DE69716092T2 - LYOCELL FIBERS AND METHOD FOR THEIR PRODUCTION - 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

This application claims priority over provisional applications with serial numbers 60/023,909 and 60/024,462, both filed on August 23, 1996.

The present invention relates to lyocell fibers having novel properties, as well as the process for their manufacture. It also relates to yarns made from the fibers, and to woven and nonwoven fabrics containing the fibers. In particular, the process comprises first dissolving cellulose in amine oxide to produce a viscous liquid. Latent fibers are then produced either by extruding the viscous liquid through small orifices into an air stream which draws the latent filaments from cellulose solution, or by expelling the viscous liquid through small orifices upon centrifugation. The fibers are then formed by regenerating the spun latent fibers in a liquid non-solvent.

Each of the processes is possible for the production of self-binding non-woven fabrics.

Background of the invention

For over a century, strong fibers have been made from regenerated cellulose by the viscose and cuprammonium processes. The latter process was first patented in 1890, and the viscose process two years later. In the viscose process, cellulose is first soaked in a corrosive, strong caustic soda solution to produce an alkaline cellulose. This is then treated with carbon disulfide to form cellulose xanthate, which is then dissolved in dilute caustic soda solution. After filtration and deaeration, the xanthate solution is extracted from submerged Spindles are forced into a regenerating bath of sulphuric acid, sodium sulphate, zinc sulphate and glucose to produce continuous filaments. The resulting rayon, called viscose rayon, is used in textiles today, and was once widely used as reinforcement in rubber articles such as tyres and transmission belts. Cellulose also dissolves in a solution of ammonic copper oxide. This property formed the basis for the production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulphuric acid to produce the fibres. After the copper is removed and washed, the fibres have great wet strength. Cuprammonium rayon is found in low denier fibres and is used almost exclusively in textiles.

Recently, other solvents for cellulose have been discovered. One of these solvents is based on a solution of nitrogen tetroxide in dimethyl formamide. Although much research has been carried out in this area, no commercial process for producing regenerated cellulose fibers using this solvent has been developed.

The usefulness of tertiary amine N-oxides as solvents for cellulose has been known for some time. Graenecher in U.S. Patent No. 2,179,181 discloses a group of amine oxide materials that are suitable solvents. However, the inventor was only able to produce solutions with low concentrations of cellulose, and solvent recovery was a major problem. Johnson in U.S. Patent No. 3,447,939 describes the use of anhydrous N-methylmorpholine N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. However, the solutions again contained only a small proportion of solid components. In his later U.S. Patent No. 3,508,941, Johnson proposed to mix a wide range of natural and synthetic polymers in solution to produce a uniform mixture with cellulose. To reduce the viscosity of the viscous liquid, a non-solvent for cellulose, such as dimethyl sulfoxide, was added. The polymer solution was spun directly in cold methanol, but the resulting filaments were of relatively low strength.

Since 1979, however, a number of patents have been published for the production of regenerated cellulose fibers, in which various amine oxides were used as solvents. In particular, N-methylmorpholine-N-oxide with a water content of about 12% proved to be a particularly favorable solvent. The cellulose was dissolved in the solvent under heat conditions, which were generally between 90ºC and 130ºC, and pressed out into the air through a large number of spinnerets with tiny openings. Filaments of the viscous cellulose liquid are mechanically drawn continuously in air by a factor of about three to ten times to achieve molecular orientation. They are then introduced into a nonsolvent, usually water, to regenerate the cellulose. Other regeneration solutions, such as lower aliphatic alcohols, have also been proposed. Examples of the process are described in detail in McCorsley and McCorsley et al. U.S. Patent 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 US Patent 5,417,909 deal specifically with the geometry of extrusion dies for spinning cellulose dissolved in NMMO. US Patent 4,426,228 to Brandner et al. is an example of a number of patents which disclose the use of various components intended to act as stabilizers to prevent degradation of the cellulose and/or solvent in the heated NMMO solution. In US Patent Nos. 4,145,532 and 4,196,282 Franks et al. deal with the difficulties encountered in both dissolving cellulose in amine oxide solvents and in achieving higher cellulose concentrations.

Cellulosic textile fibers spun from NMMO solution are called lyocell fibers. Lyocell is a common generic term for a fiber formed from cellulose precipitated from an organic solution in which no substitution of hydroxyl groups occurs and no chemical intermediates are formed. A lyocell product manufactured by Courtaulds, Ltd is currently commercially available as Tencel® fiber. These fibers are available in 0.9-2.7 denier weights and even heavier. Denier is the weight of 900 meters of the fiber in grams. Because of their fineness, yarns made from them produce fabrics that are particularly pleasant to the touch.

