Classifications

• • D—TEXTILES; PAPER
  • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
  • D01F—CHEMICAL FEATURES IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS; APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OF CARBON FILAMENTS
  • D01F8/00—Conjugated, i.e. bi- or multicomponent, artificial filaments or the like; Manufacture thereof
• • D—TEXTILES; PAPER
  • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
  • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
  • D06M13/00—Treating fibres, threads, yarns, fabrics or fibrous goods made from such materials, with non-macromolecular organic compounds; Such treatment combined with mechanical treatment
  • D06M13/10—Treating fibres, threads, yarns, fabrics or fibrous goods made from such materials, with non-macromolecular organic compounds; Such treatment combined with mechanical treatment with compounds containing oxygen
  • D06M13/12—Aldehydes; Ketones

Definitions

• This invention is concerned with individualized, crosslinked cellulosic fibers having high fluid absorption properties, and a process for making such fibers.
• the absorbent cellulosic fibers are useful in the manufacture of absorbent structures.
• individualized, crosslinked fibers refers to cellulosic fibers that have primarily intrafiber chemical crosslink bonds. That is, the crosslink bonds are primarily between cellulose molecules of a single fiber, rather than between cellulose molecules of separate fibers. Individualized, crosslinked fibers are generally regarded as being useful in absorbent product applications. In general, three categories of processes have been reported for making individualized, crosslinked fibers. These processes, described below, are herein referred to as 1) dry crosslinking processes, 2) aqueous solution crosslinking processes, and 3) substantially non-aqueous solution crosslinking processes.
• the fibers themselves and absorbent structures containing individualized, crosslinked fibers generally exhibit an improvement in at least one significant absorbency property relative to conventional, uncrosslinked fibers. Often, this improvement in absorbency is reported in terms of absorbent capacity. Additionally, absorbent structures made from individualized crosslinked fibers generally exhibit increased wet resilience and increased dry resilience relative to absorbent structures made from uncrosslinked fibers.
• the term "resilience” shall hereinafter refer to the ability of pads made from cellulosic fibers to return toward an expanded original state upon release of a compressional force. Dry resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a substantially dry condition.
• wet resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a moistened condition.
• wet resilience shall be observed and reported for an absorbent structure moistened to saturation.
• aqueous solution crosslinked fibers Processes for producing aqueous solution crosslinked fibers are disclosed, for example, in U.S.-A-3,241,553, issued to F. H. Steiger on March 22, 1966. Individualized, crosslinked fibers are produced by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers produced in this manner are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling effect of water on cellulosic fibers, aqueous solution crosslinked fibers are crosslinked while in an uncollapsed, swollen state.
• aqueous solution crosslinked fibers as disclosed in U.S.-A-3,241,553 have greater flexibility and less stiffness, and are characterized by higher fluid retention value (FRV).
• FRV fluid retention value
• Absorbent structures made from aqueous solution crosslinked fibers exhibit lower wet and dry resilience than pads made from dry crosslinked fibers.
• nonaqueous solution crosslinked fibers disclosed in U.S.-A-4,035,147 do not swell even upon extended contact with solutions known to those skilled in the art as swelling reagents. Like dry crosslinked fibers, they are highly stiffened by crosslink bonds, and absorbent structures made therefrom exhibit relatively high wet and dry resilience.
• Crosslinked fibers as described above are believed to be useful for lower density absorbent product applications such as diapers and also higher density absorbent product applications such as catamenials.
• such fibers have not provided sufficient absorbency benefits, in view of their detriments and costs, over conventional fibers to result in significant commercial success.
• Commercial appeal of crosslinked fibers has also suffered due to safety concerns.
• formaldehyde unfortunately causes irritation to human skin and has been associated with other human safety concerns. Removal of free formaldehyde to sufficiently low levels in the crosslinked product such that irritation to skin and other human safety concerns are avoided has been hindered by both technical and economic barriers.
• dry crosslinked fibers and nonaqueous solution crosslinked fibers have generally resulted in fibers of excessive stiffness and dry resiliency, thereby making them difficult to form into densified sheets for transport and subsequently refluff without fiber damage.
• pads made from these fibers have exhibited a low responsiveness to wetting. That is, once compressed in a dry state, they have not shown the ability to regain substantial amounts of their prior absorbent capacity upon wetting.
• Aqueous solution crosslinked fibers while useful for certain higher density absorbent pad applications such as surgical dressings, tampons and sanitary napkins wherein densities ordinarily are about 0.40 g/cc, are excessively flexible when in a wet state and therefore result in absorbent structures which have low wet resilience. Furthermore, upon wetting, aqueous solution crosslinked fibers become too flexible to structurally support the pad at lower fiber densities. The wetted pad therefore collapses and absorbent capacity is reduced.
• the present invention provides individualised twisted and curled crosslinked cellulosic fibers, said fibers comprising cellulosic fibers in substantially individual form, said fibers having been maintained in substantially individual form during crosslinking and drying, said crosslinking agent being selected from C2-C8 dialdehydes, acid analogues of said dialdehydes derived by having one aldehyde group of each of said dialdehydes replaced by a carboxyl group, and oligomers of said dialdehydes and said dialdehyde acid analogues; wherein said fibers have between 0.5 mole % and 3.5 mole % of crosslinking agent, calculated on a cellulose anhydroglucose basis, reacted therewith, said fibers reacted with said crosslinking agent have a moisture content of up to 30% by weight of the dry fibers and said crosslinking agent is sufficiently reacted with said fibers in intra fiber crosslink bond form to cause said fibers to have a water retention value of from 28
• the present invention provides a process for making individualised, twisted crosslinked cellulosic fibers, said process comprising the steps of:
• the fibers are crosslinked while in a highly twisted condition.
• the fibers are contacted with crosslinking agent and a catalyst in an aqueous solution, dewatered, mechanically separated into substantially individual form, and then dried and caused to crosslink under substantially unrestrained conditions.
• the dewatering, mechanical separation and drying stages allow the fibers to become highly twisted prior to crosslinking.
• the twisted condition is then at least partially but less than completely set as a result of crosslinking.
• cellulosic fibers in individualised form may be crosslinked in a substantially nonaqueous crosslinking solution containing a water-miscible polar diluent, such as acetic acid, and insufficient amount of water to cause the fibers to swell to a level greater than that corresponding to a 30% aqueous moisture content for equivalent fibers.
• a water-miscible polar diluent such as acetic acid
• Cellulosic fibers of diverse natural origin are applicable to the invention.
• Digested fibers from softwood, hardwood or cotton linters are preferably utilized. Fibers from Esparto grass, bagasse, kemp, flax, and other lignaceous and cellulosic fiber sources may also be utilized as raw material in the invention.
• the fibers may be supplied in slurry, unsheeted or sheeted form. Fibers supplied as wet lap, dry lap or other sheeted form are preferably rendered into unsheeted form by mechanically disintegrating the sheet, preferably prior to contacting the fibers with the crosslinking agent. Also, preferably the fibers are provided in a wet or moistened condition. Most preferably, the fibers are never-dried fibers. In the case of dry lap, it is advantageous to moisten the fibers prior to mechanical disintegration in order to minimize damage to the fibers.
