DE3781959T2 - METHOD FOR PRODUCING INDIVIDUALLY CROSSLINKED FIBERS. - Google Patents

Abstract

A process for making individualized, crosslinked fibers having low levels of residual crosslinking agent. The fibers are made by contacting the fibers with a crosslinking agent; reacting the crosslinking agent with the fibers to form intrafiber crosslink bonds in the substantial absence of interfiber bonds; and washing the fibers with an alkaline solution.

Description

FIELD OF INVENTION

This invention relates to methods for making individualized crosslinked fibers. In particular, this invention relates to methods for making such fibers in which reduced levels of residual crosslinking agent are obtained.

BACKGROUND OF THE INVENTION

Fibers which have been crosslinked in a substantially individualized form and various processes for making such fibers have been described in the relevant literature. The term "individualized crosslinked fibers" refers to cellulosic fibers which have primarily intrafiber chemical crosslinking bonds. That is, the crosslinking bonds are primarily between cellulose molecules of a single fiber rather than between cellulose molecules of different fibers. In general, individualized crosslinked fibers are considered useful in absorbent product applications. In general, three categories of processes have been identified to make individualized crosslinked fibers. These processes, which are described below, are referred to herein as 1) dry crosslinking processes, 2) aqueous solution crosslinking processes, and 3) substantially nonaqueous solution crosslinking processes. The fibers themselves and absorbent structures containing individualized crosslinked fibers generally exhibit an improvement in at least one significant absorbency relative to conventional uncrosslinked fibers. This improvement in absorbency is often expressed in terms of absorbency. In addition, absorbent structures made from individualized, cross-linked fibers generally exhibit increased wet resilience and increased dry resilience relative to absorbent structures made from non-cross-linked fibers. The term "resilience" shall hereinafter refer to the ability of pads made from cellulosic fibers to return to an original expanded state upon release of a compressive force. Dry resilience refers in particular to to the ability of an absorbent structure to expand upon release of compressive force applied while the fibers are in a substantially dry state. Wet resilience particularly refers to the ability of an absorbent structure to expand upon release of compressive force applied while the fibers are in a moistened state. For purposes of this invention and consistent with the disclosure, wet resilience is to be observed and reported for an absorbent structure that has been moistened to saturation.

Methods for making individualized, crosslinked fibers using a dry crosslinking technology are described in U.S. Patent No. 3,224,926, issued to L.J. Bernardin on December 21, 1965. Individualized, crosslinked fibers are made by impregnating swollen fibers in an aqueous solution with a crosslinking agent, dewatering and defibrating the fibers by mechanical action, and drying the fibers at an elevated temperature to effect crosslinking while the fibers are in a substantially individualized state. The fibers are inherently crosslinked in an unswollen, collapsed state as a result of dewatering occurring prior to crosslinking. Processes such as those exemplified in U.S. Patent No. 3,224,926, in which crosslinking is induced to occur while the fibers are in an unswollen, collapsed state, are referred to as processes for producing "dry crosslinked" fibers. Dry crosslinked fibers are characterized by low fluid retention values (FRV). U.S. Patent No. 3,440,135, issued April 22, 1969 to R. Chung, proposes soaking the fibers in an aqueous solution of a crosslinking agent to reduce interfiber bonding capacity prior to performing a dry crosslinking operation similar to that described in U.S. Patent No. 3,224,926. This time-consuming pretreatment, preferably between approximately 16 to 48 hours, is said to improve product quality by reducing the nitrate content as a result of incomplete defibration.

Methods for making aqueous solution crosslinked fibers are disclosed, for example, in U.S. Patent No. 3,241,553, issued March 22, 1966 to F.H. Steiger. Individualized crosslinked fibers are made by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers made 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. Relative to dry-crosslinked fibers, aqueous solution crosslinked fibers, as disclosed in U.S. Patent No. 3,241,553, have greater flexibility and lower stiffness and are characterized by higher fluid retention value (FRV). Absorbent structures made from aqueous solution crosslinked fibers have lower wet and dry resilience than pads made from dry-crosslinked fibers.

In U.S. Patent No. 4,035,147, issued to S. Sangenis, G. Guiroy and J. Quere on July 12, 1977, a process is disclosed whereby individualized crosslinked fibers are made by contacting dehydrated, unswollen fibers with crosslinking agent and catalyst in a substantially non-aqueous solution containing an insufficient amount of water to swell the fibers. Crosslinking occurs while the fibers are in this substantially non-aqueous solution. This type of process is hereinafter referred to as a non-aqueous solution crosslinking process; and the fibers produced thereby are referred to as non-aqueous solution crosslinked fibers. The non-aqueous solution crosslinked fibers disclosed in U.S. Patent No. 4,035,147 do not swell even upon prolonged contact with solutions known to those skilled in the art as swelling agents. Similar to dry crosslinked fibers, they are highly stiffened by crosslink bonds, and absorbent structures made therefrom exhibit relatively high wet and dry elasticity.

Crosslinked fibers as described hereinbefore are believed to be useful for applications in lower density absorbent products such as diapers and also for applications in higher density absorbent products such as sanitary napkins. However, such fibers have not offered sufficient absorbent advantages over conventional fibers in view of their disadvantages and costs to result in significant commercial success. Commercial appeal of crosslinked fibers has also suffered due to safety concerns. The most commonly cited crosslinking agent in the literature, formaldehyde, unfortunately causes human skin irritation and has been linked to other human safety concerns. Reducing free formaldehyde to sufficiently low levels in the crosslinked product so that skin irritation and other human safety concerns are avoided has been prevented by both technical and economic difficulties.

Crosslinked fibers made from crosslinking agents other than formaldehyde may also contain residual levels of crosslinking agent that are higher than those preferred for applications in the vicinity of human skin. Accordingly, it may be desirable to treat the fibers following crosslinking in such a manner as to reduce the level of unstable residual crosslinking agent on the fibers. One method of accomplishing this is to wash the fibers with water following crosslinking. This method is effective, but does not reduce the level of residual crosslinking agent to as low a level as would be desirable. In addition, such post-crosslinking washing requires additional cost and labor associated with washing and drying the fibers.

It is an object of this invention to provide commercially viable individualized, cross-linked fibers and absorbent structures made from such fibers as described above which can be safely used in the vicinity of human skin.

It is an additional object of this invention to provide a method for producing individualized, crosslinked fibers with low levels of residual crosslinking agent, in which capital investment in equipment and associated labor costs are minimized.

According to the present invention, there is provided a method for producing individualized, cross-linked cellulose-based fibers, wherein said method comprises the steps of:

a) providing cellulosic fibers and contacting said fibers in an aqueous solution with a sufficient amount of a crosslinking agent selected from C2-C8 dialdehydes, acid analogs of said dialdehydes, and oligomers of said dialdehydes and said acid analogs to yield, upon reaction, from 0.5 mole percent to 3.5 mole percent crosslinking agent on a cellulose anhydroglucose molar basis so as to impart to said reacted fibers a water retention value of from 28 to 45;

b) dewatering said fibres to a consistency of 30% to 80% by weight fibres and defibrating said fibres to a substantially individual shape;

c) air drying said fibers while they are individualized under conditions preventing fiber-to-fiber contact and reacting said crosslinking agent with said fibers to form intrafiber crosslink bonds in the substantial absence of interfiber bonds; and

d) washing said crosslinked fibers with an alkaline solution having a pH greater than 7 and containing a component for decomposing hemiacetal bonds while being neutral toward acetal bonds, thereby reducing the amount of reacted and residual crosslinking agent.

