FI94433C - Method for making crosslinked discrete cellulose fibers - Google Patents

Description

94433

The present invention relates to a process for producing crosslinked discrete cellulose fibers.

Crosslinked fibers in substantially separated form and various methods for making such fibers have been described in the literature. The term "crosslinked discrete fibers" refers to cellulosic fibers that contain predominantly intra-fiber chemical crosslinks. That is, the crosslinks are primarily between the cellulose molecules of a single fiber and not between the cellulose molecules of different fibers. Crosslinked discrete fibers are generally considered useful in absorbent articles. In general, three groups of methods for making crosslinked discrete fibers have been proposed. These methods, described below, are referred to herein as 1) dry crosslinking methods, 2) aqueous solution crosslinking methods, and 3) methods in which crosslinking occurs in a substantially anhydrous solution. The fibers themselves and the absorbent structures containing the crosslinked discrete fibers are generally superior to conventional non-crosslinked fibers for at least one significant absorbency property. This improvement in absorbency is often expressed in terms of absorption capacity *. In addition, absorbent structures made of crosslinked discrete fibers generally have higher wet and dry resilience than absorbent structures made of non-crosslinked fibers. The term "resilience" hereinafter means the ability of flat pads made of cellulosic fibers to return to their original expanded state upon cessation of the compressive force. Dry resilience specifically refers to the ability of an absorbent structure to expand upon the cessation of the compressive force applied to it when the fibers are in a substantially dry state. Wet resilience specifically refers to the ability of an absorbent structure to expand upon the cessation of the compressive force applied to it when the fibers are in a wetted state. For purposes of the main amounts of this invention and the consistency of the disclosure, wet resilience is contemplated and reported for an absorbent structure that has wetted to its saturation point.

Methods for making crosslinked discrete fibers by the dry crosslinking technique are described in U.S. Patent 3,224,926 (LJ Bernardin), issued December 21, 1965. Crosslinked discrete fibers are prepared by impregnating expanded fibers in aqueous solution with a crosslinking agent (crosslinking agent) and defibrillating fiber. mechanically and by drying the fibers at an elevated temperature to effect crosslinking, while the fibers are in a substantially separated state. The fibers are crosslinked in an inherently unexpanded, compressed state, resulting from the removal of water prior to crosslinking. The methods exemplified in U.S. Patent No. 3,224,926, in which the formation of crosslinks is caused with the fibers in an unexpanded, compressed state, are termed methods for making "dry-crosslinked" fibers. Dry crosslinked fibers are characterized by low liquid retention. U.S. Patent No. 3,440,135 (R. Chung), issued April 22, 1969, proposes soaking fibers in an aqueous solution of a crosslinking agent 30 to reduce the ability of fibers to form interfiber bonds prior to a similar dry crosslinking process. as described in U.S. Patent No. 3,224,926 This time-consuming (preferably about 16 to 48 hours) pretreatment is claimed to improve the quality of the product by reducing the fiber knob content resulting from incomplete defibration.

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Il f · Iitit III II I I I «I i l 3 94433

Methods for making crosslinked fibers in aqueous solution are described, for example, in U.S. Patent 3,241,553 (F.H. Steiger), issued March 22, 1966. Crosslinked discrete fibers are prepared by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. The fibers produced in this way are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling effect of water in the cellulose fibers, the fibers to be crosslinked in the ve-10 silicon solution are crosslinked in an uncompressed, expanded state. Compared to dry-crosslinked fibers, the aqueous-crosslinked fibers disclosed in U.S. Patent No. 3,241,553 have 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 lower resilience when wet and dry than pads made of dry crosslinked fibers.

U.S. Patent 4,035,147 (S. Sangenis, C. Guiroy, and J. Quere), issued July 12, 1977, discloses a process for making crosslinked discrete fibers by contacting non-expanded, dehydrated fibers with crosslinks. 25 with a dosing agent and a catalyst in a substantially anhydrous solution containing insufficient water to swell the fibers. The formation of crosslinks occurs when the fibers are in this substantially anhydrous solution. This type of process is hereinafter referred to as the "anhydrous solution crosslinking process" and the fibers produced therein are referred to as "anhydrous solution crosslinked" fibers. The anhydrous solution crosslinked fibers disclosed in U.S. Patent 4,035,147 do not swell even when in prolonged contact with solutions known to those skilled in the art as blowing agents. Crosslinks greatly stiffen them like dry crosslinked jacket fibers, and the absorbent structures made from them have high resilience when wet and dry.

The crosslinked fibers described are believed to be useful in lower density absorbent articles such as diapers, and also in higher density absorbent articles such as menstrual pads. However, such fibers have not provided a sufficient absorbent advantage, given their disadvantages and cost, over conventional fibers to result in significant commercial success. The commercial appeal of crosslinked fibers has also suffered due to safety concerns. The most commonly used cross-linking agent in the literature, formaldehyde, is irritating to the human skin, and has been associated with other human safety concerns. Technical and economic reasons have prevented the reduction of the amount of free formaldehyde in the crosslinked product to a sufficient level of al-20 to avoid skin irritation and other concerns for human safety.

Crosslinked fibers made from crosslinking agents other than formaldehyde may also be present in greater amounts than the crosslinking agent than is preferred for use close to human skin. Therefore, it may be desirable to treat the fibers after crosslinking in a manner that lowers the level of residual unstable crosslinker in the fibers. One method of accomplishing this is to wash the fibers with water after crosslinking. This method is. effective but does not reduce the level of residual crosslinker * 'to the desired level. In addition, such post-crosslinking washes necessarily incur additional capital and operating costs associated with fiber washing and drying.

It is an object of the present invention to provide a method for producing crosslinked discrete fibers in which the level of residual crosslinking agent is low, thereby minimizing capital investment in equipment and associated operating costs.

It has been found that the above object can be achieved and crosslinked discrete fibers in which the level of residual unstable crosslinker is reduced by using the following method, characterized in that a) wet cellulosic fibers having a moisture content of at least about 30% by weight are obtained -%; b) defibrating said fibers to render them substantially loose and drying the fibers to a moisture content of 18 to 30% by weight under conditions that substantially prevent the formation of hydrogen bonds between the fibers; c) contacting said fibers with a crosslinking solution containing an anhydrous water-miscible polar diluent in an amount insufficient to swell the fibers to a degree of expansion corresponding to fibers having a moisture content of about 30% by weight; and a crosslinking agent, wherein the crosslinking agent is selected from the group consisting of C2-C8 dialdehydes, C2-C8 dialdehyde acid analogs having at least one aldehyde group, and oligomers of said dialdehydes and dialdehyde acid analogs, and said crosslinking agent is obtained in contact with the crosslinking solution 30 in the substantial absence of interfiber bonds,. completely to form intracellular crosslinks, and the fibers are contacted with a sufficient amount of crosslinking agent so that 0.5 mole to 3.5 mole percent of the crosslinking agent, based on the molar amount of cellulose anhydroglucose, - • 4 Ä 94433 6 goi fibers to form intra-fiber crosslinks; and d) defibrating the fibers to a substantially discrete form and drying the discrete fibers under conditions that substantially prevent the formation of hydrogen bonds between the fibers, said discrete fibers having a water retention value of 25 to 60.

The fibers are preferably washed with an alkaline solution having a pH greater than about 9. Likewise, preferably the fibers are contacted with a sufficient amount of a preferred crosslinking agent to provide an amount of about 0.5 to 3.5 mol% of crosslinking agent that reacts with said fibers. based on the anhydroglucose molar amount of cellulose, and said 15 fibers have a water retention value of less than about 60.

Surprisingly, the fibers made in accordance with this invention have a higher water retention value than fibers having the same degree of crosslinking and otherwise made by similar crosslinking methods, except that they use fully bleached fibers. Corresponding absorbent structures made from the fibers of this invention have improved absorption properties, including wet resilience and wetting response.

