MXPA96004295A - Cellulosic fibers with high content of lignin, treated termicame - Google Patents

Abstract

The present invention relates to a method for preparing high cellulose fibers, heat-treated in air, having at least about 10% by weight of lignin content on a dry basis, which are free of portions of crosslinking agents and which they have a water retention value ranging from 90 to 135, and a dry elasticity defined by a 5K density ranging from 0.08 to 0.20 g / cm3, characterized in the method because it comprises the steps of: a) providing high cellulosic fibers lignin content at a consistency of 40 to 80%, which are free from mixing with cross-linking agent, b) subjecting the fibers to defibration, c) heating said cellulose fibers with high lignin content in air at atmospheric pressure to remove any moisture content and thermally treat the cellulose fibers with high content of lignin, free of moisture, resulting, at a temperature between approximately 120 ° C at approximately 280 ° C, for at least 5 seconds

Description

CELLULOSE FIBERS WITH HIGH LIGNIN CONTENT.

TREATED THERMALLY FIELD OF THE INVENTION The present invention is directed to modified high-lignin cellulosic fibers, to absorbent structures containing these fibers, and to methods for modifying high-lignin cellulosic fibers for use in absorbent structures.

BACKGROUND OF THE INVENTION Cellulosic fibers with high lignin have the advantages of being non-expensive and relatively chemical-free compared to bleached kraft pulp fibers. However, these are not useful as major constituents in absorbent structures, for example, diapers and catamenial products, because of their high hydrophobicity due to the presence of such large amounts of hydrophobic lignin. The patent application of S.A. Naieni and C. Herrón, entitled "Esterified High Lignin Content Cellulosic Fibers" (esterified cellulosic fibers, with high lignin content), currently presented with the present, is directed to the modification of cellulosic fibers, with high content of lignin, with ester portions of polycarboxylic acid of C2-C9, of intrafiber, to be used in the absorbent structures. Kinsley, Jr., in the patent of E.U.A. No. 4,557,800 is directed to thermally treated cellulosic pulps in a non-oxidative, gaseous medium at a temperature exceeding approximately 400 ° F to provide a pulp without loss of hemimellulose. Barbe et al., In the patent of E.U.A. No. 4,431,479 is directed to mechanical subjection, high performance pulp or ultra high performance to mechanical action in high consistencies (15 to 35%) to make the curled fibers and subject it to subject the curled pulp to high consistency (15 to 35% ) to heat treatment at high pressures without appreciable drying of the pulp.

BRIEF DESCRIPTION OF THE INVENTION It has now been discovered that cellulose fibers with high lignin content, thermally treated in air, that are free of portions of crosslinking agents, develop unexpectedly well in absorbent applications. One embodiment of the present invention is directed to cellulosic fibers with high lignin content, thermally treated in air, which are free of portions of crosslinking agents, and have a water retention value ranging from 90 to 135, and an elasticity in dry defined by a density after pressure (that is, a 5K density) that varies from 0.10 to 0.20 gr / cm3. In practice, fibers often have a water retention value that varies from 110 to 125, a dry elasticity defined by a density of 5K that varies from 0.12 to 0.18 gr / cm3, a wet elasticity defined by a compressibility in wet which varies from approximately 7.2 to 8.2 cm3 / gr and a drip capacity that varies from approximately 5.5 to 12.0 gr / gr. A second embodiment of the present invention is directed to an absorbent structure comprising said cellulosic fibers with high lignin content, heat treated in air. A third embodiment of the present invention is directed to a method for preparing high-lignin cellulosic fibers, heat-treated in air, which are free of portions of cross-linking agents and have a water retention value ranging from 90 to 135. , and a dry elasticity defined by a 5K density ranging from 0.08 to 0.20 g / cm 3, and comprises the steps of (a) providing cellulosic fibers with a high content of lignin at a consistency of 40 to 100%, which are free from mixture with the crosslinking agent; (b) subjecting the fibers to defibration; and (c) heating in air at atmospheric pressure to remove any moisture content and thermally treat the cellulose fibers with high content of lignin, free of moisture, for at least 5 seconds, thereby producing said cellulose fibers with high content of lignin, heat treated in air. Preferably, the mixture of step (a) has a consistency ranging from 45 to 80%, most preferably from 50 to 70%. The heating of step (c) can be carried out in two stages, a first drying stage (for example, instant drying) to obtain a consistency of at least 60%, if this consistency is not already present or to increase the consistency yes a consistency of at least 60% is already present, for example, at 85-95% or even 100% consistency, and a second stage to remove any remaining moisture content and heat treat the cellulose fibers with high content of humidity, free of moisture, for example, heating for 5 seconds to 2 hours at an air temperature in the heating apparatus from 120 ° C to 280 ° C, preferably for 2 to 75 minutes at an air temperature in the apparatus of heating from 150 ° C to 190 ° C. A fourth embodiment of the present invention is directed to a method for preparing high-lignin cellulosic fibers, heat-treated in air, which are free of portions of cross-linking agents and have a water retention value ranging from 90 to 135, and a dry elasticity defined by a 5K density that varies from 0.08 to 0.20 g / cm3, and comprises the stage of heating a sheet (from 0 to 40% moisture content) of cellulosic fibers with high lignin content in air at atmospheric pressure to remove any moisture content and thermally treat the cellulose fibers with high lignin content, free of moisture, for at least 5 seconds, for example, heating for 5 seconds to 2 hours at an air temperature in the apparatus heating from 120 ° C to 280 ° C, preferably for 2 to 75 minutes at an air temperature in the heating apparatus from 150 ° C to 190 ° C, and optionally the additional stage of defibration on The fibers resulting from the methods herein are optionally moistened to protect them from damage in subsequent handling or processing to make absorbent products. The high-lignin cellulosic fibers, heat-treated in air, prepared as described above are ready to be packed or used. The term "high lignin content" is used herein to mean 10 to 25% by weight of lignin, on a dry basis. The "water retention values" (referred to in the Examples herein as WRV), set forth herein, are determined by the following procedure: a sample of about 0.3 g to about 0.4 g of fibers (i.e., about 0.3 g. approximately a 0.4 g portion of the fibers for which the water retention value is being determined is soaked in a container covered with approximately 10 ml of distilled or deionized water at room temperature for about 15 and about 20 hours. fibers soaked in a filter and transferred to a wire mesh basket of 80, supported approximately 3.81 cm above a lower part of 60 mesh screen of a centrifugal tube.The tube is covered with a plastic cover and centrifuged sample 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 heavy. The heavy fibers are dried at a constant weight at 105 ° C and weighed again. The water retention value (WRV) is calculated as follows: WRV = (W-D) X 100 D where, W = the wet weight of the centrifuged fibers; D = the dry weight of the fibers; and W-D = the weight of the water absorbed.