• the optimum fiber source utilized in conjunction with this invention will depend upon the particular end use contemplated. Generally, pulp fibers made by chemical pulping processes are preferred. Completely bleached, partially bleached and unbleached fibers are applicable. It may frequently be desired to utilize bleached pulp for its superior brightness and consumer appeal. In one novel embodiment of the invention, hereinafter more fully described, the fibers are partially bleached, crosslinked, and then bleached to completion. For products such as paper towels and absorbent pads for diapers, sanitary napkins, catamenials, and other similar absorbent paper products, it is especially preferred to utilize fibers from southern softwood pulp due to their premium absorbency characteristics.
• Crosslinking agents applicable to the present development are C2 - C8 dialdehydes, as well as acid analogues of such dialdehydes wherein the acid analogue has one aldehyde group, and oligomers of such dialdehydes and acid analogues. These compounds are capable of reacting with at least two hydroxyl groups in a single cellulose chain or on proximately located cellulose chains in a single fiber.
• the dialdehyde crosslinking agents described above will be present, or may react in a variety of forms, including the acid analogue and oligomer forms identified above. All such forms are meant to be included within the scope of the invention.
• crosslinking agent refers to that particular crosslinking agent as well as other forms as may be present in an aqueous solution.
• Particular crosslinking agents contemplated for use with the invention are glutaraldehyde, glyoxal, and glyoxylic acid.
• Glutaraldehyde is especially preferred, since it has provided fibers with the highest levels of absorbency and resiliency, is believed to be safe and non-irritating to human skin when in a reacted, crosslinked condition, and has provided the most stable, crosslink bonds.
• Monoaldehydic compounds not having an additional carboxylic group, such as acetaldehyde and furfural have not been found to provide absorbent structures with the desired levels of absorbent capacity, resilience, and responsiveness to wetting.
• the crosslinking agent is contacted with the fibers in a liquid medium, under such conditions that the crosslinking agent penetrates into the interior of the individual fiber structures.
• other methods of crosslinking agent treatment including spraying of the fibers while in individualized, fluffed form, are also within the scope of the invention.
• the fibers will also be contacted with an appropriate catalyst prior to crosslinking.
• the type, amount, and method of contact of catalyst to the fibers will be dependent upon the particular crosslinking process practiced. These variables will be discussed in more detail below.
• the crosslinking agent is caused to react with the fibers in the substantial absence of interfiber bonds, i.e., while interfiber contact is maintained at a low degree of occurrence relative to unfluffed pulp fibers, or the fibers are submerged in a solution that does not facilitate the formation of interfiber bonding, especially hydrogen bonding. This results in the formation of crosslink bonds which are intrafiber in nature. Under these conditions, the crosslinking agent reacts to form crosslink bonds between hydroxyl groups of a single cellulose chain or between hydroxyl groups of proximately located cellulose chains of a single cellulosic fiber.
• crosslinking agent reacts with the hydroxyl groups of the cellulose to form hemiacetal and acetal bonds.
• the formation of acetal bonds believed to be the desirable bond types providing stable crosslink bonds, is favored under acidic reaction conditions. Therefore, acid catalyzed crosslinking conditions are highly preferred for the purposes of this invention.
• the fibers are mechanically defibrated into a low density, individualized, fibrous form known as "fluff" prior to reaction of the crosslinking agent with the fibers.
• Mechanical defibration may be performed by a variety of methods which are presently known in the art or which may hereinafter become known. Mechanical defibration is preferably performed by a method wherein knot formation and fiber damage are minimized.
• One type of device which has been found to be particularly useful for defibrating the cellulosic fibers is the three stage fluffing device described in US-A-3,987,968, issued to D. R. Moore and O. A. Shields on October 26, 1976.
• the fluffing device described in US-A-3,987,968 subjects moist cellulosic pulp fibers to a combination of mechanical impact, mechanical agitation, air agitation and a limited amount of air drying to create a substantially knot-free fluff.
• the individualized fibers have imparted thereto an enhanced degree of curl and twist relative to the amount of curl and twist naturally present in such fibers. It is believed that this additional curl and twist enhances the resilient character of absorbent structures made from the finished, crosslinked fibers.
• defibrating the cellulosic fibers include, but are not limited to, treatment with a Waring blender and tangentially contacting the fibers with a rotating disk refiner or wire brush.
• a Waring blender tangentially contacting the fibers with a rotating disk refiner or wire brush.
• an air stream is directed toward the fibers during such defibration to aid in separating the fibers into substantially individual form.
• the fibers are preferably mechanically treated while initially containing at least 20% moisture, and preferably containing between 40% and 60% moisture.
• Mechanical refining of fibers at high consistency or of partially dried fibers may also be utilized to provide curl or twist to the fibers in addition to curl or twist imparted as a result of mechanical defibration.
• the fibers made according to the present invention have unique combinations of stiffness and resiliency, which allow absorbent structures made from the fibers to maintain high levels of absorptivity, and exhibit high levels of resiliency and an expansionary responsiveness to wetting of a dry, compressed absorbent structure.
• the crosslinked fibers are characterized by having water retention values (WRV's) lying between 28 and 45.
• the WRV of a particular fiber is indicative of the level of crosslinking and the degree of swelling of the fiber at the time of crosslinking. Those skilled in the art will recognize that the more swollen a fiber is at the time of crosslinking, the higher the WRV will be for a given level of crosslinking.
• Very highly crosslinked fibers such as those produced by the prior known dry crosslinking processes previously discussed, have been found to have WRV's of less than 25, and generally less than 20.
• the particular crosslinking process utilized will, of course, affect the WRV of the crosslinked fiber.
• Applicable methods of crosslinking include dry crosslinking processes and nonaqueous solution crosslinking processes as generally discussed in the Background Of The Invention. Certain preferred dry crosslinking and nonaqueous solution crosslinking processes, within the scope of the present invention, will be discussed in more detail below.
• Aqueous solution crosslinking processes wherein the solution causes the fibers to become highly swollen will result in fibers having WRV's which are in excess of 45.
• individualized, crosslinked fibers may be produced from such a process by providing a quantity of cellulosic fibers, contacting a slurry of the fibers with a type and amount of crosslinking agent as described above, mechanically separating, e.g., defibrating, the fibers into substantially individual form, and drying the fibers and causing the crosslinking agent to react with the fibers in the presence of a catalyst to form crosslink bonds while the fibers are maintained in substantially individual form.
• the defibration step apart from the drying step, is believed to impart additional curl.
• Subsequent drying is accompanied by twisting of the fibers, with the degree of twist being enhanced by the curled geometry of the fiber.
• fiber “curl” refers to a geometric curvature of the fiber about the longitudinal axis of the fiber.
