Effect of carbon nanotubes implantation on electrical

Composite Interfaces

ISSN: 0927-6440 (Print) 1568-5543 (Online) Journal homepage: http://www.tandfonline.com/loi/tcoi20

Effect of carbon nanotubes implantation on electrical properties of sisal fibre–epoxy composites U. K. Dwivedi, M. Trihotri, S. C. Gupta, Fozia H. Khan, M. M. Malik & M. S. Qureshi To cite this article: U. K. Dwivedi, M. Trihotri, S. C. Gupta, Fozia H. Khan, M. M. Malik & M. S. Qureshi (2016): Effect of carbon nanotubes implantation on electrical properties of sisal fibre–epoxy composites, Composite Interfaces, DOI: 10.1080/09276440.2016.1192314 To link to this article: http://dx.doi.org/10.1080/09276440.2016.1192314

Published online: 16 Jun 2016.

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Date: 17 June 2016, At: 21:13

Composite Interfaces, 2016 http://dx.doi.org/10.1080/09276440.2016.1192314

Effect of carbon nanotubes implantation on electrical properties of sisal fibre–epoxy composites U. K. Dwivedia, M. Trihotrib,c, S. C. Guptaa, Fozia H. Khanb, M. M. Malikb and M. S. Qureshib Downloaded by [University of California Santa Barbara] at 21:13 17 June 2016

a

Department of Applied Sciences, Amity University, Jaipur, India; bDepartment of Physics, Maulana Azad National Institute of Technology, Bhopal, India; cDepartment of Physics, Lakshmi Narain College of Technology, Bhopal, India

ABSTRACT

In this paper, the effects of carbon nanotubes (CNT) implantation and sisal fibre size on the electrical properties of sisal fibre-reinforced epoxy composites are reported. For this purpose, the epoxy composites reinforced with CNT-implanted sisal fibre of 5 mm and 10 mm lengths were prepared by hand moulding and samples characterized for their electrical properties, such as dielectric constant (ε′), dielectric dissipation factor (tan δ) and AC conductivity (σac) at different temperatures and frequencies. It was observed that the dielectric constant increases with increase in temperature and decreases with increase in frequency from 500 Hz to 5 KHz. Interestingly, the sample having CNT-implanted sisal fibre of 5 mm length exhibited the highest value of dielectric constant than the one with length 10 mm. This is attributed to the increased surface area of sisal fibre and enhancement of the interfacial polarization. At a constant volume and a length of 5 mm of the fibres, the number of interfaces per unit volume element is high and results in a higher interfacial polarization. The interfaces decrease as the fibre length increases, and therefore, the value of ε′ decreases at 10 mm fibre length. The peak value of the dielectric constant decreases with increasing frequency. A continuous decrease in dissipation factor (tan δ) with increasing frequency for all samples was observed, while at lower temperatures, the values of tan δ remains approximately same. The AC conductivity for 5 mm length sisal epoxy composite and 10 mm length sisal fibre–epoxy composites is higher than that of pure epoxy at all the frequencies.

ARTICLE HISTORY

Received 2 April 2016 Accepted 18 May 2016 KEYWORDS

Electrical properties; interfacial polarization; CNT; sisal fibre–epoxy composite

1. Introduction High-performance electrical and mechanical properties, low cost and processing technologies have lead to significant increase in studies on composite materials.[1–5] Natural fibre-reinforced composites being light in weight and environment-friendly are potential materials for applications in areas such as aerospace, automobile and electromagnetic shielding.[6] Compared to other lignocellulosic fibres, sisal composites are of particular

CONTACT U. K. Dwivedi

[email protected]

© 2016 Informa UK Limited, trading as Taylor & Francis Group

Downloaded by [University of California Santa Barbara] at 21:13 17 June 2016

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U. K. Dwivedi et al.

