Effect of silver coating on electrical properties of sisal ...

Effect of silver coating on electrical properties of sisal fibre-epoxy composites

Manindra Trihotri, Deepak Jain, U. K. Dwivedi, Fozia Haque Khan, M. M. Malik & M. S. Qureshi Polymer Bulletin ISSN 0170-0839 Volume 70 Number 12 Polym. Bull. (2013) 70:3501-3517 DOI 10.1007/s00289-013-1036-7

1 23

Your article is protected by copyright and all rights are held exclusively by SpringerVerlag Berlin Heidelberg. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.

1 23

Author's personal copy Polym. Bull. (2013) 70:3501–3517 DOI 10.1007/s00289-013-1036-7 ORIGINAL PAPER

Effect of silver coating on electrical properties of sisal fibre-epoxy composites Manindra Trihotri • Deepak Jain • U. K. Dwivedi • Fozia Haque Khan • M. M. Malik • M. S. Qureshi

Received: 6 March 2013 / Revised: 6 June 2013 / Accepted: 19 August 2013 / Published online: 30 August 2013 Ó Springer-Verlag Berlin Heidelberg 2013

Abstract In this paper, the effect of silver coating and size of fibre on electrical properties of sisal fibre-reinforced epoxy composites has been reported. For this purpose, epoxy composites reinforced with silver-coated sisal (of 5 and 10 mm length) prepared by hand moulding and samples were characterized for their electrical properties, such as dielectric constant (e0 ), dielectric dissipation factor (tan d) and AC conductivity (rac), at different temperatures and frequencies. It was observed that dielectric constant increases with increase in temperature and decreases with increase in frequency from 500 Hz to 5 kHz. The peak height at the transition temperature decreases with increasing frequency. Interestingly, sample having silver-coated fibre of 5 mm length exhibited higher value of dielectric constant as compared to the sample having 10 mm of fibre length, which is attributed to the increased surface area of coated fibre. This behaviour of the material can be explained in terms of interfacial polarization. At a constant volume of fibres and at a length of 5 mm, the number of interfaces per unit volume element is high and this results in high interfacial polarization. The number of interfaces decreases as the fibre length increases and therefore the value of e0 decreases at 10 mm fibre length. To study the changes in structure of samples, Fourier transform infrared spectrometry and scanning electron microscopy of the samples were carried out.

M. Trihotri (&) F. H. Khan M. M. Malik M. S. Qureshi Department of Physics, Maulana Azad National Institute of Technology, Bhopal 462051, MP, India e-mail: [email protected] D. Jain Department of Research and Development, Permali Wallace Pvt. Ltd., Bhopal 462023, MP, India U. K. Dwivedi Department of Physics, Amity University, Jaipur 302006, Rajasthan, India

123

Author's personal copy 3502

Polym. Bull. (2013) 70:3501–3517

Keywords Electrical properties Interfacial polarization Interface Fibre-epoxy composite

Introduction Study of the properties and applications of fibre-reinforced polymer composite materials is a very fast growing area of research nowadays. With natural fibres, the interest arises due to high performance in electrical properties, mechanical properties, low cost and significant processing advantages of the composite material [1–5]. The reason is, natural fibres are cheaper, renewable, environment friendly, light in weight and possess no health hazards, which makes them smart materials with versatile applications in different areas like aerospace, automobile, electromagnetic shielding etc. In recent years, natural fibre-reinforced polymer composites have attracted more and more research interests. As a result, natural fibres are considered as replacements in place of glass or carbon fibres [6]. Compared to other lignocellulosic fibres sisal is of particular interest because its composites have high impact strength with moderate tensile and flexural properties. Figure 1 shows the chemical structure of sisal fibre. The electrical properties such as dielectric constant (e0 ), dielectric dissipation factor (tan d), AC conductivity (rac) of sisal fibre-reinforced polymer composites have also been studied by several researchers. The electrical properties of the composites have been analysed with special reference to the effect of fibre length, fibre concentration and fibre treatment. Properties of natural fibre-reinforced polymer composites like fibre length, dispersion; fibre loading and fibre to matrix adhesion are, changed by many factors [1, 7–13]. The study of dielectric constant and dielectric loss as a function of temperature and frequency, is one of the most convenient and sensitive methods of studying polymeric structure. The electrical

