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US20020088581A1 - Crosslinked cellulosic product formed by extrusion process - Google Patents

Crosslinked cellulosic product formed by extrusion process Download PDF

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Publication number
US20020088581A1
US20020088581A1 US10/002,844 US284401A US2002088581A1 US 20020088581 A1 US20020088581 A1 US 20020088581A1 US 284401 A US284401 A US 284401A US 2002088581 A1 US2002088581 A1 US 2002088581A1
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composite
fibers
foam
crosslinked
cellulosic fibers
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Peter Graef
Terry Grant
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Weyerhaeuser Co
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Individual
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    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
    • D21H11/00—Pulp or paper, comprising cellulose or lignocellulose fibres of natural origin only
    • D21H11/16—Pulp or paper, comprising cellulose or lignocellulose fibres of natural origin only modified by a particular after-treatment
    • D21H11/20—Chemically or biochemically modified fibres
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
    • D21F11/00—Processes for making continuous lengths of paper, or of cardboard, or of wet web for fibre board production, on paper-making machines
    • D21F11/002—Processes for making continuous lengths of paper, or of cardboard, or of wet web for fibre board production, on paper-making machines by using a foamed suspension
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21J—FIBREBOARD; MANUFACTURE OF ARTICLES FROM CELLULOSIC FIBROUS SUSPENSIONS OR FROM PAPIER-MACHE
    • D21J3/00—Manufacture of articles by pressing wet fibre pulp, or papier-mâché, between moulds

Definitions

  • the present invention relates generally to a cellulosic fibrous product and, more particularly, to a crosslinked cellulosic fibrous product formed by an extrusion process.
  • Crosslinked cellulosic fibers are advantageously incorporated into a variety of fibrous products to enhance product bulk, resilience, and dryness.
  • Absorbent articles such as diapers, are typically formed from fibrous composites that include absorbent fibers such as wood pulp fibers, and can additionally include crosslinked cellulosic fibers.
  • absorbent fibers such as wood pulp fibers
  • crosslinked cellulosic fibers When incorporated into absorbent articles, such fibrous composites can provide a product that offers the advantages of high liquid acquisition rate and high liquid wicking capacity imparted by the crosslinked fibers and absorbent fibers, respectively.
  • fibrous composites that include relatively high percentages of crosslinked fibers suffer from low sheet strength.
  • cellulose structures have been formed from water based sheet forming processes. Over time, airlaid and foam-forming processes were developed to use new materials and impart improved properties of the resulting web. Foam-forming processes showed improved capability to utilize long fibers of natural or synthetic origin and provided a bulky web. Airlaid processes provided bulk and softness but limited strength without a high binder content.
  • Crosslinked fibers have been used in the above mentioned processes to improve web properties such as resilience, bulk and acquisition. Crosslinked fibers have previously only been available as individually crosslinked fibers packaged in a bale. Numerous patents speak of processes to make and to use individually crosslinked fibers. Therefore there is a need to develop a process that eliminates the step of individually crosslinking the cellulose fibers.
  • Personal care absorbent products for example, infant diapers, adult incontinence products, and feminine care products, include liquid acquisition and/or distribution layers that serve to rapidly acquire and then distribute acquired liquid to a storage core for retention.
  • these layers may include crosslinked cellulosic fibers, which impart bulk and resilience to the layers.
  • webs that include high proportions of crosslinked fibers suffer from a lack of structural integrity.
  • the problem of loss of structural integrity is traditionally addressed by sandwiching webs that include crosslinked fibers between either tissues and nonwoven sheets and secured with an adhesive. Such structures are required to seek to maintain web integrity.
  • the present invention provides a crosslinked cellulosic fibrous product that includes in situ crosslinked cellulosic fibers.
  • the product further includes a bonding agent.
  • the product can optionally include other fibers alone, absorbent materials alone, and other fibers and absorbent materials.
  • the product is formed using a screw extrusion device. In another embodiment, the product is formed using a rotary mixing extrusion device.
  • the present invention provides absorbent articles that include the bonded cellulosic fibrous product.
  • the product can be combined with one or more other layers to provide structures that can be incorporated into absorbent articles such as infant diapers, adult incontinence products, and feminine care products.
  • FIG. 1 is a graph illustrating liquid drainage from foam as a function of time for foams having densities of 137 g/L, 95 g/L, 66 g/L, and 41 g/L, respectively;
  • FIG. 2 is a flow diagram illustrating a representative method for making an extruded composite in accordance with the present invention
  • FIG. 3A is an image of a representative mixing device useful in the method of the present invention.
