Study on insulating properties of polyethylene terephthalate/montmorillonite nanocomposites

Owing to the rapid development of the modern economy and the continuous exploitation and utilization of energy, natural resources are becoming increasingly scarce. Photovoltaic (PV) power generation has become the development trend of clean energy power generation in China,¹ and solar energy is one of the most valuable renewable and clean energies. Therefore, solar photovoltaic panels are a significant part of photovoltaic power generation systems. The overall structure of the solar panel is shown in Fig. 1.² Polyethylene terephthalate (PET) is the main material for the photovoltaic backsheet. Because the molecular chain of PET material has an extensional chain configuration with the lowest energy conformation, its molecular structure has a high degree of symmetry and stability; thus, PET has excellent optical properties, easy molding, environmental aging resistance and chemical resistance, etc. Although PET has good comprehensive performance, the slow crystallization rate leads to its poor processing performance and relatively weak insulation performance, thus, limiting its application in engineering plastic. With the continuous development of the solar power generation technology and photovoltaic industry, solar photovoltaic power generation systems also play a vital role in coping with the global energy crisis. However, the photovoltaic backsheet will face various challenges in harsh environments such as high temperature, high humidity, and strong ultraviolet light. Partial discharge (PD) is one of the challenges that will lead to poor insulation and mechanical properties of PET, thus, seriously affecting the long-term reliability of the photovoltaic backsheet and photovoltaic modules.³ Therefore, modified PET is usually used in the industry to improve its comprehensive performance.

There are four methods to modify PET, namely, glass fiber (GF) modification, filling modification, polyester modification, and acrylonitrile–butadiene–styrene (ABS) copolymer modification. Filling modification is to conduct a special process to blend one or two different inorganic materials or modified organic materials with polyester PET, which is one of the most widely used PET modification methods. The most commonly used inorganic powder is layered silicate (PLS), which mainly includes talc, pyrophyllite, bolus alba, montmorillonite (MMT), hydromica, muscovite, etc.⁴ At present, the T-O-T structure of layered silicate is the most widely studied, and MMT is the most representative one. The structure diagram of MMT is shown in Fig. 2. In polymer/MMT composites, MMT is dispersed in the polymer in the order of nanometers in a small proportion, and MMT materials have good fire resistance and aging resistance. At the same time, MMT can also act as a nucleating agent. After being blended with the polymer, the modified material, with enhanced stability and a greatly improved crystallization rate, is easy to process and shape.^5,6

Zhaojun Chen found that PET films made of composite materials still have the same transparency as pure PET.⁷ Xiaowei Meng found that MMT coating makes PET film have better flame retardation and oxygen resistance.⁸ Lim Ji Woo found that when MMT was mixed with PET, the oxygen permeability of PET film decreased.⁹ Jialiang Zhou from Zhengzhou University found that PET/MMT nanocomposites and new copolyesters and their nanocomposites improved the thermal stability and barrier properties of polyesters.¹⁰ Xiaohua Gu and Xunhai Zhang from Qiqihar University found that MMT was successfully modified by diphenylmethane diisocyanate (MDI), which was more compatible with the PET matrix.⁸ However, the partial discharge mechanism and insulating properties of PET/MMT materials have not been studied in the past. Therefore, the insulating properties of PET/MMT nanocomposites are studied in this paper, which has a reference value for the extension of the service life of solar panels and the application of nanocomposites in the field of green energy power generation systems.

Due to the chemical and physical properties of the used polymer matrix, as well as the modification methods and ion exchange capacity of the selected MMT particles, the preparation processes of the polymer/MMT are different. Currently, the main industrial methods for preparing PET/MMT nanocomposites include the solution method, in-situ polymerization, and the melt blending method.⁹ 

There are not many systems suitable for the solution intercalation method; in addition, the selected solvents should not only dissolve various polymers but also make the MMT well dispersed,¹⁰ and it is difficult to meet these requirements. Due to its relatively high cost, the use of in-situ polymerization is limited.

Therefore, PET/MMT composites were prepared by the melt blending method in this paper. First, PET was dried at 120 °C under vacuum for 48 h, and organic MMT was dried at 80 °C under vacuum for 6 h. PET and MMT in different proportions were mixed in a high-speed mixer for high-speed mixing. The proportions are shown in Table I. The granulation is made by the twin screw extruder. The extruder-mixed granules were dried at 120 °C for 12 h. Finally, untreated PET and PET/MMT composites were molded into sheet samples by an injection molding machine under a pressure of 50 MPa and temperature of 250–260 °C, respectively.¹¹ The length and width of the samples were 2 cm, and the thickness was 0.13 mm.

In this paper, the breakdown voltage of PET/MMT composites was measured by the CJ2678 voltage withstand tester in the Nanjing Yangtze River Wireless Power Plant. The schematic diagram of the breakdown voltage test device is shown in Fig. 3. The sample was placed between spherical electrodes, and the voltage was increased until the dielectric breakdown occurred in the sample. The voltage boost rate was 500 V/2 s, the electrode diameter was 3 mm, DC mode was adopted, and the leakage current was 5 mA. The effects of MMT doping on the dielectric strength of PET materials were studied by testing the breakdown voltage of the materials.

The schematic diagram of the experimental device of the PD measurement system is shown in Fig. 4. The experimental equipment includes a high-reliability AC power source, transformer, incubator, coupling capacity, high voltage differential probe, mixed signal oscilloscope, and computer. The AC power source is used to generate the required voltage waveform applied to both sides of the sample, which becomes parallel to the sample after the voltage was boosted by the transformer. The thermostatic chamber is used to maintain the temperature of the sample test. The voltage range of the sample is detected by the high-voltage differential probe, and the partial discharge intensity is measured by the digital oscilloscope. A resistor is added between the power source and the measurement loop to act as a low-pass filter, while a low-impedance channel is formed by increasing the coupling capacitance, which makes the high-frequency pulse current flow and block the low-frequency signal. This method has the following two advantages: (1) it allows a section of the tested sample to be grounded, which greatly improves the safety of the experiment. (2) For the tested sample with large capacitance, large current flowing directly through the branch to be tested can be avoided.

The sample is processed before the partial discharge test. First, the sample was cleaned with anhydrous ethanol and dried with an ionization blower. Then, the sample was placed on the grounded copper electrode in the thermostatic chamber. In order to study the partial discharge characteristics of PET/MMT nanocomposites with different contents of MMT, the film samples of PET-0, PET-1, PET-3, and PET-5 with a length and width of 2 cm and a thickness of 0.13 mm were discharged at 30 °C for 60 min. During the experiment, the initial voltage of Pd was 805 V, the applied voltage was 886 V, and the number of times and intensity of partial discharge of PET/MMT composites with different MMT contents at the same temperature and discharge time were compared. Then the microstructure of the film after partial discharge was analyzed by scanning electron microscopy (SEM).

The breakdown voltage values of PET-0, PET-1, PET-3, and PET-5 are shown in Table II. It can be seen from the table that the breakdown voltages of the materials doped with MMT are significantly higher than those of pure PET. Moreover, when the content of MMT is 3%, the PET/MMT breakdown voltage is the highest, which is 30% higher than that of pure PET. Compared with PET-3, the breakdown voltage of PET-1 and PET-5 is lower, but it is also more than 15% higher than that of pure PET. Therefore, the addition of MMT significantly improves the breakdown voltage of PET material.¹² 

The further analysis is as follows: Because MMT is a layered silicate mineral whose average particle size is in micrometers and it has a large specific surface area and good barrier performance, a small amount of MMT stops the current from flowing through the PET/MMT composite and leads the discharge channel to extend along the surface of the material, thus improving the discharge distance, delaying the penetrating electrode insulation breakdown, and improving the power frequency breakdown electric field strength. In addition, the addition of MMT makes it difficult for the electron in the composite to migrate and accelerate in the electric field, which weakens the impact of electrons on the PET molecular chain, resulting in the difficulty in forming the discharge channel inside the material.

MMT is uniformly dispersed in PET materials and strongly interacts with PET molecules, which, to a certain extent, restricts the movement of polar groups in PET materials. The addition of MMT occupies a part of the internal volume of PET material and increases the distance between PET molecules, resulting in more intermolecular gaps in PET material, which enhances the scattering of electrons. At the same time, MMT can act as a nucleating agent to form microcrystals in the PET matrix, which to a certain extent hinders the migration of charge carriers. Thus, the breakdown voltage of PET/MMT material is increased. Similarly, MMT addition reduces the relative dielectric constant of PET material, improves the electric field distribution inside the insulating material, and significantly increases the breakdown voltage of PET/MMT. In summary, because MMT has a good barrier and improves the distribution of the electric field inside PET material, the insulation performance of PET material is improved.