Cellulosic fibers of various natural origins are suitable for the present invention. Preferably, decomposed fibers obtained from softwood, hardwood or cotton liners are utilized. Fibers from esparto grass, bagasse, hard hair, flax, and other sources of ligin and cellulose fibers can also be used as a raw material in this invention. The fibers can be obtained in the form of a slurry, in a non-sheet form or in a sheet form. Fibers obtained in the form of a wet or dry wet-folded pulp sheet or in another sheet-like form are preferably converted to a non-sheet-like form • · <1: «a t- * 144 i> 4 -a *> '' before the fibers are contacted with the crosslinking agent. The fibers are also preferably obtained in a wet or humidified state. Most preferably, the fibers are never undried. In the case of a dry wet-folded pulp sheet, it is preferred to moisten the fibers prior to mechanical disintegration to minimize fiber damage.

The most preferred fiber source to be used in connection with the present invention depends on the intended use in each case. Pulp fibers made by a chemical pulping process are generally preferred. Fully bleached, partially bleached and unbleached fibers are useful. It may often be desirable to use bleached pulp because of its higher degree of whiteness and a more pleasing appearance to the consumer. In a new embodiment of the invention, described in more detail below, the fibers are partially bleached, crosslinked and then completely bleached. For products such as Paper Towels 20 and absorbent pads for diapers, sanitary napkins, menstrual pads, and other similar absorbent paper products, it is particularly advantageous to use fibers derived from southern softwood pulp because of their improved absorption properties.

Suitable crosslinking agents for the developed process include C 2-8 dialdehydes as well as acid analogs of dialdehydes containing at least one aldehyde group, and oligomers of such dialdehydes and their acid analogs. These compounds can; react with at least two hydroxyl groups located in one cellulose chain or in closely spaced cellulose chains in one fiber. Those skilled in the art of crosslinking agents will recognize that dial-dehyde crosslinking agents exist or may react in many different forms, including the acid analog and oligomeric forms mentioned above. All such forms are intended to be included within the scope of the invention. Thus, a particular crosslinking agent is hereinafter referred to as both the crosslinking agent and other forms thereof that may be present in aqueous solution. Individual crosslinking agents that can be used in the context of the invention include Glu taraldehyde, glyoxal and glyoxylic acid. Glutaraldehyde is particularly preferred because it provides fibers with the highest levels of absorbency and elasticity, is believed to be safe and non-irritating to the human skin in a reacted, crosslinked state, and has formed the most stable crosslinks. Monoal-15 dehyde compounds that do not additionally contain a carboxyl group, such as acetaldehyde and furfural, have not been found to provide absorbent structures with the desired level of absorbency, resilience, and wetting response.

Surprisingly, it has been found that better suction pad properties can be achieved with degrees of crosslinking that are significantly lower than previously used. In general, unexpectedly good results are obtained with air cushions made of crosslinked discrete fibers in which the amount of crosslinking agent reacted with the fibers is about 0.5 to 3.5 mol% based on the molar amount of cellulose anhydroglucose.

The crosslinking agent is preferably contacted with the fibers in the liquid medium 30 under conditions such that the crosslinking agent. the substance penetrates the interior of the individual fibrous structures.

Other methods of carrying out the crosslinking agent treatment, such as spraying the fibers in a separated fluff form, are within the scope of the invention.

11 I Ml I'ltl III ·· - · • · 9 94433

The fibers are generally also treated with a suitable catalyst prior to crosslinking. The type and amount of catalyst and the method by which the catalyst is contacted with the fibers depend on the crosslinking-5 method used in each case. These variations are discussed in more detail below.

When the fibers are treated with a crosslinking agent and a catalyst, the crosslinking agent reacts substantially completely with the fibers in the absence of interfiber bonds, i.e., maintaining little interflow contact with the untreated pulp fibers, or the fibers are embedded in a non-fibrous solution. especially the formation of hydrogen bonds. This results in the formation of crosslinks that are intracellular in nature. Under these conditions, the crosslinking agent reacts to form crosslinks between the hydroxyl groups of a single cellulose chain or between the hydroxyl groups of the cellulose chains located in the single cellulose fiber.

The crosslinking agent is believed to react with the hydroxyl groups of the cellulose to form hemi-acetal and acetal bonds, although this has not been demonstrated and is not intended to limit the scope of the invention.

Acid reaction conditions favor the formation of acetal bonds, which are believed to be the desired type of bond producing stable crosslinks. Therefore, acid-catalytic crosslinking conditions are highly preferred for the purposes of this invention.

The fibers are preferably mechanically defibrated into a low density discrete fiber form known as "fluff" prior to the reaction of the crosslinking agent with the fibers. Mechanical defibration can be accomplished by a variety of methods that are currently known in the art and are described below. Preferably, the mechanical defibration is carried out by a method in which the formation of fiber buds and damage to the cults is kept to a minimum. One type of equipment that has been found to be particularly well suited for defibrating cellulosic fibers is the three-stage fluff-forming apparatus disclosed in U.S. Patent 3,987,968 (DR Moore and OA Shields), issued October 26, 1976, which is hereby incorporated by reference. to the explanation as a reference. In the fluff making apparatus 10 disclosed in U.S. Patent No. 3,987,968, wet pulp fibers are subjected to a combination of mechanical shock, mechanical agitation, air mixing, and limited air drying to form a substantially fiber-free fluff. The discrete fibers have given it a higher degree of curl and twist than is naturally present in such fibers. This increased curl and twist is believed to enhance the resilient nature of absorbent structures made of finished crosslinked fibers.

Other useful methods for defibrating cellulosic fibers include, but are not limited to, treatment with a Waring mixer and treatment of the fibers with a tangentially rotating plate grinder or wadding brush. It is preferred to direct an air stream to the fibers during such defibration to facilitate separation of the fibers into a substantially loose form.

* Regardless of the mechanical device used to form the fluff in each case, the fibers are preferably mechanically treated to initially contain at least about 20% and preferably about 40 to 60% moisture.

. Mechanical grinding of high density fibers or partially dried fibers can also be utilized to curl or twist the fibers in addition to the curl or twist caused by mechanical de-fibrillation.

The fibers made in accordance with this invention have a uniquely combined stiffness and resilience, which allows the absorbent structures made of the fibers to maintain a high level of absorbency, and have high resilience and the ability of the dry compressed absorbent structure to expand into wetting. In addition to the degree of crosslinking of the crosslinked fibers within the stated limits, they are characterized by a water retention value (WRV) of less than about 60, preferably about 28 to 45, in the case of conventional chemically fiberized papermaking fibers. The WRV of a particular fiber provides an indication of the degree of crosslinking and the degree of expansion of the fiber at the time of crosslinking. Anunat-ti people in the art know that the more swollen the fiber at the time of crosslinking, the higher the WRV at a given degree of crosslinking. Fibers with a very high degree of crosslinking, such as those obtained by the previously known dry crosslinking methods described above, have been found to have a WRV of less than about 25 and generally less than about 20. The crosslinking method used naturally affects the WRV of the crosslinked fiber. However, all methods that result in the degree of crosslinking and WRV within the stated limits are believed to be within and are intended to be within the scope of this invention. Useful crosslinking methods include dry crosslinking methods generally discussed at the beginning of the application and methods in which crosslinking occurs in an anhydrous solution. Some preferred dry crosslinking methods within the scope of this invention and methods in which crosslinking occurs in an anhydrous solution are discussed in more detail below. Aqueous solution crosslinking processes in which the solution causes the fibers to swell strongly result in fibers having a WRV greater than about 60. The stiffness and resilience provided by these fibers are insufficient for the purposes of this invention.

• · 12 94433

Specifically with respect to dry crosslinking processes, crosslinked discrete fibers can be prepared by such a process by obtaining a certain amount of cellulosic fibers, contacting the culturate with a crosslinking agent of the type and amount set forth above, separating the fibers mechanically, e.g. fibers and reacting the crosslinking agent with the fibers in the presence of a catalyst to form crosslinks while maintaining the fibers in a substantially loose form. The defibration step, regardless of the drying step, is believed to increase the curl. Subsequent drying is followed by twisting of the fibers as the curled shape of the fibers increases the degree of twisting.