Preferably, the alkaline solution has a pH of more than 9.

Surprisingly, fibers produced according to the present invention have higher water retention values than fibers with the same level of crosslinking produced by other equivalent crosslinking processes carried out on fully bleached fibers. Corresponding absorbent structures made from the fibers of the present invention have higher absorbent capabilities including wet resilience and responsiveness to wetting.

DETAILED DESCRIPTION OF THE INVENTION

Cellulosic fibers of various natural origins are applicable to the invention. Pulped fibers from softwood, hardwood or cotton linters are preferably used. Fibers from esparto grass, bagasse, wool hair, flax and other woody and cellulosic fiber sources can also be used as raw materials in the invention. The fibers can be supplied in slurry, unbound or sheet form. Fibers supplied as wet wrap, dry wrap or other sheet form are preferably brought into unbound form by mechanically disintegrating the sheet material, preferably before the fibers are contacted with the crosslinking agent. Also, the fibers are preferably supplied in a wet or moistened state. The fibers are most preferably never-dried fibers. In the case of dry winding material, it is advantageous to moisten the fibers before mechanical disintegration in order to minimize damage to the fibers.

The optimum fiber source used in connection with this invention will depend on the particular end use contemplated. In general, pulp fibers produced by chemical pulping processes are preferred. Fully bleached, partially bleached and unbleached fibers are applicable. It may often be desirable to use bleached pulp because of its superior brightness and consumer appeal. In a new embodiment of the invention, which will be described more fully hereinafter, the fibers are partially bleached, crosslinked and then bleached to completion. For products such as paper napkins and absorbent For pads for diapers, sanitary napkins, sanitary towels and other similar absorbent paper products, it is particularly preferred to use fibers from southern softwood pulp due to their excellent absorption properties.

Crosslinking agents applicable to the present invention include C2-C8 dialdehydes, as well as acid analogs of such dialdehydes in which the acid analog has at least one aldehyde group, and oligomers of such dialdehydes and acid analogs. These compounds are capable of reacting with at least two hydroxyl groups in a single cellulose chain or with closely spaced cellulose chains in a single fiber. Those skilled in the art of crosslinking agents will recognize that the dialdehyde crosslinking agents described above will exist or react in a variety of forms, including the acid analogs and oligomer forms identified above. All such forms are intended to be included within the scope of the invention. Reference to a particular crosslinking agent shall therefore hereinafter be made to that particular crosslinking agent as well as to other forms which may be present in an aqueous solution. Particular crosslinking agents intended for use in the invention are glutaraldehyde, glyoxal and glyoxylic acid. Glutaraldehyde is particularly preferred because it has yielded fibers with the highest levels of absorbency and elasticity, is believed to be safe and non-irritating to human skin when in a reacted crosslinked state, and has yielded the most stable crosslink bonds. Mono-aldehyde compounds which do not contain an additional carboxyl group, such as acetaldehyde and furfural, have not been found to yield absorbent structures with the desired levels of absorbency, elasticity and responsiveness to wetting.

Unexpectedly, it has been found that excellent absorbent pad performance can be obtained at crosslinking levels which are substantially lower than the crosslinking levels which have been previously practiced. In general, unexpectedly good results were obtained for absorbent pads made from individualized, cross-linked fibers with between 0.5 mol% and 3.5 mol% cross-linking agent, based on a molar cellulose anhydroglucose basis, reacted with the fibers.

Preferably, the crosslinking agent is contacted with the fibers in a liquid medium under conditions such that the crosslinking agent penetrates into the interior of the individual fiber structures. However, other methods of crosslinking agent treatment, including spraying the fibers while they are in individualized flaked form, are also within the scope of the invention.

Generally, the fibers are also contacted with a suitable catalyst prior to crosslinking. The type, amount and method of contacting the catalyst with the fibers will depend on the particular crosslinking processes practiced. These variables will be discussed in more detail below.

Once the fibers have been treated with crosslinking agent and catalyst, the crosslinking agent is caused to react with the fibers in the marked absence of interfiber bonds, i.e. while maintaining interfiber contact at a low level of occurrence relative to unflocculated pulp fibers, or the fibers are immersed in a solution which does not facilitate the formation of interfiber bonds, particularly hydrogen bonds. This results in the formation of crosslink bonds which are intrafiber bonds 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 adjacently located cellulose chains of a single cellulosic fiber.

Although it is not stated or intended to limit the scope of the invention, it is believed that the crosslinking agent reacts with the hydroxyl groups of the cellulose to form hemiacetal and acetal bonds. The formation of Acetal linkages, which are considered to be the desirable type of linkages yielding stable crosslinking bonds, are favored among acidic reaction linkages. Accordingly, for the purposes of this invention, acid-catalyzed crosslinking bonds are highly preferred.

The fibers are preferably mechanically defibrated prior to reaction of the crosslinking agent with the fibers into an individualized, low density fibrous form known as a "fluff." Mechanical defibration can be accomplished by a variety of methods currently known in the art or which may hereafter become known. Mechanical defibration is preferably accomplished by a method in which knot formation and fiber damage are minimized. One type of device that has been found to be particularly useful for defibrating cellulosic fibers is the three-stage fluffing device described in U.S. Patent No. 3,987,968, issued October 26, 1976 to D.R. Moore and O.A. Shields. The fluffing apparatus described in U.S. Patent No. 3,987,968 subjects wet cellulosic pulp fibers to a combination of mechanical impact, mechanical agitation, air agitation and a limited amount of air drying to produce a substantially knot-free fluff. The individualized fibers have been imparted with an increased 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 elasticity of absorbent structures made from the finished, crosslinked fibers.

Other applicable methods for 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. Preferably, during such defibration, a stream of air is directed against the fibers to assist in separating the fibers into substantially individual shapes.

Regardless of the particular mechanical device used to form the flake, the fibers are preferably mechanically treated while initially containing at least 30% moisture, and preferably between 40% and 60% moisture.

Mechanical refining of fibers at high consistency or partially dried fibers may also be used to impart curl or twist to the fibers in addition to the curl or twist imparted as a result of mechanical defibration.

The fibers made according to the present invention have unique combinations of stiffness and resilience which allow absorbent structures made from the fibers to maintain high levels of absorbency and exhibit high levels of resilience and expansion responsiveness to wetting of a dry, compressed absorbent structure. In addition to having crosslinking levels within the stated ranges, the crosslinked fibers are characterized as having Water Retention Values (WRV's) between 28 and 45 for conventional chemically pulped papermaking fibers. The WRV of a particular fiber is indicative of the crosslinking level 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 crosslinking level. Very highly crosslinked fibers, such as those produced by the prior art dry crosslinking processes discussed above, have been found to have WRV's of less than 25, and generally less than 20. The particular crosslinking process used will, of course, affect the WRV of the crosslinked fiber. However, any process which will result in crosslinking levels and WRV's within the specified limits is believed to be and should be within the scope of this invention. Applicable crosslinking processes include dry crosslinking processes and Non-aqueous solution crosslinking processes, as discussed generally in the Background of the Invention. Certain preferred dry crosslinking processes and non-aqueous solution crosslinking processes within the scope of the present invention will be discussed in more detail below. Aqueous solution crosslinking processes in which the solution causes the fibers to swell greatly will result in fibers having WRV's which are on the order of about 60. These fibers will provide insufficient stiffness and elasticity for the purposes of the present invention.

