thermoplastic elastomer nanocomposites with improved wear properties for 3D printing

Journal Pre-proof Exfoliated graphene/thermoplastic elastomer nanocomposites with improved wear properties for 3D printing Hyerin Jeon, Youn Kim, Woong-Ryeol Yu, Jea Uk Lee PII:

S1359-8368(19)33254-8

DOI:

https://doi.org/10.1016/j.compositesb.2020.107912

Reference:

JCOMB 107912

To appear in:

Composites Part B

Received Date: 9 July 2019 Revised Date:

16 October 2019

Accepted Date: 23 February 2020

Please cite this article as: Jeon H, Kim Y, Yu W-R, Lee JU, Exfoliated graphene/thermoplastic elastomer nanocomposites with improved wear properties for 3D printing, Composites Part B (2020), doi: https:// doi.org/10.1016/j.compositesb.2020.107912. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Exfoliated graphene/thermoplastic elastomer nanocomposites with improved wear properties for 3D printing Hyerin Jeon a, b, †, Youn Kim b, †, Woong-Ryeol Yu a,*, Jea Uk Lee b,* a

Department of Materials Science and Engineering, Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea

b

Carbon Industry Frontier Research Center, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, South Korea

Abstract

Although three-dimensional (3D) thermoplastic elastomer printing has been studied, the unsatisfactory mechanical properties of 3D-printed elastomers, especially their substandard wear characteristics, make it difficult to use them in industrial products or processes. In this study, thermoplastic elastomer nanocomposites with improved wear properties were fabricated using thermoplastic polyether elastomer (TPEE), with surface-modified carbon black (CB), or electrochemically exfoliated graphene through multiple extrusion processes. †

These authors contributed equally.

*

Corresponding author: Tel: +82-2-880-9096. E-mail: [email protected] (W. R. Yu)

*

Corresponding author: Tel: +82-42-860-7392. E-mail: [email protected] (J. U. Lee)

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The surface-modified CB/TPEE composite showed about four times more wear resistance and 26% improvement in tensile strength as compared to bare TPEE resin. The graphene/TPEE composite with only 1 wt% graphene exhibited an elevenfold increase in wear resistance and 43% improvement in the tensile strength owing to the high dispersibility and lubricating effect of the two-dimensional graphene filler. Graphene/TPEE composites were extruded into filaments for 3D printing. Three-dimensional printed products made from the nanocomposites have much higher wear resistance than 3D products of bare TPEE resin, demonstrating that graphene and TPEE nanocomposites are well suited for manufacturing a wide variety of complex electronic and mechanical components with excellent wear characteristics.

Keywords: Polymer-matrix composites (PMCs); Thermoplastic resin; Mechanical properties; 3D printing

1. Introduction Three-dimensional (3D) printing, also known as additive manufacturing (AM) or rapid prototyping (RP), has attracted enormous interest in recent years because this technology allows one to design a wide range of prototypes or create functional components with complex geometries that are very difficult to manufacture by conventional methods [1, 2]. Since Charles Hull first invented the 3D printing technique in 1984, several approaches such as stereo lithography (SLA), fused deposition modeling (FDM), and selective laser

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sintering (SLS) have been developed [3]. Among these methods, FDM, which was invented and developed by Stratasys, Inc. in the early 1990s, is the most widely used technique because of its reliability, simplicity, and cost effectiveness. FDM is a filament-based printing process in which materials in a filament form are fed into a heating nozzle where the filament is melted, extruded, and deposited onto a build plate to generate 3D features in a layer-by-layer fashion [4]. Until now, only a limited number of materials such as thermoplastics and select engineering plastics have been utilized as feedstock filaments for FDM. Despite the potential industrial demand, 3D printing with thermoplastic elastomers has not had much study. Thus, the mechanical properties and extended applications for 3Dprinted elastomers remain limited [5, 6]. In recent years, two main processes have improved the mechanical properties of fabricated FDM products: (i) blending in different polymer resins and (ii) adding reinforcing fillers into the polymer. At first approach, blending two or more polymers is a well-established practice for creating new materials with differing physical properties. For example, Advincula et al. [4] blended thermoplastic polyurethane (TPU) with poly(lactic acid) (PLA) by a solvent mixing process and obtained a more mechanically robust material for printing 3D biomaterial scaffolds for tissue engineering applications. However, in the case of immiscible polymer-blends, the development of FDM products and mass production of polymer-blend filaments requires the coextrusion of multipolymer materials. Unfortunately, this is a time-consuming and technically complex process, especially due to the immiscibility. The second approach involves the addition of reinforcing fibers or particles to the thermoplastic matrix in order to improve the mechanical properties of 3D-printed products.