interest because these have high impact strength with moderate tensile and flexural properties. Many workers have studied their electrical properties such as dielectric constant (ε′), dielectric dissipation factor (tan δ) and AC conductivity (σac). Sisal fibre composites have been found to process anisotropic electrical behaviour.[7] Dinesh et al. [8] have prepared nanocomposites of high-density polyethylene/carbon black/multiwalled carbon nanotubes by melt mixing and examined their dielectric properties and electromagnetic interference shielding effectiveness at the composite interfaces. Lin et al. [9] fabricated multiwalled, bamboo-like carbon nanotube/methyl vinyl silicone composites by liquid blending method and investigated the electrical conductivity (σ) and shielding effectiveness (SE) of the composites. Zhuang et al. [10] studied jute fibres epoxy composites by depositing multiwalled carbon nanotubes (MWCNTs) on the surfaces of jute fibres or fabrics and found a significant improvement in dielectric properties of the MWCNT-jute/epoxy composites compared to neat jute/epoxy composites. Gao et al. [11] reported an innovative approach to enhance electrical conductivity of fibre composites based on non-conductive fibre and polymer matrix, which resulted enhancement in electrical conductivities of 2–3 orders of magnitude. Yan Li et al. [12] observed in their study that sisal/low-density polyethylene/carbon black composites can be used in antistatic applications to dissipate static charge. Paul et al. investigated the effect of frequency, fibre content and fibre length on electrical properties of sisal fibre/low-density polyethylene (LDPE) composites and compared with those of carbon black and glass fibre composites of LDPE.[1,12–19] The dielectric constant was found to decrease with the increase of fibre length and frequency. Their studies on surface treatment of sisal fibre LDPE composites revealed that the dielectric constant decreases with the decrease in hydrophilicity of the composites.[15] The studies on dewaxed sisal fibre-reinforced epoxy composite and raw sisal fibre-reinforced epoxy composite indicate that a good correlation between dielectric behaviour and mechanical properties of epoxy reinforced with sisal fibre. Besides these, properties of the composites have been correlated with the structural parameters of the reinforced fibre.[20] The complex impedance spectroscopy studies on cured polyester matrix and sisal fibre-reinforced polyester composites have been investigated by Patra and Bisoyi [21] in the frequency range from 180 Hz to 1 MHz and the temperature between room temperature to 200 °C. They found that the incorporation of sisal fibre increases the dielectric constant, dissipation factor and AC conductivity. But with increasing frequency, dielectric constant and dissipation factor decrease.[21] Amora et al. studied [22] polymer composites of a polyester resin matrix filled with short palm fibres by means of dielectric spectroscopy in the frequency range 0, 1–100 kHz and temperature interval from 40 to 200 °C. The effect of CNT implantation on sisal fibre reinforced with epoxy matrix having different lengths of fibre is not studied yet. The aim of this work was to analyse the electrical properties of CNT-implanted sisal epoxy composites at different temperatures and frequencies.

2. Materials and methods The thermosetting matrix used in this study was provided by Atul Pvt. Ltd. Valsad India. The density of the resin, cured at room temperature was 1.15 g/cm3. The sisal fibres were collected from Bilaspur, India. The density of the sisal fibre was 1.45 g/cm3, and the diameter was 100–200 mm. Industrial grade multiwalled carbon nanotube (MWCNT)(1205YJ) with purity more than 95% with specifications, outside diameter 10–20 nm, inside diameter



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5–10 nm, length 10–30 μm, specific surface area 180–230 m2/g, bulk density 0.04–0.05 g/cm3, was supplied by Nanostructured & Amorphous Materials, Inc. USA. The carbon nanotubes (1 wt%) were added to acetone and sonicated for 1 h for complete dispersion. Then, chopped sisal fibres were immersed in this CNT dispersion. Some of the CNT get into the pores of sisal fibres while others stick to the surface of the sisal fibre. The implanted sisal fibres were dried at 80 °C for 2 h in an air-circulating oven. This was then added to the epoxy followed by the addition of hardener. The resin/hardener ratio was maintained 10:1, and the weight fraction of sisal fibres in the prepared composite was kept in the ratio of 10%. Two different composites having 5 and 10 mm lengths of randomly oriented sisal fibre were prepared. The test samples were cut in the form of circular discs of 1 mm thickness and 10 mm diameter from the sheets. The sample disc was polished to get uniformity in the surface. The sample disc was coated both sides using air-drying silver conducting paint such that both the surfaces should not connect electrically with each other. The test samples were heated at 60 °C for 10 min to remove the solvent of conducting paste and then kept in between the electrodes of the sample holder for various measurements. Capacitance (C) and tan δ values were measured using a Wayne Kerr 6500B Impedance Analyzer in the temperature range from 35 to 180 °C at different frequencies (0.5–5 KHz) keeping the heating rate constant at 2 °C/min. Dielectric constant (ε′) of the composite has been calculated using the following relation

