Full-life-cycle eco-friendly polymeric insulating materials: research progress and future prospects

Polymeric insulating materials is the basis of electric power system and has been widely employed in various electric power system apparatus. With the emergence of net-zero carbon emission policies by 2050–2060, the eco-friendly polymeric insulation is urgent and promising in the R&D of advanced dielectric materials. This paper reviews the current progress of eco-friendly upgrade in each lifecycle stages of polymeric insulating materials, i.e. raw material, fabricating, operating, and retiring. A series of interesting and fundamental results have been summarized. Drawbacks of the current researches are discussed, and outlooks are provided for the future development of eco-friendly polymeric insulating materials. This paper is hoped to inspire some novel ideas for the development of advanced insulating materials suitable for the promotion of net-zero carbon emission technologies.


Introduction
To confront with the global warming and the increasing frequency of extreme climate, 'carbon peaking' and 'carbon * Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. neutrality' have become important demands of mankind [1], which put forwards new requirements for the eco-friendly developments of power and energy system. Aiming at this object, scholars and engineers are devoted to construct ecofriendly electric power systems by actively exploring ecofriendly power generation technologies, enhancing renewable energy connections, and developing eco-friendly power equipment. A lot of attention has been paid on the 'green', innovative system of power generation and transmission, e.g. elevating the power conversion efficiency of solar cells [2], optimizing the structure of wind turbines to increase efficiency [3], harvesting high altitude wind energy [4], developing battery systems that are more suitable for energy storage [5], designing more reasonable trading rules for power delivery [6]. Another important issue in the power system decarbonization is the eco-friendly modification of power equipment, for instance, the electrical insulating equipment. Current research on the eco-friendly electrical insulation mainly focus on the gas and liquid insulating materials [7], such as the use of mixed C 4 F 7 N gas, compressed air or even vacuum to replace SF 6 gas, which has an extremely high global warming potential (GWP, GWP SF6 = 23 800, whereas GWP CO2 = 1) [8]. As for the eco-friendly insulating liquids, various of natural ester oil has been developed as a sustainable alternate to mineral insulating oil, and has been employed in on-site high-voltage power transformers [9]. These achievements have significantly promoted the development of green energy and facilitate the realization of 2050 carbon-net-zero ambitions.
In contrast, the polymeric insulating materials has a relatively slow rate of the eco-friendly transformation. Currently, polymeric insulating materials play an important role in solid insulating materials in power systems, and they are dominated by thermosetting materials, e.g. epoxy resins (EPs), phenol-formaldehyde resins (PF), cross-linked polyethylene (XLPE), silicone rubber (SR), etc. Some thermoplastic materials, such as polyethylene terephthalate, polyvinyl chloride, and polypropylene (PP) are also involved. Due to the universality and massiveness of electric power system, the employment of polymeric insulation is also extremely huge. Taken EP as an example, it is estimated 300 000 tons of EP are used worldwidely for electrical insulation and electronic packaging in 2017 [10]. XLPE is also widely used in high-voltage cables. Up to 2021, the length of high-voltage cables of 66 kV and above in operation in China have exceeded 37 000 km [11]. These thermosetting solid insulating materials have excellent insulating strength, mechanical strength, and thermal stability. However, these materials also cause great pressure on the environment, which can be manifested in the following four aspects: (1) Aspect of raw material: Existing polymeric insulating materials are mainly produced by petroleum-based raw materials. However, these fossil-based resources are not sustainable and are likely to be mostly depleted within a few decades. The carbon dioxide emissions caused by fossil fuel resources using are also steadily increasing, which could cause irreversible damages to the environment and climate [12,13]. (2) Aspect of processing: Currently, thermosetting insulating materials are mainly processed by thermal (heat) curing, in which high temperature (100 • C-200 • C) is utilized. Despite its high technical matureness, heat-curing has many gaps such as high energy consumption, low efficiency, and easy to produce volatile organic compounds (VOCs) [14], which easily leads to heavy energy waste and serious air pollution problems.
(3) Aspect of operation: During their life-time of on-site operation, conventional polymeric insulating materials are susceptible to discharge failures due to the electric field concentration at electrode tip and electrode/insulation interface [15], which not only lead to early retirement of insulating components, but also cause disruption of power supply and huge economic losses. (4) Aspect of retirement: Due to the 3D networked molecular structure, thermosetting insulating materials are usually difficult to be degraded, melted, and dissolved, which has enormous difficulties of recycling after retirement. Most of these scrapped polymeric insulating materials is abandoned, landfilled or incinerated, which results in significant environmental pressure.
The above four aspects correspond to raw material, processing, operation and retirement, which composes the full life cycle of polymeric insulating materials. Thus, to completely solve the environmental modification of polymeric insulating materials, it is necessary to address these problems. Recently, some researchers have recognized the environmental issues of conventional polymeric insulating materials, and have achieved certain progresses in these four aspects. In this paper, we propose the concept of full-life-cycle eco-friendly polymeric insulation based on these four aspects, and discuss the current status and recent progress in the development of full-life-cycle eco-friendly polymeric insulating materials. A series of representative studies are summarized, gaps of current researches are discussed, and outlooks on the future development of full-life-cycle eco-friendly polymeric insulating materials is provided, which can provide a reference for the net-zero carbon emission policies of electric power systems. It should be noted that this article is mainly focused on technical methods. Some evaluation methods, such as the life cycle assessment (LCA), are not the focus of this article, although they are also important for the development of full-life-cycle eco-friendly polymeric insulating materials.

