Acrylonitrile butadiene styrene/multi-walled carbon nanotubes nanocomposite foams for electromagnetic interference shielding with optimized performance

Abstract

The advancement of polymeric nanocomposite foams for electromagnetic interference (EMI) shielding can be credited to two main factors: the multiple deflections of the incoming electromagnetic waves within the shield and the formation of conductive pathways by the nanofillers. In this research, chemical foaming is employed in injection molding machine to create foams made of acrylonitrile butadiene styrene (ABS) and multi-walled carbon nanotubes (MWCNTs). By incorporating a 1 wt% concentration of MWCNT, applying a pressure time of 2 s, and allowing for a cooling time of 60 s, foamed nanocomposite samples demonstrated a remarkable total EMI shielding effectiveness (SE) of SE_T = 16.25 dB. This SE value surpassed the EMI SE values of the remaining samples across the X-band frequency range. Upon comparing foamed samples of pure and nanocomposite materials with identical cell density, it was observed that the enhancement of SE_T for the nanocomposite foamed sample reached 21.2% in contrast to the pure foamed sample operating at 11.52 GHz. The research revealed that incorporating a microcellular structure had a notable impact on the electrical conductivity, relative permittivity, dielectric loss, relative permeability, and magnetic loss in ABS/MWCNT nanocomposites. Furthermore, the nanocomposite foams demonstrated significantly greater EMI SE in comparison to their solid counterparts.

Introduction

EMI, also known as electromagnetic interference, refers to the unwanted noise or disruption in electrical circuits caused by external sources. When electronic devices are poorly operated, they can malfunction or even cease to function altogether due to the presence of EMI. The increase in EMI is primarily attributed to human-made sources such as the growing use of electronic circuits, power lines, radar, radio, and wireless communications. To mitigate EMI, various shielding mechanisms are employed, including absorption, reflection, and multi-scattering.^1–3

A wide range of materials were utilized for EMI shielding such as iron (Fe) based materials like ferrites, ⁴ magnetite (Fe₃O₄), ⁵ and carbonyl iron (CI) ⁶ ; conductive polymers (CPs) as polyaniline, ⁷ polypyrrole, ⁸ and polythiophene ⁹ ; nonconducting nanocomposite polymers such as polyvinylidene fluoride ¹⁰ ; carbonaceous materials like graphite/expanded graphite, ¹¹ graphene , ¹² and carbon nano tubes (CNTs). ¹³

Researchers have recently shown great interest in polymeric nanocomposites that incorporate conductive particles. These conductive fillers typically consist of metal or magnetic nanoparticles, as well as carbon-based materials like CNTs, graphene, carbon black, and graphite. It is important to note that several factors need to be considered when fabricating the nanocomposites, including the processing method, the polymer matrix, the aspect ratio of the fillers, the dielectric and magnetic properties of additives, the physical geometry of the fillers, and the thickness of the samples. All of these aspects play a crucial role in achieving effective EMI shielding.

Jia et al. ¹⁴ studied the electrical conductivity and EMI shielding property of polyethylene (PE) nanocomposites with different CNT networks. They fabricated three diverse kinds of PE/CNT structures, segregated structure (s-PE/CNT), partially segregated structure (p-CNT/PE), and randomly distributed structure (r-Pe/CNT). Their attempts revealed that the s-PE/CNT nanocomposite possessed superior electrical conductivity, up to two orders of magnitude, compare to the p-PE/CNT and r-PE/CNT nanocomposites, at the same CNT loading. Furthermore, their studies showed that the s-PE/CNT composite had excellent EMI shielding character, 20% and 46% higher than that for p-PE/CNT and r-PE/CNT nanocomposites, respectively. Al-Saleh et al. ¹⁵ fabricated three types of ABS nanocomposite containing MWCNT, carbon nanofibers (CNF), and high structure carbon black (CB) using solution mixing method. Their attempts revealed that ABS/MWCNT nanocomposites possessed the lowest percolation threshold concentration, the critical amount of nanoparticle inside an insulative polymer that converts polymer to the conductive one. This factor was occurred for ABS/CB nanocomposite between 4 and 5 wt%, for ABS/CNF nanocomposite between 1.25 and 1.5 wt%, and below 0.5 wt% for ABS/MWCNT nanocomposite. Moreover, they investigated the effect of nano filler type and nano particle concentration on the EMI SE behaviors of ABS nanocomposites. They showed that the EMI SE of ABS nanocomposites was promoted by increasing the nano filler amount, for example, this character for ABS/CB increased from 8.2 dB to 21.1 dB with increasing CB concentration from 7.5 wt% to 15 wt%. Furthermore, they clarified that the EMI SE for CNT, CNF, and CB nanocomposites with the same loading of 10 wt%, were 40.7 dB, 26.1 dB and 15.3 dB, respectively. Also, their study specified that the real permittivity of ABS/MWCNT nanocomposites with 15 wt% was 590 at 8.2 GHz and the imaginary permittivity of ABS/MWCNT nanocomposite was 76 times that of ABS/CB and 5 times that of ABS/CNF with the same nano particles loading of 5 wt%.

Reducing reflection involves allowing a significant portion of the incoming microwave to enter the shield and be absorbed rather than reflected. This characteristic of shields is due to the porous nature of foamed nanocomposites, which is governed by the phenomenon of impedance matching. Impedance matching enables electromagnetic waves to penetrate the shield to the maximum extent possible. ¹⁶ Additionally, when the microwave enters the microcellular structure, it may undergo multiple reflections and scattering, resulting in the dissipation of electromagnetic energy through the introduction of the cellular structure.

Zhao et al. ¹⁷ analyzed the EMI shielding of thermoplastic polyurethane (TPU)/MWCNTs nanocomposites. They reported that the electrical conductivity enhanced with an increase of MWCNT content, also foamed samples exhibited higher conductivity and microwave energy dissipation compared to solid specimen which was attributed to porous structure. Their investigation revealed that TPU/CNT foam with 4 wt% and thickness of 1.3 mm possessed strong EM dissipation and lowest reflection loss (RL) value of −30.4 dB.

