PT85181B - PROCESS FOR THE MANUFACTURE OF INDIVIDUALIZED FIBERS RETICULATED WITH REDUCED WASTE - 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

Description of the object of the invention that

THE BUCKEYE CELLULOSE CORPORATION North American, (State of Ohio), industrial, headquartered in Memphis, PO Box 8407, State of Tennessee 38108, United States of America, intends to obtain in Portugal, a PROCESS FOR THE MANUFACTURE OF INDIVIDUALIZED FIBERS RETICU LADAS WITH REDUCED WASTE.

The present invention relates to processes for the manufacture of individualized crosslinked fibers. More specifically, the present invention relates to processes for the manufacture of such fibers in which reduced levels of residual crosslinking agent are obtained.

Crosslinked fibers have been described in the art substantially individually and various methods for making them. The term individualized crosslinked fibers refers to cellulosic fibers that mainly have intrafiber crosslinking links. That is, the crosslinking links are mainly between cellulose molecules and a single fiber, rather than between cellulose molecules of separate fibers. Individualized crosslinked fibers are generally considered to be useful for application in absorbent products. In general, three categories of processes have been described for the manufacture of individualized crosslinked fibers. Such processes, described below, will be referred to here as 1) dry crosslinking processes, 2) aqueous solution crosslinking processes and 3) substantially non-aqueous solution crosslinking processes. Fibers and absorptive structures

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Case WMP 35 ^ 5 which contains the individualized crosslinked fibers, show an improvement in at least one absorption property, compared to conventional non-crosslinked fibers. Often, this improvement in absorption is reported in terms of absorption capacity or absorbent capacity. In addition, absorbent structures made with individualized crosslinked fibers generally have an increased wet elasticity and an equally increased dry elasticity, relative to absorbent structures made with non-crosslinked fibers. The term elasticity will refer hereinafter given the ability of cellulosic fiber pads to return to their original expanded state when released from a compressive force, dry elasticity specifically refers to the ability that an absorbent structure has to expand when released from a compressive force, applied while the fibers are in a substantially dry state. Wet elasticity refers specifically to the ability of an absorbent structure to expand when released from an applied compressive force while the fibers are in a wet state. For the purposes of the present invention and consistency of description, wet elasticity should be observed and described for an absorbent structure moistened to saturation.

Processes for the manufacture of individualized crosslinked fibers, using dry crosslinking technology, are described in US Patent No. 3,224,926 issued to L, J, Bernardin on December 21, 1965. Individualized crosslinked fibers are produced by impregnation. dilated fibers with an aqueous solution, with the crosslinking agent, dehydration and fiber defibration by means of mechanical action and drying the fibers at an elevated temperature to carry out the cross-linking while the fibers are in a substantially individual state. The fibers are inherently cross-linked in an undilated state,

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Depressed WMP 3545 Case, resulting from being dehydrated prior to labeling. The processes exemplified in U.S. Patent No. 3,224,926, in which the crosslinking is caused to take place while the fibers are in an undilated, depressed state, are referred to as processes for making a dry crosslinking of the fibers, which consequently are straight fibers. dry cooked. Dry crosslinked fibers are characterized by low liquid retention values (FRV). In US Patent No. 3,440,135, issued to R. Chung on April 22, 1969, it is suggested that the fibers be soaked in an aqueous solution of the crosslinking agent to reduce the inter-fiber bonding capacity before taking a dry crosslinking operation similar to that described in US Patent No. 3,224,926. This lengthy pre-treatment, preferably between 16 and 48 hours, is considered to improve the quality of the product by reducing the entanglement content resulting from incomplete defibration.

Processes for the production of crosslinked fibers in aqueous solution are described, for example, in U.S. Patent No. 3,241,553, assigned to F.H. Steiger on March 22, 1966. Individualized crosslinked fibers are produced by crosslinking the fibers in an aqueous solution containing the crosslinking agent and a catalyst. Fibers produced in this way are hereinafter referred to as crosslinked fibers in aqueous solution. Due to the swelling effect of water on cellulosic fibers, the fibers crosslinked in aqueous solution are crosslinked while in an unpressed, swollen state. With respect to dry crosslinked fibers, the fibers described in U.S. Patent No. 3,241,553, crosslinked in aqueous solution, have greater flexibility and less rigidity and are characterized by a higher liquid retention value (FRV). Absorbent structures made with cross-linked fibers in aqueous solution have lower wet and dry elasticities than pads made with dry cross-linked fibers.

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In the U.S. Patent No. 4,035 147, attributed to S. Sangenis, G. Guiroy and J. Quere, on 12 July 1977, a method is presented for the production of individualized crosslinked fibers, making contact with dehydrated, undilated fibers with crosslinking agent and catalyst, in a substantially non-aqueous solution, which contains an insufficient amount of water to cause the fibers to swell. Crosslinking takes place while the fibers are within the substantially non-aqueous solution. This type of process will hereinafter be referred to as a non-aqueous crosslinking process; and the fibers produced by means of it, will be referred to as fibers crosslinked by non-aqueous solution. The non-aqueous 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 swelling reagents. Like dry crosslinked fibers, they are highly stiffened by the crosslinking connections and the absorbent structures made with them have a relatively high wet and dry elasticity.

The crosslinked fibers described above are considered to be useful for application in low density absorbent products such as catamenics. However, such fibers have not provided sufficient absorption benefits, in view of their drawbacks and costs, in relation to conventional fibers, to result in significant commercial success. The commercial appeal of crosslinked fibers has also suffered from security concerns. The cross-linking agent most widely reported in the literature, formaldehyde, unfortunately causes irritation to human skin and has been associated with other safety concerns. The removal of free formaldehyde to sufficiently low levels in the cross-linked product to avoid this skin irritation and other human safety concerns has been hampered by both technical and technical barriers.

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Cross-linked fibers made with agents other than formaldehyde, may also have residual levels of cross-linking agent that are higher than desired for applications in the vicinity of human skin. For this reason, it may be desirable to treat the fibers after crosslinking in such a way as to reduce the level of unstable, residual crosslinking agent in the fibers. One method to achieve this is to wash the fibers with water after crosslinking. This method is effective but does not reduce the level of reti agent. residual concentration to a point as low as desirable. In addition, such post-crosslinking washes require cap. such additional cost and additional operating costs associated with washing and drying the fibers.

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

It is another object of the present invention to provide a process for manufacturing individualized crosslinked fibers, with low levels of residual crosslinking agent, in which capital investments for equipment and related operating costs are minimized.

It was found that the objectives identified above can be achieved and the crosslinked fibers, individualized with reduced levels of unstable, residual crosslinking agent, can be obtained, through the practice of the following process, which comprises the steps of:

a) supply cellulosic fibers and put them in contact with a crosslinking agent;

b) make the crosslinking react with the fibers in the virtual absence of bonds

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inter-fibers, to form intra-fiber cross-links; and

c) wash the fibers with an alkaline solution.

Crosslinking agent is preferably an agent chosen from the group consisting of Cg-Cg dialdehydes, Cg-Cg dialdehyde analogues that contain at least one aldehyde group and oligomers of said dialdehydes and dialdehyde analogues.

Preferably, the fibers are washed with an alkaline solution having a pH greater than about 9 · Also preferably, the fibers are contacted with a sufficient amount of one of the preferred cross-linking agents such as that between about 0.5 moles % and about 3.5 moles% of crosslinking agent, calculated on a molar base of anhydroglucose cellulose, which react with said fibers and said fibers have a water retention value of less than about 60.

Surprisingly, fibers made in accordance with the present invention have higher water retention values than fibers, with the same level of crosslinking, made by processes, equivalent for the rest, executed on completely bleached fibers. The corresponding absorbent structures made with the fibers of the present invention have higher absorbency properties, including wet elasticity and responsiveness to wetting.

