FI94435C - Cross - linked, twisted and twisted discrete cellulose fibers and method for their production - Google Patents

Abstract

Individualized, crosslinked fiber, and process for making such fibers. The individualized, crosslinked fibers have between about 0.5 mole % and about 3.5 mole % crosslinking agent, calculated on a cellulose anhydroglucose molar basis, reacted with fibers in the form of intrafiber crosslink bonds, wherein the crosslinking agent is selected from the group consisting of C2 - C8 dialdehydes, C2 - C8 dialdehyde acid analogues having at least one aldehyde functionality, and oligomers of such C2 - C8 dialdehydes, and dialdehyde acid analogues. Preferably, the crosslinking agent is glutaraldehyde, and between about 0.75 mole % and about 2.5 mole % crosslinking agent react to form the intrafiber crosslink bonds. The individualized crosslinked fibers are useful in a variety of absorbent sturcture applications.

Description

94435

This invention relates to cellulosic fibers having good liquid absorption properties, and to methods for making such fibers. More particularly, this invention relates to absorbent cellulosic fibers and methods of making such fibers utilizing cellulosic fibers in a discrete, crosslinked form.

Fibers that have been crosslinked in a substantially separate form, as well as various methods for making such fibers, have been described in the art. The term "discrete, crosslinked fibers" refers to cellulose-15 fibers having predominantly intra-fiber chemical crosslinking bonds. That is, the crosslinking bonds are primarily between the cellulose molecules of a single fiber and not between the cellulose molecules of the individual fibers. Separate, crosslinked fibers - 20 and are generally considered useful in absorbent article applications. In general, three categories of methods have been proposed for making discrete, crosslinked fibers. These methods described below are referred to herein as: 1) dry crosslinking methods, 2) aqueous crosslinking methods, and 3) substantially anhydrous crosslinking methods. The fibers themselves and the absorbent structures containing discrete, crosslinked fibers are superior in at least one significant absorption property compared to conventional, non-crosslinked fibers. Often, this improvement in absorbency is expressed as absorption capacity. In addition, absorbent structures made of discrete, crosslinked fibers have improved wet flexibility and improved dry flexibility compared to non-crosslinked fibers. 94435 2 made of absorbent structures. The term "flexibility" as used herein refers to the ability of "pads" made from cellulosic fibers to return to an expanded original state after being released from the crimp. By dry resilience is meant in particular the ability of an absorbent structure to expand after the release of a compressive force applied to the structure when the fibers are in a substantially dry state. Wet elasticity refers to the ability of an absorbent structure to expand after the release of a compressive force applied to it when the fibers are in a wetted state. For purposes of this invention and the consistency of the disclosure, wet flexibility is contemplated and reported for the absorbent structure when it is wetted to the point of saturation.

Methods for making discrete crosslinked fibers using dry crosslinking technology are described in U.S. Patent 3,224,926, published to L.J. In the name of Bernardin, December 21, 1965. Separate, crosslinked fibers are made by impregnating swollen fibers in an aqueous solution with a crosslinking agent, dewatering them, and mechanically defibrating the fibers and drying the fibers at elevated temperature to provide crosslinking with substantially separate fibers. The fibers are inherently crosslinked in a non-swollen, compressed state as a result of being dewatered prior to crosslinking. Methods exemplified in U.S. Patent 3,224,926, in which crosslinking is effected with the fibers in a non-swollen, compressed state, are termed methods for making "dry crosslinked" fibers. Dry crosslinked fibers are characterized by low fluid retention values (FRV). U.S. Patent 3,440,135, issued to R. Chung on April 22, 1969, proposes that the fibers be soaked in an aqueous solution of a crosslinking agent to reduce the ability of the fibers to bind between the fibers before performing a dry crosslinking treatment in a manner similar to U.S. Patent 3,224,926 discloses. This time-consuming pretreatment, which preferably lasts about 16 to 48 hours, is claimed to improve product quality by reducing the number of small fiber bundles resulting from incomplete defibration.

Methods for making crosslinked fibers in aqueous solution are described, for example, in U.S. Patent 10,241,553, issued to F.H. In the name of Steiger 22.

March 1966. Separate, crosslinked fibers are prepared by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers prepared in this manner are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling effect of water on the cellulosic fibers, the fibers crosslinked in the aqueous solution crosslink when they are in a non-compressed swollen state. Compared to dry crosslinked fibers, aqueous crosslinked fibers have, as described in U.S. Patent 3,241,553, greater flexibility and lower stiffness, and are characterized by a higher fluid retention value (FRV). Absorbent structures made of crosslinked fibers in aqueous solution have a lower wet and dry spring capacity than pads made of dry crosslinked fibers.

U.S. Patent 4,035,147, issued to S. Sennis, G. Guiroy, and J. Quere on July 12, 1977, discloses a process for making discrete, crosslinked fibers by subjecting dehydrated, non-swollen fibers. contact with the crosslinking agent and the catalyst in a substantially anhydrous solution containing an amount of water insufficient to swell the fibers. Crosslinking occurs with the fibers in this substantially anhydrous solution. This type of method is hereinafter referred to as an anhydrous solution crosslinking process, and the fibers thus prepared are referred to as anhydrous solution crosslinked fibers. The fibers crosslinked in the anhydrous solution described in U.S. Patent 4,035,147 do not swell even after prolonged contact with solutions known to those skilled in the art to be edema reagents. Like dry crosslinked fibers, they are strongly stiffened by crosslinking bonds and the absorbent structures made from them have a relatively high wet and dry elasticity.

The crosslinked fibers described above are believed to be useful for low density absorbent product applications, such as baby diapers, as well as for higher density absorbent product applications such as sanitary napkins. However, taking into account their disadvantages and costs compared to conventional fibers, such products have not received sufficient absorption benefits to result in significant commercial success in the '20s. The commercial appeal of crosslinked fibers has also suffered in terms of safety. Unfortunately, the most widely used cross-linking agent in the literature, formaldehyde, causes irritation to human skin and has been associated with other human safety concerns. The removal of free formaldehyde to sufficiently low levels in a crosslinked product to avoid skin irritation and other human safety concerns has been hampered by both technical and economic barriers.

For example, dry crosslinked fibers and fibers crosslinked in an anhydrous solution have generally resulted in fibers having excessive stiffness and dry elasticity, making them difficult to convert to compact sheets for transport and subsequently re-fluffed without damaging the fibers. In addition, when compressed in a dry state, pads made of these fibers have had a low susceptibility to wetting. That is, once compressed in a dry state, they have not shown the ability to recover a substantial amount of their previous absorbency upon wetting.

Another difficulty encountered when it comes to dry and non-aqueous solution of crosslinked fibers is that the fibers rapidly form flocs 10 when deposited on a water-permeable forming wire. This has prevented the formation of absorbent, water-layered structures as well as the formation of dense sheets, which would facilitate the economical transport of the fibers to the processing plant.

15 Although aqueous crosslinked fibers are useful for certain high-density absorbent pad applications, such as wound dressings, tampons, and sanitary napkins, which typically have densities of about 0.40 g / cm 3, they are too flexible in a wetted • 70 state and therefore result in absorbent structures with low wet elasticity. In addition, upon wetting, the fibers crosslinked in the aqueous solution become too flexible to structurally support the pad when the fiber density is low. The wetted pad 25 therefore collapses and loses its absorbency.

It is an object of the present invention to provide discrete, crosslinked fibers. Absorbent structures made from such crosslinked fibers have high absorption results compared to absorbent structures made from non-crosslinked fibers, have higher wet flexibility and lower dry elasticity than structures made from previously known dry crosslinked and anhydrous solution. 35 structural integrity than structures made of previously known aqueous solution crosslinked fibers.

It is a further object of this invention to provide discrete crosslinked fibers as described above having improved wettability compared to previously known crosslinked fibers and conventional non-crosslinked fibers.

It is a further object of this invention to provide commercially useful discrete crosslinked fibers as described above. Absorbent structures made of such fibers can be used without concern in the vicinity of human skin.

It is a further object of this invention to provide a method of forming discrete, crosslinked fibers in the form of sheets layered in water.

The present invention thus relates to crosslinked, twisted and curled discrete cellulose fibers, the fibers comprising cellulose fibers in a substantially discrete form and kept substantially discrete during drying and crosslinking, wherein the crosslinking agent is selected from the group consisting of C2-C8 dialdehydes, acid analogues of said dialdehydes, wherein one aldehyde group of each of said dialdehydes is replaced by a carboxyl group, and oligomers of said dialdehydes and dialdehyde acid analogs, the fibers being characterized in that the fibers react with 0.5 mole to 3.5 mole mole of crosslinking agent based on cellulose anhydride that the moisture content of the fibers reacting with the crosslinking agent is not more than 30% by weight of the dry fibers and that the crosslinking agent has reacted sufficiently with the fibers in the form of intramibrous cross-linked bonds to give the fibers a water retention. horn 28 - 45.

The process according to the invention for the production of these twisted, crosslinked discrete cellulose fibers

Il «Η <im; i I i M: - 1 to 94435 7 is characterized in that the drying step d) is carried out to obtain partially swollen fibers having a moisture content of not more than 30% by weight, that the reaction step e) is carried out in such a way that 0.5 mol% to 3, 5 mol% of crosslinking agent, based on the molar amount of cellulose anhydroglucose, is reacted with the fibers to form intra-fiber crosslinks, the water retention value of the fibers after the drying and crosslinking steps being 28-45.

The fibers are preferably crosslinked in a highly helical state. In the most preferred embodiments, the fibers are contacted with the crosslinking agent and catalyst in aqueous solution, dewatered, mechanically separated into a substantially separate form, and then dried and crosslinked under substantially unobstructed conditions. The dewatering step, the mechanical separation step, and the drying step allow the fibers to rotate greatly before crosslinking. The helical state is then preserved, at least in part, though less than completely, as a result of networking. Alternatively, the cellulosic fibers may be crosslinked in a separate form in a substantially anhydrous crosslinking solution containing a water-miscible polar solvent such as acetic acid and an amount of water insufficient to swell the fibers to more than 30% water at a moisture content of the fibers.

