METHOD AND EQUIPMENT FOR EXTRACTING CARBON MATERIALS FROM PLASTICS

Abstract

The invention provides a method and equipment for extracting carbon materials from plastics. Particularly, the method produces nanostructured carbon materials by heating of at least one salt (e.g., NaCl) and at least one plastic material (e.g.polyethylene terephthalate) to a temperature greater than the melting point of the said salt, in which molten state of the said salt protects the carbonaceous material from oxidation. Moreover, molten salt promotes the graphitization of carbon materials. The product is in the form of graphenenano-flakes with high conductivity and high surface area. This method provides a simple, economical and efficient approach for producing conductive carbon materials. It also has a significant positive impact on the environment through the transformation of virtually non-degradable plastic wastes into high-value conductive carbon materials.

Claims

1. A method for extracting carbon materials from plastics, wherein nanostructured carbon materials are produced by heating a mixture consisting of at least one plastic and at least one metal halide salt to a heating temperature in the range defined as: the melting point of the saltheating temperaturethe boiling point of the salt+50 C.

2. The method for extracting carbon materials from plastics according to claim 1, wherein the salt is a hydrated metal halide salt.

3. The method for extracting carbon materials from plastics according to claim 1, wherein the heating temperature is above the melting point of the metal halide salts, and less than the boiling temperature of the salts, by which a mixture of nanostructured carbon materials and salts is produced; after the mixture is cooled down, the salts are dissolved by water followed by filtration to retrieve the nanostructured carbon materials and the solution of the water and the salts; the nanostructured carbon materials are collected after drying and the salts are recycled from the solution.

4. The method for extracting carbon materials from plastics according to claim 1, wherein the conductivity of the synthesized nanostructured carbon material is greater than 1000 S m.sup.1 or the value of Raman I.sub.D/I.sub.G is less than 0.5.

5. The method for extracting carbon materials from plastics according to claim 1, wherein the salt is one metal halide salt or a mixture of metal halide salts selected from the group consisting of LiCl, NaCl, KCl, MgCl.sub.2, CaCl.sub.2, NaF and ZnCl.sub.2; the salt can be a hydrated metal halide salt or a mixture of more than one hydrated metal halide salt selected from the group consisting of hydrated forms of LiCl, NaCl, KCl, MgCl.sub.2, CaCl.sub.2, NaF and ZnCl.sub.2.

6. The method for extracting carbon materials from plastics according to claim 1, wherein the plastics include at least one of polyethylene, polypropylene, polyethylene terephthalate, polystyrene, polyvinyl chloride, polylactide, polycarbonate, acrylic acid, nylon and ABS resin or synthetic rubbers.

7. The method for extracting carbon materials from plastics according to claim 1, wherein the heating is carried out in air, inert gas atmosphere, nitrogen atmosphere or vacuum conditions; when the heating atmosphere is an inert gas or nitrogen, the atmosphere contains H.sub.2 above 0.1% volume fraction.

8. The method for extracting carbon materials from plastics according to claim 1, wherein the nanostructured carbon produced has one or more of the following properties: surface area of greater than 500 m.sup.2 g.sup.1; capacitance of greater than 70 F g.sup.1; having a graphitic structure; having a symmetrical Raman 2D band; containing graphene nanolayers with the number of layers of less than 20 layers, with a flake thickness of less than 10 nm.

9. The method for extracting carbon materials from plastics according to claim 1, wherein the metal halide salt is NaCl and the plastic is polyethylene terephthalate (PET), and the heating temperature is above 1100 C.

10. An equipment by which the method for extracting carbon materials from plastics according to claim 1 is implemented, wherein the equipment includes a tunnel furnace with a moving load bracket; the moving load bracket is made of refractory materials or covered with alumina panels fitted on a metallic rail; the upper part of the tunnel furnace is provided with heating elements installed on refractory materials; the heating element can be gas or electricity powdered, providing temperature needed for the reaction; the upper part of the tunnel furnace is provided with a hole connected to a gas extraction system for collecting the gaseous substances released during the reaction process; a refractory container is placed on the moving load bracket, in which the salt and the plastic are loaded, and moves with the moving load bracket during the reaction from one end of the tunnel furnace to the other end, within which the temperature increases and then decreases, and finally the refractory container comes out of the tunnel furnace; a post-treatment and the recycling device is used to treat the reaction products obtained from the refractory container by water after the heating process, by which the salt is dissolved, followed by filtering the suspended nanostructured carbon materials, and dry out the final product; the waste heat from the tunnel furnace is used to evaporate the excess water of the filtration liquid to recover the reactant salt for reuse.

