M. A. ERDEN et al.: MECHANICAL PROPERTIES OF GRAPHENE-NANOPARTICLE AND CARBON- ... 785–789 MECHANICAL PROPERTIES OF GRAPHENE-NANOPARTICLE AND CARBON-NANOTUBE-REINFORCED PE-MATRIX NANOCOMPOSITES MEHANSKE LASTNOSTI NANODELCEV GRAFENA IN OGLJIKOVIH NANOCEV^IC KOT OJA^ITVE V POLIETILENSKI MATRICI NANOKOMPOZITOV Mehmet Akif Erden 1* , Yasin Akgul 2 , Ozge Kayabas 1 , Hayrettin Ahlatci 2 , Kerim Cetinkaya 3 , Fatih Huzeyfe Ozturk 3 1 Karabuk University, Manufacturing Engineering, 78050 Karabuk Merkez, Turkey 2 Karabuk University, Metallurgical and Materials Engineering, 78050 Karabuk Merkez, Turkey 3 Karabuk University, Industrial Design Engineering, 78050 Karabuk Merkez, Turkey Prejem rokopisa – received: 2019-04-01; sprejem za objavo – accepted for publication: 2019-05-17 doi:10.17222/mit.2019.071 In this study, graphene-nanoplatelet (GNP) and carbon-nanotube (CNT) reinforced nanocomposites were produced with pressure molding. 1 w/% of GNPs and 1 w/% of CNTs were separately added to a polyethylene (PE) matrix and 0.5 w/% of both reinforcements was also jointly added to the PE matrix. In this manner, the effects of GNPs and CNTs on the mechanical properties of PE were compared and the synergistic effect was investigated. In order to examine the mechanical properties, a tensile test, a hardness test and a wear test were applied to the produced samples. Also, fracture surfaces and wear surfaces were investigated with a scanning electron microscope (SEM). It was observed that the graphene was distributed homogeneously in the polyethylene matrix. Thus, the GNP-containing samples showed better mechanical properties than the CNT-containing samples. Keywords: polymer-matrix composites, compression molding, high-density polyethylene, graphene, carbon nanotube V {tudiji avtorji opisujejo izdelavo polimernih kompozitov, oja~anih z grafenskimi nanoplo{~icami (GNPs) in ogljikovimi nanocevkami (CNTs). Postopek izdelave je potekal z oblikovanjem v kovinskem modelu pod tlakom. V prvo polietilensko (PE) matrico so dodali 1 mas. % GNPs, v drugo 1 mas. % CNTs in v tretjo po 0,5 mas. % obeh oja~itvenih faz. Nato so dolo~ili mehanske lastnosti izdelanih kompozitov in ugotavljali vpliv dodatka GNPs in CNTs ter raziskovali sinergijski u~inek obeh. V ta namen so na izdelanih preizku{ancih izvedli natezni preizkus, meritve trdote in odpornosti proti obrabi. Prelomne in obrabne povr{ine so prav tako pregledali pod vrsti~nim elektronskim mikroskopom (SEM). Rezultati raziskave so pokazali, da je grafen enakomerno porazdeljen po PE matrici. Zato so imeli vzorci, ki so vsebovali GNPs bolj{e mehanske lastnosti kot vzorci, ki so vsebovali CNTs. Klju~ne besede: kompoziti s polimerno matrico, oblikovanje pod tlakom, polietilen z visoko gostoto, grafen, ogljikove nanocevke 1 INTRODUCTION Thermoset resins are widely used in commercial applications in the fabrication of polymer-matrix compo- sites. However, these materials have disadvantages such as the lack of recycling and a low shaping capacity. 1 For this reason, commercial applications involving thermo- plastic-matrix composites have been started in recent years. Therefore, many academic studies on the subject have been conducted. 2 Nano-sized reinforcements have become popular for these thermoplastic composites because they allow us to change the properties of the matrix with very low loadings (<2 /%). 3 Graphene nanoplatelets (GNPs) and carbon nano- tubes (CNTs) are promising nano-sized reinforcement materials due to their unique mechanical properties. 4 GNPs, which are two-dimensional materials with a one-atom-thick planar sheet of sp 2 -bonded carbon atoms, are densely packed in a honeycomb crystal lattice. 5 They have high Young’s modulus (1 TPa) and high fracture strength (125 GPa). 6 CNTs, which were discovered by K. Lau et al., 7 have diameters ranging from 1 nm to 100 nm, lengths of up to millimeters and a density 8 of 1.3 g/cm 3 . CNTs are created by rolling a single graphene sheet seamlessly to form a cylinder. 