15 "Curl" of a fiber is used herein to mean the curvature of the fiber about its longitudinal axis. By "twisting" is meant the rotation of a fiber about a cross section perpendicular to its longitudinal axis. By way of example only, and without expressly limiting the scope of the invention, it may be mentioned that crosslinked discrete fibers within the scope of the invention have been noted which contain an average of about six turns per millimeter.

Preserving the fibers during drying and crosslinking in a substantially loose form allows the # 25 fibers to be twisted during drying and thus crosslinked in such a twisted and curled state. Drying the fibers under conditions that can twist and curl is called drying the fibers under substantially non-limiting conditions. In contrast, drying the fibers in sheet form produces; resulting in dried fibers that are not twisted • · «· and curled like fibers dried in a substantially loose form. Hydrogen bonds between the fibers are thought to "limit" the relative twisting of the fibers and the curl-35.

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Il lll-t joined I I «-M · 1 13 94433

There are a variety of methods by which fibers can be contacted with a crosslinking agent and a catalyst. In one embodiment, the fibers are contacted with a solution containing, starting from the beginning, both a crosslinking agent and a catalyst. In another embodiment, the fibers are contacted with an aqueous solution of a crosslinking agent and allowed to soak before the catalyst is added.

The catalyst is then added. In a third embodiment, the crosslinking agent and the catalyst are added to the aqueous slurry of cellulose fibers. Other methods in addition to those disclosed herein will be apparent to those skilled in the art and are intended to be within the scope of this invention. Regardless of the particular method of contacting the fibers 15 with the crosslinking agent and catalyst, the cellulosic fibers, crosslinking agent, and catalyst are preferably mixed and / or soaked sufficiently to ensure thorough contact with and saturation of the loose fibers 20.

In general, any substance that catalyzes the mechanism of crosslinking can be used. Suitable catalysts are organic acids and acid salts. Particularly preferred catalysts are salts such as aluminum, magnesium, zinc and calcium chlorides, nitrates * and 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 and organic acids. The selected catalyst can be used as the sole catalyst or in combination with one or more other catalysts. The use of combinations of acid salts and organic acids as catalysts is believed to provide the most efficient crosslinking reaction. The reaction has been found to be surprisingly complete with catalyst combinations of zinc nitrate salts and organic acids such as sitic acid, and such combinations are preferred. Mineral acids are suitable for adjusting the pH of the fibers by contacting them with a crosslinking agent in solution, but are preferably not used as the primary catalyst.

The most preferred amount of crosslinking agent and catalyst to be used depends on the particular crosslinking agent used, the reaction conditions and the future use of the particular product.

The amount of catalyst preferably used depends, of course, on the type and amount of crosslinking agent in question and on the reaction conditions, in particular temperature and pH. In view of the technical and economic considerations, the amount of catalyst is generally preferably about 10 to 60% by weight of the amount of crosslinking agent to be added to the cellulosic fibers. For example, in the case where the catalyst used is zinc nitrate hexahydrate and the crosslinking agent is glutaraldehyde, the preferred amount of catalyst is about 30% by weight of the amount of glutaraldehyde to be added. Most preferably, an organic acid such as citric acid is also added as a catalyst in an amount of about 5-30% by weight of glutaraldehyde. In addition, it is desirable to adjust the pH of the aqueous portion of the cellulosic fibrous slurry or crosslinking agent solution to a target value * of about 2-5, more preferably about 2.5 to 3.5, during contact between the crosslinking agent and the fibers.

Cellulose fibers should generally be dewatered and possibly dried. Suitable and most preferred con-. the resistance varies depending on the type of fluffin ·· · »forming device used. In preferred embodiments, the fibers are dewatered and optimally dried to a consistency of about 30-80%. Even more preferably, the fibers are dewatered and dried to a consistency of about 40-60%. Drying the fibers to a moisture content within these preferred limits generally facilitates defibrating the fibers to a discrete form without excessive fiber knob formation, which is associated with higher moisture contents, and without major fiber damage, which is associated with lower moisture contents.

The dewatering can be carried out, for example, by methods such as mechanical compression of the pulp, centrifugation and air drying. Further drying is performed by methods known in the art as air drying or flake drying, under conditions such that long-term utilization of high temperature is not necessary. Too high a temperature at this stage of the process may lead to premature onset of silylation. Temperatures higher than about 160 eC are preferably not maintained for more than 2-3 seconds. The mechanical defibration is carried out as described above.

The defibrated fibers are then heated to a suitable temperature for a period of time sufficient to cause the crosslinking agent to cure, i.e., react with the cellulosic 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 when crosslinking. The crosslinking of fibers with a certain initial moisture content at a given temperature takes place at a higher rate when it is accompanied by continuous drying by blowing air, than when drying / heating takes place in a static oven. Those skilled in the art know that the temperature and time of curing a crosslinking agent can depend on each other in many ways. Conventional temperatures used to dry paper (e.g., 50 to 35 65 eC) for a period of about 30 to 60 minutes under static air conditions generally result in sufficient curing power at a fiber moisture content of less than about 5%. Those skilled in the art will also appreciate that higher temperatures and airflow will reduce the time required for co-wetting. However, the curing temperature is preferably kept below about 160 ° C, because at such temperatures above about 160 ° C, the fibers may turn yellow or otherwise damaged.

10 The maximum degree of crosslinking is achieved when the fibers are substantially dry (with a moisture content of less than about 5%). Due to this lack of water, the fibers crosslink in a substantially unexpanded, compressed state. Therefore, they are characterized by an al-15 low fluid retention value (FRV) compared to the values applicable to this invention. FRV means the amount of liquid (calculated on the basis of dry fibers) that remains absorbed into the fiber sample after the sample has been soaked and then centrifuged to remove the inter-fiber liquid. (The FRV is further defined and the method for determining the FRV is described.) The amount of liquid that crosslinked fibers are able to absorb depends on their ability to swell when saturated, i.e., their internal diameter or volume when expanded to their maximum level. This in turn depends on the degree of crosslinking. With the fiber and process remaining unchanged, the FRV of the fiber decreases until the fiber does not swell at all when tested, with an increase in the degree of introfibrous crosslinking. The FRV value of a fiber thus structurally describes the physical state of the fiber at the point of fiber saturation. Unless explicitly stated otherwise, the FRV data presented herein are given as fiber retention values (WRV). Other fluids such as brine and synthetic urine can also be advantageously used as the liquid medium in the analysis. The FRV of a particular fiber crosslinked by methods in which curing is highly dependent on drying, such as the method of this invention, generally depends primarily on the crosslinking agent and the degree of crosslinking. The WRV of fibers crosslinked by this dry crosslinking process using amounts of crosslinking agent suitable for the present invention is generally less than about 50 and greater than about 25, and preferably about 28-45. The WRV of bleached SSK fibers containing about 0.5 to 2.5 mole percent glutaraldehyde reacted with the fibers, based on the anhydroglucose molar amount of cellulose, has been found to range from about 40 to about 28, respectively. The degree of bleaching and the bleaching steps after crosslinking have been found to affect WRV. This effect is discussed in more detail below. Southern haw-15 wood-derived kraft fibers (SSK fibers) prepared by the dry crosslinking methods known prior to this invention have a higher degree of crosslinking than described herein and a WRV of less than about 25. As previously mentioned, such fibers have been found to be too rigid and they have been found to have a lower absorption capacity than the fibers of this invention.