A limitation of currently manufactured lyocell fibers is a function of their geometry. They are continuously formed and typically have fairly uniform, generally round or oval cross-sections; when spun, they lack crimp and have relatively smooth, shiny surfaces. Therefore, they are not very suitable as chopped fibers because it is difficult to achieve uniform separation in the carding process, which can result in inconsistent blending and irregular yarn. To eliminate this problem with the straight fibers, man-made chopped fibers are almost always crimped in a second process before they are cut to length. Examples 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 stuffing box and heated with dry steam. It must It should also be noted that fibers with a continuous, uniform cross-section and a shiny surface produce yarns that tend to look "plastic." Yarns made from thermoplastic polymers often must contain matting additives, such as titanium dioxide, added prior to spinning. Wilkes et al., U.S. Patent 5,458,835, teach the production of viscose rayon fibers having cruciform or other cross-sections. U.S. Patent No. 5,417,909 to Michels et al. discloses the use of profiled spinnerets to produce lyocell fibers with cross-sections that are not round, but these inventors are not aware of any commercial use of this method.

Kaneko et al., in U.S. Patent 3,833,438, teach the preparation of nonwoven materials from self-binding cellulose produced by the cuprammonium rayon process. Self-binding nonwoven lyocell fabrics have not been described to the best of the inventors' knowledge and belief.

Fibers of low denier synthetic polymers are produced by several extrusion processes. Three of these processes are relevant to the present invention. One is generally referred to as "melt blowing." The molten polymers are forced through a series of small diameter holes into a stream of air flowing generally parallel to the extruded fibers. This draws or stretches the fibers as they cool. Stretching serves two purposes. It induces some degree of longitudinal molecular orientation and reduces the final fiber diameter. A relatively similar process is referred to as "spunbond bonding," in which the fiber is forced into a tube and stretched by a stream of air through the tube created by a vacuum at the opposite end. Generally, spunbond fibers are continuous, whereas meltblown fibers are generally often of shorter length. The other process, called "centrifugal spinning," differs from the others in that the molten polymer is forced out of openings in the side walls of a high-speed spinning drum. The fibers are somewhat drawn by the air resistance created during the rotation of the drum. However, there is usually no strong air flow, as is the case with meltblowing. All three processes can be used in the manufacture of nonwoven fabrics. There are many patents and general technical literature on these processes, as they have been commercially important for many years. Examples of patents for meltblowing are Weber et al. US Patent No. 3,959,421 and Milligan et al. US Patent No. 5,075,068. The Weber et al. patent uses water sprayed into the gas stream to rapidly cool the fibers. A somewhat similar process is described in PCT publication WO 91/18682 which relates to a process for coating paper by modified melt blowing. Recommended coating materials are aqueous liquids such as "an aqueous solution of starch, carboxymethylcellulose, polyvinyl alcohol, latex, a suspension of bacterial cellulose, or any other aqueous material, solution or emulsion." However, these processes atomize the extruded material rather than converting it into latent fibers. Zikeli et al. in U.S. Patent Nos. 5,589,125 and 5,607,639 direct a stream of air horizontally over strands of extruded viscous lyocell liquid as they exit the spinnerets. This stream of air serves only to cool and does not cause any stretching of the filaments.

Centrifugal spinning is illustrated in U.S. Patent Nos. 5,242,633 and 5,326,241 to Rook et al., and U.S. Patent No. 4,440,700 to Okada et al. describes a process for centrifugally spinning thermoplastic materials. Once the material has been spun out, the fibers are collected on an annular die surrounding the spinning head and then carried downward by a curtain of flowing, cooling liquid. The list of polymers suitable for this process includes polyvinyl alcohol and polyacrylonitrile. When these two materials are used, they are spun "wet," i.e., in solution, and the curtain of cooling liquid is replaced by a "coagulating bath."

With the exception of the above-mentioned Kaneko et al. patent, processes equivalent to meltblowing, spunbonding and centrifugal spinning have never been used for cellulosic materials because cellulose itself is essentially infusible.

