Expanded Graphite Filler Research Articles

The effect of expanded graphite and paraffin wax on the morphological, thermal, and rheological properties of PP/EVA blend

The study investigated the effect of paraffin wax, expanded graphite, and their dispersion on the properties of the blend of polypropylene (PP) and ethylene vinyl acetate (EVA). The idea is to reduce interfacial tension in blends of PP and EVA while enhancing the properties of blends in the process. The chosen optimum contents of the paraffin wax and EG in the 80/20 PP/EVA blend were 10 and 6 wt.%, respectively. The morphological features show that the expanded graphite (EG) particles had high affinity for each other. Moreover, EG filler particles are localized inside the minor EVA phase and some at the interface. Rheological properties revealed that the flow of the binary composites increased when 10 wt.% wax is added suggesting that wax enhanced the processability of the composites. The Cole-Cole plots deviated from the semi-circular arc suggesting heterogeneity in the blend and PP/EVA/Wax/EG composite. The addition of paraffin wax and EVA delayed crystallization of PP, while earlier crystallization of PP was promoted by the presence of EG. Generally, all samples caught fire after 10 s, started dripping and are thus given a V-2 rating according to the Underwriters Laboratories test standard 94 results (UL-94).

Thermally expanded graphite polyetherimide composite with superior electrical and thermal conductivity

Thermally expanded graphite (EG) polymer composite has high potential for application in the modern thermo-electronic industry due to cost-effectiveness and excellent electrical and thermal properties. In this study, we report electrical and thermal conductivity enhancement of expanded graphite/polyetherimide (EG/PEI) composite fabricated by a simple solution mixing technique. Expanded graphite has the potential to enable larger enhancement in properties relative to graphene nanoplatelets, through the creation of continuous graphitic networks in the composite. Solution casting technique enables preservation of worm structure of expanded graphite in the composite thus enabling the development of continuous graphitic networks throughout the composite, leading to superior property enhancement. Measurements reveal ∼19 orders of magnitude increase in electrical conductivity of composite (yielding values up to 969 Sm-1) through the inclusion of 10 wt% of EG filler, relative to pure polyetherimide (electrical conductivity ∼ 1.2 × 10−17 Sm−1). Theoretical prediction using an effective medium approach reveals the impact of continuous graphitic network on the electrical conductivity of the composites. We also find that composite with 10 wt% EG filler composition exhibits a thermal conductivity (k) value of 7.3 Wm−1K−1, representing a significant enhancement of ∼3070% compared to pure PEI (k ∼ 0.23 Wm−1K−1). Field Emmission Environment Scanning Electron Microscopy (FE-ESEM) reveals the preserved interconnected structure of EG within the composite allowing the development of a conductive interpenetrating network of polymer and filler. Raman spectroscopy, X-ray diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) analysis are further used to characterize the fillers and EG/PEI nanocomposites. Our work identifies the solution casting method as a promising approach for developing high-performance EG/PEI composites in this work.

Investigation of the thermal conductivity enhancement mechanism of polymer composites with carbon-based fillers by scanning thermal microscopy

In order to elucidate the mechanism of enhancement of heat transfer in polymer composites, in this work, we investigated two types of polymer-carbon filler composites. This investigation was made using scanning thermal microscopy (SThM) with the Wollaston microprobe operated in active mode as a function of the carbon filler weight fraction within the polymer matrix. Samples consist of high-density polyethylene (HDPE) filled with 50 µm expanded graphite (EG) and polyvinylidene difluoride (PVDF) containing multiwall carbon nanotubes (MWCNTs). For HDPE/EG samples, SThM images allow the detection of zones with a thermal conductance larger than that of the matrix for the highest studied filler concentration. These zones correspond to EG filler agglomerations within the polymer and explain the observed enhancement of the thermal conductivity k of the HDPE/EG composite. For PVDF/MWCNTs samples, it is found that k increases from 0.25 W m−1 K−1 for pristine PVDF to 0.37 W m−1 K−1 for PVDF nanocomposites filled with 8 wt. % MWCNTs. This k variation vs filler concentration is found in good correspondence with that of the β phase relative percentage in the PVDF nanocomposites. This suggests that the observed heat transfer enhancement is rather due to the formation of β phase for PVDF/MWCNTs samples, resulting from the addition of MWCNTs than the addition of MWCNTs itself. Thus, tuning the thermophysical properties of polymer-based nanocomposites can establish new design laws to confer them specific thermal properties.

Effect of expanded graphite surface modification on phase change materials nanocomposites thermal protection efficiency

AbstractIn order to investigate phase change materials (PCMs) thermal energy storage capacity as well as shape stability, PCM nanocomposites were fabricated by employing modified expanded graphite (EG) as a conductive filler and polyethylene glycol (PEG) PCM. To improve PCM composite shape stability, EG surface has been modified by utilizing Toluene 2 and 4 diisocyanate (TDI), ethylene glycol and PEG 600, respectively. Findings have illustrated that for PEG PCM/modified EG nanocomposites, shape stability was achieved at 5 wt.% of modified EG filler with less than 10% of melt leakage for all prepared samples as a consequence of surface adsorption capacity increment as well as adequate compatibility. Furthermore, phase change efficiency was calculated as 98% and in addition, thermal diffusion at PCM melting temperature range was negligible and approximately equal to zero. To conclude, it could be declared that, fabricated phase change material composite had desirable characteristics along with adequate shape stability for thermal energy storage applications.

Study on thermal conductivities and mechanical properties of silicone/expanded graphite composites

In this study, series of expanded graphite (EG)-filled silicone composites were prepared by the bulk mixing and the solution methods to investigate the effect of thermal conductive EG particles and preparation methods on thermal conductivity of resulting silicone composites. With increasing EG contents and sizes, the thermal conductivity increased, and the solution mixing method showed higher thermal conductivity than the bulk mixing method. At 10 wt% of EG 140 μm-filled silicone composites, the solution mixing method showed the thermal conductivity of 0.789 W/m·K, which was about 147% over one prepared by the bulk mixing method and 538% increase over the pure silicone polymer. FE-SEM experiment was carried out to explore the dispersion and network formation of EG fillers in the silicone matrix. Tensile and dynamic mechanical analysis were also performed to investigate the mechanical reinforcement of the silicone composites. All the results suggested that the network structure of EG with larger size could improve mechanical reinforcement by increasing the modulus and optimize mechanical properties while improving the thermal conductivity of composites.

Synthesis and Characterization of Graphite Composite Foams for Oil Spill Recovery Application

The aim of this paper is the synthesis and characterization of a composite silicone foam filled with expanded graphite (EG) for oil spill recovery applications. The EG foams were obtained using a foaming slurry consisting of a mixture of siloxane compounds as the matrix with an EG filler. The effect of the filler content’s performance on an innovative composite silicone-based foam was investigated. All the obtained samples exhibited an open cell morphology. Each foam was evaluated in four commonly used oils (kerosene, pump oil, naphtha and crude oil). Additionally, kinetics was studied in order to investigate the physical, chemical and mass transport mechanisms that act during the absorption phenomenon and uptake evolution of the contaminants. Foam filled with 3% of EG exhibited the highest absorption capacity, particularly with light oils kerosene and virgin naphtha (854 and 1016 wt.%, respectively). Furthermore, the kinetic study showed that pseudo-second order mechanisms better fitted the composite absorption performances, suggesting that the oil sorption into EG filled polydimethylsiloxane (PDMS) foams could be related to chemisorption mechanism. The results evidenced a good oil sorption capability and water/oil selectivity indicating this class of materials as a potentially applicable material for oil spill remediation.

Fabrication of high thermal conductive epoxy composite by adding hybrid of expanded graphite, iron (III) oxide, and silver flakes

Thermally conductive epoxy adhesive composites have been fabricated by adding a tri-filler mixture of expanded graphite (EG), iron (III) oxide (Fe3O4), and silver (Ag) flakes with increasing concentration from 20 to 50 weight percent. The epoxy composite containing mixed filler of EG/Fe/Ag at 50 wt% has demonstrated diminutive higher thermal conductivity (TC) (4.85 W/mK) when compared to composite filled with EG/Fe hybrid (4.82 W/mK) at equivalent filler % which was about 25 fold improvement than epoxy unaided. The crystallography of the graphite flakes (GF), EG, neat epoxy, EG/Fe/Ag-epoxy, and EG/Fe-epoxy adhesive composites has been analyzed by X-ray diffraction (XRD) technique. The morphology of filler (EG and EG/Fe/Ag) and fractured surface analysis of corresponding epoxy composite has been carried out by scanning electron microscopy. The bonding strength through lap shear test of epoxy adhesives with the addition of 50 wt% of EG/Fe/Ag mixed and EG/Fe hybrid fillers has been displayed a decreased value as compared to neat epoxy. At the primary phase analysis of thermal dilapidation, the thermo-gravimetric analysis (TGA) concluded a negative effect on the thermal constancy of epoxy composites at higher assimilation (> 35 wt%) of EG, Fe3O4, and Ag mixed fillers. The damping factor (tan δ) (0.44 and 0.43) and glass transition temperature of composite demonstrated lower value (70 and 64 °C) when 50 wt% of EG/Fe and EG/Fe/Ag were added.

Decomposed Copper(II) Acetate Over Expanded Graphite (EG) as Hybrid Filler to Fabricate Epoxy Based Thermal Interface Materials (TIMs)

In this investigation, the fabrication of thermal conductive epoxy composite with the assimilation of synthesized expanded graphite (EG) decorated with copper compound nanoparticles [copper (Cu), copper(II) oxide (CuO) and copper(I) oxide (Cu2O) nanoparticles], has been reported wherein, Cu-compound was attached to EG surface by solid-state pyrolysis of copper(II) acetate (CA) monohydrate. The prepared hybrid filler was characterized by an x-ray diffraction technique and the Cu-compound nanoparticles size was 40.8 ± 17.67 nm. The microstructure and morphology of the distributed Cu-compound nanoparticles over the EG surface were characterized by transmission electron microscopy (TEM) and scanning electron microscopy. The Cu-compound nanoparticles decorated EG hybrid filler at 10 wt.% loading ((EG-CA (4)/Ep)10) demonstrated the thermal conductivity (TC) which is 11.8 times higher than the neat epoxy due to the formation 3D percolation heat-conducting networks. Further, decoration of Cu-compound on the EG surface resulted in higher TC as measured using a guarded heat flow meter technique. Lap shear strength of (EG-CA (4)/Ep)10 composite was tuned to 5.93 ± 0.27 MPa as characterized by a universal testing machine. The porosity of fabricated composites was decreased as Cu-compound attachment increases on the EG substrate. The thermo-gravimetric analysis revealed enhanced thermal stability of (EG-CA (4)/Ep)10 composite to 407°C at 50% weight loss consideration. The electrical resistivity of the composite was reduced with the addition of the EG filler system as confirmed from the super megaohmmeter.

Volume control of expanded graphite based on inductively coupled plasma and enhanced thermal conductivity of epoxy composite by formation of the filler network

Expanded graphite (EG) is noticed as a thermally conductive carbon filler because of its advantages of a unique structure in which graphite sheets are intercalated and relatively a low price among carbon fillers. Nevertheless, the relationship between the volume expansion (≈interlayer spacing) of EG and the thermal conductivity of EG filled composites has been rarely reported on. We designed and developed an inductively coupled plasma (ICP) for the rapid expansion of a expandable graphite precursor. Greater extent of volume expansion of EG was observed at higher ICP treatment temperature and the thermal conductivity of the composites was increased with the expanded volume of the filler, at the same filler content. Based on a quantitative analysis on filler size within the composites using non-destructive micro-computed tomography, larger size of three dimensional (3D) thermally conductive filler networks with respect to the volume expansion were obviously confirmed. Excellent thermal conductivity (the highest value of 10.77 W/m·K) and heat dissipation characteristic of the composites were derived when the 3D thermally conductive EG filler network was generated at a larger size. These findings can contribute to realizing effective and low-priced thermally conductive carbon filler based polymer composites.

Largely improved thermal conductivity of HDPE/expanded graphite/carbon nanotubes ternary composites via filler network-network synergy

Utilizing the synergistic effect of various fillers is an efficient strategy to enhance the thermal conductivity of polymer composites, in which the key is to modulate their dispersion and network formation in polymer matrix. In this work, expanded graphite (EG) was individually added into high density polyethylene (HDPE) to fabricate first the binary composites through melt blending. The electrical conductivity of the prepared composites was measured to determine the percolation threshold for HDPE/EG composites. Then HDPE/EG composites with three compositions, representing below percolation, just percolation and above percolation, respectively, were chosen as matrix and melt mixed with carbon nanotubes (CNTs) to make HDPE/EG/CNTs ternary composites. It was found that adding CNTs results in a linear increase of thermal conductivity for HDPE/EG composites with composition below percolation, along the line by adding the same amount of EG. While a jump of thermal conductivity was observed by adding CNTs for HDPE/EG composites with composition just and above percolation. The electrical conductivity and rheology property were measured and SEM experiment was carried out to explore the filler dispersion and their network formation in HDPE matrix. All the results suggested a possible location CNTs in EG filler network for HDPE/EG composites with composition just and above percolation. Thus the formation of CNTs network within EG network is attributed to the main reason for the largely enhanced thermal property. This work endows a new enlightenment to fabricate the composites with a great thermal conductivity.

Development and characterization of expanded graphite filled‐PET/PVDF blend: thermodynamic and kinetic effects

Expanded graphite (EG) filled polyethylene terephthalate (PET)/polyvinylidene fluoride (PVDF) blend composites were prepared via melt blending by applying different mixing protocols: either premixing EG in the PET phase, premixing EG in the PVDF phase, or simultaneous mixing of the components. The blends were characterized for their thermal, rheological, mechanical, and electrical properties as well as microstructure analysis. The mixing protocol had a substantial effect on the electrical resistivity of the composites. When EG was premixed within PET phase, the blend exhibited a well‐defined co‐continuous structure which was characterized by a three orders of magnitude decreased electrical resistivity relative to the simultaneous mixing procedure. The improvement in the electrical properties of the blend can be attributed to the double percolation effect. Morphology analysis showed that EG was selectively confined and dispersed in the PET continuous phase of the blend. Melt rheological behavior of the composites made evidence of preferable interactions between EG and PET molecules as well as effective filler dispersion, whereas EG practically did not interact with PVDF molecules. Morphology analysis showed good correlation with the electrical and rheological properties of the blend and revealed that EG was selectively confined and dispersed in the PET continuous phase of the blend. It can be deduced that both thermodynamic and kinetic drives encourage the selective localization of EG filler in PET phase. Copyright © 2016 John Wiley & Sons, Ltd.

Synergistic enhancement of thermal conductivity in polymer composites filled with self-hybrid expanded graphite fillers

The size of expanded graphite (EG) was controlled using a high-speed crusher because a hybrid of differently sized fillers can induce a synergistic enhancement of the thermal conductivity in polymer composites. We found that the thermal conductivity of a polymer composite filled with both 10wt% EG and 10wt% high-speed crusher treated EG (wEG) was synergistically improved by 12.0 and 20.7% compared to that of polymer composites filled with 20wt% EG and 20wt% wEG alone, respectively. A three-dimensional (3D) non-destructive analysis using X-ray micro-computed tomography (micro-CT) was applied to explain the synergistic enhancement and to identify the dispersion and 3D network of EG fillers in the composites accurately. According to the non-destructive analysis results, the synergistic enhancement was caused by the formation of efficient thermally conductive pathways due to the hybrid of the differently sized EG and wEG fillers.

