WO1991011322A1

WO1991011322A1 - Rigid rod polymer fibers and process for spinning fibers from rigid rod polymer containing solutions

Info

Publication number: WO1991011322A1
Application number: PCT/US1991/000459
Authority: WO
Inventors: Kigen Kawai
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-01-29
Filing date: 1991-01-29
Publication date: 1991-08-08
Anticipated expiration: 1992-07-29

Abstract

Improved fibers and a process for making oriented rigid rod polymer fibers (e.g. aramid) having improved properties including compressive strength and/or a substantially regular microstructure across a cross-section of oriented fiber where such improved fibers may be produced in a dry jet-wet spinning process utilizing large diameter spinneret orifices with long air gaps and increased pre-coagulation drafting.

Description

RIGID ROD POLYMER FIBERS AND

PROCESS FOR SPINNING FIBERS FROM

RIGID ROD POLYMER CONTAINING SOLUTIONS

BACKGROUND OF THE INVENTION The present invention relates to improved rigid rod polymer fibers and an improved process whereby improved rigid rod fibers may be formed.

The discovery of rigid rod polymers and the spinning of rigid rod polymers into fibers provided the world with synthetic materials which exhibit exceptional tensile properties, even under severe conditions. These materials have found use in a variety of applications, such as tire cord, armor, sports equipment and aerospace components. In general, however, the compressive strength of rigid rod fibers is not commensurate with their superior tensile strength. Many known, commercially available rigid rod fibers have a tendency to split into fibrils when stress is applied, thereby contributing to compressive failure. As a consequence the usefulness of these fibers is often limited in applications wherein good compressive as well as tensile strength are required.

Attempts have been made to improve the compressive strength of rigid rod fibers by modifying the rigid rod polymer to add branching to the polymer and thereby prevent fibril formation by the fiber when compressive force is applied. However, this branching may disturb the crystal structure of the fiber, thereby causing the fiber to have diminished tensile strength. Although attempts have been made to improve the compressive strength of rigid rod fiber materials by forming a mixed weave of rigid rod fibers with carbon fibers or blends with other polymers, these materials may have other disadvantages such as a relatively high cost or unfavorable physical properties which diminish their utility.

Given that the relative inferiority of polyaramids with respect to compressive strength has remained an obdurate problem, a polyara idwith improved compressive strength would offer significant practical advantages over many polyaramids known in the art. A process for making such improved polyaramids would be particularly advantageous if the compressive strength of polyaramid fibers could be improved without adding additional steps to the conventional fiber making process.

SUMMARY OF THE INVENTION

A process is provided for spinning a fiber from a rigid rod polymer solution. The process comprises extruding a rigid rod polymer solution through a spinneret orifice and into a noncoagulating fluid to form an extruded solution stream, imparting an extensional stretch to said extruded stream while in the noncoagulating fluid so that the polymer in the extruded stream acquires a relatively high degree of lengthwise orientation, and passing the stream through a coagulating fluid so that the stream is coagulated to form a fiber having a highly oriented, substantially regular structure.

Solutions of polyaramids are preferred. It is preferred that solutions of polyaramids be extruded through a spinneret having one or more orifices of a diameter of at least 0.25 mm. For polyaramids it is preferred that the extensional stretch be sufficient to produce a fiber having a spin draft of at least 15.

The orifice preferably has a diameter of at least .25 mm to about 6 mm. , with diameters of about 1 mm to about 4 mm being more preferred.

For polyaramids it is also preferred that the extruded solution stream pass through at least 5 cm, and more preferably at least 10 cm of the noncoagulating fluid. It is further preferred that the extruded stream pass through 50 cm or less of the noncoagulating fluid.

In the preferred process for spinning polyaramids the fiber has a spin draft of at least 50, with spin drafts of at least 100, and more preferably 400 or 500, being particularly preferred.

In the spinning of polyaramids, the solution preferably is extruded through the orifice at a rate of about 0.1 cm³/min. to about 3 cm³/min, with the solution preferably having a viscosity of about 100 poise to about 15,000 poise at the temperature at which it is spun.

The temperature of the spinneret is maintained above the gel temperature of the solution, but below the boiling point of the solvent and the temperature at which the polymer decomposes. For polyaramids, the spinneret temperature preferably is about 70'C to about 100^*C. The noncoagulating fluid is maintained at a temperature which is the same as or is below the temperature of the spinneret. For polyaramids, it is preferred that the noncoagulating fluid being maintained at a temperature which is at or below this maximum, but is at least 40^βC, and more preferably at least about 60^βC. In spinning polyaramids, the spinning solution preferably has a rigid rod polymer concentration in a solvent of about 2-20% by weight. Preferably the extruded stream is stretched while undergoing coagulation.

The invention includes the product of the above process, and is also directed to a rigid rod polymer fiber which has a highly oriented structure throughout the fiber, as evidenced by both the inside and outside of the fiber having a substantially regular or "skin" morphology. Preferably at least 70% by cross-sectional area or by volume, and more preferably at least 80%, of the fiber has a skin morphology.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of an apparatus for performing the process of the invention.

Fig. 2 is a schematic cross-sectional representation of a spinneret orifice and the velocity curve of an extruded solution stream.

Fig. 3 is a graph of orifice diameter plotted against the center velocity of an extruded liquid.

Fig. 4 is a photograph, taken from the side, of a fiber after being broken by tension.

Fig. 5 is a photograph, taken from the side, of a fiber after being broken by tension.

Fig. 6 is a photograph, taken from the side, of a fiber after being broken by tension.

Fig. 7 is a micrograph of a cut end of a fiber.

Fig. 8 is a micrograph of a cut end of a fiber. Fig. 9 is a micrograph of a fiber after being broken by tension.

Fig. 10 is a micrograph of a fiber after being broken by tension.

DETAILED DESCRIPTION OF THE INVENTION

Rigid rod polymers generally are liquid crystalline polymers with unique physical characteristics. Due to their unique physical characteristics, rigid rod polymers are processed by being dissolved in a suitable solvent, followed by spinning the polymer containing solution to obtain a fiber. At low concentrations such polymers may form solutions wherein the polymer is randomly oriented, while solutions containing relatively large amounts of polymer may be lyotropic or liquid crystalline. The present invention contemplates that either isotropic or anisotropic solutions may be spun to form highly oriented fibers using the process of the invention.