Extremely fine fibers, referred to as "microdenier fibers," are generally considered to be those with a denier of less than 1.0. Meltblown fibers made from various synthetic polymers such as polypropylene, nylons, or polyesters are available in diameters as small as 0.4 µm (approximately 0.001 denier). But the strength and "toughness" of most of these fibers tend to be low, and their low water absorption is a negative property when used in fabrics for clothing. Microdenier cellulose fibers, with a denier as low as 0.5, have so far only been produced by the viscose process.

The present process produces a new lyocell fiber that overcomes many of the limitations of fibers made from synthetic polymers, rayons and currently available lyocell fibers. It enables the formation of low denier fibers with a distribution of deniers. At the same time, each fiber has a rippled surface, a cross-section of varying shape and diameter. throughout its length, and significant natural crimp. All of these are desirable properties found in most natural fibers, but are absent in lyocell fibers currently produced commercially.

Summary of the invention

The present invention is concerned with a process for producing regenerated cellulosic fibers and fabrics, and with the fibers and fabrics produced in this manner. The terms "cellulose" and "regenerated cellulose" as used herein are intended to be broad enough to encompass mixtures of cellulose with other natural and synthetic polymers which are mutually soluble in a spinning solvent in which cellulose is the major component by weight. It is particularly concerned with low denier fibers produced from cellulose solutions in amine N-oxides by processes analogous to meltblowing or centrifugal spinning. When the terms "melt blowing", "spunbond" and "centrifugal spinning" are used, it is meant that they refer to processes that are similar or analogous to those used in the production of thermoplastic fibers, although the cellulose is in solution and the spinning temperature is only slightly increased. The term "continuously drawn" refers to the commercial process currently used for the production of lyocell fibers, where they are drawn mechanically, first through an air hole to cause elongation and molecular orientation, then through the regenerating bath.

The process involves dissolving cellulose raw material in amine oxide, preferably N-methylmorpholine-N-oxide (NMMO) in the presence of some water. This viscous liquid or cellulose solution in NMMO can be produced using known technologies; i.e., as discussed in any of the previously mentioned McCorsley or Franks et al. patents. In the present process, the viscous liquid is then transferred to the spinning apparatus at a slightly elevated temperature by a pump or extruder at about 90°C to 130°C. The viscous liquid is immediately passed through a plurality of small orifices into the air. When meltblowing is used, the extruded strands of viscous cellulosic liquid are caught in a turbulent gas stream flowing relatively parallel to the path of the filaments. Once the cellulose solution has been expelled through the orifice, the liquid strands or latent filaments are pulled (or significantly reduced in diameter and extended in length) during the continued pull after leaving the orifices. The turbulence causes a natural Crimp and some variability in the final fiber diameter, both between individual fibers and along the length of the individual fiber. This is in extreme contrast to continuously drawn fibers, where the diameter is uniform and the crimp is either absent or must be introduced in a process downstream of spinning. The crimp is irregular and has an amplitude between two maxima that is greater than one fiber diameter and has a period that is greater than about five fiber diameters.

Spunbond bonding can be considered a type of meltblowing in that the fibers are picked up and drawn in an air stream without being mechanically drawn. In the context of the present invention, meltblowing and spunbond bonding can be considered functionally equivalent.

When the fibers are produced by centrifugal spinning, the strands of viscous liquid are expelled into the air through narrow openings and pulled by the inertia provided by the spin heads. The filaments are then passed into a regenerating solution, or a regenerating solution is sprayed onto the filaments. Regenerating solutions are non-solvents such as water, lower aliphatic alcohols, or mixtures of both. The NMMO used as a solvent can then be recovered from the regenerating bath for reuse.

It is believed that turbulence and oscillations in the air around the latent fiber strands are responsible for their uniform geometry when manufactured by meltblowing or centrifugal spinning.

Filaments having an average size as small as 0.1 denier or even smaller can be readily produced. The denier can be controlled by a number of factors including, but not limited to, the diameter of the orifices, the velocity of the gas flow, the speed of the spinning heads and the viscosity of the viscous liquid. The viscosity of the viscous liquid is then primarily a factor of the cellulose DP and concentration. Fiber length can be similarly controlled by the type and velocity of the air flow surrounding the extrusion orifices. Continuous fibers or relatively short chopped fibers can be produced, depending on the spinning conditions. The equipment can be readily modified to produce individual fibers or to lay them in a mass of nonwoven cellulosic fabric. In the latter case, the mass can be formed and self-bonded even before regeneration of the cellulose has taken place. The fibres are then recovered from the regeneration medium, further washed, bleached if necessary, dried and treated in the usual way from that point in the process onwards.