  • FIG. 3B is an image of the stator of the mixing device illustrated in FIG. 3A;
  • FIG. 3C is an image of the mixing device of FIG. 3A with the rotor removed;
  • FIG. 4 is a schematic illustration of a representative extrusion device useful in the present invention illustrating positions at which various materials may be added;
  • FIG. 5 is a graph comparing the density of fibrous composites made from (a) southern pine fibers and (b) fibers crosslinked with a blend of citric and polyacrylic acids by four methods: foam laid, airlaid, wetlaid, and the extrusion process of the invention;
  • FIG. 6 is a graph illustrating pore size distribution for a representative extruded composite formed in accordance with the present invention that includes cellulose fibers crosslinked with a blend of polyacrylic and citric acids (13% by weight crosslinked based on fiber);
  • FIG. 7 is a graph illustrating pore size distribution for a representative foam laid composite that includes cellulose fibers crosslinked with a blend of polyacrylic and citric acids;
  • FIG. 8 is a graph illustrating pore size distribution for a representative airlaid composite that includes cellulose fibers crosslinked with a blend of polyacrylic and citric acids;
  • FIG. 9 is a graph illustrating pore size distribution for a representative wetlaid composite that includes cellulose fibers crosslinked with a blend of polyacrylic and citric acids;
  • FIG. 10 is a graph illustrating absorption and desorption curves obtained using an autoporosimeter for an airlaid composite that includes southern pine fibers;
  • FIG. 11 is a graph that compares the mid-point desorption pressure (MDP) of representative extruded composites formed in accordance with the present invention (“pine fiber” refers to a composite that includes southern pine fibers, “pre-crosslinked Chemistry B composite” refers to a composite that includes citric acid crosslinked fibers, and “in situ Chemistry B composite” refers to a composite that includes cellulose fibers treated with citric acid and cured in the composite);
  • MDP mid-point desorption pressure
  • FIG. 12 is a graph illustrating the effect on crosslinking chemistry on the mid-point desorption pressure for representative extruded composites formed in accordance with the present invention
  • pre-crosslinked Chemistry A refers to a composite that includes cellulose fibers crosslinked with a blend of polyacrylic and citric acids
  • pre-crosslinked Chemistry B refers to a composite that includes citric acid crosslinked fibers
  • in situ Chemistry B refers to a composite that includes cellulose fibers that had been treated with citric acid and cured in the composite
  • in situ Chemistry C refers to a composite that includes cellulose fibers treated with polyacrylic acid and cured in the composite
  • FIG. 13 is a graph illustrating the effect of latex content on the mid-point desorption pressure as a function of crosslinking chemistry for representative extruded composites formed in accordance with the present invention
  • pre-crosslinked Chemistry B refers to a composite that includes citric acid crosslinked fibers
  • pre-crosslinked Chemistry B plus 5 percent latex refers to a composite that includes citric acid crosslinked fibers and 5 percent by weight latex
  • in situ Chemistry B refers to a composite that includes fibers treated with citric acid and cured in the composite
  • in situ Chemistry B plus 5 percent latex refers to a composite that includes fibers treated with citric acid and cured in the composite and 5 percent by weight latex
  • FIG. 14 is a graph illustrating the effect of in situ crosslinking on the tensile strength of representative extruded composites formed in accordance with the present invention that include citric acid crosslinked fibers and polyacrylic acid crosslinked fibers (“2 percent in situ” refers to a composite that includes fibers treated with 2 percent by weight crosslinking agent, “6 percent in situ” refers to a composite that includes fibers treated with 6 percent by weight crosslinking agent, and “6 percent pre-crosslinked” refers to a composite that includes fibers crosslinked with 6 percent by weight crosslinking agent);
  • FIG. 15 is a graph illustrating the effect of latex and fiber type on the strength of representative composites formed in accordance with the present invention (“pine” refers to a composite that includes southern pine fibers, “blend” refers to a composite that includes a 50/50 blend of southern pine fibers and fibers crosslinked with a blend of polyacrylic and citric acids, and “Chemistry A” refers to a composite that includes fibers crosslinked with a blend of polyacrylic and citric acids);
  • FIG. 16 is a graph illustrating the effect of in situ crosslinking chemistry and latex on the tensile strength of representative extruded composites formed in accordance with the present invention (“Chem B” refers to a composite that includes citric acid crosslinked fibers and “Chem C” refers to a composite that includes polyacrylic acid crosslinked fibers);
  • FIG. 17 is a graph illustrating pore size distribution for a foam laid composite
  • FIG. 18 is a graph illustrating the mid-point desorption pressure for the composite of FIG. 17;
  • FIG. 19 is a graph illustrating pore size distribution for a foam laid composite
  • FIG. 20 is a graph illustrating the mid-point desorption pressure for the composite of FIG. 19;
  • FIG. 21 is a graph illustrating the pore size distribution for a representative composite formed in accordance with the present invention (a composite including southern pine fibers);
  • FIG. 22 is a graph illustrating the mid-point desorption pressure for the composite of FIG. 21;
  • FIG. 23 is a graph illustrating pore size distribution for a representative extruded composite formed in accordance with the present invention ( a composite including southern pine fibers and 5 percent latex);
  • FIG. 24 is a graph illustrating pore size distribution for a representative extruded composite of the present invention (a composite including fibers crosslinked with a blend of citric and polyacrylic acids with 5 percent latex);
  • FIG. 25 is a graph illustrating pore size distribution for a representative extruded composite of the present invention (a composite including a 50:50 blend of southern pine fibers and fibers crosslinked with a blend of citric and polyacrylic acids with 5 percent latex);
  • FIG. 26 is a graph illustrating the fourth gush acquisition rate of representative extruded composites formed in accordance with the present invention compared to control composites;
  • FIG. 27 is a graph illustrating the mid-point desorption pressure for a representative extruded composite formed in accordance with the present invention (a composite that includes a blend of citric and polyacrylic acid crosslinked fibers with 15 percent latex);
  • FIG. 28 is a graph illustrating the mid-point desorption pressure for a representative extruded composite formed in accordance with the present invention (a composite that includes a blend of citric and polyacrylic acid crosslinked fibers )
  • FIG. 29 is a graph illustrating the mid-point desorption pressure for a foam laid composite;
  • FIG. 30 is a graph illustrating the mid-point desorption pressure for a wetlaid composite.
  • FIG. 31 is a graph illustrating the mean desorption pressure for airlaid composites.
  • the present invention provides a bonded cellulosic fibrous product that includes in situ crosslinked cellulosic fibers.
  • in situ crosslinked cellulosic fibers refers to cellulosic fibers that have been crosslinked during the formation of the web. Therefore, the product of the invention is distinguished from conventional webs that include crosslinked cellulosic fibers that are first formed and then introduced to the web during the web formation process.
  • the product includes in situ crosslinked cellulosic fibers. Because the fibers are crosslinked during the web formation process (i.e., in situ), the product includes intrafiber crosslinked cellulosic fibers (i.e., fibers having crosslinks within each fiber) and interfiber crosslinked cellulosic fibers (i.e., fibers having crosslinks between fibers).
  • the product has a bonded structure and includes intrafiber crosslinked cellulosic fibers that are further crosslinked to adjacent fibers through interfiber crosslinks.
  • the product possesses the advantageous properties of bulk and resiliency associated with intrafiber crosslinked fibers and the advantage of structural integrity imparted to the structure by the bonding between fibers.
  • the product is a bonded web in which the crosslinked fibers and the bonded structure of the web itself contribute to the resiliency and liquid acquisition performance of the web.
  • the product can be produced by (1) forming a web of cellulosic fibers, at least some of which having been treated with a crosslinking agent and, if necessary, crosslinking catalyst; (2) drying the web; and (3) heating the web at a temperature and for a time sufficient to effect crosslinking.
