Abstract:
A method of forming carbon fibers having internal cavities. The method includes applying a polymer material to a tooling component to form carbon fiber precursor hollow tubes, oxidizing the carbon fiber precursor hollow tubes, and carbonizing the carbon fiber hollow tubes to form carbon fibers, each having a hollow inner cavity

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a division of U.S. application Ser. No. 14/221,577 filed Mar. 21, 2014, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a method of forming carbon fibers having internal cavities. 
       BACKGROUND 
       [0003]    Composite panels are commonly used to manufacture structural and body panels for vehicles and in other products. Composite panels are typically made of polymeric resins that are reinforced with carbon fibers, glass fibers, natural fibers, or the like which are dispersed in the matrix. Composite panels are typically strong, light weight and may be used in a wide variety of product applications. 
       SUMMARY 
       [0004]    According to one embodiment, a method of forming carbon fibers having internal cavities is disclosed. The method includes applying a polymer material to a tooling component to form carbon fiber precursor hollow tubes, oxidizing the carbon fiber precursor hollow tubes, and carbonizing the carbon fiber hollow tubes to form carbon fibers, each having a hollow inner cavity. 
         [0005]    According to another embodiment, a method of forming carbon fibers having internal cavities is disclosed. The method includes applying a polymer material to a tooling component having half-circle features to form carbon fiber precursor hollow tubes, oxidizing the carbon fiber precursor hollow tubes, and carbonizing the carbon fiber hollow tubes to form carbon fibers, each having a hollow inner cavity. 
         [0006]    According to yet another embodiment, a method of forming carbon fibers having internal cavities is disclosed. The method includes applying a polymer material to a tooling component having circular inner cavities to form carbon fiber precursor hollow tubes, oxidizing the carbon fiber precursor hollow tubes, and carbonizing the carbon fiber hollow tubes to form carbon fibers, each having a hollow inner cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a fragmentary perspective view of a carbon fiber having an internal cavity. 
           [0008]      FIGS. 2A, 2B and 2C  are fragmentary perspective views of a carbon fiber having an internal cavity, each view duplicating a shaping variation. 
           [0009]      FIGS. 3A and 3B  are a perspective view associated with a method for making polymer precursors for carbon fibers having internal cavities. 
           [0010]      FIG. 4A  is a perspective view associated with another method for making polymer precursors for carbon fibers having internal cavities. 
           [0011]      FIG. 4B  is a perspective view of comb-like micro pins used in the method of making polymer precursors of  FIG. 4A . 
           [0012]      FIG. 4C  is a side view of a tooling plate associated with the method of making polymer precursors for of  FIG. 4A . 
           [0013]      FIG. 5A  is a cross section view of a gas delivery tube for a method of making polymer precursors from a liquid polymer material. 
           [0014]      FIG. 5B  is an end view of a bushing for gas delivery tubes as shown in  FIG. 5A . 
           [0015]      FIGS. 6A and 6B  are a schematic of the steps in the method of making the carbon fibers having an internal cavity. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0017]    Carbon fiber (CF) reinforced polymeric composites are gaining increasing interest in the automotive industry as a promising light weight material to meet governmental corporate average fuel economy (CAFE) requirements and to meet customer expectations for fuel economy. To meet the economical requirements of high volume production of automotive composites, low cost manufacturing processes and low cost materials are being developed. 
         [0018]    Incorporation of carbon fiber into composite structures has been met with challenges because carbon fiber process methods are labor intensive and yield porous carbon fibers and are not suited to automotive production volumes. Such labor-intensive process methods include vacuum bag autoclaving of pre-impregnated carbon fiber composite laminates. Some attempts have been made to adapt the processing methods of composites that were developed around glass fiber reinforcements to that of carbon fiber reinforcements. These attempts have been met with challenges. The diameter of carbon fiber is typically half that of glass fiber. Accordingly, for an equivalent fiber volume loading, four times as many carbon fibers may be required to fill the same volume as compared to when using glass fiber. Particularly for random fiber composites, an increase in fiber quantity adds complexity to chopping processes due to the intimate interaction of the fibers and sizing formulation (thin layer of polymer coating) developed for carbon fibers. This fiber interaction may make the fibers clump during processing and result in inadequate dispersion of fibers. This will cause degradation in load transfer of the fibers and greatly reduce the composite mechanical properties. Hollow carbon fibers produced by a partial sulfonation process have graphene structure only near the fiber outer surface. Most of the content of the fibers produced by sulfonation are amorphous carbon, and are porous. This low crystallinity and high porosity may lead to lower strength and modulus than required for carbon reinforcing fibers. 