The term "dry elasticity" is used herein to refer to the capacity of the structure made from the fibers herein, to expand upon release of the applied compression force, while the fibers are in a substantially dry condition . The dry elasticity is defined by a density after applying pressure that is a measure of the hardness of the fibers and is determined here in the 5K density test according to the following procedure: a square pad placed in four-inch air is prepared by four inches, which has a mass of approximately 7.5 g of the fibers for which the dry elasticity is determined, and compressed in a dry state by a hydraulic press at a pressure of 5,000 psi, and the pressure is released quickly. The pad is reversed and the pressure is repeated and released. The thickness of the pad is measured after pressing with an unloaded gauge (the thickness was tested in Ames thickness). Five thickness readings are taken, one at the center and 0.001 inches at each of the four corners and the five values are averaged. The pad is cut to four inches by four and then measured. The density after pressing is then calculated as mass / (area x thickness). This density is denoted as the 5K density in the present. The lowest of the values of the 5K density test, that is, the density after applying the pressure, the highest of the fiber stiffness and the highest of the dry elasticity. The term "wet elasticity" is used herein to refer to the ability of a structure to expand upon releasing the compression force, while the fibers are wetted to saturation. The wet elasticity is defined by an empty volume after the reduction of the compression load, it is a measure of the volume emptied in moisture and is determined here in the "wet compressibility pod" by the following procedure: a pad is prepared square four by four inches placed in air, leaving approximately 7.5 g, from the fibers that are tested. The density to the pad is adjusted to 0.2 g / cc with a press. The pad is loaded with synthetic urine at ten times its dry weight, or up to the point of saturation, whichever is less. A compression load of 0.1 PSI is applied to the pad. After about 60 seconds, during which time the pad is balanced, then the compression load is increased to 1.1 PSI. The pad is allowed to reach equilibrium and then the compression load is reduced to 0.1 PSI. The pad is then allowed to reach equilibrium, and the thickness is measured. The density for the pad in the second charge of 0.1 PSI is calculated, i.e., based on the thickness measured after the pad is balanced, after the pressure load is reduced to 0.1 PSI. The empty volume reported in cm / g, is then determined. The empty volume is the reciprocal of the density of the pad in number minus the volume of fiber (0.95 cm3 / g). This empty volume is denoted as compressibility in the present. Very high values indicate a greater response to humidity. The drip capacity test herein provides a combined measure of absorbency and absorbent capacity and is carried out here by the following procedure: a square pad placed in air of four inches by four inches having a mass is prepared of approximately 7.5 gr, from the fibers for which the drip capacity is being determined and is placed on a screen mesh. Synthetic urine is applied to the center of the pad at a rate of 8 ml / sec. The flow of synthetic urine stops when the first drops, of synthetic urine, escape from the bottom or side of the pad. The drip capacity of the difference in mass of the pad before and subsequent to the introduction of synthetic urine divided by the mass of the fibers, on dry granulated base. The majority of the drip capacity is, the best of the absorbing properties. The term "synthetic urine" is used herein to mean a solution prepared from water and 10 g of sodium chloride per liter of water and 0.52 ml of a 1% aqueous solution of Triton X100 per liter of water. taking.

Synthetic urine should be at 25 ± 1% ° C when used. The term "defibration" and "defiberation" are used herein to refer to any process that can be used to mechanically separate fibers into substantially individual forms, although they are already in that form, ie the stage or steps of treating mechanically the fibers, either individually or in a more compacted form, where the treatment or treatments of fiber separation in a substantially individual form if they are not already in that form, and / or impart curling and twisting to the fibers in a dry state . The term "fibers herein" refers to cellulosic fibers with high lignin content, thermally treated in air, which are free of portions of crosslinking agents, and have a water retention value ranging from 90 to 135, and a dry elasticity defined by a density after pressure varying from 0.08 to 0.20 g / cm3. The term "heat treated" is used herein to mean heating in the absence of moisture.

