HIGH DENSITY POLYETHYLENE-EXFOLIATED GRAPHENE NANOPLATELET NANOCOMPOSITES FOR AUTOMOTIVE FUEL LINE AND FUEL TANKS APPLICATIONS

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1 HIGH DENSITY POLYETHYLENE-EXFOLIATED GRAPHENE NANOPLATELET NANOCOMPOSITES FOR AUTOMOTIVE FUEL LINE AND FUEL TANKS APPLICATIONS F. Vautard, T. Honaker-Schroeder, L. T. Drzal Composite Materials and Structures Center Michigan State University 2100 Engineering Building East Lansing, MI L. Sui Hyundai-Kia America Technical Center, Inc Geddes Road Superior Township, MI Abstract High density polyethylene - exfoliated graphene nanoplatelet (GnP) composites were produced by melt extrusion and injection molding. Bulk specimens were manufactured for mechanical testing (tensile, flexural and Izod impact tests) and films were produced for the measurement of the permeation to oxygen and fuel. 3 different grades of graphene nanoplatelets with different diameters (from 15 to 0.3 micron), thicknesses (from 6 to 2 nm, respectively) and surface areas (from 100 to 750 m 2 /g, respectively) were investigated over a range of concentrations from 0.2 wt. % to 7.5 wt. %. The stiffness of the GnP nanocomposites was greatly improved without a loss of the strength. The impact resistance decreased. A clear decrease of the permeation to both oxygen and fuel was measured. An increase of the crystallinity was induced by the nucleation sites provided by the surface of the platelets. The thermal stability was also notably improved. The maximum concentration of platelets tested in this study did not result in any increase in electrical conductivity. The percolation threshold was not reached because of the limited dispersion of the particles in the polymer melt. The evolution of the properties depended on the type of platelets, their concentration and their dispersion. I. Introduction The combined goals of reducing vehicle weight along with reducing fuel evaporative emissions (hydrocarbon vapors that escape from a vehicle fuel system) will require the increased use of lightweight materials with better barrier properties for the manufacture of fuel lines and fuel tanks. Currently, fuel lines and tanks are made of co-extruded laminates consisting of semicrystalline thermoplastics such as High Density PolyEthylene (HDPE) and films of PolyAmide 6 (PA 6) or Ethylene Vinyl alcohol (EVOH) that insure good barrier properties. The advent of platelet shaped nanoparticles offers a promising new alternative. Because of their platelet morphology, they force gases that penetrate the polymer to take a tortuous path, which slows down their diffusivity, as shown in Figure 1. HDPE-clay [1] and HDPE-layered silicates [2-4] nanocomposites have been investigated and a significant decrease of the permeability to oxygen and carbon dioxide was obtained. Exfoliated Graphene nanoplatelets (GnP) have the potential to improve the barrier properties of HDPE through the same mechanism. Because Page 1

2 graphene has high electrical and heat conductivity, exfolidated GnP can also increase the electrical and heat conductivity of the material, thus allowing the dissipation of potential static charges and improving its thermal stability. To the best of our knowledge, no work on the influence of the incorporation of nanoparticles in HDPE on the permeation to fuel has been reported in the literature. The nanocomposites also need to show a combination of acceptable mechanical properties (stiffness and impact resistance). Figure 1: Principle of the barrier properties provided by platelet-shaped fillers. This study focuses on extruded and injection molded HDPE-GnP nanocomposites. The evolution of the mechanical properties (tensile, flexural properties and Izod impact resistance), degree of crystallinity of the HDPE matrix, thermal stability and permeation to oxygen and fuel (Californian Air Resource Board, CARB, phase II) will be studied as a function of the platelet size, their concentration and their degree of dispersion in the matrix. II. Materials and methods II.1 Materials HDPE with the trade name K was supplied by INEOS Olefins & Polymers USA in pellet form. It has a density of g.cm -3 (ASTM D4883) and a melt index (190 ºC/ g) of 4.2 g/10 min (ASTM D1238). Graphene nanoplatelets (GnP), obtained from XG Sciences (Lansing, MI, USA), were heated in air for 1 hour at 450 ºC in order to remove any volatile forming compound. Three grades of platelets were tested; their dimensions and surface area are displayed in Table 1. Table 1: Properties of the different grades of GnP used in this study. GnP reference Average diameter Average thickness (μm) (nm) Surface area (m 2.g- 1 ) GnP-M GnP-M GnP-C II.2 Manufacture of the nanocomposites A DSM Micro 15cc extruder (vertical, co-rotating, twin-screws) was operated at 220 ºC (HDPE was really exposed to a temperature of 210 ºC) and the melt mixing was performed with a screw speed of rpm for 5 minutes before the first transfer (Figure 2). The shear force was kept around 6000 N. The composite melt was then transferred to a Daca Micro-injector with the temperature of the barrel and the temperature of the mold being at 210 C and 110 C, respectively. The pressure applied for the injection molding of flexural, tensile and Izod impact Page 2