Natural fibers of different origin are applicable in the invention. Digested fibers of soft wood, hard wood or cotton fibers are used preferentially. Spar fibers, bagasse, rough wool, flax fibers and other sources of woody or cellulosic fibers can also be used as the raw material of the present invention. The fibers can be supplied in the form of a paste, organized or not in sheets. Fibers supplied in the form of plates

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/ damp, dried sheets or other forms of sheets are preferably reduced to a non-sheet form by disintegrating the sheet by mechanical means, preferably before the fibers are brought into contact with the crosslinking agent. Also preferably, the fibers are supplied in a wet or damp state. More preferably, the fibers are fabulous and have never been dried. In the case of dry board, it is advantageous to moisten the fibers before mechanical disintegration, in order to minimize damage to the fibers.

The optimum source of fibers to be used in conjunction with the present invention will depend on the specific end use that is desired. Generally, fibers made using chemical pulping processes are preferred. Fully bleached, partially bleached or unbleached fibers are applicable. Often it may be desirable to use bleached pulp due to its brightness and attractiveness to superior consumers. In a new embodiment of the invention, which will be more fully described below, the fibers are partially bleached, reticulated and then just bleached. For products such as paper towels and absorbent diaper pads, sanitary napkins, catamenics and other similar absorbent paper products, the use of southern softwood fibers in pulp is especially preferred, due to their superior absorption characteristics »

Crosslinking agents applicable in the present development include the Cde dialdehydes, as well as the acid analogues of such dialdehydes wherein the acid analog has at least one aldehyde group and oligomers of such dialdehydes and acid analogs. These compounds are capable of reacting with at least two hydroxyl groups in a single cellulose chain or in cellulose chains located close to a single cellulose fiber. Those skilled in the art of crosslinking agents will recognize that the dialdehyde crosslinking agents described above will be present, or may react.

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Case WMP 3545 rotates in a variety of ways, including the acid analog and oligomer forms identified above. All of these forms are considered to fall within the scope of the present invention. The reference to a particular crosslinking agent should therefore and henceforth be considered to encompass that particular crosslinking agent, as well as other forms that may be present in an aqueous solution. Particular cross-linking agents considered for use with the present invention are glutaraldehyde, glyoxal, and glyoxalic acid. Glutaraldehyde is especially preferred, as it has provided fibers with the highest levels of absorption and elasticity, it is considered safe and non-irritating to human skin when in a reacted, reticulated state and it has provided the most stable cross-link bonds. Monoaldehyde compounds that do not have an additional carboxylic group, such as acetaldehyde and furfural, have not been shown to provide absorbent structures with the desired levels of absorbency, elasticity and responsiveness to wetting.

Unexpectedly it has been found that superior pad absorption performance can be achieved at levels of crosslinking that are substantially lower than previously practiced crosslinking levels. In general, unexpectedly good results are obtained for absorbent pads made of individualized fibers, crosslinked with between about 0.5 moles% and about 3.5 moles $ of crosslinking agent, calculated on a molar base of cellulose anhydroglucose, reacted with the fibers.

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

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Generally, the fibers will also be contacted with an appropriate catalyst before crosslinking. The type, amount and method of contacting the catalyst with the fibers will depend on the particular cross-linking process practiced. These variables will be discussed in more detail below.

Once the fibers are treated with the crosslinking agent and the catalyst, the crosslinking agent is reacted with the fibers in a substantial absence of inter-fiber bonds, that is, while the inter-fiber contact is maintained at a lower degree of occurrence with respect to non-woolly pulp fibers, or the fibers are submerged in a solution that does not facilitate the formation of inter-fiber bonds, especially hydrogen bonds. This results in the formation of cross links that are intrafibral in nature. Under these conditions, the cross-linking agent reacts to form cross-linkages between hydroxyl groups on a single cellulose chain or between hydroxyl groups on cellulose chains located close together in a single cellulosic fiber.

Although this is not presented or neglected to limit the scope of the present invention, it is believed that the cross-linking agent reacts with the hydroxyl groups of the cellulose to form semiacetic or acetic bonds. The formation of acetic bonds, which are believed to be the type of bonds desirable to provide stable crosslinks, and favored under acidic reaction conditions. For this reason, acid-catalyzed cross-linking conditions are highly preferred for the purposes of the present invention.

Preferably, the fibers are mechanically defibrated to a fibrous, individualized, low-density form known as fluff, prior to the reaction of the cross-linking agent with the fibers. Mechanical defibration can be performed using a variety of methods currently known in the art or that may be

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Case WNP 35 ^ 5 known. Mechanical defibration is preferably carried out by means of a method in which the formation of knots and damage to fibers are minimized. One type of device that has been found to be particularly useful for the defibration of cellulose fibers is the three-phase carding device described in U.S. Pat. 3,987,968, assigned to D.R. Moore and O.A, Shieldss on October 26, 1976, the aforementioned patent being incorporated into this description expressly for reference. The carding device described in Patent N? 3 9θ7 968 subjects wet cellulose fibers to a combination of mechanical impact, mechanical agitation, pneumatic agitation and a limited amount of air drying to create a substantially free lint. In addition, the individualized fibers provided an improved degree of curl and torsion over the curl and torsion naturally present in such fibers. This additional curl and twist is considered to improve the elastic character of absorbent structures made of finished, crosslinked fibers.

Other methods for defibrating applicable cellulose fibers include, but are not limited to, treatment with a Waring mixer and tangentially contacting the fibers with a rotating refining disc or a rotating wire brush. Preferably, a stream of air is directed at the fibers during such defibration, to assist the separation of the fibers in a substantially individual form.

Regardless of the particular mechanical device used to form the fluff, the fibers are preferably mechanically treated, while initially containing at least about 20% moisture and preferably while containing between about 40% and 6θ% moisture.

The mechanical refining of fibers in a

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Case WMP 35 ^ 5 í 25 high consistency or partially dry fibers can also be used to provide curling and torsion to the fibers, in addition to the curl and torsion provided as a result of mechanical defibration.

The fibers made according to the present invention have unique combinations of rigidity and elasticity, which allows the absorbent structures made with these fibers to maintain high levels of elasticity and have high levels of absorption and an expansive responsiveness to the wetting of a compressed absorbent structure. and dry. In addition to having the crosslink levels within the indicated limits, the crosslinked fibers are characterized by having water retention values (WRV) of less than about 60 and preferably between about 28 and 45, for chemically papermaking fibers conventional pulp. The WRV of a particular fiber is indicative of the level of crosslinking and the degree of expansion of the fiber at the time of crosslinking. Those skilled in the art will recognize that the longer a fiber is at the moment of crosslinking, the higher the WRV will be for a given crosslinking level. Very crosslinked fibers, such as those produced by the previously known dry crosslinking processes, have been found to have WRVs less than about 25 and generally less than about 20. The particular crosslinking process used will, of course, affect the WRV of the crosslinked fiber. However, any process that results in levels of crosslinking and WRV within the indicated limits is considered and is intended to be, as falling within the scope of the present invention. Crosslinking methods include dry crosslinking processes and non-aqueous solution crosslinking processes as described at the beginning of the present description. Some dry crosslinking and non-aqueous crosslinking processes, falling within the scope of the present invention, will be discussed in more detail below. Rereading processes

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Case WMP 35 ^ 5 aqueous solution bonding, where the solution causes the fibers to be extremely dilated, will cause the fibers to have WRVs that exceed about 60. These fibers will provide insufficient stiffness and elasticity for the purposes of the present invention.

With specific reference to dry crosslinking processes, individualized crosslinked fibers can be produced by such processes by supplying a quantity of cellulosic fibers, contacting a pulp of said fibers with a type and quantity of crosslinking agent as described above, separating mechanically, i.e., defibrating the fibers to a substantially individual shape and drying the fibers in the presence of a catalyst while reacting with the crosslinking agent, to form the crosslinking links while the fibers are maintained substantially individually. The defibration step, separate from the drying step, is considered to provide additional curl. Drying is accompanied by the torsion of the fibers, with the degree of torsion being increased by the curly geometry of the fiber. Confox me is used here the expression curling refers to the geometric curvature of the fiber around its longitudinal axis. Torsion refers to the rotation of the fiber around its cut perpendicular to the longitudinal axis of the fiber. By way of example only and without wishing to specifically limit the scope of the present invention, individualized crosslinked fibers within the scope of the invention have been observed with an average of about six (6) twists per millimeter.

