FI94436B - Method for making discrete crosslinked cellulosic fibers - Google Patents

Abstract

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

Description

94436

The present invention relates to a process for producing crosslinked discrete cellulose fibers. More specifically, this invention relates to a method of achieving a reduced level of residual crosslinking agent for making such fibers.

Fibers crosslinked in a 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 15 are primarily between the cellulose molecules of a single fiber rather than between the cellulose molecules of different fibers. Crosslinked discrete fibers are generally considered suitable for use 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) water-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, uncrosslinked fibers in at least one significant absorbent 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 refers to 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 consistency with the objects of this invention and the disclosure, wet resilience is contemplated and reported for an absorbent structure that is 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 (L. J. Bernardin), issued December 21, 1965. The crosslinked discrete fibers are prepared by impregnating the expanded fibers in aqueous solution with a crosslinking agent (crosslinking agent), removing water and defibrating the fibers mechanically, and drying the fibers at an elevated temperature to effect crosslinking while keeping the fibers substantially separated. The fibers are inherently crosslinked in an unexpanded compressed state as a result of the water being removed from the fibers prior to crosslinking. The methods exemplified in U.S. Patent No. 3,224,926, in which the formation of crosslinks is induced with the fibers in an unexpanded, compressed state, are termed methods for making "dry-crosslinked" fibers. Dry-crosslinked fibers are characterized by a low liquid-30 retention value (FRV). U.S. Patent 3,440,135 (R.

. Chung), published April 22, 1969, it is proposed to soak the fibers in an aqueous solution of a crosslinking agent to reduce the formation of interfiber bonds prior to performing a dry crosslinking process similar to that described in U.S. Patent No. 3,224,926.

94436 3 This time-consuming (preferably about 16 to 48 hours) pretreatment is claimed to improve product quality by reducing the fiber knob content resulting from incomplete defibration.

Methods for making crosslinked fibers in aqueous solution are described in U.S. Patent 3,241,553 (F. H. Steiger), published 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 crosslinked 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 to be crosslinked in aqueous solution are crosslinked in an uncompressed, expanded state.

Compared to dry-crosslinked fibers, the aqueous-crosslinked fibers disclosed in U.S. Patent 3,241,553 have greater flexibility and lower stiffness, and are characterized by a higher fluid retention value (FRV). Absorbent structures made from crosslinked fibers in aqueous solution have lower resilience when wet and dry than pads made from 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 a crosslinking agent and a catalyst. with a substantially anhydrous solution containing insufficient water to swell-30 behind 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 'process and the fibers produced by it are referred to as the' anhydrous solution cross-linked 'fibers.

The anhydrous solution-crosslinked fibers disclosed in U.S. Patent No. 4,044,364 do not swell even when in prolonged contact with solutions known to those skilled in the art as swelling reagents. Crosslinks greatly stiffen them, as do dry-crosslinked fibers, and the absorbent structures made from them have a relatively high resilience when wet and dry.

The crosslinked fibers described above 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 sufficient absorbent advantages, 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 reported cross-linking agent in the literature, formal-20 dehyde, is unfortunately irritating to human skin, and has been associated with other human safety concerns.

Both technical and economic reasons have prevented the reduction of the amount of free formaldehyde in the crosslinked product to a sufficiently low level to avoid skin irritation 25 and other concerns for human safety.

Also, crosslinked fibers made from crosslinking agents other than formaldehyde may contain crosslinking agent in greater amounts than is advantageous for use near human skin. Therefore, it may be desirable-. a way 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 efficient but does not reduce the amount of residual crosslinking agent to the desired level. In addition, such post-crosslinking washes necessarily incur additional capital and operating costs associated with washing and drying the fibers.

It is an object of the present invention to provide a process for producing crosslinked discrete fibers in which the level of residual crosslinking agent is low and in which the capital and operating costs associated with the equipment are minimized.

It has been found that the above object can be achieved and crosslinked discrete fibers having a reduced level of residual unstable crosslinking agent can be obtained by using the following method, characterized in that a) obtaining cellulosic fibers and contacting the fibers in an aqueous solution with sufficient an amount of a crosslinking agent selected from the group consisting of C2-8 dialdehydes, acid analogs of said dialdehydes, and oligomers of said dialdehydes and acid analogs to give, after the reaction, 0.5 mol% to 3.5 mol% of a crosslinking agent based on the molar amount of cellulose anhydroglucose, wherein said reacted fibers have a water retention value of 28 to 45; B) dewatering the fibers until the consistency is 30 to 80% by weight of the fiber and defibering the fibers to a substantially discrete form; c) air-drying the fibers as they are separated, under conditions that avoid fiber-to-fiber contact, reacting the crosslinking agent with the fibers. with the formation of intra-fiber crosslinks ♦ ”* in the substantial absence of inter-fiber bonds; and d) washing the crosslinked fibers with an alkaline solution having a pH greater than 7 and containing an ingredient 35 which breaks the hemiacetal bonds when neutral to the acetal bonds 94436 6, thereby reducing the amount of unreacted and residual crosslinking agent.

