CN1478164A - Cellulose fibers having low water retention value and low capillary desorption pressure - Google Patents

Abstract

The present invention provides cellulose fibers having low median desorption pressures and low water retention values (WRV), which exhibit improved drainage and fluid flow properties. These fibers are particularly well suited for use in acquisition, distribution, and acquisition-distribution layers, or in absorbent core structures. One embodiment of the invention is a method for preparing cellulose fibers by refining cellulose fibers to a freeness ranging from about 300 to about 700 ml CSF and crosslinking the refined fibers. Another embodiment of the invention is fibers crosslinked with at least one saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl discarboxylic acid, bifunctional monocarboxylic acid, or amine carboxylic acid. A crosslinking facilitator, such as oxalic acid, may be present during the crosslinking reaction to improve the efficacy of the crosslinking agent. Yet another embodiment of the invention is an absorbent core comprising SAP particles and reversible crosslinked fibers.

Description

Cellulose fibers having low water retention and low capillary desorption pressures

The rights in U.S. provisional application No.60/247,078, filed on 10.11.2000 and U.S. provisional application No.60/286,298, filed on 25.4.2001, both of which are incorporated herein by reference.

Technical Field

The present invention relates to cellulosic fibers having low water retention values and low median desorption pressures (as measured in a capillary absorption-desorption cycle), methods of making these fibers, and absorbent structures containing these fibers.

Background

Absorbent structures are important in a variety of disposable absorbent articles, including infant diapers, adult incontinence products, feminine sanitary napkins and other feminine hygiene products, and the like. Absorbent cores are commonly provided with these and other absorbent articles to contain and retain bodily fluids. In one conventional absorbent structure, the absorbent core is positioned between a liquid-permeable topsheet, which functions to allow liquid to pass through to the core, and a liquid-impermeable backsheet, which functions to contain the fluid and prevent it from passing through the absorbent article to the garment of the wearer of the absorbent article.

Absorbent cores for diapers, adult incontinence pads and feminine hygiene products generally comprise a fibrous mat or web of defibered, loose, soft, hydrophilic cellulosic fibers. Such fiber mats form a substrate that can absorb and retain certain liquids. However, their ability to do so is limited. Therefore, superabsorbent polymer (SAP) particles, granules, flakes or fibers (agglomerated particles) capable of absorbing many times their own weight of liquid are often included in absorbent cores to increase the absorbent capacity of the core, but not necessarily to significantly increase the volume of the core. In an absorbent core comprising matrix fibers and SAP particles, the fibers physically separate the SAP particles, provide structural integrity to the absorbent core, and provide channels for fluid flow through the core.

Absorbent cores containing SAP-particles have been successful and in recent years there has been an increasing market demand for thinner, more absorbent and more comfortable absorbent articles. As the absorbent capacity of the core material increases, the ability of the core material to rapidly draw fluid from the topsheet of the absorbent article has become important in maintaining a dry environment between the skin of the wearer of the absorbent article and the topsheet of the article.

The ability of an absorbent core to draw fluid from the layer located immediately above it in an absorbent structure is controlled by the following factors: gravity, the number, size and spatial orientation of the unoccupied volume (voids or pores) in the absorbent core and core component characteristics that affect fluid flow, such as wettability of the component by the received fluid as indicated by the contact angle, surface tension of the received fluid and viscosity of the received fluid. In the absorbent structure, an acquisition layer, a distribution layer, or an acquisition and distribution layer may be included between the topsheet and the absorbent core to facilitate the introduction of fluids into the absorbent core.

For optimal performance of the absorbent structure in terms of fluid capacity and core utilisation, it is critical that the fluid received by the absorbent core rapidly migrates from the wet zone to the dry zone of the core. Permeability can be used to describe the ability of an absorbent core to transfer fluid from the wet zone of the core to the dry zone of the core. The permeability of an absorbent core is defined as the ability of liquid to flow through the absorbent core.

The ability of a first substrate (e.g., an absorbent core) to draw fluid from a second substrate (e.g., an acquisition and distribution layer) primarily through capillary forces is referred to as the distribution characteristics of the substrates.

It is known to those skilled in the art that absorbent structures comprising an absorbent core having good fluid distribution properties also exhibit poor fluid permeability. Also, absorbent structures having good fluid permeability exhibit poor fluid distribution characteristics. It is therefore important that the fibers used in the acquisition layer have a higher stiffness or resilience (as measured by dry compression) than conventional fibers used in the absorbent core under the weight of the diaper wearer. This resilience allows the interfiber voids or pores in the acquisition layer to be maintained when the diaper is worn, so that fluid can be rapidly absorbed into the absorbent structure of the diaper through the liquid permeable topsheet.

It is also important that the core fibers, when wet, are not so dense as to restrict fluid flow into and through the absorbent core. In addition, the core fibers must have sufficient physical integrity to maintain separation of the wet SAP particles in the absorbent core, thereby minimizing or eliminating gel blocking as the particles swell.

One way to increase the stiffness and resilience of the fibers is to crosslink them. Cellulose fibers can be stiffened by intrafiber cross-linking (i.e., cross-linking between two different portions of the same fiber), and to a lesser extent by interfiber cross-linking (i.e., cross-linking between two different fibers).

Use is disclosed in U.S. Pat. No.5,190,563 to Herron et alCertain aliphatic and cycloaliphatic C₂～C₉Intramolecular cross-linking of the polycarboxylic acid. "C" as defined by Herron et al₂～C₉A polycarboxylic acid "is an organic acid comprising two or more carboxyl groups and 2 to 9 carbon atoms in the chain or ring connecting the carboxyl groups. Suitable C₂～C₉The polycarboxylic acids contain at least three carboxyl groups or two carboxyl groups, and a carbon-carbon double bond is present in the alpha and beta positions of one or two carboxyl groups. When two carboxyl groups are separated by a carbon-carbon double bond or both are attached to the same ring, the two carboxyl groups must be in the cis configuration. Examples of such polycarboxylic acids include: citric acid, 1, 2, 3-propanetricarboxylic acid, 1, 2,3, 4-butanetetracarboxylic acid (BTCA) and oxydisuccinic acid. Herron et al also found that cellulose fibers cross-linked with an aliphatic alkane containing 4 carboxyl groups, BTCA, had lower water retention values than those containing 3 carboxyl groups, citric acid and 1, 2, 3-alkanetricarboxylic acid. Generally, fibers with lower water retention values are stiffer than those with higher water retention values.

Unlike cellulosic fibers having intrafiber crosslink points, cellulosic fibers having interfiber crosslink points (e.g., those found in most papers) are stiff when dry, but do not necessarily retain their stiffness when wet. The interfiber crosslinking of paper with citric acid and 1, 2,3, 4-butanetetracarboxylic acid and of fabrics with maleic acid, citric acid and 1, 2,3, 4-butanetetracarboxylic acid is disclosed in the following documents: caulfield, TAPPI j., 77 (3): 205-212 (1994); hcrie & c.j. biermann, TAPPI j., 77 (8): 135-140 (1994); y.j.zhou, p.luner & p.caluwe, journal of applied polymer science (j.appl.polymer Sci.), 58: 1523 to 1534 (1995); and d.d.gagliardi and f.b.shippe, U.S. dye reports (am.dyeeff Reptr.), 52: 300(1963).

Zhou et al (supra) investigated the wet strength of paper crosslinked (interfiber) with certain polycarboxylic acids. Generally, interfiber crosslinking increases the wet strength of paper fibers. Zhou et al found that the wet strength of the paper increased with increasing polycarboxylic acid functionality (i.e., the number of carboxylic groups in the polycarboxylic acid). For example, 1, 2,3, 4-butanetetracarboxylic acid (BTCA) (4 carboxyl groups) was found to be more effective than tricarballylic acid (TCA) (3 carboxyl groups), which in turn was found to be significantly more effective than butanedioic acid (2 carboxyl groups). Paper treated with succinic acid showed little wet strength.

H.j.campbell and t.francis, journal of Textile research (Textile res.j.), 35: 260(1965) cotton cellulose is crosslinked with a specific polycarboxylic acid. The reaction is catalyzed by trifluoroacetic anhydride (TFAA) and requires the use of a non-aqueous solvent (benzene in this case) to prevent hydrolysis of TFAA. Campbell and Francis reported that succinic and glutaric acids show only slight activity on cotton cellulose. In addition, they reported that esterification (or crosslinking) did not occur with oxalic acid. It was found that malonic acid reacts readily with cotton cellulose, producing a fabric with a degree of yellowing that depends on the degree of reaction.

Typically, the crosslinked cellulosic fibers are produced at a location remote from where they are incorporated into the absorbent structure. Since the crosslinked fibers are bulky and the fiber-to-fiber contact is small, they do not bond well to each other. Thus, the sheet formed from the crosslinked fibers is easily unraveled. Therefore, the crosslinked cellulose fibers are typically shipped in bales. This increases the cost of shipping the crosslinked fibers and the cost of producing the absorbent structure. Therefore, it is desirable to prepare a sheet of cellulose fibers containing a crosslinking agent.

International publication No. WO 00/65146 discloses "cross-linkable" cellulose fibers that are processed into rolls or sheets. The crosslinkable product is produced by: the cross-linking agent is applied to the mat of cellulose fibers and the treated mat is dried (but not heated to a temperature sufficient to cure the cross-linking agent) so that substantially no cross-linking occurs and the product is substantially free of cross-linking sites.

Us patent No.6,059,924 discloses a method of enhancing the dry compression and capillary properties of fluffed pulp. The method includes suitably refining the chemical pulp prior to forming the fluffed pulp sheet.

There is a continuing need for improved cellulose fibers: they have low water retention values and low median desorption pressures and can be incorporated into acquisition, distribution and acquisition-distribution layers. There is also a need for a core or matrix fiber that facilitates fluid flow into and through the absorbent core and maintains sufficient physical integrity while minimizing or eliminating gel blocking of the swollen SAP particles. Finally, there is a need for a process for preparing a sheet of crosslinkable cellulosic fibers.

Summary of The Invention

The present invention provides cellulosic fibers having low median desorption pressure (as measured in a capillary absorption-desorption cycle) and low Water Retention Value (WRV), which exhibit improved drainage and fluid flow properties. These fibers are particularly useful in acquisition, distribution and acquisition-distribution layers, and in absorbent core structures.

According to one embodiment, the fibers of the present invention are crosslinked and have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 15cm or less. Preferably, the cellulose fibers also have a WRV of 45% or less. These fibers can be prepared by: the cellulosic fibers are refined to a freeness in the range of from about 300 to about 700ml Canadian Standard Freeness (CSF), and the refined fibers are crosslinked. According to a preferred embodiment, the fibres are cross-linked with citric acid after refining.

Another embodiment of the invention is a fiber crosslinked with at least one saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl dicarboxylic acid, difunctional monocarboxylic acid, or amine carboxylic acid. During the crosslinking reaction, a crosslinking promoter such as oxalic acid may be present to improve the efficiency of the crosslinking agent. According to a preferred embodiment, the cellulose fibres are refined before crosslinking in order to further harden them.

Another embodiment is a method of making a crosslinkable cellulosic fiber comprising the steps of: (a) crosslinking cellulosic fibers with at least one crosslinking agent selected from the group consisting of saturated dicarboxylic acids, aromatic dicarboxylic acids, cycloalkyl dicarboxylic acids, difunctional monocarboxylic acids, and amine carboxylic acids, and (b) uncrosslinking the crosslinked cellulosic fibers. Preferably, the crosslinking agent in this embodiment contains 4 or fewer carbon atoms. Two preferred crosslinking agents are oxalic acid and sodium chloroacetate. The crosslinkable fibers can be processed into sheets to facilitate their transportation. In addition, the crosslinkable fibers can be re-crosslinked by curing the uncrosslinked cellulosic fibers.

Yet another embodiment of the present invention is an acquisition layer, a distribution layer, or an acquisition and distribution layer comprising the cellulose fibers of the present invention.

Yet another embodiment is an absorbent core comprising the cellulosic fibers of the present invention. The absorbent core exhibits improved fluid flow properties into and through the core. According to a preferred embodiment, the absorbent core comprises SAP-particles and reversibly crosslinked fibres. The reversibly crosslinked fibers separate the SAP particles and provide channels for fluid to flow from the wet to the dry regions of the absorbent core bypassing the SAP particles. In addition, reversibly crosslinked fibers facilitate the absorption of large amounts of urine (or other fluids) in a short period of time (e.g., one gush). Once urine or other fluid enters the absorbent core, the crosslinked fibers begin to decrosslink. Uncrosslinked fibers hold and retain urine or other fluids to a greater extent than fibers that are permanently crosslinked. As a result, the absorbent core has improved initial surge capacity over absorbent cores containing conventional loose fibers and improved rewet characteristics over absorbent cores containing permanently crosslinked fibers.