• “Twist” refers to a rotation of the fiber about the perpendicular cross-section of the longitudinal axis of the fiber.
• individualized, crosslinked fibers within the scope of the invention having an average of 6 (six) twists per millimeter of fiber have been observed.
• Maintaining the fibers in substantially individual form during drying and crosslinking allows the fibers to twist during drying and thereby be crosslinked in such twisted, curled state. Drying fibers under such conditions that the fibers may twist and curl is referred to as drying the fibers under substantially unrestrained conditions. On the other hand, drying fibers in sheeted form results in dried fibers which are not twisted and curled as fibers dried in substantially individualized form. It is believed that interfiber hydrogen bonding "restrains" the relative occurrence of twisting and curling of the fiber.
• the fibers may be contacted with the crosslinking agent and catalyst.
• the fibers are contacted with a solution which initially contains both the crosslinking agent and the catalyst.
• the fibers are contacted with an aqueous solution of crosslinking agent and allowed to soak prior to addition of the catalyst.
• the catalyst is subsequently added.
• the crosslinking agent and catalyst are added to an aqueous slurry of the cellulosic fibers.
• the cellulosic fibers, crosslinking agent and catalyst are preferably mixed and/or allowed to soak sufficiently with the fibers to assure thorough contact with and impregnation of the individual fibers.
• any substance which catalyzes the crosslinking mechanism may be utilized.
• Applicable catalysts include organic acids and acid salts. Especially preferred catalysts are salts such as aluminum, magnesium, zinc and calcium salts of chlorides, nitrates or sulfates. One specific example of a preferred salt is zinc nitrate hexahydrate.
• Other catalysts include acids such as sulfuric acid, hydrochloric acid and other mineral and organic acids. The selected catalyst may be utilized as the sole catalyzing agent, or in combination with one or more other catalysts. It is believed that combinations of acid salts and organic acids as catalyzing agents provide superior crosslinking reaction efficiency.
• crosslinking agent and catalyst utilized will depend upon the particular crosslinking agent utilized, the reaction conditions and the particular product application contemplated.
• the amount of catalyst preferably utilized is, of course, dependent upon the particular type and amount of crosslinking agent and the reaction conditions, especially temperature and pH. In general, based upon technical and economic considerations, catalyst levels of between 10 wt. % and 60 wt. %, based on the weight of crosslinking agent added to the cellulosic fibers, are preferred. For exemplary purposes, in the case wherein the catalyst utilized is zinc nitrate hexahydrate and the crosslinking agent is glutaraldehyde, a catalyst level of 30 wt. %, based upon the amount of glutaraldehyde added, is preferred.
• aqueous portion of the cellulosic fiber slurry or crosslinking agent solution is also added as a catalyst. It is additionally desirable to adjust the aqueous portion of the cellulosic fiber slurry or crosslinking agent solution to a target pH of between pH 2 and pH 5, more preferably between pH 2.5 and pH 3.5, during the period of contact between the crosslinking agent and the fibers.
• the cellulosic fibers should generally be dewatered and optionally dried.
• the workable and optimal consistencies will vary depending upon the type of fluffing equipment utilized.
• the cellulosic fibers are dewatered and optimally dried to a consistency of between 30% and 80%. More preferably, the fibers are dewatered and dried to a consistency level of between 40% and 60%. Drying the fibers to within these preferred ranges generally will facilitate defibration of the fibers into individualized form without excessive formation of knots associated with higher moisture levels and without high levels of fiber damage associated with lower moisture levels.
• dewatering may be accomplished by such methods as mechanically pressing, centrifuging, or air drying the pulp. Additional drying is preferably performed by such methods, known in the art as air drying or flash drying, under conditions such that the utilization of high temperature for an extended period of time is not required. Excessively high temperature at this stage of the process may result in the premature initiation of crosslinking. Preferably, temperatures in excess of 160°C are not maintained for periods of time in excess of 2 to 3 seconds. Mechanical defibration is performed as previously described.
• the defibrated fibers are then heated to a suitable temperature for an effective period of time to cause the crosslinking agent to cure, i.e., to react with the cellulosic fibers.
• the rate and degree of crosslinking depends upon dryness of the fibers, temperature, amount and type of catalyst and crosslinking agent and the method utilized for heating and/or drying the fibers while crosslinking is performed.
• Crosslinking at a particular temperature will occur at a higher rate for fibers of a certain initial moisture content when accompanied by a continuous air through drying than when subjected to drying/heating in a static oven.
• Those skilled in the art will recognize that a number of temperature-time relationships exist for the curing of the crosslinking agent.
• the FRV refers to the amount of fluid calculated on a dry fiber basis, that remains absorbed by a sample of fibers that have been soaked and then centrifuged to remove interfiber fluid. (The FRV is further defined and the Procedure For Determining FRV, is described below.)
• the amount of fluid that the crosslinked fibers can absorb is dependent upon their ability to swell upon saturation or, in other words, upon their interior diameter or volume upon swelling to a maximum level. This, in turn, is dependent upon the level of crosslinking. As the level of intrafiber crosslinking increases for a given fiber and process, the FRV of the fiber will decrease until the fiber does not swell at all upon wetting.
• the FRV value of a fiber is structurally descriptive of the physical condition of the fiber at saturation. Unless otherwise expressly indicated, FRV data described herein shall be reported in terms of the water retention value (WRV) of the fibers. Other fluids, such as salt water and synthetic urine, may also be advantageously utilized as a fluid medium for analysis.
• WRV water retention value
• Other fluids such as salt water and synthetic urine, may also be advantageously utilized as a fluid medium for analysis.
• the FRV of a particular fiber crosslinked by procedures wherein curing is largely dependent upon drying, such as the present process will be primarily dependent upon the crosslinking agent and the level of crosslinking.
• the WRV's of fibers crosslinked by this dry crosslinking process at crosslinking agent levels applicable to this invention are generally between 28 and 45.
• SSK fibers having between 0-5 mole % and 2.5 mole % glutaraldehyde reacted thereon, calculated on a cellulose anhydroglucose molar basis, have been observed to have WRV's respectively ranging from 40 to 28.
• the degree of bleaching and the practice of post-crosslinking bleaching steps have been found to affect WRV. This effect will be explored in more detail below.
• Southern softwood Kraft (SSK) fibers prepared by dry crosslinking processes known prior to the present invention have levels of crosslinking higher than described herein, and have WRV's less than 25. Such fibers, as previously discussed, have been observed to be exceedingly stiff and to exhibit lower absorbent capabilities than the fibers of the present invention.
• crosslinking processes applicable to the present invention is nonaqueous solution cure crosslinking processes.
• the same types of fibers applicable to dry crosslinking processes may be used in the production of nonaqueous solution crosslinked fibers.
• the fibers are treated with a sufficient amount of crosslinking agent such that between 0.5 mole % and 3.5 mole % crosslinking agent subsequently react with the fibers, wherein the level of crosslinking agent reacted is calculated subsequent to said crosslinking reaction, and with an appropriate catalyst.