11. The method for extracting carbon materials from plastics according to claim 2, wherein the heating temperature is above the melting point of the metal halide salts, and less than the boiling temperature of the salts, by which a mixture of nanostructured carbon materials and salts is produced; after the mixture is cooled down, the salts are dissolved by water followed by filtration to retrieve the nanostructured carbon materials and the solution of the water and the salts; the nanostructured carbon materials are collected after drying and the salts are recycled from the solution.

12. The method for extracting carbon materials from plastics according to claim 2, wherein the conductivity of the synthesized nanostructured carbon material is greater than 1000 S m.sup.1 or the value of Raman I.sub.D/I.sub.G is less than 0.5.

13. The method for extracting carbon materials from plastics according to claim 2, wherein the salt is one metal halide salt or a mixture of metal halide salts selected from the group consisting of LiCl, NaCl, KCl, MgCl.sub.2, CaCl.sub.2, NaF and ZnCl.sub.2; the salt can be a hydrated metal halide salt or a mixture of more than one hydrated metal halide salt selected from the group consisting of hydrated forms of LiCl, NaCl, KCl, MgCl.sub.2, CaCl.sub.2, NaF and ZnCl.sub.2.

14. The method for extracting carbon materials from plastics according to claim 2, wherein the plastics include at least one of polyethylene, polypropylene, polyethylene terephthalate, polystyrene, polyvinyl chloride, polylactide, polycarbonate, acrylic acid, nylon and ABS resin or synthetic rubbers.

15. The method for extracting carbon materials from plastics according to claim 2, wherein the heating is carried out in air, inert gas atmosphere, nitrogen atmosphere or vacuum conditions; when the heating atmosphere is an inert gas or nitrogen, the atmosphere contains H.sub.2 above 0.1% volume fraction.

16. The method for extracting carbon materials from plastics according to claim 2, wherein the nanostructured carbon produced has one or more of the following properties: surface area of greater than 500 m.sup.2 g.sup.1; capacitance of greater than 70 F g.sup.1; having a graphitic structure; having a symmetrical Raman 2D band; containing graphene nanolayers with the number of layers of less than 20 layers, with a flake thickness of less than 10 nm.

17. The method for extracting carbon materials from plastics according to claim 2, wherein the metal halide salt is NaCl and the plastic is polyethylene terephthalate (PET), and the heating temperature is above 1100 C.

Description

DESCRIPTION OF DRAWINGS

[0025] FIG. 1 shows the XRD pattern of the PET water bottle. The broad diffraction peak in the spectrum corresponds to the (100) crystal plane of the triclinic structure of C.sub.10H.sub.8O.sub.4.

[0026] FIG. 2 shows the XRD Pattern of the PET plastic bottle heated overnight at 250 C. The diffraction peaks are indexed to crystalline PET (anorthic) structure.

[0027] FIG. 3 shows the DSC and TGA thermograms of PET heated at the heating rate of 40 C. min.sup.1 in an air flow of 100 ml min.sup.1.

[0028] FIG. 4 shows the (a) XRD pattern of PET material heated in air to 620 C. and 850 C.; (b) SEM images of PET material heated to (c) 620 C. and (d) 850 C.

[0029] FIG. 5 shows (a) XRD patterns and (b) Raman spectra of products obtained by heating of PET with NaCl up to 1100 C. and 1300 C., followed by the cooling and washing process.

[0030] FIG. 6 shows a SEM image of the carbon material obtained by the heating of PET and NaCl to 1100 C. (a) The edge of a smooth and irregular particle is exhibited; (b) The presence of graphene nano-flakes on carbon particles is shown; (c) Exhibits a low magnification image demonstrating the graphitisation zones scattered on the surface of irregularly shaped carbon particles; (d) Some areas are highly crystallized into graphene nano-flakes.

[0031] FIG. 7 shows an optical photograph of the carbon and NaCl obtained by heating of the mixture of PET and NaCl to 1300 C. (a) The mixture inside the alumina crucible; (b) The mixture removed from the crucible.

[0032] FIG. 8 shows the SEM images of nanostructured carbon materials obtained by heating a mixture of PET and NaCl to 1300 C. (a) The presence of graphitic layers is shown; (b) The surface of the carbon material is being peeled off to form graphene nano-flakes; (c) The surface of the graphitic layers of the carbon material and the formation of graphene nano-flakes can be seen more clearly at a higher magnification; (d) The graphene nano-flakes can be seen more clearly at a higher magnification.

[0033] FIG. 9 shows transmission electron (TEM) micrograph image of the carbon material obtained by heating PET and NaCl to 1300 C. (a) A low magnification image showing the hierarchical morphology of the material consisting of a mixture of nano-flakes and fragmented nano-flakes is exhibited; (b) A high magnification image showing the morphology of the fragmented nano-flakes showing the crystalline fringes of the nano-flakes. The Fast Fourier Transform (FFT) analysis recorded on a number of sheet fragments indicated in the figure can be seen as the inset in (b); (c) A high-resolution TEM image of the carbon sheets exist in the sample with a high crystalline structure. The FFT pattern recorded on a carbon sheet, indicated by black rectangle in the micrograph is shown as the inset in (c), exhibiting spots corresponding to the graphitic nanostructure. (d) The thickness of two graphene nanosheets is identified to be 5.6 nm and 8.5 nm in the micrograph.

