CA2264180C - Lyocell fibers and process for their preparation - Google Patents

Abstract

The invention is lyocell fiber characterized by a pebbled surface as seen at high magnification and having a variable cross section and diameter along and between fibers. The fiber is produced by centrifugal spinning, melt blowing or its spunbonding variation. The fibers can be made in the microdenier range with average weights as low as one denier or less. The fibers have inherently low gloss and can be formed into tight yarns for making fabrics of very soft hand. Alternatively, the fibers can be formed into selfbonded nonwoven fabrics.

Description

CA 02264180 1999-02-22W0 98l079ll PCT/US97/14762LYOCELL FIBERS AND PROCESSFOR THEIR PREPARATIONThis application claims priority from Provisional Applications Serial Nos.5 60/023,909 and 60/024,462, both filed August 23, 1996.l01520253035The present invention is directed to lyocell fibers having novel characteris-tics and to the method for their preparation. It is also directed to yarns produced fromthe fibers, and to woven and nonwoven fabrics containing the fibers. In particular, themethod involves first dissolving cellulose in an amine oxide to form a dope. Latent fi-bers are then produced either by extrusion of the dope through small apertures into anair stream which draws the latent filaments of cellulose solution or by centrifiigally ex-pelling the dope through small apertures. The fibers are then formed by regenerating thespun latent fibers in a liquid nonsolvent. Either process is amenable to the production ofself bonded nonwoven fabrics.BACKGROUND OF THE INVENTIONFor over a century strong fibers of regenerated cellulose have been pro-duced by the viscose and cuprammonium processes. The latter process was first pat-ented in 1890 and the viscose process two years later. In the viscose process cellulose isfirst steeped in a mercerizing strength caustic soda solution to form an alkali cellulose.This is reacted with carbon disulfide to form cellulose xanthate which is then dissolvedin dilute caustic soda solution. After filtration and deaeration the xanthate solution isextruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodiumsulfate, zinc sulfate, and glucose to form continuous filaments. The resulting so-calledviscose rayon is presently used in textiles and was formerly widely used as reinforcing inrubber articles such as tires and drive belts.Cellulose is also soluble in a solution of ammoniacal copper oxide. Thisproperty formed the basis for production of cuprammonium rayon. The cellulose solu-tion is forced through submerged spinnerets into a solution of 5% caustic soda or dilutesulfuric acid to form the fibers. Afler decoppering and washing the resulting fibers havegreat wet strength. Cuprammonium rayon is available in fibers of very low deniers andis used almost exclusively in textiles.More recently other cellulose solvents have been explored. One such sol-vent is based on a solution of nitrogen tetroxide in dimethyl formamide. While much re-search was done, no commercial process has resulted for forming regenerated cellulosefibers using this solvent.W0 98/0791 1101520253035CA 02264180 1999-02-22PCT/US97/14762-2-The usefiilness of tertiary amine—N oxides as cellulose solvents has beenknown for a considerable time. Graenacher, in U.S. Patent No. 2,179,181, discloses agroup of amine oxide materials suitable‘ as solvents. However, the inventor was onlyable to fonn solutions with low concentrations of cellulose and solvent recovery pre-sented a major problem. Johnson, in U.S. Patent No. 3,447,939, describes the use ofanhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as sol-vents for cellulose and many other natural and synthetic polymers. Again the solutionswere of relatively low solids content. In his later U.S. Patent No. 3,508,941, Johnsonproposed mixing in solution a wide variety of natural and synthetic polymers to form in-timate blends with cellulose. A nonsolvent for cellulose such as dimethylsulfoxide wasadded to reduce dope viscosity. The polymer solution was spun directly into coldmethanol but the resulting filaments were of relatively low strength.However, beginning in 1979 a series of patents were issued to preparationof regenerated cellulose fibers using various amine oxides as solvents. In particular, N-methylmorpholine~N-oxide with about 12% water present proved to be a particularlyusefiil solvent. The cellulose was dissolved in the solvent under heated conditions, usu-ally in the range of 90°C to 130°C, and extruded from a multiplicity of fine aperturedspinnerets into air. The filaments of cellulose dope are continuously mechanically drawnin air by a factor in the range of about three to ten times to cause molecular orientation.They are then led into a nonsolvent, usually water, to regenerate the cellulose. Otherregeneration solvents, such as lower aliphatic alcohols, have also been suggested. Ex-amples of the process are detailed in McCorsley and McCorsley et al. U.S. Patents Nos.4,142,913; 4,144,080; 4,211,574, 4,246,221, and 4,416,698 and others. Jurkovic et al.,in U.S. Patent No 5,252,284 and Michels et al,, in U.S. Patent 5,417,909 deal especiallywith the geometry of extrusion nozzles for spinning cellulose dissolved in NMMO.Brandner et al, in U.S. Patent 4,426,228, is exemplary of a considerable number of pat-ents that disclose the use of various compounds to act as stabilizers in order to preventcellulose and/or solvent degradation in the heated NMMO solution. Franks et al., inU.S. Patent Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellu-lose