PROCESS AND APPARATUS FOR CRIMPING FIBERS

Abstract: An apparatus for crimping and permanently heat setting an inorganic or polymeric precursor fiber (18) comprising a mechanical crimping means (13), a conveying means (11) which receives the crimped fiber (18), and a heating or irradiation zone (16A, 16B) through which the conveying means (11) and fiber (18) pass without applying stress or tension to the fiber.

Description: PROCESS AND APPARATUS FOR CRIMPING FIBERS The present invention is directed to a process and an apparatus for producing nonlinear permanently heat set polymeric fibers without imparting stress or tension to the fibers. More particularly, the invention relates to an apparatus and a process for providing a loop, coil or sinusoidal configuration to polymeric precursor fibers by heat treating the fibers without subjecting the fibers to stress or tension either before or during crimping. The process and apparatus of the invention is relatively inexpensive and simple and does not require the prior formation of a knitted fabric. The apparatus is especially useful to produce crimped fibers utilizing a multiplicity of precursor fibers of from 40,000 to 320,000 fibers (40K to 320K) which are assembled in the form of large size tows. The crimped fibers formed by the present invention when dyed possess good dye uniformity. The term "crimp" as used herein can be defined as the nonlinearity or waviness of a fiber expressed as crimps per unit length. For most of the man-made fibers employed in carpet manufacture, the crimp or bend in the fiber is induced by thermal/mechanical techniques, e.g. a gear crimping mechanism or a stuffer box technique. The crimping of fibers is important in the manufacture of carpets because it provides bulk to the fibers by preventing two or more fibers from lying parallel to one another. As a result, the tufts of a carpet have greater covering power, appear softer, and provide greater resistance to wear and abrasion, among other benefits. Crimping is also useful in the processing of 10 staple fibers and in the processing of high modulus fibers which are difficult to work with because of slipperiness. The crimp which is placed in most fibers, using 15 a stuffer box, is rarely uniform. The stuffer box technique produces fibers having a wavy, random zigzag type crimp which is V-shaped having sharp bends or kinks. The randomness of the crimp which is obtained 20 causes the fibers to have a non-uniform crimp. However, when several fibers are viewed, it can be seen that the crimp produced by this method is regular or consistently irregular. ry Crimping of fibers in a stuffer box is achieved by passing the fibers into a uniformly heated chamber which is at the temperature required to heat set the fibers in their crimped or nonlinear configuration. As the fibers are forced into the chamber by feed rolls, 30 they are pushed against fibers which are already in the chamber, thereby causing the fibers to bend and buckle (crimp) . A weighted tube fitted into the top of the stuffer box governs the flow and quantity of fibers into the stuffer box. The frequency (crimps per unit length) and the crimp amplitude of the fibers are controlled by regulating the speed of the feed rolls to that of the take up rolls as well as the weight of the tube. Crimp setting by this techniques can be done for single fibers or tows having multiple fiber ends using a spunize technique. The crimps are generally characterized by numerous sharp bends in the fiber. In order to obtain a crimp in the fiber by present methods and apparatus, the fiber must undergo severe bending stresses. During bending two types of stress modes are simultaneously developed in the fiber. A tensile stress is developed along the outer curvature of the bent fiber, while a compressive stress is acting on the inner portion of the bend. A recent study of the effects of crimping on polyester fibers showed that severe bending as in a V-type crimp, can result in extensive fiber damage. Even when the fibers had a more rounded V-type bend, the fibers exhibited compression ridges on the underside of the crimp. Severely crimped fibers (with sharp V-type bends) therefore exhibit reduced mechanical properties due to weakness in the fiber created by tensile and, compressive forces operating within the fiber. Such fibers usually failed by breaking due to tensile and compressive forces operating within the fiber. It has also been found that fibers that are crimped in a stuffer box tend to take up dye preferentially on the underside of the bend and can be the cause for optical streaking. Such streaking occurs because the knee of the bend projects toward the surface of the fiber and hence is more visible to the eye. Since the underside of the fiber bend will contain more dye the