Abstract:
A method for providing intercalated graphite flake with improved exfoliation characteristics, specifically, improved expansion pressure, is presented. The method includes treating graphite flakes with an oxidizing intercalant solution to provide intercalated graphite flakes; washing the intercalated graphite flakes; and adding a surfactant to the washed graphite flakes. The invention also includes the intercalated graphite flakes thusly prepared.

Description:
TECHNICAL FIELD 
     This present invention relates to intercalated graphite flakes having improved expansion characteristics when exposed to elevated temperatures, and a process for preparing such flakes. 
     Intercalated graphite flake has many useful applications. One common application is to exfoliate the intercalated graphite particles into vermicular-like structures that are then compressed into sheets conventionally referred to as flexible graphite, for use in the manufacture of products such as gaskets, thermal management materials or fuel cell components. Intercalated graphite flake is also used in a variety of products that take advantage of the expansion characteristics of intercalated graphite flake when exposed to high temperature. Examples of such uses include using the intercalated flake in combination with polymer foams to form seat cushions and furniture upholstery in aircraft and incorporation of the flake in resin-based coatings for wallboards, lumber and other construction materials. Upon exposure to fire, the high temperature will cause the particles of intercalated graphite to exfoliate which minimizes or prevents the formation of toxic gases from the polymer foam and may, of itself, smother a fire. 
     Since it is important to suppress, i.e. retard, a fire before it has begun to spread, it would be a substantial advantage for an intercalated graphite flake product to exhibit a very high degree of exfoliation upon exposure to temperatures as low as 600° C. and even lower. Moreover, since in some applications the graphite flake is incorporated in another material, such as a polymer foam or a resin, the foam or resin can tend to physically inhibit expansion, i.e., there is a load on the flake when it expands. In such circumstances, not only is the degree of expansion important, so is the expansion pressure. Insufficient expansion pressure will result in reduced expansion under the load to which the flake is subjected, reducing the fire-suppression effectiveness of the intercalated graphite flake. 
     It has been discovered in accordance with the present invention that the treatment of intercalated graphite flake with surfactant, following intercalation of the graphite flake with an oxidizing intercalant solution results in a material which exhibits enhanced exfoliation characteristics, such as expansion pressures, at exfoliation temperatures as low as 600° C. and even lower. 
     BACKGROUND OF THE INVENTION 
     Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion. 
     Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation. 
     As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Particles, or flakes, of graphite can be treated by a process referred to as intercalation to form intercalated graphite flake whereby, after exposure to elevated temperatures, the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for forming particles of intercalated graphite flake having enhanced exfoliation characteristics, specifically, expansion pressure, at temperatures as low as 600° C. and even lower by treating particles of graphite with an oxidizing intercalant solution to provide intercalated graphite flake; washing the intercalated graphite flake; and thereafter contacting the intercalated graphite flake with a surfactant. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Graphite is a crystalline form of carbon comprising atoms covalently bonded in flat layered planes with weaker bonds between the planes. By treating particles of graphite, such as natural graphite flake, with an intercalant of, e.g. a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated particles of graphite are hereafter referred to as “particles of intercalated graphite” or intercalated graphite flake. Upon exposure to high temperature, the intercalant within the graphite decomposes and volatilizes, causing the particles of intercalated graphite to expand in dimension as much as about 80 or more times its original volume in an accordion-like fashion in the “c” direction, i.e. in the direction perpendicular to the crystalline planes of the graphite. The exfoliated graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes. Alternatively, the intercalated graphite flake can itself be used in, e.g., fire or flame retardant compositions or coatings. 
     Graphite starting materials suitable for use in the present invention include highly graphitic carbonaceous materials capable of intercalating organic and inorganic acids as well as halogens and then expanding when exposed to heat. These highly graphitic carbonaceous materials most preferably have a degree of graphitization of about 1.0. As used in this disclosure, the term “degree of graphitization” refers to the value g according to the formula:        g   =       3.45   -     d        (   002   )         0.095                            
     where d(002) is the spacing between the graphitic layers of the carbons in the crystal structure measured in Angstrom units. The spacing d between graphite layers is measured by standard X-ray diffraction techniques. The positions of diffraction peaks corresponding to the (002), (004) and (006) Miller Indices are measured, and standard least-squares techniques are employed to derive spacing which minimizes the total error for all of these peaks. Examples of highly graphitic carbonaceous materials include natural graphites from various sources, as well as other carbonaceous materials such as carbons prepared by chemical vapor deposition and the like. Natural graphite is most preferred. 
     The graphite starting materials used in the present invention may contain non-carbon components so long as the crystal structure of the starting materials maintains the required degree of graphitization and they are capable of exfoliation. Generally, any carbon-containing material, the crystal structure of which possesses the required degree of graphitization and which can be exfoliated, is suitable for use with the present invention. Such graphite preferably has an ash content of less than six weight percent. More preferably, the graphite employed for the present invention will have a purity of at least about 99%. In the most preferred embodiment, the graphite employed will have a purity of at least about 99.9%. 