in amine oxide solvents and of achieving higher concentrations of cellulose.Cellulose textile fibers spun from NMMO solution are referred to aslyocell fibers. Lyocell is an accepted generic term for a fiber composed of cellulose pre-cipitated from an organic solution in which no substitution of hydroxyl groups takesplace and no chemical intermediates are fonned. One lyocell product produced byCourtaulds, Ltd. is presently commercially available as Tencel” fiber. These fibers areavailable in 0.9-2.7 denier weights and heavier. Denier is the weight in grams of 9000meters of a fiber. Because of their fineness, yarns made from them produce fabrics hav-ing extremely pleasing hands.W0 98/07911l0I520253035CA 02264180 1999-02-22PCT/US97/14762-3-One limitation of the lyocell fibers made presently is a function of their ge-ometry. They are continuously formed and typically have quite uniform, generally circu-lar or oval cross sections, lack crimp as spun, and have relatively smooth, glossysurfaces. This makes them less than ideal as staple fibers since it is difficult to achieveuniform separation in the carding process and can result in non-uniform blending and un-even yarn. In part to correct the problem of straight fibers, man made staple fibers arealmost always crimped in a secondary process prior to being chopped to length. Exam-ples of crimping can be seen in U.S. Patent Nos. 5,591,388 or 5,601,765 to Sellars et al.where the fiber tow is compressed in a stuifer box and heated with dry steam. It mightalso be noted that fibers having a continuously uniform cross section and glossy surfaceproduce yarns tending to have a "plastic" appearance. Yarns made from thermoplasticpolymers frequently must have delustering agents, such as titanium dioxide, added priorto spinning. Vlfrlkes et al., in U.S. Patent 5,458,835, teach the manufacture of viscoserayon fibers having cruciform and other cross sections. U.S. Patent No. 5,417,909 toMichels et al. discloses the use of profiled spinnerets to produce lyocell fibers havingnon-circular cross sections but the present inventors are not aware of any commercialuse of this method.Kaneko et al. in U.S. Patent 3,833,438 teach preparation of self bondedcellulose nonwoven materials made by the cuprammonium rayon process. Self bondedlyocell nonwoven webs have not been described to the best of the present inventors’knowledge.Low denier fibers fiom synthetic polymers have been produced by a num-ber of extrusion processes. Three of these are relevant to the present invention. One isgenerally termed "melt blowing". The molten polymers are extruded through a series ofsmall diameter orifices into an air stream flowing generally parallel to the extruded fi-bers. This draws or stretches the fibers as they cool. The stretching serves two pur-poses. It causes some degree of longitudinal molecular orientation and reduces theultimate fiber diameter. A somewhat similar process is called "spunbonding" where thefiber is extruded into a tube and stretched by an air flow through the tube caused by avacuum at the distal end. In general, spunbonded fibers are continuous while meltblown fibers are more usually in discrete shorter lengths. The other process, termed"centrifiigal spinning", differs in that the molten polymer is expelled from apertures inthe sidewalls of a rapidly spinning drum. The fibers are drawn somewhat by air resis-tance as the drum rotates. However, there is not usually a strong air stream present asin meltblowing. All three processes may be used to make nonwoven fabric materials.There is an extensive patent and general technical literature on the processes since theyhave been commercially important for many years. Exemplary patents to meltblowingare Weber et al., U.S. Patent No. 3,959,421, and Milligan et al., U.S. Patent No.W0 98I0791ll01520253035CA 02264180 1999-02-22PCT/US97/14762-4-5,075,068. The Weber et al. patent uses a water spray in the gas stream to rapidly coolthe fibers. A somewhat related process is described in PCT Publication W0 91/ 18682which is directed to a method for coating paper by modified meltblowing. Coating ma-terials suggested are aqueous liquids such as "an aqueous solution of starch, carboxy-methylcellulose, polyvinyl alcohol, latex, a suspension of bacterial cellulose, or anyaqueous material, solution or emulsion". However, this process actually atornizes theextruded material rather than forms it into latent fibers. Zikeli et al., in U.S. PatentNos. 5,589,125 and 5,607,639, direct a stream of air transversely across strands of ex-truded lyocell dope as they leave the spinnerets. This air stream serves only to cool anddoes not act to stretch the filaments.Centrifiigal spinning is exemplified in U.S. Patents Nos. 5,242,633 and5,326,241 to Rook et al. Olcada et al., in U.S. Patent No. 4,440,700 describe a centrifi.r-gal spinning process for thermoplastic materials. As the material is ejected the fibers arecaught on an annular form surrounding the spinning head and moved downward by acurtain of flowing cooling liquid. Included among the list of polymers suited to theprocess are polyvinyl alcohol and polyacrylonitrile. In the case of these two materialsthey are spun "wet"; i.e., in solution, and a "coagulation bath" is substituted for the cur-tain of cooling liquid.Vlfrth the exception of the Kaneko et al. patent noted above, processesanalogous to melt blowing, spunbonding and centrifiigal spinning have never been usedwith cellulosic materials since cellulose itself is basically infusible.Extremely fine