US20030219594A1

US20030219594A1 - Meltblown absorbent fibers and composites and method for making the same

Info

Publication number: US20030219594A1
Application number: US10/154,607
Authority: US
Inventors: Jian Qin; James Wang; Anthony Wisneski; Fu-Jya Tsai
Original assignee: Kimberly Clark Worldwide Inc
Current assignee: Kimberly Clark Worldwide Inc
Priority date: 2002-05-23
Filing date: 2002-05-23
Publication date: 2003-11-27
Also published as: BR0310007A; EP1506024A1; CN1652827A; WO2003099345A9; WO2003099345A1; KR20050008710A; AU2003220036A1

Abstract

An absorbent fiber is produced from a melt processable polymer. An absorbent composite includes the absorbent fiber in addition to natural fibers and superabsorbent material. A method of producing the superabsorbent fiber and absorbent composite is also disclosed.

Description

FIELD OF THE INVENTION

This invention relates to absorbent materials having improved absorbent properties. More specifically, this invention relates to absorbent fibers produced from a melt processable polymer and to absorbent composites containing the absorbent fibers. This invention also relates to a method for making the absorbent fibers and absorbent composites.

BACKGROUND OF THE INVENTION

Absorbent materials are useful in disposable personal care absorbent products such as diapers, training pants, feminine pads, adult incontinence products and professional health care products for absorbing and retaining fluids. Absorbent or superabsorbent materials are often combined with water-insoluble fibers to create an absorbent composite for use in an absorbent core of a disposable personal care absorbent product according to methods known in the art.

Particulate superabsorbents are widely used as superabsorbent material in disposable personal care absorbent products. However, superabsorbent particles are sometimes difficult to use because they do not remain stationary during the manufacturing of the disposable personal care absorbent product and may shift position in the disposable personal care absorbent product.

The use of absorbent or superabsorbent fibers instead of superabsorbent particles in disposable personal care absorbent products is potentially advantageous because fibers can provide improved product integrity, better containment, reduced product bulkiness and improved absorbent properties, such as rapid fluid absorption and fluid distribution properties. Furthermore, the use of fibers may also lead to improved product attributes, such as thinner and softer products that provide better fit, less gel migration, and potential simplification of the manufacturing process of the absorbent product.

Currently available commercial absorbent fibers are produced by solution spinning processes, such as a solution dry spinning process used for textile fiber manufacturing or a solution blowing spinning process used for nonwoven fabric manufacturing. Solution spinning processes start with an aqueous polymer solution. The polymer solution is then spun into fibers and cross-linked to form water-swellable, water-insoluble fibers. However, such solution spinning processes have low productivity and are expensive because of the presence of excess water in the polymer solution. For these reasons, absorbent fibers are not widely used for the absorbent material in disposable personal care absorbent products.

Some polymers can be processed into different shapes or forms when the temperature is above a certain point and can be referred to as “melt processable.” Other polymers upon reaching a certain elevated temperature will degrade, rather than melt, and can be referred to as “non-melt processable.”

Fibers produced from melt processable polymers such as polyethylene (PE) or polypropylene (PP) are widely used in nonwoven industries at low cost. However, they cannot be used in absorbent composites of disposable personal care absorbent products due to their high hydrophobicity which causes poor fluid handling properties such as non-wetting and no wicking.

U.S. Pat. No. 5,280,079 issued Jan. 18, 1994 to Allen et al., U.S. Pat. No. 5,147,956 issued Sep. 15, 1992 to Allen, U.S. Pat. No. 4,962,172 issued Oct. 09, 1990 to Allen et al., U.S. Pat. No. 5,151,465 issued Sep. 29, 1992 to Le-Khac, and U.S. Pat. No. 5,066,742 issued Nov. 19, 1991 to Gupta each describe absorbent fibers. However, the absorbent fibers described therein are produced by a solution spinning process. In addition, the water soluble polymers used to form the fibers are non-melt processable. Furthermore, superabsorbent staple fibers are commerically available from Camelot Superabsorbent Ltd. in Calgary, Canada under the trade designation FIBERDRI®, an isobutylene-maleic anhydride copolymer based absorbent fiber, or from Technical Absorbents in Grimsby, United Kingdom under the trade designation OASIS® 101, a sodium polyacrylate based absorbent fiber. These commerically available superabsorbent fibers are also produced by solution spinning of non-melt processable water soluble polymers. These staple fibers are made from textile fiber dry spinning process. They can be incorporated into absorbent cores through an air laying process but do not form bonds with other components of the core, such as superabsorbent particles and fluff fiber. The absorbent core from the process is not a stabilized structure.

There is a need for absorbent fibers that can be manufactured in high productivity and at low cost, and for absorbent composites containing such absorbent fibers. There is also a need for stabilized absorbent composites that can exhibit improved dry and wet integrities. There is also a need for a high productivity, low cost method for making the absorbent fibers and absorbent composites.

SUMMARY OF THE INVENTION

This invention relates to absorbent fibers produced from melt processable polymers and to absorbent composites containing the absorbent fibers. This invention also relates to a method for making the absorbent fibers and absorbent composites.

In one embodiment of this invention, the absorbent fibers include a melt processable, water soluble polymer which is meltblown and then cross-linked to form the water-swellable but water insoluble absorbent fibers. The resulting absorbent fibers have an absorbency under zero load value of at least about 5 grams fluid per gram fiber (g/g). The melt processable, water soluble polymer may be a non-ionic homopolymer, such as for example, polyethylene oxide, polypropylene oxide, hydroxy propyl cellulose, methyl cellulose, ethyl cellulose, methyethyl cellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid, polyacrylamide, and combinations thereof. The melt processable, water soluble polymer may also be a copolymer of monomers of at least one ionic and one non-ionic monomer such as sodium acrylate (currently used in commercial superabsorbent materials) and methyl methacrylate (currently used in commercial melt processable polymers).

