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

FIELD OF THE INVENTION

The present invention relates to the art of graphene materials and, in particular, to an environmentally benign and cost-effective method of producing graphene-reinforced polymer matrix composites.

BACKGROUND

A single-layer graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Multi-layer graphene is a platelet composed of more than one graphene plane. Individual single-layer graphene sheets and multi-layer graphene platelets are herein collectively called nano graphene platelets (NGPs) or graphene materials. NGPs include pristine graphene (essentially 99% of carbon atoms), slightly oxidized graphene (<5% by weight of oxygen), graphene oxide (≥5% by weight of oxygen), slightly fluorinated graphene (<5% by weight of fluorine), graphene fluoride ((≥5% by weight of fluorine), other halogenated graphene, and chemically functionalized graphene.

NGPs have been found to have a range of unusual physical, chemical, and mechanical properties. For instance, graphene was found to exhibit the_highest intrinsic strength and highest thermal conductivity of all existing materials. Although practical electronic device applications for graphene (e.g., replacing Si as a backbone in a transistor) are not envisioned to occur within the next 5-10 years, its application as a nanofiller in a composite material and an electrode material in energy storage devices is imminent. The availability of processable graphene sheets in large quantities is essential to the success in exploiting composite, energy, and other applications for graphene.

Our research group was among the first to discover graphene [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGP nanocomposites were recently reviewed by us [Bor Z. Jang and A Zhamu, “Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: A Review,” J. Materials Sci. 43 (2008) 5092-5101]. Our research has yielded a process for chemical-free production of isolated nano graphene platelets that is novel in that is does not follow the established methods for production of nano graphene platelets outlined below. In addition, the process is of enhanced utility in that it is cost effective, and provided novel graphene materials with significantly reduced environmental impact. Four main prior-art approaches have been followed to produce NGPs. Their advantages and shortcomings are briefly summarized as follows:

Production of Isolated Graphene Sheets
Approach 1: Chemical Formation and Reduction of Graphite Oxide (GO) Platelets

The first approach (FIG. 1) entails treating natural graphite powder with an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or, actually, graphite oxide (GO). [William S. Hummers, Jr., et al., Preparation of Graphitic Oxide, Journal of the American Chemical Society, 1958, p. 1339.] Prior to intercalation or oxidation, graphite has an inter-graphene plane spacing of approximately 0.335 nm (L_d=½ d₀₀₂=0.335 nm). With an intercalation and oxidation treatment, the inter-graphene spacing is increased to a value typically greater than 0.6 nm. This is the first expansion stage experienced by the graphite material during this chemical route. The obtained GIC or GO is then subjected to further expansion (often referred to as exfoliation) using either a thermal shock exposure or a solution-based, ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to a high temperature (typically 800-1,050° C.) for a short period of time (typically 15 to 60 seconds) to exfoliate or expand the GIC or GO for the formation of exfoliated or further expanded graphite, which is typically in the form of a “graphite worm” composed of graphite flakes that are still interconnected with one another. This thermal shock procedure can produce some separated graphite flakes or graphene sheets, but normally the majority of graphite flakes remain interconnected. Typically, the exfoliated graphite or graphite worm is then subjected to a flake separation treatment using air milling, mechanical shearing, or ultrasonication in water. Hence, approach 1 basically entails three distinct procedures: first expansion (oxidation or intercalation), further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded or exfoliated GO powder is dispersed in water or aqueous alcohol solution, which is subjected to ultrasonication. It is important to note that in these processes, ultrasonification is used after intercalation and oxidation of graphite (i.e., after first expansion) and typically after thermal shock exposure of the resulting GIC or GO (after second expansion). Alternatively, the GO powder dispersed in water is subjected to an ion exchange or lengthy purification procedure in such a manner that the repulsive forces between ions residing in the inter-planar spaces overcome the inter-graphene van der Waals forces, resulting in graphene layer separations.

