Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2000-01-28
2002-04-16
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S447100, C204S155000
Reexamination Certificate
active
06372192
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
FIELD OF THE INVENTION
The present invention relates to the production of carbon and/or graphite fibers. More particularly, the present invention relates to a method for carbonizing and/or graphitizing carbon fiber precursors using plasma technology and electromagnetic irradiation.
BACKGROUND OF THE INVENTION
Carbon and graphite fibers are commonly used as reinforcement materials in advanced structural composites. Advanced structural composites are generally lightweight and possess superior strength and elasticity over most metals. Because of these characteristics, highly advanced composites are now regularly utilized as structural members in the aerospace industry and in high-tech space applications. The use of these composites in other commercial industries, however, has seen limited application due to the high material costs associated with carbon and graphite fibers and the lack of rapid and efficient techniques for their manufacture. Currently, only moderate-cost fibers have found common application in broad consumer markets. These markets typically include the construction of items such as tennis rackets, fishing poles, and golf clubs.
Carbon and graphite fibers are produced through the controlled pyrolysis of fibrous organic carbon precursors such as polyacrylonitrile (PAN), pitch (petroleum or coal tar), or rayon. Generally, rayon-based precursors are used to produce low modulus carbon fibers (fibers having a modulus ≦50 GPa, or 7×10
6
PSI) while PAN or liquid crystalline (mesophase) pitch precursors are used to make the higher modulus carbon fibers (fibers having a modulus ≧200 GPa, or 7×10
6
PSI) used in advanced composites. Of these precursors, the PAN precursor is generally preferred due to its high carbon yield and unique mechanical properties which intrinsically avoid the need for an expensive final “graphitization” step.
The process for manufacturing carbon and graphite fibers is generally a lengthy and expensive process. The conventional process begins by spinning the carbon precursor into a fiber form using any one of several different spinning techniques. Once set in fiber form, the carbon fiber precursor is typically subjected to a stabilization step wherein the fiber is heat-treated in air and at relatively low temperatures (approximately 200° C. to 350° C. or higher). As a result of this stabilization step the outer layers or regions of the fiber are converted to an infusible and thermally stable structure capable of withstanding the high processing temperatures necessary for carbonization to a carbon or graphite form. Depending upon the stabilization process conditions employed, the stabilization process may also result in the conversion of the entire fiber to a fully stabilized form.
To form carbon or graphite fibers, the stabilized carbon fiber precursor is fired in an inert atmosphere at extremely high temperatures while under tension. Carbon fibers are generally achieved by firing at temperatures between 1000° C. and 2000° C., while higher modulus carbon fibers (graphite) normally require firing at temperatures in excess of 2500° C. The high temperatures cause the initial organic material in the fiber to convert into carbon while the fiber's noncarbon elements are expelled in the form of volatile gases. This off-gas stream is toxic and includes substantial amounts of HCN, NH
3
, N
2
, and H
2
O with lesser amounts of low molecular-weight nitriles, CO
2
, CH
4
, CO and H
2
. Because of the toxic nature of the off-gas stream, treatment by liquid-phase scrubbing or catalytic combustion is required before venting. Typically, the entire carbon/graphite manufacturing process is performed in multiple and sequential conventional graphite brick-lined furnaces and may require hours to complete.
Following carbonization, the carbon or graphite fiber is usually surface treated to enhance its ability to adhere to a sizing agent and a matrix material, usually a polymeric resin. The matrix serves to bind the fibers together, forming a coherent structure and providing a medium for transferring applied stresses from one fiber to another. The matrix material affects the composites high temperature mechanical properties, transverse strength and moisture resistance, as well as other properties, and is a key factor in toughness, shear strength, and oxidation and radiation resistance. The matrix system also strongly influences the fabrication process and associated parameters for forming intermediate and final products from the composite materials.
Untreated carbon and/or graphite fiber surfaces usually have low surface energies which limit their ability to form strong adhesive bonds with matrix materials. Surface treatments applied to these fibers are able to overcome this limitation by increasing the fiber's surface activity and surface energy. These treatments typically include surface modification processes such as anodic oxidation, electrodeposition, wet and dry oxidation, plasma etchings, coatings, ion implantations, and more. Of these processes, low pressure plasma processing has offered a very attractive and efficient method for modifying the fiber's surface activity without affecting its bulk properties.
Plasma surface treatment of fully processed (fully carbonized) fibers is a well known technology previously discussed at length by J. C. M. Peng et al., “Surface Treatment of Carbon Fibers,”
Carbon Fibers, Third Edition
, 180-187 (J. B. Donnet et al., ed., 1998); L. H. Peebles, “Plasma Treatment,”
Carbon Fibers Formation, Structure, and Properties
, 128-135 (1995); and J. Delmonte, “Surface Treatment of Carbon/Graphic Fibers,”
Technology of Carbon and Graphite Fiber Components
, 189-191 (1981), incorporated herein by reference. In the typical plasma treatment, the surface concentration of polar (oxygen-containing) groups on the filly carbonized or filly or partially graphitized fiber surface are increased by exposure to an oxygen-plasma treatment. The increased polarity, in turn, leads to both higher epoxy adhesive wetability and stronger intrinsic adhesion across the adhesive/composite interface. Under normal processing conditions, the plasma surface treatment results in extensive modifications to the outmost few atomic layers of the substrate while leaving the bulk properties of the fiber intact.
Currently, over 30,000 tons of carbon fibers are produced annually throughout the world. Although this number may seem substantial, the commercial industry has yet to realize the potential widespread use of carbon fibers because of the high costs associated with their production as compared to other materials. The most significant cost factors include the high cost of carbon precursors (45-50% of production costs), the high cost of equipment and energy consumption (20-25% of production costs), and the time expense associated with producing a quality product. In regards to the latter factor, attempts to speed the process has often resulted in the rapid burn off of the noncarbon elements which, in turn, creates bubbles and cracks in the fiber. These bubbles and cracks substantially weaken the fiber's mechanical properties such that the fibers are rendered incapable of use for their desired purpose.
U.S. Pat. No. 4,197,282, discloses a technology which is intended to reduce the costs associated with producing carbon fibers from natural organic materials, such as petroleum distillation residues or coal. In this process, carbonized and/or graphitized fibers are manufactured from natural organic precursors using a preparatory thermal treatment step and microwave irradiation. In its application, the natural organic material is spun into a fibrous carbon precursor and then heat treated in an inert atmosphere at a temperature between 300° C. and 1500° C. in a conventional furnace. The preparatory thermal treatment produces an initial carbonization which allows an interaction between the microwaves and the fibers. As with the conventional process, the inert atmosphere is obtained by
Meek Thomas T.
Paulauskas Felix L.
Yarborough Kenneth D.
Hendrickson Stuart L.
Quarles & Brady LLP
UT-Battelle, Inc.
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