**Properties of Injection Molded High Density Polyethylene Nanocomposites Filled with Exfoliated Graphene Nanoplatelets**

Xian Jiang and Lawrence T. Drzal

*Michigan State University, Composite Materials and Structures Center, Department of Chemical Engineering and Materials Science, East Lansing, Michigan, USA* 

#### **1. Introduction**

250 Some Critical Issues for Injection Molding

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enhancing the performance of wood fibre-polypropylene composites. *Composites Part A: Applied Science and Manufacturing*, Vol.35, No.12, (December 2004), pp. This book chapter investigated the potential of using exfoliated graphene nanoplatelets, GNP, as the multifunctional reinforcement in high density polyethylene (HDPE) matrix. HDPE/GNP nanocomposites were fabricated by the conventional compounding method of melt-extrusion followed with injection molding. The mechanical properties, crystallization behaviors, thermal stability, thermal conductivity, and electrical conductivity of the resulting HDPE/GNP nanocomposites were evaluated as a function of GNP concentration. Results showed that HDPE/GNP nanocomposites exhibit equivalent flexural modulus and strength to HDPE composites filled with other commercial reinforcements such as carbon fibers (CF), carbon black (CB) and glass fibers (GF). But they have superior impact strength. By investigating the crystallization behavior of HDPE/GNP nanocomposites, it was found that GNP is a good nucleating agent at low loading levels and as a result can significantly increase crystallization temperature and crystallinity of HDPE. At high GNP loadings, however, the close proximity of GNP particles retards the crystallization process. The thermal stability and thermal conductivity of HDPE/GNP nanocomposites were significantly enhanced as a function of GNP concentration due to the excellent thermal properties of GNP. Meanwhile, results indicated that the percolation threshold of these nanocomposites prepared by the conventional melt - extrusion and injection molding is relatively high at around 10-15 vol% GNP loading. The high percolation threshold is mainly due to the sever GNP aggregation and platelets alignment during the processing conditions as verified by the morphology. To enhance their electrical conductivity and lower the percolation threshold, a wax coating method was introduced in this study which is proved to be efficient in improving the dispersion of GNP in HDPE which is responsible for the better electrical and mechanical properties in the resulting nanocomposites.

Polymeric nanocomposites have attracted research interest both in industry and in academia in recent years, they can be found useful in many applications such as electromagnetic interference (EMI) shielding devices, low power rechargeable batteries, electronic devices, light emitting diodes (LEDs), gas sensors, super capacitors and photovoltaic cells (Hussain et al., 2006; Vaia, 2003). Polymeric nanocomposites have represented a radical alternative to

Properties of Injection Molded High Density Polyethylene

thickness but the diameter is around 15 μm.

properties of these materials are detailed in Table 1.

molding.

**2. Experimental** 

**2.2 Processing methods** 

**2.1 Materials** 

Nanocomposites Filled with Exfoliated Graphene Nanoplatelets 253

GNP in HDPE by a wax coating method which is suitable for melt-extrusion and injection

In this book chapter, HDPE pellets with the trade name Marlex® HXM 50100 (Density 0.948 g/cm3, MW~ 230,000) were obtained from Chevron Phillips Chemical Company. Paraffin wax (max C30, Density 0.92 g/cm3, MW~ 500) with the melting point of 55°C was purchased from Sigma-Aldrich. GNP nanoplatelets were obtained from XG Science, Inc (www.xgsciences.com). There are two kinds of GNP particles used in this study. GNP-1 has the thickness of around 5-10nm and a platelet diameter of 1 μm; while GNP-15 has the same

Several commercial reinforcements and fillers were also combined with HDPE to make composites for comparison to the HDPE/GNP nanocomposites. They are: (1) CF-PAN based carbon fiber (PANEX 33 MC Milled Carbon Fibers, Zoltek Co), (2) CB-nanosize 'High Structure' carbon black (KETJENBLACK EC-600 JD, Akzo Novel Polymer Chemicals LLC), and (3) GF-chopped glass fiber (StarStran® LCF, Johns Manville Co.) . The physical

**(m2/g)** 

**Density (g/cm3)** 

**Filler Length (μm) Diameter (μm) Aspect Ratio Surface Area** 

Table 1. Geometrical and surface characteristics of various fillers.