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Many research groups have incorporated carbon or glass-based short fibers into the resin matrix to serve as an FDM feedstock [5, 7]. However, during the preparation of composite filaments and the 3D printing step, breakage of the short fibers occurs. This affects the mechanical properties of the final composite part because the properties of the short fiberreinforced composites depend significantly on the fiber length distribution and fiber orientation in the final product [5, 8, 9]. Other researchers have applied carbon-based nanomaterials in FDM technology to improve the mechanical properties or achieve new functions of 3D-printed products. The addition of 10 wt% carbon nanofiber [10] or 10 wt% multiwalled carbon nanotubes [11] resulted in a 39 and 7.5% increase in the tensile strength of 3D printing composites, respectively, compared with those of bare polymer products. However, both composite products exhibited reduced elongation and were more brittle [3]. Lin et al. [12] reported a 62.2 and 12.8% increase in the tensile strength and elongation at the break, respectively, by adding only 0.2% graphene oxide to a photopolymer for SLA printing. Although basic mechanical properties of the printed composites such as the strength, modulus, and elongation at the break were improved by controlling the dispersion and orientation of the reinforcing carbon fillers in the host matrix, only a few studies have investigated their wear properties [13, 14], which are some of the most critical factors that determine the lifetime of 3D printing components in electronic and mechanical equipment. Conventionally, various wear/scratch-resistant materials have been developed, including carbon black [15], diamond-like carbon films [16], carbon nanotubes [17], wollastonite [18], talc [19], and nanoclay [20, 21]. Among them, carbon black has been widely used as an wear-resistant reinforcing filler, and the surface modification of carbon black and dispersing agents has been studied [22, 23]. Although it is possible to improve the

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dispersibility of a carbon filler in a polymer matrix through surface treatment, in order to optimize the wear performances of composites, it is also necessary to study the structural aspects of the carbon filler as well as its dispersion state. In this work, nanocomposites of a thermoplastic polyether elastomer (TPEE) and various carbon fillers with excellent wear properties were prepared via a mass-producible multiextrusion processes to serve as 3D printing feedstock (Scheme 1). A commercially available thermoplastic polyether elastomer, which has been widely used in industry but rarely applied as an FDM filament, was chosen as the resin matrix. To improve the wear resistance of the TPEE matrix, pristine carbon black, surface modified carbon black, and electrochemically exfoliated graphene (EEG) were prepared and incorporated as reinforcing fillers. Carbon/TPEE composites with only 1 wt% carbon loading, which is an ideal concentration of the carbon filler for high-speed 3D printing that works without clogging the printer nozzle with nanocarbon aggregates [24, 25], were prepared in order to observe the dispersion and structural effects of the carbon fillers on the mechanical and wear properties of the nanocomposites. Finally, the EEG/TPEE composite was extruded into filaments and printed to 3D structures to examine its applicability to small components of electronic and mechanical equipment.

2. Materials and method 2.1. Materials Carbon black with particle size of 30–50 nm, which was manufactured for high wear resistance, was obtained from OCI Inc. (N 330, South Korea). Graphite foil (99.8%) with a thickness of 0.5 mm was purchased from Alfa Aesar (United States). The stainless steel

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plate was obtained from 4Science (South Korea). Thermoplastic polyether elastomer with a melt flow index of 11 g/10 min and melting point of 210 °C was provided by Samyang, Inc. (TRIEL 5550, South Korea). All other reagents were purchased from Samchun, Inc. (South Korea) and used without further purification.

2.2. Surface modification of carbon black A 1,000 mL two-neck flask connected to a condenser was filled with 10 wt% aqueous nitric acid solution (500 mL), carbon black (5 g), and a stir bar. The mixture was stirred for 24 h at 150 °C to ensure the surface modification of the carbon black. After this treatment, the carbon black was thoroughly washed with deionized water until a neutral pH was achieved. The product was dried in a vacuum oven at 120 °C for 24 h.

2.3. Electrochemical exfoliation of graphite Graphite was electrochemically exfoliated following a procedure previously reported by our group (Fig. S1) [26]. Briefly, graphite foil (35 mm × 75 mm × 0.5 mm) and a stainlesssteel plate (100 mm × 150 mm × 0.5 mm) were used as working and counter electrodes, respectively, and 0.5 M aqueous ammonium sulfate solution was used as the electrolyte. Both electrodes were placed in parallel at a constant distance of 2 cm throughout the exfoliation process. A positive voltage (+10 V) was applied to the graphite electrode using a DC power supply (ITECH IT6952A, South Korea) for 5 min at room temperature. After the exfoliation of graphite was completed, the product was collected by vacuum filtration

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and thoroughly washed with deionized water to remove the residual salts. The resultant EEG was dried in a vacuum oven at 120 °C for 24 h.