There are several major problems associated with this conventional chemical production process:

- (1) The process requires the use of large quantities of several undesirable chemicals, such as sulfuric acid, nitric acid, and potassium permanganate or sodium chlorate.
- (2) The chemical treatment process requires a long intercalation and oxidation time, typically 5 hours to five days.
- (3) Strong acids consume a significant amount of graphite during this long intercalation or oxidation process by “eating their way into the graphite” (converting graphite into carbon dioxide, which is lost in the process). It is not unusual to lose 20-50% by weight of the graphite material immersed in strong acids and oxidizers.
- (4) The thermal exfoliation requires a high temperature (typically 800-1,200° C.) and, hence, is a highly energy-intensive process.
- (5) Both heat- and solution-induced exfoliation approaches require a very tedious washing and purification step. For instance, typically 2.5 kg of water is used to wash and recover 1 gram of GIC, producing huge quantities of waste water that need to be properly treated.
- (6) In both the heat- and solution-induced exfoliation approaches, the resulting products are GO platelets that must undergo a further chemical reduction treatment to reduce the oxygen content. Typically even after reduction, the electrical conductivity of GO platelets remains much lower than that of pristine graphene. Furthermore, the reduction procedure often involves the utilization of toxic chemicals, such as hydrazine.
- (7) Furthermore, the quantity of intercalation solution retained on the flakes after draining may range from 20 to 150 parts of solution by weight per 100 parts by weight of graphite flakes (pph) and more typically about 50 to 120 pph. During the high-temperature exfoliation, the residual intercalate species retained by the flakes decompose to produce various species of sulfuric and nitrous compounds (e.g., NO_xand SO_x), which are undesirable. The effluents require expensive remediation procedures in order not to have an adverse environmental impact.
  
  The present invention was made to overcome the limitations outlined above.

Approach 2: Direct Formation of Pristine Nano Graphene Platelets

In 2002, our research team succeeded in isolating single-layer and multi-layer graphene sheets from partially carbonized or graphitized polymeric carbons, which were obtained from a polymer or pitch precursor [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. Mack, et al [“Chemical manufacture of nanostructured materials” U.S. Pat. No. 6,872,330 (Mar. 29, 2005)] developed a process that involved intercalating graphite with potassium melt and contacting the resulting K-intercalated graphite with alcohol, producing violently exfoliated graphite containing NGPs. The process must be carefully conducted in a vacuum or an extremely dry glove box environment since pure alkali metals, such as potassium and sodium, are extremely sensitive to moisture and pose an explosion danger. This process is not amenable to the mass production of NGPs. The present invention was made to overcome the limitations outlined above.

Approach 3: Epitaxial Growth and Chemical Vapor Deposition of Nano Graphene Sheets on Inorganic Crystal Surfaces

Small-scale production of ultra-thin graphene sheets on a substrate can be obtained by thermal decomposition-based epitaxial growth and a laser desorption-ionization technique. [Walt A. DeHeer, Claire Berger, Phillip N. First, “Patterned thin film graphite devices and method for making same” U.S. Pat. No. 7,327,000 B2 (Jun. 12, 2003)] Epitaxial films of graphite with only one or a few atomic layers are of technological and scientific significance due to their peculiar characteristics and great potential as a device substrate. However, these processes are not suitable for mass production of isolated graphene sheets for composite materials and energy storage applications. The present invention was made to overcome the limitations outlined above.

Approach 4: The Bottom-Up Approach (Synthesis of Graphene from Small Molecules)

Yang, et al. [“Two-dimensional Graphene Nano-ribbons,” J. Am. Chem. Soc. 130 (2008) 4216-17] synthesized nano graphene sheets with lengths of up to 12 nm using a method that began with Suzuki-Miyaura coupling of 1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid. The resulting hexaphenylbenzene derivative was further derivatized and ring-fused into small graphene sheets. This is a slow process that thus far has produced very small graphene sheets. The present invention was made to overcome the limitations outlined above.