With particular reference to dry crosslinking processes, individualized, crosslinked fibers can be prepared from such a process by providing a quantity of cellulosic fibers, contacting a slurry of the fibers with a type and amount of crosslinking agents as described above, mechanically separating, e.g., defibrating, the fibers to substantially individual shape, 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 maintaining the fibers in substantially individual shape. The defibrating step, separate from the drying step, is believed to impart additional crimp. Subsequent drying is accompanied by twisting of the fibers, the degree of twist being increased by the curled geometry of the fiber. When used herein, fiber "crimp" refers to a geometric curvature of the fiber about the long axis of the fiber. "Twist" refers to a rotation of the fiber about the perpendicular cross-section of the long axis of the fiber. For exemplary purposes only, and without intending to particularly limit the scope of the invention, individualized, cross-linked fibers within the scope of the invention have been observed to have an average of six (6) closed twists per millimeter.

Maintaining the fibers in substantially individual shape during drying and crosslinking allows the fibers to twist during drying and thereby become crosslinked in such a twisted, curled state. Drying fibers under such conditions that the fibers can twist and curl is hereinafter referred to as drying the fibers under substantially unrestrained conditions. On the other hand, drying the fibers in sheet form results in dried fibers which are not twisted and curled like fibers dried in substantially individualized form. It is believed that interfiber hydrogen bonding "restrains" the relative occurrence of fiber twist and curl.

There are several methods by which the fibers can be contacted with the crosslinking agent and catalyst. In one embodiment, the fibers are contacted with a solution initially containing both the crosslinking agent and the catalyst. In another embodiment, the fibers are contacted with an aqueous solution of crosslinking agent and allowed to soak prior to adding the catalyst. The catalyst is then added. In a third embodiment, the crosslinking agent and catalyst are added to an aqueous slurry of the cellulosic fibers. Other methods in addition to those described herein will be apparent to those skilled in the art and are intended to be included within the scope of this invention. Regardless of the particular method by which the fibers are contacted with the crosslinking agent and catalyst, the cellulosic fibers, crosslinking agent and catalyst are preferably mixed and/or soaked with the fibers sufficiently to ensure continuous contact therewith and impregnation of the individual fibers.

In general, any substance which catalyzes the cross-linking mechanism can be used. Applicable catalysts include organic acids and acidic salts. Particularly preferred catalysts are salts such as aluminum, magnesium, zinc and calcium salts of chlorides, nitrates or sulfates. A 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 used as the sole catalyzing agent or in combination with one or more other catalysts. It is believed that combinations of acidic salts and organic salts as catalyzing agents give better crosslinking reaction efficiency. Unexpectedly high levels of reaction completion have been observed with catalyst combinations of zinc nitrate salts and organic acids such as citric acid and the use of such combinations is preferred. Mineral acids are useful for adjusting the pH of the fibers while they are contacted with the crosslinking agent in solution but are preferably not used as the main catalyst.

The optimum amount of crosslinker and catalyst used will depend on the particular crosslinker used, taking into account the reaction conditions and the particular product application.

The amount of catalyst preferably used will, of course, depend on the particular type and amount of crosslinking agent and the reaction conditions, particularly temperature and pH. Based on technical and economic considerations, catalyst levels of between 10% and 69% by weight, based on the weight of crosslinking agent added to the cellulosic fibers, are generally preferred. For exemplary purposes, in the case where the catalyst used is zinc nitrate hexahydrate and the crosslinking agent is glutaraldehyde, a catalyst level of 30% by weight, based on the amount of glutaraldehyde added, is preferred. Most preferably, between 5% and 30%, based on the weight of the glutaraldehyde, of an organic acid, such as citric acid, is also added as a catalyst. It is additionally desirable to adjust the aqueous portion of the cellulose-based fiber slurry or crosslinking agent solution to a target pH 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. These feasible and optimal consistencies will vary depending on the type of flocculation equipment used. For the purposes of the invention, the cellulosic fibers are dewatered and optimally dried to a consistency between 30% and 80%. More preferably, the fibers are dewatered and dried to a consistency level between 40% and 60%. Drying the fibers to within these preferred ranges will generally facilitate defibration of the fibers to individualized shape without excessive formation of knots associated with higher moisture levels and without high levels of fiber damage associated with lower moisture levels.

For example, dewatering may be carried out by such methods as mechanically pressing, centrifuging or air drying the pulp. Additional drying is preferably carried out by such methods known in the art as air drying or flash drying under conditions such that the use of high temperature over an extended period of time is not required. Excessively high temperature at this stage of the process may result in premature onset of crosslinking. Preferably, temperatures exceeding 160°C are not maintained for periods exceeding 2 to 3 seconds. Mechanical defibration is carried out as previously described.

The defibrated fibers are then heated to an appropriate temperature for an effective period of time to cause the crosslinking agent to harden, i.e. to react with the cellulosic fibers. The rate and degree of crosslinking will depend on the dryness of the fibers, the temperature, amount and type of catalyst and crosslinking agent, and the method used to heat and/or dry the fibers while crosslinking is carried out. Crosslinking at a particular temperature will occur to a greater extent for fibers having a particular initial moisture content. when accompanied by continuous air drying than when subjected to drying/heating in a static oven. Those skilled in the art will appreciate that a number of temperature-time relationships exist for curing the crosslinking agent. Conventional paper drying temperatures (e.g., 120°C to about 150°C) for a period of between 30 minutes and 60 minutes, under static atmospheric conditions, will generally provide acceptable curing efficiencies for fibers having moisture contents of less than 5%. Those skilled in the art will also recognize that higher temperatures and air convection will reduce the time required for curing. However, curing temperatures are preferably maintained at less than 160°C since exposure of the fibers to such high temperatures in excess of 160°C may result in yellowing or other damage to the fibers.

The maximum level of crosslinking will be achieved when the fibers are substantially dry (less than 5% moisture). Due to this lack of water, the fibers are crosslinked while in a substantially unswollen, collapsed state. Consequently, they characteristically have low fluid retention values (FRV) within the range applicable to this invention. The FRV refers to the amount of fluid, on a dry fiber basis, that remains absorbed by a sample of fibers that have been aspirated and then centrifuged to remove interfiber fluid. (The FRV is further defined and the 'Method of Determining the FRV' is described below.) The amount of fluid that the crosslinked fibers can absorb depends on their ability to swell to saturation, or in other words, their internal diameter or volume after swelling to a maximum level. This in turn depends on the level of crosslinking. For a given fiber and process, as the level of intrafiber crosslinking increases, the FRV of the fiber will decrease until the fiber does not swell at all after wetting. Thus, the FRV value of a fiber is structurally descriptive of the physical Condition of the fiber at saturation. Unless otherwise expressly stated, the FRV data described herein are referred to in terms of the water retention value (WRV) of the fibers. Other fluids, such as salt water and synthetic urine, may also be advantageously used as a fluid medium for analysis. In general, the FRV of a particular fiber crosslinked by processes in which hardness is highly dependent on drying, such as the present process, will depend primarily on 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 will be primarily between 28 and 45. Bleached SSK fibers with between 0.5 mole percent and 2.5 mole percent glutaraldehyde reacted thereon, based on a cellulose anhydroglucose molar basis, have been observed to have WRV's in the range of 40 to 28, respectively. The degree of bleaching and the practice of post-crosslinking bleaching steps have been found to affect WRV. This effect is discussed in detail below. Southern Softwood Kraft (SSK) fibers made by dry crosslinking processes known prior to the present invention have higher crosslinking levels than described herein and have WRV's of less than 25. Such fibers, as previously discussed, have been observed to be extremely stiff and have lower absorbency capabilities than the fibers of the present invention.