In another embodiment of this invention, an absorbent composite includes a melt processable, water soluble polymer which is meltblown with hydrophilic fibers (such as wood pulp fluff, cotton, cotton linter, other cellulose fibers, regenerated cellulose fibers, natural fibers or modified or spun staple fibers, and hydrophilic synthetic fibers, such as those available from Allied Corporation in Morristown, N.J., USA, under the trade designation HYDROFIL®, and combinations thereof) and commercially available superabsorbent material. The polymer is cross-linked to form water-swellable but water insoluble absorbent fibers. The resulting absorbent composite has an absorbency under zero load value of at least 5 grams fluid per gram composite (g/g) and may also be superabsorbent, exhibiting an absorbency under zero load of at least about 10 g/g or up to about 50 g/g. As in the previous embodiment, the melt processable, water soluble polymer may be a non-ionic homopolymer, such as for example, polyethylene oxide, polypropylene oxide, hydroxy propyl cellulose, methyl cellulose, ethyl cellulose, methylethyl cellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid, polyacrylamide, and combinations thereof, or may be a copolymer of monomers of at least one ionic and one non-ionic monomer such as sodium acrylate and methyl methacrylate.

In either embodiment, the cross-linking agent can be sprayed onto the surface of the meltblown fibers. The cross-linking agent must have at least two functional groups capable of reacting with the functional groups on the surface of the melt processable polymer. In order to initiate the cross-linking reaction, a post treatment such as heat,treatment, microwave radiation, electron beam (e-beam) radiation, ultraviolet (UV) radiation, steam treatment or vapor treatment is required.

This invention also relates to a method for making absorbent fibers and absorbent composites including the steps of melting a melt processable, water soluble polymer, extruding the polymer, spinning the polymer to form fibers, adding a cross-linking agent and curing the resulting fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a diaper with an absorbent core containing absorbent material FIGS. [0015] 2A-2C show photographs of an absorbent composite according to one embodiment of the invention.
FIG. 3 is a schematic representation of a method and apparatus for producing absorbent fibers and absorbent composites according to one embodiment of the invention. [0016]

DEFINITIONS

Within the context of this specification, each term or phrase below will include the following meaning or meanings. [0017]
“Polymer” includes, but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc., and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic and atactic symmetries. [0018]
“Nonwoven fabric or web” means a web having a structure of individual fibers or threads which are interlaid, but not in a regular or identifiable manner as in a knitted fabric. Nonwoven fabrics or webs have been formed from many processes such as, for example, meltblowing processes, spunbonding processes, air laying processes, and bonded carded web processes. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters useful are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91.). [0019]
“Spunbonded fibers” refers to small diameter fibers which are formed by extruding molten thermoplastic material as filaments from a plurality of fine capillaries of a spinnerette having a circular or other configuration, with the diameter of the extruded filaments then being rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 to Appel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartmann, U.S. Pat. No. 3,502,538 to Petersen, and U.S. Pat. No. 3,542,615 to Dobo et al., each of which is incorporated herein in its entirety by reference. Spunbond fibers are quenched and generally not tacky when they are deposited onto a collecting surface. Spunbond fibers are generally continuous and often have average diameters larger than about 7 microns, more particularly, between about 10 and 30 microns. [0020]
“Meltblown fibers” means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into converging high velocity heated gas (e.g., air) streams which attenuate the filaments of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed for example, in U.S. Pat. No. 3,849,241 to Butin et al. Meltblown fibers are microfibers which may be continuous or discontinuous, are generally smaller than 10 microns in diameter, and are generally self bonding when deposited onto a collecting surface. Meltblown fibers used in the present invention are preferably substantially continuous in length. [0021]
“Coform material” refers to a product produced by combining separate polymer and additive streams into a single deposition stream in forming the nonwoven webs. Such a process is taught, for example, by U.S. Pat. No. 4,100,324 to Anderson et al. which is hereby incorporated by reference. U.S. Pat. No. 4,818,464 to Lau discloses the introduction of superabsorbent material as well as wood pulp fluff, cellulose, or staple fibers through a centralized chute in an extrusion die for combination with resin fibers in a nonwoven web. The wood pulp fluff, staple fibers, or other material are added to vary the characteristics of the resulting web, for example, strength and absorbency. [0022]
“Pulp fibers” refers to fibers from natural sources such as woody and non-woody plants. Woody plants include, for example, deciduous and coniferous trees. Non-woody plants include, for instance, cotton, flax, esparto grass, milkweed, straw, jute hemp, and bagasse. [0023]
“Cross-linked” refers to any means for effectively rendering normally water-soluble materials substantially water insoluble but swellable. Such means can include, for example, physical entanglement, crystalline domains, covalent bonds, ionic complexes and associations, hydrophilic associations, such as hydrogen bonding, and hydrophobic associations or Van der Waals forces. [0024]
“Hydrophilic” describes fibers or the surfaces of fibers which are wettable by the aqueous liquids in contact with the fibers. The degree of wetting of the materials can, in turn, be described in terms of the contact angles and the surface tensions of the liquids and materials involved. Equipment and techniques suitable for measuring the wettability of particular fiber materials or blends of fiber materials can be provided by a Cahn SFA-222 Surface Force Analyzer System, or a substantially equivalent system. When measured with this system, fibers having contact angles less than 90° are designated “wettable” or hydrophilic, while fibers having contact angles greater than 90° are designated “nonwettable” or hydrophobic. [0025]
“Superabsorbent material” refers to a water-swellable, water-insoluble organic or inorganic material capable, under the most favorable conditions, of absorbing at least about 10 times its weight, preferably at least about 20 times its weight in an aqueous solution containing 0.9% by weight sodium chloride. Superabsorbent material can comprise a form including particles, fibers, nonwovens, films, coforms, printings, coatings, other structural forms, and combinations thereof. “Water-swellable, water-insoluble” refers to the ability of a material to swell to a equilibrium volume in excess water but not dissolve into the water. The water-swellable, water-insoluble material generally retains its original identity or physical structure, but in a highly expanded state upon the absorption of water. [0026]
“Absorbency Under Zero Load (AUZL)” refers to the result of a test which measures the amount in grams of an aqueous 0.9% by weight sodium chloride solution that a gram of material can absorb in 1 hour under negligible applied load (about 0.01 pound per square inch). [0027]
“Water-soluble” refers to materials which substantially dissolve in excess water to form a solution, thereby losing its initial form and becoming essentially molecularly dispersed throughout the water solution. As a general rule, a water-soluble material will be free from a substantial degree of cross-linking, as cross-linking tends to render a material water insoluble. A material that is “water insoluble” is one that is not water soluble according to this definition. [0028]
“Melt processable” refers to either a crystalline or semicrystalline polymer that has a melting point or an amorphous polymer that has a softening point and, therefore, can be thermally processed into different shapes or forms, for example, meltblown fibers. In order to be considered as a melt processable polymer, the crystalline or semicrystalline polymers have to have a melting point as well as reasonable thermal stability and melt processability such as adequate melt rheology. For amorphous polymers, they have to have a softening point as well as reasonable thermal stability and melt processability such as adequate melt rheology. [0029]
“Solvent” refers to a substance, particularly in liquid form, that is capable of dissolving a polymer used herein to form a substantially uniformly dispersed mixture at the molecular level. [0030]
The term “absorbent product” includes without limitation diapers, training pants, swim wear, absorbent underpants, baby wipes, adult incontinence products, feminine hygiene products, medical garments, underpads, bandages, absorbent drapes, and medical wipes, as well as industrial work wear garments. [0031]
These terms may be defined with additional language in the remaining portions of the specification. [0032]

