LINEAR CARBONACEOUS FIBER WITH IMPROVED ELONGATABILITY

Abstract: A process for preparing a nonflammable, linear, nongraphitic carbonaceous fiber having improved elongatability by heat treating a linear, stabilized polymeric precursor material fiber in a relaxed condition and without subjecting the fiber to tension or stress in an inert non-oxidizing atmosphere at a temperature above 525 DEG C to irreversibly heat set said fiber, and a linear carbonaceous fiber having improved elongatability of from 3 to 9 percent and a tenacity of from 2 to 7 g/d.

Description: LINEAR CARBONACEOUS FIBER WITH IMPROVED ELONGATABILITY The present invention relates to a linear carbonaceous fiber and to a process for preparing the linear fiber. More particularly, the invention relates to nongraphitic linear carbonaceous fibers having improved elongatability of from 3 to 9 percent and to textile structures prepared therefrom. In processing fibers into textile structures, the fibers undergo a variety of stresses and strains which oftentimes cause fiber breakage. Accordingly, fibers having improved elongatability are better able to withstand such stresses and strains. Moreover, fabrics which contain fibers having a greater elongatability have the advantage of being stretchable and wrinkle resistant. When carbon and graphitic fibers are produced from a stabilized acrylic polymer precursor fiber, the elongatability or percent extension of such a fiber is typically in the range of from 1.25 to 1.9 percent, depending upon the extent of heat treatment, i.e. the time and temperature and, therefore, the degree of carbonization or graphitization and modulus of the fiber. Typically, linear graphitic or carbon fibers are produced by passing tows of from 1,000 to 320,000 (1 K to 320 K) filaments through a furnace which gradually increases the heat treatment of the fiber tows in a temperature range of from 300°C up to 1100°C. This treatment is generally followed by a subsequent heat treatment in a high temperature furnace where the fiber tow is taken up to a temperature of from 1400°C to 2400°C. The heat treatment is carried out under tension even during treatment of the fiber in a low temperature furnace. That is, the fibers in a fiber tow are suspended and passed through the furnace with sufficient tension to pull the fiber tow through the furnace to keep the fiber tow off the floor or bottom of the furnace. It is especially advantageous if one desires to perform textile processing to have a percent fiber elongatability of from 3 to 9 percent, or greater. When a partially carbonized fiber, i.e. a fiber which still has a nitrogen content of from 10 to 20 percent, is heat treated at a temperature of from 550°C to 650°C under tension, the elongatability of such a fiber is only 2.5 percent, or less. This low elongatability is insufficient for textile processing without encountering considerable fiber breakage. U.S. Patent No. 4,347,297 to Mishima et al., discloses a process for the preparation of carbon fibers by two preoxidation treatments of polyacrylonitrile fibers under tension and the carbonizing of the oxidized fibers under tension. U.S. Patent No. 4,279,612 to Saji et al., discloses a method for producing carbon fibers which includes the step of thermally stabilizing the fibers under tension before heat treatment to carbonize the fibers. U.S. Patent No. 3,541,582 to Fainborough et al discloses the preparation of a woven carbon cloth by first oxidizing continuous yarns of polymeric fibers while under tension. The carbonization step is performed either while the fibers are under tension or without tension. However, a woven cloth inherently places the fibers under tension. U.S. Patent No. 4,837,076 to McCullough et al. discloses a process for preparing nonlinear carbonaceous fibers and tows having a reversible deflection ratio of greater than 1.2:1. The conditions for heat treatment described in the patent can be used to provide similar electrical conductivity characteristics to the linear fibers of the present invention. However, there is not disclosed any improvement in elongatability other than as a result of the nonlinear configuration. European Publication No. 88/02695, published April 21, 1988, to McCullough et al., discloses a thermal insulating and/or sound absorbing structure of nonlinear carbonaceous fibers blended with other fibers, including linear carbonaceous fibers. However, there is no teaching in the application to provide the linear carbonaceous fiber with elongatability. The term "carbonaceous fiber or fibers" is understood to mean fibers, or a plurality of fibers in the form of a tow, which have been heated to increase