Keeping the fibers in substantially individual shape during drying and cross-linking allows the fibers to twist during drying and then to be cross-linked in that curly twisted state. Drying the fibers in such conditions that the fibers can twist and curl is referred to as drying the fibers under substantive conditions.

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Case WMP 3545 especially unrestricted. On the other hand, drying the sheet fibers results in dry fibers that are not twisted or curled like dried fibers in a substantially individualized form. Inter-fiber hydrogen bonding is considered to restrict ”the relative occurrence of fiber torsion and curling.

There are several methods by which the fibers can be brought into contact with the crosslinking agent and the catalyst. In one embodiment, the fibers are brought into contact with a solution that initially contains both the crosslinking agent and the catalyst. In another embodiment, the fibers were contacted with an aqueous solution of the crosslinking agent and allowed to soak before adding the catalyst. The catalyst is added later. In a third embodiment, the crosslinking agent and the catalyst are added to a pulp of cellulosic fibers. Methods other than those described above will become apparent to those skilled in the art and are intended to be included within the scope of the present invention. Regardless of the particular method by which the fibers are brought into contact with the crosslinking agent and the catalyst, the cellulosic fibers, the crosslinking agent and the catalyst are preferably mixed and / or allowed to sufficiently impregnate with the fibers so that ensure complete contact and impregnation of the individual fibers.

In general, any substance that catalyzes the crosslinking mechanism can be used. Applicable catalysts include organic acid acids and salts. Especially preferred catalysts are the chloride, nitrate or sulfate salts of aluminum, magnesium, zinc and calcium. A specific example of a preferred salt is zinc nitrate hexahydrate. Other catalysts include acids such as sulfuric acid, hydrochloric acid and other mineral and organic acids. The chosen catalyst can be used as a single catalyst agent, or in combination13

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with one or more other catalysts. Combinations of acid salts and organic acids as catalyst agents are believed to provide superior crosslinking efficiency. Unexpectedly high levels of reaction completion have been observed for catalyst combinations of zinc nitrate salts and organic salts, such as citric acid and the use of these combinations is preferred. Mineral acids are useful for adjusting the pH of the fibers while being contacted with the crosslinking agent in solution, but are preferably not used as the main catalyst,

The optimum amount of catalyst and crosslinking agent used will depend on the particular crosslinking agent used, the reaction conditions and the particular application envisaged for the product.

The amount of catalyst preferably used is, of course, dependent on the type and amount of the crosslinking agent and the reaction conditions, especially temperature and pH. In general, based on technical and economic considerations, catalyst levels between about 10% by weight and about 60% by weight, based on the weight of crosslinking agent added to the cellulosic fibers, are preferred. For exemplary purposes, in the case where the catalyst used is zinc nitrate hexahydrate and the crosslinking agent is glutaraldehyde, a catalyst level of about 30% by weight, based on the amount of glutaraldehyde added, is considered preferable, More preferably, they are also added as a catalyst; between about 58% and about 30%, based on the weight of the glutaraldehyde, of an organic acid such as citric acid. In addition, it is desirable to adjust the aqueous portion of the cellulosic fiber slurry or crosslinker solution to a target pH of between about pH 2 and about pH 5, more preferably between about pH 2.5 and about pH 3 , 5 'during the contact period between the crosslinking agent and

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Case WMP 35 ^ 5 the fibers.

Generally, cellulosic fibers should be dehydrated and, optionally, dried. The work and optimum consistencies will vary according to the type of carding equipment used. In the preferred embodiments, the cellulosic fibers are dehydrated and optimally dried to a consistency of between about 30% and about 80%. More preferably, the fibers are dehydrated and dried to a consistency level of between about 40% and about 61% _o Drying the fibers up to these limits will generally facilitate the fiber defibration to an individualized form without excessive knot formation with higher levels of moisture and without high levels of damage stop the fibers associated with lower levels of moisture.

For example, dehydration can be carried out by means such as mechanical pressure, centrifugation or air drying of the pulp. The additional drying is preferably carried out by methods known in the art as air drying or air jet drying, in such conditions that the use of high temperatures for a prolonged period of time is not necessary. Excessively high temperature at this stage of the process can result in premature crosslinking. Preferably, temperatures above about 16θ ° C are not maintained for periods of more than 2 to 3 seconds. Mechanical defibration is performed as previously described.

The defibrated fibers are then heated to an appropriate temperature for a period of time effective to cause the crosslinking agent to cure, that is, its reaction with the cellulosic fibers. The rate and degree of crosslinking depends on the dryness of the fibers, the temperature, quantity and type of the catalyst and the crosslinking agent and the method used to heat and / or dry the fibers while the crosslinking is carried out. Crosslinking to a particular

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Case WMP 3545 temperature will occur at a higher rate for fibers with a certain initial moisture content, when accompanied by continuous drying by means of a continuous air stream, than when subjected to drying / heating in an oven static. Those skilled in the art will recognize that there are a large number of temperature-time relationships to cure the crosslinking agent. Conventional paper drying temperatures (For example, from 56.6 C (120 ° F) to 70.7 ° C (150 ° F)), for periods between about 30 minutes and 6θ minutes, under static atmospheric conditions, will provide generally acceptable curing efficacies for fibers having a moisture content of less than about 5 / · Those skilled in the art will also appreciate that higher temperatures and air convection shorten the time required for curing. However, curing temperatures are preferably maintained at less than about 16 ° C, since exposure of the fibers to such temperatures higher than about 160 ° C can lead to yellowing and other damage to the fibers.

maximum level of crosslinking will be achieved when the fibers are essentially dry (having less than about 5% moisture). Due to this absence of water, the fibers are crosslinked in a substantially undilated, depressed state. Consequently they have characteristic low liquid retention (FRV) values, relative to the limits applicable to this invention. The FRV refers to the amount of liquid calculated on a dry fiber basis, which remains absorbed by the fiber samples that have been impregnated and then centrifuged to remove the inter-fiber liquid. (FRV is described in more detail below, as well as the FRV Determination Process). The amount of liquid that the crosslinked fibers can absorb is dependent on their ability to swell when saturated or, in other words, on their inner volume or diameter when swollen to their maximum level.

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Case WMP 35 ^ 5 j 25 ximo, This, in turn, is dependent on the level of crosslinking. As the level of intra-fiber crosslinking increases for a given fiber and process, the FRV of the fiber will decrease until the fiber no longer swells when moistened. Thus, the FRV value of a fiber is structurally descriptive of the physical condition of the fiber when saturated. Unless expressly stated otherwise, the FRV data described here will be communicated in terms of the water retention value (WRV) of the fibers. Other liquids, such as salt water and synthetic urine, can also be used to advantage as a liquid medium for analysis. Generally, the FRV of a particular fiber, crosslinked by processes in which curing is largely dependent on drying, like the present process, will be primarily dependent on the crosslinking agent and the level of crosslinking. The WRVs of the fibers cross-linked by this dry cross-linking process at levels of binding agent applicable to this invention, are generally less than about 5θ, greater than about 25 and are preferably between about 28 and about 45 Bleached SSK fibers with between about 0.5 moles% and about 2.5 moles% of glutaraldehyde in them, calculated on a molar base of cellulose anhydroglucose, revealed to have WRVs that vary, respectively, between about 40 and about 28 The degree of bleaching and the practice of post-crosslinking bleaching steps have been shown to affect WRV. This effect will be explored in more detail below. Southern Kraft (SSK) softwood fibers prepared by means of crosslinking processes known prior to the present invention, have higher crosslinking levels than those described herein and have a WRV of less than about 25. Such fibers, as discussed previously, it has been shown by observation that they are too rigid and have lower absorption capacities than those of the fibers according to the present invention.

