- Email: [email protected]
Investigation of the dielectric properties of natural fibre and conductive filler reinforced polymer composites
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 22 (2020) 162–171
www.materialstoday.com/proceedings
2018 2nd International Conference on Nanomaterials and Biomaterials, ICNB 2018, 10–12 December 2018, Barcelona, Spain
Investigation of the dielectric properties of natural fibre and conductive filler reinforced polymer composites Elammaran Jayamania*,Govind Anil Naira, KokHeng Soona a
a
[email protected], [email protected], [email protected]
Faculty of Engineering, Computing and Science, Swinburne University of Technology Sarawak Campus, Jalan Simpang Tiga, Kuching,93350 Sarawak, Malaysia
Abstract Bio-composites are gaining popularity as materials used in the production of semi-conductors. Dielectric materials are important energy storage devices in an electric circuit due to their ability to get polarized upon the application of an electric field. An investigation of dielectric properties was conducted by preparing specimens consisting of various amounts of coconut fibre, copper powder and HDPE or PLA as the polymer matrix. The dielectric testing revealed the increase of the dielectric constant after the addition of copper to the neat matrix of either polymer. The dielectric loss and the dissipation factor exhibited an increase with the increased loading of fillers. The dielectric constant decreased, along with the dissipation factor and the dielectric loss after the addition of coconut fibre. The results obtained align with existing literature. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials. Keywords: Dielectric; Natural Fiber;Polymer;Composites;PLA;HDPE;Coir;Copper
* Corresponding author. Tel.: +60-0060165774867. E-mail address: [email protected] 1876-6102 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials.
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Introduction Dielectric properties of materials are of great interest to researchers in the present as it is an essential factor in the engineering of energy storage devices and semiconductors. Dielectric materials are often used in circuit boards as capacitors, in hybrid vehicles in the automotive industry and several other small and large scale electronic devices. Traditionally, polymer composites have been utilized along with ceramic powders for creating materials with desirable dielectric properties. Polymers exhibit excellent mechanical properties in addition to thermal and chemical stability. These are properties of interest in applications where the polymers are subject to temperatures at extremes and exposed to various chemicals. Ceramics however are brittle and hence large quantities in the polymer matrix can compromise the structural integrity of the composite. Other limiting factors of ceramic polymer composites include poor adhesion with the polymer matrix, high processing temperatures and toxicity. The use of metal powders and natural fibres as fillers in the polymer matrix enables the creation of dielectric polymer composites. The dielectric properties are present in these composites due to the natural dielectric traits of natural fibres and the polymer matrix. Metal powders and natural fibres can be produced at low costs and are environment friendly and biodegradable. The development of polymer composites with natural fibres and conductive fillers are a necessity as there is growing demand for efficient and sustainable dielectric materials in various industries. As devices reduce in size, the components should be able to store large amounts of energy while taking up the least space in the circuit. Procedure Natural fibre treatment: Coir fibres of long lengths were obtained to be used as the natural fibre for this experiment. The fibres were coarse with varying lengths. These fibres were washed under distilled water until the drain contained no impurities. The fibres were then spread out in a tray and heat treated in an open convention oven for 6 hours at 160°C. This facilitated the removal of wax and moisture, which enabled better resistivity to heat . Removal of the OH group in the fibres enabled better adhesion between the hydrophobic polymer matrixes. The fibres were then cut by hand to lengths not exceeding 5 mm. 2.1. Mold preparation A steel frame with circular holes measuring 50mm in diameter and 5mm in thickness was used to prepare the composites. A steel flat plate wrapped in aluminum foil was kept beneath frame. The aluminum foil ensured uniform heating of the bottom plate. A thin layer of metal polish was applied to the naked metal surfaces to prevent the molten material from sticking to the surface. A layer of polymer according to the desired volume fraction was loaded into each cavity, followed by the loadings of natural fibre and copper powder. The remaining polymer was loaded into the cavities. A second frame of the same dimensions were placed on top and the cavities were topped with extra loadings of polymer to account for any leakages. Another flat plate wrapped in aluminum foil was plated on top. 2.2 Sample compositions Samples prepared consisted of the polymer matrix (HDPE or PLA), natural fibre (coconut fibre) and conductive filler (copper powder). Separate samples containing either of the polymers with copper powder was prepared initially, with a minimum loading of 2%vol of copper (3.5g) were prepared, with increments of 2% till a maximum loading of 8%vol (7.0g). Samples prepared with natural fibres contained loadings of 1%vol ,3%vol ,5%vol and 7%vol. 2.3 Compression molding A Molding Press was used to fabricate the composites. GT-7014-H30C, from GOTECH, at a pressure of 20 Tons was used to prepare the samples. Specimens of PLA were heated at 160℃ for 20 minutes while specimens consisting of HDPE were heated at 190℃ for 30 minutes. Specimens of HDPE and PLA were cooled naturally by
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turning off the machine and leaving the samples between the plates for 16 hours. This technique was found to be more effective in cooling the samples as compared to the water quenching method as it prevented water from entering the sample through the material defects. Presence of water inside the samples could lead to increased dielectric constant readings as well as the cracks when the water expands. 2.4 Dielectric measurements The samples obtained after cooling, were then dusted to remove impurities and leftover copper powder from the surface. The presence of the conductive powder can lead to erroneous readings when the dielectric parameters are measured. E4980A precision LCR meter and 16451B dielectric test fixture provided by Keysight Technologies were used to measure the capacitance and the dissipation factor of the composites. The capacitance and dissipation factor were measured at a frequency range from 1 kHz to 2 MHz according to the ASTM D-150-18 standards. To get accurate readings and account for surface impurities, the readings were take once, removed from the fixture, flipped and a second reading was taken. When commutating the results, a mean of the two values were used. 3. Equations 3.1 Determining dielectric constant and dielectric loss The dielectric apparatus determined the capacitance (Cp) and the dissipation factor (D) for every specimen. Using the measured capacitance value of the material under test (MUT), the dielectric constant was calculated using the equation below [1]:
𝜀 =
(1)
Where Cp is the measured capacitance of the MUT and C0 is calculated from the formula: 𝐶 =𝜀
(2)
Where A is the area of the circular electrode, t is the thickness of the specimen and 𝜀 is the permittivity of vacuum measured at 8.854x10-12 F/m. Combining equations 1 and 2, the dielectric constant of the MUT is commutated by the formula: ε =
(3)
The dielectric loss of the MUT is calculated using the formula 𝜀 = 𝜀 𝑡𝑎𝑛𝛿
(4)
Where tan δ is the measured dissipation factor.