[0034] FIG. 10 shows the Nitrogen adsorption-desorption isotherms of carbon materials obtained by heating PET and NaCl to 1300 C.

[0035] FIG. 11 shows the electrical and electrochemical properties of the carbon nanostructure produced by the heating of PET with NaCl to 1300 C.: (a) V-I relationship, (b) The relationship between the electrical conductivity and the density of the carbon material with the pressure applied, (c) The cyclic voltammetry (CV) characteristics at different scanning rates, (d) The constant current charge and discharge performance of the electrode made of the carbon material at different current densities.

[0036] FIG. 12 shows the Raman spectrum of the carbon material obtained by heating of the PET plastic bottle with NaF to 1300 C. in air, followed by cooling and washing process.

[0037] FIG. 13 shows Raman spectrum of the carbon material obtained by heating of PET plastic bottle with MgCl.sub.2.6H.sub.2O to 1300 C. in vacuum, followed by cooling and washing process.

[0038] FIG. 14 shows a process flow diagram of the preferred process for the continuous production of nanostructured graphitic carbon. Referring to FIG. 14, 1 indicates the moving load bracket; 2 indicates the upper part of tunnel furnace; 3 indicates a hole connected to a gas extraction system; 4 shows the temperature profile inside the tunnel furnace; 5 shows the salt recovered from the mixture of nanostructured carbon and salt; 6 shows a refractory container; 7 shows a crucible loaded with a mixture of plastics and salts; 8 indicates the melting of plastic materials; 9 shows the solid salt particles; 10 shows the nanostructured carbon materials dispersed in molten salts; 11 shows the crucible which is sufficiently cooled, and 12 shows the nanostructured carbon materials.

DETAILED DESCRIPTION

[0039] In order for the invention to be proved, easier to understand and easy to implement by those skilled in the art, the present invention is described by unrestricted examples combined with the corresponding pictures, charts and micrographs, where:

[0040] The preferred reactor, as shown in FIG. 14, comprises a tunnel furnace with a moving load bracket. The moving load bracket 1 is made of refractory materials, such as aluminium oxide panels on a metal rail. A controller can adjust its moving speed inside the tunnel furnace. The upper part of the tunnel furnace contains heating elements mounted on the upper refractory panels. The heating elements can be gas-powered or electric. The upper part of the tunnel furnace is provided with a hole connected to a gas extraction system 3. As shown in the temperature profile inside the tunnel furnace 4, the temperature can gradually be increased from the room temperature (T.sub.1) to an adjustable maximum temperature of T.sub.max. The maximum temperature is preferentially more than 1100 C., and more preferentially more than 1200 C. or more than 1300 C. The temperature is gradually reduced from the maximum temperature by moving the load bracket towards the other end of the tunnel furnace to the temperature of T.sub.2, which is less than 500 C., or preferentially less than 400 C., or more preferentially less than 300 C.

[0041] The salt recovered from the mixture of nanostructured carbon and salt is shown as (5). The recovered salt is loaded in the refractory container 6, together with pieces of plastic materials. The salt is preferentially a metal halide or a mixture of metal halides. More preferentially the salt is NaCl or contains NaCl. It is because NaCl is cheap and highly available. Moreover, NaCl has an appropriate melting and boiling point of about 800 and 1400 C., indicating that molten NaCl can protect the carbonaceous materials from severe oxidation at high temperatures greater than 900 C. Also, NaCl has a high solubility in water, and therefore can easily be removed from the system and recovered. The refractory container 6 can be a ceramic crucible such as alumina (Al.sub.2O.sub.3). It can also be a carbon crucible such a graphite crucible, provided if the process is carried out under a protective atmosphere, such as argon or nitrogen, to avoid oxidation of the carbon crucible. To start the heat treatment process, a mixture of plastics and salt is loaded into the crucible. The crucible loaded by a mixture of plastics and salts 7 is then moved into the tunnel furnace by the application of the moving load bracket 1. The temperature inside the tunnel furnace is controlled by the heating elements positioned, preferentially, on the upper part of the tunnel furnace 2. The temperature inside the tunnel furnace is gradually increased as indicated in temperature profile inside the tunnel furnace 4. When the temperature inside the tunnel furnace exceeds the melting point of the plastic materials inside the crucible, the melting of plastic materials 8 occurs. By further moving of the crucible forward in the tunnel furnace, the temperature exceeds the decomposition point of the plastic materials. Consequently, the plastic materials decompose into a gas phase which exits the crucible and solid carbonaceous particles mixed with the solid salt particles 9. The gas phase is evacuated from the tunnel furnace through the hole connected to a gas extraction system 3. Whilst the crucible moves forward, the temperature exceeds the melting point of the salt. By further moving of the crucible in the tunnel furnace, the carbonaceous material undergoes an enhanced graphitization, promoted by the molten salt. At T.sub.max, the graphitization process is accelerated, and the carbonaceous material turn into a nanostructured carbon material dispersed into the molten salt 10. By further moving of the crucible through the tunnel furnace, the temperature decreases to the temperature of T.sub.1 at the exit location. T.sub.1 is less than 500 C., or preferentially less than 400 C., or more preferentially less than 300 C. When the crucible is sufficiently cold 11, its content is exposed to distilled water. It can be done by adding water into the crucible. Consequently, the salt is dissolved into water and a suspension comprising the nanostructured carbon and a liquid phase comprising water and salt is formed. The nanostructured carbon can be extracted by filtration of the suspension through a filter paper. A skilled person knows how to separate the nanostructured carbon materials 12 from the suspension. The liquid phase can then be used to recover the salt. It can be done by the evaporation of water to retrieve the salt. The waste heat generated by the tunnel furnace may be used for the evaporation of water. The salt recovered from the mixture of nanostructured carbon and salt 5 is then transferred to the starting point to be mixed with plastic materials.