The sheen and luster of the fibers are far less than that of continuously drawn lyocell fibers, which have not been matted so that they do not look like "plastic." It is believed that the reason for this is their uniformly "pebbled" surface, which is evident in high-resolution microscopic images.

If the spinning conditions are very well controlled, the fibers can be produced with varying cross-sectional shapes and a relatively restricted distribution of fiber diameters. Some variation in diameter and type of cross-sectional shape typically occurs along the entire length of each fiber and between different fibers. The fibers are unique to regenerated cellulose and resemble many natural fibers in their morphology.

Fibers produced by either the meltblowing or centrifugal spinning processes have a natural crimp that has nothing in common with that produced in the stuffing box. Crimp produced in the stuffing box is relatively regular, has a relatively small amplitude, usually less than one fiber diameter, and a short period from peak to peak, usually not more than two or three fiber diameters. That of the present fibers has an irregular amplitude, larger than one fiber diameter, often much larger, and an irregular period exceeding five fiber diameters, which are characteristics of fibers with a curly or wavy appearance.

The properties of the fibers of the present invention are very well suited to carding and spinning in conventional textile manufacturing processes. Although the fibers have many of the characteristics of natural fibers, they can be manufactured with microdenier diameters not found in nature. It is possible to directly produce self-binding fabrics or tightly twisted yarns from many individual filaments.

A particular advantage of the present invention is the ability to produce mixtures of cellulose with polymer materials that are otherwise considered incompatible with each other. The amine oxides are particularly effective solvents that are able to dissolve many polymers other than cellulose. It is thus possible to produce mixtures of cellulose with materials such 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 and many others. Each of these materials, when homogeneously mixed with cellulose, can produce fibers with novel and unique properties.

It is a purpose of the present invention to provide a process by which low denier regenerated cellulosic fibers or fibers from cellulosic blends can be produced from solution in an amine oxide water medium by processes analogous to meltblowing, spunbonding or centrifugal spinning.

Another purpose is to provide low denier cellulosic fibers that have favorable geometry and surface properties for yarn manufacture.

It is also a purpose to provide fibers with natural curl and low luster.

Another purpose is to provide regenerated cellulose fibers that have many properties equal to or better than natural fibers.

It is a further purpose to provide a method for forming fibers of the type described above using a process in which all chemicals used in production can be readily recovered and recycled.

Another purpose is to provide self-binding non-woven fabrics made of lyocell.

These purposes and many others will become readily apparent to those familiar with this technique after reading the following detailed description in conjunction with reference to the accompanying drawings.

Short description of the drawings

Figure 1 is a block diagram of the steps used in carrying out the present invention.

Fig. 2 is a perspective view of a partial section through a typical centrifugal spinning equipment as used in the invention.

Figure 3 is a perspective view of a partial section through a typical meltblowing equipment as adapted for use with the present invention.

Fig. 4 is a longitudinal cross-sectional view of a typical extrusion head as might be used with the meltblowing apparatus shown above.

FIGS. 5 and 6 are scanning electron micrographs of a commercially available lyocell fiber at 100x and 10,000x magnification.

FIGS. 7 and 8 are scanning electron micrographs of a lyocell fiber obtained by centrifugal spinning at 200x and 10,000x magnification, respectively.

FIGS. 9 and 10 are scanning electron micrographs at 2000x magnification showing cross sections along a single centrifugally spun fiber.

FIGS. 11 and 12 are scanning electron micrographs of a lyocell fiber produced by melt blowing at 100x and 10,000x magnification.

Fig. 13 is a drawing showing the production of a self-bonding nonwoven lyocell fabric by the melt blowing process.

Fig. 14 is a similar drawing illustrating the production of a self-bonding nonwoven lyocell fabric by the process of centrifugal spinning.

Detailed description of the preferred embodiments

The type of cellulose raw material used in the present invention is not critical. It can be either bleached or unbleached wood pulp, which can be produced by various processes, exemplified by wrapping paper, prehydrolyzed wrapping paper or sulfite. Many other cellulose raw materials, such as purified cotton gauze, are equally suitable. Before dissolving in the amine oxide solvent, the cellulose, if in layers, is torn into fine fluffs to promote rapid dissolution.

