Abstract:
This invention is for a novel method of treating fiber batts. Carbon fiber is a very unique material and its processing requires high temperatures which is provided by heating using forced heated air through the carbon fiber batt from and to rotating drum rollers with porous, perforated or slotted exterior surfaces. Such rollers allow controlled gas flow and controlled handling of the carbon fiber batt through the processing stage while reducing crimping, breakage and friction created fiber loss.

Description:
CROSS REFERENCE RELATED TO APPLICATION  
         [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/309,109 filed on Jul. 31, 2001  
         FIELD OF THE INVENTION  
         [0002]    This invention relates to a method of processing non-rigid materials and more specifically, a method for converting carbon fiber precursors into carbon fiber, and the apparatus used in the process.  
         BACKGROUND OF THE INVENTION  
         [0003]    Carbon fibers represent one of the most unique products having the strength of steel, the weight of aluminum and the conductivity of copper. These properties are extremely desirable for use today in building and construction, i.e., the automotive industry and in electronics and telecommunications products. They possess excellent thermal and electrical conductivity and because of their lightweight property, are ideal for the aircraft industry.  
           [0004]    However, because of the relatively high cost of synthetic-based carbon fibers, their use has been limited. Only high priced goods such as aircraft, sporting goods and other expensive items could afford the use of carbon fibers. Other applications that require strength, stiffness, low weight and good fatigue characteristics had to use other less expensive and less effective products than carbon fiber. Because of these drawbacks or disadvantages, there is a real need for a carbon fiber that can economically compete with these cheaper and less effective materials. A process that will reduce production costs of carbon fibers and make them more universally available is very desirable. Any improved process to create stronger and lighter products of petroleum based carbon fibers will also create new business and redefine how business is done in the carbon fiber industry.  
           [0005]    In the art processes for treating carbon fiber materials such as those described in U.S. Pat. Nos. 5,283,113 (Nishimura et al) and 5,967,770 (Heine et al), exist. Other products not concerned with carbon fiber but directed to treatment of fibrous materials U.S. Pat. Nos. 4,676,445; 4,504,344; 5,915,613; 5,979,731; 6,003,750; 6,004,432; and 6,050,469. Other patents directed to conveyor type processing of web materials include U.S. Pat. Nos. 4,718,543; 4,911,286; 5,791,455; and 5,848,890.  
           [0006]    These patents concerned with the unique product carbon fiber both disclose the basic process for producing carbon fiber felt. For instance, U.S. Pat. No. 5,283,113 issued to Nishimura discloses a process for continuously producing a pitch based carbon fiber felt starting with the raw materials and produces a pitch based carbon fiber felt having uniform unit weight.  
           [0007]    U.S. Pat. No. 5,967,770 issued to Heine patent discloses a continuous treatment of polyacrylonitrile precursor fibers that are heated and processed to produce a carbon fiber strand for use in the plastics arts.  
           [0008]    The previous technologies have relied upon use of conveyor belts to move the carbon fiber precursor material through the heating apparatus to provide the thermal pyrolysis of the precursor to form the desired carbon fiber. It should be noted that the precursor material is fragile and can be broken during the handling process. Furthermore, the prior systems use radiant heating energy to the material which means that where woven, or non-woven webs are employed the pyrolysis is uneven across the cross-section of the material.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention relates to a novel process for processing carbon fibers that employs a plurality of processing drums with porous outer surfaces that allow a furnace vent disposed internally in said drum to exhaust heated gas through the exterior of said drum onto and through the carbon fiber batt. This process allows the batt to be transported more readily and to be heated more effectively than was previously possible.  
           [0010]    The invention can be practiced by processing the fiber batt over a plurality of such rollers where several of the rollers provide a source of heated air. Alternately, a plurality of drum rollers with porous exteriors and internal sources of heated gas can be employed in conjunction with a series of drum rollers with porous surfaces and low pressure (or relatively negative pressure) intakes. In this embodiment the continuous carbon fiber precursor batt can be fed in serpentine fashion through the apparatus.  
       
    
    
     GENERAL DESCRIPTION OF THE FIGURES  
       [0011]    [0011]FIG. 1 Shows a schematic of an embodiment with both positive airflow drum rollers and intake drum rollers.  
         [0012]    [0012]FIG. 2 Shows a schematic of an embodiment with an intake inside a belt roller and an external blower source.  
         [0013]    [0013]FIG. 3 Shows a schematic of a three drum embodiment with three negative pressure intake drum rollers and at least one external heated gas source.  
         [0014]    [0014]FIG. 4 Shows a schematic of an embodiment with a negative pressure intake drum and an external heat source where said drum is clad with a durable insulation layer.  
         [0015]    [0015]FIG. 5 shows a cross-section of an embodiment with three drum rollers each with axial gas outflow and external heated gas flow source.  
         [0016]    [0016]FIG. 6 shows a side view of the cross-section shown in  5   a.    