Another category of crosslinking processes applicable to the present invention is that of non-aqueous solution curing crosslinking processes. The same types of fibers that are useful in dry crosslinking processes can be used in the preparation of non-aqueous solution crosslinked fibers. The fibers are treated with a sufficient amount of crosslinking agent and with a suitable catalyst such that between about 0.5 mole percent and 3.5 mole percent of crosslinking agent reacts sequentially with the fibers, the level of reacted crosslinking agent being calculated following said crosslinking reaction. The crosslinking agent is caused to react while the fibers are immersed in a solution which does not induce any significant levels of swelling of the fibers. However, the fibers may contain up to 30% water or they may be otherwise swollen in the crosslinking solution to a degree equivalent to fibers having a moisture content of 30%. Such partially swollen fiber geometry has been found to yield additional unexpected advantages as will be discussed more fully hereinafter. The crosslinking solution contains a non-aqueous, 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 their salts. Crosslinking solution systems suitable for use as a crosslinking medium include those disclosed in U.S. Patent No. 4,035,147, issued July 12, 1977 to S. Sangenis, G. Guiroy and J. Quere.

The crosslinking solution may contain some water or other fiber-swelling liquid, but the amount of water is preferably insufficient to induce a level of swelling corresponding to that charged to 70% consistency pulp fibers (30% aqueous moisture content). More preferably, crosslinking solution water contents of less than 10% of the total volume of the solution, excluding the fibers, are preferred. Water levels in the crosslinking solution exceeding this amount will decrease the efficiency and rate of crosslinking.

Absorption of crosslinking agent by the fibers can be carried out in the crosslinking solution itself or in an earlier stage of treatment including, but not limited to, saturation of the fibers with either an aqueous or non-aqueous solution containing the crosslinking agent. Preferably, the fibers are mechanically defibrated to individual shape. This mechanical treatment can be accomplished by prior The fiber flocking process described above can be carried out in conjunction with the dry crosslinking process described above.

It is particularly preferred to include in the production of fluff a mechanical treatment which causes the wet cellulosic fibres 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 fibres. This can be accomplished by beginning by providing fibres for fluffing which are in a wet condition, subjecting the fibres to a mechanical treatment such as the previously described process for defibrating the fibres to substantially individual shape, and at least partially drying the fibres.

The relative amounts of curl and twist imparted to the fibers depend in part on the moisture content of the fibers. Without limiting the scope of the invention, it is believed that the fibers will naturally twist upon drying under conditions where fiber-to-fiber contact is low, i.e. when the fibers are in an individualized form. Likewise, mechanical treatment of wet fibers initially causes the fibers to become curled. When the fibers are subsequently dried or partially dried under substantially untensioned conditions, they will become twisted with the degree of twist enhanced by the additional amount of mechanically imparted curl. The defibration-flocculation steps are preferably carried out on wet high consistency pulp or pulp which has been dewatered to a fiber consistency of 45% to 55% (predetermined before initiating defibration).

Following defibration, the fibers should be dried to between 0% and 30% moisture content before contacting them with the crosslinking solution, unless the defibration step has already provided fibers with moisture contents within this range. The drying step should be carried out while the fibers are under substantially unstressed conditions. This means that fiber-to-fiber contact should be minimized so that the inherent twisting of the fibers during drying is not hindered. Both air drying and flash drying processes are suitable for this purpose.

The individualized fibers are next contacted with a crosslinking solution containing a water-miscible, non-aqueous 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%.

A fiber wadding which has not been mechanically defibrated can also be brought into contact with a cross-linking solution as previously described.

The amounts of crosslinking agent and acid catalyst used will depend on 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 desired degree of crosslinking. Preferably, the amount of crosslinking agent used is in the range of 0.2 wt.% to 10 wt.% (based on the total fiber-free weight of the crosslinking solution). Preferred acid catalyst content is further dependent on the acidity of the catalyst in the crosslinking solution. Good results can generally be obtained for catalyst content, including hydrochloric acid, between 0.3 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. Fiber and crosslinking solution slurries having fiber consistencies of less than 10% by weight are preferred for crosslinking in conjunction with the crosslinking solution described above.

The crosslinking reaction can be carried out at ambient temperatures or, for accelerated reaction rates, at elevated temperatures, preferably less than 40ºC.

There are a variety of methods by which the fibers can be contacted with and crosslinked in the crosslinking solution. In one embodiment, the fibers are contacted with the solution initially containing both the crosslinking agent and the acid catalyst. The fibers are soaked in the crosslinking solution, during which time crosslinking occurs. In another embodiment, the fibers are contacted with the diluent and soaked prior to addition of the acid catalyst. The acid catalyst is then 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.

Preferably, the crosslinking agent and the conditions under which crosslinking is carried out are chosen to facilitate intrafiber crosslinking. Accordingly, it is advantageous for the crosslinking reaction to occur in substantial part after the crosslinking agent has had sufficient time to penetrate the fibers. Reaction conditions are preferably chosen to avoid immediate crosslinking without the crosslinking agent having already penetrated the fibers. Reaction periods during which crosslinking has been substantially fully completed, over a period of about 30 minutes, are preferred. Longer reaction periods are believed to provide minimal marginal benefits in fiber performance. However, both shorter periods, including substantially immediate crosslinking, and longer periods are considered to be within the scope of this invention.

It is also intended to react only partially during the residence time in solution and then to complete the crosslinking reaction later in the process by drying or heat treatments.

Following the cross-linking step, the fibres are dewatered and washed. Preferably, a sufficient amount of basic substance, such as a caustic solution, is added during the washing process to neutralize any acid remaining in the slurry. After washing, the fibers are defluidized and dried to completion. Preferably, between the defluidization and drying steps, the fibers are subjected to a second mechanical defibration step which causes the crosslinked fibers to curl, e.g., fluffing by defibration. Upon drying, the curled state of the fibers imparts additional twist, as previously described in connection with the curling treatment prior to contact with the crosslinking solution. The same devices and methods for inducing twist and curl described in connection with the first mechanical defibration step are applicable to this second mechanical defibration step. As used herein, the term "defibration" is intended to refer to any of the processes which may be used to mechanically divide the fibers into substantially individual form, even though the fibers may already be in such form. "Defibration" thus refers to the step of mechanically treating the fibers, either in individual form or in a more compact form, to a mechanical treating step which would a) divide the fibers into substantially individual form if they were not already in such form, and b) impart curl and twist to the fibers by drying.

It has been found that this second defibration treatment after the fibers have been crosslinked enhances the curled, twisted character of the pulp. This enhancement in the twisted, curled configuration of the fibers results in increased elasticity of the absorbent structure and responsiveness to wetting. A second defibration treatment can be carried out on any of the crosslinked fibers described herein which are in a wet state. However, a particular advantage of the non-aqueous solution crosslinking process is that a second defibration step is possible without requiring an additional drying step. This is due to the fact that the solution in which the Fibers are cross-linked, which keep the fibers flexible following cross-linking, even if they do not cause the fibers to assume an undesirable, highly swollen state.