[0042] Unless otherwise specified in the examples, the characterization of the materials was performed according to the following methods: electron microscopy evaluations were carried out using a Nova Nano-SEM 450 equipped with energy dispersive x-ray analyser (EDX) and a 200 kV FEI Tecnai F20 field emission gun high resolution TEM (HRTEM). X-ray diffraction (XRD) patterns were recorded on a Philips 1710 X-ray diffractometer (XRD) with CuK.sub. radiation (k=1.54 A with a step size and a dwell time of 0.05 2 and 5 s, respectively. The XRD patterns were then analysed using the X'Pert High Score Plus program. Raman spectroscopy was conducted using a Renishaw 1000 Ramanscope with a HeNe ion laser of a wavelength of 633 nm (red, 1.96 eV). Thermal gravimetry (TG) and differential scanning calorimetry (DSC) were carried out simultaneously using a thermal analyser model SDT-Q600 equipped with alumina crucibles at a heating rate of 40 C. min.sup.1 under a constant air flow rate of 100 ml min.sup.1 through the sample chamber. Brunauer-Emmett-Teller (BET) surface area analysis was performed by recording nitrogen adsorption/desorption isotherms using a static volumetric technique with a Micromeritics TriStar 3000 V6.04 Aanalyser at 196 C. The electrical conductivity measurement was conducted by compressing 0.5 g of the carbon material into an acrylic tube (ID=20.05 mm, H=45.37 mm) using a brass piston (D=20.05 mm, H=85.36 mm) on a copper holder. The carbon powder was compacted using a hydraulic press to build up a pressure on the carbon power up to about 6 MPa. At different pressures, various values of electric currents in the range 0.16-3 A was conducted between the brass piston and the copper holder, and the corresponding potentials were recorded using the four-probe DC method at 20 C. The electrical resistivity of compressed carbon powder was calculated using the equation:

=(SV)/(IH) (1)

[0043] Where is the resistivity ( m), S is the surface area of the acrylic tube's hole (mm.sup.2), V is the potential difference (mV), I is the current (A) and H is the height of the compressed powder (mm) Electrical conductivity was calculated as the reciprocal of electrical resistivity.

[0044] The electrochemical capacitance performance of the carbon product was evaluated using a three-electrode system, in which the working electrode was prepared by mixing the carbon materials with 10% conductive carbon (SP45 with a BET surface area of 45 m.sup.2 g.sup.1) and 10% binder (polytetrafluoroethylene, PTFE). The mixture was loaded on a Ni plate of 1.2 cm in diameter, with a mass loading of 3.3 mg cm.sup.2. The electrolyte was 6 M KOH. A platinum wire and a saturated calomel electrode (Hg/HgCl n saturated KCl) were employed as the counter and the reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the electrochemical performance. The specific capacitance (F g.sup.1) of the supercapacitor fabricated was calculated from the equation:

C.sub.s=(I t)/(m V) (2)

[0045] where I is the discharge current, t is discharge time, in is the mass of active material and V represents the voltage window.

EXAMPLE 1

[0046] Characterization of PET Material

[0047] The XRD diffraction pattern recorded on small pieces of a water bottle is shown in FIG. 1. The pattern can be characterized by a broad peak centered at 2=25.4, indicating the short range (100) crystalline domains of PET with an anorthic crystalline configuration (C.sub.10H.sub.8O.sub.4, JCPDS card No. 50-2275). Overall the XRD pattern of FIG. 1 represents a low crystalline PET structure.

