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The Production of Value-added Composites from AluminumPlastic Package Waste via Solid State Shear Milling Process Shuangqiao Yang, Shibing Bai, Wenfeng Duan, and Qi Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04733 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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ACS Sustainable Chemistry & Engineering

The Production of Value-added Composites from Aluminum-Plastic Package Waste via Solid State Shear Milling Process Shuangqiao Yanga, Shibing Bai*a, Wenfeng Duanb, Qi Wanga a

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute

of Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu 610065, China b

Beijing Oriental Yuhong Waterproof Technology Co.,Ltd, State Key Laboratory of

Special Functional Waterproof Materials, No.2 Shaling Section, Shunping Road, Beijing 100020, China Email of corresponding author: S. Bai (E-mail:[email protected]) Fax:

+86-28-85402465

Tel:

+86-28-85463909

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Aluminum plastic multilayer film has been widely used in packaging industry, but the recycling of such material is a great challenging task. Herein, we reported a facile route to produce value-added composites with high thermal conductive and high electrical insulation from aluminum plastic package waste (APPW) for the first time. Briefly, solid state shear milling (S3M) technology was applied to prepare ultrafine APPW powder and exfoliate expandable graphite (EG) into graphite nanoplatelets (GNPs) with lateral dimension of 1-5µm which constructed the conductive network. To block the electron transfer through the aluminum/GNPs network in composites, in-situ oxidation process employed for aluminum flakes to form nano-Al2O3 electronic insulation layer around aluminum flakes, whereas phonon can transport through this barrier layer easily. As a result, the APPW/GNPs composites exhibit an excellent balance of thermal conductivity and electrical insulation and well above all reported data to date on graphite or graphene/polymer composites, with thermal conductivity of 1.7W/mK and electrical conductivity of 10-10 S/cm. Furthermore, enhanced mechanical strength and stiffness are achieved in APPW/GNPs composites. Such materials could have potential applications in the electronic industry and turn environmental pollutant into a valuable resource. Keywords: Aluminum-Plastic Package Waste; Composites; Solid State Shear Milling; Thermal Conductive; Insulating Performance

Introduction Aluminum plastic multilayer package has been widely used for food, medicine, chemicals and electronic packaging around the world due to the special properties, including barrier to water vapor, gas, flavor or light, as well as convenient use and low cost1. With the increase of their applications in everyday-life, the volume of these multilayer package in municipal solid waste has continuously increased, especially in electronic package (e.g. in China: over 67 million m2/year). Aluminum-plastic package waste (APPW) often consists of several laminated layers including linear low-density polyethylene (LLDPE,70%), aluminium (Al,15%), and poly(ethylene


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terephthalate) (PET,15%)2. The separation and classification of these multilayer package into elemental components is technical difficult, thus such kind of material are main disposed of in incineration and landfill without any material cycling 3, which may cause serious environmental pollution such as the contamination of soil and ground water. Recently, several lab techniques have been developed for the recycling of APPW such as thermal treatment to recycle the energy and Al metal4, 5, mechanical or chemical technology to exfoliate and dissolve polymer from Al with the aid of chemical solvent and followed by materials recovery6, 7. Although some potential values may present in these methods, the risk of secondary pollution, high cost, troubles in treatment of the waste solvent and lack of commercial value of incomplete separation outcomes is still problems for industrial scale production. Therefore, there is still an urgent issue in developing environmentally benign technology for production of value-added products from APPW. So far, very little research has been paid to investigating the direct use of APPW in production of high value-added products. With the miniaturization, integration and functionalization of electronics, thermal dissipation of electrical equipment has become a challenging problem8. In general, two strategies are employed to improve the thermal conductivity of polymer composites. One is the introduction of electrically conductive fillers (e.g. graphite nanoplatelets9, graphene10, or metal like Al11) with high intrinsic thermal conductivity into the polymer matrix. Though these fillers can notably increase thermal conductivity of composites at low loadings, the electrical conductivity has a sharp increase to above 10-6 S/cm at the same time, which did not meet the requirements of electric insulation for electronic packaging materials. An alternative method is the addition of electrically insulating ceramic fillers (e.g. Al2O312, AlN13, BN14), which can improve the thermal conductivity and meet the stringent requirements for electrical insulation. But to achieve favorable thermal conductivity, a large amount of ceramic fillers is usually required, which in return increase the cost and deterioration mechanical properties12. Thus, the fabrication of high thermal conductivity and insulation polymer composites without sacrifice of mechanical strength in a low-cost ACS Paragon Plus Environment


