US20070270070A1 - Chemically Stiffened Fibers In Sheet Form - Google Patents

Abstract

A method for making stiffened cellulosic fiber in sheet form, and the resultant stiffened cellulosic fiber. The method of making the stiffened fibers involves impregnating a cellulosic base fiber in sheet form with a treatment composition solution, drying and curing the impregnated sheet, and thereafter adding water or an aqueous solution of odor removing agent to the cured sheet to produce a sheet of stiffened cellulosic fiber having a moisture content of at least about 6.0 weight %. When defiberized, the stiffened fiber has a low content of fines and knots and nits. The stiffened fiber also exhibits a low degree of yellowing and is substantially free of a burnt-like odor. The stiffened fiber may be used in an absorbent article, such as in a liquid acquisition layer or absorbent core of a baby diaper.

Description

BACKGROUND
• 1. Field
• The embodiments relate, in general to stiffened cellulosic fibers and a process for manufacturing stiffened fibers. More particularly, the embodiments relate to a process that provides stiffened fibers in sheet form useful for making a liquid acquisition layer suitable for use in absorbent products. The stiffened fibers of the embodiments can be described as having rapid liquid absorption and acquisition, high liquid absorbent capacity, low liquid retention capacity, and low contents of fines and knots and nits. In addition, the fiber is substantially free of a burnt-like odor. The embodiments also relate to personal care products that use the stiffened fibers. The personal care products have improved acquisition and reduced rewet.
• 2. Description of Related Art
• Absorbent articles intended for personal care products, such as adult incontinent pads, feminine care products, and infant diapers typically are comprised of at least a top sheet, a back sheet, a storage core disposed between the top sheet and back sheet, and an optional liquid acquisition layer disposed between the top sheet and the storage core. The liquid acquisition layer usually is incorporated in the absorbent articles to provide better distribution of liquid, increased rate of liquid absorption, reduced gel blocking, and improved surface dryness. A wide variety of fibers useful for making acquisition layers are known in the art. Included among these are synthetic fibers, a composite of cellulosic fibers and synthetic fibers, and cellulosic fibers. A cellulosic fiber is preferred because it is made from a natural fiber that is abundant, biodegradable, and relatively inexpensive. Preferably, the cellulosic fiber is cross-linked cellulosic fiber, because cross-linked cellulosic fibers provide a bulky low-density acquisition structure that allows the layer to quickly absorb fluid and then quickly release it to the core.
• Cross-linked cellulosic fibers and processes for making them have been described in the literature for many years (see, for example, G. C. Tesoro,Cross-Linking of Cellulosics, in Vol. II of Handbook of Fiber Science and Technology, pp. 1-46 (M. Lewin and S. B. Sello eds., Mercel Dekker, New York, 1983)). The cross-linked cellulosic fibers typically are prepared by reacting cellulose with polyfunctional agents that are capable of covalently bonding to at least two hydroxyl groups of the anhydroglucose repeat unit of cellulose in neighboring chains simultaneously.
• Cellulosic fibers typically are cross-linked in individualized form. Processes for making cross-linked fiber in individualized form comprise dipping swollen or non-swollen fiber in an aqueous solution of cross-linking chemicals and a catalyst. The fiber so treated usually is cross-linked by heating it at elevated temperature 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. 0,427,316 B1, the disclosures of each of which are incorporated by reference herein in their entirety.
• Cellulosic fiber also can be cross-linked in non-aqueous solution. A process for making cross-linked fiber in non-aqueous solution is shown in U.S. Pat. No. 4,035,147 by Sangenis, et al. (this disclosure of which is incorporated by reference herein in its entirety). The patent discloses that cellulosic fibers can be cross-linked by contacting dehydrated, non-swollen fibers with crosslinking agent and a catalyst in a substantially non-aqueous solution that contains an insufficient amount of water to cause the fiber to swell.
• Despite the commercial availability and practicality, cross-linked cellulosic fibers have not been widely adopted in absorbent products, seemingly because of the difficulty of successfully cross-linking cellulosic fibers in sheet form. More specifically, it has been found that cross-linked fiber in sheet form tends to become more difficult to defiberize without causing substantial problems with the fibers. These problems include severe fiber breakage and increased amounts of knots and nits (hard fiber clumps). Another short-coming is that upon wetting, the cross-linked fiber tends to emit a strong, burnt-like odor that is objectionable to most manufacturers of personal care products. This odor has been found to be common in fibers that have been heated to relatively high temperatures. The odor becomes stronger when the fiber is heated while in contact with a conventional cross-linking agent such as, for example, citric acid. It is believed that the odor may be related to compounds formed from cellulose and cross-linking agents during the heating process. These compounds can include aldehydes, ketones, acids, and some other organic materials. These disadvantages render cross-linked fiber unsuitable for applications in absorbent articles intended for body waste management.
• Efforts to make cross-linked fibers in sheet form have met with limited success. For example, Chatterjee, et al., showed in U.S. Pat. No. 3,932,209 (the disclosure of which is incorporated herein by reference in its entirety) that fiber with modified morphology and having low contents of hemicellulose and lignin, such as mercerized fibers, can be cross-linked in sheet form without substantial formation of knots and nits. Unfortunately, the use of mercerized fiber to produce cross-linked fiber in sheet form is relatively expensive.
• In previous work, (e.g., U.S. patent application Ser. No. 10/683,164 (Publication No. 2005-0079361 A1) entitled “Materials Useful In Making Cellulosic Acquisition Fibers In Sheet Form” filed Oct. 10, 2003, the disclosure of which is incorporated herein by reference in its entirety) it was shown that conventional fibers in sheet form can be successfully cross-linked using modified cross-linking agents. The modified cross-linking agent acted as a cross-linking agent and as a wedge that lowered the inter-fiber bonding and increases fiber bulkiness. This minimized the formation of knots and nits during fiber cross-linking. The resultant cross-linked fibers showed similar or better performance characteristics than conventional cellulose fibers cross-linked in individualized from.
• The description herein of certain advantages and disadvantages of known cellulosic fibers, treatment compositions, and methods of their preparation, is not intended to limit the scope of the embodiments. Indeed, the embodiments may include some or all of the methods, fibers and compositions described above without suffering from the same disadvantages.
SUMMARY
• In view of the difficulties presented by methods for making cross-linked fibers, there remains a need for a simple, commercially feasible process suitable for making cross-linked or stiffened fiber in sheet form. A need also exists for stiffened fibers in sheet form that, upon defiberizing, produce fiber having low contents of fines, and knots and nits. In addition, a need exists for a stiffened fiber that is substantially free of a burnt-like odor and that is capable of neutralizing the odor released from body fluid. The stiffened fiber preferably has high liquid absorbent capacity and low liquid retention capacity. The embodiments described herein desire to fulfill these needs and to provide further related advantages.
• It is therefore a feature of an embodiment to provide a method for making stiffened fibers in sheet form. The method involves providing a treatment composition, providing a cellulosic base fiber in sheet form, impregnating the sheet with the treatment composition, drying and curing the impregnated sheet to produce stiffened fiber in sheet form, and thereafter adding water to the cured sheet to increase the moisture content of the fiber to over 6.0 weight %. The moisture content represents the amount of moisture contained in the fiber, expressed as parts by weight of water per 100 parts of stiffened fibers.
• An alternative method of making stiffened cellulosic fibers in sheet form involves providing cellulosic base fiber, providing a treatment composition, forming a suspension of the cellulosic base fibers in the treatment composition, converting the suspension into a wet-laid sheet, pressing the wet-laid sheet until it has a wet pick-up of about 50 weight % to about 200 weight %, drying and curing the wet-laid sheet to form a sheet of stiffened cellulosic fibers, and then applying water to the sheet of stiffened cellulosic fibers to increase its moisture content to no more than about 6.0 weight %.
• It also is a feature of an embodiment to provide a stiffened fiber made by one of the above-described methods. It also is a feature of an embodiment to provide an airlaid structure, and an absorbent article comprising the stiffened fiber.
• These and other objects, features and advantages will appear more fully from the following detailed description of the preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a photograph at 1000× magnification of a commercially available cross-linked cellulosic fiber.
• FIG. 2 is a photograph at 500× magnification of a cellulosic fiber cross-linked in sheet form.
• FIG. 3 shows the absorbent capacity and centrifuge retention capacity (CRC) values of stiffened cellulosic fiber (SF), conventional un-crosslinked cellulosic fluff pulp (Rayfloc-J-LDE), mechanically-treated conventional cellulosic fluff pulp (CTMP), and commercially-available cross-linked cellulosic fibers (STCC).
• FIG. 4 shows the acquisition time and rewet for diaper samples having various acquisition layers, after a second fluid insult, as described in Example 4.
