FI90678B - A process for preparing wet-assembled compositions containing individual cured fibers - Google Patents

Description

1 90678

A process for preparing wet-assembled compositions containing individual cured fibers

The present invention relates to a process for preparing wet-assembled compositions from single-cured fibers.

Mainly single form cured fibers and various methods of making such fibers have been described in the art. The term "single, cured, cross-linked fiber" refers to cellulosic fibers having primarily chemical crosslinks within the fibers. This means that the crosslinks are mainly between the cellulose molecules of a single fiber and not between the cellulose molecules of different fibers. Single crosslinked fibers and other single hardened fibers are generally considered useful for use in absorbent articles. In general, three classes of methods have been proposed for making single, cured fibers by forming internal crosslinks. These methods, hereinafter described, are referred to herein as 1) a dry crosslinking process, 2) an aqueous crosslinking process, and 3) a substantially anhydrous solution crosslinking process. The fibers themselves and the compositions containing the individual cured fibers generally provide an improvement in at least one significant absorbent property over conventional, non-crosslinked fibers. Often, this improvement in absorbency is presented as absorption capacity. In addition, compositions made from individual crosslinked fibers have higher wet and dry resilience compared to compositions made from non-crosslinked fibers. The term "resilience" as used herein refers to the ability of cellulosic fiber pads to return to their wider, original shape upon release from the compressive force. By dry resilience is meant the ability of an absorbent composition to expand upon removal of the compressive force when the fibers are predominantly in dry conditions. Wet resilience refers in particular to the ability of an absorbent composition to expand after removal of the compressive force when the fibers are in humid conditions. For the purposes of this invention and the purposes of the disclosed composition, wet wettability should be detectable and presented for an absorbent composition moistened to saturation point 10.

Methods for preparing individual crosslinked fibers using the dry crosslinking process are disclosed in U.S. Patent No. 3,224,926 to L.J. Bernardinilie December 15, 21, 1965. Individual, crosslinked fibers are produced by impregnating swollen fibers in an aqueous solution with a crosslinking agent, removing water and defibering the fibers mechanically, and drying the fibers at elevated temperatures to provide early crosslinking, while the fibers are predominant. mode. The fibers naturally form internal crosslinks in a non-swellable, assembled state due to being dehydrated prior to crosslinking. A process such as that disclosed in U.S. Patent No. 3,224,926, in which crosslinking is performed with the fibers in an unswollen assembled form, is referred to as a method of making "dry crosslinked" fibers. Dry crosslinked fibers are characterized by poor fluid retention (FRV) values. U.S. Patent No. 3,440,135, issued to R. Chung on April 22, 1969, discloses immersing fibers in an aqueous solution of a crosslinking agent to reduce the internal binding capacity of the fibers before performing 35 dry crosslinking in the same manner as in U.S. Patent No. 3,906,778. 3,224,926. This time-consuming pretreatment, preferably 16 to 48 hours, is shown to improve the quality of the product by reducing the concentration of short portions due to incomplete defibering.

Methods for producing fibers crosslinked in aqueous solution are disclosed, for example, in U.S. Patent No. 3,241,553 to F.H. To Steiger on March 22, 1966. Individual, crosslinked fibers are prepared by crosslinking the fibers with 10 in an aqueous solution of a crosslinking agent and a catalyst. Fibers formed in this manner are referred to herein as "crosslinked" fibers in aqueous solution. Because water exerts a swelling effect on cellulosic fibers, the fibers crosslinked in the aqueous solution are crosslinked in an unassembled, swollen state. Compared to dry crosslinked fibers, the fibers disclosed in U.S. Patent 3,241,553 are more flexible and less rigid and are characterized by greater oral fluid retention (FRV). Compositions made from crosslinked fibers in aqueous solution have poorer wet and dry resilience than wads made from dry crosslinked fibers.

U.S. Patent No. 4,035,147, issued July 12, 1977 to S. Sangenis, G. Guiroy, and J. Quer, discloses a method of producing individual crosslinked fibers by joining together dehydrated, non-swellable fibers to form crosslinks. and catalyst in a substantially anhydrous solution containing so little water that it is not sufficient to cause the fibers to swell. The formation of crosslinks occurs while the fibers are essentially in an anhydrous solution. Such a method is referred to as the method of forming crosslinks 4 90678 in this anhydrous solution; and the fibers thus prepared are referred to as fibers crosslinked in an anhydrous solution. Like fibers crosslinked dry, crosslinked fibers in an anhydrous solution are well cured by crosslinking, and absorbent compositions made using them have relatively good wet and dry resilience.

The crosslinked fibers shown above are shown to have use in low density absorbent applications such as diapers, and also in higher density absorbent applications such as menstrual pads. However, such fibers have not had sufficient absorption properties in view of their disadvantages and costs compared to ordinary fibers, and thus have not achieved significant commercial success.

One difficulty observed with single, crosslinked fibers, especially fibers crosslinked in a dry and anhydrous solution, is that the fibers rapidly flocculate when deposited wet on a porous formation line. This has prevented the production of absorbent wet-laid compositions as well as compacted sheets, which would allow for the economical transfer of fibers to the paper mill.

It is an object of the present invention to provide improved methods for preparing individual, crosslinked fibers for use in wet-laid compositions.

The invention thus relates to a continuous process for producing a sheet containing crosslinked cellulosic fibers which have a high tendency to flocculate when an aqueous suspension of said fibers is deposited on a forming wire in a papermaking type apparatus, wherein a) a fibrous slurry containing individual, b) depositing the slurry on a porous forming wire, c) directing a plurality of liquid streams downwardly onto the forming layered fibers and draining the liquid out of the fibers, the streams being arranged parallel to the machine direction folder of the forming wire; preventing the formation of flocs of fibers, said fibers being obtained in a substantially non-flocculated form, and d) placing said fibers in a sheet form after step 15 c) when the fibers are in a substantially non-flocculated form; in that form.

