Polymeric nanocomposites in which the nanosize fillers are evenly distributed in the polymer material attract attention as an insulating material due to their ability to enhance the materials performance properties of electrical and mechanical. For high voltage (HV) insulation application, one of the targets is to obtain new insulators with improved dielectric properties. This paper presents the outcome of an experimental study to determine the conductivity level of the linear low-density polyethylene- (LLDPE-)natural rubber (NR) compound, filled with different amount of SiO2 and TiO2 nanofiller by using the polarization and depolarization current (PDC) measurement technique. linear low-density polyethylene (LLDPE) and natural rubber (NR) with the ratio composition of 80 : 20 are selected as a base polymer. The experiment was conducted to find PDC pattern and conductivity variations of each of the LLDPE-NR/SiO2 and LLDPE-NR/TiO2 samples. The results show that the addition of SiO2 filler exhibited less conductivity compared to TiO2 filler with certain percentage. From the study, it can be concluded that LLDPE-NR/SiO2 is a better insulator compared with LLDPE-NR/TiO2.
Polyethylene (PE) such as linear low-density polyethylene (LLDPE) is produced via the copolymerization of ethylene with various alpha olefins. They have grown in importance because of the specific properties that can be obtained by varying comonomer content and polymerization condition [
With the addition of nanoscale fillers into polymers, robust materials can potentially be produced due to the synergistic effects (cooperating for enhanced effects) arising from the blending process. The fillers added to the polymer-based matrix are just in small quantity, typically less than 10 wt%. All nanocomposites show significantly enhanced thermal stability compared to virgin LLDPE due to the increases of the effective activation energy during degradation process. Furthermore, according to [
It is known that the dispersion of a very low ratio of inorganic particles having at least one dimension smaller than 100 nm can create a network of chemical-physical interactions inside an organic matrix, leading to a dramatic change in the macroscopic properties of the material [
Most previous researches performed morphology studies, breakdown strength test, space charge formation, and dielectric loss test to determine the electrical properties of polymer nanocomposite [
Condition monitoring technique such as polarization and depolarization current (PDC) measurement has been used to predict the remaining life of the electrical apparatus [
LLDPE was used as the base matrix of the sample. The nanofiller used were silicon oxide (SiO2) and titanium oxide (TiO2). The LLDPE was mixed with natural rubber (NR) grade SMR CV 60 along with the nanofiller. Detailed specifications of the materials were summarized in Table
Detailed properties of materials.
Material | LLDPE | NR-SMR CV 60 | Silicon oxide (SiO2) | Titanium oxide (TiO2) |
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Manufacturer | Titan Chemical | Taiko Plantation | Hongwu Nanometer, China | Hongwu Nanometer, China |
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Properties | Density of 0.918 g/cm3 |
Melting temperature: 160°C |
Form: hydrophilic solid powder |
Form: hydrophobic solid rutile |
The LLDPE sample used in this study is commercially manufactured by Titan Chemical, Malaysia. It has a density of 0.918 g/cm3 and a melt index of 25 g/10 min. The SiO2 and TiO2 nanofillers were sourced from China. It has an approximately spherical shape with particle size of less than 50 nm. It has specific surface area of about 100 m2/g. The filler was dried before use. Natural rubber grade SMR CV 60 supplied by Taiko Plantations is mixed and blended with the LLDPE and nanofiller. Polyethylene nanocomposites were prepared by melt mixing at 165°C using a Brabender mixer with a chamber size of 50 cm3. The mixer has a high shear force; the screw speed was controlled at 35 rpm, and the mixing time was 2 min for each sample.
Test specimens for PDC tests were prepared by using Hydraulic Hot Press Genesis brand manufactured by Wabash MPI USA. The nanocomposites were finally prepared into square shape with the thickness of 3 mm by hot melt pressing at 1 tone pressure at 170°C for 10 min. The cooling process has a significant influence on the morphological features of the sample which may alter the dielectric characteristics. To maintain consistent morphology, the moulded samples were immersed into distilled cool water (quenching) after 10 min of moulding process. Four types of polyethylene nanocomposite square shape with a dimension of 10 cm × 10 cm were prepared, with concentrations of nanofiller of 1, 3, 5, and 7 wt%, respectively. Table
Compound formulations and designation.
Test sample | Constituent composition wt% | Designation | ||
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LLDPE | Natural rubber (SMR CV 60) | Nanofiller | ||
LLDPE + natural rubber | 80 | 20 | 0 | A0 |
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LLDPE + natural rubber + SiO2 | 80 | 20 | 1 | A1 |
80 | 20 | 3 | A3 | |
80 | 20 | 5 | A5 | |
80 | 20 | 7 | A7 | |
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LLDPE + natural rubber + TiO2 | 80 | 20 | 1 | B1 |
80 | 20 | 3 | B3 | |
80 | 20 | 5 | B5 | |
80 | 20 | 7 | B7 |
The nanostructures of the polymer nanocomposites are investigated by using a field emission scanning electron microscope (FESEM), model JEOL JSM-7600F. A 40 mA sputtering current with 60 seconds of coating time is used during the platinum coating process. The electron gun of FESEM is energised at 10 kV and 3000 times of magnification is used to capture the nanograph of the polymer.
By applying a dc voltage step on the dielectric material the polarization currents measurement is performed. For depolarization current, it is measured by removing the dc voltage source incorporating with a switch turn onto short circuit at the undertested objects. The dc voltage applied was 1000 V for about 10,000 seconds for polarization and depolarization time. The voltage and currents were recorded automatically by the control software that was developing in the LabVIEW environment. The PDC measurement principal is shown in Figure
PDC measurement concept.
PDC measurement setup.
The estimation of the conductivity for HV insulation under polarization and depolarization test result can be expressed from the PDC value [
Figures
FESEM images of the samples. (a) Sample A1, (b) sample A3, (c) sample A5, and (d) sample A7.
FESEM images of the samples. (a) Sample B1, (b) sample B3, (c) sample B5, and (d) sample B7.
The interaction between the matrix and the fillers is related to the chemical properties of the filler surface and the interfacial area between the matrix and the fillers. The stronger the interaction and the larger the interfacial area, the greater the volume of the bound part. Because the molecules of the bound part are restricted to the surface of the fillers, the molecular motion is greatly limited [
The results for polarization and depolarization currents measured for sample in group A are shown in Figures
Polarization current values for samples A0, A1, A3, A5, and A7.
Depolarization current values for samples A0, A1, A3, A5, and A7.
Sample A7 shows the highest polarization current compared with others in group A. Agglomeration of nanofillers in the composite sample tends to increase the moving charges that can contribute to higher polarization current and conductivity. Dielectric permittivity of micrometric agglomerate was higher than that of particles material. There exist hydroxyl group in SiO2 nanofillers and molecularly absorbed water on the surface of nanofillers which can introduce more charge carriers and consequently cause higher polarization current.
The results for polarization and depolarization currents measured for samples in group B are shown in Figures
Polarization current values for samples B1, B3, B5, and B7.
Depolarization current values for samples B1, B3, B5, and B7.
Polarization and depolarization current measurement allows for an estimation to be made on the condition (moisture and ageing) of the insulation with different conductivities. From (
Conductivity variations for LLDPE-NR/SiO2 samples for different amount of nanofiller are shown in Figure
Conductivity variations for sample LLDPE-NR/SiO2 at different amount of nanofiller.
The higher value of conductivity of A7 as compared to A5 indicates that the SiO2 nanofiller of more than 5 wt% has reversed the improvement on the dielectric properties of LLDPE. This is because the filler will agglomerate in both composite samples. Those observed in the sample are clearly larger size of filler for higher wt% of nanofiller. Agglomeration of nanofillers in the composite sample tends to increase the moving charges that can contribute to higher polarization current and conductivity. More improvement can be done by addition of SiO2 5 wt% as filler for dielectric properties and lower conductivity level as compared to other composition. The properties of SiO2 nanofiller as dielectric filler tend to improve the dielectric properties as a good insulator.
Figure
Conductivity variations for sample LLDPE-NR/TiO2 at different amount of nanofiller.
Morphology and conductivity of LLDPE-NR/SiO2 and LLDPE-NR/TiO2 samples have been conducted and studied in this paper. Morphology results show that, for the LLDPE-NR/SiO2 and LLDPE-NR/TiO2 composites, there are some particles contacts in agglomerates that produced a larger number of SiO2 and TiO2 particles, while observations on agglomeration on both composite samples show that they are larger in size when higher percentage (wt%) of nanofiller is used.
Based on the PDC measurement results, it was found that adding SiO2 as nanofiller at certain percentage will improve the dielectric properties of LLDPE. However, additional TiO2 nanofiller will make it worse. The LLDPE-NR/SiO2 at 5 wt% has been found to be the best composition for HV insulation in terms of the lowest polarization and depolarization current values as well as the lowest conductivity level. Furthermore, the trends of the conductivity variation were found to be dependent on the polarization and depolarization currents values. These trends can be used to evaluate the condition of the HV insulation.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the Universiti Tun Hussein Onn Malaysia (UTHM), Universiti Teknologi Malaysia (UTM), Malaysia Ministry of Education (MOE), and Ministry of Science, Technology and Innovation (MOSTI), under Grant vote nos. GUP:08H65 and 04H67, ERGS:4L133, FRGS:4F515, and eSCIENCE:4S101, for financial support, TNB Research Sdn Bhd for equipment support, and Taiko Plantation Sdn Bhd as a supplier of natural rubber.