Hence, an urgent need exists to have a graphene production process that requires a reduced amount of undesirable chemical (or elimination of these chemicals all together), shortened process time, less energy consumption, lower degree of graphene oxidation, reduced or eliminated effluents of undesirable chemical species into the drainage (e.g., sulfuric acid) or into the air (e.g., SO₂and NO₂). The process should be able to produce more pristine (less oxidized and damaged), more electrically conductive, and larger/wider graphene sheets. These graphene sheets are particularly effective in reinforcing polymer matrix materials.

Applications and Importance of Graphene Polymer Nanocomposites

Potential applications of graphene reinforced polymer matrix composites (also hereinafter referred to as graphene-polymer nanocomposites or simply graphene nanocomposites) take advantage of 4 major areas of property enhancement: electrical conductivity, thermal conductivity, mechanical properties, and barrier properties. Examples of specific applications include tires, electronic housings, EMI shielding, fuel lines, sensors, UV resistant polymer articles and flexible circuits. Graphene nanocomposites provide a major opportunity for weight reduction in automotive and aircraft body panels.

In the instant specification, NGPs or graphene sheets can refer to pristine graphene, graphene oxide (GO), reduced graphene oxide (RGO), graphene fluoride, and chemically functionalized graphene. Four main prior-art approaches have been followed to produce graphene/polymer and graphene oxide/polymer nanocomposites. Their advantages and shortcomings are briefly summarized as follows:

Approach 1: Solution Mixing to Produce Polymer/Graphene Nanocomposites

Small-scale production of well dispersed polymer/graphene and polymer/graphene oxide nanocomposites can be produced via solution mixing, as shown in FIG. 2 [Lopez-Manchado et al, “Graphene Filled Polymer Nanocomposites”, J. Mater. Chem. Vol. 21 Issue 10, pp. 3301-3310]. In the most common method a solvent suspension of graphene platelets (1) is created by shearing, cavitation or application of ultrasound energy. The solvent chosen must suitable for the selected polymer. The polymer is added to solvent solution, and energy is applied by shear or ultrasound to create a graphene/polymer solution (2). The solvent is then removed, commonly by filtration, evaporation or a combination, creating polymer wrapped graphene particles. (3) The particles are then melt compounded, pressed or sintered to create a polymer/graphene composite. Alternately, an anti-solvent can be added to the polymer/graphene solution (2) to cause precipitation of polymer wrapped graphene (3). This material may require filtration, washing and drying before further processing.

The solution mixing technique can also be carried out with graphene oxide (GO) platelets in a colloidal solvent suspension (5). Following a similar process to that used for the solvent suspension of graphene platelets, addition of a polymer creates a polymer/graphene oxide solution (6). That solution can be reduced by chemical, thermal, light or electrolytic methods commonly known in the art, creating a polymer/graphene solution (2). Alternately, the polymer/graphene oxide solution (6) can be processed by removing solvent or adding anti-solvent to create polymer wrapped graphene oxide (7). The polymer wrapped graphene oxide can be reduced to create polymer wrapped graphene (3) or processed by melt compounding or other methods to create a polymer/graphene oxide nanocomposite (8). This nanocomposite can be the final product or can be reduced by commonly known means to create a polymer/graphene nanocomposite. Because of the thermal instability of graphene oxide, a polymer/graphene oxide nanocomposite can only be created with polymers having a process temperature less than 100-150° C.

The solution mixing process is advantageous in allowing the creation of finely dispersed polymer/graphene nanocomposites. It also facilitates high loading levels of graphene. However, this process has several major limitations:

- 1) Dissolution of polymers requires significant energy input via shear or ultrasound, even for well-matched polymer/solvent systems such as ABS/acetone. Use of higher cost powdered polymers or reactor spheres can reduce but not eliminate the need for this process step.
- 2) Many solvents required for polymer dissolution have adverse health effects, safety risks, adverse environmental impact, or some combination of the above. In addition to acetone, common solvents for polymers include methyl ethyl ketone, hexane, toluene, and xylene.
- 3) Solvent usage required for solution mixing is a significant cost for production scale up. For example, production of 1 kg of ABS could require 10 kg or more of acetone. Solvent recovery equipment for industrial scale production by solution mixing represents significant energy and equipment costs.
- 4) Some polymers, for example polyimide and PEEK, are poorly soluble or insoluble in known solvents. Additionally, the solvent must be selected such that the graphene or graphene oxide can be dispersed in it. Poor compatibility of the solvent with graphene or graphene oxide results in a low quality dispersion.
  