• the crosslinking agent is caused to react while the fibers are submerged in a solution which does not induce any substantial levels of swelling of the fibers.
• the fibers may contain up to 30% water, or be otherwise swollen in the crosslinking solution to a degree equivalent to fibers having a 30% moisture content.
• the crosslinking solution contains a nonaqueous, water-miscible, polar diluent such as, but not limited to, acetic acid, propanoic acid, or acetone.
• Preferred catalysts include mineral acids, such as sulfuric acid, and halogen acids, such as hydrochloric acid. Other applicable catalysts include salts of mineral acids and halogen acids, organic acids and salts thereof.
• Crosslinking solution systems applicable for use as a crosslinking medium also include those disclosed in US-A-4,035,147, issued to S. Sangenis, G. Guiroy, and J. Quere, on July 12, 1977.
• the crosslinking solution may include some water or other fiber swelling liquid, however, the amount of water is insufficient to cause a level of swelling corresponding to that incurred by 70% consistency pulp fibers (30% aqueous moisture content). Additionally, crosslinking solution water contents less than 10% of the total volume of the solution, exclusive of the fibers are preferred. Levels of water in the crosslinking solution in excess of this amount decrease the efficiency and rate of crosslinking.
• Absorption of crosslinking agent by the fibers may be accomplished in the crosslinking solution itself or in a prior treatment stage including, but not limited to, saturation of the fibers with either an aqueous or nonaqueous solution containing the crosslinking agent.
• the fibers are mechanically defibrated into individual form. This mechanical treatment may be performed by methods previously described for fluffing fibers in connection with the previously described dry crosslinking process.
• a mechanical treatment which causes the moist cellulosic fibers to assume a curled or twisted condition to a degree in excess of the amount of curl or twist, if any, of the natural state of the fibers. This can be accomplished by initially providing fibers for fluffing which are in a moist state, subjecting the fibers to a mechanical treatment such as those previously described methods for defibrating the fibers into substantially individual form, and at least partially drying the fibers.
• the relative amounts of curl and twist imparted to the fibers is in part dependent upon the moisture content of the fibers. Without limiting the scope of the invention, it is believed that the fibers naturally twist upon drying under conditions wherein fiber to fiber contact is low, i.e., when the fibers are in an individualized form. Also, mechanical treatment of moist fibers initially causes the fibers to become curled. When the fibers are then dried or partially dried under substantially unrestrained conditions, they become twisted with the degree of twist being enhanced by the additional amount of curl mechanically imparted.
• the defibration fluffing steps are preferably practiced on high consistency moist pulp or pulp which has been dewatered to fiber consistency of 45% to 55% (determined prior to initialization of defibration).
• the fibers should be dried so as to have up to 30% moisture content prior to being contacted with the crosslinking solution, if the defibration step has not already provided fibers having moisture contents within that range.
• the drying step should be performed while the fibers are under substantially unrestrained conditions. That is, fiber to fiber contact should be minimized so that the twisting of the fibers inherent during drying is not inhibited. Both air drying and flash drying methods are suitable for this purpose.
• the individualized fibers are next contacted with a crosslinking solution which contains a water-miscible, nonaqueous diluent, a crosslinking agent and a catalyst.
• the crosslinking solution may contain a limited amount of water.
• the water content of the crosslinking solution should be less than 18% and is preferably less than 9%.
• crosslinking agent and acid catalyst utilized will depend upon such reaction conditions as consistency, temperature, water content In the crosslinking solution and fibers, type of crosslinking agent and diluent in the crosslinking solution, and the amount of crosslinking desired.
• the amount of crosslinking agent utilized ranges from .2 wt % to 10 wt % (based upon the total, fiber-free weight of the crosslinking solution).
• Preferred acid catalyst content is additionally dependent upon the acidity of the catalyst in the crosslinking solution.
• catalyst content including hydrochloric acid, between .3 wt % and 5 wt % (fiber-free crosslinking solution weight basis) in crosslinking solutions containing an acetic acid diluent, preferred levels of glutaraldehyde, and a limited amount of water. Slurries of fibers and crosslinking solution having fiber consistencies of less than 10 wt % are preferred for crosslinking in conjunction with the crosslinking solutions described above.
• the crosslinking reaction may be carried out at ambient temperatures or, for accelerated reaction rates, at elevated temperatures preferably less than 40°C.
• the fibers may be contacted with, and crosslinked in, the crosslinking solution.
• the fibers are contacted with the solution which initially contains both the crosslinking agent and the acid catalyst.
• the fibers are allowed to soak in the crosslinking solution, during which time crosslinking occurs.
• the fibers are contacted with the diluent and allowed to soak prior to addition of the acid catalyst.
• the acid catalyst subsequently is added, at which time crosslinking begins.
• Other methods in addition to those described will be apparent to those skilled in the art, and are intended to be within the scope of this invention.
• the crosslinking agent and the conditions at which crosslinking is performed are chosen to facilitate intrafiber crosslinking.
• the crosslinking reaction it is advantageous for the crosslinking reaction to occur in substantial part after the crosslinking agent has had sufficient time to penetrate into the fibers.
• Reaction conditions are preferably chosen so as to avoid instantaneous crosslinking unless the crosslinking agent has already penetrated into the fibers.
• Periods of reaction during which time crosslinking is substantially completed over a period of about 30 minutes are preferred. Longer reaction periods are believed to provide minimal marginal benefit in fiber performance. However, both shorter periods, including substantially instantaneous crosslinking, and longer periods are meant to be within the scope of this invention.
• the fibers are drained and washed.
• a sufficient amount of a basic substance such as caustic is added in the washing step to neutralize any acid remaining in the pulp.
• the fibers are defluidized and dried to completion.
• the fibers are subjected to a second mechanical defibration step which causes the crosslinked fibers to curl, e.g, fluffing by defibration, between the defluidizing and drying steps.
• the curled condition of the fibers imparts additional twist as previously described in connection with the curling treatment prior to contact with the crosslinking solution.
• the term “defibration” shall refer to any of the procedures which may be used to mechanically separate the fibers into substantially individual form, even though the fibers may already be provided in such form. "Defibration” therefore refers to the step of mechanically treating the fibers, in either individual form or in a more compacted form, to a mechanical treatment step which a) would separate the fibers into substantially individual form if they were not already in such form, and b) imparts curl and twist to the fibers upon drying.
• This second defibration treatment after the fibers have been crosslinked, has been found to increase the twisted, curled character of the pulp. This increase in the twisted, curled configuration of the fibers leads to enhanced absorbent structure resiliency and responsiveness to wetting.
• a second defibration treatment may be practiced upon any of the crosslinked fibers described herein which are in a moist condition.
• a second defibration step is possible without necessitating an additional drying step. This is due to the fact that the solution in which the fibers are crosslinked keep the fibers flexible subsequent to crosslinking even though not causing the fibers to assume an undesirable, highly swollen state.