The fibers are preferably washed with an alkaline solution having a pH greater than about 9. Also 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 molar amount of anhydroglucose of cellulose 10 and said 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 prepared by similar crosslinking methods, except that fully bleached fibers are used. Corresponding absorbent structures made from the fibers of this invention have improved absorption properties, including wet resilience and wetting response.

A variety of cellulosic fibers of natural origin are suitable for this invention. Preferably, the decomposed fibers obtained from softwood, hardwood or cotton linter are used. Fibers from esparto grass, 2'.S bagasse, hard hair, flax and other sources of lignin 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 other sheet-like form are preferably converted to a non-sheet-like form * by mechanical disintegration of the sheet, preferably 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 94436 7 undried fibers. 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 more consumer-friendly appearance. 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 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 due to their better absorption properties.

Suitable crosslinking agents for the developed method include C2-8 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 are capable of reacting with at least two hydroxyl groups located in a single cellulose chain or in closely spaced cellulose chains in a single fiber. Those skilled in the art of crosslinking agents will recognize that the dialdehyde crosslinking agents described above 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 refers to both that crosslinking agent and its 94436 8 other forms that may be present in aqueous solution. Individual crosslinking agents that can be used in the context of the invention include glutaraldehyde, 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 human skin in a reacted, crosslinked state, and has formed the most stable crosslinks. Monoaldehyde 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 absorbent capacity, resilience, and wetting response.

15 It has surprisingly been found that a better performance of the suction pad can be achieved with degrees of crosslinking which are considerably lower than previously used. In general, unexpectedly good results are obtained with absorbent pads 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 a liquid medium under conditions such that the crosslinking agent penetrates the interior of the discrete fiber structures. However, other methods for carrying out the crosslinking agent treatment, such as spraying the fibers in a separated fluff form, are within the scope of the invention.

In general, the fibers are also treated with a suitable catalyst. account before bridging. The type and amount of catalyst and the method of contacting the catalyst with the fibers are determined by the crosslinking method used. These variables are discussed in more detail 35 below.

94436 9 When treating fibers with a crosslinking agent and a catalyst, the crosslinking agent reacts with the fibers in the substantial absence of interfiber bonds, i.e., while maintaining contact between the fibers 5 as a small fluff compared to 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 cellulose chains located closest to each other in a single cellulose fiber.

The crosslinking agent is believed to react with the hydroxyl groups of 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. Acidic reaction conditions promote the formation of acetal bonds, which are believed to be a desirable type of bond that produces sta-20 bile crosslinks. Therefore, acid-catalytic crosslinking conditions are highly preferred for the purposes of this invention.

The fibers are preferably mechanically defibrated to a known low fiber form of "fluf-25 fina" of low density 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 or will be apparent below. Preferably, the mechanical de-30 fibrillation is carried out by a method with fiber knobs. formation and damage to the fibers is kept to a minimum. One type of device 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 No. 3,987,968 to D. R. Moore and 0. A.

10 94436

Shields), published October 26, 1976, which is hereby incorporated by reference. In the fluff forming apparatus 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 provide 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 thought 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 cotton brush. It is advantageous to direct an air flow into the fibers during such defibration to make it easier to separate 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 40-60% moisture.

Mechanical grinding of high consistency 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 defibration.

The fibers of the present 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 high resilience, as well as the dry compressed absorbent structure. swelling response to wetting.

Il Mi joined IMU »u 94436

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 dried 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. Those skilled in the art will recognize that the more expanded the fiber at the time of crosslinking, the higher the WRV at a given degree of crosslinking. For fibers with a very high degree of crosslinking, such as those obtained by the previously known dry crosslinking methods described above, the WRV has been found to be less than about 25 and generally less than about 20. The crosslinking method used naturally affects the WRV of the crosslinked fiber. All methods that result in the degree of crosslinking and WRV within the stated limits are believed to be within the scope of this invention. Useful crosslinking methods include the generally described dry crosslinking methods 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.

As regards dry - crossing methods in particular,. crosslinked discrete fibers can be prepared by such a method of obtaining a certain amount of cellulosic fibers, contacting the fibrous slurry with a crosslinking agent of the type and amount set forth above, mechanically separating the fibers, e.g., de-94436 12 fibrillating, substantially loosening and drying the fibers, and crosslinking react with the fibers in the presence of a catalyst to form crosslinks while the fibers remain 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. By "curl" of a fiber is meant herein a curve of the fiber from 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 should be noted that crosslinked discrete fibers within the scope of the invention have been noted which contain an average of about six turns per millimeter.