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4. Results
(a) (b) Fig.1. (a) Dielectric constant; (b) Dissipation factor of conductive filler reinforced HDPE composites
4.1 Neat polymer 4.1.1 Dielectric constant and Dissipation factor The dielectric constants of HDPE and PLA were measured at various frequencies from 1 kHz to 2Mhz. HDPE had maximum dielectric constant (ɛ’) of 1.43 at a 1 kHz while PLA had a maximum of 1.82 at 10 kHz. Both the polymers did not exhibit any serious deviation in the values of the dielectric constant as the frequency was increased from 1 kHz to 2 MHz. The dissipation factor for HDPE drops from 0.0295 to 0.0165 as the frequency is increased from 1 kHz to 2 MHz. From the range of 10 kHz to 1 MHz, the dissipation factor remains steady before experiencing slight a rise from 1 MHz to 2MHz. The fall in the dissipation factor is a result of the ionic conduction, especially at lower frequencies. As the charges exhibit increased mobility due to increased frequency applied, the conductivity increases. The increase in conductivity leads to an increase in the dissipation factor [2]. PLA exhibits an increase in the dissipation factor from 0.0054 at 1 kHz to 0.012 at 2 MHz as the frequency is increased. 4.1.2 Dielectric loss HDPE experiences a sudden drop in the dielectric loss as the frequency is increased from 1 kHz to 10 kHz. From 10 kHz, there is no increase in the dielectric loss even as the applied frequency is incremented. This can be explained by the presence of frequency dependent α-relaxation that is observed above the 100 Hz region [2]. As the relaxation occurs, there is an increase in the permittivity. The increased permittivity leads to a decrease in the dielectric loss. PLA on the other hand exhibits contradicting behavior to HDPE in the region from 1 kHz to 10 Hz as the dielectric loss increases. This result segment can be discarded as the result is an anomaly. It could be due to presence of impurities on the electrode fixture or microscopic surface defects. An increase in dielectric loss is observed in frequency region of 1kHz to 10 kHz before plateauing at the frequency range of 10 kHz to 2 MHz, which is in line with previous literature [3]. From the testing of the neat samples, it is observed that the polymers exhibit highest dielectric constants at the lowest frequency spectrum of 1 kHz. PLA has the highest dielectric constant of 1.82 at 10 kHz. At this frequency range, the dielectric loss and dissipation factor is lowest for PLA when compared to HDPE.
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4.2 Polymer with conductive filler 4.2.1 HDPE For HDPE composites reinforced with conductive filler, the highest dielectric constant (𝜀 =2.87) is observed at 8% volume loading of copper at 1 kHz (Fig. 1a). The composites exhibit an increase in the dielectric constant as the copper content is increased. Similar results were reported when investigating the relationship of the dielectric properties in response to the addition of the fillers [4-6]. The high dielectric constant is attributed to the electron tunnel effect. It is also observed that the high dielectric constant is in the low frequency of 1 kHz. Compared to the high frequency at 1 MHz and 2 MHz, the dielectric constants of all filler loadings are higher in the 1 kHz and 10 kHz spectrum. This is because at low frequency, the presence of interfacial and space charge polarization is more prominent [7]. At 1 MHz and 2 MHz, there is a decrease in the dielectric constant as the filler loading is at maximum (8 % volume). This behavior is explained by the vibration of the conductive fillers at high frequency. As they vibrate and accumulate in the composite, the aspect ratio increases. As the aspect ratio increases, the percolation threshold decreases considerably for a two-phase composite. As suggested by [8] , the critical volume is about 0.09. This could vary depending on the size and shape of the conductive filler. Hence, the decrease in the dielectric constant can be due to the transition of the composite from a dielectric towards a conductor at high frequency and maximum loadings. Fig. 1b shows the dissipation factor or loss tangent (tan δ) of HDPE composites with conductive fillers. The dissipation factor increases as the concentration of copper is increased. This is explained by the numerous conductive networks that are formed when the copper is dispersed into the composite. More the volume of copper, more conductive networks and hence greater dissipation. The dissipation values are greater in the frequency range of 1 kHz and 1 MHz and lowest at the highest frequency of 2 MHz. At this range, it is observed to be consistent with no significant change. This high dissipation factor for low frequencies (1 kHz, 10 kHz and 1 MHz) is due to the alignment of the fillers. At low frequencies, the alignment of fillers can lead to a percolating network that is responsible for the high dissipation factor values observed. 4.2.2 PLA The dielectric constant of PLA composites exhibit similar trends to the HDPE samples (Fig.2a). However, the dielectric constant obtained from the PLA samples is lower. The maximum dielectric constant (ɛ‘=2.64) obtained was at the frequency of 10 kHz for the sample with 8% loading of copper. As the PLA is in powdered form, better dispersion was observed during the fabrication of the samples. As the frequency increases, the dielectric constant shows a decrease with an increase in filler content. The same phenomenon observed in the HDPE samples is responsible for the dielectric behavior of PLA composites as well. Fig.2b shows the results from the dissipation factor measurements of PLA composites with copper fillers at 1 kHz, 10 kHz, 1 MHz and 2 MHz. The dissipation factor has displayed dual behaviour for PLA composites. At the frequency of 1 kHz and 10 kHz, the dissipation factor increases with an increase in the loading of copper. This is explained by the presence of ionic and electron polarization at low frequencies in the polymer matrix. At high frequency, the decrease in dissipation factor was observed for loadings till 4% at 1 MHz and 6% at 2 MHz. As the loading of the conductive filler reaches the maximum value of 8%, the dissipation factor rebounds to the initial levels. The up and down trend of the dissipation factor can be due the presence of surface defects, resulting in excess dissipation. Other errors such as the air gap capacitance present between the electrodes of the dielectric test fixture can lead to the odd trend as well [9]. The dielectric loss increases from 1 kHz to 10 kHz and remains steady from 1 MHz to 2 MHz for both the polymers. It was observed that the samples with HDPE exhibited more consistent behaviour with literature than those composites prepared with PLA. A study on conductivity was not conducted due to equipment limitations. Analysing the conductivity values can offer a better explanation of the odd behaviour of PLA composites. A study on the dielectric relaxation could offer a better explanation as the polarization at high frequencies show frequency dependent behaviour. This has been reported in conductive filler reinforced polymer composites by [6]. After the addition of conductive fillers, the dielectric constant has increased for the examined polymer composites when compared with the neat polymer specimens. A high dielectric constant of 2.87 was observed for the HDPE composite with 8% copper at 1 kHz. PLA with 8% copper showed the highest dielectric of 2.64 at 1 kHz.