• Fibers having higher FRV's correspond to fibers which have been crosslinked while in a more swollen state relative to fibers crosslinked while in a less swollen state, all other factors being equal.
• FRV fluid retention value
• Fibers having higher FRV's correspond to fibers which have been crosslinked while in a more swollen state relative to fibers crosslinked while in a less swollen state, all other factors being equal.
• partially swollen, crosslinked fibers with increased FRV's have greater wet resilience and responsiveness to wetting than fibers which have been crosslinked while in an unswollen state. Fibers having this increase in wet resilience and responsiveness to wetting are more readily able to expand or untwist when wetted in an attempt to return to their natural state.
• Numerical FRV data described herein in connection with partially swollen crosslinked fibers shall be water retention values (WRV). As the WRV increases beyond approximately 60, the stiffness of the fibers is believed to become insufficient to provide the wet resilience and responsiveness to wetting desired to support a saturated absorbent structure.
• the fibers are first soaked in an aqueous or other fiber swelling solution, defluidized, dried to a desired level and subsequently submersed in a water-miscible crosslinking solution containing a catalyst and crosslinking agent as previously described.
• the fibers are mechanically defibrated into fluff form subsequent to defluidization and prior to additional drying, in order to obtain the benefits of enhanced twist and curl as previously described.
• Mechanical defibration practiced subsequent to contacting the fibers with the crosslinking agent is less desirable, since such defibration would volatilize the crosslinking agent thus, possibly leading to atmospheric contamination by, or high air treatment investments due to, the crosslinking agent.
• the fibers are defibrated and then presoaked in a high concentration solution of crosslinking agent and a fiber-swelling diluent, preferably water.
• the crosslinking agent concentration is sufficiently high to inhibit water-induced swelling of fibers.
• aqueous solutions of the crosslinking agents of this invention preferably, glutaraldehyde, have been found to be useful solutions for presoaking the fibers.
• the presoaked fibers are defluidized and submerged in a crosslinking solution containing a water-miscible, polar diluent, a catalyst, and a limited amount of water, and then crosslinked as previously described.
• the crosslinked fibers may be defluidized and subjected to a second mechanical defibration step prior to further processing into a sheet or absorbent structure.
• Presoaking the fibers with crosslinking agent in an aqueous solution prior to causing the crosslinking agent to react provides unexpectedly high absorbency properties for absorbent pads made from the crosslinked fibers, even relative to pads made from crosslinked fibers of the prior described nonaqueous solution cure processes wherein the fibers were not presoaked with a solution containing crosslinking agent.
• the crosslinked fibers formed as a result of the preceding dry crosslinking and nonaqueous solution crosslinking processes are the product of the present invention.
• the crosslinked fibers of the present invention may be utilized directly in the manufacture of air laid absorbent cores. Additionally, due to their stiffened and resilient character, the crosslinked fibers may be wet laid into an uncompacted, low density sheet which, when subsequently dried, is directly useful without further mechanical processing as an absorbent core.
• the crosslinked fibers may also be wet laid as compacted pulp sheets for sale or transport to distant locations.
• the individualized, crosslinked fibers may be dry laid and directly formed into absorbent structures, or wet laid and formed into absorbent structures or densified pulp sheets.
• the fibers of the present invention provide a variety of substantial performance advantages. However, it is difficult to form such fibers into a smooth, wet laid sheet by conventional wet sheet formation practices. This is because individualized, crosslinked fibers rapidly flocculate when in solution. Such flocculation may occur both in the headbox and upon deposition into the foraminous forming wire. Attempts to sheet individualized, crosslinked fibers by conventional pulp sheeting methods have been found to result in the formation of a plurality of clumps of flocced fibers.
• individualized, crosslinked fibers according to the invention which tend to flocculate in solution, can be sheeted by a process wherein individualized, crosslinked fibers are initially deposited from a slurry on a foraminous forming wire, such as a Fourdrinier wire, in a manner similar to conventional pulp sheeting processes.
• a foraminous forming wire such as a Fourdrinier wire
• these fibers are deposited on the forming wire in a plurality of clumps of fibers.
• At least one stream of fluid, preferably water is directed at the deposited, clumped fibers.
• a series of showers are directed at the fibers deposited on the forming wire, wherein successive showers have decreasing volumetric flow rates.
• the showers should be of sufficient velocity such that the impact of the fluid against the fibers acts to inhibit the formation of flocculations of the fibers and to disperse flocculations of fibers which have already formed.
• the fiber setting step is preferably performed with a cylindrical screen, such as a dandy roll, or with another apparatus analogous in function which is or may become known in the art. Once set, the fibrous sheet may then be dried and optionally compacted as desired.
• the spacing of the showers will vary depending upon the particular rate of fiber floccing, line speed of the forming wire, drainage through the forming wire, number of showers, and velocity and flow rate through the showers. Preferably, the showers are close enough together so that substantial levels of floccing are not incurred.
• the fluid showered onto the fibers also compensates for the extremely fast drainage of individualized, crosslinked fibers, by providing additional liquid medium in which the fibers may be dispersed for subsequent sheet formation.
• the plurality of showers of decreasing volumetric flow rates facilitates a systematic net increase in slurry consistency while providing a repetitive dispersive and inhibiting effect upon flocculations of the fibers. This results in the formation of a relatively smooth and even deposition of fibers which are then promptly, i.e., before reflocculation, set into sheeted form by allowing the fluid to drain and pressing the fibers against the foraminous wire.
• Pulp sheets containing stiffened, crosslinked fibers preferably contain between 5% and 90% uncrosslinked, cellulosic fibers, based upon the total dry weight lof the sheet, mixed with the individualized, crosslinked fibers. It is especially preferred to include between 5% and 30% of highly refined, uncrosslinked cellulosic fibers, based upon the total dry weight of the sheet.
• Such highly refined fibers are refined or beaten to a freeness level less than about 300 ml CSF, and preferably less than 100 ml CSF.
• the uncrosslinked fibers are preferably mixed with an aqueous slurry of the individualized, crosslinked fibers. This mixture may then be formed into a densified pulp sheet for subsequent defibration and formation into absorbent pads.
• the incorporation of the uncrosslinked fibers eases compression of the pulp sheet into a densified form, while imparting a surprisingly small loss in absorbency to the subsequently formed absorbent pads.
• the uncrosslinked fibers additionally increase the tensile strength of the pulp sheet and to absorbent pads made either from the pulp sheet or directly from the mixture of crosslinked and uncrosslinked fibers. Regardless of whether the blend of crosslinked and uncrosslinked fibers are first made into a pulp sheet and then formed into an absorbent pad or formed directly into an absorbent pad, the absorbent pad may be air-laid or wet-laid as previously described.
• Sheets or webs made from the individualized, crosslinked fibers, or from mixtures also containing uncrosslinked fibers will preferably have basis weights of less than 800 g/m2 and densities of less than 0.60 g/cm3.