• FIG. 5 shows the acquisition time and rewet for diaper samples having various acquisition layers, after a third fluid insult, as described in Example 4.
• FIG. 6 shows the 2^ndand 3^rdacquisition times, measured according to the SART test method, for various diaper samples as described in Example 5.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
• Various embodiments described herein provide a stiffened cellulosic fiber in sheet form that, when defiberized, produces fibers having low contents of fines, knots and knits. In addition, the stiffened fibers of the embodiments are substantially free of a burnt-like odor and have a high degree of brightness.
• The stiffened cellulosic fibers of the embodiments have the same or better performance characteristics as conventional individualized cross-linked cellulose fibers. However, the stiffened cellulosic fibers of the embodiments differ from conventional cross-linked fibers in several ways. First, the stiffened fibers are not curled or twisted in any manner like commercial cross-linked cellulosic fibers are (see, for example, the fiber shown inFIG. 1). Because of the twisted and curled nature of conventional cross-linked fiber (such as fibers marketed under the name STCC, developed by the Weyerhaeuser Company, Federal Way, Wash.) it is difficult to produce a uniform air-laid structure from the fibers. In contrast, stiffened cellulosic fibers of the embodiments are stiff and linear as shown inFIG. 2, which is a photograph of a linear cellulosic fiber that has been cross-linked in sheet form. Other exemplary linear fibers that have been cross-linked in sheet form may be found in U.S. patent application Ser. No. 10/683,164 (Publication No. US 2005-0079361 A1), filed on Oct. 14, 2003 by Othman Hamed, et al., which is incorporated by reference herein in its entirety. The stiffened fibers have a fiber length and kink angle that are similar to conventional base cellulosic fiber, such as Rayfloc®-JLD-E fiber, manufactured by Rayonier Inc., Jesup, Ga. Because of the stiff and linear configuration of the stiffened cellulosic fibers, a uniform and stabilized air-laid structure can be produced from the fibers.
• In addition, stiffened cellulosic fiber of the embodiments is available in roll form, rather than bale form as conventional cross-linked fibers are provided. The bale form of conventional cross-linked fibers is difficult to handle and produces a lot of dust. Therefore, processors of the conventional cross-linked fibers must use special and expensive handling equipment so successfully process the fibers. In contrast, material provided in roll form is much easier to handle, and does not produce dust.
• Furthermore, the inventors have unexpectedly found that the stiffened cellulosic fibers of the embodiments have relatively low contents of knots and nits and fines when compared to commercially available cross-linked fiber made in individualized form. Like conventional cross-linked cellulosic fibers, stiffened cellulosic fibers have an increased potential for inter-fiber cross-linking that may lead to knots and nits and high contents of fines, resulting in poor performance in certain applications. Thus, it was unexpected to find that stiffened fibers made in accordance with the embodiments actually yields a fiber that has fewer knots and nits than commercial cellulosic fibers cross-linked in individualized form.
• The stiffened fibers of embodiments should be useful in absorbent articles, and in particular, should be useful in forming acquisition-distribution layers or absorbent cores in absorbent articles. The particular construction of the absorbent article is not critical, and any absorbent article can benefit from the embodiments. 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 stiffened fiber of the embodiments in absorbent garments, cores, acquisition layers, and the like, using the guidelines provided herein.
• As used herein, the terms and phrases “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.
• Embodiments 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 embodiments will be understood to encompass, without limitation, all classes and types of absorbent garments, including those described herein.
• 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.
• Throughout this description, the term “impregnated” insofar as it relates to a treatment composition impregnated in a fiber, denotes an intimate mixture of treatment composition and cellulosic fiber, whereby the treatment composition 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 embodiments described herein does not necessarily mean that the treatment composition is physically disposed beneath the surface of the fibers.
• Throughout this description, the expression “stiffened fiber” as used herein refers to a cross-linked cellulosic fiber suitable for use in the acquisition-distribution layer of an absorbent article intended for body waste management. The stiffened fiber imparts bulk and resilience to the layer and provides the layer with a generally open structure that is rapidly absorb the liquid from the point of insult and distributes it over a large area in the storage layer.
• The expression “pulp sheet” as used herein refers to cellulosic fiber sheets formed using a wet-laid process. The sheets typically have a basis weight of about 200 to about 800 gsm and density of about 0.3 g/cc to about 0.6 g/cc. The pulp sheets are subsequently defiberized in a hammermill to convert them into fluff pulp before being used in an absorbent product. Pulp sheets can be differentiated from tissue paper or paper sheets by their basis weights. Typically, tissue paper has a basis weight of from about 5 to about 50 gsm and paper sheets have basis weights of from about 47 to about 103 gsm, both lower than that of pulp sheets.
• Various embodiments described herein provide a method for making the stiffened cellulosic fibers. An exemplary method includes providing a treatment composition, providing a cellulosic base fiber in sheet form, impregnating the fiber with the treatment composition, drying and curing the impregnated sheet, and thereafter adding water to the cured sheet to increase the moisture content of the fiber to over 6.0 wt %.
• In accordance with embodiments, the treatment composition that is useful in making stiffened cellulosic fiber in sheet form includes an aqueous solution of cross-linking agent and, optionally, a catalyst.
• Any cross-linking agent known in the art that is capable of cross-linking the cellulosic fibers can be used in the treatment composition of the embodiments described herein. Suitable cross-linking agents include, for example, alkane polycarboxylic acids, polymeric polycarboxylic acids, aldehydes, and urea-based derivatives. Suitable alkane polycarboxylic acids include, for example, aliphatic and alicyclic polycarboxylic acids containing at least two carboxylic acid groups. The aliphatic and alicyclic polycarboxylic acids could be either saturated or unsaturated, and they might also contain other heteroatoms such as sulfur, nitrogen or halogen. Examples of suitable polycarboxylic acids include: 1,2,3,4-butanetetracarboxylic acid, 1,2,3-propanetricarboxylic acid, oxydisuccinic acid, citric acid, itaconic acid, maleic acid, tartaric acid, glutaric acid, iminodiacetic acid, citraconic acid, tartrate monosuccinic acid, benzene hexacarboxylic acid, cyclohexanehexacarboxylic acid, maleic acid, and any combinations or mixtures thereof.
• Suitable polymeric polycarboxylic acid cross-linking agents include, for example, those formed from monomers and/or co-monomers that contain carboxylic acid groups or functional groups that can be converted into carboxylic acid groups. Such monomers include, for example, acrylic acid, vinyl acetate, maleic acid, maleic anhydride, carboxy ethyl acrylate, itanoic acid, fumaric acid, methacrylic acid, crotonic acid, aconitic acid, tartrate monosuccinic acid, acrylic acid ester, methacrylic acid ester, acrylic amide, methacrylic amide, butadiene, styrene, or any combinations or mixtures thereof.
• Examples of suitable polymeric polycarboxylic acids include polyacrylic acid and polyacrylic acid copolymers such as, for example, poly(acrylamide-co-acrylic acid), poly(acrylic acid-co-maleic acid), poly(ethylene-co-acrylic acid), and poly(1-vinylpyrolidone-co-acrylic acid), as well as other polyacrylic acid derivatives such as poly(ethylene-co-methacrylic acid) and poly(methyl methacrylate-co-methacrylic acid). Other examples of suitable polymeric polycarboxylic acids include polymaleic acid and polymaleic acid copolymers such as, for example, poly(methyl vinyl ether-co-maleic acid), poly(styrene-co-maleic acid), and poly(vinyl chloride-co-vinyl acetate-co-maleic acid). The representative polycarboxylic acid copolymers noted above are commercially available in various molecular weights.
• Suitable aldehyde cross-linking agents include, for example, formaldehyde, glyoxal, glutaraldehyde, glyoxylic acid, and glyceraldehydes. Suitable urea-based derivatives for use in the embodiments include, for example, urea based-formaldehyde addition products, such as, for example, methylolated ureas, methylolated cyclic ureas, methylolated lower alkyl cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, and lower alkyl substituted cyclic ureas. Especially preferred urea-based crosslinking agents include dimethyldihydroxy urea (DMDHU, or 1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone), dimethyloldihydroxyethylene urea (DMDHEU, or 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea (DMU, or bis[N-hydroxymethyl]urea), dihydroxyethylene urea (DHEU, or 4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (DMEU, or 1,3-dihydroxymethyl-2-imidazolidinone), and dimethyldihydroxyethylene urea (DDI, or 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone). Other suitable substituted ureas include glyoxal adducts of ureas, polyhydroxyalkyl urea disclosed in U.S. Pat. No. 6,290,867, and hydroxyalkyl urea and β-hydroxyalkyl amide disclosed in U.S. Pat. No. 5,965,466 (the disclosures of these patents are incorporated herein by reference in their entireties).
• Alternatively, a cross-linking agent suitable for use herein may be comprised of any combination or mixture of two or more of the above mentioned cross-linking agents.
