Materials Processes of Graphite Nanostructured Composites Using ...

Materials and Manufacturing Processes, 20: 159–166, 2006 Copyright © Taylor & Francis Group, LLC ISSN: 1042-6914 print/1532-2475 online DOI: 10.1081/AMP-200068659

Materials Processes of Graphite Nanostructured Composites Using Ball Milling Shing-Chung Wong1 , Eric M. Sutherland2 , and Fawn M. Uhl3 2

1 Department of Mechanical Engineering, University of Akron, Akron, Ohio, USA Department of Mechanical Engineering, North Dakota State University, Fargo, North Dakota, USA 3 Owens Corning, Science & Technology Center, Granville, Ohio, USA

Polymer nanocomposites are an emerging class of multifunctional materials that have not been optimized for their functional potential. As a part of a series of studies conducted by Wong and collaborators [1–6], a novel processing design for polymer nanocomposites using ball milling and nanoscale graphite platelets (NGPs) was recently investigated. Processes for carbon nanotube (CNT), despite its promise in maneuvering varied functionality, are prohibitively expensive at present for widespread composite applications. Instead of trying to discover lower-cost processes for CNT, we investigated the possibility of using ball milling to produce nanoscale reinforcements. The studied nanocomposites are distinct from existing sp3 carbon black, CNT, and nanoclay reinforcements. Materials processes to produce nanostructured polymer composites using the ball milling method are reported. It was found that ball milling primarily reduced particle sizes to smaller platelets. No other critical advantages were noted. A bimodal distribution of 100 and 400 nm particles was observed using a particle analyzer. The ultimate objective is to develop an alternative cost-effective nanoscale carbon material with comparable properties like carbon nanotubes for future composite applications. Keywords Ball milling; Coatings; Graphite; Nanocomposites; Processes.

of nanostructured components, functional coatings, and substrates for microelectronic devices and structural components. However, attempts to produce CNT in larger quantities to facilitate load-bearing composite applications have been fraught with challenges due to poor yield and costly fabrication and purifying processes. Instead of trying to discover lower cost processes for nanotubes, we seek to develop an alternative nanoscale carbon material with comparable properties that can be produced cost-effectively and in larger quantities. This new class of nano material is herein referred to as Nanoscale Graphite Platelet (NGP). This NGP material is distinctly different from the existing carbon black, CNT, and smectite clay reinforcements in terms of its unique processing techniques for nano-reinforcement, cost-effectiveness, and functionality. An NGP is a nanoscale platelet composed of one or multiple layers of sp2 graphene plane (basal plane). In a graphene plane, carbon atoms occupy a 2-D hexagonal lattice. These carbon atoms are bonded together through strong covalent bonds lying on this plane. In the c-axis direction, several graphene planes may be weakly bonded together through van der Waals forces. An NGP may be viewed as a flattened version of a carbon nanotube (CNT). Although NGP and CNT are geometrically different in architecture, some preliminary studies by Wong and Yerramaddu [6] indicated comparable mechanical properties (in-plane stiffness and strength). Recent studies on cost-effective polymer nanocomposites have focused on the development of smectite clay systems. Enhancement in strength and stiffness for nanoscale reinforcement is well documented in the literature [7, 8]. Mechanical properties can be improved with proper surface functionalization to promote interaction between the filler and polymer molecules. Barrier properties in the

Introduction In a series of studies, Wong and coworkers [1–3] investigated the opportunities and advantages of graphitebased nanocomposites in the last few years. The efforts included using expanded graphite (EG) for dispersion in polymer matrices. Both solution blending and mass compounding routes were attempted [1, 3]. It was found that the filler form factor and percolation threshold for electrical conductivity critically depended on the acid treatment prior to rapid thermal expansion. As a result of the earlier findings, we extended our efforts to examine the advantages of ball milling as a secondary step to thermal expansion of EG in the hope to produce nanoscale graphite platelets (NGPs) and their subsequent nanocomposite coatings [4, 5]. Some interesting properties were observed. In this paper we seek to critically examine the use of ball milling in the fabrication of NGP and the formation of polymer nanocomposites. Polymer nanocomposites possess unique features and functions unavailable in conventional fiber-reinforced and unreinforced polymers. The keys to nanocomposite technology lie in the nanoscale phenomena, which changed the engineering design mindset for materials development. Nanocomposites promote synergisms in structural integrity and functionality, versatility, and cost-effective fabrication. One revolutionary filler development in recent years is the carbon nanotube (CNT), which is expected to play a significant role in the design and manufacture Received February 24, 2005; Accepted March 31, 2005 Address correspondence to Shing-Chung Wong, Department of Mechanical Engineering, University of Akron, Akron, OH 44325-3903, USA; E-mail: [email protected]