Various cellulosic fibers of natural origin are applicable to the invention. Preferably, fibers obtained by softening softwood, hardwood or cotton linters are used. Fibers derived from esparto grass, bagasse, Kemp-hair, flax, and other sources of wood and cellulose fibers can also be used as a raw material in the invention. The fibers can be supplied in slurry, non-sheeted or sheeted form. Fibers delivered as a wet pulp sheet, dry pulp sheet, or other sheeted form are preferably brought into a non-sheeted form by mechanically disintegrating the sheet, preferably prior to contacting the fibers with the crosslinking agent. The fibers are preferably also placed in a wet or humidified state. It is most preferred that the 5 fibers have never been dried before. In the case of a dry pulp sheet, it is preferable to moisten the fibers before mechanical disintegration to minimize damage to the fibers.

The optimal fiber source used in connection with this invention depends on the particular end use intended. Pulp fibers made by chemical pulping methods are generally preferred. Fully bleached, partially bleached and unbleached fibers are suitable for use. It may often be desirable to use bleached pulp because it has excellent brightness and appeals to the consumer. In a new embodiment of the invention, which will be described in more detail below, the fibers are partially bleached, crosslinked and then finally bleached. For products such as paper towels and absorbent pads for baby diapers, wound dressings, sanitary napkins and other similar absorbent paper products, it is particularly advantageous to use fibers derived from southern softwood pulp due to their excellent absorption properties.

Crosslinking agents suitable for use in the present invention include C2-C8 dialdehydes as well as acid analogs of such di-aldehydes having at least one aldehyde group in the acid analogs, and oligomers of such dialdehydes and acid analogs. Such compounds are capable of reacting with at least two hydroxyl groups present in a single cellulose chain or in adjacent cellulose chains in a single fiber. It will be appreciated by those skilled in the art of crosslinking agents that the dialdehyde crosslinking agents described above may exist or react in a variety of forms, which are the acid analog and oligomeric forms identified above. All such forms are intended to be included within the scope of the invention. Therefore, the reference below to a particular crosslinking agent refers to this particular crosslinking agent as well as other forms that may be present in aqueous solution. Particular crosslinking agents for use in the invention include glutaraldehyde, glyoxal and glyoxylic acid. Glutaraldehyde is particularly preferred because it provides fibers with the highest absorbency and elasticity values, is believed to be harmless and non-irritating to human skin when reacted, in a crosslinked state, and provides the strongest crosslinking bonds. Monoaldehyde compounds without an additional carboxyl group, such as acetaldehyde and furfural, have not been found to produce absorbent structures with the desired absorbency, flexibility, and wettability values.

Surprisingly, it has been found that a better effective absorption pad can be achieved by using amounts of crosslinking agent 20 that are significantly less than the amounts of crosslinking agent previously used. In general, unexpectedly good results are obtained with absorbent pads made of discrete, crosslinked fibers containing from about 0.5 mol% to about 3.5 mol% of crosslinking agent, calculated on a molar basis of cellulose anhydroglucose, reacted with the fibers.

The crosslinking agent is preferably contacted with the fibers in a liquid medium under conditions such that the crosslinking agent penetrates the individual fibrous structures. Other methods of treatment with a crosslinking agent, including spraying the fibers in a separate, fluffed state, are also within the scope of the invention.

The fibers are also generally contacted with a suitable catalyst prior to crosslinking. The type, amount and method of contacting the catalyst with the fibers depends on the particular crosslinking method used. These options are described in more detail below.

After the fibers have been treated with a crosslinking agent and a catalyst, the crosslinking agent is reacted with the fibers with the fibers in a state such that there are essentially no bonds between the fibers, i. wherein the interfiber contacts 10 are kept low compared to pulp fibers which have not been fluffed, or the fibers are immersed in a solution which does not facilitate the formation of interfiber bonds, in particular hydrogen bonds. This results in the formation of crosslinking bonds which are fibrous in nature. Under these conditions, the crosslinking agent reacts to form crosslinking bonds between the hydroxyl groups of a single cellulose chain or between the hydroxyl groups of adjacent cellulose chains in a single cellulose fiber.

Although not 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 bonds, which are believed to be the desired type of bonds that provide strong cross-linking bonds, is favored by acidic reaction conditions. Therefore, acid-catalyzed crosslinking conditions are highly preferred for the purposes of this invention.

The fibers are preferably mechanically defibrated to a low density, formed by discrete fibers. before the crosslinking agent is reacted with the fibers. Mechanical defibration can be performed by a variety of methods currently known in the art or which may become known subsequently. The mechanical defibration is preferably carried out by a method in which the formation of cultivars and damage to the fibers are kept to a minimum. One type of device that has been found to be particularly useful for defibrulating cellulosic fibers is the three-stage flocculation device described in U.S. Patent 3,987,968 to D.R. Moore and O.A. In the name of Shields on October 26, 1976, which patent is incorporated herein by reference. The fluffing device described in U.S. Patent 3,987,968 applies to wet cellulosic pulp fibers a treatment combination that includes mechanical shocks, mechanical agitation, air agitation, and limited air drying to provide a substantially fibrous button-free fluff in the fiber. The discrete fibers have been given an increased degree of curl and twist compared to the degree of curl and twist inherent in such fibers. It is believed that this additional curl and twist improves the flexibility property of the absorbent structures made of crosslinked fibers thus treated.

Other useful methods for defibrating cellulosic fibers include, but are not limited to, treatment with a Waring Blender and tangential contact of the fibers with a rotary disc grinder 25 or pad. It is preferred that the air flow be directed against the fibers during such defibration to facilitate separation of the fibers substantially separately.

Regardless of the particular mechanical device used to form the fluffy pulp, the fibers are preferably mechanically treated, initially containing at least about 20% moisture and preferably about 40% to about 60% moisture.

Mechanical grinding of very thick pulp or partially dried fibers can also be used to obtain 12,94435 curl or twist in the fibers in addition to the curl or twist provided as a result of mechanical defibration.

The fibers of the present invention have a unique combination of rigidity and flexibility that allows the absorbent structures made of the fibers to maintain good absorbency and high flexibility values, as well as the susceptibility to expansion for wetting the dry, compressed absorbent structure. In addition to the degrees of crosslinking within said limits, the crosslinked fibers are characterized by water retention values (WRV) of less than about 60 and preferably between about 28 and 45 in the case of conventional chemically fiberized pulp fibers.

15 The WRV of a particular fiber indicates the degree of crosslinking and the degree of fiber swelling at the time of crosslinking. It will be appreciated by those skilled in the art that the more swollen the fiber at the time of crosslinking, the higher the WRV for a given degree of crosslinking. Very strongly crosslinked fibers, such as those made by the previously known dry crosslinking methods mentioned above, have been found to have WRV numbers less than about 25 and generally less than about 20. The particular crosslinking method used naturally affects the WRV of the 25 crosslinked fibers. Any method that results in degrees of networking and WRV numbers within the stated limits is believed to be within, and is intended to be, within the scope of this invention. Useful crosslinking methods include dry crosslinking methods and crosslinking methods performed in an anhydrous solution, as generally described in connection with the description of the background of the invention. Some of the preferred dry crosslinking methods within the scope of the present invention and crosslinking methods performed in an anhydrous solution are described in more detail below. Networking in aqueous solution

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94435 13 methods in which the solution causes the fibers to swell strongly result in fibers having WRVs greater than about 60. The stiffness and flexibility of such fibers are insufficient for the purposes of the present invention.

With particular reference to dry crosslinking methods, discrete, crosslinked fibers can be prepared by such a method by contacting an amount of cellulosic fibers, contacting the fiber slurry with a certain type and amount of the crosslinking agent described above, separating the fibers mechanically, e.g. react with the fibers in the presence of a catalyst to form crosslinking bonds 15 while keeping the fibers substantially separate. The defibration step is assumed to produce additional curl, regardless of the drying step. Subsequent drying is followed by twisting of the fibers, whereby the curled geometry of the fiber improves the degree of twisting. As used herein, "curl" of a fiber means the geometric curvature of the fiber about the longitudinal axis of the fiber. "Thread" means the rotation of a fiber about a longitudinal cross-section perpendicular to the longitudinal axis of the fiber. It should be mentioned only by way of example and without in any way specifically limiting the scope of the invention that the discrete, crosslinked fibers within the scope of the invention have been found to have an average of about six turns per mm of fiber.

Keeping the fibers substantially separate during drying and crosslinking allows the fibers to rotate during drying and thus crosslink in such a twisted, curled state. By drying the fibers in such a way that the fibers can twist and curl is meant drying the fibers under substantially unobstructed conditions. On the other hand, drying the fibers in the sheeted form 35 results in dried fibers that are not 94435 14 twisted and curled like substantially separately dried fibers. It is believed that the hydrogen bond between the fibers "prevents" the relative twisting and curling of the fiber.

There are methods by which the fibers can be contacted with a crosslinking agent and a catalyst. In one embodiment, the fibers are contacted with a solution that initially contains both a crosslinking agent and a catalyst. In another embodiment, the fibers are contacted with an aqueous solution of the crosslinking agent and allowed to soak prior to the addition of the catalyst. The catalyst is then added. In a third embodiment, a crosslinking agent and a catalyst are added to the aqueous slurry of cellulose fibers. Other methods of addition in addition to those set forth above 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 allowed to soak sufficiently with the fibers to ensure thorough contact with and impregnate the individual fibers.

