MXPA96006405A - Individualized cellulose fibers, reticulated with polyacryl acid polymers - Google Patents

Abstract

The present invention relates to individualized crosslinked cellulosic fibers characterized in that said fibers have an effective amount of a polymeric polyacrylic acid crosslinking agent, which reacts with said fibers in the form of an intrafiber lattice bond, wherein said crosslinking agent is polyacrylic acid. polymer is selected from polyacrylic acid polymers, acrylic acid copolymers, and mixtures thereof.

Description

INDIVIDUALIZED CELLULOSE FIBERS. RETICULATED WITH POLYMER POLYMERS FIELD OF THE INVENTION The present invention relates to cellulosic fibers having high fluid absorption properties, absorbent structures made from said cellulosic fibers, and processes for making such fibers and structures. More specifically, the present invention relates to individualized crosslinked cellulosic fibers, to processes for making such fibers, and to absorbent structures containing cellulosic fibers that are in a reticulated, indivualized form.

BACKGROUND OF THE INVENTION Crosslinked fibers in substantially individualized form and various methods for making such fibers have been described in the art. The term "individualized, crosslinked fibers" refers to cellulosic fibers having primarily intrafiber crosslinking chemical bonds. That is, the crosslink bonds are primarily between the cellulose molecules of a single fiber, rather than between the separated fiber molecules. The individualized crosslinked fibers are generally considered to be useful in absorbent article applications. The fibers themselves and the absorbent structures containing individualized crosslinked fibers generally exhibit an improvement in at least one property of significant absorbency relative to conventional, non-crosslinked fibers. Often, the improvement in absorbency is reported in terms of the absorbent capacity. Additionally, the absorbent structures made of individualized crosslinked fibers generally exhibit increased moisture elasticity and increased dry elasticity relative to the absorbent structures made of uncrosslinked fibers. The term "elastic" should henceforth refer to the ability of pads made from cellulosic fibers to return to an expanded original seat by releasing a force of compression. Dry elasticity specifically refers to the ability of an absorbent structure to expand upon release of the applied compression force, while the fibers are in a substantially dry condition. The wet elasticity specifically refers to the ability of an absorbent structure to expand upon release of the applied compression force, while the fibers are in a wet condition. For the purposes of this invention and for consistency of the description, wet elasticity should be observed and reported for an absorbent structure moistened to saturation. In general, three categories of processes have been reported to produce reticulated, individualized fibers. These processes, described below, are referred to herein as dry cross-linking processes, cross-linking processes in aqueous solution, and cross-linking processes in substantially non-aqueous solution. Processes for making individualized, cross-linked fibers with dry technology are described in U.S. Patent No. 3,224,925, LJ Bernardin, issued December 21, 1965. Individualized cross-linked fibers are produced by impregnating the swollen fibers in a aqueous solution with crosslinking agent, drying and fiber defibering by mechanical action, and drying the fibers at elevated temperatures to effect crosslinking, while the fibers are in a substantially individual state. The fibers are inherently crosslinked in a collapsed, non-swollen state, as a result of being dehydrated prior to crosslinking. Processes as exemplified in U.S. Patent No. 3,224,926, where cross-linking is motivated to occur while the fibers are in a collapsed, non-swollen state, are referred to as processes for making "dry-crosslinked" fibers. " The dry crosslinked fibers are generally highly stiffened by the crosslink bonds, and structures made from these exhibit relatively high dry and wet elasticity. The dry-crosslinked fibers are further characterized by low fluid retention (VRF) values. The processes for producing the crosslinked fibers in aqueous solution are described, for example, in U.S. Patent No. 3,241,553, F.H. Steiger, granted on March 22, 1966. The individualized crosslinked fibers are produced by crosslinking the fibers in an aqueous solution containing a crosslinking agent and catalyst. The fibers produced in this way are hereinafter referred to as "crosslinked in aqueous solution" fibers. Due to the swelling effect of the water on the cellulose fibers, the fibers crosslinked in aqueous solution are crosslinked even in a swollen, non-collapsed state. In relation to the dry crosslinked fibers, the crosslinking fibers in aqueous solution as described in U.S. Patent No. 3,241,553 have much greater flexibility and lower stiffness, and are characterized by high fluid retention values (VRF). . Absorbent structures made from cross-linked fibers in aqueous solution exhibit lower dry and wet elasticity than structures made from dry-crosslinked fibers.

In U.S. Patent No. 4,035,147, Sangenis et al., Issued July 12, 1977, describes a method for producing individualized, crosslinked fibers by contacting dehydrated, non-swollen fibers with crosslinking agent and catalyst. in a substantially non-aqueous solution containing an insufficient amount of water to cause the fibers to swell. The crosslinking occurs while the fibers are in this substantially non-aqueous solution. This type of process in the following must be referred to as a non-aqueous solution crosslinking process; "And the fibers produced should be referred to as cross-linked fibers in non-aqueous solution. The non-aqueous solution crosslinking fibers described in US Patent No. 4, 035,147 do not yet swell upon contact with solutions known to those skilled in the art as swelling reagents. Like the dry-crosslinked fibers, they are highly stiffened by crosslinking bonds, and absorbent structures thereof can be made which exhibit relatively high dry and wet elasticity. The crosslinked fibers as described above are believed to be useful for uses in absorbent products such as diapers and catamenial products. However, such fibers have not provided sufficient absorbency benefits, in view of their detriments and costs, over conventional fibers resulting in significant commercial success. The commercial request for the crosslinked fibers have also suffered due to safety issues. The most widely reported crosslinking agents in the literature are formaldehyde and formaldehyde addition products known as N-methylol or N-methylolamide agents, which, unfortunately, cause irritation to human skin and have been associated with other issues of concern. human security The removal of free formdehyde lowers the levels in the product sufficiently to avoid irritation of the skin and other safety issues in humans, which has been hampered by technical and economic barriers. As mentioned above, the use of formaldehyde and various formaldehyde addition products to crosslink cellulosic fibers is known in the art. See, for example, United States Patent No. 3,224,926 Bernardin, issued December 21, 1965; U.S. Patent No. 3,241,553 Steiger, issued March 22, 1966; U.S. Patent No. 3,932,209, Chatterjee, issued January 13, 1976; U.S. Patent No. 4,035,147, Sangenis et al. issued July 12, 1977 and U.S. Patent No. 3756,913, Wodka expedia on September 4, 1973. Unfortunately, the irritation effect of the Formaldehyde vapor over eyes and skin is a marked disadvantage of such references. A need is evident for crosslinking agents of cellulose fibers that do not require formaldehyde or its unstable derivatives. Other references describe the use of dialdehyde crosslinking agents. See, for example, U.S. Patent No. 4,689,118, Makoui et al., Issued August 25, 1987; and U.S. Patent No. 4,822,453 Dean et al., issued April 18, 1989. The reference by Dean et al. describes absorbent structures containing individualized crosslinked fibers wherein the crosslinking agent of the group consisting of dialdehydes is selected. of C2-C8, with glutaraldehyde which is preferred. These references appear to overcome many of the disadvantages associated with formaldehyde and / or formaldehyde addition products. However, the cost associated with the production of crosslinked fibers with dialdehyde crosslinking agents, such as glutaldehyde, may be too high to result in significant commercial success. Therefore, there is a need to find cellulosic fiber crosslinking agents that are both safe to use on human skin as well as commercially viable. The use of polycarboxylic acids to impart wrinkle resistance to cotton fabrics is known in the art. See, for example, United States Patent No. -. 3,526,048 Roland et al., Issued September l, 1970; U.S. Patent No. 2,971,815 Bulloc et al., issued February 14, 1961, and U.S. Pat. 4,820,307 Welch et al., Issued April 11, 1989. All of these references pertain to the treatment of textile cotton fabrics with polycarboxylic acids and specific curing catalysts to improve the wrinkle resistance and durability properties of the treated fabrics. The use of specific monomeric polycarboxylic acids for crosslinking cellulosic fibers is known in the art. For example, U.S. Patent 5,137537, Heron et al. Issued August 11, 1992, discloses absorbent structures containing individualized cellulosic fibers crosslinked with a C2-C9 polycarboxylic acid. The ester crosslink bonds formed by the polycarboxylic acid crosslinking agents are different from the crosslink bonds resulting from the mono- and dialdehyde crosslinking agents, which form crosslinked acetal bonds. Importantly, the C2-C9 polycarboxylic acids described for use in U.S. Patent No. 5,137,537, are non-toxic, other than formaldehyde and formaldehyde addition products commonly used in the art. Unfortunately, the preferred C2-C9 crosslinking agent, citric acid, can cause discoloration (i.e., yellowing) of the white cellulosic fibers. In addition, the unpleasant odor may also be associated with the use of alpha-hydroxy carboxylic acids such as citric acid. In addition, relatively low pH is required to promote more efficient crosslinking. These low pHs increase the capital cost of the process by increasing the cost of equipment that can withstand acid corrosion at low pHs. Also, it is known that citric acid is not stable at temperatures that promote the highest crosslinking efficiency. It is believed that the acid derivatives of the citric acid decomposition do not form intrafiber crosslinks as efficiently as the polymeric polyacrylic crosslinking agents. The applicant has found that the polymeric polyacrylic crosslinking agents described hereinafter particularly suitable for forming ester crosslinking bonds with the cellulosic fibers. Importantly, the ester crosslinked fibers tend to be brighter than those crosslinked with alpha-hydroxy acids such as citric acid. In addition, the polymeric polyacrylic crosslinking agents are stable at higher temperatures, thereby promoting crosslinking more efficiently. In addition, the absorbent structures made from these individualized cellulose fibers crosslinked with a polymeric polyacrylic acid exhibit increased dry elasticity and wet elasticity, and improve the response to moisture in relation to structures containing non-crosslinked fibers. It is an object of the present invention to provide individualized fibers crosslinked with a crosslinking agent of polyacrylic acid and absorbent structures made of such fibers, wherein the absorbent structure made of crosslinked fibers has higher levels of absorbent capacity in relation to absorbent structures made of non-crosslinked fibers, and exhibit higher moisture elasticity and higher dry elasticity than structures made of non-crosslinked fibers. It is also an object of this invention to provide individualized crosslinked fibers with a crosslinking agent of polyacrylic acid and absorbent structures made of said fibers, as described above, which have a superior balance of absorbency properties. It is further an object of this invention to provide crosslinked, individualized, commercially viable fibers and absorbent structures made from said fibers, as described above, which can be used safely in proximity to the human skin. It is additionally an object of the present invention to provide individualized crosslinked fibers exhibiting a higher level of brilliance in relation to the previously known crosslinked fibers.