2.4. Preparation of carbon/TPEE nanocomposites The TPEE was dried in a vacuum oven at 70 °C for 24 h to avoid moisture-induced degradation. Then, nanocomposites loaded with 1 wt% carbon filler were prepared via a multi-extrusion process (Fig. 1). The melt-compounding was carried out using a single screw extruder (3D Factory, Inc., South Korea, ϕ = 20 mm, L/D = 27) and twin-screw extruder (BA-19, Bautek, Inc., ϕ = 19 mm, L/D = 40). First, each carbon filler such as carbon black, surface-modified carbon black (a-CB), and EEG was premixed with TPEE using the single screw extruder. The temperature profile of the extruder from Zone 1 to Zone 4 was set at 80, 220, 250, and 240 °C, respectively. The screw speed was fixed at 150 rpm. Then, the product strands were chopped with a pelletizer (3D Factory, Inc., South Korea) and compounded through a twin-screw extruder to evenly disperse the carbon filler in the polymer matrix. The temperature profile of the extruder from Zone 1 to Zone 6 was set at 120, 235, 245, 250, 245, and 240 °C, respectively. The screw speed was fixed at 110 rpm. The extruded strands (average diameter = 1.75 mm) were cooled in a water bath and then used as such for 3D printing or pelletized for fabricating specimens. For abrasion tests, disc-shaped nanocomposite specimens with a diameter of 100 mm were prepared by compression molding.

2.5. Characterization of carbon filler and nanocomposites

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Functional groups of the carbon fillers were investigated by Fourier-transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Fisher Scientific, Inc.) in the range of 500– 4000 cm-1. The C/O ratio and the content of functional groups in the carbon fillers were determined by X-ray photoelectron spectroscopy (XPS, Kratos, Axis Nova) using monochromatic Al Kα radiation. The carbon fillers were investigated by Raman spectrometry (Bruker, Senterra). The thickness and shape of the EEG sheets were determined by atomic force microscopy (AFM, Bruker, Nanoscope). The surface morphology of the carbon fillers and nanocomposites were investigated by scanning electron microscopy (SEM, COXEM, CX-200TA) and field-emission SEM (FE-SEM, Tescan Mira 3 LMU FEG). Abrasion tests of the nanocomposites were carried out using a universal ring-plate abrasion tester (Taber, Model 5132) following ASTM D4060; two abrasive wheels (CS-17) were used as the counterparts. The nanocomposite specimens were fastened on the tester as the countertops. All experiments were conducted under ambient conditions (room temperature, humidity: 8–15%, total cycle: 1000 r, rotational speed: 60 r/min, normal load: 1000 g). Then, the wear loss of the nanocomposites was evaluated by calculating the weight difference before and after the test. The 3D profiles and surface roughnesses of the nanocomposites after the abrasion test were obtained using an Alpha step profilometer (Bruker, DektakXT Stylus Profiler). The tensile strength of the nanocomposites was measured by a tensile test based on ASTSM D638V using a universal tensile machine (5882, Instron) with a load cell of 100 kN. Five specimens of each nanocomposite were prepared by punching the nanocomposite disk in a standard dogbone shape. The reported mechanical parameters of the nanocomposites were the average values from five specimens of each composite. Pencil scratch tests were

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performed using ASTM D3360. 6H to 6B pencils were sharpened and mounted on the holder at an angle of 45° and affixed by a screw. The 3D printed samples (70 × 70 mm2) were fixed to the table with double-sided tape, and scratches of more than 5 cm length were obtained by manually pushing the holder forward. All scratches were investigated by optical microscopy, and samples of interest were further analyzed by SEM.

2.6. Three-dimensional printing of carbon/TPEE nanocomposites Three-dimensional products were printed by feeding the extruded filaments into a commercial desktop FDM (Edison Pro, Rokit, Inc., South Korea). During printing, the nozzle temperature, bed temperature, printing speed, and layer height were set at 260 °C, 120 °C, 100 mm/s, and 0.15 mm/layer, respectively. The 3D wheel was printed with the quality settings of 100% for the infill parameter, which is a characteristic of the solidity of the object. The thickness of the shells was set to 0.8 mm. During printing, the nozzle temperature, bed temperature, printing speed, and layer height were set as 260 °C, 120 °C, 100 mm/s, and 0.15 mm/layer, respectively.