affect is a darker streak in the fiber. At the same time, because the dye tends to concentrate at these points, the remaining portion of the fiber tends to be deficient in dye and therefore has a lighter colored appearance. It has been shown that crimp permanency after loading can differ between fiber producers and even among various types (e.g., bright and semidull) made by the same producer. Since the application of some tension on fibers inevitably occurs during normal fiber processing, it is to be expected that some loss in crimp definition will occur. This loss must be near identical from spindle to spindle, twister, to twister, etc., otherwise the fibers will appear to be different since crimped fibers differ in appearance from uncrimped fibers as a result of the reduced bulking factor. At the same time some fiber elongation is obtained during crimp removal which would tend to order the fiber microstructure. This could influence dyeing since a more ordered microstructure will take up dye differentially than fibers which have not undergone any elongation. European Publication No. 0199567, published October 29, 1990, of McCullough et al., discloses a method for preparing nonlinear carbonaceous fibers having physical characteristics resulting from heat treating stabilized polymeric precursor fibers in the form of a knitted fabric. There is described a process wherein the knitted fabric is substantially irreversible heat set under conditions free of stress and tension. In order to obtain individual fibers or tows, which are nonlinear, it is necessary to knit and then deknit the fabric. However, the knitting and deknitting of a fabric to obtain nonlinear fibers substantially increases the cost in producing the fibers. U.S. Patent No. 2,245,874 to Robinson, discloses a method for forming curled fibers by passing the fibers over cold rollers under conditions to bend and stretch the fibers beyond their elastic limits. Such a process cannot be used to produce the stress free, nonlinear fibers with the physical properties produced by the present invention. U.S. Patent No. 2,623,266 to Hemmi discloses the mechanical preparation of sinusoidal or spirally crimped fibers. The fibers are heated and passed through a series of bars which impart a meander-like crimp. However, the fibers are formed in a crimped and stretched state, i.e. a stress induced state. More particularly, the invention resides in an apparatus for crimping and heat setting at least one polymeric precursor fiber, comprising a crimping mechanism for providing said fiber with a temporary nonlinear configuration which is free of sharp bends and without the application of tension or stress on the fiber, and conveyor means for transporting the nonlinear fiber without the application of tension or stress through a heating zone to provide said nonlinear fiber with a permanent heat set. The invention also resides in a process for crimping and heat setting at least one polymeric fiber, comprising the steps of supplying a polymeric precursor fiber to a crimping mechanism operated at a temperature sufficient to impart a temporary nonlinear configuration to said fiber without applying stress or tension to the fiber, transporting said nonlinear fiber without the application of stress or tension through a heating zone for heating said fiber to a temperature sufficient to impart a permanent heat set to said fiber, and cooling said fiber. The invention further resides in a process for producing a nonlinear carbonaceous fiber, comprising the steps of supplying an oxidized polymeric precursor fiber having a diameter to a pair of crimping members at least one of which is a cylindrical gear member having apertures with rounded gear surfaces, heating at least one of said crimping members and introducing said fiber into the apertures of the gear member to provide the fiber with a nonlinear temporary heat set without applying stress or tension to the fiber, conveying the nonlinear fiber with the temporary heat set through a heating zone without applying stress or tension to the fiber, heating the fiber in an inert atmosphere in said unstressed condition to a temperature sufficient to impart a permanent set and to form a nonlinear carbonaceous fiber which is free of sharp bends or deformations, wherein said carbonaceous fiber has a diameter of from 4 to 20 micrometer, a reversible deflection ratio of greater than 1.2:1, and a carbon content of greater than 65 percent. The term "polymer" or "polymeric precursor material" used herein applies to organic polymers as defined in Hawley's Condensed Chemical Dictionary, Eleventh Edition, published by Van Nostrand Rheinhold Company. The organic polymers generally include: 1) natural polymers, such as cellulose, and the like; 2) synthetic polymers such as