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 oC for two months.
A. 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 (TiO2 and SiO2). The evaluation is carried out before and after thermal aging.
Figure 12 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 polystyrene-silicone rubber blends. The same behavior is obtained after the thermal aging, where the breakdown strength is maximum 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 polystyrene-silicone 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 life time 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 TiO2 nanoparticles. The breakdown strength of polystyrene-silicone rubber nanocomposites is decreased by the thermal aging for all filler loadings of TiO2. The results show that, the thermal aging is not affecting the optimal filler loading (2%wt) of TiO2. 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 SiO2 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 kV/mm to 28.1 kV/mm with a percentage reduction of 37%. The maximum improvement in breakdown strength before aging is obtained by adding SiO2 compared to TiO2. However, after thermal aging, the maximum enhancement is obtained by adding TiO2 compared to SiO2.
From the obtained results, the breakdown strength of the 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 to polystyrene-silicone rubber. Adding nanofillers to the polystyrene-silicone rubber can withstand the weakening molecular bonds under thermal aging.
B. Dielectric Response under Thermal Aging
In this section, the effect of thermal aging on the relative permittivity and dissipation factor of polystyrene-silicone 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 TiO2 and SiO2. The measurement of the relative permittivity and the dissipation factor is carried out after each 14 days.
The effect of thermal aging on the relative permittivity of polystyrene-silicone rubber blends is shown in Fig. 14. Under thermal aging, the relative permittivity of polystyrene-silicone 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 stabilized to day 28. Then, the dissipation factor increases by increasing the aging time considering all polystyrene concentration levels. Also, the dissipation factor is enhanced by adding 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 TiO2 is declared in Fig. 16. From this figure, it can be seen that, the relative permittivity of polystyrene-silicone rubber nanocomposites decreases under thermal aging until 28 days. Then, it increases by increasing the thermal aging time. The results show that, adding TiO2 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 TiO2 nanofillers with 3%wt filler loading. The same behavior is obtained when adding SiO2 nanofillers as declared in Fig. 17. The relative permittivity of polystyrene-silicone rubber nanocomposites decreases under thermal aging until 28 days and then increases by increasing the aging time. Adding SiO2 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 TiO2 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 stabilized to day 28. Then, the dissipation factor increases by increasing the aging time considering all filler loadings. Figure 19 shows the effect of thermal aging on the dissipation factor of polystyrene-silicone rubber nanocomposites considering SiO2. The dissipation factor of polystyrene-silicone rubber nanocomposites decreases by increasing the aging time until 14 days and stabilized to day 28. Then, the dissipation factor increases by increasing the aging time considering all filler loadings. The dissipation factor is low when adding SiO2 nanoparticles with filler loadings of 2 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 decrease in the relative permittivity is attributed to the rearrangement of molecules. Also, the increase of thermal aging time can reduce the polymer viscosity which results in enhancing the molecular polarization. This increases the relative permittivity with increasing the thermal aging time. 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. However, the increase in dissipation factor when the aging time exceeds 28 days is attributed to the increase in electrical conductivity with the increase in the thermal aging time.