way remains still a challenge. Nowadays, conductive/insulative hybrid nano-fillers has been reported in producing insulating thermal conductivity composites and have attracted great interest. For instance, Cao et al.

15

fabricated MWCNTs/SiC

nanoparticles hybrid network in composites where SiC nanoparticles can act as electrical current barriers to prevent the composite from forming an electrically conductive network with multi-walled carbon nanotubes (MWCNTs). Dai et al.16 prepared SiC nanowires/graphene hybrid fillers in composites, similarly the SiC nanowires that grown on graphene can make composites exhibited relatively low electrical conductivity and good thermal conductivity. Qian et al17. successfully coated Al2O3 layer on the surface of graphene that retain the high electrical insulation of the polymer matrix. However, the use of carbon nanotubes or graphene in polymer not only resulted the limited improvement of thermal conductivity and the high cost but also faces the challenge of dispersion of nano-fillers in commercial production. On the other hand, little attention has been focused on the use of micro-metal fillers (e.g. Al18) or graphite that are rich in resources to resolve this problem. Our research group has previously reported that state shear milling (S3M) technology can be used in ultrafine polymer powders production19, tire rubber decrosslinking20, carbon nanotube nanoscale cutting21, montmorillonite nanoscale exfoliation22 and graphene nanoscale dispersion

23

. Recently, we find that S3M

technology may show potential application for recycling ordinarily immiscible waste plastic into value-added products without sorting by type or color24, 25. To address the environmental issue of APPW and resolve the problem presented above, a new strategy combining S3M and the in-situ oxidation of Al was proposed to produce value-added APPW/GNPs composites with high thermal conductive and high electrical resistivity for the first time. As far as we know, this is the first attempt to fabricate oxidized Al/graphite nanoplatelets hybrid network in thermal conductive composite with high electrical insulation. In this study, S3M technology was applied to prepare ultrafine APPW powder and exfoliate expandable graphite (EG) into graphite nanoplatelets (GNPs). And then, in-situ oxidation process employed for aluminum flakes to form Al2O3 insulation layer, which can block the electron transfer ACS Paragon Plus Environment

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through the filler network in composites. Our study offers an effective method for the production of value-added composites from APPW with potential application as electronic packaging materials.

Materials and methods Materials The multilayer aluminum-plastic package waste (70% LLDPE, 15% Al and 15% PET, as reported by the supplier) was scraps during the production and supplied by Shenkai packaging Co., Ltd (China). Linear low-density polyethylene (DFDA-7042) with density of 0.92 g/cm3 and melt flow index (MFI) of 2.0 g/10min was obtained from Lanzhou Petrochemical Company. The commercially available expandable graphite (EG-150) with particle size of 150 mesh was received from Qingdao Yanhai Tan Calliao Co., Ltd (China). Here, EG is a sulfuric acid and nitric acid based graphite intercalated compound with onset exfoliation temperature at 220 °C. All raw materials in this research were used without further purification. Preparation of composites

Fig.1 Schematic representation of the preparation process of APPW/GNPs composite

The schematic of the preparation process for APPW/GNPs composite with high thermal conductive and high electrical resistivity is illustrated in Fig.1. Before the preparation of composites, as-received APPW (Fig.S1c, Supporting Information) was pretreated in self-designed pan-mill equipment (aluminum-plastic package waste has been processed at Shichuan University with a commercial scale apparatus at production capacity exceeding 150 kg/h) and the details of equipment is shown in Supporting Information (Fig. S1a and S1b) which can also be found in our previous