𝜀� =

C Co

(1)

where C and CO are the capacitance with and without dielectric, respectively; Co in pF is given by

Co =

(0.08854)A pf d

where A (cm2) is the area of the electrodes and d (cm) the thickness of the sample. Dielectric dissipation factor (tan δ) is defined as follows:

tan 𝛿 =

𝜀�� 𝜀�

(2)

where ε″ is the dielectric loss. Measurements of capacitance and conductance are used to calculate, (i) real part of permittivity (apparent permittivity) ε′, which is proportional to the capacitance and measures the alignment of dipoles, (ii) dissipation factor, tan δ = ε″/ ε′ and (iii) AC conductivity (σac) calculated from the relation

𝜎ac = 𝜀0 𝜔𝜀� tan 𝛿

(3)

where ε0 is the permittivity of free space, tan δ the dielectric dissipation factor and ω the angular frequency of the applied electric field. SEM images of the prepared samples were taken by JSM 6390A (JEOL Japan) at different magnifications. The samples were coated with gold in a vacuum coating unit prior to the examination. Images of the fractured surfaces were taken and discussed.

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3. Results and discussions The conductivity of fibre-reinforced composites depends on various factors such as moisture content, crystalline and amorphous component present, chemical composition, and cellular structure. The dielectric constant of polymeric materials depends on the contribution of interfacial, dipole, electronic and atomic polarizations. Figure 1(a)–(e) shows the variation of dielectric constant (ε′) with temperature (T) for pure epoxy, epoxy composite reinforced with 5 and 10 mm length pure sisal fibre and MWCNT-implanted sisal fibre at different frequencies 0.5, 3, 5, 8 and 10 kHz, respectively. The observations clearly show an increase in the dielectric constant (ε′) of MWCNT-implanted sisal fibre–epoxy composites. Highest values of dielectric constant (ε′) are found in Figure 1(c) for 5 mm length MWCNTimplanted sisal fibre–epoxy composites. Results reveal that the values of dielectric constant for MWCNT-implanted sisal fibre– epoxy composites of 5 mm length are much higher than the sisal fibre–epoxy composites of the same length. The insertion of the MWCNT in the micrometre diameter pores of sisal fibre seems to be responsible for this behaviour. The implantation of MWCNT through entrapment and coating of sisal fibre with MWCNT enhances the dielectric constant of the samples. The ε′ value of 5 mm length pure sisal fibre is approximately 6 at 0.5 kHz at 110 °C, while the ε′ value of implanted sisal fibre–epoxy sample is approximately eight at the same frequency and temperature as shown in Figure 1(a) and (d). Similar behaviour is found in the 10 mm length sisal fibre–epoxy composite samples, but the enhancement in the ε′ value is less as compared to the 5-mm-length sisal fibre–epoxy sample values. Dielectric constant increases with the increase in temperature and decrease with increase in frequency. The peak height at the transition temperature decreases with increasing frequency. At low frequencies, all the dipole groups in the epoxy molecular chains can orient themselves, resulting in higher dielectric constant. When the frequency of AC voltage increases, the polarization fails to settle itself completely and the values of the dielectric constant of epoxy resin begin to drop, when approaching higher frequencies. Interestingly, the dielectric constant ε′ of the 5 mm length MWCNT-implanted sisal fibre–epoxy samples is observed higher than the 10-mm-length MWCNT-implanted sisal fibre–epoxy samples. This is because of the higher concentration of MWCNTs in the micrometre diameter pores of sisal fibre presented in the 5-mm-length composite specimen than the 10-mm-length composite. It is well understood that the surface area of the smaller size sisal fibre (5 mm length sisal fibre) will be more compared to larger size sisal fibre (10 mm length sisal fibre) in case of constant volume % of sisal fibre present in fibre–epoxy composite. This behaviour of the material may be explained in terms of interfacial polarization. At a length of 5 mm, the number of interfaces per unit volume element is high at a constant volume % of fibres resulting in higher interfacial polarization. But, at 10 mm fibre length, the number of interfaces per unit volume decreases resulting in a decrease of the value of dielectric constant ε′.[23] It is also observed that the ε′ values of pure sisal fibre–epoxy samples show temperature dependency while those of implanted sisal fibre composites are almost independent of the temperature. Figure 2(a)–(e) shows the variation of dissipation factor (tan δ) with temperature (T) for pure epoxy, epoxy composite reinforced with 5 and 10 mm length pure sisal fibre and MWCNT-implanted sisal fibre at frequencies 0.5, 3, 5, 8 and 10 kHz, respectively. It is seen from the figure that the dissipation factor (tan δ) is low for the samples having MWCNTimplanted sisal fibre. The tan δ values of 5 mm length MWCNT-implanted sisal fibre are