The process according to the invention is characterized in that in step (a) a fibrous slurry containing said fibers and water is prepared, the fibers being air-dried and crosslinked when they are single and unobstructed with a crosslinking agent selected from the group consisting of C2-C8 dialdehydes , C3-C8 dialdehyde acid analogs having at least one aldehyde group, and oligomers of said dialdehydes and dialdehyde acid analogs, wherein the fibers are contacted with a sufficient amount of a crosslinking agent such that 0.5 mol% to 3.5 mol% the crosslinking agent, based on the amount of anhydroglucose mole of cellulose, has reacted with the fibers to form intramibular crosslinks and such that the fibers have a water retention value of 25 to 60; in step (c) directing a series of water jets over said slurry as water runoff from the fibers proceeds, the series of jets being arranged in transversely arranged rows 35 and each row located in an adjacent β 90678 machine direction and progressively decreasing and flow rates so as to substantially disintegrate the flocs formed by said fibers and prevent further formation of the flocs of the fibers, and that in step (d) said fibers are pressed against the forming wire by a cylindrical screen roll.

The step of forming the fibers into a sheet shape can be performed by pressing the fibers against a forming line, such as against a cylindrical Dandy Roll. Preferably, a plurality of fluid streams are applied to the fibers, the streams having a gradually decreasing volume flow rate. The individual, cured fibers can also be mixed with ordinary, uncured fibers or with well-ground, uncured fibers in the form of a slurry.

Individual cured fibers made from cellulose fibers from a variety of natural sources can be used in the invention. Preferably, the decomposed fibers of conifers, hardwoods or cotton fibers are used. Sources of esparto grass, bagasse, hard hair, flax and other wood fibers or cellulosic fibers can also be used as a raw material for the invention. The fibers can be used as a slurry, layered or non-layered. The fibers used as a wet-folded pulp sheet, a dry-folded pulp sheet, or other layered form are preferably converted to a non-layered form by mechanically disintegrating the sheet, preferably before the fibers are combined with the crosslinking agent. The fibers are preferably also used in wet or wetted form. Preferably, the fibers are of a never-dried quality. In the case of a dry-folded pulp sheet, it is preferable to moisten the fibers before mechanical disintegration in order to minimize damage to the fibers. Also useful in the present invention are individual fibers of synthetic origin which tend to join together in solution by the interaction of the chemistry, geometry of the fibers, or these or other factors.

The best cellulosic fiber used in this invention depends on the intended end use. In general, pulp fibers prepared by conventional cellulose production methods are preferred. Useful are 10 fully bleached, partially bleached and unbleached fibers. Sometimes it may be desirable to use bleached pulp because it is very clear and pleasant for the consumer. In a new embodiment of the invention, which will be described in more detail later, the fibers are partially bleached, crosslinked and then completely bleached. For paper towels and absorbent layers for diapers, cellulose wadding, menstrual pads and other similar absorbent paper products, it is particularly advantageous to use fibers from the pulp of the pulp of southern conifers because they have better absorption properties. All individual, cured fibers bonded together in solutions are within the scope of this invention. These include fibers prepared by the dry crosslinking method and the anhydrous solvent crosslinking method previously described. Also included are individual cured fibers treated with resin or other polymeric compounds, as disclosed in U.S. Patent No. 3,819,470 to Shaw, DL, et al., Issued June 25, 1974, and U.S. Patent No. No. 3,756,913 issued to EA To Wodka, Sept. 4, 197 3. The present invention is believed to be most applicable to individual, cured fibers having helical, curly configurations. The following are methods for making such fibers with monomeric crosslinking agents.

Crosslinking agents that can be used in this process are preferably C 2-8 di-aldehydes, as well as acid analogs of such dialdehydes having at least one aldehyde group, and oligomers of such dialdehydes and acid analogs. Such compounds may react with at least two hydroxyl groups in a single cellulose chain or in adjacent cellulose chains in a single fiber. Those skilled in the art will recognize that the above dialdehydes that form crosslinks may be present in various forms or may react in various forms, including the acid analogs and oligomeric forms set forth above. All such forms are intended to be included within the scope of the preferred embodiments. When reference is made herein to a particular crosslinking agent, it is meant that particular crosslinking agent as well as other forms that may be present in aqueous solution. Particular crosslinking agents suitable for use in this invention include glutaraldehyde, glyoxal, and glyoxylic acid. Glutaraldehyde is particularly preferred because it has been used to obtain fibers with the highest levels of absorption and elasticity, is believed to be safe and non-irritating to human skin when in a reacted, cross-linked state, and has provided the most permanent cross-links.

Surprisingly, it has been found that the best absorption results are obtained with amounts of crosslinks that are substantially lower than the amounts of crosslinks used previously. In general, surprisingly good results have been obtained for absorbent pads made from single, crosslinked fibers in which about 0.5 to 3.5 mole percent crosslinking agent, based on the molecular weight of cellulose anhydroglucose, has reacted with the fibers. .

Preferably, the crosslinking agent is combined with the fibers in a liquid medium under conditions in which the crosslinking agent penetrates the individual fibrous structures. However, other treatments with a crosslinking agent are also within the scope of the invention, such as spraying the fibers in a fluffy form containing individual fibers.

In general, the fibers are also combined with a suitable catalyst before crosslinking. The type, amount, and method of catalyst to be incorporated into the fibers will depend on the crosslinking method used. These variations are presented in more detail later.

When the fibers are treated with a crosslinking agent and a catalyst, the crosslinking agent is reacted with the fibers in such a way that no significant interfiber bonds are formed, i.e., low interfiber contact is kept compared to non-fluffy pulp fibers, or the fibers are immersed in a solution that does not favor fibers. the formation of inter-bonds, in particular the formation of hydrogen bonds. This results in crosslinks that are intrinsic 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 adjacent cellulose chains of a single cellulose fiber.

Although not wishing to limit the scope of the invention in any way, it is believed that the crosslinking agent reacts with the hydroxyl groups of the cellulose to form hemiacetal and acetal bonds. Acid reaction conditions are preferred for the formation of acetal bonds, which are believed to be the desired type of bond for stable crosslinks. Thus, acid-catalyzed crosslinking conditions are highly advantageous to this invention.