Fig. 1 Chemical structure of sisal

123

Author's personal copy Polym. Bull. (2013) 70:3501–3517

3503

properties of sisal fibre-reinforced composites showed that the composite has electric anisotropic behaviour [14]. The electrical properties of sisal fibrereinforced, low-density polyethylene composite have been compared with those of carbon black and glass fibre filled low-density polyethylene composites. Paul et al. in their study considered the effect of frequency, fibre content and fibre length on various electrical properties [15]. They have also noted that dielectric constant decreased with increase of fibre length and frequency. The composite with 1 mm fibres and 30 % fibre content had the highest value of dielectric constant at all frequencies. Paul et al. did the investigation on the effect of surface treatment on electrical properties of low-density polyethylene composite reinforced with short sisal fibres. The dielectric strength of composite materials is found to decrease with decrease in hydrophilicity of the composite, when the samples are treated with alkali, steric acid, peroxide, acetylation and permanganate [10]. Li et al. [7] observed in their study that sisal/low-density polyethylene composites containing 5 % carbon black could be used in antistatic applications to dissipate static charge. The studies on dewaxed sisal fibre-reinforced epoxy composite (DSFREC) and raw sisal fibre-reinforced epoxy composite (RSFREC) indicate that there exists a good correlation between dielectric behaviour and mechanical properties of epoxy reinforced by sisal fibre. Beside this, both electrical and mechanical properties of the composites have been correlated with the structural parameters of the reinforced fibre [16]. Frequency and temperature dependence of dielectric constant (e0 ), dielectric loss (tan d), AC conductivity (rac) and complex impedance spectroscopy studies on cured polyester matrix and sisal fibre-reinforced polyester composites (SFRPC) have been investigated in the frequency range from 180 Hz to 1 MHz and temperature range from room temperature to 200 °C. The experimental results showed that with the incorporation of sisal fibre, the values of e0 , tan d and rac are found to increase. It is also found that the values of e0 and tan d for both cured polyester matrix and SFRPC are decreased with increasing frequency, which indicate that the major contribution to the polarization may come from interfacial polarization and orientation polarization [17]. There has been a growing interest in utilizing natural fibres in polymer composite for making low cost construction materials, in recent years. Natural fibres are prospective reinforcing materials and their use, until now, has been more traditional than technical. They have long served many useful purposes but the application of the material technology for the utilization of natural fibres as reinforcement in polymer matrix took place in comparatively recent years [2]. Researchers investigated the electrical properties of sisal fibre-reinforced epoxy composite, but they never studied the effect of the silver-coated sisal fibrereinforced epoxy composite with different lengths and at different parameters. The aim of this work is to analyse the electrical properties of silver-coated sisal epoxy composites at different temperatures and frequencies. In this study, the length of the silver-coated sisal fibre was 5 and 10 mm. The effect of length of the sisal fibre and temperature on the dielectric constant (e0 ), dielectric dissipation factor (tan d) and AC conductivity (rac) has been studied and reported here.

