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Flexible and efficient electromagnetic interference shielding materials from ground tire rubber
Carbon 121 (2017) 267e273
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Carbon journal homepage: www.elsevier.com/locate/carbon
Flexible and efficient electromagnetic interference shielding materials from ground tire rubber Li-Chuan Jia a, Yi-Ke Li b, Ding-Xiang Yan a, c, * a
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China Chengdu Shude High School, Chengdu, 610031, Sichuan, China c School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2017 Received in revised form 22 May 2017 Accepted 31 May 2017 Available online 2 June 2017
Low-cost flexible EMI shielding materials have great market potential due to the rapid development of flexible electronics such as wearable health monitoring systems, flexible displays, and flexible power supply systems. Here, by fully exploiting its three-dimensional (3D) cross-linked structure, the ground tire rubber (GTR) in industrial waste was converted into a valuable and high-performance electromagnetic interference (EMI) shielding material. A carbon nanotube (CNT)/GTR composite with typical segregated structure was designed, with CNTs selectively localized at the boundaries of GTR domains. The composite containing only 5.0 wt% CNT exhibits a high electrical conductivity of 109.3 S/m and an EMI shielding effectiveness (SE) of 66.9 dB, superior to most of the reported CNT/polymer composites. Additionally, the CNT/GTR composite shows excellent flexibility and stability with 93% retention of EMI SE even after repeatedly bending to the radius of 2.0 mm for 5000 times. Our flexible EMI shielding material will benefit the fast-growing next-generation flexible electronic devices by providing low cost and reliable protection. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Waste tire rubber (WTR) has become a heavy burden, the socalled “Black Pollution”, to the environment due to the enormous amount of tire consumption (more than 17 million tons per year) [1e7]. The irreversible vulcanized structure and the incorporation of various stabilizers make the management or recycling of WTR a serious technical, economic and ecological problem [5]. Compared to the traditional devulcanization of WTR, downsizing of WTR into ground tire rubber (GTR) for use as an additive material in asphalt paving mixtures, Portland cement concrete, or polymers (such as rubbers, thermoplastics and thermosets) is considered to be an effective waste treatment method [8e15]. Nevertheless, the utilization of GTR as a simple additive does not impart the host material with high economic value added, and sometimes even leads to a sacrifice in the performance or inferior quality of the product. For instance, only 10 phr GTR added in polyethylene resulted in 13%
* Corresponding author. College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China. E-mail address: [email protected] (D.-X. Yan). http://dx.doi.org/10.1016/j.carbon.2017.05.100 0008-6223/© 2017 Elsevier Ltd. All rights reserved.
and 16% decrease in tensile strength and elongation at break compared to the virginal polyethylene [11]. Given the inefficiency in utilizing GTR, a considerable advance may be obtained by developing GTR into advanced multifunctional materials with high value. Interestingly, even at a high temperature, GTR possesses intrinsic three-dimensional (3D) cross-linked structure similar to that of a high-viscosity gel. After mixing with conductive nanofillers, this feature would restrict the penetration of the nanofillers into GTR interior and give rise to the selective distribution of the nanofillers at the interfaces of GTR domains to form a segregated structure. The segregated structure is well-known to construct highly dense conductive networks that improve the electrical and electromagnetic interference (EMI) shielding performance of polymer composites [16,17]. Moreover, the reinforcing carbon black (CB) in GTR could construct additional conductive networks and serve as conductive centers that interact with the incoming electromagnetic waves, also promoting the EMI shielding performance. To demonstrate those concepts, by fully exploiting the 3D crosslinked structure and CB in GTR, we facilely design a valuable and high-performance EMI shielding material, i.e., carbon nanotube (CNT)/GTR composite with a typical segregated structure via
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mechanical blending and hot compaction method (Fig. 1). In this structure, the combination of CNTs selectively distributed among the GTR domains and CB in the GTR domains endow the CNT/GTR based composite with compelling electrical and EMI shielding performance. The composite containing only 5.0 wt% CNT achieves an ultra-high EMI shielding effectiveness (EMI SE) of 66.9 dB (meaning 99.9998% attenuation of electromagnetic radiation), the highest value among the reported CNT/polymer composites at such a low CNT loading [18e30]. The composite shows sufficient flexibility and highly reliable EMI shielding performance under mechanical deformation, with 93% retention of EMI SE even after repeatedly bending to the radius of 2.0 mm for 5000 times. 2. Experimental 2.1. Materials and fabrication The raw materials include GTR as the polymer matrix and CNT as the conductive filler. GTR (60 mesh size) produced from passenger car and light truck tires was kindly supplied by the Sichuan Zhongneng Rubber Co. LTD., China, with the composition of 62.8 wt % hydrocarbon, 13.4 wt% carbon black, and 23.8% ash (Fig. S1 and S2b). CNTs are the NC 7000 series (average diameter of 9.5 nm, average length of 1.5 mm, surface area of 250e300 m2/g) purchased from Nanocyl S.A., Belgium. We used a facile, efficient, economic and green method to process the segregated CNT/GTR composite (denoted as s-GTR composite), i.e., mechanical blending and compression molding. CNT and GTR were first mechanically mixed in a high-speed mixer (24,000 rpm, 2 min) at room temperature to obtain CNT-coated GTR complex granules. The complex granules were then compression-molded into composite sheets with various thicknesses (0.4, 1.0, 1.5, 2.0, 2.6 mm), under the pressure of 50 MPa at 170 C, after preheating for 10 min. For comparison, the pristine GTR (without the addition of CNT) was also fabricated under the same processing conditions. 2.2. Characterization The morphologies of the s-GTR composite and pristine GTR were analyzed by high-resolution field emission scanning electron
microscopy (FESEM, Inspect-F, FEI, Finland) at the accelerating voltage of 15 kV. The specimens for SEM observations were quickly cryo-fractured after immersing in liquid nitrogen for 30 min and the freshly fractured surfaces were coated with gold. Thermogravimetric analysis (TGA) was carried out from 30 to 700 C using a thermogravimetric analyzer (TG209 F1, NETZSCH, Germany) at the heating rate of 10 C/min, under both nitrogen and ambient atmosphere. Electrical performance was characterized with a Keithley electrometer model 4200-SCS (USA) and a four-point probe instrument (RTS-8, Guangzhou Four-Point Probe Technology Co., Ltd., China). The real-time variation of electrical resistance under mechanical deformation was recorded using the Keithley electrometer model. EMI shielding characteristics were studied using an Agilent N5230 vector network analyzer, with the APC-7 connector as the coaxial test cell. Scattering parameters (S11, S21) were measured in the X-band frequency range (8.2e12.4 GHz) to obtain the reflected power (R), transmitted power (T), and absorbed power (A). Then, the EMI SE (SEtotal), microwave reflection (SER), and microwave absorption (SEA) can be calculated (detailed calculations seen in Supporting Information).