Yet another embodiment is an absorbent structure comprising an acquisition layer, a distribution layer or an acquisition and distribution layer of the present invention and/or an absorbent core of the present invention. Preferably, the absorbent structure comprises: a top layer (acquisition layer, distribution layer, or acquisition and distribution layer), and a bottom layer (storage layer) in fluid communication with the top layer. The absorbent structure exhibits superior distribution properties from the acquisition layer and/or the distribution layer to the storage layer compared to conventional absorbent structures.

Yet another embodiment is an absorbent article comprising the absorbent structure of the present invention.Detailed Description Definition of

The term "capillary absorption-desorption cycle" (also known as capillary adsorption cycle or CSC)) Refers to a method for determining the relationship between the pore volume of an absorbent structure and capillary pressure during absorption of liquid into the absorbent structure and subsequent drainage of liquid from the absorbent structure. The capillary absorption-desorption cycle dictates the ability of the absorbent structure to attract, retain, and distribute fluid within the pores between the fibers of the absorbent structure. The absorbent structure may undergo such cycling by systematically lowering or raising the capillary pressure within the narrow gap, as determined, for example, by the methods described in the examples of the present application and in the following documents: "capillary sorption equilibrium in fibrous material", a.a. burgeni and c.kapur, journal of textile research, 37: 356 to 366 (1967); and a P.K.Chatterjee,absorbency ofTextile Science and Technology 7(Textile Science and Technology 7), Chapter II, pp.63-65, Elsevier Science Publishers (1985), which are incorporated herein by reference.

The "median desorption pressure" measured during the capillary-desorption cycle represents the ability of the water-swollen cellulose fibers to release water. For example, cellulose fiber samples that retain water strongly exhibit a much higher median desorption pressure than swollen cellulose fiber samples that release water easily. The median desorption pressures discussed herein were determined by the methods described in the examples of the present application and in the following documents: "capillary absorption balance in fibrous material", a.a. burgeni and c.kapur, journal of textile research, 37: 356 to 366 (1967); and a P.K.Chatterjee,absorbency ofTextile science and technology 7, Chapter II, pp.63-65, Elsevier sciences publishers (1985), which are incorporated herein by reference. This test method measures the ability of water-swollen cellulose fibers to retain water under hydrostatic pressure.

The "water retention value" (WRV) of cellulose fibers can be determined by the methods described in the following documents: TAPPI Useful Methods, UM 256, and p.k. chatterjee,absorbency ofTextileScience and Technology Z, Chapter II, pp.62-63, Elsevier sciences publishers (1985), which are incorporated herein by reference. This test measures the weight of water retained by a sample of water-saturated cellulose fibers after centrifugation and is based on the fiberWeight percent of dry weight expresses this amount. The WRV of a cellulose fiber is related to its water-draining ability.

Any "cellulosic fiber" known in the art, including any naturally derived cellulosic fiber (e.g., those derived from wood pulp), can be used as the feedstock in the process of the present invention. Preferred cellulosic fibers include, but are not limited to, cooked fibers, such as kraft pulp fibers derived from softwood, hardwood, or linters, prehydrolyzed kraft pulp fibers, soda pulp fibers, sulfite pulp fibers, chemi-thermomechanically treated fibers, and thermomechanically treated fibers. More preferred cellulosic fibers include, but are not limited to, kraft-cooked fibers, including pre-hydrolyzed phosphate-cooked fibers.

Generally, cellulose fibers having thicker walls are preferred because they are coarser and stiffer than similar fibers having thinner walls. The fiber walls of the fibers are defined by the lumen of the fibers (i.e., the hollow interior of the fibers) and the outer surface of the fibers. For example, fibers derived from southern softwood are preferred because the fiber walls of southern softwood are on average thicker than those of northern softwood. More preferably, the cellulosic fibers are derived from softwood, such as pine, fir, and spruce.

Other suitable cellulosic fibers include those derived from Esparto grass (Esparto grass), bagasse, ramie, flax, and other sources of wood and cellulosic fibers. The cellulosic fibers may be provided in the form of a slurry, an unpressed sheet, or a sheeted sheet.

The optimum fiber source for use in connection with the present invention will depend on the particular end use contemplated. Generally, pulp fibers prepared by chemical pulping methods are preferred. Fully bleached, partially bleached and unbleached fibers may be used. It may often be desirable to utilize bleached pulp because of its excellent whiteness and consumer appeal. In the case of products such as paper towels and absorbent pads for diapers, sanitary napkins, and other similar absorbent paper articles, the use of cellulose fibers derived from southern softwood pulp is particularly preferred because of their superior absorbent properties.

More preferred cellulosic fibers include, but are not limited to, those under the trade name Foley Fluff^TMCommercially available bleached sulfate southern pine fibers available from Buckeye technologies inc.

The cellulosic fibers can have any fiber length. In general, longer fibers produce crosslinked cellulosic fibers having lower desorption pressure and water retention values than those produced from shorter fibers.

Refined and crosslinked fibers

The present inventors have surprisingly and unexpectedly found that when cellulosic fibers are refined and crosslinked, the resulting fibers have a low median desorption pressure (as measured in the capillary absorption-desorption cycle) and low Water Retention Values (WRVs). Further, these fibers exhibit improved drainage in the acquisition layer and/or distribution layer as compared to similar unrefined fibers.

The cellulose fibers are crosslinked and have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 15cm or less. Without being bound by any theory, the inventors believe that this property is a result of intrafiber crosslinking within the cellulose fibers. More desirably, the cellulosic fibers of the present invention have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 14cm or less; it is further desirable that the fibers of the present invention have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 13cm or less; more desirably, the fibers of the present invention have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 12cm or less.

Refined and crosslinked cellulosic fibers typically have a WRV of 45% or less; more desirably 38% or less; more desirably 30% or less.

The above cellulose fibers can be prepared by: refining the cellulosic fibers to a freeness in the range of about 300 to about 700ml CSF, and then crosslinking the refined fibers. According to a preferred embodiment, the starting cellulosic fibers being refined are wet lap. According to another preferred embodiment, the cellulose fibres are bleached and/or dispersed before refining. The refined fibers may be crosslinked by any method known in the art, for example, by reacting the fibers with a crosslinking agent.

Fibers with improved median desorption pressure and water retention values can be prepared by: the fibers are first refined and then crosslinked with any of a variety of crosslinking agents. Suitable crosslinking agents include, but are not limited to, those described below as well as other polycarboxylic acids, such as aliphatic and cycloaliphatic C₂～C₉A polycarboxylic acid. The term "C" as used herein₂～C₉The polycarboxylic acid "means an organic acid containing two or more carboxyl groups (COOH) and 2 to 9 carbon atoms located on a chain or ring connecting the carboxyl groups. Carboxyl groups are not included in determining the number of carbon atoms in the chain or ring. For example, 1, 2, 3-propanetricarboxylic acid should be regarded as C containing three carboxyl groups₃A polycarboxylic acid. Likewise, 1, 2,3, 4-butanetetracarboxylic acid should be regarded as C containing four carboxyl groups₄A polycarboxylic acid.

C suitable for use as a cellulose crosslinker in the present invention₂～C₉The polycarboxylic acids preferably comprise ethylenically saturated or unsaturated aliphatic and cycloaliphatic acids containing at least three, preferably a plurality of carboxyl groups per molecule, if carbon-carbon double bonds are present in the alpha, beta positions of one or both carboxyl groups. In addition, in order to be reactive in esterifying the hydroxyl groups of cellulose, a given carboxyl group in the aliphatic or alicyclic polycarboxylic acid is preferably separated from the second carboxyl group by not less than two carbon atoms and not more than three carbon atoms. Without being bound by theory, it appears that in order for a carboxyl group to be active, it must be capable of forming a cyclic 5 or 6 membered anhydride ring with the adjacent carboxyl group in the polycarboxylic acid molecule. In the case where two carboxyl groups are separated by a carbon-carbon double bond or where both carboxyl groups are attached to the same ring, the two carboxyl groups must be in the cis configuration with respect to each other if they are to interact in this manner.

Novel crosslinked fibers

Another embodiment of the present invention is a cellulosic fiber crosslinked with at least one saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl dicarboxylic acid, difunctional monocarboxylic acid, or amine carboxylic acid that exhibits a low median desorption pressure (measured in a capillary absorption-desorption cycle) and a low water retention value. These crosslinked fibers exhibit improved liquid drainage in the acquisition layer and/or the distribution layer and improved permeability in the absorbent core.

Typically, these crosslinked cellulosic fibers have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 25cm or less. Without being bound by any theory, it is believed that this property is a result of intrafiber crosslinking within the cellulose fibers.

More desirably, the crosslinked cellulosic fibers have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 20cm or less; more desirably, the fibers have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 18cm or less; it is further desirable that the fibers have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 15cm or less; more desirably, the fibers have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 14cm or less; it is still further desirable that the fibers have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 13cm or less; yet still further desirably, the fibers have a median desorption pressure (measured in a capillary absorption-desorption cycle) of 12cm or less.

These crosslinked cellulosic fibers typically have a WRV of 50% or less; more desirably 45% or less; further desirably 38% or less; and more desirably 30% or less.

The crosslinked cellulosic fibers typically have a saturation capacity of at least 10 grams of saline per gram of sample (g/g), as exemplified herein; burgeri et al, supra; and Chatterjee et al, determined by the methods described previously. According to a preferred embodiment, the crosslinked cellulosic fibers have a saturation capacity of at least 11, 12, 13, 14 or 15 g/g.

The crosslinked cellulosic fibers of the present invention are prepared by crosslinking cellulosic fibers with one or more crosslinking agents of the present invention. The median desorption pressure and water retention value of the crosslinked fibers can be reduced by refining the fibers prior to crosslinking. In addition, the crosslinking reaction may be carried out in the presence of one or more crosslinking promoters of the present invention to improve the efficiency of the crosslinking agent.Refining

Refining may be carried out by any method known in the art, including mechanical refining. Pulp refining involves work on the fibers, usually in an aqueous slurry, but not exclusively. For example, they may be refined by cutting the fibers, thereby reducing the average fiber length. Alternatively, the fibers may be refined by rubbing the fibers against each other and against irregular surfaces under force or pressure. This causes the external fiber surface area to increase due to scratching and abrasion of the surface. In addition, work input to the fibers during refining causes delamination of the inner and surface of the fibers. The result is a thinning of the fiber wall, allowing the fiber to absorb more water and swell to a greater extent than unrefined fibers. The never-dried refined fiber is also softer than a similar never-dried fiber. Further, when compacted into a sheet, refined fibers produce greater strength and stiffness in the sheet after drying than do dried unrefined fiber sheets.

At J.d' A. Clark,pulping Technology and treatment of paper (Pulp Technology and) Treatment for Paper) Methods of refining cellulosic fibers, including but not limited to pulping and fibrillation, are described in 2 nd edition, Chapter 8, Chapter 160-183, Chapter 12, Chapter 277-305, Chapter 13, Chapter 306-355, Chapter 14, Chapter 356-407, Miller Freeman pub., San Francisco (1985). One preferred method of refining cellulose fibers is fibrillation. The cellulosic fibers can be refined using, for example, a disc refiner or a Valley beater available from Valley Mill Corporation of Lee, MA. The purification is usually carried out at normal temperature and pressure. For example, the cellulose fibers may be refined by flowing an aqueous slurry of cellulose fibers through a Valley beater for 15 minutes until the desired freeness is obtained.

Typically, the fibers are wetted or wetted prior to refining the fibers. According to a preferred embodiment, the cellulose fibres are bleached before refining.

The cellulose fibers are generally refined to a freeness of from about 300 to about 700ml CSF, preferably to a freeness of from about 500 to about 700ml CSF. According to a preferred embodiment, the cellulose fibers are refined to a freeness of from about 650 to about 700ml CSF. The freeness of the cellulose fibers discussed herein is determined by the TAPPI T-227 method.Cross-linking

Refined or unrefined cellulose fibers are stiffened by intra-fiber covalent crosslinking. Preferably, the cellulosic fibers are wetted or wetted prior to reacting with the crosslinking agent and crosslinking facilitator. Desirably, in certain embodiments, the cellulosic fibers are never-dried cellulosic fibers.

