waterborne polyurethane composite films for efficient electromagnetic interference shielding

waterborne polyurethane composite films for efficient electromagnetic interference shielding

Composites Part A 121 (2019) 411–417 Contents lists available at ScienceDirect Composites Part A journal homepage: www.elsevier.com/locate/composite...

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Composites Part A 121 (2019) 411–417

Contents lists available at ScienceDirect

Composites Part A journal homepage: www.elsevier.com/locate/compositesa

High conductive and mechanical robust carbon nanotubes/waterborne polyurethane composite films for efficient electromagnetic interference shielding ⁎

Hui Lia, Du Yuanb, Pengcheng Lia, , Chaobin Heb,c,



Key Laboratory for Green Chemical Process of Ministry of Education, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, 117574 Singapore, Singapore c Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 117602 Singapore, Singapore b



Keywords: CNT/WPU composites Filtration process High conductive Mechanical property Electromagnetic interference

Highly conductive multi-walled nanotubes (MWCNT)/waterborne polyurethane (WPU) composites were prepared using facile latex technology without surfactant, in which the solid composites were achieved by filtration process to prevent the re-aggregation of MWCNT during drying process. MWCNT was distributed along the interfaces of polyurethane particles, constructing segregated structure without insulating surfactant. Therefore, high conductivity of 362.6 ± 23.1 S m−1 was obtained at CNT content of 10.6 wt%, resulting in excellent electromagnetic interference (EMI) shielding effectiveness (SE) of 24.7 dB at thin film thickness of 0.4 mm in Xbands. Considering lightweight and thin thickness, outstanding specific shielding effectiveness (SSE) of 537 dB cm2 g−1 was achieved, higher than most values of CNT composites ever reported. Furthermore, the composites exhibited desirable mechanical properties with high elongation at break of 62%, demonstrating promising applications for commercial EMI shielding, especially for flexible and stretchable devices.

1. Introduction With the increasing application of electrical devices, wireless communication, and automation, electromagnetic radiation has become a serious problem. Every electronic device could generate electromagnetic waves which may cause malfunctioning of the devices and electromagnetic pollution on the surrounding environment and human body [1,2]. As a result, electromagnetic interference (EMI) shielding materials are greatly required for reliable applications of electronic devices. Conventional metals were the first choice for industry until the development of electrically conducting polymer composites (CPCs). Compared with traditional metals which are suffering from high mass density, poor flexibility, and corrosion, CPCs have become a promising candidate for EMI shielding, due to the advantages of flexibility, lightweight, excellent mechanical property, and tunable electrical properties [3–5]. In general, EMI shielding efficiency (SE) value of 20 dB is required to satisfy commercial application which implies 99% of incident microwave radiation can be dissipated [6]. Since EMI SE is strongly dependent on the electronic conductivity of the materials, thus

the materials which possess high conductivity are attracting for EMI shielding applications [1,7,8]. In order to satisfy the practical applications, the composites with high content of conductive filler were usually required to achieve high conductivity. However, this would result in high cost, inferior mechanical property, and processing difficulties [6,9]. Therefore, achieving high conductivity composite at relative low content of conductive filler is a promising strategy for practical applications. Carbon nanotubes (CNT), with the unique structure, high aspect ratio, good mechanical strength, and high electric conductivity, are considered as one of the most promising candidates as the conductive filler for CPCs [10–12]. The dispersion of conductive filler in the polymer matrix has significant effect on the conductive behaviour of the composites. With strong Van Der Waals force, CNT tend to aggregate as bundles, making it difficult to be uniformly dispersed in polymer matrix to form network. The poor dispersion in the polymer matrix would increase the contact resistance and on the other hand deteriorate the mechanical performance of the composites. In recent years, great efforts have been made to improve the dispersion of

Corresponding authors at: Key Laboratory for Green Chemical Process of Ministry of Education, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China (P. Li). Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, 117574 Singapore, Singapore (C. He). E-mail addresses: [email protected] (P. Li), [email protected] (C. He). https://doi.org/10.1016/j.compositesa.2019.04.003 Received 8 January 2019; Received in revised form 27 March 2019; Accepted 5 April 2019 Available online 05 April 2019 1359-835X/ © 2019 Published by Elsevier Ltd.

