Cover glass adhering device

Abstract

Graphite flake is annealed at a temperature of at least about 3000° C. prior to intercalation. This annealing process results in enhanced expansion of intercalated graphite flake and provides uniform expansion of intercalated graphite flake derived from variety of sources.

Description

FIELD OF THE INVENTION

[0001] This invention relates to intercalated graphite flake having increased exfoliation volume.

BACKGROUND OF THE INVENTION

[0002] Graphite is a crystalline form of carbon comprising atoms 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 intercalated graphite flake. Upon exposure to elevated temperatures the particles of intercalated graphite expand in dimension in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the graphite.

[0003] Intercalated graphite flake has many useful applications. A common application is to exfoliate the intercalated graphite particles into vermicular-like structures which are then compressed into sheets of flexible graphite for use in the manufacture of gaskets or as packing material. Intercalated graphite flake is also used in a variety of products which take advantage of the high expansion which such flakes undergo when exposed to high temperature. One such example is for use in combination with polymer foams to form seat cushions and furniture upholstery in aircraft. 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.

[0004] Also, as is appreciated in the art, graphites from various mines, when intercalated and subjected to exfoliation, exhibit greatly varying degrees of expansion. For this reason, naturally occurring graphite from certain sources which exhibits poor expansion upon exfoliation is excluded as a source material for many applications.

[0005] It has been discovered in accordance with the present invention that when graphite flake is subjected to a preliminary annealing step, which involves heating to a temperature of at least 3000° C., the graphite flake upon intercalation and subsequent exfoliation exhibits enhanced expansion, and that graphite flake from sources previously considered unacceptable by reason of poor expansion characteristics, achieves expansion comparable to that demonstrated by the flake from superior sources. Heretofore, such an extreme annealing temperature has not been employed in the art. Borkowski, U.S. Pat. No. 4,102,960, teaches annealing in the presence of boron, but Borkowski's process was conducted at a lower temperature, preferably 2750° C. and was designed to achieve boron permeation of the graphite flake, which involves a process unrelated to applicant's invention.

SUMMARY OF THE INVENTION

[0006] It is an object of the invention to provide graphite flake which when intercalated will exhibit excellent expansion properties.

[0007] It is another object of the invention to provide graphite flake which when intercalated exhibits an enhanced and uniform degree of expansion.

[0008] It is a further object of the invention to provide a method for treating graphite flake prior to intercalation, which treatment facilitates the subsequent intercalation of flakes which heretofore had been considered too small to provide satisfactory expansion upon subsequent intercalation and exfoliation.

[0009] It is a further object of the invention to provide a method for treating graphite flake prior to intercalation, which treatment will result in enhanced expansion of flake, even when such flake is derived from natural sources which heretofore had been found to produce flake with an inferior degree of expansion.

[0010] These and other objects are achieved by the invention which provides a method forming particles of intercalated graphite flake having enhanced exfoliation volumes. The method involves annealing graphite flake at a temperature in the range of 3000° C. prior to intercalation and exfoliation.

DETAILED DESCRIPTION OF THE INVENTION

[0011] Intercalated graphite flake is conventionally formed by treating particles of natural graphite with agents that intercalate into the crystal structure of the graphite to form a compound of graphite and the intercalant of expansion in the c-direction, i.e. the direction perpendicular to the crystalline planes of the graphite, when heated to a high temperature of above 700° C. and preferably above 1000° C. The intercalated graphite flake is washed and dried prior to exfoliation. Exfoliated graphite particles are vermiform in appearance and are commonly referred to as “worms”.

[0012] The conventional method for the intercalation and expansion, or exfoliation, of graphite flake 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.

[0013] 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. The intercalation solution may also 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.

[0014] The quantity of intercalation solution may range from about 20 to about 150 pph and more typically about 50 to about 120 pph. After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are water-washed. Alternatively, the quantity of the intercalation solution may be limited to between about 10 and about 50 pph, which permits the washing step to be eliminated as taught and described in U.S. Pat. No. 4,895,713, the disclosure of which is also herein incorporated by reference.

[0015] The particles of graphite flake treated with intercalation solution can optionally be contacted, e.g. by blending with a reducing organic agent selected from alcohols, sugars, aldehydes and esters which are reactive with the surface film of oxidizing intercalating solution at temperatures in the range of 25° C. and 125° C. Such a process is taught in U.S. Pat. No. 6,149,972 to Greinke, the disclosure of which is incorporated herein by reference. Suitable specific organic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol, decylalcohol, 1,10 decanediol, decylaldehyde, 1-propanol, 1,3 propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, glycerol monostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethyl formate, ascorbic acid and lignin-derived compounds, such as sodium lignosulfate. The amount of organic reducing agent is suitably from about 0.5 to 4% weight of the particles of graphite flake.

[0016] 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 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.

