Improved Electromagnetic Interference Shielding Properties of MWCNT–PMMA Composites Using Layered Structures
© to the authors 2009
Received: 26 September 2008
Accepted: 30 December 2008
Published: 17 January 2009
Electromagnetic interference (EMI) shielding effectiveness (SE) of multi-walled carbon nanotubes–polymethyl methacrylate (MWCNT–PMMA) composites prepared by two different techniques was measured. EMI SE up to 40 dB in the frequency range 8.2–12.4 GHz (X-band) was achieved by stacking seven layers of 0.3-mm thick MWCNT–PMMA composite films compared with 30 dB achieved by stacking two layers of 1.1-mm thick MWCNT–PMMA bulk composite. The characteristic EMI SE graphs of the composites and the mechanism of shielding have been discussed. SE in this frequency range is found to be dominated by absorption. The mechanical properties (tensile, flexural strength and modulus) of the composites were found to be comparable or better than the pure polymer. The studies therefore show that the composite can be used as structurally strong EMI shielding material.
Electromagnetic interference (EMI) shielding is very important in today’s world of electronic devices and components [1–4]. EMI shielding in the range of 8.2 to 12.4 GHz (the so-called X-band) is more important for military and commercial applications. Doppler, weather radar, TV picture transmission, and telephone microwave relay systems lie in X-band . The use of carbon nanotubes (CNTs) as a conductive additive for plastics in the electronics, automotive and aerospace sectors with potential uses as EMI shielding materials, coatings for enclosures, ESD composites, antistatic materials, conductive coatings, etc. is emerging as a major application area of CNTs in plastics industry [3, 6–9]. Compared to conventional metal-based EMI shielding materials carbon-based conducting polymer composites are becoming attractive because of their light weight, resistance to corrosion, flexibility and processing advantages [4, 10]. Amongst the carbon fillers (e.g., graphite, carbon black, or carbon fibers) carbon black is commonly used as conducting filler in polymer composites [1–3, 10–12]. The EMI shielding efficiency of a composite material depends on factors such as the filler’s intrinsic conductivity and aspect ratio . At higher filler loadings, composite system suffers from poor mechanical properties due to poor filler–matrix interactions . In using carbon black as filler, a major disadvantage is the high amount of carbon black that is required up to 30–40% to achieve desired conductivity levels, which results in deterioration in the mechanical properties of the polymer . A major advantage of using CNTs is that conductive composites can be formed at low loading of CNTs due to low percolation thresholds . The small diameter, high aspect ratio, high conductivity, and mechanical strength of CNTs make them an excellent option for creating conductive composites for high-performance EMI shielding materials at low filling [13–17]. For effective EMI shielding systems, light weight and mechanically strong materials are more desirable.
Several studies have been reported on the EMI shielding properties of CNT-based polymer composites [4, 5, 9, 13, 17–21]. Yang et al.  studied the EMI shielding applications of CNT–PS foam composites and obtained a value of about 20 dB at 7 wt% loading. The composites were more reflective than absorptive to electromagnetic radiation. Yang et al.  studied the effect of various contents of carbon nanofiber and CNTs within PS matrix on the EMI shielding effectiveness (SE) and found that with the addition of 1 wt% CNTs into a 10 wt% carbon nanofiber–polystyrene composite, a SE value of 20.3 dB was obtained for a 1-mm thick sample. Kim et al.  studied the EMI shielding properties of MWCNT–PMMA films in the range 50 MHz−13.5 GHz and reported up to 27 dB SE of MWCNT–PMMA composite films for high CNT loadings of about 40 wt%. Yuen et al.  studied the effect of processing conditions on the EMI shielding properties of MWCNT–PMMA composites prepared by in situ polymerization and ex situ fabrication methods. They found that the SE was higher for in situ fabricated composites and also found that the EMI SE of composites prepared by stacking 10 layers of 0.1-mm MWCNT–PMMA films was higher than a single 1-mm thick piece of bulk 4.76 wt% MWCNT–PMMA composite, suggesting the composite stacking process as a better fabrication method. Huang et al.  fabricated SWCNT–epoxy composites using long, short, and annealed SWCNT, with different aspect ratios and wall integrities. Very low percolation volumes and 20–30 dB EMI SE were obtained in the X-band range for 15 wt% SWCNT loading. Liu et al.  obtained an EMI SE up to 17 dB in 8.2–12.4 GHz band for PU/SWCNT composites with 20 wt% SWCNT loading. It is quite evident that EMI shielding values reported so far for SWCNT- or MWCNT-polymer composites mostly vary between 20 and 30 dB only in the X-band frequency region. Higher values have been reported at frequencies other than the X-band [13, 21].