In another method for producing crosslinked discrete fibers by the dry crosslinking method, the cellulosic fibers are contacted with a solution containing a crosslinking agent as described above. The fibers are arranged in a plate-like form either before or after contact with the crosslinking agent. The solution containing the crosslinking agent preferably also contains a catalyst suitable for dry crosslinking processes, such as those described above. The fibers in sheet form are preferably dried and crosslinked by heating to a temperature of about 120-160 ° C. After crosslinking, the fibers are mechanically separated into a substantially loose form. This is preferably accomplished by treating the fibers with a fluff-forming pad, such as the device disclosed in U.S. Patent 3,987,968, or may be accomplished by other fiber defibration methods known in the art. The crosslinked discrete fibers prepared by this sheet crosslinking process are treated with a sufficient amount of crosslinking agent to provide an amount of crosslinking agent that reacts with the fibers to form intramibular crosslinks of about 0.5 to 3.5 mole percent based on the anhydroglucose mole of cellulose and after defibration. Another consequence of drying and crosslinking the fibers in plate-like form is that the bonding between the fibers prevents the fibers from twisting and curling with further drying. Compared to crosslinked monofilaments made by a process in which the fibers are dried under substantially non-limiting conditions and then crosslinked in a twisted and curled form, it would be expected that absorbent structures made from relatively untwisted fibers made by the sheet curing method described above would has lower wet elasticity and poorer reactivity to wet wetting of the absorbent structure.

Another group of crosslinking methods suitable for this invention are crosslinking methods in which the. The hydrogenation takes place in an anhydrous solution. The same types of fibers (· · · suitable for dry crosslinking processes) can be used in the preparation of crosslinked fibers in an anhydrous solution. The fibers are treated with a sufficient amount of crosslinking agent to provide a crosslinking agent reacting with the fibers of about 0.5 to 3.5 mole percent. calculated after the crosslinking reaction, and 1 * with a suitable catalyst The crosslinking agent is reacted when the fibers are immersed in a solution which does not cause substantial swelling of the fibers, however, the fibers may contain up to about 30% water or be otherwise expanded in the crosslinking solution. fibers with a moisture content of about 30% The shape of such partially expanded fibers has been found to provide unexpected additional benefits, which are discussed in more detail below 5. The crosslinking solution contains an anhydrous, water-miscible, polar diluent weft. n acetic acid, propanoic acid or acetone, but suitable diluents are not limited thereto. 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 crosslinking media also include those disclosed in U.S. Patent 4,035,147 (S. Sangenis, G. Guiroy, and J. Quere), issued July 12, 1977, which is hereby incorporated by reference. However, the crosslinking solution may contain some water or other fiber-expanding liquid when the amount of water is insufficient to produce an expansion corresponding to the expansion of pulp fibers having a consistency of 70% (water content 30%). The water content of the crosslinking solution is preferably less than about 10% of the total volume of the solution, excluding fibers. Higher amounts of water in the crosslinking solution reduce the crosslinking efficiency and the crosslinking rate.

The impregnation of the crosslinking agent into the fibers may 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 solution containing the crosslinking agent or an anhydrous solution. Preferably, the fibers are mechanically defibrated to a loose form. This mechanical treatment can be accomplished by the methods previously described for 35 min fluff formation from fibers in connection with the dry crosslinking process described above.

• 20,94433

It is particularly advantageous to include in the manufacture of the fluff a mechanical treatment that causes the moist cellulosic fibers to assume a twisted and curled state that exceeds its possible twist and curl-5, which is the natural state of the fibers. This can be accomplished by initially obtaining wet fibers for fluff formation by mechanically treating them, e.g., by the methods described above, to defibrate the fibers to a substantially loose form and at least partially drying the fibers.

The relative amount of twist and curl applied to the fibers depends in part on the moisture content of the fibers. Without limiting the scope of the invention, the fibers are believed to rotate naturally in drying conditions where there is little contact between the fibers, i.e., the fibers are in a separated form. Mechanical treatment of wet fibers also initially causes the fibers to curl. When the fibers are then completely or partially dried under substantially non-limiting conditions, they rotate mechanically as the additional curl provided increases the degree of rotation. In the steps of forming a fluff by defibering, a high consistency moist pulp is preferably used, i.e. a pulp from which water has been removed so that the fiber-25 has a resistance of about 45 to 55% (as determined before the start of defibration).

After defibration, the fibers should be dried to a moisture content of about 0 to 30% before the fibers are contacted with the crosslinking solution 30, unless the defibration step has already resulted in fibers having a moisture content within said limits. The drying step should be performed with the fibers under substantially non-limiting conditions. In other words, the contact between the fibers should be minimized so that the inherent rotation of the fibers during drying is not prevented. Both 21 94433 air drying and flake drying methods are suitable for this purpose.

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

A bundle of fibers that is not mechanically defibrated can also be contacted with the crosslinking solution described above.

The amount of crosslinking agent and acid catalyst used depends on the reaction conditions such as consistency, temperature, water content of the crosslinking solution and fibers, the type of crosslinking agent and diluent contained in the crosslinking solution, and the degree of crosslinking desired. The amount of silicone to be used is preferably about 0.2 to 10% by weight of the total mass of the crosslinking solution, excluding the fibers. The preferred concentration of acid catalyst further depends on the acidity of the catalyst in the crosslinking solution. In general, good results can be obtained with a catalyst content of about 0.3 to 5% by weight (based on the mass of the crosslinking solution without fibers) of crosslinking solutions containing acetic acid, glutaraldehyde in preferred concentrations and a limited amount of water as a diluent. Slurries containing fibers and a crosslinking solution having a fiber consistency of less than about 10% by weight are preferred for crosslinking using the crosslinking solutions described above.

The crosslinking reaction may be carried out at ambient temperature or, to increase the rate of the reaction, at an elevated temperature, preferably less than about 40 ° C.

22 94433

There are a variety of methods by which fibers can be contacted with and crosslinked in a crosslinking solution. In one embodiment, the fibers are contacted with a solution comprising, from the beginning, both a crosslinking agent and an acid catalyst. The fibers are allowed to soak in the crosslinking solution,

During which time cross-linking occurs. In another embodiment, the fibers are contacted with a diluent and allowed to soak prior to the addition of the acid catalyst. After that, an acid catalyst is added, whereupon the formation of crosslinks begins. Other methods in addition to those disclosed herein will be apparent to those skilled in the art and are intended to be within the scope of this invention.

The crosslinking agent and crosslinking conditions are preferably selected to promote the formation of intramibular crosslinks. Thus, it is preferred that the crosslinking reaction occur substantially after the crosslinking agent has penetrated the fibers. The reaction conditions are more preferably selected to avoid immediate crosslinking unless the crosslinking agent has already penetrated the fibers. Reaction times of about 30 minutes, during which time crosslinking is substantially complete, are preferred. Longer reaction times are believed to provide only a negligible advantage in terms of fiber properties.

However, both shorter times, including the relatively immediate formation of crosslinks, and longer times are intended to be within the scope of the invention.

It is also possible to carry out the curing only in solution and then terminate the crosslinking reaction process; at a later stage by drying or heating treatments.

After the crosslinking step, the fibers are drained and washed. In the washing step, it is preferable to add a sufficient amount of a basic substance, such as sodium hydroxy-94433 23 dia, to neutralize any acid remaining in the pulp. After washing, the fibers are dehydrated and dried completely. Preferably, the fibers are mechanically defibrated a second time, which second defibration step 5 causes the crosslinked fibers to curl, for example to form a fluff by defibrating, between the dewatering and drying steps. When drying, the curly state of the fibers increases the twisting, as previously described in connection with the curling treatment to be carried out before the fibers are brought into contact with the crosslinking solution folder. The same devices and methods described in connection with the first mechanical defibration step for causing twisting and curling are useful in this second mechanical defibration step. As used herein, the term "defibrating" means any procedure that can be used to mechanically separate fibers into a substantially loose form, although fibers may already be obtained in such a form. "Defibrillation" therefore means a mechanical treatment step which a) separates the fibers into a substantially loose form, unless they are already in such a form, and b) causes the fibers to curl and twist in drying.

This second defibration treatment, which follows the crosslinking of the fibers, has been found to increase the twisted and curled nature of the pulp 25. This increase in the twisted and curled shape of the fibers results in greater resilience of the absorbent structures and a better response to wetting. The second defibroin treatment can be applied to all of the fibers described herein that are in a wet state. A particular advantage of the process in which the crosslinking takes place in an anhydrous solution is: * · that a second defibration step is possible without necessarily requiring an additional drying step.

