Polymer nanocomposites have been used for various important industrial applications. The preparation of high density polyethylene composed with Na-montmorillonite nanofiller using melt compounding method for different concentrations of clay-nanofiller of 0%, 2%, 6%, 10%, and 15% has been successfully done. The morphology of the obtained samples was optimized and characterized by scanning electron microscope showing the formation of the polymer nanocomposites. The thermal stability and dielectric properties were measured for the prepared samples. Thermal gravimetric analysis results show that thermal stability in polymer nanocomposites is more than that in the base polymer. It has been shown that the polymer nanocomposites exhibit some very different dielectric characteristics when compared to the base polymer. The dielectric breakdown strength is enhanced by the addition of clay-nanofiller. The dielectric constant (εr) and dissipation factor (Tan δ) have been studied in the frequency range 200 Hz to 2 MHz at room temperature indicating that enhancements have been occurred in εr and Tan δ by the addition of clay-nanofiller in the polymer material when compared with the pure material.
Polymers play an important role for many applications due to their unique properties which can be classified as heat sensitive, flexible, electrically insulating, amorphous, or semi-crystalline materials. For that reason, poly- mers are the most commonly used dielectrics because of their reliability, availability, ease of fabrications, and low cost. The selection of the proper dielectric polymer for a desired application depends on the requirements and operating conditions of the applied system [
The present work focuses on the dielectric properties of polyethylene (PE) nanocomposites. Polyethylene is one of the thermoplastic polyolefin which is traditionally one of the most widely used polymer classes with ap- plications in structural, textile, and packaging industries, and their nanocomposites have found multiple applica- tions for the same uses. This paper shows the preparation and characterization of high density polyethylene com- posed with Na-montmorillonite clay-nanofiller (HDPE/clay) with different concentrations of clay-nanofiller as 0%, 2%, 6%, 10% and 15%. Then the dielectric properties, such as dielectric constant, dissipation factor, dielec- tric breakdown, and insulation resistance, of the prepared samples will be discussed and compared to the base polymer material.
HDPE with melt flow rate of 0.75 g/min and density of 960 kg/m3 is chosen as the base polymer material for the current study. It was manufactured by the International Company for Manufacturing Plastic Products. Sodium montmorillonite clay K10 (MMT) was acquired from fluka chemika company. Hexadecyl Trimethyl Ammo- nium Bromide, modifier or surfactant material, was obtained from Merck KGaA, Darmstadt, Germany.
The preparation of polymer/clay nanocomposites with good dispersion of clay layers within the polymer matrix is not possible by physical mixing of polymer and clay particles. It is not easy to disperse nanolayers in most polymers due to the high face to face stacking of layers in agglomerated tactoids and their intrinsic hydrophilis- ity which make them incompatible with hydrophobic polymers. The intrinsic incompatibility of hydrophilic clay layers with hydrophobic polymer chains prevents the dispersion of clay nanolayers within polymer matrix and causes to the weak interfacial interactions. Modification of clay layers with hydrophobic agents is necessary in order to render the clay layers more compatible with polymer chains, and result in a larger interlayer spacing. In addition, modification process improves the strength of the interface between the inorganic clay and the polymer matrix. So, sodium montmorillonite (Na-MMT) clay was modified with the compatiblizer of Hexadecyl Trimethyl Ammonium Bromide [
100 g of clay was dispersed into 1000 ml of methanol solvent and placed on hot plate with magnetic stirrer to allow continuous stirring for 2 hours. On the other hand, 100 g of hexadecyl trimethyl ammonium bromide was dissolved in 500 ml of methanol. Then the solution was added to clay dispersion. The stirring continued for 72 hours. After that, the modified clay was filtered and collected. Finally the filtrate was dried in a vacuum oven at 70˚C for 6 hours [
The concentrations of modified clay-nanofiller were added as 0%, 2%, 6%, 10%, and 15% into the base polymer material. HDPE/clay nanocomposites were prepared by melt compounding method (master batch method) using twin screw extruder (TSE) at zones temperature 163˚C, 167˚C, and 167˚C, for Zone 1, Zone 2, and Zone 3 re- spectively. The screw speed was maintained at 30 rpm. After extrusion, the dried pellets of nanocomposites were preheated using Morgan Press Injection unit at 160˚C for 30 min and injected to produce test samples with dimensions 7.5 cm × 7.5 cm × 0.25 cm for dielectric measurements [
The prepared samples were characterized by the scanning electron microscope (SEM). SEM is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons in- teract with atoms in the sample, producing various signals that can be detected and that contain information about the sample’s surface topography and composition. The scanning electron microscope images were carried out by using SEM, model Quanta 250 FEG (Field Emission Gun) attached with EDX unit (Energy Dispersive X-ray Analyses), with accelerating voltage 30 kV, magnification 14× up to 1,000,000×, and a resolution of 1 nm.
Thermal stability was measured by using thermo gravimetric analysis (TGA). TGA experiments were done by a shimadzu TA-50 thermal analyzer using scanning rate of 5˚C/min under N2 with 20 ml/min flow rate, from room temperature to 600˚C.