• wet-laid sheets having basis weights between 300 g/m2 and 600 g/m2 and densities between 0.15 g/cm3 and 0.30 g/cm3 are especially contemplated for direct application as absorbent cores in disposable articles such as diapers, tampons, and other catamenial products.
• the crosslinked fibers are disposed next to or in the vicinity of a person's skin, it is desirable to further process the fibers to remove excess, unreacted crosslinking agent.
• the level of unreacted crosslinking agent is reduced to at least below 0.03%, based on the dry weight of the cellulosic fibers.
• One series of treatments found to successfully remove excess crosslinking agent comprise, in sequence, washing the crosslinked fibers, allowing the fibers to soak in an aqueous solution for an appreciable time, screening the fibers, dewatering the fibers, e.g., by centrifuging, to a consistency of between 40% and about 80%, mechanically defibrating the dewatered fibers as previously described and air drying the fibers. This process has been found to reduce residual free crosslinking agent content to between 0.01% and 0.15%.
• alkaline washes In another method for reducing residual crosslinking agent, readily extractable crosslinking agent is removed by alkaline washes.
• Alkalinity may be introduced by basic compounds such as sodium hydroxide, or alternatively in the form of oxidizing agents such as those chemicals commonly utilized as bleaching agents, e.g., sodium hypochlorite, and amino-containing compounds, e.g. ammonium hydroxide, which hydrolyze hemiacetal bonds to form Schiff bases.
• the pH is preferably maintained at a level of at least pH 7, and more preferably at least pH 9, to inhibit reversion of the acetal crosslink bond. It is preferred to induce decomposition of hemiacetal bonds, while being neutral towards acetal bonds.
• the source fibers are subjected to a conventional multistage bleaching sequence, but at a midpoint during the sequence the bleaching process is interrupted and, the fibers are crosslinked in accordance with the present invention. Subsequent to curing, the remainder of the bleaching sequence is completed. It has been found that acceptably low crosslinking agent residual levels of less than 0.006% can be obtained in this manner.
• This method is believed to embody the preferred manner of producing crosslinked fibers, since the capital expense and processing inconvenience of additional washing and extraction equipment and additional process steps are avoided due to merger of the bleaching and residual reduction steps.
• the bleaching sequences practiced and the point of interruption in the sequences for crosslinking may vary widely, as will be evident to one of ordinary skill in the art, However, multi-stage bleaching sequences, wherein DEP* or DEH* stages follow crosslinking, have been found to provide desirable results.
• the post-crosslinking bleaching sequence stages are preferably alkaline treatements performed at pH greater than pH 7, and more preferably greater than pH 9.
• FRV fluid retention value
• crosslinked fibers herein described are useful for a variety of absorbent articles including, but not limited to, tissue sheets, disposable diapers, catamenials, sanitary napkins, tampons, and bandages wherein each of said articles has an absorbent structure containing the individualized, crosslinked fibers described herein.
• tissue sheets disposable diapers, catamenials, sanitary napkins, tampons, and bandages
• each of said articles has an absorbent structure containing the individualized, crosslinked fibers described herein.
• a disposable diaper or similar article having a liquid permeable topsheet, a liquid impermeable backsheet connected to the topsheet, and an absorbent structure containing individualized, crosslinked fibers lis particularly contemplated.
• Such articles are described generally in US-A-3,860,003, issued to Kenneth B. Buell on January 14, 1975.
• absorbent cores for diapers and catamenials are made from unstiffened, uncrosslinked cellulosic fibers, wherein the absorbent cores have dry densities of 0.06 g/cc and 0.12 g/cc. Upon wetting, the absorbent core normally displays a reduction in volume.
• crosslinked fibers of the present invention can be used to make absorbent cores having substantially higher fluid absorbing properties including, but not limited to, absorbent capacity and wicking rate relative to equivalent density absorbent cores made from conventional, uncrosslinked fibers or prior known crosslinked fibers. Furthermore, these improved absorbency results may be obtained in conjunction with increased levels of wet resiliency. For absorbent cores having densities of between 0.06 g/cc and 0.15 g/cc which maintain substantially constant volume upon wetting, it is especially preferred to utilize crosslinked fibers having crosslinking levels of between 2.0 mole % and 2.5 mole % crosslinking agent, based upon a dry cellulose anhydroglucose molar basis.
• Absorbent cores made from such fibers have a desirable combination of structural integrity, i.e., resistance to compression, and wet resilience.
• wet resilience in the present context, refers to the ability of a moistened pad to spring back towards its original shape and volume upon exposure to and release from compressional forces.
• the absorbent cores made from the fibers of the present invention will regain a substantially higher proportion of their original volumes upon release of wet compressional forces.
• the individualized, crosslinked fibers are formed into either an air laid or wet laid (and subsequently dried) absorbent core which is compressed to a dry density less than the equilibrium wet density of the pad.
• the equilibrium wet density is the density of the pad, calculated on a dry fiber basis when the pad is fully saturated with fluid.
• Pads made from the fibers of the present invention have equilibrium wet densities which are substantially lower than pads made from conventional fluffed fibers.
• the fibers of the present invention can be compressed to a density higher than the equilibrium wet density, to form a thin pad which, upon wetting, will expand, thereby increasing absorbent capacity, to a degree significantly greater than obtained for uncrosslinked fibers.
• Such fibers are formed into absorbent cores having dry densities greater than their equilibrium wet densities.
• the absorbent cores are compressed to densities of between 0.12 g/cc and 0.60 g/cc, wherein the corresponding equilibrium wet density is less than the density of the dry compressed pad.
• the absorbent cores are compressed to a density of between 0.12 g/cc and 0.40 g/cc, wherein the corresponding equilibrium wet densities are between 0.08 g/cc and 0.12 g/cc, and are less than the densities of the dry, compressed cores.
• the former fibers are less stiff, thereby making them more suitable for compression to the higher density range.
• the former fibers also have higher responsiveness to wetting in that upon wetting they spring open at a faster rate and to a greater degree than do fibers having crosslinking levels within the 2.0 mole % to 2.5 mole % range, have higher wet resiliency, and retain almost as much absorbent capacity. It should be recognized, however, that absorbent structures within the higher density range can be made from crosslinked fibers within the higher crosslinking level range, as can lower density absorbent structures be made from crosslinked fibers having lower levels of crosslinking. Improved performance relative to prior known individualized, crosslinked fibers is obtained for all such structures.
• a sample of 0.3 g to 0.4 g of fibers is soaked in a covered container with 100 ml distilled or deionized water at room temperature for between 15 and 20 hours.
• the soaked fibers are collected on a filter and transferred to an 80-mesh (0.177 mm opening) wire basket supported 38 mm above a 60-mesh (0.25 mm opening) screened bottom of a centrifuge tube.
• the tube is covered with a plastic cover and the sample is centrifuged at a relative centrifuge force of 1500 to 1700 gravities for 19 to 21 minutes.
• the centrifuged fibers are then removed from the basket and weighed.
• the weighed fibers are dried to a constant weight at 105°C and reweighed.