DETAILED DESCRIPTION OF THE INVENTION The modified high-lignin fibers herein can be of various origins. Preferably, the original sources are softwood and hardwood. Other sources include herbs from Esparto, bagasse, hemp, flax and other sources of cellulosic fibers with a high lignin content. The fibers with high lignin content that are esterified to provide the fibers used in the present invention are, for example, chemothermalomechanical pulps from the above sources, and recycled fiber streams from bags and kraft boxes, where the content of Lignin in the fiber is 10% or more, on a dry basis. Unbleached cellulosic chemical pulps can also have a level of lignin content of 10 to 25% and constitute fibers with a high lignin content. Chemothermomechanical pulps can be prepared in a conventional manner, for example, by chemical treatment of the pieces of the source material (eg, wood chips) with, for example, sodium sulfite and / or sodium metabisulfate and an agent of chelating, for example diethylenetriamine pentacetic acid (DTPA) followed by processing through a disk refiner. The thermo-cracked pulps can be prepared, conventionally, for example, by steam treatment (eg, at 34 psi and 265 ° F for 20 minutes) of the pieces of source material (e.g. wood), and then process the steam treated material through a disk refiner. The recycled fiber streams are obtained from bags and boxes of kraft paper, for example, by stirring them in water and then dehydrating them. The quimicotermomecánica pulp of softwood of the north is a preferred starting material, since it is easily available commercially. We now turn to the method of the third embodiment of the present, (ie, the method comprising the steps of (a) providing cellulosic fibers with high lignin content at a consistency of 40 to 100%, which are free of admixture with the crosslinking agent, (b) subjecting the fibers to deflection, and (c) heating the product of step (b) in air to an atmospheric pressure to remove any moisture content and thermally treat the cellulose fibers with high lignin content, moisture-free, for at least 5 seconds We first went to stage (a) of the method of the third embodiment of the present, that is to say the stage of providing cellulosic fibers with a high content of lignin at a consistency of 40 to 100%, They are free from mixing with the crosslinking agent.This step is easily carried out for the fibers in an unlimited condition or for leaf-shaped fibers.For low moisture contents, ie from 0 to at about 10%, step (a) may simply involve the assembly fibers in the sheet form or in the unlimited form obtained with this moisture content. For high moisture contents, for example, consistencies of about 40 to 90%, step (a) involves the formation of a mixture of fibers and water. The pH of the mixture can vary, for example, from 2.5 to 9, and is a parameter that affects the dry elasticity, the wet elasticity and the drip capacity obtained in the fiber modified product (ie the result of stage (c)). The values of the dry elasticity (5K density) and the wet elasticity (wet compressibility) obtained are better when the pH are lower, for example, 2.5 to 4.0 are used. The drip capacities are better when the pH is medium, for example, 6.0 to 7.0 are used. The natural pH of the mixture is typically around 9. The pH adjustment downwards is easily carried out with acid, preferably sulfuric acid. The hydrochloric acid is preferably not used since it is preferred to obtain modified fibers that are free of chlorine. The uniform consistency and uniform distribution of any pH adjusting additive is easily obtained for a fiber sheet, for example, by transporting the fiber sheet (for example, initially at a moisture content of 0% to 10%) to through a body of said aqueous composition comprising water and any pH adjusting agent, contained in a press cylinder holder (for example, cylinders of 30.35 cm in diameter and 182.10 cm in width) and through said fastener for impregnating to said fiber sheet with said aqueous composition and to produce on the outlet side of the fastener a sheet impregnated with fibers containing said aqueous composition in an amount that provides 30% to 80% or more (eg, equal to 85% or more). 90% or equal to 95%), preferably 50% to 70% consistency. In a less preferred alternative, the fiber sheet is impregnated with the aqueous composition to provide the above mentioned consistencies, by spraying. In any case, if the consistency is less than 40% or more, the liquid removal is optionally carried out to achieve consistency, for example, by dehydrating (i.e., mechanically removing the liquid, for example, by centrifuging or pressing) ) and / or drying under conditions such that the use of high temperatures is not required for a prolonged period of time, for example, by a method known in the art as air drying. For example, a fiber sheet of 6% moisture content is transported through a body of aqueous composition and the press rolls to produce a sheet impregnated with fibers of 60% consistency or 80% consistency, which is ready for treatment in step (b) or to produce a sheet impregnated with fibers of 40% consistency, which is optionally subjected to a liquid removal step or steps as described above, for example to provide a consistency of 60%, before the treatment in stage (b). The uniform consistency and uniform distribution of any pH adjusting additive is easily obtained for the fibers in the unlimited form, for example, by soaking the fibers in the unlimited form in a body of said aqueous composition. The soaking is easily carried out, for example, by forming a suspension of the fibers in an unlimited form in water, with the adjustment of the pH, if desired, to provide a consistency varying from 0.1% to 20%, of prefernecia varying from 2 to 15%, and keeping them there for approximately 1 to 240 minutes, preferably for 5 to 60 minutes. By forming a suspension of fibers in an unlimited form in water, it is easily carried out either by mixing the fibers in an unlimited form with water or causing a fiber sheet (eg, dry-spliced) to disintegrate in the water. At consistencies of 0.1 to 20%, one or more liquid removal and / or drying steps are required to provide consistencies of 40 to 100%, described for step (a). Preferably, these comprise dehydrating (i.e., mechanically removing the liquid, for example, by centrifuging or pressing) to provide a consistency between 40 and 80%, for example, 40 to 50%, and optionally after drying, under conditions such that the use of a high temperature for a prolonged period of time is not required, for example, by a method known in the art as air drying, for a consistency of 50 to 80% or up to 100%, preferably to a consistency that varies from 50 to 70%. We now turn to step (b) of the method of the third embodiment of the present, that is, the step of subjecting the fibers of step (a) to defibration, sometimes referred to as fluffing. Preferably, the defibration is done by a method where knot formation and distress are minimized, and fiber damage. Typically, a commercially available disk refiner is used. Another type of device that has been found to be useful for defibrating cellulosic fibers is the three-stage sponge device, described in US Pat. No. 3,987,968, issued to D.R. Moore and 0. A. Shields, October 26, 1976, said patent being incorporated herein by reference in this disclosure. The sponge device described in described in the patent E.U.A. No. 3,987,968 subjected the wet cellulosic pulp fibers to a combination of mechanical impact, mechanical agitation, agitation with air and a limited amount of air drying to create a substantially knot-free fluff. The fibers have imparted to it an improved degree of curling in relation to the amount of crimping normally present in said fibers. It is believed that this additional curl improves the elastic character of the structures made from the modified fibers herein. Other applicable methods of defibration include, but are not limited to, treatment in a Waring softener, which makes tangential contact with the fibers with a rotating wire brush and a hammer mill. Preferably, a stream of air is directed to the fibers during said defibration to assist in the separation of the fibers in substantially individualized form. Despite the particular mechanical device used to form the fluff, the fibers are preferably mechanically treated as initially at a consistency of at least 40%. Preferably, the defibration in the fibers is carried out at a consistency ranging from 50 to 70%. The defibration can be carried out even on fibers of 100% consistency. Although, the defibración to consistencies that exceed 80% can cause damages to the fibers, degrading the development. We now turn to step (c) of the method of the third embodiment of the present, that is, to the step of heating the product of step (b) in air at atmospheric pressure to remove any moisture content and heat-treat the fibers cellulose, resulting, with high