  • Suitable fibers useful in forming the product of the invention include cellulosic fibers that have been treated with a crosslinking agent and, if necessary, crosslinking catalyst and then dried without curing the crosslinking agent. These dried and treated fibers can be introduced into the forming device for subsequent product formation.
  • crosslinking agents and crosslinking catalysts can be used to provide the product of the invention.
  • the following is a representative list of useful crosslinking agents and catalysts.
  • Each of the patents noted below is expressly incorporated herein by reference in its entirety.
  • Suitable urea-based crosslinking agents include substituted ureas such as methylolated ureas, methylolated cyclic ureas, methylolated lower alkyl cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, and lower alkyl substituted cyclic ureas.
  • Specific urea-based crosslinking agents include dimethyldihydroxy urea (DMDHU, 1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone), dimethyloldihydroxyethylene urea (DMDHEU, 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea (DMU, bis[N-hydroxymethyl]urea), dihydroxyethylene urea (DHEU, 4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (DMEU, 1,3-dihydroxymethyl-2-imidazolidinone), and dimethyldihydroxyethylene urea (DDI, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).
  • DMDHU dimethyldihydroxy urea
  • DMDHEU 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone
  • DMU dimethylol urea
  • DMU bis[N-hydroxymethyl]ure
  • Suitable crosslinking agents include dialdehydes such as C 2 -C 8 dialdehydes (e.g., glyoxal), C 2 -C 8 dialdehyde acid analogs having at least one aldehyde group, and oligomers of these aldehyde and dialdehyde acid analogs, as described in U.S. Pat. Nos. 4,822,453; 4,888,093; 4,889,595; 4,889,596; 4,889,597; and 4,898,642.
  • Other suitable dialdehyde crosslinking agents include those described in U.S. Pat. Nos. 4,853,086; 4,900,324; and 5,843,061.
  • crosslinking agents include aldehyde and urea-based formaldehyde addition products. See, for example, U.S. Pat. Nos. 3,224,926; 3,241,533; 3,932,209; 4,035,147; 3,756,913; 4,689,118; 4,822,453; 3,440,135; 4,935,022; 3,819,470; and 3,658,613.
  • Suitable crosslinking agents include glyoxal adducts of ureas, for example, U.S. Pat. No. 4,968,774, and glyoxal/cyclic urea adducts as described in U.S. Pat. Nos. 4,285,690; 4,332,586; 4,396,391; 4,455,416; and 4,505,712.
  • crosslinking agents include carboxylic acid crosslinking agents such as polycarboxylic acids.
  • Polycarboxylic acid crosslinking agents e.g., citric acid, propane tricarboxylic acid, and butane tetracarboxylic acid
  • catalysts are described in U.S. Pat. Nos. 3,526,048; 4,820,307; 4,936,865; 4,975,209; and 5,221,285.
  • C 2 -C 9 polycarboxylic acids that contain at least three carboxyl groups e.g., citric acid and oxydisuccinic acid
  • crosslinking agents is described in U.S. Pat. Nos. 5,137,537; 5,183,707; 5,190,563; 5,562,740, and 5,873,979.
  • Polymeric polycarboxylic acids are also suitable crosslinking agents.
  • Suitable polymeric polycarboxylic acid crosslinking agents are described in U.S. Pat. Nos. 4,391,878; 4,420,368; 4,431,481; 5,049,235; 5,160,789; 5,442,899; 5,698,074; 5,496,476; 5,496,477; 5,728,771; 5,705,475; and 5,981,739.
  • Polyacrylic acid and related copolymers as crosslinking agents are described U.S. Pat. Nos. 6,306,251; 5,549,791; and 5,998,511.
  • Polymaleic acid crosslinking agents are described in U.S. Pat. No. 5,998,511.
  • polycarboxylic acid crosslinking agents include citric acid, tartaric acid, malic acid, succinic acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid, polymethylvinylether-co-maleate copolymer, polymethylvinylether-co-itaconate copolymer, copolymers of acrylic acid, and copolymers of maleic acid.
  • Suitable catalysts can include acidic salts, such as ammonium chloride, ammonium sulfate, aluminum chloride, magnesium chloride, magnesium nitrate, and alkali metal salts of phosphorous-containing acids.
  • the crosslinking catalyst is sodium hypophosphite.
  • the crosslinking agent is applied to the cellulosic fibers in an amount sufficient to effect intrafiber crosslinking and interfiber crosslinking as described above.
  • the amount applied to the cellulosic fibers can be from about 1 to about 10 percent by weight based on the total weight of fibers. In one embodiment, crosslinking agent in an amount from about 4 to about 6 percent by weight based on the total weight of fibers.
  • Suitable cellulosic fibers for forming the product of the invention include those known to those skilled in the art and include any fiber or fibrous mixture that can be crosslinked and from which a fibrous web or sheet can be formed.
  • 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. The preferred pulp fiber is produced by chemical methods. Groundwood 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. For example, suitable cellulose 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 wood pulp fibers useful in the present invention can also be pretreated prior to use.
  • This pretreatment may include physical treatment, such as subjecting the fibers to steam, or chemical treatment.
  • pretreating fibers include the application of surfactants or other liquids, which modify the surface chemistry of the fibers.
  • Other pretreatments include incorporation of antimicrobials, pigments, dyes and densification or softening agents.
  • Fibers pretreated with other chemicals, such as thermoplastic and thermosetting resins also may be used. Combinations of pretreatments also may be employed. Similar treatments can also be applied after formation of the fibrous product in post-treatment processes.
  • Cellulosic fibers treated with particle binders and/or densification/softness aids known in the art can also be employed in accordance with the present invention.
  • the particle binders serve to attach other materials, such as superabsorbent polymers, as well as others, to the cellulosic fibers.
  • Cellulosic fibers treated with suitable particle binders and/or densification/softness aids and the process for combining them with cellulose fibers are disclosed in the following U.S. patents: (1) Pat. No. 5,543,215, entitled “Polymeric Binders for Binding Particles to Fibers”; (2) Pat. No.
  • synthetic fibers including polymeric fibers, such as polyolefin, polyamide, polyester, polyvinyl alcohol, polyvinyl acetate fibers, can also be incorporated into the product.
  • Suitable synthetic fibers include, for example, polyethylene terephthalate, polyethylene, polypropylene, nylon, and rayon fibers.