3 specimens was 1.0 MPa (150 psi). HDPE and GnP were simply manually mixed and transferred in the feeder. Figure 2: (a) DSM Micro 15cc extruder, (b) Daca Micro-injector. For each formulation, a set of 7 tensile, 7 flexural and 7 Izod impact resistance specimens was prepared, as displayed in Figure 3. Figure 3: Sets of specimens for mechanical characterization. Left: Neat HDPE, Right: HDPE + GnP-M-15 at 0.5 wt. %. Within the same set, from left to right: flexural test coupons, tensile test coupons, Izod impact test coupons. II.3 Mechanical testing Flexural properties were measured according to ASTM D790 with a UTS SFM-20 testing machine (United Calibration Corp.) equipped with a 100 lbs loading cell. The thickness to span ratio was 1/16 and the displacement speed of the loading nose was 1.3 mm.min -1 (0.05 in.min -1 ). For tensile testing, a 1000 lbs load cell was used and the extension of the sample was measured with reflective tape and a laser extensometer, following the directions of ASTM D638. The speed of displacement was 13 mm.min -1 (0.5 in.min -1 ). Izod impact resistance testing was performed according to ASTM D256 with a TMI impact testing device and a 1 lb hammer. The notch was made with a single tooth motorized notch cutter 24 hours before testing. II.4 Measurement of the crystallinity The crystallinity of HDPE and its composites was measured by Differential Scanning Calorimetry (DSC) (Q2000 from TA Instruments). Non-isothermal crystallization under nitrogen was studied using the following protocol. In order to erase the thermal history of the sample, it Page 3

4 was first heated to 160 C at a rate of 20 C.min -1 and isothermal conditions were maintained at 160 C for 5 minutes. Then, the sample was cooled to 40 C at a rate of 20 C.min -1. For the measurement of the melting enthalpy, the sample was heated again to 160 C at a rate of 20 C.min -1 (Figure 4). TA Universal Analysis software was used for the analysis of the graphs. Figure 4: Measurement of the melting enthalpy of HDPE and its composites by DSC. The degree of crystallization was calculated according to Equation (1). (1) With x% the degree of crystallinity of the matrix (in %), wt. % the weight percentage of GnP in the composite, δh m the melting enthalpy of the sample measured by DSC and δh 0 m the theoretical melting enthalpy of a 100 % crystalline matrix (288 J/g according to Mirabella et al. [5]). Two different samples were analyzed per formulation. II.5 Thermal stability The thermal stability of HDPE and its composites was assessed by ThermoGravimetric Analysis (TGA) (Q500 from TA Instruments). Tests were performed in air where the specimen was heated from room temperature to 600 C with a heating rate of 10 C.min -1. II.6 Electrical conductivity The electrical conductivity of the nanocomposites was determined using potentiostatic Electro- Impedance Spectroscopy (EIS) through the thickness direction (through-plane resistivity, normal to the flow direction) at room temperature with the two-probe method and using a Gamry Instruments EIS Framework v 4.21 (AC voltage = 100 mv, frequency range = 0.1 Hz-100 khz). Samples were cut from flexural specimens. Top and bottom surfaces were first etched in oxygen plasma (50 % of O 2, 14 min, 375 W) to remove a layer rich in polymer and covered with conductive carbon paste to ensure good contact with copper tape that was connected to the Page 4