Full-life-cycle eco-friendly polymeric insulating materials
In the definition of systems engineering, the life cycle usually includes the typical phases of definition, design, industrialization (testing and manufacturing), distribution, operation and retirement (recycling) [16]. As figure 1 shows, for the fulllife-cycle eco-friendly polymeric insulating materials, its life cycle mainly includes the following four stages: raw materials, processing, operation, and retirement. In the aspect of raw materials, full-life-cycle eco-friendly polymeric insulating materials use renewable, easily degradable bio-based raw materials to replace the non-renewable petroleum-based raw materials used in conventional polymeric insulating materials. In processing, it uses energy saving, efficient light-curing method instead of the energy-intensive, inefficient heat-curing method. In operation, it uses highly reliable, long-life functionally graded insulation (FGI) instead of conventional insulation. In retirement, it uses the recyclable dynamic reversible network rather than the non-recyclable thermosetting crosslinking network. The following sections provide an overview on the progresses of these four aspects of the full-life-cycle eco-friendly polymeric insulating materials.
Meanwhile, since polymeric insulating materials are mainly used in support insulation, cables, and power electronics packaging, which require excellent electrical properties (including dielectric permittivity (ε r ), dielectric loss (tanδ), resistivity, breakdown strength, etc), thermal properties (including glass transition temperature, thermal conductivity, etc), and mechanical properties (including strength, modulus, creep resistance, etc) under the actual operation conditions. These properties need to be focused on and deeply investigated in the construction of full-life-cycle eco-friendly polymeric insulating materials. At present, most of the polymeric insulating components in electric power equipment are composed of thermosetting polymers such as PF, EP, XLPE and polyurethane (PU). Examples of typical polymeric insulating materials utilized in electrical power systems are listed in table 1 [17][18][19][20][21][22]. Although these materials have excellent electrical, mechanical, and thermal properties, their main raw materials are non-renewable petroleum-based materials. Consider the example of bisphenol A diglycidyl ether (DGEBA), its raw materials, bisphenol A and epichlorohydrin, are mainly prepared from phenol-acetone condensation and high temperature chlorination of propylene [23], which are derived from fossil resources such as coal, oil and gas, and some toxic substances can be produced in manufacturing process.
Compared to the petroleum-based materials, bio-based materials are made from renewable, biological resources such as commercial crops, agricultural waste, and bacterial/algal metabolites [24], etc, which have the significant advantages of being widely available (e.g. global production of lactic acid has reached 400 000 tons annually [25]), and highly renewable. Also, most bio-based polymers can be degraded or decomposed, which undoubtedly reduces the pressure on the environment. Therefore, the application prospects of bio-based materials are significantly greater than those of petroleum-based materials considering the implementation of carbon emission reduction policies and the growing global warming. Currently, the bio-based polymer materials toward commercial application are mainly thermoplastic products such as polylactic acid (PLA), poly (butyleneadipate-co-terephthalate), starch-based plastics, and polyhydroxyalkanoate, which are widely used in packaging, Table 1. Polymer insulation materials commonly used in the power industry.
textile, agriculture, automobile, electronic, medicine and other fields [26]. Meanwhile, the study of thermosetting bio-based polymers has also received a lot of attention [27]. Some studies have shown that biomass materials, including vegetable oils, cellulose and its derivatives, lignin and its derivatives, gallic acid, itaconic acid, tannin, cashew phenol, soy glycosides, sucrose, and terpenes (abietic acid and limonene), can be employed as raw materials for thermosetting EP, PF, and PU resins, making them eco-friendly and regenerable [28]. The dielectric, mechanical, and thermal properties of some biobased materials and their possible applications are introduced in following paragraphs. PLA, which is derived from corn, sorghum, and plant roots, is a bio-based material with good electrical performance. To assess the feasibility of PLA as a dielectric insulation, the volume resistivity, relative permittivity, and dielectric loss tanδ of PLA are measured in the work of Nakagawa et al [29]. They found that the insulating properties of PLA at room temperature is almost the same as that of XLPE, which is currently used for cables and electric wires. The volume resistivity (4.9-5.5 × 10 17 Ω·cm), dielectric constant (3-3.8), and tanδ (0.02-0.022) of PLA are slightly greater than those of XLPE. The average pulse breakdown strength of PLA is about 30% higher than XLPE, as shown in figure 2(a). However, due to its relatively low glass transition temperature (60 • C-80 • C), the thermal stability of PLA should be improved before it is used in actual electric insulating components.
Poly(butylene succinate), PBS, can be mass-produced from various renewable resources, such as cellulose and starch, and is a sustainable alternative to petrochemical-derived plastics, with good mechanical and thermal properties comparable to those of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and PP [30]. The melting temperature (115 • C) of PBS is between that of LDPE and HDPE, while the tensile strength (30 MPa) is slightly higher than that of both, as demonstrated in figure 2(b). However, it is still uncertain whether the insulating performance of PBS is comparable to that of polyethylene or PP materials.
The above two materials can represent the majority of thermoplastic bio-based polymers, and their insulating properties are comparable to some thermosetting insulating materials. However, the disadvantages of thermoplastic bio-based polymers are their relatively poor mechanical and thermal stability, which makes it difficult to replace the conventional thermosetting insulating materials.

Bio-based thermosetting polymers in polymeric insulation.
Compared to thermoplastic, the high breakdown intensity, mechanical strength, and glass transition temperature of thermosetting materials are the reasons why they currently occupy most of the insulating material market. Since they are difficult to be decomposed or degraded, some bio-based thermosetting polymers, e.g. those synthesized by epoxy acrylic (EA) soybean oil, eugenol, dehydroabietic acid, cashew nut husk tannin, and itaconic acid, are reported, and their dielectric properties are investigated. These materials are expected to replace conventional thermosetting materials in the future, and the dielectric properties of these bio-based materials will be introduced as follows: (1) EA soybean oil thermoset Soybean oil is a kind of vegetable oil that can alleviate the energy shortage, and has a wide range of resources and a low cost. EA soybean oil prepared from soybean oil and epoxy group has the characteristics of low viscosity, and its processability has been significantly elevated. Hong and Wool [31] synthesized a bio-based composite resin by employing EA soybean oil (figure 3(a)) and keratin feather fibers (figure 3(b)), which are derived from triglycerides (the main component of animal and vegetable oils) and discarded feathers. They found that the hollow keratin fibers in the composite result in a low relative permittivity (1.7-2.7), because a large amount of air is retained, and the thermal expansion coefficient of epoxy/fiber (67.4 ppm • C −1 ) is equivalent to that of polyimide in printed circuit boards when the feather fiber content is 30 wt%. Besides, the mechanical properties of the resin are improved, but the glass transition temperature of around 70 • C (figure 3(c)) may limit its wide application in electronic devices as an insulating material.
Additionally, bio-based resins prepared through 3D printing technology are obtained. Zhang et al [32] developed a novel bio-based material based on plant-based soybean oil acrylate, EP, and polysulfide rubber. The prepared 3D printing dual-cured resin has a tensile strength of 83 MPa and a flexural strength of 129 MPa, as depicted in figure 4(a). Its mechanical properties are superior to those of DGEBA EP and other commercial photocurable resins, and the recovery rate of this resin can reach 98% under heating conditions. Meanwhile, the dielectric constant (figure 4(b)) of 3D printed dual-cure resin samples at different frequencies is less than 4.3, and the tanδ (figure 4(c)) is less than 0.1, however, the relative permittivity and tanδ of the recycled resin samples are higher than those of the original dual-cured resin, which is because there are irreversible broken cross-linked chains in the recycled polymer network, showing higher interfacial polarization.
(2) Eugenol based thermoset Eugenol also comes from a wide variety of sources, which is mainly derived from natural plants, and is distilled from lilac flower buds. Li et al [33] introduced methylsiloxane and phenylsiloxane linkers with different chain lengths into   [33], (b) relative permittivity test result of epoxy resins polymerized with different epoxy monomers and cured by DDS [33]. Reprinted with permission from [33]. Copyright (2018) American Chemical Society. the molecular skeleton of eugenol-based derivatives, obtaining bio-based epoxy monomers named as SIEEP2, SIEEP4, and SIEPEP, and the molecular structures of SIEEP2, SIEEP4, and SIEPEP are shown in figure 5(a). The dielectric constant of the resin becomes smaller after adding silicone because the siloxane chain is hard to polarize under a strong electronic field [34]. The result indicates that the relative permittivity (<3.75 from 10 to 10 6 Hz) of the resin cured with 3,3 ′diaminodiphenyl sulfones (DDS) is lower than that of DGEBA EP, as illustrated in figure 5(b), and this kind of bio-based resins with excellent flame retardancy have the potential to be employed in power electronics packaging.
Besides, to obtain bio-based resins with high renewable carbon content, Miao et al [35] synthesized a bio-based epoxy monomer, trifunctional allyl compound tri (4-allyl-2-methoxyphenyl) phosphate (TEUP-EP) with 100% renewable carbon content by renewable eugenol, and the TEUP-EP was then blended with 4,4 ′ -diaminodiphenylmethane (DDM) to develop a new bio-based EP (TEUP-EP/DDM). The molecule structure of epoxy monomer TEUP-EP is depicted in figure 6(a). Contrasted with DGEBA/DDM, the T g of TEUP-EP/DDM increase by 25.7 • C, reaching 203.7 • C, and its flexural strength and storage modulus increase by 24.4% and 31.4%, respectively. It can be seen from figure 6(b) that its dielectric constant is lower than 3.7 at frequencies from 10 2 to 10 6 Hz.
Another eugenol based material is benzoxazine resin, which is an attractive alternative to conventional phenolic resin, EP, and polyimide due to its flame retardancy, chemical resistance, and excellent thermo-mechanical properties. Hence, to study the feasibility of this alternative, bio-based polybenzoxazines are synthesized using eugenol, furfurylamine (FBz), and stearylamine through solventless synthetic process routes and are further reinforced with varying percentages (1, 3, 5, and 10 wt%) functionalized bio-silica (FBS) to obtain composites [36]. The study results indicate that the T g  (figure 7(a)) of these composite materials is above 150 • C, the relative permittivity (figure 7(b)) is between 2 and 4.25, and the tanδ (figure 7(c)) is small, so that they also have the potential to be applied to microelectronic devices.
(3) Dehydroabietic acid based thermoset Dehydroabietic acid is a resin acid that isolated from rosin and has a tricyclic diterpene structure. Its internal hydrogenated phenanthrene ring structure is beneficial for hydrophobicity and thermal stability. Fu et al [37] utilized dehydroabietic acid to synthesize a rosin-based benzo cyclobutene monomer, and the benzo cyclobutene reaction mechanism is depicted in figure 8(a). The thermogravimetric analysis result demonstrates that the temperature at five percent mass loss reaches 406 • C, which means that the resin has perfect thermal stability. Meanwhile, due to the introduction of the hydrogenated phenanthrene ring structure, the resin has good hydrophobicity (water contact angle 106 • C) and dielectric properties; its average relative permittivity is 2.51 and the tanδ is less than 5 × 10 −3 at frequencies from 0.1 to 18 MHz at room temperature. It is worth mentioning that the T g of the resin reaches 261 • C (figure 8(b)), which is not available in conventional resins, and this benzo cyclobutene resin may be applied  [37]. Reproduced from [37] with permission from the Royal Society of Chemistry.
to the aerospace and microelectronics industries in the future because of its outstanding comprehensive properties.
(4) Cashew nut husk tannin based thermoset Cashew nut shell is a biomaterial waste obtained from the cashew industry, while tannin is a polyol that is composed of 80% of the cashew nut shell. Sunija et al [38] considered utilizing PU (numbered PU1 and PU2) prepared from cashew nut husk tannin as electronic packaging, in which PU1 is PU without extender and PU2 is PU in the presence of extender 1,4-butanediol. They also studied the effect of frequency and temperature on the electrical performance of PUs. They found that the dielectric loss factor (approximately 0.2 at 150 kHz and 80 • C) and AC conductivity of PU2 are greater than those of PU1. Even if the temperature reaches 150 • C, the relative permittivity ε r of PU2 is still lower than 6.5, and the dielectric loss is still lower than 0.7, which prove that PU2 have excellent electrical properties and are suitable for using as dielectric materials in laminates of printed circuit boards. Nevertheless, their mechanical strengths need to be improved, and the hygroscopic property needs to be weakened.
(5) Itaconic acid based thermoset Itaconic acid is one of the products of the citric acid distillation process, which has attracted a lot of attention recently, and the U.S. Department of Energy chose it as one of the top 12 biomass compounds. To assess the potential of itaconic acid thermoset to replace conventional insulating material, Liu's team prepared itaconic acid epoxy resin (EIA, figure 9(a)) and measured its dielectric parameters and tensile strength, as well as its ability to withstand electrical trees [39,40]. In contrast with DGEBA resin, EIA is slightly inferior in mechanical properties, thermal stability, and electrical strength. The test results show that the leakage current of EIA is about 13.4% higher than that of DGEBA EP, the breakdown strength of EIA is 12.5% lower than that of DGEBA, and the average tensile strength and flexural strength of the EIA are respectively 15.3% and 28.5% lower than those of DGEBA, as shown in figures 9(b)-(d). Simultaneously, the EIA resin has a low initiation voltage of the electrical tree (9.2 kV at 50 Hz AC voltage) due to its flexible long-chain molecular structure; however, its subsequent tree growth is slow, which can be found in figures 9(e) and (f). In summary, EIA is still qualified for insulation in high voltage systems and more excellent in degradable characteristics.