  Alternative methods to remove solvent from a solution mixed polymer are spray drying and film casting. Or, the polymer/graphene mixture can be directly sprayed onto the final coated surface. These methods all share the disadvantages of solvent cost, solvent safety and costly solvent recovery. The present invention was made to overcome the limitations outlined above.

Approach 2: In Situ Polymerization to Produce Polymer/Graphene Nanocomposites

Small-scale production of well dispersed polymer/graphene and polymer/graphene oxide nanocomposites can be produced via in situ polymerization, as shown in FIG. 3 [Lopez-Manchado et al, “Graphene Filled Polymer Nanocomposites”, J. Mater. Chem. Vol. 21 Issue 10, pp. 3301-3310]. In the most common method, graphite or graphene platelets (9) are added to a solution of monomer or monomers. Shear forces or ultrasonic energy are applied, and the monomer is polymerized. This creates a polymer/graphene solution or polymer/graphite intercalation compound solution (10). Solvent is removed or a non-solvent is added, resulting in solid particles of polymer wrapped graphene or polymer/graphite intercalation compound (11). The material is processed by melt compounding, pressing or sintering, creating a polymer/graphene nanocomposite (12). Similarly, graphene oxide (13) can be processed through in situ polymerization, creating a polymer/graphene oxide solution (14) which can be reduced to create polymer wrapped graphene solution (10) or further processed by solvent removal to create polymer wrapped graphene oxide (15). This is then processed via melt compounding or other methods known in the art to create a polymer/graphene oxide composite (16).

In situ polymerization produces a very well dispersed polymer/graphene or polymer/graphene oxide nanocomposite. However, this method has significant disadvantages that make scale up to industrial scale production challenging.

- 1) Many monomers required in the in situ polymerization have adverse health effects, safety risks, adverse environmental impact, or some combination of the above.
- 2) Solvent usage required in situ polymerization is a significant cost for production scale up. Solvent recovery equipment for industrial scale production represents significant energy and equipment costs.
- 3) Poor compatibility of the monomers with graphene or graphene oxide results in a low quality dispersion.
- 4) The use of graphene oxide creates process chemistry challenges. Graphene oxide can covalently bond with many monomers, which is sometimes desirable. However, the oxygen content of graphene oxide can vary based on process conditions, storage conditions and material supplier. The inherent variability of the input material is problematic for industrial scale production.
  
  The present invention was made to overcome the limitations outlined above.

Approach 3: Dry Blending to Produce Polymer/Graphene Nanocomposites

Small-scale production of well dispersed polymer/graphene and polymer/graphene oxide nanocomposites can be produced via dry blending, as shown in FIG. 4 [Lopez-Manchado et al, “Graphene Filled Polymer Nanocomposites”, J. Mater. Chem. Vol. 21 Issue 10, pp. 3301-3310]. In the most common method, graphene platelets (17) are added to a mixing device containing polymer pellets. An additive may be included to aid in adhesion of graphene to the polymer pellets. After operation of the mixing device, polymer pellets loosely coated with graphene (18) are fed into a melt compounder to create a polymer/graphene nanocomposite. Alternately, graphene oxide (20) can be sprayed onto polymer pellets, creating GO coated polymer pellets (21). These can be melt compounded at a low temperature to create a polymer/graphene oxide nanocomposite. Alternately, they can be reduced via one of several methods, and then melt compounded to create a polymer/graphene nanocomposite.

Melt compounding is the most scalable of the commonly used methods to create polymer/graphene composites. Solvents and monomers are not required, reducing health, safety and environmental risks. However, this method has several major disadvantages impacting industrial scale up.

