CHEMICAL-FREE PRODUCTION OF GRAPHENE-POLYMER PELLETS AND GRAPHENE-POLYMER NANOCOMPOSITE PRODUCTS

Abstract

Provided is a method of producing pellets of a graphene-polymer composite, the method comprising: (a) mixing multiple particles of a graphitic material and multiple particles of a solid polymer carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; (b) operating the energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from the graphitic material particles and transferring the graphene sheets to surfaces of the solid polymer carrier material particles to produce graphene-coated polymer particles inside the impacting chamber; and (c) feeding multiple graphene-coated polymer particles into an extruder to produce filaments of an extruded graphene-polymer composite and operating a cutter or pelletizer to cut the filaments into pellets of graphene-polymer composite. The process is fast (hours as opposed to days of conventional processes), environmentally benign, cost effective, and highly scalable.

Claims

1. A method of producing pellets of a graphene-polymer composite, said method comprising: (a) placing multiple particles of a graphitic material and multiple particles of a solid polymer carrier material into an impacting chamber of an energy impacting apparatus; (b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said graphitic material and transferring said graphene sheets to surfaces of said solid polymer carrier material particles to produce graphene-coated polymer particles inside said impacting chamber; and (c) recovering graphene-coated polymer particles from said impacting chamber and feeding multiple particles of said graphene-coated polymer into an extruder to produce filaments of an extruded graphene-polymer composite and operating a cutter or pelletizer to cut said filaments into pellets of said graphene-polymer composite, wherein graphene sheets are dispersed in the polymer as a matrix.

2. The method of claim 1, wherein a plurality of impacting balls or media are added to the impacting chamber of said energy impacting apparatus.

3. The method of claim 2, wherein a magnet is used to separate the impacting balls or media from the graphene-coated polymer particles during said step of recovering said graphene-coated polymer particles.

4. The method of claim 1, wherein said impacting chamber of said energy impacting apparatus further contains a protective fluid.

5. The method of claim 1, wherein said solid polymer material particles include plastic or thermoplastic elastomer beads, pellets, spheres, wires, fibers, filaments, discs, ribbons, or rods, having a diameter or thickness from 100 nm to 10 mm.

6. The method of claim 5, wherein said diameter or thickness is from 1 m to 1 mm.

7. The method of claim 1 wherein said graphitic material comprises material selected from the group consisting of natural graphite, synthetic graphite, highly oriented pyrolytic graphite, graphite fiber, graphitic nanofiber, graphite fluoride, oxidized graphite, chemically modified graphite, exfoliated graphite, recompressed exfoliated graphite, expanded graphite, mesocarbon microbead, and combinations thereof; or wherein said graphitic material comprises material containing a non-intercalated and non-oxidized graphitic material that has never been previously exposed to a chemical or oxidation treatment prior to said mixing step.

8. The method of claim 1, wherein the energy impacting apparatus is a vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, cryo ball mill, micro ball mill, tumbler ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, freezer mill, vibratory sieve, bead mill, nano bead mill, ultrasonic homogenizer mill, centrifugal planetary mixer, or resonant acoustic mixer.

9. The method of claim 1, further comprising a step (d) of combining, melting, shaping, and consolidating multiple pellets of said graphene-polymer composite to form a graphene-reinforced polymer matrix composite component or structure.

10. The method of claim 9, wherein said step (d) includes melting said multiple pellets of graphene-polymer composite to form a polymer melt mixture with graphene sheets dispersed therein and extruding said melt mixture into a sheet or film form, spinning said melt mixture into a fiber form, or casting said melt mixture into an ingot form.

11. The method of claim 1, further comprising a step of sintering said multiple pellets of said graphene-polymer composite into a desired shape of a graphene-reinforced polymer matrix composite.

12. The method of claim 1 wherein said graphene sheets comprise graphene selected from the group consisting of single-layer graphene sheets, few-layer graphene having no greater than 10 graphene planes, pristine graphene, oxidized graphene with less than 5% oxygen content by weight, graphene fluoride, graphene fluoride with less than 5% fluorine by weight, graphene with a carbon content no less than 95% by weight, chemically modified graphene, and combinations thereof.