thermoplastic or thermosetting elastomers; and 3) Semisynthetic cellulosics. The term "fiber" used herein is intended to include an assembly of a multiplicity of fibers such as can be found in a fiber tow. The term "oxidized" used herein applies to fibers that have been oxidized at a temperature of typically less than 250°C for acrylic fibers. It will be understood that, in some instances, the fibers can also be oxidized by chemical oxidants at a lower temperature. The term "permanent heat set" used herein applies to nonlinear carbonaceous fibers which have been heat treated until they possess a degree of irreversibility where the nonlinear fibers, when stretched to a substantially linear shape, without exceeding their internal tensile strength, will revert to their original nonlinear configuration once the stress on the fiber is released. Accordingly, what is meant by "permanently set" is that a fiber possesses a degree of resiliency which manifests itself in a "reversible deflection" of the fiber when it is placed under stress such that the fiber is substantially linear in shape. When the stress is relieved, the fiber will return to its unstressed and nonlinear condition. The term "reversible deflection" defines the minimum limit of stretching the fiber which is expressed as a ratio of 1.2:1 where the fiber in the stretched condition is at least 1.2 times the length of the fiber in its relaxed or unstretched condition. The carbonaceous fibers are prepared from a suitable polymeric precursor fiber, which is stabilized, as for example by oxidation at a temperature which is typically less than 250°C for acrylic fibers. The stabilized fiber is then heat treated, in a relaxed and unstressed condition and in an inert atmosphere for a period of time sufficient to produce a heat induced thermoset reaction wherein additional cross-linking and/or a cross-chain cyclization reactions occur between the original polymer chains. The carbonaceous fiber of the invention can be classified into three groups depending upon the particular use of the fiber and the environment in which the fiber is used. In a first group, the carbonaceous fiber is partially carbonized and has a carbon content of greater than 65 percent but less than 85 percent, is electrically nonconductive and does not possess any electrostatic dissipating characteristics, i.e., the fiber is not able to dissipate an electrostatic charge. The term "electrically nonconductive" as utilized in the present invention relates to a resistance of greater than 4 x 10 6 ohms/cm when measured on a 6K (6000 filaments) tow of fibers in which the individual fibers each have a diameter of from 7 to 20 microns. The specific resistivity of the carbonaceous fiber is greater than about 10 -1 ohm-cm and is calculated from measurements as described in WO Publication No. 88/02695. When the fiber is a stabilized and heat set acrylic fiber it has been found that a nitrogen content of 18 percent or higher results in an electrically nonconductive fiber. In a second group, the carbonaceous fiber is classified as having a low electrical conductivity, i.e. being partially electrically conductive, and having a carbon content of greater than 65 percent but less than 85 percent. Low conductivity means that a 6K tow of fibers has a resistance of from 4 x 10 6 to 4 x 10 3 ohms/cm. Preferably, the carbonaceous fiber is derived from a stabilized acrylic fiber and possesses a percentage nitrogen content of from 16 to 22 percent, preferably from 16 to 18.8 percent. Such a fiber finds particular use in sound absorbing and thermal barrier structures. In a third group, the fiber has a carbon content of at least 85 percent and a nitrogen content of less than 10 percent. The fiber is characterized as having a high electrical conductivity. That is, the fiber is substantially graphitic and has an electrical resistance is less than 4 x 10 3 ohms/cm. Correspondingly, the electrical resistivity of the fiber is less than 10 "1 ohm-cm. This fiber is useful as furnace insulation or in applications where electrical grounding or shielding is desired. The carbonaceous fiber of the third group can have imparted to it an electrically conductive property on the order of that of a metallic conductor by heating the fiber to a temperature above 1000°C in a non- oxidizing atmosphere. The electroconductive property can be obtained from selected starting materials such as pitch (petroleum or coal tar), polyacetylene, acrylic materials, e.g., a polyacrylonitrile copolymer such as PANOX™ (trademark of R.K. Textiles) or GRAFIL-01™ (trademark of E.I. du Pont de Nemours & Co.), polyphenylene, polyvinylidene chloride (SARAN™, a trademark of The Dow Chemical Company) , and the like. Advantageously, the apparatus of the invention is utilized to produce carbonaceous