3.1 Thermal cycling and heating of test specimens
Fifty thermal cycling from 77 K (temperature of liquid nitrogen) to 300 K (room temperature) of test specimens made from modified E-51 epoxy resin system were performed, when the liquid nitrogen was missing due to heat absorption, liquid nitrogen was dipped into the cryostat again, as shown in Fig. 6. To compare the properties change of modified E-51 epoxy resin system after thermal cycling and thermal cycling & heating, some test specimens having been experienced thermal cycling were heated at 105℃ for 120-480h in air environment [17], as shown in Fig. 7.
The test specimens after thermal cycling and thermal cycling & heating are as shown in Fig. 8, the corresponding numbers are 1 and 2. To The test specimen after 50 thermal cycling & heating at 105℃ for 120h. To observe the micro-structure of modified E-51 epoxy resin, some test specimens after thermal cycling and thermal cycling & heating were brittle fractured by liquid nitrogen. Number 3 in Fig. 8 is the brittle fracture cross section of one test specimen after thermal cycling and thermal cycling & heating, the color is close to the test specimen after thermal cycling but before thermal cycling, however, the surface color of this test specimen is much deeper than interior. So much attention should be paid on the influence of high temperature on the micro-structure change and properties change.
3.2 Micro-structure observation
The micro-structure observation of cured modified E-51 epoxy resin system was performed by using scanning electron microscope (SEM), and the micro-structure of cured system before thermal cycling and heating are as shown in Fig .9 (a) and (b), which indicate the inactive toughening agent in cured system is sea-island structure. The micro-structure of cured system after thermal cycling are as shown in Fig. 10 (a) and (b), compared with the micro-structure before thermal cycling, which indicate there is no crack and no obvious micro-structure change. Therefore, as bonding or filling insulation material for safety warning helmet or super- conducting application, the cured modified E-51 epoxy resin system can be used at temperature of liquid nitrogen or lower temperature.
However, for the cured modified E-51 epoxy resin system having been performed thermal cycling, further heating has significant influence on the stability of micro-structure. When the test specimens after thermal cycling were heated at 105℃ for 480 h, agglomeration phenomenon can be seen in Fig. 11, which indicates high temperature can lead to phase migration. Especially, residual chemical compositions and inactive toughing agent can migrate more easily in high temperature environment.
When the cured modified E-51 epoxy resin system was used at cryogenic temperature, migration of residual chemical compositions and inactive toughing agent is very difficult. However, higher temperature will accelerate phase migration, the surface color of test specimens became yellow after heating at 105℃ for 120 h, but the elements distribution inside cured system is in good order. Because the cured system is sensitive to ultraviolet rays and infrared rays, some bonds of E-51 epoxy resin can’t endure high temperature, so the cured system will easily become yellow in high temperature and atmospheric environment.
3.3 Energy dispersive spectroscopy (EDS) analysis of cured modified E-51 epoxy resin system
Energy dispersive spectroscopy (EDS) analysis of modified E-51 epoxy resin system was performed by using SEM, the element content datum of defined line is as shown in Fig. 12. Because the atomic weight of hydrogen atom is 1, it is much lower than the atomic weight of other elements in cured system, which can’t be searched by SEM. As a result, there is no H element in Fig. 13, the atomic weight of other elements are as shown in Figs. 14–17. On the molecular constitution, the toughening agent is made from polyester, polyurethane and polyether connected by epoxy group (-CH(O)CH-), hydroxyl group (-OH), carboxyl group (-COOH) and so on, the content of C element is higher. According to the chemical formula of E-51 epoxy resin, silane-coupling agents (KH-550) and aromatic condensation amine (GY-051), the content of C element is higher too, and the content of Si element is much lower than other elements.
3.4 Infrared spectrum of cured modified E-51 epoxy resin system
The infrared test results indicate there is no obvious difference in the infrared spectrum of the cured system from room temperature to 300°C, as shown in Fig. 18, and the infrared peak of the cured system changes from 350°C to 450°C. The infrared spectra of 5 specimens at 275°C-450°C were magnified and compared.
From the test results as shown in Fig. 19, it can be seen that there is no obvious difference between the infrared spectra of the specimens after treatment at 275°C and 300°C, which indicate there is no large-scale fracture of the chemical bonds, and the chemical bonds of the specimens treated at 350°C, 400°C and 450°C have been broken on a large-scale.
From the test results, it can be seen that the cured system has the following characteristic peaks: the broad peak at 3387cm− 1 is the stretching vibration of O-H bond, the absorption peak at 2869cm− 1~2965cm− 1 is the stretching vibration of saturated C-H bond, several peaks at 1455cm− 1, 1506cm− 1 and 1606cm− 1 are the stretching vibration of C = C bonds on the benzene ring, and the absorption peaks at 1238cm− 1 belong to the stretching vibration of Ar-OC of the ether bond aromatic part. The stretching vibration of ArO-C at 1029cm− 1 bond belongs to the aliphatic ether-oxygen bond, the absorption peak at 1297cm− 1 is stretching vibration of C-C bond, and the absorption peak at 828cm− 1 is bending vibration of C-H bond on multi-substituted benzene ring.
When the cured system was treated at 350°C, the peak at 1033cm− 1 (ArO-C) almost disappeared, which indicates most of the ether bonds were broken, and the peak at 1297cm− 1 was almost disappearing, which indicates the C-C was fractured, and the peak intensity of the remaining peaks was weakened to some extent, which indicates other chemical bonds were partially broken.
When the cured system was treated at 400°C, the O-H peak at 3387cm− 1 almost completely disappeared, which indicates most of the O-H was fractured, and the peak intensity of the characteristic peak of C = C on the benzene ring decreased to a certain extent, which indicates the benzene ring began to decompose.
When the cured system is heated to 450°C, only the peak at 1606cm− 1 is remained, and the rest had been carbonized.
3.5 Mechanical tensile properties of cured modified E-51 epoxy resin system
The test specimens for mechanical tensile were fabricated according to GB/T 2567 − 2021, as shown in Fig. 20. To investigate the influence of defects on the tensile strength, test specimens with defects were made from cured modified E-51 epoxy resin system, as shown in Fig. 21. The test specimens without defects experienced 50 thermal cycling & heating at 105℃ for 120 h are as shown in Fig. 22, which indicate the surface color is light yellow.
The average ultimate tensile strength of test specimens with defects before thermal cycling is 35.30 MPa, the force-displacement curve is as shown in Fig. 23. To reveal the influence of heating time on the mechanical tensile strength, two groups of test specimens without defects after thermal cycling were heated at 105℃ for 120 h and 240 h respectively, the corresponding average ultimate tensile strengths are 49.07 MPa and 42.38 MPa respectively, the decline rate is about 13.63%. As a result, longer heating time can lead to reduced average ultimate tensile strength, the force-displacement curves are as shown in Figs. 24 and 25.
3.6 Discussion
Development of insulating component with modified E-51 epoxy resin is system engineering, the service performance of insulating component was influenced by multiple factors, such as chemical compositions, fabricating process, operating condition and so on.
Measures should be taken to reduce the defects in cured system, which include how to improve the homogeneity and reduce the void.
The micro structure of cured modified E-51 epoxy resin system in cryogenic temperature environment is stable, which is suitable for superconducting application too.
Long time heating of cured modified E-51 epoxy resin system can lead to micro structure change and reduced mechanical tensile strength, so it is necessary to limit the operating temperature.