Enhanced Dielectric Constant for Efficient Electromagnetic Shielding Based on Carbon-Nanotube-Added Styrene Acrylic Emulsion Based Composite
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An efficient electromagnetic shielding composite based on multiwalled carbon nanotubes (MWCNTs)-filled styrene acrylic emulsion-based polymer has been prepared in a water-based system. The MWCNTs were demonstrated to have an effect on the dielectric constants, which effectively enhance electromagnetic shielding efficiency (SE) of the composites. A low conductivity threshold of 0.23 wt% can be obtained. An EMI SE of ~28 dB was achieved for 20 wt% MWCNTs. The AC conductivity (σ ac) of the composites, deduced from imaginary permittivity, was used to estimate the SE of the composites in X band (8.2–12.4 GHz), showing a good agreement with the measured results.
KeywordsMultiwalled carbon nanotubes Electromagnetic interference shielding Complex permittivity Styrene acrylic emulsion
Due to the quick growth in the utilization of commercial, industrial and military applications, EMI has become a serious concern in modern society. Light-weight EMI shielding is needed to protect the environment and workplace from EMI due to unwanted electromagnetic waves, especially for the building containing power transformers and other electronic facilities, which will radiate electromagnetic wave to the environment. Electrically conductive polymer composites containing carbon-based fillers have been extensively investigated recently for EMI shielding applications [1–3]. Compared to conventional metal-based EMI shielding materials, they are light weight, resistant to corrosion and flexible .
Recently, multiwalled carbon nanotubes (MWCNTs)/polymer conductive composites have received extensive considerable attentions in both fundamental and applied research fields. Previous studies on the MWCNTs and their diversified applications show that MWCNTs possess excellent electrical, mechanical properties and unique one-dimensional structure [5–10], which make them an ideal option to create overlapping conductive network for high-performance EMI shielding at low loadings [11–13]. Many MWCNTs/polymer composites in typical solutions or melt-based systems have been studied with various polymer matrix, including epoxy , shape memory polymer , poly(methyl methacrylate) (PMMA) , polyurethane , for various applications such as effective and light-weight EMI shielding, microwave absorption, high charge storage capacitors. However, the polymer composite materials with MWCNTs in water-based systems have been largely unexplored and more environmentally friendly, for there is no or negligible content of volatile organic compounds. It has been reported that the defective arc-made MWCNTs have a high dielectric constants , and the theory prediction shows that the lattice defects can create localized states near the Fermi level , which can give rise to large microwave absorption. The defects on the MWCNTs can be introduced by a special purification process [19–21].
Styrene acrylic emulsion is widely used in the field of the architectural paints for its prominent properties such as high resistance to UV light, oxygen, water, various types of solvents and excellent durability . However, styrene acrylic emulsion-based polymer alone has a low conductivity and provides no shielding property. It seems reasonable to prepare styrene acrylic emulsion-based composites for building shielding materials by incorporating WMCNTs with all the excellent properties mentioned earlier.
In this study, we prepared a styrene acrylic emulsion-based composite with well-dispersed MWCNTs for building EMI shielding applications. The structural characteristics of the MWCNTs and composites were investigated through field-emission scanning microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy experiments. The DC conductivity (σ dc), complex permittivity and EMI SE of the composites in 8.2–12.4 GHz were reported. The AC conductivity (σ ac) of the composites, calculated from imaginary permittivity, was used to estimate the SE of the composites in the far field.
The styrene acrylic emulsion (trade name HBC-03) used in this study was provided by Beijing Huyi Co., Ltd. This emulsion was approximately 48 ± 2 wt% solids in water. The minimum film formation temperature for this system is 15°C, and the glass transition temperature (T g) is 34°C. The raw MWCNTs were supplied by Shanghai Jiaotong University (Shanghai, China). The diameter of the raw MWCNTs was 5–10 nm, the length was 5–10 μm, and the purity was 95%. They were synthesized by a chemical vapor deposition method using Ni as the main catalyst. The raw MWCNTs were subjected to chain-scission in a 3:1 mixture of sulfuric acid and nitric acid at 80°C for 2 h in a reflux system. The purified MWCNTs were then heated to 100°C under nitrogen atmosphere to evaporate excess acid and water. Anionic surfactant sodium dodecylbenzene sulfonate (SBDS) was introduced to stabilize the MWCNTs to obtain better dispersion.