This is because the solution in which the fibers are crosslinked 35 keeps the fibers flexible to the formation of crosslinks even though it does not cause undesired strong expansion of the fibers.

In addition, it has surprisingly been found that greater swelling of absorbent structures can be achieved when the compressed pads are wetted when the structures are made of fibers crosslinked in a twisted but partially expanded state than fibers that have been completely dried from water prior to crosslinking.

Improved results are obtained with crosslinked discrete fibers dried to a water content of approximately 18-30% before the fibers are contacted with the crosslinking solution. When the fibers are completely dried before they are brought into contact with the crosslinking solution, they are in an unexpanded, compressed state. The fibers do not swell when in contact with the crosslinking solution due to the low water content of the solution. As previously mentioned, the crucial feature of the crosslinking solution is that it does not cause substantial swelling of the fibers. When the diluent-diluent of the crosslinking solution is absorbed into the already expanded fiber, the fiber actually "dries" out of the water but remains in the previous, partially expanded state.

Again, to describe the degree of fiber expansion, it is practical to refer to the fluid retention value (FRV) of the fiber after crosslinking. Fibers crosslinked in a more expanded state have a higher FRV than fibers crosslinked in a less expanded state, all other factors being the same. Without limiting the scope of the invention, it is believed that partially expanded crosslinked fibers having an elevated FRV have greater wet resilience and better wettability than fibers crosslinked in the unexpanded state. Fibers with this increased wet resilience and improved wettability are able to swell or, when wetted, more easily unwind when attempting to return to their natural state. However, due to the stiffness provided by the crosslinking, the fibers are still able to provide structural support to the impregnated cushion made of fibers. The numerical FRV values associated with partially expanded crosslinked fiber-5 fractions presented herein are water retention values (WRV). As the WRV rises above about 60, the stiffness of the fibers is believed to be insufficient to provide the wet and wetting resilience that is desirable to support the impregnated absorbent structure.

In an alternative method of crosslinking the fibers in solution, the fibers are first soaked in an aqueous solution or other fiber swelling solution, dewatered and dried to the desired extent, and then immersed in a water-miscible crosslinking solution containing a catalyst and crosslinking agent as described above. agent. After removal of the liquid and before further drying, the fibers are preferably mechanically defibrated into a fluff to achieve the above advantages of increased twist and curl. Mechanical defibration after contacting the fibers with the crosslinking agent is not particularly desirable, as such defibration would evaporate the crosslinking agent and thus potentially lead to air pollution by the crosslinking agent or large air treatment investments due to the crosslinking agent.

In a variation of the method just described, the fibers are defibrated and then pre-soaked. in a concentrated solution containing a crosslinking agent and a fiber swelling diluent, preferably water. The concentration of the crosslinking agent is high enough to prevent water-induced swelling of the fibers. Aqueous solutions of 50% by weight of the crosslinking agents according to the invention, preferably glutaraldehyde, have been found to be suitable solutions for pre-soaking the fibers. The pre-soaked fibers are dewatered and immersed in a crosslinking solution containing a water-miscible polar diluent, a catalyst and a limited amount of water, and then crosslinked as described above. The crosslinked fibers can be dewatered and mechanically defibrated a second time before further processing into a sheet or absorbent structure, which dewatering and defibration is also performed as described above.

Pre-soaking the fibers with the crosslinking agent in aqueous solution before reacting the crosslinking agent gives the crosslinked fiber absorbent pads unexpectedly good absorption properties even compared to pads made by the above methods in which curing is performed in anhydrous solution from crosslinked fibers. in a solution containing 20 crosslinking agents.

The crosslinked fibers formed as a result of the above-described dry crosslinking methods and methods in which crosslinking takes place in an anhydrous solution are the products of this invention. The fibers of the invention can be used directly to make air-laid absorbent cores. In addition, due to their rigid and resilient nature, the crosslinked fibers can be wet-laid into an uncompacted sheet having a low density which, after drying, is useful directly as an absorbent core without further mechanical treatment. The crosslinked fibers can also be wet-laid into compact, commercial or long-distance pulp sheets.

Once the crosslinked discrete fibers have been made, they 35 can be dry-laid and formed directly into absorbent structures or wet layers and formed into absorbent structures or dense pulp sheets. The fibers of this invention offer a large number of significant operational benefits. However, it is difficult to form uniform wet-laid sheets from such fibers by conventional wet sheet-forming methods. This is due to the rapid flocculation of the crosslinked discrete fibers in solution. Such a flocculation hour can occur both in the headbox and when deposited on the perforated forming wire.

Attempts to form sheets from crosslinked discrete fibers by methods conventionally used to form sheets from pulp 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 number of bonds between the fibers, and the high permeability of the fibers deposited on the sheeting wire. Therefore, there is considerable commercial concern that a useful method of forming sheets from crosslinked discrete fibers should be provided, which method can be used to form wet-laid absorbent structures and dense pulp sheets for transport and subsequent defibering.

Thus, new methods have been developed for forming sheets of crosslinked discrete fibers that tend to flocculate in solution, in which a slurry containing crosslinked discrete fibers is first deposited on a perforated forming wire, such as a flat wire machine wire, in the same manner as conventional methods for forming sheets from pulp. Due to their crosslinked discrete nature, these fibers are deposited on the forming wire in a large number of fiber agglomerates. At least one liquid stream, preferably a water stream, is applied to the layered ♦ stacked fibers. Preferably, a series of jets 35 are applied to the fibers deposited on the forming wire so that the successive jets are decreasing. · ·: 28 94433 volumetric flow rate. The jets should have a sufficient velocity so that the impact of the liquid on the fibers prevents the fibers from flocculating and disintegrates the fiber flocs already formed. The step of settling the fibers is preferably carried out with a cylindrical wire, such as a wire roll, or some other similar device known in the art or possibly becoming such. After settling, the fiberboard can then be dried and possibly compressed more tightly as desired. The spacing of the jets varies depending on the respective flocculation rate of the fibers, the speed of the forming wire line, the dewatering through the forming wire and the number of jets, as well as their speed and the volume flow through them. The showers are preferably close enough to each other to avoid significant flocculation.

In addition to preventing the flocculation of the fibers and breaking up the fiber flocs, the liquid sprayed onto the fibers also compensates for the very rapid infiltration of the crosslinked discrete fibers by providing additional liquid medium into which the fibers can be dispersed for subsequent sheeting. The large number of jets with decreasing volume flow rate facilitates a systematic absolute increase in the consistency of the slurry, while providing a repetitive effect of disintegrating fiber flocs and preventing fiber flocculation. This results in the formation of a relatively uniform layer of fibers, which fibers are then placed immediately, i.e. before re-flocculation, in a sheet-like form, allowing the liquid to drain away and pressing the fibers against the perforated wire.

Pulp sheets made from the crosslinked fibers of this invention are more difficult to compress to have a density of conventional pulp sheets 35 than conventional, non-crosslinked cellulosic materials.

• · I

29 94433 Pulp of saku fibers. Therefore, it may be advantageous to combine non-crosslinked fibers with crosslinked fibers, such as fibers conventionally used in the manufacture of absorbent cores. The pulp containing rigid crosslinked fibers preferably contains about 5-90% of non-crosslinked cellulosic fibers, based on the total dry weight of the sheet, mixed with the crosslinked discrete fibers. It is particularly preferred to include about 5-30% of the highly ground non-crosslinked cellulosic fibers based on the total dry weight of the sheet. Such highly ground fibers are ground or ground to a degree of milling of less than about 300 ml CSF and preferably less than about 100 ml CSF. The uncrosslinked fibers are preferably mixed with an aqueous slurry of crosslinked discrete fibers. This mixture can then be formed into a dense pulp sheet for later defibrillation and molding into suction pads. The inclusion of uncrosslinked fibers in the pulp sheet facilitates its compression, while surprisingly reducing the absorbency of the suction pads to be formed later. In addition, uncrosslinked fibers increase the tensile strength of the pulp sheet and suction pads made either from it or directly from a mixture of crosslinked and non-crosslinked fibers. Regardless of whether the mixture of cross-linked and non-crosslinked fibers is first made into a pulp sheet and formed into a suction pad or whether the mixture is directly formed into a suction pad, the suction pad may be air-laid or wet-laid, as described above.