  • Other suitable synthetic fibers include those made from thermoplastic polymers, cellulosic and other fibers coated with thermoplastic polymers, and multicomponent fibers in which at least one of the components includes a thermoplastic polymer.
  • Single and multicomponent fibers can be manufactured from polyester, polyethylene, polypropylene, and other conventional thermoplastic fibrous materials. Single and multicomponent fibers are commercially available.
  • Suitable bicomponent fibers include CELBOND fibers available from Hoechst-Celanese Company.
  • the product can also include combinations of natural and synthetic fibers.
  • the product further includes a bonding agent.
  • the bonding agent serves to further enhance the structural integrity of the product.
  • Suitable bonding agents include thermoplastic materials, such as bicomponent fibers and latexes, and wet strength agents.
  • the bonding agent is a thermoplastic fiber
  • the fiber can be combined with cellulosic fibers and then formed into the web to be treated with the crosslinking agent.
  • the bonding agent is a wet strength agent, the bonding agent can be applied to the web prior to subjecting the web to fiber crosslinking conditions.
  • Suitable thermoplastic fibers include cellulosic and other fibers coated with thermoplastic polymers, and multicomponent fibers in which at least one of the components includes a thermoplastic polymer.
  • Single and multicomponent fibers can be manufactured from polyester, polyethylene, polypropylene, and other conventional thermoplastic fibrous materials. Single and multicomponent fibers are commercially available.
  • Suitable bicomponent fibers include CELBOND fibers available from Hoechst-Celanese Company.
  • Suitable wet strength agents include cationic modified starch having nitrogen-containing groups (e.g., amino groups) such as those available from National Starch and Chemical Corp., Bridgewater, N. J.; latex; wet strength resins, such as polyamide-epichlorohydrin resin (e.g., KYMENE 557LX, Hercules, Inc., Wilmington, Del.), and polyacrylamide resin (see, e.g., U.S. Pat. No.
  • nitrogen-containing groups e.g., amino groups
  • wet strength resins such as polyamide-epichlorohydrin resin (e.g., KYMENE 557LX, Hercules, Inc., Wilmington, Del.), and polyacrylamide resin (see, e.g., U.S. Pat. No.
  • the product can include other fibers.
  • Other fibers include, for example, the cellulosic fibers, particularly the wood pulp fibers described above, as well as hemp, bagasse, cotton, groundwood, bleached and unbleached pulp, recycled or secondary fibers.
  • the product can further include absorbent material (e.g., superabsorbent polymer particles).
  • absorbent material refers to a material that absorbs liquid and that generally has an absorbent capacity greater than the cellulosic fibrous component of the composite.
  • the absorbent material is a water-swellable, generally water-insoluble polymeric material capable of absorbing at least about 5, desirably about 20, and preferably about 100 times or more its weight in saline (e.g., 0.9 percent saline).
  • the absorbent material can be swellable in the dispersion medium utilized in the method for forming the composite.
  • the absorbent material is untreated and swellable in the dispersion medium.
  • the absorbent material is a coated absorbent material that is resistant to absorbing water during the product formation process.
  • the amount of absorbent material present in the product can vary greatly depending on the product's intended use.
  • the amount of absorbent material present in an absorbent article, such as an absorbent core for an infant's diaper, is suitably present in the composite in an amount from about 2 to about 80 weight percent, preferably from about 30 to about 60 weight percent, based on the total weight of the composite.
  • the absorbent material may include natural materials such as agar, pectin, and guar gum, and synthetic materials, such as synthetic hydrogel polymers.
  • Synthetic hydrogel polymers include, for example, carboxymethyl cellulose, alkaline metal salts of polyacrylic acid, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulphonic acid, polyacrylates, polyacrylamides, and polyvinyl pyridine among others.
  • the absorbent material is a superabsorbent material.
  • a “superabsorbent material” refers to a polymeric material that is capable of absorbing large quantities of fluid by swelling and forming a hydrated gel (i.e., a hydrogel). In addition to absorbing large quantities of fluids, superabsorbent materials can also retain significant amounts of bodily fluids under moderate pressure.
  • Superabsorbent materials generally fall into three classes: starch graft copolymers, crosslinked carboxymethylcellulose derivatives, and modified hydrophilic polyacrylates.
  • absorbent polymers include hydrolyzed starch-acrylonitrile graft copolymers, neutralized starch-acrylic acid graft copolymers, saponified acrylic acid ester-vinyl acetate copolymers, hydrolyzed acrylonitrile copolymers or acrylamide copolymers, modified crosslinked polyvinyl alcohol, neutralized self-crosslinking polyacrylic acids, crosslinked polyacrylate salts, carboxylated cellulose, and neutralized crosslinked isobutylene-maleic anhydride copolymers.
  • Superabsorbent materials are available commercially, for example, polyacrylates from Clariant of Portsmouth, Virginia. These superabsorbent polymers come in a variety of sizes, morphologies, and absorbent properties (available from Clariant under trade designations such as IM 3500 and IM 3900). Other superabsorbent materials are marketed under the trademarks SANWET (supplied by Sanyo Kasei Kogyo Kabushiki Kaisha), and SXM77 (supplied by Stockhausen of Greensboro, N.C.). Other superabsorbent materials are described in U.S. Pat. No.4,160,059; U.S. Pat. No. 4,676,784; U.S. Pat. No.
  • Suitable superabsorbent materials useful in the product include superabsorbent particles and superabsorbent fibers.
  • the product includes a superabsorbent material that swells relatively slowly for the purposes of product manufacturing and yet swells at an acceptable rate so as not to adversely affect the absorbent characteristics of the product or any construct containing the product.
  • a superabsorbent material that swells relatively slowly for the purposes of product manufacturing and yet swells at an acceptable rate so as not to adversely affect the absorbent characteristics of the product or any construct containing the product.
  • the smaller the absorbent material the more rapidly the material absorbs liquid.
  • bonded cellulosic fibrous product methods for forming the bonded cellulosic fibrous product.
  • the bonded cellulosic fibrous product can be formed by an extrusion process.
  • the methods for forming the product include introducing the components that provide the product into a forming device.