The fibers are crosslinked by reacting them with the crosslinking agents of the present invention and optionally crosslinking promoters such as those described below. Preferably, the fibers are crosslinked with a high degree of twist. Typically, this reaction step is carried out under essentially unrestricted conditions, i.e., each fiber is free to move without interaction with adjacent fibers and without being under any substantial tension or pressure. The fibers may be reacted with the crosslinking agent and optional crosslinking promoter by curing the fibers in the presence of the crosslinking agent and optional crosslinking promoter.

Typically, the fibers are crosslinked by: (i) mixing them with the crosslinking agent of the present invention and optionally a crosslinking promoter, and (ii) curing the fibers under conditions sufficient to cause intrafiber crosslinking. An effective amount of a crosslinking agent and optionally a crosslinking facilitator to cause intrafiber crosslink bond formation is typically admixed with the cellulosic fibers. Preferably, an effective amount of a crosslinking facilitator is admixed with the cellulosic fibers to increase the number and rate of formation of intrafiber crosslinking bonds formed by reaction of the fibers with the crosslinking agent. Generally, from about 0.5 to about 40 mole percent, preferably from about 1 to about 30 mole percent, of the crosslinking agent and crosslinking facilitator, calculated on a molar basis of the cellulose glucoside, are mixed with the fibers. When the crosslinking agent is a dicarboxylic acid crosslinking agent, typically from about 5 to about 21 mole percent of the crosslinking agent, based on the moles of cellulose glucoside, is mixed with the fiber. Typically, from about 1.8 to about 9 mole percent of a crosslinking promoter, calculated on a cellulose glucoside mole basis, is mixed with the fibers. The mixture comprising fibers and crosslinking agent preferably contains from about 5 to about 10 weight percent crosslinking agent based on dry weight of fibers.

After the cross-linking agent is mixed with the fibers, the fibers are preferably separated and individualized, for example by defibering or fragmenting and defibering. By separating the fibers, intrafiber cross-linking is maximized and interfiber cross-linking is minimized. Preferably, the fibers are crosslinked by the formation of covalent bonds within the fibers.

Cellulose fibers provided in the form of wet rolls, dry rolls, or other compressed sheets can be separated by mechanically breaking them into a loose form. In the case of dry lap, it is advantageous to wet the fibers prior to mechanical disintegration, for example, to 40% moisture (60% solids, based on the total weight of fiber and water) in order to plasticize the fibers and minimize damage to the fibers.

If the crosslinking agent is applied to the fibers in an aqueous solution, the fibers are dried prior to curing. The fibers are preferably dried to remove all of the water from the fibers and then allowed to solidify to produce intrafiber crosslinks. Drying may be carried out by any method known in the art. Typically, drying is carried out by heating the fibers at a temperature of from about 50 to about 225 ℃. Preferably, the drying is carried out at a temperature of about 105 to about 175 ℃. The fiber is typically dried to a constant weight. Regardless of the temperature at which the fibers are dried during the drying process, the temperature of the fibers generally does not exceed 100 ℃ (the boiling point of water) until all of the water has evaporated from the fibers. Such as the t.lindstrom,paper Structure and Properties (Paper Structure and) Properties) Drying of cellulose fibers, as discussed in International Fiber and technology Series 8, Chapter 5, pp 104-105, Marcel Dekker Inc., New York (1986), generally results in an irreversible decrease in the swelling capacity of the fibers upon rewetting. This phenomenon is often referred to as keratinization.Without being bound by any theory, it is believed that the microfibers within the fiber walls are bonded together during the drying process, thereby reducing the size of the pores in the fiber walls. This results in a stiffened fiber (compared to the fiber before drying). The subsequent curing step promotes the formation of intra-fiber covalent bonds, which locks the dry fiber stiffness and geometry.

Curing is typically carried out at a temperature sufficient to cause the formation of covalent bonds within the fibers. Curing is generally carried out at a temperature of from about 105 to about 225 ℃. Preferably, the cellulosic fibers are cured at a temperature of from about 150 to about 190 ℃. More preferably, they are cured at a temperature of from about 160 to about 175 ℃. Curing may be carried out for 15, 30, 45 or 60 minutes or more.

According to a preferred embodiment, the fibers are crosslinked by: (i) contacting an aqueous solution of a crosslinking agent and optionally a crosslinking facilitator with an aqueous mixture comprising cellulosic fibres, (ii) removing water from the aqueous mixture, (iii) mechanically separating the fibres into substantially individual form, (iv) drying the fibres, and (v) reacting the fibres with the crosslinking agent to cause crosslinking in the fibres. Typically, step (ii) involves removing a substantial portion of the water from the aqueous mixture. Preferably, sufficient water is removed from the aqueous mixture to obtain a mixture having from about 40 to about 80 weight percent solids (based on 100 percent total weight of fiber and water). According to a more preferred embodiment, step (ii) involves removing water from the aqueous mixture to obtain a mixture having about 60 wt% solids (based on 100% total weight of fiber and water). The water removal, separation and drying steps cause the fibers to become highly distorted. The distorted state is usually at least partially, but not completely, permanently fixed by a crosslinking reaction.

For example, the fibers can be crosslinked by the methods described in U.S. Pat. No.5,190,563, which is incorporated herein by reference, replacing the crosslinking agent in U.S. Pat. No.5,190,563 with the crosslinking agent and crosslinking facilitator of the present invention. In U.S. Pat. No.5,190,563, cellulose fibers are blended with a blend containing C₂～C₉A solution of a polycarboxylic acid crosslinking agent. The fibers are then mechanically separated into substantially individual forms and driedAnd reacting with a crosslinking agent while remaining substantially individual in form so as to form intrafiber crosslinking bonds. Individualized cellulosic fibers are contacted with a crosslinking agent in an amount sufficient to cause the fibers to form intrafiber crosslink bonds. Preferably, from about 0.5 mol% to about 6.0 mol% of the crosslinking agent, calculated on a cellulose glucoside mole basis, is contacted with the fibers.

When the crosslinking agent contains an amino group or an amine group which participates in the reaction, it is preferable to activate the crosslinking agent before or at the same time as the crosslinking reaction. The term "activation" as used herein means that the crosslinking agent is modified so that the nitrogen atom of the amino or amine group is in a more reactive state, i.e., more reactive. The cross-linking agent may be activated by any method known in the art. For example, a crosslinking agent containing an amine or amino group may be reacted with nitrous acid to activate the nitrogen atom of the amine or amino group.

The fibers may be crosslinked in the presence of a reducing agent (antioxidant) to prevent the fibers from yellowing during the crosslinking reaction. Suitable reducing agents include, but are not limited to, hypophosphites, such as sodium hypophosphite; sodium bisulfite; sodium phosphite; and any combination of any of the foregoing. A preferred reducing agent is sodium hypophosphite.

The fibers may be bleached during or after the crosslinking reaction to improve their appearance. For example, fibers can be bleached by reacting them with a bleaching agent. Any bleaching agent known in the art may be utilized. Suitable bleaching agents include, but are not limited to, hydrogen peroxide.

For example, the bleaching agent may be included in an aqueous solution containing a cross-linking agent applied to the fibers. Preferably, the aqueous solution contains sufficient bleach such that the mixture resulting from the addition of the aqueous solution to the fibers comprises from about 2.5 wt% to about 5 wt% bleach based on the dry weight of the fibers.Saturated dicarboxylic acid crosslinking agent

The term "saturated dicarboxylic acid" denotes dicarboxylic acids which do not contain any carbon-carbon double or triple bonds. The saturated dicarboxylic acids may comprise linear or branched aliphatic chains, i.e. they are acyclic. Preferred saturated dicarboxylic acids include, but are not limited to, C₂～C₈A saturated dicarboxylic acid. The term "C₂～C₈The saturated dicarboxylic acid "refers to a dicarboxylic acid in which the total number of carbon atoms (including those in the carboxyl group) is 2 to 8. C₂～C₈Non-limiting examples of saturated dicarboxylic acids are: oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, and suberic acid. Mention may be made in particular of C₂～C₆Saturated dicarboxylic acids and C₂～C₄A saturated dicarboxylic acid.

According to a preferred embodiment, C₃And higher saturated dicarboxylic acids such as C₃～C₈The saturated dicarboxylic acid is applied to the cellulose fibres in combination with a crosslinking promoter, such as oxalic acid.

Another class of saturated dicarboxylic acids is the saturated hydroxydicarboxylic acids. The term "saturated hydroxy dicarboxylic acid" denotes a saturated dicarboxylic acid comprising at least one hydroxy substituent. Suitable saturated hydroxy dicarboxylic acids include, but are not limited to, C₂～C₈Hydroxy saturated dicarboxylic acids (i.e., those containing 2 to 8 carbon atoms). Of particular interest is C₂～C₈Polyhydroxy saturated dicarboxylic acids. C₂～C₈Non-limiting examples of hydroxy saturated dicarboxylic acids are: tartaric acid, malic acid, glucaric acid, and mucic acid.Aromatic dicarboxylic acid crosslinking agents

The term "aromatic dicarboxylic acid" denotes an aromatic compound of the formula HOOC-R-COOH, wherein R is a substituted or unsubstituted phenyl group. The term "substituted" as used herein includes, but is not limited to, at least one of the following substituents: hydroxy, C₁～C₄Alkoxy radical, C₁～C₄Alkyl groups, amino groups, halogen atoms and nitro groups.

Preferred aromatic dicarboxylic acids have the formula:wherein R is¹、R²、R³And R⁴Independently of one another is hydrogen, hydroxy, C₁～C₄Alkoxy radical, C₁～C₄Alkyl, amino, halogen atom or nitro.The preferred aromatic dicarboxylic acid is phthalic acid.Cycloalkanedicarboxylic acid crosslinking agents

The term "cycloalkyl dicarboxylic acid" denotes cycloalkyl dicarboxylic acids which do not contain a carbon-carbon double bond in the alpha or beta position of the carboxyl group. According to one embodiment, the cycloalkyl dicarboxylic acid has the formula:wherein,

R⁶、R⁷、R¹⁰and R¹¹Independently of one another is hydrogen, hydroxy, halogen, C₁～C₄Alkoxy radical, C₁～C₄Alkyl, amino, or nitro; and

R⁸and R⁹Independently of one another is hydrogen, a halogen atom, C₁～C₄Alkoxy or C₁～C₄An alkyl group.

The preferred cycloalkyl dicarboxylic acid is 1, 2, 5, 6-tetrahydrophthalic acid.Bifunctional monocarboxylic acid crosslinking agents

"bifunctional monocarboxylic acid" means an organic acid having (a) only one carboxyl group and (b) one functional group that is not a carboxyl group but is capable of reacting with a carboxyl, carboxylic acid, amino, or hydroxyl group of a polymer. Preferably, the difunctional monocarboxylic acid comprises only two functional groups, namely one carboxyl group and a second functional group.

Suitable difunctional monocarboxylic acids include, but are not limited to, amino acids, salts of haloacetic acids, hydroxy monocarboxylic acids, and acid derivatives of hydroxy monocarboxylic acids (e.g., acid esters of hydroxy monocarboxylic acids).

The preferred salt of haloacetic acid is sodium chloroacetate. Without being bound by any theory, it is believed that when the aqueous mixture of sodium chloroacetate and cellulose fibers is dried and cured, an ether is formed by the reaction of the cellulose hydroxyl groups with the chlorine-containing carbon atoms of the sodium chloroacetate molecules. This etherification reaction releases a hydrochloric acid molecule which is immediately neutralized by the sodium salt of the newly formed cellulose-based acid. At higher temperatures, the acid can be used to esterify the nearest hydroxyl groups within the fiber, with the concomitant release of water. After ether and ester formation, sodium chloride remains as a by-product.

Suitable hydroxy monocarboxylic acids and their acid derivatives include, but are not limited to, glycolic acid, mesylate esters of glycolic acid, and p-toluenesulfonate esters of glycolic acid.Amine carboxylic acid crosslinking agents

Suitable amine carboxylic acids include, but are not limited to, primary amines, secondary amines, tertiary amines, and aromatic amines. Preferred primary amines include, but are not limited to, amino acids. Of particular interest are amino acids having the formula:

H₂N-CH₂-R¹²-C (O) OH wherein R¹²Is a single bond, C₁～C₁₂Alkyl, or by carboxyl, hydroxy, C₁～C₄Alkoxy radical, C₁～C₄C substituted by one or more of alkyl, amino and nitro₁～C₁₂An alkyl group.