Fibers obtained by spinning rigid rod polymer solutions typically are composed of a central core surrounded by a skin. As disclosed in the articles, incorporated herein by reference, "ON THE MORPHOLOGY OF AROMATIC POLYAMIDE FIBERS (KEVLAR, KEVIAR-49, AND PRD-49)", by S. Li, L.F. Allard and W.C. Bigelow, Journal of Macromolecular Science - Physics, B22(2), p. 269-290 (1983), and; "MORPHOLOGY OF POLY(p-PHENYLENE TEREPHTHALAMIDE) FIBERS", by M. Panar, P. Avakian, R.C. Blume, K.H. Gardner, T.D. Gierke, and H.H. Yang, Journal of Polymer Science: Polymer Physics Edition, vol. 21, p. 1955-1969 (1983) , the core of rigid rod polymer fibers typically comprises rods or crystallites, which may be arranged in bundles or fragments of rings, and are interrupted across the width of the fiber and at various points along the fiber's length by areas of discontinuity or irregularity. This is in contrast to the morphology, of the fiber skin, wherein the crystallites typically are largely regular and continuous along the fiber's length, with relatively few voids or areas of irregularity, if at all. For example, the article cited above by Panar et al. (page 1968) indicates that continuous fibers are not dyeable, but that damaged fibers will accept dye, and that the dye goes selectively to the fiber core. Also, Panar et al. indicate that in order to obtain skin-core differentiation in heavily etched fibers, the cut ends of the fiber must be exposed to the plasma, whereupon the core is preferentially degraded. This suggests that the skin structure is predominantly continuous and regular so that dye or plasma can not penetrate the skin, in contrast to the core wherein voids or areas of irregularity exist which will admit plasma or dye. While it is an object of the present invention to provide an improved process for spinning rigid rod polymer solutions and improved rigid rodpolymer fibers, the Applicant believes his improved results may be due to the fiber obtained possibly having a morphology throughout which may be characterized as being "skin"; that is, the microstructure of the fiber predominantly is believed to be continuous and regular along the fiber's length, such as may be provided by a plurality of continuous fibrils oriented substantially parallel to the fiber's long axis.

The improved fiber forming process may be generally described as a process which comprises extruding a rigid rod polymer solution through a spinneret orifice and into a noncoagulating fluid to form an extruded solution stream, imparting an extensional stretch to the extruded stream while the stream is in the noncoagulating fluid, and passing the stream through a coagulating fluid so that the stream is coagulated to form a fiber having a predominantly regular, continuous structure, such as when the fiber comprises substantially continuous fibrils or crystalline bundles.

The process of the invention employs the conventional spinning process of U.S. Patent No. 3,767,756 to Herbert Blades, which is incorporated herein by reference, with the modifications described herein.

Polymers useful in the improved fibers and the improved process of the invention are liquid crystalline, rigid rod polymers which may form spinnable solutions. Polymers which may be characterized as aromatic polyamides or aramids, and polybenzazoles are generally preferred. Ara id polymers are most preferred.

The preferred aromatic polyamides are those which include repeating units ' consistent with one of the following formulas:

(I) (II)

wherein R and R' are selected from the group consisting of substituted or unsubstituted m-phenylene or p- phenylene.

These polyamides may also include units consistent with formula III below: o o

II II - c— R" - c -

(III)

wherein R" is selected from the group consisting of m-phenylene and p-phenylene. When units consistent with formula III are present in the polyamide, it is preferred that the units consistent with formula 1 be present in approximately equimolar amounts with units consistent with formula I and/or II. Polyaramids which consist essentially of units selected from units consistent with formulas I, II and III are preferred.

Examples of polyaramids useful in the present invention include poly(m-phenyleneisophthalamide) (MPD-I) , poly(p- benzamide) (PBA) , and poly(p-phenyleneterephthalamide) (PPTA) . Blends of the above polyaramids with each other or with other polymeric materials may also be used in the process of the invention, but are generally less preferred.

Although polyaramids are preferred, the polymers useful in the present invention also include polybenzazoles. These polybenzazoles include repeating units consistent with one of the following formulas:

(VI)

wherein Z is S (polybenzothiazoles or "PBT"'s or "PBZ"'ε), or 0 (polybenzoxazoles or "PB0"*s). Additionally or alternatively, the units may be isomers of the units of formulas IV - VI, such as, for example, poly(benzo[1,2-d: ,5-d' ]bisthiazole-2,6-diyl-l,4- phenylene) (trans-PBT) and poly(2,5-benzoxazole) (2,5- PBO) .

The polybenzazole may be a homopolymer. of the units described above, such as poly(benzo[l,2-d:5,4- d' ]bisazole-2,6-diyl) (cis-PBZ) , poly(2,6-benzazole) (2,6-PBZ) and poly(6,6'- bibenzazole-2,2'-diyl) (2,2'- PBZ) . However, other repeating units may also be included, so that the polybenzazole is an articulated rigid rod copolymer, ordered block copolymer, or random block copolymer. When other repeating units are included such units preferably are present in approximately equimolar amounts with units consistent with formulas IV-VI, above. For example, in the preferred embodiment units consistent with formula IV are copolymerized with units consistent with formula III, such as in poly(p-phenylene benzobisthiazole) .

Blends of the above polybenzazoles with each other or with other polymeric materials may also be used in the process of the invention.

Many rigid rod polymers, including aramid and benzazole polymers consistent with the above description, are known in the art. Such polymers are available commercially, or may be made by means known in the art. Generally polymers having, higher inherent viscosities are preferred over polymers having lower inherent viscosities, as long as the polymers may be dissolved in an appropriate solvent and spun.

The present invention contemplates that the above polymers will be spun into fibers from a solution. Sulfuric acid, chlorosulfuric acid, fluorosulfuric acid or mixtures thereof may be used as a solvent for spinning rigid rod polyaramids into fibers. These acids preferably are used at a concentration of at least 96% by weight, and more preferably of about 98% ^• concentration, although the solvent may have a concentration of 100% or more when fuming sulfuric acid is employed. Sulfuric acid, preferably at a concentration of about 98%, is most preferred for spinning the polyaramids discussed above. Various additives may also be present in the spinning solution, as is known in the art, but preferably are present, if at all, in an amount less than about 30% by weight of the total weight of solvent plus additives.