Conductive nanoparticles within polymeric foam have two primary functions. Firstly, they convert an insulating polymer into a conductive one. Secondly, they act as nucleating agents, increasing the cell density and improving the dissipation of incident electromagnetic wave energy.^18,19

Hamidinejad et al. ²⁰ investigated the electrical and electromagnetic interference shielding properties of high-density PE (HDPE)/graphene nanoplatelet (Gnp) nanocomposite foams. Their efforts revealed that the electrical conductivity was substantially boosted and the percolation threshold was considerably diminished by microcellular structure. They reported the through- plan conductivity advanced up to a maximum of nine orders of magnitude and the percolation threshold reduced up to 62% for foamed samples compared to unfoamed one. Furthermore, their study disclosed that a higher dielectric constant and a higher EMI SE were achieved compared to their regular injection-molded, unfoamed, counterparts. They achieved a real permittivity of 106.4 for 9.8 vol% Gnp and 6.2 for solid nanocomposites. Also, the fabricated HDPE-19 vol% Gnp composites exhibited an EMI SE of 31.6 dB, which was 45% superior than of the unfoamed counterpart. Tran et al. ²¹ studied the foaming parameters such as foaming temperature and foaming pressure effects on the morphology of polypropylene (PP)/CNT and their impact on the electrical conductivity. Finally, they survived the relationship between foam morphology and the EMI shielding performance. They reported that the foams with a volume expansion of >25, containing 0.1 vol% of CNTs have opportunity to absorb more than 90% of the incident radiation between 25 and 40 GHz. Xu et al. ²² fabricated HDPE/CNT nanocomposite foam by compression molding plus salt-leaching. They showed that the electrical percolation threshold in foamed samples is lower than that of the solid composites. Also, the specific EMI SE of foamed specimens was 2.2 times higher than that of their solid counterpart. Furthermore, they cited that the electrical conductivity and EMI SE advancement occurred with an increase in cell diameter. Owing to the formation of more conductive channels in the foam composite matrix.

The structural property governs the EMI shielding behaviors of nanocomposite foamed samples. The restriction of foam expansion can be achieved by increasing the viscosity of the polymer. ²³ Hence, the objective of this study is to investigate the processing parameters that can influence foam expansion, which were not previously considered by other researchers, to study their influences on the EMI shielding behavior using an industrial method for fabricating nanocomposite foamed EMI shielding. The cooling process is responsible for increasing the viscosity of the polymer, which is why the cooling time is chosen as an important process parameter in this study. Additionally, we evaluate other parameters that have the potential to limit the expansion of the foam. It is important to note that, based on our research, these parameters have not been considered as processing parameters before in a study focusing on the EMI shielding performance. The primary objective of this study is to enhance the EMI shielding capabilities of foamed ABS/MWCNT nanocomposites by optimizing the processing conditions, including holding pressure time, cooling time, and the amount of MWCNT. The Taguchi approach will be employed to achieve this goal. Additionally, we will investigate the intrinsic magnetic and dielectric properties of both solid and foamed ABS/MWCNT nanocomposites.

Experimental Section

Materials and Equipment

In this study, ABS, with trade name of GBPC 50N and density of 1.04 g/cm³ was utilized as the polymeric matrix. MWCNT (US Research Nanomaterials Inc.) with carbon purity of 90%, was used as additive. This nanofiller specifications were, the average inner diameter of 5-10 nm, average external diameter of 10-30 nm, and length of 10-30 µm. The melt compounding method was chosen for producing the nanocomposite of ABS- 1 wt% MWCNT by using a ZSK-25 (Coperion Werner & Pfleidere) twin-screw extruder with an L/D = 40. Finally, an NBM HXF-128 injection molding machine with L/D = 21.1, D = 37 which was equipped by a mold with dimensions of 40 × 40 × 4.58 mm³ was used for fabricating the solid and foamed samples of ABS/MWCNT nanocomposite.

Moreover, in this study azodicarbonamide as chemical foaming agent and paraffin oil as softening agent were used. Scanning electron microscopy (SEM) tests were performed using a Leo 440i. An Agilent E8363 C vector network analyzer (VNA) was utilized to carry out the EMI SE characterization of specimens using waveguide method over the X-band (8-12 GHz) frequency. In order to weight the samples at room temperature, a JEWELRY BALANCE FX-300GD balance with accuracy of 0.001 g was used.

Preparation of Specimens

At first, neat ABS dehumanization was conducted at 85°C for 2 h by a simple laboratory oven. Then, the neat ABS was physically compounded with 1 wt% MWCNT. A masterbatch granules with 1 wt% multi- walled carbon nanotube was obtained at the melt temperature of 170-180- 185-190-210°C and the screw speed of 250 r/min by ZSK-25 twin-screw extruder machine. In order to achieve granules with 0.5 wt% multi-walled carbon nanotube, sufficient amount of pure ABS was added to obtained masterbatch granules with 1 %wt. Solid samples of ABS/MWCNT with 0,0.5 and 1 wt% MWCNT were coded as, S-0, S-0.5 and S-1, respectively. The Solid ABS/MWCNT samples were molded at 40 MPa injection pressure, 190 °C–240°C melt temperature, 80°C mold temperature, 8 s cooling time, 40 Mpa holding pressure, and 2 s holding pressure time by injecting pellets into a mold with dimensions of 40 × 40 × 4.58 mm.

Saving the research cost and time was done by using the design of experiments (DOE). Taguchi method is a powerful tool in DOE which divides parameters into two groups, signals (controllable factors) and noises (uncontrollable factors). Signal to noise ratio (S/N) means that the process was governed by controllable factors or uncontrollable factors. The optimum condition of process was occurred in high S/N value that was computed by a statistical software, Minitab software. As well as, the analysis of variance (ANOVA) approach was utilized to calculate the contribution percentage of each processing parameter on the EMI shielding characterizations.^24,25 Moreover, foamed ABS/MWCNT nanocomposite samples were fabricated according to the L9 orthogonal array of Taguchi approach by injection molding machine.