Preferred amino acids include, but are not limited to, those having the formula:

wherein R is⁵Is linear or branched C₁～C₈An alkyl group. According to a preferred embodiment, R⁵Is C₂～C₄An alkyl group. Non-limiting examples of suitable amino acids include aspartic acid and glutamic acid.

Other suitable amine carboxylic acid crosslinking agents include, but are not limited to, ethylene dinitrilotetraacetic acid (EDTA).Crosslinking accelerator

The crosslinking facilitator of the present invention increases the efficiency of the crosslinking agent. A preferred crosslinking promoter is oxalic acid. Without being bound by any theory, it is believed that oxalic acid (pKa ═ 1.23) can act as an acid catalyst for the esterification reaction of the crosslinker. Alternatively, oxalic acid can form a mixed anhydride with the crosslinker, which then facilitates esterification of the cellulose fibers.Reversibility of crosslinking

Fibers crosslinked with the short crosslinking agents of the present invention, i.e., those containing 4 or fewer carbon atoms (e.g., 3 or fewer carbon atoms), such as oxalic acid and sodium chloroacetate, can be uncrosslinked and then re-crosslinked. The crosslinking of such fibers is generally substantially reversible, i.e., at least about 50% by weight of the crosslinked fibers can be uncrosslinked. According to one embodiment, at least about 60, 70, 80, 90, or 95 wt% of the crosslinked fibers may be uncrosslinked.

The crosslinked fibers can be uncrosslinked by soaking the crosslinked fibers in water for a time sufficient to uncrosslink them. Typically, the crosslinked fibers are soaked for about 0.5 to about 4 hours. According to a preferred embodiment, the fibers are soaked for about 2 hours. The crosslinked fibers may also be uncrosslinked by subjecting the crosslinked fibers to a capillary absorption-desorption cycle as described in the examples of the present application, Burgeni et al (supra), and Chatterjee et al (supra).

The fibers may be re-crosslinked by drying them or drying and curing them. U.S. patent nos.5,137,537; this phenomenon was not observed with the covalently crosslinked fibers disclosed in 5,183,707 and 5,190,563.

Without being bound by any theory, it is believed that as the cross-linked fibers absorb water and swell, the cross-links are tightened as the cellulose polymer chains move apart to accommodate the absorbed water. When the cross-linked molecular length is short, as is the case with fibers treated with oxalic acid and sodium chloroacetate (two carbon atoms between the hydroxyl groups located on adjacent cellulose polymer chains), the stress of swelling the fiber is sufficient to promote hydrolysis and breaking of one of the two covalent bonds of the cross-linked fiber. In contrast, as described in U.S. patent nos.5,137,537; the citric acid crosslinked fibers disclosed in 5,183,707 and 5,190,563 have much longer molecules bridging the cellulose polymer chains (four or five carbon atoms between the hydroxyl groups located on adjacent cellulose chains). Thus, as the fiber absorbs water and swells, the stress on the longer cross-linked molecules is insufficient to promote the breaking of one of the cross-linked covalent bonds.

Since the fibers of the present invention can be uncrosslinked, they can be dried and shipped or stored in sheet form rather than in loose or baled form. This reduces shipping and storage costs. The fibers can be re-crosslinked at the destination or when desired, for example by separating and curing them. Once the fibers are re-crosslinked, they may be incorporated into an absorbent structure, for example.

As used herein, the terms "reversibly crosslinked fibers" and "reversibly crosslinked cellulosic fibers" refer to crosslinked fibers or crosslinked cellulosic fibers wherein at least about 50, 60, 70, 80, 90, or 95 percent by weight of the crosslinked fibers are uncrosslinked after soaking in water for up to 4 hours, and wherein at least 50, 60, 70, 80, 90, or 95 percent by weight of the uncrosslinked fibers can be re-crosslinked by drying the fibers at a temperature of 105 ℃ or more.

The reversibly crosslinked fibers of the present invention are particularly useful in absorbent cores containing superabsorbent polymer (SAP) particles. The cross-linked fibers separate the SAP particles and provide a channel for fluid to flow around the SAP particles from the wet zone to the dry zone of the absorbent core. In addition, reversibly crosslinked fibers facilitate the absorption of large amounts of urine (or other fluids) in a short period of time (e.g., a gush). Once urine or other fluid enters the absorbent core, the crosslinked fibers begin to decrosslink. According to one embodiment, a substantial portion of the wet fibers are uncrosslinked after 0.5 to 4.0 hours of exposure to urine or other fluid. Uncrosslinked fibers hold and retain urine or other fluids to a greater extent than permanently crosslinked fibers. As a result, the absorbent core has an improved initial gush capacity compared to an absorbent core comprising conventional loose fibers and improved rewet characteristics compared to an absorbent core comprising permanently crosslinked fibers.Absorbent structure

The cellulosic fibers of the present invention can be incorporated into any disposable or non-disposable absorbent structure intended to absorb and retain body exudates and which are placed or held in proximity to the body of the wearer. Such absorbent structures are commonly used in disposable and non-disposable absorbent articles. Examples of disposable absorbent articles include, but are not limited to, infant diapers, adult incontinence products, training pants, sanitary napkins, and other feminine hygiene products. Examples of absorbent structures in which the cellulosic fibers of the present invention may be incorporated include, but are not limited to, those described in the following patents: international publication No. WO 98/47456, WO 99/63906, WO 99/63922, WO 99/63923, WO 99/63925, WO 00/20095, WO 00/38607, WO 00/41882, WO 00/71790 and WO 00/74620, and U.S. Pat. No.5,695,486, all of which are incorporated herein by reference.Acquisition and distribution layers

The cellulosic fibers of the present invention may be incorporated into an acquisition layer, a distribution layer, or an acquisition-distribution layer. Such layers are commonly used in absorbent structures contained in disposable absorbent articles. The acquisition and/or distribution layer may be prepared by any method known in the art and incorporated into an absorbent structure. According to one embodiment, the absorbent structure comprises a topsheet, which comprises the acquisition layer and/or distribution layer of the present invention, and a storage backsheet (also referred to as an absorbent core). The acquisition and distribution layer may be one or two separate layers, i.e. an upper acquisition layer and a lower distribution layer. The underlying distribution layer rapidly draws fluid from the acquisition layer and distributes the fluid into the storage layer.

The acquisition layer of the present invention generally comprises from about 90 to about 100 weight percent of the cellulosic fibers of the present invention, based on the total weight of the distribution layer taken as 100 percent. The density of the acquisition layer is generally from about 0.04 to about 0.07g/cm³Within the range.Absorbent core

The cellulosic fibers of the present invention may be incorporated into an absorbent core (also referred to as a storage layer). The absorbent core may comprise any liquid-absorbent material known in the art. Suitable materials include, but are not limited to, fiber mats or webs comprised of defibered, loose, fluffed, and/or hydrophilic cellulosic fibers or fibers of the present invention; superabsorbent polymer (SAP) particles, granules, flakes or fibers (aggregated particles); and any combination of the foregoing. Typically, SAP-particles are capable of absorbing many times their weight in liquid and considerably increase the absorbent capacity of the absorbent core without substantially increasing the bulk of the layer.

The term "superabsorbent polymer particles" or "SAP particles" is intended to include superabsorbent polymer in any particulate form, including irregular particles, spherical particles (beads), powders, flakes, rayon, and other elongated particles. SAP means a generally water-soluble polymer that has been crosslinked to render it substantially water-insoluble, typically capable of absorbing at least ten times, preferably at least fifteen times, its own weight in a physiological salt solution. The SAP particles may be of any size or shape. Many examples of superabsorbents and methods of making the same can be found, for example, in U.S. patent nos.4,102,340; 4,467,012; 4,950,264; 5,147,343, respectively; 5,328,935, respectively; 5,338,766; 5,372,766, respectively; 5,849,816, respectively; 5,859,077 and Re.32,649. Examples of suitable SAP particles include, but are not limited to, starch graft copolymers, such as hydrolyzed starch-acrylate graft copolymers; crosslinked carboxymethylcellulose and derivatives thereof; and modified hydrophilic polyacrylates such as saponified acrylate-vinyl copolymers, neutralized crosslinked polyacrylic acids, and crosslinked polyacrylates.

Preferably, the SAP particles form a hydrogel upon absorption of fluid. More preferably, the SAP particles have a high gel volume or a high gel strength (as measured by the shear modulus of the hydrogel). Such SAP particles generally contain a relatively low content of polymers that can be extracted by contact with synthetic urine (so-called extractables). An example of such SAP particles is starch grafted polyacrylic acid salt hydrogel, available as IM1000^Obtained from Hoechst-Celanese of Portsmouth, Va. Other examples of hydrogels containing SAP particles include, but are not limited to, Sanyo Kasei Kogyo Kabushiki, Japan, under the trademark SANWET^TMThose sold; sumitomo KagakuKabushiki Haishi in Japan under the trademark SUMIKA GEL^TMThose sold; and Stockhausen of Garyville, LA under the trademark FAVOR^TMThose sold; and BASF of Aberdeen, MS under the trademark ASAP^TMThose sold.

According to a preferred embodiment, the absorbent core comprises: (a) SAP particles, and (b) loose fibers, matrix fibers, fibers of the present invention, or any combination thereof. The fibers provide structural integrity and provide channels for fluid flow through the absorbent core.

According to another embodiment, the absorbent core comprises from about 30 to about 70 wt% SAP particles and from about 70 to about 30 wt% of the present inventionBased on 100% of the total weight of the absorbent core. Typically, the absorbent core has a weight density of from about 0.15 to about 0.25g/cm³。

According to yet another embodiment, the absorbent core comprises SAP-particles and the reversibly crosslinked fibers of the present invention. According to a preferred embodiment, the reversibly crosslinked fibers are crosslinked with oxalic acid, sodium chloroacetate or mixtures thereof. Typically, the absorbent core comprises about 30 to about 70 wt% of SAP-particles and about 70 to about 30 wt% of the reversibly crosslinked fibers of the present invention, based on the total weight of the absorbent core taken as 100%. According to another preferred embodiment, the acquisition layer and/or the distribution layer and the absorbent core comprise fibers cross-linked with oxalic acid.

The absorbent structures of the present invention can be incorporated into disposable and non-disposable absorbent articles, such as paper towels, adult incontinence pads, and feminine hygiene products. The absorbent article may comprise a liquid permeable topsheet positioned over the acquisition and/or distribution layer, which functions to allow fluid to flow to the acquisition and/or distribution layer, and a liquid impermeable backsheet, which functions to contain the fluid and prevent it from flowing through the absorbent article and onto the garments of the wearer of the absorbent article.

According to one embodiment, the absorbent structure of the present invention is incorporated into a disposable infant diaper, which generally comprises: a front waistband zone, a back waistband zone, and a crotch therebetween. The structure of the diaper generally includes: a liquid permeable topsheet, a liquid impermeable backsheet, an absorbent structure, an elastic member, and a fastening tab. Representative disposable diaper designs can be found, for example, in U.S. Pat. Nos.4,935,022 and 5,149,335.

According to another embodiment, the absorbent structure of the present invention is incorporated into a feminine hygiene pad, such as the one described in U.S. Pat. No.5,961,505.

The following examples illustrate the invention without limiting it. All parts and percentages are given by weight unless otherwise indicated. All crosslinkers, crosslinkers used in these examplesThe co-promoters and other chemicals are available from Aldrich Chemical Company of Milwaukee, Wis.Determination of capillary and desorption pressures and saturation capacities

Both capillary absorption and desorption pressures are measured in terms of "capillary absorption equilibrium in fibrous material", a.a. burgeni and c.kapur, journal of textile research, 37: 356 to 366 (1967). The method is described in detail as follows:

0.75g of individualized fibers are processed into round pieces with a diameter of about 55 to 60 mm. The sample was placed on a sintered sheet of a 150ml coarse sintered pyrex funnel (Corning No.36060, available from VWR of Suwanee, GA). A piece of weight of diameter comparable to the sample diameter sufficient to provide 0.22psi pressure was placed on the sample. The bottom of the funnel was connected to a decreasing diameter adapter and an approximately 2 foot long R-3603 Tygon was placed^One end of the tube was connected to a transfer adapter and the other end was connected to a fluid reservoir placed on an electronic scale capable of weighing up to 0.01 g. The tube is attached to the bottom of the side of the reservoir. The fluid reservoir contained a 0.9% saline solution. The height of the saline solution in the fluid reservoir is about 1 inch or more at the tube connection. Brine is injected into the tube, and brine is also injected into the funnel below the sinter, so that the sinter is wetted by the brine, but no brine is above the sinter. The brine column is continuous from the reservoir to the sintering site, without any air in the column.