13. The method of claim 1 wherein said impacting chamber further comprises a modifier filler selected from the group consisting of a carbon fiber, ceramic fiber, glass fiber, carbon nanotube, carbon nanofiber, metal nanowire, metal particle, ceramic particle, glass powder, carbon particle, graphite particle, organic particle, and combinations thereof.

14. The method of claim 13 wherein said modifier filler is ferromagnetic or paramagnetic.

15. The method of claim 1 wherein said polymer is selected from a thermoplastic resin, thermoplastic elastomer, semi-penetrating network polymer, or a combination thereof.

16. The method of claim 1 wherein said impacting chamber further contains a functionalizing agent and said graphene sheets contain chemically functionalized graphene.

17. The method of claim 16 wherein said functionalizing agent contains a chemical functional group selected from the group consisting of alkyl or aryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group, amine group, sulfonate group (SO.sub.3H), aldehydic group, quinoidal, fluorocarbon, amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof or wherein said functionalizing agent contains an oxygenated group selected from the group consisting of hydroxyl, peroxide, ether, keto, and aldehyde.

18. The method of claim 16 wherein said functionalizing agent contains an azide compound selected from the group consisting of 2-azidoethanol, 3-azidopropan-1-amine, 4-(2-azidoethoxy)-4-oxobutanoic acid, 2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate, azidocarbonate, dichlorocarbene, carbene, aryne, nitrene, (R-)-oxycarbonyl nitrenes, where R=any one of the following groups, ##STR00002## and combinations thereof.

19. The method of claim 16 wherein said functionalizing agent contains a functional group selected from the group consisting of SO.sub.3H, COOH, NH.sub.2, OH, RCHOH, CHO, CN, COCl, halide, COSH, SH, COOR, SR, SiR.sub.3, Si(OSiR.sub.2)OR, R, Li, AlR.sub.2, HgX, TlZ.sub.2 and MgX; wherein y is an integer equal to or less than 3, R is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate, and combinations thereof or wherein said functionalizing agent contains a functional group selected from OY, NHY, OCOY, PCNRY, OCSY, OCY, CRl-OY, NY or CY, and Y is a functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from ROH, RNR.sub.2, RSH, RCHO, RCN, RX, RN.sup.+(R).sub.3X.sup., RSiR.sub.3, RSi(OR).sub.yR.sub.3-y, RSi(OSiR.sub.2) OR, RR, RNCO, (C.sub.2H.sub.4O).sub.wH, (C.sub.3H.sub.6O).sub.wH, (C.sub.2H.sub.4O).sub.wR, (C.sub.3H.sub.6O).sub.wR, R, and w is an integer greater than one and less than 200.

20. The method of claim 9 further comprising a step of heat-treating graphene-reinforced polymer matrix composite component or structure to carbonize said polymer matrix or to carbonize and graphitize the polymer matrix at a temperature of 350 C. to 3000 C. to create a graphene-reinforced carbon matrix composite or graphite matrix composite component or structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] FIG. 1 A flow chart showing the most commonly used prior art process of producing highly oxidized NGPs that entails tedious chemical oxidation/intercalation, rinsing, and high-temperature exfoliation procedures.

[0079] FIG. 2 A flow chart showing the commonly used prior art process of solution mixing to produce polymer/graphene and polymer/graphene oxide composites.

[0080] FIG. 3 A flow chart showing the commonly used prior art process of in situ polymerization to produce polymer/graphene and polymer/graphene oxide composites.

[0081] FIG. 4 A flow chart showing the commonly used prior art process of melt compounding to produce polymer/graphene and polymer/graphene oxide composites.

[0082] FIG. 5 A flow chart showing the presently invented process for producing graphene-polymer composites via an energy impacting apparatus.

[0083] FIG. 6 A diagram showing the presently invented process for producing graphene-reinforced polymer matrix composites via an energy impacting apparatus.

[0084] FIG. 7 A diagram showing the presently invented process for producing graphene-reinforced polymer matrix composites via a continuous ball mill.

[0085] FIG. 8 Another diagram showing the presently invented process for producing graphene-reinforced polymer matrix composites via a continuous ball mill.

[0086] FIG. 9(A) Transmission electron micrograph of graphene sheets produced by conventional Hummer's route (much smaller graphene sheets, but comparable thickness).

[0087] FIG. 9(B) Transmission electron micrograph of graphene sheets produced by the presently invented impact energy method.