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

This invention relates to absorbent fibers produced from a melt processable polymer and to absorbent composites containing the absorbent fibers. The absorbent composites can be used in absorbent cores for disposable personal care absorbent products. The absorbent composites are useful in absorbent articles such as diapers, training pants, swim wear, adult incontinence articles, feminine care products, and medical absorbent products. This invention also relates to a method for making the absorbent fibers and absorbent composites. [0033]
FIG. 1 illustrates an exploded perspective view of a disposable diaper. Referring to FIG. 1, [0034] disposable diaper 10 includes outer cover 12, body-side liner 14, and absorbent core 40 located between body-side liner 14 and outer cover 12. Absorbent core 40 can include the absorbent fibers or an absorbent composite according to this invention. Body-side liner 14 and outer cover 12 are constructed of conventional non-absorbent materials. By “non-absorbent” it is meant that these materials, excluding the pockets filled with superabsorbent, have an absorptive capacity not exceeding 5 grams of 0.9% aqueous sodium chloride solution per gram of material.
Body-[0035] side liner 14 is constructed from highly liquid pervious materials. This layer functions to transfer liquid from the wearer to absorbent core 40. Suitable liquid pervious materials include porous woven materials, porous nonwoven materials, films with apertures, open-celled foams, and batting. Other examples of suitable body-side liner materials include, without limitation, any flexible porous sheets of polyolefin fibers, such as polypropylene, polyethylene or polyester fibers; webs of spunbonded polypropylene, polyethylene or polyester fibers; webs of rayon fibers; bonded carded webs of synthetic or natural fibers or combinations thereof. U.S. Pat. No. 5,904,675, issued May 18, 1999 to Laux et al. and incorporated by reference, provides further examples of suitable surge materials. This layer may also be an apertured plastic film. Suitable batting includes certain air formed thermochemical and chemithermomechanical wood pulps. The various layers of article 10 have dimensions which vary depending on the size and shape of the wearer.
[0036] Outer cover material 12 should be breathable to water vapor. Generally outer cover 12 will have a moisture vapor transmission rate (MVTR) of at least about 300 grams/m²-24 hours, desirably at least about 1000 grams/m²-24 hours, or at least about 3000 grams/m²-24 hours, measured using INDA Test Method IST-70.4-99, herein incorporated by reference.
Attached to [0037] outer cover 12 are waist elastics 26, fastening tapes 28 and leg elastics 30. The leg elastics 30 typically have a carrier sheet 32 and individual elastic strands 34. The diaper of FIG. 1 is a general representation of one basic diaper embodiment. Various modifications can be made to the design and materials of diaper parts.
Construction methods and materials of an embodiment of a diaper such as illustrated in FIG. 1, are set forth in greater detail in commonly assigned U.S. Pat. No. 5,509,915, issued Apr. 23, 1996 in the name of Hanson et al., incorporated herein by reference. Possible modifications to the diaper illustrated in FIG. 1 are set forth in commonly assigned U.S. Pat. No. 5,509,915 and in commonly assigned U.S. Pat. No. 5,364,382, issued Nov. 15, 1994 to Latimer et al. [0038]
According to one embodiment of this invention, the absorbent fibers comprise a melt processable, water soluble polymer which is meltblown and then cross-linked to form water-swellable but water insoluble absorbent fibers. Suitable melt processable, water soluble polymers include non-ionic homopolymers such as polyethylene oxide, polypropylene oxide, hydroxyl propyl cellulose, methyl cellulose, ethyl cellulose, methylethyl cellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid, polyacrylamide and combinations of the foregoing. In order to enhance their melt processabilities, some degree of modification may be needed. These modifications include, but are not limited to, additions of low percentage of additives, blends, and/or comonomers. The modified polyvinyl alcohol used herein is available commercially from Nippon Gohsei located in Osaka, Japan. [0039]
Although a non-ionic water soluble and melt processable polymer is absorbent, it is not superabsorbent due to lack of ionic charge groups on its macromolecular chains. Current commercial particulate superabsorbent materials are made of ionic polyacrylate. However, pure ionic water soluble polymers in general are not melt processable. [0040]
Another melt processable, water soluble polymer may be a copolymer of both ionic and non-ionic monomers. Suitable monomers for the copolymer include an ionic monomer, such as sodium acrylate, which is commerically available from Aldrich Chemical Co. in Milwaukee, Wis., USA, and a non-ionic monomer, such as methyl methacrylate, which is commerically available from Aldrich Chemical Co. in Milwaukee, Wis., USA. In order to achieve both water solubility and melt processability of the copolymer, the ratio of the monomers on a dry weight basis is critical. Preferably, the ratio of the monomers on a dry weight basis should be from about 30:70 to about 70:30. Polymerization to form the copolymer can be carried out according to conventional methods known in the art. Because of the addition of an ionic comonomer, the water soluble and melt processable copolymers have a higher absorbency than the non-ionic homopolymers. [0041]
Whether a homopolymer or a copolymer, the molecular weight of the polymer is important. The molecular weight of the polymer must be at least about 10,000 in order to have a high fluid absorbency. However, the molecular weight of the polymer cannot exceed about 1,000,000 because the meltblowing equipment can not handle too high of a viscosity. Suitable polymers have a molecular weight between about 50,000 to 1,000,000, desirably between about 100,000 to 1,000,000, or between about 100,000 and 500,000. [0042]
Following formation of the fibers, the fibers are still water soluble. A solution containing a cross-linking agent is sprayed onto the surface of the meltblown fibers to form water-swellable, but water insoluble fibers instantly or after the curing step depending on the nature of the cross-linking agent used. The suitable cross-linking agent can be either reactive or latent. The reactive cross-linking agent will cross-link the fibers in the spinning process. The latent cross-linking agent does not cross-link the fibers and normally requires some activation energy to trigger cross-linking, such as heating. The cross-linking agent desirably has at least two functional groups capable of reacting with the pendant functional groups on the melt processable polymer. Suitable cross-linking agents include diols, polyols, diamines, polyamines, dicarboxylic acids, polycarboxylic acids, dialdehydes, polyaldehydes, butandiol, diethylene triamine, citric acid, glutaric dialdehyde and ethylene glycol diglycidyl ether, tri-valent or tetra-valent metal ions, and combinations of the foregoing. [0043]