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160 nanocomposites derived from layered silicates, intercalated or exfoliated likewise, were investigated in recent years [9–12]. However, nanoclay-reinforced polymers do not possess as good electrical conductivity and dielectric properties as functional composites such as sp3 carbon black- [13, 14], metallic powder- [15–17], polyaniline- [18], and graphite- [19] containing polymers. A nanocomposite that contains a minimal filler concentration for reduced costs and weight is lacking. Since there are no reactive ion groups on the graphite layers, it is difficult to prepare the polymer/graphite nanocomposites via ion exchange reaction in order to intercalate the monomers into the graphite sublayers. The expanded graphite, however, contains abundant multipores ranging from 2 nm to 10 µm. Average size of the pores is about 2 µm. In graphite-based nanocomposites, the monomer was first introduced into the pores of the expanded graphite to be followed by polymerization. The graphite maintains the layered structure similar to natural flake graphite but with larger layer spacing [20–23]. It was reported that markedly lower volume fractions of expanded graphite was able to reach the percolation threshold for electrical conductivity in nylon 6, PS, and PMMA nanocomposites by in situ polymerization of polymer matrices [1–3, 24, 25]. This paper reports the materials and processing issues pertaining to nanostructured polymer composites using the ball milling method. The ultimate goal is develop a paradigm to establish cost-effective substitutes for highly functional fillers such as CNT for composite applications. Materials processes and experimental work The NGP material is characterized by a nanoscale architecture that comprises one or several layers of twodimensional fused aromatic rings (graphite-like planar molecules or sp2 graphene sheets). It has been demonstrated that NGP can be readily produced by the following procedures: (1) exfoliating and expanding the carbon- or graphite structures using acid treatments, and (2) mechanical ball milling of the exfoliated structures into nanoscale powders. The intercalant is a solution of sulphuric acid or sulphuric acid and phosphoric acid, and an oxidizing agent. The heat treatment temperature and time can be varied to generate, by design, various NGP materials with a wide range of graphene platelet thickness, width, and length values. Ball milling is a mass production process, which allows NGPs to be produced in large quantities. In this paper, we focus on the effects of ball milling on platelet sizes and interlayer distance. Graphite Expansion Graphite expansion refers to the process of allowing the acid, the intercalant, to diffuse into the graphite interlayers, followed by rapid expansion using oven or microwave heating. Expandable graphite (Grafguard® Expandable Graphite Flake) was supplied by Graftech International in the form of acid-treated materials, washed to a specified pH, and dried. The manufacturer’s information indicates that expansion begins at a temperature of 160 C. The size of the flakes is less then 177 µm, and has a neutral surface pH.

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Expansion was done by placing one gram of expandable graphite into a clean crucible. A Thermolyne 48000 Furnace was heated to the desired testing temperature. Then the crucible containing the expandable graphite was placed into the oven for a controlled period of time, between 30 s to 5 min. The time and temperature varied depending on the test being conducted. The temperature inside the oven was kept constant with care. Expansion using microwave heating was also experimented using a commercially available microwave (Panasonic 1350W). The graphite was rapidly expanded inside the microwave between 5–30 s. Caution was exercised to prevent specimens catching fire inside the microwave due to the acid content. Four different sets of specimens were prepared with the processing conditions being described in Table 1. Natural Flake Graphite For a different set of samples the graphite was obtained from Sigma-Aldrich. It arrived in the form of natural flake graphite. Two concentrated acids, 98% concentrated sulfuric acid and 99.5% concentrated nitric acid, were used for graphite expansion of natural flake graphite. The acids were mixed in a 4:1 volume ratio. Nitric acid serves as an oxidizer and sulfuric acid is an intercalant. A magnetic stirrer was used to enhance mixing in a flask. The flask was placed in a container of ice to cool the sample during exothermic reaction. Ball Milling This paper addresses the materials processes for preparing graphite nanostructured polymer composites via balling milling. Balling milling had been used to provide nanotubes by Chen et al. [26, 27] prior to this study. Carbon nanotubes were synthesized by milling 98% pure hexagonal graphite powder and annealing the resulting fine powder [26].