     A common method for manufacturing particles of intercalated graphite is described by Shane et al. in U.S. Pat. No. 3,404,061, the disclosure of which is incorporated herein by reference. In the typical practice of the Shane et al. method, natural graphite flakes are intercalated by dispersing the flakes in a solution containing e.g., a mixture of nitric and sulfuric acid, advantageously at a level of about 20 to about 300 parts by weight of intercalant solution per 100 parts by weight of graphite flakes (pph). The intercalation solution contains oxidizing and other intercalating agents known in the art. Examples include those containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, e.g. trifluoroacetic acid, and a strong oxidizing agent soluble in the organic acid. Alternatively, an electric potential can be used to bring about oxidation of the graphite. Chemical species that can be introduced into the graphite crystal using electrolytic oxidation include sulfuric acid as well as other acids. 
     In a preferred embodiment, the intercalating agent is a solution of a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or periodic acids, or the like. Although less preferred, the intercalation solutions may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halide, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic solvent. The quantity of intercalation solution may range from about 20 to about 150 pph and more typically about 50 to about 120 pph. 
     The use of an expansion aid applied prior to, during or immediately after intercalation can also provide improvements. Among these improvements can be reduced exfoliation temperature and increased expanded volume (also referred to as “worm volume”). An expansion aid in this context will advantageously be an organic material sufficiently soluble in the intercalation solution to achieve an improvement in expansion. More narrowly, organic materials of this type that contain carbon, hydrogen and oxygen, preferably exclusively, may be employed. Carboxylic acids have been found especially effective. A suitable carboxylic acid useful as the expansion aid can be selected from aromatic, aliphatic or cycloaliphatic, straight chain or branched chain, saturated and unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids which have at least 1 carbon atom, and preferably up to about 15 carbon atoms, which is soluble in the intercalation solution in amounts effective to provide a measurable improvement of one or more aspects of exfoliation. Suitable organic solvents can be employed to improve solubility of an organic expansion aid in the intercalation solution. 
     Representative examples of saturated aliphatic carboxylic acids are acids such as those of the formula H(CH 2 ) n COOH wherein n is a number of from 0 to about 5, including formic, acetic, propionic, butyric, pentanoic, hexanoic, and the like. In place of the carboxylic acids, the anhydrides or reactive carboxylic acid derivatives such as alkyl esters can also be employed. Representative of alkyl esters are methyl formate and ethyl formate. Sulfuric acid, nitric acid and other known aqueous intercalants have the ability to decompose formic acid, ultimately to water and carbon dioxide. Because of this, formic acid and other sensitive expansion aids are advantageously contacted with the graphite flake prior to immersion of the flake in aqueous intercalant. Representative of dicarboxylic acids are aliphatic dicarboxylic acids having 2-12 carbon atoms, in particular oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid, 1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid and aromatic dicarboxylic acids such as phthalic acid or terephthalic acid. Representative of alkyl esters are dimethyl oxylate and diethyl oxylate. Representative of cycloaliphatic acids is cyclohexane carboxylic acid and of aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- and p-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoic acids and, acetamidobenzoic acids, phenylacetic acid and naphthoic acids. Representative of hydroxy aromatic acids are hydroxybenzoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid, 5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and 7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids is citric acid. 
     The intercalation solution will be aqueous and will preferably contain an amount of expansion aid of from about 1 to 10%, the amount being effective to enhance exfoliation. In the embodiment wherein the expansion aid is contacted with the graphite flake prior to or after immersing in the aqueous intercalation solution, the expansion aid can be admixed with the graphite by suitable means, such as a V-blender, typically in an amount of from about 0.2% to about 10% by weight of the graphite flake. 
     After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are washed, specifically water-washed. 
     After being water-washed, a surfactant is then added to the intercalated graphite flakes, such as by coating, spraying or applying the surfactant on the flakes. By surfactant is meant a surface-active agent capable of reducing interfacial tension between a liquid and a solid, such as a wetting agent. Particularly useful surfactants in the practice of the present invention include hexadecanol; lignins, such as lignin sulfonates like sodium lignosulfonate, commercially available as Marasperse N-22 from Lignotech USA, Inc. of Rothschild, Wis.; glycols, such as nonylphenol polyethylene glycol ether, commercially available as Tergitol NP-10 from Union Carbide Chemicals and Plastics Company Inc. of Danbury, Conn.; and organosilicones like polyalkyleneoxide modified polydimethylsiloxane, commercially available as Silwet L-7200 from OSi Specialites, Inc. of Danbury, Conn. Typically, the surfactant is added to the intercalated flakes at a level of about 0.25 to about 5 pph. 
     Upon exposure to high temperature, e.g. about 600° C. to 1000° C. and higher, the particles of intercalated graphite expand as much as about 80 to 1000 or more times its original volume in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the constituent graphite particles. Indeed, expansion of the graphite particles can be initiated at temperatures as low as about 150° C., which is particularly useful in fine suppression applications are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various. 
     The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible variations and modifications that will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention that is defined by the following claims. The claims are intended to cover the indicated elements and steps in any arrangement or sequence that is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.