SYNTHESIS AND CHARACTERIZATION OF MICROCELLULAR INJECTION AND INJECTION-COMPRESSION MOLDED PPgMA/GRAPHENE NANOCOMPOSITES

Shyh-shin Hwang1*, Jui-Pin Yang1, Ching-hsin Hu1, Peiming Hsu2

1Chien-hsin University of Science and Technology, Chung-Li, Taiwan.

2Fareast University, Tainan,Taiwan.

E-mail: stanhwang@uch.edu.tw

Abstract Maleated polypropylene (PPgMA) and Graphene (GP)

nanocomposites were prepared directly by microcellular injection and injection-compression molding. Synthesized PPgMA/GP nanocomposite dispersion morphology was confirmed by X-ray diffraction (XRD) and Transmission Electron Microscope (TEM) analysis. Thermo-mechanical and electric properties of PPgMA/GP composites were also reported. Furthermore, electromagnetic interference (EMI) shielding effectiveness was investigated by injection molding (IM), foamed injection molding (FIM), injection-compression molding (ICM), and foamed injection compression molding (FICM) methods.

Introduction

Two-dimensional, GP materials have attracted tremendous research interest due to their remarkable physiochemical properties including electrical, mechanical and thermal. Thus, a combination of different beneficial properties makes GP an ideal candidate for polymer nanocomposites and nanotechnology. GP dramatically improves the properties of polymer based composites at a very low loading point and the most fascinating attribute is the very high surface conductivity leading to the formation of numerous electrically conductive polymer composites [1-3].

Polymer/GP nanocomposites are formed when

PPgMA and GP nanofiller (graphene oxides (GO), reduced graphene oxides (RGO), chemically reduced graphene oxide (CRGO) and thermal reduced graphene oxides (TRGO) etc.) are mixed via various methods such as in situ polymerization, solution blending, and melt blending. Among these different processes, we chose to produce PPgMA/GP nanocomposites by melt blending because it is a commercially feasible, lower cost, environmentally friendly and straightforward approach that does not involve any solvents or monomers [4]. Melt blending is accepted to be the most researched and developed method within the industry. It is very important to make full use of the outstanding properties of graphene compared with GO, RGO, CRGO and TRGO, because graphene has a higher surface-to-volume ratio making them potentially more favorable for improving the properties of polymer matrices. Therefore, GP-based polymer composites have attracted

both academic and industrial interest [5-8].

To date, all presented IM and ICM systems have been designed for the processing of polymers. ICM [9] can provide some advantages; one being reduced residual stress. If the parts have residual stress, this will reduce lifetime and cause birefringence in the optical parts. In this study, PPgMA/GP nanocomposites were obtained by solid, microcellular injection and injection-compression molding. In the obtained nanocomposites dispersion morphology, thermal/electrical conductivity, tensile strength, and EMI shielding effectiveness were investigated.

Materials and Experiments

PPgMA (Maleic anhydride graft ratio 1 wt%) blended with GP (diameter of 8 µm and thickness of 100 nm). The nanocomposites prepared in a master-batch approach using twin-screw extruder, the content of nanofiller graphene 7 wt% with respect to PPgMA. Obtained PPgMA/GP-7 wt% nanocomposites were diluted conventionally into 0.5 wt% 1 wt% 2 wt% and 3 wt% in a dumbbell shape (for tensile test) and 3 wt%, 5wt% and 7 wt% for disc shape samples (for EMI and electrical conductivity test) molding. Dumbbell shape sample was molded by IM and FIM. Disk shape sample was molded by IM, FIM, ICM and FICM. Finally, obtained nanocomposites are represented as PPgMA/GP-0.5 wt%, PPgMA/GP-1.0 wt%, PPgMA/GP-2.0 wt%, PPgMA/GP-3.0 wt%, PPgMA/GP-5.0 wt% and PPgMA/GP-7.0 wt%. Results and Discussion Morphology of PPgMA/GP nanocomposite

The PPgMA/GP nanocomposite morphology was confirmed by XRD and TEM. XRD patterns of the PPgMA nanocomposites containing 3 wt%, 2 wt%, 1 wt% and 0.5 wt% graphene are displayed in Fig. 1 together with pristine PPgMA and GP. The characteristic peaks of graphene, PPgMA peak observed at 2θ = 26.2, 36.3 respectively. Both GP and PPgMA peak is observed without change in 2θ value, indicating that these nanocomposites may form either intercalated or exfoliated dispersion. However, XRD patterns cannot be considered as conclusive evidence of monolayer exfoliation. Therefore, TEM is the best way to describe the morphologies of nanocomposites. For this

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analysis we chose 1 wt%, 3 wt% loaded nanocomposites. Fig.2 shows the TEM images of PPgMA/GP nanocomposites with different magnification. The dark areas represent the graphene sheet re-stacks during melting, and the gray areas represent the PP within the polymer composite matrix.

Fig. 1 Wide angle XRD patterns of foamed PPgMA/GP nanocomposites.

Fig. 2 TEM micrograph of unfoamed PPgMA/GP nanocomposites. Thermo-mechanical properties of PPgMA/GP nanocomposites Fig. 3 shows the load-displacement curves of microcellular injection molded PPgMA/GP nanocomposites. It is observed that the displacement continued by increasing the loading content of GP. However, 1 wt% loaded GP hybrid composite response leveled off due to partial aggregation of GP layers in the polymer matrix. But in the case of 3 wt% GP loaded hybrid composite, tensile strength decreased while also producing better elongation, which can lead to slip formation on the interface of both the polymer matrix and GP. These bonds are responsible for stress distribution. Fig. 4 shows the tensile strength comparison of solid and foamed PPgMA/GP nanocomposites. Tensile strength increases with the increasing of GP content.