fibers, termed "microdenier fibers" generally are regardedas those having a denier of 1.0 or less. Meltblown fibers produced from various syn-thetic polymers, such as polypropylene, nylons, or polyesters are available with diame-ters as low as 0.4 pm (approximately 0.001 denier). However, the strength or"tenacity" of most of these fibers tends to be low and their generally poor water absorb-ency is a negative factor when they are used in fabrics for clothing. Microdenier cellu-lose fibers, as low as 0.5 denier, have been produced before the present only by theviscose process.The present process produces a new lyocell fiber that overcomes many ofthe limitations of the fibers produced from synthetic polymers, rayons, and the presentlyavailable lyocell fibers. It allows formation of fibers of low denier and with a distribu-tion of deniers. At the same time each fiber has a pebbled surface, a cross section ofvarying shape and diameter along its length, and significant natural crimp. All of theseare desirable characteristics that are found in most natural fibers but are missing inlyocell fibers produced commercially to the present.W0 98/0791 1l015202535CA 02264180 1999-02-22PCT/US97/14762-5-SUMMARY OF THE INVENTIONThe present invention is directed to a process for production of regener-ated cellulose fibers and webs and to the fibers and webs so produced. The terms "cel-lulose" and "regenerated cellulose" as used here should be construed sufliciently broadlyto encompass blends of cellulose with other natural and synthetic polymers, mutuallysoluble in a spinning solvent, in which cellulose is the principal component by weight. Inparticular it is directed to low denier fibers produced fi'om cellulose solutions in amineN-oxides by processes analogous to melt blowing or centrifiigal spinning. Where theterms "melt blowing", "Spunbonding", and "centrifugal spinning" are used it will be un-derstood that these refer to processes that are similar or analogous to the processes usedfor production of thermoplastic fibers, even though the cellulose is in solution and thespinning temperature is only moderately elevated. The term "continuously drawn" refersto the present commercial process for manufacture of lyocell fibers where they are me-chanically pulled, first through an air gap to cause elongation and molecular orientationthen through the regeneration bath.The processes involve dissolving a cellulosic raw material in an amine ox-ide, preferably N-methylmorpholine-N-oxide (NMMO) with some water present. Thisdope, or cellulose solution in NMMO, can be made by known technology; e.g., as is dis-cussed in any of the McCorsley or Franks et al. patents aforenoted. In the present proc-ess, the dope is then transferred at somewhat elevated temperature to the spinningapparatus by a pump or extruder at about 90°C to 130°C. Ultimately the dope is di-rected through a multiplicity of small orifices into air. In the case of melt blowing, the -extruded threads of cellulose dope are picked up by a turbulent gas stream flowing in agenerally parallel direction to the path of the filaments. As the cellulose solution isejected through the orifices the liquid strands or latent filaments are drawn (or signifi-cantly decreased in diameter and increased in length) during their continued trajectoryafter leaving the orifices. The turbulence induces a natural crimp and some variability inultimate fiber diameter both between fibers and along the length of individual fibers.This is in marked contrast to continuously drawn fibers where diameters are uniform andcrimp is lacking or must be introduced as a post spinning process. The crimp is irregularand will have a peak to peak amplitude greater than about one fiber diameter and a pe-riod greater than about five fiber diameters.Spunbonding can be regarded as a species of meltblowing in that the fibersare picked up and drawn in an airstream without being mechanically pulled. In the con-text of the present invention meltblowing and spunbonding should be regarded as fi]I‘lC-tional equivalents.Where the fibers are produced by centrifiigal spinning, the dope strandsare expelled through small orifices into air and are drawn by the inertia imparted by theW0 98l07911101520253035CA 02264180 1999-02-22PCT/US97/14762-6-spinning head. The filaments are then directed into a regenerating solution or a regener-ating solution is sprayed onto the filaments. Regenerating solutions are nonsolventssuch as water, lower aliphatic alcohols, or mixtures of these. The NMMO used as thesolvent can then be recovered from the regenerating bath for reuse.Turbulence and oscillation in the air around the latent fiber strands is be-lieved to be responsible for their unique geometry when made either by the melt blowingor centrifugal spinning process.Filaments having an average size as low as 0.1 denier or even less can bereadily formed. Denier can be controlled by a number of factors including but not lim-ited to orifice diameter, gas stream speed, spinning head speed, and dope viscosity.Dope viscosity is, in turn, largely a factor of cellulose D.P. and concentration. Fiberlength can be similarly controlled by design and velocity of the air stream surroundingthe extrusion orifices. Continuous fibers or relatively short staple fibers can be produceddepending on spinning conditions. Equipment can be readily modified to form individualfibers or to lay them into a mat of nonwoven cellulosic fabric. In the latter case the matmay be formed and become self bonded prior to regeneration of the cellulose. The fi-bers are then