10 90678

The fibers are preferably mechanically defibered into a low density, fibrous form containing individual fibers, known as "fluff", prior to reaction with the crosslinking agent. Mechanical fiber formation can be performed by a variety of methods known in the art or disclosed herein. The mechanical defibering is preferably carried out by a method in which the formation of fiber bundles and the damage to the fibers are kept to a minimum. One type of machine that has been found to be particularly useful for defibering cellulosic fibers is the three-stage fluffing device disclosed in U.S. Patent No. 3,987,968 to D.R. Moorelle and O.A. To Shields on October 26, 1976, this patent is hereby incorporated by reference into this specification. The fluffing device disclosed in U.S. Patent No. 3,987,968 performs a number of mechanical operations, mechanical mixing, air mixing, and, to a limited extent, air drying on wet cellulosic pulp fibers to obtain substantially 20 fiber-free fluffy fibers. The individual fibers have thus been given more curl and twist compared to the curl and twist commonly found in such fibers. It is believed that this increased curl and twist improves the elastic properties of absorbent compositions made from finished, crosslinked fibers. It is also believed that increased curl and helicity increase the degree of interconnection of individual, crosslinked fibers.

Another useful method of defibering cellulosic fibers, without limiting the methods, is by treatment in a Waring mixer and combining the fibers with a tangentially rotating plate grinder or wire brush. Preferably, during such defibering, a stream of air is applied to the fibers to facilitate separation of the fibers into a substantially single fiber form.

Although a special apparatus is used to form the fluffy fibers, the fibers are preferably mechanically treated to contain at least about 20% moisture 5 and preferably about 40-60% moisture.

High or partially dried fibers can also be mechanically ground to provide curl and twist to the fibers in addition to the curl and twist obtained by mechanical fiberization.

The fibers made in accordance with the present invention have a unique combination of hardness and flexibility and therefore compositions made from the fibers can retain high absorbency and can be very flexible and easily expandable when wetted from a dry compressed absorbent composition. In addition to the amounts of crosslinking in a given range, crosslinking fibers are characterized by water retention values (WRV) of less than 60 and preferably about 28-45 for ordinary chemically treated papermaking fibers. The WRV value for the fiber in question indicates the number of crosslinks and the degree of swelling of the fibers during crosslinking. One skilled in the art will recognize that the more swollen the fiber during crosslinking, the higher the WRV for a given amount of crosslinked dosage. For highly crosslinked fibers, such as those prepared by the previously known dry crosslinking methods described above, WRV values have been found to be less than about 25 and generally less than about 20. Of course, the crosslinking pattern used in each case affects the WRV of the crosslinked fiber. values. However, all methods that produce the number of crosslinks and WRVs within the given limits are believed to be within the scope of the invention. The crosslinking methods used include dry crosslinking methods and crosslinking methods in anhydrous solutions, as generally described above. Certain preferred dry crosslinking methods and crosslinking methods in anhydrous solutions, which are within the scope of the invention, will be described in more detail below. Crosslinking in aqueous solution, in which the solution causes considerable swelling of the fibers, results in fibers having WRVs greater than about 60. These fibers have insufficient hardness and flexibility for the purposes of the present invention.

With particular reference to dry crosslinking methods, individual crosslinked fibers can be produced using such methods using a certain amount of cellulosic fibers, incorporating the above amount and the above type of crosslinking agent into the fiber slurry, mechanically separating, e.g., defibering, substantially drying the fibers and drying. The bonding agent reacts in the presence of a catalyst to form crosslinks while maintaining the fibers in a predominantly single fiber form. The defibering step, separate from the drying step, is believed to increase curl. The subsequent drying is performed by twisting the fibers, whereby the twisting is increased by the curl geometry of the fibers. As used herein, the term "curly" refers to the geometric curvature of a fiber with respect to the longitudinal axis of the fiber. "Rotation" refers to the rotation of a fiber toward a perpendicular section of the longitudinal axis of the fiber. For example, without limiting the invention, individual crosslinked fibers within the scope of the invention have been found to have an average of six turns per millimeter of fiber.

When the fibers are kept substantially individual during drying and crosslinking, the fibers are allowed to rotate during drying and thus can be crosslinked in a twisted, curled state. Drying the fibers under conditions such that the fibers can twist and curl is referred to as drying the fibers under substantially unrestricted conditions. On the other hand, drying the fibers in a layered form results in dried fibers that are not twisted and curled, such as fibers dried in a form containing mainly single fibers. It is believed that the hydrogen bonds between the fibers "prevent" the corresponding occurrence of fiber twisting and curl.

There are various methods for combining fibers with a crosslinking agent and a catalyst. In one embodiment, the fibers are combined with a solution of both a crosslinking agent and a catalyst. In another embodiment, the fibers are combined with an aqueous solution of the crosslinking agent and allowed to immerse before the catalyst is added. The catalyst is then added. In a third embodiment, the crosslinking agent and the catalyst are added to the aqueous mixture of cellulosic fibers. Other methods of addition in addition to those set forth herein are known to those skilled in the art and other methods are intended to be within the scope of the invention. According to the method used, in which the fibers are combined with the crosslinking agent and the catalyst, the cellulosic fibers, the crosslinking agent and the catalyst are preferably mixed and / or sufficiently absorbed with the fibers to obtain complete contact and saturation of the individual fibers.

in general, all substances that catalyze the mechanism of crosslinking can be used. Useful catalysts include organic acids and acid salts. Particularly preferred catalysts are salts, for example the aluminum, magnesium, zinc and calcium salts of chlorides, nitrates or sulphates. A specific 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 ha-5 pot. The selected catalyst may be used as the sole catalyst or in combination with one or more other catalysts. It is believed that combinations of acid salts and organic acids as catalysts provide the best crosslinking efficiency. Surprisingly high degrees of reaction completeness have been observed with catalyst combinations of zinc nitrate salts and organic acids such as citric acid, and the use of such combinations is preferred. Mineral acids are useful in adjusting the pH of the fibers when in contact with the crosslinking agent in solution, but are preferably not used as the main catalyst.

The optimum amount of crosslinking agent and catalyst to be used depends on the particular crosslinking agent used, the reaction conditions and the intended application of the product.