3. Results and discussion 3.1. Preparation and characterization of carbon fillers The morphological features of CB, a-CB, and EEG were observed by SEM (Fig. 2). Pristine CB had a branched structure composed of spherical particles, with a particle size of approximately 30–50 nm (Fig. 2a). a-CB showed a slightly smoother surface without significant damage to the particle surface despite the acid treatment (inset of Fig. 2a, b). On

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the other hand, the EEG sheet had a two-dimensional crumpled sheet structure with a lateral size of ~2–50 µm (Fig. 2c). To evaluate the thickness and roughness of the EEG sheets, several EEG sheets were selected randomly and investigated by AFM (Fig. 2d). A representative AFM image shows that EEG has a wrinkled and crumpled sheet structure with a thickness in the range of 5–9 nm. This revealed that EEG is composed of 6 to 11 layers of graphene sheets, considering that the thickness of monolayered graphene is ~0.8 nm [27]. The chemical structures of CB, a-CB, and EEG were identified by their FT-IR spectra (Fig. 3). All three samples exhibited strong bands at ~3440 and 1630 cm-1 (corresponding to O–H stretching and C=C stretching, respectively). Except for these two bands, CB showed almost no characteristic peaks in the spectrum, whereas new bands were observed at 1670, 1479, and 1269 cm-1 (corresponding to C=O stretching in the carboxyl acid group, O-H bending vibration, and C–O stretching vibration, respectively) in the spectrum of a-CB. It can be deduced that the surface of CB is functionalized with hydrophilic groups such as – COOH and –OH upon treatment with nitric acid. These hydrophilic groups induce electrostatic repulsion and steric hindrance, which are advantageous for disassembling large agglomerates into small particles in a polar medium [28]. Therefore, a-CB can be expected to have better dispersibility in a polar polymer matrix than pristine CB owing to the oxygen-containing functional groups introduced on its surface. Meanwhile, EEG showed bands at 1713 cm-1 (C=O stretching of carboxylic acid) as well as 1265 cm-1 (C–O stretching vibration), similar to a-CB. Furthermore, EEG exhibited a new band at 1041 cm-1 (C–O of phenolic groups) that was not found in CB or a-CB. It can

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therefore be inferred that EEG has a planar graphitic structure in which the aromatic rings incorporating hydrophilic functional groups are continuously connected. The carbon fillers were also analyzed by Raman spectroscopy to probe their carbon structures (Fig. 4). The Raman spectra of carbon materials have characteristic peaks: a D (disorder or defect) peak and a G (graphite) peak, which designate the breathing mode of the sp3 carbon atoms and in-plane vibrations of the graphene lattice, respectively. In general, the D and G peaks appear at approximately 1350 and 1580 cm-1, respectively, and the ratio of the intensity of these two peaks, ID/IG, can be used to evaluate the microstructure of the carbon material [29]. The spectra of CB and a-CB were similar, and both exhibited a D peak at ~1380 cm-1 and a G peak at ~1590 cm-1. In both spectra, the D and G peaks were broad and overlap; that is, they were not clearly separated, which is a commonly observed feature of the Raman spectrum of amorphous carbon. The ID/IG of CB was 1.0, and that of a-CB was slightly higher at 1.1, which suggests the presence of functional groups on the surface of CB and that its structural order decreased slightly owing to the acid treatment [30]. Unlike these two samples, in the Raman spectrum of EEG, the G peak at 1583 cm-1 was clearly separated from the D peak at 1355 cm-1, and ID/IG was 0.24, indicating that EEG has a highly ordered structure compared to those of CB and a-CB. In addition, EEG showed a 2D peak at ~2710 cm-1, which is a peak that appears in the Raman spectrum of multilayered materials owing to various scattering processes caused by several energy bands. In previous studies, Li et al. reported that the number of graphene layers can be determined by calculating the intensity ratio of the 2D-to-G peak, I2D/IG [31]. Their results indicated that I2D/IG of monolayered graphene is 1.5 or higher, and that of bilayered graphene is ~1.

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Further, the I2D/IG of multilayered graphene with four or more layers is 0.6 or less. From the Raman spectrum, we confirmed that I2D/IG of EEG is 0.38, which suggests that multilayered graphene was obtained through the electrochemical exfoliation of graphite. An XPS analysis was carried out to investigate the chemical elements on the surface of the carbon fillers. The XPS survey scan spectra of CB, a-CB, and EEG exhibited a strong C1s peak at 284.5 eV and O1s peak at 532.3 eV (Fig. 5a). Owing to the surface treatment of CB, the intensity of the C1s peak of a-CB decreased slightly, whereas that of the O1s peak increased as compared to those of CB. In fact, the C/O ratio of CB decreased from 34.0 to 9.98 after acid treatment (for a-CB), indicating that the proportion of oxygen-containing functional groups on the CB surface increased after acid treatment. Meanwhile, the C/O ratio of EEG (13.7) lay between those of CB and a-CB. The C1s peak of each carbon filler was deconvoluted into four peaks to determine which oxygen-containing functional groups existed on the surface (Fig. 5b–d). The four peaks deconvoluted from the C1s peak are assigned to C=C–C (284.5 eV), C–O (285.9 eV), C=O (288.0 eV), and O–C=O (289.8 eV) [32]. The atomic percentage of each functional group is summarized in Table 1. The C1s spectrum of a-CB shows an overlapped structure with an extended tail at a higher binding energy (288–290 eV), while that of CB shows a small tail. Clearly, the relative area of the carboxyl group (O–C=O) increased from 6.0 for CB to 11.0 for a-CB after surface functionalization, suggesting that acid treatment can significantly affect the chemical composition of CB, especially the number of carboxyl groups. This is consistent with the results of the FT-IR and Raman analysis. These hydrophilic groups are expected to increase the dispersibility of the carbon filler in the polymer matrix. On the other hand, EEG shows a relatively sharp C=C bond and a weak tail at higher binding