Summary.
In general, bio-based materials have become a major technology to deal with the depletion of petroleum and environmental pollution problems, and show great potential to replace petroleum-based materials in electrical insulation. The petroleum industry is, however, very developed, and from the extraction of crude oil to the processing of final chemicals, a large-scale and effective production system has formed, making it economically quite competitive. Another problem is how to select biomass material from diverse sources of natural raw materials and prepare insulating materials with excellent performance. Current bio-based materials are costly to produce and have poor thermal or mechanical properties, which need to be modified to improve their comprehensive performance and meet the requirements for electrical insulation.

Light-curing instead of heat-curing
2.2.1. Photocurable processing technology. Thermosetting insulating materials are widely used in electrical power equipment due to the excellent electrical, thermal, and mechanical properties. The EP used in supporting insulators and bushings [41], the XLPE used in high-voltage cables [42], and the SR used in outdoor composite insulation [43] are all thermosetting materials. Conventionally, high temperature environment is utilized to activate the cross-linking reaction during the processing thermosetting insulating materials. Common polymeric insulating materials, such as EP [44], XLPE [45] and SR [46] are all fabricated by heat curing: (1) As for the heat-curing of EP, methyl hexahydrophthalic anhydride is widely used as the curing agent in the EP of electrical and electronic devices [47]. However, this agent usually requires a high reaction temperature (>200 • C) without catalyst. Even if a catalyst (such as the accelerator 2,4,6-tris-(dimethylaminomethyl) phenol, DMP-30) is used, a curing temperature of ∼140 • C is still needed [48]. In addition, curing process of EP in the industrial situations usually needs to  [39]. (e) and (f) Electrical trees of DGEBA and EIA resins [39,40]. Reprinted with permission from [40]. Reprinted with permission from [40]. go through two stages (pre-curing and post-curing) to regulate the curing rate and release the residual thermal stress. Each stage needs the temperature >100 • C and takes 10-20 h [48].
(2) As for the thermal cross-linking of XLPE, usually some peroxide added to PE as the crosslinking agent, requires high temperatures to decompose, thus forming radicals to initiate the cross-linking reaction. Backens et al [49] suggested that the curing process should be conducted at 200 • C for 45 min to obtain the XLPE with good properties. This crosslinking process requires a relatively high temperature and a long time.
(3) As for the high temperature vulcanization of SR, it also needs to be heated to 150 • C and kept for 2 h in processing [50].
From the above discussions, it can be seen that the heatcuring process of typical polymeric insulating materials usually requires large amount of energy and takes a long time, reducing the financial and temporal efficiency of the processing stage. Moreover, sometimes the heat-curing process also cause strong air pollutions due the releasement of VOCs [51].
These issues can be solved by the introduction of lightcuring (or photo-curing) technology. Similar to heat-curing, the light-curing is the polymerization process of macromolecular materials. The difference is that photon radiation (with 250-420 nm wavelengths) rather than elevated temperature is used to trigger the polymerization reactions. The main components of light-curing polymers are oligomers, reactive monomers (used to reduce viscosity if required), and photoinitiators (PIs). As shown in figure 10(a), the PI absorbs incident photons and generates reactive intermediates such as free radicals or cations. These intermediates can initialize the chain polymerization reactions of oligomers or reactive monomers, resulting in a rapid increase in molecular weight and the formation of a cross-linking network in the solidified material [52]. Up to now, light-curing has been applied to manufacture a variety of polymeric materials in coatings, adhesives, inks, microelectronics and dental fillings. It is also a main fabrication technique in the emerging field of additive manufacturing (3D printing) [53]. As figure 10(b) shows, comparing to heat-curing materials, light-curing materials have the following unique technical advantages: (1) efficient, the curing efficiency of light-curing is high, the liquid-solid transition only takes a few seconds to a few minutes [54]. (2) Enabling, the applicability of light-curing is wide, which can be used in coating, bonding, 3D printing and many other fields [55]. (3) Economical, the manufacturing time of light-curing for actual components is greatly reduced, the production efficiency is high [56]. (4) Energy saving, the light-curing has mild reaction conditions and room temperature curing ability [57]. (5) Eco-friendly, the light-curing produces almost zero VOC and has low air pollution [58].

