Abstract
The electrical properties like AC conductivity, AC resistivity, and dielectric constant of the palm fiber-reinforced acrylonitrile butadiene styrene composites were investigated using impedance analyzer at room and variable temperatures. Palm fiber was collected from 10 trees of different age group from Comilla region in Bangladesh. Three sets of samples were prepared for three different wt (%) (5%, 10%, and 20%) of fiber contents using Injection Moulding Machine. Each set has 10 composite samples in which first five of them are made with 11 to 20 years-aged palm fiber and latter five of them are made with 5 to 10 years-aged palm fiber. The AC electric conductivity increases with the increase of frequency, wt (%) of palm fiber in the PF-ABS composites, and with the increase of temperature. The AC resistivity decreases with the increase of frequency, with the wt (%) of fiber content in the PF-ABS composites and with the increase of temperature. Moreover, the comparison between the PF-ABS composites with 11 to 20 years-aged palm tree shows better result than 5 to 10 years-aged palm tree. The dielectric constant of the composites decreases with the increase of frequency. With the addition of palm fiber content in composites, dielectric constant increases. The dielectric constant also increased with the increase of temperature.
Introduction
Composite materials reinforced with cellulosic fibers have received interest in recent years and are widely using in many fields such as civil, industrial, military, space craft, and biomedical application mainly because of their excellent thermo mechanical properties, biodegradability, low density, and non-toxicity.1–9
It is well known that composites can be produced exhibiting enhanced properties that the constituent materials may not exhibit. For instance, from the combination of different number or fillers with polymer matrices one can produce polymer-matrix composites, a material important to the electronic industry for its dielectric properties in the use of capacitors. The effective utilization of filled polymers depends strongly on the ability to disperse the filler homogeneously throughout the matrix. The interface properties also strongly affect the characteristics and performance of these composites. One of the most attractive features of these filled composites is that their dielectric properties can be widely changed by choice of shape, size, and the conductivity of filled constituents in the polymeric matrix.10–13
Fiber-reinforced plastic materials act as effective insulators and also provide mechanical support for field carrying conductors. The incorporation of fibrous reinforcements in polymeric matrices leads to composite materials having very good mechanical properties and electrical application (electrical devices and dielectric materials). They can be used as terminals, connectors, industrial and house hold plugs, and printed circuit boards. The electrical application of composite material makes the study of composite materials important. 1
The various properties of composite materials are essentially the result of the chemical nature and physical characteristics of their constituents as well as the size and shape of the particles which make up the filler material. 14
The electric properties like AC conductivity, AC resistivity, and dielectric constant of the composites have been studied worldwide. The dielectric constant of natural fiber graft co-polymer reinforced phenol formaldehyde composites was found to decrease with frequency that was due to decrease of polarization. The dissipation factor showed first increase and then decrease with increase in frequency at different voltage. The volume resistivity of composite also decreased with frequency. The dielectric loss factor was observed to be high at lower frequency which was due to polarization of fiber at lower frequency. 1 Dielectric properties of industrial polymer composite materials have been studied by Akram et al. 10 The experimental results showed that dielectric constant and dielectric loss increased with the addition of fiber in polyester resin. The value of dielectric constant decreased with increasing frequency, which indicates that the major contribution to the polarization comes from orientation polarization. Dielectric loss peaks were also observed in the composite materials at high temperature due to Tg of polyester. The value of dielectric constant increased with increasing temperature. Electric properties of pineapple-reinforced polyethylene composites have been studied by Jayamol et al. 15 They observed that the increase in the dielectric constant of composite with fiber loading was due to increased orientation and interfacial polarization. Nigrawal and Chand 16 showed that on increasing the filler content, dielectric constant increased, it was mainly due to motion of the dipolar groups of the interfacial polarization occurring in the composites. The DC and AC electrical properties of PMMA-Al2O3 composites were studied by Hussien. 17 The dielectric constant, dielectric loss, AC electrical conductivity are changed with change in the concentration of the filler and frequency of applied electrical field. Dielectric property of wood-polymer composites was studied by Notingher et al. 14 The results show that the dielectric properties are heavily influenced by the filler content with very high registered values in the case of low frequency electric fields and high temperatures. In this research work, palm fiber is one of the particular interests because palm fiber (palmyra palm) grows plenty in Bangladesh and all over the world, and composites made of palm fibers have moderate electrical properties compared with other natural fibers.