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Fig. 1. (a) Size distribution of WPU particles; SEM images of MWCNT/WPU composites with CNT content of 0.75 wt% (b) and 1.6 wt% (c), inset is the cross-section image of the composite film; (d) Schematic illustrations of latex technology for the fabrication of segregated MWCNT/WPU composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

conductive fillers and conductivity of the composites [13], such as melt mixing, surface chemical modification, and surfactant method [14–17]. However, relatively large amount of conductive filler is required to achieve high conductivity as most of CNT do not take part in the formation of conductive network, which may sacrifice the mechanical properties of the composites [3,18]. Selective localization of CNT in one preferred phase of biphasic blends provides an alternate way to improve both the electrical property and the mechanical property of conducting composites [19–21]. However, this approach requires precise control on the biphasic mixing conditions, such as mixing time and biphasic ratios. Recently, constructing segregated structure has become a promising strategy for forming well-established conductive network with minimal filler loading [6,19,22]. In this morphology, conductive fillers distribute along the boundary of the polymer domains to form conductive pathways, which efficiently increases the utilization ratio of the conductive filler to form conducting network and increase conductivity [23–25]. For instance, Zeng et al. fabricated segregated MWCNT/high density polyethylene (HDPE) composites by alcohol-assisted dispersion and hot-pressing process, which exhibited low percolation thresholds of 0.15 vol% [18]. Segregated MWCNT/poly(vinylidene fluoride) (PVDF) composites were prepared by mechanical mixing and hot compaction, which possessed high conductivity and SE of 6 S m−1 and 30.9 dB respectively, with 7 wt% filler content [26]. The significant results promote the improvement of CPCs. However, most of the fabrication processes are complicated, which need melt processing or hot compaction [27–29]. In contrast, facile and environment friendly latex technology has attracted extensive attention to synthesize segregated CPCs [30–32]. The emulsions create excluded volume and essentially push conductive filler lie on the interfaces of polymer particles, resulting in segregated conductive network with high conductivity. Grunlan et al. incorporated SWCNT into poly(vinyl acetate) (PVAc) latex with gum Arabic(GA) as stabilizer to enhance the dispersion of conductive filler. With CNT content of 4 wt%, the composites exhibited high electrical conductivity of ∼32 S m−1 [33]. Loos et al. prepared conductive MWCNT/PS films by latex technology with

sodium dodecyl sulfate(SDS) as stabilizer, which exhibited low percolation threshold about 1.5 wt% [34]. However, the insulating surfactants would deteriorate the electrical property of the composites [34]. Moreover, most of the films were obtained by freeze-drying and dropcasting process [31,33], which complicate the fabrication processing. In this work, we developed a facile latex technology to fabricate high conductive MWCNT/waterborne polyurethane (WPU) films by filtration process without surfactant. WPU emulsions were used as polymer matrix, due to the advantages of environmental friendly and excellent elastic property [35]. As CNT are relatively hydrophobic, stabilizer becomes crucial to achieve stable suspension and prevent reaggregation of CNT for the typical drying process of drop-casting films and freeze drying. In order to preserve the segregated network without stabilizer, we propose a facile strategy of filtration process to achieve solid composites. Although various CNT/polymer composites have been studied for EMI shielding, there is few work to fabricate films directly by filtration process, especially for CNT/WPU composites. Technically, this process is superior to most of the reported latex technologies which require complicated freeze-drying and compaction processing [26,31]. Moreover, segregated network without insulating surfactant was beneficial for the electrical property of composites. Finally, high conductive MWCNT/WPU composites were achieved by this facile latex process. Besides, the elastic property of WPU enables the composites to possess superior flexible and stretchable feature, making it promising composites for application in commercial EMI shielding. These results demonstrate the feasibility of this approach to achieve high conductive CPCs. With facile process, high conductivity, and desirable mechanical property, this material demonstrated potential applications especially for stretchable devices and wearable electronics.