The absorption cycle is as follows. Starting at a height, e.g., 20, 30 or 80cm, above the level of saline in the reservoir, the sample is allowed to absorb saline to an equilibrium or steady state. The steady state was determined to be no more than 0.04g change in weight of saline displayed on the electronic scale below the reservoir in one minute. When steady state was reached, the sample was lowered 5cm closer to the level of saline in the reservoir and held there until equilibrium was reached. The sample was lowered another 5cm and the above procedure was repeated. The sample was subjected to a desorption cycle by reversing the above operation, i.e. by moving the sample upwards in 5cm increments, when the sample was in equilibrium at the same height as the level of the saline in the reservoir.

The weight of saline in the sample at the same level of saline in the reservoir is the saturation capacity of the sample. The height of the sample above the level of the saline in the reservoir (reported in cm) at 50% of the saturation capacity in the downward (absorption) cycle (median absorption pressure) and the corresponding value of the upward (desorption) curve (median desorption pressure) were determined by interpolation. The saturated capacity values are reported in the table as grams of saline per gram of sample.

Examples 1 to 8

Examples 1 to 8 in Table 1 were prepared as follows.

An unrefined cellulose fiber (available as Foley Fluff) having a freeness of 740ml CSF^TMFibers from Buckeye Technologies inc. of Memphis, TN) were pulped in water and refined to the appropriate freeness with a Valley beater at ambient temperature and pressure. The fibers were centrifuged, separated by hand, and air dried to 60% solids. The fibers were crosslinked with citric acid (dry fiber base) at an appropriate concentration by spraying the fibers with an aqueous solution of citric acid at a concentration sufficient to dilute the fiber pieces to 40% solids. The fibers were then air dried to 60% solids, fluffed, dried to constant weight, and heated at the temperature shown in table 1 for an additional 30 minutes.

The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the cured cellulose fibers were determined. The water retention values of the consolidated cellulose fibers were determined according to the procedure described in TAPPI Useful Methods, UM 256. The capillary absorption and desorption pressures were determined as described above.

The results are shown in table 1.

Comparative example 9

Dry beating at 100 ℃ of unrefined cellulose fibres of 740ml CSF (available as Foley Fluff)^TMFibers from Buckeye Technologies inc. Water retention of the cured cellulose fibers was determined as described in example 1Value, saturation capacity, capillary absorption pressure and capillary desorption pressure.

The results are shown in table 1 below.

Comparative example 10

Spraying an unrefined cellulose fiber (available as Foley Fluff) having a freeness of 740ml CSF with sufficient water^TMFibers were obtained from Buckeye Technologies Inc.) to yield a solids content of 40%. The fiber was air dried to 60% solids, mechanically fluffed, and dried to constant weight at 150 ℃. Then, the fiber was heated at the same temperature for another 30 minutes. The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the cured cellulose fibers were measured as described in example 1.

The results are shown in table 1.

Comparative example 11

An unrefined cellulose fiber (available as Foley Fluff) having a freeness of 740ml CSF^TMFibers were obtained from Buckeye Technologies Inc.) pulped in water and refined with a Valley beater to a freeness of about 500ml CSF. The fibers were centrifuged, hand separated, air dried to 60% solids, fluffed, and dried to constant weight at 150 ℃. The fiber was then heated at 150 ℃ for an additional 30 minutes. The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the cured cellulose fibers were measured as described in example 1.

The results are shown in table 1.

Comparative example 12

An unrefined cellulose fiber (available as Foley Fluff) having a freeness of 740ml CSF^TMFibers were obtained from Buckeye Technologies Inc.) pulped in water and refined with a Valley beater to a freeness of about 500ml CSF. Centrifuging the fiber, separating by hand, and air drying to 60% solids. In treating the fibers with aqueous citric acid, the cellulose fibers were sprayed with aqueous sulfuric acid at pH3 to 40% solids to adjust the pH of the fiber-water mixture to the same pH observed in example 1. The fiber was air dried to 60% solids, fluffed and dried to constant weight at 150 ℃. The fiber was then heated at 150 ℃ for an additional 30 minutes. The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the cured cellulose fibers were measured as described in example 1.

The results are shown in table 1.

Comparative example 13

The fibrous sheet was brought to 40% solids by spraying the fibers with an aqueous citric acid solution of sufficient dilution, and the unrefined cellulose fibers (available as Foley Fluff) having a freeness of 740ml CSF were crosslinked with 5% citric acid (dry fiber basis)^TMFibers were obtained from buckeye technologies Inc.). The fiber sheet was air dried to 60% solids, mechanically fluffed, and dried to constant weight at 150 ℃. The sheet was then heated at 150 ℃ for a further 30 minutes. The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the cured cellulose fibers were measured as described in example 1.

The results are shown in table 1.

Comparative example 14

The procedure described in comparative example 13 was repeated, except that the crosslinking was performed with 10% citric acid instead of 5% citric acid.

The results are shown in table 1.

TABLE 1

Examples	Beating degree (ml CSF)	Cross-linking (% citric acid) (dry fiber base)	Curing temperature (. degree.C.)	WRV(％)	Saturated capacity (g/g)	Absorption pressure (cm) at 50% of saturation capacity	Desorption pressure (cm) at 50% of saturation capacity
								Example 1	500	5	150	44.8	8.2	3.9	13.4
Example 2	500	10	150	38.1	8.7	4.4	13.5
								Example 3	300	5	150	42.7	8.2	5.3	14.7
Example 4	300	10	150	41.5	8.6	5.1	14.9
								Example 5	500	5	175	36.7	8.5	4.4	14.0
Example 6	500	10	175	29.5	8.3	3.1	11.9
								Example 7	300	5	175	33.4	7.9	4.2	15.0
Example 8	300	10	175	29.9	7.5	3.0	13.3
								Comparative example 9	740	-	100	83.0	9.3	12.3	30.5
Comparative example 10	740	-	150	73.6	11.0	12.0	27.4
								Comparative example 11	500	-	150	92.1	8.2	8.1	20.7
Comparative example 12	500	-	150	78.6	8.3	5.3	16.3
								Comparative example 13	740	5	150	43.3	12.3	-	17.9
Comparative example 14	740	10	150	38.4	12.1	6.7	18.4

As shown by the results in table 1, refined and crosslinked fibers exhibit lower WRVs and desorption pressures than similar unrefined and crosslinked fibers.

Example 15 Refining

Preparing a never-dried Foley Fluff product containing 2.75-3.25 wt%^TMA water slurry of fibers (available from Buckeye Technologies Inc.). The aqueous slurry was pumped through the refiner at ambient temperature and pressure using a Bauermodel No.444, 24 "pump. Bauer refiner plate is No. A24313.

The refiner was operated at 178 amps current and a slurry flow rate of 255 gallons per minute. These conditions produced a load of 30-60 hp/hr per dry short ton of fiber. The fibers produced had a freeness of 680ml CSF.Demineralization

The refined pulp is pumped to an auxiliary bottom tank at a consistency of 2.75-3.25%. While stirring the pulp, sulfuric acid was added until a nominal pH of 2.0 was reached. After stirring for at least 10 minutes, the water is allowed to standThe slurry was passed through a secondary bottom screen for a minimum of 3 hours to dewater. Then, the slurry is diluted with sodium-softened water to a consistency of 2.0% and the pH is adjusted to 4.5-5.0.Tabletting

Pulp sheeting was performed on a paper machine available from Sandy Hill Corporation of Hudson Falls, NY. The frame for stabilizing the paper (board width) is 36 inches at the maximum. The 2% pulp was pumped through the headbox and dosing valve and into the white water pit at a controlled flow rate. Directly introducing steam to raise the temperature in the cellar to 130-150 ℃ F, and diluting the pulp in the cellar to 1.0-1.25% of consistency by using white water.

The stock is then fed into the headbox of the paper machine and transferred to a moving wire in the Fordrinier section of the paper machine. Natural and vacuum assisted drainage was carried out until a forming plate of approximately 32% consistency was discharged from the cross-flow extruder. After forming the board but before the cross-over extruder, the wet board was deburred to a width of 24 inches using two water jets. The fiberboard having a consistency of about 32% is then passed through two wet presses where water is further removed to densify the board. After exiting the second wet press, the fiberboard entered the first dryer section at a consistency of about 48%. In the first dryer section, the pulp board passes through thirteen rotary steam generators of about 300-325 ° F. The pulp board then passes through eight rotary steam generators in the second dryer section and the moisture content on exiting the dryer is 4-8%. The fiber sheet was wound into a roll having a width of about 23 inches. The coil had a basis weight of about 0.126 pounds per square foot and a density of about 0.60g/cm³。Longitudinal cutting

The pulp roll was rewound onto a new core and slit into smaller rolls each 10 inches wide.Chemical application

A 10 inch wide roll of pulp was unwound and slowly passed through a puddling press. The nip of the puddling press was flooded with an aqueous solution of citric acid and sodium hypophosphite. Sodium hypophosphite mitigates the blackening of the pulp at high temperatures. The weight concentrations of citric acid and sodium hypophosphite in the flooded nip were about 14% and 7%, respectively. By mixing and pressingA machine, said pulp board having absorbed a sufficient amount of aqueous solution to achieve a moisture content of about 40%.Pulp sheet crushing and loosening

After the masticator press, the fiberboard is torn into smaller pieces by shredders, pre-crushers, and pickers. The pulverized pulp was then blown into the inlet of a Sunds Defibrator Model 3784RO fluor (available from Sunds Defibrator, AB of Sundsvall, Sweden) with the nip set at 5.5 mm. The defibrator combs the fiber meal into separate fibers. The evacuated fiber pulp is swept out of the RO flow with a high velocity stream of hot air at about 380 ° F.Drying and curing

The hot air stream exiting the RO flumffer transport evacuated fibers is accelerated by a blower through a flash dryer where all or nearly all of the water in the fibers is evaporated. The dried fibrous pulp falls onto a mechanical input belt where it forms a low density highly expanded bed. Then, the fiber is fed into a Proctor&Schwartz K16476 Tunnel dryer (available from Proctor)&Schwartz, inc. of Horsham, PA). The evacuated fiber bed is passed through three chambers in the dryer by a series of heated circulating air streams, heated and then cooled. In the chamber 1, the bed temperature reaches 325-330 DEG F. In chamber 2, the bed temperature is raised to 385-390 ° F. In chamber 3, the bed temperature is reduced to 355-360 ° F. The total time in the tunnel dryer was about 11.5 minutes.Packaging the mixture

The crosslinked fibers exiting the exit end of the tunnel dryer exit the conveyor into a 3445-type Baler (available from American Baler Company of Bellevue, OH) where the material is compressed into bales weighing about 70-80 pounds.

Example 16

The samples in table 2 were prepared as follows.

An unrefined cellulose fiber (available as Foley Fluff) having a freeness of 740ml CSF^TMFibers from Buckeye Technologies inc. of Memphis, TN) were pulped in water,and refined to the appropriate freeness by pumping through a refiner at normal temperature and pressure using a Bauer Model No.444, 24 "pump. Optionally, the fibers are washed with dilute sulfuric acid (acid wash) to remove minerals. The refined fibres were tableted and dried.

A piece of the fiber sheet was dipped into a tray containing a cross-linking agent and an oxalic acid solution. The sheet is then inverted and dipped into a second solution of the crosslinking agent and oxalic acid. The total solution contained 10% crosslinker (dry fiber basis) and 5% oxalic acid (dry fiber basis). These solutions had sufficient dilution to make the piece of fibrous sheet 40% solids. The fibrous sheet was stored in a sealed polyethylene bag for 1 hour. The fiber was air dried to 60% solids, fluffed, dried to constant weight, and heated at 175 ℃ for 30 minutes.

The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the cured cellulose fibers were determined. The water retention values of the consolidated cellulose fibers were determined according to the procedure described in TAPPI useful methods, UM 256. The capillary absorption and desorption pressures were determined as described above.

