US20040177935A1 - Method for making chemically cross-linked cellulosic fiber in the sheet form - Google Patents

Abstract

The present invention is directed to cross-linked cellulosic fiber in the sheet from, obtainable by cross-linking a blend of mercerized pulp and conventional pulp. The method includes heating treated cellulosic fibers to promote intra-fiber cross-linking. The cross-linked fibers are characterized by an improved acquisition rate, resiliency, absorbency, and absorbency under load. Moreover, the inventive cross-linked fibers exhibit a reduction in centrifuge retention capacity, and have low knots, nits and fines contents. The cross-linked cellulosic fibers of the invention are useful in the acquisition layer and/or absorbent core of absorbent articles.

Description

FIELD OF THE INVENTION
• The present invention is directed to a method of making chemically cross-linked cellulosic fiber in the sheet form and to the product resulting from the process.[0001]
DESCRIPTION OF RELATED ART
• Absorbent articles intended for personal care, such as adult incontinent pads, feminine care products, and infant diapers typically are comprised of at least a top sheet, a back sheet, an absorbent core disposed between the top sheet and back sheet, and an optional acquisition layer disposed between the top sheet and the absorbent core. The acquisition layer comprised of, for example, acquisition fibers, usually is incorporated in the absorbent articles to provide better distribution of liquid, increase the rate of liquid absorption, and reduce gel blocking. A wide variety of acquisition fibers are known in the art. Included among these are synthetic fibers, a composite of cellulosic fibers and synthetic fibers, and cross-linked cellulosic fibers. Cross-linked cellulose fiber is preferred because it is abundant, it is biodegradable, and it is relatively inexpensive.[0002]
• Cross-linked cellulose fibers and processes for making them have been described in the literature for many years (see, for example G. C. Tesoro, Cross-Linking of Cellulose, in[0003]Handbook of Fiber Science and technology, Vol. II, M. Lewis and S. B. Sello eds. pp 1-46, Mercel Decker, New York (1993)). The cross-linked cellulose fibers are typically made by reacting cellulose with polyfunctional agents that are capable of reacting with the hydroxyl groups of the anhydroglucose repeating units of the cellulose either in the same chain, or in neighboring chains simultaneously. Cross-linked cellulose fibers generally are characterized by their high absorbent capacity, and their high resiliency in the wet and dry states.
• Cellulosic fibers typically are cross-linked in fluff form. Processes for making cross-linked fiber in the fluff form comprise dipping swollen or non-swollen fiber in an aqueous solution of cross-linking agent, catalyst, and softener. The fiber so treated, usually is then cross-linked by heating it at elevated temperatures in the swollen state as described in U.S. Pat. No. 3,241,553, or in the collapsed state after defiberizing it as described in U.S. Pat. No. 3,224,926, and European Patent No. 0,427,361 B1, the disclosures of each of which are incorporated by reference herein in their entirety.[0004]
• The art has proposed many solutions to overcome some of the problems of cross-linking fiber in sheet form. One alleged solution to this problem is to minimize the contact between fibers in the dry state. For example, Graef et al. in U.S. Pat. No. 5,399,240, the disclosure of which is incorporated herein by reference in its entirety, describe a method of treating fiber in the sheet form with a cross-linking agent and a de-bonder. Fiber while in the sheet form is then cured at elevated temperatures. The de-bonder tends to interfere with the hydrogen bonding between fibers and thus minimizes the contact between fibers. As a result, fiber is produced with a relatively low content of knots and nits. In addition, the long aliphatic chains tend to reduce the fibers' absorbency and acquisition rate, thus rendering the fibers unsuitable for applications where high rate of absorbency and fast acquisition are important, such as in absorbent articles.[0005]
• Bernardin et al. in U.S. Pat. No. 3,434,918 disclose a method of treating fiber in sheet form with a cross-linking agent and a catalyst. The treated sheet then is wet-aged to render the cross-linking agent insoluble. The wet-aged fibers are re-dispersed before curing and mixed with untreated fiber, and then sheeted and cured. Other documents describing methods of treating fiber in sheet form include, for example, U.S. Pat. Nos. 4,204,054; 3,844,880; and 3,700,549 (the disclosures of which are incorporated by reference herein in their entirety).[0006]
• The above-described approaches complicate the process of cross-linking fiber in sheet form, and they render the process time consuming, and costly.[0007]
• In previous work (U.S. patent application entitled: “Chemically Cross-Linked Cellulosic Fiber and Method of Making the Same, filed on Jun. 11, 2002, attorney docket number 60892.000002, and Ser. No. 09/832,634, entitled “Cross-Linked Pulp and Method of Making Same, filed Apr. 10, 2001 it was shown that mercerized fiber can be successfully cross-linked in sheet form. The produced cross-linked fiber showed similar or better performance characteristics than conventional individualized cross-linked cellulose fibers. Also, the fiber showed less discoloration and reduced amounts of knots and nits compared to conventional individualized cross-linked fiber.[0008]
• Fiber mercerization, which is a treatment of fiber with an aqueous solution of sodium hydroxide (caustic), is one of the earliest known modifications of fiber. It was invented 150 years ago by John Mercer (see British Patent 1369, 1850). The process generally is used in the textile industry to improve cotton fabric's tensile strength, dyeability, and luster (see, for example, R. Freytag, J.-J. Donze, Chemical Processing of Fibers and Fabrics, Fundamental and Applications, Part A, in[0009]Handbook of Fiber Science and Technology Vol. IM. Lewis and S. B. Sello eds. pp. 1-46, Mercell Decker, New York (1983)).
• In addition to the above advantages, mercerization adds to fibers several other properties. Among these are: (1) mercerized fibers have high α-cellulose content, since caustic removes residuals such as lignin and hemicellulose from fiber leftover from pulping and bleaching processes; (2) mercerized fibers have a round, circular shape (rather than the flat, ribbon-like shape of conventional fibers) that reduces the contact and weakens the hydrogen-bonding between fibers in the sheet form; and (3) mercerization converts cellulose chains from their native structure form, cellulose I, to a more thermodynamically-stable and less crystalline form, cellulose II. The cellulosic chains in cellulose II are found to have an anti-parallel orientation rather than parallel orientation as in cellulose I (see, for example, R. H. Atalla, Comprehensive Natural products Chemistry,[0010]Carbohydrates And Their Derivatives Including Tannins, Cellulose, and Related Lignins Vol. III, D. Barton and K. Nakanishi eds. pp 529-598, Elsevier Science, Ltd., Oxford, U.K. (1999)).
• The description herein of certain advantages and disadvantages of known cross-linked cellulosic fibers, and methods of their preparation, is not intended to limit the scope of the present invention. Indeed, the present invention may include some or all of the methods and chemical reagents described above without suffering from the same disadvantages.[0011]
SUMMARY OF THE INVENTION
• One feature of an embodiment of the present invention provides cross-linked fibers with enhanced bulking characteristics, porosity and rate of acquisition. An additional aspect of the present invention is to provide cross-linked cellulosic fiber having long shelf-life and high stability. Further, another aspect of an embodiment of the present invention provides fibers useful in an acquisition layer and/or in an absorbent core of absorbent products. Various aspects of the present invention also provide absorbent articles comprising the cross-linked fiber of the present invention.[0012]
• In accordance with these and other aspects and features of embodiments of the invention, there is provided a cross-linked sheet of a blend of fibers, and a method for cross-linking cellulose fibers in sheet form. In one aspect of the invention, the cellulose fibers are a blend of mercerized fibers and conventional fibers that are cross-linked. In another aspect of the invention, the cross-linked fibers formed in accordance with the present invention can be easily defiberized without serious fiber breakage and with low knot-content and low nit-content. It will be appreciated, however, that knots and nits are advantageous for some applications, and accordingly, the present invention is not in any way limited to producing cross-linked cellulosic fibers substantially free of knots.[0013]
• In accordance with the method, a wet laid sheet of a blend of mercerized fibers and cellulose fibers are formed, and then treated a cross-linking agent to form a sheet impregnated with the cross-linking agent. The cross-linking agent then is dried and cured to form intra-fiber cross-links.[0014]
• These and other objects, features, and advantages of the present invention will appear more fully from the following detailed description of the preferred embodiments of the invention, and the attached drawings.[0015]
BRIEF DESCRIPTION OF THE DRAWINGS
• The drawings show electron microscope photographs of representative cross-linked fibers of the present invention.[0016]
• FIGS. 1[0017]aand1bare photographs at 100× and 200× magnifications, respectively, of blends of fibers obtained as shown in Example 2 from Rayfloc®-J-LD (southern pine Kraft pulp commercially available from Rayonier Performance Fibers Division, Jesup, Ga.) and mercerized fibers in about 1:1 weight ratio using glyoxylic acid (2%) cross-linking agent.