2. Experimental 2.1. Materials MWCNT were purchased from Nanocyl (NC-3100) with the average 412

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particles was obtained by dynamic light scattering (DLS) measurements (BI-200SM-3 DLS system) with a He-Ne laser operating at wavelength of 632.8 nm. The conductivity of the composite was measured by a fourprobe method with a Keithley 2400 source/meter. Tensile tests were performed using an Instron Universal Tester 5569 at a crosshead speed of 10 mm min−1. Rectangle shape tensile specimens were cut with gauge length of 10 mm and width of 3 mm. At least 5 specimens were measured for tensile test. Young’s modulus was calculated by fitting linear relationship between stress and strain, and the slope of the stressstrain curve at low strain was calculated to be Young’s modulus. EMI shielding measurements were conducted using Agilent/HP 8510C Vector Network Analyzer (VNA) over the frequency range of 8.2–12.4 GHz (X-band). 3. Results and discussion Waterborne polyurethane (WPU), with excellent mechanical property and environmental friendly, has attracted extensive attention for practical applications as polymer matrix [3,35]. WPU emulsions consisted of polymer particles prior to film formation which were around 85 nm and 250 nm (Fig. 1a). Since MWCNT is difficult to penetrate into the particles, they are excluded to lie on the interfaces of particles as evidenced by the scanning electron microscope (SEM) images. Fig. 1b showed that CNT was dispersed well in the composites without any large aggregates. Meanwhile, as WPU domains (marked with yellow lines) create excluded volume, CNT were pushed to enrich around the polymer domains and form connected network rather than uniformly distributed within the composites, thus constructing segregated structure. These results indicate the feasibility of the filtration process for the construction of segregated structure. In the typical slow drying process of drop-casting, CNT prone to re-aggregated during the solution evaporating. Therefore, insulating stabilizer was crucial to achieve stable suspension and disperse CNT along the interfaces of emulsion particles. In this work, filtration process could remove water solution in a few minutes and preserve the segregated structure without large CNT aggregates. With increasing the content of MWCNT, the bright domains were increased and MWCNT were interconnected to enhance charge transport within the composites, which indicates the outstanding electrical properties of the composites. Furthermore, as shown in the crosssectional SEM images, the obtained film is dense without porosity, beneficial for the desirable mechanical performance and electrical property. Electrical conductivity of the composites as a function of MWCNT volume fractions was investigated and shown in Fig. 2. Considering that the conductivity of WPU was in the order of 10-14 S m−1, the conductivity of the composites displayed a dramatic increase with increasing MWCNT loadings of 0.53 wt%, indicating a typical percolation transition behaviour [26,36,37]. Further increasing the content of MWCNT lead to a slightly increase of the conductivity of the composites. The experimental data could be fitted using the following equation to estimate the percolation threshold concentration (φ c) [38],

Fig. 2. (a) Electrical conductivity of MWCNT/WPU composites with different CNT loadings. The straight line in the inset is a fit to the data according to statistical percolation theory; (b) Comparison of the conductivity of the CNT composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

diameter of 9.5 nm, average length of 1.5 μm, and purity > 95 wt%. Waterborne polyurethane (WPU) was obtained from Taiwan PU Corporation (WPU-372D) with concentration of 50 wt%. The WPU suspension consists PU particles of both 85 nm and 250 nm as shown in Fig. 1. 2.2. Preparation of MWCNT/WPU films Firstly, MWCNT were dispersed in H2O with the concentration of 2.5 mg g−1 by probe ultrasonic on SCIENTZ ultrasonic homogenizer JY92-IIN (650 W) with ultrasonic power of 30% for 10 min. Then CNT dispersion was blended with WPU suspension, and the blended dispersion was vacuum filtrated using Advantec mixed cellulose ester filter membrane with pore size of 0.20 μm for about 3 min. After that, the composites were cured in the oven at 70 °C for 2 h and the films were obtained. The weight ratios of MWCNT were estimated by the initial weight of MWCNT and final weight of the composites, and the films with various MWCNT weight ratios of 0.53–21.1 wt% were obtained. The film thickness was varied from 0.05 mm to 0.4 mm by tuning the amount of mixture.