recovered from the regenerating medium, further washed, bleached if nec-essary, dried, and handled conventionally from that point in the process.Gloss or luster of the fibers is considerably lower than continuously drawnlyocell fiber lacking a delusterant so they do not have a "plastic" appearance. This is be-lieved to be due to their unique "pebbled" surface apparent in high magnificationmicrographs.By properly controlling spinning conditions the fibers can be formed withvariable cross sectional shape and a relatively narrow distribution of fiber diameters.Some variation in diameter and cross sectional configuration will typically occur alongthe length of individual fibers and between fibers. The fibers are unique for regeneratedcellulose and similar in morphology to many natural fibers.Fibers produced by either the melt blowing or centrifugal spinning proc-esses possess a natural crimp quite unlike that imparted by a stuffer box. Crimp im-parted by a stuffer box is relatively regular, has a relatively low amplitude usually lessthan one fiber diameter, and short peak-to-peak period normally not more than two orthree fiber diameters. That of the present fibers has an irregular amplitude greater thanone fiber diameter, usually much greater, and an irregular period exceeding about fivefiber diameters, a characteristic of fibers having a curly or wavy appearance.Properties of the fibers of the present invention are well matched for card-ing and spinning in conventional textile manufacturing processes. The fibers, while hav-ing many of the attributes of natural fibers, can be produced in microdenier diametersW0 98/0791!l0l520253035CA 02264180 1999-02-22PCT/US97/14762-7-unavailable in nature. It is possible to directly produce self bonded webs or tightlywound multi-ply yams.A particular advantage of the present invention is the ability to form blendsof cellulose with what might otherwise be considered as incompatible polymeric materi-als. The amine oxides are extremely powerfiil solvents and can dissolve many otherpolymers beside cellulose. It is thus possible to form blends of cellulose with materialssuch as lignin, nylons, polyethylene oxides, polypropylene oxides, poly(acrylonitrile),poly(vinylpyrrolidone), poly(acrylic acid), starches, poly(vinyl alcohol), polyesters,polyketones, casein, cellulose acetate, amylose, amylopectins, cationic starches, andmany others. Each of these materials in homogeneous blends with cellulose can producefibers having new and unique properties.It is an object of the present invention to provide a method of forming lowdenier regenerated cellulose fibers or cellulose blend fibers from solution in an amineoxide—water medium by processes analogous to melt blowing, spunbonding, or centrifi1-gal spinning. _It is a fiirther object to provide low denier cellulose fibers having advanta-geous geometry and surface characteristics for forming into yarns.It is still an object to provide fibers having natural crimp and low luster.It is also an object to provide regenerated cellulose fibers having manyproperties similar or superior to natural fibers.It is yet an object to provide a method of forming fibers of the above typesby a process in which all production chemicals can be readily recovered and reused.It is another object to provide self bonded nonwoven lyocell fabrics.These and many other objects will become readily apparent to those skilledin the art upon reading the following detailed description in conjunction with referral tothe drawings.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a block diagram of the steps used in practice of the presentprocess.FIG. 2 is a partially cut away perspective representation of typical centrifi1-gal spinning equipment used with the invention.FIGS. 3 is a partially cut away perspective representation of melt blowingequipment adapted for use with the present invention.FIG. 4 is a cross sectional view of a typical extrusion head that might beused with the above melt blowing apparatus.FIGS. 5 and 6 are scanning electron micrographs of a commercially avail-able lyocell fiber at IOOX and l0,000x magnification respectively.WO 98/0791 1101520253035CA 02264180 1999-02-22PCT/US97/14762-3-FIGS. 7 and 8 are scanning electron micrographs of a lyocell fiber pro-duced by centrifiigal spinning at ZOOX and l0,000x magnification respectively.FIGS. 9 and 10 are scanning electron micrographs at 2,000X showingcross sections along a single centrifiigally spun fiber .FIGS. 11 and l2 are scanning electron micrographs of a melt blown lyocellfiber at IOOX and l0,000x magnification respectively.FIG. 13 is a drawing illustrating production of a self bonded nonwovenlyocell fabric using a melt blowing process.FIG. 14 is a similar drawing illustrating production of a self bonded non-woven lyocell fabric using a centrifiigal spinning process.