- 1) Input materials cost: The cost of raw materials for both graphene and polymer is a substantial disadvantage. The quality of dispersion is strongly affected by particle size of the polymer pellets. Commodity polymer pellets are commonly rough cylinders 2-3 mm in diameter and 2-5 mm in length. With conventional polymer pellets, the maximum loading achievable is about 5% nano graphene platelets. Use of high surface area reactor powder or ground polymer powder can increase the available surface area for dispersion, however this significantly increases input materials costs.
- 2) Uncertainty of graphene loading: Because graphene powder is loosely adhered to the polymer carrier, an unknown amount of the material may be lost during transfer to melt compounding. This results in uncertainty of the actual loading level of graphene, as well as unnecessary dust exposure to the operator.
- 3) Limitation to maximum graphene loading level: Solid state mixing is limited to the amount of material that can be loosely adhered to the polymer surface by electrostatic forces or by an adhesion aid. This is limited to about 50% for nano graphene platelets with a thickness of about 10 nm, and about 2 percent for high surface area, few layer graphene. One way around the maximum loading limitation is to melt compound, pelletize, and re-coat with graphene, followed by additional melt compounding. Repeated melt compounding is undesirable due to thermal and mechanical degradation of the polymer matrix.
  
  The present invention was made to overcome the limitations outlined above.

Approach 4: Solid State Shear Pulverization to Produce Polymer/Graphene Nanocomposites

Small-scale production of well dispersed polymer/graphene and polymer/graphene oxide nanocomposites can be produced via solid state shear pulverization (SSSP) [Torkelson, et al “Polymer-graphite nanocomposites via solid state shear pulverization” U.S. Pat. No. 8,303,876 (Nov. 6, 2012]. In the most common method, graphite material is mixed with polymer pellets and added to a melt compounding device. Pulverization, kneading and mixing elements are used to break down the graphite into graphene dispersed in a polymer matrix. This method has several major challenges impacting industrial scale up.

- 1) Thermal degradation and heat history of the polymer matrix: It is well known to those skilled in the art that aggressive, high temperature or extended time melt compounding of polymers causes reduced mechanical strength. SSSP to create well dispersed graphene is expected to cause degradation of mechanical properties and even temperature induced color changes in the polymer matrix.
- 2) Wear of melt compounding equipment: The use of a melt compounder to knead, mix and pulverize graphite is expected to cause significant wear to the screw elements. Replacement of screw elements causes equipment down time and significant expenses. Because of screw element wear, the process may change over time, creating an undesirable decrease in the quality of dispersion.
- 3) Energy and water usage: SSSP requires cooling to dissipate heat generated by exfoliation of graphite to create graphene.
- 4) Limitations of particle size: graphite must be reduced below a certain particle size to be processed via SSSP. This is a cost and energy intensive process.
  
  The present invention was made to overcome the limitations outlined above.

SUMMARY OF THE INVENTION

The present invention provides a strikingly simple, fast, scalable, environmentally benign, and cost-effective method of producing graphene-reinforced polymer matrix composite materials. This method meets the aforementioned needs. This method entails producing single-layer or few layer graphene sheets directly from a graphitic or carbonaceous material (a graphene source material) and immediately transferring these graphene sheets onto surfaces of polymer particles (herein referred to as solid carrier material particles) to form graphene-coated solid polymer particles. The graphene-coated polymer particles are then melted and extruded into pellets of a graphene-polymer composite, wherein graphene sheets are well-dispersed in the polymer as a matrix. In other words, the polymer-coated particles are further converted into pellets of graphene sheets dispersed in the polymer matrix. These pellets are typically similar in size to commonly used plastic pellets in plastic industry, but can be larger or smaller.

These pellets can then be conveniently and readily molded into various composite shapes. This procedure solves the problem encountered by plastic molders that typically find it challenging to well-disperse graphene sheets in a polymer matrix. The molders can now directly feed these graphene-polymer pellets into their injection molding machine, compression-molding press, blow-molding machines, etc. without having to worry about poor graphene sheet dispersion in a polymer.