In the embodiment wherein the rigid rod polymer is a polybenazole, the solvent preferably is a poly(phosphoric acid) or "PPA". PPA'ε may be conceptualized as a mixture of H₃P0₄ - H₂0 or as P₂0₅ + H₂0 and are condensed phosphoric acid oligomers of the general formula H_n+2P_n0_3rγ4.₁ or:

0

HO- (-P 0-)_n-H

0 H

wherein the average value of n depends on the ratio of water to P₂0₅, and may be as high as 14 or more. The H₃P0_A (or P₂0₅) content of a PPA is defined as the weight of the H₃P0₄ (or P₂0₅) in the PPA divided by the total weight of the PPA. Accordingly, PPA's having a P₂0₅ content of about 82% or more, and more preferably about 84% or less, are preferred.

The method of preparing the spinning solution or "dope" is not critical to the invention, and may be done by any of the methods known in the art for preparing spinnable solutions, such as, for aramid polymers, that disclosed by U.S. Patent No. 3,767,756. In the preferred embodiment the solution is prepared so as to have minimal Water content, preferably on the order of 2% or .less and more preferably 1% or less, as greater amounts of water may contribute to decomposition of the polymer, thereby lowering viscosity so that the solution is unsuitable for spinning and contributing to low tensile strength in the finished fiber. The water content of the solution may be minimized by using both a solvent and a polymer which contain little or no water, and by preparing the solution under conditions whereby contact of the solution with water is minimized or avoided altogether.

As is known in the art, rigid rod polymer containing spinning solutions or "dopes" should be prepared at temperatures which are sufficient to maintain the solution in a liquid state, yet are below the temperature at which the dissolved polymer will quickly degrade. Exposure of aramid solutions to temperatures significantly above about 90^βC preferably should be minimized. It is in part for this reason that the process preferably is continuous, whereby, once formed, the dope is passed immediately to the spinning apparatus. However, means for storing rigid rod polymer solutions and suitable batch processes are known in the art.

The concentration of aramid polymer in the spinning dope preferably is at least 2% by weight. Although very large polymer molecules may be spun at low concentrations such as 2%, spinning at such low concentrations may not be economical. It is in part for this reason that concentrations of 4% by weight or more, and especially 6% by weight or more, generally are preferred. The maximum concentration of polymer in the solution will be limited by the amount of polymer which may be dissolved in the solvent while maintaining a solution viscosity which is suitable for spinning. For dopes of aramid polymers, concentrations of less than 20% by weight are usually preferred.

At the spinning temperature (temperature of the dope at the spinneret orifice) the dope preferably has a viscosity of about 100 poise to about 15,000 poise. Solutions with a viscosity of about 300 poise to about 2,000 poise are preferred.

According to the invention, the above described dopes may be spun using conventional spinning equipment well known in the art, especially as modified as described below. The individual components of this equipment are readily available from commercial sources or may be made from readily available parts using known techniques.

Referring now to Fig. 1, the typical fiber production apparatus 1 generally includes a spinning apparatus 12, a coagulating bath 13, and a take up apparatus 14.

Spinning apparatus 12 includes an intake tube 15 attached to a spinning head 16. Intake tube 15 has a generally circular cross-sectional configuration transverse to that shown so that tube 15 has an outer wall surface 17 and an inner wall surface 18. Inner wall surface 18 defines a central reservoir 19 capable of receiving a spinning dope in a continuous fashion from a continuous feed apparatus (not shown) such as a pump external to spinning apparatus 12. Spinning head 16 includes a spinneret 20 attached to head 16 on the opposite side of head 16 from reservoir 19. A noncoagulating fluid 21, which preferably is a gaseous medium such as air, is on the opposite side of spinneret 20 from reservoir 19. Spinneret 20 has at least one orifice (not shown) which communicates between reservoir 19 and noncoagulating fluid 21. Intake tube 15 feeds the dope to the orifice(s) in spinneret 20. A spinning tube 22 is attached to spinning head 16 on the side of spinneret 20 opposite from reservoir 19. Spinning tube 22 has a tube wall 23 attached to spinning head 16. Opening 24 in wall 23 of tube 22 at the opposite end from spinneret 20 permits solution stream 25, extruded from the orifice in spinneret 20, and the noncoagulating fluid to pass through tube 22. Spinning apparatus 12 is usually positioned in a generally upright orientation as shown in Fig. 1. In the embodiments wherein the spinning tube is very long, such as on the order of 2-3 meters, opening 24 may be partially closed (not shown) by means known in the art such as an annular disk or cone attached to the end of tube 22 at the opposite end from spinneret 20 and positioned around extruded stream 25 to minimize turbulence at the lower end of tube 22. Similarly, means known in the art such as a fenestrated mesh screen shelter (not shown) , such as shown in Japanese Patent No. 588,288 (February 20, 1969) , may be attached to spinneret 20 on the opposite side of the orifice from reservoir 19 inside tube wall 23 and around extruded stream 25 to control turbulence at the upper end of tube 22 when the turbulence is sufficient to disturb extruded stream 25.

Tube wall 23 defines a lumen 26 which contains the noncoagulating fluid 21. The noncoagulating fluid enters tube 22 at opening 24 and passes into lumen 26. The noncoagulating fluid 21 flows in a generally upward direction within lumen 26 and may exit tube 22 through outlets such as coagulating fluid outlet 27 in the upper end of tube wall 23, as indicated by arrows in Fig. 1, in order to prevent saturation of the noncoagulating fluid with solvent vapors. (Means for introducing the noncoagulating fluid into the top of the tube so that the general direction of flow of the noncoagulating fluid is reversed are known in the art but are generally less preferred) . Intake tube 15 and spinning head 16 typically are heated so as to maintain the dope contained therein at the desired temperature. This may be accomplished by means known in the art, such as by the intake tube and spinning head having hollow jackets, or by being surrounded by heating coils through which a heating medium such as hot air, oil or water may be circulated or which are electric resistance heaters (not shown) . Similarly, the temperature ofthenoncoagulating fluid 21 inside tube 22 is maintained by heating of tube wall 23 by any of the means mentioned above and as indicated in Fig. 1 by .heating medium intake 28 and outlet 29 to permit the circulation of a temperature controlled fluid through spinning tube wall 23 in the direction of the arrows shown.

