Materials Selection , Synthesis , and Dielectrical Properties of PVC Nanocomposites

1 Electrical Engineering Department, King Abdulaziz University, Rabigh 21911, Saudi Arabia 2 Electrical Engineering Department, Faculty of Energy Engineering, Aswan University, Sahari City 81528, Egypt 3 Chemical and Materials Engineering Department, King Abdulaziz University, Rabigh 21911, Saudi Arabia 4Chemical Engineering Department, Higher Technological Institute, Tenth of Ramadan City 11111, Egypt 5 Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia


Introduction
Nanocomposites represent a very attractive route to upgrade and diversify properties of the polymers.Nanofiller-filled polymers might be differentiated from microfiller-filled polymers in three major aspects that the nanocomposites normally contain smaller amounts, are in range of nanometers in size, and have tremendously large specific surface area.All these characteristics are reflected in their material properties [1][2][3].Fillers are added to polymeric materials in order to enhance physical and mechanical properties [4][5][6][7].Over the past few years, there have been few numbers of researches on the effect of fillers on di-electric properties of polymers [8][9][10].The shift from ceramic electrical insulating materials (e.g., porcelain and glass) and from oil-paper insulations to polymeric materials has been the major change in the field of high voltage insulation technology [11][12][13].Today polymers are widely used in most of the high voltage equipment, for example, power transformers, insulators, capacitors, reactors, surge arresters, current and voltage sensors, bushings, power cables, and terminations.The huge scenarios of new polymer composites in high voltage technology inspire the researchers of the field to innovate new materials and to study their properties [14][15][16].There is a need for developing a range of compact devices and accessories, for both outdoor and indoor conditions, in which novel and more reliable insulation systems will play the key role [17][18][19].Nanomaterials, in form of polymeric nanocomposites, are foreseen as excellent candidates which are able to fulfill the new requirements [20][21][22].Elemental properties are usually integrated over macroscopic volumes to reach explanations for macroscopic properties [23][24][25][26].The new developments in nanoscience and technology stop short of the final integration and consider what special properties are present at the nanometric level and how they might be exploited [26,27].PVC is widely used in industrial applications.Chemically, PVC has a structure which is similar to that of PE but instead of several hydrogen atoms, it has chlorine atoms, which are attached to the molecular chains at the side in a random manner [28][29][30].Also, it has excellent forming properties, is suitable for extreme thermoforming requirements, chemically resistant, UV-stabilized, weather resistant, and has increased impact resistance [31][32][33].PVC is stronger and more rigid than other general purpose thermoplastic materials.It has a high tensile strength and modulus of elasticity.Additives are used to further specific end use, such as thermal stabilizers, lubricity, impact modifiers, and pigmentation.PVC is basically tough and strong, resists water and abrasion, and is an excellent electrical insulator [34][35][36].

Materials Selection for Electrical Insulation and Mechanical Properties
Using CES Program.Effect of silica and clay particles on the performance of PVC can be governed by the mechanical and electrical properties.CES program (Granta Design Company) was initially used to predict the desired properties of PVC composites using different fillers (microscale)/matrix mixing ratio.Synthesis and manufacturing of PVC composites were carried out based on the obtained results using CES program.

Raw Materials and Equipment. Polyvinylchloride (PVC)
was received from petrochemical company (Sabic, Saudi Arabia).Physical and mechanical properties of PVC are listed in Table 1.Treated nano-montmorillonite clay was purchased from Sigma Aldrich.It is montmorillonite clay (Nanomer 1.30E), clay surface modified with 25-30 wt% octadecylamine.Spherical particle shape is the most important characteristic of nanoclay for polymer applications.The platy nature of clay fillers has a greater effect on properties such as viscosity, stiffness, and strength.Using clay as nanofiller gives high levels of flame retardancy to produced composite.Nanofumed silica powder was obtained from Sigma Aldrich.

Synthesis of Polyvinylchloride
Composites.Two sets of PVC composites were prepared.In the first set, PVC was composited with nano-fumed silica (1-10% wt/wt).Fumed silica was mixed and heated up to 200 ∘ C for 8 min using corotating twin-screw extruder (Berstorff ZE25A, Hannover, Germany) at 300 rpm.The compounded materials were ground and rolled at 185 ∘ C to obtain thin film (thickness of 1 ± 0.01 mm).PVC-fumed silica composites were obtained under 25 MPa and 185 ∘ C for 5 min using hot press.In the second set, polyvinylchloride nanomontmorillonite clay was obtained using typical techniques and operating conditions.