Dissolution of the cellulose can be accomplished in a known manner; e.g., as taught in McCorsley, U.S. Patent No. 4,246,221, where the cellulose is wet in a nonsolvent mixture of about 40% NMMO and 60% water. The ratio of cellulose to wet NMMO is about 1:5.1 by weight. The mixture is mixed in a double-armed sigma blade mixer for about 1.3 hours under vacuum at about 120°C until enough water has been distilled out to leave about 12-14%, based on NMMO, to form a cellulose solution. The resulting viscous liquid contains approximately 30% cellulose. Alternatively, NMMO with an appropriate water content can be used initially to avoid the need for vacuum distillation. This is a practical way to prepare viscous spinning fluids in the laboratory, where commercially available NMMO, in concentrations of about 40-60%, is mixed with the pure laboratory reagent NMMO, which contains only about 3% water. can be mixed to produce a cellulose solution containing 7-15% water. Moisture, which is normally present in cellulose, should be taken into account when measuring the amount of water required to be present in the solution. As a reference for the preparation of viscous cellulose liquid in NMMO-water solvents in the laboratory, the articles by Chanzy, H. and A. Peguy, Journal of Polymer Science, Polymer Physics Ed. 18: 1137-1144 (1980) and by Navard, P. and JM Haudin British Polymer Journal, p. 174, Dec. 1980 can be used.

Reference to Fig. 1 shows a block diagram of the present process. As already noted, the preparation of viscous cellulosic liquid in aqueous NMMO is common practice. What is not common is the manner in which this viscous liquid is spun. The cellulose solution is forced through extrusion orifices into a turbulent air stream, rather than directly into the regeneration bath, as is the case with viscose and cuprammonium rayon. The latent fibers are merely regenerated later. However, the present process also differs from the conventional process for producing lyocell fibers because the viscous liquid is not continuously drawn linearly downward as unbroken threads through an air hole and into a regenerating bath.

Fig. 2 illustrates a process of centrifugal spinning. The heated viscous cellulosic liquid 1 is passed into a heated, generally hollow cylinder or drum 2 having a closed bottom and a plurality of small openings 4 in the side walls 6. As the cylinder rotates, the viscous liquid is forced out horizontally through the openings as thin strands 8. When these strands encounter the resistance of the surrounding air, they are pulled or stretched by a large factor. The amount by which they are stretched depends on easily controlled factors such as the speed of rotation of the cylinder, size of the holes, and viscosity of the viscous liquid. The strands of viscous liquid either fall downward by gravity or are gently forced downward by a stream of air into a non-solvent 10 located in a basin 12 where they are coagulated into individually oriented fibers having lengths of about 1 to 25 cm. Alternatively, the strands of viscous liquid 8 may be regenerated either partially or completely by a water spray from a ring of spray nozzles 16 fed from a source of regenerating solution 18. As will be described later, they may also be formed into a nonwoven fabric both before and during regeneration. Water is used as the preferred non-solvent, but ethanol or water-ethanol mixtures are also useful. From this point, the fibers are collected and can be washed to remove any remaining NMMO, bleached if necessary, and dried. Example 2, which follows, provides specific details of the production of fibers that are centrifugally spun in the laboratory.

FIGS. 3 and 4 show details of a typical meltblowing process. As can be seen from Fig. 3, a supply of viscous liquid, not shown, is fed into an extruder 32 which drives the cellulose solution to a die head 34 having a plurality of holes 36. Air or other gas is introduced through conduits 38 and surrounds and transports extruded solution strands 40. A bath or tank 42 contains a regenerating solution 44 in which the strands are regenerated from the solution in the solvent into cellulose fibers. Alternatively, the latent fibers can be sprayed with a water spray to regenerate them, or at least partially regenerate them. The amount of tension or stretch depends on easily controlled factors such as hole size, viscosity of the viscous fluid, cellulose concentration in the viscous fluid, and air velocity and configuration of the nozzles.

Fig. 4 shows a typical extrusion hole. The hole plate 20 is drilled with a plurality of holes 36. It is attached to the body of the extrusion head 22 by a series of cap screws 18. An internal member 24 forms the extrusion passages 26 for the cellulose solution. It is surrounded by air passages 28 which surround the extruded solution filaments 40 and which both ensure that they are drawn and also assist in their transport to the regeneration medium. The following Example 3 shows specific details of fiber preparation by melt blowing on a laboratory scale.

The scanning electron micrographs shown in FIGS. 5-6 show lyocell fibers produced by the conventional continuous drawing process. It is noted that these are fairly uniform in diameter and essentially straight. The surface, viewed at 10,000x magnification in FIG. 6, is remarkably smooth.

