**2. Experimental**

#### **2.1 Materials**

252 Some Critical Issues for Injection Molding

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

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

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

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

dimension in the nanometer range.

properties of the polymer matrix.

applications of CNTs to date (Kim & Drzal, 2009).

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 thickness but the diameter is around 15 μm.

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 properties of these materials are detailed in Table 1.


Table 1. Geometrical and surface characteristics of various fillers.

#### **2.2 Processing methods**

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

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

Properties of Injection Molded High Density Polyethylene

at 5vol% as a reference.

the following equation:

**2.3 Characterization techniques** 

5vol% and the weight ratio between wax and GNP-15 is 20:80.

melting enthalpy of the polymer matrix if it is 100% crystalline.

Nanocomposites Filled with Exfoliated Graphene Nanoplatelets 255

HDPE/wax coated GNP-15 (HDPE/WaxGNP-15) composite melts were injection molded into flexural coupons for mechanical and electrical properties test. The sample nomenclature for HDPE/WaxGNP-15 nanocomposites with different wax to GNP-15 ratios is: 5vol% HDPE/WaxGNP-15 (20:80wt%) means the actual GNP-15 loading in this nanocomposite is

To exclude any artifact due to the sonication, GNP-15 was added into xylene with the same sonication time of 30 minutes but without the addition of wax. After complete evaporation of xylene, GNP-15 was melt mixed with HDPE by the same melt - extrusion and injection molding process to obtain the HDPE/GNP-15 (sonic.) nanocomposites with GNP-15 loading

Flexural tests were performed with a UTS SFM-20 machine (United Calibration Corp.) at room temperature by following the ASTM D790 standard test method (3-point bending mode). The test was performed at a flexural rate of 0.05 in/min. Impact strength tests (Izod

The crystallinity and crystallization temperature were measured by using Dynamic Scanning Calorimetry (DSC Q2000, TA instrument). The samples used were 5–10 mg and non-isothermal crystallization was studied using the following experimental conditions: the sample was heated to 160°C at a rate of 20°C/min. The prior thermal history of the sample was erased by maintaining isothermal conditions at 160°C for 5 minutes. Then the sample was cooled down to 40°C at a rate of 20°C/min, held isothermally for 5 minutes and reheated at 20°C/min to 160°C and cooled back to 40°C again. The data of melting enthalpy (*ΔHm)* and crystallization peak temperature (*Tc*) were collected during the second cycle. The degree of crystallization was calculated by the following equation, where χ% is the percent crystallinity of the matrix, wt% is the weight percentage of GNP, and *ΔHm0* is the theoretical

> <sup>1</sup> % 1 %

The thermal stability of HDPE/GNP nanocomposites was determined from the thermogravimetric analysis (TGA), which was carried out on a TA instrument (TGA 2950) at a heating rate of 20°C/min under nitrogen from 30°C to 800°C. Thermal diffusivity (*α*, m2/s) of GNP nanocomposites (round disks) was measured by a LFA Nanoflash 447 Light flash system. To calculate the thermal conductivity, the bulk density of the samples (*ρ*, kg/m3) was obtained by dividing the mass over the volume, and the specific heat capacity (*Cp*: J/(kg· K)) was measured through the Dynamic Scanning Calorimetry (DSC Q2000, TA instrument). The thermal conductivity (*κ,* W/(m·K)*)* of GNP samples was then calculated by

The electrical resistivity of HDPE/GNP nanocomposites was measured both along the material flow direction (in-plane resistivity) and through the sample thickness direction (through-plane resistivity, normal to the flow direction), using the impedance spectroscopy

0

*wt <sup>H</sup>* (1)

*κ* = *α* × *ρ* × *Cp* (2)

*m m H*

impact type) were performed following the ASTM D256 standard test method.