The corresponding phase resolved partial discharge (PRPD) patterns obtained by local exposure of PET-0, PET-1, PET-3, and PET-5 at 30 °C for 60 min are shown in Fig. 5. As can be seen from the figure, with the addition of MMT and the increase in content, the red part on the PRPD map became loose, PD points significantly decreased, the discharge times of PET/MMT gradually decreased, and the discharge intensity significantly decreased. The partial discharge patterns of PET-3 and PET-5 composites are basically similar, indicating that the addition of 3% and 5% MMT has almost the same inhibitory effect on the discharge intensity of PET. Probably the uneven dispersion of MMT sheets in PET-5 leads to the less obvious inhibitory effect on the partial discharge than that of PET-3. In conclusion, compared with pure PET, the addition of MMT significantly reduces the number of times of partial discharge and weakens the intensity of partial discharge in PET/MMT composites.¹³ 

On the one hand, this is because PET is a polar material with crystalline and amorphous regions. The doped MMT first disperses into the amorphous zone of PET material, which is compatible with the PET matrix, making the amorphous zone structure of PET/MMT more refined than that of the original PET material, changing the overall crystalline phase structure of the material, and introducing multiple-trap energy to the material. Multiple-trap energy reduces the effective carrier mobility and carrier concentration, thus inhibiting the partial discharge activity of PET/MMT nanocomposites.¹⁴ 

On the other hand, the PET and MMT layers in PET/MMT nanocomposites are bonded by ionic bonds, and there is a great interaction force between the two layers. In the process of partial discharge, due to the interaction force between the two interfaces, the MMT layer will scatter the generated electron, which reduces the bombardment of high-energy electrons on the PET matrix and inhibits the influence of partial discharge on the PET matrix. In conclusion, the addition of organic MMT particles weakens the intensity of partial discharge and reduces the local discharge in PET/MMT nanocomposites.¹⁵ 

SEM test was conducted for PET films with different MMT doping concentrations after the partial discharge test, and the results are shown in Fig. 6. As can be seen from the figure, undoped PET films begin to agglomerate. With the increase in doping concentration, the agglomeration phenomenon is further intensified. However, when the doping concentration is 3%, there is no obvious agglomeration phenomenon on the surface of the film, and the film is compact. With the further increase in the doping concentration, the film begins to agglomerate again. The results show that when the MMT doping concentration is 3%, the film has good density and no obvious agglomeration phenomenon on the surface.¹⁶ 

Based on the results of the partial discharge characteristic test and SEM test, it can be concluded that when the MMT doping concentration is 3%, the film of composite insulating material has good compactness and there is no obvious agglomeration phenomenon on the surface. At the same time, the addition of MMT improves the electric field distribution inside the material and significantly reduces the partial discharge, and the insulation performance is the best.¹⁷ 

In this paper, PET/MMT nanocomposites with an MMT content of 1%, 3%, and 5% were prepared by the melt blending method, and their insulating properties were studied. The following conclusions were obtained:

1. The addition of MMT can improve the effect of the electric field distribution inside the PET material and improve the breakdown voltage of PET/MMT composites.

2. The addition of MMT can reduce the number of times of partial discharge and weaken the intensity of partial discharge in PET/MMT composites to a certain extent.

3. When the mass fraction of MMT was 3%, its breakdown voltage was the highest, which was 30% higher than that of pure PET. In addition, the intensity of partial discharge is the weakest compared with that of the other three. After partial discharge, the film has good compactness and no obvious agglomeration on the surface.

However, the research on melt blending technology is not deep enough in this paper. Melting time and melting temperature in the process of melt blending have a great impact on the final structure of MMT nanocomposites, and, therefore, these parameters still need to be studied further.

The authors would like to thank the National Natural Science Foundation of China (Grant No. 51507135) and the Open Research Fund of Jiangsu Collaborative Innovation Center for Smart Distribution Network, Nanjing Institute of Technology (Grant No. XTCX202107), for providing assistance and support for this research.

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article.