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

Abstract

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

What is claimed is:  
     
         1 . Cross-linked cellulosic fibers comprising a blend of mercerized cellulosic fibers and conventional fibers having an absorbent capacity of at least about 8.0 grams saline/gram of fiber.  
     
     
         2 . The cross-linked fibers of  claim 1  wherein the fibers have a centrifuge retention capacity of less than about 0.6 grams of a 0.9% by weight saline solution per gram of fiber.  
     
     
         3 . The cross-linked fibers of  claim 1 , wherein the fibers have an absorption capacity of at least about 9.0 g saline/g fiber.  
     
     
         4 . The cross-linked fibers of  claim 1 , wherein the fibers have a centrifuge retention of not more than 0.5 g saline/g fiber.  
     
     
         5 . The cross-linked fibers of  claim 1 , wherein the fibers have a free swell of at least about 10.0 g saline/g fiber.  
     
     
         6 . The cross-linked fiber of  claim 1 , wherein the fibers have knots and nits contents of less than about 10%.  
     
     
         7 . The cross-linked fiber of  claim 1 , wherein the fibers have fines contents of less than about 6.5%.  
     
     
         8 . A method of making a cross-linked blend of cellulosic fibers comprising: 
 forming a wet laid sheet of a blend of mercerized and conventional cellulosic fibers;    supplying a cross-linking agent to the sheet of fibers to form a sheet impregnated with the cross-linking agent; and    drying and curing the cross-linking agent on the impregnated sheet of cellulosic fibers to form intra-fiber cross-links.    
     
     
         9 . The method of  claim 8 , wherein the mercerized cellulose fiber is a conventional cellulose fiber that has been contacted with an alkali metal, washed, neutralized, and optionally dried.  
     
     
         10 . The method of  claim 8 , wherein the conventional cellulose fiber is a wood pulp fiber selected from the group consisting of hardwood pulp, softwood cellulose pulp obtained from a Kraft or sulfite chemical process, and combinations or mixtures thereof.  
     
     
         11 . The method of  claim 10 , wherein the hardwood cellulose pulp is selected from the group consisting of gum, maple, oak, eucalyptus, poplar, beech, aspen, and combinations and mixtures thereof.  
     
     
         12 . The method of  claim 10 , wherein the soft cellulose pulp is selected from the group consisting of Southern pine, White pine, Caribbean pine, Western hemlock, spruce, Douglas fir, and mixtures and combinations thereof.  
     
     
         13 . The method of  claim 10 , wherein the conventional cellulose fiber is derived from one or more components selected from the group consisting of cotton linters, bagasse, kemp, flax, grass, and combinations and mixtures thereof.  
     
     
         14 . The method of  claim 9 , wherein the mercerized fibers are prepared by treating conventional cellulosic fibers with an aqueous solution of sodium hydroxide, washing the fibers, and neutralizing the treated fibers.  
     
     
         15 . The method of  claim 14 , wherein treating the conventional cellulosic fibers comprises contacting the fibers with an aqueous solution containing about 4% to about 40% by weight sodium hydroxide, based on the total weight of the solution.  
     
     
         16 . The method of  claim 14 , wherein treating the cellulosic fibers with caustic is carried out in an aqueous solution of sodium hydroxide at 2 to 10% consistency.  
     
     
         17 . The method of  claim 14 , wherein washing and neutralizing is carried out until the residual water has a pH of between 3 and 8.  
     
     
         18 . The method of  claim 9 , wherein the cellulosic fibers are caustic treated in sheet or roll form.  
     
     
         19 . The method of  claim 9 , wherein the caustic treated cellulosic fiber is in the dry or wet state.  
     
     
         20 . The method of  claim 9 , wherein the α-cellulose content of the caustic treated cellulosic fibers is greater than 65%.  
     
     
         21 . The method of  claim 20 , wherein the α-cellulose content is at least 90%.  
     
     
         22 . The method of  claim 8 , wherein the cross-linking agent is selected from one or more acid aldehyde organic molecules containing aldehyde and carboxylic acid functional groups.  
     
     
         23 . The method of  claim 23 , wherein the acid aldehyde cross-linking agent is selected from the group consisting of glyoxylic acid, succinic semialdehyde, and mixtures and combinations thereof.  
     
     
         24 . The method of  claim 8 , further comprising contacting the fibers with a catalyst.  
     
