Recyclable Dielectric Polymer Nanocomposites with Voltage Stabilizer

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Recyclable Dielectric Polymer Nanocomposites with Voltage Stabilizer Interface: Towards New Generation of High Voltage Direct Current Cable Insulation Yahan Gao, Xingyi Huang, Daomin Min, Sheng-Tao Li, and Pingkai Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04070 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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ACS Sustainable Chemistry & Engineering

Recyclable Dielectric Polymer Nanocomposites with Voltage Stabilizer Interface: Towards New Generation of High Voltage Direct Current Cable Insulation Yahan Gao1, Xingyi Huang1*, Daomin Min2, Shengtao Li2, Pingkai Jiang1 1Department

of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical Insulation

and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240, China *Corresponding author, E-mail: [email protected] (X.Y. Huang) 2State

Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University,

China ABSTRACT: Long-distance and large-capacity transmission systems have been strongly demanded in modern society, which require the development of high voltage direct current (HVDC) cables. However, the currently used crosslinked polyethylene (XLPE) insulated cables not only can’t fulfill the environmental sustainability but also can’t withstand HVDC electrical stress. In this study, recyclable cable insulation which can withstand HVDC electrical stress was successfully prepared by incorporating newly synthesized voltage stabilizer functionalized silica nanoparticles into a thermoplastic isotactic polypropylene. The introduction of voltage stabilizer functionalized nanoparticles resulted in significantly enhanced breakdown strength, suppressed space charge injection and greatly improved thermal stability. For example, the breakdown strength was increased by about 46% by adding 4 wt% voltage stabilizer functionalized silica. Surface potential decay, crystallinity and supramolecular structure of the polypropylene nanocomposites were measured and the improved electrical properties were mainly attributed to the shallow traps introduced by the voltage stabilizer functionalized silica. This investigation paved a new way for developing environmental1 / 31

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friendly HVDC cable insulation. KEYWORDS: Cable insulation, Nanocomposites, Polypropylene, Voltage stabilizer, Breakdown strength, environmental-friendly insulation INTRODUCTION Nowadays, a great amount of power plants locates far away from densely populated areas because the coal is increasingly replaced by renewable energy like wind, water and solar, which require the development of long-distance power transmission system. In addition, with the acceleration of urbanization, the demand of electricity continues to increase in densely populated area, which bring the requirement of transmission system with large capacity. To satisfy these expectations, the high voltage direct current (HVDC) transmission systems have been greatly developed. It is reported that twice the transmission voltage can reduce the Joule loss to a quarter of the original1, leading to the improvement of capacity and efficiency. However, the limited insulation thickness results in higher electric stress as the voltage level continuously increases, leading to accelerated electrical ageing of the cable insulation and significantly decreased lifetime of the cables. So far, significant efforts have been made to insure the stable and long-term operation of HVDC system, and many studies focused on enhancing the DC breakdown strength and/or suppressing the space charges of insulation.2-5 Cross-linked polyethylene (XLPE) has been widely used as power cable insulation because of the excellent thermo-mechanical and dielectric property. As HVDC cable insulation, XLPE has the intrinsic disadvantage because that the cross-linking by-products can cause significant space charge accumulation and greatly decreased electrical lifetime of cable insulation. Degassing process can reduce the by-products, but this process not only is time and cost consuming but also can’t totally remove the by-products because of the large insulation thickness of HVDC cables. More severely, 2 / 31


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XLPE is a thermoset plastic and can’t be recycled and reused once the cables were retired, which lead to high cost of reutilization and environmental problems. Therefore, high performance recyclable thermoplastic cable insulation is highly desirable. Polypropylene (PP) is a general polymer and could be recycled and reused because of its thermoplastic nature. Compared with XLPE, PP not only has comparable electrical property like high electrical resistivity and high breakdown strength6, but also has better thermal and mechanical properties like higher melting temperature and high-temperature mechanical modulus, which can ensure the high operation temperature of cables. Therefore, more attention should give PP based cable insulation. However, there exist a large amount of space charges in PP after applying high DC electrical field7. In addition, the space charges are difficult to be dissipated in PP when the cable was shortcircuited. The space charges can cause a significant negative impact on the operation and lifetime of PP based HVDC cables. Therefore, strategies should be developed to improve the thermal stability and to suppress the space charges in PP based HVDC cable insulation. So far, there are several approaches used to improve the electrical properties of PP. The first is tailoring the crystalline structure. It is found that syndiotactic structure7, the formation of β phase dominated8, the decrease of crystallinity and the decrease of spherulite size9 are beneficial to enhance the breakdown strength. Second, modifying the macromolecular chain structure by grafting (i.e., maleic anhydride)10 and copolymerization (e.g., ethylene as co-monomer)11, which can suppress the space charge and enhance the breakdown strength to different degree. Besides, it is a common approach to blend PP with other polymers such as propylene-ethylene