Coagulating bath 13 is positioned proximal to spinning apparatus 12 and includes a container 30 having a base 31 connected to an upstanding wall 32, coagulating fluid 33, guide means such as snag pins (not shown) or freely turning roller guides 34 a,b immersed in fluid 33, and means for circulating fluid 33 and maintaining it at a proper temperature, such as fluid intake 35 and outlet 36 which permits circulation of fluid 33 through the bath 13 in the direction of the arrows shown and maintenance of the bath composition in a continuous fashion. A gap 37 may exist between opening 24 and coagulating fluid 33. The size of this gap is not critical; similarly, the temperature within the gap is not critical and may be ambient under the spinning conditions.

Take up apparatus 14 is external to both spinning apparatus 12 and coagulating bath 13, and may be any of a variety of means known in the art for winding up coagulated fiber 38 or transferring the fiber directly to other apparatus (not shown) for post-coagulation processing. For example, this means may be a reel or a roller guide, such as reel 39 which rotates in the direction of the arrow shown thereon.

In the process of the invention a solution containing a rigid rod polymer, as described above, is extruded through an orifice in spinneret 20 to form a solution stream 25 which has an outer surface 40 and a center, depicted by broken line 41. Upon exiling the orifice extruded solution stream 25 passes through a noncoagulating fluid 21 such as air in tube 22 and exits tube 22 through opening 24. The extruded stream then enters a coagulating fluid 33 such as a water bath and is immersed in the coagulating fluid by passing under fixed snag pins or rods (not shown) or roller guides 34 a,b. Upon contact with the coagulating fluid, solvent diffuses out of the extruded stream and causes the stream to coagulate, thereby forming a fiber such as 38. It is preferred that, during coagulation, the fiber undergo additional stretching to increase the amount of lengthwise orientation of molecule chains in the fiber. For example, in the embodiment wherein both 34a and 34b are freely turning roller guides, tension is applied to fiber 38 by apparatus, external to bath 13, such as take up apparatus 14. This application of force creates friction between fiber 38 and roller guides 34 a,b and coagulating fluid 33, causing fiber 38 to undergo additional stretching while in bath 13. A process for stretching a fiber while removing solvent therefrom is disclosed for flexible polymers by U.S. Patent No. 4,344,908 to Smith et al. , incorporated herein by reference.

Fiber 38 exits coagulating fluid 33 and is wound up by reel or roller 39 or_r preferably, transferred thereby directly to post-coagulation processing including stretching the fiber to several times its length (such as 2-3 times its length) , if desired, while undergoing washing to remove residual solvent, and heat treating the fiber, such as by exposing the fiber to a (temperature of approximately 500"C for 6 seconds. (Suitable washing and heat treatment steps are well known in the art and do not form a critical part of the invention) . The extruded stream 25 is drafted immediately after exiting the orifice by the application of tension to the extruded stream. Means for applying tension to an extruded stream are known in the art. In the preferred embodiment tension is applied by the take up apparatus such as reel or roller 39.

Fig. 2 is a schematic depiction of the flow of an extruded solution stream showing a. segment of a spinneret 42, an orifice 43 and a velocity profile 44 of an extruded solution stream showing V₀, the velocity of the extruded liquid at the center of the extruded liquid stream. As is apparent from velocity profile 44 in Fig. 2, the center of the solution stream (such as 41 in Fig. 1) moves at a greater velocity than the outside of the stream (such as 40 in Fig. 1) , which travels relatively slowly. The drafting distribution of the extruded stream approximates an in version of the extrusion velocity profile 44, so that the outside of the solution stream has a larger draft than the stream's center. Although riot wishing to be bound by theory. Applicant hypothesizes that the skin/core structures typical of known rigid rod polymer fibers are the result of the difference in velocity of the outer part and the inner part of an extruded solution stream being too great during extrusion and drafting, so that shearing occurs within the stream, with the result that only the polymer in the outer or "skin" part of the stream becomes continuously highly aligned. Applicant hypothesizes that the improved, "all skin" fibers obtainable by the improved process of the invention are created by extruding a solution stream so that the stream may be drafted or stretched to cause alignment of the rigid rod polymer molecules to create a regular crystalline microstructure throughout the fiber, without the difference between the velocity of the inside and the outside of the solution stream being so great that the stream undergoes shearing when it is stretched and a less oriented core is created.

When an extruded liquid is assumed to follow Newtonian flow, the theoretical center velocity or V₀ of the liquid, such as a polymer solution, may be calculated using the following equation:

Q = R²V 2

wherein:

Q = the extrusion rate (cm3/min.) R = the radius of the orifice (cm.)

V_o = the center velocity of the extruded liquid

Fig. 3 is a graphic representation of V_o, the center velocity of an extruded liquid, plotted as a function of orifice diameter according to the above equation. The graph in Fig. 3 includes a portion wherein the graph is substantially vertical and a portion wherein the graph is substantially horizontal, connected by a portion of the graph wherein the slope is transitional. In Fig. 3 the vertical portion of the graph is included in an area of the graph marked "shea stretch zone", and the horizontal portion of the graph is included in an area marked "extensional stretch zone". Applicant hypothesizes that stretching an extruded stream having an orifice diameter and V„ in the shear stretch zone to the extent required to align polymer within the stream causes shearing within the stream, and a fiber is created which has an outer highly oriented skin, derived fro the outer areas of the stream, and an inner less oriented core, derived from the inner areas of the stream. However, by extruding a solution so that significant shearing of the stream is avoided,_.such as. by using an orifice diameter and V₀ on the horizontal portion of the graph in the area marked "extensional stretch zone", and stretching the stream to the extent required to cause alignment of the polymer, it may be possible to minimize or avoid shear within the extruded stream so that a fiber wherein substantially all of the fiber is highly oriented may be obtained. Although it is not necessary to determine the point of transition between shear and extensional stretch in order to practice the invention, the point of transition between shear and extensional stretch may be determined without undue experimentation, such as by plotting orifice diameter versus V₀, spinning fibers using orifice diameters and V₀'s of several points along the graph, particularly in the transitional part of the graph, and examining the resulting fibers.

Accordingly, extrusion rates and orifice diameters are selected according to the invention so that the extruded solution stream may be drafted to the extent required to cause alignment of the polymer within the stream while undergoing extensional stretch, so that a highly oriented fiber of the desired diameter is created by securing this alignment by coagulation of the stream in the bath. (Additional alignment or orientation may take place during further stretching in the bath and during post-coagulation processing.)