It is another object of this invention to provide absorbent structures having improved absorbent and impregnation capacity which, in current use, provide high levels of dryness of the wearer's skin.

BRIEF DESCRIPTION OF THE INVENTION It has been found that the objects identified above can be accomplished by the individualized crosslinked fibers, and by the incorporation of these fibers into the absorbent structures, as described herein. In general, these objects and other benefits are achieved by the individualized crosslinked fibers having an effective amount of a polyacrylic acid crosslinking agent, preferably between about 1% by weight and about 10% by weight, more preferably between about 3% by weight and approximately 7% by weight of crosslinking agent, calculated on a dry fiber weight basis, which reacts with the fibers in the form of intrafiber crosslink bonds. The polyacrylic acid crosslinking agent is preferably selected from the group consisting of polyacrylic acid polymers, acrylic acid copolymers and mixtures thereof. Particularly preferred crosslinking agents include copolymers of acrylic acid and maleic acid. Other preferred polyacrylic acid crosslinking agents include low molecular weight monoalkyl substituted phosphonate and phosphinate copolymers, described in U.S. Patent 5,256,746 Blankensip et al., Issued October 26, 1993, and incorporated herein by reference. reference. The copolymers and polyacrylic acid polymers used herein may also be mixed with monomeric carboxylic acids, such as citric acid. The crosslinking agent is reacted with the fibers in an intrafiber crosslinking form. Said fibers, which are characterized by having water retention values (VRA) of from about 25 to about 60, have been found to meet the identified objects related to the individualized, crosslinked fibers, and which provides a good, unexpected absorbent capacity. in applications of absorbent structures. The individualized crosslinked fibers are, without limiting the scope of the invention, preferably formed into compressed absorbent structures that expand upon wetting. The absorbent structures may additionally contain hydrogel-forming material. Significantly, improved skin dryness and absorbent capacity can be obtained, and dry skin on the user's skin with the use of hydrogel-forming materials with individualized cross-linked fibers. Significantly, improved absorbent capacity and impregnation are obtained by using individualized crosslinked fibers with hydrogel-forming material relative to conventional, non-crosslinked cellulose fibers with hydrogel-forming material. Surprisingly, such improved results can be obtained by following the use of lower levels of hydrogel-forming material, calculated on a weight basis, for pads containing individualized cross-linked fibers compared to conventional cellulosic fiber pads.

DETAILED DESCRIPTION OF THE INVENTION The cellulosic fibers used for the present invention will normally be derived from sources of wood pulp. Useful wood pulps include chemical pulps such as Kraft, sulphate and sulphite pulps also known as mechanical pulps including, for example, wood residues, thermomechanical pulps and chemically modified thermomechanical pulps (PQTM). Digested fibers of softwood or hardwood are preferably used. Other fibers of cellulosic fibrous pulp, such as esparto grass, bagasse, kemp, flax, cotton waste, and other sources of cellulosic fibers and lignaceas can also be used., as raw material in the invention. The fibers can be supplied in the form of a suspension, not laminated, or in a laminated form. The fibers supplied as wet overlays, superposed in dry or other laminated form, are preferably made in an unstable form by mechanically disintegrating the sheet, typically after it contacts the fibers with the crosslinking agent. Most preferably, the fibers are supplied as dry superimposed. In the case of dry overlap, it is advantageous to apply the appropriate amount of crosslinking agent before mechanical disintegration in order to minimize damage to the fibers. The optimum fiber source used in combination with this invention will depend on the particular end use contemplated. Generally, wood pulp fibers made by chemical pulping processes (e.g., Kraft, sulfite or sulfate) are preferred. The fibers can be completely bleached, partially bleached or unbleached. It may be desirable to use bleached pulp for its superior brightness and consumer appeal. Chlorine-based bleaching processes can also be used, as well as chlorine-free bleaching processes (eg, oxygen-based). For products such as paper towels and absorbent cores for diapers, sanitary napkins, catamenial products, and other similar absorbent paper products, it is especially preferred to use Southern softwood pulp fibers due to their superior absorbency characteristics.