12

or ethylene-octene

copolymer13, which can result in enhancement of both dielectric and mechanical properties. It has been reported that the introduction of inorganic nanoparticles can improve the dielectric properties of PP14-15. 3 / 31



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The introduced nanoparticles produce numerous traps capable of capturing space charges16-18. These traps limit the motion of space charges and change the depth distribution of trapped charges, resulting in the enhancement of DC breakdown strength. It has been reported by Cao et al19 that DC breakdown strength of PP increases about 30% after the incorporation of 1wt% MgO nanoparticles. Zhou et al20 documented the effect of four kinds of nanoparticle (MgO, TiO2, Al2O3 and ZnO) on electrical properties of PP. Different improvement of dielectric properties can be observed and mechanism was suggested from the viewpoint of space charge suppression, and the MgO and TiO2 nanoparticles performed better in comparison with the other two types, which increased 29% and 43% of the breakdown strength, respectively. Our previous work has documented that the SiO2 nanoparticles had a significant role in the space charge behavior of PP21. In spite of nanoparticles, it is found that some organic compounds (i.e., polycyclic aromatic compounds) can decrease the energy of energetic electrons and enhance the breakdown strength of dielectric polymers22. Such compounds are usually named as voltage stabilizer. Thanks to the existence of delocalized π-electrons, the voltage stabilizers possess lower ionizing potential and higher electron affinity23-24, making them be capable to capture the energetic electrons by collision and to dissipate the energy by producing relatively stable anions and cation radicals25. As shown in Figure 1, these radicals can react with each other or with hot electron, leading to the regeneration of voltage stabilizer by transforming the radicals to their ground state26. Jarvid et al27 reported seven benzil-type voltage stabilizers with alkyl chains of various length, where the alkyl chains are connected with benzil core by different functional groups like ester, ether or amine. All kinds of voltage stabilizer with benzil core could increase the AC breakdown strength of XLPE at a low concentration about 10 mmol/kg-1. In addition, the highest enhancement in AC breakdown strength of more than 70% compared with the 4 / 31


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reference XLPE was obtained when the voltage stabilizer possesses short alkyl chain and ester group. Jarvid et al also used thioxanthone28 and fullerene derivatives29 as voltage stabilizer to respectively increase the AC breakdown strength of XLPE by 55% and 30% at the concentration of 0.3 wt% and 0.5 mmol/ kg-1, respectively. However, it is found that the voltage stabilizer induced electrical performance enhancement couldn’t be maintained in the long-term service of polymers due to the physical loss or chemical consumption30. On the other hand, low-weight molecules have the tendency to migrate towards the surface of polymers during high-temperature processing and the long-term service. One promising way of limiting the voltage stabilizer migration is to link them with largeweight molecules like polymers or nanoparticles31-32.

Figure 1. Mechanism of voltage stabilizers in enhancing breakdown strength of polymers In order to use the synergistic effect of nanoparticles and voltage stabilizers and overcome both limitation, a voltage stabilizer functionalized silica nanoparticle was synthesized through thio-ene click chemistry after a series of organic reaction and modification. It was found that the introduction of voltage stabilizer functionalized silica nanoparticles resulted in significantly enhanced breakdown strength and thermal stability in PP. 5 / 31



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RESULTS AND DISCUSSION Synthesis of the voltage stabilizer (BVA). The synthesis process of the voltage stabilizer and the voltage stabilizer functionalized nanoparticles was shown in Figure 2. The hydroxylation of the methoxy benzil (BMD) was first carried out in the acidic high temperature condition, and the hydroxyl benzil (BHD) was obtained. Then, esterification reaction of BHD and methyl acryloyl chloride was done with the weight ratio of 1:1 to get BHV. The ratio of functional groups of BHD and methyl acryloyl chloride is 2:1. Therefore, only one side of the BHD was modified. Finally, the esterification reaction of BHV and acetic anhydride was performed with the weight ratio of 1:1 to obtain BVA. Two factors were considered to design the voltage stabilizer. First, the benzil-type voltage stabilizers with ester group show higher electron affinity and lower ionizing potential, which indicating the greater ability to capture the high energy electron and protect the PP from electrical ageing23-24; Second, the vinyl groups can react with thio group via click chemistry, leading to covalent bonding between the voltage stabilizer and the nanoparticles.

