution in any given plane substantially parallel to said top face and said back face is homogenous.

18. An absorbent structure according to Claim 1 having an area of low hydrogel concentration immediately adjacent to said front face.

19. An absorbent structure according to Claim 18 wherein the weight ratio of water insoluble hydrogel to hydrophillic fibers if from about 0:100 to about 10:90.

20. An absorbent structure according to Claim 19 wherein said area of low hydrogel concentration is followed by a gradually increasing hydrogel concentration.

21. An absorbent structure according to Claim 18 additionally comprising an area of low hydrogel concentration immediately adjacent to said back face.

22. An absorbent article according to Claim 21 wherein the hydrogel/airfelt ratio of both areas of low hydrogel concentration are between 0/100 and about 10/90.

23. An absorbent article according to Claim 22 wherein said area of low hydrogel concentration adjacent said top face is followed by a gradually increasing hydrogel concentration.

24. An absorbent structure according to Claim 23 wherein said back face half contains 67% of the water-insoluble hydrogel material.

25. An absorbent article according to Claim 22 comprising from 3% to 40%, by dry weight of the structure, of water-insoluble hydrogel.

26. An absorbent structure according to Claim 25 wherein the water-insoluble hydrogel is selected from the group consisting of hydrolized acrylonitrile grafted starch, acrylic acid grafted starch, polyacrylates, copolymers of isobutylene and maleic anhydride, and mixtures thereof.

27. An absorbent structure according to Claim 26 wherein the hydrophilic fibers are cellulosic fibers, preferably wood pulp fibers.

28. An absorbent structure according to Claim 27 wherein the hydrogel particles have an average particle size of from 30 micrometer to 4 mm.

29. An absorbent article according to Claim 28 comprising from 8% to 20%, by weight of the structure, of water-insoluble hydrogel material.

30. An absorbent structure according to Claim 1 wherein the hydrogel distribution in any given plane substantially parallel to said top face and said back is homogenous.

31. An absorbent structure according to claim 2 having a thin area of low hydrogel concentration immediately adjacent to the back face.

32. A disposable diaper or catamenial product comprising:
(a) a liquid impervious backing sheet;
(b) a hydrophobic, liquid pervious top sheet; and (c) an absorbent structure according to Claims 1, 8, 10, 17, 18, 21, 24, 30 or 31 wherein the backing sheet overlays the back face of the absorbent structure and the top sheet overlays the top face of the absorbent structure.

33. An absorbent structure having a top face and a back face and comprising a continuous matrix of hydrophilic fibers having dispersed therein discrete particles of a water-insoluble hydrogel, characterized in that the hydrogel concentration gradually increases from said top face to said back face so that the distribution of hydrogel particles is skewed towards said back face of the absorbent structure.

34. The absorbent structure according to claim 33 wherein the hydrogel distribution in any given plane substantially parallel to said top face and said back face is homogenous.

35. The absorbent structure according to Claim 34 additionally comprising an area of low hydrogel concentration adjacent to said front face and an area of low hydrogel concentration adjacent to said back face.

36. An absorbent structure according to Claim 35 wherein said areas of low hydrogel concentration have a hydrogel concentration less than about 10%.

37. An absorbent structure according to Claim 35 wherein said areas of low hydrogel concentration have a hydrogel concentration less than about 5%.

38. An absorbent structure according to Claim 37 wherein the hydrogel particles have an average particle size of from 30 micrometer to 4 mm.