Polystyrene/silicone rubber blends with improved dielectric properties

In this study, the dielectric properties of unfilled as well as nanofilled polystyrene-silicone rubber blends are evaluated. Accordingly, the preparation of polystyrene-silicone rubber blends is carried out considering three different concentrations of polystyrene (3, 7 and 10%wt). Also, the preparation of polystyrene-silicone rubber nanocomposites is carried out considering different two types of nanoparticles. These nanoparticle types are TiO2 and SiO2 with the same average particle size of 20 nm. Different nanoparticle concentration levels of 1.5, 2 and 3%wt are studied. The effect of thermal aging on the dielectric properties of unfilled as well as nanofilled polystyrene-silicone rubber samples is evaluated. The samples are subjected to thermal aging for two months at a temperature of 130 °C. The average breakdown strength as well as the breakdown strength at 10% and 50% probabilities is evaluated before and after the thermal aging process. The dielectric response includes relative permittivity and dissipation factor which measured and evaluated under thermal aging. The obtained results show that adding small amount of polystyrene to silicone rubber improves its dielectric properties. Finally, adding small amounts of TiO2 and SiO2 nanoparticles to polystyrene-silicone rubber blends give more improvements in its dielectric properties.


Introduction
In recent years polymer blend technologies have attracted an increased amount of attention. This is predominantly due to two main reasons. Firstly, with the polymer blend technologies, materials can be manufactured with the desired properties that are not easy to be achieved with intrinsic polymers. The second reason is the possibility to obtain the required materials with a reduced research and a minimum cost. Furthermore, the polymer blend technologies can develop a wide range of materials with enhanced electrical, mechanical, thermal and other properties [1][2][3].
In fact, nanotechnology is extensively used to improve dielectric properties of polymer materials. In [4][5][6], SiO 2 nanoparticles are added to intrinsic Polyvinyl Chloride B Amr M. Abd-Elhady am_elbaraka@yahoo.com 1 High Voltage and Dielectric Materials Lab., Faculty of Engineering, Menoufia University, Shebin El_Kom 32511, Egypt 2 Faculty of Technological Industry and Energy, Delta Technological University, Quwaysna, Egypt 3 South Delta for Electricity Distribution Company, Cairo, Egypt (PVC) and its breakdown strength is improved. Another properties for PVC are evaluated in [7,8] with the addition of titanium oxide (TiO 2 ) nanofillers to the base matrix. These properties include electrical conductivity, material permittivity and the breakdown strength. From these researches, a reduction in PVC conductivity as well as material permittivity is noticed. Also, the breakdown strength of the intrinsic PVC is increased. Regarding the preparation process, it is found that application of horizontal as well as vertical electric field during PVC nanocomposite preparation gives more improvements in its dielectric properties [9]. However, it is noticed that application of horizontal field gives a better improvement compared to the vertical one. The paper authors refer this effect to the role of the applied electric field in giving more uniform distribution of nanofillers in the polymer matrix. Also, in [10][11][12], it is found that adding nanofillers to epoxy resin gives an increased breakdown strength compared to microfillers. Considering the researches [13][14][15], it is found that adding Magnesium oxide (MgO) and zirconium oxide (ZrO 2 ) nanofillers to polyethylene improves its dielectric properties. In [16,17], it is found that the breakdown strength for both polyamide and polyvinylidene fluoride (PVDF) are improved when mica nanosheets are added their matrices. In [18,19], a better performance of silicone rubber (SiR) considering its dielectric properties as well as the tree propagation through its matrix is achieved through the addition of nanofillers.
On the other hand, the rapid growth of nanotechnology opens a new opportunity to improve the properties of polymer blends. Adding nanoparticles to polymer blends (polymer blends nanocomposites) can change the polymer-polymer interface. This can affect the dielectric properties of the polymer blends. In fact, very few researches have studied the polymer blends nanocomposites. In [20], the electrical properties of Silicone rubber/EPDM/clay have been evaluated. They have shown that adding clay nanoparticles to silicone rubber/EPDM blends enhances the polarization capability, breakdown strength, and volume resistivity of the composite. The mechanical properties of PVC/poly (ethylmethacrylate) nanocomposites have been evaluated in [21]. They have shown that modifying PVC with small amounts of poly (ethylmethacrylate) and montmorillonite nanoparticles significantly improves its mechanical properties.
Silicone rubber is one of the most important insulating materials in a lot of engineering fields. It is widely used in high voltage cable accessories due to its excellent electrical, mechanical, thermal and other properties [19]. The use of such materials with enhanced dielectric properties can acquire many benefits which are very important in electric cable applications. Therefore, the development of silicone rubber with enhanced dielectric materials by polymer blend technology incorporated with the nanosized particles is the main objective of this study. Practically, power cables as well as its accessories are subjected to thermal aging under loading or heavy loading conditions. So, the evaluation of the dielectric properties of the developed materials under thermal aging is very important.
In this study, the evaluation of dielectric properties of unfilled as well as nanofilled polystyrene-silicone rubber blends is carried out. The evaluation is carried out by adding small amounts of polystyrene to silicone rubber. Different concentrations of polystyrene (3, 7, 10%wt) are considered and evaluated in this study. Also, dielectric properties evaluation of nanofilled polystyrene-silicone rubber blends is carried out considering two different types of nanoparticles (TiO 2 and SiO 2 ) with the same average particle size of 20 nm. The impact of nanoparticle concentrations is investigated considering different concentration levels of 1.5, 2, and 3%wt. All the prepared samples are subjected to thermal aging for two months at a temperature of 130°C. The relative permittivity and dissipation factor for the prepared samples are measured after each two weeks. Finally, average breakdown and breakdown strength at 10% and 50% probabilities are evaluated before and after the thermal aging.

