Electromagnetic shielding and mechanical properties of thermally stable poly(ether ketone)/multi-walled carbon nanotube composites prepared using a twin-screw extruder equipped with novel fractional mixing elements

Abstract

In the current study, a twin screw extruder equipped with novel fractional mixing elements [FME] (for enhanced mixing) was used to prepare a thermally stable, mechanically strong and light weight electromagnetic interference (EMI) shielding material based on poly(ether ketone) [PEK]/multiwalled carbon nanotube [MWCNT] composites. SEM and TEM images showed a uniform dispersion of MWCNTs in the PEK/MWCNT composites even at a high loading of 6.4 vol%. This improved and efficient dispersion of nanotubes in the PEK matrix is reflected in the formation of an electrically conductive network at a very low percolation threshold value of 0.74 vol%. Achievement of an electromagnetic interference (EMI) shielding effectiveness (SE) value of −24 dB (>99% attenuation) at a loading of 6.4 vol% along with high thermal stability [i.e. degradation temperature at 10% mass loss (T_0.1) of 582 °C] and good mechanical properties (tensile strength-119 MPa and tensile modulus-6084 MPa) demonstrates its potential application as high performance EMI shields for demanding applications such as aerospace and defense where in addition to EMI shielding, mechanical strength and thermal stability are also important parameters.

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1. Introduction

Electromagnetic interference (EMI) occurs when radiated and/or conducted electromagnetic signals emitted from electrical circuits that are under operation perturb the proper functioning of surrounding electrical equipment.^1,2 With the fast growth of electronic and communication systems in terms of development of advanced technology and products, EMI has become a serious concern in modern society.³ Mutual interference among devices such as TVs, mobile phones, computers and radio degrade the performance of devices.⁴ So it has become an essential issue to protect electronic devices and circuits against EMI with shielding materials. EMI shielding is also required to protect and secure confidential military secrets.^5–7 EMI shielding materials in the frequency range of 8–12.4 GHz have been widely used for many commercial and military applications such as space research and satellites.^8–11 The most important military application is in stealth technology which is devoted to the reduction in detectability of a target by canceling reflection of radar signal incident on its surface. Metals are best applicable material for many EMI shielding applications, but they have limitations like heavy weight, wear, poor mechanical flexibility, difficult to process, prone to corrosion and expensive.^12–17 Moreover, metals cannot be used in applications where absorption is prime requisite because they mainly reflect the radiations.¹⁸ Polymer composites containing conductive fillers such as carbon nanotubes (CNTs) or carbon black are proved to be advantageous over metals because of its light weight, low density, resistant to corrosion and easy processability.¹⁹ Compared to conventional metal based materials, light weight EMI shielding material were developed by various researchers by loading of various conductive fillers such as carbon black, carbon nanotubes (CNTs) etc. in polymer. When using carbon black as filler, major hindrance is high weight % (upto 30 to 40%) is required to achieve optimum conductivity, which results in deterioration of mechanical properties of polymer.²⁰ On the other hand, high aspect ratio (ca. 300–1000), low mass density (1.3–2.1 g cm⁻³) of CNTs and their exceptional thermal, mechanical and electrical properties make them excellent nanofillers in improving the properties of composites.^17,21–26 Due to low cost and more suitability for large scale industrialization, multiwalled carbon nanotubes (MWCNTs) are preferred over single walled carbon nanotubes (SWCNTs). Therefore, MWCNTs have been used as reinforcement filler in various polymer matrices such as polystyrene (PS), epoxy, poly(methyl methacrylate) (PMMA), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polycarbonate (PC), polyethylene (PE), polyurethane (PU), poly(vinylidene fluoride) (PVDF) etc. for the preparation of light weight and effective EMI shielding material.^27–40 Careful examination of the above literature revealed that most of the studies in the past dealt with EMI shielding response of polymer composites using matrix based on conventional polymers. However, general properties of these conventional polymers such as low heat and radiation resistance, poor mechanical strength, short service time, low continuous use temperature restricted their use in more demanding applications like aerospace and defense.