content of lignin, free of moisture, for at least 5 seconds. As indicated above, this stage can be carried out in two stages, a first stage of drying (for example, instant drying) to obtain a consistency of at least 60%, if this consistency is not already present or to increase the consistency yes a consistency of at least 60% is already present, for example, at 85-95% or even 100% consistency, and a second stage to remove any remaining moisture content and thermally treat cellulose fibers with high moisture content , free from moisture, for example, heating for 5 seconds to 2 hours, from 120 ° C to 280 ° C (air temperature in the heating apparatus), preferably for 2 to 75 minutes, from 150 ° C to 190 ° C (air temperature in the heating device). The first stage is omitted, yes the fibers introduced in the stage (c) are at 100% consistency. The first step is preferably carried out by a method known in the art as instant drying. This is carried out by transporting the defibrated fibers in a stream of hot air at an air intake temperature ranging from 200 to 750 ° F, preferably at an air intake temperature ranging from 300 to 550 ° F, up to that objective consistency is achieved. This imparts additional curling to the fibers as the water is removed from them. Although the amount of water removed by this drying step can be varied, it is believed that instant drying at higher consistencies on the scale of 60% to 100%, provides a higher level of crimped fibers that instantly dry to a consistency in the goes down from 60% to 100%. In the preferred embodiments, the fibers are dried at about 85% to 95% consistency. Instant drying of the fibers to a consistency, such as 85% to 95%, at the top of the 60% to 100% scale, reduces the amount of drying that must be achieved in the second stage. We now turn to the second stage, where any remaining moisture content is removed and the high-lignin cellulosic fibers, free of moisture, are treated thermally for at least 5 seconds. As indicated above, this step can be carried out by heating for 5 seconds to 2 hours, from 120 ° C to 280 ° C (air temperature in the heating apparatus). If more than a minimum amount of moisture is present, for example, more than about 1% moisture, the heating must be carried out for more than 5 seconds to obtain the thermal treatment of at least 5 seconds required, for example, by at least 1 minute In a preferred process, the second step is carried out on a dry product of step (b) which initially has a consistency ranging from 85% to 95%, and the heating in the second stage is carried out for 2 to 75 minutes, from 150 ° C to 190 ° C (air temperature in the heating appliance) to remove any moisture content and thermally treat the cellulose fibers with high content of lignin, free of moisture, resulting, for at least 1 minute . If the fibers treated in the second stage are not initially present in the second stage at a consistency of at least 60%, the removal of the water can not be obtained to normally provide fibers free of moisture, so the limitation of treating thermally moisture free fibers, which allows obtaining appropriately hardened fibers suitable for producing highly porous, high volume structures at atmospheric pressure and without the use of a non-oxidizing atmosphere. The second stage is easily carried out in a continuous air passage heating apparatus (air heated perpendicularly through the fiber traveling belt is passed), or in a static oven (the fibers are maintained and the air in a static manner in a containment housing, with a static heating means). The second stage can also be carried out by routing the effluent from the instant dryer of the first stage (at 90 to 100% consistency) to a cyclone separator that separates the air from the air / fiber mixture from the instant dryer, discharging the fibers from the cyclone separator to a stream of hot air (eg 400 ° F) in a duct containing at least one U-shaped part, which carries the fibers through the duct, thereby providing a travel path that provides sufficient residence time to give rise to the removal of any moisture content and the required heat treatment, and discharged from the duct to a cyclone separator to separate the heat-treated fibersand, if appropriate, carry out additional heat treatment, for example, in a subsequent air passage oven or in a static oven. The apparatuses for the instant drying of the first stage can also be of the same type of apparatus, that is to say, a cyclone separator of lateral entrance, a duct of treatment of hot air and cyclone separator, in such a way that two or more of these devices, due to the need to flange in fresh air during the course of thermal treatment and drying. We now turn to the method of the fourth modality comprising the stage of heating a dry leaf (from 0 to 40% moisture content) of cellulose fibers with high lignin content in air at atmospheric pressure to remove any moisture content and heat treatment the cellulose fibers with high lignin content, free of moisture, for at least 5 seconds, for example, heating for 5 seconds to 2 hours, from 120 ° C to 280 ° C (air temperature in the heating apparatus). The starting materials for the method of the fourth embodiment can be, for example, a commercially obtained fiber sheet, for example, dry splicing with a high content of lignin, preferably dry chemico-mechanical splicing of softwood from the north, which normally it consists of less than about 10% moisture (eg, from 0 to 10% moisture). If desired, dry splicing or other sheet-like fibers can be adjusted by humidity and / or adjusted by pH, for example, by transporting the fiber sheet through a body of the aqueous composition comprising water, contained in the fastener of the clamping roll (for example rolls of 1 foot in diameter and 6 feet in width) and through said fastener to produce, at the lateral exit of the fastener, a sheet impregnated with fibers containing the aqueous composition in an amount which provides a consistency of 30 to 80%, or more, (for example, still up to 85% or 90% or even 95%), preferably a consistency of 60 to 80%. In a less preferred alternative, the fiber sheet is impregnated with the aqueous composition to adjust the moisture content and / or pH by spraying. In any case, if the consistency is less than 60%, the removal of the liquid is carried out to raise the consistency to this last level, for example, by dehydrating (i.e., removing the liquid mechanically, for example, by centrifuging or pressing) and / or drying under conditions, such that the use of elevated temperatures for a prolonged period of time is not required, for example, by a method known in the art as air drying. For example, a fiber sheet at a moisture content of 60% can be passed through the body of the aqueous composition and the roller fastener to produce a sheet impregnated with fibers of 60% consistency or 80% consistency, which is ready for the treatment in the heating step of the fourth embodiment, or to produce a sheet impregnated with fibers of a consistency of 40% which is subjected to a liquid removal step or steps as described above, for example, for provide a consistency of 60%, before the treatment in the heating stage of the fourth mode. If more than a minimum amount of moisture is present in the sheet of the starting material for the heating step in the fourth embodiment, the heating must be carried out for more than 5 seconds to obtain at least the heat treatment of 5 seconds, required, for example, for at least 1 minute. In a preferred heating step of the fourth embodiment, a sheet of cellulose fibers, with a high content of lignin, is heated to a consistency of 85-100%, for 2 minutes to 75 minutes, from 150 ° C to 190 ° C ( temperature of the air in the heating apparatus) to remove any moisture content and thermally treat the fibers with high lignin content, free of moisture, for at least 1 minute. The heating step of the fourth embodiment is easily carried out in an air passage heating apparatus as described above or a static oven as described above. The resulting heat-treated fiber sheet is preferably subjected to defibration by any of the defibration methods described hereinbefore to produce fibers in the unlimited form. The thermally treated fiber sheet is preferably wetted at 40 to 80% consistency, for example, by spraying or passing through a body of water in the fastener for the fastener rollers, for defibration. The heating steps in the methods of the third and fourth modes should be such that the temperature of the fibers does not exceed about 227 ° C (440 ° F), since the fibers can ignite at this temperature. The dry fibers resulting from the methods of the third and fourth embodiments are optionally moistened, for example, by spraying with water to provide a moisture content of 5 to 15%. This makes the fibers resistant to the damage that is of risk that occurs, due to the subsequent handling or due to the processing to elaborate structures