It has further been unexpectedly found that increased levels of expansion of the absorbent structure after wetting compressed pads can be obtained for structures made from fibers that have been crosslinked in a state that is twisted but partially swollen relative to fibers that have been thoroughly dried by water prior to crosslinking.

Improved results are obtained for individualized crosslinked fibers that have been crosslinked under conditions where the fibers have been dried to a water content between 18% and 30% prior to contact with the crosslinking solution. In the case where a fiber has been dried to completion prior to contact with the crosslinking solution, it is in a non-swollen, collapsed state. The fiber will not swell upon contact with the crosslinking solution due to the low water content of the solution. As previously discussed, an important aspect of the crosslinking solution is that it does not induce any significant swelling of the fibers. However, if the diluent of the crosslinking solution is absorbed by an already swollen fiber, the fiber is "dried" in the effect of water, but the fiber retains its pre-existing, partially swollen state.

To describe the degree to which the fiber is swollen, it is useful to refer to the fiber's Fluid Retention Value (FRV) following crosslinking. Fibers with higher FRV's correspond to fibers that were crosslinked in a more swollen state relative to fibers that were crosslinked in a less swollen state, all other factors being equal. Without limiting the scope of the invention, it is believed that partially swollen, crosslinked fibers with increased FRV's have greater wet resiliency and responsiveness to wetting than fibers which have been crosslinked while in an unswollen state. Fibers with this increase in wet resilience and wetting responsiveness are more easily able to expand or untwist after wetting in an attempt to return to their natural state. However, due to the stiffness imparted by crosslinking, the fibers are still capable of providing the structural support for a saturated pad made from the fibers. Numerical FRV data described herein in connection with partially swollen crosslinked fibers will be water retention values (WRV). As the water retention value increases beyond approximately 60, it is believed that the stiffness of the fibers becomes insufficient to achieve the wet resilience and wetting responsiveness desired to support a saturated absorbent structure.

In an alternative method of crosslinking the fibers in solution, the fibers are first soaked in an aqueous or other fiber swelling solution, defluidized, dried to a desired level, and then immersed in a water-miscible crosslinking solution containing a catalyst and a crosslinking agent as previously described. Following defluidization and prior to additional drying, the fibers are preferably mechanically defibrated to fluff to achieve the benefits of increased twist and curl as previously described. Mechanical defibration, which is carried out upon contacting the fibers with the crosslinking agent, is less desirable because such defibration would volatilize the crosslinking agent, thus potentially leading to atmospheric contamination by the crosslinking agent or to high air treatment investments due to the crosslinking agent.

In a modification of the process described immediately above, the fibres are defibrated and then immersed in a solution with a high concentration of cross-linking agent and a fibre-swelling agent. diluent, preferably water. The crosslinking agent concentration is sufficiently high to inhibit water-induced swelling of fibers. It has been found that 50% by weight aqueous solutions of the crosslinking agent of this invention, preferably glutaraldehyde, are useful solutions for pre-soaking the fibers. The presoaked fibers are defluidized and immersed in a crosslinking solution containing a water-miscible polar diluent, a catalyst and a limited amount of water and then crosslinked as previously described. Also as previously described, the crosslinked fibers can 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 before reacting the crosslinking agent achieves unexpectedly high absorption properties for absorbent pads made from the crosslinked fibers, even relative to pads made from crosslinked fibers by the nonaqueous solution curing processes described above, in which the fibers have not been presoaked with a solution containing crosslinking agent.

The crosslinked fibers formed as a result of the foregoing dry crosslinking and non-aqueous solution crosslinking processes are the product of the present invention. The crosslinked fibers of the present invention can be used directly in the manufacture of air-laid absorbent cores. Due to their stiffened and resilient character, the crosslinked fibers can be wet-laid in addition to a low density, non-compact sheet which, when subsequently dried, is directly useful as an absorbent core without further mechanical treatment. The crosslinked fibers can also be wet-laid as compacted pulp sheets for sale or transportation to remote locations.

Once the individualized, crosslinked fibers are prepared, they can be dry-laid and formed directly into absorbent structures, or they can be wet-laid and formed into absorbent structures or densified pulp sheets. The fibers of the present invention provide a variety of significant performance advantages. However, it is difficult to form such fibers into a soft, wet-laid sheet using conventional wet-sheet forming processes. This is because individualized, crosslinked fibers rapidly flocculate. Such flocculation can occur both in the headbox and after deposition into the foraminous forming grid. Attempts to stratify individualized, crosslinked fibers by conventional pulp-stratifying processes have been found to result in the formation of a multitude of clumps of flocculated fibers. This is due to the stiff, twisted character of the fibers, a low level of fiber-to-fiber bonding and the high dewaterability of the fibers once deposited on a layer-forming grid. It is therefore of significant commercial importance that a viable process is provided for layering individualized, cross-linked fibers to form wet-layered absorbent structures and densified pulp sheets for transition and subsequent defibration.

Accordingly, a new method for forming layers of individualized, cross-linked fibers which tend to flocculate in solution has been developed in which a slurry containing individualized, cross-linked fibers is initially deposited on a foraminous forming grid, such as a Fourdrinier grid, in a manner similar to conventional pulp sheet forming processes. However, due to the nature of individualized, cross-linked fibers, these fibers are deposited on the forming grid in a plurality of fiber clumps. At least one fluid stream, preferably water, is directed against the deposited clumped fibers. Preferably, a series of pours are directed onto the fibers deposited on the forming grid, with successive pours having decreasing volumetric flow rates. The casts should be of sufficient velocity so that the impact of the fluid against the fibers acts to prevent the formation of fiber flocs and to disperse fiber flocs which have already formed. The fiber settling step is preferably carried out with a cylindrical screen such as a Dandy roll or other functionally analogous device which is or may become known in the art. Once set, the fibrous sheet can then be dried and optionally densified if desired. The spacing of the casts will vary depending on the particular rate of fiber floc, linear velocity of the forming grid, drainage through the forming grid, number of casts, and speed and flow rate through the casts. Preferably, the casts are close enough together so that substantial levels of floc will not occur.

In addition to preventing the formation and dispersing fiber flocs, the fluid poured onto the fibers also compensates for the extremely rapid drainage of individualized, cross-linked fibers by providing additional liquid medium in which the fibers can be dispersed for subsequent layer formation. The plurality of pours of decreasing volumetric flow rates facilitates a systematic mesh increase in slurry consistency while achieving a repeated dispersing and preventing effect against fiber flocs. This results in the formation of a relatively soft and uniform deposit of fibers which are then promptly consolidated into sheet form, i.e. before reflocculation, by allowing the fluid to drain and pressing the fibers against the foraminous mesh.