Poly(ether ketone) is recently commercialized high performance polymer which posses a high glass transition temperature (T_g) of 152 °C, melting temperature (T_m) of 367 °C, high thermo-oxidative stability, excellent flame retardancy, good mechanical properties and high heat and radiation resistance.⁴¹ Due to the excellent combination of thermal and mechanical properties along with its ability to retain properties at high temperature, PEK based composites gain importance for high tech EMI shielding applications. Though it is one of the most important and least explored high performance polymers, reports on EMI shielding properties of PEK/MWCNT are still not available due to: (i) extremely high viscosity of polymer melt combined with poor dispersion/agglomeration problem of nanotubes in PEK matrix; (ii) marginal improvement in mechanical and conductive properties of composites.⁴¹ Several methods have been reported for efficient dispersion of nanotubes into polymer matrix such as solution mixing, sonication, in situ polymerization and melt blending.^42,43 However, processing via solution techniques is not suitable for PEK due to its insolubility in common organic solvent at room temperature. Also their commercial viability is still low due to time and energy issues arising from limitation of small batch processing and environmental hazards involved with handling of solvent. Therefore, the melt blending process is the most versatile and industrially viable technique because of its several advantages like low cost, energy saving, speed, shorter design cycle, simplicity and feasibility in large scale production. However, application of melt blending is limited to low CNTs loading in polymer matrices due to the strong tendency of CNTs to cluster together leading to deterioration of mechanical properties at high CNT loading.^44,45 So the key factor to develop mechanically strong EMI shields is homogenous dispersion of nanotubes in PEK matrix which can be achieved by using efficient mixing techniques. In processing of polymer/CNT composites usually two types of mixing are distinguished i.e. distributive mixing and dispersive mixing. Distributive mixing also called simple or extensive mixing aims to improve simple and spatial distribution of the CNTs in matrix where as in dispersive or intensive mixing cohesive forces among CNT agglomerates have to overcome.⁴⁶ Use of twin screw extruder equipped with novel fractional elements in the form of kneading blocks not only helps in achieving efficient mixing of both types but also results in higher wetting (adhesion between PEK and CNT) due to multi lobe geometry, which is effective in establishment of uniform shear on processing material, hence intensifying the shearing effect. Stresses of shear and elongational origin brings about dispersion of CNT agglomerates in the PEK melt by lowering cohesive strength due to van der waals forces of attraction between CNTs. Additionally, fractional mixing elements (FME) also helps in achieving efficient distributive mixing by eliminating meta radial shear.^47,48 The detailed dynamic mechanical analysis of PEK/MWCNT composites (prepared by the same method) was performed and the characteristic parameters such as storage modulus (E′), loss modulus (E′′) and damping factor (tan δ) were explored in detail to investigate the adhesion factor (A), strength factor (B), coefficient of reinforcement factor (C) and entanglement density (N). An improved interfacial adhesion between the CNT and the matrix was observed which is reported in our previous paper.⁴⁹

This article presents the report on the successful realization of uniform dispersion of CNTs at high loading (upto 6.4 vol%) in PEK matrix via co-rotating twin screw extruder equipped with fractional mixing elements. Also this is the first time we are introducing (PEK) in the family of EMI shielding materials. PEK was choosen as a matrix as it has excellent thermal/thermo-oxidative stability, intrinsic flame resistance and good mechanical properties. The combination of high performance PEK matrix, high CNT loading and excellent processing technique brings commercial viability element into the system that was missing in the previous reports. This work systematically investigates the effect of incorporation of varying amounts of MWCNTs on the properties (such as tensile, thermal, electrical conductivity and EMI shielding) of PEK. The extent of dispersion of nanotubes in PEK matrix was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM). Shielding mechanism of these composites was studied by quantifying the contribution of absorption and reflection loss to total EMI shielding effectiveness. Since shielding performance of a material also depends on dielectric behavior of the material so dielectric properties of PEK/MWCNT composites in terms of real permittivity (ε′), imaginary (ε′′) and tan δ were also studied. The correlation among these parameters such as absorption loss, reflection loss, electrical conductivity, tan δ was also investigated. It is also desirable that conducting plastic with good EMI shielding properties should have proper balance of thermal and mechanical properties. Therefore, the influence of CNTs on mechanical and thermal properties was also evaluated. Herein, an attempt has been made to demonstrate the potential of PEK/MWCNT composites as an efficient EMI shielding material by carrying out a consolidated study dealing with mechanical, thermal and EMI shielding response of PEK/MWCNT composites. The importance of selected processing technique, matrix material and filler combination has been highlighted to demonstrate the practical utility and commercial viability.