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(a)
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(b)
Fig.2 (a) Dielectric constant; (b) Dissipation factor of conductive filler reinforced PLA composites.
4.3 Polymer matrix with conductive filler and natural fibre 4.3.1 HDPE In comparison with the previous dielectric study on conductive filler reinforced polymer composites (Fig.1 and Fig.2), the dielectric constant displayed a decreasing trend in composites upon the inclusion of coconut fibres (Fig.3a). The filler size, dispersion and orientation play a role for the low values obtained. The increased loading of fillers can lead to the presence of increased relaxations in the composite. This leads to increased losses as well as the presence of other relaxations observed in similar experiments including Maxwell-Wagner-Sillars (MWS) effect, ionic conduction and interfacial polarization. Dielectric relaxation is phenomenon where the dielectric constant is delayed in composites with two or more phases [10]. A relaxation due to the direct current (DC) conductivity was also observed by [2] at low temperature and low frequency (f). Low temperatures were characterized as lower than 70 degrees and low frequency spectrum defined as f <10 MHz. The experimental conditions in this research recorded a temperature of about 25°C in the test environment and the maximum frequency did not exceed 2 MHz. The dielectric constant is highest at 1 kHz and is lowest at 2 kHz. The decrease in dielectric constant with increasing frequency has been observed in several studies and is explained by the presence of interfacial and oriental polarization which are frequency dependent. Theoretically the dielectric constant is supposed to increase as the fibre loading is increased [11]. However, it follows the opposite trend in this research. This is due to the increase in copper loadings. The copper loadings are increased with the increase in fibre loadings. Hence, the expected increase in dielectric constant is not as evident as the losses increases. It has also been observed in experiments conducted by various authors, the increase in losses as the total filler volume increases. The presence of water molecules in the coir fibres can contribute to the relaxation associated with the polarization of the cellulose fibres. This could also explain the decreasing trend of the dielectric constant as the fibre loading is increased. As the specimens were prepared using manual techniques in environments that are not controlled, the presence of impurities can lead to the MWS effect. Existing literature attributed ionic conduction due to the presence of conductive fillers, especially in the low frequency region [2, 3, 12, 13]. HDPE being a thermoplastic polymer with thermal conductivity might have pellets that were not completely melted, as was observed from the prepared specimens. These can lead to the charges being trapped on the interfaces between the polymer and the fillers, thereby creating dielectric hotspots.
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Fig.3 (a) Dielectric constant of natural fibre and conductive filler reinforced HDPE composites.
The improper dispersion of the fillers could have result in the accumulation of these charges at specific spots, thereby not reflecting on the increased dielectric constant of the specimen. These also impact the dissipation factor of composites. As seen in Fig.3b, the dissipation factor increases as the loadings of the fillers is increased.
Fig.3.(b) Dissipation factor of conductive filler and natural fibre reinforced polymer composites.
The most noticeable change is seen for composites tested at 10 kHz and 1 MHz. It is also observed that the dissipation factor is stable at the frequency of 2 MHz, where there is no noticeable changes even as the loadings increase. The increase of dissipation factor for HDPE composites at 10 kHz and 1 MHz can be due to the increase in
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ionic conduction at low frequencies as well as the presence of dielectric relaxation. However, at the frequency of 2 MHz, the stability is explained by the increase of molecular dipoles. As the dielectric relaxation is frequency dependent, it is assumed that the relaxations for three phase polymer composites such as those investigated in this research is observed at frequencies greater than 2 MHz. The increase in relaxation will lead to lower dissipation values [2]. 4.3.2 PLA
Fig.4 (a) Dielectric constant of conductive filler and natural fibre reinforced PLA composites.