• The treatment composition also may include a catalyst to accelerate the reaction between hydroxyl groups of cellulose and cross-linking agent functional groups. Any catalyst known in the art to accelerate the formation of an ester bond between hydroxyl group and carboxylic acid group or ether bond between the hydroxyl group and aldehyde group may be used. Suitable catalysts for use in the embodiments 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 is sodium hypophosphite. Preferably the weight ratio of catalyst to cross-linking agent in the treatment composition is from about 1:1 to about 1:10, more preferably from about 1:2 to about 1:10, and even more preferably from about 1:3 to about 1:6. (In other words, the percentage of catalyst to cross-linking agent in the treatment composition is about 10 weight % to 100 weight %, more preferably from about 10 weight % to about 50 weight %, and even more preferably from about 16.7 weight % to about 33.3 weight %.)
• Preferably, the catalyst is applied to the cellulosic fiber with the cross-linking agent. Alternatively, the catalyst may be applied to the fiber separately, before the addition of the cross-linking agent, or after the addition of cross-linking agent to the cellulosic fiber.
• The treatment composition may be prepared by any suitable and convenient procedure. For example, the treatment composition may be prepared as a solution, suspension, emulsion, or dispersion.
• Preferably, the treatment composition contains from about 2% to about 10% cross-linking agent. Preferably the treatment composition contains sufficient cross-linking agent to provide from about 0.5 weight % to about 10.0 weight % of cross-linking agent on fiber, more preferably from about 2.0 weight % to about 7.0 weight %, and even more preferably from about 3.0 weight % to about 6.0 weight %. By way of example, 7 weight % of cross-linking agent is equal to 7 grams of cross-linking agent per 100 grams oven dried fiber.
• Preferably, the pH of the treatment composition is adjusted to about 1.5 to about 4.5, more preferably from about 1.5 to about 4.2, and even more preferably from about 1.5 to about 3.5. The pH can be adjusted using alkaline solutions such as, for example, sodium hydroxide or sodium carbonate.
• Applicants have discovered that acidic stiffened fibers are particularly advantageous when used in a personal care product. The phrase “acidic stiffened fiber” as used herein refers to stiffened fiber with a pH below 4.2 (more preferably below 3.5) as determined according the methods provided herein. Acidic stiffened fiber is capable of neutralizing the odor produced by bacteria in bodily fluids such as urine. As such, acidic stiffened fiber may advantageously be used to make an acquisition layer for personal care products. Applicants have discovered that in certain embodiments, acidic stiffened fiber can be made using the treatment composition as-is (i.e., without neutralization). Preferably a treatment composition having low pH (<2.0) does not include a catalyst. Alternatively, the stiffened fiber may be treated with an acidic solution to lower the pH.
• According to the various embodiments, the method of producing the stiffened fibers includes impregnating a cellulosic base fiber with the treatment composition. The cellulosic base fiber may be any conventional or other cellulosic fiber, so long as it is capable of providing the physical characteristics described herein. Suitable cellulosic fiber for use in the embodiments includes that primarily derived from wood pulp. Suitable wood pulp can be obtained from any of the conventional chemical processes, such as the Kraft and sulfite processes. Preferred fiber is that 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. Fiber obtained from hardwood pulp sources, such as gum, maple, oak, eucalyptus, poplar, beech, and aspen, or mixtures and combinations thereof also can be used in the embodiments. Other cellulosic fiber derived from cotton linter, bagasse, kemp, flax, and grass also may be used in the embodiments. The cellulosic base fiber can be comprised of a mixture of two or more of the foregoing cellulosic pulp products. Particularly preferred fibers for use in forming the stiffened cellulosic fiber are those derived from wood pulp prepared by the Kraft and sulfite-pulping processes. In addition, the cellulosic base fiber may be non-bleached, partially bleached or fully bleached cellulosic fiber.
• The cellulosic base fibers can be provided in any of a variety of forms. For example, one embodiment contemplates using cellulosic base fibers in sheet or roll form. In another embodiment, the fiber can be provided in a mat of non-woven material. Fibers in mat form are not necessarily rolled up in a roll form, and typically have a density lower than fibers in sheet form. In yet another feature of an embodiment, the cellulosic base fiber is provided in a wet or dry state. It is preferred that the cellulosic base fibers be provided dry in a sheeted roll form. Alternatively, the sheet of cellulosic base fiber is formed using a wet-laid process, and has a basis weight of about 200 grams per square meter (gsm) to about 800 gsm, and a density of about 0.15 grams per cubic centimeter (g/cc) to about 0.6 g/cc.
• The cellulosic base fiber that is treated in accordance with various embodiments while in the sheet form can be any of wood pulp fibers or fiber from any other source described previously.
• Any method of applying the treatment composition to the fiber in sheet form may be used, so long as it is capable of impregnating the fiber, providing an effective amount of cross-linking agent to the fiber. Preferably, the application method provides about 10% to about 150% by weight of treatment composition on fiber, based on the total weight of the fiber. Acceptable methods of application include, for example, spraying, dipping, impregnation, and the like. Preferably, the fiber is impregnated with an aqueous treatment composition solution. Impregnation typically creates a uniform distribution of treatment composition on the sheet and provides better penetration of treatment composition into the interior part of the sheet. Preferably, the treatment composition is applied to the cellulosic fibers in an amount sufficient to provide about 2% to about 7% by weight, and more preferably about 3% to about 6% by weight of cross-linking agent on fiber, based on the total weight of the fiber.
• In one embodiment, a sheet of cellulosic fibers in roll form is conveyed through a treatment zone where the treatment composition is applied on both surfaces by conventional methods such as spraying, rolling, dipping, knife-coating, slot-coating, or any other manner of impregnation. A preferred method of applying the treatment composition to the fiber in roll form is by puddle press or size press.
• In the various embodiments, after the fiber is impregnated with the treatment composition, the fiber preferably is dried and cured. Such drying and curing removes water from the fiber, thereupon inducing the formation of a linkage between hydroxyl groups of the cellulosic fibers and cross-linking agent.
• In one embodiment, the impregnated fiber in sheet or roll form is transported by a conveying device such as a belt or a series of driven rollers through a three-zone oven for drying and curing. For example, curing typically is conducted in a forced draft oven. Any curing temperature and time can be used so long as they produce the desired effects described herein. Using the present disclosure, persons having ordinary skill in the art can determine suitable drying and curing temperatures and times, depending on the type of fiber, the type of treatment of the fiber, and the desired bonding density of the fiber.
• Preferably the drying and curing is done in a two-stage process, and more preferably a one-stage process. In one preferred embodiment, the impregnated cellulosic fiber is dried and cured in a one-step process at temperatures within the range of 130° C. to about 225° C., for a period of time ranging from about 3 minutes to about 15 minutes. In another embodiment, the drying and curing is conducted in a two-step process. In this embodiment, the drying step dries the impregnated cellulosic fiber, and the dried cellulosic fiber then is cured to form stiffened fibers. In one embodiment where the curing and drying are carried out in a two-step process, the drying step is carried out at a temperature below the curing temperature (e.g., between room temperature and about 150° C.) before the curing step. The curing step is then carried out, for example, for about 1 to 10 minutes at a temperature within the range of 150° C. to about 225° C. Alternately, the curing step may be carried out for about 0.5 minutes to about 5 minutes at a temperature range of about 130° C. to about 225° C.
• In another embodiment, the treatment composition is applied to the wood pulp during or after the bleaching process, preferably after bleaching. Wood pulp bleaching operations generally entail a series of steps that separate the pulp into individual fibers and remove lignin and other wood ingredients. After bleaching, the fiber slurry is transferred to a headbox using special equipment. The fiber slurry is injected or deposited on the forming screen or other foraminous forming surface, usually with the assistance of a vacuum supply, in order to form a wet-laid web with low water content. In a preferred embodiment, the treatment composition is added to the fiber slurry in the headbox prior to the formation of the wet fibrous web. After web formation the web preferably is pressed to produce a sheet which after drying preferably has a density ranging from about 0.15 g/cc to about 0.6 g/cc. After pressing, the sheet preferably has a wet pick-up of from about 50 weight % to about 200 weight percent. By way of example, a 100 weight % wet pick-up means approximately 100.0 grams of treatment composition per 100.0 grams of oven dried cellulosic fibers. The pressed sheet preferably contains about 3.0 wt % to about 7.0 wt % of cross-linking agent and about 0.3 wt % to about 2.0 wt % catalyst. The pressed sheet is then dried and cured in a one-step process or dried and then cured in a two-step process as mentioned previously.