fibers from polymeric precursor material fibers without subjecting the fibers to a knit/deknit step. The apparatus comprises a means for gear crimping the fiber, without the application of compression forces, preferably at a temperature of from 100°C to 250°C, to provide the fiber with a nonlinear temporary set. A conveying means is provided to receive the nonlinear set fiber and transport it without tension or stress through a heating zone comprising one or more heating units. One heating unit may comprise a fiber oxidation or stabilization zone. Another heating unit may comprise a heating means for substantially irreversibly heat setting the fiber in an inert atmosphere to produce a carbonaceous fiber having a carbon content of greater than 65 percent. Fibers that are derived from nitrogen containing polymeric materials, such as acrylic based polymers, generally have a nitrogen content of from 5 to 35 percent, preferably from 16 to 25 percent, and more preferably from 18 to 20 percent. In the preferred operation, the fiber is passed through a rounded gear crimper wherein the temporary crimp is imparted to the fiber. Preferably, the crimper is heated so as to soften the fiber. The crimped fiber is then placed in a relaxed and unstressed condition on a conveyor where it is transported through the heating zone at a temperature and speed sufficient to heat set and/or carbonize the fiber. Preferably, the polymeric precursor fibers employed in the present invention are the high performance fibers such as oxidizied acrylic fiber (OPF), aramid fibers, PBI fibers, etc. Preferably, the polymeric precursor fibers are acrylic fibers selected from acrylonitrile homopolymers, acrylonitrile copolymers and acrylonitrile terpolymers, wherein said copolymers and terpolymers contain at least 85 mole percent acrylic units and up to 15 mole percent of one or more monovinyl units copolymerized with another polymer. The apparatus is particularly suited to prepare carbonaceous fibers as disclosed in the aforementioned European Publication No. 0199567. A more complete understanding of the invention will be had by referring to the following description and claims of a preferred embodiment, taken in conjunction with the accompanying drawings, wherein like reference members refer to similar parts throughout the several views. Figure 1 is a perspective view, partly in section of a crimping apparatus of the invention; Figure 2 is an elevational view showing a section of the crimping unit of Figure 1; Figure 3 is a side elevation of the apparatus; and Figure 4 illustrates a gear crimping apparatus which can be utilized in the invention. Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the invention selected for illustration in the drawings and are not intended to define or limit the scope of the invention. As seen in Figure 1, the apparatus 10 of the invention comprises an endless conveying belt 11 which travels around drive rolls 14, 14 * . The conveyor belt 11 extends through a closure or housing 12 which contains one or more compartments for heating and, optionally, cooling. For example, two heating chambers 16A and 16B with heaters 17 and 17', respectively, are provided through which the fiber 18 passes. The heating chambers are preferably followed by a cooling chamber 20 having one or more cooling fans 21. The fiber 18 is first passed through a crimper mechanism 13 which is preferably heated. The crimper mechanism comprises a pair of cylindrical crimping rolls or drums 13A and 13B where the fiber 18 is made pliable and provided with a temporary crimp or heat set while in an unstressed condition, i.e., without subjected the fiber to tension or compression such as occurs, for example, in gear crimping mechanism employed in the industry. The nonlinear fiber, provided with a temporary set, is then placed in a relaxed condition on the conveyor belt 11 so as to be in an unstressed condition and without tension during the final heat setting procedure. The conveyor belt is constructed of any suitable material that is unaffected by and resistant to the elevated temperature in the heating chambers. After passage through the housing 12 the fiber 18 is taken up on take up roll 26. As seen in Figure 2, the crimper mechanism 13 comprises a rotatable cylindrical drum 13A having a plurality of spaced fingers 22 or longitudinally extending peripheral ridges (not illustrated) with rounded end surfaces 24. The fingers are mounted on an outer surface of the drum. The cylindrical drum 13A is mounted in an opposing relationship to a cylindrical gear 13B which has a plurality of rounded gear teeth 15. The cylindrical drum 13A is spaced from the cylindrical gear at a distance sufficient to allow the fingers or ridges to enter the spaces between the gears 15 thereby pushing