APPARATUS AND PROCESS FOR CRIMPING FIBERS

Abstract: An apparatus and process for crimping and permanently heat setting a fiber or tow without imparting stress or tension on the fiber or tow comprising a conveying means (11) having a multiplicity of openings, means for supplying the fiber or tow to the conveying means (28), means for inserting (13) the fiber or tow (18) into the openings of the conveying means whereby the fiber is retained in a nonlinear shape in the openings without stress or tension, and a heating zone (17) through which the conveying means and fiber or tow pass for heat setting the fibers or tow.

Description: APPARATUS AND PROCESS FOR CRIMPING FIBERS The present invention is directed to a process and an apparatus for crimping and permanently heat setting 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" can generally be defined as a nonlinearity or waviness of a fiber or, more particularly, as the waviness of a fiber expressed as crimps per unit length. For most of the man-made fibers employed in the manufacture of carpeting, the crimp or bend in the fiber is induced by thermal/mechanical techniques, e.g. a stuffer box. 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 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 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 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. 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, 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 technique can be done for single fibers or tows having multiple fiber ends using the 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 developed in the fiber simultaneously. 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 demonstrated that severe bending, as in a V-type crimp, can result in extensive fiber damage. Even where the fibers had a more rounded V-type bend, the fibers exhibited compression ridges on the underside of the crimp. Severely crimped fibers having sharp V-type bends therefore exhibit reduced mechanical properties and easily break due to weaknesses in the fibers created by the 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 -li¬ the cause of 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 effect is a darker streak in viewing 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 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 fiber tows which are nonlinear, it is necessary to knit and then deknit the fabric. However, the knitting and deknitting of a fabric to obtain the 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 limit. Such a process cannot be used to produce the stress free, nonlinear fibers having the physical properties of the 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. Generally, the apparatus of the invention comprises a conveying means in the form of a belt having a plurality of openings. A precursor polymeric fiber or fiber tow is supplied to the conveying means and a crimping mechanism is provided for inserting the fibers into the openings in the conveying means such that the fiber assumes a nonlinear configuration. The fiber positioned within the openings of the conveying means is transported substantially without stress or tension through at least one heating zone at a temperature and at a rate of speed sufficient to provide the fiber with a temporary or permanent heat set, and/or to carbonize the fiber. Each heating zone can comprise one or more heating units with one heating unit serving as a fiber oxidation or stabilization zone. Another heating unit serving as a means for substantially irreversibly or permanently heat set the fiber in an inert atmosphere. More particularly, the invention resides in an apparatus for crimping and heat setting at least one polymeric precursor fiber, comprising a conveying means having a planar surface with a multiplicity of openings, means for supplying the fiber to said conveying means, means for inserting said fiber into the openings in the conveying means and for retaining said fiber in a nonlinear configuration in the openings substantially without imparting stress or tension on the fiber, and a heating zone through which said conveying means and fiber pass to heat set said fiber. The invention also resides in a process for crimping and heat setting at least one polymeric precursor fiber, comprising the steps of supplying the fiber to an apertured conveying means having a planar surface, inserting said fiber into at least two apertures of the conveying means so that the fiber is maintained in a nonlinear configuration without substantially imparting stress or tension on the fiber while the fiber is being held within said apertures, passing said nonlinear fiber in said unstressed condition through a heating zone at a temperature and at a rate of speed sufficient to provide the fiber with a temporary or permanent heat set, and/or to carbonize the fiber, and then cooling said fiber while in said nonlinear configuration. 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 one or more fibers or filaments as well as an assembly of a multiplicity of fibers in the form of a 0 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 5 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 0 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 _τ 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 -re ¬ 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. 0 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 5 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 Q 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 c 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. The fibers may comprise any polymeric precursor material capable of being heat set in the apparatus of the invention. 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. 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 conveying means which is provided with a multiplicity of openings into which the fibers are inserted to provide the fiber with a nonlinear shape, i.e. for crimping the fiber, without the application of tensile stress to the fiber. The conveying means transports the fiber 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. At a temperature of from 100°C to 250°C, the fiber is provided with a nonlinear temporary set. 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. 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 numbers refer to similar parts throughout the several views. Figure 1 is a perspective view, partly in section of a crimping mechanism of the invention; Figure 2 is an elevation view showing a section of the crimping unit of Figure 1; and Figure 3 is a side elevation of the apparatus. 