GNP-1 <0.01 1 <100 100 2.1 GNP-15 <0.01 15 ~1500 40 2.1 PAN CF 175 7.2 ~24 16 1.8 GF 51 (mm) 13 ~4000 NA 2.6 CB 0.4–0.5 0.4–0.5 1 1400 1.8

Melt - extrusion of HDPE/GNP nanocomposites was carried out in a DSM Micro 15cc Compounder, (Vertical, co-rotating, twin-screws micro-extruder) operating at 220°C for 5 minutes at a screw speed of 100rpm. The composite melt was then transferred to a Daca Micro injector with the Tbarrel=220°C and Tmold=90°C. The injection pressure applied for the injection molding of flexural coupons was at 0.6MPa. Round disks (thickness ~1.5mm, diameter ~25mm) were also injection molded for thermal conductivity test under the same injection

To enhance the dispersion of GNP in the HDPE matrix, a wax coating technique was applied in this study, which uniformly coated the surface of GNP with wax. For this method, wax was first dissolved in xylene at around 60°C and GNP was added afterwards. Sonication was then applied for 30 minutes at 100W to initially break down the GNP aggregates and to ensure a uniform wax coating. The resultant mixture was poured into an aluminum pan and left in a hood at room temperature to evaporate the solvent. After xylene was completely evaporated, the wax coated GNP was further dried in a vacuum oven

conditions. The melt - extrusion and injection molding systems are shown in the Fig. 1.

conventional filled polymers or polymer blends. The difference between the conventional fillers and nano-fillers can be explained that nano-reinforcements must have at least one dimension in the nanometer range.

The advantages of using nano-fillers have been summarized in the work by Griffith and Weibull (Griffith, 1920; Weibull, 1951). They claimed that the smaller the reinforcement is, the stronger it becomes. It is believed that the failure of macroscopic specimens is mainly due to the presence of critical size defects. As the material size decreases, the probability of critical size flaws also reduces which allows the material to approach its intrinsic strength. Piggot and Hussain (Hussain et al., 2006; Piggott, 1980) concluded that nano-fillers are more effective reinforcements than their conventional counterparts because a smaller amount of nanoparticles could result in a larger enhancement in the mechanical, electrical, and thermal properties of the polymer matrix.

Recently, carbon nanotubes (CNTs) have been extensively explored as the nanoreinforcement in polymers due to their exceptional mechanical, electrical and thermal properties (Hermant et al., 2009). Many papers have appeared in literature discussing the CNTs-filled polymeric nanocomposites with excellent mechanical, electrical and thermal properties for numerous applications (Balasubramanian & Burghard, 2006; Kim et al., 2005; Singh et al., 2006; Yao et al., 2006). However due to the poor yield and costly fabrication and purifying process, the price of CNTs in the market is still high, which limits the commercial applications of CNTs to date (Kim & Drzal, 2009).

To search for an alternative nano-filler which exhibits the superior properties of CNTs but have a low cost and easy processing, graphite based materials are gaining more and more research attention. Polycrystalline graphite is a material that consists of extended networks of sp2-hybridized carbons in a planar layered structure (graphene), leading to excellent thermal and electrical conductivity within this graphitic basal plate. It is found that exfoliation of these graphite layers and dispersion into polymers offers the potential to bring multifunctionality to the host polymers. Furthermore, research has shown that fully exfoliated graphite nanosheets are as effective in conductivity enhancement as CNTs due to their two-dimensional lattice of sp2-bond carbon and extremely high aspect ratio (Xie et al., 2008). Based on this principal, a new form of graphite based nano-filler, exfoliated graphene nanoplatelets, has been under investigation in the Drzal group for several years (Fukushima, 2003; Kalaitzidou, 2006). Research has shown that this nano-particle is a potential alternative to other nano-reinforcements such as nano-clays and CNTs since it combines the low cost and layered structure of nano-clays and the superior thermal and electrical properties of CNTs (Jiang & Drzal, 2010,2009,2011; Kalaitzidou et al., 2007). The objective of this book chapter is to: (1) determine the mechanical properties, i.e., flexural strength, flexural modulus, and impact strength of HDPE/GNP nanocomposites made by melt-extrusion and injection molding and their comparison to the HDPE composites reinforced by commercially available fillers such as glass fiber, carbon fiber and carbon black; (2) investigate the crystallization behavior of HDPE with the presence of GNP; (3) explore the thermal stability, thermal conductivity, and electrical conductivity of injection molded HDPE/GNP nanocomposites; (4) observe the morphology of HDPE/GNP nanocomposites to determine the dispersion and orientation of the nano-reinforcement under the melt-extrusion and injection molding conditions; (5) enhance the dispersion of GNP in HDPE by a wax coating method which is suitable for melt-extrusion and injection molding.