123


Polym. Bull. (2013) 70:3501–3517

Experimental Materials The thermosetting matrix used in this study was unmodified epoxy resin provided by Atul Pvt. Ltd. Valsad India cured at room temperature. Figure 2a and b shows the structure of unmodified epoxy pre-polymer resin and structure of a hardener. The density of the resin, cured at room temperature was 1.15 g/cm3. The sisal fibres used in the present study were collected from Bilaspur, India. Density of the sisal fibre was 1.45 g/cm3. Fibre diameter used in this study was 100–200 mm. Composite preparation Composite is prepared using a resin/hardener ratio of 10:1. Sisal fibres were first coated with silver conducting paint and the coated fibres were dried at 80 °C for 2 h in an air-circulating oven. In the coated fibres, weight fractions in the composites were kept in the ratio of 10:90. The pressure applied was 1 MPa. Table 1 lists the density and types of composites prepared. Preparation of test sample Sample sheets having two different lengths of randomly oriented sisal fibre with epoxy were prepared. Test samples were cut from the sheets in the form of circular

Fig. 2 a Structure of unmodified epoxy pre-polymer resin. b Structure of a hardener Table 1 Types of composites

123

S. no.

Samples

Density (g/cm3)

1

Pure epoxy resin (EP-00)

1.15

2

Epoxy composites filled with silver-coated sisal of length 5 mm (EP-05)

1.22

3

Epoxy composites filled with silver-coated sisal of length 10 mm (EP-10)

1.30


3505

discs of 1 mm thickness and 10 mm diameter. Uniformity of surface was obtained by polishing the sample. Both sides of the sample were coated using air-drying conducting paint such that both the surfaces should not connect electrically with each other. The test samples were then heated at 60 °C for 10 min, to remove the solvent of the silver conducting paste, and then kept in between the electrodes of the sample holder, for various measurements.

Characterization Fourier transform infrared spectroscopy (FTIR) analysis FTIR analysis of the sample was carried out using Bruker ALPHA FT-IR Spectrometer. Scanning electron microscope (SEM) analysis SEM images of the prepared samples were taken by JSM 6390A (JEOL Japan) at different magnifications. The prepared samples were coated with gold in a vacuum coating unit prior to the examination. Images of the samples were taken along the two surfaces, fractured surfaces of the samples and plane polished surfaces of the sample pellets. Electrical measurements The dielectric properties of materials play a key role on the practical performances of integrated circuits. A basic understanding of dielectric properties is therefore needed for engineers and scientists working in semiconductor industries. One important property of dielectric materials is the dielectric constant (permittivity). Dielectric constant (e0 ) is a measure of the ability of a material to be polarized by an electric field, and is closely related to the capacitance (C) i.e. the ability to store electric charge. Capacitance (C) and tan d 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 (e0 ) of the composite has been calculated using the following relation e0 ¼

C Co

ð1Þ

where C and CO are the capacitance with and without dielectric, respectively; CO in pF is given by Co ¼

ð0:08854ÞA pF d

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

123


Polym. Bull. (2013) 70:3501–3517

tan d ¼

e00 e0

ð2Þ

where e00 is the dielectric loss. In dielectric analysis, the sample is placed between two parallel electrodes. By applying a sinusoidal voltage, an alternating electric field is created, due to which polarization is produced in the sample, which oscillates at the same frequency as the electric field, but has a phase angle shift. The phase angle shift is measured by comparing the applied voltage, with the measured current, which is separated into capacitive and conductive components [18]. Measurements of capacitance and conductance are used to calculate, (1) real part of permittivity (apparent permittivity) e0 , which is proportional to the capacitance and measures the alignment of dipoles, (2) dielectric dissipation factor, tan d = e00 /e0 and (3) AC conductivity (rac) calculated from the relation rac ¼ e0 xe0 tan d

ð3Þ

where e0 is the permittivity of free space, tan d the dielectric dissipation factor and x the angular frequency of the applied electric field. At lower and intermediate frequencies e0 and tan d values in sisal fibre-reinforced composites are due to the contributions of orientation, space charge and interfacial polarization. Contribution of orientation polarization decreases at high frequency because molecules do not have time for orientation which is indicated by the decrease in e0 and tan d of composites with frequency.