Sheets or webs made from crosslinked discrete fibers or blends that also contain non-crosslinked fibers preferably have a basis weight of less than about 800 g / m 2 and a density of preferably less than about 0.60 g / cm 3. Without intending to limit the scope of the invention in any way, particularly suitable for use as direct absorbent interiors in disposable products such as diapers, tampons and other menstrual pads are wet-laid sheets having a basis weight of about 300-600 g / m2 and a density of about 0, 15 - 0.30 g / cm 3. Structures with a higher basis weight and density than this level are considered to be best suited for subsequent grinding and air or wet layering to form a structure with a lower density and basis weight that is more useful in applications requiring absorbency.

10 However, structures with a higher basis weight and density have a surprisingly good absorbency and wettability. Other possible applications for the fibers of this invention are tissue paper sheets which may have a density of less than 0.10 g / cm 3.

In products in which the crosslinked fibers are located against or close to human skin, for further use, it is desirable to further treat the fibers to remove excess, unreacted crosslinking agent. It is preferred to reduce the concentration of unreacted crosslinking agent to below at least about 0.03% by dry weight of the cellulosic fibers. One treatment series that has been found to be successful includes washing successively the crosslinked fibers, soaking the fibers in an aqueous solution for quite a long time, lifting the fibers through a sieve, removing water from the fibers, for example by centrifugation, to a consistency of about 40-80%. water, mechanical defibration as described above, and air drying of the fibers. This process has been found to reduce the residual free crosslinker content to between about 0.01% and 0.15%.

·. In another method, to reduce the amount of crosslinking agent remaining, the easily extractable crosslinking agent is removed by alkaline washing. The basicity can be generated with basic compounds such as sodium hydroxide, or alternatively with oxidizing agents, such as chemicals commonly used as bleaching agents such as sodium hypochlorite, or amino compounds, for example, ammonium hydroxide, which hydrolyze the compounds to hemiacidize them. bases. The pH is preferably maintained at at least 7, and more preferably at least about 9 to prevent recovery of acetal crosslinks. It is advantageous to cause cleavage of the hemiacetal bonds while not affecting the acetal bonds. Therefore, extractants that operate under strongly basic conditions are preferred. A single wash with 0.01 N and 0.1 N ammonium hydroxide was found to reduce the residual crosslinker content to between about 0.0008% and about 0.0023% with a soaking time of 30 minutes to two hours. A soak time of about 30 to 15 minutes and about 0.01 N more concentrated ammonium hydroxide solutions are believed to produce only a negligible 1i radius.

Both single-step and multi-step oxidation have been found to be effective methods for removing residual crosslinker. A single step wash with sodium hypochlorite at an amount of available chlorine (av. Cl) of about 0.1 to 0.8% of the dry weight of the fibers was found to reduce the amount of residual crosslinker between about 0.0015% and about 0.0025%.

In a new method for producing crosslinked discrete fibers, the raw material fibers are bleached by a conventional multi-stage chain process, but in the middle of the chain the bleaching process is interrupted and the fibers are crosslinked in accordance with the present invention. After curing, the bleaching process is completed. In this way, it has been found that a sufficiently low level of residual crosslinker (less than about 0.006%) can be achieved. This method is believed to be an advantageous way to produce crosslinked fibers because the capital and processing difficulties caused by additional washing and extraction equipment and additional process steps are avoided due to the combination of the bleaching step and the residue removal step. The bleaching chain used and the point at which the chain is cut for crosslinking may vary from mouth to grate, which will be apparent to those skilled in the art. However, multi-stage bleaching chains with cross-linked DEP * or DEH * steps have been found to produce the desired results (* D - chlorine dioxide, E alkali extraction, P - peroxide, H - sodium hypochlorite). The steps following crosslinking of the bleach chain 10 are preferably basic treatments performed at a pH above about 7 and more preferably above about 9.

In addition to effectively reducing the amount of residual crosslinking agent after post-crosslinking, they have been found to facilitate the production of fibers with a higher FRV (liquid retention value) at the same degree of crosslinking. Fibers with a higher FRV have lower dry elasticity, i.e., they are easier to compact in the dry state, while retaining substantially the same wet elasticity and moisture reactivity as fibers that are otherwise similar but crosslinked after bleaching. out. This was particularly surprising given that the higher FRV has so far led to a deterioration in absorption properties.

* The crosslinked fibers disclosed herein are suitable for, but are not limited to, a variety of absorbent articles such as, but not limited to, disposable diapers, menstrual pads, sanitary napkins, temples, and bandages, each having an absorbent structure comprising the crosslinked discrete fibers disclosed herein. For example, a disposable diaper or other similar product having a liquid-permeable surface layer, a liquid-impermeable backing layer attached to the surface layer, and an absorbent structure comprising crosslinked discrete fibers is particularly contemplated. Such products are generally described in U.S. Patent 3,860,003 (Kenneth B. Buell), issued January 14, 1975, which is hereby incorporated by reference.

Absorbent cores for diapers and menstrual pads are conventionally made of unstiffened uncrosslinked cellulosic fibers, with a dry density of the absorbent cores of about 0.06 g / cm 3 and about 0.12 g / cm 3. As the absorbent core wets 10, its volume usually decreases.

It has been found that the crosslinked fibers of this invention can be used to make absorbent cores with significantly better liquid absorption properties, such as absorbent capacity and capillary absorption rate, compared to absorbent cores having the same density but made from conventional, uncrosslinked or previously known crosslinked fibers, but the better properties are not limited to those mentioned. Moreover, these pa-20 better absorbency results can be achieved while achieving higher wet resilience. For absorbent cores having a density of about 0.06 to 0.15 g / cm 3 and a substantially constant volume when wetted, it is particularly preferred to use crosslinked fibers having a degree of crosslinking such that they contain about 2.0 to 2 , 5 mol% of crosslinking agent reacted with the fibers of the anhydroglucose mole of dry cellulose. Absorbent cores made of such fibers combine structural integrity as desired, i. compressive strength, and resilience when wet. In the context of the present invention, the term "wet resilience" means the ability of a wetted pad to return to its original shape and volume when subjected to a compressive force and ceases to have an effect. The absorbent cores made from the fibers of this invention 35 regain a significantly greater portion of their original volume after the wet compression forces have ceased than the cores made from untreated and previously known crosslinked fibers.

In another preferred embodiment, the crosslinked discrete fibers are formed into either an air-laid or wet-laid (and then dried) absorbent core, which is compressed so that its dry density is 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 fibers, when the pad is completely impregnated with liquid. When forming an absorbent core from fibers having a dry density less than the wet density at equilibrium, the core compresses to its saturation point 15 after wetting until it reaches its wet density at equilibrium. Instead of forming the fibers into an absorbent core having a density in the dry state greater than the wet density at equilibrium, the core expands to its saturation point after wetting until it reaches its wet density at equilibrium. The equilibrium wet density of pads made from the fibers of this invention is significantly lower than that of pads made from conventional non-crosslinked fibers. The fibers of this invention can be compressed at a density greater than the equilibrium density to form a thin pad that expands upon wetting, increasing the suction capacity to a much greater extent than is achievable with non-crosslinked fibers.