In another process for making individualized crosslinked fibers by a crosslinking process

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dry, the cellulosic fibers are contacted with a solution containing a crosslinking agent as described above. Both before and after being brought into contact with the crosslinking agent, the fibers are formed, shaped into a sheet. Preferably, the solution containing the crosslinking agent also contains one of the catalysts applicable to the dry crosslinking processes also described above. The fibers, while in the form of leaves, are dried and made crosslinked, preferably by heating them to a temperature between about 120 ° C and about 160 ° C. After crosslinking, the fibers are mechanically separated into a substantially individual shape. This is preferably done by means of a carding device similar to that described in U.S. Patent No. 3 9θ7 968 or by other methods to defibrate fibers that may be known in the art. The individualized crosslinked fibers made according to this sheet crosslinking process are treated with an amount of crosslinking agent such that between about 0.5 moles $ ® about 3.5 moles% of crosslinking agent, calculated on a basis molar anhydroglucose cellulose and measured after defibration, are reacted with the fibers in the form of intrafiber cross-links. Another effect of crosslinking and drying the fibers while in sheet form is that the fiber-to-fiber bonds prevent the fibers from curling and twisting with increased drying. In comparison with individualized crosslinked fibers made according to a process in which the fibers are dried under substantially unrestricted conditions and then crosslinked in a twisted, curly configuration, the absorbent structures made of relatively untwisted fibers constituted by the sheet curing process described above, it is expected that they have a wet elasticity and a lower responsiveness to the wetting of a dry absorbent structure.

Another category of crosslinking processes

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Case WMP 35 ^ 5 applicable to the present invention are the crosslinking curing processes by non-aqueous solution. The same types of fibers applicable to dry crosslinking processes can be used in the production of crosslinked fibers by non-aqueous solution. The fibers are treated with an amount of crosslinking agent such that it is sufficient, being between about 0.5 moles% and about 3.5 moles%, to later react with the fibers, in which the level of reacted cross-linking agent is calculated after said cross-linking reaction and with a suitable catalyst. The cross-linking agent is made while the fibers are submerged in a solution that does not cause any substantial level of expansion of the fibers. The fibers may, however, contain up to about 30% water, or be swollen in any other way in the crosslinking solution, to a degree equivalent to that of fibers containing about 30% moisture content. Such geometry of partially dilated fibers has been shown to provide exceptional additional benefits as discussed in more detail below. The crosslinking solution contains a non-aqueous polar diluent, miscible in water, such as, although not limited to, acetic acid, or even, propanoic acid or acetone. Preferred catalysts include mineral acids and halogen acids, such as sulfuric acid and hydrochloric acid, respectively. Other applicable catalysts include salts of mineral acids and halogen acids, organic acids and their salts. Crosslinking solution systems applicable for use as a crosslinking medium also include those described in U.S. Patent, N? ^ .035.1 ^ 7 'attributed to S, Sangenis, G, Guiroy and J, Quere, on 12 July 1977, which is incorporated herein by reference. The cross-linking solution may include some water or other fiber-swelling fluid; however, the amount of water is preferably insufficient to cause a level of expansion corresponding to that recorded in pulp fibers with a consistency of 70% (30% aqueous moisture content).

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In addition, water content of the crosslinking solution less than about 10% of the total volume of the solution, excluding fibers, is preferred. Water levels in the crosslinking solution, which exceed this amount, decrease efficiency and the rate of crosslinking.

The absorption of the crosslinking agent by the fibers can be achieved in the crosslinking solution itself or in a previous treatment step that includes, without limitation, saturation of the fibers with a solution, aqueous or not, containing the crosslinking agent. . Preferably, the fibers are mechanically defibrated to an individual shape. This mechanical treatment can be carried out by the methods previously described for carding fibers, in connection with the dry crosslinking process previously described.

It is especially preferred to include in the production of fluff, a mechanical treatment that causes the moist cellulosic fibers to assume a curly or twisted state, to a degree that exceeds the amount of curl or twist, if any, of the natural state of the fibers. This can be achieved by initially providing fibers to be carded that are in a moist state, subjecting these fibers to mechanical treatment such as the methods previously described for defibrating the fibers in a substantially individual form and drying the fibers at least partially.

The relative amounts of curl and twist associated with the fibers are, in part, dependent on the moisture content of the fibers. Without limiting the scope of the invention, it is considered that the fibers naturally twist when they are dried under conditions where the fibers have little contact with each other, that is, when the fibers are in an individualized form, the mechanical treatment it also causes wet fibers to become curly at first. When the fibers are then dried under

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Case WMP 3545 substantially ηδο restricted, become twisted and the degree of twist is improved by the amount of additional curl mechanically provided. The defibration carding steps are preferably performed on moist pulp of great consistency, or on pulp that has been dehydrated to a fiber consistency of about 45% to about 55% (determined before the start of the defibration).

After defibration, the fibers should be dried to between 0% and about 30% moisture content, before being brought into contact with the crosslinking solution, if the defibration step has not already provided fibers with a moisture content within those limits. The drying step should be carried out while the fibers are in substantially unrestricted conditions. That is, the fiber-to-fiber contact should be minimized so that the twisting of the fibers, inherent to drying, is not inhibited. Both drying by continuous air flow and drying by means of intermittent air jets are suitable methods for this purpose.

The individualized fibers are then brought into contact with a crosslinking solution containing a non-aqueous, water-miscible diluent, cross-linking agent and a catalyst. The crosslinking solution can contain a limited amount of water. The water content of the crosslinking solution should be less than about 18% and is preferably less than about 9%.

A wad of fibers that have not been mechanically defibrated can also be brought into contact with a crosslinking solution as previously described.

The amounts of crosslinking agent and acid catalyst used will depend on reaction conditions such as consistency, temperature, water content in the crosslinking solution and fibers, type of crosslinking agent and diluent in the crosslinking solution and quantity ^

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Case WMF 35 ^ 5 of desired crosslinking. Preferably, the amount of crosslinking agent used ranges from about 2% by weight to about 10% by weight (based on the total weight of the crosslinking solution without the fibers). The preferred content of acid catalyst is, moreover, dependent on the acidity of the catalyst in the crosslinking solution. Generally, good results can be obtained for a percentage of catalyst content, including hydrochloric acid, between about yfr by weight and about 5% by weight (based on the weight of the fiber-free crosslinking solution) in crosslinking containing an acetic acid diluent, preferred levels of glutaraldehyde and a limited amount of water. Fiber binders and crosslinking solution with fiber consistencies of less than about 10% by weight, are preferred for reticulation, in conjunction with the crosslinking solutions described above,

The crosslinking reaction can be carried out at ambient temperatures or, for accelerated reaction rates, at elevated temperatures, preferably below about 4 ° C.

There are a variety of methods by which fibers can be brought into contact and cross-linked with the cross-linking solution. In one embodiment, the fibers are brought into contact with the solution that initially contains the crosslinking agent and the acid catalyst. The fibers are allowed to soak in the crosslinking solution, during which time the crosslinking takes place. In another embodiment, the fibers are brought into contact with the diluent and allowed to impregnate before adding the acid catalyst. Subsequently, the acid catalyst is added, at which point crosslinking begins. Other methods, in addition to those described, will become apparent to those skilled in the art and are considered to fall within the scope of the present invention. Preferably, the cross-linking agent and the conditions under which cross-linking is carried out are chosen to facilitate cross-linking

58,248

Case WbíP 35 ^ 5 tion intrafiber. Thus, it is advantageous that the crosslinking reaction occurs, in substantial part, after the crosslinking agent has had sufficient time to penetrate the fibers. Preferably, the reaction conditions are chosen so as to avoid instant crosslinking, unless the crosslinking agent has already penetrated the fibers. Reaction periods of about 30 minutes, during which the crosslinking is practically completed, are preferred. Longer reaction periods are believed to provide only marginal benefits in fiber performance. However, both shorter periods, which include virtually instantaneous crosslinking, and longer periods are considered to be within the scope of the invention.