[0088] FIG. 10(A) A photo of graphene-coated pellets.

[0089] FIG. 10(B) A photo of graphene-polymer pellets (graphene sheets dispersed in the polymer matrix) produced from graphene-coated pellets via extrusion and pelletizing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0090] Carbon materials can assume an essentially amorphous structure (glassy carbon), a highly organized crystal (graphite), or a whole range of intermediate structures that are characterized in that various proportions and sizes of graphite crystallites and defects are dispersed in an amorphous matrix. Typically, a graphite crystallite is composed of a number of graphene sheets or basal planes that are bonded together through van der Waals forces in the c-axis direction, the direction perpendicular to the basal plane. These graphite crystallites are typically micron- or nanometer-sized. The graphite crystallites are dispersed in or connected by crystal defects or an amorphous phase in a graphite particle, which can be a graphite flake, carbon/graphite fiber segment, carbon/graphite whisker, or carbon/graphite nanofiber.

[0091] One preferred specific embodiment of the present invention is a method of producing a nano graphene platelet (NGP) material that is essentially composed of a sheet of graphene plane (hexagonal lattice of carbon atoms) or multiple sheets of graphene plane stacked and bonded together (typically, on an average, less than five sheets per multi-layer platelet). Each graphene plane, also referred to as a graphene sheet or basal plane comprises a two-dimensional hexagonal structure of carbon atoms. Each platelet has a length and a width parallel to the graphite plane and a thickness orthogonal to the graphite plane. By definition, the thickness of an NGP is 100 nanometers (nm) or smaller, with a single-sheet NGP being as thin as 0.34 nm. However, the NGPs produced with the instant methods are mostly single-layer graphene with some few-layer graphene sheets (<5 layers). The length and width of a NGP are typically between 200 nm and 20 m, but could be longer or shorter, depending upon the sizes of source graphite material particles.

[0092] The present invention provides a strikingly simple, fast, scalable, environmentally benign, and cost-effective process that avoids essentially all of the drawbacks associated with prior art processes. As schematically illustrated in FIG. 5-FIG. 8, one preferred embodiment of this method entails placing source graphitic material particles and carrier material particles (plus optional impacting balls, if so desired) in an impacting chamber. After loading, the resulting mixture is immediately exposed to impacting energy, which is accomplished by rotating the chamber to enable the impacting of the carrier particles (and optional impacting balls) against graphite particles. These repeated impacting events (occurring in high frequencies and high intensity) act to peel off graphene sheets from the surfaces of graphitic material particles and directly transfer these graphene sheets to the surfaces of carrier particles. This is a direct transfer process (i.e. no externally added milling media).

[0093] Alternatively, in the impacting chambers containing impacting balls (e.g. stainless steel or zirconia beads), graphene sheets are peeled off by the impacting balls and tentatively transferred to the surfaces of impacting balls first. When the graphene-coated impacting balls impinge upon the carrier material particles, the graphene sheets are transferred to surfaces of the carrier material particles. This is an indirect transfer process.

[0094] In less than four hours, most of the constituent graphene sheets of source graphite particles are peeled off, forming mostly single-layer graphene and few-layer graphene (mostly less than 5 layers). Following the direct or indirect transfer process (coating of graphene sheets on carrier material particles), the graphene sheets can be separated from the carrier material particles using a broad array of methods if so desired. For instance, the carrier material (e.g. plastic or organic material) is ignited, burning away the carrier material and leaving behind isolated nano graphene platelets. The polymer carrier material may be dissolved in a benign solvent (e.g. water, if the carrier is a water soluble material). There are many water soluble polymers (e.g. polyacrylamide and polyvinyl alcohol) that can be used for this purpose. In the present invention, the resulting graphene-coated or graphene-embraced polymer particles (e.g. those shown in FIG. 10(A)) are recovered from the impacting device (e.g. ball mill pots) and fed into a plastic extruder, which melts, mixes, and extrudes out filaments or rods of polymer-graphene composite. The solidified filaments or rods are then cut or pelletized into pellets of graphene-polymer composite (e.g. FIG. 10(B)) which contains discrete graphene sheets dispersed in the polymer matrix.