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publications26. The key part of this unique equipment is a pair of inlaid pans including one stationary pan and another movable pan. Each inlaid pan is divided into equal sectors by several bevels, and the ridges of the bevels are parallel with the dividing lines. Due to its unique design, APPW which fed into the center of the pan was driven by shear force and move along a spiral route toward the edge of the pan till APPW powder come out from the outlet. After S3M process we find ultrafine APPW-S3M powders (Fig.S1d, Supporting Information) can be easy to recycle by extrusion or injection molding process. To prepare functional powder the desired EG content (10-35 vol%) and APPW powder was further co-milled together in pan-mill equipment (rotating speed of 100 rpm, pressure of 20KN, at 25°C). After co-milling process

homogeneous

black

APPW/EG-S3M

powder

(Fig.S1e, Supporting

Information) was obtained. Expandable graphite (EG) used here is made from natural flaked graphite by the intercalation of chemicals such as sulfuric or nitric acid, which can expand up to a hundred times in volume at temperature above 220°C (Fig.S2, Supporting Information) due to the thermal expansion of the evolved gases (carbon dioxide) caused by decomposition of oxidized groups in EG 27. Then APPW/EG-S3M powder was extruded into thread with a twin-screw extruder (SHJ-20, Nanjing Giant Machinery Co. Ltd., China) and the obtained composites were coded as APPW/GNPs. The twin-screw extruder has a screw diameter of 25mm and a length to diameter ratio (L/D) of 33/1. The extrusion process was controlled at screw speed of 100 rpm under temperature of 240-250°C for the oxidation of Al from APPW. For comparison, composite samples of LLDPE with the same content of EG were prepared by twin-screw extruder at temperature of 180-200°C to avoid the in-situ expansion of EG which may lead the sharp increase of electrical conductivity. The test specimens are prepared by a MA1200/370 injection-molding machine (Haitian Plastic Machinery Co. Ltd., China) at temperature of 200°C. Measurements Scanning electron microscope (SEM) (FEI Instrument Co. Ltd., USA) equipped with energy dispersive X-ray (EDX) elemental composition analyzer was used to ACS Paragon Plus Environment

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observe the morphology and elemental maps. Before SEM evaluation, the samples were sputter-coated with gold to prevent charging during the test. Transmission electron microscopy (TEM) was performed on a transmission electron microscope (JEOL JEM-100CX, Japan). Composites were cut into 80-100nm thin sections at a temperature of -100°C using a LEICA EM FC6 frozen ultramicrotome, and samples were then placed on the copper grids. In-situ Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 FT-IR spectrometer (Thermal Scientific, USA) equipped with a heated transmission cell (HT-32). In situ spectra were collected over the wavenumber range 4000-400 cm-1 from 40°C to 260°C with 5°C/min. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Kratos Nova spectroscopy (Kratos Analytical, UK) equipped with a monochromatic Al kα X-Ray source (hν=1486.6eV). X-ray diffraction (XRD) analyses were carried out on a DX-1000 diffractometer (Dandong Fangyuan Instrument, China). The samples were continuously scanned from 5° to 50° at a rate of 0.06°/s. Rheological properties were performed on an AR 2000ex stress-controlled rheometer (TA instrument, USA) with 25mm parallel-plate fixtures. A dynamic frequency sweep from 0.01 to 100 rad/s was employed and samples were maintained at 240°C. The thermal conductivity of the samples was measured by a Hot Disk thermal analyzer (Hot Disk 2500-OT, Sweden), using the transient place source (TPS) method based on a transient technique. The electrical conductivity of the samples was measured with a Keithley 6487 picoammeter (Keithley, USA) under a constant voltage of 1 V. The test specimens were prepared by hot press molding with diameter of 25mm and thickness of 10mm. The tensile strength and flexural strength of various samples were tested using a universal testing machine (Instron 5567, USA) at room temperature with a cross head speed of 50 and 2mm/min according to ASTM D638 and ASTM D790 standards, respectively. ACS Paragon Plus Environment


Results and discussion Morphology developments of APPW and APPW/EG during S3M process

Fig.2 SEM images of (a,d) as-received APPW film, (b,e) APPW-S3M powder and (c,f) APPW/EG-S3M powder with 17vol% EG content. SEM images of (g,h) as-received EG and (i) GNPs extracted from APPW/EG-S3M powder with boiled xylene.