Results and discussions Generally, the dielectric constant of a composite material depends on polarization of molecules and the dielectric constant increases with increase in polarizability. The different types of polarizations possible in a composite material are (a) Electronic polarization (b) Atomic polarization and (c) Orientation polarization due to the orientation of dipoles parallel to the applied field [18]. The prepared samples contain pure epoxy with sisal fibres embedded in it. Epoxy is also known as polyepoxide. Epoxy is a copolymer; that is, it is formed from two different chemicals. These are referred to as the ‘‘resin’’ or ‘‘compound’’ and the ‘‘hardener’’ or ‘‘activator’’. The resin consists of monomers or short chain polymers with an epoxide group at either end. The hardener consists of polyamine monomers. When these compounds are mixed, the amine groups react with the epoxide groups to form a covalent bond. Each NH group can react with an epoxide group from distinct pre-polymer molecules, so that the resulting polymer is heavily crosslinked, and is thus rigid and strong. Sisal fibre is obtained from the leaves of the plant Agave Sislana. The chemical constituents of the sisal fibre are cellulose 66–72 %, lignin 10–14 %, hemicellulose 12 % and moisture 10 %. The FTIR spectra of the three samples are shown in Fig. 3a pure epoxy (EP-00), Fig. 3b 5 mm sisal epoxy composite (EP-05) and Fig. 3c 10 mm sisal epoxy composite (EP-10). It shows the peaks at 3,628, 3,224 cm-1 in EP-00,

123


3507

Fig. 3 a FTIR spectra of pure epoxy (EP-00). b FTIR spectra of 5 mm length silver-coated sisal epoxy composite (EP-05). c FTIR spectra of 10 mm length silver-coated sisal epoxy composite (EP-10)

123


Polym. Bull. (2013) 70:3501–3517

3,683, 3,261 cm-1 in EP-05 and 3,639, 3,210 cm-1 in EP-10 correspond to characteristic OH stretching vibration of the water, and alcohol group in epoxy which form the polymer base in case of all the three samples. Peaks at 3,318 cm-1 in EP-00, 3,330 cm-1 in EP-05 and 3,306 cm-1 in EP-10 correspond to the NH stretching of primary amine. The peaks at 1,699 cm-1 in EP-00, 1,648 cm-1 in EP-05 and at 1,643 cm-1 in EP-10 can be attributed to stretching of carbonyl group of lignin and the peaks at 1,457 cm-1 in EP-00, 1,459 cm-1 in EP-05 and at 1,437 cm-1 in EP-10 corresponds to aromatic ring skeletal vibrations. There are peaks at 1,026 cm-1 in EP00, 1,024 cm-1 in EP-05 and 996 cm-1 in EP-10, which are from the stretching of methyl groups and vibrations of the benzene structure. Stretching of C–O–C of oxirane group is seen at peak 822 cm-1 in EP-00, 822 cm-1 in EP-05 and 818 cm-1 in EP-10. Stretching bands in the region of 1,024–1,232 cm-1, in all the three samples, belong to C–O–C functional group. The bands observed around 2,366 cm-1 in EP-10, 2,365 cm-1 in EP-05 and 2,340 cm-1 in EP-00 are might be due to the presence of double CO2 band. The peaks at 1,510, 1,511 and 1,541 cm-1 in all the three samples EP-00, EP-05 and EP10 ,respectively, shows the N–H deformation of primary amine and denotes the presence of primary amine due to hardener used in pure epoxy. All these findings in the FTIR spectra lead to the conclusion of the use of pure epoxy in the samples.