It is also contemplated the partial cure while still in the solution and later the completion of the crosslinking reaction at a later stage of the process, through drying or heating treatments.

Following the cross-linking step, the fibers are drained and washed. Preferably, a sufficient amount of a basic substance, such as caustic soda, is added to the washing step to neutralize any acid that remains in the pulp. After washing, the fibers are de-fluidized and dried to completion. Preferably, the fibers are subjected to a second mechanical defibration step which causes the crosslinked fibers to curl, for example, carding by defibration, between the defluidification and drying steps. When drying, the curly condition of the fibers provides additional twisting, as previously described in relation to the curling treatment prior to contact with the crosslinking solution: The same devices and methods for applying twist and curling, which have been described in relation to to the first mechanical defibration step, they are applicable to this second mechanical defibration step. As used herein, the term defibration ”refers to any

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Case WMP 3545

It is one of the processes that can be used to mechanically separate the fibers into a substantially individual form, even though the fibers may have already been supplied in that form. Defibration ”therefore refers to the step of mechanically treating the fibers, both individually and in a more compact form, for a mechanical treatment step that a) separates the fibers in a substantially individual way if they are not already in such a way form, and b) provide curling and twisting to the fibers, when drying.

This second defibration treatment, after the fibers have been cross-linked, has been shown to increase the twisted, curly character of the pulp. This increase in the twisted, curly configuration of the fibers leads to an improvement in elasticity and responsiveness to the wetting of the absorbent structure. A second defibration treatment can be practiced on any of the crosslinked fibers described herein that are in a moist state. However, it is a particular advantage of the non-aqueous solution crosslinking method, that a defibration step s and secondary is possible without the need for an additional drying step. This is due to the fact that the solution in which the fibers are crosslinked keeps the fibers flexible after crosslinking, even if it does not cause them to assume a highly dilated, undesirable state.

It has also been found, unexpectedly, that increased degrees of expansion of the absorbent structure, when wetting compressed pads, can be obtained by structures made of fibers that have been cross-linked while in a state that is twisted but partially dilated relative to the fibers. that were completely dried out of water before crosslinking.

Improved results were obtained for fi. cross-linked individualized strands that have been cross-linked under conditions where the fibers are dried before about 18% to about 30% water content, before contact with the

58.2!

Case WMP 3545

J 25 crosslinking solution. In the present case, a fiber is dried to completion, before being brought into contact with the crosslinking solution, it is in an undilated, depressed state. The fiber does not expand on contact with the crosslinking solution due to the low water content of the solution. As previously discussed, a critical aspect of the crosslinking solution is that it does not cause any substantial expansion of the fibers. However, when the diluent of the crosslinking solution is absorbed by an already swollen fiber, the fiber is effectively dried out of water, but retains its condition. pre-existing partially dilated condition.

To describe the degree of expansion to which the fiber reaches, it is again useful to refer to the fluid retention value (FRV) of the fiber, after crosslinking. Higher FRV fibers correspond to fibers that have been cross-linked while entering a more dilated state, compared to cross-linked fibers while being in a less dilated state, provided that all other factors are the same. Without limiting the scope of the invention, cross-linked, partially swollen fibers with increased FRV are considered to have greater elasticity and responsiveness to wetting than fibers that have been cross-linked while in an undilated state. Fibers with this increase in wet elasticity and responsiveness to wetting are more easily able to expand or distort when moistened in an attempt to return to their natural state. However, due to the stiffness provided by the crosslinking, the fibers can still provide the structural support for a saturated pad made of fibers. The numerical FRV data described here in connection with partially dilated crosslinked fibers should be water retention values (WRV). As the WRV increases beyond approximately 60, the fiber stiffness is judged to be insufficient to provide wet elasticity and responsiveness to wetting

58,248

Case WMP 35 ^ 5 designed to support a saturated absorbent structure.

In an alternative method for crosslinking fibers in solution, the fibers are first impregnated in a non-aqueous solution or other solution that does not swell, defluidized, dried to a desired level and then submerged in a water miscible crosslinking solution, containing a catalyst and a crosslinking agent as previously described. Preferably, the fibers are mechanically defibrated until they are in a fluff form, after defluidification and before further drying, in order to obtain the improved twisting and curling benefits described above. The mechanical defibration applied after contacting the fibers with the crosslinking agent is less desirable, since such defibration would latilize the crosslinking agent, thus possibly leading to contamination of the atmosphere by said agent or high investments with air treatment.

In a modification of the process described immediately above, the fibers are defibrated and then pre-impregnated in a solution with a high concentration of cross-linking agent and a diluent preferably water. The concentration of the cross-linking agent is sufficiently high to inhibit the expansion of fibers caused by water. Fifty percent by weight of cross-linking agents in aqueous solution, preferably glutaraldehyde, proved to be useful solutions for pre-impregnating the fibers. The pre-impregnated fibers are defluidified and submerged in a crosslinking solution containing a polar diluent, water miscible, a catalyst and a limited amount of water and then crosslinked as previously described. Also as described above, the crosslinked fibers can be de-fluidized and subjected to a second mechanical defibration, before further processing in sheet or absorbent structure.

Pre-impregnation of the fibers with cross-linking agent in an aqueous solution, before reacting the

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Case WMF 3545

crosslinking agent, provides unexpectedly high absorption properties to absorbent pads made of, 25 crosslinked fibers, even in relation to crosslinked fibers as described by processes woven with a solution curing crosslinking solution in which the fibers containing a pad made of aqueous previously were not pre-impression containing the

Crosslinked fibers formed as a result of dry crosslinking processes or by non-aqueous dry solution are the product of the present invention. The crosslinked fibers of the present invention used in the manufacture of cores can be directly absorbed with aeration. In addition, due to their rigid and elastic character, the crosslinked fibers can be laid on a low density, non-compacted sheet which, when dried, is directly useful without any further mechanical processing as an absorbent core. Reticulated fibers can also be wet laid in the form of compacted pulp sheets for sale or transport to distant locations.

Once the individualized, cross-linked fibers are made, they can be laid in a damp and directly formed into absorbent structures or densified pulp sheets. The fibers of the present invention provide advantages c. substantial performance.

However, it is difficult to form such fibers on a smooth, wet sheet by means of conventional wet sheet forming practices. This is due to the fact that individual crosslinked fibers turn into flakes very quickly when in solution. Such flocculation can take place both in the upper box and in the deposition on the porous forming wire,

It was found that attempts to transform individualized, reticulated fibers into sheet by means of

58,248

Case WMF · 3545 of conventional pulp leafing methods, resulted in the formation of a plurality of flaked fiber lumps. This results from the rigid, twisted character of the fibers, a low level of fiber-to-fiber bonding and the high drainability of the fibers once deposited on a sheet forming wire. It is therefore a significant commercial concern that a practicable process is provided to turn individualized, reticulated fibers into sheets, by which moist-based absorbent structures and densified pulp sheets for transit and subsequent defibration can be formed.

Concomitantly, a new process for transforming individualized, reticulated fibers into sheet, which tend to form flakes in solution, was developed, a process through which a paste containing individualized, reticulated fibers is initially deposited on one. porous forming wire, such as a Fourdrinier wire, in a manner similar to that of conventional pulp sheet forming processes, of individualized fibers, deposited on the

However, due to the crosslinked nature, these fibers are formed in a plurality of fiber lumps. At least one stream of fluid, preferably water, is directed to the deposited fibers, agglomerated. Preferably, a series of showers is directed towards the fibers deposited on the forming wire, a series in which the successive showers have decreasing volumetric flow rates. The showers must be of sufficient speed so that the impact of the liquid against the fibers, acts in order to inhibit the formation of the fibers and to disperse the flocculations that have been formed. The settling phase of the flocculation fibers that already fibers are carried out, preferably, with a roller sieve, such as a dardy roller, one with. another similar function apparatus which is or may be known in the art. Once installed, the fibrous sheet can then be dried and, optionally

58,248

Case WMP 3545) 25

J'Jh1987 „ίΟ-Λ / / compact as desired. The spacing of the showers will vary according to the particular rate of flocculation of the fibers, the linear speed of the formation wire draining through the formation wire, number of showers and speed and flow rate through the showers. Preferably the showers are close enough to each other, so that substantial levels of flocculation do not occur.

