Chemical-free production of graphene-reinforced polymer matrix composites

Abstract

Provided is a simple, fast, scalable, and environmentally benign method of producing a graphene-reinforced polymer matrix composite directly from a graphitic material, 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 and transferring the graphene sheets to surfaces of solid polymer carrier material particles to produce graphene-coated or graphene-embedded polymer particles inside the impacting chamber; and (c) forming graphene-coated or graphene-embedded polymer particles into the graphene-reinforced polymer matrix composite. Also provided is a mass of the graphene-coated or graphene-embedded polymer particles produced by this method.

Claims

1. A method of producing a graphene-reinforced polymer matrix composite directly from a graphitic material, said 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; and (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 or graphene-embedded polymer particles inside said impacting chamber, and recovering said graphene-coated or graphene-embedded polymer particles from said impacting chamber.

2. The method of claim 1, further comprising a step (c) of forming said graphene-coated or graphene-embedded polymer particles into a graphene-reinforced polymer matrix composite.

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

4. The method of claim 3, wherein a magnet is used to separate the impacting balls or media from the graphene-coated or graphene-embedded polymer particles.

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

6. The method of claim 5, wherein said diameter or thickness is from 1 m to 100 m.

7. The method of claim 1, wherein said solid polymer carrier material includes micron- or nanometer-scaled particles that can be dissolved in a solvent or melted above a melting temperature, and said method includes a step of dissolving or melting said solid polymer carrier material for forming said polymer matrix composites.

8. The method of claim 1 wherein said graphitic material is selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, graphite fiber, graphitic nano-fiber, graphite fluoride, oxidized graphite, chemically modified graphite, exfoliated graphite, recompressed exfoliated graphite, expanded graphite, meso-carbon micro-bead, or a combination thereof.

9. 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, vacuum ball mill, or resonant acoustic mixer.

10. The method of claim 1, wherein said graphitic material contains 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.

11. The method of claim 2, wherein said step (c) includes melting said polymer particles to form a polymer melt mixture with graphene sheets dispersed therein, forming said polymer melt mixture into a desired shape and solidifying said shape into said graphene-reinforced polymer matrix composite.

12. The method of claim 2, wherein said step (c) includes dissolving said polymer particles in a solvent to form a polymer solution mixture with graphene sheets dispersed therein, forming said polymer solution mixture into a desired shape, and removing said solvent to solidify said shape into said graphene-reinforced polymer matrix composite.

13. The method of claim 2, wherein said step (c) includes melting said polymer particles to form a polymer melt mixture with graphene sheets dispersed therein and extruding said mixture into a rod form or sheet form, spinning said mixture into a fiber form, spraying said mixture into a powder form, or casting said mixture into an ingot form.

14. The method of claim 2, wherein said step (c) includes dissolving said polymer particles in a solvent to form a polymer solution mixture with graphene sheets dispersed therein and extruding said solution mixture into a rod form or sheet form, spinning said solution mixture into a fiber form, spraying said solution mixture into a powder form, or casting said solution mixture into an ingot form, and removing said solvent.

15. The method of claim 14, wherein said polymer solution mixture is sprayed to create a nano graphene reinforced polymer matrix composite coating.

16. The method of claim 2, wherein said step (c) includes sintering said graphene-coated polymer particles into a desired shape of said graphene-reinforced polymer matrix composite, wherein said sintering occurs in a selective laser sintering apparatus.

17. The method of claim 1 wherein said graphene sheets contain single-layer graphene sheets.

18. The method of claim 1 wherein said graphene sheets contain at least 80% single-layer graphene or at least 80% few-layer graphene having no greater than 10 graphene planes.

19. The method of claim 1 wherein said graphene sheets contain 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, or chemically modified graphene.

20. The method of claim 1 wherein said impacting chamber further contains a modifier filler selected from a carbon fiber, ceramic fiber, glass fiber, carbon nanotube, carbon nano-fiber, metal nano wire, metal particle, ceramic particle, glass powder, carbon particle, graphite particle, organic particle, or a combination thereof.

21. The method of claim 1 wherein said polymer is selected from a thermoplastic polymer, thermosetting resin, rubber or elastomer, semi-penetrating network polymer, penetrating network polymer, wax, gum, mastic or a combination thereof.

22. The method of claim 1 wherein said impacting chamber further contains a functionalizing agent and said step (b) of operating said energy impacting apparatus act to chemically functionalize said graphene sheets with said functionalizing agent.