Storing the fibers during drying and crosslinking in a substantially loose form allows the fibers to twist during drying and thus crosslink them 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-like form results in dried fibers that are not twisted and curled like fibers dried in substantially loose form. Hydrogen bonds between the fibers are believed to "limit" the relative twisting and twisting of the fibers.

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, from the outset, 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, 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 specific method by which the fibers are contacted with the crosslinking agent and the catalyst, the cellulosic fibers, the crosslinking agent, and the catalyst are preferably mixed and / or allowed to soak sufficiently to ensure thorough contact with and saturation of the loose fibers.

In general, any substance that catalyzes the mechanism of crosslinking can be utilized. 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 complete surprisingly well with catalyst combinations of zinc nitrate salts and organic acids such as citric acid. and the use of such combinations is preferred. Mineral acids are 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.

14 94436

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 will, of course, depend on the type and amount of crosslinking agent in question and on the reaction conditions, in particular temperature and pH. From a technical and economic point of view, the amount of catalyst is generally 10 to preferably about 10 to 60% by weight of the amount of crosslinking agent 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 catalyst is about 30% by weight of the amount of glutar-15 aldehyde added. Most preferably, an organic acid, such as citric acid, is also added as a catalyst in an amount of about 5 to 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 to 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. The appropriate and most preferred consistency will vary depending on the type of fluffin-25 forming device used. In preferred embodiments, the fibers are dewatered and optimally dried to a consistency of about 30-80%. More preferably, the fibers are dewatered and dried to a consistency of about 40-60%. Drying of the fibers to a moisture content within these preferred limits. ; I generally facilitate defibrating the fibers into a discrete form without ϊ excessive fiber knob formation associated with higher moisture contents, and without major fiber damage associated with lower moisture-3b concentrations.

il '»» · · mk Tiia *! · <94436 15

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

The defibrated fibers are then heated to a suitable temperature for a time sufficient to cause the crosslinking agent to cure, i.e., to react with the 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 when crosslinking. The crosslinking of fibers having a certain initial moisture content at a given temperature occurs 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 will recognize 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., about 50 to 65 ° C) for a period of about 30 to 60 minutes under static atmospheric conditions generally result in a sufficiently effective cure at a lower moisture content of the fibers. than about 5%. Those skilled in the art will also appreciate that * higher temperatures and airflow reduce the time required for curing. 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.

94436 16

The maximum degree of crosslinking is achieved when the fibers are substantially dry (containing less than about 5% moisture). Due to this lack of water, the fibers are crosslinked in a substantially unexpanded, compressed state. As a result, they are characterized by a low fluid retention value (FRV) compared to values applicable to the present invention. FRV means the amount of liquid, calculated on the basis of dry fibers, which remains absorbed into the fiber sample after the sample has been soaked and then centrifuged to remove the inter-fiber liquid. (The FRV is defined in more detail below and the method for determining it is shown.) The amount of liquid that crosslinked fibers can absorb depends on their ability to swell when saturated, that is, their inner diameter or volume when expanded to their maximum. 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 wetted, with an increase in the degree of crosslinking within the fiber.

20 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 here are expressed as water retention values (WRV) of the fibers. Other liquids, such as brine and synthetic urine, can be advantageously used as the liquid medium in the assay. 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. For fibers crosslinked by this dry crosslinking process using amounts of crosslinking agent suitable for the present invention, the WRV is generally less than about 50 and greater than about 25 and preferably about 28 to 45. Bleached SSK fibers containing from about 0.5 to about Based on the molar amount of cellulose anhydroglucose, 94436 17 glutaraldehyde reacted with 2.5 mol% of fibers, the WRV has been found to range from about 40 to about 28, respectively. The use of bleaching stages and post-crosslinking bleaching steps has been found to affect 5 WRVs. This effect is discussed in more detail below. Southern softwood kraft fibers (SSK fibers) prepared by the prior art dry crosslinking methods have a higher degree of crosslinking than disclosed herein and a WRV of less than about 25. As previously mentioned, such fibers have been found to be too rigid and have been found to have have a lower absorption capacity than the fibers of this invention.

In another method for producing crosslinked separate 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 sheet-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 cross-linking the fibers are mechanically separated into substantially individual form. This is preferably accomplished by treating the fibers with a fluff-forming device, such as that disclosed in U.S. Patent No. 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 method are treated with a sufficient amount of crosslinking agent to provide a reactive amount of crosslinking agent to the fibers of about 0.5 to 3.5 mole percent based on the molar amount of cellulose anhydroglucose and measured after defibration. Another consequence of drying and crosslinking the fibers in sheet form is that the intermediate bonding of the fibers prevents the fibers from twisting and curling with further drying. Compared to crosslinked discrete fibers 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 the relatively untwisted sheet curing method described above of which fibers, have lower wet resilience and poorer ability to respond to wetting of the dry absorbent structure.