Figure 2. Scheme for the preparation of BVA and SiO2-sh-BVA nanoparticles 1H

NMR spectra was used to demonstrate the successful reaction of hydroxylation and

esterification. Figure 3 presented the 1H NMR spectra of BMD, BHD, BHV and BVA. As for the 1H 6 / 31


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NMR spectrum of BMD, the chemical shifts in 7.95 ppm (peak A) and in 7.95 ppm (peak B) are corresponding to the H atom on benzene ring, which move to high field in BHD, BHV and BVA due to smaller electron cloud density caused by extra electron-withdrawing groups. In addition, the chemical shift in 3.91 ppm (peak C) was detected for methoxy (-OCH3). In the 1H NMR spectra of BHD, peak C disappeared, and a characteristic peak D corresponding to the hydroxyl (-OH) appears in 10.84 ppm, indicating that the BMD is completely hydroxylated to BHD. In the 1H NMR spectra of BHV, the peak D disappears and there are two possible reasons. One is half of the hydroxyl react with the acyl chloride and another is that, as an active hydrogen, it is easy to exchange with the solvent leading to the weak peak. The chemical shifts in 6.34, 5.97 and 2.03 ppm are corresponding to the H atoms in double bond (-C=CH2) and methyl (-CH3). The chemical environment of the two H atoms on the double bond is different, the H atoms closed to benzil core had lower electron cloud density, and the chemical shift moved to the high field (peak E). In the 1H NMR spectra of BVA, the chemical shift in 2.33 ppm (peak H) is corresponding to the methyl from acetic anhydride.

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Figure 3. 1H NMR spectrum of BMD (a) in CDCl3, and BHD (b), BHV (c) and BVA (d) in DMSO

FT-IR spectra (Figure S1) were also provided to illustrate the chemical reaction shown in Figure 2, the peak at 3400 cm-1 corresponds to the H-O stretching vibration of the hydroxyl, which wasn’t observed in the FT-IR spectra of BMD. Compared with BHD, the peak of BHV at 3400 cm-1 became weak, indicated that the hydroxyl was partially reacted in first step of esterification, and the peak at 3400 cm-1 in BVA almost disappeared because the most of the hydroxyl were partially reacted in the second step of esterification. The FT-IR spectra of BHV and BVA exhibit the characteristic peak at 3100 cm-1 representing the stretching vibration of C-H bonds of the vinyl groups, which supports the existence of double bonds in BHV and BVA. In addition, the peak at 1750 cm-1 in the FT-IR spectra of BHV and BVA is a stretching vibration peak of C=O double bond of the ester groups, indicating the presence of esters in BHV and BVA. In short, the appearing and disappearing of feature peaks prove the successful synthesis of the voltage stabilizers. Synthesis of the voltage stabilizer functionalized nanoparticles. The silane couple agent (γmercaptopropyltriethoxysilane) was first self-polymerized on the surfaces of nanoparticles to obtain surface modified SiO2 (SiO2-sh), which introduced a large amount of thiol groups onto the nanoparticle 8 / 31


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surface. Initialized by AIBN, the thio groups on SiO2-sh was reacted with vinyl groups from BVA through click chemistry, which is simple, facile and fast. Under the click chemistry reaction condition, one side of the double bond could produce radical polymerization while another side of the double bond has a large space steric hindrance, which can effectively decrease the possibilities of radical polymerization. Figure S2 presents the FT-IR spectra of SiO2, SiO2-sh and SiO2-sh-BVA. The peak between 3500 and 3400 cm-1 corresponds to the H-O stretching vibration of the hydroxyl groups (-OH), which can react with γ-mercaptopropyltriethoxysilane to form covalent bonding (i.e., C-O-C). After the sol-gel and click chemistry reactions, the nanoparticles still contain some hydroxyl groups. Compared with FT-IR spectrum of SiO2, the peak at 2935 cm-1 in the FT-IR spectrum of SiO2-sh is about C-H stretching vibration in methylene, which comes from γ-mercaptopropyltriethoxysilane. Thus, one can conclude that the nanoparticles were modified successfully by the silane coupling agent. In contrast with the FT-IR spectrum of SiO2-sh, the spectrum of SiO2-sh-BVA has a peak at 1690 cm-1 corresponding to the C=O stretching vibration in ester group from BVA, which confirmed successful click chemistry reaction between SiO2-sh and BVA. Figure S2 presents the thermogravimetric analysis (TGA) curves of SiO2, SiO2-sh and SiO2-shBVA. The weight loss of SiO2 is about 4.4%, which mainly results from the loss of absorbed water and hydrophilic functional groups on the nanoparticle surface. The weight loss of SiO2-sh was around 9.1%, which came from the modified layer and a small amount of unreacted hydrophilic group on the surface. The weight loss of SiO2-sh-BVA is 17.2%, which should be from the outer layer of the functionalized SiO2 including BVA, silane couple agent and a small amount of unreacted hydrophilic groups. A simple calculation showed that the content of BVA in SiO2-sh-BVA is about 8 wt%. Such 9 / 31