23. The method of claim 22 wherein said functionalizing agent contains a chemical functional group selected from functional group is selected from alkyl or aryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group, amine group, sulfonate group (SO.sub.3H), aldehydic group, quinoidal, fluorocarbon, or a combination thereof.

24. The method of claim 22 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, and combinations thereof. ##STR00003##

25. The method of claim 22 wherein said functionalizing agent contains an oxygenated group selected from the group consisting of hydroxyl, peroxide, ether, keto, and aldehyde.

26. The method of claim 22 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(OR).sub.yR.sub.3-y, 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.

27. The method of claim 22 wherein said functionalizing agent contains a functional group is 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), polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof.

28. The method of claim 22 wherein said functionalizing agent contains a functional group selected from OY, NHY, OCOY, PCNRY, OCSY, OCY, CR1OY, 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.

29. The method of claim 2, wherein said step of operating said energy impacting apparatus is conducted in a continuous manner using a continuous energy impacting device.

30. A mass of graphene-coated or graphene-embedded polymer particles produced by the method of claim 1, wherein a graphene proportion is from 0.01% to 80% by weight based on the total weight of graphene and polymer combined.

31. The mass of graphene-coated or graphene-embedded polymer particles of claim 30, which is fed into an extruder, a molding machine, or a selective laser sintering apparatus to make a graphene-reinforced polymer composite part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] 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.

[0078] FIG. 2 A flow chart showing the presently invented process for producing graphene-reinforced polymer matrix composites via an energy impacting apparatus.

[0079] 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.

[0080] FIG. 4 A flow chart showing the commonly used prior art process of solution mixing to produce polymer/graphene and polymer/graphene oxide composites.

[0081] FIG. 5 A flow chart showing the commonly used prior art process of melt compounding to produce polymer/graphene and polymer/graphene oxide composites.

[0082] FIG. 6 A diagram showing the presently invented process for producing graphene-reinforced polymer matrix composites via an energy impacting apparatus.

[0083] FIG. 7 A diagram showing the presently invented process for producing graphene-reinforced polymer matrix composites via a continuous ball mill.

[0084] FIG. 8(A) Transmission electron micrograph of graphene sheets produced by conventional Hummer's route (much smaller graphene sheets, but comparable thickness).

[0085] FIG. 8(B) Transmission electron micrograph of graphene sheets produced by the presently invented impact energy method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0086] 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 nano-fiber.

[0087] One preferred specific embodiment of the present invention is a method of producing a nano graphene platelet (NGP) material and its polymer matrix composite. An NGP 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.

[0088] 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 of producing graphene sheets, which are quickly transferred to particles of a desired polymer intended to become a matrix of a composite. As schematically illustrated in FIG. 2, one preferred embodiment of this method entails placing source graphitic material particles and particles of a solid polymer carrier material (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, for instance, 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 polymer carrier particles (if no impacting balls are present) to form graphene-coated polymer particles. Some of the graphene platelets may become embedded into the polymer particles. This is a direct transfer process.

[0089] Alternatively, in the impacting chambers containing impacting balls (e.g. stainless steel or zirconia beads), graphene sheets are also 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 polymer carrier material particles, the graphene sheets are transferred to surfaces of the polymer carrier material particles to form graphene-coated polymer particles. This is an indirect transfer process.

[0090] In less than two 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 impacting balls (if present) or residual graphite particles (if present) are separated from the graphene-coated polymer particles using a broad array of methods. Separation or classification of graphene-coated polymer particles from impacting balls and residual graphite particles (if any) can be readily accomplished based on their differences in weight or density, particle sizes, magnetic properties, etc. The resulting graphene-coated polymer particles are already a composite or two-component material already; i.e. they are already mixed. The two-component material is then thermally or solution-processed into a shape of composite material.

[0091] In other words, production of graphene sheets and mixing of graphene sheets with a polymer matrix are essentially accomplished concurrently in one operation. This is in stark contrast to the traditional processes of producing graphene sheets first and then subsequently mixing the graphene sheets with a polymer matrix in the conventional production of graphene-reinforced polymer matrix composite.

[0092] In this conventional process, as shown in FIG. 1, the prior art chemical processes for producing graphene sheets or platelets alone 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, as herein prepared) 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.

[0093] In the conventional process of producing graphene-reinforced polymer matrix composite, graphene sheets produced must then be mixed with a polymer matrix to form into a composite according to one of the four approaches discussed earlier in the Background section: (1) In situ polymerization; (2) solution mixing; (3) dry blending; and (4) solid state shear pulverization to produce polymer/graphene nanocomposites.