Thus, in certain embodiments, the invention provides 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) recovering graphene-coated polymer particles from the impacting chamber and 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 for graphene sheet production alone), environmentally benign, cost effective, and highly scalable

The polymer particles may be selected from a thermoplastic (e.g. pellets of PE, PP, nylon, ABS, engineering plastics, etc.), thermoplastic elastomers (e.g. uncured EPR, EPDM, thermoplastic polyurethane, etc.), semi-penetrating network polymer, penetrating network polymer, or a combination thereof.

In a preferred embodiment, this method comprises subjecting a mixture of graphitic material, particles of a polymer-based solid carrier material, and, optionally, impacting balls to mechanical agitation via a ball mill or a similar energy impacting device for a length of time sufficient for peeling off graphene layers (planes of hexagonally arranged carbon atoms) from the source graphite material, and coating these peeled-off graphene layers onto surfaces of the solid polymer carrier material particles. With the presence of impacting balls, graphene sheets can be peeled off from the source graphite particles and tentatively deposited onto the surfaces of impacting balls. When these graphene sheet-coated impacting balls subsequently impinge upon solid carrier particles, the graphene sheets are transferred to surfaces of carrier particles to produce graphene-coated polymer particles. Subsequently, the graphene-coated polymer particles are recovered from the impacting device and some or all of these particles are fed into an extruder and pelletizer to form pellets of a graphene-polymer composite.

In certain embodiments, a plurality of impacting balls or media are added to the impacting chamber of the energy impacting apparatus if the solid polymer carriers are not sufficiently hard and rigid. In a preferred embodiment, a magnet is used to separate the impacting balls or media from the graphene-coated polymer particles prior to step of forming the graphene-coated polymer particles into the graphene-reinforced polymer matrix composite.

Preferably, the starting material (graphitic or carbonaceous material as a graphene source material) has never been previously intercalated or chemically oxidized. This starting material is not a graphite intercalation compound (GIC) or graphite oxide (GO). Preferably, the source graphitic material is 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, vein graphite, or a combination thereof.

In some embodiments, the impacting chamber of the energy impacting apparatus further contains a protective fluid; e.g. inert gas, non-reactive liquid, water, etc.

This process is of low cost and highly scalable. In less than 2 hours of process time (less than 20 minutes in many cases), graphene sheets are peeled off from graphite particles and re-deposited onto surfaces of polymer particles. The resulting graphene-coated polymer particles are then fed into an extruder and then pelletized into pelletizers containing discrete graphene sheets dispersed in a polymer matrix.

The resulting graphene-polymer pellets can then be later processed by injection molding, compression molding, blow molding, or casting, to name a few, into graphene-reinforced polymer composite parts of various shapes. In a period of 20 minutes-2 hours one could produce graphene-polymer nanocomposite components directly from a source graphite material. This process is stunningly fast and simple, considering the notion that the production of graphene sheets from graphite by using most of the known processes would take 4-120 hours just for intercalation and oxidation, plus times for repeated rinsing and drying, and subsequent thermal exfoliation. Furthermore, the dispersion of graphene sheets in a polymer matrix is also known to be a highly challenging task. The present invention combines the graphene production, graphene-polymer mixing (graphene dispersion), and composite processing into a single operation.

A preferred embodiment of the present invention is a method of directly mixing a graphitic material and a carrier material into an energy impacting device, such as a ball mill, and submitting the mixture to a sufficiently long treatment time to peel off graphene layers from the source graphitic material and transfer these graphene layers immediately to the carrier material surfaces. These graphene sheets are typically single-layer or few-layer graphene sheets (typically <5 layers; mostly single-layer graphene). Following this step, the graphene-coated polymer particles are formed into pellets of polymer-graphene composite, which can then be made into a composite shape using a broad array of composite processing techniques.