In this study, processing parameters in three levels investigated which included MWCNT concentration of 0, 0.5, and 1 wt%, holding pressure time of 1, 2, and 3 s, and cooling time of 60, 80, and 100 s. Foamed ABS/MWCNT nanocomposite samples were fabricated according to Table 1 and were denoted as F_i. The constant process parameters included injection pressure of 40 Mpa, melting temperature of 190°C–240°C, holding pressure of 40 Mpa, and injection time of 5 s. Moreover, azodicarbonamide with 1 wt% as chemical agent and paraffin oil with 1 wt% as softening agent were added to ABS/MWCNT nanocomposite granules. In order to investigate the EMI SE characterization, solid and foamed samples with the thickness of 4.58 mm were cut into rectangular shape (22.86 × 10.16 mm²) in the middle of specimens.

Table 1.

L9 orthogonal DOE of samples.

Trial no.	MWCNT (wt%)	Holding pressure time (s)	Cooling time (s)
F1	0	1	60
F2	0	2	80
F3	0	3	100
F4	0.5	1	80
F5	0.5	2	100
F6	0.5	3	60
F7	1	1	100
F8	1	2	60
F9	1	3	80

Characterization

The density calculation of solid and foamed specimens was assessed by water displacement method based on the ASTEM-D792. The ratio of the foamed sample density (ρ _f ) to the solid sample density (ρ _s ) is verified as relative density (ρ _rel ). The other foam properties, expansion ratio ( $\phi$ ) and void fraction (V _f ), are calculated from relative density as equations (1)–(3). ²⁶

${\rho }_{rel}=\frac{{\rho }_f}{{\rho }_s}$

(1)

$\phi =\frac{{\rho }_s}{{\rho }_f}=\frac{1}{{\rho }_{rel}}$

(2)

$V_f=\left(1-{\rho }_{rel}\right)\times 100$

(3)

The ratio of the cell thickness (h) and the cell radius (r) in an ideal sphere pore shape can be expressed by the equation (4). ²⁷

$\frac{h}{r}=\sqrt[3]{\frac{V_h}{V_r}+1}-1$

(4)

Where V_h and V_r are the volume fractions of ABS/MWCNT and cells. The structural property of foamed ABS/MWCNT nanocomposite was probed by the computation of cell density (N) and cell size. The cell density was determined as equation (5). That n is the number of cells in 2D SEM picture and A is the area of picture. Also, the cell size of samples was verified by the average of measured cells size in 2D SEM picture. ²⁶

$N={\left(\frac{n}{A}\right)}^{3/2}$

(5)

The EMI SE investigation was performed by using waveguide method and samples were adjusted between holder surfaces. After that the S-parameters (S₁₁, S₁₂, S₂₂, S₂₁) were obtained over the X-band frequency range for all solid and foamed ABS/MWCNT nanocomposite samples. The complex permittivity and complex permeability were derived from S-parameters by using a MATLAB code based on Nicolson-Ross-Weir (NRW) method.^15,28 Meanwhile, the dielectric loss, tanδ_e, and magnetic loss, tanδ_m, that are the ratio of imaginary part to real part of complex permittivity and permeability, the relative permittivity, ε_r, the relative permeability, µ_r, and the ac conductivity, ${\sigma }_{ac}$ (S/m), were achieved according to equations (6)–(10). ²⁹

$\begin{array}{ll}{\mathrm{\epsilon }}_{\mathrm{r}}=\frac{\mathrm{\epsilon }}{{\mathrm{\epsilon }}_0}={\epsilon }_r^{\prime }-{j\epsilon }_r^{″}& {\mathrm{\epsilon }}_0=8.85\times {10}^{-12}\hspace{0.17em}\left(\frac{\mathrm{F}}{\mathrm{m}}\right)\end{array}$

(6)

${\tan \hspace{0.17em}\delta }_{\mathrm{e}}=\frac{{\epsilon }_r^{″}}{{\epsilon }_r^{\prime }}$

(7)

$\begin{array}{ll}{\mathrm{\mu }}_{\mathrm{r}}=\frac{\mathrm{\mu }}{{\mathrm{\mu }}_0}={\mu }_r^{\prime }-{j\mu }_r^{″}& {\mathrm{\mu }}_0=4\mathrm{\pi }\times {10}^{-7}\hspace{0.17em}\left(\frac{\mathrm{H}}{\mathrm{m}}\right)\end{array}$

(8)

${\tan \hspace{0.17em}\mathrm{\delta }}_{\mathrm{m}}=\frac{{\mu }_r^{″}}{{\mu }_r^{\prime }}$

(9)

${\mathrm{\sigma }}_{\text{ac}}=2\pi \mathrm{f}{\mathrm{\epsilon }}_0{\epsilon }_r^{″}$

(10)

The effective relative permittivity of foamed samples (ε_eff) was survived from Maxwell-Garnett theory as equation (11). According to Maxwell-Garnett formulation,^30,31 ε₁ and ε₂ are the relative permittivity of the polymer matrix and the gas state, respectively. In this survey, the relative permittivity of solid samples was used to calculate the relative permittivity of corresponding foam samples, S-0 for F1, F2, and F3 also S-0.5 for F4, F5, and F6 in addition S-1 for F7, F8, and F9. The relative permittivity of air (ε₂ = 1) was utilized owing to the air replaces the gas in the foamed structure over the time. The V_f is the void fraction of foamed specimen.