5 electrodes. The resistance of the samples was measured and converted to resistivity using the cross sectional area of the sample. II.7 Permeation to oxygen Films of HDPE and its composites were made from a flexural test specimen using a Carver hydraulic press heated at 180 ºC. Specimens were sandwiched between two steel plates with a mirror finish surface and an applied pressure of 3.8 MPa (550 psi). The oxygen transmission rate through the films was measured with an OX-TRAN 2/20 ML device from Mocon. The reported value is the average of 10 measurements performed on each sample. The films were conditioned for 6 hours before the test. The transmission rate was normalized with respect to the film thickness, which was the average of four measurements made with a caliper. II.8 Permeation to fuel The permeation to fuel (CARB phase II) was done according to the cup weight loss method, following the SAE International J2665 test procedure and using the vapometer system from the Thwing-Albert Instrument Company (Figure 5). Films of HDPE and its composites were made with the same method as the one used for the permeation to oxygen. CARB phase II fuel was provided by the Haltermann Solutions Company. The permeation cups were sealed with the film and contained 100 ml of fuel at the beginning of the test. They were kept at 60 ºC in an oven constantly purged with nitrogen. The weight of the cups was measured every 24 hours for 8 days. The fuel transmission rate was normalized with respect to the film thickness, which was the average of four measurements with a caliper. Figure 5: Permeation cups used for the measurement of the permeation to fuel. II.9 Observations at a micro scale by Scanning Electron Microscopy Dispersion of the GnP in the HDPE matrix was characterized by Scanning Electron Microscopy (SEM), using a Zeiss EVO LS25 microscope. The surface of a flexural test specimen was first etched in oxygen plasma (50 % of O 2, 14 min, 375 W) and then coated with a 3 nm film of tungsten with a Leica EM MED020 sputter-coater. The same method was applied to observe the cross-section of the films made for permeation experiments. The acceleration voltage was 4 kv. The fracture surface of Izod impact resistance specimens was also observed by SEM using the same conditions for tungsten coating and characterization. Page 5

6 III. Results and discussion III.1 Characterization of the dispersion of the platelets by SEM SEM pictures of the surfaces of HDPE-GnP-M-15 and HDPE-GnP-C-750 composites that were etched by oxygen plasma are displayed in Figure 6. Overall, it is obvious that the platelets were evenly dispersed in the HDPE matrix. In the case of GnP-M-15, the size of some particles was in the range of μm, as expected, but many particles with diameters clearly below 10 μm could be seen as well. Size reduction of the platelets occurs during the extrusion process because of the shear forces applied to the particles. In the case of GnP-M-5, the dispersion was homogeneous as well, without any size reduction. For GnP-C-750, large aggregates of particles remained in the sample. Their size ranged between a few microns and 10 μm. Because of its higher surface area and the smaller particle size, this grade of graphene platelets was more difficult to disperse. During mixing, the applied shear energy is not high enough to overcome the physical interactions between the particles (π-π interactions). This point highlights the limits of the extrusion process, especially for HDPE in pellet form. The mixing step before the extrusion process is the determining step when it comes to the final level of particle dispersion. Previous studies have shown that the dispersion of carbon nanoparticles without any pre-surface treatment in HDPE by extrusion process is very challenging if no pre-mixing is involved. This issue has also been reported in the case of carbon nanotubes [6]. Figure 6: Dispersion of GnP-M-15 and GnP-C-750 at a concentration of 5 wt. % in HDPE matrix. Page 6