• Alternatively, the application of the treatment composition may occur at any point between the point of initial dewatering of the wet fibrous web to the point the dried fibrous web is wound on the reel or baled for transport to the paper machines. According to the embodiments, the treatment composition may be added at any one of a variety locations in a paper machine. For example, the treatment composition may be added to the locations where the pulps are in aqueous dispersion such as the head box, the machine chest, or stuff box. The treatment composition may be sprayed onto a wet paper web, or applied to a dried paper web. The treatment composition may be effectively applied to the fibers during the drying process, or subsequent to the drying process, such as spraying the treatment composition onto the calendar rolls. A person of ordinary skill in the art would recognize the various places that the treatment composition may be applied to the fibers in a paper machine, using the guidelines provided herein.
• According to the various embodiments, after drying and curing, the sheet of fiber preferably is treated with water. Preferably, the fiber is treated with enough water to increase the moisture content of the acquisition fiber to about 5% to about 10% based on the fiber weight, more preferably greater than about 6%. Preferably, after the cured sheet is treated with water, the oven-dried fiber weight is no greater than 94.0 weight %. Preferably, the water is added to the fiber by spraying, rolling, slot-coating or printing.
• Applicants have found that treating the fiber with water significantly reduces the burnt-like odor associated with the fiber. Cellulosic fibers treated with conventional cross-linking agent(s) and heated at high temperature for curing (typically around 195° C. for 10-15 minutes) tend to have a yellowish discoloration and to have a strong burnt-like odor. Applicants have found that the burnt-like odor produced in the fiber when it is heated during the curing process can be substantially removed by treating the fiber with water after the curing step. Furthermore, Applicants have found that treating the cured fiber with water significantly reduces the contents of knots, nits and fines in the fiber.
• The stiffened fiber optionally may be treated with an odor removing agent. The odor removing agent may be applied to the fiber with the water treatment, or may be separately applied to the fiber. Suitable odor removing agents include oxidizing agents used in wood pulping and bleaching processes such as for example, hydrogen peroxide, chlorine dioxide, peracetic acid, perbenzioc acid, chlorine, chlorine dioxide, ozone, sodium hypochorite and any combination thereof. Other suitable odor removing agents for use in the embodiments include those commercial odor removers used to remove odors such as pet odor, smoke, sweat and the like, from carpets, kitchen, vehicles, bathrooms, garbage pails and disposals, laundry shoes, sport equipment, sewer. Examples of these commercial odor removers include baking soda, talc powder, cyclodextrin, ethylenediamine tetra-acetic acid or other chelating agents, zeolites, activated silica, activated carbon granules, and odor-removing formulas such as DOUBLE-O® and UN-DUZ-IT® (manufactured by II Rep-Z Inc., Coraopolis, Pa.), X-O® (manufactured by X-O Corporation, Dallas, Tex.), and NOK-OUT® (manufactured by Amazing Concepts, LLC. Beaverton, Mich.).
• Preferably the odor removing agent is one of the following: oxidizing agents used in wood pulping and bleaching, cyclodextrin, UN-DUZ-IT, X-O®, or a combination or mixture of thereof. More preferably the odor removing agent is hydrogen peroxide or β-cyclodextrin (β-CD). Hydrogen peroxide and β-CD seem to perform dual functions—in addition to removing the burnt-like odor, they enhance the pulp brightness.
• When hydrogen peroxide is used as an odor removing agent, it preferably is combined with an activating agent such as, for example, a transition metal complex, or N,N,N′N′-tetraacetylethylene diamine reagent.
• Preferably, the odor removing agent is applied to the stiffened fiber in an aqueous solution to provide about 0.001 weight % to about 1.0 weight % of odor removing agent on fiber, based on the weight of the fiber. More preferably, the agent is applied to provide about 0.005 weight % to about 0.50 weight % of odor removing agent on fiber, and even more preferably to provide about 0.01 weight % to about 0.40 weight % of odor removing agent on fiber.
• The application of odor removing agent to stiffened fiber in sheet form may be performed in a number of ways. For example, the odor removing agent may be applied by dipping the stiffened fiber into an odor removing solution, pressing the dipped fiber to remove excess solution, and drying the fiber at a temperature below 320° F. to a moisture content of about 5.0% to about 10.0%. Alternative methods of applying the odor removing agent to the stiffened fiber include spraying, rolling, slot coating, or printing. Yet another alternative application method is spraying the odor removing solution onto defiberized stiffened fibers during the manufacturing of an absorbent core. Preferably, the odor-removing solution is sprayed onto stiffened fiber while in sheet form and immediately after curing. It should be noted that application of an odor removing agent to the acquisition fiber is not limited to application in solution, and may also include application in an emulsion, suspension or dispersion thereof.
• After treatment with the odor removing agent, the stiffened fiber preferably has an ISO Brightness of greater than about 75%, and OD of less than 95%.
• As discussed above, the stiffened cellulosic fiber of the various embodiments is not curled or twisted in any manner.FIG. 1 is a photograph of a commercially-available cross-linked cellulosic fiber, that has a twisted configuration. In comparison,FIG. 2 is a photograph of a cellulosic fiber that has been cross-linked in sheet form. As a result of being cross-linked in sheet form, the stiffened cellulosic fiber is stiff and has a linear fiber configuration, as shown inFIG. 2. Other exemplary linear fibers that have been cross-linked in sheet form may be found in U.S. patent application Ser. No. 10/683,164 (Publication No. US 2005-0079361 A1), filed on Oct. 14, 2003 by Othman Hamed, et al., which is incorporated herein by reference in its entirety.
• The stiffened cellulosic fiber has fiber length and kink angles that are similar to that of conventional cellulosic base fiber. Preferably the stiffened fiber has a fiber length of more than 2.1 mm, a kink angle of less than 55°, and kinks per mm of less than 1.0. More preferably the stiffened fiber has a fiber length of more than 2.3 mm, a kink angle of less than 52°, and kinks per mm of less than 0.9. Even more preferably the stiffened fiber has a fiber length of more than 2.4 mm, a kink angle of less than 50°, and kinks per mm of less than 0.8.
• The applicants have unexpectedly discovered that the stiffened fiber prepared in accordance with the embodiments, has a reduced amount of knots and nits, and fines. Cross-linked cellulosic fibers are expected to have an increased potential for inter-fiber cross-linking which leads to “knots” and “nits” and high contents of fines, resulting in poor performance in certain applications. Thus, it was unexpected to find that stiffened fibers of the embodiments, which have been cross-linked in sheet form then treated with water actually yields knots and nits that are fewer than commercial cellulosic fibers cross-linked in individualized form.
• While the presence of knots and nits is generally not preferable in the stiffened fibers of the embodiments, the presence of knots and nits at a certain limited amounts tends to enhance the acquisition properties of the stiffened fibers, reducing the acquisition time and rewet value of the fiber. Preferably the stiffened fiber of the embodiments has at least about 7.0% knots. Preferably, the knot content of the stiffened fiber is less than about 30.0% knots, more preferably less than about 25.0% knots, and even more preferably less than about 18.0% knots. The stiffened fiber of the embodiments preferably has less than about 12.0% fines, more preferably less than about 10.0% fines, and even more preferably less than about 8.0% fines. The stiffened fiber of the embodiments preferably has more than about 70% accepts. Preferably, the stiffened fiber of the embodiments has between about 7% to about 30% knots and nits, less than about 10% fines, and more than about 70% accept.
• Preferably, the stiffened cellulose fibers of the various embodiments exhibit the same or better performance characteristics as conventional individualized cross-linked cellulose fibers. One way to characterize and assess the potential performance of the stiffened fibers is to measure its absorbent capacity and centrifuge retention capacity (CRC). The value of the absorbent capacity and CRC indicates how much fluid the fiber will absorb and hold on to. The absorbent capacity measures the ability of the fiber to absorb fluid without being subjected to a confining or restraining pressure. The centrifuge retention capacity measures the ability of the fiber to retain fluid against a centrifugal force. Ideally, stiffened cellulosic fiber will typically absorb large quantity of fluid and retain very little. Preferably the stiffened fiber has a centrifuge retention capacity of less than about 0.65 grams of synthetic saline per gram of oven dried (OD) fibers (hereinafter “g/g OD”) and absorbent capacity of greater than about 13.0 g/g OD.
• FIG. 3 shows a comparison between the absorbent capacity and CRC values of stiffened cellulosic fiber (SF), conventional (un-crosslinked) cellulosic fluff pulp (Rayfloc-J-LDE), mechanically-treated conventional cellulosic fluff pulp (CTMP), and commercially-available cross-linked cellulosic fibers (STCC). The results inFIG. 3 demonstrate that the CRC values of conventional fluff and CTMP are higher than the other fibers. This indicates that the conventional fluff pulp and CTMP tend to retain fluids, and do not perform well at surrendering fluid to a storage layer. Therefore, these materials are not desirable for an acquisition layer. In comparison, the CRC values for the SF and STCC fibers are relatively low, indicating that these fibers perform well at surrendering fluid to a storage layer. In addition, the SF and STCC fibers have high absorbent capacity values. Thus, the SF and STCC fibers are capable of absorbing large amounts of fluid, but allow for almost all of this absorbed fluid to be transferred to a storage layer. As a result, the SF and STCC fibers are capable of absorbing subsequent fluid insults.