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energies (Fig. 5d). Functional groups with C–O bonds, such as epoxide or phenolic groups, are most abundantly found on the surface of EEG (Table 1).

3.2. Characterization of carbon/TPEE composites Cryogenically fractured surfaces of neat TPEE and each of the nanocomposites were observed by SEM to evaluate the dispersion state of the carbon fillers in the polymer matrix (Fig. 6). The morphology of bare TPEE polymer showed a relatively flat and smooth fractured surface, indicating typical brittle failure in the crystalline polymer (Fig. 6a) [33]. However, in the CB/TPEE composite with untreated CB, the fracture surface was lumpy owing to the formation of CB agglomerates (Fig. 6b). Large clusters of CB aggregates of various shapes and sizes were observed, suggesting poor interaction between CB and the polymer matrix [34]. In a previous study, a similar phenomenon was observed in silicanitrile butadiene rubber (NBR) composites without any surface modifications [35]. On the other hand, a-CB was dispersed uniformly in the TPEE matrix and showed a drastic reduction in the size of the agglomerates (Fig. 6c). This is attributed to the modification of the CB surface with functional groups, which increased the interfacial affinity between TPEE and the carbon filler. This is consistent with the expectations based on the aforementioned results. Meanwhile, in the EEG/TPEE composites, agglomerates of exfoliated graphene sheets were not observed, implying that the sheets were well dispersed (Fig. 6d). In addition, the filler embedded in the polymer matrix maintained its two-dimensional crumpled shape without damage, although it was subjected to multiple melt mixing processes. Lahiri et al. reported that such crumpled graphene sheets significantly improve the mechanical

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properties of composites owing to their mechanical strength and high flexibility because they easily deform when force is applied [36]. It can be inferred that EEG has a strong interfacial affinity with TPEE because the graphene sheets were not pulled out or dropped from the polymer matrix after fracturing [37]. Li and Jeong conducted a similar experiment using thermally exfoliated graphite [33]. Unlike our observation, they found agglomerates of graphite in their thermally exfoliated graphite/PET composites at 1 wt% loading. The improved dispersibility of our EEG/TPEE composite can be attributed to the moderate degree of oxidation of EEG during the electrochemical exfoliation [38]. Various hydrophilic functional groups such as carboxyl, epoxide, and phenolic groups contribute to the uniform dispersion of the EEG filler in the TPEE polymer matrix.

3.3 Wear and mechanical properties of carbon/TPEE composites An abrasion test was conducted to investigate the effect of carbon fillers of different dimensional structures on the wear properties of the nanocomposites (Fig. 7). The weight loss from bare TPEE and the composites was determined by measuring the weight of the specimens before and after the test. The average weight loss of the bare TPEE, CB/TPEE, a-CB/TPEE, and EEG/TPEE composites was about 25.1, 16.4, 6.2, and 2.3 mg, respectively. Generally, at a low filler content, the wear characteristics of the material are strongly dependent on the homogeneity of the dispersion of the nanofillers in the polymer matrix [39]. Therefore, a-CB/TPEE, in which a-CB was dispersed more uniformly in the TPEE matrix, exhibited a lower weight loss than CB/TPEE. In addition, EEG/TPEE exhibited the lowest weight loss: ~11 times lower than that of bare TPEE.

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The wear properties of the polymer composites could be enhanced by improving the composites’ hardness, stiffness, and compressive strength or by incorporating a lubricating transfer layer in the polymer matrix. Previous studies investigated the wear rates of composites composed of polyvinylchloride (PVC) and multilayer graphene; the wear rates of PVC composites with 1.2 wt% multilayer graphene were found to decrease by 56% compared to neat PVC [36]. It was rationalized that the multilayer graphene, which has a few layers of graphene sheets, can act as a solid lubricant and significantly increase the wear resistance of the composite. Likewise, EEG showed low friction characteristics owing to its layered structure with weak interlayer bonding that facilitates low shear resistance between atomic layers [40]. When abrasion occurred between the nanocomposites and the abrasive wheel, the composites were removed from the track, leaving wear debris. The debris formed a lubricating film on the worn surface of the composite owing to the low shear resistance of EEG present in the wear debris. This lubricating film contributed to the protection of the composite surface from wear, thus leading to improvements in the wear properties. The worn surface of the nanocomposites after the abrasion test was investigated by SEM (Fig. 8). Bare TPEE exhibited the roughest worn surface among the four materials: large chunks of wear debris were found on the surface owing to its poor wear resistance (Fig. 8a). During the abrasion tests, cracks formed at the weak points of the materials, and the wear debris broke off from the surface because of the adhesive force [41]. With the incorporation of CB and a-CB, the worn surfaces of the TPEE composites were less rough as compared to bare TPEE, and the size and amount of the wear debris decreased (Fig. 8b, c). In the case of the EEG/TPEE composite, the scratches on the worn surface were shallow and fine, and the