As used herein, the terms "polyacrylic acid polymers" and "polymeric polyacrylic acid" refer to polymerized acrylic acid (ie, polyacrylic acid), as well as to copolymers of acrylic acid, including, but not limited to, , polyacrylic acid and maleic acid copolymers, and the monoalkyl-substituted low molecular weight phosphinate and phosphinate copolymers described below, and mixtures thereof. The Applicant has found that the crosslinking agents applicable to the present invention include polyacrylic acid polymers, acrylic acid copolymers, and mixtures thereof. Particularly preferred polyacrylic acid crosslinking agents include copolymers of polyacrylic acid and maleic acid, and the low molecular weight monoalkyl substituted phosphinate and phosphinate copolymers described in U.S. Patent No. 5,256,746, Blankenship et al. October 26, 1993, and incorporated herein by reference. These polymers are preferred for their ability to crosslink individualized cellulosic fibers as described in this invention and their non-negative effects on the brightness of cellulose, when used in the successive cross-linking processes described. In particular, polyacrylic acid polymers suitable for use in the present invention have molecular weights on the scale from about 500 to 40,000, preferably, molecular weights of from about 1,000 to 20,000. The polyacrylic acid polymers are made by the polymerization of acrylic acid. CH2 = CH I c = o OH to form the repetitive chain CH, CH CH, CH C = 0 C = 0 OM OM wherein M is an alkali metal of ammonium or hydrogen ion. Polymers of this type useful in the present invention are available from Rohm and Haas Company. Other polymers that are applicable to this invention are the copolymers of polyacrylic acid and maleic acid. Preferably, the molecular weights of these copolymers range from 500 to 40,000, more preferably from about 1,000 to about 20,000. The weight ratio of acrylic acid to maleic acid can vary from about 10: 1 to about 1: 1, more preferably from about 5: 1 to 1.5: 1. A particularly preferred copolymer contains about 65% by weight of acrylic acid and 35% by weight of maleic acid. Another group of acrylic acid copolymers, which are applicable to this invention, are low molecular weight monoalkyl substituted phosphonate and phosphinate copolymers, described in U.S. Patent No. 5,256,746, Blankenship et al., Issued October 26, 1993, and which is incorporated herein by reference. The copolymers described in U.S. Patent No. 5,256,746 are especially preferred, because they provide fibers with high levels of absorbency, elasticity and brilliance, and are safe and do not irritate human skin. These copolymers are prepared with hypophosphorous acid and its salts (commonly sodium hypophosphite) and / or phosphoric acid as chain transfer agents. The molecular weights of these types of copolymers are preferably below 20,000, and more preferably, below 3,000, and more preferably between about 1,000 and 2,000. The polyacrylic acid polymers and copolymers described above can be used alone or in combination with other polycarboxylic acids such as citric acid. Those skilled in the art of polymeric polyacrylic acids will recognize that the polymeric crosslinking agents of polyacrylic acids described above can be present in a variety of forms, such as the free acid form, salts and the like. Although the free acid form is preferred, all forms must be included within the scope of this invention. The reticulated fibers, individualized, of the present invention have an effective amount of polymeric polyacrylic acid crosslinking agent that reacts with the fibers in the form of intrafiber crosslink bonds. As used herein "effective amount of crosslinking agents" refers to a sufficient amount of crosslinking agent to provide an improvement in at least one property of significant absorbency of the fibers by themselves and / or absorbent structures containing the fibers of cross-linking, individualized. An example of a property of significant absorbency is the runoff capacity, which is a combined measure of the fluid absorbent capacity and the absorbency rate of the fluid of the absorbent structure. A detailed description of the method for determining the runoff capacity is provided hereinafter. In particular, unexpected good results are obtained for absorbent pads made of individualized, crosslinked fibers, having between about 1.0% by weight and about 10.0% by weight, more preferably between about 3.0% by weight and about 7.0% by weight, most preferably between 4.0% by weight and 6.0% by weight of crosslinking agent, calculated on a dry fiber basis, which reacts with the fibers. Preferably, the crosslinking agent is contacted with the fibers in a liquid medium, under such conditions that the crosslinking agent penetrates into the individual fiber structures. However, other methods of treatment with crosslinking agent, including spraying or spraying and pressing, draining and pressing, etc. of the fibers, although the sponge, individualized, or laminated form are also within the scope of the invention. The applicant has discovered that the crosslinking reaction can be achieved at practical speeds without a catalyst, provided that the pH is maintained within a particular scale (which is discussed in more detail below). This is contrary to the prior art which teaches that specific catalysts are needed to provide sufficient fast esterification and crosslinking of the cellulose fibers by polycarboxylic acid crosslinking agents which is commercially feasible. See, for example, U.S. Patent 4,820,307 Welch et al., Issued April 11, 1989. However, if desired, fibers can also be contacted with an appropriate catalyst prior to crosslinking. . The Applicant has found that the type, amount and method of contacting the catalyst to the fibers will be dependent on the particular process of crosslinking practiced. These variables will be discussed in more detail later. One skilled in the art would appreciate that residues of the catalysts may be present from the polymerization processes such as those described in U.S. Patent No. 5,256,746. Once the fibers have been treated with the crosslinking agent (and catalyst if one is used), the crosslinking agent causes the reaction with the fibers in the substantial absence of interfiber bonds, i.e. while interfiber contact is maintained. a low degree of occurrence in relation to the non-fluffed pulp fibers, or the fibers are immersed in a solution that does not facilitate the formation of bonding of the fibers, especially hydrogen bonds. This results in the formation of crosslink bonds that are intrafiber in nature. Under these conditions, the crosslinking agent reacts to form crosslink bonds between the hydroxyl groups of a single cellulose chain or between hydroxyl chain groups of closely located celluloses of a single cellulose fiber. Although it is not intended or intended to limit the scope of the invention, it is believed that the carboxylic groups in the polymeric polyacrylic acid crosslinking agent react with the hydroxyl groups of the cellulose to form ester linkages. The formation of ester bonds, it is believed to be the desirable type of bond that provides stable crosslink links, being favored under acidic reaction conditions. Therefore, acidic crosslinking conditions, i.e., pH scales of about 1.5 to about 5, are preferred for the purposes of this invention. The fibers are preferably mechanically defibrated into a fibrous, individualized, low density form known as "foamed" prior to the reaction of the crosslinking agent with the fibers. Mechanical defibration can be performed by a variety of methods, which are now known in the art or which may be known hereinafter. Preferably, the mechanical defibration is performed by a method where knot formation and damage to the fiber are minimized. One type of device that has been found to be particularly useful for defibrating cellulosic fibers is the three-stage foaming device described in U.S. Patent 3,987,968 issued to DR Moore and 0. A. Shileds on October 26, 1976 , said patent being hereby expressly incorporated by reference in this disclosure. The foaming device described in US Pat. No. 3,987,968 subjects the wet cellulosic pulp fibers to a combination of mechanical impact, mechanical agitation, agitation with air and a limited amount of air drying to create substantially a lint. The individualized fibers have thereby imparted an increased degree of kinking and twisting in relation to the amount of kink and twist naturally present in said fibers. It is believed that this additional twisting and curling increases the elastic character of the absorbent structures made from reticulated, finished fibers. Other applicable methods for defibrating the cellulosic fibers include, but are not limited to, treatment with a Waring blender and tangentially contacting the fibers with a rotary disc refiner, hammer mill or wire brush. Preferably, a stream of air is directed to the fibers during said defibration to assist in the separation of the fibers in substantially individual fashion. In spite of the particular mechanical device used to form the fluff, the fibers are preferably mechanically treated, although initially containing at least about 20% moisture, more preferably containing between about 20% and about 60% moisture. The mechanical refining of high consistency fibers or partially dried fibers can also be used to provide curling or twisting to the fibers in addition to the curling or twisting imparted as a result of mechanical defibration. The fibers made according to the present invention have unique combinations of stiffness and elasticity which allow the structures made of the fibers to maintain high levels of absorbent capacity, and exhibiting high levels of elasticity and an expansionary response to the wetting of a compressed absorbent structure. , dry. In addition to having the levels of crosslinking within the established scales, the crosslinked fibers are characterized by having water retention values (VRA) of less than about 60, more preferably between about 25 to about 50, and most preferably between about 30 and about 45, for paper fibers, chemically pulped, conventional. The VRA of a particular fiber is indicative of the level of crosslinking for a particular chemistry and crosslinking method. Very highly crosslinked fibers, such as those produced by many of the crosslinking processes known in the prior art discussed above, have found that VRA of less than about 25, and generally less than about 20. The particular crosslinking process used, of course, will affect the VRA of the reticulated fiber. However, any process that will result in levels of crosslinking and VRA within the established limits is believed to be, and is intended to be within the scope of the invention. Applicable crosslinking methods include dry crosslinking processes and non-aqueous solution crosslinking processes as generally discussed in the background of the invention. Certain non-aqueous solution and dry crosslinking processes preferred for preparing the individualized crosslinked fibers of the present invention will be discussed in more detail below. Cross-linking processes in aqueous solution where the solution causes the fibers to become highly swollen, will result in fibers having VRAs that are in excess of about 60. These fibers will provide insufficient elasticity and rigidity for the purposes of the present invention. Specifically referring to the processes of dry crosslinking, the individualized crosslinked fibers can be produced from said process by providing a quantity of cellulosic fibers, contacting the fibers with a type of crosslinking agent amount as described above, separating mechanically, for example by shredding the fibers in substantially individual form, and drying the fibers and causing the crosslinking agent to react with the fibers in the presence of a catalyst, if desired, to form lattice bonds while keeping the fibers in substantial form. individual. The defibration stage, separate from the drying step, is believed to impart additional curling. The subsequent drying is accompanied by the twisting of the fibers, with the degree of twisting that is improved by the curled geometry of the fiber. As used herein, "curl" of fiber refers to a geometric curvature of the fiber about the longitudinal axis of the fiber. "Twist refers to a rotation of the fiber around the perpendicular cross section of the longitudinal axis of the fiber." The fibers of the preferred embodiment of the present invention are crosslinked, individualized, in the form of intrafiber bonding, and are highly crimped and twisted. As used herein, the term "torsion count" refers to the number of torsion nodes present at a certain length of the fiber. Torsion counting is used as a measure of the degree to which a fiber is rotated about its longitudinal axis. The term "torsion node" refers to a substantially axial rotation of 180a about the longitudinal axis of the fiber, where a part of the fiber (ie "node") appears dark relative to the rest of the fiber when viewed under a microscope with transmitted light. The distance between the nodes corresponds to an axial rotation of 180fl. Those skilled in the art will recognize that the occurrence of a torsion node as described above is primarily visual rather than a physical phenomenon. However, the number of torsion nodes in a certain length of the fibers (i.e., the torsion count) is directly indicative of the fiber torsion degree, which is a physical parameter of the fiber. The appearance and quantity of the torsion nodes will vary depending on whether the fiber is a summer wood fiber or a spring wood fiber. The torsional knots and the total torsion count are determined by a Torque Counting Image Analysis Method, which is described in the Experimental Method section of the description. The average torsion count referred to in the description of the fibers of the present invention is determined in an appropriate manner by the aforementioned twist count method. When counting torsion nodes, the fiber portions darken due to fiber damage or fiber compression must be distinguished from the portions of the fibers that appear obscured due to fiber twisting. The current twist count of any fiber sample will vary depending on the ratio of the spring wood fibers to the summer wood fibers. The twist count of any of the summer wood fibers or of particular spring wood fibers will also vary from fiber to fiber. Notwithstanding the foregoing, the limitations of the average twist count are useful in defining the present invention, and those limitations apply despite the particular combination of summer wood fibers and spring wood fibers. That is, any fiber mass having a torsion count encompassed by the limitations of the established torsion count means that they are encompassed within the scope of the present invention while satisfying the other claimed limitations. In the measurement of the torsion count for a sample of fibers, it is important that a significant amount of fibers be examined in order to correctly represent the average level of torsion of the individual, variable fiber torsion levels. It is suggested that at least 12.7 centimeters of accumulated fiber length of a representative sample of a fiber mass be tested in order to provide a representative count of fiber torsion. The wet fiber torsion count is described and measured analogously to the dry fiber torsion count, said method varying only in that the fiber is wetted with water before being treated and then the still wet torsion nodes are counted according to the Method of Image Analysis of Torsion Counts. In addition to being twisted, the fibers of the present invention are crimped, the curl of the fiber can be described as a short fraction due to twists, twists and / or bends in the fiber. For the purposes of this description, the fiber curl should be measured in terms of a two-dimensional field. The level of the fiber curl should be referred to in terms of a fiber curl factor. The fiber curl factor, a two-dimensional measure of the curl, is determined by looking at the fiber in a two-dimensional plane, measuring the projected length of the fiber as the longest dimension of a rectangle covering the fiber, LR, and the length current of the LA fiber, and then the fiber curl factor is calculated from the following equation: (1) curl factor = (LA / LR) - 1 An Image Index analysis method is used fibers to measure LR and LA. This method is described in the Experimental Methods section of this description. It is the background information for this method is described in the International Paper Physics Conference Symposium 1979, Harrison Hotel, Harrison Hot Springs, British Columbia, September 17-19, 1979, in a paper entitled "Image Analysis Application for the Characterization of the Fiber Pulp ": Part 1, by BD Jordán and DH Page, pp. 104 to 114, Canadian Pulp and Paper Association (Montreal, Quebec, Canada), said reference being incorporated in this description by reference. By keeping the fibers in substantially individual form during drying and crosslinking, the fibers are allowed to twist during drying and thereby to be crosslinked in said crimped, twisted state. By drying the fibers under such conditions such that the fibers can twist and curl, it is referred to as the drying of the fibers under substantially unrestricted conditions. On the other hand, the fibers dried in laminated form result in dried fibers that are not as highly twisted and curled as the fibers dried in substantially individualized form. It is believed that the interfiber hydrogen bond "restricts" the relative occurrence of twisting and crimping the fiber. There are several methods by which the fibers can be contacted with the crosslinking agent and the catalyst (if a catalyst is used). In one embodiment, the fibers are contacted with a solution that initially contains both the crosslinking agent and the catalyst. In another embodiment, the fibers are contacted with an aqueous solution of crosslinking agent and allowed to soak before the addition of the catalyst. The catalyst is subsequently added. In a third modality, the crosslinking agent and the catalyst are added to an aqueous suspension of the cellulosic fibers. Other methods in addition to those described herein will be apparent to those skilled in the art, and are intended to be included within the scope of the invention. In spite of the particular method by which the fibers are contacted with the crosslinking agent and the catalyst (if a catalyst is used), the cellulosic fibers, the crosslinking agent and the catalyst are preferably mixed and / or allowed to soak. sufficiently with the fibers to ensure complete contact with and impregnation of the individual fibers. The Applicant has discovered that the crosslinking reaction can be carried out without the use of a catalyst, if the pH of the solution containing the crosslinking agent is maintained within the ranges specified below. In particular, the aqueous portion of the cellulosic fiber suspension or the crosslinking agent solution should be adjusted to a target pH of between about pH 1.5 to about pH 5, more preferably between about pH 2 and about pH 4.5 , during the period of contact between the crosslinking agent and the fibers. The pH of the crosslinking agent solution can be adjusted with either the addition of an acid, for example hydrochloric acid or a base, for example, sodium hydroxide as appropriate. Notwithstanding the foregoing, in general any substance that can catalyze the crosslinking mechanism can be used. Applicable catalysts include alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphates, alkali metal phosphonates, and alkali metal sulfates. Especially preferred catalysts are alkali metal hypophosphites, alkali metal phosphonates and alkali metal sulfates. A more complete list of catalysts useful herein can be found in U.S. Patent No. 4,820,307, Welch et al., Issued April 11, 1989, incorporated herein by reference. The selected catalyst can be used as the single catalyst agent, or in combination with one or more other catalysts, or in combination with the same polymer as in U.S. Patent No. 5,256,746, Blankenship et al., Issued on October 26, 1993, incorporated herein by reference. The amount of catalyst preferably used is, of course, dependent on the particular type and amount of crosslinking agent and the reaction conditions, especially temperature and pH. In general, based on technical and economic considerations, catalyst levels of between about 5% by weight and about 80% by weight, based on the weight of the crosslinking agent added to the cellulosic fibers, are preferred. It is further desirable to adjust the aqueous portion of the cellulosic fiber suspension or the crosslinking agent solution to a target pH of between about pH of 1.5 and about pH of 5, more preferably between about pH of 2 and about pH of 4.5, during the period of contact between the crosslinking agent and the fibers.