         [0019]    Referring now to  FIG. 1 , the hollow carbon fiber  10  (having an internal cavity or void  11 ) has been formed as a cylinder tube having an outer diameter  12  and wall thickness  14  defining a cross sectional area  16 . The outer diameter  12  of the fiber  10  may be from 7 μm to 30 μm, preferably from 10 to 20 μm and most preferably from 13 to 15 μm. Both the outer diameter  12  and the cross-sectional area  16  can be adjusted and may vary along the length of the tube. The cross-sectional area  16  may be from 30% to 80% of the total fiber cross sectional area. The total fiber cross sectional area is π(d/2) 2  where d is the outer diameter  12 . The wall thickness  14  may be from 1 μm to 10 μm, preferably from 2 μm to 5 μm and most preferably from 2 μm to 4 μm. The outer diameter  12  of the hollow carbon fiber  10  is selected to address the issue of fiber dispersion and wet out in random fiber processing. Carbon fibers using the dimensions set forth in one or more embodiments results in a low density of fibers for reinforcement in polymeric resins. (Fibers  10 , and fewer fibers required for a volume of polymeric resin, results in less interaction of the fibers and clumping.) The design having an internal cavity, sometimes referred to as a hollow core design reduces the oxidation and diffusion pathway within the polymer precursor tube thus keeping approximately the same stabilization and oxidation time as current CF manufacturing for fibers that are approximately 7 μm in diameter. One or more embodiments provide carbon fibers  10  that can be produced with a diameter similar to glass fiber. The fibers of one or more embodiments may have a diameter that is 2× or greater than standard carbon fibers. The fibers of one or more embodiments of this disclosure may have a crystalline or graphene structure and are non-porous. High crystallinity and low porosity results in good mechanical tensile strength and tensile modulus. Specific tensile strength is the tensile strength divided by density and acceleration of gravity (g). 
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         [0020]    The specific tensile strength for the fibers of this disclosure may range from 5×10 4  m to 50×10 4  m, more preferably from 10×10 4  m to 40×10 4  m, and most preferably 20×10 4  m to 30×10 4  m. The specific tensile modulus is the tensile modulus divided by density and acceleration of gravity (g). The specific tensile modulus for the fibers of this disclosure may range from 5×10 6  m to 20×10 6  m, and more preferably from 10×10 6  m to 18×10 6  m, and most preferably 12×10 6  m to 15×10 6  m. 
         [0021]    One or more embodiments provide a relatively low cost material and manufacturing process for carbon fiber reinforced materials that are crystalline and are not porous. Other embodiments of this disclosure may provide a hollow carbon fiber that is a non-cylindrical shape, for example a shape having a square or oblong cross section, or any other suitable profile. 
         [0022]    The outer and inner diameters  12  and  18  of the hollow carbon fibers  10  may vary along the length. The wall thickness  14  and cross sectional area  16  may also vary. Referring to  FIG. 2A , the outer diameter  12  of the hollow carbon fiber  20  varies linearly along the length. The inner diameter  18  may also vary to maintain essentially the same cross sectional area  16  or it may remain the same, or it may vary in a manner different from the outer diameter  18 . Referring to  FIG. 2B , a carbon fiber tube  22  is shown where the inner diameter  18  and the outer diameter  12  vary and the cross-sectional area  16  remains the same along the length of the carbon fiber  22 . Referring to  FIG. 2C , a carbon fiber tube  24  is shown where the outer diameter  12  and the inner diameter  18  vary together in an accordion style. 
         [0023]    The carbon fiber may be in a shape defining a hollow structure other than a tube or cylinder. The cross section of the hollow structure may be a square, oblong, rectangular or other shape. A first cross section taken at a first position along the length of the hollow carbon fiber and a second cross section taken at a second position along the length have cross sectional areas that are substantially the same or may vary 80%, 50%, 20%, 6% or 0.5%. 
         [0024]    CFs are manufactured from their polymer precursors via a series of tensioning, stabilization, carbonization processing etc. The precursor shrinks over these processing by about half. One or more embodiments provides CF precursors that have the same hollow design but with all the dimensions doubled. The benefits of this design include material savings and lower fiber density. The hollow core design can save a substantial amount of material and make the fiber even lighter. 
         [0025]    One or more embodiments involve different manufacturing methods for producing the hollow polymer precursor for the hollow carbon fiber. The embodiments may be continuous processes so as to meet the demand of high volume manufacturing for automotive and other applications. Once the polymer precursor is formed, the hollow polymer precursor is oxidized and stabilized at 200° C. to 300° C. for ˜2 hours at atmospheric pressure. The polymer precursor is then carbonized at 1200° C. to 2900° C. depending on the grade of the carbon fiber. The diameter of the polymer precursor decreases during the carbonization process. The outer diameter of the polymer precursor may vary from 100 μm to 10 μm to form the hollow carbon fiber. 