Another group of crosslinking methods that can be applied to the present invention are crosslinking methods in which curing takes place in an anhydrous solution. The same types of fibers suitable for dry crosslinking methods are useful in the manufacture of fibers crosslinked in an anhydrous solution. The fibers are treated with a sufficient amount of crosslinking agent to provide an amount of crosslinking agent that reacts with the fibers of about 0.5 to 3.5 mole percent, calculated after said crosslinking reaction, and a suitable catalyst. The crosslinking agent is reacted with the fibers immersed in a solution that 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 to the extent that they correspond to fibers having a moisture content of about 30 to 30%. The shape of such partially expanded fibers has been found to provide unexpected additional benefits, which are discussed in more detail below. The crosslinking solution includes, but is not limited to, an anhydrous, water-miscible, polar diluent such as acetic acid, propanoic acid, or acetone. Preferred catalysts 94436 19 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 can be accomplished in the crosslinking solution itself or in a pretreatment step, which includes, but is not limited to, impregnating 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 forming a fluff from fibers in connection with the dry crosslinking process described above.

It is particularly advantageous to include in the preparation 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, which is the natural state of the fibers. This can be accomplished by initially obtaining wet fibers for fluff formation, mechanically treating them, e.g., by the methods described above, to defibrate the fibers to a substantially loose form, and drying the fibers at least partially.

The relative amount of curl and twist 5 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 dried, in whole or in part, under substantially non-limiting conditions, they rotate as the additional curl obtained mechanically 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 consistency has become about 45-55% (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, 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 air drying and flake drying methods are suitable for this purpose.

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

The fiber bundle, which is not mechanically defibrated, can also be contacted with the silage solution described above.

The amounts of crosslinking agent and acid catalyst used depend 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 crosslinking agent used is preferably about 0.2 to 10% by weight of the total weight 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 0.3-5% by weight (based on the mass of the crosslinking solution without fibers) in 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 below about 40 ° C.

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 containing a substrate. starting with both a crosslinking agent and an • acid catalyst. The fibers are allowed to soak in the crosslinking solution, during which time 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 to begin the formation of crosslinks. 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, the crosslinking reaction is preferably carried out substantially after the crosslinking agent has penetrated the fibers. The reaction conditions are preferably chosen so as to avoid the immediate formation of crosslinks, unless the crosslinking agent has already penetrated into 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 approximately immediate crosslinking, and longer times are intended to be within the scope of this invention.

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

25 After the crosslinking step, the liquid is drained from the fibers and washed. In the washing step, it is advantageous to add sufficient basic substance, such as sodium hydroxide, to neutralize any acid remaining in the pulp. After washing, the fibers are dehydrated and dried completely. Preferably, the fibers are defibrated. mechanically a second time, each second defibrillation step * · causes the crosslinked fibers to curl, for example by forming a fluff by defibrating, between the dewatering and drying steps. When drying, the curly space 35 of the fibers increases twisting, as previously described prior to contacting the fibers with the crosslinking solution during the curling treatment. The same devices and methods described in connection with the first mechanical defibration step for effecting 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 during 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. This increase in the twisted and curled structure 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 crosslinked fibers described herein that are in a wet state. However, a particular advantage of the process in which crosslinking takes place in an anhydrous solution is that a second defib-.25 concentration step is possible without necessarily requiring an additional drying step. This is because the solution in which the fibers are crosslinked keeps the fibers flexible after crosslinking, without, however, causing undesired strong expansion of the fibers.

3G In addition, it has surprisingly been found that greater swelling of absorbent structures can be achieved when compressed pads are wetted when the structures are made of fibers crosslinked in a twisted but partially expanded state than fibers dried completely from water prior to crosslinking.

24 94436

Better results are obtained with crosslinked discrete fibers which have been dried to a water content of approximately 18-30% before the fibers are contacted with the crosslinking solution. When the fiber is completely dried before it is brought into contact with the crosslinking solution, it is in an unexpanded, compressed state. The fiber does not swell upon contact with the crosslinking solution due to the low water content of the solution. As previously mentioned, the crucial 10 in the crosslinking solution is that it does not cause substantial swelling of the fibers. However, when the diluent of the crosslinking solution is absorbed into the already expanded fiber, the fiber actually "dries" out of the water but remains in its previous, partially expanded state.

15 As regards the degree of expansion of the fiber, it is again practical to refer to the fluid retention value (FRV) of the fiber after crosslinking. Fibers crosslinked in the expanded state have a higher FRV than fibers crosslinked in the less expanded state, all other factors being the same. Without limiting the scope of the invention, it is believed that partially expanded crosslinked fibers with elevated FRV have greater wet resilience and better wettability than fibers crosslinked in the unexpanded state. Fibers with this increased resilience when wet and better responsiveness to wetting are able to swell and, when wetted, more easily unwind when attempting to return to their natural state. However, due to the stiffness provided by the silage, 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 4 V fibers 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 wettability and wettability that are 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, dehydrated and dried to the desired extent, and then immersed in a water-miscible crosslinking solution containing a catalyst such as that described above. and a crosslinking agent. After removal of the liquid and before further drying, the fibers are preferably mechanically defibrated into a fluff to achieve the advantages of increased torsion and curl described above. Mechanical defibration after contacting the fibers with the crosslinking agent is less desirable because such defibration would evaporate the crosslinking agent and thus potentially lead to atmospheric contamination by the crosslinking agent or to 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. 50% by weight aqueous solutions 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 94436 26 can be mechanically defibrated a second time before further processing into a sheet or absorbent structure, which dewatering and defibration is also performed as described above.