The cellulosic fibers should be generally dried and optionally dried. The optimal and operable consistencies will vary depending on the type of fluffing equipment used. In the preferred embodiments, the cellulosic fibers are dried and optimally dried to a consistency of between about 20% and about 80%. More preferably, the fibers are dried and dried to a consistency level of between about 40% and about 80%. Drying the fibers within these scales will generally facilitate the defibration of the fibers in an individualized manner without excessive knot formation associated with high levels of moisture and without high levels of fiber damage associated with low moisture levels. For example purposes, desiccation can be achieved by such methods as mechanical pressing, centrifuging, or pulp air drying. Further drying of the fibers within the 40% -80% consistency scale previously described is optional, however, it is preferably carried out by a method known in the art as air drying, under conditions such that the use is not required. of elevated temperatures for an extended period of time. Excessively high temperatures and time at this stage can result in drying of the fibers beyond 60% consistency, thereby possibly causing excessive fiber damage during the followed defibration step. After drying, the fibers are then mechanically defibrated as previously described. The defibrated fibers are then dried to between about 60% and 100% consistency by a method known in the art as instant drying. This stage imparts additional curl and twist to the fibers as the water is removed from them. Although the amount of water removed by this additional drying step can be varied, it is believed that instant drying for very high consistencies provides a higher level of curl and twist of the fibers than instant drying to a consistency in the smaller part of the 60% scale - 100%. In the preferred embodiments, the fibers are dried to approximately 90% -95% consistency. It is believed that this level of instant drying provides the desired level of curl and twist of the fiber without requiring higher retention times and instant drying temperatures to achieve 100% consistency. Instantaneous drying of the fibers to a consistency, such as 90% -95%, in the highest portion of the 60% -100% scale also reduces the amount of drying that must be accompanied in the curing stage followed by instant drying . Instantly dried fibers are then heated to a suitable temperature for an effective period of time to cause the crosslinking agent to cure, i.e., to react with the cellulosic fibers. The speed and degree of crosslinking depends on the fiber dryness, temperature, pH, time, amount and type of catalyst and crosslinking agent, and the method used to heat and / or dry the fibers while crosslinking is performed. Cross-linking, at a particular temperature, will occur more quickly for fibers with a certain initial moisture content when carried out by a continuous, air-pass dryer than when subjected to drying / heating in a static oven. Those skilled in the art will recognize that there are a number of temperature-time relationships for drying and curing the crosslinking agent. Drying temperatures of about 145 ° C to about 165 ° C for periods of between about 30 minutes and 60 minutes, under static atmospheric conditions will generally provide acceptable cure efficiencies for fibers having moisture contents of less than about 10%. Those skilled in the art will also appreciate that higher temperatures and forced air convection decrease the time required for curing. In this way, drying temperatures from about 170 ° C to about 190 ° C for periods of between about 2 minutes and 20 minutes, in an air-pass oven, will generally also provide acceptable cure efficiencies for fibers that have lower moisture contents from 10%. Curing temperatures should be maintained at less than about 225 ° C, preferably less than about 200 ° C, because the exposure of the fibers to such high temperatures can result in darkening or other damage to the fibers. Following the crosslinking step, the fibers are washed, if desired. After washing, the fibers are defluidized and dried. The fibers, although still in wet condition, can be subjected to a second stage of mechanical defibration which causes the crosslinked fibers to twist and curl between the defluidization and drying stages. The same apparatuses and methods previously described for defibrating the fibers are applicable to this second stage of mechanical defibration. As used in this paragraph, the term "defibration" refers to any of the methods that can be used to mechanically separate the fibers into the substantially individual form, although the fibers may already be provided in that form. "Defibration" therefore refers to the step of mechanically treating the fibers, in any indvidual form or in a more compacted form, wherein said mechanical treatment step a) separates the fibers into the substantially individual form, if they had not already in that form, and b) imparts twist and curl to the fibers on drying. This second defibration treatment, after the fibers have been crosslinked, is believed to increase the twisted and curled character of the pulp. This increase in the curled, twisted configuration of the fibers induces to improve the response and elasticity of the absorbent structure to wetting. The maximum level of crosslinking will be achieved when the fibers are essentially dry (having less than about 5% moisture). Due to this absence of water, the fibers are crosslinked even in a collapsed, substantially non-swollen state. Consequently, they characteristically have low VRF fluid retention values in relation to the scale applicable for this invention. The VRF refers to the amount of fluid calculated on a dry fiber basis, which remains absorbed by a sample of fiber that has been soaked and then centrifuged to remove the interfiber fluid. (The VRFs are also defined from the procedure to determine the VRF described below). The amount of fluids that the crosslinked fibers can absorb depends on their capacity to swell when saturation occurs or, in other words, on their volume or inner diameter when inflating at a maximum level. This, in turn, is dependent on the level of cross-linking. As the level of intrafiber crosslinking increases for a given process and fiber, the VRF of the fibers will increase. In this way, the VRF value of a fiber is structurally descriptive of the physical condition of the fiber in saturation. Unless otherwise stated, the VRF data described here should be reported in terms of the water retention value (VRA) of the fibers. Other fluids, such as salt water and synthetic urine, can also be advantageously used as a fluid medium for analysis. Generally, the VRF of a particular fiber crosslinked by processes where the jury is largely dependent on drying, such as the present process, will be primarily dependent on the crosslinking agent and the level of crosslinking. The VRA of the fibers crosslinked by this dry crosslinking process at crosslinking agent levels applicable to this invention are generally less than about 60, greater than about 25., preferably less than about 50, and more preferably between about 30 and about 45. Bleached SSK fibers having between about 4% by weight and about 6% by weight of polyacrylic acid reacted therein, calculated on a basis by weight fiber dried, has been observed to have VRA respectively ranging from about 25 to about 50. The post-crosslinking treatment of the fibers such as the degree of bleaching and the practice of the post-crosslinking bleaching stages have been found to affect the VRA. The Southern Softwood Kraft fibers (SSK) prepared by any of the crosslinking processes known in the state of the art have higher crosslinking levels than those described herein, and have VRAs less than about 25. Such fibers, as previously discussed, they have been observed to exceed stiffness and require less absorbent capacity than the fibers of the present invention. In another process for making individualized crosslinked fibers by a dry crosslinking process, the cellulosic fibers are contacted with a solution containing a crosslinking agent as described above. Either before or after being contacted with the crosslinking agent, the fibers are provided in a sheet form. The fibers, although in the laminated form, are dried and preferably to be crosslinked by heating the fibers to a temperature of between about 1202 and about 1602. Subsequent to crosslinking, the fibers are mechanically separated into substantially individual form. This is preferably accomplished by treatment with a fiber foaming apparatus as described in U.S. Patent No. 3,987,968 or can be performed with other methods for defibrating the fibers as may be known in the art. The individualized crosslinked fibers made in accordance with this sheet crosslinking process are treated with a sufficient amount of crosslinking agent such that an effective amount of crosslinking agent, preferably between about 4% by weight and about 6% by weight of the crosslinking agent. crosslinking agent, calculated on the basis of the weight of the dry fiber and measured subsequent to the defibration are reacted with the fibers in the form of intrafiber crosslink bonds. Another effect of drying and crosslinking the fibers still in sheet form is that the fiber bond restricts the twist and curl fibers with increased drying. Compared to individualized, crosslinked fibers, made according to the process where the fibers are dried under substantially unrestricted conditions and subsequent crosslinking in absorbent structures, the curly, twisted configuration, which they contain to the relatively undistorted fibers made by a process of Leaf curing described above would be expected to exhibit elasticity in lower numbers and minor responses to wetting. It is also contemplated to mechanically separate the fibers in substantially individual form between the drying and crosslinking steps. That is, the fibers are brought into contact with the crosslinking agent and subsequently dried even in the sheet form. Before crosslinking, the fibers are individualized to facilitate intrafiber crosslinking. This alternate method of crosslinking, as well as other variations that will be apparent to those skilled in the art, are intended to be within the scope of this invention. The crosslinked fibers of the present invention are preferably prepared according to the previously described dry crosslinking process. The crosslinked fibers of the present invention can be used directly in the manufacture of absorbent cores placed in air. Additionally, due to its elastic and rigid nature, the crosslinked fibers can be wet laid on a low density sheet, not compacted, which when subsequently dried, is directly useful without additional mechanical processing as an absorbent core. The crosslinked fibers may also be wet laid as compacted pulp sheets for sale or transportation to distant locations. In relation to pulp sheets made from conventional, non-crosslinked cellulosic fibers, the sheets of pulps made from the crosslinked fibers of the present invention are more difficult to compress at conventional pulp sheet densities. Therefore, it may be desired to combine crosslinked fibers with non-crosslinked fibers, such as those conventionally used in the manufacture of absorbent cores. The pulp sheets containing reticulated, regidized fibers preferably contain between about 5% and about 90% uncrosslinked cellulosic fibers, based on the total dry weight of the sheet, mixed with the individualized crosslinked fibers. It is especially preferred to include between about 5% and about 30% uncrosslinked, highly refined cellulosic fibers, or synthetic fibers, based on the total dry weight of the sheet. Such highly refined fibers are refined or whipped to a level of freedom of less than about 300ml. of CSF, and preferably less than 100 ml. of CSF. The uncrosslinked fibers are preferably mixed with an aqueous suspension of the individualized crosslinked fibers. This mixture can then be formed into a sheet of densified pulp for subsequent defibration and formation into absorbent mats. The incorporation of the uncrosslinked fibers facilitates the compression of the pulp sheet in a densified form, while imparting a surprisingly small loss in absorbency to the subsequently formed pads. The uncrosslinked fibers further increase the strength and tension of the pulp sheet and the absorbent pads made of any of the pulp sheets or directly from the blend of uncrosslinked and crosslinked fibers. Although if the mixture of the uncrosslinked and crosslinked fibers is first made in a pulp sheet and then formed in an absorbent pad or formed directly in an absorbent pad, the absorbent pad may be placed in air or wet laid.