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSThe type of cellulosic raw material used with the present invention is notcritical. It may be bleached or unbleached wood pulp which can be made by variousprocesses of which kraft, prehydrolyzed krafi, or sulfite would be exemplary. Manyother cellulosic raw materials, such as purified cotton linters, are equally suitable. Priorto dissolving in the amine oxide solvent the cellulose, if sheeted, is normally shreddedinto a fine fluif to promote ready solution.The solution of the cellulose can be made in a known manner; e.g., astaught in McCorsley U.S. Patent No. 4,246,221. Here the cellulose is wet in a non-solvent mixture of about 40% NMMO and 60% water.NMMO is about 125.1 by weight. The mixture is mixed in a double arm sigma blademixer for about 1.3 hours under vacuum at about 120°C until sufiicient water has beendistilled off to leave about 12-14% based on NMMO so that a cellulose solution isAlternatively,The ratio of cellulose to wetformed. The resulting dope contains approximately 30% cellulose.NMMO of appropriate water content may be used initially to obviate the need for thevacuum distillation. This is a convenient way to prepare spinning dopes in the labora-tory where commercially available NMMO of about 40-60% concentration can be mixedwith laboratory reagent NMMO having only about 3% water to produce a cellulose sol-vent having 7-15% water. Moisture normally present in the cellulose should be ac-counted for in adjusting necessary water present in the solvent. Reference might bemade to articles by Chanzy, H. and A. Peguy, Journal of Polymer Science, PolymerPhysics Ed. 18: l137-1144 (1980) and Navard, P. and J. M. Haudin British PolymerJournal, p 174, Dec. 1980 for laboratory preparation of cellulose dopes in NMMO-water solvents.Reference to FIG. 1 will show a block diagram of the present process. Aswas noted, preparation of the cellulose dopes in aqueous NMMO is conventional. Whatis not conventional is the way these dopes are spun. The cellulose solution is forcedW0 98/0791 1101520253035CA 02264180 1999-02-22PCT/US97/14762-9-from extrusion orifices into a turbulent air stream rather than directly into a regenerationbath as is the case with viscose or cuprammonium rayon. Only later are the latent fila-ments regenerated. However, the present process also differs from the conventionalprocesses for forming lyocell fibers since the dope is not continuously drawn linearlydownward as unbroken threads through an air gap and into the regenerating bath.FIG. 2 is illustrative of a centtifiigal spinning process. The heated cellu-lose dope 1 is directed into a heated generally hollow cylinder or drum 2 with a closedbase and a multiplicity of small apertures 4 in the sidewalls 6. As the cylinder rotates,dope is forced out horizontally through the apertures as thin strands 8. As these strandsmeet resistance from the surrounding air they are drawn or stretched by a large factor.The amount of stretch will depend on readily controllable factors such as cylinder rota-tional speed, orifice size, and dope viscosity. The dope strands either fall by gravity orare gently forced downward by an air flow into a non—solvent 10 held in a basin 12where they are coagulated into individual oriented fibers having lengths from about 1 to25 cm. Alternatively, the dope strands 8 can be either partially or completely regener-ated by a water spray from a ringof spray nozzles 16 fed by a source of regenerating so-lution 18. Also, as will be described later, they can be formed into a nonwoven fabricprior to or during regeneration. Water is the preferred coagulating non—solvent althoughethanol or water-ethanol mixtures are also useful. From this point the fibers are col-lected and may be washed to remove any residual NMMO, bleached as might be neces-sary, and dried. Example 2 that will follow gives specific details of laboratorycentrifiigally spun fiber preparation. 0FIGS. 3 and 4 show details of a typical melt blowing process. As seen inFIG. 3, a supply of dope, not shown, is directed to an extruder 32 which forces the cel-lulose solution to an orifice head 34 having a multiplicity of orifices 36. Air or anothergas is supplied through lines 38 and surrounds and transports extruded solution strands40. A bath or tank 42 contains a regenerating solution 44 in which the strands are re-generated from solution in the solvent to cellulose fibers. Alternatively, the latent fiberscan be showered with a water spray to regenerate or partially regenerate them. Theamount of draw or stretch will depend on readily controllable factors such as orificesize, dope viscosity, cellulose concentration in the dope, and air speed and nozzleconfiguration.FIG. 4 shows a typical extrusion orifice. The orifice plate 20 is bored witha multiplicity of orifices 36. It is held to the body of the extrusion head 22 by a series ofcap screws 18. An internal member 24 forms the extrusion ports 26 for the cellulose so-lution. It is embraced by air passages 28 that surround the extruded solution filaments40 causing them to be drawn and to assist in their transport to the regenerating medium.WO 98107911101520253035CA 02264180 1999-02-22PCT/US97/14762-10-Example 3 that follows will give specific details of laboratory scale fiber preparation bymelt blowing.The scanning electron micrographs shown in FIGS. 5-6 are of lyocell fi-bers made by the conventional continuously drawn process. It is noteworthy that theseare of quite uniform diameter and are essentially straight. The surface seen at l0,000Xmagnification in FIG. 6 is remarkably smooth.FIGS. 7-10 are of fibers made by a centrifugal spinning process of the pre-sent invention. The fibers seen in FIG. 7 have a range of diameters and tend to be some-what curly giving them a natural crimp. This natural crimp is quite unlike the regularsinuous configuration obtained in a stuffer box. Both amplitude and period are irregularand are at least several fiber diameters in height and length. Most of the fibers are some-what flattened and some show a significant amount of twist. Fiber diameter varies be-tween extremes of about 1.5 pm and 20 pm (<0.l - 3.1 denier), with most of the fibersclosely grouped around a 12 um diameter average (Q. l denier).FIG. 8 shows the fibers of FIG. 7 at