7 III.2 Evolution of the crystallinity of the HDPE matrix The evolution of crystallinity of the HDPE matrix as a function of GnP grade and concentration is reported in Figure 7. The crystallinity of neat HDPE, with the thermal history applied during the DSC analysis, was around 54 %. By adding graphene nanoplatelets, the crystallinity of HDPE was significantly increased to values comprised between 58 % and 63 %, even at low concentrations such as 0.2 wt. %. This was observed for all three grades of GnP. The nucleation effect from the surface of graphene nanoplatelets can be observed from the increase in HDPE crystallinity. The degree of crystallinity was slightly higher for low concentrations of GnP (up to 2 wt. %) in comparison to a concentration of 5 wt. %. Two counteracting mechanisms take place when it comes to the development of crystallites [7]. The surface of the platelets provide nucleating sites but the particles spacing hinder the diffusion of the HDPE chains which limits the growth of those crystallites. At high GnP concentrations, the lack of mobility/diffusion of HDPE chains may explain why the degree of crystallinity does not increase further, whereas the density of nucleation sites increases. Because some of the GnP-C-750 was in the form of agglomerates, its high surface area (and therefore its higher density of nucleating sites) did not lead to higher degrees of the crystallinity above a concentration of 2 wt. %. Figure 7: Evolution of the crystallinity of the HDPE matrix in function of GnP concentration and GnP grade. III.3 Izod impact resistance The influence of GnP grade and concentration on the impact resistance of the composites is displayed in Figure 8. A sharp drop (- 46 %) is observed even at low concentration (0.2 wt. %) of GnP-M-15 and GnP-M-5. The value of impact resistance decreases steadily as the concentration of platelets increases. A significant difference between GnP-M-15 and GnP-M-5 was noticed for a concentration of 7.5 wt. %, the smaller particles led to higher impact resistance. GnP-C-750 platelets also resulted in a decrease of the impact resistance, but this Page 7

8 was less pronounced in comparison to the two other grades of GnP. A size effect was clearly demonstrated, even if partial agglomeration occurred for GnP-C-750. With better dispersion of GnP-C-750, the loss in impact resistance would have certainly been even less pronounced. Figure 8: Evolution of the Izod impact resistance as a function of GnP concentration and GnP grade. Figure 9: SEM observation of the fracture surface after Izod impact testing for neat HDPE. Left: overview of the fracture at the notch. Middle: high magnification of the fracture surface in the immediate vicinity of the notch. Right: high magnification of the fracture surface in the middle of the specimen. Page 8

9 Figure 10: SEM observation of the fracture surface of HDPE-GnP composites after Izod impact testing. Top: HDPE- GnP-M-15 at 5 wt. %, bottom: HDPE-GnP-C-750 at 5 wt. %. Left: overview of the fracture surface. Middle: high magnification of the fracture surface in the immediate vicinity of the notch showing a cell structure. Right: Zoom on the GnP particles centered in each cell structure. SEM observations of the fracture surface, close to the notch, are displayed in Figure 9 (neat HDPE) and Figure 10 (HDPE-GnP-M-15 and HDPE-GnP-C-750 composites). The difference in the appearance of the fracture surfaces between neat HDPE and its composites was clear. For neat HDPE, the fracture surface was smooth in the vicinity of the notch, where the propagation of the fracture was the most energetic, and some wave-shaped features could be found further away from the notch. In the case of HDPE-GnP composites, a honeycomb-like structure (or cell structure) was observed in every case (for all three grades of GnP and concentrations). A platelet or an agglomerate of platelets was found at the center of each cell and in the plane of the fracture. At high magnification, it looked as though the particle initiated the fracture. In the case of GnP-C-750, aggregates of platelets were seen, giving them a ball of wool appearance. It was also obvious that the interfacial adhesion between the platelets (or their aggregates) and the matrix was weak, as the surface of the fillers was not covered by any matrix. More work is necessary to identify the source of the generation of the cell structures. One possibility could be that they correspond to crystallites, the walls being constituted of some amorphous polymer that forms the boundary between crystallites. III.4 Flexural properties The influence of the adding of GnP on the flexural properties of HDPE can be seen in Figure 11. The modulus increased as the concentration of GnP increased. A clear difference was observed between the different grades of GnP for concentrations above 2 wt. %. Formulations processed with GnP-C-750 clearly suffered from the agglomeration of the platelets, as the modulus was significantly lower compared to GnP-M-15 and GnP-M-5. GnP-M-15 and GnP-M-5 gave similar properties up to a concentration of 5 wt. %, which may be due to the size reduction of GnP-M- 15 that was observed by SEM. GnP-M-5 led to the highest value at a concentration of 7.5 wt. %, with an increase of + 90 % in comparison to neat HDPE. This value was higher than for the HDPE-GnP-M-15 composites, which may be due to a better dispersion of the platelets. Page 9

10 Figure 11: Flexural properties of HDPE and its composites (top: flexural modulus, bottom: flexural strength at yield). Page 10

