Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/22—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons containing macromolecular materials
  • A61L15/28—Polysaccharides or their derivatives
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B15/00—Preparation of other cellulose derivatives or modified cellulose, e.g. complexes
  • C08B15/005—Crosslinking of cellulose derivatives
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B5/00—Preparation of cellulose esters of inorganic acids, e.g. phosphates
  • C08B5/14—Cellulose sulfate

Definitions

• the present invention relates to a modified cellulosic fiber having superabsorbent properties and, more particularly, to a crosshnked and sulfated cellulosic fiber having a structure substantially identical to the fiber from which it is derived.
• Personal care absorbent products such as infant diapers, adult incontinent pads, and feminine care products, typically contain an absorbent core that includes superabsorbent in a fibrous matrix.
• Superabsorbents are water-swellable, generally water-insoluble absorbent materials having a liquid absorbent capacity of at least about 10, preferably of about 20, and often up to about 100 times their weight in water. While the core's liquid retention or storage capacity is due in large part to the superabsorbent, the core's fibrous matrix provides the essential functions of liquid wicking, pad strength and integrity, and some amount of absorbency under load.
• the matrix includes cellulosic fibers, typically wood pulp fluff in fiber form.
• U.S. southern pine fluff pulp is used almost exclusively and is recognized worldwide as the preferred fiber for absorbent products.
• the preference is based on the fluff pulp's advantageous high fiber length (about 2.8 mm) and its relative ease of processing from a wetlaid pulp sheet to an airlaid web.
• these fluff pulp fibers can absorb only about 2-3 g/g of liquid (e.g.. water or bodily fluids) within the fibers' cell walls.
• Most of the fibers' liquid holding capacity resides in the interstices between fibers. For this reason, a fibrous matrix readily releases acquired liquid on application of pressure.
• the tendency to release acquired liquid can result in significant skin wetness during use of an absorbent product that includes a core formed exclusively from cellulosic fibers. Such products also tend to leak acquired liquid because liquid is not effectively retained in such a fibrous absorbent core.
• absorbent materials in a fibrous matrix and their incorporation into personal care products is known.
• the incorporation of superabsorbent materials into these products has had the effect of reducing the products' overall bulk while at the same time increasing its liquid absorbent capacity and enhancing skin dryness for the products' wearers.
• a variety of materials have been described for use as absorbent materials in personal care products. Included among these materials are natural-based materials such as agar. pectin, gums, carboxyalkyl starch and carboxyalkyl cellulosic fiber, such as carboxymethyl cellulose, as well as synthetic materials such as polyacrylates. polyacrylamides. and hydrolyzed polyacrylonitriles. Although natural-based absorbing materials are well known, these materials have not gained wide usage in personal care products because of their relatively inferior absorbent properties compared to synthetic absorbent materials such as polyacrylates. The relatively high cost of these materials has also precluded their use in consumer absorbent products. Furthermore, many natural-based materials tend to form soft, gelatinous masses when swollen with a liquid. The presence of such gelatinous masses in a product's core tends to limit liquid transport and distribution within the core and prevents subsequent liquid insults from being efficiently and effectively absorbed by the product.
• synthetic absorbent materials are generally capable of absorbing large quantities of liquid while maintaining a relatively non-gelatinous form.
• Synthetic absorbent materials often referred to as superabsorbent polymers (SAP)
• SAP superabsorbent polymers
• Superabsorbent polymers are generally supplied as particles having a diameter in the range from about 20-800 microns. Due to their high absorbent capacity under load, absorbent products that include superabsorbent polymer particles provide the benefit of skin dryness. Because superabsorbent polymer particles absorb about 30 times their weight in liquid under load, these particles provide the further significant advantages of thinness and wearer comfort.
• superabsorbent polymer particles are about half the cost per gram of liquid absorbed under load compared to fluff pulp fibers. For these reasons it is not surprising that there is a growing trend toward higher superabsorbent particle levels and reduced levels of fluff pulp in consumer absorbent products. In fact, some infant diapers include 60 to 70 percent by weight superabsorbent polymer in their liquid storage core. From a cost perspective, a storage core made from 100 percent superabsorbent particles is desirable. However, as noted above, such a core would fail to function satisfactorily due to the absence of any significant liquid wicking and distribution of acquired liquid throughout the core. Furthermore, such a core would also lack strength to retain its wet and/or dry structure, shape, and integrity.
• Cellulosic fibers provide absorbent products with critical functionality that has, to date, not been duplicated by particulate superabsorbent polymers.
• Superabsorbent materials have been introduced in synthetic fiber form seeking to provide a material having the functionality of both fiber and superabsorbent polymer particle.
• these superabsorbent fibers are difficult to process compared to fluff pulp fibers and do not blend well with fluff pulp fibers.
• synthetic superabsorbent fibers are significantly more expensive than superabsorbent polymer particles and. as a result, have not competed effectively for high volume use in personal care absorbent products.
• Cellulosic fibers have also been rendered highly absorptive by chemical modification to include ionic groups such as carboxylic acid, sulfonic acid, and quaternary ammonium groups that impart water swellability to the fiber. Although some of these modified cellulosic materials are soluble in water, some are water- insoluble. However, none of these highly absorptive modified cellulosic materials possess the structure of a pulp fiber, rather, these modified cellulosic materials are typically granular or have a regenerated fibril form.
• the present invention seeks to fulfill these needs and provides further related advantages.
• the present invention provides a modified cellulosic fiber having superabsorbent properties.
• the modified fiber formed in accordance with the present invention has a fibrous structure substantially identical to the cellulosic fiber from which it is derived. More importantly, the modified fiber is a water-swellable, water-insoluble fiber that substantially retains its fibrous structure in its expanded, water-swelled state.
• the modified fiber is a sulfated and crosshnked cellulosic fiber having a liquid absorption capacity of at least about 4 g/g.
• the modified fiber is an individual, crosshnked, sulfated cellulosic fiber.
• the invention provides a rollgood that includes the modified fiber.
• the rollgood includes other materials such as fibrous, binder, and absorbent materials.
• the rollgood can be directly inserted as an absorbent core into an absorbent article.
• a sulfated cellulosic fiber is crosshnked to an extent sufficient to render the fiber substantially insoluble in water.
• a crosshnked cellulosic fiber is sulfated to provide the modified fiber.
• the sulfated cellulosic fiber can be prepared by reacting the fiber with sulfuric acid in an organic solvent.
• the invention provides methods for using the modified fiber and absorbent composites and articles incorporating the modified fiber are also provided.
• the invention provides an absorbent core having a liquid capacity of at least about 22 g/g.
• the absorbent core can be advantageously incorporated into an absorbent article.
• FIGURES 1A-C are scanning electron microscope (SEM) photographs of representative fluff pulp fibers (bleached kraft southern pine fibers commercially available from Weyerhaeuser Company under the designation NB416) at 100X magnification (FIGURE 1A), at 300X magnification (FIGURE IB), and at 1000X magnification (FIGURE 1 C);
• FIGURES 2A-C are SEM photographs of representative modified fibers formed in accordance with the present invention from bleached kraft southern pine fibers (NB416) at 100X magnification (FIGURE 2A), at 300X magnification (FIGURE 2B).
• FIGURES 3A and 3B are optical microscope photographs of representative modified fibers formed in accordance with the present invention, FIGURE 3 A illustrates modified fibers before contact with water and FIGURE 3B illustrates modified fibers after contact with water; and FIGURE 4 is a graph illustrating the absorbent capacity for representative modified fibers formed in accordance with the present invention as a function of weight percent crosslinking applied to the fibers and sulfation reaction time (25 minutes, +; 35 minutes, ⁇ ; 45 minutes, ⁇ ).
• the present invention provides a modified cellulosic fiber having superabsorbent properties.
• the modified fiber formed in accordance with the present invention has a fibrous structure substantially identical to the cellulosic fiber from which it is derived. More importantly, the modified fiber is a water-swellable, water-insoluble fiber that substantially retains its fibrous structure in its expanded, water-swelled state.
• the cellulosic fiber formed in accordance with the invention is modified cellulosic fiber that has been sulfated and crosshnked. Water swellability is imparted to the cellulosic fiber through sulfation and intrafiber crosslinking renders the cellulosic fiber substantially insoluble in water.
• the modified cellulosic fiber has a degree of sulfate group substitution effective to provide advantageous water swellability.
• the modified cellulosic fiber is crosshnked to an extent sufficient to render the fiber water insoluble.
• the modified cellulosic fiber has a liquid absorption capacity that is increased compared to unmodified fluff pulp fibers.
• the modified fibers have a liquid absorption capacity of at least about 4 g/g.
• Cellulosic fibers suitable for use in forming the modified fiber of the present invention are substantially water-insoluble and not highly water-swellable. After sulfation and crosslinking in accordance with the present invention, the resulting modified fiber has the desired absorbency characteristics, is water-swellable and water-insoluble, and substantially retains the fibrous structure of the cellulosic fiber from which it is derived.
• the modified fiber of the invention has the structure of a pulp fiber including a cell wall structure. In one embodiment, the modified fiber has the structure of a wood pulp fiber.
• the modified fiber includes a lumen (i.e., central cavity) surrounded by a wall surface having four concentric layers. In addition to an outermost primary wall (commonly denoted P).
• the cell wall includes secondary walls (commonly denoted S I -S3).
• the secondary walls include an outer layer (SI ) adjacent the primary wall, an inner layer (S3) adjacent the lumen, and a middle layer (S2) intermediate the outer and inner secondary layers.
• the modified fiber's structure also includes long bundles of cellulosic fibrillar structures, referred to as macrofibrils, fibrils, microfibrils, and elementary fibrils, having varying diameters. The diameter of fibrillar material depends on the extent of fiber processing.
• Cellulose is a principal component of delignified cell walls.
• the secondary cell wall can include unbranched cellulose chains having a degree of polymerization up to about 17,000.
• the modified fiber of the invention is primarily cellulosic in nature having cellulose as its principal chemical component.
• Cellulose can be considered to be a polymer containing repeating anhydroglucose units.
• the term "anhydroglucose” refers to the repeating unit in cellulose that is formed by loss of water from glucose on condensation to form the polymer.
• the degree of polymerization (DP) for a given cellulose molecule is the number of anhydroglucose repeating units in the molecule. The DP for a particular cellulose will depend on its source and the extent of polymer degradation on processing.