9 Their ultimate break stress is nearly 200 GPa and elastic modulus is 1 TPa. 10 A HDPE (high-density polyethylene)/GNP blend was mixed homogeneously by A. J. Bourque et al. 11 using a solution during the blending. Mechanical properties of the GNP-HDPE composites were significantly improved up to 15 w/% GNP loadings. W. Tang et al. 12 fabricated HDPE/CNT composites with (0, 1, 3 and 5) % nanotube amounts. It was found that the stiffness and peak load increase with an increasing MWCNT amount. For another study, HDPE/CNT composites were produced by Materiali in tehnologije / Materials and technology 53 (2019) 6, 785–789 785 UDK 67.017:661.666:620.3 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(6)785(2019) *Corresponding author's e-mail: makiferden@karabuk.edu.tr (Mehmet Akif Erden) S. Kanagaraj et al. 13 via injection molding. Results showed that although the melting point and oxidation temperature of the composites were not affected by the addition of CNTs, the crystallinity was increased. In this study, PE-matrix nanocomposites were fabri- cated with compression molding. The effects of GNPs and CNTs on the mechanical and tribological properties of PE were compared. Also, the synergistic effect of these reinforcements was investigated. 2 EXPERIMENTAL PART The graphene nanoparticles used in this study had a surface area of 750 m 2 /g and a thickness of 5–8 nm while the multi-walled carbon nanotubes had a thickness of 9.5 nm and a surface area of 250 m 2 /g. Carbonaceous reinforcements were purchased from Nanografi (Turkey). High-density polyethylene (HDPE), used as the matrix in the study, had a density of 0.91 g/cm 3 and was supplied from PETKÝM. The granular polymer and reinforcements (in powder form) were being stirred in a turbula mixer for 1 hour. In this step, the granular polymer used as the matrix material and the nanopowders used as the reinforcement material were solid, so the mixing process can be called solid-solid mixing. The mixture was filled into a mold and the mold was heated to 170 °C. This mold tempera- ture was then kept constant for 5 min to provide for homogenous heating of the mold. Then, pressing was applied at 20 MPa. The mold was cooled in air and the samples (Figure 1) were removed. Tensile tests were conducted with Shimadzu Trapezium Single with a 50-kN capacity at a 5-mm/min speed. A UTS-10 Tribometer test device was used for the corresponding wear tests. During a wear test, an AISI 52100 steel ball was used and the stroke distance was kept at 10 mm. The applied loads were 30 Nand 60 N. The Brinell hardness test was performed on a QNESS Q250M hardness measurement device under a 2500-g load. In order to determine the hardness of the material, five measurements were taken for each sample. The fractured surfaces and wear surfaces were examined with a Zeiss UltraPlus SEM after coating the samples with the Au-Pd alloy. 3 RESULTS AND DISCUSSION Stress-strain graphs of the samples are shown in Fig- ure 2. It can be concluded that both carbon-based reinforcements increased the strength of pure PE. Comparing GNPs with CNTs, it was observed that GNPs increased the tensile strength more than CNTs. Figure 3c shows that carbon nanoparticles agglome- rated in the samples containing CNTs according to the fracture surfaces. This adversely affected the mechanical properties. This can be attributed to the facts that the samples were prepared with solid-solid mixing and the CNTs agglomerated due to their van der Vaals bonds. 14 From Figure 3a, it is clear that there is no porosity or homogenous distribution in the PE/GNP sample compared to the other samples. The reason for this is the fact that graphene has weaker van der Vaals bonds, so the GNPs show less agglomeration in the matrix. Also, it can be said that there are some porosities, which cause a decrease in the mechanical properties of the matrix of the PE/GNPs+CNTs sample (Figure 3b). As shown in Table 1, although carbon-based rein- forcements increased the strength of PE, they caused a decrease in the elongation values of PE. This is because M. A. ERDEN et al.: MECHANICAL PROPERTIES OF GRAPHENE-NANOPARTICLE AND CARBON- ... 