11 The flexural strength at yield increased as the concentration of GnP increased, for all grades of GnP. At low GnP concentration (<2 wt. %), the flexural strength of for all three grades of the composite are approximately the same at each GnP concentration. However, at high GnP concentration (>2 wt. %), GnP-C-750 had noticeably lower flexural strength than GnP-M-5 and GnP-M-15. This may be a result of higher agglomeration for GnP-C-750 particles. The effect of aspect ratio was the most apparent at a GnP concentration of 7.5 wt. % for GnP-M-5 and GnP- M-15. Higher strength was indeed obtained with GnP-M-5, which is expected since larger particles concentrate higher local stress and thus lead to a higher probability of failure for a given load. At lower concentrations, because of the size reduction of GnP-M-15 during processing, the values of strength at yield for GnP-M-15 and GnP-M-5 were close. Overall, the strength at yield of HDPE was increased from 26 MPa to 36 MPa (+ 38 %) with 7.5 wt% of GnP- M-5. III.5 Tensile properties The tensile properties of HDPE and its composites are displayed in Figure 12. The tensile modulus increased when increasing the GnP concentration for all three grades of GnP. The values of tensile modulus were similar when comparing the different grades of GnP, therefore aspect ratio had little effect on the moduli. With a concentration of 7.5 wt. %, the modulus was increased by 160 %. When comparing the tensile strength at yield, higher values were obtained with GnP-C-750 for concentrations up to 2 wt. %. At this concentration or less, the aspect ratio of the platelets had a significant influence on the tensile properties despite the presence of some aggregates. This was not the case for GnP-M-15 and GnP-M-5, the tensile strength at yield was similar or lower to neat HDPE. For a concentration of 7.5 wt. %, the higher density of agglomerated GnP-C-750 affected the strength in comparison to GnP-M-15 and GnP-5. The highest value was obtained with GnP-M-5, which again highlights the influence of the aspect ratio of the platelets on the tensile strength of the composites, when it is not counteracted by agglomeration issues. III. 6 Thermal stability The TGA analysis of neat HDPE and GnP-M-15 and GnP-C-750 based composites revealed that the incorporation of graphene nanoplatelets increased the thermal stability of HDPE (Figure 13), as graphene nanoplatelets are known to be a very good heat conductor [8]. Graphene nanoplatelets degrade at a higher temperature in comparison to HDPE. The heat being dissipated faster by the platelets, the degradation of the HDPE matrix is delayed. Moreover, as shown hereafter, the platelets affect the diffusion of oxygen in the matrix, which can slow down the combustion process as well. Curves obtained with GnP-M-5 were similar to the ones obtained with GnP-M-15. For clarity purposes only the curves obtained with GnP-M-15 and GnP-C-750 are reported. Page 11

12 Figure 12: Tensile properties of HDPE and its composites (top: tensile modulus, bottom: tensile strength at yield). Page 12

13 Figure 13: TGA analysis of HDPE and its composites with GnP-M-15 (top) and GnP-C-750 (bottom). Page 13

14 III.7 Electrical conductivity The through-plane electrical conductivity was measured for all formulations but increases in electrical conductivity were not found. This suggests the percolation threshold for electro conductivity corresponds to a concentration higher than 7.5 wt. %, for the three grades of GnP and under this processing condition. This is directly related to the lack of dispersion of the platelets, which tend to stay in aggregates form, and the orientation of the platelets in the flow direction. If the platelets are perfectly parallel to each other and do not touch each other, no conductive path is created within in the polymer matrix and the composite is not conductive. Indeed, when the platelets are pre-mixed with HDPE using a solution method (sonication of the platelets in HDPE dissolved in xylene) or using solid state ball milling, a better dispersion of the platelets in the matrix is achieved. In this case, electrical conductivity can be obtained at concentrations of about 6 wt. % for solid state ball milling [9] and even less than 1 wt. % with solution mixing of GnP-M-15 [10]. III.8 SEM characterization of the films The cross-section of films was imaged using SEM for GnP-M-15 and GnP-C-750 at a concentration of 7.5 wt. % (Figure 14). Because of the generation of heat at the interface platelet/hdpe, the matrix was removed partially by oxygen plasma etching, therefore revealing the organization of the platelets in the film. An alignment of the GnP-M-15 platelets perpendicularly to the thickness of the film was observed (direction indicated by an arrow in the pictures). It has already been reported that the platelets could be aligned in the flow direction during injection molding [11] but the compression molding process could also induce an alignment in the plane constituted by the film. Of course, this alignment will decrease the permeation to lower values than what they may be for specimens that have not been compressed. For the GnP-C-750 grade film, large ball-shaped agglomerates of several microns were observed throughout the cross-section of the film. Figure 14: SEM pictures of the cross-section of HDPE-composite films with GnP-M-15 and -C-750. Page 14