Thermal Conductivity of Polymer Composites With Geometric Characteristics of Carbon Allotropes

Effect of the sizes and shapes of the diverse carbon allotropes such as carbon black, carbon nanotube, graphene nanoplatelet, graphite, pitch‐based carbon fiber (PCF), and expanded graphite (EG) on the thermal conductivity of polymer composites is investigated. Effective improvement of the thermal conductivity of composites with microsized one‐dimensional (1D) PCF and EG including the intercalation spacing is observed due to efficient formation of thermal transfer networks and little phonon scattering at the interfaces by the reduction of the interfaces. The excellent bulk and in‐plane thermal conductivity of the composites with 20 wt% EG fillers of 1.9 and 6.7 Wm−1K−1 is observed.

Electrical conductivity properties of expanded graphite-polycarbonatediol polyurethane composites

Conductive polymer composites of segmented polycarbonatediol polyurethane and expanded graphite (EG) have been synthesized with different amounts of EG conductive filler (from 0 to 50 wt%). SEM, X-ray diffraction measurements, Fourier transform infrared and Raman spectroscopies demonstrated a homogeneous dispersion of the EG filler in the matrix. The dielectric permittivity of the composites showed an insulator to conductor percolation transition with increase in EG content. Significant changes in the dielectric permittivity take place when the weight fraction of EG is in the range 20–30 wt%. Special attention has been paid to the dependence of the conductivity on frequency, temperature and EG content. The addition of EG to the matrix causes a dramatic increase in the electrical conductivity of 10 orders of magnitude, which is an indication of percolative behavior. A percolation threshold of ca 30 wt% was evaluated by using the scaling law of percolation theory. © 2014 Society of Chemical Industry

Experimental and multiscale modeling of thermal conductivity and elastic properties of PLA/expanded graphite polymer nanocomposites

We developed experimental as well as theoretically based hierarchical multiscale procedures for the evaluation of effective elastic modulus and thermal conductivity of poly-lactide (PLA)/expanded graphite (EG) nanocomposites. The incorporation of EG fillers into PLA was carried out by a twin-screw micro-extruder. The dispersion/delamination of EG in PLA was studied using Raman spectroscopy, SEM and TEM. In the multiscale modeling, the thermal conductivity constants and stiffness tensor of EG were first acquired by the means of molecular dynamics (MD) simulations. Using the fillers’ properties obtained by the MD, we developed finite elements (FE) models to evaluate the effective thermal conductivity and elastic modulus of PLA/EG nanocomposites. Our results, for a wide range of temperatures revealed the efficiency in thermal and mechanical reinforcement of PLA by incorporation of EG nanoparticles.

Mechanical Properties and Morphology of Composites Based on the EVA Copolymer Filled with Expanded Graphite

Polymeric composites were prepared using ethylene-vinylacetate (EVA) matrix and expanded graphite (EG). Mechanical properties were investigated and it was found that Young's modulus and the yield stress of EVA-EG composites significantly increased with graphite content. However, for higher EG content the composites became brittle. The dependence of stress at the break of composites reached maximum at 30 vol.% of the filler. The morphology of both EG filler and composites were investigated using transmission electron microscopy and Raman spectroscopy. All these structural characterizations indicated partial exfoliation of EG within EVA matrix, which leads to the increase of the reinforcement effect.