l0,000x magnification. The surface isuniformly pebbly in appearance, quite unlike the commercially available fibers. This re-sults in lower gloss and improved spinning characteristics.FIGS. 9 and 10 are scanning micrographs of fiber cross sections takenabout 5 mm apart on a single centrifugally spun fiber. The variation in cross section anddiameter along the fiber is dramatically shown. This variation is characteristic of boththe centrifiigally spun and melt blown fiber. _FIGS. 1] and 12 are low and high magnification scanning micrographs ofmelt blown fiber. Fiber diameter, while still variable, is less so than the centrifiigallyspun fiber. However, crimp of these samples is significantly greater. The micrograph atl0,000X of FIG. 12 shows a pebbly surface remarkably like that of the centrifugallyspun fiber.The overall morphology of fibers from both processes is highly advanta-geous for forming fine tight yarns since many of the features resemble those of naturalfibers. This is believed to be unique for the lyocell fibers of the present invention.FIG. 13 shows one method for making a self bonded lyocell nonwovenmaterial using a modified melt blowing process. A cellulose dope 50 is fed to extruder52 and fi'om there to the extrusion head 54. An air supply 56 acts at the extrusion ori-fices to draw the dope strands 58 as they descend from the extrusion head. Process pa-rameters are preferably chosen so that the resulting fibers will be continuous rather thanrandom shorter lengths. The fibers fall onto an endless moving forarninous belt 60 sup-ported and driven by rollers 62, 64. Here they form a latent nonwoven fabric mat 66. Atop roller, not shown, may be used to press the fibers into tight contact and ensurebonding at the crossover points. As mat 66 proceeds along its path while still supportedW0 98/0791 11015202530CA 02264180 1999-02-22I 1 PCT/US97/14762on belt 60, a spray of regenerating solution 68 is directed downward by sprayers 70.The regenerated product 72 is then removed from the end of the belt where it may befiirther processed; e. g., by further washing, bleaching, and drying.FIG 14 is an alternative process for forming a self bonded nonwoven webusing centrifugal spinning. A cellulose dope 80 is fed into a rapidly rotating drum 82having a multiplicity of orifices 84 in the sidewalls. Latent fibers 86 are expelledthrough orifices 84 and drawn, or lenghtened, by air resistance and the inertia impartedby the rotating drum. They impinge on the inner sidewalls of a receiver surface 88 con-centtically located around the drum. The receiver may optionally have a frustroconicallower portion 90. A curtain or spray of regenerating solution 92 flows downward fromring 94 around the walls of receiver 88 to partially coagulate the cellulose mat impingedon the sidewalls of the receiver. Ring 94 may be located as shown or moved to a lowerposition if more time is needed for the latent fibers to self bond into a nonwoven web.The partially coagulated nonwoven web 96 is continuously mechanically pulled from thelower part 90 of the receiver into a coagulating bath 98 in container 100. As the webmoves along its path it is collapsed from a cylindrical configuration into a planar two plynonwoven structure. The web is held within the bath as it moves under rollers 102, 104.A takeout roller 106 removes the now fully coagulated two ply web 108 from the bath.Any or all of rollers 100, 102, or 104 may be driven. The web 108 is then continuouslydirected into a wash and/or bleaching operation, not shown, following which it is dried.for storage. It may be split and opened into a single ply nonwoven or maintained as atwo ply material as desired.Example 1Cellulose Dope PreparationThe cellulose pulp used in this and the following examples was a standardbleached kraft southern sofiwood market pulp, Grade NB 416, available from Weyer-haeuser Company, New Bern, North Carolina. It has an alpha cellulose content of about88-89% and a D.P. of about 1200. Prior to use, the sheeted wood pulp was run througha flufier to break it down into essentially individual fibers and small fiber clumps. Into a250 mL three necked glass flask was charged 5.3 g of fluffed cellulose, 66.2 g of 97%NMMO, 24.5 g of 50% NMMO, and 0.05 g propyl gallate. The flask was immersed inan oil bath at 120°C, a stirrer inserted, and stirring continued for about 0.5 hr. A readilyflowable dope resulted that was directly suitable for spinning.W0 98/0791 1l01520253035CA 02264180 1999-02-22PCT/US97/ 14762-12-Example 2Fiber Preparation by Centrifiigal SpinningThe spinning device used was a modified "cotton candy" type, similar tothat shown in U.S. Patent No. 5,447,423 to Fuisz et al. The rotor, preheated to 120°Cwas 89 mm in diameter and revolved at 2800 rpm. The number of orifices could bevaried between 1 and 84 by blocking olf orifices. Eight orifices 700 pm in diameterwere used for the following trial. Cellulose dope, also at 120°C, was poured onto thecenter of the spinning rotor. The thin strands of dope that emerged were allowed to fallby gravity into room temperature water contained in the basin surrounding the rotor.Here they were regenerated. While occasional fibers would bond to each other most re-mained individualized and were several centimeters in length.In addition to the process just described, very similar rnicrodenier fiberswere also successfully made from bleached and unbleached krafi pulps, sulfite pulp, mi-crocrystalline cellulose, and blends of cellulose with up to 30% corn starch orpoly(acrylic acid).Diameter (or denier) of the fibers could