the carbon content of the fiber to greater than 65 percent as a result of an irreversible chemical reaction in the polymeric precursor material. The term "graphitic" as used herein relates to those carbonaceous materials having an elemental carbon content of at least 92 percent, preferably at least 98 percent, and as further defined in U.S. Patent No. 4,005,183 to Singer. The term "stabilized" use 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. It is to be understood that all of the percentages stated herein relate to percentage by weight of the total composition unless stated otherwise. In accordance with the invention there is provided a process for preparing a linear, nonflammable, nongraphitic carbonaceous fiber having improved elongatability, comprising the step of heat treating a linear, stabilized, polymeric precursor fiber without subjecting the fiber to tension or stress in an inert, non-oxidizing atmosphere at a temperature above 525°C to irreversibly heat set said fiber. In accordance with the present invention, a linear, stabilized, polymeric precursor fiber is processed into a nongraphitic carbonaceous fiber. The thus produced linear, nongraphitic carbonaceous fiber of the invention is provided with an improved percent extensibility or elongatability. It has been found that contrary to the prior art methods of preparing carbon fibers by placing the polymeric precursor material fibers under tension during heat treatment to prevent shrinkage, the eliminating of any tension or stress during heat treatment of the fibers results in a substantially improved elongatability for the resulting nongraphitic carbonaceous fibers. The linear, nongraphitic, carbonaceous fibers of the invention are nonflammable and ignition resistant and have a Limited Oxygen Index (LOI) of greater than 40 when the fibers are tested according to the test method of ASTM D 2863-77. The test method is also known as "oxygen index" or "limited oxygen index" (LOI). With this procedure the concentration of oxygen in mixtures of 0 2 and N 2 is determined at which a vertically mounted specimen is ignited at its upper end and just barely continues to burn. The width of the specimen is from 0.65 to 0.3 cm with a length of from 7 to 15 cm. The LOI value is calculated according to the equation: C0 ] LOI = [O + [N In preparing the carbonaceous fibers of this invention, suitably stabilized or oxidized linear polymeric precursor material fibers are heat treated, in an inert atmosphere and without tension or stress to convert the fibers into irreversibly heat set, linear carbonaceous fibers. Preferably, the stabilized polymeric precursor material used in the present invention is derived from oxidatively stabilized polyacrylonitrile (PAN) filaments. Stabilized acrylic filaments which are advantageously utilized in preparing the carbonaceous fibers of the invention are selected from one or more of the following: acrylonitrile homopolymers, acrylonitrile copolymers and acrylonitrile terpolymers. The copolymers preferably contain at least about 85 mole percent of acrylonitrile units and copolymerized therewith up to 15 mole percent of one or more monovinyl units e.g. ,styrene, methylacrylate, methyl methacrylate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like. The acrylic filaments may also comprise terpolymers wherein the acrylonitrile units are represented by at least about 85 mole percent. The polymeric precursor materials are typically prepared by melt spinning, dry or wet spinning the precursor materials in a known manner to yield a monofilament or multifiber tow. The fibers are then heated to a temperature and for a period of time as described in U.S. Patent No. 4,837,076. The polyacrylonitrile (PAN) precursor fibers have a normal nominal diameter of from 4 to 25 micrometers and can be collected as an assembly of a multiplicity of continuous filaments in tows. The fibers are then stabilized, for example by oxidation or any other conventional method of stabilization. The stabilized fibers are thereafter heat treated, according to the present invention, in a relaxed and unstressed condition, at elevated temperatures in an inert non- oxidizing atmosphere for a period of time to produce a heat induced thermoset reaction in the polymer wherein additional cross linking and/or a cross chain cyclization reactions occur between the original polymer chains while maintaining a nitrogen content of from 5 to 35 percent. At a temperature range of from 150°C to 525°C, the fibers are generally provided with a varying proportion of temporary to permanent set while in an upper range of temperatures of from 525°C and above, the fibers