Experimental work
In this section, the preparation of polystyrene-silicone rubber blend samples is carried out considering different polystyrene concentration levels. Also, the preparation of nanofilled polystyrene-silicone rubber blends considering different nanofiller types at different nanofiller concentrations is carried out. The characterization of the prepared nanocomposite samples is carried out using Field Emission Scanning Electron Microscopy (FESEM). Finally, the breakdown strength and dielectric response test are presented. Figure 1 shows the preparation procedures of unfilled and nanofilled polystyrene-silicone rubber samples. Firstly, an amount of liquid silicone rubber (25 g) is mixed with its hardener. At the same time, an amount of polystyrene granules is added to the mixture. In order to reduce the viscosity of the mixture, Ethyl Methyl Ketone solvent is added. The mixture is stirred for 60 min at 1000 rev/min. The mixture is molded in a petri-dish and leaved in a vacuum chamber at room temperature for about 20 days. The above procedures are repeated considering the different concentrations of polystyrene (3,7, and 10%wt). The properties of the adopted raw materials are shown in Tables 1 and 2 for silicone rubber and polystyrene, respectively.

Sample preparation
In order to study the effect of nanoparticle types on the dielectric properties of polystyrene-silicone rubber blends, the preparation of polystyrene-silicone rubber nanocomposite samples is carried out. The preparation of polystyrenesilicone rubber nanocomposite samples is carried out considering only polystyrene-silicone blend with a polystyrene concentration level of 3% which gives the maximum percentage improvement in breakdown strength as will be shown in the next sections. An amount of nanoparticles is added to the mixture of polystyrene (3%), liquid silicone, hardener and Ethyl Methyl Ketone solvent. In this study, the adopted nanoparticle types are TiO 2 and SiO 2 with the same average particle size of 20 nm and they have relative permittivities of 100 and 4.3, respectively [19]. The adopted nanoparticles and chemicals are purchased from Sigma-Aldrich Company. The mixture is stirred for 60 min. Then, it is subjected to ultrasonic waves for other 30 min using an ultrasonic homogenizer, Model UP400S. The adopted ultrasonic homogenizer rating is 400 watts, 24 kHz. The device has a sonotrode having a tip diameter 22 mm. The preparation of the nanocomposite is carried out by adjusting the homogenizer on a full cycle mode. The nanocomposite samples are prepared considering different concentrations of TiO 2 and SiO 2 at concentrations of 1.5, 2 and 3%wt. Finally, the prepared samples are subjected to thermal aging for two months at a temperature of 130°C.