Fig. 1. (a) A DSM Micro 15cc Compounder, (Vertical, co-rotating, twin-screws microextruder); (b) A Daca Micro injector.

overnight at 30°C. In this study, four different wax coated GNP-15 samples were prepared having wax to GNP-15 ratios of 5:95, 10:90, 20:80, and 30:70wt%. This procedure of producing wax coated GNP is schematically shown in the Fig. 2.

Fig. 2. The procedure of producing wax coated GNP-15.

Then wax coated GNP-15 was re-dispersed in HDPE by melt - extrusion in the DSM Micro 15cc Compounder with the same processing parameters described above. The actual loading of GNP-15 in the final nanocomposites was kept at 5vol%. After extrusion, the resulting HDPE/wax coated GNP-15 (HDPE/WaxGNP-15) composite melts were injection molded into flexural coupons for mechanical and electrical properties test. The sample nomenclature for HDPE/WaxGNP-15 nanocomposites with different wax to GNP-15 ratios is: 5vol% HDPE/WaxGNP-15 (20:80wt%) means the actual GNP-15 loading in this nanocomposite is 5vol% and the weight ratio between wax and GNP-15 is 20:80.

To exclude any artifact due to the sonication, GNP-15 was added into xylene with the same sonication time of 30 minutes but without the addition of wax. After complete evaporation of xylene, GNP-15 was melt mixed with HDPE by the same melt - extrusion and injection molding process to obtain the HDPE/GNP-15 (sonic.) nanocomposites with GNP-15 loading at 5vol% as a reference.

#### **2.3 Characterization techniques**

254 Some Critical Issues for Injection Molding

Fig. 1. (a) A DSM Micro 15cc Compounder, (Vertical, co-rotating, twin-screws

producing wax coated GNP is schematically shown in the Fig. 2.

Fig. 2. The procedure of producing wax coated GNP-15.

overnight at 30°C. In this study, four different wax coated GNP-15 samples were prepared having wax to GNP-15 ratios of 5:95, 10:90, 20:80, and 30:70wt%. This procedure of

Then wax coated GNP-15 was re-dispersed in HDPE by melt - extrusion in the DSM Micro 15cc Compounder with the same processing parameters described above. The actual loading of GNP-15 in the final nanocomposites was kept at 5vol%. After extrusion, the resulting

microextruder); (b) A Daca Micro injector.

Flexural tests were performed with a UTS SFM-20 machine (United Calibration Corp.) at room temperature by following the ASTM D790 standard test method (3-point bending mode). The test was performed at a flexural rate of 0.05 in/min. Impact strength tests (Izod impact type) were performed following the ASTM D256 standard test method.

The crystallinity and crystallization temperature were measured by using Dynamic Scanning Calorimetry (DSC Q2000, TA instrument). The samples used were 5–10 mg and non-isothermal crystallization was studied using the following experimental conditions: the sample was heated to 160°C at a rate of 20°C/min. The prior thermal history of the sample was erased by maintaining isothermal conditions at 160°C for 5 minutes. Then the sample was cooled down to 40°C at a rate of 20°C/min, held isothermally for 5 minutes and reheated at 20°C/min to 160°C and cooled back to 40°C again. The data of melting enthalpy (*ΔHm)* and crystallization peak temperature (*Tc*) were collected during the second cycle. The degree of crystallization was calculated by the following equation, where χ% is the percent crystallinity of the matrix, wt% is the weight percentage of GNP, and *ΔHm0* is the theoretical melting enthalpy of the polymer matrix if it is 100% crystalline.