Light-curing materials.
The significant advantages and promising prospects of light-curing technology have also led to the study of its application in electrical insulation. In the early stages, researchers mainly focused on the electrical properties of commercial photocurable resins and purely photosensitive polymer. Bouanga et al [59] prepared a light-curing resin containing alicyclic epoxy monomers, glycidyl ether reactive diluents, and cationic photoinitiators, and characterized its glass transition temperature, dielectric spectra, breakdown field strength, and space charge distribution. This material has good insulating properties with breakdown strengths of more than 48 kV mm −1 (the sample thickness is about 0.7 mm). Liu et al [60] synthesized a novel ultraviolet (UV)curable EP modified with cholic acid and glycidyl methacrylate, which has high glass transition temperature (144 • C) and low dielectric permittivity (2.51) and is promising to apply for high-frequency dielectric packaging. Monzel et al [61] tested the breakdown field strength of various commercial light-curing resins, in which VeroClear resin reached a breakdown strength of 31.3 kV mm −1 (sample thickness 1 mm). Li et al [62] also indicated that stereolithographic (SLA) 3D printed components had smooth surfaces, dense structures, and high breakdown strength, which can be used to manufacture polymeric supporting insulating components.
As the research progressed, researchers began to construct light-cured composite insulating materials with excellent properties. Bian et al [63] prepared two UV-cured EA based composites filled with irregular alumina (i-Al 2 O 3 ) and spherical alumina (s-Al 2 O 3 ), respectively. The effect of alumina (Al 2 O 3 ) shapes on dielectric properties of composites were investigated, and the dielectric spectra test results show that compared with s-Al 2 O 3 /EA, i-Al 2 O 3 /EA have larger permittivity and lower dielectric loss. According to Weibull distribution, the results suggest that the composites of i-Al 2 O 3 /EA exhibited better breakdown performance than s-Al 2 O 3 /EA. Yang et al [64] constructed an SLA 3D printed meta-material structure with efficient and deterministic heat conduction through combining the 2D boron nitride (BN) with nanodiamond (DM) bridging. The research of thermal conductivity and dielectric properties exhibit that the nanosized diamondbridged and oriented 2D BN endows efficient heat transfer, and maintains low dielectric loss with low filler loading. The composites load with 19 wt% BN platelets and 1 wt% DM have the highest thermal conductivity of 3.687 W (m•K) −1 in the heat flow orientation, and the composite with 15 wt% BN and 5 wt% DM has a dielectric loss lower than 0.02. This study shows the latent capacity of light-curing materials for critical device component protection and heat administration applications in electronic devices and electric equipment. Li et al [65] fabricated barium titanate (BT) microspheres (BT-S) by radiofrequency thermal plasma spheroidization technology. It can be used as an excellent filler to improve the dielectric properties and stereolithography ability of high dielectric permittivity polymer matrix composites (high-k PMCs). The experimental results show that the prepared BT-S particles have spherical shape, smooth surface, large particle size, improved mechanical strength and higher purity, which lead to the enhancement in the 3D printability of UV curable high-k PMCs. The employment of BT-S significantly reduces the apparent viscosity and yield stress, and increases the UV curing depth by 542% maximum. It also increases the dielectric permittivity compared to the UV curable composite filled with irregular particle shape BaTiO 3 filler. This research promotes the usage of UV curable high-k PMCs in advanced electrical and electronic components. As figure 11 shows, Wang et al [66] constructed a polymeric insulating coating material based on fluorocarbon-modified light-curing resin, which can improve the insulating strength along the surface up to 170% through the suppression of secondary electron multiplication and surface charge accumulation. The analysis of these results shows that the changes in the energy band structure, charge trap parameters and migration properties of the fluorocarbon modified material are the key points to the performance improvement. To sum up, light-curing technology provides a new solution to the manufacturing processing in polymeric insulating materials, and the development of various light-curing insulating materials has also provided the basis for the application of light-curing in the processing of insulating components.

Light-curing applications.
Currently, light-curing technology has been studied and applied in photocurable support insulators and photo-crosslinked power cables, some typical results are summarized as follows.
(1) Photocurable support insulator Existing research believe that light-curing resin a promising insulating material for photocurable support insulator [67]. As figure 12(a) shows, Kurimoto et al [68] investigated the layer interface effect on the dielectric breakdown strength of 3D printed rubber insulators. They fabricated acrylic rubber insulators in different laminate directions using a SLA 3D printer, and evaluated the morphology of the layer interface and the surrounding hardness using microscopic and nano-indentation analyses. The breakdown test reveals that a layer interface with a 0.1 mm laminating pitch reducing the AC breakdown strength of the insulators, and the hardness test results show that the hardness between the layer interface and the other part is different. Therefore, it can be concluded that owing to insufficient curing around the layer interfaces, the difference in the material structure promote dielectric breakdown. Consequently, to minimize the layer interface effect on the breakdown strength, a thinner layer and 3D cylindrical printings is proposed. As figure 12(b) shows, Liu et al [69] used SLA method preparing the cylindrical insulators with sheds and truncated cone insulators. The flashover voltages (FOVs) of the insulators in a high vacuum environment (10 −4 Pa) were tested, and the effect of shape and heat treatment of insulators on the FOV were analyzed. The results show that the light curing technology can precisely manufacture insulators with different surface structures, and improve the insulators FOV in vacuum environment by designing microstructures on the surface. Qi et al [70] prepared a graphene/photosensitive resin composite insulator using an SLA 3D printing and constrained sacrificial layer device built by themselves. Compared to the uniform photosensitive resin, the dielectric permittivity is increased by about 100%, the resistivity is reduced by 93.3%, and the insulating strength along the surface is enhanced by more than 14%. This light-curing 3D printing process provides a low-cost and efficient solution for the preparation of high electrical properties polymer-based supporting insulators.
(2) Photo-crosslinked power cable In addition to photocurable insulators, photo-crosslinked power cables have also been widely studied and applied. Fu et al [71] studied the UV light-initiated crosslinking reaction mechanism and the electrical breakdown performance of XLPE. Figure 13(a) shows the representative UV-initiated crosslinking reactions of polyethylene molecules by using photon initiator and auxiliary crosslinking agent, which indicates that the auxiliary crosslinking agent in the form of monomers or homopolymers represents higher ability in the participation of forming polyethylene crosslinking nodes, and improving the photon-initiated crosslinking efficiency. The UV-initiated crosslinking process of polyethylene molecules is fulfilled, taking advantage of the UV transparency of LDPE fluid at the temperature higher than melting point in actual experiments and UV-XLPE productions (figure 13(b) Figure 13. Photo-crosslinked power cables. (a) Schematic of the representative UV-initiated crosslinking reactions of polyethylene molecules by using photon initiator and auxiliary crosslinking agent [71]. Reproduced from [71]. CC BY 4.0. (b) The industrial UV irradiation apparatus of photo-crosslinked power cables [72]. © [2009] IEEE. Reprinted, with permission, from [72]. (c) The breakdown strength of photo-crosslinked XLPE, which were composed of LDPE, 0.2 wt% photon initiators (benzophenone (BP) or 4-hydroxybenzophenone laurate (BPL)) and 3 wt% auxiliary crosslinking agent (triallyl isocyanurate (TAIC) or dioleyl-2,2-,4-,4-tetraallyl isocyanurate (STAIC)), and were named as XLPE-BP-TAIC and XLPE-BPL-STAIC [71]. Reproduced from [71]. CC BY 4.0. [72]). The test results (figure 13(c)) show that the breakdown strength of photo-crosslinked XLPE can reach 95 kV mm −1 (the sample thickness is about 0.1 mm). Zhao et al [73] successfully grafted maleic anhydride (MAH) onto polyethylene molecules through UV irradiation process, and manufacture graft-modified XLPE (XLPE-g-MAH) based on this method. Compared with pure XLPE, the remarkably suppressed space charge accumulations at high temperatures have been achieved in XLPE-g-MAH. The polar groups on the grafted MAH can provide deep traps in XLPE-g-MAH, which will increase charge injection barrier by forming a charged layer of Coulomb-potential screening near electrodes, and simultaneously reduce the electrical mobility of charge carriers by trapcarrier scattering, resulting in an appreciable suppression of space charge accumulations inside material. Zhang et al [74] developed the funcionalized-SiO 2 /XLPE nanocomposites by chemically grafting auxiliary crosslinkers trimethylolpropane triacrylate (TMPTA) onto nano-SiO 2 surfaces. UV-initiated polyethylene crosslinking reactions are efficiently stimulated by TMPTA grafted onto surfaces of SiO 2 nanofillers, averting thermal migrations out of polyethylene matrix. Since surfacemodified SiO 2 nanofillers evidently elongate the circuitous routes of electrical-tree growth to be restricted from directly developing toward ground electrode, the functionlized-SiO 2 /XLPE nanocomposite has larger fractal dimension and shorter length of electrical-trees compared with XLPE and neat-SiO 2 /XLPE nanocomposite. Polar-groups on the modified nano-SiO 2 surfaces inhibit electrical-tree growth and simultaneously introduce deep traps impeding charge injections, accounting for the significant improvements of electrical-tree resistance and dielectric breakdown strength. These studies show that as one type of the light-curing technology, photocrosslinking can be used in various ways to prepare XLPE with excellent electrical properties, which is promising to be used in high-voltage cables.

