Elsevier

Composites Part B: Engineering

Volume 89, 15 March 2016, Pages 424-431
Composites Part B: Engineering

Polymer composites made of multi-walled carbon nanotubes and graphene nano-sheets: Effects of sandwich structures on their electromagnetic interference shielding effectiveness

https://doi.org/10.1016/j.compositesb.2015.11.014Get rights and content

Abstract

This study proposes producing polymer composites with a sandwich structure. The surface layers are identical with two alternative interlayers. The surface layers are composites that are composed of polypropylene and multi-walled carbon nanotubes. Interlayers are polyvinyl alcohol (PVA)/MWCNTs composites and PVA/graphene nano-sheets composites. Maleic anhydride grafted polypropylene (PP-g-MA) serves as the coupling agent for the surface layers and the interlayer, in order to create sandwich-structured composites. The polymer composites that are constructed with different combinations are then evaluated for electromagnetic interference shielding effectiveness (EMI SE). The test results show that the incorporation of PP-g-MA reinforces the interfacial adhesion between the sandwich-structured composites, and thereby fortifies their tensile strength. When consisting of a PVA/MWCNTs interlayer, the polymer composites have an optimal electrical conductivity of 5.8 S/cm, and EMI SE of −28.60 dB for 0.0003GHz–1 GHz, −32.89 dB for 1 GHz–2 GHz, and −33.52 dB for 2 GHz–3 GHz. The sandwich-structured polymer composites that have good mechanical properties, good flexibility and high EMI SE can be achieved by using a simple and effective manufacturing method.

Introduction

Electromagnetic interference (EMI) is a troublesome problem for electronic industries as it results in malfunction of electronic components, while electromagnetic radiation could cause harm to the human body. As a result, electromagnetic interference shielding effectiveness (EMI SE) has become a dominant consideration for the production and operation of electronic equipment and integrated circuits, and subsequently, EMI SE materials are thus promising in their pervasive applications [1], [2], [3]. Polymer-based EMI SE materials are made by blending conductive fillers and matrices, and serve as an effective shielding to dissipate the energy by means of absorbing or reflecting the EMI [4], [5], [6].

Traditional EMI SE materials have good electrical conductivity and EMI SE, but they are primarily composed of metallic material, and thus have the disadvantages of having a high cost, less ease of processing, corrosion possibility, and an unmanageable EMI SE [7], [8], [9], [10]. Therefore, polymer-based EMI SE materials exclude these disadvantages and protect the precision instruments from EMI [11], [12], [13], [14], [15].

CNTs/graphene-based polymer composites are increasingly used as an EMI SE material, as they are light weight and are easy to be processed, and are also electrically conductive [6], [16], [17], [18], [19], [20], [21], [22]. For example, Hsiao et al. modified S-GNSS by using a cationic surfactant, after which S-GNSS was combined with water-borne polyurethane (WPU) in order to form flexible, light weight, and EMI shielding GNS/WPU composites. The test results indicated that a good dispersion of S-GNSS in WPU, and the composites thus yielded a high conductivity and EMI SE. Moreover, pure carbon materials are used as EMI SE materials [23]. For example, Shanov et al. synthesized graphene pellets by using a chemical vapor deposition (CVD), and further compressed them into graphene paper. The graphene paper had an electrical conductivity of 1136 S/cm and EMI SE of 60 dB [24]. In addition, laminated conducive composites can also be incorporated to form EMI SE materials. For example, Leng et al. had epoxy as matrix, to which MWCNTs, Fe3O4, and Fe are added for EMI shielding, in order to form trilayer-type laminated nanocomposites. Such a structure had Fe3O4 and Fe as a matching layer, 5 wt% CNT as an absorbing layer, and 10 wt% CNT as a reflecting layer. The test results indicated that the laminated nanocomposites have EMI SE of 40 dB at a frequency between 13 GHz and 40 GHz [25]. The manufacturing methods to prepare polymer composites include in-situ polymerization method, melt compounding method, and solution mixing method [26], [27], [28]. Three methods for making EMI SE materials have shortcomings. The solution mixing method requires an extra step of modification [23], carbon materials have a high cost for raw materials, and laminated composites are often stiff and restricted from their applications.

This study aims to develop an economical, eco-friendly, and low cost manufacturing method to prepare sandwich-structured composites that have desirable mechanical properties, flexibility, electrical conductivity, and EMI SE. The solution mixing method and the melt compounding method are both economical and eco-friendly. In this study, the solution mixing method does not require organic solvents, and can effectively distribute conductive fillers [29], [30], and thereby reinforces the electrical conductivity of the sandwich-structured composite. In addition, the melt compounding method produces different layers that can be simply hot pressed into sandwich-structured composite.

According to previous studies, the electrically conductive laminated composites composed only metallic base layers. In addition, the final materials were stiff and thus had limited applications [25]. In contrast, this study proposes sandwich-structured composite consisting of three layers of conductive polymer-based materials. PP/MWCNTs composites serve as surface layers, and PVA/MWCNTs or PVA/GNs composite serve as the interlayer. PP has satisfactory mechanical properties, high heat resistance, low cost, ease of processing, and recycling properties. PP is used as the matrices for the surface layers of sandwiches in order to mechanically improve the PVA-based interlayer. The interlayers are flexible due to the use of plasticized PVA as the matrices [31], and the sandwich-structured composite thus are expected to be mechanically strong, flexible, and have EMI shielding. Furthermore, PP-g-MA serves as the coupling agent that strengthens the interfacial adhesion between layers. The polymer composites are finally tested for their tensile strength, interfacial adhesion, electrical conductivity, and EMI SE.

Section snippets

Materials

Polypropylene (PP; YUNGSOX 1080; Formosa Plastics Corporation, Taiwan, ROC) is a homopolymer with a melt flow rate (MFR) of 10 g/10 min (ISO1133). Maleic anhydride grafted polypropylene (PP-g-MA; DuPont Fusabond P613, DuPont, US) has a MFR of 120 g/10 min (ISO1133) and a grafting level of 0.5%. Polyvinyl alcohol (PVA; Feng Xiang Materials, Taiwan, ROC) has a melt index (MI) of 1 g/10 min, and is the commercially available PVA (BF-17, Chang Chun Group., Taiwan, ROC) that is modified by using a

Tensile property

Tensile strength of the sandwich-structured composites is indicated in Fig. 3. The different surface layers or the interlayer have their corresponding tensile strengths: 28.5 MPa (PP), 16.4 MPa (PVA), 31.9 MPa (PPC), 34.8 MPa (PVAC), and 27.4 MPa (PVAG), as indicated in Fig. 3(a). The sandwich-structured composites have their tensile strength as 22.9 MPa (PP-PVA-PP), 25.1 MPa (PP-PVAC-PP), 21.3 MPa (PPC-PVA-PPC), 24.9 MPa (PPC-PVAC-PPC), 24.3 MPa (PP-PVAG-PP), and 22.3 MPa (PPC-PVAG-PPC), as

Conclusion

This study proposes synthesizing PP/MWCNTs surface layers with a PVA/MWCNTs interlayer or a PVA/GNs interlayer in order to form sandwich-structured composites that have good mechanical properties, flexibility, and EMI SE. The test results indicate that the incorporation of PP-g-MA can improve the compatibility between surface layers and an interlayer, and thereby provides the composites with a higher tensile strength. PPC-PVAC-PPC and PPC-PVAG-PPC composites have optimal electrical conductivity

Acknowledgments

The authors would especially like to thank Ministry of Science and Technology of Taiwan, for financially supporting this research under Contract MOST 103-2221-E-035-027.

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