[0094] For instance, FIG. 3 shows a flow chart illustrating the commonly used prior art process of in situ polymerization to produce polymer/graphene and polymer/graphene oxide composites. In the most common process, previously produced graphene sheets or platelets are added to a monomer solution. Energy is applied via shear mixing or ultrasound to disperse graphene platelets or sheets in the monomer solution. The monomer is polymerized with the graphene platelets in situ, creating a solution of graphene wrapped polymer platelets. Solvent is then removed, or material is precipitated by adding a non-solvent, creating a graphene wrapped polymer that can be further processed. The same in situ process can be carried out with a suspension of graphene oxide as the starting material. With the graphene oxide process, reduction can take place at any of the process steps. Well known methods of reduction include chemical reduction, thermal reduction, light energy reduction and electrolytic reduction. The end result of the graphene oxide process is polymer/graphene oxide nanocomposite, or a polymer/graphene nanocomposite with partial or complete reduction. A mixture of graphene and graphene oxide is also a possible starting material.

[0095] The disadvantages of in situ polymerization are obvioussolvent usage and recovery; solvent hazards; identification of co-solvents for monomer, polymer and graphene; and (for graphene oxide) the possibility of damaging the polymer while reducing graphene oxide.

[0096] Shown in FIG. 4 is a commonly used prior art process of solution mixing. In the most common method, previously produced graphene platelets are added to polymer/solvent solution. Energy is applied by shear or ultrasound to fully disperse graphene sheets and dissolve the polymer, followed by a process to remove the solvent. One method for solvent removal involves adding a non-solvent to induce precipitation. The product, polymer-wrapped graphene platelets, is then further processed. A similar process can use graphene oxide as the starting material. The process steps can be modified to include reduction of the graphene oxide to graphene (reduced graphene oxide, RGO), if desired. A mixture of graphene and graphene oxide is also a possible starting material.

[0097] FIG. 5 shows the commonly used prior art process of melt compounding. In the most common process, previously produced graphene platelets are added to a mixing device and blended with polymer pellets. This graphene-polymer mixture is then melt-compounded (e.g. in an extruder) to create a polymer/graphene nanocomposite. Alternately, graphene oxide can be mixed with polymer pellets and subsequently melt-compounded.

[0098] In all these prior art processes for producing graphene-reinforced polymer matrix composite, graphene sheets must be exfoliated and separated first as a separate process. This is then followed by a blending or mixing process with a polymer or monomer. The resulting mixture is then formed into a composite shape via melting-solidification or solvent dissolution-solvent removal.

[0099] In contrast, the presently invented impacting process entails combining graphene production, functionalization (if desired), and graphene-polymer mixing in a single operation. This fast and environmentally benign process not only avoids significant chemical usage, but also produces a higher quality reinforcement materialpristine graphene as opposed to thermally reduced graphene oxide, as produced by the prior art process. Pristine graphene enables the creation of composite materials with higher electrical and thermal conductivity.

[0100] 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 4 hours (typically 10 minutes to 2 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.

[0101] 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, meso-carbon micro-bead, graphite fiber, graphitic nano-fiber, graphite oxide, graphite fluoride, chemically modified graphite, exfoliated graphite, or a combination thereof. By contrast, graphitic material for used for the prior art chemical production 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 several mm or cm in size or larger has been successfully processed to create graphene-coated or graphene-embedded polymer particles. The only size limitation is the chamber capacity of the energy impacting device; but this chamber can be very large (industry-scaled).

[0102] The presently invented process is capable of producing single-layer graphene sheets well-dispersed in a polymer matrix. 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.

[0103] 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.

[0104] 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.

[0105] Another preferred embodiment of this method is melt compounding of the graphene-coated or graphene-embedded particles to create graphene/polymer nanocomposites. The melted polymer-graphene (graphene sheets dispersed in a polymer matrix) can be extruded to create nanocomposite polymer pellets, sheets, rods, or fibers. As a unique application, the melted polymer, with graphene sheets dispersed therein, can be extruded to create continuous filaments for additive manufacturing (e.g. fused deposition modeling or FDM). The melted polymer may also be directly formed into a desired shape and solidified into a graphene-reinforced polymer matrix nanocomposite.

[0106] Another embodiment of this invention is melting the coated polymer particles to spin into a fiber form, spray into a powder form, or cast into an ingot. Another preferred embodiment of this method is heated pressing of the coated pellets with minimal added shear or mixing to directly form into a desired shape which is then solidified into a graphene-polymer composite.