FIGS. 7-10 show fibers produced by a centrifugal spinning process of the present invention. The fibers shown in FIG. 7 have a range of diameters and tend to be somewhat curly, giving them a natural curl. This natural curl is very unlike the regular, wavy shape produced in the stuffing box. Both amplitude and period are irregular, and are several fiber diameters in height and length. Most of the fibers are flattened in some manner, and some have a considerable amount of twist. Fiber diameters vary between extremes such as 1.5 µm and 20 µm (<0.1-3.1 Denier), with most of the fibers having a diameter that is close to an average value of 12 μm (about 1 denier).

Fig. 8 shows the fibers from Fig. 7 at 10,000x magnification. The surface is uniformly crinkled in appearance, which is very dissimilar to commercially available fibers. This results in lower gloss and improved spinning properties.

FIGS. 9 and 10 are scanning electron micrographs of fiber cross sections taken at a distance of approximately 5 mm from a single centrifugally spun fiber. The variations in cross section and diameter along the fiber are dramatically evident. This variation is characteristic of both centrifugally spun and meltblown fibers.

FIGS. 11 and 12 are scanning electron micrographs of meltblown fibers at low and high resolution. Although the fiber diameter still varies, it is less pronounced than in the centrifugally spun fibers. However, the crimping of these samples is significantly stronger. The micrograph at 10,000x magnification in Fig. 12 shows a crimped surface that bears a remarkable resemblance to the centrifugally spun fiber.

The overall morphology of fibers from both processes is very advantageous for producing fine, dense yarns, as many of the properties resemble those of natural fibers. This is considered unique to the lyocell fibers of the present invention.

Fig. 13 shows a process for making a self-bonding nonwoven material from lyocell using a modified melt blowing process. A viscous cellulosic liquid 50 is fed into an extruder 52 and from there into the extrusion head 54. An air supply line 56 acts on the extrusion holes to pull the strands 58 of viscous liquid as they exit the extrusion head. The parameters for the process are preferably chosen so that the resulting fibers are continuous rather than having random, shorter lengths. The fibers fall onto an endless, orificed conveyor belt 60 supported and driven by rollers 62, 64. Here they produce a latent non-woven fabric felt 66. An upper roller, not shown, can be used to press the fibers into close contact with each other and ensure bonding at the intersection points. As the felt 66 continues its journey, still supported by the conveyor belt 60, a spray of regenerating solution 68 is directed downwards from sprayers 70. The regenerated product 72 is then taken off the end of the conveyor belt where it can be further processed, e.g. by further washing, bleaching and drying.

Fig. 14 is an alternative method for producing a self-bonding nonwoven fabric using centrifugal spinning. A viscous cellulosic liquid 80 is fed into a rapidly rotating drum 82 having a plurality of holes 84 in the side walls. Latent fibers 86 are expelled through the holes 84 and drawn or elongated by the air flow and inertia provided by the rotating drum. They strike the inner side walls of a collecting surface 88 disposed concentrically around the drum. The collecting surface may optionally have a frustroconical lower portion 90. A curtain or spray of regenerating solution 92 flows down from ring 94 around the walls of catcher 88 to partially coagulate the cellulose felt that has impacted the side walls of the catcher. Ring 94 may be located where shown or moved to a lower location if more time is needed for the latent fibers to self-bond into a nonwoven web. The partially bonded nonwoven web 96 is continuously, mechanically drawn from the lower portion 90 of the catcher into a coagulating bath 98 in vessel 100. As the web moves along its path, it is collapsed from a cylindrical shape into a flat, two-ply nonwoven structure. The web is held under rollers 102, 104 within the bath as it moves. A take-out roller 106 takes the now fully coagulated, two-ply web 108 out of the bath. Any of the rollers 100, 102 or 104 may be driven. The web 108 is then continuously fed to a washing and/or bleaching process, not shown, after which it is dried for storage. It may be split and opened into a single-ply nonwoven material, or left as a two-ply if desired.

example 1 Production of viscous cellulose liquid

The cellulose pulp used in this and the following examples was a standard bleached southern softwood kraft paper market pulp, grade NB 416, available from Weyerhaeuser Company, New Bern, North Carolina. It has an alpha cellulose content of about 88-89% and a DP of about 1200. Prior to use, the stratified wood pulp was passed through a flaker to break it down into essentially individual fibers and small fiber clumps. Into a 250 ml three-neck glass flask were charged 5.3 g of the flaked cellulose, 66.2 g of 97% NMMO, 24.5 g of 50% NMMO, and 0.05 g of propyl gallate. The flask was then charged with Stirrer, immersed in an oil bath at 120ºC, and stirred for about 0.5 hour. A free-flowing, viscous liquid was formed, which was directly suitable for spinning.