Fig. 4 a, b Fractured surface of EP-00

123


3509

A Peak at 2,860 and 2,860 cm-1 that is present in the EP-05 and EP-10 respectively corresponds to CH stretching in cellulose and hemicelluloses of sisal fibres. Peaks observed in the frequency range of 540–657 cm-1 correspond to C–C bond due to aromatic rings in sisal fibre, which are again not present in the case of sample EP-00 pure epoxy. This confirms the presence of sisal fibres in case of EP-05 and EP-10 and absence of these peaks confirms absence of sisal fibres in EP-00. Figures 4, 5 and 6 show the SEM micrographs of pure epoxy and silver-coated sisal epoxy composites. Fractured surface of EP-00 sample can be seen in Fig. 4a, b. SEM micrographs shown in Figs. 5a–d and 6a–c for Sisal epoxy sample exhibit the gap between sisal fibre and epoxy matrix interface due to silver coating which shows the hydrophobic nature of coated sisal fibre surface. Fibre is not completely debonded but is in poor contact with the matrix. Silver-coated sisal fibre surface could not adhere well with epoxy matrix, hence interfacial bonding is poor. The conductivity of fibre-reinforced composites depends on many factors such as the moisture content, crystalline and amorphous component present, chemical composition, cellular structure etc. Fibres having elongated shapes affect the electrical conductivity due to the contact surface area. The moisture content present in the fibre results in the increase of the conductivity of composite. The hydrophilic property of cellulose fibre is the main cause for greater conductivity of the composite. An increase in the conductivity of the resin is due to the hydroxyl groups in the hydrophilic fibre, which can absorb moisture. The dielectric constant of polymeric materials depends on the contribution of interfacial, dipole, electronic and atomic polarizations. The interfacial polarization can explain the behaviour at low frequencies. This type of polarization is present due to the heterogeneity present

Fig. 5 a–d SEM micrographs of silver-coated sisal epoxy composites (EP-05)

123


Polym. Bull. (2013) 70:3501–3517

Fig. 6 a–c SEM micrographs of silver-coated sisal epoxy composites (EP-10)

as impurity in the composite material. Interfacial relaxation occurs when charge carriers are trapped at the interfaces of heterogeneous systems. Interfacial polarization decreases with increasing frequency and it influences the low frequency dielectric properties. The dielectric constant of the material directly depends upon the polarizability. The greater the polarizability of the molecule, the higher the dielectric constant. Therefore, the polarizability decreases with increase in volume of fibres, i.e. due to the decreased number of polar groups [2]. Figure 7a–c shows the variation of dielectric constant (e0 ) with temperature (T) for pure epoxy (EP-00), epoxy composite filled with 5 mm length silver-coated sisal fibre (EP-05) and epoxy composite filled with 10 mm length silver-coated sisal fibre (EP-10) measured at 0.5, 1, 2, 4 and 5 kHz, respectively. Figure 7a shows that dielectric constant increases with increase of temperature from 35 to 185 °C and it decreases with increase in frequency from 0.5 to 5 kHz. 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 dielectric constant of epoxy resin begin to drop, when approaching at the higher frequencies. At lower temperatures, e0 values at different frequencies have merged. Figure 7b shows that dielectric constant increases with increase in temperature and decreases with increase of frequency from 0.5 to 5 kHz. In this case, the dielectric constant (e0 ) is greater than that of pure epoxy. This increase in e0 is due to the incorporation of silver conducting coated sisal fibre in the epoxy matrix. It is also observed that the e0

123


3511

Fig. 7 a–c Shows the variation of dielectric constant (e0 ) with temperature (T) for pure epoxy (EP-00), epoxy composite filled with 5 mm length silver-coated sisal fibre (EP-05) and epoxy composite filled with 10 mm length silver-coated sisal fibre (EP-10) measured at 0.5, 1, 2, 4 and 5 kHz respectively