For instance, this step of composite forming can include melting the graphene-polymer pellets to form a mixture of polymer melt and graphene sheets dispersed therein, forming the polymer melt-graphene mixture into a desired shape and solidifying the shape into the graphene-reinforced polymer matrix composite component or structure. In certain embodiments, the process includes melting the graphene-polymer pellets to form a polymer melt mixture with graphene sheets dispersed therein and subsequently extruding the melt mixture into a sheet or film form, spinning the melt mixture into a fiber form, or casting the mixture into an ingot form.

In an embodiment, the composite forming step includes sintering the graphene-polymer pellets (wherein graphene sheets are already well-dispersed in the polymer matrix), rather than graphene-coated polymer particles, into a desired shape of the graphene-reinforced polymer matrix composite.

It may be noted that the graphene production step per se (peeling off graphene sheets directly from graphite particles and immediate or concurrent transfer of graphene sheets to polymer particle surfaces) is quite surprising, considering the fact that prior researchers and manufacturers have focused on more complex, time intensive and costly methods to create graphene in industrial quantities. In other words, it has been generally believed that chemical intercalation and oxidation is needed to produce bulk quantities of graphene platelets. The present invention defies this expectation in many ways:

- (1) Unlike the chemical intercalation and oxidation (which requires expansion of inter-graphene spaces, further expansion or exfoliation of graphene planes, and full separation of exfoliated graphene sheets), the instant method directly removes graphene sheets from a source graphitic material and transfers these graphene sheets to surfaces of carrier material particles. No undesirable chemicals (e.g. sulfuric acid and nitric acid) are used.
- (2) Unlike oxidation and intercalation, pristine graphene sheets can be transferred onto the carrier material. The sheets being free of oxidation damage allow the creation of graphene containing products with higher electrical and thermal conductivity.
- (3) Unlike bottom up production methods, large continuous platelets can be produced with the instant method.
- (4) Contrary to common production methods, strong acids and oxidizers are not needed to create the graphene coating.
- (5) Contrary to the commonly used prior art production methods, a washing process requiring substantial amounts of water is not needed.
- (6) The original carrier polymer particles embraced with graphene sheets are further processed to produce pellets that contain discrete graphene sheets already well-dispersed in the polymer matrix. These graphene-polymer pellets can then be directly used in various composite processing procedures for making composite components or structures without having to go through a dispersion step. Good dispersion of graphene sheets in a polymer (particularly thermoplastic polymer) is known to be challenging and most of the plastic molders are not equipped to conduct such a difficult dispersion job.

Original polymer carrier materials can be in the form of polymer pellets, filament, fibers, powder, reactor spheres, or other forms.

The energy impacting apparatus may be selected from a ball mill, vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, vacuum ball mill, freezer (SPEX) mill, vibratory sieve, ultrasonic homogenizer mill, resonant acoustic mixer, or shaker table.

The presently invented process is capable of producing and dispersing single-layer graphene sheets in a thermoplastic resin or thermoplastic elastomer. 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 with less than 5% fluorine by weight, graphene with a carbon content no less than 95% by weight, or functionalized graphene.

In certain embodiments, the impacting chamber further contains a modifier filler selected from 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, or a combination thereof. The modifier filler can improve chemical, mechanical, and physical (electric, thermal, optical, and/or magnetic) properties of the resulting composites. For instance, the modifier filler is ferromagnetic or paramagnetic.

Another surprising and highly advantageous feature of the presently invented process is the notion that graphene sheet production and chemical functionalization can be accomplished concurrently in the same impacting chamber. The impact-induced kinetic energy experienced by the carrier particles are of sufficient energy and intensity to chemically activate the edges and surfaces of graphene sheets coated on carrier particle surfaces; e.g. creating highly active sites or free radicals). Desired functional groups can be imparted to graphene edges and/or surfaces, provided that selected chemical species (functionalizing agents) containing desired chemical function groups (e.g. —NH₂, Br—, etc.) are dispersed in the impacting chamber. Chemical functionalization reactions can occur in situ as soon as the reactive sites or active radicals are formed. Different functional groups are desired in different polymer matrix materials for the purpose of enhancing interfacial bonding between graphene sheets and a polymer matrix. For instance, —NH₂groups are desirable in epoxy resin and polyimide matrix, and —COOH groups or —OH groups are useful in polyvinyl alcohol.