Example 2 Production of fibres by centrifugal spinning

The spinning device used was a modified "cotton candy" type, similar to that shown in U.S. Patent No. 5,447,423 by Fuiz et al. The rotor, preheated to 120ºC, had a diameter of 89 cm and rotated at 2800 rpm. The number of holes can be varied from 1 to 84 by blocking holes. For the following experiment, eight holes of 700 µm diameter were used. Viscous cellulose liquid, also at 120ºC, was poured onto the center of the spinning rotor. The thin strands of viscous liquid that emerged were allowed to fall by gravity into room temperature water in the container surrounding the rotor. There they were regenerated. While isolated fibers bonded together, most of them remained single and were several centimeters long.

In addition to the process just described, very similar microdenier fibers have been successfully produced from bleached and unbleached kraft paper pulp, sulfite pulp, microcrystalline cellulose, and blends of cellulose with up to 30% corn starch or poly(acrylic acid).

Diameters (or deniers) of the fibers could be reliably controlled by various means. Higher viscosity of the viscous fluid resulted in heavier fibers being formed. Viscosity of the viscous fluid could in turn be controlled by means including the cellulose solids content or the degree of polymerization of the cellulose. Smaller sizes of the spinning holes or higher rotation speed of the drum produced fibers with narrower diameters. Fibers with diameters of 5-20 µm (0.2-3.1 denier) could be reproducibly produced. Heavier fibers, on the order of 20-50 µm diameters (3.1-19.5 denier), could also be easily produced. Fiber lengths varied between about 0.5-25 cm and depended to a considerable extent on the geometry and operating parameters of the system.

Example 3 Production of fibers by melt blowing

The viscous liquid prepared as in Example 1 was kept at 120ºC and fed into an apparatus originally designed for the production of melt-blown synthetic polymers. The total length of the holes was about 50 mm with a diameter of 635 µm, tapering to 400 µm at the discharge end. After a transport distance in air of about 20 cm in a turbulent air jet, the fibers dropped into a water bath where they regenerated. The length of the regenerated fibers varied. Some short fibers were formed, but most were several centimeters to ten centimeters long. Variations in the extrusion parameters made it possible to produce continuous fibers. Surprisingly, the cross-section of many of these fibers was not uniform along the fiber length. This property is expected to be particularly advantageous in spinning dense yarns using the microdenier material of the invention, since the fibers are more similar in overall morphology to natural fibers.

In a variation of the process described above, the fibers were allowed to hit a moving conveyor belt of stainless steel mesh before being directed into a regenerating bath. A well-bonded, non-woven felt was produced.

It should be clear that the nonwoven fabrics made of lyocell do not have to be self-bonding. They may be only partially self-bonding or not self-bonding at all. In these cases they may be bonded by any known method, including but not limited to hydro-crosslinking, the use of adhesive binders such as starch or various polymer emulsions, or combinations of these methods.

Example 4 Use of microcrystalline cellulose fibers to produce meltblown Lyocell

The procedure of Example 1 was repeated using microcrystalline fibers instead of wood pulp to increase the solids content of the viscous fluid. The product used was Avicel® Type PH-101 microcrystalline cellulose available from FMC Corp., Newark, Delaware. Viscous fluids were prepared using 15 g and 28.5 g of microcrystalline cellulose (dry weight) with 66.2 g of 97% NMMO, 24.5 g of 50% NMMO and 0.05 g of propyl gallate. The procedure was otherwise as described in Example 1. The resulting viscous fluids contained about 14% and 24% cellulose, respectively. These were melt blown as described in Example 3. The resulting fibers were morphologically essentially identical to those from Examples 2 and 3.

It should be understood that fiber denier is dependent on many controllable factors. Among these are solution solids content, solution pressure and temperature at the extrusion head, hole diameter, air pressure, and other variables well known to those familiar with meltblowing and centrifugal spinning technology. Lyocell fibers, with an average denier of 0.5 or even less, can be produced consistently by both the meltblowing and centrifugal spinning processes. A 0.5 denier fiber corresponds to an average diameter (estimated on the basis of an equivalent round cross-sectional area) of about 7-8 µm.