Sheets or webs made from reticulated, individualized or mixed fibers also containing uncrosslinked fibers will preferably have base weights of less than about 800g / m2 and densities of less than about 0.60g / cm3. Although no attempt is made to limit the scope of the invention, wet laid sheets having basis weights of between 300 g / m2 and between approximately 600 g / m2 and densities of between 0.07 g / cm3 and approximately 0.30 g / cm3 are especially contemplated for direct applications such as absorbent cores in disposable items such as diapers, tampons, and other catamenial products. Structures that have a basis weight and densities greater than these levels are believed to be most useful for subsequent measurement and placement in air or wet laid to form a lower density and weight basis structure that is more useful for absorbent applications. In addition, said higher density and basis weight structures also exhibit surprisingly greater absorbency and response to wetting. Other contemplated applications for the fibers of the present invention include sheets of low density fabrics having densities that may be less than about 0.03 g / cm 3. If desired, the crosslinked fibers can also be processed to remove excess unreacted crosslinking agent. A series of treatments found to successfully remove the excess crosslinking agent comprises, in sequence, washing the crosslinked fibers, allowing the fibers to soak an aqueous solution for a considerable time, sifting the fibers, drying the fibers, for example, by centrifugation at a consistency of between about 40% and about 80%, mechanically defibrate the dried fibers as described above and air dry the fibers. A sufficient amount of an acidic substance can be added to the wash solution, if necessary, to maintain the wash solution at a pH of less than about 7. Without being bound by theory, it is believed that the ester crosslinks do not they are stable under alkaline conditions and that maintaining the washing treatment at a pH in acidic scale, inhibits the reversion of the ester crosslinks that have been formed. The acidity can be introduced by mineral acids such as sulfuric acid, or alternatively in the form of acidic bleaching chemicals such as chlorine dioxide. The crosslinked fibers described herein are useful for a range of absorbent articles including, but not limited to, tissue sheets, disposable diapers, catamenial products, sanitary napkins, tampons, and webs wherein each of said articles has an absorbent structure containing the individualized crosslinked fibers described herein. For example, a disposable diaper or similar articles having a liquid pervious top sheet, a liquid impermeable back sheet connected to the top sheet, and an absorbent structure, disposed between the top sheet and the back sheet, containing individualized crosslinked fibers is particularly contemplated. Such articles are generally described in U.S. Patent No. 3,860,003, issued to Kenneth B. Buell on January 14, 1975, hereby incorporated by reference in this disclosure. The crosslinked fibers described herein are also useful for making articles such as filter media. Conventionally, the absorbent cores for diapers and cantameñales products are made from non-crosslinked, non-rigid cellulosic fibers, wherein the absorbent cores have dry densities of approximately 0.06 g / cm 3 and approximately 0.12 g / cm 3. When moistening, the absorbent core usually displays a reduction in volume. It has been found that the crosslinked fibers of the present invention can be used to make absorbent cores that have substantially higher fluid-absorbing properties including, but not limited to, absorbent capacity and rate of impregnation relative to absorbing cores of equivalent densities made from conventional non-crosslinked fibers. In addition, these results of improved abosrbencia can be obtained in combination with increased levels of elasticity in number. For absorbent nuclei having densities of between about 0.05 g / cm3 and about 0.15 g / cm3 which keep the volume substantially constant upon wetting, it is especially preferable to use crosslinked fibers having crosslinking levels of about 4% by weight and about 6% by weight. weight of crosslinking agent, based on a basis weight of dry cellulose fiber. Absorbent cores made from such fibers have a desirable combination of structural integrity, i.e., compression resistant and wet elasticity. The term "wet elasticity" in the present context refers to the ability of a moistened pad to return to its original volume and shape upon exposure to and release from compression forces. By comparing the cores made from untreated fibers, the absorbent cores made from the fibers of the present invention will recover a substantially greater proportion of their original volume by releasing wet compression forces. In another preferred embodiment, the individualized crosslinked fibers are formed in either air-laid or wet-laid absorbent cores (and substantially dry) which is compressed to a dry density less than the wet equilibrium density of the pad. The wet equilibrium density is the density of the pad, calculated on a dry fiber basis when the pad is completely saturated with fluid. When the fibers are formed into an absorbent core having a dry density less than the wet equilibrium density, upon wetting to saturation, the core will collapse at the wet equilibrium density. Alternatively, when the fibers are formed in an absorbent core having a dry density greater than the wet equilibrium density when moistening the saturation, the core will expand to the wet equilibrium density. The pads made from the fiber of the present invention have wet equilibrium densities that are substantially lower than pads made from conventional lint fibers. The fibers of the present invention can be compressed at a density higher than the equilibrium density in wet, to form a thin pad which, upon wetting will expand, thereby increasing the absorbent capacity, to a significantly greater extent than that objected by fibers without reticular. In another preferred embodiment, the highly absorbent properties, the wet elasticity and the wetting response can be obtained for crosslinking levels of between about 3% weight and about 6% by weight calculated on a dry fiber weight basis. Preferably, such fibers are formed into absorbent cores having dry densities greater than their wet equilibrium densities. Preferably, the absorbent cores are compressed at densities of between about 0.12 g / cm 3 and about 0.60 g / cm 3, wherein the wet equilibrium density is less than the density of the dry compressed pad. Also, preferably the absorbent cores are compressed to a density of between about 0.12 g / cm 3 and about 0.40 g / cm 3, wherein the corresponding wet equilibrium densities are between about 0.08 g / cm 3 and about 0.12 g / cm 3, and They are smaller than the densities of dry compressed cores. It should be recognized, however, that the absorbent structures within the higher density scale can be made from reticulated fibers having higher crosslinking levels, how can absorbent structures of lower densities be made from crosslinked fibers having lower levels of crosslinking. Although the above discussion involves preferred embodiments for high and low density absorbent structures, a variety of combinations of absorbent structure densities and levels of crosslinking agent among the scales described herein will be recognized to provide superior absorbency and structural integrity characteristics. absorbent in relation to conventional and known cellulosic fibers and previously known crosslinked fibers. It will be understood that such embodiments are included within the scope of the present invention. Absorbent structures made from individualized crosslinked fibers may additionally contain discrete particles of hydrogel-forming material, substantially insoluble to water. Hydrogel-forming materials are chemical compounds capable of absorbing fluids and retaining them under moderate pressures. Suitable hydrogel-forming materials can be inorganic materials such as sillic gels or organic compounds such as cross-linked polymers. It should be understood that crosslinking, when referred to in connection with hydrogel-forming materials, assumes a broad signficance that contemplates in relation to the reaction of cross-linking agents with the cellulosic fibers to form individualized cross-linked fibers. The crosslinked hydrogel-forming polymers can be crosslinked by Van der Waals hydrogen bonds, ionic or covalent. Examples of the hydrogel-forming materials include polyacrylamomers, polyvinyl alcohol, anhydromaleic-ethylene copolymers, polyvinyl ethers, hydroxypropyl cellulose, carboxymethyl cellulose, polyvinyl morphoglycine, polymers and copolymers of vinyl sulphonic acid, polyacrylates, polyacrylates, polyvinyl pyridine and the like. Other suitable hydrogel-forming materials are those described in Assarsson et al., U.S. Patent No. 3,901,236, dated August 26, 1975, a description of which is incorporated herein by reference. The hydrogel-forming polymers particularly precursor for use in the absorbent core are grafted starch of hydrolyzed acrylonitrile, grafted starch of acrylic acid, polyacrylates, and anhydromyalkylene isobutylene copolymers, or mixtures thereof. Examples of hydrogel-forming materials that can be used are Aqualic L-73, a partially neutralized polyacrylic acid made by Nippon Shokubai Co., Japan, and San et IM 1000, a grafted starch of partially neutralized acrylic acid made by Sanyo Co., Ltd. Japan. Hydrogel-forming materials having relatively high gel strength, as described in U.S. Patent Application Serial No. 746,152 filed June 18, 1985, hereby incorporated by reference, are preferred for use with individualized crosslinking fibers. Processes for preparing hydrogel-forming materials are described in Masuda et al., U.S. Patent No. 4,076,663, issued February 28, 1978; in Tsubakimoto and others; U.S. Patent No. 4,286,082 issued August 25, 1981; and further in U.S. Patent No. 3,734,876, 3,661,815, 3,670,731, 3665,343, 3,783,871 and Belgian Patent 785,850, descriptions of which are all incorporated herein by reference. The hydrogel-forming material may be distributed throughout the length of an absorbent structure containing individualized crosslinked fibers, or be limited to the distribution throughout a particular layer or section of the absorbent structure. In another embodiment, the hydrogel-forming material is adhered or laminated onto a sheet or film that is juxtaposed against a fibrous absorbent structure, which may include individualized crosslinked fibers. Such a sheet or film may be multilayer such that a hydrogel-forming material is contained between the layers. In another embodiment, the hydrogel-forming material can be adhered directly onto the surface of the fibers of the absorbent structure. Surprisingly, a large increase in skin dryness has been observed for absorbent structures combining the individualized crosslinked fibers of the present invention and hydrogel-forming materials, according to the level of skin moisture measured by a subsequent evaporimeter that makes contact with structures absorbents moistened to the skin of the human. This improvement is believed to be due to the high impregnation capacity of the individualized crosslinked crosslinking fibers in relation to conventional fibers and to the increased absorptive capacity of the structure. The unique impregnation capacity of the structures made from individualized crosslinked fibers results from the natural stiffness of the fibers and the resulting relatively large hollow spaces therefrom. However, excessively high levels of crosslinking agents, such as may be present in certain previously known individualized crosslinked fibers, may reduce impregnation due to the hydrophobic characteristics of the crosslinking agent. Another important advantage has been observed with respect to the absorbent structures made from individualized crosslinked fibers having dry densities that are higher than their corresponding wet equilibrium densities (calculated on a dry fiber basis). Specifically, this type of absorbent structure spreads the volume when wetting. As a result of this expansion, the interfiber capillary fiber network also enlarges. In conventional ablosive structures having hydrogel-forming material mixed therein, the hydrogel-forming material expands in volume due to fluid abosrbción, and can block or reduce the size of the capillary routes for the abstraction of the fluid before the use of the potential Absorption of total fluid from the structure. This phenomenon is known as gel blocking. The capillary enlargement due to the expansion of the fibrous network of the abosrbent structure reduces the occurrence of the gel block. This allows higher proportions of the fluid absorbency potential of the structure that is used and allows higher levels of hydrogel-forming materials (and if desired) to be incorporated in the aborbent structure, without significant levels of gel blocking. Absorbent structures containing individualized crosslinked fibers and hydrogel-forming material for diaper core applications preferably have dry densities of between about 0.15 g / cm 3 and about 0.40 g / cm 3 and preferably contain about 20% to about 50%, by weight of a hydrogel-forming material, calculated on a dry fiber weight basis. Most preferably, the individualized crosslinked fibrs have between about 3% by weight and about 7% by weight of crosslinking agent, calculated on a dry fiber weight basis, which is reacted with it in the intrafiber lattice bond form wherein the fibers are formed in a relatively thin absorbent structure in a sufficiently compressed dry state, such that the structure can expand when moistening The hydrogel formation material can be homogeneously dispersed throughout the length or part of the absorbent structure. For a diaper structure as described in U.S. Patent No. 3,860,003, having an absorbent core containing the individualized crosslinked fibers, it has a dry density of about 0.20 g / cm 3, and also contains hydrogel-forming material scattered throughout the nucleus. It is currently believed that an optimal balance of diaper impregnation, total absorbent capacity, skin moisture, and economic viability is obtained for contents of between about 20 to about 50% by weight, based on the total weight of the dry core, of a hydrogel-forming material such as Aqualic L-73. The hydrogel-forming material is preferably homogeneously blended with the absorbent cores that contain individualized crosslinked fibers in products as described in U.S. Patent No. 3,860,003. The absorbent structures described above may also include foamed, conventional, or highly refined fibers, wherein the amount of the hydrogel-forming material is based on the total weight of the fibers as discussed previously. The embodiments described herein are exemplary in nature and are not intended to imply that they limit the scope of application of the hydrogel-forming material with the individualized cross-linked fibers.

