Rigid amorphous fraction caused by isothermal crystallization in ATH filled silicone rubber

Alumina trihydrate (ATH) filled silicone rubber is widely used as materials of outdoor insulators in transmission lines. The lowest temperature of regions covered by transmission lines in some extreme cold areas can be below the melt crystallization temperature of ATH filled silicone rubber. In this paper, broadband dielectric spectroscopy (BDS) was used to observe internal structures of ATH filled silicone rubber during and after crystallization. Relaxations related to molecular segments constrained by lamella crystal and motions of mobile amorphous fraction were confirmed. With the decrease of the temperature, the content of rigid amorphous fraction and mobile amorphous fraction decreased. The fragility index of mobile amorphous fraction was higher than rigid amorphous fraction, indicating that the cooperative rearrangement ability in mobile amorphous fraction was greater. With the increase of isothermal crystallization time, epsilon' at 50 Hz increased and then became stable due to growth of crystallization, while epsilon" at 50 Hz decreased due to increased difficulty for the ions to accumulate on the ATH surface. It was also found that the glass transition temperature was about -105 at 50 Hz. This paper can provide guidelines for outdoor insulators used in extreme cold conditions.