Sample characterization
In this section, the characterization of unfilled and nanofilled polystyrene-silicone rubber samples is carried out. This is carried out to examine the dispersion of polystyrene as well as nanoparticles in the prepared polystyrene-silicone rubber samples. The characterization is performed using Field Emission Scanning Electron Microscopy (FESEM). Different samples are selected to be characterized. The selected samples are polystyrene-silicone rubber with polystyrene concentration levels of 3%wt and 10%wt. Also, two specimens of nanofilled polystyrene-silicone rubber samples considering 2%wt filler loading of TiO 2 and SiO 2 are selected. Figure 2 shows FESEM images of the selected samples. From this figure, it can be seen that a quietly good dispersion of polystyrene in silicone rubber is achieved as declared in Fig. 2a. However, agglomerations are observed by increasing the polystyrene concentration level as shown in Fig. 2b considering a polystyrene level of 10%wt. In both polystyrene concentrations edged particles are recognizable which may be attributed to the styrene phase. In the case of 2%wt filler loading of TiO 2 and SiO 2 , a good dispersion is achieved as shown in Fig. 2c and d, respectively.

Breakdown strength test
AC Breakdown strength test of polystyrene-silicone rubber samples is carried out according to ASTM D149 standard. The applied voltage rate rise is kept at 500 V/s. The frequency of the applied voltage is 50 Hz. The sample is inserted between two mushroom electrodes. In order to prevent creeping flashover, the sample is immersed in transformer oil during breakdown strength measurement. The average breakdown strength of ten breakdown values is computed. The breakdown strengths at 10% and 50% probabilities are also The breakdown strength test of the samples before and after thermal aging is carried out at room temperature.

Dielectric response test
Relative permittivity and dissipation factor are important dielectric properties. These two properties determine the condition of the insulating materials. Therefore, the dielectric response includes both relative permittivity and dissipation factor of the unfilled and nanofilled polystyrene-silicone rubber blends is evaluated. The measurements of relative permittivity and dissipation factor are taken at 2 V considering frequencies up to 2 MHz. The measurements of both relative permittivity and dissipation factor are taken using an accurate inductance, capacitance, and resistance (LCR) meter Model Keysight E4980A. The basic accuracy of the adopted LCR meter is 0.05%. The measurements of relative permittivity and dissipation factor are taken according to ASTM D150-98 which uses a standard three-electrode test cell.

Evaluation of polystyrene-silicone rubber blends
In this section, breakdown strength of polystyrene-silicone rubber (SiR) blends is evaluated. Also, dielectric response of polystyrene-silicone rubber blends considering the relative permittivity and dissipation factor is presented. The evaluation is carried out considering the effect of polystyrene concentration levels.