The aim of this study is to find out the different electric properties of palm fiber-reinforced acrylonitrile butadiene styrene (PF-ABS) composites at room and variable temperature and to compare these results with other’s work.
Materials and method
Raw materials
Palm leaves of 10 different aged trees (five of them are above 10 years and another five of them are below 10 years) were collected from Burura region of Comilla district. Dividing ends of the middle hard part of the leaves were hammered. Hard part of the leaves was immersed in water for 20 days to rot. Rotten materials were cleaned and fiber were then separated, dried under sun light and kept at 100℃ for 24 h for partial removal of moisture. ABS was collected from local market of old Dhaka of Bangladesh.
Composite preparation
Palm fiber was cut to 1–2 mm in sized. ABS and small palm fiber were dried in dryer at 50℃ for 24 h. Fine palm fiber and ABS polymer put into the injection moulding machine. This mixture was heated at 150℃ inside the injection molded machine, and molten mixture become composite and come out of the injection moulding machine. This composite was poured into different shape of die for different test. Three sets of composite samples (with 5%, 10%, and 20% fiber content) were prepared to carry out this research. After preparation of the composite samples, they are named as C1Sx, C2Sx, and C3Sx (x = 1, 2, 3,……,10); where C1, C2, and C3 represent 5%, 10%, and 20% fiber content in the PF-ABS composites. A total of 30 composite samples (each set contain 10) were prepared. In each set, first five of PF-ABS composites were made with fiber collected from 11 to 20 years-aged palm trees and next five of them were prepared with palm fiber collected from 5 to 10 years old palm trees. Two types of composites were prepared, one for tensile test and another for flexural test. From flexural test sample, the electrical test sample was cut and fined with paper. The average dimension of composite samples is 13.24 mm× 12.43 mm× 3.96 mm.
Measurement of electrical properties
The electrical properties of fabricated PF-ABS composites were measured using Wayne Kerr Precision Impedance Analyzer (Model No. 6500B) in the IFRD laboratory, BCSIR, Dhaka. Various properties like conductance (Gx), resistance (Rx), and capacitance (Cx) were measured at room and variable temperature. The measured values were used to calculate the AC conductivity, AC resistivity, and dielectric constant. The frequency dependent AC conductivity and dielectric measurement were taken at room temperature in the frequency range 1 kHz–1 MHz and frequency dependent AC resistivity measurement was taken at room temperature in the frequency range 100 kHz–1 MHz. The electrical properties were calculated as follows:
AC conductivity (σac)
Conductivity is a measure of material’s ability to conduct an electric current. Conductivity was calculated using the following relation
AC resistivity (ρ)
Resistivity is defined as the electrical resistance through a cube of insulating material. It was calculated using the relation
Dielectric constant (ɛ)
Dielectric constant relates to the permittivity of the material. The permittivity expresses the ability of a material to polarize in response to an applied field. It is the ratio of the permittivity of the dielectric to the permittivity of a vacuum. Physically, it means the greater the polarization developed by a material in an applied field of given strength, the greater the dielectric constant will be. It is calculated by using the following relation
Results and discussions
The electrical properties like AC electric conductivity, AC resistivity, and dielectric constant were measured using Wine Kerry AC electrometer at room temperature. Temperature-dependent measurement was done at 30℃, 40℃, 50℃, 60℃, and 70℃, respectively. The findings of the experiments are presented below:
AC electrical conductivity
The AC electrical conductivity vs. frequency curve for PF-ABS composites for different wt (%) of fiber content at room temperature is shown in Figure 1. The Figure 1 shows that the AC electrical conductivity of the composites is almost increasing nature with the increase of frequency and fiber content in the composites.
AC conductivity (at room temperature) vs. frequency curve for different wt (%) of palm fiber in PF-ABS composite. ABS: acrylonitrile butadiene styrene; PF-ABS: palm fiber-reinforced acrylonitrile butadiene styrene.
Figure 2 shows that the variation of conductivities with respect to different wt (%) of fiber content in PF-ABS composites at different frequencies. This figure symbolizes that the conductivity of the composites increases with the increase of frequency. This figure also implies that addition of palm fiber increased the conductivity of the composites.
Conductivity (at room temperature) vs. wt (%) of palm fiber in PF-ABS composites at different frequencies.
Pure ABS has very low electrical conductivity. The moisture content in fibers increased the conductivity. 18 The cellulose and hemicellulose structure in palm fiber contain polar group. The addition of palm fiber in ABS may significantly increase the electrical conductivity. It might be due to increase of polar groups in the composites. It is clear that as the polar molecule concentration increases, the number of electrical charge carrier increases resulting in the higher conductivity. In polymeric materials, most of the current flow through the crystalline regions and non-crystalline region allows current to pass through it mainly when moisture is present. The hydroxyl groups in the hydrophilic fiber can absorb moisture and hence the presence of the natural fiber increases the conductivity of the resin. Similar result was reported by Umemura et al. 19
The AC electrical conductivity increases with the increase of frequency at low frequency region due to increase of number of interfaces, which increase the interfacial polarization. On the other hand, the AC electrical conductivity at the higher frequency region increases due to electronic polarization as well as the hopping of charge carrier in the composites. Similar frequency dependence of the conductivity in (PMMA-Al2O3) and (Zn-PC) composites has been reported by Islam et al. 20 and Hussien, 17 respectively.
Figures 3 to 5 show the conductivity vs. frequency curves for three sets of composites in order to show the age effect of palm fiber on conductivity. These figures indicate that the conductivity of composites containing 11 to 20 years-aged palm fiber is greater than that of 5 to 10 years-aged group. Eleven to 20 year-aged palm fiber may contain more polar group than the 5 to 10 year-aged palm tree, which increased the conductivity of the composites.
Effect of age of palm fiber on AC conductivity (at room temperature) of 5% PF-ABS composites. Effect of age of palm fiber on AC conductivity (at room temperature) of 10% PF-ABS composites. Effect of age of palm fiber on AC conductivity (at room temperature) of 20% PF-ABS composites.
The variation of AC conductivity with respect to frequency at different temperature for 5% PF-ABS composite (C1S1) sample is shown in Figure 6. The conductivity of the composite is increased with the increase of frequency as well as temperature. Figure 7 shows the variation of conductivity with respect to temperature at different selected frequencies. This figure also reveals that the conductivity of the composites increased with the increase of temperature. There may be charge carrier in the composites. With the increase of temperature, charge carrier would be transported by hopping through the defect sites along the polymer chain, which would increase the conductivity of the composites.
16
Variation of AC conductivity of 5% PF-ABS composites (C1S1) with respect to frequency at different temperatures. Variation of AC conductivity of 5% PF-ABS composites (C1S1) with respect to temperature at different frequencies.
AC resistivity
The AC resistivity of the pure ABS and PF-ABS composites with respect to frequency is shown in Figure 8. The Figure 8 illustrates that the resistivity of the pure ABS polymer is higher than the PF-ABS composites. The addition of palm fiber to ABS polymer drastically decreased the resistivity of the composites.