The appropriate functional groups on the cross-linking agent depends upon the melt processable polymer. For example, if polyvinyl alcohol is used as the melt processable polymer, suitable functional groups on the cross-linking agent include carboxylic acid groups (forming ester linkages with the hydroxyl groups on the polyvinyl alcohol), aldehyde groups (forming acetal linkages with the hydroxyl groups on the polyvinyl alcohol), or epoxy groups (forming ether linkages with the hydroxyl groups on the polyvinyl alcohol). However, if the melt processable polymers have different types of functional groups, such as amino, or carboxylic acid, or others, the appropriate functional groups on the cross-linking agent will be different. For example, if the polymer has carboxylic acid functional groups, suitable functional groups on the cross-linking agent include hydroxyl groups (forming ester linkages with the carboxylic acid groups on the polymer), amino groups (forming amide linkages with the carboxylic acid groups on the polymer), or tri-valent or tetra-valent metal ions (forming ionic bonds with the carboxylic acid groups on the polymer). [0044]
One example of a commercially available cross-linking agent is KYMENE® available from Hercules Incorporated located in Wilmington, Del. KYMENE® contains functional groups which are capable of reacting with hydroxyl groups on polyvinyl alcohol. KYMENE® is widely used to cross-link cellulose fibers. However, the chemical composition is proprietary. [0045]
When a latent cross-linking agent is used, in order to initiate the cross-linking reaction, a posttreatment process is required. Such post treatment processes include heat treatment, microwave radiation, e-beam radiation, UV radiation, steam treatment or vapor treatment. [0046]
According to another embodiment of this invention, the absorbent composite includes a melt processable, water soluble polymer which is meltblown with hydrophilic fibers and commercially available superabsorbent material to form a coform material. The polymer is cross-linked to form water-swellable but water insoluble absorbent fibers. [0047]
Any of the previously described homopolymers or copolymers with a molecular weight between about 10,000 to 1,000,000 can be suitable for the melt processable, water soluble polymer for this embodiment of the invention. [0048]
The superabsorbent material can be any commercially available superabsorbent material, such as superabsorbent particles or superabsorbent fibers. Examples of commercially available particulate superabsorbents include SANWET® IM 3900 and SANWET® IM-5000P, available from Hoescht Celanese located in Portsmouth, Va., DRYTECH® 2035 available from Dow Chemical Co. located in Midland, Mich., and FAVOR® SXM 880, SXM 9543, available from Stockhausen, located in Greensboro, N.C. Any of the previously described commercially available superabsorbent staple fibers can be suitable superabsorbent fibers. [0049]
The hydrophilic fibers are preferably wood pulp fluff commerically available from US Alliance Forest Products Corporation in Coosa Pine, Ala., USA, under the trade designation Coosa CR 1654. [0050]
The resulting absorbent composite, which is a coform material, as shown in FIGS. [0051] 2A-2C, includes the absorbent fibers 36 made in this instance from polyvinyl alcohol, hydrophilic fibers in this instance wood pulp fluff 38, and superabsorbent material 39 in this instance superabsorbent particles.
The invention also includes a method for making absorbent fibers and an absorbent composite. Referring to FIG. 3, a [0052] hopper 50 contains pellets of a melt processable, water soluble polymer. A single or twin screw extruder 52 melts the pellets by a conventional heating arrangement to form a molten extrudable composition which is extruded through a melt-blowing die 54 by the action of a turning extruder screw (not shown) located within the extruder 52. The extrudable composition is fed through the die 54. The die 54 and the gas supply fed therethrough are heated by a conventional arrangement (not shown). Besides the spinning die diameter, the air velocity can also be adjusted to control fiber diameter.
In order to produce a nonwoven material including only the absorbent fibers of the invention, next, a solution containing the cross-linking agent is sprayed onto the gas borne stream of [0053] fibers 56 by a sprayer represented by stream 60. The absorbent fibers are then directed onto a forming wire 64 including a belt 66 and rollers 68 by vacuum 67 to air form the nonwoven material which may then be dried and treated by a post treatment process to initiate the cross-linking reaction. Use of the vacuum box 67 underneath the forming wire 64 can help the fibers form a uniform web onto the forming wire 64. The post treatment may be heat treatment, microwave treatment, e-beam radiation, UV radiation, steam treatment or vapor treatment. The nonwoven material is then wound and collected onto a winder 70.
In order to produce the absorbent composite of the invention, which is a coform material, the gas borne stream of [0054] fibers 56 is merged with a secondary gas stream 58 containing individualized hydrophilic fibers, preferably wood pulp fibers, so as to integrate the different fibrous materials in a single step. A solution containing the cross-linking agent is sprayed onto the gas borne stream of fibers 56 by a sprayer represented by stream 60. The superabsorbent material may be added simultaneously with the hydrophilic fibers and cross-linking agent via an additional gas stream 62. The integrated air stream is then directed onto a forming wire 64 including a belt 66 and rollers 68 by vacuum 67 to air form the coform material. The air may be supplied by any conventional means as, for example, a blower (not shown).
Any of the previously described melt processable polymers, superabsorbent materials, hydrophilic fibers and cross-linking agents can be used to make the absorbent fibers and/or absorbent composites. [0055]
Following formation of the coform material, the coform material is dried and then treated by a post treatment process in order to initiate the cross-linking reaction. Such post treatment is sometimes referred to as “curing.” Such treatment may be any one of heat treatment, microwave treatment, e-beam radiation, UV radiation treatment, steam treatment or vapor treatment. The coform material is then wound and collected onto a [0056] winder 70.
When heat treatment is used as the post treatment process for the coform material, undesirable discoloration of the coform material sometimes occurs. For example, when a coform material including wood pulp fiber is heat cured at a temperature higher than 140° C. for more than 2 hours, the cured coform material has a dark color ranging from yellow to brown because of the oxidation of the wood pulp fiber. Such discoloration will also occur when the meltblown fibers are made from polyvinyl alcohol. [0057]
Effective ways to minimize or eliminate the discoloration include, but are not limited to: (1) reducing the curing temperature by using catalysts or a low temperature curable cross-linking agent; (2) curing the coform material using a different curing method, such as microwave radiation or e-beam radiation; (3) using different types of melt processable polymers, such as hydroxy propyl cellulose; (4) using a self-cross-linkable polymer, such as silane grafted polyethylene oxide, which is capable of cross-linking itself induced by moisture; and (5) using an antioxidant. [0058]
During the preparation of the coform material, spraying water during the fiber spinning and in the superabsorbent material/wood pulp fluff/superabsorbent fiber mixing zone can help to enhance inter-fiber or inter-superabsorbent particle bonding. Therefore, the concentration of the cross-linking agent can play a role in controlling the structure and integrity of absorbent composites. If more water is needed, a more dilute solution of cross-linking agent can be prepared, or vice versa. On the other hand, the concentration of the cross-linking agent affects the shell thickness of the surface of the cross-linked fiber with more dilute concentrations resulting in a thicker cross-linked surface shell layer. [0059]