[0107] Another preferred embodiment of this method is sintering of the coated pellets to directly form them into a desired shape. This sintering may be done with pressure to reduce void formation. Laser sintering of the coated polymer particles may be used to create near net shape articles in a selective laser sintering apparatus.

[0108] One significant advantage of the present invention as compared to prior art is flexibility of selecting carrier materials. A wide range of polymers can be processed with this process, into composites of various form factors, including pellets, powder, continuous filaments, and various shapes according to mold/tooling shapes.

[0109] In a desired embodiment, the presently invented method is carried out in an automated and/or continuous manner. For instance, as illustrated in FIG. 6 and FIG. 7, the mixture of graphite particles 1 and solid carrier particles 2 (plus optional impacting balls) is delivered by a conveyer belt 3 and fed into a continuous ball mill 4. 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 4 into a screening device (e.g. a rotary drum 5) 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 6 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 powder classifier, a cyclone, and or an electrostatic separator. The particles may be further processed by melt compounding 7, pressing 8, or grinding/pelletizing apparatus 9. These procedures can be fully automated. The process may include characterization or classification of the output material and recycling of insufficiently processed material into the continuous energy impacting device. The process may include weight monitoring of the load in the continuous energy impacting device to optimize material properties and throughput.

[0110] Another preferred embodiment of this invention is polymer dissolving in a solvent to form a polymer solution mixture with graphene sheets dispersed therein. The solution in then formed into a desired shape, for example by extruding into a mold. The solvent is then removed to create a graphene-reinforced polymer matrix composite. Another preferred embodiment of this method is dissolving of the coated polymer pellets and spraying them into a surface to create a graphene/polymer nanocomposite coating.

[0111] One significant advantage of the present invention as compared to prior art is flexibility of selecting the polymer carrier material. Virtually any polymer can be used as a solid carrier material to make graphene/polymer nanocomposites. Ground recycled plastic can be used without pelletizing or other melt processing. This reduces the thermal degradation experienced by the polymer, enabling higher improved mechanical properties.

Chemical Functionalization

[0112] 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 nano platelets 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.

[0113] 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.

[0114] 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(O).sub.yR.sub.3-y, 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.

[0115] For NGPs to be effective reinforcement fillers in epoxy resin, the functional 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. Such an arrangement provides a good interfacial bonding between the NGP (graphene sheets) and the matrix of a composite material.

[0116] 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 material at one or two other ends.

[0117] Alternatively, the 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.

[0118] 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, CR1OY, 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.

[0119] 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.

[0120] The functionalized NGPs of the instant invention can be prepared by sulfonation, electrophilic addition to deoxygenated platelet surfaces, or metalation. 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.

[0121] 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 O- 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. 96, 1839 (1965), 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.

[0122] 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: NGP (Graphene Sheets) from Flake Graphite Via Polypropylene Powder-Based Carrier

[0123] In an experiment, 1 kg of polypropylene (PP) 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 high-energy 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. A sample of the 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. The remaining coated carrier material was then melt compounded, pelletized, and injection molded to create tensile test bars.

[0124] In a separate experiment, the same batch of PP pellets and flake graphite particles (without the impacting steel particles) were placed in the same type high-energy ball mill container and the ball mill was operated under the same conditions for the same period of time. The results were compared with those obtained from impacting ball-assisted operation.

[0125] Although polypropylene (PP) is herein used as an example, the carrier material for graphene reinforced polymer matrix composite materials is not limited to PP. It could be any polymer (thermoplastic, thermoset, rubber, wax, mastic, gum, organic resin, etc.) provided the polymer can be made into a particulate form. It may be noted that un-cured or partially cured thermosetting resins (such as epoxide and imide-based oligomers or rubber) can be made into a particle form at room temperature or lower (e.g. cryogenic temperature).

Example 2: NGP from Expanded Graphite (>100 nm in Thickness) Via ABS Polymer

[0126] 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. A small sample of 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. This sample was examined by optical microscopy and found to be graphene. The remaining pellets were extruded to create a 1.75 mm filament used for fused filament fabrication

Example 3: Functionalized Graphene from Meso-Carbon Micro Beads (MCMBs) Via PLA

[0127] 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. After the process was completed, 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 subsequently ground and sintered using a selective laser sintering apparatus.

[0128] 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, 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, polyamide, 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; and what appears to be graphene sheets were completely embedded in a resin matrix.