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a percentage can realize the concentration of BVA range from 0.05 wt% to 0.2 wt% in the final nanocomposites, which can fulfill the requirements of additive concentrations in industrial products (about 0.2 wt%). Microstructure and morphology of the nanocomposites. In the case of uniform dispersion state, the large specific surface area makes nanoparticles have the advantages of formation of high volume fraction of interfacial region in nanocomposites4, 14, 33. On the other hand, the high surface energy of the nanoparticles brings some disadvantages like aggregation in the polymer matrix, resulting in significantly decreased interfacial region and undesirable properties. Therefore, the nanoparticle dispersion is a vital factor to predict the properties of nanocomposites. Figure 4 presents TEM images of different nanocomposite microtome sections. One can see that the unmodified nanoparticles exhibit serious agglomeration in PP, and the surface modification by sliane couple agent improved the dispersion of nanoparticles and the aggregation size decreased much. It is also founded that SiO2-shBVA have the best dispersion capability among the three kinds of nanoparticles and the aggregations show the smallest size in PP. The improved dispersion of sliane modified nanoparticles originated from the enhanced compatibility and the reduced the nanoparticle surface energy. Apart from these two factors, the stronger interaction between the grafted BVA and PP additionally account for the optimal dispersion of SiO2-sh-BVA.

Figure 4. TEM images of PP/SiO2-1 (a), PP/SiO2-sh-1 (b) and PP/SiO2-sh-BVA-1 (c) 10 / 31


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The crystal type, spherulite size and crystallinity of semi-crystalline polymers can affect their electrical properties. Here the crystallization behavior of different nanocomposites was explored to understand the electrical properties of the nanocomposites. First, the crystal type of PP in different nanocomposites were investigated using XRD and the results were shown in Figure S3. One can see that the diffraction peaks appear at 2θ = 14.2° (110), 2θ = 17.0° (040), 2θ = 18.67 ° (130), 2θ = 20.2° and 21.3 ° (131), which are all corresponding to α phase. The number, position and intensity of the four samples have no difference, indicating that the SiO2, SiO2-sh and SiO2-sh-BVA have no apparent influence on the crystal type of PP. Figure 5 shows the POM images of PP and different nanocomposites isothermally crystallized at 140 °C. It could be obviously observed that PP possesses the maximum size of the spherulite about 30 μm. In the image of PP/SiO2, the size of the spherical crystal was greatly reduced around to 15 μm and the number of spherulite increased, indicating that SiO2 acted as a nucleating agent. PP/SiO2-sh and PP/SiO2-sh-BVA have the similar spherulite size, which become further smaller (about 10 μm) in comparison with PP/SiO2 because that the better dispersed nanoparticles have stronger nucleating capability. Figure 5 shows the POM image of as prepared PP and PP/SiO2-sh-BVA. One can see that compared with the isothermally crystallized samples, both as prepared PP and PP/SiO2-sh-BVA exhibit imperfect spherulite and decreased spherulite size. In the case of PP/SiO2-sh-BVA, the spherulites are too small to be recognized but their quantity was significantly increased when compared with the isothermally crystallized sample. Such a phenomenon is desirable because that smaller spherulites are beneficial to increase the dielectric properties of PP, especially the breakdown strength [10]. Crystallinity Xc of PP in different nanocomposites were calculated according to the DSC melting curves using following equation 11 / 31



ΔH

Xc = 𝛥 𝐻𝑓

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(1)

where Δ H is the melting enthalpy of the samples and Δ Hf (209.0 J/g)34 represents the melting enthalpy of 100% crystallized PP. It was found from Figure S3 that the addition of nanoparticles decreased the crystallinity of PP, in particular the addition of SiO2-sh and SiO2-sh-BVA. The crystallinity of pure PP is 44.3%, while the crystallinity decreased to 41.7%, 40.3% and 40.5% for PP/SiO2 PP/SiO2-sh and PP/SiO2-sh-BVA nanocomposites, respectively. The nanoparticles acted as nucleating agent, which accelerated the crystallization but led to the incomplete growth of spherulites, as observed in Figure 5.