Summary.
Overall, the advent of light-curing materials and light-curing technologies have provided a solution to the high energy consumption, low efficiency, and high pollution problems in the processing of heat-curing in polymeric insulating components. Its research and application in photocrosslinked cables, photocurable insulators and photocurable insulating coatings have proved the feasibility of applications in the insulating materials processing. In addition, studies have shown that light-curing technology can be combined with bio-based materials to build light-curing bio-based polymers [75,76]. However, in practical applications, light-curing still have some disadvantages such as expensive cost and the comprehensive properties need to be improved, which need to be further researched and developed.

FGI with high reliability.
For the conventional uniform insulation, serious partial electric field concentration can be caused by the action of electrical, thermal, and mechanical stress, defects, and external contamination during the operation. For example, Yang et al [77] simulated the electric field distribution of insulators under the combined effect of electric-thermal-SF 6 fluid, and the results show that the insulator temperature and electric field distribution are substantially different from previous studies when the fluid behavior of SF 6 gas is considered. In particular, the post insulator above the high voltage (HV) conductor suffers from the concentration of high normal electric field, high tangential electric field, and high temperature near the metal shell side, which greatly threatens the long-term insulation performance of the insulator and increases the risk of flashover. Guo et al [78] found that the air gap generated at the metal conductor-insulator interface can distort the electric field at the top of the air gap, resulting in a partial electric field concentration, which is probably causing discharges and reducing the FOV of the insulator, and can even occurring the insulating failure of gas insulated transmission line (GIL). Liu et al [79] made a study focusing on the influence of fine metal particles on the insulating properties of outdoor insulators. The results reveal that the particle size smaller than 28 µm and particle amount larger than 40 mg in contact with the non-uniform distribution can cause a significant distortion and intensification of the electric field resulting in a higher risk of surface discharges leading to flashover.
In summary, the electric field stress concentration is a major factor leading to the failure of insulating components during operation [80], which makes uniform insulation has a low reliability. Thus, homogenizing the electric field stress distribution has become a key solution to enhance the reliability of high-voltage electrical equipment, extend the service life of the polymeric insulating materials. In recent years, functional graded materials have been widely used in aerospace, biomedical and automotive industries due to their unique characteristics and the advantage of significantly homogenizing the stress distribution [81]. To weak the electrical stress concentration of conventional uniform insulation, researchers have been inspired by functional graded materials, and proposed to construct FGI solving this problem. As figure 14 shows, its principle is similar to the bamboo in nature, bamboo timber obtains high mechanical strength through the non-uniform distribution of vascular bundle content. Unlike uniform structure, FGI has a non-uniform dielectric permittivity or conductivity distribution, which can be used to effectively homogenize the electric field of polymeric insulating through the optimized Figure 15. ε-FGI components (a) ε-FGI in electronic packaging. Specifically, it is the comparison of the structure, electric field and εr distribution under different packages [83]. © [2021] IEEE. Reprinted, with permission, from [83]. (b) ε-FGI insulator in gas insulated switchgears (GISs) and gas insulated transmission lines (GILs) (ε-FGI insulator used in 245 kV GIS and the flashover test results) [88]. © [2020] IEEE. Reprinted, with permission, from [88]. configuration of material dielectric parameters to improve the reliability and extend service life [79].

FGI in polymeric insulating material.
The research of FGI was initiated by Nagoya University in Japan 2006, Kato et al described the applicability of the FGI spacer to gas insulated power equipment, and presented a fabrication technique of the FGI spacer sample by use of centrifugal force. The flashover tests carried out under lightning impulse voltage applications show that the FOV increased about 26.5% at 0.1 MPa SF 6 [82]. After this time, the research associated with FGI has been widely noticed. According to the graded parameter (permittivity or conductivity), FGI can be mainly classified into ε-FGI and σ-FGI, which is introduced as follows.