TEST METHOD—ABSORBENCY UNDER ZERO LOAD

The Absorbency Under Zero Load (AUZL) is a test which measures the ability of an absorbent material to absorb a liquid (such as a 0.9 weight percent solution of sodium chloride in distilled water) while under a negligible load or restraining force. About 0.16 g of meltblown web or coform discs (about 1 inch in diameter) of each sample were weighed and placed into a plastic sample cup. The sample cup consists of a plastic cylinder having a 1 inch inside diameter and an outside diameter of 1.25 inches. The bottom of the sample cup is formed by adhering a 100 mesh metal screen having 150 micron openings to the end of the cylinder by heating the screen above the melting point of the plastic and pressing the plastic cylinder against the hot screen to melt the plastic and bond the screen to the plastic cylinder. The sample is then covered with a plastic spacer disc, weighing 4.4 grams, which generated a pressure of about 0.01 pound per square inch. The sample cup is placed in a Petri dish which contains about 25 ml of 0.9% by weight sodium chloride solution. After one hour, the cup was taken out and placed on multiple layers of paper towels to blot the interstitial fluid of the web or coform. The blotting is continued by moving the cup to the area with dry paper towel until there is no fluid mark visible on the paper towel. The weight difference of the cup between wet and dry presents total amount of fluid absorbed by the web or coform and is used to calculate AUZL. [0060]