σ = σ0 (φ − φc )t where σ is the conductivity of the composite, σ0 is a constant related to the intrinsic conductivity of MWCNT, φ is the volume fraction of MWCNT, φc is the volume fraction of conductive filler at percolation threshold, and t is the critical exponent reflecting the system dimensionality of the composites. The best fitted value for the percolation threshold was 0.277 vol% (0.53 wt%), lower than most values of MWCNT/polymer composites (0.5 vol% for MWCNT/TPU composites prepared by solution cast [39], 1.0 wt% for MWCNT/TPU composites prepared through melt blending and compression moulded [40], 2.0 wt % for MWCNT/HDPE composites prepared by solution precipitation [41]), and even lower than SWCNT/polymer composites (4 wt% for SWCNT/HDPE by melt processing [42], 1.8 vol% for SWCNT/PMMA composites prepared using coagulation and compression processing

2.3. Characterizations The morphology of the synthesized MWCNT/WPU composite was investigated under Hitachi SU3500 scanning electron microscope (SEM) at an accelerating voltage of 15 kV. The size distribution of WPU 413

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Fig. 3. EMI curves of MWCNT/WPU composites with various MWCNT contents(a), 5.3 wt% MWCNT/WPU composites with various thickness(c), 10.6 wt% MWCNT/ WPU composites with various thickness (e); corresponding shielding efficiency of the composites with various MWCNT contents(b), 5.3 wt% MWCNT/WPU composites with various thickness (d), 10.6 wt% MWCNT/WPU composites with various thickness (f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

10 min before filtration to enhance the dispersion of CNT. It exhibited much lower conductivity of 2.2 ± 0.67 S m−1 and 77.2 ± 19.1 S m−1 with CNT mass content of 5.3 wt% and 10.6 wt%, respectively. These results indicate the unique segregated structure of CPCs and feasibility of latex technology with filtration process to construct high conductive CPCs, which demonstrates potential application for EMI shielding. To demonstrate the potential of the MWCNT/WPU composites for EMI shielding, the EMI shielding efficiency (SE) which characterizes the ability of an EMI shielding material to attenuate electromagnetic radiation were investigated over the frequency range of 8.2–12.4 GHz. EMI shielding mechanism mainly consists of reflection, absorption, and multiple internal reflections, which are largely dependent on mobile

[43]). Besides, the critical exponent was estimated to be 1.74, which means two-dimensional conductive network was constructed [44]. Furthermore, the composites displayed high conductivity of 123.8 ± 18.2 S m−1 and 362.6 ± 23.1 S m−1 with CNT mass content of 5.3 wt% and 10.6 wt%, respectively, higher than most of the reported values of the composites with comparable conductive filler content (20 S m−1 for 15 wt% SWCNT/epoxy [45,46], 12.4 S m−1 for 10 wt% MWCNT/TPU [47], 45.6 S m−1 for 10 wt% MWCNT/TPU [40]). The low percolation threshold and high electric conductive could be attributed to an existence of segregated structure. For comparison, MWCNT/WPU composite with CNT better dispersed within the matrix was prepared by ultrasonic treatment of the MWCNT/WPU mixture for 414

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Table 1 Comparison of various CNT composites for EMI shielding. Materials



Density/g cm−3

Conductivity/S m−1


SSE/dB cm2 g−1



10 5 10 10 7 10 10 10 15 15 20

2 2 2 2 2 1.1 1.5 2 2 1.5 4.5

∼1.05 ∼1 ∼1 1.1 ∼1.8 1.05 ∼1.3 ∼1.3 1.3 ∼2 ∼1.2

10 7.1 0.001 ∼100 6 100 12.4 45.6 20 20 2

50 17.2 20 27 33.2 40.7 29 22 25 20 30

238 86 ∼100 123 ∼92 ∼352 ∼149 ∼85 96.2 ∼67 ∼56

[52] [53] [54] [55] [26] [56] [47] [40] [45] [46] [43]


5.3 10.6



123.8 362.6

21.1 24.7

459 537

Our work

absorption of the electromagnetic wave is expressed as the following equations [35,49],

SE(R) = 39.5 + 10log(σ/(2πfμ))