be reliably controlled by severalmeans. Higher dope viscosities tended to form heavier fibers. Dope viscosity could, inturn, be controlled by means including cellulose solids content or degree of polymeriza-tion of the cellulose. Smaller spinning orifice size or higher drum rotational speed pro-duces smaller diameter fibers. Fibers having diameters from about 5-20 pm (0.2-3.ldenier) were reproducibly made. Heavier fibers in the 20-50 um diameter range(3.1-19.5 denier) could also be easily formed. Fiber length varies between about 0.5-25cm and depended considerably on the geometry and operational parameters of thesystem.Example 3Fiber Preparation by Melt BlowingThe dope as prepared in Example 1 was maintained at 120°C and fed to anapparatus originally developed for forming melt blown synthetic polymers. Overall ori-fice length was about 50 mm with a diameter of 635 pm which tapered to 400 pm at thedischarge end. Afier a transit distance in air of about 20 cm in the turbulent air blast thefibers dropped into a water bath where they were regenerated. Regenerated fiber lengthvaried. Some short fibers were formed but most were several centimeters to tens ofcentimeters in length. Variation of extrusion parameters enabled continuous fibers to beformed. Quite surprisingly, the cross section of many of the fibers was not uniformalong the fiber length. This feature is expected to be especially advantageous in spinningtight yams using the microdenier material of the invention since the fibers more closelyresemble natural fibers in overall morphology.W0 98/0791 11015202530CA 02264180 1999-02-22PCT/US97/14762-13-In a variation of the above process, the fibers were allowed to impinge ona traveling stainless steel mesh belt before they were directed into the regeneration bath.A well bonded nonwoven mat was formed. _It will be understood that the lyocell nonwoven fabrics need not be selfbonded. They may be only partially self bonded or not self bonded at all. In these casesthey may be bonded by any of the well known methods including but not limited to by-droentangling, the use of adhesive binders such as starch or various polymer emulsionsor some combination of these methods.Example 4Use of Microcrystalline Cellulose Furnish to Prepare Melt Blown LyocellThe process of Example 1 was repeated using a microcrystalline filI'I'llSl’lrather than wood pulp in order to increase solids content of the dope. The product usedwas Avicel® Type PH-101 microcrystalline cellulose available from FMC Corp, New-ark, Delaware. Dopes were made using 15 g and 28.5 g of the microcrystalline cellulose(dry weight) with 66.2 g of 97% NMMO, 24.5 g of 50% NMMO and 0.05g propyl gal-late. The procedure was otherwise as described in Example 1. The resulting dopes con-tained respectively about 14% and 24% cellulose. These were meltblown as describedin Example 3. The resulting fiber was morphologically essentially identical to that ofExamples 2 and 3.It will be understood that fiber denier is dependent on many controllablefactors. Among these are solution solids content, solution pressure and temperature atthe extruder head, orifice diameter, air pressure, and other variables well known to thoseskilled in meltblowing and centrifugal spinning technology. Lyocell fibers having an av-erage 0.5 denier or even lower may be consistently produced by either the melt blowingor centrifiigal spinning processes. A 0.5 denier fiber corresponds to an average diameter(estimated on the basis of equivalent circular cross sectional area) of about 7-8 pm.The fibers of the present invention were studied by x-ray analysis to deter-mine degree of crystallinity and crystallite type. Comparisons were also made with someother cellulosic fibers as shown in the following table. Data for the microdenier fibersare taken from the centrifiigally spun material of Example 2.CA 02264180 1999-02-22W0 93/07911 PCT/US97I 14762-14-Table ICrystalline Properties of Different Cellulose FibersMicrodenier Cellulose GenericFibers of Present Invention Lvocell Tencel“ CottonCrystallinity Index 67% 65% 70% 85%Crystallite Cellulose II Cellulose ll Cellulose II Cellulose ISome difiiculty was encountered in measuring tensile strength of the individual fi-bers so the numbers given in the following table for tenacity are estimates. Again, the5 microdenier fibers of the present invention are compared with a number of other fibers.Table 2Fiber Physical Property MeasurementsCentrifiigallyEiber_s Cotton So. Pine Ravonl” S_ill<_ Spun Lvocell TencelTypical Length, cm 4 0.5 40 >10‘ 5-25 VariableTypical Diam., um 20 40 l6 l0 5 I2Tenacity, g/d 2.5-3.0 —-- 0.7-3.2 2.8-5.2 2.1 4.5-5.0“’ Viscose processThe centrufiigally spun lyocell with an average diameter of about 5 pm corre-10 sponds to fibers of about 025 denier.The pebbled surface of the fibers of the present invention result in a desir-able lower gloss without the need for any internal delustering agents. While gloss orluster is a difiicult property to measure the following test will be exemplary of the differ-ences between a fiber sample made by the method of Example 2 and a commerciall5 lyocell fiber. Small wet formed handsheets were made from the respective fibers andlight reflectance was determined. Reflectance of the Example 2 material was 5.4%while that of the commercial fiber was 16.9%.The inventors have herein described the best present mode of practicingtheir invention. It will be evident to others skilled in the art that many variations that20 have not been exemplified should be included within the broad scope of the invention.