EXAMPLES

Example 1

Water soluble polyvinyl alcohol Ecomaty® AX-10000, available from Nippon Gohsei, Osaka, Japan, was melt blown into continuous filaments through a Killion line (2″ die tip, 0.35 mm die, 20 holes/in, other parameters listed in Table 1). Air pressure was in a range from 4 to 8 psi. Air temperature was controlled to as close to the die temperature as possible (412° F. in this case) so that no effect of cooling off or heating up on die tip occurred. Vacuum was about 8 inch water. The filament was collected on a moving conveyor belt having a foraminous surface to form a melt blown nonwoven. The nonwoven was still water soluble and dipped into a solution of 86.5% methanol, 12.3% water, and 1.2% ethylene glycol diglycidyl ether at a weight ratio of 1 g of fiber to 30 g of solution. The nonwoven was then removed out of the solution and blotted by paper towel. The wet nonwoven was dried at 80° C. and then cured at 130° C. for 20 hours. The cured nonwoven was water-swellable but water-insoluble and exhibited an Absorbency Under Zero Load (AUZL) value in 0.9% NaCl saline of about 7.5 g/g. [0061]

TABLE 1

Heat 1 Barrel Die Extruder

Temp. Heat 2 Temp. Heat 3 Temp. Die Temp. Pressure Pressure Screw

(° F.) (° F.) (° F.) (° F.) (psi) (psi) Speed (rpm)

269 304 410 412 1020 340 11

Example 2

Several solutions including different cross-linking agents were prepared as described in Table 2. The polyvinyl alcohol nonwoven prepared in Example 1 was treated separately by the solution and heat cured at different temperatures for certain times. The cured nonwovens were subjected to the AUZL test in saline. Both the AUZL value and the color of the cured nonwoven were recorded in Table 2 below:

TABLE 2


		Ratio of	Curing
Recipe #	Composition	Fiber/solution	T(° C.)/time(hr)	Discoloration	AUZL (g/g)

1	98% methanol, 2%	1/50	130/15	Golden	5.5
	Kymene (557LX)
2	84.5% methanol, 14%	1/25	110/70	Light Golden	6.0
	water, 1.5% citric acid
3	88% methanol, 11.5%	1/15	140/20	Yellow	7.1
	water, 0.5% glutaric
	dialdehyde
4	85% methanol, 12.5%	1/30	130/24	White	8.2
	water, 2.5% ethylene
	glycol diglycidyl ether

Example 3

Polyvinyl alcohol was melt blown into continuous filaments using the Killion line. Solutions including 1.25%, 2.5% or 5% citric acid or KYMENE® were separately sprayed onto the surface of the fibers at a location near the die tip. A coform material with wood pulp fluff CR 1654 at a ratio of 50% polyvinyl alcohol and 50% wood pulp fluff was also prepared. The polyvinyl alcohol fiber spinning throughput was about 30 grams per minute (gpm), and the solution spraying throughput was about 10 gpm. Process conditions are listed in Table 3. Air pressure was in a range from 4 to 8 psi. Air temperature was controlled to as close to the die temperature as possible so that no effect of cooling off or heating up on die tip occurred. Vacuum was about 8 inch water. The nonwovens obtained were heated in a 130° C. oven for up to 4 days. The cured nonwovens were completely cross-linked and become water insoluble. Again discoloration was found in all the cured nonwovens. AUZL values of the treated nonwovens were around 7 to 8 g/g.

TABLE 3


					Extruder	Spin Pump	Die
Cross-linker	Heater
1	Heater 2	Heater 3	Die	Screw Speed	Speed	Pressure
Solution	(° F.)	(° F.)	(° F.)	(° F.)	(rpm)	(rpm)	(psi)

1.25% citric	306	361	440	440	25	20	396
acid
2.5% citric	306	341	440	442	24	20	382
acid
5.0% citric	303	342	440	438	28	20	390
acid
1.25%	306	360	440	439	21	20	402
Kymene
2.5%	305	360	440	440	21	19.7	408
Kymene
5.0%	305	360	440	440	22	20	394
Kymene

Example 4

The uncured nonwovens surface sprayed by either 5% citric acid or KYMENE® solution and prepared from Example 3 were treated in a microwave oven (GE Model JE125OGW, 1.5 kW, Vac/Hz 120/60) at an intensity level of 1 (lowest of the machine) for at least 10 hours. The intensity level of the microwave oven could not be greater than 1, otherwise the nonwoven fibers would be molten due to too high temperature reached locally. The microwave treated nonwovens were completely white (no discoloration) and water-swellable, water-insoluble. [0064]

Example 5

Polyvinyl alcohol was melt blown into continuous filaments using the Killion line. A solution including 5% KYMENE® and 0.5% surfactant Rhodamox LO, available from Rhone-Poulenc Inc., was sprayed onto the surface of fibers at a location near the die tip. A coform material with both commercial superabsorbent particles FAVOR® SXM 880 and wood pulp fluff CR 1654 at a ratio of 48% superabsorbent particles, 26% polyvinyl alcohol and 26% wood pulp fluff was also prepared. Process parameters are listed in Table 4. Air pressure, temperature and vacuum were the same as specified in the examples before. The basis weight of the coform material was 484 gsm. A solution including 5% KYMENE® and 0.5% surfactant Rhodamox LO was sprayed onto the surface of the coform material at a location near the tie tip. The coform material was heat cured at 150 ° C. for 3 hours. Surprisingly, the cured coform material had almost no discoloration probably due to the presence of Rhodamox LO surfactant. The coform material exhibited an AUZL value in 0.9% NaCl saline as high as 23 g/g. [0065]

TABLE 4

Extruder

Heater

1 Heater 2 Heater 3 Die Die Pressure Screw Speed Spin Pump

(° F.) (° F.) (° F.) (° F.) (psi) (rpm) Speed (rpm)

321 401 440 443 900 27 20
While the embodiments of the invention described herein are presently preferred, various modifications and improvements can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated by the appended claims, and all changes that fall within the meaning and range of equivalents are intended to be embraced therein. [0066]

Claims

We claim:

1. An absorbent fiber, comprising:

a melt processable, water soluble polymer; and

a cross-linking agent;

wherein the absorbent fiber has an absorbency under zero load of at least about 5 g/g.