         [0017]    [0017]FIG. 7 a  shows a three quarters view of a compact convection furnace section.  
         [0018]    [0018]FIG. 7 b  shows a schematic of a furnace assembly. 
     
    
     DETAILED DESCRIPTION  
       [0019]    The present invention relates to methods and apparatus for processing fragile fiber materials that are in the form of felts, batts or woven fabrics. Many of the processes for treating such fibers are high temperature applications and are practiced in the state of the art using conveyor belts. High temperature drum rollers are costly and fragile. If such drum rollers are made of carbon or graphite, they are subject to oxidation if not operated in an inert atmosphere. Portions of the process may require an oxidization atmosphere, which would not allow the use of carbon or graphite drum rollers or components to transport the material. The high temperature drum rollers should be used only in the required area, and other means for transporting the fiber material or batt could be used before and after that area. Thus to the extent that a method and apparatus can be employed that does not employ conveyor belts, certain advantages can be realized.  
         [0020]    The drum roller of the present invention comprises an exterior surface that is generally cylindrical in shape and is porous to gas flow, either from the exterior inward or from a gas supply means outward. This porosity can be achieved by use of a material that by itself is porous, or by the provision for small holes or passageways through the material of the cylindrical outer portion of the drum. Inside the drum roller and cooperating with the porous exterior surface is a manifold or plenum chamber that provides to said exterior surface in at least a portion of said exterior surface a supply of gas. Alternately, the plenum or manifold might provide to the exterior surface a relative vacuum so that external sources of gas might be drawn into said drum roller.  
         [0021]    In cases where gas is supplied to said surface, the gas will flow out of said drum roller and provide a cushion to any fibrous material disposed on the external surface of said roller. Thus material could be transported across a series of such rollers without the same amount of friction as a series of rollers without such a gas supply.  
         [0022]    In another form of drum roller where the surface is supplied with a relative negative gas pressure, material disposed on the bottom of the drum will be urged to follow the rotation of the drum and be transported along said drum. In either embodiment, the external flow embodiment, or the suction embodiment, drum rollers can be used to transport fibrous materials along desired pathways in a fashion that is controlled and which minimize folding or kinking of the fibrous materials. When used in this fashion the drum rollers provide a transport function.  
         [0023]    Such rollers can be used to provide higher temperature gas flows through the fibrous materials and act as processing units, if gases with the desired temperature and chemical characteristics are provided. For instance, if a drum roller with an internal manifold connected to a source of high temperature air is used and carbon fiber precursor pad is passed over said drum roller and a means for exhausting said heated gas the drum roller will process the precursor providing a degree of pyrolysis that can be controlled. In such high temperature applications the external surface of the drum roller should be made from temperature resistant materials such as carbon-carbon composites known in the art, or advanced ceramic materials. Temperature ranges for such processes are within the range of 800° to 2700° C., therefore parts subject to such materials must be heat resistant and designed to minimize stress.  
         [0024]    The means for exhausting such a high-temperature apparatus can be a second drum roller that has an internal vacuum manifold, or a separate ducting means for directing the heated gas away from the fiber matt surface opposite the process drum roller.  
         [0025]    Exit purge boxes are used to allow control of gasses from one drum roller to the next. For example, the first drum roller could have air as a gas while the second drum roller could have nitrogen as a gas. The exit purge box shown has provisions for a nitrogen purge pipe, which sprays nitrogen across the moving material. This could be in the entrance purge box also, if desired. The purge boxes have movable tops, which can be externally adjusted up or down to suit the height of the material. The boxes are designed to be remotely closed to compress the material and reduce gas flow from one drum roller to the other, or closed for safety reasons, as needed.  
         [0026]    In one embodiment of this invention, the large drum in the process of this invention has a non-rotating inner spool, which is hollow. The suction side of a blower is connected to this inner opening of the spool. The spool is secured from rotating by being locked in the wall. There are slots cut in the spool to allow gas to flow through holes in the drum hub and into the appropriate segments created by vanes. As the drum rotates, vane sections become “opened” and “closed” to the suction of the inner spool.  
         [0027]    A fiber batt ( 3 ), FIG. 1, is supplied by a feed belt ( 1 ) to a first drum roller ( 4 ) with a porous or perforated exterior surface ( 6 ) and a non-rotating plenum that provides a relative vacuum to the fiber batt and guides it to a processing drum roller ( 5 ) that blows heated air or another reactive gas through the fiber batt. The heated batt then is guided to the third drum roller ( 7 ) which is the second transport drum roller in this embodiment, that has an internal vacuum source that guides the processed fiber batt onto the discharge belt.  
         [0028]    In another embodiment, FIG. 2, roller means ( 20 ) comprises a first belt roller ( 22 ) and a second belt roller ( 23 ), a perforated belt ( 21 ) rolling around said first belt roller and said second belt roller, and a manifold ( 24 ) disposed within said belt that provides a relative vacuum to the upper surface of a fiber batt that produces an up-flow of gas across said fiber batt.  