[0095] In contrast, as shown in FIG. 1, the prior art chemical processes typically involve immersing graphite powder in a mixture of concentrated sulfuric acid, nitric acid, and an oxidizer, such as potassium permanganate or sodium perchlorate, forming a reacting mass that requires typically 5-120 hours to complete the chemical intercalation/oxidation reaction. Once the reaction is completed, the slurry is subjected to repeated steps of rinsing and washing with water and then subjected to drying treatments to remove water. The dried powder, referred to as graphite intercalation compound (GIC) or graphite oxide (GO), is then subjected to a thermal shock treatment. This can be accomplished by placing GIC in a furnace pre-set at a temperature of typically 800-1100 C. (more typically 950-1050 C.). The resulting products are typically highly oxidized graphene (i.e. graphene oxide with a high oxygen content), which must be chemically or thermal reduced to obtain reduced graphene oxide (RGO). RGO is found to contain a high defect population and, hence, is not as conducting as pristine graphene. We have observed that that the pristine graphene paper (prepared by vacuum-assisted filtration of pristine graphene sheets) exhibit electrical conductivity values in the range of 1,500-4,500 S/cm. In contrast, the RGO paper prepared by the same paper-making procedure typically exhibits electrical conductivity values in the range of 100-1,000 S/cm.

[0096] It is again critically important to recognize that the impacting process not only avoids significant chemical usage, but also produces a higher quality final productpristine graphene as opposes to thermally reduced graphene oxide, as produced by the prior art process. Pristine graphene enables the creation of materials with higher electrical and thermal conductivity.

[0097] Although the mechanisms remain incompletely understood, this revolutionary process of the present invention appears to essentially eliminate the required functions of graphene plane expansion, intercalant penetration, exfoliation, and separation of graphene sheets and replace it with an entirely mechanical exfoliation process. The whole process can take less than 5 hours, and can be done with no added chemicals. This is absolutely stunning, a shocking surprise to even those top scientists and engineers or those of extraordinary ability in the art.

[0098] Another surprising result of the present study is the observation that a wide variety of carbonaceous and graphitic materials can be directly processed without any particle size reduction or pre-treatment. This material may be selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, mesocarbon microbead, graphite fiber, graphitic nanofiber, graphite oxide, graphite fluoride, chemically modified graphite, exfoliated graphite, or a combination thereof. By contrast, graphitic material for used for the prior art chemical formation and reduction of graphene oxide requires size reduction to 75 um or less average particle size. This process requires size reduction equipment (for example hammer mills or screening mills), energy input, and dust mitigation. By contrast, the energy impacting device method can accept almost any size of graphitic material. Starting material of mm or cm in size or larger has been successfully processed to create graphene. The only size limitation is the chamber capacity of the energy impacting device.

[0099] The presently invented process is capable of producing single-layer graphene sheets coated on plastic particles. In many examples, the graphene material produced contains at least 80% single-layer graphene sheets. The graphene produced can contain pristine graphene, oxidized graphene with less than 5% oxygen content by weight, graphene fluoride, graphene oxide with less than 5% fluorine by weight, graphene with a carbon content no less than 95% by weight, or functionalized graphene. These graphene-embraced plastic particles are then converted into graphene-polymer pellets containing graphene sheets dispersed in a matrix of polymer.

[0100] The presently invented process does not involve the production of GIC and, hence, does not require the exfoliation of GIC at a high exfoliation temperature (e.g. 800-1,100 C.). This is another major advantage from environmental protection perspective. The prior art processes require the preparation of dried GICs containing sulfuric acid and nitric acid intentionally implemented in the inter-graphene spaces and, hence, necessarily involve the decomposition of H.sub.2SO.sub.4 and HNO.sub.3 to produce volatile gases (e.g. NO.sub.x and SO.sub.x) that are highly regulated environmental hazards. The presently invented process completely obviates the need to decompose H.sub.2SO.sub.4 and HNO.sub.3 and, hence, is environmentally benign. No undesirable gases are released into the atmosphere during the combined graphite expansion/exfoliation/separation process of the present invention.

[0101] One preferred embodiment of the present invention is the inclusion of impacting balls or media to the impacting chamber, as illustrated in FIG. 2. The impact media may contain balls of metal, glass, ceramic, or organic materials. The size of the impacting media may be in the range of 1 mm to 20 mm, or it may be larger or smaller. The shape of the impacting media may be spherical, needle like, cylindrical, conical, pyramidal, rectilinear, or other shapes. A combination of shapes and sizes may be selected. The size distribution may be unimodal Gaussian, bimodal or tri-modal.