Preparation of the Composites
The conventional solution process described below was used to prepare the composites. The purified MWMTs were first added into de-ionized water with 2.0 wt% SBDS in an ultrasonic bath with oscillation frequency of 42 kHz for 2 h at room temperature, and then the styrene acrylic emulsion was added to the suspension of purified MWNTs with designed weight ratios. The mixture was again sonicated for 2 h. The highest concentration of purified MWNTs in emulsion was mixed first, and lower concentrations were obtained by diluting the mixture with more emulsion and de-ionized water. The prepared mixture was coated onto concrete panels (60 cm × 60 cm × 5 cm) to measure the EMI SE. Some of the mixture was poured into casting molds. The samples were consolidated at ~50°C for 24 h. Then the samples were dried at room temperature until a constant weight was achieved. The thickness of the composite films was 1.5 mm. A series of styrene acrylic emulsion-based composite films were prepared with different mass concentrations of purified MWCNTs. The prepared samples were cut into the desired sizes for further measurements.
The morphology and structure of the samples were studied with field-emission scanning electron microscope (FESEM, HITACHI S-4300), high-resolution transmission electron microscope (HRTEM, JEOL-2010) and Raman spectroscopy (Ramascope-1000, 514 nm laser). The composites were freeze-fractured in liquid nitrogen and gold coated before imagining on FESEM.
Results and Discussion
Raman and Morphological Analysis
In this system, the percolation threshold for the MWCNTs-filled styrene acrylic emulsion-based composites is ~0.23 wt%, while for the similar carbon black- or carbon fiber-filled polymer system, its percolation threshold is near 4 wt% . Various percolation thresholds have been reported for different CNT–polymer systems. For example, 0.04 wt% for SWNT/PVAc emulsion-based polymer system , the 0.62 wt% for SWCNT–epoxy system , 0.3 wt% for purified MWCNT/PMMA system . Our value for the percolation threshold is in the same range with the systems mentioned earlier.
The relatively low percolation threshold value can be attributed to the one-dimensional large aspect ratio (>100), efficient dispersion of MWCNTs and the action of microscopic solid polymer particles in the emulsion, which may push the nanotubes into an interstitial network during drying process of the emulsion-based composite .
EMI Shielding Efficiency
where f is the frequency of radiation in Hz, μ is the magnetic permeability of the sample, μ = μ 0 μ r , μ 0 = 4π × 10−7 Hm−1 is the absolute permeability of free space, since the MWCNTs wall and styrene acrylic polymer are diamagnetic, we have μ r = 1. σ is the AC conductivity of the sample in S/m. ɛ 0 is the permittivity of the free space, ɛ 0 = 8.854 × 10−12 F/m. When t/δ ≥ 1.3, the material is called ‘electrically thick’ and ‘electrically thin’ in the contrary case .
In summary, we have seen that MWCNTs, with some defects after purification, favor uniform dispersion into an environmentally friendly, water-based styrene acrylic emulsion to form an efficient electromagnetic shielding composite and have a major influence on the dielectric and electromagnetic properties. An EMI shielding effectiveness of ~28 dB in X band was achieved for 20 wt% MWCNTs. The AC conductivity (σ ac) of the composites, deduced from imaginary permittivity (ɛ′′), was used to estimate the SE and the electromagnetic shielding mechanism was explained by a parallel resistor–capacitor model. We suggest this composite can be applied for building shielding applications in high frequencies.
This work is supported by Specialized Research Fund for the Doctoral Program of Higher Education ‖SRFDP) Grant No. 200802481028, Shanghai-Applied Materials Research and Development Fund Grant No. 08520741500, National Natural Science Foundation of China Grant No. 60807008 and National Natural Science Foundation of China Grant No. 50730008.
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