123


Polym. Bull. (2013) 70:3501–3517

increases initially with temperature up to 120 °C after that it decreases up to 150 °C and again increases up to 185 °C. Fig. 7c shows that the dielectric constant (e0 ) increases initially with temperature up to 115 °C and then decreases with temperature until it increased to 140 °C. Dielectric constant (e0 ) decreases with increase of frequency from 0.5 to 5 kHz. This initial increase of e0 is due to the increased mobility of water dipoles. When the water content reduced, the value of e0 decreased. The dielectric constant of 5 mm length of sisal epoxy composite (EP-05) was observed higher than that of 10 mm length sisal epoxy composite (EP-10). This is because of the higher concentration of silver particles in form of coating presented in the 5 mm length composite specimen than the 10 mm length sisal epoxy composite. It is well understood that, 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 can be explained in terms of interfacial polarization. At a constant volume (%) of fibres and at a length of 5 mm, the number of interfaces per unit volume element is high and this results in high interfacial polarization. The number of interface decreases as the fibre length increases and therefore the value of e0 decreases at 10 mm fibre length. This observation is similar to the work of Prasantha et al. [19] in which they have shown that the surface area of the smaller size sisal fibre will be more compared to larger size sisal fibre in case of constant volume (%) of sisal fibre present in composite. We have observed the similar trend in dissipation factor (tan d) and AC conductivity (rac). Figure 8a–c shows the plots of tan d with temperature for pure epoxy, 5 and 10 mm length silver-coated sisal epoxy composite measured at 0.5, 1, 2, 4 and 5 kHz, respectively. Dissipation factor (tan d) increases with increase in temperature. Dissipation factor (tan d) is the ratio of the electrical power dissipated in a material to the total power circulating in the circuit. In polymers or their composites, tan d is a function of the electrical conductivity (which depends on the charge carrier mobility) and the applied excitation frequency. There are two different interacting processes, which might influence tan d behaviour in composites. The first one is the number of charge carriers available for electrical conduction and the other is the number of interfaces and polymer chain entanglements in the bulk [20]. The plots show that in all the three samples, there is a continuous decrease in tan d values with increasing frequency for all filler lengths and at lower temperatures the values of tan d is approximately same, there is an increase in the values of tan d with increase in temperature. The most likely reason for this observation is a decrease in electrical conductivity in the epoxy composites with increasing frequency, which may be caused by the inability of the charge carriers to traverse the thickness of the material at the higher frequencies. At high frequencies, the motion of charge carriers contributing to the conductivity primarily occurs along polymer chains [21]. A barrier to the charge transport in polymers (causing reduction in electrical conductivity) can occur due to defects, inter-chain charge transport and transport through interfaces. The influence of temperature on conductivity has been explained by considering the mobility of charge carriers responsible for hopping. As temperature increases, the mobility of hopping ions also increases thereby

123


3513

Fig. 8 a–c Shows the plots of tan d with temperature for pure epoxy (EP-00), 5 mm length silver-coated sisal epoxy composite (EP-05) and 10 mm length silver-coated sisal epoxy composite (EP-10) measured at 0.5, 1, 2, 4 and 5 kHz respectively

increasing conductivity. The electrons that are involved in hopping are responsible for electronic polarization in these composites. The conductivity increases up to a temperature and further increase of temperature reduces the conductivity. This decrease in conductivity at higher temperature is based on the thermal expansion of polymer. At higher temperatures, the polymer density reduced by thermal expansion, reduces the conductivity [22]. Probably, in composites, the presence of a large number of interfaces and polymer chain entanglements inhibit the motion of charges in the system, which in turn causes a reduction in the electrical conductivity (hence a lower tan d value). Figure 9a–c show the plots of AC conductivity (rac) with Temperature (T) for pure epoxy, 5 and 10 mm length sisal epoxy composite measured at 0.5, 1, 2, 4 and 5 kHz, respectively. These plots show that the AC conductivity increased with

123


Polym. Bull. (2013) 70:3501–3517

Fig. 9 a–c Shows the plots of AC conductivity (rac) with Temperature (T) for pure epoxy (EP-00), 5 mm length silver-coated sisal epoxy composite (EP-05) and 10 mm length silver-coated sisal epoxy composite (EP-10) measured at 0.5, 1, 2, 4 and 5 kHz respectively

increasing temperature. 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 frequencies. This is due to hydrophilicity of the lignocellulosic sisal fibres present in the composites. It was observed that AC conductivity (rac) of all the three samples increases with the increase in temperature and this 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. Figure 10a–c show the variation of e0 , tan d and (rac) with log f (frequency) for pure epoxy, 5 and 10 mm length sisal epoxy composite measured at 35 °C. It was