[0048] The low crystalline PET material was heated in a resistance furnace at 260 C., above the melting point of PET, overnight. After cooling down to the room temperature, the heat treated material obtained, in form of white colour, large and irregular shaped crystalline particles, was subjected to XRD analysis. The diffraction peaks observed in the pattern can be indexed according to the crystalline PET anorthic structure. The most intense (100) reflection peak is observed at 2 theta=26.00 degree. The SEM morphology of the crystalline PET is shown as the inset in FIG. 2, in which a large size particle with dimensions in excess of 0.5 mm, sharp edges and smooth surfaces can be observed. The EDX analysis of the crystalline PET (FIG. 1S) demonstrated a C:O value of 1.8, which is smaller than the theoretical value of atomic C:O ratio of the repeat unit for PET (2.5).

[0049] Gonzalez et al. measured the C:O ratio of virgin and plasma-treated PET to be 3 and 1.7, respectively using XPS analysis (E. Gonzalez II, M. D. Barankin, P. C. Guschl, and R. F. Hicks, Remote atmospheric-pressure plasma activation of the surfaces of polyethylene terephthalate and polyethylene naphthalate, Langmuir 24 (2008) 12636-12643). The higher value of C:O measured on virgin PET was attributed to the presence on surface contaminations. On the other hand, the lower value of C:O in plasma treated PET was explained by the presence of more CO and CO bonds on the surface of PET induced by the plasma treatment. In our case, the lower value of C:O ratio observed based on the EDX analysis can be attributed to the surface electron irradiation of PET occurred during the microscopy. These observations confirm that the crystallisation of PET occurred during its solidification, and that the plastic bottle was made of pure PET. Such material with a high carbon content and no inorganic component is an attracting source for the preparation of solid carbon materials.

[0050] The TGA and DSC thermograms recorded on a small number of PET pieces in the temperature range 25-900 C. can be seen in FIG. 3. Three endothermic events can be distinguished in the DSC curve observed. The first endothermic peak at 254.1 C. is attributed to the melting of PET. The second endothermic peak at 466.8 C. is due to the decomposition of PET, which is accompanied by a mass reduction of 84.17%, according the TGA curve. It can also be seen that the decomposition of PET began at about 390 C. The last endothermic peak at 791.2 C. might be assigned to the partial graphitization of the residual carbon material.

[0051] In order to investigate this endothermic peak, small pieces of a PET plastic was heated in a resistance furnace to 650 C. and 850 C., and the black carbon materials obtained were subjected to XRD and SEM analyses. FIG. 4a exhibits the XRD patterns of the samples. The XRD pattern of the sample heated to 620 C. shows two broad reflections centred at 2=21.2 and 43.4. The first peak, corresponding to an interplanar spacing d.sub.(002) of 4.18 , can be assigned to the (002) crystalline planes in short-range ordered hexagonal arrays of turbostratic carbon, which is also called disordered graphite or amorphous carbon, produced by the heat treatment of PET at 620 C. The second broad peak extending from 2=40-45 with a maxima at 43.4 corresponds to overlapping (100) and (101) reflections.

[0052] It should be mentioned that in the graphite hexagonal structure (JCPDS card No. 13-0148), the (002) reflection appears at the 2=26.6, indicating an interplanar spacing of 3.35 . The reflection peaks (100) and (101) appear at the 2 values of around 42.4 and 44.4, respectively. The large deviation of d.sub.(002) towards a higher value in the turbostratic carbon material produced in comparison to that of graphite indicates the poorly crystalline nature of the material.

[0053] Li et. al identified the turbostratic carbon as a variant of hexagonal graphite, in which the (002) carbon layers may randomly translate to each other and rotate about the normal of the layers.

[0054] In the XRD pattern of PET heat treated at 850 C. (FIG. 4a), the (002) and the (100)/(101) overlapping diffraction peaks exhibit maxima at the 2 value of 25.2 (corresponding to an interlayer spacing value of 3.54 ) and 43.2. The increase in the intensity of the (002) diffraction peak and also its shift towards a larger value, in comparison to the sample prepared at 620 C., is indicative of the increase in the crystalline order of the material, hence the occurrence of the graphitization onset.

[0055] The endothermic peak appeared in the DSC curve of FIG. 3 at 791.2 C. can be attributed to the onset of the graphitization of the turbostratic carbon. It can be seen from the TGA micrograph of FIG. 3 that about 10% of the material still remained at 900 C., where the analysis was terminated.