${\mathrm{\epsilon }}_{\text{eff}}={\mathrm{\epsilon }}_1\frac{\left({\mathrm{\epsilon }}_2+2{\mathrm{\epsilon }}_1\right)+2{\mathrm{V}}_{\mathrm{f}}\left({\mathrm{\epsilon }}_2-{\mathrm{\epsilon }}_1\right)}{\left({\mathrm{\epsilon }}_2+2{\mathrm{\epsilon }}_1\right)-{\mathrm{V}}_{\mathrm{f}}\left({\mathrm{\epsilon }}_2-{\mathrm{\epsilon }}_1\right)}$

(11)

Indeed, the obtained S-parameters (S₁₁, S₂₁) from vector network analyzer (VNA) test are in dB So they were converted to linear mode or power system as equations (12), and (13). The power coefficient of the reflection (R), transmission (T), and absorption (A) were calculated from the converted S-parameter as equations (14)–(16). For all solid and foamed ABS/MWCNT nanocomposite specimens the microwave incident power was 1 mw, P_I = 1 mw. ³²

$\begin{array}{l}\left|{\mathrm{S}}_{11}\right|\\ ={10}^{\frac{\left|{\mathrm{S}}_{11\left(\text{dB}\right)}\right|}{10}}\end{array}$

(12)

$\begin{array}{l}\left|S_{21}\right|\\ {=10}^{\frac{\left|S_{21\left(dB\right)}\right|}{10}}\end{array}$

(13)

$\mathrm{R}={\left|{\mathrm{S}}_{11}\right|}^2={\mathrm{P}}_{\mathrm{R}}/{\mathrm{P}}_{\mathrm{I}}$

(14)

$\mathrm{T}={\left|{\mathrm{S}}_{21}\right|}^2={\mathrm{P}}_{\mathrm{T}}/{\mathrm{P}}_{\mathrm{I}}$

(15)

$\mathrm{A}=1-\mathrm{R}-\mathrm{T}$

(16)

It is noteworthy that, the Bel (B) is the ratio of outer power to inner power in logarithmic mode, thereby the dB, dB = 0.1 B, is found as equations (17), and (18). According to Figure S.1, the total EMI shielding (SE_T) in decibel (dB), containing the shielding by absorption (SE_A) and the reflection (SE_R), were extracted as equations (19)–(21).

$\mathrm{B}=\log \frac{P_{out}}{P_{in}}$

(17)

$\begin{array}{l}\mathrm{d}B\\ =10\hspace{0.17em}\mathit{log}\frac{P_{in}}{P_{out}}\end{array}$

(18)

$\begin{array}{ll}\text{from}\hspace{0.17em}\text{surface}\hspace{0.17em}\mathrm{a}:& {\text{SE}}_{\mathrm{R}}\left(\text{dB}\right)=10\hspace{0.17em}\log \frac{{\mathrm{P}}_{\text{in}}}{{\mathrm{P}}_{\text{out}}}=10\hspace{0.17em}\log \frac{1}{1-\mathrm{R}}\end{array}$

(19)

$\begin{array}{ll}\text{from}\hspace{0.17em}\text{surface}\hspace{0.17em}\mathrm{b}:& {\text{SE}}_{\mathrm{A}}\left(\text{dB}\right)=10\hspace{0.17em}\log \frac{{\mathrm{P}}_{\text{in}}}{{\mathrm{P}}_{\text{out}}}=10\hspace{0.17em}\log \frac{1-\mathrm{R}}{\mathrm{T}}\end{array}$

(20)

$\begin{array}{ll}\text{from}\hspace{0.17em}\text{surface}\hspace{0.17em}\mathrm{a}-\mathrm{b}:& {\text{SE}}_{\mathrm{T}}\left(\text{dB}\right)=10\hspace{0.17em}\log \frac{{\mathrm{P}}_{\text{in}}}{{\mathrm{P}}_{\text{out}}}=10\hspace{0.17em}\log \frac{1}{\mathrm{T}}\end{array}$

(21)

The specific EMI SE, the ratio of SE_T to density of samples was investigated. Furthermore, the percentage of attenuation was computed as equation (22) which was originated from transmitted coefficient power. Based on the EMI shielding theory, the effect of the conductivity on the EMI shielding was computed as equation (23). ²⁸

$\text{EMI}\hspace{0.17em}\text{SE}\%=\left(1-\mathrm{T}\right)\times 100$

(22)

$\text{EMI}\hspace{0.17em}\text{SE}=20\hspace{0.17em}\log \left(1+\frac{1}{2}\sigma t{Z}_0\right)$

(23)

where σ is the electrical conductivity, t is the sample thickness, and Z₀ is the free space impedance (constant: 377 S⁻¹).

Results and Discussion

Structural Properties of Solid and Foamed ABS/MWCNT Nanocomposites

The microcellular structure of solid and foamed ABS/MWCNT nanocomposite specimens were investigated. The densities for solid samples, S-0, S-0.5, and S-1 were elucidated 1.04, 1.0437 and 1.046 g.cm⁻³, respectively. The structural characterizations for foamed ABS/MWCNT nanocomposite samples were displayed in Table 2.

Table 2.

The structural properties of foamed ABS/MWCNT nanocomposites.