Our designed fabrication procedure involves the preparation of ultrafine APPW powder and exfoliation of GNPs from EG by S3M process and the in-situ oxidation of Al during melt extrusion. The present study evaluated changes in morphology of APPW during S3M process by scanning electron micrographs (SEM) observation. Fig.2a-b present SEM morphology of APPW before and after exposed to mechanical shear force in S3M process. It can be seen that the as-received APPW film consist of three layers (LLDPE, Al and PET)2 and the thickness value of APPW film can be visually estimated to be less than 100µm (Fig.2a). Moreover, in high magnification SEM images (Fig.2d), it can be clearly seen that the thickness of Al layer is about ACS Paragon Plus Environment

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3µm. Due to the strong shearing and compressing forces exerted by S3M process, APPW film break into irregularly rod-shaped particles with a size of about 5-30µm in diameter after S3M pretreatment and the three different layers in APPW completely separated from each other (Fig.2e). However, the APPW particles bond together when further co-milled with EG (Fig.2c, f), and it is indistinguishable between the embedded EG flake and other chemical constituents in APPW due to the repeated fragmentation and bonding of polymer during S3M process. To observe the morphology development of EG during S3M process, the SEM of as-received EG and GNPs extracted from APPW/EG-S3M powder with boiled xylene was compared in Fig.2g-2i. The efficient exfoliation of micron EG to nanoscale GNPs by mechanical process can be clearly identified in high resolution SEM image (Fig.2i). In this research, it is important to note that the presence of aluminum layer in APPW makes it impossible for direct recycling by traditional plastic processing equipment. Obviously, S3M technique is an effective pretreatment for preparing ultrafine APPW powders which make it easy to recycle by extrusion and injection molding processes. In-situ oxidation of aluminum in composites



Al metal

(g)

(h)

APPW

♦ ♦ Graphite α -Al2O3

(a) APPW (b) APPW/GNPs (c) APPW/EG-S3M powder (d) LLDPE/EG

Al Al metal

APPW/EG-S3M powder

Intensity

Counts (cps)

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Al metal

♦

(d) (c)

APPW/GNPs composite

(b)

Al Oxide

(a) 70

75

80

10

20

Binding energy (ev)

30

40

50

60

2θ

Fig.3 (a)TEM images of Al from APPW/EG-S3M powder and (b) oxidized Al from APPW/GNPs composites. (c)The selected area for SEM-EDX mapping and the SEM-EDX mapping of (d) Al element, (e) O element and (f) S element in APPW/GNPs composites. (g) Al2p XPS spectrum and (h) XRD data for composites (EG in composites is fixed at 17vol%).

Aluminum particle is a kind of electric conductive filler for polymer composites, whereas the Al2O3 platelets can still keep electrical insulation for thermal conductive composite even deposited with some silver nanoparticles28. To keep the high electrical insulation, in this work, we in-situ oxidized aluminum flakes to form nano-Al2O3 shell in APPW/GNPs composite during melt extrusion. The TEM and elemental analysis was carried out to investigate the formation of oxidation layer that coated on the surface of aluminum flakes. As shown in Fig. 3a, almost no Al2O3 shell structure (less than 1nm) could be recognized in the surface of Al flak in APPW/EG-S3M powder. In Fig. 3b, however, we observed that the nano-Al2O3 shell thickness around Al platelet is about 18nm, which is three times thicker than the reported value by self-passivated method29. In element map for aluminum (Fig.3d), oxygen (Fig.3e) and sulfur (Fig.3f), specific color dots indicate the presence of a specific element while black indicates the absence of an element. The aluminum element is easily identifiable as a long and narrow area with tens of microns in length which indicates the effective breakup of aluminum film from packaging waste via S3M process. By comparing with the oxygen element map, one can easily find that oxygen element appears on almost all aluminum-rich regions. This might be attributed to the in-situ passivated of aluminum flakes, and thus nano-Al2O3 shell structure was formed around micro-Al flak. In addition, it should be noted that some oxygen atom spots also appear in other regions


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of composite, which can be explained by the presence of oxygen atom in EG and polyethylene terephthalate from multilayer APPW matrix. Furthermore, the S elemental map (Fig.3f) and EDS analysis (Fig.S3, Supporting Information) corresponding to the area in Fig.3c show a uniform distribution of S element (0.3wt%) in composites, indicating the existence of minimal residual H2SO4 in matrix and the possible oxidation reaction (or passivation reaction) of Al during melt extrusion may as follows30-32 2Al+3H2SO4→Al2O3+3H2O+3SO2 (minor)

(1)

2Al+H2SO4→Al2O3+S+H2O (major)

(2)

On the other hand, the decomposition of oxygen containing groups in GNPs may occur with sulfuric acid acting as a catalyst during melt extrusion33. The products of this decomposition were reported to be carbon dioxide and small amounts of water27, 34

which agree well with the FTIR results in our research (Fig.S4, Supporting

Information). Another possible oxidation reaction of Al at high temperature during melt extrusion may be given by 4Al + 3CO2→2Al2O3 + 3C

(3)

In order to gain further information about the in-situ oxidation of aluminum flakes in composites, we conducted X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) experiments. High resolution data in the energy range of 70 eV to 80 eV is shown in Fig. 3g. In the Al 2p spectral regions of APPW, strong peak observed at 72.8 eV correspond to Al-Al bond of Al metal35, 36. For APPW/GNPs composite, major peak in 72.8 eV can be observed as that of APPW and APPW/EG-S3M powder, but a shoulder peak at 74.5 eV can also be found, suggesting the existence of Al-O bond. This binding energy value matched well with the reported data37, 38 for Al2O3 and confirmed the in-situ oxidation of aluminum in APPW/GNPs composites during melt extrusion. Fig.3h shows the XRD patterns of APPW, APPW/EG-S3M powder, and APPW/GNPs LLDPE/EG composites. The XRD patterns of all APPW, APPW/EG-S3M powder and APPW/GNPs composite shows reflection peak at 38.5° 2θ which presents the Al characteristic peak of (111)29. However, an extra characteristic diffraction peak of Al2O3 at 42.5° 2θ can only be ACS Paragon Plus Environment


detected in APPW/GNPs composite. Details of XRD patterns implied that Al metal flakes presented in all of APPW, APPW/EG-S3M powder and APPW/GNPs samples, but only Al in APPW/GNPs composite was oxidized into Al2O3 due to the presence of EG and melt extrusion process. By the way, there were no any characteristic diffraction peaks of Al or Al2O3 can be observed in LLDPE/EG composite. Fig.3h also shows two XRD peaks at 21.8° and 24.1° 2θ in all samples, consistent well with the (110) and (200) Bragg reflections of LLDPE crystal unit cell23. When compared with LLDPE/EG composite a dramatically decrease in graphite characteristic peak at 26.3° 2θ (corresponding to the inter-graphene sheet spacing of 0.338 nm) can be noticed for both APPW/EG-S3M powder and APPW/GNPs composite. This strongly suggests that significant exfoliation and dispersion of graphite nanoplatelets is achieved during S3M process even with a high graphite loading as much as 17vol%. These result are similar to the previous work39, 40, where a significant exfoliation of graphite nanoplatelets in polypropylene was reported in mechanochemistry process. In-situ FTIR analysis of composites 0.2810

0.3

250

(b)

(a)

260°C

Absorbance

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0.04626

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0.02313

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0.000

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1642 30°C

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Fig.4 In-situ FTIR contour map and the corresponding spectra in the range 1200-1850cm−1 of


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APPW (a,b) and APPW/EG-S3M powder with 17vol% EG (c,d).