2. Experimental section

2.1. Materials

The superfine commercial PEK powders (Grade-1200 G) supplied by Gharda chemical Ltd under trade name G-PAEK™ 1200G was used as host matrix. Density of PEK is 1.3 g cm⁻³. Chemical vapor deposition (CVD) grown multiwalled carbon nanotubes (NC 7000) were purchased from Nanocyl Ltd Belgium. The average outer diameter of nanotubes was about 9.5 nm, length about 1.5 μm and percentage purity greater than 90%. SEM and TEM of MWCNTs are shown in ESI (Fig. S1†).

2.2. Preparation of composites

PEK and MWCNTs were dried in vaccum oven at 150 °C for 12 h for removal of adsorbed moisture. MWCNTs and PEK were mixed mechanically with the help of tumbler mixer and compounding was done using STEER Alpha-18 co-rotating twin screw extruder equipped with novel fractional mixing elements [FME] operating in the temperature range of 360–400 °C with screw speeds of 300 rpm. The details of configuration/specifications of the extruder are given in Table S1 (see ESI†).

Samples for tensile testing were prepared by using ARBURG 320 C injection moulding machine. The details of processing conditions used for compounding (using extruder) and injection moulding are given in Table 1.

Table 1 Details of processing parameters used for compounding [using extruder] and injection moulding

For compounding using STEER Alpha-18 co-rotating twin screw extruder
Zones	Feed	Z-1	Z-2	Z-3	Z-4	Z-5	Z-6	Die
Temperature (°C)	220	360	370	380	385	390	400	400
Screw speed (rpm)	300

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For sample preparation by injection moulding using ARBURG 320 C
Injection pressure			700 bar
Injection time			6 s
Cooling time			20 s
Injection speed			25 cm^3 s^−1
Zones	Feed	Z-1	Z-2	Z-3	Z-4	Nozzle
Temperature (°C)	220	390	395	400	405	410

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MWCNT loading was varied from 0.15 vol% to 6.4 vol% and samples prepared have been designated as PEK-0, PEK-0.15, PEK-0.3, PEK-0.6, PEK-1.2, PEK-1.9, PEK-3.2 and PEK-6.4. The numerical suffix represents the volume % of MWCNTs. The volume percent of MWCNTs was calculated using density of MWCNTs as 2.1 g cm⁻³.⁵⁰ The schematic diagram for preparation of PEK and PEK/MWCNT composites is shown in Fig. 1.

Fig. 1 Schematic diagram for the preparation of PEK and PEK/MWCNT composites.

2.3. Characterization

Morphology of PEK/MWCNT composites was investigated using scanning electron microscopy (SEM) EVO-50 operated at accelerating voltage of 20 kV. For SEM, cryogenically fractured, gold sputtered samples were used.

For transmission electron microscopy (TEM) ultra thin sections of thickness 100 nm were cut using cryo ultra microtome and then analysed by using Jeol TEM, 2010 (Japan).

The dc conductivity of PEK/MWCNT composites was determined using standard two probe method on Keithley 6517B electrometer. For this purpose, injection moulded samples were precisely cut into rectangular pellets of dimensions 22.86 × 10.14 × 2 mm³. The silver paste in conjunction with copper electrode on each end of sample was used to make electrical contacts and to reduce the contact resistance between sample and electrode. One pin of two probes was connected with the current terminal and the other pin was connected with the voltage terminal and the resistance of the samples at room temperature was determined. Conductivity (σ) was calculated from measured resistance (R) by using following equation.

where L is length (cm) of specimen, A is area of cross section (cm²) and R is resistance (ohm). The unit for conductivity (σ) is S cm⁻¹. Reported electrical conductivity value is the average of three samples for each composition.

