Molecular relaxation and glass transition properties of epoxy resin at high temperature
Authors: Lin Sheng-Jun  , Huang Yin  , Xie Dong-Ri  , Min Dao-Min  , Wang Wei-Wang  , Yang Liu-Qing  , Li Sheng-Tao  ,   High Voltage Switchgear Insulating Materials Laboratory of State Grid, Pingao Group Co. LTD., Pingdingshan 467001, China;,   State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Epoxy resin is widely used as a polymeric insulating material in power equipment, such as gas-insulated switchgear and gas-insulated lines. The motions of molecular chains or segmental chains in a polymeric insulating material can affect the material properties, such as dielectric relaxation, charge transport, breakdown, and glass transition temperature. Molecular or segmental chains may form dipoles, and their motions can contribute to dielectric relaxation properties. Molecular or segmental chains with different scales have different relaxation time constants. Their motions affect dielectric relaxation processes in different frequency ranges. The motions of molecular or segmental chains are also affected by temperature, since the magnitudes of motions are restricted by free volume in a polymeric insulating material. However, the effects of motions of molecular or segmental chains in epoxy resin on electrical properties have not been very clear to date. Therefore, it is important to investigate the relations between the motion of molecular or segmental chains and dielectric relaxation properties, the temperature and molecular scale dependence of the motions, and their effects on charge transport of epoxy resin.
In this paper, the properties of dielectric relaxation and glass transition of epoxy resin are measured. Before the experimental tests, samples of pure epoxy resin are prepared by using epoxy raw materials supplied by Pinggao Group, and the curing temperature is 130 ℃. The glass transition temperature is around 105 ℃ measured by a differential scanning calorimetry (DSC). As for the dielectric relaxation measurement with Novocontrol broadband dielectric relaxation spectroscopy, the sample is processed into a disk with a diameter of 50 mm and a thickness of 1 mm. The measurement temperature and frequency are in ranges of 100-180 ℃ and 10-1-107 Hz, respectively. The results reveal that there are two relaxation processes at high temperature. In addition, above glass transition temperature, a relaxation peak occurs at high frequencies due to the motions of molecular chains or segmental chains, and a direct current (DC) conductivity resulting from the migration of charge carriers appears at low frequencies. Besides, molecular chains with different scales have different relaxation times. It is found that epoxy resin has a very broad distribution of relaxation times. The distributions of relaxation times at various temperatures are calculated. The results show that the temperature dependence of molecular relaxation and DC conductivity satisfy Vogel-Tammann-Fulcher equation. Through fitting the experimental results, the Vogel temperatures and strength parameters of molecular relaxation and DC conductivity are obtained. From the Vogel temperatures, the glass transition temperature is estimated to be 102 ℃, which is consistent with the DSC result. It means that free volume in epoxy resin increases with the increase of temperature, which facilitates the motions of molecular chains and the migration of charge carriers.