2. The absorbent fiber of claim 1, wherein the polymer comprises a polymer selected from polyethylene oxide, polypropylene oxide, hydroxyl propyl cellulose, methyl cellulose, ethyl cellulose, methylethyl cellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid, polyacrylamide, and combinations thereof.

3. The absorbent fiber of claim 1, wherein the polymer comprises a copolymer.

4. The absorbent fiber of claim 3, wherein at least one monomer of the copolymer is sodium acrylate.

5. The absorbent fiber of claim 3, wherein at least one monomer of the copolymer is methyl methacrylate.

6. The absorbent fiber of claim 3, wherein the copolymer includes a ratio on a dry weight basis of a first monomer to a second monomer of from about 30:70 to about 70:30.

7. The absorbent fiber of claim 3, wherein the copolymer comprises sodium acrylate and methyl methacrylate.

8. The absorbent fiber of claim 1, wherein the polymer has a molecular weight in a range of about 10,000 to about 1,000,000.

9. The absorbent fiber of claim 1, wherein the polymer has a molecular weight in a range of about 50,000 to about 1,000,000.

10. The absorbent fiber of claim 1, wherein the polymer has a molecular weight in a range of about 100,000 to about 500,000.

11. The absorbent fiber of claim 1, wherein the cross-linking agent comprises at least two functional groups capable of reacting with functional groups on the polymer.

12. The absorbent fiber of claim 11, wherein the at least two functional groups of the cross-linking agent comprise a functional group selected from carboxylic acid group, epoxy group, hydroxyl group, amino group, aldehyde group, tri-valent metal ions, and tetra-valent metal ions.

13. The absorbent fiber of claim 1, wherein the cross-linking agent comprises a compound selected from diols, polyols, diamines, polyamines, dicarboxylic acids, polycarboxylic acids, dialdehydes, polyaldehydes, butandiol, diethylene triamine, ethylene glycol diglycidyl ether, citric acid, glutaric dialdehyde, and combinations thereof.

14. The superabsorbent fiber of claim 1, wherein a cross-linking reaction is initiated by a treatment selected from heat treatment, microwave radiation, e-beam radiation, UV radiation, steam treatment, and vapor treatment.

15. An absorbent composite, comprising:

a melt processable, water soluble polymer;

a superabsorbent material;

hydrophilic fibers; and

a cross-linking agent;

wherein the absorbent composite has an absorbency under zero load of at least about 10 g/g.

16. The absorbent composite of claim 15, wherein the superabsorbent material comprises superabsorbent particles.

17. The absorbent composite of claim 15, wherein the superabsorbent material comprises superabsorbent fibers.

18. The absorbent composite of claim 15, wherein the hydrophilic fibers comprise fibers selected from wood pulp fluff, cotton, cotton linter, other cellulose fibers, regenerated cellulose fibers, staple fibers, synthetic hydrophilic fibers, and combinations thereof.

19. The absorbent composite of claim 15, wherein the polymer comprises a polymer selected from polyethylene oxide, polypropylene oxide, hydroxyl propyl cellulose, methyl cellulose, ethyl cellulose, methylethyl cellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid, polyacrylamide, and combinations thereof.

20. The absorbent composite of claim 15, wherein the polymer comprises a copolymer.

21. The absorbent composite of claim 20, wherein at least one monomer of the copolymer is sodium acrylate.

22. The absorbent composite of claim 20, wherein at least one monomer of the copolymer is methyl methacrylate.

23. The absorbent composite of claim 20, wherein the copolymer includes a ratio on a dry weight basis of a first monomer to a second monomer of from about 30:70 to about 70:30.

24. The absorbent composite of claim 20, wherein the copolymer comprises sodium acrylate and methyl methacrylate.

25. The absorbent composite of claim 15, wherein the polymer has a molecular weight in a range of about 10,000 to about 1,000,000.

26. The absorbent composite of claim 15, wherein the polymer has a molecular weight in a range of about 50,000 to about 1,000,000.

27. The absorbent composite of claim 15, wherein the polymer has a molecular weight in a range of about 100,000 to about 500,000.

28. The absorbent composite of claim 15, wherein the cross-linking agent comprises at least two functional groups capable of reacting with functional groups on the polymer.

29. The absorbent composite of claim 28, wherein the at least two functional groups of the cross-linking agent comprise a functional group selected from carboxylic acid group, epoxy group, hydroxyl group, amino group, aldehyde group, tri-valent metal ions, and tetra-valent metal ions.

30. The absorbent composite of claim 15, wherein the cross-linking agent comprises a compound selected from diols, polyols, diamines, polyamines, dicarboxylic acids, polycarboxylic acids, dialdehydes, polyaldehydes, butandiol, diethylene triamine, ethylene glycol diglycidayl ether, citric acid, glutaric dialdehyde, and combinations thereof.

31. The absorbent composite of claim 15, wherein a cross-linking reaction is initiated by a treatment selected from heat treatment, microwave, e-beam radiation, UV radiation, steam treatment, and vapor treatment.

32. The absorbent composite of claim 15, wherein the absorbent composite is a superabsorbent material.

33. The absorbent composite of claim 15, wherein the absorbent composite has an absorbency under zero load of at least 15 g/g.

34. The absorbent composite of claim 15, wherein the absorbent composite has an absorbency under zero load of up to about 50 g/g.