In our earlier studies on MWCNT–PMMA composite films prepared by solvent casting method, we have shown that by dispersing about 10 vol.% MWCNT in PMMA, electrical conductivity of about 1.37 S cm−1 and EMI SE about 18 dB (X-band) could be achieved . The composite showed promise for EMI shielding applications primarily as EMI absorption materials. We report here EMI SE of 40 dB of 10 vol.% MWCNT–PMMA composite. The mechanism of EMI shielding has been investigated by comparing the contribution of reflection and absorption to the total EMI SE. In most of the previous studies on the use of CNTs as EMI shielding materials in polymer composites, no emphasis has been paid on the mechanical properties of the composite material though it remains an important factor. This parameter has also been addressed in this work.
Fabrication of MWCNT–PMMA Composite Film
MWCNT used in this study were synthesized by CVD method using toluene as the hydrocarbon source and ferrocene as iron catalyst precursor [18, 22, 23]. The purity of the as-synthesized MWCNT was about 90% and the uniform diameter was in the range of 60–70 nm, and length up to 50–100 µm.
MWCNT–PMMA composite films were fabricated by solvent casting method (Method A) . As-synthesized MWCNT were ultrasonically dispersed in toluene for 2 h to obtain a stable suspension of CNTs in toluene. The suspension was then mixed with a solution of PMMA in chloroform to obtain a mixture of CNT/PMMA containing 10 vol.% of MWCNT in PMMA. The mixture was again ultrasonicated for 2 h to obtain a uniform dispersion of CNTs in PMMA. Thin polymer film was cast from this solution by pouring the solution into a Teflon spray-coated Petri dish (diameter 4″) and allowing the solvent to evaporate over several days followed by drying in an oven. The resulting film had a thickness of about 0.25–0.30 mm.
Fabrication of MWCNT–PMMA Bulk Composite
MWCNT–PMMA bulk composite was prepared by a two-step method of solvent casting followed by compression molding (Method B) . In this method, solvent-casted films from the aforementioned method were cut into pieces and stacked in a mold (60 mm × 20 mm × 1 mm) and compression molded at 165 °C and 100 kg/cm2 pressure. The resulting composite bar had a thickness of about 1 to 1.1 mm.
SEM analysis was carried out to study the dispersion of CNTs in the matrix and also the fracture surface of the composites (film as well as bulk composite) using Model Leo S-440, scanning electron microscope.
The electrical conductivity of the composite films was measured by 4-point contact method . The polymer composite film was cut into rectangular strips of size 70 mm in length and 10 mm in width. Current was supplied using Kiethley 224 programmable current source and the voltage drop was measured using Keithley 197 A auto ranging micro volt DMM.