• FIGS. 2[0018]aand2bare photographs at 100× and 200× magnification, respectively, of a blend of fibers of the present invention prepared from Rayfloc®-J-LD and mercerized fibers in 1:1 ratio and cross-linked in accordance with Example 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• As used herein, the terms “absorbent garment,” “absorbent article” or simply “article” or “garment” refer to mechanisms that absorb and contain body fluids and other body exudates. More specifically, these terms refer to garments that are placed against or in proximity to the body of a wearer to absorb and contain the various exudates discharged from the body. A non-exhaustive list of examples of absorbent garments includes diapers, diaper covers, disposable diapers, training pants, feminine hygiene products and adult incontinence products. Such garments may be intended to be discarded or partially discarded after a single use (“disposable” garments). Such garments may comprise essentially a single inseparable structure (“unitary” garments), or they may comprise replaceable inserts or other interchangeable parts.[0019]
• The present invention may be used with all of the foregoing classes of absorbent garments, without limitation, whether disposable or otherwise. Some of the embodiments described herein provide, as an exemplary structure, a diaper for an infant, however this is not intended to limit the claimed invention. The invention will be understood to encompass, without limitation, all classes and types of absorbent garments, including those described herein.[0020]
• The term “component” can refer, but is not limited, to designated selected regions, such as edges, corners, sides or the like; structural members, such as elastic strips, absorbent pads, stretchable layers or panels, layers of material, or the like.[0021]
• Throughout this description, the term “disposed” and the expressions “disposed on,” “disposed above,” “disposed below,” “disposing on,” “disposed in,” “disposed between” and variations thereof are intended to mean that one element can be integral with another element, or that one element can be a separate structure bonded to or placed with or placed near another element. Thus, a component that is “disposed on” an element of the absorbent garment can be formed or applied directly or indirectly to a surface of the element, formed or applied between layers of a multiple layer element, formed or applied to a substrate that is placed with or near the element, formed or applied within a layer of the element or another substrate, or other variations or combinations thereof.[0022]
• Throughout this description, the terms “top sheet” and “back sheet” denote the relationship of these materials or layers with respect to the absorbent core. It is understood that additional layers may be present between the absorbent core and the top sheet and back sheet, and that additional layers and other materials may be present on the side opposite the absorbent core from either the top sheet or the back sheet.[0023]
• Throughout this description, the expressions “upper layer,” “lower layer,” “above” and “below,” which refer to the various components included in the absorbent material are used to describe the spatial relationship between the respective components. The upper layer or component “above” the other component need not always remain vertically above the core or component, and the lower layer or component “below” the other component need not always remain vertically below the core or component. Other configurations are contemplated within the context of the present invention.[0024]
• Throughout this description, the term “impregnated” insofar as it relates to a cross-linking agent impregnated in a fiber, denotes an intimate mixture of cross-linking agents and cellulosic fiber, whereby the cross-linking agent may be adhered to the fibers, adsorbed on the surface of the fibers, or linked via chemical, hydrogen or other bonding (e.g., Van der Waals forces) to the fibers. Impregnated in the context of the present invention does not necessarily mean that the cross-linking agent is physically disposed beneath the surface of the fibers.[0025]
• The present invention concerns chemically cross-linked blends of fibers that are useful in absorbent articles, and in particular, that are useful in forming acquisition layers or absorbent cores in the absorbent article. The particular construction of the absorbent article is not critical to the present invention, and any absorbent article can benefit from this invention. Suitable absorbent garments are described, for example, in U.S. Pat. Nos. 5,281,207, and 6,068,620, the disclosures of each of which are incorporated by reference herein in their entirety including their respective drawings. Those skilled in the art will be capable of utilizing the chemically cross-linked cellulosic fibers of the present invention in absorbent garments, cores, acquisition layers, and the like, using the guidelines provided herein.[0026]
• Cross-linking of fibers in fluff form is believed to improve the physical and chemical properties of the fibers in many ways, such as improving the stiffness, increasing resiliency (in the dry and wet state), increasing the absorbency, reducing wrinkling, and improving shrinkage resistance. Unfortunately, it has been found that such cross-linking, if carried out on a fiber in sheet form, may create problems in the fiber which render it unsuitable for many applications. These problems include severe fiber breakage and increased amounts of knots and nits (hard fiber clumps). These problems are attributed to the inter-fiber (fiber-to-fiber) cross-linking that occurs between fibers in close contact during the curing process. Usually, fibers get into close contact in the dry state due to (a) mechanical entanglement; (b) hydrogen bonding between fibers; and (c) pulping and bleaching residuals such as lignin and hemicellulose. As a result, when fibers treated with a cross-linking agent are heated for curing, fibers in close contact tend to form inter-fiber cross-links rather than intra-fiber cross-links (chain-to-chain within the single fiber).[0027]
• Thus, there is a need for a simple, relatively inexpensive process for cross-linking fiber in sheet form that will provide cross-linked fibers with low liquid retention, enhanced rates of acquisition, and reduced amounts of knots and nits. In another embodiment, the present invention is directed to a method of making cross-linked fibers in sheet form. The method preferably comprises treating cellulose fibers in sheet or roll form with an aqueous solution of a polyfunctional cross-linking agent, followed by drying and curing at sufficient temperature for adequate time to accelerate the formation of covalent bonding between hydroxyl groups of cellulose fibers and functional groups of the cross-linking agent.[0028]
• The method of an embodiment of the invention preferably comprises reacting a blend of fibers in sheet form with one or more reagents selected from organic molecules having carboxylic acid, an aldehyde and carboxylic acid (e.g., an acid aldehyde), or epoxy functional groups. In one embodiment the method of the present invention provides cross-linked fibers in sheet form that can be readily defiberized with low knot-content and without significant fiber breakage. In another embodiment the method of the present invention provides cross-linked fibers that are characterized by an enhanced acquisition rate, resiliency, and absorbency under load. Moreover, cross-linked fibers of the present invention display a reduction in centrifuge retention capacity which makes the fiber especially suited for use in acquisition, distribution and acquisition-distribution layers in absorbent articles intended for fluid management.[0029]
• In one aspect of the present invention, the fiber in sheet form comprises a blend of mercerized fibers and conventional fibers. Throughout this description, the expression “conventional fibers” denotes cellulose fibers of diverse origins, especially those primarily derived from wood pulp. Suitable wood pulp can be obtained from any of the chemical processes known by those of ordinary skill in the art such as Kraft, and sulfite processes. Preferred fibers are those obtained from various soft wood pulp such as Southern pine, White pine, Caribbean pine, Western hemlock, various spruces, (e.g. Sitka Spruce), Douglas fir or mixtures and combinations thereof. Fibers obtained from hardwood pulp sources, such as gum, maple, oak, eucalyptus, poplar, beech, and aspen, or mixtures and combinations thereof can also be used in the present invention. Other cellulose fibers derived form cotton linter, bagasse, kemp, flax, and grass may also be used in the present invention. The fiber can be comprised of a mixture of two or more of the foregoing cellulose pulp products. Particularly preferred “conventional” fibers for use in forming the cross-linked fibers of the present invention are those derived from wood pulp prepared by Kraft and sulfite-pulping processes.[0030]
• The fibers of the present invention preferably have a high surface purity of cellulose, but it is not necessarily required that the cellulosic fibers have a high cellulose bulk purity. It is preferred that the cellulosic fiber be cross-linked in the sheet form, and more preferably, be fiber with “high cellulose purity.” The high cellulose purity refers to the surface purity of the cellulosic fibers. Throughout this description, the expression “high cellulose purity” refers to pulp comprising at least about 65%, preferably at least 75%, and most preferably, at least about 90% α-cellulose.[0031]
• The preferred fibers in sheet form that are cross-linked in accordance with the present method are blends of conventional cellulose and “mercerized fibers.” Throughout this description, the expression “mercerized fibers” denotes any of wood pulp fibers or fibers from any source described, previously treated with an aqueous solution of alkali metal. Fiber mercerization can be carried out by any method known in the art such as those described in, for example,[0032]Cellulose and Cellulose Derivatives, Vol. V, Part 1, Ott, Spurlin, and Grafllin, eds., Interscience Publisher (1954). Fiber mercerization can be performed by mixing pulp in an aqueous solution of alkali metal (i.e. sodium hydroxide), washing, neutralizing, or washing and neutralizing, and optionally drying the pulp.