Breakdown strength
Here, the evaluation of breakdown strength of polystyrenesilicone rubber blends is carried out considering different concentration levels of polystyrene. The studied concentration levels are 3, 7 and 10%wt. The breakdown strength is evaluated considering average breakdown strength as well as breakdown strengths at 10% and 50% probabilities. Figure 3 shows the effect of polystyrene concentration on the average breakdown strength of polystyrene-silicone rubber blends. The results show that adding polystyrene to silicone rubber significantly affects the average breakdown strength. Hence, the breakdown strength is increased from 23.4 kV/mm for base silicone rubber to 30.6 kV/mm by adding 3%wt polystyrene. The percentage increase in average breakdown strength is 30.7% at 3%wt of polystyrene concentration level. The average breakdown strength is then decreased by increasing polystyrene concentration level as declared in Fig. 3. The breakdown strengths at 10% and 50% probabilities of polystyrene-silicone rubber blends are  increased from 20.9 and 23 kV/mm to 24 and 31 kV/mm, respectively, by adding 3%wt polystyrene as reported in Table 3. When the concentration level of polystyrene is beyond 3%wt, the breakdown strengths at 10% and 50% are also decreased by increasing polystyrene concentration. From the obtained results, the maximum breakdown strength improvement occurs at a polystyrene concentration level of 3%wt, and then it decreases by increasing the polystyrene level. The increase in breakdown strength by adding polystyrene is due to the fact that the breakdown strength of polystyrene is higher than the base silicone rubber. However, the decrease in breakdown strength by increasing the polystyrene concentration comes due to the agglomeration of polystyrene which occurs at high polystyrene concentration levels. From this figure, it can be seen that low agglomeration of polystyrene occurs at 3%wt polystyrene concentration level as shown in Fig. 2a. However, the agglomeration is increased by increasing the polystyrene concentration level as illustrated in Fig. 2b considering 10%wt polystyrene level. The reduction in breakdown strength at high concentration levels of polystyrene comes due to the distortion in electric filed inside the material as a result of agglomerations effect. The distortion in electric field comes due to the difference in permittivities between SiR and polystyrene. Hence, the difference in permittivities

Dilectric response
In this section, relative permittivity and dissipation factor of polystyrene-silicone rubber blends are evaluated. The evaluation is carried out considering the adopted concentration levels of polystyrene. Figure 4 shows the effect of polystyrene concentration on the relative permittivity of polystyrenesilicone rubber blends. From this figure, it is observed that adding polystyrene to silicone rubber decreases its relative permittivity. Relative permittivity of polystyrene-silicone rubber blends is decreased by increasing polystyrene concentration levels at the adopted frequency range. The reason behind this comes as polystyrene has a relative permittivity much lower than the base silicone rubber which decreases the polarization in the polymer blends. Therefore, the relative permittivity of the blend decreases by increasing the polystyrene level in the silicone rubber. Figure 5 shows the effect of polystyrene concentration on dissipation factor of polystyrene-silicone rubber blends. The results show that the concentration level of polystyrene is quietly affecting the dissipation factor of polystyrene-silicone rubber blends especially at 7% and 10% concentration levels. However, the dissipation factor at high frequencies increases by adding 3% polystyrene. This comes due to the increase of conductive pathway by adding polystyrene to silicone rubber. Hence, it is well known that the increase in conductive pathway in polymer blends increases the dissipation factor. From the aforementioned results reported in Subsect. 3.1, adding polystyrene with a concentration level of 3%wt gives the maximum percentage improvement in breakdown strength. In our opinion, further improvement in breakdown strength of the blend can be achieved using nanofillers. Therefore, the effect of adding nanofillers to polystyrenesilicone rubber blend is evaluated considering the 3%wt concentration level. This point is covered in the next section.

Evaluation of polystyrene-silicone rubber nanocomposites
In this section, breakdown strength of polystyrene-silicone rubber nanocomposites is evaluated considering two different nanofiller types at different nanofiller loadings. Also, the dielectric response of polystyrene-silicone rubber nanocomposites is evaluated. The polystyrene concentration is kept at 3%wt.