The EMI shielding and dielectric measurements were carried out on an Agilent N5224A Vector Network Analyzer in a microwave range of 8.2–12.4 GHz (X-band). For this study injection moulded samples were precisely cut into rectangular pellets of dimensions 22.86 (l) × 10.14 (w) × 2 (t) mm³ and then inserted in copper sample holder connected between the wave-guide flanges of network analyzer. The measured scattering parameters were S₁₁ (the forward reflection co-efficient), S₂₁ (the forward transmission coefficient), S₁₂ (the reverse transmission co-efficient) and S₂₂ (the reverse reflection co-efficient). The unit for S parameters is decibel (dB). Full two-port calibration was performed along with the sample holder to neglect any loss and power redistribution due to sample holder. For each sample 51 data points were taken within the specified frequency range.

Instron Universal Tester Model-3369 was used to measure the tensile properties. The samples were tested in accordance with ASTM D638 test procedure at cross head speed of 5 mm min⁻¹, gauge length of 100 mm and load cell of 50 kN. At least five samples of each composition were tested and the average values are reported. All the tests were performed at room temperature (25 ± 1 °C) and 50 ± 5% relative humidity.

Investigation on thermal stability of PEK and PEK/MWCNT composites was done by recording thermogravimetric (TG)/derivative thermogravimetric (DTG) traces (TA instruments Q-50 TG) in nitrogen atmosphere (flow rate 60 cm³ min⁻¹). In each experiment, 8 ± 2 mg of sample was heated at a heating rate of 20 °C min⁻¹.

3. Results and discussion

3.1. Morphological characterization

Morphology is very important factor to analyze the dispersion of MWCNTs in PEK matrix which in turn controls the electrical conductivity or EMI shielding properties. In order to observe electrical network formation of CNTs in PEK matrix and to qualitatively visualize morphology of composites both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) has been carried out. Fig. 2a and c show SEM micrographs of cryo-fractured surface of samples PEK-1.2 and PEK-6.4, respectively. MWCNTs are found to be homogenously dispersed in the polymer matrix and most of the CNTs are embedded within the polymer matrix however fibrillation of polymer makes identification of specific details difficult.⁵¹ Transmission electron microscopy (TEM) provides clear picture that CNTs are well dispersed in PEK matrix even at high loading (6.4 vol%) which could be due to the processing method i.e. presence of fractional mixing elements (FME) which apply uniform shearing force during melt mixing (Fig. 2b and d). This improved dispersion by use of efficient mixing technique led to the formation of electrically conductive junctions of CNTs even at low CNT concentration suggesting that electrical percolation must be achieved below 1.2 vol% loading. As loading of CNTs increases formation of conductive network increases and a close pack of conductive network responsible for intercepting electromagnetic radiations is observed at 6.4 vol% CNTs loading (Fig. 2d). Hence good EMI shielding properties are expected.

Fig. 2 SEM & TEM micrographs of PEK/MWCNT composites: PEK-1.2 (a & b), PEK-6.4 (c & d).

3.2. DC conductivity

The electrical conductivity (σ) values of PEK/MWCNT composites as a function of MWCNT content obtained at room temperature are summarized in Fig. 3. With increase in MWCNT content, conductivity of PEK/MWCNT increases and displayed sharp rise (four orders of magnitude) at around 0.74 vol% which indicates formation of 3D percolating network within PEK matrix. The conductive network in the composites were further analyzed with regard to the critical concentration of fillers by fitting the experimental data to the classical percolation theory which is given by eqn (2).^52–54

σ = σ_o(V − V_c)^β (2)

where σ_o is the conductivity scale factor related to the intrinsic conductivity of the filler, V is the volume percentage of conductive filler, V_c is the percolation threshold and β is the critical exponent reflecting dimensional stability of the composite. In the present work, theoretical values of percolation concentration V_c and the critical exponent β of the composites (plot shown in the inset in Fig. 3) is determined by least square curve fitting of the experimental data. It can be seen (inset Fig. 3) that the slope (i.e. the critical exponent β) of 5.1 at 0.74 vol% MWCNT loading shows that percolation threshold lies at 0.74 vol%. Such a low percolation threshold concentration could be due to the combined effect of large aspect ratio of MWCNTs and efficient dispersion of MWCNTs in PEK matrix.⁵⁵ The theoretical values of critical exponent (β) for a three dimensional percolating systems varies from 1.6 to 2 but the experimental values for different polymer/CNT composites ranged from 1.3 to 5.3.^55–57 Thus the critical exponent obtained in the present composite system is in reasonable agreement with experimental predictions.