In general, the orifice used to extrude a solution stream which may undergo extensional stretch is large relative to current commercial practice. For aramid polymer solutions orifices having a diameter of at least 0.25 mm usually are used. The maximum permissible orifice diameter is as large as that with which the spinning solution has a uniform concentration, as indicated by its viscosity, as it is extruded from the orifice so that the resulting stream stretches uniformly when highly drafted. However, it is preferred that the orifice diameter be small enough so that, instead of the polymer solution flowing freely from the orifice, positive pressure can be applied to extrude the solution whilemaintaining the stream'svelocity sufficiently low for extensional stretch to occur. It is in part for this reason that orifice diameters of less than 8 mm are preferred for most aramid solutions. For aramid solutions orifice diameters of 0.25 to about 6 mm are preferred, with diameters of about 1 mm to about 4 mm being more preferred.

For most solutions orifices having a ratio of capillary length to diameter of the hole (L/D ratio) of about 1 to about 9 are preferred, although the L/D ratio is not critical to the invention. Although the shape of the orifice is not critical to the invention, orifices which are generally round or circular in configuration are preferred.

Although the spinneret ' may have only one orifice consistent with the invention, it is preferred that the spinneret have more than one orifice as described above so that multi-filament fibers may be obtained. The distance between adjacent orifices is not a critical part of the present invention, but may be any distance sufficient to prevent sticking together of adjacent streams when turbulence occurs in the region of the orifice. This distance is usually determined by the boundary layer of the solvent in the solution stream. For most aramid solutions it is preferred that the orifices be about 0.5 mm to about 2 mm apart. The rate at which the polymer solution is extruded according to the invention will depend on a variety of factors including orifice size, polymer concentration, size of the gap between the orifice and coagulating bath, amount of drafting, and the diameter of fiber desired. However, for aramid solutions extrusion rates of about 0.1 cc/min. to about 3 cc/min. are usually preferred, with rates of about 0.25 cc/min. to about 1.5 cc/min. being more preferred.

The temperature at which the polymer solution is extruded should be above the gel temperature of the solution, but below the temperature at which the solvent boils or the polymer quickly degrades. For most aramid solutions it is preferred that the spinning dope have a temperature of about 70°C to about 100"C. In the spinning of manypolyaramids, such as PPTA, temperatures of about 70°C to about 90"C are most preferred. This is usually accomplished by maintaining the spinninghead and spinneret at the desired temperature, and by heating the dope to the desired temperature before it enters the spinning head.

According to the invention the solution stream, upon exiting the orifice, immediately passes through a noncoagulating fluid positioned between the outer face of the spinneret and the coagulating bath. This fluid may be composed of a gas, such as air, or a liquid, such as toluene, heptane, or some other noncoagulating organic material. However, for spinning most rigid rod polymers air is preferred.

It is important that the distance the solution stream

^•travels through the noncoagulating fluid and the temperature of the noncoagulating fluid be sufficient to permit drafting of the solution stream to the desired extent. This may be accomplished by attaching a spinning tube such as 22 in Fig. 1 to the spinning block, with the tube having the length required to draft the stream to the desired extent and being capable of having the temperature and other conditions (such as fluid circulation) inside the tube controlled so as to permit the desired drafting.

It is preferred that the distance the solution stream passes through the noncoagulating fluid - that is, the distance in which the fluid and conditions are such that substantial cooling of the stream does not occur - be at least 5 cm. Usually no more than a distance of 50 cm will be required. Distances of about 10 cm to about 20 cm are usually preferred. Referring to Fig. 1, in the preferred embodiment this distance is typicallymeasured from the point of attachment of tube 22 to spinning head 16 to the distal end of tube 22.

The temperature of the noncoagulating fluid should be sufficient to maintain the solution stream in a draftable state for a significant distance after exiting the orifice so that the extruded stream may be drafted to the extent desired, but be below the boiling point of the solvent or the temperature at which the polymer decomposes. Preferably the temperature of the noncoagulating fluid is the same as or slightly less, on the order of about 5 to about 40 degrees less (Centigrade) , than the temperature at which the solution is extruded. For most aramid solutions it is preferred that the noncoagulating fluid have a temperature of at least 40'C, and more preferably at least 60"C. The temperature of the noncoagulating fluid at the lower end of the tube may be slightly less, on the order of about 5-10 degrees (Centigrade) , than the temperature of the noncoagulating fluid in the upper end of the tube in the vicinity of the orifice, as most of the drafting of the extruded stream occurs near the orifice. The noncoagulating fluid is usually maintained at the appropriate temperature by heating the spinning tube and/or preheating the noncoagulating fluid before it enters the tube.

For the purposes of the present invention the extrusion or "jet" velocity is defined as the average velocity of the spinning solution in the spinneret as calculated from the volume of the spinning solution passing through an orifice per unit of time, and from the cross- sectional area of the orifice. The spin draft is defined for the purpose of the present invention as the ratio of the velocity of the fiber at the point at which tension is first applied to the extrusion velocity of the solution stream. The fiber may be drafted according to the present invention by means of force applied to the solution stream, such as by rotating reel 39 in Fig. 1 at a faster rate than the extrusion speed of the solution stream, or by other means known in the art.

It is critical to the process of the invention that the solution stream be drawn or drafted while in the noncoagulating fluid so that the polymer in the extruded stream acquires a relatively continuous, regular lengthwise orientation, or substantially "all skin" morphology, as evidence by the structure of the fiber upon coagulation. For aramids, fibers resulting from the process should have a spin draft of at least 15. However, spin drafts of at least 50 are preferred, with drafts of at least 100 being more preferred. Spin drafts of at least 400, and more preferably at least 500, are particularly preferred, while aramid fibers may have spin drafts of 1,000 or more consistent with the invention. The maximum draft for the process of the invention will depend on a variety of factors, such as the orifice diameter, fiber diameter desired, extrusion rate, and solution viscosity. However, most aramid fibers will have spin drafts of less than 2,500, with many improved aramid fibers being produced using spin drafts of 2,000 or less.

The coagulating bath may employ an organic fluid or a water-based fluid to coagulate the fiber. These fluids are generally liquid. Organic coagulating baths may be based on methanol, methylene chloride, N,N'-dimethyl- formamide, N,N'-dimethylacetaminde, ormixtures thereof as known in the art. However, water-based coagulating baths are usually preferred for aramids, and may consist essentially of pure water or may include significant amounts of other organic or inorganic components such as, for example, sulfuric acid, ammonium hydroxide, salts such as sodium chloride, calcium chloride or potassium carbonate, methanol and ethylene glycol. Many suitable coagulating baths are known in the art and may be formed readily using known techniques from commercially available components.

