Graphite Nanoplatelets as Multifunctional Reinforcements for Polymer Composites

Hiroyuki Fukushima and Lawrence T. Drzal Department of Chemical Engineering and Materials Science

Composite Materials and Structures Center Michigan State University

East Lansing, MI 48824-1226 drzal@egr.msu.edu

ABSTRACT

Natural crystalline graphite based graphite intercalated compounds [GICs] were exfoliated and milled into sub-micron graphite flakes. SEM and TEM images showed that the average size of graphite became 0.86 um and the thickness became around 5 to 10nm. The BET measurement showed the surface area of this material was over 100m2/g. The cost of this new nano-size graphite material was estimated around $5/lb or less. XPS measurement suggested that this new graphite nanoflakes were made of highly crystalline graphene sheets. Since its superior mechanical, electrical, thermal properties and cost effectiveness, this material was proved to be superior reinforcements in nanocomposite systems. In both thermoset and thermoplastic polymer composite systems, this material showed superior reinforcing effect and also good electrical properties. These nanocomposite systems are expected to be used in many applications such as automobile and electrical/electronics industry. Also this material could be used in other applications such as hydrogen fuel storage and anode material for butteries. 1. INTRODUCTION

Nanocomposites composed of polymer matrices with reinforcements of less than 100 nm in size, are being considered for many applications such as interior and exterior parts for automobiles, structural components for electronic devices, and films for food packaging. [1,2] Since 1987 when a research group in Toyota introduced the concept for the first time, many researchers have been focused on nanocomposites with a variety of polymer systems.

While most nanocomposite researches have focused on exfoliated clay platelets as reinforcements, the same nanoreinforcement concept can be applied to another layered material, graphite, to produce nanoplatelets and nanocomposites. Graphite is the stiffest material found in nature (Young’s Modulus = 1060 MPa), having a modulus several times that of clay, but also with excellent strength, electrical and thermal conductivity. The key to utilizing graphite as a platelet nanoreinforcement is in the ability to exfoliate Graphite Intercalated Compounds [GICs]. [3] Since late 1990’s, some research groups

reported graphite nanocomposite systems using intercalated, expanded, or exfoliated graphite flakes in polymers such as nylon6, polystyrene, polypropylene, and polyisoprene. [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]

Since the cost of GICs based on natural crystalline graphite is quite cheap, around $1.5-1.6/lb, the cost of exfoliated graphite nanoplatelets is expected to be $5/lb or less. This is significantly less expensive than carbon nanotubes (~$100/g) or VGCF ($40-50/lb), yet the properties of crystalline graphite flakes are somewhat comparable to those of NT and VGCF. If the appropriate surface treatment can be found for graphite nanoflakes, its exfoliation and dispersion in a polymer matrix will result in a composite with not only excellent mechanical properties but electrical properties as well, opening up many new structural applications as well as non-structural ones where electromagnetic shielding and high thermal conductivity are requirements.

In this research, a special thermal treatment was applied to the graphite flakes to produce exfoliated graphite reinforcements. The graphite nanocomposite materials were fabricated by combining the exfoliated graphite flakes with an nylon 66 or an amine-epoxy resin. X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), and BET surface area measurement were used to assess the degree of exfoliation of the graphite platelets. The mechanical properties of these composites were investigated by flexural testing. The electrical conductivity was investigated by impedance measurements using the 2-probe method. 2. Experimental Materials

The graphite nanoflakes were fabricated from graphite-intercalated compounds [GICs] offered by UCAR Carbon Co. The grade of the graphite was GrafGuard 160-50A, which contains about 20 wt% of acid mixtures (sulfuric and nitric acid) intercalated into the galleries of graphite layers. Upon rapid heating, these acid contents were vaporized and forced graphite layers apart, forming worm-like rods. By applying ultrasonic process, these expanded graphite rods were pulverized into flakes. The average size of these flakes was around 15 um at this point. Figure 1 shows the SEM images of as-received, expanded, and pulverized graphite materials. The average size of the graphite flakes became less than 1 um after milling. The BET measurements gave the surface area data of the pulverized and milled graphite as 105 m2/g and 94 m2/g, respectively. The thickness of these flakes was calculated as around 10 nm based on the surface area data, which is well agreed to the TEM images of milled graphite flakes in Figure 2. Three commercially available carbon materials were used as comparison, which were PAN based carbon fiber (PANEX 33 MC Milled Carbon Fibers, average length: 175 um, average diameter: 7.2 um, specific gravity: 1.81 g/cm3, Zoltek Co.), VGCF (Pyrograf III, PR-19 PS grade, Length: 50~100um, Average diameter: 150nm, Specific gravity: 2.0 g/cm3, Pyrograf Products, Inc.), and nanosize carbon black (KETJENBLACK EC-600 JD, Average diameter: 10-30 nm, Specific gravity: 1.8 g/cm3, Akzo Novel Polymer Chemicals LLC). The detailed characterization of the exfoliated graphite materials and other carbon materials are summarized in Table 1.