Fig. 3 Plot of electrical conductivity vs. MWCNT vol%.

The electrical conductivity obtained in case of polymer composites filled with different conductive fillers such as metal powders⁵⁸ (nickel, copper, zinc, iron silicon, silver and aluminium), carbon black,⁵⁹ carbon fiber,⁶⁰ graphite⁶¹ and carbon nanofiber⁶² are in the range of 10⁻² to 10², 10⁻¹⁰ to 10⁻², 10⁻¹² to 10¹, 10⁻⁸ to 10¹, 10⁻⁸ to 10² respectively. Electrical conductivity of polymer composites depends on many factors such as nature, size, percentage loading, aspect ratio, connectivity (percolation) and dispersion. Among these parameters aspect ratio of filler is one of the most important criterion in imparting conductivity to insulating matrix. Higher is the aspect ratio lower will be the value of percolation. Due to the high aspect ratio of nanotubes compared to the above mentioned conducting fillers, comparable conductivity can be obtained at a much lower loading.

Electrical conductivity of PEK/MWCNT composite at 6.4 vol% MWCNT loading was around 2.1 × 10⁻³ S cm⁻¹ which lies within the range of electrical conductivity (10⁻⁶ to 10³) reported for various CNT/polymer composites.⁶³ Comparison of electrical conductivity and percolation threshold of PEK/MWCNT composites with various polymer/CNT composites such as epoxy,⁶⁴ PE,^65,66 PC,^37,67 PP,⁶⁸ PS,²⁷ PMMA,²⁷ PU³⁹ and EMA⁶⁹ at 10 wt% loading processed using different methods are summarized in Table 2.

Table 2 Comparison of results for electrical conductivity, percolation threshold, mechanical properties and EMI shielding properties of various polymer/CNT composites at 10 wt% loading processed using different methods

Composite	Preparation method	Conductivity (S cm^−1)	Percolation (wt%)	EMI SE (dB)	Tensile strength (MPa)	Tensile modulus (MPa)
Epoxy/MWCNT^64	Solution mixing	5 × 10^−3	0.6	—	—	—
PE/MWCNT^65	Melt blending	1 × 10^−2	7.5	—	∼12	—
PE/MWCNT^66	Ball milling	3	2.0	—	∼16	—
PC/MWCNT^67	Melt blending	1 × 10^−1	1.9	—	∼30	∼1300
PP/MWCNT^68	Melt blending	5 × 10^−1	2.0	—	—	∼500
PC/MWCNT^37	Melt blending (back flow channel)	1.3 × 10^−2	2.0	−27	∼72	∼1587
PS/MWCNT^27	Solvent casting	9 × 10^−1	0.5	−17	∼14	∼1420
PMMA/MWCNT^27	Solvent casting	1.37	0.5	−18	∼30	∼1780
PU/MWCNT^39	Solvent casting	1.24 × 10^−1	—	−29	—	∼1504
EMA/MWCNT^69	Solution casting + melt mixing in rheomix	1 × 10^−5	—	−20	—	—
PEK/MWCNT	Melt blending (FME)	2.1 × 10^−3	1.28 (0.72 vol%)	−24	119	6084

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It is important to point out that PEK-6.4 display conductivity value of 2.1 × 10⁻³ S cm⁻¹ which is close to the conductivity required for microwave shielding and therefore good EMI shielding properties are expected. Table S2 (see ESI†) shows data scatter information (±) of multiple specimens testing.

3.3. EMI SE and shielding mechanism of PEK/MWCNT composites

Three shielding mechanism have been described that could result in attenuation of EMI: reflection of wave from the shield; absorption of wave as it passes through the shield and re-reflections i.e., the multiple reflections of the waves at various surfaces or interfaces within the shield.⁷⁰ Multiple reflections require the presence of large surface areas (e.g., a porous or foam material) or interfaces (e.g. composite containing fillers with large surface areas).⁷¹ Hence total EMI SE (SE_T) is determined by summation of shielding effectiveness due to reflection (SE_R), shielding effectiveness due to absorption (SE_A) and multiple internal reflections (SE_M). These three losses are interrelated as follows: SE_T = SE_R + SE_A + SE_M (dB).⁷²

Electromagnetic interference (EMI) shielding effectiveness (SE) of a material can be represented by the eqn (3).³⁹

where P_t and P_i are the transmitted and incident electromagnetic powers respectively. If transmitted power is equal to incident power it means there is no shielding property in the material. As P_t is always less than P_i, shielding effectiveness is a negative quantity. A shift towards more negative value indicates an increment in magnitude of shielding effectiveness. The loss associated with multiple reflection can be ignored when shielding by absorption is more than −10 dB and it is assumed that SE_T ≈ SE_R + SE_A (dB) where SE_T (dB) represents the reduction in electromagnetic field after shield is inserted.⁷³

Shielding effectiveness due to reflection and absorption can be described as SE_R = 10 log(1 – R) and SE_A = 10 log[T/(1 – R)].⁷⁴ Using these equations the total shielding effectiveness was resolved into absorption and reflection loss. Fig. 4a and b shows the effect of frequency and MWCNT loading on EMI SE of PEK/MWCNT composites. It was observed that EMI SE of PEK/MWCNT composites are almost independent of frequency in the measured frequency range; however EMI SE increases with increase in MWCNT content. EMI SE value of ∼−24 dB (>99% attenuation) was obtained at 6.4 vol% MWCNT loading. This increment in EMI SE is attributed to the formation of conductive network due to the uniform dispersion of nanotubes in the insulating PEK matrix.

Fig. 4 (a) EMI SE of PEK/MWCNT composites in the frequency range from 8.2 to 12.4 GHz. (b) Effect of MWCNT on EMI SE and skin depth (mm) of PEK/MWCNT composites at 8.2 GHz, (c) contribution of reflection loss and absorption loss to EMI SE in PEK/MWCNT composites.

The effect of MWCNT loading on SE_A and SE_R is shown in Fig. 4c. We observed that with increasing loading of MWCNTs, both SE_A and SE_R increased but rate of increase of SE_A is higher than that of SE_R. On going from pure PEK to PEK-6.4, increment in reflection loss is 65% where as increment in absorption loss is more than 99%. From these results we can infer that the primary shielding mechanism in PEK/MWCNT composites is absorption rather than reflection in the measured frequency range (8.2–12.4 GHz). Significant increment in tan δ also confirms microwave absorbing property of these composites. This increment in absorption loss can also be explained in terms of skin effect which is defined as penetration of EM radiation at the surface of electrically conducting materials upto only thin region.⁷⁵ The skin depth can be defined as the distance (δ) over which the amplitude of a traveling plane wave drops to 1/e of the incident value and is represented by eqn (4).^75,76

where f is the frequency, σ is the electrical conductivity, μ is the magnetic permeability and t is the thickness of the material. Fig. 4b shows the variation of skin depth with the increase in vol% of MWCNTs in the PEK matrix. Skin depth decreases from 12.8 mm (PEK-0.15) to 0.92 mm (PEK-6.4) which can be ascribed to increment in electrical conductivity and absorption loss with increase in MWCNT loading. It suggests that at higher vol% loading thin sample is sufficient to block desired level of electromagnetic radiations.

