Study on properties of nano-epoxy resin based-low temperature plasma

. In high-voltage power equipment, the traditional solid insulating dielectric is mainly polymer, epoxy resin (EP) is widely used because of its low price, superior performance, and easy processing. KH550 and low-temperature plasma fluorination in conjunction with KH550 modification in the paper is applied to improve nano-epoxy resin. EP samples with different treatment methods and different contents are prepared, and the insulation properties of EP materials before and after thermal aging are tested. It proves that the research results of the paper have a positive effect on the thermal aging characteristics of EP.


Introduction
Polymer surface fluorination technology can significantly improve the electrical properties of materials, and it is also the most used fluorination method in practice [1]. However, few people have studied the fluorination modification of nano-inorganic particles as fillers in nanocomposites. The effect of direct fluorination technology on nanofillers is remarkable. The plasma fluorination method is to use plasma technology to assist the introduction of F elements to the surface of the material, which should have the same effect on micro and nano inorganic particles. In response to this problem, according to published articles, the research team of Lu et al [2] has carried out research on it, and some results have been obtained. The research group analyzed plasma fluorination to improve the insulation performance of micro-AlN/EP materials. Obtained by SEM, XPS and FT-IR, F element can be successfully grafted to the surface of AlN particles. It is found that after 45 minutes of fluorination, the particle size of AlN decreases by 26%, and the content of F element is the largest, which can reach 38.55%. And in the form of CF2 groups, this method can increase the charge dissipation rate of epoxy resin and increase the flashover voltage, and the optimal effect is 39.9% higher than that of pure EP [3].
Both the surface fluorination of high molecular polymers and the fluorination of micro and nano fillers can significantly improve the electrical properties of the material. This is attributed to the F element is the most electronegative element. The introduction of this element into the material can adsorb free electrons passing through the material and improve its insulation properties [4]. Therefore, Plasma fluorination technology in the paper is applied to assist the modification of nano-materials and prepare nano-epoxy composites. It is hoped to improve its electrical performance to enrich the comprehensive application performance improvement methods of polymer insulating materials, so as to meet the needs of high performance, low loss and extreme operating environments.

Characterization and analysis of microscopic properties of modified nano-epoxy resin
The surface morphology, element content, functional group and particle size of the material will change after modification by various physical and chemical methods, which in turn will change certain properties of the material, such as electrical, thermal conductivity and mechanical properties. Therefore, testing these changes is particularly important for material modification, which is an important way to prove the success of the method and whether it is good or not, and to analyze the reasons for the changes in material properties. The commonly used test methods include X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) and energy spectroscopy. In the experiment, these methods will be used to characterize the modification results [5].

XPS analysis of modified nano-epoxy resin
In the experiment, XPS is applied to test the surface elements of the modified epoxy resin powder. The XPS instrument parameters and experimental conditions used are shown in table 1. The XPS spectra obtained after modification under different conditions are shown in figure 1. seen from the figure that the F1 s peak appears in the XPS spectrum after plasma fluorination, and the F element is successfully introduced into the surface of the epoxy resin modified by KH550. It preliminarily shows that the method used in the experiment is feasible and has a certain effect, and with the increase of voltage and pressure, the peak value of element F has increased significantly [6].
In order to further demonstrate the modification effect, the content of the surface elements of the epoxy resin is tested, and the results are shown in Table 2. In Table 2, after plasma fluorination for 10 minutes, the F element reached about 8-10%.
At the same time, since the F element is introduced into the surface of the epoxy resin, the content of S i , O, and N elements on the surface of the epoxy resin modified by KH550 will decrease [7]. However, the content of C element increased instead. This is because the main functional components in CF 4 /N 2 plasma are F, CF 2 , CF 3 , etc., and the F element introduced to the surface of the epoxy resin includes the introduction of F-containing methyl groups. Therefore, the content of C element will increase. Under the same modification time, when the air pressure is 13.5kPa and the voltage is 7kV, the most F element is introduced, which is 10.05%. Therefore, the air pressure is 13.5kPa and the voltage is 7kV [8].

FT-IR analysis of modified nano-epoxy resin
In order to characterize the changes of the epoxy resin surface functional groups after plasma fluorination, FT-IR analysis is performed on the epoxy resin modified by plasma fluorination (13.5kPa ､ 7kV) and KH550. The tablet method is applied. The dried KBr powder and nanoepoxy resin are mixed and grinded in proportion, then a small amount of KBr/epoxy resin mixture is taken to press tablets, and it is put into the instrument (FTIR-8900, SHIMADZU) for testing. The results of the KBr tablet as a background test are shown in Figure 2. According to the analysis in Figure 2, after KH550 modification, a new absorption peak appears on the surface of the nano-epoxy resin. Corresponding to -CH3 bending vibration peak is at but because the powder is spread on the dielectric plate, and the plasma generated by DBD is not complete Uniform, so the KH550 structure on the epoxy surface will not be completely destroyed. Therefore, the surface of the fluorinated powder still contains a large number of functional groups in KH550. From the above analysis, it can be seen that the epoxy resin treated by plasma fluorination modified KH550 is feasible [10].

Energy spectrum analysis of modified nanoepoxy resin
Using 7kV as the applied voltage, under the conditions of 10kPa and 13.5kPa, the epoxy resin with different fluorination time is analyzed by energy spectrum. The results are shown in Figure 3 In Figure 3, in the early stage of fluorination, the lower the pressure, the greater the power of the plasma, so in the first 3 minutes, the content of F on the epoxy resin surface at 10kPa is more than that at 13.5kPa. After 3 minutes, due to the decrease of the F content in the plasma, the fluorination effect at 13.5kPa is better than that at 10kPa. Under the two pressure conditions, the F element on the surface of the epoxy resin grows slowly after 10 minutes. This is because the content of F element in the plasma is greatly reduced, which reduces the fluorination efficiency. Therefore, the fluorination time selected in this experiment is 10 minutes.

SEM analysis of modified nano-epoxy resin
The SEM test of epoxy resin powder before and after fluorination was carried out, and the results are shown in Figure 4. Observing the SEM images before and after the nanoepoxy resin powder treatment shows that the agglomeration problem of the epoxy resin powder after fluorination has been significantly improved. This is because during plasma fluorination treatment, the energy generated by gas ionization can destroy the van der Waals force and chemical bond between the epoxy resin particles, which can greatly reduce the agglomeration of the epoxy resin and make it better dispersed.

Conclusion
The paper introduces the platform and method of lowpressure plasma fluorination technology, and analyzes the feasibility and effect of the method in terms of plasma parameter control and microscopic. The conclusions are as follows: (1) Based on the discharge characteristics and emission spectrum diagnostic technology, the discharge characteristics and distribution characteristics of CF4/N2 plasma are studied. The results show that the plasma distribution under the pressure and voltage conditions used is uniform, and the minimum energy generated is 0.497eV, which can break the C-F bond and provide conditions for the modification of epoxy resin.
(2) The analysis of XPS and FT-IR proves that the method is feasible. After fluorination for 10 minutes, the maximum content of F reached 10.05%, and the element existed in the form of CF2, which can increase the content of C､F and decrease the content of Si, O, N, and this method could not achieve complete coating of epoxy resin.
(3) Using SEM and energy spectrum analysis, it is found that the best treatment time of low-pressure plasma fluorination is 10min. After fluorination for 10 minutes, the agglomeration problem of epoxy resin is significantly improved, and the dispersion is improved.