123


3515

Fig. 10 a–c Shows the variation of e0 , tan d and (rac) with log f (frequency) for pure epoxy (EP-00), 5 mm length silver-coated sisal epoxy composite (EP-05)and 10 mm length silver-coated sisal epoxy composite (EP-10) measured at 35 °C

123


Polym. Bull. (2013) 70:3501–3517

observed that the e0 and tan d decreased with increasing frequency and a.c. conductivity increased with increasing frequency. The change of e0 at lower frequency region is higher than that of at high frequency. The atomic and electronic polarizations are instantaneous polarization components, the effect of which is seen only at high frequencies. The dipole or orientation polarization occurs due to the presence of polar groups in the material. The interfacial polarization arises due to heterogeneity, which is highest at lower frequency. Hence, the higher values of e0 at low frequency can be explained in terms of interfacial polarization. The behaviour of tan d with frequency is very much similar to e0 , i.e. with increase in frequency tan d value also decreases. The value of tan d in all the three samples at low frequency region becomes high due to free motion of dipoles within the material. This value of tan d is very high for the EP 10 sample. The behaviour of frequency dependence of a.c. conductivity (rac) of sisal epoxy composite (EP 05, EP 10) is similar to pure epoxy sample (EP 00) i.e. with increase in temperature, rac increases and frequency independent plateau is observed at lower frequency. It is clear from the Fig. 10c that the rac of EP 05 and EP 10 is higher than the EP 00, and that may be due to the incorporation of more polar molecules because of hydroxyl groups present in the fibre. Again, the addition of fibres enhances the flow of current through the amorphous region due to their ability to absorb moisture. Paul et al. [10] reported that the dielectric constant of sisal fibre LDPE composite increased with increase in fibre loading. The increase is higher at low and medium frequencies and lower at higher frequencies, which has been explained by considering the interfacial polarization and orientation polarization.

Conclusions The study concludes that the incorporation of conducting silver coating in sisal epoxy composite significantly enhances the dielectric properties. Sample having silver-coated fibre of 5 mm length exhibited higher value of dielectric constant as compared to the sample having 10 mm of fibre length, which is attributed to the increased surface area of coated fibre. The dielectric constant increases with increase of temperature and decreases with increase of frequency from 0.5 to 5 kHz. The peak height at the transition temperature decreases with increasing frequency. A continuous decrease in tan d values with increasing frequency for all fibre lengths and at lower temperatures the values of tan d is approximately same. The AC conductivity for 5 mm length sisal epoxy composite and 10 mm length sisal fibreepoxy composites is higher than that of pure epoxy at all frequencies. FTIR results confirm the structure present in the samples. Silver-coated sisal fibre surface could not adhere with epoxy matrix, which is observed in SEM micrographs. Acknowledgments The author would like to acknowledge the support of the Director (Dr. Appu Kuttan K.K.), Maulana Azad National Institute of Technology Bhopal-462051(M.P.) India for providing basic facilities in the institute. The support of the Dr. Rajnish Kurchania (Head) Department of Physics, Maulana Azad National Institute of Technology Bhopal-462051(M.P.) India is kindly acknowledged.