786 Materiali in tehnologije / Materials and technology 53 (2019) 6, 785–789 Figure 2: Tensile properties of the composites and pure HDPE Figure 1: Produced samples when the material is under load, the nanoparticles restrict the movement of the polymer chains and cause the load to be transferred from the polymer to these nanofillers. 15 As mentioned above, there are less agglomeration and fewer pore defects in the structure of the PE/GNP sample. Thus, the PE/GNP sample has higher elongation values compared to the other composites. Table 1: Mechanical properties of the samples Sample Ultimate tensile strength (MPa) Elongation (%) Hardness (Brinell) PE 18 490 6.7±.0.48 PE/CNT 23 28 7.2±0.30 PE/GNP 28 75 7.5±0.36 PE/CNT+GNP 24 32 6.8±0.25 M. A. ERDEN et al.: MECHANICAL PROPERTIES OF GRAPHENE-NANOPARTICLE AND CARBON- ... Materiali in tehnologije / Materials and technology 53 (2019) 6, 785–789 787 Figure 4: Wear rates of the samples under: a) 30-N load, b) 60-N load Figure 3: Fracture surfaces of: a) PE/GNPs, b) PE/CNTs+GNPs, c) PE/CNTs Figure 5: SEM images of worn surfaces of the samples The hardness results for the samples are given in Table 1. The addition of 1 w/% of GNPs to the matrix increased the hardness value of polyethylene by 12 % while the addition of 1 w/% of CNTs to the matrix increased the hardness value of polyethylene by 7.5 %. This increase in the hardness can be explained with the addition of harder particles to the matrix. 16 The hardness of the sample containing 0.5 % of GNPs and 0.5 % of CNTs is almost equal to the hardness value of pure polyethylene due to the agglomeration of CNTs and the porosity in the matrix. Wear rates of the composites under 30-N and 60-N loads are given in Figure 4. The results of the wear and hardness tests were directly proportional. This can be explained with the fact that as the hardness of the material increases, the resistance to abrasion increases. 17 The wear rate of PE/GNPs with the best abrasion resistance was 5.9 mm 3 /104 m at the 30-N load. However, this value was increased to 7.3 mm 3 /104 m for the 60-N load. The increase in the wear rate was observed for all the samples as a result of the increased load. This can be attributed to the fact that repeated higher loads on the surfaces of samples can form higher friction forces. 18 As seen in Figure 5, as a result of the wear test, there are pieces ruptured from the samples. Therefore, it can be said that the abrasive-wear mechanism was formed. When the load was increased to 60 N, parallel scratches were formed on the sample. However, the resulting straches are more clearly seen for the PE/CNTs+GNPs sample. The reason for this is that wider scratches can be seen on the materials with a low wear resistance. 19 This is also indicative of the abrasive-wear mechanism. 4 CONCLUSIONS In this study, carbon-nanoparticle-reinforced poly- ethylene-matrix nanocomposites were successfully produced with pressure molding. The results of the characterization of the produced samples can be sum- marized as follows: The additions of GNPs and CNTs led to an increase in the tensile strength of HPDE. However, the compari- son showed a greater increase in GNPs/PE compared to CNTs/PE. The reason for this is that during solid-solid mixing, CNTs form clumps due to van der Waals bonds. Carbon nanoparticles were found to be agglomerated in the samples containing CNTs when fracture surfaces were examined. GNPs showed a more homogenous dis- tribution in the PE matrix than CNTs. The addition of 1% of GNPs and 1% CNTs to the PE matrix increased the hardness of pure polyethylene by 12 % and 7.5 %, respectively. 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