  • fibers are introduced into the device as a pulp slurry (i.e., a dispersion of fibers in a dispersion medium).
  • the pulp slurry can include dried pulp, never-dried pulp, treated pulp, or mixtures thereof.
  • the pulp slurry can have a relatively high consistency, for example, in one embodiment from about 15 to about 50 percent by weight solids, and in another embodiment from about from about 20 to about 35 percent by weight solids.
  • the pulp introduced into the mixing/extrusion device can then be combined with other components.
  • crosslinking agent and, if necessary, crosslinking catalyst, for embodiments that do not include the introduction of crosslinking agent-treated pulp into the device.
  • the bonding agent can be added to the pulp slurry in the device.
  • Surfactant and air can also be added to the pulp in the device to provide a foam-forming medium.
  • Other components, such as absorbent material, can be added as necessary to provide the desired product.
  • the product's components are combined and mixed in the device and then extruded from the device.
  • the extruded cellulosic material is then dried and the product ultimately formed by heat treatment.
  • the product has an advantageous low density, as low as about 0.02 g/em 3 .
  • the product has a density in the ranges from about 0.02 to about 0.20 g/cm 3 .
  • the product of the invention is formed by subjecting a web that includes cellulosic fibers to which crosslinking agent and, if necessary, crosslinking catalyst, and bonding agent, if included, to a temperature and for a time sufficient to effect crosslinking (i.e., curing) and fiber bonding.
  • the curing of the crosslinking agent to provide the product can be performed by several methods. Crosslinking typically requires a relatively high temperature (180° C.) and long reaction times (greater than 4 minutes).
  • the product is formed by heating in a curing oven in which high temperature and large volumes of air are drawn through the web.
  • curing takes place after the webs have been placed in boxes for shipping. In this embodiment, boxes containing the treated webs are passed through a dryer (e.g., a kiln dryer) to complete the crosslinking reaction.
  • the product of the present invention can be formed as an extended web or sheet that has structural integrity and sheet strength sufficient to permit the fibrous web to be rolled, transported, and used in rolled form in subsequent processes.
  • the product of the present invention can be supplied in a fibrous rolled form and readily incorporated into subsequent processes.
  • the product 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, tampons, and pant liners; adult incontinence products; toweling; surgical and dental sponges; bandages; food tray pads; and the like.
  • the process of the invention provides a bonded composite having intrafiber crosslinked fibers and interfiber crosslinked fibers that impart improved performance of the resulting composite.
  • the process is a foam-forming process that enables forming composites at high solids content (i.e., high consistency) without the need for the use of a forming wire and drainage.
  • air content and foam density are characteristics of the foam in the process. Foam can be classified as either stable or unstable. However, overall, foams are relatively unstable thermodynamically. Foam stability depends on many factors including surfactant type and concentration, temperature, stabilizer concentration, and the presence of solids.
  • Foam collapse occurs when liquid in foam moves to the bottom of the foam bubble by the force of gravity causing the lamellae on top of or between the bubbles to thin to the point of failure. See, Porter, M. R. Handbook of Surfactants, 2 nd Ed., Blackie Academic & Professional (Chapman & Hall), 1994 pp 65-69. In the present process, the foam is considered to be stable.
  • Wiggins-Teape teaches foams having an air content of at least 65% by volume (see, U.S. Pat. Nos. 3,716,449 and 3,938,782). To effect dispersion, foams having an air content of 55-75% by volume (see U.S. Pat. No. 3,871,952) or 50-70% by volume (see U.S. Pat. No. 3,937,273). Outside this air content range (either higher or lower), solids in the foam, such as glassfiber, cellulose fiber, synthetic fiber, particulates, and the like, agglomerate. However, foam provides greater dispersion than a web or handsheet made from an aqueous slurry at the same solids content. Ahlstrom describes an improved process that uses an air content of 25-75% by volume (see U.S. Pat. No. 5,904,809).
  • the high air content foam is a stable foam acts like a semi-solid and drainage of the liquid is slow.
  • the process of the invention makes use of high air content foam, typically greater than about 75% by volume. In one embodiment, the air content is greater than about 90% by volume, and in another embodiment, greater than about 98% by volume.
  • foam collapse is initiated by temperature and/or by hygroscopic materials in the system that absorb liquid sufficient to effect foam bubble collapse.
  • Foam density is closely correlated with air content.
  • foam densities in the range of 250-500 g/l at 1 bar pressure are described.
  • lower foam densities can be used to form fibrous webs (for example, foam densities from about 20 to about 100 g/l yield air contents of from about 90 to about 98% by volume).
  • foam densities between about 20 to about 200 g/l at 1 bar are useful.
  • the elimination of the forming step in the process greatly reduces the equipment necessary. Because liquid is not drained from the foamed materials during web formation, there is no typical whitewater or reclaimed liquid or foam that needs to be reprocessed and reused.
  • the reduced liquid/foam load also reduces the amount of liquid in the product leading to more economical drying. This is illustrated by examining the forming solids or consistency in the process.
  • the Ahlstrom patents describe a foam-forming process that uses a fiber consistency of up to 12% is possible in a foam having an air content of 25-75% by volume.
  • the process of the invention makes use of fiber consistencies ranging from about 15 to about 50% by volume. In certain embodiments, fiber consistencies are between about 30 to about 50%. In other embodiments, fiber consistencies are between about 20 to about 35%. Dewatering of fiber at such high consistencies can be achieved. High solids fiber sludge can be further dewatered in an extruder with the addition of slip aids. Also, reclaimed fiber can be extruded at high consistencies with the slip aids in order to reclaim the fibers and fillers typically found in printing papers.
  • foam is used to fluidize the fiber as opposed to adding a slip aid.
  • the foam is generated by the action of high speed screws in a counter or co-rotating extruder or the high speed rotor of a rotary mixer/foamer or pump on a suitable surfactant.
  • the foam used in the process includes a surfactant.
  • a suitable surfactant is one which generates a foam density of about 100 g/l and an air content of about 90% with a surfactant concentration between about 0.01 to about 5% by weight of the composite.
  • the surfactant is Incronam 30 produced by Croda.
  • High solids fiber are used in the process of the invention.
  • High solids fiber can be produced from wet pulp that has been dewatered to greater than about 20% consistency or from dry fibers obtained via a hammermill and adding water.