The results are shown in table 2.

TABLE 2

Sample (I)	Crosslinking agent	Beating degree (ml CSF)	Acid pickling	WRV(％)	Saturated capacity (g/g)	Absorption pressure (cm) at 50% of saturation capacity	Desorption pressure (cm) at 50% of saturation capacity	Carboxy (meq/kgm)
									a	Succinic acid	740	No	31.5	13.9	6.8	14.7	343.1
b	Succinic acid	570	No	36.3	13.5	7.2	17.3	294.1
									c	Succinic acid	570	Yes	36.0	13.7	7.2	16.1	303.9
d	Adipic acid	740	No	36.0	13.5	7.4	16.4	228.6
									e	Adipic acid	570	No	39.1	12.6	7.0	16.4	190.6
f	Adipic acid	570	Yes	40.0	13.1	7.8	18.3	179.3
									g	Pimelic acid	740	No	35.4	14.7	6.4	17.5	231.4
h	Pimelic acid	570	No	39.4	13.9	6.9	17.7	209.0
									i	Pimelic acid	570	Yes	37.9	12.2	7.3	17.3	200.0
j	Malonic acid	740	No	32.8	16.1	7.2	18.9	75.6
									k	Malonic acid	570	No	36.5	11.6	6.1	16.8	78.2
l	Malonic acid	570	Yes	37.7	12.7	7.1	15.8	85.5

Example 17

The samples in table 3 were prepared as follows.

An unrefined cellulose fiber (available as Foley Fluff) having a freeness of 740ml CSF^TMFibers from Buckeye Technologies inc. of Memphis, TN) were pulped in water and refined through a refiner at normal temperature and pressure to the appropriate freeness using a Bauer Model No.444, 24 "pump. The refined fibres were tableted and dried. If the fibers are wet lap, they are then centrifuged.

A piece of the fiber sheet is dipped into a tray containing a solution of a cross-linking agent, sodium hypophosphite and optionally oxalic acid. The sheet is then inverted and dipped again in a solution of a second portion of cross-linking agent, sodium hypophosphite and optionally oxalic acid. The total solution contained 10% crosslinker (dry fiber basis), 5% sodium hypophosphite (dry fiber basis) and optionally 1% oxalic acid (dry fiber basis). These solutions had sufficient dilution to make the fibrous sheet 40% solids. The piece of fiber sheet was stored in a sealed polyethylene bag for 1 hour. The fiber was air dried to 60% solids, fluffed, dried to constant weight, and heated at 175 ℃ for an additional 30 minutes.

The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the cured cellulose fibers were determined. The water retention values of the consolidated cellulose fibers were determined according to the procedure described in TAPPI useful methods, UM 256. The capillary absorption and desorption pressures were determined as described above.

The results are shown in table 3.

TABLE 3

Sample (I)	Crosslinking agent	Concentration of oxalic acid (% w/w)	Beating degree (ml CSF)	WRV(％)	Saturated capacity (g/g)	Absorption pressure (cm) at 50% of saturation capacity	Desorption pressure (cm) at 50% of saturation capacity	Carboxy (meq/kgm)
									a	Oxalic acid	-	740	48.3	12.0	6.2	18.9	292.5
b	Maleic acid	-	740	33.8	15.3	4.7	16.4	529.5
									c	Succinic acid	-	740	38.2	14.0	5.3	16.7	414.8
d	Adipic acid	-	740	45.7	13.4	5.7	17.2	343.9
									e	Succinic acid	1	740	37.8	14.5	4.6	14.9	432.2
f	Adipic acid	1	740	45.3	13.0	6.1	17.8	356.3
									g	Citric acid	-	740	33.1	15.6	4.4	15.2	442.0
h	Oxalic acid	-	680	47.3	10.5	6.5	18.4	287.7
									i	Maleic acid	-	680	34.2	14.4	4.2	15.0	530.2
j	Succinic acid	-	680	38.0	12.8	5.8	16.6	436.8
									k	Adipic acid	-	680	49.6	11.8	5.4	15.6	351.3
l	Succinic acid	1	680	37.0	14.0	4.4	14.3	424.6
									m	Adipic acid	1	680	45.8	12.3	6.3	17.7	358.5
n	Citric acid	-	680	31.3	15.8	3.7	13.5	450.3
									o	Oxalic acid	-	570 (Wet cotton roll)	59.9	3.2	5.2	12.7	432.0
p	Maleic acid	-	570 (Wet cotton roll)	41.6	5.1	3.8	7.7	474.0
									q	Succinic acid	-	570 (Wet cotton roll)	34.8	8.8	3.2	8.3	466.1
r	Adipic acid	-	570 (Wet cotton roll)	50.1	9.0	3.4	8.7	487.5
									s	Succinic acid	1	570 (Wet cotton roll)	36.1	8.8	3.3	8.7	415.4
t	Adipic acid	1	570 (Wet cotton roll)	44.2	8.6	3.3	8.9	422.1
									u	Citric acid	-	570 (Wet cotton roll)	35.0	9.7	3.3	9.2	468.6
v	Oxalic acid	-	570	49.6	11.7	5.3	15.8	405.8
									w	Maleic acid	-	570	41.5	12.4	5.9	16.7	450.7
x	Succinic acid	-	570	37.2	13.4	4.8	15.1	510.1
									y	Adipic acid	-	570	44.9	11.3	5.3	16.0	328.0
z	Succinic acid	1	570	40.7	12.0	5.4	16.2	455.5
									aa	Adipic acid	1	570	48.6	11.4	5.9	16.7	371.0
bb	Citric acid	-	570	39.9	13.7	3.8	12.9	481.1

Example 18

The samples in Table 4 were prepared as follows.

Stock solutions of the crosslinking agents and crosslinking accelerators indicated in table 4 were prepared by dissolving the indicated amounts of crosslinking agent and crosslinking accelerator in 22.5g of distilled water.

The samples were treated with the appropriate amount of solution as follows: 15g (dry basis) of compressed, unrefined cellulose fibres with a freeness of 740ml CSF (available as Foley Fluff) are treated with appropriate amounts of stock solutions containing a crosslinking agent and a crosslinking facilitator^TMFibers were obtained from Buckeye technologies inc. This treatment reduced the fiber solids content of the mixture to 40%. The fibers were placed in a sealed container at room temperature for 60 minutes and then air dried to 60% solids. The fibers were mechanically separated, individualized, fluffed in an experimental fluffer and dried to constant weight in a forced hot air drying oven at 175 ℃. The fibers were cured at the same temperature for 30 minutes.

The control was prepared as follows: a dry, unrefined cellulose fibre board (available as a Foley Fluff) with a freeness of 740ml CSF is treated with water^TMFibers from buckeye technologies inc. of Memphis, TN) were diluted to 40% solids. The pH of the mixture was adjusted to 3 with sulfuric acid. The stock solution applied to the fibers in the above procedure typically has a pH of 3. The mixture was then placed in a sealed container at room temperature for 60 minutes and then air dried to 60% solids. The fibers were mechanically separated, individualized, fluffed in an experimental fluffer and dried to constant weight in a forced hot air drying oven at 175 ℃. The fiber was heated at the same temperature for an additional 30 minutes.

Obtaining from Pampers^The fibers of the acquisition-distribution layer of a disposable diaper (available from Proctor and Gamble of cincinnati, OH) were used as a second control.

The water retention value, saturation capacity, capillary absorption pressure and capillary desorption pressure of the above fibers were measured. The water retention values of cellulose fibers were determined according to the procedure described in TAPPI Useful Methods, UM 256. The capillary absorption and desorption pressures and saturation capacities were determined as described above.

The results are shown in table 4.

TABLE 4

Sample (I)	Crosslinking agent	Amount of crosslinking agent in stock solution (g)	Crosslinking accelerator	Amount (g) of crosslinking accelerator in stock solution	WRV(％)	Saturated capacity (g/g)	Absorption pressure (cm) at 50% of saturation capacity	Desorption pressure (cm) at 50% of saturation capacity
									a	None (contrast)	-	Is free of	-	59.4	13.6	10.7	24.6
b	Pampers(comparison)	-	-	-	44.9	6.7	4.4	18.2
									c	Chloroacetic acid sodium salt	0.75	Is free of	-	49.8	12.7	9.5	22.2
d	Chloroacetic acid sodium salt	1.5	Is free of	-	44.6	11.1	8.2	18.5
									e	Oxalic acid	0.15	Is free of	-	47.2	12.9	9.1	23.6
f	Oxalic acid	0.75	Is free of	-	42.9	13.6	8.6	21.5
									g	Oxalic acid	1.5	Is free of	-	38.5	15.3	7.6	17.8
h	Succinic acid	1.5	Is free of	-	39.8	14.2	7.5	20.4
									i	Succinic acid	1.5	Oxalic acid	0.15	33.8	14.3	7.7	16.6
j	Succinic acid	1.5	Oxalic acid	0.75	31.5	13.9	6.8	14.7
									k	Adipic acid	1.5	Is free of	-	50.5	14.1	8.0	22.3
l	Adipic acid	1.5	Oxalic acid	0.15	38.9	13.7	8.6	19.8
									m	Adipic acid	1.5	Oxalic acid	0.75	36.0	13.5	7.4	16.4
n	Malonic acid	1.5	Is free of	-	39.5	13.5	7.8	22.8
									o	Malonic acid	1.5	Oxalic acid	0.75	32.8	16.1	7.2	18.9
p	Glutaric acid	1.5	Is free of	-	37.1	15.2	7.5	23.4
									q	Glutaric acid	1.5	Oxalic acid	0.75	33.0	15.9	7.3	21.9
r	Pimelic acid	1.5	Is free of	-	42.7	14.8	7.4	23.1
									s	Pimelic acid	1.5	Oxalic acid	0.75	35.4	14.7	6.4	17.5
t	Suberic acid	1.5	Is free of	-	56.0	13.7	8.3	25.7
									u	Suberic acid	1.5	Oxalic acid	0.75	41.0	14.2	7.2	21.4
v	Phthalic acid	1.5	Is free of	-	55.4	13.9	8.4	26.3
									w	Phthalic acid	1.5	Oxalic acid	0.75	42.5	13.0	9.0	24.6
x	Tetrahydrophthalic acid	1.5	Is free of	-	53.4	12.6	8.4	22.5
									y	Tetrahydrophthalic acid	1.5	Oxalic acid	0.75	39.4	13.3	7.4	19.9
z	Fumaric acid	1.5	Is free of	-	44.5	13.9	9.1	24.1
									aa	Fumaric acid	1.5	Oxalic acid	0.75	41.5	12.7	8.4	19.2
bb	Glycolic acid	1.5	Is free of	-	47.8	13.3	8.7	22.0
									cc	Glycolic acid	1.5	Oxalic acid	0.75	39.6	14.0	7.3	17.4
dd	Tartaric acid	1.5	Is free of	-	34.3	13.7	7.3	18.5
									ee	Tartaric acid	1.5	Oxalic acid	0.75	32.2	13.1	7.3	18.3
ff	Malic acid	1.5	Is free of	-	31.6	14.5	6.9	19.0
									gg	Malic acid	1.5	Oxalic acid	0.75	30.1	14.0	6.6	18.7
hh	Glucaric acid	1.5	Is free of	-	49.6	13.4	9.4	22.8
									ii	Glucaric acid	1.5	Oxalic acid	0.75	41.1	11.4	8.2	19.4
jj	Mucic acid	1.5	Is free of	-	55.9	12.4	9.9	19.0
									kk	Mucic acid	1.5	Oxalic acid	0.75	40.3	11.9	8.4	17.1
ll	Aspartic acid	1.5	Is free of	-	55.2	13.9	9.5	26.6
									mm	Aspartic acid	1.5	Oxalic acid	0.75	37.5	14.6	6.5	16.5
nn	Glutamic acid	1.5	Is free of	-	52.8	13.8	7.8	25.1
									oo	Glutamic acid	1.5	Oxalic acid	0.75	37.4	14.2	7.0	17.3
pp	EDTA	1.5	Is free of	-	50.7	12.1	8.6	21.9
									qq	EDTA	1.5	Oxalic acid	0.75	39.7	12.1	8.0	18.4

Example 19

A sample of crosslinked fiber was prepared by the method described in example 18, using 1.5g of sodium chloroacetate. A second sample was prepared using 1.5g of oxalic acid instead of sodium chloroacetate.