         [0029]    The apparatus drum rollers could also be placed inside a furnace, FIG. 3, with an entry ( 32 ) and an exit ( 33 ), or controlled atmosphere chamber, to provide gas flow across the fiber batt ( 3 ). A means for feeding the fiber batt ( 31 ) into the furnace or chamber would provide the material to the first drum roller ( 35 ) which would have a relative vacuum and draw the reactive gas through the batt and present the fiber batt to the second roller ( 36 ) which would draw the fiber batt to the third drum roller ( 37 ). The fiber batt would then be guided to the discharge means ( 33 ). Control and retention of heat in fiber treatment processes is often critical to the industrial application of a materials processing step. With this in mind the roller drum ( 41 ), FIG. 4, used to transport gas away from the fiber batt ( 3 ), is covered with a porous insulation material ( 42 ) that is made of a material with a similar heat retention profile as the material being processed. This porous insulation layer retain heat from the heated gas source ( 44 ) so that less heat is transferred to the vacuum plenum ( 43 ) in the insulated drum roller.  
         [0030]    Gas flows ( 56 ) can also be used to support the fiber batt ( 3 ) on the drum rollers ( 52 ) as shown in cross section of a three roller system with two transport roller drums ( 53 ),( 54 ) and a larger diameter processing drum ( 55 ) as shown in FIG. 5. Material is transported to the roller furnace by a transport belt ( 51 ) and the fiber batt ( 3 ) is transported using a blower ( 54 ) to a first transport drum roller ( 53 ). The material ( 3 ) is removed using a second blower ( 58 ) to a transfer belt ( 53 ) The system can also be used to provide a greater processing length into a smaller footprint than typical conveyor belt systems by providing a series of roller drums that operate as both transport drums and as processing drums simultaneously. A plurality of drum units ( 61 ) where each drum roller has a gas intake ( 62 ) and perforated or slotted drum ( 63 ) and a gas exhaust passage, can be arranged in a linear series of overlapping rows to provide a serpentine path that is greater by several magnitudes than the length or height of the multi-roller drum assembly.  
         [0031]    [0031]FIG. 7 b  shows how a plurality of these units ( 65 , 66 , 67 , 68 , 69 ) could be linked to one another in series to form a processing furnace of great length that is quite compact.  
         [0032]    In FIG. 3 an apparatus and method are shown having three basic sections: an entrance transfer section  1 , a carbonizing or graphitizing section  2 , and exit transfer to cooling section  3 . In section  1 , a carbon fiber mat  3  is fed into the system via belt at feed entrance. The transfer of lightweight fibrous materials such as carbon fibers from the feed conveyor to the transfer drum roller can be difficult if the material is fragile and not able to stay intact. Transfer section  1  uses gas flow from a blower to float the material  3  across the gap between the feed conveyor and the first drum roller. A drum or other moving conveyor above the gap allows the material to be blown or drawn up to the drum The pressure under the mat or material  3  caused by the air flow from blower keeps the material or mat from falling and delaminating as it crosses gap. Additional ducts may be employed to provide for blow-off through the drum which also aids in the removal of the material (carbon fiber) from the drum. This blow off acts as a self-cleaning device to remove fibers sucked into the drum perforations. Adjacent the feed drum may be purge boxes, an entrance purge box  15  and exit purge box  16 . These boxes enclose the material  3  being transferred and allow control of gasses from one conveyor area to the other.  
         [0033]    If high temperatures are required, alloy drum rollers are not suitable. Common refractory materials can withstand heat but are generally weak in tension and are brittle. High temperature materials could be used to support the fiber mats  3  while other materials could take the pull forces. The material of construction of the drum rollers and drive mechanisms should be made of metal, ceramic (including graphite), intermetallic, composite and/or other materials suitable for the furnace environment. “Intermetallics” are metals that exhibit ceramic like properties. Examples of intermetallics are iron aluminide and molybdenum disilicide. “Composites” are materials made up of dissimilar materials. Examples of composites are silicon carbide reinforced aluminum and silicon carbide reinforced alumina. Alternatively, the drum rollers may be made up of plates, mesh, screen or a composite of forms.  
         [0034]    From heating section  2  (carbonizing or graphitizing section) the carbon-fiber batt  3  is transported to the exit transfer cooling section  3 . As in section  1 , a top drum  10  having internal non-rotating ducts  12  is used. The carbon-fiber mat is cooled in section  3  and ready to be taken out of the system for shipping, packing and use or may continue for further processing.  
         [0035]    Various embodiments of the present invention have been described herein and shown in the accompanying drawings to illustrate the underlying principles of the invention, but it is to be understood that numerous modifications and ramifications may be made without departing from the spirit and scope of this invention.