In general, any substance that catalyzes the crosslinking mechanism can be used. Useful catalysts include organic acids and acid salts. Particularly preferred catalysts are the aluminum, magnesium, zinc and calcium salts of chlorides, nitrates or sulphates. A particular example of a preferred salt is zinc nitrate hexahydrate. Other catalysts include acids such as sulfuric acid, hydrochloric acid and other mineral acids, as well as organic acids. The selected catalyst can be. may be used as the sole catalyst or in combination with one or more other catalysts. Combinations of acid salts and organic acids as catalysts are believed to produce an excellent crosslinking reaction result. The reaction is unexpectedly advanced to catalyst combinations of zinc nitrate xiols 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 when in contact with the crosslinking agent in solution, but are not preferred for use as the primary catalyst.

The optimal amount of crosslinking agent and catalyst used depends on the particular crosslinking agent used, the reaction conditions, and the intended use of the product in question.

The amount of catalyst preferably used will, of course, depend on the type and amount of crosslinking agent in question, as well as the reaction conditions, in particular temperature and pH. From a technical and economic point of view, amounts of catalyst ranging from about 10% to about 60% by weight of the crosslinking agent added to the cellulosic fibers are generally preferred. By way of example, in the case where the catalyst used is zinc nitrate hexahydrate and the crosslinking agent is glutaraldehyde, the amount of catalyst is preferably about 30% by weight based on the amount of glutaraldehyde added. Most preferably, about 5% to about 30% of an organic acid, such as citric acid, based on the weight of glutaraldehyde, is also added as a catalyst. In addition, it is desirable to adjust the aqueous portion of the cellulosic fiber slurry or crosslinking agent solution to a desired pH between about pH 2 and about pH 5, more preferably between about pH 2.5 and about pH 3.5, during contact between the crosslinking agent and the fibers.

Cellulose fibers should generally be dewatered and possibly dried. The processable and optimal consistencies vary depending on the type of fluffing device used. In preferred embodiments, the cellulose fibers are dewatered and dried to an optimum consistency of between about 30% and about 80%. More preferably, the fibers are dewatered and dried to a degree of consistency of between about 40% and about 60%. Drying the fibers within these preferred limits 5 facilitates defibrating the fibers into a discrete form without excessive fiber knob formation associated with high moisture contents and without high fiber damage associated with low moisture contents.

Dewatering can be performed, for example, by methods such as mechanical compression, centrifugation, or air drying of the pulp. Further drying is preferably performed by methods known in the art as air drying or flash drying under conditions that do not require the use of high temperatures for extended periods of time. Using too high a temperature at this stage of the process can result in premature onset of crosslinking.

It is preferred that temperatures above about 160 ° C are not maintained for more than 2-3 seconds. Mechanical defibration is performed as described above.

The defibrated fibers are then heated to a suitable temperature for an effective length of time to effect curing of the crosslinking agent, i. to react with cellulose fibers. The rate and degree of crosslinking depends on the dryness of the fibers, the temperature, the amount and type of catalyst and crosslinking agent, and the method used to heat and / or dry the fibers during crosslinking. The crosslinking at a certain temperature takes place at a higher rate, in the case of fibers with a certain initial moisture content, by continuously passing air during drying than by carrying out drying / heating in a static oven. It will be appreciated by those skilled in the art that there are numerous thermal-35 space / time relationships for the curing agent to cure. customary

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94435 17 paper drying temperatures (e.g., 49 ° C to about 66 ° C) with treatment times between about 30 minutes and 60 minutes provide generally acceptable static, atmospheric curing results for question 5 for fibers having a moisture content of less than about 5%. Those skilled in the art will also appreciate that higher temperatures and airflow reduce the time required for curing. However, curing temperatures are preferably kept below about 160 ° C, as exposure of the fibers to temperatures as high as about 160 ° C can result in yellowing or other damage to the fibers.

The maximum degree of crosslinking is achieved when the fibers are substantially dry (having a moisture content of less than about 5%). Due to this lack of water, the fibers crosslink in a substantially non-swollen, compressed state. As a result, they typically have low fluid retention (FRV) values compared to the range useful in this invention. The FRV number refers to the amount of liquid, calculated from the dry fibrous material, that remains absorbed in a fiber sample that has been soaked and then centrifuged to remove the interfiber liquid. (The term FRV is defined in more detail below and the method for determining the FRV is described). The amount of liquid that can be absorbed by the crosslinked fibers depends on their ability to swell when saturated, i.e., their inner diameter or volume as they swell to a maximum. This in turn depends on the degree of networking. As the degree of internal crosslinking of the fiber increases to 30 for a given fiber and method, the FRV of the fiber decreases until the fiber does not swell at all. In this case, the FRV number of the fiber structurally describes the physical state of the fiber at the saturation point. Unless explicitly stated otherwise, the FRV results shown below 35 are reported as fiber retention values (WRV) for the fibers. Other liquids, such as brine and synthetic urine, may also be advantageously used as a liquid medium for analysis. In methods where curing is highly dependent on drying, such as the present method, the FRV of a particular crosslinked fiber is generally primarily dependent on the crosslinking agent and the degree of crosslinking. The WRVs of the fibers crosslinked by this dry crosslinking process, with amounts of crosslinking agent useful in the present invention, are generally less than about 50, greater than about 25, and preferably between about 28 and about 45. Bleached SSK fibers having about 0 , 5 mole percent to about 2.5 mole percent glutaraldehyde, calculated on a molar basis of cellulose-15 anhydroglucose, has been found to have WRV numbers in the range of about 40 to about 28, respectively. The degree of bleaching and post-crosslinking bleaching apart. This effect is examined in more detail below. Southern coniferous kraft pulp (SSK) fibers prepared by the crosslinking methods prior to the present invention have higher degrees of crosslinking than disclosed herein and have WRV numbers less than about 25. As previously mentioned, such fibers have been found to be highly rigid and found to have lower absorbency than the fibers of the present invention.

In another method for producing discrete crosslinked fibers by the dry crosslinking method, the cellulose fibers are contacted with a solution containing a crosslinking agent as described above. Either before or after contact with the crosslinking agent, the fibers are formed into a sheet. The crosslinking agent solution preferably also contains a catalyst useful in dry crosslinking processes such as those described above. The fibers are dried in the form of a sheet and are preferably crosslinked by heating the fibers at a temperature between about 120 ° C and about 160 ° C. After crosslinking, the fibers are mechanically separated into a substantially separate shape. This is preferably accomplished by treating the fibers with a fluffing device, such as that disclosed in U.S. Patent 3,987,968, or by other methods known in the art for defibrating fibers. The discrete, crosslinked fibers prepared by this sheet crosslinking method are treated with a sufficient amount of crosslinking agent so that about 0.5 mole percent to about 3.5 mole percent crosslinking agent 15, calculated on a molar basis of cellulose anhydroglucose and measured after defibration, reacts with the fibers to form intracellular bonds. Another effect of drying and crosslinking the fibers in the form of a sheet is that the inter-fiber si-20 dos prevents the fibers from twisting and curling as drying increases. Compared to discrete, crosslinked fibers made by a process in which the fibers are dried under substantially unobstructed conditions and then crosslinked in a twisted, curled configuration, it could be expected that absorbent structures made from the sheet twisted sheets prepared by the sheet-drawn stretching method described above has lower wet flexibility and lower wettability of the dry absorbent structure.

Another category of crosslinking methods useful in the present invention are curing crosslinking methods performed in an anhydrous solution. Fibers of the same type useful in dry crosslinking methods can be used to make fibers crosslinked in anhydrous solution. The fibers are treated with a sufficient amount of crosslinking agent so that about 0.5 mole percent to about 3.5 mole percent crosslinking agent is then reacted with the fibers to calculate the amount of reacted crosslinking agent after said crosslinking reaction, and using a suitable catalyst. The web-forming agent is reacted while the fibers are immersed in a solution that does not cause any substantial swelling of the fibers. However, the fibers may contain up to about 30% water or be otherwise swollen in the crosslinking-10 solution to the extent that this corresponds to fibers with a moisture content of about 30%. Such a partially swollen fiber geometry has been found to have unexpected additional benefits, as will be explained in more detail below. The crosslinking solution contains, but is not limited to, an anhydrous, water-miscible, polar diluent such as, for example, acetic acid, propionic acid, or acetone. Preferred catalysts are mineral acids such as sulfuric acid, and halogen acids such as hydrochloric acid. Other useful catalysts include salts of mineral acids and halogen acids, organic acids and their salts. Crosslinking solution systems suitable for use as a crosslinking medium also include those disclosed in U.S. Patent 4,035,147, issued to S. Sangenis, G. Guiroy, and J. Quere, July 12, 1977. which patent is incorporated herein by reference. The crosslinking solution may contain some water or other fiber swelling liquid, although the amount of water is preferably insufficient to provide an amount of swelling corresponding to a consistency of 70% (30% water content) of the pulp. Water contents of the crosslinking solution that are less than about 10% of the total female volume of the solution, excluding fibers, are preferred. Higher water concentrations in the crosslinking solution reduce the crosslinking efficiency and rate.

• 94435 21

The absorption of the crosslinking agent into the fibers can be accomplished in the crosslinking solution itself or in a pretreatment step, which includes, but is not limited to, impregnation of the fibers with either an aqueous or non-aqueous solution containing the crosslinking agent. The fibers are preferably mechanically defibrated into a separate shape. This mechanical treatment can be performed by the methods previously described for fluffing fibers in connection with the dry crosslinking method previously described.

It is particularly advantageous to include in the manufacture of the fluffy pulp a mechanical treatment which causes the moist cellulosic fibers to assume a curled or twisted space in an amount greater than the amount of curl or twist that the fibers have in their natural state, if any. This can be accomplished by initially obtaining fibers for fluffing that are in a moist state, subjecting the fibers to a mechanical treatment as described above to de-fibrousize the fibers to a substantially separate shape, and dry the fibers at least in part.