Sample	Density (gr/cm3)	Relative density	%Void fraction	h/r	Expansion ratio
F1	0.946	0.909	9.04	1.23	1.10
F2	0.981	0.943	5.70	1.60	1.06
F3	0.991	0.953	4.70	1.77	1.05
F4	0.972	0.931	6.87	1.44	1.07
F5	1.006	0.964	3.60	2.02	1.04
F6	1.004	0.962	3.80	1.97	1.04
F7	0.951	0.909	9.09	1.22	1.10
F8	0.976	0.933	6.69	1.46	1.07
F9	0.963	0.921	7.94	1.32	1.09

Sample	Density (gr/cm3)	Relative density	%Void fraction	Expansion ratio
F1	0.946	0.909	9.04	1.10
F2	0.981	0.943	5.70	1.06
F3	0.991	0.953	4.70	1.05
F4	0.972	0.931	6.87	1.07
F5	1.006	0.964	3.60	1.04
F6	1.004	0.962	3.80	1.04
F7	0.951	0.909	9.09	1.10
F8	0.976	0.933	6.69	1.07
F9	0.963	0.921	7.94	1.09

According to the data of Table 2, the void fraction in foamed ABS/MWCNT nanocomposite with 1 wt% is higher than the other samples and the maximum value was attained for F1 and F7 which are the same, approximately. The SEM pictures for foamed ABS/MWCNT nanocomposite specimens were illustrated in Figure S.2. Cell density and cell size values were extracted from SEM picture and depicted in Figure 1. According to the results, similar to the void fraction, F1 and F7 with the same cell density, 42,500 cell/cm³, possess different cell size. Furthermore, the cell size was diminished for foamed ABS/MWCNT nanocomposite with 1 %wt compared to the others. It was attributed to the MWCNT presence.^18,33 As well as, it is apparent that foamed ABS/MWCNT nanocomposite with 0.5 wt%, F6, is an undesirable sample in terms of cell density. Regarding the foam type of the samples of this research study, polymeric foams are categorized into different types respect to their cell density and cell size,^34,35 Conventional polymeric foams which possess cell size larger than 100 µm and cell density lower than 10⁶ cell/cm³, Fine-celled polymeric foams with cell size between 10 and 100 µm and cell density between 10⁶ to 10⁹ cells/cm³, Microcellular polymeric foams which have cell size between 1 and 10 µm and cell density between 10⁹ to 10¹² cells/cm³, and Nanocellular polymeric foams with cell size less than 1 µm and cell density higher than 10¹² cell/cm³. Based on this categorization the specimens in this study were specified as the Conventional polymeric foams.

Figure 1.

Structural property of foamed ABS/MWCNT nanocomposite, cell size (a) and cell density (b).

Intrinsic Magnetic and Dielectric Properties of Solid and Foamed ABS/MWCNT Nanocomposite

The complex permittivity and complex permeability were computed from S-parameters by using a MATLAB code based on Nicolson-Ross-Weir (NRW) method. After that, the other inherent properties of solid and foamed ABS/MWCNT nanocomposite such as conductivity, the dielectric and magnetic loss were obtained from the complex permittivity and complex permeability results. Indeed, 200 microwave signals with different frequency were applied on the surface of samples over X-band frequency during VNA test. Also, 200 corresponding results were obtained for each parameter. The average of this results was considered for each property. The data for complex permittivity and permeability are presented in Figure 2.

Figure 2.

Real (a) and imaginary (b) part of relative permittivity, and the real (c) and the imaginary (d) part of permeability of solid and foamed ABS/MWCNT nanocomposites.

As aforementioned, the dielectric permittivity is in complex function, a real part, ${\epsilon }^{\prime }$ , and an imaginary part, ${\epsilon }^{″}$ . The real part is connected to the charge displacement, which attributed to polarization within the shield. ²⁰ According to Figure 2(a), the real part of permittivity was decreased by increasing of MWCNT amount in solid specimen. Moreover, ${\epsilon }^{\prime }$ was declined in foamed samples compare to their corresponding solid samples. As can be seen in Figure 2(a), the most reduction of the real part was ascribed to F8. By contrast, it can be seen in Figure 2(b) that the imaginary part of permittivity for F8 is superior compare to the other samples. The magnetic properties, real and imaginary parts of relative permeability were illustrated in Figure 2(c) and (d). Both parts of permeability are improved by foaming of ABS/MWCNT nanocomposite specimens.

As well as, the effective relative permittivity, obtained from Maxwell-Garnett theory, of foamed ABS/MWCNT samples (ε_eff) compared with their relative permittivity and they are exhibited in Figure 3. According to Figure 3(a), for all foamed samples, the effective relative permittivity is lower than the relative permittivity. The comparison of the dielectric loss and magnetic loss for solid and foamed ABS/MWCNT nanocomposite was depicted in Figure 3(b). The dielectric loss, tan (δ_e), is increased by adding MWCNTs in solid samples and S-1 has 102% and 87.4% progress compared to S-0 and S-0.5, respectively.

Figure 3.

Effective relative permittivity and relative permittivity of foamed ABS/MWCNT nanocomposite samples (a) and the comparison of the dielectric loss and magnetic loss (b) for solid and foamed ABS/MWCNT nanocomposites.

Also, the dielectric loss decreases in foamed neat ABS compared to solid pure ABS, on the contrary, it develops in foamed ABS/MWCNT nanocomposites compared to their solid counterparts. The highest value of dielectric loss belongs to F8, 148% and 22.8% improvement compared to S-0 and S-1, respectively. Indeed, foamed ABS/MWCNT nanocomposites possess more random distribution of MWCNTs in ABS matrix.^31,36 Also, this guided to the formation of MWCNT conductive networks. Thus, a higher ohmic loss was achieved.^36,37 On the other hand, both the dielectric permittivity and dielectric loss of the ABS/MWCNT nanocomposites were increased by the introduction of microcellular structure. ³⁶ The magnetic loss, tan (δ_m), in pure ABS was attenuated and in ABS/MWCNT nanocomposite was improved by foaming. Moreover, it was clarified that the dielectric loss is predominate phenomenon in attenuation of incident power rather than magnetic loss and this variation is high for foamed ABS/MWCNT nanocomposite with 1 wt% as illustrated in Figure 3(b).

In order to survey the effect of MWCNTs presence in EM wave attenuation inside foamed ABS structure, two samples with the same cell density, F1 and F7 with cell density of 42,500 cell/cm³, were compared. The dielectric loss and magnetic loss for F7 were increased by 3.6-fold and 13-fold, respectively. This is attributed to the creation of conductive channels in ABS matrix by MWCNTs.