The EMI SE measurements of the MWCNT–PMMA composites were carried out on an Agilent E8362B Vector Network Analyzer in the frequency range of 8.2 to 12.4 GHz (X-band). The SE of two layers of bulk composite and various layers of composite film (stacked using an insulating adhesive between each layer) was measured using sample specimen size of 21.32 mm × 10.66 mm to fit waveguide sample holder. The thickness of each layer of the film was 0.3 mm and the total thickness of seven layers of stacked composite films was 2.1 mm. The stacked composite films are hereafter referred to as SCF1 (one layer), SCF2 (2 layers), SCF3 (3 layers), SCF4 (4 layers), SCF5 (5 layers), SCF6 (6 layers), and SCF7 (7 layers of composite films). For the bulk composite, the thickness of each layer of bulk composite was 1.1 mm and the total thickness of two layers of bulk composite was 2.2 mm. The stacked bulk composite is hereafter referred to as SCB2 (two layers of bulk composite).
Flexural modulus and flexural strength (ASTM D790) of the bulk composites (size 50 mm × 5 mm × 2 mm) were measured on Instron Tensile Testing Machine Model 4411. Sample span to depth ratio was kept as 20 and cross-head speed was maintained at 0.5 mm/min.
Results and Discussion
EMI Shielding Effectiveness
For a single layer of shielding material, when SEA is ≥10 dB, then SEM → 0 and can be neglected .
Effect of MWCNT Content on SERand SEAof MWCNT–PMMA Composites
The primary mechanism of EMI shielding is usually a reflection of the electromagnetic radiation incident on the shield, which is a consequence of the interaction of EMI radiation with the free electrons on the surface of the shield . As a result, the shield has to be electrically conducting although a high conductivity is not required. The SER term increases with increase in CNT loading due to increase in conductivity, but the increase is quite gradual due to the moderate increase in conductivity of the films (SER increases from about 0.16 dB at 0.5 vol.% to about 3.85 dB at 10 vol.% loading). Absorption is usually a secondary mechanism of EMI shielding whereby electric dipoles in the shield interact with the electromagnetic fields in the radiation . The increasing difference between SER and SEA with increase in CNT loading may therefore be due to interfacial polarization of PMMA by CNT which increases the absorption component (SEA increases from about 2 dB at 0.5 vol.% to about 13 dB at 10 vol.% loading). The 10 vol.% MWCNT–PMMA film is primarily an EMI absorbing composite material.
EMI SE of 10 vol.% MWCNT–PMMA Composite (Stacked Films and Bulk Composite)
Effect of Stacking Method on SERand SEAof MWCNT–PMMA Composites
Mechanical, electrical and thermal properties of MWCNT–PMMA composites
Tensile strength (MPa)a
Tensile modulus (GPa)a
Flexural strength (MPa)
Flexural modulus (GPa)
Electrical conductivity (S/cm)
Thermal stability (°C)a
10% MWCNT–PMMA film (0.25–0.30 mm)
18 dB–40 dB in seven layers
10% MWCNT–PMMA bulk (1.0–2.0 mm)
30 dB in two layers
There are fewer chances of occurrence of the multiple reflections phenomena that can contribute to SEAwhen two thick and compact layers are stacked together as in the case of SCB2. Lower SEAin SCB2 (Fig. 5) results in total EMI SE to be lower in the bulk composite due to lesser number of multiple reflections even though the total amount of CNTs and thickness is the same as the SCF7 system.
Characterization of MWCNT–PMMA Composites
A composite system with desirable mechanical, electrical and thermal properties has been developed, suitable for EMI shielding applications as effective light weight and strong EMI absorption materials. We conclude that stacked layers of thin films or multiple layer composite system of films is more effective than the bulk composite for absorbing EM waves. In the bulk composite, the structure is more compact due to compression molding and the stacked layers are thicker. The EMI SE value of 40 dB of our composite system at 10 vol.% loading of MWCNT in polymer matrix is so far the highest reported in the X-band frequency region.
The authors are grateful to Prof. Vikram Kumar, Director NPL, for his permission to publish the research work. One of us (Shailaja Pande) is grateful to DST for providing research grant to carry out the studies under the “Women Scientist Scheme” (Grant no. SR/WOS-A/CS-89/2004).
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