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 drawing 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 apertured endless conveying belt 11 which travels around drive rolls 14, 14' and which extends through a closure or housing 12. The conveying belt can be in the form of a wire grid, screen or apertured belt. The housing 12 can contain one or more compartments for heating and, optionally, cooling. For example, there is provided a heating chamber 16 containing one or more heaters 17, 17' through which a fiber or tow 18 passes followed by a cooling chamber 20 with one or more cooling fans 21. The fiber 18 is first passed between a crimping mechanism 13 and the apertured conveyor belt and is pushed by a plurality of finger members 22 on the crimping-mechanism into the apertures of the conveyer belt 11. After passage through the housing 12, the fiber 18 is taken up on a take up roll 26. In operation, the fiber 18 is introduced into the apertures of the conveyor belt 11 by the crimping mechanism 13 where they are held in a relaxed condition and without the application of tension on the fiber during conveyance through the heating chamber 16. As seen in Figure 2, the crimper mechanism 13 comprises a plurality of the fingers 22 which are slideably mounted in sockets 15 extending from a rigid, reciprocating board. The lengths of the fingers 22 can be adjusted by sliding the fingers inwardly or outwardly of the sockets and by securing them in the desired position by means of adjustment screws 27. By adjusting the length of the fingers 22, the depth of a loop of the fiber 18 extending through the apertures of the conveying belt can be adjusted. Thus, the configuration (amplitude of crimp) of the fiber 18 is determined by the length of the fiber loop extending through the apertures. It will be apparent that with a uniform length of the fingers 22, the fiber 18 will be provided with a uniform amplitude of a generally sinusoidal configuration. Similarly, if the fingers 22 are of a nonuniform or different lengths, the fiber will be provided with a corresponding nonuniform amplitude of sinusoidal configuration. It will be understood, that the flat, reciprocating board, illustrated in the figure, can be replaced with a cylindrical drum shaped member that is rotatably mounted above the conveying belt. Rotation of the cylindrical member allows the fingers mounted on the outer peripheral surface of the cylindrical member to contact the fiber and urge it into and through the apertures of the conveying belt. The fingers can be generally tubular in shape, as illustrated in the drawing, or they can be in the form of relatively short, longitudinally extending, rib-like members. When the fingers are of a rib-like configuration, the openings provided in the conveying belt have correspondingly shaped rectangular openings to allow the ribs to enter the openings. In the side view of Figure 3, it can be seen that the fiber 18 is delivered from a supply roll 28 onto the apertured conveying belt 11. The reciprocating crimping device 13 with its adjustable fingers 22, inserts or pushes the fiber 18 into the apertures of the belt 11 so that the fiber 18 is formed into a generally sinusoidal configuration-. After insertion of the fiber 18 into the apertures, it is conveyed into the housing 12 without the application of any stress or tension on - 1 li¬ the fiber while maintaining its sinusoidal configuration. Housing 12 can contain one or more heating chambers 16. Where a preoxidized or stabilized fiber 18 is being carbonized, the heating chamber 16 is filled with an inert gas. The carbonization of the fiber 18 may be conducted by means of radiant heaters 17, by irradiation with a high energy source, or by any other means known in the art. The fiber 18, once it is heat set in chamber 16 in a nonlinear configuration is then, preferably, cooled in chamber 20 by cooling means 21 and carried out of the housing to be taken up on roll 26. The speed of the conveying belt 11 and rolls 26, 28 are synchronized so that the fiber placed on the conveyor belt 11 is not pulled out of the openings of the conveying belt or placed under stress or tension while passing through the heating chamber 16. In the case where the fiber comprises a stabilized or oxidized polyacrylonitrile fiber and heat setting and/or carbonization is to be effected, the oxidized fiber is heated to a temperature of from 250°C to 1500°C in a nonoxidizing atmosphere such as nitrogen, argon, helium or hydrogen. The heating zone can be a single or multigradient furnace comprising a number of heating zones. The inert gases can be supplied to the heating zone through an opening 19 in the housing or may be injected at various points along the path of the fiber through a conduit into the housing. The fiber residence time in the heating zone is dependent upon the particular fiber utilized, the degree of heat set desired, and the temperat re(s) utilized. Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made only be way of example and that numerous changes in the details of construction and the combination and arrangement of parts may 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 conveying means having a planar surface with a multiplicity of openings, means for supplying the fiber to said conveying means, crimping means for inserting said precursor fiber into the openings in the conveying means, said conveying means retaining said fiber in a nonlinear configuration in the openings without imparting stress or tension on the fiber, and a heating zone through which said conveying means and fiber pass to heat set said fiber. 2. The apparatus of Claim 1, wherein said conveying means comprises an apertured belt, wire grid or screen, and wherein said crimping means for inserting the fiber into the openings of said conveying means comprises a plurality of fingers or elongated ribs. 3. The apparatus of Claim 1 or 2, wherein said crimping means for inserting the fiber into the openings of said conveying means comprises a rotatable, cylindrical drum having a plurality of fingers or elongated ribs extending from the peripheral surface of the cylindrical drum. 4. The apparatus of Claim 1, 2 or 3, wherein said fingers or ribs are adjustable in length. 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 means for providing an inert gas to said heating zone. 0 6. The apparatus of any one of the preceding claims, wherein said polymeric precursor fiber is an acrylic fibers selected from acrylonitrile homopolymers, acrylonitrile copolymers and acrylonitrile terpolymers, 5 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. 0 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 c and said fiber supplying and take-up means are synchronized. 8. A process for crimping and heat setting at least one polymeric precursor fiber, comprising the 0 steps of supplying the fiber to an apertured conveying means having a planar surface, inserting said fiber into at least two apertures of the conveying means so that the fiber is maintained in a nonlinear configuration without imparting stress or tension on the fiber within said apertures, passing said nonlinear fiber in said unstressed condition through a heating zone to heat said fiber to a temperature to heat set said fiber, and then cooling said fiber while in said nonlinear configuration. 9. The process of Claim 8, wherein the precursor fiber is oxidized and then passed through the heating zone to heat the fiber 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. 10. A process for producing a nonlinear carbonaceous fiber, comprising the steps of supplying an oxidized polymeric precursor fiber to an apertured conveying means having a planar surface, inserting said fiber into at least two apertures of the conveying means so that the fiber is maintained in a nonlinear configuration without imparting stress or tension on the fiber within said apertures, conveying the nonlinear fiber through a heating zone without applying stress or tension to the fiber, heating the fiber in an inert atmosphere to a temperature of from 300°C to 1400°C for carbonizing the fiber while in said unstressed condition 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.

Inventors: MCCULLOUGH FRANCIS P JR, MCCULLOUGH, FRANCIS, P., JR.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1990
Main classification: D02G1
IPC codes: D01F9, D02G1