Breakdown strength
The evaluation of breakdown strength of polystyrenesilicone rubber nanocomposites is carried out considering the effect of nanofiller types. The adopted nanofiller types are TiO 2 and SiO 2 . Different nanofiller loadings are studied. These filler loadings are 1.5, 2 and 3%wt. The evaluation is carried out considering average breakdown strength as well as the breakdown strengths at 10% and 50% probabilities.  Figure 6 shows the effect of nanofiller types as well as nanofiller loadings on the breakdown strength of polystyrene-silicone rubber nanocomposites. From this figure, it can be seen that the average breakdown strength of base polystyrene-silicone rubber (3%wt polystyrene) is 30.6 kV/mm. The average breakdown strength is increased by increasing the filler loading until the optimal filler loading of 2%wt for both nanofiller types (TiO 2 and SiO 2 ) is achieved. Then, the average breakdown strength is decreased by increasing the filler loading. The optimal average breakdown strengths of polystyrene-silicone rubber modified by TiO 2 and SiO 2 are 42.4 and 44.5 kV/mm, respectively, at the same optimal filler loading of 2%wt. Therefore, the maxima percentage increases in average breakdown strengths are 38.5% and 45.4% considering TiO 2 and SiO 2 , respectively. Comparing the average breakdown strength of the base silicone rubber (23.4 kV/mm), the maximum percentage increases in average breakdown strengths are 81% and 90% when adding TiO 2 and SiO 2 , respectively, in addition to adding 3%wt of polystyrene. Therefore, the maximum enhancement is obtained by adding SiO 2 compared to TiO 2 .
The breakdown strengths of polystyrene-silicone rubber nanocomposites at 10% and 50% probabilities are reported in Tables 4 and 5. The results show that the breakdown strength at 10% probability is increased by increasing the filler loading until the optimal filler loading of 2%wt for both nanofiller types as reported in Table 4. Then, the breakdown strength is decreased by increasing the filler loadings of TiO 2 and SiO 2 . The breakdown strength at 50% probability is increased by increasing the filler loadings until the same optimal loading of 2%wt is achieved as reported in Table 5. Then, the breakdown strength is decreased by increasing the filler loading for the two nanofiller types. From the obtained results, the maximum improvement in breakdown strength is obtained by adding SiO 2 compared to TiO 2 considering average breakdown strength as well as the breakdown strengths at 10% and 50% probabilities. From the obtained results, the breakdown strength is increased by increasing the filler loading until the optimal filler loading of 2%wt, and then it decreases by increasing the filler loading. The increase in breakdown strength by increasing the filler loading is related to the charge trapping process of the nanofillers [22,23]. The decrease in breakdown strength when the filler loading exceeds the optimal value is due to the agglomeration of nanoparticles that occurred when the filler loading is increased [19,22]. Also, the results show that the maximum improvement in breakdown strength is obtained by adding SiO 2 compared to TiO 2 . This is related the relative permittivity of the nanoparticle itself. The relative permittivity of SiO 2 nanoparticle is much lower than the relative permittivity of TiO 2 . The presence of nanoparticle with high relative permittivity increases the electric field strength at the nanoparticle/polymer interface. As a result, local discharges at the nanoparticle/polymer interface occur. This gives a lower percentage improvement in breakdown strength with adding nanoparticles with high relative permittivity such as TiO 2 . On the other hand, adding nanoparticles with low relative permittivities decreases the electric field strength at nanoparticle/polymer interface. So, the electric field at nanoparticle/solid interface is more uniform and this results in reduced local discharges at these interfaces. This gives a higher improvement in breakdown strength with adding nanoparticles with low relative permittivities such as SiO 2 .