EMI SE of ABS/MWCNT Nanocomposites

The EMI SE is the representative of material’s ability to reduce and dissipate the electromagnetic wave intensity. It is notable that, the main electromagnetic attenuation mechanisms are the wave reflection (SE_R) and the wave absorption (SE_A).^38–40 Indeed, the total EMI SE (SE_T) is the sum of SE_R and SE_A. The reflection mechanism is governed by the impedance mismatch between material and the air which is attributed to the charge carriers for instance electrons and holes.^41–43 However, the absorption mechanism is related to the ohmic ⁴⁴ and polarization losses. ⁴⁵ The total EMI SE in foamed ABS/MWCNT nanocomposites was improved by (i) the multiple reflection on diverse surfaces so the entering microwave scattered in microcellular structure for numerous times,^45,46 (ii) the interconnection of MWCNT, ⁴¹ and (iii) the exfoliation of MWCNT caused by foaming process. ⁴⁷ The SE_T, SE_R and SE_A of the solid and foamed ABS/MWVNT nanocomposites over the X-band frequency range (8-12 GHz) were accomplished.

The SE_R variations for solid and foamed ABS/MWCNT nanocomposites specimens were illustrated in Figure S.3. The SE_R for F8 is higher than others over the X-band frequency and it is maximum at 8.74 GHz frequency. At this frequency, the EM wave was reflected from the solid and foamed ABS/MWCNT nanocomposites with 1 wt% more than others. In foamed ABS/MWCNT nanocomposites with 1 wt%, F8 possess high SE_R over 8-12 GHz as depicted in Figure S.3(b). In solid ABS/MWCNT nanocomposites, the SE_R was improved by enhancing MWCNT wt% over X-band frequency as depicted in Figure S.3(c). Also, the SE_R diagram for F1 and F7 with the same cell density (42,500 cell/cm³) was studied and it was clarified that F7 is predominant sample in reflection as displayed in Figure S.3(d). The improvement of SE_R for F7 was 1.6 times that of F1 at 8.74 GHz. This improvement of SE_R for F7 was attributed to the MWCNT conductivity channels that were created in porous structure of F7.

The shielding effectiveness of absorption (SE_A) was attained for solid and foamed ABS/MWCNT nanocomposite and depicted in Figure S.4. The SE_A for solid and foamed specimens were showed in Figure S.4(a). It clarifies that the SE_A progression was occurred for F8 over the X-band frequency range (8-12 GHz) and the maximum value was attained at 11.52 GHz. The SE_A for foamed samples at 8-12 GHz was depicted in Figure S.4(b). As it specifies, the foamed ABS/MWCNT nanocomposite sample with 1 wt%, F8, has strong ability to absorb EM wave energy compare to the other foamed specimen. Hence, the foamed sample with 1 wt% MWCNT, F8, is a good candidate for microwave energy absorption.

The variations of SE_A for solid trials were represented in Figure S.4(c). It reveals that the EM wave energy for solid ABS/MWCNT nanocomposite sample with 1 wt%, S-1, dissipates more than S-0 and S-0.5 samples. The advancement of SE_A in solid samples at 11.52 GHz was achieved for S-1, 12.2% and 5.3% compared with S-0 and S-0.5, respectively. Also, the SE_A development for F8 is 6.5%, 12.4% and 19.5% compared with S-1, S-0.5 and S-0, respectively.

The foamed neat ABS with 42,500 cell/cm³, F1, was compared to the foamed ABS/MWCNT nanocomposite with 1 wt% and with 42,500 cell/cm³, F7, as displayed in Figure S.4(d). It was clarified that the high value of SE_A ascribed to F7 over the X-band frequency and the SE_A enhancement at 11.52 GHz was 23.5% for F7 compared to F1. This improvement of SE_A for solid and foamed ABS/MWCNT nanocomposite was attributed to MWCNTs presence in ABS as matrix.

The total EMI SE (SE_T) of solid and foamed ABS/MWCNT nanocomposite samples was probed and was demonstrated in Figure 4. As mentioned before, the total EMI SE (SE_T) is sum of the shielding effectiveness of reflection (SE_R) and absorption (SE_A), thereby, the SE_T results are similar to both SE_A and SE_R results. Furthermore, it can be seen that the EMI SE of all samples with different cell size is almost depend on frequency. According to Figure 4(a), the outstanding sample for protection against EM wave over the X-band frequency is F8. The maximum SE_T value, 16.25 dB, is occurred at 11.52 GHz for F8. The SE_T at this frequency for F8 %22.18, %11.7, and %8 was elevated compare to S-0, S-0.5, and S-1, respectively.

Figure 4.

Total EMI SE (SE_T) of solid and foamed (a), foamed (b), and solid (c) ABS/MWCNT nanocomposite samples and for F1 and F7 specimens (d) at 8-12 GHz.

In foamed ABS/MWCNT nanocomposite, F8 with cell density of 17,000 cell/cm³, cell size of 175 µm and h/r value of 1.46 is an effectual shield over 8-12 GHz as depicted in Figure 4(b). Also, the SE_T for F2, F3, F4, F5 and F6 with the average cell size of 410, 310, 325, 300 and 375 µm, respectively, was decreased but for F1, F7, F8, F9 with the average cell size of 225, 150, 175 and 170 µm, respectively, it was promoted. Thereby, it can be concluded that the EMI shielding of ABS/MWCNT was augmented by the diminution of cell size. ²⁷ Moreover, the EMI SE behavior of samples can be attributed to the h/r value. The aggregation of MWCNT was increased by the enhancing of h/r value, by contrast the MWCNT content was declined by the reduction of h/r value. The effectual conductive channels were diminished in both of them. Therefore, it could be concluded that there is an optimum value that more conductive channels were performed. For current study it is 1.46 for F8.