• The stiffened cellulosic fiber provides manufacturing advantages over conventional individualized cross-linked fibers. For example, conventional cross-linked fibers are available only in bale form which makes it dusty and therefore requires special and expensive equipment for processing. In comparison, the stiffened cellulosic fiber is available in roll form, which is easy to handle and process, without additional equipment. Furthermore, because of their linear geometry the stiffened cellulosic fibers of the embodiments can produce a more uniform and stabilized air-laid structure than conventional cross-linked fibers.
• The characteristics of the stiffened fiber mentioned herein make the stiffened fiber of the embodiments particularly suitable for use, for example, as a bulking material in the manufacturing of high bulk specialty fiber that requires good absorbency and porosity. The stiffened fiber can be used, for example, in non-woven, fluff absorbent products. The stiffened fiber may be used independently, or combined with other cellulosic fibers to form blends using conventional techniques, such as air laying techniques.
• One aspect of the embodiments concerns a process for making a material suitable for use in an absorbent article, such as a liquid acquisition layer or absorbent core. In one exemplary embodiment, the process comprises defiberizing the sheet of stiffened cellulosic fiber to produce individualized stiffened fibers, and forming a liquid acquisition material from the individualized fibers. The stiffened fibers may be defiberized by any technique known or later-developed in the art, such as, for example, by passing the sheet through a hammer mill or by dispersing it in an aqueous medium. Preferably the sheet of stiffened fiber is defiberized using a hammermill. The individualized stiffened fiber is then air-laid to form a material suitable for use in an absorbent article.
• In one embodiment, the stiffened fiber can be converted into a stabilized roll good that may be converted into a liquid acquisition layer or absorbent core, or the like. Preferably the stiffened fiber is converted into a stabilized roll good by a thermal bonding process, wherein the stiffened fiber is wet-laid or air-laid with a low melting point synthetic fiber such as, for instance, bi-constituent fibers. Suitable bi-constituent fibers have two or more fiber-forming polymers such as polypropylene, polyethylene, polyethyleneterephthalate (PET), and the like. The air-laid or wet-laid structure then is dried (if necessary) and heated to form a bond between the fibers. Preferably the stabilized roll good material contains about 1.0 wt % to about 25 wt % of the bi-constituent fiber, based on the total weight of the stabilized roll good material.
• The stiffened cellulosic fiber of the various embodiments may be incorporated into various absorbent articles. For example, the stiffened fibers may be incorporated into absorbent articles intended for body waste management such as adult incontinent pads, feminine care products, and infant diapers. In these absorbent articles, the stiffened fiber can be used as a liquid acquisition layer, or it can be utilized in the absorbent core. The stiffened fibers also may be used in towels and wipes and other absorbent products such as filters.
• In one embodiment, the stiffened fiber is incorporated into an acquisition layer of an absorbent article. The resultant absorbent article exhibits the same or better performance characteristics as an absorbent article with an acquisition layer having commercially-available fibers cross-linked in individualized form. In one embodiment, an absorbent article having an acquisition layer with the stiffened fiber has acquisition and rewet properties and good wet resiliency upon multiple fluid insults.
• The liquid acquisition layer of the embodiment preferably has a basis weight from about 60 to about 640 gsm, more preferably from about 80 to about 480, even more preferably from about 100 gsm to about 400 gsm, and even more preferably from about 120 gsm to about 320 gsm. In a preferred embodiment, the liquid acquisition layer has a density from about 0.02 to about 0.12 g/cm³, and more preferably from about 0.04 to about 0.1 g/cm³.
• The liquid acquisition layer preferably is disposed at least partially between the topsheet layer and the absorbent core in the absorbent article. Alternatively, the liquid acquisition layer may be disposed at least partially between the absorbent core and the backsheet layer.
• The liquid acquisition layer and the absorbent core may have the same overall length and/or the same overall width. Alternately, the liquid acquisition layer may have a length that is longer or shorter than the length of the absorbent core. Preferably, the length of the acquisition layer is 20% to 100% the length of the absorbent core. The acquisition layer may have a width that is wider or narrower than the width of the absorbent core. Preferably, the width of the acquisition layer is 20% to 100% the width of the absorbent core. Preferably, the length of the liquid acquisition layer is not more than 40% of the length of the absorbent core and the width is not more than 80% of the width of the absorbent core. Preferably the acquisition layer is disposed on the absorbent core at or near the area of fluid insult.
• The liquid acquisition layer optionally may include superabsorbent polymer. The expression “superabsorbent polymer” or “SAP” as used herein refers to a polymeric material that is capable of absorbing large quantities of fluid by forming a hydrated gel. Superabsorbent materials are well-known to those skilled in the art as substantially water-insoluble, absorbent polymeric compositions that are capable of absorbing large amounts of fluid (e.g., 0.9% solution of NaCl in water, or blood) in relation to their weight and forming hydrogel upon such absorption. An absorbent core of the embodiments may comprise any SAP known in the art. The SAP can be in the form of particulate matter, flakes, fibers and the like. Exemplary particulate forms include granules, pulverized particles, spheres, aggregates and agglomerates. Exemplary and preferred superabsorbent materials include salts of crosslinked polyacrylic acid such as sodium polyacrylate. Preferably, the SAP is present in an amount ranging from about 8% to about 30% based on the total weight of the liquid acquisition layer.
• The liquid acquisition layer of the embodiments may comprise a mixture of stiffened cellulosic fiber and conventional cellulosic fiber. In a preferred embodiment, the liquid acquisition layer includes from about 1% to about 70% by weight conventional cellulosic fiber, based on the total weight of the acquisition layer. More preferably, the conventional cellulosic fiber is included in an amount ranging from about 5% to about 60% by weight, and even more preferably included in an amount ranging from about 10% to about 50% by weight. Any conventional cellulosic fiber may be used in combination with the stiffened fiber of the embodiments. Suitable conventional cellulosic fibers include any of the wood fibers mentioned previously herein, such as mercerized fibers, rayon, cotton linters, and mixtures and combinations thereof. Preferably the conventional cellulosic fiber in the liquid acquisition-distribution layer is mercerized fiber.
• In another embodiment, the stiffened cellulosic fiber may be incorporated into an absorbent core of an absorbent article. The absorbent core may be used as a component of consumer products such as diapers, feminine hygiene products or incontinence products. One of ordinary skill in the art would appreciate the various ways that absorbent cores can be designed to enhance fluid distribution and retention properties. For example, the absorbent core may include one or more of the following materials, in addition to the stiffened cellulosic fibers: conventional cellulosic fibers, other synthetic or natural fibers, and SAP.
• An absorbent core made in accordance with various embodiments preferably contains SAP in an amount of from about 20% to about 60% by weight, based on the total weight of the composite absorbent core, and more preferably from about 30% to about 60% by weight, based on the total weight of the composite. The SAP may be distributed throughout an absorbent core within the voids in the fiber. Alternatively, the superabsorbent polymer may be attached to the stiffened fiber via a binding agent. Suitable binding agents include, for example, a material capable of attaching 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).
• Any method known or later-developed in the art may be used to form the absorbent core of the embodiments. For example, one method of making an absorbent core comprising stiffened fiber includes forming a pad of stiffened fiber or a mixture of stiffened fiber and other fiber, and incorporating particles of superabsorbent polymer into the pad. The pad can be wet laid or air-laid. Preferably the pad is air-laid. It is preferred that the SAP and the stiffened fiber (or a mixture of stiffened fiber and other fiber) are air-laid together. The fibers and SAP may be formed into individual pads, or formed into a stabilized roll good material, and later separated into individual pads for use in an absorbent article.
• In a preferred embodiment, the absorbent core has about 10 weight % to about 80 weight % stiffened fiber, based on the total weight of the absorbent core. More preferably, the absorbent core has about 20 weight % to about 60 weight % stiffened fiber.
• An absorbent core containing stiffened fiber and superabsorbent polymer preferably has a dry density of about 0.1 g/cc to about 0.50 g/cc, and more preferably from about 0.2 g/cc to about 0.4 g/cc.