In addition to inhibiting the formation and dispersing of fiber flocculations, the liquid sprayed on the fibers also compensates for the extremely rapid drainage of the individualized, cross-linked fibers, by providing an additional liquid medium in which the fibers can be dispersed for later leaf formation. The plurality of showers with decreasing volumetric flows facilitates a systematic liquid increase in the consistency of the paste, while providing a repetitive dispersive and inhibiting effect on fiber flocculation. This results in the formation of a relatively smooth and uniform deposit of fibers which are then readily, that is, prior to refloculation, laid in the form of a sheet by allowing the liquid to stretch and compress the fibers against the porous wire.

With respect to pulp sheets made of conventional non-cross-linked cellulosic fibers, pulp sheets made with the cross-linked fibers of the present invention are more difficult to compress to densities up to conventional pulp sheet densities. For this reason, it may be desirable to combine cross-linked fibers with non-cross-linked fibers, such as those conventionally used in the manufacture of absorbent cores:

Pulp sheets containing stiffened, cross-linked fibers preferably contain between about 5% and about 90% of non-cross-linked cellulosic fibers, based on the total dry weight of the sheet, mixed with the individual cross-linked fibers. Such highly refined fibers are refined or beaten to a level of freedom of

58,248

Case WMP 3545 less than about 300 ml CSF, and preferably less than about 100 ml CSF. The non-crosslinked fibers are preferably mixed with an aqueous paste of the individual crosslinked fibers. This mixture can then be formed into a densified pulp sheet for subsequent defibration and formation and formation in absorbent pads. The incorporation of the non-crosslinked fibers facilitates the compression of the pulp sheet in a densified form, while providing a surprisingly small loss of absorbency to the subsequently formed absorbent pads. In addition, the non-crosslinked fibers increase the tensile strength of the pulp sheet and absorbent pads made both with the pulp sheet and directly from the crosslinked and non-crosslinked fiber blend. Regardless of whether the mixture of crosslinked and non-crosslinked fibers is made first in the form of a sheet of pulp and then formed on an absorbent pad or directly formed on an absorbent pad, that absorbent pad can be laid dry or wet as previously described.

Sheets or webs of individualized reticulated fibers, or blends that also contain fibers will preferably not have basic weights less than about 800 g / m cm. Although not crosslinked, is based leaves about 300 g / m ~ to about between about 0.15 g / cm almente ^J contemplated for lower densities and about O / $ Og / intend to limit the scope of the wet invention, situated between base weights of 6C0 g / m and densities located and about 0.30 g / cir /, they are directly applicable as absorbent cores in disposable articles such as breads and other catamenic articles. Diaper structures, also with basic weights and densities higher than these levels are considered to be more and dry laying or useful for wet, in terms of density and weight of subsequent crushing to form a lower base structure, which is more useful

58,248

Case WMP 3545 for absorbent applications. However, such structures with higher base weight and higher densities also have surprisingly high absorbency and responsiveness to wetting. Other applications contemplated for the fibers of the present invention include sheets of err fabric, the density of which may be less than 0.10 g / cc.

For application in products where the crosslinked fibers are arranged in the vicinity of a person's skin, it is desirable to further process the fibers to remove excess unreacted crosslinking agent. Preferably, the level of unreacted crosslinking agent is reduced to at least below about 0.03%, based on the dry weight of the cellulosic fibers.

A number of treatments that have been found to successfully remove excess crosslinking agent include, in sequence. the washing of the crosslinked fibers, allowing them during a dehydration space of up to an 80% consistency, defibration and previously if the washing of the crosslinked fibers, fibers soak in an aqueous solution for an appreciable time, sieving of the fibers, for example, by centrifugation, the one between about 40% and about the dehydrated fibers as described in the process reduces about 0 fibers to air. It was found that this for extractable crosslinking agent and drying of the 01% free residual crosslinking agent and about 15%.

In another method for reducing the residual, the immediate cross-linking agent is removed by means of alkaline washes. Alkalinity can be introduced by means of basic compounds such as sodium hydroxide, or alternatively in the form of oxidizing agents such as, for example, sodium hypochlorite, or other chemicals commonly used as bleaching agents and amine-containing compounds, for example, ammonium hydroxide, which hydrolyze hemi-acetic bonds to form Schiff bases. The pH is preferably maintained at a level of at least pH 7

58,248

Case WMP 355 and more preferably at least about pH 9 to inhibit acetic crosslink reversal. For this reason, extraction agents that operate in highly alkaline conditions are preferred. Unique wash treatments with 0.01N and O, IN of ammonium hydroxide concentrations, have been found to reduce the waste content to between about 0.0008 $ and about 0.0023 / ½ during 30-minute impregnation periods minutes to two (2) hours. It is believed that there may be an additional minimal benefit in impregnation times of more than about 30 minutes and for ammonium hydroxide concentrations greater than about 0.01N.

Both the oxidation of the single phase and the oxide. multiphase determination have proven to be effective methods for extracting the residual crosslinking agent. A single-stage wash with 0.1% available chlorine (Cl available) to about 0.8% Cl available based on the dry weight of the fibers, supplied in the form of sodium hypochlorite, noted it was found to reduce the levels of residual crosslinking agent to between about 0.0015 / ½ and about 0.0025%.

In a new approach to the production of reticulated, individualized fibers, the source of the fibers is subject to a conventional bleaching sequence, but never

midpoint of the sequence is interrupted and the present invention. After being reduced to about 0.006 $ they can be »

the fibers are completed crosslinked agent bleaching agent according to the rest of the white curing sequence,

Residual crosslinking levels lower than those obtained in this way have been found to be acceptable. It is believed that the method embodies the preferred way of producing these crosslinked fibers, since the capital expenditure and the inconveniences of processing additional washing and extraction equipment, steps being avoided due to the combination of the bleaching steps and waste reduction The practiced bleaching sequences and the point of interruption of the sequences for crosslinking, can vary lar32

58,248

Case WMP 35 ^ 5

as will become evident to any technician in the field. However, multi-step bleaching sequences5 in which DEP or DEH steps follow crosslinking have been shown to provide desirable results. (D - chlorine dioxide, E - caustic extraction, P - peroxide, H - sodium hypochlorite). The post-crosslink bleaching sequence steps are preferably alkaline treatments performed at a pH greater than about pH 7 θ more preferably greater than about pH 9 ·

In addition to providing an effective reduction of the residual cross-linking agent, the post-cross-linking alkaline treatments have been shown to facilitate the development of higher fibers FRV to equivalent levels of cross-linking. The higher the FRV (liquid retention value) the fibers have, the lower the dry elasticity, that is, they are easier to densify while in a dry state, while retaining the same wet elasticity and responsiveness to moisture. there equivalent fibers, cross-linked after completion of bleaching. This, more than FRV, was especially surprising, considering that high levels resulted in reduced absorption properties.

The crosslinked fibers described herein are useful for a wide variety of absorbent products which include, but are not limited to, disposable diapers, catamenics, sanitary napkins, tampons and bandages, in what. Each of these articles has an absorbent structure that contains individualized, crosslinked fibers, described herein. For example, a disposable diaper or similar article that has a liquid-permeable top sheet, a liquid-impermeable bottom sheet and is attached to the top sheet and an absorbent structure containing individualized, cross-linked fibers is contemplated by the present invention.