  • high solids compositions with appropriate chemical additives including binders, latex, wet strength agents, dry strength agents, crosslinking agents, acids, bases, dyes, powders, pigments, polymers, can be mixed and foamed in a mixing device.
  • FIG. 2 An overall diagram of a representative method of the invention is illustrated in FIG. 2.
  • the foaming and mixing operation can be achieved by a shear inducing mixing device.
  • the shear inducing mixing device a mixer/foamer, for example, Model 8 M Mixer/Foamer available from E. T. Oakes Corporation, Hauppauge, N.Y.
  • This device is a rotary mixer and is illustrated in FIGS. 3 A-C.
  • the fiber at roughly 30% solids is added to the device with a piston that pushes the fiber toward the rotor.
  • surfactant is injected.
  • the foamed fiber exits into a tube that feeds a die. Referring to FIGS.
  • the shear inducing mixing device is an extrusion device.
  • One such extruder is the ZSK 58, a mega compounder available from Coperion Corporation, Ramsey N.J.
  • One possible configuration of the extruder is shown in FIG. 4.
  • pulp is added into the extruder at about 20 to about 40% solids.
  • surfactant is added to initiate foaming.
  • Additional chemicals are added downstream of the foaming, but could be added prior to foaming.
  • Crosslinking agent (e.g., citric acid and catalyst) can be added and mixed with the fibers in the extruder making fibers that are prepared for crosslinking during subsequent drying of the product.
  • Binders such as KYMENE (available from Hercules, Wilmington Del.) and latex (available from DuPont, Midland Mich. or HB Fuller, St. Paul Minn.) can also be used. Chemicals and other binders in solid, liquid, or gaseous forms can be added. Air can also be added if needed to increase foaming and/or air content.
  • KYMENE available from Hercules, Wilmington Del.
  • latex available from DuPont, Midland Mich. or HB Fuller, St. Paul Minn.
  • Air can also be added if needed to increase foaming and/or air content.
  • the foamed fiber and additives are extruded through a die to form a sheet or composite web. It is also possible to generate other shapes.
  • the foam composite is extruded onto a solid conveyor belt, wire, or nonwoven carrier fabric to transport the web to the dryer.
  • the foamed composite is then dried (and/or optionally cured) using techniques such as convection drying, through air drying, impingement, microwave, radio frequency, and the like.
  • the composites formed by the processes of the invention include acquisition and storage composites.
  • the process of the invention enables the formation composites composed of any fiber, composites having low densities, and composites including in situ crosslinked fibers.
  • crosslinking agent can be added to the fibers before the fibers are added to the extruder.
  • crosslinking agent can be added to the mixing device during the extrusion process.
  • acquisition composites have a relatively low density.
  • the acquisition composite also has suitable absorption and desorption characteristics, referred to herein as the mid-point desorption pressure (MDP).
  • MDP mid-point desorption pressure
  • FIG. 5 illustrates the difference between the present process and the process for two fiber furnishes.
  • a pine only furnish can be produced typically at about 0.14 g/cm 3 with a foam process for forming cellulosic composites described in PCT/US99/26560, Reticulated Absorbent Composite, and PCT/US99/05997, Methods For Forming A Fluted Composite.
  • density can range from 0.1-1.4 g/cm 3 depending on the grade (see Handbook of Pulp and Paper Technology by K. Britt, 1970, page 669).
  • the present process can form a composite from the same fiber (pine) at a density of 0.037 g/cm 3 . This density is less than the density at which foam laid, or high dilution wetlaid processes can form composites including crosslinked fiber.
  • Density is related to pore size distribution.
  • the present process by nature of both the agglomeration effects and the dispersion effects of foam, creates a web with a wide range of pore sizes.
  • the pore sizes are measured by the autoporosimeter and confirmed by measurements taken from photomicrographs of the foam.
  • the pore size distribution in the range less than 750 un can easily be measured by an autoporosimeter available from TRI/Princeton, Princeton, N.J.
  • the porosimeter tests a sample through an absorption and desorption cycle.
  • a presaturated sample is placed in the instrument.
  • the pressure in the sample chamber is increased which causes the liquid in the sample to drain out from the largest pores first followed by the smaller pores.
  • a computer interfaced with a balance monitors the amount of liquid leaving the sample at each pressure applied. After the last pressure point is tested, the system can run in reverse causing the sample to absorb liquid which is again tracked with pressure. From this data, pore size distributions and absorption/desorption hysteresis can be determined.
  • Mode pore size radius, ⁇ m for similar webs made on several processes is shown in Table 1. Mode is defined as the pore size with the most volume which indicates the highest frequency of occurrence. These values indicate that the mode pore sizes in composites formed by the present extrusion method (pore size contributing the greatest volume) are larger than those produced by other processes. TABLE 1 Comparison of Mode Pore Size Radius ( ⁇ m). Method Extrusion Foam Airlaid Wetlaid Absorption Curve 330 225 210 250 Desorption Curve 150 88 98 75
  • FIGS. 6 - 9 illustrates the pore size distribution charts for representative composites formed by the present extrusion process, foam-formed, airlaid, and wetlaid processes, respectively. Each composite had a target basis weight of 300 g/m 2 .
  • mode pore sizes for absorption are indicated.
  • Mode desorption pore sizes are not marked, but can be determined as for the absorption.
  • Capillary desorption pressure is defined as the head pressure at which 50% of the liquid in a saturated sample has been drained from the sample.
  • CDP capillary desorption pressure
  • a value of capillary desorption pressure of 8-40 cm H 2 O (8-25 cm H 2 O typical) is advantageous for a synthetic foam to acquire fluid (see, e.g., U.S. Pat. No. 5,571,849; U.S. Pat. No. 5,550,167; U.S. Pat. No. 5,851,648).
  • values of 12-50 cm H 2 O (20-40 cm H 2 O typical) for CDP are desired for good performance (see U.S. Pat. No. 6,013,589).
  • the capillary desorption pressure can be determined from a plot of percent saturation vs. applied pressure obtained from the autoporosimeter. Absorption (uptake) and desorption (retention) curves over a range of pressure give an indication of a material's ability to acquire and retain liquid. Acquisition materials for use in absorbent products such as infant diapers must be able to quickly absorb liquid and also efficiently release it again to the diaper core.