ε-functionally graded insulation (ε-FGI).
ε-FGI is mainly used to solve the electrical stress concentration under AC or pulsed voltage. Diaham et al [83] presented an innovative way to build advanced FGI based on epoxy/SrTiO 3 composites tailored by electrophoresis for field grading in power electronics. As figure 15(a) shows, this method is applied in the context of power modules for DBC substrate encapsulation. The results show a significant mitigation of the electrical stress at the triple point since the increase of permittivity, while breakdown of FGI is increased by two times compared to neat epoxy. Nardi et al [84] synthesized a material exhibiting a gradient in permittivity through the application of an external magnetic field to a suspension of Fe 3 O 4 @TiO 2 nanoparticles in an epoxy matrix. It shows that the magnetic field not only induces the magnetophoretic motion of the particles but also causes their alignment in high aspect ratio structures. The combinations of these two effects give rise to graded nanocomposites exhibiting gradients in permittivity, which goes beyond the ones predicted for nanocomposites with homogeneously distributed and isotropic inclusions. Moreover, the simulations showed that this FGI can efficiently reduce the electrical field stress at the interface between the Figure 16. Pristine epoxy insulator and surface coated with ZnO/epoxy coating insulators doped with different concentrations, after being pre-charged with a voltage of +20 kV for 30 min, the surface potential and charge distribution were measured after dissipating for 0 min, 30 min, and 60 min, (a) insulator surface potential distribution; (b) insulator surface charge distribution [93]. © [2020] IEEE. Reprinted, with permission, from [93]. electrode and the insulator, maximum electric field reduction of more than 50%. Zhang et al [85] used 3D printing fabricated PP based FGI insulators with layered graded permittivity. The results show that this layered FGI insulator has an improvement on AC FOV, which is 17.5% higher than that of pure PP insulator (0.4 MPa SF 6 ). Du et al [86] fabricated continuously graded high permittivity layer by depositing BaTiO 3 on the insulator surface, and built an insulator with surface FGI structure. An iterative method was introduced to optimize the thickness distribution of the BaTiO 3 layer. The results show that comparing with the conventional insulator, the optimized surface FGI s-FGI insulator can significantly improve the uniformity of the electric field distribution along the insulator surface, which increases the AC FOV by about 20% in atmospheric air. Shen et al [87] developed low partial discharge (PD), high FOV BT/EP insulating composites with graded permittivity, which is assembled by the in situ electric field. The PD of the samples with 2.0 vol% BT and electric field treatment is significantly reduced, and the FOV is increased by 31.8% compared with the pure EP sample. Moreover, actual sized FGI components were also designed and manufactured. As figure 15(b) shows, Hayakawa et al [88] designed and fabricated a cone-type ε-FGI spacer samples with the relative permittivity ε r distribution between 10 and 4 for 245 kV gas insulated switchgear (GIS), and obtained the higher FOV than that of the spacer samples with uniform permittivity. Wang et al [89] designed a 126 kV disk-type FGI spacer with actual size. To manufacture this FGI insulator, a fabrication approach combining SLA 3D printing and vacuum casting was proposed, which provided an essential step toward the industrial application of FGI in GIS/GIL.

σ-functionally graded insulation (σ-FGI). σ-FGI
is mainly used to solve the electrical stress concentration under DC voltage. Li et al [90] designed a conductivity non-uniform insulator by a modified genetic algorithm, and fabricated it by using 3D printing. The DC surface flashovers of this insulator were tested in 0.1-0.3 MPa SF 6 [91] proposed a novel method for graded conductivity insulation using surface modification via argon (Ar + ) ion implantation. The flashover tests of the virgin, homogeneously ion-implanted, and graded ion-implanted samples are conducted in air and vacuum at negative DC voltage, and the FOV of samples with graded conductivity layers is improved by 27.71% in air and 28.90% in vacuum compared to the virgin samples. In addition, some studies have also constructed the new σ-FGI structures with charge and electric field adaptive ability by introducing nonlinear conductive materials (e.g. ZnO composite and SiC composite). Since these insulating materials can adapt their conductivity to the spatial electric field to avoid insulating failure due to electrical stress concentration, it can be called as 'self-adaptive dielectric' (SAD) [92]. For instance, as figure 16 shows, Wang et al [93] coated the nano ZnO/epoxy on DC insulators to promote surface charge dissipation and relieve the electric field concentration. The ZnO/epoxy coating introduced numerous shallow traps and promoted surface charge dissipation through its nonlinear conductivity. The test results showed that the FOV was increased by approximately 27% in atmospheric air. Li et al [94] designed a novel high-voltage direct-current (HVDC) spacer based on the concept of adaptively controlling surface charges using nonlinear materials, and investigated its control and regulation abilities of surface charge. The results show that the proposed spacer can restrict surface charge accumulation in the charge adaptive control region of the spacer. Doping the nonlinear material can facilitate surface charge decay in the charge adaptive control region when the electric field exceeds a threshold. The surface charge decay rate is determined to a large extent by the SiC doping weight ratio and the electric field strength. When the doping weight ratio reached 30% and the sample is under electric field of 6 kV mm −1 , the maximum surface potential value can be reduced to almost 50% of its original value. Yang et al [95] reported a polymer nanocomposite composed of a dielectric polymer embedded with aligned core-shell structured nanowires for highly efficient distributed electrostatic discharge protection. The dielectric nanocomposite is capable of self-adaptive charge release, stemming from the nonlinear interface built in the Bi/Co oxide coated ZnO nanowires that leads to a 'hand-in-hand' double-Schottky barrier. Through the alignment of the nanowires in a dielectric polymer, the resultant SAD nanocomposites exhibited reliable self-adaptive charge release properties at very low filler concentration, for example, 0.5 vol %. The low filling rate endowed the SAD nanocomposite with low relative permittivity (∼4) and high optical transmittance in the visible light wavelength range (75%), which is desirable in packaging materials and display coatings for portable electronics.

Summary.
As the research is further progressed, FGI has achieved better effects in relieving the electric stress concentration, improving the insulating strength, and enhancing the long-term operation reliability. In addition to the above mentioned studies, in recent years, a new FGI structure with both ε-FGI and σ-FGI [96], and a new FGI structure with both spatial graded insulation and surface graded insulation [97] have also been proposed. These new structures have extended the applicability of FGI components, which can improve the component insulating strength under DC, AC, and impulse voltages simultaneously.
In addition, through the efforts of many researchers, FGI is gradually getting into applications. As figure 17 shows, Lv [98] reported a 10 kV FGI switchgear insulator that satisfies the application standards of industrial electrical equipment and has been on-site operated in Anji, China, which has a higher PD initial voltage. It has already operated stably since October 2021. In general, the appearance of FGI provides a new technical concept and response to improve the electrical strength and extend the service life of insulating components. If introduce the FGI structure in bio-based light-curing insulating materials, it can effectively optimize the electrical stress distribution within and along the surface area of the component, further enhancing the comprehensive performance of fulllife-cycle eco-friendly insulating components, reducing the probability of early retirement, and extending the service life. However, FGI insulated components still have some problems such as complex manufacturing methods, which may cause some negative effects on the environment, and the mechanical and thermal properties of FGI also need to be further investigated. Thermosetting polymers occupy a large proportion of the polymeric insulating materials used in power equipment. The three-dimensional network structure composed of highly cross-linked oligomers gives excellent electrical properties, mechanical strengths, and thermal stability to polymers. Nonetheless, this network structure is the principal reason for polymers' difficulty in melting or dissolving, which makes them difficult to recycle and brings significant environmental pressure when the product fails or expires. Polymeric insulating materials produce a large amount of solid waste annually. For example, only in 2018, about 400 000 tons cable insulating materials were discarded in Western Europe [99]. In addition to incineration and landfill, the normal treatment and recycling methods of the waste thermosetting polymers include mechanical crushing, high-temperature pyrolysis, supercritical fluid degradation, acid degradation, oxidative degradation, electrochemical degradation, etc [100][101][102][103]. However, these methods have problems, such as harsh conditions, low product value, and the ability to easily cause secondary pollution. To address these problems, some teams have tried to improve the recycling and degradation methods of thermosetting polymers.

