Compositions – Electrically conductive or emissive compositions
Reexamination Certificate
2001-03-21
2004-02-10
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
C252S500000, C252S502000, C252S062200, C264S126000, C264S029100, C264S029500, C264S029600, C264S101000, C264S105000, C208S022000, C208S039000, C208S044000, C423S44500R, C423S447400, C429S245000, C521S180000
Reexamination Certificate
active
06689295
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a light porous carbon material having exfoliated or expanded graphite as a principal component, and to a method of manufacture thereof.
BACKGROUND OF THE INVENTION
Usually, non-woven fabrics, felt and the like materials composed mainly of highly porous carbon fiber are used for heat insulation at ultrahigh temperatures of 2000° C. or higher. These heat-insulating materials are highly porous, but because they have no closed pores or cells they are permeable to radiant heat and cannot exhibit the superior heat insulating characteristics that are expected for the high porosities.
Japanese Unexamined Patent Publication No. 33551/1995 discloses a method of manufacturing a porous graphite or carbon material for use as a heat insulator for vacuum furnaces. The method comprises the steps of foaming phenol resin with foaming agent and foam-stabilizing agent, and baking the resulting foam at 2000° C. or more in an inert gas atmosphere. However, the porous material produced by this method is very non-uniform in pore diameter, and since the carbonization yield after heat-treatment is about 70%, and the carbonization shrinkage rate may reach about 20% in one direction and the volumetric shrinkage can be as much as about 50%, it has very poor dimensional stability. Moreover, when heat-treating a foamed article with large volume, it is necessary to heat it carefully over a long period of time, taking care not to cause cracking in the article, and the resulting high-cost product is not suited for practical use. In addition, the porous material obtained after heating or baking is hard and fragile, with low resistance to mechanical shock. These defects may be attributable to glassy carbon which forms on graphitization of phenol resin in the foam baking step because it is a dense and hard carbon material, and it is therefore difficult to graphitize even when heat-treated at a temperature of 2000° C. or higher in a subsequent processing.
SUMMARY OF THE INVENTION
Therefore, it is a principal object of the present invention to provide a foamed carbon material excellent in high-temperature insulation and thermal shock-resistance, and a method of manufacturing such a foamed carbon material.
The inventors have found that when a mixture of phenol resin and powdered and/or crushed exfoliated or expanded graphite is used as the starting material for manufacturing a foamed carbon product, the following phenomena occur:
(1) By suitably selecting a range of grain size for the powdered and/or crushed exfoliated or expanded graphite (referred to hereinafter as “exfoliated graphite powder” except when necessary) and a ratio of exfoliated graphite powder and phenol resin in the mixture, it is possible to control the pore diameter and porosity in a resin foam formed by heating the mixture;
(2) Since phenol resin foams easily when heated, the foaming volume of the resin foam that is the intermediate product can easily be controlled through the heating temperature;
(3) Baking time can be reduced when the resulting resin foam is subjected to heating at 2000° C. or more to produce a carbon foam or foamed carbon body;
(4) When the pore diameter and porosity of the resin foam are controlled, it is dimensionally stable during the baking step and shows little shrinkage. Thus, by controlling the parameters in (1) above the pore diameter and porosity in the carbon foam as the final product can be controlled as required.
More specifically, according to the present invention, the pore diameter in the carbon foam can be controlled with use of the exfoliated graphite powder having adjusted particle size by sieving. Thus, with the proportion of exfoliated graphite powder in the starting mixture fixed, the resin foam that is the intermediate product will have a smaller pore size if the exfoliated graphite powder grains are small in diameter, making it less porous and therefore denser. Conversely, if the exfoliated graphite powder grains are large in diameter, the resin foam will have larger pore sizes, making it more porous and consequently less dense.
Alternatively, with the grain diameter of the exfoliated graphite powder in the starting blend fixed, a greater ratio of exfoliated graphite powder will result in the intermediate resin foam and the final carbon foam which are more porous and less dense. Conversely, a smaller proportion of exfoliated graphite powder will result in less porous and denser resin foam and carbon foam.
Furthermore, the controlled porosity and density described above for the resin foam can be reproduced in much the same degree in the carbon foam.
The present invention was achieved based on the new findings described above and provides the following resin foam, carbon foam and methods of manufacture thereof.
1. An exfoliated graphite/phenol resin composite foam with a porosity of 50-95% and a density of 0.1-0.8 g/cm
3
, formed by molding a mixture of 100 parts by weight of powdered and/or crushed exfoliated graphite with a grain diameter of 5-3000 &mgr;m and 40-240 parts by weight of phenol resin under contact pressure or reduced pressure at a temperature of 140-200° C.
2. A method of manufacturing an exfoliated graphite/phenol resin composite foam with a porosity of 50-95% and a density of 0.1-0.8 g/cm
3
, the method comprising molding a composition of 100 parts by weight of powdered and/or crushed exfoliated graphite with a grain diameter of 5-3000 &mgr;m and 40-240 parts by weight of phenol resin under contact pressure or reduced pressure at a temperature of 140-200° C.
3. An exfoliated graphite/glassy carbon composite foam, the composite foam being obtainable by baking the exfoliated graphite/phenol resin composite foam described in item 1 above in a non-oxidizing atmosphere at a temperature of 600-2000° C., and having a volumetric shrinkage of 10% or less.
DETAILED DESCRIPTION OF THE INVENTION
The phenol resin used in the invention is not limited to the narrowly-defined phenol resin obtained by the reaction of phenol and formaldehyde, but encompasses a broader range of phenol resins designated by such terms as cresol resin, alkylphenol resin, etc., corresponding to respective starting materials. In more detail, the nucleus of the resin structure can be formed from a wide range of phenols including not only phenol but also o-cresol, p-cresol, m-cresol, resorcinol, 1,3,5-trimethylphenol, o-xylene, p-xylene, m-xylene, phenylphenol, etc. Likewise, besides formaldehyde, benzaldehyde, terephthalaldehyde, paraxylene glycol, etc. can be used to bridge the phenols.
Although usable phenols are not limited, preferable are those having a carbonization yield of 50wt. % or more, more preferable are those having a carbonization yield of 70 wt. % or more when such a phenol resin is heated to 1000° C. under a non-oxidizing atmosphere. Using a phenol resin with a higher carbonization yield results in stronger bonds between the exfoliated graphite powder grains.
Other resins with high carbonization yields such as furan resins and polyimide resins can be used, as well as pitch and tar, but phenol resins are more desirable from the standpoint of lower cost and ease of handling.
The phenol resin used may be in liquid, powdered or solid form, but from the standpoint of productivity (ease of handling, production speed, process simplification, etc.) the powdered form is most preferable. Also, when considering the quality stability and reproducibility of the exfoliated graphite/phenol resin composite foam and exfoliated graphite/glassy carbon composite foam, the particle diameter of the phenol resin powder is preferably the same as or close to that of the exfoliated graphite powder as described below.
The properties of the powdered or crushed exfoliated graphite (referred to hereinafter simply as “exfoliated graphite powder”) used in the present invention are not limited. The exfoliated graphite powder is usually about 5-3000 &mgr;m (preferably 50-3000 &mgr;m, and more preferably 100-1000 &mgr;m) in particle diameter and 0.004-0.2 g/cm
3
(preferably 0.05-0.1 g/cm
3
)
Hirohata Takeshi
Kawakami Shinya
Armstrong Kratz Quintos Hanson & Brooks, LLP
Kopec Mark
Osaka Prefectural Government
Vijayakumar Kallambella
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