the fiber into the spaces between the gears and holding them in such position without imparting a compression or shearing force to the fiber. Optionally, the finger like members 22 are slideably and telescopically mounted in sockets 19 extending from the peripheral surface of the cylindrical drum 13A. Thus, the fingers can be adjusted in length by sliding them into or out of the sockets and by securing them in any desired position by means of the screws 23 to thereby adjust the spacing of the rounded surface of the finger with respect to the rounded surface of the gear to allow for different size fibers or fiber tows. Adjustment of the fingers with respect to the rounded gear surfaces therefor prevents tensile stress or compression of the fiber between the surfaces as the fiber is being provided with a temporary heat set by the heated drum or drums. The fiber is therefore provided with a temporary heat set while it is being conveyed, in a unstressed condition, to the conveyor. The fingers (or ribs) 22 can also be replaced with fingers of different length and width depending on the size of the fiber or fiber tow. It will be apparent that the invention can also be practiced with a pair of opposing cylindrical gears in which the teeth of the gears are rounded and spaced from each other at a distance sufficient to accommodate a fiber or fiber tow without subjecting the fiber or tow to tension or compression forces, shown in Figure 4. As seen in Figure 2, the fiber 18 is crimped by the crimping mechanism where it assumed a uniform sinusoidal configuration. It will be apparent to persons skilled in the art that the crimping mechanism can be designed with fingers or elongated ribs of a different length and width and matching gear to provide a fiber or tow with a non-uniform crimp. The operation of the apparatus is more clearly shown in Figures 1 and 3. The fiber 18 is delivered from a supply roll 28 to a heated crimper 13. After applying a temporary set to the fiber 18, the nonlinear fiber is deposited onto the conveyor 11 and then transported into housing 12 without placing any stress or tension on the fiber while maintaining its crimped configuration. The housing 12 may contain one ore more heating chambers 16A and 16B where a preoxidized or stabilized fiber 18 is provided with a permanent heat set. The heating chambers 16A, 16B are filled with an inert gas.The heat setting of the fiber 18 can be conducted by means of radiant heaters 17, 17' or by irradiation with a high energy source such as, for example, lasers. Suitable heating means are well known in the art. The fiber which is permanently heat set in chambers 16A, 16B, has a carbon content of greater than 65 percent and is thus defined as a carbonaceous fiber. The fiber is then cooled in chamber 20 by cooling means 21 such as, for example, a fan, and carried out of housing to be taken up on roll 26. The conveyor 11 and rolls 26, 28 are synchronized so that the fiber or tow placed on the conveyor 11 is not placed under stress or tension in the heating chambers 16A, 16B. It will be apparent that a plurality of fibers 18 can be processed simultaneously by the apparatus 10 utilizing a plurality of supply and take-up rolls 28, 26. In the case where the fiber comprises a stabilized or oxidized polyacrylonitrile fiber and heat setting is to be effected, the oxidized fiber is heated to a temperature of from 300°C to 1400°C in a non- oxidizing atmosphere such as nitrogen, argon, helium or hydrogen. The heating zone can contain a single or multigradient furnace. The inert gas can be supplied through any openings or conduits in the housing leading to the heating zone, and it can be injected at any point along the path of the fiber passing through the heating zone. The fiber residence time in the heating zone is dependent upon the particular fiber utilized, the degree of carbonization desired, and the temperature(s) utilized. The following example is illustrative of the present invention. A 160K tow of oxidized polyacrylonitrile fiber (OPF) manufactured under the trade name PANOX™ by R.K. Textiles, Scotland, United Kingdom, was passed through a rounded gear crimping mechanism having five gears per inch (2 gears per cm) at a temperature of from 100°C to 250°C. This resulting nonlinear OPF tow, provided with a temporary set, was allowed to fall in a relaxed state onto a moving belt conveyor which transported the fiber tow through a graduated hot zone with a final temperature of from 400°C to 1000°C to permanently heat set the fiber tow to a carbonaceous nonlinear fiber tow. Although the invention has been described with a certain degree of particularity, it is understood that changes in construction and the arrangement of parts can be resorted to without departing from the scope of the invention.