Fig. 3 Load-Displacement curves of microcellular injection molded PPgMA/GP nanocomposites.

Fig. 4 Tensile strength- GP content curves of solid and foamed PPgMA/GP composites.

TGA curves of nanocomposites are shown in Fig. 5, indicating PPgMA nanocomposites containing 3 wt%, 5wt% and 7 wt% GP. The degradation temperature of pristine PPgMA, PPgMA/GP-3 wt%, PPgMA/GP-5 wt% and PPgMA/GP-7 wt% is 401.1 ℃, 476.0, 476.2 and 473.9 ℃ respectively. The degradation temperature of nanocomposites increased with increasing the content of GP into the PPgMA matrix.

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Fig. 5 TGA curves of PPgMA/GP nanocomposites Morphology of Microcellular Foam Fig. 6 (A)–(D) show SEM images of a disc shape (for EMI test) PPgMA/GP nanocomposites. The cell size of neat PPgMA is rough and is around 100 µm. The cell size decreased to 50, 40, and 30 µm with GP loading of 3, 5, and 7 wt% respectively. However, 3 wt% GP loaded samples showed uniform cell size distribution. It appears that the addition of GP creates a nucleation site on the interface between the GP and PPgMA, thus decreasing cell size.

Fig. 6 SEM images of the fracture surface perpendicular to melt flow direction of various GP loading for microcellular injection molding. Electrical, Thermal Conductivity of PPgMA/GP nanocomposite Fig. 7 shows the volume electrical impedance of PPgMA/GP nanocomposites at various GP contents by unfoamed injection molding (IM), foamed injection molding (FIM), injection-compression molding (ICM), and foamed injection compression molding (FICM) respectively. From Fig.7, It is observed that the impedance

value decreases with increasing GP content. All the GP based PPgMA nanocomposites showed in Table 1. As can be observed, the nanocomposites generated from PPgMA show lower resistivity than the counterparts based on PPgMA at all GP concentrations. This is evidence that at higher content, GP aggregated in the PPgMA matrix.

Fig. 7 Bulk electrical conductivity of PPgMA/GP nanocomposites. The results of thermal conductivity, heat diffusivity, and heat capacity of PPgMA/GP nanocomposites are summarized in Table 1. The thermal conductivity, heat diffusivity, and heat capacity of PPgMA/GP nanocomposites increased as compared to pristine PPgMA. Thermal conductivity has been explained by effective medium theory of polymer and nanofiller. Interfacial thermal conductivity and thermal diffusivity increased with increasing nanofiller content; large aspect ratio and thickness of nanofiller provided a higher diffusion pathway in the polymer matrix. Therefore, thermal conductivity increases with increasing GP content. Table 1. Thermal conductivity of PPgMA/GP nanocomposites.

Sample Thermal Conductivity (W/mK)

Diffusivity (mm2/s)

Heat Capacity (MJ/m3K)

Pristine PPgMA 0.2436 0.1792 1.359 PPgMA/GP-5 wt% 0.3138 0.3403 0.9223 PPgMA/GP-7 wt% 0.3203 0.3446 0.9295

EMI Shielding Effectiveness

The variation of EMI over frequency range 30~1800 MHz for PPgMA/GP-7 wt% nanocomposite with various process condition samples (unfoamed molding, foamed molding, unfoamed and foamed ICM). It is observed over the entire frequency range. Fig. 8 shows the variation of EMI SE values of PPgMA/GP nanocomposites. The unfoamed ICM PPgMA had the largest SE value, which is mainly attributed to the formation of conductive, interconnected GP based sheet networks in the insulating PP matrix. However, the current GP loading is 7 wt% and its SE value is low over the high frequency range. GP

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loading higher than 7 wt% (I. E., 20 wt%, 25wt%, and 30 wt% ) will be investigated in the future.

Fig. 8 EMI-SE of PPgMA/GP-7 wt% with different processing methods. Flame Retardance

The fire-resistance properties of PPgMA/GP composites are listed in Table 2. From the Limiting Oxygen Index (LOI) results it is observed that; after the incorporation of GP in the PPgMA, the LOI value increased from 18 in the pure PPgMA to 19.5 in the 3 wt% PPgMA/GP composite. It is observed that LOI increases with increasing GP content.

Table 2. LOI results of PPgMA/GP nanocomposites.

PPgMA/GP

0

(wt%)

0.5

(wt%)

1

(wt%)

2

(wt%)

3

(wt%)

LOI 18 18 18.5 19 19.5

Conclusions

In this study, PPgMA/GP nanocomposites were prepared directly by microcellular injection and injection-compression molding methods. The effects of GP on PPgMA/GP nanocomposites were studied. Specifically, the dispersion morphology, thermo-mechanical, electrical conductivity, EMI SE and tensile properties of the microcellular injection/injection-compression molded PPgMA/GP nanocomposites were investigated. We observed that the addition of GP to PPgMA results in an increase in tensile strength, LOI, and thermal conductivity properties. Acknowledgement: This study was supported by Ministry of Science and Technology, Taiwan under grant MOST 105-2221-E-231-001 References 1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang ,

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Keywords: Maleated polypropylene, Injection-compression Molding, Graphene, Microcellular, Nanocomposites

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