The relative amounts of curl and twist provided to the fibers are in part dependent on the moisture content of the fibers. Without limiting the scope of the invention, it is believed that the fibers are inherently rotated upon drying under conditions such that contact between the fibers is minimal, i. when the fibers are in a separate form. Mechanical treatment of wet fibers also initially causes the fibers to curl. If the fibers are then dried or partially dried under substantially unobstructed conditions, they will rotate as the degree of twisting increases due to the additional mechanical curl applied to the fibers. The defibrating flocculation steps are preferably performed on a very thick, moist pulp, i.e., pulp from which so much water has been separated that the fiber density is between about 45% and about 55% (determined prior to initiating defibration).

• 94435 22

After defibration, the fibers should be dried to a moisture content of between 0% and about 30% before being contacted with the crosslinking solution, unless the defibration step has already produced fibers having a moisture content within said range. The drying step should be performed with the fibers under substantially unobstructed conditions. In other words, the contact between the fibers should be kept to a minimum so that the inherent rotation of the fibers is not prevented during drying.

10 Both air drying and flake drying methods are suitable for this purpose.

The discrete fibers are then contacted with a crosslinking solution containing a water-miscible, anhydrous diluent, crosslinking agent 15 and catalyst. The crosslinking solution may contain a limited amount of water. The water content of the crosslinking solution should be less than about 18% and preferably less than about 9%.

The fibrous sheet, which is not mechanically defibrated, can also be contacted with the crosslinking solution described above.

The amounts of crosslinking agent and acid catalyst used depend on reaction conditions such as consistency, temperature, water content in the crosslinking solution and fibers, the type of crosslinking agent and diluent in the crosslinking solution, and the degree of crosslinking desired. The amount of crosslinking agent used is preferably from about 0.2% to about 10% by weight (based on the total fiber-free weight of the crosslinking solution). In addition, the concentration of the preferred acid catalyst is dependent. acidity of the catalyst in the crosslinking solution. Good results can generally be obtained with a catalyst content, such as hydrochloric acid, from about 0.3% to about 5% by weight (based on the fiber-free crosslinking solution) in crosslinking solutions containing acetic acid. 94435 23 as an excipient, preferred amounts of glutaraldehyde and a limited amount of water. Sludges formed by fibers and a crosslinking agent solution having fiber contents of less than about 10% by weight are preferred for crosslinking in connection with the crosslinking solutions described above.

The crosslinking reaction can be carried out at ambient temperatures or at elevated temperatures to accelerate reaction rates, which are preferably below about 40 ° C.

There are many methods by which fibers can be contacted and crosslinked with a crosslinking solution. In one embodiment, the fibers are contacted with a solution that initially contains both a crosslinking agent and an acid catalyst. The fibers are allowed to soak in the crosslinking solution, during which time the crosslinking occurs. In another embodiment, the fibers are contacted with a diluent and allowed to soak prior to the addition of the acid catalyst. An acid catalyst is then added, at which point crosslinking begins. In addition to those described, there are other methods that 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 under which the crosslinking is performed are preferably selected to facilitate intra-fiber crosslinking. Thus, it is preferred for the crosslinking reaction that it occurs substantially after the crosslinking agent has had sufficient time to penetrate the fibers. The reaction conditions 30 are preferably selected so as to avoid immediate crosslinking unless the crosslinking agent has already penetrated the fibers. Reaction times in which crosslinking occurs substantially completely within about 30 minutes are preferred. Longer reaction times are expected to provide minimal benefit in the properties of the fibers. Both shorter processing times, including substantially immediate networking, and longer processing times are considered to be within the scope of this invention.

5 It is also possible to carry out only partial curing in solution and then to complete the crosslinking reaction later in the process by means of drying or heating treatments.

After the crosslinking step, the fibers are stripped of nes-10 te and washed. A sufficient amount of a basic substance, such as sodium hydroxide, is preferably added in the washing step to neutralize any acid remaining in the pulp. After washing, the fibers are separated into a liquid and finally dried. The fibers are preferably subjected to a second mechanical defibrillation step which causes the crosslinked fibers to curl, e.g. by fluffing by means of defibering between the liquid separation and the drying step. In drying, the curly state of the fibers provides additional twisting, as described above in connection with the curling treatment, prior to contacting the fibers with the crosslinking solution. The same devices and methods for producing 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 "defibrating" refers to any method that can be used to mechanically separate fibers into a substantially separate shape, even if the fibers may have already taken such a shape. "Defibrillation" therefore means a step of mechanically treating the fibers, either in separate form or in a fused form, a mechanical treatment step which a) separates the fibers into a substantially separate form if they were not already in such form, and b) gives curl and twist to the fibers. le during drying.

25 94435 This second defibration treatment, after the fibers have been crosslinked, has been found to increase the helical, curly nature of the pulp. This addition in the twisted, curled configuration of the fibers improves the flexibility and wettability of the absorbent structure. The second defibration treatment can be performed on all of the crosslinked fibers described herein that are in a moist state. However, one particular advantage of the crosslinking process performed in the anhydrous solution is that a second defibration step is possible without the need for an additional drying step. This is because the solution in which the fibers are crosslinked keeps the fibers flexible after crosslinking, although it does not cause the fibers to assume an undesirable, highly swollen state.

15 Furthermore, it has surprisingly been found that when wetting compressed pads, increased swelling of the absorbent structure can be obtained when the structures are made of fibers crosslinked in a state of twisting but partially swollen compared to fibers thoroughly dried from water prior to crosslinking.

Improved results are obtained for discrete, crosslinked fibers crosslinked under conditions in which the fibers are dried to a water content of between about 18% and about 30% before being contacted with the crosslinking solution. In the case where the fiber is completely dried before it is brought into contact with the crosslinking solution, it is not in a compressed state. The fiber does not swell upon contact with the crosslinking solution due to the low water content of the solution. As described above, a critical feature of the crosslinking solution is that it does not cause any substantial swelling of the fibers. However, in the case where the crosslinking solution diluent is ab-35 adsorbed on the already swollen fiber, the fiber is actually "dried" from water, but the fiber retains its pre-existing, partially swollen state.

To describe the degree of expansion to which the fiber is swollen, it is useful to refer again to the fluid-5 retention value (FRV number) of the fiber after crosslinking. Fibers with higher FRV numbers correspond to fibers crosslinked in a more swollen state than fibers 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 FRVs have greater wet flexibility and wettability than fibers crosslinked in the non-swollen state. Fibers with this improved wet flexibility and susceptibility to wetting are more easily able to swell or open when helically wetted in an attempt to return to their natural state. Due to the stiffness caused by the crosslinking, the fibers are still able to provide structural support for the impregnated cushion made of fibers.

The numerical FRV values presented herein for partially swollen, crosslinked fibers are water retention values (WRV figures). As the WRV numbers increase to approximately greater than 60, the stiffness of the fibers is assumed to be insufficient to provide the desired wet elasticity and wettability to support the impregnated 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, separated from the fibers, dried to the desired dryness, and then immersed with water in a crosslinking solution containing a catalyst and crosslinking agent. is described. The fibers 35 are preferably mechanically defibrated to a fluffy form 27,94435 after liquid separation and prior to further drying to provide the benefits of increased twist and curl, as described above. Mechanical defibration performed after the fibers have been brought into contact with the crosslinking agent is less desirable because such defibration could evaporate the crosslinking agent and potentially result in atmospheric contamination by the crosslinking agent or the resulting high levels of air treatment investment.

In a variation of the method just described, the fibers are defibrated and then pre-soaked in a highly concentrated solution of a crosslinking agent and a fiber swelling diluent, preferably water. The concentration of Verk-15 curling agent is high enough to prevent water-induced swelling of the fibers. 50% by weight aqueous solutions of the crosslinking agents of this invention, preferably glutaraldehyde, have been found to be useful solutions for pre-soaking fibers. A liquid is separated from the pre-extracted fibers and the fibers are immersed in a crosslinking solution containing a water-miscible polar diluent, a catalyst and a limited amount of water, followed by crosslinking as described above. As also above, it-. 25, a liquid can be separated from the crosslinked fibers and the fibers can be subjected to a second mechanical defibration step before further processing the fibers into an sheet-like or absorbent structure.

Pre-soaking the fibers with the crosslinking agent in a second solution of water-30 before reacting the crosslinking agent produces surprisingly high absorption properties for absorbent pads made of crosslinked fibers, even compared to pads made of crosslinked fibers obtained with the non-aqueous solutions described above. , in which the fibers were not pre-soaked in a solution containing a crosslinking agent.

The crosslinked fibers formed by the above dry crosslinking method and the anhydrous solution crosslinking method 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 core structures. In addition, due to the stiffened and spring-like nature of the crosslinked fibers, the fibers can be wet-laminated into an incompressible, low-density sheet which, when dried, is directly usable as an absorbent core structure without further mechanical treatment. The crosslinked fibers can also be wet-15 layered into compressed pulp sheets for sale or transportation elsewhere.

After the discrete, crosslinked fibers are made, they can be dry-laid and directly formed into absorbent structures, or wet-laid and formed into absorbent structures or compacted pulp sheets. The fibers of the present invention provide many substantial advantages in use. However, it is difficult to form such fibers into a flat, wet-laid sheet by conventional wet-sheet forming methods. This is because the discrete, crosslinked fibers rapidly flocculate in solution. Such flocculation can occur both in the headbox and when depositing fibers on a permeable forming wire. Attempts to form sheets from discrete, crosslinked fibers by conventional pulp sheeting methods have been found to result in a large number of agglomerates of flocculated fibers. This is due to the rigid, twisted nature of the fibers, the low amount of bonds between the fibers, and the high permeability of the fibers after they are deposited on the sheeting wire. Therefore, 29,94435 is of considerable commercial importance in providing a useful method of forming discrete, crosslinked fibers into a sheet for forming wet-laid absorbent structures and compacted pulp sheets for transport and subsequent defibration.