• Reagents suitable for mercerization include but are not limited to, alkali metal hydroxides, such as sodium hydroxide, potassium hydroxide, calcium hydroxide, and rubidium hydroxide, lithium hydroxide, and benzyltrimethylammonium hydroxide. Sodium hydroxide is a particularly preferred reagent for use in fiber mercerization in accordance with the present invention. The pulp of the invention preferably is treated with an aqueous solution containing from about 8 to about 30% by weight sodium hydroxide, more preferably from about 12 to about 20%, and most preferably 13% to about 16%. Mercerization may be performed during or after bleaching, purification, and drying. Preferably mercerization is carried out during bleaching and or drying processes. After mercerization, the cellulose fibers preferably contain at least about 80% by weight α-cellulose, preferably at least about 90% by weight, more preferably at least about 95% by weight, and even more preferably at least about 97% by weight α-cellulose.[0033]
• Commercially available caustic extractive pulp (i.e., mercerized pulp”) suitable for use in the present invention includes, for example, Porosanier-J-HP, available from Rayonier Performance Fibers Division (Jesup, Ga.), and Buckeye's HPZ products, available from Buckeye Technologies (Perry, Fla.). In one aspect of the present invention, it is preferred that the pulp fibers be in sheet or roll form.[0034]
• In accordance with the invention, the sheet or roll form of cellulose preferably is a blend of mercerized fiber and conventional fiber containing about 10% to about 60% by weight conventional fiber, more preferably from about 20% to about 60% by weight conventional fiber, and most preferably from about 30% to about 50% by weight conventional fiber, based on the total weight of the mixture of fibers.[0035]
• In another aspect of the invention, the fiber can be used in wet or dry state. It is preferred in the present invention that the cellulosic fibers be employed in the dry state.[0036]
• Cross-linking agents suitable for use in the present invention are polyfunctional molecules. As used herein, the expression “polyfunctional molecule” refers to a polymeric or monomeric molecule able to form a bridge between adjacent cellulose chains. Accordingly, any material capable of reacting with more than one hydroxyl group of cellulose chains can be suitable for use in the present invention. Particularly suitable cross-linking agents for use in the present invention are those carrying functional groups such as, for example, carboxyl, aldehyde and epoxy. Preferably, the cross-linking agents include dialdehydes, acid aldehydes, polycarboxylic acids, and polyepoxides.[0037]
• Cross-linking agents preferred for use in the present invention include acid aldehydes. As used herein, “acid aldehyde” refers to organic molecules having carboxylic acid and aldehyde functional groups, such as, for example, glyoxylic acid and succinic semialdehyde. A particularly preferred acid aldehyde cross-linking agent is glyoxylic acid.[0038]
• Other suitable cross-linking agents include polycarboxylic acids. Especially suitable polycarboxylic acids are those having at least two carboxyl groups, such as, 1,2,3,4-butanetetracarbocylic acid, 1,2,3-propanetricarboxylic acid, oxydisuccinic acid, citric acid, itaconic acid, maleic acid, tartaric acid, glutaric acid. Particularly preferred polycarboxylic acids are 1,2,3,4-butanetetracarbocylic acid and citric acid.[0039]
• Other suitable polycarboxylic acids include polymeric polycarboxylic acids such as those specially prepared from monomers such as, for example, acrylic acid, vinyl acetate, maleic acid, maleic anhydride, carboxy ethyl acrylate, itanoic acid, fumaric acid, methacrylic acid, crotonic acid, aconitic acid, acrylic acid ester, methacrylic acid ester, acrylic amide, and methacrylic amide, butadiene, styrene, or combinations thereof.[0040]
• Commercially available examples of these polymers and co-polymers include polyacrylic acid, polymaleic acid, polyitaconic acid, polyaspartic acid, polymethacrylic acid, poly(acrylic acid-co-maleic acid), poly(acrylamide-co-acrylic acid), poly(etheylene-co-acrylic acid), and poly(styrene-co-maleic acid). Particularly preferred polycarboxylic acids are polymaleic acid, polyacrylic acid, and a co-polymer of acrylic acid and maleic acid.[0041]
• Other cross-linking agents suitable for use in the present invention include polyepoxides, particularly those containing hydrophobic saturated, unsaturated, branched and un-branched alkyls. Examples of these include 1,4-cyclohexanedimethanol diglycidyl ether, diglycidyl 1,2-cyclohexanedicrboxylate, N,N-diglycidylaniline, N,N-diglcidyl-4-glycidyloxyaniline, and diglycidyl 1,2,3,4-tetrahydrophthalate and glycerol propoxylate triglycidyl ether. A particularly preferred polyepoxide is 1,4-cyclohexanedimethanol diglycidyl ether.[0042]
• In another aspect of the invention, a mixture or combination of cross-linking agents may be used. In another aspect, the present invention provides chemically cross-linked fibers in sheet form, that are cross-linked with a blend of cross-linking agents including those described above.[0043]
• In one embodiment of the invention, the cross-linking agent may be applied to the cellulose fiber in an aqueous solution. Preferably the aqueous solution has a pH of from about 1 to about 5, more preferably from about 2 to about 3. The present inventors have discovered that an aqueous solution of acid aldehyde cross-linking agent can be used as is without any adjacent or additional pH control agent.[0044]
• In another embodiment of the invention, a water insoluble cross-linking agent, e.g. polyexpoxide, may be used. When such a water insoluble cross-linking agent is used, it is preferred to add a minor amount of surfactant (e.g., a few drops—less than 1% by weight) to emulsify the cross-linking agent prior to fiber application. The cross-linking agent may then be applied to the fiber as a dispersion, instead of an aqueous solution.[0045]
• In general, any type of surfactant capable of forming a dispersion with the water insoluble cross-linking agent can be used. Suitable surfactants include nonionic, anionic, or cationic surfactant or mixtures and combinations of surfactants that are compatible with each other. Preferably the surfactant is selected from Triton X-100 (Rohm and Haas), Triton X-405 (Rohm and Haas), sodium lauryl sulfate, and lauryl bromoethyl ammonium chloride, ethoxylated nonylphenols, polyoxyethylene alkyl ethers, polyoxyethylene alkyl ethers, and polyoxyethylene fatty acid esters.[0046]
• The cellulosic fiber preferably is treated with an effective amount of cross-linking agent to achieve the absorbent properties and physical characteristics described herein. Generally, the concentration of the cross-linking agent in aqueous solution is sufficient to provide from about 0.5 to 10.0 weight percent cross-linking agent on fiber, more preferably from about 1 to 6 weight percent, and most preferably from about 2 to 5 weight percent.[0047]
• Optionally, the method of forming the cellulosic fiber in accordance with the invention includes a catalyst to accelerate the formation of an ester linkage between the hydroxyl groups of the cellulose and the carboxyl groups of the polycarboxylic acid and acid aldehyde cross-linking agents. A catalyst also may be used to accelerate the formation of acetal cross-links between hydroxyl groups of cellulose and aldehyde functional groups of acid aldehyde cross-linking agents. When an acid aldehyde is used as the cross-linking agent, however, a catalyst is not required. To the extent that a catalyst is used, any catalyst known in the art that is capable of accelerating the formation of an ester cross-link between a hydroxyl group and an acid group, or capable of accelerating the formation of an acetal cross-link between a hydroxyl group of cellulose and an aldehyde group could be used in the present invention. Suitable catalysts for use in the present invention to accelerate the formation of ester cross-links include alkali metal salts of phosphorous containing acids such as alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, and alkali metal sulfonates. A particularly preferred catalyst of this type is sodium hypophosphite.[0048]
• Suitable catalysts for use in the present invention to accelerate the formation of acetal cross-links are Lewis acids consisting of a metal and a halogen, such as for example FeCl[0049]₃, AlCl₃, and MgCl₂.
• A catalyst may also be used to promote the reaction between polyepoxides and cellulose hydroxyl groups, to the extent a cross-linking agent containing polyepoxide groups is used as a cross-linking agent. Any catalyst known in the art to accelerate the formation of an ether bond or linkage between the hydroxyl groups of cellulose and an epoxide group can be used in the present invention. Preferably, the catalyst is a Lewis acid selected from aluminum sulfate, magnesium sulfate, and any Lewis acid consisting of a metal and a halogen, including, for example FeCl[0050]₃, AlCl₃, and MgCl₂. The catalyst can be applied to the fiber as a mixture with the cross-linking agent, before the addition of the cross-linking agent, or after the addition of cross-linking agent to cellulosic fiber. Preferably, the ratio of catalyst to cross-linking agent is, for example, from about 1:2 to 1:10, more preferably from about 1:4 to 1:8.
• Optionally, in addition to the cross-linking agent, other finishing agents such as water repellent, softening, and wetting agents also can be used to treat the cellulosic fiber. Examples of softening agents include fatty alcohols, fatty acids amides, polyglycol ethers, fatty alcohols sulfonates, and N-stearyl-urea compounds. Examples of wetting agents include fatty amines, salts of alkylnaphthalenesulfonic acids, alkali metal salts of dioctyl sulfosuccinate, and the like.[0051]
• Any method of applying the cross-linking agent(s) to the fiber can be used in carrying out the cross-linking method of the invention. Acceptable methods include, for example, spraying, dipping, impregnation, and the like. Preferably, the fiber is impregnated with an aqueous solution of cross-linking agent. Impregnation usually creates a uniform distribution of cross-linking agent on the sheet and provides a better penetration of cross-linking agent into the interior part of the fiber.[0052]
• In one embodiment of the invention, a sheet including a blend of mercerized and conventional fibers in roll form is conveyed through a treatment zone where a cross-linking agent(s) is applied on both surfaces by conventional methods such as spraying, rolling, dipping, knife-coating or other manners of impregnation. A preferred method of applying the aqueous solution of the cross-linking agent(s) to fiber in roll form is by puddle press, size press, and blade coater.[0053]
• In one embodiment of the present invention, a blend of fibers in sheet or roll form after having been treated with a solution of cross-linking agent then preferably is transported by a conveying device such as a belt or a series of driven rollers through a two-zone oven for drying and curing.[0054]
• Fibers in roll or sheet form after treatment with the cross-linking agent preferably are dried and cured in a two stage process, and even more preferably dried and cured in a one stage process. Such drying and curing removes water from the fiber, thereupon believed to induce the formation of an ester and an ether linkage between hydroxyl groups of fiber and cross-linking agent(s). Curing usually is carried out in a forced draft oven preferably from about 300° F. to about 450° F., and more preferably from about 320° F. to about 430° F., and most preferably from about 350° F. to about 420° F. Curing preferably is carried out for a certain period of time that permits complete fiber drying and efficient cross-linking. It is preferred that the cellulosic fiber is cured for a period of time ranging from about 5 min to about 25 min, and more preferably from about 7 min to about 20 min, most preferably from about 10 min to about 15 min.[0055]
• The blend of cellulosic fibers cross-linked in accordance with the present invention can be characterized as having an absorbent capacity within the range of about 7.0 g/g to about 12.0 g/g. Preferably, the fibers have an absorbent capacity of at least 8.0 g/g, more preferably at least 9.0 g/g, even more preferably at least 10.0 g/g or higher. The cross-linked cellulosic fibers can have a centrifuge retention as determined by the Hanging Cell Test method within the range of about 0.4 g/g to about 0.6 g/g. Preferably, the fibers have a centrifuge retention of less than about 0.6 g/g, more preferably less than about 0.55 g/g, and most preferably less than about 0.5 g/g.[0056]
• The cellulosic fibers cross-linked in accordance with the present invention preferably possess characteristics that are desirable in absorbent articles. For example, the cross-linked cellulosic fibers preferably have a centrifuge retention capacity of less than about 0.6 grams of synthetic saline per gram of fiber (hereinafter “g/g”). The chemically cross-linked cellulose fibers also have desirable properties, such as a free swell of greater than about 10 g/g, an absorbent capacity of greater than about 8.0 g/g, an absorbency under load of greater than about 8.0 g/g, less than about 10% of knots, less than about 6.5% of fines, and an acquisition rate upon the third insult (or third insult strike-through) of less than about 13.0 seconds. The particular characteristics of the cross-linked cellulosic fibers of the invention are determined in accordance with the procedures described in more detail in the examples.[0057]
• The centrifuge retention capacity measures the ability of the fiber to retain fluid against a centrifugal force. It is preferred that the blend of fibers of the invention have a centrifuge retention capacity of less than about 0.6 g/g, when cross-linked with any cross-linking agent, more preferably, less than about 0.55 g/g, even more preferably less than 0.5 g/g. The cross-linked cellulosic fibers of the present invention can have a centrifuge retention capacity as low as about 0.40 g/g. It also is preferred that the fibers of the invention have a centrifuge retention capacity of less than about 0.60 g/g when cross-linked with an acid aldehyde cross-linking agent, more preferably, less than about 0.55 g/g, even more preferably less than 0.50 g/g, and most preferably less than about 0.45 g/g.[0058]
• It is preferred that the fibers of the invention have an absorbent capacity of more than about 8.0 g/g, more preferably, greater than about 8.5 g/g, even more preferably greater than about 9.0 g/g, and most preferably greater than about 10.0 g/g.[0059]
• The absorbency under load measures the ability of the fiber to absorb fluid against a restraining or confining force of 0.3 psi over a given period of time. It is preferred that the blend of fibers of the invention have an absorbency under load of greater than about 7.0 g/g, more preferably, greater than about 7.5 g/g, and most preferably, greater than about 8.0 g/g.[0060]
• The third insult strikethrough measures the ability of the fiber to acquire fluid, and is measured in terms of seconds. It is preferred that the fibers of the invention have a third insult strike-through of less than about 15.0 seconds, more preferably, less than about 14 seconds, even more preferably less than 13 seconds, and most preferably less than about 12 seconds.[0061]
• The cross-linked fibers of the present invention preferably have less than about 10% of knots, more preferably less than about 8% knots, and most preferably, less than about 5% knots. The cross-linked fibers of the present invention also preferably have less than about 8.0% of fines, preferably less than about 7.0% fines, and most preferably, less than about 6.0% fines.[0062]
• In addition to being more economical, there are several other advantages for cross-linking fibers in sheet form in accordance with the present invention. Fibers cross-linked in sheet form have typically been expected to have an increased potential for inter-fiber cross-linking which leads to “knots” and “nits” resulting in poor performance in some applications. For instance, when a standard purity fluff pulp, Rayfloc-J, is cross-linked in sheet form, the “knot” content increases substantially, indicating increased deleterious inter-fiber bonding. Examination of these “knots” recovered by classification showed they contained true “nits” (hard fiber bundles). Surprisingly, it was found that a blend of mercerized pulp and conventional pulp cross-linked in sheet or roll form actually yields fewer knots and nits than control pulps having conventional cellulose purity cross-linked under the same conditions. Significantly, a blend of fibers in sheet or roll form that were cross-linked in accordance with the present invention were found to contain fewer knots than commercial individualized cross-linked cellulose fibers, like those produced by the Weyerhaeuser Company, commonly referred to as HBA (for high-bulk additive), and by The Proctor & Gamble Company (“P&G”).[0063]
• In another aspect of the invention, it also has been discovered that the presence of knots to a certain level in the cross-linked fiber of the present invention enhances the performance of the cross-linked fiber when used as an acquisition layer in hygiene products. In this instance, the knots present in the cross-linked fiber are within the range of 2% to 15%, more preferably from 3% to 12%, and most preferably from 5% to 10%.[0064]
• Another benefit of the present invention is that cellulose fibers cross-linked in sheet form in accordance with the present invention enjoy equal performance characteristics to conventional individualized cross-linked cellulose fibers, but avoid the processing problems associated with dusty individualized cross-linked fibers.[0065]
• Scanning electron microscope (S360 Leica Cambridge Ltd., Cambridge, England) photographs illustrated in FIGS. 1A and 1B represent cross-linked fibers of the present invention obtained by cross-linking a blend of conventional fiber (Rayfloc®-J-LD) and Porosanier, in 1:1 ratio by weight, using glyoxylic acid (2%) as the cross-linking agent. The photographs were taken at 100× magnification for FIG. 1A, and 200× magnification for FIG. 1B.[0066]
• Scanning Electron Microscope (SEM) photographs illustrated in FIGS. 2A and 2B represent cross-linked fibers of the present invention obtained by cross-linking a blend of conventional fiber (Rayfloc®-J-LD) and Porosanier, in 1:1 ratio by weight, using DP-60 (5%) as the cross-linking agent. The photographs were taken at 100× magnification for FIG. 2A, and 200× magnification for FIG. 2B.[0067]
• As shown in these figures, the cross-linked fibers of the present invention are twisted and curled. The photographs show a mixture of round, circular-shaped fibers and flat, ribbon-like fibers. The round, circular-shaped fibers represent the mercerized fibers while the flat, ribbon-like fibers represent the conventional fiber Rayfloc®-J-LD.[0068]
• Cross-linked cellulosic fibers prepared in accordance with the present invention can be utilized, for example, as a bulking material in the manufacture of high bulk specialty fiber applications that require good absorbency and porosity. The cross-linked fibers can be used, for example, in non-woven, fluff absorbent applications. The fibers can be used independently, or preferably can be incorporated with other cellulosic fibers to form blends using conventional techniques, such as air laying. In an airlaid process, the cross-linked fibers of the present invention, either alone or blended with other fibers, are blown onto a forming screen. A wet laid process may also be used, combining the cross-linked fibers of the invention with other cellulosic fibers to form sheets or webs of blends.[0069]
• The cross-linked fibers of the present invention can be incorporated into various absorbent articles intended for body waste management such as adult incontinent pads, feminine care products, and infant diapers. The cross-linked fibers can be used in an acquisition layer in the absorbent articles. The cross-linked fibers also can be utilized in the absorbent core of the absorbent articles. Towels, wipes and other absorbent products such as filters also may be made with the cross-linked fibers of the present invention. Accordingly, an additional feature of the present invention is to provide an absorbent core and an absorbent article that includes the chemically cross-linked fibers of the present invention.[0070]
• As known in the art, absorbent cores typically are prepared using fluff pulp to wick the liquid, and an absorbent polymer (oftentimes a superabsorbent polymer (“SAP”)) to store liquid. As noted previously, the cross-linked blend of fibers of the present invention have high resiliency and high free swell capacity. Furthermore, the blend of cross-linked fibers are highly porous. Accordingly, the cross-linked fibers of the present invention can be used in combination with the SAP to make an absorbent composite (or core) having improved porosity, bulk, resiliency, wicking, softness, absorbent capacity, absorbency under load, low third insult strikethrough, low centrifuge retention capacity, and the like. The absorbent composite could be used as an absorbent core in absorbent articles intended for body waste management.[0071]
• It is preferred in the present invention that the cross-linked fibers be present in the absorbent composite in an amount ranging from about 10% to about 80% by weight, based on the total weight of the composite. More preferably, the cross-linked fibers are present in an amount ranging from about 20% to about 60% by weight. A mixture of cellulosic fibers and cross-linked fibers of the present invention along with the SAP may also be used to make the absorbent composite. Preferably, the cross-linked fibers of the present invention are present in the mixture in an amount ranging from about 1% to about 70% by weight, based on the total weight of the fiber mixture, and more preferably present in an amount from about 10% to about 40% by weight. Suitable additional conventional cellulosic fibers include any of the wood fibers mentioned previously, cold caustic treated fibers, conventional fibers, mercerized fibers and mixtures and combinations thereof.[0072]
• Any suitable superabsorbent polymer, or other absorbent material, can be used, to form the absorbent composite. The SAP can be in the form of, for example, fiber, flakes, or granules capable of absorbing several times its weight of saline (0.9% solution of NaCl in water) and/or blood. The SAP also preferably is capable of retaining the liquid when it is subjected to load. Non-limiting examples of superabsorbent polymers applicable for use in the present invention include any SAP presently available on the market, including, but not limited to, polyacrylate polymers, starch graft copolymers, cellulose graft copolymers, and cross-linked carboxymethylcellulose derivatives, and mixtures and combinations thereof.[0073]
• An absorbent composite made in accordance with the present invention preferably contains superabsorbent polymer in an amount from about 20% to about 60% by weight, based on the total weight of the composite, and more preferably from about 30% to about 60% by weight. The SAP may be distributed throughout an absorbent composite within the voids in the cross-linked fiber or the mixture of cross-linked fibers and cellulosic fibers. In another embodiment, the SAP is attached to the fiber via a binding agent that includes, for example, a material capable of cross-linking the SAP to the fiber via hydrogen bonding (see, for example, U.S. Pat. No. 5,614,570, the disclosure of which is incorporated by reference herein in its entirety).[0074]
• A method of making an absorbent composite of the present invention may include forming a pad comprising cross-linked fibers or a mixture of cross-linked fibers and cellulosic fibers and incorporating particles of superabsorbent polymer in the pad. The pad can be wet laid or airlaid, preferably the pad is airlaid. It also is preferred that the superabsorbent polymer and cross-linked fibers, or mixture of cross-linked fibers and cellulosic fibers, are air laid together.[0075]
• Absorbent cores containing cross-linked fibers and superabsorbent polymer preferably have dry densities ranging from about 0.1 g/cm[0076]³to 0.50 g/cm³and more preferably from about 0.2 g/cm³to 0.4 g/cm³. The absorbent core can be incorporated into a variety of absorbent articles intended for body waste management, such as diapers, training pants, adult incontinence, feminine care products, and toweling (e.g. wet and dry wipes).
• To evaluate the various attributes of the present invention, several tests were used to characterize the cross-linked fibers' performance improvements resulting from the presently described method.[0077]
• The invention will be illustrated but not limited by the following examples.[0078]
• In the examples, all percentages are by weight and all temperatures in degrees Celsius, unless otherwise noted. Also, when referring to pulp weight, the measurement includes equilibrium moisture content. When subjected to testing, all fiber contains about 5% to 7% moisture.[0079]
EXAMPLES
• The following test methods were used to measure and determine various physical characteristics of the inventive cross-linked cellulosic fibers.[0080]
Test Methods
• The Absorbency Test Method[0081]
• The absorbency test method was used to determine the absorbency under load, absorbent capacity, and centrifuge retention capacity of the cross-linked fibers of the present invention. The absorbency test was carried out in a one inch inside diameter plastic cylinder having a 100-mesh metal screen adhering to the cylinder bottom “cell,” containing a plastic spacer disk having a 0.995 inch diameter and a weight of about 4.4 g. In this test, the weight of the cell containing the spacer disk was determined to the nearest 0.0001 g, and then the spacer was removed from the cylinder and about 0.35 g of cross-linked fibers having a moisture content within the range of from about 4% to about 8% by weight were air-laid into the cylinder. The spacer disk then was inserted back into the cylinder on the fiber, and the cylinder group was weighed to the nearest 0.0001 g. The fiber in the cell was compressed with a load of 4 psi for 60 seconds, the load then was removed and the fiber pad was allowed to equilibrate for 60 seconds. The pad thickness was measured, and the result was used to calculate the dry bulk of the cross-linked fiber.[0082]
• A load of 0.3 psi then was applied to the fiber pad by placing a 100 g weight on the top of the spacer disk, and the pad was allowed to equilibrate for 60 seconds, after which the pad thickness was measured. The cell and its contents then were hanged in a Petri dish containing a sufficient amount of saline solution (0.9% by weight saline) to touch the bottom of the cell. The cell was allowed to stand in the Petri dish for 10 minutes, and then it was removed and hung in another empty Petri dish and allowed to drip for 30 seconds. While the pad still was under the load, its thickness was measured. The 100 g weight then was removed and the weight of the cell and contents was determined. The weight of the saline solution absorbed per gram fiber then was determined and expressed as the absorbency under load (g/g).[0083]
• The absorbent capacity of the cross-linked fiber was determined in the same manner as the test used to determine absorbency under load above, except that this experiment was carried out under a load of 0.01 psi. The results are used to determine the weight of the saline solution absorbed per gram fiber and expressed as the absorbent capacity (g/g).[0084]
• The cell from the absorbent capacity experiment then was centrifuged for 3 min at 1400 rpm (Centrifuge Model HN, International Equipment Co., Needham HTS, USA), and weighed. The results obtained were used to calculate the weight of saline solution retained per gram fiber, and expressed as the centrifuge retention capacity (g/g).[0085]
• Fiber Quality[0086]
• Fiber quality evaluations were carried out on an Op Test Fiber Quality Analyzer (Op Test Equipment Inc., Waterloo, Ontario, Canada) and Fluff Fiberization Measuring Instruments (Model 9010, Johnson Manufacturing, Inc., Appleton, Wis., USA).[0087]
• Op Test Fiber Quality Analyzer is an optical instrument that has the capability of measuring the average fiber length, kink, curl, and fines content.[0088]
• Fluff Fiberization Measuring Instrument is used to measure nits and fine contents of fiber. In this instrument, a sample of fiber in fluff form was continuously dispersed in an air stream. During dispersion, loose fibers passed through a 16 mesh screen (1.18 mm) and then through a 42 mesh (0.36 mm) screen. Pulp bundles (knots) which remained in the dispersion chamber and those that were trapped on the 42-mesh screen were removed and weighed. The former are called “knots” and the latter “accepts.” The combined weight of these two was subtracted from the original weight to determine the weight of fibers that passed through the 0.36 mm screen. These fibers were referred to as “fines.”[0089]
Example 1
• This example illustrates a method for making mercerized fibers and a method for forming handsheets from a blend of mercerized and Rayfloc®-J-LD fibers.[0090]
• Fiber mercerization was carried out as follows: A sample of Rayfloc®-J-LD (never dried) was obtained as a 33.7% solid wet lap from the Rayonier mill at Jesup, Ga. Rayfloc®-J-LD is an untreated southern pine Kraft pulp commercially available from Rayonier Performance Fibers Division, Jesup, Ga. A 70.0 g (dry weight base) sample of Rayfloc®-J-LD was treated with an aqueous solution of 16% (w/w) sodium hydroxide at room temperature at a consistency of about 3.5%. The mixture was agitated for about 10 min, then excess NaOH was removed by suction filtration or centrifuge. The resulting mercerized pulp then was washed with excess water, neutralized to a pH of 6.4 with acetic acid solution (0.01 M) at a consistency of about 3.5%, and optionally sheeted and dried.[0091]
• Handsheets of blends of fibers then were formed by thoroughly agitating a slurry of Rayfloc®-J-LD and mercerized fiber in water at consistency of about 3% for about 10 min. The blend of fibers then was formed into sheets on a standard 12×12 inch laboratory sheet mold. The wet sheets were optionally dried. The formed sheets had approximately the same basis weight (762 gsm) and had a density in the range of 3.2 to 4.6 g/cm[0092]³.
• The sheets prepared in accordance with this example 1 then were cross-linked as described in the following examples.[0093]
Example 2
• This example illustrates a method for cross-linking a blend of fibers in the sheet form prepared in the manner described above in example 1. In order to determine the effect of increasing the amount of conventional fiber on absorbent properties of fibers cross-linked in the sheet form, sheets with various proportions of Rayfloc®-J-LD and mercerized fibers were prepared as described in example 1 and used in this example. Sheets were soaked in a bath of 2% solution of glyoxylic acid (obtained as 50% solution in water commercially available from Clarinet Corporation, Charlotte, N.C.) for about 1 to 2 min and then pressed to a pick-up that affords about 2% of glyoxylic acid on fiber. Sheets then were dried and cured in one step process at 190° C. for 15 min.[0094]
• The absorbent capacity, absorbency under load, centrifuge retention, knots and fine contents of the cross-linked sheets were determined. The results are summarized below in Table 1.[0095]

  TABLE 1
  	Ab-
  	sorben-	Ab-	Cen-
  	cy Under	sorbent	trifuge		Knots
  Fiber Blend	Load	Ca-	Re-		and

  Rayfloc ®-	Mercerized	(0.3 psi)	pacity	tention	Fines	nits
  J-LD (%)	Fiber (%)	(g/g)	(g/g)	(g/g)	(%)	(%)
  0	100	9.8	8.3	0.46
  30	70	10.3	8.6	0.43	7.2	0.47
  50	50	10.0	8.7	0.46

Example 3
• This example illustrates a method for cross-linking a blend of fibers in sheet form using DP-60 as a cross-linking agent (Belclene® DP-60 is a mixture of polymaleic acid terpolymer with the maleic acid monomeric unit predominating (molecular weight of about 1000) and citric acid commercially available from BioLab Industrial Water Additives Division). Sheets with various proportions of Rayfloc®-J-LD and mercerized fibers were prepared as described above in Example 1, and then were soaked in a bath of DP-60 solution (5%) for about 1 to 2 min and then pressed to a pick-up that affords about 5% of DP-60 on fibers. Sheets were then dried and cured in a one step process at 190° C. for about 15 min.[0096]
• The absorbent capacity, absorbency under load, centrifuge retention, knots and fine contents of cross-linked sheets were determined. The results are summarized in Table 2.[0097]

  TABLE 2
  	Ab-
  	sorben-	Ab-	Cen-
  	cy Under	sorbent	trifuge	Knots
  Fiber Blend	Load	Ca-	Re-	and

  Rayfloc ®-	Mercerized	(0.3 psi)	pacity	tention	nits	Fines
  J-LD (%)	(%)	(g/g)	(g/g)	(g/g)	(%)	(%)
  0	100	9.1	10.8	0.52	1.7	5.7
  30	70	9.0	11.0	0.52	0.3	4.7
  40	60	9.4	11.0	0.52	2.6	4.7
  50	50	9.6	11.0	0.51	4.4	4.7
  60	40	8.4	10.0	0.50	15.5	4.8

• The results of examples 2 and 3 demonstrate that conventional fibers (for example Rayfloc®-J-LD) can be used in an amount of up to about 50% in a blend with mercerized fibers to produce cross-linked fiber in the sheet form with properties similar to those obtained using mercerized fiber alone. As shown in Table 2, the presence of Rayfloc®-J-LD in amounts up to amount 50% showed little or no effect on the absorbent properties of cross-linked fibers and revealed a slight increase in the amount of knots and nits.[0098]
Example 4
• This example illustrates the effect of curing temperature on the absorbent properties of representative cross-linked fibers formed in accordance with the present invention.[0099]
• Sheet were formed as described above in example 1 using a blend of Porosanier-J-HP 70% by weight (mercerized fiber available from Rayonier Performance Fibers Division (Jesup, Ga.)) and Rayfloc®-J-LD 30% by weight. Cross-linking was carried out using glyoxylic acid in the presence and in the absence of a catalyst. Sheets were soaked in a bath of an aqueous solution of glyoxylic acid (2%) and magnesium chloride hexahydrate (0.25%) for about 1 to 2 min and then pressed to a pick-up that affords about 2% of glyoxylic acid and 0.25% of magnesium chloride hexahydrate on fiber.[0100]
• Sheets after treatment were dried in a 50° C. oven to about 5 to 7% moisture contents then cured at various temperatures for about 5 min. The above experiment was repeated without a catalyst.[0101]
• The absorbent capacity, absorbency under load, centrifuge retention, knots and fine contents of the cross-linked fibers, and sheet density were determined. The results are summarized below in Table 3.[0102]

  TABLE 3
  	Absorbency
  	Under
  	Load	Absorbent	Centrifuge
  Cure	(0.3 psi) (g/g)	Capacity (g/g)	Retention (g/g)

  Temperature	No	With	No	With	No	With
  ° C.	Catalyst	Catalyst	catalyst	Catalyst	catalyst	catalyst
  130	8.6	7.5	10.0	8.7	0.63	0.52
  150	8.7	6.8	10.0	8.4	0.57	0.45
  1901	8.7	6.7	9.9	8.0	0.54	0.46
  190	8.5	7.4	10.2	8.6	0.43	0.40

• The results summarized in Table 3 demonstrate that cure temperature has little or no effect on absorbency under load and absorbent capacity of cross-linked fibers, whereas centrifuge retention capacity decreases with increasing cure temperature. These results indicate that higher temperatures are preferred to attain higher degrees of cross-linking. In addition, the results in Table 3 illustrate the effect of using a catalyst on cross-linking efficiency. As shown in Table 3, fiber cross-linked in the presence of a catalyst showed lower centrifuge retention compared to those cross-linked in the absence of a catalyst, especially when cross-linking is carried out at temperatures below 190° C. However, at cure temperatures of about 190° C., the presence or absence of catalyst had little or no effect on cross-linking efficiency.[0103]
Example 5
• This example illustrates the effect of curing time on absorbent properties of representative cross-linked fibers formed in accordance with the present invention.[0104]
• Cross-linking was carried out as described above in Example 2, except that, in this example a catalyst, magnesium chloride hexahydrate (0.25%) was used in addition to the cross-linking agent. The fiber was cured at 150° C. for various curing times. Sheets used in this experiment were formed as described in Example 1 using Rayfloc®-J-LD and Porosanier-J-HP in a ratio of 1:1. The results are summarized below in Table 5.[0105]

  TABLE 5
  	Absorbency
  Cure Time	Under Load (0.3 psi)	Absorbent	Centrifuge
  (min)	(g/g)	Capacity (g/g)	Retention (g/g)
  51	7.3	8.6	0.50
  5	7.3	8.6	0.45
  7	6.9	8.3	0.45
  10	7.3	8.6	0.44

• As shown in Table 5, cure times ranging from about 5 to about 10 minutes had little or no effect on absorbency under load and absorbent capacity. However, the centrifuge retention decreasing by increasing curing time indicates that cross-linking efficiency can be increased by increasing the curing time.[0106]
Example 6
• This example illustrates the effect of using various amounts of cross-linking agent on absorbent properties of produced fibers.[0107]
• Sheets used in this experiment were formed as described above in example 1 using 70% by weight Porosanier-J-HP and 30% by weight Rayfloc®-J-LD. Fiber treatment was carried out as shown in Example 2 except that in this example various amounts of cross-linking agents were used. Glyoxylic acid was used as a cross-linking agent in the presence of a constant amount of the catalyst magnesium chloride hexahydrate (0.25%). Treated sheets were dried at 50° C. then cured at 150° C. for 5 min. The results are summarized below in Table 6.[0108]

  TABLE 6
  Cross-linking			Centrifuge
  agent	Absorbency Under	Absorbent	Retention
  (%)	Load (0.3 psi) (g/g)	Capacity (g/g)	(g/g)
  1.0	7.5	7.8	0.48
  2.0	6.8	8.4	0.45
  3.0	7.4	8.4	0.44
  4.0	7.8	8.5	0.42

Example 7
• This example illustrates the effect of using varying amounts of catalyst on the absorbent properties of cross-linked fiber. The sheets used in this example were prepared as described in example 1 using 70% Porosanier-J-HP by weight and 30% Rayfloc®-J-LD by weight.[0109]
• Fiber treatment was carried out as described above in Example 2 except that in this example, glyoxylic acid (2%) was used as a cross-linking agent in the presence of various amounts of magnesium chloride hexahydrate. Treated sheets were dried at 50° C., and then cured at 150° C. for 5 min.[0110]
• The results are summarized below in Table 7.[0111]

  TABLE 7
  Catalyst			Centrifuge
  MgCl2.6H2O	Absorbency Under	Absorbent	Retention
  (%)	Load (0.3 psi) (g/g)	Capacity (g/g)	(g/g)

  0.00	8.7	10.0	0.57
  0.05	9.4	8.8	0.46
  0.10	8.2	8.9	0.46
  0.25	6.8	8.4	0.45
  0.40	7.5	8.4	0.45

• The results summarized in Table 7 indicate that increasing the amount of catalyst showed little or only a slight effect on cross-linking efficiency, since as can be seen in Table 7, increasing the amount of catalyst from 0.05% to about 0.4% showed minimal changes in fiber absorbent properties.[0112]
Example 8
• This example illustrates an airlaid method for forming a representative absorbent structure of the present invention.[0113]
• An airlaid absorbent core formed in accordance with the present invention was prepared using an airlaid apparatus known to experts in the art. Fiber (Rayfloc®-J-LD) and superabsorbent particles (P-02-055-01 obtained from BASF), were loaded into the airlaying apparatus. Vacuum then was applied, fibers and superabsorbent particles traveled through plastic tubing and were combined through an air vortex into a plastic cylinder having a 100-mesh metal screen adhering to the cylinder bottom. After fibers and superabsorbent particles were completely combined, the vacuum was discontinued and the resulting pad was removed from the cylinder. The pad was then compressed by a hydraulic press to a pressure of 700 PSI for 3 min. The pressure was then released, and pad was allowed to equilibrate for 60 seconds. The pad thickness was measured before and after pressing and the density was calculated. The total weight of the pad is about 3.0 g composed of 55% by weight superabsorbent particles and 45% by weight fiber.[0114]
Example 9
• This example illustrate the acquisition times of cross-linked fibers made in accordance with the present invention.[0115]
• The acquisition time was determined by the SART test method. The test evaluates the performance of cross-linked fibers as an acquisition layer in absorbent article. The test measures the time required for a dose of saline to be absorbed completely into the absorbent article. The test is conducted on a sample of absorbent core made in accordance with the method described in example 8. The core included about 45% by weight fiber (Rayfloc®-J-LD) and 55% by weight superabsorbent particles (P-02-055-01 obtained from BASF) based on the total weight of the core. The core had a circular shape with a diameter of 60.0 mm, a density of 0.2, and weighed about 3.0 g (+0.1 g).[0116]
• In this test a sample of cross-linked fibers made in accordance with the present invention was airlaid into a 60.0 mm pad. The pad weighed about 0.7 g and was compressed with a load of a 7.6 PSI for 60 seconds before being tested. The core was placed into the testing cell, which consisted of a plastic base and a funnel cup. The base used to hold the sample was a plastic cylinder with an inside diameter of 60.0 mm. The funnel cup was a plastic cylinder with a star shape hole, the outside diameter of the funnel cup was 58 mm. The funnel cup was placed inside the plastic base on top of the sample and a load of 0.6 PSI having a donut shape was placed on top of the funnel cup.[0117]
• The cell and contents were placed on a level surface and dosed with three successive 9.0 mL insults of saline solution; the time interval between doses was 20 min. The doses were added with a Master Flex Pump (Cole Parmer Instrument, Barrington, Ill., USA) to the funnel cup, and the time in seconds required for each dose of saline solution to disappear from the funnel cup was recorded and expressed as an acquisition time.[0118]
• The results are summarized in Table 8.[0119]

  TABLE 8
  		% of Cross-	3rdInsult
  Fiber	Cross-linking Agent	linking agent	(sec)
  Porosanier			21.0
  Porosanier Fiber (sheet	DP-60	5.0	14.0
  form)
  Blend (Rayfloc 50% and	DP-60	5.0	12.0
  Porosanier 50%)
  Blend (Rayfloc 50% and	Glyoxylic acid	2.0	14.2
  Porosanier 50%)
  Blend (Rayfloc 40% and	Glyoxlic acid	3.0	10.3
  Porosanier 60%)
  Porosanier	Glyoxylic acid	2.0	10.5
  Blend (Rayfloc 30% and	Glyoxylic acid	2.0	10.2
  Porosanier 70%)

• The above examples reveal that crosslinked blends of mercerized fibers and conventional cellulose fibers have improved absorbency, absorbency under load, retention capacity, and acquisition times. The blends of the invention therefore preferably are useful in absorbent articles as an acquisition layer and/or an absorbent core.[0120]
• While the invention has been described with reference to particularly preferred embodiments and examples, those skilled in the art recognize that various modifications may be made to the invention without departing from the spirit and scope thereof.[0121]

Claims (63)

What is claimed is:

1. Cross-linked cellulosic fibers comprising a blend of mercerized cellulosic fibers and conventional fibers having an absorbent capacity of at least about 8.0 grams saline/gram of fiber.

2. The cross-linked fibers ofclaim 1 wherein the fibers have a centrifuge retention capacity of less than about 0.6 grams of a 0.9% by weight saline solution per gram of fiber.

3. The cross-linked fibers ofclaim 1, wherein the fibers have an absorption capacity of at least about 9.0 g saline/g fiber.

4. The cross-linked fibers ofclaim 1, wherein the fibers have a centrifuge retention of not more than 0.5 g saline/g fiber.

5. The cross-linked fibers ofclaim 1, wherein the fibers have a free swell of at least about 10.0 g saline/g fiber.

6. The cross-linked fiber ofclaim 1, wherein the fibers have knots and nits contents of less than about 10%.

7. The cross-linked fiber ofclaim 1, wherein the fibers have fines contents of less than about 6.5%.

8. A method of making a cross-linked blend of cellulosic fibers comprising:

forming a wet laid sheet of a blend of mercerized and conventional cellulosic fibers;

supplying a cross-linking agent to the sheet of fibers to form a sheet impregnated with the cross-linking agent; and

drying and curing the cross-linking agent on the impregnated sheet of cellulosic fibers to form intra-fiber cross-links.

9. The method ofclaim 8, wherein the mercerized cellulose fiber is a conventional cellulose fiber that has been contacted with an alkali metal, washed, neutralized, and optionally dried.

10. The method ofclaim 8, wherein the conventional cellulose fiber is a wood pulp fiber selected from the group consisting of hardwood pulp, softwood cellulose pulp obtained from a Kraft or sulfite chemical process, and combinations or mixtures thereof.

11. The method ofclaim 10, wherein the hardwood cellulose pulp is selected from the group consisting of gum, maple, oak, eucalyptus, poplar, beech, aspen, and combinations and mixtures thereof.

12. The method ofclaim 10, wherein the soft cellulose pulp is selected from the group consisting of Southern pine, White pine, Caribbean pine, Western hemlock, spruce, Douglas fir, and mixtures and combinations thereof.

13. The method ofclaim 10, wherein the conventional cellulose fiber is derived from one or more components selected from the group consisting of cotton linters, bagasse, kemp, flax, grass, and combinations and mixtures thereof.

14. The method ofclaim 9, wherein the mercerized fibers are prepared by treating conventional cellulosic fibers with an aqueous solution of sodium hydroxide, washing the fibers, and neutralizing the treated fibers.

15. The method ofclaim 14, wherein treating the conventional cellulosic fibers comprises contacting the fibers with an aqueous solution containing about 4% to about 40% by weight sodium hydroxide, based on the total weight of the solution.

16. The method ofclaim 14, wherein treating the cellulosic fibers with caustic is carried out in an aqueous solution of sodium hydroxide at 2 to 10% consistency.

17. The method ofclaim 14, wherein washing and neutralizing is carried out until the residual water has a pH of between 3 and 8.