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size of the wear debris was too small to be observed (Fig. 8d). These results indicate that EEG is able to generate a lubricant film with low shear strength on the surface of the nanocomposite, which can prevent the material from being damaged by abrasion-induced crack formation [42, 43]. Misra and co-workers examined the scratch features of various nanocomposites using field emission scanning electron microscope [44, 45]. The scratched surface of neat polymers showed periodically parabolic and ripple-like scratch tracks with partial tearing of the materials, while the nanocomposites with high crystallinity, modulus, and yield strength exhibited discontinuous and zig-zag morphology with reduced tearing of the materials. This morphological change of the scratch tracks from parabolic and ripple-like to zig-zag shape was explained by the stick-slip motion between the tip and the surface of the material [46]. In our nanocomposites, a similar scratch behavior was observed; the wear debris (tearing of the polymers) and the surface roughness were significantly reduced. In addition, the scratches on the worn surface of the EEG/TPEE composites were shallow and fine. However, the morphological change from parabolic to zig-zag scratch tracks was not observed in our nanocomposites. In the previous scratch studies, the extent of scratch damage and resistance was quantified in terms of percentage elastic recovery using the scratch width, scratch hardness using the scratch depth, stress whitening, and average surface roughness [47, 48]. Since the measurement of the surface roughness is the most facile method in the laboratory, the worn surfaces of the nanocomposites after the abrasion test were also analyzed by Alpha step profilometer. The 3D profile images of the worn surface and the root mean square (RMS) roughness are shown in Fig. 9. From the 3D profile images, it can be deduced that the worn

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surface gradually turned smoother in the following order: bare TPEE, CB/TPEE, aCB/TPEE, and EEG/TPEE (Fig. 9a–d). These results show the same tendency as the SEM results. The RMS roughness of the nanocomposites decreased steadily in the same order (Fig. 9e). The RMS roughness of the EEG/TPEE composite was measured to be 0.436 nm, which is ~20% lower than that of bare TPEE, which has a roughness of 0.566 nm. From the 3D profiling analyses and RMS roughness of the worn surfaces after the abrasion test, it was further confirmed that EEG improves the wear properties of the TPEE elastomer owing to its self-lubricating performance. Tensile tests were performed on disc-shaped specimens of the nanocomposites to further analyze the effect of the carbon filler on the mechanical properties of the material. The stress and strain values at the break points as well as the stress-strain curves for bare TPEE and carbon/TPEE composites are presented in Fig. 10. The addition of 1 wt% of CB to the TPEE resin led to an increase in the tensile strength from 23 to 27 MPa and an increase from the strain at the break from 90 to 130%. The a-CB/TPEE composite showed a higher strength of 29 MPa and a strain at the break of 155% because the functional groups of a-CB could act as crosslinking agents in the TPEE matrix. The most improved strength (33 MPa) and strain at the break (180%) were observed for the EEG/TPEE composite because EEG satisfies most of the requirements for a reinforcing filler that enhances the mechanical properties of a composite. These properties include a high aspect ratio, high strength, uniform distribution in the host matrix, and good interfacial adhesion between the filler and matrix, which facilitate good load transfer from the matrix to the filler [49]. It has been reported that the reinforcement of polypropylene polymer with small amount (4–8 wt.%) of nanoclay results in an increase in crystallinity because of higher nucleation

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density induced by the clay particles, and thus improves the scratch-resistance of the resultant nanocomposites [45]. The higher scratch-resistance of high crystalline polymers is related to their high modulus and yield stress characteristics of the polymeric materials [50]. It has been also proposed that the resistance to mechanical deformation during the scratch deformation is strongly dependent on the mechanical properties [47]; during scratch deformation, when the tensile stress behind the scratch is greater than the tensile strength of the polymeric material, the plastic deformation occurs. Therefore, the increase of elastic modulus and yield strength leads to higher strain energy in the material that should be overcome by the applied load for elastic/plastic deformation to proceed, and has a positive influence on scratch resistance [51]. This phenomenon can be observed in metals where the plastic deformation is prevalent in a material with low modulus and yield strength and the following elastic recovery is low. In the same line of thinking, the EEG/TPEE composite can endure higher stress before scratch deformation due to its higher strength than the bare TPEE and other nanocomposites. This fact can explain why EEG filler added at only 1 wt% significantly improves the wear properties as well as the mechanical properties of the nanocomposites.