Characterization of PVC Composites.
Electrical and surface analysis of PVC-nanostructured material specimen were carried out.This was achieved by measuring dielectric properties losses, electrical resistivity, and SEM analysis.These tests are able to identify the best combination of polymersnanofillers in addition to the optimum fillers loading, in terms of improved dielectric strength and smaller space charge accumulation.

Electrical Properties.
Dielectric spectroscopy is a powerful experimental method to investigate the dynamical behavior of a sample through the analysis of its frequencydependent dielectric response.This technique is based on the measurement of the dielectric loss constants as a function of frequency of a sample sandwiched between two electrodes.The tan  and susceptance (B) were measured as a function of frequency in the range from 10 Hz to 50 kHz at 25 ∘ C for all test specimens.The measurements were made using high resolution dielectric spectroscopy.

SEM Analysis.
The morphology and dimensions of the PVC composites were tested using scanning electron microscopy.Specimens were cut in liquid nitrogen and then coated with nanogold layers using a sputter coater to make them conductive.

Predictable Mechanical and Electrical
Behavior of PVC-Silica Composites. Figure 1 illustrates the electrical and mechanical properties of PVC-silica composites using CES software.Addition of silica to PVC leads to improving the electrical resistivity of PVC.It is detected that the electrical resistivity can be increased up to 5.0 × 10 10 Ohm⋅m (mean value) using 70% silica wt/wt.This can be attributed to the high electrical resistivity of silica (1.0 × 10 12 -1.0 × 10 13 Ohm⋅m) comparing with lower electrical resistivity value of PVC matrix (3.16 × 10 9 -3.16 × 10 9 Ohm⋅m).Modulus of elasticity of PVC was increased from 3 GPa to 72 GPa in the presence of 70% wt/wt silica.

Predictable Electrical and Mechanical
Behavior of PVC-Clay Composites. Figure 2 illustrates the electrical and mechanical properties of PVC-clay composites using CES software.The initial results using the predictable model (CES software) showed that addition of clay particles to PVC can cause an increase in the electrical resistivity and modulus of elasticity.The clay fillers loading is between 1 and 70% wt/wt.Electrical resistivity was increased from 1.1 × 10 9 to 9 × 10 10 Ohm⋅m (mean value).Modulus of elasticity was improved significantly from 3.190 GPa to 93.30 GPa with respect to clay fillers loading 1 to 70% wt/wt.PVC-nano fumed silica at room temperature (25 ∘ C).It is depicted that the loss tangent of PVC nanocomposites increased with increasing the fumed silica loading.The addition of nano-fumed silica from 5 to 10% wt/wt effectively improved the loss tangent values.However, it can be observed that the loss tangent does not enhance below 5 wt%.This can be attributed to the presence of voids at PVC-nano fumed silica interphase due to low melt flow around the nanofumed silica [36].Electrical resistivity of PVC-nano fumed silica showed higher values than the CES program results (see Figure 3(b)).This can be attributed to the influence of using nanoscale fillers (larger specific area).

Effect of Nanomontmorillonite Clay on PVC Electrical
Properties. Figure 4(a) shows loss tangent as a function of frequency for PVC-nanomontmorillonite clay composites at room temperature (25 ∘ C).It is clear that the loss tangent of PVC nanocomposites decreases with the increasing frequency, while it increases with increasing clay percentage of nanofillers up to 10% wt.This can be attributed to low response of the PVC dipole to follow the system variations at high frequency [37].Figure 4(b) shows resistivity as a function of nano-montmorillonite clay loading at room temperature 25 ∘ C. The electrical resistivity of PVCnanomontmorillonite clay composites increases with increasing nanoclay.It is noticed that the resistivity was decreased below 1% nanomontmorillonite clay which has the same behavior as PVC-nano fumed silica composite.

SEM Analysis.
Microstructure studies were carried out in order to detect voids or agglomerates which can be formed through polymer composite processing.SEM images

Conclusions
As the electrical insulation of PVC composites contribute to its tan delta value, the variation of tan delta value in net PVC nanocomposites in lower frequency range may result in the electrical insulation of the nanocomposites having been affected by the presence of nanosize fillers.As this study was carried out under constant temperature, the influence of the relaxation time of the charge carriers on the electrical insulation of PVC nanocomposites can be ignored.Thus, the number of charge carriers and applied frequency become dominating factors of the electrical insulation of PVC nanocomposites.The presence of nanosize fillers inside PVC will restrict the chain mobility and result in increasing electric insulation as such restriction limited the generation of mobile charge and the movement of charge carriers in polymer dielectrics, especially at a lower frequency range where the insulation will play an important role.Thus, the variation of tan delta value at low frequency range may be due to the influence of inorganic fillers' electrical insulation.

Table 1 :
Physical and mechanical properties of polyvinyl chloride.