5 Pre-soaking the fibers with the crosslinking agent in aqueous solution before reacting the crosslinking agent gives the air cushions made of crosslinked fibers unexpectedly good absorption properties even compared to cushions made by the above methods of curing non-crosslinked non-crosslinked solution. in a solution containing the constituent.

The crosslinked fibers formed as a result of the dry crosslinking processes described above and the processes in which the crosslinking takes place in an anhydrous solution are the products of this invention. The crosslinked fibers of this invention can be used directly to make air-layered absorbent cores. In addition, due to their rigid and elastic 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 25 wet layers in dense pulp sheets for sale or long distance transport.

When making crosslinked discrete fibers, they can be dry layered and formed directly into absorbent structures or wet layered and formed into absorbent structures or dense pulp sheets. The fibers of this invention provide a number of significant operational benefits. However, it is difficult to form uniform wet-laid sheets from such fibers following conventional wet sheeting practice. This is due to the rapid flocculation of the crosslinked discrete fibers in solution. Such flocculation 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 agglomerations 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 great 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 defibrillation.

Thus, a new method has been developed for forming a sheet of crosslinked discrete fibers that tend to flocculate in solution, in which a slurry containing crosslinked discrete fibers is first deposited on a 20-hole forming wire, such as a flat wire machine, in the same manner as conventional methods for forming sheets from pulp. Due to their crosslinked discrete nature, these fibers deposit on the 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 are directed at the fibers deposited on the forming wire so that successive jets have a decreasing volume 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 aligning the fibers is preferably carried out by means of a cylindrical wire, such as a wire roll, or some other similar device known in the art or possibly known in the art. The fiberboard of the leveling residue 94436 28 can then be dried and possibly compressed as desired. The distance between the jets varies depending on the respective Flok wear rate of the fibers, the line speed of the forming wire, 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.

10 In addition to preventing the flocculation of the fibers and breaking up the fiber flocs, the liquid sprayed into the fibers also compensates for the very rapid infiltration of the crosslinked discrete fibers by producing additional liquid medium into which the fibers can be dispersed for subsequent sheet-15 formation. 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 20 relatively even fibrous layers, which are then placed immediately, i.e. before re-flocculation, into a sheet-like shape by 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 in density to conventional pulp sheets than pulp sheets made from conventional non-crosslinked cellulosic fibers. Therefore, it may be advantageous to combine non-crosslinked fibers with crosslinked fibers. and, such as fibers conventionally used in the manufacture of absorbent cores. The pulp sheets containing rigid crosslinked fibers preferably contain from about 5 to 90% 3b of uncrosslinked 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 to 30% of 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 10 for later defibrillation or shaping into a suction pad. The inclusion of uncrosslinked fibers in the pulp sheet facilitates its compression, while surprisingly slightly 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 from a mixture of directly crosslinked and non-crosslinked fibers. Regardless of whether the mixture of crosslinked and uncrosslinked fibers is first made into a pulp sheet and formed into a suction pad, or whether the suction pad is formed directly from the mixture, 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 uncrosslinked 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 directly absorbent interiors in disposable products such as diapers, tampons and other menstrual pads are wet-laid sheets having a basis weight of about 300 to * 600 g / m 2 and a density of about 0.15 to 0.30 g / cm 3. Structures having a basis weight and density higher than this level are considered to be best suited for subsequent grinding and air or wet layering to form a structure having a lower density and basis weight that is more useful in applications requiring absorbency. However, structures with a higher square meter density and density have a surprisingly good absorbency and reactivity for wetting. 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 where the crosslinked fibers are located against or close to human skin, it is desirable to further treat the fibers for use to remove excess, unreacted crosslinking agent. It is preferred to reduce the content of unreacted crosslinking agent to at least about 0.03% based on the dry weight of the cellulosic fibers. One treatment series that has been found to be successful includes, in succession, washing the crosslinked fibers, soaking the fibers in an aqueous solution for quite some time, sieving the fibers, dewatering the fibers, for example by centrifugation to a consistency of about 40-80%, dewatering the fibers. , 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 residual crosslinker, the easily extractable crosslinker is removed by alkaline washing. Alkalinity can be generated by basic compounds such as sodium hydroxide, or alternatively by oxidizing agents such as chemicals commonly used as bleaching agents, for example sodium hypochlorite, and amino compounds, for example ammonium hydroxide, which hydrolyze the hemiacetal oxides to give . The pH is preferably maintained at at least 7, and more preferably at least about 9 to prevent recovery of acetal cross-reactions. In the case of hemiacetal bonds, it is advantageous to obtain the addition of acetyl 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 concentration between about 0.0008% and about 0.0023% with a soaking time of 30 minutes to two hours. Soaking times longer than about 30 minutes and ammonium hydroxide solutions concentrated to about 0.01 N are believed to provide only a negligible additional benefit.