3.4. Three-dimensional printing of carbon/TPEE nanocomposites To use the nanocomposites in 3D printing, continuous filaments of bare TPEE and EEG/TPEE composite with a uniform diameter of 1.75 mm were fabricated. Fig. 11 shows photographs and SEM images of each filament. TPEE filament is white with a very smooth surface, while the filament of the EEG/TPEE composite is black, although a very small amount of EEG was loaded (~1 wt%). As seen in Fig. 11d, the filament based on the

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EEG/TPEE composite also exhibited a homogeneous surface without any defects and EEG aggregates owing to the good dispersibility of EEG in the TPEE matrix. A uniform diameter and smooth surface without defects and cracks are critical factors that enable a stable 3D printing process and ensure good quality 3D-printed products. Additionally, achieving a homogeneous dispersion of the carbon filler within the polymer matrix is a key factor in the development of printable nanocomposite filaments that do not clog the printer nozzle during high-speed 3D printing [52, 53]. As there is no information about the printing conditions for the TPEE homopolymer and carbon/TPEE composites, several printing runs were performed to determine the optimal printing conditions. TPEE exhibited a high degree of shrinkage during the printing process, resulting in its distortion and stripping from the heating bed of the 3D printer (Fig. S2). By varying printing conditions such as the nozzle temperature, heating bed temperature, and inner temperature of the printing space, we could stably print 3D products with both bare TPEE and EEG/TPEE composite at a very high printing speed of 100 mm/s. Since the EEG content in the nanocomposites was very low (1 wt%), the optimal printing conditions of bare TPEE were applicable to the printing of EEG/TPEE composites as well. Fig. 12 shows photographs and SEM images of 3D products fabricated with TPEE and EEG/TPEE composites. In order to evaluate the wear properties of the 3D-printed products, 3D wheel features were printed with filaments of bare TPEE and EEG/TPEE composite. Despite the high printing speed of 100 mm/s, no distortion or delamination were observed in either product (Fig. 12a, c). In the SEM images, it is observed that the EEG/TPEE filaments were well fused with each other and formed a solid 3D structure (Fig. 12d). Furthermore, as with the filament surface, no noticeable aggregates or defects were

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observed on the surface of the 3D-printed products fabricated with EEG/TPEE composites (inset image in Fig. 12d). This confirms that the EEG filler, which is highly compatible with the TPPE matrix, maintains its well-dispersed state under the high temperature and shear force of the 3D printing process. To evaluate the wear characteristics of the 3D products printed with EEG/TPEE composite filament, an abrasion test was performed by modifying the Taber abrasion test equipment: the two abrasive wheels were replaced with the 3D printing wheels, and sandpaper was fastened on the tester. Fig. 13 shows the features of the 3D wheels before and after the modified abrasion test. After 1000 cycles of testing, the 3D wheel printed with bare TPEE suffered significant wear, and its cogwheels were almost removed. In addition, the diameter of the 3D wheel of bare TPEE decreased from 5.0 to 4.6 cm owing to severe wear. On the other hand, the 3D wheel printed with EEG/TPEE composite filament experienced low wear even after the same number of cycles in the abrasion test. The cogwheels of the composite wheel remained intact, and the reduction in the wheel diameter was much less (1.6 mm) than that of the 3D wheel of bare TPEE. A similar result was observed in the pencil scratch test (Fig. 14). Three-dimensional printed samples were scratched with pencils ranging from 6H (hard) to 6B (soft) at an angle of 45° (Fig. 14a), and investigated by optical microscopy and SEM. Three-dimensional products printed with bare TPEE had a gouge hardness of 4B, indicating that the surface was removed by 6H-3B pencils. The OM and SEM images in Fig. 14b show the exemplary scratches by the 3B pencil. By contrast, 3D products printed with EEG/TPEE composite had a higher tolerance to pencil scratching: the hardness was found to be B, which means

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that only 6H-HB pencils could scratch the surface of the 3D products. An SEM image of the scratch test with a 3B pencil on the surface of the EEG/TPEE composite showed no abraded lead (Fig. 14c). Finally, optimized 3D printing conditions can produce small components for electronic and mechanical equipment using the EEG/TPEE composite (Fig. 15).