The dielectric constant of PLA shows an increase with the increased loadings of copper and natural fibre (Fig.4a). PLA is an organic polymer that is usually obtained from the processing of green plants or fibres. Due to the water content in these fibres, the dielectric constants are usually higher for PLA than other synthetic polymers. However, depending on the quality of processing and the raw materials used to produce PLA, the dielectric properties can vary. As measured at the beginning of the experiments, neat PLA displayed a dielectric constant if 1.82, which was higher than neat HDPE. The dielectric constant increases as the fibre loading is increased. This increases the number of polar bodies present in the composite. As the loading of copper is increases simultaneously, the conductive path increases within the composite. This results in the polarization of the bodies in the polar particles of the polymer matrix as well as the natural fibres. Due to the increase in polarizations, the dielectric constants display an increase. Different types of polarizations have been observed in various research conducted by different authors. The presence of ionic polarization, space-charge polarization and interfacial polarization has been observed at all frequencies as the loadings of fillers increase [6, 14-16]. The dielectric constant also experiences an increase with an increase in frequency, unlike the HDPE composites. As seen in Fig.4a, the dielectric constant of PLA composites are higher at 2 MHz frequency. This could be attributed to the aligning of the fillers at high frequency in a favorable manner to aid in increased polarizations across the composite. The effect of filler dispersion and arrangement on dielectric constant and dissipation factor at high frequencies has been investigated by [17], which supports the observed trend in this dielectric study. The PLA composites exhibited a gradual increase in the dielectric constant as the frequency and filler loadings were increased. The dissipation factor follows similar trends for increased loadings (Fig.4b). However, in terms of frequency, the specimens in the higher frequency spectrum of 1 MHz and 2 MHz have the lowest dissipation factor values. The highest dissipation factor is observed at 1 kHz followed by 10 kHz. Literature suggests the inclusion of natural fibre into the polymer matrix reduces the dissipation factor as the polarized group increases and the lack of any orientation polarization [18]. The dissipation factor is relatively stable at 0.01 for samples despite the increase in frequency. (HDPE) and PLA (0.018-0.021) increases with an increase in frequency as well as increasing filler loadings for PLA. The dissipation factor increases slightly at all frequencies towards the
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maximum loadings of fillers. This could be due to the increase in copper loadings as observed in specimens tested with the polymer matrix and conductive filler. As the polymer melts, the coir fibres get mixed with the molten matrix to form spherulites. Literature suggests the spherulites align in a lateral direction, forming a network of crystals that aid conductivity [19]. This can explain the increased dissipation factor of the samples.
Fig.4 (b) Dissipation factor of natural fibre and conductive filler reinforced PLA polymer composites.
Highest dielectric constant in the overall experiment was obtained for polymer composites with HDPE and copper at filler loading of 8% and frequency of 1 kHz (2.87). The highest dissipation fac tor was measured for the same samples at the same frequency (0.06). Hence for applications with high dielectric constants and high losses, as is the requirement for materials in the energy storage applications, HDPE composites with conductive fillers can be utilized. 5. Conclusion Compression molding technique was used to obtain the dielectric samples. The dielectric study carried out displays the potential in obtaining high quality dielectric specimens utilizing a basic compression molding press and manual hand-laying of the fillers. This process has the potential to produce dielectric samples using a cost-effective method. The dielectric constant was found to increase for composites with the increased loading of conductive filler and at the lowest frequency. The dissipation factor has shown an increase with the addition of fillers and the increasing volume of the fillers across both the composites. The increase in dielectric constants at certain frequencies and increased filler loadings have been attributed to various factors such as alignment of fibres, formation of spherulites and appearance of polarizations. Future work can focus on improving the layup technique as well as using polymer blends to observe dielectric characteristics. Polymer blends can also impart additional properties such as fire retardancy, toughness, flexibility etc. References [1] [2] [3]
G. Panomsuwan and H. Manuspiya, "A comparative study of dielectric and ferroelectric properties of sol–gel-derived BaTiO3 bulk ceramics with fine and coarse grains," Applied Physics A, journal article vol. 124, no. 10, p. 713, September 24 2018. I. Ben Amor, H. Rekik, H. Kaddami, M. Raihane, M. Arous, and A. Kallel, "Studies of dielectric relaxation in natural fiber–polymer composites," Journal of Electrostatics, vol. 67, no. 5, pp. 717-722, 2009/09/01/ 2009. A. Triki, M. Guicha, M. Ben Hassen, M. Arous, and Z. Fakhfakh, "Studies of dielectric relaxation in natural fibres reinforced unsaturated polyester," Journal of Materials Science, journal article vol. 46, no. 11, pp. 3698-3707, June 01 2011.
