Study on the Effect of SiO2 Content on the Properties of Epoxy Resin

With the development of energy layout in western China, the construction and operation of hydro-generator units in high-altitude and cold regions such as Tibet and Qinghai have increased rapidly, which puts forward stricter requirements for the insulation and anti-corona technology of high-altitude units. Improving the intrinsic properties of the matrix material is a highly efficient anti-corona method. In the paper, epoxy resin composites doped with different contents of micron silica were prepared. The effects of 0.5 wt%, 1 wt%, and 1.5 wt% SiO2 on the thermal conductivity, dielectric properties, and breakdown field strength of the matrix resin were researched. The results in this paper show that the doping of different contents of SiO2 has little effect on the thermal conductivity of the matrix resin, but improves the dielectric properties of the matrix resin to a certain extent. With the increase of SiO2 content, the breakdown field strength of the composites increased primarily and then decreased.


Introduction
The generator is the core equipment of electric power production.Its basic structure has not changed significantly due to its inception.Stator winding has always been the most critical component of the generator.Its stable operation is an important factor to ensure the reliable operation of the generator and determine the life of the generator.However, large generators will be affected by the synergistic effects of multiple physical fields such as electricity, heat, environmental stress, and mechanical stress during operation, so the weak links of stator winding insulation (such as burrs, air gaps, and other defects between the end binding structures) produce corona, which harms insulation.The corona discharge in the motor is a glow discharge caused by the ionization of the air near the winding insulation surface as a result of the presence of the non-uniform distributed electric field in some parts of the winding insulation surface and the excessive local field strength.It is a partial discharge.It is one of the important reasons for the final breakdown of the stator bar insulation damage [1][2][3].Long-term corona discharge will cause electrical corrosion, insulation aging, thermal cracking, and other phenomena in the insulation layer, resulting in insulation failure, reducing the service life of the generator, and posing a pretty serious threat to the safe and stable operation of the motor [4].With the continuous expansion of high-voltage motor capacity, the corona prevention and control of motor stator windings has become a worldwide technical problem that needs to be solved urgently [5,6].Especially for the motor running at high altitudes, the special geographical environment of high cold and low pressure will bring about the reduction of surface discharge voltage of generator stator windings, which makes the problem of corona prevention more prominent.
At present, the anti-corona material used at the end of the motor is mainly composed of base material, semi-conductive material, and other doped materials.Anti-scorching paint is divided into low, medium, and high-resistance anti-scorching paint according to its resistance value.It can also be divided into epoxy paint and alkyd paint according to the type of substrate material [7].Epoxy resin (EP) has been widely used in the fields of electrical electronics and photoelectric information due to its excellent insulation and economy.The doping of insulating inorganic fillers is of great significance in improving the dielectric properties, corona resistance, and breakdown field strength of epoxy resin [8][9][10][11].Therefore, the intrinsic properties of the matrix resin can be controlled to achieve the purpose of corona prevention, that is, adding insulating inorganic fillers to the epoxy resin to improve the surface potential distribution and uniform electric field distribution, thereby inhibiting partial discharge and achieving the anti-corona effect.
In this paper, micron SiO 2 was used as an insulating inorganic filler and mixed with matrix resin to prepare micron-filled resins with different SiO 2 contents.The effects of micron SiO 2 content on the properties of epoxy resin were analyzed by comparing it with matrix resin.It provides ideas for the subsequent research and development of new anti-corona materials applied to high-altitude generator sets.

Surface modification of micron SiO 2 .
In this study, the surface of micron SiO 2 was modified by chemical methods to enhance the bonding force between micron particles and matrix resin and improve the dispersion of particles in matrix resin.The specific surface modification process is shown in Figure 1.Firstly, KH550 and deionized water were mixed at a ratio of 1:1, and then 49 ml of anhydrous ethanol was added.The KH550 hydrolysate was obtained after magnetic stirring for 30 min at a speed of 500 r/min.5 g micron SiO 2 and 200 ml anhydrous ethanol were mixed with KH550 hydrolysate and ultrasonically oscillated for 30 min; the mixed solution was put into a water bath pot at 90℃ for 6 h, and the solution was magnetically stirred at 600 r/min to make SiO 2 and KH550 fully react.The modified micron SiO 2 particles were obtained by centrifugation, washing, and drying the fully reacted mixture.