Relative to pulp sheets made from conventional, uncrosslinked cellulosic fibers, pulp sheets made from the crosslinked fibers of the present invention are more difficult to compress to conventional pulp sheet densities. Accordingly, it may be desirable to combine crosslinked fibers with uncrosslinked fibers, such as those commonly used in the manufacture of absorbent cores. Pulp sheets containing stiffened crosslinked fibers preferably contain between 5% and 90% uncrosslinked cellulosic fibers, based on the total dry weight of the sheet, mixed with the individualized crosslinked fibers. It is particularly preferred to include between 5% and 30% highly refined uncrosslinked cellulosic fibers, based on the total dry weight of the sheet. Such highly refined fibers are refined or beaten to a degree of freedom of less than 300 ml CSF, and preferably less than 100 ml CSF. The uncrosslinked fibers are preferably mixed with an aqueous slurry of individualized crosslinked fibers. This mixture can then be formed into a densified pulp sheet for subsequent defibration and formation into absorbent pads. The incorporation of the uncrosslinked fibers facilitates compression of the pulp sheet into a densified form while imparting surprisingly little loss of absorbency to the subsequent absorbent pads. The uncrosslinked fibers additionally increase the tensile strength of the pulp sheet and absorbent pads made either from the pulp sheet or directly from the blend of crosslinked and uncrosslinked fibers. Regardless of whether the blend of crosslinked and uncrosslinked fibers is first formed into a pulp sheet and then into an absorbent pad or whether it is formed directly into an absorbent pad, the absorbent pad can be airlaid or wetlaid as previously described.

Sheets or webs made from the individualized, crosslinked fibers, or from blends also containing uncrosslinked fibers, will preferably have basis weights of less than 800 g/m² and densities of less than 0.60 g/cm³. Although not intended to limit the scope of the invention, 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 particularly suitable for direct application to absorbent cores in disposable articles such as diapers, tampons and other catamenial products. Structures having basis weights and densities higher than these levels are believed to be most useful for subsequent reduction and air-laying or wet-laying to form a lower density and basis weight structure which is more useful for absorbent uses. Although such higher basis weight and density structures also exhibit surprisingly high absorbency and responsiveness to wetting. Other applications contemplated for the fibers of the present invention include fabric webs, where the density of such webs may be less than 0.10 g/cm3.

For product applications where the crosslinked fibers are located closest to or in the vicinity of a person's skin, it is desirable to further process the fibers to remove excess, unaddressed crosslinking agent. Preferably, the level of unaddressed crosslinking agent is reduced to at least below 0.03%, based on the dry weight of the cellulosic fibers. A series of treatments found to be effective for removing excess crosslinking agent include, in sequence, washing the crosslinked fibers, allowing the fibers to soak in the aqueous solution for an acceptable time, sieving the fibers, dewatering the fibers, e.g., by centrifugation, to a consistency of between 40% and 80%, mechanically defibrating the dewatered fibers as previously described, and air drying the fibers. This process was found to reduce residual free crosslinker content to between 0.01% and 0.15%.

In accordance with the present invention, readily extractable crosslinking agent is removed by caustic washes. Alkalinity can be introduced by basic compounds such as sodium hydroxide, or alternatively in the form of oxidizing agents such as those chemicals commonly used as bleaches, e.g. sodium hydrochlorite and amino-containing compounds such as 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 prevent reversal of the acetal crosslink bond. It is preferred to induce decomposition of hemiacetal bonds while remaining neutral to acetal bonds. Accordingly, those extractive agents which operate under highly alkaline conditions are preferred. It has been observed that single wash treatments with 0.01 N and 0.1 N ammonium hydroxide concentrations are sufficient to reduce residual levels to between 0.0008% and 0.0023% for immersion times of 30 minutes to two (2) hours. Minimal additional benefit is believed to occur for immersion times greater than 30 minutes and for ammonium hydroxide concentrations greater than 0.01 N.

Both single-stage oxidation and multi-stage oxidation have been found to be effective methods for extracting residual crosslinking agent. Single-stage washing with 0.1% available chlorine (av.Cl) to 0.8% av.Cl, supplied in the form of sodium hypochlorite, based on the dry weight of the fibers, was found to reduce residual crosslinking agent levels to 0.0015% and 0.0025%.

In a new attempt to produce crosslinked individualized fibers, the fiber starting materials are subjected to a conventional multi-step bleaching sequence, but during the sequence the bleaching process is interrupted in the middle and the fibers are crosslinked in accordance with the present invention. Following curing, the remainder of the bleaching sequence is completed. It has been found that acceptably low residual crosslinking agent levels of less than about 0.006% can be obtained in this manner. This process is believed to represent the preferred manner of producing crosslinked fibers because, due to the merging of the bleaching and residual reduction steps, capital costs and process inconveniences of additional washing and extraction equipment and additional processing steps are avoided. The bleaching sequences carried out and the The timing of interruption of these sequences to effect cross-linking can be varied widely, as will be readily appreciated by those skilled in the art. Multi-stage bleaching sequences in which DEP* or DEH* steps follow cross-linking have been found to give desirable results (*D-chlorine dioxide, E-liquor extraction, P-peroxide, H-sodium hypochlorite). The bleaching sequences after cross-linking are preferably alkaline treatments carried out at a pH of more than pH7, and more preferably more than pH9.

In addition to achieving effective reduction of residual crosslinking agent, it was observed that alkaline treatments after crosslinking facilitate the development of higher FRV (fluid retention value) fibers for equivalent crosslinking levels. The higher fluid retention value fibers have lower dry resilience, i.e. they are easier to densify in a dry state, while maintaining essentially the same wet resilience and moisture responsiveness as the other equivalent fibers crosslinked following completion of bleaching. This was particularly surprising considering that, until now, higher FRV resulted in reduced absorbency.

The crosslinked fibers described herein are useful in a variety of absorbent articles including, but not limited to, disposable diapers, sanitary napkins, sanitary napkins, tampons and bandages, each of which has an absorbent structure containing the individualized crosslinked fibers described herein. For example, a disposable diaper or similar article having a liquid permeable topsheet, a liquid impermeable backsheet bonded to the topsheet, and an absorbent structure containing individualized crosslinked fibers is particularly contemplated. Such articles are generally described in U.S. Patent No. 3,860,003, issued January 14, 1975 to Kenneth B. Buell.

Typically, absorbent cores for diapers and menstrual products are made from unstiffened, non-crosslinked cellulosic fibers, where the absorbent cores have dry densities of 0.06 g/cm³ and 0.12 g/cm³. After wetting, the absorbent core typically exhibits a reduction in volume.

It has been found that the crosslinked fibers of the present invention can be used to produce absorbent cores having substantially higher fluid absorption properties, including, but not limited to, absorbent capacity and wicking rate, relative to absorbent cores of equivalent density made from conventional uncrosslinked fibers or crosslinked fibers in accordance with the prior art. Furthermore, these improved absorption results can be obtained in conjunction with increased levels of wet resiliency. For absorbent cores having densities between 0.06 g/cc and 0.15 g/cc which maintain substantially constant volume after wetting, it is particularly preferred to use crosslinked fibers having crosslinking levels between 2.0 mole percent and 2.5 mole percent crosslinking agent on 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. The term "wet resilience" in the present context refers to the ability of a moistened pad to recover to its original shape and volume after exposure to and release of compression forces. Compared to cores made from untreated fibers and crosslinked fibers of the prior art, absorbent cores made from the fibers of the present invention will recover a substantially higher proportion of their original volumes after release of wet compression forces.