15 III.8 Permeation to oxygen The values corresponding to oxygen permeation after the addition of GnP are presented in Figure 15. The addition of the three grades of GnP led to a decrease of the permeation to oxygen and the platelets with a higher aspect ratio led to a more pronounced decrease at one concentration. With 5 wt. % of GnP-M-15, the permeation to oxygen was cut in half in comparison to neat HDPE. A concentration of 7.5 wt. % did not lead to a significant decrease in comparison to a concentration of 5 wt. %, which may be due to a lack of dispersion of the platelets or a counteracting effect induced by a potential reduction of the size of the crystallites [12]. At equivalent wt. %, the permeation to oxygen was significantly higher for GnP-C-750, which is related to the lower aspect ratio of the platelets and higher agglomeration, than for GnP-M-15 or GnP-M-5. Also, the size of the crystallites is potentially affected by the type of GnP: smaller particles may have led to smaller crystals, thus increasing the permeation of HDPE itself [12]. III.9 Permeation to fuel Figure 15: Permeation to oxygen of HDPE and its composites. Typical curves obtained by the weight loss method are displayed in Figure 16. High linear correlation coefficient validated the fact that the steady state was established. The permeation to fuel of the tested formulation is reported in Table 2. The permeation to CARB phase II fuel decreased by 25 % at 7.5 wt. % of GnP-M-15. A difference was noticed between GnP-M-15 and GnP-M-5, whereas this was not the case with the oxygen permeation experiments. The efficiency of GnP-C-750 was clearly limited (-5 % at a concentration of 7.5 wt. %). Several causes can explain this phenomenon. The low aspect ratio GnP-C-750 is intrinsically less efficient than the higher aspect ratio of the bigger platelets like GnP-M-15. Another limiting factor may be from the formation of aggregates; this further reduces the aspect ratio of GnP-C- 750 particles to a ball-like shaped particle as opposed to a plate-like shaped particle. The existence of those aggregates itself is obviously an issue to solve in order to decrease the permeability to oxygen and fuel. While this is particularly true for GnP-C-750, these theories can be extended to explain permeation in all three grades of GnP. Lastly, the possibility of different Page 15

16 HDPE crystal sizes for different GnP grades can also play an important role on the permeation to fuel through the matrix itself. The influence of the concentration of the platelets was confirmed as the permeation to fuel decreased with an increase of the concentration. In order to further decrease the permeation to both oxygen and fuel, it is imperative to obtain a better dispersion and orientation of the platelets. It may also be necessary to increase their concentration. Figure 16: Fuel weight loss as a function of time for HDPE and GnP-M-15-HDPE composites. Table 2: Permeation to fuel of HDPE and its composites. Permeation (g.mm/m 2.day) Neat HDPE 22.0 HDPE-GnP-M-15 5 wt. % 18.8 HDPE-GnP-M wt. % 16.5 HDPE-GnP-M wt. % 19.6 HDPE-GnP-C wt. % 20.8 IV. Conclusions This study was undertaken to assess the potential use of graphene nanoplatelets as a filler to tailor the properties of HDPE for fuel tank and fuel line applications. First results obtained with industrial grade nanoplatelets showed that there was a clear reduction in the permeation to oxygen and fuel. The barrier properties (permeation to oxygen) were at least as good as for Page 16