• the modified fiber can include hemicellulose and lignin. While cellulose is a linear polysaccharide formed from glucose, hemicellulose can be either an unbranched or branched polysaccharide that includes sugars other than glucose. Unlike cellulose and hemicellulose, which are carbohydrate polymers having repeating saccharide units, lignin is a highly branched, three-dimensional polymer composed of aromatic units. Lignin is amorphous in structure and not an integral part of the fiber's fibrillar system of carbohydrate polymers.
• lignin content is greatest in the outer layers of the cell wall and decreases rapidly to the layer adjacent the lumen.
• cellulose content is lowest in the primary wall and increases significantly toward the inner fiber regions. Hemicellulose content tends to increase gradually from the outer to the inner regions of the fiber.
• the chemical composition of the modified fiber of the invention depends, in part, on the extent of processing of the cellulosic fiber from which the modified fiber is derived.
• the modified fiber of the invention is derived from a fiber that has been subjected to a pulping process (i.e., a pulp fiber).
• Pulp fibers are produced by pulping processes that seek to separate cellulose from lignin and hemicellulose leaving the cellulose in fiber form. The amount of lignin and hemicellulose remaining in a pulp fiber after pulping will depend on the nature and extent of the pulping process.
• the fiber of the invention is a modified pulp fiber that retains the basic chemical and structural characteristics of a pulp fiber.
• the modified fiber has a multiwalled macrostructure as described above and is composed of primarily of cellulose and can include some hemicellulose and lignin.
• the modified fiber is substantially insoluble in water.
• a material will be considered to be water-soluble when it substantially dissolves in excess water to form a solution, losing its fiber form and becoming essentially evenly disbursed throughout a water solution.
• a sufficiently sulfated cellulosic fiber that is free from a substantial degree of crosslinking will be water-soluble, whereas the modified cellulosic fiber of the invention, a sulfated and crosshnked fiber, is water- insoluble.
• the modified fiber is a water-swellable, water-insoluble fiber. As used herein, the term "water-swellable.
• water-insoluble refers to a material that, when exposed to an excess of an aqueous medium (e.g., bodily fluids such as urine or blood, water, synthetic urine, or 0.9 weight percent solution of sodium chloride in water), swells to an equilibrium volume but does not dissolve into solution.
• an aqueous medium e.g., bodily fluids such as urine or blood, water, synthetic urine, or 0.9 weight percent solution of sodium chloride in water
• the water-swellable. water-insoluble modified cellulosic fibers of the invention retain their original fibrous structure, but in a highly expanded state, during liquid absorption and have sufficient structural integrity to resist flow and fusion with neighboring materials.
• a modified fiber of the invention is effectively crosshnked to be substantially insoluble in water while being capable of absorbing at least about 4 times its weight of a 0.9 weight percent solution of sodium chloride in water under an applied load of about 0.3 pound per square inch.
• Cellulosic fibers are a starting material for preparing the superabsorbent cellulosic fiber product of the invention. Although available from other sources, suitable cellulosic fibers are derived primarily from wood pulp. Suitable wood pulp fibers for use with the invention can be obtained from well-known chemical processes such as the kraft and sulfite processes, with or without subsequent bleaching. Pulp fibers can also be processed by thermomechanical, chemithermomechanical methods, or combinations thereof. Caustic extractive pulp such as TRUCELL, commercially available from Weyerhaeuser Company, is also a suitable wood pulp fiber. The preferred pulp fiber is produced by chemical methods. Ground wood fibers, recycled or secondary wood pulp fibers, and bleached and unbleached wood pulp fibers can be used.
• Softwoods and hardwoods can be used. Details of the selection of wood pulp fibers are well-known to those skilled in the art. These fibers are commercially available from a number of companies, including Weyerhaeuser Company, the assignee of the present invention.
• suitable cellulosic fibers produced from southern pine that are usable with the present invention are available from Weyerhaeuser Company under the designations CF416, NF405, PL416, FR516, and NB416.
• the cellulosic fiber useful in making the modified fiber of the invention is a southern pine fiber commercially available from Weyerhaeuser Company under the designation NB416.
• the cellulosic fiber can be selected from among a northern softwood fiber, a eucalyptus fiber, a rye grass fiber, and a cotton fiber.
• Cellulosic fibers having a wide range of degree of polymerization are suitable for forming the modified cellulosic fiber of the invention.
• the cellulosic fiber has a relatively high degree of polymerization, greater than about 1000, and in another embodiment, about 1500.
• the modified fiber has an average length greater than about 1.0 mm. Consequently, the modified fiber is suitably prepared from fibers having lengths greater than about 1.0 mm. Fibers having lengths suitable for preparing the modified fiber include southern pine, northern softwood, and eucalyptus fibers, the average length of which is about 2.8 mm. about 2.0 mm, and about 1.5 mm, respectively. Fibers with average lengths less than about 1.0 mm have relatively poorer wicking properties and provide composites having diminished pad integrity.
• the modified cellulosic fiber of the invention is a sulfated cellulosic fiber.
• sulfated cellulosic fiber refers to a cellulosic fiber that has been sulfated by reaction of a cellulosic fiber with a sulfating agent. It will be appreciated that the term “sulfated cellulosic fiber” includes free acid and salt forms of the sulfated fiber. Suitable metal salts include sodium, potassium, and lithium salt, among others.