The chemical structure development of the polymer matrix is a key factor which affects the interface compatibility between fillers and polymer, and improve the physical performance of the composite. Herein, in-situ FTIR was carried out to get a detailed insight into the chemistry and structural changes that happened to LLDPE in APPW during the oxidation of Al. Fig. 4a displays the in-situ FTIR contour map of APPW during the heating process from 20 to 270ºC. Main characteristic bands of LLDPE found at 2918, 2850, 1465, and 720cm−1 were assigned to asymmetric and symmetric stretching vibration, bending vibration and wag vibration of methylene groups (-CH2), respectively41. It is also possible to verify the weak characteristic bands of carbonyl group (C=O) and ester group (C-O) at 1720 and 1240cm−1. Such groups ascribed to the presence of PET in matrix and could be also generated at the LLDPE surface by oxidation due to the environmental exposition 2, 42. As can be seen in Fig. 4a-b, except for the disappearance of broad hydroxyl groups (-OH) for slightly adsorbed water between 3400 to 3600cm−1, no significant changes could be detected for other chemical groups in both LLDPE and PET during the heating process. Comparing with the weak -OH band in Fig. 4a, sharper peaks at 3450 and 1642cm−1 due to stretching and bending vibration of hydroxyl groups in water molecules appears in Fig. 4c-d and gradually disappears with the increase of temperature from 20 to 270ºC, which indicate the water evaporation in EG. From Fig. 4c-d it is interesting to note that the absorption band of the carbonyl group (C=O) around 1720cm−1 and ester bond (C-O) around 1240cm−1 becomes much stronger, indicating that the LLDPE main chain is being partially oxidized. Oxidized LLDPE layer about 4nm around Al2O3 layer can be confirmed in Fig.3b. This can also in agreement with the DSC results (Fig.S5, Supporting Information) where the melt temperature of LLDPE decreased from 123.8ºC to 122.1ºC. The slightly oxidized polyethylene backbone can decrease the mobility of LLDPE chains during the crystallization process and thus result in formation of imperfect crystallites thinner lamellar thickness, which leading the decrease in the melting temperature. These oxygen functional groups on the LLDPE may facilitate chemical interactions with the ACS Paragon Plus Environment


functional fillers in composites. A similar oxidation effect has already been observed by Xie et al.

41, 43

in thermal degradation of LLDPE/EG blend at 300ºC. However, in

this study no obvious intensities decrease of CH2 band at 2918, 2850 and 1465cm−1 can be detected in S3M pretreated APPW/EG powder at a proper temperature range. These results revealed that the C-C main molecular chains of LLDPE were not significantly broken in APPW/GNPs composite during melt processing. From Fig S6 (Supporting Information) it is seen that LLDPE/EG composite exhibit degraded thermal stability with value of T5% (onset thermal degradation temperature) decrease from 432.2ºC for neat LDPE to 412.5ºC for the LLDPE/EG17vol% composite, whereas a comparable T5% can be observed for APPW and APPW/GNPs17vol% composite. This comparable thermal stability could be attributed to the increased interfacial interactions between the LLDPE matrix and excellently dispersed GNPs which can act as thermal and transport barriers in the polymer matrix44. It must be underlined that APPW/GNPs composites are thermally stable at temperatures below 350˚C, which meeting the selected process conditions without risking thermal degradation. Morphology of composites


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Fig.5 SEM images of the fracture surface of (a,d) APPW-S3M composite, (b,e,g) APPW/GNPs composite and (c,f) LLDPE/EG composite. (h,i)TEM images of APPW/GNPs composite (EG in composites is fixed at 17vol%).

Fig.5 shows the morphology of cryo-fractured surface for APPW-S3M, APPW/GNPs and LLDPE/EG composite. From Fig. 5a it can be clearly seen that Al metal flakes larger than 50µm is existent in matrix (red lines) and the voids can be observed around the Al metal flakes (Fig. 5d), indicating the poor compatibility and relative weak interfacial adhesion between Al flakes and matrix11. Besides, smooth spherical holes presented in the matrix are related to the PET particles that were pulled out from the fractured surface. For composite co-milled with EG (Fig.5b), Al metal flakes are excellently dispersed and embedded in matrix so that it is hard to distinguish flake graphite and Al flakes in this low magnification observation. No voids could be observed around the surface of Al flakes, which suggest a strong interfacial adhesion between Al flakes and the matrix (Fig.5e). This is most probably