are provided with a substantially permanent or irreversible heat set. It is to be understood that the fibers can be initially heat treated at the higher temperature so long as the heat treatment is conducted while the fibers are in a relaxed or unstressed state and under an inert, non-oxidizing atmosphere, including under a reduced pressure atmosphere. As a result of the higher temperature treatment of 525°C and above, a substantially permanently or irreversible heat set is imparted to the fibers. The resulting fibers can be used per se or formed into a wool-like fluff. Such wool-like fluff has a bulk density of from 2.4 to 32 kg/m 3 . A number of methods known in the art can be used to create a fluff or batting-like body of considerable loft. The linear, polymeric precursor fibers can be prepared from well-known materials such as pitch (petroleum or coal tar), polyacetylene, acrylonitrile based materials, e.g., a polyacrylonitrile copolymer such as GRAFIL-Ol™ (trademark of E. I. du Pont de Nemours & Co.), polyphenylene, polyvinylidene chloride (PVC), polyaromatic amides such as KEVLAR™ (trademark of E. I. du Pont de Nemours & Co.), polybenzimide resin, and the like. The improved linear carbonaceous fibers of this invention can be classified into three groups. In a first group, the carbonaceous fibers have a carbon content of greater than 65 percent but less than 85 percent and are electrically nonconductive and do not possess any antistatic characteristics, i.e., they are 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 or fibers having a single fiber diameter of from 4 to 20 microns. The preferred fibers of this group have an elongation of from 3 to 9 percent and a tenacity of from 2 to 6 grams/denier (g/d). When the fiber is a stabilized and heat set acrylic fiber, it has been found that a nitrogen content of greater than 18 percent results in an electrically nonconductive fiber. In a second group, the carbonaceous fibers are classified as being partially electrically conductive (i.e., having a low electrical conductivity), have static dissipating characteristics and have a carbon content of greater than 65 percent but less than 85 percent. Low electrical conductivity means that a 6K tow of fibers, in which a single precursor fiber has a diameter of from 4 to 20 microns, has a resistance of from 4 x 10 6 ohms/cm to 4 x 10 3 ohms/cm. The preferred fibers of this group have an elongation of from 3 to 6 percent and a tenacity of from 3 to 7 g/d. In a third group are fibers having a carbon content of at least 85 percent but less than 92 percent and a nitrogen content of at least 5 percent. These fibers are characterized as having a high electrical conductivity and a specific resistivity of less than 10 "1 ohm-cm. The specific resistivity of the fibers is calculated from measurements as described in WO Publication No. 88/02695. The preferred fibers of the third group have an elongation of from 2 to 4 percent and a tenacity of from 4 to 9 g/d. The nongraphitic carbonaceous fibers of the three groups can be fluorinated as disclosed in U.S. Patent No. 4,857,404 to McCullough et al., so as to provide linear fibers of different electrical conductivity having an electrically nonconductive surface. Generally, textile structures, such as knitted or woven fabrics, formed from the fibers of the invention are lightweight, have low moisture absorbency, good strength and elongatability together with good appearance and handle. The linear carbonaceous fibers of this invention may be used in substantially any desired fabricated form which will depend on the purpose for which the fabricated form is to be used. The carbonaceous fibers can be readily stretch broken and formed by conventional equipment into spun yarn and then into cloth, such as herringbone weave cloth, twill weave tape, tubular woven fabric, nonwoven structures such as batting, blankets, roving yarn, cord and rope. The linear carbonaceous fibers can be used alone or blended with other synthetic or natural fibers. Examples of other fibers include linear or nonlinear fibers selected from natural or polymeric fibers, other carbonaceous fibers, ceramic fibers, glass fibers, or metal or metal coated fibers. Particular natural and/or synthetic polymeric fibers that can be included into a blend with the linear carbonaceous fibers of the invention are cotton, wool, polyester, polyolefin, nylon, rayon, fibers of silica, silica alumina, potassium titanate, silicon carbide, silicon nitride, boron nitride, boron, acrylic fibers, tetrafluoro- ethylene fibers, polyamide fibers, vinyl fibers, protein fibers, and oxide fibers derived from boron, thoria or zirconia. Exemplary