Table 1.—Four treatment conditions in the composite preparation. Graphite samples

A

B

C

D

Treatment

The graphite was obtained from Graftech, under the name GRAFGUARD Expandable Graphite Flake, Grade 160-80N. The graphite was acid treated in advance with sulfuric and nitric acids prior to delivery. It was expanded using an oven at 1000 C in our laboratory. The graphite flakes were separated into individual flakes from the expanded “worm” (Fig. 3b) using a sonication bath. After the individual flakes were dried, a second acid treatment was performed using sulfuric and nitric acids. The graphite was then expanded for a second time using a microwave. The graphite flakes looked like fine powder. The graphite was obtained from Sigma-Aldrich. It arrived in the form of natural flake graphite. The graphite was ball milled using a Fritsch Pulverisette 6 planetary ball mill. It was treated using sulfuric and nitric acids in a 4:1 ratio. Microwave expansion was then employed. The graphite was delivered as in A. It was expanded using an oven at 1000 C in our laboratory. The expanded graphite “worms” were placed in a Fritsch Pulverisette 6 planetary ball mill where they were milled to form fine powder. The graphite was obtained from Sigma-Aldrich. It arrived in the form of natural flake graphite. The graphite was ball milled using a Fritsch Pulverisette 6 planetary ball mill.

MATERIALS PROCESSES OF NANOSTRUCTURED COMPOSITES

The powder was milled dry in inert argon to minimize chemical reactions. Following ball milling, the graphite powder was placed in an oven for annealing. Minute nanotube growth was observed near the bowl. This process was described as a solid-state crystal growth, in which the crystal (nanotube) was grown from the amorphous and turbstratic phases of the carbon powder. They were able to produce nanoporous carbon structures by two different milling processes: 1) vertical planetary ball mill and 2) vibrating frame grinder [27]. However, attempting to produce CNT using such processes is time-intensive and results in low yield. Mechanical attrition using the ball milling process in this study was conducted to simply reduce the size of the highly expanded graphite flakes. A planetary ball mill (Fritsh Planetary Mono Mill Pulverisette 6) was used to provide milling for small sample runs while reducing the size of the particles to submicron and nanoscale. Figure 1 shows the planetary mono mill used in this study. To facilitate the smallest particle size, a tungsten carbide grinding bowl, lid, and balls were used. Tungsten carbide also offered a high resistance to abrasion, which would keep the contamination due to wear to a minimum. A 250 mL tungsten carbide bowl with matching lid was therefore chosen. Fifty 10 mm tungsten carbide grinding balls were used. All fifty grinding balls were placed into the clean grinding bowl. The sample material was placed on top of the ball. The grinding bowl was filled to at least one-third of its volume to reduce the wear of the grinding balls and bowl, which would lead to contamination of the sample being ground. To ensure that the sample was effectively ground the bowl was not filled past three quarters of the volume.

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The closed grinding bowl was placed into the planetary ball mill and secured using the “safe lock.” The counter weight was adjusted based on the weight of the bowl and sample to reduce the vibration of the milling operation. The time per milling cycle was restricted by the heat that was generated by the grinding process. Therefore, the milling time was limited to 2 h or less depending on the test being conducted. The cooling time was 1.5 h between each milling cycle. One repetition is a milling cycle plus the cooling time. The number of repetitions was set to achieve the desired milling time. Scanning Electron Microscopy (SEM) Scanning electron microscopy was used to examine the processing temperature and expansion time. Expanded samples were placed on aluminum sample studs using double-sided carbon tape. To avoid electron charging and ensure quality imaging, SEM samples were sputter coated with gold prior to observation. The edge of a flake was of special interest, so a 40,000X magnification image was captured. With the digital images imported into AutoCAD, the flake surface dimension and flake thickness were measured using the image analysis function. The scale bar on each image was measured and a ratio was created to determine what the actual length of a line across a desired section of the image was. Multiple flake sections were measured to determine an average flake dimension. This method was used to measure the interdistance between parallel flakes. Particle Size Measurements A Nicomp model 380 particle sizer was used to measure the sizes of the milled samples. Distilled water and methyl alcohol solution were mixed at a 50:50 ratio followed by dispersion of milled powder. To prevent the particles from agglomeration prior to measurements, the graphite solution was sonicated. The solution was then diluted with more water-alcohol solution so that the solution had only a slight gray tint, but yet was transparent. The diluted graphite solution in a test tube was placed into the particle sizer sample container for measurements. Coatings Preparation Polymer coating formulations utilized in this study were composed of epoxy 6110 with an epoxide equivalent weight of 131–140; Tone polyol 301, which has a hydroxyl equivalent weight of 100, and photoinitiator, UVI-6974. These were mixed to obtain an R value of 5; R value calculation is shown below. R=

Figure 1.—The Fritsh Planetary Mono Mill Pulverisette 6 used in this study.

g epoxides/epoxide equivalent weight g polyol/hydroxyl equivalent weight

(0.1)