     
         25 . The method of  claim 24 , wherein the catalyst is Lewis acid selected from the group consisting of iron(III) Chloride, boron trifluoride, tin (IV) chloride, aluminum potassium sulfate, magnesium chloride, zinc chloride, aluminum sulfate, ammonium chloride, zirconium oxychloride, magnesium nitrate, zinc nitrate, aluminum chloride sodium, potassium bisulfate, and a like.  
     
     
         26 . The method of  claim 8 , wherein the cross-linking agent is selected from one or more alkane polycarboxylic acid organic molecules containing at least two acid functional groups.  
     
     
         27 . The method of  claim 26 , wherein the alkane polycarboxylic acid cross-linking agent is selected from the group consisting of 1,2,3,4-butantetracarboxylic acid, 1,2,3-propanetricarboxylic acid, oxydisuccinic acid, citric acid, itaconic acid, maleic acid, tartaric acid, glutaric acid and mixtures and combinations thereof.  
     
     
         28 . The method of  claim 8 , wherein the cross-linking agent is selected from one or more polymeric polycarboxylic acid organic molecules.  
     
     
         29 . The method of  claim 28 , wherein the polymeric polycarboxylic acid cross-linking agent is one or more polymer(s) and copolymer(s) prepared from monomers selected from the group consisting of acrylic acid, vinyl acetate, maleic acid, maleic anhydride, carboxy ethyl acrylate, itanoic acid, fumaric acid, methacrylic acid, crotonic acid, aconitic acid, acrylic acid ester, methacrylic acid ester, acrylic amide, methacrylic amide, butadiene, styrene, or combinations and mixtures thereof.  
     
     
         30 . The method of  claim 8 , wherein the cross-linking agent is supplied as an aqueous solution additionally comprising a catalyst.  
     
     
         31 . The method of  claim 30 , wherein the catalyst is selected form the group consisting of alkali metal salts of phosphorous containing acids such as alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, and alkali metal sulfonates  
     
     
         32 . The method of  claim 8 , wherein the cross-linking agent is selected from one or more polyepoxides having a substituent selected from the group consisting of hydrogen; hydrophobic saturated, unsaturated, cyclic saturated, cyclic unsaturated, branched, and unbranched alkyl groups; and mixtures and combinations thereof.  
     
     
         33 . The method of  claim 26 , wherein the alkane polycarboxylic acid cross-linking agent is selected from the group consisting of 1,4 cyclohexanedimethanol diglycidyl ether, diglycidyl 1,2-cyclohexanedicrboxylate, N,N-diglycidylaniline, N,N-diglcidyl-4-glycidyloxyaniline, diglycidyl 1,2,3,4-tetrahydrophthalate, glycerol propoxylate triglycidyl ether, and mixtures and combinations thereof.  
     
     
         34 . The method of  claim 32 , wherein the one or more polyepoxide cross-linking agents are supplied as an aqueous solution additionally comprising a surfactant.  
     
     
         35 . The method of  claim 34 , Wherein the surfactant is selected from the group consisting of nonionic, anionic, cationic surfactant, or combinations and mixtures thereof.  
     
     
         36 . The method of  claim 8 , wherein the cross-linking agent is supplied to the cellulosic fibers in an amount from about 0.5 to 10% by weight based on the total weight of the fiber.  
     
     
         37 . The method of  claim 8 , wherein the cross-linking agent is supplied to the cellulosic fibers in an amount from about 2 to 5% by weight based on the total weight of the fiber.  
     
     
         38 . The method of  claim 8 , wherein the cross-linking agent comprises an aqueous solution of acid aldehyde having a pH from about 1.5 to about 4.0.  
     
     
         39 . The method of  claim 8 , wherein the cross-linking agent comprises an aqueous solution of alkane polycarboxylic acid having a pH from about 1.5 to about 4.0.  
     
     
         40 . The method of  claim 8 , wherein the cross-linking agent comprises an aqueous solution of polymeric polycarboxylic acid having a pH from about 1.5 to about 4.0.  
     
     
         41 . The method of  claim 34 , wherein the surfactant is added in an amount of from about 0.01 to 5% by weight, based on the total weight of the cross-linking agent.  
     
     
         42 . The method of  claim 8 , wherein drying and curing is conducted at a temperature within the range of from about 280° F. to about 435° F.  
     