absorbent of the fibers. We now turn to the uses of the high-lignin cellulosic fibers, heat-treated in air, of the present. High-lignin cellulosic fibers, heat-treated in air, find application in the production of a variety of absorbent structures including, but not limited to, paper towels, sheets of toilet paper, disposable diapers, training pants, catamenial pads , sanitary napkins, tampons and bandages, wherein each of said articles has an absorbent structure containing said fibers. For example, a disposable diaper or similar article has a liquid-permeable top sheet, a liquid-impermeable back sheet connected to the topsheet, and an absorbent structure containing the high-lignin cellulosic fibers, heat-treated in air , of the present, is particularly contemplated. Said articles are generally described in the patent E.U.A. No. 3,860,003, issued to B. Buell on January 14, 1975, is hereby incorporated by reference in this disclosure. The fibers of the present can be used directly in the manufacture of absorbent cores placed in air. Additionally, due to its hardness and elasticity characteristics, the fibers of the present may be wet-laid on a low density, non-compacted sheet, which when subsequently dried, is directly useful without additional mechanical processing as an absorbent core. . The fibers of the present may also be wet-laid as compacted pulp sheets for sale or transport to distant locations. In relation to the pulp sheets made from conventional cellulosic fibers, the pulp sheets made from fibers of the present invention are more difficult to compress than the densities of conventional pulp sheets. Therefore, it may be desirable to combine the fibers herein with conventional fibers, such as those conventionally used in the manufacture of absorbent cores. The pulp sheets contain hardened fibers preferably contain from about 5% to about 90% uncured cellulosic fibers, based on total dry weight of the sheet, mixed with the fibers herein. It is especially preferred to include between about 5% and about 30% of the highly refined, non-hardened, cellulosic fibers based on the total dry weight of the sheet. Said highly refined fibers are refined or stirred to freeness levels of less than about 300 ml CSF, and preferably less than 100 ml CSF. The non-hardened fibers are preferably mixed with an aqueous suspension of fibers of the present invention. This mixture can then be formed into a pulp sheet, densified, for subsequent defibration and formation in absorbent pads. The incorporation of the non-hardened fibers facilitates the compression of the pulp sheet in a densified form, while imparting a surprisingly low loss in absorbency to the subsequently formed cushions. The non-hardened fibers additionally increase the tensile strength of the pulp sheet and of the absorbent pads made either of the pulp sheet or directly from the mixture of the fibers herein and the uncured fibers. Although the mixture of the fibers here and the non-hardened fibers is made first in a pulp sheet and then formed in an absorbent pad or formed directly in an absorbent pad, the absorbent pad can be placed wet or placed in air. Sheets or webs made from the fibers herein, or from blends that also contain fibers of the present, will preferably have base weights of less than 800 g / m2 and densities of less than about 0.60 g / cm 3. Although no attempt is made to limit the scope of the invention, wet laid sheets having basis weights of between 300 g / m2 and approximately 600 g / m2, and densities of between 0.07 g / cm3 and approximately 0.30 g / cm3, are especially contemplated for direct applications such as absorbent cores in disposable articles such as diapers, tampons and other catamenial products. Structures that have higher base weights and density than those levels, are believed to be more useful for crushing and placed in air and placed in wet to form a base weight and lower density structure that is more useful for absorbent applications. Said higher density and basis weight structures also exhibit surprisingly superior moisture absorption and correspondence. Other applications of patterned absorbent structures for the fibers of the present invention include sheets of low density toilet paper having densities that may be less than about 0.03 g / cm 3. In an application for absorbent structures, the fibers herein are formed in either an absorbent core placed in air or placed wet (and subsequently dried) which is compressed to the pad form at a lower dry density than the density in Wet balance pad. The equilibrium wet density is the density of the pad, calculated on a dry fiber basis, when the pad is completely saturated with fluid. When the fibers are formed into an absorbent core having a dry density less than the equilibrium wet density, upon wetting to saturation, the core will collapse towards the equilibrium wet density. Alternately, when the fibers are formed into an absorbent core having a dry density greater than the equilibrium wet density, upon wetting to saturation, the core will expand toward the equilibrium wet density. The pads made from fibers of the present invention have equilibrium wet densities that are substantially lower than pads made from conventional sponge fibers. The fibers of the present invention can be compressed to a density greater than the equilibrium wet density, to form a thin alhomadilla which, upon wetting, will expand, thereby increasing the absorbent capacity, to a significantly greater extent than the obtained for unhardened fibers. The absorbent structures can also be made from blends of fibers of the present and unhardened cellulosic fibers with crosslinking agents, such as those known to be the subject of the co-filed patent application Naieni and Herrón, mentioned above. Absorbent structures made from fibers herein may additionally contain discrete particles of hydrogel-forming materials, substantially insoluble in water. Hydrogel forming materials are chemical compounds capable of absorbing fluids and retaining them under moderate pressures. Suitable hydrogel forming materials may be inorganic materials such as silicon gels or organic compounds such as cross-linked polymers. The crosslinked hydrogel-forming polymers can be crosslinked by covalent, ionic, van der Waals, or hydrogen bonds. Examples of hydrogel-forming materials include polyacrylamines, polyvinyl alcohol, maleic anhydride-ethylene copolymers, polyvinyl ethers, hydroxypropyl cellulose, carboxylmethyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine and the like. Other suitable hydrogel-forming materials are those described in Assarsson et al., In the E.U.A. No. 3,901,236, issued August 26, 1975, description of which is incorporated herein by reference. Particularly preferred hydrogel-forming polymers for use in an absorbent core here, are acrylonityl grafted starch, hydrolyzed; grafted starch of acrylic acid, or of acrylates, and maleic anhydride-isobutylene copolymers, or mixtures thereof. Examples of hydrogel-forming materials that can be used are Aqualic L-73, a partially neutralized polyacrylic acid made by Nippon Shokubai CO., Japan, and Sanwet IM 1000, a grafted starch of partially neutralized polyacrylic acid made by Sanyo Co. , Ltd, Japan. The hydrogel-forming materials, which have relatively high gel strengths as described in the U.S.A. No. 4,654,039, issued March 31, 1987, herein incorporated by reference, are preferred for use with the fibers herein. Processes for the preparation of hydrogel-forming materials are described in Masuda et al., In U.S. Pat. No. 4,076,663, issued February 28, 1978, in Tsubakimoto et al., In the US Patent. Do not. 4,286,082, issued August 25, 1981; and also in US Patents. Nos. 3,734,876; 3,661,815; 3,670,731; 3,664,343; 3,783,871 and in Belgian Patent No. 785,850, descriptions of which are hereby incorporated by reference in their entirety. The hydrogel-forming materials may be distributed throughout the length of an absorbent structure containing fibers herein, or is limited to the distribution throughout the entire length of a particular stage or section of the absorbent structure. In another embodiment, the hydrogel-forming material is adhered or laminated to a sheet or film that is juxtaposed against a fibrous, absorbent structure, which may include fibers herein. Said sheet or film can be multi-layered such that the hydrogel-forming material is contained between the layers. In another embodiment, the hydrogel-forming material can be adhered directly onto the surface fibers of the absorbent structure. An important advantage has been observed with respect to absorbent structures made from fibers of the present, which have dry densities that are higher than their corresponding equilibrium wet densities (calculated on a dry fiber basis). Specifically, this type of absorbent structures expand in volume at the time of wetting. As a result of this expansion, the capillary fiber interfiber network also enlarges.