Thus, a new method for forming discrete, crosslinked fibers with a tendency to flocculate in solution has been developed, in which method a slurry containing discrete, crosslinked fibers is initially deposited on a permeable forming wire such as a flat wire in the same manner as in conventional pulp sheeting methods. However, due to the nature of the individual, crosslinked fibers, these fibers are deposited on the forming wire in a plurality of fiber bundles. At least one liquid jet, preferably a water jet, is directed at the layered, stacked fibers. It is preferred that a plurality of jet sets be directed at the fibers deposited on the forming wire, with successive jets having decreasing volume flow rates. The speed of the jets must be sufficient so that the impact of the liquid against the fibers acts in such a way as to prevent the formation of fiber flocs and to disperse the fiber flocs which have already had time to form. The step of settling the fibers is preferably performed by means of a 75 cylindrical wire, such as a wire roll, or some other device analogous thereto, which is known or will become known in the art. After settling, the fibrous sheet can then be dried and, if desired, compressed if desired. The distance between the jets may vary depending on the rate of flocculation of the fibers in question, the line speed of the forming wire, the removal of water through the forming wire, the number of jets and the velocity and flow rate of the liquid coming through the jets. The jets are preferably sufficiently close together that substantial flocculation cannot occur.

30 94435

In addition to preventing and disintegrating the fiber flocs, the water sprayed onto the fibers compensates for the excessively rapid separation of the liquid from the discrete, crosslinked fibers by producing additional liquid media where the fibers can disperse for subsequent sheeting. The number of jets with decreasing volume flow rates facilitates a systematic net increase in slurry consistency while applying a repetitive disintegrating and preventing effect to the fiber flocs. This results in a calm and even deposition of the fibers, which then suddenly, i.e. before re-flocculating, bind to the shape of the sheet, allowing the liquid to separate and the fibers to compress against the permeable wire.

Compared to pulp sheets made from conventional non-crosslinked cellulosic fibers, pulp sheets made from the crosslinked fibers of the present invention are much more difficult to compress to the densities of a conventional pulp sheet. Therefore, it may be desirable to combine crosslinked fibers with non-crosslinked fibers, such as those conventionally used to make absorbent cores. The pulp sheets containing stiffened crosslinked fibers preferably contain from about 5% to about 90% non-crosslinked cellulosic fibers, based on the total dry weight of the sheet, mixed with the individual crosslinked fibers. It is particularly preferred to include from about 5% to about 30% of highly ground, non-crosslinked cellulosic fibers, based on the total dry weight of the sheet 30. Such highly ground fibers are ground or ground to a degree of grinding of less than about 300 ml of CSF and preferably less than 100 ml of CSF. The non-crosslinked fibers are preferably mixed with an aqueous slurry of discrete, crosslinked fibers.

35 This mixture can then be formed into a compact pulp sheet for subsequent defibration and forming into absorbent pads. 31 i M tl I I 1'M> <t 94435 31 sheets. The inclusion of non-crosslinked fibers facilitates the compression of the pulp into a sheet in a compact form, while causing a surprisingly low loss of absorbency to the subsequently formed absorbent pads. In addition, the non-crosslinked fibers increase the tensile strength of the pulp sheet and the absorbent pads, which pads are made either of a pulp sheet or a mixture of directly crosslinked and non-crosslinked fibers. Regardless of whether the mixture of crosslinked and non-crosslinked fibers is first formed into a pulp sheet and then formed into an absorbent pad or formed directly into an absorbent pad, the absorbent pad can be made by layering the fibers in air or wet.

Sheets or webs made of discrete, crosslinked fibers or also blends containing non-crosslinked fibers preferably have m 2 weights of less than about 800 g / m 2 and densities of less than about 0.60 g / cm 3. Although not intended to limit the scope of the invention, especially wet-laid sheets having basis weights between 300 g / m 2 and about 600 g / m 2 and densities between 0.15 g / cm 3 and about 0.30 g / cm 3 are suitable for direct use. absorbent si-. 25 susps in disposable articles such as baby nappies, tampons and other sanitary napkins. Structures having higher basis weights and densities than these amounts are believed to be most useful for subsequent comminution and dry deposition or wet deposition to form a lower density and basis weight structure that is more useful in absorbent applications. Such structures with a high basis weight and density still also have a surprisingly good absorption capacity and wettability. Other applications contemplated for the fibers of the present invention include low density gauze sheets, which may have densities of less than 0.10 g / cm 3.

For product applications where the crosslinked fibers come into contact with or near human skin, it is desirable to further treat the fibers to remove excess, unreacted crosslinking agent. The amount of unreacted crosslinking agent is preferably reduced to less than 0.03%, based on the dry weight of the cellulosic fibers. A series of treatments that have been found to be successful in removing excess crosslinking agent include washing the crosslinked fibers sequentially, soaking the fibers in aqueous solution for a substantial period of time, screening the fibers, dewatering the fibers, e.g., by centrifugation, to a consistency of about 40% to about 80%. from which water has been separated, mechanical defibration as described above, and air drying of the fibers. This method has been found to reduce the concentration of residual free crosslinking agent to between about 0.01% and about 0.15%.

In another method to reduce the residual amount of crosslinking agent, the easily extractable crosslinking agent is removed by alkali washing. The alkalinity can be introduced by the addition of basic compounds, such as sodium hydroxide, or alternatively in the form of oxidizing agents, for example chemicals commonly used as bleaching agents, e.g. sodium hypochlorite, and amino group-containing compounds, e.g. ammonium hydroxide, which hydrolyze hemiacetalene-30 Schiff's bases. The pH is preferably maintained at at least about pH 7 and more preferably at least at about pH 9 to prevent recovery of the acetal crosslinking bond. It is preferred to effect degradation of the hemiacetal bonds while not affecting the acetal bonds. Therefore, such extractants are preferred additives which act under very temporal conditions. One wash treatment, using ammonium hydroxide concentrations of 0.01 N and 0.1 N, was found to reduce the residual concentration from about 0.0008% to about 5 0.0023% with soaking times of 30 minutes to two hours. A slight additional advantage is believed to be achieved with soaking times longer than about 30 minutes and ammonium hydroxide concentrations greater than about 0.01 N.

Both single-step oxidation and multi-step oxidation were found to be effective methods for removing residual crosslinking agent. One-step washing, using available chlorine in an amount of from 0.1% to about 0.8%, based on the dry weight of the fibers, and added in the form of sodium hypochlorite, was found to reduce residual crosslinking agents from about 0.0015% to about 0.0025 %.

In a new method of making crosslinked, discrete fibers, the fibers in the starting material are subjected to the conventional multi-stage bleaching treatment, but in the middle stages of the treatment the bleaching process is interrupted and the fibers are crosslinked in accordance with the present invention. After the curing treatment, the bleaching treatment is completed. It has been found that in this way, acceptable residues of crosslinking agent of less than about 0.006% can be achieved. This method is believed to be an advantageous way to produce crosslinked fibers because combining the bleaching step and the residual chemical reduction step avoids the capital cost and process inconvenience associated with additional washing and extraction equipment and processing steps. The bleaching treatments used and the treatment break point for crosslinking can vary widely, as will be appreciated by those skilled in the art. However, multi-step bleaching treatments in which the DEP * or DEH * steps follow crosslinking have been found to provide desirable results. (* D - chlorine dioxide, E - extraction with sodium hydroxide, P - peroxide, H - sodium hypochlorite).

The bleaching treatment steps performed after crosslinking are preferably temporal treatments performed at a pH greater than about pH 7, and more preferably greater than about pH 9.

In addition to effectively reducing the residual amount of crosslinking agent, post-crosslinking and alkaline treatments have been found to facilitate the production of fibers with higher FRVs (fluid retention values) with the same degrees of crosslinking. Fibers with higher FRV numbers have lower dry-15 elasticity, i. they are more easily compacted when dry, while retaining substantially the same wet elasticity and wettability as otherwise similar fibers crosslinked after bleaching. This was particularly pronounced above 20, given that higher FRVs had previously resulted in impaired absorption properties.

The crosslinked fibers described herein are useful in a variety of absorbent articles, including, but not limited to, gauze sheets, disposable diapers, sanitary napkins, wound dressings, tampons, and bandages, each said article having an absorbent structure comprising the separate and crosslinked articles described herein. In this case, we mean in particular, for example, a disposable baby diaper or a similar article having a liquid-permeable cover sheet, a liquid-impermeable backing sheet combined with a cover sheet, and an absorbent structure comprising separate, crosslinked fibers. Such articles are generally described in U.S. Patent 3,860,003, 94,435, issued to Kenneth B. Buell on January 14, 1975, which is incorporated herein by reference.

Usually, the absorbent liners for baby diapers 5 and sanitary napkins are made of unstiffened, non-crosslinked cellulosic fibers, with dry densities of the absorbent liners ranging from about 0.06 g / cm 3 to about 0.12 g / cm 3. When wetted, the volume of the absorbent core usually decreases.

It has been found that the crosslinked fibers of the present invention can be used to make absorbent cores having substantially better liquid absorption properties, such as, but not limited to, absorbency and wicking rate, compared to absorbent cores of similar density, which are made of conventional non-crosslinked fibers or previously known crosslinked fibers. In addition, these improved absorption results can be obtained in conjunction with improved wet flexibility values. In the case of absorbent cores having densities between about 0.06 g / cm 3 and about 0.15 g / cm 3 and which maintain a substantially constant volume in their moisture content, it is particularly advantageous to use crosslinked fibers having a degree of crosslinking between about 2 and about 2. .0 mole% to about 2.5 mole% crosslinking agent, calculated on a molar basis of dry cellulose anhydroglucose. Absorbent interiors made of such fibers desirably have both structural integrity, i.e. resistance to 30 compression against that wet elasticity. By the term wet resilience in the present specification is meant the ability of a wetted pad to return to its original shape and volume after compression when the pad is compressed and released therefrom. Compared to the inserts 35 made of untreated fibers and previously known crosslinked fibers, the absorbent inserts made of the fibers of the present invention achieve a substantially higher proportion of their original volume after the release of the wet compression forces.