Figure 5. Polarized optical micrographs of isothermally crystallized PP (a), PP/SiO2-1(b), PP/SiO2sh-1 (c) and PP/SiO2-sh-BVA-1 (d); POM image of as prepared PP (e) and PP/SiO2-sh-BVA-1(f) Thermal stability of nanocomposites. The thermal stability property of the nanocomposites was evaluated by thermogravimetry (TGA) and differential thermogravimetry (DTG). Figure S4 provides the TGA and DTG curves of PP and PP nanocomposites with 1% nanoparticles. The corresponding initial decomposition temperature Tini and the maximum decomposition temperature Tmax of different 12 / 31


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samples were shown in Figure S5. The Tini is defined by the decomposition temperature when mass loss reaches 5 wt% and Tmax is regarded as the temperature when the rate of weight loss become the largest. The Tini and Tmax of the nanocomposites was greatly increased about 80 ℃ and 75 ℃, respectively, and the increase is almost independent on the surface chemistry of the nanoparticles. These facts demonstrated that the addition of SiO2 nanoparticles can significant enhanced the thermal stability of PP and the improvement mainly originated from the intrinsic property of the nanoparticles. The improved thermal stability may be attributed to the trapping effect of the nanoparticles on free radicals generated at high temperatures. The improved thermal stability property of the nanocomposites was highly desirable for HVDC cable insulation because of their high work temperature (about 100 ± 10 ℃). Space charge behavior. High direct current electric field can cause injection and accumulation of large amounts of space charges in insulating materials, which brings severe electric field distortion and leads to significant decreased lifetime of the insulation. This is why most of the conventional HVAC cable insulation can’t be used in the HVDC condition. In the current work, the space charge behavior under 50 kV/mm dc electric field was characterized and the results were shown in Figure 6. One can see that in the case of pure PP, a large amount of hetero-charges appears near both electrodes and the negative charges slowly move toward the cathode. Hetero-charges are usually generated by ionization of internal impurities or additives under electric field. Here, the appearance of a large amount of heterocharges indicates the existence of impurities or additives in the commercial PP. In PP/SiO2 nanocomposites, the number of hetero-charges was reduced but the charge density was increased during the first hour polarization. In addition, there also exist injected homo-charges near the cathode. For the nanocomposites of PP/SiO2-sh and PP/SiO2-sh-BVA, one can see that both hetero-charges and 13 / 31



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homo-charges were suppressed. No apparent migration and accumulation of hetero-charges and injection of homo-charges were observed, indicating the surface modified SiO2 nanoparticles play an important role in suppressing the accumulation of space charges in PP nanocomposites.

Figure 6. Space charge distributions at room temperature under a DC electric field of 50 kV/mm in (a) PP, (b) PP/SiO2-1, (c) PP/SiO2-sh-1, (d) PP/SiO2-sh-BVA-0.5, (e) PP/SiO2-sh-BVA-1, (g) PP/SiO2-sh-BVA-4. Cathode is located at the left dot line and the anode is located at the right dot line.

Figure 7. Space charge distributions during depolarization at room temperature under a DC electric field of 50 kV/mm in (a) PP, (b) PP/SiO2-1, (c) PP/SiO2-sh-1, (d) PP/SiO2-sh-BVA-0.5, (e) PP/SiO214 / 31


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sh-BVA-1, (g) PP/SiO2-sh-BVA-4. Cathode is located at the left dot line and the anode is located at the right dot line. After applying a dc electric field of 50 kV/mm for 1.5 h, the samples were short-circuited and the decay behavior of space charges was recorded. As shown in Figure 7, a large amount of hetero-charges appear near both electrodes and the charges decay at a slow rate. In the case of nanocomposite of PP/SiO2, a large amount of homo-charges appear near both electrodes and the charges decay at a higher rate. For the PP/SiO2-sh and PP/SiO2-sh-BVA nanocomposites, only limited number of space charges can be observed after short-circuited for 10 s, and the space charges decay quickly to the minimal number in each sample. According to Poisson’s equation35, the space charge density of 1 C/m3 can cause the distortion field of 50 kV/mm to directly damage of the insulation. Therefore, it is essential to calculate the space charge density inside the insulating material, which can help to quantitatively analyze the tendency of space charge decay. The model proposed by Montanari et al36-37 is used to obtain the average space charge density since the experimental condition is consistent with the assumption. Accordingly the average volume space charge density is calculated by the following equation38 Q (t) =

1 𝐿

𝐿

∫0ǀ𝜌 (𝑥,𝑡)ǀ Sdx

(2)

where ρ(x, t) is the space charge density of any point, L the thickness of sample, x the distance to the negative electrode, t the polarization time and S represents the electrode area of the samples. The calculated results were shown in Figure 8. One can see that PP has the highest volume space charge density up to 1.44 C/m3 at 10 s short circuit and 1.02 C/m3 at 1800 s short circuit. In the case of nanocomposites with 1 wt% nanoparticles, all the samples show the significantly decreased volume space charge density, however, the charge density shows the order of PP/SiO2 > PP/SiO2-sh > PP/SiO215 / 31



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sh-BVA. This result indicates that the surface modification shows positive effect on suppressing the space charge accumulation in the nanocomposites and it is concluded that the voltage stabilizer shows the highest capability in suppressing the space charge accumulation. The effect of nanoparticle concentration on suppressing the space charge accumulation was also investigated and results are also shown in Figure 8. It can be seen that the average volume space charge density decreased with the increase of nanoparticle loading up to 2 wt%. At a SiO2-sh-BVA concentration of 2 wt%, the nanocomposite shows the minimum charge density of 0.11 C/m3. Starting from 2 wt%, the nanocomposites show increased average volume space charge density. For instance, the average volume space charge density of the nanocomposites with 4 wt% PP/SiO2-sh-BVA is 0.29 C/m3, indicating that highly filled nanocomposites may introduce more defects or impurities.