After the graphite was processed and dispersed into the UV curable epoxy, it was transferred to a glass slide, using a plastic pipette. To the formulation was added 0.5%, 1%, 2%, or 3%, wt. graphite. Formulations were then stirred, followed by 8 h of sonication. Films were cast onto glass and aluminum panels using a #1 Gardco casting bar with

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a gap of 4 mil. The glass coated with the film was placed under the UV source for 4 min for curing. The samples were allowed to sit for 24 h to ensure that the epoxy was completely cured. Using a razor blade, the coating was removed from the glass slide in one piece. Distilled water was used to lubricate the razor blade, reducing the amount of force needed to remove the film, which could lead to deformation of the film’s thickness. The samples were then allowed to dry before mechanical testing. Mechanical Testing and Glass Transition Mechanical testing on the coatings was attempted using an Instron 5542. A minimum of three specimens removed from glass slides for each coating composition were tested. Glass transition temperature was measured using a parallel DMTA from Symyx Technologies Inc. Results and discussion Microstructures Figure 2 presents a schematic that summarizes the processing of graphite intercalated compounds (GICs) and their polymer nanocomposites. The schematic illustrates the objectives in our processing. Expanded GICs are further ground to smaller platelets by ball milling for effective dispersion in polymers. Figure 3 juxtaposes the SEM photomicrographs of the a) unexpanded and b) expanded graphite flakes. Figure 3b is the wormlike expanded structures commonly observed in expanded GICs. It is clear the graphitic domains appear in stacks of many sublayers. Figure 4 focuses the electron beam on the nanoscale feature of the plate thickness of graphite platelet following acid treatment methods. The structure of expanded graphite was strongly affected by experimental conditions such as temperature, oxidizer’s concentration, and intercalating time. Evidently, the graphitic domains have been drastically reduced in thickness, from several micrometers to 100 nanometers or less. Temperature Effect on Graphite Size To produce NGP for easy dispersion in polymers, we surmise that the planar size of the platelets plays an important role. We characterized the size of platelets using a particle analyzer. We hereby emphasize the observed

Figure 3.—SEM photomicrographs of the a) unexpanded and b) expanded graphite flakes.

trends in the materials studied. Figure 5 shows the effect of temperature on particle size at a given time. Notably, high expansion temperature leads to smaller platelet sizes. However, when the platelet size was measured as a function of expansion time at a constant temperature (1000 C), it is shown that the size increases with expansion time. The size increase reaches a steady-state value shortly over

Figure 2.—Schematic describing the expansion process of graphite intercalation compounds (GICs).


Figure 4.—The electron beam of an SEM is focused on the nanoscale feature of the plate edge, showing thickness of graphite platelet less than 100 nm.

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derived from the network structures of the fillers whereby the resistivity increases and decreases as the percolation distance for electrons between the platelets increases and decreases, respectively. The understanding of piezoresistivity for NGP polymer nanocomposites is an important aspect of nanocomposite development and awaits more investigations. Figure 6 plots the interlayer distance between the graphite flakes vs. the expansion oven time and temperature. The interlayer distance shows a gentle decrease with expansion time and temperature. The results indicate that for sensitive piezoresistive behavior it is desired to characterize the interlayer distance, which is likely to be indicative of the filler form factor and the percolation network structure, as functions of expansion time and temperature. It is believed the monitoring of the interlayer distance can contribute to a better control of the percolation threshold for electrical conductivity, and piezoresistive and dielectric properties of the polymer-matrix composites. It is envisioned the NGP polymer nanocomposites will emerge

several minutes. This is attributed to the annealing effect that promotes the aggregation and graphitization for the platelets due to prolonged temperature treatment. Our results therefore suggest that temperature effect is evident and an optimal temperature controlling graphite sizes in expansion should be characterized prior to dispersion in polymer matrices. Interlayer Distance The NGP polymer nanocomposites can be studied and applied as piezoresistive (pressure-dependent resistivity) functional materials. The transport mechanisms of graphite polymer nanocomposites were discussed elsewhere [2]. In the percolation study for electrical conductivity of expanded graphite PMMA, we reasoned that the filler form factor [1–3] was a dominant factor in the transport mechanisms. Likewise, piezoresistive mechanisms are

Figure 5.—Effect of temperature on graphite flake domain size due to thermal expansion for 60 s.

Figure 6.—Interlayer distance between the graphite flakes vs. the oven expansion a) time and b) temperature.