$$\mathcal{X}\% = \frac{1}{1 - wt\%} \frac{\Delta H\_m}{\Delta H\_m^0} \tag{1}$$

The thermal stability of HDPE/GNP nanocomposites was determined from the thermogravimetric analysis (TGA), which was carried out on a TA instrument (TGA 2950) at a heating rate of 20°C/min under nitrogen from 30°C to 800°C. Thermal diffusivity (*α*, m2/s) of GNP nanocomposites (round disks) was measured by a LFA Nanoflash 447 Light flash system. To calculate the thermal conductivity, the bulk density of the samples (*ρ*, kg/m3) was obtained by dividing the mass over the volume, and the specific heat capacity (*Cp*: J/(kg· K)) was measured through the Dynamic Scanning Calorimetry (DSC Q2000, TA instrument). The thermal conductivity (*κ,* W/(m·K)*)* of GNP samples was then calculated by the following equation:

$$
\kappa = a \times \rho \times \mathbb{C}\_p \tag{2}
$$

The electrical resistivity of HDPE/GNP nanocomposites was measured both along the material flow direction (in-plane resistivity) and through the sample thickness direction (through-plane resistivity, normal to the flow direction), using the impedance spectroscopy

Properties of Injection Molded High Density Polyethylene

Fig. 4. Flexural modulus of various HDPE composites.

5vol%) counterparts.

Nanocomposites Filled with Exfoliated Graphene Nanoplatelets 257

level where the DSM extruder could not generate sufficient pressure to extrude the mix properly. For the flexural strength shown in the Fig. 3, HDPE/CF composites exhibit the highest improvement at all filler loading levels followed by HDPE/GF composites. At the highest loading of 15vol%, HDPE/CF and HDPE/GF result in ~220% and ~170% improvement in flexural strength compared to the neat HDPE respectively. The great enhancement in the flexural strength for HDPE/CF and HDPE/GF composites is largely due to the high aspect ratio and excellent flexural properties of these carbon and glass fibers (Mai et al., 1994). HDPE/GNP nanocomposites also exhibit a significant increase in flexural strength with the increasing of GNP content. At 15vol% GNP loading, HDPE/GNP-1 and HDPE/GNP-15 nanocomposites result in ~116% and ~90% improvement in flexural strength respectively. Meanwhile, it is detected that GNP-1 nanocomposites are superior to GNP-15 counterparts in flexural strength at every GNP loading. At low CB concentrations up to 5vol%, HDPE/CB composites have the flexural strength value close to those of HDPE/GNP-1 samples. And as seen for the flexural modulus of these composites presented in the Fig. 4, HDPE/CF composites display the greatest enhancement in flexural modulus. HDPE/GNP nanocomposites are competitive to their HDPE/GF and HDPE/CB (up to

The Izod impact strength of various HDPE composites up to a filler loading of 15vol% is displayed in the Fig. 5. (Jiang & Drzal, 2010). A reduction in impact strength is observed in all HDPE composites compared to the neat HDPE which is the case normally accompanies incorporation of a rigid filler into a relatively tough polymer (Wakabayashi et al., 2008). However, it is noted that HDPE/GNP nanocomposites exhibit the smallest reduction, which implies the advantage of using GNP as the reinforcement. At the filler loadings from 1vol% to 15vol%, overall HDPE/GNP-1 nanocomposites have the highest impact strength

by applying a two-probe method at room temperature. Samples with dimensions of 10.0 x 3.2 x 12.2mm (Length x Thickness x Width) were cut from the middle portion of flexural coupons. The two surfaces connected to the electrodes were first treated with O2 plasma (14mins, 375W) in order to remove the top surface layers which are rich in polymer and then conductive silver paste was applied to the surface to ensure a good contact with the electrodes. The resistance of samples was measured and converted to resistivity by taking the sample dimensions into account.

The preparation of SEM samples in this study included epoxy mounting, grinding, polishing and etching steps. First, specimens were mounted with epoxy in cylindrical sample holders to maintain a flat surface over the entire grinding and polishing area. After epoxy was fully cured, samples were carefully grounded and polished. O2 plasma etching (25mins, 375W) was then applied at the last step to remove the polymer in top surface allowing the GNP platelets to stand out under the SEM observation. A JEOL (model JSM-6400) SEM was then used to characterize the dispersion of GNP in HDPE. Samples were also gold coated to avoid charging.