The amount of catalyst used will, of course, depend on the type and amount of crosslinking agent and the reaction conditions, in particular temperature and pH. In general, based on technical and economic requirements, preferred amounts of catalyst are from 10 to 60% by weight based on the amount of crosslinking agent added to the cellulosic fibers. An example is the case where the catalyst used is zinc nitrate hexahydrate and the crosslinking agent is glutaraldehyde, and where the preferred amount of catalyst is about 30% by weight based on the amount of glutaraldehyde added. Even more preferably, 5 to 30% by weight of the added glutaraldehyde is added to an organic acid, such as citric acid, also as a catalyst. In addition, it is further desirable to adjust the pH of the aqueous portion of the cellulosic fiber slurry or crosslinking agent solution to 2-5, more preferably to about 2.5 to 3.5 during the contacting step between the crosslinking agent and the fibers.

5 Cellulose fibers should normally be water-free and possibly dried. The usable and optimal consistency will vary depending on the flocculation method used. In preferred embodiments, the cellulosic fibers are dewatered and optimally dried to a consistency of about 30-80%. Even more preferably, the fibers are dewatered and dried to a consistency of about 40-60%. Drying the fibers to these preferred areas is generally advantageous for fiberizing the fibers to a single fiber form without significant fiber knot formation associated with higher moisture contents and without significant fiber damage associated with lower moisture contents.

The dewatering can be performed, for example, by mechanically pressing, centrifuging or air-drying the pulp. Further drying is preferably performed according to methods known in the art, such as air drying or flake drying under conditions that do not require high temperatures for long periods of time. Too high a temperature at this stage of the process may cause premature onset of crosslinking. Preferably, temperatures higher than 160 ° C are not maintained for more than 2-3 seconds. Mechanical defibering is performed as described above.

The defibered fibers are then heated to a suitable temperature for a time sufficient for the crosslinking agent to cure, i.e., react with the cellulosic fibers. The rate and amount of crosslinking depends on the dryness of the fibers, the temperature and the amount and type of catalyst, and the method used for heating and / or the drying of the fibers during crosslinking. The formation of crosslinks at a given temperature occurs at a higher rate for fibers with a certain initial moisture content when continuously air-dried than when they are dried / heated in a static oven. One skilled in the art will recognize that curing the crosslinking agent involves a number of temperature-time relationships. At ordinary paper drying temperatures (e.g., about 49 to 66 ° C) for 30 to 60 minutes under static, atmospheric conditions, a satisfactory curing effect of the fibers is usually obtained for fibers having a moisture content of less than about 5%. It will also be apparent to one skilled in the art that higher temperatures and airflow reduce the time required for curing. However, curing temperatures are preferably kept below 160 ° C, as exposure of the fibers to high temperatures higher than about 160 ° C can lead to yellowing or damage to the fibers.

The maximum degree of crosslinking 20 is achieved when the fibers are substantially dry (with less than 5% moisture). Since there is thus no water in the fibers, crosslinks are formed in the fibers in a substantially non-swellable, compressed state. As a result, they typically have low fluid retention values (FRVs) compared to the values useful in this invention. FRV refers to the amount of liquid, calculated on the basis of dry fibers, which remains absorbed in a fiber sample that is immersed and then centrifuged to remove interfiber liquid. (In Edem-30, the FRV is determined in more detail and the method for determining the FRV is shown). The amount of liquid that the crosslinked fibers can absorb depends on their ability to swell when impregnated, i.e., their internal diameter or volume as they swell to their maximum value. This 35 in turn depends on the number of crosslinks. As the number of internal crosslinks in the fibers 17 90678 increases with a given fiber and method, the FRV of the fiber decreases until the fiber does not swell at all upon wetting. Thus, the FRV value of a fiber structurally describes the physical state of the fiber in the ai-5 saturation hen. Unless otherwise specifically indicated, FRV data are presented herein as fiber retention value (WRV). Other fluids, such as brine and synthetic urea, may also be well used as the liquid medium for analysis. In general, the FRV depends on certain fibers rendered cross-linked by methods in which curing is highly dependent on drying, as in the present method, mainly on the crosslinking agent and the amount of crosslinking. The WRVs of the fibers crosslinked by this dry crosslinking process at the amounts of crosslinking agent useful in this invention are usually less than about 50, greater than about 25, and are preferably between 28 and 45. Bleached SSK fibers having about 0.5 to 2.5 mole percent glutaraldehyde reacted with them, based on the molar amount of cellulose anhydroglucose, has been found to vary from about 40 to 28 WRV values, respectively. The degree of bleaching and post-crosslinking bleaching has been found to affect WRV ratio. This effect is presented in more detail 25 min later. Southern conifer kraft pulp (SSK) fibers prepared by the dry crosslinking process known prior to the present invention have higher levels of crosslinking than those disclosed herein and have WRVs of less than 25. Such fibers have been found to be as follows: as described above, are very hard and have lower absorption capacities than the fibers of the present invention.

In another method of making individual crosslinked fibers by a dry crosslinking process, the cellulose fibers are combined with a solution containing a crosslinking agent as described above. Either before or after the fibers are combined with the crosslinking agent, the fibers are sheeted. Preferably, the solution containing the crosslinking agent also contains a catalyst suitable for the dry crosslinking process also described previously. The fibers, in sheet form, are dried and crosslinked, preferably by heating the fibers to a temperature of about 120-10 ° C. After cross-linking, the fibers are mechanically separated into a form containing mainly single fibers. This is preferably accomplished by treating the fibers with a fluffing device, such as that disclosed in U.S. Patent No. 3,987,968, or may be accomplished by other fiber defibering methods known in the art. The individual crosslinked fibers prepared according to this sheet crosslinking method are treated with a sufficient amount of crosslinking agent to provide about 0.5 to 3.5 mole percent crosslinking agent, based on the molar amount of cellulose anhydroglucose and measured after fiber molding. react with the fibers in the form of intra-fiber crosslinks. Another effect of drying and crosslinking the fibers 25 when they are in sheet form is that the fiber-fiber bonds prevent the fibers from twisting and curling further upon drying. Compared to single, crosslinked fibers made by a process in which the fibers are dried under substantially unrestricted conditions and then crosslinked in a twisted, curly configuration, one would expect absorbent compositions made with the above unthreaded fibers made from have a lower wet resilience of 35 and a lower dry absorbent composition.