Although there are no studies on EMI shielding properties of MWCNT/PEK composites but lot of research has been carried out to understand EMI shielding performance of CNT filled polymer composites. Anju et al. studied EMI shielding behavior of MWCNT/PTT nanocomposites and achieved shielding effectiveness of −23 dB in the X band at 4.6 vol% MWCNT loading.⁷⁷

Yang et al. demonstrated that MWCNT/PS composites containing 7 wt% loading displays EMI SE ∼ −20 dB in the X band.⁷⁸ Liu et al. studied single wall CNTs (SWCNTs) based polyurethane (PU) composite and obtained an EMI SE of −17 dB at 20 wt% SWCNT loading in the frequency range of 8.2–12.4 GHz.⁷⁹ Mathur et al. reported EMI SE value of −18 dB at 10 vol% loading of MWCNTs in PMMA where as N. Joseph et al. achieved EMI SE value of only −12 dB in the X band for butyl rubber (BR)/SWCNT (8 wt%) composites system.^27,80

The EMI SE value of ∼−24 dB (>99% attenuation) obtained at 6.4 vol% MWCNT loading in the present work is comparable or higher than reported values and also the obtained value is higher than the value required for commercial applications i.e. −20 dB.

Further, increment in SE_A is found to be higher as compared to SE_R with increasing MWCNT loading. Shielding efficiency of a material not only depends on the electrical conductivity but strongly depends on dielectric behavior of the material. So dielectric properties of PEK/MWCNT composites were also studied.

Theory of complex permittivity stated that the conduction current and displacement current are two types of electric currents which get induced when electromagnetic radiations are incident on a shielding material. The former arises due to the presence of free electrons for conduction current and give imaginary part of permittivity (ε′′). The later arises due to bound charges, i.e., polarization effect and give real part of permittivity (ε′). Enhancement in real part of permittivity with increase in MWCNTs vol% is attributed to the presence of lattice defects in CNTs which act as active centers for the interaction of polymeric chains on the surface of CNTs and an increase in imaginary part is attributed to the enhanced electrical conductivity of composites with increasing MWCNT content.⁸¹

The dielectric behavior of PEK/MWCNT composites was investigated by measuring room temperature complex permittivity of the composites in the frequency range of 8.2–12.4 GHz. Fig. 5a and b show the plots of complex permittivity versus frequency for composites having varying amounts of MWCNTs. It is noted that the real and imaginary part of complex permittivity exhibit a dramatic increase when MWCNT loading was higher than 0.6 vol%. The maximum values of real and imaginary parts of complex permittivity for the composite containing 6.4 vol% of MWCNTs were 37 and 47 respectively, in the measured frequency range. The plot of tan δ versus frequency (8.2–12.4 GHz) for PEK/MWCNT composites at different MWCNT loading is shown in Fig. 5c. tan δ is defined as the ratio of imaginary part to real part of complex permittivity and is a measure of the ability of material to convert applied energy into heat i.e. material with high tan δ can be used as microwave absorbing material and in stealth technology.⁸² The tan δ value increased as MWCNT loading increased. Maximum value of tan δ at 6.4 vol% CNT was found to be 2.4. This increment is attributed to the phase transition of material, i.e., conversion from insulator to conducting material.⁸³

Fig. 5 Plot of (a) real (ε′) (b) imaginary (ε′′) part of permittivity and (c) tan δ vs. frequency in PEK/MWCNT composites.

3.4. Mechanical properties

Any shield material should be strong in desired application range. Nanofillers like CNTs have strong tendency to cluster together because of strong van der waals forces of attraction. So it is very difficult to translate their nanoscopic properties into filled polymer matrix composites. The key requirement to develop structurally strong composite is improved dispersion and de-agglomeration of CNTs. Therefore, the effect of MWCNT incorporation on the tensile properties of PEK was investigated. The influence of CNTs loading on tensile strength and tensile modulus of PEK nanocomposites has been depicted in Fig. 6. Table S2 (see ESI†) shows data scatter information (±) of multiple specimens testing. At 6.4 vol% loading of MWCNTs in PEK, tensile strength and tensile modulus increased by 48% (80.4 MPa to 119 MPa) and 39% (4370 MPa to 6084 MPa) respectively relative to neat PEK matrix. This augmentation of the modulus (strength) is ascribed to conventional reinforcement effect and homogeneous dispersion of nanotubes within the PEK matrix which allows efficient transfer of load from matrix to CNTs (Fig. 2). Pull out phenomenon of nanotubes which increases the force needed for initiation and propagation of crack can be regarded as another important reason for the improved mechanical properties.⁸⁴

Fig. 6 Variation of tensile strength and tensile modulus of PEK/MWCNT composites as function of MWCNT loading (vol%).