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M. L. Verma and H. D. Sahu, "Ionic conductivity and dielectric behavior of PEO-based silver ion conducting nanocomposite polymer electrolytes," Ionics, journal article vol. 21, no. 12, pp. 3223-3231, December 01 2015. G. M. Tsangaris, N. Kouloumbi, and S. Kyvelidis, "Interfacial relaxation phenomena in particulate composites of epoxy resin with copper or iron particles," Materials Chemistry and Physics, vol. 44, no. 3, pp. 245-250, 1996/06/01/ 1996. Z.-M. Dang, Y.-H. Zhang, and S. C. Tjong, "Dependence of dielectric behavior on the physical property of fillers in the polymermatrix composites," Synthetic Metals, vol. 146, no. 1, pp. 79-84, 2004/10/14/ 2004. G. Faussurier and M. S. Murillo, "Gibbs-Bogolyubov inequality and transport properties for strongly coupled Yukawa fluids," Physical Review E, vol. 67, no. 4, p. 046404, 04/28/ 2003. Z. M. Dang, L. Z. Fan, Y. Shen, and C. W. Nan, "Dielectric behavior of novel three-phase MWNTs/BaTiO3/PVDF composites," Materials Science and Engineering: B, vol. 103, no. 2, pp. 140-144, 2003/10/15/ 2003. M. Arjmand, M. Mahmoodi, S. Park, and U. Sundararaj, "An innovative method to reduce the energy loss of conductive filler/polymer composites for charge storage applications," Composites Science and Technology, vol. 78, pp. 24-29, 2013/04/01/ 2013. K. J. Andrew, "Dielectric relaxation in solids," Journal of Physics D: Applied Physics, vol. 32, no. 14, p. R57, 1999. A. Paul, K. Joseph, and S. Thomas, "Effect of surface treatments on the electrical properties of low-density polyethylene composites reinforced with short sisal fibers," Composites Science and Technology, vol. 57, no. 1, pp. 67-79, 1997/01/01/ 1997. A. Khan, M. A. Ahmad, S. Joshi, and V. Lyashenko, "Synthesis of Alumina Fibre by Annealing Method Using Coir Fibre," 2014. F. M. Al-Oqla and S. M. Sapuan, "Natural fiber reinforced polymer composites in industrial applications: feasibility of date palm fibers for sustainable automotive industry," Journal of Cleaner Production, vol. 66, pp. 347-354, 2014/03/01/ 2014. I. Ben Amor, M. Arous, and A. Kallel, "Effect of maleic anhydride on dielectric properties of natural fiber composite," Journal of Electrostatics, vol. 72, no. 2, pp. 156-160, 2014/04/01/ 2014. E. Netnapa, M. Mariatti, Z. A. A. Hamid, M. Todo, and L. Banhan, "Dielectric Breakdown Strength and Flammability Properties of Flame Retardant Filler/PLLA-PLA Microsphere/Kenaf Fiber Composites," Procedia Chemistry, vol. 19, pp. 290-296, 2016/01/01/ 2016. J. Naik and S. Mishra, Studies on Electrical Properties of Natural Fiber: HDPE Composites. 2005, pp. 687-693. N. Tagami et al., "Effects of curing and filler dispersion methods on dielectric properties of epoxy nanocomposites," in 2007 Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, 2007, pp. 232-235. K. Kwong Siong, H. Sinin, and R. Md. Rezaur, "Comparative Study of Dielectric Properties of Chicken Feather/Kenaf Fiber Reinforced Unsaturated Polyester Composites," BioResources, Article vol. 8, no. 2, pp. 1591-1603, 2013. Y. Song, Y. Shen, H. Liu, Y. Lin, M. Li, and C.-W. Nan, "Improving the dielectric constants and breakdown strength of polymer composites: effects of the shape of the BaTiO3 nanoinclusions, surface modification and polymer matrix," Journal of Materials Chemistry, 10.1039/C2JM32579A vol. 22, no. 32, pp. 16491-16498, 2012.
ScienceDirect Materials Today: Proceedings 22 (2020) 162–171
www.materialstoday.com/proceedings
2018 2nd International Conference on Nanomaterials and Biomaterials, ICNB 2018, 10–12 December 2018, Barcelona, Spain
Investigation of the dielectric properties of natural fibre and conductive filler reinforced polymer composites Elammaran Jayamania*,Govind Anil Naira, KokHeng Soona a
a
[email protected], [email protected], [email protected]
Faculty of Engineering, Computing and Science, Swinburne University of Technology Sarawak Campus, Jalan Simpang Tiga, Kuching,93350 Sarawak, Malaysia
Abstract Bio-composites are gaining popularity as materials used in the production of semi-conductors. Dielectric materials are important energy storage devices in an electric circuit due to their ability to get polarized upon the application of an electric field. An investigation of dielectric properties was conducted by preparing specimens consisting of various amounts of coconut fibre, copper powder and HDPE or PLA as the polymer matrix. The dielectric testing revealed the increase of the dielectric constant after the addition of copper to the neat matrix of either polymer. The dielectric loss and the dissipation factor exhibited an increase with the increased loading of fillers. The dielectric constant decreased, along with the dissipation factor and the dielectric loss after the addition of coconut fibre. The results obtained align with existing literature. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials. Keywords: Dielectric; Natural Fiber;Polymer;Composites;PLA;HDPE;Coir;Copper
* Corresponding author. Tel.: +60-0060165774867. E-mail address: [email protected] 1876-6102 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials.
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Introduction Dielectric properties of materials are of great interest to researchers in the present as it is an essential factor in the engineering of energy storage devices and semiconductors. Dielectric materials are often used in circuit boards as capacitors, in hybrid vehicles in the automotive industry and several other small and large scale electronic devices. Traditionally, polymer composites have been utilized along with ceramic powders for creating materials with desirable dielectric properties. Polymers exhibit excellent mechanical properties in addition to thermal and chemical stability. These are properties of interest in applications where the polymers are subject to temperatures at extremes and exposed to various chemicals. Ceramics however are brittle and hence large quantities in the polymer matrix can compromise the structural integrity of the composite. Other limiting factors of ceramic polymer composites include poor adhesion with the polymer matrix, high processing temperatures and toxicity. The use of metal powders and natural fibres as fillers in the polymer matrix enables the creation of dielectric polymer composites. The dielectric properties are present in these composites due to the natural dielectric traits of natural fibres and the polymer matrix. Metal powders and natural fibres can be produced at low costs and are environment friendly and biodegradable. The development of polymer composites with natural fibres and conductive fillers are a necessity as there is growing demand for efficient and sustainable dielectric materials in various industries. As devices reduce in size, the components should be able to store large amounts of