For comparison, a sample of crosslinked fibers was prepared by the method described in example 18 using a stock solution containing 10 wt% citric acid.

These samples were subjected to a first capillary absorption-desorption cycle by the procedure described previously. These samples were then subjected to a second capillary absorption-desorption cycle by the same operating method. The absorption and desorption pressures observed for the two cycles are shown in table 5 below.

With uncrosslinked Foley Fluff^TMThe test was repeated for the fibers.

TABLE 5

	Crosslinking agent
					Is free of	Oxalic acid	Chloroacetic acid sodium salt	Citric acid
	First cycle
Saturated capacity (g/g)						13.6	15.3	12.2	18.5
	Absorption pressure (cm) at 50% of saturation Capacity desorption pressure (cm) at 50% of saturation Capacity	10.7	7.6	10.5	7.0
24.6						17.8	18.2	13.5
		Second circulation
Saturated Capacity (g/g) absorption pressure at 50% of saturated Capacity (cm) desorption pressure at 50% of saturated Capacity (cm)	11.8						13.1	9.6	16.5
		＞30	＞30	＞30	15.3
	25.8					29.3	24.7	18.4

Example 20

The sample described in example 19 was prepared and subjected to two capillary absorption-desorption cycles. These samples were dried overnight in a forced hot air drying oven at 105 ℃. Alternatively, the sample was dried to constant weight at 105 ℃ and heated for a further 30 minutes at 175 ℃ (cure). The saturation capacity and the absorption and desorption pressures of the sample were determined.

The results are shown in table 6.

TABLE 6

	Crosslinking agent
					Is free of	Oxalic acid	Chloroacetic acid sodium salt	Citric acid
	Drying at 105 deg.C
Saturated Capacity (g/g) absorption pressure at 50% of saturated Capacity (cm) desorption pressure at 50% of saturated Capacity (cm)						12.8	12.3	10.3	17.1
	12.2	8.6	11.5	7.1
					24.7	18.1	19.8	14.3
	Curing at 175 DEG C
Saturated Capacity (g/g) absorption pressure at 50% of saturated Capacity (cm) desorption pressure at 50% of saturated Capacity (cm)						11.3	11.2	9.5	14.4
	13.0	6.8	12.6	7.2
					24.6	18.1	19.6	13.7

Example 21 Chemical application

An aqueous solution of oxalic acid and sodium hypophosphite was prepared by mixing 151 pounds of a 10 weight percent solution of oxalic acid, 15 pounds of a 50 weight percent solution of sodium hypophosphite and 1 pound of water.

Foley Fluff 10 inches wide^TMThe fiber rolls (available from buckeye technologies Inc.) were unwound and slowly passed through a puddling press. The nip of the puddling press is flooded with an aqueous solution of oxalic acid and sodium hypophosphite. The sodium hypophosphite reduces the blackening of the fiber meal at high temperature. By the puddling press, the fiberboard absorbed sufficient aqueous solution to achieve a moisture content of about 47%, based on the total weight of the dry fibers taken as 100%. The treated board also contained about 10 wt% oxalic acid and 5 wt% sodium hypophosphite, based on the total dry fiber weight of 100%.Crushing and loosening of fiberboard

After the masticator press, the fiberboard is torn into smaller pieces by shredders, pre-crushers, and pickers. The pulverized pulp was then blown into the inlet of a Sunds Defibrator Model 3784RO fluor (available from Sunds Defibrator, AB of Sundsvall, Sweden) with the nip set at 5.5 mm. The fiberThe fiber meal is combed into separated fibers by a fiber separator. The fluffed fiber pulp was swept out of the RO flumer with a high velocity stream of hot air at about 380 ° F.Drying and curing

The hot air stream from the RO flumer conveying fluffed fibers is accelerated by a blower through a flash dryer where all of the water in the fibers is evaporated. The dried fibrous pulp falls onto a mechanical input belt where it forms a low density highly expanded bed. Then, the fiber is fed into a Proctor&Schwartz K16476 Tunnel dryer (available from Proctor)&Schwartz, inc. of Horsham, PA). The fluffed fiber bed was heated by a series of hot circulating air streams through three chambers in the dryer. In chamber 1, the bed temperature reaches 330-340 DEG F. In chamber 2, the bed temperature is raised to 375-385 ° F. In chamber 3, the bed temperature is reduced to 355-360 ° F. After the three heating zones, the fiber bed passed through the last insulated chamber without additional heating. The total time in the tunnel dryer was about 11.5 minutes.Packaging the mixture

The crosslinked fibers exiting the exit end of the tunnel dryer exit the conveyor into a 3445-type Baler (available from American Baler Company of Bellevue, OH) where the material is pressed into bales weighing about 85-100 pounds.

All documents cited herein are incorporated by reference. To the extent that a conflict may exist between the present specification and a reference, the language disclosed herein controls.

Claims (138)

1. Cellulose fibers having a median desorption pressure of 15cm or less as measured in a capillary absorption-desorption cycle.

2. The cellulose fibers of claim 1, wherein the cellulose fibers have a median desorption pressure of 14cm or less.

3. The cellulose fibers of claim 1, wherein the cellulose fibers have a median desorption pressure of 13cm or less.

4. The cellulose fibers of claim 1, wherein the cellulose fibers have a median desorption pressure of 12cm or less.

5. The cellulose fibers of any one of claims 1-4, wherein the cellulose fibers have a water retention value of 45% or less.

6. The cellulose fibers of any one of claims 1-4, wherein the cellulose fibers have a water retention value of 38% or less.

7. The cellulose fibers of any one of claims 1-4, wherein the cellulose fibers have a water retention value of 30% or less.

8. The cellulose fibers of any one of claims 1-7, wherein the cellulose fibers are crosslinked.

9. An acquisition and distribution layer comprising the cellulosic fibers of any of claims 1 to 8.

10. An acquisition layer comprising the cellulosic fiber of any of claims 1 to 8.

11. A distribution layer comprising the cellulosic fiber of any of claims 1 to 8.

12. An absorbent structure comprising:

(a) a surface layer comprising cellulosic fibers having a median desorption pressure of 15cm or less as measured in a capillary absorption-desorption cycle; and

(b) a bottom layer comprising SAP particles, the second layer being in fluid communication with the first layer.

13. The absorbent structure of claim 12, wherein the cellulosic fibers have a median desorption pressure of 14cm or less.

14. The absorbent structure of claim 13, wherein the cellulosic fibers have a median desorption pressure of 13cm or less.

15. The absorbent structure of claim 14, wherein the cellulosic fibers have a median desorption pressure of 12cm or less.

16. The absorbent structure of any of claims 12-15, wherein the cellulosic fibers have a water retention value of 45% or less.

17. The absorbent structure of any of claims 12-15, wherein the cellulosic fibers have a water retention value of 38% or less.

18. The absorbent structure of any of claims 12-15, wherein the cellulosic fibers have a water retention value of 30% or less.

19. An absorbent structure comprising the cellulosic fibers of any one of claims 1-8.

20. An absorbent structure comprising the acquisition and distribution layer of claim 9.

21. An absorbent structure comprising the acquisition layer of claim 10.

22. An absorbent structure comprising the distribution layer of claim 11.

23. A method of making cellulose fibers comprising the steps of:

(a) refining the cellulose fibers to a freeness of from about 300 to about 700ml CSF; and

(b) crosslinking the refined cellulose fibers.

24. The method of claim 23, wherein the cellulose fibers refined in step (a) are wet lap.

25. The method of claim 23, wherein step (a) comprises refining the cellulose fibers to a freeness of from about 500 to about 700ml CSF.

26. The method of claim 25, wherein step (a) comprises refining the cellulose fibers to a freeness of from about 650 to about 700ml CSF.

27. The method of claim 23, wherein step (b) comprises:

(i) mixing the refined cellulose fibers with a crosslinking agent; and

(ii) curing the cellulosic fibers in the mixture.

28. The method of claim 23, wherein step (b) comprises:

(i) mixing the refined cellulose fibers with a crosslinking agent;

(ii) evacuating the cellulose fibers in the mixture; and

(iii) curing the cellulosic fibers in the mixture.

29. The method of claim 28, wherein step (b) (iii) comprises drying the cellulose fibers and curing the dried cellulose fibers.

30. The method of claim 28, wherein curing is carried out at a temperature of from about 150 ℃ to about 175 ℃.

31. Cellulose fibres produced by the process of any one of claims 23 to 29.

32. A method of making an absorbent structure comprising:

(a) preparing cellulose fibres by the process of any one of claims 23 to 29; and

(b) incorporating the above-described cellulose fibers into an absorbent structure.

33. Cellulosic fibers crosslinked with at least one crosslinking agent selected from the group consisting of saturated dicarboxylic acids, aromatic dicarboxylic acids, cycloalkyl dicarboxylic acids, difunctional monocarboxylic acids, and amine carboxylic acids, and having a median desorption pressure, as measured in a capillary absorption-desorption cycle, of 25cm or less.

34. The cellulosic fibers of claim 33 wherein the saturated dicarboxylic acid has 2 to 8 carbon atoms.

35. The cellulosic fibers of claim 34 wherein the saturated dicarboxylic acid has 2 to 6 carbon atoms.

36. The cellulose fibers of claim 35, wherein the saturated dicarboxylic acid has 2 to 4 carbon atoms.

37. The cellulosic fibers of claim 34, wherein the saturated dicarboxylic acid is selected from the group consisting of: oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, and any combination of any of the foregoing.

38. The cellulosic fibers of claim 33, wherein the saturated dicarboxylic acid is a saturated hydroxycarboxylic acid.

39. The cellulosic fibers of claim 38 wherein the saturated hydroxycarboxylic acid has 2 to 8 carbon atoms.

40. The cellulosic fibers of claim 39, wherein the hydroxy-saturated dicarboxylic acid is selected from the group consisting of: glycolic acid, tartaric acid, malic acid, glucaric acid, mucic acid, and any combination of any of the foregoing carboxylic acids.

41. The cellulosic fibers of claim 33, wherein the aromatic dicarboxylic acid has the formula:wherein R is¹、R²、R³And R⁴Independently of one another is hydrogen, hydroxy, C₁～C₄Alkoxy radical, C₁～C₄Alkyl, amino, halogen atom or nitro.

42. The cellulosic fibers of claim 41, wherein the aromatic dicarboxylic acid is phthalic acid.

43. The cellulosic fibers of claim 33, wherein the cycloalkyl dicarboxylic acid has the formula:wherein,

R⁶、R⁷、R¹⁰and R¹¹Independently of one another is hydrogen, hydroxy, halogen, C₁～C₄Alkoxy radical, C₁～C₄Alkyl, amino, or nitro; and

R⁸and R⁹Independently of one another is hydrogen, a halogen atom, C₁～C₄Alkoxy or C₁～C₄An alkyl group.

44. The cellulose fibers of claim 43, wherein the cycloalkyl dicarboxylic acid is 1, 2, 5, 6-tetrahydrophthalic acid.

45. The cellulosic fibers of claim 33, wherein the difunctional monocarboxylic acid is selected from the group consisting of: salts of haloacetic acids, hydroxy monocarboxylic acids, acid derivatives of hydroxy monocarboxylic acids, and any combination of any of the foregoing.

46. The cellulose fibers of claim 45, wherein the salt of the haloacetic acid is sodium chloroacetate.

47. The cellulose fibers of claim 33, wherein the amine carboxylic acid is an amino acid.

48. The cellulosic fibers of claim 47, wherein the amino acid has the formula:

H₂N-CH₂-R¹²-C (O) OH wherein R¹²Is a single bond, C₁～C₁₂Alkyl, or by carboxyl, hydroxy, C₁～C₄Alkoxy radical, C₁～C₄C substituted by one or more of alkyl, amino and nitro₁～C₁₂An alkyl group.

49. The cellulosic fibers of claim 47, wherein the amino acid has the formula:

wherein R is⁵Is linear or branched C₁～C₈An alkyl group.

50. The cellulosic fiber of claim 49 wherein R⁵Is C₂～C₄An alkyl group.

51. The cellulose fibers of claim 47, wherein the amino acid is selected from the group consisting of aspartic acid, glutamic acid, and any combination of any of the foregoing.

52. The cellulose fibers of claim 33, wherein the amine carboxylic acid is ethylene dinitrilotetraacetic acid.

53. The cellulose fibers of any one of claims 33 to 52, wherein the cellulose fibers are crosslinked with about 5 to about 21 mole percent of the crosslinking agent, based on the moles of cellulose glucoside.