Such articles are generally described in US patent <3,860,003, issued to Kenneth B. Buell on January 14, 1975, hereby incorporated by reference.

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Case ν.’ΧΡ 35 ^ 5

Conventionally, diaper and catamenic cores are made of non-stiff cellulosic fibers that the absorbent cores have and about 0.12g / cc, the absorbent core normally has dry and non-crosslinked densities of about 0.06 g / When a reduction in volume is wetted, the crosslinked fibers of the present invention can be used to make absorbent cores with properties substantially more than those that include, absorbent city without however limiting themselves, the cover and drip rate, relative to absorbent cores of equivalent density made with previously known crosslinked fibers

In addition, these improved absorbency results can be obtained in conjunction with increased wet elasticity levels, absorbent cores about 0.15 g / cc, constant when wet, fiber sizing at 2.0 mole% levels and about 2 Based on a molar base of dry cellulose anhydroglucose, absorbent oils made from such fibers have structural integrity, strength, and wet elasticity. The term non-reconPara with densities of about 0.06 g / cc and which maintains a volume substantially is especially preferred the use between about crosslinking of moles% crosslinking agent in this context, refers to its combination form in compression damp, a damp pad to return to its volume, compared to absorbents made with

In a desirable resistance to elasticity, the ability to initiate and respect when exposed and released from compressive forces with untreated fiber cores, the fiber cores of the present invention will recover a substantially higher proportion of their original volumes when released from wet compressive forces.

In another, the individualized fibers, core, which can be based on the preferred embodiment, cross-linked are formed nmm dry or wet (and post34

58,248

Case WMP 35 ^ 5 riorrnente dry), absorbent, which is compressed to a dry density below the wet equilibrium density of the pad. The wet equilibrium density of the cushion is the density of the cushion, calculated on a dry fiber basis when the cushion is saturated with liquid. When fibers are formed in an absorbent core with a dry density less than the wet equilibrium density, when wet to saturation, the core will yield until it reaches the wet equilibrium density. Alternatively, when the fibers are formed in absorbent cores with a dry density higher than the wet equilibrium density, when wetted to saturation, the cores will expand until they reach the wet equilibrium density. Pads made with the fibers of the present invention have wet balance densities that are substantially lower than pads made with conventional, non-crosslinked fibers. The fibers of the present invention can be compressed to a higher density than balance, to form a thin pad that, when wet, will expand, thus increasing the absorbent capacity to a significantly greater degree than that obtained from the non-crosslinked fibers.

absorbency med and cement between about

Especially high properties of wet elasticity and responsiveness to wetness can be obtained with crosslinking levels of 0.75 moles / and about 1.25 moles /, calculated on a dry cellulose molar base.

Preferably, such fibers are formed in absorbent cores with densities in their wet densities, the absorbent cores are normal than the density of the tablets to

Pre-densities, in wet equilibrium, are mechanically compressed. Also of a dryness greater than a ratio between about 0.12 g / cc and about 0.6 g / cc than the corresponding density density of the dry core, the absorbent cores are compressed to about 0.12 g / cc and about 0.40 g / cc,

58,248

Case WMP 35 ^ 5 in which the corresponding wet equilibrium densities are between about 0.08 g / cc and about 0.12 g / cc. Regarding the crosslinked fibers with crosslinking levels of between 2.0 moles% and about 2.5 moles the first fibers are less rigid, therefore making them more suitable for compression to higher density levels. The first fibers also have a higher responsiveness to wetting, because when they are wet they open instantly at a faster rate and to a greater degree than the fibers that have crosslinking levels within the limits of 2.0 moles% and about 2.5%, have a higher wet elasticity and retain almost equal absorbency. However, it should be recognized that absorbent structures within the higher density limits can be made with the cross-linked fibers with the cross-linking limits located at the higher levels, just as absorbent structures with lower density can be made with the fibers. lattices that have lower levels of lattice. For all these structures, an improved performance is obtained in relation to the previously known reticulated fibers.

Although the above discussion involves preferred embodiments, for high and low density absorbent structures, it should be recognized that a wide variety of combinations of density of absorbent structures is possible, as well as levels of crosslinking agent, within the limits presented herein, which will provide superior absorbency and structural integrity characteristics over conventional cellulosic fibers and previously known crosslinked fibers. Such embodiments are considered to fall within the scope of the present invention.

PROCESS FOR DETERMINING D0 NET RETENTION VALUE

The following process was used to

- 36 58,248

Case WMP 3545

determine the water retention value of cellulosic fibers.

A sample of about 0.3 to about 0.4 g of fibers is impregnated, in a covered container, with about 100 ml of distilled or deionized water, at room temperature, for about 15 to about 20 hours. The impregnated fibers are collected in a filter and transferred to a wire basket with an 80 mesh, supported about 3.81 cm (1.5 inches) above the bottom covered with 60 mesh of a centrifugal tube. The tube is covered with a plastic cover and the sample is centrifuged at a relative centrifugal force of 1,500 to 1,700 gravities for 19 to 21 minutes. The centrifuged fibers are then removed from the basket and weighed. The heavy fibers are dried to a constant weight at 105 ° C and re-weighed. The water retention value is calculated as follows:

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

D where,

W = wet weight of the centrifuged fibers;

D = dry weight of the fibers; and

W-D = weight of absorbed water

PROCESS FOR DETERMINING DRIPPING CAPACITY

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

An absorbent pad measuring 10.16 cm x 10.16 cm (4 x 4 ’’) weighing about 7.5 g is placed on a mesh screen. Synthetic urine is applied to the center of the pad at a rate of 8 / ml / s. The flow of synthetic urine is suspended when the first drop of synthetic urine escapes

58,248

Case WMP 3545 from the bottom or sides of the cushion. The dripping capacity is calculated by the difference between the mass of the fairy before and after the introduction of the synthetic urine, divided by the mass of the fibers on a weight basis of totally dry fibers

PROCESS FOR DETERMINING COMPRESSIBILITY IN MOIST

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

A 10.16 cm (4 x) square cushion on the side, airy, with a mass of about 7.5, is prepared and compressed, in a dry state, by means of a hydraulic press to a pressure of 5500 lbs / 16 square inches. The cushion is inverted and the pressure is repeated. The thickness of the pad is measured before and after pressing with an unloaded gauge. The density before and after the pressure is then calculated as mass / (area x thickness). Larger differences between density before and after pressure, indicate lower dry elasticity,

PROCESS FOR THE DETERMINATION OF THE GLUTARALDEHYTE LEVEL REACTED WITH CELLULOSIC FIBERS The following process was used to determine the level of glutaraldehyde that reacted to form the intra-fiber cross-links with the cellulosic component of the individualized fibers, cross-linked by glutaraldehyde.

A sample of individualized, crosslinked fibers is extracted with 0.1N HCl. The extract is separated from the fibers and the same extraction / separation process is then

58,248

Case WMP 35 ^ 5 repeated, for each sample, three more times. The extract from each of the extractions is separately mixed with an aqueous solution of 2,4-dinitrophenylhydrazone (DNPH). The reaction is then allowed to proceed for 15 minutes, after which a volume of chloroform is added to the mixture. The reaction mixture is mixed for another 45 minutes: The chlorine and aqueous layers are separated with a separating funnel. The level of glutaraldehyde is determined by analyzing the chloroform layer using high pressure liquid chromatography (HPLC) to detect the DNPH derivative.

The chromatographic conditions for the HPLC analysis were -Column: C-18 inverted phase; Detector: UV at 30 mm; Mobile phase: 80:20 methanol: water; Flow rate: 1 mi / min; measurement made: maximum height. A maximum height and glutaraldehyde content calibration curve was developed by measuring the maximum HPLC of five solutions pa. that had known levels of glutaraldehyde between D and 25 ppm.

Each of the four chloroform phases for each fiber sample was analyzed using HPLC, the maximum height measured and the corresponding level of glutaraldehyde determined from the calibration curve. The glutaraldehyde concentrations for each of the extractions were then added and divided by the weight of the fiber sample (dry fiber base) to provide the glutaraldehyde content on a dry fiber weight basis.