In another preferred embodiment, the discrete, crosslinked fibers are formed into either an air-laid or wet-laid (and then dried) absorbent core, which is compressed to a dry-density less than the wet density of the pad at equilibrium. Wet density at equilibrium is the density of the pad calculated on the basis of dry fibrous material when the pad is completely saturated with liquid. When the fibers are made into an absorbent core having a dry density less than the wet hand density at equilibrium, in wetting to saturation, the core compresses to the wet density of the equilibrium state. Alternatively, when the fibers are formed into an absorbent core having a dry density greater than the wet density at equilibrium, the core will swell to the wet density of the equilibrium state as it experiences saturation. Pads made from the fibers of the present invention have equilibrium wet densities that are substantially lower than pads made from conventional fluffed fibers. The fibers of the present invention can be compressed to a density greater than the wet density equilibrium to form a thin pad that, when tested, expands, thereby increasing the absorbency to a much greater extent than is obtained with non-crosslinked fibers.

Particularly good absorption properties, wet resilience and wettability can be obtained with degrees of crosslinking ranging from about 0.75 mol% to about 1.25 mol%, calculated on a molar basis of dry cellulose. Such fibers are preferably formed into absorbent cores having dry densities greater than their teas. 14 · »ΙΊ4 · t i t ai t 37 94435 wet densities at equilibrium. The absorbent cores are preferably compressed to densities ranging from about 0.12 g / cm 3 to about 0.60 g / cm 3, with the corresponding equilibrium wet density being less than the density of the dry compressed pad. It is also preferred that the absorbent cores be compressed to a density between about 0.12 g / cm 3 and about 0.40 g / cm 3, with corresponding equilibrium wet densities ranging from about 0.08 g / cm 3 to about 0.12 g / cm 3. and are less than the densities of dry, compressed-10 interiors. Compared to crosslinked fibers with degrees of crosslinking ranging from 2.0 mole percent to about 2.5 mole percent, the former fibers are less rigid, making them more suitable for compression to higher densities. The former fibers also have a greater susceptibility to mouth-15 in that, when wetted, they pop open at a higher rate and swell to a greater extent than fibers with crosslinking rates of 2.0 mole percent to 2.5 mole percent, and have greater wet flexibility. and they retain nearly equal to 20 high absorption capacities. It should be noted, however, that higher density absorbent structures can be made from crosslinked fibers with a higher degree of crosslinking, as well as lower density absorbent structures can be made from crosslinked fibers. fibers with lower degrees of crosslinking. All such structures provide improved performance over previously known discrete, crosslinked fibers.

Although the foregoing description includes preferred embodiments for high density and low density absorbent structures, it should be noted that many combinations of absorbent structure densities and crosslinking agent amounts within the limits described herein provide better absorption properties and absorbent structure integrity over conventional cellulosic materials. known crosslinked fibers. Such embodiments are intended to be included within the scope of this invention.

Method for determining the water retention value 5 The following method was used to determine the water retention value of cellulose fibers.

A sample comprising about 0.3 g to about 0.4 g of fibers is soaked in a lid vessel in about 100 ml of distilled or deionized water at room temperature for about 15 to about 20 hours. The soaked fibers are collected as a filter and transferred to an 80 mesh wire basket supported about 3.81 cm above the bottom of a centrifuge tube with a 60 mesh screen. The tubes are covered with a plastic lid and the sample is centrifuged at a relative centrifugal force of 1500 to 1700 times gravity for 19 to 21 minutes. The centrifuged fibers are then removed from the basket and weighed. The weighed fibers are dried to constant weight at 105 ° C and reweighed. The water retention value is calculated as follows: 20

(1) WRV = (W-D) x 100 D

where 25 W = wet weight of the centrifuged fibers, D = dry weight of the fibers and W-D = weight of water absorbed.

Method for determining the dripping capacity

The following method was used to determine the drip capacity of the absorbent cores. Dripping capacity is used as a common measure of the absorbency and absorption rate of the inner parts.

An absorbent pad measuring 10.16 cm x 10.16 cm and weighing about 7.5 g is placed on a sieve.

35 Synthetic urine is added to the center of the pad at a rate of 8 ml / second. The addition of synthetic urine is stopped, 39 94435 when the first drop of synthetic urine leaks from the bottom or sides of the pad. The dripping capacity is calculated from the difference in the mass of the pad obtained by subtracting the mass of the pad after the addition of synthetic urine before said addition, and dividing the difference by the mass of fibers calculated from the oven-dry weight.

Method for the determination of wet compressibility

The following method was used to determine the wet compressibility of absorbent structures. Wet compressibility was used as a measure of the wet compression resistance, wet structure integrity and wet flexibility of the absorbent cores.

A pillow measuring 10.16 cm x 15.16 cm and weighing 7.5 g is prepared, its thickness is measured and its density is calculated. The pad is loaded with synthetic urine so that its dry weight becomes ten times or that its saturation point is reached, whichever is lower. A compressive force of 7 g / cm2 20 (0.1 PSI) is applied to the pad. After about 60 seconds, during which the pillow is balanced, the thickness of the pillow is measured. The compressive load is then increased to 77 g / cm2 (1.1 PSI), the pad is allowed to equilibrate and the thickness is measured. The compressive load is then reduced to 25 g / cm 2 (0.1 PSI), the pad is allowed to equilibrate and the thickness is measured again. The densities are calculated for the pad with an initial load of 7 g / cm2, a load of 77 g / cm2 and a second load of 7 g / cm2, denoted 0.1 PSIR (PSI rebound, kimmah-30 dus). The pore volume, expressed in cm3 / g, is then determined for each compression load support in question. Pore volume is the inverse of the wet pad density minus fiber volume (0.95 cm 3 / g). Pore volumes with loads of 7 g / cm 2 and 77 g / cm 2 are useful indications of wet compression resistance and wet structure integrity. Higher pore volumes for standard pad initial densities indicate greater resistance to wet compression and greater wet structure integrity. The difference in pore volumes obtained between 0.1 PSI and 0.1 PSIR is useful for comparing the wet elasticity of absorbent pads. A smaller difference between a pore volume of 0.1 PSI and a pore volume of 0.1 PSIR indicates greater wet flexibility.

10 The difference in thicknesses obtained for the dry pad and the impregnated pad before they were compressed was also found to be a useful indication of the wettability of the pads.

Method for Determining Dry Compressibility The following method was used to determine the dry compressibility of absorbent cores. Dry compressibility was used as a measure of the dry elasticity of the inner parts.

An air-laid pad measuring 10.16 cm x 10.16 cm and weighing about 7.5 g is prepared and dry pressed with a hydraulic press at a compression pressure of 24 kg / cm 2 (5500 pounds / 16 square inches). The pillow is turned and the pressing is repeated. The thickness of the pad is measured before and after pressing with a non-load-measuring gauge. The density before and after compression is then calculated as the mass / (area x thickness). Larger differences between pre- and post-compression densities indicate less dry flexibility.

Method for Determining the Amount of Alutaraldehyde Reacted with Cellulose Fiber The following method was used to determine the amount of glutaraldehyde that reacted to form intracellular crosslinks with the cellulose component of discrete fibers crosslinked with glutaraldehyde.

41 94435 41 A sample of the individual cross-linked fibers is extracted with 0.1 N HCl. The extract is separated from the fibers and the same extraction / separation treatment is then repeated 3 more times for each sample. The extract 5 obtained from each extraction is mixed separately with an aqueous solution of 2,4-dinitrophenylhydrazone (DNPH). The reaction is allowed to proceed for 15 minutes, after which a volume of chloroform is added to the mixture. The reaction mixture is then stirred for 45 minutes. The chloroform and water layers are separated by means of a separatory funnel 10. The amount of glutaraldehyde is determined by analyzing the chloroform layer by high performance liquid chromatography (HPLC) for the DNPH derivative.

The chromatographic conditions used for HPLC analysis were as follows: column: C-18 reverse phase; detector: 15 UV at 360 nm? mobile phase: methanol / water, 80:20; flow rate; 1 ml / minute; measurement performed; peak height. The calibration curve for peak height and glutaraldehyde content was developed by measuring the heights of peak 20 obtained by HPLC of five standard solutions known to have glutaraldehyde levels between 0 and 25 ppm.

All four chloroform phases obtained with each fiber sample were analyzed by HPLC, the height of the peak was measured, and the corresponding amount of glutaraldehyde was determined from 25 calibration curves. The glutaraldehyde concentrations of each extract were summed and divided by the weight of the fiber sample (calculated from dry fiber) to obtain the glutaraldehyde content on a dry fiber basis.

There were two glutaraldehyde peaks in each HPLC chromatogram. Either spike can be used as long as the spike is used throughout the method.

Example 1 This example illustrates the effect of varying amounts of a crosslinking agent, glutarose-35 aldehyde, on the absorbency and flexibility of absorbent pads made of discrete 42,94435 crosslinked fibers. Separate, crosslinked fibers were prepared by the dry crosslinking method.