In conventional absorbent structures having hydrogel-forming material mixed therein, the hydrogel-forming material expands in volume due to the absorption of the fluid, and can block or reduce in size the capillary routes for fluid absorption prior to the utilization of the potential Total absorbent of the fluid of the structure. This phenomenon is known as blocking the gel. The capillary enlargement due to the expansion of the fibrous network of the absorbent structures that use the fibers of the present, reduces the occurrence of blocking of the gel. This allows large proportions of the fluid absorbency potential of the structure that is used and allows higher levels of hydrogel-forming material (if desired), to be incorporated in the absorbent structure, without significant levels of gel blocking. The structures containing the fibers herein, and hydrogel-forming materials for diaper core applications, preferably have a dry density of between 0.15 g / cm 3 and about 0.40 g / cm 3, and preferably contain less than about 20 g / cm 3. % of hydrogel-forming material, calculated on a dry fiber weight basis. The hydrogel-forming material may be homogeneously dispersed throughout the length or part of the absorbent structure. For a diaper structure as described in Patent E.U.A. No. 3,860,003, which has an absorbent core containing the fibers of the present, has a dry density of about 0.20 g / cm 3, and also contains hydrogel-forming material dispersed throughout the length of the core, it is presently believed that an optimum balance of diaper runoff, total absorbent capacity, skin moisture, and economic viability is obtained by contents of between about 5% by weight and about 20% by weight, based on the total weight of the dry absorbent core, of a matrix forming material. hydrogel such as Aqualic L-73. Of between about 8% by weight and about 10% by weight of hydrogel-forming material is preferably homogeneously mixed with absorbent cores containing fibers herein, in products as described in US Pat. No. 3,860,003. The absorbent structures described above may also include conventional foamed fibers, or highly refined fibers, wherein the amount of hydrogel-forming material is based on the total weight of the fibers as discussed previously. The embodiments described herein are examples of nature and are not intended to limit the scope of the application of hydrogel-forming materials with fibers herein. The invention herein is illustrated by the following specific examples. In the examples and reference examples hereinafter, the results are evaluated in terms of WRV, 5K density, drip capacity, and wet compressibility.