  • the capillary desorption pressure measured by the autoporosimeter is called the mid-point desorption pressure (MDP).
  • MDP mid-point desorption pressure
  • the composites of the invention have an MDP value less than about 14 cm H 2 O. In another embodiment, the composites have an MDP value less than about 12 cm H 2 O. In a further embodiment, the composites have an MDP value less than about 10 cm H 2 O.
  • crosslinked cellulosic fibers in personal care absorbent products such as infant diapers has improved the performance of current diapers in the market.
  • Several airlaid pads were made from two crosslinked fibers were tested for MDP.
  • a pad made from citric acid crosslinked fibers (Chemistry B) had an MDP of 24.2 cm H 2 O.
  • a pad made from polyacrylic acid/citric acid crosslinked fibers (Chemistry A) had an MDP of 14.4-15.9 cm H 2 O.
  • Chemistry C refers to polyacrylic acid crosslinked fibers
  • the process of the invention provides webs having improved MDP values.
  • an extruded pine fiber web formed by the Oakes mixer/foamer without using a crosslinking chemistry provides an MDP value of 18.5 cm H 2 O (compare to 44 cm H 2 O for airlaid).
  • the MDP value of an extruded web formed from fibers crosslinked with a blend of polyacrylic and citric acids was measured to be 10.5 cm H 2 O (compare to 15.2 cm H 2 O for airlaid).
  • the MDP value of an extruded web formed from citric acid crosslinked fibers was measured to be 12.0 cm H 2 O (compare to 24.2 cm H 2 O for airlaid).
  • FIG. 11 is a bar graph that demonstrates the reduction in mid-point desorption pressure as a function of crosslinking chemistry for the present process.
  • Extruded composites formed from pine fibers, citric acid crosslinked fibers, and in situ citric acid crosslinked fibers i.e., fibers added to mixing device that were treated, but not cured, with citric acid
  • FIG. 12 is a graph that shows the effect of increasing crosslinking agent on MDP for extruded composites including fibers crosslinked with a blend of polyacrylic and citric acids (pre-crosslinked Chemistry A), citric acid crosslinked fibers (pre-crosslinked Chemistry B), in situ citric acid crosslinked fibers (in situ Chemistry B), and in situ polyacrylic acid crosslinked fibers (in situ Chemistry C).
  • pre-crosslinked Chemistry A citric acid crosslinked fibers
  • pre-crosslinked Chemistry B in situ citric acid crosslinked fibers
  • in situ Chemistry C in situ polyacrylic acid crosslinked fibers
  • FIG. 13 illustrates that latex alone and latex combined with in situ crosslinking reduce MDP.
  • the pre-crosslinked data is considered the control to show the effects of in situ crosslinking (previously demonstrated as from 12 to 9.8 cm H 2 O).
  • the effect of latex without in situ crosslinking is a drop in MDP from 12 to 9.5 cm H 2 O.
  • the combined effect of in situ crosslinking and latex addition changes the MDP from 9.8 to 8.3 cm H 2 O.
  • the extruded composites of the invention also exhibit advantageous tensile strength.
  • Tensile was determined by a horizontal tensile method because many of the samples tested are very weak (e.g., air laid fluff pads).
  • the method uses a horizontal jig fixed to the lower cross head of a constant rate extension tensile machine like those provided by Instron. A 10 cm ⁇ 10 cm sample is clamped into the jig. The load cell is re-zeroed for each sample. The sample is then pulled by the tensile machine. The machine measures the elongation and failure load for each sample.
  • FIG. 14 illustrates the effect on in situ crosslinking on extruded composite tensile strength.
  • Latex can be used to increase the strength of extruded composite structures and webs.
  • the effect of latex and fiber blend on the strength of representative extruded composites is illustrated in FIG. 15.
  • the effect of latex and crosslinking chemistry on the strength of representative extruded composites is illustrated in FIG. 16.
  • a composite with 5% latex and 6% in situ cross linking has a tensile of 1065 g/in (208 gsm), which is more than ten times greater than either treatment alone.
  • the present invention provides absorbent articles that include the crosslinked cellulosic fibrous product.
  • the product can be combined with one or more other layers to provide structures that can be incorporated into absorbent articles such as infant diapers, adult incontinence products, and feminine care products.
  • the extruder units are compact and, depending on the screw diameters, can be installed on a concrete slab without footings or pilings.
  • the small size and simplicity of the process also translates into low capital and engineering costs compared to typical paper industry.
  • the ability to add SAP directly to the equipment in dry form is an advantage.
  • a nine barrel ZSK 58 mm twin screw extruder (KW&P) was used for the trials.
  • a diagram of the extruder set up is given in FIG. 4.
  • Four Acrison single screw feeders were used to feed the raw materials to the extruder.
  • a total of five different screw designs were used to disperse the fibers and generate foam. The different screw designs are described in the following section.
  • the device shown in FIG. 4 shows several features worth noting.
  • the first is the relative size of the equipment.
  • the extruder was contained with an area approximately 25′ ⁇ 25′.
  • the extruder was on the main floor while the feeders were on the second floor.
  • Each barrel is less than 1 foot long. In a commercial design for absorbent products, a nine barrel system is not needed (note: counting from the left, the first and last barrels are not necessary).
  • the third barrel may also be optional.
  • the third design reinserted a limited number of higher shear elements to improve the fiberization. These few shear elements increased the temperature.
  • the fourth screw design (similar to the second design but with more kneading blocks) allowed feed up to 225 lb. OD fiber/hr (2.5 ton/day). Processing fiber only, this screw design appeared to break up the fiber chunks. Feed was reduced to 0.5 ton/day so that the fiber dispersion could be more easily seen.
  • the slot die was then installed which produced a web of approximately 500 g/m 2 and 0.05 g/cc (26.6% “couch” solids).
  • SR1001 was added at about the 50% level.
  • the side feeder worked perfectly with no SAP feed issues.
  • This composite (BW ⁇ 1100 g/m 2 , density ⁇ 0.11 g/cc) was extruded through the die head and samples were taken (39% “couch” solids). Capacity values were 16 g/g.