The SE_T of solid samples was detected and discovered that S-1 is a predominate trial over 8-12 GHz as displayed in Figure 4(c). The variations of SE_T over X-band frequency for F1 and F7 with the same cell density and high cell density compared to other samples was displayed in Figure 4(d). As it is explicit, F7 was a dominant EM wave shield over 8-12 GHz frequency. The advancement of SE_T for F7 was %21.2 compared with F1 at 11.52 GHz which ascribed to the creation of conductive networks in F7.

A schematic picture of the cell density effect on the creation of conductive networks into polymer matrix was depicted in Figure 5. It is well known that the nano additive was exfoliated and was dispersed by hollow structure in polymer matrix, impressively.^38,48 So, it can be inferred from this point that the nano fillers were not exfoliated in polymer matrix by the reduction of cell density as depicted in Figure 5(a). Therefore, the least conductive channels constructed.

Figure 5.

Schematic diagram of the effect of cell density on the creation of conductive channels, low cell density (a), high cell density (b) and optimal cell density (c).

It should be taken into account that nano fillers are driven from cell walls by increasing the cell density at high value. Indeed, the MWCNT amount was declined in the cell walls of foamed ABS/MWCNT nanocomposite by increasing the cell density and correspondingly decreasing cell wall thickness.^35,49,50 Thereby, the effectual conductive channels diminished and it decreases the total EMI SE of samples. This can be observed from Figure 5(b).

With respect to that the sample with high cell density, for instance F1 and F7 with cell density of 42,500 cell/cm³, did not possess the maximum SE_T, it is apparent that there was an optimal cell density which the total EMI SE was increased significantly as can be seen in Figure 5(c), owing to the creation more effectual conductive channels in polymer matrix. Thus, the opportunities to interconnectivity of MWCNT particles increased during the cell growth.^36,38 As aforementioned, conductive networks possess synergistic effect on the total EMI SE. It is well established that the multi scattering of entering EM wave inside micro structure is the basic mechanism of microwave attenuation in foamed samples. In contrast to unfoamed specimen which the entering EM wave scattered only between two surfaces; the microwave scattered numerous times between various faces inside porous structure in foamed sample.

The contribution of absorption and reflection for F8 at 11.52 GHz frequency to total EMI SE is 96.31% and 3.7%, respectively. That means the EMI SE behavior is governed by absorption rather than reflection as depicted in Figure 6(a). Furthermore, the contribution of absorption loss and reflection loss to total EMI for all solid and foamed samples over 8-12 GHz that were derived from the average results, was displayed in Figure 6(b). Again, it is obvious that absorption loss is predominate phenomenon rather than reflection loss.^48,51 This behavior of solid and foamed ABS/MWCNT nanocomposite was attributed to the complex permeability, the good impedance match and conductivity. Possible explanations for the enhancement in SE_A are the conductivity of ABS/MWCNT, the lower conductivity means that the highest volume of micro wave penetrates to the shield because of the lack of the free charge carriers. As well as, the advancement of absorption ability, the reduction in reflection of EM wave, was promoted in porous structure due to the improvement of the impedance match that happens between the incident micro wave and the surfaces of ABS/MWCNT.

Figure 6.

Contribution of SE_R and SE_A to SE_T at 11.52 GHz (a) and the average results of contribution of SE_R and SE_A to SE_T at 8-12 GHz frequency (b).

Taguchi Analysis

As mentioned, the maximum SE_T was found at 11.52 GHz. To find the effect of process parameters and adding MWCNTs on the total EMI SE at this frequency, the results for each trial was investigated by Minitab software. Finally, the S/N plot was attained for total EMI SE and result was illustrated in Figure 7. It can be observed that the most variation of S/N ratio value, difference between maximum and minimum value of the average S/N ratios, was attained for MWCNT, after that this value for holding pressure time is more than cooling time. Therefore, the sequence of the effective processing parameters on high EMI shielding effectiveness was attained as, MWCNT, holding pressure time, and cooling time, respectively. As it can be seen in Figure 7, the cooling time does not have a noteworthy variation of S/N ratio. Also, the ranking for MWCNT, holding pressure time, and cooling time was computed to 1, 2, and 3, respectively. The Taguchi prediction for SE_T of a trial under the optimal conditions was 15.564 (dB).

Figure 7.

Main effects plot for S/N of SE_T at 11.52 GHz of samples.

The maximum value of signal to noise ratio for each level was reported as optimal level of parameter. Thereby, the optimal level for MWCNT, holding pressure time, and cooling time was achieved 1 wt%, 2 s, and 60 s, respectively. Moreover, the participation percentage of each parameter in total EMI SE at 11.52 GHz was attained by using the analysis of variation (ANOVA) approach and the data was reported in Table 3. The contribution percentage of MWCNT, holding pressure time, and cooling time were achieved 67.3%, 10.2%, and 7.1% at 11.52 GHz, respectively.

Table 3.

ANOVA result for the total EMI SE of samples at 11.52 GHz frequency.

Source	DF	Sum of square (SS)	Variance (SS/DF)	F (Vi/Ve)	P% (ss/st *100)
MWCNT (wt%)	2	7.812	3.906	4.34	67.3
Holding time (s)	2	1.182	0.591	0.657	10.2
Cooling time (s)	2	0.82	0.41	0.456	7.1
Error	2	1.8	0.9	1	15.4
Total	8	11.614	5.807	6.453	100

The effect of processing parameters on the total EMI shielding behavior of foamed ABS/MWCNT nanocomposite was investigated in Figure 8. The effect of MWCNT on total EMI SE was illustrated in Figure 8(a). It can be seen that SE_T was promoted by increasing MWCNT amount, owing to the conductive networks performed into micro cellular structure of foamed ABS/MWCNT nanocomposite. The effect of holding pressure time on SE_T was showed in Figure 8(b) and it can be observed that the total EMI shielding effectiveness has an increasing behavior fallowed by decreasing behavior by increasing the holding pressure time. Cell nucleation was restricted by using all available gas and increasing the melt viscosity. ²³ Possibly the available gas was finished by increasing the holding pressure time, thus the EMI shielding effectiveness of ABS/MWCNT nanocomposite declined. Also, the effect of cooling time on SE_T was survived as Figure 8(c), total EMI shielding effectiveness is significantly decreased by increasing cooling time from 60 s to 80 s, but its increasing with enhancing cooling time from 80 s to 100 s is negligible. The melt viscosity was declined by advancement of cooling time, cell nucleation diminished, which was responsible for SE_T of ABS/MWCNT nanocomposite reduction.