Two glutaraldehyde maxims were present for each of the HPLC chromatograms. Each of these maximums can be used, as long as the same maximum is always used throughout the program.

EXAMPLE 1

This example shows the effect of varying levels of a cross-linking agent, glutaraldehyde, on the absorbency and elasticity of absorbent pads made

58,248

Case WMP 3545 individual, crosslinked fiber posts. Individualized, cross-linked fibers are made using a dry cross-linking process.

For each sample, a given amount of pulp (SSK) of soft kraft wood was provided. The fibers had a moisture content of about 62.4% (equivalent to a consistency of 37.6%). A paste was formed by adding to the fibers a solution containing a chosen amount of 50% aqueous glutaraldehyde solution, 30% (based on the weight of the glutaraldehyde) zinc hexahydrate nitrate, demineralized water and a sufficient amount of IN HC1 to reduce the pH of the paste to about 3.7 »The fibers were impregnated in the paste for a period of 20 minutes and then the water was extracted, until they had a fiber consistency of about 34% at about 35%, P ^or centrifugation. Then, the dehydrated fibers were air-dried to a fiber consistency of about 55% to about 56% with a blow dryer that used room temperature. The air-dried fibers were defibrated by means of a three-phase carding apparatus as described in US Patent 3.9θ7 · 968. The defibrated fibers were placed on trays and set at 145 ° C in an essentially static drying oven for a period of 45 minutes. The crosslinking was completed during the period in the oven. The individualized crosslinked fibers were placed on a mesh screen and washed with water at about 20 ° C, impregnated to a consistency of 1% for one (1) hour in water at 60 ° C, passed through the screen, washed with water at about 20 ° C a second time, centrifuged to a fiber consistency of 60%, defibrated in a three-phase card as previously described and dried to completion in a static oven at 105 ° C for four (4) hours. The dry fibers were aerated to form absorbent pads. The pads were compressed with a hydraulic press until

58,248

Case WMP 35 ^ 5 a density of 0.10 g / cc, The pads were tested for absorbency, elasticity, and quantity of reacted glutaraldehyde, according to the previously defined processes. The reacted glutaraldehyde is indicated in moles% calculated on an anhydroglucose cellulose dry fiber basis. The results are shown in Table 1.

58,248

Case WMP 35 ^ 5

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Case WMP 35 ^ 5)

EXAMPLE 2

The purpose of this example is to demonstrate that low levels of extractable crosslinking agent can be obtained by subjecting the fibers to steps in a bleaching sequence after crosslinking. The level of extractable crosslinking agent was determined by impregnating a sample of the fibers in deionized water at 40 ° C, at a consistency of 2.5%, for one (1) hour. 0 glutaral. dehydrate extracted by water was measured by HPLC and reported with glutaraldehyde on a dry fiber weight basis. The fibers were cross-linked using a dry cross-linking process.

Southern softwood kraft pulp (SSK) was supplied. The pulp fibers were partially bleached using the following bleaching sequence: chlorination (c) - 3 - ^ paste +% consistency treated with about 5% available chlorine (Cl disp) at about pH 2 , 5 and about 38 ° C for 30 minutes; caustic extraction - paste with 12% consistency treated with 1.4 g / 1 NaOH at about 7 ° C for 60 minutes; and treatment with hypochlorite (h) - paste with k2% consistency treated with sufficient sodium hypochlorite at 11-11.5 pH, between 38 ° C and 60 ° C, for 6 minutes, to provide a 60-65 Elretho shine and a viscosity of 15.5 - 16.5 cp. The partially bleached fibers were processed into individualized fibers, cross-linked using glutaraldehyde as a cross-linking agent, according to the process described in Example 1. The fibers retained 2.29 mole% glutaraldehyde, calculated on a molar basis of cellulose anhydroglucose cellulose fiber of dry fibers. Typically such fibers have extractable levels of glutaraldehyde of about 1000 ppm (0.1%).

bleaching of partially bleached fibers, individualized, was then continued and supplemented with chlorine dioxide (ϋ), extraction (E) and hypochlorite

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Case WMP 3545

J of sodium (H) in sequence (DEH). In the chlorine dioxide (D) phase, individualized, cross-linked fibers were impregnated in an aqueous mass with a consistency of 10% that also contained a sufficient amount of sodium hypochlorite to provide 2% of available chlorine on a fiber weight basis. dry. After mixing, the pH of the paste was reduced to about pH 4.4 by the addition of NaOH. The pulp paste was then placed in an oven at 70 ° C for 2.5 hours, passed through the sieve, rinsed with water until it reached a neutral pH and centrifuged to a consistency of 61.4%. In the extraction phase, an aqueous paste of 10% consistency of the dehydrated fibers was treated with 0.33 g NaOH / liter of water for 1.5 hours at 40 ° C. The fibers were then sieved, rinsed with water to a neutral pH and centrifuged to a consistency of 62.4%.

Finally, in the sodium hypochlorite (h) phase, a paste with 10% fiber consistency, containing sufficient sodium hypochlorite to provide 1.5% of available chlorine on a dry fiber weight basis, was prepared. The paste was mixed for one hour and heated in an oven at 50 ° C. The fibers were then sieved, rinsed to pH 5, θ θ centrifuged to a consistency of 62.4%. The dehydrated fibers were air-dried, carded and dried to completion in an oven at 105 ° C for one (1) hour. The level of extractable glutaraldehyde from the completely bleached, individualized, cross-linked fibers was 25 ppm (0.0025%). This is well below the level of extractable glutaraldehyde that is believed to be acceptable for applications in which. fibers are used in close proximity to human skin.

It was also found that pads made from fibers that were partially bleached, cross-linked and then finished bleaching, had an unexpectedly higher liquid retention value and at least a drip capacity, a torque capacity

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Case WMP 35 ^ 5)

and wet elasticity, than those of the individual fibers that have been cross-linked after being completely bleached. However, as a result of the higher WRV, the crosslinked fibers at an intermediate point in the bleaching sequence were more compressive in a dry state.

Substantially equivalent results were obtained when a peroxide bleaching phase (?) Was replaced by the final hypochlorite phase phase (h). In phase P, a paste of 10% consistency was treated with 0.5% hydrogen peroxide, based on fiber weight at 11-11.5 ® 8o 9 C, respectively for 9θ minutes.

The filing of the first application for the invention described above was made in the United States of America on June 27, 1986 under N® 879.672

Claims (6)

- CLAIMS 1? - Process for the manufacture of individual reticulated fibers, characterized by comprising the following steps; The. supply of cellulosic fibers by contacting them with a crosslinking agent; B. reacting the crosslinking agent with the fibers to form intra-fiber cross-links with substantial absence of inter-fiber links; and ç. washing these fibers with an alkaline solution. 2. A process according to claim 1, characterized in that the cross-linking agent is selected from the group consisting of ^C 2 -Cg dialdehydes, analogs of ^C 2 “ ^C 8 dialdehyde acid containing at least one aldehyde group and oligomers of said dialdehydes and dialdehyde acid analogs. 3- - Process according to the claims - 44 58,248 Case WMP 3545) sections 1 or 2, characterized in that the alkaline solution has a pH greater than 9. 4? Process according to one of the preceding claims, characterized in that the fibers are contacted with a sufficient amount of cross-linking agent, between about 0.5 mole% and about 3.5 moles% of crosslinking agent, calculated on a molar base of anhydroglucose cellulose, making them react, and because the fibers have a water retention value of less than about 60. Process according to any one of the preceding claims, characterized in that the alkaline solution comprises an aqueous medium and a compound selected from the group consisting of sodium hypochlorite, ammonium hydroxide, hydrogen peroxide and sodium bisulfite. 6? Process according to any one of the preceding claims, characterized in that the cellulosic fibers of step a. partially bleached by at least one step of a multi-step bleaching sequence.