[0056] Further information can be obtained from the SEM morphology of the samples shown in FIGS. 4b-d. The SEM micrograph of the turbostratic carbon material obtained at 620 C. can be characterized by the presence of large irregular shaped particles with sharp edges and a size up to several hundreds of micrometers. As seen, despite heating in air, there is no sign of oxidation on the material, indicating a high oxidation resistance. The EDX analysis of the sample revealed a C:O ratio of 5.2, which is subsequently higher than that of observed on crystallized PET (1.8). Nevertheless, the presence of a relatively high amount of oxygen in the turbostratic carbon produced at 620 C. is perhaps the main barrier towards full graphitization of the material. The SEM micrographs of the carbon material obtained at 850 C. (FIGS. 4c and d) show approximately the same features as that observed on the material obtained at 620 C., characterized by irregular-shaped large particles. However, the high magnification images of FIG. 4d shows the presence of small pitting holes on the surface which can be due to the occurrence of minor oxidation. Nevertheless, these micrographs demonstrate the very high oxidation resistance of the material, even at a high temperature of 850 C.

[0057] It is known that the intensive oxidation of carbon materials in air begins at temperatures over 500 C., depending on the properties of the carbon material including its degree of graphitization, particle size and porosity (V. Zh. Shemet, A. P. Pomytkin and V. S. Neshpor, High temperature oxidation behavior of carbon materials in air, Carbon 31 (1993) 1-6). Thermal oxidation of highly oriented pyrolytic graphite was reported by Hahn to occur at 550-950 C. (J. R. Hahn, H. Kang S. M. Lee, Y. H. Lee, Mechanistic study of defect-induced oxidation of graphite, J. Phys Chem B 103 (1999) 9944-51). At temperatures lower than 875 C., the oxidation process was found to be dominated by the formation of pits at defect sites. At higher temperature, however, the oxidation takes place on both defects and basal planes. It is also known that the rate of oxidation of amorphous carbon is higher than that of graphitic carbon materials. Without being limited by mechanism, the high oxidation resistance of the carbon material produced by the pyrolysis of PET in air can be assigned to its large particle sizes, low porosity and perhaps its low density of surface defects.

EXAMPLE 2

[0058] Heat treatment of PET in NaCl was investigated in this example. A plastic water bottle was cut into small pieces (around 105 mm) using a scissor. 9.83 g of plastic pieces was placed into an alumina crucible with an approximate internal diameter and height of 50 mm and 100 mm, respectively. Then, 50.80 g sodium chloride (NaCl, Aladdin C111533, purity 99.5%) was added to the crucible. The crucible was placed into a resistance furnace and heated in the air atmosphere of the furnace at 20 C. min.sup.1 to 1100 C., and then immediately cooled down with an approximately same heating rate to the room temperature. The black solid mixture of solidified salt and carbon product was placed in sufficient amount of distilled water, in which the salt was dissolved. The carbon product was then recovered from the suspension by vacuum filtration using a filter paper, and left to be dried at 80 C.

[0059] FIG. 5 shows the XRD and Raman spectra of the PET-NaCl mixture heated to 1100 C. and 1300 C. (above the melting point of NaCl, 801 C.) followed by cooling and washing the NaCl off the product. The XRD pattern of the black carbon product obtained at 1100 C. reveals the presence of diffraction peaks corresponding to the hexagonal carbon and cubic NaCl. The latter is the residual of NaCl which has been remained with the carbon product even after washing process. On the other hand, the (002) reflection of hexagonal carbon structure appeared at 2=25.13 representing an interplanar spacing of 3.54 . The broad peak with the maxima at 2=43.18 corresponds to overlapping (100) and (101) reflections.

[0060] The Raman spectrum of the carbon product produced by the heat treatment of PET in molten NaCl at 1100 C., shown in the lower panel of FIG. 5b, provides information about the graphitic structure of the material. The so-called defect-induced D band and the G band which corresponds to the stretching vibrations of the basal graphene layers in carbon material appeared at the Raman shift values of 1372, 1599 cm.sup.1, respectively. Moreover a low intensity 2D band, which is the over tone of D band, can be observed at 2703 cm.sup.1. The intensity ratio I.sub.D/I.sub.G that corresponds to the level of defects or inversely to the graphitization degree of the carbon material was measured to be 0.94. Furthermore, the intensity ratio I.sub.2D/I.sub.G which represents the quality of the graphene sheets exist in the product is 0.23. Overall, The XRD and Raman analysis demonstrate the presence of nanocrystalline-graphitised domains in the sample. The SEM morphology of the carbon product produced at 1100 C. is presented in FIG. 6. FIG. 6a shows the edge of an irregular particle with mostly smooth surfaces. This morphology seems overall similar to that observed in the sample prepared by the air heating of PET to 850 C., with this difference that a number of graphitization zones are present on the smooth surface of the carbon material produced in molten NaCl. FIG. 6b clearly shows the presence of graphite flakes appeared on the carbon particle. FIG. 6c exhibits a lower magnification image demonstrating that these graphitization zones are scattered on the surface of irregular shaped carbon particles. Some areas were highly crystallized into graphene nanosheets such as shown in the SEM micrograph of FIG. 6d. This finding is interesting since it demonstrated that graphitized carbon nanostructures can be formed by simple heating of PET in NaCl to a nearly low temperature of 1100 C.