Preparation of micron-filled resin. The mass ratio of epoxy resin A glue (EP-A) and epoxy resin B glue (EP-B
) is 3:0.87.To research the effect of different contents of micron SiO 2 on the performance of matrix resin, micron-filled resins with micron SiO 2 contents of 0.5 wt%, 1 wt%, and 1.5 wt% were prepared by the blending method.Firstly, a certain proportion of EP-A adhesive was mixed with modified micron SiO 2 , and the primary mixture was obtained by magnetic stirring at 60℃ for 1 h and ultrasonic oscillation for 1 h.An appropriate amount of EP-B was added to the primary mixture, and the final mixture was obtained after magnetic stirring for 10 min at room temperature.The final mixture was put into a vacuum drying chamber and vacuumed at room temperature to eliminate bubbles in the liquid, and then the bubbles-free mixture was poured into the mold.After 24 hours at room temperature, the epoxy resin was completely cured to obtain the sample.Figure 2 clearly shows the specific preparation process.

Characterization
Firstly, the micro-morphology of micron SiO 2 was observed by Thermoscientific Helios 5 CX Focused ion/ Electron double-beam electron microscopy, and the cross-section micro-morphology of three micro-filled resins with different SiO 2 contents was observed by the scanning electron microscope to analyze the particle dispersion of micron SiO 2 in the matrix resin.Secondly, the heat conductivity factor of each sample was measured by LFA467HT laser thermal conductivity meter produced by NETZSCH Geraetebau GmbH in Germany, and the effect of micron SiO 2 content on the thermal conductivity of the micro-filled resin was analyzed.Thirdly, the dielectric properties (including dielectric constant and dielectric loss factor) of micron-filled resins were analyzed and tested by Novocontrol GmbH Germany's broadband dielectric spectroscopy/thermally stimulated current test system, and the variation of dielectric constant and dielectric loss factor of each material in the frequency range of 10 -1 ~10 6 Hz at room temperature was studied.The reference standard for testing dielectric properties is GB/T 31838.6-2021Solid insulating materials-Dielectric and resistive properties-Part 6: Dielectric properties (AC method)-Relative permittivity and dielectric dissipation factor (frequencies 0.1 Hz~10 MHz).Finally, referring to the standard GB/T 1408.Electrical strength test method for insulating materials-Part 1: Power frequency test, the breakdown field strength of each sample was tested by using the 20 mm diameter column electrode, the change of breakdown field strength between micron filled resins with different SiO 2 content was observed, and the influence of micron SiO 2 content on the breakdown field strength of filled resin was analyzed.

Microstructure
The microscopic morphology of micron SiO 2 is shown in Figure 3. From this figure, we can discover that the SiO 2 particles are spherical in shape with a very uniform size of about 3 μm.Figure 4 is the cross-section microscopic diagram of three kinds of micro-filled resins with different SiO 2 contents.It can be seen from the diagram that micron SiO 2 is well dispersed in the matrix resin, and there is no obvious agglomeration of micro-particles.To characterize the distribution of SiO 2 fillers in the matrix resin, EDS was used to analyze the Si element in the cross-section.From Figure 5, it can be seen that the points of Si element concentration are roughly the same as the positions of SiO 2 fillers observed in the corresponding electronic images, and with the increase of SiO 2 filler content, the number of points of Si element concentration increases, which is the same as the expected observation results.

Thermal conductivity
Because the operating temperature of the generator end is generally about 50℃, the thermal conductivity of each sample at 50℃ is tested, and the tested results are displayed in Figure 6.It can be found from the figure that when the SiO 2 content is 1 wt%, the thermal conductivity of the micronfilled resin is the highest, which is 0.014 W/(m*k) higher than that of the matrix resin; when the content of SiO 2 is 0.5 wt%, the thermal coefficient of the micro filled resin is the lowest, which is 0.011 W/(m*k) lower than that of the matrix resin.But on the whole, the thermal conductivity difference between the four materials is very small.It can be inferred that when the additional amount of micron SiO 2 is 0~1.5 wt%, it has little effect on the thermal conductivity of the matrix resin at 50℃.