Table 1. Specifications of Carbon/Graphite Materials

Length/ Diameter

Diameter/ Thickness

Aspect Ratio

Density (g/cm3)

Surface Area

(m2/g) As-received

Graphite 300 um 5.2 um 58 2.0 0.2

Pulverized Graphite 15 um 9.5 nm 1579 2.0 105 Milled Graphite 0.86 um 10.9 nm 79 2.0 94 Carbon Fiber 175 um 7.2 um 24 1.81 16

VGCF 50-100 um 160 nm 312-625 2.0 25 Carbon Black 10-30 nm 10-30 nm ~ 1 1.8 1400

Acid Intercalated Graphite (Scale Bar=300 um)

Exfoliated Graphite(Scale Bar=100 um)

Expanded Graphite (Scale Bar=500 um)

Figure 1. SEM Images of As-received, Expanded, and Exfoliated Graphite

Figure 2. TEM Images of Exfoliated and Milled Graphite Flakes (First two images were offered by Professor Rodney S. Ruoff,

Department of Mechanical Engineering, Northwestern University)

(Scale Bar=50 nm) (Scale Bar=5 nm)(Scale Bar=2 nm)

Composite fabrication Nylon 66 [Zytel 101 NC 010, DuPont] and epoxy [Diglycidyl ether of bisphenol A

(Epon 828), Shell Chemical Co. and Jeffamine T403 , Huntsman Petrochemical] were used as matrices for the graphite nanocomposites. Nylon 66 composites were fabricated by mixing the matrix with reinforcements in DSM mini-extruder and injection molder. The process temperature was 290°C. Epoxy samples were made by adding reinforcements to DGEBA and mixed with the aid of an ultrasonic homogenizer for 5 minutes. Then stoichiometric amount of Jeffamine T403 were added and mixed at room temperature. The ratio of DGEBA/Jeffamine was 100/45 by weight. The system was outgassed to reduce the voids and cured at 85ºC for 2 hours, followed by post curing at 150ºC for 2 hours. The density of graphite flakes was assumed as 2.0 g/cm3. The densities of other carbon materials were obtained from manufactures. Th4e density of nylon 66 was 1.14 g/cm3. The density of the epoxy matrix was measured as 1.159 g/cm3. Using these values, the volume fraction of graphite platelets in composite samples was calculated.

3. RESULTS AND DISCUSSION Nylon 66 Composite

Composites reinforced with up to 20 vol% of exfoliated nanographite or carbon fiber did not show any difficulty in the injection molding process. Composite with 15 vol% VGCF exhibited an increased viscosity, but the composition was still moldable. Composite with 10 vol% carbon black showed considerable increase in viscosity and difficulty to make an injection-molded sample. Composite with 15 vol% carbon black could not be fabricated.

Figure 3 (A) shows the results of the flexural modulus of the composite samples. The exfoliated and milled nanographite showed the best improvement and the effect was considerably better than the commercially available carbon materials. The composite with 15 vol% of exfoliated graphite produced a modulus of about 8.4 GPa, which was almost 300% of that of the control nylon 66. Carbon fiber showed the improvement effect of about 2/3 of the nanographite. VGCF showed less than half of the improvement than that of the nanographite did. The improvement of modulus by carbon black was significantly lower than the others. These results suggest that the exfoliated graphite has much higher modulus than other carbon materials, indicating that the nanographite platelets are made of highly crystalline graphene sheets.

Figure 3 (B) showed the results of the flexural strength of the composite samples. Up to 5 vol% loading, composites reinforced with CF, VGCF, and the exfoliated graphite showed almost the same improvement. But CF and VGCF showed better improvement than the exfoliated graphite at higher loading level. These results suggest that the surface condition of the exfoliated graphite has not been optimized for Nylon66 system. Also the process conditions for better dispersion in the melt need to be optimized. Carbon black showed the lowest strength.

Figure 4 showed the results of the resistivity of the nylon 66 composite samples. All samples were made from cutting the middle section of flexural bar samples. The resistivity measurement was done in the injection direction. In this case, composites with carbon black and specially treated exfoliated nanographite showed superior results. Both showed almost the same percolation threshold, around 2 vol%. Pulverized graphite percolated around 7 vol% in nylon 66, but other reinforcements showed percolation threshold of more than 10 vol%.