Dielectric response
In this section, the effect of nanofiller types on relative permittivity and dissipation factor of polystyrene-silicone rubber nanocomposites is evaluated. The studied nanofiller types are also TiO 2 and SiO 2 . The different adopted filler loadings are investigated. The evaluation is carried out at a polystyrene concentration level of 3%wt. Figure 7 shows the effect of nanofiller loading of TiO 2 on the relative permittivity of polystyrene-silicone rubber nanocomposites. From this figure, it can be seen that adding TiO 2 nanofillers to polystyrene-silicone rubber decreases its relative permittivity especially at low filler loadings (e.g., 1.5%wt). Then, the relative permittivity is increased by increasing the filler loading of TiO 2 . Similarly, the relative permittivity of polystyrene-silicone rubber is decreased by adding SiO 2 at low filler loading of 1.5%wt as declared in Fig. 8. However, the relative permittivity is decreased by increasing the filler loading. The decrease in relative permittivity at low filler loadings comes due to the restriction of molecular movement results from the presence of interaction zones between the nanoparticles and polystyrene-silicone rubber blends. However, the increase in relative permittivity at the higher filler loadings is related to the effect of the relative permittivity of nanoparticles itself. The relative permittivities of the adopted nanoparticles are higher than relative permittivity of polystyrene-silicone rubber blends. Therefore, the relative permittivity of polystyrene-silicone rubber nanocomposites is high at the higher filler loadings. Figure 9 shows the effect of filler loadings of TiO 2 on the dissipation factor of polystyrene-silicone rubber nanocomposites. The results show that adding TiO 2 to polystyrenesilicone rubber decreases its dissipation factor. The dissipation factor is decreased by increasing the filler loading as  Fig. 9 Effect of adding TiO 2 on the dissipation factor of polystyrenesilicone rubber nanocomposites declared in Fig. 9. However, adding SiO 2 nanoparticles to polystyrene-silicone rubber increases its dissipation factor as illustrated in Fig. 10. The increase or the decrease in dissipation factor by increasing the nanoparticles filler loadings of SiO 2 and TiO 2 comes due to the effect of the conductivity of the blend. Adding SiO 2 (semiconducting nanoparticles) increases the conductivity of the polystyrene-silicone rubber as compared to TiO 2 . The increase in nanoparticle conductivity results in an increase in the conductivity of blends as well as increasing the dissipation factor as occurred by adding SiO 2 . In contrast, the decrease in nanoparticle conductivity results in a decrease in the conductivity of blends as well as decreasing the dissipation factor as declared by adding TiO 2 . Practically, the loaded or overloaded power cables are subjected to thermal stresses. Under long time, thermal stresses can deteriorate the insulation of power cables as well as its accessories. Therefore, the current proposed insulating materials is evaluated under thermal aging in the next section.

Evaluation of polystyrene-silicone rubber blends and nanocomposites under thermal aging
In this section, the dielectric properties include breakdown strength as well as the dielectric response of unfilled and nanofilled polystyrene-silicone rubber blends are evaluated under thermal aging. The evaluation is carried out considering different polystyrene concentration, nanofiller types, and nanofiller loadings. Finally, the thermal aging is carried out considering a temperature of 130°C for two months.

Breakdown strength under thermal aging
The effect of thermal aging on the average breakdown strength of unfilled and nanofilled polystyrene-silicone rubber blends as well as polystyrene-silicone rubber nanocomposites is evaluated considering the adopted two nanofiller types (TiO 2 and SiO 2 ). The evaluation is carried out before and after thermal aging. Figure 11 shows the effect of thermal aging on the breakdown strength of polystyrene-silicone rubber blends considering different concentration levels of polystyrene. From this figure, it is observed that the thermal aging is significantly affecting breakdown strength of polystyrenesilicone rubber blends. The same behavior is obtained after the thermal aging, where the breakdown strength is higher at the concentration of 3%wt polystyrene. Then, the breakdown strength is decreased by increasing the concentration of polystyrene. The average breakdown strengths before and after thermal aging at the optimal concentration of polystyrene (3%wt) are 30.6 and 23.6 kV/mm, respectively. The percentage reduction in average breakdown strength after aging is 22.9%. Also, the breakdown strength of the base silicone rubber before aging (23.4 kV/mm) and polystyrenesilicone rubber blends with 3%wt polystyrene (23.6 kV/mm) after thermal aging is almost the same. This means that adding 3%wt polystyrene to silicone rubber can increase the lifetime of the polystyrene-silicone rubber blends as well as electric cable accessories. Figure 12 shows the effect of thermal aging on the breakdown strength of polystyrene-silicone rubber nanocomposites considering TiO 2 nanoparticles. The breakdown strength of polystyrene-silicone rubber nanocomposites is decreased by the thermal aging for all filler loadings of TiO 2 . The results show that the thermal aging is not affecting the optimal filler loading (2%wt) of TiO 2 . The breakdown strength at the optimal filler loading is decreased from 42.4 kV/mm (before thermal aging) to 37.8 kV/mm (after thermal aging). So, the percentage reduction in breakdown strength at the optimal filler loading is 10.8%. Figure 13 shows the effect of thermal aging on the breakdown strength of polystyrene-silicone rubber nanocomposites considering SiO 2 nanoparticles. From this figure, it can be seen that the optimal filler loading is 2%wt before thermal aging and is changed to 1.5%wt after thermal aging. The breakdown strength at 2%wt is significantly decreased after thermal aging. It is decreased from 44.5 to 28.1 kV/mm with a percentage reduction of 37%. The maximum improvement in breakdown strength before aging is obtained by adding SiO 2 compared to TiO 2 . However, after thermal aging, the maximum enhancement is obtained by adding TiO 2 compared to SiO 2 . From the obtained results, the breakdown strength of polystyrene-silicone rubber blends as well as nanocomposites decreases after thermal aging. This is observed considering all polystyrene concentration levels as well as the two nanofiller types and at all concentration levels. The decrease in breakdown strength is caused by weakening molecular bonds of the polymer samples under thermal aging. Also, the results show that the percentage reduction in breakdown strength is lower than it in the nanofilled polystyrene-silicone rubber compared to the unfilled polystyrene-silicone rubber. This can be attributed to the effect of adding nanofillers

