Plastic and nonmetallic article shaping or treating: processes – Carbonizing to form article
Reexamination Certificate
2001-05-22
2003-06-10
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Carbonizing to form article
C264S029600, C264S042000, C423S44500R, C423S448000
Reexamination Certificate
active
06576168
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the manufacture of pitch foams, to the subsequent conversion of pitch foam to carbon and graphite foam and to improvements in the manufacturing process to enhance the properties of the end products.
2. Description of the Prior Art
This invention deals with carbon in its various forms and, particularly to carbon “foams”. Carbon foams are a relatively recent area of commercial interest, although carbon fibers have been used commercially in industry for many years. Carbon fibers are known to exhibit extra ordinary mechanical properties due to the unique graphitic morphology of the extruded filaments. Advanced structural composites have been created which advantageously exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Pitch based carbon foams can be considered such an interconnected network of ligaments or struts. As such, pitch based carbon foams represent a potential alternative as a reinforcement in structural composite materials.
Additionally, current applications of carbon fibers have evolved from such structural reinforcement applications to thermal or heat sink applications. For example, heat sinks have been utilized in the aerospace industry to absorb energy in applications such as missiles and aircraft where rapid heat generation is found. A material with a large specific heat capacity is placed in contact with the object that is being heated. During the heating process, heat is transferred to the heat sink from the hot object and, as the heat sink's temperature rises, it “stores” the heat more rapidly than can be dissipated to the environment through normal convections. Carbon foams have been considered for use as such heat sink materials.
These and other applications have stimulated research into novel reinforcements and composite processing methods for carbon foams. High thermal conductivity, low weight and low coefficient of thermal expansion are of primary concern in such designs. For thermal management applications, certain designs which have been considered included sandwich type approaches in which a low density structural core material, such as a honeycomb or foam, is sandwiched between a high thermal conductivity face sheet. Structural cores of these type are generally limited to low density materials to insure that the weight limits are not exceeded. At the present time, carbon foams and carbon honeycomb materials have generally been the only available materials for use in high temperature applications (exceeding 60° C.). High thermal conductivity carbon honeycomb materials have been extremely expensive to manufacture, however, as compared to low conductivity honeycomb materials. Attempts have been made to overcome these shortcomings through the production of pitch based carbon foam materials.
Typical prior art foaming processes utilized a “blowing” technique to produce a foam of the pitch precursor. The pitch is melted and pressurized, and then the pressure is reduced. Thermodynamically, this produces a “Flash,” thereby causing the low molecular weight compounds in the pitch to vaporize (the pitch boils), resulting in a pitch foam. See Hagar, Joseph W. and Max L. Lake, “Novel Hybrid Composites Based on Carbon Foams,”
Mat. Res. Soc. Symp.
, Materials Research Society, 270:29-34 (1992), Hagar, Joseph W. and Max L. Lake, “Formulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor,”
Mat Res. Soc. Symp.
, Materials Research Society, 270:35-40 (1992), Gibson, L. J. and M. F. Ashby,
Cellular Solids: Structures and Properties
, Pergamon Press, New York (1988), Gibson, L. J., Mat Sci and Eng A110, 1 (1989), Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36 a (4), (1976), and Bonzom, A., P. Crepaur and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Additives can be added to promote, or catalyze, the foaming, such as dissolved gases (like carbon dioxide, or nitrogen), talc powder, freons, or other standard blowing agents used in making polymer foams.
Then, unlike polymer foams, the pitch based foam must generally be oxidatively stabilized by heating in air (or oxygen) for many hours, thereby, cross-linking the structure and “setting” the pitch so it does not melt, and deform the structure, during carbonization. See Hagar, Joseph W. and Max L. Lake, “Formulation for Mathematical Process Model for the Foaming of a Mesophase Carbon Precursor, ”
Mat. Res. Soc. Symp.
, Materials Research Society, 270:35-40 (1992) and White, J. L., and P. M. Shaeffer,
Carbon,
27:697 (1989). This is a time consuming step and can be an expensive step depending on the part size and equipment required.
Next, the “set” or oxidized pitch foam is then carbonized in an inert atmosphere to temperatures as high as 1100° C. Then, a final heat treatment can be performed at temperatures as high as 3000° C. to fully convert the structure to carbon and produce a carbon foam suitable for structural reinforcement. The previously described prior art processes resulted in foams which exhibited low thermal conductivities, however.
Other techniques may utilize a polymeric precursor, such as a phenolic, urethane, or blends of these with pitch. See Hagar, Joseph W. and Max L. Lake, “Idealized Strut Geometries for Open-Celled Foams, ”
Mat. Res. Soc. Symp.
, Materials Research Society, 270:41-46 (1992), Aubert, J. W., (MRS Symposium Proceedings, 207:117-127 (1990), Cowlard F. C. and J. C. Lewis,
J. of Mat. Sci.,
2:507-512 (1967) and Noda, T., Inagaki and S. Yamada,
J. of Non
-
Crystalline Solids,
1:285-302, (1969). However, these precursors produce a “glassy” or vitreous carbon which does not exhibit graphitic structure and, thus, has a very low thermal conductivity and low stiffness as well. See, Hagar, Joseph W. and Max L. Lake, “Idealized Strut Geometries for Open-Celled Foams, ”
Mat. Res. Soc. Symp.
, Materials Research Society, 270:41-46 (1992).
An improvement to the previously prescribed prior art techniques is described in now issued U.S. Pat. No. 6,033,506, issued Mar. 7, 2000 to Klett and in issued U.S. Pat. No. 6,037,032, issued Mar. 14, 2000, to Klett et al. The process described in these later patents is less time consuming than the techniques previously described, thereby lowering production and fabrication costs. Perhaps more importantly, the Klett process is unique in providing carbon foams with high thermal conductivities, generally greater than 58 W/mK.
Although the Klett process was an improvement in pitch based carbon foaming processes, the Klett process utilized a static pressure during the formation of the green artifact (billet). Routinely, this static pressure selected was about 1000 psig. Graphite artifacts made in this manner have shown a significant density gradient, generally ranging from about 0.25 g/cc at the top of a production billet to about 0.60 g/cc at the bottom of the billet. Such variations can be undesirable, depending upon the particular end application.
A need exists, therefore, for further improvements in pitch based carbon foams and products produced therefrom in which density gradients are reduced.
A need also exists for such a carbon foam exhibiting reduced pore/bubble sizes within the foam during processing.
A need exists for such a process which prevents or reduces thermally induced stresses in the final product.
A need also exists for an improved process for producing a pitch based carbon foam which allows the foam to set faster and which provides an improved ability to manipulate the viscosity of the material during the process stage in which the material is in the liquid/foaming state.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a pitch based carbon foam having a more uniform density gradient profile, with reduced shrinkage and with less tendency to crack as a finished product.
Density variations in currently produced products are thought to occur between the foaming and solidification steps of t
Dingus David F.
Hardcastle Leland A.
Sheppard Rex G.
Bracewell & Patterson L.L.P.
Kuhns Allan R.
Poco Graphite, Inc.
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