54. The cellulosic fibers of any one of claims 33 to 53, wherein the cellulosic fibers are crosslinked in the presence of a crosslinking facilitator.

55. The cellulosic fibers of claim 54, wherein the crosslinking facilitator and the crosslinking agent are different.

56. The cellulosic fibers of claim 54 or 55, wherein the crosslinking facilitator is oxalic acid.

57. The cellulose fibers of any one of claims 54-56, wherein the cellulose fibers are crosslinked in the presence of about 1.8 to about 9 mole percent of a crosslinking facilitator, based on the moles of cellulose glucoside.

58. The cellulose fibers of any one of claims 54-57, wherein the cellulose fibers are crosslinked with from about 0.5 to about 40 mole percent, based on the moles of cellulose glucoside, of a crosslinking agent and a crosslinking promoter.

59. The cellulose fibers of any one of claims 54-58, wherein the cellulose fibers are crosslinked with from about 1 to about 30 mole percent, based on the moles of cellulose glucoside, of a crosslinking agent and a crosslinking promoter.

60. The cellulose fibers of any one of claims 33 to 59, wherein the cellulose fibers are derived from wood pulp.

61. The cellulose fibers of any one of claims 33-60, wherein the cellulose fibers are refined prior to crosslinking.

62. The cellulose fibers of claim 61, wherein the cellulose fibers are refined to a freeness of from about 300 to about 700ml CSF prior to crosslinking.

63. The cellulose fibers of claim 62, wherein the cellulose fibers are refined to a freeness of from about 500 to about 700ml CSF prior to crosslinking.

64. The cellulose fibers of claim 63, wherein the cellulose fibers are refined to a freeness of from about 650 to about 700ml CSF prior to crosslinking.

65. The cellulose fibers of any one of claims 33 to 64, wherein the cellulose fibers are cured at a temperature of about 105 ℃ to about 225 ℃.

66. The cellulose fibers of claim 65, wherein the cellulose fibers are cured at a temperature of from about 150 ℃ to about 190 ℃.

67. The cellulose fibers of claim 66, wherein the cellulose fibers are cured at a temperature of from about 160 ℃ to about 175 ℃.

68. The cellulose fibers of any one of claims 33-67, wherein the cellulose fibers are cured in the presence of a reducing agent.

69. The cellulose fibers of claim 68, wherein the reducing agent is hypophosphite.

70. The cellulose fibers of claim 69, wherein the reducing agent is sodium hypophosphite.

71. The cellulose fibers of any one of claims 33-70, wherein the cellulose fibers have a water retention value of 50% or less.

72. The cellulose fibers of claim 71, wherein the cellulose fibers have a water retention value of 45% or less.

73. The cellulose fibers of claim 72, wherein the cellulose fibers have a water retention value of 38% or less.

74. The cellulose fibers of claim 73, wherein the cellulose fibers have a water retention value of 30% or less.

75. The cellulose fibers of any one of claims 33-74, wherein the cellulose fibers have a median desorption pressure of 20cm or less as measured in a capillary absorption-desorption cycle.

76. The cellulose fibers of claim 75, wherein the median desorption pressure of the cellulose fibers, as measured in a capillary absorption-desorption cycle, is 18cm or less.

77. The cellulose fibers of claim 76, wherein the median desorption pressure of the cellulose fibers, as measured in a capillary absorption-desorption cycle, is 15cm or less.

78. The cellulosic fibers of any one of claims 33 to 77, wherein the crosslinking is substantially reversible.

79. The cellulosic fibers of any one of claims 33 to 78, wherein the crosslinking agent is oxalic acid and the crosslinking is substantially reversible.

80. Uncrosslinked cellulose fibers prepared by uncrosslinking the cellulose fibers of any one of claims 33-79.

81. The uncrosslinked cellulosic fibers of claim 80, wherein said crosslinking agent contains 4 or fewer carbon atoms.

82. The uncrosslinked cellulosic fiber of claim 81, wherein said crosslinking agent is oxalic acid.

83. The uncrosslinked cellulosic fibers of claim 81, wherein said crosslinking agent is sodium chloroacetate.

84. The uncrosslinked cellulosic fiber of any one of claims 80 to 83, wherein the step of uncrosslinking comprises soaking the cellulosic fiber in water.

85. The uncrosslinked cellulosic fibers of claim 84, wherein the step of decrosslinking comprises soaking the cellulosic fibers in water for about 0.5 to about 4 hours.

86. A sheet comprising the uncrosslinked cellulosic fiber of any one of claims 80 to 85.

87. An absorbent structure comprising the fiber of any of claims 33-79.

88. A process for the preparation of crosslinked cellulosic fibers which comprises intrafiber crosslinking of cellulosic fibers with at least one saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl dicarboxylic acid, difunctional monocarboxylic acid or amine carboxylic acid.

89. The method of claim 88, wherein the saturated dicarboxylic acid has 2 to 8 carbon atoms.

90. The method of claim 89, wherein said saturated dicarboxylic acid has 2 to 6 carbon atoms.

91. The method of claim 90, wherein the saturated dicarboxylic acid has 2 to 4 carbon atoms.

92. The method of claim 89, wherein the saturated dicarboxylic acid is selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, and any combination of any of the foregoing.

93. The method of claim 88, wherein the saturated dicarboxylic acid is a saturated hydroxycarboxylic acid.

94. The method of claim 93, wherein the saturated hydroxycarboxylic acid has 2 to 8 carbon atoms.

95. The method of claim 94, wherein, said C₂～C₈The hydroxy-saturated dicarboxylic acid is selected from the group consisting of glycolic acid, tartaric acid, malic acid, glucaric acid, mucic acid, and any combination of any of the foregoing.

96. The method of claim 88, wherein the aromatic dicarboxylic acid has the formula:wherein R is¹、R²、R³And R⁴Independently of one another is hydrogen, hydroxy, C₁～C₄Alkoxy radical, C₁～C₄Alkyl, amino, halogen atom or nitro.

97. The method of claim 96, wherein the aromatic dicarboxylic acid is phthalic acid.

98. The method of claim 88, wherein the cycloalkyl dicarboxylic acid has the formula:wherein,

R⁶、R⁷、R¹⁰and R¹¹Independently of one another is hydrogen, hydroxy, halogen, C₁～C₄Alkoxy radical, C₁～C₄Alkyl, amino, or nitro; and

R⁸and R⁹Independently of one another is hydrogen, a halogen atom, C₁～C₄Alkoxy or C₁～C₄An alkyl group.

99. The method of claim 98, wherein the cycloalkyl dicarboxylic acid is 1, 2, 5, 6-tetrahydrophthalic acid.

100. The method of claim 88, wherein the difunctional monocarboxylic acid is selected from the group consisting of: salts of haloacetic acids, hydroxy monocarboxylic acids, acid derivatives of hydroxy monocarboxylic acids, and any combination of any of the foregoing.

101. The method of claim 100, wherein the salt of a haloacetic acid is sodium chloroacetate.

102. The method of claim 88, wherein the amine carboxylic acid is an amino acid.

103. The method of claim 102, wherein the amino acid has the formula:

H₂N-CH₂-R¹²-C (O) OH wherein R¹²Is a single bond, C₁～C₁₂Alkyl, or by carboxyl, hydroxy, C₁～C₄Alkoxy radical, C₁～C₄C substituted by one or more of alkyl, amino and nitro₁～C₁₂An alkyl group.

104. The method of claim 102, wherein the amino acid has the formula:wherein R is⁵Is linear or branched C₁～C₈An alkyl group.

105. The method of claim 104, wherein R⁵Is C₂～C₄An alkyl group.

106. The method of claim 102, wherein said amino acid is selected from the group consisting of aspartic acid, glutamic acid, and any combination of any of the foregoing.

107. The method of claim 88, wherein the amine carboxylic acid is ethylene dinitrilotetraacetic acid.

108. The process of any one of claims 88 to 107 wherein the molar percentage of crosslinking agent is about 5 to about 21 mol% based on the moles of cellulose glucoside.

109. The process of any one of claims 88 to 108, wherein the crosslinking step is carried out in the presence of a crosslinking promoter.

110. The method of claim 109, wherein the crosslinking agent is different from the crosslinking facilitator.

111. The method of claim 109 or 110, wherein the crosslinking facilitator is oxalic acid.

112. The method of any of claims 109 to 111 wherein the mole percent of crosslinking facilitator is from about 1.8 to about 9 mole percent, calculated on a cellulose glucoside mole basis.

113. The process of any one of claims 109 to 112 wherein the mole percent of crosslinking agent and crosslinking facilitator is from about 0.05 to about 40, calculated on a cellulose glucoside mole basis.

114. The process of any one of claims 109 to 113 wherein the mole percent of crosslinking agent and crosslinking facilitator is from about 1 to about 30, calculated on a cellulose glucoside mole basis.

115. The method of any of claims 88 to 114, wherein the crosslinking step comprises:

(i) mixing cellulose fibers with a crosslinking agent; and

(ii) curing the cellulosic fibers in the mixture.

116. The method of any one of claims 88 to 115, wherein the crosslinking step comprises:

(i) mixing cellulose fibers with a crosslinking agent;

(ii) evacuating the cellulose fibers in the mixture; and

(iii) curing the cellulosic fibers in the mixture.

117. The method of claim 116, wherein step (iii) comprises drying the cellulosic fibers and curing the dried cellulosic fibers.

118. The process of any one of claims 115 to 117, wherein curing is carried out at a temperature of about 150 ℃ to about 175 ℃.

119. The cellulosic fiber of any of claims 88 to 118, wherein the fiber is crosslinked in the presence of a reducing agent.

120. The cellulose fibers of claim 119, wherein the reducing agent is hypophosphite.

121. The cellulose fibers of claim 120, wherein the reducing agent is sodium hypophosphite.

122. The method of any of claims 88 to 121, wherein the cellulose fibers are refined prior to the crosslinking step.

123. The method of claim 122, wherein the cellulose fibers are refined to a freeness of from about 500 to about 700ml CSF.

124. The method of claim 123, wherein the cellulose fibers are refined to a freeness of from about 650 to about 700ml CSF.

125. Cellulose fibres produced by the process of any one of claims 88 to 124.

126. A method of making a uncrosslinked fiber comprising the steps of: intrafiber crosslinking of cellulosic fibers with at least one saturated dicarboxylic acid, aromatic dicarboxylic acid, cycloalkyl dicarboxylic acid, difunctional monocarboxylic acid or amine carboxylic acid; and de-crosslinking the crosslinked cellulosic fibers.

127. The method of claim 126, wherein the crosslinking agent contains 4 or fewer carbon atoms.

128. The method of claim 127, wherein the crosslinking agent is oxalic acid.

129. The method of claim 127, wherein the crosslinking agent is sodium chloroacetate.

130. The method of any one of claims 126 to 129, wherein the step of decrosslinking comprises soaking the crosslinked cellulosic fibers in water.

131. The method of claim 130, wherein the step of decrosslinking comprises soaking the crosslinked cellulosic fibers in water for about 0.5 to about 4 hours.

132. A method of making a sheet of uncrosslinked cellulosic fibers comprising the steps of: preparing uncrosslinked cellulose fibers by the method of any one of claims 126 to 131, and processing the uncrosslinked cellulose fibers into a sheet.

133. A method of making crosslinked cellulosic fibers comprising the steps of:

(a) preparing a uncrosslinked cellulosic fiber by the method of any one of claims 126 to 131; and

(b) crosslinking the cellulose fibers.

134. A method of making an absorbent structure comprising:

(a) preparing cellulose fibers by the method of any one of claims 88 to 124 and 133; and

(b) incorporating the above-described cellulose fibers into an absorbent structure.

135. An absorbent core comprising superabsorbent polymer particles and reversibly crosslinked cellulosic fibers.

136. The absorbent core of claim 135 wherein the reversibly crosslinked cellulosic fibers are crosslinked with oxalic acid, sodium chloroacetate, or mixtures thereof.

137. The absorbent core of claim 136, wherein the reversibly crosslinked cellulosic fibers are crosslinked with oxalic acid.

138. An absorbent core according to any of claims 135 to 137, wherein the absorbent core comprises from about 30 to about 70 wt% of the high-absorbency particles and from about 70 to about 30 wt% of the reversibly crosslinked fibers, based on the total weight of the absorbent core taken as 100%.