环氧树脂高温分子链松弛与玻璃化转变特性

环氧树脂是电力设备中广泛应用的一种绝缘材料, 其介电性能受到分子链运动特性的影响. 本文制备了直径为50 mm、厚度为1 mm的环氧树脂试样, 采用差示扫描量热仪和宽频介电谱仪测试了环氧树脂的玻璃化转变温度和介电特性. 实验结果表明, 环氧树脂的玻璃化转变温度为105 ℃, 在玻璃化转变温度以上, 高频段出现了由分子链段运动造成的松弛过程, 低频段出现了由载流子在材料中迁移造成的直流电导过程. 发现环氧树脂不同尺寸分子链段的松弛时间不同, 其松弛时间分布较宽, 计算得到了分子链段在不同温度下的松弛时间分布特性. 分子链松弛峰频率和直流电导随温度的变化关系服从Vogel-Tammann-Fulcher公式. 拟合实验结果得到分子链松弛峰频率和直流电导的Vogel温度和强度系数. 由Vogel温度计算得到了与差示扫描量热测试结果一致的玻璃化转变温度, 约为102 ℃. 结果表明玻璃化转变温度以上环氧树脂的自由体积增大, 分子链段有足够的空间来响应外电场从而产生分子链松弛极化, 载流子有足够的能量在材料中迁移形成电导.

Molecular relaxation and glass transition properties of epoxy resin at high temperature

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.