• a sulfated cellulosic fiber can be produced by reacting a sulfating agent with a hydroxyl group of the cellulosic fiber to provide a cellulose sulfate ester (i.e., a carbon-to-oxygen-to-sulfur ester).
• the sulfated cellulosic fiber formed in accordance with the present invention differs from other sulfur-containing cellulosic compounds in which the sulfate sulfur atom is attached directly to a carbon atom on the cellulose chain as. for example, in the case of sulfonated cellulose: or cellulosic compounds in which the sulfate sulfur atom is attached indirectly to a carbon atom on the cellulose chain as, for example, in the case of cellulose alkyl sulfonates.
• the modified cellulosic fiber of the invention can be characterized as having an average degree of sulfate group substitution of from about 0.1 to about 2.0. In one embodiment, the modified cellulosic fiber has an average degree of sulfate group substitution of from about 0.2 to about 1.0. In another embodiment, the modified cellulosic fiber has an average degree of sulfate group substitution of from about 0.3 to about 0.5. As used herein, the "average degree of sulfate group substitution" refers to the average number of moles of sulfate groups per mole of glucose unit in the modified fiber. It will be appreciated that the fibers formed in accordance with the present invention include a distribution of sulfate modified fibers having an average degree of sulfate substitution as noted above.
• Example 1 A representative method for preparing sulfated fibers is described in Example 1.
• the modified cellulosic fiber of the invention is an intrafiber crosshnked cellulosic fiber.
• Crosshnked cellulosic fibers and methods for their preparation are disclosed in U.S. Patents Nos. 5,437,418 and 5,225,047 issued to Graef et al., expressly incorporated herein by reference.
• Crosshnked fibers can be prepared by treating fibers with a crosslinking agent.
• Suitable crosslinking agents useful in producing the modified cellulosic fiber are generally soluble in water and/or alcohol.
• Suitable cellulosic fiber crosslinking agents include aldehyde, dialdehyde, and related derivatives (e.g., formaldehyde, glyoxal, glutaraldehyde, glyceraldehyde), and urea-based formaldehyde addition products (e.g., N-methylol compounds). See, for example, U.S. Patents Nos.
• Cellulosic fibers can also be crosshnked by carboxylic acid crosslinking agents including polycarboxylic acids.
• carboxylic acid crosslinking agents including polycarboxylic acids.
• Suitable urea-based crosslinking agents include methylolated ureas, methylolated cyclic ureas, methylolated lower alkyl substituted cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, and lower alkyl substituted cyclic ureas.
• urea-based crosslinking agents include dimethylol urea (DMU, bis[N-hydroxymethyl]urea), dimethylolethylene urea (DMEU, 1 ,3-dihydroxymethyl-2-imidazolidinone), dimethyloldihydroxyethylene urea (DMDHEU, l ,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), di- mefhylolpropylene urea (DMPU), dimethylolhydantoin (DMH), dimethyldihydroxy urea (DMDHU). dihydroxyethylene urea (DHEU. 4,5-dihydroxy-2 ⁇ imidazolidinone), and dimethyldihydroxyethylene urea (DMeDHEU, 4,5-dihydroxy-l ,3-dimefhyl-2- imidazolidinone).
• DMU dimethylol urea
• DMEU dimethylolethylene urea
• DMDHEU dimethylolethylene urea
• Suitable polycarboxylic acid crosslinking agents include citric acid, tartaric acid, malic acid, succinic acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic acid, 1.2,3-propane tricarboxylic acid, 1.2,3,4- butanetetracarboxylic acid, all-cis-cyclopentane tetracarboxylic acid, tetrahydrofuran tetracarboxylic acid, 1 ,2,4,5-benzenetetracarboxylic acid, and benzenehexacarboxylic acid.
• Other polycarboxylic acids crosslinking agents include polymeric polycarboxylic acids such as poly(acrylic acid).
• poly(maleic acid), poly(methylvinylefher-co-maleate) copolymer, poly(methylvinylether-co- itaconate) copolymer, copolymers of acrylic acid, and copolymers of maleic acid are described in U.S. Patent No. 5,998,51 1 , assigned to Weyerhaeuser Company and expressly incorporated herein by reference in its entirety.
• crosslinking agents include diepoxides such as, for example, vinylcyclohexene dioxide, butadiene dioxide, and diglycidyl ether; sulfones such as, for example, divinyl sulfone. bis(2-hydroxyethyl)sulfone, bis(2-chloroethyl)sulfone, and disodium tris( ⁇ -sulfatoethyl)sulfonium inner salt; and diisocyanates. Mixtures and/or blends of crosslinking agents can also be used.
• the crosslinking agent can include a catalyst to accelerate the bonding reaction between the crosslinking agent and cellulosic fiber.
• Suitable catalysts include acidic salts, such as ammonium chloride, ammonium sulfate, aluminum chloride, magnesium chloride, and alkali metal salts of phosphorous-containing acids.
• the modified cellulosic fiber of the invention is a crosshnked cellulosic fiber.
• the amount of crosslinking agent applied to the fiber is suitably the amount necessary to render the modified fiber substantially insoluble in water.