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

In a new method of making crosslinked discrete fibers, the raw material fibers are bleached by a conventional multi-stage chain process, but in the middle the chain 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 provide an advantageous way to produce crosslinked fibers because the combination of bleaching step and residue removal step 30 avoids capital costs and processing difficulties caused by additional washing extraction equipment and additional process steps. The bleaching chain used and the point at which the chain is cut for crosslinking can vary widely, as will be apparent to one skilled in the art. However, multi-stage bleaching chains with DEP * or DEH * steps followed by crosslinking have been found to produce 35 desired results (* D - chlorine dioxide, E extraction time 94436 32, P - peroxide, H - sodium hypochlorite). The steps following the crosslinking of the bleaching chain are preferably base treatments performed at a pH above about 7, and more preferably above about 9.

In addition to effectively reducing crosslinking agent residues, post-crosslinking alkaline treatments have been found to facilitate the production of fibers with a higher FRV (fluid retention value) at the same degree of crosslinking. Fibers with a higher FRV have a lower dry-elasticity, i.e., they are easier to compact in the dry state, while retaining substantially the same wet elasticity and reactivity to moisture 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, sanitary napkins, sanitary napkins, temples, and wound dressings, 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 (Ken-30 neth B. Buell), issued January 14, 1975, which is hereby incorporated by reference.

Absorbent liners for diapers and menstrual pads are usually made of non-stiffened non-crosslinked cellulosic fibers having a dry absorbent liner density of about 0.06 94436 33 g / cm 3 and about 0.12 g / cm 3. As the absorbent core wets, its volume normally decreases.

It has been found that the crosslinked fibers of this invention can be used to make absorbent liners having significantly better fluid absorption properties, such as absorbent capacity and capillary absorption rate, than absorbent liners having the same density but made from conventional, uncrosslinked or non-crosslinked 10 of the crosslinked fibers, but the better properties are not limited to those mentioned. In addition, these better absorbency results can be achieved simultaneously with higher wet resilience, for absorbent cores having a density of about 0.06 to 0.15 g / cm 3 and having a substantially constant volume when wetted, it is particularly advantageous to use crosslinked fibers having such a degree of crosslinking. that they contain about 2.0 to 2.5 mol% of crosslinking agent reacted with the fibers, based on the molar amount of dry cellulose anhydroglucose. Absorbent inner parts made of such fibers combine structural integrity as desired, i.e. 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 regain a significantly greater portion of their original volume after the wet compression force has 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 density 94436 34 when dry 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 5 having a dry density less than the wet density at equilibrium, the core compresses to its saturation point after wetting until it reaches its wet density at equilibrium. Instead of forming the fibers into an absorbent core having a dry density 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 to a density greater than the equilibrium density to form a thin pad that expands upon testing, increasing the suction capacity significantly greater than is achievable with non-crosslinked fibers.

Particularly high absorbency and resilience when wet, as well as good reactivity to wetting, can be achieved with a degree of crosslinking of about 0.75 to 1.25 mol%, based on dry cellulose. Such fibers are preferably formed into absorbent cores having a 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 aforementioned 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 to a higher density. 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 the same absorption capacity. It should be noted, however, that crosslinked fibers with a higher degree of crosslinking can be made into absorbent structures with a higher density, as well as crosslinked fibers with a lower degree of crosslinking can be made into absorbent structures with a lower density. All such structures achieve improved performance compared to previously known crosslinked discrete fibers.

Although preferred embodiments of high and low density absorbent structures have been discussed above, it should be understood that combinations of absorbent structures of different densities and different concentrations of crosslinking agent, wherein said densities and concentrations are within the limits set forth herein, provide better absorption properties. and greater integrity of the absorbent structure compared to conventional cellulosic fibers and previously known crosslinked fibers. Such embodiments are intended to be within the scope of this invention.

Method for determining the water retention value The following procedure was used to determine the water retention value of cellulosic fibers.

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 15-20 hours. The soaked fibers are collected on a filter and transferred to a metal wire basket having a mesh number of 80 and placed on supports about 3.8 cm (about 1% of an inch) above the mesh base of the centrifuge tubes of 35 mesh. Tubes 94436 36 are covered with plastic, and the sample is centrifuged for 19 to 21 minutes at a relative acceleration of 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 D

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

15 Method for determining 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 the absorption capacity and absorption rate of the inner parts.