SE(A) = 8.7t πfμσ while f is the frequency of the electromagnetic wave, μ is the magnetic permeability, σ is the conductivity, and t is the sample thickness. It is apparent that both shielding by reflection and absorption represent a positive correlation with its electrical conductivity. And SE(A) is also dependent on the thickness of materials. The EMI curves for the composites with varying MWCNT contents and thickness over the frequency range of 8.2–12.4 GHz were shown in Fig. 3. With MWCNT contents of 3.2 wt%, SE of the composites was only 2.5 dB at 10 GHz. In general, CNT attenuate electromagnetic (EM) radiations into thermal and internal electrical energies to dissipate EM waves, contributing to higher SE(A) with increasing CNT contents [50]. While SE(R) relies on the interactions between incoming EM waves and mobile charge carriers on the surface of the materials, which related with the conductivity of the materials [3]. Therefore, with increasing CNT contents, more CNT resulted in large amount of charge carriers to interact with incoming waves and dissipate EM energies, leading to improved EMI shielding performance. Consequently, SE(R) and SE(A) exhibited a significant increase and SE of 8.0 dB and 12.8 dB were obtained for MWCNT/WPU composites with MWCNT content of 5.3 wt% and 10.6 wt%, respectively. Furthermore, as the MWCNT content increased to 21.1 wt%, SE of 24.5 dB was achieved with thin film thickness of only 0.1 mm (as shown in Fig. 3b). The EMI SE for the MWCNT/WPU composites with MWCNT content of 5.3 wt% and 10.6 wt% with various thicknesses was investigated and shown in Fig. 3(c)–(f). SE was greatly increased with increasing the thickness of the composites from 0.1 mm to 0.4 mm. Samples with higher thickness provided more conductive fillers to attenuate electromagnetic waves, resulting in enhanced EMI shielding performance. With the film thickness of 0.4 mm, SE values of 21.1 dB and 24.7 dB were obtained for 5.3 wt% and 10.6 wt% MWCNT/WPU respectively, which was satisfy for practical applications. For comparison, most of reported materials for EMI SE applications required higher thickness about 1–2 mm or higher content of conductive fillers (as summarized in Table 1), which inevitably increase the weight of the electronic devices and deteriorate the flexibility feature of the composites [51]. In order to evaluate the shielding efficiency of materials more reality, specific shielding effectiveness (SSE = SE/ρt) is derived to compare the effectiveness of shielding materials taking into account of film thickness (t) and density (ρ). As shown in Fig. 4, the MWCNT/WPU composites with CNT content of 10.6 wt% exhibited high SSE of 537 dB cm2 g−1, which is higher than most value of the reported CNT/polymer films (20–400 dB cm2 g−1, Table 1). This outstanding EMI shielding performance is attributed to optimum segregated network structure of the

Fig. 4. Comparison of SSE of the CNT composites ever reported as a function of thickness. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Stress-strain curves of MWCNT/WPU films with various MWCNT content of 0, 1.1 wt%, 5.3 wt%, and 10.6 wt%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

charge carrier, electric or magnetic dipoles, and interfaces in the shielding materials, respectively [3]. Total EMI SE is usually the sum of reflection (SE(R)) and absorption (SE(A)), as the multiple reflections (SE(M)) of electromagnetic radiation can be neglected when SE is greater than 10 dB [48]. For conductive materials, the reflection and 415

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composites with interconnected conductive network, which leading to high conductivity at relative low conductive filler content. Moreover, the excellent segregated structure and electrical property demonstrate the feasibility of the latex technology to fabricate high conductive CPCs. With the facile process, low cost, and environmental friendly, this approach could well be applied to other conductive composites. The mechanical property of WPU and MWCNT/WPU composites was shown in Fig. 5. With addition of MWCNT, the Young’s modulus increased from 9.3 MPa of pristine WPU to 110 MPa of MWCNT/WPU with CNT content of 10.6 wt%, attributing to the stiffness of conductive fillers and good interfacial interactions between the CNT and polymer [57,58]. Meanwhile, the elongation at break decreased with the increasing of CNT loading. At CNT content of 10.6 wt%, the composite exhibited elongation of 62%. The superior flexibility and stretchability demonstrate its potential application on commercial industries, such as sensors, actuators, and wearable electronics which have been intensively studied to realize smart practical applications.

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