Recycling methods for conventional thermosetting
materials. Taking the waste EP as an example, Jiang et al [104] utilized methyl tetrahydrophthalic anhydride to cure the EIA EP and studied the hydrolysis of the cured product under alkaline conditions compared with two commercial EPs. The experimental results show that itaconic acid EP has good degradability, and it can be completely degraded by reflux in a 10% sodium hydroxide aqueous solution for 75 min. However, the author only investigates the degradation characteristics of EIA EP and does not explain its usage after degradation, and the degradation conditions are still harsh. In contrast, Zhao's team [105] successfully degraded EP under mild conditions, as shown in figure 18. Different from the previous studies, the 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30), which is usually utilized as an accelerator to decrease the curing temperature, is employed as the catalyst of the transesterification reaction to degrade EP. It is found that glutaric anhydridecured EP can be degraded with DMP-30 and ethylene glycol at 180 • C in a short time. Furthermore, the degraded resins can also be employed as raw materials in the preparation of new EPs. However, the maximal T g (<88 • C) and tensile strength (<66 MPa) of the resin prepared in their work are low; thereby, modification is still needed to improve its thermal and mechanical properties.
Whereas the conditions for polyolefin degradation are harsher than those for EPs. The study has shown that the siloxane bonds in XLPE can be broken in supercritical methanol at 300 • C-340 • C for 30 min [106]. Goto et al [100] proposed to improve the process of XLPE recycling via supercritical fluid and studied the selective decomposition of siloxane bonds to produce thermoplastic recycled PE. The appearance of the continuous process for recycling XLPE is illustrated in figure 19. The mechanical properties of recycled PE (approximately 25 MPa tensile strength and 630% elongation at break) satisfy the requirements for use as a wire and cable insulating material.

Intrinsically recyclable thermosetting materials.
With the development of chemistry and polymer science, a new type of dynamic chemical balance process-dynamic covalent bonds-has been discovered. Dynamic covalent bonding is a type of chemical bond that can exchange under certain stimulations (e.g. light, heat, PH stimulation, etc) [107]. Some polymer materials containing dynamic covalent bonds are called 'vitrimers'. Unlike conventional thermosetting materials, vitrimer has internal crosslinking points for reversible dissociation-reconfiguration, enabling easily network reconfiguration under the stimulation of catalysts, temperature, light, and mechanical forces, etc. Vitrimer not only maintains an intact cross-linking structure with excellent electrical, mechanical, and thermal properties, but also can be reprocessed many times, exhibiting repairable and recyclable abilities. Some high-performance, recyclable, and repairable vitrimers that are expected to be employed as insulating materials will be described in the following.
Liu et al [108] produced an epoxy monomer derived from eugenol, which was then reacted in various amounts with succinic anhydride to create vitrimers under the catalysis of zinc acetylacetonate hydrate. This vitrimer has shape memory and repair properties, can be completely decomposed at 160 • C Figure 19. (a) Decomposition reaction of the siloxane bond in supercritical alcohol, (b) appearance of the continuous process for recycling silane-XLPE by using supercritical alcohol and a twin-screw extruder [100]. Reprinted with permission from [100]. Copyright (2011) American Chemical Society. within 7 h, and re-obtained from the degraded polymers after heating 190 • C for 3 h, as depicted in figure 20. It can also can be recycled by physical and chemical methods. Meanwhile, the cracks on the surface of the material can be repaired at high temperatures based on the transesterification reaction mechanism, whereas the low T g (only 53 • C-58 • C) of this bio-based EP vitrimer limits its further application.
A high-performance EP vitrimer has also been prepared in [109]. Its thermal and mechanical properties were compared with those of commercial resins in the aviation industry. Firstly, Bisphenol F diglycidyl ether EPs, N,N,N ′ ,N ′ -tetraglycidyl-4,4 ′ -methylenebisbenzonamine, and 4-aminophenyl disulfide are blended and cured at a high temperature to obtain a disulfide-containing epoxy vitrimer with a high T g (175 • C), as shown in figure 21(a). Moreover, figure 21(b) illustrates that this resin vitrimer can be reprocessed at 200 • C by physical hot pressing, and it is certain that the vitrimer does not decompose at this temperature. Mechanical properties test results show that this vitrimer is comparable to commercial EP HexFlow ® RTM6 in tensile strength, while the bending strength of the vitrimer is slightly less than the latter. In a word, this aerospace-grade epoxy vitrimer has excellent mechanical and thermal properties, but whether it can be applied to the power or microelectronics industries remains to be verified.
For PE materials, Ji et al [110] successfully prepared HDPE vitrimers by reactive blending with a twin-screw microextruder, and the synthesis route is shown in figure 22(a). Typically, glycidyl methacrylate-grafted HDPE (HDPE-GMA) and hydroxyl-terminated polytetrahydrofuran or polycaprolactone were mixed in a twin-screw microextruder, after which 1,5,7-triazabicyclo [4.4.0]dec-5-ene and phenothiazine were added. The HDPE blends were cured at 180 • C for 1 h by hot pressing, and the obtained HDPE vitrimers exhibit outstanding reprocessability enabled by the transesterification reaction occurring in the network, the highest T g of which can reach 120 • C. Notwithstanding, its T g varies widely, and the lowest T g is only 20 • C, indicating the thermal stability of this HDPE vitrimer needs to be improved. The mechanical properties of PE vitrimer after several recyclings were investigated by Montoya et al. They [111] used 4,4 ′ -dithiodianiline to dynamically crosslink with maleic anhydride-grafted polyethylene, preparing XLPE vitrimers that can be reprocessed several times. As figures 22(b) and (c) show, the mechanical test result shows that the yield strength of vitrimer marginally declines after the second reprocessing cycle; however, this property remained constant after the third reprocessing cycle, which demonstrates the robustness of the dynamic cross-linker.
It should be noted that the processing of the vitrimers mentioned above is mainly using high temperature heat-curing  method. The high temperature is related to the processing of conventional insulating materials, and it is mainly used to initiate cross-linking reactions in polymers. This high temperature curing is not related to the introduction of dynamic covalent bonds and is not necessary for the processing of vitrimers. Currently, some vitrimers have been fabricated using ecofriendly light-curing techniques [112,113]. This makes vitrimer more promising to be used for constructing the full-lifecycle eco-friendly polymeric insulating materials.