The fibers of the present invention were examined by X-ray analysis to determine the degree of crystallinity and the crystallite type. Comparisons were also made with some other cellulosic fibers as shown in the table below. Data for the microdenier fibers were taken from the centrifugally spun material of Example 2. Table 1 Crystalline Properties of Various Cellulosic Fibers

There were some difficulties in measuring the tensile strength of individual fibers and therefore the tensile strength figures given in the following table are only estimates. Again, the microdenier fibers of the present invention are compared to a number of other fibers. Table 2 Measurement of physical fiber properties

The centrifugally spun Lyocell, with an average diameter of about 5 um, corresponds to fibers of about 0.25 denier.

The crunchy surface of the fibers of the present invention results in a desirable lower gloss without the need for any internal matting agents. Since gloss or sheen is a difficult property to measure, the following test is intended to be exemplary of the difference between a fiber sample made by the method of Example 2 and a commercial lyocell fiber. Small, wet, molded towels were made from the respective fibers and their light reflectance was determined. The reflectance of the Example 2 material was 5.4%, whereas that of the commercial fiber was 16.9%.

The inventors have herein described the best mode presently contemplated for carrying out their invention. It will be apparent to others skilled in the art that many variations not exemplified herein should be included within the broad scope of the invention.

Claims (23)

1. Lyocell fibers, which are characterized by variability in the diameter of the cross-section and in the shape of the cross-section along the length of the fiber and from fiber to fiber. 2. The lyocell fibers of claim 1 comprise fibers having a uniformly crunchy surface when viewed at 10,000x magnification and as can be seen from Figs. 8 and 12. 3. The lyocell fibers of claim 1 have an irregular crimp with an amplitude that is greater than about one fiber diameter and with a period that is greater than about five fiber diameters. 4. If Lyocell fibres have been produced by a centrifugal spinning process, these fibres are characterised by variability in the diameter of the cross-section and in the shape of the cross-section along the length of the fibre and from fibre to fibre. 5. The lyocell fibers of claim 4 comprise fibers having a uniformly crunched surface when viewed at 10,000x magnification and as seen in Fig. 8. 6. The lyocell fibers of claim 4 have an irregular crimp with an amplitude that is greater than about one fiber diameter and with a period that is greater than about five fiber diameters. 7. When Lyocell fibres are produced by the melt-blowing process, these fibres are characterised by variability in the diameter of the cross-section and in the shape of the cross-section along the length of the fibre and from fibre to fibre. 8. The lyocell fibres of claim 7 comprise fibres having a uniformly crunched surface when viewed at 10,000x magnification, as shown in Fig. 12. 9. The lyocell fibers of claim 7 have an irregular crimp with an amplitude that is greater than about one fiber diameter and with a period that is greater than about five fiber diameters. 10. A nonwoven fabric made from lyocell which has been produced by depositing a plurality of strands of cellulose solution on a receiving surface and then regenerating the cellulose. 11. The lyocell nonwoven fabric of claim 10, wherein the fibers are self-bonding. 12. The lyocell nonwoven fabric of claim 10, wherein the fibers are bonded together by a process consisting of hydrocrosslinking, adhesive binders, and a combination of these processes. 13. The lyocell fabric of claim 10, wherein the strands of cellulose solution are formed by the meltblowing process. 14. The lyocell fabric of claim 10, wherein the strands of cellulose solution are formed by the process of centrifugal spinning. 15. A process for producing lyocell fibers comprising: dissolving the cellulose in an amine oxide solvent to produce a viscous cellulose liquid, meltblowing the viscous liquid to produce and elongate latent fiber strands, and regenerating the strands to produce lyocell fibers. 16. The process of claim 15, wherein the solvent is an aqueous solution of N-methylmorpholine N-oxide. 17. The method of claim 15, wherein the fibers are produced by spunbonding. 18. A process for producing lyocell fibers comprising: dissolving the cellulose in an amine oxide solvent to produce a viscous cellulose liquid, centrifugally spinning the viscous liquid to produce and elongate latent fiber strands, and regenerating the strands to produce lyocell fibers. 19. The process of claim 18, wherein the solvent is an aqueous solution of N-methylmorpholine N-oxide. 20. Lyocell fibers having an average denier not greater than about 0.5. 21. The lyocell fibers of claims 1, 4 or 7, wherein the fiber comprises a mixture of diameters in which at least a portion of said fibers are less than 1 denier. 22. The lyocell fibers of claims 1, 4 or 7, which comprise a mixture of cellulose with non-cellulose polymers. 23. A spun yarn comprising a plurality of the fibers of claims 1, 4 or 7.