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Figure 1(a–e). Variation of dielectric constant (ε′) versus temperature (T) at different frequencies 0.5, 3, 5, 8 and 10 kHz for (a) pure epoxy, (b) epoxy composite reinforced with 5 mm length sisal fibre, (c) epoxy composite reinforced with 10 mm length sisal fibre, (d) epoxy composite reinforced with 5 mm length CNT-implanted sisal fibre and (e) epoxy composite reinforced with 10 mm length CNT-implanted sisal fibre.

less than the 10-mm-length MWCNT-implanted sisal fibre–epoxy composites. The tan δ value depends on the value of dielectric constant. Dielectric constant (ε′) values of 5 mm length MWCNT-implanted sisal fibre are greater than 10 mm length MWCNT-implanted sisal fibre–epoxy composite.


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Figure 2(a–e). Variation of dielectric dissipation factor (tan δ) versus temperature (T) at different frequencies 0.5, 3, 5, 8 and 10 kHz for (a) pure epoxy, (b) epoxy composite reinforced with 5 mm length sisal fibre, (c) epoxy composite reinforced with 10 mm length sisal fibre, (d) epoxy composite reinforced with 5 mm length CNT-implanted sisal fibre and (e) epoxy composite reinforced with 10 mm length CNT-implanted sisal fibre.

Dissipation factor (tan δ) depends on the electrical conductivity of the epoxy composites. The electrical conductivity, in turn, depends on the number of charge carriers in the bulk of the material, the relaxation time of the charge carriers and the frequency of the applied electric field.[24] Dielectric dissipation factor (tan δ) values increase with increase in temperature and decreases with the increase in the frequency. It is also clear from Figure 4.3.2 that tan δ values are small for MWCNT-implanted sisal fibre–epoxy composites as



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Figure 3(a–e). Variation of AC conductivity (σac) versus temperature (T) at different temperature and at different frequencies 0.5, 3, 5, 8 and 10 kHz for (a) pure epoxy, (b) epoxy composite reinforced with 5 mm length sisal fibre, (c) epoxy composite reinforced with 10 mm length sisal fibre, (d) epoxy composite reinforced with 5 mm length CNT-implanted sisal fibre, (e) epoxy composite reinforced with 10 mm length CNT-implanted sisal fibre.

compared to the pure sisal fibre–epoxy composites. For 5 and 10 mm pure sisal fibre–epoxy composites, the values are almost same at 3, 5, 8 and 10 kHz. frequency and at all measured temperatures. On the other hand, for 5 mm length MWCNT-implanted sisal fibre–epoxy samples, the tan δ values are minimum. Dielectric constant and dissipation factor (tan δ) for any particular composition decrease with increase in frequency. It has been mentioned that with increase in frequency, the dipoles get very less time to orient themselves in the



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Figure 4. (a, b, c) Variation of ε′, tan δ and (σac) with log f (frequency) for various studied samples measured at 110 °C.



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Figure 5. (a, b) SEM micrographs of the fractured surface of pure sisal fibre–epoxy composite.

direction of applied electric field, and hence, both dielectric constant and dissipation factor (tan δ) decrease with increase in frequency.[25] Both dielectric constant and loss increase with filler (pure sisal fibre) loading at any particular frequency, mainly due to space charge interfacial polarization. With MWCNT-implanted sisal fibre loading, the dielectric constant increases but the dissipation factor (tan δ) decreases. Dissipation factor (tan δ) values of 5 and 10 mm length pure sisal fibre–epoxy composites are almost same, while there is a marginal difference in the dissipation factor values of MWCNT-implanted sisal fibre–epoxy composites of both lengths. Variation of AC conductivity (σac) as a function of temperature (T) for pure epoxy, epoxy composite reinforced with 5 and 10 mm length pure sisal fibre and MWCNT-implanted sisal fibre at different frequencies 0.5, 3, 5, 8 and 10 kHz, respectively, are shown in Figure 3(a)–(e). The value of AC conductivity (σac) depends on both the temperature and frequency as shown in figure. With the increase in temperature, the value of σac increases. For the higher frequencies, i.e. 10 kHz, it is comparably high for all wt% samples. AC conductivity (σac) of the prepared specimens is shown in Figure 3. The AC conductivity (σac) is calculated using the empirical relation, σac = ωεrε0tanδ, where ε0 is the permittivity of free space, and ω is the angular frequency. The σac patterns show a frequency independent