  • SXM-77 was tried, all the free moisture was removed and dry chunks of SAP and fiber were generated. This is the same result seen in a previous trial (1050-XNP).
  • Capacity under load values of about 11-17g/g were measured on samples containing SAP contents of 37-46%. Capacity values were reasonable considering the high basis weights and the increasing density due to a constant die slot size.
  • FIGS. 17 - 20 above show the PSD and MDP for two lower basis weight samples produced during this trial.
  • the 1051-XNP samples are very similar to those produced on the Oakes during laboratory study described in Example 2.
  • MDP data does not show any differences either between the two studies.
  • the MDP values for runs 17 and 19 were measured to be 17.2 cm H 2 O and 16.2 cm H 2 O, respectively, while the value for 100% pine in the Oakes process was 16.5 cm H 2 O.
  • Table 6 shows a comparison of materials in accordance with the invention versus foam laid made materials. Notable differences are the density and the resulting effect on vertical wicking. Capacity values are similar despite differences in wood fluff pulp (southern pine) content. Even without optimization, the 1051-XNP material is comparable to the foam laid made materials. TABLE 6 Comparison of 1051-XNP and Foam Laid (943 and 946) Samples.
  • a plate mixer/extrusion process is one alternative to traditional forming techniques used to make composite core materials.
  • the Oakes Mixer/Foamer (Oakes Continuous Mixing Head, E. T. Oakes Corporation, Hauppauge, N.Y.) was used to generate an extrudable foam from which webs were made by extrusion through a simple die head.
  • Three fiber furnishes were used: southern pine pulp fibers (CPine); citric acid-treated cellulosic fibers (XLA); and a 50:50 blend of these two fibers.
  • PD8161 latex from H. B. Fuller Company was used as the binding and resiliency aid in levels of 5, 10, and 15% by weight.
  • EXPRO refers to the extrusion process of the invention and the products formed by the process.
  • the device and method can form pine and treated fibers into webs at densities lower than other current processes.
  • the average pore size radii for these products are larger for extruded webs than webs formed by traditional processes.
  • the products have improved acquisition over current materials.
  • the products exhibit much lower median desorption pressure values than current materials formed with traditional processes.
  • the extrusion process has minimal waste of raw materials or product
  • the desired pulp fiber along with 2% by weight Polyox (4 million molecular weight polyethylene oxide) as a slip aid, was fed into the mixer/foamer via a piston feeder at 30% consistency.
  • Other additives such as air, surfactant and latex were added via piston feeders to injection ports located on the Oakes device.
  • the current system was a batch operation which operates on a mass in, mass out basis with essentially no waste.
  • the target basis weight of the samples (300 g/m 2 ) was controlled by the pulp feed rate and the conveyor speed.
  • Pore size distribution is measured with the TRI autoporosimeter. Several examples are discussed here. The three samples chosen here represent the differences seen among three furnishes: pine (sample2, FIG. 23), XLA (sample 6, FIG. 24), and the 50:50 blend (sample 10, FIG. 25).
  • FIG. 4 note the shift in the curves to higher average radii of each curve. This is indicative of the change from pine to crosslinked fiber.
  • FIG. 25, the blend sample shows an intermediate mode value between those seen in FIGS. 23 and 24.
  • the larger average radius is a result of the stiffer crosslinked fibers forming a less dense web and also due to support by the structure.
  • the area under the curve is indicative of the total volume in the sample and thus the capacity of the sample. This arises from the fact that the PSD curves are the derivative of the cumulative volume chart obtained from the autoporosimeter.
  • fiber furnishes can be evaluated based on their impact on pore size which in turn impacts the volume or capacity of the structure.
  • Composites formed from citric acid crosslinked fibers have an MDP of approximately 20-22 cm H 2 O.
  • Composites formed from fibers such as XLA have only been able to achieve MDP values of 16-19 cm H 2 O.
  • FIG. 29 shows an example where XLA was used to produced the samples in 1012-XNP. Here the MDP was 17 cm H 2 O.
  • FIGS. 29, 30, and 31 show the MDP of composites formed by three different processes.
  • FIG. 31 shows airlaid pads made from two batches of XLA fiber: identified as 46-01 and 46-02. These pads have a density of 0.06 g/cm 3 but do not have any surface active agents to inhibit the MDP (14-16 cm H 2 O).
  • FIG. 29 as noted previously is a foam-formed composite. Despite the lower density, the MDP is slightly higher than the airlaid samples.
  • FIG. 30 shows the MDP for a wetlaid composite. This material has an MDP of 18.5 and a similar density to the foam-formed sample. As seen in FIGS. 27 and 28, samples formed by the extrusion process exhibit much lower MDP values.
  • the result can be summarized as follows.
  • the density of the extruded webs is lower than other processes of forming absorbent webs, when pine or crosslinked fibers are used. Average pore size radii are larger for extruded webs providing enhanced acquisition performance.
  • the extrusion process is capable of producing webs with improved MDP using pine and crosslinked fibers which indicates this material will release liquid into a storage core more easily.
  • the extrusion process is capable of adding latex and other additives to a web without spraying and without waste.
  • MUP medium uptake pressure
  • MDP medium desorption pressure
  • caliper of representative products of the invention were determined and compared to conventional airlaid webs.
  • the normalized results are presented in Table 10.
  • XLA refers to an airlaid web of cellulosic fibers crosslinked with citric acid
  • XLB refers to an airlaid web of cellulosic fibers crosslinked with a combination of citric and polyacrylic acid fibers
  • EXPRO-D refers to a composite of celluloisc fibers crosslinked with citric acid
  • EXPRO-E refers to a product of the invention formed from cellulosic fibers treated with citric acid
  • EXPRO-F refers to a product of the invention formed from cellulosic fibers treated with a combination of citric and polyacrylic acid and latex.
  • XLA refers to an airlaid web of cellulosic fibers crosslinked with citric acid
  • XLB refers to an airlaid web of cellulosic fibers crosslinked with a combination of citric and polyacrylic acid fibers
  • EXPRO-D refers to a foam-formed composite of celluloisc fibers crosslinked with citric acid
  • EXPRO-E refers to a product of the invention formed from cellulosic fibers treated with citric acid. All samples tested at 300 gsm basis weight.

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