• The absorbent core may comprise one or more layers, each of which may comprise stiffened fiber. In one embodiment, one or more layers of the absorbent core comprises a mixture of stiffened fiber, conventional cellulosic fibers, and SAP. Preferably, the layer comprises about 1 weight % to about 70 weight % by weight stiffened fiber, based on the total weight of the fiber mixture, and more preferably about 10 weight % to about 40 weight % stiffened fiber. Any conventional cellulosic fiber may be used in combination with the stiffened fiber in the layer. Suitable conventional cellulosic fibers include any of the wood fibers mentioned previously herein, such as caustic-treated fibers, rayon, cotton linters, and mixtures and combinations thereof.
• The absorbent core, or any layer thereof may comprise a homogeneous composition, where the stiffened fiber is uniformly dispersed throughout the layer. Alternatively, the stiffened fiber may be concentrated in one or more areas of an absorbent core layer. In one embodiment, a single layer absorbent core contains a surface-rich layer of the stiffened fiber. Preferably, the surface-rich layer has a basis weight of about 40 gsm to about 400 gsm. Preferably, the surface-rich layer has an area that is about 30% to about 70% of the total area of the absorbent core.
• In order that the present invention may be more fully understood, the embodiments will be illustrated, but not limited, by the following examples. No specific details contained therein should be understood as a limitation to the embodiments except insofar as may appear in the appended claims.
Test Methods:ISO Brightness
• ISO Brightness evaluations were carried out on various samples of the acquisition fiber in sheet and fluff form, using TAPPI test methods T272 and T525. Selected samples of the stiffened fiber in sheet form were defiberized by feeding them through a hammermill, and then about 5.0 g of the defiberized fluff was airlaid into a circular test sample having approximately a 60 mm diameter. The resultant samples were compressed to a density of about 0.1 g/cm³then evaluated for ISO brightness.
Fiber Quality
• 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).
• The Fluff Fiberization Measuring Instrument is used to measure knots, nits and fine contents of fibers. In this test, a sample of stiffened fibers was defiberized then shredded in a blender (Waring Commercial, Torrington, Conn.) for about 15 seconds, 3.000 g of produced fluff was then introduced into the dispersion chamber of the classification instrument were it is continuously dispersed in an air stream. During dispersion, loose fibers pass through a 16 mesh screen (1.18 mm) and then through a 42 mesh (0.36 mm) screen. Pulp bundles that remained in the dispersion chamber (“knots”) and those that were trapped on the 42-mesh screen (“accepts”) were removed and weighed. The combined weight of these two was subtracted from the original weight of the fluff sample to determine the weight of fibers that passed through the 0.36 mm screen (“fines.”)
• The Op Test Fiber Quality Analyzer (Op Test Equipment Inc., Waterloo, Ontario, Canada) is used to measure average fiber length, kink, curl, and fines content.
The Absorbency Test Method
• This test method was used to determine the absorbent capacity and the centrifuge retention capacity (CRC) of the stiffened fibers.
• The measurements were carried out using a plastic cylinder with one inch inside diameter having a 100-mesh metal screen attached to the base of the cylinder. Into the cylinder was inserted a plastic spacer disk having a 1.0 inch diameter and a weight of about 4.4 g. The weight of the cylinder assembly was determined to the nearest 0.001 g (W₀), and then the spacer was removed from the cylinder and about 0.35 g (dry weight basis) of acquisition fiber was air-laid into the cylinder. The spacer disk then was inserted back into the cylinder on the air-laid fibers, and the cylinder assembly was weighed to the nearest 0.001 g (W₁). Fibers in the cell were compressed with a load of 4.0 psi for 60 seconds, the load then was removed and the fiber pad was allowed to equilibrate for 60 seconds.
• The cell and its contents then were hanged in a Petri dish containing sufficient amount of saline solution (0.9% by weight NaCl) to touch the bottom of the cell and the fiber was allowed to stay in contact with the saline solution for 10 minutes. Then it was removed and hanged in another empty Petri dish and allowed to drain for one minute. The weight of the cell and contents was then determined (W₂). The weight of the saline solution absorbed per gram fibers then was calculated according to Equation (1) below, the result of which was expressed as the “absorbent capacity” (g/g).
• $\begin{array}{cc}\frac{W_2-W_1}{W_1-W_0}& \left(1\right)\end{array}$
• The cell then was centrifuged for 3 minutes at 2400 rpm (Centrifuge Model HN, International Equipment Co., Needham HTS, USA), and the weight of the cell and contents is reported (W₃). The centrifuge retention capacity (CRC) was then calculated according to Equation (2) below, the result of which was expressed as the “centrifuge retention capacity” (g/g).
• $\begin{array}{cc}\frac{W_3-W_0}{W_1-W_0}& \left(2\right)\end{array}$
Specific Absorption Rate Test (SART)
• The SART test method was used to evaluate the performance of the stiffened fibers in an absorbent article. SART measures the ability of the stiffened fiber to wick the fluid in the Z-direction. In this test the time taken for a dose of saline to be absorbed completely by storage core was recorded as an acquisition time.
• In this test a sample having a circular shape with a 60 mm diameter was cut from an absorbent article having an absorbent core and an acquisition layer. The test sample was placed into a testing apparatus (obtained from Portsmouth Tool and Die Corp., Portsmouth, Va., USA) consisting of a plastic base and a funnel cup. The base is a plastic cylinder having an inside diameter of 60.0 mm that is used to hold the sample. The funnel cup is a plastic cylinder having a hole with a star shape at the bottom, the outside diameter of which is 58 mm. The test sample was placed inside the plastic base, and the funnel cup was placed inside the plastic base over the test sample. A load of about 0.6 psi having a donut shape was placed on top of the funnel cup.
• The apparatus and its contents were placed on a leveled surface and the sample was insulted with three successive doses of 9.0 ml of saline solution, (0.9% by weight NaCl), the time interval between doses being 20 minutes. The doses were added using a Master Flex Pump (Cole Parmer Instrument, Barrington, Ill., USA) to the funnel cup, and the time taken for each dose to be absorbed completely is recorded and expressed as “acquisition time,” or “strikethrough.”
EXAMPLESExample 1
• This example illustrates a representative method for making stiffened fibers in sheet form.
• In this method a treatment composition solution containing citric acid (3.2 weight %), polymaleic acid (0.8 weight %), and a catalyst sodium hypophosphite (0.8 weight %) was prepared. The pH of the solution was adjusted to about 3.0 with an aqueous solution of NaOH (50 weight %).
• The treatment composition solution then used to treat hand sheets of fluff pulp obtained from a jumbo roll of Rayfloc®-J-LD (conventional wood fluff pulp, commercially available from Rayonier, Inc., Jesup, Ga.). The hand sheets each had dimensions of 11 inches by 11 inches, a basis weight of about 680 gsm (g/m²), and density of 0.42 g/cc. Each hand sheet was dipped in the treatment composition solution and pressed to achieve the desired level of treatment composition solution (100% wet pick-up). The treated sheets were then dried and cured at about 186° C. in an air-driven laboratory oven for about 10 minutes. After curing, the sheets were treated with an aqueous solution of odor removing agent (hydrogen peroxide, 0.5 wt %) by spraying to obtain about 8 wt % add-on based on the total weight of the sheet. The sheets of stiffened fibers were then defiberized using a hammermill (Kamas Mill H01, Kamas Industries AB, Vellinge, Sweden).
• The absorbent properties, fiber quality, ISO brightness, and fiber length, kink angle, and kinks per mm, were determined for the resultant stiffened fibers, as well as for commercially-available cross-linked fiber, and a control (uncrosslinked fiber). The results of this testing are presented in Table 1, below.
Example 2
• This example illustrates an alternative method for making stiffened fibers in sheet form.
• A 12 inch by 12 inch hand sheet of stiffened fibers with a basis weight of about 650 gsm was made in the following manner: Rayfloc®-J-LDE (65.0 g, moisture content about 7%) was dispersed in water then added to a handsheet chamber. An aqueous solution of citric acid, PMA, and sodium hypophosphite was added to the fiber suspension in the hand sheet chamber. More water was then added so that the final volume of the mixture was 10.0 L, and the mixture contained 3.2 wt % citric acid, 0.8 wt % PMA, and 0.7 wt % sodium hypophosphite. The pH of the solution was adjusted to about 3.0 by adding sodium hydroxide solution (50%). The suspended fibers in the handsheet mold were then gently agitated with a standard perforated mixing plate that was inserted into the slurry and moved up and down about 12 times, then removed. The water was then drained through the forming screen of the handsheet former. The handsheet former was then opened; a 14 inch by 14 inch, 4 mesh nylon screen was placed on top of the handsheet and two layers of blotting paper were placed on the top of the nylon screen. A roller, having the equivalent of about 2.3 pounds of pressure per linear inch, was moved back and forth thrice on each of the left side, the right side, and the center of the formed handsheet. The blotting papers were then lifted off the screen and the sheet was sandwiched between two nylon screens and placed between two blotting papers. The blotting paper, handsheet, and screens were then pressed (at 100 psi) by passing them through an M/K sheet press (Motor Master 20000-series, M/K System Inc., Glendale, Calif.). After being pressed, the handsheet had a wet-pick up of about 100 wt %. The handsheet was then dried and cured in an oven at about 185° C. for about 11 minutes.
• The absorbent properties, fiber quality, ISO brightness, and fiber length, kink angle, and kinks per mm, were determined for the resultant stiffened fibers, as well as for commercially-available cross-linked fiber, and a control (uncrosslinked fiber). The results of this testing are presented in Table 1, below.