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Figure 6. (a, b) SEM micrographs of fractured surface of MWCNT-implanted sisal fibre–epoxy composite.

plateau in the low-frequency region and exhibit dispersion at higher frequencies. It is observed that AC conductivity (σac) of all the five samples increases with the increase in temperature and confirms the positive coefficient of conductivity with temperature. This behaviour also suggests that the electrical conduction increases at the higher temperature, which may be again due to the increase in the segmental mobility of the polymer molecules. The conductivity decreases with the increase in frequency as shown in Figure 3(a–e). In the low-temperature region, the AC conductivity depends significantly on the frequency. But, there is negligible change in the AC conductivity with increase in temperature. As dielectric relaxation sets in with increase in temperature, the dependency of the conductivity on frequency gets reduced. Figure 3(a) shows that the conductivity of pure epoxy sample increases abruptly after 140 °C, while the conductivity of pure sisal fibre (5 and 10 mm lengths samples) epoxy composites changes abruptly after 120 °C for all measured frequencies as can be observed in Figure 3(b) and (d). The variation of σac with temperature and frequency of both lengths MWCNT-implanted sisal fibre–epoxy composites is shown in Figure 3(c) and (e). It is seen that in the low-temperature regions, the σac values are almost constant, but there is a sudden increase after 140 °C for both the samples.



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In Figure 4(a, b, c), variation of dielectric constant ε′, dissipation factor (tan δ) and AC conductivity (σac) with log frequency is shown, respectively. It is clear from Figure 4(a) that the dielectric constant of 5 mm length MWCNT-implanted sisal fibre is higher than other prepared samples. The values of ε′ of all the specimens decrease with the increase in frequency. The same trend is observed (Figure 4(b)) in the values of dielectric loss ( dissipation factor) for all the samples. At lower frequency (i.e. 0.5 kHz), the loss values are higher, but at higher frequencies, these loss values decrease. In Figure 4(c), the variation of AC conductivity with log (f) is shown and a reverse trend is observed showing that the σac values of the specimens increased with the increase in frequency. Figures 5(a, b) and 6(a, b) show the SEM micrographs of fractured surface of pure sisal fibre–epoxy composite and MWCNT-implanted sisal fibre–epoxy composite, respectively. Pores of microsize in sisal fibres are clearly visible in these figures. In the pores, MWCNT might be entrapped provided the enhancement in dielectric properties. In the composites with pure sisal fibre, better adhesion between the sisal fibres and the polymer matrices is clearly observed. In the case of pure sisal fibre–epoxy composite (Figure 5), the fibres are completely embedded in the matrix, which indicates good wetting with the polymer. In contrast, the MWCNT-implanted sisal fibre–epoxy composite samples (Figure 6(a, b)) exhibited that the interaction between fibres and matrix is poor due to poor wettability. Fibre is not completely debonded but in poor contact with the matrix could not adhere well to the epoxy matrix; hence, interfacial bonding is poor. Although all the composites were prepared under the same conditions, the ability of the polymer to wet the fibres seems to depend strongly on the polymer morphology.

4. Conclusions This study concludes that the implantation with CNT of sisal fibres in epoxy composites brings about significant enhancement in its dielectric properties. The sample having CNTimplanted sisal fibre of 5 mm length exhibited a higher value of the dielectric constant as compared to the sample with 10 mm fibre length. This is attributed to the increased surface area of the fibre. The dielectric constant increases with the increase in temperature and decreases with increase in frequency from 0.5 to 5 KHz. The peak value of the dielectric constant decreases with increasing frequency. A continuous decrease in dissipation factor tan δ values with increase in frequency for all the samples was observed, while at lower temperatures, the values of tan δ are approximately same. The AC conductivity for 5 mm length sisal–epoxy composite and 10 mm length sisal fibre–epoxy composites is higher than that of pure epoxy at all the frequencies.

Disclosure statement No potential conflict of interest was reported by the authors.

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