EXAMPLE 3

[0061] In order to investigate the effect of temperature, the mixture of PET-NaCl in the same weight ratio as that of Example 2, and the mixture was heated to 1300 C. by the same heating rate of 20 C. min.sup.1, and then cooled down to the room temperature. FIG. 7a, shows crucible and the mixture of salt and carbon product. In order to evaluate the distribution of the carbon phase in the solidified NaCl, the alumina crucible was broken, by which the mixture of salt and carbon was easily retrieved from the crucible (FIG. 7b). It can be seen that the carbon material is entirely distributed into the solidified NaCl. This observation is interesting, demonstrating the high dispensability of the carbon product in molten NaCl. The solid mixture was added to 500 ml distilled water. Upon the dissolution of NaCl, the carbon material was floated on the surface of water showing its low density. The suspension obtained was stirred for 20 min and then filtered. The carbon material remained on the filter paper was then dried overnight. The XRD analysis of the sample obtained (FIG. 5a), shows the presence of the (002) peak of hexagonal graphite at 2=25.9 representing an interlayer spacing of 3.44 . The broad peak with a maxima at 2=42.5, corresponds to overlapping (100) and (101) reflections. As can be seen, the diffraction peaks related to NaCl have nearly vanished from the pattern, which can be due to the washing procedure. The Raman spectrum of the sample (shown in FIG. 5down panel) provides interesting information about quality of the carbon material produced. The spectrum exhibits a relatively small D band at 1364 cm.sup.1, and a sharp and distinguished G band at 1590 cm.sup.1. The value of I.sub.D/I.sub.G ratio can be measured from the spectrum to be low at 0.47. This observation reveals the presence of crystalline carbon domains with a low defect density. In addition to this, a symmetric 2D band can also be observed in the spectrum at 2723 cm.sup.1, and the value of I.sub.2D/I.sub.G ratio was found to be 0.52. These observations confirm that the carbon product consisted of carbon entities of few layer graphene.

[0062] FIG. 8 shows the SEM micrographs of the nanostructured carbon material obtained. The presence of graphitic layers can clearly be observed from FIGS. 8a and 8d. FIGS. 8b and 8c, interestingly, demonstrate the surface exfoliation of the graphitic layers of the carbon material, leading to the formation of graphene-like nanosheets. The EDX analysis recorded on the carbon material showed a high C:O atomic ratio of 28.4.

[0063] TEM microscopy (FIG. 9) provided further evidence for the graphitized carbon nanostructure produced. FIG. 9a, exhibits a low magnification image showing the hierarchical morphology of the material consisting of a mixture of nanosheets and fragmented nanosheets. FIG. 9b exhibits a high magnification image taken on the area with the latter morphology, showing the crystalline fringes of the nanosheets. The Fast Fourier Transform (FFT) analysis recorded on a number of sheet fragments indicated in the figure can be seen as the inset in FIG. 9a. The halo ring in the FFT pattern indicates an interplanar spacing of 3.5 , corresponding to the (002) crystalline planes of hexagonal carbon. The micrograph confirms the nanocrystalline nature of the carbon material produced in molten NaCl. The down panels in FIG. 9 show high resolution TEM images of the carbon sheets exist in the sample with a high crystalline structure. The FFT pattern recorded on a carbon sheet, indicated in FIG. 9c by blackrectangle, exhibits spots corresponding to the graphitic nano structure. FIG. 9d shows several graphitic nanosheets, two of which with thickness of 5.6 nm and 8.5 nm have been identified in the micrograph. The careful examination of the carbon product revealed that graphitic sheets consisted of 4-20 layers with a thickness of less than 10 nm.

[0064] The surface properties of the nanostructured carbon material were studied through the nitrogen adsorption-desorption technique, and the recorded isotherm is presented in FIG. 10. According to the IUPAC classification, this curve displays a type-II isotherm and a type-H4 hysteresis loop, suggesting the presence of non-porous or macroporous surfaces with narrow slit like pores. The BET surface area of carbon material was found to be 522 m.sup.2 g.sup.1.

[0065] The results obtained confirm that the heat treatment of PET in molten NaCl leads to the preparation of a carbon nanostructure consisting crystalline graphitic nanosheets and sheet fragments with a thickness of less than 10 nm, and a high surface area.