• Drip capacity was utilized as a combined measure of absorbent capacity and absorbency rate of the cores.
• a 100 mm by 100 mm absorbent pad weighing 7.5 g is placed on a screen mesh.
• Synthetic urine is applied to the center of the pad at a rate of 8 ml/s.
• the flow of synthetic urine is halted when the first drop of synthetic urine escapes from the bottom or sides of the pad.
• the drip capacity is calculated by the difference in mass of the pad prior to and subsequent to introduction of the synthetic urine divided by the mass of the fibers, bone dry basis.
• wet compressibility was utilized as a measure of resistance to wet compression, wet structural integrity and wet resilience of the absorbent cores.
• a 100 mm by 100 mm square pad weighing 7.5 g is prepared, its thickness measured and density calculated.
• the pad is loaded with synthetic urine to ten times its dry weight or to its saturation point, whichever is less.
• a 70.3 kg/m2 compressional load is applied to the pad. After about 60 seconds, during which time the pad equilibrates, the thickness of the pad is measured.
• the compressional load is then increased to 773.38 kg/m2 the pad is allowed to equilibrate, and the thickness is measured.
• the compressional load is then reduced to 70.31 kg/m2 the pad allowed to equilibrate and the thickness is again measured.
• the densities are calculated for the pad at the original 70.31 kg/m2 load, the 773.38 kg/m2 load and the second 70.31 kg/m2 load, referred to as 70.31 kg/m2 rebound load.
• the void volume reported in cc/g, is then determined for each respective pressure load.
• the void volume is the reciprocal of th wet pad density minus the fiber volume (0.95 cc/g).
• the 70.31 kg/m2 and 773.38 kg/m2 void volumes are useful indicators of resistance to wet compression and wet structural integrity. Higher void volumes for a common initial pad densities indicate greater resistance to wet compression and greater wet structural integrity.
• the difference between 70.37 kg/m2 and 70.37 kg/m2 rebound void volumes is useful for comparing wet resilience of absorbent pads.
• Dry compressibility was utilized as a measure of dry resilience of the cores.
• a 100 mm by 100 mm square air laid pad having a mass of 7.5 g is prepared and compressed, in a dry state, by a hydraulic press to a pressure of 241680 kg/m2. The pad is inverted and the pressing is repeated. The thickness of the pad is measured before and after pressing with a no-load caliper. Density before and after pressing is then calculated as mass/(area X thickness). Larger differences between density before and after pressing indicate lower dry resilience.
• a sample of individualized, crosslinked fibers is extracted with 0.1N HCl at 60°C for 1 hour.
• the extract is separated from the fibers, and the same extraction/separation procedure is then repeated for each sample an additional three times.
• the extract from each extraction is separately mixed with an aqueous solution of 2,4-dinitrophenylhydrazone (DNPH).
• DNPH 2,4-dinitrophenylhydrazone
• the reaction is allowed to proceed for 15 minutes after which a volume of chloroform is added to the mixture.
• the reaction mixture is mixed for an additional 45 minutes.
• the chloroform and aqueous layers are separated with a separatory funnel.
• the level of glutaraldehyde is determined by analyzing the chloroform layer by high pressure liquid chromatography (HPLC) for DNPH derivative.
• the chromatographic conditions for HPLC analysis utilized were - Column: C-18 reversed phase; Detector: UV at 360 mm; Mobile phase 80:20 methanol: water; Flow rate: 1 ml/min.; measurement made: peak height.
• a calibration curve of peak height and glutaraldehyde content was developed by measuring the HPLC peak heights of five standard solutions having known levels of glutaraldehyde between 0 and 25 ppm.
• Each of the four chloroform phases for each fiber sample was analyzed by HPLC, the peak height measured, and the corresponding level of glutaraldehyde determined from the calibration curve.
• the glutaraldehyde concentrations for each extraction were then summed and divided by the fiber sample weight (dry fiber basis) to provide glutaraldehyde content on a fibers weight basis.
• This example shows the effect of varying levels of a crosslinking agent, glutaraldehyde, on the absorbency and resiliency of absorbent pads made from individualized, crosslinked fibers.
• the individualized, crosslinked fibers were made by a dry crosslinking process.
• a quantity of never dried, southern softwood kraft (SSK) pulp were provided.
• the fibers had a moisture content of 62.4% (equivalent to 37.6% consistency).
• a slurry was formed by adding the fibers to a solution containing a selected amount of 50% aqueous solution of glutaraldehyde, 30% (based upon the weight of the glutaraldehyde) zinc nitrate hexahydrate, demineralized water and a sufficient amount of 1N HCl to decrease the slurry pH to 3.7.
• the fibers were soaked in the slurry for a period of 20 minutes and then dewatered to a fiber consistency of 34% to 35% by centrifuging.
• the dewatered fibers were air dried to a fiber consistency of 55% to 56% with a blow through dryer utilizing ambient temperature air.
• the air dried fibers were defibrated utilizing a three-stage fluffing device as described in US-A-3,987,968.
• the defibrated fibers were placed in trays and cured at 145°C in an essentially static drying oven for a period of 45 minutes. Crosslinking was completed during the period in the oven.
• the crosslinked, individualized fibers were placed on a mesh screen and washed with 20°C water, soaked at 1% consistency for one (1) hour in 60°C water, screened, washed with 20°C water for a second time, centrifuged to 60% fiber consistency, defibrated in a three stage fluffer as previously described, and dried to completion in a static drying oven at 105°C for four (4) hours.
• the dried fibers were air laid to form absorbent pads.
• the pads were compressed with a hydraulic press to a density of 0.10 g/cc.
• the pads were tested for absorbency, resiliency, and amount of glutaraldehyde reacted according to the procedures herein defined.
• Glutaraldehyde reacted is reported in mole % calculated on a dry fiber cellulose anhydroglucose basis. The results are reported in Table 1. TABLE 1 Sample # Glutaraldehyde (mole %) Added/Reacted WRV (%) Drip Cap.
• Samples 1 and 2 have amounts of reacted glutaraldehyde outside the range 0.5 to 3.5 mole % and Samples 1 to 4 and 10 have WRVs outside the range 28 to 45.
• the purpose of the example is to illustrate the effect of reacted glutaraldehyde and the results of these Samples are presented for purposes of comparison and not as examples of the invention.
• Example 2 The individualized, crosslinked fibers of Example 1 were formed into dry laid absorbent pads having a dry fiber density of 0.20g/cc. The pads were allowed to expand under unrestrained conditions upon wetting with synthetic urine during execution of the drip capacity procedure. The pads were subsequently tested for absorbency resiliency and structural integrity according to the previously outlined wet compressibility procedure. The results are reported in Table 2. Drip capacity and wet compressibility increased significantly at 0.50 mole % glutaraldehyde.
• the purpose of this example is to show that low levels of extractable crosslinking agent may be obtained by subjecting the fibers to bleaching sequence steps subsequent to crosslinking.