energy while taking up the least space in the circuit. Procedure Natural fibre treatment: Coir fibres of long lengths were obtained to be used as the natural fibre for this experiment. The fibres were coarse with varying lengths. These fibres were washed under distilled water until the drain contained no impurities. The fibres were then spread out in a tray and heat treated in an open convention oven for 6 hours at 160°C. This facilitated the removal of wax and moisture, which enabled better resistivity to heat . Removal of the OH group in the fibres enabled better adhesion between the hydrophobic polymer matrixes. The fibres were then cut by hand to lengths not exceeding 5 mm. 2.1. Mold preparation A steel frame with circular holes measuring 50mm in diameter and 5mm in thickness was used to prepare the composites. A steel flat plate wrapped in aluminum foil was kept beneath frame. The aluminum foil ensured uniform heating of the bottom plate. A thin layer of metal polish was applied to the naked metal surfaces to prevent the molten material from sticking to the surface. A layer of polymer according to the desired volume fraction was loaded into each cavity, followed by the loadings of natural fibre and copper powder. The remaining polymer was loaded into the cavities. A second frame of the same dimensions were placed on top and the cavities were topped with extra loadings of polymer to account for any leakages. Another flat plate wrapped in aluminum foil was plated on top. 2.2 Sample compositions Samples prepared consisted of the polymer matrix (HDPE or PLA), natural fibre (coconut fibre) and conductive filler (copper powder). Separate samples containing either of the polymers with copper powder was prepared initially, with a minimum loading of 2%vol of copper (3.5g) were prepared, with increments of 2% till a maximum loading of 8%vol (7.0g). Samples prepared with natural fibres contained loadings of 1%vol ,3%vol ,5%vol and 7%vol. 2.3 Compression molding A Molding Press was used to fabricate the composites. GT-7014-H30C, from GOTECH, at a pressure of 20 Tons was used to prepare the samples. Specimens of PLA were heated at 160℃ for 20 minutes while specimens consisting of HDPE were heated at 190℃ for 30 minutes. Specimens of HDPE and PLA were cooled naturally by
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turning off the machine and leaving the samples between the plates for 16 hours. This technique was found to be more effective in cooling the samples as compared to the water quenching method as it prevented water from entering the sample through the material defects. Presence of water inside the samples could lead to increased dielectric constant readings as well as the cracks when the water expands. 2.4 Dielectric measurements The samples obtained after cooling, were then dusted to remove impurities and leftover copper powder from the surface. The presence of the conductive powder can lead to erroneous readings when the dielectric parameters are measured. E4980A precision LCR meter and 16451B dielectric test fixture provided by Keysight Technologies were used to measure the capacitance and the dissipation factor of the composites. The capacitance and dissipation factor were measured at a frequency range from 1 kHz to 2 MHz according to the ASTM D-150-18 standards. To get accurate readings and account for surface impurities, the readings were take once, removed from the fixture, flipped and a second reading was taken. When commutating the results, a mean of the two values were used. 3. Equations 3.1 Determining dielectric constant and dielectric loss The dielectric apparatus determined the capacitance (Cp) and the dissipation factor (D) for every specimen. Using the measured capacitance value of the material under test (MUT), the dielectric constant was calculated using the equation below [1]:
𝜀 =
(1)
Where Cp is the measured capacitance of the MUT and C0 is calculated from the formula: 𝐶 =𝜀
(2)
Where A is the area of the circular electrode, t is the thickness of the specimen and 𝜀 is the permittivity of vacuum measured at 8.854x10-12 F/m. Combining equations 1 and 2, the dielectric constant of the MUT is commutated by the formula: ε =
(3)
The dielectric loss of the MUT is calculated using the formula 𝜀 = 𝜀 𝑡𝑎𝑛𝛿
(4)
Where tan δ is the measured dissipation factor.
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4. Results
(a) (b) Fig.1. (a) Dielectric constant; (b) Dissipation factor of conductive filler reinforced HDPE composites
4.1 Neat polymer 4.1.1 Dielectric constant and Dissipation factor The dielectric constants of HDPE and PLA were measured at various frequencies from 1 kHz to 2Mhz. HDPE had maximum dielectric constant (ɛ’) of 1.43 at a 1 kHz while PLA had a maximum of 1.82 at 10 kHz. Both the polymers did not exhibit any serious deviation in the values of the dielectric constant as the frequency was increased from 1 kHz to 2 MHz. The dissipation factor for HDPE drops from 0.0295 to 0.0165 as the frequency is increased from 1 kHz to 2 MHz. From the range of 10 kHz to 1 MHz, the dissipation factor remains steady before experiencing slight a rise from 1 MHz to 2MHz. The fall in the dissipation factor is a result of the ionic conduction, especially at lower frequencies. As the charges exhibit increased mobility due to increased frequency applied, the conductivity increases. The increase in conductivity leads to an increase in the dissipation factor [2]. PLA exhibits an increase in the dissipation factor from 0.0054 at 1 kHz to 0.012 at 2 MHz as the frequency is increased. 4.1.2 Dielectric loss HDPE experiences a sudden drop in the dielectric loss as the frequency is increased from 1 kHz to 10 kHz. From 10 kHz, there is no increase in the dielectric loss even as the applied frequency is incremented. This can be explained by the presence of frequency dependent α-relaxation that is observed above the 100 Hz region [2]. As the relaxation occurs, there is an increase in the permittivity. The increased permittivity leads to a decrease in the dielectric loss. PLA on the other hand exhibits contradicting behavior to HDPE in the region from 1 kHz to 10 Hz as the dielectric loss increases. This result segment can be discarded as the result is an anomaly. It could be due to presence of impurities on the electrode fixture or microscopic surface defects. An increase in dielectric loss is observed in frequency region of 1kHz to 10 kHz before plateauing at the frequency range of 10 kHz to 2 MHz, which is in line with previous literature [3]. From the testing of the neat samples, it is observed that the polymers exhibit highest dielectric constants at the lowest frequency spectrum of 1 kHz. PLA has the highest dielectric constant of 1.82 at 10 kHz. At this frequency range, the dielectric loss and dissipation factor is lowest for PLA when compared to HDPE.