Particularly high absorbency and resilience when wet, as well as good wettability, can be achieved by crosslinking. with a degree of incorporation of about 0.75 to 1.25 mol%, based on dry pulp. Such fibers are preferably formed into absorbent cores having a dry density greater than their wet density at equilibrium. The absorbent cores are preferably compressed to a density of about 0.12 to 0.60 g / cm 3, with the corresponding wet density at equilibrium being less than the density of the dry compressed core. The absorbent cores are also preferably compressed to a density of about 0.12 to 0.40 g / cm 3, with a corresponding wet density at equilibrium of about 0.08 to 0.12 g / cm 3. The above-mentioned fibers are not as rigid as crosslinked fibers with a degree of crosslinking of about 2.0 to 2.5 mol%, which makes them more suitable for compression at higher densities. The above-mentioned fibers also have a better reactivity to wetting in that they swell faster and to a greater extent than fibers with a degree of crosslinking of 2.0 to 2.5 mol%, have a higher wet elasticity and retain almost equal absorption capacity.

However, it should be noted that crosslinked fibers with a higher degree of crosslinking can be made into absorbent structures with a higher density, as can crosslinked fibers with a lower degree of crosslinking be made into absorbent structures with a lower density. All such structures achieve better performance in relation to previously known crosslinked discrete fibers.

Although the preferred embodiments of high and low density absorbent structures are discussed above, it should be understood that combinations of different density absorbent structures and different crosslinker concentrations, wherein said densities and concentrations are within the ranges set forth herein, provide better absorption properties. and integrating the absorbent structure into conventional cellulosic fibers and in the past. compared to known crosslinked fibers. Such embodiments are intended to be included within the scope of the invention.

Method for Determining the Liquid Retention Value 35 The following procedure was used to determine the water retention value of cellulosic fibers.

»· 36 94433

A fiber sample of about 0.3 to 0.4 g is soaked in a lid container in about 100 ml of distilled or deionized water at room temperature for about 15 to 20 hours. The soaked fibers are collected on a filter and transferred to a wire mesh 5 having a mesh number of 80 and supported about 3.8 cm (about 1½ inch) above the mesh base of a centrifuge tube having a mesh number of 60. The tubes are covered with plastic and the sample is centrifuged for 19 to 21 minutes at 1500 to 1700 x g. 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:

(1) WRV = (W-D) x 100 15 D

where W = wet weight of the centrifuged fibers, D = dry weight of the fibers and 20 W-D = amount of water absorbed

Method for determining the seepage capacity

The following procedure was used to determine the seepage capacity of the absorbent cores. Drip capacity was used as a common measure of gut absorption capacity and absorption rate.

A suction pad measuring 10 cm x 10 cm and weighing about 7.5 g is placed on a sieve net. Synthetic urine is dropped into the center of the pad at a rate of 8 ml / s. The flow of synthetic urine is interrupted when the first drop of synthetic urine leaks from the bottom or 30 sides of the pad.

Drip capacity is calculated by dividing the synthetic current by *. the difference between the mass of the pad before wetting the hall and the mass of its stiff pad with the dry weight of the fibers.

Method for Determining Wet Compressibility 35 The following procedure was used to determine the wet compressibility of absorbent structures. Wet compressibility was used as a measure of the wet compressive strength, structural wet integrity, and wet resilience of absorbent cores.

A square cushion measuring 10 cm by 5 x 10 cm and weighing 7.5 g is prepared, its thickness is measured and its density is calculated. Synthetic urine is absorbed into the pad ten times its dry weight or until it reaches its saturation point, whichever is lower. A pressure of 0.7 kPa (0.1 psi) is applied to the pad. The thickness of the pad 10 is measured after about 60 seconds, during which time the pad is in equilibrium. The compressive load is then raised to 7.6 kp (1.1 psi), the pad is allowed to equilibrate, and its thickness is measured. The compressive load is then reduced to 0.7 kPa (0.1 psi), allowed to equilibrate, and its thickness measured again. Calculate the density of the pad with an initial load of 0,7 kPa, a load of 7,6 kPa and a second load of 0,7 kPa (0,1 psir) called the elastic load of 0,7 kPa. The pore space-20 volume (cm3 / g) is then determined for each different load. Pore volume is the difference between the inverse of the wet density of the pad and the fiber volume (0.75 cm 3 / g). Pore volume at loads of 0.7 kPa and 7.6 kPa are useful indicators of wet compressive strength and structural wet integrity. The higher wetting volume of huo-25 at the normal initial density of the pad is an indication of higher wet compressive strength and higher structural wet integrity. The pore volume difference at loads of 0.7 kPa and 7.6 kPa is suitable for comparing the wet resilience of absorbent pads. The smaller pore volume-30 difference at loads of 0.7 kPa and 7.6 kPa is an indication of a larger. wet resilience. The difference in thickness between the dry pad and the impregnated pad before compression has also been found to be a useful indicator of the wettability of the pads.

35. · ·.

38 94433

Method for the determination of angular compressibility

Absorbency The following procedure was used to determine the angular compressibility of sten cores. Dry compressibility was used as a measure of the dry resilience of the interiors.

5 An air-laid pad measuring 10 cm x 10 cm and weighing about 7.5 g is prepared and pressed in a dry state with a hydraulic press at a pressure of 2.4 MPa. The pad is inverted and the compression is repeated. The thickness of the pad is measured before and after pressing so that the pad is not subjected to a load at the same time.

The density is then calculated before and after compression by dividing the mass by the product of area and thickness. Larger differences in pre- and post-compression densities are indicative of lower dry resilience.

Method for determining the amount of alu-taraldehvdl reacted with cellulose fibers

The following procedure was used to determine the amount of glutaraldehyde that reacted with the cellulosic component of the individual fibers crosslinked with glutaraldehyde to form introfibrous crosslinks.

A sample of crosslinked discrete fibers is extracted with 0.1 N HCl. The extract is separated from the fibers, and the same extraction / separation procedure is then repeated for each sample -... 25 le 3 more times. An aqueous solution of 2,4-dinitrophenylhydrazone (DNPH) is mixed separately with the extract * obtained from each extract. The reaction is allowed to proceed for 15 minutes, after which chloroform is added to the mixture. The reaction mixture is stirred for an additional 45 minutes. The chloroform and water layers 30 are separated by a separatory funnel. The amount of glutaraldehyde is determined by analysis of the chloroform layer with a DNPH derivative by high performance liquid chromatography (HPLC).

The chromatographic conditions used in the HPLC analysis were: column - C-18 reverse phase; detector - UV, wavelength 35 nm 360 nm; mobile phase - methanol / water mixture 39: 3433 in 80:20; flow rate - 1 ml / min; measurement - peak height. A calibration curve describing the relationship between peak height and glutaraldehyde content was generated by measuring the height of HPLC peaks from five standard solutions with known glutaraldehyde content in the range of 0-25 ppm.

Each of the four chloroform phases obtained with each fiber sample was analyzed by HPLC, the height of the peaks was measured, and the corresponding glutaralde-10 hyd content was determined from the calibration curve. The glutaraldehyde contents obtained for each extraction were then summed and the sum divided by the mass of the fiber sample (dry fiber mass) to obtain the glutaraldehyde content based on the mass of the fibers.

There were two glutaraldehyde peaks in each HPLC chromatogram. Either glutaraldehyde peak can be used as long as the same peak is used throughout the procedure.

Example 1 This example demonstrates the effect of different concentrations of the crosslinking agent, glutaraldehyde, on the absorbency and resilience of pads made of crosslinked discrete fibers. The crosslinked discrete fibers were prepared by the dry crosslinking method.

A certain amount of never-dried southern softwood kraft pulp (SSK pulp) was used for each sample. 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 certain amount of 50% aqueous glutaraldehyde, 30% (by weight of glutaraldehyde) zinc nitrate hexahydrate, de-mineralized water, and sufficient 1 N HCl to lower the pH of the slurry to about 3.7. The fibers were soaked in the slurry for 20 minutes and then dewatered by centrifugation to obtain a fiber consistency.