3. Results and discussion Fig. 2a, b show the surface morphology of the CNT/GTR complex granules. Numerous CNTs are deposited onto the surfaces of GTR granules. The specific surface areas of the GTR granules are greatly enlarged due to the irregular shapes with rough surfaces (shown in Fig. S2a), encouraging the efficient adsorption of CNTs. Through the compression molding, the CNT-coated GTR complex granules were consolidated into s-GTR composites with predetermined shapes that exhibit excellent flexibility, as shown in Fig. 2cef. The compression pressure induces the intimate contact of the GTR granules and the heat during the processing provides the energy to break the polysulfidic crosslinks [31]. The broken cross-link bonds can create new links, thereby “sintering” the GTR granules into a bulk material. The compressive creep of GTR molecules also facilitates the fabrication of such bulk material. Additionally, the survived 3D cross-linked structure within the GTR domains endows the s-GTR composite with fascinating flexibility. The composite can maintain its initial form even after enduring a rigorous bending-
Fig. 1. Schematic for the fabrication of flexible CNT/GTR composite with segregated structure. (A colour version of this figure can be viewed online.)
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Fig. 2. (a) SEM micrographs of 3.0 wt% CNT coated GTR complex granules; (b) magnified image of (a); (c) Digital images of the s-GTR composites with wafer shape; (d), (e) and (f) Digital images of the s-GTR composites with a bending-release cycle (radius of ~2.0 mm). (g) SEM images of the 3.0 wt% s-GTR composite; (h) and (i) are the magnified images of (g). (A colour version of this figure can be viewed online.)
release cycle with the radius of 2.0 mm (Fig. 2def). The SEM images of the fractured surface of the s-GTR composite were taken to characterize the detailed microstructure. A typical segregated structure is observed, with the CNTs squeezed along specific paths (bright lines) surrounding the GTR domains (dotted cycle) to form continuous conducting networks (Fig. 2g). The magnified images (Fig. 2h, i) further indicate that adjacent CNTs overlap to develop dense paths along the boundaries of the GTR domains. The movement of GTR molecular chains is extremely confined due to the original 3D cross-linked structure suppressing the penetration of the CNTs into interior of the GTR domains and guaranteeing the construction of a segregated structure. Additionally, much CB is visually shown in the interior of the GTR domains (Fig. 2i). The merit of the unique segregated structure combined with the presence of CB is expected to impart outstanding electrical and EMI shielding performance to the s-GTR composites. Fig. 3a shows the electrical conductivity of the pristine GTR and the s-GTR composite as a function of CNT loading. Due to the presence of CB, the pristine GTR exhibits an electrical conductivity of 4.8 10 7 S/m. Excitingly, only 1.0 wt% CNT loading gives the sGTR composite a high electrical conductivity (2.6 S/m) that already exceeds the target value (1.0 S/m) for commercial EMI shielding applications [16]. Increasing CNT loading to 5.0 wt% results in a much higher electrical conductivity of 109.3 S/m, among the highest values ever reported in CNT/polymer composites at the similar CNT loading level (Table 1) [18e30]. The electrical conductivity of the s-GTR composite is even superior to those of various CNT/polymer (such as CNT/ultrahigh molecular weight polyethylene, CNT/polystyrene, and CNT/polyethylene) composites with segregated structure [32e34]. In addition to the contribution of the segregated structure, the CB embedded in the GTR domains also contributes to constructing additional conductive networks and thus results in the unprecedented electrical performance. This result means that the use of percolated or conductive polymer domains as matrix to construct a segregated structure can facilitate the fabrication of more conductive CNT/polymer composites. The outstanding electrical conductivity is beneficial for the development of high-performance EMI shielding materials. Fig. 3b shows the increased EMI SE with CNT loading in the s-GTR composites
over the measured frequency range (X band, 8.2e12.4 GHz). A higher CNT loading not only improves the electrical conductivity but also provides a larger amount of free electrons to attenuate the incident electromagnetic waves inside the s-GTR composite. We note that the s-GTR composite containing only 1.0 wt% CNT already exhibits an EMI SE of 24.5 dB that satisfies the requirements of commercial EMI shielding application (20 dB). As the CNT loading rises to 5.0 wt%, the composite achieves an ultra-high EMI SE of 66.9 dB, far superior to the highest values ever reported in CNT/ polymer composites (Table 1) [18e30]. In conventional CNT/polymer composites, CNTs are randomly distributed throughout the polymer matrices and therefore high loadings are always required to realize the target EMI SE (20 dB) for commercial application. For instance, the average EMI SE of 21.0 dB for the CNT/epoxy composite was obtained at 15 wt% CNT content [28]. Elaborate manipulation of specific conductive networks (i.e., double-percolated and segregated structure) can improve the EMI shielding performance [19,30,35,36]. Only 3.0 wt% CNT addition in the double percolated polycarbonate/poly(styrene-co-acrylonitrile) blends realized an EMI SE of 25 dB [19]. In our recent work, the segregated structure has been demonstrated to significantly increase EMI SE. The segregated CNT/polyethylene composite with 5.0 wt% CNT achieved an excellent EMI SE as high as 46.4 dB, 46% higher than the value for a conventional material [30]. The exceptional EMI SE (66.9 dB) for the s-GTR composite in this work far exceeds even the values reported in the CNT/polymer composites with a double percolated structure or a segregated structure at the similar CNT loading. It was well-established that the formation of the segregated structure is beneficial for the densification of the effective CNTs to create additional conducting pathways, giving rise to high electrical conductivity and thus EMI SE. The CB in the GTR domains also functions as conductive centers that interact with the incident electromagnetic microwaves and further enhance the EMI SE [37]. The EMI shielding mechanism was examined to understand the excellent EMI shielding performance. Fig. 3c presents a comparison of the microwave reflection (SER) and microwave absorption (SEA) of the s-GTR composites and pristine GTR. The SEA rises noticeably with the increase of CNT loading while the SER changes slightly, which means that contribution of SEA to the SEtotal dominates that of SER.
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Fig. 3. Electrical conductivity (a) and EMI SE (b) of the pristine GTR and the s-GTR composites as a function of CNT loading; (c) Microwave reflection (SER) and microwave absorption (SEA) versus CNT loading for the s-GTR composite and the pristine GTR; (d) Schematic representation of the EMI shielding mechanism for the s-GTR composites. (A colour version of this figure can be viewed online.)
Table 1 Average EMI SE in X-band frequency range for the presented s-GTR composites and the reported CNT/polymer composites. Aside from the existing CNT at the interfaces among GTR domains, the 2.0 and 5.0 wt% s-GTR also contains 13.1 and 12.7 wt% CB, respectively. Matrix
CNT content (wt%)
Electrical conductivity (S/m)
Thickness (mm)
EMI SE (dB)
Specific EMI SE (dB/mm)
Flexible
Reference
GTR GTR GTR UHMWPEa PC/PVDFa PC/SANa PVDFa WPUa PSa PTTa ABSa PEa PEa PSa BRa PUa PPa Epoxy WPUa PUa
2.0 5.0 5.0 2.0 2.0 3.0 3.0 4.8 5.0 5.0 5.0 5.0 5.0 5.5 7.4 10.0 10.0 15.0 16.7 20
12.9 109.3 109.3 2.0 6 10 5 5 10 4 0.03 10 5 1.0 3.0 10.0 13.7 0.8 1.0 6 10 5 12.4 8 10 3 20.0 0.2 2.2 10
2.6 2.6 0.4 e 5.0 5.0 e 1.0 2.0 2.0 2.8 2.1 e e 1.0 1.5 2.8 2.0 0.6 2.0
36.8 66.9 22.4 e 18.0 25.0 26.0 7.5 23.0 23.5 38.0 31.7 e e 11.0 29.0 25.0 21.0 17.0 17.5
14.2 25.7 56.0 e 3.6 5.0 e 7.5 11.5 11.8 13.6 15.1 e e 11.0 19.3 8.9 10.5 28.3 8.8
Yes Yes Yes No Yes No Yes Yes No No No No No No Yes No No No Yes No
This work This work This work [32] [18] [19] [20] [21] [22] [23] [24] [30] [34] [33] [25] [26] [27] [28] [21] [29]
2
a UHMWPE, PC, PVDF, SAN, WPU, PS, PTT, ABS, PE, BR, PU, and PP are ultrahigh molecular weight polyethylene, polycarbonate, poly(vinylidene fluoride), poly(styreneacrylonitrile), waterborne polyurethane, polystyrene, poly(trimethylene terephthalate), acrylonitrileebutadieneestyrene, polyethylene, butyl rubber, polyurethane and polypropylene, respectively.
In the case of the composite loaded with 5.0 wt% CNT at the frequency of 10.3 GHz, the SEtotal, SEA, and SER are 67.6, 60.0 and 7.6 dB, respectively, showing that microwave absorption (89%) is the dominant shielding mechanism. The absorption-dominant shielding mechanism should mainly originate from the synergistic effect
of the unique segregated CNT conducting networks and the CB in the GTR domains, as represented schematically in Fig. 3d. The segregated composite can be regarded as a shielding material composed of an extremely larger number of core shell particles with GTR domains as the cores and highly conductive CNT layers as
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continuous shells. Such a unique structure is expected to create a huge interface area between the GTR domains and CNT layers for reflecting, scattering and absorbing the incident electromagnetic microwaves many times inside the composites. Furthermore, the many CB sites in the GTR domains also act as attenuating centers that interact with the electromagnetic microwaves, leading to a further enhancement of SEA and the outstanding EMI shielding performance. Then we systematically studied the mechanical performance of s-GTR composites, which is of great importance for practical applications. Fig. S3a shows the representative stressstrain curves for the pristine GTR and s-GTR composites with varying CNT content. As illustrated in Fig. S3b, the Young's modulus increases obviously with the addition of CNT, which is mainly attributed to the highly stiffness of CNTs. The tensile strength and elongation at break decrease as the CNT content rises, yet still maintain the relatively satisfied values of 1.5 MPa and 52% in 5.0 CNT-loaded composite. The cyclic stretching-releasing tests were also performed to evaluate the elasticity of the s-GTR composite (Fig. S3c). These results show a small plastic deformation and no evident strength degradation after 100 cycles, indicating excellent mechanical robustness. The good mechanical performance makes the s-GTR composite an ideal EMI shielding candidate for flexible devices. In addition to the conductive networks, the shield thickness also plays an important role in the EMI shielding performance for a shielding material [16,38e41]. The EMI SE of the 5.0 wt% s-GTR composite specimens with varying thickness is thus characterized, as presented in Fig. 4a. Specimens with larger thickness display more striking EMI shielding performance, due to the increased amount of CNTs and CB that interact with the incident electromagnetic microwaves. It is worth noting that the s-GTR composite at only 0.4 mm thickness already shows an EMI SE of 22.4 dB. This means that the s-GTR composite that starts to fulfill the commercial EMI shielding application requires the critical thickness of only 0.4 mm and the CNT loading of only 5.0 wt%. Both are the lowest values among the reported CNT/polymer composites (0.6 mm @ 16.7 wt% for the CNT/PU composite (17 dB) [21], 2.0 mm @ 15 wt% for the CNT/epoxy composite (21 dB) [28], 2.0 mm @ 5.0 wt% for the CNT/PS composite (23 dB) [22]). In addition, the specific EMI SE (EMI SE divided by material thickness) of the s-GTR composites is much higher than those of reported CNT based composites at similar filler loadings. These results further highlight the advantage of s-GTR composites for efficient EMI shielding. As shown in Fig. 4b, it should be emphasized that the SEA of the composite increases with the increase of thickness while SER remains almost constant. The adsorption-dominated shielding mechanism is proven again
271
Fig. 5. EMI SE of the 5.0 wt% CNT loaded composite (0.4 mm thickness) before and after repeated bending to the radius of 2.0 mm for 5000 times. Left inset shows variation of the normalized resistance (R/R0) of the 5.0 wt% s-GTR composite as a function of the bending radius. Right inset shows the R/R0 as a function of bending cycles (radius of 2.0 mm). (A colour version of this figure can be viewed online.)
because of the dominant contribution of SEA that is as high as 81% of the SEtotal, for the thicknesses varying from 0.4 to 2.6 mm. A major feature of the s-GTR composite is its flexible nature, which is extremely valuable for its practical applications in the next-generation flexible electronics [42e46]. EMI SE reliability under mechanical deformation is a crucial indicator for the evaluation of the flexibility of a shielding material. Fig. 5 presents the EMI SE of the 5.0 wt% s-GTR composite before and after repeated bending to the radius of 2.0 mm for 5000 times. It is apparent that the EMI SE only shows very limited decreases over the entire frequency range, with the average EMI SE changing from 22.4 to 20.9 dB (93% retention). Excitingly, the EMI SE of the s-GTR composites after the bending deformation still satisfies the requirements for commercial EMI shielding. The effect of bending deformation on the electrical performance of the composite is investigated to elucidate the underlying mechanism. As shown in the left inset of Fig. 5, the resistance is almost unchanged even with the bend radius of 2.0 mm during the first bending-release cycle. The cyclic bending-release test (right inset of Fig. 5) further demonstrates that the composite resistance maintains its high stability, with only 4% change even after 5000 cycles. The high cyclic
Fig. 4. (a) EMI SE for the s-GTR composites with specimen thicknesses. (b) Comparison of SEtotal, SEA, and SER at the frequency of 10.3 GHz. (A colour version of this figure can be viewed online.)
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reliability of electrical performance is responsible for the superior EMI SE reliability of the s-GTR composites. It is worth mentioning that the electrical conductivity of CNT and the flexibility of GTR are effectively combined in the s-GTR composite to achieve high EMI SE and EMI SE reliability. This indicates the great potential of such composites as flexible shielding materials in next-generation flexible electronics, especially for use on curved surfaces and movable parts. Compared to the conventional method utilizing GTR as an additive, the direct utilization of GTR into an advanced multifunctional material for EMI shielding achieves a much higher additional value, as realized here for the first time. 4. Conclusions GTR is first utilized to develop highly valuable EMI shielding materials that also show low active material loading, which has tremendous scientific interest for the minimization of chemical waste and environmental benefits. With only 5.0 wt% CNT, the sGTR composite exhibits an EMI SE as high as 66.9 dB, the highest value ever reported in CNT/polymer composites. The critical thickness for the composite necessary to fulfill the requirements of commercial EMI shielding applications (20 dB) is only 0.4 mm. Furthermore, the excellent flexibility imparts reliable EMI SE to the composite that is almost unchanged even after repeated bending to the radius of 2.0 mm for 5000 times. Our work demonstrates the efficient utilization of GTR into a flexible and high-performance EMI shielding material for use in next-generation flexible electronics through a facile, scalable, and sustainable approach. We believe that the conductive s-GTR material will also provide a new pathway for applications in other fields, such as sensors and interconnects. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51673134, 51273131, and 51473102), the Innovation Team Program of Science and Technology Department of Sichuan Province (Grant No. 2014TD0002), and the China Postdoctoral Science Found (Grant No. 2015M572474, 2016T90848). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2017.05.100. References [1] B. Adhikari, D. De, S. Maiti, Reclamation and recycling of waste rubber, Prog. Polym. Sci. 25 (7) (2000) 909e948. [2] Z. Liu, X. Li, X. Xu, X. Wang, C. Dong, F. Liu, et al., Devulcanizaiton of waste tread rubber in supercritical carbon dioxide: operating parameters and product characterization, Polym. Degrad. Stabil. 119 (2015) 198e207. [3] H.P. Xiang, H.J. Qian, Z.Y. Lu, M.Z. Rong, M.Q. Zhang, Crack healing and reclaiming of vulcanized rubber by triggering the rearrangement of inherent sulfur crosslinked networks, Green Chem. 17 (8) (2015) 4315e4325. [4] E.P. Rhodes, Z. Ren, D.C. Mays, Zinc leaching from tire crumb rubber, Environ. Sci. Technol. 46 (23) (2012) 12856e12863. [5] S. Ramarad, M. Khalid, C.T. Ratnam, A.L. Chuah, W. Rashmi, Waste tire rubber in polymer blends: a review on the evolution, properties and future, Prog. Mater. Sci. 72 (2015) 100e140. [6] B. Sripornsawat, S. Saiwari, S. Pichaiyut, C. Nakason, Influence of ground tire rubber devulcanization conditions on properties of its thermoplastic vulcanizate blends with copolyester, Eur. Polym. J. 85 (2016) 279e297. [7] K. Aoudia, S. Azem, N. Aït Hocine, M. Gratton, V. Pettarin, S. Seghar, Recycling of waste tire rubber: microwave devulcanization and incorporation in a thermoset resin, Waste Manag. 60 (2017) 471e481. [8] M. Sienkiewicz, K. Borze˛ dowska-Labuda, A. Wojtkiewicz, H. Janik, Development of methods improving storage stability of bitumen modified with ground tire rubber: a review, Fuel Process Technol. 159 (2017) 272e279.
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Flexible and efficient electromagnetic interference shielding materials from ground tire rubber Li-Chuan Jia a, Yi-Ke Li b, Ding-Xiang Yan a, c, * a
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China Chengdu Shude High School, Chengdu, 610031, Sichuan, China c School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2017 Received in revised form 22 May 2017 Accepted 31 May 2017 Available online 2 June 2017
Low-cost flexible EMI shielding materials have great market potential due to the rapid development of flexible electronics such as wearable health monitoring systems, flexible displays, and flexible power supply systems. Here, by fully exploiting its three-dimensional (3D) cross-linked structure, the ground tire rubber (GTR) in industrial waste was converted into a valuable and high-performance electromagnetic interference (EMI) shielding material. A carbon nanotube (CNT)/GTR composite with typical segregated structure was designed, with CNTs selectively localized at the boundaries of GTR domains. The composite containing only 5.0 wt% CNT exhibits a high electrical conductivity of 109.3 S/m and an EMI shielding effectiveness (SE) of 66.9 dB, superior to most of the reported CNT/polymer composites. Additionally, the CNT/GTR composite shows excellent flexibility and stability with 93% retention of EMI SE even after repeatedly bending to the radius of 2.0 mm for 5000 times. Our flexible EMI shielding material will benefit the fast-growing next-generation flexible electronic devices by providing low cost and reliable protection. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Waste tire rubber (WTR) has become a heavy burden, the socalled “Black Pollution”, to the environment due to the enormous amount of tire consumption (more than 17 million tons per year) [1e7]. The irreversible vulcanized structure and the incorporation of various stabilizers make the management or recycling of WTR a serious technical, economic and ecological problem [5]. Compared to the traditional devulcanization of WTR, downsizing of WTR into ground tire rubber (GTR) for use as an additive material in asphalt paving mixtures, Portland cement concrete, or polymers (such as rubbers, thermoplastics and thermosets) is considered to be an effective waste treatment method [8e15]. Nevertheless, the utilization of GTR as a simple additive does not impart the host material with high economic value added, and sometimes even leads to a sacrifice in the performance or inferior quality of the product. For instance, only 10 phr GTR added in polyethylene resulted in 13%
* Corresponding author. College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China. E-mail address: [email protected] (D.-X. Yan). http://dx.doi.org/10.1016/j.carbon.2017.05.100 0008-6223/© 2017 Elsevier Ltd. All rights reserved.
and 16% decrease in tensile strength and elongation at break compared to the virginal polyethylene [11]. Given the inefficiency in utilizing GTR, a considerable advance may be obtained by developing GTR into advanced multifunctional materials with high value. Interestingly, even at a high temperature, GTR possesses intrinsic three-dimensional (3D) cross-linked structure similar to that of a high-viscosity gel. After mixing with conductive nanofillers, this feature would restrict the penetration of the nanofillers into GTR interior and give rise to the selective distribution of the nanofillers at the interfaces of GTR domains to form a segregated structure. The segregated structure is well-known to construct highly dense conductive networks that improve the electrical and electromagnetic interference (EMI) shielding performance of polymer composites [16,17]. Moreover, the reinforcing carbon black (CB) in GTR could construct additional conductive networks and serve as conductive centers that interact with the incoming electromagnetic waves, also promoting the EMI shielding performance. To demonstrate those concepts, by fully exploiting the 3D crosslinked structure and CB in GTR, we facilely design a valuable and high-performance EMI shielding material, i.e., carbon nanotube (CNT)/GTR composite with a typical segregated structure via
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mechanical blending and hot compaction method (Fig. 1). In this structure, the combination of CNTs selectively distributed among the GTR domains and CB in the GTR domains endow the CNT/GTR based composite with compelling electrical and EMI shielding performance. The composite containing only 5.0 wt% CNT achieves an ultra-high EMI shielding effectiveness (EMI SE) of 66.9 dB (meaning 99.9998% attenuation of electromagnetic radiation), the highest value among the reported CNT/polymer composites at such a low CNT loading [18e30]. The composite shows sufficient flexibility and highly reliable EMI shielding performance under mechanical deformation, with 93% retention of EMI SE even after repeatedly bending to the radius of 2.0 mm for 5000 times. 2. Experimental 2.1. Materials and fabrication The raw materials include GTR as the polymer matrix and CNT as the conductive filler. GTR (60 mesh size) produced from passenger car and light truck tires was kindly supplied by the Sichuan Zhongneng Rubber Co. LTD., China, with the composition of 62.8 wt % hydrocarbon, 13.4 wt% carbon black, and 23.8% ash (Fig. S1 and S2b). CNTs are the NC 7000 series (average diameter of 9.5 nm, average length of 1.5 mm, surface area of 250e300 m2/g) purchased from Nanocyl S.A., Belgium. We used a facile, efficient, economic and green method to process the segregated CNT/GTR composite (denoted as s-GTR composite), i.e., mechanical blending and compression molding. CNT and GTR were first mechanically mixed in a high-speed mixer (24,000 rpm, 2 min) at room temperature to obtain CNT-coated GTR complex granules. The complex granules were then compression-molded into composite sheets with various thicknesses (0.4, 1.0, 1.5, 2.0, 2.6 mm), under the pressure of 50 MPa at 170 C, after preheating for 10 min. For comparison, the pristine GTR (without the addition of CNT) was also fabricated under the same processing conditions. 2.2. Characterization The morphologies of the s-GTR composite and pristine GTR were analyzed by high-resolution field emission scanning electron
microscopy (FESEM, Inspect-F, FEI, Finland) at the accelerating voltage of 15 kV. The specimens for SEM observations were quickly cryo-fractured after immersing in liquid nitrogen for 30 min and the freshly fractured surfaces were coated with gold. Thermogravimetric analysis (TGA) was carried out from 30 to 700 C using a thermogravimetric analyzer (TG209 F1, NETZSCH, Germany) at the heating rate of 10 C/min, under both nitrogen and ambient atmosphere. Electrical performance was characterized with a Keithley electrometer model 4200-SCS (USA) and a four-point probe instrument (RTS-8, Guangzhou Four-Point Probe Technology Co., Ltd., China). The real-time variation of electrical resistance under mechanical deformation was recorded using the Keithley electrometer model. EMI shielding characteristics were studied using an Agilent N5230 vector network analyzer, with the APC-7 connector as the coaxial test cell. Scattering parameters (S11, S21) were measured in the X-band frequency range (8.2e12.4 GHz) to obtain the reflected power (R), transmitted power (T), and absorbed power (A). Then, the EMI SE (SEtotal), microwave reflection (SER), and microwave absorption (SEA) can be calculated (detailed calculations seen in Supporting Information).
3. Results and discussion Fig. 2a, b show the surface morphology of the CNT/GTR complex granules. Numerous CNTs are deposited onto the surfaces of GTR granules. The specific surface areas of the GTR granules are greatly enlarged due to the irregular shapes with rough surfaces (shown in Fig. S2a), encouraging the efficient adsorption of CNTs. Through the compression molding, the CNT-coated GTR complex granules were consolidated into s-GTR composites with predetermined shapes that exhibit excellent flexibility, as shown in Fig. 2cef. The compression pressure induces the intimate contact of the GTR granules and the heat during the processing provides the energy to break the polysulfidic crosslinks [31]. The broken cross-link bonds can create new links, thereby “sintering” the GTR granules into a bulk material. The compressive creep of GTR molecules also facilitates the fabrication of such bulk material. Additionally, the survived 3D cross-linked structure within the GTR domains endows the s-GTR composite with fascinating flexibility. The composite can maintain its initial form even after enduring a rigorous bending-
Fig. 1. Schematic for the fabrication of flexible CNT/GTR composite with segregated structure. (A colour version of this figure can be viewed online.)
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269
Fig. 2. (a) SEM micrographs of 3.0 wt% CNT coated GTR complex granules; (b) magnified image of (a); (c) Digital images of the s-GTR composites with wafer shape; (d), (e) and (f) Digital images of the s-GTR composites with a bending-release cycle (radius of ~2.0 mm). (g) SEM images of the 3.0 wt% s-GTR composite; (h) and (i) are the magnified images of (g). (A colour version of this figure can be viewed online.)
release cycle with the radius of 2.0 mm (Fig. 2def). The SEM images of the fractured surface of the s-GTR composite were taken to characterize the detailed microstructure. A typical segregated structure is observed, with the CNTs squeezed along specific paths (bright lines) surrounding the GTR domains (dotted cycle) to form continuous conducting networks (Fig. 2g). The magnified images (Fig. 2h, i) further indicate that adjacent CNTs overlap to develop dense paths along the boundaries of the GTR domains. The movement of GTR molecular chains is extremely confined due to the original 3D cross-linked structure suppressing the penetration of the CNTs into interior of the GTR domains and guaranteeing the construction of a segregated structure. Additionally, much CB is visually shown in the interior of the GTR domains (Fig. 2i). The merit of the unique segregated structure combined with the presence of CB is expected to impart outstanding electrical and EMI shielding performance to the s-GTR composites. Fig. 3a shows the electrical conductivity of the pristine GTR and the s-GTR composite as a function of CNT loading. Due to the presence of CB, the pristine GTR exhibits an electrical conductivity of 4.8 10 7 S/m. Excitingly, only 1.0 wt% CNT loading gives the sGTR composite a high electrical conductivity (2.6 S/m) that already exceeds the target value (1.0 S/m) for commercial EMI shielding applications [16]. Increasing CNT loading to 5.0 wt% results in a much higher electrical conductivity of 109.3 S/m, among the highest values ever reported in CNT/polymer composites at the similar CNT loading level (Table 1) [18e30]. The electrical conductivity of the s-GTR composite is even superior to those of various CNT/polymer (such as CNT/ultrahigh molecular weight polyethylene, CNT/polystyrene, and CNT/polyethylene) composites with segregated structure [32e34]. In addition to the contribution of the segregated structure, the CB embedded in the GTR domains also contributes to constructing additional conductive networks and thus results in the unprecedented electrical performance. This result means that the use of percolated or conductive polymer domains as matrix to construct a segregated structure can facilitate the fabrication of more conductive CNT/polymer composites. The outstanding electrical conductivity is beneficial for the development of high-performance EMI shielding materials. Fig. 3b shows the increased EMI SE with CNT loading in the s-GTR composites
over the measured frequency range (X band, 8.2e12.4 GHz). A higher CNT loading not only improves the electrical conductivity but also provides a larger amount of free electrons to attenuate the incident electromagnetic waves inside the s-GTR composite. We note that the s-GTR composite containing only 1.0 wt% CNT already exhibits an EMI SE of 24.5 dB that satisfies the requirements of commercial EMI shielding application (20 dB). As the CNT loading rises to 5.0 wt%, the composite achieves an ultra-high EMI SE of 66.9 dB, far superior to the highest values ever reported in CNT/ polymer composites (Table 1) [18e30]. In conventional CNT/polymer composites, CNTs are randomly distributed throughout the polymer matrices and therefore high loadings are always required to realize the target EMI SE (20 dB) for commercial application. For instance, the average EMI SE of 21.0 dB for the CNT/epoxy composite was obtained at 15 wt% CNT content [28]. Elaborate manipulation of specific conductive networks (i.e., double-percolated and segregated structure) can improve the EMI shielding performance [19,30,35,36]. Only 3.0 wt% CNT addition in the double percolated polycarbonate/poly(styrene-co-acrylonitrile) blends realized an EMI SE of 25 dB [19]. In our recent work, the segregated structure has been demonstrated to significantly increase EMI SE. The segregated CNT/polyethylene composite with 5.0 wt% CNT achieved an excellent EMI SE as high as 46.4 dB, 46% higher than the value for a conventional material [30]. The exceptional EMI SE (66.9 dB) for the s-GTR composite in this work far exceeds even the values reported in the CNT/polymer composites with a double percolated structure or a segregated structure at the similar CNT loading. It was well-established that the formation of the segregated structure is beneficial for the densification of the effective CNTs to create additional conducting pathways, giving rise to high electrical conductivity and thus EMI SE. The CB in the GTR domains also functions as conductive centers that interact with the incident electromagnetic microwaves and further enhance the EMI SE [37]. The EMI shielding mechanism was examined to understand the excellent EMI shielding performance. Fig. 3c presents a comparison of the microwave reflection (SER) and microwave absorption (SEA) of the s-GTR composites and pristine GTR. The SEA rises noticeably with the increase of CNT loading while the SER changes slightly, which means that contribution of SEA to the SEtotal dominates that of SER.
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Fig. 3. Electrical conductivity (a) and EMI SE (b) of the pristine GTR and the s-GTR composites as a function of CNT loading; (c) Microwave reflection (SER) and microwave absorption (SEA) versus CNT loading for the s-GTR composite and the pristine GTR; (d) Schematic representation of the EMI shielding mechanism for the s-GTR composites. (A colour version of this figure can be viewed online.)
Table 1 Average EMI SE in X-band frequency range for the presented s-GTR composites and the reported CNT/polymer composites. Aside from the existing CNT at the interfaces among GTR domains, the 2.0 and 5.0 wt% s-GTR also contains 13.1 and 12.7 wt% CB, respectively. Matrix
CNT content (wt%)
Electrical conductivity (S/m)
Thickness (mm)
EMI SE (dB)
Specific EMI SE (dB/mm)
Flexible
Reference
GTR GTR GTR UHMWPEa PC/PVDFa PC/SANa PVDFa WPUa PSa PTTa ABSa PEa PEa PSa BRa PUa PPa Epoxy WPUa PUa
2.0 5.0 5.0 2.0 2.0 3.0 3.0 4.8 5.0 5.0 5.0 5.0 5.0 5.5 7.4 10.0 10.0 15.0 16.7 20
12.9 109.3 109.3 2.0 6 10 5 5 10 4 0.03 10 5 1.0 3.0 10.0 13.7 0.8 1.0 6 10 5 12.4 8 10 3 20.0 0.2 2.2 10
2.6 2.6 0.4 e 5.0 5.0 e 1.0 2.0 2.0 2.8 2.1 e e 1.0 1.5 2.8 2.0 0.6 2.0
36.8 66.9 22.4 e 18.0 25.0 26.0 7.5 23.0 23.5 38.0 31.7 e e 11.0 29.0 25.0 21.0 17.0 17.5
14.2 25.7 56.0 e 3.6 5.0 e 7.5 11.5 11.8 13.6 15.1 e e 11.0 19.3 8.9 10.5 28.3 8.8
Yes Yes Yes No Yes No Yes Yes No No No No No No Yes No No No Yes No
This work This work This work [32] [18] [19] [20] [21] [22] [23] [24] [30] [34] [33] [25] [26] [27] [28] [21] [29]
2
a UHMWPE, PC, PVDF, SAN, WPU, PS, PTT, ABS, PE, BR, PU, and PP are ultrahigh molecular weight polyethylene, polycarbonate, poly(vinylidene fluoride), poly(styreneacrylonitrile), waterborne polyurethane, polystyrene, poly(trimethylene terephthalate), acrylonitrileebutadieneestyrene, polyethylene, butyl rubber, polyurethane and polypropylene, respectively.
In the case of the composite loaded with 5.0 wt% CNT at the frequency of 10.3 GHz, the SEtotal, SEA, and SER are 67.6, 60.0 and 7.6 dB, respectively, showing that microwave absorption (89%) is the dominant shielding mechanism. The absorption-dominant shielding mechanism should mainly originate from the synergistic effect
of the unique segregated CNT conducting networks and the CB in the GTR domains, as represented schematically in Fig. 3d. The segregated composite can be regarded as a shielding material composed of an extremely larger number of core shell particles with GTR domains as the cores and highly conductive CNT layers as
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continuous shells. Such a unique structure is expected to create a huge interface area between the GTR domains and CNT layers for reflecting, scattering and absorbing the incident electromagnetic microwaves many times inside the composites. Furthermore, the many CB sites in the GTR domains also act as attenuating centers that interact with the electromagnetic microwaves, leading to a further enhancement of SEA and the outstanding EMI shielding performance. Then we systematically studied the mechanical performance of s-GTR composites, which is of great importance for practical applications. Fig. S3a shows the representative stressstrain curves for the pristine GTR and s-GTR composites with varying CNT content. As illustrated in Fig. S3b, the Young's modulus increases obviously with the addition of CNT, which is mainly attributed to the highly stiffness of CNTs. The tensile strength and elongation at break decrease as the CNT content rises, yet still maintain the relatively satisfied values of 1.5 MPa and 52% in 5.0 CNT-loaded composite. The cyclic stretching-releasing tests were also performed to evaluate the elasticity of the s-GTR composite (Fig. S3c). These results show a small plastic deformation and no evident strength degradation after 100 cycles, indicating excellent mechanical robustness. The good mechanical performance makes the s-GTR composite an ideal EMI shielding candidate for flexible devices. In addition to the conductive networks, the shield thickness also plays an important role in the EMI shielding performance for a shielding material [16,38e41]. The EMI SE of the 5.0 wt% s-GTR composite specimens with varying thickness is thus characterized, as presented in Fig. 4a. Specimens with larger thickness display more striking EMI shielding performance, due to the increased amount of CNTs and CB that interact with the incident electromagnetic microwaves. It is worth noting that the s-GTR composite at only 0.4 mm thickness already shows an EMI SE of 22.4 dB. This means that the s-GTR composite that starts to fulfill the commercial EMI shielding application requires the critical thickness of only 0.4 mm and the CNT loading of only 5.0 wt%. Both are the lowest values among the reported CNT/polymer composites (0.6 mm @ 16.7 wt% for the CNT/PU composite (17 dB) [21], 2.0 mm @ 15 wt% for the CNT/epoxy composite (21 dB) [28], 2.0 mm @ 5.0 wt% for the CNT/PS composite (23 dB) [22]). In addition, the specific EMI SE (EMI SE divided by material thickness) of the s-GTR composites is much higher than those of reported CNT based composites at similar filler loadings. These results further highlight the advantage of s-GTR composites for efficient EMI shielding. As shown in Fig. 4b, it should be emphasized that the SEA of the composite increases with the increase of thickness while SER remains almost constant. The adsorption-dominated shielding mechanism is proven again
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Fig. 5. EMI SE of the 5.0 wt% CNT loaded composite (0.4 mm thickness) before and after repeated bending to the radius of 2.0 mm for 5000 times. Left inset shows variation of the normalized resistance (R/R0) of the 5.0 wt% s-GTR composite as a function of the bending radius. Right inset shows the R/R0 as a function of bending cycles (radius of 2.0 mm). (A colour version of this figure can be viewed online.)
because of the dominant contribution of SEA that is as high as 81% of the SEtotal, for the thicknesses varying from 0.4 to 2.6 mm. A major feature of the s-GTR composite is its flexible nature, which is extremely valuable for its practical applications in the next-generation flexible electronics [42e46]. EMI SE reliability under mechanical deformation is a crucial indicator for the evaluation of the flexibility of a shielding material. Fig. 5 presents the EMI SE of the 5.0 wt% s-GTR composite before and after repeated bending to the radius of 2.0 mm for 5000 times. It is apparent that the EMI SE only shows very limited decreases over the entire frequency range, with the average EMI SE changing from 22.4 to 20.9 dB (93% retention). Excitingly, the EMI SE of the s-GTR composites after the bending deformation still satisfies the requirements for commercial EMI shielding. The effect of bending deformation on the electrical performance of the composite is investigated to elucidate the underlying mechanism. As shown in the left inset of Fig. 5, the resistance is almost unchanged even with the bend radius of 2.0 mm during the first bending-release cycle. The cyclic bending-release test (right inset of Fig. 5) further demonstrates that the composite resistance maintains its high stability, with only 4% change even after 5000 cycles. The high cyclic
Fig. 4. (a) EMI SE for the s-GTR composites with specimen thicknesses. (b) Comparison of SEtotal, SEA, and SER at the frequency of 10.3 GHz. (A colour version of this figure can be viewed online.)
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reliability of electrical performance is responsible for the superior EMI SE reliability of the s-GTR composites. It is worth mentioning that the electrical conductivity of CNT and the flexibility of GTR are effectively combined in the s-GTR composite to achieve high EMI SE and EMI SE reliability. This indicates the great potential of such composites as flexible shielding materials in next-generation flexible electronics, especially for use on curved surfaces and movable parts. Compared to the conventional method utilizing GTR as an additive, the direct utilization of GTR into an advanced multifunctional material for EMI shielding achieves a much higher additional value, as realized here for the first time. 4. Conclusions GTR is first utilized to develop highly valuable EMI shielding materials that also show low active material loading, which has tremendous scientific interest for the minimization of chemical waste and environmental benefits. With only 5.0 wt% CNT, the sGTR composite exhibits an EMI SE as high as 66.9 dB, the highest value ever reported in CNT/polymer composites. The critical thickness for the composite necessary to fulfill the requirements of commercial EMI shielding applications (20 dB) is only 0.4 mm. Furthermore, the excellent flexibility imparts reliable EMI SE to the composite that is almost unchanged even after repeated bending to the radius of 2.0 mm for 5000 times. Our work demonstrates the efficient utilization of GTR into a flexible and high-performance EMI shielding material for use in next-generation flexible electronics through a facile, scalable, and sustainable approach. We believe that the conductive s-GTR material will also provide a new pathway for applications in other fields, such as sensors and interconnects. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51673134, 51273131, and 51473102), the Innovation Team Program of Science and Technology Department of Sichuan Province (Grant No. 2014TD0002), and the China Postdoctoral Science Found (Grant No. 2015M572474, 2016T90848). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2017.05.100. References [1] B. Adhikari, D. De, S. Maiti, Reclamation and recycling of waste rubber, Prog. Polym. Sci. 25 (7) (2000) 909e948. [2] Z. Liu, X. Li, X. Xu, X. Wang, C. Dong, F. Liu, et al., Devulcanizaiton of waste tread rubber in supercritical carbon dioxide: operating parameters and product characterization, Polym. Degrad. Stabil. 119 (2015) 198e207. [3] H.P. Xiang, H.J. Qian, Z.Y. Lu, M.Z. Rong, M.Q. Zhang, Crack healing and reclaiming of vulcanized rubber by triggering the rearrangement of inherent sulfur crosslinked networks, Green Chem. 17 (8) (2015) 4315e4325. [4] E.P. Rhodes, Z. Ren, D.C. Mays, Zinc leaching from tire crumb rubber, Environ. Sci. Technol. 46 (23) (2012) 12856e12863. [5] S. Ramarad, M. Khalid, C.T. Ratnam, A.L. Chuah, W. Rashmi, Waste tire rubber in polymer blends: a review on the evolution, properties and future, Prog. Mater. Sci. 72 (2015) 100e140. [6] B. Sripornsawat, S. Saiwari, S. Pichaiyut, C. Nakason, Influence of ground tire rubber devulcanization conditions on properties of its thermoplastic vulcanizate blends with copolyester, Eur. Polym. J. 85 (2016) 279e297. [7] K. Aoudia, S. Azem, N. Aït Hocine, M. Gratton, V. Pettarin, S. Seghar, Recycling of waste tire rubber: microwave devulcanization and incorporation in a thermoset resin, Waste Manag. 60 (2017) 471e481. [8] M. Sienkiewicz, K. Borze˛ dowska-Labuda, A. Wojtkiewicz, H. Janik, Development of methods improving storage stability of bitumen modified with ground tire rubber: a review, Fuel Process Technol. 159 (2017) 272e279.
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