[0102] One significant advantage of the present invention as compared to prior art is flexibility of selecting carrier materials. There are many opportunities to use pre-consumer or post-consumer waste material as the carrier, diverting this material from disposal by landfill or incineration. Recycled plastics, such as ground co-mingled recycled plastic particles, are all possible cost effective carrier materials for the production of graphene-coated polymer particles and subsequently converted graphene-polymer pellets.

[0103] In a desired embodiment, the presently invented method is carried out in an automated and/or continuous manner. For instance, as illustrated in FIG. 3, the mixture of graphite particles and solid carrier particles (plus optional impacting balls) is delivered by a conveyer belt 12 and fed into a continuous ball mill 14. After ball milling to form graphene-coated solid carrier particles, the product mixture (possibly also containing some residual graphite particles and optional impacting balls) is discharged from the ball mill apparatus 14 into a screening device (e.g. a rotary drum 16) to separate graphene-coated solid carrier particles from residual graphite particles (if any) and impacting balls (if any). This separation operation may be assisted by a, magnetic separator 18 if the impacting balls are ferromagnetic (e.g. balls of Fe, Co, Ni, or Mn-based metal). The graphene-coated carrier particles may be delivered into a combustion chamber 20, if the solid carrier can be burned off (e.g. plastic beads, rubber particles, and wax particles, etc.). Alternatively, these particles can be discharged into a dissolving chamber for dissolving the carrier particles (e.g. plastic beads). The product mass can be further screened in another (optional) screening device 22, a powder classifier or cyclone 24, and/or an electrostatic separator 26. These procedures can be fully automated.

[0104] Preferred mode of Chemical Functionalization Graphene sheets transferred to carrier solid particle surfaces have a significant proportion of surfaces that correspond to the edge planes of graphite crystals. The carbon atoms at the edge planes are reactive and must contain some heteroatom or group to satisfy carbon valency. There are many types of functional groups (e.g. hydroxyl and carboxylic) that are naturally present at the edge or surface of graphene nanoplatelets produced through transfer to a solid carrier particle. The impact-induced kinetic energy experienced by the carrier particles are of sufficient energy and intensity to chemically activate the edges and even surfaces of graphene sheets coated on carrier particle surfaces (e.g. creating highly active sites or free radicals). Provided that certain chemical species containing desired chemical function groups (e.g. NH.sub.2, Br, etc.) are included in the impacting chamber, these functional groups can be imparted to graphene edges and/or surfaces. In other words, production and chemical functionalization of graphene sheets can be accomplished concurrently by including appropriate chemical compounds in the impacting chamber. In summary, a major advantage of the present invention over other processes is the simplicity of simultaneous production and modification of surface chemistry.

[0105] Graphene platelets derived by this process may be functionalized through the inclusion of various chemical species in the impacting chamber. In each group of chemical species discussed below, we selected 2 or 3 chemical species for functionalization studies.

[0106] In one preferred group of chemical agents, the resulting functionalized NGP may broadly have the following formula(e): [NGP]R.sub.m, wherein m is the number of different functional group types (typically between 1 and 5), R is selected from SO.sub.3H, COOH, NH.sub.2, OH, RCHOH, CHO, CN, COCl, halide, COSH, SH, COOR, SR, SiR.sub.3, Si(OSiR.sub.2)OR, R, Li, AlR.sub.2, HgX, TlZ.sub.2 and MgX; wherein y is an integer equal to or less than 3, R is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate.

[0107] For NGPs to be effective reinforcement fillers in epoxy resin, the function group NH.sub.2 is of particular interest. For example, a commonly used curing agent for epoxy resin is diethylenetriamine (DETA), which has three NH.sub.2 groups. If DETA is included in the impacting chamber, one of the three NH.sub.2 groups may be bonded to the edge or surface of a graphene sheet and the remaining two un-reacted NH.sub.2 groups will be available for reacting with epoxy resin later. Such an arrangement provides a good interfacial bonding between the NGP (graphene sheets) and the matrix resin of a composite material.