Recyclable thermosetting polymers in polymeric insu-
lating. Vitrimer's repairability prolongs its service life, and the material can be reused through some external stimuli due to the unique exchange properties of dynamic covalent bonds. Researchers have started to investigate the repairable and recyclable abilities of the vitrimers used as electrical insulating materials. For minor damage, vitrimers can be repaired to restore their insulating properties and return to service [114]. When the vitrimer suffers extensive electrical damage that is difficult to repair, it can be recycled, thereby alleviating the environmental pressure caused by the waste solid insulating material.
Sun et al [115] developed a new epoxy network with electrical breakdown healing ability for power equipment insulation. The dynamic networks are generated by amineterminated, ring-opened epoxies and bi-functional isocyanates, which endow good reprocessing and healing abilities, as shown in figure 23(a). To test the ability to repair electrical damage, the electrical breakdown hole damage was healed for 10 min at different temperatures (160 • C, 180 • C, and 200 • C) and pressures (1 MPa, 3 MPa, and 5 MPa). The figure 23(b) demonstrates that the thru-hole caused by electrical breakdown can be repaired after healing within 10 min. A high healing efficiency of about 90% based on breakdown strength is observed. With temperature and pressure stimulation, the network dissociated reversibly to rejoin the separated surface of the breakdown holes. Meanwhile, flowing chain segments squeeze and isolate the carbon residue generated by electrical discharge. Additionally, less carbon residue is found after electrical breakdown, and better healing abilities on morphology and insulating properties are found when DGEBA EP is substituted with a lower ratio between carbon and hydrogen elements (C/H ratio) of hydrogenated EP.
Xie et al [116] reported a self-healing polymer that can be reversibly switched between the glassy state and the highelastic state, as enabled by the coexistence of reversible and permanent crosslinking sites in the synthesized dynamic crosslinker. This reversibly convertible molecular network is primarily composed of curing agents containing reversible Diels-Alder bonds and DGEBA EP, as illustrated in figure 24(a). The synthesis of a dynamic crosslinker containing a reversible Diels-Alder bond and three extra reactive groups that can be permanently bonded to the epoxy molecular chain. The reversible bond in the dynamic crosslinker was introduced by the Diels-Alder reaction of 3-furoic acid and maleic anhydride, which have one and two reaction sites with the epoxy molecular chain, respectively. This design allows for the complete repair of internal defects multiple times while maintaining the dimensional stability of mechanically robust polymers during the healing process. Furthermore, this selfhealing material is readily prepared by curing the commercially available epoxy monomer with the synthesized dynamic crosslinker, which endows this approach with great potential for industrial applications. The results show that for the electrical tree inception voltage, after three aging-healing cycles, the self-healing sample still recovered to 92% of the initial value; even after five aging-healing cycles, the tree inception  [115]. Reprinted from [115], Copyright (2022), with permission from Elsevier. voltage of the self-healing sample remained at 82% of the initial value ( figure 24(b)). This method can restore the ability of the damaged region to withstand electrical stress. However, both methods of Sun and Xie cannot eliminate the carbon residue caused by insulating failure, which is the main reason for the decrease in breakdown strength of the repaired polymeric insulating materials.
In addition to the epoxy system vitrimer, researchers also studied the insulating properties of other systems vitrimer [117]. Zhao et al [118] successfully obtained LDPE vitrimers cross-linked by β-hydroxy ester bonds via reactive processing between polyethylene-co-glycidyl methacrylate (PE-GMA) and decanedioic acid. The PE vitrimer they reported exhibits an excellent tensile strength of 20.41 MPa and an elongation at break of 800.16%; however, the mechanical properties of the recycled PE were reduced by more than 50%. In addition, PE vitrimer has good creep resistance, heat resistance, and electrical insulating properties, whose magnitude order of AC conductivity is 10 -15 S cm −1 at 0.01 Hz, hence demonstrating a high potential for usage in the field of cable insulation. Sun et al [119] developed a fluorine-substituted healable silicone elastomer as an actuator dielectric material. This silicone possessed an electrical breakdown strength >65 kV mm −1 (the thickness of the samples is 0.1 mm) and an intrinsic permittivity >4.3 at 50 Hz. Through fluorinestimulated carbamate bond exchange, the networks of reprocessed silicones can be recovered with 80% of its mechanical and insulating properties. Results have shown that the electrical breakdown damage can be repaired with 90% efficiency. Wan et al [120] designed a smart dielectric film which crosslinked by the low-molecular-weight polyimide gene unit and polyimide ligase. Due to the variability of gene unit and ligase combinations, the polyimide films combining hardness with softness are designed into three forms via a 'Mimosa-like' bionic strategy to adapt to different application scenarios. The films have good degradation efficiency, excellent recyclability, and can be self-healable, which makes them can be reused. Dai et al developed a triboelectric nanogenerator (TENG) that can effectively promote self-healing by absorbing infrared thermal radiation from human skin [121]. The TENG consists of three Figure 24. Characterization of the self-healing epoxy resin, (a) cleavage and reconstruction of the dynamic Diels-Alder bond in the self-healing epoxy resin molecular chain, (b) the optical images of the self-healing samples and normal epoxy samples before and after healing. (c) The apparent discharge magnitudes and tree inception voltages of the self-healing samples with small tree defects during the multiple aging-healing cycles [116]. Reproduced from [116] with permission from the Royal Society of Chemistry. layers; its insulating layer consists of a polysiloxane material containing imine bonds, which can reconstruct the network due to the synergistic effect of imine bonds and hydrogen bonds, allowing the damage to recover rapidly.

Summary.
The recycling of traditional thermosetting insulating materials still faces some challenges, but with the improvement of chemical processing technology, the possibility of waste conversion into high-value raw materials is gradually increasing. Vitrimers have a lot of potential for the future development of polymeric insulating because the appearance and wide research of dynamic covalent bonds have been provided for the recovery, repair, and re-use of electrical insulating materials in the retirement stage. While the recovery of mechanical properties is still the main focus of current research on dynamic covalent bonds, electrical properties receive less attention. Whether the introduction of a dynamic covalent structure will cause a change or even deterioration of insulating properties needs to be focused on in the subsequent research.

Outlook
The construction of full-life-cycle eco-friendly polymeric insulating materials is an effective method that is promising to completely address the eco-friendly transformation of polymeric insulating materials. However, it is worth noting that the present technologies still have some distances from fully realizing the construction, and they still have the rooms for further development. In this chapter, several shortcomings of the current studies are discussed, and the targeted outlooks are presented.

About bio-based insulating materials
Different from the petroleum-based raw materials, bio-based raw materials are obtained from a variety of sources, and they also have more complex structures. To obtain the bio-based polymers with excellent electrical, thermal and mechanical properties for solid insulating applications, the selection and modification of bio-based raw materials with different properties still needs to be further investigated. On the one hand, it is crucial to build high-throughput experimental techniques to comprehensively characterize the performance of bio-based insulating materials, increasing the efficiency in the research process. On the other hand, molecular simulation combined with machine learning can be used to guide the selection and modification of bio-based materials, further increase the research efficiency. Just as this method has achieved in the field of drug design [122].

About light-curing insulating materials
Up to now, light-curing technology has been widely used in aerospace, automobile, medicine and other industry fields. Its ability in the preparation of insulating materials have been experimentally verified. However, for the actual electrical insulating components, there are still some problems need to be solved such as the difficulties of printing composite insulating materials or large insulating components, and the metal inserts cannot be easily installed. To solve these issues, the following solutions can be considered, including using larger light source, adopting a segmented light-curing and splicing manufacturing method, reducing the ability of fillers to scatter curing light and trying to cure insulating materials directly on the metal inserts.

About functionally graded insulating materials
FGI structures can significantly improve the electrical strength of insulating components, reduce the probability of insulating accidents, and extend the service life of equipment. However, compared to the conventional homogeneous insulating structures, FGI components are usually composed of multiple composites, which introduce many interfaces. Unfortunately, electrical, thermal, and mechanical stresses usually tend to be concentrated at the interfaces [123,124]. There is still a lack of research on the effects of interfaces on the mechanical, thermal, and long-term operational stability of FGI components. Moreover, the manufacturing methods of FGI components are often more complex than conventional insulating components, which may lead to some environmental problems such as increased energy costs and reduced manufacturing efficiency. Therefore, in the subsequent research, while verifying the improved insulating strength of FGI, more in-depth research on its comprehensive performance (especially in the interfacial region) and simplification of the manufacturing process should be carried out [89,125].

About vitrimer-based recyclable insulating materials
Since Lieber's report in 2011 [126], vitrimer has obtained widespread attentions due to the unique advantages such as recyclability, repairability, and reshapeability. However, research on vitrimers used in electrical insulation is still in the initial stage. How to ensure the performance of reused vitrimers after recycling or reparation still needs to be studied in depth. In addition, insulating failures often generate the impurities such as residual carbons, which will greatly affect the insulating properties of reused vitrimer if they are not cleaned up during recovery or repairation. Thus, a technology that can clean up the impurities introduced by electrical damage while repairing or recovery should also be developed.

About the application of full-life-cycle eco-friendly insulating materials
Despite the unsolved issues in the four aspects (raw materials, processing, operation and recycling) of full-life-cycle eco-friendly polymeric insulating materials, the integration of these four aspects to form a complete material system is also an important issue. This not only requires the collaborative research contribution on solving the technical issues, but also needs effective and quantitative evaluation on the environmental impact of the above techniques. Some evaluation tools, such as LCA [127] and carbon foot printing [128], should be customized for the eco-friendly insulating materials.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).