PROCEDURE TO DETERMINE THE VALUE OF FLUID RETENTION The following procedure can be used to determine the water retention value of the cellulosic fibers. A sample of approximately 0.03 gr. to 0.4 gr. of fibers is soaked in a covered container with approximately 100 ml. of deionized or distilled water at room temperature for between about 15 and about 20 hours. * The soaked fibers are collected on a filter and transferred to an 80-mesh wire basket supported approximately 3.81 cm. or above a sieved bottom of 60 meshes of a centrifugal tube. The tube is covered with a plastic cover and the sample is centrifuged at a relative centrifugal force of 1500 to 1700 gravities for 19 to 21 minutes. The centrifuged fibers are then removed from the basket and weighed. The heavy fibers are dried to a constant weight at 1052 and reweighed. The water retention value is calculated as follows: (1) WRV = (W-D) x 100 D where, W = wet weight of the centrifuged fibers D = dry weight of the fibers, and W-D = weight of the absorbed water PROCEDURE TO DETERMINE OCCUPANCY CAPACITY The following procedure can be used to determine the drainage capacity of the absorbent cores. Runoff capacity is used as a combined measure of the absorptive capacity and the absorbance evaluation of the cores. An absorbent pad of 10.14 cm by 10.14 cm. , weighing approximately 7.5 gr. It is placed on a screen mesh. Synthetic urine is applied to the center of the pad at a speed of 8mm per second. The flow of synthetic urine is interrupted when the first drop of synthetic urine escapes from the bottom or sides of the pad, the runoff capacity is calculated by the difference in mass of the pad before and after the introduction of the synthetic urine divided by the mass of the fibers, on a completely dry basis.

PROCEDURE TO DETERMINE THE COMPRESSIBILITY IN HUMID The following procedure can be used to determine the wet compressibility of absorbent structures. Wet compressibility is used as a measure of the wet compressive strength, the wet structural integrity and the wet elasticity of the absorbent cores. A pad of 10.14 cm. by 10.14 cm2 weighing approximately 7.5 gr. it is prepared, its thickness is measured and its density is calculated. The pad is loaded with synthetic urine at 10 times its dry weight or until its saturation point, whichever is less. A compression load of 0.1 PSI is applied to the pad. After approximately 60 seconds, during which time the pad balances, the thickness of the pad is measured. The compressional load is then increased to 1.1 PSI, the pad is allowed to equilibrate, and the thickness is measured. The compressional load is then reduced to 0.1 PSI, the pad is allowed to equilibrate and the thickness is again measured. The densities are calculated for pad in the original charge of 0.1 PSI the charge of 1.1 PSI and the second charge of 0.1 PSI, referred to as a charge of 0.1 PSIR (bounce PSI). The hollow volume reported in cm3 / gr, is then determined for each respective pressure load. The hollow volume is the reciprocal of the pad of the density of the wet pad minus the volume of fiber (0.95 cm3 / g). Hollow volumes of 0.1 PSI and 1.1 PSI are useful indicators of wet compressive strength and wet structural integrity. Higher hollow volumes for common initial pad densities indicate higher wet compressive strength and greater wet structural integrity. The difference between the hollow volumes between 0.1 PIS and 0.1 PSIR is useful to compare the wet elasticity of the abosrbent pads. A smaller difference between the hollow volume 0.1 PSI and the hollow volume 0.1 PSIR indicate a higher wet elasticity.

Also, the difference in caliber between the dry pad and the saturated pad before compression is found to be a useful indicator of the pad wetting response.

PROCEDURE TO DETERMINE DRY COMPRESSIBILITY The following procedure can be used to determine the dry compressibility of abosorbent cores. Dry compressibility is used as a measure of the dry elasticity of the cores. A pad placed in dry 10.14 cm. by 10.14 cm2 that has a mass of approximately 7.5 gr. it is prepared and compressed, in a dry state, by means of a hydraulic press at a pressure of 5500 lbs / 16in2. The pad is reversed and the pressure is repeated. The thickness of the pad is measured before and after pressing with an unloaded gauge. The density before and after pressing is then calculated as mass / (area X thickness). The largest differences between the density before and after the pressure indicate the lowest dry elasticity).