• the amount of crosslinking agent applied to the cellulosic fiber will depend on the particular crosslinking agent and is suitably in the range of from about 0.01 to about 8.0 percent by weight based on the total weight of cellulosic fiber. In one embodiment, the amount of crosslinking agent applied to the fibers is in the range from about 0.20 to about 5.0 percent by weight based on the total weight of fibers.
• the crosslinking agent can be applied to the cellulosic fibers as an aqueous alcoholic solution. Water is present in the solution in an amount sufficient swell the fiber to an extent to allow for crosslinking within the fiber's cell wall. However, the solution does not include enough water to dissolve the fiber. Suitable alcohols include those alcohols in which the crosslinking agent is soluble and the fiber to be crosshnked (i.e., unmodified or sulfated cellulosic fiber) is not.
• Representative alcohols include alcohols that include from 1 to 5 carbon atoms, for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, s- butanol, and pentanols.
• the crosslinking agent can be applied to the fibers as an ether solution (e.g., diethyl ether).
• the modified fiber of the invention can have a distribution of sulfate and/or crosslinking groups along the fiber's length and through the fiber's cell wall.
• sulfate and/or crosslinking groups along the fiber's length and through the fiber's cell wall.
• Surface crosslinking may be advantageous to improve modified fiber dryness and provide a better balance of total absorbent capacity and surface dryness.
• Fiber swelling and soak time can also effect the sulfation and crosslinking gradients. Such gradients may be due to the fiber structure and can be adjusted and optimized through control of sulfation and/or crosslinking reaction conditions.
• FIGURES 1A-C A representative method for crosslinking sulfated fibers is described in Example 2.
• Scanning electron microscope (SEM) photographs of bleached kraft southern pine fibers (NB416) at 100X, 300X, and 1000X magnification are illustrated in FIGURES 1A-C, respectively.
• SEM photographs of representative modified fibers formed from NB416 fibers in accordance with the invention at 100X, 300X, and 1000X magnification are illustrated in FIGURES 2A-C. respectively.
• the modified fibers are ribbon-like and are twisted and curled, and have a structure substantially identical to the fiber from which they are derived.
• the modified fiber of the invention has a liquid absorbent capacity of at least about 4 g/g as measured by the centrifuge capacity test described in Example 3. In one embodiment, the modified fiber has a capacity of at least about 10 g/g. In another embodiment, the fiber has a capacity of at least about 15 g/g, and in a further embodiment, the fiber has a capacity of at least about 20 g/g.
• the absorbent capacity of representative modified fibers formed in accordance with the present invention is described in Example 3. As noted above, the modified fiber retains the structure of a fiber.
• FIGS. 3A and 3B are optical microscope photographs of representative modified fibers formed in accordance with the invention before and after contact with water.
• FIGURE 3A shows representative modified fibers that have not been contacted with water. Referring to FIGURE 3A, these fibers are ribbon-like and are twisted and curled.
• FIGURE 3B shows representative modified fibers that have been contacted with water. Referring to FIGURE 3B, these swelled fibers have retained their fiber structure and have expanded diameters that are from about 3 to about 6 times their original diameter.
• cellulosic fibers are sulfated and crosshnked to provide superabsorbent fibers.
• cellulosic fibers are sulfated and then crosshnked.
• sulfated cellulosic fibers are treated with an amount of crosslinking agent sufficient to render the resulting modified cellulosic fibers substantially insoluble in water.
• cellulosic fibers are crosshnked then sulfated.
• crosshnked cellulosic fibers are sulfated to render the resulting modified cellulosic fibers highly water absorptive.
• the modified cellulosic fiber formed by either method is highly water absorptive, water-swellable, water-insoluble, and retains the fibrous structure of the fibers from which it is derived.
• the modified fiber of the invention is a sulfated cellulosic fiber. Sulfated cellulosic fibers can be made by reacting cellulosic fibers (e.g., cellulosic fibers that are crosshnked or noncrosslinked) with a sulfating agent.
• Suitable sulfating agents include concentrated sulfuric acid (95-98%), fuming sulfuric acid (oleum), sulfur trioxide and related complexes including sulfur trioxide/dimethylformamide and WO 01/52911 n PCT/USOl/01883
• the sulfating agent is concentrated sulfuric acid.
• the sulfating agent is preferably applied to the fibers as a solution in an organic solvent.
• Suitable organic solvents include alcohols, pyridine, dimethylformamide. acetic acid including glacial acetic acid, and dioxane.
• the organic solvent is an alcohol having up to about 6 carbon atoms.
• Suitable alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, s-butanol, pentanols, and hexanols.
• the alcohol is selected from among isopropanol and isobutanol.
• the molar ratio of sulfuric acid to alcohol in the solution can be varied from about 1 : 1 to about 4: 1.
• the molar ratio of sulfuric acid to alcohol is about 2.4: 1, for example, an 80:20 (weight/weight) solution of sulfuric acid in isopropanol.
• the weight ratio of sulfuric acid to cellulosic fibers in the sulfation reaction can be varied from about 5:1 to about 30:1. At low sulfuric acid ratios the reaction is slow and incomplete and at high sulfuric acid ratios significant cellulose polymer degradation can occur.
• the weight ratio of sulfuric acid to pulp fiber is from about 10:1 to about 25:1. In another embodiment, the weight ratio of sulfuric acid to pulp fiber is about 24: 1.
• the present invention provides methods for making sulfated cellulose fibers without significant cellulose hydrolysis.
• cellulose fiber degradation i.e., degree of polymerization reduction
• a sulfating agent in a nonaqueous environment and/or at low temperature (e.g., at or below about 4°C).
• a dehydrating agent to absorb water can be added to the sulfating reaction mixture.
• Suitable dehydrating agents include, for example, sulfur trioxide, magnesium sulfate, acetic anhydride, and molecular sieves.
• cellulosic fibers are reacted with the sulfating agent at a temperature of about 4°C and both the cellulosic fibers and the sulfating agent are cooled to about 4°C prior to reaction.
• cellulosic fibers, including cooled fibers are reacted with the sulfating agent in the presence of a dehydrating agent.
• the fibers and sulfating agent are reacted for a period of time of from about 10 to about 60 minutes. Following this reaction period and prior to neutralizing the resulting sulfated fibers, the sulfated fibers are separated from excess sulfating agent. In one embodiment, the sulfated fibers are washed with an alcohol prior to neutralization.
• the fibers Prior to crosslinking the sulfated cellulosic fibers to provide the modified fibers of the invention, the fibers can be at least partially neutralized with a neutralizing agent.
• the neutralizing agent is suitably soluble in the sulfation solvent.
• the neutralizing agent is a base such as, for example, an alkaline base (e.g., lithium, potassium, sodium or calcium hydroxide; lithium, potassium, or sodium acetate).
• the neutralizing agent can include a multivalent metal salt.
• Suitable metal salts include cerium, magnesium, calcium, zirconium, and aluminum salts such as ammonium cerium nitrate, magnesium sulfate, magnesium chloride, calcium chloride, zirconium chloride, aluminum chloride, and aluminum sulfate, among others.
• the use of multivalent metal salts as neutralizing agents also offers the advantage of intrafiber crosslinking.
• the sulfated cellulosic fiber can be partially neutralized and partially crosshnked. Fibers so treated can be further crosshnked with other crosslinking agents including those described above.
• the extent of fiber sulfation is dependent on a number of reaction conditions including reaction time. For example, in a series of representative sulfation reactions, a 25 minute reaction time provided a fiber that included about 3.8 percent by weight sulfur; a 35 minute reaction time provided a fiber that included about 4.9 percent by weight sulfur; and a 45 minute reaction time provided a fiber that included about 6.4 percent by weight sulfur. However, in these experiments, the extended sulfation reaction time had an adverse effect on fiber length (i.e.. cellulose hydrolysis occurred under the prolonged reaction conditions).
• the sulfated fibers produced by the 25 and 35 minute reaction conditions provide cellulose solutions classified as having a Gardner-Holt bubble tube H viscosity (i.e., about 200 Centistokes), while the sulfated fibers produced by the 45 minute reaction provided cellulose solutions classified as having C viscosity (i.e., about 85 Centistokes).
• the results indicate that at extended reaction times, significant fiber degradation can occur.
• the absorbent capacity of modified fibers prepared from these sulfated fibers is described in Example 3.
• Example 1 A representative method for preparing sulfated fibers is described in Example 1.
• the at least partially neutralized sulfated cellulosic fibers can then be crosshnked by applying a crosslinking agent to the fibers.
• the crosslinking agent is applied to the fibers as an aqueous alcoholic solution.
• the crosslinking agent solution includes water sufficient to swell but not dissolve the fibers. Above about 95 percent by weight alcohol, the crosslinking agent does not penetrate the fiber cell wall sufficiently and the result is a crosshnked fiber having nonuniform crosslinking and low absorbent capacity.
• the aqueous alcoholic solution includes from about 10 to about 50 percent by weight water and from about 50 to about 90 percent by weight alcohol.
• the crosslinking agent solution is an aqueous ethanol solution (88 percent by weight ethanol).
• the crosslinking agent is cured by, for example, heating the treated fibers, to provide intrafiber crosshnked fibers.
• Example 2 A representative method for crosslinking sulfated fibers is described in Example 2.
• the method of Example 2 describes crosslinking sulfated fibers that have been isolated and dried.
• sulfated fibers formed as described above and in Example 1 may be directly crosshnked, after neutralization, without drying the fibers.
• the present invention provides a method for making cellulosic fibers having superabsorbent properties that includes the step of reacting cellulosic fibers with a sulfating agent, at least partially neutralizing the sulfated fibers to provide fibers suitable for crosslinking, applying a crosslinking agent to the sulfated fibers, and then curing the crosslinking agent to provide the modified fibers.
• the crosslinking reaction when it is desirable to produce the modified fiber in individual fiber form, relatively less water is used in the crosslinking reaction. Conversely, when it is desired that the modified fiber be produced as a sheet or web (e.g., rollgood), the crosslinking reaction includes a relatively greater amount of water. It has been found that water present during the crosslinking reaction effects bonding between the individual, modified fibers. When the water content is sufficiently high in the crosslinking reaction, interfiber bonding can occur to provide a structure having sufficient strength and integrity to provide a fibrous web or sheet of the modified fiber suitable for the formation of a rollgood. Where it is desirable to form the modified fiber in individual form, the modified fiber can be baled for shipping and subsequent processing.
• interfiber bonding and loss of individual fiber structure occurs when more than about 50 percent by weight water is present in the crosslinking reaction. Between from about 50 and about 90 percent by weight alcohol, interfiber bonding occurs without the loss of individual fiber structure.
• the method described above can further include other steps to optimize the production of the modified fibers of the invention.
• the cellulosic fibers can be dried prior to the sulfation reaction.
• the fibers can be dried by any one of a number of drying methods including heating and chemical methods.
• the fibers can be dried by heating in a drying oven; solvent exchange with a suitable solvent; solvent exchange with a suitable solvent followed by heating; or treatment with a dehydrating agent such sulfur trioxide or acetic anhydride.
• a never-dried fiber can be dried by solvent exchange using a suitable solvent.
• cellulosic fibers can be swelled prior to sulfation using a swelling agent.
• Suitable swelling agents include, for example, water, glacial acetic acid, acetic anhydride, zinc chloride, sulfuric acid, sulfur trioxide, and ammonia.
• the fibers can be swelled by mixing the fibers with the swelling agent followed by removing excess swelling agent prior to reacting the fibers with the sulfating agent.
• the present invention provides a method for making cellulosic fibers having superabsorbent properties that includes the steps of swelling cellulosic fibers, including dry fibers, with a swelling agent; separating excess swelling agent from the swelled fibers; reacting the swelled fibers with a sulfating agent; separating excess sulfating agent from the fibers; at least partially neutralizing the sulfated fibers to provide fibers suitable for crosslinking; applying a crosslinking agent to the sulfated fibers; and then curing the crosslinking agent to provide intrafiber crosshnked, sulfated cellulosic fibers.
• the modified cellulosic fibers of the invention can be formed by crosslinking then sulfating the cellulosic fibers.
• the modified fibers can be prepared by applying a crosslinking agent to cellulosic fibers; curing the crosslinking agent to provide crosshnked fibers; reacting the crosshnked cellulosic fibers with a sulfating agent; at least partially neutralizing the sulfated, crosshnked fibers; and then drying the sulfated, crosshnked cellulosic fibers.
• the modified fiber of the invention is formed by methods that do not include dissolving the fiber in solution. In this way, the modified fiber retains the structure of the fiber from which it is derived.
• the structure of the modified fiber of the invention is in contrast to other fibrous materials that lack fiber structure and that are prepared by regeneration from solutions (i.e., formed, for example, by precipitation, from solutions containing dissolved cellulosic materials).
• the modified fiber formed in accordance with the present invention has superabsorbent properties while, at the same time, has the structure of the cellulosic pulp fiber from which it is derived.
• the modified fiber of the invention can be produced as an individual fiber or as sheet or web (e.g.. rollgood) of fibers. The nature of the modified fiber produced depends on the use for which the fiber is ultimately intended.
• the modified fibers can be incorporated into a personal care absorbent product.
• the modified fibers can be formed into a composite for incorporation into a personal care absorbent product.
• Composites can be formed from the modified fibers alone or by combining the modified fibers with other materials, including fibrous materials, binder materials, other absorbent materials, and other materials commonly employed in personal care absorbent products.
• Suitable fibrous materials include synthetic fibers, such as polyester, polypropylene, and bicomponent binding fibers; and cellulosic fibers, such as fluff pulp fibers, crosshnked cellulosic fibers, cotton fibers, and CTMP fibers.
• Suitable absorbent materials include natural absorbents, such as sphagnum moss, and synthetic superabsorbents, such as polyacrylates (e.g., SAPs).
• the modified fiber is further treated with a compatible material to provide a coated modified fiber.
• the modified fiber can be coated with a variety of materials including those noted above as well as binders, pH control agents, and odor reducing agents, among others.
• Webs that include the modified fibers can be prepared in any one of a variety of methods known in the web-forming art.
• the methods include airlaid and wet forming methods.
• wet-formed webs that include the modified fibers can be formed by, for example, adding water in an amount sufficient to bond the crosshnked sulfated fibers to an extent sufficient to provide a web with structural integrity.
• Other materials, such as fibrous and absorbent materials, can also be included in these webs. In some instances, when intended for use in a personal care absorbent product, the rollgood form of the modified fiber is desired.
• modified fiber in rollgood form can be directly incorporated as received by a diaper manufacturer by cutting the rollgood into the desired shape and size, and inserting the shaped and sized web into an absorbent article. In this way, the modified fiber in rollgood form can be directly utilized in a diaper manufacturing line.
• the rollgood containing the modified fiber can also include any one or more of a variety of other useful materials such as those identified above.
• Absorbent composites derived from or that include the modified fibers of the invention can be advantageously incorporated into a variety of absorbent articles such as diapers including disposable diapers and training pants; feminine care products including sanitary napkins, and pant liners; adult incontinence products; toweling; surgical and dental sponges; bandages: food tray pads; and the like.
• the present invention provides absorbent composites and absorbent articles that include the modified fiber.
• the modified fiber of the invention has a fiber structure that, like other pulp fibers, provides for liquid wicking. Like superabsorbent materials, the modified fiber has a high liquid absorbent capacity.
• the modified fiber can be useful in absorbent products such as, for example, an infant diaper, where liquid wicking and liquid storage are required. Because of its unique, liquid wicking and capacity properties, the modified fiber can be formed into a composite and utilized as a storage core in a diaper. Such a core may only include the modified fiber. For a modified fiber having an absorbent capacity of at least about 22 g/g, the resulting core has an absorbent capacity of at least about 22 g/g.
• Conventional, commercial diaper storage cores typically include two components: (1) fluff pulp fibers to wick liquid, and (2) superabsorbent material to store acquired liquid. The core typically consists of minimally about 25 percent by weight fluff pulp fibers and maximally about 75 percent by weight superabsorbent material.
• Superabsorbent materials generally have an absorbent capacity of about 28 g/g and fluff pulp fibers generally have an absorbent capacity of about 2 g/g. Therefore, such a core has a capacity of about 22 g/g.
• Cores prepared from a modified fiber having a capacity of at least about 22 g/g can exceed the performance characteristics of conventional absorbent composites.
• the modified fibers of the invention provide advantages related to the manufacture of absorbent cores.
• the pulp Prior to sulfation, the pulp was activated with acetic acid. Ten grams of fiberized bleached kraft southern yellow pine fluff pulp (NB416. Weyerhaeuser Company, Federal Way, WA) that had been oven dried at 105°C was disbursed in 600 mL of glacial acetic acid. The pulp/acid slurry was then placed in a vacuum chamber and the air was evacuated. The slurry was allowed to stand under vacuum for 30 minutes after which time the chamber was repressurized to atmospheric pressure. The slurry was then allowed to stand at ambient conditions for 45 minutes before being resubjected to a vacuum for an additional 30 minutes. After the second application of a vacuum the slurry was again allowed to stand for 45 minutes at atmospheric pressure.
• the slurry was then poured into a Buchner funnel where the pulp was collected and pressed until the weight of the residual acetic acid was equal to twice the weight of the oven dry pulp (i.e., total weight of the collected pulp was 30 g.)
• the collected pulp was placed inside a plastic bag and cooled to -10°C in a freezer.
• the sulfation liquor was prepared by mixing 240 g concentrated sulfuric acid with 60 g isopropanol and 0.226 g magnesium sulfate.
• the liquor was prepared by pouring isopropanol into a beaker that was maintained at 4°C in an ice bath. Magnesium sulfate was then added to the isopropanol and the mixture chilled to 4°C.
• Sulfuric acid was weighed into a beaker and separately chilled to 9°C before being slowly mixed into the isopropanol and magnesium sulfate mixture. The resulting sulfating liquor was then allowed to cool to 4°
• the cooled acetic acid activated pulp (-10°C) was stirred into the cooled sulfation liquor (4°C).
• the resulting slurry of pulp and sulfation liquor was allowed to react for 35 minutes with constant stirring.
• the pulp/sulfation liquor slurry was poured into a Buchner funnel and the sulfated pulp was collected and washed over a vacuum with cooled isopropanol (-10°C).
• the collected pulp was then slurried with cooled isopropanol (-10°C) in a Waring blender and poured back into the Buchner funnel where the pulp was again washed with cooled isopropanol (-10°C).
• the nature and quality of the modified fiber formed in accordance with the invention can depend on the washing step.
• the acid is preferably washed from the pulp as quickly as possible to prevent continued and/or accelerated cellulose degradation.
• the cool temperature of the pulp is preferably maintained to prevent cellulose degradation.
• the acid is preferably washed from the pulp as thoroughly as possible before neutralization to prevent the formation of difficult to remove inorganic salts during the neutralization step. These salts can adversely impact modified fiber absorbency.
• the washed sulfated pulp was next slurried in cooled isopropanol (-10°C) and an ethanolic sodium hydroxide solution was added dropwise until the slurry was neutralized.
• the slurry was then poured into a Buchner funnel where the neutralized sulfated pulp was washed with room temperature isopropanol.
• the neutralized sulfated pulp was then agitated to remove any inorganic salts that may have been crusted on the fiber surfaces after which the neutralized sulfated pulp was again washed with isopropanol in a Buchner funnel. Finally the collected sulfated pulp was allowed to air dry.
• a catalyzed urea-formaldehyde system was used to crosslink the sulfated cellulosic fibers.
• the catalyst included magnesium chloride and the sodium salt of dodecylbenzenesulfonic acid dissolved in 88%) ethanol/water.
• the catalyst solution served as a diluent for the crosslinking agent.
• the crosslinking agent was obtained by dissolving urea in 37 percent (w/w) aqueous formaldehyde.
• the crosslinking agent was combined with the catalyst solution and applied to the sulfated fibers. The treated fibers were then cured by placing in a 105°C oven for 60 minutes.
• Tea bag preparation unroll tea bag material (Dexter #1234T heat- sealable tea bag material) and cut cross ways into 6 cm pieces. Fold lengthwise, outside-to-outside. Heatseal edges 1 /8 inch with an iron (high setting), leave top end
• Net wet weight sample - Net dry weight sample g/g capacity 25 Net wet weight is the centrifuge weight less the dry weight of the tea bag and fiber sample. Net dry weight is the dry weight of the fiber sample.
• absorbent capacity increases with increasing sulfation. However, at the point where sulfation results in fiber degradation, absorbent capacity decreases. The results also demonstrate that absorbent capacity also increases with increasing crosslinking to a point. At higher levels of crosslinking. absorbent capacity decreases.

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