Claims (14)

Claims:

1. Lyocell fibers characterized by variability in cross sectional diameter and cross sectional configuration along the fiber length and from fiber to fiber, and further characterized by a pebbled surface and a natural crimp

2. The lyocell fibers of claim 1 having a uniformly pebbled surface.

3. The lyocell fibers of claim 1 having an irregular crimp with an amplitude greater than about one fiber diameter and a period greater than about five fiber diameters.

4. Lyocell fibers produced by a centrifugal spinning process, the fibers being characterized by variability in cross sectional diameter and cross sectional configuration along the fiber length and from fiber to fiber, and further characterized by a pebbled surface and a natural crimp.

5. The lyocell fibers of claim 4 having a uniformly pebbled surface.

6. The lyocell fibers of claim 4 having an irregular crimp with an amplitude greater than about one fiber diameters and a period greater than about five fiber diameters.

7. Lyocell fibers produced by a melt blowing process, the fibers being characterized by variability in cross sectional diameter and cross sectional configuration along the fiber length and from fiber to fiber, and further characterized by a pebbled surface and a natural crimp.

8. The lyocell fibers of claim 7 having a uniformly pebbled surface.

9. The lyocell fibers of claim 7 having an irregular crimp with an amplitude greater than about one fiber diameters and a period greater than about five fiber diameters.

10. The lyocell fibers of claims 1, 4, or 7 in which the fiber comprises a mixture of diameters with at least a portion of said fibers being less than about 1 denier.

11. The lyocell fibers of claims 1, 4, or 7 comprising a mixture of cellulose with noncellulosic polymers.

12. A spun yarn comprising a multiplicity of the fibers of claims 1, 4, or 7.

13. The lyocell fibers of claims 1, 4, or 7 wherein the fibers are further characterized by reduced gloss compared with continuously drawn lyocell fibers lacking delusterants.

14. The lyocell fibers of claims 1, 4, or 7 wherein the fibers are further characterized as individualized and continuous.