For each sample, a certain amount of never-dried, southern conifer kraft pulp (SSK-mas) was obtained. The moisture content of the fibers was about 62.4% (corresponding to a consistency of 37.6%). A slurry was formed by adding the fibers to a solution containing a selected amount of a 50% aqueous solution of glutaraldehyde, 30% (based on the weight of glutar-10 aldehyde) zinc nitrate hexahydrate, demineralized water, and sufficient 1 N HCl to lower the pH of the slurry. 3.7. The fibers were soaked in the slurry for 20 minutes, and then dewatered by centrifugation to a fiber density of about 34% to about 35%. The water-separated fibers were then air dried to a fiber density of about 55% to about 56% by blowing air at ambient temperature through a dryer. The air-dried fibers were defibrated using a three-stage fluffing device 20 as described in U.S. Patent 3,987,968. The defibrated fibers were placed in trays and cured at 145 ° C in a substantially static drying oven for 45 minutes. The networking took place completely during the oven treatment. The crosslinked, individual fibers were placed on a wire screen 2.5 and washed with water at about 20 ° C, soaked at 1% consistency for one hour in water at 60 ° C, screened, washed a second time with water at about 20 ° C, centrifuged To a fiber consistency of 60%, was defibrated in a three-stage fluffing apparatus as described above, and finally dried .. in a static drying oven at 105 ° C for 4 hours. The dried fibers were layered in air to form absorbent pads. The pads were compressed with a hydraulic press to a density of 0.10 g / cm 3. The pads were tested for absorbance, flexibility, and the amount of reacted glutaraldehyde 43,94435 d according to the methods described herein. The reacted glutaraldehyde is expressed in mole% based on dry fiber cellulose anhydroglucose. The results are shown in Table 1.

5

table 1

Dripping Capacity Wet Compressibility Sample Glutaraldesicity cos- (cm3 / g) No. hyd (mol%)

10 added / WRV Sat 8 ml / s 7g / cm3 77g / cm2 7g / cm2R

_reacted (%) (σ / s) _ 1 0/0 79.2 N / A 10.68 6.04 6.46 2 1.73 / 0.44 51.0 6.98 11.25 5.72 6 , 57

3 * N / A / 0.50 48.3 N / A N / A N / A N / A

15 4 2.09 / 0.62 46.7 N / A 11.25 6.05 6.09 5 3.16 / 0.99 36.3 15.72 12.04 6.09 6.86 6 4, 15 / 1.54 35.0 15.46 13.34 6.86 8.22 7 6.46 / 1.99 32.8 12.87 13.34 6.93 8.31 8 8.42 / 2, 75 33.2 16.95 13.13 7.38 8.67 20 9 8.89 / 2.32 29.2 13.59 12.56 6.51 7.90 10 12.60 / 3.32 27, 7 13.47 12.04 6.63 7.82 * Taken from a different fiber sample (N / A) - not available 2b Example 2

The individual crosslinked fibers of Example 1 were formed into dry-laid absorbent pads with a dry fiber density of 0.20 g / cm 3. The pads were allowed to swell under unobstructed conditions with a Moisturizer-50 covering them with synthetic urine during the performance of the drip capacity method. The pads were then tested for absorbency, flexibility and structural integrity according to the wet compressibility method described above. The results are shown in Table 2. The drop-35 capacity and wet compressibility were significantly increased with a glutaraldehyde content of 0.50 mol%.

44 94435

Table 2

Dripping Capacities - Wet Compressibility (cm3 / g)

The wetting rate was 7 g / cm2 77 g / cm2 7 g / cm2R

number della 8 m / s (a / a) __ 5 1 4.56 8.95 5.38 5.90 2 7.84 8.31 4.80 5.72 3 * 11.05 11.71 6.63 7.31 4 9.65 8.90 5.11 6.10 10 5 12.23 11.87 6.35 7.52 6 13.37 10.54 6.04 7.25 7 11.09 9.80 5.67 6.92 8 12.04 9.69 5.72 6.86 9 7.99 9.80 5.50 6.74 15 10 3.57 9.25 5.50 6.46 * Taken from a different fiber sample

Example 3 The purpose of this example is to show that low amounts of extractable crosslinking agent can be obtained by performing successive bleaching treatments on the fibers after crosslinking. The amount of extractable crosslinking agent was determined by soaking a fiber sample at 40 ° C in deionized water at a consistency of 2.5% for one hour. The amount of glutaraldehyde extracted in water was measured by HPLC and the extractable glutaraldehyde. the amount of rialdehyde was reported on a dry fiber basis.

The fibers were crosslinked using the dry crosslinking method.

Kraft-30 pulp (SSK) made from southern softwood was procured. The pulp fibers were partially bleached by the following successive bleaching steps: chlorination (C) - a 3-4% slurry was treated with about 5% available chlorine at about pH 2.5 and about 38 eC for 30 minutes; extraction with sodium hydroxide A -35 slurry of 12% consistency was treated with NaOH (1.4 g / liter) at about 74 ° C for 60 minutes; and hypochlorite treatment The slurry with a (H) consistency of 12% was treated with a sufficient amount of sodium hypochlorite at pH 11 to 11.5 at 38 ° C to 60 ° C for 60 minutes to obtain an Elretho brightness value between 60 ° C and 60 ° C. -5 65 and a viscosity between 15.5 and 16.5 mPa * s (cp). Separate bleached fibers were made into discrete, crosslinked fibers using glutaraldehyde as a crosslinking agent according to the method described in Example 1. 2.29 mole% glutaraldehyde, based on the molar basis of dry fiber cellulose anhydroglucose, was retained in the fibers. The extractable amount of glutaraldehyde in such fibers is typically about 1000 parts per million (0.1%).

The bleaching of the partially bleached, discrete fibers was then continued and supplemented with a series of treatments (DEH) comprising treatment with chlorine dioxide (D), extraction (E) and treatment with sodium hypochlorite (H). In the chlorine dioxide treatment (D), discrete, crosslinked fibers were soaked in a 10% aqueous slurry, which also contained a sufficient amount of sodium hypochlorite, so that the available chlorine was 2% by weight of the dry fiber. After stirring, the pH of the slurry was lowered to approximately pH 2.5 by the addition of HCl and then raised to pH 4.4 by the addition of NaOH. The pulp slurry was then placed in a 70 ° C oven for 2.5 hours, screened, rinsed with water to neutral pH, and centrifuged to a consistency of 61.4%.

In the extraction step, a 10% aqueous slurry of fibers from which water had been separated was treated with a solution of 0.33 g NaOH / liter of water for 1.5 hours at 40 ° C. The fibers were then screened, rinsed with water to neutral pH, and centrifuged to a consistency of 62.4%.

46 94435

Finally, for the sodium hypochlorite step (H), a fiber slurry with a consistency of 10% was prepared containing sufficient sodium hypochlorite in the presence of 1.5% available chlorine, calculated on a dry fiber basis. The slurry was stirred and heated at 50 ° C in an oven for one hour. The fibers were then screened, rinsed to pH 5.0, and thickened to a consistency of 62.4%. The water-separated fibers were air-dried, fluffed, and finally dried in an oven at 105 ° C for one hour. The amount of glutaraldehyde extractable from fully bleached, discrete, crosslinked fibers was 25 ppm (0.0025%). This is well below the maximum value of extractable glutaraldehyde, which is believed to be acceptable for applications where the fibers are used in the vicinity of human skin.

It was also found that pads made of fibers that had been partially bleached, reticulated and then finally bleached surprisingly had higher fluid retention and wic-king rates and at least the same dripping capacity and wet flexibility as the individual fibers, after they were completely val-25 lane. However, due to the higher WRV number, fibers that were crosslinked in the middle stages of the bleaching treatment series were more compressible in the dry state.

Substantially similar results were obtained when the last 30 hypochlorite steps (H) were replaced by a peroxide bleaching step (P). In step P, a slurry with a consistency of 10% was treated with 0.5% hydrogen peroxide, based on the weight of the fiber, at a pH between 11 and 11.5 and 80 ° C for 90 minutes.

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47 94435

Example 4 This example shows the effect of mixing an organic acid with an inorganic salt catalyst on the progress of the crosslinking reaction. The fibers mesh-5 were assembled by the dry crosslinking method.

A first sample of discrete, crosslinked fibers was prepared as described in Example 1, with 4.0 mole percent glutaraldehyde retained after water separation. Analytical measurements of the fibers after crosslinking showed that the amount of glutaraldehyde reacted with the fibers was 1.58 mol%, corresponding to about 37% of the complete reaction.

A second sample of individual crosslinked fibers was prepared in the same manner as the first sample described in this example, except that in addition to the zinc nitrate catalyst, an amount of citric acid corresponding to 10% by weight of glutaraldehyde was mixed with zinc nitrate in the pulp slurry as an additional catalyst. Analytical measurements of the fibers after crosslinking showed that the amount of glutaraldehyde reacted with the fibers was 2.45 mol%, corresponding to about 61% of the complete reaction (on a molar basis) and a 55.1% increase in reaction progress compared to the sample in which only zinc-25 was used as a catalyst. stearate.

Example 5 This example illustrates the use of low amounts of a glyoxylic acid, a dialdehyde acid analog having a single aldehyde group, in the dry crosslinking method described in Example 1.

A fibrous slurry was prepared from a never dried SSK pulp * containing a sufficient amount of glyoxylic acid to react with an estimated 1.2% glyoxalic acid to react with cellulose fibers, calculated on a molar basis of cellulose anhydroglucose, and zinc 48,94435 nitrate hexahydrate hexahydrate. The centrifuged fibers had a fiber consistency of about 38% and the fibers contained about 1.06% by weight of glyoxylic acid on a dry fiber basis. The ratio of catalyst to crosslinking agent was about 0.30. The pH of the slurry at the start of crosslinking was about 2.16. The fibers were separated and crosslinked according to the procedures described in Example 1.

In another sample, about 0.53% by weight of glyoxylic acid, based on dry fiber weight, was added to the fibers to react an estimated amount of glyoxylic acid of about 0.6 mol%, calculated on a molar basis of cellulose anhydroglucose. Separate, crosslinked fibers were otherwise prepared according to the sample just described, except that the pH of the slurry at the start of crosslinking was about 2.35.