Dielectric response under thermal aging
In this section, the effect of thermal aging on the relative permittivity and dissipation factor of polystyrenesilicone rubber blends as well as polystyrene-silicone rubber nanocomposites is evaluated. The evaluation is carried out considering different polystyrene concentration levels and different nanofiller loadings of TiO 2 and SiO 2 . The measurements of the relative permittivity and the dissipation factor are taken after each 14 days. The recorded values are captured at a frequency of 1 MHz to compare the performance of all samples at the same frequency. The effect of thermal aging on relative permittivity of polystyrene-silicone rubber blends is shown in Fig. 14. Under thermal aging, the relative permittivity of polystyrenesilicone rubber blends for all concentrations of polystyrene decreases until 28 days. Then, the relative permittivity of polystyrene-silicone rubber blends increases by increasing the thermal aging time. Also, the relative permittivity of polystyrene-silicone rubber blends decreases by increasing the polystyrene concentration for all aging period as declared in Fig. 14. The effect of thermal aging on the dissipation factor of polystyrene-silicone rubber blends is illustrated in Fig. 15. The results show that the dissipation factor of polystyrene-silicone rubber blends decreases by increasing the aging time until 14 days and increases again to the end of the test period considering all polystyrene concentration levels. Also, the dissipation factor is enhanced by adding  16 Effect of thermal aging on the relative permittivity @ 1 MHz of polystyrene-silicone rubber nanocomposites considering TiO 2 nanoparticles polystyrene compared to the base silicone rubber considering all aging period. Regarding the polystyrene-silicone rubber nanocomposites, the effect of thermal aging on the relative permittivity considering TiO 2 is declared in Fig. 16. From this figure, it can be seen that the relative permittivity of polystyrenesilicone rubber nanocomposites decreases under thermal aging until 28 days. Then, it increases by increasing the thermal aging time. The results show that adding TiO 2 nanfillers with concentrations of 1.5 and 2%wt is low compared to the base polystyrene-silicone rubber blends considering all aging period. However, the relative permittivity is quiet high by adding TiO 2 nanofillers with 3%wt filler loading. The same behavior is obtained when adding SiO 2 nanofillers as  Fig. 18 Effect of thermal aging on the dissipation factor @ 1 MHz of polystyrene-silicone rubber nanocomposites considering TiO 2 declared in Fig. 17. The relative permittivity of polystyrenesilicone rubber nanocomposites decreases under thermal aging until 28 days and then increases by increasing the aging time. Adding SiO 2 nanfillers with concentrations of 1.5 and 3%wt is low compared to the base polystyrene-silicone rubber blends considering all aging period. However, the relative permittivity is quiet increased especially in the last aging period.
The effect of thermal aging on the dissipation factor of polystyrene-silicone rubber nanocomposites considering TiO 2 is illustrated in Fig. 18. From this figure, it can be seen that the dissipation factor of polystyrene-silicone rubber nanocomposites decreases by increasing the aging time until 14 days and increases again to the end of the test period considering all filler loadings. Figure 19 shows the effect of thermal aging on the dissipation factor of polystyrenesilicone rubber nanocomposites considering SiO 2 . The dissipation factor of polystyrene-silicone rubber nanocomposites decreases by increasing the aging time until 14 days. Then, the dissipation factor increases by increasing the aging time considering all filler loadings. The dissipation factor is low when adding SiO 2 nanoparticles with filler loadings of 2%wt and 3%wt compared to the base polystyrene-silicone rubber blends especially at the end of aging period. From the aforementioned results, the relative permittivity decreases at the first 28 days of thermal aging then, it increases by increasing the thermal aging time. This is declared in all the prepared samples. The obtained behavior of the relative permittivity comes due to the process of depolarization and repolarization during thermal aging. At the first period of aging, the degree of depolarization is higher than the degree of repolarization. However, this process is reversed at the last period of aging [24]. In the case of dissipation factor, the aging time decreases the dissipation factor until 28 days then, it increases by increasing the aging time. The decrease in dissipation factor at the first 28 days comes due to the decrease in moisture content by increasing the aging time. The decrease in moisture content in polymeric materials with thermal aging is reported in [23]. Also, during the initial stages of thermal aging, low molecular weight materials gradually precipitate under the influence of heat, and internal re-crosslinking reactions of the material predominate. This causes a reduction in the number and mobility of carriers, which would result in a decrease in material conductivity [25]. However, the main chain, side chain, crosslinking bonds of composite, as well as a significant amount of free radicals and polar groups, are broken as the thermal aging process is prolonged, increasing the degree of thermal deterioration inside the composite material group. This significantly increases the material conductivity and significantly increases the number and mobility of carriers [25].