35. A method of producing an absorbent fiber, comprising the steps of:

melting a melt processable, water soluble polymer;

extruding the polymer;

spinning the polymer to form fibers;

adding a cross-linking agent to the fibers; and

curing the fibers.

36. The method of claim 35, wherein the polymer comprises a polymer selected from polyethylene oxide, polypropylene oxide, hydroxyl propyl cellulose, methyl cellulose, ethyl cellulose, methyethyl cellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid, polyacrylamide, and combinations thereof.

37. The method of claim 35, wherein the polymer comprises a copolymer.

38. The method claim 37, wherein at least one monomer of the copolymer is sodium acrylate.

39. The method of claim 37, wherein at least one monomer of the copolymer is methyl methacrylate.

40. The method of claim 37, wherein the copolymer includes a ratio on a dry weight basis of a first monomer to a second monomer of from about 30:70 to about 70:30.

41. The method of claim 37, wherein the copolymer comprises sodium polyacrylate and methyl methacrylate.

42. The method of claim 35, wherein the polymer has a molecular weight in a range of about 10,000 to about 1,000,000.

43. The method of claim 35, wherein the polymer has a molecular weight in a range of about 50,000 to about 1,000,000.

44. The method of claim 35, wherein the polymer has a molecular weight in a range of about 100,000 to about 500,000.

45. The method of claim 35, wherein the cross-linking agent comprises at least two functional groups capable of reacting with functional groups on the polymer.

46. The method of claim 45, wherein the at least two functional groups of the cross-linking agent comprise a functional group selected from carboxylic acid group, epoxy group, hydroxyl group, amino group, aldehyde group, tri-valent metal ions, and tetra-valent metal ions.

47. The method of claim 35, wherein the cross-linking agent comprises a compound selected from diols, polyols, diamines, polyamines, dicarboxylic acids, polycarboxylic acids, dialdehydes, polyaldehydes, butandiol, diethylene triamine, ethylene glycol diglycidyl ether, citric acid, glutaric dialdehyde, and combinations thereof.

48. The method of claim 35, wherein a cross-linking reaction is initiated by a treatment selected from heat treatment, microwave, e-beam radiation, UV radiation, steam treatment, and vapor treatment.

49. A diaper comprising the absorbent fiber produced according to the method of claim 35.

50. Training pants comprising the absorbent fiber produced according to the method of claim 35.

51. Swim wear comprising the absorbent fiber produced according to the method of claim 35.

52. An adult incontinence garment comprising the absorbent fiber produced according to the method of claim 35.

53. A feminine hygiene product comprising the absorbent fiber produced according to the method of claim 35.

54. A medical absorbent product comprising the absorbent fiber produced according to the method of claim 35.

55. A method of producing an absorbent composite, comprising the steps of:

melting a melt processable, water soluble polymer;

extruding the polymer;

spinning the polymer to form fibers;

adding hydrophilic fibers to the fibers;

adding a superabsorbent material to the fibers;

adding a cross-linking agent to the fibers; and

curing the fibers.

56. The method of claim 55, wherein the hydrophilic fibers comprise fibers selected from wood pulp fluff, cotton, cotton linter, other cellulose fibers, regenerated cellulose fibers, staple fibers, synthetic hydrophilic fibers, and combinations thereof.

57. The method of claim 55, wherein the polymer comprises a polymer selected from polyethylene oxide, polypropylene oxide, hydroxyl propyl cellulose, methyl cellulose, ethyl cellulose, methylethyl cellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid, polyacrylamide, and combinations thereof.

58. The method of claim 55, wherein the polymer comprises a copolymer.

59. The method claim of 58, wherein at least one monomer of the copolymer is sodium acrylate.

60. The method of claim 58, wherein at least one monomer of the copolymer is methyl methacrylate.

61. The method of claim 58, wherein the copolymer includes a ratio on a dry weight basis of a first monomer to a second monomer of from about 30:70 to about 70:30.

62. The method of claim 58, wherein the copolymer comprises sodium acrylate and methyl methacrylate.

63. The method of claim 55, wherein the polymer has a molecular weight in a range of about 10,000 to about 1,000,000.

64. The method of claim 55, wherein the polymer has a molecular weight in a range of about 50,000 to about 1,000,000.

65. The method of claim 55, wherein the polymer has a molecular weight in a range of about 100,000 to 500,000.

66. The method of claim 55, wherein the cross-linking agent comprises at least two functional groups capable of reacting with functional groups on the polymer.

67. The method of claim 66, wherein the at least two functional groups of the cross-linking agent comprise a functional group selected from carboxylic acid group, epoxy group, hydroxyl group, amino group, aldehyde group, tri-valent metal ions, and tetra-valent metal ions.

68. The method of claim 55, wherein the cross-linking agent comprises a compound selected from diols, polyols, diamines, polyamines, dicarboxylic acids, polycarboxylic acids, dialdehydes, polyaldehydes, butandiol, diethylene triamine, ethylene glycol diglycidyl ether, citric acid, glutaric dialdehyde, and combinations thereof.

69. The method of claim 55, wherein a cross-linking reaction is initiated by a treatment selected from heat treatment, microwave, e-beam radiation, UV radiation, steam treatment, and vapor treatment.

70. A diaper comprising the absorbent composite produced according to the method of claim 55.

71. Training pants comprising the absorbent composite produced according to the method of claim 55.

72. Swim wear comprising the absorbent composite produced according to the method of claim 55.

73. An adult incontinence garment comprising the absorbent composite produced according to the method of claim 55.

74. A feminine hygiene product comprising the absorbent composite produced according to the method of claim 55.

75. A medical absorbent product comprising the absorbent composite produced according to the method of claim 55.