20 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. Synthetic urine flow is interrupted when the first drop of synthetic urine leaks from the bottom or sides of the pad. The dripping capacity is calculated by dividing the difference between the mass of the pad before and after the wetting of the synthetic urine by the mass of the fibers (= parched dry fibers).

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

33 A square pillow measuring 10 cm x 10 cm and weighing 7.5 g is made, its thickness is measured and no one 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. After about 60 seconds to 5 seconds, during which the pad reaches equilibrium, the thickness of the pad is measured. The compressive load is then increased to 7.6 kPar (1.1 psi), the pad is given time to reach equilibrium, and its thickness is measured. The compressive load is then reduced to 0.7 kPa (0.1 psi), the pad 10 is allowed to reach equilibrium, and its thickness is 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, called the elastic load of 0,7 kPa (0,1 psir). ). The pore volume (expressed in cm3 / g) is then determined for each different load. The 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 volumes at loads of 0.7 kPa and 7.6 kPa are useful indicators of wet compressive strength and structural wet integrity. A higher pore volume with a normal pad density is indicative of higher wet compressive strength and greater 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 suction pads. The smaller difference in pore volumes at loads of 0.7 kPa and 7.6 kPa is a sign of higher wet resilience.

The difference in thickness between the dry pad and the impregnated pad before compression was also found to be a useful indicator of the wettability of the pads.

::: Method for the determination of dry compressibility

The following procedure was used to determine the dry compressibility of absorbent cores. Dry wood-35 crosslinkability was used as a measure of the dry resilience of the interior parts.

94436 38

An air-layered pad measuring 10 cm x 10 cm and weighing about 7.5 g is prepared and pressed dry 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 area and thickness by fire. Larger differences between pre- and post-compression densities are indicative of lower dry resilience.

Method for Determining the Amount of Glutaraldehyde Reacted with Cellulose Fiber The amount of glutaraldehyde that reacted with the cellulose component 15 of the individual fibers crosslinked with glutaraldehyde to form intra-fiber crosslinks was determined using the following procedure.

A sample of crosslinked discrete fibers is extracted with 0,1 N Helium. The extract is separated from the fibers, and the same extraction / separation procedure is then repeated 3 more times for each sample. An aqueous solution of 2,4-dinitrophenylhydrazone (DNPH) is mixed separately with the extract 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 25 are separated by means of a separatory funnel. The amount of glutaraldehyde is determined by analyzing the DNPH derivative of the chloroform layer using high performance liquid chromatography (HPLC).

The chromatographic conditions used in the HPLC analysis were: column - C-18 reverse phase; detector - UV, wavelength wavelength 360 nm 360 nm; mobile phase - methanol / water mixture ...: 80:20; flow rate - 1 ml / min; measurement performed • · - 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 of known glutaraldehyde content in the range of 0-25 ppm.

94436 39

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 glutaraldehyde content was determined from the calibration curve. Thereafter, the glutaraldehyde contents obtained for each extract were summed and the sum was divided by the mass of the fiber sample (dry fiber mass) to obtain the glutaraldehyde content based on the mass of the fibers.

Two Glu-10 taraldehyde peaks appeared 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 a crosslinking agent, glutar-15 aldehyde, on the absorbency and resilience of absorbent pads made of crosslinked discrete fibers. The crosslinked discrete fibers were prepared by the dry crosslinking method.

For each sample, a certain amount of 20 undried, south coniferous kraft pulp (SSK pulp) was reserved. 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 a 50% aqueous solution of glutaraldehyde, 30% (by weight of glutaraldehyde-25) zinc nitrate hexahydrate, demineralized 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 until the fiber consistency became about 34-35%. The dewatered ... water-dried fibers were then air-dried in a blow-drying 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 as described in U.S. Patent No. 3,987,968. were placed on plates and cured in a substantially 40 94436 static drying oven at 145 ° C for 45 minutes, crosslinking was completed during the oven holding period, and the crosslinked discrete fibers were placed on a sieve and washed with water at about 20 ° C, soaked for one hour at 60 ° C. 1% and washed, washed a second time with water at about 20 ° C, centrifuged to a fiber consistency of about 60%, defibrated in a three-stage fluffing apparatus as described above, and dried for 10 hours in a static oven for four hours. in an s-oven at 105 ° C. The dried fibers were air-laid to form absorbent pads. The pads were compressed with a hydraulic press to a density of 0.10 g / cm 3. The pads were then tested for absorbency, resilience and the amount of glutaraldehyde reacted according to the procedures described. The amount of glutaraldehyde reacted is expressed as a molar percentage of the anhydroglucose molar amount of dry fiber cellulose. The results are reported in Table 1.