Electrically conductive nanocomposites of aromatic polydisulfide/expanded graphite

Expanded graphite (EG) with different expansion ratios were prepared by the exfoliation of expandable graphite under microwave irradiation. Nanocomposite precursor was fabricated by direct solution blending of cyclic(arylene disulfide) oligomers with the EG fillers. Subsequently, the nanocomposite precursor was hot-molded at 200 °C to carry out simultaneously the in situ ring-opening polymerization (ROP) of these oligomers via free radical mechanism to prepare aromatic polydisulfide/EG nanocomposites. The resulting aromatic polydisulfide/EG nanocomposite exhibited a completely intercalated nanostructure as evidenced by TEM observation. Highly electrical conductivity of the composite was measured by a four-point probe resistivity determiner. Three-point bending tests were used to determine the flexural strength of the composites. The nanocomposite can be used as potential materials to prepare bipolar plate of polymer electrolyte membrane fuel cell due to its high electrical conductivity, recyclable properties of the matrix and nanoscale effect in gas barrier performance.

Electrical and mechanical properties of expanded graphite‐reinforced high‐density polyethylene

AbstractHigh‐density polyethylene (HDPE) was reinforced with expanded and untreated graphite in a melt‐compounding process. Viscosity increased upon addition of graphite phase, with the expanded graphite (EG) showing more dramatic rise than the untreated graphite (UG) in viscosity. The increase in viscosity was attributed to the increased surface‐to‐volume ratio for the EG filler after acid treatment. Electrical conductivity also increased from that pertaining to an insulator to one characteristic of a semiconductor. The EG system showed a lower percolation threshold for transition in conductivity compared to that in the UG system. DSC results indicated that the fillers acted as a nucleating agent in inducing the crystallization of HDPE in the composites. However, the overall degree of crystallinity and melting temperature of HDPE decreased with the addition of EG and UG. Mechanical properties improved as a function of filler content but the overall enhancement was not impressive. It was conjectured that the filler–matrix interface was not optimized in the melt‐mixing process. However, the role of EG as a reinforcement phase for both electrical and mechanical properties was unambiguously established. The EG composites demonstrated potentially useful attributes for antistatic, barrier, mechanical, electrical, and cost‐effective applications. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:2781–2788, 2004

Electrical conductivity and dielectric properties of PMMA/expanded graphite composites

PMMA/expanded graphite (EG) composites were prepared by direct solution blending of PMMA with the expanded graphite filler. Electrical conductivity and dielectric properties of the composites were measured by a four-point probe resistivity determiner and a dielectric analyzer (DEA). Interestingly, only 1 wt.% filler content was required to reach the percolation threshold ( φ) of transition in electrical conductivity from an insulator to a semiconductor using PMMA/EG. The thickness of the interlayer of the expanded graphite was shown to be close to the nanometer scale. The reported filler content was much lower than that required for conventional PMMA/carbon black (8 wt.% carbon) and PMMA/graphite (3.5 wt.% graphite) composites. The improvements in both electrical conductivity and structural integrity were attributed to the difference in filler geometry (aspect ratio and surface area) and the formation of conductive networks in the composites.

Transport behavior of PMMA/expanded graphite nanocomposites

Polymethylmethacrylate (PMMA)/expanded graphite (EG) and PMMA/untreated graphite (UG) composites were prepared by direct solution blending of PMMA with EG and UG fillers. A four-point resistivity probe system was used to measure the electrical conductivity of the composites. With the increase of filler content, the electrical conductivity of the composites showed the transition from an insulator to a semiconductor. The transition can be described by classic percolation theory with a critical exponent of 2.1±0.1 for PMMA/EG and 1.8±0.1 for PMMA/UG composites. Interestingly, only 0.6 vol% filler content was required to reach the percolation threshold of transition in electrical conductivity using PMMA/EG. The thickness of the EG sheet was found to be at the nanometer scale. The filler content necessary to reach the percolation threshold in PMMA/EG was found to be much lower than those required for PMMA/UG (2.0 vol% graphite) and conventional PMMA/carbon black (4.5 vol% CB) composites. Evidence was presented in this to demonstrate the improvement in electrical conductivity which was effected by the increase in filler form factor and their enhanced dispersion.