[0066] It should be mentioned that, together with surface area and crystallinity, the electrical conductivity is one of the most important parameters which determine the performance of carbon materials in practical applications such as supercapacitors, electromagnetic shielding, catalysts, and metal-ion batteries. However, in carbon materials, often the electrical conductivity decreases with the increase in surface area.

EXAMPLE 4

[0067] Nanostructured graphite was produced as explained in Example 3. The electrical conductivity of the carbon material produced was evaluated at various compression pressures using the four probe method. FIG. 11a shows the current-voltage response of the carbon material at different pressures, in the range 0.01-6.13 MPa, applied to 0.5 g sample, showing a perfect Ohmic response. The values of density, calculated by considering the column height of the compressed powder, and those of electrical conductivity of the sample were plotted as the function of the pressure applied, and the result is exhibited in FIG. 11b. As it can be seen, the density of the carbon powder under a light pressure of 0.10 MPa is 0.10 g cm.sup.3 exhibiting an electrical conductivity of 12.53 S m.sup.1. By increasing the pressure to 4.14 MPa, the density and conductivity increase considerably to 0.89 g cm.sup.3 and 1071.24 S m.sup.1, respectively. As can be depicted from the plot, the density increased to 1.04 g cm.sup.3 by a further increase of pressure to 5.47 MPa, but the corresponding conductivity slightly decreased to 1058.43 S m.sup.1. As can be observed from FIG. 11a, the resistance of the sample, indicated by the slope of the I-V curve decreased from 8.76 m to 6.79 m by the same increase in pressure. The decrease in the value of electrical conductivity by the increase of pressure observed can be explained by the contribution of the reduced column height of the compressed powder to the value of electrical conductivity (equation 1). It can be seen that the values of density and conductivity increased to 1.06 g cm.sup.3 and 1150.15 S m.sup.1 by the increase of the pressure to 6.13 MPa. The conductivity obtained is rather impressive.

EXAMPLE 5

[0068] Nanostructures graphite was produced as explained in Example 3. The Electrochemical behavior of the carbon product was evaluated using a three-electrode system with 6 M KOH as the electrolyte. The CV profiles, recorded at various scan rates of 5-200 mV s.sup.1, shown in FIG. 11c, exhibit nearly rectangular shapes, indicating that the carbon product stores charge mainly via an electrochemical double layer capacitive mechanism, with a reasonable rate performance and no pseudocapacitive effect. It further confirms the high carbon purity and electrical conductivity of the sample.

[0069] FIG. 11d shows the potential-time profiles measured at deferent current densities in the range 0.2 to 20 A g.sup.1. As can be seen, the discharge time is roughly equal to the charging time, and the curves are almost isosceles triangle, indicating a high reversibility. According to the galvanostatic charge/discharge curves, the specific capacitances could be calculated to be 90.2, 78.6, 73.0, 58.0, 50.0 and 40.0 F g.sup.1 at current densities of 0.2, 0.5, 1.0, 5.0, 10.0 and 20 A g.sup.1, respectively.

EXAMPLE 6

[0070] 100 g Sodium fluoride (NaF) was mixed with 20 g PET plastic pieces. The mixture was placed in an alumina crucible and heated to a maximum temperature of 1300 C., and held at this temperature for 1 h in an electric furnace in air. The crucible was then allowed to be cooled down, and the salt was washed off using sufficient amount of water followed by the filtration of the suspension obtained. The black carbon material obtained on the filter paper was dried at 80 C. for 2 h. The Raman spectrum of the carbon material obtained is shown in FIG. 12. The Raman spectrum clearly show the presence of D, G and 2D bands at 1322, 1571 and 2640 cm.sup.1, respectively, characteristic of graphitic structures. The intensity ratio I.sub.D/I.sub.G can be calculated from the spectrum to be 0.5, representing a high degree of graphitization. The 2D band in the Raman spectrum is intense and symmetric, representing the presence of few layer graphene.

EXAMPLE 7

[0071] 50 g MgCl.sub.2.6H.sub.2O was mixed with 10 g PET plastic pieces, and the mixture was placed in an alumina crucible and treated according to the heat treatment and washing process explained in the previous Example 6, with the only difference that the heating process was conducted in a tube furnace under a vacuum of 10.sup.2 mbar. The Raman spectrum of the carbon material obtained is shown in FIG. 13. The D, G and 2D band, characteristic of graphitic carbon materials was observed at 1307, 1567 and 2620 cm.sup.1, respectively. The Raman ration I.sub.D/I.sub.G in the carbon material was calculated from the spectrum to be 0.3, representing a high degree of graphitization.

[0072] The process of producing conductive nanostructured graphitized carbon can be carried out in a continuous manner in the tunnel furnace, as shown in FIG. 14. The tunnel furnace provides continuous operation for the preparation of nanostructured carbon.