Flexural Modulus

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Milled Gr.Pulverized Gr.CFVGCFCB

(A) Flexural Modulus

Flexural Strength

020406080

100120140160180200

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ngth

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(B) Flexural Strength

Resistivity

1.00E+001.00E+011.00E+021.00E+031.00E+041.00E+051.00E+061.00E+071.00E+081.00E+091.00E+101.00E+111.00E+121.00E+13

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[ohm

*cm

]

Exfoliated Gr.Milled.Gr.Pulverized Gr.CFVGCFCB

Figure 4. Resistivity of Nylon 66 Composites

Figure 3. Flexural Properties of Nylon 66 Composites

Epoxy Composite Graphite nanoplatelets treated by O2 plasma, amine grafting, and acrylamide

grafting were prepared and used as reinforcements to fabricate epoxy composites with 1.0, 2.0 and 3.0 vol% of graphite flakes. The flexural strength of each sample are summarized in Figure 5. The results indicate that the acrylamide grafting was the most effective surface treatment in terms of both strength and modulus enhancements. This is supported by XPS data that showed largest N/C ratio for acrylamide grafting. These data suggest that the amine groups grafted on graphite nanoplatelets improve the compatibility between the graphite nanoplatelets and the matrix and form a bond with the epoxy matrix and improve mechanical properties

Epoxy composites reinforced with PAN based carbon fibers, VGCFs, and nanosize carbon blacks were also fabricated. The flexural properties of these composites were measured and compared with those of composites with acrylamide-grafted nanographite. The results are shown in Figure 6. Here acrylamide-grafted nanographite showed the best results in strength enhancement. This implies that the acrylamide grafting treatment is a very effective surface treatment for graphite nanoplatelets.

Effect of Surface Treatments on Modulus

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Effect of Surface Treatments on trength

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(A) Flexural Modulus (B) Flexural Strength Figure 5. Effect of Surface Treatment on Flexural Properties of Epoxy Composites

Flexural Modulus

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(A) Flexural Modulus (B) Flexural Strength Figure 6. Flexural Properties of Epoxy Composites

The electrical resistivity of the epoxy composites with various reinforcement contents were determined. The size of each composite sample was about 30 x 12 x 8 mm. Each sample was polished and gold was deposited on the surface to insure good electrical contacts. The results are summarized in Figure 7. The VGCF, carbon black and graphite microplate percolated at less than 3 wt% (2 vol%) while conventional carbon fiber and graphite nanoplatelet showed percolation threshold of about 9 wt% (6 vol%). Among the former three reinforcements, graphite microplatels and carbon blacks produced composites with the lowest resistivity, which reached around 10-1.5 ohm*cm. Thus, the exfoliated graphite sample also showed excellent electrical property as reinforcement in polymer matrix. 4. CONCLUSIONS

A newly developed graphite nanoflakes were used as reinforcements in epoxy matrix. The acrylamide grafting was found to be very effective to enhance the interaction with epoxy matrix. The acrylamide treated graphite nanoflake reinforced composites showed better flexural and tensile properties than those reinforced with commercially available carbon materials. The electrical conductivity of composites was also investigated. The results revealed the exfoliated graphite with high aspect ratio was an excellent conductive filler, which showed comparable or better percolation threshold and conductivity than conventional fillers. From these results and predicted low cost ($ 5/lb), this new type of graphite material has realistic possibility to replace conventional carbon materials or newly developed carbon nanotubes. The possible application fields are broad, including automobile, electrical/electronics, and aerospace.

Resistivity of Epoxy Composites

1.00E+001.00E+011.00E+021.00E+031.00E+041.00E+051.00E+061.00E+071.00E+081.00E+091.00E+101.00E+111.00E+121.00E+13

0 5 10 15 20 25 30 35Reinforcement Content [Wt%]

[ohm

*cm

]

Milled GraphitePulverized Graphite CFVGCFCarbon Black

Figure 7. Resistivity of Epoxy Composites

5. ACKNOWLEDGEMENT

The authors would like to thank UCAR Co. for providing the acid intercalated graphite compounds and DuPont Co. for providing nylon 66 sample.We also thank Professor Thomas J. Pinnavaia, Department of Chemistry, Michigan State University for providing BET surface area data and Professor Rodney S. Ruoff, Department of Mechanical Engineering, Northwestern University for providing TEM images of the nano-size graphite flakes.

Partial support for this research was provided by a grant from NASA LaRC, Graphite Nanoreinforcements for Aerospace Nanocomposites NAG1-01004, Thomas Gates, Program Director.

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