20

table 1

Dripping- Wet Compressibility Sample- Glutaraldesicity to water (cm3 / g) at hyd (mol%) introduction rate 0.7kPa 7.6kPa 0.7kPa 25 No. added / WRV Sat 8 ml / s (0.1 (1, 1 (0.1 _reacted (%) (a / s) _psi) psi) psir) 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

30 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 35 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 Λ1 94436 41

Example 2 The purpose of this example is to show that a low level of extractable crosslinker can be achieved with a series of bleaching steps applied to the fibers after crosslinking. 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.

10 The fibers were crosslinked by the dry crosslinking method.

Kraft pulp (SSK pulp) made from southern softwood was procured. The pulp fibers were partially bleached by the following sequential bleaching steps: the chlorination (C) slurry, which had a consistency of 3-4%, was treated orally at pH 2.5 and about 38 ° C for 30 minutes so that the amount of chlorine available (av. Cl) was about 5%; the alkali extraction slurry with a consistency of 12% was treated at about 74 ° C with NaOH at a concentration of 1.4 g / l for 60 minutes; and hypochlorite treatment (H) - 20 slurries with a consistency of 12% were treated at pH 11-11.5 and 38-60 ° C for 60 minutes with sufficient sodium hypochlorite to achieve Elretho whiteness of 60-65 and 15 , Viscosity from 5 to 16.5 mPa * s (cP). Crosslinked fibers were prepared from partially bleached fibers according to the method described in Example 1 using glutaraldehyde as a crosslinking agent. 2.29 mol% glutaraldehyde remained in the fibers, based on the anhydroglucose molar amount of dry fiber cellulose. The amount of extractable glutaraldehyde 30 in such fibers is typically about 1000 ppm ... (0.1%).

> · · ·· The bleaching of the partially bleached discrete fibers was then continued and completed with a chain (DEH) containing chlorine dioxide treatment (D), extraction (E) and sodium hypochlorite-35 treatment (H). The individual fibers crosslinked with chlorine dioxide-94436 42 in step (D) were soaked in an aqueous slurry having a consistency of 10% and also containing enough 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 an oven at 70 ° C 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 dewatered fibers was treated with an aqueous NaOH solution having a concentration of 0.33 g NaOH / liter 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%.

Finally, for the sodium hypochlorite step (H), a 10% consistency fiber slurry was prepared containing sufficient sodium hypochlorite to provide 1.5% chlorine by 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 dewatered fibers were air dried, fluffed and dried by keeping them in an oven at 105 ° C for one hour. The amount of extractable glutaraldehyde in the fully bleached crosslinked discrete fibers was 25 ppm (0.0025%). This amount is clearly less than the amount of extracted glutaraldehyde that is believed to be 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 35 had a surprisingly higher liquid retention value and capillary absorption rate, as well as at least the same dripping capacity and wet resilience as the individual fibers crosslinked after complete bleaching. However, due to the higher WRV, the fibers crosslinked in the middle of the casting chain were more compressible in the dry state.

Approximately similar results were obtained when replacing the last hypochlorite step (H) with a peroxide bleach step (P). In step P, a 10% 10% slurry 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.

• ·.

. <t · 4 »·

Claims (5)

A process for preparing cross-linked individual cellulose fibers, characterized in that a) cellulose fibers are abolished and the fibers are contacted in an aqueous solution with a sufficient amount of a crosslinking agent selected from the group C oligomers of said dialdehydes and acid analogs, to provide after the reaction 0.5 mole% - 3.5 mole% of the crosslinking agent calculated on the cellulose anhydroglucose mole amount, whereby a water binding value of said reactor fiber is obtained. 28 - 45; B) the fibers are dewatered until the consistency is 30-80% by weight fiber and the fibers are defibrated to a substantially individual form; c) the fibers are air dried while individualized, in the conditions under which fiber-fiber contact is avoided, the cross-linking agent is reacted with the fibers, the cross-links within the fibers being formed in substantial absence of bonds between the fibers; and d) the crosslinked fibers are washed with an alkaline solution having a pH greater than 7 and containing a constituent which disintegrates hemiacetal bonds while neutral to acetal bonds, reducing the amount of non-reaction and residual crosslinking agent. Process according to claim 1, characterized in that the alkaline solution has a pH greater than 9. 3. A process according to claim 1 or 2, characterized in that the fibers are contacted with a sufficient amount of a crosslinking agent to provide 0.5 mole% - 2.5 mole% of the crosslinking after the reaction. of the cellulose anhydroglucose mole amount of cellulose. il ia.s a.ii: ι 11: ι i 94436 Method according to any of claims 1-3, characterized in that the cellulose fibers mentioned in step a) are partially bleached with at least one step of a multi-stage bleaching chain a. Process according to any of claims 1-4, characterized in that the component of the alkaline solution in step d) contains some of the compounds sodium hypochlorite, ammonium hydroxide, hydrogen peroxide and sodium hydrogen sulfite. "· ·> * L f · ·