In another preferred embodiment, the individualized, cross-linked fibers are either 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. If fibers are processed into an absorbent core having a dry density less than the equilibrium wet density, the core will collapse to the equilibrium wet density upon wetting to saturation. Alternatively, if fibers are processed into an absorbent core having a dry density greater than the equilibrium wet density, the core will expand to the equilibrium wet density upon wetting to saturation. Pads made from the fibers of the present invention have equilibrium wet densities which are substantially less than pads made from conventional uncrosslinked 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 will expand upon wetting, thereby increasing the absorbency to a substantially greater degree than obtained from uncrosslinked fibers.

Particularly high absorbent properties, wet resilience and wetting responsiveness can be obtained by crosslinking levels between 0.75 mole percent and 1.25 mole percent on a dry cellulose molar basis. Preferably, such fibers are formed into absorbent cores having dry densities greater than their equilibrium wet densities. Preferably, the absorbent cores are compressed to densities between 0.12 g/cm³ and 0.60 g/cm³, with the corresponding equilibrium wet density being less than the density of the dry, compressed core. Also preferably, the absorbent cores are compressed to a density between 0.12 g/cm³ and 0.40 g/cm³, with the corresponding equilibrium wet densities being between 0.08 g/cm³ and 0.12 g/cm³. Regarding crosslinked fibers with crosslinking levels between 2.0 mol.% and 2.5 mol.%, the earlier fibers are less stiff, thereby making them more suitable for compression to the higher density range. The earlier fibers also have higher Responsiveness to wetting in that they pop open after wetting at a faster rate and to a greater extent than do fibers with crosslinking levels within the 2.0 mole percent to 2.5 mole percent range, have higher wet resilience, and retain nearly as much absorbent capacity. It should be noted, however, that absorbent structures within the higher density range can be made from crosslinked fibers within the higher crosslinking level range, just as lower density absorbent structures can be made from crosslinked fibers with lower crosslinking levels. Improved performance relative to previously known individualized crosslinked fibers is achieved for all such structures.

While the foregoing discussion relates to preferred embodiments for high and low density absorbent structures, it should be noted that varying combinations of absorbent structure densities and crosslinking agent levels between the ranges disclosed herein will yield superior absorbent properties and integrity of the absorbent structure relative to conventional cellulosic-based fibers and previously known crosslinked fibers. Such embodiments are believed to be included within the scope of this invention.

METHOD FOR DETERMINING FLUID RETENTION VALUE

The following procedure was used to determine the water retention value of cellulose-based fibers. A sample of 0.3 g to 0.4 g of fibers is soaked in a covered container with 100 mL of 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 basket supported 38 mm above a 60-mesh (0.25 mm opening) sieve bottom of a centrifuge tube. The tube is sealed with a plastic cap and the sample is centrifuged at a relative centrifuge force of 1500 to 1700 G for 19 to 21 minutes. The centrifuged fibers are then removed from the basket and weighed. The weighed fibers are dried at 105º C to a constant weight and weighed again. The water retention value is calculated as follows:

(1) WRV = (W-D)/D · 100

where

W = wet weight of centrifuged fibres;

D = dry weight of fibres; and

W-D = weight of water absorbed.

METHOD FOR DETERMINING DRIP CAPACITY

The following procedure was used to determine the drip capacity of absorbent cores. Drip capacity was used as a combined measure of absorbency and absorption rate of the cores.

An absorbent pad, four inches by four inches, weighing 7.5 g, is placed on a mesh screen. Synthetic urine is applied to the center of the pad at a rate of 8 ml/sec. The flow of synthetic urine is stopped when the first drop of synthetic urine emerges from the bottom or sides of the pad. The drip capacity is calculated by the difference in the mass of the pad before and after the introduction of the synthetic urine divided by the mass of the fibers on a dry basis.

METHOD FOR DETERMINING WET COMPRESSIBILITY

The following procedure was used to determine the wet compressibility of absorbent structures. Wet compressibility was used as a measure of resistance to wet compression, wet structural integrity, and wet resilience of the absorbent cores.

A four-inch by four-inch square pillow weighing 7.5 g is prepared, its thickness measured and its density calculated. The pillow is filled with ten times its dry weight or to its saturation point, whichever is whichever is less, with synthetic urine. A compression load of 70.31 kg/m² is applied to the pad. After approximately 60 seconds, during which time the pad is allowed to equilibrate, the thickness of the pad is measured. The compression load is then increased to 773.38 kg/m², the pad is allowed to equilibrate, and the thickness is measured. The compression load is then reduced to 70.31 kg/m², the pad is allowed to equilibrate, and the thickness is measured again. 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 the 70.31 kg/m² rebound load. The interstitial volume, expressed in cm³/g, is then determined for each compression load. The interstitial volume is calculated as the reciprocal of the wet density of the pad minus the fiber volume (0.75 cc/g). The 70.31 kg/m² and 773.38 kg/m² interstitial volumes are valuable indicators of wet compression resistance and wet structural integrity. Higher interstitial volumes for an overall initial pad density indicate greater wet compression resistance and greater wet structural integrity. The difference between 70.31 kg/m² volume and 70.31 kg/m² rebound volume is useful for comparing wet resiliency of absorbent pads. A smaller difference between 70.31 kg/m² interstitial volume and 70.31 kg/m² rebound volume indicates higher wet resiliency.

The difference in grip height between the dry cushion and the saturated cushion before compression has also been shown to be a useful indicator of the responsiveness to wetting of the cushions.

METHOD FOR DETERMINING DRY COMPRESSIBILITY

The following procedure was used to determine the dry compressibility of absorbent cores. Dry compressibility was used to determine dry resilience of the cores as a measure of dry resilience of the cores.

A 100 mm x 100 mm square air-laid pad with a mass of 7.5 g is prepared and compressed in a dry state by a hydraulic press at a pressure of 241680 kg/m2. The pad is turned over and the pressing is repeated. The thickness of the pad is measured with an unloaded caliper before and after pressing. The density before and after pressing is then calculated as mass/(zone x thickness). Larger differences between the density before and after pressing indicate lower dry elasticity.

METHOD FOR DETERMINING LEVELS OF GLUTARALDEHYDE WHICH HAS REACTED WITH CELLULOSE-BASED FIBERS

The following procedure was used to determine the level of glutaraldehyde which reacted to form intrafiber crosslink bonds with the cellulosic component of the individualized glutaraldehyde crosslinked fibers.

A sample of individualized cross-linked fibers is extracted with 0.1 N HCl at 60ºC for one hour. The extract is separated from the fibers and the same extraction/separation procedure is then repeated three additional times for each sample. The extract from each extraction is mixed separately with an aqueous solution of 2,4-dinitrophenylhydrazone (DNPH). The reaction is allowed to continue for 15 minutes, after which one 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 performance liquid chromatography (HPLC) for the DNPH derivative.

The chromatography conditions for the HLPC analysis used were: column: C-18 reversed phase; detector: UV at 360 mm; mobile phase 80:20 methanol:water; flow rate 1 ml/min; measurement taken: peak height. A calibration curve of peak height and glutaraldehyde content was by measuring the HPLC peak heights of five standard solutions with known glutaraldehyde levels between 0 ppm and 25 ppm.

Each of the four chloroform phases for each fiber sample was analyzed by HPLC, the peak height measured, and the corresponding glutaraldehyde level determined from the calibration curve. The glutaraldehyde concentrations for each extraction were then summed and divided by the fiber sample weight (based on dry fiber) to give glutaraldehyde content based on fiber weight.

There were two glutaraldehyde peaks for each of the HPLC chromatograms. Any peak can be used as long as the same peak is used throughout the procedure.