3.5. Thermogravimetric analysis

Thermal stability is also very important for some specific applications where highly thermally stable shields are required. The characteristic degradation temperatures of the different samples are given in ESI.† Table S3 (see ESI†) and thermo-gravimetric traces in N₂ atmosphere are shown in Fig. 7. Thermal stability of samples was compared by analyzing temperatures at 10% (T_0.1), 20% (T_0.2), and 30% (T_0.3) mass loss, temperature at which rate of mass loss is maximum (T_max) and char yield (%) at 800 °C. It can be seen from the results that there is enhancement in values of thermal degradation temperatures and char yield (%) of neat material on incorporation of higher vol% of CNTs. This exceptional enhancement has been attributed to two factors: firstly very good dispersion of MWCNTs within the matrix effectively hinders the diffusion of degradation products hence slowing down the decomposition process; secondly increasing MWCNTs loading strengthen the barrier effect and increase the thermal conductivity which facilitates the heat dissipation within the composite and hence higher degradation temperatures.⁸⁵ With increasing MWCNT loading the barrier effect becomes stronger and the thermal conductivity rises, resulting in higher degradation temperatures. Results of TGA traces are shown in ESI (Table S3†).

Fig. 7 (a) TGA curve for PEK/MWCNT composites (b) zoomed plot between 640–800 °C.

In short, aeronautic industry require material with low density, high strength to weight ratio, superior mechanical properties and ability to meet the needs of harsh environment along with high EMI shielding effectiveness to protect aircraft parts against lightening and other electromagnetic radiation. In recent past, PEK have gained wide acceptance to be used as matrix for high performance polymer composites for aerospace structural applications. Further on incorporation of nanotubes in PEK, we obtained excellent improvement in EMI shielding, mechanical and thermal properties. We observed EMI shielding of −24 dB (>99% attenuation) at 6.4 vol% loading which is higher than the required value for commercial applications (−20 dB). In addition to this, an excellent balance of thermal and mechanical properties was achieved in the present studies which are superior to other polymer/CNT composites (PC/PMMA/PS-CNT) at equal (10 wt%) loading. Table 2 show the comparison of existing literature and present study where mechanical properties of PEK based composites are much higher as compared to other polymer based materials, however EMI shielding at equal loading is comparable.^27,37 Improved mechanical properties (tensile strength-119 MPa and tensile modulus-6084 MPa) and high thermal stability (T_0.1 ∼ 582 °C and char yield of ∼60% at 800 °C) enhances the suitability of such composite systems for demanding applications where in addition to EMI shielding, mechanical strength and thermal stability is also a prime requisite. Herein, we successfully developed a light weight, mechanically strong and thermally stable, efficient EMI shielding materials using an industrially viable melt blending approach using a twin screw extruder equipped with fractional mixing elements for enhanced mixing.

4. Conclusions

Thermally stable and mechanically strong PEK/MWCNT composites with MWCNT loading upto 6.4 vol% have been successfully prepared by melt compounding using twin screw extruder equipped with novel fractional mixing elements (for enhanced mixing). These composites display very low percolation threshold (0.74 vol%) which indicates good dispersion of MWCNTs in PEK matrix. SEM and TEM images have confirmed the good dispersion of CNTs in PEK matrix. The composite having 6.4 vol% loading shows EMI SE of −24 dB (>99% attenuation) along with good mechanical properties (tensile strength-119 MPa and tensile modulus-6084 MPa) and high thermal stability (T_0.1 ∼ 582 °C) that justifies the use of these nanocomposites as potentially high performance materials for EMI shielding applications. Such a light weight, mechanically strong and thermally stable efficient EMI shielding material can be considered as potential candidate for aeronautic industries.

Acknowledgements

Ministry of Human Resource Development (MHRD), Government of India for providing financial assistance to one of the author (Mr Sampat Singh Chauhan) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22969g

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