[0108] Other useful chemical functional groups or reactive molecules may be selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), hexamethylenetetramine, polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof. These functional groups are multi-functional, with the capability of reacting with at least two chemical species from at least two ends. Most importantly, they are capable of bonding to the edge or surface of graphene using one of their ends and, during subsequent epoxy curing stage, are able to react with epoxide or epoxy resin at one or two other ends.

[0109] The above-described [NGP]-R.sub.m may be further functionalized. This can be conducted by opening up the lid of an impacting chamber after the R.sub.m groups have been attached to graphene sheets and then adding the new functionalizing agents to the impacting chamber and resuming the impacting operation. The resulting graphene sheets or platelets include compositions of the formula: [NGP]-A.sub.m, where A is selected from OY, NHY, OCOY, PCNRY, OCSY, OCY, CRl-OY, NY or CY, and Y is an appropriate functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from ROH, RNR.sub.2, RSH, RCHO, RCN, RX, RN.sup.+(R).sub.3X.sup., RSiR.sub.3, RSi(OR).sub.yR.sub.3-y, RSi(OSiR.sub.2)OR, RR, RNCO, (C.sub.2H.sub.4O).sub.wH, (C.sub.3H.sub.6O).sub.wH, (C.sub.2H.sub.4O).sub.wR, (C.sub.3H.sub.6O).sub.wR, R, and w is an integer greater than one and less than 200.

[0110] The NGPs may also be functionalized to produce compositions having the formula: [NGP]-[R-A].sub.m, where m, R and A are as defined above. The compositions of the invention also include NGPs upon which certain cyclic compounds are adsorbed. These include compositions of matter of the formula: [NGP]-[XR.sub.a].sub.m, where a is zero or a number less than 10, X is a polynuclear aromatic, polyheteronuclear aromatic or metallopolyheteronuclear aromatic moiety and R is as defined above. Preferred cyclic compounds are planar. More preferred cyclic compounds for adsorption are porphyrins and phthalocyanines. The adsorbed cyclic compounds may be functionalized. Such compositions include compounds of the formula, [NGP]-[X-A.sub.a].sub.m, where m, a, X and A are as defined above.

[0111] The functionalized NGPs of the instant invention can be prepared by sulfonation, electrophilic addition to deoxygenated platelet surfaces, or metallation. The graphitic platelets can be processed prior to being contacted with a functionalizing agent. Such processing may include dispersing the platelets in a solvent. In some instances the platelets may then be filtered and dried prior to contact. One particularly useful type of functional group is the carboxylic acid moieties, which naturally exist on the surfaces of NGPs if they are prepared from the acid intercalation route discussed earlier. If carboxylic acid functionalization is needed, the NGPs may be subjected to chlorate, nitric acid, or ammonium persulfate oxidation.

[0112] Carboxylic acid functionalized graphitic platelets are particularly useful because they can serve as the starting point for preparing other types of functionalized NGPs. For example, alcohols or amides can be easily linked to the acid to give stable esters or amides. If the alcohol or amine is part of a di- or poly-functional molecule, then linkage through the 0- or NH-leaves the other functionalities as pendant groups. These reactions can be carried out using any of the methods developed for esterifying or aminating carboxylic acids with alcohols or amines as known in the art. Examples of these methods can be found in G. W. Anderson, et al., J. Amer. Chem. Soc. 86, 1839 (1964), which is hereby incorporated by reference in its entirety. Amino groups can be introduced directly onto graphitic platelets by treating the platelets with nitric acid and sulfuric acid to obtain nitrated platelets, then chemically reducing the nitrated form with a reducing agent, such as sodium dithionite, to obtain amino-functionalized platelets.

[0113] The following examples serve to provide the best modes of practice for the present invention and should not be construed as limiting the scope of the invention:

Example 1: Isolated NGP (Graphene Sheets) from Flake Graphite Via Polypropylene Powder-Based Carrier

[0114] In an experiment, 1 kg of polypropylene pellets, 50 grams of flake graphite, 50 mesh (average particle size 0.18 mm; Asbury Carbons, Asbury N.J.) and 250 grams of magnetic stainless steel pins (Raytech Industries, Middletown Conn.) were placed in a ball mill container. The ball mill was operated at 300 rpm for 4 hours. The container lid was removed and stainless steel pins were removed via a magnet. The polymer carrier material was found to be coated with a dark carbon layer. Carrier material was placed over a 50 mesh sieve and a small amount of unprocessed flake graphite was removed. Coated carrier material was then placed in a crucible in a vented furnace at 600 C. After cooling down, the furnace was opened to reveal a crucible full of isolated graphene sheet powder.