EXAMPLE I REFERENCE Dry splice sheets of chemothermomechanical pulp fibers (CTMP), from commercial softwood, (Sphinx), which has approximately 20% lignin content, were dispersed, immersed and mixed with a paddle wheel mixer in a citric acid solution and water at a pH of 3.0 to produce a mixture of 10% consistency. The mixture was centrifuged to provide a dewatered cake of approximately 50% consistency. The dewatered cake, which contains 6% by weight of citric acid on a fiber basis, was air dried at approximately 60% consistency, swollen in a laboratory disk refiner and dried instantly at * approximately 90% consistency. The test indicated a WRV of 131, a density of 5K of 0.235 gr / cm3, a drip density of 5.9 gr / gr and a wet compressibility of 7.0 cm3 / gr.

EXAMPLE I Dry splicing sheets of commercially available chemicotemporaneous pulp (SSTX) fibers (CTMP), having approximately 20% lignin content, were dispersed, immersed and mixed with a paddle wheel mixer in water at a pH of 8.9 to produce a mixture of 10% consistency. The mixture was centrifuged to provide a dewatered cake of approximately 50% consistency. The dewatered cake was air dried at approximately 60% consistency, swollen in a laboratory disk refiner, dried instantly to approximately 90% consistency, and heated in a laboratory oven at an air temperature of 165 ° C. for 60 minutes. The test indicated a density of 5K of 0.158 gr / cm3, a drip density of 5.9 gr / gr and a wet compressibility of 7.3 cm3 / gr.