18. The method ofclaim 9, wherein the cellulosic fibers are caustic treated in sheet or roll form.

19. The method ofclaim 9, wherein the caustic treated cellulosic fiber is in the dry or wet state.

20. The method ofclaim 9, wherein the α-cellulose content of the caustic treated cellulosic fibers is greater than 65%.

21. The method ofclaim 20, wherein the α-cellulose content is at least 90%.

22. The method ofclaim 8, wherein the cross-linking agent is selected from one or more acid aldehyde organic molecules containing aldehyde and carboxylic acid functional groups.

23. The method ofclaim 23, wherein the acid aldehyde cross-linking agent is selected from the group consisting of glyoxylic acid, succinic semialdehyde, and mixtures and combinations thereof.

24. The method ofclaim 8, further comprising contacting the fibers with a catalyst.

25. The method ofclaim 24, wherein the catalyst is Lewis acid selected from the group consisting of iron(III) Chloride, boron trifluoride, tin (IV) chloride, aluminum potassium sulfate, magnesium chloride, zinc chloride, aluminum sulfate, ammonium chloride, zirconium oxychloride, magnesium nitrate, zinc nitrate, aluminum chloride sodium, potassium bisulfate, and a like.

26. The method ofclaim 8, wherein the cross-linking agent is selected from one or more alkane polycarboxylic acid organic molecules containing at least two acid functional groups.

27. The method ofclaim 26, wherein the alkane polycarboxylic acid cross-linking agent is selected from the group consisting of 1,2,3,4-butantetracarboxylic acid, 1,2,3-propanetricarboxylic acid, oxydisuccinic acid, citric acid, itaconic acid, maleic acid, tartaric acid, glutaric acid and mixtures and combinations thereof.

28. The method ofclaim 8, wherein the cross-linking agent is selected from one or more polymeric polycarboxylic acid organic molecules.

29. The method ofclaim 28, wherein the polymeric polycarboxylic acid cross-linking agent is one or more polymer(s) and copolymer(s) prepared from monomers selected from the group consisting of acrylic acid, vinyl acetate, maleic acid, maleic anhydride, carboxy ethyl acrylate, itanoic acid, fumaric acid, methacrylic acid, crotonic acid, aconitic acid, acrylic acid ester, methacrylic acid ester, acrylic amide, methacrylic amide, butadiene, styrene, or combinations and mixtures thereof.

30. The method ofclaim 8, wherein the cross-linking agent is supplied as an aqueous solution additionally comprising a catalyst.

31. The method ofclaim 30, wherein the catalyst is selected form the group consisting of alkali metal salts of phosphorous containing acids such as alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, and alkali metal sulfonates

32. The method ofclaim 8, wherein the cross-linking agent is selected from one or more polyepoxides having a substituent selected from the group consisting of hydrogen; hydrophobic saturated, unsaturated, cyclic saturated, cyclic unsaturated, branched, and unbranched alkyl groups; and mixtures and combinations thereof.

33. The method ofclaim 26, wherein the alkane polycarboxylic acid cross-linking agent is selected from the group consisting of 1,4 cyclohexanedimethanol diglycidyl ether, diglycidyl 1,2-cyclohexanedicrboxylate, N,N-diglycidylaniline, N,N-diglcidyl-4-glycidyloxyaniline, diglycidyl 1,2,3,4-tetrahydrophthalate, glycerol propoxylate triglycidyl ether, and mixtures and combinations thereof.

34. The method ofclaim 32, wherein the one or more polyepoxide cross-linking agents are supplied as an aqueous solution additionally comprising a surfactant.

35. The method ofclaim 34, Wherein the surfactant is selected from the group consisting of nonionic, anionic, cationic surfactant, or combinations and mixtures thereof.

36. The method ofclaim 8, wherein the cross-linking agent is supplied to the cellulosic fibers in an amount from about 0.5 to 10% by weight based on the total weight of the fiber.

37. The method ofclaim 8, wherein the cross-linking agent is supplied to the cellulosic fibers in an amount from about 2 to 5% by weight based on the total weight of the fiber.

38. The method ofclaim 8, wherein the cross-linking agent comprises an aqueous solution of acid aldehyde having a pH from about 1.5 to about 4.0.

39. The method ofclaim 8, wherein the cross-linking agent comprises an aqueous solution of alkane polycarboxylic acid having a pH from about 1.5 to about 4.0.

40. The method ofclaim 8, wherein the cross-linking agent comprises an aqueous solution of polymeric polycarboxylic acid having a pH from about 1.5 to about 4.0.

41. The method ofclaim 34, wherein the surfactant is added in an amount of from about 0.01 to 5% by weight, based on the total weight of the cross-linking agent.

42. The method ofclaim 8, wherein drying and curing is conducted at a temperature within the range of from about 280° F. to about 435° F.

43. The method ofclaim 8, wherein drying and curing is conducted for a period of time of from about 3 minutes to about 15 minutes at temperatures within the range of from about 320° F. to about 435° F.

44. The method ofclaim 8, wherein the treated fibers are first dried then cured.

45. The method ofclaim 44, wherein the fiber is dried at a temperature below the curing temperature, and curing is conducted for about 1 to about 10 min at a temperature within the range of from about 300° F. to about 435° F.

46. The method ofclaim 45, wherein drying is conducted at temperatures within the range of from about 150 to about 300° F., and curing is conducted for about 0.5 to about 5 minutes at temperatures within the range of from about 320° F. to about 435° F.

47. An absorbent article comprising the cross-linked fiber ofclaim 1.

48. The absorbent article ofclaim 47, wherein the absorbent article is at least one article selected from the group consisting of infant diapers, feminine care products, training pants, and adult incontinence briefs.

49. The absorbent article ofclaim 47, further comprising a liquid penetrable top sheet, a liquid impenetrable back sheet, an acquisition layer, and an absorbent structure, wherein the acquisition layer is disposed beneath the top sheet, and the absorbent structure is disposed between the acquisition layer and the back sheet.

50. The absorbent article ofclaim 49, wherein the acquisition layer comprises the cross-linked fibers.

51. The absorbent article ofclaim 49, wherein the absorbent structure comprises a composite of superabsorbent polymer and cellulosic fibers.

52. The absorbent article ofclaim 51, wherein the superabsorbent polymer is selected from the group consisting of polyacrylate polymers, starch graft copolymers, cellulose graft copolymers, cross-linked carboxymethylcellulose derivatives, and mixtures and combinations thereof.

53 The absorbent article ofclaim 51, wherein the superabsorbent polymer is in the form of fiber, flakes, or granules.

54. The absorbent article ofclaim 51, wherein the superabsorbent polymer is present in an amount of from about 20 to about 60% by weight, based on the total weight of the absorbent structure.

55. The absorbent article ofclaim 51, wherein the cellulosic fibers comprise the cross-linked cellulosic fibers.

56. The absorbent article ofclaim 51, wherein the cellulosic fibers comprises a mixture of the cross-linked cellulosic fibers and cellulosic fibers.

57. The absorbent article ofclaim 56, wherein the cellulosic fiber is a wood pulp fiber selected from the group consisting of hardwood pulp, softwood cellulose pulp obtained from a Kraft or sulfite chemical process, mercerized, rayon, cotton linters, and combinations or mixtures thereof.

58. The absorbent article ofclaim 57, wherein the cross-linked cellulosic fiber is present in the mixture of fibers in an amount of from about 1 to 70% by weight, based on the total weight of the mixture of fibers.

59. The absorbent article ofclaim 58, wherein the cross-linked cellulosic fiber is present in an amount of from about 10 to 40% by weight, based on the total weight of the mixture of fibers.

60. The absorbent article ofclaim 56, wherein the mixture of cross-linked cellulosic fibers and cellulosic fibers is present in an amount of from about 10 to about 80% by weight, based on the total weight of the absorbent structure.

61. The absorbent articleclaim 60, wherein the mixture is present in an amount of from about 20 to about 60% by weight, based on the total weight of the absorbent structure.

62. The method ofclaim 8, wherein the cross-linking agent is a mixture of cross-linking agents selected from the group consisting of: a mixture of glyoxylic acid and citric acid; a mixture of glyoxylic acid and polymaleic acid; and a mixture of glyoxylic acid, citric acid, and polymaleic acid.

63. The method ofclaim 8, wherein the cross-linking agents is a mixture cross-linking agents selected from the group consisting of: a mixture of glyoxylic acid and citric acid; a mixture of glyoxylic acid and a terpolymer of maleic acid, vinyl acetate, and ethyl acrylate; and a mixture of glyoxylic acid, citric acid, and a terpolymer of maleic acid, vinyl acetate, and ethyl acrylate.

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