Another class of crosslinking methods suitable for use in the present invention are crosslinking methods in an anhydrous solution. The same types of fibers as in the dry crosslinking process can be used in the preparation of fibers crosslinked in an anhydrous solution. The fibers are treated with a sufficient amount of crosslinking agent so that 0.5 to 3.5 to 10 mole percent of the crosslinking agent reacts sequentially with the fibers to calculate the amount of crosslinking agent after that crosslinking reaction, and with a suitable catalyst. The crosslinking agent is reacted by immersing the fibers in Liu-15, which does not cause significant swelling of the fibers.

However, the fibers may contain up to about 30% water or may otherwise be swollen crosslinks formed in solution of the substance to the extent that they correspond to fibers with a moisture content of 30%. Such swollen fiber geometry of the sections has been found to provide additional unexpected advantages, as will be discussed in more detail below. The crosslinking solution contains, but is not limited to, an anhydrous, water-miscible, polar diluent such as acetic acid, propanoic acid, or acetone. Preferred catalysts are mineral acids such as sulfuric acid and halogen acids such as hydrochloric acid. Other useful catalysts include salts of mineral acids and halogen acids, organic acids and their salts. Crosslinking solution systems useful as a crosslinking medium also include the systems disclosed in U.S. Patent No. 4,035,147 to S. Sangenis, G. Cuiroy, and J. Quer, issued July 12, 1977, which is hereby incorporated by reference. The crosslinking solution may contain some water or other fibrous swelling fluid, however, the amount of water is preferably insufficient to cause a degree of swelling corresponding to a degree of consistency obtained with 70% pulp (30% water moisture content). In addition, the water of the crosslinking solution 5 preferably constitutes less than 10% of the total volume of the solution, excluding the fibers. Higher water concentrations in the crosslinking solution reduce the efficiency and rate of crosslinking.

Absorption of the fiber crosslinking agent can be performed in the crosslinking solution itself or in an earlier treatment step, including, but not limited to, impregnation of the fibers with either an aqueous or non-aqueous solution containing the crosslinking agent. Preferably, the fibers are mechanically defibered into a form containing individual fibers. This mechanical treatment can be performed by the previously described fiber fluffing methods and the previously described dry crosslinking method.

It is particularly preferred to use a mechanical treatment in the production of the fluff that gives the moist cellulosic fibers a curly or helical shape to the extent that it exceeds the curl or helicity of the natural state of the fibers, if any. This is accomplished by initially using fibers in the moist state for fluffing, subjecting the fibers to mechanical treatments previously described as methods for defibering fibers to a form comprising substantially 30 individual fibers, and finally always partially drying the fibers.

The curl and twist obtained in the fibers depend in part on the moisture content of the fibers. Without limiting the invention in any way, it is believed that the fibers are inherently rotated upon drying under conditions in which the fiber-to-fiber contact is low, i.e., if the fibers are in a form containing individual fibers. Mechanical treatment of wet fibers also causes the fibers to curl. When the fibers are then dried or partially dried under essentially unrestricted conditions, the fibers become helical, whereby the helicity is further increased by the amount of mechanically induced curl. The flocculation fluffing steps are preferably performed on a very thick moist cellulosic pulp or dewatered cellulosic pulp to a fiber consistency of about 45-55% (determined prior to the commencement of the fiberization).

After defibering, the fibers should be dried to 0-30% moisture before combining with the crosslinking agent, unless the defibering step 15 has already produced fibers with a moisture content in this range. The drying step should be performed with the fibers under substantially unrestricted conditions. This means that the fiber-to-fiber contact should be kept to a minimum so as not to prevent the fibers from twisting during drying. Both air drying and flake drying are suitable for this purpose.

Next, the individual fibers are combined 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% and is preferably less than 9%.

A sheet of fibers that have not been mechanically defibered can also be combined with the crosslinking solution, as described above.

The amounts of crosslinking agent and catalyst used depend on the reaction conditions such as consistency, temperature, water content in the crosslinking solution and fibers, the type of crosslinking agent and diluent in the crosslinking agent, and the amount of crosslinking desired. Preferably, the amount of crosslinking agent used is 0.2 to 10% by weight (based on the total weight of the crosslinking solution without fibers). The preferred acid catalyst concentration further depends on the acidity of the catalyst in the crosslinking solution. Good results are usually obtained with catalyst concentrations, including hydrochloric acid, of between 0.3 and 5% by weight (based on the weight of the crosslinking solution without fibers) in crosslinking solutions with acetic acid diluent, preferred amounts of glutaraldehyde and limited water. Mixtures of fibers and crosslinking agent having a fiber content of less than 10% by weight are preferred for crosslinking using the crosslinking solutions described above.

The crosslinking reaction can be carried out at ambient temperature or at accelerated reaction times at elevated temperatures, preferably below 40 ° C.

There are various methods by which fibers can be combined and crosslinked with a crosslinking agent. In one embodiment, the fibers are combined in a solution that initially contains both a crosslinking agent and an acid catalyst. The fibers are allowed to immerse in the crosslinking solution, during which crosslinking occurs. In another embodiment, the fibers are combined with a diluent and allowed to immerse prior to the addition of the acid catalyst. The acid catalyst is then added, whereupon the formation of crosslinks begins. Other methods of addition are known to those skilled in the art and are within the scope of the invention.

Preferably, the crosslinking agent and the conditions under which the crosslinking is performed are selected to favor the formation of crosslinks within the fibers. Thus, it is preferable for the crosslinking reaction to take place in such a way that the crosslinking agent has had sufficient time to be absorbed into the fibers. The reaction conditions are preferably chosen so as to avoid the immediate formation of crosslinks unless the crosslinking agent has already been absorbed into the fibers. The reaction times during which the formation of cross-sections have mainly taken place are preferably about 30 minutes. Longer reaction times are believed to provide minimal marginal advantages in fiber fabrication. However, both shorter reaction times, including substantially immediate crosslinking, and longer reaction times are within the scope of the invention.

It is also intended to only partially cure the fibers in solution and then to terminate the crosslinking reaction later in the process by drying or heating treatments.