• TABLE 1
  Absorbent properties and fiber quality of stiffened fiber of Example 1

  		Commercial	Stiffened	Stiffened
  		cross-linked	Fiber/	Fiber/
  	Control1	fiber2	Example 1	Example 2

  Absorbent	11.4	19.2	17.1	18.7
  Capacity (g/g
  OD)
  CRC (g/g OD)	0.93	0.56	0.57	0.57
  Accept (%)	85.1	66.8	71.0	92.3
  Knots and nits	9.9	29.0	22.4	3.53
  (%)
  Fines (%)	4.7	4.0	6.5
  ISO brightness1	87.0	75.0	79.0	82.0
  Fiber Length	2.6	2.1	2.4	2.38
  (mm)
  Kink Angle	43.6°	110.5°	42.5°	54.8°
  Kinks per mm	0.60	1.37	0.66	1.5
  1Untreated Rayfloc ®-J-LDE.
  2Extracted from Pampers ® Cruiser (stage 4) product acquisition layer, produced by Procter & Gamble Company, Cincinnati, OH. This acquisition layer is representative of commercially-available individualized cross-linked cellulose fiber.

• The results in Tables 1 demonstrate that stiffened fibers have physical characteristics comparable to those of the untreated cellulose base fiber, but have absorbent properties more similar to commercially cross-linked fiber. For example, the fiber length, kink angle, and brightness are comparable to those of the base fiber. However, the stiffened fiber has much higher absorbent capacity and much lower Centrifuge Retention Capacities than the base fiber, which indicates that the fiber is desirable for use as acquisition/distribution fiber.
Example 3
• This example illustrates a method for making acidic stiffened fiber.
• In this example, the procedure described in Example 1 was followed, except that in this experiment after curing the sheet of stiffened fiber was treated with an aqueous solution of citric acid. Three sample sheets were produced, one having 1.0 weight % citric acid, a second having 1.5 weight % citric acid, and a third having 2.0 weight % citric acid, based on the to total weight of the stiffened fiber sheet.
• The pH of the acidic stiffened fiber samples was measured using the following method. 10.0 g of stiffened cellulosic fiber was saturated with distilled water (50.0 g). The produced mixture was left for about 10.0 minutes, after which about 10.0 grams of liquid was squeezed out of the fiber. The pH of the squeezed liquid was measured and used as the pH of the fiber. The results are summarized in Table 2, below.

• TABLE 2
  pH of acidic stiffened fibers prepared as
  shown in Example 3

  	Acquisition fiber
  	Citric acid add on (%)	pH
  	1.0	3.12
  	1.5	2.61
  	2.0	2.60

Example 4
• Stiffened fiber made in accordance with the foregoing examples was evaluated for acquisition and rewet performance. The acquisition and rewet test is a useful tool for understanding fluid acquisition performance in the x-y plane.
• Airlaid pads were made from the stiffened fibers produced in Example 1. The airlaid pads had a basis weight of 260 gsm, a density 0.07 g/cc, and dimensions of 180 mm×70 mm. These airlaid pads were used as acquisition layers in baby diaper samples that were tested for acquisition and rewet properties, as specified in Table 3 below. Sample D1 was a Pampers® Baby Dry® brand baby diaper, size 4 (large). The Pampers® diaper had an acquisition layer made of STCC fibers, located between the topsheet and the absorbent core. In sample D2, the acquisition layer of the Pampers® diaper was removed, and replaced with an acquisition layer made from the stiffened fibers of Example 1. Sample D3 was a HUGGIES® brand baby diaper, size 4 (large). The HUGGIES® baby diaper had an acquisition layer made of synthetic fibers, located between the topsheet and the absorbent core. In sample D4, the acquisition layer of the HUGGIES® diaper was removed, and replaced with an acquisition layer made from STCC fibers extracted from the Pampers® diaper. In sample D5, the acquisition layer of the HUGGIES® diaper was removed, and replaced with an acquisition layer made from the stiffened fibers of Example 1. The characteristics of the diapers are summarized in Table 3.

• TABLE 3
  Diaper Sample	Specifications
  D1	Pampers ® Baby Dry size 4
  D2	Storage core: Pampers ® Baby Dry size 4
  	Acquisition layer: stiffened fiber, basis weight: 260 gsm,
  	density 0.07 cc; dimensions: 180 mm × 70 mm
  D3	HUGGIES ® size 4
  D4	Storage core: HUGGIES ® size 4
  	Acquisition layer: STCC fiber, basis weight: 260 gsm,
  	density 0.067 cc; dimensions: 180 mm × 70 mm
  D5	Storage core: HUGGIES ® size 4
  	Acquisition layer: stiffened fiber, basis weight: 260 gsm,
  	density 0.07 cc; dimensions: 180 mm × 70 mm

• In this test method, the test diaper was stretched out flat. A weighted apparatus (0.1 psi load), into which a tube of specific diameter was inserted, was placed on top of the diaper. Fluid (100 mL 0.9% saline) was poured into the tube, and the time taken for the fluid to be absorbed into the core of the diaper was measured and recorded as “acquisition time.” After a pre-determined time, a number of blotter papers were placed on the insult area, and a load (0.5 psi, 2.5 kg) was placed on top of the blotter papers for 2.0 minutes. The liquid absorbed by the blotter papers is measured and recorded as the “rewet” value. The procedure was repeated three times. The results of the 2^ndacquisition and rewet values are shown inFIG. 4, and the results of the 3^rdacquisition time and rewet are summarized inFIG. 5.
• FIG. 4 shows that the diapers containing an acquisition layer made from stiffened fiber has significantly lower 2^ndacquisition times and rewet values, than those for the commercial diapers.
• FIG. 5 shows that the Pampers® diaper samples, containing an acquisition layer made from stiffened fibers, have lower acquisition times and rewet values than the Pampers® diaper containing the acquisition layer with STCC. On the HUGGIES® diaper samples, the diaper containing the acquisition layer with stiffened fiber had much lower rewet values than both the diaper with the acquisition layer with STCC fiber and the HUGGIES® synthetic fiber acquisition layer. However, the STCC fiber appears to provide lower acquisition times than the other two acquisition layers.
Example 5
• In this example, the SART test method was used to measure the acquisition time of the diaper samples described in Example 4 (Table 3).
• The results of the 2^ndand 3^rdacquisition time are summarized inFIG. 6. The results inFIG. 6 show that the diaper samples having acquisition layers made from stiffened fibers have lower acquisition times than those having commercial acquisition layers. In the Pampers® samples, stiffened fibers outperformed the STCC acquisition layer. However, in the HUGGIES® samples, the stiffened fiber and STCC acquisition layers had comparable performance.
• 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 thereto without departing from the spirit and scope thereof.

Claims (50)

1. A method of making stiffened cellulosic fiber in sheet form having a low degree of yellowing and low odor, said method comprising:

providing a treatment composition comprising an aqueous solution of a cross-linking agent and a catalyst;

providing cellulosic base fiber in sheet form;

applying the treatment composition to the cellulosic base fiber to impregnate the cellulosic base fiber;

drying and curing the impregnated fiber; and

applying water to the cured sheet of fiber to increase its moisture content to more than about 6.0 weight %.

2. The method ofclaim 1, wherein the cross-linking agent is selected from the group consisting of: a polycarboxylic acid, an aldehyde, a urea-based derivative, and combinations and mixtures thereof.