Figure 8. Time dependent average volume space charge densities of PP and its nanocomposite with different fillers (a) and PP/SiO2-sh-BVA nanocomposite with different content (b) during the depolarization process. Owing to the heating of loaded conductor, the power cables usually work at a high temperature (e.g., >70 ºC). In order to investigated whether the nanocomposite can perform well under the cable operation environment, the space charge behavior of the nanocomposites films was tested under 20 kV/mm DC electric field at 80 ºC, which is closed to the operation condition. As shown in Figure 9, 16 / 31


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A large amount of homo-charges were injected into the pure iPP from both electrodes and moved to the opposite electrodes with time. In PP/SiO2, the charge density of both electrodes increases during the early stage of the voltage application, indicating the existence of hetero-charges near both electrodes. In the cases of PP/SiO2-sh-BVA and PP/SiO2-sh, there is only a small amount of homocharges near both electrodes and the space charge migration is not apparent, particularly in PP/SiO2sh-BVA. Therefore, one can conclude that the voltage stabilizer functionalized SiO2 nanoparticles show the most apparent suppression effect on the space charge injection and accumulation in the PP nanocomposites at 80 ºC.

Figure 9. Space charge distributions at 80 ℃ under a DC electric field of 20 kV/mm in (a) PP, (b) PP/SiO2-2, (c) PP/SiO2-sh-2, (d) PP/SiO2-sh-BVA-2. Cathode is located at the left dot line and the anode is located at the right dot line. 17 / 31



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After applying a 20 kV/mm DC electric field at 80 ºC for 1.5 h, the samples were short-circuited and the decay behavior of space charges was presented in Figure 10. For the pure PP, a large amount of charges accumulated inside PP, and the decay rate is low. Unlike the PP polarized at room temperature, a large amount of homo-charges were observed near both electrodes, indicating that injection is the main mechanism at high temperature. In the case of nanocomposite, many homocharges can be observed near both electrodes and most of the space charges decayed to the minimal density at a rapid rate and only a small amount of space charges can be detected after 10 s depolarization. Figure 11 provides the time dependent average volume space charge densities calculated according to Equation 2 of PP and its nanocomposite during the depolarization at 80 ºC. PP has the highest space charge density up to 2.14 C/m3 after 10 s short circuit, which is much larger than that of the shortcircuited PP under 50 kV/mm at room temperature. In the case of nanocomposites, the space charge density decreased significantly and it shows the order of PP/SiO2 > PP/SiO2-sh > PP/SiO2-sh-BVA. In a brief summary, the voltage stabilizer functionalized SiO2 are capable of effectively suppressing the injection and accumulation of space charges no matter at high temperature or room temperature.

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Figure 10. Space charge distributions during depolarization at 80 ℃ under a DC electric field of 20 kV/mm in (a) PP, (b) PP/SiO2-1, (c) PP/SiO2-sh-1, (d) PP/SiO2-sh-BVA-0.5, (e) PP/SiO2-sh-BVA-1, (g) PP/SiO2-sh-BVA-4. Cathode is located at the left dot line and the anode is located at the right dot line.

Figure 11. Time dependent average volume space charge densities of PP and its nanocomposite with different fillers during the depolarization process at 80 ºC. Breakdown strength of the nanocomposites. The breakdown strength is one of the most important electrical parameters to evaluate the property of insulating materials. In order to explore the influence of SiO2-sh-BVA on DC breakdown strength, the characteristic breakdown strength of the different samples with thickness of 0.05 ± 0.005 mm were presented in Figure 12. Comparing with pure PP, the nanocomposites with 1 wt% unmodified SiO2 exhibit slightly lower breakdown strength, while the nanocomposites with 1 wt% silane modified SiO2 exhibit slightly higher breakdown strength. In the case of nanocomposites with voltage stabilizer modified SiO2, each nanocomposite shows apparently 19 / 31



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enhanced breakdown strength and the breakdown strength increases with the increase of SiO2-sh-BVA. At a SiO2-sh-BVA concentration of 4 wt%, the characteristic breakdown increased to 493 kV/mm from 337 kV/mm of the pure PP, resulting in an enhancement of 46%. Figure 12 also compared the breakdown strength enhancement of this work with the reported values. The reported highest enhancement of breakdown strength of PP or PE was achieved by the addition of surface modified TiO2 nanoparticles, which is up to 43%. One can see that compared with the reported nanocomposites, the SiO2-sh-BVA exhibits the higher enhancement of breakdown strength of PP or PE. Considering that SiO2 shows much lower electrical conductivity or dielectric constant mismatch with PP in comparison with TiO2 (see Table S4), one can expect the better enhanced long-time performance of the PP/SiO2-sh-BVA nanocomposites.