After the crosslinking step, the fibers are dried and washed. Preferably, a sufficient amount of a basic agent, such as quicklime, is added to the washing step to neutralize any acidity remaining in the cellulose. After washing, the fibers are dehydrated and dried completely. Preferably, the fibers are subjected to a second mechanical defibering step which causes the crosslinked fibers to curl, e.g., fluffed by defibering between the liquid removal step and the drying step. During drying, the fibers curled are subjected to additional twisting in a curling treatment, as previously described, prior to combining with the crosslinking agent. In this second mechanical defibering step, the same equipment and methods can be used to effect twisting and curling as were used in the first mechanical defibering step. The term "defibering" refers to all methods that can be used to mechanically separate fibers into a form that contains predominantly individual fibers, although the fibers may already be in such a form. "Fibrillation" thus refers to the mechanical treatment of fibers, either in single-fiber form or in a more compact form, referring to a step of mechanical treatment that a) separates the fibers into a predominantly single-fiber form, unless they are already in such a form; threading to the fibers during drying.

This second defibering step, which is performed after the fibers have been crosslinked, increases the helicity and curl of the cellulose. This increase in the twist and curl of the fiber configuration results in improved resilience and wetting response of the absorbent composition. The second defibering step can be performed on all of the crosslinked fibers disclosed herein that are in a moist state. However, a particular advantage of the Kui-20 crosslinking process in anhydrous solution is that the second defibering step can be performed without the need for a further drying step. This is due to the fact that the solution in which the fibers have been crosslinked keeps the fibers flexible after crosslinking, without, however, causing an undesired, highly swollen state for the fibers.

Furthermore, it has surprisingly been found that higher degrees of expansion of the absorbent composition by wetting the compressed layers can be observed with compositions made of fibers crosslinked in a state of helical but partially swollen compared to fibers completely dried from water before cross-linking.

35 Better results have been obtained for single, crosslinked 90678 branched fibers that have been crosslinked under conditions in which the fibers have been dried to a water content of about 18-30% prior to blending with the crosslinking agent. In the case where the fiber has been completely dried before combining with the crosslinking agent, it is in a non-swellable, compressed state. The fiber does not swell when in contact with the crosslinking solution due to the low water content of the solution. As previously discussed, a critical factor in the crosslinking solution is that it does not cause substantial swelling of the fibers. However, if the already swollen fiber absorbs the diluent of the crosslinking solution, the fiber is actually "dried" from water, but the fiber retains its former partially swollen state.

To describe the degree to which the fiber is swollen, it is useful to refer again to the fluid retention value (FRV) of the fiber after crosslinking. Fibers with a higher FRV correspond to fibers made crosslinked in the more swollen state compared to fibers made crosslinked in the less swollen state, all other values being the same. Without wishing to limit the scope of the invention, it is believed that partially swollen, crosslinked fibers with higher FRVs have a higher wet resilience and wetting response than fibers made crosslinked in the non-swellable state. Fibers with increased wet resilience and response to wetting will expand more easily or become unthreaded when wetted in an attempt to restore them to their natural state. However, due to the hardness obtained by forming crosslinks, the fibers are still able to maintain structural support for the impregnated wadding made of 35 fibers. The numerical FRV data presented here for partially 26 90678 swollen crosslinked fibers are water retention values (WRV). As the WRV grows to greater than about 60, the hardness of the fibers is believed to be insufficient to provide wet resilience and wetting response, which is required to support the impregnated absorbent composition.

In another crosslinking alternative, in operation, the fibers are first immersed in an aqueous solution or other fiber swelling solution, the liquid is removed, dried to the desired level, and then immersed in a water-miscible crosslinking solution having a catalyst and crosslinking agent such as Fig. The fibers are preferably mechanically fiberized to a fluffy form after removal of the liquid 15 and prior to further drying to achieve the benefits of increased twist and curl, as previously described. Mechanical defibering, which is performed after the fibers have been joined together with a crosslinking agent, is less desirable because such defibering would evaporate the crosslinking agent and thus cause atmospheric contamination or high air handling costs due to the crosslinking agent.

In a variation of the above method, the fibers are fiberized and then pre-immersed in a concentrated solution of the 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. A 50% by weight aqueous solution of the crosslinking agent, preferably a glutaraldehyde solution, has been found to be a useful solution for pre-immersion of fibers. The pre-immersed fibers are dewatered and immersed in a crosslinking solution of a water-miscible polar diluent, catalyst and a limited amount of water, and then crosslinked as described above. As also described above, the crosslinked fibers can be dewatered and a second mechanical defibering step performed before the sheet or absorbent composition is further prepared.

By pre-immersing the fibers with the crosslinking agent in aqueous solution before reacting the crosslinking agent, unexpectedly good absorption properties are obtained for pads made of crosslinked fibers, even compared to wads made of crosslinked fibers from the above anhydrous solutions. with a crosslinking agent.

The crosslinked fibers obtained from the above methods crosslinked dry and crosslinked in anhydrous solutions are the products of the present invention. The crosslinked fibers of the present invention can be used directly in the manufacture of air-laid absorbent cores. In addition, due to their cured and resilient nature, the crosslinked fibers can be laminated as a wet uncompressed, low density sheet which, when subsequently dried, is directly useful as an absorbent core without further mechanical processing. The crosslinked fibers can also be laminated as wet compressed pulp sheets for sale or transportation to distant destinations.

Once the individual crosslinked fibers are made, they are deposited dry and formed directly into absorbent compositions, or they are deposited wet and formed into absorbent compositions or compacted pulp sheets. However, such fibers are difficult to form into a uniform wet sheet by conventional wet sheet forming methods. This is because the individual, crosslinked fibers readily adhere to each other when in solution. Such coalescence can occur both in the headbox and when deposited on a porous forming line. Attempts to use conventional cellulose sheeting methods to form single, crosslinked fibers into sheets have been found to result in the formation of numerous lumps of interconnected fibers. Without limiting the invention, it is believed that this is due to the hard, helical nature of the fibers, the low amount of fiber-fiber bonds, and the high dryness of the fibers as soon as they are deposited on the forming line. Therefore, it is of great economic importance to be able to obtain a useful method for sheeting individual crosslinked fibers, whereby wet-laid absorbent compositions and compacted pulp sheets can be prepared for transfer and subsequent defibering.