of the present invention are the following examples: Example 1 An oxidized acrylonitrile based precursor fiber, sold under the name PANOX™ by R.K. Textiles having a density of from 1.356 to 1.39 g/cc and at least 85 mole percent of acrylonitrile units, was heat treated in a low temperature furnace at a peak temperature of from 525°C to 650°C in a purged nitrogen environment so as to produce a partially carbonized fiber. In lieu of a purged nitrogen atmosphere, the system can be purged with nitrogen and then evacuated. The precursor fiber was first chopped into staple fibers having a length of from 3.75 to 7.5 cm and then placed (loosely and without applying any tension) on a fine mesh belt and conveyed through the furnace operated at the above indicated temperature to produce staple partially carbonized fibers having an elongation of from 4 to 7 percent. Example 2 Samples of Example 1 were blended with 60 percent KODEL™ 435 (trademark of Tennessee Eastman Company) polyester staple fiber and 20 percent KODEL™ 410 binder fiber with 20 percent of the improved linear partially carbonized staple fibers on a randomizing carding machine. The blended fibers were then placed in a Rando B nonwoven web former and a 4 oz/yd 2 (135 g/m 2 ) nonwoven batting was produced. This resulting batting had fire resistant characteristics and passed the vertical burn test according to FTM-5903 and FAR25.853b. Example 3 Following the procedure disclosed in U.S. Patent No. 4,857,404, the carbonaceous fibers of Example 1 were placed in a monel reaction vessel. The reaction vessel was evacuated and fluorine gas diluted with helium gas was flowed into the reactor. The amount of fluorine reacted was from 0.1 to 2.5 moles per mole of carbon and typically from 1 mole of fluorine per mole of carbon. The percent fluorine reacted was from 1 to 75 percent. The reaction time was from 5 minutes to one hour.

Claims: CLAIMS 1. A process for preparing a nonflammable, linear, nongraphitic carbonaceous fiber having improved elongatability, comprising the step of heat treating a linear stabilized polymeric precursor fiber in a relaxed condition, and without subjecting the fiber to tension or stress, in an inert non-oxidizing atmosphere at a temperature above 525°C to irreversibly heat set said fiber, said carbonaceous fiber having an elongation of from 3 to 9 percent and a tenacity of from 2 to 7 g/d. 2. The process of Claim 1, wherein said polymeric precursor fiber is selected from acrylonitrile homopolymers, acrylonitrile copolymers and acrylonitrile terpolymers. 3. The process of Claim 2, 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. 4. The process of any one of the preceding claims, including the further step of fluorinating the carbonaceous fiber. 5. A nonflammable, linear, nongraphitic carbonaceous fiber prepared from a polymeric precursor fiber, said carbonaceous fiber having a nitrogen content of from 5 to 35 percent, and a carbon content of greater than 65 percent, characterized in that the carbonaceous fiber has an elongatability of from 3 to 9 percent and a tenacity of from 2 to 7 g/d. 6. The fiber of Claim 5, wherein the carbonaceous fiber has a carbon content of less than 85 percent, is electrically nonconductive, and does not possess any electrostatic dissipating characteristics, said fiber having an electrical resistance of greater than 4 x 10 3 ohms/cm when measured on a 6K tow of fibers having a fiber diameter of from 4 to 20 microns. 7. The fiber of Claim 5, wherein the carbonaceous fiber has a carbon content of less than 85 percent, low electrical conductivity and electrostatic dissipating characteristics, and an electrical resistance of from 4 x 10 6 to 4 x 10 3 ohms/cm when measured on a 6K tow of fibers having a fiber diameter of from 4 to 20 microns. 8. The fiber of Claim 5, wherein the carbonaceous fiber has a carbon content of at least 85 percent, is electrically conductive and has an electrical resistance of less than 4 x 10 3 ohms/cm when measured on a 6K tow of fibers having a fiber diameter of from 4 to 20 microns. 9. The fiber of any one of Claims 5 to 7, wherein the outer surface of said fiber is fluorinated. 10. The fiber of any one of Claims 5 to 8, wherein said polymeric precursor fiber is selected from acrylonitrile homopolymers, acrylonitrile copolymers and acrylonitrile terpolymers. 11. The process of Claim 9, 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.

Inventors: MCCULLOUGH FRANCIS P JR, SNELGROVE R VERNON, MCCULLOUGH, FRANCIS, P., JR., SNELGROVE, R., VERNON
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1990
Main classification: D02G3
IPC codes: D01F9, D01F11, D02G3, D06M11