It is preferred that the coagulating bath be maintained at a temperature significantly below the temperature at which the solution stream is spun from the spinneret, so that the speed of coagulation is reduced sufficiently to permit further stretching (such as 2-4 times) of the polymer in the coagulation bath. It is in part for this reason that the temperature of the coagulation bath for aramids preferably is maintained at about -25^βC to about 50°C, with bath temperatures of about 0'C to about 25"C being more preferred.

Once formed the fiber may be subjected to post- coagulation processing. Many post-coagulation processes are known in the art, and may include washing to remove solvent, treatment at relatively high temperatures, and further stretching. The present invention is also directed to the product of she process disclosed above, which may result in fibers having superior properties, particularly where compressive attributes are concerned, and to rigid rod polymer fibers as a composition of matter. These fibers may be produced by the process described above, and are preferably made from polyaramids or polybenzazoles as described above in relation to the process of the invention. Fibers made from polyaramids, and from p-phenyleneterephthalamide polymers in particular, are most preferred.

Fibers of the present invention may be distinguished from conventional fibers known in the art in that fibers of the invention have a predominantly highly oriented or "skin" morphology throughout the fiber, as opposed to having a highly oriented skin surrounding a less oriented core. The presence of skin versus core structures in a fiber may be determined by means known in the art.

It is preferred that at least 70% by cross-sectional area or by volume of the fibers of the invention exhibit a high degree of lengthwise orientation or "skin" morphology. Fibers having at least 80% by area or by volume, and more preferably at least 90% by area or by volume, highly oriented or "skin" morphology are preferred, while fibers may comprise 95% or more "skin" structures consistent with the invention.

The process, product by process and composition of the present invention may also be understood by reference to the following illustrative examples.

SPECIFIC EMBODIMENTS

The relatively poor compressive strength of many rigid rod polymer fibers may be caused, at least in part, by the tendency of such fibers to split into fibrils when force is applied.

Fig. 4 is a photograph of a KEVLAR-49 brand fiber, a polyaramid fiber obtained from the E.I. du Pont de Nemours and Company in Wilmington, Delaware, after the fiber had been broken by the application of tensile force. Fig. 5 is a photograph of a VECTRAN brand fiber, a rigid rod polymer fiber obtained from the Celanese Company, after the fiber had been broken by tension using the same procedure as used to break the fiber in Fig. 4. Fig. 6 is a photograph of a fiber of the present invention, designated "ARAMID-K", which was an aramid fiber produced using the process of the invention and which was broken by tension using the same procedure as that used to break the fibers shown in Figs. 4 and 5. The photographs in Figs. 4, 5 and 6 were taken at lOOx magnification under a light microscope. As is readily apparent from Figs. 4 and 5, both the KEVLAR-49 and VECTRAN fibers split into fibrils. However the ARAMID-K fiber of the invention depicted in Fig. 6 showed very little fibril formation.

Although not wishing to be bound by theory, it is believed that the majority of the compressive and tensile strength of conventional rigid rod polymer fibers is contributed by the fiber skin, with only relatively small contributions being made, if at all, by the fiber core. It is believed the improved properties of the fibers of the invention are due to fibers having a highly oriented or "skin" morphology throughout the fiber (although relatively small, unsubstantial amounts of less oriented or "core" material may be present consistent with the invention) so that, for fibers of the same diameter, fibers of the invention have a higher proportion of force bearing structures. For example, Fig. 7 is a picture taken by a scanning electron microscope, at a magnification of 2500x, of the cut ends of a multifilament, KEVLAR-49 brand aramid fiber. Fig. 8 is a picture taken by a scanning electron microscope, at 1300x magnification, of the cut end of a single filament aramid fiber produced according to the process of the invention. The cut ends of both fibers are traversed by approximately parallel lines, believed to be caused by cutting of the fibers. However, the cut ends of the KEVLAR fiber filaments in Fig. 7 show a dark central region surrounded by a bright annulus, believed to be indicative of different electron densities caused by a difference in fiber morphology between the two areas. The relative darkness of the fiber end in Fig. 8 is a function of the electron density used to obtain the picture, akin to the light level used to take a picture using an optical microscope. However the fiber end in Fig. 8, except for contrast caused by the cut marks, is virtually all the same shade, which is believed to indicate that the fibermorphology is relatively uniform across the cut end of the fiber.

Fig. 9 is a picture taken by a scanning electron microscope, at a magnification of 6000x, of the broken end of a single filament of a KEVLAR-49 brand fiber, obtained by stretching the fiber until it split into filaments, after which the filament was broken under tensile force. This filament was broken using relatively fast deformation, which caused the skin part of the filament to explosively split into fibrils reselling ribbons. The skin appears at the end to have neatly separated from the core, suggesting that the adhesion between skin and core was not strong. The diameter of this filament was measured to be 14.4 microns, while the core was measured to be 12.5 microns wide. The percent area of the filament which had a skin structure was calculated from these measurements to be 24.6%.

Fig. 10 is a picture taken by a scanning electron microscope, at a magnification of 900x, of the end of a fiber which was broken by tension using the same procedure as the fiber in Fig. 9. This fiber was obtained by dissolving KEVLAR 49 in a solvent and spinning the solution according to the process of the invention to obtain a fiber consistent with the invention. Fig. 10 shows that, upon breaking, the end of the fiber of the invention split, but did not break into small fibrils like the KEVLAR-49 fiber shown in Fig. 9. No core structure is shown in Fig. 10.

Additional experiments were performed to demonstrate various aspects of the process of the invention, either by embodying the invention or by providing a basis for comparison. In all of the following examples the rigid rod polymer resins were formed into fibers according to a dry jet-wet spinning process as described by U.S. Patent No. 3,767,756, which process is modified by the present invention as described above and below.

EXAMPLES 1-4 In Examples 1-4 an extrudable dope of a rigid rod polymer in a solvent was prepared by mixing an aramid with a concentrated mineral acid according to conventional means. The dope comprised 12% by weight of commercially available KEVLAR-49 brand aramid fiber in concentrated (98%) sulfuric acid. The dope was extruded at a rate of 0.5 cc/min. with the temperature of the spinneret and the spinning tube both set at 85"C. The spinning tube was 10 cm. long. The spinneret diameter was different for each of the four examples while the extrusion rate was kept the same. Therefore the speed of extrusion (jet velocity) was different for each spinneret diameter as shown in Table I.

