Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2001-01-25
2003-11-11
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S460000
Reexamination Certificate
active
06645456
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for producing expandable graphite intercalation compounds by reacting crystalline graphite particles having a laminar structure with an intercalation agent which contains a strong protonic acid, an oxidizing agent and a phosphoric acid. The graphite intercalation compounds can be used to produce expanded graphite and to form graphite foils, graphite laminates and graphite seals. The invention also relates to a graphite foil providing an improved fluid tight graphite seal.
Graphite intercalation compounds in which the graphite is a carrier of positive charges and anions are intercalated between layer planes of a crystal lattice of the graphite have become increasingly more significant technically over the last 25 years because of their capacity to expand when heat is supplied. Graphite bisulphates and graphite nitrates in particular are important starting products for producing expanded graphite, or graphite expandate, which is in turn processed further by compression to form molded bodies of a wide variety of types, in particular plates and foils. A plurality of products for sealing technology, such as flat seals, stuffing box packings or spherical cap seals, are produced among other items, from plates and foils of that type, as well as directly from graphite expandate. In addition to optimizing the properties of sealing materials such as resilience behavior, chemical resistance, sliding properties or the prevention of corrosion on the sealing surfaces, because of the continuously increasing demands on sealing materials for reasons of environmental protection and the protection of health and safety standards at work, attempts have been made to minimize the permeability of sealing materials and to increase their strength. The improvement of the oxidation stability is an additional target for high-temperature uses. A key to pursuing those targets lies in the manner of producing the starting product, namely the graphite intercalation compound.
According to known methods for producing sealing materials, good crystalline flake graphite is reacted with a mixture of concentrated sulfuric acid and nitric acid, with fuming nitric acid (U.S. Pat. No. 3,404,061), or with a mixture of hydrogen peroxide and concentrated sulfuric acid (U.S. Pat. No. 4,091,083). Excess acid is removed by washing with water and a graphite salt which is obtained in that way is expanded by rapid heating to temperatures of more than 800° C., after a drying step. The expandate, which has a very low bulk weight, is then processed further to form sealing materials or other products. According to another method (U.S. Pat. No. 4,895,713), the process is carried out as anhydrously as possible with such a small amount of intercalation agent that the reaction mixture does not have an excess of liquid and the graphite intercalation compound no longer has to be washed but can instead be expanded directly. A phosphate or phosphoric acid can be added to the reaction mixture in order to improve the oxidation stability of the products produced from the graphite expandate. A disadvantage of the above-mentioned methods is that, despite the use of mainly concentrated acids in the intercalation mixture, generally substantial amounts of water are always present. It is known that salt-type graphite intercalation compounds are hydrolyzed by water. Therefore, if water is present in the reaction mixture, the largest possible intercalation effect cannot be achieved. The structure of the graphite expandate produced from the graphite intercalation compound obtained in that way is admittedly quite good, but is still not optimal. Graphite intercalation compounds produced in accordance with U.S. Pat. No. 3,333,941 do not have an optimal structure either. According to the method which is taught therein, it is possible to work with both very small amounts and very large amounts of intercalation agents (range: 0.25 to 4 g intercalation agent to 1 g graphite). A fundamental characteristic of that method, however, is the addition of phosphorus pentoxide to the intercalation mixture in amounts of 2 to 500 percent by weight with respect to the amount of graphite being used. Washing with water must not take place after the end of the intercalation reaction. An addition of phosphoric acid to the reaction mixture is ineffective. However, it was not the aim of the invention to produce graphite intercalation compounds having a great expanding capacity. The products obtained according to that method are intended for use as mulch in farming, as flame-retardant additives or as pH-value-regulating measures. They could not, therefore be used for the production of high-grade sealing materials. The expansion factor of those graphite intercalation compounds, which is at 1 to a maximum of 200, is accordingly comparatively low. Tests have shown that a serious, practical problem when working according to that method is the handling of the phosphorus pentoxide and the working with that substance. Apart from dusting with the aggressive powder all at once, it cannot be mixed-in homogeneously, or it can only be mixed-in homogeneously with difficulty, particularly when only working with small amounts of intercalation liquid. The mixture heats up and lumps start to form even when mixing phosphorus pentoxide with natural graphite. That happens even if phosphorus pentoxide is mixed with a reaction mixture made up of liquid intercalation agent and natural graphite. Such a coalesced reaction mixture is difficult to handle. An addition of water, which could solve that problem, is counter-productive, because it results in the reaction mixture heating up further and uncontrollably and the actual aim of working as anhydrously as possible is ruined.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for producing expandable graphite intercalation compounds using phosphoric acids, and a graphite foil, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and products of this general type.
Therefore, it is an object of the invention to develop a method for producing salt-type graphite intercalation compounds. In this method, graphite intercalation compounds can be produced with small amounts of intercalation agents and a substantial exclusion of water, which have a very great expansion capacity and which, moreover, have a phosphorus content that gives the products produced from the graphite intercalation compound through the stage of the graphite expandate a high level of fluid tightness, high oxidation stability and an improved strength property. It is noted that the term “fluids” as used herein refers to both liquid and gaseous media.
Another object of the invention was to achieve this target while using as small an amount of intercalation agent as possible.
Furthermore, it was an object of the invention to make available an expanded graphite which is suitable for producing products such as graphite foils, graphite laminates, graphite gaskets, graphite packings, graphite packing rings and graphite packing yarns. The underlying object of the invention was, furthermore, to develop a graphite seal having excellent fluid tightness.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing an expandable graphite intercalation compound, which comprises reacting crystalline graphite particles having a laminar structure, in a temperature range from −10° C. to +80° C., without an additional supply of water, with a reaction mixture formed of at least one strong concentrated acid, an oxidizing agent for oxidizing the graphite, and a thermal polyphosphoric acid.
All naturally occurring or synthetically obtained grades of graphite that have a laminar or plate-like structure can be used for the production of the graphite intercalation compounds. Sulfuric acid in concentrations from 90% by weight, nitric acid in concentrations from 60% by weight, or phosphoric acid
Malik Hubert
Öttinger Oswin
Greenberg Laurence A.
Hendrickson Stuart L.
Lish Peter J
Mayback Gregory L.
SGL Technik GmbH
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