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4.2 Polymer with conductive filler 4.2.1 HDPE For HDPE composites reinforced with conductive filler, the highest dielectric constant (𝜀 =2.87) is observed at 8% volume loading of copper at 1 kHz (Fig. 1a). The composites exhibit an increase in the dielectric constant as the copper content is increased. Similar results were reported when investigating the relationship of the dielectric properties in response to the addition of the fillers [4-6]. The high dielectric constant is attributed to the electron tunnel effect. It is also observed that the high dielectric constant is in the low frequency of 1 kHz. Compared to the high frequency at 1 MHz and 2 MHz, the dielectric constants of all filler loadings are higher in the 1 kHz and 10 kHz spectrum. This is because at low frequency, the presence of interfacial and space charge polarization is more prominent [7]. At 1 MHz and 2 MHz, there is a decrease in the dielectric constant as the filler loading is at maximum (8 % volume). This behavior is explained by the vibration of the conductive fillers at high frequency. As they vibrate and accumulate in the composite, the aspect ratio increases. As the aspect ratio increases, the percolation threshold decreases considerably for a two-phase composite. As suggested by [8] , the critical volume is about 0.09. This could vary depending on the size and shape of the conductive filler. Hence, the decrease in the dielectric constant can be due to the transition of the composite from a dielectric towards a conductor at high frequency and maximum loadings. Fig. 1b shows the dissipation factor or loss tangent (tan δ) of HDPE composites with conductive fillers. The dissipation factor increases as the concentration of copper is increased. This is explained by the numerous conductive networks that are formed when the copper is dispersed into the composite. More the volume of copper, more conductive networks and hence greater dissipation. The dissipation values are greater in the frequency range of 1 kHz and 1 MHz and lowest at the highest frequency of 2 MHz. At this range, it is observed to be consistent with no significant change. This high dissipation factor for low frequencies (1 kHz, 10 kHz and 1 MHz) is due to the alignment of the fillers. At low frequencies, the alignment of fillers can lead to a percolating network that is responsible for the high dissipation factor values observed. 4.2.2 PLA The dielectric constant of PLA composites exhibit similar trends to the HDPE samples (Fig.2a). However, the dielectric constant obtained from the PLA samples is lower. The maximum dielectric constant (ɛ‘=2.64) obtained was at the frequency of 10 kHz for the sample with 8% loading of copper. As the PLA is in powdered form, better dispersion was observed during the fabrication of the samples. As the frequency increases, the dielectric constant shows a decrease with an increase in filler content. The same phenomenon observed in the HDPE samples is responsible for the dielectric behavior of PLA composites as well. Fig.2b shows the results from the dissipation factor measurements of PLA composites with copper fillers at 1 kHz, 10 kHz, 1 MHz and 2 MHz. The dissipation factor has displayed dual behaviour for PLA composites. At the frequency of 1 kHz and 10 kHz, the dissipation factor increases with an increase in the loading of copper. This is explained by the presence of ionic and electron polarization at low frequencies in the polymer matrix. At high frequency, the decrease in dissipation factor was observed for loadings till 4% at 1 MHz and 6% at 2 MHz. As the loading of the conductive filler reaches the maximum value of 8%, the dissipation factor rebounds to the initial levels. The up and down trend of the dissipation factor can be due the presence of surface defects, resulting in excess dissipation. Other errors such as the air gap capacitance present between the electrodes of the dielectric test fixture can lead to the odd trend as well [9]. The dielectric loss increases from 1 kHz to 10 kHz and remains steady from 1 MHz to 2 MHz for both the polymers. It was observed that the samples with HDPE exhibited more consistent behaviour with literature than those composites prepared with PLA. A study on conductivity was not conducted due to equipment limitations. Analysing the conductivity values can offer a better explanation of the odd behaviour of PLA composites. A study on the dielectric relaxation could offer a better explanation as the polarization at high frequencies show frequency dependent behaviour. This has been reported in conductive filler reinforced polymer composites by [6]. After the addition of conductive fillers, the dielectric constant has increased for the examined polymer composites when compared with the neat polymer specimens. A high dielectric constant of 2.87 was observed for the HDPE composite with 8% copper at 1 kHz. PLA with 8% copper showed the highest dielectric of 2.64 at 1 kHz.
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Fig.2 (a) Dielectric constant; (b) Dissipation factor of conductive filler reinforced PLA composites.
4.3 Polymer matrix with conductive filler and natural fibre 4.3.1 HDPE In comparison with the previous dielectric study on conductive filler reinforced polymer composites (Fig.1 and Fig.2), the dielectric constant displayed a decreasing trend in composites upon the inclusion of coconut fibres (Fig.3a). The filler size, dispersion and orientation play a role for the low values obtained. The increased loading of fillers can lead to the presence of increased relaxations in the composite. This leads to increased losses as well as the presence of other relaxations observed in similar experiments including Maxwell-Wagner-Sillars (MWS) effect, ionic conduction and interfacial polarization. Dielectric relaxation is phenomenon where the dielectric constant is delayed in composites with two or more phases [10]. A relaxation due to the direct current (DC) conductivity was also observed by [2] at low temperature and low frequency (f). Low temperatures were characterized as lower than 70 degrees and low frequency spectrum defined as f <10 MHz. The experimental conditions in this research recorded a temperature of about 25°C in the test environment and the maximum frequency did not exceed 2 MHz. The dielectric constant is highest at 1 kHz and is lowest at 2 kHz. The decrease in dielectric constant with increasing frequency has been observed in several studies and is explained by the presence of interfacial and oriental polarization which are frequency dependent. Theoretically the dielectric constant is supposed to increase as the fibre loading is increased [11]. However, it follows the opposite trend in this research. This is due to the increase in copper loadings. The copper loadings are increased with the increase in fibre loadings. Hence, the expected increase in dielectric constant is not as evident as the losses increases. It has also been observed in experiments conducted by various authors, the increase in losses as the total filler volume increases. The presence of water molecules in the coir fibres can contribute to the relaxation associated with the polarization of the cellulose fibres. This could also explain the decreasing trend of the dielectric constant as the fibre loading is increased. As the specimens were prepared using manual techniques in environments that are not controlled, the presence of impurities can lead to the MWS effect. Existing literature attributed ionic conduction due to the presence of conductive fillers, especially in the low frequency region [2, 3, 12, 13]. HDPE being a thermoplastic polymer with thermal conductivity might have pellets that were not completely melted, as was observed from the prepared specimens. These can lead to the charges being trapped on the interfaces between the polymer and the fillers, thereby creating dielectric hotspots.