         [0026]      FIGS. 3A and 3B  depict a perspective view associated with the initial step of forming polymer precursors from polymer materials in a sheet or film form. Polymer material  30  is produced in a sheet or film from CF precursor pellets. The polymer material  30  is pulled against and across one or more tooling plates  32 . The tooling plates have a series of half circle features  34  extending in a longitudinal direction where the features transition to a flat shape. The film initially approaches the tool at the flat end and is tensioned and gradually formed into corrugated half-tube structures as it is pulled. The film direction may change from 10 degrees to 90 degrees from where the film touches the tool plate  36  to the end of the tool plate  38 . Two half-tube films are finally hot pressed together to form complete tubes and split into individual hollow filaments. The hollow polymer precursor tubes may be collected by a spool  40 . 
         [0027]    Referring now to  FIG. 4A , a method of direct die casting of hollow tubes is provided. Polymer material  42  is heated to become re-formable and then pressed together by one or more tooling plates  32  with the half circle features  34 . Comb-like micro pins  44 , shown in  FIG. 4B , are placed at the center of the tooling plate  32  to ensure that a hollow polymer precursor is formed. The hollow fibers are formed and may be collected by a spool. The films may be continuously pressed. The polymer material  42  and the polymer precursor may be pulled across the tooling plate  32 . Referring to  FIG. 4C , a cross section of the tooling plate  32  with half circle features  34  and comb-like micro pins  44  is shown. 
         [0028]    Referring now to  FIG. 5A and 5B , a method of making a polymer precursor is provided from a polymer material that flows and can be pumped. It may be a liquid, in a solution or as pellets. Carbon fiber materials are melted or dissolved in solutions and pumped through a bushing  50 . An air, or gas, delivery tube  52  in the center of bushing holes  54  make the polymer precursors form a hollow tube structure after they are drawn out from the bushing  50  and solidified. The delivery tube prevents the hollow polymer precursor tube wall from collapse and can be a hollow tube or manifold or can be made from a porous steal rod. It can be used with or without gas flowing. It can also be a solid tube. When gas is introduced through the gas delivery tube  52 , it flows out of a manifold outlet  56 . 
         [0029]    The method of forming the polymer precursor for the hollow carbon fiber may utilize mating of two sections, or partial tubes, having unequal size. A polymer precursor is formed on a tooling plate sized to produce a portion of the polymer precursor having a cross section that is more than half the cross section to be formed, with a portion that is less than half of the final cross section. The complete cross sectional shape is then formed by joining partial tubes that are not each half of the carbon fiber to be formed. The method of forming the carbon fiber may include tooling plates and bushings shaped to produce polymer precursors of different cross sectional shapes such as square or rectangular hollow fibers. 
         [0030]    Referring now to  FIGS. 6A and 6B , a flow chart of the steps in the method of making hollow carbon fibers is provided. A polymer material is provided in step  1  in the form of a sheet  60 , a liquid  62 , a solution or pellets. A tooling plate component is provided in step  2  with features  64  on one or both sides. The features may be half round or a portion of the final tube to be formed. The features may be flat on an end of the tooling plate and transition to the desired shape at a second end of the tooling plate. Alternatively, the tooling plate component may be shaped with holes  54  or openings to provide the features by having the polymer material provided in step  1  be a material which is a liquid  62  and flows through the openings. In step  3 , portions of the polymer precursor are joined by hot pressing to form the final polymer precursor shape with walls. In step  4 , shaped walls that are connected, are split into discrete polymer precursors. Step  3  and step  4  would not be required for the tooling plate component designed for a polymer material that flows. If desired, the polymer precursor can be wound onto spools in step  5 . Winding on spools provides a method of transporting and feeding the polymer precursor in the next step, step  6 , where the polymer precursor is oxidized and stabilized. Oxidation in step  6  may be done at atmospheric pressure and 200 to 300° C. for approximately two hours. The oxidized and stabilized polymer precursor is then carbonized in step  7  at 1,200 to 2,900° C. The required temperature depends on the quality of the polymer precursor used. Finally, in step  8  a non-porous, crystalline hollow carbon fiber  10  is formed that has a smaller diameter than the polymer precursor. The fiber diameter is reduced in steps  6  and  7  and may decrease in size by up to a factor of two. 
         [0031]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.