Absorbent structures with densities of 0.1 g / cm 3 and 0.2 g / cm 3 were made from discrete, crosslinked fibers as shown in Example 2.

20 The dripping capacities, wet compressibility values of 0.1 PSI, 1.1 PSI and 0.1 PSIR, and fluid transfer capacity of the pads were significantly higher than those of absorbent structures of similar density made of conventional non-crosslinked fibers.

Example 6 This example illustrates a process for making discrete, crosslinked fibers by an anhydrous solution curing crosslinking process in which the fibers are crosslinked in a substantially non-swollen, compressed state.

Never dried, bleached SSK fibers are obtained and dried to a fiber density of about 67%. The fibers are mechanically defibrated using a three-stage fluffing device such as that disclosed in U.S. Patent 3,987,358. The defibrated fibers are then completely dried at 49,94435 at 105 ° C for four hours. The dried fibers are then made into a slurry of fibers and a crosslinking solution having a consistency of 10%, wherein the crosslinking solution contains from about 0.5% to about 6.0% by weight of a 50% glutaraldehyde solution, with an additional amount of water of about 1. 5% to about 13% by weight, about 0.3% to about 3.0% by weight of an acid catalyst (HCl or H 2 SO 4) and the remainder acetic acid. The fibers are kept in the crosslinking solution for a period of 0.5 hours to 6 hours at a temperature of about 10 to 25 ° C, during which time the primary intramibular crosslinking bonds are formed. The fibers are then washed with cold water and centrifuged to a fiber density of about 60% to about 65% by weight, defibrated in a three-stage fluffing apparatus, and dried at 105 ° C for four hours. Such fibers contain from about 0.5 mole percent to about 3.5 mole percent crosslinking agent, based on the molar basis of cellulose anhydro-glucose. The dried fibers can be layered to form air-absorbent structures 20 and compressed to a density of 0.10 g / cm 3 or 0.20 g / cm 3 by means of a hydraulic press into pads similar to those formed in Examples 1 and 2, or other densities if desired.

Example 7 This example illustrates a method of forming discrete, crosslinked fibers by a curing crosslinking process in an anhydrous solution in which the crosslinking is partially but not completely swollen.

The following method is similar to that described in Example 6, except that the never-dried SSK fibers are initially dried to a fiber density of 50 to 55% by weight prior to defibration, and the defibrated fibers are dried to a moisture content of about 18% by weight to about 18% by weight. 30% by weight, 35 as a result of such defibration and, if desired, an additional drying step is carried out. The fibers with a partially swollen configuration are then crosslinked, washed, centrifuged, defibrated and dried as described in Example 6. Compared to the crosslinked fibers of Example 6, the partially swollen crosslinked fibers of this example, which contain substantially equivalent amounts of glutaraldehyde, have higher WRV numbers and can be made into absorbent structures with higher dripping capacity and wet purity.

Example 8 This example illustrates a process for preparing discrete, crosslinked fibers by an anhydrous solution crosslinking process in which the fibers are pre-soaked in a highly concentrated aqueous solution containing glutaraldehyde prior to crosslinking in a substantially anhydrous crosslinking solution.

The never-dried SSK fibers are mechanically separated by the defibrator described in U.S. Patent No. 3,987,968 and pre-soaked in an aqueous solution containing 50% by weight glutaraldehyde and 50% by weight water for a treatment time of about 2 minutes to about 30 minutes. The fibers are then mechanically compressed to obtain partially swollen fibers impregnated with glutaraldehyde. The fibers are then crosslinked in the presence of a catalyst, washed, centrifuged, defibrated and dried as described in Example 6. Compared to the crosslinked fibers of Examples 6 or 7, in which the degrees of crosslinking are the same, the absorbent structures made of the fibers of the present Example 30 have higher dripping capacities and wet compressibility.

Example 9

Separate, crosslinked fibers were prepared by the method described in Example 7. The crosslinking solutions ai itzi mu t i i al i 51 94435 contained: 2% glutaraldehyde, 1.29% H 2 SO 4, 3% water, the remainder acetic acid for samples 1 and 2; and 0.5% glutaraldehyde, 0.6% H 2 SO 4, 1.2% water, the remaining acetic acid for samples 3 and 4. The moisture content of the fibers 5 added to the crosslinking solution was 30% for samples 1 and 2, and 18% for samples 3 and 4. Glutaraldehyde reacts to form cross-linking bonds with the fibers. WRV, dripping capacity and recovered wet compressibility (7 g / cm 2 R, 0.1 PSIR) were measured and the results are shown in Table 3 below.

Table 3

Fiber Reacted Drip-Wet Press Moisture Glutar Capacity Sample Concentrated Aldehyde Density WRV 8 ml / s (cm3 / g) No. 15 (%) (mol-%) (g / cm3) (%) (a / a ) _ (7 g / cm 2 R) 1 30 3.2 0.10 55 N / A 8.4 2 30 3.2 0.20 55 14.4 7.7

3 18 1.6 0.10 46 N / A N / A

20 4 18 1.6 0.20 46 12.6 7.2 N / A - not available

Example 10 The purpose of this example is to illustrate a method of making separate wet-laid sheets containing crosslinked fibers.

A 0.55% fiber blend slurry containing 90% discrete, crosslinked fibers prepared according to the method described in Example 1 and 10% of 30 conventional non-crosslinked fibers with a freeness degree of less than 100 CSF was deposited as flocculated, bundled fibers for a standard 84 mesh planing wire. The outflow rate from the paper machine headbox was 430 kg / minute. Immediately after deposition 3b, five water streams were applied to the fibers, the flow rates of which decreased in the direction of travel of the wire. The total flow ratio produced by the five water streams was 85 kg water / kg oven dry fiber. The spray nozzles were all arranged at a distance of approximately one meter from each other parallel to the direction of travel of the forming wire. A jet of water was sprayed onto the fibers through a series of circular openings with an inside diameter of 3.2 mm (1/8 ") and spacing of 12.7 mm (1/2") extending across the width of the forming wire. across. The approximate% flow rate, calculators) of the total flow rate and the flow rate through the orifices for each spray nozzle were as follows: spray 1:37% of total flow, 170 m / min; shower 2:36% of total flow, 165 m / minute; shower 3:13% of total flow, 61 m / minute; shower 4: 9% of total flow-15, 41 m / min; shower 5: 5% of total flow, 20 m / minute. Immediately after the fifth spray, the fibers were placed by treatment with a cylindrical roll of wire known in the art as a knife. The egutheran pressed the fibers, which at the time of placing were in the form of a very thick thick slurry, against the forming wire, placing the fibers in the form of a wet sheet. The sheet is similar in appearance to conventional pulp sheets.

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Claims (10)

1. Crosslinked, twisted and crimped individual cellulose fibers comprising cellulose fibers in a substantially individual form and which have been maintained in a substantially individual form during drying and crosslinking, wherein the crosslinking agent is selected from the group C2-C8 dialdehydes, in which an aldehyde group of each of said dialdehydes is replaced by a carboxylic group, and oligomers of said dialdehydes and dialdehyde acid analogs, characterized in that the fibers have 0.5 mol% - 3.5 mol% of reacted crosslinking agent calculated on the anhydroglucose mole of the cellulose, that the moisture content of the fibers reacting with the crosslinking agent is at most 30% calculated on the weight of the dry fibers and that the crosslinking agent is sufficiently extruded into the fibers, the crosslinking agent is sufficiently reacted with the fibers -th of 28 - 45. 2. Crosslinked individual fibers according to claim 1, characterized in that the fibers have 0.75 mole% - 2.5 mole% of reacted crosslinking agent calculated on the anhydroglucose mole amount of the cellulose in the form of crosslinks within the fibers. Crosslinked individual fibers according to claim 1 or 2, characterized in that the crosslinking agent is selected from the group of glutaraldehyde, glyoxal and glyoxylic acid. A process for producing twisted, crosslinked individual cellulose fibers, wherein a) abolishes cellulose fibers; "b) said fibers are contacted with a solution containing a crosslinking agent selected from the group C2-8 dialdehydes, C2-8 dialdehyde acid analogs in which an aldehyde group of each of said dialdehydes is replaced by a carboxyl group, and oligomers of c) the fibers are mechanically separated into a substantially individual form; d) the fibers are dried and e) the crosslinking agent is reacted with the fibers while the fibers are in a substantially individual dried form to form crosslinks within the fibers, characterized therein. that the drying step d) is carried out to produce partially swollen fibers having a moisture content of not more than 30% by weight, that the reaction step e) is carried out such that 0.5 mole% - 3.5 mole% of the crosslinking agent calculated on the cellulose anhydroglucose mole amount is reacted with the fibers to form cross-links within the fibers, the water bond value of the fibers after drying and the crosslinking steps are 28 - 45. 5. A process according to claim 4, characterized in that the cross-linking agent is selected from the group of glutaraldehyde, glyoxal and glyoxylic acid. 20 6. A process according to claim 4 or 5, characterized in that 0.75 mole% - 2.5 mole% of the crosslinking agent calculated on the anhydroglucose mole amount of the cellulose is reacted with the fibers to effect cross-links within the fibers. 25 Process according to any one of claims 4-6, characterized in that the crosslinking agent is reacted with the fibers in the presence of at least one catalyst selected from the group of mineral acids, halogen acids, organic acids, mineral salts, halogen salts and organic salts. 30 Method according to any of claims 4-7, characterized in that the fibers are contacted with a solution containing the crosslinking agent and at least one of said catalysts. 9. A process according to claim 8, characterized in that the solution contains a mineral salt catalytic acid and an organic acid catalytic acid. 94435 58 10. A process according to claim 9, characterized in that the mineral salt is a zinc nitrate salt and the organic acid is citric acid and the pH is 2-5. «I i.