21
Resistivity vs. frequency curve of pure ABS and PF-ABS composites at room temperature. PF-ABS: palm-fiber reinforced acrylonitrile butadiene styrene; ABS: acrylonitrile butadiene styrene. Resistivity vs. frequency curve of PF-ABS composites at room temperature. PF-ABS: palm-fiber reinforced acrylonitrile butadiene styrene. Effect of age of palm fiber on AC resistivity of 5% PF-ABS composites at room temperature. Effect of age of palm fiber on AC resistivity of 10% PF-ABS composites at room temperature. Effect of age of palm fiber on AC resistivity of 20% PF-ABS composites at room temperature.
Figure 9 represents the variation of resistivity with respect to frequency for PF-ABS composites. Since the resistivity of the pure ABS is too much higher than the PF-ABS composites, the effect of fiber loading was not clear in the Figure 8. Figure 9 shows that the resistivity of the composites decreases with the frequency and the fiber loading. The 5% PF-ABS composites show higher resistivity than other composites.
The resistivity of fiber-reinforced composites depends on the moisture content, crystalline and amorphous component present, presence of impurities, chemical composition, cellular structure, microfibrillar angle, etc. The shapes of reinforcement determine the interparticle contact, which affect the conductivity of the system. Fibers and flakes having elongated shapes affect the electrical conductivity. 18 Pure ABS shows higher resistivity. But the fiber-reinforced composites show comparatively lower resistivity than pure ABS composites. The addition of fiber in composites increases the polar group, which would increase the conductivity and inversely decrease the resistivity. Moreover, with the increase of frequency interfacial polarization occurs because of heterogeneity of the system, which intern increases the resistivity of the composites. The similar effect of resistivity is observed in natural fiber graft co-polymer-reinforced phenol formaldehyde composites by Pathania et al. 1
Figures 10 to 12 show the effect of age of palm fiber on resistivity of PF-ABS composites for 5%, 10%, and 20% fiber content. These figures show that the resistivity of 11 to 20 years age group is lower than the 5 to 10 years age group. The resistivity depends on the polar group of the fiber. The higher the value of polar molecule, the lower is the resistivity. The resistivity of 11 to 20 years age group composites is lower because these palm fiber contains higher amount of polar molecule than 5 to 10 years age group.
The effect of increase of temperature with different frequencies on resistivity on 5% PF-ABS composites (C1S1) is presented in the Figure 13. The Figure 13 shows that the resistivity decreases with the increase of frequency and temperature. The higher is the value of temperature; the lower is the value of resistivity.
Resistivity vs. frequency curve at different temperature.
Dielectric constant
The variation of dielectric constant with respect to log frequency at room temperature is shown in Figure 14. The dielectric constant decreases with increasing frequency.14,20 Moreover, it is also evident that the decrement of dielectric constant at low frequency region is faster than the higher frequency region.
Frequency dependence of dielectric constant at room temperature for different composites. PF-ABS: palm-fiber reinforced acrylonitrile butadiene styrene; ABS: acrylonitrile butadiene styrene.
A larger difference in the dielectric constant value is observed at lower frequency region than higher frequency region. Since palm fiber is insulating material like ABS, it may behave as dielectric material by generating space charge polarization at interface. The interfacial polarization occurs at lower frequency due to the number of interfaces between palm fiber and ABS matrix. At lower frequency, the dipole is much responsive to the applied filed, resulting in the higher dielectric value. 22
The palm fiber consists of 38.4% α-Cellulose, 30.9% hemicellulose, and 27.3% lignin. 23 Due to presence of α-Cellulose and hemicellulose, palm fiber consists of polar group. At higher frequency, the rotational motion of polar molecules of dielectric is not sufficient for the attainment of equilibrium with the field, and hence dielectric constant decreases with increase in frequency. 24 The dielectric constant of polymer materials depends on interfacial, dipole, and atomic polarization. The greater is the polarizability of the molecule, higher will be the dielectric constant. The electronic and atomic polarization occur at high frequency is due to presence of polar group in the material. The interfacial polarization arises due to heterogeneity, which is highest at lower frequency. 1
Figure 15 also reveals that the dielectric constant increases with the increase of fiber content in the composites. In the PF-ABS composites, palm fiber acts as an intermediate plasticizer in which fiber can be able to penetrate the molecular bundle of ABS polymer, leading to chain elongation. This may be one of the reasons of the increase of dielectric constant. The space charge polarization plays a major role in increasing dielectric constant of composite. The space charge polarization arises from the ABS/Palm fiber interfaces. The dielectric constant increases with weight fraction of palm fiber. The increase in dielectric constant with weight fraction of palm fiber supports the fact of the space charge polarization contribution. Similar results were also observed for pure polypropylene and polypropylene banana fiber composites by Islam et al.