Dielectric breakdown refers to a rapid reduction in the resistance of an electrical insulator when the voltage ap- plied across it exceeds the breakdown voltage. Dielectric breakdown measurements were performed using AC Dielectric Test Set. The samples were sandwiched between two electrodes and tested at room temperature under an ac voltage ramp of 750 V/sec. The ac voltage was increased with a rate of 750 V/Sec until breakdown occurred.
Dielectric constant is called relative permittivity which is a parameter that indicates the relative charge storage capability of dielectrics in the presence of an electric field. The used instrument is an Agilent E4980A LCR me- ter with dielectric sample holder. The equivalent parallel capacitance (Cp) was measured directly by the LCR meter, then the dielectric constant is calculated as shown below in the results section.
Dissipation factor is called loss tangent or Tan δ. It represents the energy loss in the dielectrics and it is pre- ferred to be smaller for insulation materials. It was measured directly by an Agilent E4980A LCR meter with di- electric sample holder in the frequency range 200 Hz to 2 MHz at room temperature.
Also, insulation resistance was measured directly by LCR meter at the same conditions.
The morphology of the SEM images for HDPE with 2% clay, 6% clay, 10% clay, and 15% clay composites is shown in figures 1-4 respectively. Each sample has two images with different magnifications. All SEM images for all samples revealed that, clay was dispersed in polymer matrix very well and there wasn’t any accumulation of clay-nanofiller in it. An important observation is that the thickness of clay content is still in nano-size range (1 - 100 nm). This means that the samples were successfully prepared.
The thermal stability of the prepared samples was measured using thermo-gravimetric analyzer (TGA). In this
technique, the weight loss of the material due to the formation of volatile compounds under degradation because of the heating and temperature rise is monitored.
The data available from TGA is tabulated in table 1 and graphed in figure 5 including T10% (onset tempera- ture), the temperature at which 10% degradation from the sample occurs, T50%, the temperature at which 50% degradation occurs, Tmax, the temperature at which maximum degradation occurs, and residual loss at 600˚C.
According to TGA results as shown in figure 5, the incorporation of MMT to HDPE improved the thermal stability at higher degradation temperature ranges compared to pure HDPE. The temperature of the 10% degra- dation of HDPE 2%, HDPE 6% and HDPE 10% has been shifted to lower temperatures relative to HDPE 0%, while the 10% degradation temperature of HDPE 15% shifted to higher temperatures compared to HDPE 0%. The 50% and maximum degradation temperatures have been shifted to higher temperatures compared to HDPE 0%. This means that, thermal stability has been occurred with increasing the concentration of MMT composed to HDPE. The residual weight of the samples at 600˚C increased with increasing the concentration of clay com- posed to HDPE. Thus, thermal stability of HDPE/clay has been improved compared to pure HDPE.
The dielectric breakdown strength of the composites is analyzed using an AC dielectric test set at room tempera- ture. The test was repeated 5 times for each sample and the average value was recorded and plotted as shown in figure 6.
Samples | T10% (˚C) | T50% (˚C) | Tmax (˚C) | Residual Weight (mg) at 600˚C |
---|---|---|---|---|
HDPE 0% | 403.6 | 451.4 | 478.3 | 0.32 |
HDPE 2% | 403.5 | 451.6 | 479.3 | 0.42 |
HDPE 6% | 396.7 | 459.0 | 481.2 | 1.24 |
HDPE 10% | 399.1 | 463.1 | 484.2 | 1.77 |
HDPE 15% | 405.3 | 464.8 | 485.1 | 4.34 |
Measured quantity was the equivalent parallel capacitance (Cp) of the samples in the frequency range of 200 Hz to 2 MHz, then the dielectric constant (εr) was calculated by the following equations [
where
HDPE/clay composites are prepared by melt compounding method (Master Batch method). Morphology struc- ture (clay dispersion in polymer matrix and the thickness of clay-nanofiller) of the prepared samples is investi- gated by SEM. SEM images show that clay content is well dispersed in the polymer matrix indicating samples are successfully prepared. Thermal stability and dielectric properties are investigated for the prepared samples. TGA results show that HDPE nanocomposites have thermal stability more than unfilled polymer material. Di- electric breakdown strength is improved by the addition of clay-nanofillers. Dielectric constant and dissipation factor are studied at room temperature in the frequency range 200 Hz to 2 MHz. The experimental results show that there is an enhancement in both εr and Tan δ due to the unique behavior of clay-nanofiller when incorpo-
rated into the polymer base matrix HDPE. Also insulation resistance has been improved by the addition of clay- nanofiller. From all results, it can be noticed that 6% filler concentration is the optimum clay content for HDPE/ clay system.
The authors would like to thank the Fire and Explosion Protection Lab at NIS for giving access to the TSE facil- ity used in this research work. The authors are also grateful to Prof. Dr. Mostafa, in Thermometry Lab at NIS, for his help in the TGA measurements.