Example 4: ABS Composite Via Freezer Mill

[0129] In one experiment, 10 grams of ABS pellets 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 and a magnetic stainless steel impactor. 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 an acetone bath. An ultrasound horn was operated for 15 minutes to dissolve the ABS carrier. The resulting solution was sprayed onto a metal substrate, creating a graphene/polymer composite coating.

Example 5: NGP from Natural Graphite Particles Via Polyethylene (PE) and Nylon 6/6 Beads and Ceramic Impacting Balls or Glass Beads

[0130] In an experiment, 0.5 kg of PE or nylon 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 graphene layer. Carrier material was placed over a 50 mesh sieve and a small amount of unprocessed flake graphite was removed. A sample of the 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), as shown in FIG. 8(B). The remaining graphene-coated PE or nylon beads were separately melt-compounded and injection molded to create flexural test bars and discs for electrical conductivity measurements. In a separate experiment, glass beads were used as the impacting balls; other ball-milling operation conditions remained the same.

Comparative Example 1: Graphene Via Hummer's Process and Polymer Composite

[0131] 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). A sample of 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), as shown in FIG. 8(A). Surface area was measured via nitrogen adsorption BET.

[0132] This material was then transferred to a furnace pre-set at 650 C. for 4 minutes to for exfoliation and then heated in an inert atmosphere furnace at 1200 C. for 4 hours to create a low density powder comprised of few layer graphene. This powder was subsequently dry mixed at a 1%-25% loading level with ABS, PE, PP, and nylon pellets, respectively, and compounded using a 25 mm twin screw extruder.

Example 6: Summary of Testing Results

[0133] Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, flexural strength test (both long beam test for flexural strength and modulus determination and short beam shear test for inter-laminar or interfacial bonding assessment), BET test for determination of specific surface area (SSA), electrical conductivity (4-point probe) test, and thermal conductivity (laser flash) test were conducted to measure structure and properties of both polymer matrix composites and the matrix-free isolated graphene sheets recovered after all-milling. The following are a summary of some of the more significant results: [0134] 1) In general, the addition of impacting balls helps to accelerate the process of peeling off graphene sheets from graphite particles. However, this option necessitates the separation of these impacting balls after graphene-coated polymer particles are made. [0135] 2) When no impacting particles (ceramic, glass, metal balls, etc.) are used, harder polymer particles (e.g. PE, PP, nylon, ABS, polystyrene, high impact polystyrene, etc. and their filler-reinforced versions) are more capable of peeling off graphene sheets from graphite particles, as compared to soft polymer particles (e.g. rubber, PVC, polyvinyl alcohol, latex particles). [0136] 3) Without externally added impacting balls, softer polymer particles tend to result in graphene-coated or embedded particles having 0.01% to 5% by weight of graphene (mostly single-layer graphene sheets) and harder polymer balls tend to lead to graphene-coated particles having 0.1% to 30% by weight of graphene (mostly single-layer and few layer graphene sheets). [0137] 4) With externally added impacting balls, all polymer balls are capable of supporting from 0.01% to approximately 80% by weight of graphene sheets (mostly few-layer graphene, <10 layers if over 30% by weight). [0138] 5) The graphene-reinforced polymer matrix composites (graphene/polymer nanocomposites) produced by the presently invented method typically exhibit a significantly higher flexural strength as compared to their counterparts produced by the conventional, prior art methods. SEM examination of fractures surfaces reveals much more uniform dispersion of graphene in the presently invented graphene/polymer nanocomposites. Agglomeration of nano-fillers can be sources of crack initiation in a composite material. [0139] 6) The graphene/polymer nanocomposites produced by the presently invented method also have a significantly lower percolation threshold. The percolation threshold is the critical volume fraction or weight fraction of a conducting filler that enables the formation of a network of electron-conducting paths in an otherwise non-conducting polymer matrix. This is typically characterized by a sudden jump, by 1-5 orders of magnitude, in an electrical conductivity-vs.-filler fraction curve. For instance, the presently invented graphene/ABS nanocomposites can exhibit a percolation threshold as low as 0.3%, but the same type of composites require approximately 2.5% by weight of graphene sheets to achieve the percolation threshold [0140] 7) The graphene/polymer nanocomposites containing chemically functionalized graphene sheets exhibit a significantly higher short-beam shear strength as compared with those containing non-functionalized graphene sheets. This demonstrates the surprising effectiveness of the presently invented method of combined graphene production/functionalization.