• the level of extractable crosslinking agent was determined by soaking a sample of the fibers in 40°C deionized water at 2.5% consistency for one (1) hour.
• the glutaraldehyde extracted by the water was measured by HPLC, and reported as extractable glutaraldehyde on a dry fiber weight basis.
• the fibers were crosslinked by a dry crosslinking process.
• Southern softwood kraft pulp SSK was provided.
• the pulp fibers were partially bleached by the following bleaching sequence stages: chlorination (C) - 3-4% consistency slurry treated with about 5% available chlorine (av. Cl) at about pH 2.5 and 38°C for 30 minutes; caustic extraction - 12% consistency slurry treated with 1.4 g/l NaOH at about 74°C for 60 minutes; and hypochlorite treatment (H) - 12% consistency slurry treated with sufficient sodium hypochlorite, at 11-11.5 pH between 38°C and 60°C for 60 minutes, to provide a 60-65 Elrepho brightness and a viscosity of 0.0155-0.0165 Pa.s (15.5 - 16.5 cp).
• the partially bleached fibers were processed into individualized, crosslinked fibers utilizing glutaraldehyde as the crosslinking agent in accordance with the process described in Example 1.
• the fibers retained 2.29 mole % glutaraldehyde, calculated on a dry fiber cellulosic anhydroglucose molar basis.
• such fibers have extractable glutaraldehyde levels of 1000 ppm (0.1%).
• a 10% consistency aqueous slurry of the dewatered fibers were treated with 0.33 g NaOH/liter water for 1.5 hours in a 40°C.
• the fibers were then screened, rinsed with water to neutral pH and centrifuged to 62.4% consistency.
• a 10% consistency slurry of the fibers containing sufficient sodium hypochlorite to provide 1.5% available chlorine on a dry fiber weight basis was prepared.
• the slurry was mixed and heated in 50°C oven for one (1) hour.
• the fibers were then screened, rinsed to pH 5.0 and centrifuged to 62.4% consistency.
• the dewatered fibers were air dried, fluffed and dried to completion in a 105°C oven for one (1) hour.
• the level of extractable glutaraldehyde of the fully bleached, individualized, crosslinked fibers was 25 ppm (0.0025%). This is well below the maximum level of extractable glutaraldehyde believed to be acceptable for applications wherein the fibers are utilized in proximity to human skin.
• pads made from the fibers which were partially bleached, crosslinked and then bleached to completion had unexpectedly higher fluid retention value and wicking rate and at least equivalent drip capacity and wet resilience as individualized fibers which were crosslinked subsequent to being fully bleached.
• the fibers crosslinked at an intermediate point of the bleaching sequence were more compressible in a dry state.
• This example shows the effect of mixing an organic acid with an inorganic salt catalyst on the level of crosslinking reaction completion.
• the fibers were crosslinked by a dry crosslinking process.
• a first sample of individualized, crosslinked fibers was prepared as described in Example 1, wherein 4.0 mole % glutaraldehyde was retained subsequent to dewatering. Analytical measurements of the fibers subsequent to crosslinking indicated that the level of glutaraldehyde reacted on the fibers was 1.58 mole %, corresponding to a reaction completion percentage of about 37%.
• a second sample of individualized, crosslinked fibers was prepared the same way as the first sample described in this example, except that in addition to the zinc nitrate catalyst, a quantity of citric acid equivalent to 10 wt. % of the glutaraldehyde mixed with the zinc nitrate in the pulp slurry as an additional catalyst.
• Analytical measurements of the fibers subsequent to crosslinking indicated that the level of glutaraldehyde reacted on the fibers was 2.45 mole %, corresponding to a reaction completion percentage of 61%, (molar basis) a 55.1% increase in reaction completion relative to the unmixed zinc nitrate catalyst sample.
• This example disclosed the use of low levels of glyoxylic acid, a dialdehyde acid analogue having one aldehyde group, in a dry crosslinking process as described in Example 1.
• a fibrous slurry of never dried SSK containing a sufficient amount of glyoxylic acid to provide an estimated 1.2% glyoxylic acid reacted with cellulosic fibers, on a cellulose anhydroglucose molar basis and a zinc nitrate hexahydrate catalyst was prepared.
• the centrifuged fibers had a fiber consistency of 38% and contained 1.06 wt % glyoxylic acid, on a dry fiber basis.
• the catalyst to crosslinking agent ratio was 0.30.
• the pH of the slurry at the start of crosslinking was 2.16.
• the fibers were individualized and crosslinked according to the procedures described in Example 1.
• glyoxylic acid based on a dry fiber weight basis
• 0.53 wt % glyoxylic acid, based on a dry fiber weight basis was added to the fibers to provide an estimated level of glyoxylic acid reacted with the fibers of 0.6 mole %, calculated on a cellulose anhydroglucose molar basis.
• the individualized, crosslinked fibers were otherwise prepared in accordance with the sample described immediately above, except that the slurry pH at the start of crosslinking was 2.35.
• Absorbent structures of 0.1 g/cc acid 0.2 g/cc densities were made from the individualized, crosslinked fibers as described in Example 2.
• the drip capacities, the wet compressibilities at 70.31 kg/m2, 73.38 kg/m2 and 70.31 kg/m2 rebound and the wicking of the pads were significantly greater than for similar density absorbent structures made from conventional, uncrosslinked fibers.
• the purpose of this example is to exemplify a process for making wet-laid sheets containing individualized, crosslinked fibers.
• a 0.55% consistency slurry of a blend of fibers containing 90% individualized, crosslinked fibers made according to the process described in Example 1 and 10% conventional, uncrosslinked fibers having a freeness of less than 100 CSF were deposited in flocculated, clumped fibers on a conventional 84-mesh Fourdrinier forming wire.
• the papermaking flow rate out of the headbox was 430 kg/min.
• a series of five streams of water of sequentially decreasing flow rates were directed upon the fibers.
• the five streams of water provided a cumulative flow ratio 85 kg. water/kg. bone dry (b.d.) fiber.
• the showers were all spaced within an approximately 1 meter long area parallel to the direction of travel of the forming wire.
• Each stream of water was showered onto the fibers through a linear series of 1/8" (3.2 mm) ID circular apertures spaced 1/2" (12.7 mm) apart and extending across the width of the forming wire.
• the approximate percentage of flow, based upon the total flow rate, and velocity of flow through the aperatures for each of the showers was as follows: Shower 1-37% of total flow, 170 m/min.; Shower 2-36% of total flow, 165 m/min.; shower 3-13% of total flow, 61 m/min.; shower 4-9% of total flow, 41 m/min.; shower 5-5% of total flow, 20 m/min.
• the fibers were set by treatment with a cylindrical, screened roll known in the art as a Dandy Roll.
• the Dandy Roll pressed the fibers, which at the time of setting were in a high consistency slurry form, against the forming wire to set the fibers to form of a wet sheet.
• the sheet was similar in appearance to conventional fibrous pulp sheets.

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