     
         43 . The method of  claim 8 , wherein drying and curing is conducted for a period of time of from about 3 minutes to about 15 minutes at temperatures within the range of from about 320° F. to about 435° F.  
     
     
         44 . The method of  claim 8 , wherein the treated fibers are first dried then cured.  
     
     
         45 . The method of  claim 44 , wherein the fiber is dried at a temperature below the curing temperature, and curing is conducted for about 1 to about 10 min at a temperature within the range of from about 300° F. to about 435° F.  
     
     
         46 . The method of  claim 45 , wherein drying is conducted at temperatures within the range of from about 150 to about 300° F., and curing is conducted for about 0.5 to about 5 minutes at temperatures within the range of from about 320° F. to about 435° F.  
     
     
         47 . An absorbent article comprising the cross-linked fiber of  claim 1 .  
     
     
         48 . The absorbent article of  claim 47 , wherein the absorbent article is at least one article selected from the group consisting of infant diapers, feminine care products, training pants, and adult incontinence briefs.  
     
     
         49 . The absorbent article of  claim 47 , further comprising a liquid penetrable top sheet, a liquid impenetrable back sheet, an acquisition layer, and an absorbent structure, wherein the acquisition layer is disposed beneath the top sheet, and the absorbent structure is disposed between the acquisition layer and the back sheet.  
     
     
         50 . The absorbent article of  claim 49 , wherein the acquisition layer comprises the cross-linked fibers.  
     
     
         51 . The absorbent article of  claim 49 , wherein the absorbent structure comprises a composite of superabsorbent polymer and cellulosic fibers.  
     
     
         52 . The absorbent article of  claim 51 , wherein the superabsorbent polymer is selected from the group consisting of polyacrylate polymers, starch graft copolymers, cellulose graft copolymers, cross-linked carboxymethylcellulose derivatives, and mixtures and combinations thereof.  
     
     
         53  The absorbent article of  claim 51 , wherein the superabsorbent polymer is in the form of fiber, flakes, or granules.  
     
     
         54 . The absorbent article of  claim 51 , wherein the superabsorbent polymer is present in an amount of from about 20 to about 60% by weight, based on the total weight of the absorbent structure.  
     
     
         55 . The absorbent article of  claim 51 , wherein the cellulosic fibers comprise the cross-linked cellulosic fibers.  
     
     
         56 . The absorbent article of  claim 51 , wherein the cellulosic fibers comprises a mixture of the cross-linked cellulosic fibers and cellulosic fibers.  
     
     
         57 . The absorbent article of  claim 56 , wherein the cellulosic fiber is a wood pulp fiber selected from the group consisting of hardwood pulp, softwood cellulose pulp obtained from a Kraft or sulfite chemical process, mercerized, rayon, cotton linters, and combinations or mixtures thereof.  
     
     
         58 . The absorbent article of  claim 57 , wherein the cross-linked cellulosic fiber is present in the mixture of fibers in an amount of from about 1 to 70% by weight, based on the total weight of the mixture of fibers.  
     
     
         59 . The absorbent article of  claim 58 , wherein the cross-linked cellulosic fiber is present in an amount of from about 10 to 40% by weight, based on the total weight of the mixture of fibers.  
     
     
         60 . The absorbent article of  claim 56 , wherein the mixture of cross-linked cellulosic fibers and cellulosic fibers is present in an amount of from about 10 to about 80% by weight, based on the total weight of the absorbent structure.  
     
     
         61 . The absorbent article  claim 60 , wherein the mixture is present in an amount of from about 20 to about 60% by weight, based on the total weight of the absorbent structure.  
     
     
         62 . The method of  claim 8 , wherein the cross-linking agent is a mixture of cross-linking agents selected from the group consisting of: a mixture of glyoxylic acid and citric acid; a mixture of glyoxylic acid and polymaleic acid; and a mixture of glyoxylic acid, citric acid, and polymaleic acid.  
     
     
         63 . The method of  claim 8 , wherein the cross-linking agents is a mixture cross-linking agents selected from the group consisting of: a mixture of glyoxylic acid and citric acid; a mixture of glyoxylic acid and a terpolymer of maleic acid, vinyl acetate, and ethyl acrylate; and a mixture of glyoxylic acid, citric acid, and a terpolymer of maleic acid, vinyl acetate, and ethyl acrylate.