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because that intercalating agents from EG oxidized both Al and LLDPE matrix during melt extrusion, and consequently improved the compatibility between oxidized Al and LLDPE. From the high magnification SEM image (Fig.5e), it can be clearly seen that GNPs with lateral dimensions of 1-5µm excellently dispersed in composite and no graphite agglomerates are visible to the naked eye. As revealed in an edge-on view of isolated GNPs (Fig.5g), the thickness less than 20 nm can be observed. The TEM image (Fig. 5h) shows that Al flakes randomly dispersed in matrix while the well-dispersed GNPs with thickness less than 20nm are too thin to be observed unequivocally in a relatively large field of view. In a high magnification TEM image (Figure 5i), we even observed a transparent sheet which consists of numbers of graphene layers (in the 100nm thick ultracryotomy TEM specimens, lateral dimensions of GNPs should not be determined). In the previous work, the effectively exfoliation of graphite into graphite nanoplatelets and a desired dispersion of graphite nanoplatelets in polymers has been mentioned by S3M44 and other solid-state mechanical methods with strong mechanical shear force

39

. Interestingly, when

considering the distribution of both oxidized Al flakes and graphite in the present case, we noticed that the homogeneously dispersed graphite nanoplatelets separated by oxidized Al flakes and a graphite/aluminum hybrid network is established in APPW/GNPs composite. However, as shown in Fig.5c and Fig.5f, a poor dispersion of graphite is observed in LLDPE/EG composite and continuous graphite network may not be easily formed by the agglomerates of graphite platelets at length scales of tens to hundreds of microns. The thermal and electrical conductivity of composites


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4

10 Agari Model Coefficients

LLDPE/EG APPW/GNPs Agari fit(LLDPE/EG) Agari fit(APPW/GNPs) Maxwell-Eucken model

3

C1 0.98 0.96

6

(b)

C2 0.74 0.86

10

5

10

4

10

3

10

2

0.03

G' (Pa)

Thermal conductivity (W/mK)

(a)

2

0.59 LLDPE LLDPE/EG (16.5vol %) APPW-S3M APPW/EG-S3M (16.5vol%)

0.39

1 10

1

10

0

1.07

0

0

5

10

15

20

EG content (vol%)

25

30

-2

-1

10

35

10

0

10

1

10

2

10

Frenquency (rad/s)

-1

10

4

(c)

(d)

-3

10

Thermal conductivity (W/mK)

Electrical conductivity (S/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5

10

Conductive

-7

10

-9

10

Insulation

-11

10

LLDPE/EG APPW/GNPs

-13

10

-15

10

0

5

10

15

20

25

30

35

3

2

LLDPE/EG APPW/GNPs LDPE/LTEG[38] HDPE/EG[39] HDPE/graphene nanoplatelet [40] HDPE/xEG[41]

Insulation

1 Conductive

0 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 10 10 10 10 10 10

EG content (vol%)

Electrical conductivi (S/cm)

Fig.6 (a) The model fitting curves and the corresponding thermal conductivity of LLDPE/EG and APPW/GNPs composites. (b) Dynamic storage modulus(G’) of composites. (c) Electrical conductivity of LLDPE/EG and APPW/GNPs composites, and (d) comparison of the thermal conductivity and electrical conductivity values reported for polyethylene/EG composites45-48(EG in composites is fixed at 17vol%).

In this section, the thermal and electrical conductivity of composites is presented. Fig.6a shows the experimental and theoretical model predicted thermal conductivity for LLDPE/EG and APPW/GNPs composites with different EG loading. It is evident that the thermal conductivity of composites increases steadily with the increasing of EG content in both cases. Meanwhile, we note that the thermal conductivity values of APPW/GNPs composites were higher than those of LLDPE/EG composites at all EG loadings, for example reaching 1.7W/mK and 1.2W/mK with 17vol% EG, respectively. Based on the suspension of spherical non-interacting particles in a diluent matrix, the Maxwell-Eucken model

49

is found to in good agreement with

experimental values of LLDPE/EG composites at low filler content (

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