• I

40,94433 li about 34-35%. The dewatered fibers were then dried in a blow dryer using ambient air until the fiber consistency was about 55-56%. The air-dried fibers were defibrated using a three-stage fluff forming apparatus such as that described in U.S. Patent No. 3,987,968. The defibrated fibers were placed on plates and cured in a substantially static drying oven at 145 ° C for 45 minutes. Silylation was complete while in the oven. The crosslinked 10 single fibers were placed on a sieve and washed with water at about 20 ° C, soaked for 1 hour in water at 60 ° C with a consistency of 1%, lifted through a sieve, washed a second time with water at about 20 ° C, centrifuged to remove The resistance to -15 became 60%, was defibrated in a three-stage fluff-forming apparatus as described above, and dried by drying for four (4) hours in a static drying oven at 105 ° C. The dried fibers were air-layered to form absorbent pads. Type-20 was now compressed with a hydraulic press to a density of 0.10 g / cm 3. The pads were then tested for absorbency, resilience and the amount of glutaraldehyde reacted according to the procedures described. The amount of reacted Glu taraldehyde is expressed as a molar percentage of the amount of dry cellulose anhydroglucose in the fibers. The results are reported in Table 1.

• · • · 41 94433

table 1

Dripping Capacity- Wet Compressibility Sample- Glutaraldesicity to Water (cm3 / g) Tea Hydr (Mol-%) Introduction Rate- 0.7kPa 7.6kPa 0.7kPa 5 No. Added / WRV Sat 8 ml / s (0.1 (1, 1 (0.1 _reacted (%) (a / s) _psi) psi) psi) 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

10 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 15 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 batch of fibers (N / A) - not available 20 Example 2 The purpose of this example is to show that by performing a series of bleaching steps on the fibers after crosslinking, a low level of extractable crosslinker can be achieved. . The level of extractable crosslinker was determined by soaking a fiber sample in deionized water at 40 ° C for one hour at a consistency of 2.5%. The amount of glutaraldehyde extracted in the water was determined by HPLC and is expressed as the dry weight of the fibers. The fibers were crosslinked by the dry crosslinking method.

Southern softwood kraft pulp (SSK pulp) was used. The pulp fibers were partially bleached with a bleaching chain consisting of the following steps: chlorination (C) * A slurry with a consistency of 3-4% was treated at about pH 2.5 and about 38 ° C for 30 minutes so that the available the amount of chlorine (av. Cl) was about 5%; an alkali extraction slurry having a consistency of • <42 94433 12% was treated with NaOH (1.4 g / l) at about 74 ° C for 60 minutes; and a hypochlorite treatment (H) slurry with a consistency of 12% was treated at pH 11-11.5 and 38-60 ° C for 60 minutes with sufficient sodium hypochlorite to achieve an Elretho whiteness of 60-65 and 15. , 5 to 16.5 mPa.s (15.5 to 16.5 cp) viscosity. From the partially bleached fibers, crosslinked monofilaments were prepared according to the procedure described in Example 1 and using glutaraldehyde as a crosslinking agent. 2.29 mol% of glutaraldehyde was retained in the fibers, based on the molar amount of cellulose anhydroglucose in the dry fibers. The extractable amount of glutaraldehyde in such fibers is typically about 1000 ppm (0.1%).

Bleaching of the partially bleached individual fibers was then continued and completed with a chain of chlorine dioxide treatment (D), extraction (E) and sodium hypochlorite treatment (H) (DEH). In the chlorine dioxide step (D), the crosslinked discrete fibers were lyophilized in an aqueous slurry having a consistency of 10% and also containing a sufficient amount of sodium hypochlorite to provide 2% of the available chlorine by dry weight of the fibers. After stirring, the pH of the slurry was lowered to about 2.5 by the addition of HCl and then raised to 4.4 by the addition of NaOH. The pulp slurry was then placed in a 70 ° C Crise oven for 2.5 hours, filtered, rinsed with water until neutral, and centrifuged to a consistency of 61.4%.

In the extraction step, a 10% aqueous slurry of dehydrated fibers was treated with NaOH (0.33 g / l) at 40 ° C for 1.5 hours. The ** fibers were then sieved, rinsed with water until neutral, and centrifuged to a consistency of 62.4%.

11; Iti »01 (11 I I 4« 0

., 94453 43

Finally, for the sodium hypochlorite step (H), a fiber slurry having a consistency of 10% and containing enough sodium hypochlorite was prepared so that the amount of chlorine available was 1.5% of the dry weight of the fibers. The slurry was stirred and heated in a 50 ° C oven for one hour. The fibers were then sieved, rinsed until pH 5.0, and centrifuged to a consistency of 62.4%. The dehydrated fibers were air-dried, fluffed, and dried to 10 ° C for one hour. The amount of extractable glutaraldehyde in the fully bleached crosslinked discrete fibers was 25 ppm (0.0025%). This is well below the maximum amount of glutaraldehyde that can be extracted, which is considered acceptable in applications where the fibers are used close to human skin.

It was also found that pads made of partially bleached, crosslinked and subsequently bleached fibers had a surprisingly higher nes-20 tea retention value and capillary absorption rate and always the same dripping capacity and wet resilience as those crosslinked after complete bleaching. However, due to the higher WRV, the fibers crosslinked in the middle of the bleaching chain were more compressible in the dry state.

Substantially similar results were obtained when the last hypochlorite step (H) was replaced by a peroxide bleaching step (P). In step (P), a slurry having a consistency of 10% was treated with hydrogen peroxide in an amount of 0.5% by weight of the fibers at pH 11-11.5 and 80 ° C for 90 minutes.

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Claims (2)

A process for producing cross-linked individual cellulose fibers, characterized in that a) moistened cellulose fibers having a moisture content of at least about 30% by weight are abolished; b) said fibers are defibrated to bring them into a substantially individual shape and the fibers are dried to a moisture content of 18-30% by weight under conditions which substantially prevent the formation of hydrogen bonds between the fibers; c) said fibers are contacted with a crosslinking solution containing an anhydrous, water miscible polar diluent, such an amount of water insufficient to swell the fibers to an degree of expansion corresponding to fibers having a moisture content of about 30% by weight, and a cross-linking agent, wherein the cross-linking agent is selected from C bonds between the fibers, wherein crosslinks within the fibers are formed, and the fibers are contacted with a sufficient amount of a crosslinking agent such that 0.5 mole% - 3.5 mole% of the crosslinking agent, calculated on the number of cellulose anhydroglucose mols. reacts with the fibers to form cross-links within the fibers; and d) the fibers are defibrated to a substantially individual form and the individual fibers are dried under conditions which substantially prevent the formation of hydrogen bonds between the fibers, said individual fibers having a water bond value of 25-60. lii »- · 94433 2. A process for producing cross-linked individual cellulose fibers, characterized in that a) moistened cellulose fibers having a moisture content of at least about 30% by weight are removed; b) said fibers are defibrated to bring them into a substantially individual form and said fibers are dried to a moisture content of 18-30% by weight under conditions which substantially prevent formation of hydrogen bonds between the fibers; c) said fibers are contacted with a crosslinking agent in an aqueous solution, the crosslinking agent being selected from C2-C8 dialdehydes, C2-C8 dialdehyde acid analogs with at least one aldehyde group, and oligomers of said dialdehyde and dialdehyde acid analogs; d) liquid is removed from the fibers and dried to a moisture content of less than 30% by weight; e) the fibers are contacted with a solution containing an anhydrous, water miscible polar diluent and such amount of water which is insufficient to swell the fibers to an degree of expansion corresponding to fibers having a moisture content of about 30% by weight; f) said cross-linking agent is reacted with the fibers where the fibers are in contact with said solution in the substantial absence of bonds between the fibers, thereby forming cross-links within the fibers; and g) the fibers are defibrated to a substantially individual form and the fibers are dried under conditions which substantially prevent formation of hydrogen bonds between the fibers, the individual fiber containing 0.5 mole% - 3.5 mole% of the crosslinking agent calculated on the basis of the fiber. the anhydroglucose mols of the cellulose reacted with the individual fibers and the individual fibers having a water-binding value of 25-60.