EXAMPLE 1

This example shows the effect of varying levels of a cross-linking agent, glutaraldehyde, on the absorbency and resiliency of absorbent pads made from individualized cross-linked fibers. The individualized cross-linked fibers were made by a dry cross-linking process.

For each sample, a quantity of never-dried southern softwood kraft (SSK) pulp was provided. The fibers had a moisture content of 62.4% (equivalent to a 37.6% consistency). A slurry was formed by adding the fibers to a solution containing a selected amount of 50% aqueous glutaraldehyde solution, 30% (by weight of glutaraldehyde) zinc nitrate hexahydrate, demineralized water, and a sufficient amount of 1 N HCl to reduce 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 centrifugation. Next, the dewatered fibers were air dried to a fiber consistency of 55% to 56% using a through-air dryer using room temperature air. The air dried fibers were defibrated using a three-stage fluffing apparatus as described in U.S. Patent No. 3,987,968. The defibrated fibers were placed on pallets and allowed to cure at 345ºC in a substantially static drying oven for a period of 45 minutes. Crosslinking was completed during the dwell period in the oven. The crosslinked, individualized fibers were placed on a mesh screen and washed with 20ºC water, soaked in 60ºC water to a 1% consistency for one (one) hour, washed a second time with 20ºC water, centrifuged to 60% fiber consistency, defibrated in a three-stage fluffing apparatus as previously described, completely dried in a static drying oven at 105ºC 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/cm2. The pads were tested for absorbency, resiliency and amount of reacted glutaraldehyde according to the procedures defined herein. Reacted glutaraldehyde is reported in mole percent calculated on a dry fiber cellulose anhydroglucose basis. The results are presented in Table 1. TABLE 1 Sample Glutaraldehyde (mol.% added/addressed WRV (%) Drop Capacity 8 ml/s (g/g) Wet Compressibility (cc/g) * obtained from a separate sample of fibers (N/A) - not available (a) = 70.31 kg/m² (b) = 773.38 kg/m² (c) = 70.31 kg/m² Rebound

EXAMPLE 2

The purpose of this example is to demonstrate that low levels of extractable crosslinker can be obtained by subjecting the fibers to bleaching steps following crosslinking. The level of extractable crosslinker was determined by soaking a sample of the fibers in 40ºC deionized water at 2.5% consistency for one (1) hour. The glutaraldehyde extracted by the water was measured by HPLC and referred to as extractable glutaraldehyde based on dry fiber weight. The fibers were crosslinked by a dry crosslinking process.

Southern Softwood Kraft (SSK) pulp was provided. The pulp fibers were partially bleached by the following bleaching sequence steps:

Chlorination (C) - 3-4% consistency slurry treated with 5% available chlorine (av. Cl) at pH 2.5 and approximately 38ºC for 30 minutes; caustic extraction - 12% consistency slurry treated with 1.4 g/l NaOH at approximately 74ºC for 60 minutes; and hypochlorite treatment (H)

- 12% consistency slurry treated with sufficient sodium hypochlorite at 11-11.5 pH between 38ºC and 60ºC for 60 minutes to give a 60-65 Elrepho brightness and a 1.55-1.65 10-2 Pa s viscosity. The partially bleached fibers were processed into individualized crosslinked fibers using glutaraldehyde as the crosslinking agent in accordance with the procedure described in Example 1. The fibers retained 2.29 mole percent glutaraldehyde calculated on a dry fiber molar cellulosic anhydroglucose basis. Typically such fibers have extractable glutaraldehyde levels of 1000 ppm (0.1%).

Bleaching of the partially bleached individualized fibers was then continued and completed with a chlorine dioxide (D), extraction (E) and sodium hypochlorite (H) sequence (DEH). In the chlorine dioxide step (D), individualized cross-linked fibers were suspended in a 10% consistency aqueous slurry which also contained a sufficient amount of sodium hypochlorite to give 2% available chlorine on a dry fiber weight basis. After mixing, the pH of the slurry was reduced to pH 2.5 by adding HCl and then raised to pH 4.4 by adding NaOH. The pulp slurry was then placed in a 70ºC oven for 2.5 hours, sieved, flushed with water to neutral pH and centrifuged to 61.4% consistency.

In the extraction step, a 10% consistency aqueous slurry of the dewatered fibers was treated with 0.33 NaOH/liter water for 1.5 hours at 40ºC. The fibers were then sieved, washed with water to neutral pH and centrifuged to 62.4% consistency.

Finally, for the sodium hypochlorite step (H), a 10% consistency slurry of the fibers containing enough sodium hypochlorite to yield 1.5% available chlorine based on dry fiber weight was prepared. The slurry was mixed and heated in a 50ºC oven for one hour. The fibers were then sieved, floated to pH 5.0 and centrifuged to 62.4% consistency. The dewatered fibers were air dried, flaked and completely dried in a 105ºC oven for one hour. The extractable glutaraldehyde level 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 where the fibres are used in proximity to human skin.

It was also found that pads made from fibers that had been partially bleached, crosslinked and then fully bleached had unexpectedly higher fluid retention value and wicking rate and at least equivalent drip capacity and wet resilience as individualized fibers that had been crosslinked following full bleaching. As a result of the higher water retention value, however, the fibers that had been the bleaching sequence are more compressible in a dry state.

Substantially equivalent results were obtained when a peroxide bleaching step (P) was substituted for the final hypochlorite step (H). In the P step, a 10% consistency slurry was treated with 0.5% hydrogen peroxide by fiber weight at 11-11.5 pH and 80ºC for 90 minutes.

Claims (5)

1. A process for producing individualized, cross-linked cellulose-based fibers, wherein said process comprises the steps of: a) providing cellulosic fibers and contacting said fibers in an aqueous solution with a sufficient amount of a crosslinking agent selected from C2-C8 dialdehydes, acid analogs of said dialdehydes, and oligomers of said dialdehydes and said acid analogs to yield, upon reaction, from 0.5 mole % to 3.5 mole % crosslinking agent on a cellulose anhydroglucose molar basis so as to impart to said reacted fibers a water retention value of from 28 to 45; b) dewatering said fibres to a consistency of 30% to 80% fibres by weight and defibrating said fibres to a substantially individual shape; c) air drying said fibers while they are being individualized under conditions preventing fiber-to-fiber contact, reacting said crosslinking agent with said fibers to form intrafiber crosslink bonds in the substantial absence of interfiber bonds; and d) washing said crosslinked fibers with an alkaline solution having a pH greater than 7 and containing a component for decomposing hemiacetal bonds while being neutral toward acetal bonds, thereby reducing the amount of unreacted and residual crosslinking agent. 2. A method according to claim 1, wherein said alkaline solution has a pH greater than pH 9. 3. A process according to any one of claims 1 and 2, wherein said fibers are contacted with a sufficient amount of crosslinking agent to yield upon reaction from 0.5 mole % to 2.5 mole % crosslinking agent, calculated on a molar cellulose anhydroglucose basis. 4. A process according to any one of claims 1 to 3, wherein said cellulosic fibers of step a) are partially bleached by at least one step of a multi-stage bleaching sequence. 5. A process according to any one of claims 1 to 4, wherein the component of said alkaline solution of step d) comprises a compound selected from sodium hypochlorite, ammonium hydroxide, hydrogen peroxide and sodium bisulfite.