3. The method ofclaim 1, wherein the cross-linking agent is a polycarboxylic acid comprising an alkanepolycarboxylic acid selected from the group consisting of: 1,2,3,4-butanetetracarboxylic acid, 1,2,3-propanetricarboxylic acid, oxydisuccinic acid, citric acid, itaconic acid, maleic acid, tartaric acid, glutaric acid, iminodiacetic acid, citraconic acid, tartarate monsuccininc acid, benzene hexacarboxylic acid, cyclohexanehexacarboxylic acid, and mixtures and combinations thereof.

4. The method ofclaim 1, wherein the cross-linking agent is a polymeric polycarboxylic acid prepared from one or more 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 amid, butadiene, styrene, and combinations and mixtures thereof.

5. The method ofclaim 1, wherein the cross-linking agent is a polycarboxylic acid comprising a combination of polymeric polycarboxylic acid and alkanepolycarboxylic acid.

6. The method ofclaim 1, wherein the cross-linking agent is an aldehyde selected from the group consisting of: formaldehyde, glyoxal, glyoxylic acid, glutaraldehyde, glyceraldehydes, and combinations and mixtures thereof.

7. The method ofclaim 1, wherein the cross-linking agent is a urea-based derivative selected from the group consisting of: urea based-formaldehyde addition products, methylolated ureas, methylolated cyclic ureas, methylolated lower alkyl cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, lower alkyl substituted cyclic ureas, dimethyldihydroxy urea (1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea (bis[N-hydroxymethyl]urea), dihydroxyethylene urea (4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (1,3-dihydroxymethyl-2-imidazolidinone), glyoxal adducts of urea, polyhydroxyalkyl urea, hydroxyalkyl urea, β-hydroxyalkyl amide, and combinations and mixtures thereof.

8. The method ofclaim 1, wherein the treatment composition has a pH of about 1.5 to about 4.5.

9. The method ofclaim 1, wherein applying the treatment composition to cellulosic base fiber comprises spraying, dipping, rolling, or applying with a puddle press, size press or a blade-coater.

10. The method ofclaim 1, wherein the treatment composition is applied to the cellulosic based fiber to provide from about 10 weight % to about 150 weight % of treatment composition on fiber, based on the total weight of the fiber.

11. The method ofclaim 1, wherein the treatment composition is applied to the cellulosic base fiber to provide from about 2 weight % to about 7 weight % of the cross-linking agent on fiber, and from about 0.3 weight % to about 2.0 weight % of catalyst on fiber, based on the total weight of the fiber.

12. The method ofclaim 1, wherein the catalyst is an alkali metal salt of phosphorous containing an acid selected from the group consisting of: alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, alkali metal sulfonates, and combinations and mixtures thereof.

13. The method ofclaim 1, wherein the treatment composition comprises from about 2 weight % to about 10 weight % cross-linking agent.

14. The method ofclaim 1, wherein the weight of the catalyst in the treatment composition is about 10% to about 50% of the weight of the cross-linking agent in the treatment composition.

15. The method ofclaim 1, wherein the cellulosic base fiber is a conventional cellulose fiber derived from hardwood cellulose pulp, softwood cellulose pulp, cotton linters, bagasse, kemp, flax, grass, or combinations or mixtures thereof.

16. The method ofclaim 15, wherein the hardwood cellulose pulp is selected from the group consisting of: gum, maple, oak, eucalyptus, poplar, beech, aspen, and combinations and mixtures thereof.

17. The method ofclaim 15, wherein the softwood 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.

18. The method ofclaim 1, wherein the sheet of cellulosic base fiber is formed using a wet-laid process, and has a basis weight of about 200 grams per square meter (gsm) to about 800 gsm and a density of about 0.15 grams per cubic centimeter (g/cc) to about 0.6 g/cc.

19. The method ofclaim 1, wherein the drying and curing occurs in a one-step process conducted for about 3 minutes to about 15 minutes at a temperature within the range of about 130° C. to about 225° C.

20. The method ofclaim 1, wherein the drying and curing is a two-step process comprising:

first drying the impregnated cellulosic fiber at a temperature below curing temperature; and

curing the dried cellulosic fiber for about 1 to 10 minutes at a temperature within the range of about 150° C. to about 225° C.

21. The method ofclaim 1, wherein applying the water to the cured sheet of fiber comprises spraying, dipping, rolling, printing, or applying with a puddle press, size, press, or a blade-coater.

22. The method ofclaim 1, wherein an odor removing agent is applied with the water to the cured sheet of fiber.

23. The method ofclaim 22, wherein the odor removing agent is selected from the group consisting of hydrogen peroxide, chlorine dioxide, peracetic acid, perbenzoic acid, chlorine, chlorine dioxide, ozone, sodium hypochlorite, baking soda, talc powder, cyclodextrin, ethylenediamine tetra-acetic acid or other chelating agents, zeolites, activated silica, activated carbon granules, DOUBLE-O, UN-DUZ-IT, X-O, NOK-OUT, and combinations and mixtures thereof.

24. The method ofclaim 22, wherein the odor removing agent is applied to the stiffened fiber to provide from about 0.001 weight % to about 1.0 weight % of odor removing agent on fiber, based on the total weight of the fiber.

25. Stiffened cellulosic fiber having low degree of yellowing and low odor produced by the method ofclaim 1.

26. The stiffened cellulosic fiber ofclaim 25, having an ISO Brightness of greater than 77%.

27. The stiffened cellulosic fiber ofclaim 25, having a pH of less than about 4.2.

28. The stiffened cellulosic fiber ofclaim 25, having an average kink angle of less than 55°.

29. The stiffened cellulosic fiber ofclaim 25, having less than about 1.0 kink per millimeter.

30. The stiffened cellulosic fiber ofclaim 25, having less than about 30% knots and nits when defiberized.

31. The stiffened cellulosic fiber ofclaim 25, having at least about 70% accepts and not more than about 25% knots and nits when defiberized.

32. An airlaid structure comprising the stiffened cellulosic fiber ofclaim 25 and synthetic fibers.

33. The airlaid structure ofclaim 32, having a basis weight of about 60 gsm to about 640 gsm.

34. The airlaid structure ofclaim 32, having a density of about 0.02 g/cc to about 0.12 g/cc.

35. The airlaid structure ofclaim 32, wherein the synthetic fiber is selected from the group consisting of polypropylene, polyethylene, bi-constituent fibers, and any combination or mixture thereof.

36. The airlaid structure ofclaim 32, wherein the bi-constituent fiber is a polyethylene/polyester fiber.

37. The airlaid structure ofclaim 32, wherein the airlaid structure is a thermally-bonded stabilized roll good material.

38. The airlaid structure ofclaim 32, wherein the airlaid material is a stabilized roll good material that contains about 1.0 weight % to about 25.0 weight % of a bi-constituent fiber, based on the total weight of the stabilized roll good material.

39. An absorbent article comprising the stiffened cellulosic fiber ofclaim 25.

40. The absorbent article ofclaim 39, wherein the absorbent article is a disposable baby diaper, an adult incontinent device, or a feminine hygiene product.

41. The absorbent article ofclaim 39, wherein the absorbent article has a top sheet, a back sheet, and a storage layer disposed at least partially between the top sheet and the backsheet.

42. The absorbent article ofclaim 39, wherein the absorbent article further comprises a liquid acquisition layer disposed at least partially between the top sheet and the storage layer.

43. The absorbent article ofclaim 42, wherein the liquid acquisition layer comprises stiffened cellulosic fiber.

44. The absorbent article ofclaim 43, wherein the liquid acquisition layer comprises a mixture of SAP and the stiffened cellulosic fiber.

45. The absorbent article ofclaim 44, wherein the liquid acquisition layer contains about 8 weight % to about 30 weight % SAP, based on total weight of the acquisition layer.

46. The absorbent article ofclaim 43, wherein the liquid acquisition layer is an thermally-bonded airlaid structure comprising the stiffened cellulosic fiber and synthetic fibers.

47. A method of making stiffened cellulosic fiber in sheet form having a low degree of yellowing and low odor, said method comprising:

providing cellulosic base fibers;

providing a treatment composition comprising an aqueous solution of a cross-linking agent and a catalyst;

forming a suspension of the cellulosic fibers in the treatment composition solution;

converting the suspension into a wet-laid sheet;

pressing the wet-laid sheet until the sheet has a wet pick-up of about 50 weight % to about 200 weight %;

drying and curing the pressed sheet to form a sheet of stiffened cellulosic fiber;

applying water to the cured stiffened cellulosic fiber sheet to increase its moisture content to more than about 6.0 weight %.

48. The method of47, wherein the cellulosic base fibers are provided in sheet or fluff form.

49. The method of47, wherein the cellulosic base fibers are provided in dry or wet state.

50. The method of47, wherein the sheet of stiffened cellulosic fibers has a density of at least about 0.15 g/cc.

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