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Fig.3 (a) Dielectric constant of natural fibre and conductive filler reinforced HDPE composites.
The improper dispersion of the fillers could have result in the accumulation of these charges at specific spots, thereby not reflecting on the increased dielectric constant of the specimen. These also impact the dissipation factor of composites. As seen in Fig.3b, the dissipation factor increases as the loadings of the fillers is increased.
Fig.3.(b) Dissipation factor of conductive filler and natural fibre reinforced polymer composites.
The most noticeable change is seen for composites tested at 10 kHz and 1 MHz. It is also observed that the dissipation factor is stable at the frequency of 2 MHz, where there is no noticeable changes even as the loadings increase. The increase of dissipation factor for HDPE composites at 10 kHz and 1 MHz can be due to the increase in
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ionic conduction at low frequencies as well as the presence of dielectric relaxation. However, at the frequency of 2 MHz, the stability is explained by the increase of molecular dipoles. As the dielectric relaxation is frequency dependent, it is assumed that the relaxations for three phase polymer composites such as those investigated in this research is observed at frequencies greater than 2 MHz. The increase in relaxation will lead to lower dissipation values [2]. 4.3.2 PLA
Fig.4 (a) Dielectric constant of conductive filler and natural fibre reinforced PLA composites.
The dielectric constant of PLA shows an increase with the increased loadings of copper and natural fibre (Fig.4a). PLA is an organic polymer that is usually obtained from the processing of green plants or fibres. Due to the water content in these fibres, the dielectric constants are usually higher for PLA than other synthetic polymers. However, depending on the quality of processing and the raw materials used to produce PLA, the dielectric properties can vary. As measured at the beginning of the experiments, neat PLA displayed a dielectric constant if 1.82, which was higher than neat HDPE. The dielectric constant increases as the fibre loading is increased. This increases the number of polar bodies present in the composite. As the loading of copper is increases simultaneously, the conductive path increases within the composite. This results in the polarization of the bodies in the polar particles of the polymer matrix as well as the natural fibres. Due to the increase in polarizations, the dielectric constants display an increase. Different types of polarizations have been observed in various research conducted by different authors. The presence of ionic polarization, space-charge polarization and interfacial polarization has been observed at all frequencies as the loadings of fillers increase [6, 14-16]. The dielectric constant also experiences an increase with an increase in frequency, unlike the HDPE composites. As seen in Fig.4a, the dielectric constant of PLA composites are higher at 2 MHz frequency. This could be attributed to the aligning of the fillers at high frequency in a favorable manner to aid in increased polarizations across the composite. The effect of filler dispersion and arrangement on dielectric constant and dissipation factor at high frequencies has been investigated by [17], which supports the observed trend in this dielectric study. The PLA composites exhibited a gradual increase in the dielectric constant as the frequency and filler loadings were increased. The dissipation factor follows similar trends for increased loadings (Fig.4b). However, in terms of frequency, the specimens in the higher frequency spectrum of 1 MHz and 2 MHz have the lowest dissipation factor values. The highest dissipation factor is observed at 1 kHz followed by 10 kHz. Literature suggests the inclusion of natural fibre into the polymer matrix reduces the dissipation factor as the polarized group increases and the lack of any orientation polarization [18]. The dissipation factor is relatively stable at 0.01 for samples despite the increase in frequency. (HDPE) and PLA (0.018-0.021) increases with an increase in frequency as well as increasing filler loadings for PLA. The dissipation factor increases slightly at all frequencies towards the
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maximum loadings of fillers. This could be due to the increase in copper loadings as observed in specimens tested with the polymer matrix and conductive filler. As the polymer melts, the coir fibres get mixed with the molten matrix to form spherulites. Literature suggests the spherulites align in a lateral direction, forming a network of crystals that aid conductivity [19]. This can explain the increased dissipation factor of the samples.
Fig.4 (b) Dissipation factor of natural fibre and conductive filler reinforced PLA polymer composites.
Highest dielectric constant in the overall experiment was obtained for polymer composites with HDPE and copper at filler loading of 8% and frequency of 1 kHz (2.87). The highest dissipation fac tor was measured for the same samples at the same frequency (0.06). Hence for applications with high dielectric constants and high losses, as is the requirement for materials in the energy storage applications, HDPE composites with conductive fillers can be utilized. 5. Conclusion Compression molding technique was used to obtain the dielectric samples. The dielectric study carried out displays the potential in obtaining high quality dielectric specimens utilizing a basic compression molding press and manual hand-laying of the fillers. This process has the potential to produce dielectric samples using a cost-effective method. The dielectric constant was found to increase for composites with the increased loading of conductive filler and at the lowest frequency. The dissipation factor has shown an increase with the addition of fillers and the increasing volume of the fillers across both the composites. The increase in dielectric constants at certain frequencies and increased filler loadings have been attributed to various factors such as alignment of fibres, formation of spherulites and appearance of polarizations. Future work can focus on improving the layup technique as well as using polymer blends to observe dielectric characteristics. Polymer blends can also impart additional properties such as fire retardancy, toughness, flexibility etc. References [1] [2] [3]
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