[0017] Representative examples of saturated aliphatic carboxylic are acids such as those of the formula H(CH2)nCOOH 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 of 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 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-pentanedicarbosylic 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. Representatives of alkyl esters are dimethyl oxylate and diethyl oxylate. Representatives of alkyl esters are dimethyl oxylate and diethyl oxylate. Representative of cycloaliphatic acids is cyclohexane carboxylic acid and of aromatic carobxylic 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 napthoic acids. Representative of hydroxy aromatic acids are hydrobenzoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-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.

[0018] 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. If 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 a V-blender, typically in an amount of from about 0.2% to about 10% by weight of the graphite flake.

[0019] After intercalating the graphite flake, and following the blending of the intercalant coated intercalated graphite flake with the organic reducing agent, the blend may be exposed to temperatures in the range of 25° to 125° C. to promote reaction of the reducing agent and intercalant coating. The heating period is up to about 20 hours, with shorter heating period, e.g., at least about 10 minutes, for higher temperatures in the above-noted range. Times of one half hour or less, e.g., on the order of 10 to 25 minutes, can be employed at the higher temperatures.

[0020] The thus treated particles of graphite are sometimes referred to as “particles of intercalated graphite.” Typically the intercalated flake is subjected to a water wash to remove surface acid and other impurities. For some applications, it may be desirable to then contact the intercalated graphite flake with a surfactant as is taught in U.S. Pat. No. 5,376,450 to Greinke et al , the disclosure of which is incorporated herein by reference. This surfactant treatment reduces the level of surface acid below the level achievable with a simple water wash, and may result in enhanced expansion.

[0021] Upon exposure to high temperature, e.g., temperatures of at least about 160° C. and especially about 700° C. to 1000° C. and higher, the thus prepared particles of intercalated graphite expand as much as about 80 to 10000 or more times their 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. The expanded, i.e. exfoliated, graphite particles are vermiform in appearance, and are therefore commonly referred to as worms, and are sometimes referred herein as “particles of expanded graphite.” The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact as hereinafter described.

[0022] The present invention is based on the discovery that the above described methods for intercalating and exfoliating graphite flake may beneficially be augmented by a pretreatment of the graphite flake at graphitization temperatures, i.e. temperatures in the range of about 3000° C. and above. This initial heating, or annealing, of the graphite flake results in significantly increased expansion (i.e., increase in expansion volume of up to 300% or greater) when the flake is subsequently subjected to intercalation and exfoliation. Indeed, desirably, the increase in expansion is at least about 50%, as compared to similar processing without the annealing step. The temperatures employed for the annealing step should not be significantly below 3000° C., because temperatures even 100° C. lower result in substantially reduced expansion.

[0023] As is appreciated in the art, natural graphite from many mines has heretofore been considered unsatisfactory, or of marginal utility, for many applications, for the graphite flake from those sources did not undergo satisfactory expansion when subjected to conventional intercalation and exfoliation. When subjected to the above described annealing, however, graphite flake from sources previously considered unsatisfactory becomes usable. In fact, when subjected to the annealing procedure of the instant invention, graphite flake from a full range of natural sources achieves a uniform and unexpectedly enhanced degree of expansion.

[0024] In addition to providing uniform and enhanced expansion of graphite from all sources, annealing permits satisfactory intercalation and exfoliation of smaller particle size natural graphite then had heretofore had been deemed suitable for intercalation and exfoliation owing to poor expansion. Following the annealing process of the present invention, intercalating and exfoliation of small graphite flake can be made to yield a unique expanded graphite product.

[0025] The annealing of the present invention is performed for a period of time sufficient to result in a flake having an enhanced degree of expansion upon intercalation and subsequent exfoliation. Typically the time required will be 1 hour or more, preferably 1 to 3 hours and will most advantageously proceed in an inert environment. For maximum beneficial results, the annealed graphite flake will also be subjected to other processes known in the art to enhance the degree expansion—namely intercalation in the presence of an organic reducing agent, an intercalation aid such as an organic acid, and a surfactant wash following intercalation. Moreover, for maximum beneficial results, the intercalation step may be repeated.

[0026] The annealing step of the instant invention may be performed in an induction furnace or other such apparatus as is known and appreciated in the art of graphitization; for the temperatures here employed, which are in the range of 3000° C., are at the high end of the range encountered in graphitization processes.

Claims

1. A process for enhancing the expansion of intercalated graphite flake comprising annealing the graphite flake at a temperature of at least 3000° C. prior to intercalation.

2. The process claim of claim 1 wherein the annealing is performed for a period in excess of 1 hour.

3. The process of claim 2 wherein the annealing is performed in an inert environment.

4. A process for achieving expansion of graphite flake comprising:

(1) annealing the flake at a temperature of at least 3000° C. for at least about 1 hour; and

(2) treating the flake with an intercalant solution to provide intercalated graphite flake.

5. The process of claim 4 wherein the annealing is performed in an inert environment.

6. The process of claim 4 wherein the intercalated flake is blended with an organic reducing agent.

7. The process of claim 4 wherein an expansion aid is included in the intercalant solution.