Accordingly, a new method has been developed to form sheets of single crosslinked fibers that tend to bond together in solutions, with the slurry having individual crosslinked fibers first deposited on a porous forming line such as the Fourdrinier line in the same manner as in conventional pulp sheeting methods. . However, due to the nature of the individual, crosslinked fibers, these fibers are deposited on the forming line so that the fibers form numerous lumps. At least one liquid stream, preferably a water jet, is applied to the layered, lumpy fibers. Preferably, a series of jets are applied to the fibers deposited on the forming line, with successive jets having decreasing volume flow rates. The jets should have a sufficient velocity so that the impact of the liquid against the fibers has a preventive effect on the formation of fiber entanglements and 29 90678 disperses the fiber entanglements which have already formed. The step of depositing the fibers is preferably performed with a cylindrical screen, such as a wire roll, or other device having an analogous function, which is or will become known in the art. Once the fibrous sheet is formed, it can then be dried and, if desired, compacted. The jet spacing varies depending on the speed of fiber joining, the line speed of the forming line, the drying through the forming line, the number and speed of the jets 10, and the flow rate of the jets. Preferably, the sprays are so close together that no significant amount of fiber entanglement occurs.

The liquid jets prevent the fibers from coalescing and disperse the coalescences, and in addition, the liquid jets, when applied to the fibers, compensate for the rapid drying of the individual crosslinked fibers, providing additional liquid medium into which the fibers can disperse during subsequent sheeting. The set of jets, in which the jets have decreasing volume flow rates, causes a systematic net increase in the consistency of the slurry while providing a repetitive dispersing and inhibiting effect on the coalescence of the fibers. This results in a relatively even and smooth deposition of the fibers, which are then formed into a sheet immediately, i.e. before re-joining, by allowing the liquid to dry and pressing the fibers against the line of formation.

Compared to pulp sheets made from ordinary, non-crosslinked cellulose fibers, pulp sheets made from single, crosslinked fibers are more difficult to compress to regular pulp sheet densities. Therefore, it may be desirable to combine crosslinked fibers with non-crosslinked fibers, such as those commonly used in the manufacture of absorbent cores. Pulp sheets containing cured, crosslinked fibers preferably contain from about 5% to about 90% non-crosslinked cellulosic fibers, based on the total dry weight of the sheet, mixed with the individual crosslinked fibers. It is particularly preferred to use about 5 to 30% of well-ground crosslinked cellulosic fibers based on the total dry weight of the sheet. Such well-ground fibers are ground or chopped to a degree of grinding of less than about 300 ml CSF, and more preferably less than 100 ml CSF. The non-crosslinked fibers are preferably mixed with an aqueous mixture of individual crosslinked fibers. This mixture can then be formed into a compacted pulp sheet for subsequent defibering and forming into an absorbent wadding. The addition of non-crosslinked fibers facilitates the compression of the pulp sheet into a compacted form while causing surprisingly small losses in the absorbency of the absorbent pads made therefrom. In addition, the non-crosslinked fibers increase the tensile strength and tensile strength of the pulp sheet in absorbent pads made either from the pulp sheet or directly from a mixture of crosslinked and non-crosslinked fibers. The mixture of crosslinked and non-crosslinked fibers can first be made into a pulp sheet and then ground to form an absorbent pad or formed directly for use as an absorbent pad. The fibers, if ground, can be deposited in air or deposited wet, as described above.

Wet-laid sheets or fabrics made from single, crosslinked fibers or blends that also contain non-crosslinked fibers preferably have a basis weight of less than about 800 g / m 2 and densities of less than about 0.60 g / cm 3. Although not intended to limit the scope of the invention, sheets having a basis weight of about 300 to 600 g / m 2 and I; 3i 90678 with densities of about 0.15 to 0.3 g / cm * are particularly suitable for use as direct absorbent cores in disposable products such as diapers, tampons and other menstrual pads. Compositions having weights and densities greater than these values per square meter are believed to be most useful for grinding and air deposition or wet deposition, resulting in a low density and low basis weight composition that is more useful for absorbent applications. Other applications for which the fibers of the present invention are suitable are low density tissue sheets, which may have densities of less than 0.10 g / cm 2.

Absorbent compositions prepared by the disclosed method are useful for a variety of absorbent articles, including, but not limited to, tissue sheets, disposable diapers, menstrual pads, cellulose pads, tampons, and dressings, all of which have an absorbent composition containing the individual absorbents set forth herein. , cross-20 dox fibers. In particular, for example, disposable diapers or the like having a liquid-permeable surface layer, a liquid-impermeable backing layer attached to the surface layer, and an absorbent composition containing individual, crosslinked fibers are included. Such products are disclosed, in particular, in U.S. Patent 3,860,003, issued to Kenneth B. Buell on January 14, 1975, which is hereby incorporated by reference into the present invention.

Determination of Water Retention Value 30 The following method was used to determine the water retention value of cellulose fibers.

A sample of about 0.3 to 0.4 g of fibers is immersed in a covered container of about 100 ml of distilled or deionized water at room temperature for 15 to 20 hours. The embedded fibers are collected on a filter and transferred to a 32 90678 80-mesh fabric basket placed at a height of approximately 3.81 cm (1 1/2 inches) above a 60-mesh screen-based centrifuge tube. The tubes are covered with a plastic lid and the sample is centrifuged at a relative centrifugal force of 1,500 to 1,700 times the force of gravity for 19 to 21 minutes. The centrifuged fibers are then removed from the basket and weighed. The weighed fibers are dried to constant weight at 105 ° C and reweighed. The water retention capacity is calculated as follows: 10 WRV = x 100 ^ where 15 W = wet weight of the centrifuged fibers; D = dry weight of the fibers; and W-D = weight of water absorbed

Method. Determining the Amount of Alutar Aldehyde Reacted with Cellulose Fibers The following method was used to determine the amount of glutaraldehyde that reacted to form intracellular crosslinks with the cellulose portion of individual glutaraldehyde crosslinked fibers.

A sample of individual crosslinked fibers is extracted with 0.1 N HCl. The extract is separated from the fibers and the same extraction / separation method is repeated three more times for each sample. The product from each extract is mixed separately with an aqueous solution of 2,4-dinitrophenylhydrazone (DNPH). The reaction is allowed to proceed for 15 minutes to 30 minutes, after which a certain amount of chloroform is added to the mixture. The mixture is then stirred for a further 45 minutes. The chloroform and aqueous layer are separated in a separatory funnel. The amount of glutaraldehyde is determined by analyzing the chloroform layer by HPLC for the DNPH derivative.