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as functional materials for microelectronics and microelectro-mechanical systems. Particle Size and TEM In the particle analysis of the ball-milled expanded graphite, the size ranges from 50–100 nm. Figure 7 shows the NGP being embedded in polymer medium. It can be seen that the NGP aggregates to form larger domains of multiple layers or clumps, typical of carbon black TEM images. The dimensions of both in SEM and TEM studies are consistent with particle analysis results shown in Fig. 8. The particle analysis of milled natural graphite shows a bimodal distribution of size domains. The majority of the material in the studied sample shows an average diameter of 400 nm. What is interesting is the smaller concentration of particles presents an average size about 100 nm. When observing the TEM image of NGP embedded in polymers, it is clear that the smaller 100-nm particles are individual particles that form aggregates in larger structures, accounting for the larger diameter particle size. If this is the case, it is logical to assume that smaller diameter samples would have a smaller distribution, which is apparent in the particle analysis results. In view of the irregular shape of aggregated particles, the distribution of the 400-nm particles would tend to be wider, the result of which is consistent with Fig. 8. Based on these findings, the advantages of ball milling appear to be only size reduction of readily expanded graphite platelets. Further advantages remain unclear. When the platelet dimensions of NGP diminish, it also introduces problems of agglomeration, and as a result, the dispersion of NGP in polymer matrices would become less effective. Ball milling does not give rise to further reduced thickness of the platelet edge as noted in the consistency of the observed 100-nm range of thickness in Figs. 4 and 8.

Figure 7.—TEM photomicrograph of NGP being embedded in a polymer medium.

Figure 8.—Bimodal distributions of NGP particle sizes in particle analyzer.

Mechanical Properties and Glass Transition Mechanical properties of expanded graphite nanocomposite films are critically influenced by the degree of dispersion and aspect ratio of the nanofillers in the polymers. Figure 9 shows the elastic modulus, E, vs. the graphite content in nanocomposite films prepared using the coatings methods. The glass transition temperature, Tg , of the nanocomposite samples is plotted in Fig. 10. From Fig. 9, Samples A and B provide a stiffening effect at higher graphite content while Samples C and D both show decreasing trends. Note that both Samples A and B were microwave expanded, whereas Samples C and D were simply expanded and ball milled. The results strongly suggest microwave expansion provides more potent expansion and benefit in enhancing the reinforcement effect arising from NGP in an epoxy matrix. The outcome could be attributed to the extra heating on the surface of the graphite platelet due to the microwave expansion. This leads to a stronger reinforcement-matrix interface for stress transfer. The minimal reinforcement outcome is attributed to poor

Figure 9.—Elastic modulus of NGP composite coating vs. graphite content.

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nanocomposites were more ideal for future development of functional applications with good thermal stability and dielectric properties [5]. It is noted that the processing parameters at present have not yet been optimized. Acknowledgments This project was supported by the National Science Foundation under SGER Grant # CMS 0335390. EMS acknowledges a graduate assistantship funded by NSF. Supply of the expandable graphite by Graftech was greatly appreciated. Some assistance with preparing the manuscript provided by Ms. Shiyue Qu was greatly appreciated. References

Figure 10.—Glass transition temperature, Tg , vs. graphite content.

dispersion of the graphite fillers and reduced filler form factor due to the milling influence. Figure 10 indicates the inclusion of graphite does not alter the Tg of the matrix polymers. Such results point us to the conclusion that the highly expanded graphite nanocomposite films are suitable for non-load bearing but temperature sensitive applications. This is particularly relevant for the composite polymers to be used in a microelectronic packaging environment such as flexible substrates. Conclusions In this paper, we broadly reported the processes for nanoscale graphite plates (NGPs) using the ball milling approach. It has been demonstrated that the ball milling method was only effective in reducing the sizes of the highly expanded graphite intercalated compounds (GICs). Both particle analysis and microscopic techniques indicated a bimodal distribution of particle sizes of 100 nm and 400 nm. Our data did not suggest ball milling could further reduce the thickness dimension of the readily expanded NGP. The bimodal observation was clearly caused by the individual NGP in the range of 100 nm in thickness and particle dimensions and their aggregates, which provided a broader distribution of 400-nm-wide particles. In both cases, it is assumed that the NGP has been highly expanded with one dimension in the range of 100 nm in thickness as shown in Fig. 4. For functional properties, it is suggested that the interlayer distance be monitored as functions of expansion time and temperature. This would allow the design of NGP polymer nanocomposites for piezoeresistive and dielectric applications, both of which critically depend on the filler form factor and the NGP network structures being reflected from the interlayer distance. Microwave expansion also provided notable advantages in enhancing the reinforcement effectiveness of the NGP-containing polymer films. The increase in stiffness was attributed to the microwave heating on the NGP surface, which enhances the reinforcementmatrix interface for stress transfer. Overall, the increase in mechanical properties of the ball-milled NGP composites was unimpressive. The results suggest graphite polymer

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