20
and for PMMA-Al2O3 composites by Hussien.
17
Variation of dielectric constant with respect to wt (%) of fiber in composite at different frequencies.
Figure 16 depicts the variation of dielectric constant with respect to log frequencies at different temperature for 5% PF-ABS composites (C1S1) sample. The dielectric constant increases with the increase of temperature. Dielectric constant only changes due to change of frequency.
Effect of temperature on dielectric constant at different frequencies in 5% PF-ABS composite (C1S1) sample.
Temperature affects dielectric properties. As the temperature is increased, the intermolecular forces between polymer chains are broken which enhance thermal agitation. The polar group will be free to orient allowing it to keep up with the changing electric field. At lower temperature, the segmental motion of the chain is practically freezed and this will reduce the dielectric constant. At sufficiently higher temperature, the dielectric constant is again reduced due to strong thermal motion which disturbs the orientation of the dipoles. At this latter stage, the polarization effectively contributes minimal dielectric constant.
The increase in dielectric constant with temperature is due to greater freedom of movement of dipole molecular chain of palm fiber in composites at high temperature. At lower temperature, as the dipoles are rigidly fixed in the dielectric, the field cannot change the condition of dipoles. As the temperature increases, the dipoles comparatively become free and they respond to the applied field. Thus polarization increased and hence dielectric constant is also increased with the increase of temperature. Similar result was found in industrial polymer composite materials by Akram et al. 10
Conclusion
Using impedance analyzer, the AC electric conductivity, AC resistivity, and dielectric constant of the pure ABS and PF-ABS composites for different wt (%) of palm fiber was measured. The AC conductivity of the PF-ABS composites increases with the increase of palm fiber content along with the increase of frequency. The AC conductivity of PF-ABS composites having 11 to 20 years-aged palm fiber is more than that of 5 to 10 years-aged palm fiber. This result is same for three sets of samples. With the increase of temperature, the AC conductivity increases. On the other hand, the AC resistivity of PF-ABS composites is decreased with the increase of frequency as well as with the increase of fiber content in PF-ABS composites. In addition, the AC resistivity of PF-ABS composites is decreased with the increase of frequency and with the increase of fiber content in PF-ABS composites. Moreover, PF-ABS composites having 11 to 20 years-aged palm fiber shows better result than 5 to 10 years-aged palm fiber. With the increase of temperature, the AC resistivity decreased. The dielectric constant of the PF-ABS composites decreases with the increase of frequency. This decrease is more prominent at low frequency region. With the addition of palm fiber content in composites, dielectric constant increases due to increase of polar group in the composite. The more the polar group, the more is the polarization, which ultimately increased the dielectric constant. The dielectric constant also increased with the increase of temperature due to the increase of the movement of polar molecule in the composites. As a better dielectric constant was obtained in PF-ABS composite, it would be a suitable replacement of conventional dielectric materials. It can also be used in electrical applications such as terminals, connectors, insulator, etc. which would be environmentally friendly.
Footnotes
Conflict of interest
None Declared.
Funding
The research work has done under the financial support of Bangabandhu Fellowship on Science & ICT, Ministry of Science and Technology, Government of the People’s Republic of Bangladesh, and Comilla University research Grand.