EXAMPLE II Example I was repeated, except that the pH of the water is adjusted to 6.5 using sulfuric acid. The test indicated a WRV of 120, a 5k density of 0.178 gr / cm3, a drip capacity of 7.6 gr / gr, and a wet compressibility of 7.9 cm3 / gr.

EXAMPLE III Example I was repeated, except that the pH of the water is adjusted to 3.0 using sulfuric acid. The test indicated a 5k density of 0.135 gr / cm3, a drip capacity of 6.4 gr / gr, and a wet compressibility of 8.0 cm3 / gr.

EXAMPLE IV Dry, spliced sheets of chemically-thermo-mechanical pulp fibers (Sphinx), having approximately 20% lignin content, and a moisture content of 6%, were heated in an air-pass oven for 6 minutes at a temperature of air of 350 ° F. The resulting sheets are defibrated using a disk refiner. The resulting fibers have the significantly improved 5K density.

EXAMPLE V Dry splicing sheets of commercial chemicotemporaneous pulp fibers (Sphinx) are processed as in Example IV except that the sheet is tranported through the body of the aqueous composition (pH adjusted to 6.5 with sulfuric acid) in the fasteners the clamping rollers (the rollers are one foot in diameter by 6 feet in width) and through the clamping rollers to produce on the exit side of the fastener a sheet impregnated with fibers of 80% consistency (residence time in the composition aqueous 0.1 seconds), and heated in an air-pass oven for 20 minutes at the air temperature of 350 ° F. The resulting sheet is moistened to 20% moisture content by rolling it with water and then defibrated using a disk refiner. The resulting fibers have significantly improved 5K density, wet compressibility and drip capacity.

EXAMPLE VI The heat treated fibers prepared as in any of the examples I to V, are placed in air in absorbent pads, and compressed with a hydraulic press to a density of approximately 0.1 gr / cm3, with a basis weight of approximately 0.13 gr / in2 . The pads are cut 15"by 3" to be used as absorbent pads for sanitary napkins. Variations will be obvious to those skilled in the art. Therefore, the invention is defined by the scope of the claims.

Claims (13)

NOVELTY OF THE INVENTION CLAIMS

1. The high-lignin cellulosic fibers, thermally treated in air, which are free of portions of cross-linking agents and have a water retention value ranging from 90 to 135, and a dry elasticity defined by a 5K density that it varies from 0.08 to 0.20 gr / cm3.

2. The cellulose fibers with high lignin content, thermally treated in air, according to claim 1, further characterized in that said fibers have a water retention value that varies from 110 to 125, a dry elasticity defined by a density 5K which varies from 0.12 to 0.18 gr / cm3, a wet elasticity defined by a wet compressibility ranging from approximately 7.2 to 8.2 cm3 / gr, and a drip capacity ranging from approximately 5.5 to 12.0 gr / gr.

3. The cellulose fibers with a high content of lignin, thermally treated in air, according to claim 2, characterized in that they are chemically thermomechanical pulp fibers, made of softwood from the north.

4. An absorbent structure comprising high-lignin cellulosic fibers, thermally treated in air, according to claim 1.

5. A method for preparing high-lignin cellulosic fibers, heat-treated in air, which are free of portions of crosslinking agents and having a water retention value ranging from 90 to 135, and a dry elasticity defined by a 5K density ranging from 0.08 to 0.20 g / cm 3, said method comprising the steps of: a ) provide cellulosic fibers with a high content of lignin at a consistency of 40 to 80%, which are free from mixing with crosslinking agent; b) subjecting the fibers to defibration; c) heating in air at atmospheric pressure to remove any moisture content and thermally treat the cellulose fibers with high content of lignin, free of moisture, resulting, for at least 5 seconds.

6. The method for preparing high-lignin cellulosic fibers, heat-treated in air, according to claim 5, further characterized in that the heating is carried out for 5 seconds up to 2 hours at an air temperature of 120 ° C at 280 ° C.

7. The method for preparing high-lignin cellulosic fibers, heat-treated in air, according to claim 6, further characterized in that the mixture of stage a) has a consistency ranging from 50 to 70%, and the stage c) comprises an instantaneous drying up to a consistency that varies from 86 to 95%, and then heat for 2 to 75 minutes at an air temperature of 150 to 190 ° C, to remove the remaining moisture content and thermally treat the cellulosic fibers with high content of lignin, free of humidity, resulting, for at least 1 minute.

8. The method for preparing high-lignin cellulosic fibers, heat-treated in air, according to claim 7, further characterized in that the resulting high-lignin fibers are wetted to provide a moisture content of 5 to 15% .

9. A method for preparing high-lignin cellulosic fibers, heat treated in air, which are free of portions of crosslinking agents and having a water retention value ranging from 90 to 135, and a defined dry elasticity by a 5K density that varies from - * 0.08 to 0.20 gr / cm3, said method comprising the step of heating a sheet of cellulosic fibers with high moisture content of moisture content ranging from 0 to 40% in air at atmospheric pressure to remove any moisture content and heat treated cellulose fibers with high content of lignin, moisture-free, resulting, for at least 5 seconds.

10. The method to prepare cellulose fibers with high lignin content, heat treated in air, according to claim 9, further characterized in that the thermally treated is subjected to defibration.

11. The method for preparing high-lignin cellulosic fibers, heat-treated in air, according to claim 10, further characterized in that the heating is carried out for 5 seconds to 2 hours at an air temperature of 120 ° C at 280 ° C.

12. The method for preparing high-lignin cellulosic fibers, heat-treated in air, according to claim 11, further characterized in that the heating is carried out for 2 to 75 minutes at an air temperature of 150 to 190 °. C, to remove the remaining moisture content and thermally treat the cellulose fibers with high content of lignin, free of moisture, resulting, for at least 1 minute.

13. The method for preparing high-lignin cellulosic fibers, heat-treated in air, according to claim 9, further characterized in that the resulting high-lignin fibers are wetted to provide a moisture content of 5 to 15% . EXTRACT OF DISCLOSURE High-lignin cellulosic fibers, heat-treated in air, which are free of portions of crosslinking agents, for use in absorbent structures, are prepared by sponging the fibers with high lignin content at a consistency of when minus 40%, and heating in air at atmospheric pressure at a temperature ranging from 120 ° C to 280 ° C, from the water-fiber mixture having a consistency of at least 60% moisture-free fluffed fibers, to remove any moisture content and thermally treat fibers with high lignin content, moisture-free, resulting for at least 5 seconds, or by heating a sheet of high-lignin, dry fibers (moisture content from 0 to 40%) using these same heating conditions and then sponge.