In some embodiments, functionalizing agents contain 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 (—SO₃H), aldehydic group, quinoidal, fluorocarbon, or a combination thereof.

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,

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and combinations thereof.

In certain embodiments, the functionalizing agent contains an oxygenated group selected from the group consisting of hydroxyl, peroxide, ether, keto, and aldehyde. In certain embodiments, the functionalizing agent contains a functional group selected from the group consisting of SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃, Si(—OR′—)_yR′_3-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂and Mg—X; 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.

The functionalizing agent may contain 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.

In some embodiments, the functionalizing agent contains a functional group selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′l-OY, NY or C′Y, 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 R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_yR′_3-y, R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_wH, (—C₃H₆O—)_w—R′, (—C₂H₄O)_w—R′, (—C₃H₆O)_w—R′, R′, and w is an integer greater than one and less than 200.

The procedure of operating the energy impacting apparatus may be conducted in a continuous manner using a continuous energy impacting device. This process can be automated.

The composite forming step from graphene-polymer pellets may be followed by heat-treating the graphene-reinforced polymer matrix composite to carbonize the polymer matrix or to carbonize and graphitize the polymer matrix at a temperature of 350° C. to 3000° C. to convert the graphene-reinforced polymer matrix composite into a graphene-reinforced carbon matrix composite or graphite matrix composite.

BRIEF DESCRIPTION OF THE DRAWINGS

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.

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

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.

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

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

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

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

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

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

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

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

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

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.

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 μm, but could be longer or shorter, depending upon the sizes of source graphite material particles.

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).

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.

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.

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.

It is again critically important to recognize that the impacting process not only avoids significant chemical usage, but also produces a higher quality final product—pristine 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.

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.

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.

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.

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 H₂SO₄and HNO₃to produce volatile gases (e.g. NO_xand SO_x) that are highly regulated environmental hazards. The presently invented process completely obviates the need to decompose H₂SO₄and HNO₃and, hence, is environmentally benign. No undesirable gases are released into the atmosphere during the combined graphite expansion/exfoliation/separation process of the present invention.

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.

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.

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.

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. —NH₂, 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.

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.

In one preferred group of chemical agents, the resulting functionalized NGP may broadly have the following formula(e): [NGP]—R_m, wherein m is the number of different functional group types (typically between 1 and 5), R is selected from SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂and Mg—X; 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.

For NGPs to be effective reinforcement fillers in epoxy resin, the function group —NH₂is of particular interest. For example, a commonly used curing agent for epoxy resin is diethylenetriamine (DETA), which has three —NH₂groups. If DETA is included in the impacting chamber, one of the three —NH₂groups may be bonded to the edge or surface of a graphene sheet and the remaining two un-reacted —NH₂groups 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.

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.

The above-described [NGP]-R_mmay be further functionalized. This can be conducted by opening up the lid of an impacting chamber after the —R_mgroups 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_m, where A is selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′l-OY, N′Y or C′Y, 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 R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_yR′_3-y, R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_wH, (—C₃H₆O—)_wH, (—C₂H₄O)_w—R′, (C₃H₆O)_w—R′, R′, and w is an integer greater than one and less than 200.

The NGPs may also be functionalized to produce compositions having the formula: [NGP]-[R′-A]_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]-[X—R_a]_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_a]_m, where m, a, X and A are as defined above.

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.

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.

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

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.

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

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

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.

In separate experiments, the following functional group-containing species were introduced to the graphene sheets produced: an amino acid, sulfonate group (—SO₃H), 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

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

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

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

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

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.

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

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.

	Number	Date	Country
Parent	14757193	Dec 2015	US
Child	15991714		US

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

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CROSS REFERENCE TO RELATED APPLICATIONS

Continuation in Parts (1)