Dielectric property
The test results of the dielectric constant are shown in Figure 7(a).In general, the dielectric constants of the four materials decrease with the increase in frequency.The dielectric constant of the matrix resin without SiO 2 is the highest.With the increase of SiO 2 content, the dielectric constant of the epoxy resin decreases gradually.The dielectric constants of each material under power frequency (f=50 Hz) are 4.76, 4.70, 4.61, and 4.61 respectively.It can be seen from this result that the addition of micron SiO 2 can decrease the dielectric constant of the matrix resin to a certain extent.This may be because micron SiO 2 is dispersed in the resin as an insulating inorganic filler, which enhances the internal structure of the matrix resin, limits the orientation movement of polar groups, and reduces the surface polarization of EP, thereby reducing the dielectric constant of the material.The test results of the dielectric loss factor are shown in Figure 7(b).In general, the dielectric loss factors of the four materials decrease first and then increase with the applied frequency increasing.For the same material, the dielectric loss factor changes obviously at low frequencies (f=0.1~10Hz).The change rate is small at f=1~10 4  Hz; the change rate increases at high frequencies (f=10 4 ~10 6 Hz).The dielectric loss of the four materials is also different at low and high frequencies, but the difference is very small at f=10 3 ~10 5 Hz.Following the increase of SiO 2 content, the dielectric loss tangent of each material at 50 Hz is 0.00654, 0.00694, 0.00504, and 0.00431 respectively.It can be seen from this result that the addition of micron SiO 2 can decrease the dielectric constant of the matrix resin and reduce the dielectric loss of the material to a certain extent.

Breakdown field strength
The test results of the power frequency breakdown field strength of four materials are shown in Figure 8.It can be obviously observed from Figure 8 that the breakdown field strength of the three microfilled resins filled with SiO 2 is higher than that of the matrix resin in different degrees.When the content of SiO 2 is 1 wt%, the breakdown strength of the micron-filled resin is the highest, which is 17.4 kv/mm higher than that of the matrix resin.In general, with the increase of SiO 2 content, the breakdown field strength of micron-filled resin increases first and then decreases.The reason for this phenomenon may be that when an appropriate amount of SiO 2 is added to the matrix resin, there is an interface effect between the micron particles and the resin, which enhances the binding ability of the electrons and hinders the formation of the conductive path, thereby increasing the breakdown field strength.When the content of SiO 2 is 1.5 wt%, due to a large number of micron particles, the phenomenon of overlapping and uneven dispersion may occur in the matrix resin, which reduces the interface bonding force between SiO 2 and the matrix resin, resulting in the weakening of the interface effect between the two, thus reducing the breakdown field strength of the material.

Conclusion
In this paper, the effects of different contents (0 ~ 1.5 wt %) of micron SiO 2 on the basic properties of epoxy resin were researched.The results in this paper show that: (1) The micron SiO2 modified by KH550 has good dispersion in the matrix resin, and no obvious agglomeration occurs.(2) At 50℃, micron SiO 2 has little effect on the heat conductivity factor of the matrix resin.This may be because SiO 2 is not a high thermal conductivity material, it does not significantly improve the thermal conductivity of the matrix resin.(3) Following the increase of micron SiO 2 content, the permittivity of the four materials shows a decreasing trend.At power frequency, the maximum reduction rate of the dielectric constant is about 3.15% compared with the matrix resin.In a certain frequency range, the dielectric loss tangent of the micron-filled resin also decreases to a certain extent compared with the matrix resin.This may be due to the fact that micron SiO 2 enhances the internal structure of the epoxy resin and limits the orientation movement of the polar groups, thereby reducing the dielectric loss tangent of the material.(4) The introduction of micron SiO 2 increases the breakdown strength of the matrix resin.With the increase of micron SiO 2 content, the breakdown field strength increases primarily and then decreases, which may be related to the interface effect between micron particles and epoxy resin.When the SiO 2 content is 1 wt%, the breakdown field strength of the material is the largest, which is about 37.43% higher than that of the matrix resin.In summary, micro SiO 2 can improve the electrical properties of the matrix resin to a certain extent, which has a certain enlightenment for the development of new anti-corona paints for generator ends.The micro-filled resin may be used as anti-corona paint to improve the anti-corona performance of motor end insulation.Further research on the anti-corona performance of micro-filled resin (such as corona onset voltage) can be carried out.

Figure 5 .
Figure 5. EDS diagram of sample section.