[0115] Although polypropylene (PP) is herein used as an example, the carrier material for making isolated graphene sheets is not limited to PP or any polymer (thermoplastic, thermoset, rubber, etc.). The carrier material can be a glass, ceramic, metal, or other organic material, provided the carrier material is hard enough to peel off graphene sheets from the graphitic material (if the optional impacting balls are not present).

Example 2: NGP from Expanded Graphite Via ABS Polymer

[0116] In an experiment, 100 grams of ABS pellets, as solid carrier material particles, were placed in a 16 oz plastic container along with 5 grams of expanded graphite. This container was placed in an acoustic mixing unit (Resodyn Acoustic mixer), and processed for 30 minutes. After processing, carrier material was found to be coated with a thin layer of carbon. Carrier material was placed in acetone and subjected to ultrasound energy to speed dissolution of the ABS. The solution was filtered using an appropriate filter and washed four times with additional acetone. Subsequent to washing, filtrate was dried in a vacuum oven set at 60 C. for 2 hours.

Example 3: Functionalized Graphene from Mesocarbon Micro Beads (MCMBs) Via PLA

[0117] In one example, 100 grams of PLA pellets (carrier material) and 2 grams of MCMBs (China Steel Chemical Co., Taiwan) were placed in a vibratory ball mill, which also contains particles of magnetic stainless steel impactor and processed for 2 hours. Subsequently, DETA was added and the material mixture was processed for an additional 2 hours. The vibratory mill was then opened and the carrier material was found to be coated with a dark coating of graphene. The magnetic steel particles were removed with a magnet. The carrier material was rinsed with isopropyl alcohol and placed on a vacuum filter. The vacuum filter was heated to 160 C. and vacuum was applied, resulting in removal of the PLA.

[0118] In separate experiments, the following functional group-containing species were introduced to the graphene sheets produced: an amino acid, sulfonate group (SO.sub.3H), 2-Azidoethanol, polyamide (caprolactam), and aldehydic group. In general, these functional groups were found to impart significantly improved interfacial bonding between resulting graphene sheets and epoxy, polyester, polyimide, and vinyl ester matrix materials to make stronger polymer matrix composites. The interfacial bonding strength was semi-quantitatively determined by using a combination of short beam shear test and fracture surface examination via scanning electronic microscopy (SEM). Non-functionalized graphene sheets tend to protrude out of the fractured surface without any residual matrix resin being attached to graphene sheet surfaces. In contrast, the fractured surface of composite samples containing functionalized graphene sheets do not exhibit any bare graphene sheets; any what appears to be graphene sheets were completely embedded in a resin matrix.

Example 4: NGP from HOPG Via Glass Beads and SPEX Mill

[0119] In an experiment, 5 grams of glass beads were placed in a SPEX mill sample holder (SPEX Sample Prep, Metuchen, N.J.) along with 0.25 grams of HOPG derived from graphitized polyimide. This process was carried out in a 1% dry room to reduce the condensation of water onto the completed product. The SPEX mill was operated for 10 minutes. After operation, the contents of the sample holder were transferred to a water bath subjected to ultrasonication, which helps to separate graphene sheets from glass bead surfaces. The remaining material in the weight dish was a mixture of single layer graphene (86%) and few layer graphene.

Example 5: Production of Few Layer Graphene Via Wax-Based Carrier

[0120] In one example, 100 grams of hard machining wax in pellet form (F-14 green, Machinablewax.com, Traverse City Michigan, Hardness 55, Shore D) was mixed with 10 grams of vein graphite (40 mesh size, Asbury Carbons, Asbury N.J.) and loaded into a vibratory mill. The material was processed for 4 hours, and the vibratory mill was opened. The wax pellets were found to be carbon coated. These pellets were removed from the mill, melted, and re-pelletized, resulting in 103 grams of graphene-loaded wax pellets. The wax pellets were placed again in the vibratory mill with an additional 10 grams of vein graphite, and processed for 4 hours. The resultant material was pelletized and processed with an additional 10 grams of vein graphite, creating a wax/graphene composite with a graphene filler level of about 8.9%. The wax carrier material was then dissolved in hexane and transferred into acetone via repeated washing, then separated from acetone via filtration, producing isolated, pristine NGP. The graphene platelets were dried in a vacuum oven at 60 C. for 24 hours, and then surface area was measured via nitrogen adsorption BET.

