BACKGROUND OF THE INVENTION 1. Field of the Invention This invention resides in the field of polymeric fibers and processes for their manufacture.

2. Description of the Prior Art The synthetic fiber industry has evolved over the years into a high volume mature industry world-wide. Innovations in polymer chemistry as well as advances in equipment design and production have been made to meet consumer expectations and fabric design applications.

Despite such growth, acrylic fiber based on non-thermoplastic acrylonitrile (AN) monomer, has shown little growth over the past several years. The properties of acrylic fiber are unique, but producers are finding it more difficult to persuade buyers to use acrylic fiber due to the higher price of acrylic resin compared to many other resin types. The reason for the high price of acrylic is its higher cost of production.

Acrylic fibers are made by preparing the resin in a polar solvent, pumping the resulting"liquor"through a spinneret having many holes of the desired fiber diameter, then separating the fiber from its syrup by either dry spinning using a hot air chamber or wet spinning by coagulating the fiber in a solvent tank and drawing the wet fiber from the tank bath, followed by drying. In either method, the process is rather slow, and requires recovery of the solvent for reuse in subsequent batches for economic reasons. Normally, acrylic resins have an acrylonitrile (AN) monomer content of about 85%, the remainder consisting of one or more"neutral"co-monomers such as ethyl acrylate or vinyl acetate.

These co-monomers are useful in forming coloring dye sites and in improving the hot melt tensile strength of the fiber to prevent breakage of the fiber as it emerges from the spinneret. Solvent and unused reactant liquor are recovered, and the solvent is purified and reused in subsequent batches.

Because acrylic fibers lose solvent during production, the circular profile of the fiber collapses into various other shapes such"dog bone", boomerang, etc., and the departing solvent leaves microscopic pores in the fiber which allow the fiber to"breathe" or to absorb moisture. This gives makes the fiber a"handle"or feel that is very similar to natural wool. Acrylic fiber has a high resistance to cleaning agents and to ultraviolet light, as well as high temperature tolerance and good tensile properties. Because of these qualities, acrylic fiber is an excellent blending fiber in wool blends and a good substitute for wool. Acrylic fiber is also unique as a precursor for PAN (polyacrylonitrile) carbon fiber. PAN carbon fiber sales are expected to surpass 90 million pounds in the year 2000.

PAN carbon fiber has found many high-technology uses, especially as reinforcements for cement in construction, and as materials in sporting goods, aircraft tail empanage and other critical body panels. PAN carbon fiber is fulfilling a host of applications in aerospace, in Texas tower oil rigs to deeper depths, and in other new concepts. PAN carbon fiber has 10 times the strength of steel, pound for pound. It is believed that if the price of PAN carbon fiber could be lowered to less than $5.00 per pound, the fiber would offer an exceptional growth potential to the industry.

SUMMARY OF THE INVENTION It has now been discovered that synthetic fibers can be manufactured by extruding a polymer melt into a sheet, passing the sheet between a pair of rollers whose contacting surfaces are contoured with circumferential ridges to form longitudinal grooves on each side of the sheet, then stretching the grooved sheet longitudinally with the effect of deepening the grooves and drawing the sheet apart at the grooves with the result that grooves on opposing sides of the sheet join and the sheet is separated at the grooves into a plurality of fibers. The invention is particularly well suited to the formation of fibers from polyacrylonitrile, but applicable to other polymeric materials as well, particularly non-thermoplastic polymers that are prepared with solvents that must be removed in the fiber extrusion process.

This invention offers the fiber industry an alternative to using a spinneret, especially for non-thermoplastic polymers that are dissolved in a solvent prior to extrusion into fibers. This invention also offers a lower cost means of production of solvent-based polymer types of fibers, of which acrylic fibers are one example. For acrylic fibers, the invention offers a means of producing PAN fibers with very high levels of AN monomer (as high as to 99.6% AN monomer, for example) without the need for neutral co-monomers which impede the oxidation and carbonization time when the PAN fibers are converted to PAN carbon fibers. The invention is also useful in the production of very high nitrile fibers at high production rates and with infinite controls and yet with a minimum of fiber breakage during production. In certain embodiments, this invention provides for changing the fiber diameters without the need to shut down the production line. The invention also allows for much greater freedom of control over the production rate and the desired degree of tensilizing, as compared with the prior art, and permits production to occur in a non-toxic plant environment. A PAN polymer can be used that has been cleaned of any residual polymerizing liquor, then washed and dried prior to dry blending with its non-toxic fugitive plasticizer. A further advantage of this invention is that during the shaping of the polymer web by the rollers that form the grooves, the pressure exerted on the web by the rollers condenses the fibers. This reduces or eliminates microvoids in the fibers, thereby producing a high quality fiber for any application.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of a fiber fabrication unit in accordance with this invention.

FIG. 2 is a front elevation view of a pair of rollers that form part of the unit of FIG. 1 and whose function is to impress grooves into the moving polymer melt sheet.

FIG. 3 is an end view of a roller mount with three sets of rollers for use in the unit of FIG. 1.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS While this invention is susceptible to a wide range of embodiments and implementations, the attached figures are directed to certain specific embodiments, an

examination of which will promote an understanding of the concepts and features of the invention as a whole.

FIG. 1 is a schematic drawing of a fiber processing unit. The first component of the unit is a sheet extrusion line 11 which extrudes the polymer melt through a slot die 12 into a continuous melt-fused polymeric sheet or web 13. The sheet proceeds through a standard gauge control/chill roll stack 14, with an optional third bottom roll 15, followed by a guide roll 16 to provide accuracy in feeding the sheet into the downstream components. The sheet then enters a heated chamber 17 where the sheet passes through a pair of groove-cutting rolls 18,19 that are contoured as described above to form grooves in the sheet. (FIG. 2, described below, presents a more detailed view of these rolls.) The grooved sheet then proceeds into a series of rolls 20 that perform tensilizing and stretching of the sheet, which cause the sheet to divide into fibers 21.

The heated chamber 17 exposes the sheet to an elevated temperature, preferably one that increases either continuously or in a stepwise manner along the direction of movement of the sheet. When the temperature increase is stepwise, the sheet passes through a series of at least three, and if desired, four zones of successively increasing temperature. For example, a three-zone temperature arrangement may consist of a first zone at 120-125°C, a second zone at 150-160°C, and a third zone at 180-200°C.

When four temperature zones are used, examples of suitable temperatures are 80-120°C for the first zone, 120-140°C for the second, 140-160°C for the third, and 160-200°C for the fourth. While FIG. 1 shows only a single chamber housing, multiple successive chambers or ovens can be used for the successive temperature zones.

When exposed to these elevated temperatures, the solvent included in the polymer melt is removed by volatilization, and a fugitive plasticizer when present is removed as well. The optimal temperatures in any particular application will depend on the nature of the polymer, the running speed, the desired rate of stretch, and similar production factors, all of which will be readily apparent to those skilled in the art. The resulting fibers 21 are then transferred to further units for such options as crimping, dyeing, cutting for yarn making, or other desired fiber processing.

FIG. 2 is a front view of the groove-cutting rolls 18,19, with their roller bearing assembly 31. The edges 32,33 of the rollers are wide enough to contact each other to set the spacing of the cutter teeth 34,35 that form the grooves in the two sides of the sheet. The cutter teeth 34,35 are shown in a magnified view in the insert. The

cutting surfaces of the two rollers are spaced apart 36 by a preselected distance which determines how deep the grooves that are formed by the cutting teeth will be cut. The result is a contiguous thin but grooved sheet (or web) with a parallel row of grooves extending the width of the groove-cutting rollers. The continuity of the sheet reduces the risk of breakage at high running speeds. In the embodiment shown in FIG. 2, each contoured surface of the rollers has a profile in the shape of a row of arcs or circles, which will result in fibers of circular cross section. Alternative contours may be used as well, to form fiber of hexagonal cross section, hour-glass cross section, or other shapes.

FIG. 3 illustrates a particular embodiment of the invention in which the groove-cutting rollers 18,19 are mounted on rotatable supports 41,42 to which are also mounted two other pairs of groove-cutting rollers 43,44,45,46, each pair having cutting teeth of different dimensions or spacings for use as alternatives to the first pair 18,19.

Any matched pair can be swung into cutting position by rotating the supports on their axes 47,48 while the sheet 49 continues to run. A change in the diameter of the fibers can therefore be completed without shutting down the production line. Sheet extrusion thickness and speed (running feet per minute) can be computer-controlled and can also be changed by simply rotating a different pair of cutting rollers into position. Although only three pairs of rollers are shown, any number of pairs can be included on a single pair of rotating supports.

This invention provides a high-speed, non-toxic means of producing synthetic fibers from extruded flat polymeric sheet rather than using a spinneret to form such fibers. The invention is especially useful for the production of fibers made of acrylic polymers and other polymers, which are fabricated by dissolving the polymers in a suitable solvent to form a thick"syrup,"then melt fusing the syrup into a flat sheet having the thickness of the diameter of the fiber being made. The polymer solution or melt may be of any composition known or used in prior art processes involving spinnerets. An example of one such melt composition is that disclosed in U. S. Patent 5,304,590, the disclosure of which is incorporated herein by reference.

In the process described in U. S. Patent No. 5,304,590, a polyacrylonitrile (PAN) polymer is formed by polymerizing a mixture containing 99.6% acrylonitrile (AN) monomer and 0.4% chain extender/cross linking agent (all by weight). The resulting polymer is collected from the polymerizing solution, washed several times and rinsed of the washing agents, then dried to form a very fine white powder. In some processes, a small amount of water is added to the dry blend to produce micropores.

The PAN resin is then dry blended with ethylene carbonate, which is referred to as a non-toxic fugitive plasticizer because it leaves the product through volatilization by the processing heat. The dry blend is 60% PAN to 40% plasticizer, both by weight. At this stage, certain desired additives may be included such as flame retardants, coloring dyes, ultra violet inhibitors, and the like. No neutral co-monomers are used, nor are any metallic ions or sulfonic acid groups which can cause longer periods of time to oxidize and carbonize such fibers in converting the PAN fibers to PAN carbon fibers.

To use the dry blend prepared in this manner in the present invention, the dry blend is melt fused in a low shear compounder/extruder at standard production rates, and extruded either directly through a slot die to form the desired sheet, or through a pelletizing unit for re-extrusion at a later time or other location. When re-extrusion is performed, it can be accomplished by a single screw extruder with a standard work screw such as that used for polypropylene polymers. The extrusion die is held at a relatively low temperature to provide a smooth, clear polymer melt.

The extruded polymer sheet is fed through a two-or three-roll standard sheet take-off roll stack 14, as shown in FIG. 1, which assures exact thickness and uniformity control, while optimizing the temperature of the sheet for tensilizing. After passing through a guide roll for perfect sheet alignment, the sheet enters the heated oven 17 as described above. Control of the temperature in the various zones allows for choosing a balance of settings for optimum tensilizing with respect to throughput speed, fiber diameter, degree of stretch desired, rate of extraction of the fugitive plasticizer from the fibers, and residual plasticizer in the final fiber, if any.

The roller-cutter pair 18,19 consists of rolls whose surfaces are face- ground and contoured to approximate the forms of the top and bottom, respectively, of a fiber profile by forming parallel grooves on each side of the sheet. Each contoured surface preferably forms a row of arcs of circles to approximate fibers of circular profile.

The grooves are formed by ridges on the roller-cutter faces that press into the top and bottom surfaces of the sheet, each ridge preferably impressing a groove to a depth of at least 40% of the sheet thickness. This leaves thin connecting portions (at the base of each groove) between fiber areas, each portion having a thickness of about 20% of the original sheet thickness. These connecting portions prevent fiber breakage until the tensilizing stage, which increases the fiber tensile properties by aligning their molecular chains. The

applied heat contributes to an increase in tensile properties as well by causing fugitive plasticizer present in the melt to escape by volatilization.

The contoured surfaces of the roller-cutter pair 18,19 will also contribute to the character of the fibers being formed. For example, the use of highly polished surfaces will result in fibers with a shiny appearance, whereas the finishing of the contoured surfaces with a higher grit grinding stone will produce of fiber with a matte finish.

The machined lips or flanges at the ends of roller-cutters (32,33 of FIG. 2) are in constant running contact with each other during operation of the rollers, and their machined thicknesses establish the thickness of the fiber profiles and the depths of the grooves. These flanges also encase the edges of the polymer web to prevent loss of polymer laterally across the roller edges. Optionally, a sheet edge trimmer can also be used at each edge, such trimmers being used with most conventional sheet casting equipment, to insure the integrity of the fibers formed at each edge. Other means to achieve the same result can also be used, such as the end bearing design arrangements commonly used in sheet casting or calender roll assemblies.

The groove-cutting rollers are preferably idler rollers, i. e., those that require no power of their own for rotation. These rollers can also be operated with no internal heat source, as their desired temperature can be maintained through direct contact with the sheet that they emboss and from the oven heat as well. Once the system is running, the temperature will reach a steady state, and can be controlled by the oven temperature for optimization. Control of the temperature of the fibers emerging from the heated chamber 17 (FIG. 1) for purposes of further tensilizing to be performed downstream can be achieved by appropriate selection of the temperatures of the successive temperature zones in the oven. The PAN polymer described above will withstand a final heat quench temperature of 205°C (400°F), which will set the fiber for optimal dimensional stability and UV resistance, and will achieve full crosslinking within the polymer structure while establishing crystalline domains for permanence.

Tensilizing the partially sliced sheet in the heated chamber 17 narrows the diameter of each formed fiber, which in turn causes each fiber to separate from its adjacent fibers. For example, a fiber diameter of 0. inch, when tensilized by 10 times will have a diameter of only 0.001 inch.

When the temperature in the heated chamber is increased by stages in the direction of movement of the sheet, as described above, partial removal of the plasticizer occurs in the first stage together with the onset of stretching, and further removal of plasticizer as well as further stretching of the fiber occur progressively in the succeeding temperature stages. Crosslinking also occurs as the temperature rises, increasing the tensile properties of the fibers even more. The last and highest heating stage imparts high dimensional stability to the fibers.

Very high nitrile PAN fibers that are made in accordance with this invention will serve as excellent precursor fibers for the production of PAN carbon fibers.

As more of the bulk weight of the fibers is converted into pure carbon fiber, there is less material that must be burned out of the fibers during oxidization and carbonization. In addition, PAN polymer fibers made in accordance with this invention are rapidly oxidized and carbonized, a feature which reduces production costs and thereby contributes to lower prices which will attract higher sales volume.

When PAN polymers are crosslinked with a crosslinking agent and the application of heat, the molecular chains of the polymers join more tightly together while their molecular weight increases. This conversion enhances the formation of the fibers by helping to separate the grooved web into fibers along the thin connecting portions at the base of each groove at tensilizing temperatures, and reducing or eliminating the occurrences of transverse breakages in the individual fibers.

The groove-cutting rollers may be mounted on a rotating support (FIG. 3) to equip the apparatus with more than just one set of such rollers. This provides the operator with a choice of fiber diameter profiles, and allows for a quick fiber diameter change-over without shutting down the production line. By such action, a great deal of time is saved by simply rotating a different set of groove-cutting rollers into place"on the fly."The sheet thickness to accommodate a different fiber diameter cutter can be controlled by computer technology available for modem sheet extrusion lines to tolerances of 0.0002 inch deviation.

Tensilization of the sheet in the formation of the fibers in the practice of this invention can be done at conventional tensilization pressures. The selection of particular pressures or pressure ranges will vary with the speed, temperature, solvent, polymer, and final fiber diameter, and the selection of the appropriate pressure for any particular set of conditions and materials is well within the routine skill of the experienced fiber technician.

The process and apparatus of the present invention can also serve to provide a"back pressure"upstream from the tensilizing steps of production. This is especially useful for low tensile type polymers having a cross linking agent. This back pressure may permit greater control over tensilizing operations that utilize very rapid stretching, a distinct advantage for low tensile-type polymers. High stretch rates can also benefit in the production of micro fibers from such types of polymers, as they allow for higher ratios of stretching.

The foregoing descriptions are offered primarily for purposes of illustration. Further modifications, variations, and embodiments that will be readily apparent to those skilled in the art are included within the scope of this invention.