Claims: Claims 1. An apparatus for crimping and heat setting at least one polymeric precursor fiber, comprising a crimping mechanism for providing said fiber with a temporary nonlinear configuration which is free of sharp bends and without the application of tension or stress on the fiber, and conveyor means for transporting the nonlinear fiber without the application of tension or stress through a heating zone to provide said nonlinear fiber with a permanent heat set. 0 2. The apparatus of Claim 1, wherein said crimping mechanism comprises a pair of mating cylindrical gears having rounded teeth, wherein said gears are spaced at a distance from each other 5 sufficient to hold the fiber between the gears without the application of compression forces to the fiber, and means for heating at least one of said gears to a temperature of from 100°C to 250°C to heat set said 0 fiber and to provide the fiber with said temporary nonlinear configuration. 3. The apparatus of Claim 1, wherein said crimping mechanism comprises a cylindrical drum having a c plurality of finger members with rounded end surfaces mounted on an outer surface of the drum, a cylindrical gear member having rounded teeth positioned in a mating relationship to the cylindrical drum, wherein said plurality of finger members are adapted to protrude into the recesses of adjacent gears of the cylindrical gear c - member for a distance sufficient to hold the fiber between the drum and the gear without the application of compression forces to the fiber, and means for heating at least one of said drum or gear member to a temperature of from 100°C to 250°C to heat set said 10 fiber and to provide the fiber with said temporary nonlinear configuration. 4. The apparatus of Claim 3, wherein said fingers are adjustable in length and replaceable. 15 5. The apparatus of any one of the preceding claims, wherein said heating zone comprises at least one heating unit for carbonizing the nonlinear precursor fiber at a temperature of from 300°C to 1400°C, and 20 means for providing an inert gas to said heating zone. 6. The apparatus of any one of the preceding claims, wherein said polymeric precursor fibers are acrylic fibers selected from acrylonitrile homopolymers, 25 acrylonitrile copolymers and acrylonitrile terpolymers, wherein said copolymers and terpolymers contain at least 85 mole percent acrylic units and up to 15 mole percent of one or more monovinyl units copolymerized with another polymer. 30 7. The apparatus of any one of the preceding claims, including means for supplying a plurality of fibers to said conveying means, and a plurality of fiber take up means, wherein the speed of said conveying means and said fiber supplying and take up means are synchronized. 8. A process for crimping and heat setting at least one polymeric fiber, comprising the steps of supplying a polymeric precursor fiber to a crimping mechanism operated at a temperature sufficient to impart a temporary nonlinear configuration to said fiber without applying stress or tension to the fiber, transporting said nonlinear fiber without the application of stress or tension through a heating zone for heating said fiber to a temperature sufficient to impart a permanent heat set to said fiber, and cooling said fiber. 9. A process for producing a nonlinear carbonaceous fiber, comprising the steps of supplying an oxidized polymeric precursor fiber to a pair of crimping members at least one of which is a cylindrical gear member having apertures with rounded gear surfaces, heating at least one of said crimping members and introducing said fiber into the apertures of the gear member to provide the fiber with a nonlinear temporary heat set without applying stress or tension to the fiber, conveying the nonlinear fiber with the temporary heat set through a heating zone without applying stress or tension to the fiber, heating the fiber in an inert atmosphere in said unstressed condition to a temperature sufficient to impart a permanent set and to form a nonlinear carbonaceous fiber which is free of sharp bends or deformations, wherein said carbonaceous fiber has a diameter of from 4 to 20 micrometer, a reversible deflection ratio of greater than 1.2:1, and a carbon content of greater than 65 percent. 10. The process according to Claim 8 or 9, wherein said crimping mechanism is heated to a temperature of from 100°C to 250°C. 11. The process of Claim 8 or 9, wherein the fiber is heated in the heating zone to a temperature of from 300°C to 1400°C for carbonizing the fiber, and means for providing an inert gas to said heating zone.

Inventors: MCCULLOUGH FRANCIS P JR, PATTON ROBERT T, SNELGROVE R VERNON, MCCULLOUGH, FRANCIS, P., JR., PATTON, ROBERT, T., SNELGROVE, R., VERNON
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1990
Main classification: D02G1
IPC codes: D01F9, D02G1