PROCEDURE TO DETERMINE THE LEVEL OF POLYACRIL ACID THAT HAS REACTED WITH CELLULOSE FIBERS There is a range of suitable analytical methods to determine the level of crosslinked polyacrylic acid with cellulosic fibers: Any suitable method can be used. For the purpose of determining the level of the preferred polymeric polyacrylic acid (such as polymeric monoalkyl phosphonate and polymeric monalkyl phosphinate, ie polymers containing a percent by weight known of a certain inorganic element chemically bonded to the polymer), which reacts to form intrafiber lattice bonds with the cellulose component of the individualized crosslinked fibers in the examples of the present invention, the following procedure is used. First, a sample of crosslinked fibers is washed with enough hot water to remove any crosslinking chemicals or unreacted catalyst. Next, the fibers are dried to a moisture equilibrium content. The completely dry weight of the sample is then determined with a moisture balance or other suitable equipment. Then, the mixture is burned or "made ashes", in an oven at a suitable temperature to remove all the organic material from the sample. The inorganic material remanating from the sample is dissolved in a strong acid, such as parloric acid. This acid solution is then analyzed to determine the mass of the inorganic element that was present in the initial polymer (in a ratio of a known mass of (total polymer) / (inorganic element) applied to the cellulosic fibers. Inductively coupled plasma (ICP AES) is a method that can be used to analyze this solution.The amount of polymer that is crosslinked on the cellulosic fibers can then be calculated by the following formula: Crosslinking level (weight%) = W, R x 100 Wc Where Wj = mass of the inorganic element of the sample bound to the polymer that is crosslinked to the cellulose fibers, measured as described above (in grams) R = is the ratio defined by: the mass of the total polymer divided by the mass of the element inorganic bound to the polymer Wc = is the totally dry mass of the cellulosic fiber sample that is analyzed (in grams) For the purposes of determining the level of preferred polymeric polyacrylic acid (for example, polyacrylic acid with a molecular weight of about 1000, or an acrylic / maleic copolymer containing 65% by weight of acrylic acid and 35% by weight of maleic acid having a molecular weight of about 9000) which reacts to form the intrafiber lattice bonds with the cellulose component of the individualized crosslinked fibers in In the examples of the present invention, the following procedure is used. Reaction efficiency (defined as the polymer percent applied to the cellulosic fibers that react with the cellulose to form the intrafiber lattice bonds) of a similar polymer containing an inorganic element is determined by the ICP AES method described above. This reaction efficiency is then assumed to be applicable to the polymer in question. The applied amount of the polymer in question is then multiplied by the reaction efficiency to determine the amount of the polymer in question that is reacted with the cellulosic fibers to form intrafiber crosslink bonds. Typically the reaction efficiency is about 0.75.

PROCEDURE TO DETERMINE THE TORSION COUNT The following method can be used to determine the torsion count of the fibers analyzed in the description. The dried fibers are placed on a stage covered with a thin film of immersion oil, and then covered with a sliding cover. The effect of the oil immersion was to make the fiber transparent without inducing swelling and thereby assist in the identification of the torsion nodes (described below). The wet fibers are placed on a stage by spilling a low consistency suspension of the fibers on the stage then covered with a slide cover. The water makes the fibers transparent in such a way that the identification of the torsion nodes is facilitated. A computer-controlled microscope, a video camera, a video screen and a computer charger with QUIPS software, available from Cambridge Instruments Limited (Cambridge, England; Buffalo, New York), is used to determine the torsion count. The total length of the fibers within a particular area of the microscope stage at a magnification of 200X is measured by the image analyzer. The torsion knots are identified and marked by an operator. This procedure is continued by measuring the length of fibers and marking the torsional knots until 1200 mm of the length of the fiber is analyzed. The numbers of torsion nodes per millimeter are calculated from this data by dividing the total length of the fiber by the total number of torsion nodes marked.

PROCEDURE TO DETERMINE THE RIZO FACTOR The following method can be used to measure the fiber's curl index. The dried fibers are placed on a microscope slide. A sliding cover is placed over the fibers and sticks on the edges. The current length LA and the maximum projected length LR (equivalent to the length of the longest side of a rectangle encompassing the fiber) are measured using an image analyzer comprising a software-controlled microscope, a video camera, a video monitor and a computer. The software used is the same as that described in the Torsion Counting Image Analysis Method above. Once the LA and LR are obtained, the curl factor is calculated according to the equation (1) shown above. The curl factor for each fiber sample is calculated by at least 250 individual fibers and then averaged to determine the mean curl factor for the sample. Fibers having LA less than 0.25 mm are excluded from the calculation. The following examples illustrate the practice of the present invention but are not intended to be limiting thereof. EXAMPLE 1 Individualized crosslinked fibers of the present invention are made by a dry crosslinking process using an acrylic / maleic copolymer (containing 65% by weight of acrylic acid and 35% by weight of maleic acid having a molecular weight of about 9000) as the crosslinking agent. The process used to produce the acrylic / maleic acid copolymer crosslinked fibers is as follows: 1. For each sample, 1735 grams of a fresh Southern Wood Softwood Kraft pulp (SSK) is provided. The fibers have a moisture content of about 7% (equivalent to 93% consistency). 2. A suspension is formed by adding the fibers to an aqueous solution containing approximately 2942 g. of the acrylic / maleic copolymer and 410 ml. of 50% sodium hydroxide solution in 59,323 gr. of water. The fibers are soaked in the suspension for approximately 60 minutes. This stage is also referred to as "soaking". The pH of the stage is about 3.0. 3. The fibers are then dried by centrifuging them to a consistency ranging from about 40% to about 50%. The consistency of the centrifuged suspension of that step combined with the concentration of carboxylic acid in the suspension filtered in step 2 establishes the amount of cross-linking agents present in the fibers after centrifugation. In that example, about 6% by weight of the acrylic / maleic copolymer on a dry fiber cellulose anhydroglucose base, is present on the fibers after the initial centrifugation. In practice, the concentration of the crosslinking agent in the suspension filtrate is calculated assuming a target drying consistency and a desired level of chemicals in the fibers. Next, the dried fibers are defibrated using a Sprout-Waldron 12-inch disc refiner (model number 105-A) whose plates are set in a space that produces substantially individualized fibers but with a minimal amount of fiber damage. As the individualized fibers leave the refiner, they are instantly dried with hot air in two vertical tubes in order to provide curl and twist of the fiber. The fibers contain approximately 10% moisture when leaving these tubes and are ready to be cured. If the moisture content of the fibers is greater than about 10% when leaving the flash drying tubes, then the fibers are dried with air at room temperature until the moisture content is about 10%.

. The almost dry fibers are then placed in trays and cured in an air-pass drying oven for a duration of time and at a temperature in practice depending on the amount of copolymer added, dryness of the fibers, etc. In this example, the samples are cured at a temperature of about 188a for a period of about 8 minutes. The crosslinking is complemented during the period in the furnace. 6. Individually crosslinked fibers are placed in a mesh screen and rinsed with approximately 20% water, soaked at a consistency of 1% for one hour in approximately 60a water, sieved, rinsed with approximately 20a water for a second time , centrifuged at approximately 60% of the fiber consistency, and dried at an equilibrium moisture content of about 8% with air at room temperature. The resulting individualized crosslinked cellulosic fibers have a VRA of 43 and contain 4.6% by weight of the maleic acid / acrylic acid copolymer, calculated on a dry fiber weight basis, which reacted with the fibers in the form of intrafiber crosslink bonds.

Importantly, the resultant individualized crosslinked fibers have a better response to wetting relative to the conventional non-crosslinked fibers and the previously known crosslinked fibers, and can be safely used in the vicinity of human skin.

EXAMPLE II The individualized crosslinked fibers of the present invention are made by a dry crosslinking process using polyacrylic acid with a molecular weight of about 1000 as the crosslinking agent. The individualized crosslinked fibers are produced according to the process described here above of Example I with the following modifications: The suspension to stage 2 of Example I contains 150 g. of dried pulp, 1186 gr. of water, 63.6 gr. of polyacrylic acid, and 4 gr. of sodium hydroxide. In step 5, the fibers are cured at a temperature of about 190a for a period of about 30 min. The resultant individualized crosslinked cellulosic fibers have a VRA of 38 and contain 4.2% by weight of polyacrylic acid calculated on a dry weight basis of fiber, which reacted with the fibers in the form of intrafiber lattice bonds. Importantly, the resulting individualized crosslinked fibers have an improved wetting response relative to conventional uncrosslinked fibers and known crosslinked fibers, and can be used safely in the vicinity of human skin.

EXAMPLE III The individualized crosslinked fibers of the present invention are made by a dry crosslinking process using a copolymer of acrylic acid and maleic acid, and citric acid. The total applied percentage of the copolymer and citric acid is 4.53%. The concentration of citric acid is 33% of the total. The ratio of acrylic acid to maleic acid is 63:35. The molecular weight of the copolymer is 9,000. The individualized crosslinked fibers are produced according to the process described here above of Example I with the following modifications: The suspension in step 2 of Example I contains 150 g. of dry pulp, 1,113 gr. of water, 28 gr. of mixture of citric acid and copolymer. In step 5, the fibers are cured at a temperature of approximately 190a for a period of approximately 30 minutes. The resulting individualized crosslinked cellulosic fibers have a VRA of 38 and contain 3.4% by weight of citric acid / maleic acid / acrylic acid crosslinking agent calculated on a dry weight basis of fiber, which reacted with the fibers in the form of bonds of intrafiber reticulum. Importantly, the resulting individualized crosslinked fibers have an improved response to wetting relative to fibers without conventional reticules and known crosslinked fibers, and can be used safely in the vicinity of human skin.

EXAMPLE IV The individualized crosslinked fibers of the present invention are made by a dry crosslinking process using sodium hyposphite acrylic acid / acrylic acid having a molecular weight of about 1600 as the crosslinking agent. The total percentage applied of - • copolymer is 5.92%. The weight ratio of acrylic acid to maleic acid to sodium hyposphosphite is 51/26/23. The individualized crosslinked fibers are produced according to the process described here above of Example I with the following modifications: The suspension in step 2 of Example I contains 333 g. of dried pulp, 230 gr. of water and 15 gr. of polymer. The pH of the polymer solution is adjusted to 3 using hydrochloric acid. In step 5, the fibers are cured in an air-pass oven at 178a for a period of about 6 min. The resulting individualized crosslinked cellulosic fibers have a VRA of 42 and 4.4% by weight of the copolymer calculated on a dry fiber weight basis, which reacted with the fibers in the form of intrafiber lattice bonds. Importantly, the resulting individualized crosslinked fibers have an improved response to wetting relative to conventional fibers and reticules and to previously known crosslinked fibers, and can be used safely in the vicinity of human skin.