4. Conclusions We successfully prepared 3D nanocomposites composed of a thermoplastic polyether elastomer and three kinds of carbon fillers (CB, a-CB, and EEG) with excellent wear properties via scalable (multi-extrusion processes), cost-effective (only 1 wt% carbon filler loading), and fast 3D printing (100 mm/s) methods. From a variety of spectroscopic analyses, we confirmed that the surfaces of a-CB and EEG contained hydrophilic groups, resulting in their better dispersion in the TPEE resin than pristine CB. The surface-modified CB/TPEE composite showed about four times’ higher wear resistance and a 26% enhancement in the tensile strength compared to bare TPEE resin owing to the uniform dispersion of CB particles. The EEG/TPEE composite with only 1 wt% EEG loading exhibited about an elevenfold higher wear resistance and a 43% enhancement in the tensile strength. Furthermore, the wear resistance of the 3D products printed with the EEG/TPEE composite was much higher than that of the 3D products of bare TPEE resin. The improved wear and mechanical properties of the EEG/TPEE composite are attributed to the high dispersibility and lubricating effect of the robust two-dimensional EEG filler. Thus, 3D printing with EEG/TPEE composite can be used to fabricate various small components of

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electronic and mechanical equipment, which require sophisticated structures and excellent wear properties.

Acknowledgement This work was supported by the Principal Research Program (KK1913-10) of the Korea Research Institute of Chemical Technology (KRICT) and Technology Innovation Program (TS197-13R, Research on carbon/biocompatible polymer composites and 3D printer for antibacterial/abrasion resistive sports insole) funded by the Ministry of Trade, Industry & Energy (MI, Korea). This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP) (No. NRF-2015R1A5A1037627).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.compositesa.2016.xx.xxx.

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Scheme 1. Preparation of carbon/TPEE composites for application as 3D printing feedstock.

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Fig. 1. Preparation of nanocomposite of TPEE and carbon filler: (a) premixing and compounding via multi-extrusion process. Photographs of carbon filler (b) before and (c) after premixing with single screw extruder, (d) product obtained after compounding with twin screw extruder, and (e) disc-shaped nanocomposite specimen for abrasion test.

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Fig. 2. SEM images of carbon fillers: (a) CB, (b) a-CB, and (c) EEG. (d) AFM image of EEG sheets prepared by electrochemical exfoliation, which are in form of multilayered graphene.

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Fig. 3. FT-IR spectra of carbon fillers: CB, a-CB, and EEG.

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Fig. 4. Raman spectra and ID/IG and I2D/IG ratios of carbon fillers: CB, a-CB, and EEG.

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Fig. 5. (a) XPS survey scans of CB, a-CB, and EEG. Deconvolution of C1s peak of (b) CB, (c) a-CB, and (d) EEG.

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Fig. 6. SEM images of fracture surfaces of (a) bare TPEE, (b) CB/TPEE, (c) a-CB/TPEE, and (d) EEG/TPEE composites.

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Fig. 7. Weight loss of bare TPEE and nanocomposites filled with CB, a-CB, and EEG. Inset images show bare TPEE and nanocomposite specimens used for abrasion test.

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Fig. 8. SEM images of worn surfaces of nanocomposites: (a) bare TPEE, (b) CB/TPEE, (c) a-CB/TPEE, and (d) EEG/TPEE.

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Fig. 9. Surface 3D profile images of worn surfaces of nanocomposites: (a) bare TPEE, (b) CB/TPEE, (c) a-CB/TPEE, and (d) EEG/TPEE. (e) RMS roughness values of composites in comparison with that of TPEE.

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Fig. 10. (a) Stress vs. strain, (b) strain at break, and (c) stress at break values for bare TPEE, CB/TPEE, a-CB/TPEE, and EEG/TPEE composites obtained from tensile test results.

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Fig. 11. Photographs and SEM images of filaments of (a), (b) bare TPEE, and (c), (d) EEG/TPEE composite.

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Fig. 12. Photographs and SEM images of 3D-printed (a), (b) TPEE wheel and (c), (d) EEG/TPEE wheel. Inset images in (b) and (d) are optical microscope images of each wheel.

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Fig. 13. Photographs of 3D-printed (a) TPEE wheel and (b) EEG/TPEE wheel after modified abrasion test. Insets show wheels before test.

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Fig. 14. (a) Photograph of pencil scratch test. OM and SEM images of surface of 3Dprinted (b) TPEE and (c) EEG/TPEE samples scratched by 3B pencil.

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Fig. 15. Three-dimensional-printed gears for mechanical equipment based on EEG/TPEE composite.

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Table 1. Atomic percentage of oxygen functional groups of CB, a-CB, and EEG C=C–C at.% C–O at.% C=O at.% O–C=O at.% Sample (284.5 eV) (285.9 eV) (288.0 eV) (289.8 eV) CB 67.6 17.3 9.1 6.0 a-CB 65.4 16.8 6.8 11.0 EEG 66.57 20.94 6.06 6.43

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Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: