Polypropylene

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Materials and Interfaces

Injection molded segregated carbon nanotube/polypropylene composite for efficient electromagnetic interference shielding Hong-Yuan Wu, Yun-Peng Zhang, Li-Chuan Jia, Ding-Xiang Yan, Jie-feng Gao, and Zhong-Ming Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02293 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Industrial & Engineering Chemistry Research

Injection

molded

segregated

carbon

nanotube/polypropylene

composite for efficient electromagnetic interference shielding

Hong-Yuan Wu,† Yun-Peng Zhang,† Li-Chuan Jia,† Ding-Xiang Yan,*,‡ Jie-Feng Gao,§ Zhong-Ming Li†

†

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, Chengdu 610065, China ‡

School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China

§

The College of Chemistry and Chemical Engineering, Yangzhou University,

Yangzhou, 225009, China

ABSTRACT: Constructing segregated structure in conductive polymer composite (CPC) is effective to achieve high electromagnetic interference shielding effectiveness (EMI SE) at low filler loading. Nevertheless, the present segregated CPCs were only fabricated via compression molding technique, which limits their practical application. In this work, the injection molding technique was utilized for the first time to develop a carbon nanotube (CNT)/isotactic polypropylene (iPP)/atactic polypropylene (aPP) composite with typical segregated structure. The injection molded segregated CNT/PP composite exhibits an excellent EMI SE of 43.1 dB, which is 67% higher than the CNT/PP composite with randomly distributed CNTs, at the same CNT loading of 5.0 wt%. Such EMI SE is also comparable to the value for segregated CNT/PP

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

composites prepared by compression molding. Our work provides a promising strategy to develop segregated CPCs via an efficient injection molding technique, in comparison to compression molding technique.

KEYWORDS: carbon nanotube, polypropylene, injection molding, segregated structure, EMI shielding

1. INTRODUCTION Conductive polymer composites (CPCs) are promising candidates for electromagnetic interference (EMI) shielding because of low cost, light weight, corrosion resistance, and good processability.1-9 The formation of segregated structure has been demonstrated to be effective to achieve high EMI shielding effectiveness (EMI SE) in CPCs with low conductive filler loading.10-14 For instance, only 1.1 vol% graphene already endowed the poly(methyl methacrylate) composite with an EMI SE (20.7 dB) that exceeded the commercial EMI shielding level.10 Very recently, Yu et al. developed the MXene/polystyrene composite with a much higher EMI SE of 54 dB at 1.90 vol% MXene loading.15 In our previous work, comprehensive investigations were performed on CPCs with segregated and conventional structures, demonstrating that segregated structure was in favor of improving the effective concentration of conductive filler to form more perfect conductive networks and thus improving the EMI shielding performance.16-20 Till now, segregated CPCs with high EMI SE were significantly developed for efficient EMI shielding, based on various conductive


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fillers (e.g., carbon nanoparticles and metal nanoparticles) and polymer matrices (e.g., ultrahigh molecular polyethylene, conventional semicrystalline polymers, amorphous polymers, and elastomeric polymers).21-28 Nevertheless, it should be noted that the present segregated CPCs were manufactured via compression molding, under relatively low processing temperature or pressure. This is because that low shear force and high melt viscosity during processing are always required to avoid the migration of conductive fillers into the interior of polymer regions.17 The development of segregated CPCs via more efficient methods, e.g., injection molding or extrusion molding, is significantly important to expand their practical applications. Nevertheless, it is still a great challenge, due to the fact that the high shear effect during the injection molding or extrusion molding process would inevitably cause the diffusion of conductive fillers into polymer interior and disturb the selective distribution of conductive fillers. Herein, a segregated carbon nanotube (CNT)/polypropylene (PP) composite was fabricated via injection molding for the first time, with CNT as conductive filler, isotactic PP (iPP) as matrix, and atactic PP (aPP) as flow accelerator, as schematically shown in Figure 1. During injection, iPP granules only partly melted and maintained semi-solid state, while aPP with very low viscosity promoted the composite flowability. CNTs were selectively distributed in aPP to evolve CNT/aPP continuous phase, which surrounded isolated iPP regions to form segregated structure in the CNT/PP composite. To improve the effective CNT concentration to acquire high EMI SE, the aPP amount should be as low as possible, whereas, reducing aPP amount was



adverse to the composite flowability. Moreover, the distributed CNTs in aPP would increase its viscosity and also influence the composite flowability. To solve the above issues to make the CNT/PP composite injectable, we precoated CNTs on the surfaces of iPP granules before being mixed with aPP. In this case, CNTs were in-situ migrated into aPP during injection instead of being predisposed in aPP, which is beneficial to maintain the promoting effect of aPP on the composite flowability. Thus only 20 wt% aPP was sufficient to enable the injection of the segregated CNT/PP composite and significantly improved the effective CNT concentration. The composite with only 5.0 wt% CNT exhibits an excellent EMI SE of 43.1 dB, which is much higher than the value (25.8 dB) for conventionally injection molded CNT/PP composite, and also comparable to the value (45.0 dB) for segregated CNT/PP composite prepared by compression molding. This work provides insight into the facile preparation of segregated CPCs via injection molding, showing huge industrial potential for efficient EMI shielding materials.

Figure 1. Schematic for preparation of injection molded segregated CNT/PP composites.


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2. EXPERIMENTAL SECTION 2.1 Materials CNT (Nanocyl NC7000) was supplied by Nanocyl S.A., Belgium, with the average diameter of 9.5 nm, length of 1.5 µm, surface area of 250–300 m2/g, and carbon purity of 90%. The transmission electron microscope (TEM) image shows that CNTs have larger aspect ratio and are densely entangled with each other (Figure S1). The iPP (T30S) was purchased from Dushanzi Petroleum Chemical Co. (China), with melt flow rate (MFR) of 3 g/10 min (230 °C, 21.6 N), Mw of 39.9 × 104 g/mol, Mw/Mn of 4.6, and melting temperature (Tm) of 163°C (10 °C/min). Detailed information for iPP granules could be found in our previous work.19 The DSC heating curve and size distribution of neat PP granules were also displayed in Figure S2. The aPP (trade mark 399) was supplied by Exxon Mobil with, with Mw of 5.2 × 104 g/mol and Mw/Mn of 4.1. The rheological measurement indicates that aPP shows much lower complex viscosity (15 ~ 17 Pa·s, 160oC) than iPP (700 ~ 6000 Pa·s, 180 oC) even at lower temperature (Figure S3). Distilled water, xylene (AR grade), and alcohol (AR grade) were purchased from Chengdu Kelong Chemical Reagent Factory (China) and used as received. 2.2 Fabrication of the CNT/PP composites The schematic for the preparation of the segregated CNT/PP composites via injection molding was presented in Figure 1. Firstly, CNTs were dispersed in ethanol and iPP



granules were dispersed in distilled water/ethanol mixture under ultrasonication, respectively. The two suspensions were then mixed together under ultrasonication for 30 min, followed by filtrating and drying processes to prepare CNT coated iPP complex granules. Meanwhile, aPP was dissolve in xylene at 90 ºC for 60 min. Subsequently, the CNT coated iPP complex granules were added into aPP/xylene solution (20 mg/mL) under continuous stirring (120 rpm) for 10 min. After that, abundant ethanol was added to flocculate the mixture and the floccules were filtrated and dried in an oven (60 °C, 12 h) to remove solvent. Finally, the obtained CNT/iPP/aPP mixture was injection molded into dumbbell bars (dimension of narrow section is 25 × 4 × 2 mm3) via a HAAKE Mini Jet (Thermo Fisher Scientific, USA). The injection temperatures and mold temperatures were 160 °C and 100 °C. The injection pressure and holding pressure are 750 bar and 500 bar. And the injection time and dwell time were 5 s and 10 s. The injection molded CNT/iPP/aPP composite with segregated structure was marked as s-CNT/PP-IM. A series of s-CNT/PP-IM with various CNT contents (0.05, 0.1, 0.3, 0.5, 1.0, 2.0, 3.0 and 5.0 wt%) and constant aPP content (20 wt%) were prepared. For comparison, conventionally injection molded CNT/iPP/aPP composite with 5.0 wt% CNT and 20 wt% aPP was also prepared. The composite was marked as r-CNT/PP-IM. First, CNTs were dispersed in xylene under ultrasonication, while iPP and aPP was dissolved in xylene under strong stirring. Then the CNT/xylene and iPP/aPP/xylene solution were mixed, flocculated by ethanol, filtrated and dried. The obtained mixture was injection molded into dumbbell bars at 180 ºC to prepare r-CNT/PP-IM. We also prepared a segregated


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CNT/iPP composite with 5.0 wt% CNT using compression molding, according to our previous work.19 The composite was marked as s-CNT/PP-CM. 2.3 Characterization Transmission electron microscopy (TEM) observation was conducted on a FEI Tecnai F20 instrument at 200 kV. The morphologies of the CNT/PP composites were characterized using optical microscopy (OM, Olympus BX51) and field emission scanning electron microscope (FE-SEM, FEI Sirion 200). A microtome was utilized to prepare the 5 µm-thick specimens and the specimens were placed between two coverslips for OM observation. The specimens for SEM observation were cryofractured after being immersed in liquid nitrogen for 60 min. Before SEM observation, the fractured surface of the specimens, neat iPP granules, and CNT coated complex iPP granules were all coated with Au using a sputter coater. Electrical performance was measured using a Keithley electrometer model (4200-SCS, USA). The test voltage was fixed at 1.0 V and the specimens for electrical measurements are rectangular sheets with three-dimensional sizes of 12.0 mm in length, 5.0 mm in width and 2.0 mm in thickness. Silver paste was coated on both ends of the rectangular specimen to ensure good contact between specimen and contacted electrodes. The electrical conductivity is calculated according to the following equation:

= /

(1)

where l is the length, Re is the resistance, w is the thickness, and t is the width of the specimens. EMI shielding performance was conducted on an Agilent N5247A vector



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network analyzer. The specimens held the diameter in 13.0 mm and 2.0 mm in thickness. The S-parameters (S11, S21) in the frequency range of 8.2 - 12.4 GHz were collected. The EMI SE (SEtotal), microwave reflection (SER), and microwave absorption (SEA) was calculated using the following equations.

= | |

(2)

= | |

(3)

= 1−−

(4)

= −10Log1 −

(5)

= −10Log ⁄1 −

(6)

= !" = + + $

(7)

Where R, T, and A are power coefficients (reflectivity, transmissivity, and absorptivity), respectively. SEM is the microwave multiple internal reflections, which can be negligible when SEtotal ≥ 10 dB.29,30

3. RESULTS AND DISCUSSION 3.1 Morphology The morphologies of neat iPP granules and CNT coated iPP complex granules were performed. As shown in Figure 2a and b, neat iPP granules are spherical-like granules with average diameter of ~ 40 µm and the granules show rough surfaces (Figure 2c), which would benefit the absorption of CNTs. The observation in Figure 2d-f indicates that iPP granules are indeed covered with numerous CNTs, even at only 1.0 wt% CNT loading. This is the precondition for constructing segregated conductive networks in


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the CNT/PP composites.

Figure 2. SEM images of (a-c) the neat iPP granules and (d-f) the 1.0 wt% CNT coated iPP granules. The OM images of s-CNT/PP-IM with various CNT contents are shown in Figure 3a-c. As expected, typical segregated structure was formed, with CNT/aPP continuous phase (dark regions) surrounding isolated iPP regions (bright regions). Interconnected conductive networks already developed in s-CNT/PP-CM with only 0.5 wt% CNT. As CNT content increases to 1.0 wt % and 5.0 wt%, denser CNT/aPP grids were observed, indicative of perfect segregated conductive networks (Figure 3b and c). For s-CNT/PP-CM, segregated conductive networks were also achieved (Figure 3d), in line with our previous work.19 When it comes to r-CNT/PP-IM, segregated dark regions are observed instead of interconnected conductive networks (Fig 3e), which was derived from the CNT aggregates during the conventional mixing and injection process.



Figure 3. OM images of s-CNT/PP-IM with (a) 0.5 wt% CNT, (b) 1.0 wt% CNT, and (c) 5.0 wt% CNT. (d) OM images of s-CNT/PP-CM with 1.0 wt% CNT. (e) OM images of r-CNT/PP-IM with 1.0 wt% CNT. SEM observations were further performed to ascertain the detailed microstructure of the s-CNT/PP-IM composites. Figure 4 clearly shows the selective distribution of CNTs along specific paths to form segregated conductive networks, which is in accordance with OM results. The formation of segregated structure could be understood according to the unique preparation process of s-CNT/PP-IM. The injection temperature (160 ºC) is around the initial Tm of iPP and far surpasses the viscous flow temperature (Tf) of aPP. Thus, iPP granules only partly melted and maintained high viscosity while aPP held high flowability (Figure S3). This would lead to the easy migration of CNTs to the interior of aPP, rather than the interior iPP domains. Moreover, the high flowability aPP made CNT/aPP to form a continuous phase among the whole system. Then segregated structure was well-organized in s-CNT/PP-IM. Similar CNTs distribution was also found in s-CNT/PP-CM (Figure


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S4a and b), because of the solid phase of iPP domain during elevated processing pressure and temperature. The SEM images of r-CNT/PP-IM (Figure S4c and d) display that CNTs are randomly distributed in the composite with some aggregates. The microstructure of conductive networks in these CNT/PP composites would inevitably influence their electrical and EMI shielding performance.

Figure 4. SEM images of s-CNT/PP-IM with (a, b) 0.5 wt% CNT, (c, d) 1.0 wt% CNT, and (e, f) 5.0 wt% CNT. 3.2 Electrical conductive and EMI shielding performance Figure 5a shows the electrical conductivity of s-CNT/PP-IM as a function of CNT content. A dramatic increase (nearly 11 order of magnitudes) in electrical conductivity is observed from 10-14 S/m for pure PP to 4.1×10-3 S/m for s-CNT/PP-IM with only



0.1 wt% CNT, demonstrating the formation of percolated networks at such a low CNT loading. The classical percolation theory (detail discussions about the percolation model are shown in Supporting Information), σ = & ' − '( ) , was introduced to evaluate the relationship between electrical conductivity (σ) and CNT volume fraction (φ), where & is a constant related to the intrinsic conductivity of CNT, φc is the percolation threshold of s-CNT/PP-IM, t is a parameter related to the dimensionality of conductive networks.31 The fitted '( is as low as 0.021 vol% (Figure 5a insert), superior to most previously reported CNT/polymer composites and graphene/polymer composites, regardless of segregated structure or randomly distributed structure, as listed in Table 1..3,11,20 The slop of the fitted line gave t value of 1.9, indicating three-dimensional conductive networks in s-CNT/PP-IM.20 As CNT loading gradually increases, the electrical conductivity of s-CNT/PP-IM increases. Only 0.5 wt% CNTs endows s-CNT/PP-IM with an electrical conductivity of 3.0 S/m, already exceeding the target value (1 S/m) for commercial EMI shielding.32 Increasing CNT content to 5.0 wt% results in a much higher electrical conductivity of 86.3 S/m, which is far over 3.3 S/m for 5.0 wt% r-CNT/PP-IM and also comparable to 117.0 S/m for 5.0 wt% s-CNT/PP-CM. The outstanding electrical performance indicates that the shear force during injection molding did not disturb the segregated conductive networks in s-CNT/PP-IM. It is noted that this is for the first time a highly electrical conductive CNT/polymer composite with segregated structure was successfully manufactured via injection molding method. Among the CNT/iPP/aPP mixture for the preparation of s-CNT/PP-IM, CNTs were pre-coated on the surfaces of iPP granules rather than


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dispersed into aPP interior, thus the high flowability aPP was maintained and 20% aPP is sufficient to make the CNT/iPP/aPP mixture injection. The low aPP content in s-CNT/PP-IM is helpful to improve the effective CNT concentration to develop interconnected conductive networks, which result in excellent electrical performance.

Figure 5. (a) Electrical conductivity versus CNT content for s-CNT/PP-IM. The insert shows a log–log plot of the conductivity as a function of φ-φc. (b) EMI SE in X-band frequency range of s-CNT/PP-IM with various CNT contents. (c) EMI SE of s-CNT/PP-IM, s-CNT/PP-CM, and r-CNT/PP-IM with 5.0 wt% CNT. (d) Comparison of SETotal, SEA, and SER of s-CNT/PP-IM. The high-performance electrical conductivity stimulates us to explore the EMI shielding application of s-CNT/PP-IM. Figure 5b displays the EMI SE (X-band frequency range) of s-CNT/PP-IM with various CNT contents. Since EMI SE exhibits



weak frequency dependence on the whole frequency range, average EMI SE was used to assess the EMI shielding effect. At only 1.0 wt% CNT content, s-CNT/PP-IM achieves an EMI SE of 21.9 dB that exceeds the commercial EMI shielding level (20 dB).33 As CNT content increases to 5.0 wt%, EMI SE rises to 43.1 dB, meaning that 99.995% electromagnetic wave was blocked by s-CNT/PP-IM. Figure 5c also shows the EMI shielding performance of r-CNT/PP-IM and s-CNT/PP-CM with 5.0 wt% CNT for comparison. It is observed that among the whole frequency range, EMI SE of s-CNT/PP-IM is much higher than that for r-CNT/PP-IM and comparable to that for s-CNT/PP-CM, which is analogous to the results of electrical conductivity. The EMI SE achieved in s-CNT/PP-IM is also outstanding in comparison to previously reported CNT/polymer composites and graphene/polymer composites for EMI shielding, as presented in Table 1.3,11,20,34-47 For instance, Al-Saleh, et al. reported an EMI SE of 25.0 dB for 5.0 wt% CNT/PP composite.34 In our s-CNT/PP-IM, the excellent EMI shielding performance should be attributed to selective distribution of CNTs in CNT/aPP continuous phase instead of the whole system, even under shear force during injection molding. The formation of segregated conductive networks is in favor of the improved electrical conductivity and thus EMI SE. The EMI shielding mechanism was then investigated to understand the excellent EMI shielding performance of s-CNT/PP-IM. Figure S5 showed the power coefficients as a function of CNT loading. The reflectivity (R) exhibits a monotonous increase, because of the dielectric mismatch at interfaces with the increased CNT content. Whereas, the absorptivity (A) decreases, which could be attributed to the less


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electromagnetic wave entering the composite as a result of the better reflection at high CNT loading. It is noted that power coefficients are quantitative characteristics of power balance, while EMI SE is a relative quantity and has nothing to do with the absolute power value. Actually, A is a value describing the ratio of power dissipated by the shielding materials toward the overall incident power, while SEA is a measure of the ability to attenuate the electromagnetic power that has entered into the sample.34 Then the dependences of EMI SE (also denoted as SETotal), SEA, and SER on CNT content are evaluated and shown in Figure 5d. It can be seen that SETotal and SEA significantly increase with increased CNT content, while SER changes slightly. Obviously, SER is much lower than SEA among all the CNT contents, revealing that the improvement of SEtotal is mainly ascribed to the improvement of SEA. For instance, the SEtotal, SEA and SER of 5.0 wt% s-CNT/PP-IM are 43.1, 38.1, and 4.9 dB, respectively, which means that SEA contributes 88.4% to SEtotal, suggesting an absorption dominant shielding mechanism. A cell-like configuration can be found in s-CNT/PP-IM with conductive CNT/aPP layer act as cell wall,10 which provides numerous interfaces that reflect, scatter, and absorb the microwaves.12,16 The incident electromagnetic waves entering the interior of s-CNT/PP-IM could be trapped in numerous unique cell, and continue to bounce off the CNT/aPP cell walls until being attenuated, and thus the SEA is enhanced, revealing an absorption dominant shielding mechanism. Table 1. Comparison of percolation threshold and EMI SE for s-CNT/PP-IM and the previously reported CNT/polymer composites and graphene/polymer composites.



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Materials

Percolation threshold (vol%)

CNT content (wt%)

EMI SE (dB)

Thickness (mm)

Ref.

CNT/PPa

0.021

5.0

43.1

2.0

this work

CNT/PPb

~

5.0

25.0

2.8

34

CNT/NRa

0.068

5.0

39.2

2.2

3

CNT/SEBSb

0.454

15

30.1

2.0

35

~

5.0

23.5

2.5

36

CNT/PVDFa

0.052

7.0

30.9

2.0

11

CNT/PUb

0.598

10.0

21.0

2.0

37

CNT/PSb

~

20.0

30.0

2.0

38

CNT/PTTa

0.645

10.0

38.0

1.7

39

CNT/EVAb

~

10.0

22.0

3.5

40

CNT/UHMWPEa

~

10.0

50.0

1.0

41

Graphene/PSa

0.09

7.0

2.5

45.1

20

Graphene/PETb

0.87

~

~

~

42

Graphene/PMMAb

0.62

9.6

25.0

2.0

43

Graphene/PSb

~

7.0

8.0

~

44

Graphene/PUb

0.44

7.0

21.0

3.0

45

Graphene/WPUb

~

7.7

35.0

2.0

46

Graphene/PLAb

~

15

14.0

1.0

47

CNT/PSa

NR: nature rubber; EP: epoxy; PU: polyurethane; EVA: ethylene-vinyl acetate copolymer; PTT: Poly(trimethylene terephthalate). UHMWPE: ultra high molecular weight polyethylene; PS: polystyrene; PET: polyethylene terephthalate; PMMA: poly(methyl methacrylate); WPU: waterborne polyurethane; PLA: polylactide. a CNT or graphene based composites with segregated structure.


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b

CNT or graphene based composites with randomly distributed structure.

Figure 6. The Young’s modulus (a) and tensile strength (b) of s-CNT/PP-IM and r-CNT/PP-IM with 5.0 wt% CNT. We further investigated the mechanical performance of s-CNT/PP-IM, as shown in Figure 6. The mechanical performance of r-CNT/PP-IM was also provided. It is found that s-CNT/PP-IM shows comparable Young’s modulus to r-CNT/PP-IM, though tensile strength is lower than that for r-CNT/PP-IM. This phenomenon should be attributed to the fact that the precoated CNTs on the surface of iPP granules would suppress the diffusion between aPP molecular chain and iPP molecular chain, during the mixing and injection processing. The mechanical performance of s-CNT/PP-IM is still need to be improved, which is our focused research in the next step. 4. CONCLUSION A segregated CNT/PP composite was developed via injection molding for the first time. The pre-coated CNT on iPP surfaces enabled the successful injection of the composite at only 20 wt% aPP addition, which significantly improved the effective CNTs concentration to form perfect conductive networks. The segregated CNT/PP composite with only 5.0 wt% CNT achieves an excellent EMI SE of 43.1 dB, much



higher than the CNT/PP composite with randomly distributed CNT (25.8 dB), and even compared to conventional segregated CNT/PP composite prepared by compression molding. This work provides insight into a more efficient processing method to prepare segregated CPCs with outstanding EMI shielding performance, which is a great progress for developing segregated CPCs with potential industrial applications.

ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: (1) The morphology of CNTs; (2) Size distribution and DSC heating curve of neat iPP granules; (3) Complex viscosity of aPP and iPP; SEM images of s-CNT/PP-CM and r-CNT/PP-IMwith 5 wt% CNT; (4) SEM images of s-CNT/PP-CM and r-CNT/PP-IMwith 5 wt% CNT; (5) Percolation Model for s-CNT/PP-IM; (6) The power coefficients of the s-CNT/PP-IM composites (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51673134, 21704070, and 51421061), the Programme of Introducing Talents of Discipline to Universities (B13040), and the Science and Technology Department of Sichuan Province (Grant No. 2017GZ0412 and 2018RZ0041).

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Polypropylene

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