17 other types of nanoparticles (clay, layered silicates). Platelets with a large aspect ratio (large diameter to thickness) will be more effective. In theory, the mechanical properties (flexural, tensile, and Izod impact resistance) should be better with smaller platelets, as their ability to concentrate stress is lower compared to large platelets. But our study showed that smaller platelets can be subjected to more agglomeration and result in lower modulus and strength. The physical properties were clearly limited by the quality of the dispersion obtained with melt mixing. Other mixing methods may have to be considered in order to achieve a better dispersion. Some of which include the use of HDPE in powder form, mixing in solution with sonication or solid state ball milling. With improvements in particle dispersion, improvements in the mechanical and permeation properties can be achieved as well. This work will also be cognizant of the scalability of this process to insure the transferability to an industrial scale mixing processed provided by a compounder. Another strategy can be to coat the platelets with a polymer layer that is compatible with the matrix [13] to reduce aggregation. This approach is currently under investigation. An improved dispersion will also improve the thermal stability and decrease the percolation threshold (value of the concentration of platelets in order to achieve electrical conductivity). Finally, another parameter to be monitored will be the alignment of the platelets in the composite, well aligned platelets in the flow direction will decrease the permeation to fuel even further. V. Acknowledgements INEOS Olefins & Polymers USA and XG Sciences are gratefully acknowledged for the providing of the HDPE and the GnP graphene nanoplatelets, respectively. Per Askeland, Mike Rich, Edward Drown and Brian Rook from the Composite Materials and Structures Center of Michigan State University are sincerely thanked for sharing their expertise and providing trainings for the manufacture of the samples and for the analytical techniques. VI. Bibliography 1. Swain S. K., Isayev A. I. Effect of ultrasound on HDPE/clay nanocomposites: Rheology, structure and properties. Polymer 48 (2007) Carrera M. C., Erdmann E., and Destéfanis A. H. Barrier Properties and Structural Study of Nanocomposite of HDPE/Montmorillonite Modified with Polyvinylalcohol. Journal of Chemistry 2013 (2013), Article ID Monsiváis-Barrón A. J., Bonilla-Rios J., Ramos de Valle L. F., Palacios E. Oxygen permeation properties of HDPE-layered silicate nanocomposites. Polymer Bulletin 70 (2013) Osman A. M., Rupp J. E. P. and Suter U. W. Gas permeation properties of polyethylene-layered silicate nanocomposites. Journal of Materials Chemistry 15 (2005) Mirabella F. M., Bafna A.. Determination of the Crystallinity of Polyethylene/α-Olefin Copolymers by Thermal Analysis: Relationship of the Heat of Fusion of 100% Polyethylene Crystal and the Density. Journal of Polymer Science: Part B: Polymer Physics 40 (2002) Wang M. -W., Hsu T. -C., Zheng J. -R. Sintering Process and Mechanical Property of MWCNTs/HDPE Bulk Composite. Polymer-Plastics Technology and Engineering 48 (2009) Kalaitzidou K., Fukushima H., Askeland P., Drzal L. T. The nucleating effect of exfoliated graphite nanoplatelets and their influence on the crystal structure and electrical conductivity of polypropylene nanocomposites. Journal of Materials Science 43 (2008) Bui K., Duong H. M., Striolo A., Papavassiliou D. V. Effective Heat Transfer Properties of Graphene Sheet Nanocomposites and Comparison to Carbon Nanotube Nanocomposites. Journal of Physical Chemistry C 115 (2011) Page 17

18 9. Jiang X., Drzal L. T. Reduction in Percolation Threshold of Injection Molded High-Density Polyethylene/Exfoliated Graphene Nanoplatelets Composites by Solid State Ball Milling and Solid State Shear Pulverization. Journal of Applied Polymer Science 124 (2012) Kalaitzidou K., Fukushima H., Drzal L. T. A new compounding method for exfoliated graphitepolypropylene nanocomposites with enhanced flexural properties and lower percolation threshold. Composites Science and Technology 67 (2007) Kalaitzidou K., Fukushima H., Drzal L. T. A Route for Polymer Nanocomposites with Engineered Electrical Conductivity and Percolation Threshold. Materials 3 (2010) Kalaitzidou K., Fukushima H., Drzal L. T. Multifunctional polypropylene composites produced by incorporation of exfoliated graphite nanoplatelets. Carbon 45 (2007). 13. Jiang X., Drzal L. T. Improving electrical conductivity and mechanical properties of high density polyethylene through incorporation of paraffin wax coated exfoliated graphene nanoplatelets and multi-wall carbon nano-tubes. Composites: Part A 42 (2011) Page 18