Conclusions
In this study, the evaluation of unfilled and nanofilled polystyrene-silicone rubber blends has been carried out. Different concentrations of polystyrene (3, 7 and 10%wt) have been evaluated. Also, relative permittivity and dissipation factor have been studied. The evaluation of nanofilled polystyrene-silicone rubber blends has been carried out considering two different types of nanoparticles (TiO 2 and SiO 2 ) at different filler loadings. Finally, the effect of thermal aging on the dielectric properties of the prepared samples has been investigated. The studied aging period is about two months at aging temperature of 130°C. From the obtained results, the following points are concluded.
1. Adding small amount of polystyrene (3%wt) to silicone rubber gives the maximum improvement in breakdown strength. 2. The relative permittivity of polystyrene-silicone rubber decreases by increasing the polystyrene concentration levels. Also, the dissipation factor quietly increases by adding polystyrene to silicone rubber. 3. Adding nanoparticles (TiO 2 or SiO 2 ) to polystyrenesilicone rubber results in an increase in breakdown strength. The maximum improvement in breakdown strength is obtained by adding SiO 2 compared to TiO 2 considering the non-aged polystyrene-silicone rubber nanocomposites. 4. The relative permittivity of polystyrene-silicone rubber nanocomposites decreases at low filler loadings of TiO 2 as well as SiO 2 . 5. The dissipation factor is significantly decreased by adding TiO 2 nanoparticles. However, adding SiO 2 to polystyrene-silicone rubber nanocomposites results in an increase in the dissipation factor especially at the frequencies below 1.5 MHz. 6. Under thermal aging, the breakdown strength of polystyrene-silicone rubber blends decreases. Before and after thermal aging, the maximum percentage improvement is obtained at 3%wt concentration level of polystyrene. 7. Relative permittivity decreases by increasing the aging time until 28 days then, it increases by increasing the aging time for the studied two nanoparticle types (TiO 2 and SiO 2 ). Considering the studied aging period, the relative permittivities by adding TiO 2 and SiO 2 are low compared to the base polystyrene-silicone rubber blends.
8. Under thermal aging, the maximum improvement in breakdown strength is obtained by adding TiO 2 compared to SiO 2 .
Author contributions AMA-E, MEI and AS wrote the main manuscript text and Amira Sleet prepared figures. All authors reviewed the manuscript Funding No funding is applicable.
Data availability This declaration is not applicable.