Example 6: Low Temperature Metal Particles as the Carrier Material

[0121] In one example, 100 grams of tin (45 micron, 99.9% purity, Goodfellow Inc; Coraopolis, Pa.) was mixed with 10 grams of vein graphite (40 mesh size, Asbury Carbons, Asbury N.J.) and loaded into a vibratory mill. The material was processed for 2 hours, and the vibratory mill was opened. The tin powder was found to be carbon coated. These pellets were removed from the mill with tin being melted by heat and filtered using a vacuum filter. The specific surface area of the resulting graphene material was measured via nitrogen adsorption BET. A similar procedure was conducted using zinc particles as the solid carrier material.

Example 7: Isolated NGP from Natural Graphite Particles Via Polyethylene (PE) Beads and Ceramic Impacting Balls

[0122] In an experiment, 0.5 kg of PE beads (as a solid carrier material), 50 grams of natural graphite (source of graphene sheets) and 250 grams of zirconia powder (impacting balls) were placed in containers of a planetary ball mill. The ball mill was operated at 300 rpm for 4 hours. The container lid was removed and zirconia beads (different sizes and weights than graphene-coated PE beads) were removed through a vibratory screen. The polymer carrier material was found to be coated with a dark carbon layer. Carrier material was placed over a 50 mesh sieve and a small amount of unprocessed flake graphite was removed. Coated carrier material was then placed in a crucible in a vented furnace at 600 C. After cooling down, the furnace was opened to reveal a crucible full of isolated graphene sheet powder (>95% single-layer graphene).

Comparative Example 1: NGP Via Hummer's Process

[0123] Graphite oxide as prepared by oxidation of graphite flakes with sulfuric acid, nitrate, and permanganate according to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The graphite oxide was repeatedly washed in a 5% solution of HCl to remove most of the sulphate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was neutral. The slurry was spray-dried and stored in a vacuum oven at 60 C. for 24 hours. The interlayer spacing of the resulting laminar graphite oxide was determined by the Debey-Scherrer X-ray technique to be approximately 0.73 nm (7.3 A). This material was subsequently transferred to a furnace pre-set at 650 C. for 4 minutes for exfoliation and heated in an inert atmosphere furnace at 1200 C. for 4 hours to create a low density powder comprised of few layer reduced graphene oxide (RGO). Surface area was measured via nitrogen adsorption BET.

[0124] The RGO sheets were made into a disc of RGO paper 1 mm thick using a vacuum-assisted filtration procedure. The electrical conductivity of this disc of RGO paper was measured using a 4-point probe technique. The conductivity of this RGO disc was found to be approximately 550 S/cm. In contrast, the graphene paper discs made from the pristine graphene sheets with the presently invented chemical-free process exhibits an electrical conductivity in the range of 1,500 to 4,500 S/cm. The differences are quite dramatic.

Example 8: Production of Graphene-Polymer Pellets from Graphene-Embraced Particles

[0125] Upon completion of the impact procedure, graphene sheets are basically coated on or wrapped around polymer carrier particle surfaces (e.g. graphene-coated PET particles or pellets, FIG. 10(A)). These graphene sheets are typically not embedded inside the polymer and not dispersed in the polymer. These graphene-coated pellets were then fed into a twin-screw extruder that melts the plastic at a temperature higher than the melting point or glass transition point of a plastic, mixes the plastic melt with graphene sheets, well-disperses the graphene sheets in the plastic melt. The graphene sheet-containing plastic melt was then continuously extruded out to form one or multiple filaments or thin rods, which were cooled and then cut or pelletized into solid pellets of graphene-polymer composite containing discrete graphene sheets dispersed in the polymer matrix (e.g. graphene-PET nanocomposite pellets shown in FIG. 10(B)). Extrusion is well-known in plastic industry. One may optionally allow these pellets to go through the extruder one or more times if deemed necessary.