EXAMPLE V The individualized crosslinked fibers are produced according to the procedure described hereinabove of Example I with the following modifications: In step 2 of Example I, the solution containing the crosslinking agent is rubbed directly onto a fiber sheet. The consistency of the fibers of the sprayed sheet vary from about 50% to about 80%, by weight. The consistency of the sheet combined with the crosslinking concentration establishes the amount of crosslinking agents (and base if needed) present in the fibers. The concentration of the crosslinking agent (and the base) are calculated by determining an object consistency and a desired level of chemicals in the fibers.

The resulting individualized crosslinked cellulosic fibers have a VRA of 43 and contain 4.2% by weight of maleic acid / acrylic acid copolymers calculated on a dry weight basis of fiber, which reacted with the fibers in the form of intrafiber lattice bonds. Importantly, the resulting individualized crosslinked fibers have improved response to humidification relative to conventional fibers and reticles 0 to previously known crosslinked fibers, and can be used safely in the vicinity of human skin.

EXAMPLE VI The crosslinked fibers, individualized from the example 1 are placed in air to form absorbent pads, and compressed with a hydraulic press at a density of 0.10 g / cm3 and 0.20 g / cm3. The pads are subsequently tested for absorbency, elasticity, structural integrity according to previously summarized procedures. The results are reported in table 1 and are compared to an absorbent pad made from conventional cellulosic and reticular fibers.

TABLE 1 Sample # VRA CAP Crosslinking Agent. DRIPPING Compressibility (weight%) (%) a) 8 ml / s 0.1 / 0.2 / gcc Reacted g / g) Density Tested Pad 1 0 79.2 4.56 6.04 / 5.38 4.6 43 1.58 7.75 / 6.24 As can be seen from Table 1, the absorbent pads containing individualized, cross-linked fibers with maleic / acrylic acid (ie sample 2) have drip capabilities and significantly higher wet compressibilities in both test pad densities 0.10 g / cm3 and 0.20 g / cm3 relative to pads containing conventional non-crosslinked fibers (ie, sample 1). In addition to having an improved response to wetting in relation to conventional non-crosslinked fibers, the absorbent pads containing the crosslinked fibers with maleic / acrylic acid copolymer can be used safely in the vicinity of human skin. EXAMPLE VII The indvidualized crosslinked fibers of Example II are placed in air to form absorbent pads and compressed with a hydraulic press at a density of 0.10 g / cm3 and 0.20 g / cm3. The pads are subsequently tested for abosrbence, elasticity, and structural integrity according to the procedure previously summarized. The results are reported in Table 2 and compared to an absorbent pad made from conventional untyped cellulosic fibers.

TABLE 2 Sample # VRA CAP Crosslinking Agent. DRIPPING Compressibility (weight%) (%) a) 8 ml / s 0.l / 0.2 / gcc Reacted g / g) Density Tested Pad 1 0 79.2 4.56 6.04 / 5.38 3 4.2 38 8.44 6.92 / 5.99 As can be seen from Table 2, the absorbent pads containing individualized polyacrylic crosslinked fibers, (ie sample 2), have significantly higher dripping capacities and wet compressibilities at both test pad densities at 0.10 g. / cm3 and 0.20 g / cm3 in relation to pads containing conventional non-crosslinked fibers (ie, sample 1). In addition to having an improved response to wetting relative to conventional uncrosslinked fibers, absorbent pads containing fibers crosslinked with polyacrylic acid can be used safely in the vicinity of human skin.

EXAMPLE VIII The individualized crosslinked fibers of the example III are placed in air to form absorbent pads, and compressed with a hydraulic press at a density of 0.10 g / cm3 and 0.20 g / cm3. The pads are subsequently tested for abosrbence, elasticity, and structural integrity in accordance with the procedures outlined previously. The results are reported in Table III and an absorbent pad made from conventional untyped cellulosic fibers is compared.

TABLE 3 Sample # VRA CAP Crosslinking Agent. DRIPPING Compressibility (weight%) (%) a) 8 ml / s 0.1 / 0.2 / gcc Reacted g / g) Density Tested Pad 1 0 79. 2 4 .56 6. 04/5 38 4 3 .4 38 11 .54 7. 82/6 .46 As can be seen from Table 3, the absorbent pads containing fibers crosslinked with citric acid / maleic acid / acrylic acid, individualized (ie), sample 4) have significantly high wetting and wetting capabilities at both test pad densities 0.10 g / cm3 and 0.20 / g / cm3 relative to conventional pads containing uncrosslinked fibers (i.e. sample 1). In addition to having an improved response to wetting relative to conventional non-crosslinked fibers, absorbent pads containing crosslinked fibers of acidic / maleic / acrylic acid copolymers can be used safely in the vicinity of human skin.

EXAMPLE IX The individualized crosslinked fibers of Example IV are placed in air to form absorbent pads and compressed with a hydraulic press at a density of 0.10 g / cm 3 and 02.0 g / cm 3. The pads are subsequently tested for absorbency, elasticity and structural integrity according to the procedures outlined above. The results are reported in table IV and compared to an absorbent pad made from cross-linked cellulose fibers, the conventional untyped cellulosic fibers. TABLE 4 Sample # VRA CAP Crosslinking Agent. DRIPPING Compressibility (weight%) (%) a) 8 ml / s 0.1 / 0.2 / gcc Reacted g / g) Density Tested Pad 1 0 79.2 4.56 6.04 / 5.38 5 4.4 42 13.56 8.06 / 6.80 As can be seen from Table 4, the absorbent pads containing fibers crosslinked with sodium hypophosphite / maleic acid / acrylic acid, individualized (ie sample 5), have significantly higher wetting and drainability capabilities in both test pad densities 0.10 g / cm3 and 0.20 g / cm3 relative to pads containing conventional non-crosslinked fibers (ie, sample 1). In addition to having an improved response to wetting relative to conventional non-crosslinked fibers, absorbent pads containing fibers cross-linked with sodium hypophosphite / maleic acid / acrylic acid can be safely used in the vicinity of human skin.

Claims (10)

1. CLAIMS 1.- Crosslinked, individualized cellulosic fibers, said fibers having an effective amount of a polymeric polyacrylic acid crosslinking agent, which reacts with said fibers in the form of intrafiber lattice bond, wherein said polymeric polyacrylic acid crosslinking agent is selects polyacrylic acid polymers, acrylic acid copolymers, and mixtures thereof.
2. 2. The individualized crosslinked fibers according to claim 1, further characterized in that said fibers have between 1.0% by weight and 10.0% by weight, preferably between 3% by weight and 7% by weight, of crosslinking agent , calculated on a dry fiber basis, which reacts with this in the form of intrafiber crosslink bond, and wherein said fibers have a water retention value of from 25 to 60, preferably from 30 to 45.
3. 3.- The fibers crosslinked, individualized, according to claim 1 or 2, further characterized in that said crosslinking agent is a polyacrylic acid polymer having a molecular weight of from 500 to 40,000, preferably 1,000 to 20,000.
4. 4. The crosslinked fibers, individualized according to any of claims 1 to 3, further characterized in that said polymeric polyacrylic acid crosslinking agent is a copolymer of acrylic acid and maleic acid.
5. 5. The crosslinked fibers, individualized according to claim 4, further characterized in that the weight ratio of acrylic acid to maleic acid is from 10: 1 to 1: 1, preferably from 5: 1 to 1.5: 1.
6. 6. The crosslinked fibers, individualized according to any of claims 1 to 5, further characterized in that an effective amount of citric acid is mixed with said polymeric polyacrylic acid crosslinking agent.
7. 7. The individualized crosslinked cellulosic fibers, said fibers having having an effective amount of crosslinking agent of a polymer mixture reacted with said fibers in the intrafiber lattice bond form, wherein said polymer blend crosslinking agent. It comprises polymeric monoalkyl phosphonates and polymeric monoalkyl phosphinates.
8. 8. The individualized crosslinked fibers according to claim 7, further characterized in that said fibers have between 1.0% by weight and 10.0% by weight, preferably between 3% by weight and 7% by weight, of crosslinking agent, calculated on a weight basis of dry fiber, which reacts with this in the form of intrafiber lattice bonds, and wherein said crosslinked fibers have a water retention value of from 25 to 60, preferably from 30 to 45.
9. 9. The individualized crosslinked fibers, according to claim 7 or 8, further characterized in that said polymer blend crosslinking agent has a molecular weight of less than 5,000.
10. 10. The individualized crosslinked fibers according to any of claims 7 to 9, further characterized in that an effective amount of citric acid is mixed with said crosslinking agent of polymer blend. ABSTRACT Individualized, crosslinked fibers are described, processes for making such fibers, and absorbent structures containing the fibers. The individualized crosslinked fibers have a crosslinking agent of polymeric polyacrylic acid which reacts with the fibers in the form of intrafiber crosslink bonds. Preferably, the crosslinking agent is a copolymer of acrylic acid and maleic acid and preferably, between 1% by weight and about 10.0% by weight of the crosslinking agent reacts with the individualized fibers to form the intrafiber crosslink bonds. The individualized crosslinked fibers are useful in a variety of applications of absorbent structures. The absorbent structures may also contain hydrogel-forming material.