123


3517

References 1. Sreekumar PA, Saiter JM, Joseph K, Unnikrishnan G, Thomas S (2012) Electrical properties of short sisal fibre reinforced polyester composites fabricated by resin transfer molding. Compos A 43:507–511 2. Pathania D, Singh D (2009) A review on electrical properties of fibre reinforced polymer composites. Int J Theor Appl Sci 1(2):34–37 3. Ronga MZ, Zhang MQ, Liu Y, Yang GC, Zeng HM (2001) The effect of fibre treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites. Compos Sci Technol 61:1437–1447 4. Devi LU, Bhagawan SS, Thomas S (1997) Mechanical properties of pineapple leaf fibre-reinforced polyester composites. J Appl Polym Sci 64:1739 5. Bledzki AK, Gassan J (1999) Composites reinforced with cellulose based fibre. Prog Polym Sci 24:221 6. Joseph S, Sreekala MS, Oommen Z, Koshy P, Thomas S (2002) A comparison of the mechanical properties of phenol formaldehyde composites reinforced with banana fibres and glass fibres. Compos Sci Technol 62:1857–1868 7. Li Y, Yiu WM, Lin YE (2000) Sisal fibre and its composites: a review of recent developments. Compos Sci Technol 60:2037–2055 8. Reid JD, Lawrence WH, Buck RP (1986) Dielectric properties of an epoxy resin and its composite I. Moisture effects on dipole relaxation. J Appl Polym Sci 30:1771–1784 9. Yang GC, Zery HM, Li JJ, Jian NB, Zhang WB (1996) Relation of modification of tensile properties of sisal fibres. Acta Sci Nat Univ Synyatseni 35:55 10. Paul A, Joseph K, Thomas S (1997) Effect of surface treatments on the electrical properties of lowdensity polyethylene composites reinforced with short sisal fibres. Compos Sci Technol 51:67–79 11. Abdelmouleh M, Boufi S, Belgacem MN, Dufresne A, Gandini A (2007) Short natural-fibre reinforced polyethylene and natural rubber composites: effect of silane coupling agents and fibres loading. Compos Sci Technol 67:1627–1639 12. Chand N, Dwivedi UK (2007) Influence of fiber orientation on high stress wear behavior of sisal fiber-reinforced epoxy composites. Polym Compos. doi:10.1002/pc.20286 13. Dwivedi UK, Chand Navin (2009) Influence of MA-g-PP on abrasive wear behaviour of chopped sisal fibre reinforced polypropylene composites. J Mater Process Technol 209:5371–5375 14. Chand N, Jain D (2005) Effect of sisal fibre orientation on electrical properties of sisal fibre reinforced epoxy composites. Compos A 36:594–602 15. Paul A, Thomas S (1997) Electrical properties of natural-fibre-reinforced low density polyethylene composites: a comparison with carbon black and glass-fibre filled low density polyethylene composites. J Appl Polym Sci 63:247–266 16. Patra A, Bisoyi DK (2011) Investigation of the electrical and mechanical properties of short sisal fibre-reinforced epoxy composite in correlation with structural parameters of the reinforced fibre. J Mater Sci 46:7206–7213 17. Patra A, Bisoyi DK (2010) Dielectric and impedance spectroscopy studies on sisal fibre-reinforced polyester composite. J Mater Sci 45:5742–5748 18. Ben Amora I, Rekik H, Kaddami H, Raihane M, Arous M, Kallel A (2009) Studies of dielectric relaxation in natural fibre–polymer composites. J Electrostat 67:717–722 19. Prasantha Kumar R (1999) Short natural fibre reinforced elastomer composites from sisal fibre and styrene-butadiene rubber, Ph. D Thesis, School of Chemical Sciences Mahatma Gandhi University, Kottayam, Kerala, India 20. Singha S, Thomas MJ (2008) Permittivity and tan delta characteristics of epoxy nanocomposites in the frequency range of 1 MHz–1 GHz. IEEE Trans Dielectr Electr Insul 15(1):2–11 21. Prins P, Grozema FC, Schins JM, Siebbeles LDA (2006) Frequency dependent mobility of charge carriers along polymer chains with finite length. Phys Status Solidi (b) 243:382–386 22. Chand N, Jain D (2004) Evaluation of a.c. conductivity behaviour of graphite filled polysulphide modified epoxy composites. Bull Mater Sci 27(3):227–233

123