Figure 12. Characteristic breakdown strength of PP and PP nanocomposites (a) and a comparison of breakdown enhancement between PP/SiO2-sh-BVA-4 and the reported PP nanocomposites10, 20, 39-41 (b) Leakage current. Leakage current is an important parameter for evaluating the electrical property of HVDC cable insulation. Figure 13 shows the electric field dependent leakage current density of PP and the PP nanocomposites at room temperature and 90 ℃. One can see that at room temperature, compared with the pure PP, PP/SiO2-2, PP/SiO2-sh-2 and PP/SiO2-sh-BVA-2 show higher, 20 / 31


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comparable and lower leakage current densities, respectively. In addition, all the PP/SiO2-sh-BVA nanocomposites investigated in this work show lower leakage current densities. At 90 ℃, PP/SiO2-2 and PP/SiO2-sh-2 have comparable leakage current densities in comparison with the pure PP, while the leakage current density of PP/SiO2-sh-BVA is lower than that of PP when SiO2-sh-BVA is higher than 0.5%. These results indicate that the SiO2-sh-BVA nanoparticles show the most apparent suppressing effect on the leakage current in PP under high electrical field. Since the leakage currents mainly originate from the charge injection from electrodes, the suppressed leakage current in PP/SiO2sh-BVA means lower charge injection, which is consistent with the aforementioned space charge behavior of the nanocomposites.

Figure 13. Leakage current density of PP, PP/SiO2-1, PP/SiO2-sh-1, PP/SiO2-sh-BVA-0.5, PP/SiO2sh-BVA-1, PP/SiO2-sh-BVA-2 and PP/SiO2-sh-BVA-4 in room temperature and 90 ℃ 21 / 31



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Trap energy and trap density. The trap characteristics were useful to understand the electrical properties of dielectrics under electric field. In this work, the surface potential decay behavior of PP and the nanocomposites were measured to characterize the trap energy and trap density42-43. The samples were charged by positive corona and negative corona, respectively. Figure S6 presents the surface potential decay behavior of PP and the nanocomposites. Accordingly, the characteristics of hole and electron traps were obtained, respectively. Figure 14 presents the trap energy versus trap density of different samples and Tables S5 and S6 list the corresponding peak trap energy and peak trap density. One can see that no matter charged by positive corona or negative corona, the trap density and trap energy displayed the same tendency. First, in the case of nanocomposites with SiO2-sh, apart from the decrease of deep trap energy, a large number of shallow traps with a peak trap energy near 0.95 eV appeared. Second, the addition of SiO2-sh-BVA apparently increased the density of the shallow trap density of the nanocomposites. At last, both the deep trap energy and the deep trap density were significantly decreased in the nanocomposites of SiO2-sh-BVA. On the other hand, the introduction of SiO2 nanoparticles results in the decrease of deep trap energy after charged by positive corona while the deep trap energy increased after charged by negative corona. According to the multicore model proposed by Tanaka44, the interface between nanoparticles and dielectric polymer matrix can be considered as a multi-core structure, which was composed of bonded layer, bound layer and loose layers form the nanoparticle surface to the outer matrix. Deep traps were usually introduced in bonded layer (the first layer) and bound layer (the second layer) and shallow traps were generated in loose layers (the third layer). Hence, it can conclude that the voltage stabilizer functionalized SiO2 can tailor the third layer in the interface between nanoparticles and PP. Namely, the voltage stabilizer functionalized SiO2 nanoparticle have a strong interaction with the macromolecular matrix, resulting 22 / 31


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in significantly increased volume fractions of loose layers.