For all examples fibers were spun through a first fluid and then through a second fluid, followed by winding or reeling of the spun filament on a bobbin. The first fluid is generally termed in the art as being a noncoagulating fluid and the second is termed a coagulating fluid. In the present examples the purpose of the first fluid is to provide a zone whereby a filament may be drawn to the desired degree of draft; a purpose of the second (coagulating) fluid is to further stretch the fiber and maintain as well as possible the fiber draft or orientation created during drafting and stretching.

In Examples 1-4 the first fluid was air having a temperature of 85*C; the second fluid was water having a temperature of about 20"C. The apparatus and procedure was similar to that described above with respect to Fig. 1, with the spin draft being based on the speed of the roller corresponding to 39, and a 3-4 times stretch being assumed to occur in the coagulating bath. It should be understood that the first (noncoagulating) fluid extends from the spinneret opening through the length of the spinning tube (10 cm for each of Examples 1-4) . The distance between the bottom of the spinning tube and the surface of the second fluid (water) is beneficially kept to a minimum and for all of the examples was about 2 cm.

The maximum draft of the fiber is reported in Table I and is measured by placing a reel below the spinning tube and above the coagulating bath so that the fiber is wound onto the reel instead of entering the bath. The speed of rotation of the reel is progressively increased while maintaining the extrusion rate constant until the fiber breaks. The speed of the reel at breakage of the fiber is used to calculate the draft of the fiber, which is designated the fiber's "maximum draft".

Afterthedrawn, substantiallyuncoagulated fiberleaves the spinning tube it is conveyed through a coagulating fluid, which in Examples 1-4 is a tank of circulating water held at about 20"C. While in the coagulating bath the fiber is stretched, removed from the water bath by using a snag pin or roller and wound on a reel or bobbin. The fibers in the examples were subjected to an approximately 4:1 stretch through the coagulation bath as measured by the velocity of the fiber on the take up bobbin relative to the velocity of the fiber entering the coagulation bath. The bobbin take up speed was adjusted to obtain a fiber having a diameter of approximately 16 microns (2 denier) . This take up speed of about 50.6 meters per minute was held constant, as was the extrusion rate, for Examples 1-4. The fibers were then washed and heat treated at 500^βC for 6 seconds.

Referring now to Table I the effect of varying the diameter of the spinneret orifice was examined with the extrusion speed being as reported. Maximum draft and total spin draft for each example was calculated as reported. Physical properties of the spun fibers after coagulation were measured. The tensile strength and tensile modulus were measured according to ASTM Method D 3379 - 75, "Standard Test Method For Tensile Strength And Young's Modulus Of High Modulus Single Filament Materials", (1975), and reported in Table I.

As shown by the data reported in Table I, the larger the diameter of the spinneret the greater the orientation imparted to the fibers produced, with a concomitant improvement in physical properties such as tensile strength and tensile modulus.

TABLE I

EXAMPLES 5-8 Examples 5-8 were conducted to demonstrate the effect of spinning tube length on spin draft. In the following Examples 5-8 the procedures and parameters described in Examples 1-4 were followed except as noted below. The equipment, coagulating and noncoagulating fluids, temperature, aramid polymer resin and resin solvent were as described for Examples 1-4. For Examples 5-8 the spinneret orifice diameter was 3 mm and the extrusion rate was 0.5 cm³/min.

The spinning tube was as for Examples 1-4 except that tubes of varying length were used as shown in Table II. No tube was used for Example 8. The speed of the take up bobbin was set to provide a final coagulated fiber having a diameter of about 16 microns for Examples 5-6 and about 17.5 for Example 7.

For Example 7 and particularly Example 8, a 16 micron diameter fiber could not be obtained as the maximum draft was too low to permit sufficient drafting of the fibers to obtain a diameter of 16 microns. Use of the large diameter (3 mm) spinneret orifice with a short spinning tube length (3 cm for Example 7) or without any spinning tube (Example 8) caused the extruded dope to cool down too soon, thereby substantially increasing the viscosity of the extruded stream and reducing the obtainable draft. For Example 8 the extruded dope was cooled before any significant draft could be imparted in the air gap (noncoagulating fluid) . This limited the draft of Example 8 to drafting after cooling, which substantially reduced the draft obtained. The speed of the take up bobbin had to be slowed for Examples 7 and 8 to form fibers and avoid breakage. This slowing resulted in a spun fiber diameter for Examples 7 and 8 of 17.5 and 62 microns, respectively, following coagulation and a reduced total draft including a substantially reduced total draft for Example 8 of 184 relative to Examples 5, 6 and 7. Physical properties of the fibers were examined and the fibers having a high draft had greater tensile strength and modulus values relative to Example 8. Therefore, use of the spinning tube is shown by the examples to be very important for spinning of small diameter fibers when using a large diameter spinneret orifice, due to the fact that the duration time of the extruded dope near the orifice is very long compared with processes using spinnerets having smaller diameter orifices, with the long time contributing to cooling of the dope due to the ambient temperature of the noncoagulating fluid. This cooling in the noncoagulating fluid reduces the maximum draft obtainable and therefore, the overall spin draft. The reduction in fiber orientation has a deleterious effect upon the physical properties of the spun fiber such as tensile strength and tensile modulus.

For a 3 mm diameter spinneret orifice with an extrusion rate of 0.5 cc/min the duration time of the extruded dope from the surface of the spinneret orifice to 1 cm away from the surface is about 1 second, while for a 0.05 mm spinneret orifice and a 0.12 cc/min extrusion rate the duration time is about 0.01 seconds. Therefore according to the teachings of U.S. Patent No. 3,767,756 the preferred air gap between the spinneret orifice^' and the coagulating bath is between about 0.5 to 1.0 cm, which does not provide enough time to substantially draft the extruded dope.

TABLE II

EXAMPLES 9-13 In Examples 9-13, the effect of extrusion rate on spin draft was demonstrated. Examples 9-13 followed the procedures and parameters of Examples 1-4, except as noted below. The polymer concentration, solvent, noncoagulating fluid, coagulating fluid, spinneret temperature, spinning tube length and temperature were all as described for Examples 1-4. The spinneret orifice diameter was 3 mm. The extrusion rate was varied as reported in Table III. Physical properties were measured and the tensile strength and tensile modulus are reported below in Table III.