The chromatographic conditions used in the HPLC analysis were 33 90 678 teas: column: C-18 reverse phase; detectors: UV 360 mm; mobile phase 80:20 methanol / water; flow rate: 1 ml / min; measurement method: peak height. A calibration curve for peak height and glutaraldehyde content was performed by measuring peak heights for five standard solutions with certain known amounts of glutaraldehyde between 0-25 ppm.

All four chloroform phases from each fiber sample were analyzed by HPLC, peak heights were measured at 10, and the corresponding amount of glutaraldehyde was determined from the calibration curve. The glutaraldehyde concentrations of each extract were summed and divided by the weight of the fiber sample (calculated from dry fiber) to obtain the glutaraldehyde content based on the weight of the fibers.

There were two glutaraldehyde peaks for each HPLC chromatogram. Either peak can be used as long as the same peak is used all the time during the method.

Example 1 This example shows a preferred method for making single, crosslinked fibers. Individual, crosslinked fibers were prepared dry by the crosslinking method.

For each sample, a certain amount of as-25 undamaged southern softwood (SSK) cellulose pulp was used. The moisture content of the fibers was about 62.4% (corresponding to a consistency of 37.6%). A slurry was formed by adding fibers to a solution of a selected amount of a 50% aqueous solution of glutaraldehyde, 30% (based on 30 amounts of glutaraldehyde) zinc nitrate hexahydrate, demineralized water, and sufficient 1 N HCl to lower the pH of the slurry to about 3.7. The fibers were immersed in the slurry for 20 minutes and then dewatered by centrifugation until the fiber consistency was about 34-35%. Next, the dehydrated fibers were air dried to a fiber consistency of about 55-56% by blow drying using ambient air. The air-dried fibers were fiberized using a three-stage fluffing apparatus, as disclosed in U.S. Patent 3,987,968. The fiberized fibers were placed on trays and cured at 145 ° C primarily statically by drying in an oven for 45 minutes. Cross-linking occurred completely during this time in the furnace. The cross-linked, individual fibers were placed on a mesh screen and washed with water at about 20 ° C, immersed in water at 60 ° C for 1 hour at 1% consistency, screened, washed a second time with water at 20 ° C, centrifuged To a fiber consistency of 60%, was defibered in a three-stage pulverizer as previously described, and dried completely in a static drying oven at 105 ° C for four hours. The fibers contained 0-3.3 mol% glutaraldehyde reacted in the form of crosslinks. The corresponding WRVs ranged from 51% to 28%.

Example 2 The purpose of this example is to illustrate a method of making wet-laid sheets with individual crosslinked fibers.

A slurry having a consistency of 0.55% and a mixture of fibers containing 90% of individual, crosslinked fibers prepared by the crosslinking method of Example 1 and 10% of ordinary non-crosslinked fibers having a degree of grinding of less than 100 CSF, were deposited as interconnected, lumpy fibers on a standard 84-mesh Fourdinier formation line. The flow rate of papermaking in the headbox was 430 kg / roin. Immediately after deposition, the fibers were subjected to five water jets with successive decreasing flow rates. Five water jets gave a cumulative flow rate of 85 kg water / kg oven dry (b.d.) fiber. The jets were all directed over an area of about 1 m in length parallel to the direction of movement of the forming line. Each water jet was applied to the fibers through a linear series of 3.2 mm (1/8 inch) ID circular apertures spaced 12.7 mm (1/2 inch) from the shape line and extending across the width of the formation line. The approximate flow percentages calculated based on the total flow rate and the flow rate through the orifices for each jet were as follows: jet 1 to 37% of the total flow, 170 m / min; shower 2 10 - 36% of total flow, 165 m / min; shower 3 - 13% of total flow, 61 m / min; shower 4 - 9% of total flow, 41 m / min; shower 5 - 5% of the total flow, 20 m / min. Immediately after the fifth spray, the fibers were fixed by treatment with a cylindrical screen-15 roll known in the art as the Dandy Roll. The wire roll compressed the fibers that were in the form of a high-consistency slurry at the time of attachment, against the formation line, attaching the fibers to form a wet sheet. The sheet was similar in appearance to the ordinary 20-sheet fibrous pulp sheets.

Claims (2)

A continuous process for producing a sheet containing crosslinked cellulose fibers with a high tendency to flocculate, wherein an aqueous suspension of said fibers is deposited on a forming wire in a papermaking type device, wherein: a) a fibrous slurry containing individual , cured fibers and a liquid medium are prepared b) the sludge is deposited on a porous forming wire, c) a plurality of fluid streams are directed downwardly onto the fibers deposited on the forming wire and the liquid is allowed to drain from the fibers, the streams being arranged parallel to the machine direction of the forming wire, wherein the liquid streams disintegrate the floc formed by the fibers and prevent flocculation in the fibers, said fibers being obtained in substantially non-flocculated form, and d) said fibers being placed in sheet form after step c), where the fibers are substantially non-flocculating. - flocculated shape, characterized in that in step (a) a fibrous is produced slurry containing said fibers and water, the fibers being air-dried and crosslinked, where they are single and unobstructed, with a crosslinking agent selected from the group C2-C8 dialdehydes, C2-25 C8 dialdehyde acid analogs with at least one aldehyde group and oligomers said dialdehydes and dialdehyde acid analogs, wherein the fibers have been contacted with a sufficient amount of the crosslinking agent such that 0.5 mole% 3.5 mole% of the crosslinking agent, calculated on the cellulose anhydroglucose mole amount, has reacted with the fibers to form cross-links within the fibers and such that the water-binding value of the fibers is 25-60; in step (c), a series of water jets is directed to said sludge as the water drain from the fibers advances, the series of jets being arranged in rows positioned in the transverse direction 1 39. 0 678 and each row are arranged in the machine direction relative to the adjacent row, whereby the flow rate and flow rate of the successive beams decrease progressively while having sufficient flow rates and flow rates, so as to substantially decompose fibers form the flock and prevent further flocculation in the fibers, and by pressing the fibers mentioned in step (d) against the forming wire with a cylindrical sieve roll. 2. A method according to claim 1, characterized in that the water bond value of the crosslinked fibers is 28 - 45.