Figure 14. Trap energy versus trap density of PP, PP/SiO2-1, PP/SiO2-sh-1, PP/SiO2-sh-BVA-1, and PP/SiO2-sh-BVA-2 after charged by positive corona (a) and negative corona (b) Discussion on the breakdown strength enhancement. The breakdown strength of PP was influenced by several factors, including the supermolecular structure, crystallinity, molecular weight and its distribution, additives, etc. Here, the variation of molecular weight was not considered since all the samples were subjected into the same processing procedure. The introduction of any nanoparticle resulted in slight decrease of crystallinity and thus the influence of crystallinity on breakdown strength can also be ignored. Regarding the supermolecular structure, the introduction of any nanoparticle brought about apparently decreased size and increased number of spherulites, which should be beneficial to enhance the breakdown strength of the corresponding nanocomposites. However, the nanocomposite with original SiO2 nanoparticles show decreased breakdown strength, indicating the spherulite size does not play the critical role in the breakdown enhancement of the nanocomposites. Therefore, the interface between the nanoparticles and PP should play the critical role in enhancing the breakdown strength. First, the decreased breakdown strength of the nanocomposite with original SiO2 should be mainly attributed to their poor compatibility with PP, which not only leads to large 23 / 31



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sized aggregations shown in Figure 4 but also causes a large number of interfacial defects where breakdown is easy to occur. The defects can elongate the mean free path to produce high-energy carriers, which damage the polymer chains via collision. Thanks to the improved compatibility between the modified nanoparticles and PP, the PP/SiO2-sh and PP/SiO2-sh-BVA nanocomposites exhibit enhanced breakdown strength and the enhancement can be understood by the trap characteristics and role of voltage stabilizer. According to the hopping model, the carriers would be trapped by deep traps first and then be captured by shallow traps when a large amount of charge carriers were injected into the dielectrics at high electric field. It is believed that the shallow traps introduced by the addition of nanoparticles play an important role in the enhancement of breakdown strength. Figure 14 shows that the density of shallow traps has the order of PP/SiO2-sh-1 < PP/SiO2-sh-BVA-1, which is consistent with the enhancement trend of breakdown strength. In addition, in the case of nanocomposites with SiO2-shBVA, the shallow trap density enhances with the increasing content of SiO2-sh-BVA, which is also consistent with the enhancement trend of breakdown strength. Injected or already existed charge carriers hopped more easily in shallow traps, resulted in faster migration and nonlinear conductivity in the nanocomposites, which suppressed the accumulation of space charges and the local electrical field distortion. There are much more shallow traps in the PP/SiO2-sh-BVA nanocomposites, which result in the highest enhancement of breakdown strength. The benzil-type voltage stabilizer, BVA, with ester groups have high electron affinity (~1.70 eV) and low ionizing potential (~8.05 eV)

24,

which have strong capability to capture the high energy

charges and protect the PP from collision with charges, resulting in enhanced breakdown strength of the corresponding nanocomposites. Overall, the enhanced breakdown strength of the PP/SiO2-sh-BVA 24 / 31


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nanocomposites should be mainly determined by improved interfacial compatibility, the introduction of shallow traps and the high electron affinity and low ionizing potential of the voltage stabilizer. The decrease of spherulite size may helpful to enhance the breakdown strength of PP nanocomposites. However, the enhancement can realize only under the condition that the nanoparticles have excellent interfacial compatibility with the PP matrix. CONCLUSIONS By introducing newly synthesized voltage stabilizer functionalized silica nanoparticles into an isotactic polypropylene, we successfully fabricated recyclable nanocomposite insulation with significantly improved thermal stability and HVDC properties. All the nanocomposites show the similar improved initial decomposition temperature and the similar improved maximum decomposition temperature, and thus the improved thermal stability has been attributed to the trapping effect of the nanoparticles on free radicals generated at high temperatures. The PP/SiO2-sh-BVA exhibit much higher enhancement of breakdown strength and the enhancement increases with the SiO2sh-BVA content. We attributed the higher enhancement to the introduced shallow traps and the high electron affinity and low ionizing potential of the voltage stabilizer grafted on SiO2-sh-BVA. It is also suggested that the excellent interfacial compatibility is the basic requirement for realizing the enhancement of breakdown strength in the nanocomposites. In spite of the significantly enhanced electrical property, the PP/SiO2-sh-BVA nanocomposites cannot be directly used as HVDC cable insulation because of their low flexibility. Further investigation is needed to improve their flexibility while remaining their excellent electrical property.

ASSOCIATED CONTENT 25 / 31



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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Material, synthesis and preparation methods, characterizations, breakdown strength and β data, FT-IR spectrum, XRD curves, crystallization and decomposition data, electrical conductivity and dielectric constant data, trap energy and trap density data. (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Special Fund of the National Priority Basic Research of China (2014CB239503), the National Natural Science Foundation of China (Nos. 51572703, 51477096), and State Key Laboratory of Electrical Insulation and Power Equipment (EIPE17206). The authors thank Dr. Jiandong Wu for space charge measurement.

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TOC

Polypropylene based nanocomposites with voltage stabilizer functionalized nanoparticles can withstand high dc electrical stress and suppress space charges, showing promising potential for recyclable high voltage direct current cable insulation.

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Recyclable Dielectric Polymer Nanocomposites with Voltage Stabilizer

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