The extrusion rate was varied from 0.3 to 1.5 cc/min for a spinneret orifice of 3 mm in diameter, and fibers which were approximately 16 microns in diameter were obtained. As shown in Table III, the maximum draft varied slightly, but not substantially. The extrusion speeds varied because the orifice diameter was fixed at 3 mm and therefore the take up reel (bobbin) speed was adjusted for each example so that the overall spin draft did not vary. The reported physical properties were similar. All extrusion rates of Examples 9-13 are suitable, with extrusion rates of about 0.5 to about 1 cc/min. being preferred. Especially preferred for increased production is an extrusion rate of about 1 cc/min.

TABLE III

EXAMPLES 14-17 In Examples 14-17, the effect of the temperature of the spinning tube (and hence the noncoagulating fluid) on spin draft and physical properties of the resulting fiber was demonstrated. Examples 14-17 followed the procedures and parameters of Examples 1-4, except as noted below. The polymer concentration, solvent, extrusion rate, noncoagulating fluid, coagulating fluid, spinneret temperature, and spinning tube length were all as described for Examples 1-4. The spinneret orifice diameter was 3 mm. The temperature of the spinning tube was varied as reported in Table IV, with the temperature of the noncoagulating fluid being maintained as closely as possible to that of the spinning tube. Physical properties were measured and the tensile strength and tensile modulus are reported below in Table IV. The spinning tube temperature should be such that a rapid cooling of the extruded solution stream, as occurs in Example 17, should be avoided, as cooling causes an increase in viscosity which, if sufficiently great, may reduce the amount of draft which may be obtained. In Examples 14-16 the take up speed was adjusted so that fibers of comparable diameter were obtained. However, the draft of the fiber in Example 17 was the maximum obtainable under the spinning conditions.

TABLE IV

EXAMPLES 18-21

In Examples 18-21, the effect of the temperature of the spinneret on spin draft and physical properties of the resulting fiber was; demonstrated. Examples 18-21 followed the procedures and parameters of Examples 1-4, except as noted below. The polymer concentration, solvent, extrusion rate, noncoagulating fluid, coagulating fluid, spinning tube temperature, and spinning tube length were all as described for Examples 1-4. The spinneret orifice diameter was 3 mm. The temperature of the spinneret was varied as reported in Table V. Physical properties were measured and the tensile strength and tensile modulus are reported below in Table V.

The effect of spinneret temperature changes on fiber properties is relatively small. For the aramid and solvent systems used an upper limit preferably would be about 90"C due to volatilization of the solvent, while a lower limit of 70^βC would be preferred due to gelling of the spinning solution. TABLE V

EXAMPLES 22-24 In Examples 22-24, the effect of the concentration of aramid resin in the spinning dope on spin draft and physical properties of the resulting fiber was examined. The data for Example 24 came from product brochure E46813 regarding KEVLAR-49, obtained from E.I. du Pont. This brochure did not state how the KEVLAR-49 fiber was produced or tested. Examples 22-23 followed the procedures and parameters of Examples 1-4, except as noted below. The solvent, extrusion rate, noncoagulating fluid, coagulating fluid, spinneret temperature, spinning tube temperature, and spinning tube length were all as described for Examples 1-4. The spinneret orifice was 3 mm. The concentration of polyaramid in the spinning dope was varied as reported in Table VI. Physical properties were measured and the tensile strength and tensile modulus are reported below in Table VI.

Comparing Examples 22 and 23 of the invention it is demonstrated that higher concentrations produce fibers with higher tensile strengths and modulus when fibers are otherwise made by the same process of the invention. The tensile modulus values for both examples of the invention are significantlyhigher than for the reported value for prior art KEVLAR-49. Concentrations of about 6-15% are suitable for the method of the present invention and it is expected that fibers may also be formed from solutions of lesser and greater concentration.

TABLE VI

In view of the above examples, the written description and the drawing, different embodiments, modifications and changes will be apparent to those skilled in the art and all suchmodifications, embodiments, and changes are deemed to be within the scope of the invention as defined by the following claims.

Claims

I CLAIM:

1. An improved fiber forming process comprising extruding a solution containing a rigid rod polymer through a spinneret orifice and into a noncoagulating fluid to form an extruded solution stream, imparting an extensional stretch to said extruded stream while in said noncoagulating fluid, and passing said stream through a coagulating fluid so that said stream is coagulated to form a fiber having a predominantly skin morphology.

2. The process of claim 1 wherein said polymer is a polyaramid.

3. The process of claim 2 wherein said solution is extruded through a spinneret having at least one orifice of a diameter of at least 0.25 mm, and said extensional stretch is sufficient to produce a spin draft of at least 15 in said fiber.

4. The process of claim -2 wherein said orifice has a diameter of 0.25 mm to about 6 mm.

5. The process of claim 4 wherein said orifice has a diameter of about 1 mm to about 4 mm.

6. The process of claim 2 wherein said solution stream passes through at least 5 cm of said noncoagulating fluid.

7. The process of claim 6 wherein said solution stream passes through at least 10 cm of said noncoagulating fluid.

8. The process of claim 6 wherein said solution stream passes through 50 cm or less of said noncoagulating fluid.

9. The process of claim 2 wherein said fiber has a spin draft of at least 100.

10. The process of claim 2 wherein said solution is extruded through said orifice at a rate of about 0.1 cm³/min. to about 3 cm³/min.

11. The process of claim 1 wherein said solution has a viscosity at the temperature of the spinneret orifice of about 100 poise to about 15,000 poise.

12. The process of claim 2 wherein said solution is extruded at a temperature of about 70°C to about 100°C.

13. The process of claim 2 wherein said noncoagulating fluid is maintained at a temperature of at least 40^βC.

14. The process of claim 2 wherein said solution has a polymer concentration of 20% by weight or less.

15. The process of claim 2 wherein said polyaramid is a polymer of p-phenyleneterephthalamide.

16. The process of claim 1 wherein extruded stream is stretched while undergoing coagulation.

17. The product of the process of claim 1.

18. A rigid rod polymer fiber, wherein said fiber has a predominantly skin morphology.

19. The rigid rod polymer fiber of claim 18, wherein at least 70% by volume of said fiber has a skin morphology.

20. The rigid rod polymer fiber of claim 19, wherein at least 80% by volume of said fiber has a skin morphology.

21. The rigid rod polymer fiber of claim 18 wherein said polymer is a polyaramid.