Figure 8.

Effects of processing parameters on total EMI SE, MWCNT concentration (a), holding pressure time (b), and cooling time (c).

The interaction plot of processing parameters for total EMI SE of foamed ABS/MWCNT nanocomposite was displayed in Figure 9. It can be observed in Figure 9(a) that the variation of SE_T for foamed neat ABS and ABS/MWCNT nanocomposites with 0.5 wt% is negligible with increasing of holding pressure time but it is significant for foamed ABS/MWCNT nanocomposite with 1 wt%. The maximum SE_T was occurred for this sample at 2 s of holding pressure time. It can be seen from Figure 9(b) that total EMI SE was augmented by increasing MWCNTs at all levels of cooling time. Also, maximum SE_T was occurred for foamed ABS/MWCNT nanocomposite with 1 wt% at 60 s of cooling time. As well as the simultaneous effect of holding pressure time and cooling time on the total EMI shielding effectiveness was illustrated in Figure 9(c). According to the findings, the SE_T possess an independent behavior with holding pressure time of 3 s at all levels of cooling time, approximately. Moreover, the total EMI SE for holding pressure time of 1 s and 2 s at cooling time of 100 s has an equal value.

Figure 9.

Interaction plot of processing parameters for total EMI SE, MWCNT-holding pressure time (a), MWCNT-cooling time (b), and holding pressure time-cooling time(c).

EMI Shielding Performance of ABS/MWCNT Nanocomposite Foams

The percentage of EM wave attenuation was probed. The average of results was illustrated as Figure 10(a). It can be observed that the EM wave passed from F1 easily and without any difficulty, in contrary to F8, the EM wave transmitted from shield hardly. Foamed pure ABS, F1, and foamed ABS/MWCNT nanocomposite with 1 wt%, F7, with the same cell density, 42,500 cell/cm³, possess different action toward to the incident EM wave. As it can be seen in Figure 10(a), the attenuation percentage of EM wave for F7 is superior than F1. The advancement of the dissipation percentage for F7 is attributed to the existence of MWCNT and conductive channel in ABS matrix.

Figure 10.

Attenuation percentage of EM wave intensity (a) and the specific total EMI SE of solid and foamed ABS/MWCNT nanocomposite at 11.52 GHz frequency.

Moreover, in order to investigate the EMI characterization at the same weight, the specific total EMI SE of solid and foamed ABS/MWCNT nanocomposite at 11.52 GHz, the ratio of SE_T to density, was surveyed and reported as Figure 10(b). At this frequency, the specific SE_T was elevated by increasing MWCNT particles in solid and foamed samples and F8 has the utmost specific SE_T value.

The average results of conductivity for solid and foamed ABS/MWCNT nanocomposite over the X-band was probed and was depicted as Figure 11(a). As displayed, the conductivity was decreased in foamed neat ABS compared to S-0 but it was enhanced in foamed ABS/MWCNT nanocomposite. The maximum value of conductivity was attained for F8. The efficacy of micro cellular structure with appropriate structural property and adding MWCNT was achieved by comparison of F8 and S-0, the conductivity for F8 was enhanced by 2.3-fold. Moreover, the influence of MWCNT particles on conductivity was clarified by comparison of F1, the foamed neat ABS, and F7, the foamed ABS/MWCNT with 1 wt%, that possess the same cell density, 42,500 cell/cm³. The conductivity of F7 was attained 6 times that of F1.

Figure 11.

Average results of the conductivity (a) and the EMI SE derived from conductivity (b) at 8-12 GHz frequency.

The effect of conductivity on the total EMI SE was explored as Figure 11(b). In solid ABS/MWCNT nanocomposite, the EMI SE was enhanced by increasing MWCNT so that the improvement of the EMI SE for S-1 is 22.5% and 34.6% compared to S-0.5 and S-0, respectively. Also, in foamed samples, the similar results were found. The betterment of the EMI SE for F8 was 74.6% attained higher compared to the foamed pure ABS, F1.

Conclusions

The melt mixing method was chosen to fabricate the ABS/MWCNT nanocomposite samples. After that, the solid and foamed ABS/MWCNT nanocomposite specimens were manufactured via an injection molding process. The foamed ABS/MWCNT nanocomposite disclosed high EM wave absorption ability compared with unfoamed counterparts. This valuable behavior of foamed ABS/MWCNT nanocomposite is attributed to the performing conductive networks during foaming process. MWCNTs were exfoliated in ABS matrix and were arranged to make the effectual electrical conductivity channels by cell growth during foaming. The promotion of EMI SE characterizations was governed by absorption mechanism rather than reflection mechanism. Also, the absorption ability of samples attributed to the ohmic loss, the polarization loss, and the multiple scattering in foamed specimens are due to porous structure. This study has revealed that the foamed ABS/MWCNT nanocomposite with 1 wt% of MWCNT, 175 µm as cell size, 6.69% as void fraction, and 17,000 cell/cm³ as cell density, F8, possess the utmost value of the total EMI shielding, 16.25 dB, at 11.52 GHz frequency. The superior EMI shielding specification of foamed ABS/MWCNT nanocomposite with 1 wt%, F8, was belonged to the unique arrangement of the MWCNTs into porous structure of ABS matrix. This survey has proposed an industrial and an inexpensive method to fabricate the solid and foamed ABS/MWCNT nanocomposite shields. Also, the inherent properties of the ABS/MWCNT nanocomposite as an electromagnetic wave shield were extracted in details.

Supplemental Material

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